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April, 26th. 2006:
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GCC
Section: GNU (1) Updated: 2016-06-03 Index
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NAME
gcc - GNU project C and C++ compiler
SYNOPSIS
gcc [ -c| -S| -E] [ -std=standard]
[ -g] [ -pg] [ -Olevel]
[ -Wwarn...] [ -Wpedantic]
[ -Idir...] [ -Ldir...]
[ -Dmacro[= defn]...] [ -Umacro]
[ -foption...] [ -mmachine-option...]
[ -o outfile] [@ file] infile...
Only the most useful options are listed here; see below for the
remainder. g++ accepts mostly the same options as gcc.
DESCRIPTION
When you invoke GCC, it normally does preprocessing, compilation,
assembly and linking. The ``overall options'' allow you to stop this
process at an intermediate stage. For example, the -c option
says not to run the linker. Then the output consists of object files
output by the assembler.
Other options are passed on to one stage of processing. Some options
control the preprocessor and others the compiler itself. Yet other
options control the assembler and linker; most of these are not
documented here, since you rarely need to use any of them.
Most of the command-line options that you can use with GCC are useful
for C programs; when an option is only useful with another language
(usually C++), the explanation says so explicitly. If the description
for a particular option does not mention a source language, you can use
that option with all supported languages.
The gcc program accepts options and file names as operands. Many
options have multi-letter names; therefore multiple single-letter options
may not be grouped: -dv is very different from -d -v.
You can mix options and other arguments. For the most part, the order
you use doesn't matter. Order does matter when you use several
options of the same kind; for example, if you specify -L more
than once, the directories are searched in the order specified. Also,
the placement of the -l option is significant.
Many options have long names starting with -f or with
-W---for example,
-fmove-loop-invariants, -Wformat and so on. Most of
these have both positive and negative forms; the negative form of
-ffoo is -fno-foo. This manual documents
only one of these two forms, whichever one is not the default.
OPTIONS
Option Summary
Here is a summary of all the options, grouped by type. Explanations are
in the following sections.
- Overall Options
-
-c -S -E -o file -no-canonical-prefixes
-pipe -pass-exit-codes
-x language -v -### --help[=class[,...]] --target-help
--version -wrapper @file -fplugin=file -fplugin-arg-name=arg
-fdump-ada-spec[-slim] -fada-spec-parent=unit -fdump-go-spec=file
- C Language Options
-
-ansi -std=standard -fgnu89-inline
-aux-info filename -fallow-parameterless-variadic-functions
-fno-asm -fno-builtin -fno-builtin-function
-fhosted -ffreestanding -fopenacc -fopenmp -fopenmp-simd
-fms-extensions -fplan9-extensions -trigraphs -traditional -traditional-cpp
-fallow-single-precision -fcond-mismatch -flax-vector-conversions
-fsigned-bitfields -fsigned-char
-funsigned-bitfields -funsigned-char
- C++ Language Options
-
-fabi-version=n -fno-access-control -fcheck-new
-fconstexpr-depth=n -ffriend-injection
-fno-elide-constructors
-fno-enforce-eh-specs
-ffor-scope -fno-for-scope -fno-gnu-keywords
-fno-implicit-templates
-fno-implicit-inline-templates
-fno-implement-inlines -fms-extensions
-fno-nonansi-builtins -fnothrow-opt -fno-operator-names
-fno-optional-diags -fpermissive
-fno-pretty-templates
-frepo -fno-rtti -fsized-deallocation
-fstats -ftemplate-backtrace-limit=n
-ftemplate-depth=n
-fno-threadsafe-statics -fuse-cxa-atexit
-fno-weak -nostdinc++
-fvisibility-inlines-hidden
-fvtable-verify=[std|preinit|none]
-fvtv-counts -fvtv-debug
-fvisibility-ms-compat
-fext-numeric-literals
-Wabi=n -Wabi-tag -Wconversion-null -Wctor-dtor-privacy
-Wdelete-non-virtual-dtor -Wliteral-suffix -Wnarrowing
-Wnoexcept -Wnon-virtual-dtor -Wreorder
-Weffc++ -Wstrict-null-sentinel
-Wno-non-template-friend -Wold-style-cast
-Woverloaded-virtual -Wno-pmf-conversions
-Wsign-promo
- Objective-C and Objective-C++ Language Options
-
-fconstant-string-class=class-name
-fgnu-runtime -fnext-runtime
-fno-nil-receivers
-fobjc-abi-version=n
-fobjc-call-cxx-cdtors
-fobjc-direct-dispatch
-fobjc-exceptions
-fobjc-gc
-fobjc-nilcheck
-fobjc-std=objc1
-fno-local-ivars
-fivar-visibility=[public|protected|private|package]
-freplace-objc-classes
-fzero-link
-gen-decls
-Wassign-intercept
-Wno-protocol -Wselector
-Wstrict-selector-match
-Wundeclared-selector
- Language Independent Options
-
-fmessage-length=n
-fdiagnostics-show-location=[once|every-line]
-fdiagnostics-color=[auto|never|always]
-fno-diagnostics-show-option -fno-diagnostics-show-caret
- Warning Options
-
-fsyntax-only -fmax-errors=n -Wpedantic
-pedantic-errors
-w -Wextra -Wall -Waddress -Waggregate-return
-Waggressive-loop-optimizations -Warray-bounds -Warray-bounds=n
-Wbool-compare
-Wno-attributes -Wno-builtin-macro-redefined
-Wc90-c99-compat -Wc99-c11-compat
-Wc++-compat -Wc++11-compat -Wc++14-compat -Wcast-align -Wcast-qual
-Wchar-subscripts -Wclobbered -Wcomment -Wconditionally-supported
-Wconversion -Wcoverage-mismatch -Wdate-time -Wdelete-incomplete -Wno-cpp
-Wno-deprecated -Wno-deprecated-declarations -Wno-designated-init
-Wdisabled-optimization
-Wno-discarded-qualifiers -Wno-discarded-array-qualifiers
-Wno-div-by-zero -Wdouble-promotion -Wempty-body -Wenum-compare
-Wno-endif-labels -Werror -Werror=*
-Wfatal-errors -Wfloat-equal -Wformat -Wformat=2
-Wno-format-contains-nul -Wno-format-extra-args -Wformat-nonliteral
-Wformat-security -Wformat-signedness -Wformat-y2k
-Wframe-larger-than=len -Wno-free-nonheap-object -Wjump-misses-init
-Wignored-qualifiers -Wincompatible-pointer-types
-Wimplicit -Wimplicit-function-declaration -Wimplicit-int
-Winit-self -Winline -Wno-int-conversion
-Wno-int-to-pointer-cast -Wno-invalid-offsetof
-Winvalid-pch -Wlarger-than=len -Wunsafe-loop-optimizations
-Wlogical-op -Wlogical-not-parentheses -Wlong-long
-Wmain -Wmaybe-uninitialized -Wmemset-transposed-args -Wmissing-braces
-Wmissing-field-initializers -Wmissing-include-dirs
-Wno-multichar -Wnonnull -Wnormalized=[none|id|nfc|nfkc]
-Wodr -Wno-overflow -Wopenmp-simd
-Woverlength-strings -Wpacked -Wpacked-bitfield-compat -Wpadded
-Wparentheses -Wpedantic-ms-format -Wno-pedantic-ms-format
-Wpointer-arith -Wno-pointer-to-int-cast
-Wredundant-decls -Wno-return-local-addr
-Wreturn-type -Wsequence-point -Wshadow -Wno-shadow-ivar
-Wshift-count-negative -Wshift-count-overflow
-Wsign-compare -Wsign-conversion -Wfloat-conversion
-Wsizeof-pointer-memaccess -Wsizeof-array-argument
-Wstack-protector -Wstack-usage=len -Wstrict-aliasing
-Wstrict-aliasing=n -Wstrict-overflow -Wstrict-overflow=n
-Wsuggest-attribute=[pure|const|noreturn|format]
-Wsuggest-final-types -Wsuggest-final-methods -Wsuggest-override
-Wmissing-format-attribute
-Wswitch -Wswitch-default -Wswitch-enum -Wswitch-bool -Wsync-nand
-Wsystem-headers -Wtrampolines -Wtrigraphs -Wtype-limits -Wundef
-Wuninitialized -Wunknown-pragmas -Wno-pragmas
-Wunsuffixed-float-constants -Wunused -Wunused-function
-Wunused-label -Wunused-local-typedefs -Wunused-parameter
-Wno-unused-result -Wunused-value -Wunused-variable
-Wunused-but-set-parameter -Wunused-but-set-variable
-Wuseless-cast -Wvariadic-macros -Wvector-operation-performance
-Wvla -Wvolatile-register-var -Wwrite-strings
-Wzero-as-null-pointer-constant
- C and Objective-C-only Warning Options
-
-Wbad-function-cast -Wmissing-declarations
-Wmissing-parameter-type -Wmissing-prototypes -Wnested-externs
-Wold-style-declaration -Wold-style-definition
-Wstrict-prototypes -Wtraditional -Wtraditional-conversion
-Wdeclaration-after-statement -Wpointer-sign
- Debugging Options
-
-dletters -dumpspecs -dumpmachine -dumpversion
-fsanitize=style -fsanitize-recover -fsanitize-recover=style
-fasan-shadow-offset=number -fsanitize-undefined-trap-on-error
-fcheck-pointer-bounds -fchkp-check-incomplete-type
-fchkp-first-field-has-own-bounds -fchkp-narrow-bounds
-fchkp-narrow-to-innermost-array -fchkp-optimize
-fchkp-use-fast-string-functions -fchkp-use-nochk-string-functions
-fchkp-use-static-bounds -fchkp-use-static-const-bounds
-fchkp-treat-zero-dynamic-size-as-infinite -fchkp-check-read
-fchkp-check-read -fchkp-check-write -fchkp-store-bounds
-fchkp-instrument-calls -fchkp-instrument-marked-only
-fchkp-use-wrappers
-fdbg-cnt-list -fdbg-cnt=counter-value-list
-fdisable-ipa-pass_name
-fdisable-rtl-pass_name
-fdisable-rtl-pass-name=range-list
-fdisable-tree-pass_name
-fdisable-tree-pass-name=range-list
-fdump-noaddr -fdump-unnumbered -fdump-unnumbered-links
-fdump-translation-unit[-n]
-fdump-class-hierarchy[-n]
-fdump-ipa-all -fdump-ipa-cgraph -fdump-ipa-inline
-fdump-passes
-fdump-statistics
-fdump-tree-all
-fdump-tree-original[-n]
-fdump-tree-optimized[-n]
-fdump-tree-cfg -fdump-tree-alias
-fdump-tree-ch
-fdump-tree-ssa[-n] -fdump-tree-pre[-n]
-fdump-tree-ccp[-n] -fdump-tree-dce[-n]
-fdump-tree-gimple[-raw]
-fdump-tree-dom[-n]
-fdump-tree-dse[-n]
-fdump-tree-phiprop[-n]
-fdump-tree-phiopt[-n]
-fdump-tree-forwprop[-n]
-fdump-tree-copyrename[-n]
-fdump-tree-nrv -fdump-tree-vect
-fdump-tree-sink
-fdump-tree-sra[-n]
-fdump-tree-forwprop[-n]
-fdump-tree-fre[-n]
-fdump-tree-vtable-verify
-fdump-tree-vrp[-n]
-fdump-tree-storeccp[-n]
-fdump-final-insns=file
-fcompare-debug[=opts] -fcompare-debug-second
-feliminate-dwarf2-dups -fno-eliminate-unused-debug-types
-feliminate-unused-debug-symbols -femit-class-debug-always
-fenable-kind-pass
-fenable-kind-pass=range-list
-fdebug-types-section -fmem-report-wpa
-fmem-report -fpre-ipa-mem-report -fpost-ipa-mem-report -fprofile-arcs
-fopt-info
-fopt-info-options[=file]
-frandom-seed=string -fsched-verbose=n
-fsel-sched-verbose -fsel-sched-dump-cfg -fsel-sched-pipelining-verbose
-fstack-usage -ftest-coverage -ftime-report -fvar-tracking
-fvar-tracking-assignments -fvar-tracking-assignments-toggle
-g -glevel -gtoggle -gcoff -gdwarf-version
-ggdb -grecord-gcc-switches -gno-record-gcc-switches
-gstabs -gstabs+ -gstrict-dwarf -gno-strict-dwarf
-gvms -gxcoff -gxcoff+ -gz[=type]
-fno-merge-debug-strings -fno-dwarf2-cfi-asm
-fdebug-prefix-map=old=new
-femit-struct-debug-baseonly -femit-struct-debug-reduced
-femit-struct-debug-detailed[=spec-list]
-p -pg -print-file-name=library -print-libgcc-file-name
-print-multi-directory -print-multi-lib -print-multi-os-directory
-print-prog-name=program -print-search-dirs -Q
-print-sysroot -print-sysroot-headers-suffix
-save-temps -save-temps=cwd -save-temps=obj -time[=file]
- Optimization Options
-
-faggressive-loop-optimizations -falign-functions[=n]
-falign-jumps[=n]
-falign-labels[=n] -falign-loops[=n]
-fassociative-math -fauto-profile -fauto-profile[=path]
-fauto-inc-dec -fbranch-probabilities
-fbranch-target-load-optimize -fbranch-target-load-optimize2
-fbtr-bb-exclusive -fcaller-saves
-fcheck-data-deps -fcombine-stack-adjustments -fconserve-stack
-fcompare-elim -fcprop-registers -fcrossjumping
-fcse-follow-jumps -fcse-skip-blocks -fcx-fortran-rules
-fcx-limited-range
-fdata-sections -fdce -fdelayed-branch
-fdelete-null-pointer-checks -fdevirtualize -fdevirtualize-speculatively
-fdevirtualize-at-ltrans -fdse
-fearly-inlining -fipa-sra -fexpensive-optimizations -ffat-lto-objects
-ffast-math -ffinite-math-only -ffloat-store -fexcess-precision=style
-fforward-propagate -ffp-contract=style -ffunction-sections
-fgcse -fgcse-after-reload -fgcse-las -fgcse-lm -fgraphite-identity
-fgcse-sm -fhoist-adjacent-loads -fif-conversion
-fif-conversion2 -findirect-inlining
-finline-functions -finline-functions-called-once -finline-limit=n
-finline-small-functions -fipa-cp -fipa-cp-clone -fipa-cp-alignment
-fipa-pta -fipa-profile -fipa-pure-const -fipa-reference -fipa-icf
-fira-algorithm=algorithm
-fira-region=region -fira-hoist-pressure
-fira-loop-pressure -fno-ira-share-save-slots
-fno-ira-share-spill-slots -fira-verbose=n
-fisolate-erroneous-paths-dereference -fisolate-erroneous-paths-attribute
-fivopts -fkeep-inline-functions -fkeep-static-consts
-flive-range-shrinkage
-floop-block -floop-interchange -floop-strip-mine
-floop-unroll-and-jam -floop-nest-optimize
-floop-parallelize-all -flra-remat -flto -flto-compression-level
-flto-partition=alg -flto-report -flto-report-wpa -fmerge-all-constants
-fmerge-constants -fmodulo-sched -fmodulo-sched-allow-regmoves
-fmove-loop-invariants -fno-branch-count-reg
-fno-defer-pop -fno-function-cse -fno-guess-branch-probability
-fno-inline -fno-math-errno -fno-peephole -fno-peephole2
-fno-sched-interblock -fno-sched-spec -fno-signed-zeros
-fno-toplevel-reorder -fno-trapping-math -fno-zero-initialized-in-bss
-fomit-frame-pointer -foptimize-sibling-calls
-fpartial-inlining -fpeel-loops -fpredictive-commoning
-fprefetch-loop-arrays -fprofile-report
-fprofile-correction -fprofile-dir=path -fprofile-generate
-fprofile-generate=path
-fprofile-use -fprofile-use=path -fprofile-values
-fprofile-reorder-functions
-freciprocal-math -free -frename-registers -freorder-blocks
-freorder-blocks-and-partition -freorder-functions
-frerun-cse-after-loop -freschedule-modulo-scheduled-loops
-frounding-math -fsched2-use-superblocks -fsched-pressure
-fsched-spec-load -fsched-spec-load-dangerous
-fsched-stalled-insns-dep[=n] -fsched-stalled-insns[=n]
-fsched-group-heuristic -fsched-critical-path-heuristic
-fsched-spec-insn-heuristic -fsched-rank-heuristic
-fsched-last-insn-heuristic -fsched-dep-count-heuristic
-fschedule-fusion
-fschedule-insns -fschedule-insns2 -fsection-anchors
-fselective-scheduling -fselective-scheduling2
-fsel-sched-pipelining -fsel-sched-pipelining-outer-loops
-fsemantic-interposition
-fshrink-wrap -fsignaling-nans -fsingle-precision-constant
-fsplit-ivs-in-unroller -fsplit-wide-types -fssa-phiopt
-fstack-protector -fstack-protector-all -fstack-protector-strong
-fstack-protector-explicit -fstdarg-opt -fstrict-aliasing
-fstrict-overflow -fthread-jumps -ftracer -ftree-bit-ccp
-ftree-builtin-call-dce -ftree-ccp -ftree-ch
-ftree-coalesce-inline-vars -ftree-coalesce-vars -ftree-copy-prop
-ftree-copyrename -ftree-dce -ftree-dominator-opts -ftree-dse
-ftree-forwprop -ftree-fre -ftree-loop-if-convert
-ftree-loop-if-convert-stores -ftree-loop-im
-ftree-phiprop -ftree-loop-distribution -ftree-loop-distribute-patterns
-ftree-loop-ivcanon -ftree-loop-linear -ftree-loop-optimize
-ftree-loop-vectorize
-ftree-parallelize-loops=n -ftree-pre -ftree-partial-pre -ftree-pta
-ftree-reassoc -ftree-sink -ftree-slsr -ftree-sra
-ftree-switch-conversion -ftree-tail-merge -ftree-ter
-ftree-vectorize -ftree-vrp
-funit-at-a-time -funroll-all-loops -funroll-loops
-funsafe-loop-optimizations -funsafe-math-optimizations -funswitch-loops
-fipa-ra -fvariable-expansion-in-unroller -fvect-cost-model -fvpt
-fweb -fwhole-program -fwpa -fuse-linker-plugin
--param name=value
-O -O0 -O1 -O2 -O3 -Os -Ofast -Og
- Preprocessor Options
-
-Aquestion=answer
-A-question[=answer]
-C -dD -dI -dM -dN
-Dmacro[=defn] -E -H
-idirafter dir
-include file -imacros file
-iprefix file -iwithprefix dir
-iwithprefixbefore dir -isystem dir
-imultilib dir -isysroot dir
-M -MM -MF -MG -MP -MQ -MT -nostdinc
-P -fdebug-cpp -ftrack-macro-expansion -fworking-directory
-remap -trigraphs -undef -Umacro
-Wp,option -Xpreprocessor option -no-integrated-cpp
- Assembler Option
-
-Wa,option -Xassembler option
- Linker Options
-
object-file-name -fuse-ld=linker -llibrary
-nostartfiles -nodefaultlibs -nostdlib -pie -rdynamic
-s -static -static-libgcc -static-libstdc++
-static-libasan -static-libtsan -static-liblsan -static-libubsan
-static-libmpx -static-libmpxwrappers
-shared -shared-libgcc -symbolic
-T script -Wl,option -Xlinker option
-u symbol -z keyword
- Directory Options
-
-Bprefix -Idir -iplugindir=dir
-iquotedir -Ldir -specs=file -I-
--sysroot=dir --no-sysroot-suffix
- Machine Dependent Options
-
AArch64 Options
-mabi=name -mbig-endian -mlittle-endian
-mgeneral-regs-only
-mcmodel=tiny -mcmodel=small -mcmodel=large
-mstrict-align
-momit-leaf-frame-pointer -mno-omit-leaf-frame-pointer
-mtls-dialect=desc -mtls-dialect=traditional
-mfix-cortex-a53-835769 -mno-fix-cortex-a53-835769
-mfix-cortex-a53-843419 -mno-fix-cortex-a53-843419
-march=name -mcpu=name -mtune=name
Adapteva Epiphany Options
-mhalf-reg-file -mprefer-short-insn-regs
-mbranch-cost=num -mcmove -mnops=num -msoft-cmpsf
-msplit-lohi -mpost-inc -mpost-modify -mstack-offset=num
-mround-nearest -mlong-calls -mshort-calls -msmall16
-mfp-mode=mode -mvect-double -max-vect-align=num
-msplit-vecmove-early -m1reg-reg
ARC Options
-mbarrel-shifter
-mcpu=cpu -mA6 -mARC600 -mA7 -mARC700
-mdpfp -mdpfp-compact -mdpfp-fast -mno-dpfp-lrsr
-mea -mno-mpy -mmul32x16 -mmul64
-mnorm -mspfp -mspfp-compact -mspfp-fast -msimd -msoft-float -mswap
-mcrc -mdsp-packa -mdvbf -mlock -mmac-d16 -mmac-24 -mrtsc -mswape
-mtelephony -mxy -misize -mannotate-align -marclinux -marclinux_prof
-mepilogue-cfi -mlong-calls -mmedium-calls -msdata
-mucb-mcount -mvolatile-cache
-malign-call -mauto-modify-reg -mbbit-peephole -mno-brcc
-mcase-vector-pcrel -mcompact-casesi -mno-cond-exec -mearly-cbranchsi
-mexpand-adddi -mindexed-loads -mlra -mlra-priority-none
-mlra-priority-compact mlra-priority-noncompact -mno-millicode
-mmixed-code -mq-class -mRcq -mRcw -msize-level=level
-mtune=cpu -mmultcost=num -munalign-prob-threshold=probability
ARM Options
-mapcs-frame -mno-apcs-frame
-mabi=name
-mapcs-stack-check -mno-apcs-stack-check
-mapcs-float -mno-apcs-float
-mapcs-reentrant -mno-apcs-reentrant
-msched-prolog -mno-sched-prolog
-mlittle-endian -mbig-endian
-mfloat-abi=name
-mfp16-format=name
-mthumb-interwork -mno-thumb-interwork
-mcpu=name -march=name -mfpu=name
-mtune=name -mprint-tune-info
-mstructure-size-boundary=n
-mabort-on-noreturn
-mlong-calls -mno-long-calls
-msingle-pic-base -mno-single-pic-base
-mpic-register=reg
-mnop-fun-dllimport
-mpoke-function-name
-mthumb -marm
-mtpcs-frame -mtpcs-leaf-frame
-mcaller-super-interworking -mcallee-super-interworking
-mtp=name -mtls-dialect=dialect
-mword-relocations
-mfix-cortex-m3-ldrd
-munaligned-access
-mneon-for-64bits
-mslow-flash-data
-masm-syntax-unified
-mrestrict-it
AVR Options
-mmcu=mcu -maccumulate-args -mbranch-cost=cost
-mcall-prologues -mint8 -mn_flash=size -mno-interrupts
-mrelax -mrmw -mstrict-X -mtiny-stack -nodevicelib -Waddr-space-convert
Blackfin Options
-mcpu=cpu[-sirevision]
-msim -momit-leaf-frame-pointer -mno-omit-leaf-frame-pointer
-mspecld-anomaly -mno-specld-anomaly -mcsync-anomaly -mno-csync-anomaly
-mlow-64k -mno-low64k -mstack-check-l1 -mid-shared-library
-mno-id-shared-library -mshared-library-id=n
-mleaf-id-shared-library -mno-leaf-id-shared-library
-msep-data -mno-sep-data -mlong-calls -mno-long-calls
-mfast-fp -minline-plt -mmulticore -mcorea -mcoreb -msdram
-micplb
C6X Options
-mbig-endian -mlittle-endian -march=cpu
-msim -msdata=sdata-type
CRIS Options
-mcpu=cpu -march=cpu -mtune=cpu
-mmax-stack-frame=n -melinux-stacksize=n
-metrax4 -metrax100 -mpdebug -mcc-init -mno-side-effects
-mstack-align -mdata-align -mconst-align
-m32-bit -m16-bit -m8-bit -mno-prologue-epilogue -mno-gotplt
-melf -maout -melinux -mlinux -sim -sim2
-mmul-bug-workaround -mno-mul-bug-workaround
CR16 Options
-mmac
-mcr16cplus -mcr16c
-msim -mint32 -mbit-ops
-mdata-model=model
Darwin Options
-all_load -allowable_client -arch -arch_errors_fatal
-arch_only -bind_at_load -bundle -bundle_loader
-client_name -compatibility_version -current_version
-dead_strip
-dependency-file -dylib_file -dylinker_install_name
-dynamic -dynamiclib -exported_symbols_list
-filelist -flat_namespace -force_cpusubtype_ALL
-force_flat_namespace -headerpad_max_install_names
-iframework
-image_base -init -install_name -keep_private_externs
-multi_module -multiply_defined -multiply_defined_unused
-noall_load -no_dead_strip_inits_and_terms
-nofixprebinding -nomultidefs -noprebind -noseglinkedit
-pagezero_size -prebind -prebind_all_twolevel_modules
-private_bundle -read_only_relocs -sectalign
-sectobjectsymbols -whyload -seg1addr
-sectcreate -sectobjectsymbols -sectorder
-segaddr -segs_read_only_addr -segs_read_write_addr
-seg_addr_table -seg_addr_table_filename -seglinkedit
-segprot -segs_read_only_addr -segs_read_write_addr
-single_module -static -sub_library -sub_umbrella
-twolevel_namespace -umbrella -undefined
-unexported_symbols_list -weak_reference_mismatches
-whatsloaded -F -gused -gfull -mmacosx-version-min=version
-mkernel -mone-byte-bool
DEC Alpha Options
-mno-fp-regs -msoft-float
-mieee -mieee-with-inexact -mieee-conformant
-mfp-trap-mode=mode -mfp-rounding-mode=mode
-mtrap-precision=mode -mbuild-constants
-mcpu=cpu-type -mtune=cpu-type
-mbwx -mmax -mfix -mcix
-mfloat-vax -mfloat-ieee
-mexplicit-relocs -msmall-data -mlarge-data
-msmall-text -mlarge-text
-mmemory-latency=time
FR30 Options
-msmall-model -mno-lsim
FRV Options
-mgpr-32 -mgpr-64 -mfpr-32 -mfpr-64
-mhard-float -msoft-float
-malloc-cc -mfixed-cc -mdword -mno-dword
-mdouble -mno-double
-mmedia -mno-media -mmuladd -mno-muladd
-mfdpic -minline-plt -mgprel-ro -multilib-library-pic
-mlinked-fp -mlong-calls -malign-labels
-mlibrary-pic -macc-4 -macc-8
-mpack -mno-pack -mno-eflags -mcond-move -mno-cond-move
-moptimize-membar -mno-optimize-membar
-mscc -mno-scc -mcond-exec -mno-cond-exec
-mvliw-branch -mno-vliw-branch
-mmulti-cond-exec -mno-multi-cond-exec -mnested-cond-exec
-mno-nested-cond-exec -mtomcat-stats
-mTLS -mtls
-mcpu=cpu
GNU/Linux Options
-mglibc -muclibc -mbionic -mandroid
-tno-android-cc -tno-android-ld
H8/300 Options
-mrelax -mh -ms -mn -mexr -mno-exr -mint32 -malign-300
HPPA Options
-march=architecture-type
-mdisable-fpregs -mdisable-indexing
-mfast-indirect-calls -mgas -mgnu-ld -mhp-ld
-mfixed-range=register-range
-mjump-in-delay -mlinker-opt -mlong-calls
-mlong-load-store -mno-disable-fpregs
-mno-disable-indexing -mno-fast-indirect-calls -mno-gas
-mno-jump-in-delay -mno-long-load-store
-mno-portable-runtime -mno-soft-float
-mno-space-regs -msoft-float -mpa-risc-1-0
-mpa-risc-1-1 -mpa-risc-2-0 -mportable-runtime
-mschedule=cpu-type -mspace-regs -msio -mwsio
-munix=unix-std -nolibdld -static -threads
IA-64 Options
-mbig-endian -mlittle-endian -mgnu-as -mgnu-ld -mno-pic
-mvolatile-asm-stop -mregister-names -msdata -mno-sdata
-mconstant-gp -mauto-pic -mfused-madd
-minline-float-divide-min-latency
-minline-float-divide-max-throughput
-mno-inline-float-divide
-minline-int-divide-min-latency
-minline-int-divide-max-throughput
-mno-inline-int-divide
-minline-sqrt-min-latency -minline-sqrt-max-throughput
-mno-inline-sqrt
-mdwarf2-asm -mearly-stop-bits
-mfixed-range=register-range -mtls-size=tls-size
-mtune=cpu-type -milp32 -mlp64
-msched-br-data-spec -msched-ar-data-spec -msched-control-spec
-msched-br-in-data-spec -msched-ar-in-data-spec -msched-in-control-spec
-msched-spec-ldc -msched-spec-control-ldc
-msched-prefer-non-data-spec-insns -msched-prefer-non-control-spec-insns
-msched-stop-bits-after-every-cycle -msched-count-spec-in-critical-path
-msel-sched-dont-check-control-spec -msched-fp-mem-deps-zero-cost
-msched-max-memory-insns-hard-limit -msched-max-memory-insns=max-insns
LM32 Options
-mbarrel-shift-enabled -mdivide-enabled -mmultiply-enabled
-msign-extend-enabled -muser-enabled
M32R/D Options
-m32r2 -m32rx -m32r
-mdebug
-malign-loops -mno-align-loops
-missue-rate=number
-mbranch-cost=number
-mmodel=code-size-model-type
-msdata=sdata-type
-mno-flush-func -mflush-func=name
-mno-flush-trap -mflush-trap=number
-G num
M32C Options
-mcpu=cpu -msim -memregs=number
M680x0 Options
-march=arch -mcpu=cpu -mtune=tune
-m68000 -m68020 -m68020-40 -m68020-60 -m68030 -m68040
-m68060 -mcpu32 -m5200 -m5206e -m528x -m5307 -m5407
-mcfv4e -mbitfield -mno-bitfield -mc68000 -mc68020
-mnobitfield -mrtd -mno-rtd -mdiv -mno-div -mshort
-mno-short -mhard-float -m68881 -msoft-float -mpcrel
-malign-int -mstrict-align -msep-data -mno-sep-data
-mshared-library-id=n -mid-shared-library -mno-id-shared-library
-mxgot -mno-xgot
MCore Options
-mhardlit -mno-hardlit -mdiv -mno-div -mrelax-immediates
-mno-relax-immediates -mwide-bitfields -mno-wide-bitfields
-m4byte-functions -mno-4byte-functions -mcallgraph-data
-mno-callgraph-data -mslow-bytes -mno-slow-bytes -mno-lsim
-mlittle-endian -mbig-endian -m210 -m340 -mstack-increment
MeP Options
-mabsdiff -mall-opts -maverage -mbased=n -mbitops
-mc=n -mclip -mconfig=name -mcop -mcop32 -mcop64 -mivc2
-mdc -mdiv -meb -mel -mio-volatile -ml -mleadz -mm -mminmax
-mmult -mno-opts -mrepeat -ms -msatur -msdram -msim -msimnovec -mtf
-mtiny=n
MicroBlaze Options
-msoft-float -mhard-float -msmall-divides -mcpu=cpu
-mmemcpy -mxl-soft-mul -mxl-soft-div -mxl-barrel-shift
-mxl-pattern-compare -mxl-stack-check -mxl-gp-opt -mno-clearbss
-mxl-multiply-high -mxl-float-convert -mxl-float-sqrt
-mbig-endian -mlittle-endian -mxl-reorder -mxl-mode-app-model
MIPS Options
-EL -EB -march=arch -mtune=arch
-mips1 -mips2 -mips3 -mips4 -mips32 -mips32r2 -mips32r3 -mips32r5
-mips32r6 -mips64 -mips64r2 -mips64r3 -mips64r5 -mips64r6
-mips16 -mno-mips16 -mflip-mips16
-minterlink-compressed -mno-interlink-compressed
-minterlink-mips16 -mno-interlink-mips16
-mabi=abi -mabicalls -mno-abicalls
-mshared -mno-shared -mplt -mno-plt -mxgot -mno-xgot
-mgp32 -mgp64 -mfp32 -mfpxx -mfp64 -mhard-float -msoft-float
-mno-float -msingle-float -mdouble-float
-modd-spreg -mno-odd-spreg
-mabs=mode -mnan=encoding
-mdsp -mno-dsp -mdspr2 -mno-dspr2
-mmcu -mmno-mcu
-meva -mno-eva
-mvirt -mno-virt
-mxpa -mno-xpa
-mmicromips -mno-micromips
-mfpu=fpu-type
-msmartmips -mno-smartmips
-mpaired-single -mno-paired-single -mdmx -mno-mdmx
-mips3d -mno-mips3d -mmt -mno-mt -mllsc -mno-llsc
-mlong64 -mlong32 -msym32 -mno-sym32
-Gnum -mlocal-sdata -mno-local-sdata
-mextern-sdata -mno-extern-sdata -mgpopt -mno-gopt
-membedded-data -mno-embedded-data
-muninit-const-in-rodata -mno-uninit-const-in-rodata
-mcode-readable=setting
-msplit-addresses -mno-split-addresses
-mexplicit-relocs -mno-explicit-relocs
-mcheck-zero-division -mno-check-zero-division
-mdivide-traps -mdivide-breaks
-mmemcpy -mno-memcpy -mlong-calls -mno-long-calls
-mmad -mno-mad -mimadd -mno-imadd -mfused-madd -mno-fused-madd -nocpp
-mfix-24k -mno-fix-24k
-mfix-r4000 -mno-fix-r4000 -mfix-r4400 -mno-fix-r4400
-mfix-r10000 -mno-fix-r10000 -mfix-rm7000 -mno-fix-rm7000
-mfix-vr4120 -mno-fix-vr4120
-mfix-vr4130 -mno-fix-vr4130 -mfix-sb1 -mno-fix-sb1
-mflush-func=func -mno-flush-func
-mbranch-cost=num -mbranch-likely -mno-branch-likely
-mfp-exceptions -mno-fp-exceptions
-mvr4130-align -mno-vr4130-align -msynci -mno-synci
-mrelax-pic-calls -mno-relax-pic-calls -mmcount-ra-address
MMIX Options
-mlibfuncs -mno-libfuncs -mepsilon -mno-epsilon -mabi=gnu
-mabi=mmixware -mzero-extend -mknuthdiv -mtoplevel-symbols
-melf -mbranch-predict -mno-branch-predict -mbase-addresses
-mno-base-addresses -msingle-exit -mno-single-exit
MN10300 Options
-mmult-bug -mno-mult-bug
-mno-am33 -mam33 -mam33-2 -mam34
-mtune=cpu-type
-mreturn-pointer-on-d0
-mno-crt0 -mrelax -mliw -msetlb
Moxie Options
-meb -mel -mmul.x -mno-crt0
MSP430 Options
-msim -masm-hex -mmcu= -mcpu= -mlarge -msmall -mrelax
-mhwmult= -minrt
NDS32 Options
-mbig-endian -mlittle-endian
-mreduced-regs -mfull-regs
-mcmov -mno-cmov
-mperf-ext -mno-perf-ext
-mv3push -mno-v3push
-m16bit -mno-16bit
-misr-vector-size=num
-mcache-block-size=num
-march=arch
-mcmodel=code-model
-mctor-dtor -mrelax
Nios II Options
-G num -mgpopt=option -mgpopt -mno-gpopt
-mel -meb
-mno-bypass-cache -mbypass-cache
-mno-cache-volatile -mcache-volatile
-mno-fast-sw-div -mfast-sw-div
-mhw-mul -mno-hw-mul -mhw-mulx -mno-hw-mulx -mno-hw-div -mhw-div
-mcustom-insn=N -mno-custom-insn
-mcustom-fpu-cfg=name
-mhal -msmallc -msys-crt0=name -msys-lib=name
Nvidia PTX Options
-m32 -m64 -mmainkernel
PDP-11 Options
-mfpu -msoft-float -mac0 -mno-ac0 -m40 -m45 -m10
-mbcopy -mbcopy-builtin -mint32 -mno-int16
-mint16 -mno-int32 -mfloat32 -mno-float64
-mfloat64 -mno-float32 -mabshi -mno-abshi
-mbranch-expensive -mbranch-cheap
-munix-asm -mdec-asm
picoChip Options
-mae=ae_type -mvliw-lookahead=N
-msymbol-as-address -mno-inefficient-warnings
PowerPC Options
See RS/6000 and PowerPC Options.
RL78 Options
-msim -mmul=none -mmul=g13 -mmul=rl78
-m64bit-doubles -m32bit-doubles
RS/6000 and PowerPC Options
-mcpu=cpu-type
-mtune=cpu-type
-mcmodel=code-model
-mpowerpc64
-maltivec -mno-altivec
-mpowerpc-gpopt -mno-powerpc-gpopt
-mpowerpc-gfxopt -mno-powerpc-gfxopt
-mmfcrf -mno-mfcrf -mpopcntb -mno-popcntb -mpopcntd -mno-popcntd
-mfprnd -mno-fprnd
-mcmpb -mno-cmpb -mmfpgpr -mno-mfpgpr -mhard-dfp -mno-hard-dfp
-mfull-toc -mminimal-toc -mno-fp-in-toc -mno-sum-in-toc
-m64 -m32 -mxl-compat -mno-xl-compat -mpe
-malign-power -malign-natural
-msoft-float -mhard-float -mmultiple -mno-multiple
-msingle-float -mdouble-float -msimple-fpu
-mstring -mno-string -mupdate -mno-update
-mavoid-indexed-addresses -mno-avoid-indexed-addresses
-mfused-madd -mno-fused-madd -mbit-align -mno-bit-align
-mstrict-align -mno-strict-align -mrelocatable
-mno-relocatable -mrelocatable-lib -mno-relocatable-lib
-mtoc -mno-toc -mlittle -mlittle-endian -mbig -mbig-endian
-mdynamic-no-pic -maltivec -mswdiv -msingle-pic-base
-mprioritize-restricted-insns=priority
-msched-costly-dep=dependence_type
-minsert-sched-nops=scheme
-mcall-sysv -mcall-netbsd
-maix-struct-return -msvr4-struct-return
-mabi=abi-type -msecure-plt -mbss-plt
-mblock-move-inline-limit=num
-misel -mno-isel
-misel=yes -misel=no
-mspe -mno-spe
-mspe=yes -mspe=no
-mpaired
-mgen-cell-microcode -mwarn-cell-microcode
-mvrsave -mno-vrsave
-mmulhw -mno-mulhw
-mdlmzb -mno-dlmzb
-mfloat-gprs=yes -mfloat-gprs=no -mfloat-gprs=single -mfloat-gprs=double
-mprototype -mno-prototype
-msim -mmvme -mads -myellowknife -memb -msdata
-msdata=opt -mvxworks -G num -pthread
-mrecip -mrecip=opt -mno-recip -mrecip-precision
-mno-recip-precision
-mveclibabi=type -mfriz -mno-friz
-mpointers-to-nested-functions -mno-pointers-to-nested-functions
-msave-toc-indirect -mno-save-toc-indirect
-mpower8-fusion -mno-mpower8-fusion -mpower8-vector -mno-power8-vector
-mcrypto -mno-crypto -mdirect-move -mno-direct-move
-mquad-memory -mno-quad-memory
-mquad-memory-atomic -mno-quad-memory-atomic
-mcompat-align-parm -mno-compat-align-parm
-mupper-regs-df -mno-upper-regs-df -mupper-regs-sf -mno-upper-regs-sf
-mupper-regs -mno-upper-regs
RX Options
-m64bit-doubles -m32bit-doubles -fpu -nofpu
-mcpu=
-mbig-endian-data -mlittle-endian-data
-msmall-data
-msim -mno-sim
-mas100-syntax -mno-as100-syntax
-mrelax
-mmax-constant-size=
-mint-register=
-mpid
-mno-warn-multiple-fast-interrupts
-msave-acc-in-interrupts
S/390 and zSeries Options
-mtune=cpu-type -march=cpu-type
-mhard-float -msoft-float -mhard-dfp -mno-hard-dfp
-mlong-double-64 -mlong-double-128
-mbackchain -mno-backchain -mpacked-stack -mno-packed-stack
-msmall-exec -mno-small-exec -mmvcle -mno-mvcle
-m64 -m31 -mdebug -mno-debug -mesa -mzarch
-mtpf-trace -mno-tpf-trace -mfused-madd -mno-fused-madd
-mwarn-framesize -mwarn-dynamicstack -mstack-size -mstack-guard
-mhotpatch=halfwords,halfwords
Score Options
-meb -mel
-mnhwloop
-muls
-mmac
-mscore5 -mscore5u -mscore7 -mscore7d
SH Options
-m1 -m2 -m2e
-m2a-nofpu -m2a-single-only -m2a-single -m2a
-m3 -m3e
-m4-nofpu -m4-single-only -m4-single -m4
-m4a-nofpu -m4a-single-only -m4a-single -m4a -m4al
-m5-64media -m5-64media-nofpu
-m5-32media -m5-32media-nofpu
-m5-compact -m5-compact-nofpu
-mb -ml -mdalign -mrelax
-mbigtable -mfmovd -mhitachi -mrenesas -mno-renesas -mnomacsave
-mieee -mno-ieee -mbitops -misize -minline-ic_invalidate -mpadstruct
-mspace -mprefergot -musermode -multcost=number -mdiv=strategy
-mdivsi3_libfunc=name -mfixed-range=register-range
-mindexed-addressing -mgettrcost=number -mpt-fixed
-maccumulate-outgoing-args -minvalid-symbols
-matomic-model=atomic-model
-mbranch-cost=num -mzdcbranch -mno-zdcbranch
-mcbranch-force-delay-slot
-mfused-madd -mno-fused-madd -mfsca -mno-fsca -mfsrra -mno-fsrra
-mpretend-cmove -mtas
Solaris 2 Options
-mclear-hwcap -mno-clear-hwcap -mimpure-text -mno-impure-text
-pthreads -pthread
SPARC Options
-mcpu=cpu-type
-mtune=cpu-type
-mcmodel=code-model
-mmemory-model=mem-model
-m32 -m64 -mapp-regs -mno-app-regs
-mfaster-structs -mno-faster-structs -mflat -mno-flat
-mfpu -mno-fpu -mhard-float -msoft-float
-mhard-quad-float -msoft-quad-float
-mstack-bias -mno-stack-bias
-munaligned-doubles -mno-unaligned-doubles
-muser-mode -mno-user-mode
-mv8plus -mno-v8plus -mvis -mno-vis
-mvis2 -mno-vis2 -mvis3 -mno-vis3
-mcbcond -mno-cbcond
-mfmaf -mno-fmaf -mpopc -mno-popc
-mfix-at697f -mfix-ut699
SPU Options
-mwarn-reloc -merror-reloc
-msafe-dma -munsafe-dma
-mbranch-hints
-msmall-mem -mlarge-mem -mstdmain
-mfixed-range=register-range
-mea32 -mea64
-maddress-space-conversion -mno-address-space-conversion
-mcache-size=cache-size
-matomic-updates -mno-atomic-updates
System V Options
-Qy -Qn -YP,paths -Ym,dir
TILE-Gx Options
-mcpu=CPU -m32 -m64 -mbig-endian -mlittle-endian
-mcmodel=code-model
TILEPro Options
-mcpu=cpu -m32
V850 Options
-mlong-calls -mno-long-calls -mep -mno-ep
-mprolog-function -mno-prolog-function -mspace
-mtda=n -msda=n -mzda=n
-mapp-regs -mno-app-regs
-mdisable-callt -mno-disable-callt
-mv850e2v3 -mv850e2 -mv850e1 -mv850es
-mv850e -mv850 -mv850e3v5
-mloop
-mrelax
-mlong-jumps
-msoft-float
-mhard-float
-mgcc-abi
-mrh850-abi
-mbig-switch
VAX Options
-mg -mgnu -munix
Visium Options
-mdebug -msim -mfpu -mno-fpu -mhard-float -msoft-float
-mcpu=cpu-type -mtune=cpu-type -msv-mode -muser-mode
VMS Options
-mvms-return-codes -mdebug-main=prefix -mmalloc64
-mpointer-size=size
VxWorks Options
-mrtp -non-static -Bstatic -Bdynamic
-Xbind-lazy -Xbind-now
x86 Options
-mtune=cpu-type -march=cpu-type
-mtune-ctrl=feature-list -mdump-tune-features -mno-default
-mfpmath=unit
-masm=dialect -mno-fancy-math-387
-mno-fp-ret-in-387 -msoft-float
-mno-wide-multiply -mrtd -malign-double
-mpreferred-stack-boundary=num
-mincoming-stack-boundary=num
-mcld -mcx16 -msahf -mmovbe -mcrc32
-mrecip -mrecip=opt
-mvzeroupper -mprefer-avx128
-mmmx -msse -msse2 -msse3 -mssse3 -msse4.1 -msse4.2 -msse4 -mavx
-mavx2 -mavx512f -mavx512pf -mavx512er -mavx512cd -msha
-maes -mpclmul -mfsgsbase -mrdrnd -mf16c -mfma -mprefetchwt1
-mclflushopt -mxsavec -mxsaves
-msse4a -m3dnow -mpopcnt -mabm -mbmi -mtbm -mfma4 -mxop -mlzcnt
-mbmi2 -mfxsr -mxsave -mxsaveopt -mrtm -mlwp -mmpx -mmwaitx -mthreads
-mno-align-stringops -minline-all-stringops
-minline-stringops-dynamically -mstringop-strategy=alg
-mmemcpy-strategy=strategy -mmemset-strategy=strategy
-mpush-args -maccumulate-outgoing-args -m128bit-long-double
-m96bit-long-double -mlong-double-64 -mlong-double-80 -mlong-double-128
-mregparm=num -msseregparm
-mveclibabi=type -mvect8-ret-in-mem
-mpc32 -mpc64 -mpc80 -mstackrealign
-momit-leaf-frame-pointer -mno-red-zone -mno-tls-direct-seg-refs
-mcmodel=code-model -mabi=name -maddress-mode=mode
-m32 -m64 -mx32 -m16 -mlarge-data-threshold=num
-msse2avx -mfentry -mrecord-mcount -mnop-mcount -m8bit-idiv
-mavx256-split-unaligned-load -mavx256-split-unaligned-store
-malign-data=type -mstack-protector-guard=guard
x86 Windows Options
-mconsole -mcygwin -mno-cygwin -mdll
-mnop-fun-dllimport -mthread
-municode -mwin32 -mwindows -fno-set-stack-executable
Xstormy16 Options
-msim
Xtensa Options
-mconst16 -mno-const16
-mfused-madd -mno-fused-madd
-mforce-no-pic
-mserialize-volatile -mno-serialize-volatile
-mtext-section-literals -mno-text-section-literals
-mtarget-align -mno-target-align
-mlongcalls -mno-longcalls
zSeries Options
See S/390 and zSeries Options.
- Code Generation Options
-
-fcall-saved-reg -fcall-used-reg
-ffixed-reg -fexceptions
-fnon-call-exceptions -fdelete-dead-exceptions -funwind-tables
-fasynchronous-unwind-tables
-fno-gnu-unique
-finhibit-size-directive -finstrument-functions
-finstrument-functions-exclude-function-list=sym,sym,...
-finstrument-functions-exclude-file-list=file,file,...
-fno-common -fno-ident
-fpcc-struct-return -fpic -fPIC -fpie -fPIE
-fno-jump-tables
-frecord-gcc-switches
-freg-struct-return -fshort-enums
-fshort-double -fshort-wchar
-fverbose-asm -fpack-struct[=n] -fstack-check
-fstack-limit-register=reg -fstack-limit-symbol=sym
-fno-stack-limit -fsplit-stack
-fleading-underscore -ftls-model=model
-fstack-reuse=reuse_level
-ftrapv -fwrapv -fbounds-check
-fvisibility=[default|internal|hidden|protected]
-fstrict-volatile-bitfields -fsync-libcalls
Options Controlling the Kind of Output
Compilation can involve up to four stages: preprocessing, compilation
proper, assembly and linking, always in that order. GCC is capable of
preprocessing and compiling several files either into several
assembler input files, or into one assembler input file; then each
assembler input file produces an object file, and linking combines all
the object files (those newly compiled, and those specified as input)
into an executable file.
For any given input file, the file name suffix determines what kind of
compilation is done:
- file.c
-
C source code that must be preprocessed.
- file.i
-
C source code that should not be preprocessed.
- file.ii
-
C++ source code that should not be preprocessed.
- file.m
-
Objective-C source code. Note that you must link with the libobjc
library to make an Objective-C program work.
- file.mi
-
Objective-C source code that should not be preprocessed.
- file.mm
-
- file.M
-
Objective-C++ source code. Note that you must link with the libobjc
library to make an Objective-C++ program work. Note that .M refers
to a literal capital M.
- file.mii
-
Objective-C++ source code that should not be preprocessed.
- file.h
-
C, C++, Objective-C or Objective-C++ header file to be turned into a
precompiled header (default), or C, C++ header file to be turned into an
Ada spec (via the -fdump-ada-spec switch).
- file.cc
-
- file.cp
-
- file.cxx
-
- file.cpp
-
- file.CPP
-
- file.c++
-
- file.C
-
C++ source code that must be preprocessed. Note that in .cxx,
the last two letters must both be literally x. Likewise,
.C refers to a literal capital C.
- file.mm
-
- file.M
-
Objective-C++ source code that must be preprocessed.
- file.mii
-
Objective-C++ source code that should not be preprocessed.
- file.hh
-
- file.H
-
- file.hp
-
- file.hxx
-
- file.hpp
-
- file.HPP
-
- file.h++
-
- file.tcc
-
C++ header file to be turned into a precompiled header or Ada spec.
- file.f
-
- file.for
-
- file.ftn
-
Fixed form Fortran source code that should not be preprocessed.
- file.F
-
- file.FOR
-
- file.fpp
-
- file.FPP
-
- file.FTN
-
Fixed form Fortran source code that must be preprocessed (with the traditional
preprocessor).
- file.f90
-
- file.f95
-
- file.f03
-
- file.f08
-
Free form Fortran source code that should not be preprocessed.
- file.F90
-
- file.F95
-
- file.F03
-
- file.F08
-
Free form Fortran source code that must be preprocessed (with the
traditional preprocessor).
- file.go
-
Go source code.
- file.ads
-
Ada source code file that contains a library unit declaration (a
declaration of a package, subprogram, or generic, or a generic
instantiation), or a library unit renaming declaration (a package,
generic, or subprogram renaming declaration). Such files are also
called specs.
- file.adb
-
Ada source code file containing a library unit body (a subprogram or
package body). Such files are also called bodies.
- file.s
-
Assembler code.
- file.S
-
- file.sx
-
Assembler code that must be preprocessed.
- other
-
An object file to be fed straight into linking.
Any file name with no recognized suffix is treated this way.
You can specify the input language explicitly with the -x option:
- -x language
-
Specify explicitly the language for the following input files
(rather than letting the compiler choose a default based on the file
name suffix). This option applies to all following input files until
the next -x option. Possible values for language are:
c c-header cpp-output
c++ c++-header c++-cpp-output
objective-c objective-c-header objective-c-cpp-output
objective-c++ objective-c++-header objective-c++-cpp-output
assembler assembler-with-cpp
ada
f77 f77-cpp-input f95 f95-cpp-input
go
java
- -x none
-
Turn off any specification of a language, so that subsequent files are
handled according to their file name suffixes (as they are if -x
has not been used at all).
- -pass-exit-codes
-
Normally the gcc program exits with the code of 1 if any
phase of the compiler returns a non-success return code. If you specify
-pass-exit-codes, the gcc program instead returns with
the numerically highest error produced by any phase returning an error
indication. The C, C++, and Fortran front ends return 4 if an internal
compiler error is encountered.
If you only want some of the stages of compilation, you can use
-x (or filename suffixes) to tell gcc where to start, and
one of the options -c, -S, or -E to say where
gcc is to stop. Note that some combinations (for example,
-x cpp-output -E) instruct gcc to do nothing at all.
- -c
-
Compile or assemble the source files, but do not link. The linking
stage simply is not done. The ultimate output is in the form of an
object file for each source file.
By default, the object file name for a source file is made by replacing
the suffix .c, .i, .s, etc., with .o.
Unrecognized input files, not requiring compilation or assembly, are
ignored.
- -S
-
Stop after the stage of compilation proper; do not assemble. The output
is in the form of an assembler code file for each non-assembler input
file specified.
By default, the assembler file name for a source file is made by
replacing the suffix .c, .i, etc., with .s.
Input files that don't require compilation are ignored.
- -E
-
Stop after the preprocessing stage; do not run the compiler proper. The
output is in the form of preprocessed source code, which is sent to the
standard output.
Input files that don't require preprocessing are ignored.
- -o file
-
Place output in file file. This applies to whatever
sort of output is being produced, whether it be an executable file,
an object file, an assembler file or preprocessed C code.
If -o is not specified, the default is to put an executable
file in a.out, the object file for
source.suffix in source.o, its
assembler file in source.s, a precompiled header file in
source.suffix.gch, and all preprocessed C source on
standard output.
- -v
-
Print (on standard error output) the commands executed to run the stages
of compilation. Also print the version number of the compiler driver
program and of the preprocessor and the compiler proper.
- -###
-
Like -v except the commands are not executed and arguments
are quoted unless they contain only alphanumeric characters or "./-_".
This is useful for shell scripts to capture the driver-generated command lines.
- -pipe
-
Use pipes rather than temporary files for communication between the
various stages of compilation. This fails to work on some systems where
the assembler is unable to read from a pipe; but the GNU assembler has
no trouble.
- --help
-
Print (on the standard output) a description of the command-line options
understood by gcc. If the -v option is also specified
then --help is also passed on to the various processes
invoked by gcc, so that they can display the command-line options
they accept. If the -Wextra option has also been specified
(prior to the --help option), then command-line options that
have no documentation associated with them are also displayed.
- --target-help
-
Print (on the standard output) a description of target-specific command-line
options for each tool. For some targets extra target-specific
information may also be printed.
- --help={class|[^]qualifier}[,...]
-
Print (on the standard output) a description of the command-line
options understood by the compiler that fit into all specified classes
and qualifiers. These are the supported classes:
-
- optimizers
-
Display all of the optimization options supported by the
compiler.
- warnings
-
Display all of the options controlling warning messages
produced by the compiler.
- target
-
Display target-specific options. Unlike the
--target-help option however, target-specific options of the
linker and assembler are not displayed. This is because those
tools do not currently support the extended --help= syntax.
- params
-
Display the values recognized by the --param
option.
- language
-
Display the options supported for language, where
language is the name of one of the languages supported in this
version of GCC.
- common
-
Display the options that are common to all languages.
-
These are the supported qualifiers:
- undocumented
-
Display only those options that are undocumented.
- joined
-
Display options taking an argument that appears after an equal
sign in the same continuous piece of text, such as:
--help=target.
- separate
-
Display options taking an argument that appears as a separate word
following the original option, such as: -o output-file.
-
Thus for example to display all the undocumented target-specific
switches supported by the compiler, use:
--help=target,undocumented
The sense of a qualifier can be inverted by prefixing it with the
^ character, so for example to display all binary warning
options (i.e., ones that are either on or off and that do not take an
argument) that have a description, use:
--help=warnings,^joined,^undocumented
The argument to --help= should not consist solely of inverted
qualifiers.
Combining several classes is possible, although this usually
restricts the output so much that there is nothing to display. One
case where it does work, however, is when one of the classes is
target. For example, to display all the target-specific
optimization options, use:
--help=target,optimizers
The --help= option can be repeated on the command line. Each
successive use displays its requested class of options, skipping
those that have already been displayed.
If the -Q option appears on the command line before the
--help= option, then the descriptive text displayed by
--help= is changed. Instead of describing the displayed
options, an indication is given as to whether the option is enabled,
disabled or set to a specific value (assuming that the compiler
knows this at the point where the --help= option is used).
Here is a truncated example from the ARM port of gcc:
% gcc -Q -mabi=2 --help=target -c
The following options are target specific:
-mabi= 2
-mabort-on-noreturn [disabled]
-mapcs [disabled]
The output is sensitive to the effects of previous command-line
options, so for example it is possible to find out which optimizations
are enabled at -O2 by using:
-Q -O2 --help=optimizers
Alternatively you can discover which binary optimizations are enabled
by -O3 by using:
gcc -c -Q -O3 --help=optimizers > /tmp/O3-opts
gcc -c -Q -O2 --help=optimizers > /tmp/O2-opts
diff /tmp/O2-opts /tmp/O3-opts | grep enabled
- -no-canonical-prefixes
-
Do not expand any symbolic links, resolve references to /../
or /./, or make the path absolute when generating a relative
prefix.
- --version
-
Display the version number and copyrights of the invoked GCC.
- -wrapper
-
Invoke all subcommands under a wrapper program. The name of the
wrapper program and its parameters are passed as a comma separated
list.
gcc -c t.c -wrapper gdb,--args
This invokes all subprograms of gcc under
gdb --args, thus the invocation of cc1 is
gdb --args cc1 ....
- -fplugin=name.so
-
Load the plugin code in file name.so, assumed to be a
shared object to be dlopen'd by the compiler. The base name of
the shared object file is used to identify the plugin for the
purposes of argument parsing (See
-fplugin-arg-name-key=value below).
Each plugin should define the callback functions specified in the
Plugins API.
- -fplugin-arg-name-key=value
-
Define an argument called key with a value of value
for the plugin called name.
- -fdump-ada-spec[-slim]
-
For C and C++ source and include files, generate corresponding Ada specs.
- -fada-spec-parent=unit
-
In conjunction with -fdump-ada-spec[-slim] above, generate
Ada specs as child units of parent unit.
- -fdump-go-spec=file
-
For input files in any language, generate corresponding Go
declarations in file. This generates Go "const",
"type", "var", and "func" declarations which may be a
useful way to start writing a Go interface to code written in some
other language.
- @file
-
Read command-line options from file. The options read are
inserted in place of the original @file option. If file
does not exist, or cannot be read, then the option will be treated
literally, and not removed.
Options in file are separated by whitespace. A whitespace
character may be included in an option by surrounding the entire
option in either single or double quotes. Any character (including a
backslash) may be included by prefixing the character to be included
with a backslash. The file may itself contain additional
@file options; any such options will be processed recursively.
Compiling C++ Programs
C ++ source files conventionally use one of the suffixes .C,
.cc, .cpp, .CPP, .c++, .cp, or
.cxx; C ++ header files often use .hh, .hpp,
.H, or (for shared template code) .tcc; and
preprocessed C ++ files use the suffix .ii. GCC recognizes
files with these names and compiles them as C ++ programs even if you
call the compiler the same way as for compiling C programs (usually
with the name gcc).
However, the use of gcc does not add the C++ library.
g++ is a program that calls GCC and automatically specifies linking
against the C++ library. It treats .c,
.h and .i files as C++ source files instead of C source
files unless -x is used. This program is also useful when
precompiling a C header file with a .h extension for use in C++
compilations. On many systems, g++ is also installed with
the name c++.
When you compile C++ programs, you may specify many of the same
command-line options that you use for compiling programs in any
language; or command-line options meaningful for C and related
languages; or options that are meaningful only for C++ programs.
Options Controlling C Dialect
The following options control the dialect of C (or languages derived
from C, such as C ++, Objective-C and Objective-C ++) that the compiler
accepts:
- -ansi
-
In C mode, this is equivalent to -std=c90. In C++ mode, it is
equivalent to -std=c++98.
This turns off certain features of GCC that are incompatible with ISO
C90 (when compiling C code), or of standard C++ (when compiling C++ code),
such as the "asm" and "typeof" keywords, and
predefined macros such as "unix" and "vax" that identify the
type of system you are using. It also enables the undesirable and
rarely used ISO trigraph feature. For the C compiler,
it disables recognition of C++ style // comments as well as
the "inline" keyword.
The alternate keywords "__asm__", "__extension__",
"__inline__" and "__typeof__" continue to work despite
-ansi. You would not want to use them in an ISO C program, of
course, but it is useful to put them in header files that might be included
in compilations done with -ansi. Alternate predefined macros
such as "__unix__" and "__vax__" are also available, with or
without -ansi.
The -ansi option does not cause non-ISO programs to be
rejected gratuitously. For that, -Wpedantic is required in
addition to -ansi.
The macro "__STRICT_ANSI__" is predefined when the -ansi
option is used. Some header files may notice this macro and refrain
from declaring certain functions or defining certain macros that the
ISO standard doesn't call for; this is to avoid interfering with any
programs that might use these names for other things.
Functions that are normally built in but do not have semantics
defined by ISO C (such as "alloca" and "ffs") are not built-in
functions when -ansi is used.
- -std=
-
Determine the language standard. This option
is currently only supported when compiling C or C++.
The compiler can accept several base standards, such as c90 or
c++98, and GNU dialects of those standards, such as
gnu90 or gnu++98. When a base standard is specified, the
compiler accepts all programs following that standard plus those
using GNU extensions that do not contradict it. For example,
-std=c90 turns off certain features of GCC that are
incompatible with ISO C90, such as the "asm" and "typeof"
keywords, but not other GNU extensions that do not have a meaning in
ISO C90, such as omitting the middle term of a "?:"
expression. On the other hand, when a GNU dialect of a standard is
specified, all features supported by the compiler are enabled, even when
those features change the meaning of the base standard. As a result, some
strict-conforming programs may be rejected. The particular standard
is used by -Wpedantic to identify which features are GNU
extensions given that version of the standard. For example
-std=gnu90 -Wpedantic warns about C++ style //
comments, while -std=gnu99 -Wpedantic does not.
A value for this option must be provided; possible values are
-
- c90
-
- c89
-
- iso9899:1990
-
Support all ISO C90 programs (certain GNU extensions that conflict
with ISO C90 are disabled). Same as -ansi for C code.
- iso9899:199409
-
ISO C90 as modified in amendment 1.
- c99
-
- c9x
-
- iso9899:1999
-
- iso9899:199x
-
ISO C99. This standard is substantially completely supported, modulo
bugs and floating-point issues
(mainly but not entirely relating to optional C99 features from
Annexes F and G). See
<http://gcc.gnu.org/c99status.html> for more information. The
names c9x and iso9899:199x are deprecated.
- c11
-
- c1x
-
- iso9899:2011
-
ISO C11, the 2011 revision of the ISO C standard. This standard is
substantially completely supported, modulo bugs, floating-point issues
(mainly but not entirely relating to optional C11 features from
Annexes F and G) and the optional Annexes K (Bounds-checking
interfaces) and L (Analyzability). The name c1x is deprecated.
- gnu90
-
- gnu89
-
GNU dialect of ISO C90 (including some C99 features).
- gnu99
-
- gnu9x
-
GNU dialect of ISO C99. The name gnu9x is deprecated.
- gnu11
-
- gnu1x
-
GNU dialect of ISO C11. This is the default for C code.
The name gnu1x is deprecated.
- c++98
-
- c++03
-
The 1998 ISO C++ standard plus the 2003 technical corrigendum and some
additional defect reports. Same as -ansi for C++ code.
- gnu++98
-
- gnu++03
-
GNU dialect of -std=c++98. This is the default for
C++ code.
- c++11
-
- c++0x
-
The 2011 ISO C++ standard plus amendments.
The name c++0x is deprecated.
- gnu++11
-
- gnu++0x
-
GNU dialect of -std=c++11.
The name gnu++0x is deprecated.
- c++14
-
- c++1y
-
The 2014 ISO C++ standard plus amendments.
The name c++1y is deprecated.
- gnu++14
-
- gnu++1y
-
GNU dialect of -std=c++14.
The name gnu++1y is deprecated.
- c++1z
-
The next revision of the ISO C++ standard, tentatively planned for
2017. Support is highly experimental, and will almost certainly
change in incompatible ways in future releases.
- gnu++1z
-
GNU dialect of -std=c++1z. Support is highly experimental,
and will almost certainly change in incompatible ways in future
releases.
-
- -fgnu89-inline
-
The option -fgnu89-inline tells GCC to use the traditional
GNU semantics for "inline" functions when in C99 mode.
Using this option is roughly equivalent to adding the
"gnu_inline" function attribute to all inline functions.
The option -fno-gnu89-inline explicitly tells GCC to use the
C99 semantics for "inline" when in C99 or gnu99 mode (i.e., it
specifies the default behavior).
This option is not supported in -std=c90 or
-std=gnu90 mode.
The preprocessor macros "__GNUC_GNU_INLINE__" and
"__GNUC_STDC_INLINE__" may be used to check which semantics are
in effect for "inline" functions.
- -aux-info filename
-
Output to the given filename prototyped declarations for all functions
declared and/or defined in a translation unit, including those in header
files. This option is silently ignored in any language other than C.
Besides declarations, the file indicates, in comments, the origin of
each declaration (source file and line), whether the declaration was
implicit, prototyped or unprototyped (I, N for new or
O for old, respectively, in the first character after the line
number and the colon), and whether it came from a declaration or a
definition (C or F, respectively, in the following
character). In the case of function definitions, a K&R-style list of
arguments followed by their declarations is also provided, inside
comments, after the declaration.
- -fallow-parameterless-variadic-functions
-
Accept variadic functions without named parameters.
Although it is possible to define such a function, this is not very
useful as it is not possible to read the arguments. This is only
supported for C as this construct is allowed by C++.
- -fno-asm
-
Do not recognize "asm", "inline" or "typeof" as a
keyword, so that code can use these words as identifiers. You can use
the keywords "__asm__", "__inline__" and "__typeof__"
instead. -ansi implies -fno-asm.
In C++, this switch only affects the "typeof" keyword, since
"asm" and "inline" are standard keywords. You may want to
use the -fno-gnu-keywords flag instead, which has the same
effect. In C99 mode (-std=c99 or -std=gnu99), this
switch only affects the "asm" and "typeof" keywords, since
"inline" is a standard keyword in ISO C99.
- -fno-builtin
-
- -fno-builtin-function
-
Don't recognize built-in functions that do not begin with
__builtin_ as prefix.
GCC normally generates special code to handle certain built-in functions
more efficiently; for instance, calls to "alloca" may become single
instructions which adjust the stack directly, and calls to "memcpy"
may become inline copy loops. The resulting code is often both smaller
and faster, but since the function calls no longer appear as such, you
cannot set a breakpoint on those calls, nor can you change the behavior
of the functions by linking with a different library. In addition,
when a function is recognized as a built-in function, GCC may use
information about that function to warn about problems with calls to
that function, or to generate more efficient code, even if the
resulting code still contains calls to that function. For example,
warnings are given with -Wformat for bad calls to
"printf" when "printf" is built in and "strlen" is
known not to modify global memory.
With the -fno-builtin-function option
only the built-in function function is
disabled. function must not begin with __builtin_. If a
function is named that is not built-in in this version of GCC, this
option is ignored. There is no corresponding
-fbuiltin-function option; if you wish to enable
built-in functions selectively when using -fno-builtin or
-ffreestanding, you may define macros such as:
#define abs(n) __builtin_abs ((n))
#define strcpy(d, s) __builtin_strcpy ((d), (s))
- -fhosted
-
Assert that compilation targets a hosted environment. This implies
-fbuiltin. A hosted environment is one in which the
entire standard library is available, and in which "main" has a return
type of "int". Examples are nearly everything except a kernel.
This is equivalent to -fno-freestanding.
- -ffreestanding
-
Assert that compilation targets a freestanding environment. This
implies -fno-builtin. A freestanding environment
is one in which the standard library may not exist, and program startup may
not necessarily be at "main". The most obvious example is an OS kernel.
This is equivalent to -fno-hosted.
- -fopenacc
-
Enable handling of OpenACC directives "#pragma acc" in C/C++ and
"!$acc" in Fortran. When -fopenacc is specified, the
compiler generates accelerated code according to the OpenACC Application
Programming Interface v2.0 <http://www.openacc.org/>. This option
implies -pthread, and thus is only supported on targets that
have support for -pthread.
Note that this is an experimental feature, incomplete, and subject to
change in future versions of GCC. See
<https://gcc.gnu.org/wiki/OpenACC> for more information.
- -fopenmp
-
Enable handling of OpenMP directives "#pragma omp" in C/C++ and
"!$omp" in Fortran. When -fopenmp is specified, the
compiler generates parallel code according to the OpenMP Application
Program Interface v4.0 <http://www.openmp.org/>. This option
implies -pthread, and thus is only supported on targets that
have support for -pthread. -fopenmp implies
-fopenmp-simd.
- -fopenmp-simd
-
Enable handling of OpenMP's SIMD directives with "#pragma omp"
in C/C++ and "!$omp" in Fortran. Other OpenMP directives
are ignored.
- -fcilkplus
-
Enable the usage of Cilk Plus language extension features for C/C++.
When the option -fcilkplus is specified, enable the usage of
the Cilk Plus Language extension features for C/C++. The present
implementation follows ABI version 1.2. This is an experimental
feature that is only partially complete, and whose interface may
change in future versions of GCC as the official specification
changes. Currently, all features but "_Cilk_for" have been
implemented.
- -fgnu-tm
-
When the option -fgnu-tm is specified, the compiler
generates code for the Linux variant of Intel's current Transactional
Memory ABI specification document (Revision 1.1, May 6 2009). This is
an experimental feature whose interface may change in future versions
of GCC, as the official specification changes. Please note that not
all architectures are supported for this feature.
For more information on GCC's support for transactional memory,
Note that the transactional memory feature is not supported with
non-call exceptions (-fnon-call-exceptions).
- -fms-extensions
-
Accept some non-standard constructs used in Microsoft header files.
In C++ code, this allows member names in structures to be similar
to previous types declarations.
typedef int UOW;
struct ABC {
UOW UOW;
};
Some cases of unnamed fields in structures and unions are only
accepted with this option.
Note that this option is off for all targets but x86
targets using ms-abi.
- -fplan9-extensions
-
Accept some non-standard constructs used in Plan 9 code.
This enables -fms-extensions, permits passing pointers to
structures with anonymous fields to functions that expect pointers to
elements of the type of the field, and permits referring to anonymous
fields declared using a typedef. This is only
supported for C, not C++.
- -trigraphs
-
Support ISO C trigraphs. The -ansi option (and -std
options for strict ISO C conformance) implies -trigraphs.
- -traditional
-
- -traditional-cpp
-
Formerly, these options caused GCC to attempt to emulate a pre-standard
C compiler. They are now only supported with the -E switch.
The preprocessor continues to support a pre-standard mode. See the GNU
CPP manual for details.
- -fcond-mismatch
-
Allow conditional expressions with mismatched types in the second and
third arguments. The value of such an expression is void. This option
is not supported for C++.
- -flax-vector-conversions
-
Allow implicit conversions between vectors with differing numbers of
elements and/or incompatible element types. This option should not be
used for new code.
- -funsigned-char
-
Let the type "char" be unsigned, like "unsigned char".
Each kind of machine has a default for what "char" should
be. It is either like "unsigned char" by default or like
"signed char" by default.
Ideally, a portable program should always use "signed char" or
"unsigned char" when it depends on the signedness of an object.
But many programs have been written to use plain "char" and
expect it to be signed, or expect it to be unsigned, depending on the
machines they were written for. This option, and its inverse, let you
make such a program work with the opposite default.
The type "char" is always a distinct type from each of
"signed char" or "unsigned char", even though its behavior
is always just like one of those two.
- -fsigned-char
-
Let the type "char" be signed, like "signed char".
Note that this is equivalent to -fno-unsigned-char, which is
the negative form of -funsigned-char. Likewise, the option
-fno-signed-char is equivalent to -funsigned-char.
- -fsigned-bitfields
-
- -funsigned-bitfields
-
- -fno-signed-bitfields
-
- -fno-unsigned-bitfields
-
These options control whether a bit-field is signed or unsigned, when the
declaration does not use either "signed" or "unsigned". By
default, such a bit-field is signed, because this is consistent: the
basic integer types such as "int" are signed types.
Options Controlling C++ Dialect
This section describes the command-line options that are only meaningful
for C ++ programs. You can also use most of the GNU compiler options
regardless of what language your program is in. For example, you
might compile a file firstClass.C like this:
g++ -g -frepo -O -c firstClass.C
In this example, only -frepo is an option meant
only for C++ programs; you can use the other options with any
language supported by GCC.
Here is a list of options that are only for compiling C++ programs:
- -fabi-version=n
-
Use version n of the C++ ABI. The default is version 0.
Version 0 refers to the version conforming most closely to
the C++ ABI specification. Therefore, the ABI obtained using version 0
will change in different versions of G++ as ABI bugs are fixed.
Version 1 is the version of the C++ ABI that first appeared in G++ 3.2.
Version 2 is the version of the C++ ABI that first appeared in G++
3.4, and was the default through G++ 4.9.
Version 3 corrects an error in mangling a constant address as a
template argument.
Version 4, which first appeared in G++ 4.5, implements a standard
mangling for vector types.
Version 5, which first appeared in G++ 4.6, corrects the mangling of
attribute const/volatile on function pointer types, decltype of a
plain decl, and use of a function parameter in the declaration of
another parameter.
Version 6, which first appeared in G++ 4.7, corrects the promotion
behavior of C++11 scoped enums and the mangling of template argument
packs, const/static_cast, prefix ++ and --, and a class scope function
used as a template argument.
Version 7, which first appeared in G++ 4.8, that treats nullptr_t as a
builtin type and corrects the mangling of lambdas in default argument
scope.
Version 8, which first appeared in G++ 4.9, corrects the substitution
behavior of function types with function-cv-qualifiers.
Version 9, which first appeared in G++ 5.2, corrects the alignment of
"nullptr_t".
See also -Wabi.
- -fabi-compat-version=n
-
On targets that support strong aliases, G++
works around mangling changes by creating an alias with the correct
mangled name when defining a symbol with an incorrect mangled name.
This switch specifies which ABI version to use for the alias.
With -fabi-version=0 (the default), this defaults to 2. If
another ABI version is explicitly selected, this defaults to 0.
The compatibility version is also set by -Wabi=n.
- -fno-access-control
-
Turn off all access checking. This switch is mainly useful for working
around bugs in the access control code.
- -fcheck-new
-
Check that the pointer returned by "operator new" is non-null
before attempting to modify the storage allocated. This check is
normally unnecessary because the C++ standard specifies that
"operator new" only returns 0 if it is declared
"throw()", in which case the compiler always checks the
return value even without this option. In all other cases, when
"operator new" has a non-empty exception specification, memory
exhaustion is signalled by throwing "std::bad_alloc". See also
new (nothrow).
- -fconstexpr-depth=n
-
Set the maximum nested evaluation depth for C++11 constexpr functions
to n. A limit is needed to detect endless recursion during
constant expression evaluation. The minimum specified by the standard
is 512.
- -fdeduce-init-list
-
Enable deduction of a template type parameter as
"std::initializer_list" from a brace-enclosed initializer list, i.e.
template <class T> auto forward(T t) -> decltype (realfn (t))
{
return realfn (t);
}
void f()
{
forward({1,2}); // call forward<std::initializer_list<int>>
}
This deduction was implemented as a possible extension to the
originally proposed semantics for the C++11 standard, but was not part
of the final standard, so it is disabled by default. This option is
deprecated, and may be removed in a future version of G++.
- -ffriend-injection
-
Inject friend functions into the enclosing namespace, so that they are
visible outside the scope of the class in which they are declared.
Friend functions were documented to work this way in the old Annotated
C++ Reference Manual.
However, in ISO C++ a friend function that is not declared
in an enclosing scope can only be found using argument dependent
lookup. GCC defaults to the standard behavior.
This option is for compatibility, and may be removed in a future
release of G++.
- -fno-elide-constructors
-
The C++ standard allows an implementation to omit creating a temporary
that is only used to initialize another object of the same type.
Specifying this option disables that optimization, and forces G++ to
call the copy constructor in all cases.
- -fno-enforce-eh-specs
-
Don't generate code to check for violation of exception specifications
at run time. This option violates the C++ standard, but may be useful
for reducing code size in production builds, much like defining
"NDEBUG". This does not give user code permission to throw
exceptions in violation of the exception specifications; the compiler
still optimizes based on the specifications, so throwing an
unexpected exception results in undefined behavior at run time.
- -fextern-tls-init
-
- -fno-extern-tls-init
-
The C++11 and OpenMP standards allow "thread_local" and
"threadprivate" variables to have dynamic (runtime)
initialization. To support this, any use of such a variable goes
through a wrapper function that performs any necessary initialization.
When the use and definition of the variable are in the same
translation unit, this overhead can be optimized away, but when the
use is in a different translation unit there is significant overhead
even if the variable doesn't actually need dynamic initialization. If
the programmer can be sure that no use of the variable in a
non-defining TU needs to trigger dynamic initialization (either
because the variable is statically initialized, or a use of the
variable in the defining TU will be executed before any uses in
another TU), they can avoid this overhead with the
-fno-extern-tls-init option.
On targets that support symbol aliases, the default is
-fextern-tls-init. On targets that do not support symbol
aliases, the default is -fno-extern-tls-init.
- -ffor-scope
-
- -fno-for-scope
-
If -ffor-scope is specified, the scope of variables declared in
a for-init-statement is limited to the "for" loop itself,
as specified by the C++ standard.
If -fno-for-scope is specified, the scope of variables declared in
a for-init-statement extends to the end of the enclosing scope,
as was the case in old versions of G++, and other (traditional)
implementations of C++.
If neither flag is given, the default is to follow the standard,
but to allow and give a warning for old-style code that would
otherwise be invalid, or have different behavior.
- -fno-gnu-keywords
-
Do not recognize "typeof" as a keyword, so that code can use this
word as an identifier. You can use the keyword "__typeof__" instead.
-ansi implies -fno-gnu-keywords.
- -fno-implicit-templates
-
Never emit code for non-inline templates that are instantiated
implicitly (i.e. by use); only emit code for explicit instantiations.
- -fno-implicit-inline-templates
-
Don't emit code for implicit instantiations of inline templates, either.
The default is to handle inlines differently so that compiles with and
without optimization need the same set of explicit instantiations.
- -fno-implement-inlines
-
To save space, do not emit out-of-line copies of inline functions
controlled by "#pragma implementation". This causes linker
errors if these functions are not inlined everywhere they are called.
- -fms-extensions
-
Disable Wpedantic warnings about constructs used in MFC, such as implicit
int and getting a pointer to member function via non-standard syntax.
- -fno-nonansi-builtins
-
Disable built-in declarations of functions that are not mandated by
ANSI/ISO C. These include "ffs", "alloca", "_exit",
"index", "bzero", "conjf", and other related functions.
- -fnothrow-opt
-
Treat a "throw()" exception specification as if it were a
"noexcept" specification to reduce or eliminate the text size
overhead relative to a function with no exception specification. If
the function has local variables of types with non-trivial
destructors, the exception specification actually makes the
function smaller because the EH cleanups for those variables can be
optimized away. The semantic effect is that an exception thrown out of
a function with such an exception specification results in a call
to "terminate" rather than "unexpected".
- -fno-operator-names
-
Do not treat the operator name keywords "and", "bitand",
"bitor", "compl", "not", "or" and "xor" as
synonyms as keywords.
- -fno-optional-diags
-
Disable diagnostics that the standard says a compiler does not need to
issue. Currently, the only such diagnostic issued by G++ is the one for
a name having multiple meanings within a class.
- -fpermissive
-
Downgrade some diagnostics about nonconformant code from errors to
warnings. Thus, using -fpermissive allows some
nonconforming code to compile.
- -fno-pretty-templates
-
When an error message refers to a specialization of a function
template, the compiler normally prints the signature of the
template followed by the template arguments and any typedefs or
typenames in the signature (e.g. "void f(T) [with T = int]"
rather than "void f(int)") so that it's clear which template is
involved. When an error message refers to a specialization of a class
template, the compiler omits any template arguments that match
the default template arguments for that template. If either of these
behaviors make it harder to understand the error message rather than
easier, you can use -fno-pretty-templates to disable them.
- -frepo
-
Enable automatic template instantiation at link time. This option also
implies -fno-implicit-templates.
- -fno-rtti
-
Disable generation of information about every class with virtual
functions for use by the C++ run-time type identification features
("dynamic_cast" and "typeid"). If you don't use those parts
of the language, you can save some space by using this flag. Note that
exception handling uses the same information, but G++ generates it as
needed. The "dynamic_cast" operator can still be used for casts that
do not require run-time type information, i.e. casts to "void *" or to
unambiguous base classes.
- -fsized-deallocation
-
Enable the built-in global declarations
void operator delete (void *, std::size_t) noexcept;
void operator delete[] (void *, std::size_t) noexcept;
as introduced in C++14. This is useful for user-defined replacement
deallocation functions that, for example, use the size of the object
to make deallocation faster. Enabled by default under
-std=c++14 and above. The flag -Wsized-deallocation
warns about places that might want to add a definition.
- -fstats
-
Emit statistics about front-end processing at the end of the compilation.
This information is generally only useful to the G++ development team.
- -fstrict-enums
-
Allow the compiler to optimize using the assumption that a value of
enumerated type can only be one of the values of the enumeration (as
defined in the C++ standard; basically, a value that can be
represented in the minimum number of bits needed to represent all the
enumerators). This assumption may not be valid if the program uses a
cast to convert an arbitrary integer value to the enumerated type.
- -ftemplate-backtrace-limit=n
-
Set the maximum number of template instantiation notes for a single
warning or error to n. The default value is 10.
- -ftemplate-depth=n
-
Set the maximum instantiation depth for template classes to n.
A limit on the template instantiation depth is needed to detect
endless recursions during template class instantiation. ANSI/ISO C++
conforming programs must not rely on a maximum depth greater than 17
(changed to 1024 in C++11). The default value is 900, as the compiler
can run out of stack space before hitting 1024 in some situations.
- -fno-threadsafe-statics
-
Do not emit the extra code to use the routines specified in the C++
ABI for thread-safe initialization of local statics. You can use this
option to reduce code size slightly in code that doesn't need to be
thread-safe.
- -fuse-cxa-atexit
-
Register destructors for objects with static storage duration with the
"__cxa_atexit" function rather than the "atexit" function.
This option is required for fully standards-compliant handling of static
destructors, but only works if your C library supports
"__cxa_atexit".
- -fno-use-cxa-get-exception-ptr
-
Don't use the "__cxa_get_exception_ptr" runtime routine. This
causes "std::uncaught_exception" to be incorrect, but is necessary
if the runtime routine is not available.
- -fvisibility-inlines-hidden
-
This switch declares that the user does not attempt to compare
pointers to inline functions or methods where the addresses of the two functions
are taken in different shared objects.
The effect of this is that GCC may, effectively, mark inline methods with
"__attribute__ ((visibility ("hidden")))" so that they do not
appear in the export table of a DSO and do not require a PLT indirection
when used within the DSO. Enabling this option can have a dramatic effect
on load and link times of a DSO as it massively reduces the size of the
dynamic export table when the library makes heavy use of templates.
The behavior of this switch is not quite the same as marking the
methods as hidden directly, because it does not affect static variables
local to the function or cause the compiler to deduce that
the function is defined in only one shared object.
You may mark a method as having a visibility explicitly to negate the
effect of the switch for that method. For example, if you do want to
compare pointers to a particular inline method, you might mark it as
having default visibility. Marking the enclosing class with explicit
visibility has no effect.
Explicitly instantiated inline methods are unaffected by this option
as their linkage might otherwise cross a shared library boundary.
- -fvisibility-ms-compat
-
This flag attempts to use visibility settings to make GCC's C++
linkage model compatible with that of Microsoft Visual Studio.
The flag makes these changes to GCC's linkage model:
-
- 1.
-
It sets the default visibility to "hidden", like
-fvisibility=hidden.
- 2.
-
Types, but not their members, are not hidden by default.
- 3.
-
The One Definition Rule is relaxed for types without explicit
visibility specifications that are defined in more than one
shared object: those declarations are permitted if they are
permitted when this option is not used.
-
In new code it is better to use -fvisibility=hidden and
export those classes that are intended to be externally visible.
Unfortunately it is possible for code to rely, perhaps accidentally,
on the Visual Studio behavior.
Among the consequences of these changes are that static data members
of the same type with the same name but defined in different shared
objects are different, so changing one does not change the other;
and that pointers to function members defined in different shared
objects may not compare equal. When this flag is given, it is a
violation of the ODR to define types with the same name differently.
- -fvtable-verify=[std|preinit|none]
-
Turn on (or off, if using -fvtable-verify=none) the security
feature that verifies at run time, for every virtual call, that
the vtable pointer through which the call is made is valid for the type of
the object, and has not been corrupted or overwritten. If an invalid vtable
pointer is detected at run time, an error is reported and execution of the
program is immediately halted.
This option causes run-time data structures to be built at program startup,
which are used for verifying the vtable pointers.
The options std and preinit
control the timing of when these data structures are built. In both cases the
data structures are built before execution reaches "main". Using
-fvtable-verify=std causes the data structures to be built after
shared libraries have been loaded and initialized.
-fvtable-verify=preinit causes them to be built before shared
libraries have been loaded and initialized.
If this option appears multiple times in the command line with different
values specified, none takes highest priority over both std and
preinit; preinit takes priority over std.
- -fvtv-debug
-
When used in conjunction with -fvtable-verify=std or
-fvtable-verify=preinit, causes debug versions of the
runtime functions for the vtable verification feature to be called.
This flag also causes the compiler to log information about which
vtable pointers it finds for each class.
This information is written to a file named vtv_set_ptr_data.log
in the directory named by the environment variable VTV_LOGS_DIR
if that is defined or the current working directory otherwise.
Note: This feature appends data to the log file. If you want a fresh log
file, be sure to delete any existing one.
- -fvtv-counts
-
This is a debugging flag. When used in conjunction with
-fvtable-verify=std or -fvtable-verify=preinit, this
causes the compiler to keep track of the total number of virtual calls
it encounters and the number of verifications it inserts. It also
counts the number of calls to certain run-time library functions
that it inserts and logs this information for each compilation unit.
The compiler writes this information to a file named
vtv_count_data.log in the directory named by the environment
variable VTV_LOGS_DIR if that is defined or the current working
directory otherwise. It also counts the size of the vtable pointer sets
for each class, and writes this information to vtv_class_set_sizes.log
in the same directory.
Note: This feature appends data to the log files. To get fresh log
files, be sure to delete any existing ones.
- -fno-weak
-
Do not use weak symbol support, even if it is provided by the linker.
By default, G++ uses weak symbols if they are available. This
option exists only for testing, and should not be used by end-users;
it results in inferior code and has no benefits. This option may
be removed in a future release of G++.
- -nostdinc++
-
Do not search for header files in the standard directories specific to
C++, but do still search the other standard directories. (This option
is used when building the C++ library.)
In addition, these optimization, warning, and code generation options
have meanings only for C++ programs:
- -Wabi (C, Objective-C, C++ and Objective-C++ only)
-
When an explicit -fabi-version=n option is used, causes
G++ to warn when it generates code that is probably not compatible with the
vendor-neutral C++ ABI. Since G++ now defaults to
-fabi-version=0, -Wabi has no effect unless either
an older ABI version is selected (with -fabi-version=n)
or an older compatibility version is selected (with
-Wabi=n or -fabi-compat-version=n).
Although an effort has been made to warn about
all such cases, there are probably some cases that are not warned about,
even though G++ is generating incompatible code. There may also be
cases where warnings are emitted even though the code that is generated
is compatible.
You should rewrite your code to avoid these warnings if you are
concerned about the fact that code generated by G++ may not be binary
compatible with code generated by other compilers.
-Wabi can also be used with an explicit version number to
warn about compatibility with a particular -fabi-version
level, e.g. -Wabi=2 to warn about changes relative to
-fabi-version=2. Specifying a version number also sets
-fabi-compat-version=n.
The known incompatibilities in -fabi-version=2 (which was the
default from GCC 3.4 to 4.9) include:
-
- *
-
A template with a non-type template parameter of reference type was
mangled incorrectly:
extern int N;
template <int &> struct S {};
void n (S<N>) {2}
This was fixed in -fabi-version=3.
- *
-
SIMD vector types declared using "__attribute ((vector_size))" were
mangled in a non-standard way that does not allow for overloading of
functions taking vectors of different sizes.
The mangling was changed in -fabi-version=4.
- *
-
"__attribute ((const))" and "noreturn" were mangled as type
qualifiers, and "decltype" of a plain declaration was folded away.
These mangling issues were fixed in -fabi-version=5.
- *
-
Scoped enumerators passed as arguments to a variadic function are
promoted like unscoped enumerators, causing "va_arg" to complain.
On most targets this does not actually affect the parameter passing
ABI, as there is no way to pass an argument smaller than "int".
Also, the ABI changed the mangling of template argument packs,
"const_cast", "static_cast", prefix increment/decrement, and
a class scope function used as a template argument.
These issues were corrected in -fabi-version=6.
- *
-
Lambdas in default argument scope were mangled incorrectly, and the
ABI changed the mangling of "nullptr_t".
These issues were corrected in -fabi-version=7.
- *
-
When mangling a function type with function-cv-qualifiers, the
un-qualified function type was incorrectly treated as a substitution
candidate.
This was fixed in -fabi-version=8, the default for GCC 5.1.
- *
-
"decltype(nullptr)" incorrectly had an alignment of 1, leading to
unaligned accesses. Note that this did not affect the ABI of a
function with a "nullptr_t" parameter, as parameters have a
minimum alignment.
This was fixed in -fabi-version=9, the default for GCC 5.2.
-
It also warns about psABI-related changes. The known psABI changes at this
point include:
- *
-
For SysV/x86-64, unions with "long double" members are
passed in memory as specified in psABI. For example:
union U {
long double ld;
int i;
};
"union U" is always passed in memory.
-
- -Wabi-tag (C++ and Objective-C++ only)
-
Warn when a type with an ABI tag is used in a context that does not
have that ABI tag. See C++ Attributes for more information
about ABI tags.
- -Wctor-dtor-privacy (C++ and Objective-C++ only)
-
Warn when a class seems unusable because all the constructors or
destructors in that class are private, and it has neither friends nor
public static member functions. Also warn if there are no non-private
methods, and there's at least one private member function that isn't
a constructor or destructor.
- -Wdelete-non-virtual-dtor (C++ and Objective-C++ only)
-
Warn when "delete" is used to destroy an instance of a class that
has virtual functions and non-virtual destructor. It is unsafe to delete
an instance of a derived class through a pointer to a base class if the
base class does not have a virtual destructor. This warning is enabled
by -Wall.
- -Wliteral-suffix (C++ and Objective-C++ only)
-
Warn when a string or character literal is followed by a ud-suffix which does
not begin with an underscore. As a conforming extension, GCC treats such
suffixes as separate preprocessing tokens in order to maintain backwards
compatibility with code that uses formatting macros from "<inttypes.h>".
For example:
#define __STDC_FORMAT_MACROS
#include <inttypes.h>
#include <stdio.h>
int main() {
int64_t i64 = 123;
printf("My int64: %"PRId64"\n", i64);
}
In this case, "PRId64" is treated as a separate preprocessing token.
This warning is enabled by default.
- -Wnarrowing (C++ and Objective-C++ only)
-
Warn when a narrowing conversion prohibited by C++11 occurs within
{ }, e.g.
int i = { 2.2 }; // error: narrowing from double to int
This flag is included in -Wall and -Wc++11-compat.
With -std=c++11, -Wno-narrowing suppresses the diagnostic
required by the standard. Note that this does not affect the meaning
of well-formed code; narrowing conversions are still considered
ill-formed in SFINAE context.
- -Wnoexcept (C++ and Objective-C++ only)
-
Warn when a noexcept-expression evaluates to false because of a call
to a function that does not have a non-throwing exception
specification (i.e. "throw()" or "noexcept") but is known by
the compiler to never throw an exception.
- -Wnon-virtual-dtor (C++ and Objective-C++ only)
-
Warn when a class has virtual functions and an accessible non-virtual
destructor itself or in an accessible polymorphic base class, in which
case it is possible but unsafe to delete an instance of a derived
class through a pointer to the class itself or base class. This
warning is automatically enabled if -Weffc++ is specified.
- -Wreorder (C++ and Objective-C++ only)
-
Warn when the order of member initializers given in the code does not
match the order in which they must be executed. For instance:
struct A {
int i;
int j;
A(): j (0), i (1) { }
};
The compiler rearranges the member initializers for "i"
and "j" to match the declaration order of the members, emitting
a warning to that effect. This warning is enabled by -Wall.
- -fext-numeric-literals (C++ and Objective-C++ only)
-
Accept imaginary, fixed-point, or machine-defined
literal number suffixes as GNU extensions.
When this option is turned off these suffixes are treated
as C++11 user-defined literal numeric suffixes.
This is on by default for all pre-C++11 dialects and all GNU dialects:
-std=c++98, -std=gnu++98, -std=gnu++11,
-std=gnu++14.
This option is off by default
for ISO C++11 onwards (-std=c++11, ...).
The following -W... options are not affected by -Wall.
- -Weffc++ (C++ and Objective-C++ only)
-
Warn about violations of the following style guidelines from Scott Meyers'
Effective C++ series of books:
-
- *
-
Define a copy constructor and an assignment operator for classes
with dynamically-allocated memory.
- *
-
Prefer initialization to assignment in constructors.
- *
-
Have "operator=" return a reference to *this.
- *
-
Don't try to return a reference when you must return an object.
- *
-
Distinguish between prefix and postfix forms of increment and
decrement operators.
- *
-
Never overload "&&", "||", or ",".
-
This option also enables -Wnon-virtual-dtor, which is also
one of the effective C++ recommendations. However, the check is
extended to warn about the lack of virtual destructor in accessible
non-polymorphic bases classes too.
When selecting this option, be aware that the standard library
headers do not obey all of these guidelines; use grep -v
to filter out those warnings.
- -Wstrict-null-sentinel (C++ and Objective-C++ only)
-
Warn about the use of an uncasted "NULL" as sentinel. When
compiling only with GCC this is a valid sentinel, as "NULL" is defined
to "__null". Although it is a null pointer constant rather than a
null pointer, it is guaranteed to be of the same size as a pointer.
But this use is not portable across different compilers.
- -Wno-non-template-friend (C++ and Objective-C++ only)
-
Disable warnings when non-templatized friend functions are declared
within a template. Since the advent of explicit template specification
support in G++, if the name of the friend is an unqualified-id (i.e.,
friend foo(int)), the C++ language specification demands that the
friend declare or define an ordinary, nontemplate function. (Section
14.5.3). Before G++ implemented explicit specification, unqualified-ids
could be interpreted as a particular specialization of a templatized
function. Because this non-conforming behavior is no longer the default
behavior for G++, -Wnon-template-friend allows the compiler to
check existing code for potential trouble spots and is on by default.
This new compiler behavior can be turned off with
-Wno-non-template-friend, which keeps the conformant compiler code
but disables the helpful warning.
- -Wold-style-cast (C++ and Objective-C++ only)
-
Warn if an old-style (C-style) cast to a non-void type is used within
a C++ program. The new-style casts ("dynamic_cast",
"static_cast", "reinterpret_cast", and "const_cast") are
less vulnerable to unintended effects and much easier to search for.
- -Woverloaded-virtual (C++ and Objective-C++ only)
-
Warn when a function declaration hides virtual functions from a
base class. For example, in:
struct A {
virtual void f();
};
struct B: public A {
void f(int);
};
the "A" class version of "f" is hidden in "B", and code
like:
B* b;
b->f();
fails to compile.
- -Wno-pmf-conversions (C++ and Objective-C++ only)
-
Disable the diagnostic for converting a bound pointer to member function
to a plain pointer.
- -Wsign-promo (C++ and Objective-C++ only)
-
Warn when overload resolution chooses a promotion from unsigned or
enumerated type to a signed type, over a conversion to an unsigned type of
the same size. Previous versions of G++ tried to preserve
unsignedness, but the standard mandates the current behavior.
Options Controlling Objective-C and Objective-C++ Dialects
( NOTE: This manual does not describe the Objective-C and Objective-C ++
languages themselves.
This section describes the command-line options that are only meaningful
for Objective-C and Objective-C++ programs. You can also use most of
the language-independent GNU compiler options.
For example, you might compile a file some_class.m like this:
gcc -g -fgnu-runtime -O -c some_class.m
In this example, -fgnu-runtime is an option meant only for
Objective-C and Objective-C++ programs; you can use the other options with
any language supported by GCC.
Note that since Objective-C is an extension of the C language, Objective-C
compilations may also use options specific to the C front-end (e.g.,
-Wtraditional). Similarly, Objective-C++ compilations may use
C++-specific options (e.g., -Wabi).
Here is a list of options that are only for compiling Objective-C
and Objective-C++ programs:
- -fconstant-string-class=class-name
-
Use class-name as the name of the class to instantiate for each
literal string specified with the syntax "@"..."". The default
class name is "NXConstantString" if the GNU runtime is being used, and
"NSConstantString" if the NeXT runtime is being used (see below). The
-fconstant-cfstrings option, if also present, overrides the
-fconstant-string-class setting and cause "@"..."" literals
to be laid out as constant CoreFoundation strings.
- -fgnu-runtime
-
Generate object code compatible with the standard GNU Objective-C
runtime. This is the default for most types of systems.
- -fnext-runtime
-
Generate output compatible with the NeXT runtime. This is the default
for NeXT-based systems, including Darwin and Mac OS X. The macro
"__NEXT_RUNTIME__" is predefined if (and only if) this option is
used.
- -fno-nil-receivers
-
Assume that all Objective-C message dispatches ("[receiver
message:arg]") in this translation unit ensure that the receiver is
not "nil". This allows for more efficient entry points in the
runtime to be used. This option is only available in conjunction with
the NeXT runtime and ABI version 0 or 1.
- -fobjc-abi-version=n
-
Use version n of the Objective-C ABI for the selected runtime.
This option is currently supported only for the NeXT runtime. In that
case, Version 0 is the traditional (32-bit) ABI without support for
properties and other Objective-C 2.0 additions. Version 1 is the
traditional (32-bit) ABI with support for properties and other
Objective-C 2.0 additions. Version 2 is the modern (64-bit) ABI. If
nothing is specified, the default is Version 0 on 32-bit target
machines, and Version 2 on 64-bit target machines.
- -fobjc-call-cxx-cdtors
-
For each Objective-C class, check if any of its instance variables is a
C++ object with a non-trivial default constructor. If so, synthesize a
special "- (id) .cxx_construct" instance method which runs
non-trivial default constructors on any such instance variables, in order,
and then return "self". Similarly, check if any instance variable
is a C++ object with a non-trivial destructor, and if so, synthesize a
special "- (void) .cxx_destruct" method which runs
all such default destructors, in reverse order.
The "- (id) .cxx_construct" and "- (void) .cxx_destruct"
methods thusly generated only operate on instance variables
declared in the current Objective-C class, and not those inherited
from superclasses. It is the responsibility of the Objective-C
runtime to invoke all such methods in an object's inheritance
hierarchy. The "- (id) .cxx_construct" methods are invoked
by the runtime immediately after a new object instance is allocated;
the "- (void) .cxx_destruct" methods are invoked immediately
before the runtime deallocates an object instance.
As of this writing, only the NeXT runtime on Mac OS X 10.4 and later has
support for invoking the "- (id) .cxx_construct" and
"- (void) .cxx_destruct" methods.
- -fobjc-direct-dispatch
-
Allow fast jumps to the message dispatcher. On Darwin this is
accomplished via the comm page.
- -fobjc-exceptions
-
Enable syntactic support for structured exception handling in
Objective-C, similar to what is offered by C++ and Java. This option
is required to use the Objective-C keywords @try,
@throw, @catch, @finally and
@synchronized. This option is available with both the GNU
runtime and the NeXT runtime (but not available in conjunction with
the NeXT runtime on Mac OS X 10.2 and earlier).
- -fobjc-gc
-
Enable garbage collection (GC) in Objective-C and Objective-C++
programs. This option is only available with the NeXT runtime; the
GNU runtime has a different garbage collection implementation that
does not require special compiler flags.
- -fobjc-nilcheck
-
For the NeXT runtime with version 2 of the ABI, check for a nil
receiver in method invocations before doing the actual method call.
This is the default and can be disabled using
-fno-objc-nilcheck. Class methods and super calls are never
checked for nil in this way no matter what this flag is set to.
Currently this flag does nothing when the GNU runtime, or an older
version of the NeXT runtime ABI, is used.
- -fobjc-std=objc1
-
Conform to the language syntax of Objective-C 1.0, the language
recognized by GCC 4.0. This only affects the Objective-C additions to
the C/C++ language; it does not affect conformance to C/C++ standards,
which is controlled by the separate C/C++ dialect option flags. When
this option is used with the Objective-C or Objective-C++ compiler,
any Objective-C syntax that is not recognized by GCC 4.0 is rejected.
This is useful if you need to make sure that your Objective-C code can
be compiled with older versions of GCC.
- -freplace-objc-classes
-
Emit a special marker instructing ld(1) not to statically link in
the resulting object file, and allow dyld(1) to load it in at
run time instead. This is used in conjunction with the Fix-and-Continue
debugging mode, where the object file in question may be recompiled and
dynamically reloaded in the course of program execution, without the need
to restart the program itself. Currently, Fix-and-Continue functionality
is only available in conjunction with the NeXT runtime on Mac OS X 10.3
and later.
- -fzero-link
-
When compiling for the NeXT runtime, the compiler ordinarily replaces calls
to "objc_getClass("...")" (when the name of the class is known at
compile time) with static class references that get initialized at load time,
which improves run-time performance. Specifying the -fzero-link flag
suppresses this behavior and causes calls to "objc_getClass("...")"
to be retained. This is useful in Zero-Link debugging mode, since it allows
for individual class implementations to be modified during program execution.
The GNU runtime currently always retains calls to "objc_get_class("...")"
regardless of command-line options.
- -fno-local-ivars
-
By default instance variables in Objective-C can be accessed as if
they were local variables from within the methods of the class they're
declared in. This can lead to shadowing between instance variables
and other variables declared either locally inside a class method or
globally with the same name. Specifying the -fno-local-ivars
flag disables this behavior thus avoiding variable shadowing issues.
- -fivar-visibility=[public|protected|private|package]
-
Set the default instance variable visibility to the specified option
so that instance variables declared outside the scope of any access
modifier directives default to the specified visibility.
- -gen-decls
-
Dump interface declarations for all classes seen in the source file to a
file named sourcename.decl.
- -Wassign-intercept (Objective-C and Objective-C++ only)
-
Warn whenever an Objective-C assignment is being intercepted by the
garbage collector.
- -Wno-protocol (Objective-C and Objective-C++ only)
-
If a class is declared to implement a protocol, a warning is issued for
every method in the protocol that is not implemented by the class. The
default behavior is to issue a warning for every method not explicitly
implemented in the class, even if a method implementation is inherited
from the superclass. If you use the -Wno-protocol option, then
methods inherited from the superclass are considered to be implemented,
and no warning is issued for them.
- -Wselector (Objective-C and Objective-C++ only)
-
Warn if multiple methods of different types for the same selector are
found during compilation. The check is performed on the list of methods
in the final stage of compilation. Additionally, a check is performed
for each selector appearing in a "@selector(...)"
expression, and a corresponding method for that selector has been found
during compilation. Because these checks scan the method table only at
the end of compilation, these warnings are not produced if the final
stage of compilation is not reached, for example because an error is
found during compilation, or because the -fsyntax-only option is
being used.
- -Wstrict-selector-match (Objective-C and Objective-C++ only)
-
Warn if multiple methods with differing argument and/or return types are
found for a given selector when attempting to send a message using this
selector to a receiver of type "id" or "Class". When this flag
is off (which is the default behavior), the compiler omits such warnings
if any differences found are confined to types that share the same size
and alignment.
- -Wundeclared-selector (Objective-C and Objective-C++ only)
-
Warn if a "@selector(...)" expression referring to an
undeclared selector is found. A selector is considered undeclared if no
method with that name has been declared before the
"@selector(...)" expression, either explicitly in an
@interface or @protocol declaration, or implicitly in
an @implementation section. This option always performs its
checks as soon as a "@selector(...)" expression is found,
while -Wselector only performs its checks in the final stage of
compilation. This also enforces the coding style convention
that methods and selectors must be declared before being used.
- -print-objc-runtime-info
-
Generate C header describing the largest structure that is passed by
value, if any.
Options to Control Diagnostic Messages Formatting
Traditionally, diagnostic messages have been formatted irrespective of
the output device's aspect (e.g. its width, ...). You can use the
options described below
to control the formatting algorithm for diagnostic messages,
e.g. how many characters per line, how often source location
information should be reported. Note that some language front ends may not
honor these options.
- -fmessage-length=n
-
Try to format error messages so that they fit on lines of about
n characters. If n is zero, then no line-wrapping is
done; each error message appears on a single line. This is the
default for all front ends.
- -fdiagnostics-show-location=once
-
Only meaningful in line-wrapping mode. Instructs the diagnostic messages
reporter to emit source location information once; that is, in
case the message is too long to fit on a single physical line and has to
be wrapped, the source location won't be emitted (as prefix) again,
over and over, in subsequent continuation lines. This is the default
behavior.
- -fdiagnostics-show-location=every-line
-
Only meaningful in line-wrapping mode. Instructs the diagnostic
messages reporter to emit the same source location information (as
prefix) for physical lines that result from the process of breaking
a message which is too long to fit on a single line.
- -fdiagnostics-color[=WHEN]
-
- -fno-diagnostics-color
-
Use color in diagnostics. WHEN is never, always,
or auto. The default depends on how the compiler has been configured,
it can be any of the above WHEN options or also never
if GCC_COLORS environment variable isn't present in the environment,
and auto otherwise.
auto means to use color only when the standard error is a terminal.
The forms -fdiagnostics-color and -fno-diagnostics-color are
aliases for -fdiagnostics-color=always and
-fdiagnostics-color=never, respectively.
The colors are defined by the environment variable GCC_COLORS.
Its value is a colon-separated list of capabilities and Select Graphic
Rendition (SGR) substrings. SGR commands are interpreted by the
terminal or terminal emulator. (See the section in the documentation
of your text terminal for permitted values and their meanings as
character attributes.) These substring values are integers in decimal
representation and can be concatenated with semicolons.
Common values to concatenate include
1 for bold,
4 for underline,
5 for blink,
7 for inverse,
39 for default foreground color,
30 to 37 for foreground colors,
90 to 97 for 16-color mode foreground colors,
38;5;0 to 38;5;255
for 88-color and 256-color modes foreground colors,
49 for default background color,
40 to 47 for background colors,
100 to 107 for 16-color mode background colors,
and 48;5;0 to 48;5;255
for 88-color and 256-color modes background colors.
The default GCC_COLORS is
error=01;31:warning=01;35:note=01;36:caret=01;32:locus=01:quote=01
where 01;31 is bold red, 01;35 is bold magenta,
01;36 is bold cyan, 01;32 is bold green and
01 is bold. Setting GCC_COLORS to the empty
string disables colors.
Supported capabilities are as follows.
-
- error=
-
SGR substring for error: markers.
- warning=
-
SGR substring for warning: markers.
- note=
-
SGR substring for note: markers.
- caret=
-
SGR substring for caret line.
- locus=
-
SGR substring for location information, file:line or
file:line:column etc.
- quote=
-
SGR substring for information printed within quotes.
-
- -fno-diagnostics-show-option
-
By default, each diagnostic emitted includes text indicating the
command-line option that directly controls the diagnostic (if such an
option is known to the diagnostic machinery). Specifying the
-fno-diagnostics-show-option flag suppresses that behavior.
- -fno-diagnostics-show-caret
-
By default, each diagnostic emitted includes the original source line
and a caret '^' indicating the column. This option suppresses this
information. The source line is truncated to n characters, if
the -fmessage-length=n option is given. When the output is done
to the terminal, the width is limited to the width given by the
COLUMNS environment variable or, if not set, to the terminal width.
Options to Request or Suppress Warnings
Warnings are diagnostic messages that report constructions that
are not inherently erroneous but that are risky or suggest there
may have been an error.
The following language-independent options do not enable specific
warnings but control the kinds of diagnostics produced by GCC.
- -fsyntax-only
-
Check the code for syntax errors, but don't do anything beyond that.
- -fmax-errors=n
-
Limits the maximum number of error messages to n, at which point
GCC bails out rather than attempting to continue processing the source
code. If n is 0 (the default), there is no limit on the number
of error messages produced. If -Wfatal-errors is also
specified, then -Wfatal-errors takes precedence over this
option.
- -w
-
Inhibit all warning messages.
- -Werror
-
Make all warnings into errors.
- -Werror=
-
Make the specified warning into an error. The specifier for a warning
is appended; for example -Werror=switch turns the warnings
controlled by -Wswitch into errors. This switch takes a
negative form, to be used to negate -Werror for specific
warnings; for example -Wno-error=switch makes
-Wswitch warnings not be errors, even when -Werror
is in effect.
The warning message for each controllable warning includes the
option that controls the warning. That option can then be used with
-Werror= and -Wno-error= as described above.
(Printing of the option in the warning message can be disabled using the
-fno-diagnostics-show-option flag.)
Note that specifying -Werror=foo automatically implies
-Wfoo. However, -Wno-error=foo does not
imply anything.
- -Wfatal-errors
-
This option causes the compiler to abort compilation on the first error
occurred rather than trying to keep going and printing further error
messages.
You can request many specific warnings with options beginning with
-W, for example -Wimplicit to request warnings on
implicit declarations. Each of these specific warning options also
has a negative form beginning -Wno- to turn off warnings; for
example, -Wno-implicit. This manual lists only one of the
two forms, whichever is not the default. For further
language-specific options also refer to C++ Dialect Options and
Objective-C and Objective-C++ Dialect Options.
Some options, such as -Wall and -Wextra, turn on other
options, such as -Wunused, which may turn on further options,
such as -Wunused-value. The combined effect of positive and
negative forms is that more specific options have priority over less
specific ones, independently of their position in the command-line. For
options of the same specificity, the last one takes effect. Options
enabled or disabled via pragmas take effect
as if they appeared at the end of the command-line.
When an unrecognized warning option is requested (e.g.,
-Wunknown-warning), GCC emits a diagnostic stating
that the option is not recognized. However, if the -Wno- form
is used, the behavior is slightly different: no diagnostic is
produced for -Wno-unknown-warning unless other diagnostics
are being produced. This allows the use of new -Wno- options
with old compilers, but if something goes wrong, the compiler
warns that an unrecognized option is present.
- -Wpedantic
-
- -pedantic
-
Issue all the warnings demanded by strict ISO C and ISO C++;
reject all programs that use forbidden extensions, and some other
programs that do not follow ISO C and ISO C++. For ISO C, follows the
version of the ISO C standard specified by any -std option used.
Valid ISO C and ISO C++ programs should compile properly with or without
this option (though a rare few require -ansi or a
-std option specifying the required version of ISO C). However,
without this option, certain GNU extensions and traditional C and C++
features are supported as well. With this option, they are rejected.
-Wpedantic does not cause warning messages for use of the
alternate keywords whose names begin and end with __. Pedantic
warnings are also disabled in the expression that follows
"__extension__". However, only system header files should use
these escape routes; application programs should avoid them.
Some users try to use -Wpedantic to check programs for strict ISO
C conformance. They soon find that it does not do quite what they want:
it finds some non-ISO practices, but not all---only those for which
ISO C requires a diagnostic, and some others for which
diagnostics have been added.
A feature to report any failure to conform to ISO C might be useful in
some instances, but would require considerable additional work and would
be quite different from -Wpedantic. We don't have plans to
support such a feature in the near future.
Where the standard specified with -std represents a GNU
extended dialect of C, such as gnu90 or gnu99, there is a
corresponding base standard, the version of ISO C on which the GNU
extended dialect is based. Warnings from -Wpedantic are given
where they are required by the base standard. (It does not make sense
for such warnings to be given only for features not in the specified GNU
C dialect, since by definition the GNU dialects of C include all
features the compiler supports with the given option, and there would be
nothing to warn about.)
- -pedantic-errors
-
Give an error whenever the base standard (see -Wpedantic)
requires a diagnostic, in some cases where there is undefined behavior
at compile-time and in some other cases that do not prevent compilation
of programs that are valid according to the standard. This is not
equivalent to -Werror=pedantic, since there are errors enabled
by this option and not enabled by the latter and vice versa.
- -Wall
-
This enables all the warnings about constructions that some users
consider questionable, and that are easy to avoid (or modify to
prevent the warning), even in conjunction with macros. This also
enables some language-specific warnings described in C++ Dialect
Options and Objective-C and Objective-C++ Dialect Options.
-Wall turns on the following warning flags:
-Waddress
-Warray-bounds=1 (only with -O2)
-Wc++11-compat -Wc++14-compat
-Wchar-subscripts
-Wenum-compare (in C/ObjC; this is on by default in C++)
-Wimplicit-int (C and Objective-C only)
-Wimplicit-function-declaration (C and Objective-C only)
-Wcomment
-Wformat
-Wmain (only for C/ObjC and unless -ffreestanding)
-Wmaybe-uninitialized
-Wmissing-braces (only for C/ObjC)
-Wnonnull
-Wopenmp-simd
-Wparentheses
-Wpointer-sign
-Wreorder
-Wreturn-type
-Wsequence-point
-Wsign-compare (only in C++)
-Wstrict-aliasing
-Wstrict-overflow=1
-Wswitch
-Wtrigraphs
-Wuninitialized
-Wunknown-pragmas
-Wunused-function
-Wunused-label
-Wunused-value
-Wunused-variable
-Wvolatile-register-var
Note that some warning flags are not implied by -Wall. Some of
them warn about constructions that users generally do not consider
questionable, but which occasionally you might wish to check for;
others warn about constructions that are necessary or hard to avoid in
some cases, and there is no simple way to modify the code to suppress
the warning. Some of them are enabled by -Wextra but many of
them must be enabled individually.
- -Wextra
-
This enables some extra warning flags that are not enabled by
-Wall. (This option used to be called -W. The older
name is still supported, but the newer name is more descriptive.)
-Wclobbered
-Wempty-body
-Wignored-qualifiers
-Wmissing-field-initializers
-Wmissing-parameter-type (C only)
-Wold-style-declaration (C only)
-Woverride-init
-Wsign-compare
-Wtype-limits
-Wuninitialized
-Wunused-parameter (only with -Wunused or -Wall)
-Wunused-but-set-parameter (only with -Wunused or -Wall)
The option -Wextra also prints warning messages for the
following cases:
-
- *
-
A pointer is compared against integer zero with "<", "<=",
">", or ">=".
- *
-
(C++ only) An enumerator and a non-enumerator both appear in a
conditional expression.
- *
-
(C++ only) Ambiguous virtual bases.
- *
-
(C++ only) Subscripting an array that has been declared "register".
- *
-
(C++ only) Taking the address of a variable that has been declared
"register".
- *
-
(C++ only) A base class is not initialized in a derived class's copy
constructor.
-
- -Wchar-subscripts
-
Warn if an array subscript has type "char". This is a common cause
of error, as programmers often forget that this type is signed on some
machines.
This warning is enabled by -Wall.
- -Wcomment
-
Warn whenever a comment-start sequence /* appears in a /*
comment, or whenever a Backslash-Newline appears in a // comment.
This warning is enabled by -Wall.
- -Wno-coverage-mismatch
-
Warn if feedback profiles do not match when using the
-fprofile-use option.
If a source file is changed between compiling with -fprofile-gen and
with -fprofile-use, the files with the profile feedback can fail
to match the source file and GCC cannot use the profile feedback
information. By default, this warning is enabled and is treated as an
error. -Wno-coverage-mismatch can be used to disable the
warning or -Wno-error=coverage-mismatch can be used to
disable the error. Disabling the error for this warning can result in
poorly optimized code and is useful only in the
case of very minor changes such as bug fixes to an existing code-base.
Completely disabling the warning is not recommended.
- -Wno-cpp
-
(C, Objective-C, C++, Objective-C++ and Fortran only)
Suppress warning messages emitted by "#warning" directives.
- -Wdouble-promotion (C, C++, Objective-C and Objective-C++ only)
-
Give a warning when a value of type "float" is implicitly
promoted to "double". CPUs with a 32-bit ``single-precision''
floating-point unit implement "float" in hardware, but emulate
"double" in software. On such a machine, doing computations
using "double" values is much more expensive because of the
overhead required for software emulation.
It is easy to accidentally do computations with "double" because
floating-point literals are implicitly of type "double". For
example, in:
float area(float radius)
{
return 3.14159 * radius * radius;
}
the compiler performs the entire computation with "double"
because the floating-point literal is a "double".
- -Wformat
-
- -Wformat=n
-
Check calls to "printf" and "scanf", etc., to make sure that
the arguments supplied have types appropriate to the format string
specified, and that the conversions specified in the format string make
sense. This includes standard functions, and others specified by format
attributes, in the "printf",
"scanf", "strftime" and "strfmon" (an X/Open extension,
not in the C standard) families (or other target-specific families).
Which functions are checked without format attributes having been
specified depends on the standard version selected, and such checks of
functions without the attribute specified are disabled by
-ffreestanding or -fno-builtin.
The formats are checked against the format features supported by GNU
libc version 2.2. These include all ISO C90 and C99 features, as well
as features from the Single Unix Specification and some BSD and GNU
extensions. Other library implementations may not support all these
features; GCC does not support warning about features that go beyond a
particular library's limitations. However, if -Wpedantic is used
with -Wformat, warnings are given about format features not
in the selected standard version (but not for "strfmon" formats,
since those are not in any version of the C standard).
-
- -Wformat=1
-
- -Wformat
-
Option -Wformat is equivalent to -Wformat=1, and
-Wno-format is equivalent to -Wformat=0. Since
-Wformat also checks for null format arguments for several
functions, -Wformat also implies -Wnonnull. Some
aspects of this level of format checking can be disabled by the
options: -Wno-format-contains-nul,
-Wno-format-extra-args, and -Wno-format-zero-length.
-Wformat is enabled by -Wall.
- -Wno-format-contains-nul
-
If -Wformat is specified, do not warn about format strings that
contain NUL bytes.
- -Wno-format-extra-args
-
If -Wformat is specified, do not warn about excess arguments to a
"printf" or "scanf" format function. The C standard specifies
that such arguments are ignored.
Where the unused arguments lie between used arguments that are
specified with $ operand number specifications, normally
warnings are still given, since the implementation could not know what
type to pass to "va_arg" to skip the unused arguments. However,
in the case of "scanf" formats, this option suppresses the
warning if the unused arguments are all pointers, since the Single
Unix Specification says that such unused arguments are allowed.
- -Wno-format-zero-length
-
If -Wformat is specified, do not warn about zero-length formats.
The C standard specifies that zero-length formats are allowed.
- -Wformat=2
-
Enable -Wformat plus additional format checks. Currently
equivalent to -Wformat -Wformat-nonliteral -Wformat-security
-Wformat-y2k.
- -Wformat-nonliteral
-
If -Wformat is specified, also warn if the format string is not a
string literal and so cannot be checked, unless the format function
takes its format arguments as a "va_list".
- -Wformat-security
-
If -Wformat is specified, also warn about uses of format
functions that represent possible security problems. At present, this
warns about calls to "printf" and "scanf" functions where the
format string is not a string literal and there are no format arguments,
as in "printf (foo);". This may be a security hole if the format
string came from untrusted input and contains %n. (This is
currently a subset of what -Wformat-nonliteral warns about, but
in future warnings may be added to -Wformat-security that are not
included in -Wformat-nonliteral.)
- -Wformat-signedness
-
If -Wformat is specified, also warn if the format string
requires an unsigned argument and the argument is signed and vice versa.
- -Wformat-y2k
-
If -Wformat is specified, also warn about "strftime"
formats that may yield only a two-digit year.
-
- -Wnonnull
-
Warn about passing a null pointer for arguments marked as
requiring a non-null value by the "nonnull" function attribute.
-Wnonnull is included in -Wall and -Wformat. It
can be disabled with the -Wno-nonnull option.
- -Winit-self (C, C++, Objective-C and Objective-C++ only)
-
Warn about uninitialized variables that are initialized with themselves.
Note this option can only be used with the -Wuninitialized option.
For example, GCC warns about "i" being uninitialized in the
following snippet only when -Winit-self has been specified:
int f()
{
int i = i;
return i;
}
This warning is enabled by -Wall in C++.
- -Wimplicit-int (C and Objective-C only)
-
Warn when a declaration does not specify a type.
This warning is enabled by -Wall.
- -Wimplicit-function-declaration (C and Objective-C only)
-
Give a warning whenever a function is used before being declared. In
C99 mode (-std=c99 or -std=gnu99), this warning is
enabled by default and it is made into an error by
-pedantic-errors. This warning is also enabled by
-Wall.
- -Wimplicit (C and Objective-C only)
-
Same as -Wimplicit-int and -Wimplicit-function-declaration.
This warning is enabled by -Wall.
- -Wignored-qualifiers (C and C++ only)
-
Warn if the return type of a function has a type qualifier
such as "const". For ISO C such a type qualifier has no effect,
since the value returned by a function is not an lvalue.
For C++, the warning is only emitted for scalar types or "void".
ISO C prohibits qualified "void" return types on function
definitions, so such return types always receive a warning
even without this option.
This warning is also enabled by -Wextra.
- -Wmain
-
Warn if the type of "main" is suspicious. "main" should be
a function with external linkage, returning int, taking either zero
arguments, two, or three arguments of appropriate types. This warning
is enabled by default in C++ and is enabled by either -Wall
or -Wpedantic.
- -Wmissing-braces
-
Warn if an aggregate or union initializer is not fully bracketed. In
the following example, the initializer for "a" is not fully
bracketed, but that for "b" is fully bracketed. This warning is
enabled by -Wall in C.
int a[2][2] = { 0, 1, 2, 3 };
int b[2][2] = { { 0, 1 }, { 2, 3 } };
This warning is enabled by -Wall.
- -Wmissing-include-dirs (C, C++, Objective-C and Objective-C++ only)
-
Warn if a user-supplied include directory does not exist.
- -Wparentheses
-
Warn if parentheses are omitted in certain contexts, such
as when there is an assignment in a context where a truth value
is expected, or when operators are nested whose precedence people
often get confused about.
Also warn if a comparison like "x<=y<=z" appears; this is
equivalent to "(x<=y ? 1 : 0) <= z", which is a different
interpretation from that of ordinary mathematical notation.
Also warn about constructions where there may be confusion to which
"if" statement an "else" branch belongs. Here is an example of
such a case:
{
if (a)
if (b)
foo ();
else
bar ();
}
In C/C++, every "else" branch belongs to the innermost possible
"if" statement, which in this example is "if (b)". This is
often not what the programmer expected, as illustrated in the above
example by indentation the programmer chose. When there is the
potential for this confusion, GCC issues a warning when this flag
is specified. To eliminate the warning, add explicit braces around
the innermost "if" statement so there is no way the "else"
can belong to the enclosing "if". The resulting code
looks like this:
{
if (a)
{
if (b)
foo ();
else
bar ();
}
}
Also warn for dangerous uses of the GNU extension to
"?:" with omitted middle operand. When the condition
in the "?": operator is a boolean expression, the omitted value is
always 1. Often programmers expect it to be a value computed
inside the conditional expression instead.
This warning is enabled by -Wall.
- -Wsequence-point
-
Warn about code that may have undefined semantics because of violations
of sequence point rules in the C and C++ standards.
The C and C++ standards define the order in which expressions in a C/C++
program are evaluated in terms of sequence points, which represent
a partial ordering between the execution of parts of the program: those
executed before the sequence point, and those executed after it. These
occur after the evaluation of a full expression (one which is not part
of a larger expression), after the evaluation of the first operand of a
"&&", "||", "? :" or "," (comma) operator, before a
function is called (but after the evaluation of its arguments and the
expression denoting the called function), and in certain other places.
Other than as expressed by the sequence point rules, the order of
evaluation of subexpressions of an expression is not specified. All
these rules describe only a partial order rather than a total order,
since, for example, if two functions are called within one expression
with no sequence point between them, the order in which the functions
are called is not specified. However, the standards committee have
ruled that function calls do not overlap.
It is not specified when between sequence points modifications to the
values of objects take effect. Programs whose behavior depends on this
have undefined behavior; the C and C++ standards specify that ``Between
the previous and next sequence point an object shall have its stored
value modified at most once by the evaluation of an expression.
Furthermore, the prior value shall be read only to determine the value
to be stored.''. If a program breaks these rules, the results on any
particular implementation are entirely unpredictable.
Examples of code with undefined behavior are "a = a++;", "a[n]
= b[n++]" and "a[i++] = i;". Some more complicated cases are not
diagnosed by this option, and it may give an occasional false positive
result, but in general it has been found fairly effective at detecting
this sort of problem in programs.
The standard is worded confusingly, therefore there is some debate
over the precise meaning of the sequence point rules in subtle cases.
Links to discussions of the problem, including proposed formal
definitions, may be found on the GCC readings page, at
<http://gcc.gnu.org/readings.html>.
This warning is enabled by -Wall for C and C++.
- -Wno-return-local-addr
-
Do not warn about returning a pointer (or in C++, a reference) to a
variable that goes out of scope after the function returns.
- -Wreturn-type
-
Warn whenever a function is defined with a return type that defaults
to "int". Also warn about any "return" statement with no
return value in a function whose return type is not "void"
(falling off the end of the function body is considered returning
without a value), and about a "return" statement with an
expression in a function whose return type is "void".
For C++, a function without return type always produces a diagnostic
message, even when -Wno-return-type is specified. The only
exceptions are "main" and functions defined in system headers.
This warning is enabled by -Wall.
- -Wshift-count-negative
-
Warn if shift count is negative. This warning is enabled by default.
- -Wshift-count-overflow
-
Warn if shift count >= width of type. This warning is enabled by default.
- -Wswitch
-
Warn whenever a "switch" statement has an index of enumerated type
and lacks a "case" for one or more of the named codes of that
enumeration. (The presence of a "default" label prevents this
warning.) "case" labels outside the enumeration range also
provoke warnings when this option is used (even if there is a
"default" label).
This warning is enabled by -Wall.
- -Wswitch-default
-
Warn whenever a "switch" statement does not have a "default"
case.
- -Wswitch-enum
-
Warn whenever a "switch" statement has an index of enumerated type
and lacks a "case" for one or more of the named codes of that
enumeration. "case" labels outside the enumeration range also
provoke warnings when this option is used. The only difference
between -Wswitch and this option is that this option gives a
warning about an omitted enumeration code even if there is a
"default" label.
- -Wswitch-bool
-
Warn whenever a "switch" statement has an index of boolean type.
It is possible to suppress this warning by casting the controlling
expression to a type other than "bool". For example:
switch ((int) (a == 4))
{
...
}
This warning is enabled by default for C and C++ programs.
- -Wsync-nand (C and C++ only)
-
Warn when "__sync_fetch_and_nand" and "__sync_nand_and_fetch"
built-in functions are used. These functions changed semantics in GCC 4.4.
- -Wtrigraphs
-
Warn if any trigraphs are encountered that might change the meaning of
the program (trigraphs within comments are not warned about).
This warning is enabled by -Wall.
- -Wunused-but-set-parameter
-
Warn whenever a function parameter is assigned to, but otherwise unused
(aside from its declaration).
To suppress this warning use the "unused" attribute.
This warning is also enabled by -Wunused together with
-Wextra.
- -Wunused-but-set-variable
-
Warn whenever a local variable is assigned to, but otherwise unused
(aside from its declaration).
This warning is enabled by -Wall.
To suppress this warning use the "unused" attribute.
This warning is also enabled by -Wunused, which is enabled
by -Wall.
- -Wunused-function
-
Warn whenever a static function is declared but not defined or a
non-inline static function is unused.
This warning is enabled by -Wall.
- -Wunused-label
-
Warn whenever a label is declared but not used.
This warning is enabled by -Wall.
To suppress this warning use the "unused" attribute.
- -Wunused-local-typedefs (C, Objective-C, C++ and Objective-C++ only)
-
Warn when a typedef locally defined in a function is not used.
This warning is enabled by -Wall.
- -Wunused-parameter
-
Warn whenever a function parameter is unused aside from its declaration.
To suppress this warning use the "unused" attribute.
- -Wno-unused-result
-
Do not warn if a caller of a function marked with attribute
"warn_unused_result" does not use
its return value. The default is -Wunused-result.
- -Wunused-variable
-
Warn whenever a local variable or non-constant static variable is unused
aside from its declaration.
This warning is enabled by -Wall.
To suppress this warning use the "unused" attribute.
- -Wunused-value
-
Warn whenever a statement computes a result that is explicitly not
used. To suppress this warning cast the unused expression to
"void". This includes an expression-statement or the left-hand
side of a comma expression that contains no side effects. For example,
an expression such as "x[i,j]" causes a warning, while
"x[(void)i,j]" does not.
This warning is enabled by -Wall.
- -Wunused
-
All the above -Wunused options combined.
In order to get a warning about an unused function parameter, you must
either specify -Wextra -Wunused (note that -Wall implies
-Wunused), or separately specify -Wunused-parameter.
- -Wuninitialized
-
Warn if an automatic variable is used without first being initialized
or if a variable may be clobbered by a "setjmp" call. In C++,
warn if a non-static reference or non-static "const" member
appears in a class without constructors.
If you want to warn about code that uses the uninitialized value of the
variable in its own initializer, use the -Winit-self option.
These warnings occur for individual uninitialized or clobbered
elements of structure, union or array variables as well as for
variables that are uninitialized or clobbered as a whole. They do
not occur for variables or elements declared "volatile". Because
these warnings depend on optimization, the exact variables or elements
for which there are warnings depends on the precise optimization
options and version of GCC used.
Note that there may be no warning about a variable that is used only
to compute a value that itself is never used, because such
computations may be deleted by data flow analysis before the warnings
are printed.
- -Wmaybe-uninitialized
-
For an automatic variable, if there exists a path from the function
entry to a use of the variable that is initialized, but there exist
some other paths for which the variable is not initialized, the compiler
emits a warning if it cannot prove the uninitialized paths are not
executed at run time. These warnings are made optional because GCC is
not smart enough to see all the reasons why the code might be correct
in spite of appearing to have an error. Here is one example of how
this can happen:
{
int x;
switch (y)
{
case 1: x = 1;
break;
case 2: x = 4;
break;
case 3: x = 5;
}
foo (x);
}
If the value of "y" is always 1, 2 or 3, then "x" is
always initialized, but GCC doesn't know this. To suppress the
warning, you need to provide a default case with assert(0) or
similar code.
This option also warns when a non-volatile automatic variable might be
changed by a call to "longjmp". These warnings as well are possible
only in optimizing compilation.
The compiler sees only the calls to "setjmp". It cannot know
where "longjmp" will be called; in fact, a signal handler could
call it at any point in the code. As a result, you may get a warning
even when there is in fact no problem because "longjmp" cannot
in fact be called at the place that would cause a problem.
Some spurious warnings can be avoided if you declare all the functions
you use that never return as "noreturn".
This warning is enabled by -Wall or -Wextra.
- -Wunknown-pragmas
-
Warn when a "#pragma" directive is encountered that is not understood by
GCC. If this command-line option is used, warnings are even issued
for unknown pragmas in system header files. This is not the case if
the warnings are only enabled by the -Wall command-line option.
- -Wno-pragmas
-
Do not warn about misuses of pragmas, such as incorrect parameters,
invalid syntax, or conflicts between pragmas. See also
-Wunknown-pragmas.
- -Wstrict-aliasing
-
This option is only active when -fstrict-aliasing is active.
It warns about code that might break the strict aliasing rules that the
compiler is using for optimization. The warning does not catch all
cases, but does attempt to catch the more common pitfalls. It is
included in -Wall.
It is equivalent to -Wstrict-aliasing=3
- -Wstrict-aliasing=n
-
This option is only active when -fstrict-aliasing is active.
It warns about code that might break the strict aliasing rules that the
compiler is using for optimization.
Higher levels correspond to higher accuracy (fewer false positives).
Higher levels also correspond to more effort, similar to the way -O
works.
-Wstrict-aliasing is equivalent to -Wstrict-aliasing=3.
Level 1: Most aggressive, quick, least accurate.
Possibly useful when higher levels
do not warn but -fstrict-aliasing still breaks the code, as it has very few
false negatives. However, it has many false positives.
Warns for all pointer conversions between possibly incompatible types,
even if never dereferenced. Runs in the front end only.
Level 2: Aggressive, quick, not too precise.
May still have many false positives (not as many as level 1 though),
and few false negatives (but possibly more than level 1).
Unlike level 1, it only warns when an address is taken. Warns about
incomplete types. Runs in the front end only.
Level 3 (default for -Wstrict-aliasing):
Should have very few false positives and few false
negatives. Slightly slower than levels 1 or 2 when optimization is enabled.
Takes care of the common pun+dereference pattern in the front end:
"*(int*)&some_float".
If optimization is enabled, it also runs in the back end, where it deals
with multiple statement cases using flow-sensitive points-to information.
Only warns when the converted pointer is dereferenced.
Does not warn about incomplete types.
- -Wstrict-overflow
-
- -Wstrict-overflow=n
-
This option is only active when -fstrict-overflow is active.
It warns about cases where the compiler optimizes based on the
assumption that signed overflow does not occur. Note that it does not
warn about all cases where the code might overflow: it only warns
about cases where the compiler implements some optimization. Thus
this warning depends on the optimization level.
An optimization that assumes that signed overflow does not occur is
perfectly safe if the values of the variables involved are such that
overflow never does, in fact, occur. Therefore this warning can
easily give a false positive: a warning about code that is not
actually a problem. To help focus on important issues, several
warning levels are defined. No warnings are issued for the use of
undefined signed overflow when estimating how many iterations a loop
requires, in particular when determining whether a loop will be
executed at all.
-
- -Wstrict-overflow=1
-
Warn about cases that are both questionable and easy to avoid. For
example, with -fstrict-overflow, the compiler simplifies
"x + 1 > x" to 1. This level of
-Wstrict-overflow is enabled by -Wall; higher levels
are not, and must be explicitly requested.
- -Wstrict-overflow=2
-
Also warn about other cases where a comparison is simplified to a
constant. For example: "abs (x) >= 0". This can only be
simplified when -fstrict-overflow is in effect, because
"abs (INT_MIN)" overflows to "INT_MIN", which is less than
zero. -Wstrict-overflow (with no level) is the same as
-Wstrict-overflow=2.
- -Wstrict-overflow=3
-
Also warn about other cases where a comparison is simplified. For
example: "x + 1 > 1" is simplified to "x > 0".
- -Wstrict-overflow=4
-
Also warn about other simplifications not covered by the above cases.
For example: "(x * 10) / 5" is simplified to "x * 2".
- -Wstrict-overflow=5
-
Also warn about cases where the compiler reduces the magnitude of a
constant involved in a comparison. For example: "x + 2 > y" is
simplified to "x + 1 >= y". This is reported only at the
highest warning level because this simplification applies to many
comparisons, so this warning level gives a very large number of
false positives.
-
- -Wsuggest-attribute=[pure|const|noreturn|format]
-
Warn for cases where adding an attribute may be beneficial. The
attributes currently supported are listed below.
-
- -Wsuggest-attribute=pure
-
- -Wsuggest-attribute=const
-
- -Wsuggest-attribute=noreturn
-
Warn about functions that might be candidates for attributes
"pure", "const" or "noreturn". The compiler only warns for
functions visible in other compilation units or (in the case of "pure" and
"const") if it cannot prove that the function returns normally. A function
returns normally if it doesn't contain an infinite loop or return abnormally
by throwing, calling "abort" or trapping. This analysis requires option
-fipa-pure-const, which is enabled by default at -O and
higher. Higher optimization levels improve the accuracy of the analysis.
- -Wsuggest-attribute=format
-
- -Wmissing-format-attribute
-
Warn about function pointers that might be candidates for "format"
attributes. Note these are only possible candidates, not absolute ones.
GCC guesses that function pointers with "format" attributes that
are used in assignment, initialization, parameter passing or return
statements should have a corresponding "format" attribute in the
resulting type. I.e. the left-hand side of the assignment or
initialization, the type of the parameter variable, or the return type
of the containing function respectively should also have a "format"
attribute to avoid the warning.
GCC also warns about function definitions that might be
candidates for "format" attributes. Again, these are only
possible candidates. GCC guesses that "format" attributes
might be appropriate for any function that calls a function like
"vprintf" or "vscanf", but this might not always be the
case, and some functions for which "format" attributes are
appropriate may not be detected.
-
- -Wsuggest-final-types
-
Warn about types with virtual methods where code quality would be improved
if the type were declared with the C++11 "final" specifier,
or, if possible,
declared in an anonymous namespace. This allows GCC to more aggressively
devirtualize the polymorphic calls. This warning is more effective with link
time optimization, where the information about the class hierarchy graph is
more complete.
- -Wsuggest-final-methods
-
Warn about virtual methods where code quality would be improved if the method
were declared with the C++11 "final" specifier,
or, if possible, its type were
declared in an anonymous namespace or with the "final" specifier.
This warning is
more effective with link time optimization, where the information about the
class hierarchy graph is more complete. It is recommended to first consider
suggestions of -Wsuggest-final-types and then rebuild with new
annotations.
- -Wsuggest-override
-
Warn about overriding virtual functions that are not marked with the override
keyword.
- -Warray-bounds
-
- -Warray-bounds=n
-
This option is only active when -ftree-vrp is active
(default for -O2 and above). It warns about subscripts to arrays
that are always out of bounds. This warning is enabled by -Wall.
-
- -Warray-bounds=1
-
This is the warning level of -Warray-bounds and is enabled
by -Wall; higher levels are not, and must be explicitly requested.
- -Warray-bounds=2
-
This warning level also warns about out of bounds access for
arrays at the end of a struct and for arrays accessed through
pointers. This warning level may give a larger number of
false positives and is deactivated by default.
-
- -Wbool-compare
-
Warn about boolean expression compared with an integer value different from
"true"/"false". For instance, the following comparison is
always false:
int n = 5;
...
if ((n > 1) == 2) { ... }
This warning is enabled by -Wall.
- -Wno-discarded-qualifiers (C and Objective-C only)
-
Do not warn if type qualifiers on pointers are being discarded.
Typically, the compiler warns if a "const char *" variable is
passed to a function that takes a "char *" parameter. This option
can be used to suppress such a warning.
- -Wno-discarded-array-qualifiers (C and Objective-C only)
-
Do not warn if type qualifiers on arrays which are pointer targets
are being discarded. Typically, the compiler warns if a
"const int (*)[]" variable is passed to a function that
takes a "int (*)[]" parameter. This option can be used to
suppress such a warning.
- -Wno-incompatible-pointer-types (C and Objective-C only)
-
Do not warn when there is a conversion between pointers that have incompatible
types. This warning is for cases not covered by -Wno-pointer-sign,
which warns for pointer argument passing or assignment with different
signedness.
- -Wno-int-conversion (C and Objective-C only)
-
Do not warn about incompatible integer to pointer and pointer to integer
conversions. This warning is about implicit conversions; for explicit
conversions the warnings -Wno-int-to-pointer-cast and
-Wno-pointer-to-int-cast may be used.
- -Wno-div-by-zero
-
Do not warn about compile-time integer division by zero. Floating-point
division by zero is not warned about, as it can be a legitimate way of
obtaining infinities and NaNs.
- -Wsystem-headers
-
Print warning messages for constructs found in system header files.
Warnings from system headers are normally suppressed, on the assumption
that they usually do not indicate real problems and would only make the
compiler output harder to read. Using this command-line option tells
GCC to emit warnings from system headers as if they occurred in user
code. However, note that using -Wall in conjunction with this
option does not warn about unknown pragmas in system
headers---for that, -Wunknown-pragmas must also be used.
- -Wtrampolines
-
Warn about trampolines generated for pointers to nested functions.
A trampoline is a small piece of data or code that is created at run
time on the stack when the address of a nested function is taken, and is
used to call the nested function indirectly. For some targets, it is
made up of data only and thus requires no special treatment. But, for
most targets, it is made up of code and thus requires the stack to be
made executable in order for the program to work properly.
- -Wfloat-equal
-
Warn if floating-point values are used in equality comparisons.
The idea behind this is that sometimes it is convenient (for the
programmer) to consider floating-point values as approximations to
infinitely precise real numbers. If you are doing this, then you need
to compute (by analyzing the code, or in some other way) the maximum or
likely maximum error that the computation introduces, and allow for it
when performing comparisons (and when producing output, but that's a
different problem). In particular, instead of testing for equality, you
should check to see whether the two values have ranges that overlap; and
this is done with the relational operators, so equality comparisons are
probably mistaken.
- -Wtraditional (C and Objective-C only)
-
Warn about certain constructs that behave differently in traditional and
ISO C. Also warn about ISO C constructs that have no traditional C
equivalent, and/or problematic constructs that should be avoided.
-
- *
-
Macro parameters that appear within string literals in the macro body.
In traditional C macro replacement takes place within string literals,
but in ISO C it does not.
- *
-
In traditional C, some preprocessor directives did not exist.
Traditional preprocessors only considered a line to be a directive
if the # appeared in column 1 on the line. Therefore
-Wtraditional warns about directives that traditional C
understands but ignores because the # does not appear as the
first character on the line. It also suggests you hide directives like
"#pragma" not understood by traditional C by indenting them. Some
traditional implementations do not recognize "#elif", so this option
suggests avoiding it altogether.
- *
-
A function-like macro that appears without arguments.
- *
-
The unary plus operator.
- *
-
The U integer constant suffix, or the F or L floating-point
constant suffixes. (Traditional C does support the L suffix on integer
constants.) Note, these suffixes appear in macros defined in the system
headers of most modern systems, e.g. the _MIN/_MAX macros in "<limits.h>".
Use of these macros in user code might normally lead to spurious
warnings, however GCC's integrated preprocessor has enough context to
avoid warning in these cases.
- *
-
A function declared external in one block and then used after the end of
the block.
- *
-
A "switch" statement has an operand of type "long".
- *
-
A non-"static" function declaration follows a "static" one.
This construct is not accepted by some traditional C compilers.
- *
-
The ISO type of an integer constant has a different width or
signedness from its traditional type. This warning is only issued if
the base of the constant is ten. I.e. hexadecimal or octal values, which
typically represent bit patterns, are not warned about.
- *
-
Usage of ISO string concatenation is detected.
- *
-
Initialization of automatic aggregates.
- *
-
Identifier conflicts with labels. Traditional C lacks a separate
namespace for labels.
- *
-
Initialization of unions. If the initializer is zero, the warning is
omitted. This is done under the assumption that the zero initializer in
user code appears conditioned on e.g. "__STDC__" to avoid missing
initializer warnings and relies on default initialization to zero in the
traditional C case.
- *
-
Conversions by prototypes between fixed/floating-point values and vice
versa. The absence of these prototypes when compiling with traditional
C causes serious problems. This is a subset of the possible
conversion warnings; for the full set use -Wtraditional-conversion.
- *
-
Use of ISO C style function definitions. This warning intentionally is
not issued for prototype declarations or variadic functions
because these ISO C features appear in your code when using
libiberty's traditional C compatibility macros, "PARAMS" and
"VPARAMS". This warning is also bypassed for nested functions
because that feature is already a GCC extension and thus not relevant to
traditional C compatibility.
-
- -Wtraditional-conversion (C and Objective-C only)
-
Warn if a prototype causes a type conversion that is different from what
would happen to the same argument in the absence of a prototype. This
includes conversions of fixed point to floating and vice versa, and
conversions changing the width or signedness of a fixed-point argument
except when the same as the default promotion.
- -Wdeclaration-after-statement (C and Objective-C only)
-
Warn when a declaration is found after a statement in a block. This
construct, known from C++, was introduced with ISO C99 and is by default
allowed in GCC. It is not supported by ISO C90.
- -Wundef
-
Warn if an undefined identifier is evaluated in an "#if" directive.
- -Wno-endif-labels
-
Do not warn whenever an "#else" or an "#endif" are followed by text.
- -Wshadow
-
Warn whenever a local variable or type declaration shadows another
variable, parameter, type, class member (in C++), or instance variable
(in Objective-C) or whenever a built-in function is shadowed. Note
that in C++, the compiler warns if a local variable shadows an
explicit typedef, but not if it shadows a struct/class/enum.
- -Wno-shadow-ivar (Objective-C only)
-
Do not warn whenever a local variable shadows an instance variable in an
Objective-C method.
- -Wlarger-than=len
-
Warn whenever an object of larger than len bytes is defined.
- -Wframe-larger-than=len
-
Warn if the size of a function frame is larger than len bytes.
The computation done to determine the stack frame size is approximate
and not conservative.
The actual requirements may be somewhat greater than len
even if you do not get a warning. In addition, any space allocated
via "alloca", variable-length arrays, or related constructs
is not included by the compiler when determining
whether or not to issue a warning.
- -Wno-free-nonheap-object
-
Do not warn when attempting to free an object that was not allocated
on the heap.
- -Wstack-usage=len
-
Warn if the stack usage of a function might be larger than len bytes.
The computation done to determine the stack usage is conservative.
Any space allocated via "alloca", variable-length arrays, or related
constructs is included by the compiler when determining whether or not to
issue a warning.
The message is in keeping with the output of -fstack-usage.
-
- *
-
If the stack usage is fully static but exceeds the specified amount, it's:
warning: stack usage is 1120 bytes
- *
-
If the stack usage is (partly) dynamic but bounded, it's:
warning: stack usage might be 1648 bytes
- *
-
If the stack usage is (partly) dynamic and not bounded, it's:
warning: stack usage might be unbounded
-
- -Wunsafe-loop-optimizations
-
Warn if the loop cannot be optimized because the compiler cannot
assume anything on the bounds of the loop indices. With
-funsafe-loop-optimizations warn if the compiler makes
such assumptions.
- -Wno-pedantic-ms-format (MinGW targets only)
-
When used in combination with -Wformat
and -pedantic without GNU extensions, this option
disables the warnings about non-ISO "printf" / "scanf" format
width specifiers "I32", "I64", and "I" used on Windows targets,
which depend on the MS runtime.
- -Wpointer-arith
-
Warn about anything that depends on the ``size of'' a function type or
of "void". GNU C assigns these types a size of 1, for
convenience in calculations with "void *" pointers and pointers
to functions. In C++, warn also when an arithmetic operation involves
"NULL". This warning is also enabled by -Wpedantic.
- -Wtype-limits
-
Warn if a comparison is always true or always false due to the limited
range of the data type, but do not warn for constant expressions. For
example, warn if an unsigned variable is compared against zero with
"<" or ">=". This warning is also enabled by
-Wextra.
- -Wbad-function-cast (C and Objective-C only)
-
Warn when a function call is cast to a non-matching type.
For example, warn if a call to a function returning an integer type
is cast to a pointer type.
- -Wc90-c99-compat (C and Objective-C only)
-
Warn about features not present in ISO C90, but present in ISO C99.
For instance, warn about use of variable length arrays, "long long"
type, "bool" type, compound literals, designated initializers, and so
on. This option is independent of the standards mode. Warnings are disabled
in the expression that follows "__extension__".
- -Wc99-c11-compat (C and Objective-C only)
-
Warn about features not present in ISO C99, but present in ISO C11.
For instance, warn about use of anonymous structures and unions,
"_Atomic" type qualifier, "_Thread_local" storage-class specifier,
"_Alignas" specifier, "Alignof" operator, "_Generic" keyword,
and so on. This option is independent of the standards mode. Warnings are
disabled in the expression that follows "__extension__".
- -Wc++-compat (C and Objective-C only)
-
Warn about ISO C constructs that are outside of the common subset of
ISO C and ISO C++, e.g. request for implicit conversion from
"void *" to a pointer to non-"void" type.
- -Wc++11-compat (C++ and Objective-C++ only)
-
Warn about C++ constructs whose meaning differs between ISO C++ 1998
and ISO C++ 2011, e.g., identifiers in ISO C++ 1998 that are keywords
in ISO C++ 2011. This warning turns on -Wnarrowing and is
enabled by -Wall.
- -Wc++14-compat (C++ and Objective-C++ only)
-
Warn about C++ constructs whose meaning differs between ISO C++ 2011
and ISO C++ 2014. This warning is enabled by -Wall.
- -Wcast-qual
-
Warn whenever a pointer is cast so as to remove a type qualifier from
the target type. For example, warn if a "const char *" is cast
to an ordinary "char *".
Also warn when making a cast that introduces a type qualifier in an
unsafe way. For example, casting "char **" to "const char **"
is unsafe, as in this example:
/* p is char ** value. */
const char **q = (const char **) p;
/* Assignment of readonly string to const char * is OK. */
*q = "string";
/* Now char** pointer points to read-only memory. */
**p = 'b';
- -Wcast-align
-
Warn whenever a pointer is cast such that the required alignment of the
target is increased. For example, warn if a "char *" is cast to
an "int *" on machines where integers can only be accessed at
two- or four-byte boundaries.
- -Wwrite-strings
-
When compiling C, give string constants the type "const
char[length]" so that copying the address of one into a
non-"const" "char *" pointer produces a warning. These
warnings help you find at compile time code that can try to write
into a string constant, but only if you have been very careful about
using "const" in declarations and prototypes. Otherwise, it is
just a nuisance. This is why we did not make -Wall request
these warnings.
When compiling C++, warn about the deprecated conversion from string
literals to "char *". This warning is enabled by default for C++
programs.
- -Wclobbered
-
Warn for variables that might be changed by "longjmp" or
"vfork". This warning is also enabled by -Wextra.
- -Wconditionally-supported (C++ and Objective-C++ only)
-
Warn for conditionally-supported (C++11 [intro.defs]) constructs.
- -Wconversion
-
Warn for implicit conversions that may alter a value. This includes
conversions between real and integer, like "abs (x)" when
"x" is "double"; conversions between signed and unsigned,
like "unsigned ui = -1"; and conversions to smaller types, like
"sqrtf (M_PI)". Do not warn for explicit casts like "abs
((int) x)" and "ui = (unsigned) -1", or if the value is not
changed by the conversion like in "abs (2.0)". Warnings about
conversions between signed and unsigned integers can be disabled by
using -Wno-sign-conversion.
For C++, also warn for confusing overload resolution for user-defined
conversions; and conversions that never use a type conversion
operator: conversions to "void", the same type, a base class or a
reference to them. Warnings about conversions between signed and
unsigned integers are disabled by default in C++ unless
-Wsign-conversion is explicitly enabled.
- -Wno-conversion-null (C++ and Objective-C++ only)
-
Do not warn for conversions between "NULL" and non-pointer
types. -Wconversion-null is enabled by default.
- -Wzero-as-null-pointer-constant (C++ and Objective-C++ only)
-
Warn when a literal '0' is used as null pointer constant. This can
be useful to facilitate the conversion to "nullptr" in C++11.
- -Wdate-time
-
Warn when macros "__TIME__", "__DATE__" or "__TIMESTAMP__"
are encountered as they might prevent bit-wise-identical reproducible
compilations.
- -Wdelete-incomplete (C++ and Objective-C++ only)
-
Warn when deleting a pointer to incomplete type, which may cause
undefined behavior at runtime. This warning is enabled by default.
- -Wuseless-cast (C++ and Objective-C++ only)
-
Warn when an expression is casted to its own type.
- -Wempty-body
-
Warn if an empty body occurs in an "if", "else" or "do
while" statement. This warning is also enabled by -Wextra.
- -Wenum-compare
-
Warn about a comparison between values of different enumerated types.
In C++ enumeral mismatches in conditional expressions are also
diagnosed and the warning is enabled by default. In C this warning is
enabled by -Wall.
- -Wjump-misses-init (C, Objective-C only)
-
Warn if a "goto" statement or a "switch" statement jumps
forward across the initialization of a variable, or jumps backward to a
label after the variable has been initialized. This only warns about
variables that are initialized when they are declared. This warning is
only supported for C and Objective-C; in C++ this sort of branch is an
error in any case.
-Wjump-misses-init is included in -Wc++-compat. It
can be disabled with the -Wno-jump-misses-init option.
- -Wsign-compare
-
Warn when a comparison between signed and unsigned values could produce
an incorrect result when the signed value is converted to unsigned.
This warning is also enabled by -Wextra; to get the other warnings
of -Wextra without this warning, use -Wextra -Wno-sign-compare.
- -Wsign-conversion
-
Warn for implicit conversions that may change the sign of an integer
value, like assigning a signed integer expression to an unsigned
integer variable. An explicit cast silences the warning. In C, this
option is enabled also by -Wconversion.
- -Wfloat-conversion
-
Warn for implicit conversions that reduce the precision of a real value.
This includes conversions from real to integer, and from higher precision
real to lower precision real values. This option is also enabled by
-Wconversion.
- -Wsized-deallocation (C++ and Objective-C++ only)
-
Warn about a definition of an unsized deallocation function
void operator delete (void *) noexcept;
void operator delete[] (void *) noexcept;
without a definition of the corresponding sized deallocation function
void operator delete (void *, std::size_t) noexcept;
void operator delete[] (void *, std::size_t) noexcept;
or vice versa. Enabled by -Wextra along with
-fsized-deallocation.
- -Wsizeof-pointer-memaccess
-
Warn for suspicious length parameters to certain string and memory built-in
functions if the argument uses "sizeof". This warning warns e.g.
about "memset (ptr, 0, sizeof (ptr));" if "ptr" is not an array,
but a pointer, and suggests a possible fix, or about
"memcpy (&foo, ptr, sizeof (&foo));". This warning is enabled by
-Wall.
- -Wsizeof-array-argument
-
Warn when the "sizeof" operator is applied to a parameter that is
declared as an array in a function definition. This warning is enabled by
default for C and C++ programs.
- -Wmemset-transposed-args
-
Warn for suspicious calls to the "memset" built-in function, if the
second argument is not zero and the third argument is zero. This warns e.g.@
about "memset (buf, sizeof buf, 0)" where most probably
"memset (buf, 0, sizeof buf)" was meant instead. The diagnostics
is only emitted if the third argument is literal zero. If it is some
expression that is folded to zero, a cast of zero to some type, etc.,
it is far less likely that the user has mistakenly exchanged the arguments
and no warning is emitted. This warning is enabled by -Wall.
- -Waddress
-
Warn about suspicious uses of memory addresses. These include using
the address of a function in a conditional expression, such as
"void func(void); if (func)", and comparisons against the memory
address of a string literal, such as "if (x == "abc")". Such
uses typically indicate a programmer error: the address of a function
always evaluates to true, so their use in a conditional usually
indicate that the programmer forgot the parentheses in a function
call; and comparisons against string literals result in unspecified
behavior and are not portable in C, so they usually indicate that the
programmer intended to use "strcmp". This warning is enabled by
-Wall.
- -Wlogical-op
-
Warn about suspicious uses of logical operators in expressions.
This includes using logical operators in contexts where a
bit-wise operator is likely to be expected.
- -Wlogical-not-parentheses
-
Warn about logical not used on the left hand side operand of a comparison.
This option does not warn if the RHS operand is of a boolean type. Its
purpose is to detect suspicious code like the following:
int a;
...
if (!a > 1) { ... }
It is possible to suppress the warning by wrapping the LHS into
parentheses:
if ((!a) > 1) { ... }
This warning is enabled by -Wall.
- -Waggregate-return
-
Warn if any functions that return structures or unions are defined or
called. (In languages where you can return an array, this also elicits
a warning.)
- -Wno-aggressive-loop-optimizations
-
Warn if in a loop with constant number of iterations the compiler detects
undefined behavior in some statement during one or more of the iterations.
- -Wno-attributes
-
Do not warn if an unexpected "__attribute__" is used, such as
unrecognized attributes, function attributes applied to variables,
etc. This does not stop errors for incorrect use of supported
attributes.
- -Wno-builtin-macro-redefined
-
Do not warn if certain built-in macros are redefined. This suppresses
warnings for redefinition of "__TIMESTAMP__", "__TIME__",
"__DATE__", "__FILE__", and "__BASE_FILE__".
- -Wstrict-prototypes (C and Objective-C only)
-
Warn if a function is declared or defined without specifying the
argument types. (An old-style function definition is permitted without
a warning if preceded by a declaration that specifies the argument
types.)
- -Wold-style-declaration (C and Objective-C only)
-
Warn for obsolescent usages, according to the C Standard, in a
declaration. For example, warn if storage-class specifiers like
"static" are not the first things in a declaration. This warning
is also enabled by -Wextra.
- -Wold-style-definition (C and Objective-C only)
-
Warn if an old-style function definition is used. A warning is given
even if there is a previous prototype.
- -Wmissing-parameter-type (C and Objective-C only)
-
A function parameter is declared without a type specifier in K&R-style
functions:
void foo(bar) { }
This warning is also enabled by -Wextra.
- -Wmissing-prototypes (C and Objective-C only)
-
Warn if a global function is defined without a previous prototype
declaration. This warning is issued even if the definition itself
provides a prototype. Use this option to detect global functions
that do not have a matching prototype declaration in a header file.
This option is not valid for C++ because all function declarations
provide prototypes and a non-matching declaration declares an
overload rather than conflict with an earlier declaration.
Use -Wmissing-declarations to detect missing declarations in C++.
- -Wmissing-declarations
-
Warn if a global function is defined without a previous declaration.
Do so even if the definition itself provides a prototype.
Use this option to detect global functions that are not declared in
header files. In C, no warnings are issued for functions with previous
non-prototype declarations; use -Wmissing-prototypes to detect
missing prototypes. In C++, no warnings are issued for function templates,
or for inline functions, or for functions in anonymous namespaces.
- -Wmissing-field-initializers
-
Warn if a structure's initializer has some fields missing. For
example, the following code causes such a warning, because
"x.h" is implicitly zero:
struct s { int f, g, h; };
struct s x = { 3, 4 };
This option does not warn about designated initializers, so the following
modification does not trigger a warning:
struct s { int f, g, h; };
struct s x = { .f = 3, .g = 4 };
In C++ this option does not warn either about the empty { }
initializer, for example:
struct s { int f, g, h; };
s x = { };
This warning is included in -Wextra. To get other -Wextra
warnings without this one, use -Wextra -Wno-missing-field-initializers.
- -Wno-multichar
-
Do not warn if a multicharacter constant ('FOOF') is used.
Usually they indicate a typo in the user's code, as they have
implementation-defined values, and should not be used in portable code.
- -Wnormalized[=<none|id|nfc|nfkc>]
-
In ISO C and ISO C++, two identifiers are different if they are
different sequences of characters. However, sometimes when characters
outside the basic ASCII character set are used, you can have two
different character sequences that look the same. To avoid confusion,
the ISO 10646 standard sets out some normalization rules which
when applied ensure that two sequences that look the same are turned into
the same sequence. GCC can warn you if you are using identifiers that
have not been normalized; this option controls that warning.
There are four levels of warning supported by GCC. The default is
-Wnormalized=nfc, which warns about any identifier that is
not in the ISO 10646 ``C'' normalized form, NFC. NFC is the
recommended form for most uses. It is equivalent to
-Wnormalized.
Unfortunately, there are some characters allowed in identifiers by
ISO C and ISO C++ that, when turned into NFC, are not allowed in
identifiers. That is, there's no way to use these symbols in portable
ISO C or C++ and have all your identifiers in NFC.
-Wnormalized=id suppresses the warning for these characters.
It is hoped that future versions of the standards involved will correct
this, which is why this option is not the default.
You can switch the warning off for all characters by writing
-Wnormalized=none or -Wno-normalized. You should
only do this if you are using some other normalization scheme (like
``D''), because otherwise you can easily create bugs that are
literally impossible to see.
Some characters in ISO 10646 have distinct meanings but look identical
in some fonts or display methodologies, especially once formatting has
been applied. For instance "\u207F", ``SUPERSCRIPT LATIN SMALL
LETTER N'', displays just like a regular "n" that has been
placed in a superscript. ISO 10646 defines the NFKC
normalization scheme to convert all these into a standard form as
well, and GCC warns if your code is not in NFKC if you use
-Wnormalized=nfkc. This warning is comparable to warning
about every identifier that contains the letter O because it might be
confused with the digit 0, and so is not the default, but may be
useful as a local coding convention if the programming environment
cannot be fixed to display these characters distinctly.
- -Wno-deprecated
-
Do not warn about usage of deprecated features.
- -Wno-deprecated-declarations
-
Do not warn about uses of functions,
variables, and types marked as deprecated by using the "deprecated"
attribute.
- -Wno-overflow
-
Do not warn about compile-time overflow in constant expressions.
- -Wno-odr
-
Warn about One Definition Rule violations during link-time optimization.
Requires -flto-odr-type-merging to be enabled. Enabled by default.
- -Wopenmp-simd
-
Warn if the vectorizer cost model overrides the OpenMP or the Cilk Plus
simd directive set by user. The -fsimd-cost-model=unlimited
option can be used to relax the cost model.
- -Woverride-init (C and Objective-C only)
-
Warn if an initialized field without side effects is overridden when
using designated initializers.
This warning is included in -Wextra. To get other
-Wextra warnings without this one, use -Wextra
-Wno-override-init.
- -Wpacked
-
Warn if a structure is given the packed attribute, but the packed
attribute has no effect on the layout or size of the structure.
Such structures may be mis-aligned for little benefit. For
instance, in this code, the variable "f.x" in "struct bar"
is misaligned even though "struct bar" does not itself
have the packed attribute:
struct foo {
int x;
char a, b, c, d;
} __attribute__((packed));
struct bar {
char z;
struct foo f;
};
- -Wpacked-bitfield-compat
-
The 4.1, 4.2 and 4.3 series of GCC ignore the "packed" attribute
on bit-fields of type "char". This has been fixed in GCC 4.4 but
the change can lead to differences in the structure layout. GCC
informs you when the offset of such a field has changed in GCC 4.4.
For example there is no longer a 4-bit padding between field "a"
and "b" in this structure:
struct foo
{
char a:4;
char b:8;
} __attribute__ ((packed));
This warning is enabled by default. Use
-Wno-packed-bitfield-compat to disable this warning.
- -Wpadded
-
Warn if padding is included in a structure, either to align an element
of the structure or to align the whole structure. Sometimes when this
happens it is possible to rearrange the fields of the structure to
reduce the padding and so make the structure smaller.
- -Wredundant-decls
-
Warn if anything is declared more than once in the same scope, even in
cases where multiple declaration is valid and changes nothing.
- -Wnested-externs (C and Objective-C only)
-
Warn if an "extern" declaration is encountered within a function.
- -Wno-inherited-variadic-ctor
-
Suppress warnings about use of C++11 inheriting constructors when the
base class inherited from has a C variadic constructor; the warning is
on by default because the ellipsis is not inherited.
- -Winline
-
Warn if a function that is declared as inline cannot be inlined.
Even with this option, the compiler does not warn about failures to
inline functions declared in system headers.
The compiler uses a variety of heuristics to determine whether or not
to inline a function. For example, the compiler takes into account
the size of the function being inlined and the amount of inlining
that has already been done in the current function. Therefore,
seemingly insignificant changes in the source program can cause the
warnings produced by -Winline to appear or disappear.
- -Wno-invalid-offsetof (C++ and Objective-C++ only)
-
Suppress warnings from applying the "offsetof" macro to a non-POD
type. According to the 2014 ISO C++ standard, applying "offsetof"
to a non-standard-layout type is undefined. In existing C++ implementations,
however, "offsetof" typically gives meaningful results.
This flag is for users who are aware that they are
writing nonportable code and who have deliberately chosen to ignore the
warning about it.
The restrictions on "offsetof" may be relaxed in a future version
of the C++ standard.
- -Wno-int-to-pointer-cast
-
Suppress warnings from casts to pointer type of an integer of a
different size. In C++, casting to a pointer type of smaller size is
an error. Wint-to-pointer-cast is enabled by default.
- -Wno-pointer-to-int-cast (C and Objective-C only)
-
Suppress warnings from casts from a pointer to an integer type of a
different size.
- -Winvalid-pch
-
Warn if a precompiled header is found in
the search path but can't be used.
- -Wlong-long
-
Warn if "long long" type is used. This is enabled by either
-Wpedantic or -Wtraditional in ISO C90 and C++98
modes. To inhibit the warning messages, use -Wno-long-long.
- -Wvariadic-macros
-
Warn if variadic macros are used in ISO C90 mode, or if the GNU
alternate syntax is used in ISO C99 mode. This is enabled by either
-Wpedantic or -Wtraditional. To inhibit the warning
messages, use -Wno-variadic-macros.
- -Wvarargs
-
Warn upon questionable usage of the macros used to handle variable
arguments like "va_start". This is default. To inhibit the
warning messages, use -Wno-varargs.
- -Wvector-operation-performance
-
Warn if vector operation is not implemented via SIMD capabilities of the
architecture. Mainly useful for the performance tuning.
Vector operation can be implemented "piecewise", which means that the
scalar operation is performed on every vector element;
"in parallel", which means that the vector operation is implemented
using scalars of wider type, which normally is more performance efficient;
and "as a single scalar", which means that vector fits into a
scalar type.
- -Wno-virtual-move-assign
-
Suppress warnings about inheriting from a virtual base with a
non-trivial C++11 move assignment operator. This is dangerous because
if the virtual base is reachable along more than one path, it is
moved multiple times, which can mean both objects end up in the
moved-from state. If the move assignment operator is written to avoid
moving from a moved-from object, this warning can be disabled.
- -Wvla
-
Warn if variable length array is used in the code.
-Wno-vla prevents the -Wpedantic warning of
the variable length array.
- -Wvolatile-register-var
-
Warn if a register variable is declared volatile. The volatile
modifier does not inhibit all optimizations that may eliminate reads
and/or writes to register variables. This warning is enabled by
-Wall.
- -Wdisabled-optimization
-
Warn if a requested optimization pass is disabled. This warning does
not generally indicate that there is anything wrong with your code; it
merely indicates that GCC's optimizers are unable to handle the code
effectively. Often, the problem is that your code is too big or too
complex; GCC refuses to optimize programs when the optimization
itself is likely to take inordinate amounts of time.
- -Wpointer-sign (C and Objective-C only)
-
Warn for pointer argument passing or assignment with different signedness.
This option is only supported for C and Objective-C. It is implied by
-Wall and by -Wpedantic, which can be disabled with
-Wno-pointer-sign.
- -Wstack-protector
-
This option is only active when -fstack-protector is active. It
warns about functions that are not protected against stack smashing.
- -Woverlength-strings
-
Warn about string constants that are longer than the ``minimum
maximum'' length specified in the C standard. Modern compilers
generally allow string constants that are much longer than the
standard's minimum limit, but very portable programs should avoid
using longer strings.
The limit applies after string constant concatenation, and does
not count the trailing NUL. In C90, the limit was 509 characters; in
C99, it was raised to 4095. C++98 does not specify a normative
minimum maximum, so we do not diagnose overlength strings in C++.
This option is implied by -Wpedantic, and can be disabled with
-Wno-overlength-strings.
- -Wunsuffixed-float-constants (C and Objective-C only)
-
Issue a warning for any floating constant that does not have
a suffix. When used together with -Wsystem-headers it
warns about such constants in system header files. This can be useful
when preparing code to use with the "FLOAT_CONST_DECIMAL64" pragma
from the decimal floating-point extension to C99.
- -Wno-designated-init (C and Objective-C only)
-
Suppress warnings when a positional initializer is used to initialize
a structure that has been marked with the "designated_init"
attribute.
Options for Debugging Your Program or GCC
GCC has various special options that are used for debugging
either your program or GCC:
- -g
-
Produce debugging information in the operating system's native format
(stabs, COFF, XCOFF, or DWARF 2). GDB can work with this debugging
information.
On most systems that use stabs format, -g enables use of extra
debugging information that only GDB can use; this extra information
makes debugging work better in GDB but probably makes other debuggers
crash or
refuse to read the program. If you want to control for certain whether
to generate the extra information, use -gstabs+, -gstabs,
-gxcoff+, -gxcoff, or -gvms (see below).
GCC allows you to use -g with
-O. The shortcuts taken by optimized code may occasionally
produce surprising results: some variables you declared may not exist
at all; flow of control may briefly move where you did not expect it;
some statements may not be executed because they compute constant
results or their values are already at hand; some statements may
execute in different places because they have been moved out of loops.
Nevertheless it proves possible to debug optimized output. This makes
it reasonable to use the optimizer for programs that might have bugs.
The following options are useful when GCC is generated with the
capability for more than one debugging format.
- -gsplit-dwarf
-
Separate as much dwarf debugging information as possible into a
separate output file with the extension .dwo. This option allows
the build system to avoid linking files with debug information. To
be useful, this option requires a debugger capable of reading .dwo
files.
- -ggdb
-
Produce debugging information for use by GDB. This means to use the
most expressive format available (DWARF 2, stabs, or the native format
if neither of those are supported), including GDB extensions if at all
possible.
- -gpubnames
-
Generate dwarf .debug_pubnames and .debug_pubtypes sections.
- -ggnu-pubnames
-
Generate .debug_pubnames and .debug_pubtypes sections in a format
suitable for conversion into a GDB index. This option is only useful
with a linker that can produce GDB index version 7.
- -gstabs
-
Produce debugging information in stabs format (if that is supported),
without GDB extensions. This is the format used by DBX on most BSD
systems. On MIPS, Alpha and System V Release 4 systems this option
produces stabs debugging output that is not understood by DBX or SDB.
On System V Release 4 systems this option requires the GNU assembler.
- -feliminate-unused-debug-symbols
-
Produce debugging information in stabs format (if that is supported),
for only symbols that are actually used.
- -femit-class-debug-always
-
Instead of emitting debugging information for a C++ class in only one
object file, emit it in all object files using the class. This option
should be used only with debuggers that are unable to handle the way GCC
normally emits debugging information for classes because using this
option increases the size of debugging information by as much as a
factor of two.
- -fdebug-types-section
-
When using DWARF Version 4 or higher, type DIEs can be put into
their own ".debug_types" section instead of making them part of the
".debug_info" section. It is more efficient to put them in a separate
comdat sections since the linker can then remove duplicates.
But not all DWARF consumers support ".debug_types" sections yet
and on some objects ".debug_types" produces larger instead of smaller
debugging information.
- -gstabs+
-
Produce debugging information in stabs format (if that is supported),
using GNU extensions understood only by the GNU debugger (GDB). The
use of these extensions is likely to make other debuggers crash or
refuse to read the program.
- -gcoff
-
Produce debugging information in COFF format (if that is supported).
This is the format used by SDB on most System V systems prior to
System V Release 4.
- -gxcoff
-
Produce debugging information in XCOFF format (if that is supported).
This is the format used by the DBX debugger on IBM RS/6000 systems.
- -gxcoff+
-
Produce debugging information in XCOFF format (if that is supported),
using GNU extensions understood only by the GNU debugger (GDB). The
use of these extensions is likely to make other debuggers crash or
refuse to read the program, and may cause assemblers other than the GNU
assembler (GAS) to fail with an error.
- -gdwarf-version
-
Produce debugging information in DWARF format (if that is supported).
The value of version may be either 2, 3, 4 or 5; the default version
for most targets is 4. DWARF Version 5 is only experimental.
Note that with DWARF Version 2, some ports require and always
use some non-conflicting DWARF 3 extensions in the unwind tables.
Version 4 may require GDB 7.0 and -fvar-tracking-assignments
for maximum benefit.
- -grecord-gcc-switches
-
This switch causes the command-line options used to invoke the
compiler that may affect code generation to be appended to the
DW_AT_producer attribute in DWARF debugging information. The options
are concatenated with spaces separating them from each other and from
the compiler version. See also -frecord-gcc-switches for another
way of storing compiler options into the object file. This is the default.
- -gno-record-gcc-switches
-
Disallow appending command-line options to the DW_AT_producer attribute
in DWARF debugging information.
- -gstrict-dwarf
-
Disallow using extensions of later DWARF standard version than selected
with -gdwarf-version. On most targets using non-conflicting
DWARF extensions from later standard versions is allowed.
- -gno-strict-dwarf
-
Allow using extensions of later DWARF standard version than selected with
-gdwarf-version.
- -gz[=type]
-
Produce compressed debug sections in DWARF format, if that is supported.
If type is not given, the default type depends on the capabilities
of the assembler and linker used. type may be one of
none (don't compress debug sections), zlib (use zlib
compression in ELF gABI format), or zlib-gnu (use zlib
compression in traditional GNU format). If the linker doesn't support
writing compressed debug sections, the option is rejected. Otherwise,
if the assembler does not support them, -gz is silently ignored
when producing object files.
- -gvms
-
Produce debugging information in Alpha/VMS debug format (if that is
supported). This is the format used by DEBUG on Alpha/VMS systems.
- -glevel
-
- -ggdblevel
-
- -gstabslevel
-
- -gcofflevel
-
- -gxcofflevel
-
- -gvmslevel
-
Request debugging information and also use level to specify how
much information. The default level is 2.
Level 0 produces no debug information at all. Thus, -g0 negates
-g.
Level 1 produces minimal information, enough for making backtraces in
parts of the program that you don't plan to debug. This includes
descriptions of functions and external variables, and line number
tables, but no information about local variables.
Level 3 includes extra information, such as all the macro definitions
present in the program. Some debuggers support macro expansion when
you use -g3.
-gdwarf-2 does not accept a concatenated debug level, because
GCC used to support an option -gdwarf that meant to generate
debug information in version 1 of the DWARF format (which is very
different from version 2), and it would have been too confusing. That
debug format is long obsolete, but the option cannot be changed now.
Instead use an additional -glevel option to change the
debug level for DWARF.
- -gtoggle
-
Turn off generation of debug info, if leaving out this option
generates it, or turn it on at level 2 otherwise. The position of this
argument in the command line does not matter; it takes effect after all
other options are processed, and it does so only once, no matter how
many times it is given. This is mainly intended to be used with
-fcompare-debug.
- -fsanitize=address
-
Enable AddressSanitizer, a fast memory error detector.
Memory access instructions are instrumented to detect
out-of-bounds and use-after-free bugs.
See <https://github.com/google/sanitizers/wiki/AddressSanitizer> for
more details. The run-time behavior can be influenced using the
ASAN_OPTIONS environment variable. When set to "help=1",
the available options are shown at startup of the instrumended program. See
<https://github.com/google/sanitizers/wiki/AddressSanitizerFlags#run-time-flags>
for a list of supported options.
- -fsanitize=kernel-address
-
Enable AddressSanitizer for Linux kernel.
See <https://github.com/google/kasan/wiki> for more details.
- -fsanitize=thread
-
Enable ThreadSanitizer, a fast data race detector.
Memory access instructions are instrumented to detect
data race bugs. See <https://github.com/google/sanitizers/wiki#threadsanitizer> for more
details. The run-time behavior can be influenced using the TSAN_OPTIONS
environment variable; see
<https://github.com/google/sanitizers/wiki/ThreadSanitizerFlags> for a list of
supported options.
- -fsanitize=leak
-
Enable LeakSanitizer, a memory leak detector.
This option only matters for linking of executables and if neither
-fsanitize=address nor -fsanitize=thread is used. In that
case the executable is linked against a library that overrides "malloc"
and other allocator functions. See
<https://github.com/google/sanitizers/wiki/AddressSanitizerLeakSanitizer> for more
details. The run-time behavior can be influenced using the
LSAN_OPTIONS environment variable.
- -fsanitize=undefined
-
Enable UndefinedBehaviorSanitizer, a fast undefined behavior detector.
Various computations are instrumented to detect undefined behavior
at runtime. Current suboptions are:
-
- -fsanitize=shift
-
This option enables checking that the result of a shift operation is
not undefined. Note that what exactly is considered undefined differs
slightly between C and C++, as well as between ISO C90 and C99, etc.
- -fsanitize=integer-divide-by-zero
-
Detect integer division by zero as well as "INT_MIN / -1" division.
- -fsanitize=unreachable
-
With this option, the compiler turns the "__builtin_unreachable"
call into a diagnostics message call instead. When reaching the
"__builtin_unreachable" call, the behavior is undefined.
- -fsanitize=vla-bound
-
This option instructs the compiler to check that the size of a variable
length array is positive.
- -fsanitize=null
-
This option enables pointer checking. Particularly, the application
built with this option turned on will issue an error message when it
tries to dereference a NULL pointer, or if a reference (possibly an
rvalue reference) is bound to a NULL pointer, or if a method is invoked
on an object pointed by a NULL pointer.
- -fsanitize=return
-
This option enables return statement checking. Programs
built with this option turned on will issue an error message
when the end of a non-void function is reached without actually
returning a value. This option works in C++ only.
- -fsanitize=signed-integer-overflow
-
This option enables signed integer overflow checking. We check that
the result of "+", "*", and both unary and binary "-"
does not overflow in the signed arithmetics. Note, integer promotion
rules must be taken into account. That is, the following is not an
overflow:
signed char a = SCHAR_MAX;
a++;
- -fsanitize=bounds
-
This option enables instrumentation of array bounds. Various out of bounds
accesses are detected. Flexible array members, flexible array member-like
arrays, and initializers of variables with static storage are not instrumented.
- -fsanitize=alignment
-
This option enables checking of alignment of pointers when they are
dereferenced, or when a reference is bound to insufficiently aligned target,
or when a method or constructor is invoked on insufficiently aligned object.
- -fsanitize=object-size
-
This option enables instrumentation of memory references using the
"__builtin_object_size" function. Various out of bounds pointer
accesses are detected.
- -fsanitize=float-divide-by-zero
-
Detect floating-point division by zero. Unlike other similar options,
-fsanitize=float-divide-by-zero is not enabled by
-fsanitize=undefined, since floating-point division by zero can
be a legitimate way of obtaining infinities and NaNs.
- -fsanitize=float-cast-overflow
-
This option enables floating-point type to integer conversion checking.
We check that the result of the conversion does not overflow.
Unlike other similar options, -fsanitize=float-cast-overflow is
not enabled by -fsanitize=undefined.
This option does not work well with "FE_INVALID" exceptions enabled.
- -fsanitize=nonnull-attribute
-
This option enables instrumentation of calls, checking whether null values
are not passed to arguments marked as requiring a non-null value by the
"nonnull" function attribute.
- -fsanitize=returns-nonnull-attribute
-
This option enables instrumentation of return statements in functions
marked with "returns_nonnull" function attribute, to detect returning
of null values from such functions.
- -fsanitize=bool
-
This option enables instrumentation of loads from bool. If a value other
than 0/1 is loaded, a run-time error is issued.
- -fsanitize=enum
-
This option enables instrumentation of loads from an enum type. If
a value outside the range of values for the enum type is loaded,
a run-time error is issued.
- -fsanitize=vptr
-
This option enables instrumentation of C++ member function calls, member
accesses and some conversions between pointers to base and derived classes,
to verify the referenced object has the correct dynamic type.
-
While -ftrapv causes traps for signed overflows to be emitted,
-fsanitize=undefined gives a diagnostic message.
This currently works only for the C family of languages.
- -fno-sanitize=all
-
This option disables all previously enabled sanitizers.
-fsanitize=all is not allowed, as some sanitizers cannot be used
together.
- -fasan-shadow-offset=number
-
This option forces GCC to use custom shadow offset in AddressSanitizer checks.
It is useful for experimenting with different shadow memory layouts in
Kernel AddressSanitizer.
- -fsanitize-recover[=opts]
-
-fsanitize-recover= controls error recovery mode for sanitizers
mentioned in comma-separated list of opts. Enabling this option
for a sanitizer component causes it to attempt to continue
running the program as if no error happened. This means multiple
runtime errors can be reported in a single program run, and the exit
code of the program may indicate success even when errors
have been reported. The -fno-sanitize-recover= option
can be used to alter
this behavior: only the first detected error is reported
and program then exits with a non-zero exit code.
Currently this feature only works for -fsanitize=undefined (and its suboptions
except for -fsanitize=unreachable and -fsanitize=return),
-fsanitize=float-cast-overflow, -fsanitize=float-divide-by-zero and
-fsanitize=kernel-address. For these sanitizers error recovery is turned on by default.
-fsanitize-recover=all and -fno-sanitize-recover=all is also
accepted, the former enables recovery for all sanitizers that support it,
the latter disables recovery for all sanitizers that support it.
Syntax without explicit opts parameter is deprecated. It is equivalent to
-fsanitize-recover=undefined,float-cast-overflow,float-divide-by-zero
Similarly -fno-sanitize-recover is equivalent to
-fno-sanitize-recover=undefined,float-cast-overflow,float-divide-by-zero
- -fsanitize-undefined-trap-on-error
-
The -fsanitize-undefined-trap-on-error option instructs the compiler to
report undefined behavior using "__builtin_trap" rather than
a "libubsan" library routine. The advantage of this is that the
"libubsan" library is not needed and is not linked in, so this
is usable even in freestanding environments.
- -fcheck-pointer-bounds
-
Enable Pointer Bounds Checker instrumentation. Each memory reference
is instrumented with checks of the pointer used for memory access against
bounds associated with that pointer.
Currently there
is only an implementation for Intel MPX available, thus x86 target
and -mmpx are required to enable this feature.
MPX-based instrumentation requires
a runtime library to enable MPX in hardware and handle bounds
violation signals. By default when -fcheck-pointer-bounds
and -mmpx options are used to link a program, the GCC driver
links against the libmpx runtime library and libmpxwrappers
library. It also passes '-z bndplt' to a linker in case it supports this
option (which is checked on libmpx configuration). Note that old versions
of linker may ignore option. Gold linker doesn't support '-z bndplt'
option. With no '-z bndplt' support in linker all calls to dynamic libraries
lose passed bounds reducing overall protection level. It's highly
recommended to use linker with '-z bndplt' support. In case such linker
is not available it is adviced to always use -static-libmpxwrappers
for better protection level or use -static to completely avoid
external calls to dynamic libraries. MPX-based instrumentation
may be used for debugging and also may be included in production code
to increase program security. Depending on usage, you may
have different requirements for the runtime library. The current version
of the MPX runtime library is more oriented for use as a debugging
tool. MPX runtime library usage implies -lpthread. See
also -static-libmpx. The runtime library behavior can be
influenced using various CHKP_RT_* environment variables. See
<https://gcc.gnu.org/wiki/Intel%20MPX%20support%20in%20the%20GCC%20compiler>
for more details.
Generated instrumentation may be controlled by various
-fchkp-* options and by the "bnd_variable_size"
structure field attribute and
"bnd_legacy", and "bnd_instrument" function attributes. GCC also provides a number of built-in
functions for controlling the Pointer Bounds Checker.
- -fchkp-check-incomplete-type
-
Generate pointer bounds checks for variables with incomplete type.
Enabled by default.
- -fchkp-narrow-bounds
-
Controls bounds used by Pointer Bounds Checker for pointers to object
fields. If narrowing is enabled then field bounds are used. Otherwise
object bounds are used. See also -fchkp-narrow-to-innermost-array
and -fchkp-first-field-has-own-bounds. Enabled by default.
- -fchkp-first-field-has-own-bounds
-
Forces Pointer Bounds Checker to use narrowed bounds for the address of the
first field in the structure. By default a pointer to the first field has
the same bounds as a pointer to the whole structure.
- -fchkp-narrow-to-innermost-array
-
Forces Pointer Bounds Checker to use bounds of the innermost arrays in
case of nested static array access. By default this option is disabled and
bounds of the outermost array are used.
- -fchkp-optimize
-
Enables Pointer Bounds Checker optimizations. Enabled by default at
optimization levels -O, -O2, -O3.
- -fchkp-use-fast-string-functions
-
Enables use of *_nobnd versions of string functions (not copying bounds)
by Pointer Bounds Checker. Disabled by default.
- -fchkp-use-nochk-string-functions
-
Enables use of *_nochk versions of string functions (not checking bounds)
by Pointer Bounds Checker. Disabled by default.
- -fchkp-use-static-bounds
-
Allow Pointer Bounds Checker to generate static bounds holding
bounds of static variables. Enabled by default.
- -fchkp-use-static-const-bounds
-
Use statically-initialized bounds for constant bounds instead of
generating them each time they are required. By default enabled when
-fchkp-use-static-bounds is enabled.
- -fchkp-treat-zero-dynamic-size-as-infinite
-
With this option, objects with incomplete type whose
dynamically-obtained size is zero are treated as having infinite size
instead by Pointer Bounds
Checker. This option may be helpful if a program is linked with a library
missing size information for some symbols. Disabled by default.
- -fchkp-check-read
-
Instructs Pointer Bounds Checker to generate checks for all read
accesses to memory. Enabled by default.
- -fchkp-check-write
-
Instructs Pointer Bounds Checker to generate checks for all write
accesses to memory. Enabled by default.
- -fchkp-store-bounds
-
Instructs Pointer Bounds Checker to generate bounds stores for
pointer writes. Enabled by default.
- -fchkp-instrument-calls
-
Instructs Pointer Bounds Checker to pass pointer bounds to calls.
Enabled by default.
- -fchkp-instrument-marked-only
-
Instructs Pointer Bounds Checker to instrument only functions
marked with the "bnd_instrument" attribute. Disabled by default.
- -fchkp-use-wrappers
-
Allows Pointer Bounds Checker to replace calls to built-in functions
with calls to wrapper functions. When -fchkp-use-wrappers
is used to link a program, the GCC driver automatically links
against libmpxwrappers. See also -static-libmpxwrappers.
Enabled by default.
- -fdump-final-insns[=file]
-
Dump the final internal representation (RTL) to file. If the
optional argument is omitted (or if file is "."), the name
of the dump file is determined by appending ".gkd" to the
compilation output file name.
- -fcompare-debug[=opts]
-
If no error occurs during compilation, run the compiler a second time,
adding opts and -fcompare-debug-second to the arguments
passed to the second compilation. Dump the final internal
representation in both compilations, and print an error if they differ.
If the equal sign is omitted, the default -gtoggle is used.
The environment variable GCC_COMPARE_DEBUG, if defined, non-empty
and nonzero, implicitly enables -fcompare-debug. If
GCC_COMPARE_DEBUG is defined to a string starting with a dash,
then it is used for opts, otherwise the default -gtoggle
is used.
-fcompare-debug=, with the equal sign but without opts,
is equivalent to -fno-compare-debug, which disables the dumping
of the final representation and the second compilation, preventing even
GCC_COMPARE_DEBUG from taking effect.
To verify full coverage during -fcompare-debug testing, set
GCC_COMPARE_DEBUG to say -fcompare-debug-not-overridden,
which GCC rejects as an invalid option in any actual compilation
(rather than preprocessing, assembly or linking). To get just a
warning, setting GCC_COMPARE_DEBUG to -w%n-fcompare-debug
not overridden will do.
- -fcompare-debug-second
-
This option is implicitly passed to the compiler for the second
compilation requested by -fcompare-debug, along with options to
silence warnings, and omitting other options that would cause
side-effect compiler outputs to files or to the standard output. Dump
files and preserved temporary files are renamed so as to contain the
".gk" additional extension during the second compilation, to avoid
overwriting those generated by the first.
When this option is passed to the compiler driver, it causes the
first compilation to be skipped, which makes it useful for little
other than debugging the compiler proper.
- -feliminate-dwarf2-dups
-
Compress DWARF 2 debugging information by eliminating duplicated
information about each symbol. This option only makes sense when
generating DWARF 2 debugging information with -gdwarf-2.
- -femit-struct-debug-baseonly
-
Emit debug information for struct-like types
only when the base name of the compilation source file
matches the base name of file in which the struct is defined.
This option substantially reduces the size of debugging information,
but at significant potential loss in type information to the debugger.
See -femit-struct-debug-reduced for a less aggressive option.
See -femit-struct-debug-detailed for more detailed control.
This option works only with DWARF 2.
- -femit-struct-debug-reduced
-
Emit debug information for struct-like types
only when the base name of the compilation source file
matches the base name of file in which the type is defined,
unless the struct is a template or defined in a system header.
This option significantly reduces the size of debugging information,
with some potential loss in type information to the debugger.
See -femit-struct-debug-baseonly for a more aggressive option.
See -femit-struct-debug-detailed for more detailed control.
This option works only with DWARF 2.
- -femit-struct-debug-detailed[=spec-list]
-
Specify the struct-like types
for which the compiler generates debug information.
The intent is to reduce duplicate struct debug information
between different object files within the same program.
This option is a detailed version of
-femit-struct-debug-reduced and -femit-struct-debug-baseonly,
which serves for most needs.
A specification has the syntax[dir:|ind:][ord:|gen:](any|sys|base|none)
The optional first word limits the specification to
structs that are used directly (dir:) or used indirectly (ind:).
A struct type is used directly when it is the type of a variable, member.
Indirect uses arise through pointers to structs.
That is, when use of an incomplete struct is valid, the use is indirect.
An example is
struct one direct; struct two * indirect;.
The optional second word limits the specification to
ordinary structs (ord:) or generic structs (gen:).
Generic structs are a bit complicated to explain.
For C++, these are non-explicit specializations of template classes,
or non-template classes within the above.
Other programming languages have generics,
but -femit-struct-debug-detailed does not yet implement them.
The third word specifies the source files for those
structs for which the compiler should emit debug information.
The values none and any have the normal meaning.
The value base means that
the base of name of the file in which the type declaration appears
must match the base of the name of the main compilation file.
In practice, this means that when compiling foo.c, debug information
is generated for types declared in that file and foo.h,
but not other header files.
The value sys means those types satisfying base
or declared in system or compiler headers.
You may need to experiment to determine the best settings for your application.
The default is -femit-struct-debug-detailed=all.
This option works only with DWARF 2.
- -fno-merge-debug-strings
-
Direct the linker to not merge together strings in the debugging
information that are identical in different object files. Merging is
not supported by all assemblers or linkers. Merging decreases the size
of the debug information in the output file at the cost of increasing
link processing time. Merging is enabled by default.
- -fdebug-prefix-map=old=new
-
When compiling files in directory old, record debugging
information describing them as in new instead.
- -fno-dwarf2-cfi-asm
-
Emit DWARF 2 unwind info as compiler generated ".eh_frame" section
instead of using GAS ".cfi_*" directives.
- -p
-
Generate extra code to write profile information suitable for the
analysis program prof. You must use this option when compiling
the source files you want data about, and you must also use it when
linking.
- -pg
-
Generate extra code to write profile information suitable for the
analysis program gprof. You must use this option when compiling
the source files you want data about, and you must also use it when
linking.
- -Q
-
Makes the compiler print out each function name as it is compiled, and
print some statistics about each pass when it finishes.
- -ftime-report
-
Makes the compiler print some statistics about the time consumed by each
pass when it finishes.
- -fmem-report
-
Makes the compiler print some statistics about permanent memory
allocation when it finishes.
- -fmem-report-wpa
-
Makes the compiler print some statistics about permanent memory
allocation for the WPA phase only.
- -fpre-ipa-mem-report
-
- -fpost-ipa-mem-report
-
Makes the compiler print some statistics about permanent memory
allocation before or after interprocedural optimization.
- -fprofile-report
-
Makes the compiler print some statistics about consistency of the
(estimated) profile and effect of individual passes.
- -fstack-usage
-
Makes the compiler output stack usage information for the program, on a
per-function basis. The filename for the dump is made by appending
.su to the auxname. auxname is generated from the name of
the output file, if explicitly specified and it is not an executable,
otherwise it is the basename of the source file. An entry is made up
of three fields:
-
- *
-
The name of the function.
- *
-
A number of bytes.
- *
-
One or more qualifiers: "static", "dynamic", "bounded".
-
The qualifier "static" means that the function manipulates the stack
statically: a fixed number of bytes are allocated for the frame on function
entry and released on function exit; no stack adjustments are otherwise made
in the function. The second field is this fixed number of bytes.
The qualifier "dynamic" means that the function manipulates the stack
dynamically: in addition to the static allocation described above, stack
adjustments are made in the body of the function, for example to push/pop
arguments around function calls. If the qualifier "bounded" is also
present, the amount of these adjustments is bounded at compile time and
the second field is an upper bound of the total amount of stack used by
the function. If it is not present, the amount of these adjustments is
not bounded at compile time and the second field only represents the
bounded part.
- -fprofile-arcs
-
Add code so that program flow arcs are instrumented. During
execution the program records how many times each branch and call is
executed and how many times it is taken or returns. When the compiled
program exits it saves this data to a file called
auxname.gcda for each source file. The data may be used for
profile-directed optimizations (-fbranch-probabilities), or for
test coverage analysis (-ftest-coverage). Each object file's
auxname is generated from the name of the output file, if
explicitly specified and it is not the final executable, otherwise it is
the basename of the source file. In both cases any suffix is removed
(e.g. foo.gcda for input file dir/foo.c, or
dir/foo.gcda for output file specified as -o dir/foo.o).
- --coverage
-
This option is used to compile and link code instrumented for coverage
analysis. The option is a synonym for -fprofile-arcs
-ftest-coverage (when compiling) and -lgcov (when
linking). See the documentation for those options for more details.
-
- *
-
Compile the source files with -fprofile-arcs plus optimization
and code generation options. For test coverage analysis, use the
additional -ftest-coverage option. You do not need to profile
every source file in a program.
- *
-
Link your object files with -lgcov or -fprofile-arcs
(the latter implies the former).
- *
-
Run the program on a representative workload to generate the arc profile
information. This may be repeated any number of times. You can run
concurrent instances of your program, and provided that the file system
supports locking, the data files will be correctly updated. Also
"fork" calls are detected and correctly handled (double counting
will not happen).
- *
-
For profile-directed optimizations, compile the source files again with
the same optimization and code generation options plus
-fbranch-probabilities.
- *
-
For test coverage analysis, use gcov to produce human readable
information from the .gcno and .gcda files. Refer to the
gcov documentation for further information.
-
With -fprofile-arcs, for each function of your program GCC
creates a program flow graph, then finds a spanning tree for the graph.
Only arcs that are not on the spanning tree have to be instrumented: the
compiler adds code to count the number of times that these arcs are
executed. When an arc is the only exit or only entrance to a block, the
instrumentation code can be added to the block; otherwise, a new basic
block must be created to hold the instrumentation code.
- -ftest-coverage
-
Produce a notes file that the gcov code-coverage utility can use to
show program coverage. Each source file's note file is called
auxname.gcno. Refer to the -fprofile-arcs option
above for a description of auxname and instructions on how to
generate test coverage data. Coverage data matches the source files
more closely if you do not optimize.
- -fdbg-cnt-list
-
Print the name and the counter upper bound for all debug counters.
- -fdbg-cnt=counter-value-list
-
Set the internal debug counter upper bound. counter-value-list
is a comma-separated list of name:value pairs
which sets the upper bound of each debug counter name to value.
All debug counters have the initial upper bound of "UINT_MAX";
thus "dbg_cnt" returns true always unless the upper bound
is set by this option.
For example, with -fdbg-cnt=dce:10,tail_call:0,
"dbg_cnt(dce)" returns true only for first 10 invocations.
- -fenable-kind-pass
-
- -fdisable-kind-pass=range-list
-
This is a set of options that are used to explicitly disable/enable
optimization passes. These options are intended for use for debugging GCC.
Compiler users should use regular options for enabling/disabling
passes instead.
-
- -fdisable-ipa-pass
-
Disable IPA pass pass. pass is the pass name. If the same pass is
statically invoked in the compiler multiple times, the pass name should be
appended with a sequential number starting from 1.
- -fdisable-rtl-pass
-
- -fdisable-rtl-pass=range-list
-
Disable RTL pass pass. pass is the pass name. If the same pass is
statically invoked in the compiler multiple times, the pass name should be
appended with a sequential number starting from 1. range-list is a
comma-separated list of function ranges or assembler names. Each range is a number
pair separated by a colon. The range is inclusive in both ends. If the range
is trivial, the number pair can be simplified as a single number. If the
function's call graph node's uid falls within one of the specified ranges,
the pass is disabled for that function. The uid is shown in the
function header of a dump file, and the pass names can be dumped by using
option -fdump-passes.
- -fdisable-tree-pass
-
- -fdisable-tree-pass=range-list
-
Disable tree pass pass. See -fdisable-rtl for the description of
option arguments.
- -fenable-ipa-pass
-
Enable IPA pass pass. pass is the pass name. If the same pass is
statically invoked in the compiler multiple times, the pass name should be
appended with a sequential number starting from 1.
- -fenable-rtl-pass
-
- -fenable-rtl-pass=range-list
-
Enable RTL pass pass. See -fdisable-rtl for option argument
description and examples.
- -fenable-tree-pass
-
- -fenable-tree-pass=range-list
-
Enable tree pass pass. See -fdisable-rtl for the description
of option arguments.
-
Here are some examples showing uses of these options.
# disable ccp1 for all functions
-fdisable-tree-ccp1
# disable complete unroll for function whose cgraph node uid is 1
-fenable-tree-cunroll=1
# disable gcse2 for functions at the following ranges [1,1],
# [300,400], and [400,1000]
# disable gcse2 for functions foo and foo2
-fdisable-rtl-gcse2=foo,foo2
# disable early inlining
-fdisable-tree-einline
# disable ipa inlining
-fdisable-ipa-inline
# enable tree full unroll
-fenable-tree-unroll
- -dletters
-
- -fdump-rtl-pass
-
- -fdump-rtl-pass=filename
-
Says to make debugging dumps during compilation at times specified by
letters. This is used for debugging the RTL-based passes of the
compiler. The file names for most of the dumps are made by appending
a pass number and a word to the dumpname, and the files are
created in the directory of the output file. In case of
=filename option, the dump is output on the given file
instead of the pass numbered dump files. Note that the pass number is
computed statically as passes get registered into the pass manager.
Thus the numbering is not related to the dynamic order of execution of
passes. In particular, a pass installed by a plugin could have a
number over 200 even if it executed quite early. dumpname is
generated from the name of the output file, if explicitly specified
and it is not an executable, otherwise it is the basename of the
source file. These switches may have different effects when
-E is used for preprocessing.
Debug dumps can be enabled with a -fdump-rtl switch or some
-d option letters. Here are the possible
letters for use in pass and letters, and their meanings:
-
- -fdump-rtl-alignments
-
Dump after branch alignments have been computed.
- -fdump-rtl-asmcons
-
Dump after fixing rtl statements that have unsatisfied in/out constraints.
- -fdump-rtl-auto_inc_dec
-
Dump after auto-inc-dec discovery. This pass is only run on
architectures that have auto inc or auto dec instructions.
- -fdump-rtl-barriers
-
Dump after cleaning up the barrier instructions.
- -fdump-rtl-bbpart
-
Dump after partitioning hot and cold basic blocks.
- -fdump-rtl-bbro
-
Dump after block reordering.
- -fdump-rtl-btl1
-
- -fdump-rtl-btl2
-
-fdump-rtl-btl1 and -fdump-rtl-btl2 enable dumping
after the two branch
target load optimization passes.
- -fdump-rtl-bypass
-
Dump after jump bypassing and control flow optimizations.
- -fdump-rtl-combine
-
Dump after the RTL instruction combination pass.
- -fdump-rtl-compgotos
-
Dump after duplicating the computed gotos.
- -fdump-rtl-ce1
-
- -fdump-rtl-ce2
-
- -fdump-rtl-ce3
-
-fdump-rtl-ce1, -fdump-rtl-ce2, and
-fdump-rtl-ce3 enable dumping after the three
if conversion passes.
- -fdump-rtl-cprop_hardreg
-
Dump after hard register copy propagation.
- -fdump-rtl-csa
-
Dump after combining stack adjustments.
- -fdump-rtl-cse1
-
- -fdump-rtl-cse2
-
-fdump-rtl-cse1 and -fdump-rtl-cse2 enable dumping after
the two common subexpression elimination passes.
- -fdump-rtl-dce
-
Dump after the standalone dead code elimination passes.
- -fdump-rtl-dbr
-
Dump after delayed branch scheduling.
- -fdump-rtl-dce1
-
- -fdump-rtl-dce2
-
-fdump-rtl-dce1 and -fdump-rtl-dce2 enable dumping after
the two dead store elimination passes.
- -fdump-rtl-eh
-
Dump after finalization of EH handling code.
- -fdump-rtl-eh_ranges
-
Dump after conversion of EH handling range regions.
- -fdump-rtl-expand
-
Dump after RTL generation.
- -fdump-rtl-fwprop1
-
- -fdump-rtl-fwprop2
-
-fdump-rtl-fwprop1 and -fdump-rtl-fwprop2 enable
dumping after the two forward propagation passes.
- -fdump-rtl-gcse1
-
- -fdump-rtl-gcse2
-
-fdump-rtl-gcse1 and -fdump-rtl-gcse2 enable dumping
after global common subexpression elimination.
- -fdump-rtl-init-regs
-
Dump after the initialization of the registers.
- -fdump-rtl-initvals
-
Dump after the computation of the initial value sets.
- -fdump-rtl-into_cfglayout
-
Dump after converting to cfglayout mode.
- -fdump-rtl-ira
-
Dump after iterated register allocation.
- -fdump-rtl-jump
-
Dump after the second jump optimization.
- -fdump-rtl-loop2
-
-fdump-rtl-loop2 enables dumping after the rtl
loop optimization passes.
- -fdump-rtl-mach
-
Dump after performing the machine dependent reorganization pass, if that
pass exists.
- -fdump-rtl-mode_sw
-
Dump after removing redundant mode switches.
- -fdump-rtl-rnreg
-
Dump after register renumbering.
- -fdump-rtl-outof_cfglayout
-
Dump after converting from cfglayout mode.
- -fdump-rtl-peephole2
-
Dump after the peephole pass.
- -fdump-rtl-postreload
-
Dump after post-reload optimizations.
- -fdump-rtl-pro_and_epilogue
-
Dump after generating the function prologues and epilogues.
- -fdump-rtl-sched1
-
- -fdump-rtl-sched2
-
-fdump-rtl-sched1 and -fdump-rtl-sched2 enable dumping
after the basic block scheduling passes.
- -fdump-rtl-ree
-
Dump after sign/zero extension elimination.
- -fdump-rtl-seqabstr
-
Dump after common sequence discovery.
- -fdump-rtl-shorten
-
Dump after shortening branches.
- -fdump-rtl-sibling
-
Dump after sibling call optimizations.
- -fdump-rtl-split1
-
- -fdump-rtl-split2
-
- -fdump-rtl-split3
-
- -fdump-rtl-split4
-
- -fdump-rtl-split5
-
These options enable dumping after five rounds of
instruction splitting.
- -fdump-rtl-sms
-
Dump after modulo scheduling. This pass is only run on some
architectures.
- -fdump-rtl-stack
-
Dump after conversion from GCC's ``flat register file'' registers to the
x87's stack-like registers. This pass is only run on x86 variants.
- -fdump-rtl-subreg1
-
- -fdump-rtl-subreg2
-
-fdump-rtl-subreg1 and -fdump-rtl-subreg2 enable dumping after
the two subreg expansion passes.
- -fdump-rtl-unshare
-
Dump after all rtl has been unshared.
- -fdump-rtl-vartrack
-
Dump after variable tracking.
- -fdump-rtl-vregs
-
Dump after converting virtual registers to hard registers.
- -fdump-rtl-web
-
Dump after live range splitting.
- -fdump-rtl-regclass
-
- -fdump-rtl-subregs_of_mode_init
-
- -fdump-rtl-subregs_of_mode_finish
-
- -fdump-rtl-dfinit
-
- -fdump-rtl-dfinish
-
These dumps are defined but always produce empty files.
- -da
-
- -fdump-rtl-all
-
Produce all the dumps listed above.
- -dA
-
Annotate the assembler output with miscellaneous debugging information.
- -dD
-
Dump all macro definitions, at the end of preprocessing, in addition to
normal output.
- -dH
-
Produce a core dump whenever an error occurs.
- -dp
-
Annotate the assembler output with a comment indicating which
pattern and alternative is used. The length of each instruction is
also printed.
- -dP
-
Dump the RTL in the assembler output as a comment before each instruction.
Also turns on -dp annotation.
- -dx
-
Just generate RTL for a function instead of compiling it. Usually used
with -fdump-rtl-expand.
-
- -fdump-noaddr
-
When doing debugging dumps, suppress address output. This makes it more
feasible to use diff on debugging dumps for compiler invocations with
different compiler binaries and/or different
text / bss / data / heap / stack / dso start locations.
- -freport-bug
-
Collect and dump debug information into temporary file if ICE in C/C++
compiler occured.
- -fdump-unnumbered
-
When doing debugging dumps, suppress instruction numbers and address output.
This makes it more feasible to use diff on debugging dumps for compiler
invocations with different options, in particular with and without
-g.
- -fdump-unnumbered-links
-
When doing debugging dumps (see -d option above), suppress
instruction numbers for the links to the previous and next instructions
in a sequence.
- -fdump-translation-unit (C++ only)
-
- -fdump-translation-unit-options (C++ only)
-
Dump a representation of the tree structure for the entire translation
unit to a file. The file name is made by appending .tu to the
source file name, and the file is created in the same directory as the
output file. If the -options form is used, options
controls the details of the dump as described for the
-fdump-tree options.
- -fdump-class-hierarchy (C++ only)
-
- -fdump-class-hierarchy-options (C++ only)
-
Dump a representation of each class's hierarchy and virtual function
table layout to a file. The file name is made by appending
.class to the source file name, and the file is created in the
same directory as the output file. If the -options form
is used, options controls the details of the dump as described
for the -fdump-tree options.
- -fdump-ipa-switch
-
Control the dumping at various stages of inter-procedural analysis
language tree to a file. The file name is generated by appending a
switch specific suffix to the source file name, and the file is created
in the same directory as the output file. The following dumps are
possible:
-
- all
-
Enables all inter-procedural analysis dumps.
- cgraph
-
Dumps information about call-graph optimization, unused function removal,
and inlining decisions.
- inline
-
Dump after function inlining.
-
- -fdump-passes
-
Dump the list of optimization passes that are turned on and off by
the current command-line options.
- -fdump-statistics-option
-
Enable and control dumping of pass statistics in a separate file. The
file name is generated by appending a suffix ending in
.statistics to the source file name, and the file is created in
the same directory as the output file. If the -option
form is used, -stats causes counters to be summed over the
whole compilation unit while -details dumps every event as
the passes generate them. The default with no option is to sum
counters for each function compiled.
- -fdump-tree-switch
-
- -fdump-tree-switch-options
-
- -fdump-tree-switch-options=filename
-
Control the dumping at various stages of processing the intermediate
language tree to a file. The file name is generated by appending a
switch-specific suffix to the source file name, and the file is
created in the same directory as the output file. In case of
=filename option, the dump is output on the given file
instead of the auto named dump files. If the -options
form is used, options is a list of - separated options
which control the details of the dump. Not all options are applicable
to all dumps; those that are not meaningful are ignored. The
following options are available
-
- address
-
Print the address of each node. Usually this is not meaningful as it
changes according to the environment and source file. Its primary use
is for tying up a dump file with a debug environment.
- asmname
-
If "DECL_ASSEMBLER_NAME" has been set for a given decl, use that
in the dump instead of "DECL_NAME". Its primary use is ease of
use working backward from mangled names in the assembly file.
- slim
-
When dumping front-end intermediate representations, inhibit dumping
of members of a scope or body of a function merely because that scope
has been reached. Only dump such items when they are directly reachable
by some other path.
When dumping pretty-printed trees, this option inhibits dumping the
bodies of control structures.
When dumping RTL, print the RTL in slim (condensed) form instead of
the default LISP-like representation.
- raw
-
Print a raw representation of the tree. By default, trees are
pretty-printed into a C-like representation.
- details
-
Enable more detailed dumps (not honored by every dump option). Also
include information from the optimization passes.
- stats
-
Enable dumping various statistics about the pass (not honored by every dump
option).
- blocks
-
Enable showing basic block boundaries (disabled in raw dumps).
- graph
-
For each of the other indicated dump files (-fdump-rtl-pass),
dump a representation of the control flow graph suitable for viewing with
GraphViz to file.passid.pass.dot. Each function in
the file is pretty-printed as a subgraph, so that GraphViz can render them
all in a single plot.
This option currently only works for RTL dumps, and the RTL is always
dumped in slim form.
- vops
-
Enable showing virtual operands for every statement.
- lineno
-
Enable showing line numbers for statements.
- uid
-
Enable showing the unique ID ("DECL_UID") for each variable.
- verbose
-
Enable showing the tree dump for each statement.
- eh
-
Enable showing the EH region number holding each statement.
- scev
-
Enable showing scalar evolution analysis details.
- optimized
-
Enable showing optimization information (only available in certain
passes).
- missed
-
Enable showing missed optimization information (only available in certain
passes).
- note
-
Enable other detailed optimization information (only available in
certain passes).
- =filename
-
Instead of an auto named dump file, output into the given file
name. The file names stdout and stderr are treated
specially and are considered already open standard streams. For
example,
gcc -O2 -ftree-vectorize -fdump-tree-vect-blocks=foo.dump
-fdump-tree-pre=stderr file.c
outputs vectorizer dump into foo.dump, while the PRE dump is
output on to stderr. If two conflicting dump filenames are
given for the same pass, then the latter option overrides the earlier
one.
- all
-
Turn on all options, except raw, slim, verbose
and lineno.
- optall
-
Turn on all optimization options, i.e., optimized,
missed, and note.
-
The following tree dumps are possible:
- original
-
Dump before any tree based optimization, to file.original.
- optimized
-
Dump after all tree based optimization, to file.optimized.
- gimple
-
Dump each function before and after the gimplification pass to a file. The
file name is made by appending .gimple to the source file name.
- cfg
-
Dump the control flow graph of each function to a file. The file name is
made by appending .cfg to the source file name.
- ch
-
Dump each function after copying loop headers. The file name is made by
appending .ch to the source file name.
- ssa
-
Dump SSA related information to a file. The file name is made by appending
.ssa to the source file name.
- alias
-
Dump aliasing information for each function. The file name is made by
appending .alias to the source file name.
- ccp
-
Dump each function after CCP. The file name is made by appending
.ccp to the source file name.
- storeccp
-
Dump each function after STORE-CCP. The file name is made by appending
.storeccp to the source file name.
- pre
-
Dump trees after partial redundancy elimination. The file name is made
by appending .pre to the source file name.
- fre
-
Dump trees after full redundancy elimination. The file name is made
by appending .fre to the source file name.
- copyprop
-
Dump trees after copy propagation. The file name is made
by appending .copyprop to the source file name.
- store_copyprop
-
Dump trees after store copy-propagation. The file name is made
by appending .store_copyprop to the source file name.
- dce
-
Dump each function after dead code elimination. The file name is made by
appending .dce to the source file name.
- sra
-
Dump each function after performing scalar replacement of aggregates. The
file name is made by appending .sra to the source file name.
- sink
-
Dump each function after performing code sinking. The file name is made
by appending .sink to the source file name.
- dom
-
Dump each function after applying dominator tree optimizations. The file
name is made by appending .dom to the source file name.
- dse
-
Dump each function after applying dead store elimination. The file
name is made by appending .dse to the source file name.
- phiopt
-
Dump each function after optimizing PHI nodes into straightline code. The file
name is made by appending .phiopt to the source file name.
- forwprop
-
Dump each function after forward propagating single use variables. The file
name is made by appending .forwprop to the source file name.
- copyrename
-
Dump each function after applying the copy rename optimization. The file
name is made by appending .copyrename to the source file name.
- nrv
-
Dump each function after applying the named return value optimization on
generic trees. The file name is made by appending .nrv to the source
file name.
- vect
-
Dump each function after applying vectorization of loops. The file name is
made by appending .vect to the source file name.
- slp
-
Dump each function after applying vectorization of basic blocks. The file name
is made by appending .slp to the source file name.
- vrp
-
Dump each function after Value Range Propagation (VRP). The file name
is made by appending .vrp to the source file name.
- all
-
Enable all the available tree dumps with the flags provided in this option.
-
- -fopt-info
-
- -fopt-info-options
-
- -fopt-info-options=filename
-
Controls optimization dumps from various optimization passes. If the
-options form is used, options is a list of
- separated option keywords to select the dump details and
optimizations.
The options can be divided into two groups: options describing the
verbosity of the dump, and options describing which optimizations
should be included. The options from both the groups can be freely
mixed as they are non-overlapping. However, in case of any conflicts,
the later options override the earlier options on the command
line.
The following options control the dump verbosity:
-
- optimized
-
Print information when an optimization is successfully applied. It is
up to a pass to decide which information is relevant. For example, the
vectorizer passes print the source location of loops which are
successfully vectorized.
- missed
-
Print information about missed optimizations. Individual passes
control which information to include in the output.
- note
-
Print verbose information about optimizations, such as certain
transformations, more detailed messages about decisions etc.
- all
-
Print detailed optimization information. This includes
optimized, missed, and note.
-
One or more of the following option keywords can be used to describe a
group of optimizations:
- ipa
-
Enable dumps from all interprocedural optimizations.
- loop
-
Enable dumps from all loop optimizations.
- inline
-
Enable dumps from all inlining optimizations.
- vec
-
Enable dumps from all vectorization optimizations.
- optall
-
Enable dumps from all optimizations. This is a superset of
the optimization groups listed above.
-
If options is
omitted, it defaults to optimized-optall, which means to dump all
info about successful optimizations from all the passes.
If the filename is provided, then the dumps from all the
applicable optimizations are concatenated into the filename.
Otherwise the dump is output onto stderr. Though multiple
-fopt-info options are accepted, only one of them can include
a filename. If other filenames are provided then all but the
first such option are ignored.
Note that the output filename is overwritten
in case of multiple translation units. If a combined output from
multiple translation units is desired, stderr should be used
instead.
In the following example, the optimization info is output to
stderr:
gcc -O3 -fopt-info
This example:
gcc -O3 -fopt-info-missed=missed.all
outputs missed optimization report from all the passes into
missed.all, and this one:
gcc -O2 -ftree-vectorize -fopt-info-vec-missed
prints information about missed optimization opportunities from
vectorization passes on stderr.
Note that -fopt-info-vec-missed is equivalent to
-fopt-info-missed-vec.
As another example,
gcc -O3 -fopt-info-inline-optimized-missed=inline.txt
outputs information about missed optimizations as well as
optimized locations from all the inlining passes into
inline.txt.
Finally, consider:
gcc -fopt-info-vec-missed=vec.miss -fopt-info-loop-optimized=loop.opt
Here the two output filenames vec.miss and loop.opt are
in conflict since only one output file is allowed. In this case, only
the first option takes effect and the subsequent options are
ignored. Thus only vec.miss is produced which contains
dumps from the vectorizer about missed opportunities.
- -frandom-seed=string
-
This option provides a seed that GCC uses in place of
random numbers in generating certain symbol names
that have to be different in every compiled file. It is also used to
place unique stamps in coverage data files and the object files that
produce them. You can use the -frandom-seed option to produce
reproducibly identical object files.
The string can either be a number (decimal, octal or hex) or an
arbitrary string (in which case it's converted to a number by
computing CRC32).
The string should be different for every file you compile.
- -fsched-verbose=n
-
On targets that use instruction scheduling, this option controls the
amount of debugging output the scheduler prints. This information is
written to standard error, unless -fdump-rtl-sched1 or
-fdump-rtl-sched2 is specified, in which case it is output
to the usual dump listing file, .sched1 or .sched2
respectively. However for n greater than nine, the output is
always printed to standard error.
For n greater than zero, -fsched-verbose outputs the
same information as -fdump-rtl-sched1 and -fdump-rtl-sched2.
For n greater than one, it also output basic block probabilities,
detailed ready list information and unit/insn info. For n greater
than two, it includes RTL at abort point, control-flow and regions info.
And for n over four, -fsched-verbose also includes
dependence info.
- -save-temps
-
- -save-temps=cwd
-
Store the usual ``temporary'' intermediate files permanently; place them
in the current directory and name them based on the source file. Thus,
compiling foo.c with -c -save-temps produces files
foo.i and foo.s, as well as foo.o. This creates a
preprocessed foo.i output file even though the compiler now
normally uses an integrated preprocessor.
When used in combination with the -x command-line option,
-save-temps is sensible enough to avoid over writing an
input source file with the same extension as an intermediate file.
The corresponding intermediate file may be obtained by renaming the
source file before using -save-temps.
If you invoke GCC in parallel, compiling several different source
files that share a common base name in different subdirectories or the
same source file compiled for multiple output destinations, it is
likely that the different parallel compilers will interfere with each
other, and overwrite the temporary files. For instance:
gcc -save-temps -o outdir1/foo.o indir1/foo.c&
gcc -save-temps -o outdir2/foo.o indir2/foo.c&
may result in foo.i and foo.o being written to
simultaneously by both compilers.
- -save-temps=obj
-
Store the usual ``temporary'' intermediate files permanently. If the
-o option is used, the temporary files are based on the
object file. If the -o option is not used, the
-save-temps=obj switch behaves like -save-temps.
For example:
gcc -save-temps=obj -c foo.c
gcc -save-temps=obj -c bar.c -o dir/xbar.o
gcc -save-temps=obj foobar.c -o dir2/yfoobar
creates foo.i, foo.s, dir/xbar.i,
dir/xbar.s, dir2/yfoobar.i, dir2/yfoobar.s, and
dir2/yfoobar.o.
- -time[=file]
-
Report the CPU time taken by each subprocess in the compilation
sequence. For C source files, this is the compiler proper and assembler
(plus the linker if linking is done).
Without the specification of an output file, the output looks like this:
# cc1 0.12 0.01
# as 0.00 0.01
The first number on each line is the ``user time'', that is time spent
executing the program itself. The second number is ``system time'',
time spent executing operating system routines on behalf of the program.
Both numbers are in seconds.
With the specification of an output file, the output is appended to the
named file, and it looks like this:
0.12 0.01 cc1 <options>
0.00 0.01 as <options>
The ``user time'' and the ``system time'' are moved before the program
name, and the options passed to the program are displayed, so that one
can later tell what file was being compiled, and with which options.
- -fvar-tracking
-
Run variable tracking pass. It computes where variables are stored at each
position in code. Better debugging information is then generated
(if the debugging information format supports this information).
It is enabled by default when compiling with optimization (-Os,
-O, -O2, ...), debugging information (-g) and
the debug info format supports it.
- -fvar-tracking-assignments
-
Annotate assignments to user variables early in the compilation and
attempt to carry the annotations over throughout the compilation all the
way to the end, in an attempt to improve debug information while
optimizing. Use of -gdwarf-4 is recommended along with it.
It can be enabled even if var-tracking is disabled, in which case
annotations are created and maintained, but discarded at the end.
By default, this flag is enabled together with -fvar-tracking,
except when selective scheduling is enabled.
- -fvar-tracking-assignments-toggle
-
Toggle -fvar-tracking-assignments, in the same way that
-gtoggle toggles -g.
- -print-file-name=library
-
Print the full absolute name of the library file library that
would be used when linking---and don't do anything else. With this
option, GCC does not compile or link anything; it just prints the
file name.
- -print-multi-directory
-
Print the directory name corresponding to the multilib selected by any
other switches present in the command line. This directory is supposed
to exist in GCC_EXEC_PREFIX.
- -print-multi-lib
-
Print the mapping from multilib directory names to compiler switches
that enable them. The directory name is separated from the switches by
;, and each switch starts with an @ instead of the
-, without spaces between multiple switches. This is supposed to
ease shell processing.
- -print-multi-os-directory
-
Print the path to OS libraries for the selected
multilib, relative to some lib subdirectory. If OS libraries are
present in the lib subdirectory and no multilibs are used, this is
usually just ., if OS libraries are present in libsuffix
sibling directories this prints e.g. ../lib64, ../lib or
../lib32, or if OS libraries are present in lib/subdir
subdirectories it prints e.g. amd64, sparcv9 or ev6.
- -print-multiarch
-
Print the path to OS libraries for the selected multiarch,
relative to some lib subdirectory.
- -print-prog-name=program
-
Like -print-file-name, but searches for a program such as cpp.
- -print-libgcc-file-name
-
Same as -print-file-name=libgcc.a.
This is useful when you use -nostdlib or -nodefaultlibs
but you do want to link with libgcc.a. You can do:
gcc -nostdlib <files>... `gcc -print-libgcc-file-name`
- -print-search-dirs
-
Print the name of the configured installation directory and a list of
program and library directories gcc searches---and don't do anything else.
This is useful when gcc prints the error message
installation problem, cannot exec cpp0: No such file or directory.
To resolve this you either need to put cpp0 and the other compiler
components where gcc expects to find them, or you can set the environment
variable GCC_EXEC_PREFIX to the directory where you installed them.
Don't forget the trailing /.
- -print-sysroot
-
Print the target sysroot directory that is used during
compilation. This is the target sysroot specified either at configure
time or using the --sysroot option, possibly with an extra
suffix that depends on compilation options. If no target sysroot is
specified, the option prints nothing.
- -print-sysroot-headers-suffix
-
Print the suffix added to the target sysroot when searching for
headers, or give an error if the compiler is not configured with such
a suffix---and don't do anything else.
- -dumpmachine
-
Print the compiler's target machine (for example,
i686-pc-linux-gnu)---and don't do anything else.
- -dumpversion
-
Print the compiler version (for example, 3.0)---and don't do
anything else.
- -dumpspecs
-
Print the compiler's built-in specs---and don't do anything else. (This
is used when GCC itself is being built.)
- -fno-eliminate-unused-debug-types
-
Normally, when producing DWARF 2 output, GCC avoids producing debug symbol
output for types that are nowhere used in the source file being compiled.
Sometimes it is useful to have GCC emit debugging
information for all types declared in a compilation
unit, regardless of whether or not they are actually used
in that compilation unit, for example
if, in the debugger, you want to cast a value to a type that is
not actually used in your program (but is declared). More often,
however, this results in a significant amount of wasted space.
Options That Control Optimization
These options control various sorts of optimizations.
Without any optimization option, the compiler's goal is to reduce the
cost of compilation and to make debugging produce the expected
results. Statements are independent: if you stop the program with a
breakpoint between statements, you can then assign a new value to any
variable or change the program counter to any other statement in the
function and get exactly the results you expect from the source
code.
Turning on optimization flags makes the compiler attempt to improve
the performance and/or code size at the expense of compilation time
and possibly the ability to debug the program.
The compiler performs optimization based on the knowledge it has of the
program. Compiling multiple files at once to a single output file mode allows
the compiler to use information gained from all of the files when compiling
each of them.
Not all optimizations are controlled directly by a flag. Only
optimizations that have a flag are listed in this section.
Most optimizations are only enabled if an -O level is set on
the command line. Otherwise they are disabled, even if individual
optimization flags are specified.
Depending on the target and how GCC was configured, a slightly different
set of optimizations may be enabled at each -O level than
those listed here. You can invoke GCC with -Q --help=optimizers
to find out the exact set of optimizations that are enabled at each level.
- -O
-
- -O1
-
Optimize. Optimizing compilation takes somewhat more time, and a lot
more memory for a large function.
With -O, the compiler tries to reduce code size and execution
time, without performing any optimizations that take a great deal of
compilation time.
-O turns on the following optimization flags:
-fauto-inc-dec
-fbranch-count-reg
-fcombine-stack-adjustments
-fcompare-elim
-fcprop-registers
-fdce
-fdefer-pop
-fdelayed-branch
-fdse
-fforward-propagate
-fguess-branch-probability
-fif-conversion2
-fif-conversion
-finline-functions-called-once
-fipa-pure-const
-fipa-profile
-fipa-reference
-fmerge-constants
-fmove-loop-invariants
-fshrink-wrap
-fsplit-wide-types
-ftree-bit-ccp
-ftree-ccp
-fssa-phiopt
-ftree-ch
-ftree-copy-prop
-ftree-copyrename
-ftree-dce
-ftree-dominator-opts
-ftree-dse
-ftree-forwprop
-ftree-fre
-ftree-phiprop
-ftree-sink
-ftree-slsr
-ftree-sra
-ftree-pta
-ftree-ter
-funit-at-a-time
-O also turns on -fomit-frame-pointer on machines
where doing so does not interfere with debugging.
- -O2
-
Optimize even more. GCC performs nearly all supported optimizations
that do not involve a space-speed tradeoff.
As compared to -O, this option increases both compilation time
and the performance of the generated code.
-O2 turns on all optimization flags specified by -O. It
also turns on the following optimization flags:
-fthread-jumps
-falign-functions -falign-jumps
-falign-loops -falign-labels
-fcaller-saves
-fcrossjumping
-fcse-follow-jumps -fcse-skip-blocks
-fdelete-null-pointer-checks
-fdevirtualize -fdevirtualize-speculatively
-fexpensive-optimizations
-fgcse -fgcse-lm
-fhoist-adjacent-loads
-finline-small-functions
-findirect-inlining
-fipa-cp
-fipa-cp-alignment
-fipa-sra
-fipa-icf
-fisolate-erroneous-paths-dereference
-flra-remat
-foptimize-sibling-calls
-foptimize-strlen
-fpartial-inlining
-fpeephole2
-freorder-blocks -freorder-blocks-and-partition -freorder-functions
-frerun-cse-after-loop
-fsched-interblock -fsched-spec
-fschedule-insns -fschedule-insns2
-fstrict-aliasing -fstrict-overflow
-ftree-builtin-call-dce
-ftree-switch-conversion -ftree-tail-merge
-ftree-pre
-ftree-vrp
-fipa-ra
Please note the warning under -fgcse about
invoking -O2 on programs that use computed gotos.
- -O3
-
Optimize yet more. -O3 turns on all optimizations specified
by -O2 and also turns on the -finline-functions,
-funswitch-loops, -fpredictive-commoning,
-fgcse-after-reload, -ftree-loop-vectorize,
-ftree-loop-distribute-patterns,
-ftree-slp-vectorize, -fvect-cost-model,
-ftree-partial-pre and -fipa-cp-clone options.
- -O0
-
Reduce compilation time and make debugging produce the expected
results. This is the default.
- -Os
-
Optimize for size. -Os enables all -O2 optimizations that
do not typically increase code size. It also performs further
optimizations designed to reduce code size.
-Os disables the following optimization flags:
-falign-functions -falign-jumps -falign-loops
-falign-labels -freorder-blocks -freorder-blocks-and-partition
-fprefetch-loop-arrays
- -Ofast
-
Disregard strict standards compliance. -Ofast enables all
-O3 optimizations. It also enables optimizations that are not
valid for all standard-compliant programs.
It turns on -ffast-math and the Fortran-specific
-fno-protect-parens and -fstack-arrays.
- -Og
-
Optimize debugging experience. -Og enables optimizations
that do not interfere with debugging. It should be the optimization
level of choice for the standard edit-compile-debug cycle, offering
a reasonable level of optimization while maintaining fast compilation
and a good debugging experience.
If you use multiple -O options, with or without level numbers,
the last such option is the one that is effective.
Options of the form -fflag specify machine-independent
flags. Most flags have both positive and negative forms; the negative
form of -ffoo is -fno-foo. In the table
below, only one of the forms is listed---the one you typically
use. You can figure out the other form by either removing no-
or adding it.
The following options control specific optimizations. They are either
activated by -O options or are related to ones that are. You
can use the following flags in the rare cases when ``fine-tuning'' of
optimizations to be performed is desired.
- -fno-defer-pop
-
Always pop the arguments to each function call as soon as that function
returns. For machines that must pop arguments after a function call,
the compiler normally lets arguments accumulate on the stack for several
function calls and pops them all at once.
Disabled at levels -O, -O2, -O3, -Os.
- -fforward-propagate
-
Perform a forward propagation pass on RTL. The pass tries to combine two
instructions and checks if the result can be simplified. If loop unrolling
is active, two passes are performed and the second is scheduled after
loop unrolling.
This option is enabled by default at optimization levels -O,
-O2, -O3, -Os.
- -ffp-contract=style
-
-ffp-contract=off disables floating-point expression contraction.
-ffp-contract=fast enables floating-point expression contraction
such as forming of fused multiply-add operations if the target has
native support for them.
-ffp-contract=on enables floating-point expression contraction
if allowed by the language standard. This is currently not implemented
and treated equal to -ffp-contract=off.
The default is -ffp-contract=fast.
- -fomit-frame-pointer
-
Don't keep the frame pointer in a register for functions that
don't need one. This avoids the instructions to save, set up and
restore frame pointers; it also makes an extra register available
in many functions. It also makes debugging impossible on
some machines.
On some machines, such as the VAX, this flag has no effect, because
the standard calling sequence automatically handles the frame pointer
and nothing is saved by pretending it doesn't exist. The
machine-description macro "FRAME_POINTER_REQUIRED" controls
whether a target machine supports this flag.
The default setting (when not optimizing for
size) for 32-bit GNU/Linux x86 and 32-bit Darwin x86 targets is
-fomit-frame-pointer. You can configure GCC with the
--enable-frame-pointer configure option to change the default.
Enabled at levels -O, -O2, -O3, -Os.
- -foptimize-sibling-calls
-
Optimize sibling and tail recursive calls.
Enabled at levels -O2, -O3, -Os.
- -foptimize-strlen
-
Optimize various standard C string functions (e.g. "strlen",
"strchr" or "strcpy") and
their "_FORTIFY_SOURCE" counterparts into faster alternatives.
Enabled at levels -O2, -O3.
- -fno-inline
-
Do not expand any functions inline apart from those marked with
the "always_inline" attribute. This is the default when not
optimizing.
Single functions can be exempted from inlining by marking them
with the "noinline" attribute.
- -finline-small-functions
-
Integrate functions into their callers when their body is smaller than expected
function call code (so overall size of program gets smaller). The compiler
heuristically decides which functions are simple enough to be worth integrating
in this way. This inlining applies to all functions, even those not declared
inline.
Enabled at level -O2.
- -findirect-inlining
-
Inline also indirect calls that are discovered to be known at compile
time thanks to previous inlining. This option has any effect only
when inlining itself is turned on by the -finline-functions
or -finline-small-functions options.
Enabled at level -O2.
- -finline-functions
-
Consider all functions for inlining, even if they are not declared inline.
The compiler heuristically decides which functions are worth integrating
in this way.
If all calls to a given function are integrated, and the function is
declared "static", then the function is normally not output as
assembler code in its own right.
Enabled at level -O3.
- -finline-functions-called-once
-
Consider all "static" functions called once for inlining into their
caller even if they are not marked "inline". If a call to a given
function is integrated, then the function is not output as assembler code
in its own right.
Enabled at levels -O1, -O2, -O3 and -Os.
- -fearly-inlining
-
Inline functions marked by "always_inline" and functions whose body seems
smaller than the function call overhead early before doing
-fprofile-generate instrumentation and real inlining pass. Doing so
makes profiling significantly cheaper and usually inlining faster on programs
having large chains of nested wrapper functions.
Enabled by default.
- -fipa-sra
-
Perform interprocedural scalar replacement of aggregates, removal of
unused parameters and replacement of parameters passed by reference
by parameters passed by value.
Enabled at levels -O2, -O3 and -Os.
- -finline-limit=n
-
By default, GCC limits the size of functions that can be inlined. This flag
allows coarse control of this limit. n is the size of functions that
can be inlined in number of pseudo instructions.
Inlining is actually controlled by a number of parameters, which may be
specified individually by using --param name=value.
The -finline-limit=n option sets some of these parameters
as follows:
-
- max-inline-insns-single
-
is set to n/2.
- max-inline-insns-auto
-
is set to n/2.
-
See below for a documentation of the individual
parameters controlling inlining and for the defaults of these parameters.
Note: there may be no value to -finline-limit that results
in default behavior.
Note: pseudo instruction represents, in this particular context, an
abstract measurement of function's size. In no way does it represent a count
of assembly instructions and as such its exact meaning might change from one
release to an another.
- -fno-keep-inline-dllexport
-
This is a more fine-grained version of -fkeep-inline-functions,
which applies only to functions that are declared using the "dllexport"
attribute or declspec
- -fkeep-inline-functions
-
In C, emit "static" functions that are declared "inline"
into the object file, even if the function has been inlined into all
of its callers. This switch does not affect functions using the
"extern inline" extension in GNU C90. In C++, emit any and all
inline functions into the object file.
- -fkeep-static-consts
-
Emit variables declared "static const" when optimization isn't turned
on, even if the variables aren't referenced.
GCC enables this option by default. If you want to force the compiler to
check if a variable is referenced, regardless of whether or not
optimization is turned on, use the -fno-keep-static-consts option.
- -fmerge-constants
-
Attempt to merge identical constants (string constants and floating-point
constants) across compilation units.
This option is the default for optimized compilation if the assembler and
linker support it. Use -fno-merge-constants to inhibit this
behavior.
Enabled at levels -O, -O2, -O3, -Os.
- -fmerge-all-constants
-
Attempt to merge identical constants and identical variables.
This option implies -fmerge-constants. In addition to
-fmerge-constants this considers e.g. even constant initialized
arrays or initialized constant variables with integral or floating-point
types. Languages like C or C++ require each variable, including multiple
instances of the same variable in recursive calls, to have distinct locations,
so using this option results in non-conforming
behavior.
- -fmodulo-sched
-
Perform swing modulo scheduling immediately before the first scheduling
pass. This pass looks at innermost loops and reorders their
instructions by overlapping different iterations.
- -fmodulo-sched-allow-regmoves
-
Perform more aggressive SMS-based modulo scheduling with register moves
allowed. By setting this flag certain anti-dependences edges are
deleted, which triggers the generation of reg-moves based on the
life-range analysis. This option is effective only with
-fmodulo-sched enabled.
- -fno-branch-count-reg
-
Do not use ``decrement and branch'' instructions on a count register,
but instead generate a sequence of instructions that decrement a
register, compare it against zero, then branch based upon the result.
This option is only meaningful on architectures that support such
instructions, which include x86, PowerPC, IA-64 and S/390.
Enabled by default at -O1 and higher.
The default is -fbranch-count-reg.
- -fno-function-cse
-
Do not put function addresses in registers; make each instruction that
calls a constant function contain the function's address explicitly.
This option results in less efficient code, but some strange hacks
that alter the assembler output may be confused by the optimizations
performed when this option is not used.
The default is -ffunction-cse
- -fno-zero-initialized-in-bss
-
If the target supports a BSS section, GCC by default puts variables that
are initialized to zero into BSS. This can save space in the resulting
code.
This option turns off this behavior because some programs explicitly
rely on variables going to the data section---e.g., so that the
resulting executable can find the beginning of that section and/or make
assumptions based on that.
The default is -fzero-initialized-in-bss.
- -fthread-jumps
-
Perform optimizations that check to see if a jump branches to a
location where another comparison subsumed by the first is found. If
so, the first branch is redirected to either the destination of the
second branch or a point immediately following it, depending on whether
the condition is known to be true or false.
Enabled at levels -O2, -O3, -Os.
- -fsplit-wide-types
-
When using a type that occupies multiple registers, such as "long
long" on a 32-bit system, split the registers apart and allocate them
independently. This normally generates better code for those types,
but may make debugging more difficult.
Enabled at levels -O, -O2, -O3,
-Os.
- -fcse-follow-jumps
-
In common subexpression elimination (CSE), scan through jump instructions
when the target of the jump is not reached by any other path. For
example, when CSE encounters an "if" statement with an
"else" clause, CSE follows the jump when the condition
tested is false.
Enabled at levels -O2, -O3, -Os.
- -fcse-skip-blocks
-
This is similar to -fcse-follow-jumps, but causes CSE to
follow jumps that conditionally skip over blocks. When CSE
encounters a simple "if" statement with no else clause,
-fcse-skip-blocks causes CSE to follow the jump around the
body of the "if".
Enabled at levels -O2, -O3, -Os.
- -frerun-cse-after-loop
-
Re-run common subexpression elimination after loop optimizations are
performed.
Enabled at levels -O2, -O3, -Os.
- -fgcse
-
Perform a global common subexpression elimination pass.
This pass also performs global constant and copy propagation.
Note: When compiling a program using computed gotos, a GCC
extension, you may get better run-time performance if you disable
the global common subexpression elimination pass by adding
-fno-gcse to the command line.
Enabled at levels -O2, -O3, -Os.
- -fgcse-lm
-
When -fgcse-lm is enabled, global common subexpression elimination
attempts to move loads that are only killed by stores into themselves. This
allows a loop containing a load/store sequence to be changed to a load outside
the loop, and a copy/store within the loop.
Enabled by default when -fgcse is enabled.
- -fgcse-sm
-
When -fgcse-sm is enabled, a store motion pass is run after
global common subexpression elimination. This pass attempts to move
stores out of loops. When used in conjunction with -fgcse-lm,
loops containing a load/store sequence can be changed to a load before
the loop and a store after the loop.
Not enabled at any optimization level.
- -fgcse-las
-
When -fgcse-las is enabled, the global common subexpression
elimination pass eliminates redundant loads that come after stores to the
same memory location (both partial and full redundancies).
Not enabled at any optimization level.
- -fgcse-after-reload
-
When -fgcse-after-reload is enabled, a redundant load elimination
pass is performed after reload. The purpose of this pass is to clean up
redundant spilling.
- -faggressive-loop-optimizations
-
This option tells the loop optimizer to use language constraints to
derive bounds for the number of iterations of a loop. This assumes that
loop code does not invoke undefined behavior by for example causing signed
integer overflows or out-of-bound array accesses. The bounds for the
number of iterations of a loop are used to guide loop unrolling and peeling
and loop exit test optimizations.
This option is enabled by default.
- -funsafe-loop-optimizations
-
This option tells the loop optimizer to assume that loop indices do not
overflow, and that loops with nontrivial exit condition are not
infinite. This enables a wider range of loop optimizations even if
the loop optimizer itself cannot prove that these assumptions are valid.
If you use -Wunsafe-loop-optimizations, the compiler warns you
if it finds this kind of loop.
- -fcrossjumping
-
Perform cross-jumping transformation.
This transformation unifies equivalent code and saves code size. The
resulting code may or may not perform better than without cross-jumping.
Enabled at levels -O2, -O3, -Os.
- -fauto-inc-dec
-
Combine increments or decrements of addresses with memory accesses.
This pass is always skipped on architectures that do not have
instructions to support this. Enabled by default at -O and
higher on architectures that support this.
- -fdce
-
Perform dead code elimination (DCE) on RTL.
Enabled by default at -O and higher.
- -fdse
-
Perform dead store elimination (DSE) on RTL.
Enabled by default at -O and higher.
- -fif-conversion
-
Attempt to transform conditional jumps into branch-less equivalents. This
includes use of conditional moves, min, max, set flags and abs instructions, and
some tricks doable by standard arithmetics. The use of conditional execution
on chips where it is available is controlled by -fif-conversion2.
Enabled at levels -O, -O2, -O3, -Os.
- -fif-conversion2
-
Use conditional execution (where available) to transform conditional jumps into
branch-less equivalents.
Enabled at levels -O, -O2, -O3, -Os.
- -fdeclone-ctor-dtor
-
The C++ ABI requires multiple entry points for constructors and
destructors: one for a base subobject, one for a complete object, and
one for a virtual destructor that calls operator delete afterwards.
For a hierarchy with virtual bases, the base and complete variants are
clones, which means two copies of the function. With this option, the
base and complete variants are changed to be thunks that call a common
implementation.
Enabled by -Os.
- -fdelete-null-pointer-checks
-
Assume that programs cannot safely dereference null pointers, and that
no code or data element resides there. This enables simple constant
folding optimizations at all optimization levels. In addition, other
optimization passes in GCC use this flag to control global dataflow
analyses that eliminate useless checks for null pointers; these assume
that if a pointer is checked after it has already been dereferenced,
it cannot be null.
Note however that in some environments this assumption is not true.
Use -fno-delete-null-pointer-checks to disable this optimization
for programs that depend on that behavior.
Some targets, especially embedded ones, disable this option at all levels.
Otherwise it is enabled at all levels: -O0, -O1,
-O2, -O3, -Os. Passes that use the information
are enabled independently at different optimization levels.
- -fdevirtualize
-
Attempt to convert calls to virtual functions to direct calls. This
is done both within a procedure and interprocedurally as part of
indirect inlining (-findirect-inlining) and interprocedural constant
propagation (-fipa-cp).
Enabled at levels -O2, -O3, -Os.
- -fdevirtualize-speculatively
-
Attempt to convert calls to virtual functions to speculative direct calls.
Based on the analysis of the type inheritance graph, determine for a given call
the set of likely targets. If the set is small, preferably of size 1, change
the call into a conditional deciding between direct and indirect calls. The
speculative calls enable more optimizations, such as inlining. When they seem
useless after further optimization, they are converted back into original form.
- -fdevirtualize-at-ltrans
-
Stream extra information needed for aggressive devirtualization when running
the link-time optimizer in local transformation mode.
This option enables more devirtualization but
significantly increases the size of streamed data. For this reason it is
disabled by default.
- -fexpensive-optimizations
-
Perform a number of minor optimizations that are relatively expensive.
Enabled at levels -O2, -O3, -Os.
- -free
-
Attempt to remove redundant extension instructions. This is especially
helpful for the x86-64 architecture, which implicitly zero-extends in 64-bit
registers after writing to their lower 32-bit half.
Enabled for Alpha, AArch64 and x86 at levels -O2,
-O3, -Os.
- -fno-lifetime-dse
-
In C++ the value of an object is only affected by changes within its
lifetime: when the constructor begins, the object has an indeterminate
value, and any changes during the lifetime of the object are dead when
the object is destroyed. Normally dead store elimination will take
advantage of this; if your code relies on the value of the object
storage persisting beyond the lifetime of the object, you can use this
flag to disable this optimization.
- -flive-range-shrinkage
-
Attempt to decrease register pressure through register live range
shrinkage. This is helpful for fast processors with small or moderate
size register sets.
- -fira-algorithm=algorithm
-
Use the specified coloring algorithm for the integrated register
allocator. The algorithm argument can be priority, which
specifies Chow's priority coloring, or CB, which specifies
Chaitin-Briggs coloring. Chaitin-Briggs coloring is not implemented
for all architectures, but for those targets that do support it, it is
the default because it generates better code.
- -fira-region=region
-
Use specified regions for the integrated register allocator. The
region argument should be one of the following:
-
- all
-
Use all loops as register allocation regions.
This can give the best results for machines with a small and/or
irregular register set.
- mixed
-
Use all loops except for loops with small register pressure
as the regions. This value usually gives
the best results in most cases and for most architectures,
and is enabled by default when compiling with optimization for speed
(-O, -O2, ...).
- one
-
Use all functions as a single region.
This typically results in the smallest code size, and is enabled by default for
-Os or -O0.
-
- -fira-hoist-pressure
-
Use IRA to evaluate register pressure in the code hoisting pass for
decisions to hoist expressions. This option usually results in smaller
code, but it can slow the compiler down.
This option is enabled at level -Os for all targets.
- -fira-loop-pressure
-
Use IRA to evaluate register pressure in loops for decisions to move
loop invariants. This option usually results in generation
of faster and smaller code on machines with large register files (>= 32
registers), but it can slow the compiler down.
This option is enabled at level -O3 for some targets.
- -fno-ira-share-save-slots
-
Disable sharing of stack slots used for saving call-used hard
registers living through a call. Each hard register gets a
separate stack slot, and as a result function stack frames are
larger.
- -fno-ira-share-spill-slots
-
Disable sharing of stack slots allocated for pseudo-registers. Each
pseudo-register that does not get a hard register gets a separate
stack slot, and as a result function stack frames are larger.
- -fira-verbose=n
-
Control the verbosity of the dump file for the integrated register allocator.
The default value is 5. If the value n is greater or equal to 10,
the dump output is sent to stderr using the same format as n minus 10.
- -flra-remat
-
Enable CFG-sensitive rematerialization in LRA. Instead of loading
values of spilled pseudos, LRA tries to rematerialize (recalculate)
values if it is profitable.
Enabled at levels -O2, -O3, -Os.
- -fdelayed-branch
-
If supported for the target machine, attempt to reorder instructions
to exploit instruction slots available after delayed branch
instructions.
Enabled at levels -O, -O2, -O3, -Os.
- -fschedule-insns
-
If supported for the target machine, attempt to reorder instructions to
eliminate execution stalls due to required data being unavailable. This
helps machines that have slow floating point or memory load instructions
by allowing other instructions to be issued until the result of the load
or floating-point instruction is required.
Enabled at levels -O2, -O3.
- -fschedule-insns2
-
Similar to -fschedule-insns, but requests an additional pass of
instruction scheduling after register allocation has been done. This is
especially useful on machines with a relatively small number of
registers and where memory load instructions take more than one cycle.
Enabled at levels -O2, -O3, -Os.
- -fno-sched-interblock
-
Don't schedule instructions across basic blocks. This is normally
enabled by default when scheduling before register allocation, i.e.
with -fschedule-insns or at -O2 or higher.
- -fno-sched-spec
-
Don't allow speculative motion of non-load instructions. This is normally
enabled by default when scheduling before register allocation, i.e.
with -fschedule-insns or at -O2 or higher.
- -fsched-pressure
-
Enable register pressure sensitive insn scheduling before register
allocation. This only makes sense when scheduling before register
allocation is enabled, i.e. with -fschedule-insns or at
-O2 or higher. Usage of this option can improve the
generated code and decrease its size by preventing register pressure
increase above the number of available hard registers and subsequent
spills in register allocation.
- -fsched-spec-load
-
Allow speculative motion of some load instructions. This only makes
sense when scheduling before register allocation, i.e. with
-fschedule-insns or at -O2 or higher.
- -fsched-spec-load-dangerous
-
Allow speculative motion of more load instructions. This only makes
sense when scheduling before register allocation, i.e. with
-fschedule-insns or at -O2 or higher.
- -fsched-stalled-insns
-
- -fsched-stalled-insns=n
-
Define how many insns (if any) can be moved prematurely from the queue
of stalled insns into the ready list during the second scheduling pass.
-fno-sched-stalled-insns means that no insns are moved
prematurely, -fsched-stalled-insns=0 means there is no limit
on how many queued insns can be moved prematurely.
-fsched-stalled-insns without a value is equivalent to
-fsched-stalled-insns=1.
- -fsched-stalled-insns-dep
-
- -fsched-stalled-insns-dep=n
-
Define how many insn groups (cycles) are examined for a dependency
on a stalled insn that is a candidate for premature removal from the queue
of stalled insns. This has an effect only during the second scheduling pass,
and only if -fsched-stalled-insns is used.
-fno-sched-stalled-insns-dep is equivalent to
-fsched-stalled-insns-dep=0.
-fsched-stalled-insns-dep without a value is equivalent to
-fsched-stalled-insns-dep=1.
- -fsched2-use-superblocks
-
When scheduling after register allocation, use superblock scheduling.
This allows motion across basic block boundaries,
resulting in faster schedules. This option is experimental, as not all machine
descriptions used by GCC model the CPU closely enough to avoid unreliable
results from the algorithm.
This only makes sense when scheduling after register allocation, i.e. with
-fschedule-insns2 or at -O2 or higher.
- -fsched-group-heuristic
-
Enable the group heuristic in the scheduler. This heuristic favors
the instruction that belongs to a schedule group. This is enabled
by default when scheduling is enabled, i.e. with -fschedule-insns
or -fschedule-insns2 or at -O2 or higher.
- -fsched-critical-path-heuristic
-
Enable the critical-path heuristic in the scheduler. This heuristic favors
instructions on the critical path. This is enabled by default when
scheduling is enabled, i.e. with -fschedule-insns
or -fschedule-insns2 or at -O2 or higher.
- -fsched-spec-insn-heuristic
-
Enable the speculative instruction heuristic in the scheduler. This
heuristic favors speculative instructions with greater dependency weakness.
This is enabled by default when scheduling is enabled, i.e.
with -fschedule-insns or -fschedule-insns2
or at -O2 or higher.
- -fsched-rank-heuristic
-
Enable the rank heuristic in the scheduler. This heuristic favors
the instruction belonging to a basic block with greater size or frequency.
This is enabled by default when scheduling is enabled, i.e.
with -fschedule-insns or -fschedule-insns2 or
at -O2 or higher.
- -fsched-last-insn-heuristic
-
Enable the last-instruction heuristic in the scheduler. This heuristic
favors the instruction that is less dependent on the last instruction
scheduled. This is enabled by default when scheduling is enabled,
i.e. with -fschedule-insns or -fschedule-insns2 or
at -O2 or higher.
- -fsched-dep-count-heuristic
-
Enable the dependent-count heuristic in the scheduler. This heuristic
favors the instruction that has more instructions depending on it.
This is enabled by default when scheduling is enabled, i.e.
with -fschedule-insns or -fschedule-insns2 or
at -O2 or higher.
- -freschedule-modulo-scheduled-loops
-
Modulo scheduling is performed before traditional scheduling. If a loop
is modulo scheduled, later scheduling passes may change its schedule.
Use this option to control that behavior.
- -fselective-scheduling
-
Schedule instructions using selective scheduling algorithm. Selective
scheduling runs instead of the first scheduler pass.
- -fselective-scheduling2
-
Schedule instructions using selective scheduling algorithm. Selective
scheduling runs instead of the second scheduler pass.
- -fsel-sched-pipelining
-
Enable software pipelining of innermost loops during selective scheduling.
This option has no effect unless one of -fselective-scheduling or
-fselective-scheduling2 is turned on.
- -fsel-sched-pipelining-outer-loops
-
When pipelining loops during selective scheduling, also pipeline outer loops.
This option has no effect unless -fsel-sched-pipelining is turned on.
- -fsemantic-interposition
-
Some object formats, like ELF, allow interposing of symbols by the
dynamic linker.
This means that for symbols exported from the DSO, the compiler cannot perform
interprocedural propagation, inlining and other optimizations in anticipation
that the function or variable in question may change. While this feature is
useful, for example, to rewrite memory allocation functions by a debugging
implementation, it is expensive in the terms of code quality.
With -fno-semantic-interposition the compiler assumes that
if interposition happens for functions the overwriting function will have
precisely the same semantics (and side effects).
Similarly if interposition happens
for variables, the constructor of the variable will be the same. The flag
has no effect for functions explicitly declared inline
(where it is never allowed for interposition to change semantics)
and for symbols explicitly declared weak.
- -fshrink-wrap
-
Emit function prologues only before parts of the function that need it,
rather than at the top of the function. This flag is enabled by default at
-O and higher.
- -fcaller-saves
-
Enable allocation of values to registers that are clobbered by
function calls, by emitting extra instructions to save and restore the
registers around such calls. Such allocation is done only when it
seems to result in better code.
This option is always enabled by default on certain machines, usually
those which have no call-preserved registers to use instead.
Enabled at levels -O2, -O3, -Os.
- -fcombine-stack-adjustments
-
Tracks stack adjustments (pushes and pops) and stack memory references
and then tries to find ways to combine them.
Enabled by default at -O1 and higher.
- -fipa-ra
-
Use caller save registers for allocation if those registers are not used by
any called function. In that case it is not necessary to save and restore
them around calls. This is only possible if called functions are part of
same compilation unit as current function and they are compiled before it.
Enabled at levels -O2, -O3, -Os.
- -fconserve-stack
-
Attempt to minimize stack usage. The compiler attempts to use less
stack space, even if that makes the program slower. This option
implies setting the large-stack-frame parameter to 100
and the large-stack-frame-growth parameter to 400.
- -ftree-reassoc
-
Perform reassociation on trees. This flag is enabled by default
at -O and higher.
- -ftree-pre
-
Perform partial redundancy elimination (PRE) on trees. This flag is
enabled by default at -O2 and -O3.
- -ftree-partial-pre
-
Make partial redundancy elimination (PRE) more aggressive. This flag is
enabled by default at -O3.
- -ftree-forwprop
-
Perform forward propagation on trees. This flag is enabled by default
at -O and higher.
- -ftree-fre
-
Perform full redundancy elimination (FRE) on trees. The difference
between FRE and PRE is that FRE only considers expressions
that are computed on all paths leading to the redundant computation.
This analysis is faster than PRE, though it exposes fewer redundancies.
This flag is enabled by default at -O and higher.
- -ftree-phiprop
-
Perform hoisting of loads from conditional pointers on trees. This
pass is enabled by default at -O and higher.
- -fhoist-adjacent-loads
-
Speculatively hoist loads from both branches of an if-then-else if the
loads are from adjacent locations in the same structure and the target
architecture has a conditional move instruction. This flag is enabled
by default at -O2 and higher.
- -ftree-copy-prop
-
Perform copy propagation on trees. This pass eliminates unnecessary
copy operations. This flag is enabled by default at -O and
higher.
- -fipa-pure-const
-
Discover which functions are pure or constant.
Enabled by default at -O and higher.
- -fipa-reference
-
Discover which static variables do not escape the
compilation unit.
Enabled by default at -O and higher.
- -fipa-pta
-
Perform interprocedural pointer analysis and interprocedural modification
and reference analysis. This option can cause excessive memory and
compile-time usage on large compilation units. It is not enabled by
default at any optimization level.
- -fipa-profile
-
Perform interprocedural profile propagation. The functions called only from
cold functions are marked as cold. Also functions executed once (such as
"cold", "noreturn", static constructors or destructors) are identified. Cold
functions and loop less parts of functions executed once are then optimized for
size.
Enabled by default at -O and higher.
- -fipa-cp
-
Perform interprocedural constant propagation.
This optimization analyzes the program to determine when values passed
to functions are constants and then optimizes accordingly.
This optimization can substantially increase performance
if the application has constants passed to functions.
This flag is enabled by default at -O2, -Os and -O3.
- -fipa-cp-clone
-
Perform function cloning to make interprocedural constant propagation stronger.
When enabled, interprocedural constant propagation performs function cloning
when externally visible function can be called with constant arguments.
Because this optimization can create multiple copies of functions,
it may significantly increase code size
(see --param ipcp-unit-growth=value).
This flag is enabled by default at -O3.
- -fipa-cp-alignment
-
When enabled, this optimization propagates alignment of function
parameters to support better vectorization and string operations.
This flag is enabled by default at -O2 and -Os. It
requires that -fipa-cp is enabled.
- -fipa-icf
-
Perform Identical Code Folding for functions and read-only variables.
The optimization reduces code size and may disturb unwind stacks by replacing
a function by equivalent one with a different name. The optimization works
more effectively with link time optimization enabled.
Nevertheless the behavior is similar to Gold Linker ICF optimization, GCC ICF
works on different levels and thus the optimizations are not same - there are
equivalences that are found only by GCC and equivalences found only by Gold.
This flag is enabled by default at -O2 and -Os.
- -fisolate-erroneous-paths-dereference
-
Detect paths that trigger erroneous or undefined behavior due to
dereferencing a null pointer. Isolate those paths from the main control
flow and turn the statement with erroneous or undefined behavior into a trap.
This flag is enabled by default at -O2 and higher.
- -fisolate-erroneous-paths-attribute
-
Detect paths that trigger erroneous or undefined behavior due a null value
being used in a way forbidden by a "returns_nonnull" or "nonnull"
attribute. Isolate those paths from the main control flow and turn the
statement with erroneous or undefined behavior into a trap. This is not
currently enabled, but may be enabled by -O2 in the future.
- -ftree-sink
-
Perform forward store motion on trees. This flag is
enabled by default at -O and higher.
- -ftree-bit-ccp
-
Perform sparse conditional bit constant propagation on trees and propagate
pointer alignment information.
This pass only operates on local scalar variables and is enabled by default
at -O and higher. It requires that -ftree-ccp is enabled.
- -ftree-ccp
-
Perform sparse conditional constant propagation (CCP) on trees. This
pass only operates on local scalar variables and is enabled by default
at -O and higher.
- -fssa-phiopt
-
Perform pattern matching on SSA PHI nodes to optimize conditional
code. This pass is enabled by default at -O and higher.
- -ftree-switch-conversion
-
Perform conversion of simple initializations in a switch to
initializations from a scalar array. This flag is enabled by default
at -O2 and higher.
- -ftree-tail-merge
-
Look for identical code sequences. When found, replace one with a jump to the
other. This optimization is known as tail merging or cross jumping. This flag
is enabled by default at -O2 and higher. The compilation time
in this pass can
be limited using max-tail-merge-comparisons parameter and
max-tail-merge-iterations parameter.
- -ftree-dce
-
Perform dead code elimination (DCE) on trees. This flag is enabled by
default at -O and higher.
- -ftree-builtin-call-dce
-
Perform conditional dead code elimination (DCE) for calls to built-in functions
that may set "errno" but are otherwise side-effect free. This flag is
enabled by default at -O2 and higher if -Os is not also
specified.
- -ftree-dominator-opts
-
Perform a variety of simple scalar cleanups (constant/copy
propagation, redundancy elimination, range propagation and expression
simplification) based on a dominator tree traversal. This also
performs jump threading (to reduce jumps to jumps). This flag is
enabled by default at -O and higher.
- -ftree-dse
-
Perform dead store elimination (DSE) on trees. A dead store is a store into
a memory location that is later overwritten by another store without
any intervening loads. In this case the earlier store can be deleted. This
flag is enabled by default at -O and higher.
- -ftree-ch
-
Perform loop header copying on trees. This is beneficial since it increases
effectiveness of code motion optimizations. It also saves one jump. This flag
is enabled by default at -O and higher. It is not enabled
for -Os, since it usually increases code size.
- -ftree-loop-optimize
-
Perform loop optimizations on trees. This flag is enabled by default
at -O and higher.
- -ftree-loop-linear
-
Perform loop interchange transformations on tree. Same as
-floop-interchange. To use this code transformation, GCC has
to be configured with --with-isl to enable the Graphite loop
transformation infrastructure.
- -floop-interchange
-
Perform loop interchange transformations on loops. Interchanging two
nested loops switches the inner and outer loops. For example, given a
loop like:
DO J = 1, M
DO I = 1, N
A(J, I) = A(J, I) * C
ENDDO
ENDDO
loop interchange transforms the loop as if it were written:
DO I = 1, N
DO J = 1, M
A(J, I) = A(J, I) * C
ENDDO
ENDDO
which can be beneficial when "N" is larger than the caches,
because in Fortran, the elements of an array are stored in memory
contiguously by column, and the original loop iterates over rows,
potentially creating at each access a cache miss. This optimization
applies to all the languages supported by GCC and is not limited to
Fortran. To use this code transformation, GCC has to be configured
with --with-isl to enable the Graphite loop transformation
infrastructure.
- -floop-strip-mine
-
Perform loop strip mining transformations on loops. Strip mining
splits a loop into two nested loops. The outer loop has strides
equal to the strip size and the inner loop has strides of the
original loop within a strip. The strip length can be changed
using the loop-block-tile-size parameter. For example,
given a loop like:
DO I = 1, N
A(I) = A(I) + C
ENDDO
loop strip mining transforms the loop as if it were written:
DO II = 1, N, 51
DO I = II, min (II + 50, N)
A(I) = A(I) + C
ENDDO
ENDDO
This optimization applies to all the languages supported by GCC and is
not limited to Fortran. To use this code transformation, GCC has to
be configured with --with-isl to enable the Graphite loop
transformation infrastructure.
- -floop-block
-
Perform loop blocking transformations on loops. Blocking strip mines
each loop in the loop nest such that the memory accesses of the
element loops fit inside caches. The strip length can be changed
using the loop-block-tile-size parameter. For example, given
a loop like:
DO I = 1, N
DO J = 1, M
A(J, I) = B(I) + C(J)
ENDDO
ENDDO
loop blocking transforms the loop as if it were written:
DO II = 1, N, 51
DO JJ = 1, M, 51
DO I = II, min (II + 50, N)
DO J = JJ, min (JJ + 50, M)
A(J, I) = B(I) + C(J)
ENDDO
ENDDO
ENDDO
ENDDO
which can be beneficial when "M" is larger than the caches,
because the innermost loop iterates over a smaller amount of data
which can be kept in the caches. This optimization applies to all the
languages supported by GCC and is not limited to Fortran. To use this
code transformation, GCC has to be configured with --with-isl
to enable the Graphite loop transformation infrastructure.
- -fgraphite-identity
-
Enable the identity transformation for graphite. For every SCoP we generate
the polyhedral representation and transform it back to gimple. Using
-fgraphite-identity we can check the costs or benefits of the
GIMPLE -> GRAPHITE -> GIMPLE transformation. Some minimal optimizations
are also performed by the code generator ISL, like index splitting and
dead code elimination in loops.
- -floop-nest-optimize
-
Enable the ISL based loop nest optimizer. This is a generic loop nest
optimizer based on the Pluto optimization algorithms. It calculates a loop
structure optimized for data-locality and parallelism. This option
is experimental.
- -floop-unroll-and-jam
-
Enable unroll and jam for the ISL based loop nest optimizer. The unroll
factor can be changed using the loop-unroll-jam-size parameter.
The unrolled dimension (counting from the most inner one) can be changed
using the loop-unroll-jam-depth parameter. .
- -floop-parallelize-all
-
Use the Graphite data dependence analysis to identify loops that can
be parallelized. Parallelize all the loops that can be analyzed to
not contain loop carried dependences without checking that it is
profitable to parallelize the loops.
- -fcheck-data-deps
-
Compare the results of several data dependence analyzers. This option
is used for debugging the data dependence analyzers.
- -ftree-loop-if-convert
-
Attempt to transform conditional jumps in the innermost loops to
branch-less equivalents. The intent is to remove control-flow from
the innermost loops in order to improve the ability of the
vectorization pass to handle these loops. This is enabled by default
if vectorization is enabled.
- -ftree-loop-if-convert-stores
-
Attempt to also if-convert conditional jumps containing memory writes.
This transformation can be unsafe for multi-threaded programs as it
transforms conditional memory writes into unconditional memory writes.
For example,
for (i = 0; i < N; i++)
if (cond)
A[i] = expr;
is transformed to
for (i = 0; i < N; i++)
A[i] = cond ? expr : A[i];
potentially producing data races.
- -ftree-loop-distribution
-
Perform loop distribution. This flag can improve cache performance on
big loop bodies and allow further loop optimizations, like
parallelization or vectorization, to take place. For example, the loop
DO I = 1, N
A(I) = B(I) + C
D(I) = E(I) * F
ENDDO
is transformed to
DO I = 1, N
A(I) = B(I) + C
ENDDO
DO I = 1, N
D(I) = E(I) * F
ENDDO
- -ftree-loop-distribute-patterns
-
Perform loop distribution of patterns that can be code generated with
calls to a library. This flag is enabled by default at -O3.
This pass distributes the initialization loops and generates a call to
memset zero. For example, the loop
DO I = 1, N
A(I) = 0
B(I) = A(I) + I
ENDDO
is transformed to
DO I = 1, N
A(I) = 0
ENDDO
DO I = 1, N
B(I) = A(I) + I
ENDDO
and the initialization loop is transformed into a call to memset zero.
- -ftree-loop-im
-
Perform loop invariant motion on trees. This pass moves only invariants that
are hard to handle at RTL level (function calls, operations that expand to
nontrivial sequences of insns). With -funswitch-loops it also moves
operands of conditions that are invariant out of the loop, so that we can use
just trivial invariantness analysis in loop unswitching. The pass also includes
store motion.
- -ftree-loop-ivcanon
-
Create a canonical counter for number of iterations in loops for which
determining number of iterations requires complicated analysis. Later
optimizations then may determine the number easily. Useful especially
in connection with unrolling.
- -fivopts
-
Perform induction variable optimizations (strength reduction, induction
variable merging and induction variable elimination) on trees.
- -ftree-parallelize-loops=n
-
Parallelize loops, i.e., split their iteration space to run in n threads.
This is only possible for loops whose iterations are independent
and can be arbitrarily reordered. The optimization is only
profitable on multiprocessor machines, for loops that are CPU-intensive,
rather than constrained e.g. by memory bandwidth. This option
implies -pthread, and thus is only supported on targets
that have support for -pthread.
- -ftree-pta
-
Perform function-local points-to analysis on trees. This flag is
enabled by default at -O and higher.
- -ftree-sra
-
Perform scalar replacement of aggregates. This pass replaces structure
references with scalars to prevent committing structures to memory too
early. This flag is enabled by default at -O and higher.
- -ftree-copyrename
-
Perform copy renaming on trees. This pass attempts to rename compiler
temporaries to other variables at copy locations, usually resulting in
variable names which more closely resemble the original variables. This flag
is enabled by default at -O and higher.
- -ftree-coalesce-inlined-vars
-
Tell the copyrename pass (see -ftree-copyrename) to attempt to
combine small user-defined variables too, but only if they are inlined
from other functions. It is a more limited form of
-ftree-coalesce-vars. This may harm debug information of such
inlined variables, but it keeps variables of the inlined-into
function apart from each other, such that they are more likely to
contain the expected values in a debugging session.
- -ftree-coalesce-vars
-
Tell the copyrename pass (see -ftree-copyrename) to attempt to
combine small user-defined variables too, instead of just compiler
temporaries. This may severely limit the ability to debug an optimized
program compiled with -fno-var-tracking-assignments. In the
negated form, this flag prevents SSA coalescing of user variables,
including inlined ones. This option is enabled by default.
- -ftree-ter
-
Perform temporary expression replacement during the SSA->normal phase. Single
use/single def temporaries are replaced at their use location with their
defining expression. This results in non-GIMPLE code, but gives the expanders
much more complex trees to work on resulting in better RTL generation. This is
enabled by default at -O and higher.
- -ftree-slsr
-
Perform straight-line strength reduction on trees. This recognizes related
expressions involving multiplications and replaces them by less expensive
calculations when possible. This is enabled by default at -O and
higher.
- -ftree-vectorize
-
Perform vectorization on trees. This flag enables -ftree-loop-vectorize
and -ftree-slp-vectorize if not explicitly specified.
- -ftree-loop-vectorize
-
Perform loop vectorization on trees. This flag is enabled by default at
-O3 and when -ftree-vectorize is enabled.
- -ftree-slp-vectorize
-
Perform basic block vectorization on trees. This flag is enabled by default at
-O3 and when -ftree-vectorize is enabled.
- -fvect-cost-model=model
-
Alter the cost model used for vectorization. The model argument
should be one of unlimited, dynamic or cheap.
With the unlimited model the vectorized code-path is assumed
to be profitable while with the dynamic model a runtime check
guards the vectorized code-path to enable it only for iteration
counts that will likely execute faster than when executing the original
scalar loop. The cheap model disables vectorization of
loops where doing so would be cost prohibitive for example due to
required runtime checks for data dependence or alignment but otherwise
is equal to the dynamic model.
The default cost model depends on other optimization flags and is
either dynamic or cheap.
- -fsimd-cost-model=model
-
Alter the cost model used for vectorization of loops marked with the OpenMP
or Cilk Plus simd directive. The model argument should be one of
unlimited, dynamic, cheap. All values of model
have the same meaning as described in -fvect-cost-model and by
default a cost model defined with -fvect-cost-model is used.
- -ftree-vrp
-
Perform Value Range Propagation on trees. This is similar to the
constant propagation pass, but instead of values, ranges of values are
propagated. This allows the optimizers to remove unnecessary range
checks like array bound checks and null pointer checks. This is
enabled by default at -O2 and higher. Null pointer check
elimination is only done if -fdelete-null-pointer-checks is
enabled.
- -fsplit-ivs-in-unroller
-
Enables expression of values of induction variables in later iterations
of the unrolled loop using the value in the first iteration. This breaks
long dependency chains, thus improving efficiency of the scheduling passes.
A combination of -fweb and CSE is often sufficient to obtain the
same effect. However, that is not reliable in cases where the loop body
is more complicated than a single basic block. It also does not work at all
on some architectures due to restrictions in the CSE pass.
This optimization is enabled by default.
- -fvariable-expansion-in-unroller
-
With this option, the compiler creates multiple copies of some
local variables when unrolling a loop, which can result in superior code.
- -fpartial-inlining
-
Inline parts of functions. This option has any effect only
when inlining itself is turned on by the -finline-functions
or -finline-small-functions options.
Enabled at level -O2.
- -fpredictive-commoning
-
Perform predictive commoning optimization, i.e., reusing computations
(especially memory loads and stores) performed in previous
iterations of loops.
This option is enabled at level -O3.
- -fprefetch-loop-arrays
-
If supported by the target machine, generate instructions to prefetch
memory to improve the performance of loops that access large arrays.
This option may generate better or worse code; results are highly
dependent on the structure of loops within the source code.
Disabled at level -Os.
- -fno-peephole
-
- -fno-peephole2
-
Disable any machine-specific peephole optimizations. The difference
between -fno-peephole and -fno-peephole2 is in how they
are implemented in the compiler; some targets use one, some use the
other, a few use both.
-fpeephole is enabled by default.
-fpeephole2 enabled at levels -O2, -O3, -Os.
- -fno-guess-branch-probability
-
Do not guess branch probabilities using heuristics.
GCC uses heuristics to guess branch probabilities if they are
not provided by profiling feedback (-fprofile-arcs). These
heuristics are based on the control flow graph. If some branch probabilities
are specified by "__builtin_expect", then the heuristics are
used to guess branch probabilities for the rest of the control flow graph,
taking the "__builtin_expect" info into account. The interactions
between the heuristics and "__builtin_expect" can be complex, and in
some cases, it may be useful to disable the heuristics so that the effects
of "__builtin_expect" are easier to understand.
The default is -fguess-branch-probability at levels
-O, -O2, -O3, -Os.
- -freorder-blocks
-
Reorder basic blocks in the compiled function in order to reduce number of
taken branches and improve code locality.
Enabled at levels -O2, -O3.
- -freorder-blocks-and-partition
-
In addition to reordering basic blocks in the compiled function, in order
to reduce number of taken branches, partitions hot and cold basic blocks
into separate sections of the assembly and .o files, to improve
paging and cache locality performance.
This optimization is automatically turned off in the presence of
exception handling, for linkonce sections, for functions with a user-defined
section attribute and on any architecture that does not support named
sections.
Enabled for x86 at levels -O2, -O3.
- -freorder-functions
-
Reorder functions in the object file in order to
improve code locality. This is implemented by using special
subsections ".text.hot" for most frequently executed functions and
".text.unlikely" for unlikely executed functions. Reordering is done by
the linker so object file format must support named sections and linker must
place them in a reasonable way.
Also profile feedback must be available to make this option effective. See
-fprofile-arcs for details.
Enabled at levels -O2, -O3, -Os.
- -fstrict-aliasing
-
Allow the compiler to assume the strictest aliasing rules applicable to
the language being compiled. For C (and C++), this activates
optimizations based on the type of expressions. In particular, an
object of one type is assumed never to reside at the same address as an
object of a different type, unless the types are almost the same. For
example, an "unsigned int" can alias an "int", but not a
"void*" or a "double". A character type may alias any other
type.
Pay special attention to code like this:
union a_union {
int i;
double d;
};
int f() {
union a_union t;
t.d = 3.0;
return t.i;
}
The practice of reading from a different union member than the one most
recently written to (called ``type-punning'') is common. Even with
-fstrict-aliasing, type-punning is allowed, provided the memory
is accessed through the union type. So, the code above works as
expected. However, this code might not:
int f() {
union a_union t;
int* ip;
t.d = 3.0;
ip = &t.i;
return *ip;
}
Similarly, access by taking the address, casting the resulting pointer
and dereferencing the result has undefined behavior, even if the cast
uses a union type, e.g.:
int f() {
double d = 3.0;
return ((union a_union *) &d)->i;
}
The -fstrict-aliasing option is enabled at levels
-O2, -O3, -Os.
- -fstrict-overflow
-
Allow the compiler to assume strict signed overflow rules, depending
on the language being compiled. For C (and C++) this means that
overflow when doing arithmetic with signed numbers is undefined, which
means that the compiler may assume that it does not happen. This
permits various optimizations. For example, the compiler assumes
that an expression like "i + 10 > i" is always true for
signed "i". This assumption is only valid if signed overflow is
undefined, as the expression is false if "i + 10" overflows when
using twos complement arithmetic. When this option is in effect any
attempt to determine whether an operation on signed numbers
overflows must be written carefully to not actually involve overflow.
This option also allows the compiler to assume strict pointer
semantics: given a pointer to an object, if adding an offset to that
pointer does not produce a pointer to the same object, the addition is
undefined. This permits the compiler to conclude that "p + u >
p" is always true for a pointer "p" and unsigned integer
"u". This assumption is only valid because pointer wraparound is
undefined, as the expression is false if "p + u" overflows using
twos complement arithmetic.
See also the -fwrapv option. Using -fwrapv means
that integer signed overflow is fully defined: it wraps. When
-fwrapv is used, there is no difference between
-fstrict-overflow and -fno-strict-overflow for
integers. With -fwrapv certain types of overflow are
permitted. For example, if the compiler gets an overflow when doing
arithmetic on constants, the overflowed value can still be used with
-fwrapv, but not otherwise.
The -fstrict-overflow option is enabled at levels
-O2, -O3, -Os.
- -falign-functions
-
- -falign-functions=n
-
Align the start of functions to the next power-of-two greater than
n, skipping up to n bytes. For instance,
-falign-functions=32 aligns functions to the next 32-byte
boundary, but -falign-functions=24 aligns to the next
32-byte boundary only if this can be done by skipping 23 bytes or less.
-fno-align-functions and -falign-functions=1 are
equivalent and mean that functions are not aligned.
Some assemblers only support this flag when n is a power of two;
in that case, it is rounded up.
If n is not specified or is zero, use a machine-dependent default.
Enabled at levels -O2, -O3.
- -falign-labels
-
- -falign-labels=n
-
Align all branch targets to a power-of-two boundary, skipping up to
n bytes like -falign-functions. This option can easily
make code slower, because it must insert dummy operations for when the
branch target is reached in the usual flow of the code.
-fno-align-labels and -falign-labels=1 are
equivalent and mean that labels are not aligned.
If -falign-loops or -falign-jumps are applicable and
are greater than this value, then their values are used instead.
If n is not specified or is zero, use a machine-dependent default
which is very likely to be 1, meaning no alignment.
Enabled at levels -O2, -O3.
- -falign-loops
-
- -falign-loops=n
-
Align loops to a power-of-two boundary, skipping up to n bytes
like -falign-functions. If the loops are
executed many times, this makes up for any execution of the dummy
operations.
-fno-align-loops and -falign-loops=1 are
equivalent and mean that loops are not aligned.
If n is not specified or is zero, use a machine-dependent default.
Enabled at levels -O2, -O3.
- -falign-jumps
-
- -falign-jumps=n
-
Align branch targets to a power-of-two boundary, for branch targets
where the targets can only be reached by jumping, skipping up to n
bytes like -falign-functions. In this case, no dummy operations
need be executed.
-fno-align-jumps and -falign-jumps=1 are
equivalent and mean that loops are not aligned.
If n is not specified or is zero, use a machine-dependent default.
Enabled at levels -O2, -O3.
- -funit-at-a-time
-
This option is left for compatibility reasons. -funit-at-a-time
has no effect, while -fno-unit-at-a-time implies
-fno-toplevel-reorder and -fno-section-anchors.
Enabled by default.
- -fno-toplevel-reorder
-
Do not reorder top-level functions, variables, and "asm"
statements. Output them in the same order that they appear in the
input file. When this option is used, unreferenced static variables
are not removed. This option is intended to support existing code
that relies on a particular ordering. For new code, it is better to
use attributes when possible.
Enabled at level -O0. When disabled explicitly, it also implies
-fno-section-anchors, which is otherwise enabled at -O0 on some
targets.
- -fweb
-
Constructs webs as commonly used for register allocation purposes and assign
each web individual pseudo register. This allows the register allocation pass
to operate on pseudos directly, but also strengthens several other optimization
passes, such as CSE, loop optimizer and trivial dead code remover. It can,
however, make debugging impossible, since variables no longer stay in a
``home register''.
Enabled by default with -funroll-loops.
- -fwhole-program
-
Assume that the current compilation unit represents the whole program being
compiled. All public functions and variables with the exception of "main"
and those merged by attribute "externally_visible" become static functions
and in effect are optimized more aggressively by interprocedural optimizers.
This option should not be used in combination with -flto.
Instead relying on a linker plugin should provide safer and more precise
information.
- -flto[=n]
-
This option runs the standard link-time optimizer. When invoked
with source code, it generates GIMPLE (one of GCC's internal
representations) and writes it to special ELF sections in the object
file. When the object files are linked together, all the function
bodies are read from these ELF sections and instantiated as if they
had been part of the same translation unit.
To use the link-time optimizer, -flto and optimization
options should be specified at compile time and during the final link.
For example:
gcc -c -O2 -flto foo.c
gcc -c -O2 -flto bar.c
gcc -o myprog -flto -O2 foo.o bar.o
The first two invocations to GCC save a bytecode representation
of GIMPLE into special ELF sections inside foo.o and
bar.o. The final invocation reads the GIMPLE bytecode from
foo.o and bar.o, merges the two files into a single
internal image, and compiles the result as usual. Since both
foo.o and bar.o are merged into a single image, this
causes all the interprocedural analyses and optimizations in GCC to
work across the two files as if they were a single one. This means,
for example, that the inliner is able to inline functions in
bar.o into functions in foo.o and vice-versa.
Another (simpler) way to enable link-time optimization is:
gcc -o myprog -flto -O2 foo.c bar.c
The above generates bytecode for foo.c and bar.c,
merges them together into a single GIMPLE representation and optimizes
them as usual to produce myprog.
The only important thing to keep in mind is that to enable link-time
optimizations you need to use the GCC driver to perform the link-step.
GCC then automatically performs link-time optimization if any of the
objects involved were compiled with the -flto command-line option.
You generally
should specify the optimization options to be used for link-time
optimization though GCC tries to be clever at guessing an
optimization level to use from the options used at compile-time
if you fail to specify one at link-time. You can always override
the automatic decision to do link-time optimization at link-time
by passing -fno-lto to the link command.
To make whole program optimization effective, it is necessary to make
certain whole program assumptions. The compiler needs to know
what functions and variables can be accessed by libraries and runtime
outside of the link-time optimized unit. When supported by the linker,
the linker plugin (see -fuse-linker-plugin) passes information
to the compiler about used and externally visible symbols. When
the linker plugin is not available, -fwhole-program should be
used to allow the compiler to make these assumptions, which leads
to more aggressive optimization decisions.
When -fuse-linker-plugin is not enabled then, when a file is
compiled with -flto, the generated object file is larger than
a regular object file because it contains GIMPLE bytecodes and the usual
final code (see -ffat-lto-objects. This means that
object files with LTO information can be linked as normal object
files; if -fno-lto is passed to the linker, no
interprocedural optimizations are applied. Note that when
-fno-fat-lto-objects is enabled the compile-stage is faster
but you cannot perform a regular, non-LTO link on them.
Additionally, the optimization flags used to compile individual files
are not necessarily related to those used at link time. For instance,
gcc -c -O0 -ffat-lto-objects -flto foo.c
gcc -c -O0 -ffat-lto-objects -flto bar.c
gcc -o myprog -O3 foo.o bar.o
This produces individual object files with unoptimized assembler
code, but the resulting binary myprog is optimized at
-O3. If, instead, the final binary is generated with
-fno-lto, then myprog is not optimized.
When producing the final binary, GCC only
applies link-time optimizations to those files that contain bytecode.
Therefore, you can mix and match object files and libraries with
GIMPLE bytecodes and final object code. GCC automatically selects
which files to optimize in LTO mode and which files to link without
further processing.
There are some code generation flags preserved by GCC when
generating bytecodes, as they need to be used during the final link
stage. Generally options specified at link-time override those
specified at compile-time.
If you do not specify an optimization level option -O at
link-time then GCC computes one based on the optimization levels
used when compiling the object files. The highest optimization
level wins here.
Currently, the following options and their setting are take from
the first object file that explicitely specified it:
-fPIC, -fpic, -fpie, -fcommon,
-fexceptions, -fnon-call-exceptions, -fgnu-tm
and all the -m target flags.
Certain ABI changing flags are required to match in all compilation-units
and trying to override this at link-time with a conflicting value
is ignored. This includes options such as -freg-struct-return
and -fpcc-struct-return.
Other options such as -ffp-contract, -fno-strict-overflow,
-fwrapv, -fno-trapv or -fno-strict-aliasing
are passed through to the link stage and merged conservatively for
conflicting translation units. Specifically
-fno-strict-overflow, -fwrapv and -fno-trapv take
precedence and for example -ffp-contract=off takes precedence
over -ffp-contract=fast. You can override them at linke-time.
It is recommended that you compile all the files participating in the
same link with the same options and also specify those options at
link time.
If LTO encounters objects with C linkage declared with incompatible
types in separate translation units to be linked together (undefined
behavior according to ISO C99 6.2.7), a non-fatal diagnostic may be
issued. The behavior is still undefined at run time. Similar
diagnostics may be raised for other languages.
Another feature of LTO is that it is possible to apply interprocedural
optimizations on files written in different languages:
gcc -c -flto foo.c
g++ -c -flto bar.cc
gfortran -c -flto baz.f90
g++ -o myprog -flto -O3 foo.o bar.o baz.o -lgfortran
Notice that the final link is done with g++ to get the C++
runtime libraries and -lgfortran is added to get the Fortran
runtime libraries. In general, when mixing languages in LTO mode, you
should use the same link command options as when mixing languages in a
regular (non-LTO) compilation.
If object files containing GIMPLE bytecode are stored in a library archive, say
libfoo.a, it is possible to extract and use them in an LTO link if you
are using a linker with plugin support. To create static libraries suitable
for LTO, use gcc-ar and gcc-ranlib instead of ar
and ranlib;
to show the symbols of object files with GIMPLE bytecode, use
gcc-nm. Those commands require that ar, ranlib
and nm have been compiled with plugin support. At link time, use the the
flag -fuse-linker-plugin to ensure that the library participates in
the LTO optimization process:
gcc -o myprog -O2 -flto -fuse-linker-plugin a.o b.o -lfoo
With the linker plugin enabled, the linker extracts the needed
GIMPLE files from libfoo.a and passes them on to the running GCC
to make them part of the aggregated GIMPLE image to be optimized.
If you are not using a linker with plugin support and/or do not
enable the linker plugin, then the objects inside libfoo.a
are extracted and linked as usual, but they do not participate
in the LTO optimization process. In order to make a static library suitable
for both LTO optimization and usual linkage, compile its object files with
-flto -ffat-lto-objects.
Link-time optimizations do not require the presence of the whole program to
operate. If the program does not require any symbols to be exported, it is
possible to combine -flto and -fwhole-program to allow
the interprocedural optimizers to use more aggressive assumptions which may
lead to improved optimization opportunities.
Use of -fwhole-program is not needed when linker plugin is
active (see -fuse-linker-plugin).
The current implementation of LTO makes no
attempt to generate bytecode that is portable between different
types of hosts. The bytecode files are versioned and there is a
strict version check, so bytecode files generated in one version of
GCC do not work with an older or newer version of GCC.
Link-time optimization does not work well with generation of debugging
information. Combining -flto with
-g is currently experimental and expected to produce unexpected
results.
If you specify the optional n, the optimization and code
generation done at link time is executed in parallel using n
parallel jobs by utilizing an installed make program. The
environment variable MAKE may be used to override the program
used. The default value for n is 1.
You can also specify -flto=jobserver to use GNU make's
job server mode to determine the number of parallel jobs. This
is useful when the Makefile calling GCC is already executing in parallel.
You must prepend a + to the command recipe in the parent Makefile
for this to work. This option likely only works if MAKE is
GNU make.
- -flto-partition=alg
-
Specify the partitioning algorithm used by the link-time optimizer.
The value is either 1to1 to specify a partitioning mirroring
the original source files or balanced to specify partitioning
into equally sized chunks (whenever possible) or max to create
new partition for every symbol where possible. Specifying none
as an algorithm disables partitioning and streaming completely.
The default value is balanced. While 1to1 can be used
as an workaround for various code ordering issues, the max
partitioning is intended for internal testing only.
The value one specifies that exactly one partition should be
used while the value none bypasses partitioning and executes
the link-time optimization step directly from the WPA phase.
- -flto-odr-type-merging
-
Enable streaming of mangled types names of C++ types and their unification
at linktime. This increases size of LTO object files, but enable
diagnostics about One Definition Rule violations.
- -flto-compression-level=n
-
This option specifies the level of compression used for intermediate
language written to LTO object files, and is only meaningful in
conjunction with LTO mode (-flto). Valid
values are 0 (no compression) to 9 (maximum compression). Values
outside this range are clamped to either 0 or 9. If the option is not
given, a default balanced compression setting is used.
- -flto-report
-
Prints a report with internal details on the workings of the link-time
optimizer. The contents of this report vary from version to version.
It is meant to be useful to GCC developers when processing object
files in LTO mode (via -flto).
Disabled by default.
- -flto-report-wpa
-
Like -flto-report, but only print for the WPA phase of Link
Time Optimization.
- -fuse-linker-plugin
-
Enables the use of a linker plugin during link-time optimization. This
option relies on plugin support in the linker, which is available in gold
or in GNU ld 2.21 or newer.
This option enables the extraction of object files with GIMPLE bytecode out
of library archives. This improves the quality of optimization by exposing
more code to the link-time optimizer. This information specifies what
symbols can be accessed externally (by non-LTO object or during dynamic
linking). Resulting code quality improvements on binaries (and shared
libraries that use hidden visibility) are similar to -fwhole-program.
See -flto for a description of the effect of this flag and how to
use it.
This option is enabled by default when LTO support in GCC is enabled
and GCC was configured for use with
a linker supporting plugins (GNU ld 2.21 or newer or gold).
- -ffat-lto-objects
-
Fat LTO objects are object files that contain both the intermediate language
and the object code. This makes them usable for both LTO linking and normal
linking. This option is effective only when compiling with -flto
and is ignored at link time.
-fno-fat-lto-objects improves compilation time over plain LTO, but
requires the complete toolchain to be aware of LTO. It requires a linker with
linker plugin support for basic functionality. Additionally,
nm, ar and ranlib
need to support linker plugins to allow a full-featured build environment
(capable of building static libraries etc). GCC provides the gcc-ar,
gcc-nm, gcc-ranlib wrappers to pass the right options
to these tools. With non fat LTO makefiles need to be modified to use them.
The default is -fno-fat-lto-objects on targets with linker plugin
support.
- -fcompare-elim
-
After register allocation and post-register allocation instruction splitting,
identify arithmetic instructions that compute processor flags similar to a
comparison operation based on that arithmetic. If possible, eliminate the
explicit comparison operation.
This pass only applies to certain targets that cannot explicitly represent
the comparison operation before register allocation is complete.
Enabled at levels -O, -O2, -O3, -Os.
- -fcprop-registers
-
After register allocation and post-register allocation instruction splitting,
perform a copy-propagation pass to try to reduce scheduling dependencies
and occasionally eliminate the copy.
Enabled at levels -O, -O2, -O3, -Os.
- -fprofile-correction
-
Profiles collected using an instrumented binary for multi-threaded programs may
be inconsistent due to missed counter updates. When this option is specified,
GCC uses heuristics to correct or smooth out such inconsistencies. By
default, GCC emits an error message when an inconsistent profile is detected.
- -fprofile-dir=path
-
Set the directory to search for the profile data files in to path.
This option affects only the profile data generated by
-fprofile-generate, -ftest-coverage, -fprofile-arcs
and used by -fprofile-use and -fbranch-probabilities
and its related options. Both absolute and relative paths can be used.
By default, GCC uses the current directory as path, thus the
profile data file appears in the same directory as the object file.
- -fprofile-generate
-
- -fprofile-generate=path
-
Enable options usually used for instrumenting application to produce
profile useful for later recompilation with profile feedback based
optimization. You must use -fprofile-generate both when
compiling and when linking your program.
The following options are enabled: -fprofile-arcs, -fprofile-values, -fvpt.
If path is specified, GCC looks at the path to find
the profile feedback data files. See -fprofile-dir.
- -fprofile-use
-
- -fprofile-use=path
-
Enable profile feedback-directed optimizations,
and the following optimizations
which are generally profitable only with profile feedback available:
-fbranch-probabilities, -fvpt,
-funroll-loops, -fpeel-loops, -ftracer,
-ftree-vectorize, and ftree-loop-distribute-patterns.
By default, GCC emits an error message if the feedback profiles do not
match the source code. This error can be turned into a warning by using
-Wcoverage-mismatch. Note this may result in poorly optimized
code.
If path is specified, GCC looks at the path to find
the profile feedback data files. See -fprofile-dir.
- -fauto-profile
-
- -fauto-profile=path
-
Enable sampling-based feedback-directed optimizations,
and the following optimizations
which are generally profitable only with profile feedback available:
-fbranch-probabilities, -fvpt,
-funroll-loops, -fpeel-loops, -ftracer,
-ftree-vectorize,
-finline-functions, -fipa-cp, -fipa-cp-clone,
-fpredictive-commoning, -funswitch-loops,
-fgcse-after-reload, and -ftree-loop-distribute-patterns.
path is the name of a file containing AutoFDO profile information.
If omitted, it defaults to fbdata.afdo in the current directory.
Producing an AutoFDO profile data file requires running your program
with the perf utility on a supported GNU/Linux target system.
For more information, see <https://perf.wiki.kernel.org/>.
E.g.
perf record -e br_inst_retired:near_taken -b -o perf.data \
-- your_program
Then use the create_gcov tool to convert the raw profile data
to a format that can be used by GCC. You must also supply the
unstripped binary for your program to this tool.
See <https://github.com/google/autofdo>.
E.g.
create_gcov --binary=your_program.unstripped --profile=perf.data \
--gcov=profile.afdo
The following options control compiler behavior regarding floating-point
arithmetic. These options trade off between speed and
correctness. All must be specifically enabled.
- -ffloat-store
-
Do not store floating-point variables in registers, and inhibit other
options that might change whether a floating-point value is taken from a
register or memory.
This option prevents undesirable excess precision on machines such as
the 68000 where the floating registers (of the 68881) keep more
precision than a "double" is supposed to have. Similarly for the
x86 architecture. For most programs, the excess precision does only
good, but a few programs rely on the precise definition of IEEE floating
point. Use -ffloat-store for such programs, after modifying
them to store all pertinent intermediate computations into variables.
- -fexcess-precision=style
-
This option allows further control over excess precision on machines
where floating-point registers have more precision than the IEEE
"float" and "double" types and the processor does not
support operations rounding to those types. By default,
-fexcess-precision=fast is in effect; this means that
operations are carried out in the precision of the registers and that
it is unpredictable when rounding to the types specified in the source
code takes place. When compiling C, if
-fexcess-precision=standard is specified then excess
precision follows the rules specified in ISO C99; in particular,
both casts and assignments cause values to be rounded to their
semantic types (whereas -ffloat-store only affects
assignments). This option is enabled by default for C if a strict
conformance option such as -std=c99 is used.
-fexcess-precision=standard is not implemented for languages
other than C, and has no effect if
-funsafe-math-optimizations or -ffast-math is
specified. On the x86, it also has no effect if -mfpmath=sse
or -mfpmath=sse+387 is specified; in the former case, IEEE
semantics apply without excess precision, and in the latter, rounding
is unpredictable.
- -ffast-math
-
Sets the options -fno-math-errno, -funsafe-math-optimizations,
-ffinite-math-only, -fno-rounding-math,
-fno-signaling-nans and -fcx-limited-range.
This option causes the preprocessor macro "__FAST_MATH__" to be defined.
This option is not turned on by any -O option besides
-Ofast since it can result in incorrect output for programs
that depend on an exact implementation of IEEE or ISO rules/specifications
for math functions. It may, however, yield faster code for programs
that do not require the guarantees of these specifications.
- -fno-math-errno
-
Do not set "errno" after calling math functions that are executed
with a single instruction, e.g., "sqrt". A program that relies on
IEEE exceptions for math error handling may want to use this flag
for speed while maintaining IEEE arithmetic compatibility.
This option is not turned on by any -O option since
it can result in incorrect output for programs that depend on
an exact implementation of IEEE or ISO rules/specifications for
math functions. It may, however, yield faster code for programs
that do not require the guarantees of these specifications.
The default is -fmath-errno.
On Darwin systems, the math library never sets "errno". There is
therefore no reason for the compiler to consider the possibility that
it might, and -fno-math-errno is the default.
- -funsafe-math-optimizations
-
Allow optimizations for floating-point arithmetic that (a) assume
that arguments and results are valid and (b) may violate IEEE or
ANSI standards. When used at link-time, it may include libraries
or startup files that change the default FPU control word or other
similar optimizations.
This option is not turned on by any -O option since
it can result in incorrect output for programs that depend on
an exact implementation of IEEE or ISO rules/specifications for
math functions. It may, however, yield faster code for programs
that do not require the guarantees of these specifications.
Enables -fno-signed-zeros, -fno-trapping-math,
-fassociative-math and -freciprocal-math.
The default is -fno-unsafe-math-optimizations.
- -fassociative-math
-
Allow re-association of operands in series of floating-point operations.
This violates the ISO C and C++ language standard by possibly changing
computation result. NOTE: re-ordering may change the sign of zero as
well as ignore NaNs and inhibit or create underflow or overflow (and
thus cannot be used on code that relies on rounding behavior like
"(x + 2**52) - 2**52". May also reorder floating-point comparisons
and thus may not be used when ordered comparisons are required.
This option requires that both -fno-signed-zeros and
-fno-trapping-math be in effect. Moreover, it doesn't make
much sense with -frounding-math. For Fortran the option
is automatically enabled when both -fno-signed-zeros and
-fno-trapping-math are in effect.
The default is -fno-associative-math.
- -freciprocal-math
-
Allow the reciprocal of a value to be used instead of dividing by
the value if this enables optimizations. For example "x / y"
can be replaced with "x * (1/y)", which is useful if "(1/y)"
is subject to common subexpression elimination. Note that this loses
precision and increases the number of flops operating on the value.
The default is -fno-reciprocal-math.
- -ffinite-math-only
-
Allow optimizations for floating-point arithmetic that assume
that arguments and results are not NaNs or +-Infs.
This option is not turned on by any -O option since
it can result in incorrect output for programs that depend on
an exact implementation of IEEE or ISO rules/specifications for
math functions. It may, however, yield faster code for programs
that do not require the guarantees of these specifications.
The default is -fno-finite-math-only.
- -fno-signed-zeros
-
Allow optimizations for floating-point arithmetic that ignore the
signedness of zero. IEEE arithmetic specifies the behavior of
distinct +0.0 and -0.0 values, which then prohibits simplification
of expressions such as x+0.0 or 0.0*x (even with -ffinite-math-only).
This option implies that the sign of a zero result isn't significant.
The default is -fsigned-zeros.
- -fno-trapping-math
-
Compile code assuming that floating-point operations cannot generate
user-visible traps. These traps include division by zero, overflow,
underflow, inexact result and invalid operation. This option requires
that -fno-signaling-nans be in effect. Setting this option may
allow faster code if one relies on ``non-stop'' IEEE arithmetic, for example.
This option should never be turned on by any -O option since
it can result in incorrect output for programs that depend on
an exact implementation of IEEE or ISO rules/specifications for
math functions.
The default is -ftrapping-math.
- -frounding-math
-
Disable transformations and optimizations that assume default floating-point
rounding behavior. This is round-to-zero for all floating point
to integer conversions, and round-to-nearest for all other arithmetic
truncations. This option should be specified for programs that change
the FP rounding mode dynamically, or that may be executed with a
non-default rounding mode. This option disables constant folding of
floating-point expressions at compile time (which may be affected by
rounding mode) and arithmetic transformations that are unsafe in the
presence of sign-dependent rounding modes.
The default is -fno-rounding-math.
This option is experimental and does not currently guarantee to
disable all GCC optimizations that are affected by rounding mode.
Future versions of GCC may provide finer control of this setting
using C99's "FENV_ACCESS" pragma. This command-line option
will be used to specify the default state for "FENV_ACCESS".
- -fsignaling-nans
-
Compile code assuming that IEEE signaling NaNs may generate user-visible
traps during floating-point operations. Setting this option disables
optimizations that may change the number of exceptions visible with
signaling NaNs. This option implies -ftrapping-math.
This option causes the preprocessor macro "__SUPPORT_SNAN__" to
be defined.
The default is -fno-signaling-nans.
This option is experimental and does not currently guarantee to
disable all GCC optimizations that affect signaling NaN behavior.
- -fsingle-precision-constant
-
Treat floating-point constants as single precision instead of
implicitly converting them to double-precision constants.
- -fcx-limited-range
-
When enabled, this option states that a range reduction step is not
needed when performing complex division. Also, there is no checking
whether the result of a complex multiplication or division is "NaN
+ I*NaN", with an attempt to rescue the situation in that case. The
default is -fno-cx-limited-range, but is enabled by
-ffast-math.
This option controls the default setting of the ISO C99
"CX_LIMITED_RANGE" pragma. Nevertheless, the option applies to
all languages.
- -fcx-fortran-rules
-
Complex multiplication and division follow Fortran rules. Range
reduction is done as part of complex division, but there is no checking
whether the result of a complex multiplication or division is "NaN
+ I*NaN", with an attempt to rescue the situation in that case.
The default is -fno-cx-fortran-rules.
The following options control optimizations that may improve
performance, but are not enabled by any -O options. This
section includes experimental options that may produce broken code.
- -fbranch-probabilities
-
After running a program compiled with -fprofile-arcs, you can compile it a second time using
-fbranch-probabilities, to improve optimizations based on
the number of times each branch was taken. When a program
compiled with -fprofile-arcs exits, it saves arc execution
counts to a file called sourcename.gcda for each source
file. The information in this data file is very dependent on the
structure of the generated code, so you must use the same source code
and the same optimization options for both compilations.
With -fbranch-probabilities, GCC puts a
REG_BR_PROB note on each JUMP_INSN and CALL_INSN.
These can be used to improve optimization. Currently, they are only
used in one place: in reorg.c, instead of guessing which path a
branch is most likely to take, the REG_BR_PROB values are used to
exactly determine which path is taken more often.
- -fprofile-values
-
If combined with -fprofile-arcs, it adds code so that some
data about values of expressions in the program is gathered.
With -fbranch-probabilities, it reads back the data gathered
from profiling values of expressions for usage in optimizations.
Enabled with -fprofile-generate and -fprofile-use.
- -fprofile-reorder-functions
-
Function reordering based on profile instrumentation collects
first time of execution of a function and orders these functions
in ascending order.
Enabled with -fprofile-use.
- -fvpt
-
If combined with -fprofile-arcs, this option instructs the compiler
to add code to gather information about values of expressions.
With -fbranch-probabilities, it reads back the data gathered
and actually performs the optimizations based on them.
Currently the optimizations include specialization of division operations
using the knowledge about the value of the denominator.
- -frename-registers
-
Attempt to avoid false dependencies in scheduled code by making use
of registers left over after register allocation. This optimization
most benefits processors with lots of registers. Depending on the
debug information format adopted by the target, however, it can
make debugging impossible, since variables no longer stay in
a ``home register''.
Enabled by default with -funroll-loops and -fpeel-loops.
- -fschedule-fusion
-
Performs a target dependent pass over the instruction stream to schedule
instructions of same type together because target machine can execute them
more efficiently if they are adjacent to each other in the instruction flow.
Enabled at levels -O2, -O3, -Os.
- -ftracer
-
Perform tail duplication to enlarge superblock size. This transformation
simplifies the control flow of the function allowing other optimizations to do
a better job.
Enabled with -fprofile-use.
- -funroll-loops
-
Unroll loops whose number of iterations can be determined at compile time or
upon entry to the loop. -funroll-loops implies
-frerun-cse-after-loop, -fweb and -frename-registers.
It also turns on complete loop peeling (i.e. complete removal of loops with
a small constant number of iterations). This option makes code larger, and may
or may not make it run faster.
Enabled with -fprofile-use.
- -funroll-all-loops
-
Unroll all loops, even if their number of iterations is uncertain when
the loop is entered. This usually makes programs run more slowly.
-funroll-all-loops implies the same options as
-funroll-loops.
- -fpeel-loops
-
Peels loops for which there is enough information that they do not
roll much (from profile feedback). It also turns on complete loop peeling
(i.e. complete removal of loops with small constant number of iterations).
Enabled with -fprofile-use.
- -fmove-loop-invariants
-
Enables the loop invariant motion pass in the RTL loop optimizer. Enabled
at level -O1
- -funswitch-loops
-
Move branches with loop invariant conditions out of the loop, with duplicates
of the loop on both branches (modified according to result of the condition).
- -ffunction-sections
-
- -fdata-sections
-
Place each function or data item into its own section in the output
file if the target supports arbitrary sections. The name of the
function or the name of the data item determines the section's name
in the output file.
Use these options on systems where the linker can perform optimizations
to improve locality of reference in the instruction space. Most systems
using the ELF object format and SPARC processors running Solaris 2 have
linkers with such optimizations. AIX may have these optimizations in
the future.
Only use these options when there are significant benefits from doing
so. When you specify these options, the assembler and linker
create larger object and executable files and are also slower.
You cannot use gprof on all systems if you
specify this option, and you may have problems with debugging if
you specify both this option and -g.
- -fbranch-target-load-optimize
-
Perform branch target register load optimization before prologue / epilogue
threading.
The use of target registers can typically be exposed only during reload,
thus hoisting loads out of loops and doing inter-block scheduling needs
a separate optimization pass.
- -fbranch-target-load-optimize2
-
Perform branch target register load optimization after prologue / epilogue
threading.
- -fbtr-bb-exclusive
-
When performing branch target register load optimization, don't reuse
branch target registers within any basic block.
- -fstack-protector
-
Emit extra code to check for buffer overflows, such as stack smashing
attacks. This is done by adding a guard variable to functions with
vulnerable objects. This includes functions that call "alloca", and
functions with buffers larger than 8 bytes. The guards are initialized
when a function is entered and then checked when the function exits.
If a guard check fails, an error message is printed and the program exits.
- -fstack-protector-all
-
Like -fstack-protector except that all functions are protected.
NOTE: When --enable-esp, this option is enabled by default
for C, C++, ObjC, and ObjC++ if neither -fno-stack-protector,
-nostdlib, -ffreestanding, -fstack-protector,
-fstack-protector-strong or -fstack-protector-all are found.
- -fstack-protector-strong
-
Like -fstack-protector but includes additional functions to
be protected --- those that have local array definitions, or have
references to local frame addresses.
NOTE: In Gentoo GCC 4.9.0 and later versions this option is enabled by default
for C, C++, ObjC, and ObjC++ if none of the options -fno-stack-protector,
-nostdlib, -ffreestanding, -fstack-protector,
-fstack-protector-strong, or -fstack-protector-all are found.
- -fstack-protector-explicit
-
Like -fstack-protector but only protects those functions which
have the "stack_protect" attribute
- -fstdarg-opt
-
Optimize the prologue of variadic argument functions with respect to usage of
those arguments.
- -fsection-anchors
-
Try to reduce the number of symbolic address calculations by using
shared ``anchor'' symbols to address nearby objects. This transformation
can help to reduce the number of GOT entries and GOT accesses on some
targets.
For example, the implementation of the following function "foo":
static int a, b, c;
int foo (void) { return a + b + c; }
usually calculates the addresses of all three variables, but if you
compile it with -fsection-anchors, it accesses the variables
from a common anchor point instead. The effect is similar to the
following pseudocode (which isn't valid C):
int foo (void)
{
register int *xr = &x;
return xr[&a - &x] + xr[&b - &x] + xr[&c - &x];
}
Not all targets support this option.
- --param name=value
-
In some places, GCC uses various constants to control the amount of
optimization that is done. For example, GCC does not inline functions
that contain more than a certain number of instructions. You can
control some of these constants on the command line using the
--param option.
The names of specific parameters, and the meaning of the values, are
tied to the internals of the compiler, and are subject to change
without notice in future releases.
In each case, the value is an integer. The allowable choices for
name are:
-
- predictable-branch-outcome
-
When branch is predicted to be taken with probability lower than this threshold
(in percent), then it is considered well predictable. The default is 10.
- max-crossjump-edges
-
The maximum number of incoming edges to consider for cross-jumping.
The algorithm used by -fcrossjumping is O(N^2) in
the number of edges incoming to each block. Increasing values mean
more aggressive optimization, making the compilation time increase with
probably small improvement in executable size.
- min-crossjump-insns
-
The minimum number of instructions that must be matched at the end
of two blocks before cross-jumping is performed on them. This
value is ignored in the case where all instructions in the block being
cross-jumped from are matched. The default value is 5.
- max-grow-copy-bb-insns
-
The maximum code size expansion factor when copying basic blocks
instead of jumping. The expansion is relative to a jump instruction.
The default value is 8.
- max-goto-duplication-insns
-
The maximum number of instructions to duplicate to a block that jumps
to a computed goto. To avoid O(N^2) behavior in a number of
passes, GCC factors computed gotos early in the compilation process,
and unfactors them as late as possible. Only computed jumps at the
end of a basic blocks with no more than max-goto-duplication-insns are
unfactored. The default value is 8.
- max-delay-slot-insn-search
-
The maximum number of instructions to consider when looking for an
instruction to fill a delay slot. If more than this arbitrary number of
instructions are searched, the time savings from filling the delay slot
are minimal, so stop searching. Increasing values mean more
aggressive optimization, making the compilation time increase with probably
small improvement in execution time.
- max-delay-slot-live-search
-
When trying to fill delay slots, the maximum number of instructions to
consider when searching for a block with valid live register
information. Increasing this arbitrarily chosen value means more
aggressive optimization, increasing the compilation time. This parameter
should be removed when the delay slot code is rewritten to maintain the
control-flow graph.
- max-gcse-memory
-
The approximate maximum amount of memory that can be allocated in
order to perform the global common subexpression elimination
optimization. If more memory than specified is required, the
optimization is not done.
- max-gcse-insertion-ratio
-
If the ratio of expression insertions to deletions is larger than this value
for any expression, then RTL PRE inserts or removes the expression and thus
leaves partially redundant computations in the instruction stream. The default value is 20.
- max-pending-list-length
-
The maximum number of pending dependencies scheduling allows
before flushing the current state and starting over. Large functions
with few branches or calls can create excessively large lists which
needlessly consume memory and resources.
- max-modulo-backtrack-attempts
-
The maximum number of backtrack attempts the scheduler should make
when modulo scheduling a loop. Larger values can exponentially increase
compilation time.
- max-inline-insns-single
-
Several parameters control the tree inliner used in GCC.
This number sets the maximum number of instructions (counted in GCC's
internal representation) in a single function that the tree inliner
considers for inlining. This only affects functions declared
inline and methods implemented in a class declaration (C++).
The default value is 400.
- max-inline-insns-auto
-
When you use -finline-functions (included in -O3),
a lot of functions that would otherwise not be considered for inlining
by the compiler are investigated. To those functions, a different
(more restrictive) limit compared to functions declared inline can
be applied.
The default value is 40.
- inline-min-speedup
-
When estimated performance improvement of caller + callee runtime exceeds this
threshold (in precent), the function can be inlined regardless the limit on
--param max-inline-insns-single and --param
max-inline-insns-auto.
- large-function-insns
-
The limit specifying really large functions. For functions larger than this
limit after inlining, inlining is constrained by
--param large-function-growth. This parameter is useful primarily
to avoid extreme compilation time caused by non-linear algorithms used by the
back end.
The default value is 2700.
- large-function-growth
-
Specifies maximal growth of large function caused by inlining in percents.
The default value is 100 which limits large function growth to 2.0 times
the original size.
- large-unit-insns
-
The limit specifying large translation unit. Growth caused by inlining of
units larger than this limit is limited by --param inline-unit-growth.
For small units this might be too tight.
For example, consider a unit consisting of function A
that is inline and B that just calls A three times. If B is small relative to
A, the growth of unit is 300\% and yet such inlining is very sane. For very
large units consisting of small inlineable functions, however, the overall unit
growth limit is needed to avoid exponential explosion of code size. Thus for
smaller units, the size is increased to --param large-unit-insns
before applying --param inline-unit-growth. The default is 10000.
- inline-unit-growth
-
Specifies maximal overall growth of the compilation unit caused by inlining.
The default value is 20 which limits unit growth to 1.2 times the original
size. Cold functions (either marked cold via an attribute or by profile
feedback) are not accounted into the unit size.
- ipcp-unit-growth
-
Specifies maximal overall growth of the compilation unit caused by
interprocedural constant propagation. The default value is 10 which limits
unit growth to 1.1 times the original size.
- large-stack-frame
-
The limit specifying large stack frames. While inlining the algorithm is trying
to not grow past this limit too much. The default value is 256 bytes.
- large-stack-frame-growth
-
Specifies maximal growth of large stack frames caused by inlining in percents.
The default value is 1000 which limits large stack frame growth to 11 times
the original size.
- max-inline-insns-recursive
-
- max-inline-insns-recursive-auto
-
Specifies the maximum number of instructions an out-of-line copy of a
self-recursive inline
function can grow into by performing recursive inlining.
--param max-inline-insns-recursive applies to functions
declared inline.
For functions not declared inline, recursive inlining
happens only when -finline-functions (included in -O3) is
enabled; --param max-inline-insns-recursive-auto applies instead. The
default value is 450.
- max-inline-recursive-depth
-
- max-inline-recursive-depth-auto
-
Specifies the maximum recursion depth used for recursive inlining.
--param max-inline-recursive-depth applies to functions
declared inline. For functions not declared inline, recursive inlining
happens only when -finline-functions (included in -O3) is
enabled; --param max-inline-recursive-depth-auto applies instead. The
default value is 8.
- min-inline-recursive-probability
-
Recursive inlining is profitable only for function having deep recursion
in average and can hurt for function having little recursion depth by
increasing the prologue size or complexity of function body to other
optimizers.
When profile feedback is available (see -fprofile-generate) the actual
recursion depth can be guessed from probability that function recurses via a
given call expression. This parameter limits inlining only to call expressions
whose probability exceeds the given threshold (in percents).
The default value is 10.
- early-inlining-insns
-
Specify growth that the early inliner can make. In effect it increases
the amount of inlining for code having a large abstraction penalty.
The default value is 14.
- max-early-inliner-iterations
-
Limit of iterations of the early inliner. This basically bounds
the number of nested indirect calls the early inliner can resolve.
Deeper chains are still handled by late inlining.
- comdat-sharing-probability
-
Probability (in percent) that C++ inline function with comdat visibility
are shared across multiple compilation units. The default value is 20.
- profile-func-internal-id
-
A parameter to control whether to use function internal id in profile
database lookup. If the value is 0, the compiler uses an id that
is based on function assembler name and filename, which makes old profile
data more tolerant to source changes such as function reordering etc.
The default value is 0.
- min-vect-loop-bound
-
The minimum number of iterations under which loops are not vectorized
when -ftree-vectorize is used. The number of iterations after
vectorization needs to be greater than the value specified by this option
to allow vectorization. The default value is 0.
- gcse-cost-distance-ratio
-
Scaling factor in calculation of maximum distance an expression
can be moved by GCSE optimizations. This is currently supported only in the
code hoisting pass. The bigger the ratio, the more aggressive code hoisting
is with simple expressions, i.e., the expressions that have cost
less than gcse-unrestricted-cost. Specifying 0 disables
hoisting of simple expressions. The default value is 10.
- gcse-unrestricted-cost
-
Cost, roughly measured as the cost of a single typical machine
instruction, at which GCSE optimizations do not constrain
the distance an expression can travel. This is currently
supported only in the code hoisting pass. The lesser the cost,
the more aggressive code hoisting is. Specifying 0
allows all expressions to travel unrestricted distances.
The default value is 3.
- max-hoist-depth
-
The depth of search in the dominator tree for expressions to hoist.
This is used to avoid quadratic behavior in hoisting algorithm.
The value of 0 does not limit on the search, but may slow down compilation
of huge functions. The default value is 30.
- max-tail-merge-comparisons
-
The maximum amount of similar bbs to compare a bb with. This is used to
avoid quadratic behavior in tree tail merging. The default value is 10.
- max-tail-merge-iterations
-
The maximum amount of iterations of the pass over the function. This is used to
limit compilation time in tree tail merging. The default value is 2.
- max-unrolled-insns
-
The maximum number of instructions that a loop may have to be unrolled.
If a loop is unrolled, this parameter also determines how many times
the loop code is unrolled.
- max-average-unrolled-insns
-
The maximum number of instructions biased by probabilities of their execution
that a loop may have to be unrolled. If a loop is unrolled,
this parameter also determines how many times the loop code is unrolled.
- max-unroll-times
-
The maximum number of unrollings of a single loop.
- max-peeled-insns
-
The maximum number of instructions that a loop may have to be peeled.
If a loop is peeled, this parameter also determines how many times
the loop code is peeled.
- max-peel-times
-
The maximum number of peelings of a single loop.
- max-peel-branches
-
The maximum number of branches on the hot path through the peeled sequence.
- max-completely-peeled-insns
-
The maximum number of insns of a completely peeled loop.
- max-completely-peel-times
-
The maximum number of iterations of a loop to be suitable for complete peeling.
- max-completely-peel-loop-nest-depth
-
The maximum depth of a loop nest suitable for complete peeling.
- max-unswitch-insns
-
The maximum number of insns of an unswitched loop.
- max-unswitch-level
-
The maximum number of branches unswitched in a single loop.
- lim-expensive
-
The minimum cost of an expensive expression in the loop invariant motion.
- iv-consider-all-candidates-bound
-
Bound on number of candidates for induction variables, below which
all candidates are considered for each use in induction variable
optimizations. If there are more candidates than this,
only the most relevant ones are considered to avoid quadratic time complexity.
- iv-max-considered-uses
-
The induction variable optimizations give up on loops that contain more
induction variable uses.
- iv-always-prune-cand-set-bound
-
If the number of candidates in the set is smaller than this value,
always try to remove unnecessary ivs from the set
when adding a new one.
- scev-max-expr-size
-
Bound on size of expressions used in the scalar evolutions analyzer.
Large expressions slow the analyzer.
- scev-max-expr-complexity
-
Bound on the complexity of the expressions in the scalar evolutions analyzer.
Complex expressions slow the analyzer.
- omega-max-vars
-
The maximum number of variables in an Omega constraint system.
The default value is 128.
- omega-max-geqs
-
The maximum number of inequalities in an Omega constraint system.
The default value is 256.
- omega-max-eqs
-
The maximum number of equalities in an Omega constraint system.
The default value is 128.
- omega-max-wild-cards
-
The maximum number of wildcard variables that the Omega solver is
able to insert. The default value is 18.
- omega-hash-table-size
-
The size of the hash table in the Omega solver. The default value is
550.
- omega-max-keys
-
The maximal number of keys used by the Omega solver. The default
value is 500.
- omega-eliminate-redundant-constraints
-
When set to 1, use expensive methods to eliminate all redundant
constraints. The default value is 0.
- vect-max-version-for-alignment-checks
-
The maximum number of run-time checks that can be performed when
doing loop versioning for alignment in the vectorizer.
- vect-max-version-for-alias-checks
-
The maximum number of run-time checks that can be performed when
doing loop versioning for alias in the vectorizer.
- vect-max-peeling-for-alignment
-
The maximum number of loop peels to enhance access alignment
for vectorizer. Value -1 means 'no limit'.
- max-iterations-to-track
-
The maximum number of iterations of a loop the brute-force algorithm
for analysis of the number of iterations of the loop tries to evaluate.
- hot-bb-count-ws-permille
-
A basic block profile count is considered hot if it contributes to
the given permillage (i.e. 0...1000) of the entire profiled execution.
- hot-bb-frequency-fraction
-
Select fraction of the entry block frequency of executions of basic block in
function given basic block needs to have to be considered hot.
- max-predicted-iterations
-
The maximum number of loop iterations we predict statically. This is useful
in cases where a function contains a single loop with known bound and
another loop with unknown bound.
The known number of iterations is predicted correctly, while
the unknown number of iterations average to roughly 10. This means that the
loop without bounds appears artificially cold relative to the other one.
- builtin-expect-probability
-
Control the probability of the expression having the specified value. This
parameter takes a percentage (i.e. 0 ... 100) as input.
The default probability of 90 is obtained empirically.
- align-threshold
-
Select fraction of the maximal frequency of executions of a basic block in
a function to align the basic block.
- align-loop-iterations
-
A loop expected to iterate at least the selected number of iterations is
aligned.
- tracer-dynamic-coverage
-
- tracer-dynamic-coverage-feedback
-
This value is used to limit superblock formation once the given percentage of
executed instructions is covered. This limits unnecessary code size
expansion.
The tracer-dynamic-coverage-feedback parameter
is used only when profile
feedback is available. The real profiles (as opposed to statically estimated
ones) are much less balanced allowing the threshold to be larger value.
- tracer-max-code-growth
-
Stop tail duplication once code growth has reached given percentage. This is
a rather artificial limit, as most of the duplicates are eliminated later in
cross jumping, so it may be set to much higher values than is the desired code
growth.
- tracer-min-branch-ratio
-
Stop reverse growth when the reverse probability of best edge is less than this
threshold (in percent).
- tracer-min-branch-ratio
-
- tracer-min-branch-ratio-feedback
-
Stop forward growth if the best edge has probability lower than this
threshold.
Similarly to tracer-dynamic-coverage two values are present, one for
compilation for profile feedback and one for compilation without. The value
for compilation with profile feedback needs to be more conservative (higher) in
order to make tracer effective.
- max-cse-path-length
-
The maximum number of basic blocks on path that CSE considers.
The default is 10.
- max-cse-insns
-
The maximum number of instructions CSE processes before flushing.
The default is 1000.
- ggc-min-expand
-
GCC uses a garbage collector to manage its own memory allocation. This
parameter specifies the minimum percentage by which the garbage
collector's heap should be allowed to expand between collections.
Tuning this may improve compilation speed; it has no effect on code
generation.
The default is 30% + 70% * (RAM/1GB) with an upper bound of 100% when
RAM >= 1GB. If "getrlimit" is available, the notion of ``RAM'' is
the smallest of actual RAM and "RLIMIT_DATA" or "RLIMIT_AS". If
GCC is not able to calculate RAM on a particular platform, the lower
bound of 30% is used. Setting this parameter and
ggc-min-heapsize to zero causes a full collection to occur at
every opportunity. This is extremely slow, but can be useful for
debugging.
- ggc-min-heapsize
-
Minimum size of the garbage collector's heap before it begins bothering
to collect garbage. The first collection occurs after the heap expands
by ggc-min-expand% beyond ggc-min-heapsize. Again,
tuning this may improve compilation speed, and has no effect on code
generation.
The default is the smaller of RAM/8, RLIMIT_RSS, or a limit that
tries to ensure that RLIMIT_DATA or RLIMIT_AS are not exceeded, but
with a lower bound of 4096 (four megabytes) and an upper bound of
131072 (128 megabytes). If GCC is not able to calculate RAM on a
particular platform, the lower bound is used. Setting this parameter
very large effectively disables garbage collection. Setting this
parameter and ggc-min-expand to zero causes a full collection
to occur at every opportunity.
- max-reload-search-insns
-
The maximum number of instruction reload should look backward for equivalent
register. Increasing values mean more aggressive optimization, making the
compilation time increase with probably slightly better performance.
The default value is 100.
- max-cselib-memory-locations
-
The maximum number of memory locations cselib should take into account.
Increasing values mean more aggressive optimization, making the compilation time
increase with probably slightly better performance. The default value is 500.
- reorder-blocks-duplicate
-
- reorder-blocks-duplicate-feedback
-
Used by the basic block reordering pass to decide whether to use unconditional
branch or duplicate the code on its destination. Code is duplicated when its
estimated size is smaller than this value multiplied by the estimated size of
unconditional jump in the hot spots of the program.
The reorder-block-duplicate-feedback parameter
is used only when profile
feedback is available. It may be set to higher values than
reorder-block-duplicate since information about the hot spots is more
accurate.
- max-sched-ready-insns
-
The maximum number of instructions ready to be issued the scheduler should
consider at any given time during the first scheduling pass. Increasing
values mean more thorough searches, making the compilation time increase
with probably little benefit. The default value is 100.
- max-sched-region-blocks
-
The maximum number of blocks in a region to be considered for
interblock scheduling. The default value is 10.
- max-pipeline-region-blocks
-
The maximum number of blocks in a region to be considered for
pipelining in the selective scheduler. The default value is 15.
- max-sched-region-insns
-
The maximum number of insns in a region to be considered for
interblock scheduling. The default value is 100.
- max-pipeline-region-insns
-
The maximum number of insns in a region to be considered for
pipelining in the selective scheduler. The default value is 200.
- min-spec-prob
-
The minimum probability (in percents) of reaching a source block
for interblock speculative scheduling. The default value is 40.
- max-sched-extend-regions-iters
-
The maximum number of iterations through CFG to extend regions.
A value of 0 (the default) disables region extensions.
- max-sched-insn-conflict-delay
-
The maximum conflict delay for an insn to be considered for speculative motion.
The default value is 3.
- sched-spec-prob-cutoff
-
The minimal probability of speculation success (in percents), so that
speculative insns are scheduled.
The default value is 40.
- sched-spec-state-edge-prob-cutoff
-
The minimum probability an edge must have for the scheduler to save its
state across it.
The default value is 10.
- sched-mem-true-dep-cost
-
Minimal distance (in CPU cycles) between store and load targeting same
memory locations. The default value is 1.
- selsched-max-lookahead
-
The maximum size of the lookahead window of selective scheduling. It is a
depth of search for available instructions.
The default value is 50.
- selsched-max-sched-times
-
The maximum number of times that an instruction is scheduled during
selective scheduling. This is the limit on the number of iterations
through which the instruction may be pipelined. The default value is 2.
- selsched-max-insns-to-rename
-
The maximum number of best instructions in the ready list that are considered
for renaming in the selective scheduler. The default value is 2.
- sms-min-sc
-
The minimum value of stage count that swing modulo scheduler
generates. The default value is 2.
- max-last-value-rtl
-
The maximum size measured as number of RTLs that can be recorded in an expression
in combiner for a pseudo register as last known value of that register. The default
is 10000.
- max-combine-insns
-
The maximum number of instructions the RTL combiner tries to combine.
The default value is 2 at -Og and 4 otherwise.
- integer-share-limit
-
Small integer constants can use a shared data structure, reducing the
compiler's memory usage and increasing its speed. This sets the maximum
value of a shared integer constant. The default value is 256.
- ssp-buffer-size
-
The minimum size of buffers (i.e. arrays) that receive stack smashing
protection when -fstack-protection is used.
NOTE: In Gentoo this is change from ``8'' to ``4'', to increase
the number of functions protected by the stack protector.
- min-size-for-stack-sharing
-
The minimum size of variables taking part in stack slot sharing when not
optimizing. The default value is 32.
- max-jump-thread-duplication-stmts
-
Maximum number of statements allowed in a block that needs to be
duplicated when threading jumps.
- max-fields-for-field-sensitive
-
Maximum number of fields in a structure treated in
a field sensitive manner during pointer analysis. The default is zero
for -O0 and -O1,
and 100 for -Os, -O2, and -O3.
- prefetch-latency
-
Estimate on average number of instructions that are executed before
prefetch finishes. The distance prefetched ahead is proportional
to this constant. Increasing this number may also lead to less
streams being prefetched (see simultaneous-prefetches).
- simultaneous-prefetches
-
Maximum number of prefetches that can run at the same time.
- l1-cache-line-size
-
The size of cache line in L1 cache, in bytes.
- l1-cache-size
-
The size of L1 cache, in kilobytes.
- l2-cache-size
-
The size of L2 cache, in kilobytes.
- min-insn-to-prefetch-ratio
-
The minimum ratio between the number of instructions and the
number of prefetches to enable prefetching in a loop.
- prefetch-min-insn-to-mem-ratio
-
The minimum ratio between the number of instructions and the
number of memory references to enable prefetching in a loop.
- use-canonical-types
-
Whether the compiler should use the ``canonical'' type system. By
default, this should always be 1, which uses a more efficient internal
mechanism for comparing types in C++ and Objective-C++. However, if
bugs in the canonical type system are causing compilation failures,
set this value to 0 to disable canonical types.
- switch-conversion-max-branch-ratio
-
Switch initialization conversion refuses to create arrays that are
bigger than switch-conversion-max-branch-ratio times the number of
branches in the switch.
- max-partial-antic-length
-
Maximum length of the partial antic set computed during the tree
partial redundancy elimination optimization (-ftree-pre) when
optimizing at -O3 and above. For some sorts of source code
the enhanced partial redundancy elimination optimization can run away,
consuming all of the memory available on the host machine. This
parameter sets a limit on the length of the sets that are computed,
which prevents the runaway behavior. Setting a value of 0 for
this parameter allows an unlimited set length.
- sccvn-max-scc-size
-
Maximum size of a strongly connected component (SCC) during SCCVN
processing. If this limit is hit, SCCVN processing for the whole
function is not done and optimizations depending on it are
disabled. The default maximum SCC size is 10000.
- sccvn-max-alias-queries-per-access
-
Maximum number of alias-oracle queries we perform when looking for
redundancies for loads and stores. If this limit is hit the search
is aborted and the load or store is not considered redundant. The
number of queries is algorithmically limited to the number of
stores on all paths from the load to the function entry.
The default maxmimum number of queries is 1000.
- ira-max-loops-num
-
IRA uses regional register allocation by default. If a function
contains more loops than the number given by this parameter, only at most
the given number of the most frequently-executed loops form regions
for regional register allocation. The default value of the
parameter is 100.
- ira-max-conflict-table-size
-
Although IRA uses a sophisticated algorithm to compress the conflict
table, the table can still require excessive amounts of memory for
huge functions. If the conflict table for a function could be more
than the size in MB given by this parameter, the register allocator
instead uses a faster, simpler, and lower-quality
algorithm that does not require building a pseudo-register conflict table.
The default value of the parameter is 2000.
- ira-loop-reserved-regs
-
IRA can be used to evaluate more accurate register pressure in loops
for decisions to move loop invariants (see -O3). The number
of available registers reserved for some other purposes is given
by this parameter. The default value of the parameter is 2, which is
the minimal number of registers needed by typical instructions.
This value is the best found from numerous experiments.
- lra-inheritance-ebb-probability-cutoff
-
LRA tries to reuse values reloaded in registers in subsequent insns.
This optimization is called inheritance. EBB is used as a region to
do this optimization. The parameter defines a minimal fall-through
edge probability in percentage used to add BB to inheritance EBB in
LRA. The default value of the parameter is 40. The value was chosen
from numerous runs of SPEC2000 on x86-64.
- loop-invariant-max-bbs-in-loop
-
Loop invariant motion can be very expensive, both in compilation time and
in amount of needed compile-time memory, with very large loops. Loops
with more basic blocks than this parameter won't have loop invariant
motion optimization performed on them. The default value of the
parameter is 1000 for -O1 and 10000 for -O2 and above.
- loop-max-datarefs-for-datadeps
-
Building data dapendencies is expensive for very large loops. This
parameter limits the number of data references in loops that are
considered for data dependence analysis. These large loops are no
handled by the optimizations using loop data dependencies.
The default value is 1000.
- max-vartrack-size
-
Sets a maximum number of hash table slots to use during variable
tracking dataflow analysis of any function. If this limit is exceeded
with variable tracking at assignments enabled, analysis for that
function is retried without it, after removing all debug insns from
the function. If the limit is exceeded even without debug insns, var
tracking analysis is completely disabled for the function. Setting
the parameter to zero makes it unlimited.
- max-vartrack-expr-depth
-
Sets a maximum number of recursion levels when attempting to map
variable names or debug temporaries to value expressions. This trades
compilation time for more complete debug information. If this is set too
low, value expressions that are available and could be represented in
debug information may end up not being used; setting this higher may
enable the compiler to find more complex debug expressions, but compile
time and memory use may grow. The default is 12.
- min-nondebug-insn-uid
-
Use uids starting at this parameter for nondebug insns. The range below
the parameter is reserved exclusively for debug insns created by
-fvar-tracking-assignments, but debug insns may get
(non-overlapping) uids above it if the reserved range is exhausted.
- ipa-sra-ptr-growth-factor
-
IPA-SRA replaces a pointer to an aggregate with one or more new
parameters only when their cumulative size is less or equal to
ipa-sra-ptr-growth-factor times the size of the original
pointer parameter.
- sra-max-scalarization-size-Ospeed
-
- sra-max-scalarization-size-Osize
-
The two Scalar Reduction of Aggregates passes (SRA and IPA-SRA) aim to
replace scalar parts of aggregates with uses of independent scalar
variables. These parameters control the maximum size, in storage units,
of aggregate which is considered for replacement when compiling for
speed
(sra-max-scalarization-size-Ospeed) or size
(sra-max-scalarization-size-Osize) respectively.
- tm-max-aggregate-size
-
When making copies of thread-local variables in a transaction, this
parameter specifies the size in bytes after which variables are
saved with the logging functions as opposed to save/restore code
sequence pairs. This option only applies when using
-fgnu-tm.
- graphite-max-nb-scop-params
-
To avoid exponential effects in the Graphite loop transforms, the
number of parameters in a Static Control Part (SCoP) is bounded. The
default value is 10 parameters. A variable whose value is unknown at
compilation time and defined outside a SCoP is a parameter of the SCoP.
- graphite-max-bbs-per-function
-
To avoid exponential effects in the detection of SCoPs, the size of
the functions analyzed by Graphite is bounded. The default value is
100 basic blocks.
- loop-block-tile-size
-
Loop blocking or strip mining transforms, enabled with
-floop-block or -floop-strip-mine, strip mine each
loop in the loop nest by a given number of iterations. The strip
length can be changed using the loop-block-tile-size
parameter. The default value is 51 iterations.
- loop-unroll-jam-size
-
Specify the unroll factor for the -floop-unroll-and-jam option. The
default value is 4.
- loop-unroll-jam-depth
-
Specify the dimension to be unrolled (counting from the most inner loop)
for the -floop-unroll-and-jam. The default value is 2.
- ipa-cp-value-list-size
-
IPA-CP attempts to track all possible values and types passed to a function's
parameter in order to propagate them and perform devirtualization.
ipa-cp-value-list-size is the maximum number of values and types it
stores per one formal parameter of a function.
- ipa-cp-eval-threshold
-
IPA-CP calculates its own score of cloning profitability heuristics
and performs those cloning opportunities with scores that exceed
ipa-cp-eval-threshold.
- ipa-cp-recursion-penalty
-
Percentage penalty the recursive functions will receive when they
are evaluated for cloning.
- ipa-cp-single-call-penalty
-
Percentage penalty functions containg a single call to another
function will receive when they are evaluated for cloning.
- ipa-max-agg-items
-
IPA-CP is also capable to propagate a number of scalar values passed
in an aggregate. ipa-max-agg-items controls the maximum
number of such values per one parameter.
- ipa-cp-loop-hint-bonus
-
When IPA-CP determines that a cloning candidate would make the number
of iterations of a loop known, it adds a bonus of
ipa-cp-loop-hint-bonus to the profitability score of
the candidate.
- ipa-cp-array-index-hint-bonus
-
When IPA-CP determines that a cloning candidate would make the index of
an array access known, it adds a bonus of
ipa-cp-array-index-hint-bonus to the profitability
score of the candidate.
- ipa-max-aa-steps
-
During its analysis of function bodies, IPA-CP employs alias analysis
in order to track values pointed to by function parameters. In order
not spend too much time analyzing huge functions, it gives up and
consider all memory clobbered after examining
ipa-max-aa-steps statements modifying memory.
- lto-partitions
-
Specify desired number of partitions produced during WHOPR compilation.
The number of partitions should exceed the number of CPUs used for compilation.
The default value is 32.
- lto-minpartition
-
Size of minimal partition for WHOPR (in estimated instructions).
This prevents expenses of splitting very small programs into too many
partitions.
- cxx-max-namespaces-for-diagnostic-help
-
The maximum number of namespaces to consult for suggestions when C++
name lookup fails for an identifier. The default is 1000.
- sink-frequency-threshold
-
The maximum relative execution frequency (in percents) of the target block
relative to a statement's original block to allow statement sinking of a
statement. Larger numbers result in more aggressive statement sinking.
The default value is 75. A small positive adjustment is applied for
statements with memory operands as those are even more profitable so sink.
- max-stores-to-sink
-
The maximum number of conditional stores paires that can be sunk. Set to 0
if either vectorization (-ftree-vectorize) or if-conversion
(-ftree-loop-if-convert) is disabled. The default is 2.
- allow-store-data-races
-
Allow optimizers to introduce new data races on stores.
Set to 1 to allow, otherwise to 0. This option is enabled by default
at optimization level -Ofast.
- case-values-threshold
-
The smallest number of different values for which it is best to use a
jump-table instead of a tree of conditional branches. If the value is
0, use the default for the machine. The default is 0.
- tree-reassoc-width
-
Set the maximum number of instructions executed in parallel in
reassociated tree. This parameter overrides target dependent
heuristics used by default if has non zero value.
- sched-pressure-algorithm
-
Choose between the two available implementations of
-fsched-pressure. Algorithm 1 is the original implementation
and is the more likely to prevent instructions from being reordered.
Algorithm 2 was designed to be a compromise between the relatively
conservative approach taken by algorithm 1 and the rather aggressive
approach taken by the default scheduler. It relies more heavily on
having a regular register file and accurate register pressure classes.
See haifa-sched.c in the GCC sources for more details.
The default choice depends on the target.
- max-slsr-cand-scan
-
Set the maximum number of existing candidates that are considered when
seeking a basis for a new straight-line strength reduction candidate.
- asan-globals
-
Enable buffer overflow detection for global objects. This kind
of protection is enabled by default if you are using
-fsanitize=address option.
To disable global objects protection use --param asan-globals=0.
- asan-stack
-
Enable buffer overflow detection for stack objects. This kind of
protection is enabled by default when using-fsanitize=address.
To disable stack protection use --param asan-stack=0 option.
- asan-instrument-reads
-
Enable buffer overflow detection for memory reads. This kind of
protection is enabled by default when using -fsanitize=address.
To disable memory reads protection use
--param asan-instrument-reads=0.
- asan-instrument-writes
-
Enable buffer overflow detection for memory writes. This kind of
protection is enabled by default when using -fsanitize=address.
To disable memory writes protection use
--param asan-instrument-writes=0 option.
- asan-memintrin
-
Enable detection for built-in functions. This kind of protection
is enabled by default when using -fsanitize=address.
To disable built-in functions protection use
--param asan-memintrin=0.
- asan-use-after-return
-
Enable detection of use-after-return. This kind of protection
is enabled by default when using -fsanitize=address option.
To disable use-after-return detection use
--param asan-use-after-return=0.
- asan-instrumentation-with-call-threshold
-
If number of memory accesses in function being instrumented
is greater or equal to this number, use callbacks instead of inline checks.
E.g. to disable inline code use
--param asan-instrumentation-with-call-threshold=0.
- chkp-max-ctor-size
-
Static constructors generated by Pointer Bounds Checker may become very
large and significantly increase compile time at optimization level
-O1 and higher. This parameter is a maximum nubmer of statements
in a single generated constructor. Default value is 5000.
- max-fsm-thread-path-insns
-
Maximum number of instructions to copy when duplicating blocks on a
finite state automaton jump thread path. The default is 100.
- max-fsm-thread-length
-
Maximum number of basic blocks on a finite state automaton jump thread
path. The default is 10.
- max-fsm-thread-paths
-
Maximum number of new jump thread paths to create for a finite state
automaton. The default is 50.
-
Options Controlling the Preprocessor
These options control the C preprocessor, which is run on each C source
file before actual compilation.
If you use the -E option, nothing is done except preprocessing.
Some of these options make sense only together with -E because
they cause the preprocessor output to be unsuitable for actual
compilation.
- -Wp,option
-
You can use -Wp,option to bypass the compiler driver
and pass option directly through to the preprocessor. If
option contains commas, it is split into multiple options at the
commas. However, many options are modified, translated or interpreted
by the compiler driver before being passed to the preprocessor, and
-Wp forcibly bypasses this phase. The preprocessor's direct
interface is undocumented and subject to change, so whenever possible
you should avoid using -Wp and let the driver handle the
options instead.
- -Xpreprocessor option
-
Pass option as an option to the preprocessor. You can use this to
supply system-specific preprocessor options that GCC does not
recognize.
If you want to pass an option that takes an argument, you must use
-Xpreprocessor twice, once for the option and once for the argument.
- -no-integrated-cpp
-
Perform preprocessing as a separate pass before compilation.
By default, GCC performs preprocessing as an integrated part of
input tokenization and parsing.
If this option is provided, the appropriate language front end
(cc1, cc1plus, or cc1obj for C, C++,
and Objective-C, respectively) is instead invoked twice,
once for preprocessing only and once for actual compilation
of the preprocessed input.
This option may be useful in conjunction with the -B or
-wrapper options to specify an alternate preprocessor or
perform additional processing of the program source between
normal preprocessing and compilation.
- -D name
-
Predefine name as a macro, with definition 1.
- -D name=definition
-
The contents of definition are tokenized and processed as if
they appeared during translation phase three in a #define
directive. In particular, the definition will be truncated by
embedded newline characters.
If you are invoking the preprocessor from a shell or shell-like
program you may need to use the shell's quoting syntax to protect
characters such as spaces that have a meaning in the shell syntax.
If you wish to define a function-like macro on the command line, write
its argument list with surrounding parentheses before the equals sign
(if any). Parentheses are meaningful to most shells, so you will need
to quote the option. With sh and csh,
-D'name(args...)=definition' works.
-D and -U options are processed in the order they
are given on the command line. All -imacros file and
-include file options are processed after all
-D and -U options.
- -U name
-
Cancel any previous definition of name, either built in or
provided with a -D option.
- -undef
-
Do not predefine any system-specific or GCC-specific macros. The
standard predefined macros remain defined.
- -I dir
-
Add the directory dir to the list of directories to be searched
for header files.
Directories named by -I are searched before the standard
system include directories. If the directory dir is a standard
system include directory, the option is ignored to ensure that the
default search order for system directories and the special treatment
of system headers are not defeated
.
If dir begins with "=", then the "=" will be replaced
by the sysroot prefix; see --sysroot and -isysroot.
- -o file
-
Write output to file. This is the same as specifying file
as the second non-option argument to cpp. gcc has a
different interpretation of a second non-option argument, so you must
use -o to specify the output file.
- -Wall
-
Turns on all optional warnings which are desirable for normal code.
At present this is -Wcomment, -Wtrigraphs,
-Wmultichar and a warning about integer promotion causing a
change of sign in "#if" expressions. Note that many of the
preprocessor's warnings are on by default and have no options to
control them.
- -Wcomment
-
- -Wcomments
-
Warn whenever a comment-start sequence /* appears in a /*
comment, or whenever a backslash-newline appears in a // comment.
(Both forms have the same effect.)
- -Wtrigraphs
-
Most trigraphs in comments cannot affect the meaning of the program.
However, a trigraph that would form an escaped newline (??/ at
the end of a line) can, by changing where the comment begins or ends.
Therefore, only trigraphs that would form escaped newlines produce
warnings inside a comment.
This option is implied by -Wall. If -Wall is not
given, this option is still enabled unless trigraphs are enabled. To
get trigraph conversion without warnings, but get the other
-Wall warnings, use -trigraphs -Wall -Wno-trigraphs.
- -Wtraditional
-
Warn about certain constructs that behave differently in traditional and
ISO C. Also warn about ISO C constructs that have no traditional C
equivalent, and problematic constructs which should be avoided.
- -Wundef
-
Warn whenever an identifier which is not a macro is encountered in an
#if directive, outside of defined. Such identifiers are
replaced with zero.
- -Wunused-macros
-
Warn about macros defined in the main file that are unused. A macro
is used if it is expanded or tested for existence at least once.
The preprocessor will also warn if the macro has not been used at the
time it is redefined or undefined.
Built-in macros, macros defined on the command line, and macros
defined in include files are not warned about.
Note: If a macro is actually used, but only used in skipped
conditional blocks, then CPP will report it as unused. To avoid the
warning in such a case, you might improve the scope of the macro's
definition by, for example, moving it into the first skipped block.
Alternatively, you could provide a dummy use with something like:
#if defined the_macro_causing_the_warning
#endif
- -Wendif-labels
-
Warn whenever an #else or an #endif are followed by text.
This usually happens in code of the form
#if FOO
...
#else FOO
...
#endif FOO
The second and third "FOO" should be in comments, but often are not
in older programs. This warning is on by default.
- -Werror
-
Make all warnings into hard errors. Source code which triggers warnings
will be rejected.
- -Wsystem-headers
-
Issue warnings for code in system headers. These are normally unhelpful
in finding bugs in your own code, therefore suppressed. If you are
responsible for the system library, you may want to see them.
- -w
-
Suppress all warnings, including those which GNU CPP issues by default.
- -pedantic
-
Issue all the mandatory diagnostics listed in the C standard. Some of
them are left out by default, since they trigger frequently on harmless
code.
- -pedantic-errors
-
Issue all the mandatory diagnostics, and make all mandatory diagnostics
into errors. This includes mandatory diagnostics that GCC issues
without -pedantic but treats as warnings.
- -M
-
Instead of outputting the result of preprocessing, output a rule
suitable for make describing the dependencies of the main
source file. The preprocessor outputs one make rule containing
the object file name for that source file, a colon, and the names of all
the included files, including those coming from -include or
-imacros command-line options.
Unless specified explicitly (with -MT or -MQ), the
object file name consists of the name of the source file with any
suffix replaced with object file suffix and with any leading directory
parts removed. If there are many included files then the rule is
split into several lines using \-newline. The rule has no
commands.
This option does not suppress the preprocessor's debug output, such as
-dM. To avoid mixing such debug output with the dependency
rules you should explicitly specify the dependency output file with
-MF, or use an environment variable like
DEPENDENCIES_OUTPUT. Debug output
will still be sent to the regular output stream as normal.
Passing -M to the driver implies -E, and suppresses
warnings with an implicit -w.
- -MM
-
Like -M but do not mention header files that are found in
system header directories, nor header files that are included,
directly or indirectly, from such a header.
This implies that the choice of angle brackets or double quotes in an
#include directive does not in itself determine whether that
header will appear in -MM dependency output. This is a
slight change in semantics from GCC versions 3.0 and earlier.
- -MF file
-
When used with -M or -MM, specifies a
file to write the dependencies to. If no -MF switch is given
the preprocessor sends the rules to the same place it would have sent
preprocessed output.
When used with the driver options -MD or -MMD,
-MF overrides the default dependency output file.
- -MG
-
In conjunction with an option such as -M requesting
dependency generation, -MG assumes missing header files are
generated files and adds them to the dependency list without raising
an error. The dependency filename is taken directly from the
"#include" directive without prepending any path. -MG
also suppresses preprocessed output, as a missing header file renders
this useless.
This feature is used in automatic updating of makefiles.
- -MP
-
This option instructs CPP to add a phony target for each dependency
other than the main file, causing each to depend on nothing. These
dummy rules work around errors make gives if you remove header
files without updating the Makefile to match.
This is typical output:
test.o: test.c test.h
test.h:
- -MT target
-
Change the target of the rule emitted by dependency generation. By
default CPP takes the name of the main input file, deletes any
directory components and any file suffix such as .c, and
appends the platform's usual object suffix. The result is the target.
An -MT option will set the target to be exactly the string you
specify. If you want multiple targets, you can specify them as a single
argument to -MT, or use multiple -MT options.
For example, -MT '$(objpfx)foo.o' might give
$(objpfx)foo.o: foo.c
- -MQ target
-
Same as -MT, but it quotes any characters which are special to
Make. -MQ '$(objpfx)foo.o' gives
$$(objpfx)foo.o: foo.c
The default target is automatically quoted, as if it were given with
-MQ.
- -MD
-
-MD is equivalent to -M -MF file, except that
-E is not implied. The driver determines file based on
whether an -o option is given. If it is, the driver uses its
argument but with a suffix of .d, otherwise it takes the name
of the input file, removes any directory components and suffix, and
applies a .d suffix.
If -MD is used in conjunction with -E, any
-o switch is understood to specify the dependency output file, but if used without -E, each -o
is understood to specify a target object file.
Since -E is not implied, -MD can be used to generate
a dependency output file as a side-effect of the compilation process.
- -MMD
-
Like -MD except mention only user header files, not system
header files.
- -fpch-deps
-
When using precompiled headers, this flag
will cause the dependency-output flags to also list the files from the
precompiled header's dependencies. If not specified only the
precompiled header would be listed and not the files that were used to
create it because those files are not consulted when a precompiled
header is used.
- -fpch-preprocess
-
This option allows use of a precompiled header together with -E. It inserts a special "#pragma",
"#pragma GCC pch_preprocess "filename"" in the output to mark
the place where the precompiled header was found, and its filename.
When -fpreprocessed is in use, GCC recognizes this "#pragma"
and loads the PCH.
This option is off by default, because the resulting preprocessed output
is only really suitable as input to GCC. It is switched on by
-save-temps.
You should not write this "#pragma" in your own code, but it is
safe to edit the filename if the PCH file is available in a different
location. The filename may be absolute or it may be relative to GCC's
current directory.
- -x c
-
- -x c++
-
- -x objective-c
-
- -x assembler-with-cpp
-
Specify the source language: C, C++, Objective-C, or assembly. This has
nothing to do with standards conformance or extensions; it merely
selects which base syntax to expect. If you give none of these options,
cpp will deduce the language from the extension of the source file:
.c, .cc, .m, or .S. Some other common
extensions for C++ and assembly are also recognized. If cpp does not
recognize the extension, it will treat the file as C; this is the most
generic mode.
Note: Previous versions of cpp accepted a -lang option
which selected both the language and the standards conformance level.
This option has been removed, because it conflicts with the -l
option.
- -std=standard
-
- -ansi
-
Specify the standard to which the code should conform. Currently CPP
knows about C and C++ standards; others may be added in the future.
standard
may be one of:
-
- c90
-
- c89
-
- iso9899:1990
-
The ISO C standard from 1990. c90 is the customary shorthand for
this version of the standard.
The -ansi option is equivalent to -std=c90.
- iso9899:199409
-
The 1990 C standard, as amended in 1994.
- iso9899:1999
-
- c99
-
- iso9899:199x
-
- c9x
-
The revised ISO C standard, published in December 1999. Before
publication, this was known as C9X.
- iso9899:2011
-
- c11
-
- c1x
-
The revised ISO C standard, published in December 2011. Before
publication, this was known as C1X.
- gnu90
-
- gnu89
-
The 1990 C standard plus GNU extensions. This is the default.
- gnu99
-
- gnu9x
-
The 1999 C standard plus GNU extensions.
- gnu11
-
- gnu1x
-
The 2011 C standard plus GNU extensions.
- c++98
-
The 1998 ISO C++ standard plus amendments.
- gnu++98
-
The same as -std=c++98 plus GNU extensions. This is the
default for C++ code.
-
- -I-
-
Split the include path. Any directories specified with -I
options before -I- are searched only for headers requested with
"#include "file""; they are not searched for
"#include <file>". If additional directories are
specified with -I options after the -I-, those
directories are searched for all #include directives.
In addition, -I- inhibits the use of the directory of the current
file directory as the first search directory for "#include "file"".
This option has been deprecated.
- -nostdinc
-
Do not search the standard system directories for header files.
Only the directories you have specified with -I options
(and the directory of the current file, if appropriate) are searched.
- -nostdinc++
-
Do not search for header files in the C++-specific standard directories,
but do still search the other standard directories. (This option is
used when building the C++ library.)
- -include file
-
Process file as if "#include "file"" appeared as the first
line of the primary source file. However, the first directory searched
for file is the preprocessor's working directory instead of
the directory containing the main source file. If not found there, it
is searched for in the remainder of the "#include "..."" search
chain as normal.
If multiple -include options are given, the files are included
in the order they appear on the command line.
- -imacros file
-
Exactly like -include, except that any output produced by
scanning file is thrown away. Macros it defines remain defined.
This allows you to acquire all the macros from a header without also
processing its declarations.
All files specified by -imacros are processed before all files
specified by -include.
- -idirafter dir
-
Search dir for header files, but do it after all
directories specified with -I and the standard system directories
have been exhausted. dir is treated as a system include directory.
If dir begins with "=", then the "=" will be replaced
by the sysroot prefix; see --sysroot and -isysroot.
- -iprefix prefix
-
Specify prefix as the prefix for subsequent -iwithprefix
options. If the prefix represents a directory, you should include the
final /.
- -iwithprefix dir
-
- -iwithprefixbefore dir
-
Append dir to the prefix specified previously with
-iprefix, and add the resulting directory to the include search
path. -iwithprefixbefore puts it in the same place -I
would; -iwithprefix puts it where -idirafter would.
- -isysroot dir
-
This option is like the --sysroot option, but applies only to
header files (except for Darwin targets, where it applies to both header
files and libraries). See the --sysroot option for more
information.
- -imultilib dir
-
Use dir as a subdirectory of the directory containing
target-specific C++ headers.
- -isystem dir
-
Search dir for header files, after all directories specified by
-I but before the standard system directories. Mark it
as a system directory, so that it gets the same special treatment as
is applied to the standard system directories.
If dir begins with "=", then the "=" will be replaced
by the sysroot prefix; see --sysroot and -isysroot.
- -iquote dir
-
Search dir only for header files requested with
"#include "file""; they are not searched for
"#include <file>", before all directories specified by
-I and before the standard system directories.
If dir begins with "=", then the "=" will be replaced
by the sysroot prefix; see --sysroot and -isysroot.
- -fdirectives-only
-
When preprocessing, handle directives, but do not expand macros.
The option's behavior depends on the -E and -fpreprocessed
options.
With -E, preprocessing is limited to the handling of directives
such as "#define", "#ifdef", and "#error". Other
preprocessor operations, such as macro expansion and trigraph
conversion are not performed. In addition, the -dD option is
implicitly enabled.
With -fpreprocessed, predefinition of command line and most
builtin macros is disabled. Macros such as "__LINE__", which are
contextually dependent, are handled normally. This enables compilation of
files previously preprocessed with "-E -fdirectives-only".
With both -E and -fpreprocessed, the rules for
-fpreprocessed take precedence. This enables full preprocessing of
files previously preprocessed with "-E -fdirectives-only".
- -fdollars-in-identifiers
-
Accept $ in identifiers.
- -fextended-identifiers
-
Accept universal character names in identifiers. This option is
enabled by default for C99 (and later C standard versions) and C++.
- -fno-canonical-system-headers
-
When preprocessing, do not shorten system header paths with canonicalization.
- -fpreprocessed
-
Indicate to the preprocessor that the input file has already been
preprocessed. This suppresses things like macro expansion, trigraph
conversion, escaped newline splicing, and processing of most directives.
The preprocessor still recognizes and removes comments, so that you can
pass a file preprocessed with -C to the compiler without
problems. In this mode the integrated preprocessor is little more than
a tokenizer for the front ends.
-fpreprocessed is implicit if the input file has one of the
extensions .i, .ii or .mi. These are the
extensions that GCC uses for preprocessed files created by
-save-temps.
- -ftabstop=width
-
Set the distance between tab stops. This helps the preprocessor report
correct column numbers in warnings or errors, even if tabs appear on the
line. If the value is less than 1 or greater than 100, the option is
ignored. The default is 8.
- -fdebug-cpp
-
This option is only useful for debugging GCC. When used with
-E, dumps debugging information about location maps. Every
token in the output is preceded by the dump of the map its location
belongs to. The dump of the map holding the location of a token would
be:
{"P":F</file/path>;"F":F</includer/path>;"L":<line_num>;"C":<col_num>;"S":<system_header_p>;"M":<map_address>;"E":<macro_expansion_p>,"loc":<location>}
When used without -E, this option has no effect.
- -ftrack-macro-expansion[=level]
-
Track locations of tokens across macro expansions. This allows the
compiler to emit diagnostic about the current macro expansion stack
when a compilation error occurs in a macro expansion. Using this
option makes the preprocessor and the compiler consume more
memory. The level parameter can be used to choose the level of
precision of token location tracking thus decreasing the memory
consumption if necessary. Value 0 of level de-activates
this option just as if no -ftrack-macro-expansion was present
on the command line. Value 1 tracks tokens locations in a
degraded mode for the sake of minimal memory overhead. In this mode
all tokens resulting from the expansion of an argument of a
function-like macro have the same location. Value 2 tracks
tokens locations completely. This value is the most memory hungry.
When this option is given no argument, the default parameter value is
2.
Note that "-ftrack-macro-expansion=2" is activated by default.
- -fexec-charset=charset
-
Set the execution character set, used for string and character
constants. The default is UTF-8. charset can be any encoding
supported by the system's "iconv" library routine.
- -fwide-exec-charset=charset
-
Set the wide execution character set, used for wide string and
character constants. The default is UTF-32 or UTF-16, whichever
corresponds to the width of "wchar_t". As with
-fexec-charset, charset can be any encoding supported
by the system's "iconv" library routine; however, you will have
problems with encodings that do not fit exactly in "wchar_t".
- -finput-charset=charset
-
Set the input character set, used for translation from the character
set of the input file to the source character set used by GCC. If the
locale does not specify, or GCC cannot get this information from the
locale, the default is UTF-8. This can be overridden by either the locale
or this command-line option. Currently the command-line option takes
precedence if there's a conflict. charset can be any encoding
supported by the system's "iconv" library routine.
- -fworking-directory
-
Enable generation of linemarkers in the preprocessor output that will
let the compiler know the current working directory at the time of
preprocessing. When this option is enabled, the preprocessor will
emit, after the initial linemarker, a second linemarker with the
current working directory followed by two slashes. GCC will use this
directory, when it's present in the preprocessed input, as the
directory emitted as the current working directory in some debugging
information formats. This option is implicitly enabled if debugging
information is enabled, but this can be inhibited with the negated
form -fno-working-directory. If the -P flag is
present in the command line, this option has no effect, since no
"#line" directives are emitted whatsoever.
- -fno-show-column
-
Do not print column numbers in diagnostics. This may be necessary if
diagnostics are being scanned by a program that does not understand the
column numbers, such as dejagnu.
- -A predicate=answer
-
Make an assertion with the predicate predicate and answer
answer. This form is preferred to the older form -A
predicate(answer), which is still supported, because
it does not use shell special characters.
- -A -predicate=answer
-
Cancel an assertion with the predicate predicate and answer
answer.
- -dCHARS
-
CHARS is a sequence of one or more of the following characters,
and must not be preceded by a space. Other characters are interpreted
by the compiler proper, or reserved for future versions of GCC, and so
are silently ignored. If you specify characters whose behavior
conflicts, the result is undefined.
-
- M
-
Instead of the normal output, generate a list of #define
directives for all the macros defined during the execution of the
preprocessor, including predefined macros. This gives you a way of
finding out what is predefined in your version of the preprocessor.
Assuming you have no file foo.h, the command
touch foo.h; cpp -dM foo.h
will show all the predefined macros.
If you use -dM without the -E option, -dM is
interpreted as a synonym for -fdump-rtl-mach.
- D
-
Like M except in two respects: it does not include the
predefined macros, and it outputs both the #define
directives and the result of preprocessing. Both kinds of output go to
the standard output file.
- N
-
Like D, but emit only the macro names, not their expansions.
- I
-
Output #include directives in addition to the result of
preprocessing.
- U
-
Like D except that only macros that are expanded, or whose
definedness is tested in preprocessor directives, are output; the
output is delayed until the use or test of the macro; and
#undef directives are also output for macros tested but
undefined at the time.
-
- -P
-
Inhibit generation of linemarkers in the output from the preprocessor.
This might be useful when running the preprocessor on something that is
not C code, and will be sent to a program which might be confused by the
linemarkers.
- -C
-
Do not discard comments. All comments are passed through to the output
file, except for comments in processed directives, which are deleted
along with the directive.
You should be prepared for side effects when using -C; it
causes the preprocessor to treat comments as tokens in their own right.
For example, comments appearing at the start of what would be a
directive line have the effect of turning that line into an ordinary
source line, since the first token on the line is no longer a #.
- -CC
-
Do not discard comments, including during macro expansion. This is
like -C, except that comments contained within macros are
also passed through to the output file where the macro is expanded.
In addition to the side-effects of the -C option, the
-CC option causes all C++-style comments inside a macro
to be converted to C-style comments. This is to prevent later use
of that macro from inadvertently commenting out the remainder of
the source line.
The -CC option is generally used to support lint comments.
- -traditional-cpp
-
Try to imitate the behavior of old-fashioned C preprocessors, as
opposed to ISO C preprocessors.
- -trigraphs
-
Process trigraph sequences.
These are three-character sequences, all starting with ??, that
are defined by ISO C to stand for single characters. For example,
??/ stands for \, so '??/n' is a character
constant for a newline. By default, GCC ignores trigraphs, but in
standard-conforming modes it converts them. See the -std and
-ansi options.
The nine trigraphs and their replacements are
Trigraph: ??( ??) ??< ??> ??= ??/ ??' ??! ??-
Replacement: [ ] { } # \ ^ | ~
- -remap
-
Enable special code to work around file systems which only permit very
short file names, such as MS-DOS.
- --help
-
- --target-help
-
Print text describing all the command-line options instead of
preprocessing anything.
- -v
-
Verbose mode. Print out GNU CPP's version number at the beginning of
execution, and report the final form of the include path.
- -H
-
Print the name of each header file used, in addition to other normal
activities. Each name is indented to show how deep in the
#include stack it is. Precompiled header files are also
printed, even if they are found to be invalid; an invalid precompiled
header file is printed with ...x and a valid one with ...! .
- -version
-
- --version
-
Print out GNU CPP's version number. With one dash, proceed to
preprocess as normal. With two dashes, exit immediately.
Passing Options to the Assembler
You can pass options to the assembler.
- -Wa,option
-
Pass option as an option to the assembler. If option
contains commas, it is split into multiple options at the commas.
- -Xassembler option
-
Pass option as an option to the assembler. You can use this to
supply system-specific assembler options that GCC does not
recognize.
If you want to pass an option that takes an argument, you must use
-Xassembler twice, once for the option and once for the argument.
Options for Linking
These options come into play when the compiler links object files into
an executable output file. They are meaningless if the compiler is
not doing a link step.
- object-file-name
-
A file name that does not end in a special recognized suffix is
considered to name an object file or library. (Object files are
distinguished from libraries by the linker according to the file
contents.) If linking is done, these object files are used as input
to the linker.
- -c
-
- -S
-
- -E
-
If any of these options is used, then the linker is not run, and
object file names should not be used as arguments.
- -fuse-ld=bfd
-
Use the bfd linker instead of the default linker.
- -fuse-ld=gold
-
Use the gold linker instead of the default linker.
- -llibrary
-
- -l library
-
Search the library named library when linking. (The second
alternative with the library as a separate argument is only for
POSIX compliance and is not recommended.)
It makes a difference where in the command you write this option; the
linker searches and processes libraries and object files in the order they
are specified. Thus, foo.o -lz bar.o searches library z
after file foo.o but before bar.o. If bar.o refers
to functions in z, those functions may not be loaded.
The linker searches a standard list of directories for the library,
which is actually a file named liblibrary.a. The linker
then uses this file as if it had been specified precisely by name.
The directories searched include several standard system directories
plus any that you specify with -L.
Normally the files found this way are library files---archive files
whose members are object files. The linker handles an archive file by
scanning through it for members which define symbols that have so far
been referenced but not defined. But if the file that is found is an
ordinary object file, it is linked in the usual fashion. The only
difference between using an -l option and specifying a file name
is that -l surrounds library with lib and .a
and searches several directories.
- -lobjc
-
You need this special case of the -l option in order to
link an Objective-C or Objective-C++ program.
- -nostartfiles
-
Do not use the standard system startup files when linking.
The standard system libraries are used normally, unless -nostdlib
or -nodefaultlibs is used.
- -nodefaultlibs
-
Do not use the standard system libraries when linking.
Only the libraries you specify are passed to the linker, and options
specifying linkage of the system libraries, such as -static-libgcc
or -shared-libgcc, are ignored.
The standard startup files are used normally, unless -nostartfiles
is used.
The compiler may generate calls to "memcmp",
"memset", "memcpy" and "memmove".
These entries are usually resolved by entries in
libc. These entry points should be supplied through some other
mechanism when this option is specified.
- -nostdlib
-
Do not use the standard system startup files or libraries when linking.
No startup files and only the libraries you specify are passed to
the linker, and options specifying linkage of the system libraries, such as
-static-libgcc or -shared-libgcc, are ignored.
The compiler may generate calls to "memcmp", "memset",
"memcpy" and "memmove".
These entries are usually resolved by entries in
libc. These entry points should be supplied through some other
mechanism when this option is specified.
One of the standard libraries bypassed by -nostdlib and
-nodefaultlibs is libgcc.a, a library of internal subroutines
which GCC uses to overcome shortcomings of particular machines, or special
needs for some languages.
In most cases, you need libgcc.a even when you want to avoid
other standard libraries. In other words, when you specify -nostdlib
or -nodefaultlibs you should usually specify -lgcc as well.
This ensures that you have no unresolved references to internal GCC
library subroutines.
(An example of such an internal subroutine is "__main", used to ensure C++
constructors are called.)
- -pie
-
Produce a position independent executable on targets that support it.
For predictable results, you must also specify the same set of options
used for compilation (-fpie, -fPIE,
or model suboptions) when you specify this linker option.
NOTE: When --enable-esp, this option is enabled by default
for C, C++, ObjC, and ObjC++ if neither -fno-pie or -fno-PIE
or -fno-pic or -fno-PIC or -nostdlib or
-nostartfiles or -shared or -pg or -p
are found.
- -rdynamic
-
Pass the flag -export-dynamic to the ELF linker, on targets
that support it. This instructs the linker to add all symbols, not
only used ones, to the dynamic symbol table. This option is needed
for some uses of "dlopen" or to allow obtaining backtraces
from within a program.
- -s
-
Remove all symbol table and relocation information from the executable.
- -static
-
On systems that support dynamic linking, this prevents linking with the shared
libraries. On other systems, this option has no effect.
- -shared
-
Produce a shared object which can then be linked with other objects to
form an executable. Not all systems support this option. For predictable
results, you must also specify the same set of options used for compilation
(-fpic, -fPIC, or model suboptions) when
you specify this linker option.[1]
- -shared-libgcc
-
- -static-libgcc
-
On systems that provide libgcc as a shared library, these options
force the use of either the shared or static version, respectively.
If no shared version of libgcc was built when the compiler was
configured, these options have no effect.
There are several situations in which an application should use the
shared libgcc instead of the static version. The most common
of these is when the application wishes to throw and catch exceptions
across different shared libraries. In that case, each of the libraries
as well as the application itself should use the shared libgcc.
Therefore, the G++ and GCJ drivers automatically add
-shared-libgcc whenever you build a shared library or a main
executable, because C++ and Java programs typically use exceptions, so
this is the right thing to do.
If, instead, you use the GCC driver to create shared libraries, you may
find that they are not always linked with the shared libgcc.
If GCC finds, at its configuration time, that you have a non-GNU linker
or a GNU linker that does not support option --eh-frame-hdr,
it links the shared version of libgcc into shared libraries
by default. Otherwise, it takes advantage of the linker and optimizes
away the linking with the shared version of libgcc, linking with
the static version of libgcc by default. This allows exceptions to
propagate through such shared libraries, without incurring relocation
costs at library load time.
However, if a library or main executable is supposed to throw or catch
exceptions, you must link it using the G++ or GCJ driver, as appropriate
for the languages used in the program, or using the option
-shared-libgcc, such that it is linked with the shared
libgcc.
- -static-libasan
-
When the -fsanitize=address option is used to link a program,
the GCC driver automatically links against libasan. If
libasan is available as a shared library, and the -static
option is not used, then this links against the shared version of
libasan. The -static-libasan option directs the GCC
driver to link libasan statically, without necessarily linking
other libraries statically.
- -static-libtsan
-
When the -fsanitize=thread option is used to link a program,
the GCC driver automatically links against libtsan. If
libtsan is available as a shared library, and the -static
option is not used, then this links against the shared version of
libtsan. The -static-libtsan option directs the GCC
driver to link libtsan statically, without necessarily linking
other libraries statically.
- -static-liblsan
-
When the -fsanitize=leak option is used to link a program,
the GCC driver automatically links against liblsan. If
liblsan is available as a shared library, and the -static
option is not used, then this links against the shared version of
liblsan. The -static-liblsan option directs the GCC
driver to link liblsan statically, without necessarily linking
other libraries statically.
- -static-libubsan
-
When the -fsanitize=undefined option is used to link a program,
the GCC driver automatically links against libubsan. If
libubsan is available as a shared library, and the -static
option is not used, then this links against the shared version of
libubsan. The -static-libubsan option directs the GCC
driver to link libubsan statically, without necessarily linking
other libraries statically.
- -static-libmpx
-
When the -fcheck-pointer bounds and -mmpx options are
used to link a program, the GCC driver automatically links against
libmpx. If libmpx is available as a shared library,
and the -static option is not used, then this links against
the shared version of libmpx. The -static-libmpx
option directs the GCC driver to link libmpx statically,
without necessarily linking other libraries statically.
- -static-libmpxwrappers
-
When the -fcheck-pointer bounds and -mmpx options are used
to link a program without also using -fno-chkp-use-wrappers, the
GCC driver automatically links against libmpxwrappers. If
libmpxwrappers is available as a shared library, and the
-static option is not used, then this links against the shared
version of libmpxwrappers. The -static-libmpxwrappers
option directs the GCC driver to link libmpxwrappers statically,
without necessarily linking other libraries statically.
- -static-libstdc++
-
When the g++ program is used to link a C++ program, it
normally automatically links against libstdc++. If
libstdc++ is available as a shared library, and the
-static option is not used, then this links against the
shared version of libstdc++. That is normally fine. However, it
is sometimes useful to freeze the version of libstdc++ used by
the program without going all the way to a fully static link. The
-static-libstdc++ option directs the g++ driver to
link libstdc++ statically, without necessarily linking other
libraries statically.
- -symbolic
-
Bind references to global symbols when building a shared object. Warn
about any unresolved references (unless overridden by the link editor
option -Xlinker -z -Xlinker defs). Only a few systems support
this option.
- -T script
-
Use script as the linker script. This option is supported by most
systems using the GNU linker. On some targets, such as bare-board
targets without an operating system, the -T option may be required
when linking to avoid references to undefined symbols.
- -Xlinker option
-
Pass option as an option to the linker. You can use this to
supply system-specific linker options that GCC does not recognize.
If you want to pass an option that takes a separate argument, you must use
-Xlinker twice, once for the option and once for the argument.
For example, to pass -assert definitions, you must write
-Xlinker -assert -Xlinker definitions. It does not work to write
-Xlinker ``-assert definitions'', because this passes the entire
string as a single argument, which is not what the linker expects.
When using the GNU linker, it is usually more convenient to pass
arguments to linker options using the option=value
syntax than as separate arguments. For example, you can specify
-Xlinker -Map=output.map rather than
-Xlinker -Map -Xlinker output.map. Other linkers may not support
this syntax for command-line options.
- -Wl,option
-
Pass option as an option to the linker. If option contains
commas, it is split into multiple options at the commas. You can use this
syntax to pass an argument to the option.
For example, -Wl,-Map,output.map passes -Map output.map to the
linker. When using the GNU linker, you can also get the same effect with
-Wl,-Map=output.map.
- -u symbol
-
Pretend the symbol symbol is undefined, to force linking of
library modules to define it. You can use -u multiple times with
different symbols to force loading of additional library modules.
- -z keyword
-
-z is passed directly on to the linker along with the keyword
keyword. See the section in the documentation of your linker for
permitted values and their meanings.
Options for Directory Search
These options specify directories to search for header files, for
libraries and for parts of the compiler:
- -Idir
-
Add the directory dir to the head of the list of directories to be
searched for header files. This can be used to override a system header
file, substituting your own version, since these directories are
searched before the system header file directories. However, you should
not use this option to add directories that contain vendor-supplied
system header files (use -isystem for that). If you use more than
one -I option, the directories are scanned in left-to-right
order; the standard system directories come after.
If a standard system include directory, or a directory specified with
-isystem, is also specified with -I, the -I
option is ignored. The directory is still searched but as a
system directory at its normal position in the system include chain.
This is to ensure that GCC's procedure to fix buggy system headers and
the ordering for the "include_next" directive are not inadvertently changed.
If you really need to change the search order for system directories,
use the -nostdinc and/or -isystem options.
- -iplugindir=dir
-
Set the directory to search for plugins that are passed
by -fplugin=name instead of
-fplugin=path/name.so. This option is not meant
to be used by the user, but only passed by the driver.
- -iquotedir
-
Add the directory dir to the head of the list of directories to
be searched for header files only for the case of "#include
"file""; they are not searched for "#include <file>",
otherwise just like -I.
- -Ldir
-
Add directory dir to the list of directories to be searched
for -l.
- -Bprefix
-
This option specifies where to find the executables, libraries,
include files, and data files of the compiler itself.
The compiler driver program runs one or more of the subprograms
cpp, cc1, as and ld. It tries
prefix as a prefix for each program it tries to run, both with and
without machine/version/.
For each subprogram to be run, the compiler driver first tries the
-B prefix, if any. If that name is not found, or if -B
is not specified, the driver tries two standard prefixes,
/usr/lib/gcc/ and /usr/local/lib/gcc/. If neither of
those results in a file name that is found, the unmodified program
name is searched for using the directories specified in your
PATH environment variable.
The compiler checks to see if the path provided by -B
refers to a directory, and if necessary it adds a directory
separator character at the end of the path.
-B prefixes that effectively specify directory names also apply
to libraries in the linker, because the compiler translates these
options into -L options for the linker. They also apply to
include files in the preprocessor, because the compiler translates these
options into -isystem options for the preprocessor. In this case,
the compiler appends include to the prefix.
The runtime support file libgcc.a can also be searched for using
the -B prefix, if needed. If it is not found there, the two
standard prefixes above are tried, and that is all. The file is left
out of the link if it is not found by those means.
Another way to specify a prefix much like the -B prefix is to use
the environment variable GCC_EXEC_PREFIX.
As a special kludge, if the path provided by -B is
[dir/]stageN/, where N is a number in the range 0 to
9, then it is replaced by [dir/]include. This is to help
with boot-strapping the compiler.
- -specs=file
-
Process file after the compiler reads in the standard specs
file, in order to override the defaults which the gcc driver
program uses when determining what switches to pass to cc1,
cc1plus, as, ld, etc. More than one
-specs=file can be specified on the command line, and they
are processed in order, from left to right.
- --sysroot=dir
-
Use dir as the logical root directory for headers and libraries.
For example, if the compiler normally searches for headers in
/usr/include and libraries in /usr/lib, it instead
searches dir/usr/include and dir/usr/lib.
If you use both this option and the -isysroot option, then
the --sysroot option applies to libraries, but the
-isysroot option applies to header files.
The GNU linker (beginning with version 2.16) has the necessary support
for this option. If your linker does not support this option, the
header file aspect of --sysroot still works, but the
library aspect does not.
- --no-sysroot-suffix
-
For some targets, a suffix is added to the root directory specified
with --sysroot, depending on the other options used, so that
headers may for example be found in
dir/suffix/usr/include instead of
dir/usr/include. This option disables the addition of
such a suffix.
- -I-
-
This option has been deprecated. Please use -iquote instead for
-I directories before the -I- and remove the -I-
option.
Any directories you specify with -I options before the -I-
option are searched only for the case of "#include "file"";
they are not searched for "#include <file>".
If additional directories are specified with -I options after
the -I- option, these directories are searched for all "#include"
directives. (Ordinarily all -I directories are used
this way.)
In addition, the -I- option inhibits the use of the current
directory (where the current input file came from) as the first search
directory for "#include "file"". There is no way to
override this effect of -I-. With -I. you can specify
searching the directory that is current when the compiler is
invoked. That is not exactly the same as what the preprocessor does
by default, but it is often satisfactory.
-I- does not inhibit the use of the standard system directories
for header files. Thus, -I- and -nostdinc are
independent.
Specifying Target Machine and Compiler Version
The usual way to run GCC is to run the executable called gcc, or
machine-gcc when cross-compiling, or
machine-gcc-version to run a version other than the
one that was installed last.
Hardware Models and Configurations
Each target machine types can have its own
special options, starting with -m, to choose among various
hardware models or configurations---for example, 68010 vs 68020,
floating coprocessor or none. A single installed version of the
compiler can compile for any model or configuration, according to the
options specified.
Some configurations of the compiler also support additional special
options, usually for compatibility with other compilers on the same
platform.
AArch64 Options
These options are defined for AArch64 implementations:
- -mabi=name
-
Generate code for the specified data model. Permissible values
are ilp32 for SysV-like data model where int, long int and pointer
are 32-bit, and lp64 for SysV-like data model where int is 32-bit,
but long int and pointer are 64-bit.
The default depends on the specific target configuration. Note that
the LP64 and ILP32 ABIs are not link-compatible; you must compile your
entire program with the same ABI, and link with a compatible set of libraries.
- -mbig-endian
-
Generate big-endian code. This is the default when GCC is configured for an
aarch64_be-*-* target.
- -mgeneral-regs-only
-
Generate code which uses only the general registers.
- -mlittle-endian
-
Generate little-endian code. This is the default when GCC is configured for an
aarch64-*-* but not an aarch64_be-*-* target.
- -mcmodel=tiny
-
Generate code for the tiny code model. The program and its statically defined
symbols must be within 1GB of each other. Pointers are 64 bits. Programs can
be statically or dynamically linked. This model is not fully implemented and
mostly treated as small.
- -mcmodel=small
-
Generate code for the small code model. The program and its statically defined
symbols must be within 4GB of each other. Pointers are 64 bits. Programs can
be statically or dynamically linked. This is the default code model.
- -mcmodel=large
-
Generate code for the large code model. This makes no assumptions about
addresses and sizes of sections. Pointers are 64 bits. Programs can be
statically linked only.
- -mstrict-align
-
Do not assume that unaligned memory references are handled by the system.
- -momit-leaf-frame-pointer
-
- -mno-omit-leaf-frame-pointer
-
Omit or keep the frame pointer in leaf functions. The former behaviour is the
default.
- -mtls-dialect=desc
-
Use TLS descriptors as the thread-local storage mechanism for dynamic accesses
of TLS variables. This is the default.
- -mtls-dialect=traditional
-
Use traditional TLS as the thread-local storage mechanism for dynamic accesses
of TLS variables.
- -mfix-cortex-a53-835769
-
- -mno-fix-cortex-a53-835769
-
Enable or disable the workaround for the ARM Cortex-A53 erratum number 835769.
This involves inserting a NOP instruction between memory instructions and
64-bit integer multiply-accumulate instructions.
- -mfix-cortex-a53-843419
-
- -mno-fix-cortex-a53-843419
-
Enable or disable the workaround for the ARM Cortex-A53 erratum number 843419.
This erratum workaround is made at link time and this will only pass the
corresponding flag to the linker.
- -march=name
-
Specify the name of the target architecture, optionally suffixed by one or
more feature modifiers. This option has the form
-march=arch{+[no]feature}*, where the
only permissible value for arch is armv8-a. The permissible
values for feature are documented in the sub-section below.
Where conflicting feature modifiers are specified, the right-most feature is
used.
GCC uses this name to determine what kind of instructions it can emit when
generating assembly code.
Where -march is specified without either of -mtune
or -mcpu also being specified, the code is tuned to perform
well across a range of target processors implementing the target
architecture.
- -mtune=name
-
Specify the name of the target processor for which GCC should tune the
performance of the code. Permissible values for this option are:
generic, cortex-a53, cortex-a57, cortex-a72,
exynos-m1, thunderx, xgene1.
Additionally, this option can specify that GCC should tune the performance
of the code for a big.LITTLE system. Permissible values for this
option are: cortex-a57.cortex-a53, cortex-a72.cortex-a53.
Where none of -mtune=, -mcpu= or -march=
are specified, the code is tuned to perform well across a range
of target processors.
This option cannot be suffixed by feature modifiers.
- -mcpu=name
-
Specify the name of the target processor, optionally suffixed by one or more
feature modifiers. This option has the form
-mcpu=cpu{+[no]feature}*, where the
permissible values for cpu are the same as those available for
-mtune.
The permissible values for feature are documented in the sub-section
below.
Where conflicting feature modifiers are specified, the right-most feature is
used.
GCC uses this name to determine what kind of instructions it can emit when
generating assembly code (as if by -march) and to determine
the target processor for which to tune for performance (as if
by -mtune). Where this option is used in conjunction
with -march or -mtune, those options take precedence
over the appropriate part of this option.
-march and -mcpu Feature Modifiers
Feature modifiers used with -march and -mcpu can be one
the following:
- crc
-
Enable CRC extension.
- crypto
-
Enable Crypto extension. This implies Advanced SIMD is enabled.
- fp
-
Enable floating-point instructions.
- simd
-
Enable Advanced SIMD instructions. This implies floating-point instructions
are enabled. This is the default for all current possible values for options
-march and -mcpu=.
Adapteva Epiphany Options
These -m options are defined for Adapteva Epiphany:
- -mhalf-reg-file
-
Don't allocate any register in the range "r32"..."r63".
That allows code to run on hardware variants that lack these registers.
- -mprefer-short-insn-regs
-
Preferrentially allocate registers that allow short instruction generation.
This can result in increased instruction count, so this may either reduce or
increase overall code size.
- -mbranch-cost=num
-
Set the cost of branches to roughly num ``simple'' instructions.
This cost is only a heuristic and is not guaranteed to produce
consistent results across releases.
- -mcmove
-
Enable the generation of conditional moves.
- -mnops=num
-
Emit num NOPs before every other generated instruction.
- -mno-soft-cmpsf
-
For single-precision floating-point comparisons, emit an "fsub" instruction
and test the flags. This is faster than a software comparison, but can
get incorrect results in the presence of NaNs, or when two different small
numbers are compared such that their difference is calculated as zero.
The default is -msoft-cmpsf, which uses slower, but IEEE-compliant,
software comparisons.
- -mstack-offset=num
-
Set the offset between the top of the stack and the stack pointer.
E.g., a value of 8 means that the eight bytes in the range "sp+0...sp+7"
can be used by leaf functions without stack allocation.
Values other than 8 or 16 are untested and unlikely to work.
Note also that this option changes the ABI; compiling a program with a
different stack offset than the libraries have been compiled with
generally does not work.
This option can be useful if you want to evaluate if a different stack
offset would give you better code, but to actually use a different stack
offset to build working programs, it is recommended to configure the
toolchain with the appropriate --with-stack-offset=num option.
- -mno-round-nearest
-
Make the scheduler assume that the rounding mode has been set to
truncating. The default is -mround-nearest.
- -mlong-calls
-
If not otherwise specified by an attribute, assume all calls might be beyond
the offset range of the "b" / "bl" instructions, and therefore load the
function address into a register before performing a (otherwise direct) call.
This is the default.
- -mshort-calls
-
If not otherwise specified by an attribute, assume all direct calls are
in the range of the "b" / "bl" instructions, so use these instructions
for direct calls. The default is -mlong-calls.
- -msmall16
-
Assume addresses can be loaded as 16-bit unsigned values. This does not
apply to function addresses for which -mlong-calls semantics
are in effect.
- -mfp-mode=mode
-
Set the prevailing mode of the floating-point unit.
This determines the floating-point mode that is provided and expected
at function call and return time. Making this mode match the mode you
predominantly need at function start can make your programs smaller and
faster by avoiding unnecessary mode switches.
mode can be set to one the following values:
-
- caller
-
Any mode at function entry is valid, and retained or restored when
the function returns, and when it calls other functions.
This mode is useful for compiling libraries or other compilation units
you might want to incorporate into different programs with different
prevailing FPU modes, and the convenience of being able to use a single
object file outweighs the size and speed overhead for any extra
mode switching that might be needed, compared with what would be needed
with a more specific choice of prevailing FPU mode.
- truncate
-
This is the mode used for floating-point calculations with
truncating (i.e. round towards zero) rounding mode. That includes
conversion from floating point to integer.
- round-nearest
-
This is the mode used for floating-point calculations with
round-to-nearest-or-even rounding mode.
- int
-
This is the mode used to perform integer calculations in the FPU, e.g.
integer multiply, or integer multiply-and-accumulate.
-
The default is -mfp-mode=caller
- -mnosplit-lohi
-
- -mno-postinc
-
- -mno-postmodify
-
Code generation tweaks that disable, respectively, splitting of 32-bit
loads, generation of post-increment addresses, and generation of
post-modify addresses. The defaults are msplit-lohi,
-mpost-inc, and -mpost-modify.
- -mnovect-double
-
Change the preferred SIMD mode to SImode. The default is
-mvect-double, which uses DImode as preferred SIMD mode.
- -max-vect-align=num
-
The maximum alignment for SIMD vector mode types.
num may be 4 or 8. The default is 8.
Note that this is an ABI change, even though many library function
interfaces are unaffected if they don't use SIMD vector modes
in places that affect size and/or alignment of relevant types.
- -msplit-vecmove-early
-
Split vector moves into single word moves before reload. In theory this
can give better register allocation, but so far the reverse seems to be
generally the case.
- -m1reg-reg
-
Specify a register to hold the constant -1, which makes loading small negative
constants and certain bitmasks faster.
Allowable values for reg are r43 and r63,
which specify use of that register as a fixed register,
and none, which means that no register is used for this
purpose. The default is -m1reg-none.
ARC Options
The following options control the architecture variant for which code
is being compiled:
- -mbarrel-shifter
-
Generate instructions supported by barrel shifter. This is the default
unless -mcpu=ARC601 is in effect.
- -mcpu=cpu
-
Set architecture type, register usage, and instruction scheduling
parameters for cpu. There are also shortcut alias options
available for backward compatibility and convenience. Supported
values for cpu are
-
- ARC600
-
Compile for ARC600. Aliases: -mA6, -mARC600.
- ARC601
-
Compile for ARC601. Alias: -mARC601.
- ARC700
-
Compile for ARC700. Aliases: -mA7, -mARC700.
This is the default when configured with --with-cpu=arc700.
-
- -mdpfp
-
- -mdpfp-compact
-
FPX: Generate Double Precision FPX instructions, tuned for the compact
implementation.
- -mdpfp-fast
-
FPX: Generate Double Precision FPX instructions, tuned for the fast
implementation.
- -mno-dpfp-lrsr
-
Disable LR and SR instructions from using FPX extension aux registers.
- -mea
-
Generate Extended arithmetic instructions. Currently only
"divaw", "adds", "subs", and "sat16" are
supported. This is always enabled for -mcpu=ARC700.
- -mno-mpy
-
Do not generate mpy instructions for ARC700.
- -mmul32x16
-
Generate 32x16 bit multiply and mac instructions.
- -mmul64
-
Generate mul64 and mulu64 instructions. Only valid for -mcpu=ARC600.
- -mnorm
-
Generate norm instruction. This is the default if -mcpu=ARC700
is in effect.
- -mspfp
-
- -mspfp-compact
-
FPX: Generate Single Precision FPX instructions, tuned for the compact
implementation.
- -mspfp-fast
-
FPX: Generate Single Precision FPX instructions, tuned for the fast
implementation.
- -msimd
-
Enable generation of ARC SIMD instructions via target-specific
builtins. Only valid for -mcpu=ARC700.
- -msoft-float
-
This option ignored; it is provided for compatibility purposes only.
Software floating point code is emitted by default, and this default
can overridden by FPX options; mspfp, mspfp-compact, or
mspfp-fast for single precision, and mdpfp,
mdpfp-compact, or mdpfp-fast for double precision.
- -mswap
-
Generate swap instructions.
The following options are passed through to the assembler, and also
define preprocessor macro symbols.
- -mdsp-packa
-
Passed down to the assembler to enable the DSP Pack A extensions.
Also sets the preprocessor symbol "__Xdsp_packa".
- -mdvbf
-
Passed down to the assembler to enable the dual viterbi butterfly
extension. Also sets the preprocessor symbol "__Xdvbf".
- -mlock
-
Passed down to the assembler to enable the Locked Load/Store
Conditional extension. Also sets the preprocessor symbol
"__Xlock".
- -mmac-d16
-
Passed down to the assembler. Also sets the preprocessor symbol
"__Xxmac_d16".
- -mmac-24
-
Passed down to the assembler. Also sets the preprocessor symbol
"__Xxmac_24".
- -mrtsc
-
Passed down to the assembler to enable the 64-bit Time-Stamp Counter
extension instruction. Also sets the preprocessor symbol
"__Xrtsc".
- -mswape
-
Passed down to the assembler to enable the swap byte ordering
extension instruction. Also sets the preprocessor symbol
"__Xswape".
- -mtelephony
-
Passed down to the assembler to enable dual and single operand
instructions for telephony. Also sets the preprocessor symbol
"__Xtelephony".
- -mxy
-
Passed down to the assembler to enable the XY Memory extension. Also
sets the preprocessor symbol "__Xxy".
The following options control how the assembly code is annotated:
- -misize
-
Annotate assembler instructions with estimated addresses.
- -mannotate-align
-
Explain what alignment considerations lead to the decision to make an
instruction short or long.
The following options are passed through to the linker:
- -marclinux
-
Passed through to the linker, to specify use of the "arclinux" emulation.
This option is enabled by default in tool chains built for
"arc-linux-uclibc" and "arceb-linux-uclibc" targets
when profiling is not requested.
- -marclinux_prof
-
Passed through to the linker, to specify use of the
"arclinux_prof" emulation. This option is enabled by default in
tool chains built for "arc-linux-uclibc" and
"arceb-linux-uclibc" targets when profiling is requested.
The following options control the semantics of generated code:
- -mepilogue-cfi
-
Enable generation of call frame information for epilogues.
- -mno-epilogue-cfi
-
Disable generation of call frame information for epilogues.
- -mlong-calls
-
Generate call insns as register indirect calls, thus providing access
to the full 32-bit address range.
- -mmedium-calls
-
Don't use less than 25 bit addressing range for calls, which is the
offset available for an unconditional branch-and-link
instruction. Conditional execution of function calls is suppressed, to
allow use of the 25-bit range, rather than the 21-bit range with
conditional branch-and-link. This is the default for tool chains built
for "arc-linux-uclibc" and "arceb-linux-uclibc" targets.
- -mno-sdata
-
Do not generate sdata references. This is the default for tool chains
built for "arc-linux-uclibc" and "arceb-linux-uclibc"
targets.
- -mucb-mcount
-
Instrument with mcount calls as used in UCB code. I.e. do the
counting in the callee, not the caller. By default ARC instrumentation
counts in the caller.
- -mvolatile-cache
-
Use ordinarily cached memory accesses for volatile references. This is the
default.
- -mno-volatile-cache
-
Enable cache bypass for volatile references.
The following options fine tune code generation:
- -malign-call
-
Do alignment optimizations for call instructions.
- -mauto-modify-reg
-
Enable the use of pre/post modify with register displacement.
- -mbbit-peephole
-
Enable bbit peephole2.
- -mno-brcc
-
This option disables a target-specific pass in arc_reorg to
generate "BRcc" instructions. It has no effect on "BRcc"
generation driven by the combiner pass.
- -mcase-vector-pcrel
-
Use pc-relative switch case tables - this enables case table shortening.
This is the default for -Os.
- -mcompact-casesi
-
Enable compact casesi pattern.
This is the default for -Os.
- -mno-cond-exec
-
Disable ARCompact specific pass to generate conditional execution instructions.
Due to delay slot scheduling and interactions between operand numbers,
literal sizes, instruction lengths, and the support for conditional execution,
the target-independent pass to generate conditional execution is often lacking,
so the ARC port has kept a special pass around that tries to find more
conditional execution generating opportunities after register allocation,
branch shortening, and delay slot scheduling have been done. This pass
generally, but not always, improves performance and code size, at the cost of
extra compilation time, which is why there is an option to switch it off.
If you have a problem with call instructions exceeding their allowable
offset range because they are conditionalized, you should consider using
-mmedium-calls instead.
- -mearly-cbranchsi
-
Enable pre-reload use of the cbranchsi pattern.
- -mexpand-adddi
-
Expand "adddi3" and "subdi3" at rtl generation time into
"add.f", "adc" etc.
- -mindexed-loads
-
Enable the use of indexed loads. This can be problematic because some
optimizers then assume that indexed stores exist, which is not
the case.
- -mlra
-
Enable Local Register Allocation. This is still experimental for ARC,
so by default the compiler uses standard reload
(i.e. -mno-lra).
- -mlra-priority-none
-
Don't indicate any priority for target registers.
- -mlra-priority-compact
-
Indicate target register priority for r0..r3 / r12..r15.
- -mlra-priority-noncompact
-
Reduce target regsiter priority for r0..r3 / r12..r15.
- -mno-millicode
-
When optimizing for size (using -Os), prologues and epilogues
that have to save or restore a large number of registers are often
shortened by using call to a special function in libgcc; this is
referred to as a millicode call. As these calls can pose
performance issues, and/or cause linking issues when linking in a
nonstandard way, this option is provided to turn off millicode call
generation.
- -mmixed-code
-
Tweak register allocation to help 16-bit instruction generation.
This generally has the effect of decreasing the average instruction size
while increasing the instruction count.
- -mq-class
-
Enable 'q' instruction alternatives.
This is the default for -Os.
- -mRcq
-
Enable Rcq constraint handling - most short code generation depends on this.
This is the default.
- -mRcw
-
Enable Rcw constraint handling - ccfsm condexec mostly depends on this.
This is the default.
- -msize-level=level
-
Fine-tune size optimization with regards to instruction lengths and alignment.
The recognized values for level are:
-
- 0
-
No size optimization. This level is deprecated and treated like 1.
- 1
-
Short instructions are used opportunistically.
- 2
-
In addition, alignment of loops and of code after barriers are dropped.
- 3
-
In addition, optional data alignment is dropped, and the option Os is enabled.
-
This defaults to 3 when -Os is in effect. Otherwise,
the behavior when this is not set is equivalent to level 1.
- -mtune=cpu
-
Set instruction scheduling parameters for cpu, overriding any implied
by -mcpu=.
Supported values for cpu are
-
- ARC600
-
Tune for ARC600 cpu.
- ARC601
-
Tune for ARC601 cpu.
- ARC700
-
Tune for ARC700 cpu with standard multiplier block.
- ARC700-xmac
-
Tune for ARC700 cpu with XMAC block.
- ARC725D
-
Tune for ARC725D cpu.
- ARC750D
-
Tune for ARC750D cpu.
-
- -mmultcost=num
-
Cost to assume for a multiply instruction, with 4 being equal to a
normal instruction.
- -munalign-prob-threshold=probability
-
Set probability threshold for unaligning branches.
When tuning for ARC700 and optimizing for speed, branches without
filled delay slot are preferably emitted unaligned and long, unless
profiling indicates that the probability for the branch to be taken
is below probability.
The default is (REG_BR_PROB_BASE/2), i.e. 5000.
The following options are maintained for backward compatibility, but
are now deprecated and will be removed in a future release:
- -margonaut
-
Obsolete FPX.
- -mbig-endian
-
- -EB
-
Compile code for big endian targets. Use of these options is now
deprecated. Users wanting big-endian code, should use the
"arceb-elf32" and "arceb-linux-uclibc" targets when
building the tool chain, for which big-endian is the default.
- -mlittle-endian
-
- -EL
-
Compile code for little endian targets. Use of these options is now
deprecated. Users wanting little-endian code should use the
"arc-elf32" and "arc-linux-uclibc" targets when
building the tool chain, for which little-endian is the default.
- -mbarrel_shifter
-
Replaced by -mbarrel-shifter.
- -mdpfp_compact
-
Replaced by -mdpfp-compact.
- -mdpfp_fast
-
Replaced by -mdpfp-fast.
- -mdsp_packa
-
Replaced by -mdsp-packa.
- -mEA
-
Replaced by -mea.
- -mmac_24
-
Replaced by -mmac-24.
- -mmac_d16
-
Replaced by -mmac-d16.
- -mspfp_compact
-
Replaced by -mspfp-compact.
- -mspfp_fast
-
Replaced by -mspfp-fast.
- -mtune=cpu
-
Values arc600, arc601, arc700 and
arc700-xmac for cpu are replaced by ARC600,
ARC601, ARC700 and ARC700-xmac respectively
- -multcost=num
-
Replaced by -mmultcost.
ARM Options
These -m options are defined for the ARM port:
- -mabi=name
-
Generate code for the specified ABI. Permissible values are: apcs-gnu,
atpcs, aapcs, aapcs-linux and iwmmxt.
- -mapcs-frame
-
Generate a stack frame that is compliant with the ARM Procedure Call
Standard for all functions, even if this is not strictly necessary for
correct execution of the code. Specifying -fomit-frame-pointer
with this option causes the stack frames not to be generated for
leaf functions. The default is -mno-apcs-frame.
This option is deprecated.
- -mapcs
-
This is a synonym for -mapcs-frame and is deprecated.
- -mthumb-interwork
-
Generate code that supports calling between the ARM and Thumb
instruction sets. Without this option, on pre-v5 architectures, the
two instruction sets cannot be reliably used inside one program. The
default is -mno-thumb-interwork, since slightly larger code
is generated when -mthumb-interwork is specified. In AAPCS
configurations this option is meaningless.
- -mno-sched-prolog
-
Prevent the reordering of instructions in the function prologue, or the
merging of those instruction with the instructions in the function's
body. This means that all functions start with a recognizable set
of instructions (or in fact one of a choice from a small set of
different function prologues), and this information can be used to
locate the start of functions inside an executable piece of code. The
default is -msched-prolog.
- -mfloat-abi=name
-
Specifies which floating-point ABI to use. Permissible values
are: soft, softfp and hard.
Specifying soft causes GCC to generate output containing
library calls for floating-point operations.
softfp allows the generation of code using hardware floating-point
instructions, but still uses the soft-float calling conventions.
hard allows generation of floating-point instructions
and uses FPU-specific calling conventions.
The default depends on the specific target configuration. Note that
the hard-float and soft-float ABIs are not link-compatible; you must
compile your entire program with the same ABI, and link with a
compatible set of libraries.
- -mlittle-endian
-
Generate code for a processor running in little-endian mode. This is
the default for all standard configurations.
- -mbig-endian
-
Generate code for a processor running in big-endian mode; the default is
to compile code for a little-endian processor.
- -march=name
-
This specifies the name of the target ARM architecture. GCC uses this
name to determine what kind of instructions it can emit when generating
assembly code. This option can be used in conjunction with or instead
of the -mcpu= option. Permissible names are: armv2,
armv2a, armv3, armv3m, armv4, armv4t,
armv5, armv5t, armv5e, armv5te,
armv6, armv6j,
armv6t2, armv6z, armv6zk, armv6-m,
armv7, armv7-a, armv7-r, armv7-m, armv7e-m,
armv7ve, armv8-a, armv8-a+crc,
iwmmxt, iwmmxt2, ep9312.
-march=armv7ve is the armv7-a architecture with virtualization
extensions.
-march=armv8-a+crc enables code generation for the ARMv8-A
architecture together with the optional CRC32 extensions.
-march=native causes the compiler to auto-detect the architecture
of the build computer. At present, this feature is only supported on
GNU/Linux, and not all architectures are recognized. If the auto-detect
is unsuccessful the option has no effect.
- -mtune=name
-
This option specifies the name of the target ARM processor for
which GCC should tune the performance of the code.
For some ARM implementations better performance can be obtained by using
this option.
Permissible names are: arm2, arm250,
arm3, arm6, arm60, arm600, arm610,
arm620, arm7, arm7m, arm7d, arm7dm,
arm7di, arm7dmi, arm70, arm700,
arm700i, arm710, arm710c, arm7100,
arm720,
arm7500, arm7500fe, arm7tdmi, arm7tdmi-s,
arm710t, arm720t, arm740t,
strongarm, strongarm110, strongarm1100,
strongarm1110,
arm8, arm810, arm9, arm9e, arm920,
arm920t, arm922t, arm946e-s, arm966e-s,
arm968e-s, arm926ej-s, arm940t, arm9tdmi,
arm10tdmi, arm1020t, arm1026ej-s,
arm10e, arm1020e, arm1022e,
arm1136j-s, arm1136jf-s, mpcore, mpcorenovfp,
arm1156t2-s, arm1156t2f-s, arm1176jz-s, arm1176jzf-s,
cortex-a5, cortex-a7, cortex-a8, cortex-a9,
cortex-a12, cortex-a15, cortex-a53,
cortex-a57, cortex-a72,
cortex-r4,
cortex-r4f, cortex-r5, cortex-r7, cortex-m7,
cortex-m4,
cortex-m3,
cortex-m1,
cortex-m0,
cortex-m0plus,
cortex-m1.small-multiply,
cortex-m0.small-multiply,
cortex-m0plus.small-multiply,
exynos-m1,
marvell-pj4,
xscale, iwmmxt, iwmmxt2, ep9312,
fa526, fa626,
fa606te, fa626te, fmp626, fa726te,
xgene1.
Additionally, this option can specify that GCC should tune the performance
of the code for a big.LITTLE system. Permissible names are:
cortex-a15.cortex-a7, cortex-a57.cortex-a53,
cortex-a72.cortex-a53.
-mtune=generic-arch specifies that GCC should tune the
performance for a blend of processors within architecture arch.
The aim is to generate code that run well on the current most popular
processors, balancing between optimizations that benefit some CPUs in the
range, and avoiding performance pitfalls of other CPUs. The effects of
this option may change in future GCC versions as CPU models come and go.
-mtune=native causes the compiler to auto-detect the CPU
of the build computer. At present, this feature is only supported on
GNU/Linux, and not all architectures are recognized. If the auto-detect is
unsuccessful the option has no effect.
- -mcpu=name
-
This specifies the name of the target ARM processor. GCC uses this name
to derive the name of the target ARM architecture (as if specified
by -march) and the ARM processor type for which to tune for
performance (as if specified by -mtune). Where this option
is used in conjunction with -march or -mtune,
those options take precedence over the appropriate part of this option.
Permissible names for this option are the same as those for
-mtune.
-mcpu=generic-arch is also permissible, and is
equivalent to -march=arch -mtune=generic-arch.
See -mtune for more information.
-mcpu=native causes the compiler to auto-detect the CPU
of the build computer. At present, this feature is only supported on
GNU/Linux, and not all architectures are recognized. If the auto-detect
is unsuccessful the option has no effect.
- -mfpu=name
-
This specifies what floating-point hardware (or hardware emulation) is
available on the target. Permissible names are: vfp, vfpv3,
vfpv3-fp16, vfpv3-d16, vfpv3-d16-fp16, vfpv3xd,
vfpv3xd-fp16, neon, neon-fp16, vfpv4,
vfpv4-d16, fpv4-sp-d16, neon-vfpv4,
fpv5-d16, fpv5-sp-d16,
fp-armv8, neon-fp-armv8, and crypto-neon-fp-armv8.
If -msoft-float is specified this specifies the format of
floating-point values.
If the selected floating-point hardware includes the NEON extension
(e.g. -mfpu=neon), note that floating-point
operations are not generated by GCC's auto-vectorization pass unless
-funsafe-math-optimizations is also specified. This is
because NEON hardware does not fully implement the IEEE 754 standard for
floating-point arithmetic (in particular denormal values are treated as
zero), so the use of NEON instructions may lead to a loss of precision.
- -mfp16-format=name
-
Specify the format of the "__fp16" half-precision floating-point type.
Permissible names are none, ieee, and alternative;
the default is none, in which case the "__fp16" type is not
defined.
- -mstructure-size-boundary=n
-
The sizes of all structures and unions are rounded up to a multiple
of the number of bits set by this option. Permissible values are 8, 32
and 64. The default value varies for different toolchains. For the COFF
targeted toolchain the default value is 8. A value of 64 is only allowed
if the underlying ABI supports it.
Specifying a larger number can produce faster, more efficient code, but
can also increase the size of the program. Different values are potentially
incompatible. Code compiled with one value cannot necessarily expect to
work with code or libraries compiled with another value, if they exchange
information using structures or unions.
- -mabort-on-noreturn
-
Generate a call to the function "abort" at the end of a
"noreturn" function. It is executed if the function tries to
return.
- -mlong-calls
-
- -mno-long-calls
-
Tells the compiler to perform function calls by first loading the
address of the function into a register and then performing a subroutine
call on this register. This switch is needed if the target function
lies outside of the 64-megabyte addressing range of the offset-based
version of subroutine call instruction.
Even if this switch is enabled, not all function calls are turned
into long calls. The heuristic is that static functions, functions
that have the "short_call" attribute, functions that are inside
the scope of a "#pragma no_long_calls" directive, and functions whose
definitions have already been compiled within the current compilation
unit are not turned into long calls. The exceptions to this rule are
that weak function definitions, functions with the "long_call"
attribute or the "section" attribute, and functions that are within
the scope of a "#pragma long_calls" directive are always
turned into long calls.
This feature is not enabled by default. Specifying
-mno-long-calls restores the default behavior, as does
placing the function calls within the scope of a "#pragma
long_calls_off" directive. Note these switches have no effect on how
the compiler generates code to handle function calls via function
pointers.
- -msingle-pic-base
-
Treat the register used for PIC addressing as read-only, rather than
loading it in the prologue for each function. The runtime system is
responsible for initializing this register with an appropriate value
before execution begins.
- -mpic-register=reg
-
Specify the register to be used for PIC addressing.
For standard PIC base case, the default is any suitable register
determined by compiler. For single PIC base case, the default is
R9 if target is EABI based or stack-checking is enabled,
otherwise the default is R10.
- -mpic-data-is-text-relative
-
Assume that each data segments are relative to text segment at load time.
Therefore, it permits addressing data using PC-relative operations.
This option is on by default for targets other than VxWorks RTP.
- -mpoke-function-name
-
Write the name of each function into the text section, directly
preceding the function prologue. The generated code is similar to this:
t0
.ascii "arm_poke_function_name", 0
.align
t1
.word 0xff000000 + (t1 - t0)
arm_poke_function_name
mov ip, sp
stmfd sp!, {fp, ip, lr, pc}
sub fp, ip, #4
When performing a stack backtrace, code can inspect the value of
"pc" stored at "fp + 0". If the trace function then looks at
location "pc - 12" and the top 8 bits are set, then we know that
there is a function name embedded immediately preceding this location
and has length "((pc[-3]) & 0xff000000)".
- -mthumb
-
- -marm
-
Select between generating code that executes in ARM and Thumb
states. The default for most configurations is to generate code
that executes in ARM state, but the default can be changed by
configuring GCC with the --with-mode=state
configure option.
- -mtpcs-frame
-
Generate a stack frame that is compliant with the Thumb Procedure Call
Standard for all non-leaf functions. (A leaf function is one that does
not call any other functions.) The default is -mno-tpcs-frame.
- -mtpcs-leaf-frame
-
Generate a stack frame that is compliant with the Thumb Procedure Call
Standard for all leaf functions. (A leaf function is one that does
not call any other functions.) The default is -mno-apcs-leaf-frame.
- -mcallee-super-interworking
-
Gives all externally visible functions in the file being compiled an ARM
instruction set header which switches to Thumb mode before executing the
rest of the function. This allows these functions to be called from
non-interworking code. This option is not valid in AAPCS configurations
because interworking is enabled by default.
- -mcaller-super-interworking
-
Allows calls via function pointers (including virtual functions) to
execute correctly regardless of whether the target code has been
compiled for interworking or not. There is a small overhead in the cost
of executing a function pointer if this option is enabled. This option
is not valid in AAPCS configurations because interworking is enabled
by default.
- -mtp=name
-
Specify the access model for the thread local storage pointer. The valid
models are soft, which generates calls to "__aeabi_read_tp",
cp15, which fetches the thread pointer from "cp15" directly
(supported in the arm6k architecture), and auto, which uses the
best available method for the selected processor. The default setting is
auto.
- -mtls-dialect=dialect
-
Specify the dialect to use for accessing thread local storage. Two
dialects are supported---gnu and gnu2. The
gnu dialect selects the original GNU scheme for supporting
local and global dynamic TLS models. The gnu2 dialect
selects the GNU descriptor scheme, which provides better performance
for shared libraries. The GNU descriptor scheme is compatible with
the original scheme, but does require new assembler, linker and
library support. Initial and local exec TLS models are unaffected by
this option and always use the original scheme.
- -mword-relocations
-
Only generate absolute relocations on word-sized values (i.e. R_ARM_ABS32).
This is enabled by default on targets (uClinux, SymbianOS) where the runtime
loader imposes this restriction, and when -fpic or -fPIC
is specified.
- -mfix-cortex-m3-ldrd
-
Some Cortex-M3 cores can cause data corruption when "ldrd" instructions
with overlapping destination and base registers are used. This option avoids
generating these instructions. This option is enabled by default when
-mcpu=cortex-m3 is specified.
- -munaligned-access
-
- -mno-unaligned-access
-
Enables (or disables) reading and writing of 16- and 32- bit values
from addresses that are not 16- or 32- bit aligned. By default
unaligned access is disabled for all pre-ARMv6 and all ARMv6-M
architectures, and enabled for all other architectures. If unaligned
access is not enabled then words in packed data structures are
accessed a byte at a time.
The ARM attribute "Tag_CPU_unaligned_access" is set in the
generated object file to either true or false, depending upon the
setting of this option. If unaligned access is enabled then the
preprocessor symbol "__ARM_FEATURE_UNALIGNED" is also
defined.
- -mneon-for-64bits
-
Enables using Neon to handle scalar 64-bits operations. This is
disabled by default since the cost of moving data from core registers
to Neon is high.
- -mslow-flash-data
-
Assume loading data from flash is slower than fetching instruction.
Therefore literal load is minimized for better performance.
This option is only supported when compiling for ARMv7 M-profile and
off by default.
- -masm-syntax-unified
-
Assume inline assembler is using unified asm syntax. The default is
currently off which implies divided syntax. Currently this option is
available only for Thumb1 and has no effect on ARM state and Thumb2.
However, this may change in future releases of GCC. Divided syntax
should be considered deprecated.
- -mrestrict-it
-
Restricts generation of IT blocks to conform to the rules of ARMv8.
IT blocks can only contain a single 16-bit instruction from a select
set of instructions. This option is on by default for ARMv8 Thumb mode.
- -mprint-tune-info
-
Print CPU tuning information as comment in assembler file. This is
an option used only for regression testing of the compiler and not
intended for ordinary use in compiling code. This option is disabled
by default.
AVR Options
These options are defined for AVR implementations:
- -mmcu=mcu
-
Specify Atmel AVR instruction set architectures (ISA) or MCU type.
The default for this option is@tie{}avr2.
GCC supports the following AVR devices and ISAs:
-
- avr2
-
``Classic'' devices with up to 8@tie{}KiB of program memory.
mcu@tie{}= "attiny22", "attiny26", "at90c8534", "at90s2313", "at90s2323", "at90s2333", "at90s2343", "at90s4414", "at90s4433", "at90s4434", "at90s8515", "at90s8535".
- avr25
-
``Classic'' devices with up to 8@tie{}KiB of program memory and with the "MOVW" instruction.
mcu@tie{}= "ata5272", "ata6616c", "attiny13", "attiny13a", "attiny2313", "attiny2313a", "attiny24", "attiny24a", "attiny25", "attiny261", "attiny261a", "attiny43u", "attiny4313", "attiny44", "attiny44a", "attiny441", "attiny45", "attiny461", "attiny461a", "attiny48", "attiny828", "attiny84", "attiny84a", "attiny841", "attiny85", "attiny861", "attiny861a", "attiny87", "attiny88", "at86rf401".
- avr3
-
``Classic'' devices with 16@tie{}KiB up to 64@tie{}KiB of program memory.
mcu@tie{}= "at43usb355", "at76c711".
- avr31
-
``Classic'' devices with 128@tie{}KiB of program memory.
mcu@tie{}= "atmega103", "at43usb320".
- avr35
-
``Classic'' devices with 16@tie{}KiB up to 64@tie{}KiB of program memory and with the "MOVW" instruction.
mcu@tie{}= "ata5505", "ata6617c", "ata664251", "atmega16u2", "atmega32u2", "atmega8u2", "attiny1634", "attiny167", "at90usb162", "at90usb82".
- avr4
-
``Enhanced'' devices with up to 8@tie{}KiB of program memory.
mcu@tie{}= "ata6285", "ata6286", "ata6289", "ata6612c", "atmega48", "atmega48a", "atmega48p", "atmega48pa", "atmega8", "atmega8a", "atmega8hva", "atmega8515", "atmega8535", "atmega88", "atmega88a", "atmega88p", "atmega88pa", "at90pwm1", "at90pwm2", "at90pwm2b", "at90pwm3", "at90pwm3b", "at90pwm81".
- avr5
-
``Enhanced'' devices with 16@tie{}KiB up to 64@tie{}KiB of program memory.
mcu@tie{}= "ata5702m322", "ata5782", "ata5790", "ata5790n", "ata5795", "ata5831", "ata6613c", "ata6614q", "atmega16", "atmega16a", "atmega16hva", "atmega16hva2", "atmega16hvb", "atmega16hvbrevb", "atmega16m1", "atmega16u4", "atmega161", "atmega162", "atmega163", "atmega164a", "atmega164p", "atmega164pa", "atmega165", "atmega165a", "atmega165p", "atmega165pa", "atmega168", "atmega168a", "atmega168p", "atmega168pa", "atmega169", "atmega169a", "atmega169p", "atmega169pa", "atmega32", "atmega32a", "atmega32c1", "atmega32hvb", "atmega32hvbrevb", "atmega32m1", "atmega32u4", "atmega32u6", "atmega323", "atmega324a", "atmega324p", "atmega324pa", "atmega325", "atmega325a", "atmega325p", "atmega325pa", "atmega3250", "atmega3250a", "atmega3250p", "atmega3250pa", "atmega328", "atmega328p", "atmega329", "atmega329a", "atmega329p", "atmega329pa", "atmega3290", "atmega3290a", "atmega3290p", "atmega3290pa", "atmega406", "atmega64", "atmega64a", "atmega64c1", "atmega64hve", "atmega64hve2", "atmega64m1", "atmega64rfr2", "atmega640", "atmega644", "atmega644a", "atmega644p", "atmega644pa", "atmega644rfr2", "atmega645", "atmega645a", "atmega645p", "atmega6450", "atmega6450a", "atmega6450p", "atmega649", "atmega649a", "atmega649p", "atmega6490", "atmega6490a", "atmega6490p", "at90can32", "at90can64", "at90pwm161", "at90pwm216", "at90pwm316", "at90scr100", "at90usb646", "at90usb647", "at94k", "m3000".
- avr51
-
``Enhanced'' devices with 128@tie{}KiB of program memory.
mcu@tie{}= "atmega128", "atmega128a", "atmega128rfa1", "atmega128rfr2", "atmega1280", "atmega1281", "atmega1284", "atmega1284p", "atmega1284rfr2", "at90can128", "at90usb1286", "at90usb1287".
- avr6
-
``Enhanced'' devices with 3-byte PC, i.e. with more than 128@tie{}KiB of program memory.
mcu@tie{}= "atmega256rfr2", "atmega2560", "atmega2561", "atmega2564rfr2".
- avrxmega2
-
``XMEGA'' devices with more than 8@tie{}KiB and up to 64@tie{}KiB of program memory.
mcu@tie{}= "atxmega16a4", "atxmega16a4u", "atxmega16c4", "atxmega16d4", "atxmega16e5", "atxmega32a4", "atxmega32a4u", "atxmega32c3", "atxmega32c4", "atxmega32d3", "atxmega32d4", "atxmega32e5", "atxmega8e5".
- avrxmega4
-
``XMEGA'' devices with more than 64@tie{}KiB and up to 128@tie{}KiB of program memory.
mcu@tie{}= "atxmega64a3", "atxmega64a3u", "atxmega64a4u", "atxmega64b1", "atxmega64b3", "atxmega64c3", "atxmega64d3", "atxmega64d4".
- avrxmega5
-
``XMEGA'' devices with more than 64@tie{}KiB and up to 128@tie{}KiB of program memory and more than 64@tie{}KiB of RAM.
mcu@tie{}= "atxmega64a1", "atxmega64a1u".
- avrxmega6
-
``XMEGA'' devices with more than 128@tie{}KiB of program memory.
mcu@tie{}= "atxmega128a3", "atxmega128a3u", "atxmega128b1", "atxmega128b3", "atxmega128c3", "atxmega128d3", "atxmega128d4", "atxmega192a3", "atxmega192a3u", "atxmega192c3", "atxmega192d3", "atxmega256a3", "atxmega256a3b", "atxmega256a3bu", "atxmega256a3u", "atxmega256c3", "atxmega256d3", "atxmega384c3", "atxmega384d3".
- avrxmega7
-
``XMEGA'' devices with more than 128@tie{}KiB of program memory and more than 64@tie{}KiB of RAM.
mcu@tie{}= "atxmega128a1", "atxmega128a1u", "atxmega128a4u".
- avrtiny
-
``TINY'' Tiny core devices with 512@tie{}B up to 4@tie{}KiB of program memory.
mcu@tie{}= "attiny10", "attiny20", "attiny4", "attiny40", "attiny5", "attiny9".
- avr1
-
This ISA is implemented by the minimal AVR core and supported for assembler only.
mcu@tie{}= "attiny11", "attiny12", "attiny15", "attiny28", "at90s1200".
-
- -maccumulate-args
-
Accumulate outgoing function arguments and acquire/release the needed
stack space for outgoing function arguments once in function
prologue/epilogue. Without this option, outgoing arguments are pushed
before calling a function and popped afterwards.
Popping the arguments after the function call can be expensive on
AVR so that accumulating the stack space might lead to smaller
executables because arguments need not to be removed from the
stack after such a function call.
This option can lead to reduced code size for functions that perform
several calls to functions that get their arguments on the stack like
calls to printf-like functions.
- -mbranch-cost=cost
-
Set the branch costs for conditional branch instructions to
cost. Reasonable values for cost are small, non-negative
integers. The default branch cost is 0.
- -mcall-prologues
-
Functions prologues/epilogues are expanded as calls to appropriate
subroutines. Code size is smaller.
- -mint8
-
Assume "int" to be 8-bit integer. This affects the sizes of all types: a
"char" is 1 byte, an "int" is 1 byte, a "long" is 2 bytes,
and "long long" is 4 bytes. Please note that this option does not
conform to the C standards, but it results in smaller code
size.
- -mn-flash=num
-
Assume that the flash memory has a size of
num times 64@tie{}KiB.
- -mno-interrupts
-
Generated code is not compatible with hardware interrupts.
Code size is smaller.
- -mrelax
-
Try to replace "CALL" resp. "JMP" instruction by the shorter
"RCALL" resp. "RJMP" instruction if applicable.
Setting -mrelax just adds the --mlink-relax option to
the assembler's command line and the --relax option to the
linker's command line.
Jump relaxing is performed by the linker because jump offsets are not
known before code is located. Therefore, the assembler code generated by the
compiler is the same, but the instructions in the executable may
differ from instructions in the assembler code.
Relaxing must be turned on if linker stubs are needed, see the
section on "EIND" and linker stubs below.
- -mrmw
-
Assume that the device supports the Read-Modify-Write
instructions "XCH", "LAC", "LAS" and "LAT".
- -msp8
-
Treat the stack pointer register as an 8-bit register,
i.e. assume the high byte of the stack pointer is zero.
In general, you don't need to set this option by hand.
This option is used internally by the compiler to select and
build multilibs for architectures "avr2" and "avr25".
These architectures mix devices with and without "SPH".
For any setting other than -mmcu=avr2 or -mmcu=avr25
the compiler driver adds or removes this option from the compiler
proper's command line, because the compiler then knows if the device
or architecture has an 8-bit stack pointer and thus no "SPH"
register or not.
- -mstrict-X
-
Use address register "X" in a way proposed by the hardware. This means
that "X" is only used in indirect, post-increment or
pre-decrement addressing.
Without this option, the "X" register may be used in the same way
as "Y" or "Z" which then is emulated by additional
instructions.
For example, loading a value with "X+const" addressing with a
small non-negative "const < 64" to a register Rn is
performed as
adiw r26, const ; X += const
ld <Rn>, X ; <Rn> = *X
sbiw r26, const ; X -= const
- -mtiny-stack
-
Only change the lower 8@tie{}bits of the stack pointer.
- -nodevicelib
-
Don't link against AVR-LibC's device specific library "libdev.a".
- -Waddr-space-convert
-
Warn about conversions between address spaces in the case where the
resulting address space is not contained in the incoming address space.
"EIND" and Devices with More Than 128 Ki Bytes of Flash
Pointers in the implementation are 16@tie{}bits wide.
The address of a function or label is represented as word address so
that indirect jumps and calls can target any code address in the
range of 64@tie{}Ki words.
In order to facilitate indirect jump on devices with more than 128@tie{}Ki
bytes of program memory space, there is a special function register called
"EIND" that serves as most significant part of the target address
when "EICALL" or "EIJMP" instructions are used.
Indirect jumps and calls on these devices are handled as follows by
the compiler and are subject to some limitations:
- *
-
The compiler never sets "EIND".
- *
-
The compiler uses "EIND" implicitely in "EICALL"/"EIJMP"
instructions or might read "EIND" directly in order to emulate an
indirect call/jump by means of a "RET" instruction.
- *
-
The compiler assumes that "EIND" never changes during the startup
code or during the application. In particular, "EIND" is not
saved/restored in function or interrupt service routine
prologue/epilogue.
- *
-
For indirect calls to functions and computed goto, the linker
generates stubs. Stubs are jump pads sometimes also called
trampolines. Thus, the indirect call/jump jumps to such a stub.
The stub contains a direct jump to the desired address.
- *
-
Linker relaxation must be turned on so that the linker generates
the stubs correctly in all situations. See the compiler option
-mrelax and the linker option --relax.
There are corner cases where the linker is supposed to generate stubs
but aborts without relaxation and without a helpful error message.
- *
-
The default linker script is arranged for code with "EIND = 0".
If code is supposed to work for a setup with "EIND != 0", a custom
linker script has to be used in order to place the sections whose
name start with ".trampolines" into the segment where "EIND"
points to.
- *
-
The startup code from libgcc never sets "EIND".
Notice that startup code is a blend of code from libgcc and AVR-LibC.
For the impact of AVR-LibC on "EIND", see the
AVR-LibC user manual ("http://nongnu.org/avr-libc/user-manual/").
- *
-
It is legitimate for user-specific startup code to set up "EIND"
early, for example by means of initialization code located in
section ".init3". Such code runs prior to general startup code
that initializes RAM and calls constructors, but after the bit
of startup code from AVR-LibC that sets "EIND" to the segment
where the vector table is located.
#include <avr/io.h>
static void
__attribute__((section(".init3"),naked,used,no_instrument_function))
init3_set_eind (void)
{
__asm volatile ("ldi r24,pm_hh8(__trampolines_start)\n\t"
"out %i0,r24" :: "n" (&EIND) : "r24","memory");
}
The "__trampolines_start" symbol is defined in the linker script.
- *
-
Stubs are generated automatically by the linker if
the following two conditions are met:
-
- -<The address of a label is taken by means of the gs modifier>
-
(short for generate stubs) like so:
LDI r24, lo8(gs(<func>))
LDI r25, hi8(gs(<func>))
- -<The final location of that label is in a code segment>
-
outside the segment where the stubs are located.
-
- *
-
The compiler emits such "gs" modifiers for code labels in the
following situations:
-
- -<Taking address of a function or code label.>
-
- -<Computed goto.>
-
- -<If prologue-save function is used, see -mcall-prologues>
-
command-line option.
- -<Switch/case dispatch tables. If you do not want such dispatch>
-
tables you can specify the -fno-jump-tables command-line option.
- -<C and C++ constructors/destructors called during startup/shutdown.>
-
- -<If the tools hit a gs() modifier explained above.>
-
-
- *
-
Jumping to non-symbolic addresses like so is not supported:
int main (void)
{
/* Call function at word address 0x2 */
return ((int(*)(void)) 0x2)();
}
Instead, a stub has to be set up, i.e. the function has to be called
through a symbol ("func_4" in the example):
int main (void)
{
extern int func_4 (void);
/* Call function at byte address 0x4 */
return func_4();
}
and the application be linked with -Wl,--defsym,func_4=0x4.
Alternatively, "func_4" can be defined in the linker script.
Handling of the "RAMPD", "RAMPX", "RAMPY" and "RAMPZ" Special Function Registers
Some AVR devices support memories larger than the 64@tie{}KiB range
that can be accessed with 16-bit pointers. To access memory locations
outside this 64@tie{}KiB range, the contentent of a "RAMP"
register is used as high part of the address:
The "X", "Y", "Z" address register is concatenated
with the "RAMPX", "RAMPY", "RAMPZ" special function
register, respectively, to get a wide address. Similarly,
"RAMPD" is used together with direct addressing.
- *
-
The startup code initializes the "RAMP" special function
registers with zero.
- *
-
If a AVR Named Address Spaces,named address space other than
generic or "__flash" is used, then "RAMPZ" is set
as needed before the operation.
- *
-
If the device supports RAM larger than 64@tie{}KiB and the compiler
needs to change "RAMPZ" to accomplish an operation, "RAMPZ"
is reset to zero after the operation.
- *
-
If the device comes with a specific "RAMP" register, the ISR
prologue/epilogue saves/restores that SFR and initializes it with
zero in case the ISR code might (implicitly) use it.
- *
-
RAM larger than 64@tie{}KiB is not supported by GCC for AVR targets.
If you use inline assembler to read from locations outside the
16-bit address range and change one of the "RAMP" registers,
you must reset it to zero after the access.
AVR Built-in Macros
GCC defines several built-in macros so that the user code can test
for the presence or absence of features. Almost any of the following
built-in macros are deduced from device capabilities and thus
triggered by the -mmcu= command-line option.
For even more AVR-specific built-in macros see
AVR Named Address Spaces and AVR Built-in Functions.
- __AVR_ARCH__
-
Build-in macro that resolves to a decimal number that identifies the
architecture and depends on the -mmcu=mcu option.
Possible values are:
2, 25, 3, 31, 35,
4, 5, 51, 6
for mcu="avr2", "avr25", "avr3", "avr31",
"avr35", "avr4", "avr5", "avr51", "avr6",
respectively and
100, 102, 104,
105, 106, 107
for mcu="avrtiny", "avrxmega2", "avrxmega4",
"avrxmega5", "avrxmega6", "avrxmega7", respectively.
If mcu specifies a device, this built-in macro is set
accordingly. For example, with -mmcu=atmega8 the macro is
defined to 4.
- __AVR_Device__
-
Setting -mmcu=device defines this built-in macro which reflects
the device's name. For example, -mmcu=atmega8 defines the
built-in macro "__AVR_ATmega8__", -mmcu=attiny261a defines
"__AVR_ATtiny261A__", etc.
The built-in macros' names follow
the scheme "__AVR_Device__" where Device is
the device name as from the AVR user manual. The difference between
Device in the built-in macro and device in
-mmcu=device is that the latter is always lowercase.
If device is not a device but only a core architecture like
avr51, this macro is not defined.
- __AVR_DEVICE_NAME__
-
Setting -mmcu=device defines this built-in macro to
the device's name. For example, with -mmcu=atmega8 the macro
is defined to "atmega8".
If device is not a device but only a core architecture like
avr51, this macro is not defined.
- __AVR_XMEGA__
-
The device / architecture belongs to the XMEGA family of devices.
- __AVR_HAVE_ELPM__
-
The device has the the "ELPM" instruction.
- __AVR_HAVE_ELPMX__
-
The device has the "ELPM Rn,Z" and "ELPM
Rn,Z+" instructions.
- __AVR_HAVE_MOVW__
-
The device has the "MOVW" instruction to perform 16-bit
register-register moves.
- __AVR_HAVE_LPMX__
-
The device has the "LPM Rn,Z" and
"LPM Rn,Z+" instructions.
- __AVR_HAVE_MUL__
-
The device has a hardware multiplier.
- __AVR_HAVE_JMP_CALL__
-
The device has the "JMP" and "CALL" instructions.
This is the case for devices with at least 16@tie{}KiB of program
memory.
- __AVR_HAVE_EIJMP_EICALL__
-
- __AVR_3_BYTE_PC__
-
The device has the "EIJMP" and "EICALL" instructions.
This is the case for devices with more than 128@tie{}KiB of program memory.
This also means that the program counter
(PC) is 3@tie{}bytes wide.
- __AVR_2_BYTE_PC__
-
The program counter (PC) is 2@tie{}bytes wide. This is the case for devices
with up to 128@tie{}KiB of program memory.
- __AVR_HAVE_8BIT_SP__
-
- __AVR_HAVE_16BIT_SP__
-
The stack pointer (SP) register is treated as 8-bit respectively
16-bit register by the compiler.
The definition of these macros is affected by -mtiny-stack.
- __AVR_HAVE_SPH__
-
- __AVR_SP8__
-
The device has the SPH (high part of stack pointer) special function
register or has an 8-bit stack pointer, respectively.
The definition of these macros is affected by -mmcu= and
in the cases of -mmcu=avr2 and -mmcu=avr25 also
by -msp8.
- __AVR_HAVE_RAMPD__
-
- __AVR_HAVE_RAMPX__
-
- __AVR_HAVE_RAMPY__
-
- __AVR_HAVE_RAMPZ__
-
The device has the "RAMPD", "RAMPX", "RAMPY",
"RAMPZ" special function register, respectively.
- __NO_INTERRUPTS__
-
This macro reflects the -mno-interrupts command-line option.
- __AVR_ERRATA_SKIP__
-
- __AVR_ERRATA_SKIP_JMP_CALL__
-
Some AVR devices (AT90S8515, ATmega103) must not skip 32-bit
instructions because of a hardware erratum. Skip instructions are
"SBRS", "SBRC", "SBIS", "SBIC" and "CPSE".
The second macro is only defined if "__AVR_HAVE_JMP_CALL__" is also
set.
- __AVR_ISA_RMW__
-
The device has Read-Modify-Write instructions (XCH, LAC, LAS and LAT).
- __AVR_SFR_OFFSET__=offset
-
Instructions that can address I/O special function registers directly
like "IN", "OUT", "SBI", etc. may use a different
address as if addressed by an instruction to access RAM like "LD"
or "STS". This offset depends on the device architecture and has
to be subtracted from the RAM address in order to get the
respective I/O@tie{}address.
- __WITH_AVRLIBC__
-
The compiler is configured to be used together with AVR-Libc.
See the --with-avrlibc configure option.
Blackfin Options
- -mcpu=cpu[-sirevision]
-
Specifies the name of the target Blackfin processor. Currently, cpu
can be one of bf512, bf514, bf516, bf518,
bf522, bf523, bf524, bf525, bf526,
bf527, bf531, bf532, bf533,
bf534, bf536, bf537, bf538, bf539,
bf542, bf544, bf547, bf548, bf549,
bf542m, bf544m, bf547m, bf548m, bf549m,
bf561, bf592.
The optional sirevision specifies the silicon revision of the target
Blackfin processor. Any workarounds available for the targeted silicon revision
are enabled. If sirevision is none, no workarounds are enabled.
If sirevision is any, all workarounds for the targeted processor
are enabled. The "__SILICON_REVISION__" macro is defined to two
hexadecimal digits representing the major and minor numbers in the silicon
revision. If sirevision is none, the "__SILICON_REVISION__"
is not defined. If sirevision is any, the
"__SILICON_REVISION__" is defined to be 0xffff.
If this optional sirevision is not used, GCC assumes the latest known
silicon revision of the targeted Blackfin processor.
GCC defines a preprocessor macro for the specified cpu.
For the bfin-elf toolchain, this option causes the hardware BSP
provided by libgloss to be linked in if -msim is not given.
Without this option, bf532 is used as the processor by default.
Note that support for bf561 is incomplete. For bf561,
only the preprocessor macro is defined.
- -msim
-
Specifies that the program will be run on the simulator. This causes
the simulator BSP provided by libgloss to be linked in. This option
has effect only for bfin-elf toolchain.
Certain other options, such as -mid-shared-library and
-mfdpic, imply -msim.
- -momit-leaf-frame-pointer
-
Don't keep the frame pointer in a register for leaf functions. This
avoids the instructions to save, set up and restore frame pointers and
makes an extra register available in leaf functions. The option
-fomit-frame-pointer removes the frame pointer for all functions,
which might make debugging harder.
- -mspecld-anomaly
-
When enabled, the compiler ensures that the generated code does not
contain speculative loads after jump instructions. If this option is used,
"__WORKAROUND_SPECULATIVE_LOADS" is defined.
- -mno-specld-anomaly
-
Don't generate extra code to prevent speculative loads from occurring.
- -mcsync-anomaly
-
When enabled, the compiler ensures that the generated code does not
contain CSYNC or SSYNC instructions too soon after conditional branches.
If this option is used, "__WORKAROUND_SPECULATIVE_SYNCS" is defined.
- -mno-csync-anomaly
-
Don't generate extra code to prevent CSYNC or SSYNC instructions from
occurring too soon after a conditional branch.
- -mlow-64k
-
When enabled, the compiler is free to take advantage of the knowledge that
the entire program fits into the low 64k of memory.
- -mno-low-64k
-
Assume that the program is arbitrarily large. This is the default.
- -mstack-check-l1
-
Do stack checking using information placed into L1 scratchpad memory by the
uClinux kernel.
- -mid-shared-library
-
Generate code that supports shared libraries via the library ID method.
This allows for execute in place and shared libraries in an environment
without virtual memory management. This option implies -fPIC.
With a bfin-elf target, this option implies -msim.
- -mno-id-shared-library
-
Generate code that doesn't assume ID-based shared libraries are being used.
This is the default.
- -mleaf-id-shared-library
-
Generate code that supports shared libraries via the library ID method,
but assumes that this library or executable won't link against any other
ID shared libraries. That allows the compiler to use faster code for jumps
and calls.
- -mno-leaf-id-shared-library
-
Do not assume that the code being compiled won't link against any ID shared
libraries. Slower code is generated for jump and call insns.
- -mshared-library-id=n
-
Specifies the identification number of the ID-based shared library being
compiled. Specifying a value of 0 generates more compact code; specifying
other values forces the allocation of that number to the current
library but is no more space- or time-efficient than omitting this option.
- -msep-data
-
Generate code that allows the data segment to be located in a different
area of memory from the text segment. This allows for execute in place in
an environment without virtual memory management by eliminating relocations
against the text section.
- -mno-sep-data
-
Generate code that assumes that the data segment follows the text segment.
This is the default.
- -mlong-calls
-
- -mno-long-calls
-
Tells the compiler to perform function calls by first loading the
address of the function into a register and then performing a subroutine
call on this register. This switch is needed if the target function
lies outside of the 24-bit addressing range of the offset-based
version of subroutine call instruction.
This feature is not enabled by default. Specifying
-mno-long-calls restores the default behavior. Note these
switches have no effect on how the compiler generates code to handle
function calls via function pointers.
- -mfast-fp
-
Link with the fast floating-point library. This library relaxes some of
the IEEE floating-point standard's rules for checking inputs against
Not-a-Number (NAN), in the interest of performance.
- -minline-plt
-
Enable inlining of PLT entries in function calls to functions that are
not known to bind locally. It has no effect without -mfdpic.
- -mmulticore
-
Build a standalone application for multicore Blackfin processors.
This option causes proper start files and link scripts supporting
multicore to be used, and defines the macro "__BFIN_MULTICORE".
It can only be used with -mcpu=bf561[-sirevision].
This option can be used with -mcorea or -mcoreb, which
selects the one-application-per-core programming model. Without
-mcorea or -mcoreb, the single-application/dual-core
programming model is used. In this model, the main function of Core B
should be named as "coreb_main".
If this option is not used, the single-core application programming
model is used.
- -mcorea
-
Build a standalone application for Core A of BF561 when using
the one-application-per-core programming model. Proper start files
and link scripts are used to support Core A, and the macro
"__BFIN_COREA" is defined.
This option can only be used in conjunction with -mmulticore.
- -mcoreb
-
Build a standalone application for Core B of BF561 when using
the one-application-per-core programming model. Proper start files
and link scripts are used to support Core B, and the macro
"__BFIN_COREB" is defined. When this option is used, "coreb_main"
should be used instead of "main".
This option can only be used in conjunction with -mmulticore.
- -msdram
-
Build a standalone application for SDRAM. Proper start files and
link scripts are used to put the application into SDRAM, and the macro
"__BFIN_SDRAM" is defined.
The loader should initialize SDRAM before loading the application.
- -micplb
-
Assume that ICPLBs are enabled at run time. This has an effect on certain
anomaly workarounds. For Linux targets, the default is to assume ICPLBs
are enabled; for standalone applications the default is off.
C6X Options
- -march=name
-
This specifies the name of the target architecture. GCC uses this
name to determine what kind of instructions it can emit when generating
assembly code. Permissible names are: c62x,
c64x, c64x+, c67x, c67x+, c674x.
- -mbig-endian
-
Generate code for a big-endian target.
- -mlittle-endian
-
Generate code for a little-endian target. This is the default.
- -msim
-
Choose startup files and linker script suitable for the simulator.
- -msdata=default
-
Put small global and static data in the ".neardata" section,
which is pointed to by register "B14". Put small uninitialized
global and static data in the ".bss" section, which is adjacent
to the ".neardata" section. Put small read-only data into the
".rodata" section. The corresponding sections used for large
pieces of data are ".fardata", ".far" and ".const".
- -msdata=all
-
Put all data, not just small objects, into the sections reserved for
small data, and use addressing relative to the "B14" register to
access them.
- -msdata=none
-
Make no use of the sections reserved for small data, and use absolute
addresses to access all data. Put all initialized global and static
data in the ".fardata" section, and all uninitialized data in the
".far" section. Put all constant data into the ".const"
section.
CRIS Options
These options are defined specifically for the CRIS ports.
- -march=architecture-type
-
- -mcpu=architecture-type
-
Generate code for the specified architecture. The choices for
architecture-type are v3, v8 and v10 for
respectively ETRAX 4, ETRAX 100, and ETRAX 100 LX.
Default is v0 except for cris-axis-linux-gnu, where the default is
v10.
- -mtune=architecture-type
-
Tune to architecture-type everything applicable about the generated
code, except for the ABI and the set of available instructions. The
choices for architecture-type are the same as for
-march=architecture-type.
- -mmax-stack-frame=n
-
Warn when the stack frame of a function exceeds n bytes.
- -metrax4
-
- -metrax100
-
The options -metrax4 and -metrax100 are synonyms for
-march=v3 and -march=v8 respectively.
- -mmul-bug-workaround
-
- -mno-mul-bug-workaround
-
Work around a bug in the "muls" and "mulu" instructions for CPU
models where it applies. This option is active by default.
- -mpdebug
-
Enable CRIS-specific verbose debug-related information in the assembly
code. This option also has the effect of turning off the #NO_APP
formatted-code indicator to the assembler at the beginning of the
assembly file.
- -mcc-init
-
Do not use condition-code results from previous instruction; always emit
compare and test instructions before use of condition codes.
- -mno-side-effects
-
Do not emit instructions with side effects in addressing modes other than
post-increment.
- -mstack-align
-
- -mno-stack-align
-
- -mdata-align
-
- -mno-data-align
-
- -mconst-align
-
- -mno-const-align
-
These options (no- options) arrange (eliminate arrangements) for the
stack frame, individual data and constants to be aligned for the maximum
single data access size for the chosen CPU model. The default is to
arrange for 32-bit alignment. ABI details such as structure layout are
not affected by these options.
- -m32-bit
-
- -m16-bit
-
- -m8-bit
-
Similar to the stack- data- and const-align options above, these options
arrange for stack frame, writable data and constants to all be 32-bit,
16-bit or 8-bit aligned. The default is 32-bit alignment.
- -mno-prologue-epilogue
-
- -mprologue-epilogue
-
With -mno-prologue-epilogue, the normal function prologue and
epilogue which set up the stack frame are omitted and no return
instructions or return sequences are generated in the code. Use this
option only together with visual inspection of the compiled code: no
warnings or errors are generated when call-saved registers must be saved,
or storage for local variables needs to be allocated.
- -mno-gotplt
-
- -mgotplt
-
With -fpic and -fPIC, don't generate (do generate)
instruction sequences that load addresses for functions from the PLT part
of the GOT rather than (traditional on other architectures) calls to the
PLT. The default is -mgotplt.
- -melf
-
Legacy no-op option only recognized with the cris-axis-elf and
cris-axis-linux-gnu targets.
- -mlinux
-
Legacy no-op option only recognized with the cris-axis-linux-gnu target.
- -sim
-
This option, recognized for the cris-axis-elf, arranges
to link with input-output functions from a simulator library. Code,
initialized data and zero-initialized data are allocated consecutively.
- -sim2
-
Like -sim, but pass linker options to locate initialized data at
0x40000000 and zero-initialized data at 0x80000000.
CR16 Options
These options are defined specifically for the CR16 ports.
- -mmac
-
Enable the use of multiply-accumulate instructions. Disabled by default.
- -mcr16cplus
-
- -mcr16c
-
Generate code for CR16C or CR16C+ architecture. CR16C+ architecture
is default.
- -msim
-
Links the library libsim.a which is in compatible with simulator. Applicable
to ELF compiler only.
- -mint32
-
Choose integer type as 32-bit wide.
- -mbit-ops
-
Generates "sbit"/"cbit" instructions for bit manipulations.
- -mdata-model=model
-
Choose a data model. The choices for model are near,
far or medium. medium is default.
However, far is not valid with -mcr16c, as the
CR16C architecture does not support the far data model.
Darwin Options
These options are defined for all architectures running the Darwin operating
system.
FSF GCC on Darwin does not create ``fat'' object files; it creates
an object file for the single architecture that GCC was built to
target. Apple's GCC on Darwin does create ``fat'' files if multiple
-arch options are used; it does so by running the compiler or
linker multiple times and joining the results together with
lipo.
The subtype of the file created (like ppc7400 or ppc970 or
i686) is determined by the flags that specify the ISA
that GCC is targeting, like -mcpu or -march. The
-force_cpusubtype_ALL option can be used to override this.
The Darwin tools vary in their behavior when presented with an ISA
mismatch. The assembler, as, only permits instructions to
be used that are valid for the subtype of the file it is generating,
so you cannot put 64-bit instructions in a ppc750 object file.
The linker for shared libraries, /usr/bin/libtool, fails
and prints an error if asked to create a shared library with a less
restrictive subtype than its input files (for instance, trying to put
a ppc970 object file in a ppc7400 library). The linker
for executables, ld, quietly gives the executable the most
restrictive subtype of any of its input files.
- -Fdir
-
Add the framework directory dir to the head of the list of
directories to be searched for header files. These directories are
interleaved with those specified by -I options and are
scanned in a left-to-right order.
A framework directory is a directory with frameworks in it. A
framework is a directory with a Headers and/or
PrivateHeaders directory contained directly in it that ends
in .framework. The name of a framework is the name of this
directory excluding the .framework. Headers associated with
the framework are found in one of those two directories, with
Headers being searched first. A subframework is a framework
directory that is in a framework's Frameworks directory.
Includes of subframework headers can only appear in a header of a
framework that contains the subframework, or in a sibling subframework
header. Two subframeworks are siblings if they occur in the same
framework. A subframework should not have the same name as a
framework; a warning is issued if this is violated. Currently a
subframework cannot have subframeworks; in the future, the mechanism
may be extended to support this. The standard frameworks can be found
in /System/Library/Frameworks and
/Library/Frameworks. An example include looks like
"#include <Framework/header.h>", where Framework denotes
the name of the framework and header.h is found in the
PrivateHeaders or Headers directory.
- -iframeworkdir
-
Like -F except the directory is a treated as a system
directory. The main difference between this -iframework and
-F is that with -iframework the compiler does not
warn about constructs contained within header files found via
dir. This option is valid only for the C family of languages.
- -gused
-
Emit debugging information for symbols that are used. For stabs
debugging format, this enables -feliminate-unused-debug-symbols.
This is by default ON.
- -gfull
-
Emit debugging information for all symbols and types.
- -mmacosx-version-min=version
-
The earliest version of MacOS X that this executable will run on
is version. Typical values of version include 10.1,
10.2, and 10.3.9.
If the compiler was built to use the system's headers by default,
then the default for this option is the system version on which the
compiler is running, otherwise the default is to make choices that
are compatible with as many systems and code bases as possible.
- -mkernel
-
Enable kernel development mode. The -mkernel option sets
-static, -fno-common, -fno-use-cxa-atexit,
-fno-exceptions, -fno-non-call-exceptions,
-fapple-kext, -fno-weak and -fno-rtti where
applicable. This mode also sets -mno-altivec,
-msoft-float, -fno-builtin and
-mlong-branch for PowerPC targets.
- -mone-byte-bool
-
Override the defaults for "bool" so that "sizeof(bool)==1".
By default "sizeof(bool)" is 4 when compiling for
Darwin/PowerPC and 1 when compiling for Darwin/x86, so this
option has no effect on x86.
Warning: The -mone-byte-bool switch causes GCC
to generate code that is not binary compatible with code generated
without that switch. Using this switch may require recompiling all
other modules in a program, including system libraries. Use this
switch to conform to a non-default data model.
- -mfix-and-continue
-
- -ffix-and-continue
-
- -findirect-data
-
Generate code suitable for fast turnaround development, such as to
allow GDB to dynamically load .o files into already-running
programs. -findirect-data and -ffix-and-continue
are provided for backwards compatibility.
- -all_load
-
Loads all members of static archive libraries.
See man ld(1) for more information.
- -arch_errors_fatal
-
Cause the errors having to do with files that have the wrong architecture
to be fatal.
- -bind_at_load
-
Causes the output file to be marked such that the dynamic linker will
bind all undefined references when the file is loaded or launched.
- -bundle
-
Produce a Mach-o bundle format file.
See man ld(1) for more information.
- -bundle_loader executable
-
This option specifies the executable that will load the build
output file being linked. See man ld(1) for more information.
- -dynamiclib
-
When passed this option, GCC produces a dynamic library instead of
an executable when linking, using the Darwin libtool command.
- -force_cpusubtype_ALL
-
This causes GCC's output file to have the ALL subtype, instead of
one controlled by the -mcpu or -march option.
- -allowable_client client_name
-
- -client_name
-
- -compatibility_version
-
- -current_version
-
- -dead_strip
-
- -dependency-file
-
- -dylib_file
-
- -dylinker_install_name
-
- -dynamic
-
- -exported_symbols_list
-
- -filelist
-
- -flat_namespace
-
- -force_flat_namespace
-
- -headerpad_max_install_names
-
- -image_base
-
- -init
-
- -install_name
-
- -keep_private_externs
-
- -multi_module
-
- -multiply_defined
-
- -multiply_defined_unused
-
- -noall_load
-
- -no_dead_strip_inits_and_terms
-
- -nofixprebinding
-
- -nomultidefs
-
- -noprebind
-
- -noseglinkedit
-
- -pagezero_size
-
- -prebind
-
- -prebind_all_twolevel_modules
-
- -private_bundle
-
- -read_only_relocs
-
- -sectalign
-
- -sectobjectsymbols
-
- -whyload
-
- -seg1addr
-
- -sectcreate
-
- -sectobjectsymbols
-
- -sectorder
-
- -segaddr
-
- -segs_read_only_addr
-
- -segs_read_write_addr
-
- -seg_addr_table
-
- -seg_addr_table_filename
-
- -seglinkedit
-
- -segprot
-
- -segs_read_only_addr
-
- -segs_read_write_addr
-
- -single_module
-
- -static
-
- -sub_library
-
- -sub_umbrella
-
- -twolevel_namespace
-
- -umbrella
-
- -undefined
-
- -unexported_symbols_list
-
- -weak_reference_mismatches
-
- -whatsloaded
-
These options are passed to the Darwin linker. The Darwin linker man page
describes them in detail.
DEC Alpha Options
These -m options are defined for the DEC Alpha implementations:
- -mno-soft-float
-
- -msoft-float
-
Use (do not use) the hardware floating-point instructions for
floating-point operations. When -msoft-float is specified,
functions in libgcc.a are used to perform floating-point
operations. Unless they are replaced by routines that emulate the
floating-point operations, or compiled in such a way as to call such
emulations routines, these routines issue floating-point
operations. If you are compiling for an Alpha without floating-point
operations, you must ensure that the library is built so as not to call
them.
Note that Alpha implementations without floating-point operations are
required to have floating-point registers.
- -mfp-reg
-
- -mno-fp-regs
-
Generate code that uses (does not use) the floating-point register set.
-mno-fp-regs implies -msoft-float. If the floating-point
register set is not used, floating-point operands are passed in integer
registers as if they were integers and floating-point results are passed
in $0 instead of $f0. This is a non-standard calling sequence,
so any function with a floating-point argument or return value called by code
compiled with -mno-fp-regs must also be compiled with that
option.
A typical use of this option is building a kernel that does not use,
and hence need not save and restore, any floating-point registers.
- -mieee
-
The Alpha architecture implements floating-point hardware optimized for
maximum performance. It is mostly compliant with the IEEE floating-point
standard. However, for full compliance, software assistance is
required. This option generates code fully IEEE-compliant code
except that the inexact-flag is not maintained (see below).
If this option is turned on, the preprocessor macro "_IEEE_FP" is
defined during compilation. The resulting code is less efficient but is
able to correctly support denormalized numbers and exceptional IEEE
values such as not-a-number and plus/minus infinity. Other Alpha
compilers call this option -ieee_with_no_inexact.
- -mieee-with-inexact
-
This is like -mieee except the generated code also maintains
the IEEE inexact-flag. Turning on this option causes the
generated code to implement fully-compliant IEEE math. In addition to
"_IEEE_FP", "_IEEE_FP_EXACT" is defined as a preprocessor
macro. On some Alpha implementations the resulting code may execute
significantly slower than the code generated by default. Since there is
very little code that depends on the inexact-flag, you should
normally not specify this option. Other Alpha compilers call this
option -ieee_with_inexact.
- -mfp-trap-mode=trap-mode
-
This option controls what floating-point related traps are enabled.
Other Alpha compilers call this option -fptm trap-mode.
The trap mode can be set to one of four values:
-
- n
-
This is the default (normal) setting. The only traps that are enabled
are the ones that cannot be disabled in software (e.g., division by zero
trap).
- u
-
In addition to the traps enabled by n, underflow traps are enabled
as well.
- su
-
Like u, but the instructions are marked to be safe for software
completion (see Alpha architecture manual for details).
- sui
-
Like su, but inexact traps are enabled as well.
-
- -mfp-rounding-mode=rounding-mode
-
Selects the IEEE rounding mode. Other Alpha compilers call this option
-fprm rounding-mode. The rounding-mode can be one
of:
-
- n
-
Normal IEEE rounding mode. Floating-point numbers are rounded towards
the nearest machine number or towards the even machine number in case
of a tie.
- m
-
Round towards minus infinity.
- c
-
Chopped rounding mode. Floating-point numbers are rounded towards zero.
- d
-
Dynamic rounding mode. A field in the floating-point control register
(fpcr, see Alpha architecture reference manual) controls the
rounding mode in effect. The C library initializes this register for
rounding towards plus infinity. Thus, unless your program modifies the
fpcr, d corresponds to round towards plus infinity.
-
- -mtrap-precision=trap-precision
-
In the Alpha architecture, floating-point traps are imprecise. This
means without software assistance it is impossible to recover from a
floating trap and program execution normally needs to be terminated.
GCC can generate code that can assist operating system trap handlers
in determining the exact location that caused a floating-point trap.
Depending on the requirements of an application, different levels of
precisions can be selected:
-
- p
-
Program precision. This option is the default and means a trap handler
can only identify which program caused a floating-point exception.
- f
-
Function precision. The trap handler can determine the function that
caused a floating-point exception.
- i
-
Instruction precision. The trap handler can determine the exact
instruction that caused a floating-point exception.
-
Other Alpha compilers provide the equivalent options called
-scope_safe and -resumption_safe.
- -mieee-conformant
-
This option marks the generated code as IEEE conformant. You must not
use this option unless you also specify -mtrap-precision=i and either
-mfp-trap-mode=su or -mfp-trap-mode=sui. Its only effect
is to emit the line .eflag 48 in the function prologue of the
generated assembly file.
- -mbuild-constants
-
Normally GCC examines a 32- or 64-bit integer constant to
see if it can construct it from smaller constants in two or three
instructions. If it cannot, it outputs the constant as a literal and
generates code to load it from the data segment at run time.
Use this option to require GCC to construct all integer constants
using code, even if it takes more instructions (the maximum is six).
You typically use this option to build a shared library dynamic
loader. Itself a shared library, it must relocate itself in memory
before it can find the variables and constants in its own data segment.
- -mbwx
-
- -mno-bwx
-
- -mcix
-
- -mno-cix
-
- -mfix
-
- -mno-fix
-
- -mmax
-
- -mno-max
-
Indicate whether GCC should generate code to use the optional BWX,
CIX, FIX and MAX instruction sets. The default is to use the instruction
sets supported by the CPU type specified via -mcpu= option or that
of the CPU on which GCC was built if none is specified.
- -mfloat-vax
-
- -mfloat-ieee
-
Generate code that uses (does not use) VAX F and G floating-point
arithmetic instead of IEEE single and double precision.
- -mexplicit-relocs
-
- -mno-explicit-relocs
-
Older Alpha assemblers provided no way to generate symbol relocations
except via assembler macros. Use of these macros does not allow
optimal instruction scheduling. GNU binutils as of version 2.12
supports a new syntax that allows the compiler to explicitly mark
which relocations should apply to which instructions. This option
is mostly useful for debugging, as GCC detects the capabilities of
the assembler when it is built and sets the default accordingly.
- -msmall-data
-
- -mlarge-data
-
When -mexplicit-relocs is in effect, static data is
accessed via gp-relative relocations. When -msmall-data
is used, objects 8 bytes long or smaller are placed in a small data area
(the ".sdata" and ".sbss" sections) and are accessed via
16-bit relocations off of the $gp register. This limits the
size of the small data area to 64KB, but allows the variables to be
directly accessed via a single instruction.
The default is -mlarge-data. With this option the data area
is limited to just below 2GB. Programs that require more than 2GB of
data must use "malloc" or "mmap" to allocate the data in the
heap instead of in the program's data segment.
When generating code for shared libraries, -fpic implies
-msmall-data and -fPIC implies -mlarge-data.
- -msmall-text
-
- -mlarge-text
-
When -msmall-text is used, the compiler assumes that the
code of the entire program (or shared library) fits in 4MB, and is
thus reachable with a branch instruction. When -msmall-data
is used, the compiler can assume that all local symbols share the
same $gp value, and thus reduce the number of instructions
required for a function call from 4 to 1.
The default is -mlarge-text.
- -mcpu=cpu_type
-
Set the instruction set and instruction scheduling parameters for
machine type cpu_type. You can specify either the EV
style name or the corresponding chip number. GCC supports scheduling
parameters for the EV4, EV5 and EV6 family of processors and
chooses the default values for the instruction set from the processor
you specify. If you do not specify a processor type, GCC defaults
to the processor on which the compiler was built.
Supported values for cpu_type are
-
- ev4
-
- ev45
-
- 21064
-
Schedules as an EV4 and has no instruction set extensions.
- ev5
-
- 21164
-
Schedules as an EV5 and has no instruction set extensions.
- ev56
-
- 21164a
-
Schedules as an EV5 and supports the BWX extension.
- pca56
-
- 21164pc
-
- 21164PC
-
Schedules as an EV5 and supports the BWX and MAX extensions.
- ev6
-
- 21264
-
Schedules as an EV6 and supports the BWX, FIX, and MAX extensions.
- ev67
-
- 21264a
-
Schedules as an EV6 and supports the BWX, CIX, FIX, and MAX extensions.
-
Native toolchains also support the value native,
which selects the best architecture option for the host processor.
-mcpu=native has no effect if GCC does not recognize
the processor.
- -mtune=cpu_type
-
Set only the instruction scheduling parameters for machine type
cpu_type. The instruction set is not changed.
Native toolchains also support the value native,
which selects the best architecture option for the host processor.
-mtune=native has no effect if GCC does not recognize
the processor.
- -mmemory-latency=time
-
Sets the latency the scheduler should assume for typical memory
references as seen by the application. This number is highly
dependent on the memory access patterns used by the application
and the size of the external cache on the machine.
Valid options for time are
-
- number
-
A decimal number representing clock cycles.
- L1
-
- L2
-
- L3
-
- main
-
The compiler contains estimates of the number of clock cycles for
``typical'' EV4 & EV5 hardware for the Level 1, 2 & 3 caches
(also called Dcache, Scache, and Bcache), as well as to main memory.
Note that L3 is only valid for EV5.
-
FR30 Options
These options are defined specifically for the FR30 port.
- -msmall-model
-
Use the small address space model. This can produce smaller code, but
it does assume that all symbolic values and addresses fit into a
20-bit range.
- -mno-lsim
-
Assume that runtime support has been provided and so there is no need
to include the simulator library (libsim.a) on the linker
command line.
FRV Options
- -mgpr-32
-
Only use the first 32 general-purpose registers.
- -mgpr-64
-
Use all 64 general-purpose registers.
- -mfpr-32
-
Use only the first 32 floating-point registers.
- -mfpr-64
-
Use all 64 floating-point registers.
- -mhard-float
-
Use hardware instructions for floating-point operations.
- -msoft-float
-
Use library routines for floating-point operations.
- -malloc-cc
-
Dynamically allocate condition code registers.
- -mfixed-cc
-
Do not try to dynamically allocate condition code registers, only
use "icc0" and "fcc0".
- -mdword
-
Change ABI to use double word insns.
- -mno-dword
-
Do not use double word instructions.
- -mdouble
-
Use floating-point double instructions.
- -mno-double
-
Do not use floating-point double instructions.
- -mmedia
-
Use media instructions.
- -mno-media
-
Do not use media instructions.
- -mmuladd
-
Use multiply and add/subtract instructions.
- -mno-muladd
-
Do not use multiply and add/subtract instructions.
- -mfdpic
-
Select the FDPIC ABI, which uses function descriptors to represent
pointers to functions. Without any PIC/PIE-related options, it
implies -fPIE. With -fpic or -fpie, it
assumes GOT entries and small data are within a 12-bit range from the
GOT base address; with -fPIC or -fPIE, GOT offsets
are computed with 32 bits.
With a bfin-elf target, this option implies -msim.
- -minline-plt
-
Enable inlining of PLT entries in function calls to functions that are
not known to bind locally. It has no effect without -mfdpic.
It's enabled by default if optimizing for speed and compiling for
shared libraries (i.e., -fPIC or -fpic), or when an
optimization option such as -O3 or above is present in the
command line.
- -mTLS
-
Assume a large TLS segment when generating thread-local code.
- -mtls
-
Do not assume a large TLS segment when generating thread-local code.
- -mgprel-ro
-
Enable the use of "GPREL" relocations in the FDPIC ABI for data
that is known to be in read-only sections. It's enabled by default,
except for -fpic or -fpie: even though it may help
make the global offset table smaller, it trades 1 instruction for 4.
With -fPIC or -fPIE, it trades 3 instructions for 4,
one of which may be shared by multiple symbols, and it avoids the need
for a GOT entry for the referenced symbol, so it's more likely to be a
win. If it is not, -mno-gprel-ro can be used to disable it.
- -multilib-library-pic
-
Link with the (library, not FD) pic libraries. It's implied by
-mlibrary-pic, as well as by -fPIC and
-fpic without -mfdpic. You should never have to use
it explicitly.
- -mlinked-fp
-
Follow the EABI requirement of always creating a frame pointer whenever
a stack frame is allocated. This option is enabled by default and can
be disabled with -mno-linked-fp.
- -mlong-calls
-
Use indirect addressing to call functions outside the current
compilation unit. This allows the functions to be placed anywhere
within the 32-bit address space.
- -malign-labels
-
Try to align labels to an 8-byte boundary by inserting NOPs into the
previous packet. This option only has an effect when VLIW packing
is enabled. It doesn't create new packets; it merely adds NOPs to
existing ones.
- -mlibrary-pic
-
Generate position-independent EABI code.
- -macc-4
-
Use only the first four media accumulator registers.
- -macc-8
-
Use all eight media accumulator registers.
- -mpack
-
Pack VLIW instructions.
- -mno-pack
-
Do not pack VLIW instructions.
- -mno-eflags
-
Do not mark ABI switches in e_flags.
- -mcond-move
-
Enable the use of conditional-move instructions (default).
This switch is mainly for debugging the compiler and will likely be removed
in a future version.
- -mno-cond-move
-
Disable the use of conditional-move instructions.
This switch is mainly for debugging the compiler and will likely be removed
in a future version.
- -mscc
-
Enable the use of conditional set instructions (default).
This switch is mainly for debugging the compiler and will likely be removed
in a future version.
- -mno-scc
-
Disable the use of conditional set instructions.
This switch is mainly for debugging the compiler and will likely be removed
in a future version.
- -mcond-exec
-
Enable the use of conditional execution (default).
This switch is mainly for debugging the compiler and will likely be removed
in a future version.
- -mno-cond-exec
-
Disable the use of conditional execution.
This switch is mainly for debugging the compiler and will likely be removed
in a future version.
- -mvliw-branch
-
Run a pass to pack branches into VLIW instructions (default).
This switch is mainly for debugging the compiler and will likely be removed
in a future version.
- -mno-vliw-branch
-
Do not run a pass to pack branches into VLIW instructions.
This switch is mainly for debugging the compiler and will likely be removed
in a future version.
- -mmulti-cond-exec
-
Enable optimization of "&&" and "||" in conditional execution
(default).
This switch is mainly for debugging the compiler and will likely be removed
in a future version.
- -mno-multi-cond-exec
-
Disable optimization of "&&" and "||" in conditional execution.
This switch is mainly for debugging the compiler and will likely be removed
in a future version.
- -mnested-cond-exec
-
Enable nested conditional execution optimizations (default).
This switch is mainly for debugging the compiler and will likely be removed
in a future version.
- -mno-nested-cond-exec
-
Disable nested conditional execution optimizations.
This switch is mainly for debugging the compiler and will likely be removed
in a future version.
- -moptimize-membar
-
This switch removes redundant "membar" instructions from the
compiler-generated code. It is enabled by default.
- -mno-optimize-membar
-
This switch disables the automatic removal of redundant "membar"
instructions from the generated code.
- -mtomcat-stats
-
Cause gas to print out tomcat statistics.
- -mcpu=cpu
-
Select the processor type for which to generate code. Possible values are
frv, fr550, tomcat, fr500, fr450,
fr405, fr400, fr300 and simple.
GNU/Linux Options
These -m options are defined for GNU/Linux targets:
- -mglibc
-
Use the GNU C library. This is the default except
on *-*-linux-*uclibc* and *-*-linux-*android* targets.
- -muclibc
-
Use uClibc C library. This is the default on
*-*-linux-*uclibc* targets.
- -mbionic
-
Use Bionic C library. This is the default on
*-*-linux-*android* targets.
- -mandroid
-
Compile code compatible with Android platform. This is the default on
*-*-linux-*android* targets.
When compiling, this option enables -mbionic, -fPIC,
-fno-exceptions and -fno-rtti by default. When linking,
this option makes the GCC driver pass Android-specific options to the linker.
Finally, this option causes the preprocessor macro "__ANDROID__"
to be defined.
- -tno-android-cc
-
Disable compilation effects of -mandroid, i.e., do not enable
-mbionic, -fPIC, -fno-exceptions and
-fno-rtti by default.
- -tno-android-ld
-
Disable linking effects of -mandroid, i.e., pass standard Linux
linking options to the linker.
H8/300 Options
These -m options are defined for the H8/300 implementations:
- -mrelax
-
Shorten some address references at link time, when possible; uses the
linker option -relax.
- -mh
-
Generate code for the H8/300H.
- -ms
-
Generate code for the H8S.
- -mn
-
Generate code for the H8S and H8/300H in the normal mode. This switch
must be used either with -mh or -ms.
- -ms2600
-
Generate code for the H8S/2600. This switch must be used with -ms.
- -mexr
-
Extended registers are stored on stack before execution of function
with monitor attribute. Default option is -mexr.
This option is valid only for H8S targets.
- -mno-exr
-
Extended registers are not stored on stack before execution of function
with monitor attribute. Default option is -mno-exr.
This option is valid only for H8S targets.
- -mint32
-
Make "int" data 32 bits by default.
- -malign-300
-
On the H8/300H and H8S, use the same alignment rules as for the H8/300.
The default for the H8/300H and H8S is to align longs and floats on
4-byte boundaries.
-malign-300 causes them to be aligned on 2-byte boundaries.
This option has no effect on the H8/300.
HPPA Options
These -m options are defined for the HPPA family of computers:
- -march=architecture-type
-
Generate code for the specified architecture. The choices for
architecture-type are 1.0 for PA 1.0, 1.1 for PA
1.1, and 2.0 for PA 2.0 processors. Refer to
/usr/lib/sched.models on an HP-UX system to determine the proper
architecture option for your machine. Code compiled for lower numbered
architectures runs on higher numbered architectures, but not the
other way around.
- -mpa-risc-1-0
-
- -mpa-risc-1-1
-
- -mpa-risc-2-0
-
Synonyms for -march=1.0, -march=1.1, and -march=2.0 respectively.
- -mjump-in-delay
-
This option is ignored and provided for compatibility purposes only.
- -mdisable-fpregs
-
Prevent floating-point registers from being used in any manner. This is
necessary for compiling kernels that perform lazy context switching of
floating-point registers. If you use this option and attempt to perform
floating-point operations, the compiler aborts.
- -mdisable-indexing
-
Prevent the compiler from using indexing address modes. This avoids some
rather obscure problems when compiling MIG generated code under MACH.
- -mno-space-regs
-
Generate code that assumes the target has no space registers. This allows
GCC to generate faster indirect calls and use unscaled index address modes.
Such code is suitable for level 0 PA systems and kernels.
- -mfast-indirect-calls
-
Generate code that assumes calls never cross space boundaries. This
allows GCC to emit code that performs faster indirect calls.
This option does not work in the presence of shared libraries or nested
functions.
- -mfixed-range=register-range
-
Generate code treating the given register range as fixed registers.
A fixed register is one that the register allocator cannot use. This is
useful when compiling kernel code. A register range is specified as
two registers separated by a dash. Multiple register ranges can be
specified separated by a comma.
- -mlong-load-store
-
Generate 3-instruction load and store sequences as sometimes required by
the HP-UX 10 linker. This is equivalent to the +k option to
the HP compilers.
- -mportable-runtime
-
Use the portable calling conventions proposed by HP for ELF systems.
- -mgas
-
Enable the use of assembler directives only GAS understands.
- -mschedule=cpu-type
-
Schedule code according to the constraints for the machine type
cpu-type. The choices for cpu-type are 700
7100, 7100LC, 7200, 7300 and 8000. Refer
to /usr/lib/sched.models on an HP-UX system to determine the
proper scheduling option for your machine. The default scheduling is
8000.
- -mlinker-opt
-
Enable the optimization pass in the HP-UX linker. Note this makes symbolic
debugging impossible. It also triggers a bug in the HP-UX 8 and HP-UX 9
linkers in which they give bogus error messages when linking some programs.
- -msoft-float
-
Generate output containing library calls for floating point.
Warning: the requisite libraries are not available for all HPPA
targets. Normally the facilities of the machine's usual C compiler are
used, but this cannot be done directly in cross-compilation. You must make
your own arrangements to provide suitable library functions for
cross-compilation.
-msoft-float changes the calling convention in the output file;
therefore, it is only useful if you compile all of a program with
this option. In particular, you need to compile libgcc.a, the
library that comes with GCC, with -msoft-float in order for
this to work.
- -msio
-
Generate the predefine, "_SIO", for server IO. The default is
-mwsio. This generates the predefines, "__hp9000s700",
"__hp9000s700__" and "_WSIO", for workstation IO. These
options are available under HP-UX and HI-UX.
- -mgnu-ld
-
Use options specific to GNU ld.
This passes -shared to ld when
building a shared library. It is the default when GCC is configured,
explicitly or implicitly, with the GNU linker. This option does not
affect which ld is called; it only changes what parameters
are passed to that ld.
The ld that is called is determined by the
--with-ld configure option, GCC's program search path, and
finally by the user's PATH. The linker used by GCC can be printed
using which `gcc -print-prog-name=ld`. This option is only available
on the 64-bit HP-UX GCC, i.e. configured with hppa*64*-*-hpux*.
- -mhp-ld
-
Use options specific to HP ld.
This passes -b to ld when building
a shared library and passes +Accept TypeMismatch to ld on all
links. It is the default when GCC is configured, explicitly or
implicitly, with the HP linker. This option does not affect
which ld is called; it only changes what parameters are passed to that
ld.
The ld that is called is determined by the --with-ld
configure option, GCC's program search path, and finally by the user's
PATH. The linker used by GCC can be printed using which
`gcc -print-prog-name=ld`. This option is only available on the 64-bit
HP-UX GCC, i.e. configured with hppa*64*-*-hpux*.
- -mlong-calls
-
Generate code that uses long call sequences. This ensures that a call
is always able to reach linker generated stubs. The default is to generate
long calls only when the distance from the call site to the beginning
of the function or translation unit, as the case may be, exceeds a
predefined limit set by the branch type being used. The limits for
normal calls are 7,600,000 and 240,000 bytes, respectively for the
PA 2.0 and PA 1.X architectures. Sibcalls are always limited at
240,000 bytes.
Distances are measured from the beginning of functions when using the
-ffunction-sections option, or when using the -mgas
and -mno-portable-runtime options together under HP-UX with
the SOM linker.
It is normally not desirable to use this option as it degrades
performance. However, it may be useful in large applications,
particularly when partial linking is used to build the application.
The types of long calls used depends on the capabilities of the
assembler and linker, and the type of code being generated. The
impact on systems that support long absolute calls, and long pic
symbol-difference or pc-relative calls should be relatively small.
However, an indirect call is used on 32-bit ELF systems in pic code
and it is quite long.
- -munix=unix-std
-
Generate compiler predefines and select a startfile for the specified
UNIX standard. The choices for unix-std are 93, 95
and 98. 93 is supported on all HP-UX versions. 95
is available on HP-UX 10.10 and later. 98 is available on HP-UX
11.11 and later. The default values are 93 for HP-UX 10.00,
95 for HP-UX 10.10 though to 11.00, and 98 for HP-UX 11.11
and later.
-munix=93 provides the same predefines as GCC 3.3 and 3.4.
-munix=95 provides additional predefines for "XOPEN_UNIX"
and "_XOPEN_SOURCE_EXTENDED", and the startfile unix95.o.
-munix=98 provides additional predefines for "_XOPEN_UNIX",
"_XOPEN_SOURCE_EXTENDED", "_INCLUDE__STDC_A1_SOURCE" and
"_INCLUDE_XOPEN_SOURCE_500", and the startfile unix98.o.
It is important to note that this option changes the interfaces
for various library routines. It also affects the operational behavior
of the C library. Thus, extreme care is needed in using this
option.
Library code that is intended to operate with more than one UNIX
standard must test, set and restore the variable "__xpg4_extended_mask"
as appropriate. Most GNU software doesn't provide this capability.
- -nolibdld
-
Suppress the generation of link options to search libdld.sl when the
-static option is specified on HP-UX 10 and later.
- -static
-
The HP-UX implementation of setlocale in libc has a dependency on
libdld.sl. There isn't an archive version of libdld.sl. Thus,
when the -static option is specified, special link options
are needed to resolve this dependency.
On HP-UX 10 and later, the GCC driver adds the necessary options to
link with libdld.sl when the -static option is specified.
This causes the resulting binary to be dynamic. On the 64-bit port,
the linkers generate dynamic binaries by default in any case. The
-nolibdld option can be used to prevent the GCC driver from
adding these link options.
- -threads
-
Add support for multithreading with the dce thread library
under HP-UX. This option sets flags for both the preprocessor and
linker.
IA-64 Options
These are the -m options defined for the Intel IA-64 architecture.
- -mbig-endian
-
Generate code for a big-endian target. This is the default for HP-UX.
- -mlittle-endian
-
Generate code for a little-endian target. This is the default for AIX5
and GNU/Linux.
- -mgnu-as
-
- -mno-gnu-as
-
Generate (or don't) code for the GNU assembler. This is the default.
- -mgnu-ld
-
- -mno-gnu-ld
-
Generate (or don't) code for the GNU linker. This is the default.
- -mno-pic
-
Generate code that does not use a global pointer register. The result
is not position independent code, and violates the IA-64 ABI.
- -mvolatile-asm-stop
-
- -mno-volatile-asm-stop
-
Generate (or don't) a stop bit immediately before and after volatile asm
statements.
- -mregister-names
-
- -mno-register-names
-
Generate (or don't) in, loc, and out register names for
the stacked registers. This may make assembler output more readable.
- -mno-sdata
-
- -msdata
-
Disable (or enable) optimizations that use the small data section. This may
be useful for working around optimizer bugs.
- -mconstant-gp
-
Generate code that uses a single constant global pointer value. This is
useful when compiling kernel code.
- -mauto-pic
-
Generate code that is self-relocatable. This implies -mconstant-gp.
This is useful when compiling firmware code.
- -minline-float-divide-min-latency
-
Generate code for inline divides of floating-point values
using the minimum latency algorithm.
- -minline-float-divide-max-throughput
-
Generate code for inline divides of floating-point values
using the maximum throughput algorithm.
- -mno-inline-float-divide
-
Do not generate inline code for divides of floating-point values.
- -minline-int-divide-min-latency
-
Generate code for inline divides of integer values
using the minimum latency algorithm.
- -minline-int-divide-max-throughput
-
Generate code for inline divides of integer values
using the maximum throughput algorithm.
- -mno-inline-int-divide
-
Do not generate inline code for divides of integer values.
- -minline-sqrt-min-latency
-
Generate code for inline square roots
using the minimum latency algorithm.
- -minline-sqrt-max-throughput
-
Generate code for inline square roots
using the maximum throughput algorithm.
- -mno-inline-sqrt
-
Do not generate inline code for "sqrt".
- -mfused-madd
-
- -mno-fused-madd
-
Do (don't) generate code that uses the fused multiply/add or multiply/subtract
instructions. The default is to use these instructions.
- -mno-dwarf2-asm
-
- -mdwarf2-asm
-
Don't (or do) generate assembler code for the DWARF 2 line number debugging
info. This may be useful when not using the GNU assembler.
- -mearly-stop-bits
-
- -mno-early-stop-bits
-
Allow stop bits to be placed earlier than immediately preceding the
instruction that triggered the stop bit. This can improve instruction
scheduling, but does not always do so.
- -mfixed-range=register-range
-
Generate code treating the given register range as fixed registers.
A fixed register is one that the register allocator cannot use. This is
useful when compiling kernel code. A register range is specified as
two registers separated by a dash. Multiple register ranges can be
specified separated by a comma.
- -mtls-size=tls-size
-
Specify bit size of immediate TLS offsets. Valid values are 14, 22, and
64.
- -mtune=cpu-type
-
Tune the instruction scheduling for a particular CPU, Valid values are
itanium, itanium1, merced, itanium2,
and mckinley.
- -milp32
-
- -mlp64
-
Generate code for a 32-bit or 64-bit environment.
The 32-bit environment sets int, long and pointer to 32 bits.
The 64-bit environment sets int to 32 bits and long and pointer
to 64 bits. These are HP-UX specific flags.
- -mno-sched-br-data-spec
-
- -msched-br-data-spec
-
(Dis/En)able data speculative scheduling before reload.
This results in generation of "ld.a" instructions and
the corresponding check instructions ("ld.c" / "chk.a").
The default is 'disable'.
- -msched-ar-data-spec
-
- -mno-sched-ar-data-spec
-
(En/Dis)able data speculative scheduling after reload.
This results in generation of "ld.a" instructions and
the corresponding check instructions ("ld.c" / "chk.a").
The default is 'enable'.
- -mno-sched-control-spec
-
- -msched-control-spec
-
(Dis/En)able control speculative scheduling. This feature is
available only during region scheduling (i.e. before reload).
This results in generation of the "ld.s" instructions and
the corresponding check instructions "chk.s".
The default is 'disable'.
- -msched-br-in-data-spec
-
- -mno-sched-br-in-data-spec
-
(En/Dis)able speculative scheduling of the instructions that
are dependent on the data speculative loads before reload.
This is effective only with -msched-br-data-spec enabled.
The default is 'enable'.
- -msched-ar-in-data-spec
-
- -mno-sched-ar-in-data-spec
-
(En/Dis)able speculative scheduling of the instructions that
are dependent on the data speculative loads after reload.
This is effective only with -msched-ar-data-spec enabled.
The default is 'enable'.
- -msched-in-control-spec
-
- -mno-sched-in-control-spec
-
(En/Dis)able speculative scheduling of the instructions that
are dependent on the control speculative loads.
This is effective only with -msched-control-spec enabled.
The default is 'enable'.
- -mno-sched-prefer-non-data-spec-insns
-
- -msched-prefer-non-data-spec-insns
-
If enabled, data-speculative instructions are chosen for schedule
only if there are no other choices at the moment. This makes
the use of the data speculation much more conservative.
The default is 'disable'.
- -mno-sched-prefer-non-control-spec-insns
-
- -msched-prefer-non-control-spec-insns
-
If enabled, control-speculative instructions are chosen for schedule
only if there are no other choices at the moment. This makes
the use of the control speculation much more conservative.
The default is 'disable'.
- -mno-sched-count-spec-in-critical-path
-
- -msched-count-spec-in-critical-path
-
If enabled, speculative dependencies are considered during
computation of the instructions priorities. This makes the use of the
speculation a bit more conservative.
The default is 'disable'.
- -msched-spec-ldc
-
Use a simple data speculation check. This option is on by default.
- -msched-control-spec-ldc
-
Use a simple check for control speculation. This option is on by default.
- -msched-stop-bits-after-every-cycle
-
Place a stop bit after every cycle when scheduling. This option is on
by default.
- -msched-fp-mem-deps-zero-cost
-
Assume that floating-point stores and loads are not likely to cause a conflict
when placed into the same instruction group. This option is disabled by
default.
- -msel-sched-dont-check-control-spec
-
Generate checks for control speculation in selective scheduling.
This flag is disabled by default.
- -msched-max-memory-insns=max-insns
-
Limit on the number of memory insns per instruction group, giving lower
priority to subsequent memory insns attempting to schedule in the same
instruction group. Frequently useful to prevent cache bank conflicts.
The default value is 1.
- -msched-max-memory-insns-hard-limit
-
Makes the limit specified by msched-max-memory-insns a hard limit,
disallowing more than that number in an instruction group.
Otherwise, the limit is ``soft'', meaning that non-memory operations
are preferred when the limit is reached, but memory operations may still
be scheduled.
LM32 Options
These -m options are defined for the LatticeMico32 architecture:
- -mbarrel-shift-enabled
-
Enable barrel-shift instructions.
- -mdivide-enabled
-
Enable divide and modulus instructions.
- -mmultiply-enabled
-
Enable multiply instructions.
- -msign-extend-enabled
-
Enable sign extend instructions.
- -muser-enabled
-
Enable user-defined instructions.
M32C Options
- -mcpu=name
-
Select the CPU for which code is generated. name may be one of
r8c for the R8C/Tiny series, m16c for the M16C (up to
/60) series, m32cm for the M16C/80 series, or m32c for
the M32C/80 series.
- -msim
-
Specifies that the program will be run on the simulator. This causes
an alternate runtime library to be linked in which supports, for
example, file I/O. You must not use this option when generating
programs that will run on real hardware; you must provide your own
runtime library for whatever I/O functions are needed.
- -memregs=number
-
Specifies the number of memory-based pseudo-registers GCC uses
during code generation. These pseudo-registers are used like real
registers, so there is a tradeoff between GCC's ability to fit the
code into available registers, and the performance penalty of using
memory instead of registers. Note that all modules in a program must
be compiled with the same value for this option. Because of that, you
must not use this option with GCC's default runtime libraries.
M32R/D Options
These -m options are defined for Renesas M32R/D architectures:
- -m32r2
-
Generate code for the M32R/2.
- -m32rx
-
Generate code for the M32R/X.
- -m32r
-
Generate code for the M32R. This is the default.
- -mmodel=small
-
Assume all objects live in the lower 16MB of memory (so that their addresses
can be loaded with the "ld24" instruction), and assume all subroutines
are reachable with the "bl" instruction.
This is the default.
The addressability of a particular object can be set with the
"model" attribute.
- -mmodel=medium
-
Assume objects may be anywhere in the 32-bit address space (the compiler
generates "seth/add3" instructions to load their addresses), and
assume all subroutines are reachable with the "bl" instruction.
- -mmodel=large
-
Assume objects may be anywhere in the 32-bit address space (the compiler
generates "seth/add3" instructions to load their addresses), and
assume subroutines may not be reachable with the "bl" instruction
(the compiler generates the much slower "seth/add3/jl"
instruction sequence).
- -msdata=none
-
Disable use of the small data area. Variables are put into
one of ".data", ".bss", or ".rodata" (unless the
"section" attribute has been specified).
This is the default.
The small data area consists of sections ".sdata" and ".sbss".
Objects may be explicitly put in the small data area with the
"section" attribute using one of these sections.
- -msdata=sdata
-
Put small global and static data in the small data area, but do not
generate special code to reference them.
- -msdata=use
-
Put small global and static data in the small data area, and generate
special instructions to reference them.
- -G num
-
Put global and static objects less than or equal to num bytes
into the small data or BSS sections instead of the normal data or BSS
sections. The default value of num is 8.
The -msdata option must be set to one of sdata or use
for this option to have any effect.
All modules should be compiled with the same -G num value.
Compiling with different values of num may or may not work; if it
doesn't the linker gives an error message---incorrect code is not
generated.
- -mdebug
-
Makes the M32R-specific code in the compiler display some statistics
that might help in debugging programs.
- -malign-loops
-
Align all loops to a 32-byte boundary.
- -mno-align-loops
-
Do not enforce a 32-byte alignment for loops. This is the default.
- -missue-rate=number
-
Issue number instructions per cycle. number can only be 1
or 2.
- -mbranch-cost=number
-
number can only be 1 or 2. If it is 1 then branches are
preferred over conditional code, if it is 2, then the opposite applies.
- -mflush-trap=number
-
Specifies the trap number to use to flush the cache. The default is
12. Valid numbers are between 0 and 15 inclusive.
- -mno-flush-trap
-
Specifies that the cache cannot be flushed by using a trap.
- -mflush-func=name
-
Specifies the name of the operating system function to call to flush
the cache. The default is _flush_cache, but a function call
is only used if a trap is not available.
- -mno-flush-func
-
Indicates that there is no OS function for flushing the cache.
M680x0 Options
These are the -m options defined for M680x0 and ColdFire processors.
The default settings depend on which architecture was selected when
the compiler was configured; the defaults for the most common choices
are given below.
- -march=arch
-
Generate code for a specific M680x0 or ColdFire instruction set
architecture. Permissible values of arch for M680x0
architectures are: 68000, 68010, 68020,
68030, 68040, 68060 and cpu32. ColdFire
architectures are selected according to Freescale's ISA classification
and the permissible values are: isaa, isaaplus,
isab and isac.
GCC defines a macro "__mcfarch__" whenever it is generating
code for a ColdFire target. The arch in this macro is one of the
-march arguments given above.
When used together, -march and -mtune select code
that runs on a family of similar processors but that is optimized
for a particular microarchitecture.
- -mcpu=cpu
-
Generate code for a specific M680x0 or ColdFire processor.
The M680x0 cpus are: 68000, 68010, 68020,
68030, 68040, 68060, 68302, 68332
and cpu32. The ColdFire cpus are given by the table
below, which also classifies the CPUs into families:
-
- Family : -mcpu arguments
-
- 51 : 51 51ac 51ag 51cn 51em 51je 51jf 51jg 51jm 51mm 51qe 51qm
-
- 5206 : 5202 5204 5206
-
- 5206e : 5206e
-
- 5208 : 5207 5208
-
- 5211a : 5210a 5211a
-
- 5213 : 5211 5212 5213
-
- 5216 : 5214 5216
-
- 52235 : 52230 52231 52232 52233 52234 52235
-
- 5225 : 5224 5225
-
- 52259 : 52252 52254 52255 52256 52258 52259
-
- 5235 : 5232 5233 5234 5235 523x
-
- 5249 : 5249
-
- 5250 : 5250
-
- 5271 : 5270 5271
-
- 5272 : 5272
-
- 5275 : 5274 5275
-
- 5282 : 5280 5281 5282 528x
-
- 53017 : 53011 53012 53013 53014 53015 53016 53017
-
- 5307 : 5307
-
- 5329 : 5327 5328 5329 532x
-
- 5373 : 5372 5373 537x
-
- 5407 : 5407
-
- 5475 : 5470 5471 5472 5473 5474 5475 547x 5480 5481 5482 5483 5484 5485
-
-
-mcpu=cpu overrides -march=arch if
arch is compatible with cpu. Other combinations of
-mcpu and -march are rejected.
GCC defines the macro "__mcf_cpu_cpu" when ColdFire target
cpu is selected. It also defines "__mcf_family_family",
where the value of family is given by the table above.
- -mtune=tune
-
Tune the code for a particular microarchitecture within the
constraints set by -march and -mcpu.
The M680x0 microarchitectures are: 68000, 68010,
68020, 68030, 68040, 68060
and cpu32. The ColdFire microarchitectures
are: cfv1, cfv2, cfv3, cfv4 and cfv4e.
You can also use -mtune=68020-40 for code that needs
to run relatively well on 68020, 68030 and 68040 targets.
-mtune=68020-60 is similar but includes 68060 targets
as well. These two options select the same tuning decisions as
-m68020-40 and -m68020-60 respectively.
GCC defines the macros "__mcarch" and "__mcarch__"
when tuning for 680x0 architecture arch. It also defines
"mcarch" unless either -ansi or a non-GNU -std
option is used. If GCC is tuning for a range of architectures,
as selected by -mtune=68020-40 or -mtune=68020-60,
it defines the macros for every architecture in the range.
GCC also defines the macro "__muarch__" when tuning for
ColdFire microarchitecture uarch, where uarch is one
of the arguments given above.
- -m68000
-
- -mc68000
-
Generate output for a 68000. This is the default
when the compiler is configured for 68000-based systems.
It is equivalent to -march=68000.
Use this option for microcontrollers with a 68000 or EC000 core,
including the 68008, 68302, 68306, 68307, 68322, 68328 and 68356.
- -m68010
-
Generate output for a 68010. This is the default
when the compiler is configured for 68010-based systems.
It is equivalent to -march=68010.
- -m68020
-
- -mc68020
-
Generate output for a 68020. This is the default
when the compiler is configured for 68020-based systems.
It is equivalent to -march=68020.
- -m68030
-
Generate output for a 68030. This is the default when the compiler is
configured for 68030-based systems. It is equivalent to
-march=68030.
- -m68040
-
Generate output for a 68040. This is the default when the compiler is
configured for 68040-based systems. It is equivalent to
-march=68040.
This option inhibits the use of 68881/68882 instructions that have to be
emulated by software on the 68040. Use this option if your 68040 does not
have code to emulate those instructions.
- -m68060
-
Generate output for a 68060. This is the default when the compiler is
configured for 68060-based systems. It is equivalent to
-march=68060.
This option inhibits the use of 68020 and 68881/68882 instructions that
have to be emulated by software on the 68060. Use this option if your 68060
does not have code to emulate those instructions.
- -mcpu32
-
Generate output for a CPU32. This is the default
when the compiler is configured for CPU32-based systems.
It is equivalent to -march=cpu32.
Use this option for microcontrollers with a
CPU32 or CPU32+ core, including the 68330, 68331, 68332, 68333, 68334,
68336, 68340, 68341, 68349 and 68360.
- -m5200
-
Generate output for a 520X ColdFire CPU. This is the default
when the compiler is configured for 520X-based systems.
It is equivalent to -mcpu=5206, and is now deprecated
in favor of that option.
Use this option for microcontroller with a 5200 core, including
the MCF5202, MCF5203, MCF5204 and MCF5206.
- -m5206e
-
Generate output for a 5206e ColdFire CPU. The option is now
deprecated in favor of the equivalent -mcpu=5206e.
- -m528x
-
Generate output for a member of the ColdFire 528X family.
The option is now deprecated in favor of the equivalent
-mcpu=528x.
- -m5307
-
Generate output for a ColdFire 5307 CPU. The option is now deprecated
in favor of the equivalent -mcpu=5307.
- -m5407
-
Generate output for a ColdFire 5407 CPU. The option is now deprecated
in favor of the equivalent -mcpu=5407.
- -mcfv4e
-
Generate output for a ColdFire V4e family CPU (e.g. 547x/548x).
This includes use of hardware floating-point instructions.
The option is equivalent to -mcpu=547x, and is now
deprecated in favor of that option.
- -m68020-40
-
Generate output for a 68040, without using any of the new instructions.
This results in code that can run relatively efficiently on either a
68020/68881 or a 68030 or a 68040. The generated code does use the
68881 instructions that are emulated on the 68040.
The option is equivalent to -march=68020 -mtune=68020-40.
- -m68020-60
-
Generate output for a 68060, without using any of the new instructions.
This results in code that can run relatively efficiently on either a
68020/68881 or a 68030 or a 68040. The generated code does use the
68881 instructions that are emulated on the 68060.
The option is equivalent to -march=68020 -mtune=68020-60.
- -mhard-float
-
- -m68881
-
Generate floating-point instructions. This is the default for 68020
and above, and for ColdFire devices that have an FPU. It defines the
macro "__HAVE_68881__" on M680x0 targets and "__mcffpu__"
on ColdFire targets.
- -msoft-float
-
Do not generate floating-point instructions; use library calls instead.
This is the default for 68000, 68010, and 68832 targets. It is also
the default for ColdFire devices that have no FPU.
- -mdiv
-
- -mno-div
-
Generate (do not generate) ColdFire hardware divide and remainder
instructions. If -march is used without -mcpu,
the default is ``on'' for ColdFire architectures and ``off'' for M680x0
architectures. Otherwise, the default is taken from the target CPU
(either the default CPU, or the one specified by -mcpu). For
example, the default is ``off'' for -mcpu=5206 and ``on'' for
-mcpu=5206e.
GCC defines the macro "__mcfhwdiv__" when this option is enabled.
- -mshort
-
Consider type "int" to be 16 bits wide, like "short int".
Additionally, parameters passed on the stack are also aligned to a
16-bit boundary even on targets whose API mandates promotion to 32-bit.
- -mno-short
-
Do not consider type "int" to be 16 bits wide. This is the default.
- -mnobitfield
-
- -mno-bitfield
-
Do not use the bit-field instructions. The -m68000, -mcpu32
and -m5200 options imply -mnobitfield.
- -mbitfield
-
Do use the bit-field instructions. The -m68020 option implies
-mbitfield. This is the default if you use a configuration
designed for a 68020.
- -mrtd
-
Use a different function-calling convention, in which functions
that take a fixed number of arguments return with the "rtd"
instruction, which pops their arguments while returning. This
saves one instruction in the caller since there is no need to pop
the arguments there.
This calling convention is incompatible with the one normally
used on Unix, so you cannot use it if you need to call libraries
compiled with the Unix compiler.
Also, you must provide function prototypes for all functions that
take variable numbers of arguments (including "printf");
otherwise incorrect code is generated for calls to those
functions.
In addition, seriously incorrect code results if you call a
function with too many arguments. (Normally, extra arguments are
harmlessly ignored.)
The "rtd" instruction is supported by the 68010, 68020, 68030,
68040, 68060 and CPU32 processors, but not by the 68000 or 5200.
- -mno-rtd
-
Do not use the calling conventions selected by -mrtd.
This is the default.
- -malign-int
-
- -mno-align-int
-
Control whether GCC aligns "int", "long", "long long",
"float", "double", and "long double" variables on a 32-bit
boundary (-malign-int) or a 16-bit boundary (-mno-align-int).
Aligning variables on 32-bit boundaries produces code that runs somewhat
faster on processors with 32-bit busses at the expense of more memory.
Warning: if you use the -malign-int switch, GCC
aligns structures containing the above types differently than
most published application binary interface specifications for the m68k.
- -mpcrel
-
Use the pc-relative addressing mode of the 68000 directly, instead of
using a global offset table. At present, this option implies -fpic,
allowing at most a 16-bit offset for pc-relative addressing. -fPIC is
not presently supported with -mpcrel, though this could be supported for
68020 and higher processors.
- -mno-strict-align
-
- -mstrict-align
-
Do not (do) assume that unaligned memory references are handled by
the system.
- -msep-data
-
Generate code that allows the data segment to be located in a different
area of memory from the text segment. This allows for execute-in-place in
an environment without virtual memory management. This option implies
-fPIC.
- -mno-sep-data
-
Generate code that assumes that the data segment follows the text segment.
This is the default.
- -mid-shared-library
-
Generate code that supports shared libraries via the library ID method.
This allows for execute-in-place and shared libraries in an environment
without virtual memory management. This option implies -fPIC.
- -mno-id-shared-library
-
Generate code that doesn't assume ID-based shared libraries are being used.
This is the default.
- -mshared-library-id=n
-
Specifies the identification number of the ID-based shared library being
compiled. Specifying a value of 0 generates more compact code; specifying
other values forces the allocation of that number to the current
library, but is no more space- or time-efficient than omitting this option.
- -mxgot
-
- -mno-xgot
-
When generating position-independent code for ColdFire, generate code
that works if the GOT has more than 8192 entries. This code is
larger and slower than code generated without this option. On M680x0
processors, this option is not needed; -fPIC suffices.
GCC normally uses a single instruction to load values from the GOT.
While this is relatively efficient, it only works if the GOT
is smaller than about 64k. Anything larger causes the linker
to report an error such as:
relocation truncated to fit: R_68K_GOT16O foobar
If this happens, you should recompile your code with -mxgot.
It should then work with very large GOTs. However, code generated with
-mxgot is less efficient, since it takes 4 instructions to fetch
the value of a global symbol.
Note that some linkers, including newer versions of the GNU linker,
can create multiple GOTs and sort GOT entries. If you have such a linker,
you should only need to use -mxgot when compiling a single
object file that accesses more than 8192 GOT entries. Very few do.
These options have no effect unless GCC is generating
position-independent code.
MCore Options
These are the -m options defined for the Motorola M*Core
processors.
- -mhardlit
-
- -mno-hardlit
-
Inline constants into the code stream if it can be done in two
instructions or less.
- -mdiv
-
- -mno-div
-
Use the divide instruction. (Enabled by default).
- -mrelax-immediate
-
- -mno-relax-immediate
-
Allow arbitrary-sized immediates in bit operations.
- -mwide-bitfields
-
- -mno-wide-bitfields
-
Always treat bit-fields as "int"-sized.
- -m4byte-functions
-
- -mno-4byte-functions
-
Force all functions to be aligned to a 4-byte boundary.
- -mcallgraph-data
-
- -mno-callgraph-data
-
Emit callgraph information.
- -mslow-bytes
-
- -mno-slow-bytes
-
Prefer word access when reading byte quantities.
- -mlittle-endian
-
- -mbig-endian
-
Generate code for a little-endian target.
- -m210
-
- -m340
-
Generate code for the 210 processor.
- -mno-lsim
-
Assume that runtime support has been provided and so omit the
simulator library (libsim.a) from the linker command line.
- -mstack-increment=size
-
Set the maximum amount for a single stack increment operation. Large
values can increase the speed of programs that contain functions
that need a large amount of stack space, but they can also trigger a
segmentation fault if the stack is extended too much. The default
value is 0x1000.
MeP Options
- -mabsdiff
-
Enables the "abs" instruction, which is the absolute difference
between two registers.
- -mall-opts
-
Enables all the optional instructions---average, multiply, divide, bit
operations, leading zero, absolute difference, min/max, clip, and
saturation.
- -maverage
-
Enables the "ave" instruction, which computes the average of two
registers.
- -mbased=n
-
Variables of size n bytes or smaller are placed in the
".based" section by default. Based variables use the $tp
register as a base register, and there is a 128-byte limit to the
".based" section.
- -mbitops
-
Enables the bit operation instructions---bit test ("btstm"), set
("bsetm"), clear ("bclrm"), invert ("bnotm"), and
test-and-set ("tas").
- -mc=name
-
Selects which section constant data is placed in. name may
be tiny, near, or far.
- -mclip
-
Enables the "clip" instruction. Note that -mclip is not
useful unless you also provide -mminmax.
- -mconfig=name
-
Selects one of the built-in core configurations. Each MeP chip has
one or more modules in it; each module has a core CPU and a variety of
coprocessors, optional instructions, and peripherals. The
"MeP-Integrator" tool, not part of GCC, provides these
configurations through this option; using this option is the same as
using all the corresponding command-line options. The default
configuration is default.
- -mcop
-
Enables the coprocessor instructions. By default, this is a 32-bit
coprocessor. Note that the coprocessor is normally enabled via the
-mconfig= option.
- -mcop32
-
Enables the 32-bit coprocessor's instructions.
- -mcop64
-
Enables the 64-bit coprocessor's instructions.
- -mivc2
-
Enables IVC2 scheduling. IVC2 is a 64-bit VLIW coprocessor.
- -mdc
-
Causes constant variables to be placed in the ".near" section.
- -mdiv
-
Enables the "div" and "divu" instructions.
- -meb
-
Generate big-endian code.
- -mel
-
Generate little-endian code.
- -mio-volatile
-
Tells the compiler that any variable marked with the "io"
attribute is to be considered volatile.
- -ml
-
Causes variables to be assigned to the ".far" section by default.
- -mleadz
-
Enables the "leadz" (leading zero) instruction.
- -mm
-
Causes variables to be assigned to the ".near" section by default.
- -mminmax
-
Enables the "min" and "max" instructions.
- -mmult
-
Enables the multiplication and multiply-accumulate instructions.
- -mno-opts
-
Disables all the optional instructions enabled by -mall-opts.
- -mrepeat
-
Enables the "repeat" and "erepeat" instructions, used for
low-overhead looping.
- -ms
-
Causes all variables to default to the ".tiny" section. Note
that there is a 65536-byte limit to this section. Accesses to these
variables use the %gp base register.
- -msatur
-
Enables the saturation instructions. Note that the compiler does not
currently generate these itself, but this option is included for
compatibility with other tools, like "as".
- -msdram
-
Link the SDRAM-based runtime instead of the default ROM-based runtime.
- -msim
-
Link the simulator run-time libraries.
- -msimnovec
-
Link the simulator runtime libraries, excluding built-in support
for reset and exception vectors and tables.
- -mtf
-
Causes all functions to default to the ".far" section. Without
this option, functions default to the ".near" section.
- -mtiny=n
-
Variables that are n bytes or smaller are allocated to the
".tiny" section. These variables use the $gp base
register. The default for this option is 4, but note that there's a
65536-byte limit to the ".tiny" section.
MicroBlaze Options
- -msoft-float
-
Use software emulation for floating point (default).
- -mhard-float
-
Use hardware floating-point instructions.
- -mmemcpy
-
Do not optimize block moves, use "memcpy".
- -mno-clearbss
-
This option is deprecated. Use -fno-zero-initialized-in-bss instead.
- -mcpu=cpu-type
-
Use features of, and schedule code for, the given CPU.
Supported values are in the format vX.YY.Z,
where X is a major version, YY is the minor version, and
Z is compatibility code. Example values are v3.00.a,
v4.00.b, v5.00.a, v5.00.b, v5.00.b, v6.00.a.
- -mxl-soft-mul
-
Use software multiply emulation (default).
- -mxl-soft-div
-
Use software emulation for divides (default).
- -mxl-barrel-shift
-
Use the hardware barrel shifter.
- -mxl-pattern-compare
-
Use pattern compare instructions.
- -msmall-divides
-
Use table lookup optimization for small signed integer divisions.
- -mxl-stack-check
-
This option is deprecated. Use -fstack-check instead.
- -mxl-gp-opt
-
Use GP-relative ".sdata"/".sbss" sections.
- -mxl-multiply-high
-
Use multiply high instructions for high part of 32x32 multiply.
- -mxl-float-convert
-
Use hardware floating-point conversion instructions.
- -mxl-float-sqrt
-
Use hardware floating-point square root instruction.
- -mbig-endian
-
Generate code for a big-endian target.
- -mlittle-endian
-
Generate code for a little-endian target.
- -mxl-reorder
-
Use reorder instructions (swap and byte reversed load/store).
- -mxl-mode-app-model
-
Select application model app-model. Valid models are
-
- executable
-
normal executable (default), uses startup code crt0.o.
- xmdstub
-
for use with Xilinx Microprocessor Debugger (XMD) based
software intrusive debug agent called xmdstub. This uses startup file
crt1.o and sets the start address of the program to 0x800.
- bootstrap
-
for applications that are loaded using a bootloader.
This model uses startup file crt2.o which does not contain a processor
reset vector handler. This is suitable for transferring control on a
processor reset to the bootloader rather than the application.
- novectors
-
for applications that do not require any of the
MicroBlaze vectors. This option may be useful for applications running
within a monitoring application. This model uses crt3.o as a startup file.
-
Option -xl-mode-app-model is a deprecated alias for
-mxl-mode-app-model.
MIPS Options
- -EB
-
Generate big-endian code.
- -EL
-
Generate little-endian code. This is the default for mips*el-*-*
configurations.
- -march=arch
-
Generate code that runs on arch, which can be the name of a
generic MIPS ISA, or the name of a particular processor.
The ISA names are:
mips1, mips2, mips3, mips4,
mips32, mips32r2, mips32r3, mips32r5,
mips32r6, mips64, mips64r2, mips64r3,
mips64r5 and mips64r6.
The processor names are:
4kc, 4km, 4kp, 4ksc,
4kec, 4kem, 4kep, 4ksd,
5kc, 5kf,
20kc,
24kc, 24kf2_1, 24kf1_1,
24kec, 24kef2_1, 24kef1_1,
34kc, 34kf2_1, 34kf1_1, 34kn,
74kc, 74kf2_1, 74kf1_1, 74kf3_2,
1004kc, 1004kf2_1, 1004kf1_1,
loongson2e, loongson2f, loongson3a,
m4k,
m14k, m14kc, m14ke, m14kec,
octeon, octeon+, octeon2, octeon3,
orion,
p5600,
r2000, r3000, r3900, r4000, r4400,
r4600, r4650, r4700, r6000, r8000,
rm7000, rm9000,
r10000, r12000, r14000, r16000,
sb1,
sr71000,
vr4100, vr4111, vr4120, vr4130, vr4300,
vr5000, vr5400, vr5500,
xlr and xlp.
The special value from-abi selects the
most compatible architecture for the selected ABI (that is,
mips1 for 32-bit ABIs and mips3 for 64-bit ABIs).
The native Linux/GNU toolchain also supports the value native,
which selects the best architecture option for the host processor.
-march=native has no effect if GCC does not recognize
the processor.
In processor names, a final 000 can be abbreviated as k
(for example, -march=r2k). Prefixes are optional, and
vr may be written r.
Names of the form nf2_1 refer to processors with
FPUs clocked at half the rate of the core, names of the form
nf1_1 refer to processors with FPUs clocked at the same
rate as the core, and names of the form nf3_2 refer to
processors with FPUs clocked a ratio of 3:2 with respect to the core.
For compatibility reasons, nf is accepted as a synonym
for nf2_1 while nx and bfx are
accepted as synonyms for nf1_1.
GCC defines two macros based on the value of this option. The first
is "_MIPS_ARCH", which gives the name of target architecture, as
a string. The second has the form "_MIPS_ARCH_foo",
where foo is the capitalized value of "_MIPS_ARCH".
For example, -march=r2000 sets "_MIPS_ARCH"
to "r2000" and defines the macro "_MIPS_ARCH_R2000".
Note that the "_MIPS_ARCH" macro uses the processor names given
above. In other words, it has the full prefix and does not
abbreviate 000 as k. In the case of from-abi,
the macro names the resolved architecture (either "mips1" or
"mips3"). It names the default architecture when no
-march option is given.
- -mtune=arch
-
Optimize for arch. Among other things, this option controls
the way instructions are scheduled, and the perceived cost of arithmetic
operations. The list of arch values is the same as for
-march.
When this option is not used, GCC optimizes for the processor
specified by -march. By using -march and
-mtune together, it is possible to generate code that
runs on a family of processors, but optimize the code for one
particular member of that family.
-mtune defines the macros "_MIPS_TUNE" and
"_MIPS_TUNE_foo", which work in the same way as the
-march ones described above.
- -mips1
-
Equivalent to -march=mips1.
- -mips2
-
Equivalent to -march=mips2.
- -mips3
-
Equivalent to -march=mips3.
- -mips4
-
Equivalent to -march=mips4.
- -mips32
-
Equivalent to -march=mips32.
- -mips32r3
-
Equivalent to -march=mips32r3.
- -mips32r5
-
Equivalent to -march=mips32r5.
- -mips32r6
-
Equivalent to -march=mips32r6.
- -mips64
-
Equivalent to -march=mips64.
- -mips64r2
-
Equivalent to -march=mips64r2.
- -mips64r3
-
Equivalent to -march=mips64r3.
- -mips64r5
-
Equivalent to -march=mips64r5.
- -mips64r6
-
Equivalent to -march=mips64r6.
- -mips16
-
- -mno-mips16
-
Generate (do not generate) MIPS16 code. If GCC is targeting a
MIPS32 or MIPS64 architecture, it makes use of the MIPS16e ASE.
MIPS16 code generation can also be controlled on a per-function basis
by means of "mips16" and "nomips16" attributes.
- -mflip-mips16
-
Generate MIPS16 code on alternating functions. This option is provided
for regression testing of mixed MIPS16/non-MIPS16 code generation, and is
not intended for ordinary use in compiling user code.
- -minterlink-compressed
-
- -mno-interlink-compressed
-
Require (do not require) that code using the standard (uncompressed) MIPS ISA
be link-compatible with MIPS16 and microMIPS code, and vice versa.
For example, code using the standard ISA encoding cannot jump directly
to MIPS16 or microMIPS code; it must either use a call or an indirect jump.
-minterlink-compressed therefore disables direct jumps unless GCC
knows that the target of the jump is not compressed.
- -minterlink-mips16
-
- -mno-interlink-mips16
-
Aliases of -minterlink-compressed and
-mno-interlink-compressed. These options predate the microMIPS ASE
and are retained for backwards compatibility.
- -mabi=32
-
- -mabi=o64
-
- -mabi=n32
-
- -mabi=64
-
- -mabi=eabi
-
Generate code for the given ABI.
Note that the EABI has a 32-bit and a 64-bit variant. GCC normally
generates 64-bit code when you select a 64-bit architecture, but you
can use -mgp32 to get 32-bit code instead.
For information about the O64 ABI, see
<http://gcc.gnu.org/projects/mipso64-abi.html>.
GCC supports a variant of the o32 ABI in which floating-point registers
are 64 rather than 32 bits wide. You can select this combination with
-mabi=32 -mfp64. This ABI relies on the "mthc1"
and "mfhc1" instructions and is therefore only supported for
MIPS32R2, MIPS32R3 and MIPS32R5 processors.
The register assignments for arguments and return values remain the
same, but each scalar value is passed in a single 64-bit register
rather than a pair of 32-bit registers. For example, scalar
floating-point values are returned in $f0 only, not a
$f0/$f1 pair. The set of call-saved registers also
remains the same in that the even-numbered double-precision registers
are saved.
Two additional variants of the o32 ABI are supported to enable
a transition from 32-bit to 64-bit registers. These are FPXX
(-mfpxx) and FP64A (-mfp64 -mno-odd-spreg).
The FPXX extension mandates that all code must execute correctly
when run using 32-bit or 64-bit registers. The code can be interlinked
with either FP32 or FP64, but not both.
The FP64A extension is similar to the FP64 extension but forbids the
use of odd-numbered single-precision registers. This can be used
in conjunction with the "FRE" mode of FPUs in MIPS32R5
processors and allows both FP32 and FP64A code to interlink and
run in the same process without changing FPU modes.
- -mabicalls
-
- -mno-abicalls
-
Generate (do not generate) code that is suitable for SVR4-style
dynamic objects. -mabicalls is the default for SVR4-based
systems.
- -mshared
-
- -mno-shared
-
Generate (do not generate) code that is fully position-independent,
and that can therefore be linked into shared libraries. This option
only affects -mabicalls.
All -mabicalls code has traditionally been position-independent,
regardless of options like -fPIC and -fpic. However,
as an extension, the GNU toolchain allows executables to use absolute
accesses for locally-binding symbols. It can also use shorter GP
initialization sequences and generate direct calls to locally-defined
functions. This mode is selected by -mno-shared.
-mno-shared depends on binutils 2.16 or higher and generates
objects that can only be linked by the GNU linker. However, the option
does not affect the ABI of the final executable; it only affects the ABI
of relocatable objects. Using -mno-shared generally makes
executables both smaller and quicker.
-mshared is the default.
- -mplt
-
- -mno-plt
-
Assume (do not assume) that the static and dynamic linkers
support PLTs and copy relocations. This option only affects
-mno-shared -mabicalls. For the n64 ABI, this option
has no effect without -msym32.
You can make -mplt the default by configuring
GCC with --with-mips-plt. The default is
-mno-plt otherwise.
- -mxgot
-
- -mno-xgot
-
Lift (do not lift) the usual restrictions on the size of the global
offset table.
GCC normally uses a single instruction to load values from the GOT.
While this is relatively efficient, it only works if the GOT
is smaller than about 64k. Anything larger causes the linker
to report an error such as:
relocation truncated to fit: R_MIPS_GOT16 foobar
If this happens, you should recompile your code with -mxgot.
This works with very large GOTs, although the code is also
less efficient, since it takes three instructions to fetch the
value of a global symbol.
Note that some linkers can create multiple GOTs. If you have such a
linker, you should only need to use -mxgot when a single object
file accesses more than 64k's worth of GOT entries. Very few do.
These options have no effect unless GCC is generating position
independent code.
- -mgp32
-
Assume that general-purpose registers are 32 bits wide.
- -mgp64
-
Assume that general-purpose registers are 64 bits wide.
- -mfp32
-
Assume that floating-point registers are 32 bits wide.
- -mfp64
-
Assume that floating-point registers are 64 bits wide.
- -mfpxx
-
Do not assume the width of floating-point registers.
- -mhard-float
-
Use floating-point coprocessor instructions.
- -msoft-float
-
Do not use floating-point coprocessor instructions. Implement
floating-point calculations using library calls instead.
- -mno-float
-
Equivalent to -msoft-float, but additionally asserts that the
program being compiled does not perform any floating-point operations.
This option is presently supported only by some bare-metal MIPS
configurations, where it may select a special set of libraries
that lack all floating-point support (including, for example, the
floating-point "printf" formats).
If code compiled with -mno-float accidentally contains
floating-point operations, it is likely to suffer a link-time
or run-time failure.
- -msingle-float
-
Assume that the floating-point coprocessor only supports single-precision
operations.
- -mdouble-float
-
Assume that the floating-point coprocessor supports double-precision
operations. This is the default.
- -modd-spreg
-
- -mno-odd-spreg
-
Enable the use of odd-numbered single-precision floating-point registers
for the o32 ABI. This is the default for processors that are known to
support these registers. When using the o32 FPXX ABI, -mno-odd-spreg
is set by default.
- -mabs=2008
-
- -mabs=legacy
-
These options control the treatment of the special not-a-number (NaN)
IEEE 754 floating-point data with the "abs.fmt" and
"neg.fmt" machine instructions.
By default or when -mabs=legacy is used the legacy
treatment is selected. In this case these instructions are considered
arithmetic and avoided where correct operation is required and the
input operand might be a NaN. A longer sequence of instructions that
manipulate the sign bit of floating-point datum manually is used
instead unless the -ffinite-math-only option has also been
specified.
The -mabs=2008 option selects the IEEE 754-2008 treatment. In
this case these instructions are considered non-arithmetic and therefore
operating correctly in all cases, including in particular where the
input operand is a NaN. These instructions are therefore always used
for the respective operations.
- -mnan=2008
-
- -mnan=legacy
-
These options control the encoding of the special not-a-number (NaN)
IEEE 754 floating-point data.
The -mnan=legacy option selects the legacy encoding. In this
case quiet NaNs (qNaNs) are denoted by the first bit of their trailing
significand field being 0, whereas signalling NaNs (sNaNs) are denoted
by the first bit of their trailing significand field being 1.
The -mnan=2008 option selects the IEEE 754-2008 encoding. In
this case qNaNs are denoted by the first bit of their trailing
significand field being 1, whereas sNaNs are denoted by the first bit of
their trailing significand field being 0.
The default is -mnan=legacy unless GCC has been configured with
--with-nan=2008.
- -mllsc
-
- -mno-llsc
-
Use (do not use) ll, sc, and sync instructions to
implement atomic memory built-in functions. When neither option is
specified, GCC uses the instructions if the target architecture
supports them.
-mllsc is useful if the runtime environment can emulate the
instructions and -mno-llsc can be useful when compiling for
nonstandard ISAs. You can make either option the default by
configuring GCC with --with-llsc and --without-llsc
respectively. --with-llsc is the default for some
configurations; see the installation documentation for details.
- -mdsp
-
- -mno-dsp
-
Use (do not use) revision 1 of the MIPS DSP ASE.
This option defines the
preprocessor macro "__mips_dsp". It also defines
"__mips_dsp_rev" to 1.
- -mdspr2
-
- -mno-dspr2
-
Use (do not use) revision 2 of the MIPS DSP ASE.
This option defines the
preprocessor macros "__mips_dsp" and "__mips_dspr2".
It also defines "__mips_dsp_rev" to 2.
- -msmartmips
-
- -mno-smartmips
-
Use (do not use) the MIPS SmartMIPS ASE.
- -mpaired-single
-
- -mno-paired-single
-
Use (do not use) paired-single floating-point instructions.
This option requires
hardware floating-point support to be enabled.
- -mdmx
-
- -mno-mdmx
-
Use (do not use) MIPS Digital Media Extension instructions.
This option can only be used when generating 64-bit code and requires
hardware floating-point support to be enabled.
- -mips3d
-
- -mno-mips3d
-
Use (do not use) the MIPS-3D ASE.
The option -mips3d implies -mpaired-single.
- -mmicromips
-
- -mno-micromips
-
Generate (do not generate) microMIPS code.
MicroMIPS code generation can also be controlled on a per-function basis
by means of "micromips" and "nomicromips" attributes.
- -mmt
-
- -mno-mt
-
Use (do not use) MT Multithreading instructions.
- -mmcu
-
- -mno-mcu
-
Use (do not use) the MIPS MCU ASE instructions.
- -meva
-
- -mno-eva
-
Use (do not use) the MIPS Enhanced Virtual Addressing instructions.
- -mvirt
-
- -mno-virt
-
Use (do not use) the MIPS Virtualization Application Specific instructions.
- -mxpa
-
- -mno-xpa
-
Use (do not use) the MIPS eXtended Physical Address (XPA) instructions.
- -mlong64
-
Force "long" types to be 64 bits wide. See -mlong32 for
an explanation of the default and the way that the pointer size is
determined.
- -mlong32
-
Force "long", "int", and pointer types to be 32 bits wide.
The default size of "int"s, "long"s and pointers depends on
the ABI. All the supported ABIs use 32-bit "int"s. The n64 ABI
uses 64-bit "long"s, as does the 64-bit EABI; the others use
32-bit "long"s. Pointers are the same size as "long"s,
or the same size as integer registers, whichever is smaller.
- -msym32
-
- -mno-sym32
-
Assume (do not assume) that all symbols have 32-bit values, regardless
of the selected ABI. This option is useful in combination with
-mabi=64 and -mno-abicalls because it allows GCC
to generate shorter and faster references to symbolic addresses.
- -G num
-
Put definitions of externally-visible data in a small data section
if that data is no bigger than num bytes. GCC can then generate
more efficient accesses to the data; see -mgpopt for details.
The default -G option depends on the configuration.
- -mlocal-sdata
-
- -mno-local-sdata
-
Extend (do not extend) the -G behavior to local data too,
such as to static variables in C. -mlocal-sdata is the
default for all configurations.
If the linker complains that an application is using too much small data,
you might want to try rebuilding the less performance-critical parts with
-mno-local-sdata. You might also want to build large
libraries with -mno-local-sdata, so that the libraries leave
more room for the main program.
- -mextern-sdata
-
- -mno-extern-sdata
-
Assume (do not assume) that externally-defined data is in
a small data section if the size of that data is within the -G limit.
-mextern-sdata is the default for all configurations.
If you compile a module Mod with -mextern-sdata -G
num -mgpopt, and Mod references a variable Var
that is no bigger than num bytes, you must make sure that Var
is placed in a small data section. If Var is defined by another
module, you must either compile that module with a high-enough
-G setting or attach a "section" attribute to Var's
definition. If Var is common, you must link the application
with a high-enough -G setting.
The easiest way of satisfying these restrictions is to compile
and link every module with the same -G option. However,
you may wish to build a library that supports several different
small data limits. You can do this by compiling the library with
the highest supported -G setting and additionally using
-mno-extern-sdata to stop the library from making assumptions
about externally-defined data.
- -mgpopt
-
- -mno-gpopt
-
Use (do not use) GP-relative accesses for symbols that are known to be
in a small data section; see -G, -mlocal-sdata and
-mextern-sdata. -mgpopt is the default for all
configurations.
-mno-gpopt is useful for cases where the $gp register
might not hold the value of "_gp". For example, if the code is
part of a library that might be used in a boot monitor, programs that
call boot monitor routines pass an unknown value in $gp.
(In such situations, the boot monitor itself is usually compiled
with -G0.)
-mno-gpopt implies -mno-local-sdata and
-mno-extern-sdata.
- -membedded-data
-
- -mno-embedded-data
-
Allocate variables to the read-only data section first if possible, then
next in the small data section if possible, otherwise in data. This gives
slightly slower code than the default, but reduces the amount of RAM required
when executing, and thus may be preferred for some embedded systems.
- -muninit-const-in-rodata
-
- -mno-uninit-const-in-rodata
-
Put uninitialized "const" variables in the read-only data section.
This option is only meaningful in conjunction with -membedded-data.
- -mcode-readable=setting
-
Specify whether GCC may generate code that reads from executable sections.
There are three possible settings:
-
- -mcode-readable=yes
-
Instructions may freely access executable sections. This is the
default setting.
- -mcode-readable=pcrel
-
MIPS16 PC-relative load instructions can access executable sections,
but other instructions must not do so. This option is useful on 4KSc
and 4KSd processors when the code TLBs have the Read Inhibit bit set.
It is also useful on processors that can be configured to have a dual
instruction/data SRAM interface and that, like the M4K, automatically
redirect PC-relative loads to the instruction RAM.
- -mcode-readable=no
-
Instructions must not access executable sections. This option can be
useful on targets that are configured to have a dual instruction/data
SRAM interface but that (unlike the M4K) do not automatically redirect
PC-relative loads to the instruction RAM.
-
- -msplit-addresses
-
- -mno-split-addresses
-
Enable (disable) use of the "%hi()" and "%lo()" assembler
relocation operators. This option has been superseded by
-mexplicit-relocs but is retained for backwards compatibility.
- -mexplicit-relocs
-
- -mno-explicit-relocs
-
Use (do not use) assembler relocation operators when dealing with symbolic
addresses. The alternative, selected by -mno-explicit-relocs,
is to use assembler macros instead.
-mexplicit-relocs is the default if GCC was configured
to use an assembler that supports relocation operators.
- -mcheck-zero-division
-
- -mno-check-zero-division
-
Trap (do not trap) on integer division by zero.
The default is -mcheck-zero-division.
- -mdivide-traps
-
- -mdivide-breaks
-
MIPS systems check for division by zero by generating either a
conditional trap or a break instruction. Using traps results in
smaller code, but is only supported on MIPS II and later. Also, some
versions of the Linux kernel have a bug that prevents trap from
generating the proper signal ("SIGFPE"). Use -mdivide-traps to
allow conditional traps on architectures that support them and
-mdivide-breaks to force the use of breaks.
The default is usually -mdivide-traps, but this can be
overridden at configure time using --with-divide=breaks.
Divide-by-zero checks can be completely disabled using
-mno-check-zero-division.
- -mmemcpy
-
- -mno-memcpy
-
Force (do not force) the use of "memcpy" for non-trivial block
moves. The default is -mno-memcpy, which allows GCC to inline
most constant-sized copies.
- -mlong-calls
-
- -mno-long-calls
-
Disable (do not disable) use of the "jal" instruction. Calling
functions using "jal" is more efficient but requires the caller
and callee to be in the same 256 megabyte segment.
This option has no effect on abicalls code. The default is
-mno-long-calls.
- -mmad
-
- -mno-mad
-
Enable (disable) use of the "mad", "madu" and "mul"
instructions, as provided by the R4650 ISA.
- -mimadd
-
- -mno-imadd
-
Enable (disable) use of the "madd" and "msub" integer
instructions. The default is -mimadd on architectures
that support "madd" and "msub" except for the 74k
architecture where it was found to generate slower code.
- -mfused-madd
-
- -mno-fused-madd
-
Enable (disable) use of the floating-point multiply-accumulate
instructions, when they are available. The default is
-mfused-madd.
On the R8000 CPU when multiply-accumulate instructions are used,
the intermediate product is calculated to infinite precision
and is not subject to the FCSR Flush to Zero bit. This may be
undesirable in some circumstances. On other processors the result
is numerically identical to the equivalent computation using
separate multiply, add, subtract and negate instructions.
- -nocpp
-
Tell the MIPS assembler to not run its preprocessor over user
assembler files (with a .s suffix) when assembling them.
- -mfix-24k
-
- -mno-fix-24k
-
Work around the 24K E48 (lost data on stores during refill) errata.
The workarounds are implemented by the assembler rather than by GCC.
- -mfix-r4000
-
- -mno-fix-r4000
-
Work around certain R4000 CPU errata:
-
- -
-
A double-word or a variable shift may give an incorrect result if executed
immediately after starting an integer division.
- -
-
A double-word or a variable shift may give an incorrect result if executed
while an integer multiplication is in progress.
- -
-
An integer division may give an incorrect result if started in a delay slot
of a taken branch or a jump.
-
- -mfix-r4400
-
- -mno-fix-r4400
-
Work around certain R4400 CPU errata:
-
- -
-
A double-word or a variable shift may give an incorrect result if executed
immediately after starting an integer division.
-
- -mfix-r10000
-
- -mno-fix-r10000
-
Work around certain R10000 errata:
-
- -
-
"ll"/"sc" sequences may not behave atomically on revisions
prior to 3.0. They may deadlock on revisions 2.6 and earlier.
-
This option can only be used if the target architecture supports
branch-likely instructions. -mfix-r10000 is the default when
-march=r10000 is used; -mno-fix-r10000 is the default
otherwise.
- -mfix-rm7000
-
- -mno-fix-rm7000
-
Work around the RM7000 "dmult"/"dmultu" errata. The
workarounds are implemented by the assembler rather than by GCC.
- -mfix-vr4120
-
- -mno-fix-vr4120
-
Work around certain VR4120 errata:
-
- -
-
"dmultu" does not always produce the correct result.
- -
-
"div" and "ddiv" do not always produce the correct result if one
of the operands is negative.
-
The workarounds for the division errata rely on special functions in
libgcc.a. At present, these functions are only provided by
the "mips64vr*-elf" configurations.
Other VR4120 errata require a NOP to be inserted between certain pairs of
instructions. These errata are handled by the assembler, not by GCC itself.
- -mfix-vr4130
-
Work around the VR4130 "mflo"/"mfhi" errata. The
workarounds are implemented by the assembler rather than by GCC,
although GCC avoids using "mflo" and "mfhi" if the
VR4130 "macc", "macchi", "dmacc" and "dmacchi"
instructions are available instead.
- -mfix-sb1
-
- -mno-fix-sb1
-
Work around certain SB-1 CPU core errata.
(This flag currently works around the SB-1 revision 2
``F1'' and ``F2'' floating-point errata.)
- -mr10k-cache-barrier=setting
-
Specify whether GCC should insert cache barriers to avoid the
side-effects of speculation on R10K processors.
In common with many processors, the R10K tries to predict the outcome
of a conditional branch and speculatively executes instructions from
the ``taken'' branch. It later aborts these instructions if the
predicted outcome is wrong. However, on the R10K, even aborted
instructions can have side effects.
This problem only affects kernel stores and, depending on the system,
kernel loads. As an example, a speculatively-executed store may load
the target memory into cache and mark the cache line as dirty, even if
the store itself is later aborted. If a DMA operation writes to the
same area of memory before the ``dirty'' line is flushed, the cached
data overwrites the DMA-ed data. See the R10K processor manual
for a full description, including other potential problems.
One workaround is to insert cache barrier instructions before every memory
access that might be speculatively executed and that might have side
effects even if aborted. -mr10k-cache-barrier=setting
controls GCC's implementation of this workaround. It assumes that
aborted accesses to any byte in the following regions does not have
side effects:
-
- 1.
-
the memory occupied by the current function's stack frame;
- 2.
-
the memory occupied by an incoming stack argument;
- 3.
-
the memory occupied by an object with a link-time-constant address.
-
It is the kernel's responsibility to ensure that speculative
accesses to these regions are indeed safe.
If the input program contains a function declaration such as:
void foo (void);
then the implementation of "foo" must allow "j foo" and
"jal foo" to be executed speculatively. GCC honors this
restriction for functions it compiles itself. It expects non-GCC
functions (such as hand-written assembly code) to do the same.
The option has three forms:
- -mr10k-cache-barrier=load-store
-
Insert a cache barrier before a load or store that might be
speculatively executed and that might have side effects even
if aborted.
- -mr10k-cache-barrier=store
-
Insert a cache barrier before a store that might be speculatively
executed and that might have side effects even if aborted.
- -mr10k-cache-barrier=none
-
Disable the insertion of cache barriers. This is the default setting.
-
- -mflush-func=func
-
- -mno-flush-func
-
Specifies the function to call to flush the I and D caches, or to not
call any such function. If called, the function must take the same
arguments as the common "_flush_func", that is, the address of the
memory range for which the cache is being flushed, the size of the
memory range, and the number 3 (to flush both caches). The default
depends on the target GCC was configured for, but commonly is either
"_flush_func" or "__cpu_flush".
- mbranch-cost=num
-
Set the cost of branches to roughly num ``simple'' instructions.
This cost is only a heuristic and is not guaranteed to produce
consistent results across releases. A zero cost redundantly selects
the default, which is based on the -mtune setting.
- -mbranch-likely
-
- -mno-branch-likely
-
Enable or disable use of Branch Likely instructions, regardless of the
default for the selected architecture. By default, Branch Likely
instructions may be generated if they are supported by the selected
architecture. An exception is for the MIPS32 and MIPS64 architectures
and processors that implement those architectures; for those, Branch
Likely instructions are not be generated by default because the MIPS32
and MIPS64 architectures specifically deprecate their use.
- -mfp-exceptions
-
- -mno-fp-exceptions
-
Specifies whether FP exceptions are enabled. This affects how
FP instructions are scheduled for some processors.
The default is that FP exceptions are
enabled.
For instance, on the SB-1, if FP exceptions are disabled, and we are emitting
64-bit code, then we can use both FP pipes. Otherwise, we can only use one
FP pipe.
- -mvr4130-align
-
- -mno-vr4130-align
-
The VR4130 pipeline is two-way superscalar, but can only issue two
instructions together if the first one is 8-byte aligned. When this
option is enabled, GCC aligns pairs of instructions that it
thinks should execute in parallel.
This option only has an effect when optimizing for the VR4130.
It normally makes code faster, but at the expense of making it bigger.
It is enabled by default at optimization level -O3.
- -msynci
-
- -mno-synci
-
Enable (disable) generation of "synci" instructions on
architectures that support it. The "synci" instructions (if
enabled) are generated when "__builtin___clear_cache" is
compiled.
This option defaults to -mno-synci, but the default can be
overridden by configuring GCC with --with-synci.
When compiling code for single processor systems, it is generally safe
to use "synci". However, on many multi-core (SMP) systems, it
does not invalidate the instruction caches on all cores and may lead
to undefined behavior.
- -mrelax-pic-calls
-
- -mno-relax-pic-calls
-
Try to turn PIC calls that are normally dispatched via register
$25 into direct calls. This is only possible if the linker can
resolve the destination at link-time and if the destination is within
range for a direct call.
-mrelax-pic-calls is the default if GCC was configured to use
an assembler and a linker that support the ".reloc" assembly
directive and -mexplicit-relocs is in effect. With
-mno-explicit-relocs, this optimization can be performed by the
assembler and the linker alone without help from the compiler.
- -mmcount-ra-address
-
- -mno-mcount-ra-address
-
Emit (do not emit) code that allows "_mcount" to modify the
calling function's return address. When enabled, this option extends
the usual "_mcount" interface with a new ra-address
parameter, which has type "intptr_t *" and is passed in register
$12. "_mcount" can then modify the return address by
doing both of the following:
-
- *
-
Returning the new address in register $31.
- *
-
Storing the new address in "*ra-address",
if ra-address is nonnull.
-
The default is -mno-mcount-ra-address.
MMIX Options
These options are defined for the MMIX:
- -mlibfuncs
-
- -mno-libfuncs
-
Specify that intrinsic library functions are being compiled, passing all
values in registers, no matter the size.
- -mepsilon
-
- -mno-epsilon
-
Generate floating-point comparison instructions that compare with respect
to the "rE" epsilon register.
- -mabi=mmixware
-
- -mabi=gnu
-
Generate code that passes function parameters and return values that (in
the called function) are seen as registers $0 and up, as opposed to
the GNU ABI which uses global registers $231 and up.
- -mzero-extend
-
- -mno-zero-extend
-
When reading data from memory in sizes shorter than 64 bits, use (do not
use) zero-extending load instructions by default, rather than
sign-extending ones.
- -mknuthdiv
-
- -mno-knuthdiv
-
Make the result of a division yielding a remainder have the same sign as
the divisor. With the default, -mno-knuthdiv, the sign of the
remainder follows the sign of the dividend. Both methods are
arithmetically valid, the latter being almost exclusively used.
- -mtoplevel-symbols
-
- -mno-toplevel-symbols
-
Prepend (do not prepend) a : to all global symbols, so the assembly
code can be used with the "PREFIX" assembly directive.
- -melf
-
Generate an executable in the ELF format, rather than the default
mmo format used by the mmix simulator.
- -mbranch-predict
-
- -mno-branch-predict
-
Use (do not use) the probable-branch instructions, when static branch
prediction indicates a probable branch.
- -mbase-addresses
-
- -mno-base-addresses
-
Generate (do not generate) code that uses base addresses. Using a
base address automatically generates a request (handled by the assembler
and the linker) for a constant to be set up in a global register. The
register is used for one or more base address requests within the range 0
to 255 from the value held in the register. The generally leads to short
and fast code, but the number of different data items that can be
addressed is limited. This means that a program that uses lots of static
data may require -mno-base-addresses.
- -msingle-exit
-
- -mno-single-exit
-
Force (do not force) generated code to have a single exit point in each
function.
MN10300 Options
These -m options are defined for Matsushita MN10300 architectures:
- -mmult-bug
-
Generate code to avoid bugs in the multiply instructions for the MN10300
processors. This is the default.
- -mno-mult-bug
-
Do not generate code to avoid bugs in the multiply instructions for the
MN10300 processors.
- -mam33
-
Generate code using features specific to the AM33 processor.
- -mno-am33
-
Do not generate code using features specific to the AM33 processor. This
is the default.
- -mam33-2
-
Generate code using features specific to the AM33/2.0 processor.
- -mam34
-
Generate code using features specific to the AM34 processor.
- -mtune=cpu-type
-
Use the timing characteristics of the indicated CPU type when
scheduling instructions. This does not change the targeted processor
type. The CPU type must be one of mn10300, am33,
am33-2 or am34.
- -mreturn-pointer-on-d0
-
When generating a function that returns a pointer, return the pointer
in both "a0" and "d0". Otherwise, the pointer is returned
only in "a0", and attempts to call such functions without a prototype
result in errors. Note that this option is on by default; use
-mno-return-pointer-on-d0 to disable it.
- -mno-crt0
-
Do not link in the C run-time initialization object file.
- -mrelax
-
Indicate to the linker that it should perform a relaxation optimization pass
to shorten branches, calls and absolute memory addresses. This option only
has an effect when used on the command line for the final link step.
This option makes symbolic debugging impossible.
- -mliw
-
Allow the compiler to generate Long Instruction Word
instructions if the target is the AM33 or later. This is the
default. This option defines the preprocessor macro "__LIW__".
- -mnoliw
-
Do not allow the compiler to generate Long Instruction Word
instructions. This option defines the preprocessor macro
"__NO_LIW__".
- -msetlb
-
Allow the compiler to generate the SETLB and Lcc
instructions if the target is the AM33 or later. This is the
default. This option defines the preprocessor macro "__SETLB__".
- -mnosetlb
-
Do not allow the compiler to generate SETLB or Lcc
instructions. This option defines the preprocessor macro
"__NO_SETLB__".
Moxie Options
- -meb
-
Generate big-endian code. This is the default for moxie-*-*
configurations.
- -mel
-
Generate little-endian code.
- -mmul.x
-
Generate mul.x and umul.x instructions. This is the default for
moxiebox-*-* configurations.
- -mno-crt0
-
Do not link in the C run-time initialization object file.
MSP430 Options
These options are defined for the MSP430:
- -masm-hex
-
Force assembly output to always use hex constants. Normally such
constants are signed decimals, but this option is available for
testsuite and/or aesthetic purposes.
- -mmcu=
-
Select the MCU to target. This is used to create a C preprocessor
symbol based upon the MCU name, converted to upper case and pre- and
post-fixed with __. This in turn is used by the
msp430.h header file to select an MCU-specific supplementary
header file.
The option also sets the ISA to use. If the MCU name is one that is
known to only support the 430 ISA then that is selected, otherwise the
430X ISA is selected. A generic MCU name of msp430 can also be
used to select the 430 ISA. Similarly the generic msp430x MCU
name selects the 430X ISA.
In addition an MCU-specific linker script is added to the linker
command line. The script's name is the name of the MCU with
.ld appended. Thus specifying -mmcu=xxx on the gcc
command line defines the C preprocessor symbol "__XXX__" and
cause the linker to search for a script called xxx.ld.
This option is also passed on to the assembler.
- -mcpu=
-
Specifies the ISA to use. Accepted values are msp430,
msp430x and msp430xv2. This option is deprecated. The
-mmcu= option should be used to select the ISA.
- -msim
-
Link to the simulator runtime libraries and linker script. Overrides
any scripts that would be selected by the -mmcu= option.
- -mlarge
-
Use large-model addressing (20-bit pointers, 32-bit "size_t").
- -msmall
-
Use small-model addressing (16-bit pointers, 16-bit "size_t").
- -mrelax
-
This option is passed to the assembler and linker, and allows the
linker to perform certain optimizations that cannot be done until
the final link.
- mhwmult=
-
Describes the type of hardware multiply supported by the target.
Accepted values are none for no hardware multiply, 16bit
for the original 16-bit-only multiply supported by early MCUs.
32bit for the 16/32-bit multiply supported by later MCUs and
f5series for the 16/32-bit multiply supported by F5-series MCUs.
A value of auto can also be given. This tells GCC to deduce
the hardware multiply support based upon the MCU name provided by the
-mmcu option. If no -mmcu option is specified then
32bit hardware multiply support is assumed. auto is the
default setting.
Hardware multiplies are normally performed by calling a library
routine. This saves space in the generated code. When compiling at
-O3 or higher however the hardware multiplier is invoked
inline. This makes for bigger, but faster code.
The hardware multiply routines disable interrupts whilst running and
restore the previous interrupt state when they finish. This makes
them safe to use inside interrupt handlers as well as in normal code.
- -minrt
-
Enable the use of a minimum runtime environment - no static
initializers or constructors. This is intended for memory-constrained
devices. The compiler includes special symbols in some objects
that tell the linker and runtime which code fragments are required.
NDS32 Options
These options are defined for NDS32 implementations:
- -mbig-endian
-
Generate code in big-endian mode.
- -mlittle-endian
-
Generate code in little-endian mode.
- -mreduced-regs
-
Use reduced-set registers for register allocation.
- -mfull-regs
-
Use full-set registers for register allocation.
- -mcmov
-
Generate conditional move instructions.
- -mno-cmov
-
Do not generate conditional move instructions.
- -mperf-ext
-
Generate performance extension instructions.
- -mno-perf-ext
-
Do not generate performance extension instructions.
- -mv3push
-
Generate v3 push25/pop25 instructions.
- -mno-v3push
-
Do not generate v3 push25/pop25 instructions.
- -m16-bit
-
Generate 16-bit instructions.
- -mno-16-bit
-
Do not generate 16-bit instructions.
- -misr-vector-size=num
-
Specify the size of each interrupt vector, which must be 4 or 16.
- -mcache-block-size=num
-
Specify the size of each cache block,
which must be a power of 2 between 4 and 512.
- -march=arch
-
Specify the name of the target architecture.
- -mcmodel=code-model
-
Set the code model to one of
-
- small
-
All the data and read-only data segments must be within 512KB addressing space.
The text segment must be within 16MB addressing space.
- medium
-
The data segment must be within 512KB while the read-only data segment can be
within 4GB addressing space. The text segment should be still within 16MB
addressing space.
- large
-
All the text and data segments can be within 4GB addressing space.
-
- -mctor-dtor
-
Enable constructor/destructor feature.
- -mrelax
-
Guide linker to relax instructions.
Nios II Options
These are the options defined for the Altera Nios II processor.
- -G num
-
Put global and static objects less than or equal to num bytes
into the small data or BSS sections instead of the normal data or BSS
sections. The default value of num is 8.
- -mgpopt=option
-
- -mgpopt
-
- -mno-gpopt
-
Generate (do not generate) GP-relative accesses. The following
option names are recognized:
-
- none
-
Do not generate GP-relative accesses.
- local
-
Generate GP-relative accesses for small data objects that are not
external or weak. Also use GP-relative addressing for objects that
have been explicitly placed in a small data section via a "section"
attribute.
- global
-
As for local, but also generate GP-relative accesses for
small data objects that are external or weak. If you use this option,
you must ensure that all parts of your program (including libraries) are
compiled with the same -G setting.
- data
-
Generate GP-relative accesses for all data objects in the program. If you
use this option, the entire data and BSS segments
of your program must fit in 64K of memory and you must use an appropriate
linker script to allocate them within the addressible range of the
global pointer.
- all
-
Generate GP-relative addresses for function pointers as well as data
pointers. If you use this option, the entire text, data, and BSS segments
of your program must fit in 64K of memory and you must use an appropriate
linker script to allocate them within the addressible range of the
global pointer.
-
-mgpopt is equivalent to -mgpopt=local, and
-mno-gpopt is equivalent to -mgpopt=none.
The default is -mgpopt except when -fpic or
-fPIC is specified to generate position-independent code.
Note that the Nios II ABI does not permit GP-relative accesses from
shared libraries.
You may need to specify -mno-gpopt explicitly when building
programs that include large amounts of small data, including large
GOT data sections. In this case, the 16-bit offset for GP-relative
addressing may not be large enough to allow access to the entire
small data section.
- -mel
-
- -meb
-
Generate little-endian (default) or big-endian (experimental) code,
respectively.
- -mbypass-cache
-
- -mno-bypass-cache
-
Force all load and store instructions to always bypass cache by
using I/O variants of the instructions. The default is not to
bypass the cache.
- -mno-cache-volatile
-
- -mcache-volatile
-
Volatile memory access bypass the cache using the I/O variants of
the load and store instructions. The default is not to bypass the cache.
- -mno-fast-sw-div
-
- -mfast-sw-div
-
Do not use table-based fast divide for small numbers. The default
is to use the fast divide at -O3 and above.
- -mno-hw-mul
-
- -mhw-mul
-
- -mno-hw-mulx
-
- -mhw-mulx
-
- -mno-hw-div
-
- -mhw-div
-
Enable or disable emitting "mul", "mulx" and "div" family of
instructions by the compiler. The default is to emit "mul"
and not emit "div" and "mulx".
- -mcustom-insn=N
-
- -mno-custom-insn
-
Each -mcustom-insn=N option enables use of a
custom instruction with encoding N when generating code that uses
insn. For example, -mcustom-fadds=253 generates custom
instruction 253 for single-precision floating-point add operations instead
of the default behavior of using a library call.
The following values of insn are supported. Except as otherwise
noted, floating-point operations are expected to be implemented with
normal IEEE 754 semantics and correspond directly to the C operators or the
equivalent GCC built-in functions.
Single-precision floating point:
-
- fadds, fsubs, fdivs, fmuls
-
Binary arithmetic operations.
- fnegs
-
Unary negation.
- fabss
-
Unary absolute value.
- fcmpeqs, fcmpges, fcmpgts, fcmples, fcmplts, fcmpnes
-
Comparison operations.
- fmins, fmaxs
-
Floating-point minimum and maximum. These instructions are only
generated if -ffinite-math-only is specified.
- fsqrts
-
Unary square root operation.
- fcoss, fsins, ftans, fatans, fexps, flogs
-
Floating-point trigonometric and exponential functions. These instructions
are only generated if -funsafe-math-optimizations is also specified.
-
Double-precision floating point:
- faddd, fsubd, fdivd, fmuld
-
Binary arithmetic operations.
- fnegd
-
Unary negation.
- fabsd
-
Unary absolute value.
- fcmpeqd, fcmpged, fcmpgtd, fcmpled, fcmpltd, fcmpned
-
Comparison operations.
- fmind, fmaxd
-
Double-precision minimum and maximum. These instructions are only
generated if -ffinite-math-only is specified.
- fsqrtd
-
Unary square root operation.
- fcosd, fsind, ftand, fatand, fexpd, flogd
-
Double-precision trigonometric and exponential functions. These instructions
are only generated if -funsafe-math-optimizations is also specified.
-
Conversions:
- fextsd
-
Conversion from single precision to double precision.
- ftruncds
-
Conversion from double precision to single precision.
- fixsi, fixsu, fixdi, fixdu
-
Conversion from floating point to signed or unsigned integer types, with
truncation towards zero.
- round
-
Conversion from single-precision floating point to signed integer,
rounding to the nearest integer and ties away from zero.
This corresponds to the "__builtin_lroundf" function when
-fno-math-errno is used.
- floatis, floatus, floatid, floatud
-
Conversion from signed or unsigned integer types to floating-point types.
-
In addition, all of the following transfer instructions for internal
registers X and Y must be provided to use any of the double-precision
floating-point instructions. Custom instructions taking two
double-precision source operands expect the first operand in the
64-bit register X. The other operand (or only operand of a unary
operation) is given to the custom arithmetic instruction with the
least significant half in source register src1 and the most
significant half in src2. A custom instruction that returns a
double-precision result returns the most significant 32 bits in the
destination register and the other half in 32-bit register Y.
GCC automatically generates the necessary code sequences to write
register X and/or read register Y when double-precision floating-point
instructions are used.
- fwrx
-
Write src1 into the least significant half of X and src2 into
the most significant half of X.
- fwry
-
Write src1 into Y.
- frdxhi, frdxlo
-
Read the most or least (respectively) significant half of X and store it in
dest.
- frdy
-
Read the value of Y and store it into dest.
-
Note that you can gain more local control over generation of Nios II custom
instructions by using the "target("custom-insn=N")"
and "target("no-custom-insn")" function attributes
or pragmas.
- -mcustom-fpu-cfg=name
-
This option enables a predefined, named set of custom instruction encodings
(see -mcustom-insn above).
Currently, the following sets are defined:
-mcustom-fpu-cfg=60-1 is equivalent to:
-mcustom-fmuls=252
-mcustom-fadds=253
-mcustom-fsubs=254
-fsingle-precision-constant
-mcustom-fpu-cfg=60-2 is equivalent to:
-mcustom-fmuls=252
-mcustom-fadds=253
-mcustom-fsubs=254
-mcustom-fdivs=255
-fsingle-precision-constant
-mcustom-fpu-cfg=72-3 is equivalent to:
-mcustom-floatus=243
-mcustom-fixsi=244
-mcustom-floatis=245
-mcustom-fcmpgts=246
-mcustom-fcmples=249
-mcustom-fcmpeqs=250
-mcustom-fcmpnes=251
-mcustom-fmuls=252
-mcustom-fadds=253
-mcustom-fsubs=254
-mcustom-fdivs=255
-fsingle-precision-constant
Custom instruction assignments given by individual
-mcustom-insn= options override those given by
-mcustom-fpu-cfg=, regardless of the
order of the options on the command line.
Note that you can gain more local control over selection of a FPU
configuration by using the "target("custom-fpu-cfg=name")"
function attribute
or pragma.
These additional -m options are available for the Altera Nios II
ELF (bare-metal) target:
- -mhal
-
Link with HAL BSP. This suppresses linking with the GCC-provided C runtime
startup and termination code, and is typically used in conjunction with
-msys-crt0= to specify the location of the alternate startup code
provided by the HAL BSP.
- -msmallc
-
Link with a limited version of the C library, -lsmallc, rather than
Newlib.
- -msys-crt0=startfile
-
startfile is the file name of the startfile (crt0) to use
when linking. This option is only useful in conjunction with -mhal.
- -msys-lib=systemlib
-
systemlib is the library name of the library that provides
low-level system calls required by the C library,
e.g. "read" and "write".
This option is typically used to link with a library provided by a HAL BSP.
Nvidia PTX Options
These options are defined for Nvidia PTX:
- -m32
-
- -m64
-
Generate code for 32-bit or 64-bit ABI.
- -mmainkernel
-
Link in code for a __main kernel. This is for stand-alone instead of
offloading execution.
PDP-11 Options
These options are defined for the PDP-11:
- -mfpu
-
Use hardware FPP floating point. This is the default. (FIS floating
point on the PDP-11/40 is not supported.)
- -msoft-float
-
Do not use hardware floating point.
- -mac0
-
Return floating-point results in ac0 (fr0 in Unix assembler syntax).
- -mno-ac0
-
Return floating-point results in memory. This is the default.
- -m40
-
Generate code for a PDP-11/40.
- -m45
-
Generate code for a PDP-11/45. This is the default.
- -m10
-
Generate code for a PDP-11/10.
- -mbcopy-builtin
-
Use inline "movmemhi" patterns for copying memory. This is the
default.
- -mbcopy
-
Do not use inline "movmemhi" patterns for copying memory.
- -mint16
-
- -mno-int32
-
Use 16-bit "int". This is the default.
- -mint32
-
- -mno-int16
-
Use 32-bit "int".
- -mfloat64
-
- -mno-float32
-
Use 64-bit "float". This is the default.
- -mfloat32
-
- -mno-float64
-
Use 32-bit "float".
- -mabshi
-
Use "abshi2" pattern. This is the default.
- -mno-abshi
-
Do not use "abshi2" pattern.
- -mbranch-expensive
-
Pretend that branches are expensive. This is for experimenting with
code generation only.
- -mbranch-cheap
-
Do not pretend that branches are expensive. This is the default.
- -munix-asm
-
Use Unix assembler syntax. This is the default when configured for
pdp11-*-bsd.
- -mdec-asm
-
Use DEC assembler syntax. This is the default when configured for any
PDP-11 target other than pdp11-*-bsd.
picoChip Options
These -m options are defined for picoChip implementations:
- -mae=ae_type
-
Set the instruction set, register set, and instruction scheduling
parameters for array element type ae_type. Supported values
for ae_type are ANY, MUL, and MAC.
-mae=ANY selects a completely generic AE type. Code
generated with this option runs on any of the other AE types. The
code is not as efficient as it would be if compiled for a specific
AE type, and some types of operation (e.g., multiplication) do not
work properly on all types of AE.
-mae=MUL selects a MUL AE type. This is the most useful AE type
for compiled code, and is the default.
-mae=MAC selects a DSP-style MAC AE. Code compiled with this
option may suffer from poor performance of byte (char) manipulation,
since the DSP AE does not provide hardware support for byte load/stores.
- -msymbol-as-address
-
Enable the compiler to directly use a symbol name as an address in a
load/store instruction, without first loading it into a
register. Typically, the use of this option generates larger
programs, which run faster than when the option isn't used. However, the
results vary from program to program, so it is left as a user option,
rather than being permanently enabled.
- -mno-inefficient-warnings
-
Disables warnings about the generation of inefficient code. These
warnings can be generated, for example, when compiling code that
performs byte-level memory operations on the MAC AE type. The MAC AE has
no hardware support for byte-level memory operations, so all byte
load/stores must be synthesized from word load/store operations. This is
inefficient and a warning is generated to indicate
that you should rewrite the code to avoid byte operations, or to target
an AE type that has the necessary hardware support. This option disables
these warnings.
PowerPC Options
These are listed under
RL78 Options
- -msim
-
Links in additional target libraries to support operation within a
simulator.
- -mmul=none
-
- -mmul=g13
-
- -mmul=rl78
-
Specifies the type of hardware multiplication support to be used. The
default is none, which uses software multiplication functions.
The g13 option is for the hardware multiply/divide peripheral
only on the RL78/G13 targets. The rl78 option is for the
standard hardware multiplication defined in the RL78 software manual.
- -m64bit-doubles
-
- -m32bit-doubles
-
Make the "double" data type be 64 bits (-m64bit-doubles)
or 32 bits (-m32bit-doubles) in size. The default is
-m32bit-doubles.
IBM RS/6000 and PowerPC Options
These -m options are defined for the IBM RS/6000 and PowerPC:
- -mpowerpc-gpopt
-
- -mno-powerpc-gpopt
-
- -mpowerpc-gfxopt
-
- -mno-powerpc-gfxopt
-
- -mpowerpc64
-
- -mno-powerpc64
-
- -mmfcrf
-
- -mno-mfcrf
-
- -mpopcntb
-
- -mno-popcntb
-
- -mpopcntd
-
- -mno-popcntd
-
- -mfprnd
-
- -mno-fprnd
-
- -mcmpb
-
- -mno-cmpb
-
- -mmfpgpr
-
- -mno-mfpgpr
-
- -mhard-dfp
-
- -mno-hard-dfp
-
You use these options to specify which instructions are available on the
processor you are using. The default value of these options is
determined when configuring GCC. Specifying the
-mcpu=cpu_type overrides the specification of these
options. We recommend you use the -mcpu=cpu_type option
rather than the options listed above.
Specifying -mpowerpc-gpopt allows
GCC to use the optional PowerPC architecture instructions in the
General Purpose group, including floating-point square root. Specifying
-mpowerpc-gfxopt allows GCC to
use the optional PowerPC architecture instructions in the Graphics
group, including floating-point select.
The -mmfcrf option allows GCC to generate the move from
condition register field instruction implemented on the POWER4
processor and other processors that support the PowerPC V2.01
architecture.
The -mpopcntb option allows GCC to generate the popcount and
double-precision FP reciprocal estimate instruction implemented on the
POWER5 processor and other processors that support the PowerPC V2.02
architecture.
The -mpopcntd option allows GCC to generate the popcount
instruction implemented on the POWER7 processor and other processors
that support the PowerPC V2.06 architecture.
The -mfprnd option allows GCC to generate the FP round to
integer instructions implemented on the POWER5+ processor and other
processors that support the PowerPC V2.03 architecture.
The -mcmpb option allows GCC to generate the compare bytes
instruction implemented on the POWER6 processor and other processors
that support the PowerPC V2.05 architecture.
The -mmfpgpr option allows GCC to generate the FP move to/from
general-purpose register instructions implemented on the POWER6X
processor and other processors that support the extended PowerPC V2.05
architecture.
The -mhard-dfp option allows GCC to generate the decimal
floating-point instructions implemented on some POWER processors.
The -mpowerpc64 option allows GCC to generate the additional
64-bit instructions that are found in the full PowerPC64 architecture
and to treat GPRs as 64-bit, doubleword quantities. GCC defaults to
-mno-powerpc64.
- -mcpu=cpu_type
-
Set architecture type, register usage, and
instruction scheduling parameters for machine type cpu_type.
Supported values for cpu_type are 401, 403,
405, 405fp, 440, 440fp, 464, 464fp,
476, 476fp, 505, 601, 602, 603,
603e, 604, 604e, 620, 630, 740,
7400, 7450, 750, 801, 821, 823,
860, 970, 8540, a2, e300c2,
e300c3, e500mc, e500mc64, e5500,
e6500, ec603e, G3, G4, G5,
titan, power3, power4, power5, power5+,
power6, power6x, power7, power8, powerpc,
powerpc64, powerpc64le, and rs64.
-mcpu=powerpc, -mcpu=powerpc64, and
-mcpu=powerpc64le specify pure 32-bit PowerPC (either
endian), 64-bit big endian PowerPC and 64-bit little endian PowerPC
architecture machine types, with an appropriate, generic processor
model assumed for scheduling purposes.
The other options specify a specific processor. Code generated under
those options runs best on that processor, and may not run at all on
others.
The -mcpu options automatically enable or disable the
following options:
-maltivec -mfprnd -mhard-float -mmfcrf -mmultiple
-mpopcntb -mpopcntd -mpowerpc64
-mpowerpc-gpopt -mpowerpc-gfxopt -msingle-float -mdouble-float
-msimple-fpu -mstring -mmulhw -mdlmzb -mmfpgpr -mvsx
-mcrypto -mdirect-move -mpower8-fusion -mpower8-vector
-mquad-memory -mquad-memory-atomic
The particular options set for any particular CPU varies between
compiler versions, depending on what setting seems to produce optimal
code for that CPU; it doesn't necessarily reflect the actual hardware's
capabilities. If you wish to set an individual option to a particular
value, you may specify it after the -mcpu option, like
-mcpu=970 -mno-altivec.
On AIX, the -maltivec and -mpowerpc64 options are
not enabled or disabled by the -mcpu option at present because
AIX does not have full support for these options. You may still
enable or disable them individually if you're sure it'll work in your
environment.
- -mtune=cpu_type
-
Set the instruction scheduling parameters for machine type
cpu_type, but do not set the architecture type or register usage,
as -mcpu=cpu_type does. The same
values for cpu_type are used for -mtune as for
-mcpu. If both are specified, the code generated uses the
architecture and registers set by -mcpu, but the
scheduling parameters set by -mtune.
- -mcmodel=small
-
Generate PowerPC64 code for the small model: The TOC is limited to
64k.
- -mcmodel=medium
-
Generate PowerPC64 code for the medium model: The TOC and other static
data may be up to a total of 4G in size.
- -mcmodel=large
-
Generate PowerPC64 code for the large model: The TOC may be up to 4G
in size. Other data and code is only limited by the 64-bit address
space.
- -maltivec
-
- -mno-altivec
-
Generate code that uses (does not use) AltiVec instructions, and also
enable the use of built-in functions that allow more direct access to
the AltiVec instruction set. You may also need to set
-mabi=altivec to adjust the current ABI with AltiVec ABI
enhancements.
When -maltivec is used, rather than -maltivec=le or
-maltivec=be, the element order for Altivec intrinsics such
as "vec_splat", "vec_extract", and "vec_insert"
match array element order corresponding to the endianness of the
target. That is, element zero identifies the leftmost element in a
vector register when targeting a big-endian platform, and identifies
the rightmost element in a vector register when targeting a
little-endian platform.
- -maltivec=be
-
Generate Altivec instructions using big-endian element order,
regardless of whether the target is big- or little-endian. This is
the default when targeting a big-endian platform.
The element order is used to interpret element numbers in Altivec
intrinsics such as "vec_splat", "vec_extract", and
"vec_insert". By default, these match array element order
corresponding to the endianness for the target.
- -maltivec=le
-
Generate Altivec instructions using little-endian element order,
regardless of whether the target is big- or little-endian. This is
the default when targeting a little-endian platform. This option is
currently ignored when targeting a big-endian platform.
The element order is used to interpret element numbers in Altivec
intrinsics such as "vec_splat", "vec_extract", and
"vec_insert". By default, these match array element order
corresponding to the endianness for the target.
- -mvrsave
-
- -mno-vrsave
-
Generate VRSAVE instructions when generating AltiVec code.
- -mgen-cell-microcode
-
Generate Cell microcode instructions.
- -mwarn-cell-microcode
-
Warn when a Cell microcode instruction is emitted. An example
of a Cell microcode instruction is a variable shift.
- -msecure-plt
-
Generate code that allows ld and ld.so
to build executables and shared
libraries with non-executable ".plt" and ".got" sections.
This is a PowerPC
32-bit SYSV ABI option.
- -mbss-plt
-
Generate code that uses a BSS ".plt" section that ld.so
fills in, and
requires ".plt" and ".got"
sections that are both writable and executable.
This is a PowerPC 32-bit SYSV ABI option.
- -misel
-
- -mno-isel
-
This switch enables or disables the generation of ISEL instructions.
- -misel=yes/no
-
This switch has been deprecated. Use -misel and
-mno-isel instead.
- -mspe
-
- -mno-spe
-
This switch enables or disables the generation of SPE simd
instructions.
- -mpaired
-
- -mno-paired
-
This switch enables or disables the generation of PAIRED simd
instructions.
- -mspe=yes/no
-
This option has been deprecated. Use -mspe and
-mno-spe instead.
- -mvsx
-
- -mno-vsx
-
Generate code that uses (does not use) vector/scalar (VSX)
instructions, and also enable the use of built-in functions that allow
more direct access to the VSX instruction set.
- -mcrypto
-
- -mno-crypto
-
Enable the use (disable) of the built-in functions that allow direct
access to the cryptographic instructions that were added in version
2.07 of the PowerPC ISA.
- -mdirect-move
-
- -mno-direct-move
-
Generate code that uses (does not use) the instructions to move data
between the general purpose registers and the vector/scalar (VSX)
registers that were added in version 2.07 of the PowerPC ISA.
- -mpower8-fusion
-
- -mno-power8-fusion
-
Generate code that keeps (does not keeps) some integer operations
adjacent so that the instructions can be fused together on power8 and
later processors.
- -mpower8-vector
-
- -mno-power8-vector
-
Generate code that uses (does not use) the vector and scalar
instructions that were added in version 2.07 of the PowerPC ISA. Also
enable the use of built-in functions that allow more direct access to
the vector instructions.
- -mquad-memory
-
- -mno-quad-memory
-
Generate code that uses (does not use) the non-atomic quad word memory
instructions. The -mquad-memory option requires use of
64-bit mode.
- -mquad-memory-atomic
-
- -mno-quad-memory-atomic
-
Generate code that uses (does not use) the atomic quad word memory
instructions. The -mquad-memory-atomic option requires use of
64-bit mode.
- -mupper-regs-df
-
- -mno-upper-regs-df
-
Generate code that uses (does not use) the scalar double precision
instructions that target all 64 registers in the vector/scalar
floating point register set that were added in version 2.06 of the
PowerPC ISA. -mupper-regs-df is turned on by default if you
use any of the -mcpu=power7, -mcpu=power8, or
-mvsx options.
- -mupper-regs-sf
-
- -mno-upper-regs-sf
-
Generate code that uses (does not use) the scalar single precision
instructions that target all 64 registers in the vector/scalar
floating point register set that were added in version 2.07 of the
PowerPC ISA. -mupper-regs-sf is turned on by default if you
use either of the -mcpu=power8 or -mpower8-vector
options.
- -mupper-regs
-
- -mno-upper-regs
-
Generate code that uses (does not use) the scalar
instructions that target all 64 registers in the vector/scalar
floating point register set, depending on the model of the machine.
If the -mno-upper-regs option is used, it turns off both
-mupper-regs-sf and -mupper-regs-df options.
- -mfloat-gprs=yes/single/double/no
-
- -mfloat-gprs
-
This switch enables or disables the generation of floating-point
operations on the general-purpose registers for architectures that
support it.
The argument yes or single enables the use of
single-precision floating-point operations.
The argument double enables the use of single and
double-precision floating-point operations.
The argument no disables floating-point operations on the
general-purpose registers.
This option is currently only available on the MPC854x.
- -m32
-
- -m64
-
Generate code for 32-bit or 64-bit environments of Darwin and SVR4
targets (including GNU/Linux). The 32-bit environment sets int, long
and pointer to 32 bits and generates code that runs on any PowerPC
variant. The 64-bit environment sets int to 32 bits and long and
pointer to 64 bits, and generates code for PowerPC64, as for
-mpowerpc64.
- -mfull-toc
-
- -mno-fp-in-toc
-
- -mno-sum-in-toc
-
- -mminimal-toc
-
Modify generation of the TOC (Table Of Contents), which is created for
every executable file. The -mfull-toc option is selected by
default. In that case, GCC allocates at least one TOC entry for
each unique non-automatic variable reference in your program. GCC
also places floating-point constants in the TOC. However, only
16,384 entries are available in the TOC.
If you receive a linker error message that saying you have overflowed
the available TOC space, you can reduce the amount of TOC space used
with the -mno-fp-in-toc and -mno-sum-in-toc options.
-mno-fp-in-toc prevents GCC from putting floating-point
constants in the TOC and -mno-sum-in-toc forces GCC to
generate code to calculate the sum of an address and a constant at
run time instead of putting that sum into the TOC. You may specify one
or both of these options. Each causes GCC to produce very slightly
slower and larger code at the expense of conserving TOC space.
If you still run out of space in the TOC even when you specify both of
these options, specify -mminimal-toc instead. This option causes
GCC to make only one TOC entry for every file. When you specify this
option, GCC produces code that is slower and larger but which
uses extremely little TOC space. You may wish to use this option
only on files that contain less frequently-executed code.
- -maix64
-
- -maix32
-
Enable 64-bit AIX ABI and calling convention: 64-bit pointers, 64-bit
"long" type, and the infrastructure needed to support them.
Specifying -maix64 implies -mpowerpc64,
while -maix32 disables the 64-bit ABI and
implies -mno-powerpc64. GCC defaults to -maix32.
- -mxl-compat
-
- -mno-xl-compat
-
Produce code that conforms more closely to IBM XL compiler semantics
when using AIX-compatible ABI. Pass floating-point arguments to
prototyped functions beyond the register save area (RSA) on the stack
in addition to argument FPRs. Do not assume that most significant
double in 128-bit long double value is properly rounded when comparing
values and converting to double. Use XL symbol names for long double
support routines.
The AIX calling convention was extended but not initially documented to
handle an obscure K&R C case of calling a function that takes the
address of its arguments with fewer arguments than declared. IBM XL
compilers access floating-point arguments that do not fit in the
RSA from the stack when a subroutine is compiled without
optimization. Because always storing floating-point arguments on the
stack is inefficient and rarely needed, this option is not enabled by
default and only is necessary when calling subroutines compiled by IBM
XL compilers without optimization.
- -mpe
-
Support IBM RS/6000 SP Parallel Environment (PE). Link an
application written to use message passing with special startup code to
enable the application to run. The system must have PE installed in the
standard location (/usr/lpp/ppe.poe/), or the specs file
must be overridden with the -specs= option to specify the
appropriate directory location. The Parallel Environment does not
support threads, so the -mpe option and the -pthread
option are incompatible.
- -malign-natural
-
- -malign-power
-
On AIX, 32-bit Darwin, and 64-bit PowerPC GNU/Linux, the option
-malign-natural overrides the ABI-defined alignment of larger
types, such as floating-point doubles, on their natural size-based boundary.
The option -malign-power instructs GCC to follow the ABI-specified
alignment rules. GCC defaults to the standard alignment defined in the ABI.
On 64-bit Darwin, natural alignment is the default, and -malign-power
is not supported.
- -msoft-float
-
- -mhard-float
-
Generate code that does not use (uses) the floating-point register set.
Software floating-point emulation is provided if you use the
-msoft-float option, and pass the option to GCC when linking.
- -msingle-float
-
- -mdouble-float
-
Generate code for single- or double-precision floating-point operations.
-mdouble-float implies -msingle-float.
- -msimple-fpu
-
Do not generate "sqrt" and "div" instructions for hardware
floating-point unit.
- -mfpu=name
-
Specify type of floating-point unit. Valid values for name are
sp_lite (equivalent to -msingle-float -msimple-fpu),
dp_lite (equivalent to -mdouble-float -msimple-fpu),
sp_full (equivalent to -msingle-float),
and dp_full (equivalent to -mdouble-float).
- -mxilinx-fpu
-
Perform optimizations for the floating-point unit on Xilinx PPC 405/440.
- -mmultiple
-
- -mno-multiple
-
Generate code that uses (does not use) the load multiple word
instructions and the store multiple word instructions. These
instructions are generated by default on POWER systems, and not
generated on PowerPC systems. Do not use -mmultiple on little-endian
PowerPC systems, since those instructions do not work when the
processor is in little-endian mode. The exceptions are PPC740 and
PPC750 which permit these instructions in little-endian mode.
- -mstring
-
- -mno-string
-
Generate code that uses (does not use) the load string instructions
and the store string word instructions to save multiple registers and
do small block moves. These instructions are generated by default on
POWER systems, and not generated on PowerPC systems. Do not use
-mstring on little-endian PowerPC systems, since those
instructions do not work when the processor is in little-endian mode.
The exceptions are PPC740 and PPC750 which permit these instructions
in little-endian mode.
- -mupdate
-
- -mno-update
-
Generate code that uses (does not use) the load or store instructions
that update the base register to the address of the calculated memory
location. These instructions are generated by default. If you use
-mno-update, there is a small window between the time that the
stack pointer is updated and the address of the previous frame is
stored, which means code that walks the stack frame across interrupts or
signals may get corrupted data.
- -mavoid-indexed-addresses
-
- -mno-avoid-indexed-addresses
-
Generate code that tries to avoid (not avoid) the use of indexed load
or store instructions. These instructions can incur a performance
penalty on Power6 processors in certain situations, such as when
stepping through large arrays that cross a 16M boundary. This option
is enabled by default when targeting Power6 and disabled otherwise.
- -mfused-madd
-
- -mno-fused-madd
-
Generate code that uses (does not use) the floating-point multiply and
accumulate instructions. These instructions are generated by default
if hardware floating point is used. The machine-dependent
-mfused-madd option is now mapped to the machine-independent
-ffp-contract=fast option, and -mno-fused-madd is
mapped to -ffp-contract=off.
- -mmulhw
-
- -mno-mulhw
-
Generate code that uses (does not use) the half-word multiply and
multiply-accumulate instructions on the IBM 405, 440, 464 and 476 processors.
These instructions are generated by default when targeting those
processors.
- -mdlmzb
-
- -mno-dlmzb
-
Generate code that uses (does not use) the string-search dlmzb
instruction on the IBM 405, 440, 464 and 476 processors. This instruction is
generated by default when targeting those processors.
- -mno-bit-align
-
- -mbit-align
-
On System V.4 and embedded PowerPC systems do not (do) force structures
and unions that contain bit-fields to be aligned to the base type of the
bit-field.
For example, by default a structure containing nothing but 8
"unsigned" bit-fields of length 1 is aligned to a 4-byte
boundary and has a size of 4 bytes. By using -mno-bit-align,
the structure is aligned to a 1-byte boundary and is 1 byte in
size.
- -mno-strict-align
-
- -mstrict-align
-
On System V.4 and embedded PowerPC systems do not (do) assume that
unaligned memory references are handled by the system.
- -mrelocatable
-
- -mno-relocatable
-
Generate code that allows (does not allow) a static executable to be
relocated to a different address at run time. A simple embedded
PowerPC system loader should relocate the entire contents of
".got2" and 4-byte locations listed in the ".fixup" section,
a table of 32-bit addresses generated by this option. For this to
work, all objects linked together must be compiled with
-mrelocatable or -mrelocatable-lib.
-mrelocatable code aligns the stack to an 8-byte boundary.
- -mrelocatable-lib
-
- -mno-relocatable-lib
-
Like -mrelocatable, -mrelocatable-lib generates a
".fixup" section to allow static executables to be relocated at
run time, but -mrelocatable-lib does not use the smaller stack
alignment of -mrelocatable. Objects compiled with
-mrelocatable-lib may be linked with objects compiled with
any combination of the -mrelocatable options.
- -mno-toc
-
- -mtoc
-
On System V.4 and embedded PowerPC systems do not (do) assume that
register 2 contains a pointer to a global area pointing to the addresses
used in the program.
- -mlittle
-
- -mlittle-endian
-
On System V.4 and embedded PowerPC systems compile code for the
processor in little-endian mode. The -mlittle-endian option is
the same as -mlittle.
- -mbig
-
- -mbig-endian
-
On System V.4 and embedded PowerPC systems compile code for the
processor in big-endian mode. The -mbig-endian option is
the same as -mbig.
- -mdynamic-no-pic
-
On Darwin and Mac OS X systems, compile code so that it is not
relocatable, but that its external references are relocatable. The
resulting code is suitable for applications, but not shared
libraries.
- -msingle-pic-base
-
Treat the register used for PIC addressing as read-only, rather than
loading it in the prologue for each function. The runtime system is
responsible for initializing this register with an appropriate value
before execution begins.
- -mprioritize-restricted-insns=priority
-
This option controls the priority that is assigned to
dispatch-slot restricted instructions during the second scheduling
pass. The argument priority takes the value 0, 1,
or 2 to assign no, highest, or second-highest (respectively)
priority to dispatch-slot restricted
instructions.
- -msched-costly-dep=dependence_type
-
This option controls which dependences are considered costly
by the target during instruction scheduling. The argument
dependence_type takes one of the following values:
-
- no
-
No dependence is costly.
- all
-
All dependences are costly.
- true_store_to_load
-
A true dependence from store to load is costly.
- store_to_load
-
Any dependence from store to load is costly.
- number
-
Any dependence for which the latency is greater than or equal to
number is costly.
-
- -minsert-sched-nops=scheme
-
This option controls which NOP insertion scheme is used during
the second scheduling pass. The argument scheme takes one of the
following values:
-
- no
-
Don't insert NOPs.
- pad
-
Pad with NOPs any dispatch group that has vacant issue slots,
according to the scheduler's grouping.
- regroup_exact
-
Insert NOPs to force costly dependent insns into
separate groups. Insert exactly as many NOPs as needed to force an insn
to a new group, according to the estimated processor grouping.
- number
-
Insert NOPs to force costly dependent insns into
separate groups. Insert number NOPs to force an insn to a new group.
-
- -mcall-sysv
-
On System V.4 and embedded PowerPC systems compile code using calling
conventions that adhere to the March 1995 draft of the System V
Application Binary Interface, PowerPC processor supplement. This is the
default unless you configured GCC using powerpc-*-eabiaix.
- -mcall-sysv-eabi
-
- -mcall-eabi
-
Specify both -mcall-sysv and -meabi options.
- -mcall-sysv-noeabi
-
Specify both -mcall-sysv and -mno-eabi options.
- -mcall-aixdesc
-
On System V.4 and embedded PowerPC systems compile code for the AIX
operating system.
- -mcall-linux
-
On System V.4 and embedded PowerPC systems compile code for the
Linux-based GNU system.
- -mcall-freebsd
-
On System V.4 and embedded PowerPC systems compile code for the
FreeBSD operating system.
- -mcall-netbsd
-
On System V.4 and embedded PowerPC systems compile code for the
NetBSD operating system.
- -mcall-openbsd
-
On System V.4 and embedded PowerPC systems compile code for the
OpenBSD operating system.
- -maix-struct-return
-
Return all structures in memory (as specified by the AIX ABI).
- -msvr4-struct-return
-
Return structures smaller than 8 bytes in registers (as specified by the
SVR4 ABI).
- -mabi=abi-type
-
Extend the current ABI with a particular extension, or remove such extension.
Valid values are altivec, no-altivec, spe,
no-spe, ibmlongdouble, ieeelongdouble,
elfv1, elfv2.
- -mabi=spe
-
Extend the current ABI with SPE ABI extensions. This does not change
the default ABI, instead it adds the SPE ABI extensions to the current
ABI.
- -mabi=no-spe
-
Disable Book-E SPE ABI extensions for the current ABI.
- -mabi=ibmlongdouble
-
Change the current ABI to use IBM extended-precision long double.
This is a PowerPC 32-bit SYSV ABI option.
- -mabi=ieeelongdouble
-
Change the current ABI to use IEEE extended-precision long double.
This is a PowerPC 32-bit Linux ABI option.
- -mabi=elfv1
-
Change the current ABI to use the ELFv1 ABI.
This is the default ABI for big-endian PowerPC 64-bit Linux.
Overriding the default ABI requires special system support and is
likely to fail in spectacular ways.
- -mabi=elfv2
-
Change the current ABI to use the ELFv2 ABI.
This is the default ABI for little-endian PowerPC 64-bit Linux.
Overriding the default ABI requires special system support and is
likely to fail in spectacular ways.
- -mprototype
-
- -mno-prototype
-
On System V.4 and embedded PowerPC systems assume that all calls to
variable argument functions are properly prototyped. Otherwise, the
compiler must insert an instruction before every non-prototyped call to
set or clear bit 6 of the condition code register ("CR") to
indicate whether floating-point values are passed in the floating-point
registers in case the function takes variable arguments. With
-mprototype, only calls to prototyped variable argument functions
set or clear the bit.
- -msim
-
On embedded PowerPC systems, assume that the startup module is called
sim-crt0.o and that the standard C libraries are libsim.a and
libc.a. This is the default for powerpc-*-eabisim
configurations.
- -mmvme
-
On embedded PowerPC systems, assume that the startup module is called
crt0.o and the standard C libraries are libmvme.a and
libc.a.
- -mads
-
On embedded PowerPC systems, assume that the startup module is called
crt0.o and the standard C libraries are libads.a and
libc.a.
- -myellowknife
-
On embedded PowerPC systems, assume that the startup module is called
crt0.o and the standard C libraries are libyk.a and
libc.a.
- -mvxworks
-
On System V.4 and embedded PowerPC systems, specify that you are
compiling for a VxWorks system.
- -memb
-
On embedded PowerPC systems, set the "PPC_EMB" bit in the ELF flags
header to indicate that eabi extended relocations are used.
- -meabi
-
- -mno-eabi
-
On System V.4 and embedded PowerPC systems do (do not) adhere to the
Embedded Applications Binary Interface (EABI), which is a set of
modifications to the System V.4 specifications. Selecting -meabi
means that the stack is aligned to an 8-byte boundary, a function
"__eabi" is called from "main" to set up the EABI
environment, and the -msdata option can use both "r2" and
"r13" to point to two separate small data areas. Selecting
-mno-eabi means that the stack is aligned to a 16-byte boundary,
no EABI initialization function is called from "main", and the
-msdata option only uses "r13" to point to a single
small data area. The -meabi option is on by default if you
configured GCC using one of the powerpc*-*-eabi* options.
- -msdata=eabi
-
On System V.4 and embedded PowerPC systems, put small initialized
"const" global and static data in the ".sdata2" section, which
is pointed to by register "r2". Put small initialized
non-"const" global and static data in the ".sdata" section,
which is pointed to by register "r13". Put small uninitialized
global and static data in the ".sbss" section, which is adjacent to
the ".sdata" section. The -msdata=eabi option is
incompatible with the -mrelocatable option. The
-msdata=eabi option also sets the -memb option.
- -msdata=sysv
-
On System V.4 and embedded PowerPC systems, put small global and static
data in the ".sdata" section, which is pointed to by register
"r13". Put small uninitialized global and static data in the
".sbss" section, which is adjacent to the ".sdata" section.
The -msdata=sysv option is incompatible with the
-mrelocatable option.
- -msdata=default
-
- -msdata
-
On System V.4 and embedded PowerPC systems, if -meabi is used,
compile code the same as -msdata=eabi, otherwise compile code the
same as -msdata=sysv.
- -msdata=data
-
On System V.4 and embedded PowerPC systems, put small global
data in the ".sdata" section. Put small uninitialized global
data in the ".sbss" section. Do not use register "r13"
to address small data however. This is the default behavior unless
other -msdata options are used.
- -msdata=none
-
- -mno-sdata
-
On embedded PowerPC systems, put all initialized global and static data
in the ".data" section, and all uninitialized data in the
".bss" section.
- -mblock-move-inline-limit=num
-
Inline all block moves (such as calls to "memcpy" or structure
copies) less than or equal to num bytes. The minimum value for
num is 32 bytes on 32-bit targets and 64 bytes on 64-bit
targets. The default value is target-specific.
- -G num
-
On embedded PowerPC systems, put global and static items less than or
equal to num bytes into the small data or BSS sections instead of
the normal data or BSS section. By default, num is 8. The
-G num switch is also passed to the linker.
All modules should be compiled with the same -G num value.
- -mregnames
-
- -mno-regnames
-
On System V.4 and embedded PowerPC systems do (do not) emit register
names in the assembly language output using symbolic forms.
- -mlongcall
-
- -mno-longcall
-
By default assume that all calls are far away so that a longer and more
expensive calling sequence is required. This is required for calls
farther than 32 megabytes (33,554,432 bytes) from the current location.
A short call is generated if the compiler knows
the call cannot be that far away. This setting can be overridden by
the "shortcall" function attribute, or by "#pragma
longcall(0)".
Some linkers are capable of detecting out-of-range calls and generating
glue code on the fly. On these systems, long calls are unnecessary and
generate slower code. As of this writing, the AIX linker can do this,
as can the GNU linker for PowerPC/64. It is planned to add this feature
to the GNU linker for 32-bit PowerPC systems as well.
On Darwin/PPC systems, "#pragma longcall" generates "jbsr
callee, L42", plus a branch island (glue code). The two target
addresses represent the callee and the branch island. The
Darwin/PPC linker prefers the first address and generates a "bl
callee" if the PPC "bl" instruction reaches the callee directly;
otherwise, the linker generates "bl L42" to call the branch
island. The branch island is appended to the body of the
calling function; it computes the full 32-bit address of the callee
and jumps to it.
On Mach-O (Darwin) systems, this option directs the compiler emit to
the glue for every direct call, and the Darwin linker decides whether
to use or discard it.
In the future, GCC may ignore all longcall specifications
when the linker is known to generate glue.
- -mtls-markers
-
- -mno-tls-markers
-
Mark (do not mark) calls to "__tls_get_addr" with a relocation
specifying the function argument. The relocation allows the linker to
reliably associate function call with argument setup instructions for
TLS optimization, which in turn allows GCC to better schedule the
sequence.
- -pthread
-
Adds support for multithreading with the pthreads library.
This option sets flags for both the preprocessor and linker.
- -mrecip
-
- -mno-recip
-
This option enables use of the reciprocal estimate and
reciprocal square root estimate instructions with additional
Newton-Raphson steps to increase precision instead of doing a divide or
square root and divide for floating-point arguments. You should use
the -ffast-math option when using -mrecip (or at
least -funsafe-math-optimizations,
-finite-math-only, -freciprocal-math and
-fno-trapping-math). Note that while the throughput of the
sequence is generally higher than the throughput of the non-reciprocal
instruction, the precision of the sequence can be decreased by up to 2
ulp (i.e. the inverse of 1.0 equals 0.99999994) for reciprocal square
roots.
- -mrecip=opt
-
This option controls which reciprocal estimate instructions
may be used. opt is a comma-separated list of options, which may
be preceded by a "!" to invert the option:
-
- all
-
Enable all estimate instructions.
- default
-
Enable the default instructions, equivalent to -mrecip.
- none
-
Disable all estimate instructions, equivalent to -mno-recip.
- div
-
Enable the reciprocal approximation instructions for both
single and double precision.
- divf
-
Enable the single-precision reciprocal approximation instructions.
- divd
-
Enable the double-precision reciprocal approximation instructions.
- rsqrt
-
Enable the reciprocal square root approximation instructions for both
single and double precision.
- rsqrtf
-
Enable the single-precision reciprocal square root approximation instructions.
- rsqrtd
-
Enable the double-precision reciprocal square root approximation instructions.
-
So, for example, -mrecip=all,!rsqrtd enables
all of the reciprocal estimate instructions, except for the
"FRSQRTE", "XSRSQRTEDP", and "XVRSQRTEDP" instructions
which handle the double-precision reciprocal square root calculations.
- -mrecip-precision
-
- -mno-recip-precision
-
Assume (do not assume) that the reciprocal estimate instructions
provide higher-precision estimates than is mandated by the PowerPC
ABI. Selecting -mcpu=power6, -mcpu=power7 or
-mcpu=power8 automatically selects -mrecip-precision.
The double-precision square root estimate instructions are not generated by
default on low-precision machines, since they do not provide an
estimate that converges after three steps.
- -mveclibabi=type
-
Specifies the ABI type to use for vectorizing intrinsics using an
external library. The only type supported at present is mass,
which specifies to use IBM's Mathematical Acceleration Subsystem
(MASS) libraries for vectorizing intrinsics using external libraries.
GCC currently emits calls to "acosd2", "acosf4",
"acoshd2", "acoshf4", "asind2", "asinf4",
"asinhd2", "asinhf4", "atan2d2", "atan2f4",
"atand2", "atanf4", "atanhd2", "atanhf4",
"cbrtd2", "cbrtf4", "cosd2", "cosf4",
"coshd2", "coshf4", "erfcd2", "erfcf4",
"erfd2", "erff4", "exp2d2", "exp2f4",
"expd2", "expf4", "expm1d2", "expm1f4",
"hypotd2", "hypotf4", "lgammad2", "lgammaf4",
"log10d2", "log10f4", "log1pd2", "log1pf4",
"log2d2", "log2f4", "logd2", "logf4",
"powd2", "powf4", "sind2", "sinf4", "sinhd2",
"sinhf4", "sqrtd2", "sqrtf4", "tand2",
"tanf4", "tanhd2", and "tanhf4" when generating code
for power7. Both -ftree-vectorize and
-funsafe-math-optimizations must also be enabled. The MASS
libraries must be specified at link time.
- -mfriz
-
- -mno-friz
-
Generate (do not generate) the "friz" instruction when the
-funsafe-math-optimizations option is used to optimize
rounding of floating-point values to 64-bit integer and back to floating
point. The "friz" instruction does not return the same value if
the floating-point number is too large to fit in an integer.
- -mpointers-to-nested-functions
-
- -mno-pointers-to-nested-functions
-
Generate (do not generate) code to load up the static chain register
("r11") when calling through a pointer on AIX and 64-bit Linux
systems where a function pointer points to a 3-word descriptor giving
the function address, TOC value to be loaded in register "r2", and
static chain value to be loaded in register "r11". The
-mpointers-to-nested-functions is on by default. You cannot
call through pointers to nested functions or pointers
to functions compiled in other languages that use the static chain if
you use -mno-pointers-to-nested-functions.
- -msave-toc-indirect
-
- -mno-save-toc-indirect
-
Generate (do not generate) code to save the TOC value in the reserved
stack location in the function prologue if the function calls through
a pointer on AIX and 64-bit Linux systems. If the TOC value is not
saved in the prologue, it is saved just before the call through the
pointer. The -mno-save-toc-indirect option is the default.
- -mcompat-align-parm
-
- -mno-compat-align-parm
-
Generate (do not generate) code to pass structure parameters with a
maximum alignment of 64 bits, for compatibility with older versions
of GCC.
Older versions of GCC (prior to 4.9.0) incorrectly did not align a
structure parameter on a 128-bit boundary when that structure contained
a member requiring 128-bit alignment. This is corrected in more
recent versions of GCC. This option may be used to generate code
that is compatible with functions compiled with older versions of
GCC.
The -mno-compat-align-parm option is the default.
RX Options
These command-line options are defined for RX targets:
- -m64bit-doubles
-
- -m32bit-doubles
-
Make the "double" data type be 64 bits (-m64bit-doubles)
or 32 bits (-m32bit-doubles) in size. The default is
-m32bit-doubles. Note RX floating-point hardware only
works on 32-bit values, which is why the default is
-m32bit-doubles.
- -fpu
-
- -nofpu
-
Enables (-fpu) or disables (-nofpu) the use of RX
floating-point hardware. The default is enabled for the RX600
series and disabled for the RX200 series.
Floating-point instructions are only generated for 32-bit floating-point
values, however, so the FPU hardware is not used for doubles if the
-m64bit-doubles option is used.
Note If the -fpu option is enabled then
-funsafe-math-optimizations is also enabled automatically.
This is because the RX FPU instructions are themselves unsafe.
- -mcpu=name
-
Selects the type of RX CPU to be targeted. Currently three types are
supported, the generic RX600 and RX200 series hardware and
the specific RX610 CPU. The default is RX600.
The only difference between RX600 and RX610 is that the
RX610 does not support the "MVTIPL" instruction.
The RX200 series does not have a hardware floating-point unit
and so -nofpu is enabled by default when this type is
selected.
- -mbig-endian-data
-
- -mlittle-endian-data
-
Store data (but not code) in the big-endian format. The default is
-mlittle-endian-data, i.e. to store data in the little-endian
format.
- -msmall-data-limit=N
-
Specifies the maximum size in bytes of global and static variables
which can be placed into the small data area. Using the small data
area can lead to smaller and faster code, but the size of area is
limited and it is up to the programmer to ensure that the area does
not overflow. Also when the small data area is used one of the RX's
registers (usually "r13") is reserved for use pointing to this
area, so it is no longer available for use by the compiler. This
could result in slower and/or larger code if variables are pushed onto
the stack instead of being held in this register.
Note, common variables (variables that have not been initialized) and
constants are not placed into the small data area as they are assigned
to other sections in the output executable.
The default value is zero, which disables this feature. Note, this
feature is not enabled by default with higher optimization levels
(-O2 etc) because of the potentially detrimental effects of
reserving a register. It is up to the programmer to experiment and
discover whether this feature is of benefit to their program. See the
description of the -mpid option for a description of how the
actual register to hold the small data area pointer is chosen.
- -msim
-
- -mno-sim
-
Use the simulator runtime. The default is to use the libgloss
board-specific runtime.
- -mas100-syntax
-
- -mno-as100-syntax
-
When generating assembler output use a syntax that is compatible with
Renesas's AS100 assembler. This syntax can also be handled by the GAS
assembler, but it has some restrictions so it is not generated by default.
- -mmax-constant-size=N
-
Specifies the maximum size, in bytes, of a constant that can be used as
an operand in a RX instruction. Although the RX instruction set does
allow constants of up to 4 bytes in length to be used in instructions,
a longer value equates to a longer instruction. Thus in some
circumstances it can be beneficial to restrict the size of constants
that are used in instructions. Constants that are too big are instead
placed into a constant pool and referenced via register indirection.
The value N can be between 0 and 4. A value of 0 (the default)
or 4 means that constants of any size are allowed.
- -mrelax
-
Enable linker relaxation. Linker relaxation is a process whereby the
linker attempts to reduce the size of a program by finding shorter
versions of various instructions. Disabled by default.
- -mint-register=N
-
Specify the number of registers to reserve for fast interrupt handler
functions. The value N can be between 0 and 4. A value of 1
means that register "r13" is reserved for the exclusive use
of fast interrupt handlers. A value of 2 reserves "r13" and
"r12". A value of 3 reserves "r13", "r12" and
"r11", and a value of 4 reserves "r13" through "r10".
A value of 0, the default, does not reserve any registers.
- -msave-acc-in-interrupts
-
Specifies that interrupt handler functions should preserve the
accumulator register. This is only necessary if normal code might use
the accumulator register, for example because it performs 64-bit
multiplications. The default is to ignore the accumulator as this
makes the interrupt handlers faster.
- -mpid
-
- -mno-pid
-
Enables the generation of position independent data. When enabled any
access to constant data is done via an offset from a base address
held in a register. This allows the location of constant data to be
determined at run time without requiring the executable to be
relocated, which is a benefit to embedded applications with tight
memory constraints. Data that can be modified is not affected by this
option.
Note, using this feature reserves a register, usually "r13", for
the constant data base address. This can result in slower and/or
larger code, especially in complicated functions.
The actual register chosen to hold the constant data base address
depends upon whether the -msmall-data-limit and/or the
-mint-register command-line options are enabled. Starting
with register "r13" and proceeding downwards, registers are
allocated first to satisfy the requirements of -mint-register,
then -mpid and finally -msmall-data-limit. Thus it
is possible for the small data area register to be "r8" if both
-mint-register=4 and -mpid are specified on the
command line.
By default this feature is not enabled. The default can be restored
via the -mno-pid command-line option.
- -mno-warn-multiple-fast-interrupts
-
- -mwarn-multiple-fast-interrupts
-
Prevents GCC from issuing a warning message if it finds more than one
fast interrupt handler when it is compiling a file. The default is to
issue a warning for each extra fast interrupt handler found, as the RX
only supports one such interrupt.
Note: The generic GCC command-line option -ffixed-reg
has special significance to the RX port when used with the
"interrupt" function attribute. This attribute indicates a
function intended to process fast interrupts. GCC ensures
that it only uses the registers "r10", "r11", "r12"
and/or "r13" and only provided that the normal use of the
corresponding registers have been restricted via the
-ffixed-reg or -mint-register command-line
options.
S/390 and zSeries Options
These are the -m options defined for the S/390 and zSeries architecture.
- -mhard-float
-
- -msoft-float
-
Use (do not use) the hardware floating-point instructions and registers
for floating-point operations. When -msoft-float is specified,
functions in libgcc.a are used to perform floating-point
operations. When -mhard-float is specified, the compiler
generates IEEE floating-point instructions. This is the default.
- -mhard-dfp
-
- -mno-hard-dfp
-
Use (do not use) the hardware decimal-floating-point instructions for
decimal-floating-point operations. When -mno-hard-dfp is
specified, functions in libgcc.a are used to perform
decimal-floating-point operations. When -mhard-dfp is
specified, the compiler generates decimal-floating-point hardware
instructions. This is the default for -march=z9-ec or higher.
- -mlong-double-64
-
- -mlong-double-128
-
These switches control the size of "long double" type. A size
of 64 bits makes the "long double" type equivalent to the "double"
type. This is the default.
- -mbackchain
-
- -mno-backchain
-
Store (do not store) the address of the caller's frame as backchain pointer
into the callee's stack frame.
A backchain may be needed to allow debugging using tools that do not understand
DWARF 2 call frame information.
When -mno-packed-stack is in effect, the backchain pointer is stored
at the bottom of the stack frame; when -mpacked-stack is in effect,
the backchain is placed into the topmost word of the 96/160 byte register
save area.
In general, code compiled with -mbackchain is call-compatible with
code compiled with -mmo-backchain; however, use of the backchain
for debugging purposes usually requires that the whole binary is built with
-mbackchain. Note that the combination of -mbackchain,
-mpacked-stack and -mhard-float is not supported. In order
to build a linux kernel use -msoft-float.
The default is to not maintain the backchain.
- -mpacked-stack
-
- -mno-packed-stack
-
Use (do not use) the packed stack layout. When -mno-packed-stack is
specified, the compiler uses the all fields of the 96/160 byte register save
area only for their default purpose; unused fields still take up stack space.
When -mpacked-stack is specified, register save slots are densely
packed at the top of the register save area; unused space is reused for other
purposes, allowing for more efficient use of the available stack space.
However, when -mbackchain is also in effect, the topmost word of
the save area is always used to store the backchain, and the return address
register is always saved two words below the backchain.
As long as the stack frame backchain is not used, code generated with
-mpacked-stack is call-compatible with code generated with
-mno-packed-stack. Note that some non-FSF releases of GCC 2.95 for
S/390 or zSeries generated code that uses the stack frame backchain at run
time, not just for debugging purposes. Such code is not call-compatible
with code compiled with -mpacked-stack. Also, note that the
combination of -mbackchain,
-mpacked-stack and -mhard-float is not supported. In order
to build a linux kernel use -msoft-float.
The default is to not use the packed stack layout.
- -msmall-exec
-
- -mno-small-exec
-
Generate (or do not generate) code using the "bras" instruction
to do subroutine calls.
This only works reliably if the total executable size does not
exceed 64k. The default is to use the "basr" instruction instead,
which does not have this limitation.
- -m64
-
- -m31
-
When -m31 is specified, generate code compliant to the
GNU/Linux for S/390 ABI. When -m64 is specified, generate
code compliant to the GNU/Linux for zSeries ABI. This allows GCC in
particular to generate 64-bit instructions. For the s390
targets, the default is -m31, while the s390x
targets default to -m64.
- -mzarch
-
- -mesa
-
When -mzarch is specified, generate code using the
instructions available on z/Architecture.
When -mesa is specified, generate code using the
instructions available on ESA/390. Note that -mesa is
not possible with -m64.
When generating code compliant to the GNU/Linux for S/390 ABI,
the default is -mesa. When generating code compliant
to the GNU/Linux for zSeries ABI, the default is -mzarch.
- -mmvcle
-
- -mno-mvcle
-
Generate (or do not generate) code using the "mvcle" instruction
to perform block moves. When -mno-mvcle is specified,
use a "mvc" loop instead. This is the default unless optimizing for
size.
- -mdebug
-
- -mno-debug
-
Print (or do not print) additional debug information when compiling.
The default is to not print debug information.
- -march=cpu-type
-
Generate code that runs on cpu-type, which is the name of a system
representing a certain processor type. Possible values for
cpu-type are g5, g6, z900, z990,
z9-109, z9-ec, z10, z196, zEC12,
and z13.
When generating code using the instructions available on z/Architecture,
the default is -march=z900. Otherwise, the default is
-march=g5.
- -mtune=cpu-type
-
Tune to cpu-type everything applicable about the generated code,
except for the ABI and the set of available instructions.
The list of cpu-type values is the same as for -march.
The default is the value used for -march.
- -mtpf-trace
-
- -mno-tpf-trace
-
Generate code that adds (does not add) in TPF OS specific branches to trace
routines in the operating system. This option is off by default, even
when compiling for the TPF OS.
- -mfused-madd
-
- -mno-fused-madd
-
Generate code that uses (does not use) the floating-point multiply and
accumulate instructions. These instructions are generated by default if
hardware floating point is used.
- -mwarn-framesize=framesize
-
Emit a warning if the current function exceeds the given frame size. Because
this is a compile-time check it doesn't need to be a real problem when the program
runs. It is intended to identify functions that most probably cause
a stack overflow. It is useful to be used in an environment with limited stack
size e.g. the linux kernel.
- -mwarn-dynamicstack
-
Emit a warning if the function calls "alloca" or uses dynamically-sized
arrays. This is generally a bad idea with a limited stack size.
- -mstack-guard=stack-guard
-
- -mstack-size=stack-size
-
If these options are provided the S/390 back end emits additional instructions in
the function prologue that trigger a trap if the stack size is stack-guard
bytes above the stack-size (remember that the stack on S/390 grows downward).
If the stack-guard option is omitted the smallest power of 2 larger than
the frame size of the compiled function is chosen.
These options are intended to be used to help debugging stack overflow problems.
The additionally emitted code causes only little overhead and hence can also be
used in production-like systems without greater performance degradation. The given
values have to be exact powers of 2 and stack-size has to be greater than
stack-guard without exceeding 64k.
In order to be efficient the extra code makes the assumption that the stack starts
at an address aligned to the value given by stack-size.
The stack-guard option can only be used in conjunction with stack-size.
- -mhotpatch=pre-halfwords,post-halfwords
-
If the hotpatch option is enabled, a ``hot-patching'' function
prologue is generated for all functions in the compilation unit.
The funtion label is prepended with the given number of two-byte
NOP instructions (pre-halfwords, maximum 1000000). After
the label, 2 * post-halfwords bytes are appended, using the
largest NOP like instructions the architecture allows (maximum
1000000).
If both arguments are zero, hotpatching is disabled.
This option can be overridden for individual functions with the
"hotpatch" attribute.
Score Options
These options are defined for Score implementations:
- -meb
-
Compile code for big-endian mode. This is the default.
- -mel
-
Compile code for little-endian mode.
- -mnhwloop
-
Disable generation of "bcnz" instructions.
- -muls
-
Enable generation of unaligned load and store instructions.
- -mmac
-
Enable the use of multiply-accumulate instructions. Disabled by default.
- -mscore5
-
Specify the SCORE5 as the target architecture.
- -mscore5u
-
Specify the SCORE5U of the target architecture.
- -mscore7
-
Specify the SCORE7 as the target architecture. This is the default.
- -mscore7d
-
Specify the SCORE7D as the target architecture.
SH Options
These -m options are defined for the SH implementations:
- -m1
-
Generate code for the SH1.
- -m2
-
Generate code for the SH2.
- -m2e
-
Generate code for the SH2e.
- -m2a-nofpu
-
Generate code for the SH2a without FPU, or for a SH2a-FPU in such a way
that the floating-point unit is not used.
- -m2a-single-only
-
Generate code for the SH2a-FPU, in such a way that no double-precision
floating-point operations are used.
- -m2a-single
-
Generate code for the SH2a-FPU assuming the floating-point unit is in
single-precision mode by default.
- -m2a
-
Generate code for the SH2a-FPU assuming the floating-point unit is in
double-precision mode by default.
- -m3
-
Generate code for the SH3.
- -m3e
-
Generate code for the SH3e.
- -m4-nofpu
-
Generate code for the SH4 without a floating-point unit.
- -m4-single-only
-
Generate code for the SH4 with a floating-point unit that only
supports single-precision arithmetic.
- -m4-single
-
Generate code for the SH4 assuming the floating-point unit is in
single-precision mode by default.
- -m4
-
Generate code for the SH4.
- -m4-100
-
Generate code for SH4-100.
- -m4-100-nofpu
-
Generate code for SH4-100 in such a way that the
floating-point unit is not used.
- -m4-100-single
-
Generate code for SH4-100 assuming the floating-point unit is in
single-precision mode by default.
- -m4-100-single-only
-
Generate code for SH4-100 in such a way that no double-precision
floating-point operations are used.
- -m4-200
-
Generate code for SH4-200.
- -m4-200-nofpu
-
Generate code for SH4-200 without in such a way that the
floating-point unit is not used.
- -m4-200-single
-
Generate code for SH4-200 assuming the floating-point unit is in
single-precision mode by default.
- -m4-200-single-only
-
Generate code for SH4-200 in such a way that no double-precision
floating-point operations are used.
- -m4-300
-
Generate code for SH4-300.
- -m4-300-nofpu
-
Generate code for SH4-300 without in such a way that the
floating-point unit is not used.
- -m4-300-single
-
Generate code for SH4-300 in such a way that no double-precision
floating-point operations are used.
- -m4-300-single-only
-
Generate code for SH4-300 in such a way that no double-precision
floating-point operations are used.
- -m4-340
-
Generate code for SH4-340 (no MMU, no FPU).
- -m4-500
-
Generate code for SH4-500 (no FPU). Passes -isa=sh4-nofpu to the
assembler.
- -m4a-nofpu
-
Generate code for the SH4al-dsp, or for a SH4a in such a way that the
floating-point unit is not used.
- -m4a-single-only
-
Generate code for the SH4a, in such a way that no double-precision
floating-point operations are used.
- -m4a-single
-
Generate code for the SH4a assuming the floating-point unit is in
single-precision mode by default.
- -m4a
-
Generate code for the SH4a.
- -m4al
-
Same as -m4a-nofpu, except that it implicitly passes
-dsp to the assembler. GCC doesn't generate any DSP
instructions at the moment.
- -m5-32media
-
Generate 32-bit code for SHmedia.
- -m5-32media-nofpu
-
Generate 32-bit code for SHmedia in such a way that the
floating-point unit is not used.
- -m5-64media
-
Generate 64-bit code for SHmedia.
- -m5-64media-nofpu
-
Generate 64-bit code for SHmedia in such a way that the
floating-point unit is not used.
- -m5-compact
-
Generate code for SHcompact.
- -m5-compact-nofpu
-
Generate code for SHcompact in such a way that the
floating-point unit is not used.
- -mb
-
Compile code for the processor in big-endian mode.
- -ml
-
Compile code for the processor in little-endian mode.
- -mdalign
-
Align doubles at 64-bit boundaries. Note that this changes the calling
conventions, and thus some functions from the standard C library do
not work unless you recompile it first with -mdalign.
- -mrelax
-
Shorten some address references at link time, when possible; uses the
linker option -relax.
- -mbigtable
-
Use 32-bit offsets in "switch" tables. The default is to use
16-bit offsets.
- -mbitops
-
Enable the use of bit manipulation instructions on SH2A.
- -mfmovd
-
Enable the use of the instruction "fmovd". Check -mdalign for
alignment constraints.
- -mrenesas
-
Comply with the calling conventions defined by Renesas.
- -mno-renesas
-
Comply with the calling conventions defined for GCC before the Renesas
conventions were available. This option is the default for all
targets of the SH toolchain.
- -mnomacsave
-
Mark the "MAC" register as call-clobbered, even if
-mrenesas is given.
- -mieee
-
- -mno-ieee
-
Control the IEEE compliance of floating-point comparisons, which affects the
handling of cases where the result of a comparison is unordered. By default
-mieee is implicitly enabled. If -ffinite-math-only is
enabled -mno-ieee is implicitly set, which results in faster
floating-point greater-equal and less-equal comparisons. The implcit settings
can be overridden by specifying either -mieee or -mno-ieee.
- -minline-ic_invalidate
-
Inline code to invalidate instruction cache entries after setting up
nested function trampolines.
This option has no effect if -musermode is in effect and the selected
code generation option (e.g. -m4) does not allow the use of the "icbi"
instruction.
If the selected code generation option does not allow the use of the "icbi"
instruction, and -musermode is not in effect, the inlined code
manipulates the instruction cache address array directly with an associative
write. This not only requires privileged mode at run time, but it also
fails if the cache line had been mapped via the TLB and has become unmapped.
- -misize
-
Dump instruction size and location in the assembly code.
- -mpadstruct
-
This option is deprecated. It pads structures to multiple of 4 bytes,
which is incompatible with the SH ABI.
- -matomic-model=model
-
Sets the model of atomic operations and additional parameters as a comma
separated list. For details on the atomic built-in functions see
__atomic Builtins. The following models and parameters are supported:
-
- none
-
Disable compiler generated atomic sequences and emit library calls for atomic
operations. This is the default if the target is not "sh*-*-linux*".
- soft-gusa
-
Generate GNU/Linux compatible gUSA software atomic sequences for the atomic
built-in functions. The generated atomic sequences require additional support
from the interrupt/exception handling code of the system and are only suitable
for SH3* and SH4* single-core systems. This option is enabled by default when
the target is "sh*-*-linux*" and SH3* or SH4*. When the target is SH4A,
this option also partially utilizes the hardware atomic instructions
"movli.l" and "movco.l" to create more efficient code, unless
strict is specified.
- soft-tcb
-
Generate software atomic sequences that use a variable in the thread control
block. This is a variation of the gUSA sequences which can also be used on
SH1* and SH2* targets. The generated atomic sequences require additional
support from the interrupt/exception handling code of the system and are only
suitable for single-core systems. When using this model, the gbr-offset=
parameter has to be specified as well.
- soft-imask
-
Generate software atomic sequences that temporarily disable interrupts by
setting "SR.IMASK = 1111". This model works only when the program runs
in privileged mode and is only suitable for single-core systems. Additional
support from the interrupt/exception handling code of the system is not
required. This model is enabled by default when the target is
"sh*-*-linux*" and SH1* or SH2*.
- hard-llcs
-
Generate hardware atomic sequences using the "movli.l" and "movco.l"
instructions only. This is only available on SH4A and is suitable for
multi-core systems. Since the hardware instructions support only 32 bit atomic
variables access to 8 or 16 bit variables is emulated with 32 bit accesses.
Code compiled with this option is also compatible with other software
atomic model interrupt/exception handling systems if executed on an SH4A
system. Additional support from the interrupt/exception handling code of the
system is not required for this model.
- gbr-offset=
-
This parameter specifies the offset in bytes of the variable in the thread
control block structure that should be used by the generated atomic sequences
when the soft-tcb model has been selected. For other models this
parameter is ignored. The specified value must be an integer multiple of four
and in the range 0-1020.
- strict
-
This parameter prevents mixed usage of multiple atomic models, even if they
are compatible, and makes the compiler generate atomic sequences of the
specified model only.
-
- -mtas
-
Generate the "tas.b" opcode for "__atomic_test_and_set".
Notice that depending on the particular hardware and software configuration
this can degrade overall performance due to the operand cache line flushes
that are implied by the "tas.b" instruction. On multi-core SH4A
processors the "tas.b" instruction must be used with caution since it
can result in data corruption for certain cache configurations.
- -mprefergot
-
When generating position-independent code, emit function calls using
the Global Offset Table instead of the Procedure Linkage Table.
- -musermode
-
- -mno-usermode
-
Don't allow (allow) the compiler generating privileged mode code. Specifying
-musermode also implies -mno-inline-ic_invalidate if the
inlined code would not work in user mode. -musermode is the default
when the target is "sh*-*-linux*". If the target is SH1* or SH2*
-musermode has no effect, since there is no user mode.
- -multcost=number
-
Set the cost to assume for a multiply insn.
- -mdiv=strategy
-
Set the division strategy to be used for integer division operations.
For SHmedia strategy can be one of:
-
- fp
-
Performs the operation in floating point. This has a very high latency,
but needs only a few instructions, so it might be a good choice if
your code has enough easily-exploitable ILP to allow the compiler to
schedule the floating-point instructions together with other instructions.
Division by zero causes a floating-point exception.
- inv
-
Uses integer operations to calculate the inverse of the divisor,
and then multiplies the dividend with the inverse. This strategy allows
CSE and hoisting of the inverse calculation. Division by zero calculates
an unspecified result, but does not trap.
- inv:minlat
-
A variant of inv where, if no CSE or hoisting opportunities
have been found, or if the entire operation has been hoisted to the same
place, the last stages of the inverse calculation are intertwined with the
final multiply to reduce the overall latency, at the expense of using a few
more instructions, and thus offering fewer scheduling opportunities with
other code.
- call
-
Calls a library function that usually implements the inv:minlat
strategy.
This gives high code density for "m5-*media-nofpu" compilations.
- call2
-
Uses a different entry point of the same library function, where it
assumes that a pointer to a lookup table has already been set up, which
exposes the pointer load to CSE and code hoisting optimizations.
- inv:call
-
- inv:call2
-
- inv:fp
-
Use the inv algorithm for initial
code generation, but if the code stays unoptimized, revert to the call,
call2, or fp strategies, respectively. Note that the
potentially-trapping side effect of division by zero is carried by a
separate instruction, so it is possible that all the integer instructions
are hoisted out, but the marker for the side effect stays where it is.
A recombination to floating-point operations or a call is not possible
in that case.
- inv20u
-
- inv20l
-
Variants of the inv:minlat strategy. In the case
that the inverse calculation is not separated from the multiply, they speed
up division where the dividend fits into 20 bits (plus sign where applicable)
by inserting a test to skip a number of operations in this case; this test
slows down the case of larger dividends. inv20u assumes the case of a such
a small dividend to be unlikely, and inv20l assumes it to be likely.
-
For targets other than SHmedia strategy can be one of:
- call-div1
-
Calls a library function that uses the single-step division instruction
"div1" to perform the operation. Division by zero calculates an
unspecified result and does not trap. This is the default except for SH4,
SH2A and SHcompact.
- call-fp
-
Calls a library function that performs the operation in double precision
floating point. Division by zero causes a floating-point exception. This is
the default for SHcompact with FPU. Specifying this for targets that do not
have a double precision FPU defaults to "call-div1".
- call-table
-
Calls a library function that uses a lookup table for small divisors and
the "div1" instruction with case distinction for larger divisors. Division
by zero calculates an unspecified result and does not trap. This is the default
for SH4. Specifying this for targets that do not have dynamic shift
instructions defaults to "call-div1".
-
When a division strategy has not been specified the default strategy is
selected based on the current target. For SH2A the default strategy is to
use the "divs" and "divu" instructions instead of library function
calls.
- -maccumulate-outgoing-args
-
Reserve space once for outgoing arguments in the function prologue rather
than around each call. Generally beneficial for performance and size. Also
needed for unwinding to avoid changing the stack frame around conditional code.
- -mdivsi3_libfunc=name
-
Set the name of the library function used for 32-bit signed division to
name.
This only affects the name used in the call and inv:call
division strategies, and the compiler still expects the same
sets of input/output/clobbered registers as if this option were not present.
- -mfixed-range=register-range
-
Generate code treating the given register range as fixed registers.
A fixed register is one that the register allocator can not use. This is
useful when compiling kernel code. A register range is specified as
two registers separated by a dash. Multiple register ranges can be
specified separated by a comma.
- -mindexed-addressing
-
Enable the use of the indexed addressing mode for SHmedia32/SHcompact.
This is only safe if the hardware and/or OS implement 32-bit wrap-around
semantics for the indexed addressing mode. The architecture allows the
implementation of processors with 64-bit MMU, which the OS could use to
get 32-bit addressing, but since no current hardware implementation supports
this or any other way to make the indexed addressing mode safe to use in
the 32-bit ABI, the default is -mno-indexed-addressing.
- -mgettrcost=number
-
Set the cost assumed for the "gettr" instruction to number.
The default is 2 if -mpt-fixed is in effect, 100 otherwise.
- -mpt-fixed
-
Assume "pt*" instructions won't trap. This generally generates
better-scheduled code, but is unsafe on current hardware.
The current architecture
definition says that "ptabs" and "ptrel" trap when the target
anded with 3 is 3.
This has the unintentional effect of making it unsafe to schedule these
instructions before a branch, or hoist them out of a loop. For example,
"__do_global_ctors", a part of libgcc
that runs constructors at program
startup, calls functions in a list which is delimited by -1. With the
-mpt-fixed option, the "ptabs" is done before testing against -1.
That means that all the constructors run a bit more quickly, but when
the loop comes to the end of the list, the program crashes because "ptabs"
loads -1 into a target register.
Since this option is unsafe for any
hardware implementing the current architecture specification, the default
is -mno-pt-fixed. Unless specified explicitly with
-mgettrcost, -mno-pt-fixed also implies -mgettrcost=100;
this deters register allocation from using target registers for storing
ordinary integers.
- -minvalid-symbols
-
Assume symbols might be invalid. Ordinary function symbols generated by
the compiler are always valid to load with
"movi"/"shori"/"ptabs" or
"movi"/"shori"/"ptrel",
but with assembler and/or linker tricks it is possible
to generate symbols that cause "ptabs" or "ptrel" to trap.
This option is only meaningful when -mno-pt-fixed is in effect.
It prevents cross-basic-block CSE, hoisting and most scheduling
of symbol loads. The default is -mno-invalid-symbols.
- -mbranch-cost=num
-
Assume num to be the cost for a branch instruction. Higher numbers
make the compiler try to generate more branch-free code if possible.
If not specified the value is selected depending on the processor type that
is being compiled for.
- -mzdcbranch
-
- -mno-zdcbranch
-
Assume (do not assume) that zero displacement conditional branch instructions
"bt" and "bf" are fast. If -mzdcbranch is specified, the
compiler prefers zero displacement branch code sequences. This is
enabled by default when generating code for SH4 and SH4A. It can be explicitly
disabled by specifying -mno-zdcbranch.
- -mcbranch-force-delay-slot
-
Force the usage of delay slots for conditional branches, which stuffs the delay
slot with a "nop" if a suitable instruction can't be found. By default
this option is disabled. It can be enabled to work around hardware bugs as
found in the original SH7055.
- -mfused-madd
-
- -mno-fused-madd
-
Generate code that uses (does not use) the floating-point multiply and
accumulate instructions. These instructions are generated by default
if hardware floating point is used. The machine-dependent
-mfused-madd option is now mapped to the machine-independent
-ffp-contract=fast option, and -mno-fused-madd is
mapped to -ffp-contract=off.
- -mfsca
-
- -mno-fsca
-
Allow or disallow the compiler to emit the "fsca" instruction for sine
and cosine approximations. The option -mfsca must be used in
combination with -funsafe-math-optimizations. It is enabled by default
when generating code for SH4A. Using -mno-fsca disables sine and cosine
approximations even if -funsafe-math-optimizations is in effect.
- -mfsrra
-
- -mno-fsrra
-
Allow or disallow the compiler to emit the "fsrra" instruction for
reciprocal square root approximations. The option -mfsrra must be used
in combination with -funsafe-math-optimizations and
-ffinite-math-only. It is enabled by default when generating code for
SH4A. Using -mno-fsrra disables reciprocal square root approximations
even if -funsafe-math-optimizations and -ffinite-math-only are
in effect.
- -mpretend-cmove
-
Prefer zero-displacement conditional branches for conditional move instruction
patterns. This can result in faster code on the SH4 processor.
Solaris 2 Options
These -m options are supported on Solaris 2:
- -mclear-hwcap
-
-mclear-hwcap tells the compiler to remove the hardware
capabilities generated by the Solaris assembler. This is only necessary
when object files use ISA extensions not supported by the current
machine, but check at runtime whether or not to use them.
- -mimpure-text
-
-mimpure-text, used in addition to -shared, tells
the compiler to not pass -z text to the linker when linking a
shared object. Using this option, you can link position-dependent
code into a shared object.
-mimpure-text suppresses the ``relocations remain against
allocatable but non-writable sections'' linker error message.
However, the necessary relocations trigger copy-on-write, and the
shared object is not actually shared across processes. Instead of
using -mimpure-text, you should compile all source code with
-fpic or -fPIC.
These switches are supported in addition to the above on Solaris 2:
- -pthreads
-
Add support for multithreading using the POSIX threads library. This
option sets flags for both the preprocessor and linker. This option does
not affect the thread safety of object code produced by the compiler or
that of libraries supplied with it.
- -pthread
-
This is a synonym for -pthreads.
SPARC Options
These -m options are supported on the SPARC:
- -mno-app-regs
-
- -mapp-regs
-
Specify -mapp-regs to generate output using the global registers
2 through 4, which the SPARC SVR4 ABI reserves for applications. Like the
global register 1, each global register 2 through 4 is then treated as an
allocable register that is clobbered by function calls. This is the default.
To be fully SVR4 ABI-compliant at the cost of some performance loss,
specify -mno-app-regs. You should compile libraries and system
software with this option.
- -mflat
-
- -mno-flat
-
With -mflat, the compiler does not generate save/restore instructions
and uses a ``flat'' or single register window model. This model is compatible
with the regular register window model. The local registers and the input
registers (0--5) are still treated as ``call-saved'' registers and are
saved on the stack as needed.
With -mno-flat (the default), the compiler generates save/restore
instructions (except for leaf functions). This is the normal operating mode.
- -mfpu
-
- -mhard-float
-
Generate output containing floating-point instructions. This is the
default.
- -mno-fpu
-
- -msoft-float
-
Generate output containing library calls for floating point.
Warning: the requisite libraries are not available for all SPARC
targets. Normally the facilities of the machine's usual C compiler are
used, but this cannot be done directly in cross-compilation. You must make
your own arrangements to provide suitable library functions for
cross-compilation. The embedded targets sparc-*-aout and
sparclite-*-* do provide software floating-point support.
-msoft-float changes the calling convention in the output file;
therefore, it is only useful if you compile all of a program with
this option. In particular, you need to compile libgcc.a, the
library that comes with GCC, with -msoft-float in order for
this to work.
- -mhard-quad-float
-
Generate output containing quad-word (long double) floating-point
instructions.
- -msoft-quad-float
-
Generate output containing library calls for quad-word (long double)
floating-point instructions. The functions called are those specified
in the SPARC ABI. This is the default.
As of this writing, there are no SPARC implementations that have hardware
support for the quad-word floating-point instructions. They all invoke
a trap handler for one of these instructions, and then the trap handler
emulates the effect of the instruction. Because of the trap handler overhead,
this is much slower than calling the ABI library routines. Thus the
-msoft-quad-float option is the default.
- -mno-unaligned-doubles
-
- -munaligned-doubles
-
Assume that doubles have 8-byte alignment. This is the default.
With -munaligned-doubles, GCC assumes that doubles have 8-byte
alignment only if they are contained in another type, or if they have an
absolute address. Otherwise, it assumes they have 4-byte alignment.
Specifying this option avoids some rare compatibility problems with code
generated by other compilers. It is not the default because it results
in a performance loss, especially for floating-point code.
- -muser-mode
-
- -mno-user-mode
-
Do not generate code that can only run in supervisor mode. This is relevant
only for the "casa" instruction emitted for the LEON3 processor. This
is the default.
- -mno-faster-structs
-
- -mfaster-structs
-
With -mfaster-structs, the compiler assumes that structures
should have 8-byte alignment. This enables the use of pairs of
"ldd" and "std" instructions for copies in structure
assignment, in place of twice as many "ld" and "st" pairs.
However, the use of this changed alignment directly violates the SPARC
ABI. Thus, it's intended only for use on targets where the developer
acknowledges that their resulting code is not directly in line with
the rules of the ABI.
- -mcpu=cpu_type
-
Set the instruction set, register set, and instruction scheduling parameters
for machine type cpu_type. Supported values for cpu_type are
v7, cypress, v8, supersparc, hypersparc,
leon, leon3, leon3v7, sparclite, f930,
f934, sparclite86x, sparclet, tsc701, v9,
ultrasparc, ultrasparc3, niagara, niagara2,
niagara3 and niagara4.
Native Solaris and GNU/Linux toolchains also support the value native,
which selects the best architecture option for the host processor.
-mcpu=native has no effect if GCC does not recognize
the processor.
Default instruction scheduling parameters are used for values that select
an architecture and not an implementation. These are v7, v8,
sparclite, sparclet, v9.
Here is a list of each supported architecture and their supported
implementations.
-
- v7
-
cypress, leon3v7
- v8
-
supersparc, hypersparc, leon, leon3
- sparclite
-
f930, f934, sparclite86x
- sparclet
-
tsc701
- v9
-
ultrasparc, ultrasparc3, niagara, niagara2, niagara3, niagara4
-
By default (unless configured otherwise), GCC generates code for the V7
variant of the SPARC architecture. With -mcpu=cypress, the compiler
additionally optimizes it for the Cypress CY7C602 chip, as used in the
SPARCStation/SPARCServer 3xx series. This is also appropriate for the older
SPARCStation 1, 2, IPX etc.
With -mcpu=v8, GCC generates code for the V8 variant of the SPARC
architecture. The only difference from V7 code is that the compiler emits
the integer multiply and integer divide instructions which exist in SPARC-V8
but not in SPARC-V7. With -mcpu=supersparc, the compiler additionally
optimizes it for the SuperSPARC chip, as used in the SPARCStation 10, 1000 and
2000 series.
With -mcpu=sparclite, GCC generates code for the SPARClite variant of
the SPARC architecture. This adds the integer multiply, integer divide step
and scan ("ffs") instructions which exist in SPARClite but not in SPARC-V7.
With -mcpu=f930, the compiler additionally optimizes it for the
Fujitsu MB86930 chip, which is the original SPARClite, with no FPU. With
-mcpu=f934, the compiler additionally optimizes it for the Fujitsu
MB86934 chip, which is the more recent SPARClite with FPU.
With -mcpu=sparclet, GCC generates code for the SPARClet variant of
the SPARC architecture. This adds the integer multiply, multiply/accumulate,
integer divide step and scan ("ffs") instructions which exist in SPARClet
but not in SPARC-V7. With -mcpu=tsc701, the compiler additionally
optimizes it for the TEMIC SPARClet chip.
With -mcpu=v9, GCC generates code for the V9 variant of the SPARC
architecture. This adds 64-bit integer and floating-point move instructions,
3 additional floating-point condition code registers and conditional move
instructions. With -mcpu=ultrasparc, the compiler additionally
optimizes it for the Sun UltraSPARC I/II/IIi chips. With
-mcpu=ultrasparc3, the compiler additionally optimizes it for the
Sun UltraSPARC III/III+/IIIi/IIIi+/IV/IV+ chips. With
-mcpu=niagara, the compiler additionally optimizes it for
Sun UltraSPARC T1 chips. With -mcpu=niagara2, the compiler
additionally optimizes it for Sun UltraSPARC T2 chips. With
-mcpu=niagara3, the compiler additionally optimizes it for Sun
UltraSPARC T3 chips. With -mcpu=niagara4, the compiler
additionally optimizes it for Sun UltraSPARC T4 chips.
- -mtune=cpu_type
-
Set the instruction scheduling parameters for machine type
cpu_type, but do not set the instruction set or register set that the
option -mcpu=cpu_type does.
The same values for -mcpu=cpu_type can be used for
-mtune=cpu_type, but the only useful values are those
that select a particular CPU implementation. Those are cypress,
supersparc, hypersparc, leon, leon3,
leon3v7, f930, f934, sparclite86x, tsc701,
ultrasparc, ultrasparc3, niagara, niagara2,
niagara3 and niagara4. With native Solaris and GNU/Linux
toolchains, native can also be used.
- -mv8plus
-
- -mno-v8plus
-
With -mv8plus, GCC generates code for the SPARC-V8+ ABI. The
difference from the V8 ABI is that the global and out registers are
considered 64 bits wide. This is enabled by default on Solaris in 32-bit
mode for all SPARC-V9 processors.
- -mvis
-
- -mno-vis
-
With -mvis, GCC generates code that takes advantage of the UltraSPARC
Visual Instruction Set extensions. The default is -mno-vis.
- -mvis2
-
- -mno-vis2
-
With -mvis2, GCC generates code that takes advantage of
version 2.0 of the UltraSPARC Visual Instruction Set extensions. The
default is -mvis2 when targeting a cpu that supports such
instructions, such as UltraSPARC-III and later. Setting -mvis2
also sets -mvis.
- -mvis3
-
- -mno-vis3
-
With -mvis3, GCC generates code that takes advantage of
version 3.0 of the UltraSPARC Visual Instruction Set extensions. The
default is -mvis3 when targeting a cpu that supports such
instructions, such as niagara-3 and later. Setting -mvis3
also sets -mvis2 and -mvis.
- -mcbcond
-
- -mno-cbcond
-
With -mcbcond, GCC generates code that takes advantage of
compare-and-branch instructions, as defined in the Sparc Architecture 2011.
The default is -mcbcond when targeting a cpu that supports such
instructions, such as niagara-4 and later.
- -mpopc
-
- -mno-popc
-
With -mpopc, GCC generates code that takes advantage of the UltraSPARC
population count instruction. The default is -mpopc
when targeting a cpu that supports such instructions, such as Niagara-2 and
later.
- -mfmaf
-
- -mno-fmaf
-
With -mfmaf, GCC generates code that takes advantage of the UltraSPARC
Fused Multiply-Add Floating-point extensions. The default is -mfmaf
when targeting a cpu that supports such instructions, such as Niagara-3 and
later.
- -mfix-at697f
-
Enable the documented workaround for the single erratum of the Atmel AT697F
processor (which corresponds to erratum #13 of the AT697E processor).
- -mfix-ut699
-
Enable the documented workarounds for the floating-point errata and the data
cache nullify errata of the UT699 processor.
These -m options are supported in addition to the above
on SPARC-V9 processors in 64-bit environments:
- -m32
-
- -m64
-
Generate code for a 32-bit or 64-bit environment.
The 32-bit environment sets int, long and pointer to 32 bits.
The 64-bit environment sets int to 32 bits and long and pointer
to 64 bits.
- -mcmodel=which
-
Set the code model to one of
-
- medlow
-
The Medium/Low code model: 64-bit addresses, programs
must be linked in the low 32 bits of memory. Programs can be statically
or dynamically linked.
- medmid
-
The Medium/Middle code model: 64-bit addresses, programs
must be linked in the low 44 bits of memory, the text and data segments must
be less than 2GB in size and the data segment must be located within 2GB of
the text segment.
- medany
-
The Medium/Anywhere code model: 64-bit addresses, programs
may be linked anywhere in memory, the text and data segments must be less
than 2GB in size and the data segment must be located within 2GB of the
text segment.
- embmedany
-
The Medium/Anywhere code model for embedded systems:
64-bit addresses, the text and data segments must be less than 2GB in
size, both starting anywhere in memory (determined at link time). The
global register %g4 points to the base of the data segment. Programs
are statically linked and PIC is not supported.
-
- -mmemory-model=mem-model
-
Set the memory model in force on the processor to one of
-
- default
-
The default memory model for the processor and operating system.
- rmo
-
Relaxed Memory Order
- pso
-
Partial Store Order
- tso
-
Total Store Order
- sc
-
Sequential Consistency
-
These memory models are formally defined in Appendix D of the Sparc V9
architecture manual, as set in the processor's "PSTATE.MM" field.
- -mstack-bias
-
- -mno-stack-bias
-
With -mstack-bias, GCC assumes that the stack pointer, and
frame pointer if present, are offset by -2047 which must be added back
when making stack frame references. This is the default in 64-bit mode.
Otherwise, assume no such offset is present.
SPU Options
These -m options are supported on the SPU:
- -mwarn-reloc
-
- -merror-reloc
-
The loader for SPU does not handle dynamic relocations. By default, GCC
gives an error when it generates code that requires a dynamic
relocation. -mno-error-reloc disables the error,
-mwarn-reloc generates a warning instead.
- -msafe-dma
-
- -munsafe-dma
-
Instructions that initiate or test completion of DMA must not be
reordered with respect to loads and stores of the memory that is being
accessed.
With -munsafe-dma you must use the "volatile" keyword to protect
memory accesses, but that can lead to inefficient code in places where the
memory is known to not change. Rather than mark the memory as volatile,
you can use -msafe-dma to tell the compiler to treat
the DMA instructions as potentially affecting all memory.
- -mbranch-hints
-
By default, GCC generates a branch hint instruction to avoid
pipeline stalls for always-taken or probably-taken branches. A hint
is not generated closer than 8 instructions away from its branch.
There is little reason to disable them, except for debugging purposes,
or to make an object a little bit smaller.
- -msmall-mem
-
- -mlarge-mem
-
By default, GCC generates code assuming that addresses are never larger
than 18 bits. With -mlarge-mem code is generated that assumes
a full 32-bit address.
- -mstdmain
-
By default, GCC links against startup code that assumes the SPU-style
main function interface (which has an unconventional parameter list).
With -mstdmain, GCC links your program against startup
code that assumes a C99-style interface to "main", including a
local copy of "argv" strings.
- -mfixed-range=register-range
-
Generate code treating the given register range as fixed registers.
A fixed register is one that the register allocator cannot use. This is
useful when compiling kernel code. A register range is specified as
two registers separated by a dash. Multiple register ranges can be
specified separated by a comma.
- -mea32
-
- -mea64
-
Compile code assuming that pointers to the PPU address space accessed
via the "__ea" named address space qualifier are either 32 or 64
bits wide. The default is 32 bits. As this is an ABI-changing option,
all object code in an executable must be compiled with the same setting.
- -maddress-space-conversion
-
- -mno-address-space-conversion
-
Allow/disallow treating the "__ea" address space as superset
of the generic address space. This enables explicit type casts
between "__ea" and generic pointer as well as implicit
conversions of generic pointers to "__ea" pointers. The
default is to allow address space pointer conversions.
- -mcache-size=cache-size
-
This option controls the version of libgcc that the compiler links to an
executable and selects a software-managed cache for accessing variables
in the "__ea" address space with a particular cache size. Possible
options for cache-size are 8, 16, 32, 64
and 128. The default cache size is 64KB.
- -matomic-updates
-
- -mno-atomic-updates
-
This option controls the version of libgcc that the compiler links to an
executable and selects whether atomic updates to the software-managed
cache of PPU-side variables are used. If you use atomic updates, changes
to a PPU variable from SPU code using the "__ea" named address space
qualifier do not interfere with changes to other PPU variables residing
in the same cache line from PPU code. If you do not use atomic updates,
such interference may occur; however, writing back cache lines is
more efficient. The default behavior is to use atomic updates.
- -mdual-nops
-
- -mdual-nops=n
-
By default, GCC inserts nops to increase dual issue when it expects
it to increase performance. n can be a value from 0 to 10. A
smaller n inserts fewer nops. 10 is the default, 0 is the
same as -mno-dual-nops. Disabled with -Os.
- -mhint-max-nops=n
-
Maximum number of nops to insert for a branch hint. A branch hint must
be at least 8 instructions away from the branch it is affecting. GCC
inserts up to n nops to enforce this, otherwise it does not
generate the branch hint.
- -mhint-max-distance=n
-
The encoding of the branch hint instruction limits the hint to be within
256 instructions of the branch it is affecting. By default, GCC makes
sure it is within 125.
- -msafe-hints
-
Work around a hardware bug that causes the SPU to stall indefinitely.
By default, GCC inserts the "hbrp" instruction to make sure
this stall won't happen.
Options for System V
These additional options are available on System V Release 4 for
compatibility with other compilers on those systems:
- -G
-
Create a shared object.
It is recommended that -symbolic or -shared be used instead.
- -Qy
-
Identify the versions of each tool used by the compiler, in a
".ident" assembler directive in the output.
- -Qn
-
Refrain from adding ".ident" directives to the output file (this is
the default).
- -YP,dirs
-
Search the directories dirs, and no others, for libraries
specified with -l.
- -Ym,dir
-
Look in the directory dir to find the M4 preprocessor.
The assembler uses this option.
TILE-Gx Options
These -m options are supported on the TILE-Gx:
- -mcmodel=small
-
Generate code for the small model. The distance for direct calls is
limited to 500M in either direction. PC-relative addresses are 32
bits. Absolute addresses support the full address range.
- -mcmodel=large
-
Generate code for the large model. There is no limitation on call
distance, pc-relative addresses, or absolute addresses.
- -mcpu=name
-
Selects the type of CPU to be targeted. Currently the only supported
type is tilegx.
- -m32
-
- -m64
-
Generate code for a 32-bit or 64-bit environment. The 32-bit
environment sets int, long, and pointer to 32 bits. The 64-bit
environment sets int to 32 bits and long and pointer to 64 bits.
- -mbig-endian
-
- -mlittle-endian
-
Generate code in big/little endian mode, respectively.
TILEPro Options
These -m options are supported on the TILEPro:
- -mcpu=name
-
Selects the type of CPU to be targeted. Currently the only supported
type is tilepro.
- -m32
-
Generate code for a 32-bit environment, which sets int, long, and
pointer to 32 bits. This is the only supported behavior so the flag
is essentially ignored.
V850 Options
These -m options are defined for V850 implementations:
- -mlong-calls
-
- -mno-long-calls
-
Treat all calls as being far away (near). If calls are assumed to be
far away, the compiler always loads the function's address into a
register, and calls indirect through the pointer.
- -mno-ep
-
- -mep
-
Do not optimize (do optimize) basic blocks that use the same index
pointer 4 or more times to copy pointer into the "ep" register, and
use the shorter "sld" and "sst" instructions. The -mep
option is on by default if you optimize.
- -mno-prolog-function
-
- -mprolog-function
-
Do not use (do use) external functions to save and restore registers
at the prologue and epilogue of a function. The external functions
are slower, but use less code space if more than one function saves
the same number of registers. The -mprolog-function option
is on by default if you optimize.
- -mspace
-
Try to make the code as small as possible. At present, this just turns
on the -mep and -mprolog-function options.
- -mtda=n
-
Put static or global variables whose size is n bytes or less into
the tiny data area that register "ep" points to. The tiny data
area can hold up to 256 bytes in total (128 bytes for byte references).
- -msda=n
-
Put static or global variables whose size is n bytes or less into
the small data area that register "gp" points to. The small data
area can hold up to 64 kilobytes.
- -mzda=n
-
Put static or global variables whose size is n bytes or less into
the first 32 kilobytes of memory.
- -mv850
-
Specify that the target processor is the V850.
- -mv850e3v5
-
Specify that the target processor is the V850E3V5. The preprocessor
constant "__v850e3v5__" is defined if this option is used.
- -mv850e2v4
-
Specify that the target processor is the V850E3V5. This is an alias for
the -mv850e3v5 option.
- -mv850e2v3
-
Specify that the target processor is the V850E2V3. The preprocessor
constant "__v850e2v3__" is defined if this option is used.
- -mv850e2
-
Specify that the target processor is the V850E2. The preprocessor
constant "__v850e2__" is defined if this option is used.
- -mv850e1
-
Specify that the target processor is the V850E1. The preprocessor
constants "__v850e1__" and "__v850e__" are defined if
this option is used.
- -mv850es
-
Specify that the target processor is the V850ES. This is an alias for
the -mv850e1 option.
- -mv850e
-
Specify that the target processor is the V850E. The preprocessor
constant "__v850e__" is defined if this option is used.
If neither -mv850 nor -mv850e nor -mv850e1
nor -mv850e2 nor -mv850e2v3 nor -mv850e3v5
are defined then a default target processor is chosen and the
relevant __v850*__ preprocessor constant is defined.
The preprocessor constants "__v850" and "__v851__" are always
defined, regardless of which processor variant is the target.
- -mdisable-callt
-
- -mno-disable-callt
-
This option suppresses generation of the "CALLT" instruction for the
v850e, v850e1, v850e2, v850e2v3 and v850e3v5 flavors of the v850
architecture.
This option is enabled by default when the RH850 ABI is
in use (see -mrh850-abi), and disabled by default when the
GCC ABI is in use. If "CALLT" instructions are being generated
then the C preprocessor symbol "__V850_CALLT__" is defined.
- -mrelax
-
- -mno-relax
-
Pass on (or do not pass on) the -mrelax command-line option
to the assembler.
- -mlong-jumps
-
- -mno-long-jumps
-
Disable (or re-enable) the generation of PC-relative jump instructions.
- -msoft-float
-
- -mhard-float
-
Disable (or re-enable) the generation of hardware floating point
instructions. This option is only significant when the target
architecture is V850E2V3 or higher. If hardware floating point
instructions are being generated then the C preprocessor symbol
"__FPU_OK__" is defined, otherwise the symbol
"__NO_FPU__" is defined.
- -mloop
-
Enables the use of the e3v5 LOOP instruction. The use of this
instruction is not enabled by default when the e3v5 architecture is
selected because its use is still experimental.
- -mrh850-abi
-
- -mghs
-
Enables support for the RH850 version of the V850 ABI. This is the
default. With this version of the ABI the following rules apply:
-
- *
-
Integer sized structures and unions are returned via a memory pointer
rather than a register.
- *
-
Large structures and unions (more than 8 bytes in size) are passed by
value.
- *
-
Functions are aligned to 16-bit boundaries.
- *
-
The -m8byte-align command-line option is supported.
- *
-
The -mdisable-callt command-line option is enabled by
default. The -mno-disable-callt command-line option is not
supported.
-
When this version of the ABI is enabled the C preprocessor symbol
"__V850_RH850_ABI__" is defined.
- -mgcc-abi
-
Enables support for the old GCC version of the V850 ABI. With this
version of the ABI the following rules apply:
-
- *
-
Integer sized structures and unions are returned in register "r10".
- *
-
Large structures and unions (more than 8 bytes in size) are passed by
reference.
- *
-
Functions are aligned to 32-bit boundaries, unless optimizing for
size.
- *
-
The -m8byte-align command-line option is not supported.
- *
-
The -mdisable-callt command-line option is supported but not
enabled by default.
-
When this version of the ABI is enabled the C preprocessor symbol
"__V850_GCC_ABI__" is defined.
- -m8byte-align
-
- -mno-8byte-align
-
Enables support for "double" and "long long" types to be
aligned on 8-byte boundaries. The default is to restrict the
alignment of all objects to at most 4-bytes. When
-m8byte-align is in effect the C preprocessor symbol
"__V850_8BYTE_ALIGN__" is defined.
- -mbig-switch
-
Generate code suitable for big switch tables. Use this option only if
the assembler/linker complain about out of range branches within a switch
table.
- -mapp-regs
-
This option causes r2 and r5 to be used in the code generated by
the compiler. This setting is the default.
- -mno-app-regs
-
This option causes r2 and r5 to be treated as fixed registers.
VAX Options
These -m options are defined for the VAX:
- -munix
-
Do not output certain jump instructions ("aobleq" and so on)
that the Unix assembler for the VAX cannot handle across long
ranges.
- -mgnu
-
Do output those jump instructions, on the assumption that the
GNU assembler is being used.
- -mg
-
Output code for G-format floating-point numbers instead of D-format.
Visium Options
- -mdebug
-
A program which performs file I/O and is destined to run on an MCM target
should be linked with this option. It causes the libraries libc.a and
libdebug.a to be linked. The program should be run on the target under
the control of the GDB remote debugging stub.
- -msim
-
A program which performs file I/O and is destined to run on the simulator
should be linked with option. This causes libraries libc.a and libsim.a to
be linked.
- -mfpu
-
- -mhard-float
-
Generate code containing floating-point instructions. This is the
default.
- -mno-fpu
-
- -msoft-float
-
Generate code containing library calls for floating-point.
-msoft-float changes the calling convention in the output file;
therefore, it is only useful if you compile all of a program with
this option. In particular, you need to compile libgcc.a, the
library that comes with GCC, with -msoft-float in order for
this to work.
- -mcpu=cpu_type
-
Set the instruction set, register set, and instruction scheduling parameters
for machine type cpu_type. Supported values for cpu_type are
mcm, gr5 and gr6.
mcm is a synonym of gr5 present for backward compatibility.
By default (unless configured otherwise), GCC generates code for the GR5
variant of the Visium architecture.
With -mcpu=gr6, GCC generates code for the GR6 variant of the Visium
architecture. The only difference from GR5 code is that the compiler will
generate block move instructions.
- -mtune=cpu_type
-
Set the instruction scheduling parameters for machine type cpu_type,
but do not set the instruction set or register set that the option
-mcpu=cpu_type would.
- -msv-mode
-
Generate code for the supervisor mode, where there are no restrictions on
the access to general registers. This is the default.
- -muser-mode
-
Generate code for the user mode, where the access to some general registers
is forbidden: on the GR5, registers r24 to r31 cannot be accessed in this
mode; on the GR6, only registers r29 to r31 are affected.
VMS Options
These -m options are defined for the VMS implementations:
- -mvms-return-codes
-
Return VMS condition codes from "main". The default is to return POSIX-style
condition (e.g. error) codes.
- -mdebug-main=prefix
-
Flag the first routine whose name starts with prefix as the main
routine for the debugger.
- -mmalloc64
-
Default to 64-bit memory allocation routines.
- -mpointer-size=size
-
Set the default size of pointers. Possible options for size are
32 or short for 32 bit pointers, 64 or long
for 64 bit pointers, and no for supporting only 32 bit pointers.
The later option disables "pragma pointer_size".
VxWorks Options
The options in this section are defined for all VxWorks targets.
Options specific to the target hardware are listed with the other
options for that target.
- -mrtp
-
GCC can generate code for both VxWorks kernels and real time processes
(RTPs). This option switches from the former to the latter. It also
defines the preprocessor macro "__RTP__".
- -non-static
-
Link an RTP executable against shared libraries rather than static
libraries. The options -static and -shared can
also be used for RTPs; -static
is the default.
- -Bstatic
-
- -Bdynamic
-
These options are passed down to the linker. They are defined for
compatibility with Diab.
- -Xbind-lazy
-
Enable lazy binding of function calls. This option is equivalent to
-Wl,-z,now and is defined for compatibility with Diab.
- -Xbind-now
-
Disable lazy binding of function calls. This option is the default and
is defined for compatibility with Diab.
x86 Options
These -m options are defined for the x86 family of computers.
- -march=cpu-type
-
Generate instructions for the machine type cpu-type. In contrast to
-mtune=cpu-type, which merely tunes the generated code
for the specified cpu-type, -march=cpu-type allows GCC
to generate code that may not run at all on processors other than the one
indicated. Specifying -march=cpu-type implies
-mtune=cpu-type.
The choices for cpu-type are:
-
- native
-
This selects the CPU to generate code for at compilation time by determining
the processor type of the compiling machine. Using -march=native
enables all instruction subsets supported by the local machine (hence
the result might not run on different machines). Using -mtune=native
produces code optimized for the local machine under the constraints
of the selected instruction set.
- i386
-
Original Intel i386 CPU.
- i486
-
Intel i486 CPU. (No scheduling is implemented for this chip.)
- i586
-
- pentium
-
Intel Pentium CPU with no MMX support.
- pentium-mmx
-
Intel Pentium MMX CPU, based on Pentium core with MMX instruction set support.
- pentiumpro
-
Intel Pentium Pro CPU.
- i686
-
When used with -march, the Pentium Pro
instruction set is used, so the code runs on all i686 family chips.
When used with -mtune, it has the same meaning as generic.
- pentium2
-
Intel Pentium II CPU, based on Pentium Pro core with MMX instruction set
support.
- pentium3
-
- pentium3m
-
Intel Pentium III CPU, based on Pentium Pro core with MMX and SSE instruction
set support.
- pentium-m
-
Intel Pentium M; low-power version of Intel Pentium III CPU
with MMX, SSE and SSE2 instruction set support. Used by Centrino notebooks.
- pentium4
-
- pentium4m
-
Intel Pentium 4 CPU with MMX, SSE and SSE2 instruction set support.
- prescott
-
Improved version of Intel Pentium 4 CPU with MMX, SSE, SSE2 and SSE3 instruction
set support.
- nocona
-
Improved version of Intel Pentium 4 CPU with 64-bit extensions, MMX, SSE,
SSE2 and SSE3 instruction set support.
- core2
-
Intel Core 2 CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3 and SSSE3
instruction set support.
- nehalem
-
Intel Nehalem CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3,
SSE4.1, SSE4.2 and POPCNT instruction set support.
- westmere
-
Intel Westmere CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3,
SSE4.1, SSE4.2, POPCNT, AES and PCLMUL instruction set support.
- sandybridge
-
Intel Sandy Bridge CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3,
SSE4.1, SSE4.2, POPCNT, AVX, AES and PCLMUL instruction set support.
- ivybridge
-
Intel Ivy Bridge CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3,
SSE4.1, SSE4.2, POPCNT, AVX, AES, PCLMUL, FSGSBASE, RDRND and F16C
instruction set support.
- haswell
-
Intel Haswell CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3,
SSE4.1, SSE4.2, POPCNT, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA,
BMI, BMI2 and F16C instruction set support.
- broadwell
-
Intel Broadwell CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3,
SSE4.1, SSE4.2, POPCNT, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA,
BMI, BMI2, F16C, RDSEED, ADCX and PREFETCHW instruction set support.
- bonnell
-
Intel Bonnell CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3 and SSSE3
instruction set support.
- silvermont
-
Intel Silvermont CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3,
SSE4.1, SSE4.2, POPCNT, AES, PCLMUL and RDRND instruction set support.
- knl
-
Intel Knight's Landing CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3,
SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA,
BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, AVX512F, AVX512PF, AVX512ER and
AVX512CD instruction set support.
- k6
-
AMD K6 CPU with MMX instruction set support.
- k6-2
-
- k6-3
-
Improved versions of AMD K6 CPU with MMX and 3DNow! instruction set support.
- athlon
-
- athlon-tbird
-
AMD Athlon CPU with MMX, 3dNOW!, enhanced 3DNow! and SSE prefetch instructions
support.
- athlon-4
-
- athlon-xp
-
- athlon-mp
-
Improved AMD Athlon CPU with MMX, 3DNow!, enhanced 3DNow! and full SSE
instruction set support.
- k8
-
- opteron
-
- athlon64
-
- athlon-fx
-
Processors based on the AMD K8 core with x86-64 instruction set support,
including the AMD Opteron, Athlon 64, and Athlon 64 FX processors.
(This supersets MMX, SSE, SSE2, 3DNow!, enhanced 3DNow! and 64-bit
instruction set extensions.)
- k8-sse3
-
- opteron-sse3
-
- athlon64-sse3
-
Improved versions of AMD K8 cores with SSE3 instruction set support.
- amdfam10
-
- barcelona
-
CPUs based on AMD Family 10h cores with x86-64 instruction set support. (This
supersets MMX, SSE, SSE2, SSE3, SSE4A, 3DNow!, enhanced 3DNow!, ABM and 64-bit
instruction set extensions.)
- bdver1
-
CPUs based on AMD Family 15h cores with x86-64 instruction set support. (This
supersets FMA4, AVX, XOP, LWP, AES, PCL_MUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A,
SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.)
- bdver2
-
AMD Family 15h core based CPUs with x86-64 instruction set support. (This
supersets BMI, TBM, F16C, FMA, FMA4, AVX, XOP, LWP, AES, PCL_MUL, CX16, MMX,
SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set
extensions.)
- bdver3
-
AMD Family 15h core based CPUs with x86-64 instruction set support. (This
supersets BMI, TBM, F16C, FMA, FMA4, FSGSBASE, AVX, XOP, LWP, AES,
PCL_MUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and
64-bit instruction set extensions.
- bdver4
-
AMD Family 15h core based CPUs with x86-64 instruction set support. (This
supersets BMI, BMI2, TBM, F16C, FMA, FMA4, FSGSBASE, AVX, AVX2, XOP, LWP,
AES, PCL_MUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1,
SSE4.2, ABM and 64-bit instruction set extensions.
- btver1
-
CPUs based on AMD Family 14h cores with x86-64 instruction set support. (This
supersets MMX, SSE, SSE2, SSE3, SSSE3, SSE4A, CX16, ABM and 64-bit
instruction set extensions.)
- btver2
-
CPUs based on AMD Family 16h cores with x86-64 instruction set support. This
includes MOVBE, F16C, BMI, AVX, PCL_MUL, AES, SSE4.2, SSE4.1, CX16, ABM,
SSE4A, SSSE3, SSE3, SSE2, SSE, MMX and 64-bit instruction set extensions.
- winchip-c6
-
IDT WinChip C6 CPU, dealt in same way as i486 with additional MMX instruction
set support.
- winchip2
-
IDT WinChip 2 CPU, dealt in same way as i486 with additional MMX and 3DNow!
instruction set support.
- c3
-
VIA C3 CPU with MMX and 3DNow! instruction set support. (No scheduling is
implemented for this chip.)
- c3-2
-
VIA C3-2 (Nehemiah/C5XL) CPU with MMX and SSE instruction set support.
(No scheduling is
implemented for this chip.)
- geode
-
AMD Geode embedded processor with MMX and 3DNow! instruction set support.
-
- -mtune=cpu-type
-
Tune to cpu-type everything applicable about the generated code, except
for the ABI and the set of available instructions.
While picking a specific cpu-type schedules things appropriately
for that particular chip, the compiler does not generate any code that
cannot run on the default machine type unless you use a
-march=cpu-type option.
For example, if GCC is configured for i686-pc-linux-gnu
then -mtune=pentium4 generates code that is tuned for Pentium 4
but still runs on i686 machines.
The choices for cpu-type are the same as for -march.
In addition, -mtune supports 2 extra choices for cpu-type:
-
- generic
-
Produce code optimized for the most common IA32/AMD64/EM64T processors.
If you know the CPU on which your code will run, then you should use
the corresponding -mtune or -march option instead of
-mtune=generic. But, if you do not know exactly what CPU users
of your application will have, then you should use this option.
As new processors are deployed in the marketplace, the behavior of this
option will change. Therefore, if you upgrade to a newer version of
GCC, code generation controlled by this option will change to reflect
the processors
that are most common at the time that version of GCC is released.
There is no -march=generic option because -march
indicates the instruction set the compiler can use, and there is no
generic instruction set applicable to all processors. In contrast,
-mtune indicates the processor (or, in this case, collection of
processors) for which the code is optimized.
- intel
-
Produce code optimized for the most current Intel processors, which are
Haswell and Silvermont for this version of GCC. If you know the CPU
on which your code will run, then you should use the corresponding
-mtune or -march option instead of -mtune=intel.
But, if you want your application performs better on both Haswell and
Silvermont, then you should use this option.
As new Intel processors are deployed in the marketplace, the behavior of
this option will change. Therefore, if you upgrade to a newer version of
GCC, code generation controlled by this option will change to reflect
the most current Intel processors at the time that version of GCC is
released.
There is no -march=intel option because -march indicates
the instruction set the compiler can use, and there is no common
instruction set applicable to all processors. In contrast,
-mtune indicates the processor (or, in this case, collection of
processors) for which the code is optimized.
-
- -mcpu=cpu-type
-
A deprecated synonym for -mtune.
- -mfpmath=unit
-
Generate floating-point arithmetic for selected unit unit. The choices
for unit are:
-
- 387
-
Use the standard 387 floating-point coprocessor present on the majority of chips and
emulated otherwise. Code compiled with this option runs almost everywhere.
The temporary results are computed in 80-bit precision instead of the precision
specified by the type, resulting in slightly different results compared to most
of other chips. See -ffloat-store for more detailed description.
This is the default choice for x86-32 targets.
- sse
-
Use scalar floating-point instructions present in the SSE instruction set.
This instruction set is supported by Pentium III and newer chips,
and in the AMD line
by Athlon-4, Athlon XP and Athlon MP chips. The earlier version of the SSE
instruction set supports only single-precision arithmetic, thus the double and
extended-precision arithmetic are still done using 387. A later version, present
only in Pentium 4 and AMD x86-64 chips, supports double-precision
arithmetic too.
For the x86-32 compiler, you must use -march=cpu-type, -msse
or -msse2 switches to enable SSE extensions and make this option
effective. For the x86-64 compiler, these extensions are enabled by default.
The resulting code should be considerably faster in the majority of cases and avoid
the numerical instability problems of 387 code, but may break some existing
code that expects temporaries to be 80 bits.
This is the default choice for the x86-64 compiler.
- sse,387
-
- sse+387
-
- both
-
Attempt to utilize both instruction sets at once. This effectively doubles the
amount of available registers, and on chips with separate execution units for
387 and SSE the execution resources too. Use this option with care, as it is
still experimental, because the GCC register allocator does not model separate
functional units well, resulting in unstable performance.
-
- -masm=dialect
-
Output assembly instructions using selected dialect. Also affects
which dialect is used for basic "asm" and
extended "asm". Supported choices (in dialect
order) are att or intel. The default is att. Darwin does
not support intel.
- -mieee-fp
-
- -mno-ieee-fp
-
Control whether or not the compiler uses IEEE floating-point
comparisons. These correctly handle the case where the result of a
comparison is unordered.
- -msoft-float
-
Generate output containing library calls for floating point.
Warning: the requisite libraries are not part of GCC.
Normally the facilities of the machine's usual C compiler are used, but
this can't be done directly in cross-compilation. You must make your
own arrangements to provide suitable library functions for
cross-compilation.
On machines where a function returns floating-point results in the 80387
register stack, some floating-point opcodes may be emitted even if
-msoft-float is used.
- -mno-fp-ret-in-387
-
Do not use the FPU registers for return values of functions.
The usual calling convention has functions return values of types
"float" and "double" in an FPU register, even if there
is no FPU. The idea is that the operating system should emulate
an FPU.
The option -mno-fp-ret-in-387 causes such values to be returned
in ordinary CPU registers instead.
- -mno-fancy-math-387
-
Some 387 emulators do not support the "sin", "cos" and
"sqrt" instructions for the 387. Specify this option to avoid
generating those instructions. This option is the default on
OpenBSD and NetBSD. This option is overridden when -march
indicates that the target CPU always has an FPU and so the
instruction does not need emulation. These
instructions are not generated unless you also use the
-funsafe-math-optimizations switch.
- -malign-double
-
- -mno-align-double
-
Control whether GCC aligns "double", "long double", and
"long long" variables on a two-word boundary or a one-word
boundary. Aligning "double" variables on a two-word boundary
produces code that runs somewhat faster on a Pentium at the
expense of more memory.
On x86-64, -malign-double is enabled by default.
Warning: if you use the -malign-double switch,
structures containing the above types are aligned differently than
the published application binary interface specifications for the x86-32
and are not binary compatible with structures in code compiled
without that switch.
- -m96bit-long-double
-
- -m128bit-long-double
-
These switches control the size of "long double" type. The x86-32
application binary interface specifies the size to be 96 bits,
so -m96bit-long-double is the default in 32-bit mode.
Modern architectures (Pentium and newer) prefer "long double"
to be aligned to an 8- or 16-byte boundary. In arrays or structures
conforming to the ABI, this is not possible. So specifying
-m128bit-long-double aligns "long double"
to a 16-byte boundary by padding the "long double" with an additional
32-bit zero.
In the x86-64 compiler, -m128bit-long-double is the default choice as
its ABI specifies that "long double" is aligned on 16-byte boundary.
Notice that neither of these options enable any extra precision over the x87
standard of 80 bits for a "long double".
Warning: if you override the default value for your target ABI, this
changes the size of
structures and arrays containing "long double" variables,
as well as modifying the function calling convention for functions taking
"long double". Hence they are not binary-compatible
with code compiled without that switch.
- -mlong-double-64
-
- -mlong-double-80
-
- -mlong-double-128
-
These switches control the size of "long double" type. A size
of 64 bits makes the "long double" type equivalent to the "double"
type. This is the default for 32-bit Bionic C library. A size
of 128 bits makes the "long double" type equivalent to the
"__float128" type. This is the default for 64-bit Bionic C library.
Warning: if you override the default value for your target ABI, this
changes the size of
structures and arrays containing "long double" variables,
as well as modifying the function calling convention for functions taking
"long double". Hence they are not binary-compatible
with code compiled without that switch.
- -malign-data=type
-
Control how GCC aligns variables. Supported values for type are
compat uses increased alignment value compatible uses GCC 4.8
and earlier, abi uses alignment value as specified by the
psABI, and cacheline uses increased alignment value to match
the cache line size. compat is the default.
- -mlarge-data-threshold=threshold
-
When -mcmodel=medium is specified, data objects larger than
threshold are placed in the large data section. This value must be the
same across all objects linked into the binary, and defaults to 65535.
- -mrtd
-
Use a different function-calling convention, in which functions that
take a fixed number of arguments return with the "ret num"
instruction, which pops their arguments while returning. This saves one
instruction in the caller since there is no need to pop the arguments
there.
You can specify that an individual function is called with this calling
sequence with the function attribute "stdcall". You can also
override the -mrtd option by using the function attribute
"cdecl".
Warning: this calling convention is incompatible with the one
normally used on Unix, so you cannot use it if you need to call
libraries compiled with the Unix compiler.
Also, you must provide function prototypes for all functions that
take variable numbers of arguments (including "printf");
otherwise incorrect code is generated for calls to those
functions.
In addition, seriously incorrect code results if you call a
function with too many arguments. (Normally, extra arguments are
harmlessly ignored.)
- -mregparm=num
-
Control how many registers are used to pass integer arguments. By
default, no registers are used to pass arguments, and at most 3
registers can be used. You can control this behavior for a specific
function by using the function attribute "regparm".
Warning: if you use this switch, and
num is nonzero, then you must build all modules with the same
value, including any libraries. This includes the system libraries and
startup modules.
- -msseregparm
-
Use SSE register passing conventions for float and double arguments
and return values. You can control this behavior for a specific
function by using the function attribute "sseregparm".
Warning: if you use this switch then you must build all
modules with the same value, including any libraries. This includes
the system libraries and startup modules.
- -mvect8-ret-in-mem
-
Return 8-byte vectors in memory instead of MMX registers. This is the
default on Solaris@tie{}8 and 9 and VxWorks to match the ABI of the Sun
Studio compilers until version 12. Later compiler versions (starting
with Studio 12 Update@tie{}1) follow the ABI used by other x86 targets, which
is the default on Solaris@tie{}10 and later. Only use this option if
you need to remain compatible with existing code produced by those
previous compiler versions or older versions of GCC.
- -mpc32
-
- -mpc64
-
- -mpc80
-
Set 80387 floating-point precision to 32, 64 or 80 bits. When -mpc32
is specified, the significands of results of floating-point operations are
rounded to 24 bits (single precision); -mpc64 rounds the
significands of results of floating-point operations to 53 bits (double
precision) and -mpc80 rounds the significands of results of
floating-point operations to 64 bits (extended double precision), which is
the default. When this option is used, floating-point operations in higher
precisions are not available to the programmer without setting the FPU
control word explicitly.
Setting the rounding of floating-point operations to less than the default
80 bits can speed some programs by 2% or more. Note that some mathematical
libraries assume that extended-precision (80-bit) floating-point operations
are enabled by default; routines in such libraries could suffer significant
loss of accuracy, typically through so-called ``catastrophic cancellation'',
when this option is used to set the precision to less than extended precision.
- -mstackrealign
-
Realign the stack at entry. On the x86, the -mstackrealign
option generates an alternate prologue and epilogue that realigns the
run-time stack if necessary. This supports mixing legacy codes that keep
4-byte stack alignment with modern codes that keep 16-byte stack alignment for
SSE compatibility. See also the attribute "force_align_arg_pointer",
applicable to individual functions.
- -mpreferred-stack-boundary=num
-
Attempt to keep the stack boundary aligned to a 2 raised to num
byte boundary. If -mpreferred-stack-boundary is not specified,
the default is 4 (16 bytes or 128 bits).
Warning: When generating code for the x86-64 architecture with
SSE extensions disabled, -mpreferred-stack-boundary=3 can be
used to keep the stack boundary aligned to 8 byte boundary. Since
x86-64 ABI require 16 byte stack alignment, this is ABI incompatible and
intended to be used in controlled environment where stack space is
important limitation. This option leads to wrong code when functions
compiled with 16 byte stack alignment (such as functions from a standard
library) are called with misaligned stack. In this case, SSE
instructions may lead to misaligned memory access traps. In addition,
variable arguments are handled incorrectly for 16 byte aligned
objects (including x87 long double and __int128), leading to wrong
results. You must build all modules with
-mpreferred-stack-boundary=3, including any libraries. This
includes the system libraries and startup modules.
- -mincoming-stack-boundary=num
-
Assume the incoming stack is aligned to a 2 raised to num byte
boundary. If -mincoming-stack-boundary is not specified,
the one specified by -mpreferred-stack-boundary is used.
On Pentium and Pentium Pro, "double" and "long double" values
should be aligned to an 8-byte boundary (see -malign-double) or
suffer significant run time performance penalties. On Pentium III, the
Streaming SIMD Extension (SSE) data type "__m128" may not work
properly if it is not 16-byte aligned.
To ensure proper alignment of this values on the stack, the stack boundary
must be as aligned as that required by any value stored on the stack.
Further, every function must be generated such that it keeps the stack
aligned. Thus calling a function compiled with a higher preferred
stack boundary from a function compiled with a lower preferred stack
boundary most likely misaligns the stack. It is recommended that
libraries that use callbacks always use the default setting.
This extra alignment does consume extra stack space, and generally
increases code size. Code that is sensitive to stack space usage, such
as embedded systems and operating system kernels, may want to reduce the
preferred alignment to -mpreferred-stack-boundary=2.
- -mmmx
-
- -msse
-
- -msse2
-
- -msse3
-
- -mssse3
-
- -msse4
-
- -msse4a
-
- -msse4.1
-
- -msse4.2
-
- -mavx
-
- -mavx2
-
- -mavx512f
-
- -mavx512pf
-
- -mavx512er
-
- -mavx512cd
-
- -msha
-
- -maes
-
- -mpclmul
-
- -mclfushopt
-
- -mfsgsbase
-
- -mrdrnd
-
- -mf16c
-
- -mfma
-
- -mfma4
-
- -mno-fma4
-
- -mprefetchwt1
-
- -mxop
-
- -mlwp
-
- -m3dnow
-
- -mpopcnt
-
- -mabm
-
- -mbmi
-
- -mbmi2
-
- -mlzcnt
-
- -mfxsr
-
- -mxsave
-
- -mxsaveopt
-
- -mxsavec
-
- -mxsaves
-
- -mrtm
-
- -mtbm
-
- -mmpx
-
- -mmwaitx
-
These switches enable the use of instructions in the MMX, SSE,
SSE2, SSE3, SSSE3, SSE4.1, AVX, AVX2, AVX512F, AVX512PF, AVX512ER, AVX512CD,
SHA, AES, PCLMUL, FSGSBASE, RDRND, F16C, FMA, SSE4A, FMA4, XOP, LWP, ABM,
BMI, BMI2, FXSR, XSAVE, XSAVEOPT, LZCNT, RTM, MPX, MWAITX or 3DNow!
extended instruction sets. Each has a corresponding -mno- option
to disable use of these instructions.
These extensions are also available as built-in functions: see
x86 Built-in Functions, for details of the functions enabled and
disabled by these switches.
To generate SSE/SSE2 instructions automatically from floating-point
code (as opposed to 387 instructions), see -mfpmath=sse.
GCC depresses SSEx instructions when -mavx is used. Instead, it
generates new AVX instructions or AVX equivalence for all SSEx instructions
when needed.
These options enable GCC to use these extended instructions in
generated code, even without -mfpmath=sse. Applications that
perform run-time CPU detection must compile separate files for each
supported architecture, using the appropriate flags. In particular,
the file containing the CPU detection code should be compiled without
these options.
- -mdump-tune-features
-
This option instructs GCC to dump the names of the x86 performance
tuning features and default settings. The names can be used in
-mtune-ctrl=feature-list.
- -mtune-ctrl=feature-list
-
This option is used to do fine grain control of x86 code generation features.
feature-list is a comma separated list of feature names. See also
-mdump-tune-features. When specified, the feature is turned
on if it is not preceded with ^, otherwise, it is turned off.
-mtune-ctrl=feature-list is intended to be used by GCC
developers. Using it may lead to code paths not covered by testing and can
potentially result in compiler ICEs or runtime errors.
- -mno-default
-
This option instructs GCC to turn off all tunable features. See also
-mtune-ctrl=feature-list and -mdump-tune-features.
- -mcld
-
This option instructs GCC to emit a "cld" instruction in the prologue
of functions that use string instructions. String instructions depend on
the DF flag to select between autoincrement or autodecrement mode. While the
ABI specifies the DF flag to be cleared on function entry, some operating
systems violate this specification by not clearing the DF flag in their
exception dispatchers. The exception handler can be invoked with the DF flag
set, which leads to wrong direction mode when string instructions are used.
This option can be enabled by default on 32-bit x86 targets by configuring
GCC with the --enable-cld configure option. Generation of "cld"
instructions can be suppressed with the -mno-cld compiler option
in this case.
- -mvzeroupper
-
This option instructs GCC to emit a "vzeroupper" instruction
before a transfer of control flow out of the function to minimize
the AVX to SSE transition penalty as well as remove unnecessary "zeroupper"
intrinsics.
- -mprefer-avx128
-
This option instructs GCC to use 128-bit AVX instructions instead of
256-bit AVX instructions in the auto-vectorizer.
- -mcx16
-
This option enables GCC to generate "CMPXCHG16B" instructions.
"CMPXCHG16B" allows for atomic operations on 128-bit double quadword
(or oword) data types.
This is useful for high-resolution counters that can be updated
by multiple processors (or cores). This instruction is generated as part of
atomic built-in functions: see __sync Builtins or
__atomic Builtins for details.
- -msahf
-
This option enables generation of "SAHF" instructions in 64-bit code.
Early Intel Pentium 4 CPUs with Intel 64 support,
prior to the introduction of Pentium 4 G1 step in December 2005,
lacked the "LAHF" and "SAHF" instructions
which are supported by AMD64.
These are load and store instructions, respectively, for certain status flags.
In 64-bit mode, the "SAHF" instruction is used to optimize "fmod",
"drem", and "remainder" built-in functions;
see Other Builtins for details.
- -mmovbe
-
This option enables use of the "movbe" instruction to implement
"__builtin_bswap32" and "__builtin_bswap64".
- -mcrc32
-
This option enables built-in functions "__builtin_ia32_crc32qi",
"__builtin_ia32_crc32hi", "__builtin_ia32_crc32si" and
"__builtin_ia32_crc32di" to generate the "crc32" machine instruction.
- -mrecip
-
This option enables use of "RCPSS" and "RSQRTSS" instructions
(and their vectorized variants "RCPPS" and "RSQRTPS")
with an additional Newton-Raphson step
to increase precision instead of "DIVSS" and "SQRTSS"
(and their vectorized
variants) for single-precision floating-point arguments. These instructions
are generated only when -funsafe-math-optimizations is enabled
together with -finite-math-only and -fno-trapping-math.
Note that while the throughput of the sequence is higher than the throughput
of the non-reciprocal instruction, the precision of the sequence can be
decreased by up to 2 ulp (i.e. the inverse of 1.0 equals 0.99999994).
Note that GCC implements "1.0f/sqrtf(x)" in terms of "RSQRTSS"
(or "RSQRTPS") already with -ffast-math (or the above option
combination), and doesn't need -mrecip.
Also note that GCC emits the above sequence with additional Newton-Raphson step
for vectorized single-float division and vectorized "sqrtf(x)"
already with -ffast-math (or the above option combination), and
doesn't need -mrecip.
- -mrecip=opt
-
This option controls which reciprocal estimate instructions
may be used. opt is a comma-separated list of options, which may
be preceded by a ! to invert the option:
-
- all
-
Enable all estimate instructions.
- default
-
Enable the default instructions, equivalent to -mrecip.
- none
-
Disable all estimate instructions, equivalent to -mno-recip.
- div
-
Enable the approximation for scalar division.
- vec-div
-
Enable the approximation for vectorized division.
- sqrt
-
Enable the approximation for scalar square root.
- vec-sqrt
-
Enable the approximation for vectorized square root.
-
So, for example, -mrecip=all,!sqrt enables
all of the reciprocal approximations, except for square root.
- -mveclibabi=type
-
Specifies the ABI type to use for vectorizing intrinsics using an
external library. Supported values for type are svml
for the Intel short
vector math library and acml for the AMD math core library.
To use this option, both -ftree-vectorize and
-funsafe-math-optimizations have to be enabled, and an SVML or ACML
ABI-compatible library must be specified at link time.
GCC currently emits calls to "vmldExp2",
"vmldLn2", "vmldLog102", "vmldLog102", "vmldPow2",
"vmldTanh2", "vmldTan2", "vmldAtan2", "vmldAtanh2",
"vmldCbrt2", "vmldSinh2", "vmldSin2", "vmldAsinh2",
"vmldAsin2", "vmldCosh2", "vmldCos2", "vmldAcosh2",
"vmldAcos2", "vmlsExp4", "vmlsLn4", "vmlsLog104",
"vmlsLog104", "vmlsPow4", "vmlsTanh4", "vmlsTan4",
"vmlsAtan4", "vmlsAtanh4", "vmlsCbrt4", "vmlsSinh4",
"vmlsSin4", "vmlsAsinh4", "vmlsAsin4", "vmlsCosh4",
"vmlsCos4", "vmlsAcosh4" and "vmlsAcos4" for corresponding
function type when -mveclibabi=svml is used, and "__vrd2_sin",
"__vrd2_cos", "__vrd2_exp", "__vrd2_log", "__vrd2_log2",
"__vrd2_log10", "__vrs4_sinf", "__vrs4_cosf",
"__vrs4_expf", "__vrs4_logf", "__vrs4_log2f",
"__vrs4_log10f" and "__vrs4_powf" for the corresponding function type
when -mveclibabi=acml is used.
- -mabi=name
-
Generate code for the specified calling convention. Permissible values
are sysv for the ABI used on GNU/Linux and other systems, and
ms for the Microsoft ABI. The default is to use the Microsoft
ABI when targeting Microsoft Windows and the SysV ABI on all other systems.
You can control this behavior for specific functions by
using the function attributes "ms_abi" and "sysv_abi".
- -mtls-dialect=type
-
Generate code to access thread-local storage using the gnu or
gnu2 conventions. gnu is the conservative default;
gnu2 is more efficient, but it may add compile- and run-time
requirements that cannot be satisfied on all systems.
- -mpush-args
-
- -mno-push-args
-
Use PUSH operations to store outgoing parameters. This method is shorter
and usually equally fast as method using SUB/MOV operations and is enabled
by default. In some cases disabling it may improve performance because of
improved scheduling and reduced dependencies.
- -maccumulate-outgoing-args
-
If enabled, the maximum amount of space required for outgoing arguments is
computed in the function prologue. This is faster on most modern CPUs
because of reduced dependencies, improved scheduling and reduced stack usage
when the preferred stack boundary is not equal to 2. The drawback is a notable
increase in code size. This switch implies -mno-push-args.
- -mthreads
-
Support thread-safe exception handling on MinGW. Programs that rely
on thread-safe exception handling must compile and link all code with the
-mthreads option. When compiling, -mthreads defines
-D_MT; when linking, it links in a special thread helper library
-lmingwthrd which cleans up per-thread exception-handling data.
- -mno-align-stringops
-
Do not align the destination of inlined string operations. This switch reduces
code size and improves performance in case the destination is already aligned,
but GCC doesn't know about it.
- -minline-all-stringops
-
By default GCC inlines string operations only when the destination is
known to be aligned to least a 4-byte boundary.
This enables more inlining and increases code
size, but may improve performance of code that depends on fast
"memcpy", "strlen",
and "memset" for short lengths.
- -minline-stringops-dynamically
-
For string operations of unknown size, use run-time checks with
inline code for small blocks and a library call for large blocks.
- -mstringop-strategy=alg
-
Override the internal decision heuristic for the particular algorithm to use
for inlining string operations. The allowed values for alg are:
-
- rep_byte
-
- rep_4byte
-
- rep_8byte
-
Expand using i386 "rep" prefix of the specified size.
- byte_loop
-
- loop
-
- unrolled_loop
-
Expand into an inline loop.
- libcall
-
Always use a library call.
-
- -mmemcpy-strategy=strategy
-
Override the internal decision heuristic to decide if "__builtin_memcpy"
should be inlined and what inline algorithm to use when the expected size
of the copy operation is known. strategy
is a comma-separated list of alg:max_size:dest_align triplets.
alg is specified in -mstringop-strategy, max_size specifies
the max byte size with which inline algorithm alg is allowed. For the last
triplet, the max_size must be "-1". The max_size of the triplets
in the list must be specified in increasing order. The minimal byte size for
alg is 0 for the first triplet and "max_size + 1" of the
preceding range.
- -mmemset-strategy=strategy
-
The option is similar to -mmemcpy-strategy= except that it is to control
"__builtin_memset" expansion.
- -momit-leaf-frame-pointer
-
Don't keep the frame pointer in a register for leaf functions. This
avoids the instructions to save, set up, and restore frame pointers and
makes an extra register available in leaf functions. The option
-fomit-leaf-frame-pointer removes the frame pointer for leaf functions,
which might make debugging harder.
- -mtls-direct-seg-refs
-
- -mno-tls-direct-seg-refs
-
Controls whether TLS variables may be accessed with offsets from the
TLS segment register (%gs for 32-bit, %fs for 64-bit),
or whether the thread base pointer must be added. Whether or not this
is valid depends on the operating system, and whether it maps the
segment to cover the entire TLS area.
For systems that use the GNU C Library, the default is on.
- -msse2avx
-
- -mno-sse2avx
-
Specify that the assembler should encode SSE instructions with VEX
prefix. The option -mavx turns this on by default.
- -mfentry
-
- -mno-fentry
-
If profiling is active (-pg), put the profiling
counter call before the prologue.
Note: On x86 architectures the attribute "ms_hook_prologue"
isn't possible at the moment for -mfentry and -pg.
- -mrecord-mcount
-
- -mno-record-mcount
-
If profiling is active (-pg), generate a __mcount_loc section
that contains pointers to each profiling call. This is useful for
automatically patching and out calls.
- -mnop-mcount
-
- -mno-nop-mcount
-
If profiling is active (-pg), generate the calls to
the profiling functions as nops. This is useful when they
should be patched in later dynamically. This is likely only
useful together with -mrecord-mcount.
- -mskip-rax-setup
-
- -mno-skip-rax-setup
-
When generating code for the x86-64 architecture with SSE extensions
disabled, -mskip-rax-setup can be used to skip setting up RAX
register when there are no variable arguments passed in vector registers.
Warning: Since RAX register is used to avoid unnecessarily
saving vector registers on stack when passing variable arguments, the
impacts of this option are callees may waste some stack space,
misbehave or jump to a random location. GCC 4.4 or newer don't have
those issues, regardless the RAX register value.
- -m8bit-idiv
-
- -mno-8bit-idiv
-
On some processors, like Intel Atom, 8-bit unsigned integer divide is
much faster than 32-bit/64-bit integer divide. This option generates a
run-time check. If both dividend and divisor are within range of 0
to 255, 8-bit unsigned integer divide is used instead of
32-bit/64-bit integer divide.
- -mavx256-split-unaligned-load
-
- -mavx256-split-unaligned-store
-
Split 32-byte AVX unaligned load and store.
- -mstack-protector-guard=guard
-
Generate stack protection code using canary at guard. Supported
locations are global for global canary or tls for per-thread
canary in the TLS block (the default). This option has effect only when
-fstack-protector or -fstack-protector-all is specified.
These -m switches are supported in addition to the above
on x86-64 processors in 64-bit environments.
- -m32
-
- -m64
-
- -mx32
-
- -m16
-
Generate code for a 16-bit, 32-bit or 64-bit environment.
The -m32 option sets "int", "long", and pointer types
to 32 bits, and
generates code that runs on any i386 system.
The -m64 option sets "int" to 32 bits and "long" and pointer
types to 64 bits, and generates code for the x86-64 architecture.
For Darwin only the -m64 option also turns off the -fno-pic
and -mdynamic-no-pic options.
The -mx32 option sets "int", "long", and pointer types
to 32 bits, and
generates code for the x86-64 architecture.
The -m16 option is the same as -m32, except for that
it outputs the ".code16gcc" assembly directive at the beginning of
the assembly output so that the binary can run in 16-bit mode.
- -mno-red-zone
-
Do not use a so-called ``red zone'' for x86-64 code. The red zone is mandated
by the x86-64 ABI; it is a 128-byte area beyond the location of the
stack pointer that is not modified by signal or interrupt handlers
and therefore can be used for temporary data without adjusting the stack
pointer. The flag -mno-red-zone disables this red zone.
- -mcmodel=small
-
Generate code for the small code model: the program and its symbols must
be linked in the lower 2 GB of the address space. Pointers are 64 bits.
Programs can be statically or dynamically linked. This is the default
code model.
- -mcmodel=kernel
-
Generate code for the kernel code model. The kernel runs in the
negative 2 GB of the address space.
This model has to be used for Linux kernel code.
- -mcmodel=medium
-
Generate code for the medium model: the program is linked in the lower 2
GB of the address space. Small symbols are also placed there. Symbols
with sizes larger than -mlarge-data-threshold are put into
large data or BSS sections and can be located above 2GB. Programs can
be statically or dynamically linked.
- -mcmodel=large
-
Generate code for the large model. This model makes no assumptions
about addresses and sizes of sections.
- -maddress-mode=long
-
Generate code for long address mode. This is only supported for 64-bit
and x32 environments. It is the default address mode for 64-bit
environments.
- -maddress-mode=short
-
Generate code for short address mode. This is only supported for 32-bit
and x32 environments. It is the default address mode for 32-bit and
x32 environments.
x86 Windows Options
These additional options are available for Microsoft Windows targets:
- -mconsole
-
This option
specifies that a console application is to be generated, by
instructing the linker to set the PE header subsystem type
required for console applications.
This option is available for Cygwin and MinGW targets and is
enabled by default on those targets.
- -mdll
-
This option is available for Cygwin and MinGW targets. It
specifies that a DLL---a dynamic link library---is to be
generated, enabling the selection of the required runtime
startup object and entry point.
- -mnop-fun-dllimport
-
This option is available for Cygwin and MinGW targets. It
specifies that the "dllimport" attribute should be ignored.
- -mthread
-
This option is available for MinGW targets. It specifies
that MinGW-specific thread support is to be used.
- -municode
-
This option is available for MinGW-w64 targets. It causes
the "UNICODE" preprocessor macro to be predefined, and
chooses Unicode-capable runtime startup code.
- -mwin32
-
This option is available for Cygwin and MinGW targets. It
specifies that the typical Microsoft Windows predefined macros are to
be set in the pre-processor, but does not influence the choice
of runtime library/startup code.
- -mwindows
-
This option is available for Cygwin and MinGW targets. It
specifies that a GUI application is to be generated by
instructing the linker to set the PE header subsystem type
appropriately.
- -fno-set-stack-executable
-
This option is available for MinGW targets. It specifies that
the executable flag for the stack used by nested functions isn't
set. This is necessary for binaries running in kernel mode of
Microsoft Windows, as there the User32 API, which is used to set executable
privileges, isn't available.
- -fwritable-relocated-rdata
-
This option is available for MinGW and Cygwin targets. It specifies
that relocated-data in read-only section is put into .data
section. This is a necessary for older runtimes not supporting
modification of .rdata sections for pseudo-relocation.
- -mpe-aligned-commons
-
This option is available for Cygwin and MinGW targets. It
specifies that the GNU extension to the PE file format that
permits the correct alignment of COMMON variables should be
used when generating code. It is enabled by default if
GCC detects that the target assembler found during configuration
supports the feature.
See also under x86 Options for standard options.
Xstormy16 Options
These options are defined for Xstormy16:
- -msim
-
Choose startup files and linker script suitable for the simulator.
Xtensa Options
These options are supported for Xtensa targets:
- -mconst16
-
- -mno-const16
-
Enable or disable use of "CONST16" instructions for loading
constant values. The "CONST16" instruction is currently not a
standard option from Tensilica. When enabled, "CONST16"
instructions are always used in place of the standard "L32R"
instructions. The use of "CONST16" is enabled by default only if
the "L32R" instruction is not available.
- -mfused-madd
-
- -mno-fused-madd
-
Enable or disable use of fused multiply/add and multiply/subtract
instructions in the floating-point option. This has no effect if the
floating-point option is not also enabled. Disabling fused multiply/add
and multiply/subtract instructions forces the compiler to use separate
instructions for the multiply and add/subtract operations. This may be
desirable in some cases where strict IEEE 754-compliant results are
required: the fused multiply add/subtract instructions do not round the
intermediate result, thereby producing results with more bits of
precision than specified by the IEEE standard. Disabling fused multiply
add/subtract instructions also ensures that the program output is not
sensitive to the compiler's ability to combine multiply and add/subtract
operations.
- -mserialize-volatile
-
- -mno-serialize-volatile
-
When this option is enabled, GCC inserts "MEMW" instructions before
"volatile" memory references to guarantee sequential consistency.
The default is -mserialize-volatile. Use
-mno-serialize-volatile to omit the "MEMW" instructions.
- -mforce-no-pic
-
For targets, like GNU/Linux, where all user-mode Xtensa code must be
position-independent code (PIC), this option disables PIC for compiling
kernel code.
- -mtext-section-literals
-
- -mno-text-section-literals
-
These options control the treatment of literal pools. The default is
-mno-text-section-literals, which places literals in a separate
section in the output file. This allows the literal pool to be placed
in a data RAM/ROM, and it also allows the linker to combine literal
pools from separate object files to remove redundant literals and
improve code size. With -mtext-section-literals, the literals
are interspersed in the text section in order to keep them as close as
possible to their references. This may be necessary for large assembly
files.
- -mtarget-align
-
- -mno-target-align
-
When this option is enabled, GCC instructs the assembler to
automatically align instructions to reduce branch penalties at the
expense of some code density. The assembler attempts to widen density
instructions to align branch targets and the instructions following call
instructions. If there are not enough preceding safe density
instructions to align a target, no widening is performed. The
default is -mtarget-align. These options do not affect the
treatment of auto-aligned instructions like "LOOP", which the
assembler always aligns, either by widening density instructions or
by inserting NOP instructions.
- -mlongcalls
-
- -mno-longcalls
-
When this option is enabled, GCC instructs the assembler to translate
direct calls to indirect calls unless it can determine that the target
of a direct call is in the range allowed by the call instruction. This
translation typically occurs for calls to functions in other source
files. Specifically, the assembler translates a direct "CALL"
instruction into an "L32R" followed by a "CALLX" instruction.
The default is -mno-longcalls. This option should be used in
programs where the call target can potentially be out of range. This
option is implemented in the assembler, not the compiler, so the
assembly code generated by GCC still shows direct call
instructions---look at the disassembled object code to see the actual
instructions. Note that the assembler uses an indirect call for
every cross-file call, not just those that really are out of range.
zSeries Options
These are listed under
Options for Code Generation Conventions
These machine-independent options control the interface conventions
used in code generation.
Most of them have both positive and negative forms; the negative form
of -ffoo is -fno-foo. In the table below, only
one of the forms is listed---the one that is not the default. You
can figure out the other form by either removing no- or adding
it.
- -fbounds-check
-
For front ends that support it, generate additional code to check that
indices used to access arrays are within the declared range. This is
currently only supported by the Java and Fortran front ends, where
this option defaults to true and false respectively.
- -fstack-reuse=reuse-level
-
This option controls stack space reuse for user declared local/auto variables
and compiler generated temporaries. reuse_level can be all,
named_vars, or none. all enables stack reuse for all
local variables and temporaries, named_vars enables the reuse only for
user defined local variables with names, and none disables stack reuse
completely. The default value is all. The option is needed when the
program extends the lifetime of a scoped local variable or a compiler generated
temporary beyond the end point defined by the language. When a lifetime of
a variable ends, and if the variable lives in memory, the optimizing compiler
has the freedom to reuse its stack space with other temporaries or scoped
local variables whose live range does not overlap with it. Legacy code extending
local lifetime is likely to break with the stack reuse optimization.
For example,
int *p;
{
int local1;
p = &local1;
local1 = 10;
....
}
{
int local2;
local2 = 20;
...
}
if (*p == 10) // out of scope use of local1
{
}
Another example:
struct A
{
A(int k) : i(k), j(k) { }
int i;
int j;
};
A *ap;
void foo(const A& ar)
{
ap = &ar;
}
void bar()
{
foo(A(10)); // temp object's lifetime ends when foo returns
{
A a(20);
....
}
ap->i+= 10; // ap references out of scope temp whose space
// is reused with a. What is the value of ap->i?
}
The lifetime of a compiler generated temporary is well defined by the C++
standard. When a lifetime of a temporary ends, and if the temporary lives
in memory, the optimizing compiler has the freedom to reuse its stack
space with other temporaries or scoped local variables whose live range
does not overlap with it. However some of the legacy code relies on
the behavior of older compilers in which temporaries' stack space is
not reused, the aggressive stack reuse can lead to runtime errors. This
option is used to control the temporary stack reuse optimization.
- -ftrapv
-
This option generates traps for signed overflow on addition, subtraction,
multiplication operations.
- -fwrapv
-
This option instructs the compiler to assume that signed arithmetic
overflow of addition, subtraction and multiplication wraps around
using twos-complement representation. This flag enables some optimizations
and disables others. This option is enabled by default for the Java
front end, as required by the Java language specification.
- -fexceptions
-
Enable exception handling. Generates extra code needed to propagate
exceptions. For some targets, this implies GCC generates frame
unwind information for all functions, which can produce significant data
size overhead, although it does not affect execution. If you do not
specify this option, GCC enables it by default for languages like
C++ that normally require exception handling, and disables it for
languages like C that do not normally require it. However, you may need
to enable this option when compiling C code that needs to interoperate
properly with exception handlers written in C++. You may also wish to
disable this option if you are compiling older C++ programs that don't
use exception handling.
- -fnon-call-exceptions
-
Generate code that allows trapping instructions to throw exceptions.
Note that this requires platform-specific runtime support that does
not exist everywhere. Moreover, it only allows trapping
instructions to throw exceptions, i.e. memory references or floating-point
instructions. It does not allow exceptions to be thrown from
arbitrary signal handlers such as "SIGALRM".
- -fdelete-dead-exceptions
-
Consider that instructions that may throw exceptions but don't otherwise
contribute to the execution of the program can be optimized away.
This option is enabled by default for the Ada front end, as permitted by
the Ada language specification.
Optimization passes that cause dead exceptions to be removed are enabled independently at different optimization levels.
- -funwind-tables
-
Similar to -fexceptions, except that it just generates any needed
static data, but does not affect the generated code in any other way.
You normally do not need to enable this option; instead, a language processor
that needs this handling enables it on your behalf.
- -fasynchronous-unwind-tables
-
Generate unwind table in DWARF 2 format, if supported by target machine. The
table is exact at each instruction boundary, so it can be used for stack
unwinding from asynchronous events (such as debugger or garbage collector).
- -fno-gnu-unique
-
On systems with recent GNU assembler and C library, the C++ compiler
uses the "STB_GNU_UNIQUE" binding to make sure that definitions
of template static data members and static local variables in inline
functions are unique even in the presence of "RTLD_LOCAL"; this
is necessary to avoid problems with a library used by two different
"RTLD_LOCAL" plugins depending on a definition in one of them and
therefore disagreeing with the other one about the binding of the
symbol. But this causes "dlclose" to be ignored for affected
DSOs; if your program relies on reinitialization of a DSO via
"dlclose" and "dlopen", you can use
-fno-gnu-unique.
- -fpcc-struct-return
-
Return ``short'' "struct" and "union" values in memory like
longer ones, rather than in registers. This convention is less
efficient, but it has the advantage of allowing intercallability between
GCC-compiled files and files compiled with other compilers, particularly
the Portable C Compiler (pcc).
The precise convention for returning structures in memory depends
on the target configuration macros.
Short structures and unions are those whose size and alignment match
that of some integer type.
Warning: code compiled with the -fpcc-struct-return
switch is not binary compatible with code compiled with the
-freg-struct-return switch.
Use it to conform to a non-default application binary interface.
- -freg-struct-return
-
Return "struct" and "union" values in registers when possible.
This is more efficient for small structures than
-fpcc-struct-return.
If you specify neither -fpcc-struct-return nor
-freg-struct-return, GCC defaults to whichever convention is
standard for the target. If there is no standard convention, GCC
defaults to -fpcc-struct-return, except on targets where GCC is
the principal compiler. In those cases, we can choose the standard, and
we chose the more efficient register return alternative.
Warning: code compiled with the -freg-struct-return
switch is not binary compatible with code compiled with the
-fpcc-struct-return switch.
Use it to conform to a non-default application binary interface.
- -fshort-enums
-
Allocate to an "enum" type only as many bytes as it needs for the
declared range of possible values. Specifically, the "enum" type
is equivalent to the smallest integer type that has enough room.
Warning: the -fshort-enums switch causes GCC to generate
code that is not binary compatible with code generated without that switch.
Use it to conform to a non-default application binary interface.
- -fshort-double
-
Use the same size for "double" as for "float".
Warning: the -fshort-double switch causes GCC to generate
code that is not binary compatible with code generated without that switch.
Use it to conform to a non-default application binary interface.
- -fshort-wchar
-
Override the underlying type for "wchar_t" to be "short
unsigned int" instead of the default for the target. This option is
useful for building programs to run under WINE.
Warning: the -fshort-wchar switch causes GCC to generate
code that is not binary compatible with code generated without that switch.
Use it to conform to a non-default application binary interface.
- -fno-common
-
In C code, controls the placement of uninitialized global variables.
Unix C compilers have traditionally permitted multiple definitions of
such variables in different compilation units by placing the variables
in a common block.
This is the behavior specified by -fcommon, and is the default
for GCC on most targets.
On the other hand, this behavior is not required by ISO C, and on some
targets may carry a speed or code size penalty on variable references.
The -fno-common option specifies that the compiler should place
uninitialized global variables in the data section of the object file,
rather than generating them as common blocks.
This has the effect that if the same variable is declared
(without "extern") in two different compilations,
you get a multiple-definition error when you link them.
In this case, you must compile with -fcommon instead.
Compiling with -fno-common is useful on targets for which
it provides better performance, or if you wish to verify that the
program will work on other systems that always treat uninitialized
variable declarations this way.
- -fno-ident
-
Ignore the "#ident" directive.
- -finhibit-size-directive
-
Don't output a ".size" assembler directive, or anything else that
would cause trouble if the function is split in the middle, and the
two halves are placed at locations far apart in memory. This option is
used when compiling crtstuff.c; you should not need to use it
for anything else.
- -fverbose-asm
-
Put extra commentary information in the generated assembly code to
make it more readable. This option is generally only of use to those
who actually need to read the generated assembly code (perhaps while
debugging the compiler itself).
-fno-verbose-asm, the default, causes the
extra information to be omitted and is useful when comparing two assembler
files.
- -frecord-gcc-switches
-
This switch causes the command line used to invoke the
compiler to be recorded into the object file that is being created.
This switch is only implemented on some targets and the exact format
of the recording is target and binary file format dependent, but it
usually takes the form of a section containing ASCII text. This
switch is related to the -fverbose-asm switch, but that
switch only records information in the assembler output file as
comments, so it never reaches the object file.
See also -grecord-gcc-switches for another
way of storing compiler options into the object file.
- -fpic
-
Generate position-independent code (PIC) suitable for use in a shared
library, if supported for the target machine. Such code accesses all
constant addresses through a global offset table (GOT). The dynamic
loader resolves the GOT entries when the program starts (the dynamic
loader is not part of GCC; it is part of the operating system). If
the GOT size for the linked executable exceeds a machine-specific
maximum size, you get an error message from the linker indicating that
-fpic does not work; in that case, recompile with -fPIC
instead. (These maximums are 8k on the SPARC and 32k
on the m68k and RS/6000. The x86 has no such limit.)
Position-independent code requires special support, and therefore works
only on certain machines. For the x86, GCC supports PIC for System V
but not for the Sun 386i. Code generated for the IBM RS/6000 is always
position-independent.
When this flag is set, the macros "__pic__" and "__PIC__"
are defined to 1.
- -fPIC
-
If supported for the target machine, emit position-independent code,
suitable for dynamic linking and avoiding any limit on the size of the
global offset table. This option makes a difference on the m68k,
PowerPC and SPARC.
Position-independent code requires special support, and therefore works
only on certain machines.
When this flag is set, the macros "__pic__" and "__PIC__"
are defined to 2.
- -fpie
-
- -fPIE
-
These options are similar to -fpic and -fPIC, but
generated position independent code can be only linked into executables.
Usually these options are used when -pie GCC option is
used during linking.
-fpie and -fPIE both define the macros
"__pie__" and "__PIE__". The macros have the value 1
for -fpie and 2 for -fPIE.
NOTE: When --enable-esp, this option is enabled by default
for C, C++, ObjC, and ObjC++ if neither -fno-pie or -fno-PIE
or -fno-pic or -fno-PIC or -nostdlib or
-nostartfiles or -shared are found.
- -fno-jump-tables
-
Do not use jump tables for switch statements even where it would be
more efficient than other code generation strategies. This option is
of use in conjunction with -fpic or -fPIC for
building code that forms part of a dynamic linker and cannot
reference the address of a jump table. On some targets, jump tables
do not require a GOT and this option is not needed.
- -ffixed-reg
-
Treat the register named reg as a fixed register; generated code
should never refer to it (except perhaps as a stack pointer, frame
pointer or in some other fixed role).
reg must be the name of a register. The register names accepted
are machine-specific and are defined in the "REGISTER_NAMES"
macro in the machine description macro file.
This flag does not have a negative form, because it specifies a
three-way choice.
- -fcall-used-reg
-
Treat the register named reg as an allocable register that is
clobbered by function calls. It may be allocated for temporaries or
variables that do not live across a call. Functions compiled this way
do not save and restore the register reg.
It is an error to use this flag with the frame pointer or stack pointer.
Use of this flag for other registers that have fixed pervasive roles in
the machine's execution model produces disastrous results.
This flag does not have a negative form, because it specifies a
three-way choice.
- -fcall-saved-reg
-
Treat the register named reg as an allocable register saved by
functions. It may be allocated even for temporaries or variables that
live across a call. Functions compiled this way save and restore
the register reg if they use it.
It is an error to use this flag with the frame pointer or stack pointer.
Use of this flag for other registers that have fixed pervasive roles in
the machine's execution model produces disastrous results.
A different sort of disaster results from the use of this flag for
a register in which function values may be returned.
This flag does not have a negative form, because it specifies a
three-way choice.
- -fpack-struct[=n]
-
Without a value specified, pack all structure members together without
holes. When a value is specified (which must be a small power of two), pack
structure members according to this value, representing the maximum
alignment (that is, objects with default alignment requirements larger than
this are output potentially unaligned at the next fitting location.
Warning: the -fpack-struct switch causes GCC to generate
code that is not binary compatible with code generated without that switch.
Additionally, it makes the code suboptimal.
Use it to conform to a non-default application binary interface.
- -finstrument-functions
-
Generate instrumentation calls for entry and exit to functions. Just
after function entry and just before function exit, the following
profiling functions are called with the address of the current
function and its call site. (On some platforms,
"__builtin_return_address" does not work beyond the current
function, so the call site information may not be available to the
profiling functions otherwise.)
void __cyg_profile_func_enter (void *this_fn,
void *call_site);
void __cyg_profile_func_exit (void *this_fn,
void *call_site);
The first argument is the address of the start of the current function,
which may be looked up exactly in the symbol table.
This instrumentation is also done for functions expanded inline in other
functions. The profiling calls indicate where, conceptually, the
inline function is entered and exited. This means that addressable
versions of such functions must be available. If all your uses of a
function are expanded inline, this may mean an additional expansion of
code size. If you use "extern inline" in your C code, an
addressable version of such functions must be provided. (This is
normally the case anyway, but if you get lucky and the optimizer always
expands the functions inline, you might have gotten away without
providing static copies.)
A function may be given the attribute "no_instrument_function", in
which case this instrumentation is not done. This can be used, for
example, for the profiling functions listed above, high-priority
interrupt routines, and any functions from which the profiling functions
cannot safely be called (perhaps signal handlers, if the profiling
routines generate output or allocate memory).
- -finstrument-functions-exclude-file-list=file,file,...
-
Set the list of functions that are excluded from instrumentation (see
the description of -finstrument-functions). If the file that
contains a function definition matches with one of file, then
that function is not instrumented. The match is done on substrings:
if the file parameter is a substring of the file name, it is
considered to be a match.
For example:
-finstrument-functions-exclude-file-list=/bits/stl,include/sys
excludes any inline function defined in files whose pathnames
contain /bits/stl or include/sys.
If, for some reason, you want to include letter , in one of
sym, write ,. For example,
-finstrument-functions-exclude-file-list=',,tmp'
(note the single quote surrounding the option).
- -finstrument-functions-exclude-function-list=sym,sym,...
-
This is similar to -finstrument-functions-exclude-file-list,
but this option sets the list of function names to be excluded from
instrumentation. The function name to be matched is its user-visible
name, such as "vector<int> blah(const vector<int> &)", not the
internal mangled name (e.g., "_Z4blahRSt6vectorIiSaIiEE"). The
match is done on substrings: if the sym parameter is a substring
of the function name, it is considered to be a match. For C99 and C++
extended identifiers, the function name must be given in UTF-8, not
using universal character names.
- -fstack-check
-
Generate code to verify that you do not go beyond the boundary of the
stack. You should specify this flag if you are running in an
environment with multiple threads, but you only rarely need to specify it in
a single-threaded environment since stack overflow is automatically
detected on nearly all systems if there is only one stack.
Note that this switch does not actually cause checking to be done; the
operating system or the language runtime must do that. The switch causes
generation of code to ensure that they see the stack being extended.
You can additionally specify a string parameter: no means no
checking, generic means force the use of old-style checking,
specific means use the best checking method and is equivalent
to bare -fstack-check.
Old-style checking is a generic mechanism that requires no specific
target support in the compiler but comes with the following drawbacks:
-
- 1.
-
Modified allocation strategy for large objects: they are always
allocated dynamically if their size exceeds a fixed threshold.
- 2.
-
Fixed limit on the size of the static frame of functions: when it is
topped by a particular function, stack checking is not reliable and
a warning is issued by the compiler.
- 3.
-
Inefficiency: because of both the modified allocation strategy and the
generic implementation, code performance is hampered.
-
Note that old-style stack checking is also the fallback method for
specific if no target support has been added in the compiler.
- -fstack-limit-register=reg
-
- -fstack-limit-symbol=sym
-
- -fno-stack-limit
-
Generate code to ensure that the stack does not grow beyond a certain value,
either the value of a register or the address of a symbol. If a larger
stack is required, a signal is raised at run time. For most targets,
the signal is raised before the stack overruns the boundary, so
it is possible to catch the signal without taking special precautions.
For instance, if the stack starts at absolute address 0x80000000
and grows downwards, you can use the flags
-fstack-limit-symbol=__stack_limit and
-Wl,--defsym,__stack_limit=0x7ffe0000 to enforce a stack limit
of 128KB. Note that this may only work with the GNU linker.
- -fsplit-stack
-
Generate code to automatically split the stack before it overflows.
The resulting program has a discontiguous stack which can only
overflow if the program is unable to allocate any more memory. This
is most useful when running threaded programs, as it is no longer
necessary to calculate a good stack size to use for each thread. This
is currently only implemented for the x86 targets running
GNU/Linux.
When code compiled with -fsplit-stack calls code compiled
without -fsplit-stack, there may not be much stack space
available for the latter code to run. If compiling all code,
including library code, with -fsplit-stack is not an option,
then the linker can fix up these calls so that the code compiled
without -fsplit-stack always has a large stack. Support for
this is implemented in the gold linker in GNU binutils release 2.21
and later.
- -fleading-underscore
-
This option and its counterpart, -fno-leading-underscore, forcibly
change the way C symbols are represented in the object file. One use
is to help link with legacy assembly code.
Warning: the -fleading-underscore switch causes GCC to
generate code that is not binary compatible with code generated without that
switch. Use it to conform to a non-default application binary interface.
Not all targets provide complete support for this switch.
- -ftls-model=model
-
Alter the thread-local storage model to be used.
The model argument should be one of global-dynamic,
local-dynamic, initial-exec or local-exec.
Note that the choice is subject to optimization: the compiler may use
a more efficient model for symbols not visible outside of the translation
unit, or if -fpic is not given on the command line.
The default without -fpic is initial-exec; with
-fpic the default is global-dynamic.
- -fvisibility=[default|internal|hidden|protected]
-
Set the default ELF image symbol visibility to the specified option---all
symbols are marked with this unless overridden within the code.
Using this feature can very substantially improve linking and
load times of shared object libraries, produce more optimized
code, provide near-perfect API export and prevent symbol clashes.
It is strongly recommended that you use this in any shared objects
you distribute.
Despite the nomenclature, default always means public; i.e.,
available to be linked against from outside the shared object.
protected and internal are pretty useless in real-world
usage so the only other commonly used option is hidden.
The default if -fvisibility isn't specified is
default, i.e., make every symbol public.
A good explanation of the benefits offered by ensuring ELF
symbols have the correct visibility is given by ``How To Write
Shared Libraries'' by Ulrich Drepper (which can be found at
<http://www.akkadia.org/drepper/>)---however a superior
solution made possible by this option to marking things hidden when
the default is public is to make the default hidden and mark things
public. This is the norm with DLLs on Windows and with -fvisibility=hidden
and "__attribute__ ((visibility("default")))" instead of
"__declspec(dllexport)" you get almost identical semantics with
identical syntax. This is a great boon to those working with
cross-platform projects.
For those adding visibility support to existing code, you may find
"#pragma GCC visibility" of use. This works by you enclosing
the declarations you wish to set visibility for with (for example)
"#pragma GCC visibility push(hidden)" and
"#pragma GCC visibility pop".
Bear in mind that symbol visibility should be viewed as
part of the API interface contract and thus all new code should
always specify visibility when it is not the default; i.e., declarations
only for use within the local DSO should always be marked explicitly
as hidden as so to avoid PLT indirection overheads---making this
abundantly clear also aids readability and self-documentation of the code.
Note that due to ISO C++ specification requirements, "operator new" and
"operator delete" must always be of default visibility.
Be aware that headers from outside your project, in particular system
headers and headers from any other library you use, may not be
expecting to be compiled with visibility other than the default. You
may need to explicitly say "#pragma GCC visibility push(default)"
before including any such headers.
"extern" declarations are not affected by -fvisibility, so
a lot of code can be recompiled with -fvisibility=hidden with
no modifications. However, this means that calls to "extern"
functions with no explicit visibility use the PLT, so it is more
effective to use "__attribute ((visibility))" and/or
"#pragma GCC visibility" to tell the compiler which "extern"
declarations should be treated as hidden.
Note that -fvisibility does affect C++ vague linkage
entities. This means that, for instance, an exception class that is
be thrown between DSOs must be explicitly marked with default
visibility so that the type_info nodes are unified between
the DSOs.
An overview of these techniques, their benefits and how to use them
is at <http://gcc.gnu.org/wiki/Visibility>.
- -fstrict-volatile-bitfields
-
This option should be used if accesses to volatile bit-fields (or other
structure fields, although the compiler usually honors those types
anyway) should use a single access of the width of the
field's type, aligned to a natural alignment if possible. For
example, targets with memory-mapped peripheral registers might require
all such accesses to be 16 bits wide; with this flag you can
declare all peripheral bit-fields as "unsigned short" (assuming short
is 16 bits on these targets) to force GCC to use 16-bit accesses
instead of, perhaps, a more efficient 32-bit access.
If this option is disabled, the compiler uses the most efficient
instruction. In the previous example, that might be a 32-bit load
instruction, even though that accesses bytes that do not contain
any portion of the bit-field, or memory-mapped registers unrelated to
the one being updated.
In some cases, such as when the "packed" attribute is applied to a
structure field, it may not be possible to access the field with a single
read or write that is correctly aligned for the target machine. In this
case GCC falls back to generating multiple accesses rather than code that
will fault or truncate the result at run time.
Note: Due to restrictions of the C/C++11 memory model, write accesses are
not allowed to touch non bit-field members. It is therefore recommended
to define all bits of the field's type as bit-field members.
The default value of this option is determined by the application binary
interface for the target processor.
- -fsync-libcalls
-
This option controls whether any out-of-line instance of the "__sync"
family of functions may be used to implement the C++11 "__atomic"
family of functions.
The default value of this option is enabled, thus the only useful form
of the option is -fno-sync-libcalls. This option is used in
the implementation of the libatomic runtime library.
ENVIRONMENT
This section describes several environment variables that affect how GCC
operates. Some of them work by specifying directories or prefixes to use
when searching for various kinds of files. Some are used to specify other
aspects of the compilation environment.
Note that you can also specify places to search using options such as
-B, -I and -L. These
take precedence over places specified using environment variables, which
in turn take precedence over those specified by the configuration of GCC.
- LANG
-
- LC_CTYPE
-
- LC_MESSAGES
-
- LC_ALL
-
These environment variables control the way that GCC uses
localization information which allows GCC to work with different
national conventions. GCC inspects the locale categories
LC_CTYPE and LC_MESSAGES if it has been configured to do
so. These locale categories can be set to any value supported by your
installation. A typical value is en_GB.UTF-8 for English in the United
Kingdom encoded in UTF-8.
The LC_CTYPE environment variable specifies character
classification. GCC uses it to determine the character boundaries in
a string; this is needed for some multibyte encodings that contain quote
and escape characters that are otherwise interpreted as a string
end or escape.
The LC_MESSAGES environment variable specifies the language to
use in diagnostic messages.
If the LC_ALL environment variable is set, it overrides the value
of LC_CTYPE and LC_MESSAGES; otherwise, LC_CTYPE
and LC_MESSAGES default to the value of the LANG
environment variable. If none of these variables are set, GCC
defaults to traditional C English behavior.
- TMPDIR
-
If TMPDIR is set, it specifies the directory to use for temporary
files. GCC uses temporary files to hold the output of one stage of
compilation which is to be used as input to the next stage: for example,
the output of the preprocessor, which is the input to the compiler
proper.
- GCC_COMPARE_DEBUG
-
Setting GCC_COMPARE_DEBUG is nearly equivalent to passing
-fcompare-debug to the compiler driver. See the documentation
of this option for more details.
- GCC_EXEC_PREFIX
-
If GCC_EXEC_PREFIX is set, it specifies a prefix to use in the
names of the subprograms executed by the compiler. No slash is added
when this prefix is combined with the name of a subprogram, but you can
specify a prefix that ends with a slash if you wish.
If GCC_EXEC_PREFIX is not set, GCC attempts to figure out
an appropriate prefix to use based on the pathname it is invoked with.
If GCC cannot find the subprogram using the specified prefix, it
tries looking in the usual places for the subprogram.
The default value of GCC_EXEC_PREFIX is
prefix/lib/gcc/ where prefix is the prefix to
the installed compiler. In many cases prefix is the value
of "prefix" when you ran the configure script.
Other prefixes specified with -B take precedence over this prefix.
This prefix is also used for finding files such as crt0.o that are
used for linking.
In addition, the prefix is used in an unusual way in finding the
directories to search for header files. For each of the standard
directories whose name normally begins with /usr/local/lib/gcc
(more precisely, with the value of GCC_INCLUDE_DIR), GCC tries
replacing that beginning with the specified prefix to produce an
alternate directory name. Thus, with -Bfoo/, GCC searches
foo/bar just before it searches the standard directory
/usr/local/lib/bar.
If a standard directory begins with the configured
prefix then the value of prefix is replaced by
GCC_EXEC_PREFIX when looking for header files.
- COMPILER_PATH
-
The value of COMPILER_PATH is a colon-separated list of
directories, much like PATH. GCC tries the directories thus
specified when searching for subprograms, if it can't find the
subprograms using GCC_EXEC_PREFIX.
- LIBRARY_PATH
-
The value of LIBRARY_PATH is a colon-separated list of
directories, much like PATH. When configured as a native compiler,
GCC tries the directories thus specified when searching for special
linker files, if it can't find them using GCC_EXEC_PREFIX. Linking
using GCC also uses these directories when searching for ordinary
libraries for the -l option (but directories specified with
-L come first).
- LANG
-
This variable is used to pass locale information to the compiler. One way in
which this information is used is to determine the character set to be used
when character literals, string literals and comments are parsed in C and C++.
When the compiler is configured to allow multibyte characters,
the following values for LANG are recognized:
-
- C-JIS
-
Recognize JIS characters.
- C-SJIS
-
Recognize SJIS characters.
- C-EUCJP
-
Recognize EUCJP characters.
-
If LANG is not defined, or if it has some other value, then the
compiler uses "mblen" and "mbtowc" as defined by the default locale to
recognize and translate multibyte characters.
Some additional environment variables affect the behavior of the
preprocessor.
- CPATH
-
- C_INCLUDE_PATH
-
- CPLUS_INCLUDE_PATH
-
- OBJC_INCLUDE_PATH
-
Each variable's value is a list of directories separated by a special
character, much like PATH, in which to look for header files.
The special character, "PATH_SEPARATOR", is target-dependent and
determined at GCC build time. For Microsoft Windows-based targets it is a
semicolon, and for almost all other targets it is a colon.
CPATH specifies a list of directories to be searched as if
specified with -I, but after any paths given with -I
options on the command line. This environment variable is used
regardless of which language is being preprocessed.
The remaining environment variables apply only when preprocessing the
particular language indicated. Each specifies a list of directories
to be searched as if specified with -isystem, but after any
paths given with -isystem options on the command line.
In all these variables, an empty element instructs the compiler to
search its current working directory. Empty elements can appear at the
beginning or end of a path. For instance, if the value of
CPATH is ":/special/include", that has the same
effect as -I. -I/special/include.
- DEPENDENCIES_OUTPUT
-
If this variable is set, its value specifies how to output
dependencies for Make based on the non-system header files processed
by the compiler. System header files are ignored in the dependency
output.
The value of DEPENDENCIES_OUTPUT can be just a file name, in
which case the Make rules are written to that file, guessing the target
name from the source file name. Or the value can have the form
file target, in which case the rules are written to
file file using target as the target name.
In other words, this environment variable is equivalent to combining
the options -MM and -MF,
with an optional -MT switch too.
- SUNPRO_DEPENDENCIES
-
This variable is the same as DEPENDENCIES_OUTPUT (see above),
except that system header files are not ignored, so it implies
-M rather than -MM. However, the dependence on the
main input file is omitted.
BUGS
For instructions on reporting bugs, see
< https://bugs.gentoo.org/>.
FOOTNOTES
- 1.
-
On some systems, gcc -shared
needs to build supplementary stub code for constructors to work. On
multi-libbed systems, gcc -shared must select the correct support
libraries to link against. Failing to supply the correct flags may lead
to subtle defects. Supplying them in cases where they are not necessary
is innocuous.
SEE ALSO
gpl(7), gfdl(7), fsf-funding(7),
cpp(1), gcov(1), as(1), ld(1), gdb(1), adb(1), dbx(1), sdb(1)
and the Info entries for gcc, cpp, as,
ld, binutils and gdb.
AUTHOR
See the Info entry for gcc, or
< http://gcc.gnu.org/onlinedocs/gcc/Contributors.html>,
for contributors to GCC.
COPYRIGHT
Copyright (c) 1988-2015 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with the
Invariant Sections being ``GNU General Public License'' and ``Funding
Free Software'', the Front-Cover texts being (a) (see below), and with
the Back-Cover Texts being (b) (see below). A copy of the license is
included in the gfdl(7) man page.
(a) The FSF's Front-Cover Text is:
A GNU Manual
(b) The FSF's Back-Cover Text is:
You have freedom to copy and modify this GNU Manual, like GNU
software. Copies published by the Free Software Foundation raise
funds for GNU development.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- OPTIONS
-
- Option Summary
-
- Options Controlling the Kind of Output
-
- Compiling C++ Programs
-
- Options Controlling C Dialect
-
- Options Controlling C++ Dialect
-
- Options Controlling Objective-C and Objective-C++ Dialects
-
- Options to Control Diagnostic Messages Formatting
-
- Options to Request or Suppress Warnings
-
- Options for Debugging Your Program or GCC
-
- Options That Control Optimization
-
- Options Controlling the Preprocessor
-
- Passing Options to the Assembler
-
- Options for Linking
-
- Options for Directory Search
-
- Specifying Target Machine and Compiler Version
-
- Hardware Models and Configurations
-
- Options for Code Generation Conventions
-
- ENVIRONMENT
-
- BUGS
-
- FOOTNOTES
-
- SEE ALSO
-
- AUTHOR
-
- COPYRIGHT
-
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