GCC(1) gcc-4.2.3 GCC(1)
GNU GNU
2008-02-01
NAME
gcc - GNU project C and C++ compiler
SYNOPSIS
gcc [-c|-S|-E] [-std=standard]
[-g] [-pg] [-Olevel]
[-Wwarn...] [-pedantic]
[-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: -dr is very different from -d -r.
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.
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
would be -fno-foo. This manual documents only one of these two forms,
whichever one is not the default.
OPTIONS
- 1 - Formatted: April 20, 2024
GCC(1) gcc-4.2.3 GCC(1)
GNU GNU
2008-02-01
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 -combine -pipe -pass-exit-codes -x language
-v -### --help --target-help --version @file
C Language Options
-ansi -std=standard -fgnu89-inline -aux-info filename -fno-asm
-fno-builtin -fno-builtin-function -fhosted -ffreestanding
-fopenmp -fms-extensions -trigraphs -no-integrated-cpp
-traditional -traditional-cpp -fallow-single-precision
-fcond-mismatch -fsigned-bitfields -fsigned-char
-funsigned-bitfields -funsigned-char
C++ Language Options
-fabi-version=n -fno-access-control -fcheck-new -fconserve-space
-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
-fno-operator-names -fno-optional-diags -fpermissive -frepo
-fno-rtti -fstats -ftemplate-depth-n -fno-threadsafe-statics
-fuse-cxa-atexit -fno-weak -nostdinc++ -fno-default-inline
-fvisibility-inlines-hidden -Wabi -Wctor-dtor-privacy
-Wnon-virtual-dtor -Wreorder -Weffc++ -Wno-deprecated
-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-call-cxx-cdtors -fobjc-direct-dispatch
-fobjc-exceptions -fobjc-gc -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-show-option
Warning Options
-fsyntax-only -pedantic -pedantic-errors -w -Wextra -Wall
-Waddress -Waggregate-return -Wno-attributes -Wc++-compat
-Wcast-align -Wcast-qual -Wchar-subscripts -Wcomment
-Wconversion -Wno-deprecated-declarations -Wdisabled-optimization
-Wno-div-by-zero -Wno-endif-labels -Werror -Werror=*
-Werror-implicit-function-declaration -Wfatal-errors
- 2 - Formatted: April 20, 2024
GCC(1) gcc-4.2.3 GCC(1)
GNU GNU
2008-02-01
-Wfloat-equal -Wformat -Wformat=2 -Wno-format-extra-args
-Wformat-nonliteral -Wformat-security -Wformat-y2k -Wimplicit
-Wimplicit-function-declaration -Wimplicit-int -Wimport
-Wno-import -Winit-self -Winline -Wno-int-to-pointer-cast
-Wno-invalid-offsetof -Winvalid-pch -Wlarger-than-len
-Wunsafe-loop-optimizations -Wlong-long -Wmain -Wmissing-braces
-Wmissing-field-initializers -Wmissing-format-attribute
-Wmissing-include-dirs -Wmissing-noreturn -Wno-multichar
-Wnonnull -Wno-overflow -Woverlength-strings -Wpacked -Wpadded
-Wparentheses -Wpointer-arith -Wno-pointer-to-int-cast
-Wredundant-decls -Wreturn-type -Wsequence-point -Wshadow
-Wsign-compare -Wstack-protector -Wstrict-aliasing
-Wstrict-aliasing=2 -Wstrict-overflow -Wstrict-overflow=n -Wswitch
-Wswitch-default -Wswitch-enum -Wsystem-headers -Wtrigraphs
-Wundef -Wuninitialized -Wunknown-pragmas -Wno-pragmas
-Wunreachable-code -Wunused -Wunused-function -Wunused-label
-Wunused-parameter -Wunused-value -Wunused-variable
-Wvariadic-macros -Wvolatile-register-var -Wwrite-strings
C-only Warning Options
-Wbad-function-cast -Wmissing-declarations -Wmissing-prototypes
-Wnested-externs -Wold-style-definition -Wstrict-prototypes
-Wtraditional -Wdeclaration-after-statement -Wpointer-sign
Debugging Options
-dletters -dumpspecs -dumpmachine -dumpversion -fdump-noaddr
-fdump-unnumbered -fdump-translation-unit[-n]
-fdump-class-hierarchy[-n] -fdump-ipa-all -fdump-ipa-cgraph
-fdump-tree-all -fdump-tree-original[-n] -fdump-tree-optimized[-n]
-fdump-tree-inlined[-n] -fdump-tree-cfg -fdump-tree-vcg
-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-mudflap[-n]
-fdump-tree-dom[-n] -fdump-tree-dse[-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-salias -fdump-tree-fre[-n]
-fdump-tree-vrp[-n] -ftree-vectorizer-verbose=n
-fdump-tree-storeccp[-n] -feliminate-dwarf2-dups
-feliminate-unused-debug-types -feliminate-unused-debug-symbols
-femit-class-debug-always -fmem-report -fprofile-arcs
-frandom-seed=string -fsched-verbose=n -ftest-coverage
-ftime-report -fvar-tracking -g -glevel -gcoff -gdwarf-2 -ggdb
-gstabs -gstabs+ -gvms -gxcoff -gxcoff+ -p -pg
-print-file-name=library -print-libgcc-file-name
-print-multi-directory -print-multi-lib -print-prog-name=program
-print-search-dirs -Q -save-temps -time
Optimization Options
- 3 - Formatted: April 20, 2024
GCC(1) gcc-4.2.3 GCC(1)
GNU GNU
2008-02-01
-falign-functions[=n] -falign-jumps[=n] -falign-labels[=n]
-falign-loops[=n] -fmudflap -fmudflapth -fmudflapir
-fbranch-probabilities -fprofile-values -fvpt
-fbranch-target-load-optimize -fbranch-target-load-optimize2
-fbtr-bb-exclusive -fcaller-saves -fcprop-registers
-fcse-follow-jumps -fcse-skip-blocks -fcx-limited-range
-fdata-sections -fdelayed-branch -fdelete-null-pointer-checks
-fearly-inlining -fexpensive-optimizations -ffast-math
-ffloat-store -fforce-addr -ffunction-sections -fgcse -fgcse-lm
-fgcse-sm -fgcse-las -fgcse-after-reload -fcrossjumping
-fif-conversion -fif-conversion2 -finline-functions
-finline-functions-called-once -finline-limit=n
-fkeep-inline-functions -fkeep-static-consts -fmerge-constants
-fmerge-all-constants -fmodulo-sched -fno-branch-count-reg
-fno-default-inline -fno-defer-pop -fmove-loop-invariants
-fno-function-cse -fno-guess-branch-probability -fno-inline
-fno-math-errno -fno-peephole -fno-peephole2
-funsafe-math-optimizations -funsafe-loop-optimizations
-ffinite-math-only -fno-toplevel-reorder -fno-trapping-math
-fno-zero-initialized-in-bss -fomit-frame-pointer
-foptimize-register-move -foptimize-sibling-calls
-fprefetch-loop-arrays -fprofile-generate -fprofile-use -fregmove
-frename-registers -freorder-blocks
-freorder-blocks-and-partition -freorder-functions
-frerun-cse-after-loop -frounding-math -frtl-abstract-sequences
-fschedule-insns -fschedule-insns2 -fno-sched-interblock
-fno-sched-spec -fsched-spec-load -fsched-spec-load-dangerous
-fsched-stalled-insns[=n] -fsched-stalled-insns-dep[=n]
-fsched2-use-superblocks -fsched2-use-traces -fsee
-freschedule-modulo-scheduled-loops -fsection-anchors
-fsignaling-nans -fsingle-precision-constant -fstack-protector
-fstack-protector-all -fstrict-aliasing -fstrict-overflow
-ftracer -fthread-jumps -funroll-all-loops -funroll-loops
-fpeel-loops -fsplit-ivs-in-unroller -funswitch-loops
-fvariable-expansion-in-unroller -ftree-pre -ftree-ccp
-ftree-dce -ftree-loop-optimize -ftree-loop-linear -ftree-loop-im
-ftree-loop-ivcanon -fivopts -ftree-dominator-opts -ftree-dse
-ftree-copyrename -ftree-sink -ftree-ch -ftree-sra -ftree-ter
-ftree-lrs -ftree-fre -ftree-vectorize -ftree-vect-loop-version
-ftree-salias -fipa-pta -fweb -ftree-copy-prop -ftree-store-ccp
-ftree-store-copy-prop -ftree-vrp -funit-at-a-time -fwhole-program
--param name=value -O -O0 -O1 -O2 -O3 -Os
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 -fworking-directory -remap -trigraphs -undef
- 4 - Formatted: April 20, 2024
GCC(1) gcc-4.2.3 GCC(1)
GNU GNU
2008-02-01
-Umacro -Wp,option -Xpreprocessor option
Assembler Option
-Wa,option -Xassembler option
Linker Options
object-file-name -llibrary -nostartfiles -nodefaultlibs
-nostdlib -pie -rdynamic -s -static -static-libgcc -shared
-shared-libgcc -symbolic -Wl,option -Xlinker option -u symbol
Directory Options
-Bprefix -Idir -iquotedir -Ldir -specs=file -I- --sysroot=dir
Target Options
-V version -b machine
Machine Dependent Options
ARC Options -EB -EL -mmangle-cpu -mcpu=cpu -mtext=text-section
-mdata=data-section -mrodata=readonly-data-section
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
-mwords-little-endian -mfloat-abi=name -msoft-float -mhard-float
-mfpe -mthumb-interwork -mno-thumb-interwork -mcpu=name
-march=name -mfpu=name -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 -mcirrus-fix-invalid-insns
-mno-cirrus-fix-invalid-insns -mpoke-function-name -mthumb -marm
-mtpcs-frame -mtpcs-leaf-frame -mcaller-super-interworking
-mcallee-super-interworking -mtp=name
AVR Options -mmcu=mcu -msize -minit-stack=n -mno-interrupts
-mcall-prologues -mno-tablejump -mtiny-stack -mint8
Blackfin Options -momit-leaf-frame-pointer
-mno-omit-leaf-frame-pointer -mspecld-anomaly -mno-specld-anomaly
-mcsync-anomaly -mno-csync-anomaly -mlow-64k -mno-low64k
-mid-shared-library -mno-id-shared-library -mshared-library-id=n
-mlong-calls -mno-long-calls
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
- 5 - Formatted: April 20, 2024
GCC(1) gcc-4.2.3 GCC(1)
GNU GNU
2008-02-01
CRX Options -mmac -mpush-args
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 -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 -malpha-as -mgas
-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
DEC Alpha/VMS Options -mvms-return-codes
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 -muclibc
H8/300 Options -mrelax -mh -ms -mn -mint32 -malign-300
- 6 - Formatted: April 20, 2024
GCC(1) gcc-4.2.3 GCC(1)
GNU GNU
2008-02-01
HPPA Options -march=architecture-type -mbig-switch
-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-big-switch
-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
i386 and x86-64 Options -mtune=cpu-type -march=cpu-type
-mfpmath=unit -masm=dialect -mno-fancy-math-387
-mno-fp-ret-in-387 -msoft-float -msvr3-shlib -mno-wide-multiply
-mrtd -malign-double -mpreferred-stack-boundary=num -mmmx -msse
-msse2 -msse3 -m3dnow -mthreads -mno-align-stringops
-minline-all-stringops -mpush-args -maccumulate-outgoing-args
-m128bit-long-double -m96bit-long-double -mregparm=num
-msseregparm -mstackrealign -momit-leaf-frame-pointer
-mno-red-zone -mno-tls-direct-seg-refs -mcmodel=code-model -m32
-m64 -mlarge-data-threshold=num
IA-64 Options -mbig-endian -mlittle-endian -mgnu-as -mgnu-ld
-mno-pic -mvolatile-asm-stop -mregister-names -mno-sdata
-mconstant-gp -mauto-pic -minline-float-divide-min-latency
-minline-float-divide-max-throughput
-minline-int-divide-min-latency -minline-int-divide-max-throughput
-minline-sqrt-min-latency -minline-sqrt-max-throughput
-mno-dwarf2-asm -mearly-stop-bits -mfixed-range=register-range
-mtls-size=tls-size -mtune=cpu-type -mt -pthread -milp32 -mlp64
-mno-sched-br-data-spec -msched-ar-data-spec
-mno-sched-control-spec -msched-br-in-data-spec
-msched-ar-in-data-spec -msched-in-control-spec -msched-ldc
-mno-sched-control-ldc -mno-sched-spec-verbose
-mno-sched-prefer-non-data-spec-insns
-mno-sched-prefer-non-control-spec-insns
-mno-sched-count-spec-in-critical-path
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 -m68000 -m68020 -m68020-40 -m68020-60 -m68030
-m68040 -m68060 -mcpu32 -m5200 -mcfv4e -m68881 -mbitfield
-mc68000 -mc68020 -mnobitfield -mrtd -mshort -msoft-float
- 7 - Formatted: April 20, 2024
GCC(1) gcc-4.2.3 GCC(1)
GNU GNU
2008-02-01
-mpcrel -malign-int -mstrict-align -msep-data -mno-sep-data
-mshared-library-id=n -mid-shared-library -mno-id-shared-library
M68hc1x Options -m6811 -m6812 -m68hc11 -m68hc12 -m68hcs12
-mauto-incdec -minmax -mlong-calls -mshort
-msoft-reg-count=count
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
MIPS Options -EL -EB -march=arch -mtune=arch -mips1 -mips2
-mips3 -mips4 -mips32 -mips32r2 -mips64 -mips16 -mno-mips16
-mabi=abi -mabicalls -mno-abicalls -mshared -mno-shared -mxgot
-mno-xgot -mgp32 -mgp64 -mfp32 -mfp64 -mhard-float
-msoft-float -msingle-float -mdouble-float -mdsp
-mpaired-single -mips3d -mlong64 -mlong32 -msym32 -mno-sym32
-Gnum -membedded-data -mno-embedded-data
-muninit-const-in-rodata -mno-uninit-const-in-rodata
-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
-mfused-madd -mno-fused-madd -nocpp -mfix-r4000 -mno-fix-r4000
-mfix-r4400 -mno-fix-r4400 -mfix-vr4120 -mno-fix-vr4120
-mfix-vr4130 -mfix-sb1 -mno-fix-sb1 -mflush-func=func
-mno-flush-func -mbranch-likely -mno-branch-likely
-mfp-exceptions -mno-fp-exceptions -mvr4130-align
-mno-vr4130-align
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 -mam33 -mno-am33
-mam33-2 -mno-am33-2 -mreturn-pointer-on-d0 -mno-crt0 -mrelax
MT Options -mno-crt0 -mbacc -msim -march=cpu-type
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 -msplit
-mno-split -munix-asm -mdec-asm
- 8 - Formatted: April 20, 2024
GCC(1) gcc-4.2.3 GCC(1)
GNU GNU
2008-02-01
PowerPC Options See RS/6000 and PowerPC Options.
RS/6000 and PowerPC Options -mcpu=cpu-type -mtune=cpu-type -mpower
-mno-power -mpower2 -mno-power2 -mpowerpc -mpowerpc64
-mno-powerpc -maltivec -mno-altivec -mpowerpc-gpopt
-mno-powerpc-gpopt -mpowerpc-gfxopt -mno-powerpc-gfxopt -mmfcrf
-mno-mfcrf -mpopcntb -mno-popcntb -mfprnd -mno-fprnd
-mnew-mnemonics -mold-mnemonics -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 -mstring -mno-string
-mupdate -mno-update -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
-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 -misel
-mno-isel -misel=yes -misel=no -mspe -mno-spe -mspe=yes -mspe=no
-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
-mwindiss -G num -pthread
S/390 and zSeries Options -mtune=cpu-type -march=cpu-type
-mhard-float -msoft-float -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
Score Options -meb -mel -mnhwloop -muls -mmac -mscore5 -mscore5u
-mscore7 -mscore7d
SH Options -m1 -m2 -m2e -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 -misize -mpadstruct -mspace -mprefergot -musermode
-multcost=number -mdiv=strategy -mdivsi3_libfunc=name
-madjust-unroll -mindexed-addressing -mgettrcost=number -mpt-fixed
-minvalid-symbols
SPARC Options -mcpu=cpu-type -mtune=cpu-type -mcmodel=code-model
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-m32 -m64 -mapp-regs -mno-app-regs -mfaster-structs
-mno-faster-structs -mfpu -mno-fpu -mhard-float -msoft-float
-mhard-quad-float -msoft-quad-float -mimpure-text
-mno-impure-text -mlittle-endian -mstack-bias -mno-stack-bias
-munaligned-doubles -mno-unaligned-doubles -mv8plus -mno-v8plus
-mvis -mno-vis -threads -pthreads -pthread
System V Options -Qy -Qn -YP,paths -Ym,dir
TMS320C3x/C4x Options -mcpu=cpu -mbig -msmall -mregparm
-mmemparm -mfast-fix -mmpyi -mbk -mti -mdp-isr-reload
-mrpts=count -mrptb -mdb -mloop-unsigned -mparallel-insns
-mparallel-mpy -mpreserve-float
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 -mv850e1 -mv850e -mv850 -mbig-switch
VAX Options -mg -mgnu -munix
x86-64 Options See i386 and x86-64 Options.
Xstormy16 Options -msim
Xtensa Options -mconst16 -mno-const16 -mfused-madd
-mno-fused-madd -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 -funwind-tables
-fasynchronous-unwind-tables -finhibit-size-directive
-finstrument-functions -fno-common -fno-ident -fpcc-struct-return
-fpic -fPIC -fpie -fPIE -fno-jump-tables -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 -fargument-alias
-fargument-noalias -fargument-noalias-global
-fargument-noalias-anything -fleading-underscore
-ftls-model=model -ftrapv -fwrapv -fbounds-check -fvisibility
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
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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 which must be preprocessed.
file.i
C source code which should not be preprocessed.
file.ii
C++ source code which 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 which 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 which should not be preprocessed.
file.h
C, C++, Objective-C or Objective-C++ header file to be turned into
a precompiled header.
file.cc
file.cp
file.cxx
file.cpp
file.CPP
file.c++
file.C
C++ source code which must be preprocessed. Note that in .cxx,
the last two letters must both be literally x. Likewise, .C
refers to a literal capital C.
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file.mm
file.M
Objective-C++ source code which must be preprocessed.
file.mii
Objective-C++ source code which should not be preprocessed.
file.hh
file.H
C++ header file to be turned into a precompiled header.
file.f
file.for
file.FOR
Fixed form Fortran source code which should not be preprocessed.
file.F
file.fpp
file.FPP
Fixed form Fortran source code which must be preprocessed (with
the traditional preprocessor).
file.f90
file.f95
Free form Fortran source code which should not be preprocessed.
file.F90
file.F95
Free form Fortran source code which must be preprocessed (with the
traditional preprocessor).
file.ads
Ada source code file which 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
Assembler code which must be preprocessed.
other
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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 c-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
f95 f95-cpp-input
java
treelang
-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 will exit 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 will instead return with
numerically highest error produced by any phase that returned an
error indication. The C, C++, and Fortran frontends 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.
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-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 which don't require preprocessing are ignored.
-o file
Place output in file file. This applies regardless 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 all command
arguments are quoted. 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.
-combine
If you are compiling multiple source files, this option tells the
driver to pass all the source files to the compiler at once (for
those languages for which the compiler can handle this). This
will allow intermodule analysis (IMA) to be performed by the
compiler. Currently the only language for which this is supported
is C. If you pass source files for multiple languages to the
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driver, using this option, the driver will invoke the compiler(s)
that support IMA once each, passing each compiler all the source
files appropriate for it. For those languages that do not support
IMA this option will be ignored, and the compiler will be invoked
once for each source file in that language. If you use this
option in conjunction with -save-temps, the compiler will generate
multiple pre-processed files (one for each source file), but only
one (combined) .o or .s file.
--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 will also be passed on to the various processes
invoked by gcc, so that they can display the command line options
they accept. If the -Wextra option is also specified then command
line options which have no documentation associated with them will
also be displayed.
--target-help
Print (on the standard output) a description of target specific
command line options for each tool.
--version
Display the version number and copyrights of the invoked GCC.
@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 or .H; 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
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program that calls GCC and treats .c, .h and .i files as C++ source
files instead of C source files unless -x is used, and automatically
specifies linking against the C++ library. 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=c89. 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, -pedantic 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 would normally be built in but do not have
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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++.
A value for this option must be provided; possible values are
c89
iso9899:1990
ISO C90 (same as -ansi).
iso9899:199409
ISO C90 as modified in amendment 1.
c99
c9x
iso9899:1999
iso9899:199x
ISO C99. Note that this standard is not yet fully supported;
see <http://gcc.gnu.org/gcc-4.2/c99status.html> for more
information. The names c9x and iso9899:199x are deprecated.
gnu89
Default, ISO C90 plus GNU extensions (including some C99
features).
gnu99
gnu9x
ISO C99 plus GNU extensions. When ISO C99 is fully
implemented in GCC, this will become the default. The name
gnu9x is deprecated.
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.
Even when this option is not specified, you can still use some of
the features of newer standards in so far as they do not conflict
with previous C standards. For example, you may use
"__restrict__" even when -std=c99 is not specified.
The -std options specifying some version of ISO C have the same
effects as -ansi, except that features that were not in ISO C90
but are in the specified version (for example, // comments and the
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"inline" keyword in ISO C99) are not disabled.
-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.
This option is accepted by GCC versions 4.1.3 and up. In GCC
versions prior to 4.3, C99 inline semantics are not supported, and
thus this option is effectively assumed to be present regardless
of whether or not it is specified; the only effect of specifying
it explicitly is to disable warnings about using inline functions
in C99 mode. Likewise, the option -fno-gnu89-inline is not
supported in versions of GCC before 4.3. It will be supported
only in C99 or gnu99 mode, not in C89 or gnu89 mode.
The preprocesor 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.
-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.
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-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 that 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 this 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 takes place in 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 takes place in 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.
-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
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Program Interface v2.5 <http://www.openmp.org/>.
-fms-extensions
Accept some non-standard constructs used in Microsoft header
files.
Some cases of unnamed fields in structures and unions are only
accepted with this option.
-trigraphs
Support ISO C trigraphs. The -ansi option (and -std options for
strict ISO C conformance) implies -trigraphs.
-no-integrated-cpp
Performs a compilation in two passes: preprocessing and compiling.
This option allows a user supplied "cc1", "cc1plus", or "cc1obj"
via the -B option. The user supplied compilation step can then
add in an additional preprocessing step after normal preprocessing
but before compiling. The default is to use the integrated cpp
(internal cpp)
The semantics of this option will change if "cc1", "cc1plus", and
"cc1obj" are merged.
-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++.
-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.
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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; but 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. Version 2 is the version of the C++
ABI that first appeared in G++ 3.4. Version 1 is the version of
the C++ ABI that first appeared in G++ 3.2. Version 0 will always
be the version that conforms most closely to the C++ ABI
specification. Therefore, the ABI obtained using version 0 will
change as ABI bugs are fixed.
The default is version 2.
-fno-access-control
Turn off all access checking. This switch is mainly useful for
working around bugs in the access control code.
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-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" will only return 0 if it is declared throw(), in
which case the compiler will always check 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).
-fconserve-space
Put uninitialized or runtime-initialized global variables into the
common segment, as C does. This saves space in the executable at
the cost of not diagnosing duplicate definitions. If you compile
with this flag and your program mysteriously crashes after
"main()" has completed, you may have an object that is being
destroyed twice because two definitions were merged.
This option is no longer useful on most targets, now that support
has been added for putting variables into BSS without making them
common.
-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, and versions of G++ before
4.1 always worked that way. However, in ISO C++ a friend function
which is not declared in an enclosing scope can only be found
using argument dependent lookup. This option causes friends to be
injected as they were in earlier releases.
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 which 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 runtime. 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 will still optimize based on the
specifications, so throwing an unexpected exception will result in
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undefined behavior.
-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++.
The default if neither flag is given 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 which 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 will 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 will cause linker
errors if these functions are not inlined everywhere they are
called.
-fms-extensions
Disable pedantic 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.
-fno-operator-names
Do not treat the operator name keywords "and", "bitand", "bitor",
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"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 will allow some nonconforming
code to compile.
-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++ runtime 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 it will generate
it as needed. The dynamic_cast operator can still be used for
casts that do not require runtime type information, i.e. casts to
"void *" or to unambiguous base classes.
-fstats
Emit statistics about front-end processing at the end of the
compilation. This information is generally only useful to the G++
development team.
-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.
-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 will only work if your C library supports
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"__cxa_atexit".
-fno-use-cxa-get-exception-ptr
Don't use the "__cxa_get_exception_ptr" runtime routine. This
will cause "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 methods where the addresses of the two
functions were 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 behaviour 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 will have no effect.
Explicitly instantiated inline methods are unaffected by this
option as their linkage might otherwise cross a shared library
boundary.
-fno-weak
Do not use weak symbol support, even if it is provided by the
linker. By default, G++ will use weak symbols if they are
available. This option exists only for testing, and should not be
used by end-users; it will result 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:
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-fno-default-inline
Do not assume inline for functions defined inside a class scope.
Note that these functions will have linkage like inline
functions; they just won't be inlined by default.
-Wabi (C++ only)
Warn when G++ generates code that is probably not compatible with
the vendor-neutral C++ ABI. 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 will be 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.
The known incompatibilities at this point include:
* Incorrect handling of tail-padding for bit-fields. G++ may
attempt to pack data into the same byte as a base class. For
example:
struct A { virtual void f(); int f1 : 1; };
struct B : public A { int f2 : 1; };
In this case, G++ will place "B::f2" into the same byte
as"A::f1"; other compilers will not. You can avoid this
problem by explicitly padding "A" so that its size is a
multiple of the byte size on your platform; that will cause
G++ and other compilers to layout "B" identically.
* Incorrect handling of tail-padding for virtual bases. G++
does not use tail padding when laying out virtual bases. For
example:
struct A { virtual void f(); char c1; };
struct B { B(); char c2; };
struct C : public A, public virtual B {};
In this case, G++ will not place "B" into the tail-padding for
"A"; other compilers will. You can avoid this problem by
explicitly padding "A" so that its size is a multiple of its
alignment (ignoring virtual base classes); that will cause G++
and other compilers to layout "C" identically.
* Incorrect handling of bit-fields with declared widths greater
than that of their underlying types, when the bit-fields
appear in a union. For example:
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union U { int i : 4096; };
Assuming that an "int" does not have 4096 bits, G++ will make
the union too small by the number of bits in an "int".
* Empty classes can be placed at incorrect offsets. For
example:
struct A {};
struct B {
A a;
virtual void f ();
};
struct C : public B, public A {};
G++ will place the "A" base class of "C" at a nonzero offset;
it should be placed at offset zero. G++ mistakenly believes
that the "A" data member of "B" is already at offset zero.
* Names of template functions whose types involve "typename" or
template template parameters can be mangled incorrectly.
template <typename Q>
void f(typename Q::X) {}
template <template <typename> class Q>
void f(typename Q<int>::X) {}
Instantiations of these templates may be mangled incorrectly.
-Wctor-dtor-privacy (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.
-Wnon-virtual-dtor (C++ only)
Warn when a class appears to be polymorphic, thereby requiring a
virtual destructor, yet it declares a non-virtual one. This
warning is also enabled if -Weffc++ is specified.
-Wreorder (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:
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struct A {
int i;
int j;
A(): j (0), i (1) { }
};
The compiler will rearrange 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.
The following -W... options are not affected by -Wall.
-Weffc++ (C++ only)
Warn about violations of the following style guidelines from Scott
Meyers' Effective C++ book:
* Item 11: Define a copy constructor and an assignment operator
for classes with dynamically allocated memory.
* Item 12: Prefer initialization to assignment in constructors.
* Item 14: Make destructors virtual in base classes.
* Item 15: Have "operator=" return a reference to *this.
* Item 23: Don't try to return a reference when you must return
an object.
Also warn about violations of the following style guidelines from
Scott Meyers' More Effective C++ book:
* Item 6: Distinguish between prefix and postfix forms of
increment and decrement operators.
* Item 7: Never overload "&&", "||", or ",".
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.
-Wno-deprecated (C++ only)
Do not warn about usage of deprecated features.
-Wstrict-null-sentinel (C++ only)
Warn also 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 not a
null pointer, it is guaranteed to of the same size as a pointer.
But this use is not portable across different compilers.
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-Wno-non-template-friend (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++ 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++ 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();
will fail to compile.
-Wno-pmf-conversions (C++ only)
Disable the diagnostic for converting a bound pointer to member
function to a plain pointer.
-Wsign-promo (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++ would try to
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preserve unsignedness, but the standard mandates the current
behavior.
struct A {
operator int ();
A& operator = (int);
};
main ()
{
A a,b;
a = b;
}
In this example, G++ will synthesize a default A& operator =
(const A&);, while cfront will use the user-defined operator =.
Options Controlling Objective-C and Objective-C++ Dialects
(NOTE: This manual does not describe the Objective-C and Objective-C++
languages themselves. See
This section describes the command-line options that are only
meaningful for Objective-C and Objective-C++ programs, but 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, will
override the -fconstant-string-class setting and cause "@"...""
literals to be laid out as constant CoreFoundation strings.
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-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 (e.g., "[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. Currently, this option is only available in
conjunction with the NeXT runtime on Mac OS X 10.3 and later.
-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 that
will run 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 that will run all such default destructors,
in reverse order.
The "- (id) .cxx_construct" and/or "- (void) .cxx_destruct"
methods thusly generated will 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 will be invoked by
the runtime immediately after a new object instance is allocated;
the "- (void) .cxx_destruct" methods will be 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
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option is unavailable in conjunction with the NeXT runtime on Mac
OS X 10.2 and earlier.
@try {
...
@throw expr;
...
}
@catch (AnObjCClass *exc) {
...
@throw expr;
...
@throw;
...
}
@catch (AnotherClass *exc) {
...
}
@catch (id allOthers) {
...
}
@finally {
...
@throw expr;
...
}
The @throw statement may appear anywhere in an Objective-C or
Objective-C++ program; when used inside of a @catch block, the
@throw may appear without an argument (as shown above), in which
case the object caught by the @catch will be rethrown.
Note that only (pointers to) Objective-C objects may be thrown and
caught using this scheme. When an object is thrown, it will be
caught by the nearest @catch clause capable of handling objects of
that type, analogously to how "catch" blocks work in C++ and Java.
A "@catch(id ...)" clause (as shown above) may also be provided to
catch any and all Objective-C exceptions not caught by previous
@catch clauses (if any).
The @finally clause, if present, will be executed upon exit from
the immediately preceding "@try ... @catch" section. This will
happen regardless of whether any exceptions are thrown, caught or
rethrown inside the "@try ... @catch" section, analogously to the
behavior of the "finally" clause in Java.
There are several caveats to using the new exception mechanism:
* Although currently designed to be binary compatible with
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"NS_HANDLER"-style idioms provided by the "NSException" class,
the new exceptions can only be used on Mac OS X 10.3 (Panther)
and later systems, due to additional functionality needed in
the (NeXT) Objective-C runtime.
* As mentioned above, the new exceptions do not support handling
types other than Objective-C objects. Furthermore, when used
from Objective-C++, the Objective-C exception model does not
interoperate with C++ exceptions at this time. This means you
cannot @throw an exception from Objective-C and "catch" it in
C++, or vice versa (i.e., "throw ... @catch").
The -fobjc-exceptions switch also enables the use of
synchronization blocks for thread-safe execution:
@synchronized (ObjCClass *guard) {
...
}
Upon entering the @synchronized block, a thread of execution shall
first check whether a lock has been placed on the corresponding
"guard" object by another thread. If it has, the current thread
shall wait until the other thread relinquishes its lock. Once
"guard" becomes available, the current thread will place its own
lock on it, execute the code contained in the @synchronized block,
and finally relinquish the lock (thereby making "guard" available
to other threads).
Unlike Java, Objective-C does not allow for entire methods to be
marked @synchronized. Note that throwing exceptions out of
@synchronized blocks is allowed, and will cause the guarding
object to be unlocked properly.
-fobjc-gc
Enable garbage collection (GC) in Objective-C and Objective-C++
programs.
-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
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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.
-gen-decls
Dump interface declarations for all classes seen in the source
file to a file named sourcename.decl.
-Wassign-intercept
Warn whenever an Objective-C assignment is being intercepted by
the garbage collector.
-Wno-protocol
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
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
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 will omit such warnings if any differences found are
confined to types which share the same size and alignment.
-Wundeclared-selector
Warn if a "@selector(...)" expression referring to an undeclared
selector is found. A selector is considered undeclared if no
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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, ...). The options
described below can be used to control the diagnostic messages
formatting algorithm, e.g. how many characters per line, how often
source location information should be reported. Right now, only the
C++ front end can honor these options. However it is expected, in the
near future, that the remaining front ends would be able to digest
them correctly.
-fmessage-length=n
Try to format error messages so that they fit on lines of about n
characters. The default is 72 characters for g++ and 0 for the
rest of the front ends supported by GCC. If n is zero, then no
line-wrapping will be done; each error message will appear on a
single line.
-fdiagnostics-show-location=once
Only meaningful in line-wrapping mode. Instructs the diagnostic
messages reporter to emit once source location information; 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-show-option
This option instructs the diagnostic machinery to add text to each
diagnostic emitted, which indicates which command line option
directly controls that diagnostic, when such an option is known to
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the diagnostic machinery.
Options to Request or Suppress Warnings
Warnings are diagnostic messages that report constructions which are
not inherently erroneous but which are risky or suggest there may have
been an error.
You can request many specific warnings with options beginning -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.
The following options control the amount and kinds of warnings
produced by GCC; for further, language-specific options also refer to
C++ Dialect Options and Objective-C and Objective-C++ Dialect Options.
-fsyntax-only
Check the code for syntax errors, but don't do anything beyond
that.
-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 will 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.
-pedantic 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 -pedantic 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
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useful in some instances, but would require considerable
additional work and would be quite different from -pedantic. 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 gnu89 or gnu99, there is a corresponding
base standard, the version of ISO C on which the GNU extended
dialect is based. Warnings from -pedantic are given where they
are required by the base standard. (It would 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
Like -pedantic, except that errors are produced rather than
warnings.
-w Inhibit all warning messages.
-Wno-import
Inhibit warning messages about the use of #import.
-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.
-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.
-Wformat
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
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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 -pedantic is used with -Wformat, warnings will be
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).
Since -Wformat also checks for null format arguments for several
functions, -Wformat also implies -Wnonnull.
-Wformat is included in -Wall. For more control over some aspects
of format checking, the options -Wformat-y2k,
-Wno-format-extra-args, -Wno-format-zero-length,
-Wformat-nonliteral, -Wformat-security, and -Wformat=2 are
available, but are not included in -Wall.
-Wformat-y2k
If -Wformat is specified, also warn about "strftime" formats which
may yield only a two-digit year.
-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 will suppress 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-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
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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=2
Enable -Wformat plus format checks not included in -Wformat.
Currently equivalent to -Wformat -Wformat-nonliteral
-Wformat-security -Wformat-y2k.
-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 which are initialized with
themselves. Note this option can only be used with the
-Wuninitialized option, which in turn only works with -O1 and
above.
For example, GCC will warn about "i" being uninitialized in the
following snippet only when -Winit-self has been specified:
int f()
{
int i = i;
return i;
}
-Wimplicit-int
Warn when a declaration does not specify a type. This warning is
enabled by -Wall.
-Wimplicit-function-declaration
-Werror-implicit-function-declaration
Give a warning (or error) whenever a function is used before being
declared. The form -Wno-error-implicit-function-declaration is
not supported. This warning is enabled by -Wall (as a warning,
not an error).
-Wimplicit
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Same as -Wimplicit-int and -Wimplicit-function-declaration. This
warning is enabled by -Wall.
-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 -Wall.
-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.
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. Only the warning for an assignment used
as a truth value is supported when compiling C++; the other
warnings are only supported when compiling C.
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, 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
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by indentation the programmer chose. When there is the potential
for this confusion, GCC will issue 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" could
belong to the enclosing "if". The resulting code would look like
this:
{
if (a)
{
if (b)
foo ();
else
bar ();
}
}
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 defines 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
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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++.
-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".
For C, also warn if the return type of a function has a type
qualifier such as "const". Such a type qualifier has no effect,
since the value returned by a function is not an lvalue. ISO C
prohibits qualified "void" return types on function definitions,
so such return types always receive a warning even without this
option.
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.
-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. 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
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provoke warnings when this option is used.
-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-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-parameter
Warn whenever a function parameter is unused aside from its
declaration.
To suppress this warning use the unused attribute.
-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. This warning is enabled by -Wall.
To suppress this warning cast the expression to void.
-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.
These warnings are possible only in optimizing compilation,
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because they require data flow information that is computed only
when optimizing. If you do not specify -O, you will not get these
warnings. Instead, GCC will issue a warning about -Wuninitialized
requiring -O.
If you want to warn about code which 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 which 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 will depend 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.
These warnings are made optional because GCC is not smart enough
to see all the reasons why the code might be correct despite
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. Here is another common
case:
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{
int save_y;
if (change_y) save_y = y, y = new_y;
...
if (change_y) y = save_y;
}
This has no bug because "save_y" is used only if it is set.
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 which 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.
-Wunknown-pragmas
Warn when a #pragma directive is encountered which is not
understood by GCC. If this command line option is used, warnings
will even be issued for unknown pragmas in system header files.
This is not the case if the warnings were 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 which 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.
-Wstrict-aliasing=2
This option is only active when -fstrict-aliasing is active. It
warns about code which might break the strict aliasing rules that
the compiler is using for optimization. This warning catches more
cases than -Wstrict-aliasing, but it will also give a warning for
some ambiguous cases that are safe.
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-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 which 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 which 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 will require, in particular when determining
whether a loop will be executed at all.
-Wstrict-overflow=1
Warn about cases which are both questionable and easy to
avoid. For example: "x + 1 > x"; with -fstrict-overflow, the
compiler will simplify this 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" will be simplified to "x > 0".
-Wstrict-overflow=4
Also warn about other simplifications not covered by the above
cases. For example: "(x * 10) / 5" will be 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" will be 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 will give a
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very large number of false positives.
-Wall
All of the above -W options combined. 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.
The following -W... options 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.
-Wextra
(This option used to be called -W. The older name is still
supported, but the newer name is more descriptive.) Print extra
warning messages for these events:
* A function can return either with or without a value.
(Falling off the end of the function body is considered
returning without a value.) For example, this function would
evoke such a warning:
foo (a)
{
if (a > 0)
return a;
}
* An expression-statement or the left-hand side of a comma
expression contains no side effects. To suppress the warning,
cast the unused expression to void. For example, an
expression such as x[i,j] will cause a warning, but
x[(void)i,j] will not.
* An unsigned value is compared against zero with < or >=.
* Storage-class specifiers like "static" are not the first
things in a declaration. According to the C Standard, this
usage is obsolescent.
* If -Wall or -Wunused is also specified, warn about unused
arguments.
* A comparison between signed and unsigned values could produce
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an incorrect result when the signed value is converted to
unsigned. (But don't warn if -Wno-sign-compare is also
specified.)
* An aggregate has an initializer which does not initialize all
members. This warning can be independently controlled by
-Wmissing-field-initializers.
* An initialized field without side effects is overridden when
using designated initializers. This warning can be
independently controlled by -Woverride-init.
* A function parameter is declared without a type specifier in
K&R-style functions:
void foo(bar) { }
* An empty body occurs in an if or else statement.
* A pointer is compared against integer zero with <, <=, >, or
>=.
* A variable might be changed by longjmp or vfork.
*<(C++ only)>
An enumerator and a non-enumerator both appear in a
conditional expression.
*<(C++ only)>
A non-static reference or non-static const member appears in a
class without constructors.
*<(C++ only)>
Ambiguous virtual bases.
*<(C++ only)>
Subscripting an array which has been declared register.
*<(C++ only)>
Taking the address of a variable which has been declared
register.
*<(C++ only)>
A base class is not initialized in a derived class' copy
constructor.
-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
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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 will not warn about unknown
pragmas in system headers---for that, -Wunknown-pragmas must also
be used.
-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 would 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 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 which
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 does not in ISO C.
* In traditional C, some preprocessor directives did not exist.
Traditional preprocessors would only consider 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 would ignore 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 would not recognize #elif, so it suggests
avoiding it altogether.
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* 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 would cause serious problems.
This is a subset of the possible conversion warnings, for the
full set use -Wconversion.
* Use of ISO C style function definitions. This warning
intentionally is not issued for prototype declarations or
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variadic functions because these ISO C features will 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.
-Wdeclaration-after-statement (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 and was
not supported by GCC versions before GCC 3.0.
-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 shadows another local variable,
parameter or global variable or whenever a built-in function is
shadowed.
-Wlarger-than-len
Warn whenever an object of larger than len bytes is defined.
-Wunsafe-loop-optimizations
Warn if the loop cannot be optimized because the compiler could
not assume anything on the bounds of the loop indices. With
-funsafe-loop-optimizations warn if the compiler made such
assumptions.
-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.
-Wbad-function-cast (C only)
Warn whenever a function call is cast to a non-matching type. For
example, warn if "int malloc()" is cast to "anything *".
-Wc++-compat
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.
-Wcast-qual
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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 *".
-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 will get a warning; when compiling
C++, warn about the deprecated conversion from string literals to
"char *". This warning, by default, is enabled for C++ programs.
These warnings will 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 will just be a nuisance; this is why we did not make
-Wall request these warnings.
-Wconversion
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.
Also, warn if a negative integer constant expression is implicitly
converted to an unsigned type. For example, warn about the
assignment "x = -1" if "x" is unsigned. But do not warn about
explicit casts like "(unsigned) -1".
-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.
-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
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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.
-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-attributes
Do not warn if an unexpected "__attribute__" is used, such as
unrecognized attributes, function attributes applied to variables,
etc. This will not stop errors for incorrect use of supported
attributes.
-Wstrict-prototypes (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 which specifies the
argument types.)
-Wold-style-definition (C only)
Warn if an old-style function definition is used. A warning is
given even if there is a previous prototype.
-Wmissing-prototypes (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. The aim is to detect global functions that
fail to be declared in header files.
-Wmissing-declarations (C only)
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.
-Wmissing-field-initializers
Warn if a structure's initializer has some fields missing. For
example, the following code would cause 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 would not trigger a warning:
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struct s { int f, g, h; };
struct s x = { .f = 3, .g = 4 };
This warning is included in -Wextra. To get other -Wextra
warnings without this one, use -Wextra
-Wno-missing-field-initializers.
-Wmissing-noreturn
Warn about functions which might be candidates for attribute
"noreturn". Note these are only possible candidates, not absolute
ones. Care should be taken to manually verify functions actually
do not ever return before adding the "noreturn" attribute,
otherwise subtle code generation bugs could be introduced. You
will not get a warning for "main" in hosted C environments.
-Wmissing-format-attribute
Warn about function pointers which might be candidates for
"format" attributes. Note these are only possible candidates, not
absolute ones. GCC will guess 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 will also warn about function definitions which might be
candidates for "format" attributes. Again, these are only
possible candidates. GCC will guess 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.
-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
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warn you if you are using identifiers which have not been
normalized; this option controls that warning.
There are four levels of warning that GCC supports. The default
is -Wnormalized=nfc, which warns about any identifier which is not
in the ISO 10646 "C" normalized form, NFC. NFC is the recommended
form for most uses.
Unfortunately, there are some characters which ISO C and ISO C++
allow in identifiers that when turned into NFC aren't allowable as
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. You would only want to do this if you were
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", will display just like a regular "n" which
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 will warn 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
is unable to be fixed to display these characters distinctly.
-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.
-Woverride-init
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.
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-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" will be
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;
};
-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 only)
Warn if an "extern" declaration is encountered within a function.
-Wunreachable-code
Warn if the compiler detects that code will never be executed.
This option is intended to warn when the compiler detects that at
least a whole line of source code will never be executed, because
some condition is never satisfied or because it is after a
procedure that never returns.
It is possible for this option to produce a warning even though
there are circumstances under which part of the affected line can
be executed, so care should be taken when removing apparently-
unreachable code.
For instance, when a function is inlined, a warning may mean that
the line is unreachable in only one inlined copy of the function.
This option is not made part of -Wall because in a debugging
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version of a program there is often substantial code which checks
correct functioning of the program and is, hopefully, unreachable
because the program does work. Another common use of unreachable
code is to provide behavior which is selectable at compile-time.
-Winline
Warn if a function can not be inlined and it was declared as
inline. Even with this option, the compiler will 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++ only)
Suppress warnings from applying the offsetof macro to a non-POD
type. According to the 1998 ISO C++ standard, applying offsetof
to a non-POD type is undefined. In existing C++ implementations,
however, offsetof typically gives meaningful results even when
applied to certain kinds of non-POD types. (Such as a simple
struct that fails to be a POD type only by virtue of having a
constructor.) 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 (C only)
Suppress warnings from casts to pointer type of an integer of a
different size.
-Wno-pointer-to-int-cast (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 default. To inhibit the
warning messages, use -Wno-long-long. Flags -Wlong-long and
-Wno-long-long are taken into account only when -pedantic flag is
used.
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-Wvariadic-macros
Warn if variadic macros are used in pedantic ISO C90 mode, or the
GNU alternate syntax when in pedantic ISO C99 mode. This is
default. To inhibit the warning messages, use
-Wno-variadic-macros.
-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.
-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 were unable to
handle the code effectively. Often, the problem is that your code
is too big or too complex; GCC will refuse to optimize programs
when the optimization itself is likely to take inordinate amounts
of time.
-Wpointer-sign
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 -pedantic, which can be disabled
with -Wno-pointer-sign.
-Werror
Make all warnings into errors.
-Werror=
Make the specified warning into an errors. 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. You can use the
-fdiagnostics-show-option option to have each controllable warning
amended with the option which controls it, to determine what to
use with this option.
Note that specifying -Werror=foo automatically implies -Wfoo.
However, -Wno-error=foo does not imply anything.
-Wstack-protector
This option is only active when -fstack-protector is active. It
warns about functions that will not be protected against stack
smashing.
-Woverlength-strings
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Warn about string constants which are longer than the "minimum
maximum" length specified in the C standard. Modern compilers
generally allow string constants which 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 C89, 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 -pedantic, and can be disabled with
-Wno-overlength-strings.
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 will probably
make 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
were already at hand; some statements may execute in different
places because they were 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.
-ggdb
Produce debugging information for use by GDB. This means to use
the most expressive format available (DWARF 2, stabs, or the
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native format if neither of those are supported), including GDB
extensions if at all possible.
-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 which 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 will increase the size of
debugging information by as much as a factor of two.
-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-2
Produce debugging information in DWARF version 2 format (if that
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is supported). This is the format used by DBX on IRIX 6. With
this option, GCC uses features of DWARF version 3 when they are
useful; version 3 is upward compatible with version 2, but may
still cause problems for older debuggers.
-gvms
Produce debugging information in VMS debug format (if that is
supported). This is the format used by DEBUG on 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 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, but no
information about local variables and no line numbers.
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 DWARF2.
-feliminate-dwarf2-dups
Compress DWARF2 debugging information by eliminating duplicated
information about each symbol. This option only makes sense when
generating DWARF2 debugging information with -gdwarf-2.
-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.
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-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.
-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.
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* 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 will match the source files more
closely, if you do not optimize.
-dletters
-fdump-rtl-pass
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. 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.
Most debug dumps can be enabled either passing a letter to the -d
option, or with a long -fdump-rtl switch; here are the possible
letters for use in letters and pass, and their meanings:
-dA Annotate the assembler output with miscellaneous debugging
information.
-dB
-fdump-rtl-bbro
Dump after block reordering, to file.148r.bbro.
-dc
-fdump-rtl-combine
Dump after instruction combination, to the file
file.129r.combine.
-dC
-fdump-rtl-ce1
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-fdump-rtl-ce2
-dC and -fdump-rtl-ce1 enable dumping after the first if
conversion, to the file file.117r.ce1. -dC and -fdump-rtl-ce2
enable dumping after the second if conversion, to the file
file.130r.ce2.
-dd
-fdump-rtl-btl
-fdump-rtl-dbr
-dd and -fdump-rtl-btl enable dumping after branch target load
optimization, to file.31.btl. -dd and -fdump-rtl-dbr enable
dumping after delayed branch scheduling, to file.36.dbr.
-dD Dump all macro definitions, at the end of preprocessing, in
addition to normal output.
-dE
-fdump-rtl-ce3
Dump after the third if conversion, to file.146r.ce3.
-df
-fdump-rtl-cfg
-fdump-rtl-life
-df and -fdump-rtl-cfg enable dumping after control and data
flow analysis, to file.116r.cfg. -df and -fdump-rtl-cfg
enable dumping dump after life analysis, to file.128r.life1
and file.135r.life2.
-dg
-fdump-rtl-greg
Dump after global register allocation, to file.139r.greg.
-dG
-fdump-rtl-gcse
-fdump-rtl-bypass
-dG and -fdump-rtl-gcse enable dumping after GCSE, to
file.114r.gcse. -dG and -fdump-rtl-bypass enable dumping
after jump bypassing and control flow optimizations, to
file.115r.bypass.
-dh
-fdump-rtl-eh
Dump after finalization of EH handling code, to file.02.eh.
-di
-fdump-rtl-sibling
Dump after sibling call optimizations, to file.106r.sibling.
-dj
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-fdump-rtl-jump
Dump after the first jump optimization, to file.112r.jump.
-dk
-fdump-rtl-stack
Dump after conversion from registers to stack, to
file.152r.stack.
-dl
-fdump-rtl-lreg
Dump after local register allocation, to file.138r.lreg.
-dL
-fdump-rtl-loop2
-dL and -fdump-rtl-loop2 enable dumping after the loop
optimization pass, to file.119r.loop2, file.120r.loop2_init,
file.121r.loop2_invariant, and file.125r.loop2_done.
-dm
-fdump-rtl-sms
Dump after modulo scheduling, to file.136r.sms.
-dM
-fdump-rtl-mach
Dump after performing the machine dependent reorganization
pass, to file.155r.mach.
-dn
-fdump-rtl-rnreg
Dump after register renumbering, to file.147r.rnreg.
-dN
-fdump-rtl-regmove
Dump after the register move pass, to file.132r.regmove.
-do
-fdump-rtl-postreload
Dump after post-reload optimizations, to file.24.postreload.
-dr
-fdump-rtl-expand
Dump after RTL generation, to file.104r.expand.
-dR
-fdump-rtl-sched2
Dump after the second scheduling pass, to file.150r.sched2.
-ds
-fdump-rtl-cse
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Dump after CSE (including the jump optimization that sometimes
follows CSE), to file.113r.cse.
-dS
-fdump-rtl-sched
Dump after the first scheduling pass, to file.21.sched.
-dt
-fdump-rtl-cse2
Dump after the second CSE pass (including the jump
optimization that sometimes follows CSE), to file.127r.cse2.
-dT
-fdump-rtl-tracer
Dump after running tracer, to file.118r.tracer.
-dV
-fdump-rtl-vpt
-fdump-rtl-vartrack
-dV and -fdump-rtl-vpt enable dumping after the value profile
transformations, to file.10.vpt. -dV and -fdump-rtl-vartrack
enable dumping after variable tracking, to file.154r.vartrack.
-dw
-fdump-rtl-flow2
Dump after the second flow pass, to file.142r.flow2.
-dz
-fdump-rtl-peephole2
Dump after the peephole pass, to file.145r.peephole2.
-dZ
-fdump-rtl-web
Dump after live range splitting, to file.126r.web.
-da
-fdump-rtl-all
Produce all the dumps listed above.
-dH Produce a core dump whenever an error occurs.
-dm Print statistics on memory usage, at the end of the run, to
standard error.
-dp Annotate the assembler output with a comment indicating which
pattern and alternative was used. The length of each
instruction is also printed.
-dP Dump the RTL in the assembler output as a comment before each
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instruction. Also turns on -dp annotation.
-dv For each of the other indicated dump files (either with -d or
-fdump-rtl-pass), dump a representation of the control flow
graph suitable for viewing with VCG to file.pass.vcg.
-dx Just generate RTL for a function instead of compiling it.
Usually used with r (-fdump-rtl-expand).
-dy Dump debugging information during parsing, to standard error.
-fdump-noaddr
When doing debugging dumps (see -d option above), 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.
-fdump-unnumbered
When doing debugging dumps (see -d option above), suppress
instruction numbers, line number note 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-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. 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. 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. The following
dumps are possible:
all Enables all inter-procedural analysis dumps; currently the
only produced dump is the cgraph dump.
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cgraph
Dumps information about call-graph optimization, unused
function removal, and inlining decisions.
-fdump-tree-switch
-fdump-tree-switch-options
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. If
the -options form is used, options is a list of - separated
options that control the details of the dump. Not all options are
applicable to all dumps, those which are not meaningful will be
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.
slim
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.
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).
stats
Enable dumping various statistics about the pass (not honored
by every dump option).
blocks
Enable showing basic block boundaries (disabled in raw dumps).
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.
all Turn on all options, except raw, slim and lineno.
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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.
inlined
Dump after function inlining, to file.inlined.
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.
vcg Dump the control flow graph of each function to a file in VCG
format. The file name is made by appending .vcg to the source
file name. Note that if the file contains more than one
function, the generated file cannot be used directly by VCG.
You will need to cut and paste each function's graph into its
own separate file first.
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.
salias
Dump structure aliasing variable information to a file. This
file name is made by appending .salias 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
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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.
mudflap
Dump each function after adding mudflap instrumentation. The
file name is made by appending .mudflap 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
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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.
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.
-ftree-vectorizer-verbose=n
This option controls the amount of debugging output the vectorizer
prints. This information is written to standard error, unless
-fdump-tree-all or -fdump-tree-vect is specified, in which case it
is output to the usual dump listing file, .vect. For n=0 no
diagnostic information is reported. If n=1 the vectorizer reports
each loop that got vectorized, and the total number of loops that
got vectorized. If n=2 the vectorizer also reports non-vectorized
loops that passed the first analysis phase
(vect_analyze_loop_form) - i.e. countable, inner-most, single-bb,
single-entry/exit loops. This is the same verbosity level that
-fdump-tree-vect-stats uses. Higher verbosity levels mean either
more information dumped for each reported loop, or same amount of
information reported for more loops: If n=3, alignment related
information is added to the reports. If n=4, data-references
related information (e.g. memory dependences, memory
access-patterns) is added to the reports. If n=5, the vectorizer
reports also non-vectorized inner-most loops that did not pass the
first analysis phase (i.e. may not be countable, or may have
complicated control-flow). If n=6, the vectorizer reports also
non-vectorized nested loops. For n=7, all the information the
vectorizer generates during its analysis and transformation is
reported. This is the same verbosity level that
-fdump-tree-vect-details uses.
-frandom-seed=string
This option provides a seed that GCC uses when it would otherwise
use random numbers. It is used to generate 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.
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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 -dS or -dR is
specified, in which case it is output to the usual dump listing
file, .sched 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 -dRS. 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
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 would produce
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.
-time
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). 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.
-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
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information).
It is enabled by default when compiling with optimization (-Os,
-O, -O2, ...), debugging information (-g) and the debug info
format supports it.
-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 @samp{-, without spaces between multiple switches. This is
supposed to ease shell-processing.
-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 will search---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 /.
-dumpmachine
Print the compiler's target machine (for example,
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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.)
-feliminate-unused-debug-types
Normally, when producing DWARF2 output, GCC will emit debugging
information for all types declared in a compilation unit,
regardless of whether or not they are actually used in that
compilation unit. Sometimes this is useful, such as 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. With this
option, GCC will avoid producing debug symbol output for types
that are nowhere used in the source file being compiled.
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 would 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. Optimization levels -O and above, in particular, enable
unit-at-a-time mode, which allows the compiler to consider information
gained from later functions in the file when compiling a function.
Compiling multiple files at once to a single output file in unit-at-
a-time 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.
-O
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-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: -fdefer-pop
-fdelayed-branch -fguess-branch-probability -fcprop-registers
-fif-conversion -fif-conversion2 -ftree-ccp -ftree-dce
-ftree-dominator-opts -ftree-dse -ftree-ter -ftree-lrs -ftree-sra
-ftree-copyrename -ftree-fre -ftree-ch -funit-at-a-time
-fmerge-constants
-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. The
compiler does not perform loop unrolling or function inlining when
you specify -O2. 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
-fcrossjumping -foptimize-sibling-calls -fcse-follow-jumps
-fcse-skip-blocks -fgcse -fgcse-lm -fexpensive-optimizations
-frerun-cse-after-loop -fcaller-saves -fpeephole2 -fschedule-insns
-fschedule-insns2 -fsched-interblock -fsched-spec -fregmove
-fstrict-aliasing -fstrict-overflow -fdelete-null-pointer-checks
-freorder-blocks -freorder-functions -falign-functions
-falign-jumps -falign-loops -falign-labels -ftree-vrp -ftree-pre
Please note the warning under -fgcse about invoking -O2 on
programs that use computed gotos.
-O2 doesn't turn on -ftree-vrp for the Ada compiler. This option
must be explicitly specified on the command line to be enabled for
the Ada compiler.
-O3 Optimize yet more. -O3 turns on all optimizations specified by
-O2 and also turns on the -finline-functions, -funswitch-loops and
-fgcse-after-reload options.
-O0 Do not optimize. 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.
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-Os disables the following optimization flags: -falign-functions
-falign-jumps -falign-loops -falign-labels -freorder-blocks
-freorder-blocks-and-partition -fprefetch-loop-arrays
-ftree-vect-loop-version
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 would be -fno-foo. In the table below, only one of the forms is
listed---the one you typically will 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-default-inline
Do not make member functions inline by default merely because they
are defined inside the class scope (C++ only). Otherwise, when
you specify -O, member functions defined inside class scope are
compiled inline by default; i.e., you don't need to add inline in
front of the member function name.
-fno-defer-pop
Always pop the arguments to each function call as soon as that
function returns. For machines which 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.
-fforce-mem
Force memory operands to be copied into registers before doing
arithmetic on them. This produces better code by making all
memory references potential common subexpressions. When they are
not common subexpressions, instruction combination should
eliminate the separate register-load. This option is now a nop and
will be removed in 4.3.
-fforce-addr
Force memory address constants to be copied into registers before
doing arithmetic on them.
-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
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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.
Enabled at levels -O, -O2, -O3, -Os.
-foptimize-sibling-calls
Optimize sibling and tail recursive calls.
Enabled at levels -O2, -O3, -Os.
-fno-inline
Don't pay attention to the "inline" keyword. Normally this option
is used to keep the compiler from expanding any functions inline.
Note that if you are not optimizing, no functions can be expanded
inline.
-finline-functions
Integrate all simple functions into their callers. The compiler
heuristically decides which functions are simple enough to be
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 if -funit-at-a-time is enabled.
-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.
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Enabled by default.
-finline-limit=n
By default, GCC limits the size of functions that can be inlined.
This flag allows the control of this limit for functions that are
explicitly marked as inline (i.e., marked with the inline keyword
or defined within the class definition in c++). n is the size of
functions that can be inlined in number of pseudo instructions
(not counting parameter handling). The default value of n is 600.
Increasing this value can result in more inlined code at the cost
of compilation time and memory consumption. Decreasing usually
makes the compilation faster and less code will be inlined (which
presumably means slower programs). This option is particularly
useful for programs that use inlining heavily such as those based
on recursive templates with C++.
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 I<n>/2.
max-inline-insns-auto
is set to I<n>/2.
min-inline-insns
is set to 130 or I<n>/4, whichever is smaller.
max-inline-insns-rtl
is set to I<n>.
See below for a documentation of the individual parameters
controlling inlining.
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.
-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 C. In C++, emit any and all inline
functions into the object file.
-fkeep-static-consts
Emit variables declared "static const" when optimization isn't
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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 the variable was 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 non-automatic
variable to have distinct location, so using this option will
result 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.
-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.
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
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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.
-fmudflap -fmudflapth -fmudflapir
For front-ends that support it (C and C++), instrument all risky
pointer/array dereferencing operations, some standard library
string/heap functions, and some other associated constructs with
range/validity tests. Modules so instrumented should be immune to
buffer overflows, invalid heap use, and some other classes of
C/C++ programming errors. The instrumentation relies on a
separate runtime library (libmudflap), which will be linked into a
program if -fmudflap is given at link time. Run-time behavior of
the instrumented program is controlled by the MUDFLAP_OPTIONS
environment variable. See "env MUDFLAP_OPTIONS=-help a.out" for
its options.
Use -fmudflapth instead of -fmudflap to compile and to link if
your program is multi-threaded. Use -fmudflapir, in addition to
-fmudflap or -fmudflapth, if instrumentation should ignore pointer
reads. This produces less instrumentation (and therefore faster
execution) and still provides some protection against outright
memory corrupting writes, but allows erroneously read data to
propagate within a program.
-fthread-jumps
Perform optimizations where we 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.
-fcse-follow-jumps
In common subexpression elimination, scan through jump
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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 will follow 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 which 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
has been 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 runtime 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
will attempt to move loads which 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 gcse is enabled.
-fgcse-sm
When -fgcse-sm is enabled, a store motion pass is run after global
common subexpression elimination. This pass will attempt 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.
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-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
cleanup redundant spilling.
-funsafe-loop-optimizations
If given, the loop optimizer will assume that loop indices do not
overflow, and that the 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. Using -Wunsafe-loop-optimizations, the
compiler will warn you if it finds this kind of loop.
-fcrossjumping
Perform cross-jumping transformation. This transformation unifies
equivalent code and save code size. The resulting code may or may
not perform better than without cross-jumping.
Enabled at levels -O2, -O3, -Os.
-fif-conversion
Attempt to transform conditional jumps into branch-less
equivalents. This include 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 "if-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.
-fdelete-null-pointer-checks
Use global dataflow analysis to identify and eliminate useless
checks for null pointers. The compiler assumes that dereferencing
a null pointer would have halted the program. If a pointer is
checked after it has already been dereferenced, it cannot be null.
In some environments, this assumption is not true, and programs
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can safely dereference null pointers. Use
-fno-delete-null-pointer-checks to disable this optimization for
programs which depend on that behavior.
Enabled at levels -O2, -O3, -Os.
-fexpensive-optimizations
Perform a number of minor optimizations that are relatively
expensive.
Enabled at levels -O2, -O3, -Os.
-foptimize-register-move
-fregmove
Attempt to reassign register numbers in move instructions and as
operands of other simple instructions in order to maximize the
amount of register tying. This is especially helpful on machines
with two-operand instructions.
Note -fregmove and -foptimize-register-move are the same
optimization.
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, -Os.
-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.
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-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-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-fsched-stalled-insns and
-fsched-stalled-insns=0 are equivalent and mean that no insns will
be moved prematurely. If n is unspecified then there is no limit
on how many queued insns can be moved prematurely.
-fsched-stalled-insns-dep
-fsched-stalled-insns-dep=n
Define how many insn groups (cycles) will be examined for a
dependency on a stalled insn that is 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 and its value is not zero.
-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, do use superblock
scheduling algorithm. Superblock scheduling allows motion across
basic block boundaries resulting on 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,
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i.e. with -fschedule-insns2 or at -O2 or higher.
-fsched2-use-traces
Use -fsched2-use-superblocks algorithm when scheduling after
register allocation and additionally perform code duplication in
order to increase the size of superblocks using tracer pass. See
-ftracer for details on trace formation.
This mode should produce faster but significantly longer programs.
Also without -fbranch-probabilities the traces constructed may not
match the reality and hurt the performance. This only makes sense
when scheduling after register allocation, i.e. with
-fschedule-insns2 or at -O2 or higher.
-fsee
Eliminates redundant extension instructions and move the non
redundant ones to optimal placement using LCM.
-freschedule-modulo-scheduled-loops
The modulo scheduling comes before the traditional scheduling, if
a loop was modulo scheduled we may want to prevent the later
scheduling passes from changing its schedule, we use this option
to control that.
-fcaller-saves
Enable values to be allocated in registers that will be 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 than would otherwise
be produced.
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.
-ftree-pre
Perform Partial Redundancy Elimination (PRE) on trees. This flag
is enabled by default at -O2 and -O3.
-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 faster than PRE, though it
exposes fewer redundancies. This flag is enabled by default at -O
and higher.
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2008-02-01
-ftree-copy-prop
Perform copy propagation on trees. This pass eliminates
unnecessary copy operations. This flag is enabled by default at
-O and higher.
-ftree-store-copy-prop
Perform copy propagation of memory loads and stores. This pass
eliminates unnecessary copy operations in memory references
(structures, global variables, arrays, etc). This flag is enabled
by default at -O2 and higher.
-ftree-salias
Perform structural alias analysis on trees. This flag is enabled
by default at -O and higher.
-fipa-pta
Perform interprocedural pointer analysis.
-ftree-sink
Perform forward store motion on trees. This flag is enabled by
default at -O and higher.
-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.
-ftree-store-ccp
Perform sparse conditional constant propagation (CCP) on trees.
This pass operates on both local scalar variables and memory
stores and loads (global variables, structures, arrays, etc).
This flag is enabled by default at -O2 and higher.
-ftree-dce
Perform dead code elimination (DCE) on trees. This flag is
enabled by default at -O and higher.
-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-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.
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2008-02-01
-ftree-loop-optimize
Perform loop optimizations on trees. This flag is enabled by
default at -O and higher.
-ftree-loop-linear
Perform linear loop transformations on tree. This flag can
improve cache performance and allow further loop optimizations to
take place.
-ftree-loop-im
Perform loop invariant motion on trees. This pass moves only
invariants that would be 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 the loop
for that 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-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-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.
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-ftree-lrs
Perform live range splitting during the SSA->normal phase.
Distinct live ranges of a variable are split into unique
variables, allowing for better optimization later. This is
enabled by default at -O and higher.
-ftree-vectorize
Perform loop vectorization on trees.
-ftree-vect-loop-version
Perform loop versioning when doing loop vectorization on trees.
When a loop appears to be vectorizable except that data alignment
or data dependence cannot be determined at compile time then
vectorized and non-vectorized versions of the loop are generated
along with runtime checks for alignment or dependence to control
which version is executed. This option is enabled by default
except at level -Os where it is disabled.
-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.
-ftracer
Perform tail duplication to enlarge superblock size. This
transformation simplifies the control flow of the function
allowing other optimizations to do better job.
-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. This option makes code larger, and may or
may not make it run faster.
-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,
-fsplit-ivs-in-unroller
Enables expressing 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.
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2008-02-01
Combination of -fweb and CSE is often sufficient to obtain the
same effect. However in cases the loop body is more complicated
than a single basic block, this is not reliable. It also does not
work at all on some of the architectures due to restrictions in
the CSE pass.
This optimization is enabled by default.
-fvariable-expansion-in-unroller
With this option, the compiler will create multiple copies of some
local variables when unrolling a loop which can result in superior
code.
-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 will use 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 will be 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.
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2008-02-01
-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.
-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 in to make this option
effective. See -fprofile-arcs for details.
Enabled at levels -O2, -O3, -Os.
-fstrict-aliasing
Allows 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;
};
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2008-02-01
int f() {
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
will work as expected. However, this code might not:
int f() {
a_union t;
int* ip;
t.d = 3.0;
ip = &t.i;
return *ip;
}
Every language that wishes to perform language-specific alias
analysis should define a function that computes, given an "tree"
node, an alias set for the node. Nodes in different alias sets
are not allowed to alias. For an example, see the C front-end
function "c_get_alias_set".
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 will
not happen. This permits various optimizations. For example, the
compiler will assume that an expression like "i + 10 > i" will
always be 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 will overflow must be written carefully to not
actually involve overflow.
See also the -fwrapv option. Using -fwrapv means that signed
overflow is fully defined: it wraps. When -fwrapv is used, there
is no difference between -fstrict-overflow and
-fno-strict-overflow. 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.
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2008-02-01
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 would align 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 will not be 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 will not be 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. The hope is that the loop will be
executed many times, which will make up for any execution of the
dummy operations.
-fno-align-loops and -falign-loops=1 are equivalent and mean that
loops will not be aligned.
If n is not specified or is zero, use a machine-dependent default.
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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 will not be aligned.
If n is not specified or is zero, use a machine-dependent default.
Enabled at levels -O2, -O3.
-funit-at-a-time
Parse the whole compilation unit before starting to produce code.
This allows some extra optimizations to take place but consumes
more memory (in general). There are some compatibility issues
with unit-at-a-time mode:
* enabling unit-at-a-time mode may change the order in which
functions, variables, and top-level "asm" statements are
emitted, and will likely break code relying on some particular
ordering. The majority of such top-level "asm" statements,
though, can be replaced by "section" attributes. The fno-
toplevel-reorder option may be used to keep the ordering used
in the input file, at the cost of some optimizations.
* unit-at-a-time mode removes unreferenced static variables and
functions. This may result in undefined references when an
"asm" statement refers directly to variables or functions that
are otherwise unused. In that case either the
variable/function shall be listed as an operand of the "asm"
statement operand or, in the case of top-level "asm"
statements the attribute "used" shall be used on the
declaration.
* Static functions now can use non-standard passing conventions
that may break "asm" statements calling functions directly.
Again, attribute "used" will prevent this behavior.
As a temporary workaround, -fno-unit-at-a-time can be used, but
this scheme may not be supported by future releases of GCC.
Enabled at levels -O, -O2, -O3, -Os.
-fno-toplevel-reorder
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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 will not be removed. This option is intended to support
existing code which relies on a particular ordering. For new
code, it is better to use attributes.
-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 will no longer stay in a
"home register".
Enabled by default with -funroll-loops.
-fwhole-program
Assume that the current compilation unit represents 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 a affect gets
more aggressively optimized by interprocedural optimizers. While
this option is equivalent to proper use of "static" keyword for
programs consisting of single file, in combination with option
--combine this flag can be used to compile most of smaller scale C
programs since the functions and variables become local for the
whole combined compilation unit, not for the single source file
itself.
-fcprop-registers
After register allocation and post-register allocation instruction
splitting, we perform a copy-propagation pass to try to reduce
scheduling dependencies and occasionally eliminate the copy.
Enabled at levels -O, -O2, -O3, -Os.
-fprofile-generate
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".
-fprofile-use
Enable profile feedback directed optimizations, and optimizations
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generally profitable only with profile feedback available.
The following options are enabled: "-fbranch-probabilities",
"-fvpt", "-funroll-loops", "-fpeel-loops", "-ftracer"
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.
-ffast-math
Sets -fno-math-errno, -funsafe-math-optimizations,
-fno-trapping-math, -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 should never be turned on by any -O option since it
can result in incorrect output for programs which depend on an
exact implementation of IEEE or ISO rules/specifications for math
functions.
-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 should never be turned on by any -O option since it
can result in incorrect output for programs which depend on an
exact implementation of IEEE or ISO rules/specifications for math
functions.
The default is -fmath-errno.
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2008-02-01
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 should never be turned on by any -O option since it
can result in incorrect output for programs which depend on an
exact implementation of IEEE or ISO rules/specifications for math
functions.
The default is -fno-unsafe-math-optimizations.
-ffinite-math-only
Allow optimizations for floating-point arithmetic that assume that
arguments and results are not NaNs or +-Infs.
This option should never be turned on by any -O option since it
can result in incorrect output for programs which depend on an
exact implementation of IEEE or ISO rules/specifications.
The default is -fno-finite-math-only.
-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 implies -fno-signaling-nans. 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 which 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
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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".
-frtl-abstract-sequences
It is a size optimization method. This option is to find identical
sequences of code, which can be turned into pseudo-procedures and
then replace all occurrences with calls to the newly created
subroutine. It is kind of an opposite of -finline-functions. This
optimization runs at RTL level.
-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 constant as single precision constant instead
of implicitly converting it to double precision constant.
-fcx-limited-range
When enabled, this option states that a range reduction step is
not needed when performing complex division. 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.
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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 the 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 mostly 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 and adds REG_VALUE_PROFILE notes
to instructions for their later usage in optimizations.
Enabled with -fprofile-generate and -fprofile-use.
-fvpt
If combined with -fprofile-arcs, it instructs the compiler to add
a 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 operation 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 will most benefit processors with lots of registers.
Depending on the debug information format adopted by the target,
however, it can make debugging impossible, since variables will no
longer stay in a "home register".
Enabled by default with -funroll-loops.
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-ftracer
Perform tail duplication to enlarge superblock size. This
transformation simplifies the control flow of the function
allowing other optimizations to do 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 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 the loops for that 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
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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 will create larger object and executable files and will
also be slower. You will not be able to 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 in 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.
-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; }
would usually calculate the addresses of all three variables, but
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if you compile it with -fsection-anchors, it will access 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 will not inline
functions that contain more that 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 given in the following table:
salias-max-implicit-fields
The maximum number of fields in a variable without direct
structure accesses for which structure aliasing will consider
trying to track each field. The default is 5
salias-max-array-elements
The maximum number of elements an array can have and its
elements still be tracked individually by structure aliasing.
The default is 4
sra-max-structure-size
The maximum structure size, in bytes, at which the scalar
replacement of aggregates (SRA) optimization will perform
block copies. The default value, 0, implies that GCC will
select the most appropriate size itself.
sra-field-structure-ratio
The threshold ratio (as a percentage) between instantiated
fields and the complete structure size. We say that if the
ratio of the number of bytes in instantiated fields to the
number of bytes in the complete structure exceeds this
parameter, then block copies are not used. The default is 75.
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max-crossjump-edges
The maximum number of incoming edges to consider for
crossjumping. 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 compile
time increase with probably small improvement in executable
size.
min-crossjump-insns
The minimum number of instructions which must be matched at
the end of two blocks before crossjumping will be performed on
them. This value is ignored in the case where all
instructions in the block being crossjumped 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 is searched, the time savings
from filling the delay slot will be minimal so stop searching.
Increasing values mean more aggressive optimization, making
the compile time increase with probably small improvement in
executable run 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
compile 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 will be
allocated in order to perform the global common subexpression
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elimination optimization. If more memory than specified is
required, the optimization will not be done.
max-gcse-passes
The maximum number of passes of GCSE to run. The default is
1.
max-pending-list-length
The maximum number of pending dependencies scheduling will
allow 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-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 will consider for inlining. This only affects
functions declared inline and methods implemented in a class
declaration (C++). The default value is 450.
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 will be investigated. To those functions, a
different (more restrictive) limit compared to functions
declared inline can be applied. The default value is 90.
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 backend. This parameter is
ignored when -funit-at-a-time is not used. The default value
is 2700.
large-function-growth
Specifies maximal growth of large function caused by inlining
in percents. This parameter is ignored when -funit-at-a-time
is not used. 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
(consider unit consisting of function A that is inline and B
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that just calls A three time. 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 inlininable
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. This parameter is ignored when
-funit-at-a-time is not used. The default value is 50 which
limits unit growth to 1.5 times the original size.
max-inline-insns-recursive
max-inline-insns-recursive-auto
Specifies maximum number of instructions out-of-line copy of
self recursive inline function can grow into by performing
recursive inlining.
For functions declared inline --param max-inline-insns-
recursive is taken into account. For function not declared
inline, recursive inlining happens only when
-finline-functions (included in -O3) is enabled and --param
max-inline-insns-recursive-auto is used. The default value is
450.
max-inline-recursive-depth
max-inline-recursive-depth-auto
Specifies maximum recursion depth used by the recursive
inlining.
For functions declared inline --param max-inline-recursive-
depth is taken into account. For function not declared
inline, recursive inlining happens only when
-finline-functions (included in -O3) is enabled and --param
max-inline-recursive-depth-auto is used. The default value is
450.
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 will recurse via given call expression. This
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parameter limits inlining only to call expression whose
probability exceeds given threshold (in percents). The
default value is 10.
inline-call-cost
Specify cost of call instruction relative to simple
arithmetics operations (having cost of 1). Increasing this
cost disqualifies inlining of non-leaf functions and at the
same time increases size of leaf function that is believed to
reduce function size by being inlined. In effect it increases
amount of inlining for code having large abstraction penalty
(many functions that just pass the arguments to other
functions) and decrease inlining for code with low abstraction
penalty. The default value is 16.
max-unrolled-insns
The maximum number of instructions that a loop should have if
that loop is unrolled, and if the loop is unrolled, it
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 should have if that loop is
unrolled, and if the loop is unrolled, it 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 should have if
that loop is peeled, and if the loop is peeled, it determines
how many times the loop code is peeled.
max-peel-times
The maximum number of peelings of a single loop.
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-unswitch-insns
The maximum number of insns of an unswitched loop.
max-unswitch-level
The maximum number of branches unswitched in a single loop.
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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
that all candidates are considered for each use in induction
variable optimizations. Only the most relevant candidates are
considered if there are more candidates, 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 number of candidates in the set is smaller than this value,
we always try to remove unnecessary ivs from the set during
its optimization when a new iv is added to the set.
scev-max-expr-size
Bound on size of expressions used in the scalar evolutions
analyzer. Large expressions slow the analyzer.
vect-max-version-checks
The maximum number of runtime checks that can be performed
when doing loop versioning in the vectorizer. See option
ftree-vect-loop-version for more information.
max-iterations-to-track
The maximum number of iterations of a loop the brute force
algorithm for analysis of # of iterations of the loop tries to
evaluate.
hot-bb-count-fraction
Select fraction of the maximal count of repetitions of basic
block in program given basic block needs to have to be
considered hot.
hot-bb-frequency-fraction
Select fraction of the maximal 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 function contain single loop
with known bound and other loop with unknown. We predict the
known number of iterations correctly, while the unknown number
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of iterations average to roughly 10. This means that the loop
without bounds would appear artificially cold relative to the
other one.
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 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 rather hokey argument, as most of the
duplicates will be 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 do have 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
Maximum number of basic blocks on path that cse considers.
The default is 10.
max-cse-insns
The maximum instructions CSE process before flushing. The
default is 1000.
global-var-threshold
Counts the number of function calls (n) and the number of
call-clobbered variables (v). If nxv is larger than this
limit, a single artificial variable will be created to
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represent all the call-clobbered variables at function call
sites. This artificial variable will then be made to alias
every call-clobbered variable. (done as "int * size_t" on the
host machine; beware overflow).
max-aliased-vops
Maximum number of virtual operands allowed to represent
aliases before triggering the alias grouping heuristic. Alias
grouping reduces compile times and memory consumption needed
for aliasing at the expense of precision loss in alias
information.
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
which 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 compile time increase with
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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 compile time increase with probably slightly better
performance. The default value is 500.
max-flow-memory-locations
Similar as max-cselib-memory-locations but for dataflow
liveness. The default value is 100.
reorder-blocks-duplicate
reorder-blocks-duplicate-feedback
Used by 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 is used only when profile
feedback is available and 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-sched-region-insns
The maximum number of insns in a region to be considered for
interblock scheduling. The default value is 100.
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. 0 - disable region extension, N - do at most N
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iterations. The default value is 0.
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 insn will be scheduled. The default value
is 40.
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.
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's.
The default value is 256.
min-virtual-mappings
Specifies the minimum number of virtual mappings in the
incremental SSA updater that should be registered to trigger
the virtual mappings heuristic defined by
virtual-mappings-ratio. The default value is 100.
virtual-mappings-ratio
If the number of virtual mappings is virtual-mappings-ratio
bigger than the number of virtual symbols to be updated, then
the incremental SSA updater switches to a full update for
those symbols. The default ratio is 3.
ssp-buffer-size
The minimum size of buffers (i.e. arrays) that will receive
stack smashing protection when -fstack-protection is used.
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 we will treat in a
field sensitive manner during pointer analysis.
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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.
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 which GCC does not
know how to 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.
-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
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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 .
-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.
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-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.
-Wimport
Warn the first time #import is used.
-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.
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-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 basename of the source file with any suffix
replaced with object file suffix. 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
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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, including
any path, deletes 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
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$$(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 take the basename of the input file 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++
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-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:
"iso9899:1990"
"c89"
The ISO C standard from 1990. c89 is the customary shorthand
for this version of the standard.
The -ansi option is equivalent to -std=c89.
"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.
"gnu89"
The 1990 C standard plus GNU extensions. This is the default.
"gnu99"
"gnu9x"
The 1999 C standard plus GNU extensions.
"c++98"
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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
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exhausted. dir is treated as a system include directory.
-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. 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.
-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.
-fdollars-in-identifiers
Accept $ in identifiers.
-fextended-identifiers
Accept universal character names in identifiers. This option is
experimental; in a future version of GCC, it will be enabled by
default for C99 and C++.
-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
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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.
-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.
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-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.
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.
-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
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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.
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-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 which GCC does not know
how to 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.
-llibrary
-l library
Search the library named library when linking. (The second
alternative with the library as a separate argument is only for
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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 will be passed to the linker. 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 will
be passed to the linker. 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
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specified.
One of the standard libraries bypassed by -nostdlib and
-nodefaultlibs is libgcc.a, a library of internal subroutines that
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. (For example, __main, used to ensure C++
constructors will be called.)
-pie
Produce a position independent executable on targets which support
it. For predictable results, you must also specify the same set
of options that were used to generate code (-fpie, -fPIE, or model
suboptions) when you specify this option.
-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 that were used to generate code (-fpic, -fPIC, or
model suboptions) when you specify this 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.
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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 will not always be 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 will link the shared version of libgcc into
shared libraries by default. Otherwise, it will take advantage of
the linker and optimize 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.
-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.
-Xlinker option
Pass option as an option to the linker. You can use this to
supply system-specific linker options which GCC does not know how
to recognize.
If you want to pass an option that takes an 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.
-Wl,option
Pass option as an option to the linker. If option contains
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commas, it is split into multiple options at the commas.
-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.
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 will be
ignored. The directory will still be 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.
-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/.
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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 was not
specified, the driver tries two standard prefixes, which are
/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 will check to see if the path provided by the -B
refers to a directory, and if necessary it will add 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
includes 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 run-time 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 will be 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 that 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 would normally search for headers in
/usr/include and libraries in /usr/lib, it will instead search
dir/usr/include and dir/usr/lib.
If you use both this option and the -isysroot option, then the
--sysroot option will apply to libraries, but the -isysroot option
will apply to header files.
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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 will still work, but
the library aspect will not.
-I- This option has been deprecated. Please use -iquote instead for
-I directories before the -I- and remove the -I-. 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-, 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 which was current when the compiler was 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. Sometimes this
is inconvenient, so GCC provides options that will switch to another
cross-compiler or version.
-b machine
The argument machine specifies the target machine for compilation.
The value to use for machine is the same as was specified as the
machine type when configuring GCC as a cross-compiler. For
example, if a cross-compiler was configured with configure arm-
elf, meaning to compile for an arm processor with elf binaries,
then you would specify -b arm-elf to run that cross compiler.
Because there are other options beginning with -b, the
configuration must contain a hyphen.
-V version
The argument version specifies which version of GCC to run. This
is useful when multiple versions are installed. For example,
version might be 4.0, meaning to run GCC version 4.0.
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The -V and -b options work by running the <machine>-gcc-<version>
executable, so there's no real reason to use them if you can just run
that directly.
Hardware Models and Configurations
Earlier we discussed the standard option -b which chooses among
different installed compilers for completely different target
machines, such as VAX vs. 68000 vs. 80386.
In addition, each of these 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.
ARC Options
These options are defined for ARC implementations:
-EL Compile code for little endian mode. This is the default.
-EB Compile code for big endian mode.
-mmangle-cpu
Prepend the name of the cpu to all public symbol names. In
multiple-processor systems, there are many ARC variants with
different instruction and register set characteristics. This flag
prevents code compiled for one cpu to be linked with code compiled
for another. No facility exists for handling variants that are
"almost identical". This is an all or nothing option.
-mcpu=cpu
Compile code for ARC variant cpu. Which variants are supported
depend on the configuration. All variants support -mcpu=base,
this is the default.
-mtext=text-section
-mdata=data-section
-mrodata=readonly-data-section
Put functions, data, and readonly data in text-section, data-
section, and readonly-data-section respectively by default. This
can be overridden with the "section" attribute.
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ARM Options
These -m options are defined for Advanced RISC Machines (ARM)
architectures:
-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 will cause the stack frames
not to be generated for leaf functions. The default is
-mno-apcs-frame.
-mapcs
This is a synonym for -mapcs-frame.
-mthumb-interwork
Generate code which supports calling between the ARM and Thumb
instruction sets. Without this option 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.
-mno-sched-prolog
Prevent the reordering of instructions in the function prolog, or
the merging of those instruction with the instructions in the
function's body. This means that all functions will 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 if functions inside an executable
piece of code. The default is -msched-prolog.
-mhard-float
Generate output containing floating point instructions. This is
the default.
-msoft-float
Generate output containing library calls for floating point.
Warning: the requisite libraries are not available for all ARM
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;
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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.
-mfloat-abi=name
Specifies which ABI to use for floating point values. Permissible
values are: soft, softfp and hard.
soft and hard are equivalent to -msoft-float and -mhard-float
respectively. softfp allows the generation of floating point
instructions, but still uses the soft-float calling conventions.
-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.
-mwords-little-endian
This option only applies when generating code for big-endian
processors. Generate code for a little-endian word order but a
big-endian byte order. That is, a byte order of the form
32107654. Note: this option should only be used if you require
compatibility with code for big-endian ARM processors generated by
versions of the compiler prior to 2.8.
-mcpu=name
This specifies the name of the target ARM processor. GCC uses
this name to determine what kind of instructions it can emit when
generating assembly code. Permissible names are: arm2, arm250,
arm3, arm6, arm60, arm600, arm610, arm620, arm7, arm7m, arm7d,
arm7dm, arm7di, arm7dmi, arm70, arm700, arm700i, arm710, arm710c,
arm7100, arm7500, arm7500fe, arm7tdmi, arm7tdmi-s, arm8,
strongarm, strongarm110, strongarm1100, 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, arm1176jz-s, arm1176jzf-s, xscale, iwmmxt, ep9312.
-mtune=name
This option is very similar to the -mcpu= option, except that
instead of specifying the actual target processor type, and hence
restricting which instructions can be used, it specifies that GCC
should tune the performance of the code as if the target were of
the type specified in this option, but still choosing the
instructions that it will generate based on the cpu specified by a
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-mcpu= option. For some ARM implementations better performance
can be obtained by using this option.
-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,
armv5te, armv6, armv6j, iwmmxt, ep9312.
-mfpu=name
-mfpe=number
-mfp=number
This specifies what floating point hardware (or hardware
emulation) is available on the target. Permissible names are:
fpa, fpe2, fpe3, maverick, vfp. -mfp and -mfpe are synonyms for
-mfpu=fpenumber, for compatibility with older versions of GCC.
If -msoft-float is specified this specifies the format of floating
point values.
-mstructure-size-boundary=n
The size of all structures and unions will be 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 the 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 will be 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 will lie outside of the 64 megabyte addressing
range of the offset based version of subroutine call instruction.
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Even if this switch is enabled, not all function calls will be
turned into long calls. The heuristic is that static functions,
functions which 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, will not be turned into long calls. The
exception to this rule is 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, will always be turned into long calls.
This feature is not enabled by default. Specifying
-mno-long-calls will restore the default behavior, as will 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.
-mnop-fun-dllimport
Disable support for the "dllimport" attribute.
-msingle-pic-base
Treat the register used for PIC addressing as read-only, rather
than loading it in the prologue for each function. The run-time
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. The default
is R10 unless stack-checking is enabled, when R9 is used.
-mcirrus-fix-invalid-insns
Insert NOPs into the instruction stream to in order to work around
problems with invalid Maverick instruction combinations. This
option is only valid if the -mcpu=ep9312 option has been used to
enable generation of instructions for the Cirrus Maverick floating
point co-processor. This option is not enabled by default, since
the problem is only present in older Maverick implementations.
The default can be re-enabled by use of the
-mno-cirrus-fix-invalid-insns switch.
-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:
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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
Generate code for the 16-bit Thumb instruction set. The default
is to use the 32-bit ARM instruction set.
-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.
-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.
-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
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"cp15" directly (supported in the arm6k architecture), and auto,
which uses the best available method for the selected processor.
The default setting is auto.
AVR Options
These options are defined for AVR implementations:
-mmcu=mcu
Specify ATMEL AVR instruction set or MCU type.
Instruction set avr1 is for the minimal AVR core, not supported by
the C compiler, only for assembler programs (MCU types: at90s1200,
attiny10, attiny11, attiny12, attiny15, attiny28).
Instruction set avr2 (default) is for the classic AVR core with up
to 8K program memory space (MCU types: at90s2313, at90s2323,
attiny22, at90s2333, at90s2343, at90s4414, at90s4433, at90s4434,
at90s8515, at90c8534, at90s8535).
Instruction set avr3 is for the classic AVR core with up to 128K
program memory space (MCU types: atmega103, atmega603, at43usb320,
at76c711).
Instruction set avr4 is for the enhanced AVR core with up to 8K
program memory space (MCU types: atmega8, atmega83, atmega85).
Instruction set avr5 is for the enhanced AVR core with up to 128K
program memory space (MCU types: atmega16, atmega161, atmega163,
atmega32, atmega323, atmega64, atmega128, at43usb355, at94k).
-msize
Output instruction sizes to the asm file.
-minit-stack=N
Specify the initial stack address, which may be a symbol or
numeric value, __stack is the default.
-mno-interrupts
Generated code is not compatible with hardware interrupts. Code
size will be smaller.
-mcall-prologues
Functions prologues/epilogues expanded as call to appropriate
subroutines. Code size will be smaller.
-mno-tablejump
Do not generate tablejump insns which sometimes increase code
size.
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-mtiny-stack
Change only the low 8 bits of the stack pointer.
-mint8
Assume int to be 8 bit integer. This affects the sizes of all
types: A char will be 1 byte, an int will be 1 byte, an long will
be 2 bytes and long long will be 4 bytes. Please note that this
option does not comply to the C standards, but it will provide you
with smaller code size.
Blackfin Options
-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 will ensure that the generated code
does not contain speculative loads after jump instructions. This
option is enabled by default.
-mno-specld-anomaly
Don't generate extra code to prevent speculative loads from
occurring.
-mcsync-anomaly
When enabled, the compiler will ensure that the generated code
does not contain CSYNC or SSYNC instructions too soon after
conditional branches. This option is enabled by default.
-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.
-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
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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
Specified the identification number of the ID based shared library
being compiled. Specifying a value of 0 will generate more
compact code, specifying other values will force the allocation of
that number to the current library but is no more space or time
efficient than omitting this option.
-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 will lie 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 will restore the default behavior. Note these
switches have no effect on how the compiler generates code to
handle function calls via function pointers.
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.
-melinux-stacksize=n
Only available with the cris-axis-aout target. Arranges for
indications in the program to the kernel loader that the stack of
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the program should be set to 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 to turn 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) arranges (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
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With -mno-prologue-epilogue, the normal function prologue and
epilogue that sets 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 variable 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.
-maout
Legacy no-op option only recognized with the cris-axis-aout
target.
-melf
Legacy no-op option only recognized with the cris-axis-elf and
cris-axis-linux-gnu targets.
-melinux
Only recognized with the cris-axis-aout target, where it selects a
GNU/linux-like multilib, include files and instruction set for
-march=v8.
-mlinux
Legacy no-op option only recognized with the cris-axis-linux-gnu
target.
-sim
This option, recognized for the cris-axis-aout and 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.
CRX Options
These options are defined specifically for the CRX ports.
-mmac
Enable the use of multiply-accumulate instructions. Disabled by
default.
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-mpush-args
Push instructions will be used to pass outgoing arguments when
functions are called. Enabled by default.
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 will create
an object file for the single architecture that it 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 targetting,
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, will only permit instructions to be used
that are valid for the subtype of the file it is generating, so you
cannot put 64-bit instructions in an ppc750 object file. The linker
for shared libraries, /usr/bin/libtool, will fail and print 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, will quietly
give 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 will be
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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.
-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.
The default for this option is to make choices that seem to be
most useful.
-mkernel
Enable kernel development mode. The -mkernel option sets -static,
-fno-common, -fno-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 turn around development. Needed
to enable gdb to dynamically load ".o" files into already running
programs. -findirect-data and -ffix-and-continue are provided for
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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 be loading the
build output file being linked. See man ld(1) for more
information.
-dynamiclib
When passed this option, GCC will produce 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
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-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:
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-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 will be 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 will 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
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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
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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. Under DEC Unix, this has the effect that
IEEE-conformant math library routines will be linked in.
-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 will output the constant as a
literal and generate code to load it from the data segment at
runtime.
Use this option to require GCC to construct all integer constants
using code, even if it takes more instructions (the maximum is
six).
You would 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.
-malpha-as
-mgas
Select whether to generate code to be assembled by the vendor-
supplied assembler (-malpha-as) or by the GNU assembler -mgas.
-mbwx
-mno-bwx
-mcix
-mno-cix
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-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 was
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,
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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 will
choose the default values for the instruction set from the
processor you specify. If you do not specify a processor type,
GCC will default 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.
-mtune=cpu_type
Set only the instruction scheduling parameters for machine type
cpu_type. The instruction set is not changed.
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-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.
DEC Alpha/VMS Options
These -m options are defined for the DEC Alpha/VMS implementations:
-mvms-return-codes
Return VMS condition codes from main. The default is to return
POSIX style condition (e.g. error) codes.
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
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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, that 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.
-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
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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
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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.
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-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 instead of uClibc. This is the default
except on *-*-linux-*uclibc* targets.
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-muclibc
Use uClibc instead of the GNU C library. This is the default on
*-*-linux-*uclibc* targets.
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.
-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 will run
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.
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-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.
-mjump-in-delay
Fill delay slots of function calls with unconditional jump
instructions by modifying the return pointer for the function call
to be the target of the conditional jump.
-mdisable-fpregs
Prevent floating point registers from being used in any manner.
This is necessary for compiling kernels which perform lazy context
switching of floating point registers. If you use this option and
attempt to perform floating point operations, the compiler will
abort.
-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 which performs faster indirect calls.
This option will 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
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.
-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
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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. The embedded
target hppa1.1-*-pro does 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.
-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 GNU ld specific options. 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 have any affect on 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`.
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This option is only available on the 64 bit HP-UX GCC, i.e.
configured with hppa*64*-*-hpux*.
-mhp-ld
Use HP ld specific options. 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 have any
affect on 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 will degrade
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.
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-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.
Intel 386 and AMD x86-64 Options
These -m options are defined for the i386 and x86-64 family of
computers:
-mtune=cpu-type
Tune to cpu-type everything applicable about the generated code,
except for the ABI and the set of available instructions. The
choices for cpu-type are:
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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 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, the code generated option
will change to reflect the processors that were most common
when that version of GCC was 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.
native
This selects the CPU to tune for at compilation time by
determining the processor type of the compiling machine.
Using -mtune=native will produce code optimized for the local
machine under the constraints of the selected instruction set.
Using -march=native will enable all instruction subsets
supported by the local machine (hence the result might not run
on different machines).
i386
Original Intel's i386 CPU.
i486
Intel's i486 CPU. (No scheduling is implemented for this
chip.)
i586, pentium
Intel Pentium CPU with no MMX support.
pentium-mmx
Intel PentiumMMX CPU based on Pentium core with MMX
instruction set support.
pentiumpro
Intel PentiumPro CPU.
i686
Same as "generic", but when used as "march" option, PentiumPro
instruction set will be used, so the code will run on all i686
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family chips.
pentium2
Intel Pentium2 CPU based on PentiumPro core with MMX
instruction set support.
pentium3, pentium3m
Intel Pentium3 CPU based on PentiumPro core with MMX and SSE
instruction set support.
pentium-m
Low power version of Intel Pentium3 CPU with MMX, SSE and SSE2
instruction set support. Used by Centrino notebooks.
pentium4, pentium4m
Intel Pentium4 CPU with MMX, SSE and SSE2 instruction set
support.
prescott
Improved version of Intel Pentium4 CPU with MMX, SSE, SSE2 and
SSE3 instruction set support.
nocona
Improved version of Intel Pentium4 CPU with 64-bit extensions,
MMX, SSE, SSE2 and SSE3 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
AMD K8 core based CPUs with x86-64 instruction set support.
(This supersets MMX, SSE, SSE2, 3dNOW!, enhanced 3dNOW! and
64-bit instruction set extensions.)
winchip-c6
IDT Winchip C6 CPU, dealt in same way as i486 with additional
MMX instruction set support.
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winchip2
IDT Winchip2 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 CPU with MMX and SSE instruction set support. (No
scheduling is implemented for this chip.)
While picking a specific cpu-type will schedule things
appropriately for that particular chip, the compiler will not
generate any code that does not run on the i386 without the
-march=cpu-type option being used.
-march=cpu-type
Generate instructions for the machine type cpu-type. The choices
for cpu-type are the same as for -mtune. Moreover, specifying
-march=cpu-type implies -mtune=cpu-type.
-mcpu=cpu-type
A deprecated synonym for -mtune.
-m386
-m486
-mpentium
-mpentiumpro
These options are synonyms for -mtune=i386, -mtune=i486,
-mtune=pentium, and -mtune=pentiumpro respectively. These
synonyms are deprecated.
-mfpmath=unit
Generate floating point arithmetics for selected unit unit. The
choices for unit are:
387 Use the standard 387 floating point coprocessor present
majority of chips and emulated otherwise. Code compiled with
this option will run almost everywhere. The temporary results
are computed in 80bit precision instead of 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 i386 compiler.
sse Use scalar floating point instructions present in the SSE
instruction set. This instruction set is supported by
Pentium3 and newer chips, in the AMD line by Athlon-4,
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Athlon-xp and Athlon-mp chips. The earlier version of SSE
instruction set supports only single precision arithmetics,
thus the double and extended precision arithmetics is still
done using 387. Later version, present only in Pentium4 and
the future AMD x86-64 chips supports double precision
arithmetics too.
For the i386 compiler, you need to 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 80bit.
This is the default choice for the x86-64 compiler.
sse,387
Attempt to utilize both instruction sets at once. This
effectively double 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 instable
performance.
-masm=dialect
Output asm instructions using selected dialect. Supported choices
are intel or att (the default one). Darwin does not support
intel.
-mieee-fp
-mno-ieee-fp
Control whether or not the compiler uses IEEE floating point
comparisons. These handle correctly 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
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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 FreeBSD,
OpenBSD and NetBSD. This option is overridden when -march
indicates that the target cpu will always have an FPU and so the
instruction will not need emulation. As of revision 2.6.1, 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 will produce
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 will be aligned differently than the
published application binary interface specifications for the 386
and will not be 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 i386
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) would prefer "long
double" to be aligned to an 8 or 16 byte boundary. In arrays or
structures conforming to the ABI, this would not be possible. So
specifying a -m128bit-long-double will align "long double" to a 16
byte boundary by padding the "long double" with an additional 32
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bit zero.
In the x86-64 compiler, -m128bit-long-double is the default choice
as its ABI specifies that "long double" is to be 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,
the structures and arrays containing "long double" variables will
change their size as well as function calling convention for
function taking "long double" will be modified. Hence they will
not be binary compatible with arrays or structures in code
compiled without that switch.
-mmlarge-data-threshold=number
When -mcmodel=medium is specified, the data greater than threshold
are placed in large data section. This value must be the same
across all object linked into the binary and defaults to 65535.
-msvr3-shlib
-mno-svr3-shlib
Control whether GCC places uninitialized local variables into the
"bss" or "data" segments. -msvr3-shlib places them into "bss".
These options are meaningful only on System V Release 3.
-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 will be generated for calls to those functions.
In addition, seriously incorrect code will result if you call a
function with too many arguments. (Normally, extra arguments are
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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.
-mstackrealign
Realign the stack at entry. On the Intel x86, the -mstackrealign
option will generate an alternate prologue and epilogue that
realigns the runtime stack. This supports mixing legacy codes
that keep a 4-byte aligned stack with modern codes that keep a
16-byte stack for SSE compatibility. The alternate prologue and
epilogue are slower and bigger than the regular ones, and the
alternate prologue requires an extra scratch register; this lowers
the number of registers available if used in conjunction with the
"regparm" attribute. The -mstackrealign option is incompatible
with the nested function prologue; this is considered a hard
error. 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).
On Pentium and PentiumPro, "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
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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 will most likely misalign 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
-mno-mmx
-msse
-mno-sse
-msse2
-mno-sse2
-msse3
-mno-sse3
-m3dnow
-mno-3dnow
These switches enable or disable the use of instructions in the
MMX, SSE, SSE2 or 3DNow! extended instruction sets. 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 have SSE/SSE2 instructions generated automatically from
floating-point code (as opposed to 387 instructions), see
-mfpmath=sse.
These options will enable GCC to use these extended instructions
in generated code, even without -mfpmath=sse. Applications which
perform runtime 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.
-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 will be computed in the function prologue. This is
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faster on most modern CPUs because of reduced dependencies,
improved scheduling and reduced stack usage when 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 Mingw32. Code that
relies 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 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 destination is
known to be aligned at least to 4 byte boundary. This enables
more inlining, increase code size, but may improve performance of
code that depends on fast memcpy, strlen and memset for short
lengths.
-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.
-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 legal depends on the operating system, and whether it maps
the segment to cover the entire TLS area.
For systems that use GNU libc, the default is on.
These -m switches are supported in addition to the above on AMD x86-64
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 and generates
code that runs on any i386 system. The 64-bit environment sets
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int to 32 bits and long and pointer to 64 bits and generates code
for AMD's x86-64 architecture. For darwin only the -m64 option
turns off the -fno-pic and -mdynamic-no-pic options.
-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 will not be 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 but symbols can be located
anywhere in the address space. Programs can be statically or
dynamically linked, but building of shared libraries are not
supported with the medium model.
-mcmodel=large
Generate code for the large model: This model makes no assumptions
about addresses and sizes of sections. Currently GCC does not
implement this model.
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
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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.
-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
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maximum throughput algorithm.
-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-dwarf2-asm
-mdwarf2-asm
Don't (or do) generate assembler code for the DWARF2 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
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.
-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.
-mt
-pthread
Add support for multithreading using the POSIX threads library.
This option sets flags for both the preprocessor and linker. It
does not affect the thread safety of object code produced by the
compiler or that of libraries supplied with it. These are HP-UX
specific flags.
-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
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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 will
result in generation of the 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 will
result in generation of the 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 will result 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'.
-msched-ldc
-mno-sched-ldc
(En/Dis)able use of simple data speculation checks ld.c . If
disabled, only chk.a instructions will be emitted to check data
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speculative loads. The default is 'enable'.
-mno-sched-control-ldc
-msched-control-ldc
(Dis/En)able use of ld.c instructions to check control speculative
loads. If enabled, in case of control speculative load with no
speculatively scheduled dependent instructions this load will be
emitted as ld.sa and ld.c will be used to check it. The default
is 'disable'.
-mno-sched-spec-verbose
-msched-spec-verbose
(Dis/En)able printing of the information about speculative
motions.
-mno-sched-prefer-non-data-spec-insns
-msched-prefer-non-data-spec-insns
If enabled, data speculative instructions will be chosen for
schedule only if there are no other choices at the moment. This
will make 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 will be chosen for
schedule only if there are no other choices at the moment. This
will make 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 will be considered during
computation of the instructions priorities. This will make the
use of the speculation a bit more conservative. The default is
'disable'.
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
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needed.
-memregs=number
Specifies the number of memory-based pseudo-registers GCC will use
during code generation. These pseudo-registers will be 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 the default
runtime libraries gcc builds.
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 will generate "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 will generate "seth/add3" instructions to load their
addresses), and assume subroutines may not be reachable with the
"bl" instruction (the compiler will generate the much slower
"seth/add3/jl" instruction sequence).
-msdata=none
Disable use of the small data area. Variables will be put into
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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 will give an error message---incorrect code
will not be 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 will be
preferred over conditional code, if it is 2, then the opposite
will apply.
-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.
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-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
will only be 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 the 68000 series. The default
values for these options depends on which style of 68000 was selected
when the compiler was configured; the defaults for the most common
choices are given below.
-m68000
-mc68000
Generate output for a 68000. This is the default when the
compiler is configured for 68000-based systems.
Use this option for microcontrollers with a 68000 or EC000 core,
including the 68008, 68302, 68306, 68307, 68322, 68328 and 68356.
-m68020
-mc68020
Generate output for a 68020. This is the default when the
compiler is configured for 68020-based systems.
-m68881
Generate output containing 68881 instructions for floating point.
This is the default for most 68020 systems unless --nfp was
specified when the compiler was configured.
-m68030
Generate output for a 68030. This is the default when the
compiler is configured for 68030-based systems.
-m68040
Generate output for a 68040. This is the default when the
compiler is configured for 68040-based systems.
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
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Generate output for a 68060. This is the default when the
compiler is configured for 68060-based systems.
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.
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" family cpu. This is the
default when the compiler is configured for 520X-based systems.
Use this option for microcontroller with a 5200 core, including
the MCF5202, MCF5203, MCF5204 and MCF5202.
-mcfv4e
Generate output for a ColdFire V4e family cpu (e.g. 547x/548x).
This includes use of hardware floating point instructions.
-m68020-40
Generate output for a 68040, without using any of the new
instructions. This results in code which 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.
-m68020-60
Generate output for a 68060, without using any of the new
instructions. This results in code which 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.
-msoft-float
Generate output containing library calls for floating point.
Warning: the requisite libraries are not available for all m68k
targets. 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. The embedded
targets m68k-*-aout and m68k-*-coff do provide software floating
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point support.
-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.
-mnobitfield
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 will be generated for calls to those functions.
In addition, seriously incorrect code will result 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.
-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 will align
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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 will be
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
Specified the identification number of the ID based shared library
being compiled. Specifying a value of 0 will generate more
compact code, specifying other values will force the allocation of
that number to the current library but is no more space or time
efficient than omitting this option.
M68hc1x Options
These are the -m options defined for the 68hc11 and 68hc12
microcontrollers. The default values for these options depends on
which style of microcontroller was selected when the compiler was
configured; the defaults for the most common choices are given below.
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-m6811
-m68hc11
Generate output for a 68HC11. This is the default when the
compiler is configured for 68HC11-based systems.
-m6812
-m68hc12
Generate output for a 68HC12. This is the default when the
compiler is configured for 68HC12-based systems.
-m68S12
-m68hcs12
Generate output for a 68HCS12.
-mauto-incdec
Enable the use of 68HC12 pre and post auto-increment and auto-
decrement addressing modes.
-minmax
-nominmax
Enable the use of 68HC12 min and max instructions.
-mlong-calls
-mno-long-calls
Treat all calls as being far away (near). If calls are assumed to
be far away, the compiler will use the "call" instruction to call
a function and the "rtc" instruction for returning.
-mshort
Consider type "int" to be 16 bits wide, like "short int".
-msoft-reg-count=count
Specify the number of pseudo-soft registers which are used for the
code generation. The maximum number is 32. Using more pseudo-
soft register may or may not result in better code depending on
the program. The default is 4 for 68HC11 and 2 for 68HC12.
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).
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-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 four 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.
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 will run 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, and
mips64. The processor names are: 4kc, 4km, 4kp, 5kc, 5kf, 20kc,
24k, 24kc, 24kf, 24kx, m4k, orion, r2000, r3000, r3900, r4000,
r4400, r4600, r4650, r6000, r8000, rm7000, rm9000, sb1, sr71000,
vr4100, vr4111, vr4120, vr4130, vr4300, vr5000, vr5400 and vr5500.
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).
In processor names, a final 000 can be abbreviated as k (for
example, -march=r2k). Prefixes are optional, and vr may be
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written r.
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
will set _MIPS_ARCH to "r2000" and define the macro
_MIPS_ARCH_R2000.
Note that the _MIPS_ARCH macro uses the processor names given
above. In other words, it will have the full prefix and will 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 will optimize for the processor
specified by -march. By using -march and -mtune together, it is
possible to generate code that will run 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.
-mips32r2
Equivalent to -march=mips32r2.
-mips64
Equivalent to -march=mips64.
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-mips16
-mno-mips16
Generate (do not generate) MIPS16 code. If GCC is targetting a
MIPS32 or MIPS64 architecture, it will make use of the MIPS16e
ASE.
-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>.
-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 will
generally make executables both smaller and quicker.
-mshared is the default.
-mxgot
-mno-xgot
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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 will only work if the
GOT is smaller than about 64k. Anything larger will cause 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. It
should then work with very large GOTs, although it will also be
less efficient, since it will take 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.
-mhard-float
Use floating-point coprocessor instructions.
-msoft-float
Do not use floating-point coprocessor instructions. Implement
floating-point calculations using library calls instead.
-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.
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-mdsp
-mno-dsp
Use (do not use) the MIPS DSP ASE.
-mpaired-single
-mno-paired-single
Use (do not use) paired-single floating-point 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.
-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 global and static items less than or equal to num bytes into
the small data or bss section instead of the normal data or bss
section. This allows the data to be accessed using a single
instruction.
All modules should be compiled with the same -G num value.
-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
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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.
-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.
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-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.
-mfused-madd
-mno-fused-madd
Enable (disable) use of the floating point multiply-accumulate
instructions, when they are available. The default is
-mfused-madd.
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.
-nocpp
Tell the MIPS assembler to not run its preprocessor over user
assembler files (with a .s suffix) when assembling them.
-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.
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-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 will
avoid 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.)
-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-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 which implement those architectures;
for those, Branch Likely instructions will not be generated by
default because the MIPS32 and MIPS64 architectures specifically
deprecate their use.
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-mfp-exceptions
-mno-fp-exceptions
Specifies whether FP exceptions are enabled. This affects how we
schedule FP instructions 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 will align 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.
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
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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
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Generate code which uses features specific to the AM33 processor.
-mno-am33
Do not generate code which uses features specific to the AM33
processor. This is the default.
-mreturn-pointer-on-d0
When generating a function which 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 would 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.
MT Options
These -m options are defined for Morpho MT architectures:
-march=cpu-type
Generate code that will run on cpu-type, which is the name of a
system representing a certain processor type. Possible values for
cpu-type are ms1-64-001, ms1-16-002, ms1-16-003 and ms2.
When this option is not used, the default is -march=ms1-16-002.
-mbacc
Use byte loads and stores when generating code.
-mno-bacc
Do not use byte loads and stores when generating code.
-msim
Use simulator runtime
-mno-crt0
Do not link in the C run-time initialization object file crti.o.
Other run-time initialization and termination files such as
startup.o and exit.o are still included on the linker command
line.
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2008-02-01
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".
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-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.
-msplit
Generate code for a system with split I&D.
-mno-split
Generate code for a system without split I&D. 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.
PowerPC Options
These are listed under
IBM RS/6000 and PowerPC Options
These -m options are defined for the IBM RS/6000 and PowerPC:
-mpower
-mno-power
-mpower2
-mno-power2
-mpowerpc
-mno-powerpc
-mpowerpc-gpopt
-mno-powerpc-gpopt
-mpowerpc-gfxopt
-mno-powerpc-gfxopt
-mpowerpc64
-mno-powerpc64
-mmfcrf
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-mno-mfcrf
-mpopcntb
-mno-popcntb
-mfprnd
-mno-fprnd
GCC supports two related instruction set architectures for the
RS/6000 and PowerPC. The POWER instruction set are those
instructions supported by the rios chip set used in the original
RS/6000 systems and the PowerPC instruction set is the
architecture of the Freescale MPC5xx, MPC6xx, MPC8xx
microprocessors, and the IBM 4xx, 6xx, and follow-on
microprocessors.
Neither architecture is a subset of the other. However there is a
large common subset of instructions supported by both. An MQ
register is included in processors supporting the POWER
architecture.
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.
The -mpower option allows GCC to generate instructions that are
found only in the POWER architecture and to use the MQ register.
Specifying -mpower2 implies -power and also allows GCC to generate
instructions that are present in the POWER2 architecture but not
the original POWER architecture.
The -mpowerpc option allows GCC to generate instructions that are
found only in the 32-bit subset of the PowerPC architecture.
Specifying -mpowerpc-gpopt implies -mpowerpc and also allows GCC
to use the optional PowerPC architecture instructions in the
General Purpose group, including floating-point square root.
Specifying -mpowerpc-gfxopt implies -mpowerpc and also 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 -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
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architecture.
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.
If you specify both -mno-power and -mno-powerpc, GCC will use only
the instructions in the common subset of both architectures plus
some special AIX common-mode calls, and will not use the MQ
register. Specifying both -mpower and -mpowerpc permits GCC to
use any instruction from either architecture and to allow use of
the MQ register; specify this for the Motorola MPC601.
-mnew-mnemonics
-mold-mnemonics
Select which mnemonics to use in the generated assembler code.
With -mnew-mnemonics, GCC uses the assembler mnemonics defined for
the PowerPC architecture. With -mold-mnemonics it uses the
assembler mnemonics defined for the POWER architecture.
Instructions defined in only one architecture have only one
mnemonic; GCC uses that mnemonic irrespective of which of these
options is specified.
GCC defaults to the mnemonics appropriate for the architecture in
use. Specifying -mcpu=cpu_type sometimes overrides the value of
these option. Unless you are building a cross-compiler, you
should normally not specify either -mnew-mnemonics or
-mold-mnemonics, but should instead accept the default.
-mcpu=cpu_type
Set architecture type, register usage, choice of mnemonics, and
instruction scheduling parameters for machine type cpu_type.
Supported values for cpu_type are 401, 403, 405, 405fp, 440,
440fp, 505, 601, 602, 603, 603e, 604, 604e, 620, 630, 740, 7400,
7450, 750, 801, 821, 823, 860, 970, 8540, ec603e, G3, G4, G5,
power, power2, power3, power4, power5, power5+, power6, common,
powerpc, powerpc64, rios, rios1, rios2, rsc, and rs64.
-mcpu=common selects a completely generic processor. Code
generated under this option will run on any POWER or PowerPC
processor. GCC will use only the instructions in the common
subset of both architectures, and will not use the MQ register.
GCC assumes a generic processor model for scheduling purposes.
-mcpu=power, -mcpu=power2, -mcpu=powerpc, and -mcpu=powerpc64
specify generic POWER, POWER2, pure 32-bit PowerPC (i.e., not
MPC601), and 64-bit PowerPC architecture machine types, with an
appropriate, generic processor model assumed for scheduling
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purposes.
The other options specify a specific processor. Code generated
under those options will run 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,
-mnew-mnemonics, -mpopcntb, -mpower, -mpower2, -mpowerpc64,
-mpowerpc-gpopt, -mpowerpc-gfxopt, -mstring, -mmulhw, -mdlmzb.
The particular options set for any particular CPU will vary
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, register usage, or
choice of mnemonics, as -mcpu=cpu_type would. The same values for
cpu_type are used for -mtune as for -mcpu. If both are specified,
the code generated will use the architecture, registers, and
mnemonics set by -mcpu, but the scheduling parameters set by
-mtune.
-mswdiv
-mno-swdiv
Generate code to compute division as reciprocal estimate and
iterative refinement, creating opportunities for increased
throughput. This feature requires: optional PowerPC Graphics
instruction set for single precision and FRE instruction for
double precision, assuming divides cannot generate user-visible
traps, and the domain values not include Infinities, denormals or
zero denominator.
-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.
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-mvrsave
-mno-vrsave
Generate VRSAVE instructions when generating AltiVec code.
-msecure-plt
Generate code that allows ld and ld.so to build executables and
shared libraries with non-exec .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.
-mspe=yes/no
This option has been deprecated. Use -mspe and -mno-spe instead.
-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
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-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 will allocate at least one TOC entry
for each unique non-automatic variable reference in your program.
GCC will also place 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 will produce 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 and -mpowerpc, 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
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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 which
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.
-mmultiple
-mno-multiple
Generate code that uses (does not use) the load multiple word
instructions and the store multiple word instructions. These
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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 the instructions usage 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 the instructions usage 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.
-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 is used.
-mmulhw
-mno-mulhw
Generate code that uses (does not use) the half-word multiply and
multiply-accumulate instructions on the IBM 405 and 440
processors. These instructions are generated by default when
targetting those processors.
-mdlmzb
-mno-dlmzb
Generate code that uses (does not use) the string-search dlmzb
instruction on the IBM 405 and 440 processors. This instruction
is generated by default when targetting those processors.
-mno-bit-align
-mbit-align
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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 would be aligned to a 4 byte
boundary and have a size of 4 bytes. By using -mno-bit-align, the
structure would be aligned to a 1 byte boundary and be one byte in
size.
-mno-strict-align
-mstrict-align
On System V.4 and embedded PowerPC systems do not (do) assume that
unaligned memory references will be handled by the system.
-mrelocatable
-mno-relocatable
On embedded PowerPC systems generate code that allows (does not
allow) the program to be relocated to a different address at
runtime. If you use -mrelocatable on any module, all objects
linked together must be compiled with -mrelocatable or
-mrelocatable-lib.
-mrelocatable-lib
-mno-relocatable-lib
On embedded PowerPC systems generate code that allows (does not
allow) the program to be relocated to a different address at
runtime. Modules compiled with -mrelocatable-lib can be linked
with either modules compiled without -mrelocatable and
-mrelocatable-lib or with modules compiled with 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.
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-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.
-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 0/1/2 to assign
no/highest/second-highest 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 which latency >= number is costly.
-minsert-sched-nops=scheme
This option controls which nop insertion scheme will be 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 which 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 adheres 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
Specify both -mcall-sysv and -meabi options.
-mcall-sysv-noeabi
Specify both -mcall-sysv and -mno-eabi options.
-mcall-solaris
On System V.4 and embedded PowerPC systems compile code for the
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Solaris operating system.
-mcall-linux
On System V.4 and embedded PowerPC systems compile code for the
Linux-based GNU system.
-mcall-gnu
On System V.4 and embedded PowerPC systems compile code for the
Hurd-based GNU system.
-mcall-netbsd
On System V.4 and embedded PowerPC systems compile code for the
NetBSD 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.
-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 Booke 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.
-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 were
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passed in the floating point registers in case the function takes
a variable arguments. With -mprototype, only calls to prototyped
variable argument functions will 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.
-mwindiss
Specify that you are compiling for the WindISS simulation
environment.
-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 to 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, do not call an
initialization function from "main", and the -msdata option will
only use "r13" to point to a single small data area. The -meabi
option is on by default if you configured GCC using one of the
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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.
-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
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forms.
-mlongcall
-mno-longcall
By default assume that all calls are far away so that a longer
more expensive calling sequence is required. This is required for
calls further than 32 megabytes (33,554,432 bytes) from the
current location. A short call will be 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" will generate "jbsr
callee, L42", plus a "branch island" (glue code). The two target
addresses represent the callee and the "branch island". The
Darwin/PPC linker will prefer the first address and generate a "bl
callee" if the PPC "bl" instruction will reach the callee
directly; otherwise, the linker will generate "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, we may cause GCC to ignore all longcall
specifications when the linker is known to generate glue.
-pthread
Adds support for multithreading with the pthreads library. This
option sets flags for both the preprocessor and linker.
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
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specified, functions in libgcc.a will be used to perform
floating-point operations. When -mhard-float is specified, the
compiler generates IEEE floating-point instructions. This is the
default.
-mlong-double-64
-mlong-double-128
These switches control the size of "long double" type. A size of
64bit 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.
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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 will run on cpu-type, which is the name of a
system representing a certain processor type. Possible values for
cpu-type are g5, g6, z900, and z990. When generating code using
the instructions available on z/Architecture, the default is
-march=z900. Otherwise, the default is -march=g5.
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-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 which 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
These arguments always have to be used in conjunction. If they
are present the s390 back end emits additional instructions in the
function prologue which trigger a trap if the stack size is
stack-guard bytes above the stack-size (remember that the stack on
s390 grows downward). 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.
Score Options
These options are defined for Score implementations:
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-meb
Compile code for big endian mode. This is the default.
-mel
Compile code for little endian mode.
-mnhwloop
Disable generate bcnz instruction.
-muls
Enable generate unaligned load and store instruction.
-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.
-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
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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.
-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.
-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 will 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.
-mfmovd
Enable the use of the instruction "fmovd".
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-mhitachi
Comply with the calling conventions defined by Renesas.
-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 except for sh-symbianelf.
-mnomacsave
Mark the "MAC" register as call-clobbered, even if -mhitachi is
given.
-mieee
Increase IEEE-compliance of floating-point code. At the moment,
this is equivalent to -fno-finite-math-only. When generating 16
bit SH opcodes, getting IEEE-conforming results for comparisons of
NANs / infinities incurs extra overhead in every floating point
comparison, therefore the default is set to -ffinite-math-only.
-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.
-mspace
Optimize for space instead of speed. Implied by -Os.
-mprefergot
When generating position-independent code, emit function calls
using the Global Offset Table instead of the Procedure Linkage
Table.
-musermode
Generate a library function call to invalidate instruction cache
entries, after fixing up a trampoline. This library function call
doesn't assume it can write to the whole memory address space.
This is the default when the target is "sh-*-linux*".
-multcost=number
Set the cost to assume for a multiply insn.
-mdiv=strategy
Set the division strategy to use for SHmedia code. strategy must
be one of: call, call2, fp, inv, inv:minlat, inv20u, inv20l,
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inv:call, inv:call2, inv:fp . "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" is a variant of "inv" where if no cse / 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 / code hoisting optimizations.
"inv:call", "inv:call2" and "inv:fp" all 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 fp operations
or a call is not possible in that case. "inv20u" and "inv20l" are
variants of the "inv:minlat" strategy. In the case that the
inverse calculation was nor 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.
-mdivsi3_libfunc=name
Set the name of the library function used for 32 bit signed
division to name. This only affect the name used in the call and
inv:call division strategies, and the compiler will still expect
the same sets of input/output/clobbered registers as if this
option was not present.
-madjust-unroll
Throttle unrolling to avoid thrashing target registers. This
option only has an effect if the gcc code base supports the
TARGET_ADJUST_UNROLL_MAX target hook.
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-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 will generally generate
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 ptabs / ptrel before a
branch, or hoist it 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 will be done before
testing against -1. That means that all the constructors will be
run a bit quicker, 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 the user specifies a specific cost with -mgettrcost,
-mno-pt-fixed also implies -mgettrcost=100; this deters register
allocation using target registers for storing ordinary integers.
-minvalid-symbols
Assume symbols might be invalid. Ordinary function symbols
generated by the compiler will always be 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 will cause
ptabs / ptrel to trap. This option is only meaningful when
-mno-pt-fixed is in effect. It will then prevent cross-basic-
block cse, hoisting and most scheduling of symbol loads. The
default is -mno-invalid-symbols.
SPARC Options
These -m options are supported on the SPARC:
-mno-app-regs
-mapp-regs
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Specify -mapp-regs to generate output using the global registers 2
through 4, which the SPARC SVR4 ABI reserves for applications.
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.
-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
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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.
-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 will not be directly in line with the
rules of the ABI.
-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 will 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.
This option is only available on SunOS and Solaris.
-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, sparclite, f930, f934,
hypersparc, sparclite86x, sparclet, tsc701, v9, ultrasparc,
ultrasparc3, and niagara.
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.
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v7: cypress
v8: supersparc, hypersparc
sparclite: f930, f934, sparclite86x
sparclet: tsc701
v9: ultrasparc, ultrasparc3, niagara
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.
-mtune=cpu_type
Set the instruction scheduling parameters for machine type
cpu_type, but do not set the instruction set or register set that
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the option -mcpu=cpu_type would.
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, f930, f934, sparclite86x, tsc701, ultrasparc,
ultrasparc3, and niagara.
-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-bit 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.
These -m options are supported in addition to the above on SPARC-V9
processors in 64-bit environments:
-mlittle-endian
Generate code for a processor running in little-endian mode. It
is only available for a few configurations and most notably not on
Solaris and Linux.
-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=medlow
Generate code for 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.
-mcmodel=medmid
Generate code for 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.
-mcmodel=medany
Generate code for the Medium/Anywhere code model: 64-bit
addresses, programs may be linked anywhere in memory, the text and
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data segments must be less than 2GB in size and the data segment
must be located within 2GB of the text segment.
-mcmodel=embmedany
Generate code for 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.
-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.
These switches are supported in addition to the above on Solaris:
-threads
Add support for multithreading using the Solaris 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.
-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.
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).
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-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.
TMS320C3x/C4x Options
These -m options are defined for TMS320C3x/C4x implementations:
-mcpu=cpu_type
Set the instruction set, register set, and instruction scheduling
parameters for machine type cpu_type. Supported values for
cpu_type are c30, c31, c32, c40, and c44. The default is c40 to
generate code for the TMS320C40.
-mbig-memory
-mbig
-msmall-memory
-msmall
Generates code for the big or small memory model. The small
memory model assumed that all data fits into one 64K word page.
At run-time the data page (DP) register must be set to point to
the 64K page containing the .bss and .data program sections. The
big memory model is the default and requires reloading of the DP
register for every direct memory access.
-mbk
-mno-bk
Allow (disallow) allocation of general integer operands into the
block count register BK.
-mdb
-mno-db
Enable (disable) generation of code using decrement and branch,
DBcond(D), instructions. This is enabled by default for the C4x.
To be on the safe side, this is disabled for the C3x, since the
maximum iteration count on the C3x is 2^{23 + 1} (but who iterates
loops more than 2^{23} times on the C3x?). Note that GCC will try
to reverse a loop so that it can utilize the decrement and branch
instruction, but will give up if there is more than one memory
reference in the loop. Thus a loop where the loop counter is
decremented can generate slightly more efficient code, in cases
where the RPTB instruction cannot be utilized.
-mdp-isr-reload
-mparanoid
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Force the DP register to be saved on entry to an interrupt service
routine (ISR), reloaded to point to the data section, and restored
on exit from the ISR. This should not be required unless someone
has violated the small memory model by modifying the DP register,
say within an object library.
-mmpyi
-mno-mpyi
For the C3x use the 24-bit MPYI instruction for integer multiplies
instead of a library call to guarantee 32-bit results. Note that
if one of the operands is a constant, then the multiplication will
be performed using shifts and adds. If the -mmpyi option is not
specified for the C3x, then squaring operations are performed
inline instead of a library call.
-mfast-fix
-mno-fast-fix
The C3x/C4x FIX instruction to convert a floating point value to
an integer value chooses the nearest integer less than or equal to
the floating point value rather than to the nearest integer. Thus
if the floating point number is negative, the result will be
incorrectly truncated an additional code is necessary to detect
and correct this case. This option can be used to disable
generation of the additional code required to correct the result.
-mrptb
-mno-rptb
Enable (disable) generation of repeat block sequences using the
RPTB instruction for zero overhead looping. The RPTB construct is
only used for innermost loops that do not call functions or jump
across the loop boundaries. There is no advantage having nested
RPTB loops due to the overhead required to save and restore the
RC, RS, and RE registers. This is enabled by default with -O2.
-mrpts=count
-mno-rpts
Enable (disable) the use of the single instruction repeat
instruction RPTS. If a repeat block contains a single
instruction, and the loop count can be guaranteed to be less than
the value count, GCC will emit a RPTS instruction instead of a
RPTB. If no value is specified, then a RPTS will be emitted even
if the loop count cannot be determined at compile time. Note that
the repeated instruction following RPTS does not have to be
reloaded from memory each iteration, thus freeing up the CPU buses
for operands. However, since interrupts are blocked by this
instruction, it is disabled by default.
-mloop-unsigned
-mno-loop-unsigned
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The maximum iteration count when using RPTS and RPTB (and DB on
the C40) is 2^{31 + 1} since these instructions test if the
iteration count is negative to terminate the loop. If the
iteration count is unsigned there is a possibility than the 2^{31
+ 1} maximum iteration count may be exceeded. This switch allows
an unsigned iteration count.
-mti
Try to emit an assembler syntax that the TI assembler (asm30) is
happy with. This also enforces compatibility with the API
employed by the TI C3x C compiler. For example, long doubles are
passed as structures rather than in floating point registers.
-mregparm
-mmemparm
Generate code that uses registers (stack) for passing arguments to
functions. By default, arguments are passed in registers where
possible rather than by pushing arguments on to the stack.
-mparallel-insns
-mno-parallel-insns
Allow the generation of parallel instructions. This is enabled by
default with -O2.
-mparallel-mpy
-mno-parallel-mpy
Allow the generation of MPY||ADD and MPY||SUB parallel
instructions, provided -mparallel-insns is also specified. These
instructions have tight register constraints which can pessimize
the code generation of large functions.
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 will always load the functions address
up into a register, and call 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
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-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.
-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 will cause r2 and r5 to be used in the code generated
by the compiler. This setting is the default.
-mno-app-regs
This option will cause r2 and r5 to be treated as fixed registers.
-mv850e1
Specify that the target processor is the V850E1. The preprocessor
constants __v850e1__ and __v850e__ will be defined if this option
is used.
-mv850e
Specify that the target processor is the V850E. The preprocessor
constant __v850e__ will be defined if this option is used.
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If neither -mv850 nor -mv850e nor -mv850e1 are defined then a
default target processor will be chosen and the relevant __v850*__
preprocessor constant will be defined.
The preprocessor constants __v850 and __v851__ are always defined,
regardless of which processor variant is the target.
-mdisable-callt
This option will suppress generation of the CALLT instruction for
the v850e and v850e1 flavors of the v850 architecture. The
default is -mno-disable-callt which allows the CALLT instruction
to be used.
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 you will
assemble with the GNU assembler.
-mg Output code for g-format floating point numbers instead of
d-format.
x86-64 Options
These are listed under
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"
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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.
-mtext-section-literals
-mno-text-section-literals
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 will be
performed. The default is -mtarget-align. These options do not
affect the treatment of auto-aligned instructions like "LOOP",
which the assembler will always align, either by widening density
instructions or by inserting no-op 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
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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
will still show direct call instructions---look at the
disassembled object code to see the actual instructions. Note
that the assembler will use an indirect call for every cross-file
call, not just those that really will be 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 would be -fno-foo. In the table below, only one of the forms
is listed---the one which 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.
-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 will
generate frame unwind information for all functions, which can
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produce significant data size overhead, although it does not
affect execution. If you do not specify this option, GCC will
enable it by default for languages like C++ which normally require
exception handling, and disable 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".
-funwind-tables
Similar to -fexceptions, except that it will just generate any
needed static data, but will not affect the generated code in any
other way. You will normally not enable this option; instead, a
language processor that needs this handling would enable it on
your behalf.
-fasynchronous-unwind-tables
Generate unwind table in dwarf2 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).
-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.
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-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
will be equivalent to the smallest integer type which 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, allocate even 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
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(without "extern") in two different compilations, you will get an
error when you link them. The only reason this might be useful is
if you wish to verify that the program will work on other systems
which always work 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.
-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 386 has no
such limit.)
Position-independent code requires special support, and therefore
works only on certain machines. For the 386, 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
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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 will be used
during linking.
-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 which 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 will not save and restore the register reg.
It is an error to used 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 will produce
disastrous results.
This flag does not have a negative form, because it specifies a
three-way choice.
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-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 will save
and restore the register reg if they use it.
It is an error to used 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 will produce
disastrous results.
A different sort of disaster will result 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 will be 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 will be 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 will indicate where,
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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 anyways,
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 will not be 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).
-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 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 must do that. The switch causes generation
of code to ensure that the operating system sees the stack being
extended.
-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 the stack would grow beyond the value, a signal is
raised. 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.
-fargument-alias
-fargument-noalias
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-fargument-noalias-global
-fargument-noalias-anything
Specify the possible relationships among parameters and between
parameters and global data.
-fargument-alias specifies that arguments (parameters) may alias
each other and may alias global storage.-fargument-noalias
specifies that arguments do not alias each other, but may alias
global storage.-fargument-noalias-global specifies that arguments
do not alias each other and do not alias global storage.
-fargument-noalias-anything specifies that arguments do not alias
any other storage.
Each language will automatically use whatever option is required
by the language standard. You should not need to use these
options yourself.
-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".
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 will be 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 ie;
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 will be "hidden". The
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default if -fvisibility isn't specified is "default", i.e., make
every symbol public---this causes the same behavior as previous
versions of GCC.
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://people.redhat.com/~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 DLL's 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 ie; 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 will 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 will be
thrown between DSOs must be explicitly marked with default
visibility so that the type_info nodes will be unified between the
DSOs.
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An overview of these techniques, their benefits and how to use
them is at <http://gcc.gnu.org/wiki/Visibility>.
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 that allow 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 would otherwise be
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_EXEC_PREFIX
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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 will attempt to figure out an
appropriate prefix to use based on the pathname it was 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 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 will search
foo/bar where it would normally search /usr/local/lib/bar. These
alternate directories are searched first; the standard directories
come next.
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
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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 will use mblen and mbtowc as defined by the default
locale to recognize and translate multibyte characters.
Some additional environments 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
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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
<http://gcc.gnu.org/bugs.html>.
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, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998,
1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006 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 1.2 or
any later version published by the Free Software Foundation; with the
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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.
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