GCC(1)
💖 GCC(1) — GNU 💖
okay bestie, buckle up, this is the manual page for gcc — the GNU project C and C++ compiler, straight from the GNU girlies themselves. it's giving legendary, it's giving foundational, it's giving "your entire build system depends on me and I know it." 👑
this one is long, so it comes in 131 parts and this is part 1 — we're doing the header, the name, the synopsis, the whole description, and the giant option summary. more parts are coming, so don't go anywhere. 🎀
✨ NAME ✨
gcc — GNU project C and C++ compiler
🌸 SYNOPSIS 🌸
gcc [-c|-S|-E] [-std=standard]
[-g] [-pg] [-Olevel]
[-Wwarn...] [-Wpedantic]
[-Idir...] [-Ldir...]
[-Dmacro[=defn]...] [-Umacro]
[-foption...] [-mmachine-option...]
[-o outfile] [@file] infile...okay so real talk: that's only the most useful options up there. the rest of them live further down this page, and there are so many more. also <cyan>g++</cyan> accepts mostly the same options as gcc, so if you're a C++ girlie you're still in the right place. 🫶
💅 DESCRIPTION 💅
When you invoke GCC, it normally does a whole little four-step routine: preprocessing, compilation, assembly, and linking. ✨ The "overall options" are the ones that let you stop that process at an intermediate stage. Like, -c says "bestie, do not run the linker" — and then your output is just object files that the assembler produced. cute, right?
Other options get passed along to one or more of those processing stages. Some options control the preprocessor, others control the compiler itself, and yet others boss around the assembler and the linker — most of those aren't documented here, because honestly you rarely need to touch any of them.
Most of the command-line options 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. And if the description for a particular option doesn't mention a source language at all? Then you can use that option with all supported languages. periodt. 💅
The usual way to run GCC is to run the executable called gcc, or machine-gcc when you're cross-compiling, or machine-gcc-version to run one specific version of GCC. When you compile C++ programs, you should invoke GCC as g++ instead. that's the vibe check, and it matters.
The gcc program accepts options and file names as operands. A lot of options have multi-letter names, which means multiple single-letter options may not be grouped together: -dv is very different from -d -v. not me learning that the hard way 😭
You can mix options and other arguments, and for the most part the order you use doesn't matter. Order does matter when you use several options of the same kind — for example, if you specify -L more than once, the directories are searched in the order specified. Also, the placement of the -l option is significant. lowkey load-bearing.
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 a positive and a negative form; the negative form of -ffoo is -fno-foo. This manual documents only one of those two forms — whichever one isn't the default. iconic efficiency honestly 👑
Some options take one or more arguments, typically separated from the option name either by a space or by the equals sign (=). Unless documented otherwise, an argument can be either numeric or a string. Numeric arguments must typically be small unsigned decimal or hexadecimal integers, and hexadecimal arguments must begin with the 0x prefix. Arguments to options that specify a size threshold of some sort may be arbitrarily large decimal or hexadecimal integers followed by a byte size suffix designating a multiple of bytes — such as "kB" and "KiB" for kilobyte and kibibyte, respectively, "MB" and "MiB" for megabyte and mebibyte, "GB" and "GiB" for gigabyte and gigibyte, and so on. Such arguments are designated by byte-size in the following text. For the full listing and explanation of the binary and decimal byte size prefixes, refer to the NIST, IEC, and other relevant national and international standards. 🌈
👑 OPTIONS 👑
🦋 Option Summary 🦋
Here is a summary of all the options, grouped by type. The explanations are in the following sections — this is just the ✨ menu ✨.
🌟 Overall Options
the main character switches — the ones that decide which stages even happen:
-c -S -E -o file -dumpbase dumpbase -dumpbase-ext auxdropsuf
-dumpdir dumppfx -x language -v -### --help[=class[,...]]
--target-help --version -pass-exit-codes -pipe -specs=file
-wrapper @file -ffile-prefix-map=old=new -fcanon-prefix-map
-fplugin=file -fplugin-arg-name=arg -fdump-ada-spec[-slim]
-fada-spec-parent=unit -fdump-go-spec=file🍓 C Language Options
the C girlies' home turf:
-ansi -std=standard -aux-info filename -fno-asm -fno-builtin
-fno-builtin-function -fcond-mismatch -ffreestanding -fgimple
-fgnu-tm -fgnu89-inline -fhosted -flax-vector-conversions
-fms-extensions -fpermitted-flt-eval-methods=standard
-fplan9-extensions -fsigned-bitfields -funsigned-bitfields
-fsigned-char -funsigned-char -fstrict-flex-arrays[=n]
-fsso-struct=endianness💜 C++ Language Options
a lot of options. she's thorough. she's committed. she ate:
-fabi-version=n -fno-access-control -faligned-new=n
-fno-assume-sane-operators-new-delete -fchar8_t -fcheck-new
-fconcepts -fconstexpr-depth=n -fconstexpr-cache-depth=n
-fconstexpr-loop-limit=n -fconstexpr-ops-limit=n
-fno-elide-constructors -fno-enforce-eh-specs -fno-gnu-keywords
-fno-immediate-escalation -fno-implicit-templates
-fno-implicit-inline-templates -fno-implement-inlines
-fmodule-header[=kind] -fmodule-only -fmodules
-fmodule-implicit-inline -fno-module-lazy
-fmodule-mapper=specification -fmodule-version-ignore
-fms-extensions -fnew-inheriting-ctors -fnew-ttp-matching
-fno-nonansi-builtins -fnothrow-opt -fno-operator-names
-fno-optional-diags -fno-pretty-templates -frange-for-ext-temps
-fno-rtti -fsized-deallocation -fstrong-eval-order[=kind]
-ftemplate-backtrace-limit=n -ftemplate-depth=n
-fno-threadsafe-statics -fuse-cxa-atexit -fno-weak -nostdinc++
-fvisibility-inlines-hidden -fvisibility-ms-compat
-fext-numeric-literals -flang-info-include-translate[=header]
-flang-info-include-translate-not -flang-info-module-cmi[=module]
-stdlib=libstdc++,libc++ -Wabi-tag -Wcatch-value -Wcatch-value=n
-Wno-class-conversion -Wclass-memaccess -Wcomma-subscript
-Wconditionally-supported -Wno-conversion-null
-Wctad-maybe-unsupported -Wctor-dtor-privacy -Wdangling-reference
-Wno-defaulted-function-deleted -Wno-delete-incomplete
-Wdelete-non-virtual-dtor -Wno-deprecated-array-compare
-Wdeprecated-copy -Wdeprecated-copy-dtor
-Wno-deprecated-enum-enum-conversion
-Wno-deprecated-enum-float-conversion -Weffc++
-Wno-elaborated-enum-base -Wno-exceptions -Wextra-semi
-Wno-global-module -Wno-inaccessible-base
-Wno-inherited-variadic-ctor -Wno-init-list-lifetime
-Winvalid-constexpr -Winvalid-imported-macros -Wno-invalid-offsetof
-Wno-literal-suffix -Wmismatched-new-delete -Wmismatched-tags
-Wmultiple-inheritance -Wnamespaces -Wnarrowing -Wnoexcept
-Wnoexcept-type -Wnon-virtual-dtor -Wpessimizing-move
-Wno-placement-new -Wplacement-new=n -Wrange-loop-construct
-Wredundant-move -Wredundant-tags -Wreorder -Wregister
-Wstrict-null-sentinel -Wno-subobject-linkage -Wtemplates
-Wno-non-template-friend -Wold-style-cast -Woverloaded-virtual
-Wno-pmf-conversions -Wself-move -Wsign-promo -Wsized-deallocation
-Wsuggest-final-methods -Wsuggest-final-types -Wsuggest-override
-Wno-template-body -Wno-template-id-cdtor -Wtemplate-names-tu-local
-Wno-terminate -Wno-vexing-parse -Wvirtual-inheritance
-Wno-virtual-move-assign -Wvolatile🧚 Objective-C and Objective-C++ Language Options 🧚
for the bracket-obsessed besties:
-fconstant-string-class=class-name -fgnu-runtime -fnext-runtime
-fno-nil-receivers -fobjc-abi-version=n -fobjc-call-cxx-cdtors
-fobjc-direct-dispatch -fobjc-exceptions -fobjc-gc -fobjc-nilcheck
-fobjc-std=objc1 -fno-local-ivars
-fivar-visibility=[public|protected|private|package]
-freplace-objc-classes -fzero-link -gen-decls -Wassign-intercept
-Wno-property-assign-default -Wno-protocol -Wobjc-root-class
-Wselector -Wstrict-selector-match -Wundeclared-selector🔥 OpenMP and OpenACC Options 🔥
parallel girlies, this is your moment:
-foffload=arg -foffload-options=arg -fopenacc -fopenacc-dim=geom
-fopenmp -fopenmp-simd -fopenmp-target-simd-clone[=device-type]🌈 Diagnostic Message Formatting Options 🌈
the ones that decide how your errors look when they inevitably happen. aesthetics matter:
-fmessage-length=n -fdiagnostics-plain-output
-fdiagnostics-show-location=[once|every-line]
-fdiagnostics-color=[auto|never|always]
-fdiagnostics-urls=[auto|never|always]
-fdiagnostics-format=[text|sarif-stderr|sarif-file|json|json-stderr|json-file]
-fdiagnostics-add-output=DIAGNOSTICS-OUTPUT-SPEC
-fdiagnostics-set-output=DIAGNOSTICS-OUTPUT-SPEC
-fno-diagnostics-json-formatting -fno-diagnostics-show-option
-fno-diagnostics-show-caret -fno-diagnostics-show-event-links
-fno-diagnostics-show-labels -fno-diagnostics-show-line-numbers
-fno-diagnostics-show-cwe -fno-diagnostics-show-rules
-fno-diagnostics-show-highlight-colors
-fdiagnostics-minimum-margin-width=width
-fdiagnostics-parseable-fixits -fdiagnostics-generate-patch
-fdiagnostics-show-template-tree -fno-elide-type
-fdiagnostics-path-format=[none|separate-events|inline-events]
-fdiagnostics-show-path-depths -fno-show-column
-fdiagnostics-column-unit=[display|byte]
-fdiagnostics-column-origin=origin
-fdiagnostics-escape-format=[unicode|bytes]
-fdiagnostics-text-art-charset=[none|ascii|unicode|emoji]okay that's the summary menu, bestie. all the real explanations are still coming — part 1 stops right here, and the next parts go deep on every single one of these. no skipping, no summarising, we're doing all of them. 💖
Warning Options ⚠️✨
okay bestie, deep breath, because this is the section where GCC shows you exactly how many opinions it has. 💅 This is the warning fam — the flags that decide how loudly your compiler yells at you about things. There's like three hundred of them. I counted. I'm not okay. 🫶
Here's the opening lineup:
-fsyntax-only -fmax-errors=n -Wpedantic -pedantic-errors
-fpermissive -w -Wextra -Wall -Wabi=n -Waddress-fsyntax-only, -fmax-errors=n, -Wpedantic, -pedantic-errors, -fpermissive, -w, -Wextra, -Wall, -Wabi=n, -Waddress — iconic ten, truly. Note how -W and -f are both just vibing in the same list. GCC does not care about your categories, she contains multitudes. 🌈
Then it just... keeps going:
-Wno-address-of-packed-member -Waggregate-return -Walloc-size
-Walloc-size-larger-than=byte-size -Walloc-zero -Walloca
-Walloca-larger-than=byte-size -Wno-aggressive-loop-optimizations
-Warith-conversion -Warray-bounds -Warray-bounds=n -Warray-compare
-Warray-parameter -Warray-parameter=n -Wno-attributes
-Wattribute-alias=n -Wno-attribute-alias -Wno-attribute-warning
-Wbidi-chars=[none|unpaired|any|ucn] -Wbool-compare
-Wbool-operation -Wno-builtin-declaration-mismatchthe way -Wbidi-chars=[none|unpaired|any|ucn] is literally an options-inside-an-option situation... nested decisions, main character energy, obsessed. 🎀 and yes bestie, lots of these have a =n or a =byte-size or a =[...] hanging off the end — that's just part of the flag, keep it attached like a little charm on a bracelet. ✨
-Wno-builtin-macro-redefined -Wc90-c99-compat -Wc99-c11-compat
-Wc11-c23-compat -Wc23-c2y-compat -Wc++-compat -Wc++11-compat
-Wc++14-compat -Wc++17-compat -Wc++20-compat -Wno-c++11-extensions
-Wno-c++14-extensions -Wno-c++17-extensions -Wno-c++20-extensions
-Wno-c++23-extensions -Wcalloc-transposed-args -Wcast-align
-Wcast-align=strict -Wcast-function-type -Wcast-qual
-Wchar-subscripts -Wclobbered -Wcommentthe whole -Wc90-c99-compat → -Wc99-c11-compat → -Wc11-c23-compat → -Wc23-c2y-compat pipeline is giving era by era, a little compatibility timeline, we love a glow-up sequence. 💖 and then the C++ one comes in with the literal ++ in the middle of the flag name. -Wc++-compat. slay. 💅
-Wcompare-distinct-pointer-types -Wno-complain-wrong-lang
-Wconversion -Wno-coverage-mismatch -Wno-cpp -Wdangling-else
-Wdangling-pointer -Wdangling-pointer=n -Wdate-time -Wno-deprecated
-Wno-deprecated-declarations -Wno-designated-init
-Wdisabled-optimization -Wno-discarded-array-qualifiers
-Wno-discarded-qualifiers -Wno-div-by-zero -Wdouble-promotion
-Wduplicated-branches -Wduplicated-cond -Wempty-body
-Wno-endif-labels -Wenum-compare -Wenum-conversion
-Wenum-int-mismatch -Werror -Werror=* -Wexpansion-to-defined
-Wfatal-errors -Wflex-array-member-not-at-end -Wfloat-conversion-Werror=* — star, wildcard, mysterious, unbothered, exactly as written. ⭐ and can we talk about -Wflex-array-member-not-at-end, that name is a whole sentence, she said what she said. 😭
-Wfloat-equal -Wformat -Wformat=2 -Wno-format-contains-nul
-Wno-format-extra-args -Wformat-nonliteral -Wformat-overflow=n
-Wformat-security -Wformat-signedness -Wformat-truncation=n
-Wformat-y2k -Wframe-address -Wframe-larger-than=byte-size
-Wno-free-nonheap-object -Wheader-guard -Wno-if-not-aligned
-Wno-ignored-attributes -Wignored-qualifiers
-Wno-incompatible-pointer-types -Whardened -Wimplicit-Wformat-y2k bestie. Y2K. it's giving 1999, it's giving dial-up, it's giving a manpage that has seen things. 🦋
-Wimplicit-fallthrough -Wimplicit-fallthrough=n
-Wno-implicit-function-declaration -Wno-implicit-int
-Winfinite-recursion -Winit-self -Winline -Wno-int-conversion
-Wint-in-bool-context -Wno-int-to-pointer-cast
-Wno-invalid-memory-model -Winvalid-pch -Winvalid-utf8
-Wno-unicode -Wjump-misses-init -Wlarger-than=byte-size
-Wleading-whitespace=kind -Wlogical-not-parentheses -Wlogical-op
-Wlong-long -Wno-lto-type-mismatch -Wmain -Wmaybe-uninitialized-Wlong-long just made me laugh out loud, it's so redundant-cute, like saying "big big". 🙈
-Wmemset-elt-size -Wmemset-transposed-args -Wmisleading-indentation
-Wmissing-attributes -Wmissing-braces -Wmissing-field-initializers
-Wmissing-format-attribute -Wmissing-include-dirs
-Wmissing-noreturn -Wmusttail-local-addr
-Wmaybe-musttail-local-addr -Wno-missing-profile -Wno-multichar
-Wmultistatement-macros -Wnonnull -Wnonnull-compare
-Wnormalized=[none|id|nfc|nfkc] -Wnull-dereference -Wno-odr
-Wopenacc-parallelism -Wopenmp -Wopenmp-simd -Wno-overflow-Wnormalized=[none|id|nfc|nfkc] again with the brackets — and -Wmaybe-musttail-local-addr is a whole mood, "maybe must tail", indecisive queen, I respect it. 💜
-Woverlength-strings -Wno-override-init-side-effects -Wpacked
-Wno-packed-bitfield-compat -Wpacked-not-aligned -Wpadded
-Wparentheses -Wno-pedantic-ms-format -Wpointer-arith
-Wno-pointer-compare -Wno-pointer-to-int-cast -Wno-pragmas
-Wno-pragma-once-outside-header -Wno-prio-ctor-dtor -Wno-psabi
-Wredundant-decls -Wrestrict -Wno-return-local-addr -Wreturn-type
-Wno-scalar-storage-order -Wsequence-point -Wshadow
-Wshadow=global -Wshadow=local -Wshadow=compatible-localthe -Wshadow trio with =global, =local, =compatible-local — a whole gradient, a family, three sisters. 👑
-Wno-shadow-ivar -Wno-shift-count-negative
-Wno-shift-count-overflow -Wshift-negative-value
-Wno-shift-overflow -Wshift-overflow=n -Wsign-compare
-Wsign-conversion -Wno-sizeof-array-argument -Wsizeof-array-div
-Wsizeof-pointer-div -Wsizeof-pointer-memaccess -Wstack-protector
-Wstack-usage=byte-size -Wstrict-aliasing -Wstrict-aliasing=n
-Wstrict-overflow -Wstrict-overflow=n -Wstring-compare
-Wno-stringop-overflow -Wno-stringop-overread
-Wno-stringop-truncation -Wstrict-flex-arrays-Wsizeof-pointer-div vs -Wsizeof-array-div vs -Wsizeof-pointer-memaccess... the sizeof girlies really said "we're a trio now". 🌸
-Wsuggest-attribute=attribute-name -Wswitch -Wno-switch-bool
-Wswitch-default -Wswitch-enum -Wno-switch-outside-range
-Wno-switch-unreachable -Wsync-nand -Wsystem-headers
-Wtautological-compare -Wtrailing-whitespace
-Wtrailing-whitespace=kind -Wtrampolines -Wtrigraphs
-Wtrivial-auto-var-init -Wno-tsan -Wtype-limits -Wundef
-Wuninitialized -Wunknown-pragmas -Wunsuffixed-float-constants-Wtrailing-whitespace showing up both plain and with =kind — she contains multitudes, again. and -Wtrigraphs?? a throwback, a relic, an artifact, I'm emotional. 😭
-Wunterminated-string-initialization -Wunused
-Wunused-but-set-parameter -Wunused-but-set-variable
-Wunused-const-variable -Wunused-const-variable=n -Wunused-function
-Wunused-label -Wunused-local-typedefs -Wunused-macros
-Wunused-parameter -Wno-unused-result -Wunused-value
-Wunused-variable -Wuse-after-free -Wuse-after-free=n
-Wuseless-cast -Wno-varargs -Wvariadic-macros
-Wvector-operation-performance -Wvla -Wvla-larger-than=byte-size
-Wno-vla-larger-than -Wvolatile-register-var -Wwrite-strings
-Wno-xor-used-as-pow -Wzero-as-null-pointer-constant
-Wzero-length-boundsand THAT, bestie, is the end of the warning alphabet. the -Wunused-* section alone is like eight flags deep, which is honestly so real of GCC — she will absolutely tell you about every single thing you didn't use. 💀
Static Analyzer Options 🔍✨
okay now we're in the analyzer section, which is where GCC puts on her detective coat and goes actually looking for problems instead of just complaining about syntax. 🕵️♀️
-fanalyzer -fanalyzer-call-summaries -fanalyzer-checker=name
-fno-analyzer-feasibility -fanalyzer-fine-grained
-fanalyzer-show-events-in-system-headers -fno-analyzer-state-merge
-fno-analyzer-state-purge -fno-analyzer-suppress-followups
-fanalyzer-transitivity -fno-analyzer-undo-inlining
-fanalyzer-verbose-edges -fanalyzer-verbose-state-changes
-fanalyzer-verbosity=level -fdump-analyzer -fdump-analyzer-callgraph
-fdump-analyzer-exploded-graph -fdump-analyzer-exploded-nodes
-fdump-analyzer-exploded-nodes-2 -fdump-analyzer-exploded-nodes-3notice the -f / -fno- pairing pattern all over this section — every "turn it on" has a little "turn it off" twin, it's giving twin flames, it's giving balance. 🫶 also -fdump-analyzer-exploded-nodes-2 and -3?? she just kept numbering them. she was NOT done. 💅
-fdump-analyzer-exploded-paths -fdump-analyzer-feasibility
-fdump-analyzer-infinite-loop -fdump-analyzer-json
-fdump-analyzer-state-purge -fdump-analyzer-stderr
-fdump-analyzer-supergraph -fdump-analyzer-untracked-fdump-analyzer-json — she's modern, she's got a JSON mode, she's adaptable. ✨
-Wno-analyzer-double-fclose -Wno-analyzer-double-free
-Wno-analyzer-exposure-through-output-file
-Wno-analyzer-exposure-through-uninit-copy
-Wno-analyzer-fd-access-mode-mismatch -Wno-analyzer-fd-double-close
-Wno-analyzer-fd-leak -Wno-analyzer-fd-phase-mismatch
-Wno-analyzer-fd-type-mismatch -Wno-analyzer-fd-use-after-close
-Wno-analyzer-fd-use-without-check -Wno-analyzer-file-leak
-Wno-analyzer-free-of-non-heap -Wno-analyzer-imprecise-fp-arithmetic
-Wno-analyzer-infinite-loop -Wno-analyzer-infinite-recursion
-Wno-analyzer-jump-through-null -Wno-analyzer-malloc-leakthe -Wno-analyzer-fd-* block is a whole girl group, they all showed up together. 🎀
-Wno-analyzer-mismatching-deallocation -Wno-analyzer-null-argument
-Wno-analyzer-null-dereference -Wno-analyzer-out-of-bounds
-Wno-analyzer-overlapping-buffers
-Wno-analyzer-possible-null-argument
-Wno-analyzer-possible-null-dereference
-Wno-analyzer-putenv-of-auto-var -Wno-analyzer-shift-count-negative
-Wno-analyzer-shift-count-overflow -Wno-analyzer-stale-setjmp-buffer
-Wno-analyzer-tainted-allocation-size
-Wno-analyzer-tainted-assertion -Wno-analyzer-tainted-array-index
-Wno-analyzer-tainted-divisor -Wno-analyzer-tainted-offset
-Wno-analyzer-tainted-size -Wanalyzer-symbol-too-complex
-Wanalyzer-too-complex -Wno-analyzer-undefined-behavior-ptrdiff-Wno-analyzer-putenv-of-auto-var is SO specific and I love that for her, she's thorough, she's detail-oriented, she notices things. 👑
-Wno-analyzer-undefined-behavior-strtok
-Wno-analyzer-unsafe-call-within-signal-handler
-Wno-analyzer-use-after-free
-Wno-analyzer-use-of-pointer-in-stale-stack-frame
-Wno-analyzer-use-of-uninitialized-value
-Wno-analyzer-va-arg-type-mismatch -Wno-analyzer-va-list-exhausted
-Wno-analyzer-va-list-leak -Wno-analyzer-va-list-use-after-va-end
-Wno-analyzer-write-to-const -Wno-analyzer-write-to-string-literaland the -Wno-analyzer-va-list-* quartet to close us out — va-list has four whole flags about her, she's booked and busy. 💖
C and Objective-C-only Warning Options 🍎✨
these ones are picky, they only work for the C and Objective-C girlies specifically. if you're a C++ bestie, these are not for you, no cap.
-Wbad-function-cast -Wdeprecated-non-prototype -Wfree-labels
-Wmissing-declarations -Wmissing-parameter-name
-Wmissing-parameter-type -Wdeclaration-missing-parameter-type
-Wmissing-prototypes -Wmissing-variable-declarations
-Wnested-externs -Wold-style-declaration -Wold-style-definition
-Wstrict-prototypes -Wtraditional -Wtraditional-conversion
-Wdeclaration-after-statement -Wpointer-signthe -Wmissing-* run in here is like six flags long, she really does keep a whole checklist of everything you forgot to declare. 😭 and then -Wtraditional and -Wtraditional-conversion at the end, giving vintage, giving nostalgia, giving "back in my day". 🌸
Debugging Options 🐛✨
and finally, the debugging fam! these are the ones that decide what kind of info gets baked into your binary so you can actually figure out what went wrong later. forensics, bestie. 🔍
-g -glevel -gdwarf -gdwarf-version -gbtf -gctf -gctflevel
-gprune-btf -gno-prune-btf -ggdb -grecord-gcc-switches
-gno-record-gcc-switches -gstrict-dwarf -gno-strict-dwarf
-gas-loc-support -gno-as-loc-support -gas-locview-support
-gno-as-locview-support -gcodeview -gcolumn-info -gno-column-info
-gdwarf32 -gdwarf64 -gstatement-frontiers -gno-statement-frontiers-g is THE most famous flag in this entire manpage and she knows it. she doesn't need a description. she's the moment. 💅
-gvariable-location-views -gno-variable-location-views
-ginternal-reset-location-views -gno-internal-reset-location-views
-ginline-points -gno-inline-points -gvms -gz[=type] -gsplit-dwarf
-gdescribe-dies -gno-describe-dies -fdebug-prefix-map=old=new-gz[=type] has optional bracket energy, -fdebug-prefix-map=old=new has the old=new thing which is genuinely so satisfying to look at, like a little equation. ⭐
-fdebug-types-section -fno-eliminate-unused-debug-types
-femit-struct-debug-baseonly -femit-struct-debug-reduced
-femit-struct-debug-detailed[=spec-list]
-fno-eliminate-unused-debug-symbols -femit-class-debug-always
-fno-merge-debug-strings -fno-dwarf2-cfi-asm -fvar-tracking
-fvar-tracking-assignments-femit-struct-debug-detailed[=spec-list] closing us out with a bracket again, because of course. she loves a bracket. 🦋
real talk bestie: this whole section is just the list of what exists. the actual explanations come later in the page, so don't panic if you don't know what -gdescribe-dies does yet — we're just doing attendance here. 💜
✨ Optimization Options ✨
okay bestie, deep breath. this is THE list. every single knob GCC has for making your code faster, smaller, or occasionally just unhinged. no explainers here — this is the roll call, the actual deep dives live further down the page. we're going in chunks so nobody faints. 🫶
-faggressive-loop-optimizations-falign-functions[=n[:m:[n2[:m2]]]]-falign-jumps[=n[:m:[n2[:m2]]]]-falign-labels[=n[:m:[n2[:m2]]]]-falign-loops[=n[:m:[n2[:m2]]]]-fmin-function-alignment=[n]-fno-allocation-dce-fallow-store-data-races-fassociative-math-fauto-profile-fauto-profile[=path]-fauto-inc-dec-fbranch-probabilities-fcaller-saves-fcombine-stack-adjustments-fconserve-stack-ffold-mem-offsets-fcompare-elim-fcprop-registers-fcrossjumping-fcse-follow-jumps-fcse-skip-blocks-fcx-fortran-rules-fcx-limited-range-fdata-sections-fdce-fdelayed-branch-fdelete-null-pointer-checks-fdevirtualize-fdevirtualize-speculatively-fdevirtualize-at-ltrans-fdse-fearly-inlining-fipa-sra-fexpensive-optimizations-ffat-lto-objects-ffast-math-ffinite-math-only-ffloat-store-fexcess-precision=style-ffinite-loops-fforward-propagate-ffp-contract=style-ffunction-sections-fgcse-fgcse-after-reload-fgcse-las-fgcse-lm-fgraphite-identity-fgcse-sm
still going. hydrate. 💧
-fhoist-adjacent-loads-fif-conversion-fif-conversion2-findirect-inlining-finline-stringops[=fn]-finline-functions-finline-functions-called-once-finline-limit=n-finline-small-functions-fipa-modref-fipa-cp-fipa-cp-clone-fipa-bit-cp-fipa-vrp-fipa-pta-fipa-profile-fipa-pure-const-fipa-reference-fipa-reference-addressable-fipa-reorder-for-locality-fipa-stack-alignment-fipa-icf-fira-algorithm=algorithm-flate-combine-instructions-flifetime-dse-flive-patching=level-fira-region=region-fira-hoist-pressure-fira-loop-pressure-fno-ira-share-save-slots-fno-ira-share-spill-slots-fisolate-erroneous-paths-dereference-fisolate-erroneous-paths-attribute-fivopts-fkeep-inline-functions-fkeep-static-functions-fkeep-static-consts-flimit-function-alignment-flive-range-shrinkage-floop-block-floop-interchange-floop-strip-mine-floop-unroll-and-jam-floop-nest-optimize-floop-parallelize-all-flra-remat-flto-flto-compression-level-flto-partition=alg-flto-incremental=path-flto-incremental-cache-size=n-fmalloc-dce-fmerge-all-constants-fmerge-constants
not even halfway, bestie. we persist. 💅
-fmodulo-sched-fmodulo-sched-allow-regmoves-fmove-loop-invariants-fmove-loop-stores-fno-branch-count-reg-fno-defer-pop-fno-fp-int-builtin-inexact-fno-function-cse-fno-guess-branch-probability-fno-inline-fno-math-errno-fno-peephole-fno-peephole2-fno-printf-return-value-fno-sched-interblock-fno-sched-spec-fno-signed-zeros-fno-toplevel-reorder-fno-trapping-math-fno-zero-initialized-in-bss-fomit-frame-pointer-foptimize-crc-foptimize-sibling-calls-fpartial-inlining-fpeel-loops-fpredictive-commoning-fprefetch-loop-arrays-fprofile-correction-fprofile-use-fprofile-use=path-fprofile-partial-training-fprofile-values-fprofile-reorder-functions-freciprocal-math-free-frename-registers-freorder-blocks-freorder-blocks-algorithm=algorithm-freorder-blocks-and-partition-freorder-functions-frerun-cse-after-loop-freschedule-modulo-scheduled-loops-frounding-math-fsave-optimization-record-fsched2-use-superblocks-fsched-pressure-fsched-spec-load-fsched-spec-load-dangerous-fsched-stalled-insns-dep[=n]-fsched-stalled-insns[=n]-fsched-group-heuristic-fsched-critical-path-heuristic-fsched-spec-insn-heuristic-fsched-rank-heuristic-fsched-last-insn-heuristic-fsched-dep-count-heuristic-fschedule-fusion-fschedule-insns-fschedule-insns2-fsection-anchors-fselective-scheduling-fselective-scheduling2-fsel-sched-pipelining-fsel-sched-pipelining-outer-loops-fsemantic-interposition-fshrink-wrap-fshrink-wrap-separate-fsignaling-nans-fsingle-precision-constant-fsplit-ivs-in-unroller-fsplit-loops-fsplit-paths-fsplit-wide-types-fsplit-wide-types-early-fssa-backprop-fssa-phiopt-fstdarg-opt-fstore-merging-fstrict-aliasing-fipa-strict-aliasing
the tree-* era is upon us. 🌳
-fthread-jumps-ftracer-ftree-bit-ccp-ftree-builtin-call-dce-ftree-ccp-ftree-ch-ftree-coalesce-vars-ftree-copy-prop-ftree-cselim-ftree-dce-ftree-dominator-opts-ftree-dse-ftree-forwprop-ftree-fre-fcode-hoisting-ftree-loop-if-convert-ftree-loop-im-ftree-phiprop-ftree-loop-distribution-ftree-loop-distribute-patterns-ftree-loop-ivcanon-ftree-loop-linear-ftree-loop-optimize-ftree-loop-vectorize-ftree-parallelize-loops=n-ftree-pre-ftree-partial-pre-ftree-pta-ftree-reassoc-ftree-scev-cprop-ftree-sink-ftree-slsr-ftree-sra-ftree-switch-conversion-ftree-tail-merge-ftree-ter-ftree-vectorize-ftree-vrp-ftrivial-auto-var-init-funconstrained-commons-funit-at-a-time-funroll-all-loops-funroll-loops-funsafe-math-optimizations-funswitch-loops-fipa-ra-fvariable-expansion-in-unroller-fvect-cost-model-fvpt-fweb-fwhole-program-fwpa-fuse-linker-plugin-fzero-call-used-regs
and then the finishers — the param channel plus the -O levels everyone actually types 👑:
--param name=value-O-O0-O1-O2-O3-Os-Ofast-Og-Oz
🩺 Program Instrumentation Options 🩺
profiling, sanitizing, hardening — this is the "i want receipts on what my program is doing" section. girlies who debug for fun, this one's for you. 💖
-p-pg-fprofile-arcs--coverage-ftest-coverage-fcondition-coverage-fpath-coverage-fprofile-abs-path-fprofile-dir=path-fprofile-generate-fprofile-generate=path-fprofile-info-section-fprofile-info-section=name-fprofile-note=path-fprofile-prefix-path=path-fprofile-update=method-fprofile-filter-files=regex-fprofile-exclude-files=regex-fprofile-reproducible=[multithreaded|parallel-runs|serial]-fsanitize=style-fsanitize-recover-fsanitize-recover=style-fsanitize-trap-fsanitize-trap=style-fasan-shadow-offset=number-fsanitize-sections=s1,s2,...-fsanitize-undefined-trap-on-error-fbounds-check-fcf-protection-fcf-protection=[full|branch|return|none|check]-fharden-compares-fharden-conditional-branches-fhardened-fharden-control-flow-redundancy-fhardcfr-skip-leaf-fhardcfr-check-exceptions-fhardcfr-check-returning-calls-fhardcfr-check-noreturn-calls=[always|no-xthrow|nothrow|never]-fstack-protector-fstack-protector-all-fstack-protector-strong-fstack-protector-explicit-fstack-check-fstack-limit-register=reg-fstack-limit-symbol=sym-fno-stack-limit-fsplit-stack-fstrub=disable-fstrub=strict-fstrub=relaxed-fstrub=all-fstrub=at-calls-fstrub=internal-fvtable-verify=[std|preinit|none]-fvtv-counts-fvtv-debug-finstrument-functions-finstrument-functions-once-finstrument-functions-exclude-function-list=sym,sym,...-finstrument-functions-exclude-file-list=file,file,...-fprofile-prefix-map=old=new-fpatchable-function-entry=N[,M]
🎀 Preprocessor Options 🎀
macro city, babes. this is where #define lives and where you go when something expands into a novel. 🌸
-Aquestion=answer-A-question[=answer]-C-CC-Dmacro[=defn]-dD-dI-dM-dN-dU-fdebug-cpp-fdirectives-only-fdollars-in-identifiers-fexec-charset=charset-fextended-identifiers-finput-charset=charset-fmacro-prefix-map=old=new-fmax-include-depth=depth-fno-canonical-system-headers-fpch-deps-fpch-preprocess-fpreprocessed-ftabstop=width-ftrack-macro-expansion-fwide-exec-charset=charset-fworking-directory-H-imacros file-include file-M-MD-MF-MG-MM-MMD-MP-MQ-MT-Mno-modules-no-integrated-cpp-P-pthread-remap-traditional-traditional-cpp-trigraphs-Umacro-undef-Wp,option-Xpreprocessor option
🛠️ Assembler Options 🛠️
short and sweet. two options, no notes. 🫡
-Wa,option-Xassembler option
🔗 Linker Options 🔗
the part where everything gets stitched together. glue era. ✨
object-file-name-fuse-ld=linker-llibrary-nostartfiles-nodefaultlibs-nolibc-nostdlib-nostdlib++-e entry--entry=entry-pie-pthread-r-rdynamic-s-static-static-pie-static-libgcc-static-libstdc++-static-libasan-static-libtsan-static-liblsan-static-libubsan-shared-shared-libgcc-symbolic-T script-Wl,option-Xlinker option-u symbol-z keyword
📁 Directory Options 📁
where does GCC even look for things? that's this list. include paths, sysroots, plugin dirs — the whole map. 🗺️
-Bprefix-Idir-I--idirafter dir-imacros file-imultilib dir-iplugindir=dir-iprefix file-iquote dir-isysroot dir-isystem dir-iwithprefix dir-iwithprefixbefore dir--embed-dir=dir-Ldir-no-canonical-prefixes--no-sysroot-suffix-nostdinc-nostdinc++--sysroot=dir
⚙️ Code Generation Options ⚙️
the "how do you want the actual machine code shaped" section. pic/pie girlies rise. 🦋
-fcall-saved-reg-fcall-used-reg-ffixed-reg-fexceptions-fnon-call-exceptions-fdelete-dead-exceptions-funwind-tables-fasynchronous-unwind-tables-fno-gnu-unique-finhibit-size-directive-fcommon-fno-ident-fpcc-struct-return-fpic-fPIC-fpie-fPIE-fno-plt-fno-jump-tables-fno-bit-tests-frecord-gcc-switches-freg-struct-return-fshort-enums-fshort-wchar-fverbose-asm-fpack-struct[=n]-fleading-underscore-ftls-model=model-fstack-reuse=reuse_level-ftrampolines-ftrampoline-impl=[stack|heap]-ftrapv-fwrapv-fvisibility=[default|internal|hidden|protected]-fstrict-volatile-bitfields-fsync-libcalls-fzero-init-padding-bits=value
🧪 Developer Options 🧪
okay so this is the backstage pass section. dumps, reports, seeds, and every -fdump- you could possibly want when you're deep in the compiler guts at 2am. unhinged in the best way. 💀✨
-dletters-dumpspecs-dumpmachine-dumpversion-dumpfullversion-fcallgraph-info[=su,da]-fchecking-fchecking=n-fdbg-cnt-list-fdbg-cnt=counter-value-list-fdisable-ipa-pass_name-fdisable-rtl-pass_name-fdisable-rtl-pass-name=range-list-fdisable-tree-pass_name-fdisable-tree-pass-name=range-list-fdump-debug-fdump-earlydebug-fdump-noaddr-fdump-unnumbered-fdump-unnumbered-links-fdump-final-insns[=file]-fdump-ipa-all-fdump-ipa-cgraph-fdump-ipa-inline-fdump-lang-all-fdump-lang-switch-fdump-lang-switch-options-fdump-lang-switch-options=filename-fdump-passes-fdump-rtl-pass-fdump-rtl-pass=filename-fdump-statistics-fdump-tree-all-fdump-tree-switch-fdump-tree-switch-options-fdump-tree-switch-options=filename-fcompare-debug[=opts]-fcompare-debug-second-fenable-kind-pass-fenable-kind-pass=range-list-fira-verbose=n-flto-report-flto-report-wpa-fmem-report-wpa-fmem-report-fpre-ipa-mem-report-fpost-ipa-mem-report-fopt-info-fopt-info-options[=file]-fmultiflags-fprofile-report-frandom-seed=string-fsched-verbose=n-fsel-sched-verbose-fsel-sched-dump-cfg-fsel-sched-pipelining-verbose-fstats-fstack-usage-ftime-report-ftime-report-details-fvar-tracking-assignments-toggle-gtoggle-print-file-name=library-print-libgcc-file-name-print-multi-directory-print-multi-lib-print-multi-os-directory-print-prog-name=program-print-search-dirs-Q-print-sysroot-print-sysroot-headers-suffix-save-temps-save-temps=cwd-save-temps=obj-time[=file]
and that's the option summary, bestie — but the page is nowhere near done 🫶
🌈 Machine-Dependent Options 🌈
okay so here's where GCC stops being one program and starts being every program 💅 every chip family got its own little love language, and this section is the master list. we're going architecture by architecture, and bestie, I am NOT skipping a single flag. not one. every dash-option in the house is coming with us. 🫶
✨ AArch64 Options (AArch64 Options) ✨
the 64-bit ARM era, iconic, Apple-Silicon-adjacent, main character energy for real. full flag roster below:
-mabi=name -mbig-endian -mlittle-endian -mgeneral-regs-only -mcmodel=tiny -mcmodel=small
-mcmodel=large -mstrict-align -mno-strict-align
-momit-leaf-frame-pointer -mtls-dialect=desc
-mtls-dialect=traditional -mtls-size=size -mfix-cortex-a53-835769
-mfix-cortex-a53-843419 -mlow-precision-recip-sqrt
-mlow-precision-sqrt -mlow-precision-div
-mpc-relative-literal-loads -msign-return-address=scope
-mbranch-protection=none|standard|pac-ret[+leaf +b-key]|bti|gcs
-mharden-sls=opts -march=name -mcpu=name -mtune=name
-moverride=string -mverbose-cost-dump -mstack-protector-guard=guard
-mstack-protector-guard-reg=sysreg
-mstack-protector-guard-offset=offset -mtrack-speculation
-moutline-atomics -mearly-ldp-fusion -mlate-ldp-fusion
-Wexperimental-fmv-target✨ Adapteva Epiphany Options (Adapteva Epiphany Options) ✨
niche? yes. still got a whole flag wall? also yes. 💀
-mhalf-reg-file -mprefer-short-insn-regs -mbranch-cost=num -mcmove
-mnops=num -msoft-cmpsf -msplit-lohi -mpost-inc -mpost-modify
-mstack-offset=num -mround-nearest -mlong-calls -mshort-calls
-msmall16 -mfp-mode=mode -mvect-double -max-vect-align=num
-msplit-vecmove-early -m1reg-reg✨ AMD GCN Options (AMD GCN Options) ✨
GPU brain time, three flags, no notes:
-march=gpu -mtune=gpu
-mstack-size=bytes✨ ARC Options (ARC Options) ✨
ARC said "hold my beverage" and produced a truly stunning number of toggles. obsessed. 🎀
-mbarrel-shifter -mjli-always -mcpu=cpu
-mA6 -mARC600 -mA7 -mARC700 -mdpfp -mdpfp-compact -mdpfp-fast
-mno-dpfp-lrsr -mea -mno-mpy -mmul32x16 -mmul64 -matomic -mnorm
-mspfp -mspfp-compact -mspfp-fast -msimd -msoft-float -mswap
-mcrc -mdsp-packa -mdvbf -mlock -mmac-d16 -mmac-24 -mrtsc
-mswape -mtelephony -mxy -misize -mannotate-align -marclinux
-marclinux_prof -mlong-calls -mmedium-calls -msdata
-mirq-ctrl-saved -mrgf-banked-regs -mlpc-width=width -G num
-mvolatile-cache -mtp-regno=regno -malign-call -mauto-modify-reg
-mbbit-peephole -mno-brcc -mcase-vector-pcrel -mcompact-casesi
-mno-cond-exec -mearly-cbranchsi -mexpand-adddi -mindexed-loads
-mlra -mlra-priority-none -mlra-priority-compact
-mlra-priority-noncompact -mmillicode -mmixed-code -mq-class
-mRcq -mRcw -msize-level=level -mtune=cpu -mmultcost=num
-mcode-density-frame -munalign-prob-threshold=probability
-mmpy-option=multo -mdiv-rem -mcode-density -mll64 -mfpu=fpu
-mrf16 -mbranch-index✨ ARM Options (ARM Options) ✨
the classic 32-bit ARM list, literally the blueprint. every vibe check you could ever want:
-mapcs-frame -mno-apcs-frame -mabi=name
-mapcs-stack-check -mno-apcs-stack-check -mapcs-reentrant
-mno-apcs-reentrant -mgeneral-regs-only -msched-prolog
-mno-sched-prolog -mlittle-endian -mbig-endian -mbe8 -mbe32
-mfloat-abi=name -mfp16-format=name -mthumb-interwork
-mno-thumb-interwork -mcpu=name -march=name -mfpu=name -mtune=name
-mprint-tune-info -mstructure-size-boundary=n -mabort-on-noreturn
-mlong-calls -mno-long-calls -msingle-pic-base
-mno-single-pic-base -mpic-register=reg -mnop-fun-dllimport
-mpoke-function-name -mthumb -marm -mflip-thumb -mtpcs-frame
-mtpcs-leaf-frame -mcaller-super-interworking
-mcallee-super-interworking -mtp=name -mtls-dialect=dialect
-mword-relocations -mfix-cortex-m3-ldrd -mfix-cortex-a57-aes-1742098
-mfix-cortex-a72-aes-1655431 -munaligned-access -mneon-for-64bits
-mslow-flash-data -masm-syntax-unified -mrestrict-it
-mverbose-cost-dump -mpure-code -mcmse -mfix-cmse-cve-2021-35465
-mstack-protector-guard=guard -mstack-protector-guard-offset=offset
-mfdpic -mbranch-protection=none|standard|pac-ret[+leaf]
[+bti]|bti[+pac-ret[+leaf]]✨ AVR Options (AVR Options) ✨
tiny microcontrollers, big feelings. 🦋
-mmcu=mcu -mabsdata -maccumulate-args
-mcvt -mbranch-cost=cost -mfuse-add=level -mfuse-move=level
-mfuse-move2 -mcall-prologues -mgas-isr-prologues -mint8 -mflmap
-mdouble=bits -mlong-double=bits -mno-call-main -mn_flash=size
-mfract-convert-truncate -mno-interrupts -mmain-is-OS_task -mrelax
-mrmw -mstrict-X -mtiny-stack -mrodata-in-ram -msplit-bit-shift
-msplit-ldst -mshort-calls -mskip-bug -muse-nonzero-bits
-nodevicelib -nodevicespecs -Waddr-space-convert -Wmisspelled-isr✨ Blackfin Options (Blackfin Options) ✨
anomalies? addressed. every one. slay.
-mcpu=cpu[-sirevision] -msim
-momit-leaf-frame-pointer -mno-omit-leaf-frame-pointer
-mspecld-anomaly -mno-specld-anomaly -mcsync-anomaly
-mno-csync-anomaly -mlow-64k -mno-low64k -mstack-check-l1
-mid-shared-library -mno-id-shared-library -mshared-library-id=n
-mleaf-id-shared-library -mno-leaf-id-shared-library -msep-data
-mno-sep-data -mlong-calls -mno-long-calls -mfast-fp -minline-plt
-mmulticore -mcorea -mcoreb -msdram -micplb✨ C6X Options (C6X Options) ✨
short, sweet, still exact:
-mbig-endian -mlittle-endian -march=cpu
-msim -msdata=sdata-type✨ CRIS Options (CRIS Options) ✨
okay bestie note this one: <green>-sim</green> and <green>-sim2</green> down there have no m, that's not a typo, that's just how they're written. copy them exactly. 📌
-mcpu=cpu -march=cpu -mtune=cpu
-mmax-stack-frame=n -metrax4 -metrax100 -mpdebug -mcc-init
-mno-side-effects -mstack-align -mdata-align -mconst-align
-m32-bit -m16-bit -m8-bit -mno-prologue-epilogue -melf -maout
-sim -sim2 -mmul-bug-workaround -mno-mul-bug-workaround✨ C-SKY Options (C-SKY Options) ✨
domestic chip royalty, and look at that, it brought both endian flags AND both short forms:
-march=arch -mcpu=cpu -mbig-endian
-EB -mlittle-endian -EL -mhard-float -msoft-float -mfpu=fpu
-mdouble-float -mfdivdu -mfloat-abi=name -melrw -mistack -mmp
-mcp -mcache -msecurity -mtrust -mdsp -medsp -mvdsp -mdiv
-msmart -mhigh-registers -manchor -mpushpop -mmultiple-stld
-mconstpool -mstack-size -mccrt -mbranch-cost=n -mcse-cc
-msched-prolog -msim✨ Cygwin and MinGW Options (Cygwin and MinGW Options) ✨
Windows-adjacent GCC, we love her, she contains multitudes:
-mconsole
-mcrtdll=library -mdll -mnop-fun-dllimport -mthread -municode
-mwin32 -mwindows -fno-set-stack-executable✨ Darwin Options (Darwin Options) ✨
the macOS pile 💻✨ and yeah, some of these names show up more than once in the original, I'm keeping it exactly as written, no deduping, no "fixing", periodt.
-all_load -allowable_client -arch
-arch_errors_fatal -arch_only -bind_at_load -bundle
-bundle_loader -client_name -compatibility_version
-current_version -dead_strip -dependency-file -dylib_file
-dylinker_install_name -dynamic -dynamiclib -exported_symbols_list
-filelist -flat_namespace -force_cpusubtype_ALL
-force_flat_namespace -headerpad_max_install_names -iframework
-image_base -init -install_name -keep_private_externs
-multi_module -multiply_defined -multiply_defined_unused
-noall_load -no_dead_strip_inits_and_terms -nodefaultrpaths
-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 (DEC Alpha Options) ✨
retro legend behavior 🌈 the IEEE flags go HARD here:
-mno-fp-regs -msoft-float
-mieee -mieee-with-inexact -mieee-conformant -mfp-trap-mode=mode
-mfp-rounding-mode=mode -mtrap-precision=mode -mbuild-constants
-mcpu=cpu-type -mtune=cpu-type -mbwx -mmax -mfix -mcix
-msafe-bwa -msafe-partial -mfloat-vax -mfloat-ieee
-mexplicit-relocs -msmall-data -mlarge-data -msmall-text
-mlarge-text -mmemory-latency=time✨ eBPF Options (eBPF Options) ✨
the kernel-filter girlies are thriving, look at this glow-up list:
-mbig-endian -mlittle-endian
-mframe-limit=bytes -mxbpf -mco-re -mno-co-re -mjmpext -mjmp32
-malu32 -mv3-atomics -mbswap -msdiv -msmov -mcpu=version
-masm=dialect -minline-memops-threshold=bytes✨ FR30 Options (FR30 Options) ✨
two flags. two. and we respect them equally. 💖
-msmall-model -mno-lsim✨ FT32 Options (FT32 Options) ✨
small but mighty:
-msim -mlra -mnodiv -mft32b
-mcompress -mnopm✨ FRV Options (FRV Options) ✨
this list is so long and I love that for it, every single one preserved:
-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=cpuheads up bestie,<cyan>-mTLS</cyan>and<cyan>-mtls</cyan>are two DIFFERENT flags up there, case matters, do not let autocorrect gaslight you. 💀
✨ GNU/Linux Options (GNU/Linux Options) ✨
libc flavor selection, it's giving pick your fighter:
-mglibc -muclibc -mmusl
-mbionic -mandroid -tno-android-cc -tno-android-ld✨ H8/300 Options (H8/300 Options) ✨
short stack, all accounted for:
-mrelax -mh -ms -mn -mexr
-mno-exr -mint32 -malign-300✨ HPPA Options (HPPA Options) ✨
HP Precision Architecture, precision flag list, we stay consistent. 🎀
-march=architecture-type
-matomic-libcalls -mbig-switch -mcaller-copies -mdisable-fpregs
-mdisable-indexing -mordered -mfast-indirect-calls -mgas -mgnu-ld
-mhp-ld -mfixed-range=register-range -mcoherent-ldcw -mjump-in-delay
-mlinker-opt -mlong-calls -mlong-load-store -mno-atomic-libcalls
-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 -msoft-mult -msio -mwsio
-munix=unix-std -nolibdld -static -threads✨ IA-64 Options (IA-64 Options) ✨
Itanium, my beloved complicated king, look at the scheduling flags, literally a whole vibe:
-mbig-endian -mlittle-endian
-mgnu-as -mgnu-ld -mno-pic -mvolatile-asm-stop -mregister-names
-msdata -mno-sdata -mconstant-gp -mauto-pic -mfused-madd
-minline-float-divide-min-latency
-minline-float-divide-max-throughput -mno-inline-float-divide
-minline-int-divide-min-latency -minline-int-divide-max-throughput
-mno-inline-int-divide -minline-sqrt-min-latency
-minline-sqrt-max-throughput -mno-inline-sqrt -mdwarf2-asm
-mearly-stop-bits -mfixed-range=register-range -mtls-size=tls-size
-mtune=cpu-type -milp32 -mlp64 -msched-br-data-spec
-msched-ar-data-spec -msched-control-spec -msched-br-in-data-spec
-msched-ar-in-data-spec -msched-in-control-spec -msched-spec-ldc
-msched-spec-control-ldc -msched-prefer-non-data-spec-insns
-msched-prefer-non-control-spec-insns
-msched-stop-bits-after-every-cycle
-msched-count-spec-in-critical-path
-msel-sched-dont-check-control-spec -msched-fp-mem-deps-zero-cost
-msched-max-memory-insns-hard-limit
-msched-max-memory-insns=max-insns✨ LM32 Options (LM32 Options) ✨
and to close out this batch, the tiniest cutest list of them all 🧚:
-mbarrel-shift-enabled -mdivide-enabled
-mmultiply-enabled -msign-extend-enabled -muser-enabledand bestie… we're not even done. 😭 more architecture flags are queued up right after this, so don't touch that dial. 💅
🦋 LoongArch Options 🦋
okay so LoongArch came in and said "I have opinions" and honestly? respect. Here's the whole flag roster, exactly as it comes, no notes from me 💖
-march=arch-type -mtune=tune-type -mabi=base-abi-type -mfpu=fpu-type
-msimd=simd-type -msoft-float -msingle-float -mdouble-float -mlsx -mno-lsx
-mlasx -mno-lasx -mbranch-cost=n -maddr-reg-reg-cost=n
-mcheck-zero-division -mno-check-zero-division -mcond-move-int
-mno-cond-move-int -mcond-move-float -mno-cond-move-float -memcpy
-mno-memcpy -mstrict-align -mno-strict-align -G num
-mmax-inline-memcpy-size=n -mexplicit-relocs=style -mexplicit-relocs
-mno-explicit-relocs -mdirect-extern-access -mno-direct-extern-access
-mcmodel=code-model -mrelax -mpass-mrelax-to-as
-mrecip -mrecip=opt -mfrecipe -mno-frecipe -mdiv32 -mno-div32 -mlam-bh
-mno-lam-bh -mlamcas -mno-lamcas -mld-seq-sa -mno-ld-seq-sa
-mtls-dialect=opt -mannotate-tablejump -mno-annotate-tablejumpsee how many of them come in -m / -mno- pairs? iconic behaviour, she gives you both sides of every story 🫶
🌸 M32C Options 🌸
short and sweet, three little icons, no drama ✨
-mcpu=cpu -msim -memregs=number🎀 M32R/D Options 🎀
a whole family of chips sharing one flag pile. group project energy but they actually finished it 💅
-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💜 M680x0 Options 💜
the 68k dynasty, the OGs, the ones who've been in the game since forever. respect the elders 😌
-march=arch -mcpu=cpu -mtune=tune -m68000 -m68020 -m68020-40 -m68020-60
-m68030 -m68040 -m68060 -mcpu32 -m5200 -m5206e -m528x -m5307 -m5407
-mcfv4e -mbitfield -mno-bitfield -mc68000 -mc68020 -mnobitfield -mrtd
-mno-rtd -mdiv -mno-div -mshort -mno-short -mhard-float -m68881
-msoft-float -mpcrel -malign-int -mstrict-align -msep-data -mno-sep-data
-mshared-library-id=n -mid-shared-library -mno-id-shared-library -mxgot
-mno-xgot -mlong-jump-table-offsets🍓 MCore Options 🍓
ngl I love how many of these are straight-up "yes / no" pairs. balanced. symmetrical. she understood the assignment. ⭐
-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🧚 MicroBlaze Options 🧚
soft float, hard float, and a whole little Xilinx-coded era at the end 💖
-msoft-float -mhard-float -msmall-divides -mcpu=cpu -mmemcpy -mxl-soft-mul
-mxl-soft-div -mxl-barrel-shift -mxl-pattern-compare -mxl-stack-check
-mxl-gp-opt -mno-clearbss -mxl-multiply-high -mxl-float-convert
-mxl-float-sqrt -mbig-endian -mlittle-endian -mxl-reorder
-mxl-mode-app-model -mpic-data-is-text-relative👑 MIPS Options 👑
bestie. BESTIE. Sit down, get a snack, hydrate, because this list is enormous and it deserves the full scroll. MIPS has been everywhere and knows everybody, so of course she has a flag for every occasion 🌈
-EL -EB -march=arch -mtune=arch -mips1 -mips2 -mips3 -mips4 -mips32
-mips32r2 -mips32r3 -mips32r5 -mips32r6 -mips64 -mips64r2 -mips64r3
-mips64r5 -mips64r6 -mips16 -mno-mips16 -mflip-mips16
-minterlink-compressed -mno-interlink-compressed -minterlink-mips16
-mno-interlink-mips16 -mabi=abi -mabicalls -mno-abicalls -mshared
-mno-shared -mplt -mno-plt -mxgot -mno-xgot -mgp32 -mgp64 -mfp32 -mfpxx
-mfp64 -mhard-float -msoft-float -mno-float -msingle-float -mdouble-float
-modd-spreg -mno-odd-spreg -mabs=mode -mnan=encoding -mdsp -mno-dsp -mdspr2
-mno-dspr2 -mmcu -mmno-mcu -meva -mno-eva -mvirt -mno-virt -mxpa -mno-xpa
-mcrc -mno-crc -mginv -mno-ginv -mmicromips -mno-micromips -mmsa -mno-msa
-mloongson-mmi -mno-loongson-mmi -mloongson-ext -mno-loongson-ext
-mloongson-ext2 -mno-loongson-ext2 -mfpu=fpu-type -msmartmips
-mno-smartmips -mpaired-single -mno-paired-single -mdmx -mno-mdmx -mips3d
-mno-mips3d -mmt -mno-mt -mllsc -mno-llsc -mlong64 -mlong32 -msym32
-mno-sym32 -Gnum -mlocal-sdata -mno-local-sdata -mextern-sdata
-mno-extern-sdata -mgpopt -mno-gopt -membedded-data -mno-embedded-data
-muninit-const-in-rodata -mno-uninit-const-in-rodata -mcode-readable=setting
-msplit-addresses -mno-split-addresses -mexplicit-relocs -mno-explicit-relocs
-mexplicit-relocs=release -mcheck-zero-division -mno-check-zero-division
-mdivide-traps -mdivide-breaks -mload-store-pairs -mno-load-store-pairs
-mstrict-align -mno-strict-align -mno-unaligned-access -munaligned-access
-mmemcpy -mno-memcpy -mlong-calls -mno-long-calls -mmad -mno-mad -mimadd
-mno-imadd -mfused-madd -mno-fused-madd -nocpp -mfix-24k -mno-fix-24k
-mfix-r4000 -mno-fix-r4000 -mfix-r4400 -mno-fix-r4400 -mfix-r5900
-mno-fix-r5900 -mfix-r10000 -mno-fix-r10000 -mfix-rm7000 -mno-fix-rm7000
-mfix-vr4120 -mno-fix-vr4120 -mfix-vr4130 -mno-fix-vr4130 -mfix-sb1
-mno-fix-sb1 -mflush-func=func -mno-flush-func -mbranch-cost=num
-mbranch-likely -mno-branch-likely -mcompact-branches=policy
-mfp-exceptions -mno-fp-exceptions -mvr4130-align -mno-vr4130-align -msynci
-mno-synci -mlxc1-sxc1 -mno-lxc1-sxc1 -mmadd4 -mno-madd4 -mrelax-pic-calls
-mno-relax-pic-calls -mmcount-ra-address -mframe-header-opt
-mno-frame-header-optfr fr do not try to memorise this. nobody memorises this. people look it up, and that's valid 💀
🦋 MMIX Options 🦋
Knuth's baby, and yes, the -mknuthdiv is right there in the list, I'm not making that up 😭✨
-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 🌟
tiny list, big -mam33 through -mam34 energy. generational upgrade, we love to see it 🎀
-mmult-bug -mno-mult-bug -mno-am33 -mam33 -mam33-2 -mam34 -mtune=cpu-type
-mreturn-pointer-on-d0 -mno-crt0 -mrelax -mliw -msetlb🫶 Moxie Options 🫶
four flags. four. a minimalist queen, truly the capsule wardrobe of option sets 💅
-meb -mel -mmul.x -mno-crt0✨ MSP430 Options ✨
lots of these end in = which means you gotta actually fill in the blank yourself. no hand-holding, just vibes ⭐
-msim -masm-hex -mmcu= -mcpu= -mlarge -msmall -mrelax -mwarn-mcu
-mcode-region= -mdata-region= -msilicon-errata= -msilicon-errata-warn=
-mhwmult= -minrt -mtiny-printf -mmax-inline-shift=🌈 NDS32 Options 🌈
endianness, register counts, cmov, and the cutest-named flag of the whole page: -mv3push. no I will not elaborate 🍓
-mbig-endian -mlittle-endian -mreduced-regs -mfull-regs -mcmov -mno-cmov
-mext-perf -mno-ext-perf -mext-perf2 -mno-ext-perf2 -mext-string
-mno-ext-string -mv3push -mno-v3push -m16bit -mno-16bit
-misr-vector-size=num -mcache-block-size=num -march=arch
-mcmodel=code-model -mctor-dtor -mrelax💀 Nvidia PTX Options 💀
three flags, zero fluff, the "I only own black clothing" of option sets 🖤
-m64 -mmainkernel -moptimize🧚 OpenRISC Options 🧚
mul and div, soft and hard, plus a whole little model-number section at the end. balance 💖
-mboard=name -mnewlib -mhard-mul -mhard-div -msoft-mul -msoft-div
-msoft-float -mhard-float -mdouble-float -munordered-float -mcmov -mror
-mrori -msext -msfimm -mshftimm -mcmodel=code-model🌸 PDP-11 Options 🌸
the vintage icon herself. -munix-asm, -mdec-asm, -mgnu-asm — choosing your assembler dialect like choosing an aesthetic 😌
-mfpu -msoft-float -mac0 -mno-ac0 -m40 -m45 -m10 -mint32 -mno-int16
-mint16 -mno-int32 -msplit -munix-asm -mdec-asm -mgnu-asm -mlra🎀 PowerPC Options 🎀
not me about to write this whole section and then getting redirected 💀 PowerPC is literally sending you to the RS/6000 block, so keep scrolling bestie, she's right there 👇
PowerPC Optionsthe actual list lives under RS/6000 and PowerPC Options below. this one is just the signpost 🪧
🦋 PRU Options 🦋
short, specific, does exactly what it says, no notes 🫶
-mmcu=mcu -minrt -mno-relax -mloop -mabi=variant💜 RISC-V Options 💜
RISC-V said "we're the future" and then handed over a list long enough to have its own weather system. obsessed with her confidence 🔥
-mbranch-cost=N-instruction -mplt -mno-plt -mabi=ABI-string -mfdiv
-mno-fdiv -mfence-tso -mno-fence-tso -mdiv -mno-div -misa-spec=ISA-spec-string
-march=ISA-string -mtune=processor-string -mpreferred-stack-boundary=num
-msmall-data-limit=N-bytes -msave-restore -mno-save-restore
-mshorten-memrefs -mno-shorten-memrefs -mstrict-align -mno-strict-align
-mcmodel=medlow -mcmodel=medany -mcmodel=large -mexplicit-relocs
-mno-explicit-relocs -mrelax -mno-relax -mriscv-attribute
-mno-riscv-attribute -malign-data=type -mbig-endian -mlittle-endian
-mstack-protector-guard=guard -mstack-protector-guard-reg=reg
-mstack-protector-guard-offset=offset -mcsr-check -mno-csr-check -mmovcc
-mno-movcc -minline-atomics -mno-inline-atomics -minline-strlen
-mno-inline-strlen -minline-strcmp -mno-inline-strcmp -minline-strncmp
-mno-inline-strncmp -mtls-dialect=desc -mtls-dialect=trad⭐ RL78 Options ⭐
-mmul=none, -mmul=g13, -mmul=g14 — pick your multiplier like you're picking a starter Pokémon 🌱🔥💧
-msim -mmul=none -mmul=g13 -mmul=g14 -mallregs -mcpu=g10 -mcpu=g13
-mcpu=g14 -mg10 -mg13 -mg14 -m64bit-doubles -m32bit-doubles
-msave-mduc-in-interrupts👑 RS/6000 and PowerPC Options 👑
and HERE she is. the main character. the one PowerPC was pointing at. This block is colossal and honestly it ate — cpu models, tuning, code models, calling conventions, struct returns, floating point, memory barriers, security features, the whole personality 🎀
-mcpu=cpu-type -mtune=cpu-type -mcmodel=code-model -mpowerpc64
-maltivec -mno-altivec -mpowerpc-gpopt -mno-powerpc-gpopt
-mpowerpc-gfxopt -mno-powerpc-gfxopt -mmfcrf -mno-mfcrf -mpopcntb
-mno-popcntb -mpopcntd -mno-popcntd -mfprnd -mno-fprnd -mcmpb
-mno-cmpb -mhard-dfp -mno-hard-dfp -mfull-toc -mminimal-toc
-mno-fp-in-toc -mno-sum-in-toc -m64 -m32 -mxl-compat
-mno-xl-compat -mpe -malign-power -malign-natural -msoft-float
-mhard-float -mmultiple -mno-multiple -mupdate -mno-update
-mavoid-indexed-addresses -mno-avoid-indexed-addresses -mfused-madd
-mno-fused-madd -mbit-align -mno-bit-align -mstrict-align
-mno-strict-align -mrelocatable -mno-relocatable -mrelocatable-lib
-mno-relocatable-lib -mtoc -mno-toc -mlittle -mlittle-endian
-mbig -mbig-endian -mdynamic-no-pic -mswdiv -msingle-pic-base
-mprioritize-restricted-insns=priority
-msched-costly-dep=dependence_type -minsert-sched-nops=scheme
-mcall-aixdesc -mcall-eabi -mcall-freebsd -mcall-linux
-mcall-netbsd -mcall-openbsd -mcall-sysv -mcall-sysv-eabi
-mcall-sysv-noeabi -mtraceback=traceback_type -maix-struct-return
-msvr4-struct-return -mabi=abi-type -msecure-plt -mbss-plt
-msplit-patch-nops -mlongcall -mno-longcall -mpltseq -mno-pltseq
-mblock-move-inline-limit=num -mblock-compare-inline-limit=num
-mblock-compare-inline-loop-limit=num -mno-block-ops-unaligned-vsx
-mstring-compare-inline-limit=num -misel -mno-isel -mvrsave
-mno-vrsave -mmulhw -mno-mulhw -mdlmzb -mno-dlmzb -mprototype
-mno-prototype -msim -mmvme -mads -myellowknife -memb -msdata
-msdata=opt -mreadonly-in-sdata -mvxworks -G num -mrecip
-mrecip=opt -mno-recip -mrecip-precision -mno-recip-precision
-mveclibabi=type -mfriz -mno-friz -mpointers-to-nested-functions
-mno-pointers-to-nested-functions -msave-toc-indirect
-mno-save-toc-indirect -mpower8-fusion -mno-mpower8-fusion -mcrypto
-mno-crypto -mhtm -mno-htm -mquad-memory -mno-quad-memory
-mquad-memory-atomic -mno-quad-memory-atomic -mcompat-align-parm
-mno-compat-align-parm -mfloat128 -mno-float128
-mfloat128-hardware -mno-float128-hardware -mgnu-attribute
-mno-gnu-attribute -mstack-protector-guard=guard
-mstack-protector-guard-reg=reg
-mstack-protector-guard-offset=offset -mprefixed -mno-prefixed
-mpcrel -mno-pcrel -mmma -mno-mmma -mrop-protect -mno-rop-protect
-mprivileged -mno-privilegedthat's the end of this chunk of machine-specific options, but there's more coming — don't close the tab 💖
💜 RX Options (RX Options) 💜
Renesas RX energy incoming, and she has options for days:
-m64bit-doubles,-m32bit-doubles,-fpu,-nofpu,-mcpu=,-mbig-endian-data,-mlittle-endian-data,-msmall-data,-msim,-mno-sim,-mas100-syntax,-mno-as100-syntax,-mrelax,-mmax-constant-size=,-mint-register=,-mpid,-mallow-string-insns,-mno-allow-string-insns,-mjsr,-mno-warn-multiple-fast-interrupts,-msave-acc-in-interrupts
🧊 S/390 and zSeries Options (S/390 and zSeries Options) 🧊
Big iron bestie!! Mainframe main character moment, no cap:
-mtune=cpu-type,-march=cpu-type,-mhard-float,-msoft-float,-mhard-dfp,-mno-hard-dfp,-mlong-double-64,-mlong-double-128,-mbackchain,-mno-backchain,-mpacked-stack,-mno-packed-stack,-msmall-exec,-mno-small-exec,-mmvcle,-mno-mvcle,-m64,-m31,-mdebug,-mno-debug,-mesa,-mzarch,-mhtm,-mvx,-mzvector,-mtpf-trace,-mno-tpf-trace,-mtpf-trace-skip,-mno-tpf-trace-skip,-mfused-madd,-mno-fused-madd,-mwarn-framesize,-mwarn-dynamicstack,-mstack-size,-mstack-guard,-mhotpatch=halfwords,halfwords
🎀 SH Options (SH Options) 🎀
SuperH said let me serve:
-m1,-m2,-m2e,-m2a-nofpu,-m2a-single-only,-m2a-single,-m2a,-m3,-m3e,-m4-nofpu,-m4-single-only,-m4-single,-m4,-m4a-nofpu,-m4a-single-only,-m4a-single,-m4a,-m4al,-mb,-ml,-mdalign,-mrelax,-mbigtable,-mfmovd,-mrenesas,-mno-renesas,-mnomacsave,-mieee,-mno-ieee,-mbitops,-misize,-minline-ic_invalidate,-mpadstruct,-mprefergot,-musermode,-multcost=number,-mdiv=strategy,-mdivsi3_libfunc=name,-mfixed-range=register-range,-maccumulate-outgoing-args,-matomic-model=atomic-model,-mbranch-cost=num,-mzdcbranch,-mno-zdcbranch,-mcbranch-force-delay-slot,-mfused-madd,-mno-fused-madd,-mfsca,-mno-fsca,-mfsrra,-mno-fsrra,-mpretend-cmove,-mtas
🌞 Solaris 2 Options (Solaris 2 Options) 🌞
Sun-kissed and unbothered:
-mclear-hwcap,-mno-clear-hwcap,-mimpure-text,-mno-impure-text,-pthreads
🌈 SPARC Options (SPARC Options) 🌈
The RISC girlies are THRIVING:
-mcpu=cpu-type,-mtune=cpu-type,-mcmodel=code-model,-mmemory-model=mem-model,-m32,-m64,-mapp-regs,-mno-app-regs,-mfaster-structs,-mno-faster-structs,-mflat,-mno-flat,-mfpu,-mno-fpu,-mhard-float,-msoft-float,-mhard-quad-float,-msoft-quad-float,-mstack-bias,-mno-stack-bias,-mstd-struct-return,-mno-std-struct-return,-munaligned-doubles,-mno-unaligned-doubles,-muser-mode,-mno-user-mode,-mv8plus,-mno-v8plus,-mvis,-mno-vis,-mvis2,-mno-vis2,-mvis3,-mno-vis3,-mvis3b,-mno-vis3b,-mvis4,-mno-vis4,-mvis4b,-mno-vis4b,-mcbcond,-mno-cbcond,-mfmaf,-mno-fmaf,-mfsmuld,-mno-fsmuld,-mpopc,-mno-popc,-msubxc,-mno-subxc,-mfix-at697f,-mfix-ut699,-mfix-ut700,-mfix-gr712rc
🍓 System V Options (System V Options) 🍓
Old-school but still cute:
-Qy,-Qn,-YP,paths,-Ym,dir
🦋 V850 Options (V850 Options) 🦋
Tiny chip, huge flag collection, we love to see it:
-mlong-calls,-mno-long-calls,-mep,-mno-ep,-mprolog-function,-mno-prolog-function,-mspace,-mtda=n,-msda=n,-mzda=n,-mapp-regs,-mno-app-regs,-mdisable-callt,-mno-disable-callt,-mv850e2v3,-mv850e2,-mv850e1,-mv850es,-mv850e,-mv850,-mv850e3v5,-mloop,-mrelax,-mlong-jumps,-msoft-float,-mhard-float,-mgcc-abi,-mrh850-abi,-mbig-switch
💾 VAX Options (VAX Options) 💾
The vintage queen herself:
-munix,-mgnu,-md,-md-float,-mg,-mg-float,-mlra
👑 Visium Options (Visium Options) 👑
Serving looks in silico:
-mdebug,-msim,-mfpu,-mno-fpu,-mhard-float,-msoft-float,-mcpu=cpu-type,-mtune=cpu-type,-msv-mode,-muser-mode
🌸 VMS Options (VMS Options) 🌸
Bestie, DEC is calling and it wants its flags back:
-mvms-return-codes,-mdebug-main=prefix,-mmalloc64,-mpointer-size=size
🧚 VxWorks Options (VxWorks Options) 🧚
Real-time realness:
-mrtp,-msmp,-non-static,-Bstatic,-Bdynamic,-Xbind-lazy,-Xbind-now
🔥 x86 Options (x86 Options) 🔥
Okay so THIS is the big one. The x86 option list is literally giving "I contain multitudes". Grab a snack, hydrate, and let's go:
-mtune=cpu-type,-march=cpu-type,-mtune-ctrl=feature-list,-mdump-tune-features,-mno-default,-mfpmath=unit,-masm=dialect,-mno-fancy-math-387,-mno-fp-ret-in-387,-m80387,-mhard-float,-msoft-float,-mno-wide-multiply,-mrtd,-malign-double,-mpreferred-stack-boundary=num,-mincoming-stack-boundary=num,-mcld,-mcx16,-msahf,-mmovbe,-mcrc32,-mmwait,-mrecip,-mrecip=opt,-mvzeroupper,-mprefer-avx128,-mprefer-vector-width=opt,-mpartial-vector-fp-math,-mmove-max=bits,-mstore-max=bits,-mnoreturn-no-callee-saved-registers,-mmmx,-msse,-msse2,-msse3,-mssse3,-msse4.1,-msse4.2,-msse4,-mavx,-mavx2,-mavx512f,-mavx512cd,-mavx512vl,-mavx512bw,-mavx512dq,-mavx512ifma,-mavx512vbmi,-msha,-maes,-mpclmul,-mfsgsbase,-mrdrnd,-mf16c,-mfma,-mpconfig,-mwbnoinvd,-mptwrite,-mclflushopt,-mclwb,-mxsavec,-mxsaves,-msse4a,-m3dnow,-m3dnowa,-mpopcnt,-mabm,-mbmi,-mtbm,-mfma4,-mxop,-madx,-mlzcnt,-mbmi2,-mfxsr,-mxsave,-mxsaveopt,-mrtm,-mhle,-mlwp,-mmwaitx,-mclzero,-mpku,-mthreads,-mgfni,-mvaes,-mwaitpkg,-mshstk,-mmanual-endbr,-mcet-switch,-mforce-indirect-call,-mavx512vbmi2,-mavx512bf16,-menqcmd,-mvpclmulqdq,-mavx512bitalg,-mmovdiri,-mmovdir64b,-mavx512vpopcntdq,-mavx512vnni,-mprfchw,-mrdpid,-mrdseed,-msgx,-mavx512vp2intersect,-mserialize,-mtsxldtrk,-mamx-tile,-mamx-int8,-mamx-bf16,-muintr,-mhreset,-mavxvnni,-mamx-fp8,-mavx512fp16,-mavxifma,-mavxvnniint8,-mavxneconvert,-mcmpccxadd,-mamx-fp16,-mprefetchi,-mraoint,-mamx-complex,-mavxvnniint16,-msm3,-msha512,-msm4,-mapxf,-musermsr,-mavx10.1,-mavx10.1-256,-mavx10.1-512,-mevex512,-mavx10.2,-mamx-avx512,-mamx-tf32,-mamx-transpose,-mmovrs,-mamx-movrs,-mcldemote,-mms-bitfields,-mno-align-stringops,-minline-all-stringops,-minline-stringops-dynamically,-mstringop-strategy=alg,-mkl,-mwidekl,-mmemcpy-strategy=strategy,-mmemset-strategy=strategy,-mpush-args,-maccumulate-outgoing-args,-m128bit-long-double,-m96bit-long-double,-mlong-double-64,-mlong-double-80,-mlong-double-128,-mregparm=num,-msseregparm,-mveclibabi=type,-mvect8-ret-in-mem,-mpc32,-mpc64,-mpc80,-mdaz-ftz,-mstackrealign,-momit-leaf-frame-pointer,-mno-red-zone,-mno-tls-direct-seg-refs,-mcmodel=code-model,-mabi=name,-maddress-mode=mode,-m32,-m64,-mx32,-m16,-miamcu,-mlarge-data-threshold=num,-msse2avx,-mfentry,-mrecord-mcount,-mnop-mcount,-m8bit-idiv,-minstrument-return=type,-mfentry-name=name,-mfentry-section=name,-mavx256-split-unaligned-load,-mavx256-split-unaligned-store,-malign-data=type,-mstack-protector-guard=guard,-mstack-protector-guard-reg=reg,-mstack-protector-guard-offset=offset,-mstack-protector-guard-symbol=symbol,-mgeneral-regs-only,-mcall-ms2sysv-xlogues,-mrelax-cmpxchg-loop,-mindirect-branch=choice,-mfunction-return=choice,-mindirect-branch-register,-mharden-sls=choice,-mindirect-branch-cs-prefix,-mneeded,-mno-direct-extern-access,-munroll-only-small-loops,-mlam=choice
🪟 x86 Windows Options 🪟
See Cygwin and MinGW Options. (<- literally just a redirect, bestie, no tea here)
⭐ Xstormy16 Options (Xstormy16 Options) ⭐
One single flag. Minimalist icon behaviour:
-msim
🌺 Xtensa Options (Xtensa Options) 🌺
Embedded baddie with range:
-mconst16,-mno-const16,-mfused-madd,-mno-fused-madd,-mforce-no-pic,-mserialize-volatile,-mno-serialize-volatile,-mtext-section-literals,-mno-text-section-literals,-mauto-litpools,-mno-auto-litpools,-mtarget-align,-mno-target-align,-mlongcalls,-mno-longcalls,-mabi=abi-type,-mextra-l32r-costs=cycles,-mstrict-align,-mno-strict-align
💙 zSeries Options 💙
See S/390 and zSeries Options. (Yeah, we already did this one above — she's just on her "as seen on TV" rerun era 💀)
✨ Options Controlling the Kind of Output ✨
Okay so compiling a whole program? It's a four-act play, and the order is non-negotiable, bestie: preprocessing, then compilation proper, then assembly, then linking. Always in that order, periodt. 🔥
GCC can preprocess and compile a bunch of files either into several assembler input files, or into just one assembler input file. After that, every assembler input file makes an object file, and then linking swoops in and combines all the object files — both the ones it just freshly compiled and the ones you handed it as input — into one executable file. It's giving assembly line but make it glamorous.
For any given input file, the file name suffix is what decides which kind of compilation happens. So like, pay attention to your extensions, bestie:
file.c— C source code that must be preprocessed.file.i— C source code that should not be preprocessed.file.ii— C++ source code that should not be preprocessed.file.m— Objective-C source code. Heads up: you must link with thelibobjclibrary to make an Objective-C program work.file.mi— Objective-C source code that should not be preprocessed.file.mm,file.M— Objective-C++ source code. Note that you must link with thelibobjclibrary to make an Objective-C++ program work. And note that.Mrefers to a literal capital M. 👀file.mii— Objective-C++ source code that should not be preprocessed.file.h— C, C++, Objective-C or Objective-C++ header file to be turned into a precompiled header (that's the default), or a C, C++ header file to be turned into an Ada spec (via the-fdump-ada-specswitch).file.cc,file.cp,file.cxx,file.cpp,file.CPP,file.c++,file.C— C++ source code that must be preprocessed. Note that in.cxx, the last two letters must both be literallyx. Likewise,.Crefers to a literal capital C.file.mm,file.M— Objective-C++ source code that must be preprocessed.file.mii— Objective-C++ source code that should not be preprocessed.file.hh,file.H,file.hp,file.hxx,file.hpp,file.HPP,file.h++,file.tcc— C++ header file to be turned into a precompiled header or Ada spec.file.f,file.for,file.ftn,file.fi— Fixed form Fortran source code that should not be preprocessed.file.F,file.FOR,file.fpp,file.FPP,file.FTN— Fixed form Fortran source code that must be preprocessed (with the traditional preprocessor).file.f90,file.f95,file.f03,file.f08,file.fii— Free form Fortran source code that should not be preprocessed.file.F90,file.F95,file.F03,file.F08— Free form Fortran source code that must be preprocessed (with the traditional preprocessor).file.cob,file.COB,file.cbl,file.CBL— COBOL source code.file.go— Go source code.file.d— D source code.file.di— D interface file.file.dd— D documentation code (Ddoc).file.ads— Ada source code file that contains a library unit declaration (a declaration of a package, subprogram, or generic, or a generic instantiation), or a library unit renaming declaration (a package, generic, or subprogram renaming declaration). Files like this are also called specs.file.adb— Ada source code file containing a library unit body (a subprogram or package body). Files like this are also called bodies.file.s— Assembler code.file.S,file.sx— Assembler code that must be preprocessed.- other — An object file to be fed straight into linking. Any file name with no recognized suffix is treated this way. Surprise! 🎁
okay so instead of just praying the filename suffix tells gcc the truth, you can straight-up announce what language you're feeding it. Enter the -x option 💅
-xlanguage — say it with your chest: this is the language for the input files that come after it, no guessing off the file name suffix allowed. This option applies to all following input files until the next-xoption shows up. The possible values for language are, in all their glory:
c c-header cpp-output
c++ c++-header c++-system-header c++-user-header c++-cpp-output
objective-c objective-c-header objective-c-cpp-output objc-cpp-output
objective-c++ objective-c++-header objective-c++-cpp-output
objc++-cpp-output
assembler assembler-with-cpp
ada adascil adawhy
cobol
d
f77 f77-cpp-input f95 f95-cpp-input
go
modula-2 modula-2-cpp-output
rust
ltolowkey important bestie:-xdoes not imply a particular language standard. Like,-x f77may also require-std=legacyfor some older source codes. Two separate things, don't get it twisted 🫶
-x none— turn off any specification of a language, so subsequent files get handled according to their file name suffixes, exactly as if-xhad never been used at all. it's giving "undo, undo" 🙈
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 -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 — iconic behavior honestly, it just vibes 💀
-c— compile or assemble the source files, but do not link. The linking stage simply is not done, periodt. 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. (they get the silent treatment ✨)
-S— stop after the stage of compilation proper; do not assemble. The output is in the form of an assembler code file for each non-assembler input file specified.
By default, the assembler file name for a source file is made by replacing the suffix .c, .i, etc., with .s.
Input files that don't require compilation are ignored.
-E— stop after the preprocessing stage; do not run the compiler proper. The output is in the form of preprocessed source code, which is sent to the standard output.
Input files that don't require preprocessing are ignored.
-ofile — place the primary output in file. This applies to whatever sort of output is being produced, whether it be an executable file, an object file, an assembler file or preprocessed C code. 🎯
If -o is not specified, the default is to put an executable file in a.out, the object file for source.suffix in source.o, its assembler file in source.s, a precompiled header file in source.suffix.gch, and all preprocessed C source on standard output.
Though -o names only the primary output, it also affects the naming of auxiliary and dump outputs. See the examples below. Unless overridden, both auxiliary outputs and dump outputs are placed in the same directory as the primary output. In auxiliary outputs, the suffix of the input file is replaced with that of the auxiliary output file type; in dump outputs, the suffix of the dump file is appended to the input file suffix. In compilation commands, the base name of both auxiliary and dump outputs is that of the primary output; in compile and link commands, the primary output name, minus the executable suffix, is combined with the input file name. If both share the same base name, disregarding the suffix, the result of the combination is that base name, otherwise, they are concatenated, separated by a dash.
gcc -c foo.c ...will use foo.o as the primary output, and place aux outputs and dumps next to it, e.g., aux file foo.dwo for -gsplit-dwarf, and dump file foo.c.???r.final for -fdump-rtl-final.
If a non-linker output file is explicitly specified, aux and dump files by default take the same base name:
gcc -c foo.c -o dir/foobar.o ...will name aux outputs dir/foobar.* and dump outputs dir/foobar.c.*.
A linker output will instead prefix aux and dump outputs:
gcc foo.c bar.c -o dir/foobar ...will generally name aux outputs dir/foobar-foo.* and dir/foobar-bar.*, and dump outputs dir/foobar-foo.c.* and dir/foobar-bar.c.*.
The one exception to the above is when the executable shares the base name with the single input:
gcc foo.c -o dir/foo ...in which case aux outputs are named dir/foo.* and dump outputs named dir/foo.c.*.
The location and the names of auxiliary and dump outputs can be adjusted by the options -dumpbase, -dumpbase-ext, -dumpdir, -save-temps=cwd, and -save-temps=obj.
-dumpbasedumpbase — this option sets the base name for auxiliary and dump output files. It does not affect the name of the primary output file. Intermediate outputs, when preserved, are not regarded as primary outputs, but as auxiliary outputs:
gcc -save-temps -S foo.csaves the (no longer) temporary preprocessed file in foo.i, and then compiles to the (implied) output file foo.s, whereas:
gcc -save-temps -dumpbase save-foo -c foo.cpreprocesses to in save-foo.i, compiles to save-foo.s (now an intermediate, thus auxiliary output), and then assembles to the (implied) output file foo.o.
Absent this option, dump and aux files take their names from the input file, or from the (non-linker) output file, if one is explicitly specified: dump output files (e.g. those requested by -fdump-* options) with the input name suffix, and aux output files (those requested by other non-dump options, e.g. "-save-temps", "-gsplit-dwarf", "-fcallgraph-info") without it.
Similar suffix differentiation of dump and aux outputs can be attained for explicitly-given -dumpbase basename.suf by also specifying -dumpbase-ext .suf.
If dumpbase is explicitly specified with any directory component, any dumppfx specification (e.g. -dumpdir or -save-temps=*) is ignored, and instead of appending to it, dumpbase fully overrides it:
gcc foo.c -c -o dir/foo.o -dumpbase alt/foo \
-dumpdir pfx- -save-temps=cwd ...creates auxiliary and dump outputs named alt/foo.*, disregarding dir/ in -o, the ./ prefix implied by -save-temps=cwd, and pfx- in -dumpdir.
When -dumpbase is specified in a command that compiles multiple inputs, or that compiles and then links, it may be combined with dumppfx, as specified under -dumpdir. Then, each input file is compiled using the combined dumppfx, and default values for dumpbase and auxdropsuf are computed for each input file:
gcc foo.c bar.c -c -dumpbase main ...creates foo.o and bar.o as primary outputs, and avoids overwriting the auxiliary and dump outputs by using the dumpbase as a prefix, creating auxiliary and dump outputs named main-foo.* and main-bar.*.
An empty string specified as dumpbase avoids the influence of the output basename in the naming of auxiliary and dump outputs during compilation, computing default values :
gcc -c foo.c -o dir/foobar.o -dumpbase " ...will name aux outputs dir/foo.* and dump outputs dir/foo.c.*. Note how their basenames are taken from the input name, but the directory still defaults to that of the output.
The empty-string dumpbase does not prevent the use of the output basename for outputs during linking:
gcc foo.c bar.c -o dir/foobar -dumpbase " -flto ...The compilation of the source files will name auxiliary outputs dir/foo.* and dir/bar.*, and dump outputs dir/foo.c.* and dir/bar.c.*. LTO recompilation during linking will use dir/foobar. as the prefix for dumps and auxiliary files.
-dumpbase-extauxdropsuf — when forming the name of an auxiliary (but not a dump) output file, drop trailing auxdropsuf from dumpbase before appending any suffixes. If not specified, this option defaults to the suffix of a default dumpbase, i.e., the suffix of the input file when-dumpbaseis not present in the command line, or dumpbase is combined with dumppfx. 🍓
gcc foo.c -c -o dir/foo.o -dumpbase x-foo.c -dumpbase-ext .c ...creates dir/foo.o as the main output, and generates auxiliary outputs in dir/x-foo.*, taking the location of the primary output, and dropping the .c suffix from the dumpbase. Dump outputs retain the suffix: dir/x-foo.c.*.
This option is disregarded if it does not match the suffix of a specified dumpbase, except as an alternative to the executable suffix when appending the linker output base name to dumppfx, as specified below:
gcc foo.c bar.c -o main.out -dumpbase-ext .out ...creates main.out as the primary output, and avoids overwriting the auxiliary and dump outputs by using the executable name minus auxdropsuf as a prefix, creating auxiliary outputs named main-foo.* and main-bar.* and dump outputs named main-foo.c.* and main-bar.c.*.
-dumpdirdumppfx — when forming the name of an auxiliary or dump output file, use dumppfx as a prefix:
gcc -dumpdir pfx- -c foo.c ...creates foo.o as the primary output, and auxiliary outputs named pfx-foo.*, combining the given dumppfx with the default dumpbase derived from the default primary output, derived in turn from the input name. Dump outputs also take the input name suffix: pfx-foo.c.*.
If dumppfx is to be used as a directory name, it must end with a directory separator:
gcc -dumpdir dir/ -c foo.c -o obj/bar.o ...creates obj/bar.o as the primary output, and auxiliary outputs named dir/bar.*, combining the given dumppfx with the default dumpbase derived from the primary output name. Dump outputs also take the input name suffix: dir/bar.c.*.
It defaults to the location of the output file, unless the output file is a special file like "/dev/null". Options -save-temps=cwd and -save-temps=obj override this default, just like an explicit -dumpdir option. In case multiple such options are given, the last one prevails:
gcc -dumpdir pfx- -c foo.c -save-temps=obj ...so like, this spits out foo.o, and the auxiliary outputs are named foo.* because -save-temps=* straight-up overrides the dumppfx that the earlier -dumpdir option set up. And bestie, it does NOT matter that =obj is the default for -save-temps, or that the output directory is lowkey just the current directory. Dump outputs? They're named foo.c.*. 💅
When you're compiling from multiple input files and -dumpbase is specified, dumpbase (minus an auxdropsuf suffix) plus a dash get appended to (or override, if they contain any directory components) an explicit or defaulted dumppfx, so every one of the multiple compilations gets its own differently-named aux and dump outputs. 🦋
gcc foo.c bar.c -c -dumpdir dir/pfx- -dumpbase main ...outputs auxiliary dumps to dir/pfx-main-foo.* and dir/pfx-main-bar.*, appending dumpbase- to dumppfx. Dump outputs keep the input file suffix: dir/pfx-main-foo.c.* and dir/pfx-main-bar.c.*, respectively. Now contrast that with a single-input compilation: 🌸
gcc foo.c -c -dumpdir dir/pfx- -dumpbase main ...That one, applying -dumpbase to a single source, does not compute and append a separate dumpbase per input file. Its auxiliary and dump outputs go in dir/pfx-main.*. ✨
When you're compiling and then linking from multiple input files, a defaulted or explicitly specified dumppfx also goes through that dumpbase- transformation above (like the compilation of foo.c and bar.c above, but without -c). If neither -dumpdir nor -dumpbase are given, the linker output base name, minus auxdropsuf if specified, or the executable suffix otherwise, plus a dash is appended to the default dumppfx instead. But note, unlike earlier cases of linking: 💖
gcc foo.c bar.c -dumpdir dir/pfx- -o main ...does not append the output name main to dumppfx, because -dumpdir is explicitly specified. The goal is that the explicitly-specified dumppfx may contain the specified output name as part of the prefix, if desired; only an explicitly-specified -dumpbase would be combined with it, in order to avoid simply discarding a meaningful option. 🫶
When compiling and then linking from a single input file, the linker output base name will only be appended to the default dumppfx as above if it does not share the base name with the single input file name. This has been covered in single-input linking cases above, but not with an explicit -dumpdir that inhibits the combination, even if overridden by -save-temps=*: 👑
gcc foo.c -dumpdir alt/pfx- -o dir/main.exe -save-temps=cwd ...Auxiliary outputs are named foo.*, and dump outputs foo.c.*, in the current working directory as ultimately requested by -save-temps=cwd. 🌈
Summing it all up for an intuitive though slightly imprecise data flow: the primary output name is broken into a directory part and a basename part; dumppfx is set to the former, unless overridden by -dumpdir or -save-temps=*, and dumpbase is set to the latter, unless overridden by -dumpbase. If there are multiple inputs or linking, this dumpbase may be combined with dumppfx and taken from each input file. Auxiliary output names for each input are formed by combining dumppfx, dumpbase minus suffix, and the auxiliary output suffix; dump output names are only different in that the suffix from dumpbase is retained. 💀
When it comes to auxiliary and dump outputs created during LTO recompilation, a combination of dumppfx and dumpbase, as given or as derived from the linker output name but not from inputs, even in cases in which this combination would not otherwise be used as such, is passed down with a trailing period replacing the compiler-added dash, if any, as a -dumpdir option to lto-wrapper; being involved in linking, this program does not normally get any -dumpbase and -dumpbase-ext, and it ignores them. 😭
When running sub-compilers, lto-wrapper appends LTO stage names to the received dumppfx, ensures it contains a directory component so that it overrides any -dumpdir, and passes that as -dumpbase to sub-compilers. 🔥
-v— prints (on standard error output) the commands executed to run the stages of compilation. It also prints the version number of the compiler driver program and of the preprocessor and the compiler proper. ✨-###— Like-vexcept the commands are not executed and arguments are quoted unless they contain only alphanumeric characters or"./-_". This is useful for shell scripts to capture the driver-generated command lines. 🎀--help— Print (on the standard output) a description of the command-line options understood bygcc. If the-voption is also specified then--helpis also passed on to the various processes invoked bygcc, so that they can display the command-line options they accept. If the-Wextraoption has also been specified (prior to the--helpoption), then command-line options that have no documentation associated with them are also displayed. 💖--target-help— Print (on the standard output) a description of target-specific command-line options for each tool. For some targets extra target-specific information may also be printed. 🌸--help={class|[^]qualifier}[,...]— Print (on the standard output) a description of the command-line options understood by the compiler that fit into all specified classes and qualifiers. These are the supported classes:optimizers— Display all of the optimization options supported by the compiler. 🔥warnings— Display all of the options controlling warning messages produced by the compiler. ⭐target— Display target-specific options. Unlike the--target-helpoption however, target-specific options of the linker and assembler are not displayed. This is because those tools do not currently support the extended--help=syntax. 👑params— Display the values recognized by the--paramoption. 🦋language— Display the options supported for language, where language is the name of one of the languages supported in this version of GCC. If an option is supported by all languages, one needs to selectcommonclass. 🌈common— Display the options that are common to all languages. 💜
These are the supported qualifiers:
undocumented— Display only those options that are undocumented. 🙈joined— Display options taking an argument that appears after an equal sign in the same continuous piece of text, such as:--help=target. 🍓separate— Display options taking an argument that appears as a separate word following the original option, such as:-o output-file. 🧚
Thus for example to display all the undocumented target-specific switches supported by the compiler, use:
--help=target,undocumentedThe sense of a qualifier can be inverted by prefixing it with the ^ character, so for example to display all binary warning options (i.e., ones that are either on or off and that do not take an argument) that have a description, use:
--help=warnings,^joined,^undocumentedThe argument to --help= should not consist solely of inverted qualifiers.
Combining several classes is possible, although this usually restricts the output so much that there is nothing to display. One case where it does work, however, is when one of the classes is target. For example, to display all the target-specific optimization options, use:
--help=target,optimizersThe --help= option can be repeated on the command line. Each successive use displays its requested class of options, skipping those that have already been displayed. If --help is also specified anywhere on the command line then this takes precedence over any --help= option. 💅
If the -Q option appears on the command line before the --help= option, then the descriptive text displayed by --help= is changed. Instead of describing the displayed options, an indication is given as to whether the option is enabled, disabled or set to a specific value (assuming that the compiler knows this at the point where the --help= option is used). 😭
Here is a truncated example from the ARM port of gcc:
% gcc -Q -mabi=2 --help=target -c
The following options are target specific:
-mabi= 2
-mabort-on-noreturn [disabled]
-mapcs [disabled]The output is sensitive to the effects of previous command-line options, so for example it is possible to find out which optimizations are enabled at -O2 by using:
-Q -O2 --help=optimizersAlternatively you can discover which binary optimizations are enabled by -O3 by using:
gcc -c -Q -O3 --help=optimizers > /tmp/O3-opts
gcc -c -Q -O2 --help=optimizers > /tmp/O2-opts
diff /tmp/O2-opts /tmp/O3-opts | grep enabled--version— Display the version number and copyrights of the invoked GCC. 💖-pass-exit-codes— Normally thegccprogram exits with the code of1if any phase of the compiler returns a non-success return code. If you specify-pass-exit-codes, thegccprogram instead returns with the numerically highest error produced by any phase returning an error indication. The C, C++, and Fortran front ends return4if an internal compiler error is encountered. 💀-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. 🌸-specs=file— Process file after the compiler reads in the standard specs file, in order to override the defaults which thegccdriver program uses when determining what switches to pass tocc1,cc1plus,as,ld, etc. More than one-specs=filecan be specified on the command line, and they are processed in order, from left to right. 🦋-wrapper— Invoke all subcommands under a wrapper program. The name of the wrapper program and its parameters are passed as a comma separated list. ✨
gcc -c t.c -wrapper gdb,--argsThis invokes all subprograms of gcc under gdb --args, thus the invocation of cc1 is gdb --args cc1 .... 👑
-ffile-prefix-map=old=new— okay so when you're compiling files that live in directory old, this records any references to them in the compilation result as if they actually lived in directory new instead. Specifying this one is literally equivalent to specifying all the individual-f*-prefix-mapoptions at once. It's the move for making reproducible builds that are location independent. 💅 Heads up though: directories referenced by directives are not affected by these options. Also see-fmacro-prefix-map,-fdebug-prefix-map,-fprofile-prefix-mapand-fcanon-prefix-map.-fcanon-prefix-map— so for the-f*-prefix-mapoptions, the comparison of old prefix against the filename that'd normally be referenced in the compilation result is usually done with a textual comparison of the prefixes, or by ignoring character case on case insensitive filesystems, and by treating slashes and backslashes as equal on DOS based filesystems. This flag makes those comparisons happen on canonicalized paths of old and the referenced filename instead. ✨-fplugin=name.so— loads the plugin code in filename.so, which is assumed to be a shared object that the compiler willdlopen. The base name of the shared object file is what identifies the plugin for argument parsing purposes (peep-fplugin-arg-name-key=valuebelow 👀). Every plugin should define the callback functions specified in the Plugins API. no cap.-fplugin-arg-name-key=value— defines an argument called key with a value of value for the plugin called name. 🫶-fdump-ada-spec[-slim]— for C and C++ source and include files, generates corresponding Ada specs. iconic.-fada-spec-parent=unit— used together with-fdump-ada-spec[-slim]above, generates Ada specs as child units of parent unit.-fdump-go-spec=file— for input files in any language, generates corresponding Go declarations in file. This spits out Go"const","type","var", and"func"declarations, which lowkey might be a useful way to start writing a Go interface to code written in some other language. 🌈@file— reads command-line options from file. The options that get read are inserted in place of the original@fileoption. If file doesn't exist, or can't be read, then the option is treated literally, and not removed.
Options inside file are separated by whitespace. A whitespace character can be included in an option by surrounding the entire option in either single or double quotes. Any character (including a backslash) can be included by prefixing the character you want with a backslash. And the file may itself contain additional @file options — any such options get processed recursively. 🌀
🦋 Compiling C++ Programs 🦋
C++ source files conventionally use one of the suffixes .C, .cc, .cpp, .CPP, .c++, .cp, or .cxx; C++ header files often use .hh, .hpp, .H, or (for shared template code) .tcc; preprocessed C++ files use the suffix .ii; and C++20 module interface units sometimes use .ixx, .cppm, .cxxm, .c++m, or .ccm.
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).
But bestie, listen — the use of gcc does not add the C++ library. 😭 g++ is a program that calls GCC and automatically specifies linking against the C++ library. It treats .c, .h and .i files as C++ source files instead of C source files unless -x is used. This program is also useful when precompiling a C header file with a .h extension for use in C++ compilations. On many systems, g++ is also installed with the name c++. 💅
When you compile C++ programs, you may specify many of the same command-line options that you use for compiling programs in any language; or command-line options meaningful for C and related languages; or options that are meaningful only for C++ programs.
👑 Options Controlling C Dialect 👑
The following options control the dialect of C (or languages derived from C, such as C++, Objective-C and Objective-C++) that the compiler accepts:
-ansi— in C mode, this is equivalent to-std=c90. In C++ mode, it's equivalent to-std=c++98. 🎀-std=— determines the language standard. This option is currently only supported when compiling C or C++.
The compiler can accept several base standards, like c90 or c++98, and GNU dialects of those standards, like gnu90 or gnu++98. When a base standard is specified, the compiler accepts all programs following that standard plus those using GNU extensions that don't contradict it. For example, -std=c90 turns off certain features of GCC that are incompatible with ISO C90, such as the "asm" and "typeof" keywords, but not other GNU extensions that don't have a meaning in ISO C90, such as omitting the middle term of a "?:") expression. On the other hand, when a GNU dialect of a standard is specified, all features supported by the compiler are enabled, even when those features change the meaning of the base standard. As a result, some strict-conforming programs may be rejected. The particular standard is used by -Wpedantic to identify which features are GNU extensions given that version of the standard. For example -std=gnu90 -Wpedantic warns about C++ style // comments, while -std=gnu99 -Wpedantic does not.
A value for this option must be provided; possible values are:
c90,c89,iso9899:1990— supports all ISO C90 programs (certain GNU extensions that conflict with ISO C90 are disabled). Same as-ansifor C code.iso9899:199409— ISO C90 as modified in amendment 1.c99,c9x,iso9899:1999,iso9899:199x— ISO C99. This standard is substantially completely supported, modulo bugs and floating-point issues (mainly but not entirely relating to optional C99 features from Annexes F and G). See<https://gcc.gnu.org/c99status.html>for more info. The namesc9xandiso9899:199xare deprecated. ⚠️c11,c1x,iso9899:2011— ISO C11, the 2011 revision of the ISO C standard. This standard is substantially completely supported, modulo bugs, floating-point issues (mainly but not entirely relating to optional C11 features from Annexes F and G) and the optional Annexes K (Bounds-checking interfaces) and L (Analyzability). The namec1xis deprecated.c17,c18,iso9899:2017,iso9899:2018— ISO C17, the 2017 revision of the ISO C standard (published in 2018). This standard is the same as C11 except for corrections of defects (all of which are also applied with-std=c11) and a new value of"__STDC_VERSION__", and so is supported to the same extent as C11.c23,c2x,iso9899:2024— ISO C23, the 2023 revision of the ISO C standard (published in 2024). The namec2xis deprecated.c2y— the next version of the ISO C standard, still under development. The support for this version is experimental and incomplete. 🙈gnu90,gnu89— GNU dialect of ISO C90 (including some C99 features).gnu99,gnu9x— GNU dialect of ISO C99. The namegnu9xis deprecated.gnu11,gnu1x— GNU dialect of ISO C11. The namegnu1xis deprecated.gnu17,gnu18— GNU dialect of ISO C17.gnu23,gnu2x— GNU dialect of ISO C23. This is the default for C code. The namegnu2xis deprecated.gnu2y— the next version of the ISO C standard, still under development, plus GNU extensions. The support for this version is experimental and incomplete. The namegnu2xis deprecated.c++98,c++03— the 1998 ISO C++ standard plus the 2003 technical corrigendum and some additional defect reports. Same as-ansifor C++ code.gnu++98,gnu++03— GNU dialect of-std=c++98.c++11,c++0x— the 2011 ISO C++ standard plus amendments. The namec++0xis deprecated.gnu++11,gnu++0x— GNU dialect of-std=c++11. The namegnu++0xis deprecated.c++14,c++1y— the 2014 ISO C++ standard plus amendments. The namec++1yis deprecated.gnu++14,gnu++1y— GNU dialect of-std=c++14. The namegnu++1yis deprecated.c++17,c++1z— the 2017 ISO C++ standard plus amendments. The namec++1zis deprecated.gnu++17,gnu++1z— GNU dialect of-std=c++17. This is the default for C++ code. The namegnu++1zis deprecated.c++20,c++2a— the 2020 ISO C++ standard plus amendments. Support is experimental, and could change in incompatible ways in future releases. The namec++2ais deprecated.gnu++20,gnu++2a— GNU dialect of-std=c++20. Support is experimental, and could change in incompatible ways in future releases. The namegnu++2ais deprecated.c++23,c++2b— the 2023 ISO C++ standard plus amendments (published in 2024). Support is experimental, and could change in incompatible ways in future releases. The namec++2bis deprecated.gnu++23,gnu++2b— GNU dialect of-std=c++23. Support is experimental, and could change in incompatible ways in future releases. The namegnu++2bis deprecated.c++2c,c++26— the next revision of the ISO C++ standard, planned for 2026. Support is highly experimental, and will almost certainly change in incompatible ways in future releases. 💀gnu++2c,gnu++26— GNU dialect of-std=c++2c. Support is highly experimental, and will almost certainly change in incompatible ways in future releases. 💀
-aux-info filename— outputs 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 the declarations themselves, bestie, that file is also doing the most in its comments — it spills the origin of every single declaration (which source file, which line, no cap 🫶), whether that declaration was implicit, prototyped or unprototyped (I, N for new or O for old, respectively — that's the first character right after the line number and the colon), and whether it came from a declaration or a definition (C or F, respectively — that's the next character after that one). 🎀
And when it's a function definition? You also get a K&R-style list of arguments followed by their declarations, sitting right there inside the comments, after the declaration. It's giving annotated receipts and honestly we love that for us. ✨
-fno-asm— okay so this one says do not recognize"asm","inline"or"typeof"as a keyword, so your code can literally just use these words as identifiers. Instead, you can use the keywords"__asm__","__inline__"and"__typeof__". In C,-ansiimplies-fno-asm. 😌
In C++, "inline" is a standard keyword and is not affected by this switch. You may want to use the -fno-gnu-keywords flag instead, which disables "typeof" but not "asm" and "inline". 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. In C23 mode (-std=c23 or -std=gnu23), this switch only affects the "asm" keyword, since "typeof" is a standard keyword in ISO C23. periodt. 💅
-fno-builtin,-fno-builtin-function— don't recognize built-in functions that do not begin with__builtin_as prefix. 🚫
GCC normally generates special code to handle certain built-in functions more efficiently; for instance, calls to "alloca" may become single instructions which adjust the stack directly, and calls to "memcpy" may become inline copy loops. The resulting code is often both smaller and faster, but since the function calls no longer appear as such, you cannot set a breakpoint on those calls, nor can you change the behavior of the functions by linking with a different library. In addition, when a function is recognized as a built-in function, GCC may use information about that function to warn about problems with calls to that function, or to generate more efficient code, even if the resulting code still contains calls to that function. For example, warnings are given with -Wformat for bad calls to "printf" when "printf" is built in and "strlen" is known not to modify global memory. 👑
With the -fno-builtin-function option only the built-in function function is disabled. function must not begin with __builtin_. If a function is named that is not built-in in this version of GCC, this option is ignored. There is no corresponding -fbuiltin-function option; if you wish to enable built-in functions selectively when using -fno-builtin or -ffreestanding, you may define macros such as:
#define abs(n) __builtin_abs ((n))
#define strcpy(d, s) __builtin_strcpy ((d), (s))-fcond-mismatch— allow conditional expressions with mismatched types in the second and third arguments. The value of such an expression isvoid. 🫠 This option is not supported for C++.-ffreestanding— assert that compilation targets a freestanding environment. This implies-fno-builtin. A freestanding environment is one in which the standard library may not exist, and program startup may not necessarily be at"main". The most obvious example is an OS kernel. 🌈 This is equivalent to-fno-hosted.-fgimple— enable parsing of function definitions marked with"__GIMPLE". This is an experimental feature that allows unit testing of GIMPLE passes. 🧪 (experimental girlie, handle with care!)-fgnu-tm— when the option-fgnu-tmis specified, the compiler generates code for the Linux variant of Intel's current Transactional Memory ABI specification document (Revision 1.1, May 6 2009). This is an experimental feature whose interface may change in future versions of GCC, as the official specification changes. Please note that not all architectures are supported for this feature. 🦋
For more information on GCC's support for transactional memory,
Note that the transactional memory feature is not supported with non-call exceptions (-fnon-call-exceptions).
-fgnu89-inline— the option-fgnu89-inlinetells GCC to use the traditional GNU semantics for"inline"functions when in C99 mode. 🎀
Using this option is roughly equivalent to adding the "gnu_inline" function attribute to all inline functions.
The option -fno-gnu89-inline explicitly tells GCC to use the C99 semantics for "inline" when in C99 or gnu99 mode (i.e., it specifies the default behavior). This option is not supported in -std=c90 or -std=gnu90 mode.
The preprocessor macros "__GNUC_GNU_INLINE__" and "__GNUC_STDC_INLINE__" may be used to check which semantics are in effect for "inline" functions. iykyk 💅
-fhosted— assert that compilation targets a hosted environment. This implies-fbuiltin. A hosted environment is one in which the entire standard library is available, and in which"main"has a return type of"int". Examples are nearly everything except a kernel. 🌸 This is equivalent to-fno-freestanding.-flax-vector-conversions— allow implicit conversions between vectors with differing numbers of elements and/or incompatible element types. > This option should not be used for new code. 🚫 (like, respectfully, it's giving legacy.)-fms-extensions— accept some non-standard constructs used in Microsoft header files. 🪟
In C++ code, this allows member names in structures to be similar to previous types declarations. Like:
typedef int UOW;
struct ABC {
UOW UOW;
};Some cases of unnamed fields in structures and unions are only accepted with this option.
Note that this option is off for all targets except for x86 targets using ms-abi.
-fpermitted-flt-eval-methods=style— ISO/IEC TS 18661-3 defines new permissible values for"FLT_EVAL_METHOD"that indicate that operations and constants with a semantic type that is an interchange or extended format should be evaluated to the precision and range of that type. These new values are a superset of those permitted under C99/C11, which does not specify the meaning of other positive values of"FLT_EVAL_METHOD". As such, code conforming to C11 may not have been written expecting the possibility of the new values. 😵💫
-fpermitted-flt-eval-methods specifies whether the compiler should allow only the values of "FLT_EVAL_METHOD" specified in C99/C11, or the extended set of values specified in ISO/IEC TS 18661-3.
style is either "c11" or "ts-18661-3" as appropriate.
The default when in a standards compliant mode (-std=c11 or similar) is -fpermitted-flt-eval-methods=c11. The default when in a GNU dialect (-std=gnu11 or similar) is -fpermitted-flt-eval-methods=ts-18661-3. 🌟
The -fdeps-* options are used to extract structured dependency information for a source. This involves determining what resources provided by other source files will be required to compile the source as well as what resources are provided by the source. This information can be used to add required dependencies between compilation rules of dependent sources based on their contents rather than requiring such information be reflected within the build tools as well. 🧚 (it's giving ✨ receipts, but for your build system.)
-fdeps-file=file— where to write structured dependency information. 📝-fdeps-format=format— the format to use for structured dependency information.p1689r5is the only supported format right now. Note that when this argument is specified, the output of-MFis stripped of some information (namely C++ modules) so that it does not use extended makefile syntax not understood by most tools. 🎯-fdeps-target=file— analogous to-MTbut for structured dependency information. This indicates the target which will ultimately need any required resources and provide any resources extracted from the source that may be required by other sources. 🔥-fplan9-extensions— accept some non-standard constructs used in Plan 9 code. 🌈
This enables -fms-extensions, permits passing pointers to structures with anonymous fields to functions that expect pointers to elements of the type of the field, and permits referring to anonymous fields declared using a typedef. This is only supported for C, not C++. 🫶
-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. 💅-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-charis equivalent to-funsigned-char.
-funsigned-char— let the type"char"be unsigned, like"unsigned char". 🎀
Each kind of machine has a default for what "char" should be. It is either like "unsigned char" by default or like "signed char" by default.
Ideally, a portable program should always use "signed char" or "unsigned char" when it depends on the signedness of an object. But many programs have been written to use plain "char" and expect it to be signed, or expect it to be unsigned, depending on the machines they were written for. This option, and its inverse, let you make such a program work with the opposite default. 🔄
The type "char" is always a distinct type from each of "signed char" or "unsigned char", even though its behavior is always just like one of those two. no cap. 💯
-fstrict-flex-arrays(C and C++ only),-fstrict-flex-arrays=level(C and C++ only) — control when to treat the trailing array of a structure as a flexible array member for the purpose of accessing the elements of such an array. The value of level controls the level of strictness. 🧐
-fstrict-flex-arrays is equivalent to -fstrict-flex-arrays=3, which is the strictest; a trailing array is treated as a flexible array member only when it is declared as a flexible array member per C99 standard onwards.
The negative form -fno-strict-flex-arrays is equivalent to -fstrict-flex-arrays=0, which is the least strict. In this case all trailing arrays of structures are treated as flexible array members. 🦋
There are two more levels chillin' between 0 and 3, and they exist purely to support older code that uses the GCC zero-length array extension ([0]) or one-element array as flexible array members ([1]). So when level is 1, the trailing array gets treated as a flexible array member when it's declared as either [], [0], or [1]. And when level is 2, the trailing array gets treated as a flexible array member when it's declared as either [], or [0]. ✨
You can control this behavior for a specific trailing array field of a structure by using the variable attribute strict_flex_array attribute. iykyk 💅
The -fstrict_flex_arrays option interacts with the -Wstrict-flex-arrays option. besties, they're literally in conversation.
-fsso-struct=endianness— sets the default scalar storage order of structures and unions to whatever endianness you picked. The accepted values arebig-endian,little-endianandnativefor the native endianness of the target (that's the default). Heads up: this option is not supported for C++. 💀
Warning: the -fsso-struct switch causes GCC to generate code that is not binary compatible with code generated without it if the specified endianness is not the native endianness of the target. so like… be careful, okay? 🫶
🎀 Options Controlling C++ Dialect 🎀
This section is for the command-line options that are only meaningful for C++ programs. But lowkey you can also use most of the GNU compiler options no matter what language your program is in. Like, for example, you might compile a file firstClass.C like this:
g++ -g -fstrict-enums -O -c firstClass.CIn this example, only -fstrict-enums is an option meant only for C++ programs; you can use the other options with any language supported by GCC. slay 💖
Some options for compiling C programs, such as -std, are also relevant for C++ programs.
Here's the list of options that are only for compiling C++ programs:
-fabi-version=n— use version n of the C++ ABI. The default is version0.
Version 0 refers to the version conforming most closely to the C++ ABI specification. Therefore, the ABI obtained using version 0 will change in different versions of G++ as ABI bugs are fixed. (not me describing an ABI that keeps shifting 😭)
Version 1 is the version of the C++ ABI that first appeared in G++ 3.2.
Version 2 is the version of the C++ ABI that first appeared in G++ 3.4, and was the default through G++ 4.9.
Version 3 corrects an error in mangling a constant address as a template argument.
Version 4, which first appeared in G++ 4.5, implements a standard mangling for vector types.
Version 5, which first appeared in G++ 4.6, corrects the mangling of attribute const/volatile on function pointer types, decltype of a plain decl, and use of a function parameter in the declaration of another parameter.
Version 6, which first appeared in G++ 4.7, corrects the promotion behavior of C++11 scoped enums and the mangling of template argument packs, const/static_cast, prefix ++ and --, and a class scope function used as a template argument.
Version 7, which first appeared in G++ 4.8, that treats nullptr_t as a builtin type and corrects the mangling of lambdas in default argument scope.
Version 8, which first appeared in G++ 4.9, corrects the substitution behavior of function types with function-cv-qualifiers.
Version 9, which first appeared in G++ 5.2, corrects the alignment of nullptr_t.
Version 10, which first appeared in G++ 6.1, adds mangling of attributes that affect type identity, such as ia32 calling convention attributes (e.g. stdcall).
Version 11, which first appeared in G++ 7, corrects the mangling of sizeof... expressions and operator names. For multiple entities with the same name within a function, that are declared in different scopes, the mangling now changes starting with the twelfth occurrence. It also implies -fnew-inheriting-ctors.
Version 12, which first appeared in G++ 8, corrects the calling conventions for empty classes on the x86_64 target and for classes with only deleted copy/move constructors. It accidentally changes the calling convention for classes with a deleted copy constructor and a trivial move constructor. (oopsie era 🙈)
Version 13, which first appeared in G++ 8.2, fixes the accidental change in version 12. we love a redemption arc 💅
Version 14, which first appeared in G++ 10, corrects the mangling of the nullptr expression.
Version 15, which first appeared in G++ 10.3, corrects G++ 10 ABI tag regression.
Version 16, which first appeared in G++ 11, changes the mangling of __alignof__ to be distinct from that of alignof, and dependent operator names.
Version 17, which first appeared in G++ 12, fixes layout of classes that inherit from aggregate classes with default member initializers in C++14 and up.
Version 18, which first appeared in G++ 13, fixes manglings of lambdas that have additional context.
Version 19, which first appeared in G++ 14, fixes manglings of structured bindings to include ABI tags.
Version 20, which first appeared in G++ 15, fixes manglings of lambdas in static data member initializers.
See also -Wabi. 👑
-fabi-compat-version=n— on targets that support strong aliases, G++ works around mangling changes by creating an alias with the correct mangled name when defining a symbol with an incorrect mangled name. This switch specifies which ABI version to use for the alias.
With -fabi-version=0 (the default), this defaults to 13 (GCC 8.2 compatibility). If another ABI version is explicitly selected, this defaults to 0. For compatibility with GCC versions 3.2 through 4.9, use -fabi-compat-version=2.
If this option is not provided but -Wabi=n is, that version is used for compatibility aliases. If this option is provided along with -Wabi (without the version), the version from this option is used for the warning. the way these two gossip with each other fr fr 🦋
-fno-access-control— turn off all access checking. This switch is mainly useful for working around bugs in the access control code. (it's giving "I have the master key to every private member" 🔥)-faligned-new— enable support for C++17newof types that require more alignment thanvoid* ::operator new(std::size_t)provides. A numeric argument such as-faligned-new=32can be used to specify how much alignment (in bytes) is provided by that function, but few users will need to override the default ofalignof(std::max_align_t).
This flag is enabled by default for -std=c++17. periodt ✨
-fno-assume-sane-operators-new— okay so the C++ standard allows replacing the globalnew,new[],deleteanddelete[]operators, though a lot of C++ programs don't replace them and just use the implementation provided version. Furthermore, the C++ standard allows omitting those calls if they are made from new or delete expressions (and by extension the same is assumed if__builtin_operator_newor__builtin_operator_deletefunctions are used). This option allows control over some optimizations around calls to those operators. With-fassume-sane-operators-new-deleteoption GCC may assume that calls to the replaceable global operators from new or delete expressions or from__builtin_operator_newor__builtin_operator_deletecalls don't read or modify any global variables or variables whose address could escape to the operators (global state; except forerrnofor thenewandnew[]operators). This allows most optimizations across those calls and is something that the implementation provided operators satisfy unlessmallocimplementation details are observable in the code or unlessmallochooks are used, but might not be satisfied if a program replaces those operators. This behavior is enabled by default. With-fno-assume-sane-operators-new-deleteoption GCC must assume all these calls (whether from new or delete expressions or called directly) may read and write global state unless proven otherwise (e.g. when GCC compiles their implementation). Use this option if those operators are or may be replaced and code needs to expect such behavior. 💀 (this one's a whole essay but it ate, honestly)-fchar8_t-fno-char8_t— enable support forchar8_tas adopted for C++20. This includes the addition of a newchar8_tfundamental type, changes to the types of UTF-8 string and character literals, new signatures for user-defined literals, associated standard library updates, and new__cpp_char8_tand__cpp_lib_char8_tfeature test macros.
This option enables functions to be overloaded for ordinary and UTF-8 strings:
int f(const char *); // #1
int f(const char8_t *); // #2
int v1 = f("text"); // Calls #1
int v2 = f(u8"text"); // Calls #2and introduces new signatures for user-defined literals:
int operator""_udl1(char8_t);
int v3 = u8'x'_udl1;
int operator""_udl2(const char8_t*, std::size_t);
int v4 = u8"text"_udl2;
template<typename T, T...> int operator""_udl3();
int v5 = u8"text"_udl3;The change to the types of UTF-8 string and character literals introduces incompatibilities with ISO C++11 and later standards. For example, the following code is well-formed under ISO C++11, but is ill-formed when -fchar8_t is specified. (glow down for your old code 😭)
const char *cp = u8"xx";// error: invalid conversion from
// `const char8_t*' to `const char*'
int f(const char*);
auto v = f(u8"xx"); // error: invalid conversion from
// `const char8_t*' to `const char*'
std::string s{u8"xx"}; // error: no matching function for call to
// `std::basic_string<char>::basic_string()'
using namespace std::literals;
s = u8"xx"s; // error: conversion from
// `basic_string<char8_t>' to non-scalar
// type `basic_string<char>' requested-fcheck-new— okay so this one makes the compiler double-check that the pointeroperator newhands back isn't null before it tries to touch the storage it just allocated. Sounds paranoid? It lowkey is, because normally you don't need it at all — the C++ standard saysoperator newonly returns0if it's declaredthrow(), and in that case the compiler is gonna check the return value for you anyway, even without this flag. In literally every other case, whenoperator newhas a non-empty exception specification, running out of memory gets signalled by throwingstd::bad_allocinstead. See alsonew (nothrow). 💅-fconcepts— turns on support for the C++ Concepts feature, the one that lets you constrain template arguments. Bestie, with-std=c++20and above, Concepts are literally part of the language standard, so-fconceptsdefaults to on. ✨
Some constructs that the earlier C++ Extensions for Concepts Technical Specification, ISO 19217 (2015), allowed but that never made it into the standard, could additionally be enabled with -fconcepts-ts. That option was deprecated in GCC 14 and removed in GCC 15 — so the vibe now is: convert your code to C++20 concepts, fr fr.
-fconstexpr-depth=n— sets the maximum nested evaluation depth for C++11 constexpr functions to n. You need some limit here so the compiler can catch endless recursion during constant expression evaluation instead of spiralling forever. The minimum specified by the standard is512. 🌸-fconstexpr-cache-depth=n— sets the maximum level of nested evaluation depth for C++11 constexpr functions that will be cached, to n. It's a heuristic that trades off compilation speed (when the cache avoids repeated calculations) against memory consumption (when the cache grows huge from super recursive evaluations). Default is8. Very few people are gonna want to touch this, but if your code does heavy constexpr math you might wanna experiment and find which value works best for you. 🧚-fconstexpr-fp-except— so Annex F of the C standard says IEC559 floating point exceptions hit at compile time should not stop compilation. C++ compilers have historically ignored that guidance, treating floating point division by zero as non-constant even though it has a well defined value. This flag tells the compiler to give Annex F priority over other rules saying a particular operation is undefined. 🦋
constexpr float inf = 1./0.; // OK with -fconstexpr-fp-except-fconstexpr-loop-limit=n— sets the maximum number of iterations for a loop inside C++14 constexpr functions to n. Again, a limit is needed to detect infinite loops during constant expression evaluation. Default is262144(that's1<<18). 🔥-fconstexpr-ops-limit=n— sets the max number of operations during a single constexpr evaluation. Even when the number of iterations of one loop is capped with the limit above, if there are several nested loops and each one has a ton of iterations but still under that limit, or if inside some loop body (or even outside a loop) way too many expressions need evaluating, the resulting constexpr evaluation might take forever. Default is33554432(that's1<<25). 💀-fcontracts— enables experimental support for the C++ Contracts feature, as briefly added to and then removed from the C++20 working paper (N4820). The implementation also includes proposed enhancements from papers P1290, P1332, and P1429. This functionality is intended mostly for people who are into experimentation towards refining the feature so it can get into shape for a future C++ standard. ✨
On violation of a checked contract, the violation handler is called. Users can replace that handler by defining:
void
handle_contract_violation (const std::experimental::contract_violation&);There are different sets of additional flags you can use together to specify which contracts will be checked and how — for N4820 contracts, P1332 contracts, or P1429 contracts. Heads up: these sets cannot be used together. 🙈
-fcontract-mode=[on|off]— controls whether any contracts have any semantics at all. Defaults toon.-fcontract-assumption-mode=[on|off]— [N4820] controls whether contracts with level axiom should have the assume semantic. Defaults toon.-fcontract-build-level=[off|default|audit]— [N4820] specifies which level of contracts to generate checks for. Defaults todefault.-fcontract-continuation-mode=[on|off]— [N4820] controls whether to let the program keep executing after a contract violation. That is, do checked contracts have the maybe semantic described below rather than the never semantic. Defaults tooff.-fcontract-role=<name>:<default>,<audit>,<axiom>— [P1332] specifies the concrete semantics for each contract level of a particular contract role.-fcontract-semantic=[default|audit|axiom]:<semantic>— [P1429] specifies the concrete semantic for a particular contract level.-fcontract-strict-declarations=[on|off]— controls whether to reject adding contracts to a function after its first declaration. Defaults tooff.
The possible concrete semantics that can be specified with -fcontract-role or -fcontract-semantic are:
"ignore"— this contract has no effect."assume"— this contract is treated like C++23[[assume]]."check_never_continue","never","abort"— this contract is checked. If it fails, the violation handler is called. If the handler returns,std::terminateis called."check_maybe_continue","maybe"— this contract is checked. If it fails, the violation handler is called. If the handler returns, execution continues normally.
-fcoroutines— enables support for the C++ coroutines extension (experimental). 🌀-fdiagnostics-all-candidates— permits the C++ front end to note all candidates during overload resolution failure, including when a deleted function is selected. 👑-fno-elide-constructors— the C++ standard lets an implementation omit creating a temporary that's 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. It also causes G++ to call trivial member functions which would otherwise be expanded inline. 🌈
In C++17, the compiler is required to omit these temporaries, but this option still affects trivial member functions.
-fno-enforce-eh-specs— don't generate code to check for violation of exception specifications at run time. This option violates the C++ standard, but it may be useful for shrinking code size in production builds, kinda like definingNDEBUG. ⚠️ It does not give user code permission to throw exceptions in violation of the exception specifications; the compiler still optimizes based on the specifications, so throwing an unexpected exception results in undefined behavior at run time. 💀-fextern-tls-init,-fno-extern-tls-init— the C++11 and OpenMP standards allowthread_localandthreadprivatevariables to have dynamic (runtime) initialization. To support that, any use of such a variable goes through a wrapper function that performs any necessary initialization. When the use and definition of the variable are in the same translation unit, this overhead can be optimized away — but when the use is in a different translation unit there's significant overhead even if the variable doesn't actually need dynamic initialization. If the programmer can be sure that no use of the variable in a non-defining TU needs to trigger dynamic initialization (either because the variable is statically initialized, or a use of the variable in the defining TU will run before any uses in another TU), they can dodge this overhead with the-fno-extern-tls-initoption. 🫶
On targets that support symbol aliases, the default is -fextern-tls-init. On targets that do not support symbol aliases, the default is -fno-extern-tls-init.
-ffold-simple-inlines,-fno-fold-simple-inlines— permits the C++ frontend to fold calls tostd::move,std::forward,std::addressofandstd::as_const. In contrast to inlining, this means no debug information will be generated for such calls. Since these functions are rarely interesting to debug, this flag is enabled by default unless-fno-inlineis active. ✨-fno-gnu-keywords— do not recognizetypeofas a keyword, so your code can use that word as an identifier. You can use the keyword__typeof__instead. This option is implied by the strict ISO C++ dialects:-ansi,-std=c++98,-std=c++11, etc. 🎀-fno-immediate-escalation— do not enable immediate function escalation, whereby certain functions can be promoted toconsteval, as specified in P2564R3. For example:
consteval int id(int i) { return i; }
constexpr int f(auto t)
{
return t + id(t); // id causes f<int> to be promoted to consteval
}
void g(int i)
{
f (3);
}This compiles in C++20: f is an immediate-escalating function (because of the auto it's a function template and it's declared constexpr) and id(t) is an immediate-escalating expression, so f gets promoted to consteval. Consequently, the call to id(t) is in an immediate context, so it doesn't have to produce a constant (that's the mechanism allowing consteval function composition). 🔥 However, with -fno-immediate-escalation, f is not promoted to consteval, and since the call to the consteval function id(t) isn't a constant expression, the compiler rejects the code.
This option is turned on by default; it's only effective in C++20 mode or later.
-fimplicit-constexpr— makes inline functions implicitlyconstexpr, if they satisfy the requirements for a constexpr function. This option can be used in C++14 mode or later. It can result in initialization changing from dynamic to static, plus other optimizations. 💜-fno-implicit-templates— bestie, this one says: never emit code for non-inline templates that got instantiated implicitly (aka by someone actually using them). Only explicit instantiations get code. So if you flip this on, you gotta be real careful and structure your code so it includes all the explicit instantiations it needs, otherwise you're getting undefined symbols at link time and the linker is gonna be screaming at you 😭-fno-implicit-inline-templates— same energy but for inline templates: don't emit code for implicit instantiations of those either. The default is to treat inlines differently, specifically so that compiles with and without optimization need the same set of explicit instantiations. ✨-fno-implement-inlines— to save space, don't emit out-of-line copies of inline functions controlled by#pragma implementation. Heads up though: this causes linker errors if those functions aren't inlined everywhere they're called. 💀-fmodules,-fno-modules— turn on support for C++20 modules. The-fno-modulesis usually not needed since that's the default anyway. And like, even though this is a C++20 feature, it is not currently implicitly enabled just by selecting that standard version. Not me learning that the hard way 🙈-fmodule-header,-fmodule-header=user,-fmodule-header=system— compile a header file to create an importable header unit. 🎀-fmodule-implicit-inline— so member functions defined in their class definitions are not implicitly inline for modular code, which is different from traditional C++ behavior, and for good reasons. But it may cause difficulty during code porting. This option makes such function definitions implicitly inline again. The catch: it generates an ABI incompatibility, so you must use it everywhere or nowhere. Periodt. (Such definitions outside of a named module remain implicitly inline regardless, iykyk.)-fno-module-lazy— disable lazy module importing and module mapper creation.-fmodule-mapper=[hostname]:port[?ident]-fmodule-mapper=|program[?ident] args...-fmodule-mapper==socket[?ident]-fmodule-mapper=<>[inout][?ident]-fmodule-mapper=<in>out[?ident]-fmodule-mapper=file[?ident]— an oracle to query for module name to filename mappings. If unspecified, theCXX_MODULE_MAPPERenvironment variable is used, and if that is unset, an in-process default is provided. 🔮-fmodule-only— only emit the Compiled Module Interface, inhibiting any object file.-fms-extensions— disableWpedanticwarnings about constructs used in MFC, like implicit int and getting a pointer to member function via non-standard syntax.-fnew-inheriting-ctors— enable the P0136 adjustment to the semantics of C++11 constructor inheritance. This is part of C++17 but also considered to be a Defect Report against C++11 and C++14. Enabled by default unless-fabi-version=10or lower is specified. 🌈-fnew-ttp-matching— enable the P0522 resolution to Core issue 150, template template parameters and default arguments: this allows a template with default template arguments as an argument for a template template parameter with fewer template parameters. Enabled by default for-std=c++17. ⭐-fno-nonansi-builtins— disable built-in declarations of functions that are not mandated by ANSI/ISO C. These includeffs,alloca,_exit,index,bzero,conjf, and other related functions.-fnothrow-opt— treat athrow()exception specification as if it were anoexceptspecification, to reduce or eliminate the text size overhead relative to a function with no exception specification. Fun fact: if the function has local variables of types with non-trivial destructors, the exception specification actually makes the function smaller, because the EH cleanups for those variables can be optimized away. The semantic effect is that an exception thrown out of a function with such an exception specification results in a call toterminaterather thanunexpected.-fno-operator-names— do not treat the operator name keywordsand,bitand,bitor,compl,not,orandxoras 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.-fno-pretty-templates— so when an error message refers to a specialization of a function template, the compiler normally prints the signature of the template followed by the template arguments and any typedefs or typenames in the signature (e.g.void f(T) [with T = int]rather thanvoid f(int)) so it's clear which template is involved. And when an error message refers to a specialization of a class template, the compiler omits any template arguments that match the default template arguments for that template. If either of these behaviors makes it harder to understand the error message rather than easier, you can use-fno-pretty-templatesto disable them. 🫶-frange-for-ext-temps— enable lifetime extension of C++ range based for temporaries. With-std=c++23and above this is part of the language standard, so the lifetime of the temporaries is extended until the end of the loop by default. This option lets you enable that behavior also in earlier versions of the standard.-fno-rtti— disable generation of information about every class with virtual functions for use by the C++ run-time type identification features (dynamic_castandtypeid). If you don't use those parts of the language, you can save some space by using this flag. Note that exception handling uses the same information, but G++ generates it as needed. Thedynamic_castoperator can still be used for casts that do not require run-time type information, i.e. casts tovoid *or to unambiguous base classes.
Mixing code compiled with-frttiwith that compiled with-fno-rttimay not work. For example, programs may fail to link if a class compiled with-fno-rttiis used as a base for a class compiled with-frtti. 😭
-fsized-deallocation— enable the built-in global declarations
void operator delete (void *, std::size_t) noexcept;
void operator delete[] (void *, std::size_t) noexcept;as introduced in C++14. This is useful for user-defined replacement deallocation functions that, for example, use the size of the object to make deallocation faster. Enabled by default under -std=c++14 and above. The flag -Wsized-deallocation warns about places that might want to add a definition. 🔥
-fstrict-enums— let the compiler optimize using the assumption that a value of enumerated type can only be one of the values of the enumeration (as defined in the C++ standard; basically, a value that can be represented in the minimum number of bits needed to represent all the enumerators). This assumption may not be valid if the program uses a cast to convert an arbitrary integer value to the enumerated type. Heads up: this option has no effect for an enumeration type with a fixed underlying type.-fstrong-eval-order,-fstrong-eval-order=kind— evaluate member access, array subscripting, and shift expressions in left-to-right order, and evaluate assignment in right-to-left order, as adopted for C++17.-fstrong-eval-orderis equivalent to-fstrong-eval-order=all, and is enabled by default with-std=c++17or later.-fstrong-eval-order=someenables just the ordering of member access and shift expressions, and is the default for C++ dialects prior to C++17.-fstrong-eval-order=noneis equivalent to-fno-strong-eval-order.
-ftemplate-backtrace-limit=n— set the maximum number of template instantiation notes for a single warning or error to n. The default value is10.-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 than17(changed to1024in C++11). The default value is900, since the compiler can run out of stack space before hitting1024in some situations. 💅-fno-threadsafe-statics— do not emit the extra code to use the routines specified in the C++ ABI for thread-safe initialization of local statics. You can use this option to reduce code size slightly in code that doesn't need to be thread-safe.-fuse-cxa-atexit— register destructors for objects with static storage duration with the__cxa_atexitfunction rather than theatexitfunction. This option is required for fully standards-compliant handling of static destructors, but only works if your C library supports__cxa_atexit.-fno-use-cxa-get-exception-ptr— don't use the__cxa_get_exception_ptrruntime routine. This causesstd::uncaught_exceptionto be incorrect, but is necessary if the runtime routine is not available. 🫠-fvisibility-inlines-hidden— this switch declares that the user does not attempt to compare pointers to inline functions or methods where the addresses of the two functions are taken in different shared objects.
The effect of this is that GCC may, effectively, mark inline methods with __attribute__ ((visibility ("hidden"))) so that they do not appear in the export table of a DSO and do not require a PLT indirection when used within the DSO. Enabling this option can have a dramatic effect on load and link times of a DSO, since it massively reduces the size of the dynamic export table when the library makes heavy use of templates. 🔥
The behavior of this switch is not quite the same as marking the methods as hidden directly, because it does not affect static variables local to the function or cause the compiler to deduce that the function is defined in only one shared object.
You may mark a method as having a visibility explicitly to negate the effect of the switch for that method. For example, if you do want to compare pointers to a particular inline method, you might mark it as having default visibility. Marking the enclosing class with explicit visibility has no effect.
Explicitly instantiated inline methods are unaffected by this option as their linkage might otherwise cross a shared library boundary. 🌸
-fvisibility-ms-compat— okay so this one is out here trying to make GCC's C++ linkage model match Microsoft Visual Studio's, using visibility settings. It's giving "we can be compatible" energy 💅
Here's what it changes in GCC's linkage model:
- It sets the default visibility to
"hidden", like-fvisibility=hidden. - Types, but not their members, are not hidden by default.
- The One Definition Rule is relaxed for types without explicit visibility specifications that are defined in more than one shared object: those declarations are permitted if they are permitted when this option is not used.
In new code it's better to use -fvisibility=hidden and export those classes that are intended to be externally visible. Unfortunately it's possible for code to rely, perhaps accidentally, on the Visual Studio behavior. 😭
Among the consequences of these changes: static data members of the same type with the same name but defined in different shared objects are different, so changing one does not change the other; and pointers to function members defined in different shared objects may not compare equal. When this flag is given, it is a violation of the ODR to define types with the same name differently. 🫶
-fno-weak— nope, do not use weak symbol support, even if the linker is offering it. By default, G++ uses weak symbols if they are available. Bestie, this option exists only for testing, and should not be used by end-users; it results in inferior code and has no benefits. This option may be removed in a future release of G++. 🚫-fext-numeric-literals(C++ and Objective-C++ only) — accept imaginary, fixed-point, or machine-defined literal number suffixes as GNU extensions. When this option is turned off these suffixes are treated as C++11 user-defined literal numeric suffixes. This is on by default for all pre-C++11 dialects and all GNU dialects:-std=c++98,-std=gnu++98,-std=gnu++11,-std=gnu++14. This option is off by default for ISO C++11 onwards (-std=c++11, ...).-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.) 🎀-flang-info-include-translate-flang-info-include-translate-not-flang-info-include-translate=header— these three inform you of include translation events. The first will note accepted include translations, the second will note declined include translations. The header form will inform of include translations relating to that specific header. If header is of the form"user"or"<system>"it will be resolved to a specific user or system header using the include path. 👑-flang-info-module-cmi-flang-info-module-cmi=module— these inform you of Compiled Module Interface pathnames. The first will note all read CMI pathnames. The module form will not reading a specific module's CMI. module may be a named module or a header-unit (the latter indicated by either being a pathname containing directory separators or enclosed in"<>"or"").-stdlib=libstdc++,libc++— when G++ is configured to support this option, it allows specification of alternate C++ runtime libraries. Two options are available:libstdc++(the default, native C++ runtime for G++) andlibc++which is the C++ runtime installed on some operating systems (e.g. Darwin versions from Darwin11 onwards). The option switches G++ to use the headers from the specified library and to emit"-lstdc++"or"-lc++"respectively, when a C++ runtime is required for linking. ✨
In addition, these warning options have meanings only for C++ programs: 💜
-Wabi-tag(C++ and Objective-C++ only) — warn when a type with an ABI tag is used in a context that does not have that ABI tag. See C++ Attributes for more information about ABI tags.-Wcomma-subscript(C++ and Objective-C++ only) — warn about uses of a comma expression within a subscripting expression. This usage was deprecated in C++20 and is going to be removed in C++23. However, a comma expression wrapped in"( )"is not deprecated. Example:
void f(int *a, int b, int c) {
a[b,c]; // deprecated in C++20, invalid in C++23
a[(b,c)]; // OK
}In C++23 it is valid to have comma separated expressions in a subscript when an overloaded subscript operator is found and supports the right number and types of arguments. G++ will accept the formerly valid syntax for code that is not valid in C++23 but used to be valid but deprecated in C++20 with a pedantic warning that can be disabled with -Wno-comma-subscript.
Enabled by default with -std=c++20 unless -Wno-deprecated, and after -std=c++23 regardless of -Wno-deprecated. Before -std=c++20, enabled with explicit -Wdeprecated.
This warning is upgraded to an error by -pedantic-errors in C++23 mode or later. 💀
-Wctad-maybe-unsupported(C++ and Objective-C++ only) — warn when performing class template argument deduction (CTAD) on a type with no explicitly written deduction guides. This warning will point out cases where CTAD succeeded only because the compiler synthesized the implicit deduction guides, which might not be what the programmer intended. Certain style guides allow CTAD only on types that specifically "opt-in"; i.e., on types that are designed to support CTAD. This warning can be suppressed with the following pattern:
struct allow_ctad_t; // any name works
template <typename T> struct S {
S(T) { }
};
// Guide with incomplete parameter type will never be considered.
S(allow_ctad_t) -> S<void>;-Wctor-dtor-privacy(C++ and Objective-C++ only) — warn when a class seems unusable because all the constructors or destructors in that class are private, and it has neither friends nor public static member functions. Also warn if there are no non-private methods, and there's at least one private member function that isn't a constructor or destructor. 🙈-Wdangling-reference(C++ and Objective-C++ only) — warn when a reference is bound to a temporary whose lifetime has ended. For example:
int n = 1;
const int& r = std::max(n - 1, n + 1); // r is danglingIn the example above, two temporaries are created, one for each argument, and a reference to one of the temporaries is returned. However, both temporaries are destroyed at the end of the full expression, so the reference "r" is dangling. This warning also detects dangling references in member initializer lists:
const int& f(const int& i) { return i; }
struct S {
const int &r; // r is dangling
S() : r(f(10)) { }
};Member functions are checked as well, but only their object argument:
struct S {
const S& self () { return *this; }
};
const S& s = S().self(); // s is danglingCertain functions are safe in this respect, for example "std::use_facet": they take and return a reference, but they don't return one of its arguments, which can fool the warning. Such functions can be excluded from the warning by wrapping them in a "#pragma":
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wdangling-reference"
const T& foo (const T&) { ... }
#pragma GCC diagnostic popThe "#pragma" can also surround the class; in that case, the warning will be disabled for all the member functions.
-Wdangling-reference also warns about code like
auto p = std::minmax(1, 2);where "std::minmax" returns "std::pair<const int&, const int&>", and both references dangle after the end of the full expression that contains the call to "std::minmax". 😭
The warning does not warn for "std::span"-like classes. We consider classes of the form:
template<typename T>
struct Span {
T* data_;
std::size len_;
};as "std::span"-like; that is, the class is a non-union class that has a pointer data member and a trivial destructor.
The warning can be disabled by using the "gnu::no_dangling" attribute.
This warning is enabled by -Wextra.
-Wdelete-non-virtual-dtor(C++ and Objective-C++ only) — warn when"delete"is used to destroy an instance of a class that has virtual functions and non-virtual destructor. It is unsafe to delete an instance of a derived class through a pointer to a base class if the base class does not have a virtual destructor. This warning is enabled by-Wall. ⚠️-Wdeprecated-copy(C++ and Objective-C++ only) — warn that the implicit declaration of a copy constructor or copy assignment operator is deprecated if the class has a user-provided copy constructor or copy assignment operator, in C++11 and up. This warning is enabled by-Wextra. With-Wdeprecated-copy-dtor, also deprecate if the class has a user-provided destructor.-Wno-deprecated-enum-enum-conversion(C++ and Objective-C++ only) — disable the warning about the case when the usual arithmetic conversions are applied on operands where one is of enumeration type and the other is of a different enumeration type. This conversion was deprecated in C++20. For example:
enum E1 { e };
enum E2 { f };
int k = f - e;-Wdeprecated-enum-enum-conversion is enabled by default with -std=c++20. In pre-C++20 dialects, this warning can be enabled by -Wenum-conversion or -Wdeprecated.
-Wno-deprecated-enum-float-conversion(C++ and Objective-C++ only) — disable the warning about the case when the usual arithmetic conversions are applied on operands where one is of enumeration type and the other is of a floating-point type. This conversion was deprecated in C++20. For example:
enum E1 { e };
enum E2 { f };
bool b = e <= 3.7;-Wdeprecated-enum-float-conversion is enabled by default with -std=c++20. In pre-C++20 dialects, this warning can be enabled by -Wenum-conversion or -Wdeprecated. 🌸
-Wdeprecated-literal-operator(C++ and Objective-C++ only) — bestie, this one clocks you if you declare a user-defined literal operator with a space before the suffix. That space is deprecated, no cap. It's on by default in C++23, or whenever you explicitly pass-Wdeprecated.
string operator "" _i18n(const char*, std::size_t); // deprecated
string operator ""_i18n(const char*, std::size_t); // preferred-Wdeprecated-variadic-comma-omission(C++ and Objective-C++ only) — this is giving "forgot the comma" energy. It warns you when you leave out the comma before the varargs...at the end of a function parameter list, because that's deprecated. Enabled by default in C++26, or with explicit-Wdeprecated.
void f1(int...); // deprecated
void f1(int, ...); // preferred
template <typename ...T>
void f2(T...); // ok
template <typename ...T>
void f3(T......); // deprecated-Wno-elaborated-enum-base— for C++11 and above, this yells at you if an (invalid) extra enum-base shows up in an elaborated-type-specifier. Like, if you use an enum with a given underlying type and no enumerator list somewhere other than just a standalone declaration of that enum. It's enabled by default, and it gets upgraded to a full-on error with-pedantic-errors. ✨-Wno-init-list-lifetime(C++ and Objective-C++ only) — okay so this turns OFF the warnings aboutstd::initializer_listuses that are likely to leave you with dangling pointers. Because the underlying array for aninitializer_listis handled like a normal C++ temporary object, it is so easy to accidentally keep a pointer to that array past the end of its lifetime. Like:- If a function returns a temporary
initializer_list, or a localinitializer_listvariable, the array's lifetime ends at the end of the return statement, so the value returned has a dangling pointer. 💀 - If a new-expression creates an
initializer_list, the array only lives until the end of the enclosing full-expression, so theinitializer_listsitting in the heap has a dangling pointer. - When an
initializer_listvariable is assigned from a brace-enclosed initializer list, the temporary array created for the right side of the assignment only lives until the end of the full-expression, so by the next statement theinitializer_listvariable has a dangling pointer.
- If a function returns a temporary
// li's initial underlying array lives as long as li
std::initializer_list<int> li = { 1,2,3 };
// assignment changes li to point to a temporary array
li = { 4, 5 };
// now the temporary is gone and li has a dangling pointer
int i = li.begin()[0] // undefined behavior- When a list constructor stores the
beginpointer from theinitializer_listargument, that does not extend the lifetime of the array, so if a class variable is constructed from a temporaryinitializer_list, the pointer is left dangling by the end of the variable declaration statement.
-Winvalid-constexpr— warns when a function never produces a constant expression. In C++20 and earlier, for everyconstexprfunction and function template, there has to be at least one set of function arguments in at least one instantiation such that an invocation of the function or constructor could be an evaluated subexpression of a core constant expression. C++23 removed that restriction, so it's totally possible to have a function or function template markedconstexprwhere no invocation satisfies the requirements of a core constant expression.
This warning is enabled as a pedantic warning by default in C++20 and earlier. In C++23, -Winvalid-constexpr can be turned on, and in that case it's just an ordinary warning. Example time:
void f (int& i);
constexpr void
g (int& i)
{
// Warns by default in C++20, in C++23 only with -Winvalid-constexpr.
f(i);
}-Winvalid-imported-macros— verifies that all imported macro definitions are valid at the end of compilation. Not enabled by default, bestie, because it requires extra processing to figure out. It can be useful when you're preparing sets of header-units and want to make sure the macros are consistent. 🎀-Wno-literal-suffix(C++ and Objective-C++ only) — do not warn when a string or character literal is followed by a ud-suffix that does not begin with an underscore. As a conforming extension, GCC treats such suffixes as separate preprocessing tokens, to keep backwards compatibility with code that uses formatting macros from<inttypes.h>. For example:
#define __STDC_FORMAT_MACROS
#include <inttypes.h>
#include <stdio.h>
int main() {
int64_t i64 = 123;
printf("My int64: %" PRId64"\n", i64);
}In this case, PRId64 is treated as a separate preprocessing token.
This option also controls warnings when a user-defined literal operator is declared with a literal suffix identifier that doesn't begin with an underscore. Literal suffix identifiers that don't begin with an underscore are reserved for future standardization. These warnings are enabled by default. 🌸
-Wno-narrowing(C++ and Objective-C++ only) — for C++11 and later standards, narrowing conversions are diagnosed by default, as the standard requires. A narrowing conversion from a constant produces an error, and a narrowing conversion from a non-constant produces a warning, but-Wno-narrowingsuppresses the diagnostic. Heads up though: this does not affect the meaning of well-formed code; narrowing conversions are still considered ill-formed in SFINAE contexts.
With -Wnarrowing in C++98, warn when a narrowing conversion prohibited by C++11 occurs within { }, like:
int i = { 2.2 }; // error: narrowing from double to intThis flag is included in -Wall and -Wc++11-compat.
-Wnoexcept(C++ and Objective-C++ only) — warns when a noexcept-expression evaluates to false because of a call to a function that doesn't have a non-throwing exception specification (i.e.throw()ornoexcept) but is known by the compiler to never throw an exception. 👑-Wnoexcept-type(C++ and Objective-C++ only) — warns if the C++17 feature that makesnoexceptpart of a function type changes the mangled name of a symbol relative to C++14. Enabled by-Wabiand-Wc++17-compat.
Example moment:
template <class T> void f(T t) { t(); };
void g() noexcept;
void h() { f(g); }In C++14, f calls f<void(*)()>, but in C++17 it calls f<void(*)()noexcept>.
-Wclass-memaccess(C++ and Objective-C++ only) — warns when the destination of a call to a raw memory function likememsetormemcpyis an object of class type, and when writing into such an object might bypass the class's non-trivial or deleted constructor or copy assignment, violate const-correctness or encapsulation, or corrupt virtual table pointers. Modifying the representation of such objects may violate invariants maintained by member functions of the class. For example, the call tomemsetbelow is undefined because it modifies a non-trivial class object, and is therefore diagnosed. The safe way to either initialize or clear the storage of objects of these types is by using the appropriate constructor or assignment operator, if one is available.
std::string str = "abc";
memset (&str, 0, sizeof str);The -Wclass-memaccess option is enabled by -Wall. Explicitly casting the pointer to the class object to void * or to a type that can be safely accessed by the raw memory function suppresses the warning.
-Wnon-virtual-dtor(C++ and Objective-C++ only) — warns when a class has virtual functions and an accessible non-virtual destructor itself or in an accessible polymorphic base class, in which case it's possible but unsafe to delete an instance of a derived class through a pointer to the class itself or the base class. This warning is automatically enabled if-Weffc++is specified. The-Wdelete-non-virtual-dtoroption (enabled by-Wall) should be preferred, because it warns about the unsafe cases without false positives.-Wregister(C++ and Objective-C++ only) — warns on uses of theregisterstorage class specifier, except when it's part of the GNU Explicit Register Variables extension. Using theregisterkeyword as a storage class specifier has been deprecated in C++11 and removed in C++17. Enabled by default with-std=c++17.-Wreorder(C++ and Objective-C++ only) — warns when the order of member initializers in your code doesn't match the order in which they must actually be executed. Like:
struct A {
int i;
int j;
A(): j (0), i (1) { }
};The compiler rearranges the member initializers for i and j to match the declaration order of the members, and emits a warning to that effect. This warning is enabled by -Wall. 🦋
-Wno-pessimizing-move(C++ and Objective-C++ only) — this warning clocks you when a call tostd::moveprevents copy elision. A typical scenario where copy elision can happen is when you return inside a function with a class return type, the expression being returned is the name of a non-volatile automatic object, it's not a function parameter, and it has the same type as the function return type.
struct T {
...
};
T fn()
{
T t;
...
return std::move (t);
}But in this example, the std::move call prevents copy elision. This warning is enabled by -Wall.
-Wno-redundant-move(C++ and Objective-C++ only) — this one warns about redundant calls tostd::move; that is, when a move operation would have been performed even without thestd::movecall. This happens because the compiler is forced to treat the object as if it were an rvalue in certain situations, such as returning a local variable, where copy elision isn't applicable. Consider:
struct T {
...
};
T fn(T t)
{
...
return std::move (t);
}bestie look at this — that std::move call up there? it's redundant. literally doing nothing. 💅
And because G++ implements Core Issue 1579, here's another lil guy:
struct T { // convertible to U
...
};
struct U {
...
};
U fn()
{
T t;
...
return std::move (t);
}So in this example, copy elision isn't applicable — because the type of the expression being returned and the function return type are different — yet G++ treats the return value as if it were designated by an rvalue. the more you know ✨
This warning is enabled by -Wextra.
🌀 -Wrange-loop-construct (C++ and Objective-C++ only)
Okay so this one warns you when a C++ range-based for-loop is creating an unnecessary copy. This can happen when the range declaration is not a reference, but probably should be. Like:
struct S { char arr[128]; };
void fn () {
S arr[5];
for (const auto x : arr) { ... }
}girl. you're copying 128 bytes five times for no reason. 🙈
It does not warn when the type being copied is a trivially-copyable type whose size is less than 64 bytes. (tiny copies get a pass, we're chill like that 🫶)
This warning also warns when a loop variable in a range-based for-loop is initialized with a value of a different type resulting in a copy. For example:
void fn() {
int arr[10];
for (const double &x : arr) { ... }
}In the example above, in every single iteration of the loop a temporary value of type double is created and destroyed, to which the reference const double & is bound. 😭 that's so much wasted work omg.
This warning is enabled by -Wall.
🏷️ -Wredundant-tags (C++ and Objective-C++ only)
Warn about redundant class-key and enum-key in references to class types and enumerated types in contexts where the key can be eliminated without causing an ambiguity. Like:
struct foo;
struct foo *p; // warn that keyword struct can be eliminatedSee how that struct is just sitting there doing absolutely nothing? iconic of it to leave. 👑
On the other hand, in this example there is no warning:
struct foo;
void foo (); // "hides" struct foo
void bar (struct foo&); // no warning, keyword struct is necessaryBecause here the keyword is lowkey load-bearing. iykyk.
🔇 -Wno-subobject-linkage (C++ and Objective-C++ only)
Do not warn if a class type has a base or a field whose type uses the anonymous namespace or depends on a type with no linkage. Here's the tea: if a type A depends on a type B with no or internal linkage, defining it in multiple translation units would be an ODR violation — because the meaning of B is different in each translation unit. 💀
If A only appears in a single translation unit, the best way to silence the warning is to give it internal linkage by putting it in an anonymous namespace as well. The compiler doesn't give this warning for types defined in the main .C file, as those are unlikely to have multiple definitions. -Wsubobject-linkage is enabled by default.
📚 -Weffc++ (C++ and Objective-C++ only)
Warn about violations of the following style guidelines from Scott Meyers' Effective C++ series of books:
- Define a copy constructor and an assignment operator for classes with dynamically-allocated memory.
- Prefer initialization to assignment in constructors.
- Have
operator=return a reference to*this. - Don't try to return a reference when you must return an object.
- Distinguish between prefix and postfix forms of increment and decrement operators.
- Never overload
&&,||, or,.
This option also enables -Wnon-virtual-dtor, which is also one of the effective C++ recommendations. However, the check is extended to warn about the lack of virtual destructor in accessible non-polymorphic bases classes too.
When selecting this option, be aware that the standard library headers do not obey all of these guidelines; use grep -v to filter out those warnings. fr fr, you'll need it.
🚫 -Wno-exceptions (C++ and Objective-C++ only)
Disable the warning about the case when an exception handler is shadowed by another handler, which can point out a wrong ordering of exception handlers. 😬
🎯 -Wstrict-null-sentinel (C++ and Objective-C++ only)
Warn about the use of an uncasted NULL as sentinel. When compiling only with GCC this is a valid sentinel, as NULL is defined to __null. Although it is a null pointer constant rather than a null pointer, it is guaranteed to be of the same size as a pointer. But this use is not portable across different compilers. 🫠
🤝 -Wno-non-template-friend (C++ and Objective-C++ only)
Disable warnings when non-template friend functions are declared within a template. In very old versions of GCC that predate implementation of the ISO standard, declarations such as friend int foo(int), where the name of the friend is an unqualified-id, could be interpreted as a particular specialization of a template function; the warning exists to diagnose compatibility problems, and is enabled by default.
🕰️ -Wold-style-cast (C++ and Objective-C++ only)
Warn if an old-style (C-style) cast to a non-void type is used within a C++ program. The new-style casts (dynamic_cast, static_cast, reinterpret_cast, and const_cast) are less vulnerable to unintended effects and much easier to search for. slay, upgrade your casts bestie 💅
👻 -Woverloaded-virtual (C++ and Objective-C++ only)
-Woverloaded-virtual=n
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); // does not override
};the A class version of f is hidden in B, and code like:
B* b;
b->f();fails to compile. oof. 💀
In cases where the different signatures are not an accident, the simplest solution is to add a using-declaration to the derived class to un-hide the base function, e.g. add using A::f; to B.
The optional level suffix controls the behavior when all the declarations in the derived class override virtual functions in the base class, even if not all of the base functions are overridden:
struct C {
virtual void f();
virtual void f(int);
};
struct D: public C {
void f(int); // does override
}This pattern is less likely to be a mistake; if D is only used virtually, the user might have decided that the base class semantics for some of the overloads are fine.
At level 1, this case does not warn; at level 2, it does. -Woverloaded-virtual by itself selects level 2. Level 1 is included in -Wall.
🔇 -Wno-pmf-conversions (C++ and Objective-C++ only)
Disable the diagnostic for converting a bound pointer to member function to a plain pointer. that's it, that's the whole thing 🙈
⚖️ -Wsign-promo (C++ and Objective-C++ only)
Warn when overload resolution chooses a promotion from unsigned or enumerated type to a signed type, over a conversion to an unsigned type of the same size. Previous versions of G++ tried to preserve unsignedness, but the standard mandates the current behavior. periodt.
📝 -Wtemplates (C++ and Objective-C++ only)
Warn when a primary template declaration is encountered. Some coding rules disallow templates, and this may be used to enforce that rule. The warning is inactive inside a system header file, such as the STL, so one can still use the STL. One may also instantiate or specialize templates.
🧩 -Wmismatched-new-delete (C++ and Objective-C++ only)
Warn for mismatches between calls to operator new or operator delete and the corresponding call to the allocation or deallocation function. This includes invocations of C++ operator delete with pointers returned from either mismatched forms of operator new, or from other functions that allocate objects for which the operator delete isn't a suitable deallocator, as well as calls to other deallocation functions with pointers returned from operator new for which the deallocation function isn't suitable.
For example, the delete expression in the function below is diagnosed because it doesn't match the array form of the new expression the pointer argument was returned from. Similarly, the call to free is also diagnosed.
void f ()
{
int *a = new int[n];
delete a; // warning: mismatch in array forms of expressions
char *p = new char[n];
free (p); // warning: mismatch between new and free
}matching your new with your delete is literally the bare minimum bestie 😭
The related option -Wmismatched-dealloc diagnoses mismatches involving allocation and deallocation functions other than operator new and operator delete.
-Wmismatched-new-delete is included in -Wall.
🏷️ -Wmismatched-tags (C++ and Objective-C++ only)
Warn for declarations of structs, classes, and class templates and their specializations with a class-key that does not match either the definition or the first declaration if no definition is provided.
For example, the declaration of struct Object in the argument list of draw triggers the warning. To avoid it, either remove the redundant class-key struct or replace it with class to match its definition.
class Object {
public:
virtual ~Object () = 0;
};
void draw (struct Object*);It is not wrong to declare a class with the class-key struct as the example above shows. The -Wmismatched-tags option is intended to help achieve a consistent style of class declarations. In code that is intended to be portable to Windows-based compilers the warning helps prevent unresolved references due to the difference in the mangling of symbols declared with different class-keys. The option can be used either on its own or in conjunction with -Wredundant-tags. ✨
-Wmultiple-inheritance<span style="color:hotpink">(C++ and Objective-C++ only)</span> — bestie, this one yells at you when a class gets defined with multiple direct base classes. Some coding rules are like "no multiple inheritance ever" and this is the flag that enforces that vibe. The warning is inactive inside a system header file, like the STL, so you can still use the STL, no problem. And you can also define classes that indirectly use multiple inheritance — that's fine too. 💅-Wvirtual-inheritance— warns when a class is defined with a virtual direct base class. Same energy as above: some coding rules disallow multiple inheritance and this enforces it. Again, the warning is inactive inside a system header file, such as the STL, so the STL stays usable. And you may also define classes that indirectly use virtual inheritance. ✨-Wno-virtual-move-assign— suppresses warnings about inheriting from a virtual base with a non-trivial C++11 move assignment operator. Okay so listen: this is dangerous 🚨 because if the virtual base is reachable along more than one path, it gets moved multiple times, which can mean both objects end up in the moved-from state. If the move assignment operator is written to avoid moving from a moved-from object, you can disable this warning.-Wnamespaces— warns when a namespace definition is opened. Some coding rules disallow namespaces, so this is how you enforce that. The warning is inactive inside a system header file, like the STL, so STL stays fine. And you may also useusingdirectives and qualified names. 🎀-Wno-template-body<span style="color:hotpink">(C++ and Objective-C++ only)</span> — disables diagnosing errors when parsing a template, and instead issues an error only upon instantiation of the template. This flag can also be used to downgrade such errors into warnings withWno-error=or-fpermissive. lowkey chaotic but okay 🫶-Wno-template-id-cdtor<span style="color:hotpink">(C++ and Objective-C++ only)</span> — disables the warning about using a simple-template-id as the declarator-id of a constructor or destructor, which became invalid in C++20 via DR 2237. Example time:
template<typename T> struct S {
S<T>(); // should be S();
~S<T>(); // should be ~S();
};-Wtemplate-id-cdtor is enabled by default with -std=c++20; it is also enabled by -Wc++20-compat.
-Wtemplate-names-tu-local— warns when a template body hides an exposure of a translation-unit-local entity. In most cases, referring to a translation-unit-local entity (like an internal linkage declaration) within an entity that is emitted into a module's CMI is an error. But — plot twist — within the initializer of a variable, or in the body of a non-inline function, this is not an exposure and no error is emitted. 😌
This can cause variable or function templates to accidentally become unusable if they reference such an entity, because other translation units that import the template will never be able to instantiate it. This warning tries to detect cases where that might happen. The presence of an explicit instantiation silences the warning.
This flag is enabled by -Wextra.
-Wno-terminate<span style="color:hotpink">(C++ and Objective-C++ only)</span> — disables the warning about a throw-expression that will immediately result in a call toterminate. 💀-Wno-vexing-parse<span style="color:hotpink">(C++ and Objective-C++ only)</span> — warns about the most vexing parse syntactic ambiguity. It warns about the cases when a declaration looks like a variable definition, but the C++ language requires it to be interpreted as a function declaration. For instance:
void f(double a) {
int i(); // extern int i (void);
int n(int(a)); // extern int n (int);
}Another example:
struct S { S(int); };
void f(double a) {
S x(int(a)); // extern struct S x (int);
S y(int()); // extern struct S y (int (*) (void));
S z(); // extern struct S z (void);
}The warning will suggest options for how to deal with such an ambiguity; e.g., it can suggest removing the parentheses or using braces instead. 🧚
This warning is enabled by default.
-Wno-class-conversion<span style="color:hotpink">(C++ and Objective-C++ only)</span> — do not warn when a conversion function converts an object to the same type, to a base class of that type, or tovoid; such a conversion function will never be called. periodt.-Wvolatile<span style="color:hotpink">(C++ and Objective-C++ only)</span> — warns about deprecated uses of thevolatilequalifier. That includes postfix and prefix++and--expressions ofvolatile-qualified types, using simple assignments where the left operand is avolatile-qualified non-class type for their value, compound assignments where the left operand is avolatile-qualified non-class type,volatile-qualified function return type,volatile-qualified parameter type, and structured bindings of avolatile-qualified type. This usage was deprecated in C++20. 🫠
Enabled by default with -std=c++20. Before -std=c++20, enabled with explicit -Wdeprecated.
-Waligned-new— warns about a new-expression of a type that requires greater alignment thanalignof(std::max_align_t)but uses an allocation function without an explicit alignment parameter. This option is enabled by-Wall.
Normally this only warns about global allocation functions, but -Waligned-new=all also warns about class member allocation functions. ✨
-Wno-placement-new-Wplacement-new=n— warns about placement new expressions with undefined behavior, like constructing an object in a buffer that is smaller than the type of the object. For example, the placement new expression below is diagnosed because it tries to construct an array of 64 integers in a buffer only 64 bytes large. 😭
char buf [64];
new (buf) int[64];This warning is enabled by default.
-Wplacement-new=1— this is the default warning level of-Wplacement-new. At this level the warning is not issued for some strictly undefined constructs that GCC allows as extensions for compatibility with legacy code. For example, the followingnewexpression is not diagnosed at this level even though it has undefined behavior according to the C++ standard, because it writes past the end of the one-element array.
struct S { int n, a[1]; };
S *s = (S *)malloc (sizeof *s + 31 * sizeof s->a[0]);
new (s->a)int [32]();-Wplacement-new=2— at this level, in addition to diagnosing all the same constructs as at level 1, a diagnostic is also issued for placement new expressions that construct an object in the last member of a structure whose type is an array of a single element and whose size is less than the size of the object being constructed. While the previous example would be diagnosed, the following construct makes use of the flexible member array extension to avoid the warning at level 2.
struct S { int n, a[]; };
S *s = (S *)malloc (sizeof *s + 32 * sizeof s->a[0]);
new (s->a)int [32]();-Wcatch-value-Wcatch-value=n<span style="color:hotpink">(C++ and Objective-C++ only)</span> — warns about catch handlers that do not catch via reference. With-Wcatch-value=1(or just-Wcatch-valuefor short) warn about polymorphic class types that are caught by value. With-Wcatch-value=2warn about all class types that are caught by value. With-Wcatch-value=3warn about all types that are not caught by reference.-Wcatch-valueis enabled by-Wall. 👑-Wconditionally-supported<span style="color:hotpink">(C++ and Objective-C++ only)</span> — warns for conditionally-supported (C++11 [intro.defs]) constructs.-Wno-defaulted-function-deleted<span style="color:hotpink">(C++ and Objective-C++ only)</span> — warns when an explicitly defaulted function is deleted by the compiler. That can happen when the function's declared type does not match the type of the function that would have been implicitly declared. This warning is enabled by default.-Wno-delete-incomplete<span style="color:hotpink">(C++ and Objective-C++ only)</span> — do not warn when deleting a pointer to an incomplete type, which may cause undefined behavior at runtime. This warning is enabled by default. 🙈-Wextra-semi<span style="color:hotpink">(C++, Objective-C++ only)</span> — warns about redundant semicolons. There are various contexts in which an extra semicolon can occur. One is a semicolon after in-class function definitions, which is valid in all C++ dialects (and is never a pedwarn):
struct S {
void foo () {};
};Another is an extra semicolon at namespace scope, which has been allowed since C++11 (therefore it is a pedwarn in C++98):
struct S {
};
;And yet another is an extra semicolon in class definitions, which has been allowed since C++11 (therefore it is a pedwarn in C++98):
struct S {
int a;
;
};-Wno-global-module<span style="color:hotpink">(C++ and Objective-C++ only)</span> — disables the diagnostic for when the global module fragment of a module unit does not consist only of preprocessor directives.-Wno-inaccessible-base<span style="color:hotpink">(C++, Objective-C++ only)</span> — this option controls warnings when a base class is inaccessible in a class derived from it due to ambiguity. The warning is enabled by default. Note that the warning for ambiguous virtual bases is enabled by the-Wextraoption. 💅
struct A { int a; };
struct B : A { };
struct C : B, A { };-Wno-inherited-variadic-ctor— suppresses warnings about use of C++11 inheriting constructors when the base class inherited from has a C variadic constructor; the warning is on by default because the ellipsis is not inherited. 🍓-Wno-invalid-offsetof<span style="color:hotpink">(C++ and Objective-C++ only)</span> — suppresses warnings from applying theoffsetofmacro to a non-POD type. According to the 2014 ISO C++ standard, applyingoffsetofto a non-standard-layout type is undefined. In existing C++ implementations, however,offsetoftypically gives meaningful results. This flag is for users who are aware that they are writing nonportable code and who have deliberately chosen to ignore the warning about it.
The restrictions on offsetof may be relaxed in a future version of the C++ standard. 🔥
-Wsized-deallocation(C++ and Objective-C++ only) — okay so bestie, this warning is all about deallocation functions matching up. It yells at you when you define an unsized deallocation function like:
void operator delete (void *) noexcept;
void operator delete[] (void *) noexcept;without a definition of the corresponding sized deallocation function like:
void operator delete (void *, std::size_t) noexcept;
void operator delete[] (void *, std::size_t) noexcept;or vice versa. It's enabled by -Wextra along with -fsized-deallocation. 💅
-Wsuggest-final-types— this one is like your code's little stylist 👑. It warns about types with virtual methods where code quality would be improved if the type were declared with the C++11finalspecifier, or, if possible, declared in an anonymous namespace. Why? Because that allows GCC to more aggressively devirtualize the polymorphic calls. This warning is more effective with link-time optimization, where the information about the class hierarchy graph is more complete. ✨-Wsuggest-final-methods— same energy but for virtual methods: it warns where code quality would be improved if the method were declared with the C++11finalspecifier, or, if possible, its type were declared in an anonymous namespace or with thefinalspecifier. This warning is more effective with link-time optimization, where the information about the class hierarchy graph is more complete. It is recommended to first consider suggestions of-Wsuggest-final-typesand then rebuild with new annotations. 🌸-Wsuggest-override— warns about overriding virtual functions that are not marked with theoverridekeyword. Like, hello? Mark it. 💅-Wno-conversion-null(C++ and Objective-C++ only) — do not warn for conversions betweenNULLand non-pointer types. Note that-Wconversion-nullis enabled by default, so this is you turning that off. 🙈
✨ Options Controlling Objective-C and Objective-C++ Dialects ✨
(NOTE: This manual does not describe the Objective-C and Objective-C++ languages themselves.
This section describes the command-line options that are only meaningful for Objective-C and Objective-C++ programs. You can also use most of the language-independent GNU compiler options. For example, you might compile a file some_class.m like this:
gcc -g -fgnu-runtime -O -c some_class.mIn 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— useclass-nameas the name of the class to instantiate for each literal string specified with the syntax@"...". The default class name isNXConstantStringif the GNU runtime is being used, andNSConstantStringif the NeXT runtime is being used (see below). On Darwin / macOS platforms, the-fconstant-cfstringsoption, if also present, overrides the-fconstant-string-classsetting and cause@"..."literals to be laid out as constant CoreFoundation strings. Note that-fconstant-cfstringsis an alias for the target-specific-mconstant-cfstringsequivalent. 🍓-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 / macOS. The macro__NEXT_RUNTIME__is predefined if (and only if) this option is used. 💅-fno-nil-receivers— assume that all Objective-C message dispatches ([receiver message:arg]) in this translation unit ensure that the receiver is notnil. This allows for more efficient entry points in the runtime to be used. This option is only available in conjunction with the NeXT runtime and ABI version 0 or 1. 🦋-fobjc-abi-version=n— use versionnof the Objective-C ABI for the selected runtime. This option is currently supported only for the NeXT runtime. In that case, Version 0 is the traditional (32-bit) ABI without support for properties and other Objective-C 2.0 additions. Version 1 is the traditional (32-bit) ABI with support for properties and other Objective-C 2.0 additions. Version 2 is the modern (64-bit) ABI. If nothing is specified, the default is Version 0 on 32-bit target machines, and Version 2 on 64-bit target machines. 🌈-fobjc-call-cxx-cdtors— for each Objective-C class, check if any of its instance variables is a C++ object with a non-trivial default constructor. If so, synthesize a special- (id) .cxx_constructinstance method which runs non-trivial default constructors on any such instance variables, in order, and then returnself. Similarly, check if any instance variable is a C++ object with a non-trivial destructor, and if so, synthesize a special- (void) .cxx_destructmethod which runs all such default destructors, in reverse order.
The - (id) .cxx_construct and - (void) .cxx_destruct methods thusly generated only operate on instance variables declared in the current Objective-C class, and not those inherited from superclasses. It is the responsibility of the Objective-C runtime to invoke all such methods in an object's inheritance hierarchy. The - (id) .cxx_construct methods are invoked by the runtime immediately after a new object instance is allocated; the - (void) .cxx_destruct methods are invoked immediately before the runtime deallocates an object instance.
As of this writing, only the NeXT runtime on Mac OS X 10.4 and later has support for invoking the - (id) .cxx_construct and - (void) .cxx_destruct methods. 😭
-fobjc-direct-dispatch— allow fast jumps to the message dispatcher. On Darwin this is accomplished via the comm page. 🔥-fobjc-exceptions— enable syntactic support for structured exception handling in Objective-C, similar to what is offered by C++. This option is required to use the Objective-C keywords@try,@throw,@catch,@finallyand@synchronized. This option is available with both the GNU runtime and the NeXT runtime (but not available in conjunction with the NeXT runtime on Mac OS X 10.2 and earlier). 💀-fobjc-gc— enable garbage collection (GC) in Objective-C and Objective-C++ programs. This option is only available with the NeXT runtime; the GNU runtime has a different garbage collection implementation that does not require special compiler flags. 🧚-fobjc-nilcheck— for the NeXT runtime with version 2 of the ABI, check for a nil receiver in method invocations before doing the actual method call. This is the default and can be disabled using-fno-objc-nilcheck. Class methods and super calls are never checked for nil in this way no matter what this flag is set to. Currently this flag does nothing when the GNU runtime, or an older version of the NeXT runtime ABI, is used. 👑-fobjc-std=objc1— conform to the language syntax of Objective-C 1.0, the language recognized by GCC 4.0. This only affects the Objective-C additions to the C/C++ language; it does not affect conformance to C/C++ standards, which is controlled by the separate C/C++ dialect option flags. When this option is used with the Objective-C or Objective-C++ compiler, any Objective-C syntax that is not recognized by GCC 4.0 is rejected. This is useful if you need to make sure that your Objective-C code can be compiled with older versions of GCC. 💖-freplace-objc-classes— emit a special marker instructingld(1)not to statically link in the resulting object file, and allowdyld(1)to load it in at run time instead. This is used in conjunction with the Fix-and-Continue debugging mode, where the object file in question may be recompiled and dynamically reloaded in the course of program execution, without the need to restart the program itself. Currently, Fix-and-Continue functionality is only available in conjunction with the NeXT runtime on Mac OS X 10.3 and later. 🎀-fzero-link— when compiling for the NeXT runtime, the compiler ordinarily replaces calls toobjc_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-linkflag suppresses this behavior and causes calls toobjc_getClass("...")to be retained. This is useful in Zero-Link debugging mode, since it allows for individual class implementations to be modified during program execution. The GNU runtime currently always retains calls toobjc_get_class("...")regardless of command-line options. 🧚-fno-local-ivars— by default instance variables in Objective-C can be accessed as if they were local variables from within the methods of the class they're declared in. This can lead to shadowing between instance variables and other variables declared either locally inside a class method or globally with the same name. Specifying the-fno-local-ivarsflag disables this behavior thus avoiding variable shadowing issues. 🌸-fivar-visibility=[public|protected|private|package]— set the default instance variable visibility to the specified option so that instance variables declared outside the scope of any access modifier directives default to the specified visibility. ✨-gen-decls— dump interface declarations for all classes seen in the source file to a file namedsourcename.decl. 📄-Wassign-intercept(Objective-C and Objective-C++ only) — warn whenever an Objective-C assignment is being intercepted by the garbage collector. 💅-Wno-property-assign-default(Objective-C and Objective-C++ only) — do not warn if a property for an Objective-C object has no assign semantics specified. 🙈-Wno-protocol(Objective-C and Objective-C++ only) — okay so if a class says it implements a protocol, GCC normally drags it for every method in that protocol the class doesn't implement. And the default is strict bestie: it warns for every method that isn't explicitly implemented in the class, even if the implementation is literally just inherited from the superclass 💀. Use-Wno-protocoland suddenly those inherited methods count as implemented, so no warning for them. Chill.-Wobjc-root-class(Objective-C and Objective-C++ only) — warns you if a class interface has no superclass. Which, like, most classes do inherit from"NSObject"(or"Object") for example 🌸. If you meant it and the class is a root class, hush the warning by marking the interface with__attribute__((objc_root_class)). Slay.-Wselector(Objective-C and Objective-C++ only) — warns when multiple methods of different types for the same selector show up during compilation 🔍. The check happens against the list of methods in the final stage of compilation. Also, a check runs for each selector that appears in a@selector(...)expression where a corresponding method for that selector was found during compilation. The catch, bestie: because these checks scan the method table only at the end of compilation, you get no warnings if that final stage is never reached — like if an error shows up during compilation, or if you're using-fsyntax-only. It's giving "you have to actually finish the race" 🏁.-Wstrict-selector-match(Objective-C and Objective-C++ only) — warns if multiple methods with differing argument and/or return types exist for a given selector when you're trying to send a message using that selector to a receiver of typeidorClass. When this flag is off (which is the default behavior), the compiler just... skips those warnings if the differences found are confined to types that share the same size and alignment. Lowkey lenient, no cap.-Wundeclared-selector(Objective-C and Objective-C++ only) — warns if a@selector(...)expression refers to an undeclared selector. A selector counts as undeclared if no method with that name was declared before the@selector(...)expression — either explicitly in an@interfaceor@protocoldeclaration, or implicitly in an@implementationsection 👀. This option always runs its checks as soon as a@selector(...)expression is found, while-Wselectoronly checks in the final stage of compilation. It also enforces the coding style convention that methods and selectors must be declared before being used. Manifesting clean code ✨.-print-objc-runtime-info— generates a C header describing the largest structure that is passed by value, if any. That's it, that's the whole job 💅.
🌈 Options Controlling OpenMP and OpenACC 🌈
GCC supports OpenMP extensions to the C, C++, and Fortran languages with the -fopenmp option. Similarly, OpenACC extensions are supported in all three languages with -fopenacc. Two whole worlds of parallelism, bestie 🧚.
-foffload=disable-foffload=default-foffload=target-list
Specify for which OpenMP and OpenACC offload targets code should be generated. The default behavior, equivalent to -foffload=default, is to generate code for all supported offload targets. The -foffload=disable form generates code only for the host fallback, while -foffload=target-list generates code only for the specified comma-separated list of offload targets.
Offload targets are specified in GCC's internal target-triplet format. You can run the compiler with -v to show the list of configured offload targets under "OFFLOAD_TARGET_NAMES". Peek behind the curtain, you deserve it 👑.
-foffload-options=options-foffload-options=target-triplet-list=options
With -foffload-options=options, GCC passes the specified options to the compilers for all enabled offloading targets. You can specify options that apply only to a specific target or targets by using the -foffload-options=target-list=options form. The target-list is a comma-separated list in the same format as for the -foffload= option.
Typical command lines are
-foffload-options='-fno-math-errno -ffinite-math-only' \
-foffload-options=nvptx-none=-latomic
-foffload-options=amdgcn-amdhsa=-march=gfx906-fopenacc— enables handling of OpenACC directives#pragma accin C/C++ and!$accin free-form Fortran, plus!$acc,c$accand*$accin fixed-form Fortran. This option implies-pthread, and thus is only supported on targets that have support for-pthread. Sorry bestie, no pthreads, no party 😭.-fopenacc-dim=geom— specify default compute dimensions for parallel offload regions that don't explicitly specify them. The geom value is a triple of:-separated sizes, in order gang, worker, and vector. A size can be omitted, to use a target-specific default value. Mix and match, it's your world 💖.-fopenmp— enables handling of OpenMP directives#pragma omp,[[omp::directive(...)]],[[omp::decl(...)]], and[[omp::sequence(...)]]in C/C++. In Fortran, it enables!$ompand the conditional compilation sentinel!$. In fixed source form Fortran, the sentinels can also start withcor*.
This option implies -pthread, and thus is only supported on targets that have support for -pthread. -fopenmp implies -fopenmp-simd. Iconic combo behavior, periodt 🔥.
-fopenmp-simd— enables handling of OpenMP's"simd","declare simd","declare reduction","assume","ordered","scan"and"loop"directive, and of combined or composite directives with"simd"as constituent with"#pragma omp","[[omp::directive(...)]]","[[omp::sequence(...)]]"and"[[omp::decl(...)]]"in C/C++ and"!$omp"in Fortran. It additionally enables the conditional compilation sentinel!$in Fortran. In fixed source form Fortran, the sentinels can also start withcor*. Other OpenMP directives are ignored 🙈. Unless-fopenmpis additionally specified, the"loop"region binds to the current task region, independent of the specified"bind"clause.-fopenmp-target-simd-clone-fopenmp-target-simd-clone=device-type
In addition to generating SIMD clones for functions marked with the "declare simd" directive, GCC also generates clones for functions marked with the OpenMP "declare target" directive that are suitable for vectorization when this option is in effect. The device-type may be one of "none", "host", "nohost", and "any", which correspond to keywords for the "device_type" clause of the "declare target" directive; clones are generated for the intersection of devices specified. -fopenmp-target-simd-clone is equivalent to -fopenmp-target-simd-clone=any and -fno-openmp-target-simd-clone is equivalent to -fopenmp-target-simd-clone=none.
At -O2 and higher (but not -Os or -Og) this optimization defaults to -fopenmp-target-simd-clone=nohost; otherwise it is disabled by default. Optimizer said "not today" 🫶.
🎀 Options to Control Diagnostic Messages Formatting 🎀
Traditionally, diagnostic messages have been formatted irrespective of the output device's aspect (e.g. its width, ...). You can use the options described below to control the formatting algorithm for diagnostic messages — like how many characters per line, how often source location information should be reported. Note that some language front ends may not honor these options. Rude, but okay 💀.
-fmessage-length=n— try to format error messages so that they fit on lines of about n characters. If n is zero, then no line-wrapping is done; each error message appears on a single line. This is the default for all front ends.
Note — this option also affects the display of the#errorand#warningpre-processor directives, and the deprecated function/type/variable attribute. It does not however affect the pragmaGCC warningand pragmaGCC errorpragmas. 👀
-fdiagnostics-plain-output— requests that diagnostic output look as plain as possible, which may be useful when running dejagnu or other utilities that need to parse diagnostics output and prefer that it remain more stable over time.-fdiagnostics-plain-outputis currently equivalent to the following options:-fno-diagnostics-show-caret-fno-diagnostics-show-line-numbers-fdiagnostics-color=never-fdiagnostics-urls=never-fdiagnostics-path-format=separate-events-fdiagnostics-text-art-charset=none-fno-diagnostics-show-event-links. In the future, if GCC changes the default appearance of its diagnostics, the corresponding option to disable the new behavior will be added to this list. Future-proofing, we love to see it ✨.-fdiagnostics-show-location=once— only meaningful in line-wrapping mode. Instructs the diagnostic messages reporter to emit source location information once; that is, in case the message is too long to fit on a single physical line and has to be wrapped, the source location won't be emitted (as prefix) again, over and over, in subsequent continuation lines. This is the default behavior. Once is enough, bestie 💅.-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. Every. Single. Line. Consistency queen 👑.-fdiagnostics-color[=WHEN]-fno-diagnostics-color
Use color in diagnostics. WHEN is never, always, or auto. The default depends on how the compiler has been configured, it can be any of the above WHEN options or also never if GCC_COLORS environment variable isn't present in the environment, and auto otherwise. auto makes GCC use color only when the standard error is a terminal, and when not executing in an emacs shell 🌈. The forms -fdiagnostics-color and -fno-diagnostics-color are aliases for -fdiagnostics-color=always and -fdiagnostics-color=never, respectively.
Ok bestie, the color lore continues — because obviously we're still deep in the diagnostics-aesthetics corner of the manual and it's giving very much main character energy. 💅
The colors are defined by the environment variable GCC_COLORS. Its value is a colon-separated list of capabilities and Select Graphic Rendition (SGR) substrings. SGR commands are interpreted by the terminal or terminal emulator. (Go read the section in your text terminal's documentation for the permitted values and what they mean as character attributes, iykyk.) These substring values are integers in decimal representation and can be concatenated with semicolons. Common values to concatenate include 1 for bold, 4 for underline, 5 for blink, 7 for inverse, 39 for default foreground color, 30 to 37 for foreground colors, 90 to 97 for 16-color mode foreground colors, 38;5;0 to 38;5;255 for 88-color and 256-color modes foreground colors, 49 for default background color, 40 to 47 for background colors, 100 to 107 for 16-color mode background colors, and 48;5;0 to 48;5;255 for 88-color and 256-color modes background colors.
The default GCC_COLORS is
error=01;31:warning=01;35:note=01;36:range1=32:range2=34:locus=01:\
quote=01:path=01;36:fixit-insert=32:fixit-delete=31:\
diff-filename=01:diff-hunk=32:diff-delete=31:diff-insert=32:\
type-diff=01;32:fnname=01;32:targs=35:valid=01;31:invalid=01;32\
highlight-a=01;32:highlight-b=01;34where 01;31 is bold red, 01;35 is bold magenta, 01;36 is bold cyan, 32 is green, 34 is blue, 01 is bold, and 31 is red. Setting GCC_COLORS to the empty string disables colors. Supported capabilities are as follows. 🌈
"error="— SGR substring forerror:markers."warning="— SGR substring forwarning:markers."note="— SGR substring fornote:markers."path="— SGR substring for colorizing paths of control-flow events as printed via-fdiagnostics-path-format=, like the identifiers of individual events and the lines indicating interprocedural calls and returns."range1="— SGR substring for the first additional range."range2="— SGR substring for the second additional range."locus="— SGR substring for location information,file:lineorfile:line:columnetc."quote="— SGR substring for information printed within quotes."fnname="— SGR substring for names of C++ functions."targs="— SGR substring for C++ function template parameter bindings."fixit-insert="— SGR substring for fix-it hints suggesting text to be inserted or replaced."fixit-delete="— SGR substring for fix-it hints suggesting text to be deleted."diff-filename="— SGR substring for filename headers within generated patches."diff-hunk="— SGR substring for the starts of hunks within generated patches."diff-delete="— SGR substring for deleted lines within generated patches."diff-insert="— SGR substring for inserted lines within generated patches."type-diff="— SGR substring for highlighting mismatching types within template arguments in the C++ frontend."valid="— SGR substring for highlighting valid elements within text art diagrams."invalid="— SGR substring for highlighting invalid elements within text art diagrams."highlight-a=","highlight-b="— SGR substrings for contrasting two different things within diagnostics, like a pair of mismatching types. See-fdiagnostics-show-highlight-colors.
-fdiagnostics-urls[=WHEN]— Use escape sequences to embed URLs in diagnostics, bestie. Like, when-fdiagnostics-show-optionemits text showing the command-line option controlling a diagnostic, this embeds a URL for documentation of that option. 🫶
WHEN is never, always, or auto. auto makes GCC use URL escape sequences only when the standard error is a terminal, and when not executing in an emacs shell or any graphical terminal which is known to be incompatible with this feature, see below.
The default depends on how the compiler has been configured. It can be any of the above WHEN options.
GCC can also be configured (via the --with-diagnostics-urls=auto-if-env configure-time option) so that the default is affected by environment variables. Under such a configuration, GCC defaults to using auto if either GCC_URLS or TERM_URLS environment variables are present and non-empty in the environment of the compiler, or never if neither are.
However, even with -fdiagnostics-urls=always the behavior is dependent on those environment variables: If GCC_URLS is set to empty or no, do not embed URLs in diagnostics. If set to st, URLs use ST escape sequences. If set to bel, the default, URLs use BEL escape sequences. Any other non-empty value enables the feature. If GCC_URLS is not set, use TERM_URLS as a fallback. Note: ST is an ANSI escape sequence, string terminator ESC \, BEL is an ASCII character, CTRL-G that usually sounds like a beep. 😭
At this time GCC tries to detect also a few terminals that are known to not implement the URL feature, and have bugs or at least had bugs in some versions that are still in use, where the URL escapes are likely to misbehave, i.e. print garbage on the screen. That list is currently xfce4-terminal, certain known to be buggy gnome-terminal versions, the linux console, and mingw. This check can be skipped with the -fdiagnostics-urls=always.
-fno-diagnostics-show-option— By default, each diagnostic emitted includes text indicating the command-line option that directly controls the diagnostic (if such an option is known to the diagnostic machinery). Specifying the-fno-diagnostics-show-optionflag suppresses that behavior.-fno-diagnostics-show-caret— By default, each diagnostic emitted includes the original source line and a caret^indicating the column. This option suppresses this information. The source line is truncated to n characters, if the-fmessage-length=noption is given. When the output is done to the terminal, the width is limited to the width given by theCOLUMNSenvironment variable or, if not set, to the terminal width.-fno-diagnostics-show-labels— By default, when printing source code (via-fdiagnostics-show-caret), diagnostics can label ranges of source code with pertinent information, like the types of expressions:
printf ("foo %s bar", long_i + long_j);
~^ ~~~~~~~~~~~~~~~
| |
char * long intThis option suppresses the printing of these labels (in the example above, the vertical bars and the "char *" and "long int" text). 🎀
-fno-diagnostics-show-event-links— By default, when printing execution paths (via-fdiagnostics-path-format=inline-events), GCC will print lines connecting related events, like the line connecting events 1 and 2 in:
3 | if (p)
| ^
| |
| (1) following `false' branch (when `p' is NULL)... ->-+
| |
| |
|+------------------------------------------------------------+
4 || return 0;
5 || return *p;
|| ~
|| |
|+-------->(2) ...to here
| (3) dereference of NULL `p'This option suppresses the printing of such connector lines.
-fno-diagnostics-show-cwe— Diagnostic messages can optionally have an associated CWE (https://cwe.mitre.org/index.html) identifier. GCC itself only provides such metadata for some of the-fanalyzerdiagnostics. GCC plugins may also provide diagnostics with such metadata. By default, if this information is present, it will be printed with the diagnostic. This option suppresses the printing of this metadata.-fno-diagnostics-show-rules— Diagnostic messages can optionally have rules associated with them, like from a coding standard, or a specification. GCC itself does not do this for any of its diagnostics, but plugins may do so. By default, if this information is present, it will be printed with the diagnostic. This option suppresses the printing of this metadata.-fno-diagnostics-show-highlight-colors— GCC can use color for emphasis and contrast when printing diagnostic messages and quoting the user's source. ✨
For example, in
demo.c: In function `test_bad_format_string_args':
../../src/demo.c:25:18: warning: format `%i' expects argument of type `int', but argument 2 has type `const char *' [-Wformat=]
25 | printf("hello %i", msg);
| ~^ ~~~
| | |
| int const char *
| %s- the
%iand"int"in the message and the"int"in the quoted source are colored using"highlight-a"(bold green by default), and - the
"const char *"in the message and in the quoted source are both colored using"highlight-b"(bold blue by default).
The intent is to draw the reader's eyes to the relationships between the various aspects of the diagnostic message and the source, using color to group related elements and distinguish between mismatching ones. Obsessed, honestly. 👑
This additional colorization is enabled by default if color printing is enabled (as per -fdiagnostics-color=), but it can be separately disabled via -fno-diagnostics-show-highlight-colors.
-fno-diagnostics-show-line-numbers— By default, when printing source code (via-fdiagnostics-show-caret), a left margin is printed, showing line numbers. This option suppresses this left margin.-fdiagnostics-minimum-margin-width=width— This option controls the minimum width of the left margin printed by-fdiagnostics-show-line-numbers. It defaults to6. 💜-fdiagnostics-parseable-fixits— ok so this one emits fix-it hints in a machine-parseable format, made for IDEs to slurp up 🧠✨ For every single fix-it, a line gets printed right after the relevant diagnostic, and it starts with the stringfix-it:. Like, for example:
fix-it:"test.c":{45:3-45:21}:"gtk_widget_show_all"The location is expressed as a half-open range, expressed as a count of bytes, starting at byte 1 for the initial column. In the example up there, bytes 3 through 20 of line 45 of test.c are to be replaced with the given string:
00000000011111111112222222222
12345678901234567890123456789
gtk_widget_showall (dlg);
^^^^^^^^^^^^^^^^^^
gtk_widget_show_allThe filename and replacement string escape backslash as \\, tab as \t, newline as \n, double quotes as \", and non-printable characters as octal (e.g. vertical tab as \013). 💅
An empty replacement string indicates that the given range is to be removed. An empty range (e.g. 45:3-45:3) indicates that the string is to be inserted at the given position. no cap, that's the whole vibe.
-fdiagnostics-generate-patch— prints fix-it hints tostderrin unified diff format, after any diagnostics are printed. For example:
--- test.c
+++ test.c
@ -42,5 +42,5 @
void show_cb(GtkDialog *dlg)
{
- gtk_widget_showall(dlg);
+ gtk_widget_show_all(dlg);
}The diff may or may not be colorized, following the same rules as for diagnostics (see -fdiagnostics-color). 🎨
-fdiagnostics-show-template-tree— in the C++ frontend, when printing diagnostics showing mismatching template types, such as:
could not convert 'std::map<int, std::vector<double> >()'
from 'map<[...],vector<double>>' to 'map<[...],vector<float>>the -fdiagnostics-show-template-tree flag enables printing a tree-like structure showing the common and differing parts of the types, such as:
map<
[...],
vector<
[double != float]>>The parts that differ are highlighted with color (double and float in this case). it's giving family tree but make it a compiler error 💀
-fno-elide-type— by default when the C++ frontend prints diagnostics showing mismatching template types, common parts of the types are printed as[...]to simplify the error message. For example:
could not convert 'std::map<int, std::vector<double> >()'
from 'map<[...],vector<double>>' to 'map<[...],vector<float>>Specifying the -fno-elide-type flag suppresses that behavior. This flag also affects the output of the -fdiagnostics-show-template-tree flag. 🦋
-fdiagnostics-path-format=KIND— specify how to print paths of control-flow events for diagnostics that have such a path associated with them.
KIND is none, separate-events, or inline-events, the default.
none means to not print diagnostic paths. 🙈
separate-events means to print a separate "note" diagnostic for each event within the diagnostic. For example:
test.c:29:5: error: passing NULL as argument 1 to 'PyList_Append' which requires a non-NULL parameter
test.c:25:10: note: (1) when 'PyList_New' fails, returning NULL
test.c:27:3: note: (2) when 'i < count'
test.c:29:5: note: (3) when calling 'PyList_Append', passing NULL from (1) as argument 1inline-events means to print the events "inline" within the source code. This view attempts to consolidate the events into runs of sufficiently-close events, printing them as labelled ranges within the source. 👑
For example, the same events as above might be printed as:
'test': events 1-3
25 | list = PyList_New(0);
| ^~~~~~~~~~~~~
| |
| (1) when 'PyList_New' fails, returning NULL
26 |
27 | for (i = 0; i < count; i++) {
| ~~~
| |
| (2) when 'i < count'
28 | item = PyLong_FromLong(random());
29 | PyList_Append(list, item);
| ~~~~~~~~~~~~~~~~~~~~~~~~~
| |
| (3) when calling 'PyList_Append', passing NULL from (1) as argument 1Interprocedural control flow is shown by grouping the events by stack frame, and using indentation to show how stack frames are nested, pushed, and popped. 🫶
For example:
'test': events 1-2
|
| 133 | {
| | ^
| | |
| | (1) entering 'test'
| 134 | boxed_int *obj = make_boxed_int (i);
| | ~~~~~~~~~~~~~~~~~~
| | |
| | (2) calling 'make_boxed_int'
|
+--> 'make_boxed_int': events 3-4
|
| 120 | {
| | ^
| | |
| | (3) entering 'make_boxed_int'
| 121 | boxed_int *result = (boxed_int *)wrapped_malloc (sizeof (boxed_int));
| | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
| | |
| | (4) calling 'wrapped_malloc'
|
+--> 'wrapped_malloc': events 5-6
|
| 7 | {
| | ^
| | |
| | (5) entering 'wrapped_malloc'
| 8 | return malloc (size);
| | ~~~~~~~~~~~~~
| | |
| | (6) calling 'malloc'
|
<-------------+
|
'test': event 7
|
| 138 | free_boxed_int (obj);
| | ^~~~~~~~~~~~~~~~~~~~
| | |
| | (7) calling 'free_boxed_int'
|
(etc)-fdiagnostics-show-path-depths— this option provides additional information when printing control-flow paths associated with a diagnostic. 🔥
If this is option is provided then the stack depth will be printed for each run of events within -fdiagnostics-path-format=inline-events. If provided with -fdiagnostics-path-format=separate-events, then the stack depth and function declaration will be appended when printing each event.
This is intended for use by GCC developers and plugin developers when debugging diagnostics that report interprocedural control flow. iykyk, it's a dev tool bestie, not a casual girlie thing 💜
-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 asdejagnu. 😭-fdiagnostics-column-unit=UNIT— select the units for the column number. This affects traditional diagnostics (in the absence of-fno-show-column), as well as JSON format diagnostics if requested. 📏
The default UNIT, display, considers the number of display columns occupied by each character. This may be larger than the number of bytes required to encode the character, in the case of tab characters, or it may be smaller, in the case of multibyte characters. For example, the character GREEK SMALL LETTER PI (U+03C0) occupies one display column, and its UTF-8 encoding requires two bytes; the character SLIGHTLY SMILING FACE (U+1F642) occupies two display columns, and its UTF-8 encoding requires four bytes. 🙃
Setting UNIT to byte changes the column number to the raw byte count in all cases, as was traditionally output by GCC prior to version 11.1.0.
-fdiagnostics-column-origin=ORIGIN— select the origin for column numbers, i.e. the column number assigned to the first column. The default value of1corresponds to traditional GCC behavior and to the GNU style guide. Some utilities may perform better with an origin of0; any non-negative value may be specified. ✨-fdiagnostics-escape-format=FORMAT— when GCC prints pertinent source lines for a diagnostic it normally attempts to print the source bytes directly. However, some diagnostics relate to encoding issues in the source file, such as malformed UTF-8, or issues with Unicode normalization. These diagnostics are flagged so that GCC will escape bytes that are not printable ASCII when printing their pertinent source lines. 🌸
This option controls how such bytes should be escaped.
The default FORMAT, unicode, displays Unicode characters that are not printable ASCII in the form <U+XXXX>, and bytes that do not correspond to a Unicode character validly-encoded in UTF-8-encoded will be displayed as hexadecimal in the form <XX>.
For example, a source line containing the string before followed by the Unicode character U+03C0 (GREEK SMALL LETTER PI, with UTF-8 encoding 0xCF 0x80) followed by the byte 0xBF (a stray UTF-8 trailing byte), followed by the string after will be printed for such a diagnostic as:
before<U+03C0><BF>afterSetting FORMAT to bytes will display all non-printable-ASCII bytes in the form <XX>, thus showing the underlying encoding of non-ASCII Unicode characters. For the example above, the following will be printed:
before<CF><80><BF>after-fdiagnostics-text-art-charset=CHARSET— some diagnostics can contain "text art" diagrams: visualizations created from text, intended to be viewed in a monospaced font. 🎀
This option selects which characters should be used for printing such diagrams, if any. CHARSET is none, ascii, unicode, or emoji. not the emoji charset omg, that's so extra and I'm obsessed 💖
none— bestie, this one just... does not print those diagrams at all. Silenced. Gone. 💀ascii— makes sure those diagrams stay pure ASCII, i.e. actual "ASCII art" 🎨unicode— allows a conservative lil sprinkle of unicode drawing characters (think box-drawing characters, the cute ones) ✨emoji— and this one goes further and adds the possibility of emoji in the output, like emittingU+26A0 WARNING SIGNfollowed byU+FE0F VARIATION SELECTOR-16to pick the emoji variant of the character 👑
The default is emoji, EXCEPT when the environment variable LANG is set to C, in which case the default is ascii. iykyk 🌸
-fdiagnostics-format=FORMAT— pick a whole different format for printing diagnostics.FORMATis one oftext,sarif-stderr,sarif-file,json,json-stderr, orjson-file.
Using this option replaces any additional "output sinks" that got added by -fdiagnostics-add-output=, or the one set by -fdiagnostics-set-output=. So like, it's a full reset of the output situation, no cap 🫶
The default is text.
sarif-stderr and sarif-file both emit diagnostics in SARIF Version 2.1.0 format — either to stderr, or to a file named source.sarif, respectively.
The various json, json-stderr, and json-file values are deprecated and refer to a legacy JSON-based output format. The json format is a synonym for json-stderr. The json-stderr and json-file formats are identical, apart from where the JSON gets emitted to. With json-stderr, the JSON goes to stderr, whereas with json-file it gets written to source.gcc.json.
-fdiagnostics-add-output=DIAGNOSTICS-OUTPUT-SPEC— adds an additional "output sink" for emitting diagnostics. More sinks, more vibes 💅
DIAGNOSTICS-OUTPUT-SPEC should specify a scheme, optionally followed by ":" and one or more KEY=VALUE pairs, in this form:
<SCHEME>
<SCHEME>:<KEY>=<VALUE>
<SCHEME>:<KEY>=<VALUE>,<KEY2>=<VALUE2>etc.
Schemes, keys, or values with a name prefixed experimental may change or be removed without notice. So don't get too attached, bestie 😭
SCHEME can be:
text
Emit diagnostics to stderr using GCC's classic text output format.
Supported keys are:
color=[yes|no]— override colorization settings from-fdiagnostics-colorfor this text output.experimental-nesting=[yes|no]— enable an experimental mode that emphasizes hierarchical relationships within diagnostics messages, displaying location information on separate lines.experimental-nesting-show-locations=[yes|no]— if"experimental-nesting=yes", then by default locations are shown; set this key to"no"to disable printing such locations. This exists for use by GCC developers, for writing DejaGnu test cases.experimental-nesting-show-levels=[yes|no]— this is a debugging option for use with"experimental-nesting=yes". Set this key to"yes"to print explicit nesting levels in the output. This exists for use by GCC developers.
sarif
Emit diagnostics to a file in SARIF format.
Supported keys are:
file=FILENAME— specify the filename to write the SARIF output to, potentially with a leading absolute or relative path. If not specified, it defaults tosource.sarif.version=[2.1|2.2-prerelease]— specify the version of SARIF to use for the output. If not specified, defaults to2.1."2.2-prerelease"uses an unofficial draft of the future SARIF 2.2 specification and should only be used for experimentation in this release.
For example,
-fdiagnostics-add-output=sarif:version=2.1,file=foo.2.1.sarif
-fdiagnostics-add-output=sarif:version=2.2-prerelease,file=foo.2.2.sarifwould add a pair of outputs, each writing to a different file, using versions 2.1 and 2.2 of the SARIF standard respectively. Two files, one glow-up ✨
In EBNF:
<diagnostics-output-specifier> = <diagnostics-output-name>
| <diagnostics-output-name>, ":", <key-value-pairs>;
<diagnostics-output-name> = "text" | "sarif";
<key-value-pairs> = <key-value-pair>
| <key-value-pair> "," <key-value-pairs>;
<key-value-pair> = <key> "=" <value>;
<key> = ? string without a '=' ? ;
<value> = ? string without a ',' ? ;-fdiagnostics-set-output=DIAGNOSTICS-OUTPUT-SPEC— works in a similar way to-fdiagnostics-add-output=except that instead of adding an additional "output sink" for diagnostics, it replaces all existing output sinks, such as from-fdiagnostics-format=,-fdiagnostics-add-output=, or a prior call to-fdiagnostics-set-output=. It's giving main character energy — out with the old, in with the new 👑-fno-diagnostics-json-formatting— by default, when JSON is emitted for diagnostics (via-fdiagnostics-format=sarif-stderr,-fdiagnostics-format=sarif-file,-fdiagnostics-format=json,-fdiagnostics-format=json-stderr,-fdiagnostics-format=json-file), GCC will add newlines and indentation to visually emphasize the hierarchical structure of the JSON. Pretty JSON, we love to see it 🌈
Use -fno-diagnostics-json-formatting to suppress this whitespace. It must be passed before the option it is to affect. Order matters, bestie, do not play with it 🙈
This is intended for compatibility with tools that do not expect the output to contain newlines, such as that emitted by older GCC releases.
✨ Options to Request or Suppress Warnings ✨
Warnings are diagnostic messages that report constructions that are not inherently erroneous but that are risky or suggest there may have been an error. So like, not technically illegal but... sus 🚩
The following language-independent options do not enable specific warnings but control the kinds of diagnostics produced by GCC.
-fsyntax-only— check the code for syntax errors, but don't do anything beyond that. Just a vibe check, no output 💅-fmax-errors=n— limits the maximum number of error messages ton, at which point GCC bails out rather than attempting to continue processing the source code. Ifnis0(the default), there is no limit on the number of error messages produced. If-Wfatal-errorsis also specified, then-Wfatal-errorstakes precedence over this option. So-Wfatal-errorsis the boss here, no questions 😤-w— inhibit all warning messages. Zero warnings. Nothing. Silence, bestie 🤫-Werror— turn all warnings into errors. All of them. Every single one. You asked for this 🔥-Werror=— turn the specified warning into an error. The specifier for a warning is appended; for example-Werror=switchturns the warnings controlled by-Wswitchinto errors. This switch takes a negative form, to be used to negate-Werrorfor specific warnings; for example-Wno-error=switchmakes-Wswitchwarnings not be errors, even when-Werroris in effect. Cherry-picking, we stan 🍓
The warning message for each controllable warning includes the option that controls the warning. That option can then be used with -Werror= and -Wno-error= as described above. (Printing of the option in the warning message can be disabled using the -fno-diagnostics-show-option flag.)
Note that specifying-Werror=fooautomatically implies-Wfoo. However,-Wno-error=foodoes not imply anything. Asymmetric, but that's just how it is 💀
-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. First error and it's out, no second chances 😭
You can request many specific warnings with options beginning with -W, for example -Wunused-variable to request warnings on declarations of variables that are never used. Each of these specific warning options also has a negative form beginning with -Wno- to turn off warnings; for example, -Wno-unused-variable. This manual lists only one of the two forms, whichever is not the default. For further language-specific options also refer to C++ Dialect Options and Objective-C and Objective-C++ Dialect Options. Additional warnings can be produced by enabling the static analyzer;
Some options, such as -Wall and -Wextra, turn on other options, such as -Wunused, which may turn on further options, such as -Wunused-variable. It's options all the way down, bestie 🌀 The combined effect of positive and negative forms is that more specific options have priority over less specific ones, independently of their position in the command line. For options of the same specificity, the last one takes effect. Options enabled or disabled via pragmas take effect as if they appeared at the end of the command line.
When an unrecognized warning option is requested (e.g., -Wunknown-warning), GCC gives an error stating that the option is not recognized. However, if the -Wno- form is used, the behavior is slightly different: no diagnostic is produced for -Wno-unknown-warning unless other diagnostics are being produced. This allows the use of new -Wno- options with old compilers, but if something goes wrong, the compiler warns that an unrecognized option is present. Sneaky but useful 🫶
The effectiveness of some warnings depends on optimizations also being enabled. For example, -Wsuggest-final-types is more effective with link-time optimization. Some other warnings may not be issued at all unless optimization is enabled. While optimization in general improves the efficacy of warnings about control and data-flow problems, in some cases it may also cause false positives. Yeah... it's giving complicated 😬
-Wpedantic-pedantic— issue all the warnings demanded by strict ISO C and ISO C++; diagnose all programs that use forbidden extensions, and some other programs that do not follow ISO C and ISO C++. This follows the version of the ISO C or C++ standard specified by any-stdoption used.
Valid ISO C and ISO C++ programs should compile properly with or without this option (though a rare few require -ansi or a -std option specifying the version of the standard). However, without this option, certain GNU extensions and traditional C and C++ features are supported as well. With this option, they are diagnosed (or rejected with -pedantic-errors).
-Wpedantic does not cause warning messages for use of the alternate keywords whose names begin and end with __. This alternate format can also be used to disable warnings for non-ISO __intN types, i.e. __intN__. 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. Don't be out here using the back door, bestie 🚪✨
Some warnings about non-conforming programs are controlled by options other than -Wpedantic; in many cases they are implied by -Wpedantic but can be disabled separately by their specific option, e.g. -Wpedantic -Wno-pointer-sign.
Where the standard you picked with -std is a GNU extended dialect of C — think gnu90 or gnu99 — there's a matching base standard, aka the version of ISO C that the GNU extended dialect is built on top of. So -Wpedantic warnings show up wherever the base standard says they're required. (And no, it wouldn't make sense to only warn about stuff that's missing from the GNU C dialect you specified — because by definition the GNU dialects of C include literally every feature the compiler supports with that option, so there'd be nothing left to warn about. 🫶)
-pedantic-errors— throws an error any time the base standard (see-Wpedantic) demands a diagnostic, plus some cases of compile-time undefined behavior, plus some other cases that don't even stop programs that are perfectly valid per the standard from compiling. Bestie, this is NOT the same as-Werror=pedantic— that other option is lowkey not useful, because it only turns into errors the diagnostics controlled by-Wpedantic, whereas this one also hits the required diagnostics that are always on or controlled by options other than-Wpedantic. 💅
If you want the required diagnostics that are warnings by default to become errors, but you do not also want the -Wpedantic diagnostics turned on, you can say -pedantic-errors -Wno-pedantic (or -pedantic-errors -Wno-error=pedantic if you want them enabled but only as warnings).
Some required diagnostics are errors by default, but you can downgrade them to warnings using -fpermissive or their own specific warning option, e.g. -Wno-error=narrowing.
Some diagnostics for non-ISO practices are controlled by specific warning options other than -Wpedantic, but -pedantic-errors also promotes them to errors. For instance:
-Wattributes(for standard attributes)-Wchanges-meaning(C++)-Wcomma-subscript(C++23 or later)-Wdeclaration-after-statement(C90 or earlier)-Welaborated-enum-base(C++11 or later)-Wimplicit-int(C99 or later)-Wimplicit-function-declaration(C99 or later)-Wincompatible-pointer-types-Wint-conversion-Wlong-long(C90 or earlier)-Wmain-Wnarrowing(C++11 or later)-Wpointer-arith-Wpointer-sign-Wincompatible-pointer-types-Wregister(C++17 or later)-Wvla(C90 or earlier)-Wwrite-strings(C++11 or later)
-fpermissive— downgrades some required diagnostics about nonconformant code from errors to warnings. So using-fpermissivelets some nonconforming code actually compile. 🌈 Some C++ diagnostics are controlled only by this flag, but it also downgrades some C and C++ diagnostics that have their own flag:-Wdeclaration-missing-parameter-type(C and Objective-C only)-Wimplicit-function-declaration(C and Objective-C only)-Wimplicit-int(C and Objective-C only)-Wincompatible-pointer-types(C and Objective-C only)-Wint-conversion(C and Objective-C only)-Wnarrowing(C++ and Objective-C++ only)-Wreturn-mismatch(C and Objective-C only)-Wtemplate-body(C++ and Objective-C++ only)
Fun fact: -fpermissive is the default for historic C language modes (-std=c89, -std=gnu89, -std=c90, -std=gnu90). Throwback vibes ✨
-Wall— turns on all the warnings about constructions that some users consider questionable, and that are easy to avoid (or tweak to kill the warning), even when macros are in the mix. It also enables some language-specific warnings described in C++ Dialect Options and Objective-C and Objective-C++ Dialect Options. 🎀
-Wall turns on the following warning flags:
-Waddress-Waligned-new(C++ and Objective-C++ only)-Warray-bounds=1(only with-O2)-Warray-compare-Warray-parameter=2-Wbool-compare-Wbool-operation-Wc++11-compat-Wc++14-compat-Wc++17compat-Wc++20compat-Wcatch-value(C++ and Objective-C++ only)-Wchar-subscripts-Wclass-memaccess(C++ and Objective-C++ only)-Wcomment-Wdangling-else-Wdangling-pointer=2-Wdelete-non-virtual-dtor(C++ and Objective-C++ only)-Wduplicate-decl-specifier(C and Objective-C only)-Wenum-compare(in C/ObjC; this is on by default in C++)-Wenum-int-mismatch(C and Objective-C only)-Wformat=1-Wformat-contains-nul-Wformat-diag-Wformat-extra-args-Wformat-overflow=1-Wformat-truncation=1-Wformat-zero-length-Wframe-address-Wimplicit(C and Objective-C only)-Wimplicit-function-declaration(C and Objective-C only)-Wimplicit-int(C and Objective-C only)-Winfinite-recursion-Winit-self(C++ and Objective-C++ only)-Wint-in-bool-context-Wlogical-not-parentheses-Wmain(only for C/ObjC and unless-ffreestanding)-Wmaybe-uninitialized-Wmemset-elt-size-Wmemset-transposed-args-Wmisleading-indentation(only for C/C++)-Wmismatched-dealloc-Wmismatched-new-delete(C++ and Objective-C++ only)-Wmissing-attributes-Wmissing-braces(only for C/ObjC)-Wmultistatement-macros-Wnarrowing(C++ and Objective-C++ only)-Wnonnull-Wnonnull-compare-Wopenmp-simd(C and C++ only)-Woverloaded-virtual=1(C++ and Objective-C++ only)-Wpacked-not-aligned-Wparentheses-Wpessimizing-move(C++ and Objective-C++ only)-Wpointer-sign(only for C/ObjC)-Wrange-loop-construct(C++ and Objective-C++ only)-Wreorder(C++ and Objective-C++ only)-Wrestrict-Wreturn-type-Wself-move(C++ and Objective-C++ only)-Wsequence-point-Wsign-compare(C++ and Objective-C++ only)-Wsizeof-array-div-Wsizeof-pointer-div-Wsizeof-pointer-memaccess-Wstrict-aliasing-Wstrict-overflow=1-Wswitch-Wtautological-compare-Wtrigraphs-Wuninitialized-Wunknown-pragmas-Wunused-Wunused-but-set-variable-Wunused-const-variable=1(only for C/ObjC)-Wunused-function-Wunused-label-Wunused-local-typedefs-Wunused-value-Wunused-variable-Wuse-after-free=2-Wvla-parameter-Wvolatile-register-var-Wzero-length-bounds
Heads up bestie: some warning flags are not implied by-Wall. Some of them warn about constructions users generally don't consider questionable but that you might occasionally want to check for; others warn about constructions that are necessary or hard to avoid in some cases, and there's no simple way to rewrite the code to silence the warning. Some of them get enabled by-Wextrabut many of them you gotta enable individually. 😭
-Wextra— enables some extra warning flags that-Walldoesn't turn on. (This option used to be called-W. The older name still works, but the newer name is more descriptive. 💖)-Wabsolute-value(only for C/ObjC)-Walloc-size-Wcalloc-transposed-args-Wcast-function-type-Wclobbered-Wdangling-reference(C++ only)-Wdeprecated-copy(C++ and Objective-C++ only)-Wempty-body-Wenum-conversion(only for C/ObjC)-Wexpansion-to-defined-Wignored-qualifiers(only for C/C++)-Wimplicit-fallthrough=3-Wmaybe-uninitialized-Wmissing-field-initializers-Wmissing-parameter-name(C/ObjC only)-Wmissing-parameter-type(C/ObjC only)-Wold-style-declaration(C/ObjC only)-Woverride-init(C/ObjC only)-Wredundant-move(C++ and Objective-C++ only)-Wshift-negative-value(in C++11 to C++17 and in C99 and newer)-Wsign-compare(C++ and Objective-C++ only)-Wsized-deallocation(C++ and Objective-C++ only)-Wstring-compare-Wtype-limits-Wuninitialized-Wunterminated-string-initialization-Wunused-parameter(only with-Wunusedor-Wall)-Wunused-but-set-parameter(only with-Wunusedor-Wall)
The option -Wextra also prints warning messages for these cases:
- A pointer is compared against integer zero with
"<","<=",">", or">=". - (C++ only) An enumerator and a non-enumerator both appear in a conditional expression.
- (C++ only) Ambiguous virtual bases.
- (C++ only) Subscripting an array that has been declared
"register". - (C++ only) Taking the address of a variable that has been declared
"register". - (C++ only) A base class is not initialized in the copy constructor of a derived class.
-Wabi(C, Objective-C, C++ and Objective-C++ only) — warns about code affected by ABI changes. This includes code that may not be compatible with the vendor-neutral C++ ABI as well as the psABI for the particular target. Those psABI warnings are also controlled separately by-Wpsabi, which-Wabiimplies. 🌸
Since G++ now defaults to updating the ABI with each major release, normally -Wabi only warns about C++ ABI compatibility problems if a check was added later in a release series for an ABI issue discovered since the initial release. -Wabi warns about more stuff if an older ABI version is selected (with -fabi-version=n).
-Wabi can also be used with an explicit version number to warn about C++ ABI compatibility with a particular -fabi-version level, e.g. -Wabi=2 to warn about changes relative to -fabi-version=2.
If an explicit version number is provided and -fabi-compat-version is not specified, the version number from this option is used for compatibility aliases. If no explicit version number is provided with this option, but -fabi-compat-version is specified, that version number is used for C++ ABI warnings.
Okay so real talk — even though 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 get emitted even though the generated code is actually compatible. 🙈
You should rewrite your code to avoid these warnings if you're concerned about the fact that code generated by G++ may not be binary compatible with code generated by other compilers.
Known incompatibilities in -fabi-version=2 (which was the default from GCC 3.4 to 4.9) include:
- A template with a non-type template parameter of reference type was mangled incorrectly:
extern int N;
template <int &> struct S {};
void n (S<N>) {2}This was fixed in -fabi-version=3.
- SIMD vector types declared using
"__attribute ((vector_size))"were mangled in a non-standard way that does not allow for overloading of functions taking vectors of different sizes.
The mangling was changed in -fabi-version=4.
"__attribute ((const))"and"noreturn"were mangled as type qualifiers, and"decltype"of a plain declaration was folded away.
These mangling issues were fixed in -fabi-version=5.
- Scoped enumerators passed as arguments to a variadic function get promoted exactly like unscoped enumerators do, which makes
va_argbig mad and complain. On most targets this genuinely does not mess with the parameter passing ABI tho, because there's literally no way to pass an argument smaller thanintanyway. 💅
Also lowkey: the ABI changed the mangling of template argument packs, const_cast, static_cast, prefix increment/decrement, and a class scope function used as a template argument.
All of that got fixed in -fabi-version=6. ✨
- Lambdas in default argument scope were being mangled wrong, and the ABI also changed the mangling of
nullptr_t.
Fixed in -fabi-version=7.
- Mangling a function type with function-cv-qualifiers treated the un-qualified function type as a substitution candidate, which was just wrong of it.
Fixed in -fabi-version=8, the default for GCC 5.1. 👑
decltype(nullptr)had an alignment of 1, which caused unaligned accesses. Note that this did not affect the ABI of a function with anullptr_tparameter, because parameters have a minimum alignment.
Fixed in -fabi-version=9, the default for GCC 5.2.
- Target-specific attributes that affect the identity of a type, like ia32 calling conventions on a function type (stdcall, regparm, etc.), didn't affect the mangled name, which led to name collisions whenever function pointers were used as template arguments. messy fr.
Fixed in -fabi-version=10, the default for GCC 6.1.
✨ more options, we're still in OPTIONS btw ✨
-Wpsabi(C, Objective-C, C++ and Objective-C++ only) —-Wpsabiturns on warnings about processor-specific ABI changes, like changes in alignment requirements or how function arguments get passed. On several targets — including AArch64, ARM, x86, MIPS, RS6000/PowerPC, and S/390 — these details have shifted between different versions of GCC and/or different versions of the C or C++ language standards in ways that affect binary compatibility of compiled code. With-Wpsabi, GCC can spot potentially incompatible usages and warn you about them. 🫶
-Wpsabi is enabled by default, and is also implied by -Wabi.
-Wno-changes-meaning(C++ and Objective-C++ only) — C++ requires that unqualified uses of a name within a class have the same meaning across the complete scope of the class, so declaring the name after you've used it is ill-formed:
struct A;
struct B1 { A a; typedef A A; }; // warning, 'A' changes meaning
struct B2 { A a; struct A { }; }; // error, 'A' changes meaningBy default, the B1 case is only a warning because the two declarations have the same type, while the B2 case is an error. Both diagnostics can be disabled with -Wno-changes-meaning. Alternately, the error case can be reduced to a warning with -Wno-error=changes-meaning or -fpermissive.
Both diagnostics are also suppressed by -fms-extensions.
-Wchar-subscripts— Warns if an array subscript has typechar. This is a common cause of error, because programmers keep forgetting that this type is signed on some machines. 😭 This warning is enabled by-Wall.-Wno-coverage-mismatch— Warns if feedback profiles don't match when you're using the-fprofile-useoption. If a source file gets changed between compiling with-fprofile-generateand with-fprofile-use, the files with the profile feedback can fail to match the source file and GCC can't use the profile feedback information. By default this warning is enabled and treated as an error.-Wno-coverage-mismatchdisables the warning, or-Wno-error=coverage-mismatchdisables the error.
Disabling the error for this warning can result in poorly optimized code and is useful only in the case of very minor changes such as bug fixes to an existing code-base. Completely disabling the warning is not recommended. 💀
-Wno-coverage-too-many-conditions— Warns if-fcondition-coverageis used and an expression has too many terms, so GCC gives up on coverage. Coverage gets given up when there are more terms in the conditional than there are bits in agcov_type_unsigned. This warning is enabled by default.-Wno-coverage-too-many-paths— Warns if-fpath-coverageis used and a function has too many paths, so GCC gives up on coverage. The giving-up part is controlled by-fpath-coverage-limit. This warning is enabled by default.-Wno-coverage-invalid-line-number— Warns in case a function ends earlier than it begins, because of invalid linenum macros. The warning is only emitted with--coverageenabled. 🎀
By default, this warning is enabled and is treated as an error. -Wno-coverage-invalid-line-number disables the warning, or -Wno-error=coverage-invalid-line-number disables the error.
-Wno-cpp(C, Objective-C, C++, Objective-C++ and Fortran only) — Suppresses warning messages emitted by#warningdirectives. shhh 🤫-Wdouble-promotion(C, C++, Objective-C and Objective-C++ only) — Gives a warning when a value of typefloatgets implicitly promoted todouble. CPUs with a 32-bit "single-precision" floating-point unit implementfloatin hardware, but emulatedoublein software. On a machine like that, doing computations withdoublevalues is much more expensive because of the overhead that software emulation needs.
It's genuinely easy to accidentally do computations with double, because floating-point literals are implicitly of type double. For example, in:
float area(float radius)
{
return 3.14159 * radius * radius;
}the compiler performs the entire computation with double, because the floating-point literal is a double. sneaky. 🙈
-Wduplicate-decl-specifier(C and Objective-C only) — Warns if a declaration has a duplicateconst,volatile,restrictor_Atomicspecifier. This warning is enabled by-Wall.-Wformat-Wformat=n— Checks calls toprintfandscanf, etc., to make sure the arguments supplied have types appropriate to the format string specified, and that the conversions specified in the format string actually make sense. This covers standard functions, plus others specified by format attributes, in theprintf,scanf,strftimeandstrfmonfamilies (wherestrfmonis an X/Open extension, not in the C standard) — or other target-specific families.
Which functions get checked without format attributes having been specified depends on the standard version selected, and such checks of functions without the attribute specified are disabled by -ffreestanding or -fno-builtin.
The formats are checked against the format features supported by GNU libc version 2.2. These include all ISO C90 and C99 features, as well as features from the Single Unix Specification and some BSD and GNU extensions. Other library implementations may not support all these features; GCC does not support warning about features that go beyond a particular library's limitations. However, if -Wpedantic is used with -Wformat, warnings are given about format features not in the selected standard version (but not for strfmon formats, since those are not in any version of the C standard).
-Wformat=1-Wformat— Option-Wformatis equivalent to-Wformat=1, and-Wno-formatis equivalent to-Wformat=0. Since-Wformatalso checks for null format arguments for several functions,-Wformatalso implies-Wnonnull. Some aspects of this level of format checking can be disabled by the options:-Wno-format-contains-nul,-Wno-format-extra-args, and-Wno-format-zero-length.-Wformatis enabled by-Wall.-Wformat=2— Enables-Wformatplus additional format checks. Currently equivalent to-Wformat -Wformat-nonliteral -Wformat-security -Wformat-y2k.
-Wno-format-contains-nul— If-Wformatis specified, don't warn about format strings that contain NUL bytes.-Wno-format-extra-args— If-Wformatis specified, don't warn about excess arguments to aprintforscanfformat 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 couldn't know what type to pass to va_arg to skip the unused arguments. However, in the case of scanf formats, this option suppresses the warning if the unused arguments are all pointers, since the Single Unix Specification says that such unused arguments are allowed.
-Wformat-overflow-Wformat-overflow=level— Warns about calls to formatted input/output functions such assprintfandvsprintfthat might overflow the destination buffer. When the exact number of bytes written by a format directive can't be determined at compile-time, it gets estimated based on heuristics that depend on the level argument and on optimization. While enabling optimization will in most cases improve the accuracy of the warning, it may also result in false positives. 😬-Wformat-overflow-Wformat-overflow=1— Level 1 of-Wformat-overflow(enabled by-Wformat) employs a conservative approach that warns only about calls that most likely overflow the buffer. At this level, numeric arguments to format directives with unknown values are assumed to have the value of one, and strings of unknown length are assumed to be empty. Numeric arguments that are known to be bounded to a subrange of their type, or string arguments whose output is bounded either by their directive's precision or by a finite set of string literals, are assumed to take on the value within the range that results in the most bytes on output. For example, the call tosprintfbelow is diagnosed because even with bothaandbequal to zero, the terminating NUL character ('\0') appended by the function to the destination buffer will be written past its end. Increasing the size of the buffer by a single byte is sufficient to avoid the warning, though it may not be sufficient to avoid the overflow.
void f (int a, int b)
{
char buf [13];
sprintf (buf, "a = %i, b = %i\n", a, b);
}-Wformat-overflow=2— Level 2 is where it gets thorough, bestie. It also warns about calls that might overflow the destination buffer when you hand it an argument of sufficient length or magnitude. At level 2, unknown numeric arguments are assumed to have the minimum representable value for signed types with a precision greater than 1, and the maximum representable value otherwise. Unknown string arguments whose length can't be assumed to be bounded either by the directive's precision, or by a finite set of string literals they may evaluate to, or the character array they may point to, are assumed to be 1 character long. 😌
At level 2, the call in the example above is again diagnosed — but this time because with a equal to a 32-bit INT_MIN the first %i directive will write some of its digits beyond the end of the destination buffer. To make the call safe regardless of the values of the two variables, the size of the destination buffer must be increased to at least 34 bytes. GCC includes the minimum size of the buffer in an informational note following the warning. 🫶
An alternative to increasing the size of the destination buffer is to constrain the range of formatted values. The maximum length of string arguments can be bounded by specifying the precision in the format directive. When numeric arguments of format directives can be assumed to be bounded by less than the precision of their type, choosing an appropriate length modifier to the format specifier will reduce the required buffer size. For example, if a and b in the example above can be assumed to be within the precision of the short int type then using either the %hi format directive or casting the argument to short reduces the maximum required size of the buffer to 24 bytes.
void f (int a, int b)
{
char buf [23];
sprintf (buf, "a = %hi, b = %i\n", a, (short)b);
}-Wno-format-zero-length— if-Wformatis on, this one tells it not to warn about zero-length formats. And no cap: the C standard literally specifies that zero-length formats are allowed. So there. 💅-Wformat-nonliteral— if-Wformatis 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 ava_list. iykyk ✨-Wformat-security— if-Wformatis specified, also warn about uses of format functions that represent possible security problems. At present, this warns about calls toprintfandscanffunctions where the format string is not a string literal and there are no format arguments, as inprintf (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-nonliteralwarns about, but in future warnings may be added to-Wformat-securitythat are not included in-Wformat-nonliteral.) ⚠️-Wformat-signedness— if-Wformatis specified, also warn if the format string requires an unsigned argument and the argument is signed and vice versa. the way it just catches the mismatch... iconic 🔍-Wformat-truncation,-Wformat-truncation=level— warn about calls to formatted input/output functions such assnprintfandvsnprintfthat might result in output truncation. When the exact number of bytes written by a format directive cannot be determined at compile-time it is estimated based on heuristics that depend on the level argument and on optimization. While enabling optimization will in most cases improve the accuracy of the warning, it may also result in false positives. Except as noted otherwise, the option uses the same logic as-Wformat-overflow. 😭-Wformat-truncation,-Wformat-truncation=1— Level 1 of-Wformat-truncation, enabled by-Wformat, employs a conservative approach that warns only about calls to bounded functions whose return value is unused and that will most likely result in output truncation.-Wformat-truncation=2— Level 2 warns also about calls to bounded functions whose return value is used and that might result in truncation given an argument of sufficient length or magnitude.
-Wformat-y2k— if-Wformatis specified, also warn aboutstrftimeformats that may yield only a two-digit year. 🗓️ the Y2K flashbacks fr fr-Wnonnull— warn about passing a null pointer for arguments marked as requiring a non-null value by thenonnullfunction attribute.
-Wnonnull is included in -Wall and -Wformat. It can be disabled with the -Wno-nonnull option.
-Wnonnull-compare— warn when comparing an argument marked with thenonnullfunction attribute against null inside the function.
-Wnonnull-compare is included in -Wall. It can be disabled with the -Wno-nonnull-compare option.
-Wnull-dereference— warn if the compiler detects paths that trigger erroneous or undefined behavior due to dereferencing a null pointer. This option is only active when-fdelete-null-pointer-checksis active, which is enabled by optimizations in most targets. The precision of the warnings depends on the optimization options used. 💀-Wno-musttail-local-addr— do not warn about passing a pointer (or in C++, a reference) to a local variable or label to an argument of amusttailcall. Those variables go out of scope before the tail call instruction. 🙈-Wmaybe-musttail-local-addr— warn when the address of a local variable can escape to amusttailcall, unless it goes out of scope already before themusttailcall.
int foo (int *);
int
bar (int *x)
{
if (x[0] == 1)
{
int a = 42;
foo (&a);
/* Without the C<musttail> attribute this call would not
be tail called, because address of the C<a> variable escapes
and the second foo call could dereference it. With the attribute
the local variables are assumed to go out of scope immediately
before the tail call instruction and the compiler warns about
this. */
[[gnu::musttail]] return foo (nullptr);
}
else
{
{
int a = 42;
foo (&a);
}
/* The C<a> variable isn't already in scope, so even when it
escaped, even without C<musttail> attribute it would be
undefined behavior to dereference it and the compiler could
turn this into a tail call. No warning is diagnosed here. */
[[gnu::musttail]] return foo (nullptr);
}
}This warning is enabled by -Wextra.
-Wnrvo(C++ and Objective-C++ only) — warn if the compiler does not elide the copy from a local variable to the return value of a function in a context where it is allowed by [class.copy.elision]. This elision is commonly known as the Named Return Value Optimization. For instance, in the example below the compiler cannot elide copies from bothv1andv2, so it elides neither. 🦋
std::vector<int> f()
{
std::vector<int> v1, v2;
// ...
if (cond) return v1;
else return v2; // warning: not eliding copy
}-Winfinite-recursion— warn about infinitely recursive calls. The warning is effective at all optimization levels but requires optimization in order to detect infinite recursion in calls between two or more functions.-Winfinite-recursionis included in-Wall. 🔁
Compare with -Wanalyzer-infinite-recursion which provides a similar diagnostic, but is implemented in a different way (as part of -fanalyzer).
-Winit-self(C, C++, Objective-C and Objective-C++ only) — warn about uninitialized variables that are initialized with themselves. Note this option can only be used with the-Wuninitializedoption.
For example, GCC warns about i being uninitialized in the following snippet only when -Winit-self has been specified:
int f()
{
int i = i;
return i;
}This warning is enabled by -Wall in C++.
-Wno-implicit-int(C and Objective-C only) — this option controls warnings when a declaration does not specify a type. This warning is enabled by default, as an error, in C99 and later dialects of C, and also by-Wall. The error can be downgraded to a warning using-fpermissive(along with certain other errors), or for this error alone, with-Wno-error=implicit-int.
This warning is upgraded to an error by -pedantic-errors.
-Wno-implicit-function-declaration(C and Objective-C only) — this option controls warnings when a function is used before being declared. This warning is enabled by default, as an error, in C99 and later dialects of C, and also by-Wall. The error can be downgraded to a warning using-fpermissive(along with certain other errors), or for this error alone, with-Wno-error=implicit-function-declaration.
This warning is upgraded to an error by -pedantic-errors.
-Wimplicit(C and Objective-C only) — same as-Wimplicit-intand-Wimplicit-function-declaration. This warning is enabled by-Wall. 👑-Whardened— warn when-fhardeneddid not enable an option from its set (for which see-fhardened). For instance, using-fhardenedand-fstack-protectorat the same time on the command line causes-Whardenedto warn because-fstack-protector-strongwill not be enabled by-fhardened.
This warning is enabled by default and has effect only when -fhardened is enabled.
-Wimplicit-fallthrough—-Wimplicit-fallthroughis the same as-Wimplicit-fallthrough=3and-Wno-implicit-fallthroughis the same as-Wimplicit-fallthrough=0. 🍓-Wimplicit-fallthrough=n— warn when a switch case falls through. For example:
switch (cond)
{
case 1:
a = 1;
break;
case 2:
a = 2;
case 3:
a = 3;
break;
}okay so this warning stays quiet when the last statement of a case cannot fall through, like when there's a return statement or a call to a function declared with the noreturn attribute. 💅
and -Wimplicit-fallthrough= also takes control flow statements into account — stuff like ifs — and only warns when it's actually appropriate. for example:
switch (cond)
{
case 1:
if (i > 3) {
bar (5);
break;
} else if (i < 1) {
bar (0);
} else
return;
default:
...
}now, because sometimes a switch case falling through is genuinely what you want, GCC hands you an attribute — __attribute__ ((fallthrough)) — that you use right alongside a null statement to suppress the warning that would normally show up:
switch (cond)
{
case 1:
bar (0);
__attribute__ ((fallthrough));
default:
...
}C++17 and C23 give you a standard way to silence the -Wimplicit-fallthrough warning too, using [[fallthrough]]; instead of the GNU attribute. and if you're on C++11 or C++14 you can use [[gnu::fallthrough]];, which is a GNU extension. 🫶
but also — instead of any of those attributes, you can just drop a fallthrough comment to hush the warning. the whole body of the C or C++ style comment has to match one of the regular expressions listed below. the option argument n decides what kind of comments get accepted:
-Wimplicit-fallthrough=0— disables the warning altogether.-Wimplicit-fallthrough=1— matches the.*regular expression, so literally any comment counts as a fallthrough comment.-Wimplicit-fallthrough=2— case insensitively matches the.*falls?[ \t-]*thr(ough|u).*regular expression.-Wimplicit-fallthrough=3— case sensitively matches one of these regular expressions:"-fallthrough""@fallthrough@""lint -fallthrough[ \t]*""[ \t.!]*(ELSE,? |INTENTIONAL(LY)? )?FALL(S | |-)?THR(OUGH|U)[ \t.!]*(-[^\n\r]*)?""[ \t.!]*(Else,? |Intentional(ly)? )?Fall((s | |-)[Tt]|t)hr(ough|u)[ \t.!]*(-[^\n\r]*)?""[ \t.!]*([Ee]lse,? |[Ii]ntentional(ly)? )?fall(s | |-)?thr(ough|u)[ \t.!]*(-[^\n\r]*)?"
-Wimplicit-fallthrough=4— case sensitively matches one of these regular expressions:"-fallthrough""@fallthrough@""lint -fallthrough[ \t]*""[ \t]*FALLTHR(OUGH|U)[ \t]*"
-Wimplicit-fallthrough=5— doesn't recognize any comments as fallthrough comments, so only attributes can disable the warning. periodt. 💀
the comment has to be followed — after optional whitespace and other comments — by the case or default keywords, or by a user label that comes before some case or default label. like:
switch (cond)
{
case 1:
bar (0);
/* FALLTHRU */
default:
...
}and heads up bestie: the -Wimplicit-fallthrough=3 warning is enabled by -Wextra. ✨
💖 -Wno-if-not-aligned (C, C++, Objective-C and Objective-C++ only)
this one controls whether warnings triggered by the warn_if_not_aligned attribute get issued. these warnings are enabled by default.
💖 -Wignored-qualifiers (C and C++ only)
warns you if the return type of a function has a type qualifier like const. for ISO C, that kind of type qualifier has literally no effect, because the value a function returns isn't an lvalue. for C++, the warning only fires for scalar types or void. and ISO C actually prohibits qualified void return types on function definitions, so those return types always get a warning even without this option. 🙈
this warning is also enabled by -Wextra.
💖 -Wno-ignored-attributes (C and C++ only)
this option controls warnings when an attribute gets ignored. it's different from the -Wattributes option because it warns whenever the compiler decides to drop an attribute — not when the attribute is unknown or used in a wrong place or whatever. this warning is enabled by default.
💖 -Wmain
warns if the type of main is suspicious. main should be a function with external linkage, returning int, and taking either zero arguments, two, or three arguments of appropriate types. this warning is enabled by default in C++ and gets enabled by either -Wall or -Wpedantic.
this warning is upgraded to an error by -pedantic-errors. 😭
💖 -Wmisleading-indentation (C and C++ only)
warns when the indentation of the code doesn't actually reflect the block structure. specifically, a warning gets issued for if, else, while, and for clauses with a guarded statement that doesn't use braces, followed by an unguarded statement with the same indentation.
in this next example, the call to bar is misleadingly indented as if it were guarded by the if conditional:
if (some_condition ())
foo ();
bar (); /* Gotcha: this is not guarded by the "if". */when you've got mixed tabs and spaces, the warning uses the -ftabstop= option to figure out if the statements line up (defaulting to 8).
the warning does not get issued for code involving multiline preprocessor logic, like this example here:
if (flagA)
foo (0);
#if SOME_CONDITION_THAT_DOES_NOT_HOLD
if (flagB)
#endif
foo (1);and the warning isn't issued after a #line directive either, since that typically means autogenerated code and you can't make any assumptions about the layout of the file that directive references.
this warning is enabled by -Wall in C and C++. 🌸
💖 -Wmissing-attributes
warns when a declaration of a function is missing one or more attributes that a related function is declared with — and whose absence might mess with the correctness or efficiency of the generated code. for example, the warning gets issued for declarations of aliases that use attributes to specify less restrictive requirements than those of their targets. this typically represents a potential optimization opportunity. by contrast, the -Wattribute-alias=2 option controls warnings issued when the alias is more restrictive than the target, which could lead to incorrect code generation. attributes considered include alloc_align, alloc_size, cold, const, hot, leaf, malloc, nonnull, noreturn, nothrow, pure, returns_nonnull, and returns_twice.
in C++, the warning is issued when an explicit specialization of a primary template declared with attribute alloc_align, alloc_size, assume_aligned, format, format_arg, malloc, or nonnull is declared without it. attributes deprecated, error, and warning suppress the warning.
you can use the copy attribute to apply the same set of attributes to a declaration as the ones on another declaration, without explicitly listing out every attribute. this attribute can be applied to declarations of functions, variables, or types.
-Wmissing-attributes is enabled by -Wall.
for example, since the declaration of the primary function template below makes use of both attribute malloc and alloc_size, the declaration of the explicit specialization of the template gets diagnosed because it's missing one of the attributes:
template <class T>
T* __attribute__ ((malloc, alloc_size (1)))
allocate (size_t);
template <>
void* __attribute__ ((malloc)) // missing alloc_size
allocate<void> (size_t);💖 -Wmissing-braces
warns if an aggregate or union initializer isn't fully bracketed. in the example below, the initializer for a isn't fully bracketed, but the one 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, Objective-C++ and Fortran only)
warns if a user-supplied include directory doesn't exist. this option is disabled by default for C, C++, Objective-C and Objective-C++. for Fortran, it's partially enabled by default — only warning for -I and -J. 👑
💖 -Wno-missing-profile
this option controls warnings about missing feedback profiles when you're using the -fprofile-use option. it diagnoses those cases where a new function or a new file gets added between compiling with -fprofile-generate and with -fprofile-use, without regenerating the profiles. in those cases, the profile feedback data files don't contain any profile feedback information for the newly added function or file. also, when profile count data (.gcda) files get removed, GCC can't use any profile feedback information at all. in all these cases, warnings are issued to let you know a profile generation step is due. ignoring the warning can result in poorly optimized code. -Wno-missing-profile can be used to disable the warning, but that is not recommended and should only be done when non-existent profile data is justified.
fr fr, don't just yeet this warning away unless you actually know what you're doing. 💀
💖 -Wmismatched-dealloc
warns for calls to deallocation functions with pointer arguments that were returned from allocation functions for which the former isn't a suitable deallocator. a pair of functions can be associated as matching allocators and deallocators by using attribute malloc. unless it's disabled by the -fno-builtin option, the standard functions calloc, malloc, realloc, and free, plus the corresponding forms of C++ operator new and operator delete, are implicitly associated as matching allocators and deallocators. in the example below, mydealloc is the deallocator for pointers returned from myalloc. 🦋
void mydealloc (void*);__attribute__ ((malloc (mydealloc, 1))) void*
myalloc (size_t);
void f (void)
{
void *p = myalloc (32);
// ...use p...
free (p); // warning: not a matching deallocator for myalloc
mydealloc (p); // ok
}Like, bestie, see how free (p) gets a lil side-eye but mydealloc (p) is totally chill? That's the whole point. Matching your allocator to its deallocator is the vibe. ✨
In C++, the related option -Wmismatched-new-delete diagnoses mismatches involving either operator new or operator delete.
Option -Wmismatched-dealloc is included in -Wall.
-Wmultistatement-macros— Warns you about unsafe multi-statement macros that look like they're guarded by a clause likeif,else,for,switch, orwhile, but where only the first statement is actually guarded after the macro expands. Sneaky!! 🐍
For example, peep this:
#define DOIT x++; y++
if (c)
DOIT;This will increment y unconditionally, not just when c holds. Rude of it, honestly. 😭 The fix is usually to wrap the macro in a do-while loop:
#define DOIT do { x++; y++; } while (0)
if (c)
DOIT;This warning is enabled by -Wall in C and C++.
-Wparentheses— Warns if parentheses are omitted in certain contexts, like when there's an assignment in a place where a truth value is expected, or when operators are nested in a way that people constantly get confused about. 😵💫
Also warns if a comparison like x<=y<=z shows up; that's equivalent to (x<=y ? 1 : 0) <= z, which is a whole different meaning from ordinary mathematical notation. Not the math girlies' version, periodt.
Also warns for dangerous uses of the GNU extension to ?: with an omitted middle operand. When the condition in the ?: operator is a boolean expression, the omitted value is always 1. Often programmers expect it to be a value computed inside the conditional expression instead. 🙈
For C++ this also warns for some cases of unnecessary parentheses in declarations, which can hint at an attempt at a function call instead of a declaration:
{
// Declares a local variable called mymutex.
std::unique_lock<std::mutex> (mymutex);
// User meant std::unique_lock<std::mutex> lock (mymutex);
}This warning is enabled by -Wall.
-Wno-self-move(C++ and Objective-C++ only) — This warning warns when a value is moved to itself withstd::move. Such astd::movetypically has no effect. It's giving "I did all that for nothing." 💀
struct T {
...
};
void fn()
{
T t;
...
t = std::move (t);
}This warning is enabled by -Wall.
-Wsequence-point— Warns about code that may have undefined semantics because of violations of sequence point rules in the C and C++ standards. 🚨
The C and C++ standards define the order in which expressions in a C/C++ program are evaluated in terms of sequence points, which represent a partial ordering between the execution of parts of the program: those executed before the sequence point, and those executed after it. These occur after the evaluation of a full expression (one which is not part of a larger expression), after the evaluation of the first operand of a &&, ||, ? : or , (comma) operator, before a function is called (but after the evaluation of its arguments and the expression denoting the called function), and in certain other places. Other than as expressed by the sequence point rules, the order of evaluation of subexpressions of an expression is not specified. All these rules describe only a partial order rather than a total order, since, for example, if two functions are called within one expression with no sequence point between them, the order in which the functions are called is not specified. However, the standards committee have ruled that function calls do not overlap.
It is not specified when between sequence points modifications to the values of objects take effect. Programs whose behavior depends on this have undefined behavior; the C and C++ standards specify that "Between the previous and next sequence point an object shall have its stored value modified at most once by the evaluation of an expression. Furthermore, the prior value shall be read only to determine the value to be stored." If a program breaks these rules, the results on any particular implementation are entirely unpredictable. 😳
Examples of code with undefined behavior are a = a++;, a[n] = b[n++] and a[i++] = i;. Some more complicated cases are not diagnosed by this option, and it may give an occasional false positive result, but in general it has been found fairly effective at detecting this sort of problem in programs.
The C++17 standard will define the order of evaluation of operands in more cases: in particular it requires that the right-hand side of an assignment be evaluated before the left-hand side, so the above examples are no longer undefined. But this option will still warn about them, to help people avoid writing code that is undefined in C and earlier revisions of C++.
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 <https://gcc.gnu.org/readings.html>.
This warning is enabled by -Wall for C and C++.
-Wno-return-local-addr— Do not warn about returning a pointer (or in C++, a reference) to a variable that goes out of scope after the function returns. Living dangerously, I see. 🫣-Wreturn-mismatch— Warns about return statements without an expression in functions which do not returnvoid. Also warns about areturnstatement with an expression in a function whose return type isvoid, unless the expression type is alsovoid. As a GNU extension, the latter case is accepted without a warning unless-Wpedanticis used.
Attempting to use the return value of a non-void function other than main that flows off the end by reaching the closing curly brace that terminates the function is undefined. 🚫
This warning is specific to C and enabled by default. In C99 and later language dialects, it is treated as an error. It can be downgraded to a warning using -fpermissive (along with other warnings), or for just this warning, with -Wno-error=return-mismatch.
-Wreturn-type— Warns whenever a function is defined with a return type that defaults toint(unless-Wimplicit-intis active, which takes precedence). Also warns if execution may reach the end of the function body, or if the function does not contain any return statement at all.
Attempting to use the return value of a non-void function other than main that flows off the end by reaching the closing curly brace that terminates the function is undefined.
Unlike in C, in C++, flowing off the end of a non-void function other than main results in undefined behavior even when the value of the function is not used. 💅
This warning is enabled by default in C++ and by -Wall otherwise.
-Wno-shift-count-negative— Controls warnings if a shift count is negative. This warning is enabled by default.-Wno-shift-count-overflow— Controls warnings if a shift count is greater than or equal to the bit width of the type. This warning is enabled by default.-Wshift-negative-value— Warns if left shifting a negative value. This warning is enabled by-Wextrain C99 (and newer) and C++11 to C++17 modes.-Wno-shift-overflow-Wshift-overflow=n— These options control warnings about left shift overflows. Pick your fighter: 👑-Wshift-overflow=1— This is the warning level of-Wshift-overflowand is enabled by default in C99 and C++11 modes (and newer). This warning level does not warn about left-shifting 1 into the sign bit. (However, in C, such an overflow is still rejected in contexts where an integer constant expression is required.) No warning is emitted in C++20 mode (and newer), as signed left shifts always wrap.-Wshift-overflow=2— This warning level also warns about left-shifting 1 into the sign bit, unless C++14 mode (or newer) is active.
-Wswitch— Warns whenever aswitchstatement has an index of enumerated type and lacks acasefor one or more of the named codes of that enumeration. (The presence of adefaultlabel prevents this warning.)caselabels that do not correspond to enumerators also provoke warnings when this option is used, unless the enumeration is marked with theflag_enumattribute. This warning is enabled by-Wall.-Wswitch-default— Warns whenever aswitchstatement does not have adefaultcase. Like, girl, always have a default. 🙏-Wswitch-enum— Warns whenever aswitchstatement has an index of enumerated type and lacks acasefor one or more of the named codes of that enumeration.caselabels that do not correspond to enumerators also provoke warnings when this option is used, unless the enumeration is marked with theflag_enumattribute. The only difference between-Wswitchand this option is that this option gives a warning about an omitted enumeration code even if there is adefaultlabel. No hiding behind defaults here! ✨-Wno-switch-bool— Do not warn when aswitchstatement has an index of boolean type and the case values are outside the range of a boolean type. It is possible to suppress this warning by casting the controlling expression to a type other thanbool. For example:
switch ((int) (a == 4))
{
...
}This warning is enabled by default for C and C++ programs.
-Wno-switch-outside-range— This option controls warnings when aswitchcase has a value that is outside of its respective type range. This warning is enabled by default for C and C++ programs.-Wno-switch-unreachable— Do not warn when aswitchstatement contains statements between the controlling expression and the first case label, which will never be executed. For example:
switch (cond)
{
i = 15;
...
case 5:
...
}Also, -Wswitch-unreachable does not warn if the statement between the controlling expression and the first case label is just a declaration:
switch (cond)
{
int i;
...
case 5:
i = 5;
...
}This warning is enabled by default for C and C++ programs. periodt.
-Wsync-nand(C and C++ only) — Warn when__sync_fetch_and_nandand__sync_nand_and_fetchbuilt-in functions are used. These functions changed semantics in GCC 4.4. 💅-Wtrivial-auto-var-init— Warn when-ftrivial-auto-var-initcannot initialize the automatic variable. A common situation is an automatic variable that is declared between the controlling expression and the first case label of aswitchstatement.-Wunused-but-set-parameter— Warn whenever a function parameter is assigned to, but otherwise unused (aside from its declaration).
To suppress this warning use the unused attribute.
This warning is also enabled by -Wunused together with -Wextra.
-Wunused-but-set-variable— Warn whenever a local variable is assigned to, but otherwise unused (aside from its declaration). This warning is enabled by-Wall.
To suppress this warning use the unused attribute.
This warning is also enabled by -Wunused, which is enabled by -Wall.
-Wunused-function— Warn whenever a static function is declared but not defined or a non-inline static function is unused. This warning is enabled by-Wall.-Wunused-label— Warn whenever a label is declared but not used. This warning is enabled by-Wall.
To suppress this warning use the unused attribute.
-Wunused-local-typedefs(C, Objective-C, C++ and Objective-C++ only) — Warn when a typedef locally defined in a function is not used. This warning is enabled by-Wall.-Wunused-parameter— Warn whenever a function parameter is unused aside from its declaration. This option is not enabled by-Wunusedunless-Wextrais also specified.
To suppress this warning use the unused attribute.
-Wno-unused-result— Do not warn if a caller of a function marked with attributewarn_unused_resultdoes not use its return value. The default is-Wunused-result.-Wunused-variable— Warn whenever a local or static variable is unused aside from its declaration. This option implies-Wunused-const-variable=1for C, but not for C++. This warning is enabled by-Wall.
To suppress this warning use the unused attribute.
-Wunused-const-variable-Wunused-const-variable=n— Warn whenever a constant static variable is unused aside from its declaration.
To suppress this warning use the unused attribute.
-Wunused-const-variable=1— Warn about unused static const variables defined in the main compilation unit, but not about static const variables declared in any header included.
-Wunused-const-variable=1 is enabled by either -Wunused-variable or -Wunused for C, but not for C++. In C this declares variable storage, but in C++ this is not an error since const variables take the place of #defines.
-Wunused-const-variable=2— This warning level also warns for unused constant static variables in headers (excluding system headers). It is equivalent to the short form-Wunused-const-variable. This level must be explicitly requested in both C and C++ because it might be hard to clean up all headers included.
-Wunused-value— Warn whenever a statement computes a result that is explicitly not used. To suppress this warning cast the unused expression tovoid. This includes an expression-statement or the left-hand side of a comma expression that contains no side effects. For example, an expression such asx[i,j]causes a warning, whilex[(void)i,j]does not.
This warning is enabled by -Wall.
-Wunused— All the above-Wunusedoptions combined, except those documented as needing to be specified explicitly.
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 and/or -Wunused-but-set-parameter.
-Wunused enables only -Wunused-const-variable=1 rather than -Wunused-const-variable, and only for C, not C++.
-Wuse-after-free(C, Objective-C, C++ and Objective-C++ only)-Wuse-after-free=n— Warn about uses of pointers to dynamically allocated objects that have been rendered indeterminate by a call to a deallocation function. The warning is enabled at all optimization levels but may yield different results with optimization than without.-Wuse-after-freeis equivalent to-Wuse-after-free=1.-Wuse-after-free=1— At level 1 the warning attempts to diagnose only unconditional uses of pointers made indeterminate by a deallocation call or a successful call torealloc, regardless of whether or not the call resulted in an actual reallocation of memory. This includes double-freecalls as well as uses in arithmetic and relational expressions. Although undefined, uses of indeterminate pointers in equality (or inequality) expressions are not diagnosed at this level.-Wuse-after-free=2— At level 2, in addition to unconditional uses, the warning also diagnoses conditional uses of pointers made indeterminate by a deallocation call. As at level 2, uses in equality (or inequality) expressions are not diagnosed. For example, the second call tofreein the following function is diagnosed at this level:
struct A { int refcount; void *data; };
void release (struct A *p)
{
int refcount = --p->refcount;
free (p);
if (refcount == 0)
free (p->data); // warning: p may be used after free
}-Wuse-after-free=3— At level 3, the warning also diagnoses uses of indeterminate pointers in equality expressions. All uses of indeterminate pointers are undefined but equality tests sometimes appear after calls toreallocas an attempt to determine whether the call resulted in relocating the object to a different address. They are diagnosed at a separate level to aid gradually transitioning legacy code to safe alternatives. For example, the equality test in the function below is diagnosed at this level:
void adjust_pointers (int**, int);
void grow (int **p, int n)
{
int **q = (int**)realloc (p, n *= 2);
if (q == p)
return;
adjust_pointers ((int**)q, n);
}To avoid the warning at this level, store offsets into allocated memory instead of pointers. This approach obviates needing to adjust the stored pointers after reallocation.
-Wuse-after-free=2 is included in -Wall.
-Wuseless-cast(C, Objective-C, C++ and Objective-C++ only) — Warn when an expression is cast to its own type. This warning does not occur when a class object is converted to a non-reference type as that is a way to create a temporary:
struct S { };
void g (S&&);
void f (S&& arg)
{
g (S(arg)); // make arg prvalue so that it can bind to S&&
}-Wuninitialized— Warn if an object with automatic or allocated storage duration is used without having been initialized. In C++, also warn if a non-static reference or non-staticconstmember appears in a class without constructors.
In addition, passing a pointer (or in C++, a reference) to an uninitialized object to a const-qualified argument of a built-in function known to read the object is also diagnosed by this warning. (-Wmaybe-uninitialized is issued for ordinary functions.)
If you want to warn about code that uses the uninitialized value of the variable in its own initializer, use the -Winit-self option.
These warnings occur for individual uninitialized elements of structure, union or array variables as well as for variables that are uninitialized 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 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.
In C++, this warning also warns about using uninitialized objects in member-initializer-lists. For example, GCC warns about b being uninitialized in the following snippet:
struct A {
int a;
int b;
A() : a(b) { }
};-Wno-invalid-memory-model— This option controls warnings for invocations of__atomicBuiltins,__syncBuiltins, and the C11 atomic generic functions with a memory consistency argument that is either invalid for the operation or outside the range of values of thememory_orderenumeration. For example, since the__atomic_storeand__atomic_store_nbuilt-ins are only defined for the relaxed, release, and sequentially consistent memory orders the following code is diagnosed:
void store (int *i)
{
__atomic_store_n (i, 0, memory_order_consume);
}-Winvalid-memory-model is enabled by default.
-Wmaybe-uninitialized— For an object with automatic or allocated storage duration, if there exists a path from the function entry to a use of the object that is initialized, but there exist some other paths for which the object is not initialized, the compiler emits a warning if it cannot prove the uninitialized paths are not executed at run time.
Also, lowkey iconic detail: handing a pointer (or in C++, a reference) to an uninitialized object over to a const-qualified function argument? Yeah, that gets flagged by this warning too. (-Wuninitialized fires for built-in functions that are known to read the object.) But if you annotate the function with the attribute access (none), that's basically saying "bestie this arg is not used to touch the object," and the warning goes away. 🫶
Okay but here's the thing — these warnings are only possible in optimizing compilation, because otherwise GCC simply isn't keeping track of the state of your variables. It's not being lazy, it just doesn't have the receipts. 😭
And they're made optional because GCC might not be able to tell when the code is actually correct even though it looks like an error. Here's an example of exactly how that happens:
{
int x;
switch (y)
{
case 1: x = 1;
break;
case 2: x = 4;
break;
case 3: x = 5;
}
foo (x);
}So if the value of y is always 1, 2 or 3, then x is always initialized — but GCC doesn't know that. To suppress the warning, you need to provide a default case with assert(0) or similar code.
This option also warns when a non-volatile automatic variable might get changed by a call to longjmp. The compiler only sees the calls to setjmp. It cannot know where longjmp will be called; fr fr, a signal handler could call it at any point in the code. So you may get a warning even when there's genuinely no problem, because longjmp can't actually be called at the place that would cause a problem.
Some spurious warnings can be avoided if you declare all the functions you use that never return as noreturn.
This warning is enabled by -Wall or -Wextra.
-Wunknown-pragmas— Warn when a#pragmadirective shows up that GCC doesn't understand. If this command-line option is used, warnings are even issued for unknown pragmas in system header files. That is not the case if the warnings are only enabled by the-Wallcommand-line option.-Wno-pragmas— Do not warn about misuses of pragmas, like incorrect parameters, invalid syntax, or conflicts between pragmas. See also-Wunknown-pragmas.-Wno-pragma-once-outside-header— Do not warn when#pragma onceis used in a file that is not a header file, such as a main file.-Wno-prio-ctor-dtor— Do not warn if a priority from 0 to 100 is used for a constructor or destructor. The use of constructor and destructor attributes lets you assign a priority to the constructor/destructor to control its order of execution beforemainis called or after it returns. The priority values must be greater than 100, since the compiler reserves priority values between 0--100 for the implementation. 👑-Wstrict-aliasing— This option is only active when-fstrict-aliasingis active. It warns about code that might break the strict aliasing rules that the compiler is using for optimization. The warning does not catch all cases, but it does attempt to catch the more common pitfalls. It is included in-Wall. It is equivalent to-Wstrict-aliasing=3.-Wstrict-aliasing=n— This option is only active when-fstrict-aliasingis active. It warns about code that might break the strict aliasing rules that the compiler is using for optimization. Higher levels correspond to higher accuracy (fewer false positives). Higher levels also correspond to more effort, similar to the way-Oworks.-Wstrict-aliasingis equivalent to-Wstrict-aliasing=3.
Level 1: Most aggressive, quick, least accurate. Possibly useful when higher levels do not warn but -fstrict-aliasing still breaks the code, as it has very few false negatives. However, it has many false positives. Warns for all pointer conversions between possibly incompatible types, even if never dereferenced. Runs in the front end only.
Level 2: Aggressive, quick, not too precise. May still have many false positives (not as many as level 1 though), and few false negatives (but possibly more than level 1). Unlike level 1, it only warns when an address is taken. Warns about incomplete types. Runs in the front end only.
Level 3 (default for -Wstrict-aliasing): Should have very few false positives and few false negatives. Slightly slower than levels 1 or 2 when optimization is enabled. Takes care of the common pun+dereference pattern in the front end: *(int*)&some_float. If optimization is enabled, it also runs in the back end, where it deals with multiple statement cases using flow-sensitive points-to information. Only warns when the converted pointer is dereferenced. Does not warn about incomplete types.
-Wstrict-overflow-Wstrict-overflow=n— This option is only active when signed overflow is undefined. 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. So this warning depends on the optimization level. 🌸
An optimization that assumes signed overflow does not occur is perfectly safe if the values of the variables involved are such that overflow never actually occurs. So this warning can easily give a false positive: a warning about code that isn't actually a problem. To help focus on important issues, several warning levels are defined. No warnings are issued for the use of undefined signed overflow when estimating how many iterations a loop requires, in particular when determining whether a loop will be executed at all.
-Wstrict-overflow=1— Warn about cases that are both questionable and easy to avoid. For example the compiler simplifiesx + 1 > xto 1. This level of-Wstrict-overflowis 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 signed integer overflow is undefined, becauseabs (INT_MIN)overflows toINT_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 > 1is simplified tox > 0.-Wstrict-overflow=4— Also warn about other simplifications not covered by the above cases. For example:(x * 10) / 5is simplified tox * 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 > yis simplified tox + 1 >= y. This is reported only at the highest warning level because this simplification applies to many comparisons, so this warning level gives a very large number of false positives. 💀
-Wstring-compare— Warn for calls tostrcmpandstrncmpwhose result is determined to be either zero or non-zero in tests for such equality, owing to the length of one argument being greater than the size of the array the other argument is stored in (or the bound in the case ofstrncmp). Such calls could be mistakes. For example, the call tostrcmpbelow is diagnosed because its result is necessarily non-zero irrespective of the contents of the arraya.
extern char a[4];
void f (char *d)
{
strcpy (d, "string");
...
if (0 == strcmp (a, d)) // cannot be true
puts ("a and d are the same");
}-Wstring-compare is enabled by -Wextra.
-Wno-stringop-overflow-Wstringop-overflow-Wstringop-overflow=type— Warn for code that can be statically determined to cause buffer overflows or memory overruns, such as calls tomemcpyandstrcpythat overflow the destination buffer. The optional argument is one greater than the type of Object Size Checking to perform to determine the size of the destination. The argument is meaningful only for string functions that operate on character arrays; raw memory functions likememcpyalways use type-zero Object Size Checking.
The option also warns for calls that specify a size in excess of the largest possible object or at most SIZE_MAX / 2 bytes.
The option produces the best results with optimization enabled, but it can detect a small subset of simple buffer overflows even without optimization in calls to the GCC built-in functions like __builtin_memcpy that correspond to the standard functions. In any case, the option warns about just a subset of buffer overflows detected by the corresponding overflow checking built-ins, such as __builtin___memcpy_chk, which can perform run-time checking if the access cannot be identified as safe at compile time.
For example, the option issues a warning for the strcpy call below because it copies at least 5 characters (the string "blue" including the terminating NUL) into the buffer of size 4.
enum Color { blue, purple, yellow };
const char* f (enum Color clr)
{
static char buf [4];
const char *str;
switch (clr)
{
case blue: str = "blue"; break;
case purple: str = "purple"; break;
case yellow: str = "yellow"; break;
}
return strcpy (buf, str); // warning here
}The effect of this option is not limited to string or memory manipulation functions. In this example, a warning is diagnosed because a 1-element array is passed to a function requiring at least a 4-element array argument:
void f (int[static 4]);
void g (void)
{
int *p = (int *) malloc (1 * sizeof(int));
f (p); // warning here
}Option -Wstringop-overflow=2 is enabled by default.
-Wstringop-overflow-Wstringop-overflow=1— this level uses type-zero Object Size Checking to figure out how big the destination objects are. At this setting it does not warn about writes past the end of subobjects of larger objects that are reached through pointers — unless the size of the biggest surrounding object is actually known. When the destination could be one of several objects, it just assumes it's the biggest one of them, bestie. And on Linux systems, when optimization is turned on at this setting, it warns on the same code as when the_FORTIFY_SOURCEmacro is defined to a non-zero value. Slay.-Wstringop-overflow=2— this level uses type-one Object Size Checking for the destination sizes. At this setting it warns about overflows when you write to members of the largest complete objects whose exact size is known. But it will not warn about excessive writes to those same members of unknown objects referenced by pointers, because those pointers might point at arrays with an unknown number of elements, and honestly, fair. This is the default setting of the option, so if you do nothing, this is what you get. 💅-Wstringop-overflow=3— this level uses type-two Object Size Checking for the destination sizes. At this setting it warns about overflowing the smallest object or data member. Heads up though: this is the most restrictive setting of the option and it may result in warnings for safe code. It's giving overprotective friend energy.-Wstringop-overflow=4— this level uses type-three Object Size Checking for the destination sizes. At this setting it warns about overflowing any data members, and when the destination is one of several objects, it uses the size of the largest of them to decide whether to fire off a warning. And just like-Wstringop-overflow=3, this setting may result in warnings for benign code. 🙈-Wno-stringop-overread— warns you about calls to string manipulation functions likememchrorstrcpythat are determined to read past the end of the source sequence. The option-Wstringop-overreadis enabled by default, so this is already watching your back, no cap.-Wno-stringop-truncation— does not warn about calls to bounded string manipulation functions likestrncat,strncpy, andstpncpythat may either truncate the copied string or leave the destination unchanged.
Okay so example time. In this next one, the call to strncat specifies a bound that's less than the length of the source string. So the copy of the source gets truncated, and the call gets diagnosed. To dodge the warning, use bufsize - strlen (buf) - 1) as the bound.
void append (char *buf, size_t bufsize)
{
strncat (buf, ".txt", 3);
}Next up: this call to strncpy ends up copying to d just the characters before the terminating NUL, and it never appends the NUL to the end. Assuming the result of strncpy is necessarily a NUL-terminated string is a super common mistake, so the call gets diagnosed. If you're not expecting the result to be NUL-terminated, call memcpy instead and the warning goes away.
void copy (char *d, const char *s)
{
strncpy (d, s, strlen (s));
}And here: the call to strncpy specifies the size of the destination buffer as the bound. If the length of the source string is equal to or greater than this size, the result of the copy will not be NUL-terminated. So yeah, diagnosed. To avoid the warning, specify sizeof buf - 1 as the bound and set the last element of the buffer to NUL.
void copy (const char *s)
{
char buf[80];
strncpy (buf, s, sizeof buf);
...
}Now, if you've got a character array that's meant to store a sequence of bytes with no terminating NUL, you can annotate that array with the attribute nonstring to silence this warning. But — and this matters — such arrays are not suitable arguments to functions that expect NUL-terminated strings. To help catch accidental misuse of those arrays, GCC issues warnings unless it can prove the use is safe. 🫶
-Wstrict-flex-arrays(C and C++ only) — warns about improper usages of flexible array members, according to the level of thestrict_flex_array (level)attribute attached to the trailing array field of a structure if it's available, otherwise according to the level of the option-fstrict-flex-arrays=level. Note that-Wstrict-flex-arraysis only effective when level is greater than 0.- When level=1, warnings are issued for a trailing array reference of a structure that has 2 or more elements, if the trailing array is referenced as a flexible array member.
- When level=2, in addition to level=1, extra warnings are issued for a trailing one-element array reference of a structure if the array is referenced as a flexible array member.
- When level=3, in addition to level=2, extra warnings are issued for a trailing zero-length array reference of a structure if the array is referenced as a flexible array member.
This option hits different when -ftree-vrp is active (that's the default for -O2 and above), but some warnings may get diagnosed even without optimization.
- *
-Wsuggest-attribute=attribute-name* — warns for cases where adding an attribute could be beneficial. The *attribute-name*s currently supported are listed below. 👇-Wsuggest-attribute=pure-Wsuggest-attribute=const-Wsuggest-attribute=noreturn-Wmissing-noreturn-Wsuggest-attribute=malloc-Wsuggest-attribute=returns_nonnull— warns about functions that might be candidates for the attributespure,const,noreturn,mallocorreturns_nonnull. The compiler only warns for functions visible in other compilation units, or (forpureandconst) if it can't prove the function returns normally. And a function returns normally if it doesn't contain an infinite loop or return abnormally by throwing, callingabort, or trapping. This analysis needs the option-fipa-pure-const, which is enabled by default at-Oand higher. Higher optimization levels make the analysis more accurate, periodt.-Wsuggest-attribute=format-Wmissing-format-attribute— warns about function pointers that might be candidates forformatattributes. Note that these are only possible candidates, not absolute ones. GCC guesses that function pointers withformatattributes that get used in assignment, initialization, parameter passing or return statements should have a correspondingformatattribute in the resulting type. Like, 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 aformatattribute to avoid the warning.
GCC also warns about function definitions that might be candidates for format attributes. Again, only possible candidates. GCC guesses that format attributes might be appropriate for any function that calls something like vprintf or vscanf, but that might not always be the case, and some functions where format attributes would be appropriate may not get detected at all. It's giving vibes-based guessing.
-Wsuggest-attribute=cold— warns about functions that might be candidates for thecoldattribute. This one's based on static detection, and generally only warns about functions that always lead to a call to anothercoldfunction — like wrappers of C++throwor fatal error reporting functions that lead toabort.
-Walloc-size— warns about calls to allocation functions decorated with the attributealloc_sizethat specify insufficient size for the target type of the pointer the result is assigned to, including calls to the built-in forms of the functionsaligned_alloc,alloca,calloc,malloc, andrealloc.-Walloc-zero— warns about calls to allocation functions decorated with the attributealloc_sizethat specify zero bytes, including calls to the built-in forms ofaligned_alloc,alloca,calloc,malloc, andrealloc. Here's the tea: the behavior of these functions when called with a zero size differs between implementations (and forreallocit's been deprecated), so relying on it may cause subtle portability bugs and should be avoided. ⚠️-Wcalloc-transposed-args— warns about calls to allocation functions decorated with the attributealloc_sizewith two arguments, which use thesizeofoperator as the earlier size argument and don't use it as the later size argument. This is a coding style warning. The first argument tocallocis documented to be the number of elements in the array, while the second argument is the size of each element, socalloc (n, sizeof (int))is preferred overcalloc (sizeof (int), n). If havingsizeofin the earlier argument and not the latter is intentional, you can suppress the warning by usingcalloc (sizeof (struct S) + 0, n)orcalloc (1 * sizeof (struct S), 4)— or just usesizeofin the later argument too. Easy. ✨-Walloc-size-larger-than=byte-size — warns about calls to functions decorated with the attributealloc_sizethat try to allocate objects larger than the specified number of bytes, or where the result of the size computation in an integer type with infinite precision would exceed the value ofPTRDIFF_MAXon the target. Note that-Walloc-size-larger-than=PTRDIFF_MAXis *enabled by default. You can turn off warnings controlled by this option either by specifying a *byte-size ofSIZE_MAXor more, or by using-Wno-alloc-size-larger-than.-Wno-alloc-size-larger-than— disables-Walloc-size-larger-than=warnings. This option is equivalent to-Walloc-size-larger-than=SIZE_MAXor larger.-Walloca— this option warns on all uses ofallocain the source. Every single one. It's watching. 👀
-Walloca-larger-than=byte-size</p>
okay so bestie, this one is the alloca snitch 💅 it yells at you when you call alloca with an integer argument whose value is either zero, or that isn't bounded by a controlling predicate that caps it at most byte-size. It also screams when the bound value is just… unknown. And here's the spicy part: arguments of non-integer types are treated as unbounded even if they look constrained to the expected range. Rude? maybe. Correct? yes. 🙈
For example, a bounded case of alloca could be:
void func (size_t n)
{
void *p;
if (n <= 1000)
p = alloca (n);
else
p = malloc (n);
f (p);
}In the above example, passing -Walloca-larger-than=1000 would not issue a warning because the call to alloca is known to be at most 1000 bytes. However, if -Walloca-larger-than=500 were passed, the compiler would emit a warning. slay, math checks out ✨
Unbounded uses, on the other hand, are uses of alloca with no controlling predicate constraining its integer argument. For example:
void func ()
{
void *p = alloca (n);
f (p);
}If -Walloca-larger-than=500 were passed, the above would trigger a warning, but this time because of the lack of bounds checking. it's giving unhinged 😭
Note, that even seemingly correct code involving signed integers could cause a warning:
void func (signed int n)
{
if (n < 500)
{
p = alloca (n);
f (p);
}
}In the above example, n could be negative, causing a larger than expected argument to be implicitly cast into the alloca call. negative numbers ruining everything as usual 💀
This option also warns when alloca is used in a loop.
-Walloca-larger-than=PTRDIFF_MAX is enabled by default but is usually only effective when -ftree-vrp is active (default for -O2 and above).
See also -Wvla-larger-than=byte-size.
-Wno-alloca-larger-than</p>
Disable -Walloca-larger-than= warnings. The option is equivalent to -Walloca-larger-than=SIZE_MAX or larger. so basically "girl, stop watching me" 🫶
-Warith-conversion
Do warn about implicit conversions from arithmetic operations even when conversion of the operands to the same type cannot change their values. This affects warnings from -Wconversion, -Wfloat-conversion, and -Wsign-conversion.
void f (char c, int i)
{
c = c + i; // warns with -Wconversion
c = c + 1; // only warns with -Warith-conversion
}lowkey the most "actually i DO care" flag in the whole list 💅
-Warray-bounds
-Warray-bounds=n
Warn about out of bounds subscripts or offsets into arrays. This warning is enabled by -Wall. It is more effective when -ftree-vrp is active (the default for -O2 and above) but a subset of instances are issued even without optimization. detective work, but make it compiler 🔍
By default, the trailing array of a structure will be treated as a flexible array member by -Warray-bounds or -Warray-bounds=n if it is declared as either a flexible array member per C99 standard onwards ([]), a GCC zero-length array extension ([0]), or an one-element array ([1]). As a result, out of bounds subscripts or offsets into zero-length arrays or one-element arrays are not warned by default.
You can add the option -fstrict-flex-arrays or -fstrict-flex-arrays=level to control how this option treat trailing array of a structure as a flexible array member:
- when level
<=1, no change to the default behavior. - when level
=2, additional warnings will be issued for out of bounds subscripts or offsets into one-element arrays; - when level
=3, in addition to level=2, additional warnings will be issued for out of bounds subscripts or offsets into zero-length arrays. -Warray-bounds=1— This is the default warning level of-Warray-boundsand is enabled by-Wall; higher levels are not, and must be explicitly requested.-Warray-bounds=2— This warning level also warns about the intermediate results of pointer arithmetic that may yield out of bounds values. This warning level may give a larger number of false positives and is deactivated by default. ⚠️ so many false positives, it's giving boy-who-cried-wolf
-Wunterminated-string-initialization (C and Objective-C only)
Warn about character arrays initialized as unterminated character sequences with a string literal, unless the declaration being initialized has the nonstring attribute. For example:
char arr[3] = "foo"; /* Warning. */
char arr2[3] __attribute__((nonstring)) = "bar"; /* No warning. */This warning is enabled by -Wextra. If -Wc++-compat is enabled, the warning has slightly different wording and warns even if the declaration being initialized has the nonstring warning, as in C++ such initializations are an error. the C++ kids are NOT playing around 😤
-Warray-compare
Warn about equality and relational comparisons between two operands of array type. This comparison was deprecated in C++20. For example:
int arr1[5];
int arr2[5];
bool same = arr1 == arr2;-Warray-compare is enabled by -Wall. periodt.
-Warray-parameter
-Warray-parameter=n
Warn about redeclarations of functions involving parameters of array or pointer types of inconsistent kinds or forms, and enable the detection of out-of-bounds accesses to such parameters by warnings such as -Warray-bounds. consistency check but make it ✨vibes✨
If the first function declaration uses the array form for a parameter declaration, the bound specified in the array is assumed to be the minimum number of elements expected to be provided in calls to the function and the maximum number of elements accessed by it. Failing to provide arguments of sufficient size or accessing more than the maximum number of elements may be diagnosed by warnings such as -Warray-bounds or -Wstringop-overflow. At level 1, the warning diagnoses inconsistencies involving array parameters declared using the T[static N] form.
For example, the warning triggers for the second declaration of f because the first one with the keyword static specifies that the array argument must have at least four elements, while the second allows an array of any size to be passed to f.
void f (int[static 4]);
void f (int[]); // warning (inconsistent array form)
void g (void)
{
int *p = (int *)malloc (1 * sizeof (int));
f (p); // warning (array too small)
...
}At level 2 the warning also triggers for redeclarations involving any other inconsistency in array or pointer argument forms denoting array sizes. Pointers and arrays of unspecified bound are considered equivalent and do not trigger a warning.
void g (int*);
void g (int[]); // no warning
void g (int[8]); // warning (inconsistent array bound)-Warray-parameter=2 is included in -Wall. The -Wvla-parameter option triggers warnings for similar inconsistencies involving Variable Length Array arguments.
The short form of the option -Warray-parameter is equivalent to -Warray-parameter=2. The negative form -Wno-array-parameter is equivalent to -Warray-parameter=0.
-Wattribute-alias=n
-Wno-attribute-alias
Warn about declarations using the alias and similar attributes whose target is incompatible with the type of the alias. identity crisis, but for functions 🦋
-Wattribute-alias=1— The default warning level of the-Wattribute-aliasoption diagnoses incompatibilities between the type of the alias declaration and that of its target. Such incompatibilities are typically indicative of bugs.-Wattribute-alias=2— At this level-Wattribute-aliasalso diagnoses cases where the attributes of the alias declaration are more restrictive than the attributes applied to its target. These mismatches can potentially result in incorrect code generation. In other cases they may be benign and could be resolved simply by adding the missing attribute to the target. For comparison, see the-Wmissing-attributesoption, which controls diagnostics when the alias declaration is less restrictive than the target, rather than more restrictive.
Attributes considered include alloc_align, alloc_size, cold, const, hot, leaf, malloc, nonnull, noreturn, nothrow, pure, returns_nonnull, and returns_twice. the whole squad, accounted for 💖
-Wattribute-alias is equivalent to -Wattribute-alias=1. This is the default. You can disable these warnings with either -Wno-attribute-alias or -Wattribute-alias=0.
-Wbidi-chars=[none|unpaired|any|ucn]
Warn about possibly misleading UTF-8 bidirectional control characters in comments, string literals, character constants, and identifiers. Such characters can change left-to-right writing direction into right-to-left (and vice versa), which can cause confusion between the logical order and visual order. This may be dangerous; for instance, it may seem that a piece of code is not commented out, whereas it in fact is. 🚨 genuinely scary stuff, this is the Trojan horse of characters 😭
There are three levels of warning supported by GCC. The default is -Wbidi-chars=unpaired, which warns about improperly terminated bidi contexts. -Wbidi-chars=none turns the warning off. -Wbidi-chars=any warns about any use of bidirectional control characters.
By default, this warning does not warn about UCNs. It is, however, possible to turn on such checking by using -Wbidi-chars=unpaired,ucn or -Wbidi-chars=any,ucn. Using -Wbidi-chars=ucn is valid, and is equivalent to -Wbidi-chars=unpaired,ucn, if no previous -Wbidi-chars=any was specified.
-Wbool-compare
Warn about boolean expression compared with an integer value different from true/false. For instance, the following comparison is always false:
int n = 5;
...
if ((n > 1) == 2) { ... }This warning is enabled by -Wall. bestie that comparison was never gonna work out 💀 always false, always heartbroken 🥀
-Wbool-operation— okay so why are you doing bitwise things to a boolean 😭 This one flags suspicious operations on expressions of a boolean type. Like, bitwise-negating a boolean? That's very likely a bug in your program, fr fr. And for C it also warns when you increment or decrement a boolean, because that rarely makes sense. (In C++ decrementing a boolean is always invalid — periodt. Incrementing one is invalid in C++17, and deprecated otherwise.) 💀
This warning is enabled by -Wall. 🫶
-Wduplicated-branches— warns you when anif-elsehas literally identical branches, bestie. Like this iconic nonsense:
if (p != NULL)
return 0;
else
return 0;It does not warn when both branches contain just a null statement, okay? And it also catches conditional operators being extra:
int i = x ? *p : *p;-Wduplicated-cond— this one clocks duplicated conditions in anif-else-ifchain 👀. For instance, warn for the following code:
if (p->q != NULL) { ... }
else if (p->q != NULL) { ... }-Wframe-address— warns when__builtin_frame_addressor__builtin_return_addressgets called with an argument greater than 0. Those calls may return indeterminate values or crash the program 🙈. The warning is included in-Wall.-Wno-discarded-qualifiers(C and Objective-C only) — tells the compiler not to warn when type qualifiers on pointers are being discarded. Typically, the compiler warns if a"const char *"variable is passed to a function that takes a"char *"parameter. This option can be used to suppress such a warning. ✨-Wno-discarded-array-qualifiers(C and Objective-C only) — do not warn if type qualifiers on arrays which are pointer targets are being discarded. Typically, the compiler warns if a"const int (*)[]"variable is passed to a function that takes a"int (*)[]"parameter. This option can be used to suppress such a warning. 🎀-Wno-incompatible-pointer-types(C and Objective-C only) — do not warn when there is a conversion between pointers that have incompatible types. This warning is for cases not covered by-Wno-pointer-sign, which warns for pointer argument passing or assignment with different signedness. 🦋
By default, in C99 and later dialects of C, GCC treats this issue as an error. The error can be downgraded to a warning using -fpermissive (along with certain other errors), or for this error alone, with -Wno-error=incompatible-pointer-types. 😭
This warning is upgraded to an error by -pedantic-errors.
-Wno-int-conversion(C and Objective-C only) — do not warn about incompatible integer to pointer and pointer to integer conversions. This warning is about implicit conversions; for explicit conversions the warnings-Wno-int-to-pointer-castand-Wno-pointer-to-int-castmay be used. 💅
By default, in C99 and later dialects of C, GCC treats this issue as an error. The error can be downgraded to a warning using -fpermissive (along with certain other errors), or for this error alone, with -Wno-error=int-conversion.
This warning is upgraded to an error by -pedantic-errors.
-Wzero-as-null-pointer-constant— warns when a literal0is used as a null pointer constant. just sayNULLbestie 😌-Wzero-length-bounds— warns about accesses to elements of zero-length array members that might overlap other members of the same object. Declaring interior zero-length arrays is discouraged because accesses to them are undefined ⚠️.
For example, the first two stores in function "bad" are diagnosed because the array elements overlap the subsequent members "b" and "c". The third store is diagnosed by -Warray-bounds because it is beyond the bounds of the enclosing object.
struct X { int a[0]; int b, c; };
struct X x;
void bad (void)
{
x.a[0] = 0; // -Wzero-length-bounds
x.a[1] = 1; // -Wzero-length-bounds
x.a[2] = 2; // -Warray-bounds
}Option -Wzero-length-bounds is enabled by -Warray-bounds. 🫶
-Wno-div-by-zero— do not warn about compile-time integer division by zero. Floating-point division by zero is not warned about, as it can be a legitimate way of obtaining infinities and NaNs. no cap, that one's on purpose ✨-Wsystem-headers— prints 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-Wallin conjunction with this option does not warn about unknown pragmas in system headers — for that,-Wunknown-pragmasmust also be used. 👑-Wtautological-compare— warns if a self-comparison always evaluates to true or false. bestie… comparing a thing to itself? This warning detects various mistakes such as:
int i = 1;
...
if (i > i) { ... }This warning also warns about bitwise comparisons that always evaluate to true or false, for instance:
if ((a & 16) == 10) { ... }will always be false. 😭
This warning is enabled by -Wall. 💖
-Wtrailing-whitespace-Wtrailing-whitespace=kind
Warn about trailing whitespace at the end of lines, including inside of comments, but excluding trailing whitespace in raw string literals. "-Wtrailing-whitespace" is equivalent to "-Wtrailing-whitespace=blanks" and warns just about trailing space and horizontal tab characters. "-Wtrailing-whitespace=any" warns about those or trailing form feed or vertical tab characters. "-Wno-trailing-whitespace" or "-Wtrailing-whitespace=none" disables the warning, which is the default. This is a coding style warning. 🎀
-Wleading-whitespace=kind— warn about style issues in leading whitespace, but not about the amount of indentation. Some projects use coding styles where only spaces are used for indentation, others use only tabs, others use zero or more tabs (for multiples of"-ftabstop=n") followed by zero or fewer than n spaces. No warning is emitted on lines which contain solely whitespace (although"-Wtrailing-whitespace="warning might be emitted), no warnings are emitted inside of raw string literals. Warnings are also emitted for leading whitespace inside of multi-line comments."-Wleading-whitespace=spaces"warns about leading whitespace other than spaces for projects which want to indent just by spaces."-Wleading-whitespace=tabs"warns about leading whitespace other than horizontal tabs for projects which want to indent just by horizontal tabs."-Wleading-whitespace=blanks"warns about leading whitespace other than spaces and horizontal tabs, or about horizontal tab after a space in the leading whitespace, or about n or more consecutive spaces in leading whitespace (where n is argument of"-ftabstop=n", 8 by default)."-Wleading-whitespace=none"disables the warning, which is the default. This is a coding style warning. 💅-Wtrampolines— warns about trampolines generated for pointers to nested functions. A trampoline is a small piece of data or code that is created at run time on the stack when the address of a nested function is taken, and is used to call the nested function indirectly. For some targets, it is made up of data only and thus requires no special treatment. But, for most targets, it is made up of code and thus requires the stack to be made executable in order for the program to work properly. 😬-Wfloat-equal— warns if floating-point values are used in equality comparisons. 🚩
The idea behind this is that sometimes it is convenient (for the programmer) to consider floating-point values as approximations to infinitely precise real numbers. If you are doing this, then you need to compute (by analyzing the code, or in some other way) the maximum or likely maximum error that the computation introduces, and allow for it when performing comparisons (and when producing output, but that's a different problem). In particular, instead of testing for equality, you should check to see whether the two values have ranges that overlap; and this is done with the relational operators, so equality comparisons are probably mistaken. so true, the vibes are off with == on floats ✨
-Wtraditional(C and Objective-C only) — warns about certain constructs that behave differently in traditional and ISO C. Also warns about ISO C constructs that have no traditional C equivalent, and/or problematic constructs that should be avoided. 🌸- Macro parameters that appear within string literals in the macro body. In traditional C macro replacement takes place within string literals, but in ISO C it does not.
- In traditional C, some preprocessor directives did not exist. Traditional preprocessors only considered a line to be a directive if the
#appeared in column 1 on the line. Therefore-Wtraditionalwarns about directives that traditional C understands but ignores because the#does not appear as the first character on the line. It also suggests you hide directives like"#pragma"not understood by traditional C by indenting them. Some traditional implementations do not recognize"#elif", so this option suggests avoiding it altogether. - A function-like macro that appears without arguments.
- The unary plus operator.
- The U integer constant suffix, or the F or L floating-point constant suffixes. (Traditional C does support the L suffix on integer constants.) Note, these suffixes appear in macros defined in the system headers of most modern systems, e.g. the _MIN/_MAX macros in
"<limits.h>". Use of these macros in user code might normally lead to spurious warnings, however GCC's integrated preprocessor has enough context to avoid warning in these cases. - A function declared external in one block and then used after the end of the block.
- A
"switch"statement has an operand of type"long". - A non-
"static"function declaration follows a"static"one. This construct is not accepted by some traditional C compilers.
- The ISO type of an integer constant has a different width or signedness than its traditional type. This warning only pops off if the base of the constant is ten. So hexadecimal or octal values — which usually are just bit patterns doing bit pattern things — get a pass, no warning, we love that for them. ✨
- Usage of ISO string concatenation gets caught. yes bestie we see you.
- Initialization of automatic aggregates. 👀
- Identifier conflicts with labels. Traditional C didn't have a separate namespace for labels, so yeah, this is a thing. 💀
- Initialization of unions. If the initializer is zero, the warning is skipped. Why? Because the assumption is that the zero initializer in user code shows up conditioned on stuff like
__STDC__to dodge missing-initializer warnings, and it's relying on default initialization to zero in the traditional C case. iykyk. - Conversions by prototypes between fixed/floating-point values and vice versa. Not having these prototypes when you compile with traditional C causes serious problems fr fr. This is only a subset of the possible conversion warnings — for the whole set use
-Wtraditional-conversion. - Use of ISO C style function definitions. This warning intentionally does not fire for prototype declarations or variadic functions, because these ISO C features show up in your code when you're using libiberty's traditional C compatibility macros,
PARAMSandVPARAMS. This warning is also skipped for nested functions, because that feature is already a GCC extension and therefore not relevant to traditional C compatibility. 🫶
✨ OPTIONS ✨ (the saga continues)
-Wtraditional-conversion(C and Objective-C only) — Warn if a prototype causes a type conversion that's different from what would happen to the same argument without a prototype. That includes conversions of fixed point to floating and vice versa, plus conversions changing the width or signedness of a fixed-point argument — except when it's the same as the default promotion. 💅-Wdeclaration-after-statement(C and Objective-C only) — Warn when a declaration shows up after a statement inside a block. This construct, known from C++, was introduced with ISO C99 and is allowed in GCC by default. ISO C90 does not support it. 🚫
This warning is upgraded to an error by -pedantic-errors.
-Wshadow— Warn whenever a local variable or type declaration shadows another variable, parameter, type, class member (in C++), or instance variable (in Objective-C) — or whenever a built-in function gets shadowed. Heads up: in C++, the compiler warns if a local variable shadows an explicit typedef, but not if it shadows a struct/class/enum. If this warning is enabled, it also includes every instance of local shadowing. Which means-Wno-shadow=localand-Wno-shadow=compatible-localare ignored when-Wshadowis used. Same as-Wshadow=global. 👑-Wno-shadow-ivar(Objective-C only) — Do not warn whenever a local variable shadows an instance variable in an Objective-C method.-Wshadow=global— Warn for any shadowing. Same as-Wshadow.-Wshadow=local— Warn when a local variable shadows another local variable or parameter.-Wshadow=compatible-local— Warn when a local variable shadows another local variable or parameter whose type is compatible with that of the shadowing variable. In C++, type compatibility here means the type of the shadowing variable can be converted to that of the shadowed variable. This flag exists (on top of-Wshadow=local) because of the idea that when a local variable shadows one of incompatible type, it's most likely intentional — not a bug or a typo — like in this example:
for (SomeIterator i = SomeObj.begin(); i != SomeObj.end(); ++i)
{
for (int i = 0; i < N; ++i)
{
...
}
...
}Since the two variables i in the example above have incompatible types, enabling only -Wshadow=compatible-local does not emit a warning. And because their types are incompatible, if a programmer accidentally uses one in place of the other, type checking is expected to catch it and emit an error or warning. Using this flag instead of -Wshadow=local can possibly reduce the number of warnings triggered by intentional shadowing. Note that this also means shadowing const char *i by char *i does not emit a warning. 🌸
This warning is also enabled by -Wshadow=local.
-Wlarger-than=byte-size— Warn whenever an object is defined whose size exceeds byte-size.-Wlarger-than=PTRDIFF_MAXis enabled by default. Warnings controlled by this option can be disabled either by specifying a byte-size ofSIZE_MAXor more, or by-Wno-larger-than. 🔥
Also warns for calls to bounded functions such as memchr or strnlen that specify a bound greater than the largest possible object, which is PTRDIFF_MAX bytes by default. These warnings can only be disabled by -Wno-larger-than.
-Wno-larger-than— Disable-Wlarger-than=warnings. Equivalent to-Wlarger-than=SIZE_MAXor larger.-Wframe-larger-than=byte-size— Warn if the size of a function frame exceeds byte-size. The computation done to figure out the stack frame size is approximate and not conservative. The actual requirements may be somewhat greater than byte-size even if you get no warning at all. On top of that, any space allocated viaalloca, variable-length arrays, or related constructs is not included by the compiler when it decides whether to warn.-Wframe-larger-than=PTRDIFF_MAXis enabled by default. Warnings controlled by this option can be disabled either by specifying a byte-size ofSIZE_MAXor more, or by-Wno-frame-larger-than.-Wno-frame-larger-than— Disable-Wframe-larger-than=warnings. Equivalent to-Wframe-larger-than=SIZE_MAXor larger.-Wfree-nonheap-object— Warn when you try to deallocate an object that either wasn't allocated on the heap, or was allocated using a pointer that wasn't returned from a prior call to the corresponding allocation function. For example, since the call tostpcpyreturns a pointer to the terminating nul character and not to the beginning of the object, the call tofreebelow gets diagnosed. 😭
void f (char *p)
{
p = stpcpy (p, "abc");
// ...
free (p); // warning
}-Wfree-nonheap-object is included in -Wall. 🫶
-Wstack-usage=byte-size— Warn if the stack usage of a function might exceed byte-size. The computation done to figure out stack usage is conservative. Any space allocated viaalloca, variable-length arrays, or related constructs is included by the compiler when deciding whether to warn. ✨
The message matches the output of -fstack-usage.
- If the stack usage is fully static but exceeds the specified amount, it's:
warning: stack usage is 1120 bytes- If the stack usage is (partly) dynamic but bounded, it's:
warning: stack usage might be 1648 bytes- If the stack usage is (partly) dynamic and not bounded, it's:
warning: stack usage might be unbounded-Wstack-usage=PTRDIFF_MAX is enabled by default. Warnings controlled by this option can be disabled either by specifying a byte-size of SIZE_MAX or more, or by -Wno-stack-usage.
-Wno-stack-usage— Disable-Wstack-usage=warnings. Equivalent to-Wstack-usage=SIZE_MAXor larger.-Wunsafe-loop-optimizations— Warn if the loop can't be optimized because the compiler can't assume anything about the bounds of the loop indices. With-funsafe-loop-optimizations, warn if the compiler does make such assumptions. 💀-Wno-pedantic-ms-format(MinGW targets only) — When used together with-Wformatand-pedanticwithout GNU extensions, this option disables the warnings about non-ISOprintf/scanfformat width specifiersI32,I64, andIused on Windows targets, which depend on the MS runtime.-Wpointer-arith— Warn about anything that depends on the "size of" a function type or ofvoid. GNU C gives these types a size of 1, for convenience in calculations withvoid *pointers and pointers to functions. In C++, also warn when an arithmetic operation involvesNULL. This warning is also enabled by-Wpedantic. 🌈
This warning is upgraded to an error by -pedantic-errors.
-Wno-pointer-compare— Do not warn if a pointer is compared with a zero character constant. Usually that means the pointer was meant to be dereferenced. For example:
const char *p = foo ();
if (p == '\0')
return 42;Note that the code above is invalid in C++11.
This warning is enabled by default.
-Wno-tsan— Disable warnings about unsupported features in ThreadSanitizer. 🧵
ThreadSanitizer does not support std::atomic_thread_fence and can report false positives. 😭
-Wtype-limits— Warn if a comparison is always true or always false because of the limited range of the data type — but don't warn for constant expressions. For example, warn if an unsigned variable is compared against zero with<or>=. This warning is also enabled by-Wextra. 👑-Wabsolute-value(C and Objective-C only) — Warn for calls to standard functions that compute the absolute value of an argument when a more appropriate standard function is available. For example, callingabs(3.14)triggers the warning, because the appropriate function to call to compute the absolute value of a double argument isfabs. The option also triggers warnings when the argument in a call to such a function has an unsigned type. This warning can be suppressed with an explicit type cast, and it's also enabled by-Wextra. ✨-Wcomment-Wcomments— 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. 💅-Wtrigraphs— Warn if any trigraphs are encountered that might change the meaning of the program. Trigraphs within comments are not warned about — except those that would form escaped newlines. 🦋
This one is implied by -Wall anyway. But like — if -Wall isn't given, this is still on, unless trigraphs are enabled. So if you want the trigraph conversion without the warning, but you still want all the other -Wall warnings, you do:
-trigraphs -Wall -Wno-trigraphsand you're golden ✨
-Wundef— yells at you if an undefined identifier gets evaluated inside an#ifdirective. Those poor little identifiers just get replaced with zero, and GCC is like "bestie... did you mean that?" 😭-Wexpansion-to-defined— warns wheneverdefinedshows up in the expansion of a macro (yes, including when the macro gets expanded by an#ifdirective). This is not portable, so GCC is doing you a solid by flagging it. Also switched on by-Wpedanticand-Wextra.-Wunused-macros— snitches on macros defined in the main file that never get used. A macro counts as used if it's expanded or tested for existence at least once. And the preprocessor also warns if the macro hadn't been used yet at the moment it gets redefined or undefined. 👀
Built-in macros, macros defined on the command line, and macros defined in include files? Not warned about. They're exempt, they have immunity, good for them 💅
Note: If a macro is actually used but only inside skipped conditional blocks, the preprocessor will still report it as unused. Rude, honestly. To dodge the warning in that case, you could improve the scope of the macro's definition — for example by moving it into the first skipped block. Or you could just feed it a dummy use, like:
#if defined the_macro_causing_the_warning
#endif-Wno-endif-labels— don't warn when an#elseor an#endifis followed by text. This happens in older programs with code shaped like:
#if FOO
...
#else FOO
...
#endif FOOThe second and third FOO should be in comments. This warning is on by default, no cap.
-Wbad-function-cast(C and Objective-C only) — warns when a function call gets cast to a type that doesn't match. Like, warn if you call a function returning an integer type and then cast it to a pointer type. Ma'am that is not the same thing 🫠-Wc90-c99-compat(C and Objective-C only) — warns about features that don't exist in ISO C90 but do exist in ISO C99. Think: variable length arrays, thelong longtype, thebooltype, compound literals, designated initializers, all of it. This option is independent of the standards mode, and warnings get disabled in the expression that follows__extension__.-Wc99-c11-compat(C and Objective-C only) — same energy but for features not present in ISO C99 that are present in ISO C11. So: anonymous structures and unions, the_Atomictype qualifier, the_Thread_localstorage-class specifier, the_Alignasspecifier, theAlignofoperator, the_Generickeyword, etc. Independent of the standards mode. Warnings disabled in the expression that follows__extension__.-Wc11-c23-compat(C and Objective-C only)-Wc11-c2x-compat(C and Objective-C only) — warns about features not present in ISO C11 but present in ISO C23. For instance: omitting the string in_Static_assert, using the[[]]syntax for attributes, using decimal floating-point types, and so on. This option is independent of the standards mode. Warnings are disabled in the expression that follows__extension__. Heads up: the name-Wc11-c2x-compatis deprecated.
And when you're not compiling in C23 mode, these warnings get upgraded to errors by -pedantic-errors. ✨
-Wc23-c2y-compat(C and Objective-C only)-Wc23-c2y-compat(C and Objective-C only) — warns about features not present in ISO C23 but present in ISO C2Y. Example:_Genericselecting with a type name instead of an expression. This option is independent of the standards mode. Warnings are disabled in the expression that follows__extension__.
When not compiling in C2Y mode, these warnings are upgraded to errors by -pedantic-errors.
-Wc++-compat(C and Objective-C only) — warns about ISO C constructs that live outside the common subset of ISO C and ISO C++, e.g. a request for implicit conversion fromvoid *to a pointer to non-voidtype. 🫶-Wc++11-compat(C++ and Objective-C++ only) — warns about C++ constructs whose meaning differs between ISO C++ 1998 and ISO C++ 2011. For example, identifiers in ISO C++ 1998 that are keywords in ISO C++ 2011. This warning turns on-Wnarrowingand is enabled by-Wall.-Wc++14-compat(C++ and Objective-C++ only) — warns about C++ constructs whose meaning differs between ISO C++ 2011 and ISO C++ 2014. Enabled by-Wall.-Wc++17-compat(C++ and Objective-C++ only) — warns about C++ constructs whose meaning differs between ISO C++ 2014 and ISO C++ 2017. Enabled by-Wall.-Wc++20-compat(C++ and Objective-C++ only) — warns about C++ constructs whose meaning differs between ISO C++ 2017 and ISO C++ 2020. Enabled by-Wall.-Wno-c++11-extensions(C++ and Objective-C++ only) — don't warn about C++11 constructs in code being compiled with an older C++ standard. Even without this option, some C++11 constructs only get diagnosed if-Wpedanticis used.-Wno-c++14-extensions(C++ and Objective-C++ only) — don't warn about C++14 constructs in code compiled with an older C++ standard. Even without this option, some C++14 constructs only get diagnosed if-Wpedanticis used.-Wno-c++17-extensions(C++ and Objective-C++ only) — don't warn about C++17 constructs in code compiled with an older C++ standard. Even without this option, some C++17 constructs only get diagnosed if-Wpedanticis used.-Wno-c++20-extensions(C++ and Objective-C++ only) — don't warn about C++20 constructs in code compiled with an older C++ standard. Even without this option, some C++20 constructs only get diagnosed if-Wpedanticis used.-Wno-c++23-extensions(C++ and Objective-C++ only) — don't warn about C++23 constructs in code compiled with an older C++ standard. Even without this option, some C++23 constructs only get diagnosed if-Wpedanticis used.-Wno-c++26-extensions(C++ and Objective-C++ only) — don't warn about C++26 constructs in code compiled with an older C++ standard. Even without this option, some C++26 constructs only get diagnosed if-Wpedanticis used.
-Wcast-qual— warns whenever a pointer is cast in a way that strips a type qualifier off the target type. Like, warn if aconst char *gets cast to a plain oldchar *. 🚩
It also warns when a cast introduces a type qualifier in an unsafe way. For example, casting char ** to const char ** is unsafe, like in this lil example:
/* p is char ** value. */
const char **q = (const char **) p;
/* Assignment of readonly string to const char * is OK. */
*q = "string";
/* Now char** pointer points to read-only memory. */
**p = 'b';yeah. that's the danger zone 💀
-Wcast-align— warns whenever a pointer is cast so that the required alignment of the target gets increased. For example, warn if achar *is cast to anint *on machines where integers can only be accessed at two- or four-byte boundaries.-Wcast-align=strict— warns whenever a pointer is cast such that the required alignment of the target is increased. For example, warn if achar *is cast to anint *, regardless of the target machine. No exceptions, no mercy.-Wcast-function-type— warns when a function pointer gets cast to an incompatible function pointer. In a cast involving function types with a variable argument list, only the types of the initial arguments that are provided are considered. Any parameter of pointer-type matches any other pointer-type. Any benign differences in integral types get ignored, likeintvs.longon ILP32 targets. Type qualifiers get ignored too, same deal. The function typevoid (*) (void)is special and matches everything, which you can use to suppress this warning. In a cast involving pointer to member types, this warning fires whenever the type cast changes the pointer to member type. Enabled by-Wextra.-Wcast-user-defined— warns when a cast to reference type doesn't involve a user-defined conversion that the programmer might expect to be called.
struct A { operator const int&(); } a;
auto r = (int&)a; // warningThis warning is enabled by default. 👑
-Wwrite-strings— when compiling C, this gives string constants the typeconst char[length], so that copying the address of one into a non-constchar *pointer produces a warning. These warnings help you find, at compile time, code that might try to write into a string constant — but only if you've been super careful about usingconstin declarations and prototypes. Otherwise it's just a nuisance. That's literally why we didn't make-Wallrequest these warnings. 🙈
When compiling C++, it warns about the deprecated conversion from string literals to char *. This warning is enabled by default for C++ programs.
And this warning gets upgraded to an error by -pedantic-errors in C++11 mode or later. ✨
-Wclobbered— warns about variables that might get changed bylongjmporvfork. Also enabled by-Wextra.-Wno-complain-wrong-lang— by default, language front ends complain when a command-line option is valid but not applicable to that front end. You can turn that off with-Wno-complain-wrong-lang, which is mostly useful when you're invoking a single compiler driver for multiple source files written in different languages, like:
$ g++ -fno-rtti a.cc b.f90The driver g++ invokes the C++ front end to compile a.cc and the Fortran front end to compile b.f90. That latter front end diagnoses f951: Warning: command-line option '-fno-rtti' is valid for C++/D/ObjC++ but not for Fortran, which may be disabled with -Wno-complain-wrong-lang.
-Wcompare-distinct-pointer-types(C and Objective-C only) — warns if pointers of distinct types get compared without a cast. This warning is enabled by default. 🦋-Wconversion— bestie, this one is the value-shift detective 🔍. It warns for implicit conversions that may alter a value. That includes real ↔ integer glow-ups, likeabs (x)whenxisdouble; signed ↔ unsigned chaos, likeunsigned ui = -1; and shrinking to smaller types, likesqrtf (M_PI). It does not warn for explicit casts likeabs ((int) x)andui = (unsigned) -1, or if the value is not changed by the conversion, like inabs (2.0). Warnings about conversions between signed and unsigned integers can be disabled by using-Wno-sign-conversion.
For C++, it also warns for confusing overload resolution for user-defined conversions; and conversions that never use a type conversion operator: conversions to void, the same type, a base class or a reference to them. Warnings about conversions between signed and unsigned integers are disabled by default in C++ unless -Wsign-conversion is explicitly enabled.
Warnings about conversion from arithmetic on a small type back to that type are only given with -Warith-conversion.
-Wdangling-else— this one is giving suspicious indentation energy 😭. It warns about constructions where there may be confusion to whichifstatement anelsebranch belongs. Here is an example of such a case:
{
if (a)
if (b)
foo ();
else
bar ();
}In C/C++, every else branch belongs to the innermost possible if statement, which in this example is if (b). This is often not what the programmer expected, as illustrated in the above example by indentation the programmer chose. When there is the potential for this confusion, GCC issues a warning when this flag is specified. To eliminate the warning, add explicit braces around the innermost if statement so there is no way the else can belong to the enclosing if. The resulting code looks like this:
{
if (a)
{
if (b)
foo ();
else
bar ();
}
}This warning is enabled by -Wparentheses.
-Wdangling-pointer-Wdangling-pointer=n— ok so this is the lifetime police 👑. It warns about uses of pointers (or C++ references) to objects with automatic storage duration after their lifetime has ended. This includes local variables declared in nested blocks, compound literals and other unnamed temporary objects. In addition, it warns about storing the address of such objects in escaped pointers. The warning is enabled at all optimization levels but may yield different results with optimization than without.-Wdangling-pointer=1— at level 1, the warning diagnoses only unconditional uses of dangling pointers.-Wdangling-pointer=2— at level 2, in addition to unconditional uses, the warning also diagnoses conditional uses of dangling pointers.
The short form -Wdangling-pointer is equivalent to -Wdangling-pointer=2, while -Wno-dangling-pointer and -Wdangling-pointer=0 have the same effect of disabling the warnings. -Wdangling-pointer=2 is included in -Wall.
This example triggers the warning at level 1; the address of the unnamed temporary is unconditionally referenced outside of its scope.
char f (char c1, char c2, char c3)
{
char *p;
{
p = (char[]) { c1, c2, c3 };
}
// warning: using dangling pointer 'p' to an unnamed temporary
return *p;
}In the following function the store of the address of the local variable x in the escaped pointer *p triggers the warning at level 1.
void g (int **p)
{
int x = 7;
// warning: storing the address of local variable 'x' in '*p'
*p = &x;
}In this example, the array a is out of scope when the pointer s is used. Since the code that sets s is conditional, the warning triggers at level 2.
extern void frob (const char *);
void h (char *s)
{
if (!s)
{
char a[12] = "tmpname";
s = a;
}
// warning: dangling pointer 's' to 'a' may be used
frob (s);
}-Wdate-time— warns when macros__TIME__,__DATE__or__TIMESTAMP__are encountered, because they might prevent bit-wise-identical reproducible compilations. reproducible-build besties, stay vigilant 🕰️.-Wempty-body— warns if an empty body occurs in anif,elseordo whilestatement. This warning is also enabled by-Wextra. empty body? not the vibe 💀.-Wno-endif-labels— do not warn about stray tokens after#elseand#endif. mind your preprocessor business, bestie ✨.-Wenum-compare— warns about a comparison between values of different enumerated types. In C++, enumerated type mismatches in conditional expressions are also diagnosed and the warning is enabled by default. In C, this warning is enabled by-Wall. enum vs enum drama 🎭.-Wenum-conversion— warns when a value of enumerated type is implicitly converted to a different enumerated type. This warning is enabled by-Wextrain C. no sneaky enum swaps 🙈.-Wenum-int-mismatch(C and Objective-C only) — warns about mismatches between an enumerated type and an integer type in declarations. For example:
enum E { l = -1, z = 0, g = 1 };
int foo(void);
enum E foo(void);In C, an enumerated type is compatible with char, a signed integer type, or an unsigned integer type. However, since the choice of the underlying type of an enumerated type is implementation-defined, such mismatches may cause portability issues. In C++, such mismatches are an error. In C, this warning is enabled by -Wall and -Wc++-compat.
-Wjump-misses-init(C, Objective-C only) — warns if agotostatement or aswitchstatement jumps forward across the initialization of a variable, or jumps backward to a label after the variable has been initialized. This only warns about variables that are initialized when they are declared. This warning is only supported for C and Objective-C; in C++ this sort of branch is an error in any case.
-Wjump-misses-init is included in -Wc++-compat. It can be disabled with the -Wno-jump-misses-init option. jumping over init? not today 🦋.
-Wsign-compare— warns when a comparison between signed and unsigned values could produce an incorrect result when the signed value is converted to unsigned. In C++, this warning is also enabled by-Wall. In C, it is also enabled by-Wextra. signed vs unsigned showdown ⚔️.-Wsign-conversion— warns for implicit conversions that may change the sign of an integer value, like assigning a signed integer expression to an unsigned integer variable. An explicit cast silences the warning. In C, this option is enabled also by-Wconversion. flip the sign, catch the warning 🔄.-Wflex-array-member-not-at-end(C and C++ only) — warns when a structure containing a C99 flexible array member as the last field is not at the end of another structure. This warning warns e.g. about
struct flex { int length; char data[]; };
struct mid_flex { int m; struct flex flex_data; int n; };flexible array member said “I must be last” 💅.
-Wfloat-conversion— warns for implicit conversions that reduce the precision of a real value. This includes conversions from real to integer, and from higher precision real to lower precision real values. This option is also enabled by-Wconversion. precision loss is real 😭.-Wno-scalar-storage-order— do not warn on suspicious constructs involving reverse scalar storage order. storage order? not my problem 🙈.-Wsizeof-array-div— warns about divisions of twosizeofoperators when the first one is applied to an array and the divisor does not equal the size of the array element. In such a case, the computation will not yield the number of elements in the array, which is likely what the user intended. This warning warns e.g. about
int fn ()
{
int arr[10];
return sizeof (arr) / sizeof (short);
}This warning is enabled by -Wall. math girlies, check your divisors 🧮.
-Wsizeof-pointer-div— warns for suspicious divisions of twosizeofexpressions that divide the pointer size by the element size, which is the usual way to compute the array size but won’t work out correctly with pointers. This warning warns e.g. aboutsizeof (ptr) / sizeof (ptr[0])ifptris not an array, but a pointer. This warning is enabled by-Wall. pointer math said “not today” 👑.-Wsizeof-pointer-memaccess— warns for suspicious length parameters to certain string and memory built-in functions if the argument usessizeof. This warning triggers for example formemset (ptr, 0, sizeof (ptr));ifptris not an array, but a pointer, and suggests a possible fix, or aboutmemcpy (&foo, ptr, sizeof (&foo));.-Wsizeof-pointer-memaccessalso warns about calls to bounded string copy functions likestrncatorstrncpythat specify as the bound asizeofexpression of the source array. For example, in the following function the call tostrncatspecifies the size of the source string as the bound. That is almost certainly a mistake and so the call is diagnosed.
void make_file (const char *name)
{
char path[PATH_MAX];
strncpy (path, name, sizeof path - 1);
strncat (path, ".text", sizeof ".text");
...
}The -Wsizeof-pointer-memaccess option is enabled by -Wall. sizeof trap detected 🚨.
-Wno-sizeof-array-argument— do not warn when thesizeofoperator is applied to a parameter that is declared as an array in a function definition. This warning is enabled by default for C and C++ programs. array parameter? leave it alone 🌸.-Wmemset-elt-size— warns for suspicious calls to thememsetbuilt-in function, if the first argument references an array, and the third argument is a number equal to the number of elements, but not equal to the size of the array in memory. This indicates that the user has omitted a multiplication by the element size. This warning is enabled by-Wall. memset forgot to multiply? caught 💅.-Wmemset-transposed-args— bestie, this one catches the classic "oops I swapped my args" moment 💀. It warns about sus calls to thememsetbuilt-in function where the second argument is not zero and the third argument is zero. Like, the callmemset (buf, sizeof buf, 0)gets diagnosed because you obviously meantmemset (buf, 0, sizeof buf). The catch tho: the diagnostic is only emitted if the third argument is a literal zero. If it's an expression that gets folded to zero, or a cast of zero to some type, then it's wayyy less likely the args got transposed, so no warning. This warning is enabled by-Wall.-Waddress— warns about suspicious uses of address expressions ✨. These include comparing the address of a function or a declared object to the null pointer constant, like this:
void f (void);
void g (void)
{
if (!f) // warning: expression evaluates to false
abort ();
}comparisons of a pointer to a string literal, like:
void f (const char *x)
{
if (x == "abc") // warning: expression evaluates to false
puts ("equal");
}and tests of the results of pointer addition or subtraction for equality to null, like:
void f (const int *p, int i)
{
return p + i == NULL;
}Such uses typically indicate a programmer error: the address of most functions and objects necessarily evaluates to true (the exception being weak symbols), so using them in a conditional might mean you're missing parentheses in a function call or missing a dereference in an array expression. The subset of the warning for object pointers can be suppressed by casting the pointer operand to an integer type such as intptr_t or uintptr_t. Comparisons against string literals result in unspecified behavior and are not portable, and they lowkey suggest the intent was to call strcmp. The warning is suppressed if the suspicious expression is the result of macro expansion. -Waddress warning is enabled by -Wall.
-Wno-address-of-packed-member— do not warn when the address of a packed member of a struct or union is taken, which usually results in an unaligned pointer value. This is enabled by default.-Wlogical-op— warns about suspicious uses of logical operators in expressions 👀. This includes using logical operators in contexts where a bit-wise operator is likely expected. Also warns when the operands of a logical operator are the same:
extern int a;
if (a < 0 && a < 0) { ... }-Wlogical-not-parentheses— warns about logical not used on the left hand side operand of a comparison. This option does not warn if the right operand is considered to be a boolean expression. Its whole purpose is to detect suspicious code like this:
int a;
...
if (!a > 1) { ... }It's possible to suppress the warning by wrapping the LHS into parentheses:
if ((!a) > 1) { ... }This warning is enabled by -Wall.
-Waggregate-return— warns if any functions that return structures or unions are defined or called. (In languages where you can return an array, this also elicits a warning.)-Wno-aggressive-loop-optimizations— do not warn if the compiler detects undefined behavior in a loop with a constant number of iterations.-Waggressive-loop-optimizationsis enabled by default.-Wno-attributes— do not warn if an unexpected__attribute__is used, such as unrecognized attributes, function attributes applied to variables, etc. This does not stop errors for incorrect use of supported attributes 🫶.
Warnings about ill-formed uses of standard attributes are upgraded to errors by -pedantic-errors.
Additionally, using -Wno-attributes=, it's possible to suppress warnings about unknown scoped attributes (in C++11 and C23). For example, -Wno-attributes=vendor::attr disables warning about the following declaration:
[[vendor::attr]] void f();It's also possible to disable warning about all attributes in a namespace using -Wno-attributes=vendor:: which prevents warning about both of these declarations:
[[vendor::safe]] void f();
[[vendor::unsafe]] void f2();Note that -Wno-attributes= does not imply -Wno-attributes.
-Wno-builtin-declaration-mismatch— warns if a built-in function is declared with an incompatible signature or as a non-function, or when a built-in function declared with a type that does not include a prototype is called with arguments whose promoted types do not match those expected by the function. When-Wextrais specified, it also warns when a built-in function that takes arguments is declared without a prototype. The-Wbuiltin-declaration-mismatchwarning is enabled by default. To avoid the warning, include the appropriate header to bring the prototypes of built-in functions into scope.
For example, the call to memset below is diagnosed by the warning because the function expects a value of type size_t as its argument but the type of 32 is int. With -Wextra, the declaration of the function is diagnosed as well.
extern void* memset ();
void f (void *d)
{
memset (d, '\0', 32);
}-Wno-builtin-macro-redefined— do not warn if certain built-in macros are redefined. This suppresses warnings for redefinition of__TIMESTAMP__,__TIME__,__DATE__,__FILE__, and__BASE_FILE__.-Wfree-labels(C and Objective-C only) — warns if a label is applied to a non-statement, or occurs at the end of a compound statement. Such labels are allowed by C23 and later dialects of C, and are available as a GCC extension in all other dialects.
This warning is also enabled by -Wc11-c23-compat. It is turned into an error if building for a C version before C23 by -pedantic-errors.
-Wheader-guard— warns if a valid preprocessor header multiple inclusion guard has a#definedirective right after#ifndefor#if !defineddirective for the multiple inclusion guard, which defines a different macro from the guard macro with a similar name, the actual multiple inclusion guard macro isn't defined at the corresponding#ifndefdirective at the end of the header, and the#definedirective defines an object-like macro with empty definition. In such a case, it often is just a misspelled guard name, either in the#ifndefor#if !defineddirective or in the subsequent#definedirective 😭. This warning is enabled by-Wall.-Wstrict-prototypes(C and Objective-C only) — warns if a function is declared or defined without specifying the argument types. (An old-style function definition is permitted without a warning if preceded by a declaration that specifies the argument types.)-Wold-style-declaration(C and Objective-C only) — warns for obsolescent usages, according to the C Standard, in a declaration. For example, warn if storage-class specifiers likestaticare not the first things in a declaration. This warning is also enabled by-Wextra.-Wold-style-definition(C and Objective-C only) — warns if an old-style function definition is used. A warning is given even if there is a previous prototype. A definition using()is not considered an old-style definition in C23 mode, because it is equivalent to(void)in that case, but it is considered an old-style definition for older standards.-Wdeprecated-non-prototype(C and Objective-C only) — warns if a function declared with an empty parameter list()is called with one or more arguments, or if a function definition with one or more parameters is encountered after such a declaration. Both cases are errors in C23 and later dialects of C 💅.
This warning is also enabled by -Wc11-c23-compat.
-Wmissing-parameter-name(C and Objective-C only) — warns if a function definition omits a parameter name, specifying only its type. This can be used to document that a parameter is unused in the definition. It's part of C23 and later dialects of C, and available as a GCC extension in all other dialects.
This warning is also enabled by -Wc11-c23-compat. It is turned into an error if building for a C version before C23 by -pedantic-errors.
-Wmissing-parameter-type(C and Objective-C only) — a function parameter is declared without a type specifier in K&R-style functions:
void foo(bar) { }This warning is also enabled by -Wextra.
-Wno-declaration-missing-parameter-type(C and Objective-C only) — do not warn if a function declaration contains a parameter name without a type. Such function declarations do not provide a function prototype and prevent most type checking in function calls.
This warning is enabled by default. In C99 and later dialects of C, it is treated as an error. The error can be downgraded to a warning using -fpermissive (along with certain other errors), or for this error alone, with -Wno-error=declaration-missing-parameter-type.
This warning is upgraded to an error by -pedantic-errors.
-Wmissing-prototypes(C and Objective-C only) — warns if a global function is defined without a previous prototype declaration. This warning is issued even if the definition itself provides a prototype. Use this option to detect global functions that do not have a matching prototype declaration in a header file. This option is not valid for C++ because all function declarations provide prototypes and a non-matching declaration declares an overload rather than conflict with an earlier declaration. Use-Wmissing-declarationsto detect missing declarations in C++.-Wmissing-variable-declarations(C and Objective-C only) — warns if a global variable is defined without a previous declaration. Use this option to detect global variables that do not have a matching extern declaration in a header file 🌸.-Wmissing-declarations— ok so bestie, this one yells at you if a global function gets defined without a previous declaration. And yes, it still yells even if the definition itself hands you a prototype 💀. The whole point is catching global functions that never made it into a header file. In C, no warnings fire for functions with previous non-prototype declarations — if that's the flavor of missing you're hunting, go use-Wmissing-prototypesinstead. In C++, silence for function templates, inline functions, and functions chilling in anonymous namespaces. ✨-Wmissing-field-initializers— warns when a structure's initializer is out here missing some fields 🙈. Like, this code triggers it, becausex.his implicitly zero:
struct s { int f, g, h; };
struct s x = { 3, 4 };In C this option does not warn about designated initializers, so this little glow-up doesn't trigger anything:
struct s { int f, g, h; };
struct s x = { .f = 3, .g = 4 };In C it also doesn't warn about the universal zero initializer { 0 }:
struct s { int f, g, h; };
struct s x = { 0 };Same energy in C++ — no warning for the empty { } initializer, for example:
struct s { int f, g, h; };
s x = { };This warning is included in -Wextra. Want the rest of the -Wextra squad without this one? Use -Wextra -Wno-missing-field-initializers. 🫶
-Wno-missing-requires— by default the compiler warns about a concept-id showing up as a C++20 simple-requirement:
bool satisfied = requires { C<T> };Here satisfied will be true if C<T> is a valid expression, which it is for all T. Presumably the user meant to write
bool satisfied = requires { requires C<T> };so satisfied is only true if concept C is satisfied for type T. This warning can be disabled with -Wno-missing-requires. 👑
-Wno-missing-template-keyword— the member access tokens.,->and::must be followed by the"template"keyword if the parent object is dependent and the member being named is a template.
template <class X>
void DoStuff (X x)
{
x.template DoSomeOtherStuff<X>(); // Good.
x.DoMoreStuff<X>(); // Warning, x is dependent.
}In rare cases it is possible to get false positives. To silence this, wrap the expression in parentheses. For example, the following is treated as a template, even where m and N are integers:
void NotATemplate (my_class t)
{
int N = 5;
bool test = t.m < N > (0); // Treated as a template.
test = (t.m < N) > (0); // Same meaning, but not treated as a template.
}This warning can be disabled with -Wno-missing-template-keyword. 💅
-Wno-multichar— do not warn if a multicharacter constant ('FOOF') is used. Usually they indicate a typo in the user's code, because they have implementation-defined values, and they should not be used in portable code. no cap. 🌸-Wnormalized=[none|id|nfc|nfkc]— in ISO C and ISO C++, two identifiers are different if they are different sequences of characters. But sometimes, when characters outside the basic ASCII character set get involved, 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 get turned into the same sequence. GCC can warn you if you're using identifiers that have not been normalized; this option controls that warning.
There are four levels of warning supported by GCC. The default is -Wnormalized=nfc, which warns about any identifier that is not in the ISO 10646 "C" normalized form, NFC. NFC is the recommended form for most uses. It is equivalent to -Wnormalized.
Unfortunately, there are some characters allowed in identifiers by ISO C and ISO C++ that, when turned into NFC, are not allowed in identifiers. That is, there's no way to use these symbols in portable ISO C or C++ and have all your identifiers in NFC. -Wnormalized=id suppresses the warning for these characters. It is hoped that future versions of the standards involved will correct this, which is why this option is not the default. 🙏
You can switch the warning off for all characters by writing -Wnormalized=none or -Wno-normalized. You should only do this if you are using some other normalization scheme (like "D"), because otherwise you can easily create bugs that are literally impossible to see. ⚠️
Some characters in ISO 10646 have distinct meanings but look identical in some fonts or display methodologies, especially once formatting has been applied. For instance "\u207F", "SUPERSCRIPT LATIN SMALL LETTER N", displays just like a regular "n" that has been placed in a superscript. ISO 10646 defines the NFKC normalization scheme to convert all these into a standard form as well, and GCC warns if your code is not in NFKC if you use -Wnormalized=nfkc. This warning is comparable to warning about every identifier that contains the letter O because it might be confused with the digit 0, and so is not the default, but may be useful as a local coding convention if the programming environment cannot be fixed to display these characters distinctly. 🦋
-Wno-attribute-warning— do not warn about usage of functions declared with"warning"attribute. By default, this warning is enabled.-Wno-attribute-warningcan be used to disable the warning or-Wno-error=attribute-warningcan be used to disable the error when compiled with-Werrorflag.-Wno-deprecated— do not warn about usage of deprecated features. 💀
In C++, explicitly specifying -Wdeprecated also enables warnings about some features that are deprecated in later language standards, specifically -Wcomma-subscript, -Wvolatile, -Wdeprecated-enum-float-conversion, -Wdeprecated-enum-enum-conversion, -Wdeprecated-literal-operator, and -Wdeprecated-variadic-comma-omission.
-Wno-deprecated-declarations— do not warn about uses of functions, variables, and types marked as deprecated by using the"deprecated"attribute.-Wno-overflow— do not warn about compile-time overflow in constant expressions.-Wno-odr— warn about One Definition Rule violations during link-time optimization. Enabled by default. 👀-Wopenacc-parallelism— warn about potentially suboptimal choices related to OpenACC parallelism.-Wno-openmp— warn about suspicious OpenMP code. 🕵️-Wopenmp-simd— warn if the vectorizer cost model overrides the OpenMP simd directive set by user. The-fsimd-cost-model=unlimitedoption can be used to relax the cost model.-Woverride-init(C and Objective-C only) — warn if an initialized field without side effects is overridden when using designated initializers.
This warning is included in -Wextra. To get other -Wextra warnings without this one, use -Wextra -Wno-override-init.
-Wno-override-init-side-effects(C and Objective-C only) — do not warn if an initialized field with side effects is overridden when using designated initializers. This warning is enabled by default.-Wpacked— warn if a structure is given the packed attribute, but the packed attribute has no effect on the layout or size of the structure. Such structures may be mis-aligned for little benefit 😬. For instance, in this code, the variable"f.x"in"struct bar"is misaligned even though"struct bar"does not itself have the packed attribute:
struct foo {
int x;
char a, b, c, d;
} __attribute__((packed));
struct bar {
char z;
struct foo f;
};-Wnopacked-bitfield-compat— the 4.1, 4.2 and 4.3 series of GCC ignore the"packed"attribute on bit-fields of type"char". This was fixed in GCC 4.4 but the change can lead to differences in the structure layout. GCC informs you when the offset of such a field has changed in GCC 4.4. For example there is no longer a 4-bit padding between field"a"and"b"in this structure:
struct foo
{
char a:4;
char b:8;
} __attribute__ ((packed));This warning is enabled by default. Use -Wno-packed-bitfield-compat to disable this warning.
-Wpacked-not-aligned(C, C++, Objective-C and Objective-C++ only) — warn if a structure field with explicitly specified alignment in a packed struct or union is misaligned. For example, a warning will be issued on"struct S", like,"warning: alignment 1 of 'struct S' is less than 8", in this code:
struct __attribute__ ((aligned (8))) S8 { char a[8]; };
struct __attribute__ ((packed)) S {
struct S8 s8;
};This warning is enabled by -Wall. ⭐
-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.-Wrestrict— warn when an object referenced by a"restrict"-qualified parameter (or, in C++, a"__restrict"-qualified parameter) is aliased by another argument, or when copies between such objects overlap. For example, the call to the"strcpy"function below attempts to truncate the string by replacing its initial characters with the last four. However, because the call writes the terminating NUL into"a[4]", the copies overlap and the call is diagnosed. 🫠
void foo (void)
{
char a[] = "abcd1234";
strcpy (a, a + 4);
...
}The -Wrestrict option detects some instances of simple overlap even without optimization but works best at -O2 and above. It is included in -Wall.
-Wnested-externs(C and Objective-C only) — warn if an"extern"declaration is encountered within a function. like, why is it in there bestie 😭-Winline— bestie, this one yells at you if a function you declared asinlinedidn't actually get inlined. BUT 💀 the compiler stays quiet about failures to inline functions declared in system headers, so don't expect it to tattle on those.
The compiler uses a whole variety of heuristics to decide whether to inline a function — like, for example, it takes into account the size of the function being inlined and the amount of inlining that's already been done in the current function. So seemingly insignificant changes in the source program can make the warnings from -Winline appear or disappear. Fr fr, it's giving chaotic.
-Winterference-size— warns you about use of C++17std::hardware_destructive_interference_sizewithout specifying its value with--param destructive-interference-size. Also warns about questionable values for that option. 🫶
This variable is intended to be used for controlling class layout, to avoid false sharing in concurrent code:
struct independent_fields {
alignas(std::hardware_destructive_interference_size)
std::atomic<int> one;
alignas(std::hardware_destructive_interference_size)
std::atomic<int> two;
};Here one and two are intended to be far enough apart that stores to one won't require accesses to the other to reload the cache line.
By default, --param destructive-interference-size and --param constructive-interference-size are set based on the current -mtune option, typically to the L1 cache line size for the particular target CPU, sometimes to a range if tuning for a generic target. So all translation units that depend on ABI compatibility for the use of these variables must be compiled with the same -mtune (or -mcpu).
If ABI stability is important, such as if the use is in a header for a library, you should probably not use the hardware interference size variables at all. Alternatively, you can force a particular value with --param.
If you are confident that your use of the variable does not affect ABI outside a single build of your project, you can turn off the warning with -Wno-interference-size. ✨
-Wint-in-bool-context— warns for suspicious use of integer values where boolean values are expected, like conditional expressions (?:) using non-boolean integer constants in boolean context, likeif (a <= b ? 2 : 3). Or left shifting of signed integers in boolean context, likefor (a = 0; 1 << a; a++);. Likewise for all kinds of multiplications regardless of the data type. This warning is enabled by-Wall. 💅-Wno-int-to-pointer-cast— suppresses warnings from casts to pointer type of an integer of a different size. In C++, casting to a pointer type of smaller size is an error.Wint-to-pointer-castis enabled by default.-Wno-pointer-to-int-cast(C and Objective-C only) — suppresses warnings from casts from a pointer to an integer type of a different size.-Winvalid-pch— warns if a precompiled header is found in the search path but cannot be used. It's giving "I see you but I can't work with you" 😭-Winvalid-utf8— warns if an invalid UTF-8 character is found. This warning is on by default for C++23 if-finput-charset=UTF-8is used and turned into error with-pedantic-errors.-Wno-unicode— don't diagnose invalid forms of delimited or named escape sequences which are treated as separate tokens.Wunicodeis enabled by default.-Wlong-long— warns iflong longtype is used. This is enabled by either-Wpedanticor-Wtraditionalin ISO C90 and C++98 modes. To inhibit the warning messages, use-Wno-long-long.
This warning is upgraded to an error by -pedantic-errors.
-Wvariadic-macros— warns if variadic macros are used in ISO C90 mode, or if the GNU alternate syntax is used in ISO C99 mode. This is enabled by either-Wpedanticor-Wtraditional. To inhibit the warning messages, use-Wno-variadic-macros.-Wno-varargs— do not warn upon questionable usage of the macros used to handle variable arguments likeva_start. These warnings are enabled by default.-Wvector-operation-performance— warns if vector operation is not implemented via SIMD capabilities of the architecture. Mainly useful for the performance tuning. 🏎️ Vector operation can be implemented "piecewise", which means that the scalar operation is performed on every vector element; "in parallel", which means that the vector operation is implemented using scalars of wider type, which normally is more performance efficient; and "as a single scalar", which means that vector fits into a scalar type.-Wvla— warns if a variable-length array is used in the code.-Wno-vlaprevents the-Wpedanticwarning of the variable-length array.
This warning is upgraded to an error by -pedantic-errors.
- *
-Wvla-larger-than=byte-size* — if this option is used, the compiler warns for declarations of variable-length arrays whose size is either unbounded, or bounded by an argument that allows the array size to exceedbyte-sizebytes. This is similar to how-Walloca-larger-than=byte-sizeworks, but with variable-length arrays.
Note that GCC may optimize small variable-length arrays of a known value into plain arrays, so this warning may not get triggered for such arrays. 🫠
-Wvla-larger-than=PTRDIFF_MAX is enabled by default but is typically only effective when -ftree-vrp is active (default for -O2 and above).
See also -Walloca-larger-than=byte-size.
-Wno-vla-larger-than— disable-Wvla-larger-than=warnings. The option is equivalent to-Wvla-larger-than=SIZE_MAXor larger.-Wvla-parameter— warns about redeclarations of functions involving arguments of Variable Length Array types of inconsistent kinds or forms, and enables the detection of out-of-bounds accesses to such parameters by warnings such as-Warray-bounds.
If the first function declaration uses the VLA form the bound specified in the array is assumed to be the minimum number of elements expected to be provided in calls to the function and the maximum number of elements accessed by it. Failing to provide arguments of sufficient size or accessing more than the maximum number of elements may be diagnosed.
For example, the warning triggers for the following redeclarations because the first one allows an array of any size to be passed to f while the second one specifies that the array argument must have at least n elements. In addition, calling f with the associated VLA bound parameter in excess of the actual VLA bound triggers a warning as well.
void f (int n, int[n]);
// warning: argument 2 previously declared as a VLA
void f (int, int[]);
void g (int n)
{
if (n > 4)
return;
int a[n];
// warning: access to a by f may be out of bounds
f (sizeof a, a);
...
}-Wvla-parameter is included in -Wall. The -Warray-parameter option triggers warnings for similar problems involving ordinary array arguments.
-Wvolatile-register-var— warns if a register variable is declared volatile. The volatile modifier does not inhibit all optimizations that may eliminate reads and/or writes to register variables. This warning is enabled by-Wall.-Wno-xor-used-as-pow(C, C++, Objective-C and Objective-C++ only) — disable warnings about uses of^, the exclusive or operator, where it appears the code meant exponentiation. Specifically, the warning occurs when the left-hand side is the decimal constant2or10and the right-hand side is also a decimal constant.
In C and C++, ^ means exclusive or, whereas in some other languages (e.g. TeX and some versions of BASIC) it means exponentiation.
This warning can be silenced by converting one of the operands to hexadecimal as well as by compiling with -Wno-xor-used-as-pow. ✨
-Wdisabled-optimization— warns if a requested optimization pass is disabled. This warning does not generally indicate that there is anything wrong with your code; it merely indicates that GCC's optimizers are unable to handle the code effectively. Often, the problem is that your code is too big or too complex; GCC refuses to optimize programs when the optimization itself is likely to take inordinate amounts of time. 😮💨-Wpointer-sign(C and Objective-C only) — warns for pointer argument passing or assignment with different signedness. This option is only supported for C and Objective-C. It is implied by-Walland by-Wpedantic, which can be disabled with-Wno-pointer-sign.
This warning is upgraded to an error by -pedantic-errors.
-Wstack-protector— this option is only active when-fstack-protectoris active. It warns about functions that are not protected against stack smashing. 🛡️-Woverlength-strings— warns about string constants that are longer than the "minimum maximum" length specified in the C standard. Modern compilers generally allow string constants that are much longer than the standard's minimum limit, but very portable programs should avoid using longer strings.
The limit applies after string constant concatenation, and does not count the trailing NUL. In C90, the limit was 509 characters; in C99, it was raised to 4095. C++98 does not specify a normative minimum maximum, so we do not diagnose overlength strings in C++.
This option is implied by -Wpedantic, and can be disabled with -Wno-overlength-strings.
-Wunsuffixed-float-constants(C and Objective-C only) — issues a warning for any floating constant that does not have a suffix. When used together with-Wsystem-headersit warns about such constants in system header files. This can be useful when preparing code to use with theFLOAT_CONST_DECIMAL64pragma from the decimal floating-point extension to C99. 🍓-Wno-lto-type-mismatch— during the link-time optimization, do not warn about type mismatches in global declarations from different compilation units. Requires-fltoto be enabled. Enabled by default.-Wno-designated-init(C and Objective-C only) — suppresses warnings when a positional initializer is used to initialize a structure that has been marked with thedesignated_initattribute.
🌈 Options That Control Static Analysis 🌈
okay bestie, buckle up 💅 this is the analyzer corner of GCC and it's a whole vibe. These are the flags that make GCC actually think about your program's flow instead of just yelling about syntax. Let's go 🦋
-fanalyzer— this one turns on a static analysis of program flow that goes hunting for "interesting" interprocedural paths through your code and drops warnings for the problems it finds sitting on them ✨
Heads up though: this analysis is way more expensive than the other GCC warnings. Like, it's giving main character energy but it's also giving runtime cost, so don't be shocked 😭
In technical terms (iykyk), it performs coverage-guided symbolic execution of the code being compiled. And here's the honest part: it is neither sound nor complete — it can have false positives and false negatives. It is a bug-finding tool, not a tool for proving program correctness. Periodt.
Also, the analyzer is only suitable for use on C code in this release 🙈
Enabling this option effectively enables the following warnings:
-Wanalyzer-allocation-size -Wanalyzer-deref-before-check
-Wanalyzer-double-fclose -Wanalyzer-double-free
-Wanalyzer-exposure-through-output-file
-Wanalyzer-exposure-through-uninit-copy
-Wanalyzer-fd-access-mode-mismatch -Wanalyzer-fd-double-close
-Wanalyzer-fd-leak -Wanalyzer-fd-phase-mismatch
-Wanalyzer-fd-type-mismatch -Wanalyzer-fd-use-after-close
-Wanalyzer-fd-use-without-check -Wanalyzer-file-leak
-Wanalyzer-free-of-non-heap -Wanalyzer-imprecise-fp-arithmetic
-Wanalyzer-infinite-loop -Wanalyzer-infinite-recursion
-Wanalyzer-jump-through-null -Wanalyzer-malloc-leak
-Wanalyzer-mismatching-deallocation -Wanalyzer-null-argument
-Wanalyzer-null-dereference -Wanalyzer-out-of-bounds
-Wanalyzer-overlapping-buffers -Wanalyzer-possible-null-argument
-Wanalyzer-possible-null-dereference -Wanalyzer-putenv-of-auto-var
-Wanalyzer-shift-count-negative -Wanalyzer-shift-count-overflow
-Wanalyzer-stale-setjmp-buffer -Wanalyzer-tainted-allocation-size
-Wanalyzer-tainted-array-index -Wanalyzer-tainted-assertion
-Wanalyzer-tainted-divisor -Wanalyzer-tainted-offset
-Wanalyzer-tainted-size -Wanalyzer-undefined-behavior-ptrdiff
-Wanalyzer-undefined-behavior-strtok
-Wanalyzer-unsafe-call-within-signal-handler
-Wanalyzer-use-after-free
-Wanalyzer-use-of-pointer-in-stale-stack-frame
-Wanalyzer-use-of-uninitialized-value
-Wanalyzer-va-arg-type-mismatch -Wanalyzer-va-list-exhausted
-Wanalyzer-va-list-leak -Wanalyzer-va-list-use-after-va-end
-Wanalyzer-write-to-const -Wanalyzer-write-to-string-literal⚠️ and one big caveat: this option is only available if GCC was configured with analyzer support enabled. If it wasn't, no analyzer for you, sorry babe 💀
-Wanalyzer-symbol-too-complex— so if-fanalyzeris on, the analyzer uses a bunch of heuristics to try and track the state of memory 🧠 but sufficiently complicated code can absolutely defeat them.
By default, the analysis just silently stops tracking values of expressions once they blow past the threshold defined by --param analyzer-max-svalue-depth=value, and it falls back to an imprecise representation for those expressions. This option, -Wanalyzer-symbol-too-complex, is the one that warns you when that happens 🫶
-Wanalyzer-too-complex— same deal, different flavor: with-fanalyzeron, the analyzer uses heuristics to try exploring the control flow and data flow in your program, but again, complicated code can defeat them.
By default, the analysis silently stops if the code is too complicated for the analyzer to fully explore and it hits an internal limit. -Wanalyzer-too-complex warns if that occurs 🔥
-Wno-analyzer-allocation-size— this warning requires-fanalyzer, which enables it; to disable it, use-Wno-analyzer-allocation-size.
This diagnostic warns for paths through the code where a pointer to a buffer gets assigned to point at a buffer whose size is not a multiple of sizeof (*pointer) 😭
See CWE-131: Incorrect Calculation of Buffer Size (https://cwe.mitre.org/data/definitions/131.html).
-Wno-analyzer-deref-before-check— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-deref-before-checkto disable it.
This diagnostic warns for paths through the code where a pointer gets checked for NULL after it has already been dereferenced 🙈 suggesting the pointer could have been NULL. Such cases suggest that the check for NULL is either redundant, or that it needs to be moved to before the pointer is dereferenced.
This diagnostic also considers values passed to a function argument marked with __attribute__((nonnull)) as requiring a non-NULL value, and thus will complain if such values are checked for NULL after returning from such a function call.
Also, lowkey: this diagnostic is unlikely to be reported when any level of optimization is enabled, because GCC's optimization logic will typically consider such checks for NULL as redundant and optimize them away before the analyzer even "sees" them. So optimization should be disabled when you're trying to trigger this one 🎀
-Wno-analyzer-double-fclose— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-double-fcloseto disable it.
This diagnostic warns for paths through the code where a FILE * can have fclose called on it more than once 💀
See CWE-1341: Multiple Releases of Same Resource or Handle (https://cwe.mitre.org/data/definitions/1341.html).
-Wno-analyzer-double-free— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-double-freeto disable it.
This diagnostic warns for paths through the code where a pointer can have a deallocator called on it more than once — either free, or a deallocator referenced by attribute malloc 😭
See CWE-415: Double Free (https://cwe.mitre.org/data/definitions/415.html).
-Wno-analyzer-exposure-through-output-file— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-exposure-through-output-fileto disable it.
This diagnostic warns for paths through the code where a security-sensitive value is written to an output file — think writing a password into a log file 🙈 iconic behavior, terrible security.
See CWE-532: Information Exposure Through Log Files (https://cwe.mitre.org/data/definitions/532.html).
-Wanalyzer-exposure-through-uninit-copy— this warning requires both-fanalyzerand the use of a plugin to specify a function that copies across a "trust boundary". Use-Wno-analyzer-exposure-through-uninit-copyto disable it.
This diagnostic warns for "infoleaks" — paths through the code where uninitialized values get copied across a security boundary. Example: code within an OS kernel that copies a partially-initialized struct on the stack to user space 😱
See CWE-200: Exposure of Sensitive Information to an Unauthorized Actor (https://cwe.mitre.org/data/definitions/200.html).
-Wno-analyzer-fd-access-mode-mismatch— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-fd-access-mode-mismatchto disable it.
This diagnostic warns for paths through code where a read on a write-only file descriptor is attempted, or vice versa 🫠
It also warns for code paths where a function with attribute fd_arg_read (N) is called with a file descriptor opened with O_WRONLY at referenced argument N, or a function with attribute fd_arg_write (N) is called with a file descriptor opened with O_RDONLY at referenced argument N.
-Wno-analyzer-fd-double-close— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-fd-double-closeto disable it.
This diagnostic warns for paths through code where a file descriptor can be closed more than once 💀
See CWE-1341: Multiple Releases of Same Resource or Handle (https://cwe.mitre.org/data/definitions/1341.html).
-Wno-analyzer-fd-leak— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-fd-leakto disable it.
This diagnostic warns for paths through code where an open file descriptor is leaked 😭
See CWE-775: Missing Release of File Descriptor or Handle after Effective Lifetime (https://cwe.mitre.org/data/definitions/775.html).
-Wno-analyzer-fd-phase-mismatch— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-fd-phase-mismatchto disable it.
This diagnostic warns for paths through code where an operation is attempted in the wrong phase of a file descriptor's lifetime 🦋 For example, it will warn on attempts to call accept on a stream socket that hasn't had listen successfully called on it yet.
See CWE-666: Operation on Resource in Wrong Phase of Lifetime (https://cwe.mitre.org/data/definitions/666.html).
-Wno-analyzer-fd-type-mismatch— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-fd-type-mismatchto disable it.
This diagnostic warns for paths through code where an operation is attempted on the wrong type of file descriptor 🙈 For example, it will warn on attempts to use socket operations on a file descriptor obtained via open, or when attempting to use a stream socket operation on a datagram socket.
-Wno-analyzer-fd-use-after-close— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-fd-use-after-closeto disable it.
This diagnostic warns for paths through code where a read or write is called on a closed file descriptor 💀
It also warns for paths through code where a function with attribute fd_arg (N) or fd_arg_read (N) or fd_arg_write (N) is called with a closed file descriptor at referenced argument N.
-Wno-analyzer-fd-use-without-check— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-fd-use-without-checkto disable it.
This diagnostic warns for paths through code where a file descriptor is used without being checked for validity 😭
It also warns for paths through code where a function with attribute fd_arg (N) or fd_arg_read (N) or fd_arg_write (N) is called with a file descriptor, at referenced argument N, without being checked for validity.
-Wno-analyzer-file-leak— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-file-leakto disable it.
This diagnostic warns for paths through the code where a <stdio.h> FILE * stream object is leaked 🫠
See CWE-775: Missing Release of File Descriptor or Handle after Effective Lifetime (https://cwe.mitre.org/data/definitions/775.html).
-Wno-analyzer-free-of-non-heap— bestie, this one needs-fanalyzerto even exist, because-fanalyzeris what turns it on in the first place; hit it with-Wno-analyzer-free-of-non-heapif you want it gone.
It flags paths through your code where free gets called on a pointer that was never on the heap — think an on-stack buffer, or a global. Rude. Not the vibe. 💀
See CWE-590: Free of Memory not on the Heap (https://cwe.mitre.org/data/definitions/590.html).
-Wno-analyzer-imprecise-fp-arithmetic— again,-fanalyzerenables it, and-Wno-analyzer-imprecise-fp-arithmeticturns it back off.
This one calls out paths where floating-point arithmetic shows up in places that genuinely need precise computation. And lowkey, right now it only warns about floating-point operands used inside the calculation of an allocation size. That's the whole scope, no cap.
-Wno-analyzer-infinite-loop—-fanalyzerturns it on,-Wno-analyzer-infinite-loopturns it off. You know the drill by now 🫶
This diagnostic warns you about paths through the code that look like they lead to an infinite loop.
Specifically, the analyzer fires this warning when it "sees" a loop where:
- no externally-visible work could be being done within the loop
- there is no way to escape from the loop
- the analyzer is sufficiently confident about the program state throughout the loop to know that the above are true
One way this warning gets emitted is when there's an execution path through a loop where taking that path on one iteration means the same path gets taken on every iteration after it. Iconic behavior, but the bad kind.
For example, consider:
while (1)
{
char opcode = *cpu_state.pc;
switch (opcode)
{
case OPCODE_FOO:
handle_opcode_foo (&cpu_state);
break;
case OPCODE_BAR:
handle_opcode_bar (&cpu_state);
break;
}
}The analyzer will complain about the above because if opcode ever matches none of the cases, the switch falls through to the implicit default case, which makes the body of the loop a total no-op with cpu_state.pc unchanged — so the same value of opcode gets used on all subsequent iterations, and that's an infinite loop, periodt. 😭
See CWE-835: Loop with Unreachable Exit Condition ('Infinite Loop') (https://cwe.mitre.org/data/definitions/835.html).
-Wno-analyzer-infinite-recursion— enabled by-fanalyzer, disabled by-Wno-analyzer-infinite-recursion.
This diagnostic warns about paths through the code that appear to lead to infinite recursion.
So here's how it works: when the analyzer "sees" a recursive call, it compares the state of memory at the entry to the new frame with the state at the entry to the previous frame of that function on the stack. If nothing in memory appears to be changing, you get the warning. Any changes it spots to parameters or globals get assumed to lead to termination of the recursion, which suppresses the warning.
Heads up: this diagnostic is likely to miss cases of infinite recursion that get converted to iteration by the optimizer before the analyzer ever "sees" them. So if you're trying to trigger it, turn optimization off.
Compare with -Winfinite-recursion, which gives you a similar diagnostic but is implemented in a totally different way.
See CWE-674: Uncontrolled Recursion (https://cwe.mitre.org/data/definitions/674.html).
-Wno-analyzer-jump-through-null—-fanalyzerenables it;-Wno-analyzer-jump-through-nulldisables it.
It warns about paths through the code where a NULL function pointer gets called. That's it, that's the whole thing, and honestly it's giving "how did we get here". 🙈
-Wno-analyzer-malloc-leak—-fanalyzerenables it;-Wno-analyzer-malloc-leakdisables it.
This diagnostic warns about paths through the code where a pointer allocated via an allocator gets leaked. And by "an allocator" we mean either malloc, or a function marked with the attribute malloc.
See CWE-401: Missing Release of Memory after Effective Lifetime (https://cwe.mitre.org/data/definitions/401.html).
-Wno-analyzer-mismatching-deallocation—-fanalyzerenables it;-Wno-analyzer-mismatching-deallocationdisables it.
This diagnostic warns about paths through the code where the wrong deallocation function gets called on a pointer value, based on which function was used to allocate that pointer value in the first place. It'll call out mismatches between free, scalar delete and vector delete[], plus anything marked as allocator/deallocator pairs using the attribute malloc.
See CWE-762: Mismatched Memory Management Routines (https://cwe.mitre.org/data/definitions/762.html).
-Wno-analyzer-out-of-bounds—-fanalyzerenables it;-Wno-analyzer-out-of-boundsdisables it.
This diagnostic warns about paths through the code where a buffer is definitely read or written out-of-bounds. It applies to cases where the analyzer can determine a constant offset, and to accesses past the end of a buffer (also with a constant capacity). And on top of that, it does limited checking for accesses past the end when both the offset and the capacity are symbolic.
See CWE-119: Improper Restriction of Operations within the Bounds of a Memory Buffer (https://cwe.mitre.org/data/definitions/119.html).
Where the analyzer is able to, it will emit a text art diagram visualizing the spatial relationship between the memory region it predicts would be accessed, versus the range of memory that's valid to access — whether they overlap, are touching, are close or far apart; which one comes before or after in memory; the relative sizes involved; the direction of the access (read vs write); and, in some cases, the values of the data involved. Absolutely unhinged levels of detail and I'm obsessed. ✨
You can suppress this diagram using -fdiagnostics-text-art-charset=none.
-Wno-analyzer-overlapping-buffers—-fanalyzerenables it;-Wno-analyzer-overlapping-buffersdisables it.
This diagnostic warns about paths through the code where overlapping buffers get passed to an API whose behavior on such buffers is undefined.
Specifically, the diagnostic fires on calls to these functions:
memcpystrcatstrcpy
— for cases where the buffers are known to overlap.
-Wno-analyzer-possible-null-argument—-fanalyzerenables it;-Wno-analyzer-possible-null-argumentdisables it.
This diagnostic warns about paths through the code where a possibly-NULL value gets passed to a function argument that's marked with __attribute__((nonnull)) as requiring a non-NULL value.
See CWE-690: Unchecked Return Value to NULL Pointer Dereference (https://cwe.mitre.org/data/definitions/690.html).
-Wno-analyzer-possible-null-dereference—-fanalyzerenables it;-Wno-analyzer-possible-null-dereferencedisables it.
This diagnostic warns about paths through the code where a possibly-NULL value gets dereferenced. Yeah. 😭
See CWE-690: Unchecked Return Value to NULL Pointer Dereference (https://cwe.mitre.org/data/definitions/690.html).
-Wno-analyzer-null-argument—-fanalyzerenables it;-Wno-analyzer-null-argumentdisables it.
This diagnostic warns about paths through the code where a value known to be NULL gets passed to a function argument marked with __attribute__((nonnull)) as requiring a non-NULL value.
See CWE-476: NULL Pointer Dereference (https://cwe.mitre.org/data/definitions/476.html).
-Wno-analyzer-null-dereference—-fanalyzerenables it;-Wno-analyzer-null-dereferencedisables it.
This diagnostic warns about paths through the code where a value known to be NULL gets dereferenced.
See CWE-476: NULL Pointer Dereference (https://cwe.mitre.org/data/definitions/476.html).
-Wno-analyzer-putenv-of-auto-var—-fanalyzerenables it;-Wno-analyzer-putenv-of-auto-vardisables it.
This diagnostic warns about paths through the code where a call to putenv gets passed a pointer to an automatic variable or an on-stack buffer. That stack frame is not gonna be there forever, bestie. 🫠
See POS34-C. Do not call putenv() with a pointer to an automatic variable as the argument (https://wiki.sei.cmu.edu/confluence/x/6NYxBQ).
-Wno-analyzer-shift-count-negative—-fanalyzerenables it;-Wno-analyzer-shift-count-negativedisables it.
This diagnostic warns about paths through the code where a shift is attempted with a negative count. It's analogous to the -Wshift-count-negative diagnostic implemented in the C/C++ front ends, except this one is implemented by analyzing interprocedural paths rather than just parsing the syntax tree. That said, the analyzer doesn't prioritize detection of these paths, so false negatives are more likely here relative to other warnings. Not the flex it could've been, ngl.
-Wno-analyzer-shift-count-overflow—-fanalyzerenables it;-Wno-analyzer-shift-count-overflowdisables it.
This diagnostic warns about paths through the code where a shift is attempted with a count greater than or equal to the precision of the operand's type. It's analogous to the -Wshift-count-overflow diagnostic implemented in the C/C++ front ends, except again this one is implemented by analyzing interprocedural paths rather than just parsing the syntax tree. And same deal as its sibling: the analyzer doesn't prioritize detection of these paths, so false negatives are more likely relative to other warnings.
-Wno-analyzer-stale-setjmp-buffer—-fanalyzerenables it;-Wno-analyzer-stale-setjmp-bufferdisables it.
This diagnostic warns about paths through the code where longjmp gets called to rewind to a jmp_buf relating to a setjmp call in a function that has already returned. That buffer is stale, babe, let it go. 💅
-Wno-analyzer-use-of-pointer-in-stale-stack-frame(the one we were just talking about, bestie) — whensetjmpgets called on ajmp_bufto record a rewind spot, it records the stack frame. But that stack frame goes invalid the second the function holding thesetjmpcall returns. So trying to rewind back to it withlongjmpwould be pointing at a stack frame that literally does not exist anymore, and that's probably gonna crash you (or, like, worse). 😭
-Wno-analyzer-tainted-allocation-size— this one needs-fanalyzer, which is what turns it on; use-Wno-analyzer-tainted-allocation-sizeto turn it off.
It warns about paths through your code where a value that could be under an attacker's control gets used as the size of an allocation without being sanitized first. Which means an attacker could inject a wildly oversized allocation and potentially cause a denial of service attack. 💀
See CWE-789: Memory Allocation with Excessive Size Value (https://cwe.mitre.org/data/definitions/789.html).
-Wno-analyzer-tainted-assertion— needs-fanalyzer, which enables it; use-Wno-analyzer-tainted-assertionto disable it.
It warns about paths where a value that could be attacker-controlled is used as part of a condition without being sanitized first, and that condition guards a call to a function marked with attribute noreturn (like the function __builtin_unreachable). Those functions usually mean the program is terminating abnormally, like for assertion failure handlers. For example:
assert (some_tainted_value < SOME_LIMIT);In those cases:
- when assertion-checking is enabled: an attacker could trigger a denial of service by injecting an assertion failure
- when assertion-checking is disabled, like by defining
NDEBUG, an attacker could inject data that subverts the process, since it presumably violates a precondition the code is assuming.
Heads up tho: when assertion-checking is disabled, the assertions are typically removed by the preprocessor before the analyzer even gets a chance to "see" them, so this diagnostic can only generate warnings on builds where assertion-checking is enabled.
For this warning's purposes, any function marked with attribute noreturn counts as a possible assertion failure handler, including __builtin_unreachable. And note that these functions sometimes get removed by the optimizer before the analyzer "sees" them. So optimization should be disabled if you're trying to trigger this diagnostic.
See CWE-617: Reachable Assertion (https://cwe.mitre.org/data/definitions/617.html).
It can also report problematic constructions like
switch (some_tainted_value) {
case 0:
/* [...etc; various valid cases omitted...] */
break;
default:
__builtin_unreachable (); /* BUG: attacker can trigger this */
}even though, strictly speaking, that isn't an assertion failure. 🫠
-Wno-analyzer-tainted-array-index— needs-fanalyzer, which enables it; use-Wno-analyzer-tainted-array-indexto disable it.
It warns about paths where a value that could be attacker-controlled gets used as the index of an array access without being sanitized, so an attacker could inject an out-of-bounds access. 🚨
See CWE-129: Improper Validation of Array Index (https://cwe.mitre.org/data/definitions/129.html).
-Wno-analyzer-tainted-divisor— needs-fanalyzer, which enables it; use-Wno-analyzer-tainted-divisorto disable it.
It warns about paths where a value that could be attacker-controlled gets used as the divisor in a division or modulus operation without being sanitized, so an attacker could inject a division-by-zero. 🙈
See CWE-369: Divide By Zero (https://cwe.mitre.org/data/definitions/369.html).
-Wno-analyzer-tainted-offset— needs-fanalyzer, which enables it; use-Wno-analyzer-tainted-offsetto disable it.
It warns about paths where a value that could be attacker-controlled gets used as a pointer offset without being sanitized, so an attacker could inject an out-of-bounds access.
See CWE-823: Use of Out-of-range Pointer Offset (https://cwe.mitre.org/data/definitions/823.html).
-Wno-analyzer-tainted-size— needs-fanalyzer, which enables it; use-Wno-analyzer-tainted-sizeto disable it.
It warns about paths where a value that could be attacker-controlled gets used as the size of an operation like memset without being sanitized, so an attacker could inject an out-of-bounds access.
See CWE-129: Improper Validation of Array Index (https://cwe.mitre.org/data/definitions/129.html).
-Wno-analyzer-undefined-behavior-ptrdiff— needs-fanalyzer, which enables it; use-Wno-analyzer-undefined-behavior-ptrdiffto disable it.
It warns about paths where a pointer subtraction happens between pointers that refer to different chunks of memory. That kind of code is relying on undefined behavior, because pointer subtraction is only defined when both pointers point within (or just after) the same array.
See CWE-469: Use of Pointer Subtraction to Determine Size (https://cwe.mitre.org/data/definitions/469.html).
-Wno-analyzer-undefined-behavior-strtok— needs-fanalyzer, which enables it; use-Wno-analyzer-undefined-behavior-strtokto disable it.
It warns about paths where a call is made to strtok with undefined behavior.
Specifically: passing NULL as the first parameter for the initial call to strtok within a process has undefined behavior. 🫥
-Wno-analyzer-unsafe-call-within-signal-handler— needs-fanalyzer, which enables it; use-Wno-analyzer-unsafe-call-within-signal-handlerto disable it.
It warns about paths where a function known to be async-signal-unsafe (like fprintf) gets called from a signal handler.
See CWE-479: Signal Handler Use of a Non-reentrant Function (https://cwe.mitre.org/data/definitions/479.html).
-Wno-analyzer-use-after-free— needs-fanalyzer, which enables it; use-Wno-analyzer-use-after-freeto disable it.
It warns about paths where a pointer is used after a deallocator got called on it: either free, or a deallocator referenced by attribute malloc.
See CWE-416: Use After Free (https://cwe.mitre.org/data/definitions/416.html).
-Wno-analyzer-use-of-pointer-in-stale-stack-frame— needs-fanalyzer, which enables it; use-Wno-analyzer-use-of-pointer-in-stale-stack-frameto disable it.
It warns about paths where a pointer gets dereferenced that points to a variable in a stale stack frame.
-Wno-analyzer-va-arg-type-mismatch— needs-fanalyzer, which enables it; use-Wno-analyzer-va-arg-type-mismatchto disable it.
It warns about interprocedural paths where the analyzer catches an attempt to use va_arg to pull out a value that was passed to a variadic call, but using a type that doesn't match the type of the expression passed to the call.
See CWE-686: Function Call With Incorrect Argument Type (https://cwe.mitre.org/data/definitions/686.html).
-Wno-analyzer-va-list-exhausted— needs-fanalyzer, which enables it; use-Wno-analyzer-va-list-exhaustedto disable it.
It warns about interprocedural paths where the analyzer catches an attempt to use va_arg to grab the next value passed to a variadic call, but all the values in the va_list have already been consumed.
See CWE-685: Function Call With Incorrect Number of Arguments (https://cwe.mitre.org/data/definitions/685.html).
-Wno-analyzer-va-list-leak— needs-fanalyzer, which enables it; use-Wno-analyzer-va-list-leakto disable it.
It warns about interprocedural paths where the analyzer catches that va_start or va_copy got called on a va_list with no matching call to va_end. 😬
-Wno-analyzer-va-list-use-after-va-end— needs-fanalyzer, which enables it; use-Wno-analyzer-va-list-use-after-va-endto disable it.
It warns about interprocedural paths where the analyzer catches an attempt to use a va_list after va_end has been called on it. va_list.
-Wno-analyzer-write-to-const— needs-fanalyzer, which enables it; use-Wno-analyzer-write-to-constto disable it.
It warns about paths where the analyzer catches an attempt to write through a pointer to a const object. But lowkey the analyzer doesn't prioritize detecting these paths, so false negatives are more likely here compared to other warnings.
-Wno-analyzer-write-to-string-literal— needs-fanalyzer, which enables it; use-Wno-analyzer-write-to-string-literalto disable it.
It warns about paths where the analyzer catches an attempt to write through a pointer to a string literal. But again, the analyzer doesn't prioritize detecting these paths, so false negatives are more likely relative to other warnings.
-Wno-analyzer-use-of-uninitialized-value— needs-fanalyzer, which enables it; use-Wno-analyzer-use-of-uninitialized-valueto disable it.
It warns about paths where an uninitialized value gets used.
See CWE-457: Use of Uninitialized Variable (https://cwe.mitre.org/data/definitions/457.html).
The analyzer has hardcoded knowledge about the behavior of the following memory-management functions:
"alloca"- the built-in functions
"__builtin_alloc","__builtin_alloc_with_align","__builtin_calloc","__builtin_free","__builtin_malloc","__builtin_memcpy","__builtin_memcpy_chk","__builtin_memset","__builtin_memset_chk","__builtin_realloc","__builtin_stack_restore", and"__builtin_stack_save" "calloc""free""malloc""memset""operator delete""operator delete []""operator new""operator new []""realloc""strdup""strndup"
…and then bestie, the ones that play with file descriptors, okay? 💅
"open""close""creat""dup","dup2"and"dup3""isatty""pipe", and"pipe2""read""write""socket","bind","listen","accept", and"connect"
and then the ones that vibe with "<stdio.h>" streams 🎀
- the built-in functions
"__builtin_fprintf","__builtin_fprintf_unlocked","__builtin_fputc","__builtin_fputc_unlocked","__builtin_fputs","__builtin_fputs_unlocked","__builtin_fwrite","__builtin_fwrite_unlocked","__builtin_printf","__builtin_printf_unlocked","__builtin_putc","__builtin_putchar","__builtin_putchar_unlocked","__builtin_putc_unlocked","__builtin_puts","__builtin_puts_unlocked","__builtin_vfprintf", and"__builtin_vprintf" "fopen""fclose""ferror""fgets""fgets_unlocked""fileno""fread""getc""getchar""fprintf""printf""fwrite"
and literally these ones too: ✨
- the built-in functions
"__builtin_expect","__builtin_expect_with_probability","__builtin_strchr","__builtin_strcpy","__builtin_strcpy_chk","__builtin_strlen","__builtin_va_copy", and"__builtin_va_start" - the GNU extensions
"error"and"error_at_line" "getpass""longjmp""putenv""setjmp""siglongjmp""signal""sigsetjmp""strcat""strchr""strlen"
Oh and also — a whole bunch of functions with an "__analyzer_" prefix have special meaning to the analyzer, and those are described over in the GCC Internals manual. iykyk 📖
The parameters that actually control how far the exploration goes are: 🦋
--param analyzer-bb-explosion-factor=value--param analyzer-max-enodes-per-program-point=value--param analyzer-max-recursion-depth=value--param analyzer-min-snodes-for-call-summary=value
The following options control the analyzer. 👑
-fanalyzer-call-summaries— this one simplifies the interprocedural analysis by computing the effect of certain calls, instead of exploring all the paths through the function from the callsite to every possible return. Way less walking, same destination, fr fr.
If it's enabled, call summaries only get used for functions with more than one call site, and that are sufficiently complicated (as per --param analyzer-min-snodes-for-call-summary=value).
-fanalyzer-checker=name— restrict the analyzer so it runs just the named checker, and enable it. One checker, one job. 💅-fanalyzer-debug-text-art-headings— bestie this one is for analyzer developers. If enabled, the analyzer adds extra annotations to any diagrams it generates. Cute little labels for the girlies who build it.-fno-analyzer-feasibility— also for analyzer developers, ngl.
By default the analyzer verifies that there's a feasible control flow path for each diagnostic it emits: that the conditions that hold are not mutually exclusive. Diagnostics where no feasible path can be found get rejected. This filtering can be suppressed with -fno-analyzer-feasibility, for debugging issues in this code.
-fanalyzer-fine-grained— yep, analyzer developers again.
Internally the analyzer builds an "exploded graph" that combines control flow graphs with data flow information.
By default, an edge in this graph can contain the effects of a run of multiple statements within a basic block. With -fanalyzer-fine-grained, each statement gets its own edge. Every single one, no sharing. ✨
-fanalyzer-show-duplicate-count— analyzer developers only: if multiple diagnostics have been detected as being duplicates of each other, it emits a note when reporting the best diagnostic, giving the number of additional diagnostics that were suppressed by the deduplication logic. so it tells you how many it ghosted 💀-fanalyzer-show-events-in-system-headers— by default the analyzer emits simplified diagnostics paths by hiding events fully located within a system header. With-fanalyzer-show-events-in-system-headerssuch events are no longer suppressed. nothing stays hidden 💖-fno-analyzer-state-merge— analyzer developers, this is your one.
By default the analyzer attempts to simplify analysis by merging sufficiently similar states at each program point as it builds its "exploded graph". With -fno-analyzer-state-merge this merging can be suppressed, for debugging state-handling issues.
-fno-analyzer-state-purge— again, for analyzer developers.
By default the analyzer attempts to simplify analysis by purging aspects of state at a program point that appear to no longer be relevant e.g. the values of locals that aren't accessed later in the function and which aren't relevant to leak analysis.
With -fno-analyzer-state-purge this purging of state can be suppressed, for debugging state-handling issues.
-fno-analyzer-suppress-followups— analyzer developers, gather round 🫶
By default the analyzer will stop exploring an execution path after encountering certain diagnostics, in order to avoid potentially issuing a cascade of follow-up diagnostics.
The diagnostics that terminate analysis along a path are:
-Wanalyzer-null-argument-Wanalyzer-null-dereference-Wanalyzer-use-after-free-Wanalyzer-use-of-pointer-in-stale-stack-frame-Wanalyzer-use-of-uninitialized-value
With -fno-analyzer-suppress-followups the analyzer will continue to explore such paths even after such diagnostics, which may be helpful for debugging issues in the analyzer, or for microbenchmarks for detecting undefined behavior. it just keeps going 😭
-fanalyzer-transitivity— this option enables transitivity of constraints within the analyzer. ✨-fno-analyzer-undo-inlining— for analyzer developers, as per usual.
-fanalyzer runs relatively late compared to other code analysis tools, and some optimizations have already been applied to the code. In particular function inlining may have occurred, leading to the interprocedural execution paths emitted by the analyzer containing function frames that don't correspond to those in the original source code.
By default the analyzer attempts to reconstruct the original function frames, and to emit events showing the inlined calls.
With -fno-analyzer-undo-inlining this attempt to reconstruct the original frame information can be disabled, which may be of help when debugging issues in the analyzer.
-fanalyzer-verbose-edges— analyzer developers only. It enables more verbose, lower-level detail in the descriptions of control flow within diagnostic paths. so much detail bestie-fanalyzer-verbose-state-changes— also for analyzer developers. It enables more verbose, lower-level detail in the descriptions of events relating to state machines within diagnostic paths.-fanalyzer-verbosity=level— this one controls the complexity of the control flow paths that are emitted for analyzer diagnostics.
The level can be one of:
- At this level, interprocedural call and return events are displayed, along with the most pertinent state-change events relating to a diagnostic. For example, for a double-
"free"diagnostic, both calls to"free"will be shown. - As per the previous level, but also show events for the entry to each function.
- As per the previous level, but also show events relating to control flow that are significant to triggering the issue (e.g.
"true path taken"at a conditional).
This level is the default. 👑
- As per the previous level, but show all control flow events, not just significant ones.
- This level is intended for analyzer developers; it adds various other events intended for debugging the analyzer.
-fdump-analyzer— dump internal details about what the analyzer is doing tofile.analyzer.txt.-fdump-analyzer-stderroverrides this option.-fdump-analyzer-stderr— dump internal details about what the analyzer is doing to stderr. This option overrides-fdump-analyzer.-fdump-analyzer-callgraph— dump a representation of the call graph suitable for viewing with GraphViz tofile.callgraph.dot.-fdump-analyzer-exploded-graph— dump a representation of the"exploded graph"suitable for viewing with GraphViz tofile.eg.dot. Nodes are color-coded based on state-machine states to emphasize state changes. 🌈-fdump-analyzer-exploded-nodes— emit diagnostics showing where nodes in the"exploded graph"are in relation to the program source.-fdump-analyzer-exploded-nodes-2— dump a textual representation of the"exploded graph"tofile.eg.txt.-fdump-analyzer-exploded-nodes-3— dump a textual representation of the"exploded graph"to one dump file per node, tofile.eg-id.txt. This is typically a large number of dump files. 💀-fdump-analyzer-exploded-paths— dump a textual representation of the"exploded path"for each diagnostic tofile.idx.kind.epath.txt.-fdump-analyzer-feasibility— dump internal details about the analyzer's search for feasible paths. The details are written in a form suitable for viewing with GraphViz to filenames of the formfile.*.fg.dot,file.*.tg.dot, andfile.*.fpath.txt.-fdump-analyzer-infinite-loop— dump internal details about the analyzer's search for infinite loops. The details are written in a form suitable for viewing with GraphViz to filenames of the formfile.*.infinite-loop.dot. 🔁-fdump-analyzer-json— dump a compressed JSON representation of analyzer internals tofile.analyzer.json.gz. The precise format is subject to change. don't get attached to it bestie ✨-fdump-analyzer-state-purge— okay so this one is literally-fdump-analyzer-supergraph's sibling 💫 it dumps a representation of the "supergraph" that's suitable for viewing with GraphViz, BUT it annotates the graph with info about what state is gonna get purged at each node. Where does it go? Intofile.state-purge.dot. iconic.-fdump-analyzer-supergraph— dumps representations of the "supergraph" suitable for viewing with GraphViz tofile.supergraph.dotand tofile.supergraph-eg.dot🦋 These show literally all of the control flow graphs in the program, with interprocedural edges for calls and returns. The second dump has annotations showing nodes in the "exploded graph" plus the diagnostics associated with them. chef's kiss.-fdump-analyzer-untracked— emits custom warnings with internal details, and bestie these are intended for analyzer developers 🧚 so if you're not one, this is not your moment.
✨ Options for Debugging Your Program ✨
So you wanna debug your program? 💅 To tell GCC to emit extra info for use by a debugger, in almost all cases you literally just need to add -g to your other options. And plot twist: some debug formats can co-exist (like DWARF with CTF) when each of them is enabled explicitly by adding the respective command line option to your other options. main character energy.
GCC allows you to use -g with -O 👑 The shortcuts taken by optimized code may occasionally be surprising, and I need you to hear me out on this:
- some variables you declared may not exist at all 😭
- flow of control may briefly move where you did not expect it
- some statements may not be executed because they compute constant results or their values are already at hand
- some statements may execute in different places because they have been moved out of loops
Nevertheless, it is possible to debug optimized output. This makes it reasonable to use the optimizer for programs that might have bugs. fr fr.
If you are not using some other optimization option, consider using -Og with -g 🌸 With no -O option at all, some compiler passes that collect information useful for debugging do not run at all, so that -Og may result in a better debugging experience. iykyk.
-g— produces debugging information in the operating system's native format (stabs, COFF, XCOFF, or DWARF). GDB can work with this debugging information.
On most systems that use stabs format, -g enables use of extra debugging information that only GDB can use 🫶 this extra information makes debugging work better in GDB but probably makes other debuggers crash or refuse to read the program. If you want to control for certain whether to generate the extra information, use -gvms (see below).
-ggdb— produces debugging information for use by GDB. This means to use the most expressive format available (DWARF, stabs, or the native format if neither of those are supported), including GDB extensions if at all possible 💖-gdwarf-gdwarf-version— produces debugging information in DWARF format (if that is supported). The value of version may be either 2, 3, 4 or 5 ⭐ the default version for most targets is 5 — with the exception of VxWorks, TPF and Darwin / macOS, which default to version 2, and AIX, which defaults to version 4.
Note that with DWARF Version 2, some ports require and always use some non-conflicting DWARF 3 extensions in the unwind tables.
Version 4 may require GDB 7.0 and -fvar-tracking-assignments for maximum benefit. Version 5 requires GDB 8.0 or higher.
GCC no longer supports DWARF Version 1 💀 which is substantially different than Version 2 and later. For historical reasons, some other DWARF-related options such as -fno-dwarf2-cfi-asm retain a reference to DWARF Version 2 in their names, but apply to all currently-supported versions of DWARF.
-gbtf— request BTF debug information. BTF is the default debugging format for the eBPF target 🌈 On other targets, like x86, BTF debug information can be generated along with DWARF debug information when both of the debug formats are enabled explicitly via their respective command line options.-gprune-btf-gno-prune-btf— prune BTF information before emission. When pruning, only type information for types used by global variables and file-scope functions will be emitted. If compiling for the BPF target with BPF CO-RE enabled, type information will also be emitted for types used in BPF CO-RE relocations 🎀 In addition, struct and union types which are only referred to via pointers from members of other struct or union types shall be pruned and replaced withBTF_KIND_FWD, as though those types were only present in the input as forward declarations.
This option substantially reduces the size of produced BTF information, but at significant loss in the amount of detailed type information. It is primarily useful when compiling for the BPF target, to minimize the size of the resulting object, and to eliminate BTF information which is not immediately relevant to the BPF program loading process.
This option is enabled by default for the BPF target when generating BTF information.
-gctf-gctflevel— request CTF debug information and use level to specify how much CTF debug information should be produced. If-gctfis specified without a value for level, the default level of CTF debug information is 2 ✨
CTF debug information can be generated along with DWARF debug information when both of the debug formats are enabled explicitly via their respective command line options.
Level 0 produces no CTF debug information at all. Thus, -gctf0 negates -gctf 🙈
Level 1 produces CTF information for tracebacks only. This includes callsite information, but does not include type information.
Level 2 produces type information for entities (functions, data objects etc.) at file-scope or global-scope only.
-gvms— produces debugging information in Alpha/VMS debug format (if that is supported). This is the format used by DEBUG on Alpha/VMS systems 💜-gcodeview— produces debugging information in CodeView debug format (if that is supported). This is the format used by Microsoft Visual C++ on Windows.-glevel-ggdblevel-gvmslevel— request debugging information and also use level to specify how much information. The default level is 2.
Level 0 produces no debug information at all. Thus, -g0 negates -g 💀
Level 1 produces minimal information, enough for making backtraces in parts of the program that you don't plan to debug. This includes descriptions of functions and external variables, and line number tables, but no information about local variables.
Level 3 includes extra information, such as all the macro definitions present in the program. Some debuggers support macro expansion when you use -g3 🌸
If you use multiple -g options, with or without level numbers, the last such option is the one that is effective 👑
-gdwarf does not accept a concatenated debug level, to avoid confusion with -gdwarf-level. Instead use an additional -glevel option to change the debug level for DWARF.
-fno-eliminate-unused-debug-symbols— by default, no debug information is produced for symbols that are not actually used. Use this option if you want debug information for all symbols 💅-femit-class-debug-always— instead of emitting debugging information for a C++ class in only one object file, emit it in all object files using the class. This option should be used only with debuggers that are unable to handle the way GCC normally emits debugging information for classes, because using this option increases the size of debugging information by as much as a factor of two 😭-fno-merge-debug-strings— direct the linker to not merge together strings in the debugging information that are identical in different object files. Merging is not supported by all assemblers or linkers. Merging decreases the size of the debug information in the output file at the cost of increasing link processing time. Merging is enabled by default ⭐-fdebug-prefix-map=old=new— when compiling files residing in directory old, record debugging information describing them as if the files resided in directory new instead 🦋 This can be used to replace a build-time path with an install-time path in the debug info. It can also be used to change an absolute path to a relative path by using.for new. This can give more reproducible builds, which are location independent, but may require an extra command to tell GDB where to find the source files. See also-ffile-prefix-mapand-fcanon-prefix-map.-fvar-tracking— run variable tracking pass. It computes where variables are stored at each position in code. Better debugging information is then generated (if the debugging information format supports this information) ✨
It is enabled by default when compiling with optimization (-Os, -O, -O2, ...), debugging information (-g) and the debug info format supports it.
-fvar-tracking-assignments— annotate assignments to user variables early in the compilation and attempt to carry the annotations over throughout the compilation all the way to the end, in an attempt to improve debug information while optimizing. Use of-gdwarf-4is recommended along with it 💖
It can be enabled even if var-tracking is disabled, in which case annotations are created and maintained, but discarded at the end. By default, this flag is enabled together with -fvar-tracking, except when selective scheduling is enabled.
-gsplit-dwarf— if DWARF debugging information is enabled, separate as much debugging information as possible into a separate output file with the extension.dwo🎀 This option allows the build system to avoid linking files with debug information. To be useful, this option requires a debugger capable of reading.dwofiles.-gdwarf32-gdwarf64— if DWARF debugging information is enabled, the-gdwarf32selects the 32-bit DWARF format and the-gdwarf64selects the 64-bit DWARF format. The default is target specific, on most targets it is-gdwarf32though 👀 The 32-bit DWARF format is smaller, but can't support more than 2GiB of debug information in any of the DWARF debug information sections. The 64-bit DWARF format allows larger debug information and might not be well supported by all consumers yet.-gdescribe-dies— add description attributes to some DWARF DIEs that have no name attribute, such as artificial variables, external references and call site parameter DIEs 🌈-gpubnames— makes DWARF.debug_pubnamesand.debug_pubtypessections. ✨-ggnu-pubnames— makes.debug_pubnamesand.debug_pubtypessections in a format that's suitable for conversion into a GDB index. Heads up bestie: this one is only useful with a linker that can produce GDB index version 7. 💅-fdebug-types-section— when you're using DWARF Version 4 or higher, type DIEs can go live in their own.debug_typessection instead of being shoved into.debug_info. Putting them in a separate comdat section is more efficient, because then the linker can just remove duplicates. 🎀 But plot twist: not all DWARF consumers support.debug_typessections yet, and on some objects.debug_typesproduces larger instead of smaller debugging info. 😭-grecord-gcc-switches-gno-record-gcc-switches— this switch appends the command-line options used to invoke the compiler that may affect code generation to theDW_AT_producerattribute in DWARF debugging information. The options get concatenated with spaces separating them from each other and from the compiler version. It's enabled by default, no cap. Also peep-frecord-gcc-switchesfor another way of storing compiler options into the object file. 👑-gstrict-dwarf— disallows using extensions of later DWARF standard versions than the one you selected with-gdwarf-version. On most targets, using non-conflicting DWARF extensions from later standard versions is allowed (so this is you being strict about it).-gno-strict-dwarf— allows using extensions of later DWARF standard versions than the one selected with-gdwarf-version.-gas-loc-support— tells the compiler that the assembler supports.locdirectives. It may then use them so the assembler generates DWARF2+ line number tables. This is generally desirable, because assembler-generated line-number tables are a lot more compact than the ones the compiler can generate itself. 🌸 This option is enabled by default if, at GCC configure time, the assembler was found to support such directives.-gno-as-loc-support— forces GCC to generate DWARF2+ line number tables internally, if DWARF2+ line number tables are to be generated.-gas-locview-support— tells the compiler that the assembler supports "view" assignment and reset assertion checking in.locdirectives. This option is enabled by default if, at GCC configure time, the assembler was found to support them. ✨-gno-as-locview-support— forces GCC to assign view numbers internally, if-gvariable-location-viewsare explicitly requested.-gcolumn-info-gno-column-info— emits location column information into DWARF debugging information, rather than just file and line. Enabled by default, bestie. 💖-gstatement-frontiers-gno-statement-frontiers— this option makes GCC create markers in the internal representation at the beginning of statements, and keep them roughly in place throughout compilation, using them to guide the output ofis_stmtmarkers in the line number table. It's enabled by default when compiling with optimization (-Os,-O1,-O2, ...), and outputting DWARF 2 debug information at the normal level.-gvariable-location-views-gvariable-location-views=incompat5-gno-variable-location-views— augments variable location lists with progressive view numbers implied from the line number table. This lets debug information consumers inspect state at certain points of the program, even if no instructions associated with the corresponding source locations are present at that point. If the assembler lacks support for view numbers in line number tables, this will cause the compiler to emit the line number table, which generally makes them somewhat less compact. The augmented line number tables and location lists are fully backward-compatible, so they can be consumed by debug information consumers that aren't aware of these augmentations — but they won't derive any benefit from them either. 🦋
This is enabled by default when outputting DWARF 2 debug information at the normal level, as long as there is assembler support, -fvar-tracking-assignments is enabled and -gstrict-dwarf is not. When assembler support is not available, this may still be enabled, but it will force GCC to output internal line number tables, and if -ginternal-reset-location-views is not enabled, that will most certainly lead to silently mismatching location views. 😭
There's a proposed representation for view numbers that is not backward compatible with the location list format introduced in DWARF 5, and it can be enabled with -gvariable-location-views=incompat5. This option may be removed in the future, is only provided as a reference implementation of the proposed representation. Debug information consumers are not expected to support this extended format, and they'd be rendered unable to decode location lists using it. 💀
-ginternal-reset-location-views-gno-internal-reset-location-views— tries to determine location views that can be omitted from location view lists. This requires the compiler to have very accurate insn length estimates, which isn't always the case, and it may cause incorrect view lists to be generated silently when using an assembler that doesn't support location view lists. The GNU assembler will flag any such error as a "view number mismatch". This is only enabled on ports that define a reliable estimation function. 🫶-ginline-points-gno-inline-points— generates extended debug information for inlined functions. Location view tracking markers are inserted at inlined entry points, so that address and view numbers can be computed and output in debug information. This can be enabled independently of location views, in which case the view numbers won't be output — but it can only be enabled along with statement frontiers, and it is only enabled by default if location views are enabled. ✨-gz[=type]— produces compressed debug sections in DWARF format, if that's supported. If type isn't given, the default type depends on the capabilities of the assembler and linker used. type may be one ofnone(don't compress debug sections), orzlib(use zlib compression in ELF gABI format). If the linker doesn't support writing compressed debug sections, the option is rejected. Otherwise, if the assembler doesn't support them,-gzis silently ignored when producing object files. 🙈-femit-struct-debug-baseonly— emits debug information for struct-like types only when the base name of the compilation source file matches the base name of the file in which the struct is defined. This option substantially reduces the size of debugging information, but at significant potential loss in type information to the debugger. Peep-femit-struct-debug-reducedfor a less aggressive option, and-femit-struct-debug-detailedfor more detailed control. This option works only with DWARF debug output. 🎀-femit-struct-debug-reduced— emits debug information for struct-like types only when the base name of the compilation source file matches the base name of the file in which the type is defined, unless the struct is a template or defined in a system header. This option significantly reduces the size of debugging information, with some potential loss in type information to the debugger. Peep-femit-struct-debug-baseonlyfor a more aggressive option, and-femit-struct-debug-detailedfor more detailed control. This option works only with DWARF debug output. 💜-femit-struct-debug-detailed[=spec-list]— specifies the struct-like types for which the compiler generates debug information. The intent is to reduce duplicate struct debug information between different object files within the same program. This option is a detailed version of-femit-struct-debug-reducedand-femit-struct-debug-baseonly, which serves for most needs. 👑
A specification has the syntax:
[dir:|ind:][ord:|gen:](any|sys|base|none)The optional first word limits the specification to structs that are used directly (dir:) or used indirectly (ind:). A struct type is used directly when it is the type of a variable, member. Indirect uses arise through pointers to structs. That is, when use of an incomplete struct is valid, the use is indirect. An example is struct one direct; struct two * indirect;.
The optional second word limits the specification to ordinary structs (ord:) or generic structs (gen:). Generic structs are a bit complicated to explain. For C++, these are non-explicit specializations of template classes, or non-template classes within the above. Other programming languages have generics, but -femit-struct-debug-detailed does not yet implement them.
The third word specifies the source files for those structs for which the compiler should emit debug information. The values none and any have the normal meaning. The value base means that the base of name of the file in which the type declaration appears must match the base of the name of the main compilation file. In practice, this means that when compiling foo.c, debug information is generated for types declared in that file and foo.h, but not other header files. The value sys means those types satisfying base or declared in system or compiler headers.
You may need to experiment to determine the best settings for your application. 🌈
The default is -femit-struct-debug-detailed=all.
This option works only with DWARF debug output.
-fno-dwarf2-cfi-asm— emits DWARF unwind info as a compiler generated.eh_framesection instead of using GAS.cfi_*directives.-fno-eliminate-unused-debug-types— normally, when producing DWARF output, GCC avoids producing debug symbol output for types that are nowhere used in the source file being compiled. Sometimes it's useful to have GCC emit debugging information for all types declared in a compilation unit, regardless of whether or not they're actually used in that compilation unit — for example if, in the debugger, you want to cast a value to a type that isn't actually used in your program (but is declared). More often, however, this results in a significant amount of wasted space. 😭
💅 Options That Control Optimization 💅
These options control various sorts of optimizations.
okay so real talk, bestie 💖 with no optimization option at all, the compiler's whole goal is to reduce the cost of compilation and to make debugging produce the expected results. Statements are independent: if you stop the program with a breakpoint between statements, you can then assign a new value to any variable or change the program counter to any other statement in the function and get exactly the results you expect from the source code. no surprises, no drama, just vibes. ✨
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. it's giving glow-up but your debugger can't recognize you anymore 😭
The compiler performs optimization based on the knowledge it has of the program. Compiling multiple files at once to a single output file mode allows the compiler to use information gained from all of the files when compiling each of them. group project where everyone actually pulls their weight, iconic. 👑
Not all optimizations are controlled directly by a flag. Only optimizations that have a flag are listed in this section.
Most optimizations are completely disabled at-O0or if an-Olevel is not set on the command line, even if individual optimization flags are specified. Similarly,-Ogsuppresses many optimization passes.
Depending on the target and how GCC was configured, a slightly different set of optimizations may be enabled at each -O level than those listed here. You can invoke GCC with -Q --help=optimizers to find out the exact set of optimizations that are enabled at each level. receipts, always. 🧾
-O,-O1— Optimize. Optimizing compilation takes somewhat more time, and a lot more memory for a large function.
With -O, the compiler tries to reduce code size and execution time, without performing any optimizations that take a great deal of compilation time.
-O is the recommended optimization level for large machine-generated code as a sensible balance between time taken to compile and memory use: higher optimization levels perform optimizations with greater algorithmic complexity than at -O.
-O turns on the following optimization flags:
-fauto-inc-dec -fbranch-count-reg -fcombine-stack-adjustments
-fcompare-elim -fcprop-registers -fdce -fdefer-pop -fdelayed-branch
-fdse -fforward-propagate -fguess-branch-probability -fif-conversion
-fif-conversion2 -finline-functions-called-once -fipa-modref
-fipa-profile -fipa-pure-const -fipa-reference
-fipa-reference-addressable -fivopts -fmerge-constants
-fmove-loop-invariants -fmove-loop-stores -fomit-frame-pointer
-freorder-blocks -fshrink-wrap -fshrink-wrap-separate
-fsplit-wide-types -fssa-backprop -fssa-phiopt -ftree-bit-ccp
-ftree-ccp -ftree-ch -ftree-coalesce-vars -ftree-copy-prop
-ftree-dce -ftree-dominator-opts -ftree-dse -ftree-forwprop
-ftree-fre -ftree-phiprop -ftree-pta -ftree-scev-cprop -ftree-sink
-ftree-slsr -ftree-sra -ftree-ter -funit-at-a-time-O2— Optimize even more. GCC performs nearly all supported optimizations that do not involve a space-speed tradeoff. As compared to-O, this option increases both compilation time and the performance of the generated code.
-O2 turns on all optimization flags specified by -O1. It also turns on the following optimization flags:
-falign-functions -falign-jumps -falign-labels -falign-loops
-fcaller-saves -fcode-hoisting -fcrossjumping -fcse-follow-jumps
-fcse-skip-blocks -fdelete-null-pointer-checks -fdevirtualize
-fdevirtualize-speculatively -fexpensive-optimizations
-ffinite-loops -fgcse -fgcse-lm -fhoist-adjacent-loads
-finline-functions -finline-small-functions -findirect-inlining
-fipa-bit-cp -fipa-cp -fipa-icf -fipa-ra -fipa-sra -fipa-vrp
-fisolate-erroneous-paths-dereference -flra-remat -foptimize-crc
-foptimize-sibling-calls -foptimize-strlen -fpartial-inlining
-fpeephole2 -freorder-blocks-algorithm=stc
-freorder-blocks-and-partition -freorder-functions
-frerun-cse-after-loop -fschedule-insns -fschedule-insns2
-fsched-interblock -fsched-spec -fstore-merging -fstrict-aliasing
-fthread-jumps -ftree-builtin-call-dce -ftree-loop-vectorize
-ftree-pre -ftree-slp-vectorize -ftree-switch-conversion
-ftree-tail-merge -ftree-vrp -fvect-cost-model=very-cheapPlease note the warning under-fgcseabout invoking-O2on programs that use computed gotos. not a drill, bestie. ⚠️
-O3— Optimize yet more.-O3turns on all optimizations specified by-O2and also turns on the following optimization flags:
-fgcse-after-reload -fipa-cp-clone -floop-interchange
-floop-unroll-and-jam -fpeel-loops -fpredictive-commoning
-fsplit-loops -fsplit-paths -ftree-loop-distribution
-ftree-partial-pre -funswitch-loops -fvect-cost-model=dynamic
-fversion-loops-for-strides-O0— Reduce compilation time and make debugging produce the expected results. This is the default. 🎀
At -O0, GCC completely disables most optimization passes; they are not run even if you explicitly enable them on the command line, or are listed by -Q --help=optimizers as being enabled by default. Many optimizations performed by GCC depend on code analysis or canonicalization passes that are enabled by -O, and it would not be useful to run individual optimization passes in isolation. like, they need the whole squad or nobody shows up. 💅
-Os— Optimize for size.-Osenables all-O2optimizations except those that often increase code size:
-falign-functions -falign-jumps -falign-labels -falign-loops
-fprefetch-loop-arrays -freorder-blocks-algorithm=stcIt also enables -finline-functions, causes the compiler to tune for code size rather than execution speed, and performs further optimizations designed to reduce code size. tiny but mighty, we love her. 🦋
-Ofast— Disregard strict standards compliance.-Ofastenables all-O3optimizations. It also enables optimizations that are not valid for all standard-compliant programs. It turns on-ffast-math,-fallow-store-data-racesand the Fortran-specific-fstack-arrays, unless-fmax-stack-var-sizeis specified, and-fno-protect-parens. It turns off-fsemantic-interposition. rules? never heard of her. 😈-Og— Optimize debugging experience.-Ogshould be the optimization level of choice for the standard edit-compile-debug cycle, offering a reasonable level of optimization while maintaining fast compilation and a good debugging experience. It is a better choice than-O0for producing debuggable code because some compiler passes that collect debug information are disabled at-O0.
Like -O0, -Og completely disables a number of optimization passes so that individual options controlling them have no effect. Otherwise -Og enables all -O1 optimization flags except for those that may interfere with debugging:
-fbranch-count-reg -fdelayed-branch -fdse -fif-conversion
-fif-conversion2 -finline-functions-called-once
-fmove-loop-invariants -fmove-loop-stores -fssa-phiopt
-ftree-bit-ccp -ftree-dse -ftree-pta -ftree-sra-Oz— Optimize aggressively for size rather than speed. This may increase the number of instructions executed if those instructions require fewer bytes to encode.-Ozbehaves similarly to-Osincluding enabling most-O2optimizations.
If you use multiple -O options, with or without level numbers, the last such option is the one that is effective. last one wins, periodt. 👑
Options of the form -fflag specify machine-independent flags. Most flags have both positive and negative forms; the negative form of -ffoo is -fno-foo. In the table below, only one of the forms is listed---the one you typically use. You can figure out the other form by either removing no- or adding it. ✨
The following options control specific optimizations. They are either activated by -O options or are related to ones that are. You can use the following flags in the rare cases when "fine-tuning" of optimizations to be performed is desired.
-fno-defer-pop— For machines that must pop arguments after a function call, always pop the arguments as soon as each function returns. At levels-O1and higher,-fdefer-popis the default; this allows the compiler to let arguments accumulate on the stack for several function calls and pop them all at once. stack of plates, cleared in one trip. 🍽️-fforward-propagate— Perform a forward propagation pass on RTL. The pass tries to combine two instructions and checks if the result can be simplified. If loop unrolling is active, two passes are performed and the second is scheduled after loop unrolling.
This option is enabled by default at optimization levels -O1, -O2, -O3, -Os.
-favoid-store-forwarding,-fno-avoid-store-forwarding— Many CPUs will stall for many cycles when a load partially depends on previous smaller stores. This pass tries to detect such cases and avoid the penalty by changing the order of the load and store and then fixing up the loaded value.
Disabled by default.
-ffp-contract=style—-ffp-contract=offdisables floating-point expression contraction.-ffp-contract=fastenables floating-point expression contraction such as forming of fused multiply-add operations if the target has native support for them.-ffp-contract=onenables floating-point expression contraction if allowed by the language standard. This is implemented for C and C++, where it enables contraction within one expression, but not across different statements.
The default is -ffp-contract=off for C in a standards compliant mode (-std=c11 or similar), -ffp-contract=fast otherwise.
-fomit-frame-pointer— Omit the frame pointer in functions that don't need one. This avoids the instructions to save, set up and restore the frame pointer; on many targets it also makes an extra register available. free real estate. 🎁
On some targets this flag has no effect because the standard calling sequence always uses a frame pointer, so it cannot be omitted.
Note that -fno-omit-frame-pointer doesn't guarantee the frame pointer is used in all functions. Several targets always omit the frame pointer in leaf functions.
Enabled by default at -O1 and higher.
-foptimize-crc— Detect loops calculating CRC (performing polynomial long division) and replace them with a faster implementation. Detect 8, 16, 32, and 64 bit CRC, with a constant polynomial without the leading 1 bit, for both bit-forward and bit-reversed cases. If the target supports a CRC instruction and the polynomial used in the source code matches the polynomial used in the CRC instruction, generate that CRC instruction. Otherwise, if the target supports a carry-less-multiplication instruction, generate CRC using it; otherwise generate table-based CRC. glow-up for your checksums. 🌈
Enabled by default at -O2 and higher.
-foptimize-sibling-calls— bestie this one optimizes sibling and tail recursive calls, it's giving ✨recursion glow-up✨
Turned on at levels -O2, -O3, -Os.
-foptimize-strlen— optimizes a bunch of standard C string functions (likestrlen,strchrorstrcpy) plus their_FORTIFY_SOURCEtwins into faster alternatives. no cap, they get a whole makeover 💅
Turned on at levels -O2, -O3.
-finline-stringops[=fn]— expands memory and string operations inline (for now, onlymemset, okay bestie we're being honest about the limits), even when the length is variable or big enough that it'd require looping. This is most useful alongside-ffreestandingand-fno-builtin🫶
In some circumstances it lets the compiler generate code that takes advantage of known alignment and length multipliers, but even then it may be less efficient than optimized runtime implementations, and it can grow code size so much that even a less performant but shared implementation runs faster thanks to better use of code caches. This option is disabled by default.
-fno-inline— do not expand any functions inline apart from those marked with thealways_inlineattribute. This is the default when you're not optimizing.
Single functions can be exempted from inlining by marking them with the noinline attribute.
-finline-small-functions— integrates functions into their callers when their body is smaller than the expected function call code (so the overall size of the program gets smaller, we love a size win 💖). The compiler heuristically decides which functions are simple enough to be worth integrating this way. This inlining applies to all functions, even those not declared inline.
Turned on at levels -O2, -O3, -Os.
-findirect-inlining— inline also indirect calls that are discovered to be known at compile time thanks to previous inlining. This option only does anything when inlining itself is turned on by the-finline-functionsor-finline-small-functionsoptions.
Turned on at levels -O2, -O3, -Os.
-finline-functions— consider all functions for inlining, even if they aren't declared inline. The compiler heuristically decides which functions are worth integrating 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.
Turned on at levels -O2, -O3, -Os. Also enabled by -fprofile-use and -fauto-profile.
-finline-functions-called-once— consider allstaticfunctions called once for inlining into their caller even if they're not markedinline. If a call to a given function is integrated, then the function is not output as assembler code in its own right.
Turned on at levels -O1, -O2, -O3 and -Os, but not -Og.
-fearly-inlining— inlines functions marked byalways_inlineand functions whose body seems smaller than the function call overhead early, before doing-fprofile-generateinstrumentation and the real inlining pass. Doing so makes profiling significantly cheaper and usually inlining faster on programs having large chains of nested wrapper functions. iconic behavior honestly 🌸
Enabled by default.
-fipa-sra— perform interprocedural scalar replacement of aggregates, removal of unused parameters, and replacement of parameters passed by reference by parameters passed by value.
Turned on at levels -O2, -O3 and -Os.
-finline-limit=n— by default, GCC limits the size of functions that can be inlined. This flag allows coarse control of this limit. n is the size of functions that can be inlined in number of pseudo instructions.
Inlining is actually controlled by a number of parameters, which may be specified individually by using --param name=value. The -finline-limit=n option sets some of these parameters as follows:
max-inline-insns-single— is set ton/2.max-inline-insns-auto— is set ton/2.
See below for documentation of the individual parameters controlling inlining and for the defaults of these parameters.
Note: there may be no value to -finline-limit that results in default behavior.
Note: pseudo instruction represents, in this particular context, an abstract measurement of a 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 another. 💀 don't @ me about it
-fno-keep-inline-dllexport— this is a more fine-grained version of-fkeep-inline-functions, which applies only to functions that are declared using thedllexportattribute or declspec.-fkeep-inline-functions— in C, emitstaticfunctions that are declaredinlineinto the object file, even if the function has been inlined into all of its callers. This switch does not affect functions using theextern inlineextension in GNU C90. In C++, emit any and all inline functions into the object file.-fkeep-static-functions— emitstaticfunctions into the object file, even if the function is never used. she's keeping them anyway, sentimental queen 😭-fkeep-static-consts— emit variables declaredstatic constwhen optimization isn't turned on, even if the variables aren't referenced.
GCC enables this option by default. If you want to force the compiler to check whether a variable is referenced, regardless of whether or not optimization is turned on, use the -fno-keep-static-consts option.
-fmerge-constants— attempt to merge identical constants (string constants and floating-point constants) across compilation units.
This option is the default for optimized compilation if the assembler and linker support it. Use -fno-merge-constants to inhibit this behavior.
Turned on at levels -O1, -O2, -O3, -Os.
-fmerge-all-constants— attempt to merge identical constants and identical variables.
This option implies -fmerge-constants. In addition to -fmerge-constants, this considers e.g. even constant initialized arrays or initialized constant variables with integral or floating-point types. Languages like C or C++ require each variable, including multiple instances of the same variable in recursive calls, to have distinct locations, so using this option results in non-conforming behavior. ⚠️ bestie that's a real warning, not a vibe, it literally breaks the rules of the language
-fmodulo-sched— perform swing modulo scheduling immediately before the first scheduling pass. This pass looks at innermost loops and reorders their instructions by overlapping different iterations.-fmodulo-sched-allow-regmoves— perform more aggressive SMS-based modulo scheduling with register moves allowed. By setting this flag, certain anti-dependence edges are deleted, which triggers the generation of reg-moves based on the life-range analysis. This option is effective only with-fmodulo-schedenabled.-fno-branch-count-reg— disable the optimization pass that scans for opportunities to usedecrement and branchinstructions on a count register instead of instruction sequences that decrement a register, compare it against zero, and 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. Note that the-fno-branch-count-regoption doesn't remove the decrement and branch instructions from the generated instruction stream introduced by other optimization passes.
The default is -fbranch-count-reg at -O1 and higher, except for -Og.
-fno-function-cse— do not put function addresses in registers; make each instruction that calls a constant function contain the function's address explicitly.
This option results in less efficient code, but some strange hacks that alter the assembler output may be confused by the optimizations performed when this option is not used. 🙈 weird little hacks, we still love you
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 that 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 except in Ada.
-fthread-jumps— perform optimizations that check to see if a jump branches to a location where another comparison subsumed by the first is found. If so, the first branch is redirected to either the destination of the second branch or a point immediately following it, depending on whether the condition is known to be true or false.
Turned on at levels -O1, -O2, -O3, -Os.
-fsplit-wide-types— when using a type that occupies multiple registers, such aslong longon a 32-bit system, split the registers apart and allocate them independently. This normally generates better code for those types, but may make debugging more difficult. 😭 a little trade-off, that's the tea
Turned on at levels -O1, -O2, -O3, -Os.
-fsplit-wide-types-early— fully split wide types early, instead of very late. This option has no effect unless-fsplit-wide-typesis turned on.
This is the default on some targets.
-fcse-follow-jumps— in common subexpression elimination (CSE), scan through jump instructions when the target of the jump is not reached by any other path. For example, when CSE encounters anifstatement with anelseclause, CSE follows the jump when the condition tested is false.
Turned on at levels -O2, -O3, -Os.
-fcse-skip-blocks— this is similar to-fcse-follow-jumps, but causes CSE to follow jumps that conditionally skip over blocks. When CSE encounters a simpleifstatement with no else clause,-fcse-skip-blockscauses CSE to follow the jump around the body of theif.
Turned on at levels -O2, -O3, -Os.
-frerun-cse-after-loop— re-run common subexpression elimination after loop optimizations are performed. round two bestie, she's back for more 💅
Turned on at levels -O2, -O3, -Os.
-fgcse— perform a global common subexpression elimination pass. This pass also performs global constant and copy propagation.
Note: when compiling a program using computed gotos, a GCC extension, you may get better run-time performance if you disable the global common subexpression elimination pass by adding -fno-gcse to the command line.
Turned on at levels -O2, -O3, -Os.
-fgcse-lm— okay so when this one's on, global common subexpression elimination is gonna try to move loads that are only ever killed by stores into themselves ✨. The way it works: a loop that's got a load/store sequence in it can get rewritten so the load happens outside the loop and the loop just does a copy/store 💅. Enabled by default whenever-fgcseis enabled, periodt.-fgcse-sm— turn this on and a store motion pass runs after global common subexpression elimination 🎀. This pass is out here trying to move stores out of loops. And when you pair it with-fgcse-lm? Chef's kiss — loops with a load/store sequence can be changed so the load happens before the loop and the store happens after it 🔥. Not enabled at any optimization level, fr fr.-fgcse-las— when-fgcse-lasis enabled, the global common subexpression elimination pass goes and eliminates redundant loads that come after stores to the same memory location 🧚 — and yes bestie, that's both partial and full redundancies. Not enabled at any optimization level. Nope, not even one.-fgcse-after-reload— with this on, a redundant load elimination pass gets performed after reload 💫. Its whole job? Cleaning up redundant spilling. Enabled by-O3,-fprofile-useand-fauto-profile. She's booked and busy.-faggressive-loop-optimizations— this option tells the loop optimizer to use language constraints to derive bounds for the number of iterations of a loop 👑. It's assuming your loop code does not invoke undefined behavior — like, no signed integer overflows, no out-of-bound array accesses 🙈. Those derived bounds then get used to guide loop unrolling and peeling and loop exit test optimizations. Enabled by default. Yes, by default, no cap.-funconstrained-commons— this one tells the compiler that variables declared in common blocks (e.g. Fortran) might later be overridden with longer trailing arrays 😭. Which prevents certain optimizations that depend on knowing the array bounds. F in the chat for those optimizations.-fcrossjumping— perform cross-jumping transformation 🦋. This transformation unifies equivalent code and saves code size. Fair warning though: the resulting code may or may not actually perform better than it did without cross-jumping 💀. Enabled at levels-O2,-O3,-Os.-fauto-inc-dec— combine increments or decrements of addresses with memory accesses ✨. This pass is always skipped on architectures that don't have instructions to support it. Enabled by default at-O1and higher on architectures that do support it 💅.-fdce— perform dead code elimination (DCE) on RTL. Enabled by default at-O1and higher. Short and sweet, iconic behavior.-fdse— perform dead store elimination (DSE) on RTL. Enabled by default at-O1and higher. Matching set with-fdce, we love to see it 🫶.-fif-conversion— attempt to transform conditional jumps into branch-less equivalents 🌈. This includes use of conditional moves, min, max, set flags and abs instructions, and some tricks doable by standard arithmetics. The use of conditional execution on chips where it's available is controlled by-fif-conversion2. Enabled at levels-O1,-O2,-O3,-Os, but not with-Og. Noted bestie.-fif-conversion2— use conditional execution (where available) to transform conditional jumps into branch-less equivalents ⭐. Enabled at levels-O1,-O2,-O3,-Os, but not with-Og. Same energy, different pass.-fdeclone-ctor-dtor— so the C++ ABI requires multiple entry points for constructors and destructors 😳: one for a base subobject, one for a complete object, and one for a virtual destructor that callsoperator deleteafterwards. For a hierarchy with virtual bases, the base and complete variants are clones, which means two copies of the function. With this option, the base and complete variants get changed to be thunks that call a common implementation 💅. Enabled by-Os.-fdelete-null-pointer-checks— assume that programs cannot safely dereference null pointers, and that no code or data element resides at address zero 💖. This option enables simple constant folding optimizations at all optimization levels. Plus, other optimization passes in GCC use this flag to control global dataflow analyses that eliminate useless checks for null pointers — these assume that a memory access to address zero always results in a trap, so that if a pointer is checked after it's already been dereferenced, it cannot be null 🧠.
Okay so heads up bestie: in some environments this assumption is just not true 😭. Use -fno-delete-null-pointer-checks to disable this optimization for programs that depend on that behavior.
This option is enabled by default on most targets. On AVR and MSP430, this option is completely disabled 💀. Passes that use the dataflow information are enabled independently at different optimization levels.
-fdevirtualize— attempt to convert calls to virtual functions to direct calls 🔥. This happens both within a procedure and interprocedurally, as part of indirect inlining (-findirect-inlining) and interprocedural constant propagation (-fipa-cp). Enabled at levels-O2,-O3,-Os.-fdevirtualize-speculatively— attempt to convert calls to virtual functions to speculative direct calls 👑. Based on the analysis of the type inheritance graph, it determines for a given call the set of likely targets. If the set is small — preferably of size 1 — it changes the call into a conditional deciding between direct and indirect calls 🦋. These speculative calls enable more optimizations, like inlining. And when they seem useless after further optimization, they get converted back into their original form. Iconic glow-up and a graceful exit.-fdevirtualize-at-ltrans— stream extra information needed for aggressive devirtualization when running the link-time optimizer in local transformation mode 🌸. This option enables more devirtualization but significantly increases the size of streamed data. Which is exactly why it's disabled by default 💅.-fexpensive-optimizations— perform a number of minor optimizations that are relatively expensive 💸. Enabled at levels-O2,-O3,-Os.-free— attempt to remove redundant extension instructions ✨. This is especially helpful for the x86-64 architecture, which implicitly zero-extends in 64-bit registers after writing to their lower 32-bit half 🧠. Enabled for Alpha, AArch64, LoongArch, PowerPC, RISC-V, SPARC, h83000 and x86 at levels-O2,-O3,-Os. Yes she named every single one, we don't cut corners here.-fno-lifetime-dse— in C++ the value of an object is only affected by changes within its lifetime 😳: when the constructor begins, the object has an indeterminate value, and any changes during the lifetime of the object are dead when the object is destroyed. Normally dead store elimination will take advantage of this. But if your code relies on the value of the object storage persisting beyond the lifetime of the object, you can use this flag to disable this optimization 🫶. To preserve stores before the constructor starts (e.g. because youroperator newclears the object storage) but still treat the object as dead after the destructor, you can use-flifetime-dse=1. The default behavior can be explicitly selected with-flifetime-dse=2. And-flifetime-dse=0is equivalent to-fno-lifetime-dse💖.-flive-range-shrinkage— attempt to decrease register pressure through register live range shrinkage 🌈. This is helpful for fast processors with small or moderate size register sets. Tiny register file? She's got you.- *
-fira-algorithm=algorithm* — use the specified coloring algorithm for the integrated register allocator 🎨. Thealgorithmargument can bepriority, which specifies Chow's priority coloring, orCB, which specifies Chaitin-Briggs coloring. Chaitin-Briggs coloring is not implemented for all architectures, but for those targets that do support it, it's the default because it generates better code 💅. - *
-fira-region=region* — use specified regions for the integrated register allocator. Theregionargument should be one of the following:all— use all loops as register allocation regions. This can give the best results for machines with a small and/or irregular register set ⭐.mixed— use all loops except for loops with small register pressure as the regions. This value usually gives the best results in most cases and for most architectures, and is enabled by default when compiling with optimization for speed (-O,-O2, ...) 🔥.one— use all functions as a single region. This typically results in the smallest code size, and is enabled by default for-Osor-O0💖.
-fira-hoist-pressure— use IRA to evaluate register pressure in the code hoisting pass for decisions to hoist expressions 🌸. This option usually results in smaller code, but it can slow the compiler down 😭. Enabled at level-Osfor all targets.-fira-loop-pressure— use IRA to evaluate register pressure in loops for decisions to move loop invariants 👑. This option usually results in generation of faster and smaller code on machines with large register files (>= 32 registers), but it can slow the compiler down 🐢. Enabled at level-O3for some targets.-fno-ira-share-save-slots— disable sharing of stack slots used for saving call-used hard registers living through a call 💀. Each hard register gets a separate stack slot, and as a result function stack frames are larger. Bigger frames, bestie, that's the trade.-fno-ira-share-spill-slots— disable sharing of stack slots allocated for pseudo-registers 🙈. Each pseudo-register that does not get a hard register gets a separate stack slot, and as a result function stack frames are larger. Same energy as the one above, different victims.-flra-remat— enable CFG-sensitive rematerialization in LRA ✨. Instead of loading values of spilled pseudos, LRA tries to rematerialize (recalculate) values if it's profitable. Enabled at levels-O2,-O3,-Os.-fdelayed-branch— if supported for the target machine, attempt to reorder instructions to exploit instruction slots available after delayed branch instructions 🦋. Enabled at levels-O1,-O2,-O3,-Os, but not at-Og.-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 actually required. She's filling the gaps so your CPU doesn't just sit there 💅.-fschedule-insns2— okay so this is the sequel ✨ like-fschedule-insnsbut it asks for one more extra pass of instruction scheduling after register allocation already went down. It's giving super useful on machines with a kinda small number of registers and where memory load instructions take more than one cycle 🫶
Slay at levels -O2, -O3, -Os.
-fno-sched-interblock— turns OFF instruction scheduling across basic blocks 💅 that stuff is normally on when you're scheduling before register allocation, i.e. with-fschedule-insnsor at-O2or higher.-fno-sched-spec— disables speculative motion of non-load instructions, which is normally enabled when scheduling before register allocation, i.e. with-fschedule-insnsor at-O2or higher 🙈-fsched-pressure— enables register pressure sensitive insn scheduling before register allocation 💖 this only makes sense when scheduling before register allocation is turned on, i.e. with-fschedule-insnsor at-O2or higher. Using this can genuinely improve the generated code and decrease its size by preventing register pressure from going above the number of available hard registers and the spills that follow in register allocation. no cap 🧚-fsched-spec-load— allow speculative motion of some load instructions 🌸 again, only makes sense when scheduling before register allocation, i.e. with-fschedule-insnsor at-O2or higher.-fsched-spec-load-dangerous— allow speculative motion of more load instructions 💀 (the name said what it said). Still only makes sense when scheduling before register allocation, i.e. with-fschedule-insnsor at-O2or 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 👑 so:-fno-sched-stalled-insnsmeans no insns get moved prematurely,-fsched-stalled-insns=0means there's no limit on how many queued insns can be moved prematurely, and-fsched-stalled-insnswithout a value is equivalent to-fsched-stalled-insns=1💅-fsched-stalled-insns-dep-fsched-stalled-insns-dep=n— define how many insn groups (cycles) get examined for a dependency on a stalled insn that's a candidate for premature removal from the queue of stalled insns 🔥 this only has an effect during the second scheduling pass, and only if-fsched-stalled-insnsis used.-fno-sched-stalled-insns-depis equivalent to-fsched-stalled-insns-dep=0, and-fsched-stalled-insns-depwithout a value is equivalent to-fsched-stalled-insns-dep=1✨-fsched2-use-superblocks— when scheduling after register allocation, use superblock scheduling 🌈 this allows motion across basic block boundaries, resulting in faster schedules. But bestie, heads up: this option is experimental 😭 not all machine descriptions used by GCC model the CPU closely enough to avoid unreliable results from the algorithm.
This only makes sense when scheduling after register allocation, i.e. with -fschedule-insns2 or at -O2 or higher.
-fsched-group-heuristic— enable the group heuristic in the scheduler 💖 this heuristic favors the instruction that belongs to a schedule group. On by default when scheduling is enabled, i.e. with-fschedule-insnsor-fschedule-insns2or at-O2or higher.-fsched-critical-path-heuristic— enable the critical-path heuristic in the scheduler 🎀 favors instructions on the critical path. On by default when scheduling is enabled, i.e. with-fschedule-insnsor-fschedule-insns2or at-O2or higher.-fsched-spec-insn-heuristic— enable the speculative instruction heuristic in the scheduler 🦋 favors speculative instructions with greater dependency weakness. On by default when scheduling is enabled, i.e. with-fschedule-insnsor-fschedule-insns2or at-O2or higher.-fsched-rank-heuristic— enable the rank heuristic in the scheduler ⭐ favors the instruction belonging to a basic block with greater size or frequency. On by default when scheduling is enabled, i.e. with-fschedule-insnsor-fschedule-insns2or at-O2or higher.-fsched-last-insn-heuristic— enable the last-instruction heuristic in the scheduler 💜 favors the instruction that's less dependent on the last instruction scheduled. On by default when scheduling is enabled, i.e. with-fschedule-insnsor-fschedule-insns2or at-O2or higher.-fsched-dep-count-heuristic— enable the dependent-count heuristic in the scheduler 🌸 favors the instruction that has more instructions depending on it. On by default when scheduling is enabled, i.e. with-fschedule-insnsor-fschedule-insns2or at-O2or higher.-freschedule-modulo-scheduled-loops— modulo scheduling happens before traditional scheduling 💫 so if a loop is modulo scheduled, later scheduling passes may change its schedule. Use this option to control that behavior.-fselective-scheduling— schedule instructions using the selective scheduling algorithm 🔥 selective scheduling runs instead of the first scheduler pass.-fselective-scheduling2— schedule instructions using the selective scheduling algorithm again, but this time it runs instead of the second scheduler pass 👑-fsel-sched-pipelining— enable software pipelining of innermost loops during selective scheduling 🎀 this option has no effect unless one of-fselective-schedulingor-fselective-scheduling2is turned on. iykyk.-fsel-sched-pipelining-outer-loops— when pipelining loops during selective scheduling, also pipeline outer loops 🌈 this option has no effect unless-fsel-sched-pipeliningis turned on.-fsemantic-interposition— okay so some object formats, like ELF, allow interposing of symbols by the dynamic linker 💅 that means for symbols exported from the DSO, the compiler can't do interprocedural propagation, inlining and other optimizations, because it has to anticipate that the function or variable in question may change. While this feature is useful — for example, to rewrite memory allocation functions by a debugging implementation — it's expensive in terms of code quality 😭
With -fno-semantic-interposition the compiler assumes that if interposition happens for functions, the overwriting function will have precisely the same semantics (and side effects). Similarly, if interposition happens for variables, the constructor of the variable will be the same 💖
The flag has no effect for functions explicitly declared inline (where it's never allowed for interposition to change semantics) and for symbols explicitly declared weak 🫶
-fshrink-wrap— emit function prologues only before the parts of the function that actually need it, rather than at the top of the function ✨ this flag is enabled by default at-Oand higher.-fshrink-wrap-separate— shrink-wrap separate parts of the prologue and epilogue separately, so those parts are only executed when needed 🦋 on by default, but has no effect unless-fshrink-wrapis also turned on and the target supports this.-fcaller-saves— enable allocation of values to registers that are clobbered by function calls, by emitting extra instructions to save and restore the registers around such calls 💖 such allocation is done only when it seems to result in better code.
This option is always enabled by default on certain machines, usually those which have no call-preserved registers to use instead 👑
Slay at levels -O2, -O3, -Os.
-fcombine-stack-adjustments— tracks stack adjustments (pushes and pops) and stack memory references, and then tries to find ways to combine them 🌸 enabled by default at-O1and higher.-fipa-ra— use caller save registers for allocation if those registers aren't used by any called function 🎀 in that case it's not necessary to save and restore them around calls. This is only possible if the called functions are part of the same compilation unit as the current function and they're compiled before it.
Enabled at levels -O2, -O3, -Os, however the option is disabled if generated code will be instrumented for profiling (-p, or -pg) or if the callee's register usage can't be known exactly (this happens on targets that don't expose prologues and epilogues in RTL) 😭
-fconserve-stack— attempt to minimize stack usage 💜 the compiler tries to use less stack space, even if that makes the program slower. This option implies setting thelarge-stack-frameparameter to100and thelarge-stack-frame-growthparameter to400.-ftree-reassoc— perform reassociation on trees 🌈 this flag is enabled by default at-O1and higher.-fcode-hoisting— perform code hoisting ✨ code hoisting tries to move the evaluation of expressions executed on all paths to the function exit as early as possible. This is especially useful as a code size optimization, but it often helps for code speed as well. Enabled by default at-O2and higher.-ftree-pre— perform partial redundancy elimination (PRE) on trees 💅 this flag is enabled by default at-O2and-O3.-ftree-partial-pre— make partial redundancy elimination (PRE) more aggressive 🔥 this flag is enabled by default at-O3.-ftree-forwprop— perform forward propagation on trees 🦋 this flag is enabled by default at-O1and higher.-ftree-fre— perform full redundancy elimination (FRE) on trees ⭐ the difference between FRE and PRE is that FRE only considers expressions that are computed on all paths leading to the redundant computation. This analysis is faster than PRE, though it exposes fewer redundancies. Enabled by default at-O1and higher.-ftree-phiprop— perform hoisting of loads from conditional pointers on trees 🎀 this pass is enabled by default at-O1and higher.-fhoist-adjacent-loads— bestie, this one speculatively hoists loads out of both branches of an if-then-else 🎀 but ONLY when those loads are from adjacent locations in the same structure and the target architecture actually has a conditional move instruction. It's on by default at-O2and higher. manifesting efficiency ✨-ftree-copy-prop— does copy propagation on trees. This pass yeets the unnecessary copy operations, periodt. Enabled by default at-O1and higher. 💅-fipa-pure-const— finds out which functions are pure or constant. Enabled by default at-O1and higher. it's giving detective work 🔍-fipa-reference— discovers which static variables do not escape the compilation unit. Enabled by default at-O1and higher.-fipa-reference-addressable— discovers read-only, write-only and non-addressable static variables. Enabled by default at-O1and higher. so thorough of her 💜-fipa-reorder-for-locality— groups call chains close together in the binary layout so the code locality is better and the jump distances between frequently called functions get minimized. Now, unlike-freorder-functions, this pass actually considers the call chains between functions and groups them together, rather than sorting all the hot/normal/cold/never-executed functions into separate sections. And unlike-fprofile-reorder-functions, it's aiming to improve code locality throughout the whole runtime of the program, not just focusing on program startup. okay range 👑
Heads up though: this option is incompatible with an explicit -flto-partition= option, because it enforces a custom partitioning scheme. If you're using it, it's recommended to also use profile feedback — but this option is not enabled by default otherwise.
-fipa-stack-alignment— reduces stack alignment on call sites if possible. Enabled by default. ✨-fipa-pta— performs interprocedural pointer analysis and interprocedural modification and reference analysis. Warning bestie: this option can cause excessive memory and compile-time usage on large compilation units 😭. It is not enabled by default at any optimization level.-fipa-profile— performs interprocedural profile propagation. Functions that are only called from cold functions get marked as cold. Also, functions that are executed once — likecold,noreturn, static constructors or destructors — get identified. Then cold functions and the loop-less parts of functions executed once get optimized for size. Enabled by default at-O1and higher. 🧊-fipa-modref— performs interprocedural mod/ref analysis. This optimization analyzes the side effects of functions (aka the memory locations that get modified or referenced) and enables better optimization across the function call boundary. Enabled by default at-O1and higher. 🌸-fipa-cp— performs interprocedural constant propagation. It analyzes the program to figure out when values passed to functions are constants, then optimizes accordingly. This can substantially increase performance if the application has constants passed to functions. Enabled by default at-O2,-Osand-O3. It's also enabled by-fprofile-useand-fauto-profile.-fipa-cp-clone— performs function cloning to make interprocedural constant propagation stronger 💪. When enabled, interprocedural constant propagation does function cloning whenever an externally visible function can be called with constant arguments. Because this optimization can create multiple copies of functions, it may significantly increase code size — see--param ipa-cp-unit-growth=value. Enabled by default at-O3. It's also enabled by-fprofile-useand-fauto-profile.-fipa-bit-cp— when enabled, performs interprocedural bitwise constant propagation. Enabled by default at-O2and by-fprofile-useand-fauto-profile. It requires that-fipa-cpis enabled. teamwork makes the dream work 🫶-fipa-vrp— when enabled, performs interprocedural propagation of value ranges. Enabled by default at-O2. It requires that-fipa-cpis enabled. ✨-fipa-icf— performs Identical Code Folding for functions and read-only variables. This optimization reduces code size and may disturb unwind stacks by replacing a function with an equivalent one that has a different name 😳. The optimization works more effectively with link-time optimization enabled.
So the behavior is similar to the Gold Linker's ICF optimization, but GCC ICF works on different levels, so the optimizations are not the same — there are equivalences found only by GCC and equivalences found only by Gold.
Enabled by default at -O2 and -Os.
-flate-combine-instructions— enables two instruction combination passes that run relatively late in the compilation process. One of the passes runs before register allocation and the other after register allocation. The main aim of the passes is to substitute definitions into all uses. Most targets enable this flag by default at-O2and-Os. 🔥-flive-patching=level— controls GCC's optimizations to produce output suitable for live-patching. okay this one is a whole situation, sit down 🍓
Here's the deal: if the compiler's optimization uses a function's body (or info extracted from its body) to optimize/change another function, then that other function is called an impacted function of the former. And if a function gets patched, its impacted functions should be patched too.
The impacted functions are determined by the compiler's interprocedural optimizations. For example, a caller is impacted when inlining a function into its caller, cloning a function and changing its caller to call this new clone, or extracting a function's pureness/constness information to optimize its direct or indirect callers, etc.
Usually, the more IPA optimizations enabled, the larger the number of impacted functions for each function. So to control the number of impacted functions and more easily compute the list of impacted functions, IPA optimizations can be partially enabled at two different levels.
The level argument should be one of the following:
inline-clone— only enables inlining and cloning optimizations, which includes inlining, cloning, interprocedural scalar replacement of aggregates and partial inlining. As a result, when patching a function, all its callers and its clones' callers are impacted, therefore they need to be patched as well.
-flive-patching=inline-clone disables the following optimization flags: -fwhole-program -fipa-pta -fipa-reference -fipa-ra -fipa-icf -fipa-icf-functions -fipa-icf-variables -fipa-bit-cp -fipa-vrp -fipa-pure-const -fipa-reference-addressable -fipa-stack-alignment -fipa-modref
inline-only-static— only enables inlining of static functions. As a result, when patching a static function, all its callers are impacted and so need to be patched as well.
In addition to all the flags that -flive-patching=inline-clone disables, -flive-patching=inline-only-static disables the following additional optimization flags: -fipa-cp-clone -fipa-sra -fpartial-inlining -fipa-cp
When -flive-patching is specified without any value, the default value is inline-clone.
This flag is disabled by default.
Note that-flive-patchingis not supported with link-time optimization (-flto). 💀
-fisolate-erroneous-paths-dereference— detects paths that trigger erroneous or undefined behavior due to dereferencing a null pointer. It isolates those paths from the main control flow and turns the statement with erroneous or undefined behavior into a trap 🪤. Enabled by default at-O2and higher, and it depends on-fdelete-null-pointer-checksalso being enabled.-fisolate-erroneous-paths-attribute— detects paths that trigger erroneous or undefined behavior due to a null value being used in a way forbidden by areturns_nonnullornonnullattribute. It isolates those paths from the main control flow and turns the statement with erroneous or undefined behavior into a trap. This is not currently enabled, but may be enabled by-O2in the future. foreshadowing ✨-ftree-sink— performs forward store motion on trees. Enabled by default at-O1and higher.-ftree-bit-ccp— performs sparse conditional bit constant propagation on trees and propagates pointer alignment information. This pass only operates on local scalar variables and is enabled by default at-O1and higher — except for-Og. It requires that-ftree-ccpis enabled. 🌈-ftree-ccp— performs sparse conditional constant propagation (CCP) on trees. This pass only operates on local scalar variables and is enabled by default at-O1and higher.-fssa-backprop— propagates information about uses of a value up the definition chain in order to simplify the definitions. For example, this pass strips sign operations if the sign of a value never matters. Enabled by default at-O1and higher. 💅-fssa-phiopt— performs pattern matching on SSA PHI nodes to optimize conditional code. Enabled by default at-O1and higher — except for-Og.-ftree-switch-conversion— performs conversion of simple initializations in a switch to initializations from a scalar array. Enabled by default at-O2and higher. 🦋-ftree-tail-merge— looks for identical code sequences. When it finds one, it replaces one with a jump to the other. This optimization is known as tail merging or cross jumping. Enabled by default at-O2and higher. The compilation time in this pass can be limited using themax-tail-merge-comparisonsparameter and themax-tail-merge-iterationsparameter. ⏱️-ftree-cselim— performs conditional store elimination on trees. Enabled by default at-O1and higher on targets that have conditional move instructions.-ftree-dce— performs dead code elimination (DCE) on trees. Enabled by default at-O1and higher. 💀✨-ftree-builtin-call-dce— performs conditional dead code elimination (DCE) for calls to built-in functions that may seterrnobut are otherwise free of side effects. Enabled by default at-O2and higher if-Osis not also specified.-ffinite-loops— bestie, this one says: assume that a loop with an exit will eventually take that exit and not just vibe in there forever ♾️. That means the compiler gets to delete loops that have no side-effects otherwise, because it's not sitting around worrying about "what if this thing loops endlessly tho". Enabled by default at-O2for C++ with-std=c++11or higher.-ftree-dominator-opts— the little clean-up crew 🧹. It does a whole variety of simple scalar cleanups (constant/copy propagation, redundancy elimination, range propagation and expression simplification) based on a dominator tree traversal. It also does jump threading (which is the iconic move of reducing jumps to jumps). This flag is enabled by default at-O1and higher.-ftree-dse— dead store elimination, aka DSE, on trees. A dead store is when you store into a memory location and then literally just overwrite it later with another store and there were no intervening loads in between 🙈. In that case the earlier store can be deleted, gone, bye. This flag is enabled by default at-O1and higher.-ftree-ch— performs loop header copying on trees. This is a slay because it increases the effectiveness of code motion optimizations, and it also saves one jump 💅. This flag is enabled by default at-O1and higher. It is not enabled for-Os, since it usually increases code size.-ftree-loop-optimize— performs loop optimizations on trees. Enabled by default at-O1and higher.-ftree-loop-linear,-floop-strip-mine,-floop-block— these three are the same vibe: perform loop nest optimizations. Same as-floop-nest-optimize. To use this code transformation, GCC has to be configured with--with-islto enable the Graphite loop transformation infrastructure.-fgraphite-identity— turns on the identity transformation for graphite ✨. For every SCoP we generate the polyhedral representation and transform it back to gimple. Using-fgraphite-identitywe can check the costs or benefits of the GIMPLE -> GRAPHITE -> GIMPLE transformation. Some minimal optimizations are also performed by the code generator isl, like index splitting and dead code elimination in loops.-floop-nest-optimize— enables the isl based loop nest optimizer. This is a generic loop nest optimizer based on the Pluto optimization algorithms. It calculates a loop structure optimized for data-locality and parallelism. This option is experimental.-floop-parallelize-all— uses the Graphite data dependence analysis to identify loops that can be parallelized. It parallelizes all the loops that can be analyzed to not contain loop carried dependences without checking that it is profitable to parallelize the loops 🫶.-ftree-coalesce-vars— while transforming the program out of the SSA representation, this attempts to reduce copying by coalescing versions of different user-defined variables, instead of just compiler temporaries. This may severely limit the ability to debug an optimized program compiled with-fno-var-tracking-assignments. In the negated form, this flag prevents SSA coalescing of user variables. Enabled by default if optimization is enabled, and it does very little otherwise.-ftree-loop-if-convert— attempts to transform conditional jumps in the innermost loops to branch-less equivalents. The intent is to remove control-flow from the innermost loops in order to improve the ability of the vectorization pass to handle these loops. Enabled by default if vectorization is enabled.-ftree-loop-distribution— performs loop distribution. This flag can improve cache performance on big loop bodies and allow further loop optimizations, like parallelization or vectorization, to take place. For example, the loop
DO I = 1, N
A(I) = B(I) + C
D(I) = E(I) * F
ENDDOis transformed to
DO I = 1, N
A(I) = B(I) + C
ENDDO
DO I = 1, N
D(I) = E(I) * F
ENDDOThis flag is enabled by default at -O3. It is also enabled by -fprofile-use and -fauto-profile.
-ftree-loop-distribute-patterns— performs loop distribution of patterns that can be code generated with calls to a library 📚. This flag is enabled by default at-O2and higher, and by-fprofile-useand-fauto-profile.
This pass distributes the initialization loops and generates a call to memset zero. For example, the loop
DO I = 1, N
A(I) = 0
B(I) = A(I) + I
ENDDOis transformed to
DO I = 1, N
A(I) = 0
ENDDO
DO I = 1, N
B(I) = A(I) + I
ENDDOand the initialization loop is transformed into a call to memset zero.
-floop-interchange— performs loop interchange outside of graphite. This flag can improve cache performance on loop nest and allow further loop optimizations, like vectorization, to take place. For example, the loop
for (int i = 0; i < N; i++)
for (int j = 0; j < N; j++)
for (int k = 0; k < N; k++)
c[i][j] = c[i][j] + a[i][k]*b[k][j];is transformed to
for (int i = 0; i < N; i++)
for (int k = 0; k < N; k++)
for (int j = 0; j < N; j++)
c[i][j] = c[i][j] + a[i][k]*b[k][j];This flag is enabled by default at -O3. It is also enabled by -fprofile-use and -fauto-profile.
-floop-unroll-and-jam— applies unroll and jam transformations on feasible loops 🍓. In a loop nest this unrolls the outer loop by some factor and fuses the resulting multiple inner loops. This flag is enabled by default at-O3. It is also enabled by-fprofile-useand-fauto-profile.-ftree-loop-im— performs loop invariant motion on trees. This pass moves only invariants that are hard to handle at RTL level (function calls, operations that expand to nontrivial sequences of insns). With-funswitch-loopsit 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— creates a canonical counter for number of iterations in loops for which determining number of iterations requires complicated analysis 😮💨. Later optimizations then may determine the number easily. Useful especially in connection with unrolling.-ftree-scev-cprop— performs final value replacement. If a variable is modified in a loop in such a way that its value when exiting the loop can be determined using only its initial value and the number of loop iterations, it replaces uses of the final value by such a computation, provided it is sufficiently cheap. This reduces data dependencies and may allow further simplifications. Enabled by default at-O1and higher.-fivopts— performs induction variable optimizations (strength reduction, induction variable merging and induction variable elimination) on trees. Enabled by default at-O1and higher.- *
-ftree-parallelize-loops=n* — parallelizes loops, i.e., splits their iteration space to run in n threads 🧵🔥. This is only possible for loops whose iterations are independent and can be arbitrarily reordered. The optimization is only profitable on multiprocessor machines, for loops that are CPU-intensive, rather than constrained e.g. by memory bandwidth. This option implies-pthread, and thus is only supported on targets that have support for-pthread. -ftree-pta— performs function-local points-to analysis on trees. This flag is enabled by default at-O1and higher, except for-Og.-ftree-sra— performs 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-O1and higher, except for-Og.-fstore-merging— performs merging of narrow stores to consecutive memory addresses. This pass merges contiguous stores of immediate values narrower than a word into fewer wider stores to reduce the number of instructions 💅. Enabled by default at-O2and higher as well as-Os.-ftree-ter— performs 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. Enabled by default at-O1and higher.-ftree-slsr— performs straight-line strength reduction on trees. This recognizes related expressions involving multiplications and replaces them by less expensive calculations when possible. Enabled by default at-O1and higher.-ftree-vectorize— performs vectorization on trees ✨. This flag enables-ftree-loop-vectorizeand-ftree-slp-vectorizeif not explicitly specified.-ftree-loop-vectorize— performs loop vectorization on trees. This flag is enabled by default at-O2and by-ftree-vectorize,-fprofile-use, and-fauto-profile.-ftree-slp-vectorize— performs basic block vectorization on trees. This flag is enabled by default at-O2and by-ftree-vectorize,-fprofile-use, and-fauto-profile.- *
-ftrivial-auto-var-init=choice* — initializes automatic variables with either a pattern or with zeroes to increase the security and predictability of a program by preventing uninitialized memory disclosure and use 🔐. GCC still considers an automatic variable that doesn't have an explicit initializer as uninitialized,-Wuninitializedand-Wanalyzer-use-of-uninitialized-valuewill still report warning messages on such automatic variables and the compiler will perform optimization as if the variable were uninitialized. With this option, GCC will also initialize any padding of automatic variables that have structure or union types to zeroes. However, the current implementation cannot initialize automatic variables that are declared between the controlling expression and the firstcaseof aswitchstatement. Using-Wtrivial-auto-var-initto report all such cases.
The three values of choice are:
uninitialized— doesn't initialize any automatic variables at all. This is C and C++'s default, no cap.pattern— initializes automatic variables with values that will likely transform logic bugs into crashes down the line, are easily recognized in a crash dump, and are not values that programmers can rely on for useful program semantics. The current value is a byte-repeatable pattern with byte"0xFE". The values used for pattern initialization might be changed in the future, so don't get too attached bestie.zero— initializes automatic variables with zeroes.
The default is uninitialized.
Okay so real talk: the initializer values, whether zero or pattern, refer to data representation (in memory or machine registers), rather than to their interpretation as numerical values. This distinction may be important in languages that support types with biases or implicit multipliers, and with such extensions as hardbool. For example, a variable that uses 8 bits to represent (biased) quantities in the "range 160..400" will be initialized with the bit patterns 0x00 or 0xFE, depending on choice, whether or not these representations stand for values in that range, and even if they do, the interpretation of the value held by the variable will depend on the bias. A hardbool variable that uses say "0X5A" and 0xA5 for "false" and "true", respectively, will trap with either choice of trivial initializer, i.e., zero initialization will not convert to the representation for "false", even if it would for a "static" variable of the same type. This means the initializer pattern doesn't generally depend on the type of the initialized variable. One notable exception is that (non-hardened) boolean variables that fit in registers are initialized with "false" (zero), even when pattern is requested. 🙈
You can control this behavior for a specific variable by using the variable attribute "uninitialized".
-fvect-cost-model=model — alter the cost model used for vectorization. The model argument should be one ofunlimited,dynamic,cheaporvery-cheap. With theunlimitedmodel the vectorized code-path is assumed to be profitable, while with thedynamicmodel a runtime check guards the vectorized code-path to enable it only for iteration counts that will likely execute faster than when executing the original scalar loop. Thecheapmodel disables vectorization of loops where doing so would be cost prohibitive — for example due to required runtime checks for data dependence or alignment — but otherwise is equal to thedynamicmodel. Thevery-cheapmodel disables vectorization of loops when any runtime check for data dependence or alignment is required, it also disables vectorization of epilogue loops, but otherwise is equal to thecheapmodel. ✨
The default cost model depends on other optimization flags and is either dynamic or cheap.
-fsimd-cost-model=model — alter the cost model used for vectorization of loops marked with the OpenMPsimddirective. The model argument should be one ofunlimited,dynamic,cheap. All values of model have the same meaning as described in-fvect-cost-modeland by default a cost model defined with-fvect-cost-modelis used.-ftree-vrp— perform Value Range Propagation on trees. This is similar to the constant propagation pass, but instead of values, ranges of values are propagated. This allows the optimizers to remove unnecessary range checks like array bound checks and null pointer checks. This is enabled by default at-O2and higher. Null pointer check elimination is only done if-fdelete-null-pointer-checksis enabled. 🦋-fsplit-paths— split paths leading to loop backedges. This can improve dead code elimination and common subexpression elimination. This is enabled by default at-O3and above.-fsplit-ivs-in-unroller— enables expression of values of induction variables in later iterations of the unrolled loop using the value in the first iteration. This breaks long dependency chains, thus improving efficiency of the scheduling passes. Iconic behaviour tbh.
A combination of -fweb and CSE is often sufficient to obtain the same effect. However, that is not reliable in cases where the loop body is more complicated than a single basic block. It also does not work at all on some architectures due to restrictions in the CSE pass. 😭
This optimization is enabled by default.
-fvariable-expansion-in-unroller— with this option, the compiler creates multiple copies of some local variables when unrolling a loop, which can result in superior code.
This optimization is enabled by default for PowerPC targets, but disabled by default otherwise.
-fpartial-inlining— inline parts of functions. This option has any effect only when inlining itself is turned on by the-finline-functionsor-finline-small-functionsoptions.
Enabled at levels -O2, -O3, -Os.
-fpredictive-commoning— perform predictive commoning optimization, i.e., reusing computations (especially memory loads and stores) performed in previous iterations of loops. 🌀
This option is enabled at level -O3. It is also enabled by -fprofile-use and -fauto-profile.
-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. So like… vibe check your benchmarks, bestie.
Disabled at level -Os.
-fno-printf-return-value— do not substitute constants for known return value of formatted output functions such as"sprintf","snprintf","vsprintf", and"vsnprintf"(but not"printf"of"fprintf"). This transformation allows GCC to optimize or even eliminate branches based on the known return value of these functions called with arguments that are either constant, or whose values are known to be in a range that makes determining the exact return value possible. For example, when-fprintf-return-valueis in effect, both the branch and the body of the"if"statement (but not the call to"snprint") can be optimized away when"i"is a 32-bit or smaller integer because the return value is guaranteed to be at most 8. 💅
char buf[9];
if (snprintf (buf, "%08x", i) >= sizeof buf)
...The -fprintf-return-value option relies on other optimizations and yields best results with -O2 and above. It works in tandem with the -Wformat-overflow and -Wformat-truncation options. The -fprintf-return-value option is enabled by default.
-fno-peephole-fno-peephole2— disable any machine-specific peephole optimizations. The difference between-fno-peepholeand-fno-peephole2is in how they are implemented in the compiler; some targets use one, some use the other, a few use both.
-fpeephole is enabled by default. -fpeephole2 enabled at levels -O2, -O3, -Os.
-fno-guess-branch-probability— do not guess branch probabilities using heuristics.
GCC uses heuristics to guess branch probabilities if they are not provided by profiling feedback (-fprofile-arcs). These heuristics are based on the control flow graph. If some branch probabilities are specified by "__builtin_expect", then the heuristics are used to guess branch probabilities for the rest of the control flow graph, taking the "__builtin_expect" info into account. The interactions between the heuristics and "__builtin_expect" can be complex, and in some cases, it may be useful to disable the heuristics so that the effects of "__builtin_expect" are easier to understand. 👑
It is also possible to specify expected probability of the expression with "__builtin_expect_with_probability" built-in function.
The default is -fguess-branch-probability at levels -O, -O2, -O3, -Os.
-freorder-blocks— reorder basic blocks in the compiled function in order to reduce number of taken branches and improve code locality.
Enabled at levels -O1, -O2, -O3, -Os.
-freorder-blocks-algorithm=algorithm — use the specified algorithm for basic block reordering. The algorithm argument can besimple, which does not increase code size (except sometimes due to secondary effects like alignment), orstc, the "software trace cache" algorithm, which tries to put all often executed code together, minimizing the number of branches executed by making extra copies of code.
The default is simple at levels -O1, -Os, and stc 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.ofiles, to improve paging and cache locality performance. 🌈
This optimization is automatically turned off in the presence of exception handling or unwind tables (on targets using setjump/longjump or target specific scheme), for linkonce sections, for functions with a user-defined section attribute and on any architecture that does not support named sections. When -fsplit-stack is used this option is not enabled by default (to avoid linker errors), but may be enabled explicitly (if using a working linker).
Enabled for x86 at levels -O2, -O3, -Os.
-freorder-functions— reorder functions in the object file in order to improve code locality. Unlike-fipa-reorder-for-localitythis option prioritises grouping all functions within a category (hot/normal/cold/never-executed) together. 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. 🎀
This option isn't effective unless you either provide profile feedback (see -fprofile-arcs for details) or manually annotate functions with "hot" or "cold" attributes.
Enabled at levels -O2, -O3, -Os.
-fstrict-aliasing— bestie, this one lets the compiler assume the strictest aliasing rules that even apply to the language you're compiling. For C (and C++, obvi) it turns on optimizations based on the type of expressions. The tea: an object of one type is assumed to never live at the same address as an object of a different type, unless the types are almost the same. Example: anunsigned intcan alias anint, but it can't alias avoid*or adouble. And a character type? She can alias any other type. 💅
Pay extra attention to code like this, bestie:
union a_union {
int i;
double d;
};
int f() {
union a_union t;
t.d = 3.0;
return t.i;
}Reading from a different union member than the one you most recently wrote to (this is called "type-punning", iykyk) is super common. And even with -fstrict-aliasing, type-punning is allowed — provided the memory is accessed through the union type. So the code above works as expected, no cap. 🫶 But this next one might not:
int f() {
union a_union t;
int* ip;
t.d = 3.0;
ip = &t.i;
return *ip;
}Same energy: accessing by taking the address, casting the resulting pointer, and dereferencing the result has undefined behavior, even if the cast uses a union type. Like this:
int f() {
double d = 3.0;
return ((union a_union *) &d)->i;
}The -fstrict-aliasing option is enabled at levels -O2, -O3, -Os. ✨
-fipa-strict-aliasing— this controls whether the rules of-fstrict-aliasingget applied across function boundaries. Quick note tho: if multiple functions get inlined into a single function, the memory accesses are no longer considered to be crossing a function boundary. The-fipa-strict-aliasingoption is enabled by default and is only effective in combination with-fstrict-aliasing. 🦋-falign-functions-falign-functions=n-falign-functions=n:m-falign-functions=n:m:n2-falign-functions=n:m:n2:m2— align the start of functions to the next power-of-two greater than or equal to n, skipping up to m-1 bytes. This ensures at least the first m bytes of the function can be fetched by the CPU without crossing an n-byte alignment boundary. It's an optimization of code performance, and alignment is ignored for functions considered cold. If alignment is required for all functions, use-fmin-function-alignmentinstead.
If m is not specified, it defaults to n.
Examples, bestie: -falign-functions=32 aligns functions to the next 32-byte boundary, -falign-functions=24 aligns to the next 32-byte boundary only if this can be done by skipping 23 bytes or less, and -falign-functions=32:7 aligns to the next 32-byte boundary only if this can be done by skipping 6 bytes or less.
That second pair of n2:m2 values lets you specify a secondary alignment: -falign-functions=64:7:32:3 aligns to the next 64-byte boundary if this can be done by skipping 6 bytes or less, otherwise it aligns to the next 32-byte boundary if this can be done by skipping 2 bytes or less. If m2 is not specified, it defaults to n2.
Some assemblers only support this flag when n is a power of two; in that case, it is rounded up. 🙈
-fno-align-functions and -falign-functions=1 are equivalent and mean that functions are not aligned.
If n is not specified or is zero, use a machine-dependent default. The maximum allowed n option value is 65536.
Enabled at levels -O2, -O3. 🌸
-flimit-function-alignment— with this on, the compiler tries to avoid unnecessarily overaligning functions. It attempts to instruct the assembler to align by the amount specified by-falign-functions, but not to skip more bytes than the size of the function. So polite of her. 🎀-falign-labels-falign-labels=n-falign-labels=n:m-falign-labels=n:m:n2-falign-labels=n:m:n2:m2— align all branch targets to a power-of-two boundary.
The parameters of this option are analogous to the -falign-functions option. -fno-align-labels and -falign-labels=1 are equivalent and mean that labels are not aligned.
If -falign-loops or -falign-jumps are applicable and are greater than this value, then their values are used instead.
If n is not specified or is zero, use a machine-dependent default which is very likely to be 1, meaning no alignment. The maximum allowed n option value is 65536.
Enabled at levels -O2, -O3. ⭐
-falign-loops-falign-loops=n-falign-loops=n:m-falign-loops=n:m:n2-falign-loops=n:m:n2:m2— align loops to a power-of-two boundary. If the loops are executed many times, this makes up for any execution of the dummy padding instructions. It's an optimization of code performance, and alignment is ignored for loops considered cold. 🧊
If -falign-labels is greater than this value, then its value is used instead.
Parameters of this option are analogous to the -falign-functions option. -fno-align-loops and -falign-loops=1 are equivalent and mean that loops are not aligned. The maximum allowed n option value is 65536.
If n is not specified or is zero, use a machine-dependent default.
Enabled at levels -O2, -O3. 💜
-falign-jumps-falign-jumps=n-falign-jumps=n:m-falign-jumps=n:m:n2-falign-jumps=n:m:n2:m2— align branch targets to a power-of-two boundary, for branch targets where the targets can only be reached by jumping. In this case, no dummy operations need be executed. It's an optimization of code performance, and alignment is ignored for jumps considered cold.
If -falign-labels is greater than this value, then its value is used instead.
Parameters of this option are analogous to the -falign-functions option. -fno-align-jumps and -falign-jumps=1 are equivalent and mean that loops are not aligned.
If n is not specified or is zero, use a machine-dependent default. The maximum allowed n option value is 65536.
Enabled at levels -O2, -O3. 🔥
-fmin-function-alignment— specify minimal alignment of functions to the next power-of-two greater than or equal to n. Unlike-falign-functionsthis alignment is applied also to all functions (even those considered cold). The alignment is also not affected by-flimit-function-alignment. 👑-fno-allocation-dce— do not remove unused C++ allocations (using operatornewand operatordelete) in dead code elimination. See also-fmalloc-dce. 💀-fallow-store-data-races— allow the compiler to perform optimizations that may introduce new data races on stores, without proving that the variable cannot be concurrently accessed by other threads. Does not affect optimization of local data. It is safe to use this option if it is known that global data will not be accessed by multiple threads.
Examples of optimizations enabled by -fallow-store-data-races include hoisting or if-conversions that may cause a value that was already in memory to be re-written with that same value. Such re-writing is safe in a single threaded context but may be unsafe in a multi-threaded context. Note that on some processors, if-conversions may be required in order to enable vectorization.
Enabled at level -Ofast. 🏎️
-funit-at-a-time— this option is left for compatibility reasons, bestie.-funit-at-a-timehas no effect, while-fno-unit-at-a-timeimplies-fno-toplevel-reorderand-fno-section-anchors.
Enabled by default.
-fno-toplevel-reorder— do not reorder top-level functions, variables, andasmstatements. Output them in the same order that they appear in the input file. When this option is used, unreferenced static variables are not removed. This option is intended to support existing code that relies on a particular ordering. For new code, it is better to use attributes when possible.
-ftoplevel-reorder is the default at -O1 and higher, and also at -O0 if -fsection-anchors is explicitly requested. Additionally -fno-toplevel-reorder implies -fno-section-anchors. 🌈
-funreachable-traps— with this option, the compiler turns calls to__builtin_unreachableinto traps, instead of using them for optimization. This also affects any such calls implicitly generated by the compiler.
This option has the same effect as -fsanitize=unreachable -fsanitize-trap=unreachable, but does not affect the values of those options. If -fsanitize=unreachable is enabled, that option takes priority over this one.
This option is enabled by default at -O0 and -Og. 🪤
-fweb— constructs webs as commonly used for register allocation purposes and assigns each web an individual pseudo register. This allows the register allocation pass to operate on pseudos directly, but also strengthens several other optimization passes, such as CSE, loop optimizer and trivial dead code remover. It can, however, make debugging impossible, since variables no longer stay in a "home register". 😭
Enabled by default with -funroll-loops.
-fwhole-program— assume that the current compilation unit represents the whole program being compiled. All public functions and variables — with the exception ofmainand those merged by attributeexternally_visible— become static functions and in effect are optimized more aggressively by interprocedural optimizers.
With -flto this option has a limited use. In most cases the precise list of symbols used or exported from the binary is known the resolution info passed to the link-time optimizer by the linker plugin. It is still useful if no linker plugin is used or during incremental link step when final code is produced (with -flto -flinker-output=nolto-rel). 🦋
-flto[=n]— this option runs the standard link-time optimizer. When invoked with source code, it generates GIMPLE (one of GCC's internal representations) and writes it to special ELF sections in the object file. When the object files are linked together, all the function bodies are read from these ELF sections and instantiated as if they had been part of the same translation unit. ✨
Okay bestie, so to actually use the link-time optimizer, -flto and your optimization options need to be specified at compile time and during the final link. Both. Yes. And the recommendation is very much: compile every file that's participating in the same link with the same options, and also specify those options at link time. Like this ✨:
gcc -c -O2 -flto foo.c
gcc -c -O2 -flto bar.c
gcc -o myprog -flto -O2 foo.o bar.oSo here's the tea 🍓 — the first two invocations to GCC save a bytecode representation of GIMPLE into special ELF sections inside foo.o and bar.o. Then the final invocation reads the GIMPLE bytecode back out of foo.o and bar.o, merges the two files into a single internal image, and compiles the result as usual. And because both foo.o and bar.o get merged into one single image, all the interprocedural analyses and optimizations in GCC now work across the two files as if they were one file. Which means, for example, the inliner can inline functions in bar.o into functions in foo.o, and vice-versa. Chef's kiss 💅.
Another (way simpler) way to enable link-time optimization is:
gcc -o myprog -flto -O2 foo.c bar.cThe above generates bytecode for foo.c and bar.c, merges them together into a single GIMPLE representation, and optimizes them as usual to produce myprog.
The important thing to keep in mind (and this one matters) is that to enable link-time optimizations, you need to use the GCC driver to perform the link step. GCC automatically performs link-time optimization if any of the objects involved were compiled with the -flto command-line option. And you can always override that automatic decision to do link-time optimization by passing -fno-lto to the link command. Easy peasy.
To make whole program optimization effective, it's necessary to make certain whole program assumptions. The compiler needs to know what functions and variables can be accessed by libraries and runtime outside of the link-time optimized unit. When it's supported by the linker, the linker plugin (see -fuse-linker-plugin) passes information to the compiler about used and externally visible symbols. When the linker plugin isn't available, -fwhole-program should be used to allow the compiler to make these assumptions — which leads to more aggressive optimization decisions. No cap.
When a file is compiled with -flto without -fuse-linker-plugin, the generated object file is larger than a regular object file, because it contains GIMPLE bytecodes and the usual final code (see -ffat-lto-objects). This means object files with LTO information can be linked as normal object files; and if -fno-lto is passed to the linker, no interprocedural optimizations get applied. Note that when -fno-fat-lto-objects is enabled, the compile stage is faster, but you cannot perform a regular, non-LTO link on them. Periodt.
When producing the final binary, GCC only applies link-time optimizations to those files that contain bytecode. So you can absolutely mix and match object files and libraries with GIMPLE bytecodes and final object code. GCC automatically selects which files to optimize in LTO mode and which files to link without further processing. Main character energy 👑.
Generally, options specified at link time override those specified at compile time — although in some cases GCC attempts to infer link-time options from the settings used to compile the input files.
If you don't specify an optimization level option -O at link time, then GCC uses the highest optimization level used when compiling the object files. But ngl, it's generally ineffective to specify an optimization level option only at link time and not at compile time, for two reasons:
- Compiling without optimization suppresses compiler passes that gather information needed for effective optimization at link time.
- Some early optimization passes can be performed only at compile time and not at link time.
There are some code generation flags preserved by GCC when generating bytecodes, since they need to be used during the final link. Currently, the following options and their settings are taken from the first object file that explicitly specifies them: -fcommon, -fexceptions, -fnon-call-exceptions, -fgnu-tm and all the -m target flags.
The following options — -fPIC, -fpic, -fpie and -fPIE — are combined based on the following scheme:
-fPIC + -fpic = -fpic
-fPIC + -fno-pic = -fno-pic
-fpic/-fPIC + (no option) = (no option)
-fPIC + -fPIE = -fPIE
-fpic + -fPIE = -fpie
-fPIC/-fpic + -fpie = -fpieCertain ABI-changing flags are required to match in all compilation units, and trying to override this at link time with a conflicting value is ignored. This includes options such as -freg-struct-return and -fpcc-struct-return.
Other options such as -ffp-contract, -fno-strict-overflow, -fwrapv, -fno-trapv or -fno-strict-aliasing are passed through to the link stage and merged conservatively for conflicting translation units. Specifically, -fno-strict-overflow, -fwrapv and -fno-trapv take precedence; and for example -ffp-contract=off takes precedence over -ffp-contract=fast. You can override them at link time. 💪
Diagnostic options such as -Wstringop-overflow are passed through to the link stage and their setting matches that of the compile-step at function granularity. Note that this matters only for diagnostics emitted during optimization. Also note that code transforms such as inlining can lead to warnings being enabled or disabled for regions of code not consistent with the setting at compile time.
When you need to pass options to the assembler via -Wa or -Xassembler, make sure to either compile such translation units with -fno-lto, or consistently use the same assembler options on all translation units. You can alternatively also specify assembler options at LTO link time.
To enable debug info generation you need to supply -g at compile time. If any of the input files at link time were built with debug info generation enabled, the link will enable debug info generation as well. Any elaborate debug info settings like the dwarf level -gdwarf-5 need to be explicitly repeated at the linker command line, and mixing different settings in different translation units is discouraged. 🙈
If LTO encounters objects with C linkage declared with incompatible types in separate translation units to be linked together (undefined behavior according to ISO C99 6.2.7), a non-fatal diagnostic may be issued. The behavior is still undefined at run time. Similar diagnostics may be raised for other languages.
Another feature of LTO is that it's possible to apply interprocedural optimizations on files written in different languages:
gcc -c -flto foo.c
g++ -c -flto bar.cc
gfortran -c -flto baz.f90
g++ -o myprog -flto -O3 foo.o bar.o baz.o -lgfortranNotice that the final link is done with g++ to get the C++ runtime libraries, and -lgfortran is added to get the Fortran runtime libraries. In general, when mixing languages in LTO mode, you should use the same link command options as when mixing languages in a regular (non-LTO) compilation.
If object files containing GIMPLE bytecode are stored in a library archive — say libfoo.a — it's possible to extract and use them in an LTO link, if you're using a linker with plugin support. To create static libraries suitable for LTO, use gcc-ar and gcc-ranlib instead of ar and ranlib; to show the symbols of object files with GIMPLE bytecode, use gcc-nm. Those commands require that ar, ranlib and nm have been compiled with plugin support. At link time, use the flag -fuse-linker-plugin to ensure that the library participates in the LTO optimization process:
gcc -o myprog -O2 -flto -fuse-linker-plugin a.o b.o -lfooWith the linker plugin enabled, the linker extracts the needed GIMPLE files from libfoo.a and passes them on to the running GCC to make them part of the aggregated GIMPLE image to be optimized. Gorgeous 🌸.
If you're not using a linker with plugin support and/or don't enable the linker plugin, then the objects inside libfoo.a are extracted and linked as usual — but they do not participate in the LTO optimization process. In order to make a static library suitable for both LTO optimization and usual linkage, compile its object files with -flto -ffat-lto-objects.
Link-time optimizations do not require the presence of the whole program to operate. If the program doesn't require any symbols to be exported, it's possible to combine -flto and -fwhole-program to allow the interprocedural optimizers to use more aggressive assumptions, which may lead to improved optimization opportunities. Use of -fwhole-program isn't needed when the linker plugin is active (see -fuse-linker-plugin).
The current implementation of LTO makes no attempt to generate bytecode that is portable between different types of hosts. The bytecode files are versioned and there's a strict version check, so bytecode files generated in one version of GCC do not work with an older or newer version of GCC. 😭
Link-time optimization does not work well with generation of debugging information on systems other than those using a combination of ELF and DWARF.
If you specify the optional n, the optimization and code generation done at link time is executed in parallel using n parallel jobs, by utilizing an installed make program. The environment variable MAKE may be used to override the program used. 🔥
You can also specify -flto=jobserver to use GNU make's job server mode to determine the number of parallel jobs. This is useful when the Makefile calling GCC is already executing in parallel. You must prepend a + to the command recipe in the parent Makefile for this to work. This option likely only works if MAKE is GNU make. Even without the option value, GCC tries to automatically detect a running GNU make's job server.
Use -flto=auto to use GNU make's job server, if available, or otherwise fall back to autodetection of the number of CPU threads present in your system. ✨
-flto-partition=alg— bestie, this one picks the partitioning algorithm that the link-time optimizer uses, and the value you pass is lowkey the whole personality of the thing ✨1to1— partitions mirror the original source files, exactly as they came 🪞balanced— chops everything into equally sized chunks whenever possible (this is the default, no cap 👑)max— makes a brand-new partition for every symbol where possible... but heads up, this one is intended for internal testing only 🙈cache— balances chunk sizes while keeping related symbols together, so incremental LTO gets better caching 🧠none— disables partitioning and streaming completely 💀one— says "use exactly one partition, bestie, we're keeping it simple"- and separately,
nonebypasses partitioning and runs the link-time optimization step directly from the WPA phase 🦋
Real talk: 1to1 can be used as a workaround for various code ordering issues. The default value is balanced.
-flto-incremental=path— enables incremental LTO, with its cache living in the given existing directory 📁. Can significantly shorten edit-compile cycles with LTO, which is giving "we ate and left no crumbs" energy 🔥- When used with LTO (
-flto), the output of translation units inside LTO gets cached 💾 - Cached translation units are likely to be encountered again when you recompile with small code changes, so recompile time goes down 📉
- Multiple GCC instances can use the same cache in parallel, so no fighting, no drama 🤝
- When used with LTO (
-flto-incremental-cache-size=n— sets the number of cache entries in incremental LTO after which old entries get pruned ✂️. It's a soft limit, so temporarily there may be more entries. It's giving "vibes-based ceiling" 🎀-flto-compression-level=n— picks the level of compression used for intermediate language written to LTO object files, and it's only meaningful in conjunction with LTO mode (-flto) 🗜️. GCC currently supports two LTO compression algorithms:- for
zstd, valid values are0(no compression) to19(maximum compression) - for
zlib, values run from0to9
- for
Values outside the range get clamped to either the minimum or the maximum of the supported values 🫠. If you don't give the option at all, a default balanced compression setting is used ✨
-fuse-linker-plugin— enables the use of a linker plugin during link-time optimization 💅. This relies on plugin support in the linker, which is available ingoldor in GNU ld 2.21 or newer.- It enables extraction of object files with GIMPLE bytecode out of library archives 📦, which improves optimization quality by exposing more code to the link-time optimizer
- That info specifies what symbols can be accessed externally (by non-LTO object or during dynamic linking) 👀
- Resulting code quality improvements on binaries (and shared libraries that use hidden visibility) are similar to
-fwhole-program. See-fltofor a description of the effect of this flag and how to use it, bestie 📖
This option is enabled by default when LTO support in GCC is enabled and GCC was configured for use with a linker supporting plugins (GNU ld 2.21 or newer, or gold) 👑
-ffat-lto-objects— fat LTO objects are object files that contain both the intermediate language and the object code 🍔. That makes them usable for both LTO linking and normal linking. This option is effective only when compiling with-flto, and it is ignored at link time.-fno-fat-lto-objectsimproves compilation time over plain LTO, but it requires the complete toolchain to be aware of LTO 😤. It needs a linker with linker plugin support for basic functionality. On top of that,nm,arandranlibneed to support linker plugins to allow a full-featured build environment (capable of building static libraries etc)- GCC provides the
gcc-ar,gcc-nm,gcc-ranlibwrappers to pass the right options to these tools. With non fat LTO, makefiles need to be modified to use them 🛠️
Note that modern binutils provide a plugin auto-load mechanism. Installing the linker plugin into$libdir/bfd-pluginshas the same effect as usage of the command wrappers (gcc-ar,gcc-nmandgcc-ranlib) 💖
The default is -fno-fat-lto-objects on targets with linker plugin support.
-fcompare-elim— after register allocation and post-register allocation instruction splitting, this identifies arithmetic instructions that compute processor flags similar to a comparison operation based on that arithmetic 🕵️. If possible, it eliminates the explicit comparison operation.- This pass only applies to certain targets that cannot explicitly represent the comparison operation before register allocation is complete
- Enabled at levels
-O1,-O2,-O3,-Os✨
-ffold-mem-offsets-fno-fold-mem-offsets— try to eliminate add instructions by folding them in memory loads/stores 🧩. Enabled at levels-O2,-O3🔥-fcprop-registers— after register allocation and post-register allocation instruction splitting, performs a copy-propagation pass to try to reduce scheduling dependencies and occasionally eliminate the copy 💅. Enabled at levels-O1,-O2,-O3,-Os.-fprofile-correction— profiles collected using an instrumented binary for multi-threaded programs may be inconsistent because of missed counter updates 😭. When this option is specified, GCC uses heuristics to correct or smooth out such inconsistencies. By default, GCC emits an error message when an inconsistent profile is detected 🚨- This option is enabled by
-fauto-profile.
- This option is enabled by
-fprofile-partial-training— with-fprofile-use, all portions of programs not executed during the train run are optimized aggressively for size rather than speed 📏. Sometimes it's just not practical to train all possible hot paths in the program (for example, the program may contain functions specific for a given hardware, and training may not cover all hardware configurations the program is run on). With-fprofile-partial-training, profile feedback is ignored for all functions not executed during the train run, which leads them to be optimized as if they were compiled without profile feedback 🦋. This leads to better performance when the train run is not representative, but also leads to significantly bigger code. Trade-offs, bestie 💀-fprofile-use-fprofile-use=path— enables profile feedback-directed optimizations, plus the following optimizations, many of which are generally profitable only with profile feedback available 👑
-fbranch-probabilities -fprofile-values -funroll-loops
-fpeel-loops -ftracer -fvpt -finline-functions -fipa-cp
-fipa-cp-clone -fipa-bit-cp -fpredictive-commoning -fsplit-loops
-funswitch-loops -fgcse-after-reload -ftree-loop-vectorize
-ftree-slp-vectorize -fvect-cost-model=dynamic
-ftree-loop-distribute-patterns -fprofile-reorder-functionsBefore you can use this option, you must first generate profiling information 📊
By default, GCC emits an error message if the feedback profiles do not match the source code. This error can be turned into a warning by using-Wno-error=coverage-mismatch. Note this may result in poorly optimized code 😬 Additionally, by default, GCC also emits a warning message if the feedback profiles do not exist (see-Wmissing-profile).
If path is specified, GCC looks at the path to find the profile feedback data files. See -fprofile-dir 📁
-fauto-profile-fauto-profile=path— enables sampling-based feedback-directed optimizations, plus the following optimizations, many of which are generally profitable only with profile feedback available ✨
-fbranch-probabilities -fprofile-values -funroll-loops
-fpeel-loops -ftracer -fvpt -finline-functions -fipa-cp
-fipa-cp-clone -fipa-bit-cp -fpredictive-commoning -fsplit-loops
-funswitch-loops -fgcse-after-reload -ftree-loop-vectorize
-ftree-slp-vectorize -fvect-cost-model=dynamic
-ftree-loop-distribute-patterns -fprofile-correctionpath is the name of a file containing AutoFDO profile information. If omitted, it defaults to fbdata.afdo in the current directory 🎀
Producing an AutoFDO profile data file requires running your program with the perf utility on a supported GNU/Linux target system 🐧 For more information, see <https://perf.wiki.kernel.org/>.
E.g.
perf record -e br_inst_retired:near_taken -b -o perf.data \
-- your_programThen use the create_gcov tool to convert the raw profile data to a format that can be used by GCC 🔄 You must also supply the unstripped binary for your program to this tool. See <https://github.com/google/autofdo>.
E.g.
create_gcov --binary=your_program.unstripped --profile=perf.data \
--gcov=profile.afdoThe following options control compiler behavior regarding floating-point arithmetic. 💖 These options trade off between speed and correctness. All must be specifically enabled — no sneaky defaults turning themselves on, okay? 🙅
-fexcess-precision=style— this option allows control over excess precision on machines where floating-point operations occur in a format with more precision or range than the IEEE standard and interchange floating-point types 🌈. An example of such a target is x87 floating point on x86 processors, which uses an 80-bit representation internally instead of the 64-bit IEEE format 😳 For most programs, the excess precision is harmless, but some programs may rely on the requirements of the C or C++ language standards for handling IEEE values.
By default, -fexcess-precision=fast is in effect; this means that operations may be carried out in a wider precision than the types specified in the source if that would result in faster code, and it is unpredictable when rounding to the types specified in the source code takes place 💀 When compiling C or C++, if -fexcess-precision=standard is specified then excess precision follows the rules specified in ISO C99 or C++; in particular, both casts and assignments cause values to be rounded to their semantic types (whereas -ffloat-store only affects assignments) 📌
This option is enabled by default for C or C++ if a strict conformance option such as -std=c99 or -std=c++17 is used. -ffast-math enables -fexcess-precision=fast by default regardless of whether a strict conformance option is used ⚡ If -fexcess-precision=16 is specified, constants and the results of expressions with types _Float16 and __bf16 are computed without excess precision 🧚
-fexcess-precision=standardis not implemented for languages other than C or C++ — sorry bestie, that's just the vibe 💀. On the x86, it has no effect if-mfpmath=sseor-mfpmath=sse+387is specified; in the former case, IEEE semantics apply without excess precision, and in the latter, rounding is unpredictable 🙈.-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 has generally been subsumed by-fexcess-precision=standard, which is more general ✨. If you do use-ffloat-store, you may need to modify your program to explicitly store intermediate computations in temporary variables, since-ffloat-storehandles rounding to IEEE format only on assignments and not casts as-fexcess-precision=standarddoes 😭.-ffast-math— sets the options-fno-math-errno,-funsafe-math-optimizations,-ffinite-math-only,-fno-rounding-math,-fno-signaling-nans,-fcx-limited-rangeand-fexcess-precision=fast🔥.
This option causes the preprocessor macro __FAST_MATH__ to be defined 👑
This option is not turned on by any -O option besides -Ofast, since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions 💀. It may, however, yield faster code for programs that do not require the guarantees of these specifications ✨.
-fno-math-errno— do not seterrnoafter calling math functions that are executed with a single instruction, e.g.,sqrt🍓. A program that relies on IEEE exceptions for math error handling may want to use this flag for speed while maintaining IEEE arithmetic compatibility 💅.
This option is not turned on by any -O option besides -Ofast, since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions 💀. It may, however, yield faster code for programs that do not require the guarantees of these specifications ✨.
The default is -fmath-errno 🎀.
On Darwin systems, the math library never sets errno 😌. There is therefore no reason for the compiler to consider the possibility that it might, and -fno-math-errno is the default on those systems 🦋.
-funsafe-math-optimizations— allow optimizations for floating-point arithmetic that (a) assume that arguments and results are valid and (b) may violate IEEE or ANSI standards 😭. When used at link time, it may include libraries or startup files that change the default FPU control word or other similar optimizations 🧚.
This option is not turned on by any -O option besides -Ofast, since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions 💀. It may, however, yield faster code for programs that do not require the guarantees of these specifications ✨. Enables -fno-signed-zeros, -fno-trapping-math, -fassociative-math and -freciprocal-math 🌈.
The default is -fno-unsafe-math-optimizations 💖.
-fassociative-math— allow re-association of operands in series of floating-point operations 🔄. This violates the ISO C and C++ language standard by possibly changing computation result 😭. NOTE: re-ordering may change the sign of zero as well as ignore NaNs and inhibit or create underflow or overflow (and thus cannot be used on code that relies on rounding behavior like(x + 2**52) - 2**52) 🙈. May also reorder floating-point comparisons and thus may not be used when ordered comparisons are required 💀. This option requires that both-fno-signed-zerosand-fno-trapping-mathbe in effect 👑. Moreover, it doesn't make much sense with-frounding-math🤷. For Fortran the option is automatically enabled when both-fno-signed-zerosand-fno-trapping-mathare in effect ✨.
The default is -fno-associative-math 🎀.
-freciprocal-math— allow the reciprocal of a value to be used instead of dividing by the value if this enables optimizations 🍓. For examplex / ycan be replaced withx * (1/y), which is useful if(1/y)is subject to common subexpression elimination 🌸. Note that this loses precision and increases the number of flops operating on the value 😬.
The default is -fno-reciprocal-math 💜.
-ffinite-math-only— allow optimizations for floating-point arithmetic that assume that arguments and results are not NaNs or +-Infs ✨.
This option is not turned on by any -O option besides -Ofast, since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions 💀. It may, however, yield faster code for programs that do not require the guarantees of these specifications 🔥.
The default is -fno-finite-math-only 🎀.
-fno-signed-zeros— allow optimizations for floating-point arithmetic that ignore the signedness of zero 😌. IEEE arithmetic specifies the behavior of distinct+0.0and-0.0values, which then prohibits simplification of expressions such asx+0.0or0.0*x(even with-ffinite-math-only) 😭. This option implies that the sign of a zero result isn't significant 💅.
The default is -fsigned-zeros 🦋.
-fno-trapping-math— compile code assuming that floating-point operations cannot generate user-visible traps 🪤. These traps include division by zero, overflow, underflow, inexact result and invalid operation 💀. This option requires that-fno-signaling-nansbe in effect 👑. Setting this option may allow faster code if one relies on "non-stop" IEEE arithmetic, for example ✨.
This option is not turned on by any -O option besides -Ofast, since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions 😭.
The default is -ftrapping-math 🎀.
Future versions of GCC may provide finer control of this setting using C99's FENV_ACCESS pragma 🌈. This command-line option will be used along with -frounding-math to specify the default state for FENV_ACCESS 💖.
-frounding-math— disable transformations and optimizations that assume default floating-point rounding behavior (round-to-nearest) 🔄. This option should be specified for programs that change the FP rounding mode dynamically, or that may be executed with a non-default rounding mode ✨. This option disables constant folding of floating-point expressions at compile time (which may be affected by rounding mode) and arithmetic transformations that are unsafe in the presence of sign-dependent rounding modes 😌.
The default is -fno-rounding-math 🎀.
This option is experimental and does not currently guarantee to disable all GCC optimizations that are affected by rounding mode 🙈. Future versions of GCC may provide finer control of this setting using C99's FENV_ACCESS pragma 🌈. This command-line option will be used along with -ftrapping-math to specify the default state for FENV_ACCESS 💜.
-fsignaling-nans— compile code assuming that IEEE signaling NaNs may generate user-visible traps during floating-point operations 🚨. Setting this option disables optimizations that may change the number of exceptions visible with signaling NaNs 💀. This option implies-ftrapping-math👑.
This option causes the preprocessor macro __SUPPORT_SNAN__ to be defined ✨.
The default is -fno-signaling-nans 🎀.
This option is experimental and does not currently guarantee to disable all GCC optimizations that affect signaling NaN behavior 😭.
-fno-fp-int-builtin-inexact— do not allow the built-in functionsceil,floor,roundandtrunc, and theirfloatandlong doublevariants, to generate code that raises theinexactfloating-point exception for noninteger arguments 🙅. ISO C99 and C11 allow these functions to raise theinexactexception, but ISO/IEC TS 18661-1:2014, the C bindings to IEEE 754-2008, as integrated into ISO C23, does not allow these functions to do so 💀.
The default is -ffp-int-builtin-inexact, allowing the exception to be raised, unless C23 or a later C standard is selected 🎀. This option does nothing unless -ftrapping-math is in effect 👀.
Even if -fno-fp-int-builtin-inexact is used, if the functions generate a call to a library function then the inexact exception may be raised if the library implementation does not follow TS 18661 😭.
-fsingle-precision-constant— treat floating-point constants as single precision instead of implicitly converting them to double-precision constants 🍓.-fcx-limited-range— when enabled, this option states that a range reduction step is not needed when performing complex division 🌀. Also, there is no checking whether the result of a complex multiplication or division isNaN + I*NaN, with an attempt to rescue the situation in that case 🙈. The default is-fno-cx-limited-range, but is enabled by-ffast-math🔥.
This option controls the default setting of the ISO C99 CX_LIMITED_RANGE pragma ✨. Nevertheless, the option applies to all languages 👑.
-fcx-fortran-rules— complex multiplication and division follow Fortran rules 🧚. Range reduction is done as part of complex division, but there is no checking whether the result of a complex multiplication or division isNaN + I*NaN, with an attempt to rescue the situation in that case 😌.
The default is -fno-cx-fortran-rules 🎀.
The following options control optimizations that may improve performance, but are not enabled by any -O options 👀. This section includes experimental options that may produce broken code 💀 — you've been warned, bestie.
-fbranch-probabilities— after running a program compiled with-fprofile-arcs, you can compile it a second time using-fbranch-probabilities, to improve optimizations based on the number of times each branch was taken 📊. When a program compiled with-fprofile-arcsexits, it saves arc execution counts to a file calledsourcename.gcdafor 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 😤. See details about the file naming in-fprofile-arcs🔍.
okay so when -fbranch-probabilities is in the chat, GCC drops a REG_BR_PROB note on literally every JUMP_INSN and CALL_INSN ✨ and those lil notes can be used to level up your optimization. Currently they're only used in one place: over in reorg.cc, instead of guessing which path a branch is most likely to take, the REG_BR_PROB values get used to figure out exactly which path gets taken more often. no vibes-based guessing here 💅
Enabled by -fprofile-use and -fauto-profile.
-fprofile-values— pair this with-fprofile-arcsand it adds code so that some data about the values of expressions in your program gets gathered 🫶
And with -fbranch-probabilities, it reads that data back (the profiling-values data) so it can be used in optimizations.
Enabled by -fprofile-generate, -fprofile-use, and -fauto-profile.
-fprofile-reorder-functions— function reordering based on profile instrumentation: it collects the first time of execution of a function and then orders those functions in ascending order, aiming to optimize program startup by loading text segments more efficiently 🌸
Enabled with -fprofile-use.
-fvpt— if you combine it with-fprofile-arcs, this option tells the compiler to add code that gathers info about the values of expressions 👑
With -fbranch-probabilities, it reads back the gathered data and actually does the optimizations based on it. Currently those optimizations include specialization of division operations using the knowledge about the value of the denominator. math but make it profile-informed fr fr
Enabled with -fprofile-use and -fauto-profile.
-frename-registers— attempts to dodge false dependencies in scheduled code by making use of registers that are left over after register allocation. This optimization most benefits processors with lots of registers 💖
⚠️ Depending on the debug information format adopted by the target, though, it can make debugging impossible, since variables no longer stay in a "home register". so like, iconic but also be careful bestie.
Enabled by default with -funroll-loops.
-fschedule-fusion— performs a target dependent pass over the instruction stream to schedule instructions of the same type together, because the target machine can execute them more efficiently when they're adjacent to each other in the instruction flow ✨
Enabled at levels -O2, -O3, -Os.
-ftracer— performs tail duplication to enlarge superblock size. This transformation simplifies the control flow of the function, which lets other optimizations do a better job 🦋
Enabled by -fprofile-use and -fauto-profile.
-funroll-loops— unrolls 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 (that's complete removal of loops with a small constant number of iterations). Heads up: this option makes code larger, and it may or may not make it run faster. it's giving tradeoff 💀
Enabled by -fprofile-use and -fauto-profile.
-funroll-all-loops— unrolls all the loops, even if their number of iterations is uncertain when the loop is entered 🙈
This usually makes programs run more slowly.
-funroll-all-loops implies the same options as -funroll-loops.
-fpeel-loops— peels loops for which there's enough information that they don't roll much (info coming from profile feedback or static analysis). It also turns on complete loop peeling (aka complete removal of loops with a small constant number of iterations) 🍓
Enabled by -O3, -fprofile-use, and -fauto-profile.
-fmalloc-dce— controls whether"malloc"(and its variants like"calloc"or"strdup") can be optimized away, provided its return value is only used as a parameter of a"free"call or compared with"NULL"💅
If -fmalloc-dce=1 is used, only calls to "free" are allowed, while with -fmalloc-dce=2 comparisons with "NULL" pointer are also considered safe to remove.
The default is -fmalloc-dce=2. See also -fallocation-dce.
-fmove-loop-invariants— enables the loop invariant motion pass in the RTL loop optimizer ✨ Enabled at level-O1and higher, except for-Og.-fmove-loop-stores— enables the loop store motion pass in the GIMPLE loop optimizer. This moves invariant stores to after the end of the loop, in exchange for carrying the stored value in a register across the iteration 🌈
ngl, for this option to have any effect, -ftree-loop-im has to be enabled as well.
Enabled at level -O1 and higher, except for -Og.
-fsplit-loops— splits a loop into two if it contains a condition that's always true for one side of the iteration space and false for the other ✂️
Enabled by -fprofile-use and -fauto-profile.
-funswitch-loops— moves branches with loop invariant conditions out of the loop, duplicating the loop on both branches (modified according to the result of the condition) 💜
Enabled by -fprofile-use and -fauto-profile.
-fversion-loops-for-strides— if a loop iterates over an array with a variable stride, this creates another version of the loop that assumes the stride is always one 👑
For example:
for (int i = 0; i < n; ++i)
x[i * stride] = ...;becomes:
if (stride == 1)
for (int i = 0; i < n; ++i)
x[i] = ...;
else
for (int i = 0; i < n; ++i)
x[i * stride] = ...;This is particularly useful for assumed-shape arrays in Fortran, where (for example) it allows better vectorization assuming contiguous accesses. This flag is enabled by default at -O3. It is also enabled by -fprofile-use and -fauto-profile 🧚
-ffunction-sections,-fdata-sections— place each function or data item into its own section in the output file, if the target supports arbitrary sections. The name of the function (or the name of the data item) determines the section's name in the output file 💅
Use these options on systems where the linker can perform optimizations to improve locality of reference in the instruction space. Most systems using the ELF object format have linkers with such optimizations. On AIX, the linker rearranges sections (CSECTs) based on the call graph. The performance impact varies 🌸
Together with a linker garbage collection (linker --gc-sections option), these options may lead to smaller statically-linked executables (after stripping).
On ELF/DWARF systems these options do not degenerate the quality of the debug information. There could be issues with other object files/debug info formats 👀
Only use these options when there are significant benefits from doing so. When you specify these options, the assembler and linker create larger object and executable files and are also slower. These options affect code generation. They prevent optimizations by the compiler and assembler using relative locations inside a translation unit, since the locations are unknown until link time. An example of such an optimization is relaxing calls to short call instructions. so like, it's giving real consequences bestie 😭
-fstdarg-opt— optimizes the prologue of variadic argument functions with respect to usage of those arguments ✨-fsection-anchors— tries to reduce the number of symbolic address calculations by using shared"anchor"symbols to address nearby objects 🫶 This transformation can help reduce the number of GOT entries and GOT accesses on some targets.
For example, the implementation of the following function "foo":
static int a, b, c;
int foo (void) { return a + b + c; }usually calculates the addresses of all three variables, but if you compile it with -fsection-anchors, it accesses the variables from a common anchor point instead. The effect is similar to the following pseudocode (which isn't valid C):
int foo (void)
{
register int *xr = &x;
return xr[&a - &x] + xr[&b - &x] + xr[&c - &x];
}Not all targets support this option.
-fzero-call-used-regs=choice— zeroes call-used registers at function return to increase program security, by either mitigating Return-Oriented Programming (ROP) attacks or preventing information leakage through registers 👑
The possible values of choice are the same as for the "zero_call_used_regs" attribute. The default is skip.
You can control this behavior for a specific function by using the function attribute "zero_call_used_regs" 💅
--param name=value— in some places, GCC uses various constants to control how much optimization gets done. For example, GCC does not inline functions that contain more than a certain number of instructions. You can control some of these constants on the command line using the--paramoption ✨
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. iykyk 💀
In order to get the minimal, maximal and default values of a parameter, use the --help=param -Q options.
In each case, the value is an integer. The following choices of name are recognized for all targets:
phiopt-factor-max-stmts-live— when factoring statements out of if/then/else, this is the max # of statements after the defining statement to be allowed to extend the lifetime of a name.predictable-branch-outcome— when a branch is predicted to be taken with probability lower than this threshold (in percent), then it is considered well predictable.max-rtl-if-conversion-insns— RTL if-conversion tries to remove conditional branches around a block and replace them with conditionally executed instructions. This parameter gives the maximum number of instructions in a block which should be considered for if-conversion. The compiler will also use other heuristics to decide whether if-conversion is likely to be profitable.file-cache-files— max number of files in the file cache. The file cache is used to print source lines in diagnostics and do some source checks like-Wmisleading-indentation.file-cache-lines— max number of lines to index into file cache. When0this is automatically sized. The file cache is used to print source lines in diagnostics and do some source checks like-Wmisleading-indentation.
max-rtl-if-conversion-predictable-cost— bestie, RTL if-conversion is literally out here trying to yeet conditional branches around a block and swap them for conditionally executed instructions 💅. These parameters hand it the maximum permissible cost for the sequence that if-conversion would generate, and that number depends on whether the branch is statically determined to be predictable or not. The units here are the same as those for the GCC internalseq_costmetric. The compiler will try to give you a reasonable default for this one using theBRANCH_COSTtarget macro.max-crossjump-edges— the maximum number of incoming edges to consider for cross-jumping. The algorithm used by-fcrossjumpingis O(N^2) in the number of edges incoming to each block 👀. So bigger values = more aggressive optimization, which means compile time goes up with probably only a tiny improvement in executable size. pick your battles fr.min-crossjump-insns— the minimum number of instructions that have to match at the end of two blocks before cross-jumping even happens on them. This value gets ignored in the case where all instructions in the block being cross-jumped from are matched.max-grow-copy-bb-insns— the max code size expansion factor when copying basic blocks instead of jumping. The expansion is measured relative to a jump instruction.max-goto-duplication-insns— the max number of instructions to duplicate to a block that jumps to a computed goto. To dodge O(N^2) behavior in a bunch of passes, GCC factors computed gotos early in compilation and unfactors them as late as possible. Only computed jumps at the end of a basic block with no more thanmax-goto-duplication-insnsget unfactored, periodt.max-delay-slot-insn-search— the max number of instructions to consider when hunting for an instruction to fill a delay slot. If more than this arbitrary number of instructions get searched, the time savings from filling the delay slot are minimal, so it stops searching. Bigger values = more aggressive optimization, so compile time increases with probably a small improvement in execution time.max-delay-slot-live-search— when filling delay slots, this is the max number of instructions to consider while searching for a block with valid live register info. Increasing this arbitrarily chosen value means more aggressive optimization, which increases compile time. Note: 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 in"kB"that can be allocated in order to do the global common subexpression elimination optimization. If more memory than specified is required, the optimization just... doesn't happen 🙈.max-gcse-insertion-ratio— if the ratio of expression insertions to deletions is larger than this value for any expression, then RTL PRE inserts or removes the expression and thus leaves partially redundant computations in the instruction stream.max-pending-list-length— the maximum number of pending dependencies scheduling allows before it flushes the current state and starts over. Large functions with few branches or calls can create excessively large lists that needlessly eat up memory and resources 😭.max-modulo-backtrack-attempts— the max number of backtrack attempts the scheduler should make when modulo scheduling a loop. Larger values can exponentially increase compilation time. be careful bestie 💀.max-inline-functions-called-once-loop-depth— maximal loop depth of a call considered by inline heuristics that tries to inline all functions called once.max-inline-functions-called-once-insns— maximal estimated size of functions produced while inlining functions called once.max-inline-insns-single— several parameters control the tree inliner used in GCC, and this number sets the maximum number of instructions (counted in GCC's internal representation) in a single function that the tree inliner considers for inlining. This only affects functions declaredinlineand methods implemented in a class declaration (C++).max-inline-insns-auto— when you use-finline-functions(which is included in-O3), a whole lot of functions that would otherwise not be considered for inlining by the compiler get investigated 👀. To those functions, a different (more restrictive) limit compared to functions declaredinlinecan be applied (--param max-inline-insns-auto).max-inline-insns-small— this is the bound applied to calls that are considered relevant with-finline-small-functions.max-inline-insns-size— this is the bound applied to calls that are optimized for size. Small growth may be desirable to anticipate optimization opportunities exposed by inlining.uninlined-function-insns— number of instructions accounted by inliner for function overhead such as function prologue and epilogue.uninlined-function-time— extra time accounted by inliner for function overhead such as time needed to execute function prologue and epilogue.inline-heuristics-hint-percent— the scale (in percents) applied toinline-insns-single,inline-insns-single-O2,inline-insns-autowhen inline heuristics hints that inlining is very profitable (i.e. it will enable later optimizations). slay ✨.uninlined-thunk-insns,uninlined-thunk-time— same as--param uninlined-function-insnsand--param uninlined-function-timebut applied to function thunks.inline-min-speedup— when the estimated performance improvement of caller + callee runtime exceeds this threshold (in percent), the function can be inlined regardless of the limit on--param max-inline-insns-singleand--param max-inline-insns-auto. main character energy tbh.large-function-insns— the limit specifying really large functions. For functions larger than this limit after inlining, inlining gets constrained by--param large-function-growth. This parameter is useful primarily to avoid extreme compilation time caused by the non-linear algorithms used by the back end.large-function-growth— specifies maximal growth of large functions caused by inlining, in percents. For example, parameter value100limits large function growth to 2.0 times the original size.large-unit-insns— the limit specifying a large translation unit. Growth caused by inlining of units larger than this limit is limited by--param inline-unit-growth. For small units this might be too tight. For example, consider a unit consisting of function A that is inline and B that just calls A three times. If B is small relative to A, the growth of the unit is300\%and yet such inlining is very sane. For very large units consisting of small inlineable functions, however, the overall unit growth limit is needed to avoid exponential explosion of code size. So for smaller units, the size is increased to--param large-unit-insnsbefore applying--param inline-unit-growth.lazy-modules— maximum number of concurrently open C++ module files when lazy loading.inline-unit-growth— specifies maximal overall growth of the compilation unit caused by inlining. For example, parameter value20limits unit growth to 1.2 times the original size. Cold functions (either marked cold via an attribute or by profile feedback) are not accounted into the unit size 🥶.ipa-cp-unit-growth— specifies maximal overall growth of the compilation unit caused by interprocedural constant propagation. For example, parameter value10limits unit growth to 1.1 times the original size.ipa-cp-large-unit-insns— the size of translation unit that the IPA-CP pass considers large.large-stack-frame— the limit specifying large stack frames. While inlining, the algorithm tries not to grow past this limit too much.large-stack-frame-growth— specifies maximal growth of large stack frames caused by inlining, in percents. For example, parameter value1000limits large stack frame growth to 11 times the original size.max-inline-insns-recursive,max-inline-insns-recursive-auto— specifies the maximum number of instructions an out-of-line copy of a self-recursive inline function can grow into by performing recursive inlining.--param max-inline-insns-recursiveapplies to functions declaredinline. For functions not declaredinline, recursive inlining only happens when-finline-functions(included in-O3) is enabled — and in that case,--param max-inline-insns-recursive-autoapplies instead.max-inline-recursive-depth,max-inline-recursive-depth-auto— specifies the maximum recursion depth used for recursive inlining.--param max-inline-recursive-depthapplies to functions declaredinline. For functions not declaredinline, recursive inlining happens only when-finline-functions(included in-O3) is enabled — and in that case,--param max-inline-recursive-depth-autoapplies instead.min-inline-recursive-probability— recursive inlining is only profitable for a function having deep recursion on average, and it can hurt for a function with little recursion depth by increasing the prologue size or the complexity of the function body to other optimizers 😭. When profile feedback is available (see-fprofile-generate), the actual recursion depth can be guessed from the probability that the function recurses via a given call expression. This parameter limits inlining only to call expressions whose probability exceeds the given threshold (in percents).early-inlining-insns— the growth budget you're handing the early inliner 💸 like, how much it's allowed to blow up. Basically it means MORE inlining for code that's carrying a big abstraction penalty.max-early-inliner-iterations— caps how many rounds the early inliner gets to loop. This is what bounds how many nested indirect calls the early inliner can resolve — anything deeper than that still gets handled, just later, by late inlining.comdat-sharing-probability— a probability, in percent, that a C++ inline function with comdat visibility ends up shared across multiple compilation units. ✨modref-max-bases,modref-max-refs,modref-max-accesses— the max number of base pointers, references, and accesses (respectively) that mod/ref analysis will stash for a single function. 📦modref-max-tests— the max* number of tests the alias oracle can run to disambiguate memory locations using mod/ref info. Heads up bestie, this one ought to be bigger than--param modref-max-basesand--param modref-max-refs.modref-max-depth— the maximum depth of the DFS walk that modref escape analysis uses. Set it to0and the analysis is fully disabled, gone, deleted. 💀modref-max-escape-points— the max number of escape points modref tracks per SSA-name.modref-max-adjustments— the maximum number of times the access range gets enlarged during modref dataflow analysis. 🔍profile-func-internal-id— controls whether function internal id is used in profile database lookup. If it's0, the compiler uses an id based on the function assembler name and filename instead — which makes old profile data more tolerant to source changes like function reordering and stuff. 🫶min-vect-loop-bound— the minimum number of iterations below which loops do NOT get vectorized when-ftree-vectorizeis in play. For vectorization to even be allowed, the iteration count after vectorization has to be greater than whatever you set here.gcse-cost-distance-ratio— the scaling factor used when calculating the max distance an expression can be moved by GCSE optimizations. Currently only supported in the code hoisting pass. The bigger the ratio, the more aggressive hoisting gets with simple expressions — meaning ones that cost less thangcse-unrestricted-cost. Set it to0and hoisting of simple expressions is off. 🚫gcse-unrestricted-cost— the cost (roughly the cost of one typical machine instruction) at which GCSE optimizations stop constraining how far an expression can travel. Also currently only supported in the code hoisting pass. The lesser the cost, the more aggressive the hoisting. Set it to0and every expression gets to travel unrestricted distances. 🛫max-hoist-depth— how deep it searches the dominator tree for expressions to hoist. This exists to dodge quadratic behavior in the hoisting algorithm. A value of0means no limit on the search — but it may slow down compilation of huge functions. 🐌max-tail-merge-comparisons— the max number of similar bbs to compare a bb against. Keeps tree tail merging from going quadratic. ⚖️max-tail-merge-iterations— the max number of iterations of the pass over the function. This one's here to limit compilation time in tree tail merging.store-merging-allow-unaligned— lets the store merging pass introduce unaligned stores, if doing so is legal. 🎀max-stores-to-merge— the max number of stores the store merging pass will try to merge into wider stores.max-store-chains-to-track— the max number of store chains tracked at the same time while trying to merge them into wider stores in the store merging pass.max-stores-to-track— the max number of stores tracked at the same time in the attemt to to merge them into wider stores in the store merging pass. (yes the original really says "attemt to to" — iconic typo behavior, we keep it 💅)max-unrolled-insns— the max number of instructions a loop may have for it to be unrolled. If a loop does get unrolled, this parameter also decides how many times the loop code gets unrolled. 🔁max-average-unrolled-insns— same idea but the instruction count is biased by the probabilities of their execution. If a loop is unrolled, this also determines how many times the loop code is unrolled.max-unroll-times— the max number of unrollings of a single loop.max-peeled-insns— the max number of instructions a loop may have for it to be peeled. If a loop is peeled, this parameter also determines how many times the loop code is peeled. 🍌max-peel-times— the max number of peelings of a single loop.max-peel-branches— the max number of branches on the hot path through the peeled sequence.max-completely-peeled-insns— the max number of insns of a completely peeled loop.max-completely-peel-times— the max number of iterations of a loop for it to be suitable for complete peeling.max-completely-peel-loop-nest-depth— the max depth of a loop nest that's suitable for complete peeling.max-unswitch-insns— the max number of insns of an unswitched loop.max-unswitch-depth— the max depth of a loop nest to be unswitched.lim-expensive— the minimum cost of an expensive expression in loop invariant motion.min-loop-cond-split-prob— when FDO profile information is available,min-loop-cond-split-probsets the minimum threshold for the probability of a semi-invariant condition statement to trigger a loop split. 🌸iv-consider-all-candidates-bound— a bound on the number of induction variable candidates; below it, ALL candidates get considered for each use in induction variable optimizations. If there are more candidates than this, only the most relevant ones get considered — to avoid quadratic time complexity. ⏱️iv-max-considered-uses— the induction variable optimizations straight up give up on loops containing more induction variable uses than this. 🙈iv-always-prune-cand-set-bound— if the number of candidates in the set is smaller than this value, always try to remove unnecessary ivs from the set when adding a new one.avg-loop-niter— average number of iterations of a loop.dse-max-object-size— max size, in bytes, of objects tracked bytewise by dead store elimination. Larger values may result in larger compilation times. ⏳dse-max-alias-queries-per-store— max number of queries into the alias oracle per store. Larger values mean larger compilation times and may result in more removed dead stores.scev-max-expr-size— bound on the size of expressions used in the scalar evolutions analyzer. Large expressions slow the analyzer down. 🐢scev-max-expr-complexity— bound on the complexity of expressions in the scalar evolutions analyzer. Complex expressions slow the analyzer down too.max-tree-if-conversion-phi-args— max number of arguments in a PHI that TREE if conversion supports — unless the loop is marked with a simd pragma. ✨vect-max-layout-candidates— the max number of possible vector layouts (like permutations) to consider when optimizing to-be-vectorized code.vect-max-version-for-alignment-checks— the max number of run-time checks that can be performed when doing loop versioning for alignment in the vectorizer.vect-max-version-for-alias-checks— the max number of run-time checks that can be performed when doing loop versioning for alias in the vectorizer.vect-max-peeling-for-alignment— the max number of loop peels to enhance access alignment for the vectorizer. A value of-1means no limit. 👑max-iterations-to-track— the max number of iterations of a loop that the brute-force algorithm for analyzing the loop's iteration count tries to evaluate.hot-bb-count-fraction— the denominator n of the fraction 1/n of the maximal execution count of a basic block in the entire program that a basic block needs to at least have to be considered hot. Default is10000, meaning a basic block counts as hot if its execution count is greater than 1/10000 of the maximal execution count.0means it's never considered hot. Used in non-LTO mode.hot-bb-count-ws-permille— the number of most executed permilles, ranging from0to1000, of the profiled execution of the entire program that a basic block's execution count must be part of to be considered hot. Default is990, meaning a basic block counts as hot if its execution count contributes to the upper 990 permilles — that's 99.0% — of the profiled execution of the entire program.0means it's never considered hot. Used in LTO mode.hot-bb-frequency-fraction— the denominator n of the fraction 1/n of the execution frequency of the entry block of a function that a basic block of that function needs to at least have to be considered hot. Default is1000, meaning a basic block counts as hot within a function if it's executed more frequently than 1/1000 of the frequency of the function's entry block.0means it's never considered hot. 🔥
unlikely-bb-count-fraction
okay so this one is the denominator n of the fraction 1/n — it's about the number of profiled runs of the whole entire program, and it's the cutoff below which a basic block's execution count has to fall for that block to get labeled unlikely executed 👀
The default is 20, so a basic block counts as unlikely executed if it runs in fewer than 1/20, aka 5%, of the program's runs. And if you set it to 0? Then it's always considered unlikely executed, no questions asked 💀
max-predicted-iterations
The max number of loop iterations we predict statically ✨
It's giving very specific-problem-solver energy: it's useful when a function contains a single loop with a known bound and another loop with an unknown bound. The known number of iterations gets predicted correctly, while the unknown number of iterations averages out to roughly 10 😭 So the loop without bounds ends up looking artificially cold next to the other one. awkward!!
builtin-expect-probability
This controls the probability of the expression having the specified value. It takes a percentage (i.e. 0 ... 100) as input 💅
builtin-string-cmp-inline-length
The maximum length of a constant string for a builtin string cmp call that's eligible for inlining 🌸
align-threshold
Selects the fraction of the maximal frequency of executions of a basic block in a function that gets that basic block aligned ✨
align-loop-iterations
A loop that's expected to iterate at least the selected number of iterations gets aligned 🎀
tracer-dynamic-coverage and tracer-dynamic-coverage-feedback
This value limits superblock formation once the given percentage of executed instructions is covered. Basically it caps unnecessary code size expansion — we love a boundary 💅
The tracer-dynamic-coverage-feedback parameter is used only when profile feedback is available. Real profiles (as opposed to the statically estimated ones) are way less balanced, which allows the threshold to be a larger value 👑
tracer-max-code-growth
Stop tail duplication once code growth has reached the given percentage 🛑
ngl this is a rather artificial limit, because most of the duplicates get eliminated later in cross jumping — so it can totally be set to much higher values than is the desired code growth. she's flexible like that 🙈
tracer-min-branch-ratio
Stop reverse growth when the reverse probability of the best edge is less than this threshold (in percent) 🦋
tracer-min-branch-probability and tracer-min-branch-probability-feedback
Stop forward growth if the best edge has probability lower than this threshold ✋
Similarly to tracer-dynamic-coverage, two parameters are provided. tracer-min-branch-probability-feedback is used for compilation with profile feedback, and tracer-min-branch-probability is used for compilation without it. The value for compilation with profile feedback needs to be more conservative (higher) in order to make tracer effective — she needs her space, okay?? 💅
stack-clash-protection-guard-size
Specify the size of the operating system provided stack guard as 2 raised to num bytes 🛡️
Higher values may reduce the number of explicit probes, but listen — a value larger than the operating system provided guard will leave your code vulnerable to stack clash style attacks. do not do that bestie 😭
stack-clash-protection-probe-interval
Stack clash protection involves probing stack space as it is allocated. This param controls the maximum distance between probes into the stack as 2 raised to num bytes 🧚
Higher values may reduce the number of explicit probes, but a value larger than the operating system provided guard will leave code vulnerable to stack clash style attacks. same warning, same stakes, same drama 💀
max-cse-path-length
The maximum number of basic blocks on a path that CSE considers 👑
max-cse-insns
The maximum number of instructions CSE processes before flushing 🚽
ggc-min-expand
So 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 — main character debugging arc fr 💅
ggc-min-heapsize
Minimum size of the garbage collector's heap before it begins bothering to collect garbage 🗑️ The first collection occurs after the heap expands by ggc-min-expand% beyond ggc-min-heapsize. Again, tuning this may improve compilation speed, and has no effect on code generation ✨
The default is the smaller of RAM/8, RLIMIT_RSS, or a limit that tries to ensure that RLIMIT_DATA or RLIMIT_AS are not exceeded, but with a lower bound of 4096 (four megabytes) and an upper bound of 131072 (128 megabytes). If GCC is not able to calculate RAM on a particular platform, the lower bound is used. Setting this parameter very large effectively disables garbage collection 💀 Setting this parameter and ggc-min-expand to zero causes a full collection to occur at every opportunity.
max-reload-search-insns
The maximum number of instructions reload should look backward for an equivalent register 🔍 Increasing values mean more aggressive optimization, making the compilation time increase with probably slightly better performance. trade-off queen 👑
max-cselib-memory-locations
The maximum number of memory locations cselib should take into account. Increasing values mean more aggressive optimization, making the compilation time increase with probably slightly better performance 🌸
max-sched-ready-insns
The maximum number of instructions ready to be issued that 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 🙈
max-sched-region-blocks
The maximum number of blocks in a region to be considered for interblock scheduling ✨
max-pipeline-region-blocks
The maximum number of blocks in a region to be considered for pipelining in the selective scheduler 🎀
max-sched-region-insns
The maximum number of insns in a region to be considered for interblock scheduling 👑
max-pipeline-region-insns
The maximum number of insns in a region to be considered for pipelining in the selective scheduler 💅
min-spec-prob
The minimum probability (in percents) of reaching a source block for interblock speculative scheduling 🦋
max-sched-extend-regions-iters
The maximum number of iterations through CFG to extend regions. A value of 0 disables region extensions 🛑
max-sched-insn-conflict-delay
The maximum conflict delay for an insn to be considered for speculative motion ⏱️
sched-spec-prob-cutoff
The minimal probability of speculation success (in percents), so that speculative insns are scheduled ✨
sched-state-edge-prob-cutoff
The minimum probability an edge must have for the scheduler to save its state across it 💾
sched-mem-true-dep-cost
Minimal distance (in CPU cycles) between a store and a load targeting the same memory locations 🎯
selsched-max-lookahead
The maximum size of the lookahead window of selective scheduling. It is a depth of search for available instructions 👀
selsched-max-sched-times
The maximum number of times that an instruction is scheduled during selective scheduling. This is the limit on the number of iterations through which the instruction may be pipelined 🔁
selsched-insns-to-rename
The maximum number of best instructions in the ready list that are considered for renaming in the selective scheduler 💅
sms-min-sc
The minimum value of stage count that swing modulo scheduler generates 🎀
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 🧠
max-combine-insns
The maximum number of instructions the RTL combiner tries to combine 🧩
max-combine-search-insns
The maximum number of instructions that the RTL combiner searches in order to find the next use of a given register definition. If this limit is reached without finding such a use, the combiner will stop trying to optimize the definition 🛑
Currently this limit only applies after certain successful combination attempts, but it could be extended to other cases in future. iykyk 🔮
integer-share-limit
Small integer constants can use a shared data structure, which reduces the compiler's memory usage and increases its speed 🌈 This sets the maximum value of a shared integer constant.
ssp-buffer-size
The minimum size of buffers (i.e. arrays) that receive stack smashing protection when -fstack-protector is used 🛡️
min-size-for-stack-sharing
The minimum size of variables taking part in stack slot sharing when not optimizing 👑
max-jump-thread-duplication-stmts
Maximum number of statements allowed in a block that needs to be duplicated when threading jumps 🔀
max-jump-thread-paths
The maximum number of paths to consider when searching for jump threading opportunities 🛤️ When arriving at a block, incoming edges are only considered if the number of paths to be searched so far multiplied by the number of incoming edges does not exhaust the specified maximum number of paths to consider. math girlie behaviour fr fr 💅
max-fields-for-field-sensitive
The max number of fields in a struct that get treated in a field sensitive way during pointer analysis. That's the ceiling, bestie. 💅
prefetch-latency
An estimate of, on average, how many instructions get executed before a prefetch actually finishes. The distance we prefetch ahead is proportional to this constant. Heads up though: cranking this number up can also mean fewer streams get prefetched (go peek at simultaneous-prefetches for that whole situation). 🦋
simultaneous-prefetches
The maximum number of prefetches that can be running at the same exact time.
l1-cache-line-size
The size of a cache line in the L1 data cache, measured in bytes. ✨
l1-cache-size
The size of the L1 data cache, in kilobytes.
l2-cache-size
The size of the L2 data cache, in kilobytes. 🎀
prefetch-dynamic-strides
Should the loop array prefetch pass drop software prefetch hints for strides that are non-constant? In some cases that's genuinely beneficial, though the fact that the stride is non-constant can make it hard to predict when there's a clear benefit to issuing those hints.
- Set to
1if the prefetch hints should be issued for non-constant strides. - Set to
0if prefetch hints should be issued only for strides that are known to be constant and belowprefetch-minimum-stride.
prefetch-minimum-stride
The minimum constant stride, in bytes, to start using prefetch hints for. If the stride is less than this threshold, no prefetch hints get issued, periodt.
This setting is a whole vibe for processors that have hardware prefetchers, because in that case there can be conflicts between the hardware prefetchers and the software prefetchers. If the hardware prefetchers have a maximum stride they can handle, that value should go here to improve the use of software prefetchers.
A value of -1 means we don't have a threshold, and therefore prefetch hints can be issued for any constant stride.
This setting is only useful for strides that are known and constant. No cap. 🌸
destructive-interference-size
constructive-interference-size
These two are the values for the C++17 variables std::hardware_destructive_interference_size and std::hardware_constructive_interference_size. Okay so:
- The destructive interference size is the minimum recommended offset between two independent concurrently-accessed objects.
- The constructive interference size is the maximum recommended size of contiguous memory accessed together.
Typically both will be the size of an L1 cache line for the target, in bytes. For a generic target covering a range of L1 cache line sizes, typically the constructive interference size will be the small end of the range and the destructive size will be the large end.
The destructive interference size is intended to be used for layout, and thus has ABI impact. The default value is not expected to be stable, and on some targets it varies with -mtune, so using this variable in a context where ABI stability matters — like the public interface of a library — is strongly discouraged. If it is used in that context, users can stabilize the value using this option.
The constructive interference size is less sensitive, since it's typically only used in a static_assert to make sure that a type fits within a cache line.
See also -Winterference-size. 👑
loop-interchange-max-num-stmts
The maximum number of stmts in a loop that's allowed to be interchanged.
loop-interchange-stride-ratio
The minimum ratio between the stride of two loops for interchange to actually be profitable. 💸
min-insn-to-prefetch-ratio
The minimum ratio between the number of instructions and the number of prefetches needed to enable prefetching in a loop.
prefetch-min-insn-to-mem-ratio
The minimum ratio between the number of instructions and the number of memory references needed to enable prefetching in a loop. 🔥
use-canonical-types
Whether the compiler should use the "canonical" type system. This should always be 1, which uses a more efficient internal mechanism for comparing types in C++ and Objective-C++. But — if bugs in the canonical type system are causing compilation failures, set this value to 0 to disable canonical types.
switch-conversion-max-branch-ratio
Switch initialization conversion refuses to create arrays that are bigger than switch-conversion-max-branch-ratio times the number of branches in the switch. Simple as that.
switch-lower-slow-alg-max-cases
Maximum number of cases for slow switch lowering algorithms to be used. 🐢
max-partial-antic-length
Maximum length of the partial antic set computed during the tree partial redundancy elimination optimization (-ftree-pre) when optimizing at -O3 and above. For some sorts of source code, the enhanced partial redundancy elimination optimization can run away, consuming all of the memory available on the host machine. 😭
This parameter sets a limit on the length of the sets that are computed, which prevents that runaway behavior. Setting a value of 0 for this parameter allows an unlimited set length.
rpo-vn-max-loop-depth
Maximum loop depth that gets value-numbered optimistically. When the limit hits, the innermost rpo-vn-max-loop-depth loops and the outermost loop in the loop nest are value-numbered optimistically, and the remaining ones are not.
sccvn-max-alias-queries-per-access
Maximum number of alias-oracle queries we perform when looking for redundancies for loads and stores. If this limit is hit, the search is aborted and the load or store is not considered redundant. The number of queries is algorithmically limited to the number of stores on all paths from the load to the function entry.
ira-max-loops-num
IRA uses regional register allocation by default. If a function contains more loops than the number given by this parameter, only at most the given number of the most frequently-executed loops form regions for regional register allocation. 💫
ira-max-conflict-table-size
Even though IRA uses a sophisticated algorithm to compress the conflict table, that table can still require excessive amounts of memory for huge functions. If the conflict table for a function could be more than the size in MB given by this parameter, the register allocator instead uses a faster, simpler, and lower-quality algorithm that doesn't require building a pseudo-register conflict table.
ira-loop-reserved-regs
IRA can be used to evaluate more accurate register pressure in loops for decisions to move loop invariants (see -O3). The number of available registers reserved for some other purposes is given by this parameter. Default of the parameter is the best found from numerous experiments. 🧪
ira-consider-dup-in-all-alts
Makes IRA consider the matching constraint (duplicated operand number) heavily in all available alternatives for the preferred register class.
- If it's set as zero, IRA only respects the matching constraint when it's in the only available alternative with an appropriate register class.
- Otherwise, IRA will check all available alternatives for preferred register class even if it has found some choice with an appropriate register class, and respect the found qualified matching constraint.
ira-simple-lra-insn-threshold
Approximate function insn number in 1K units triggering simple local RA.
lra-inheritance-ebb-probability-cutoff
LRA tries to reuse values reloaded in registers in subsequent insns. This optimization is called inheritance. EBB is used as a region to do this optimization. This parameter defines a minimal fall-through edge probability in percentage used to add BB to inheritance EBB in LRA. The default value was chosen from numerous runs of SPEC2000 on x86-64. 🏆
loop-invariant-max-bbs-in-loop
Loop invariant motion can be very expensive — both in compilation time and in amount of needed compile-time memory — with very large loops. Loops with more basic blocks than this parameter won't have loop invariant motion optimization performed on them. Sorry babes, too big. 💀
loop-max-datarefs-for-datadeps
Building data dependencies is expensive for very large loops. This parameter limits the number of data references in loops that are considered for data dependence analysis. These large loops are not handled by the optimizations that use loop data dependencies.
max-vartrack-size
Sets a maximum number of hash table slots to use during variable tracking dataflow analysis of any function.
- If this limit is exceeded with variable tracking at assignments enabled, analysis for that function is retried without it, after removing all debug insns from the function.
- If the limit is exceeded even without debug insns, var tracking analysis is completely disabled for the function.
- Setting the parameter to zero makes it unlimited.
max-vartrack-expr-depth
Sets a maximum number of recursion levels when attempting to map variable names or debug temporaries to value expressions. This trades compilation time for more complete debug information. If this is set too low, value expressions that are available and could be represented in debug information may end up not being used; setting this higher may enable the compiler to find more complex debug expressions, but compile time and memory use may grow. The eternal tradeoff, fr fr. ⚖️
max-debug-marker-count
Sets a threshold on the number of debug markers (e.g. begin stmt markers) to avoid complexity explosion at inlining or expanding to RTL. If a function has more such gimple stmts than the set limit, such stmts will be dropped from the inlined copy of a function, and from its RTL expansion. 🫶
💫 min-nondebug-insn-uid
So, uids for nondebug insns start at this number, bestie. Everything below this parameter is reserved exclusively for the debug insns that -fvar-tracking-assignments creates — but if that reserved range runs out, debug insns can still grab (non-overlapping) uids above it. No cap on the drama, just a cap on the range. 💅
💫 ipa-sra-deref-prob-threshold
IPA-SRA will swap out a pointer that's known not to be NULL for one or more new parameters — but only when the probability (in percent, relative to function entry) of it actually being dereferenced is higher than this parameter. No deref energy, no replacement. Periodt. ✨
💫 ipa-sra-ptr-growth-factor
IPA-SRA replaces a pointer to an aggregate with one or more new parameters only when their cumulative size is less than or equal to ipa-sra-ptr-growth-factor times the size of the original pointer parameter. Growth factor check: passed or it's not happening. 🎀
💫 ipa-sra-ptrwrap-growth-factor
This one's the additional maximum allowed growth of the total size of the new parameters that ipa-sra replaces a pointer to an aggregate with — but only if it points to a local variable that the caller only writes to and passes it as an argument to other functions. Very specific queen behavior. 👑
💫 ipa-sra-max-replacements
Maximum pieces of an aggregate that IPA-SRA tracks. And lowkey, as a consequence, it's also the maximum number of replacements of a formal parameter. Two limits, one parameter, iconic. 🌸
💫 sra-max-scalarization-size-Ospeed and sra-max-scalarization-size-Osize
Okay so the two Scalar Reduction of Aggregates passes (SRA and IPA-SRA) are out here trying to replace scalar parts of aggregates with uses of independent scalar variables. These parameters control the maximum size, in storage units, of aggregate that gets considered for replacement when compiling for speed (sra-max-scalarization-size-Ospeed) or size (sra-max-scalarization-size-Osize) respectively. Pick your fighter. 🦋
💫 sra-max-propagations
The maximum number of artificial accesses that Scalar Replacement of Aggregates (SRA) will track, per one local variable, to facilitate copy propagation. It's giving dedicated. 💖
💫 tm-max-aggregate-size
When making copies of thread-local variables in a transaction, this parameter specifies the size in bytes after which variables are saved with the logging functions as opposed to save/restore code sequence pairs. Heads up bestie: this option only applies when you're using -fgnu-tm. 🫶
💫 graphite-max-nb-scop-params
To avoid exponential effects in the Graphite loop transforms, the number of parameters in a Static Control Part (SCoP) is bounded. A value of zero can be used to lift the bound. And for the record — a variable whose value is unknown at compilation time and defined outside a SCoP is a parameter of the SCoP. Now you know. ✨
💫 hardcfr-max-blocks
Disables -fharden-control-flow-redundancy for functions with a larger number of blocks than the specified value. Zero removes any limit. Zero said "no constraints, we die like icons". 💀
💫 hardcfr-max-inline-blocks
Force -fharden-control-flow-redundancy to use out-of-line checking for functions with a larger number of basic blocks than the specified value. Big function? Out-of-line checking it is. 🧚
💫 loop-block-tile-size
Loop blocking or strip mining transforms — enabled with -floop-block or -floop-strip-mine — strip mine each loop in the loop nest by a given number of iterations. The strip length can be changed using the loop-block-tile-size parameter. Slice it up, bestie. 🔥
💫 ipa-jump-function-lookups
Specifies the number of statements visited during jump function offset discovery. That's it, that's the whole vibe. ⭐
💫 ipa-cp-value-list-size
IPA-CP attempts to track all possible values and types passed to a function's parameter so it can propagate them and perform devirtualization. ipa-cp-value-list-size is the maximum number of values and types it stores per one formal parameter of a function. It's collecting data like it's running a fan account. 📊
💫 ipa-cp-eval-threshold
IPA-CP calculates its own score of cloning profitability heuristics and performs those cloning opportunities with scores that exceed ipa-cp-eval-threshold. Gotta beat the threshold to get the clone. No participation trophies. 💅
💫 ipa-cp-max-recursive-depth
Maximum depth of recursive cloning for a self-recursive function. It can go down, but only this far. 🌀
💫 ipa-cp-min-recursive-probability
Recursive cloning only when the probability of the call being executed exceeds the parameter. Low probability? Not today. 🙈
💫 ipa-cp-profile-count-base
When using the -fprofile-use option, IPA-CP will consider the measured execution count of a call graph edge at this percentage position in their histogram as the basis for its heuristics calculation. Math girlies rise. 🌈
💫 ipa-cp-recursive-freq-factor
The number of times interprocedural copy propagation expects recursive functions to call themselves. Expectations set, bestie. 💜
💫 ipa-cp-recursion-penalty
Percentage penalty the recursive functions will receive when they are evaluated for cloning. Recursion tax, basically. 😭
💫 ipa-cp-single-call-penalty
Percentage penalty functions containing a single call to another function will receive when they are evaluated for cloning. One call? Small penalty. That's the rule. 🎀
💫 ipa-max-agg-items
IPA-CP is also capable of propagating a number of scalar values passed in an aggregate. ipa-max-agg-items controls the maximum number of such values per one parameter. Aggregate girlies, this one's for you. ✨
💫 ipa-cp-loop-hint-bonus
When IPA-CP determines that a cloning candidate would make the number of iterations of a loop known, it adds a bonus of ipa-cp-loop-hint-bonus to the profitability score of the candidate. Bonus points for knowing the loop count. We love a queen who rewards knowledge. 👑
💫 ipa-max-loop-predicates
The maximum number of different predicates IPA will use to describe when loops in a function have known properties. Predicate budget: spent. 🌸
💫 ipa-max-aa-steps
During its analysis of function bodies, IPA-CP employs alias analysis in order to track values pointed to by function parameters. In order not to spend too much time analyzing huge functions, it gives up and considers all memory clobbered after examining ipa-max-aa-steps statements modifying memory. It literally said "I'm tired, everything's clobbered" 😭💀
💫 ipa-max-switch-predicate-bounds
Maximal number of boundary endpoints of case ranges of a switch statement. For a switch exceeding this limit, IPA-CP will not construct the cloning cost predicate — which is used to estimate cloning benefit — for the default case of the switch statement. Too many boundaries? No predicate for you. 🚫
💫 ipa-max-param-expr-ops
IPA-CP will analyze a conditional statement that references some function parameter to estimate benefit for cloning upon a certain constant value. But if the number of operations in a parameter expression exceeds ipa-max-param-expr-ops, the expression is treated as a complicated one and is not handled by IPA analysis. Complicated girlies get skipped. 💅
💫 lto-partitions
Specify the desired number of partitions produced during WHOPR compilation. Bestie, note this: the number of partitions should exceed the number of CPUs used for compilation. More partitions than CPUs, always. 🔥
💫 lto-min-partition
Size of minimal partition for WHOPR (in estimated instructions). This prevents the expense of splitting very small programs into too many partitions. Tiny program? Not getting shredded into confetti. ✂️
💫 lto-max-partition
Size of max partition for WHOPR (in estimated instructions), to provide an upper bound for the individual size of a partition. This is meant to be used only with balanced partitioning. Only. With. Balanced. Partitioning. 🫶
💫 lto-partition-locality-frequency-cutoff
The denominator n of fraction 1/n of the execution frequency of a callee to be cloned for a particular caller. The special value of 0 dictates to always clone without a cut-off. Zero said "clone everybody, no exceptions." 💖
💫 lto-partition-locality-size-cutoff
Size cut-off for a callee — including inlined calls — to be cloned for a particular caller. Size limit, but make it clone-flavored. 🦋
💫 lto-max-locality-partition
Maximal size of a locality partition for LTO (in estimated instructions). A value of 0 results in the default value being used. Zero = default mode activated. ✨
💫 lto-max-streaming-parallelism
Maximal number of parallel processes used for LTO streaming. Parallel processes, we love a multitasking queen. 🧚
💫 cxx-max-namespaces-for-diagnostic-help
The maximum number of namespaces to consult for suggestions when C++ name lookup fails for an identifier. When you typo and it's like "did you mean…" — this is how hard it looks. 🔍
💫 sink-frequency-threshold
The maximum relative execution frequency (in percents) of the target block relative to a statement's original block to allow statement sinking of a statement. Larger numbers result in more aggressive statement sinking. And a small positive adjustment is applied for statements with memory operands, as those are even more profitable to sink. Sink responsibly. 🌊
💫 max-stores-to-sink
The maximum number of conditional store pairs that can be sunk. Set to 0 if either vectorization (-ftree-vectorize) or if-conversion (-ftree-loop-if-convert) is disabled. Both off? Set it to zero. 🚫
💫 case-values-threshold
The smallest number of different values for which it is best to use a jump-table instead of a tree of conditional branches. If the value is 0, use the default for the machine. Let the machine decide. 👑
💫 jump-table-max-growth-ratio-for-size
The maximum code size growth ratio when expanding into a jump table (in percent). The parameter is used when optimizing for size. Shrink era. 🎀
💫 jump-table-max-growth-ratio-for-speed
The maximum code size growth ratio when expanding into a jump table (in percent). The parameter is used when optimizing for speed. Zoom era. 🔥
💫 tree-reassoc-width
Set the maximum number of instructions executed in parallel in a reassociated tree. This parameter overrides the target dependent heuristics used by default if it has a non-zero value. Non-zero = you're in charge now. 💪
💫 sched-pressure-algorithm
Choose between the two available implementations of -fsched-pressure. Algorithm 1 is the original implementation and is the more likely to prevent instructions from being reordered. Algorithm 2 was designed to be a compromise between the relatively conservative approach taken by algorithm 1 and the rather aggressive approach taken by the default scheduler. It relies more heavily on having a regular register file and accurate register pressure classes. See haifa-sched.cc in the GCC sources for more details.
The default choice depends on the target. Yes, it's a mystery. Yes, it's the vibe. 🌀
💫 max-slsr-cand-scan
Set the maximum number of existing candidates that are considered when seeking a basis for a new straight-line strength reduction candidate. Candidate shopping, but make it compiler. 🛍️
asan-globals— turns on buffer overflow detection for global objects 🌍. This protection is on by default when you're using-fsanitize=address. Wanna turn off the global objects protection? Hit it with--param asan-globals=0.asan-stack— enables buffer overflow detection for stack objects 📚. On by default when you're using-fsanitize=address. To kill the stack protection, use--param asan-stack=0.asan-instrument-reads— enables buffer overflow detection for memory reads 👀. On by default when using-fsanitize=address. To disable memory reads protection, use--param asan-instrument-reads=0.asan-instrument-writes— enables buffer overflow detection for memory writes ✍️. On by default when using-fsanitize=address. To disable memory writes protection, use--param asan-instrument-writes=0.asan-memintrin— enables detection for built-in functions 🧩. On by default when using-fsanitize=address. To disable built-in functions protection, use--param asan-memintrin=0.asan-use-after-return— enables detection of use-after-return 🕵️♀️. On by default when using the-fsanitize=addressoption. To disable it, use--param asan-use-after-return=0.
Note: by default this check is disabled at run time. To actually turn it on, add"detect_stack_use_after_return=1"to the environment variableASAN_OPTIONS. No cap, it does nothing until you do that 💀.
asan-instrumentation-with-call-threshold— if the number of memory accesses in the function being instrumented is greater than or equal to this number, use callbacks instead of inline checks 🔁. E.g. to disable inline code, use--param asan-instrumentation-with-call-threshold=0.asan-kernel-mem-intrinsic-prefix— if nonzero, prefix calls to"memcpy","memset"and"memmove"with__asan_or__hwasan_for-fsanitize=kernel-addressor-fsanitize=kernel-hwaddress, respectively ✨.hwasan-instrument-stack— enables hwasan instrumentation of statically sized stack-allocated variables 📦. On by default when using-fsanitize=hwaddress, and disabled by default when using-fsanitize=kernel-hwaddress. To disable stack instrumentation use--param hwasan-instrument-stack=0, and to enable it use--param hwasan-instrument-stack=1.hwasan-random-frame-tag— when using stack instrumentation, decides tags for stack variables using a deterministic sequence beginning at a random tag for each frame 🎲. With this parameter unset, tags are chosen using the same sequence but beginning from 1. Enabled by default for-fsanitize=hwaddressand unavailable for-fsanitize=kernel-hwaddress. To disable it, use--param hwasan-random-frame-tag=0.hwasan-instrument-allocas— enables hwasan instrumentation of dynamically sized stack-allocated variables 🧺. On by default when using-fsanitize=hwaddressand disabled by default when using-fsanitize=kernel-hwaddress. To disable instrumentation of such variables use--param hwasan-instrument-allocas=0, and to enable it use--param hwasan-instrument-allocas=1.hwasan-instrument-reads— enables hwasan checks on memory reads 📖. Instrumentation of reads is on by default for both-fsanitize=hwaddressand-fsanitize=kernel-hwaddress. To disable checking memory reads use--param hwasan-instrument-reads=0.hwasan-instrument-writes— enables hwasan checks on memory writes ✏️. Instrumentation of writes is on by default for both-fsanitize=hwaddressand-fsanitize=kernel-hwaddress. To disable checking memory writes use--param hwasan-instrument-writes=0.hwasan-instrument-mem-intrinsics— enables hwasan instrumentation of builtin functions 🧱. Instrumentation of these builtin functions is on by default for both-fsanitize=hwaddressand-fsanitize=kernel-hwaddress. To disable instrumentation of builtin functions use--param hwasan-instrument-mem-intrinsics=0.use-after-scope-direct-emission-threshold— if the size of a local variable in bytes is smaller than or equal to this number, directly poison (or unpoison) shadow memory instead of using run-time callbacks ☠️.tsan-distinguish-volatile— emits special instrumentation for accesses to volatiles ⚡.tsan-instrument-func-entry-exit— emits instrumentation calls to__tsan_func_entry()and__tsan_func_exit()🚪.max-fsm-thread-path-insns— the maximum number of instructions to copy when duplicating blocks on a finite state automaton jump thread path 🔂.threader-debug—threader-debug=[none|all]enables verbose dumping of the threader solver 🐛.parloops-chunk-size— chunk size of the omp schedule for loops parallelized by parloops 🍰.parloops-schedule— schedule type of the omp schedule for loops parallelized by parloops (static,dynamic,guided,auto,runtime) 📅.parloops-min-per-thread— the minimum number of iterations per thread of an innermost parallelized loop for which the parallelized variant is preferred over the single threaded one 🧵. Heads up: for a parallelized loop nest, the minimum number of iterations of the outermost loop per thread is two.max-ssa-name-query-depth— maximum depth of recursion when querying properties of SSA names in things like fold routines 🌀. One level of recursion corresponds to following a use-def chain.max-speculative-devirt-maydefs— the maximum number of may-defs we analyze when looking for a must-def specifying the dynamic type of an object that invokes a virtual call we may be able to devirtualize speculatively 🎯.ranger-debug— specifies the type of debug output to be issued for ranges 🔍.unroll-jam-min-percent— the minimum percentage of memory references that must be optimized away for the unroll-and-jam transformation to be considered profitable 💸.unroll-jam-max-unroll— the maximum number of times the outer loop should be unrolled by the unroll-and-jam transformation 🔄.max-rtl-if-conversion-unpredictable-cost— maximum permissible cost for the sequence that would be generated by the RTL if-conversion pass for a branch that is considered unpredictable 🎢.max-variable-expansions-in-unroller— if-fvariable-expansion-in-unrolleris used, the maximum number of times that an individual variable will be expanded during loop unrolling 🫧.partial-inlining-entry-probability— maximum probability of the entry BB of the split region (in percent relative to the entry BB of the function) to make partial inlining happen 📊.max-tracked-strlens— maximum number of strings for which the strlen optimization pass will track string lengths 🧮.gcse-after-reload-partial-fraction— the threshold ratio for performing partial redundancy elimination after reload ♻️.gcse-after-reload-critical-fraction— the threshold ratio of critical edges execution count that permits performing redundancy elimination after reload ⚖️.max-loop-header-insns— the maximum number of insns in a loop header duplicated by the copy loop headers pass 📋.vect-epilogues-nomask— enables loop epilogue vectorization using a smaller vector size 🪄.vect-partial-vector-usage— controls when the loop vectorizer considers using partial vector loads and stores as an alternative to falling back to scalar code 🧷.0stops the vectorizer from ever using partial vector loads and stores.1allows partial vector loads and stores if vectorization removes the need for the code to iterate.2allows partial vector loads and stores in all loops.
This parameter only has an effect on targets that support partial vector loads and stores, iykyk 💅.
vect-inner-loop-cost-factor— the maximum factor which the loop vectorizer applies to the cost of statements in an inner loop relative to the loop being vectorized 🧠. The factor applied is the maximum of the estimated number of iterations of the inner loop and this parameter. The default value of this parameter is50.vect-induction-float— enables loop vectorization of floating point inductions 🌊.vect-force-slp— forces the use of SLP when vectorizing, and fails if that's not possible 💥.vrp-block-limit— maximum number of basic blocks before VRP switches to a lower memory algorithm 🧊.vrp-sparse-threshold— maximum number of basic blocks before VRP uses a sparse bitmap cache 🗺️.vrp-switch-limit— maximum number of outgoing edges in a switch before VRP will not process it 🚫.vrp-vector-threshold— maximum number of basic blocks for VRP to use a basic cache vector 📐.avoid-fma-max-bits— maximum number of bits for which we avoid creating FMAs 🙅♀️.fully-pipelined-fma— whether the target fully pipelines FMA instructions 🚰. If non-zero, reassociation considers the benefit of parallelizing FMA's multiplication part and addition part, assumingFMULandFMAuse the same units that can also doFADD.sms-loop-average-count-threshold— a threshold on the average loop count considered by the swing modulo scheduler 🎛️.sms-dfa-history— the number of cycles the swing modulo scheduler considers when checking conflicts using DFA ⏱️.graphite-allow-codegen-errors— whether codegen errors should be ICEs when-fchecking💣.sms-max-ii-factor— a factor for tuning the upper bound that the swing modulo scheduler uses for scheduling a loop 📈.lra-max-considered-reload-pseudos— the max number of reload pseudos which are considered during spilling a non-reload pseudo 🩹.max-pow-sqrt-depth— the maximum depth of sqrt chains to use when synthesizing exponentiation by a real constant. Keeping those chains from going feral, bestie ✨max-dse-active-local-stores— the maximum number of active local stores in RTL dead store elimination. Dead store elimination said "know your limit" 💅asan-instrument-allocas— enable asan allocas/VLAs protection. Safety girlie behavior 🫶max-iterations-computation-cost— a bound on the cost of an expression to compute the number of iterations. Budgeting, but make it compiler-core 💸max-isl-operations— the maximum number of isl operations;0means unlimited. Zero is giving "no rules, just vibes" 🦋graphite-max-arrays-per-scop— the maximum number of arrays per scop. Scop by scop, array by array 🧚max-vartrack-reverse-op-size— max. size of loc list for which reverse ops should be added. Little list, big responsibility 💜fsm-scale-path-stmts— scale factor to apply to the number of statements in a threading path crossing a loop backedge when comparing to--param=max-jump-thread-duplication-stmts. Threading path math, no cap 🔥uninit-control-dep-attempts— the maximum number of nested calls to search for control dependencies during uninitialized variable analysis. Nested calls going deep like your group chat replies 😭uninit-max-chain-len— the maximum number of predicates anded for each predicate ored in the normalized predicate chain. Anded, ored, normalized — grammar but for logic 🌈uninit-max-num-chains— the maximum number of predicates ored in the normalized predicate chain. Chain count, but make it predicate-core ⭐uninit-max-prune-work— the maximum amount of work done to prune paths where the variable is always initialized. Pruning paths like a bonsai bestie 🌸sched-autopref-queue-depth— hardware autoprefetcher scheduler model control flag. Number of lookahead cycles the model looks into; at' 'only enable instruction sorting heuristic. Lookahead cycles, literally peeking into the future 👑loop-versioning-max-inner-insns— the maximum number of instructions that an inner loop can have before the loop versioning pass considers it too big to copy. Inner loop said "don't copy me, I'm too iconic" 💅loop-versioning-max-outer-insns— the maximum number of instructions that an outer loop can have before the loop versioning pass considers it too big to copy, discounting any instructions in inner loops that directly benefit from versioning. Outer loop boundaries, but with inner-loop exceptions 🦋ssa-name-def-chain-limit— the maximum number ofSSA_NAMEassignments to follow in determining a property of a variable such as its value. This limits the number of iterations or recursive calls GCC performs when optimizing certain statements or when determining their validity prior to issuing diagnostics. SSA_NAME detective work, but with a chain limit 🔍store-merging-max-size— maximum size of a single store merging region in bytes. Merge the stores, but keep it byte-sized 🎀store-forwarding-max-distance— maximum number of instruction distance that a small store forwarded to a larger load may stall. Value'0'disables the cost checks for the avoid-store-forwarding pass. Distance check, and'0'says "turn off the cost checks" 💀hash-table-verification-limit— the number of elements for which hash table verification is done for each searched element. Verification limit, because we can't check everything forever ✨max-find-base-term-values— maximum number ofVALUEs handled during a singlefind_base_termcall.VALUEs getting handled, onefind_base_termat a time 🫶analyzer-max-enodes-per-program-point— the maximum number of exploded nodes per program point within the analyzer, before terminating analysis of that point. Exploded nodes? The analyzer is dramatic, periodt 😭analyzer-max-constraints— the maximum number of constraints per state. Constraints per state, like boundaries but for the analyzer 🧚analyzer-min-snodes-for-call-summary— the minimum number of supernodes within a function for the analyzer to consider summarizing its effects at call sites. Supernodes minimum, because summaries need a little substance 💜analyzer-max-enodes-for-full-dump— the maximum depth of exploded nodes that should appear in a dot dump before switching to a less verbose format. Dot dump depth limit, then it goes quiet 🌸analyzer-max-recursion-depth— the maximum number of times a callsite can appear in a call stack within the analyzer, before terminating analysis of a call that would recurse deeper. Recursion depth said "not me going deeper than this" 🔥analyzer-max-svalue-depth— the maximum depth of a symbolic value, before approximating the value as unknown. Symbolic value depth, then it's giving "unknown" 💅analyzer-max-infeasible-edges— the maximum number of infeasible edges to reject before declaring a diagnostic as infeasible. Infeasible edges rejected until the diagnostic says "infeasible" 🌈gimple-fe-computed-hot-bb-threshold— the number of executions of a basic block which is considered hot. The parameter is used only in GIMPLE FE. Hot basic block threshold, GIMPLE FE only ⭐analyzer-bb-explosion-factor— the maximum number of'after supernode'exploded nodes within the analyzer per supernode, before terminating analysis. Explosion factor, but for after-supernode exploded nodes 💥analyzer-text-art-string-ellipsis-threshold— the number of bytes at which to ellipsize string literals in analyzer text art diagrams. Text art string literals getting the ellipsis treatment 🎀analyzer-text-art-ideal-canvas-width— the ideal width in characters of text art diagrams generated by the analyzer. Ideal canvas width, because text art deserves a good aspect ratio 🦋analyzer-text-art-string-ellipsis-head-len— the number of literal bytes to show at the head of a string literal in text art when ellipsizing it. Head bytes on display when ellipsizing 💜analyzer-text-art-string-ellipsis-tail-len— the number of literal bytes to show at the tail of a string literal in text art when ellipsizing it. Tail bytes on display when ellipsizing ✨ranger-logical-depth— maximum depth of logical expression evaluation ranger will look through when evaluating outgoing edge ranges. Ranger looking through logical expressions, deep 🔍ranger-recompute-depth— maximum depth of instruction chains to consider for recomputation in the outgoing range calculator. Recompute depth, because sometimes you run it back 🔁relation-block-limit— maximum number of relations the oracle will register in a basic block. Oracle relation limit per basic block, iconic 👑transitive-relations-work-bound— work bound when discovering transitive relations from existing relations. Transitive relations discovery, but bounded 🧚min-pagesize— minimum page size for warning and early break vectorization purposes. Minimum page size, warning and early break vectorization business 💅openacc-kernels— specify mode of OpenACC`kernels'constructs handling. With--param=openacc-kernels=decompose, OpenACC`kernels'constructs are decomposed into parts, a sequence of compute constructs, each then handled individually. This is work in progress. With--param=openacc-kernels=parloops, OpenACC`kernels'constructs are handled by the parloops pass, en bloc. This is the current default. OpenACC kernels mode, and yes, decompose is work in progress 🚧openacc-privatization— control whether the-fopt-info-omp-noteand applicable-fdump-tree-*-detailsoptions emit OpenACC privatization diagnostics. With--param=openacc-privatization=quiet, don't diagnose. This is the current default. With--param=openacc-privatization=noisy, do diagnose. Quiet vs noisy diagnostics, current default is quiet 🤫cycle-accurate-model— specifies whether GCC should assume that the scheduling description is mostly a cycle-accurate model of the target processor the code is intended to run on, in the absence of cache misses. Nonzero means that the selected scheduling model is accurate and likely describes an in-order processor, and that scheduling should aggressively spill to try and fill any pipeline bubbles. This is the current default. Zero means the scheduling description might not be available/accurate or perhaps not applicable at all, such as for modern out-of-order processors. Cycle-accurate model tea: nonzero is current default, zero is for out-of-order modernity ⚙️
The following choices of name are available on AArch64 targets: AArch64 girlies, this one's for you 💖
aarch64-vect-compare-costs— when vectorizing, consider using multiple different approaches and use the cost model to choose the cheapest one. This includes:- Trying both SVE and Advanced SIMD, when SVE is available.
- Trying to use 64-bit Advanced SIMD vectors for the smallest data elements, rather than using 128-bit vectors for everything.
- Trying to use "unpacked" SVE vectors for smaller elements. This includes storing smaller elements in larger containers and accessing elements with extending loads and truncating stores.
aarch64-float-recp-precision— the number of Newton iterations for calculating the reciprocal for float type. The precision of division is proportional to this param when division approximation is enabled. The default value is1. Newton iterations for float reciprocal, default1🍓aarch64-double-recp-precision— the number of Newton iterations for calculating the reciprocal for double type. The precision of division is proportional to this param when division approximation is enabled. The default value is2. Double reciprocal precision, default2💜aarch64-autovec-preference— force an ISA selection strategy for auto-vectorization.default— use the default heuristics.asimd-only— use only Advanced SIMD for auto-vectorization.sve-only— use only SVE for auto-vectorization.prefer-asimd— use both Advanced SIMD and SVE. Prefer Advanced SIMD when the costs are deemed equal.prefer-sve— use both Advanced SIMD and SVE. Prefer SVE when the costs are deemed equal.
aarch64-ldp-policy— fine-grained policy for load pairs. With--param=aarch64-ldp-policy=default, use the policy of the tuning structure. This is the current default. With--param=aarch64-ldp-policy=always, emitldpregardless of alignment. With--param=aarch64-ldp-policy=never, do not emitldp. With--param=aarch64-ldp-policy=aligned, emitldponly if the source pointer is aligned to at least double the alignment of the type. Load pair policy: default, always, never, aligned — pick your fighter 💅aarch64-stp-policy— fine-grained policy for store pairs. With--param=aarch64-stp-policy=default, use the policy of the tuning structure. This is the current default. With--param=aarch64-stp-policy=always, emitstpregardless of alignment. With--param=aarch64-stp-policy=never, do not emitstp. With--param=aarch64-stp-policy=aligned, emitstponly if the source pointer is aligned to at least double the alignment of the type. Store pair policy, same energy, different letters 🎀aarch64-ldp-alias-check-limit— limit on the number of alias checks performed by the AArch64 load/store pair fusion pass when attempting to form anldp/stp. Higher values make the pass more aggressive at re-ordering loads over stores, at the expense of increased compile time. Higher values = more aggressive re-ordering, more compile time, no cap 🔥aarch64-ldp-writeback— bestie, this one decides which writeback opportunities we even bother trying to handle in the AArch64 load/store pair fusion pass. Set it to zero and writeback handling is just... off, dead, not invited. One means we try to form pairs involving one or more existing individual writeback accesses where we can. Two means we also go hunting opportunistically, folding in trailing destructive updates of the base register that a pair uses. 💅aarch64-loop-vect-issue-rate-niters— okay so some AArch64 CPUs have tuning that wants to weigh both latencies and issue rates when it's deciding whether a loop gets vectorized with SVE, vectorized with Advanced SIMD, or not vectorized at all (tragic). If you set this parameter to n, GCC will not use that heuristic for loops known to execute in fewer than n Advanced SIMD iterations. 🦋aarch64-vect-unroll-limit— the vectorizer checks the available tuning info to figure out if unrolling the main vectorized loop would be beneficial, and by how much. This parameter sets the upper bound on how much the vectorizer will unroll the main loop. Default value is four. no cap ✨
The following choices of name are available on GCN targets:
gcn-preferred-vectorization-factor— preferred vectorization factor:default,32,64. that's it that's the list 🎀
The following choices of name are available on i386 and x86_64 targets:
x86-stlf-window-ninsns— instructions number above which the STFL stall penalty can be compensated.x86-stv-max-visits— the maximum number of use and def visits when discovering a STV chain before the discovery gets aborted. 🙈
🌈 Program Instrumentation Options 🌈
GCC supports a whole lil collection of command-line options that control adding run-time instrumentation to the code it normally generates. Like, one purpose of instrumentation is collecting profiling statistics so you can find program hot spots, do code coverage analysis, or run profile-guided optimizations. Another class of program instrumentation is adding run-time checking to catch programming errors like invalid pointer dereferences or out-of-bounds array accesses — plus deliberately hostile attacks, the villain era stuff, like stack smashing or C++ vtable hijacking. And there's also a general hook you can use to implement other forms of tracing or function-level instrumentation for debug or program analysis purposes. 🧚
-p-pg— generate extra code that writes profile information suitable for the analysis programprof(that's for-p) orgprof(that's for-pg). You must use this option when compiling the source files you want data about, and you must also use it when linking. both times, bestie, don't skip the link step 😭
You can use the function attribute no_instrument_function to suppress profiling of individual functions when compiling with these options. lowkey handy.
-fprofile-arcs— add code so that program flow arcs get instrumented. During execution the program records how many times each branch and call is executed, and how many times it is taken or returns. On targets that support constructors with priority support, profiling properly handles constructors, destructors, and C++ constructors (and destructors) of classes which are used as a type of a global variable. 👑
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).
Note that if a command line directly links source files, the corresponding .gcda files will be prefixed with the unsuffixed name of the output file. E.g. gcc a.c b.c -o binary would generate binary-a.gcda and binary-b.gcda files. 📁
-fcondition-coverage— add code so that program conditions are instrumented. During execution the program records what terms in a conditional contribute to a decision, which you can use to verify that all terms in a Boolean function are tested and have an independent effect on the outcome of a decision. The result can be read withgcov --conditions. 🍓-fpath-coverage— add code so that the paths taken are tracked. During execution the program records the prime paths taken. The number of paths grows very fast with complexity, and to avoid exploding compile times GCC will give up instrumentation if the approximate number of paths exceeds the limit controlled by-fpath-coverage-limit. The result can be read withgcov --prime-paths --prime-paths-lines --prime-paths-source, 💀-fpath-coverage-limit=limit— the threshold at which point-fpath-coveragegives up on instrumenting a function. This limit is approximate and conservative, because GCC uses a pessimistic heuristic which slightly overcounts the running number of paths, and gives up if the threshold is reached before finding all the paths. This option is not for fine grained control over which functions to instrument — rather it is intended to limit the effect of path explosion and keep compile times reasonable. The default is250000.--coverage— this option is used to compile and link code instrumented for coverage analysis. It's 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-arcsplus optimization and code generation options. For test coverage analysis, use the additional-ftest-coverageoption. You do not need to profile every source file in a program. - Compile the source files additionally with
-fprofile-abs-pathto create absolute path names in the.gcnofiles. This allowsgcovto find the correct sources in projects where compilations occur with different working directories. - Link your object files with
-lgcovor-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. Unless a strict ISO C dialect option is in effect,
forkcalls are detected and correctly handled without double counting.
- Compile the source files with
Moreover, an object file can be recompiled multiple times and the corresponding .gcda file merges as long as the source file and the compiler options are unchanged. 🫶
- For profile-directed optimizations, compile the source files again with the same optimization and code generation options plus
-fbranch-probabilities. - For test coverage analysis, use
gcovto produce human readable information from the.gcnoand.gcdafiles. Refer to thegcovdocumentation 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. iconic compiler behaviour honestly.
With -fcondition-coverage, for each conditional in your program GCC creates a bitset and records the exercised boolean values that have an independent effect on the outcome of that expression.
With -fpath-coverage, GCC finds and enumerates and records the taken prime paths of each function, unless the number of paths would exceed the limit controlled by -fpath-coverage-limit. If the limit is exceeded the function is not instrumented as if -fpath-coverage was not used. A prime path is the longest sequence of unique blocks, except possibly the first and last, which is not a subpath of any other path. 🌸
-ftest-coverage— produce a notes file that thegcovcode-coverage utility can use to show program coverage. Each source file's note file is calledauxname.gcno. Refer to the-fprofile-arcsoption above for a description ofauxnameand instructions on how to generate test coverage data. Coverage data matches the source files more closely if you do not optimize. 🎀-fprofile-abs-path— automatically convert relative source file names to absolute path names in the.gcnofiles. This allowsgcovto find the correct sources in projects where compilations occur with different working directories.-fprofile-dir=path— set the directory to search for the profile data files in to path. This option affects only the profile data generated by-fprofile-generate,-ftest-coverage,-fprofile-arcsand used by-fprofile-useand-fbranch-probabilitiesand its related options. Both absolute and relative paths can be used. By default, GCC uses the current directory as path, thus the profile data file appears in the same directory as the object file. In order to prevent the file name clashing, if the object file name is not an absolute path, we mangle the absolute path of thesourcename.gcdafile and use it as the file name of a.gcdafile. See details about the file naming in-fprofile-arcs. See similar option-fprofile-note.
When an executable is run in a massive parallel environment, it is recommended to save profile to different folders. That can be done with variables in path that are exported during run-time:
%p— process ID.%q{VAR}— value of environment variableVAR👑
-fprofile-generate-fprofile-generate=path— enable options usually used for instrumenting application to produce profile useful for later recompilation with profile feedback based optimization. You must use-fprofile-generateboth when compiling and when linking your program. 💖
The following options are enabled: -fprofile-arcs, -fprofile-values, -finline-functions, and -fipa-bit-cp.
If path is specified, GCC looks at the path to find the profile feedback data files. See -fprofile-dir.
To optimize the program based on the collected profile information, use -fprofile-use.
-fprofile-info-section,-fprofile-info-section=name — registers the profile info in a section you pick instead of doing the whole constructor/destructor thing 💅. The section name is name if you pass it, otherwise it defaults to.gcov_info. A pointer to the profile info that-fprofile-arcsgenerates gets dropped into that section for every translation unit. Heads up: this option disables profile info registration via constructor and disables profile info processing via destructor. And bestie, this one is not meant for hosted environments like GNU/Linux — it's for freestanding environments (embedded systems, the resource-limited girlies) that don't support constructors/destructors or C library file I/O 🫶.
The linker could collect the input sections in a continuous memory block and define start and end symbols. A GNU linker script example which defines a linker output section follows:
.gcov_info :
{
PROVIDE (__gcov_info_start = .);
KEEP (*(.gcov_info))
PROVIDE (__gcov_info_end = .);
}The program could dump the profiling information registered in this linker set for example like this:
#include <gcov.h>
#include <stdio.h>
#include <stdlib.h>
extern const struct gcov_info *const __gcov_info_start[];
extern const struct gcov_info *const __gcov_info_end[];
static void
dump (const void *d, unsigned n, void *arg)
{
const unsigned char *c = d;
for (unsigned i = 0; i < n; ++i)
printf ("%02x", c[i]);
}
static void
filename (const char *f, void *arg)
{
__gcov_filename_to_gcfn (f, dump, arg );
}
static void *
allocate (unsigned length, void *arg)
{
return malloc (length);
}
static void
dump_gcov_info (void)
{
const struct gcov_info *const *info = __gcov_info_start;
const struct gcov_info *const *end = __gcov_info_end;
/* Obfuscate variable to prevent compiler optimizations. */
__asm__ ("" : "+r" (info));
while (info != end)
{
void *arg = NULL;
__gcov_info_to_gcda (*info, filename, dump, allocate, arg);
putchar ('\n');
++info;
}
}
int
main (void)
{
dump_gcov_info ();
return 0;
}The merge-stream subcommand of gcov-tool may be used to deserialize the data stream generated by the __gcov_filename_to_gcfn and __gcov_info_to_gcda functions and merge the profile information into .gcda files on the host filesystem.
-fprofile-note=path — if path is specified, GCC saves the.gcnofile into the path location. ⚠️ But if you combine the option with multiple source files, the.gcnofile will be overwritten. One file, bestie. That's the deal 🙈.-fprofile-prefix-path=path — this one can be used in combination withprofile-generate=profile_dir andprofile-use=profile_dir to inform GCC where the base directory of the built source tree is. By default profile_dir will contain files with mangled absolute paths of all object files in the built project. That's not desirable when the directory used to build the instrumented binary differs from the directory used to build the binary optimized with profile feedback, because then the profile data will not be found during the optimized build 😭. In such setups,-fprofile-prefix-path=path with path pointing to the base directory of the build can be used to strip the irrelevant part of the path and keep all file names relative to the main build directory. Iconic path-cleanup behaviour ✨.-fprofile-prefix-map=old=new — when compiling files residing in directory old, record profiling information (with--coverage) describing them as if the files resided in directory new instead. See also-ffile-prefix-mapand-fcanon-prefix-map🦋.-fprofile-update=method — alters the update method for an application instrumented for profile feedback based optimization. The method argument should be one ofsingle,atomicorprefer-atomic. The first one is useful for single-threaded applications, while the second one prevents profile corruption by emitting thread-safe code 🧵✨.
Warning: When an application does not properly join all threads (or creates a detached thread), a profile file can still be corrupted. 💀
Using prefer-atomic would be transformed either to atomic, when supported by a target, or to single otherwise. The GCC driver automatically selects prefer-atomic when -pthread is present in the command line, otherwise the default method is single.
If atomic is selected, then the profile information is updated using atomic operations on a best-effort basis. Ideally, the profile information is updated through atomic operations in hardware. If the target platform does not support the required atomic operations in hardware, however, libatomic is available, then the profile information is updated through calls to libatomic. If the target platform neither supports the required atomic operations in hardware nor libatomic, then the profile information is not atomically updated and a warning is issued. In this case, the obtained profiling information may be corrupt for multi-threaded applications. 🫠
For performance reasons, if 64-bit counters are used for the profiling information and the target platform only supports 32-bit atomic operations in hardware, then the performance critical profiling updates are done using two 32-bit atomic operations for each counter update. If a signal interrupts these two operations updating a counter, then the profiling information may be in an inconsistent state 😵💫.
-fprofile-filter-files=regex — instrument only functions from files whose name matches any of the regular expressions (separated by semi-colons).
For example, -fprofile-filter-files=main\.c;module.*\.c will instrument only main.c and all C files starting with 'module' 🎀.
-fprofile-exclude-files=regex — instrument only functions from files whose name does not match any of the regular expressions (separated by semi-colons).
For example, -fprofile-exclude-files=/usr/.* will prevent instrumentation of all files that are located in the /usr/ folder 🚫.
-fprofile-reproducible=[multithreaded|parallel-runs|serial]— controls the level of reproducibility of profile gathered by-fprofile-generate. This makes it possible to rebuild a program with the same outcome, which is useful, for example, for distribution packages 📦✨.
With -fprofile-reproducible=serial the profile gathered by -fprofile-generate is reproducible provided the trained program behaves the same at each invocation of the train run, it is not multi-threaded and profile data streaming is always done in the same order. Note that profile streaming happens at the end of program run but also before fork function is invoked.
Note that it is quite common that execution counts of some part of programs depends, for example, on length of temporary file names or memory space randomization (that may affect hash-table collision rate). Such non-reproducible part of programs may be annotated by no_instrument_function function attribute. gcov-dump with -l can be used to dump gathered data and verify that they are indeed reproducible 🔍.
With -fprofile-reproducible=parallel-runs collected profile stays reproducible regardless the order of streaming of the data into gcda files. This setting makes it possible to run multiple instances of instrumented program in parallel (such as with make -j). This reduces quality of gathered data, in particular of indirect call profiling 📉.
-fsanitize=address— enable AddressSanitizer, a fast memory error detector 🔥. Memory access instructions are instrumented to detect out-of-bounds and use-after-free bugs. The option enables-fsanitize-address-use-after-scope. Seehttps://github.com/google/sanitizers/wiki/AddressSanitizerfor more details. The run-time behavior can be influenced using theASAN_OPTIONSenvironment variable. When set tohelp=1, the available options are shown at startup of the instrumented program. Seehttps://github.com/google/sanitizers/wiki/AddressSanitizerFlags#run-time-flagsfor a list of supported options. The option cannot be combined with-fsanitize=threador-fsanitize=hwaddress. Note that the only targets-fsanitize=hwaddressis currently supported on are x86-64 (only with-mlam=u48or-mlam=u57options) and AArch64, in both cases only in ABIs with 64-bit pointers 👑
When compiling with -fsanitize=address, you should also use -g to produce more meaningful output. To get more accurate stack traces, it is possible to use options such as -O0, -O1, or -Og (which, for instance, prevent most function inlining), -fno-optimize-sibling-calls (which prevents optimizing sibling and tail recursive calls; this option is implicit for -O0, -O1, or -Og), or -fno-ipa-icf (which disables Identical Code Folding for functions). Using -fno-omit-frame-pointer also improves stack traces 🪜. Since multiple runs of the program may yield backtraces with different addresses due to ASLR (Address Space Layout Randomization), it may be desirable to turn ASLR off. On Linux, this can be achieved with setarch \uname -m` -R ./prog`.
-fsanitize=kernel-address— enable AddressSanitizer for the Linux kernel. Seehttps://github.com/google/kernel-sanitizersfor more details 🌸.-fsanitize=hwaddress— bestie, this one is hardware-assisted and it's giving main character energy ✨ It enables Hardware-assisted AddressSanitizer, which uses a hardware ability to ignore the top byte of a pointer so it can catch memory errors with low memory overhead. Memory access instructions get instrumented to sniff out out-of-bounds and use-after-free bugs 🦋
This option also turns on -fsanitize-address-use-after-scope. Want the full lore? See https://clang.llvm.org/docs/HardwareAssistedAddressSanitizerDesign.html 💖
Runtime behavior can be shaped with the HWASAN_OPTIONS environment variable. Set it to "help=1" and the available options pop up at startup of the instrumented program 🌸
Heads up queen: this option cannot be combined with-fsanitize=threador-fsanitize=address, and it's currently only available on AArch64. No exceptions 💅
-fsanitize=kernel-hwaddress— this is Hardware-assisted AddressSanitizer but for compiling the Linux kernel 👑 It's similar to-fsanitize=kernel-addressbut uses an alternate instrumentation method, and similar to-fsanitize=hwaddressbut with instrumentation differences that are necessary for compiling the Linux kernel. Those differences exist to avoid hwasan library initialization calls, and to account for the stack pointer having a different value in its top byte 🧚
Note: this option has different defaults to-fsanitize=hwaddress. Instrumenting the stack andallocacalls are not on by default, but you can still get them by specifying the command-line options--param hwasan-instrument-stack=1and--param hwasan-instrument-allocas=1respectively. Using a random frame tag is not implemented for kernel instrumentation. Fr fr, read that twice 📌
-fsanitize=pointer-compare— instruments comparison operations (<,<=,>,>=) that have pointer operands 🔍 You must combine this with either-fsanitize=kernel-addressor-fsanitize=address. And it cannot be combined with-fsanitize=thread😤
Note: by default the check is disabled at run time. To turn it on, add"detect_invalid_pointer_pairs=2"to the environment variableASAN_OPTIONS. Using"detect_invalid_pointer_pairs=1"detects the invalid operation only when both pointers are non-null. Iykyk 💀
-fsanitize=pointer-subtract— instruments subtraction with pointer operands ➖ Same deal as its sibling: the option must be combined with either-fsanitize=kernel-addressor-fsanitize=address, and it cannot be combined with-fsanitize=thread🚫
Note: by default the check is disabled at run time. To enable it, add"detect_invalid_pointer_pairs=2"to the environment variableASAN_OPTIONS. Using"detect_invalid_pointer_pairs=1"detects the invalid operation only when both pointers are non-null 🍓
-fsanitize=shadow-call-stack— okay this one is a whole security moment 🛡️ It enables ShadowCallStack, a security enhancement mechanism used to protect programs against return address overwrites (think stack buffer overflows, e.g.). How does it work? It saves a function's return address to a separately allocated shadow call stack in the function prologue, then restores the return address from the shadow call stack in the function epilogue. Instrumentation only happens in functions that actually need to save the return address to the stack 🌈
Currently it only supports the aarch64 platform. It's specifically designed for linux kernels that enable the CONFIG_SHADOW_CALL_STACK option. For user space programs, runtime support is not currently provided in libc and libgcc 🙈 Users who want to use this feature in user space need to provide their own runtime support. It should be noted that this may cause the ABI rules to be broken 😭
On aarch64, the instrumentation makes use of the platform register "x18". This generally means that any code that may run on the same thread as code compiled with ShadowCallStack must be compiled with the flag -ffixed-x18, otherwise functions compiled without -ffixed-x18 might clobber "x18" and so corrupt the shadow stack pointer 💅
Also — and this is important — because there is no userspace runtime support, code compiled with ShadowCallStack cannot use exception handling. Use -fno-exceptions to turn off exceptions ✨
More details live at https://clang.llvm.org/docs/ShadowCallStack.html 💜
-fsanitize=thread— enables ThreadSanitizer, a fast data race detector 🏁 Memory access instructions are instrumented to detect data race bugs. The lore is at https://github.com/google/sanitizers/wiki#threadsanitizer 📖 Runtime behavior can be influenced with theTSAN_OPTIONSenvironment variable; see https://github.com/google/sanitizers/wiki/ThreadSanitizerFlags for a list of supported options 🎀
This option cannot be combined with-fsanitize=addressor-fsanitize=leak. Pick a lane bestie 😌
When compiling with -fsanitize=thread, you should also use -g so the output is more meaningful 👑
Note that sanitized atomic builtins cannot throw exceptions when operating on invalid memory addresses with non-call exceptions (-fnon-call-exceptions) ⚠️
-fsanitize=leak— enables LeakSanitizer, a memory leak detector 💧 This option only matters for linking of executables. The executable gets linked against a library that overridesmallocand other allocator functions. More details at https://github.com/google/sanitizers/wiki/AddressSanitizerLeakSanitizer 🌊 Runtime behavior can be influenced with theLSAN_OPTIONSenvironment variable ✨
This option cannot be combined with -fsanitize=thread. Periodt 🚫
-fsanitize=undefined— enables UndefinedBehaviorSanitizer, a fast undefined behavior detector 💥 Various computations get instrumented to detect undefined behavior at runtime. Details at https://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html 📚 Runtime behavior can be influenced with theUBSAN_OPTIONSenvironment variable 🧚
Current suboptions are:
-fsanitize=shift— enables checking that the result of a shift operation is not undefined 🌀 Note that what exactly counts as undefined differs slightly between C and C++, and between ISO C90 and C99, etc. This option has two suboptions:-fsanitize=shift-baseand-fsanitize=shift-exponent✌️-fsanitize=shift-exponent— enables checking that the second argument of a shift operation is not negative and is smaller than the precision of the promoted first argument 🔢-fsanitize=shift-base— if the second argument of a shift operation is within range, check that the result of the shift operation is not undefined ✅ Note that what exactly is considered undefined differs slightly between C and C++, and between ISO C90 and C99, etc. 💭-fsanitize=integer-divide-by-zero— detects integer division by zero 🚨-fsanitize=unreachable— with this option, the compiler turns the"__builtin_unreachable"call into a diagnostics message call instead 📣 When reaching the"__builtin_unreachable"call, the behavior is undefined 💀-fsanitize=vla-bound— tells the compiler to check that the size of a variable length array is positive 📏-fsanitize=null— enables pointer checking 👆 Specifically, an application built with this option turned on will issue an error message when it tries to dereference aNULLpointer, or if a reference (possibly an rvalue reference) is bound to aNULLpointer, or if a method is invoked on an object pointed to by aNULLpointer 😵-fsanitize=return— enables return statement checking 🎯 Programs built with this option turned on will issue an error message when the end of a non-void function is reached without actually returning a value. This option works in C++ only 👑-fsanitize=signed-integer-overflow— enables signed integer overflow checking ➕ We check that the result of"+","*", and both unary and binary"-"does not overflow in the signed arithmetics. This also detects"INT_MIN / -1"signed division 🧮
Note: integer promotion rules must be taken into account. That is, the following is not an overflow:
signed char a = SCHAR_MAX;
a++;-fsanitize=bounds— enables instrumentation of array bounds 🧱 Various out of bounds accesses are detected. Flexible array members, flexible array member-like arrays, and initializers of variables with static storage are not instrumented — with the exception of flexible array member-like arrays for which"-fstrict-flex-arrays"or"-fstrict-flex-arrays="options or"strict_flex_array"attributes say they shouldn't be treated like flexible array member-like arrays 🌸-fsanitize=bounds-strict— enables strict instrumentation of array bounds 🔒 Most out of bounds accesses are detected, including flexible array member-like arrays. Initializers of variables with static storage are not instrumented ✨-fsanitize=alignment— enables checking of alignment of pointers when they are dereferenced, or when a reference is bound to an insufficiently aligned target, or when a method or constructor is invoked on an insufficiently aligned object 📐-fsanitize=object-size— enables instrumentation of memory references using the"__builtin_dynamic_object_size"function 🔍 Various out of bounds pointer accesses are detected 🎀-fsanitize=float-divide-by-zero— detects floating-point division by zero 🫧 Unlike other similar options,-fsanitize=float-divide-by-zerois not enabled by-fsanitize=undefined, since floating-point division by zero can be a legitimate way of obtaining infinities and NaNs 🌈-fsanitize=float-cast-overflow— enables floating-point type to integer conversion checking 🔄 We check that the result of the conversion does not overflow. Unlike other similar options,-fsanitize=float-cast-overflowis not enabled by-fsanitize=undefined. This option does not work well with"FE_INVALID"exceptions enabled ⚠️-fsanitize=nonnull-attribute— enables instrumentation of calls, checking whether null values are not passed to arguments marked as requiring a non-null value by the"nonnull"function attribute 🙅♀️
-fsanitize=returns-nonnull-attribute— bestie, this one puts instrumentation onreturnstatements inside functions that carry thereturns_nonnullfunction attribute, so it can catch you returning a null value from a function that pinky-promised it never would. Rude behavior, now detected. 💅-fsanitize=bool— instruments every load from abool. If the value that comes back is anything other than0/1, boom: run-time error. A bool that isn't a bool is giving identity crisis and the sanitizer is not having it. 😭-fsanitize=enum— same energy but for loads from an enum type. If the value is outside the range of values for that enum, run-time error, immediately. Not on my watch. ✨-fsanitize=vptr— this one instruments C++ member function calls, member accesses, and some conversions between pointers to base and derived classes, all to verify the referenced object actually has the correct dynamic type. It's the vibe check for your polymorphism. 👑-fsanitize=pointer-overflow— instruments pointer arithmetic. If the pointer arithmetic overflows, run-time error. No silent wraparound shenanigans, we're calling it out. 🚨-fsanitize=builtin— instruments the arguments passed to selected builtin functions. Pass an invalid value to one of those arguments and you get a run-time error. Example from the source, no cap: passing0as the argument to__builtin_ctzor__builtin_clzinvokes undefined behavior, and this option diagnoses it. 🫶
Heads up, queen: sanitizers tend to increase the rate of false positive warnings, most notably the ones hanging around-Wmaybe-uninitialized. So we recommend against combining-Werrorand [the use of] sanitizers. Pick a struggle. 💀
Also, for the record: while -ftrapv causes traps for signed overflows to be emitted, -fsanitize=undefined gives you a diagnostic message instead. And this currently works only for the C family of languages. F for everyone else, sorry. 🍓
-fno-sanitize=all— disables all previously enabled sanitizers in one clean sweep. Note the tea though:-fsanitize=allis not allowed, because some sanitizers literally cannot be used together. They don't play nice, so don't force it. 🙈-fasan-shadow-offset=number— forces GCC to use a custom shadow offset in AddressSanitizer checks. It's useful for experimenting with different shadow memory layouts in Kernel AddressSanitizer. Science! 🧪-fsanitize-sections=s1,s2,...— sanitizes global variables in selected user-defined sections. And plot twist:simay contain wildcards. Glow up your section selection. 🌈-fsanitize-recover[=opts]— okay so-fsanitize-recover=controls the error recovery mode for whichever sanitizers you name in the comma-separated list of opts. Turn it on for a sanitizer component and it'll attempt to keep running the program as if no error happened. That means multiple runtime errors can be reported in a single program run, and the exit code of the program may indicate success even when errors have been reported. 💀 Wild, I know.
The -fno-sanitize-recover= option flips that behavior: only the first detected error is reported, and then the program exits with a non-zero exit code. Much more decisive of it. ✨
Currently this feature only works for:
-fsanitize=undefined(and its suboptions except for-fsanitize=unreachableand-fsanitize=return)-fsanitize=float-cast-overflow-fsanitize=float-divide-by-zero-fsanitize=bounds-strict-fsanitize=kernel-address-fsanitize=address
For these sanitizers, error recovery is turned on by default — except -fsanitize=address, for which this feature is experimental. -fsanitize-recover=all and -fno-sanitize-recover=all are also accepted: the former enables recovery for all sanitizers that support it, the latter disables recovery for all sanitizers that support it. One toggle, all the vibes. 🎀
And listen — even if recovery mode is turned on on the compiler side, it also needs to be enabled on the runtime library side, otherwise the failures are still fatal. The runtime library defaults to halt_on_error=0 for ThreadSanitizer and UndefinedBehaviorSanitizer, while the default value for AddressSanitizer is halt_on_error=1. You can override that by setting the halt_on_error flag in the corresponding environment variable. 🔥
The syntax without an explicit opts parameter is deprecated. It's equivalent to specifying an opts list of:
undefined,float-cast-overflow,float-divide-by-zero,bounds-strict-fsanitize-address-use-after-scope— enables sanitization of local variables to detect use-after-scope bugs. Fun fact: the option sets-fstack-reusetonone. 🔍-fsanitize-trap[=opts]— the-fsanitize-trap=option tells the compiler to report undefined behavior for the sanitizers named in the comma-separated list of opts using__builtin_traprather than alibubsanlibrary routine. And if it's enabled for a certain sanitizer, it takes precedence over-fsanitizer-recover=for that sanitizer —__builtin_trapwill be emitted and be fatal regardless of whether recovery is enabled or disabled using-fsanitize-recover=. It said what it said. 💅
The advantage? The libubsan library isn't needed and isn't linked in, so this is usable even in freestanding environments. Iconic for embedded girlies. 👑
Currently this feature works with:
-fsanitize=undefined(and its suboptions except for-fsanitize=vptr)-fsanitize=float-cast-overflow-fsanitize=float-divide-by-zero-fsanitize=bounds-strict
-fsanitize-trap=all can also be specified, which enables it for undefined suboptions, -fsanitize=float-cast-overflow, -fsanitize=float-divide-by-zero and -fsanitize=bounds-strict. But — and this is the important part — if -fsanitize-trap=undefined or -fsanitize-trap=all is used and -fsanitize=vptr is enabled on the command line, the instrumentation is silently ignored, because that instrumentation always needs libubsan support. And -fsanitize-trap=vptr is not allowed. Read that again. 📢
-fsanitize-undefined-trap-on-error— the deprecated equivalent of-fsanitize-trap=all. Old, tired, but still here. 🫠-fsanitize-coverage=trace-pc— enables coverage-guided fuzzing code instrumentation. Inserts a call to__sanitizer_cov_trace_pcinto every basic block. Every. Single. One. 📍-fsanitize-coverage=trace-cmp— enables dataflow guided fuzzing code instrumentation. It inserts:- a call to
__sanitizer_cov_trace_cmp1,__sanitizer_cov_trace_cmp2,__sanitizer_cov_trace_cmp4or__sanitizer_cov_trace_cmp8for integral comparison with both operands variable, or __sanitizer_cov_trace_const_cmp1,__sanitizer_cov_trace_const_cmp2,__sanitizer_cov_trace_const_cmp4or__sanitizer_cov_trace_const_cmp8for integral comparison with one operand constant,__sanitizer_cov_trace_cmpfor__sanitizer_cov_trace_cmpdfor float or double comparisons, and__sanitizer_cov_trace_switchfor switch statements. 🌸
- a call to
🛡️ -fcf-protection=[full|branch|return|none|check] / -fcf-protection
Enables code instrumentation to increase program security by checking that target addresses of control-flow transfer instructions (like indirect function call, function return, indirect jump) are valid. This prevents diverting the flow of control to an unexpected target. It's built to protect against threats such as Return-oriented Programming (ROP), and similarly call/jmp-oriented programming (COP/JOP). We're locking the doors, bestie. 🔒
The -fcf-protection= keywords are interpreted like this:
- The value
branchtells the compiler to implement checking of validity of control-flow transfer at the point of indirect branch instructions, i.e.call/jmpinstructions. - The value
returnimplements checking of validity at the point of returning from a function. - The value
fullis an alias for specifying bothbranchandreturn. - The value
checkis used for the final link with link-time optimization (LTO). An error is issued if LTO object files are compiled with different-fcf-protectionvalues. The valuecheckis ignored at the compile time. - The value
noneturns off instrumentation.
-fcf-protection is an alias for -fcf-protection=full. To override a previous -fcf-protection option on the command line, add -fcf-protection=none and then -fcf-protection=kind. Order matters, babe. ✨
The macro __CET__ is defined when -fcf-protection is used. The first bit of __CET__ is set to 1 for the value branch, and the second bit of __CET__ is set to 1 for the return. Bits doing bits. 🎀
You can also use the nocf_check attribute to identify which functions and calls should be skipped from instrumentation. Opt-out, but make it fashion. 🦋
Currently the x86 GNU/Linux target provides an implementation based on Intel Control-flow Enforcement Technology (CET) which works for i686 processor or newer. Retro but functional. 👑
-fharden-compares— for every logical test that survives gimple optimizations and is not the condition in a conditional branch (for example, conditions tested for conditional moves, or to store in boolean variables), emit extra code to compute and verify the reversed condition, and to call__builtin_trapif the results do not match. Use it with-fharden-conditional-branchesto cover all conditionals. Double-checking the double-check. 🕵️-fharden-conditional-branches— for every non-vectorized conditional branch that survives gimple optimizations, emit extra code to compute and verify the reversed condition, and to call__builtin_trapif the result is unexpected. Use it with-fharden-comparesto cover all conditionals. Tag team. 🫶-fharden-control-flow-redundancy— emits extra code to set booleans when entering basic blocks, and to verify and trap, at function exits, when the booleans do not form an execution path that is compatible with the control flow graph. It's giving paranoid, but correctly so. 🔍
Verification takes place before returns, before mandatory tail calls (see below) and, optionally, before escaping exceptions with -fhardcfr-check-exceptions, before returning calls with -fhardcfr-check-returning-calls, and before noreturn calls with -fhardcfr-check-noreturn-calls. Tuning options --param hardcfr-max-blocks and --param hardcfr-max-inline-blocks are available. 🎛️
Tail call optimization takes place too late to affect control flow redundancy, but calls annotated as mandatory tail calls by language front-ends, and any calls marked early enough as potential tail calls, would also have verification issued before the call — however, these possibilities are merely theoretical, as these conditions can only be met when using custom compiler plugins. So, like, don't stress about it. 💜
-fhardcfr-skip-leaf— disables-fharden-control-flow-redundancyin leaf functions. Leaves get a pass. 🍃-fhardcfr-check-exceptions— so when-fharden-control-flow-redundancyis doing its thing, this one checks the recorded execution path against the control flow graph at exception escape points 💅 — like the function body got wrapped in a cleanup handler that did the check and then reraised. It's on by default, bestie. Want it off? Use-fno-hardcfr-check-exceptions.-fhardcfr-check-returning-calls— again, while-fharden-control-flow-redundancyis active, this checks the recorded execution path against the control flow graph before any function call that's immediately followed by a return of its result (if there is one), so it doesn't block tail-call optimization — whether or not it actually ends up optimized into a tail call, no cap.
This one is on by default whenever sibling call optimizations are on (peep -foptimize-sibling-calls ✨), but you can also flip it on (or off, using its negated form) explicitly, no matter what the optimizations are doing.
-fhardcfr-check-noreturn-calls=[always|no-xthrow|nothrow|never]— with-fharden-control-flow-redundancyactive, this checks the recorded execution path against the control flow graph beforenoreturncalls. And you get to pick which ones 🎀:always— literally all of them.no-xthrow— those that aren't expected to return control to the caller through an exception (the default ✨).nothrow— those that may not return control to the caller through an exception either.never— none of them, periodt.
Checking before a noreturn function that may return control to the caller through an exception can make the checking happen more than once, if the exception is caught in the caller — whether by a handler or a cleanup 😭. When -fhardcfr-check-exceptions is also enabled, the compiler will avoid associating a noreturn call with the implicitly-added cleanup handler, since that'd be redundant with the check done before the call — but other handlers or cleanups in the function, if activated, will modify the recorded execution path and check it again when another checkpoint is hit. And the checkpoint might even be another noreturn call, so the checking can end up happening multiple times 💀.
Also, various optimizers may cause calls to be marked as noreturn and/or nothrow even when the corresponding attributes aren't there, which can affect where checks get placed before calls, plus the addition of implicit cleanup handlers for them. This unpredictability, plus the fact that raising and reraising exceptions frequently amounts to implicitly calling noreturn functions, is why no-xthrow is the default here: it only excludes from the noreturn treatment those internal functions used to (re)raise exceptions, which aren't affected by these optimizations. iykyk 🧚
-fhardened— turns on a whole set of flags for C and C++ that level up the security of the generated code without affecting its ABI. The exact flags enabled can change between major GCC releases, but right now they are:
-D_FORTIFY_SOURCE=3 -D_GLIBCXX_ASSERTIONS
-ftrivial-auto-var-init=zero -fPIE -pie -Wl,-z,relro,-z,now
-fstack-protector-strong -fstack-clash-protection
-fcf-protection=full (x86 GNU/Linux only)You can get the list of options -fhardened enables by using the --help=hardened option 👑
When the system glibc is older than 2.35, -D_FORTIFY_SOURCE=2 gets used instead.
This option is meant for production builds, not just debug builds, fr fr.
Currently, -fhardened is only supported on GNU/Linux targets.
And here's the lil rule: -fhardened only enables a particular option if it wasn't already specified anywhere on the command line 💅 So for example, -fhardened -fstack-protector will only enable -fstack-protector, but not -fstack-protector-strong.
-fstack-protector— emits extra code to check for buffer overflows, like stack smashing attacks 🛡️ It does this by adding a guard variable to functions with vulnerable objects. That includes functions that callalloca, and functions with buffers larger than or equal to8bytes. The guards get initialized when a function is entered, then checked when the function exits. If a guard check fails, an error message is printed and the program exits 💀 Only variables that are actually allocated on the stack count — optimized-away variables or variables allocated in registers don't count.-fstack-protector-all— same vibe as-fstack-protector, except all functions are protected. every single one. iconic.-fstack-protector-strong— like-fstack-protectorbut it includes additional functions to be protected — namely those that have local array definitions, or have references to local frame addresses 🌸 Only variables actually allocated on the stack are considered; optimized-away variables or variables allocated in registers don't count.-fstack-protector-explicit— like-fstack-protectorbut it only protects those functions that have thestack_protectattribute ✨-fstack-check— generates code to verify that you don't go beyond the boundary of the stack. You should specify this flag if you're running in an environment with multiple threads, but you only rarely need it in a single-threaded environment, since stack overflow is automatically detected on nearly all systems when there's only one stack.
Heads up: this switch does not actually cause checking to be done — the operating system or the language runtime has to do that. The switch just generates code so they see the stack being extended 👀
You can also tack on a string parameter: no means no checking, generic forces old-style checking, and specific uses the best checking method — it's equivalent to bare -fstack-check.
Old-style checking is a generic mechanism that needs no specific target support in the compiler, but bestie, it comes with these drawbacks:
- Modified allocation strategy for large objects: they're always allocated dynamically if their size exceeds a fixed threshold. Note this may change the semantics of some code 😭
- Fixed limit on the size of the static frame of functions: when it's topped by a particular function, stack checking is not reliable and a warning is issued by the compiler.
- Inefficiency: because of both the modified allocation strategy and the generic implementation, code performance is hampered 💀
Also note that old-style stack checking is the fallback method for specific if no target support has been added in the compiler.
-fstack-check= is designed for Ada's needs to detect infinite recursion and stack overflows. specific is an excellent choice when compiling Ada code 👑 But it's not generally sufficient to protect against stack-clash attacks. For those you want -fstack-clash-protection.
-fstack-clash-protection— generates code to prevent stack clash style attacks 🛡️ When this option is enabled, the compiler only allocates one page of stack space at a time, and each page is accessed immediately after allocation. So it stops allocations from jumping over any stack guard page provided by the operating system.
Most targets don't fully support stack clash protection. But on those targets, -fstack-clash-protection will protect dynamic stack allocations. It may also provide limited protection for static stack allocations if the target supports -fstack-check=specific ✨
-fstack-limit-register=reg-fstack-limit-symbol=sym-fno-stack-limit
These generate code to make sure the stack doesn't grow beyond a certain value — either the value of a register or the address of a symbol 🌸 If a larger stack is required, a signal is raised at run time. For most targets, the signal is raised before the stack overruns the boundary, so it's 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 👀
You can locally override stack limit checking by using the no_stack_limit function attribute.
-fsplit-stack— generates code to automatically split the stack before it overflows 🦋 The resulting program has a discontiguous stack, which can only overflow if the program can't allocate any more memory. This is most useful when running threaded programs, since you no longer have to calculate a good stack size for each thread. Currently this is only implemented for the x86 targets running GNU/Linux.
When code compiled with -fsplit-stack calls code compiled without -fsplit-stack, there may not be much stack space available for the latter code to run 😭 If compiling all code — library code included — with -fsplit-stack isn't an option, then the linker can fix up these calls so that the code compiled without -fsplit-stack always has a large stack. Support for this is implemented in the gold linker in GNU binutils release 2.21 and later ✨
-fstrub=disable— disables stack scrubbing entirely, ignoring anystrubattributes. See-fstrub=strict— functions default tostrubmodedisabled, and it strictly applies the restriction that only functions associated withstrub-callablemodes (at-calls,callableandalways_inlineinternal) arecallableby functions withstrub-enabled modes (at-callsandinternal) 💅-fstrub=relaxed— restores the default stack scrub (strub) setting, meaningstrubis only enabled as required bystrubattributes associated with function and data types.Relaxedmeans strub contexts are only prevented from calling functions explicitly associated withstrubmodedisabled. This option is only useful for overriding other-fstrub=*options that come before it in the command line 🎀-fstrub=at-calls— turns on "at-calls"strubmode wherever it's actually viable. ngl bestie, the main reason this flag exists is testing — it flexes thestrubmachinery in situations that are strictly local to a single translation unit. ⚠️ Heads up tho: thisstrubmode messes with function interfaces, so any function that other translation units can see, or that has its address taken, will not be affected by this option. Optimization options can also mess with viability. Want the full tea on viability and eligibility requirements? Check thestrubattribute documentation. ✨-fstrub=internal— turns on "internal"strubmode wherever viable. Again, primarily a testing thing. This one is meant to thoroughly exercise the parts of thestrubmachinery that implement the less efficient but interface-preservingstrubmode. Functions that this option would leave untouched are, like, super uncommon. 🦋-fstrub=all— enable somestrubmode wherever viable. When both modes are viable,at-callsis the preferred one.-fdump-ipa-strubmadds function attributes that tell you which mode got picked for each function. Primary use: testing — to thoroughly exercise thestrubmachinery. 👑-fvtable-verify=[std|preinit|none]— only available when you're compiling C++ code. It flips on (or off, if you use-fvtable-verify=none) a security feature that verifies at run time, for every virtual call, that the vtable pointer the call goes through is actually valid for the type of the object — i.e. it hasn't been corrupted or overwritten. If an invalid vtable pointer gets detected at run time, an error is reported and the program is immediately halted. 💀
This option makes run-time data structures get built at program startup, and those get used to verify the vtable pointers. The std and preinit values control when those data structures get built. In both cases they're built before execution reaches main. Using -fvtable-verify=std builds the data structures after shared libraries have been loaded and initialized. -fvtable-verify=preinit builds them before shared libraries have been loaded and initialized.
If this option shows up multiple times on the command line with different values, none takes highest priority over both std and preinit; and preinit takes priority over std.
-fvtv-debug— when used alongside-fvtable-verify=stdor-fvtable-verify=preinit, it makes debug versions of the runtime functions for the vtable verification feature get called. This flag also makes the compiler log info about which vtable pointers it finds for each class. That info gets written to a file namedvtv_set_ptr_data.log, in the directory named by the environment variableVTV_LOGS_DIRif that's defined — otherwise the current working directory. 🌸
Note: this feature appends data to the log file. Want a fresh log file? Delete any existing one first, bestie. 🫶
-fvtv-counts— this is a debugging flag. Used alongside-fvtable-verify=stdor-fvtable-verify=preinit, it makes the compiler keep track of the total number of virtual calls it runs into, plus the number of verifications it inserts. It also counts the number of calls to certain run-time library functions that it inserts, and logs that info for each compilation unit. The compiler writes this to a file namedvtv_count_data.log, in the directory named by the environment variableVTV_LOGS_DIRif that's defined — otherwise the current working directory. It also counts the size of the vtable pointer sets for each class, and writes that tovtv_class_set_sizes.login the same directory. 📝
Note: this feature appends data to the log files. To get fresh log files, delete any existing ones. No cap.
-finstrument-functions— generates instrumentation calls for entry and exit to functions. Right after function entry and right before function exit, these profiling functions get called with the address of the current function and its call site. (On some platforms__builtin_return_addressdoesn't work beyond the current function, so the call site info might 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 you can look up exactly in the symbol table.
This instrumentation also happens for functions that get expanded inline in other functions. The profiling calls indicate where, conceptually, the inline function is entered and exited. Which means addressable versions of such functions must be available. If all your uses of a function get expanded inline, that can mean extra code size. And if you use extern inline in your C code, an addressable version of such functions must be provided. (Normally that's the case anyway, but if you get lucky and the optimizer always inlines the functions, you might have gotten away without providing static copies.)
A function can be given the attribute no_instrument_function, in which case this instrumentation doesn't happen. Use it for, like, the profiling functions listed above, high-priority interrupt routines, and any functions from which the profiling functions can't safely be called (maybe signal handlers, if the profiling routines generate output or allocate memory). 🙈
-finstrument-functions-once— similar to-finstrument-functions, except the profiling functions get called only once per instrumented function. i.e. the first profiling function is called after the first entry into the instrumented function, and the second profiling function is called before the exit corresponding to that first entry. 💫
The definition of "once" for this option is a little vague, because the implementation isn't protected against data races. So the implementation only guarantees the profiling functions are called at least once per process and at most once per thread — but the calls are always paired: if a thread calls the first function, then it will call the second function, unless it never reaches the exit of the instrumented function.
-finstrument-functions-exclude-file-list=file,file,...— sets the list of functions excluded from instrumentation (peep the description of-finstrument-functions). If the file containing a function definition matches one of file, then that function isn't instrumented. The match is done on substrings: if the file parameter is a substring of the file name, that counts as a match. 🎀
For example:
-finstrument-functions-exclude-file-list=/bits/stl,include/sysexcludes any inline function defined in files whose pathnames contain /bits/stl or include/sys.
If, for some reason, you want to include letter , in one of sym, write ,. For example, -finstrument-functions-exclude-file-list=',,tmp' (note the single quote surrounding the option).
-finstrument-functions-exclude-function-list=sym,sym,...— similar to-finstrument-functions-exclude-file-list, but this one sets the list of function names to be excluded from instrumentation. The function name to be matched is its user-visible name, likevector<int> blah(const vector<int> &), not the internal mangled name (e.g._Z4blahRSt6vectorIiSaIiEE). The match is done on substrings: if the sym parameter is a substring of the function name, that's a match. For C99 and C++ extended identifiers, the function name has to be given in UTF-8, not using universal character names. 🌈-fpatchable-function-entry=N[,M]— generatesNNOPs right at the beginning of each function, with the function entry point before theMth NOP. IfMis omitted, it defaults to0, so the function entry points to the address just at the first NOP. These NOP instructions reserve extra space that you can use to patch in whatever instrumentation you want at run time — provided the code segment is writable. The amount of space is controllable indirectly via the number of NOPs; the NOP instruction used corresponds to the instruction emitted by the internal GCC back-end interfacegen_nop. This behavior is target-specific and may also depend on the architecture variant and/or other compilation options. 🧚
For run-time identification, the starting addresses of these areas — which correspond to their respective function entries minus M — are additionally collected in the __patchable_function_entries section of the resulting binary.
Note that the value of__attribute__ ((patchable_function_entry (N,M)))takes precedence over the command-line option-fpatchable-function-entry=N,M. You can use this to increase the area size, or to remove it completely on a single function. IfN=0, no pad location is recorded.
The NOP instructions are inserted at — and maybe before, depending on M — the function entry address, even before the prologue. On PowerPC with the ELFv2 ABI, for a function with dual entry points, the local entry point is this function entry address by default. See the -msplit-patch-nops option to change this.
The maximum value of N and M is 65535. On PowerPC with the ELFv2 ABI, for a function with dual entry points, the supported values for M are 0, 2, 6 and 14 when not using -msplit-patch-nops.
✨ Options Controlling the Preprocessor ✨
These options control the C preprocessor, which gets run on each C source file before actual compilation. 🧠
If you use the -E option, nothing happens except preprocessing. Some of these options only make sense together with -E, because they cause the preprocessor output to be unsuitable for actual compilation.
On top of the options listed here, there are a bunch of options for controlling search paths for include files documented in Directory Options. Options to control preprocessor diagnostics are listed in Warning Options.
-D name— predefine name as a macro, with definition1. 💅-D name=definition— Okay so the contents of definition are tokenized and processed as if they appeared during translation phase three in a#definedirective. In particular, the definition is 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 should quote the option. With sh and csh, -D'name(args...)=definition' works.
-D and -U options are processed in the order they are given on the command line. All -imacros file and -include file options are processed after all -D and -U options. ✨
-U name— Cancel any previous definition of name, either built in or provided with a-Doption. 💅-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. 🎀
-undef— Do not predefine any system-specific or GCC-specific macros. The standard predefined macros remain defined. 🙈-pthread— Define additional macros required for using the POSIX threads library. You should use this option consistently for both compilation and linking. This option is supported on GNU/Linux targets, most other Unix derivatives, and also on x86 Cygwin and MinGW targets. 🧵-M— Instead of outputting the result of preprocessing, output a rule suitable formakedescribing the dependencies of the main source file. The preprocessor outputs onemakerule containing the object file name for that source file, a colon, and the names of all the included files, including those coming from-includeor-imacroscommand-line options.
Unless specified explicitly (with -MT or -MQ), the object file name consists of the name of the source file with any suffix replaced with object file suffix and with any leading directory parts removed. If there are many included files then the rule is split into several lines using \-newline. The rule has no commands.
This option does not suppress the preprocessor's debug output, such as -dM. To avoid mixing such debug output with the dependency rules you should explicitly specify the dependency output file with -MF, or use an environment variable like DEPENDENCIES_OUTPUT. Debug output is still sent to the regular output stream as normal.
Passing -M to the driver implies -E, and suppresses warnings with an implicit -w. 🔥
-MM— Like-Mbut 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 appears in -MM dependency output. 🌈
-MF file— When used with-Mor-MM, specifies a file to write the dependencies to. If no-MFswitch is given the preprocessor sends the rules to the same place it would send preprocessed output.
When used with the driver options -MD or -MMD, -MF overrides the default dependency output file.
If file is -, then the dependencies are written to stdout. ✨
-MG— In conjunction with an option such as-Mrequesting dependency generation,-MGassumes 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.-MGalso suppresses preprocessed output, as a missing header file renders this useless.
This feature is used in automatic updating of makefiles. 🛠️
-Mno-modules— Disable dependency generation for compiled module interfaces. 💀-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 errorsmakegives if you remove header files without updating the Makefile to match.
This is typical output:
test.o: test.c test.h
test.h:-MT target— Change the target of the rule emitted by dependency generation. By default CPP takes the name of the main input file, deletes any directory components and any file suffix such as.c, and appends the platform's usual object suffix. The result is the target.
An -MT option sets the target to be exactly the string you specify. If you want multiple targets, you can specify them as a single argument to -MT, or use multiple -MT options.
For example, -MT '$(objpfx)foo.o' might give
$(objpfx)foo.o: foo.c-MQ target— Same as-MT, but it quotes any characters which are special to Make.-MQ '$(objpfx)foo.o'gives
$$(objpfx)foo.o: foo.cThe default target is automatically quoted, as if it were given with -MQ. 💅
-MD—-MDis equivalent to-M -MFfile, except that-Eis not implied. The driver determines file based on whether an-ooption is given. If it is, the driver uses its argument but with a suffix of.d, otherwise it takes the name of the input file, removes any directory components and suffix, and applies a.dsuffix.
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-MDexcept mention only user header files, not system header files. 🌸-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-Cto the compiler without problems. In this mode the integrated preprocessor is little more than a tokenizer for the front ends.
-fpreprocessed is implicit if the input file has one of the extensions .i, .ii or .mi. These are the extensions that GCC uses for preprocessed files created by -save-temps. 🦋
-fdirectives-only— When preprocessing, handle directives, but do not expand macros.
The option's behavior depends on the -E and -fpreprocessed options.
With -E, preprocessing is limited to the handling of directives such as "#define", "#ifdef", and "#error". Other preprocessor operations, such as macro expansion and trigraph conversion are not performed. In addition, the -dD option is implicitly enabled.
With -fpreprocessed, predefinition of command line and most builtin macros is disabled. Macros such as "__LINE__", which are contextually dependent, are handled normally. This enables compilation of files previously preprocessed with "-E -fdirectives-only".
With both -E and -fpreprocessed, the rules for -fpreprocessed take precedence. This enables full preprocessing of files previously preprocessed with "-E -fdirectives-only". ✨
-fdollars-in-identifiers— Accept$in identifiers. 💸-fextended-identifiers— Accept universal character names and extended characters in identifiers. This option is enabled by default for C99 (and later C standard versions) and C++. 🌈-fno-canonical-system-headers— When preprocessing, do not shorten system header paths with canonicalization. 🙈-fmax-include-depth=depth— Set the maximum depth of the nested#include. The default is 200. 🌀-fsearch-include-path[=kind]— Look for input files on the#includepath, not just the current directory. This is particularly useful with C++20 modules, for which both header units and module interface units need to be compiled directly:
g++ -c -std=c++20 -fmodules -fsearch-include-path bits/stdc++.h bits/std.cckind defaults to user, which looks on the #include "..." search path; you can also explicitly specify system for the "#include <...>" search path. 🎀
-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. 📏-ftrack-macro-expansion[=level]— Track locations of tokens across macro expansions. This allows the compiler to emit diagnostic about the current macro expansion stack when a compilation error occurs in a macro expansion. Using this option makes the preprocessor and the compiler consume more memory. The level parameter can be used to choose the level of precision of token location tracking thus decreasing the memory consumption if necessary. Value 0 of level de-activates this option. Value 1 tracks tokens locations in a degraded mode for the sake of minimal memory overhead. In this mode all tokens resulting from the expansion of an argument of a function-like macro have the same location. Value 2 tracks tokens locations completely. This value is the most memory hungry. When this option is given no argument, the default parameter value is 2.
Note that -ftrack-macro-expansion=2 is activated by default. 💖
-fmacro-prefix-map=old=new— okay so when you're preprocessing files that live in directoryold, this expands the__FILE__and__BASE_FILE__macros as if those files actually lived in directorynewinstead. the way that works is you can change an absolute path into a relative one by using.fornew, which can give you more reproducible builds that are location independent. This option also affects__builtin_FILE()during compilation. see also-ffile-prefix-mapand-fcanon-prefix-map.-fexec-charset=charset— sets the execution character set, used for string and character constants. The default isUTF-8. charset can be any encoding supported by the system'siconvlibrary routine.-fwide-exec-charset=charset— sets the wide execution character set, used for wide string and character constants. The default is one ofUTF-32BE,UTF-32LE,UTF-16BE, orUTF-16LE, whichever corresponds to the width ofwchar_tand the big-endian or little-endian byte order being used for code generation. As with-fexec-charset, charset can be any encoding supported by the system'siconvlibrary routine; however, you will have problems with encodings that do not fit exactly inwchar_t.-finput-charset=charset— sets 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 isUTF-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'siconvlibrary routine.-fpch-deps— when using precompiled headers, this flag causes the dependency-output flags to also list the files from the precompiled header's dependencies. If not specified, only the precompiled header are 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-fpreprocessedis in use, GCC recognizes this#pragmaand 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#pragmain 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.-fworking-directory— enable generation of linemarkers in the preprocessor output that let the compiler know the current working directory at the time of preprocessing. When this option is enabled, the preprocessor emits, after the initial linemarker, a second linemarker with the current working directory followed by two slashes. GCC uses 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-Pflag is present in the command line, this option has no effect, since no#linedirectives are emitted whatsoever.-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.-C— do not discard comments. All comments are passed through to the output file, except for comments in processed directives, which are deleted along with the directive. you should be prepared for side effects when using-C; it causes the preprocessor to treat comments as tokens in their own right. For example, comments appearing at the start of what would be a directive line have the effect of turning that line into an ordinary source line, since the first token on the line is no longer a#.-CC— do not discard comments, including during macro expansion. This is like-C, except that comments contained within macros are also passed through to the output file where the macro is expanded. In addition to the side effects of the-Coption, the-CCoption 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-CCoption is generally used to support lint comments.-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.-traditional,-traditional-cpp— try to imitate the behavior of pre-standard C preprocessors, as opposed to ISO C preprocessors. See the GNU CPP manual for details. Note that GCC does not otherwise attempt to emulate a pre-standard C compiler, and these options are only supported with the-Eswitch, or when invoking CPP explicitly.-trigraphs— support ISO C trigraphs. 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. The nine trigraphs and their replacements are
Trigraph: ??( ??) ??< ??> ??= ??/ ??' ??! ??-
Replacement: [ ] { } # \ ^ | ~By default, GCC ignores trigraphs, but in standard-conforming modes it converts them. See the -std and -ansi options.
-remap— enable special code to work around file systems which only permit very short file names, such as MS-DOS.-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#includestack it is. Precompiled header files are also printed, even if they are found to be invalid; an invalid precompiled header file is printed with...xand a valid one with...!.-dletters— says to make debugging dumps during compilation as specified by letters. The flags documented here are those relevant to the preprocessor. Other letters are interpreted by the compiler proper, or reserved for future versions of GCC, and so are silently ignored. If you specify letters whose behavior conflicts, the result is undefined.-dM— instead of the normal output, generate a list of#definedirectives 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 filefoo.h, the command
touch foo.h; cpp -dM foo.hshows all the predefined macros.
If you use -dM without the -E option, -dM is interpreted as a synonym for -fdump-rtl-mach.
-dD— like-dMexcept that it outputs both the#definedirectives and the result of preprocessing. Both kinds of output go to the standard output file.-dN— like-dD, but emit only the macro names, not their expansions.-dI— output#includedirectives in addition to the result of preprocessing.-dU— like-dDexcept that only macros that are expanded, or whose definedness is tested in preprocessor directives, are output; the output is delayed until the use or test of the macro; and#undefdirectives are also output for macros tested but undefined at the time.
-fdebug-cpp— this option is only useful for debugging GCC. When used from CPP or with-E, it dumps debugging information about location maps. Every token in the output is preceded by the dump of the map its location belongs to. When used from GCC without-E, this option has no effect.-Wp,option— you can use-Wp,optionto 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-Wpforcibly bypasses this phase. The preprocessor's direct interface is undocumented and subject to change, so whenever possible you should avoid using-Wpand let the driver handle the options instead.-Xpreprocessor option— pass option as an option to the preprocessor. You can use this to supply system-specific preprocessor options that GCC does not recognize. If you want to pass an option that takes an argument, you must use-Xpreprocessortwice, once for the option and once for the argument.-no-integrated-cpp— perform preprocessing as a separate pass before compilation. By default, GCC performs preprocessing as an integrated part of input tokenization and parsing. If this option is provided, the appropriate language front end (cc1,cc1plus, orcc1objfor C, C++, and Objective-C, respectively) is instead invoked twice, once for preprocessing only and once for actual compilation of the preprocessed input. This option may be useful in conjunction with the-Bor-wrapperoptions to specify an alternate preprocessor or perform additional processing of the program source between normal preprocessing and compilation.
✨ Passing Options to the Assembler ✨
okay bestie, you can pass options to the assembler.
-Wa,option— pass option as an option to the assembler. If option contains commas, it is split into multiple options at the commas.-Xassembler option— pass option as an option to the assembler. You can use this to supply system-specific assembler options that GCC does not recognize. If you want to pass an option that takes an argument, you must use-Xassemblertwice, once for the option and once for the argument.
✨ Options for Linking ✨
okay bestie so these options only matter when the compiler is actually doing a link step — turning your object files into an executable. If there's no linking happening, they're just sitting there being meaningless. 💅
object-file-name— if a file name doesn't end in one of the special recognized suffixes, it's treated as naming an object file or a library. (The linker tells object files apart from libraries by peeking at the file contents, not the name, slay.) If linking is happening, these object files get used as input to the linker.-c,-S,-E— if you use any of these, the linker is not run, so don't be passing object file names as arguments. it's giving "no link step today". 🚫-flinker-output=type— this one controls code generation of the link-time optimizer. By default, the linker output is auto-determined by the linker plugin. But if you're debugging the compiler, or you want incremental linking with a non-LTO object file, it may be useful to set the type manually. Here are the types, and each one is a whole vibe:exec— code generation produces a static binary. In this case-fpicand-fpieare both disabled.dyn— code generation produces a shared library. In this case-fpicor-fPICis preserved, but not enabled automatically. This lets you build shared libraries without position-independent code on architectures where that's possible, i.e. on x86.pie— code generation produces an-fpieexecutable. This gives similar optimizations toexec, except that-fpieis not disabled if it was specified at compilation time.rel— the compiler assumes incremental linking is being done. The sections containing intermediate code for link-time optimization get merged, pre-optimized, and output to the resulting object file. Also, if-ffat-lto-objectsis specified, binary code is produced for future non-LTO linking. The object file from incremental linking is smaller than a static library built from the same object files. At link time, the result of incremental linking also loads faster than a static library — assuming the majority of objects in the library are used.nolto-rel— configures the compiler for incremental linking where code generation is forced, a final binary is produced, and the intermediate code for later link-time optimization is stripped. When multiple object files are linked together, the resulting code is better optimized than with link-time optimizations disabled (cross-module inlining happens, for example), but most of the benefits of whole program optimizations are lost. 💀
During the incremental link (that's the one done by -r), the linker plugin defaults to rel. But heads up bestie — with the current interfaces to GNU Binutils, it is not possible to incrementally link LTO objects and non-LTO objects into a single mixed object file. If any of the object files in an incremental link can't be used for link-time optimization, the linker plugin issues a warning and uses nolto-rel. To maintain whole program optimization, the recommendation is to link such objects into a static library instead. Alternatively, you can use H.J. Lu's binutils with support for mixed objects.
-fuse-ld=bfd— use the bfd linker instead of the default linker.-fuse-ld=gold— use the gold linker instead of the default linker.-fuse-ld=lld— use the LLVM lld linker instead of the default linker.-fuse-ld=mold— use the Modern Linker (mold) instead of the default linker. ✨-llibrary,-l library— search the library named library when linking. (That second form, with the library as a separate argument, is only there for POSIX compliance and is not recommended.) GCC passes-ldirectly to the linker, so go read your linker documentation for the exact details — the general description below applies to the GNU linker.
The linker searches a standard list of directories for the library. That list includes several standard system directories plus any you specify with -L.
Static libraries are archives of object files and have names like liblibrary.a. Some targets also support shared libraries, which typically have names like liblibrary.so. If both static and shared versions are found, the linker prefers linking with the shared one — unless you used -static.
And okay this part is so important: where you write this option in the command matters. The linker searches and processes libraries and object files in the order they're specified. So 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. 😭
-lobjc— you need this special case of the-loption in order to link an Objective-C or Objective-C++ program.-nostartfiles— don't use the standard system startup files when linking. The standard system libraries are still used normally, unless-nostdlib,-nolibc, or-nodefaultlibsis used.-nodefaultlibs— don't use the standard system libraries when linking. Only the libraries you specify are passed to the linker, and options specifying linkage of the system libraries — like-static-libgccor-shared-libgcc— are ignored. The standard startup files are used normally, unless-nostartfilesis used.
The compiler may generate calls to memcmp, memset, memcpy and memmove. These entries are usually resolved by entries in libc, so you should supply these entry points through some other mechanism when this option is specified.
-nolibc— don't use the C library or the system libraries tightly coupled with it when linking. It still links with the startup files, libgcc, or toolchain-provided language support libraries such as libgnat, libgfortran or libstdc++, unless options preventing their inclusion are used as well. This typically removes-lcfrom the link command line, as well as the system libraries that normally go with it and become meaningless once the absence of a C library is assumed — for example-lpthreador-lmin some configurations. This is intended for bare-board targets when there is indeed no C library available.-nostdlib— don't use the standard system startup files or libraries when linking. No startup files, and only the libraries you specify are passed to the linker, and options specifying linkage of the system libraries — like-static-libgccor-shared-libgcc— are ignored.
The compiler may generate calls to memcmp, memset, memcpy and memmove. These entries are usually resolved by entries in libc, so you should supply these entry points through some other mechanism when this option is specified.
One of the standard libraries bypassed by -nostdlib and -nodefaultlibs is libgcc.a, a library of internal subroutines which GCC uses to overcome shortcomings of particular machines, or special needs for some languages.
In most cases, you need libgcc.a even when you want to avoid other standard libraries. In other words, when you specify -nostdlib or -nodefaultlibs you should usually specify -lgcc as well. This ensures that you have no unresolved references to internal GCC library subroutines. (An example of such an internal subroutine is __main, used to ensure C++ constructors are called.) 🫶
-nostdlib++— don't implicitly link with standard C++ libraries.-e entry,--entry=entry— specify that the program entry point is entry. The argument is interpreted by the linker; the GNU linker accepts either a symbol name or an address.-pie— produce a dynamically linked position independent executable on targets that support it. For predictable results, you must also specify the same set of options used for compilation (-fpie,-fPIE, or model suboptions) when you specify this linker option.-no-pie— don't produce a dynamically linked position independent executable.-static-pie— produce a static position independent executable on targets that support it. A static position independent executable is similar to a static executable, but it can be loaded at any address without a dynamic linker. For predictable results, you must also specify the same set of options used for compilation (-fpie,-fPIE, or model suboptions) when you specify this linker option.-pthread— link with the POSIX threads library. This option is supported on GNU/Linux targets, most other Unix derivatives, and also on x86 Cygwin and MinGW targets. On some targets this option also sets flags for the preprocessor, so it should be used consistently for both compilation and linking. 👑-r— produce a relocatable object as output. Also known as partial linking.-rdynamic— pass the flag-export-dynamicto the ELF linker, on targets that support it. This instructs the linker to add all symbols to the dynamic symbol table, not just the used ones. This option is needed for some uses ofdlopen, 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 overrides-pieand prevents linking with the shared libraries. On other systems, this option has no effect.-shared— produce a shared object which can then be linked with other objects to form an executable. Not all systems support this option. For predictable results, you must also specify the same set of options used for compilation (-fpic,-fPIC, or model suboptions) when you specify this linker option.[1]-shared-libgcc,-static-libgcc— on systems that provide libgcc as a shared library, these options force the use of either the shared or static version, respectively. If no shared version of libgcc was built when the compiler was configured, these options have no effect.
There are several situations in which an application should use the shared libgcc instead of the static version. The most common 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.
So therefore, the G++ driver automatically adds -shared-libgcc whenever you build a shared library or a main executable, because C++ programs typically use exceptions, so this is the right thing to do. and honestly? iconic behaviour. 💖
Okay so real talk — if you go the other way and use the GCC driver to build shared libraries, bestie, you might notice they don't always get linked with the shared libgcc. Here's the tea: if GCC figures out at its configuration time that you've got a non-GNU linker, or a GNU linker that doesn't support the --eh-frame-hdr option, then it links the shared version of libgcc into shared libraries by default. Otherwise it takes advantage of the linker and optimizes the linking with the shared version of libgcc away, linking with the static version of libgcc by default instead. And why do we care? Because that lets exceptions propagate through such shared libraries without paying relocation costs at library load time. ✨
But — and this is the important part — if a library or main executable is supposed to throw or catch exceptions, you must link it using the G++ driver, or using the option-shared-libgcc, so that it's linked with the sharedlibgcc. No cap. 💅
-static-libasan— when-fsanitize=addressis used to link a program, the GCC driver automatically links againstlibasan. Iflibasanis available as a shared library and-staticisn't used, it links against the shared version oflibasan. The-static-libasanoption tells the GCC driver to linklibasanstatically instead, without necessarily linking the other libraries statically. Iconic behavior, honestly.-static-libtsan— when-fsanitize=threadis used to link a program, the GCC driver automatically links againstlibtsan. Iflibtsanis available as a shared library and-staticisn't used, it links against the shared version oflibtsan. The-static-libtsanoption directs the GCC driver to linklibtsanstatically, without necessarily linking other libraries statically. 🧵-static-liblsan— when-fsanitize=leakis used to link a program, the GCC driver automatically links againstliblsan. Ifliblsanis available as a shared library and-staticisn't used, it links against the shared version ofliblsan. The-static-liblsanoption directs the GCC driver to linkliblsanstatically, without necessarily linking other libraries statically. 🚰-static-libubsan— when-fsanitize=undefinedis used to link a program, the GCC driver automatically links againstlibubsan. Iflibubsanis available as a shared library and-staticisn't used, it links against the shared version oflibubsan. The-static-libubsanoption directs the GCC driver to linklibubsanstatically, without necessarily linking other libraries statically. 🌈-static-libstdc++— when theg++program is used to link a C++ program, it normally automatically links againstlibstdc++. Iflibstdc++is available as a shared library and-staticisn't used, it links against the shared version oflibstdc++. That's normally fine, lowkey. But sometimes it's genuinely useful to freeze the version oflibstdc++your program uses without going all the way to a fully static link. The-static-libstdc++option directs theg++driver to linklibstdc++statically, without necessarily linking other libraries statically. Obsessed with the flexibility tbh. 💖-symbolic— bind references to global symbols when building a shared object, and warn about any unresolved references (unless that's overridden by the link editor option-Xlinker -z -Xlinker defs). Heads up though: only a few systems support this option. 🙈-Tscript — use script as the linker script. This one is supported by most systems using the GNU linker. And on some targets — like bare-board targets with no operating system — the-Toption may be required when linking, just to avoid references to undefined symbols. 👑-Xlinkeroption — pass option as an option to the linker. You can use this to supply system-specific linker options that GCC doesn't recognize. So handy.
If you want to pass an option that takes a separate argument, you have to use -Xlinker twice — once for the option and once for the argument. For example, to pass -assert definitions, you must write:
-Xlinker -assert -Xlinker definitionsAnd it does not work to write -Xlinker "-assert definitions", because that passes the entire string as a single argument, which is not what the linker expects. Not me making that mistake... okay maybe me. 😭
When you're using the GNU linker, it's usually way more convenient to pass arguments to linker options using the option=value syntax rather than as separate arguments. For example, you can specify -Xlinker -Map=output.map rather than -Xlinker -Map -Xlinker output.map. Other linkers may not support this syntax for command-line options though — so vibe check your linker first. 🔍
-Wl,option — pass option as an option to the linker. If option contains commas, it gets split into multiple options at the commas. You can use this syntax to pass an argument to the option. For example,-Wl,-Map,output.mappasses-Map output.mapto the linker. And when you're using the GNU linker, you can also get the same effect with-Wl,-Map=output.map. Same energy, different fit. ✨-usymbol — pretend the symbol symbol is undefined, to force linking of library modules to define it. You can use-umultiple times with different symbols to force loading of additional library modules. Stack 'em up. 🎀-zkeyword —-zis passed directly on to the linker along with the keyword keyword. Go check the section in the documentation of your linker for the permitted values and what they mean, bestie — that part is on your linker, not on GCC. 🦋
🗂️ Options for Directory Search 🗂️
These options specify directories to search for header files, for libraries, and for parts of the compiler itself. Directory management, but make it fashion. 💅
-Idir,-iquotedir,-isystemdir,-idirafterdir — add the directory dir to the list of directories to be searched for header files during preprocessing. If dir begins with=or$SYSROOT, then the=or$SYSROOTis replaced by the sysroot prefix; see--sysrootand-isysroot.
Directories specified with -iquote apply only to the quote form of the directive, #include "file". Directories specified with -I, -isystem, or -idirafter apply to lookup for both the #include "file" and #include <file> directives.
You can specify any number or combination of these options on the command line to search for header files in several directories. The lookup order is as follows:
- For the quote form of the include directive, the directory of the current file is searched first.
- For the quote form of the include directive, the directories specified by
-iquoteoptions are searched in left-to-right order, as they appear on the command line. - Directories specified with
-Ioptions are scanned in left-to-right order. - Directories specified with
-isystemoptions are scanned in left-to-right order. - Standard system directories are scanned.
- Directories specified with
-idirafteroptions are scanned in left-to-right order.
You can use -I to override a system header file, substituting your own version, since these directories are searched before the standard system header file directories. However — and listen up — you should not use this option to add directories that contain vendor-supplied system header files; use -isystem for that instead. 🙅♀️
The -isystem and -idirafter options also mark the directory as a system directory, so it gets the same special treatment that's applied to the standard system directories.
If a standard system include directory, or a directory specified with -isystem, is also specified with -I, then the -I option is ignored. The directory is still searched, but as a system directory at its normal position in the system include chain. Why? To make sure GCC's procedure for fixing buggy system headers and the ordering for the #include_next directive don't get inadvertently changed. If you really need to change the search order for system directories, use the -nostdinc and/or -isystem options. Periodt. 💯
-I-— split the include path. ⚠️ This option has been deprecated. Please use-iquoteinstead for-Idirectories before the-I-, and remove the-I-option entirely.
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.
On top of that, -I- inhibits the use of the directory of the current file directory as the first search directory for #include "file". And there is no way to override this effect of -I-. It's giving irreversible, fr fr. 💀
-iprefixprefix — specify prefix as the prefix for subsequent-iwithprefixoptions. If the prefix represents a directory, you should include the final/. Don't forget the slash, bestie. 🎀-iwithprefixdir,-iwithprefixbeforedir — append dir to the prefix specified previously with-iprefix, and add the resulting directory to the include search path.-iwithprefixbeforeputs it in the same place-Iwould;-iwithprefixputs it where-idirafterwould. Same dir, different seat at the table. 🪑-isysrootdir — this option is like the--sysrootoption, but applies only to header files (except for Darwin targets, where it applies to both header files and libraries). See the--sysrootoption for more information. 🍎-imultilibdir — use dir as a subdirectory of the directory containing target-specific C++ headers.-nostdinc— do not search the standard system directories for header files. Only the directories explicitly specified with-I,-iquote,-isystem, and/or-idirafteroptions are searched (plus the directory of the current file, if appropriate). You're on your own now, and honestly? Main character energy. 👑-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.) 🦋--embed-dir=dir — append the dir directory to the list of searched directories for the#embedpreprocessing directive or the__has_embedmacro. Note: there are no default directories for#embed. 🌸
If dir begins with = or $SYSROOT, then the = or $SYSROOT is replaced by the sysroot prefix; see --sysroot and -isysroot.
-iplugindir=dir — set the directory to search for plugins that are passed by-fplugin=nameinstead of-fplugin=path/name.so. Heads up though: this option is not meant to be used by the user, but only passed by the driver. Driver-only, iykyk. 🔌-Ldir— add directory dir to the list of directories to be searched for-l. 📚-Bprefix— this option specifies where to find the executables, libraries, include files, and data files of the compiler itself. 🏠
The compiler driver program runs one or more of the subprograms cpp, cc1, as and ld. It tries prefix as a prefix for each program it tries to run, both with and without machine/version/ for the corresponding target machine and compiler version. 💅
For each subprogram to be run, the compiler driver first tries the -B prefix, if any. If that name is not found, or if -B wasn't specified at all, the driver tries two standard prefixes: /usr/lib/gcc/ and /usr/local/lib/gcc/. If neither of those results in a file name that's actually found, the unmodified program name gets searched for using the directories specified in your PATH environment variable. 🫶
The compiler checks to see if the path provided by -B refers to a directory, and if necessary it adds a directory separator character at the end of the path. ✨
-B prefixes that effectively specify directory names also apply to libraries in the linker, because the compiler translates these options into -L options for the linker. They also apply to include files in the preprocessor, because the compiler translates these options into -isystem options for the preprocessor. In this case, the compiler appends include to the prefix.
The runtime support file libgcc.a can also be searched for using the -B prefix, if needed. If it's not found there, the two standard prefixes above are tried, and that is all. The file is left out of the link if it's not found by those means. 😭
Another way to specify a prefix much like the -B prefix is to use the environment variable GCC_EXEC_PREFIX.
As a special kludge, if the path provided by -B is [dir/]stageN/, where N is a number in the range 0 to 9, then it is replaced by [dir/]include. This is to help with boot-strapping the compiler. 🧚
-no-canonical-prefixes— do not expand any symbolic links, resolve references to/../or/./, or make the path absolute when generating a relative prefix.--sysroot=dir— use dir as the logical root directory for headers and libraries. Like, if the compiler normally searches for headers in/usr/includeand libraries in/usr/lib, it instead searchesdir/usr/includeanddir/usr/lib. 🌸
If you use both this option and the -isysroot option, then the --sysroot option applies to libraries, but the -isysroot option applies to header files. Not me splitting the duties between two flags 😭
The GNU linker (beginning with version 2.16) has the necessary support for this option. If your linker does not support this option, the header file aspect of --sysroot still works, but the library aspect does not. 💀
--no-sysroot-suffix— for some targets, a suffix is added to the root directory specified with--sysroot, depending on the other options used, so that headers may for example be found indir/suffix/usr/includeinstead ofdir/usr/include. This option disables the addition of such a suffix.
✨ Options for Code Generation Conventions ✨
These machine-independent options control the interface conventions used in code generation. 🎀
Most of them have both positive and negative forms; the negative form of -ffoo is -fno-foo. In the table below, only one of the forms is listed---the one that is not the default. You can figure out the other form by either removing no- or adding it. iykyk 💅
-fstack-reuse=reuse-level— this option controls stack space reuse for user declared local/auto variables and compiler generated temporaries.
reuse_level can be all, named_vars, or none. all enables stack reuse for all local variables and temporaries, named_vars enables the reuse only for user defined local variables with names, and none disables stack reuse completely. The default value is all.
The option is needed when the program extends the lifetime of a scoped local variable or a compiler generated temporary beyond the end point defined by the language. When a lifetime of a variable ends, and if the variable lives in memory, the optimizing compiler has the freedom to reuse its stack space with other temporaries or scoped local variables whose live range does not overlap with it. Legacy code extending local lifetime is likely to break with the stack reuse optimization. 😭
For example,
int *p;
{
int local1;
p = &local1;
local1 = 10;
....
}
{
int local2;
local2 = 20;
...
}
if (*p == 10) // out of scope use of local1
{
}Another example:
struct A
{
A(int k) : i(k), j(k) { }
int i;
int j;
};
A *ap;
void foo(const A& ar)
{
ap = &ar;
}
void bar()
{
foo(A(10)); // temp object's lifetime ends when foo returns
{
A a(20);
....
}
ap->i+= 10; // ap references out of scope temp whose space
// is reused with a. What is the value of ap->i?
}The lifetime of a compiler generated temporary is well defined by the C++ standard. When a lifetime of a temporary ends, and if the temporary lives in memory, the optimizing compiler has the freedom to reuse its stack space with other temporaries or scoped local variables whose live range does not overlap with it. However some of the legacy code relies on the behavior of older compilers in which temporaries' stack space is not reused, and the aggressive stack reuse can lead to runtime errors. This option is used to control the temporary stack reuse optimization. 💀
-ftrapv— this option generates traps for signed overflow on addition, subtraction, multiplication operations. The options-ftrapvand-fwrapvoverride each other, so using-ftrapv -fwrapvon the command-line results in-fwrapvbeing effective. Note that only active options override, so using-ftrapv -fwrapv -fno-wrapvon the command-line results in-ftrapvbeing effective. 🔥-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. The options-ftrapvand-fwrapvoverride each other, so using-ftrapv -fwrapvon the command-line results in-fwrapvbeing effective. Note that only active options override, so using-ftrapv -fwrapv -fno-wrapvon the command-line results in-ftrapvbeing effective.-fwrapv-pointer— this option instructs the compiler to assume that pointer arithmetic overflow on addition and subtraction wraps around using twos-complement representation. This flag disables some optimizations which assume pointer overflow is invalid.-fstrict-overflow— this option implies-fno-wrapv -fno-wrapv-pointerand when negated implies-fwrapv -fwrapv-pointer.-fexceptions— enable exception handling. Generates extra code needed to propagate exceptions. For some targets, this implies GCC generates frame unwind information for all functions, which can produce significant data size overhead, although it does not affect execution. If you do not specify this option, GCC enables it by default for languages like C++ that normally require exception handling, and disables it for languages like C that do not normally require it. However, you may need to enable this option when compiling C code that needs to interoperate properly with exception handlers written in C++. You may also wish to disable this option if you are compiling older C++ programs that don't use exception handling. 🌈-fnon-call-exceptions— generate code that allows trapping instructions to throw exceptions. Note that this requires platform-specific runtime support that does not exist everywhere. Moreover, it only allows trapping instructions to throw exceptions, i.e. memory references or floating-point instructions. It does not allow exceptions to be thrown from arbitrary signal handlers such asSIGALRM. This enables-fexceptions. 💀-fdelete-dead-exceptions— consider that instructions that may throw exceptions but don't otherwise contribute to the execution of the program can be optimized away. This does not affect calls to functions except those with thepureorconstattributes. This option is enabled by default for the Ada and C++ compilers, as permitted by the language specifications. Optimization passes that cause dead exceptions to be removed are enabled independently at different optimization levels. ✨-funwind-tables— similar to-fexceptions, except that it just generates any needed static data, but does not affect the generated code in any other way. You normally do not need to enable this option; instead, a language processor that needs this handling enables it on your behalf. 🫶-fasynchronous-unwind-tables— generate unwind table in DWARF format, if supported by target machine. The table is exact at each instruction boundary, so it can be used for stack unwinding from asynchronous events (such as debugger or garbage collector). 👑-fno-gnu-unique— on systems with recent GNU assembler and C library, the C++ compiler uses theSTB_GNU_UNIQUEbinding to make sure that definitions of template static data members and static local variables in inline functions are unique even in the presence ofRTLD_LOCAL; this is necessary to avoid problems with a library used by two differentRTLD_LOCALplugins depending on a definition in one of them and therefore disagreeing with the other one about the binding of the symbol. But this causesdlcloseto be ignored for affected DSOs; if your program relies on reinitialization of a DSO viadlcloseanddlopen, you can use-fno-gnu-unique. 💅-fpcc-struct-return— okay so this one says 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-returnswitch is not binary compatible with code compiled with the-freg-struct-returnswitch. Use it to conform to a non-default application binary interface. 🚨
-freg-struct-return— bestie 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-returnswitch is not binary compatible with code compiled with the-fpcc-struct-returnswitch. Use it to conform to a non-default application binary interface. 💀
-fshort-enums— allocate to an"enum"type only as many bytes as it needs for the declared range of possible values. Specifically, the"enum"type is equivalent to the smallest integer type that has enough room. This option has no effect for an enumeration type with a fixed underlying type.
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-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. 🚨
-fcommon— in C code, this option controls the placement of global variables defined without an initializer, known as tentative definitions in the C standard. Tentative definitions are distinct from declarations of a variable with the"extern"keyword, which do not allocate storage.
The default is -fno-common, which specifies that the compiler places uninitialized global variables in the BSS section of the object file. This inhibits the merging of tentative definitions by the linker so you get a multiple-definition error if the same variable is accidentally defined in more than one compilation unit.
The -fcommon places uninitialized global variables in a common block. This allows the linker to resolve all tentative definitions of the same variable in different compilation units to the same object, or to a non-tentative definition. This behavior is inconsistent with C++, and on many targets implies a speed and code size penalty on global variable references. It is mainly useful to enable legacy code to link without errors.
-fno-ident— ignore the"#ident"directive. periodt.-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 compilingcrtstuff.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.
The added comments include:
- information on the compiler version and command-line options,
- the source code lines associated with the assembly instructions, in the form
FILENAME:LINENUMBER:CONTENT OF LINE, - hints on which high-level expressions correspond to the various assembly instruction operands.
For example, given this C source file:
int test (int n)
{
int i;
int total = 0;
for (i = 0; i < n; i++)
total += i * i;
return total;
}compiling to (x86_64) assembly via -S and emitting the result direct to stdout via -o -
gcc -S test.c -fverbose-asm -Os -o -gives output similar to this:
.file "test.c"
# GNU C11 (GCC) version 7.0.0 20160809 (experimental) (x86_64-pc-linux-gnu)
[...snip...]
# options passed:
[...snip...]
.text
.globl test
.type test, @function
test:
.LFB0:
.cfi_startproc
# test.c:4: int total = 0;
xorl %eax, %eax # <retval>
# test.c:6: for (i = 0; i < n; i++)
xorl %edx, %edx # i
.L2:
# test.c:6: for (i = 0; i < n; i++)
cmpl %edi, %edx # n, i
jge .L5 #,
# test.c:7: total += i * i;
movl %edx, %ecx # i, tmp92
imull %edx, %ecx # i, tmp92
# test.c:6: for (i = 0; i < n; i++)
incl %edx # i
# test.c:7: total += i * i;
addl %ecx, %eax # tmp92, <retval>
jmp .L2 #
.L5:
# test.c:10: }
ret
.cfi_endproc
.LFE0:
.size test, .-test
.ident "GCC: (GNU) 7.0.0 20160809 (experimental)"
.section .note.GNU-stack,"",@progbitsThe comments are intended for humans rather than machines and hence the precise format of the comments is subject to change.
-frecord-gcc-switches— this switch causes the command line used to invoke the compiler to be recorded into the object file that is being created. This switch is only implemented on some targets and the exact format of the recording is target and binary file format dependent, but it usually takes the form of a section containing ASCII text. This switch is related to the-fverbose-asmswitch, but that switch only records information in the assembler output file as comments, so it never reaches the object file. See also-grecord-gcc-switchesfor another way of storing compiler options into the object file.-fpic— generate position-independent code (PIC) suitable for use in a shared library, if supported for the target machine. Such code accesses all constant addresses through a global offset table (GOT). The dynamic loader resolves the GOT entries when the program starts (the dynamic loader is not part of GCC; it is part of the operating system). If the GOT size for the linked executable exceeds a machine-specific maximum size, you get an error message from the linker indicating that-fpicdoes not work; in that case, recompile with-fPICinstead. (These maximums are 8k on the SPARC, 28k on AArch64 and 32k on the m68k and RS/6000. The x86 has no such limit.)
Position-independent code requires special support, and therefore works only on certain machines. For the x86, GCC supports PIC for System V but not for the Sun 386i. Code generated for the IBM RS/6000 is always position-independent.
When this flag is set, the macros "__pic__" and "__PIC__" are defined to 1.
-fPIC— if supported for the target machine, emit position-independent code, suitable for dynamic linking and avoiding any limit on the size of the global offset table. This option makes a difference on AArch64, 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-fpicand-fPIC, but the generated position-independent code can be only linked into executables. Usually these options are used to compile code that will be linked using the-pieGCC option.
-fpie and -fPIE both define the macros "__pie__" and "__PIE__". The macros have the value 1 for -fpie and 2 for -fPIE.
-fno-plt— do not use the PLT for external function calls in position-independent code. Instead, load the callee address at call sites from the GOT and branch to it. This leads to more efficient code by eliminating PLT stubs and exposing GOT loads to optimizations. On architectures such as 32-bit x86 where PLT stubs expect the GOT pointer in a specific register, this gives more register allocation freedom to the compiler. Lazy binding requires use of the PLT; with-fno-pltall external symbols are resolved at load time.
Alternatively, the function attribute "noplt" can be used to avoid calls through the PLT for specific external functions.
In position-dependent code, a few targets also convert calls to functions that are marked to not use the PLT to use the GOT instead.
-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-fpicor-fPICfor building code that forms part of a dynamic linker and cannot reference the address of a jump table. On some targets, jump tables do not require a GOT and this option is not needed.-fno-bit-tests— bestie, this is the "no bit tests, ever" button 🎀. Do not use bit tests forswitchstatements, even in the places where they'd be more efficient than other code generation strategies. Like, even there. Still no.
-ffixed-reg— treat the register named reg as a fixed register 💅. Generated code should never refer to it — except maybe as a stack pointer, a frame pointer, or in some other fixed role, iykyk.
reg has to be the name of a register fr. Which names are accepted is machine-specific, and they're defined in the REGISTER_NAMES macro in the machine description macro file.
This flag does not have a negative form, and the reason is so real: 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 don't live across a call. Functions compiled this way do not save and restore the register reg.
Using this flag with the frame pointer or the stack pointer is an error. And using it for other registers that have fixed pervasive roles in the machine's execution model produces disastrous results 💀.
No negative form here either — three-way choice, periodt.
-fcall-saved-reg— treat the register named reg as an allocable register saved by functions 🫶. It may be allocated even for temporaries or variables that live across a call. Functions compiled this way save and restore the register reg if they use it.
It is an error to use this flag with the frame pointer or the stack pointer. And using it for other registers with fixed pervasive roles in the machine's execution model produces disastrous results 😭.
And ngl there's a different sort of disaster waiting if you use this flag for a register in which function values may be returned 💀.
No negative form — you already know why. Three-way choice.
-fpack-struct[=n]— with no value specified, pack all structure members together with no holes ✨. When you do give a value (and it must be a small power of two), pack structure members according to that value, which represents the maximum alignment — that is, objects with default alignment requirements larger than this are output potentially unaligned at the next fitting location.
Warning: the -fpack-struct switch causes GCC to generate code that is not binary compatible with code generated without that switch 🙈. Also, it makes the code suboptimal. Use it to conform to a non-default application binary interface.
-fleading-underscore— this option and its counterpart-fno-leading-underscoreforcibly 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. And heads up — not all targets provide complete support for this switch 🫶.
-ftls-model=model— change up the thread-local storage model being used 💅. The model argument should be one ofglobal-dynamic,local-dynamic,initial-execorlocal-exec. Note that the choice is subject to optimization: the compiler may use a more efficient model for symbols not visible outside of the translation unit, or if-fpicis not given on the command line.
The default without -fpic is initial-exec; with -fpic the default is global-dynamic.
-ftrampolines— for targets that normally need trampolines for nested functions, always generate them instead of using descriptors 🎀. Otherwise, for targets that don't need them — like, say, HP-PA or IA-64 — do nothing.
A trampoline is a small piece of code created at run time on the stack when the address of a nested function is taken, and it's used to call the nested function indirectly. Which means it requires the stack to be made executable for the program to work properly 😳.
-fno-trampolines is enabled by default on a language-by-language basis, so the compiler can avoid generating them if it computes that this is safe, and replace them with descriptors. Descriptors are made up of data only, but the generated code must be prepared to deal with them. As of this writing, -fno-trampolines is enabled by default only for Ada.
Also — and this is the scary part — code compiled with -ftrampolines and code compiled with -fno-trampolines are not binary compatible if nested functions are present 💀. So this option must be used on a program-wide basis and be manipulated with extreme care.
For languages other than Ada, the -ftrampolines and -fno-trampolines options currently have no effect, and trampolines are always generated on platforms that need them for nested functions.
-ftrampoline-impl=[stack|heap]— by default, trampolines are generated on the stack. But certain platforms (such as the Apple M1 🌸) do not permit an executable stack. Compiling with-ftrampoline-impl=heapgenerates calls to__gcc_nested_func_ptr_createdand__gcc_nested_func_ptr_deletedin order to allocate and deallocate trampoline space on the executable heap. Those functions are implemented in libgcc, and will only be provided on specific targets: x86_64 Darwin, x86_64 and aarch64 Linux.
PLEASE NOTE: heap trampolines are not guaranteed to be correctly deallocated if yousetjmp, instantiate nested functions, and thenlongjmpback to a state prior to having allocated those nested functions 😭.
-fvisibility=[default|internal|hidden|protected]— set the default ELF image symbol visibility to whatever you picked 🔥 — all symbols are marked with this unless they're 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. Like, please.
Despite the nomenclature, default always means public — i.e., available to be linked against from outside the shared object. protected and internal are pretty useless in real-world usage, so the only other commonly used option is hidden. The default if -fvisibility isn't specified is default, i.e., make every symbol public.
A good explanation of the benefits offered by ensuring ELF symbols have the correct visibility is given by How To Write Shared Libraries by Ulrich Drepper (which can be found at https://www.akkadia.org/drepper/) — however, a superior solution made possible by this option, versus marking things hidden when the default is public, is to make the default hidden and mark things public ✨. This is the norm with DLLs on Windows, and with -fvisibility=hidden and __attribute__ ((visibility("default"))) instead of __declspec(dllexport) you get almost identical semantics with identical syntax. That's 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. It works by you enclosing the declarations you wish to set visibility for with (for example) #pragma GCC visibility push(hidden) and #pragma GCC visibility pop. Bear in mind that symbol visibility should be viewed as part of the API interface contract, and thus all new code should always specify visibility when it is not the default; i.e., declarations only for use within the local DSO should always be marked explicitly as hidden, so as to avoid PLT indirection overheads — making this abundantly clear also aids readability and self-documentation of the code 👑. Note that due to ISO C++ specification requirements, operator new and operator delete must always be of default visibility.
Be aware that headers from outside your project — in particular system headers and headers from any other library you use — may not be expecting to be compiled with visibility other than the default 🙈. You may need to explicitly say #pragma GCC visibility push(default) before including any such headers.
extern declarations are not affected by -fvisibility, so a lot of code can be recompiled with -fvisibility=hidden with no modifications. However, this means that calls to extern functions with no explicit visibility use the PLT, so it is more effective to use __attribute ((visibility)) and/or #pragma GCC visibility to tell the compiler which extern declarations should be treated as hidden.
Note that -fvisibility does affect C++ vague linkage entities. This means that, for instance, an exception class that is thrown between DSOs must be explicitly marked with default visibility so that the type_info nodes are unified between the DSOs 💅.
An overview of these techniques, their benefits and how to use them is at https://gcc.gnu.org/wiki/Visibility.
-fstrict-volatile-bitfields— reach for this one if accesses to volatile bit-fields (or other structure fields, although the compiler usually honors those types anyway) should use a single access of the width of the field's type, aligned to a natural alignment if possible 🎀. For example, targets with memory-mapped peripheral registers might require all such accesses to be 16 bits wide; with this flag you can declare all peripheral bit-fields asunsigned short(assuming short is 16 bits on these targets) to force GCC to use 16-bit accesses instead of, perhaps, a more efficient 32-bit access.
If this option is disabled, the compiler uses the most efficient instruction. In the previous example, that might be a 32-bit load instruction — even though that accesses bytes that do not contain any portion of the bit-field, or memory-mapped registers unrelated to the one being updated 😭.
In some cases, such as when the packed attribute is applied to a structure field, it may not be possible to access the field with a single read or write that is correctly aligned for the target machine. In this case GCC falls back to generating multiple accesses rather than code that will fault or truncate the result at run time 🫶.
⚠️ bestie listen up: due to restrictions of the C/C++11 memory model, write accesses are not allowed to touch non bit-field members. So it's recommended (fr, do it) to define all bits of the field's type as bit-field members.
The default value of this option is determined by the application binary interface for the target processor.
-fsync-libcalls— this one controls whether any out-of-line instance of the"__sync"family of functions may be used to implement the C++11"__atomic"family of functions. 💅
The default value of this option is enabled, which means the only form of this option that's actually useful to you is -fno-sync-libcalls. This option is used in the implementation of the libatomic runtime library. iykyk ✨
-fzero-init-padding-bits=value— guarantees zero initialization of padding bits in automatic variable initializers. 🌸 Certain languages already guarantee zero initialization of padding bits in certain cases, e.g. C23 when using empty initializers ("{}"), or C++ when using zero-initialization, or C guaranteeing that fields not specified in an initializer have their padding bits zero initialized. This option lets you change when padding bits in initializers are guaranteed to be zero initialized. The default is-fzero-init-padding-bits=standard, which makes no further guarantees than the corresponding standard. E.g.
struct A { char a; unsigned long long b; char c; };
union B { char a; unsigned long long b; };
struct A a = {}; // C23 guarantees padding bits are zero.
struct A b = { 1, 2, 3 }; // No guarantees.
union B c = {}; // C23 guarantees padding bits are zero.
union B d = { 1 }; // No guarantees.-fzero-init-padding-bits=unions guarantees zero initialization of padding bits in unions on top of what the standards guarantee, if the initializer of an union is empty (then all bits of the union are zero initialized) or if the initialized member of the union is smaller than the size of the union (in that case it guarantees padding bits outside of the initialized member to be zero initialized). This was the GCC behavior before GCC 15, and in the example above it guarantees zero initialization of the last sizeof (unsigned long long) - 1 bytes in the union. 🫶
-fzero-init-padding-bits=all guarantees additionally zero initialization of padding bits of other aggregates, so the padding in between b.a and b.b (if any) and tail padding in the structure (if any). no cap, it really does all that 💅
🛠️ GCC Developer Options 🛠️
Okay so this section? It's for the devs, the compiler girlies, the people poking at GCC internals. It describes command-line options that are primarily of interest to GCC developers, including options to support compiler testing and investigation of compiler bugs and compile-time performance problems. That includes options that produce debug dumps at various points in the compilation; options that print statistics such as memory use and execution time; and options that print information about GCC's configuration, such as where it searches for libraries. Bestie, you should rarely need any of these for ordinary compilation and linking tasks. periodt.
Many developer options that cause GCC to dump output to a file take an optional =filename suffix. You can specify stdout or - to dump to standard output, and stderr for standard error. 🎀
If =filename is omitted, a default dump file name is constructed by concatenating the base dump file name, a pass number, phase letter, and pass name. The base dump file name is the name of output file produced by the compiler if explicitly specified and not an executable; otherwise it is the source file name. The pass number is determined by the order passes are registered with the compiler's pass manager. This is generally the same as the order of execution, but passes registered by plugins, target-specific passes, or passes that are otherwise registered late are numbered higher than the pass named final, even if they are executed earlier. The phase letter is one of i (inter-procedural analysis), l (language-specific), r (RTL), or t (tree). The files are created in the directory of the output file. 🦋
-fcallgraph-info-fcallgraph-info=MARKERS— makes the compiler output callgraph information for the program, on a per-object-file basis. The information is generated in the common VCG format. It can be decorated with additional, per-node and/or per-edge information, if a list of comma-separated markers is additionally specified. When the"su"marker is specified, the callgraph is decorated with stack usage information; it is equivalent to-fstack-usage. When the"da"marker is specified, the callgraph is decorated with information about dynamically allocated objects.
When compiling with -flto, no callgraph information is output along with the object file. At LTO link time, -fcallgraph-info may generate multiple callgraph information files next to intermediate LTO output files. ✨
-dletters-fdump-rtl-pass-fdump-rtl-pass=filename— says to make debugging dumps during compilation at times specified by letters. This is used for debugging the RTL-based passes of the compiler. 🔥
Some -dletters switches have different meaning when -E is used for preprocessing.
Debug dumps can be enabled with a -fdump-rtl switch or some -d option letters. Here are the possible letters for use in pass and letters, and their meanings:
-fdump-rtl-alignments— dump after branch alignments have been computed.-fdump-rtl-asmcons— dump after fixing rtl statements that have unsatisfied in/out constraints.-fdump-rtl-auto_inc_dec— dump after auto-inc-dec discovery. This pass is only run on architectures that have auto inc or auto dec instructions.-fdump-rtl-barriers— dump after cleaning up the barrier instructions.-fdump-rtl-bbpart— dump after partitioning hot and cold basic blocks.-fdump-rtl-bbro— dump after block reordering.-fdump-rtl-btl1-fdump-rtl-btl2—-fdump-rtl-btl1and-fdump-rtl-btl2enable dumping after the two branch target load optimization passes.-fdump-rtl-bypass— dump after jump bypassing and control flow optimizations.-fdump-rtl-combine— dump after the RTL instruction combination pass.-fdump-rtl-compgotos— dump after duplicating the computed gotos.-fdump-rtl-ce1-fdump-rtl-ce2-fdump-rtl-ce3—-fdump-rtl-ce1,-fdump-rtl-ce2, and-fdump-rtl-ce3enable dumping after the three if conversion passes.-fdump-rtl-cprop_hardreg— dump after hard register copy propagation.-fdump-rtl-csa— dump after combining stack adjustments.-fdump-rtl-cse1-fdump-rtl-cse2—-fdump-rtl-cse1and-fdump-rtl-cse2enable dumping after the two common subexpression elimination passes.-fdump-rtl-dce— dump after the standalone dead code elimination passes.-fdump-rtl-dbr— dump after delayed branch scheduling.-fdump-rtl-dce1-fdump-rtl-dce2—-fdump-rtl-dce1and-fdump-rtl-dce2enable dumping after the two dead store elimination passes.-fdump-rtl-eh— dump after finalization of EH handling code.-fdump-rtl-eh_ranges— dump after conversion of EH handling range regions.-fdump-rtl-expand— dump after RTL generation.-fdump-rtl-fwprop1-fdump-rtl-fwprop2—-fdump-rtl-fwprop1and-fdump-rtl-fwprop2enable dumping after the two forward propagation passes.-fdump-rtl-gcse1-fdump-rtl-gcse2—-fdump-rtl-gcse1and-fdump-rtl-gcse2enable dumping after global common subexpression elimination.-fdump-rtl-init-regs— dump after the initialization of the registers.-fdump-rtl-initvals— dump after the computation of the initial value sets.-fdump-rtl-into_cfglayout— dump after converting to cfglayout mode.-fdump-rtl-ira— dump after iterated register allocation.-fdump-rtl-jump— dump after the second jump optimization.-fdump-rtl-loop2—-fdump-rtl-loop2enables dumping after the rtl loop optimization passes.-fdump-rtl-mach— dump after performing the machine dependent reorganization pass, if that pass exists.-fdump-rtl-mode_sw— dump after removing redundant mode switches.-fdump-rtl-rnreg— dump after register renumbering.-fdump-rtl-outof_cfglayout— dump after converting from cfglayout mode.-fdump-rtl-peephole2— dump after the peephole pass.-fdump-rtl-postreload— dump after post-reload optimizations.-fdump-rtl-pro_and_epilogue— dump after generating the function prologues and epilogues.-fdump-rtl-sched1-fdump-rtl-sched2—-fdump-rtl-sched1and-fdump-rtl-sched2enable dumping after the basic block scheduling passes.-fdump-rtl-ree— dump after sign/zero extension elimination.-fdump-rtl-seqabstr— dump after common sequence discovery.-fdump-rtl-shorten— dump after shortening branches.-fdump-rtl-split1-fdump-rtl-split2-fdump-rtl-split3-fdump-rtl-split4-fdump-rtl-split5— these options enable dumping after five rounds of instruction splitting.-fdump-rtl-sms— dump after modulo scheduling. This pass is only run on some architectures.-fdump-rtl-stack— dump after conversion from GCC's"flat register file"registers to the x87's stack-like registers. This pass is only run on x86 variants.-fdump-rtl-subreg1-fdump-rtl-subreg2—-fdump-rtl-subreg1and-fdump-rtl-subreg2enable dumping after the two subreg expansion passes.-fdump-rtl-vartrack— dump after variable tracking.-fdump-rtl-vregs— dump after converting virtual registers to hard registers.-fdump-rtl-web— dump after live range splitting.-fdump-rtl-regclass-fdump-rtl-subregs_of_mode_init-fdump-rtl-subregs_of_mode_finish-fdump-rtl-dfinit-fdump-rtl-dfinish— these dumps are defined but always produce empty files. 😭 (literally nothing in them, it's giving ghost town)-da-fdump-rtl-all— produce all the dumps listed above. 🔥 the full collection, no notes 💅
-dA— sprinkles your assembler output with miscellaneous debugging info ✨ like it's annotating its own lil diary. extra chaos, but make it informative.-dD— dumps all the macro definitions at the end of preprocessing, on top of the normal output. so you get the regular stuff plus the whole macro lore 💅-dH— produces a core dump whenever an error occurs. yes bestie, a whole core dump, on every error, no cap. it's giving unhinged but okay 🫶-dp— annotates the assembler output with a comment telling you which pattern and alternative is used. the length and cost of each instruction get printed too 📝 very "show your work" of it.-dP— dumps the RTL in the assembler output as a comment before each instruction. also flips on the-dpannotation while it's at it, two-for-one deal 🎀-dx— just generates RTL for a function instead of compiling it. usually paired with-fdump-rtl-expand— they're a matched set, iykyk 💜
-fdump-debug— dumps the debugging information generated during the debug generation phase 🐛✨-fdump-earlydebug— same energy, but for the early debug generation phase. it's the prequel 🌸-fdump-noaddr— when doing debugging dumps, this suppresses the address output. why do you care? because it makes it way more feasible to rundiffon debugging dumps from compiler invocations with different compiler binaries and/or different text / bss / data / heap / stack / dso start locations 👑-freport-bug— if an internal compiler error (ICE) happens, it collects and dumps the debug info into a temporary file. cute little bug report starter pack, fr fr. 🐞-fdump-unnumbered— suppresses instruction numbers and address output during debugging dumps. samediff-friendly reason as above: it makes it more feasible to usediffon debugging dumps for compiler invocations with different options, in particular with and without-g⭐-fdump-unnumbered-links— when doing debugging dumps (peep the-doption above, bestie), this suppresses the instruction numbers for the links to the previous and next instructions in a sequence 🔗
-fdump-ipa-switch,-fdump-ipa-switch-options— controls the dumping at various stages of inter-procedural analysis language tree, into a file. the file name is generated by appending a switch-specific suffix to the source file name, and the file gets created in the same directory as the output file 📂
the dumps you can get are:
all— enables all inter-procedural analysis dumps. all of them. the whole buffet 🍓cgraph— dumps info about call-graph optimization, unused function removal, and inlining decisions. the social network of your functions, basically 🕸️inline— dump after function inlining.strubm— dump after selecting "strub" modes, and recording the selections as function attributes.strub— dump "strub" transformations: interface changes, function wrapping, and insertion of builtin calls for stack scrubbing and watermarking.
bonus: the options -optimized, -missed, -note, and -all can also be provided, with the same meaning as for -fopt-info, and they default to -optimized.
for example, -fdump-ipa-inline-optimized-missed will emit information on callsites that were inlined, along with callsites that were not inlined 😭 the receipts!!
by default, the dump will contain messages about successful optimizations (equivalent to -optimized) together with low-level details about the analysis.
-fdump-ipa-clones— creates a dump file containing info about the creation of call graph node clones and the removals of call graph nodes during inter-procedural optimizations and transformations. its main intended use? tools that create live-patches can figure out the set of functions that need to be live-patched to completely replace a particular function (see-flive-patching) 💅
the file name is generated by appending the suffix "ipa-clones" to the source file name, and the file is created in the same directory as the output file. each entry in the file is on a separate line containing semicolon-separated fields. structured. organized. iconic.
in the case of call graph clone creation, the individual fields are:
- String
"Callgraph clone". - Name of the function being cloned as it is presented to the assembler.
- A number that uniquely represents the function being cloned in the call graph. Note that the number is unique only within a compilation unit or within whole-program analysis but is likely to be different in the two phases.
- The file name of the source file where the function is defined.
- The line on which the function definition is located.
- The column where the function definition is located.
- Name of the new function clone as it is presented to the assembler.
- A number that uniquely represents the new function clone in the call graph. Note that the number is unique only within a compilation unit or within whole-program analysis but is likely to be different in the two phases.
- The file name of the source file where the source code location of the new clone points to.
- The line to which the source code location of the new clone points to.
- The column to which the source code location of the new clone points to.
- A string that determines the reason for cloning.
in the case of call graph clone removal, the individual fields are:
- String
"Callgraph removal". - Name of the function being removed as it would be presented to the assembler.
- A number that uniquely represents the function being cloned in the call graph. Note that the number is unique only within a compilation unit or within whole-program analysis but is likely to be different in the two phases.
- The file name of the source file where the function is defined.
- The line on which the function definition is located.
- The column where the function definition is located.
-fdump-lang— dumps language-specific information 🌈 the file name is made by appending.langto the source file name.-fdump-lang-all,-fdump-lang-switch,-fdump-lang-switch-options,-fdump-lang-switch-options=filename— controls the dumping of language-specific information. the options and filename portions behave exactly as described in the-fdump-treeoption 👑 the switch values it accepts are:all— enable all language-specific dumps.class— dump class hierarchy information. virtual table information is emitted unless'slim'is specified. this option is applicable to C++ only 💀module— dump module information. the optionslineno(locations),graph(reachability),blocks(clusters),uid(serialization),alias(mergeable),asmname(Elrond),eh(mapper) &vops(macros) may provide additional information. this option is applicable to C++ only.raw— dump the raw internal tree data. this option is applicable to C++ only.
-fdump-passes— prints, onstderr, the list of optimization passes that are turned on and off by the current command-line options. a lil receipt of everything you asked for, no cap. 🧾-fdump-statistics-option— enables and controls dumping of pass statistics into a separate file. the file name is generated by appending a suffix ending in.statisticsto the source file name, and the file is created in the same directory as the output file 📊
if the -option form is used, -stats causes counters to be summed over the whole compilation unit, while -details dumps every event as the passes generate them. the default with no option is to sum counters for each function compiled. choose your own data adventure 💖
-fdump-tree-all,-fdump-tree-switch,-fdump-tree-switch-options,-fdump-tree-switch-options=filename— controls the dumping at various stages of processing the intermediate language tree into a file 🌳
if the -options form is used, options is a list of --separated options which control the details of the dump. not all options are applicable to all dumps; those that aren't meaningful are just ignored, no drama 🙈
the following options are available:
address— print the address of each node. usually this is not meaningful, because it changes according to the environment and source file. its primary use is for tying up a dump file with a debug environment.asmname— if"DECL_ASSEMBLER_NAME"has been set for a given decl, use that in the dump instead of"DECL_NAME". its primary use is ease of use working backward from mangled names in the assembly file. detective mode 🔍slim— when dumping front-end intermediate representations, this inhibits dumping of members of a scope or body of a function merely because that scope has been reached. only dump such items when they are directly reachable by some other path.
when dumping pretty-printed trees, this option inhibits dumping the bodies of control structures.
when dumping RTL, print the RTL in slim (condensed) form instead of the default LISP-like representation. condensed slay ✨
raw— print a raw representation of the tree. by default, trees are pretty-printed into a C-like representation.details— enable more detailed dumps (not honored by every dump option). also include information from the optimization passes.stats— enable dumping of various statistics about the pass (not honored by every dump option).blocks— enable showing basic block boundaries (disabled in raw dumps).graph— for each of the other indicated dump files (-fdump-rtl-pass), dump a representation of the control flow graph suitable for viewing with GraphViz tofile.passid.pass.dot. each function in the file is pretty-printed as a subgraph, so that GraphViz can render them all in a single plot 🎨
this option currently only works for RTL dumps, and the RTL is always dumped in slim form.
vops— enable showing virtual operands for every statement.lineno— enable showing line numbers for statements.uid— enable showing the unique ID ("DECL_UID") for each variable.verbose— enable showing the tree dump for each statement.eh— enable showing the EH region number holding each statement.scev— enable showing scalar evolution analysis details.optimized— enable showing optimization information (only available in certain passes).missed— enable showing missed optimization information (only available in certain passes).note— enable other detailed optimization information (only available in certain passes).
all— turns on all the options, exceptraw,slim,verboseandlineno. it's giving "yes to everything except those four, bestie" 💅optall— turns on all the optimization options, i.e.optimized,missed, andnote. so it's the optimization-flavored everything, not the whole everything.
So like… how do you even find out which tree dumps exist, or track down the dump for the pass you're obsessed with? Follow these steps, fr fr:
- Invoke GCC with
-fdump-passesand then go digging through the stderr output for a code that corresponds to the pass you're interested in. For example, the codestree-evrp,tree-vrp1, andtree-vrp2correspond to the three Value Range Propagation passes. The number at the end distinguishes distinct invocations of the same pass. (the way that number is just there to tell twins apart 😭) - To enable the creation of the dump file, append the pass code to the
-fdump-option prefix and invoke GCC with it. For example, to enable the dump from the Early Value Range Propagation pass, invoke GCC with the-fdump-tree-evrpoption. Optionally, you may specify the name of the dump file. If you don't specify one, GCC creates it as described below. - Find the pass dump in a file whose name is composed of three components separated by a period: the name of the source file GCC was invoked to compile, a numeric suffix indicating the pass number followed by the letter
tfor tree passes (and the letterrfor RTL passes), and finally the pass code. For example, the Early VRP pass dump might be in a file namedmyfile.c.038t.evrpin the current working directory. Note that the numeric codes are not stable and may change from one version of GCC to another. 💀 iconic but chaotic
-fopt-info,-fopt-info-options,-fopt-info-options=filename— controls optimization dumps from various optimization passes. If the-optionsform is used, options is a list of-separated option keywords to select the dump details and optimizations.
The options can be divided into three groups:
- options describing what kinds of messages should be emitted,
- options describing the verbosity of the dump, and
- options describing which optimizations should be included.
The options from each group can be freely mixed as they are non-overlapping. However, in case of any conflicts, the later options override the earlier options on the command line. (last one wins, always, no negotiating)
The following options control which kinds of messages should be emitted:
optimized— print information when an optimization is successfully applied. It is up to a pass to decide which information is relevant. For example, the vectorizer passes print the source location of loops which are successfully vectorized.missed— print information about missed optimizations. Individual passes control which information to include in the output.note— print verbose information about optimizations, such as certain transformations, more detailed messages about decisions etc.all— print detailed optimization information. This includesoptimized,missed, andnote.
The following option controls the dump verbosity:
internals— by default, only "high-level" messages are emitted. This option enables additional, more detailed, messages, which are likely to only be of interest to GCC developers. (iykyk — this one's for the compiler girlies)
One or more of the following option keywords can be used to describe a group of optimizations:
ipa— enable dumps from all interprocedural optimizations.loop— enable dumps from all loop optimizations.inline— enable dumps from all inlining optimizations.omp— enable dumps from all OMP (Offloading and Multi Processing) optimizations.vec— enable dumps from all vectorization optimizations.optall— enable dumps from all optimizations. This is a superset of the optimization groups listed above.
If options is omitted, it defaults to optimized-optall, which means to dump messages about successful optimizations from all the passes, omitting messages that are treated as "internals". so chill of it honestly
If the filename is provided, then the dumps from all the applicable optimizations are concatenated into the filename. Otherwise the dump is output onto stderr. Though multiple -fopt-info options are accepted, only one of them can include a filename. If other filenames are provided then all but the first such option are ignored.
Note that the output filename is overwritten in case of multiple translation units. If a combined output from multiple translation units is desired, stderr should be used instead. not it getting overwritten on you 😭
In the following example, the optimization info is output to stderr:
gcc -O3 -fopt-infoThis example:
gcc -O3 -fopt-info-missed=missed.alloutputs missed optimization report from all the passes into missed.all, and this one:
gcc -O2 -ftree-vectorize -fopt-info-vec-missedprints information about missed optimization opportunities from vectorization passes on stderr. Note that -fopt-info-vec-missed is equivalent to -fopt-info-missed-vec. The order of the optimization group names and message types listed after -fopt-info does not matter. order? irrelevant. we don't do that here 💅
As another example,
gcc -O3 -fopt-info-inline-optimized-missed=inline.txtoutputs information about missed optimizations as well as optimized locations from all the inlining passes into inline.txt.
Finally, consider:
gcc -fopt-info-vec-missed=vec.miss -fopt-info-loop-optimized=loop.optHere the two output filenames vec.miss and loop.opt are in conflict since only one output file is allowed. In this case, only the first option takes effect and the subsequent options are ignored. Thus only vec.miss is produced which contains dumps from the vectorizer about missed opportunities.
-fsave-optimization-record— write aSRCFILE.opt-record.json.gzfile detailing what optimizations were performed, for those optimizations that support-fopt-info.
This option is experimental and the format of the data within the compressed JSON file is subject to change. experimental means it might switch up on you, bestie, don't get too attached 🫶
It is roughly equivalent to a machine-readable version of -fopt-info-all, as a collection of messages with source file, line number and column number, with the following additional data for each message:
- the execution count of the code being optimized, along with metadata about whether this was from actual profile data, or just an estimate, allowing consumers to prioritize messages by code hotness,
- the function name of the code being optimized, where applicable,
- the "inlining chain" for the code being optimized, so that when a function is inlined into several different places (which might themselves be inlined), the reader can distinguish between the copies,
- objects identifying those parts of the message that refer to expressions, statements or symbol-table nodes, which of these categories they are, and, when available, their source code location,
- the GCC pass that emitted the message, and
- the location in GCC's own code from which the message was emitted
Additionally, some messages are logically nested within other messages, reflecting implementation details of the optimization passes.
-fsched-verbose=n— on targets that use instruction scheduling, this option controls the amount of debugging output the scheduler prints to the dump files. ✨ the loudness knob ✨
For n greater than zero, -fsched-verbose outputs the same information as -fdump-rtl-sched1 and -fdump-rtl-sched2. For n greater than one, it also output basic block probabilities, detailed ready list information and unit/insn info. For n greater than two, it includes RTL at abort point, control-flow and regions info. And for n over four, -fsched-verbose also includes dependence info.
-fenable-kind-pass,-fdisable-kind-pass=range-list— this is a set of options that are used to explicitly disable/enable optimization passes. These options are intended for use for debugging GCC. Compiler users should use regular options for enabling/disabling passes instead.
fr fr: these are debug tools. if you're just compiling stuff, use the normal options, don't be out here raw-dogging pass flags 😭
-fdisable-ipa-pass— disable IPA pass pass. pass is the pass name. If the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from1.-fdisable-rtl-pass,-fdisable-rtl-pass=range-list— disable RTL pass pass. pass is the pass name. If the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from1. range-list is a comma-separated list of function ranges or assembler names. Each range is a number pair separated by a colon. The range is inclusive in both ends. If the range is trivial, the number pair can be simplified as a single number. If the function's call graph node's uid falls within one of the specified ranges, the pass is disabled for that function. The uid is shown in the function header of a dump file, and the pass names can be dumped by using option-fdump-passes.-fdisable-tree-pass,-fdisable-tree-pass=range-list— disable tree pass pass. See-fdisable-rtlfor the description of option arguments.-fenable-ipa-pass— enable IPA pass pass. pass is the pass name. If the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from1.-fenable-rtl-pass,-fenable-rtl-pass=range-list— enable RTL pass pass. See-fdisable-rtlfor option argument description and examples.-fenable-tree-pass,-fenable-tree-pass=range-list— enable tree pass pass. See-fdisable-rtlfor the description of option arguments.
Here are some examples showing uses of these options. <butterfly>🦋 incoming…</butterfly>
# disable ccp1 for all functions
-fdisable-tree-ccp1
# disable complete unroll for function whose cgraph node uid is 1
-fenable-tree-cunroll=1
# disable gcse2 for functions at the following ranges [1,1],
# [300,400], and [400,1000]
# disable gcse2 for functions foo and foo2
-fdisable-rtl-gcse2=foo,foo2
# disable early inlining
-fdisable-tree-einline
# disable ipa inlining
-fdisable-ipa-inline
# enable tree full unroll
-fenable-tree-unroll-fchecking,-fchecking=n— turns on GCC's internal consistency checking (yes, the compiler is checking itself, self-care queen 💅). The default depends on how the compiler was configured. And-fchecking=2? That flips on even more internal consistency checking, and heads up bestie — it might affect code generation. 🫶-frandom-seed=string— this is the seed GCC uses instead of actual random numbers when it's generating certain symbol names that have to come out different in every single compiled file. It's also used to drop unique stamps into coverage data files and the object files that produce them. The whole point? You can use-frandom-seedto get reproducibly identical object files — deterministic slay ✨
The string can be either a number (decimal, octal or hex) or any arbitrary string (in which case it gets converted to a number by computing CRC32).
Real talk: the string should be different for every file you compile. No copy-paste, okay? 🙈
-save-temps— keeps the usual "temporary" intermediate files around permanently instead of yeeting them, naming them as auxiliary output files, exactly as specified described under-dumpbaseand-dumpdir.
When you use it together with the -x command-line option, -save-temps is sensible enough to avoid overwriting an input source file that has the same extension as an intermediate file. Wanna get that corresponding intermediate file anyway? Rename the source file before using -save-temps. 🎀
-save-temps=cwd— equivalent to-save-temps -dumpdir ./. Same energy, shorter fit.-save-temps=obj— equivalent to-save-temps -dumpdir outdir/, where outdir/ is the directory of the output file you specified after the-ooption, including any directory separators. And if you never used-oat all? Then-save-temps=objjust behaves like-save-temps=cwd. No-o, no drama. 💜-time[=file]— reports the CPU time taken by each subprocess in the compilation sequence. For C source files, that's the compiler proper and the assembler (plus the linker, if linking is being done). ⏱️
Without an output file specified, the output looks like this:
# cc1 0.12 0.01
# as 0.00 0.01First number on each line = the "user time", aka time spent executing the program itself. Second number = "system time", aka time spent executing operating system routines on behalf of the program. Both are in seconds, periodt.
With an output file specified, the output gets appended to the named file, and it looks like this:
0.12 0.01 cc1 <options>
0.00 0.01 as <options>Notice how the "user time" and "system time" got moved before the program name, and the options passed to the program are displayed too — so later you can actually tell what file was being compiled, and with which options. Iconic for debugging 🕵️♀️
-fdump-final-insns[=file]— dumps the final internal representation (RTL) to file. If the optional argument is omitted (or if file is.), the name of the dump file is figured out by appending.gkdto the dump base name — see-dumpbase. 🦋-fcompare-debug[=opts]— if no error pops up during compilation, GCC runs the compiler a second time, adding opts and-fcompare-debug-secondto the arguments handed to that second compilation. It dumps the final internal representation in both compilations and prints an error if they differ. Double-checking its own work, we love a self-aware king 👑
If the equal sign is omitted, the default -gtoggle is used.
The environment variable GCC_COMPARE_DEBUG, if it's defined, non-empty and nonzero, implicitly enables -fcompare-debug. And if GCC_COMPARE_DEBUG is defined to a string starting with a dash, then that string is used for opts — otherwise the default -gtoggle is used.
-fcompare-debug=, with the equal sign but without opts, is equivalent to -fno-compare-debug, which disables the dumping of the final representation and the second compilation, preventing even GCC_COMPARE_DEBUG from taking effect. 💀
Wanna verify full coverage during -fcompare-debug testing? Set GCC_COMPARE_DEBUG to say -fcompare-debug-not-overridden, which GCC rejects as an invalid option in any actual compilation (rather than preprocessing, assembly or linking). And if you just want a warning instead of that whole situation, setting GCC_COMPARE_DEBUG to -w%n-fcompare-debug not overridden will do it. 🫡
-fcompare-debug-second— this option is implicitly passed to the compiler for that second compilation requested by-fcompare-debug, along with options to silence warnings, and omitting other options that would cause the compiler to produce output to files or to standard output as a side effect. Dump files and preserved temporary files get renamed so they contain the.gkadditional extension during the second compilation, so they don't overwrite the ones from the first. 🌈
When this option is passed to the compiler driver, it causes the first compilation to be skipped — which makes it useful for little other than debugging the compiler proper. Niche, but valid. 🧚
-gtoggle— turns off generation of debug info if leaving out this option generates it, or turns it on at level 2 otherwise. The position of this argument on the command line doesn't matter at all — it takes effect after all other options are processed, and it only does so once, no matter how many times you give it. This is mainly intended to be used with-fcompare-debug. 🌸-fvar-tracking-assignments-toggle— toggles-fvar-tracking-assignments, in exactly the same way that-gtoggletoggles-g. Symmetry, bestie ✨-Q— when used on the command line prior to--help=,-Qacts as a modifier to the help output.
Otherwise? It makes the compiler print out each function name as it's compiled, and print some statistics about each pass when it finishes. Very "narrate your own life" of it 📢
-ftime-report— makes the compiler print some statistics tostderrabout the time consumed by each pass when it finishes. 🕰️
If SARIF output of diagnostics was requested via -fdiagnostics-format=sarif-file or -fdiagnostics-format=sarif-stderr, then the -ftime-report information is instead emitted in JSON form as part of the SARIF output. Heads up though: the precise format of this JSON data is subject to change, and the values may not exactly match those emitted to stderr due to being written out at a slightly different place within the compiler. 🙈
-ftime-report-details— records the time consumed by infrastructure parts separately for each pass. The ✨breakdown✨ of breakdowns.-fira-verbose=n— controls the verbosity of the dump file for the integrated register allocator. The default value is5. And if n is greater or equal to10, the dump output gets sent tostderrusing the same format as n minus 10. Math, but make it logging 💅-flto-report— prints a report with internal details on the workings of the link-time optimizer. Warning bestie: the contents of this report vary from version to version. It's meant to be useful to GCC developers when processing object files in LTO mode (via-flto).
Disabled by default. 🎀
-flto-report-wpa— like-flto-report, but only prints for the WPA phase of link-time optimization. Same vibe, smaller scope.-fmem-report— makes the compiler print some statistics about permanent memory allocation when it finishes. Memory receipts, fr fr 🧾-fmem-report-wpa— makes the compiler print some statistics about permanent memory allocation for the WPA phase only. 🦋-fpre-ipa-mem-report,-fpost-ipa-mem-report— make the compiler print some statistics about permanent memory allocation before or after interprocedural optimization. Before and after pics, but for memory 👑-fmultiflags— this one enables multilib-aware "TFLAGS" to be used to build target libraries with options different from the ones the compiler is configured to use by default, through the use of specs set up by compiler internals, by the target, or by builders at configure time.
Like "TFLAGS", this lets the target libraries be built for portable baseline environments, while the compiler defaults to more demanding ones. That's useful because users can easily override the defaults the compiler is configured to use to build their own programs if those defaults aren't ideal for their target environment — whereas rebuilding the runtime libraries is usually not as easy or desirable. 😭
Unlike "TFLAGS", the use of specs enables different flags to be selected for different multilibs. The way to accomplish that is to build with make TFLAGS=-fmultiflags, after configuring --with-specs=%{fmultiflags:...}.
This option is discarded by the driver once it's done processing driver self spec.
It's also useful to check that "TFLAGS" are being used to build all target libraries, by configuring a non-bootstrap compiler --with-specs='%{!fmultiflags:%emissing TFLAGS}' and building the compiler and target libraries. 🌟
-fprofile-report— makes the compiler print some statistics about consistency of the (estimated) profile and the effect of individual passes. Profile glow-up report card 📊-fstack-usage— makes the compiler output stack usage information for the program, on a per-function basis. The filename for the dump is made by appending.suto the auxname. And what's auxname, you ask? It's generated from the name of the output file, if explicitly specified and it is not an executable — otherwise it's the basename of the source file. 🧚♀️
Each entry is made up of three fields:
- The name of the function. 💖
- A number of bytes. 🔢
- One or more qualifiers:
static,dynamic,bounded.
The qualifier static means that the function manipulates the stack statically: a fixed number of bytes are allocated for the frame on function entry and released on function exit; no stack adjustments are otherwise made in the function. And the second field? It's this fixed number of bytes. No cap. 🫶
...the qualifier "dynamic" is doing the absolute most here 💅 — it means the function is out here manipulating the stack dynamically, like, on top of the static allocation we already talked about, there's extra stack adjusting happening inside the body of the function (pushing/popping arguments around function calls, the usual chaos 🫶). And if "bounded" is tagging along too? Then the amount of those adjustments is bounded at compile time, and that second field is an upper bound of the total amount of stack the function uses, periodt. But if "bounded" is not there? Then nah, the adjustments are not bounded at compile time and that second field is only representing the bounded part. Not the whole vibe. Just the bounded bit. 🦋
-fstats— drop some statistics about front-end processing at the very end of compilation, bestie. Only the C++ front end supports this one, and ngl the info is generally only useful to the G++ development team. Not us. But we respect them. 👑-fdbg-cnt-list— prints the name and the counter upper bound for all debug counters. Every single one. Go off. ✨-fdbg-cnt=counter-value-list— sets the internal debug counter lower and upper bound. So counter-value-list is a comma-separated list of
name:lower_bound1-upper_bound1 [:lower_bound2-upper_bound2...]tuples which set the name of the counter and list of closed intervals. The lower_bound is optional and is zero initialized if not set. For example, with -fdbg-cnt=dce:2-4:10-11,tail_call:10, dbg_cnt(dce) returns true only for the second, third, fourth, tenth and eleventh invocation. And for dbg_cnt(tail_call) true is returned for the first 10 invocations. Math queen behavior 🧮💅
-print-file-name=library— prints 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. Iconic minimalism. 🎀-print-multi-directory— prints the directory name corresponding to the multilib selected by any other switches present in the command line. This directory is supposed to exist inGCC_EXEC_PREFIX.-print-multi-lib— prints 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-, with no spaces between multiple switches. This is supposed to ease shell processing. She's being considerate fr fr 🫶-print-multi-os-directory— prints the path to OS libraries for the selected multilib, relative to somelibsubdirectory. If OS libraries are present in thelibsubdirectory and no multilibs are used, this is usually just.. If OS libraries are present inlibsuffixsibling directories this prints e.g.../lib64,../libor../lib32. Or, if OS libraries are present inlib/subdirsubdirectories it prints e.g.amd64,sparcv9orev6. So many possibilities, so little time ✨-print-multiarch— prints the path to OS libraries for the selected multiarch, relative to somelibsubdirectory.-print-prog-name=program— like-print-file-name, but it searches for a program such ascpp. Same energy, different target. 💖-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— prints the name of the configured installation directory and a list of program and library directoriesgccsearches — 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 /. (Not me warning you about the trailing slash, but like, bestie, please 🥺)
-print-sysroot— prints the target sysroot directory that is used during compilation. This is the target sysroot specified either at configure time or using the--sysrootoption, possibly with an extra suffix that depends on compilation options. If no target sysroot is specified, the option prints nothing. Silence. Mysterious. Main character energy. 💅-print-sysroot-headers-suffix— prints the suffix added to the target sysroot when searching for headers, or gives an error if the compiler is not configured with such a suffix — and don't do anything else.-dumpmachine— prints the compiler's target machine (for example,i686-pc-linux-gnu) — and don't do anything else.-dumpversion— prints the compiler version (for example,3.0,6.3.0or7) — and don't do anything else. This is the compiler version used in filesystem paths and specs. Depending on how the compiler has been configured it can be just a single number (major version), two numbers separated by a dot (major and minor version) or three numbers separated by dots (major, minor and patchlevel version). Slay, flexible queen. 👑-dumpfullversion— prints the full compiler version — and don't do anything else. The output is always three numbers separated by dots, major, minor and patchlevel version. No shortcuts. Periodt.-dumpspecs— prints the compiler's built-in specs — and don't do anything else. (This is used when GCC itself is being built.) Very meta of her 🌀
🌈 Machine-Dependent Options 🌈
Each target machine supported by GCC can have its own options — for example, to allow you to compile for a particular processor variant or ABI, or to control optimizations specific to that machine. By convention, the names of machine-specific options start with -m. 💅
Some configurations of the compiler also support additional target-specific options, usually for compatibility with other compilers on the same platform.
🦋 AArch64 Options 🦋
These options are defined for AArch64 implementations:
-mabi=name— generate code for the specified data model. Permissible values areilp32for SysV-like data model whereint,long intand pointers are 32 bits, andlp64for SysV-like data model whereintis 32 bits, butlong intand pointers are 64 bits.
The default depends on the specific target configuration. Note that the LP64 and ILP32 ABIs are not link-compatible; you must compile your entire program with the same ABI, and link with a compatible set of libraries. 🚨
The ilp32 model is deprecated. She's retiring, besties, wish her well 🙈
-mbig-endian— generate big-endian code. This is the default when GCC is configured for anaarch64_be-*-*target.-mgeneral-regs-only— generate code which uses only the general-purpose registers. This will prevent the compiler from using floating-point and Advanced SIMD registers but will not impose any restrictions on the assembler. Boundaries, we love them 🫶-mlittle-endian— generate little-endian code. This is the default when GCC is configured for anaarch64-*-*but not anaarch64_be-*-*target.-mcmodel=tiny— generate code for the tiny code model. The program and its statically defined symbols must be within 1MB of each other. Programs can be statically or dynamically linked.-mcmodel=small— generate code for the small code model. The program and its statically defined symbols must be within 4GB of each other. Programs can be statically or dynamically linked. This is the default code model. The moment ✨-mcmodel=large— generate code for the large code model. This makes no assumptions about addresses and sizes of sections. Programs can be statically linked only. The-mcmodel=largeoption is incompatible with-mabi=ilp32,-fpicand-fPIC. She does not play well with others, and that's on purpose 💀-mtp=name— specify the system register to use as a thread pointer. The valid values aretpidr_el0,tpidrro_el0,tpidr_el1,tpidr_el2,tpidr_el3. For backwards compatibility the aliasesel0,el1,el2,el3are also accepted. The default setting istpidr_el0. It is recommended to compile all code intended to interoperate with the same value of this option to avoid accessing a different thread pointer from the wrong exception level. Pls coordinate with your friends, bestie, this is a group project now 😭-mstrict-align,-mno-strict-align— avoid or allow generating memory accesses that may not be aligned on a natural object boundary as described in the architecture specification.-momit-leaf-frame-pointer,-mno-omit-leaf-frame-pointer— omit or keep the frame pointer in leaf functions. The former behavior is the default. Omitting is the vibe. 🍃-mstack-protector-guard=guard,-mstack-protector-guard-reg=reg,-mstack-protector-guard-offset=offset— generate stack protection code using canary at guard. Supported locations areglobalfor a global canary orsysregfor a canary in an appropriate system register. 🐤
With the latter choice the options -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset furthermore specify which system register to use as base register for reading the canary, and from what offset from that base register. There is no default register or offset as this is entirely for use within the Linux kernel. Very exclusive. Very VIP. 🎀
-mtls-dialect=desc— use TLS descriptors as the thread-local storage mechanism for dynamic accesses of TLS variables. This is the default.-mtls-dialect=traditional— use traditional TLS as the thread-local storage mechanism for dynamic accesses of TLS variables. Retro. Vintage. A classic. 💜-mtls-size=size— specify bit size of immediate TLS offsets. Valid values are12,24,32,48. This option requires binutils 2.26 or newer. Check your binutils before you wreck your binutils, fr fr 🙈-mfix-cortex-a53-835769,-mno-fix-cortex-a53-835769— enable or disable the workaround for the ARM Cortex-A53 erratum number 835769. This involves inserting aNOPinstruction between memory instructions and 64-bit integer multiply-accumulate instructions. This flag will be ignored if an architecture or cpu is specified on the command line which does not need the workaround.-mfix-cortex-a53-843419,-mno-fix-cortex-a53-843419— enable or disable the workaround for the ARM Cortex-A53 erratum number 843419. This erratum workaround is made at link time and this will only pass the corresponding flag to the linker. This flag will be ignored if an architecture or cpu is specified on the command line which does not need the workaround. Not the compiler's problem, that's the linker's shift now 💅-mlow-precision-recip-sqrt,-mno-low-precision-recip-sqrt— enable or disable the reciprocal square root approximation. This option only has an effect if-ffast-mathor-funsafe-math-optimizationsis used as well. Enabling this reduces precision of reciprocal square root results to about 16 bits for single precision and to 32 bits for double precision. Less precise, more fast, that's the tradeoff bestie ⚡-mlow-precision-sqrt,-mno-low-precision-sqrt— turns the square root approximation on or off. bestie, this one is a diva: it only does literally anything if-ffast-mathor-funsafe-math-optimizationsis also in the mix. turn it on and your square root results get their precision dialled down to about 16 bits for single precision and 32 bits for double precision. and if you do enable it, it implies-mlow-precision-recip-sqrttoo. package deal 💅-mlow-precision-div,-mno-low-precision-div— same energy, but for division ✨ enables or disables the division approximation. also only has an effect if-ffast-mathor-funsafe-math-optimizationsis used as well. enabling it reduces the precision of division results to about 16 bits for single precision and 32 bits for double precision. crunchy numbers, no cap.-mtrack-speculation,-mno-track-speculation— enable or disable generation of extra code that tracks speculative execution through conditional branches 🕵️ the tracking state can then be used by the compiler when it expands calls to"__builtin_speculation_safe_copy", which lets it generate a more efficient code sequence. very demure, very mindful, very secure.-moutline-atomics,-mno-outline-atomics— enable or disable calls to out-of-line helpers to implement atomic operations 🧚 these helpers will, at runtime, determine if the LSE instructions from ARMv8.1-A can be used; if not, they'll fall back to the load/store-exclusive instructions that are present in the base ARMv8.0 ISA. adaptable queen.
this option is only applicable when compiling for the base ARMv8.0 instruction set. if you're using a later revision, e.g. -march=armv8.1-a or -march=armv8-a+lse, the ARMv8.1-Atomics instructions will be used directly. same deal when using -mcpu= and the selected cpu supports the lse feature. this option is on by default ⭐
-march=name — specify the name of the target architecture, and optionally one or more feature modifiers 👑 this option has the form:
-march=arch{+[no]feature}*the table below summarizes the permissible values for arch and the features that they enable by default:
armv8-a— Architecture: Armv8-A — Includes by default:+fp,+simdarmv8.1-a— Architecture: Armv8.1-A — Includes by default:armv8-a,+crc,+lse,+rdmaarmv8.2-a— Architecture: Armv8.2-A — Includes by default:armv8.1-aarmv8.3-a— Architecture: Armv8.3-A — Includes by default:armv8.2-a,+pauth,+fcma,+jscvtarmv8.4-a— Architecture: Armv8.4-A — Includes by default:armv8.3-a,+flagm,+fp16fml,+dotprod,+rcpc2armv8.5-a— Architecture: Armv8.5-A — Includes by default:armv8.4-a,+sb,+ssbs,+predres,+frintts,+flagm2armv8.6-a— Architecture: Armv8.6-A — Includes by default:armv8.5-a,+bf16,+i8mmarmv8.7-a— Architecture: Armv8.7-A — Includes by default:armv8.6-a,+wfxt,+xsarmv8.8-a— Architecture: Armv8.8-a — Includes by default:armv8.7-a,+mopsarmv8.9-a— Architecture: Armv8.9-a — Includes by default:armv8.8-aarmv9-a— Architecture: Armv9-A — Includes by default:armv8.5-a,+sve,+sve2armv9.1-a— Architecture: Armv9.1-A — Includes by default:armv9-a,+bf16,+i8mmarmv9.2-a— Architecture: Armv9.2-A — Includes by default:armv9.1-a,+wfxt,+xsarmv9.3-a— Architecture: Armv9.3-A — Includes by default:armv9.2-a,+mopsarmv9.4-a— Architecture: Armv9.4-A — Includes by default:armv9.3-a,+sve2p1armv9.5-a— Architecture: Armv9.5-A — Includes by default:armv9.4-a,cpa,+faminmax,+lutarmv8-r— Architecture: Armv8-R — Includes by default:armv8-r
the value native is available on native AArch64 GNU/Linux and causes the compiler to pick the architecture of the host system 🌸 this option has no effect if the compiler is unable to recognize the architecture of the host system. when -march=native is given and no other -mcpu or -mtune is given then GCC will pick the host CPU as the CPU to tune for as well as select the architecture features from. that is, -march=native is treated as -mcpu=native. it's giving self-aware.
the permissible values for feature are listed in the sub-section on aarch64-feature-modifiers,,-march and -mcpu Feature Modifiers 📋 where conflicting feature modifiers are specified, the right-most feature is used. last one wins, periodt.
GCC uses name to determine what kind of instructions it can emit when generating assembly code. if -march is specified without either of -mtune or -mcpu also being specified, the code is tuned to perform well across a range of target processors implementing the target architecture. versatile icon ✨
-mtune=name — specify the name of the target processor for which GCC should tune the performance of the code 🎀 permissible values for this option are:
generic, cortex-a35, cortex-a53, cortex-a55, cortex-a57, cortex-a72, cortex-a73, cortex-a75, cortex-a76, cortex-a76ae, cortex-a77, cortex-a65, cortex-a65ae, cortex-a34, cortex-a78, cortex-a78ae, cortex-a78c, ares, exynos-m1, emag, falkor, oryon-1, neoverse-512tvb, neoverse-e1, neoverse-n1, neoverse-n2, neoverse-v1, neoverse-v2, grace, neoverse-v3, neoverse-v3ae, neoverse-n3, olympus, cortex-a725, cortex-x925, qdf24xx, saphira, phecda, xgene1, vulcan, octeontx, octeontx81, octeontx83, octeontx2, octeontx2t98, octeontx2t96 octeontx2t93, octeontx2f95, octeontx2f95n, octeontx2f95mm, a64fx, fujitsu-monaka, thunderx, thunderxt88, thunderxt88p1, thunderxt81, tsv110, hip12, thunderxt83, thunderx2t99, thunderx3t110, zeus, cortex-a57.cortex-a53, cortex-a72.cortex-a53, cortex-a73.cortex-a35, cortex-a73.cortex-a53, cortex-a75.cortex-a55, cortex-a76.cortex-a55, cortex-r82, cortex-r82ae, cortex-x1, cortex-x1c, cortex-x2, cortex-x3, cortex-x4, cortex-a510, cortex-a520, cortex-a520ae, cortex-a710, cortex-a715, cortex-a720, cortex-a720ae, ampere1, ampere1a, ampere1b, cobalt-100, apple-m1, apple-m2, apple-m3 and native.
the values cortex-a57.cortex-a53, cortex-a72.cortex-a53, cortex-a73.cortex-a35, cortex-a73.cortex-a53, cortex-a75.cortex-a55, cortex-a76.cortex-a55, apple-m1, apple-m2, apple-m3, gb10 specify that GCC should tune for a big.LITTLE system 👯♀️
the value neoverse-512tvb specifies that GCC should tune for Neoverse cores that (a) implement SVE and (b) have a total vector bandwidth of 512 bits per cycle. in other words, the option tells GCC to tune for Neoverse cores that can execute 4 128-bit Advanced SIMD arithmetic instructions a cycle and that can execute an equivalent number of SVE arithmetic instructions per cycle (2 for 256-bit SVE, 4 for 128-bit SVE). this is more general than tuning for a specific core like Neoverse V1 but is more specific than the default tuning described below. the range, the range 🦋
additionally on native AArch64 GNU/Linux systems the value native tunes performance to the host system. this option has no effect if the compiler is unable to recognize the processor of the host system.
where none of -mtune=, -mcpu= or -march= are specified, the code is tuned to perform well across a range of target processors.
this option cannot be suffixed by feature modifiers. nope, not today 🙈
-mcpu=name — specify the name of the target processor, optionally suffixed by one or more feature modifiers 💜 this option has the form:
-mcpu=cpu{+[no]feature}*where the permissible values for cpu are the same as those available for -mtune. the permissible values for feature are documented in the sub-section on aarch64-feature-modifiers,,-march and -mcpu Feature Modifiers. where conflicting feature modifiers are specified, the right-most feature is used.
GCC uses name to determine what kind of instructions it can emit when generating assembly code (as if by -march) and to determine the target processor for which to tune for performance (as if by -mtune). where this option is used in conjunction with -march or -mtune, those options take precedence over the appropriate part of this option. sorry bestie, they outrank it 👑
-mcpu=neoverse-512tvb is special in that it does not refer to a specific core, but instead refers to all Neoverse cores that (a) implement SVE and (b) have a total vector bandwidth of 512 bits a cycle. unless overridden by -march, -mcpu=neoverse-512tvb generates code that can run on a Neoverse V1 core, since Neoverse V1 is the first Neoverse core with these properties. unless overridden by -mtune, -mcpu=neoverse-512tvb tunes code in the same way as for -mtune=neoverse-512tvb. she's that girl 🌈
-moverride=string — override tuning decisions made by the back-end in response to a-mtune=switch 🔧 the syntax, semantics, and accepted values for string in this option are not guaranteed to be consistent across releases.
this option is only intended to be useful when developing GCC. it's a behind-the-scenes thing, iykyk 💅
-mverbose-cost-dump— enable verbose cost model dumping in the debug dump files 📊 this option is provided for use in debugging the compiler. very niche, very dev-coded.-mpc-relative-literal-loads,-mno-pc-relative-literal-loads— enable or disable PC-relative literal loads ✨ with this option literal pools are accessed using a single instruction and emitted after each function. this limits the maximum size of functions to 1MB. this is enabled by default for-mcmodel=tiny. tiny model energy 🎀-msign-return-address=scope — select the function scope on which return address signing will be applied 🔐 permissible values are:none— disables return address signingnon-leaf— enables pointer signing for functions which are not leaf functionsall— enables pointer signing for all functions
the default value is none. this option has been deprecated by -mbranch-protection. rip bestie, she had a good run 💀
-mbranch-protection=none|standard|pac-ret[+leaf+b-key]|bti|gcs— select the branch protection features to use 🛡️noneis the default and turns off all types of branch protection.standardturns on all types of branch protection features. if a feature has additional tuning options, thenstandardsets it to its standard level.pac-ret[+leaf]turns on return address signing to its standard level: signing functions that save the return address to memory (non-leaf functions will practically always do this) using the a-key. the optional argumentleafcan be used to extend the signing to include leaf functions. the optional argumentb-keycan be used to sign the functions with the B-key instead of the A-key.btiturns on branch target identification mechanism.gcsturns on guarded control stack compatible code generation.
lock it down, queen 🔥
-mharden-sls=opts — enable compiler hardening against straight line speculation (SLS) 💪 opts is a comma-separated list of the following options:retbrblr
in addition, -mharden-sls=all enables all SLS hardening while -mharden-sls=none disables all SLS hardening. all or nothing, no in-between 🌸
-mearly-ra=scope — determine when to enable an early register allocation pass 🧠 this pass runs before instruction scheduling and tries to find a spill-free allocation of floating-point and vector code. it also tries to make use of strided multi-register instructions, such as SME2's strided LD1 and ST1. efficient and unbothered, we love to see it ✨
The possible values of scope are: all (which runs the pass on all functions), strided (which runs the pass on functions that have access to strided multi-register instructions), and none (which disables the pass). 💅
-mearly-ra=all is the default for -O2 and above, and for -Os. -mearly-ra=none is the default otherwise. No cap.
-mearly-ldp-fusion— Bestie, this enables the copy of the AArch64 load/store pair fusion pass that runs before register allocation. It's enabled by default at-Oand above. ✨-mlate-ldp-fusion— And this one enables the copy of the AArch64 load/store pair fusion pass that runs after register allocation. Also enabled by default at-Oand above. 🦋-msve-vector-bits=bits— Okay so this specifies the number of bits in an SVE vector register. Heads up: this option only has an effect when SVE is enabled. 💖
GCC supports two forms of SVE code generation: "vector-length agnostic" output that works with any size of vector register, and "vector-length specific" output that allows GCC to make assumptions about the vector length when it is useful for optimization reasons. The possible values of bits are: scalable, 128, 256, 512, 1024 and 2048. Specifying scalable selects vector-length agnostic output. At present -msve-vector-bits=128 also generates vector-length agnostic output for big-endian targets. All other values generate vector-length specific code. The behavior of these values may change in future releases and no value except scalable should be relied on for producing code that is portable across different hardware SVE vector lengths. 😭
The default is -msve-vector-bits=scalable, which produces vector-length agnostic code. Periodt.
-Wexperimental-fmv-target— This warns about use of experimental Function Multi Versioning. The Arm C Language Extension specification for Function Multi Versioning is beta and subject to change. Any usage of FMV is caveated that future behavior change and incompatibility is likely. So like, be careful. 💀
🌈 -march and -mcpu Feature Modifiers 🌈
Feature modifiers used with -march and -mcpu can be any of the following and their inverses nofeature:
crc— Enable CRC extension. This is on by default for-march=armv8.1-a.crypto— Enable Crypto extension. This also enables Advanced SIMD and floating-point instructions.fp— Enable floating-point instructions. This is on by default for all possible values for options-marchand-mcpu.simd— Enable Advanced SIMD instructions. This also enables floating-point instructions. This is on by default for all possible values for options-marchand-mcpu.sve— Enable Scalable Vector Extension instructions. This also enables Advanced SIMD and floating-point instructions.lse— Enable Large System Extension instructions. This is on by default for-march=armv8.1-a.rdma— Enable Round Double Multiply Accumulate instructions. This is on by default for-march=armv8.1-a.fp16— Enable FP16 extension. This also enables floating-point instructions.fp16fml— Enable FP16 fmla extension. This also enables FP16 extensions and floating-point instructions. This option is enabled by default for-march=armv8.4-a. Use of this option with architectures prior to Armv8.2-A is not supported.rcpc— Enable the RCpc extension. This enables the use of theLDAPRinstructions for load-acquire atomic semantics, and passes it on to the assembler, enabling inline asm statements to use instructions from the RCpc extension.dotprod— Enable the Dot Product extension. This also enables Advanced SIMD instructions.aes— Enable the Armv8-aaesandpmullcrypto extension. This also enables Advanced SIMD instructions.sha2— Enable the Armv8-asha2crypto extension. This also enables Advanced SIMD instructions.sha3— Enable thesha512andsha3crypto extension. This also enables Advanced SIMD instructions. Use of this option with architectures prior to Armv8.2-A is not supported.sm4— Enable thesm3andsm4crypto extension. This also enables Advanced SIMD instructions. Use of this option with architectures prior to Armv8.2-A is not supported.profile— Enable the Statistical Profiling extension. This option is only to enable the extension at the assembler level and does not affect code generation.rng— Enable the Armv8.5-a Random Number instructions. This option is only to enable the extension at the assembler level and does not affect code generation.memtag— Enable the Armv8.5-a Memory Tagging Extensions. Use of this option with architectures prior to Armv8.5-A is not supported.sb— Enable the Armv8-a Speculation Barrier instruction. This option is only to enable the extension at the assembler level and does not affect code generation. This option is enabled by default for-march=armv8.5-a.ssbs— Enable the Armv8-a Speculative Store Bypass Safe instruction. This option is only to enable the extension at the assembler level and does not affect code generation. This option is enabled by default for-march=armv8.5-a.predres— Enable the Armv8-a Execution and Data Prediction Restriction instructions. This option is only to enable the extension at the assembler level and does not affect code generation. This option is enabled by default for-march=armv8.5-a.sve2— Enable the Armv8-a Scalable Vector Extension 2. This also enables SVE instructions.sve2-bitperm— Enable SVE2 bitperm instructions. This also enables SVE2 instructions.sve2-sm4— Enable SVE2 sm4 instructions. This also enables SVE2 instructions.sve2-aes— Enable SVE2 aes instructions. This also enables SVE2 instructions.sve2-sha3— Enable SVE2 sha3 instructions. This also enables SVE2 instructions.sve2p1— Enable SVE2.1 instructions. This also enables SVE2 instructions.tme— Enable the Transactional Memory Extension.i8mm— Enable 8-bit Integer Matrix Multiply instructions. This also enables Advanced SIMD and floating-point instructions. This option is enabled by default for-march=armv8.6-a. Use of this option with architectures prior to Armv8.2-A is not supported.f32mm— Enable 32-bit Floating point Matrix Multiply instructions. This also enables SVE instructions. Use of this option with architectures prior to Armv8.2-A is not supported.f64mm— Enable 64-bit Floating point Matrix Multiply instructions. This also enables SVE instructions. Use of this option with architectures prior to Armv8.2-A is not supported.bf16— Enable brain half-precision floating-point instructions. This also enables Advanced SIMD and floating-point instructions. This option is enabled by default for-march=armv8.6-a. Use of this option with architectures prior to Armv8.2-A is not supported.ls64— Enable the 64-byte atomic load and store instructions for accelerators.mops— Enable the instructions to accelerate memory operations likememcpy,memmove,memset. This option is enabled by default for-march=armv8.8-aflagm— Enable the Flag Manipulation instructions Extension.flagm2— Enable the FlagM2 flag conversion instructions.pauth— Enable the Pointer Authentication Extension.cssc— Enable the Common Short Sequence Compression instructions.sme— Enable the Scalable Matrix Extension. This is only supported when SVE2 is also enabled.sme-i16i64— Enable the FEAT_SME_I16I64 extension to SME. This also enables SME instructions.sme-f64f64— Enable the FEAT_SME_F64F64 extension to SME. This also enables SME instructions.sme2— Enable the Scalable Matrix Extension 2. This also enables SME instructions.sme-b16b16— Enable the FEAT_SME_B16B16 extension to SME. This also enables SME2 and SVE_B16B16 instructions.sme-f16f16— Enable the FEAT_SME_F16F16 extension to SME. This also enables SME2 instructions.sme2p1— Enable the Scalable Matrix Extension version 2.1. This also enables SME2 instructions.fcma— Enable the complex number SIMD extensions.jscvt— Enable thefjcvtzsJavaScript conversion instruction.frintts— Enable floating-point round to integral value instructions.wfxt— Enablewfetandwfitinstructions.xs— Enable the XS memory attribute extension.lse128— Enable the LSE128 128-bit atomic instructions extension. This also enables LSE instructions.d128— Enable support for 128-bit system register read/write instructions. This also enables the LSE128 extension.gcs— Enable support for Armv9.4-a Guarded Control Stack extension.the— Enable support for Armv8.9-a/9.4-a translation hardening extension.rcpc2— Enable the RCpc2 extension.rcpc3— Enable the RCpc3 (Release Consistency) extension.fp8— Enable the fp8 (8-bit floating point) extension.fp8fma— Enable the fp8 (8-bit floating point) multiply accumulate extension.ssve-fp8fma— Enable the fp8 (8-bit floating point) multiply accumulate extension in streaming mode.fp8dot4— Enable the fp8 (8-bit floating point) to single-precision 4-way dot product extension.ssve-fp8dot4— Enable the fp8 (8-bit floating point) to single-precision 4-way dot product extension in streaming mode.fp8dot2— Enable the fp8 (8-bit floating point) to half-precision 2-way dot product extension.ssve-fp8dot2— Enable the fp8 (8-bit floating point) to half-precision 2-way dot product extension in streaming mode.faminmax— Enable the Floating Point Absolute Maximum/Minimum extension.lut— Enable the Lookup Table extension.cpa— Enable the Checked Pointer Arithmetic instructions.sve-b16b16— Enable the SVE non-widening brain floating-point (bf16) extension. This only has an effect whensve2orsme2are also enabled.
Feature crypto implies aes, sha2, and simd, which implies fp. Conversely, nofp implies nosimd, which implies nocrypto, noaes and nosha2.
🦋 Adapteva Epiphany Options 🦋
These -m options are defined for Adapteva Epiphany:
-mhalf-reg-file— Don't allocate any register in the ranger32...r63. That allows code to run on hardware variants that lack these registers.-mprefer-short-insn-regs— preferentially allocate registers that let short instructions get generated. Fair warning tho: this can bump up the instruction count, so overall code size might go down or up. It's giving chaos, honestly. 🎲-mbranch-cost=num— set the cost of branches to roughly num "simple" instructions. This is only a heuristic and there's no promise it'll produce consistent results across releases. Not a vibe you can count on, bestie.-mcmove— turn on generation of conditional moves. ✨-mnops=num— emit num NOPs before every other generated instruction. Yeah. Really. Every other one. 💀-mno-soft-cmpsf— for single-precision floating-point comparisons, emit anfsubinstruction and test the flags. This is faster than a software comparison, but it can get incorrect results when NaNs are around, or when two different small numbers get compared such that their difference comes out as zero. The default is-msoft-cmpsf, which uses slower-but-IEEE-compliant software comparisons. 🫶-mstack-offset=num— set the offset between the top of the stack and the stack pointer. So like, a value of 8 means the eight bytes in the rangesp+0...sp+7can be used by leaf functions without any stack allocation. Values other than 8 or 16 are untested and unlikely to work. Also note this option changes the ABI — compiling a program with a different stack offset than the libraries were compiled with generally does not work. It can be useful if you want to evaluate whether a different stack offset would give you better code, but to actually use a different stack offset to build working programs, the recommended move is to configure the toolchain with the appropriate--with-stack-offset=numoption. 🛠️-mno-round-nearest— make the scheduler assume the rounding mode has been set to truncating. The default is-mround-nearest.-mlong-calls— if an attribute doesn't say otherwise, assume all calls might be beyond the offset range of theb/blinstructions, and therefore load the function address into a register before performing an (otherwise direct) call. This is the default. 📞-mshort-calls— if an attribute doesn't say otherwise, assume all direct calls are in the range of theb/blinstructions, so use these instructions for direct calls. The default is-mlong-calls.-msmall16— assume addresses can be loaded as 16-bit unsigned values. This does not apply to function addresses, for which-mlong-callssemantics are in effect.-mfp-mode=mode— set the prevailing mode of the floating-point unit. This determines the floating-point mode that's provided and expected at function call and return time. Making this mode match the mode you predominantly need at function start can make your programs smaller and faster by avoiding unnecessary mode switches. 🌸
mode can be set to one of the following values:
caller— any mode at function entry is valid, and retained or restored when the function returns, and when it calls other functions. This mode is useful for compiling libraries or other compilation units you might want to incorporate into different programs with different prevailing FPU modes, and the convenience of being able to use a single object file outweighs the size and speed overhead for any extra mode switching that might be needed, compared with what would be needed with a more specific choice of prevailing FPU mode.truncate— this is the mode used for floating-point calculations with truncating (i.e. round towards zero) rounding mode. That includes conversion from floating point to integer.round-nearest— this is the mode used for floating-point calculations with round-to-nearest-or-even rounding mode.int— this is the mode used to perform integer calculations in the FPU, e.g. integer multiply, or integer multiply-and-accumulate.
The default is -mfp-mode=caller. 💅
-mno-split-lohi,-mno-postinc,-mno-postmodify— code generation tweaks that disable, respectively, splitting of 32-bit loads, generation of post-increment addresses, and generation of post-modify addresses. The defaults aremsplit-lohi,-mpost-inc, and-mpost-modify.-mnovect-double— change the preferred SIMD mode to SImode. The default is-mvect-double, which uses DImode as preferred SIMD mode.-max-vect-align=num— the maximum alignment for SIMD vector mode types. num may be 4 or 8. The default is 8. Note that this is an ABI change, even though many library function interfaces are unaffected if they don't use SIMD vector modes in places that affect size and/or alignment of relevant types.-msplit-vecmove-early— split vector moves into single word moves before reload. In theory this can give better register allocation, but so far the reverse seems to be generally the case. 🙈-m1reg-reg— specify a register to hold the constant -1, which makes loading small negative constants and certain bitmasks faster. Allowable values for reg arer43andr63, which specify use of that register as a fixed register, andnone, which means that no register is used for this purpose. The default is-m1reg-none.
🌈 AMD GCN Options 🌈
These options are defined specifically for the AMD GCN port. GPU girlies, this one's for you. 👑
-march=gpu,-mtune=gpu— set architecture type or tuning for gpu. Supported values for gpu are:gfx900— compile for GCN5 Vega 10 devices (gfx900).gfx902— compile for GCN5 Vega gfx902 devices. (Experimental)gfx904— compile for GCN5 Vega gfx904 devices. (Experimental)gfx906— compile for GCN5 Vega 20 devices (gfx906).gfx908— compile for CDNA1 Instinct MI100 series devices (gfx908).gfx909— compile for GCN5 Vega gfx909 devices. (Experimental)gfx90a— compile for CDNA2 Instinct MI200 series devices (gfx90a).gfx90c— compile for GCN5 Vega 7 devices (gfx90c).gfx9-generic— compile generic code for Vega devices, executable on the following subset of GFX9 devices:gfx900,gfx902,gfx904,gfx906,gfx909andgfx90c. (Experimental)gfx1030— compile for RDNA2 gfx1030 devices (GFX10 series).gfx1031— compile for RDNA2 gfx1031 devices (GFX10 series). (Experimental)gfx1032— compile for RDNA2 gfx1032 devices (GFX10 series). (Experimental)gfx1033— compile for RDNA2 gfx1033 devices (GFX10 series). (Experimental)gfx1034— compile for RDNA2 gfx1034 devices (GFX10 series). (Experimental)gfx1035— compile for RDNA2 gfx1035 devices (GFX10 series). (Experimental)gfx1036— compile for RDNA2 gfx1036 devices (GFX10 series).gfx10-3-generic— compile generic code for GFX10-3 devices, executable ongfx1030,gfx1031,gfx1032,gfx1033,gfx1034,gfx1035, andgfx1036. (Experimental)gfx1100— compile for RDNA3 gfx1100 devices (GFX11 series).gfx1101— compile for RDNA3 gfx1101 devices (GFX11 series). (Experimental)gfx1102— compile for RDNA3 gfx1102 devices (GFX11 series). (Experimental)gfx1103— compile for RDNA3 gfx1103 devices (GFX11 series).gfx1150— compile for RDNA3 gfx1150 devices (GFX11 series). (Experimental)gfx1151— compile for RDNA3 gfx1151 devices (GFX11 series). (Experimental)gfx1152— compile for RDNA3 gfx1152 devices (GFX11 series). (Experimental)gfx1153— compile for RDNA3 gfx1153 devices (GFX11 series). (Experimental)gfx11-generic— compile generic code for GFX11 devices, executable ongfx1100,gfx1101,gfx1102,gfx1103,gfx1150,gfx1151,gfx1152, andgfx1153. (Experimental)
-msram-ecc=on,-msram-ecc=off,-msram-ecc=any— compile binaries suitable for devices with the SRAM-ECC feature enabled, disabled, or either mode. This feature can be enabled per-process on some devices. The compiled code must match the device mode. The default isany, for devices that support it. ⚠️-mstack-size=bytes— specify how many bytes of stack space will be requested for each GPU thread (wave-front). Beware that there may be many threads and limited memory available. The size of the stack allocation may also have an impact on run-time performance. The default is 32KB when using OpenACC or OpenMP, and 1MB otherwise. 🧠-mxnack=on,-mxnack=off,-mxnack=any— compile binaries suitable for devices with the XNACK feature enabled, disabled, or either mode. Some devices always require XNACK and some allow the user to configure XNACK. The compiled code must match the device mode. The default is-mxnack=anyon devices that support Unified Shared Memory, and-mxnack=nootherwise.
🦋 ARC Options 🦋
The following options control the architecture variant for which code is being compiled:
-mbarrel-shifter— generate instructions supported by barrel shifter. This is the default unless-mcpu=ARC601or-mcpu=ARCEMis in effect.-mjli-always— force to call a function usingjli_sinstruction. This option is valid only for ARCv2 architecture. ✨-mcpu=cpu— set architecture type, register usage, and instruction scheduling parameters for cpu. There are also shortcut alias options available for backward compatibility and convenience. Supported values for cpu are:arc600— compile for ARC600. Aliases:-mA6,-mARC600.arc601— compile for ARC601. Alias:-mARC601.arc700— compile for ARC700. Aliases:-mA7,-mARC700. This is the default when configured with--with-cpu=arc700.arcem— compile for ARC EM.archs— compile for ARC HS.em— compile for ARC EM CPU with no hardware extensions.em4— compile for ARC EM4 CPU.em4_dmips— compile for ARC EM4 DMIPS CPU.em4_fpus— compile for ARC EM4 DMIPS CPU with the single-precision floating-point extension.em4_fpuda— compile for ARC EM4 DMIPS CPU with single-precision floating-point and double assist instructions.hs— compile for ARC HS CPU with no hardware extensions except the atomic instructions.hs34— compile for ARC HS34 CPU.
hs38— compile for the ARC HS38 CPU. periodt. 💅hs38_linux— compile for the ARC HS38 CPU with, like, every hardware extension turned on. go big or go home bestie ✨hs4x— compile for the ARC HS4x CPU.hs4xd— compile for the ARC HS4xD CPU.hs4x_rel31— compile for the ARC HS4x CPU, release 3.10a specifically. vintage vibes 🎀arc600_norm— compile for the ARC 600 CPU with"norm"instructions enabled.arc600_mul32x16— compile for the ARC 600 CPU with"norm"and 32x16-bit multiply instructions enabled.arc600_mul64— compile for the ARC 600 CPU with"norm"and"mul64"-family instructions enabled.arc601_norm— compile for the ARC 601 CPU with"norm"instructions enabled.arc601_mul32x16— compile for the ARC 601 CPU with"norm"and 32x16-bit multiply instructions enabled.arc601_mul64— compile for the ARC 601 CPU with"norm"and"mul64"-family instructions enabled.nps400— compile for the ARC 700 on the NPS400 chip. lowkey specific but ok ✨em_mini— compile for the ARC EM minimalist configuration, the one featuring a reduced register set. minimalism is a lifestyle 🦋
-mdpfp,-mdpfp-compact— generate double-precision FPX instructions, but tuned for the compact implementation. cute and efficient 💅-mdpfp-fast— generate double-precision FPX instructions, this time tuned for the fast implementation. zoom zoom 🔥-mno-dpfp-lrsr— disables"lr"and"sr"instructions from using FPX extension aux registers. like, hands off those aux registers bestie 🙈-mea— generate extended arithmetic instructions. rn only"divaw","adds","subs", and"sat16"are supported. heads up: this is only valid for-mcpu=ARC700. ⭐-mno-mpy— do not generate"mpy"-family instructions for ARC700. > heads up: this option is deprecated. it's giving legacy energy 💀-mmul32x16— generate 32x16-bit multiply and multiply-accumulate instructions. math girlies rise 🎀-mmul64— generate"mul64"and"mulu64"instructions. only valid for-mcpu=ARC600.-mnorm— generate"norm"instructions. this is the default if-mcpu=ARC700is in effect. the way it just defaults ✨-mspfp,-mspfp-compact— generate single-precision FPX instructions, tuned for the compact implementation.-mspfp-fast— generate single-precision FPX instructions, tuned for the fast implementation. ⚡-msimd— enable generation of ARC SIMD instructions via target-specific builtins. only valid for-mcpu=ARC700.-msoft-float— okay so ngl, this option is ignored; it's provided for compatibility purposes only. software floating-point code is emitted by default, and this default can be overridden by the FPX options:-mspfp,-mspfp-compact, or-mspfp-fastfor single precision, and-mdpfp,-mdpfp-compact, or-mdpfp-fastfor double precision. the way it's just here for the vibes 😭-mswap— generate"swap"instructions.-matomic— this enables use of the locked load/store conditional extension to implement atomic memory built-in functions. > not available for ARC 6xx or ARC EM cores. know your limits bestie 💅-mdiv-rem— enable"div"and"rem"instructions for ARCv2 cores.-mcode-density— enable code density instructions for ARC EM. this one is on by default for ARC HS. slay ✨-mll64— enable double load/store operations for ARC HS cores.-mtp-regno=regno — specify the thread pointer register number.-mmpy-option=multo — compile ARCv2 code with a multiplier design option. you can specify the option using either a string or a numeric value for multo.wlh1is the default value. the recognized values are:0,none— no multiplier available. it's giving bare minimum 🙈1,w— 16x16 multiplier, fully pipelined. the following instructions are enabled:"mpyw"and"mpyuw".2,wlh1— 32x32 multiplier, fully pipelined (1 stage). the following instructions are additionally enabled:"mpy","mpyu","mpym","mpymu", and"mpy_s".3,wlh2— 32x32 multiplier, fully pipelined (2 stages). the following instructions are additionally enabled:"mpy","mpyu","mpym","mpymu", and"mpy_s".4,wlh3— two 16x16 multipliers, blocking, sequential. the following instructions are additionally enabled:"mpy","mpyu","mpym","mpymu", and"mpy_s".5,wlh4— one 16x16 multiplier, blocking, sequential. the following instructions are additionally enabled:"mpy","mpyu","mpym","mpymu", and"mpy_s".6,wlh5— one 32x4 multiplier, blocking, sequential. the following instructions are additionally enabled:"mpy","mpyu","mpym","mpymu", and"mpy_s".7,plus_dmpy— ARC HS SIMD support.8,plus_macd— ARC HS SIMD support.9,plus_qmacw— ARC HS SIMD support.
this option is only available for ARCv2 cores. no ARCv2? no multiplier tea for you ☕
-mfpu=fpu — enables support for specific floating-point hardware extensions for ARCv2 cores. the supported values for fpu are:fpus— enables support for single-precision floating-point hardware extensions.fpud— enables support for double-precision floating-point hardware extensions. the single-precision floating-point extension is also enabled. > not available for ARC EM.fpuda— enables support for double-precision floating-point hardware extensions using double-precision assist instructions. the single-precision floating-point extension is also enabled. this option is only available for ARC EM.fpuda_div— enables support for double-precision floating-point hardware extensions using double-precision assist instructions. the single-precision floating-point, square-root, and divide extensions are also enabled. this option is only available for ARC EM.fpuda_fma— enables support for double-precision floating-point hardware extensions using double-precision assist instructions. the single-precision floating-point and fused multiply and add hardware extensions are also enabled. this option is only available for ARC EM.fpuda_all— enables support for double-precision floating-point hardware extensions using double-precision assist instructions. all single-precision floating-point hardware extensions are also enabled. this option is only available for ARC EM.fpus_div— enables support for single-precision floating-point, square-root and divide hardware extensions.fpud_div— enables support for double-precision floating-point, square-root and divide hardware extensions. this option includes optionfpus_div. > not available for ARC EM.fpus_fma— enables support for single-precision floating-point and fused multiply and add hardware extensions.fpud_fma— enables support for double-precision floating-point and fused multiply and add hardware extensions. this option includes optionfpus_fma. > not available for ARC EM.fpus_all— enables support for all single-precision floating-point hardware extensions. all of them. the whole roster 👑fpud_all— enables support for all single- and double-precision floating-point hardware extensions. > not available for ARC EM.
-mirq-ctrl-saved=register-range, blink, lp_count — specifies the general-purpose registers that the processor automatically saves/restores on interrupt entry and exit. register-range is specified as two registers separated by a dash. the register range always starts with"r0", and the upper limit is the"fp"register. blink and lp_count are optional. this option is only valid for ARC EM and ARC HS cores. interrupt bookkeeping but make it cute ✨-mrgf-banked-regs=number — specifies the number of registers replicated in the second register bank on entry to a fast interrupt. fast interrupts are interrupts with the highest priority level P0. these interrupts save only PC and STATUS32 registers to avoid memory transactions during the interrupt entry and exit sequences. use this option when you're using fast interrupts in an ARC V2 family processor. permitted values are4,8,16, and32. 🦋-mlpc-width=width — specify the width of the"lp_count"register. valid values for width are8,16,20,24,28and32bits. the default width is fixed to32bits. if the width is less than 32, the compiler does not attempt to transform loops in your program to use the zero-delay loop mechanism unless it's known that the"lp_count"register can hold the required loop-counter value. depending on the width you specify, the compiler and run-time library might continue to use the loop mechanism for various needs. this option defines the macro__ARC_LPC_WIDTH__with the value of width. iykyk 💅-mrf16— this option instructs the compiler to generate code for a 16-entry register file. this option defines the__ARC_RF16__preprocessor macro. sixteen entries, iconic ✨-mbranch-index— enable use of"bi"or"bih"instructions to implement jump tables.
the following options are passed through to the assembler, and also define preprocessor macro symbols. two-for-one, we love that 🫶
-mdsp-packa— passed down to the assembler to enable the DSP Pack A extensions. also sets the preprocessor symbol__Xdsp_packa. > this option is deprecated. 💀-mdvbf— passed down to the assembler to enable the dual Viterbi butterfly extension. also sets the preprocessor symbol__Xdvbf. > this option is deprecated.-mlock— passed down to the assembler to enable the locked load/store conditional extension. also sets the preprocessor symbol__Xlock.-mmac-d16— passed down to the assembler. also sets the preprocessor symbol__Xxmac_d16. > this option is deprecated.-mmac-24— passed down to the assembler. also sets the preprocessor symbol__Xxmac_24. > this option is deprecated.-mrtsc— passed down to the assembler to enable the 64-bit time-stamp counter extension instruction. also sets the preprocessor symbol__Xrtsc. > this option is deprecated.-mswape— passed down to the assembler to enable the swap byte ordering extension instruction. also sets the preprocessor symbol__Xswape.-mtelephony— passed down to the assembler to enable dual- and single-operand instructions for telephony. also sets the preprocessor symbol__Xtelephony. > this option is deprecated.-mxy— hands this one down to the assembler so the XY memory extension gets switched on. Also sets the preprocessor symbol"__Xxy"so your#ifdefs know what's up. ✨
The next ones are all about how your assembly gets decorated — annotated, bestie:
-misize— annotate assembler instructions with estimated addresses. Little address receipts everywhere. 🎀-mannotate-align— does literally nothing. Kept around for backward compatibility. It's giving decorative. 💀
And these ones get passed straight through to the linker:
-marclinux— passed through to the linker to specify use of the"arclinux"emulation. It's on by default in tool chains built for"arc-linux-uclibc"and"arceb-linux-uclibc"targets when profiling is not requested.-marclinux_prof— passed through to the linker to specify use of the"arclinux_prof"emulation. This one's on by default in tool chains built for"arc-linux-uclibc"and"arceb-linux-uclibc"targets when profiling is requested.
Now for the semantics of the generated code, aka the personality of your binary:
-mlong-calls— generate calls as register indirect calls, which opens up access to the whole full 32-bit address range. main character range fr. 👑-mmedium-calls— don't use less than a 25-bit addressing range for calls, which is the offset you get for an unconditional branch-and-link instruction. Conditional execution of function calls gets suppressed so you can use the 25-bit range instead of the 21-bit range with conditional branch-and-link. This is the default for tool chains built for"arc-linux-uclibc"and"arceb-linux-uclibc"targets.-G num— put definitions of externally-visible data in a small data section if that data is no bigger than num bytes. The default value of num is4for any ARC configuration, or8when we have double load/store operations.-mno-sdata— don't generate sdata references. This is the default for tool chains built for"arc-linux-uclibc"and"arceb-linux-uclibc"targets.-mvolatile-cache— use ordinarily cached memory accesses for volatile references. This is the default, so chill.-mno-volatile-cache— enable cache bypass for volatile references.
Okay and these are the fine-tuning knobs, for when you want to be so specific about codegen:
-malign-call— does nothing. Preserved for backward compatibility. Iconic of it honestly. 🙈-mauto-modify-reg— enable the use of pre/post modify with register displacement.-mbbit-peephole— does nothing. Preserved for backward compatibility.-mno-brcc— this disables a target-specific pass inarc_reorgthat generates compare-and-branch ("brcc") instructions. It has no effect on generation of these instructions driven by the combiner pass. So don't expect it to do more than it does.-mcase-vector-pcrel— use PC-relative switch case tables to enable case table shortening. This is the default for-Os.-mcompact-casesi— enable the compact"casesi"pattern. This is the default for-Os, and it's only available for ARCv1 cores. This option is deprecated.-mno-cond-exec— disable the ARCompact-specific pass that generates conditional execution instructions.
Okay so here's the tea, because this one's long: due to delay slot scheduling and interactions between operand numbers, literal sizes, instruction lengths, and the support for conditional execution, the target-independent pass that generates conditional execution is often lacking. So the ARC port has kept a special pass around that tries to find more conditional execution generation opportunities after register allocation, branch shortening, and delay slot scheduling have been done. This pass generally, but not always, improves performance and code size — at the cost of extra compilation time, which is exactly why there's an option to switch it off. And if you have a problem with call instructions exceeding their allowable offset range because they are conditionalized, you should consider using -mmedium-calls instead. 🫶
-mearly-cbranchsi— enable pre-reload use of the"cbranchsi"pattern.-mexpand-adddi— expand"adddi3"and"subdi3"at RTL generation time into"add.f","adc"etc. This option is deprecated.-mindexed-loads— enable the use of indexed loads. This can be problematic, because some optimizers then assume that indexed stores exist — and bestie, they do not exist. 😭-mlra— does nothing. Preserved for backward compatibility.-mlra-priority-none— don't indicate any priority for target registers.-mlra-priority-compact— indicate target register priority forr0..r3/r12..r15.-mlra-priority-noncompact— reduce target register priority forr0..r3/r12..r15.-mmillicode— when optimizing for size (using-Os), prologues and epilogues that have to save or restore a large number of registers are often shortened by using a call to a special function inlibgcc; this is referred to as a millicode call. Since these calls can pose performance issues, and/or cause linking issues when linking in a nonstandard way, this option is provided so you can turn millicode call generation on or off.-mcode-density-frame— this option enables the compiler to emit"enter"and"leave"instructions. These instructions are only valid for CPUs with the code-density feature, so don't get cute with them. 💅-mmixed-code— does nothing. Preserved for backward compatibility.-mq-class— this option is deprecated. Enable q instruction alternatives. This is the default for-Os.-mRcq— does nothing. Preserved for backward compatibility.-mRcw— does nothing. Preserved for backward compatibility.-msize-level=level— fine-tune size optimization with regards to instruction lengths and alignment. The recognized values for level are:0— no size optimization. This level is deprecated and treated like1.1— short instructions are used opportunistically.2— in addition, alignment of loops and of code after barriers are dropped.3— in addition, optional data alignment is dropped, and the optionOsis enabled.
This defaults to 3 when -Os is in effect. Otherwise, the behavior when this is not set is equivalent to level 1.
-mtune=cpu— set instruction scheduling parameters for cpu, overriding any implied by-mcpu=.
Supported values for cpu are:
ARC600— tune for ARC600 CPU.ARC601— tune for ARC601 CPU.ARC700— tune for ARC700 CPU with standard multiplier block.ARC700-xmac— tune for ARC700 CPU with XMAC block.ARC725D— tune for ARC725D CPU.ARC750D— tune for ARC750D CPU.core3— tune for ARCv2 core3 type CPU. This option enables usage of the"dbnz"instruction.release31a— tune for ARC4x release 3.10a.
-mmultcost=num— cost to assume for a multiply instruction, with4being equal to a normal instruction.-munalign-prob-threshold=probability— does nothing. Preserved for backward compatibility.
💀 The Deprecated Zone 💀
The following options are maintained for backward compatibility, but are now deprecated and will be removed in a future release. So like… stop using them, bestie, fr fr:
-margonaut— obsolete FPX.-mbig-endian,-EB— compile code for big-endian targets. Use of these options is now deprecated. Big-endian code is supported by configuring GCC to build"arceb-elf32"and"arceb-linux-uclibc"targets, for which big endian is the default.-mlittle-endian,-EL— compile code for little-endian targets. Use of these options is now deprecated. Little-endian code is supported by configuring GCC to build"arc-elf32"and"arc-linux-uclibc"targets, for which little endian is the default.-mbarrel_shifter— replaced by-mbarrel-shifter.-mdpfp_compact— replaced by-mdpfp-compact.-mdpfp_fast— replaced by-mdpfp-fast.-mdsp_packa— replaced by-mdsp-packa.-mEA— replaced by-mea.-mmac_24— replaced by-mmac-24.-mmac_d16— replaced by-mmac-d16.-mspfp_compact— replaced by-mspfp-compact.-mspfp_fast— replaced by-mspfp-fast.-mtune=cpu— the valuesarc600,arc601,arc700andarc700-xmacfor cpu are replaced byARC600,ARC601,ARC700andARC700-xmacrespectively. (Yes, casing matters now. glow up.) ✨-multcost=num— replaced by-mmultcost.
🌈 ARM Options 🌈
These -m options are defined for the ARM port, and they are a lot, so buckle up:
-mabi=name— generate code for the specified ABI. Permissible values are:apcs-gnu,atpcs,aapcs,aapcs-linuxandiwmmxt.-mapcs-frame— generate a stack frame that's compliant with the ARM Procedure Call Standard for all functions, even if that's not strictly necessary for correct execution of the code. Specifying-fomit-frame-pointerwith this option causes the stack frames not to be generated for leaf functions. The default is-mno-apcs-frame. This option is deprecated.-mapcs— this is a synonym for-mapcs-frameand is deprecated.-mthumb-interwork— generate code that supports calling between the ARM and Thumb instruction sets. Without this option, on pre-v5 architectures, the two instruction sets can't be reliably used inside one program. The default is-mno-thumb-interwork, since slightly larger code is generated when-mthumb-interworkis specified. In AAPCS configurations this option is meaningless.-mno-sched-prolog— prevent the reordering of instructions in the function prologue, or the merging of those instructions with the instructions in the function's body. This means all functions start with a recognizable set of instructions (or in fact one of a choice from a small set of different function prologues), and that information can be used to locate the start of functions inside an executable piece of code. The default is-msched-prolog.-mfloat-abi=name— specifies which floating-point ABI to use. Permissible values are:soft,softfpandhard.
Specifying soft causes GCC to generate output containing library calls for floating-point operations. softfp allows the generation of code using hardware floating-point instructions, but still uses the soft-float calling conventions. hard allows generation of floating-point instructions and uses FPU-specific calling conventions.
The default depends on the specific target configuration. And heads up: the hard-float and soft-float ABIs are not link-compatible — you must compile your entire program with the same ABI, and link with a compatible set of libraries. No mixing, no matching, no chaos. 🫶
-mgeneral-regs-only— bestie, this one makes the compiler generate code that only touches the general-purpose registers. So it's like "no fancy floaty regs, no Advanced SIMD regs, we're keeping it basic ✨" — but heads up, it does not put any restrictions on the assembler. The assembler can still do whatever it wants, no cap.-mlittle-endian— generates code for a processor running in little-endian mode. This is literally the default for all standard configurations, so if you do nothing you already have it. 💅-mbig-endian— generates code for a processor running in big-endian mode. And the default? Compiling for a little-endian processor. So this is the "not like the other girls" option, you have to opt in. 🦋-mbe8,-mbe32— okay so when you're linking a big-endian image, these pick between the BE8 and BE32 formats. For little-endian images the option has no effect and is just ignored, it's giving "not my problem". The default depends on the selected target architecture: for ARMv6 and later architectures the default is BE8, and for older architectures the default is BE32.
⚠️ Also: BE32 format has been deprecated by ARM. So like… it's still there but ARM said no. 🙈
-march=name[+extension...]— this specifies the name of the target ARM architecture. GCC uses this name to figure out what kind of instructions it's allowed to emit when generating assembly code. You can use this option together with the-mcpu=option, or instead of it. main character energy honestly. 👑
Permissible names are:
armv4tarmv5tarmv5tearmv6armv6jarmv6karmv6kzarmv6t2armv6zarmv6zkarmv7armv7-aarmv7vearmv8-aarmv8.1-aarmv8.2-aarmv8.3-aarmv8.4-aarmv8.5-aarmv8.6-aarmv9-aarmv7-rarmv8-rarmv6-marmv6s-marmv7-marmv7e-marmv8-m.basearmv8-m.mainarmv8.1-m.mainiwmmxtiwmmxt2
Additionally, the following architectures — which lack support for the Thumb execution state — are recognized, but support is deprecated:
armv4
🌈 extensions, aka the fun part
Many of the architectures support extensions. You add them by appending +extension to the architecture name. Here's the tea 🍓: extension options are processed in order, and capabilities accumulate. An extension will also enable any necessary base extensions that it depends on. For example, the +crypto extension will always enable the +simd extension.
The exception to this additive construction is for extensions prefixed with +no... — those extensions disable the specified option and any other extensions that may depend on that extension being present. So it's giving "no takebacks, no plus-ones". 💀
For example:
-march=armv7-a+simd+nofp+vfpv4is equivalent to writing:
-march=armv7-a+vfpv4since the +simd option is entirely disabled by the +nofp option that follows it. Order matters, bestie, it's literally a stack. 🎀
Most extension names are generically named, but they have an effect that depends on the architecture they're applied to. For example, the +simd option can be applied to both armv7-a and armv8-a architectures, but it will enable the original ARMv7-A Advanced SIMD (Neon) extensions for armv7-a, and the ARMv8-A variant for armv8-a. Same name, different vibe check depending on context. ✨
The list below shows the supported extensions for each architecture. Architectures not mentioned do not support any extensions.
armv5te, armv6, armv6j, armv6k, armv6kz, armv6t2, armv6z, armv6zk
+fp— the VFPv2 floating-point instructions. The extension+vfpv2can be used as an alias for this extension.+nofp— disable the floating-point instructions.
armv7
The common subset of the ARMv7-A, ARMv7-R and ARMv7-M architectures. 🌸
+fp— the VFPv3 floating-point instructions, with 16 double-precision registers. The extension+vfpv3-d16can be used as an alias for this extension. Note that floating-point is not supported by the base ARMv7-M architecture, but it is compatible with both the ARMv7-A and ARMv7-R architectures.+nofp— disable the floating-point instructions.
armv7-a
+mp— the multiprocessing extension.+sec— the security extension.+fp— the VFPv3 floating-point instructions, with 16 double-precision registers. The extension+vfpv3-d16can be used as an alias for this extension.+simd— the Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions. The extensions+neonand+neon-vfpv3can be used as aliases for this extension.+vfpv3— the VFPv3 floating-point instructions, with 32 double-precision registers.+vfpv3-d16-fp16— the VFPv3 floating-point instructions, with 16 double-precision registers and the half-precision floating-point conversion operations.+vfpv3-fp16— the VFPv3 floating-point instructions, with 32 double-precision registers and the half-precision floating-point conversion operations.+vfpv4-d16— the VFPv4 floating-point instructions, with 16 double-precision registers.+vfpv4— the VFPv4 floating-point instructions, with 32 double-precision registers.+neon-fp16— the Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions, with the half-precision floating-point conversion operations.+neon-vfpv4— the Advanced SIMD (Neon) v2 and the VFPv4 floating-point instructions.+nosimd— disable the Advanced SIMD instructions (does not disable floating point). 🙈+nofp— disable the floating-point and Advanced SIMD instructions.
armv7ve
The extended version of the ARMv7-A architecture with support for virtualization. 💜
+fp— the VFPv4 floating-point instructions, with 16 double-precision registers. The extension+vfpv4-d16can be used as an alias for this extension.+simd— the Advanced SIMD (Neon) v2 and the VFPv4 floating-point instructions. The extension+neon-vfpv4can be used as an alias for this extension.+vfpv3-d16— the VFPv3 floating-point instructions, with 16 double-precision registers.+vfpv3— the VFPv3 floating-point instructions, with 32 double-precision registers.+vfpv3-d16-fp16— the VFPv3 floating-point instructions, with 16 double-precision registers and the half-precision floating-point conversion operations.+vfpv3-fp16— the VFPv3 floating-point instructions, with 32 double-precision registers and the half-precision floating-point conversion operations.+vfpv4-d16— the VFPv4 floating-point instructions, with 16 double-precision registers.+vfpv4— the VFPv4 floating-point instructions, with 32 double-precision registers.+neon— the Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions. The extension+neon-vfpv3can be used as an alias for this extension.+neon-fp16— the Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions, with the half-precision floating-point conversion operations.+nosimd— disable the Advanced SIMD instructions (does not disable floating point).+nofp— disable the floating-point and Advanced SIMD instructions.
armv8-a
+crc— the Cyclic Redundancy Check (CRC) instructions.+simd— the ARMv8-A Advanced SIMD and floating-point instructions.+crypto— the cryptographic instructions.+nocrypto— disable the cryptographic instructions.+nofp— disable the floating-point, Advanced SIMD and cryptographic instructions.+sb— Speculation Barrier Instruction.+predres— Execution and Data Prediction Restriction Instructions.
armv8.1-a
+simd— the ARMv8.1-A Advanced SIMD and floating-point instructions.+crypto— the cryptographic instructions. This also enables the Advanced SIMD and floating-point instructions.+nocrypto— disable the cryptographic instructions.+nofp— disable the floating-point, Advanced SIMD and cryptographic instructions.+sb— Speculation Barrier Instruction.+predres— Execution and Data Prediction Restriction Instructions.
armv8.2-a, armv8.3-a
+fp16— the half-precision floating-point data processing instructions. This also enables the Advanced SIMD and floating-point instructions.+fp16fml— the half-precision floating-point fmla extension. This also enables the half-precision floating-point extension and Advanced SIMD and floating-point instructions.+simd— the ARMv8.1-A Advanced SIMD and floating-point instructions.+crypto— the cryptographic instructions. This also enables the Advanced SIMD and floating-point instructions.+dotprod— enable the Dot Product extension. This also enables Advanced SIMD instructions.+nocrypto— disable the cryptographic extension.+nofp— disable the floating-point, Advanced SIMD and cryptographic instructions.+sb— Speculation Barrier Instruction.+predres— Execution and Data Prediction Restriction Instructions.+i8mm— 8-bit Integer Matrix Multiply instructions. This also enables Advanced SIMD and floating-point instructions.+bf16— Brain half-precision floating-point instructions. This also enables Advanced SIMD and floating-point instructions.
armv8.4-a
+fp16— the half-precision floating-point data processing instructions. This also enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension and the half-precision floating-point fmla extension.
armv8.5-a
+fp16— The half-precision floating-point data processing instructions. This also turns on the Advanced SIMD and floating-point instructions, plus the Dot Product extension and the half-precision floating-pointfmlaextension. bundle of joy fr 💖+simd— The ARMv8.3-A Advanced SIMD and floating-point instructions, plus the Dot Product extension.+crypto— The cryptographic instructions. This also enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension.+nocrypto— Disable the cryptographic extension. bye bestie 👋+nofp— Disable the floating-point, Advanced SIMD and cryptographic instructions. all of it, gone 💀+i8mm— 8-bit Integer Matrix Multiply instructions. This also enables Advanced SIMD and floating-point instructions.+bf16— Brain half-precision floating-point instructions. This also enables Advanced SIMD and floating-point instructions.
armv8.6-a
+fp16— The half-precision floating-point data processing instructions. This also enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension and the half-precision floating-pointfmlaextension. yes it's the same deal as above, she's consistent 💅+simd— The ARMv8.3-A Advanced SIMD and floating-point instructions as well as the Dot Product extension.+crypto— The cryptographic instructions. This also enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension.+nocrypto— Disable the cryptographic extension.+nofp— Disable the floating-point, Advanced SIMD and cryptographic instructions.+i8mm— 8-bit Integer Matrix Multiply instructions. This also enables Advanced SIMD and floating-point instructions.+bf16— Brain half-precision floating-point instructions. This also enables Advanced SIMD and floating-point instructions.
armv7-r
+fp.sp— The single-precision VFPv3 floating-point instructions. The extension+vfpv3xdcan be used as an alias for this extension. aliases are so iconic ✨+fp— The VFPv3 floating-point instructions with 16 double-precision registers. The extension+vfpv3-d16can be used as an alias for this extension.+vfpv3xd-d16-fp16— The single-precision VFPv3 floating-point instructions with 16 double-precision registers and the half-precision floating-point conversion operations. say that five times fast 😭+vfpv3-d16-fp16— The VFPv3 floating-point instructions with 16 double-precision registers and the half-precision floating-point conversion operations.+nofp— Disable the floating-point extension.+idiv— The ARM-state integer division instructions.+noidiv— Disable the ARM-state integer division extension.
armv7e-m
+fp— The single-precision VFPv4 floating-point instructions.+fpv5— The single-precision FPv5 floating-point instructions.+fp.dp— The single- and double-precision FPv5 floating-point instructions.+nofp— Disable the floating-point extensions. plural extensions, she contains multitudes 🦋
armv8.1-m.main
+dsp— The DSP instructions.+mve— The M-Profile Vector Extension (MVE) integer instructions.+mve.fp— The M-Profile Vector Extension (MVE) integer and single precision floating-point instructions.+fp— The single-precision floating-point instructions.+fp.dp— The single- and double-precision floating-point instructions.+nofp— Disable the floating-point extension.+cdecp0,+cdecp1, ... ,+cdecp7— Enable the Custom Datapath Extension (CDE) on selected coprocessors according to the numbers given in the options in the range 0 to 7. pick your coprocessor, bestie 🎀+pacbti— Enable the Pointer Authentication and Branch Target Identification Extension.
armv8-m.main
+dsp— The DSP instructions.+nodsp— Disable the DSP extension.+fp— The single-precision floating-point instructions.+fp.dp— The single- and double-precision floating-point instructions.+nofp— Disable the floating-point extension.+cdecp0,+cdecp1, ... ,+cdecp7— Enable the Custom Datapath Extension (CDE) on selected coprocessors according to the numbers given in the options in the range 0 to 7.
armv8-r
+crc— The Cyclic Redundancy Check (CRC) instructions.+fp.sp— The single-precision FPv5 floating-point instructions.+simd— The ARMv8-A Advanced SIMD and floating-point instructions.+crypto— The cryptographic instructions.+nocrypto— Disable the cryptographic instructions.+nofp— Disable the floating-point, Advanced SIMD and cryptographic instructions.
-march=native makes the compiler auto-detect the architecture of the build computer. At present, this feature is only supported on GNU/Linux, and not all architectures are recognized. If the auto-detect is unsuccessful the option has no effect. so like… she tries her best, no promises 🙈
-march=unset makes the compiler ignore any -march=... options that appear earlier on the command line and behave as if the option was never passed. This is useful to avoid warnings about conflicting CPU and architecture options when the two produce different architecture specifications. un-sending the message, essentially 💅
-mtune=name
This option specifies the name of the target ARM processor for which GCC should tune the performance of the code. For some ARM implementations better performance can be obtained by using this option. Permissible names are:
arm7tdmi, arm7tdmi-s, arm710t, arm720t, arm740t, strongarm,
strongarm110, strongarm1100, strongarm1110, arm8, arm810, arm9, arm9e,
arm920, arm920t, arm922t, arm946e-s, arm966e-s, arm968e-s, arm926ej-s,
arm940t, arm9tdmi, arm10tdmi, arm1020t, arm1026ej-s, arm10e, arm1020e,
arm1022e, arm1136j-s, arm1136jf-s, mpcore, mpcorenovfp, arm1156t2-s,
arm1156t2f-s, arm1176jz-s, arm1176jzf-s, generic-armv7-a, cortex-a5,
cortex-a7, cortex-a8, cortex-a9, cortex-a12, cortex-a15, cortex-a17,
cortex-a32, cortex-a35, cortex-a53, cortex-a55, cortex-a57, cortex-a72,
cortex-a73, cortex-a75, cortex-a76, cortex-a76ae, cortex-a77,
cortex-a78, cortex-a78ae, cortex-a78c, cortex-a710, ares, cortex-r4,
cortex-r4f, cortex-r5, cortex-r7, cortex-r8, cortex-r52,
cortex-r52plus, cortex-m0, cortex-m0plus, cortex-m1, cortex-m3,
cortex-m4, cortex-m7, cortex-m23, cortex-m33, cortex-m35p, cortex-m52,
cortex-m55, cortex-m85, cortex-x1, cortex-x1c,
cortex-m1.small-multiply, cortex-m0.small-multiply,
cortex-m0plus.small-multiply, exynos-m1, marvell-pj4, neoverse-n1,
neoverse-n2, neoverse-v1, xscale, iwmmxt, iwmmxt2, ep9312, fa526,
fa626, fa606te, fa626te, fmp626, fa726te, star-mc1, xgene1.okay that's the whole roster, no cap. 🫶
Additionally, this option can specify that GCC should tune the performance of the code for a big.LITTLE system. Permissible names are:
cortex-a15.cortex-a7, cortex-a17.cortex-a7, cortex-a57.cortex-a53,
cortex-a72.cortex-a53, cortex-a72.cortex-a35, cortex-a73.cortex-a53,
cortex-a75.cortex-a55, cortex-a76.cortex-a55.-mtune=generic-arch specifies that GCC should tune the performance for a blend of processors within architecture arch. The aim is to generate code that run well on the current most popular processors, balancing between optimizations that benefit some CPUs in the range, and avoiding performance pitfalls of other CPUs. The effects of this option may change in future GCC versions as CPU models come and go. it's giving "everyone's invited to the party" 🌈
-mtune permits the same extension options as -mcpu, but the extension options do not affect the tuning of the generated code. the extensions are there, they're just not the ones calling the shots 👑
-mtune=native causes the compiler to auto-detect the CPU of the build computer. At present, this feature is only supported on GNU/Linux, and not all architectures are recognized. If the auto-detect is unsuccessful the option has no effect.
-mcpu=name[+extension...]
This specifies the name of the target ARM processor. GCC uses this name to derive the name of the target ARM architecture (as if specified by -march) and the ARM processor type for which to tune for performance (as if specified by -mtune). Where this option is used in conjunction with -march or -mtune, those options take precedence over the appropriate part of this option. the two-for-one special, and yes the explicit ones still win 💖
Many of the supported CPUs implement optional architectural extensions. Where that's the case, those architectural extensions are normally enabled by default, bestie. If implementations that lack the extension exist, then you use the extension syntax to disable the extensions that have been omitted. And for floating-point and Advanced SIMD (Neon) instructions, you also gotta think about the settings of -mfloat-abi and -mfpu: floating-point and Advanced SIMD instructions will only be used if -mfloat-abi is not set to soft; and any setting of -mfpu other than auto will override the available floating-point and SIMD extension instructions. ✨
For example, cortex-a9 can be found in three major configurations: integer only, with just a floating-point unit, or with floating-point and Advanced SIMD. The default is to enable all the instructions, but the extensions +nosimd and +nofp can be used to disable just the SIMD, or both the SIMD and floating-point instructions respectively. 🎀
Permissible names for this option are the same as those for -mtune. (iykyk 💅)
The following extension options are common to the listed CPUs:
+nodsp— Disable the DSP instructions oncortex-m33,cortex-m35p,cortex-m52,cortex-m55andcortex-m85. Also disable the M-Profile Vector Extension (MVE) integer and single precision floating-point instructions oncortex-m52,cortex-m55andcortex-m85.+nopacbti— Disable the Pointer Authentication and Branch Target Identification Extension oncortex-m52andcortex-m85.+nomve— Disable the M-Profile Vector Extension (MVE) integer and single precision floating-point instructions oncortex-m52,cortex-m55andcortex-m85.+nomve.fp— Disable the M-Profile Vector Extension (MVE) single precision floating-point instructions oncortex-m52,cortex-m55andcortex-m85.+cdecp0,+cdecp1, ... ,+cdecp7— Enable the Custom Datapath Extension (CDE) on selected coprocessors according to the numbers given in the options in the range 0 to 7 oncortex-m52,cortex-m55andstar-mc1.+nofp— Disables the floating-point instructions onarm9e,arm946e-s,arm966e-s,arm968e-s,arm10e,arm1020e,arm1022e,arm926ej-s,arm1026ej-s,cortex-r5,cortex-r7,cortex-r8,cortex-m4,cortex-m7,cortex-m33,cortex-m35p,cortex-m52,cortex-m55andcortex-m85. Disables the floating-point and SIMD instructions ongeneric-armv7-a,cortex-a5,cortex-a7,cortex-a8,cortex-a9,cortex-a12,cortex-a15,cortex-a17,cortex-a15.cortex-a7,cortex-a17.cortex-a7,cortex-a32,cortex-a35,cortex-a53andcortex-a55.+nofp.dp— Disables the double-precision component of the floating-point instructions oncortex-r5,cortex-r7,cortex-r8,cortex-r52,cortex-r52plusandcortex-m7.+nosimd— Disables the SIMD (but not floating-point) instructions ongeneric-armv7-a,cortex-a5,cortex-a7andcortex-a9.+crypto— Enables the cryptographic instructions oncortex-a32,cortex-a35,cortex-a53,cortex-a55,cortex-a57,cortex-a72,cortex-a73,cortex-a75,exynos-m1,xgene1,cortex-a57.cortex-a53,cortex-a72.cortex-a53,cortex-a73.cortex-a35,cortex-a73.cortex-a53andcortex-a75.cortex-a55.
Additionally the generic-armv7-a pseudo target defaults to VFPv3 with 16 double-precision registers. It supports the following extension options: mp, sec, vfpv3-d16, vfpv3, vfpv3-d16-fp16, vfpv3-fp16, vfpv4-d16, vfpv4, neon, neon-vfpv3, neon-fp16, neon-vfpv4. The meanings are the same as for the extensions to -march=armv7-a. 🌸
-mcpu=generic-arch is also permissible, and is equivalent to -march=arch -mtune=generic-arch. See -mtune for more information.
-mcpu=native causes the compiler to auto-detect the CPU of the build computer. At present, this feature is only supported on GNU/Linux, and not all architectures are recognized. If the auto-detect is unsuccessful the option has no effect. 😭
-mcpu=unset causes the compiler to ignore any -mcpu=... options that appear earlier on the command line and behave as if the option was never passed. This is useful to avoid warnings about conflicting CPU and architecture options when the two produce different architecture specifications. 🫶
✨ -mfpu=name ✨
This specifies what floating-point hardware (or hardware emulation) is available on the target. Permissible names are: auto, vfpv2, vfpv3, vfpv3-fp16, vfpv3-d16, vfpv3-d16-fp16, vfpv3xd, vfpv3xd-fp16, neon-vfpv3, neon-fp16, vfpv4, vfpv4-d16, fpv4-sp-d16, neon-vfpv4, fpv5-d16, fpv5-sp-d16, fp-armv8, neon-fp-armv8 and crypto-neon-fp-armv8. Note that neon is an alias for neon-vfpv3 and vfp is an alias for vfpv2. 💖
The setting auto is the default and is special. It causes the compiler to select the floating-point and Advanced SIMD instructions based on the settings of -mcpu and -march.
If the selected floating-point hardware includes the NEON extension (e.g. -mfpu=neon), note that floating-point operations are not generated by GCC's auto-vectorization pass unless -funsafe-math-optimizations is also specified. This is because NEON hardware does not fully implement the IEEE 754 standard for floating-point arithmetic (in particular denormal values are treated as zero), so the use of NEON instructions may lead to a loss of precision. ⚠️
You can also set the fpu name at function level by using the target("fpu=") function attributes or pragmas.
✨ -mfp16-format=name ✨
Specify the format of the __fp16 half-precision floating-point type. Permissible names are none, ieee, and alternative; the default is none, in which case the __fp16 type is not defined.
✨ -mstructure-size-boundary=n ✨
The sizes of all structures and unions are rounded up to a multiple of the number of bits set by this option. Permissible values are 8, 32 and 64. The default value varies for different toolchains. For the COFF targeted toolchain the default value is 8. A value of 64 is only allowed if the underlying ABI supports it.
Specifying a larger number can produce faster, more efficient code, but can also increase the size of the program. Different values are potentially incompatible. Code compiled with one value cannot necessarily expect to work with code or libraries compiled with another value, if they exchange information using structures or unions. 🧚
This option is deprecated. (rip bestie, it had a good run 💀)
✨ -mabort-on-noreturn ✨
Generate a call to the function abort at the end of a noreturn function. It is executed if the function tries to return.
✨ -mlong-calls ✨
✨ -mno-long-calls ✨
Tells the compiler to perform function calls by first loading the address of the function into a register and then performing a subroutine call on this register. This switch is needed if the target function lies outside of the 64-megabyte addressing range of the offset-based version of subroutine call instruction. 🔥
Even if this switch is enabled, not all function calls are turned into long calls. The heuristic is that static functions, functions that have the short_call attribute, functions that are inside the scope of a #pragma no_long_calls directive, and functions whose definitions have already been compiled within the current compilation unit are not turned into long calls. The exceptions to this rule are that weak function definitions, functions with the long_call attribute or the section attribute, and functions that are within the scope of a #pragma long_calls directive are always turned into long calls.
This feature is not enabled by default. Specifying -mno-long-calls restores the default behavior, as does placing the function calls within the scope of a #pragma long_calls_off directive. Note these switches have no effect on how the compiler generates code to handle function calls via function pointers. 💅
✨ -msingle-pic-base ✨
Treat the register used for PIC addressing as read-only, rather than loading it in the prologue for each function. The runtime system is responsible for initializing this register with an appropriate value before execution begins.
✨ -mpic-register=reg ✨
Specify the register to be used for PIC addressing. For standard PIC base case, the default is any suitable register determined by compiler. For single PIC base case, the default is R9 if target is EABI based or stack-checking is enabled, otherwise the default is R10.
✨ -mpic-data-is-text-relative ✨
Assume that the displacement between the text and data segments is fixed at static link time. This permits using PC-relative addressing operations to access data known to be in the data segment. For non-VxWorks RTP targets, this option is enabled by default. When disabled on such targets, it will enable -msingle-pic-base by default. ✨
✨ -mpoke-function-name ✨
Write the name of each function into the text section, directly preceding the function prologue. The generated code is similar to this:
t0
.ascii "arm_poke_function_name", 0
.align
t1
.word 0xff000000 + (t1 - t0)
arm_poke_function_name
mov ip, sp
stmfd sp!, {fp, ip, lr, pc}
sub fp, ip, #4When performing a stack backtrace, code can inspect the value of pc stored at fp + 0. If the trace function then looks at location pc - 12 and the top 8 bits are set, then we know that there is a function name embedded immediately preceding this location and has length ((pc[-3]) & 0xff000000). 🌈
✨ -mthumb ✨
✨ -marm ✨
Select between generating code that executes in ARM and Thumb states. The default for most configurations is to generate code that executes in ARM state, but the default can be changed by configuring GCC with the --with-mode=state configure option. 👑
You can also override the ARM and Thumb mode for each function by using the target("thumb") and target("arm") function attributes or pragmas.
-mflip-thumb— lowkey unhinged but ok: it flips between ARM and Thumb modes on alternating functions. This is literally only here for regression testing of mixed Thumb/ARM codegen, and it is not meant for ordinary use in compiling actual code, bestie. Like, do not.-mtpcs-frame— makes a stack frame that plays nice with the Thumb Procedure Call Standard for all non-leaf functions. (A leaf function = one that doesn't call any other functions, a true independent queen who needs nobody. 💅) The default is-mno-tpcs-frame.-mtpcs-leaf-frame— same energy, but generates a stack frame compliant with the Thumb Procedure Call Standard for all leaf functions only. (Again: leaf function = one that does not call any other functions.) The default is-mno-apcs-leaf-frame. yes, the original really says-mno-apcs-leaf-framethere, we're just reporting the lore 📖-mcallee-super-interworking— gives every externally visible function in the file being compiled an ARM instruction set header that switches to Thumb mode before it runs the rest of the function. That means these functions can get called from non-interworking code. Not valid in AAPCS configurations, because interworking is already on by default there. no notes.-mcaller-super-interworking— lets calls through function pointers (virtual functions included, periodt) execute correctly whether or not the target code was compiled for interworking. The tradeoff: there's a small overhead in the cost of executing a function pointer when this is enabled. Also not valid in AAPCS configurations, since interworking is enabled by default there.-mtp=name — picks the access model for the thread local storage pointer. Thesoftmodel generates calls to__aeabi_read_tp. Other accepted models aretpidrurw,tpidruroandtpidrprw, which fetch the thread pointer straight from the corresponding system register (supported from the **arm6k architecture and later ✨). Those system registers are reached through the CP15 co-processor interface, and the argumentcp15is also accepted as a convenience alias oftpidruro. The argumentautouses the best available method for the selected processor. The default setting isauto.-mtls-dialect=dialect — picks the dialect for accessing thread local storage. Two dialects are supported:gnuandgnu2.gnuselects the original GNU scheme for supporting local and global dynamic TLS models.gnu2selects the GNU descriptor scheme, which gives better performance for shared libraries. The GNU descriptor scheme is compatible with the original scheme, but it **does require new assembler, linker and library support. Initial and local exec TLS models are unaffected by this option and always use the original scheme. classic vs. glow-up, both valid 💜-mword-relocations— only generate absolute relocations on word-sized values (i.e.R_ARM_ABS32). This is enabled by default on targets (uClinux, SymbianOS) where the runtime loader imposes this restriction, and when-fpicor-fPICis specified. This option conflicts with-mslow-flash-data. pick a lane.-mfix-cortex-m3-ldrd— some Cortex-M3 cores can cause data corruption whenldrdinstructions with overlapping destination and base registers are used. 😭 This option avoids generating those instructions. It's enabled by default when-mcpu=cortex-m3is specified.-mfix-cortex-a57-aes-1742098-mno-fix-cortex-a57-aes-1742098-mfix-cortex-a72-aes-1655431-mno-fix-cortex-a72-aes-1655431— enable (or disable) mitigation for an erratum on Cortex-A57 and Cortex-A72 that affects the AES cryptographic instructions. 🔐 This option is enabled by default when either-mcpu=cortex-a57or-mcpu=cortex-a72is specified.-munaligned-access-mno-unaligned-access— enables (or disables) reading and writing of 16- and 32-bit values from addresses that are not 16- or 32-bit aligned. By default, unaligned access is disabled for all pre-ARMv6, all ARMv6-M and for ARMv8-M Baseline architectures, and enabled for all other architectures. If unaligned access is not enabled, then words in packed data structures are accessed a byte at a time. 🐌
The ARM attribute Tag_CPU_unaligned_access is set in the generated object file to either true or false, depending on the setting of this option. And if unaligned access is enabled, the preprocessor symbol __ARM_FEATURE_UNALIGNED is also defined. ✨
-mneon-for-64bits— this option is deprecated and has no effect. it's giving ghost town 👻-mslow-flash-data— assume loading data from flash is slower than fetching instructions. So literal loads get minimized for better performance. Only supported when compiling for ARMv7 M-profile, and off by default. It conflicts with-mword-relocations.-masm-syntax-unified— assume inline assembler is using unified asm syntax. The default is currently off, which implies divided syntax. This option has no impact on Thumb2. However, this may change in future releases of GCC. Divided syntax should be considered deprecated. 👀-mrestrict-it— restricts generation of IT blocks to conform to the rules of ARMv8-A. IT blocks can only contain a single 16-bit instruction from a select set of instructions. This option is on by default for ARMv8-A Thumb mode.-mprint-tune-info— prints CPU tuning information as a comment in the assembler file. This is an option used only for regression testing of the compiler, not intended for ordinary use in compiling code. Disabled by default. 🛠️-mverbose-cost-dump— enables verbose cost model dumping in the debug dump files. This option is provided for use in debugging the compiler. 🐛-mpure-code— do not allow constant data to be placed in code sections. Also, when compiling for ELF object format, give all text sections the ELF processor-specific section attributeSHF_ARM_PURECODE. Only available when generating non-pic code for M-profile targets.-mcmse— generate secure code as per the ARMv8-M Security Extensions: Requirements on Development Tools Engineering Specification, which lives at <https://developer.arm.com/documentation/ecm0359818/latest/>. 🔒-mfix-cmse-cve-2021-35465— mitigate against a potential security issue with theVLLDMinstruction in some M-profile devices when using CMSE (CVE-2021-365465). Enabled by default when the option-mcpu=is used withcortex-m33,cortex-m35p,cortex-m52,cortex-m55,cortex-m85orstar-mc1. The option-mno-fix-cmse-cve-2021-35465can be used to disable the mitigation. 🛡️-mstack-protector-guard=guard</br>--mstack-protector-guard-offset=offset — generate stack protection code using a canary at guard. Supported locations areglobalfor a global canary, ortlsfor a canary accessible via the TLS register. The option-mstack-protector-guard-offset=is for use with-fstack-protector-guard=tlsand **not for use in user-land code. user-land said stay out of it 🚫-mfdpic-mno-fdpic— select the FDPIC ABI, which uses 64-bit function descriptors to represent pointers to functions. When the compiler is configured forarm-*-uclinuxfdpiceabitargets, this option is on by default and implies-fPIEif none of the PIC/PIE-related options is provided. On other targets, it only enables the FDPIC-specific code generation features, and the user should explicitly provide the PIC/PIE-related options as needed.
Note that static linking is not supported, because it would still involve the dynamic linker when the program self-relocates. If such behavior is acceptable, use -static and -Wl,-dynamic-linker options.
The opposite -mno-fdpic option is useful (and required) to build the Linux kernel using the same (arm-*-uclinuxfdpiceabi) toolchain as the one used to build the userland programs. 🐧
-mbranch-protection=none|standard|pac-ret[+leaf][+bti]|bti[+pac-ret[+leaf]] — enable branch protection features (armv8.1-m.main only**).nonegenerates code without branch protection or return address signing.standard[+leaf]generates code with all branch protection features enabled at their standard level.pac-ret[+leaf]generates code with return address signing set to its standard level, which means signing all functions that save the return address to memory.leaf— when return address signing is enabled, also sign leaf functions even if they don't write the return address to memory.+bti— add landing-pad instructions at the permitted targets of indirect branch instructions. 🛬
If the +pacbti architecture extension is not enabled, then all branch protection and return address signing operations are constrained to use only the instructions defined in the architectural-NOP space. The generated code stays backwards-compatible with earlier versions of the architecture, but the additional security can be enabled at run time on processors that support the PACBTI extension. ✨
Branch target enforcement using BTI can only be enabled at runtime if all code in the application has been compiled with at least -mbranch-protection=bti.
Any setting other than none is supported only on armv8-m.main or later.
The default is to generate code without branch protection or return address signing. no armor by default, bestie 💜
🌱 AVR Options 🌱
These options are defined for AVR implementations, and they're honestly kind of adorable:
- *
-mmcu=mcu* — specify the AVR instruction set architecture (ISA) or device type. The default for this option isavr2.
The following AVR devices and ISAs are supported. Note: a complete device support consists of startup code crtmcu.o, a device header avr/io*.h, a device library libmcu.a and a device-specs (<https://gcc.gnu.org/wiki/avr-gcc#spec-files>) file specs-mcu. Only the latter is provided by the compiler according to the supported mcus below. The rest is supported by AVR-LibC (<https://github.com/avrdudes/avr-libc/>), or by means of atpack (<https://gcc.gnu.org/wiki/avr-gcc#atpack>) files from the hardware manufacturer. 🧩
avr2 : "Classic" devices with up to 8 KiB of program memory. mcu = attiny22, attiny26, at90s2313, at90s2323, at90s2333, at90s2343, at90s4414, at90s4433, at90s4434, at90c8534, at90s8515, at90s8535.
avr25 🌈
The "Classic" girlies with up to 8 KiB of program memory and the MOVW instruction. mcu = attiny13, attiny13a, attiny24, attiny24a, attiny25, attiny261, attiny261a, attiny2313, attiny2313a, attiny43u, attiny44, attiny44a, attiny45, attiny48, attiny441, attiny461, attiny461a, attiny4313, attiny84, attiny84a, attiny85, attiny87, attiny88, attiny828, attiny841, attiny861, attiny861a, ata5272, ata6616c, at86rf401.
avr3 🌈
"Classic" devices with 16 KiB up to 64 KiB of program memory. mcu = at76c711, at43usb355.
avr31 🌈
"Classic" devices with 128 KiB of program memory. mcu = atmega103, at43usb320.
avr35 🌈
"Classic" devices with 16 KiB up to 64 KiB of program memory and with the MOVW instruction. mcu = attiny167, attiny1634, atmega8u2, atmega16u2, atmega32u2, ata5505, ata6617c, ata664251, at90usb82, at90usb162.
avr4 🌈
"Enhanced" devices with up to 8 KiB of program memory. mcu = atmega48, atmega48a, atmega48p, atmega48pa, atmega48pb, atmega8, atmega8a, atmega8hva, atmega88, atmega88a, atmega88p, atmega88pa, atmega88pb, atmega8515, atmega8535, ata5795, ata6285, ata6286, ata6289, ata6612c, at90pwm1, at90pwm2, at90pwm2b, at90pwm3, at90pwm3b, at90pwm81.
avr5 🌈
"Enhanced" devices with 16 KiB up to 64 KiB of program memory. mcu = atmega16, atmega16a, atmega16hva, atmega16hva2, atmega16hvb, atmega16hvbrevb, atmega16m1, atmega16u4, atmega161, atmega162, atmega163, atmega164a, atmega164p, atmega164pa, atmega165, atmega165a, atmega165p, atmega165pa, atmega168, atmega168a, atmega168p, atmega168pa, atmega168pb, atmega169, atmega169a, atmega169p, atmega169pa, atmega32, atmega32a, atmega32c1, atmega32hvb, atmega32hvbrevb, atmega32m1, atmega32u4, atmega32u6, atmega323, atmega324a, atmega324p, atmega324pa, atmega324pb, atmega325, atmega325a, atmega325p, atmega325pa, atmega328, atmega328p, atmega328pb, atmega329, atmega329a, atmega329p, atmega329pa, atmega3250, atmega3250a, atmega3250p, atmega3250pa, atmega3290, atmega3290a, atmega3290p, atmega3290pa, atmega406, atmega64, atmega64a, atmega64c1, atmega64hve, atmega64hve2, atmega64m1, atmega64rfr2, atmega640, atmega644, atmega644a, atmega644p, atmega644pa, atmega644rfr2, atmega645, atmega645a, atmega645p, atmega649, atmega649a, atmega649p, atmega6450, atmega6450a, atmega6450p, atmega6490, atmega6490a, atmega6490p, ata5790, ata5790n, ata5791, ata6613c, ata6614q, ata5782, ata5831, ata8210, ata8510, ata5787, ata5835, ata5700m322, ata5702m322, at90pwm161, at90pwm216, at90pwm316, at90can32, at90can64, at90scr100, at90usb646, at90usb647, at94k, m3000.
avr51 🌈
"Enhanced" devices with 128 KiB of program memory. mcu = atmega128, atmega128a, atmega128rfa1, atmega128rfr2, atmega1280, atmega1281, atmega1284, atmega1284p, atmega1284rfr2, at90can128, at90usb1286, at90usb1287.
avr6 🌈
"Enhanced" devices with a 3-byte PC, i.e. with more than 128 KiB of program memory. mcu = atmega256rfr2, atmega2560, atmega2561, atmega2564rfr2.
avrxmega2 🌈
"XMEGA" devices with more than 8 KiB and up to 64 KiB of program memory. mcu = atxmega8e5, atxmega16a4, atxmega16a4u, atxmega16c4, atxmega16d4, atxmega16e5, atxmega32a4, atxmega32a4u, atxmega32c3, atxmega32c4, atxmega32d3, atxmega32d4, atxmega32e5, avr64da28, avr64da28s, avr64da32, avr64da32s, avr64da48, avr64da48s, avr64da64, avr64da64s, avr64db28, avr64db32, avr64db48, avr64db64, avr64dd14, avr64dd20, avr64dd28, avr64dd32, avr64du28, avr64du32, avr64ea28, avr64ea32, avr64ea48, avr64sd28, avr64sd32, avr64sd48.
avrxmega3 🌈
"XMEGA" devices with up to 64 KiB of combined program memory and RAM, and with program memory visible in the RAM address space. mcu = attiny202, attiny204, attiny212, attiny214, attiny402, attiny404, attiny406, attiny412, attiny414, attiny416, attiny416auto, attiny417, attiny424, attiny426, attiny427, attiny804, attiny806, attiny807, attiny814, attiny816, attiny817, attiny824, attiny826, attiny827, attiny1604, attiny1606, attiny1607, attiny1614, attiny1616, attiny1617, attiny1624, attiny1626, attiny1627, attiny3214, attiny3216, attiny3217, attiny3224, attiny3226, attiny3227, atmega808, atmega809, atmega1608, atmega1609, atmega3208, atmega3209, atmega4808, atmega4809, avr16dd14, avr16dd20, avr16dd28, avr16dd32, avr16du14, avr16du20, avr16du28, avr16du32, avr16ea28, avr16ea32, avr16ea48, avr16eb14, avr16eb20, avr16eb28, avr16eb32, avr16la14, avr16la20, avr16la28, avr16la32, avr32da28, avr32da28s, avr32da32, avr32da32s, avr32da48, avr32da48s, avr32db28, avr32db32, avr32db48, avr32dd14, avr32dd20, avr32dd28, avr32dd32, avr32du14, avr32du20, avr32du28, avr32du32, avr32ea28, avr32ea32, avr32ea48, avr32eb14, avr32eb20, avr32eb28, avr32eb32, avr32la14, avr32la20, avr32la28, avr32la32, avr32sd20, avr32sd28, avr32sd32.
avrxmega4 🌈
"XMEGA" devices with more than 64 KiB and up to 128 KiB of program memory. mcu = atxmega64a3, atxmega64a3u, atxmega64a4u, atxmega64b1, atxmega64b3, atxmega64c3, atxmega64d3, atxmega64d4, avr128da28, avr128da28s, avr128da32, avr128da32s, avr128da48, avr128da48s, avr128da64, avr128da64s, avr128db28, avr128db32, avr128db48, avr128db64.
avrxmega5 🌈
"XMEGA" devices with more than 64 KiB and up to 128 KiB of program memory and more than 64 KiB of RAM. mcu = atxmega64a1, atxmega64a1u.
avrxmega6 🌈
"XMEGA" devices with more than 128 KiB of program memory. mcu = atxmega128a3, atxmega128a3u, atxmega128b1, atxmega128b3, atxmega128c3, atxmega128d3, atxmega128d4, atxmega192a3, atxmega192a3u, atxmega192c3, atxmega192d3, atxmega256a3, atxmega256a3b, atxmega256a3bu, atxmega256a3u, atxmega256c3, atxmega256d3, atxmega384c3, atxmega384d3.
avrxmega7 🌈
"XMEGA" devices with more than 128 KiB of program memory and more than 64 KiB of RAM. mcu = atxmega128a1, atxmega128a1u, atxmega128a4u.
avrtiny 🌈
"Reduced Tiny" — Tiny core devices with only 16 general purpose registers and 512 B up to 4 KiB of program memory. mcu = attiny4, attiny5, attiny9, attiny10, attiny102, attiny104, attiny20, attiny40.
avr1 🌈
This ISA is implemented by the minimal AVR core and supported for assembler only. mcu = attiny11, attiny12, attiny15, attiny28, at90s1200.
-mabsdata 🫶
Assume that all data in static storage can be accessed by LDS / STS instructions. Heads up bestie, this option only has an effect on reduced Tiny devices like ATtiny40. See also the absdata AVR Variable Attributes, variable attribute.
-mcvt 🫶
Use a compact vector table. Some devices support a CVT with only four entries: 0=Reset, 1=NMI, 2=Prio1 IRQ, 3=Prio0 IRQs. This option will link startup code from crtmcu-cvt.o instead of the usual crtmcu.o. Apart from providing a compact vector table, the startup code will set bit CPUINT_CTRLA.CPUINT_CVT which enables the CVT on the device.
When you do not want the startup code to set CPUINT_CTRLA.CPUINT_CVT, then you can satisfy symbol __init_cvt so that the respective code is no more pulled in from libmcu.a. For example, you can link with -Wl,--defsym,__init_cvt=0.
The CVT startup code is available since AVR-LibC v2.3 (https://github.com/avrdudes/avr-libc/issues/1010).
-mdouble=bits 🫶
-mlong-double=bits 🫶
Set the size (in bits) of the double or long double type, respectively. Possible values for bits are 32 and 64. Whether or not a specific value for bits is allowed depends on the --with-double= and --with-long-double= configure options (https://gcc.gnu.org/install/configure.html#avr), and the same applies for the default values of the options.
-mgas-isr-prologues 🫶
Interrupt service routines (ISRs) may use the __gcc_isr pseudo instruction supported by GNU Binutils. If this option is on, the feature can still be disabled for individual ISRs by means of the AVR Function Attributes no_gccisr function attribute. This feature is activated per default if optimization is on (but not with -Og, see Optimize Options), and if GNU Binutils support PR21683 (https://sourceware.org/PR21683).
-mint8 🫶
Assume int to be 8-bit integer. This affects the sizes of all types: a char is 1 byte, an int is 1 byte, a long is 2 bytes, and long long is 4 bytes. Please note that this option does not conform to the C standards, but it results in smaller code size. ✨
-mmain-is-OS_task 🫶
Do not save registers in main. The effect is the same like attaching attribute AVR Function Attributes OS_task to main. It is activated per default if optimization is on.
-mno-call-main 🫶
Don't run main by means of
XCALL main
XJMP exitInstead, put main in section .init9 (https://avrdudes.github.io/avr-libc/avr-libc-user-manual/mem_sections.html#sec_dot_init) so that no call is required. By setting this option the user asserts that main will not return. 👑
Fr fr, if you flip this on, you're promising main never returns. Do not lie to the compiler bestie. 💀
This option can be used for devices with very limited resources so you can save a few bytes of code and stack space. 💾 It will work as expected since AVR-LibC v2.3 (https://github.com/avrdudes/avr-libc/issues/1012). With older versions, there will be no performance gain. Bestie, that's it, that's the whole tea.
-mno-interrupts— the generated code is not compatible with hardware interrupts 💀. The tradeoff? Code size is smaller. So like, pick your fighter.-mrelax— try to swap aCALL(resp.JMP) instruction for the shorterRCALL(resp.RJMP) instruction if that applies. Setting-mrelaxjust adds the--mlink-relaxoption to the assembler's command line and the--relaxoption to the linker's command line. ✨
Jump relaxing is performed by the linker, because jump offsets are not known before code is located. Therefore the assembler code generated by the compiler is the same, but the instructions in the executable may differ from instructions in the assembler code. 🦋
Relaxing must be turned on if linker stubs are needed — see the section on EIND and linker stubs below. 👇
-mrodata-in-ram-mno-rodata-in-ram— locate the.rodatasections for read-only data in RAM (resp. in program memory). For most devices there is no choice, and this option acts rather like an assertion. 🙈
Since v14, and for the AVR64* and AVR128* devices, .rodata is located in flash memory per default, provided the required GNU Binutils support (PR31124 (https://sourceware.org/PR31124)) is available. In that case, -mrodata-in-ram can be used to return to the old layout with .rodata in RAM. 🧚
-mtiny-stack— only change the lower 8 bits of the stack pointer. Tiny. 🎀-mfract-convert-truncate— allow to use truncation instead of rounding towards zero for fractional fixed-point types. ✂️-nodevicelib— don't link against AVR-LibC's device specific librarylibmcu.a. 🚫
Notice that since AVR-LibC v2.3, that library contains code that is essential for the correct functioning of a program. In particular, it contains parts of the startup code like:__init_sp(https://github.com/avrdudes/avr-libc/issues/1011) to initialize the stack pointer with symbol__stack,__init_cvt(https://github.com/avrdudes/avr-libc/issues/1010) to set up the hardware to use a compact vector table with-mcvt,__call_main(https://github.com/avrdudes/avr-libc/issues/1012) to callmainandexit, and__do_flmap_init(https://github.com/avrdudes/avr-libc/issues/931) to set up FLMAP according to symbol__flmap. 😭
-nodevicespecs— don't add-specs=device-specs/specs-mcuto the compiler driver's command line. The user takes responsibility for supplying the sub-processes like compiler proper, assembler and linker with appropriate command line options. This means that the user has to supply her private device specs file by means of-specs=path-to-specs-file. There is no more need for option-mmcu=mcu. 💅
This option can also serve as a replacement for the older way of specifying custom device-specs files that needed -B some-path to point to a directory which contains a folder named device-specs which contains a specs file named specs-mcu, where mcu was specified by -mmcu=mcu. 🌈
-Waddr-space-convert— warn about conversions between address spaces in the case where the resulting address space is not contained in the incoming address space. ⚠️-Wmisspelled-isr— warn if the ISR is misspelled, i.e. without__vectorprefix. Enabled by default. 👑
✨ AVR Optimization Options ✨
The following options are pure optimization options. Options -mgas-isr-prologues, -mmain-is-OS_task, -mno-call-main and -mrelax from above are only almost optimization options, since there are rare occasions where their different code generation matters. 😌
-maccumulate-args— accumulate outgoing function arguments and acquire/release the needed stack space for outgoing function arguments once in function prologue/epilogue. Without this option, outgoing arguments are pushed before calling a function and popped afterwards. See also the-fdefer-popOptimize Options,,optimization option. 🫶
Popping the arguments after the function call can be expensive on AVR, so that accumulating the stack space might lead to smaller executables, because arguments need not be removed from the stack after such a function call.
This option can lead to reduced code size for functions that perform several calls to functions that get their arguments on the stack — like calls to printf-like functions. 🍓
-mbranch-cost=cost— set the branch costs for conditional branch instructions to cost. Reasonable values for cost are small, non-negative integers. The default branch cost is0. 💯-mcall-prologues— function prologues/epilogues are expanded as calls to appropriate subroutines. Code size is smaller. 📦-mfuse-add-mno-fuse-add-mfuse-add=level— optimize indirect memory accesses on reduced Tiny devices. The default useslevel=1for optimizations-Ogand-O1, andlevel=2for higher optimizations. Valid values for level are0,1and2. 🔥-mfuse-move-mno-fuse-move-mfuse-move=level— run a post reload optimization pass that tries to fuse move instructions and to split multi-byte instructions into 8-bit operations. The default useslevel=3for optimization-O1, andlevel=23for higher optimizations. Valid values for level are in the range0 ... 23, which is a 3:2:2:2 mixed radix value. Each digit controls some aspect of the optimization. 🤯-mfuse-move2— run a post combine optimization pass that tries to fuse move instructions. 🪄-mstrict-X— use address registerXin a way proposed by the hardware. This means thatXis only used in indirect, post-increment or pre-decrement addressing. 💅
Without this option, the X register may be used in the same way as Y or Z, which then is emulated by additional instructions. For example, loading a value with X+const addressing with a small non-negative const < 64 to a register Rn is performed as
adiw r26, const ; X += const
ld <Rn>, X ; <Rn> = *X
sbiw r26, const ; X -= const-msplit-bit-shift— split multi-byte shifts with a constant offset into a shift with a byte offset and a residual shift with a non-byte offset. This optimization is turned on per default for-O2and higher, including-Osbut excluding-Oz. Splitting of shifts with a constant offset that is a multiple of 8 is controlled by-mfuse-move. ✨-msplit-ldst— split multi-byte loads and stores into several byte loads and stores. This optimization is turned on per default for-O2and higher. 🦋-muse-nonzero-bits— enable optimizations that are only possible when some bits in a register are always zero. This optimization is turned on per default for-O2and higher. 🌟
👑 EIND and Devices with More Than 128 Ki Bytes of Flash 👑
Pointers in the implementation are 16 bits wide. The address of a function or label is represented as word address, so that indirect jumps and calls can target any code address in the range of 64 Ki words. 🎯
In order to facilitate indirect jump on devices with more than 128 Ki bytes of program memory space, there is a special function register called EIND that serves as most significant part of the target address when EICALL or EIJMP instructions are used. 💫
Indirect jumps and calls on these devices are handled as follows by the compiler, and are subject to some limitations:
- The compiler never sets
EIND. 🚫 - The compiler uses
EINDimplicitly inEICALL/EIJMPinstructions, or might readEINDdirectly in order to emulate an indirect call/jump by means of aRETinstruction. 👀 - The compiler assumes that
EINDnever changes during the startup code or during the application. In particular,EINDis not saved/restored in function or interrupt service routine prologue/epilogue. 😤 - For indirect calls to functions and computed goto, the linker generates stubs. Stubs are jump pads, sometimes also called trampolines. Thus the indirect call/jump jumps to such a stub. The stub contains a direct jump to the desired address. 🛝
- Linker relaxation must be turned on so that the linker generates the stubs correctly in all situations. See the compiler option
-mrelaxand the linker option--relax. There are corner cases where the linker is supposed to generate stubs but aborts without relaxation and without a helpful error message. 💀 - The default linker script is arranged for code with
EIND = 0. If code is supposed to work for a setup withEIND != 0, a custom linker script has to be used in order to place the sections whose name start with.trampolinesinto the segment whereEINDpoints to. 🗺️ - The startup code from libgcc never sets
EIND. Notice that startup code is a blend of code from libgcc and AVR-LibC. For the impact of AVR-LibC onEIND, see the AVR-LibC user manual (https://avrdudes.github.io/avr-libc/avr-libc-user-manual/). 📚 - It is legitimate for user-specific startup code to set up
EINDearly, for example by means of initialization code located in section.init3. Such code runs prior to general startup code that initializes RAM and calls constructors, but after the bit of startup code from AVR-LibC that setsEINDto the segment where the vector table is located. 🧠
#include <avr/io.h>
static void
__attribute__((section(".init3"),naked,used,no_instrument_function))
init3_set_eind (void)
{
__asm volatile ("ldi r24,pm_hh8(__trampolines_start)\n\t"
"out %i0,r24" :: "n" (&EIND) : "r24","memory");
}The __trampolines_start symbol is defined in the linker script. 🎀
- Stubs are generated automatically by the linker if the following two conditions are met: 🔍
- <The address of a label is taken by means of the
gsmodifier> (short for generate stubs) like so:
- <The address of a label is taken by means of the
LDI r24, lo8(gs(<func>))
LDI r25, hi8(gs(<func>))- <The final location of that label is in a code segment> outside the segment where the stubs are located.
- The compiler emits such
gsmodifiers for code labels in the following situations: 👇 - Taking address of a function or code label. 🫶
- Computed goto.
- If prologue-save function is used, see the
-mcall-prologuescommand-line option. - Switch/case dispatch tables. If you do not want such dispatch tables you can specify the
-fno-jump-tablescommand-line option. 🚫 - C and C++ constructors/destructors called during startup/shutdown.
- If the tools hit a
gs()modifier explained above. - Jumping to non-symbolic addresses like this?? Not supported, bestie. Periodt. 💀
int main (void)
{
/* Call function at word address 0x2 */
return ((int(*)(void)) 0x2)();
}Instead, a stub has to be set up, i.e. the function has to be called through a symbol (func_4 in the example):
int main (void)
{
extern int func_4 (void);
/* Call function at byte address 0x4 */
return func_4();
}and the application be linked with -Wl,--defsym,func_4=0x4. Alternatively, func_4 can be defined in the linker script. ✨
💖 Handling of the RAMPD, RAMPX, RAMPY and RAMPZ Special Function Registers
Some AVR devices support memories bigger than the 64 KiB range that can be accessed with 16-bit pointers. To access memory locations outside this 64 KiB range, the content of a RAMP register is used as the high part of the address: the X, Y, Z address register is concatenated with the RAMPX, RAMPY, RAMPZ special function register, respectively, to get a wide address. Similarly, RAMPD is used together with direct addressing. 🧚
- The startup code initializes the
RAMPspecial function registers with zero. 🍓 - If a AVR Named Address Spaces, named address space other than generic or
__flashis used, thenRAMPZis set as needed before the operation. - If the device supports RAM larger than 64 KiB and the compiler needs to change
RAMPZto accomplish an operation,RAMPZis reset to zero after the operation. - If the device comes with a specific
RAMPregister, the ISR prologue/epilogue saves/restores that SFR and initializes it with zero in case the ISR code might (implicitly) use it. - RAM larger than 64 KiB is not supported by GCC for AVR targets. If you use inline assembler to read from locations outside the 16-bit address range and change one of the
RAMPregisters, you must reset it to zero after the access. Don't forget. I'm watching. 👀
🌈 AVR Built-in Macros
GCC defines several built-in macros so that the user code can test for the presence or absence of features. Almost any of the following built-in macros are deduced from device capabilities and thus triggered by the -mmcu= command-line option. 🔥
For even more AVR-specific built-in macros see AVR Named Address Spaces and AVR Built-in Functions.
__AVR_ARCH__— Build-in macro that resolves to a decimal number that identifies the architecture and depends on the-mmcu=mcuoption. Possible values are:
2, 25, 3, 31, 35, 4, 5, 51, 6for mcu="avr2", "avr25", "avr3", "avr31", "avr35", "avr4", "avr5", "avr51", "avr6",
respectively and
100, 102, 103, 104, 105, 106, 107for mcu="avrtiny", "avrxmega2", "avrxmega3", "avrxmega4", "avrxmega5", "avrxmega6", "avrxmega7", respectively. If mcu specifies a device, this built-in macro is set accordingly. For example, with -mmcu=atmega8 the macro is defined to 4. 🎀
__AVR_Device__— Setting-mmcu=devicedefines this built-in macro which reflects the device's name. For example,-mmcu=atmega8defines the built-in macro__AVR_ATmega8__,-mmcu=attiny261adefines__AVR_ATtiny261A__, etc.
The built-in macros' names follow the scheme __AVR_Device__ where Device is the device name as from the AVR user manual. The difference between Device in the built-in macro and device in -mmcu=device is that the latter is always lowercase. 🦋
If device is not a device but only a core architecture like avr51, this macro is not defined.
__AVR_DEVICE_NAME__— Setting-mmcu=devicedefines this built-in macro to the device's name. For example, with-mmcu=atmega8the macro is defined to"atmega8".
If device is not a device but only a core architecture like avr51, this macro is not defined.
__AVR_CVT__— The code is being compiled with option-mcvtto use a compact vector table.__AVR_XMEGA__— The device / architecture belongs to the XMEGA family of devices. ✨__AVR_HAVE_ADIW__— The device has theADIWandSBIWinstructions.__AVR_HAVE_ELPM__— The device has theELPMinstruction.__AVR_HAVE_ELPMX__— The device has theELPM Rn,ZandELPM Rn,Z+instructions.__AVR_HAVE_LPMX__— The device has theLPM Rn,ZandLPM Rn,Z+instructions.__AVR_HAVE_MOVW__— The device has theMOVWinstruction to perform 16-bit register-register moves.__AVR_HAVE_MUL__— The device has a hardware multiplier.__AVR_HAVE_JMP_CALL__— The device has theJMPandCALLinstructions. This is the case for devices with more than 8 KiB of program memory.__AVR_HAVE_EIJMP_EICALL__,__AVR_3_BYTE_PC__— The device has theEIJMPandEICALLinstructions. This is the case for devices with more than 128 KiB of program memory. This also means that the program counter (PC) is 3 bytes wide.__AVR_2_BYTE_PC__— The program counter (PC) is 2 bytes wide. This is the case for devices with up to 128 KiB of program memory.__AVR_HAVE_8BIT_SP__,__AVR_HAVE_16BIT_SP__— The stack pointer (SP) register is treated as 8-bit respectively 16-bit register by the compiler. The definition of these macros is affected by-mtiny-stack.__AVR_HAVE_SPH__,__AVR_SP8__— The device has the SPH (high part of stack pointer) special function register or has an 8-bit stack pointer, respectively. The definition of these macros is affected by-mmcu=and in the cases of-mmcu=avr2and-mmcu=avr25also by-msp8.__AVR_HAVE_RAMPD__,__AVR_HAVE_RAMPX__,__AVR_HAVE_RAMPY__,__AVR_HAVE_RAMPZ__— The device has theRAMPD,RAMPX,RAMPY,RAMPZspecial function register, respectively.__NO_INTERRUPTS__— This macro reflects the-mno-interruptscommand-line option.__AVR_ERRATA_SKIP__,__AVR_ERRATA_SKIP_JMP_CALL__— Some AVR devices (AT90S8515, ATmega103) must not skip 32-bit instructions because of a hardware erratum. Skip instructions areSBRS,SBRC,SBIS,SBICandCPSE. The second macro is only defined if__AVR_HAVE_JMP_CALL__is also set. ⚠️__AVR_ISA_RMW__— The device has Read-Modify-Write instructions (XCH, LAC, LAS and LAT).__AVR_SFR_OFFSET__=offset— Instructions that can address I/O special function registers directly likeIN,OUT,SBI, etc. may use a different address as if addressed by an instruction to access RAM likeLDorSTS. This offset depends on the device architecture and has to be subtracted from the RAM address in order to get the respective I/O address.__AVR_SHORT_CALLS__— The-mshort-callscommand line option is set.__AVR_PM_BASE_ADDRESS__=addr— Some devices support reading from flash memory by means ofLD*instructions. The flash memory is seen in the data address space at an offset of__AVR_PM_BASE_ADDRESS__. If this macro is not defined, this feature is not available. If defined, the address space is linear and there is no need to put.rodatainto RAM. This is handled by the default linker description file, and is currently available foravrtinyandavrxmega3. Even more convenient, there is no need to use address spaces like__flashor features like attributeprogmemandpgm_read_*. 💅__AVR_HAVE_FLMAP__— This macro is defined provided the following conditions are met:- The device has the
NVMCTRL_CTRLB.FLMAPbitfield. This applies to the AVR64* and AVR128* devices. - It's not known at assembler-time which emulation will be used.
- The device has the
This implies the compiler was configured with GNU Binutils that implement PR31124 (https://sourceware.org/PR31124).
__AVR_RODATA_IN_RAM__— This macro is undefined when the code is compiled for a core architecture.
When the code is compiled for a device, the macro is defined to 1 when the .rodata sections for read-only data is located in RAM; and defined to 0, otherwise.
__WITH_AVRLIBC__— The compiler is configured to be used together with AVR-LibC. See the--with-avrlibcconfigure option.__HAVE_SIGNAL_N__— The compiler supports the signal(num) and interrupt(num) AVR Function Attributes,, function attributes with an argument num that specifies the number of the interrupt service routine.__HAVE_DOUBLE_MULTILIB__— Defined if-mdouble=acts as a multilib option.__HAVE_DOUBLE32__,__HAVE_DOUBLE64__— Defined if the compiler supports 32-bit double resp. 64-bit double. The actual layout is specified by option-mdouble=.__DEFAULT_DOUBLE__— The size in bits ofdoubleif-mdouble=is not set. To test the layout ofdoublein a program, use the built-in macro__SIZEOF_DOUBLE__.__HAVE_LONG_DOUBLE32__,__HAVE_LONG_DOUBLE64__,__HAVE_LONG_DOUBLE_MULTILIB__,__DEFAULT_LONG_DOUBLE__— Same as above, but forlong doubleinstead ofdouble.__WITH_DOUBLE_COMPARISON__— Reflects the--with-double-comparison={tristate|bool|libf7}configure option (https://gcc.gnu.org/install/configure.html#avr) and is defined to2or3.__WITH_LIBF7_LIBGCC__,__WITH_LIBF7_MATH__,__WITH_LIBF7_MATH_SYMBOLS__— Reflects the--with-libf7={libgcc|math|math-symbols}configure option (https://gcc.gnu.org/install/configure.html#avr).
🔧 AVR Internal Options
The following options are used internally by the compiler and to communicate between device specs files and the compiler proper. You don't need to set these options by hand, in particular they are not optimization options. Using these options in the wrong way may lead to sub-optimal or wrong code. They are documented for completeness, and in order to get a better understanding of device specs (https://gcc.gnu.org/wiki/avr-gcc#spec-files) files. 🙈
-mn-flash=num— okay so this is you telling GCC "bestie the flash memory isnumtimes 64 KiB, that's just the math". That number is literally what decides which__flashNaddress spaces are even available to you. 💾✨-mflmap— this device has theFLMAPbit field, and it lives in the special function registerNVMCTRL_CTRLB. That's the whole vibe. 🎀-mrmw— assume the device supports the Read-Modify-Write instructionsXCH,LAC,LASandLAT. Slay. 🔥-mshort-calls— assume thatRJMPandRCALLcan target the whole program memory. This one is used for multilib generation and selection for the devices from architectureavrxmega3. 👑-mskip-bug— generate code with no skips (CPSE,SBRS,SBRC,SBIS,SBIC) over 32-bit instructions. Dodging the bug like 💅-msp8— treat the stack pointer register as an 8-bit register, i.e. assume the high byte of the stack pointer is zero. The compiler uses this to select and build multilibs for architecturesavr2andavr25. Those architectures mix devices with and withoutSPH, so yeah, it's giving mixed signals fr fr. 🌸
🖤 Blackfin Options 🖤
-mcpu=cpu[-sirevision]— this names the target Blackfin processor you're building for.
Right now, cpu can be one of: bf512, bf514, bf516, bf518, bf522, bf523, bf524, bf525, bf526, bf527, bf531, bf532, bf533, bf534, bf536, bf537, bf538, bf539, bf542, bf544, bf547, bf548, bf549, bf542m, bf544m, bf547m, bf548m, bf549m, bf561, bf592. Long list, we love a moment of range. ✨
The optional sirevision specifies the silicon revision of the target Blackfin processor. Any workarounds available for the targeted silicon revision get enabled. If sirevision is none, no workarounds are enabled. If sirevision is any, all workarounds for the targeted processor are enabled. The __SILICON_REVISION__ macro is defined to two hexadecimal digits representing the major and minor numbers in the silicon revision. If sirevision is none, the __SILICON_REVISION__ is not defined. If sirevision is any, the __SILICON_REVISION__ is defined to be 0xffff. If this optional sirevision isn't used at all, GCC just assumes the latest known silicon revision of the targeted Blackfin processor.
GCC defines a preprocessor macro for the specified cpu. For the bfin-elf toolchain, this option causes the hardware BSP provided by libgloss to be linked in if -msim is not given.
Without this option, bf532 is used as the processor by default.
Heads up bestie: support for bf561 is incomplete. For bf561, only the preprocessor macro is defined. 😭
-msim— tells GCC the program is gonna run on the simulator, so the simulator BSP provided by libgloss gets linked in. This option only has effect for the bfin-elf toolchain. And lowkey, some other options — like-mid-shared-libraryand-mfdpic— imply-msimanyway. 🧚-momit-leaf-frame-pointer— don't keep the frame pointer in a register for leaf functions. This skips the instructions to save, set up and restore frame pointers, and frees up an extra register in leaf functions. Iconic little optimization. 🎀-mspecld-anomaly— when enabled, the compiler makes sure the generated code doesn't contain speculative loads after jump instructions. If you use this option,__WORKAROUND_SPECULATIVE_LOADSis defined. 🛡️-mno-specld-anomaly— don't generate extra code to prevent speculative loads from occurring.-mcsync-anomaly— when enabled, the compiler makes sure the generated code doesn't contain CSYNC or SSYNC instructions too soon after conditional branches. If you use this option,__WORKAROUND_SPECULATIVE_SYNCSis defined.-mno-csync-anomaly— don't generate extra code to prevent CSYNC or SSYNC instructions from occurring too soon after a conditional branch.-mlow64k— when enabled, the compiler is free to take advantage of the knowledge that the entire program fits into the low 64k of memory. 🫶-mno-low64k— assume that the program is arbitrarily large. This is the default.-mstack-check-l1— do stack checking using information placed into L1 scratchpad memory by the uClinux kernel. Very specific, very niche, we respect it. 🧠-mid-shared-library— generate code that supports shared libraries via the library ID method. This allows for execute in place and shared libraries in an environment without virtual memory management. This option implies-fPIC. With a bfin-elf target, this option implies-msim. 💖-mno-id-shared-library— generate code that doesn't assume ID-based shared libraries are being used. This is the default.-mleaf-id-shared-library— generate code that supports shared libraries via the library ID method, but assumes this library or executable won't link against any other ID shared libraries. That lets the compiler use faster code for jumps and calls. ⚡-mno-leaf-id-shared-library— do not assume that the code being compiled won't link against any ID shared libraries. Slower code is generated for jump and call insns. 💀-mshared-library-id=n— specifies the identification number of the ID-based shared library being compiled. Specifying a value of0generates more compact code; specifying other values forces the allocation of that number to the current library, but it's no more space- or time-efficient than just omitting this option. So like,0or nothing, pick your fighter. 💅-msep-data— generate code that allows the data segment to be located in a different area of memory from the text segment. This allows for execute in place in an environment without virtual memory management by eliminating relocations against the text section. ✨-mno-sep-data— generate code that assumes that the data segment follows the text segment. This is the default.-mlong-calls,-mno-long-calls— tells the compiler to perform function calls by first loading the address of the function into a register and then performing a subroutine call on this register. This switch is needed if the target function lies outside of the 24-bit addressing range of the offset-based version of subroutine call instruction.
This feature is not enabled by default. Specifying -mno-long-calls restores the default behavior. Note that these switches have no effect on how the compiler generates code to handle function calls via function pointers. 🙈
-mfast-fp— link with the fast floating-point library. This library relaxes some of the IEEE floating-point standard's rules for checking inputs against Not-a-Number (NAN), in the interest of performance. Speed over strictness, the eternal tradeoff. 🔥-minline-plt— enable inlining of PLT entries in function calls to functions that are not known to bind locally. It has no effect without-mfdpic. 🫠-mmulticore— build a standalone application for multicore Blackfin processors. This option causes proper start files and link scripts supporting multicore to be used, and defines the macro__BFIN_MULTICORE. It can only be used with-mcpu=bf561[-sirevision]. 👑
This option can be used with -mcorea or -mcoreb, which selects the one-application-per-core programming model. Without -mcorea or -mcoreb, the single-application/dual-core programming model is used. In this model, the main function of Core B should be named as coreb_main.
If this option is not used, the single-core application programming model is used.
-mcorea— build a standalone application for Core A of BF561 when using the one-application-per-core programming model. Proper start files and link scripts are used to support Core A, and the macro__BFIN_COREAis defined. This option can only be used in conjunction with-mmulticore. 🅰️-mcoreb— build a standalone application for Core B of BF561 when using the one-application-per-core programming model. Proper start files and link scripts are used to support Core B, and the macro__BFIN_COREBis defined. When this option is used,coreb_mainshould be used instead ofmain. This option can only be used in conjunction with-mmulticore. 🅱️-msdram— build a standalone application for SDRAM. Proper start files and link scripts are used to put the application into SDRAM, and the macro__BFIN_SDRAMis defined. The loader should initialize SDRAM before loading the application. 🌈-micplb— assume that ICPLBs are enabled at run time. This has an effect on certain anomaly workarounds. For Linux targets, the default is to assume ICPLBs are enabled; for standalone applications the default is off. 🧚
✨ C6X Options ✨
-march=name— this specifies the name of the target architecture. GCC uses this name to figure out what kind of instructions it can emit when generating assembly code. Permissible names are: c62x, c64x, c64x+, c67x, c67x+, c674x. Pick your fave. 💖-mbig-endian— generate code for a big-endian target.-mlittle-endian— generate code for a little-endian target. This is the default.-msim— choose startup files and linker script suitable for the simulator. 🎮-msdata=default— put small global and static data in the.neardatasection, which is pointed to by registerB14. Put small uninitialized global and static data in the.bsssection, which is adjacent to the.neardatasection. Put small read-only data into the.rodatasection. The corresponding sections used for large pieces of data are.fardata,.farand.const. 🗂️-msdata=all— put all data, not just small objects, into the sections reserved for small data, and use addressing relative to theB14register to access them. Go big or go home. 🔥-msdata=none— make no use of the sections reserved for small data, and use absolute addresses to access all data. Put all initialized global and static data in the.fardatasection, and all uninitialized data in the.farsection. Put all constant data into the.constsection. 🎀
💜 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 arev3,v8andv10for respectively ETRAX 4, ETRAX 100, and ETRAX 100 LX. Default isv0. ✨-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. Watch out bestie, that's your early warning system. 🚨-metrax4— bestie this is literally just a synonym for-march=v3. Same vibe, different name. ✨-metrax100— and this one is a synonym for-march=v8. That's it, that's the whole thing. 💫-mmul-bug-workaround,-mno-mul-bug-workaround— works around a bug in themulsandmuluinstructions on the CPU models where that bug actually applies. Disabled by default, so don't come crying if you assumed it was on. 😭-mpdebug— turns on CRIS-specific verbose debug-related info in the assembly code. Also, fun side quest: it turns off the#NO_APPformatted-code indicator that gets handed to the assembler at the beginning of the assembly file. 🔍-mcc-init— do not use condition-code results left over from a previous instruction. Instead, always emit compare and test instructions before anything touches the condition codes. Fresh codes only, no hand-me-downs. 💅-mno-side-effects— do not emit instructions with side effects in addressing modes other than post-increment. Periodt. 🚫-mstack-align,-mno-stack-align,-mdata-align,-mno-data-align,-mconst-align,-mno-const-align— these options (and theirno-versions) arrange — or, in theno-case, eliminate the arrangements for — the stack frame, individual data and constants to be aligned for the maximum single data access size of whichever CPU model you picked. The default is to arrange for 32-bit alignment. ABI details like structure layout are not affected by any of these, so breathe. 🫶-m32-bit,-m16-bit,-m8-bit— cousins of the stack-/data-/const-align options above. These arrange for the stack frame, writable data and constants to all be 32-bit, 16-bit or 8-bit aligned. The default is 32-bit alignment. ⭐-mno-prologue-epilogue,-mprologue-epilogue— with-mno-prologue-epilogue, the normal function prologue and epilogue that set up the stack frame get omitted, and no return instructions or return sequences are generated in the code at all.
🚨 bestie, use this one only while visually inspecting the compiled code with your own two eyes: no warnings, no errors, nothing gets generated when call-saved registers must be saved or when storage for local variables needs to be allocated. The compiler will not warn you. It will simply do the thing. 💀
-melf— legacy no-op option. It does nothing, it has never done anything, it is purely decorative. Leave it alone. 🎀-sim— arranges for linking against input-output functions from a simulator library. Code, initialized data and zero-initialized data all get allocated consecutively. 🧚-sim2— like-sim, but it passes linker options to locate initialized data at0x40000000and zero-initialized data at0x80000000. Big address energy. 🔥
🌈 C-SKY Options 🌈
GCC has these options when you're compiling for C-SKY V2 processors. Let's get into it. 👑
-march=arch — pick the C-SKY target architecture. The valid values for arch are:ck801,ck802,ck803,ck807, andck810. The default isck810. 👑-mcpu=cpu — pick the C-SKY target processor. The valid values for cpu are:ck801,ck801t,ck802,ck802t,ck802j,ck803,ck803h,ck803t,ck803ht,ck803f,ck803fh,ck803e,ck803eh,ck803et,ck803eht,ck803ef,ck803efh,ck803ft,ck803eft,ck803efht,ck803r1,ck803hr1,ck803tr1,ck803htr1,ck803fr1,ck803fhr1,ck803er1,ck803ehr1,ck803etr1,ck803ehtr1,ck803efr1,ck803efhr1,ck803ftr1,ck803eftr1,ck803efhtr1,ck803s,ck803st,ck803se,ck803sf,ck803sef,ck803seft,ck807e,ck807ef,ck807,ck807f,ck810e,ck810et,ck810ef,ck810eft,ck810,ck810v,ck810f,ck810t,ck810fv,ck810tv,ck810ft, andck810ftv.
Yeah. It's a lot. It's giving ✨*extensive menu*✨. Pick your fighter carefully. 💅
-mbig-endian,-EB,-mlittle-endian,-EL— select big- or little-endian code. The default is little-endian. 🦋-mfloat-abi=name — specifies which floating-point ABI to use. Permissible values are:soft,softfpandhard.softmakes GCC generate output containing library calls for floating-point operations.softfpallows code to be generated using hardware floating-point instructions, but still uses the soft-float calling conventions.hardallows generation of floating-point instructions and uses FPU-specific calling conventions.
The default depends on the specific target configuration. And heads up, bestie: the hard-float and soft-float ABIs are not link-compatible — you must compile your entire program with the same ABI, and link with a compatible set of libraries. No mixing. Not even a little. 🚫💖
-mhard-float,-msoft-float— select hardware or software floating-point implementations. The default is soft float. 🫧-mdouble-float,-mno-double-float— when-mhard-floatis in effect, enables generation of double-precision float instructions. This is the default except when compiling forCK803. 😤-mfdivdu,-mno-fdivdu— when-mhard-floatis in effect, enables generation offrecipd,fsqrtd, andfdivdinstructions. This is the default except when compiling forCK803. (CK803 really said "not me" to all the fancy float stuff. 🙈)-mfpu=fpu — select the floating-point processor. This option can only be used with-mhard-float. Values for fpu arefpv2_sf(equivalent to-mno-double-float -mno-fdivdu),fpv2(-mdouble-float -mno-divdu), andfpv2_divd(-mdouble-float -mdivdu). 👑-melrw,-mno-elrw— enable the extendedlrwinstruction. This option defaults to on forCK801and off otherwise. 🎀-mistack,-mno-istack— enable interrupt stack instructions; the default is off.
The -mistack option is required to handle the interrupt and isr function attributes. So if you're doing interrupt stuff, you need this. No cap. 🔔
-mmp— enable multiprocessor instructions; the default is off. 🧩-mcp— enable coprocessor instructions; the default is off. ⚙️-mcache— enable coprocessor instructions; this option defaults to off. (Yes, it says "coprocessor" again. That's what the page says, bestie, I'm just the messenger. 🫠)-msecurity— enable C-SKY security instructions; this option defaults to off. 🔐-mtrust— enable C-SKY trust instructions; this option defaults to off. 🤝-mdsp,-medsp,-mvdsp— enable C-SKY DSP, Enhanced DSP, or Vector DSP instructions, respectively. All of these options default to off. 🎛️-mdiv,-mno-div— generate divide instructions. Default is off. ➗-msmart,-mno-smart— generate code for Smart Mode, using only registers numbered0-7so that 16-bit instructions can be used. This option is ignored forCK801, where this is the required behavior, and it defaults to on forCK802. For other targets, the default is off. 🧠✨-mhigh-registers,-mno-high-registers— generate code using the high registers numbered16-31. This option is not supported onCK801,CK802, orCK803, and is enabled by default for other processors. 👑-manchor,-mno-anchor— generate code using global anchor symbol addresses. ⚓-mpushpop,-mno-pushpop— generate code usingpushandpopinstructions. This option defaults to on. 🫸🫷-mmultiple-stld,-mstm,-mno-multiple-stld,-mno-stm— generate code usingstmandldminstructions. This option isn't supported onCK801, but it is enabled by default on other processors. 📦-mconstpool,-mno-constpool— create constant pools in the compiler instead of deferring that job to the assembler. This option is the default and required for correct code generation onCK801andCK802, and it's optional on other processors. So like, on those two chips, don't turn it off. Fr fr. 🚨-mstack-size,-mno-stack-size— emit.stack_sizedirectives for each function in the assembly output. This option defaults to off. 📏-mccrt,-mno-ccrt— generate code for the C-SKY compiler runtime instead oflibgcc. This option defaults to off. 🧵-mbranch-cost=n — set the branch costs to roughly n instructions. The default is1. 🌿-msched-prolog,-mno-sched-prolog— permit scheduling of function prologue and epilogue sequences.
⚠️ bestie be careful: using this option can result in code that is not compliant with the C-SKY V2 ABI prologue requirements and that cannot be debugged or backtraced. It is disabled by default. This is a "you asked for it" situation. 💀
-msim— links the librarylibsemi.awhich is in compatible with simulator. Applicable to ELF compiler only. 🖥️
🎀 Cygwin and MinGW Options 🎀
These additional options are available for Microsoft Windows targets. Windows besties, this one's for you. 🪟💖
-mconsole— specifies that a console application is to be generated, by instructing the linker to set the PE header subsystem type required for console applications. This option is available for Cygwin and MinGW targets and is enabled by default on those targets. 🖤-mcrtdll=library — preprocess, compile or link with the specified C RunTime DLL library. This option adjusts the predefined macros__CRTDLL__,__MSVCRT__,_UCRTand__MSVCRT_VERSION__for the specified CRT library, chooses the start file for that CRT library, and links with that CRT library. The recognized CRT library names for the preprocessor are:crtdll*,msvcrt10*,msvcrt20*,msvcrt40*,msvcr40*,msvcrtd*,msvcrt-os*,msvcr70*,msvcr71*,msvcr80*,msvcr90*,msvcr100*,msvcr110*,msvcr120*anducrt*.
If this option is not specified, then the default MinGW import library msvcrt is used for linking and no other adjustment for the preprocessor is done. The MinGW import library msvcrt is just a symlink to (or a copy of) another MinGW CRT import library chosen during MinGW compilation. The MinGW import library msvcrt-os is for the Windows system CRT DLL library msvcrt.dll and in most cases is the default MinGW import library.
Generally speaking, changing the CRT DLL requires recompiling the entire MinGW CRT.
🙈 This option is for experimental and testing purposes only. This option is available for MinGW targets.
-mdll— this option is available for Cygwin and MinGW targets. It specifies that a DLL — a dynamic link library — is to be generated, enabling the selection of the required runtime startup object and entry point. 🧱-mnop-fun-dllimport— this option is available for Cygwin and MinGW targets. It specifies that thedllimportattribute should be ignored. 🙅♀️-mthreads— this option is available for MinGW targets. It specifies that MinGW-specific thread support is to be used. 🧵✨-municode— this option is available for MinGW-w64 targets. It causes theUNICODEpreprocessor macro to be predefined, and chooses Unicode-capable runtime startup code. 🌍-mwin32— this option is available for Cygwin and MinGW targets. It specifies that the typical Microsoft Windows predefined macros are to be set in the pre-processor, but it does not influence the choice of runtime library/startup code. 🪟-mwindows— this option is available for Cygwin and MinGW targets. It specifies that a GUI application is to be generated by instructing the linker to set the PE header subsystem type appropriately. 🖼️💅-fno-set-stack-executable— this one's for MinGW targets only, bestie. It says: don't set the executable flag on the stack that nested functions use. Why would you ever want that? Because binaries running in kernel mode on Microsoft Windows can't use the User32 API (which is literally the thing that sets executable privileges), so there's nothing there to do the job. It's giving "we have no choice" energy. 💅-fwritable-relocated-rdata— available for MinGW and Cygwin targets. It says relocated-data that lives in a read-only section gets yeeted into the.datasection instead. Needed for older runtimes that can't handle modifying.rdatasections for pseudo-relocation. Legacy runtime support, we love to see it 🫶-mpe-aligned-commons— for Cygwin and MinGW targets. This says use the GNU extension to the PE file format that lets COMMON variables get aligned properly when generating code. It's ON by default if GCC figures out that the target assembler it found during configuration supports the feature. Auto-detected, iconic. ✨
See also under x86 Options for standard options.
🍎 Darwin Options 🍎
These options are defined for all architectures running the Darwin operating system.
FSF GCC on Darwin does not create "fat" object files — it creates an object file for the single architecture that GCC was built to target. Apple's GCC on Darwin does create "fat" files if you use multiple -arch options, and it does that by running the compiler or linker multiple times and joining the results together with lipo. Teamwork, honestly. 💖
The subtype of the file created (like ppc7400 or ppc970 or i686) is decided by the flags that specify the ISA GCC is targeting, like -mcpu or -march. The -force_cpusubtype_ALL option can override that.
The Darwin tools all behave differently when you hand them an ISA mismatch, and ngl it's messy:
- The assembler,
as, only lets you use instructions that are valid for the subtype of the file it's generating, so you cannot put 64-bit instructions in appc750object file. - The linker for shared libraries,
/usr/bin/libtool, fails and prints an error if you ask it to create a shared library with a less restrictive subtype than its input files (like trying to put appc970object file in appc7400library). - The linker for executables,
ld, quietly gives the executable the most restrictive subtype of any of its input files. Silent but deadly 💀 -Fdir— add the framework directory dir to the head of the list of directories searched for header files. These directories are interleaved with the ones you specified with-Ioptions and are scanned in a left-to-right order.
Okay so 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 minus the .framework. Headers associated with the framework live in one of those two directories, with Headers being searched first. A subframework is a framework directory that's inside 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 gets issued if you violate that. Currently a subframework cannot have subframeworks; in the future, the mechanism may be extended to support that. The standard frameworks can be found in /System/Library/Frameworks and /Library/Frameworks. An example include looks like "#include <Framework/header.h>", where Framework denotes the name of the framework and header.h is found in the PrivateHeaders or Headers directory.
-iframeworkdir— like-Fexcept the directory is treated as a system directory. The main difference between-iframeworkand-Fis that with-iframeworkthe compiler doesn't warn about constructs contained within header files found via dir. This option is valid only for the C family of languages.-gused— emit debugging information for symbols that are used. For the stabs debugging format, this enables-feliminate-unused-debug-symbols. It's ON by default.-gfull— emit debugging information for all symbols and types.-fconstant-cfstrings— this is an alias for-mconstant-cfstrings. Same girl, different name 💅-mconstant-cfstrings— when the NeXT runtime is being used (that's the default on these systems), this overrides any-fconstant-string-classsetting and makes@"..."literals get laid out as constant CoreFoundation strings.-mmacosx-version-min=version— the earliest version of MacOS X that this executable will run on is version. Typical values supported for version include12,10.12, and10.5.8.
If the compiler was built to use the system's headers by default, then the default for this option is the system version the compiler is running on; otherwise the default is to make choices compatible with as many systems and code bases as possible.
-mkernel— enable kernel development mode. The-mkerneloption sets-static,-fno-common,-fno-use-cxa-atexit,-fno-exceptions,-fno-non-call-exceptions,-fapple-kext,-fno-weakand-fno-rttiwhere applicable. This mode also sets-mno-altivec,-msoft-float,-fno-builtinand-mlong-branchfor PowerPC targets. It's a whole bundle, it's a package deal, it's a vibe. 🔥-mone-byte-bool— override the defaults forboolso thatsizeof(bool)==1. By defaultsizeof(bool)is4when compiling for Darwin/PowerPC and1when compiling for Darwin/x86, so this option has no effect on x86. Bestie, if you're on x86 you can literally skip this one.
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. Not me casually breaking ABI 💀
-mfix-and-continue-ffix-and-continue-findirect-data— generate code suitable for fast turnaround development, like letting GDB dynamically load.ofiles into already-running programs.-findirect-dataand-ffix-and-continueare provided for backwards compatibility.-all_load— loads all members of static archive libraries. Seeman ld(1)for more information.-arch_errors_fatal— cause errors about 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. Seeman ld(1)for more information.-bundle_loader executable— this option specifies the executable that will load the build output file being linked. Seeman ld(1)for more information.-dynamiclib— when you pass this option, GCC produces a dynamic library instead of an executable when linking, using the Darwinlibtoolcommand.-force_cpusubtype_ALL— this causes GCC's output file to have theALLsubtype, instead of one controlled by the-mcpuor-marchoption.-nodefaultrpaths— do not add default run paths for the compiler library directories to executables, modules or dynamic libraries. On macOS 10.5 and later, the embedded runpath is added by default unless the user adds-nodefaultrpathsto the link line. Run paths are needed (and therefore enforced) to build on macOS version 10.11 or later.-allowable_clientclient_name-client_name-compatibility_version-current_version-dead_strip-dependency-file-dylib_file-dylinker_install_name-dynamic-exported_symbols_list-filelist-flat_namespace-force_flat_namespace-headerpad_max_install_names-image_base-init-install_name-keep_private_externs-multi_module-multiply_defined-multiply_defined_unused-noall_load-no_dead_strip_inits_and_terms-nofixprebinding-nomultidefs-noprebind-noseglinkedit-pagezero_size-prebind-prebind_all_twolevel_modules-private_bundle-read_only_relocs-sectalign-sectobjectsymbols-whyload-seg1addr-sectcreate-sectobjectsymbols-sectorder-segaddr-segs_read_only_addr-segs_read_write_addr-seg_addr_table-seg_addr_table_filename-seglinkedit-segprot-segs_read_only_addr-segs_read_write_addr-single_module-static-sub_library-sub_umbrella-twolevel_namespace-umbrella-undefined-unexported_symbols_list-weak_reference_mismatches-whatsloaded— these options are passed to the Darwin linker. The Darwin linker man page describes them in detail. Not me gatekeeping the whole list, go read that one too 🎀
💜 DEC Alpha Options 💜
These -m options are defined for the DEC Alpha implementations:
-mno-soft-float-msoft-float— use (or don't use) the hardware floating-point instructions for floating-point operations. When-msoft-floatis specified, functions inlibgcc.aare used to perform floating-point operations. Unless they're replaced by routines that emulate the floating-point operations, or compiled in such a way as to call such emulation routines, these routines issue floating-point operations. If you're compiling for an Alpha without floating-point operations, you must make sure 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. Yeah, make it make sense 🫠
-mfp-reg-mno-fp-regs— generate code that uses (or does not use) the floating-point register set.-mno-fp-regsimplies-msoft-float. If the floating-point register set isn't used, floating-point operands are passed in integer registers as if they were integers, and floating-point results are passed in$0instead of$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-regsmust also be compiled with that option. Match or crash, fr fr.
A typical use of this option is building a kernel that doesn't use — and therefore doesn't need to save and restore — any floating-point registers. Kernel girlies, this one's for you 👑
-mieee— okay so the Alpha architecture has floating-point hardware that's built for maximum performance, and it's mostly IEEE-compliant, but for full compliance it needs a lil software assist 🫶. This option makes GCC generate code that's fully IEEE-compliant, except the inexact-flag isn't maintained (more on that below, bestie). When you turn this on, the preprocessor macro_IEEE_FPgets defined during compilation. The code it spits out is less efficient, but it correctly supports denormalized numbers and those exceptional IEEE values like not-a-number and plus/minus infinity. Other Alpha compilers call this one-ieee_with_no_inexact.-mieee-with-inexact— literally-mieeebut the generated code also maintains the IEEE inexact-flag ✨. Turning this on makes the generated code implement fully-compliant IEEE math. Besides_IEEE_FP, the macro_IEEE_FP_EXACTis also defined. Heads up tho: on some Alpha implementations the resulting code may run significantly slower than the default code 😭. And since very little code actually depends on the inexact-flag, you should normally not specify this option. Other Alpha compilers call this-ieee_with_inexact.- *
-mfp-trap-mode=trap-mode* — this one controls which floating-point related traps are enabled. Other Alpha compilers call it-fptmtrap-mode. The trap mode can be set to one of four values:n— the default (normal) setting. The only traps enabled are the ones that can't be disabled in software (like, division by zero trap).u— on top of the traps enabled byn, underflow traps are enabled too.su— likeu, but the instructions are marked to be safe for software completion (peep the Alpha architecture manual for the deets).sui— likesu, but inexact traps are enabled as well.
- *
-mfp-rounding-mode=rounding-mode* — picks the IEEE rounding mode. Other Alpha compilers call this-fprmrounding-mode. The rounding-mode can be one of:n— normal IEEE rounding mode. Floating-point numbers get 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 get rounded towards zero.d— dynamic rounding mode. A field in the floating-point control register (fpcr, see the Alpha architecture reference manual) controls the rounding mode in effect. The C library initializes this register for rounding towards plus infinity. So unless your program modifies thefpcr,dcorresponds to round towards plus infinity.
- *
-mtrap-precision=trap-precision* — so in the Alpha architecture, floating-point traps are imprecise 😬. That means without software assistance it's impossible to recover from a floating trap, and program execution normally needs to be terminated. GCC can generate code that assists operating system trap handlers in figuring out the exact location that caused a floating-point trap. Depending on what your application needs, you can pick different levels of precision: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 equivalent options called -scope_safe and -resumption_safe.
-mieee-conformant— this marks the generated code as IEEE conformant. 🚨 You must not use this option unless you also specify-mtrap-precision=iand either-mfp-trap-mode=suor-mfp-trap-mode=sui. Its only effect is to emit the line.eflag 48in the function prologue of the generated assembly file.-mbuild-constants— normally GCC looks at a 32- or 64-bit integer constant to see if it can construct it from smaller constants in two or three instructions. If it can't, it outputs the constant as a literal and generates code to load it from the data segment at run time. Use this option to require GCC to construct all integer constants using code, even if it takes more instructions (the max is six). You typically use this one to build a shared library dynamic loader. Itself a shared library, it has to relocate itself in memory before it can find the variables and constants in its own data segment. iykyk 💅-mbwx,-mno-bwx,-mcix,-mno-cix,-mfix,-mno-fix,-mmax,-mno-max— these 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 the-mcpu=option, or that of the CPU on which GCC was built if none is specified.-msafe-bwa,-mno-safe-bwa— indicate whether, in the absence of the optional BWX instruction set, GCC should generate multi-thread and async-signal safe code for byte and aligned word memory accesses.-msafe-partial,-mno-safe-partial— indicate whether GCC should generate multi-thread and async-signal safe code for partial memory accesses, including piecemeal accesses to unaligned data as well as block accesses to leading and trailing parts of aggregate types or other objects in memory that don't respectively start and end on an aligned 64-bit data boundary.-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 gave you no way to generate symbol relocations except via assembler macros. Using those macros doesn't allow optimal instruction scheduling 😔. GNU binutils as of version 2.12 supports a new syntax that lets the compiler explicitly mark which relocations should apply to which instructions. This option is mostly useful for debugging, since GCC detects the capabilities of the assembler when it's built and sets the default accordingly.-msmall-data,-mlarge-data— when-mexplicit-relocsis in effect, static data is accessed via gp-relative relocations. When-msmall-datais used, objects 8 bytes long or smaller get placed in a small data area (the.sdataand.sbsssections) and are accessed via 16-bit relocations off of the$gpregister. This limits the size of the small data area to 64KB, but it lets the variables be directly accessed via a single instruction. ✨
The default is -mlarge-data. With this option the data area is limited to just below 2GB. Programs that need more than 2GB of data must use malloc or mmap to allocate the data in the heap instead of in the program's data segment.
When generating code for shared libraries, -fpic implies -msmall-data and -fPIC implies -mlarge-data.
-msmall-text,-mlarge-text— when-msmall-textis used, the compiler assumes that the code of the entire program (or shared library) fits in 4MB, and is thus reachable with a branch instruction. When-msmall-datais used, the compiler can assume that all local symbols share the same$gpvalue, and thus reduce the number of instructions required for a function call from 4 to 1. The default is-mlarge-text.- *
-mcpu=cpu_type* — sets the instruction set and instruction scheduling parameters for machine type cpu_type. You can specify either the EV style name or the corresponding chip number. GCC supports scheduling parameters for the EV4, EV5 and EV6 family of processors and chooses the default values for the instruction set from the processor you specify. If you don't specify a processor type, GCC defaults to the processor on which the compiler was built.
Supported values for cpu_type are:
ev4,ev45,21064— schedules as an EV4 and has no instruction set extensions.ev5,21164— schedules as an EV5 and has no instruction set extensions.ev56,21164a— schedules as an EV5 and supports the BWX extension.pca56,21164pc,21164PC— schedules as an EV5 and supports the BWX and MAX extensions.ev6,21264— schedules as an EV6 and supports the BWX, FIX, and MAX extensions.ev67,21264a— schedules as an EV6 and supports the BWX, CIX, FIX, and MAX extensions.
Native toolchains also support the value native, which selects the best architecture option for the host processor. -mcpu=native has no effect if GCC doesn't recognize the processor.
-mtune=cpu_type — sets *only the instruction scheduling parameters for machine type *cpu_type. The instruction set is not changed, bestie.
Native toolchains also support the value native, which selects the best architecture option for the host processor. -mtune=native has no effect if GCC doesn't recognize the processor.
- *
-mmemory-latency=time* — sets the latency the scheduler should assume for typical memory references as seen by the application. This number is highly dependent on the memory access patterns used by the application and the size of the external cache on the machine.
Valid options for time are:
- number — a decimal number representing clock cycles.
L1,L2,L3,main— the compiler contains estimates of the number of clock cycles for "typical" EV4 & EV5 hardware for the Level 1, 2 & 3 caches (also called Dcache, Scache, and Bcache), as well as to main memory. Note thatL3is only valid for EV5.
🦋 eBPF Options 🦋
- *
-mframe-limit=bytes* — this specifies the hard limit for frame sizes, in bytes. Currently, the value you can specify should be less than or equal to 32767. Defaults to whatever limit is imposed by the version of the Linux kernel targeted. -mbig-endian— generate code for a big-endian target.-mlittle-endian— generate code for a little-endian target. This is the default. 💅-mjmpext,-mno-jmpext— turn on / turn off generating those extra conditional-branch instructions. It's on for CPU v2 and above. 💫-mjmp32,-mno-jmp32— turn on / turn off generating 32-bit jump instructions. On for CPU v3 and above.-malu32,-mno-alu32— turn on / turn off generating 32-bit ALU instructions. On for CPU v3 and above.-mv3-atomics,-mno-v3-atomics— turn on / turn off the instructions for general atomic operations that got introduced in CPU v3. On for CPU v3 and above.-mbswap,-mno-bswap— turn on / turn off byte swap instructions. On for CPU v4 and above.-msdiv,-mno-sdiv— turn on / turn off signed division and modulus instructions. On for CPU v4 and above.-msmov,-mno-smov— turn on / turn off sign-extending move and memory load instructions. On for CPU v4 and above.-mcpu=version— bestie this one picks which version of the eBPF ISA you're targeting. Heads up tho: newer versions might not be supported by every kernel out there. Default isv4. 💅
The values you can put in for version are:
v1— the very first stable eBPF ISA, no special features, no extensions, just vibes.v2— supports the jump extensions, same as-mjmpext.v3— literally everything fromv2, plus:- 32-bit jump operations, as in
-mjmp32 - 32-bit ALU operations, as in
-malu32 - general atomic operations, as in
-mv3-atomics
- 32-bit jump operations, as in
v4— everything fromv3, plus:- Byte swap instructions, as in
-mbswap - Signed division and modulus instructions, as in
-msdiv - Sign-extending move and memory load instructions, as in
-msmov
- Byte swap instructions, as in
-mco-re— enables BPF Compile Once - Run Everywhere (CO-RE) support. It requires-gbtfand is also implied by it. ✨-mno-co-re— disables BPF Compile Once - Run Everywhere (CO-RE) support. FYI BPF CO-RE support is enabled by default when you're generating BTF debug information for the BPF target.-mxbpf— generates code for an expanded version of BPF, one that relaxes some of the restrictions the BPF architecture normally imposes:- Save and restore callee-saved registers at function entry and exit, respectively. 🫶
-masm=dialect— outputs assembly instructions using the eBPF selected dialect. Default ispseudoc.
The values you can put in for dialect are:
normal— outputs normal assembly dialect.pseudoc— outputs pseudo-c assembly dialect.
-minline-memops-threshold=bytes— sets a size threshold in bytes at or below whichmemmove,memcpyandmemsetare always expanded inline. Anything bigger than this threshold would have to be done with a library call instead of being inlined — but since BPF doesn't allow libcalls, going over this threshold straight up results in a compile-time error. 😭 Default is1024bytes.
🎀 FR30 Options 🎀
These ones are defined specifically for the FR30 port, no cap.
-msmall-model— uses the small address space model. It can produce smaller code, but bestie it does assume that all symbolic values and addresses fit into a 20-bit range. 👀-mno-lsim— assumes that runtime support has already been provided, so there's no need to include the simulator library (libsim.a) on the linker command line.
🌸 FT32 Options 🌸
These options are defined specifically for the FT32 port.
-msim— specifies that the program will be run on the simulator. This causes an alternate runtime startup and library to be linked. 🚨 You must not use this option when generating programs that will run on real hardware; you have to provide your own runtime library for whatever I/O functions are needed.-mlra— does literally nothing. It's preserved for backward compatibility. 💀-mnodiv— do not usedivandmodinstructions.-mft32b— enables use of the extended instructions of the FT32B processor.-mcompress— compresses all code using the Ft32B code compression scheme.-mnopm— do not generate code that reads program memory.
🦋 FRV Options 🦋
-mgpr-32— only use the first 32 general-purpose registers.-mgpr-64— use all 64 general-purpose registers. 👑-mfpr-32— use only the first 32 floating-point registers.-mfpr-64— use all 64 floating-point registers.-mhard-float— use hardware instructions for floating-point operations.-msoft-float— use library routines for floating-point operations.-malloc-cc— dynamically allocate condition code registers.-mfixed-cc— do not try to dynamically allocate condition code registers, only useicc0andfcc0.-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— selects the FDPIC ABI, which uses function descriptors to represent pointers to functions. Without any PIC/PIE-related options, it implies-fPIE. With-fpicor-fpie, it assumes GOT entries and small data are within a 12-bit range from the GOT base address; with-fPICor-fPIE, GOT offsets are computed with 32 bits. With abfin-elftarget, this option implies-msim.-minline-plt— enables inlining of PLT entries in function calls to functions that aren't known to bind locally. It has no effect without-mfdpic. It's enabled by default if you're optimizing for speed and compiling for shared libraries (i.e.,-fPICor-fpic), or when an optimization option such as-O3or above is present in the command line. ✨-mTLS— assume a large TLS segment when generating thread-local code.-mtls— do not assume a large TLS segment when generating thread-local code.-mgprel-ro— enables the use ofGPRELrelocations in the FDPIC ABI for data that's known to be in read-only sections. It's enabled by default, except for-fpicor-fpie: even though it may help make the global offset table smaller, it trades 1 instruction for 4. With-fPICor-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 isn't,-mno-gprel-rocan 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-fPICand-fpicwithout-mfdpic. You should never have to use it explicitly. 🙈-mlinked-fp— follow the EABI requirement of always creating a frame pointer whenever a stack frame is allocated. This option is enabled by default and can be disabled with-mno-linked-fp.-mlong-calls— use indirect addressing to call functions outside the current compilation unit. This allows the functions to be placed anywhere within the 32-bit address space. 🌈-malign-labels— try to align labels to an 8-byte boundary by inserting NOPs into the previous packet. This option only has an effect when VLIW packing is enabled. It doesn't create new packets; it merely adds NOPs to existing ones.-mlibrary-pic— generate position-independent EABI code.-macc-4— use only the first four media accumulator registers.-macc-8— use all eight media accumulator registers.-mpack— pack VLIW instructions.-mno-pack— do not pack VLIW instructions.-mno-eflags— do not mark ABI switches ine_flags.-mcond-move— enable the use of conditional-move instructions (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version. 👀
-mno-cond-move— disable the use of conditional-move instructions.
This switch is mainly for debugging the compiler and will likely be removed in a future version. 👀
-mscc— enable the use of conditional set instructions (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version. 👀
-mno-scc— disable the use of conditional set instructions.
This switch is mainly for debugging the compiler and will likely be removed in a future version. 👀
-mcond-exec— enable the use of conditional execution (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version. 👀
-mno-cond-exec— disable the use of conditional execution.
This switch is mainly for debugging the compiler and will likely be removed in a future version. 👀
-mvliw-branch— run a pass to pack branches into VLIW instructions (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version. 👀
-mno-vliw-branch— do not run a pass to pack branches into VLIW instructions.
This switch is mainly for debugging the compiler and will likely be removed in a future version. 👀
-mmulti-cond-exec— enable optimization of&&and||in conditional execution (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version. 👀
-mno-multi-cond-exec— disable optimization of&&and||in conditional execution.
This switch is mainly for debugging the compiler and will likely be removed in a future version. 👀
-mnested-cond-exec— enable nested conditional execution optimizations (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version. 👀
-mno-nested-cond-exec— disable nested conditional execution optimizations.
This switch is mainly for debugging the compiler and will likely be removed in a future version. 👀
-moptimize-membar— this switch removes redundantmembarinstructions from the compiler-generated code. It's enabled by default. 🔥-mno-optimize-membar— this switch disables the automatic removal of redundantmembarinstructions from the generated code.-mtomcat-stats— cause gas to print out tomcat statistics. 🐱-mcpu=cpu— picks the processor type you're generating code for, bestie. The possible values arefrv,fr550,tomcat,fr500,fr450,fr405,fr400,fr300andsimple.
🌈 GNU/Linux Options 🌈
These -m options are the ones defined for GNU/Linux targets, and honestly they're giving "which libc are you loyal to" energy:
-mglibc— use the GNU C library. This is the default except on*-*-linux-*uclibc*,*-*-linux-*musl*and*-*-linux-*android*targets.-muclibc— use uClibc C library. Default on*-*-linux-*uclibc*targets. slay.-mmusl— use the musl C library. Default on*-*-linux-*musl*targets. 🧚-mbionic— use Bionic C library. Default on*-*-linux-*android*targets.-mandroid— compile code that's compatible with the Android platform. This is the default on*-*-linux-*android*targets.
When compiling, this option enables -mbionic, -fPIC, -fno-exceptions and -fno-rtti by default. When linking, this option makes the GCC driver pass Android-specific options to the linker. And finally, this option causes the preprocessor macro __ANDROID__ to be defined. three-for-one deal, we love to see it 💅
-tno-android-cc— turns off the compilation effects of-mandroid, i.e. it does not enable-mbionic,-fPIC,-fno-exceptionsand-fno-rttiby default.-tno-android-ld— turns off the linking effects of-mandroid, i.e. it passes standard Linux linking options to the linker.
🐝 H8/300 Options 🐝
These -m options are defined for the H8/300 implementations:
-mrelax— shorten some address references at link time when possible; uses the linker option-relax.-mh— generate code for the H8/300H.-ms— generate code for the H8S.-mn— generate code for the H8S and H8/300H in the normal mode. This switch must be used either with-mhor-ms.-ms2600— generate code for the H8S/2600. This switch must be used with-ms.-mexr— extended registers get stored on the stack before execution of a function with monitor attribute. Default option is-mexr. This option is valid only for H8S targets.-mno-exr— extended registers are not stored on the stack before execution of a function with monitor attribute. Default option is-mno-exr. This option is valid only for H8S targets.-mint32— makesintdata 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-300causes 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, and there are a lot of them, so buckle up buttercup:
-march=architecture-type— generate code for the specified architecture. The choices for architecture-type are1.0for PA 1.0,1.1for PA 1.1, and2.0for PA 2.0 processors. Go peek at/usr/lib/sched.modelson an HP-UX system to figure out the proper architecture option for your machine. Code compiled for lower numbered architectures runs on higher numbered architectures, but not the other way around. it's giving "you can go up but you can't come back down" 😭-mpa-risc-1-0-mpa-risc-1-1-mpa-risc-2-0— synonyms for-march=1.0,-march=1.1, and-march=2.0respectively.-matomic-libcalls— generate libcalls for atomic loads and stores when sync libcalls are disabled. This option is enabled by default. It only affects the generation of atomic libcalls by the HPPA backend.
Both the sync and libatomic libcall implementations use locking. As a result, processor stores are not atomic with respect to other atomic operations. Processor loads up to DImode are atomic with respect to other atomic operations, provided they are implemented as a single access.
The PA-RISC architecture does not support any atomic operations in hardware except for the ldcw instruction. Thus, all atomic support is implemented using sync and atomic libcalls. Sync libcall support is in libgcc.a. Atomic libcall support is in libatomic.
This option generates __atomic_exchange calls for atomic stores. It also provides special handling for atomic DImode accesses on 32-bit targets. no cap.
-mbig-switch— does nothing. Preserved for backward compatibility. it's just vibing here fr fr.-mcaller-copies— the caller copies function arguments passed by hidden reference. Use this option with care, bestie, because it is not compatible with the default 32-bit runtime. However, only aggregates larger than eight bytes are passed by hidden reference, and the option provides better compatibility with OpenMP.-mcoherent-ldcw— useldcw/ldcdcoherent cache-control hint.-mdisable-fpregs— disable floating-point registers. Equivalent to-msoft-float.-mdisable-indexing— stops the compiler from using indexing address modes. This avoids some rather obscure problems when compiling MIG generated code under MACH.-mfast-indirect-calls— generate code that assumes calls never cross space boundaries. This lets GCC emit code that performs faster indirect calls.
This option does not work in the presence of shared libraries or nested functions. 💀
-mfixed-range=register-range— generate code treating the given register range as fixed registers. A fixed register is one that the register allocator cannot use. Super 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.-mgas— enable the use of assembler directives only GAS understands.-mgnu-ld— use options specific to GNU ld. This passes-sharedto ld when building a shared library. It is the default when GCC is configured, explicitly or implicitly, with the GNU linker. This option does not affect which ld is called; it only changes what parameters are passed to that ld. The ld that is called is determined by the--with-ldconfigure option, GCC's program search path, and finally by the user'sPATH. The linker used by GCC can be printed using whichgcc -print-prog-name=ld. This option is only available on the 64-bit HP-UX GCC, i.e. configured withhppa*64*-*-hpux*.-mhp-ld— use options specific to HP ld. This passes-bto ld when building a shared library and passes+Accept TypeMismatchto ld on all links. It is the default when GCC is configured, explicitly or implicitly, with the HP linker. This option does not affect which ld is called; it only changes what parameters are passed to that ld. The ld that is called is determined by the--with-ldconfigure option, GCC's program search path, and finally by the user'sPATH. The linker used by GCC can be printed using whichgcc -print-prog-name=ld. This option is only available on the 64-bit HP-UX GCC, i.e. configured withhppa*64*-*-hpux*.-mlinker-opt— enable the optimization pass in the HP-UX linker. Heads up though: this makes symbolic debugging impossible. 😭-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 are7,600,000and240,000bytes, respectively for the PA 2.0 and PA 1.X architectures. Sibcalls are always limited at240,000bytes.
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, since it degrades performance. However, it may be useful in large applications, particularly when partial linking is used to build the application.
The types of long calls used depends on the capabilities of the assembler and linker, and the type of code being generated. The impact on systems that support long absolute calls, and long pic symbol-difference or pc-relative calls should be relatively small. However, an indirect call is used on 32-bit ELF systems in pic code, and it is quite long.
-mlong-load-store— generate 3-instruction load and store sequences, as sometimes required by the HP-UX 10 linker. This is equivalent to the+koption to the HP compilers.-mjump-in-delay— this option is ignored and provided for compatibility purposes only. yeah she's just there for the plot 🫶-mno-space-regs— generate code that assumes the target has no space registers. This lets GCC generate faster indirect calls and use unscaled index address modes.
Such code is suitable for level 0 PA systems and kernels.
-mordered— assume memory references are ordered and barriers are not needed.-mportable-runtime— use the portable calling conventions proposed by HP for ELF systems.-mschedule=cpu-type— schedule code according to the constraints for the machine type cpu-type. The choices for cpu-type are7007100,7100LC,7200,7300and8000. Go look at/usr/lib/sched.modelson an HP-UX system to determine the proper scheduling option for your machine. The default scheduling is8000.-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.-msoft-float— generate output containing library calls for floating point. Warning: the requisite libraries are not available for all HPPA targets. Normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation.
-msoft-float changes the calling convention in the output file; therefore, it is only useful if you compile all of a program with this option. In particular, you need to compile libgcc.a, the library that comes with GCC, with -msoft-float in order for this to work. periodt.
-msoft-mult— use software integer multiplication.
This disables the use of the xmpyu instruction.
-munix=unix-std— bestie, this one makes the compiler spit out the predefines for a specific UNIX standard and pick a startfile to match. The choices for unix-std are93,95and98.93works on every HP-UX version ever.95is available on HP-UX 10.10 and later.98is available on HP-UX 11.11 and later. And the defaults? They're93for HP-UX 10.00,95for HP-UX 10.10 all the way through 11.00, and98for HP-UX 11.11 and later. 💅
So like, -munix=93 gives you the exact same predefines as GCC 3.3 and 3.4. Then -munix=95 adds extra predefines for "XOPEN_UNIX" and "_XOPEN_SOURCE_EXTENDED", plus the startfile unix95.o. And -munix=98 throws in extra predefines for "_XOPEN_UNIX", "_XOPEN_SOURCE_EXTENDED", "_INCLUDE__STDC_A1_SOURCE" and "_INCLUDE_XOPEN_SOURCE_500", plus the startfile unix98.o.
⚠️ Okay this part is important, no cap: this option changes the interfaces for various library routines. It also messes with the operational behavior of the C library. So like… extreme care is needed when you use this. It's giving loaded weapon, fr fr. 😭
Library code that's meant to work with more than one UNIX standard has to test, set and restore the variable "__xpg4_extended_mask" as appropriate. And most GNU software? Doesn't provide this capability at all. 🙈
-nolibdld— stops the generation of link options that would searchlibdld.slwhen-staticis specified on HP-UX 10 and later.-static— so here's the tea 🍓: the HP-UX implementation ofsetlocalein libc has a dependency onlibdld.sl. And there is no archive version oflibdld.sl. So when you pass-static, special link options are needed to resolve that dependency.
On HP-UX 10 and later, the GCC driver adds the necessary options to link with libdld.sl whenever -static is specified. Which means… the resulting binary ends up being dynamic. 🙃 On the 64-bit port, the linkers generate dynamic binaries by default anyway. And if you want to stop the GCC driver from adding those link options, that's literally what -nolibdld is for.
-threads— adds support for multithreading with the dce thread library under HP-UX. This one sets flags for both the preprocessor and the linker, bestie. Both. 🫶
✨ IA-64 Options ✨
These are the -m options defined for the Intel IA-64 architecture. Buckle up. 🎀
-mbig-endian— generate code for a big-endian target. This is the default for HP-UX.-mlittle-endian— generate code for a little-endian target. This is the default for AIX5 and GNU/Linux.-mgnu-as,-mno-gnu-as— generate (or don't) code for the GNU assembler. This is the default.-mgnu-ld,-mno-gnu-ld— generate (or don't) code for the GNU linker. This is the default.-mno-pic— generate code that does not use a global pointer register. The result is not position independent code, and it violates the IA-64 ABI. 👀 So like, know that going in.-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, andoutregister names for the stacked registers. This may make assembler output more readable. ✨-mno-sdata,-msdata— disable (or enable) optimizations that use the small data section. This may be useful for working around optimizer bugs. 💀-mconstant-gp— generate code that uses a single constant global pointer value. This is useful when compiling kernel code.-mauto-pic— generate code that is self-relocatable. This implies-mconstant-gp. This is useful when compiling firmware code.-minline-float-divide-min-latency— generate code for inline divides of floating-point values using the minimum latency algorithm.-minline-float-divide-max-throughput— generate code for inline divides of floating-point values using the maximum throughput algorithm.-mno-inline-float-divide— do not generate inline code for divides of floating-point values.-minline-int-divide-min-latency— generate code for inline divides of integer values using the minimum latency algorithm.-minline-int-divide-max-throughput— generate code for inline divides of integer values using the maximum throughput algorithm.-mno-inline-int-divide— do not generate inline code for divides of integer values.-minline-sqrt-min-latency— generate code for inline square roots using the minimum latency algorithm.-minline-sqrt-max-throughput— generate code for inline square roots using the maximum throughput algorithm.-mno-inline-sqrt— do not generate inline code for"sqrt".-mfused-madd,-mno-fused-madd— do (don't) generate code that uses the fused multiply/add or multiply/subtract instructions. The default is to use these instructions.-mno-dwarf2-asm,-mdwarf2-asm— don't (or do) generate assembler code for the DWARF 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. It's giving maybe 💅-mfixed-range=register-range— generate code treating the given register range as fixed registers. A fixed register is one that the register allocator cannot use. This is useful when compiling kernel code. A register range is specified as two registers separated by a dash. Multiple register ranges can be specified separated by a comma. Slay, flexible.-mtls-size=tls-size— specify bit size of immediate TLS offsets. Valid values are14,22, and64.-mtune=cpu-type— tune the instruction scheduling for a particular CPU. Valid values areitanium,itanium1,merced,itanium2, andmckinley.-milp32,-mlp64— generate code for a 32-bit or 64-bit environment. The 32-bit environment setsint,longand pointer to 32 bits. The 64-bit environment setsintto 32 bits andlongand pointer to 64 bits. These are HP-UX specific flags.-mno-sched-br-data-spec,-msched-br-data-spec— (dis/en)able data speculative scheduling before reload. This results in generation of"ld.a"instructions and the corresponding check instructions ("ld.c"/"chk.a"). The default setting is disabled.-msched-ar-data-spec,-mno-sched-ar-data-spec— (en/dis)able data speculative scheduling after reload. This results in generation of"ld.a"instructions and the corresponding check instructions ("ld.c"/"chk.a"). The default setting is enabled.-mno-sched-control-spec,-msched-control-spec— (dis/en)able control speculative scheduling. This feature is available only during region scheduling (i.e. before reload). This results in generation of the"ld.s"instructions and the corresponding check instructions"chk.s". The default setting is disabled.-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-specenabled. The default setting is enabled.-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-specenabled. The default setting is enabled.-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-specenabled. The default setting is enabled.-mno-sched-prefer-non-data-spec-insns,-msched-prefer-non-data-spec-insns— if enabled, data-speculative instructions are chosen for schedule only if there are no other choices at the moment. This makes the use of the data speculation much more conservative. The default setting is disabled.-mno-sched-prefer-non-control-spec-insns,-msched-prefer-non-control-spec-insns— if enabled, control-speculative instructions are chosen for schedule only if there are no other choices at the moment. This makes the use of the control speculation much more conservative. The default setting is disabled.-mno-sched-count-spec-in-critical-path,-msched-count-spec-in-critical-path— if enabled, speculative dependencies are considered during computation of the instructions priorities. This makes the use of the speculation a bit more conservative. The default setting is disabled.-msched-spec-ldc— use a simple data speculation check. This option is on by default.-msched-control-spec-ldc— use a simple check for control speculation. This option is on by default.-msched-stop-bits-after-every-cycle— place a stop bit after every cycle when scheduling. This option is on by default.-msched-fp-mem-deps-zero-cost— assume that floating-point stores and loads are not likely to cause a conflict when placed into the same instruction group. This option is disabled by default.-msel-sched-dont-check-control-spec— generate checks for control speculation in selective scheduling. This flag is disabled by default.-msched-max-memory-insns=max-insns— a limit on the number of memory insns per instruction group, giving lower priority to subsequent memory insns attempting to schedule in the same instruction group. Frequently useful to prevent cache bank conflicts. The default value is1.-msched-max-memory-insns-hard-limit— makes the limit specified bymsched-max-memory-insnsa hard limit, disallowing more than that number in an instruction group. Otherwise, the limit is"soft", meaning that non-memory operations are preferred when the limit is reached, but memory operations may still be scheduled. 🫶
🌸 LM32 Options 🌸
These -m options are defined for the LatticeMico32 architecture:
-mbarrel-shift-enabled— enable barrel-shift instructions.-mdivide-enabled— enable divide and modulus instructions.-mmultiply-enabled— enable multiply instructions.-msign-extend-enabled— enable sign extend instructions.-muser-enabled— enable user-defined instructions.
🦋 LoongArch Options 🦋
These command-line options are defined for LoongArch targets:
-march=arch-type— okay so this one tells GCC which machine type to generate instructions for. And bestie, listen:-march=arch-typelets GCC generate code that may not run at all on processors other than the one you indicated. It's giving "committed to the bit" 💅 so make sure that's the processor you actually have.
The choices for arch-type are:
native— local processor type, detected by the native compiler. She just knows. 🔍loongarch64— generic LoongArch 64-bit processor.la464— LoongArch LA464-based processor with LSX, LASX.la664— LoongArch LA664-based processor with LSX, LASX and all LoongArch v1.1 instructions.la64v1.0— LoongArch64 ISA version 1.0.la64v1.1— LoongArch64 ISA version 1.1.
More information about LoongArch ISA versions can be found at https://github.com/loongson/la-toolchain-conventions 🌈
-mtune=tune-type— optimizes the generated code for the given processor target. Basically "who am I writing this for" 🎯
The choices for tune-type are:
native— local processor type detected by the native compiler.generic— generic LoongArch processor.loongarch64— generic LoongArch 64-bit processor.la464— LoongArch LA464 core.la664— LoongArch LA664 core.
-mabi=base-abi-type— generates code for the specified calling convention, aka how functions pass stuff to each other 🤝 base-abi-type can be one of:lp64d— uses 64-bit general purpose registers and 32/64-bit floating-point registers for parameter passing. Data model is LP64, whereintis 32 bits, whilelong intand pointers are 64 bits.lp64f— uses 64-bit general purpose registers and 32-bit floating-point registers for parameter passing. Data model is LP64, whereintis 32 bits, whilelong intand pointers are 64 bits.lp64s— uses 64-bit general purpose registers and no floating-point registers for parameter passing. Data model is LP64, whereintis 32 bits, whilelong intand pointers are 64 bits.
-mfpu=fpu-type— generates code for the specified FPU type, which can be one of:64— allow the use of hardware floating-point instructions for 32-bit and 64-bit operations.32— allow the use of hardware floating-point instructions for 32-bit operations.none,0— prevent the use of hardware floating-point instructions. 🚫
-msimd=simd-type— turns on generation of LoongArch SIMD instructions for vectorization and via builtin functions. The value can be one of:lasx— enable generating instructions from the 256-bit LoongArch Advanced SIMD Extension (LASX) and the 128-bit LoongArch SIMD Extension (LSX).lsx— enable generating instructions from the 128-bit LoongArch SIMD Extension (LSX).none— no LoongArch SIMD instruction may be generated. not even one. 🙈
-msoft-float— forces-mfpu=noneand prevents the use of floating-point registers for parameter passing. Heads up: this option may change the target ABI 👀-msingle-float— forces-mfpu=32and allows the use of 32-bit floating-point registers for parameter passing. Also may change the target ABI.-mdouble-float— forces-mfpu=64and allows the use of 32/64-bit floating-point registers for parameter passing. Also may change the target ABI. two floats, one option 💅-mlasx,-mno-lasx,-mlsx,-mno-lsx— these incrementally adjust the scope of the SIMD extensions (none / LSX / LASX) that the compiler can use for code generation. Enabling LASX withmlasxautomatically enables LSX, and disabling LSX withmno-lsxautomatically disables LASX. These are driver-only options: they act upon the finalmsimdconfiguration state and make incremental changes in the order they appear on the GCC driver's command line, deriving the final / canonicalizedmsimdoption that gets passed to the compiler proper. order matters, bestie 📋-mbranch-cost=n— sets the cost of branches to roughly n instructions.-maddr-reg-reg-cost=n— sets the cost of ADDRESS_REG_REG to the value calculated by n.-mcheck-zero-division,-mno-check-zero-divison— trap (or do not trap) on integer division by zero 😱 The default is-mcheck-zero-divisionfor-O0or-Og, and-mno-check-zero-divisionfor other optimization levels.-mcond-move-int,-mno-cond-move-int— conditional moves for integral data in general-purpose registers are enabled (disabled). The default is-mcond-move-int.-mcond-move-float,-mno-cond-move-float— conditional moves for floating-point registers are enabled (disabled). The default is-mcond-move-float.-mmemcpy,-mno-memcpy— force (or do not force) the use ofmemcpyfor non-trivial block moves. The default is-mno-memcpy, which lets GCC inline most constant-sized copies. Setting the optimization level to-Osalso forces the use ofmemcpy, but-mno-memcpymay override this behavior if explicitly specified, regardless of the order of these options on the command line. she does what she wants 💁-mstrict-align,-mno-strict-align— avoid or allow generating memory accesses that may not be aligned on a natural object boundary as described in the architecture specification. The default is-mno-strict-align.-G num— put global and static data smaller than num bytes into a small data section. The default value is0.-mmax-inline-memcpy-size=n— inline all block moves (such as calls tomemcpyor structure copies) less than or equal to n bytes. The default value of n is1024.-mcmodel=code-model— sets the code model to one of:tiny-static(Not implemented yet)tiny(Not implemented yet)normal— the text segment must be within 128MB addressing space. The data segment must be within 2GB addressing space.medium— the text segment and data segment must be within 2GB addressing space.large(Not implemented yet)extreme— this mode does not limit the size of the code segment and data segment. The-mcmodel=extremeoption is incompatible with-fpltand/or-mexplicit-relocs=none. 🚨
The default code model is normal.
-mexplicit-relocs=style— sets when to use assembler relocation operators when dealing with symbolic addresses. The alternative is to use assembler macros instead, which may limit instruction scheduling but allow linker relaxation. With-mexplicit-relocs=nonethe assembler macros are always used, with-mexplicit-relocs=alwaysthe assembler relocation operators are always used, with-mexplicit-relocs=autothe compiler will use the relocation operators where the linker relaxation is impossible to improve the code quality, and macros elsewhere. The default value for the option is determined with the assembler capability detected during GCC build-time and the setting of-mrelax:-mexplicit-relocs=noneif the assembler does not support relocation operators at all,-mexplicit-relocs=alwaysif the assembler supports relocation operators but-mrelaxis not enabled,-mexplicit-relocs=autoif the assembler supports relocation operators and-mrelaxis enabled. very "it depends" of her ✨-mexplicit-relocs— an alias of-mexplicit-relocs=alwaysfor backward compatibility.-mno-explicit-relocs— an alias of-mexplicit-relocs=nonefor backward compatibility.-mdirect-extern-access,-mno-direct-extern-access— do not use or use GOT to access external symbols. The default is-mno-direct-extern-access: GOT is used for external symbols with default visibility, but not used for other external symbols.
With -mdirect-extern-access, GOT is not used and all external symbols are PC-relatively addressed. It is only suitable for environments where no dynamic link is performed, like firmwares, OS kernels, executables linked with -static or -static-pie. And -mdirect-extern-access is not compatible with -fPIC or -fpic. 🙅
-mrelax,-mno-relax— take (or do not take) advantage of linker relaxations. If-mpass-mrelax-to-asis enabled, this option is also passed to the assembler. The default is determined during GCC build-time by detecting corresponding assembler support:-mrelaxif the assembler supports both the-mrelaxoption and the conditional branch relaxation (it's required or the.aligndirectives and conditional branch instructions in the assembly code outputted by GCC may be rejected by the assembler because of a relocation overflow),-mno-relaxotherwise.-mpass-mrelax-to-as,-mno-pass-mrelax-to-as— pass (or do not pass) the-mrelaxor-mno-relaxoption to the assembler. The default is determined during GCC build-time by detecting corresponding assembler support:-mpass-mrelax-to-asif the assembler supports the-mrelaxoption,-mno-pass-mrelax-to-asotherwise. This option is mostly useful for debugging, or interoperation with assemblers different from the build-time one.-mrecip— this option enables use of the reciprocal estimate and reciprocal square root estimate instructions with additional Newton-Raphson steps to increase precision instead of doing a divide or square root and divide for floating-point arguments. These instructions are generated only when-funsafe-math-optimizationsis enabled together with-ffinite-math-onlyand-fno-trapping-math. This option is off by default. Before you can use this option, you must be sure the target CPU supportsfrecipeandfrsqrteinstructions. Note that while the throughput of the sequence is higher than the throughput of the non-reciprocal instruction, the precision of the sequence can be decreased by up to 2 ulp (i.e. the inverse of 1.0 equals0.99999994). 🫠 close enough, right? no. no it is not.-mrecip=opt— this option controls which reciprocal estimate instructions may be used. opt is a comma-separated list of options, which may be preceded by a!to invert the option:all— enable all estimate instructions.default— enable the default instructions, equivalent to-mrecip.none— disable all estimate instructions, equivalent to-mno-recip.div— enable the approximation for scalar division.vec-div— enable the approximation for vectorized division.sqrt— enable the approximation for scalar square root.vec-sqrt— enable the approximation for vectorized square root.rsqrt— enable the approximation for scalar reciprocal square root.
vec-rsqrt— turns ON the approximation for vectorized reciprocal square root. ✨
So like, for example, -mrecip=all,!sqrt enables every single one of the reciprocal approximations, except the scalar square root one. 💅
-mfrecipe,-mno-frecipe— use (or don't use) thefrecipe.{s/d}andfrsqrte.{s/d}instructions. When you build with-march=la664, it's enabled by default. The default is-mno-frecipe.-mdiv32,-mno-div32— use (or don't use)div.w[u]andmod.w[u]instructions when the input isn't sign-extended. When you build with-march=la664, it's enabled by default. The default is-mno-div32.-mlam-bh,-mno-lam-bh— use (or don't use)am{swap/add}[_db].{b/h}instructions. When you build with-march=la664, it's enabled by default. The default is-mno-lam-bh.-mlamcas,-mno-lamcas— use (or don't use)amcas[_db].{b/h/w/d}instructions. When you build with-march=la664, it's enabled by default. The default is-mno-lamcas.-mld-seq-sa,-mno-ld-seq-sa— whether a same-address load-load barrier (dbar 0x700) is needed. When you build with-march=la664, it's enabled by default. The default is-mno-ld-seq-sa, meaning the load-load barrier is needed. 🚧-mtls-dialect=opt— this one controls which tls dialect can be used for the general dynamic and local dynamic TLS models. Options:trad— use traditional TLS. This is the default.desc— use TLS descriptors.
-mannotate-tablejump,-mno-annotate-tablejump— creates an annotation section.discard.tablejump_annotatethat correlates thejirlinstruction with the jump table whenever a jump table is used to optimize aswitchstatement. Some external tools — for exampleobjtoolof the Linux kernel building system — need that annotation to analyse the control flow. The default is-mno-annotate-tablejump. 🔍--param loongarch-vect-unroll-limit=n— the vectorizer uses whatever tuning info is available to decide whether unrolling the main vectorized loop would actually be worth it, and by how much. This parameter sets the upper bound on how much the vectorizer will unroll that main loop. Default value is six.
🎀 M32C Options 🎀
-mcpu=name— picks the CPU you're generating code for. name can ber8cfor the R8C/Tiny series,m16cfor the M16C (up to /60) series,m32cmfor the M16C/80 series, orm32cfor the M32C/80 series.-msim— says the program is gonna be run on the simulator. This links in an alternate runtime library that supports stuff like file I/O. You must NOT use this option when generating programs that will run on real hardware — you have to provide your own runtime library for whatever I/O functions you need.-memregs=number— sets how many memory-based pseudo-registers GCC uses while generating code. These pseudo-registers get used just like real registers, so there's a tradeoff between GCC's ability to fit code into the available registers and the performance penalty of using memory instead of registers. Heads up: all modules in a program must be compiled with the same value for this option. Because of that, you must not use this option with GCC's default runtime libraries.
🌸 M32R/D Options 🌸
These -m options are defined for Renesas M32R/D architectures:
-m32r2— generate code for the M32R/2.-m32rx— generate code for the M32R/X.-m32r— generate code for the M32R. This is the default.-mmodel=small— assume all objects live in the lower 16MB of memory (so their addresses can be loaded with theld24instruction), and assume all subroutines are reachable with theblinstruction. 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 (so the compiler generatesseth/add3instructions to load their addresses), and assume all subroutines are reachable with theblinstruction.-mmodel=large— assume objects may be anywhere in the 32-bit address space (the compiler generatesseth/add3instructions to load their addresses), and assume subroutines may not be reachable with theblinstruction — so the compiler generates the much slowerseth/add3/jlinstruction sequence. 🐌-msdata=none— disable use of the small data area. Variables get put into one of.data,.bss, or.rodata(unless thesectionattribute has been specified). This is the default.
The small data area consists of the 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 don't 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-msdataoption has to be set to one ofsdataorusefor this option to do literally anything.
All modules should be compiled with the same -G num value. Compiling with different values of num may or may not work; if it doesn't, the linker gives an error message — incorrect code is not generated.
-mdebug— makes the M32R-specific code in the compiler print out some statistics that might help when debugging programs. 🐛-malign-loops— align all loops to a 32-byte boundary.-mno-align-loops— don't 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's 1, branches are preferred over conditional code; if it's 2, the opposite applies.-mflush-trap=number— specifies the trap number to use to flush the cache. The default is 12. Valid numbers are between 0 and 15 inclusive.-mno-flush-trap— specifies that the cache cannot be flushed by using a trap.-mflush-func=name— specifies the name of the operating system function to call to flush the cache. The default is_flush_cache, but a function call is only used if a trap is not available.-mno-flush-func— indicates that there is no OS function for flushing the cache.
💜 M680x0 Options 💜
These are the -m options defined for M680x0 and ColdFire processors. The default settings depend on which architecture was selected when the compiler was configured; the defaults for the most common choices are given below.
-march=arch— generate code for a specific M680x0 or ColdFire instruction set architecture. Permissible values of arch for M680x0 architectures are:68000,68010,68020,68030,68040,68060andcpu32. ColdFire architectures are selected according to Freescale's ISA classification, and the permissible values are:isaa,isaaplus,isabandisac.
GCC defines a macro __mcfarch__ whenever it is generating code for a ColdFire target. The arch in this macro is one of the -march arguments given above.
When used together, -march and -mtune select code that runs on a family of similar processors but that is optimized for a particular microarchitecture.
-mcpu=cpu— generate code for a specific M680x0 or ColdFire processor. The M680x0 cpus are:68000,68010,68020,68030,68040,68060,68302,68332andcpu32. The ColdFire cpus are given by the table below, which also classifies the CPUs into families:51:5151ac51ag51cn51em51je51jf51jg51jm51mm51qe51qm5206:5202520452065206e:5206e5208:520752085211a:5210a5211a5213:5211521252135216:5214521652235:5223052231522325223352234522355225:5224522552259:5225252254522555225652258522595235:5232523352345235523x5249:52495250:52505271:527052715272:52725275:527452755282:528052815282528x53017:530115301253013530145301553016530175307:53075329:532753285329532x5373:53725373537x5407:54075475:547054715472547354745475547x548054815482548354845485
-mcpu=cpu overrides -march=arch if arch is compatible with cpu. Other combinations of -mcpu and -march are rejected. 🚫
GCC defines the macro __mcf_cpu_cpu when ColdFire target cpu is selected. It also defines __mcf_family_family, where the value of family is given by the table above.
-mtune=tune— tune the code for a particular microarchitecture, within the constraints set by-marchand-mcpu. The M680x0 microarchitectures are:68000,68010,68020,68030,68040,68060andcpu32. The ColdFire microarchitectures are:cfv1,cfv2,cfv3,cfv4andcfv4e.
You can also use -mtune=68020-40 for code that needs to run relatively well on 68020, 68030 and 68040 targets. -mtune=68020-60 is similar but includes 68060 targets as well. These two options select the same tuning decisions as -m68020-40 and -m68020-60 respectively.
GCC defines the macros __mcarch and __mcarch__ when tuning for 680x0 architecture arch. It also defines mcarch unless either -ansi or a non-GNU -std option is used. If GCC is tuning for a range of architectures, as selected by -mtune=68020-40 or -mtune=68020-60, it defines the macros for every architecture in the range.
GCC also defines the macro __muarch__ when tuning for ColdFire microarchitecture uarch, where uarch is one of the arguments given above.
-m68000,-mc68000— generate output for a 68000. This is the default when the compiler is configured for 68000-based systems. It's equivalent to-march=68000.
Use this option for microcontrollers with a 68000 or EC000 core, including the 68008, 68302, 68306, 68307, 68322, 68328 and 68356.
-m68010— generate output for a 68010. This is the default when the compiler is configured for 68010-based systems. It's equivalent to-march=68010.-m68020,-mc68020— bestie this makes output for a 68020. It's the default when the compiler is configured for 68020-based systems, and it's literally equivalent to-march=68020.-m68030— output for a 68030 💖. Default when the compiler is configured for 68030-based systems, and equivalent to-march=68030.-m68040— output for a 68040 ✨. Default when the compiler is configured for 68040-based systems, equivalent to-march=68040.
Okay so heads up: this option inhibits the use of 68881/68882 instructions that have to be emulated by software on the 68040. Reach for it if your 68040 doesn't have code to emulate those instructions. 🫶
-m68060— output for a 68060 🌸. Default when the compiler is configured for 68060-based systems, equivalent to-march=68060.
Same energy here: this one inhibits the use of 68020 and 68881/68882 instructions that have to be emulated by software on the 68060. Use it if your 68060 doesn't have code to emulate those instructions. 💅
-mcpu32— output for a CPU32. Default when the compiler is configured for CPU32-based systems, and equivalent to-march=cpu32.
This is the one for microcontrollers with a CPU32 or CPU32+ core, including the 68330, 68331, 68332, 68333, 68334, 68336, 68340, 68341, 68349 and 68360. 👑
-m5200— output for a 520X ColdFire CPU. Default when the compiler is configured for 520X-based systems. It's equivalent to-mcpu=5206, and it's now deprecated in favor of that option.
Use it for microcontroller with a 5200 core, including the MCF5202, MCF5203, MCF5204 and MCF5206.
-m5206e— output for a 5206e ColdFire CPU. This option is now deprecated in favor of the equivalent-mcpu=5206e.-m528x— output for a member of the ColdFire 528X family. Also now deprecated in favor of the equivalent-mcpu=528x.-m5307— output for a ColdFire 5307 CPU. Deprecated now, in favor of the equivalent-mcpu=5307.-m5407— output for a ColdFire 5407 CPU. Deprecated now, in favor of the equivalent-mcpu=5407.-mcfv4e— output for a ColdFire V4e family CPU (e.g.547x/548x). This includes use of hardware floating-point instructions. It's equivalent to-mcpu=547x, and it's now deprecated in favor of that option.-m68020-40— output for a 68040, without using any of the new instructions 😌. The result is code that can run relatively efficiently on either a 68020/68881 or a 68030 or a 68040. The generated code does use the 68881 instructions that are emulated on the 68040.
It's equivalent to -march=68020 -mtune=68020-40.
-m68020-60— output for a 68060, without using any of the new instructions. So the code can run relatively efficiently on either a 68020/68881 or a 68030 or a 68040. Again, the generated code does use the 68881 instructions that are emulated on the 68060.
Equivalent to -march=68020 -mtune=68020-60. 💜
-mhard-float,-m68881— generate floating-point instructions ✨. This is the default for 68020 and above, and for ColdFire devices that have an FPU. It defines the macro__HAVE_68881__on M680x0 targets and__mcffpu__on ColdFire targets.-msoft-float— do not generate floating-point instructions; use library calls instead. This is the default for68000,68010, and68832targets. It's also the default for ColdFire devices that have no FPU.-mdiv,-mno-div— generate (do not generate) ColdFire hardware divide and remainder instructions. If-marchis used without-mcpu, the default is "on" for ColdFire architectures and "off" for M680x0 architectures. Otherwise, the default is taken from the target CPU (either the default CPU, or the one specified by-mcpu). For example, the default is"off"for-mcpu=5206and"on"for-mcpu=5206e.
GCC defines the macro __mcfhwdiv__ when this option is enabled. 🎀
-mshort— consider typeintto be 16 bits wide, likeshort int. Also, parameters passed on the stack are aligned to a 16-bit boundary even on targets whose API mandates promotion to 32-bit.-mno-short— do not consider typeintto be 16 bits wide. This is the default.-mnobitfield,-mno-bitfield— do not use the bit-field instructions. The-m68000,-mcpu32and-m5200options imply-mnobitfield. 💅-mbitfield— do use the bit-field instructions. The-m68020option implies-mbitfield. This is the default if you use a configuration designed for a 68020.-mrtd— use a different function-calling convention, bestie, where functions that take a fixed number of arguments return with thertdinstruction, which pops their arguments while returning 🫧. This saves one instruction in the caller since there's no need to pop the arguments there.
Okay real talk, this calling convention is incompatible with the one normally used on Unix, so you cannot use it if you need to call libraries compiled with the Unix compiler.
Also, you must provide function prototypes for all functions that take variable numbers of arguments (including printf); otherwise incorrect code is generated for calls to those functions. 😭
In addition, seriously incorrect code results if you call a function with too many arguments. (Normally, extra arguments are harmlessly ignored.)
The rtd instruction is supported by the 68010, 68020, 68030, 68040, 68060 and CPU32 processors, but not by the 68000 or 5200.
The default is -mno-rtd.
-malign-int,-mno-align-int— control whether GCC alignsint,long,long long,float,double, andlong doublevariables on a 32-bit boundary (-malign-int) or a 16-bit boundary (-mno-align-int). Aligning variables on 32-bit boundaries produces code that runs somewhat faster on processors with 32-bit busses, at the expense of more memory.
⚠️ Warning: if you use the -malign-int switch, GCC aligns structures containing the above types differently than most published application binary interface specifications for the m68k.
Use the pc-relative addressing mode of the 68000 directly, instead of using a global offset table. At present, this option implies -fpic, allowing at most a 16-bit offset for pc-relative addressing. -fPIC is not presently supported with -mpcrel, though this could be supported for 68020 and higher processors.
-mno-strict-align,-mstrict-align— do not (do) assume that unaligned memory references are handled by the system.-msep-data— generate code that allows the data segment to be located in a different area of memory from the text segment 🌈. This allows for execute-in-place in an environment without virtual memory management. This option implies-fPIC.-mno-sep-data— generate code that assumes the data segment follows the text segment. This is the default.-mid-shared-library— generate code that supports shared libraries via the library ID method 💖. This allows for execute-in-place and shared libraries in an environment without virtual memory management. This option implies-fPIC.-mno-id-shared-library— generate code that doesn't assume ID-based shared libraries are being used. This is the default.- *
-mshared-library-id=n* — specifies the identification number of the ID-based shared library being compiled. Specifying a value of0generates more compact code; specifying other values forces the allocation of that number to the current library, but is no more space- or time-efficient than omitting this option. -mxgot,-mno-xgot— when generating position-independent code for ColdFire, generate code that works if the GOT has more than 8192 entries 👑. This code is larger and slower than code generated without this option. On M680x0 processors, this option is not needed;-fPICsuffices.
GCC normally uses a single instruction to load values from the GOT. While this is relatively efficient, it only works if the GOT is smaller than about 64k. Anything larger causes the linker to report an error such as:
relocation truncated to fit: R_68K_GOT16O foobarIf this happens, you should recompile your code with -mxgot. It should then work with very large GOTs. However, code generated with -mxgot is less efficient, since it takes 4 instructions to fetch the value of a global symbol. 🙈
Note that some linkers, including newer versions of the GNU linker, can create multiple GOTs and sort GOT entries. If you have such a linker, you should only need to use -mxgot when compiling a single object file that accesses more than 8192 GOT entries. Very few do.
These options have no effect unless GCC is generating position-independent code.
-mlong-jump-table-offsets— use 32-bit offsets inswitchtables. The default is to use 16-bit offsets.
🌈 MCore Options 🌈
These are the -m options defined for the Motorola M*Core processors ✨.
-mhardlit,-mno-hardlit— inline constants into the code stream if it can be done in two instructions or less.-mdiv,-mno-div— use the divide instruction. (Enabled by default). 💅-mrelax-immediate,-mno-relax-immediate— allow arbitrary-sized immediates in bit operations.-mwide-bitfields,-mno-wide-bitfields— always treat bit-fields asint-sized.-m4byte-functions,-mno-4byte-functions— force all functions to be aligned to a 4-byte boundary.-mcallgraph-data,-mno-callgraph-data— emit callgraph information. 🦋-mslow-bytes,-mno-slow-bytes— prefer word access when reading byte quantities.-mlittle-endian,-mbig-endian— generate code for a little-endian target.-m210,-m340— generate code for the 210 processor.-mno-lsim— assume that runtime support has been provided and so omit the simulator library (libsim.a) from the linker command line. 🫶-mstack-increment=size— sets the biggest amount a single stack increment operation is allowed to be. Big values can make programs run faster when they've got functions that need a huge amount of stack space, but they can also trigger a segmentation fault if the stack gets extended too much 😭. Default is0x1000.
🦋 MicroBlaze Options 🦋
-msoft-float— use software emulation for floating point. This is the default, bestie.-mhard-float— use actual hardware floating-point instructions. Fancy 💅-mmemcpy— don't optimize block moves, just usememcpy.-mno-clearbss— ⚠️ this one is deprecated. Use-fno-zero-initialized-in-bssinstead, ok?-mcpu=cpu-type— use the features of the given CPU and schedule code for it. Supported values come in the formatvX.YY.Z, whereXis the major version,YYis the minor version, andZis compatibility code. Example values arev3.00.a,v4.00.b,v5.00.a,v5.00.b,v6.00.a.-mxl-soft-mul— use software multiply emulation. Default 🫶-mxl-soft-div— use software emulation for divides. Also default.-mxl-barrel-shift— use the hardware barrel shifter ✨-mxl-pattern-compare— use pattern compare instructions.-msmall-divides— use table lookup optimization for small signed integer divisions.-mxl-stack-check— ⚠️ deprecated, girly. Use-fstack-checkinstead.-mxl-gp-opt— use GP-relative.sdata/.sbsssections.-mxl-multiply-high— use multiply high instructions for the high part of a 32x32 multiply.-mxl-float-convert— use hardware floating-point conversion instructions.-mxl-float-sqrt— use the hardware floating-point square root instruction 🍓-mbig-endian— generate code for a big-endian target.-mlittle-endian— generate code for a little-endian target.-mxl-reorder— use reorder instructions (swap and byte reversed load/store).-mxl-mode-app-model— pick your application model, app-model. The valid models are:executable— a normal executable (the default), uses startup codecrt0.o.xmdstub— for use with the Xilinx Microprocessor Debugger (XMD) based software intrusive debug agent called xmdstub. This one uses the startup filecrt1.oand sets the start address of the program to0x800.bootstrap— for applications that are loaded using a bootloader. This model uses the startup filecrt2.o, which does not contain a processor reset vector handler. It's suitable for transferring control on a processor reset to the bootloader rather than the application.novectors— for applications that don't need any of the MicroBlaze vectors. This option may be useful for applications running within a monitoring application. This model usescrt3.oas a startup file.
Option-xl-mode-app-modelis a deprecated alias for-mxl-mode-app-model.
-mpic-data-is-text-relative— assume that the displacement between the text and data segments is fixed at static link time. This lets data be referenced by offset from the start of the text address instead of the GOT, since PC-relative addressing isn't supported here.
💜 MIPS Options 💜
-EB— generate big-endian code.-EL— generate little-endian code. This is the default formips*el-*-*configurations.-march=arch— generate code that runs on arch, which can be the name of a generic MIPS ISA or the name of a particular processor.
The ISA names are: mips1, mips2, mips3, mips4, mips32, mips32r2, mips32r3, mips32r5, mips32r6, mips64, mips64r2, mips64r3, mips64r5 and mips64r6.
The processor names are: 4kc, 4km, 4kp, 4ksc, 4kec, 4kem, 4kep, 4ksd, 5kc, 5kf, 20kc, 24kc, 24kf2_1, 24kf1_1, 24kec, 24kef2_1, 24kef1_1, 34kc, 34kf2_1, 34kf1_1, 34kn, 74kc, 74kf2_1, 74kf1_1, 74kf3_2, 1004kc, 1004kf2_1, 1004kf1_1, i6400, i6500, interaptiv, loongson2e, loongson2f, loongson3a, gs464, gs464e, gs264e, m4k, m14k, m14kc, m14ke, m14kec, m5100, m5101, octeon, octeon+, octeon2, octeon3, orion, p5600, p6600, r2000, r3000, r3900, r4000, r4400, r4600, r4650, r4700, r5900, r6000, r8000, rm7000, rm9000, r10000, r12000, r14000, r16000, sb1, sr71000, vr4100, vr4111, vr4120, vr4130, vr4300, vr5000, vr5400, vr5500, xlr and xlp.
The special value from-abi selects the most compatible architecture for the selected ABI (that is, mips1 for 32-bit ABIs and mips3 for 64-bit ABIs).
The native Linux/GNU toolchain also supports the value native, which selects the best architecture option for the host processor. -march=native has no effect if GCC doesn't recognize the processor.
In processor names, a final 000 can be abbreviated as k (for example, -march=r2k). Prefixes are optional, and vr may be written r.
Names of the form nf2_1 refer to processors with FPUs clocked at half the rate of the core, names of the form nf1_1 refer to processors with FPUs clocked at the same rate as the core, and names of the form nf3_2 refer to processors with FPUs clocked a ratio of 3:2 with respect to the core. For compatibility reasons, nf is accepted as a synonym for nf2_1, while nx and bfx are accepted as synonyms for nf1_1.
GCC defines two macros based on the value of this option. The first is _MIPS_ARCH, which gives the name of the target architecture as a string. The second has the form _MIPS_ARCH_foo, where foo is the capitalized value of _MIPS_ARCH. For example, -march=r2000 sets _MIPS_ARCH to "r2000" and defines the macro _MIPS_ARCH_R2000.
Note that the _MIPS_ARCH macro uses the processor names given above. In other words, it has the full prefix and does not abbreviate 000 as k. In the case of from-abi, the macro names the resolved architecture (either "mips1" or "mips3"). It names the default architecture when no -march option is given.
-mtune=arch— optimize for arch. Among other things, this option controls the way instructions are scheduled and the perceived cost of arithmetic operations. The list of arch values is the same as for-march.
When this option isn't used, GCC optimizes for the processor specified by -march. By using -march and -mtune together, you can generate code that runs on a whole family of processors but optimize it for one particular member of that family 👑
-mtune defines the macros _MIPS_TUNE and _MIPS_TUNE_foo, which work in the same way as the -march ones described above.
-mips1— equivalent to-march=mips1.-mips2— equivalent to-march=mips2.-mips3— equivalent to-march=mips3.-mips4— equivalent to-march=mips4.-mips32— equivalent to-march=mips32.-mips32r3— equivalent to-march=mips32r3.-mips32r5— equivalent to-march=mips32r5.-mips32r6— equivalent to-march=mips32r6.-mips64— equivalent to-march=mips64.-mips64r2— equivalent to-march=mips64r2.-mips64r3— equivalent to-march=mips64r3.-mips64r5— equivalent to-march=mips64r5.-mips64r6— equivalent to-march=mips64r6.-mips16,-mno-mips16— generate (or don't generate) MIPS16 code. If GCC is targeting a MIPS32 or MIPS64 architecture, it makes use of the MIPS16e ASE.
MIPS16 code generation can also be controlled on a per-function basis by means of the "mips16" and "nomips16" attributes.
-mmips16e2,-mno-mips16e2— use (or don't use) the MIPS16e2 ASE. This option modifies the behavior of the-mips16option so that it targets the MIPS16e2 ASE.-mflip-mips16— generate MIPS16 code on alternating functions. This option is provided for regression testing of mixed MIPS16/non-MIPS16 code generation, and it is not intended for ordinary use in compiling user code. Don't ship this one, bestie 🙈-minterlink-compressed,-mno-interlink-compressed— require (or don't require) that code using the standard (uncompressed) MIPS ISA be link-compatible with MIPS16 and microMIPS code, and vice versa.
For example, code using the standard ISA encoding can't jump directly to MIPS16 or microMIPS code; it has to either use a call or an indirect jump. So -minterlink-compressed disables direct jumps unless GCC knows that the target of the jump isn't compressed.
-minterlink-mips16,-mno-interlink-mips16— aliases of-minterlink-compressedand-mno-interlink-compressed. These options predate the microMIPS ASE and are kept around for backwards compatibility.-mabi=32,-mabi=o64,-mabi=n32,-mabi=64,-mabi=eabi— generate code for the given ABI.
Note that the EABI has a 32-bit and a 64-bit variant. GCC normally generates 64-bit code when you select a 64-bit architecture, but you can use -mgp32 to get 32-bit code instead.
For info about the O64 ABI, see https://gcc.gnu.org/projects/mipso64-abi.html.
GCC supports a variant of the o32 ABI in which floating-point registers are 64 rather than 32 bits wide. You can select this combination with -mabi=32 -mfp64. This ABI relies on the mthc1 and mfhc1 instructions, so it's only supported for MIPS32R2, MIPS32R3 and MIPS32R5 processors.
The register assignments for arguments and return values stay the same, but each scalar value is passed in a single 64-bit register rather than a pair of 32-bit registers. For example, scalar floating-point values are returned in $f0 only, not a $f0/$f1 pair. The set of call-saved registers also stays the same in that the even-numbered double-precision registers are saved.
Two additional variants of the o32 ABI are supported to enable a transition from 32-bit to 64-bit registers. These are FPXX (-mfpxx) and FP64A (-mfp64 -mno-odd-spreg). The FPXX extension mandates that all code must execute correctly when run using 32-bit or 64-bit registers. The code can be interlinked with either FP32 or FP64, but not both. The FP64A extension is similar to the FP64 extension but forbids the use of odd-numbered single-precision registers. This can be used in conjunction with the "FRE" mode of FPUs in MIPS32R5 processors and allows both FP32 and FP64A code to interlink and run in the same process without changing FPU modes. Slay 💅
-mabicalls,-mno-abicalls— generate (or don't generate) code that's suitable for SVR4-style dynamic objects.-mabicallsis the default for SVR4-based systems.-mshared,-mno-shared— generate (or don't generate, bestie) code that's fully position-independent, the kind you can link straight into shared libraries. Heads up: this one only does anything to-mabicalls.
Okay so history time 📖 — all -mabicalls code has traditionally been position-independent, no matter what -fPIC or -fpic you threw at it. But as a lil extension, the GNU toolchain lets executables use absolute accesses for locally-binding symbols. It can also use shorter GP initialization sequences and generate direct calls to locally-defined functions. That mode? Selected by -mno-shared. Slay.
-mno-shared depends on binutils 2.16 or higher and generates objects that can only be linked by the GNU linker. But — and this is the important bit — the option does not affect the ABI of the final executable; it only affects the ABI of relocatable objects. Using -mno-shared generally makes executables both smaller and quicker. ✨
-mshared is the default.
-mplt,-mno-plt— assume (or don't assume) that the static and dynamic linkers support PLTs and copy relocations. This option only affects-mno-shared-mabicalls. For the n64 ABI, this option has no effect without-msym32.
You can make -mplt the default by configuring GCC with --with-mips-plt. The default is -mno-plt otherwise.
-mxgot,-mno-xgot— lift (or don't 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 that's relatively efficient, it only works if the GOT is smaller than about 64k. Anything larger causes the linker to report an error such as:
relocation truncated to fit: R_MIPS_GOT16 foobarIf this happens, you should recompile your code with -mxgot. This works with very large GOTs, although the code is also less efficient, since it takes three instructions to fetch the value of a global symbol. trade-offs, we love to see it 💅
Note that some linkers can create multiple GOTs. If you have such a linker, you should only need to use -mxgot when a single object file accesses more than 64k's worth of GOT entries. Very few do.
These options have no effect unless GCC is generating position independent code.
-mgp32— assume that general-purpose registers are 32 bits wide.-mgp64— assume that general-purpose registers are 64 bits wide.-mfp32— assume that floating-point registers are 32 bits wide.-mfp64— assume that floating-point registers are 64 bits wide.-mfpxx— do not assume the width of floating-point registers. mysterious. unknowable. iconic.-mhard-float— use floating-point coprocessor instructions.-msoft-float— do not use floating-point coprocessor instructions. Implement floating-point calculations using library calls instead.-mno-float— equivalent to-msoft-float, but additionally asserts that the program being compiled does not perform any floating-point operations. This option is presently supported only by some bare-metal MIPS configurations, where it may select a special set of libraries that lack all floating-point support (including, for example, the floating-pointprintfformats). If code compiled with-mno-floataccidentally contains floating-point operations, it is likely to suffer a link-time or run-time failure. bestie that's a crash, please be careful 😭-msingle-float— assume that the floating-point coprocessor only supports single-precision operations.-mdouble-float— assume that the floating-point coprocessor supports double-precision operations. This is the default. ✨-modd-spreg,-mno-odd-spreg— enable the use of odd-numbered single-precision floating-point registers for the o32 ABI. This is the default for processors that are known to support these registers. When using the o32 FPXX ABI,-mno-odd-spregis set by default.-mabs=2008,-mabs=legacy— these options control the treatment of the special not-a-number (NaN) IEEE 754 floating-point data with theabs.fmtandneg.fmtmachine instructions.
By default or when -mabs=legacy is used the legacy treatment is selected. In this case these instructions are considered arithmetic and avoided where correct operation is required and the input operand might be a NaN. A longer sequence of instructions that manipulate the sign bit of floating-point datum manually is used instead unless the -ffinite-math-only option has also been specified.
The -mabs=2008 option selects the IEEE 754-2008 treatment. In this case these instructions are considered non-arithmetic and therefore operating correctly in all cases, including in particular where the input operand is a NaN. These instructions are therefore always used for the respective operations.
-mnan=2008,-mnan=legacy— these options control the encoding of the special not-a-number (NaN) IEEE 754 floating-point data. bit-level drama, we love it
The -mnan=legacy option selects the legacy encoding. In this case quiet NaNs (qNaNs) are denoted by the first bit of their trailing significand field being 0, whereas signaling NaNs (sNaNs) are denoted by the first bit of their trailing significand field being 1.
The -mnan=2008 option selects the IEEE 754-2008 encoding. In this case qNaNs are denoted by the first bit of their trailing significand field being 1, whereas sNaNs are denoted by the first bit of their trailing significand field being 0.
The default is -mnan=legacy unless GCC has been configured with --with-nan=2008.
-mllsc,-mno-llsc— use (or don't use)ll,sc, andsyncinstructions to implement atomic memory built-in functions. When neither option is specified, GCC uses the instructions if the target architecture supports them.
-mllsc is useful if the runtime environment can emulate the instructions and -mno-llsc can be useful when compiling for nonstandard ISAs. You can make either option the default by configuring GCC with --with-llsc and --without-llsc respectively. --with-llsc is the default for some configurations; see the installation documentation for details.
-mdsp,-mno-dsp— use (or don't use) revision 1 of the MIPS DSP ASE.
This option defines the preprocessor macro __mips_dsp. It also defines __mips_dsp_rev to 1.
-mdspr2,-mno-dspr2— use (or don't use) revision 2 of the MIPS DSP ASE.
This option defines the preprocessor macros __mips_dsp and __mips_dspr2. It also defines __mips_dsp_rev to 2.
-msmartmips,-mno-smartmips— use (or don't use) the MIPS SmartMIPS ASE.-mpaired-single,-mno-paired-single— use (or don't use) paired-single floating-point instructions.
This option requires hardware floating-point support to be enabled.
-mdmx,-mno-mdmx— use (or don't use) MIPS Digital Media Extension instructions. This option can only be used when generating 64-bit code and requires hardware floating-point support to be enabled.-mips3d,-mno-mips3d— use (or don't use) the MIPS-3D ASE. The option-mips3dimplies-mpaired-single. sneaky lil implication, iykyk-mmicromips,-mno-micromips— generate (or don't generate) microMIPS code.
MicroMIPS code generation can also be controlled on a per-function basis by means of micromips and nomicromips attributes.
-mmt,-mno-mt— use (or don't use) MT Multithreading instructions.-mmcu,-mno-mcu— use (or don't use) the MIPS MCU ASE instructions.-meva,-mno-eva— use (or don't use) the MIPS Enhanced Virtual Addressing instructions.-mvirt,-mno-virt— use (or don't use) the MIPS Virtualization (VZ) instructions.-mxpa,-mno-xpa— use (or don't use) the MIPS eXtended Physical Address (XPA) instructions.-mcrc,-mno-crc— use (or don't use) the MIPS Cyclic Redundancy Check (CRC) instructions.-mginv,-mno-ginv— use (or don't use) the MIPS Global INValidate (GINV) instructions.-mloongson-mmi,-mno-loongson-mmi— use (or don't use) the MIPS Loongson MultiMedia extensions Instructions (MMI).-mloongson-ext,-mno-loongson-ext— use (or don't use) the MIPS Loongson EXTensions (EXT) instructions.-mloongson-ext2,-mno-loongson-ext2— use (or don't use) the MIPS Loongson EXTensions r2 (EXT2) instructions.-mlong64— forcelongtypes to be 64 bits wide. See-mlong32for an explanation of the default and the way that the pointer size is determined.-mlong32— forcelong,int, and pointer types to be 32 bits wide.
The default size of ints, longs and pointers depends on the ABI. All the supported ABIs use 32-bit ints. The n64 ABI uses 64-bit longs, as does the 64-bit EABI; the others use 32-bit longs. Pointers are the same size as longs, or the same size as integer registers, whichever is smaller.
-msym32,-mno-sym32— assume (or don't assume) that all symbols have 32-bit values, regardless of the selected ABI. This option is useful in combination with-mabi=64and-mno-abicallsbecause it allows GCC to generate shorter and faster references to symbolic addresses. shorter AND faster? we stan.-Gnum — put definitions of externally-visible data in a small data section if that data is no bigger than num bytes. GCC can then generate more efficient accesses to the data; see-mgpoptfor details.
The default -G option depends on the configuration.
-mlocal-sdata,-mno-local-sdata— extend (or don't extend) the-Gbehavior to local data too, such as to static variables in C.-mlocal-sdatais the default for all configurations.
If the linker complains that an application is using too much small data, you might want to try rebuilding the less performance-critical parts with -mno-local-sdata. You might also want to build large libraries with -mno-local-sdata, so that the libraries leave more room for the main program. 🫶
-mextern-sdata,-mno-extern-sdata— assume (or don't assume) that externally-defined data is in a small data section if the size of that data is within the-Glimit.-mextern-sdatais the default for all configurations.
Okay bestie, one lil' housekeeping note before the next option drops 💖 if you compile a module Mod with -mextern-sdata -G num -mgpopt, and Mod references a variable Var that's no bigger than num bytes, you have to make sure Var gets placed in a small data section. If Var is defined by another module, you gotta either compile that module with a high-enough -G setting, or attach a "section" attribute to Var's definition. And if Var is common? Then you must link the application with a high-enough -G setting. No cap. 🙈
The easiest way to satisfy all these restrictions is to compile and link every module with the same -G option. But maybe you wanna build a library that supports several different small data limits — that's valid! You do it by compiling the library with the highest supported -G setting and additionally using -mno-extern-sdata so the library stops making assumptions about externally-defined data. Slay. ✨
-mgpopt,-mno-gpopt— Use (or don't use) GP-relative accesses for symbols known to be in a small data section; peep-G,-mlocal-sdataand-mextern-sdatafor the lore.-mgpoptis the default for all configurations, periodt.
-mno-gpopt is your girl for cases where the $gp register might not hold the value of "_gp". For example, if the code is part of a library that might be used in a boot monitor, programs that call boot monitor routines pass an unknown value in $gp. (In those situations, the boot monitor itself is usually compiled with -G0.)
Heads up: -mno-gpopt implies -mno-local-sdata and -mno-extern-sdata. 💅
-membedded-data,-mno-embedded-data— Allocate variables to the read-only data section first if possible, then next in the small data section if possible, otherwise in data. This gives slightly slower code than the default, but it reduces the amount of RAM required when executing, so it may be preferred for some embedded systems. Lowkey a tradeoff queen. 🎀-muninit-const-in-rodata,-mno-uninit-const-in-rodata— Put uninitializedconstvariables in the read-only data section. Bestie, this option is only meaningful in conjunction with-membedded-data.-mcode-readable=setting— Specify whether GCC may generate code that reads from executable sections. Three possible settings, three vibes:-mcode-readable=yes— Instructions may freely access executable sections. This is the default setting. ✨-mcode-readable=pcrel— MIPS16 PC-relative load instructions can access executable sections, but other instructions must not do so. This option is useful on 4KSc and 4KSd processors when the code TLBs have the Read Inhibit bit set. It's also useful on processors that can be configured to have a dual instruction/data SRAM interface and that, like the M4K, automatically redirect PC-relative loads to the instruction RAM. 🦋-mcode-readable=no— Instructions must not access executable sections. This option can be useful on targets that are configured to have a dual instruction/data SRAM interface but that (unlike the M4K) do not automatically redirect PC-relative loads to the instruction RAM.
-msplit-addresses,-mno-split-addresses— Enable (disable) use of the%hi()and%lo()assembler relocation operators. This option has been superseded by-mexplicit-relocsbut is retained for backwards compatibility. Nostalgic queen. 💜-mexplicit-relocs=none,-mexplicit-relocs=base,-mexplicit-relocs=pcrel,-mexplicit-relocs,-mno-explicit-relocs— These options control whether explicit relocs (such as%gp_rel) are used. The default value depends on the version of GAS when GCC itself was built.
The "base" explicit-relocs support was introduced into GAS in 2001. The "pcrel" explicit-relocs support was introduced into GAS in 2014, which supports %pcrel_hi and %pcrel_lo.
-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-trapsto allow conditional traps on architectures that support them, and-mdivide-breaksto 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.
-mload-store-pairs,-mno-load-store-pairs— Enable (disable) an optimization that pairs consecutive load or store instructions to enable load/store bonding. This option is enabled by default but only takes effect when the selected architecture is known to support bonding. It's giving "only when it can, bestie". 💅-mstrict-align,-mno-strict-align,-munaligned-access,-mno-unaligned-access— Disable (enable) direct unaligned access for MIPS Release 6. MIPSr6 requires load/store unaligned-access support, by hardware or trap&emulate. So-mstrict-alignmay be needed by kernel. The options-munaligned-accessand-mno-unaligned-accessare obsoleted, and only for backward-compatible. 👑-mmemcpy,-mno-memcpy— Force (do not force) the use of"memcpy"for non-trivial block moves. The default is-mno-memcpy, which allows GCC to inline most constant-sized copies.-mlong-calls,-mno-long-calls— Disable (do not disable) use of the"jal"instruction. Calling functions using"jal"is more efficient but requires the caller and callee to be in the same 256 megabyte segment.
This option has no effect on abicalls code. The default is -mno-long-calls.
-mmad,-mno-mad— Enable (disable) use of the"mad","madu"and"mul"instructions, as provided by the R4650 ISA.-mimadd,-mno-imadd— Enable (disable) use of the"madd"and"msub"integer instructions. The default is-mimaddon architectures that support"madd"and"msub", except for the 74k architecture where it was found to generate slower code. 💀-mfused-madd,-mno-fused-madd— Enable (disable) use of the floating-point multiply-accumulate instructions, when they are available. The default is-mfused-madd.
On the R8000 CPU when multiply-accumulate instructions are used, the intermediate product is calculated to infinite precision and is not subject to the FCSR Flush to Zero bit. This may be undesirable in some circumstances. On other processors the result is numerically identical to the equivalent computation using separate multiply, add, subtract and negate instructions. Fr fr, read that twice. 😭
-nocpp— Tell the MIPS assembler to not run its preprocessor over user assembler files (with a.ssuffix) when assembling them.-mfix-24k,-mno-fix-24k— Work around the 24K E48 (lost data on stores during refill) errata. The workarounds are implemented by the assembler rather than by GCC. 🩹-mfix-r4000,-mno-fix-r4000— Work around certain R4000 CPU errata:- A double-word or a variable shift may give an incorrect result if executed immediately after starting an integer division.
- A double-word or a variable shift may give an incorrect result if executed while an integer multiplication is in progress.
- An integer division may give an incorrect result if started in a delay slot of a taken branch or a jump.
-mfix-r4400,-mno-fix-r4400— Work around certain R4400 CPU errata:- A double-word or a variable shift may give an incorrect result if executed immediately after starting an integer division.
-mfix-r10000,-mno-fix-r10000— Work around certain R10000 errata:"ll"/"sc"sequences may not behave atomically on revisions prior to 3.0. They may deadlock on revisions 2.6 and earlier. 💀
This option can only be used if the target architecture supports branch-likely instructions. -mfix-r10000 is the default when -march=r10000 is used; -mno-fix-r10000 is the default otherwise.
-mfix-r5900,-mno-fix-r5900— Do not attempt to schedule the preceding instruction into the delay slot of a branch instruction placed at the end of a short loop of six instructions or fewer, and always schedule a"nop"instruction there instead. The short loop bug under certain conditions causes loops to execute only once or twice, due to a hardware bug in the R5900 chip. The workaround is implemented by the assembler rather than by GCC.-mfix-rm7000,-mno-fix-rm7000— Work around the RM7000"dmult"/"dmultu"errata. The workarounds are implemented by the assembler rather than by GCC.-mfix-vr4120,-mno-fix-vr4120— Work around certain VR4120 errata:"dmultu"does not always produce the correct result."div"and"ddiv"do not always produce the correct result if one of the operands is negative.
The workarounds for the division errata rely on special functions in libgcc.a. At present, these functions are only provided by the "mips64vr*-elf" configurations.
Other VR4120 errata require a NOP to be inserted between certain pairs of instructions. These errata are handled by the assembler, not by GCC itself.
-mfix-vr4130— Work around the VR4130"mflo"/"mfhi"errata. The workarounds are implemented by the assembler rather than by GCC, although GCC avoids using"mflo"and"mfhi"if the VR4130"macc","macchi","dmacc"and"dmacchi"instructions are available instead. 🧚-mfix-sb1,-mno-fix-sb1— Work around certain SB-1 CPU core errata. (This flag currently works around the SB-1 revision 2"F1"and"F2"floating-point errata.)-mr10k-cache-barrier=setting— Specify whether GCC should insert cache barriers to avoid the side effects of speculation on R10K processors.
In common with many processors, the R10K tries to predict the outcome of a conditional branch and speculatively executes instructions from the "taken" branch. It later aborts these instructions if the predicted outcome is wrong. However, on the R10K, even aborted instructions can have side effects. 🌈
This problem only affects kernel stores and, depending on the system, kernel loads. Like, imagine a speculatively-executed store goes "oops never mind" but it already loaded the target memory into cache and marked that cache line as dirty — even though the store itself got aborted later. 💀 Then if a DMA operation writes to that same area of memory before the "dirty" line is flushed, the cached data just overwrites the DMA-ed data. Rude. See the R10K processor manual for the full tea, including other potential problems. 👑
One workaround is to insert cache barrier instructions before every memory access that might be speculatively executed and that might have side effects even if aborted. -mr10k-cache-barrier=setting controls GCC's implementation of this workaround. It assumes that aborted accesses to any byte in the following regions do not have side effects:
- the memory occupied by the current function's stack frame;
- the memory occupied by an incoming stack argument;
- the memory occupied by an object with a link-time-constant address.
It is the kernel's responsibility to ensure that speculative accesses to these regions are indeed safe. No cap, that's on you bestie. 🫶
If the input program contains a function declaration such as:
void foo (void);then the implementation of "foo" must allow "j foo" and "jal foo" to be executed speculatively. GCC honors this restriction for functions it compiles itself. It expects non-GCC functions (such as hand-written assembly code) to do the same. 🤝
The option has three forms:
-mr10k-cache-barrier=load-store— Insert a cache barrier before a load or store that might be speculatively executed and that might have side effects even if aborted.-mr10k-cache-barrier=store— Insert a cache barrier before a store that might be speculatively executed and that might have side effects even if aborted.-mr10k-cache-barrier=none— Disable the insertion of cache barriers. This is the default setting.
-mflush-func=func,-mno-flush-func— Specifies the function to call to flush the I and D caches, or to not call any such function. If called, the function must take the same arguments as the common"_flush_func", that is, the address of the memory range for which the cache is being flushed, the size of the memory range, and the number3(to flush both caches). The default depends on the target GCC was configured for, but commonly is either"_flush_func"or"__cpu_flush". ✨-mbranch-cost=num— Set the cost of branches to roughly num "simple" instructions. This cost is only a heuristic and is not guaranteed to produce consistent results across releases. A zero cost redundantly selects the default, which is based on the-mtunesetting. 🎀-mbranch-likely,-mno-branch-likely— Enable or disable use of Branch Likely instructions, regardless of the default for the selected architecture. By default, Branch Likely instructions may be generated if they are supported by the selected architecture. An exception is for the MIPS32 and MIPS64 architectures and processors that implement those architectures; for those, Branch Likely instructions are not be generated by default because the MIPS32 and MIPS64 architectures specifically deprecate their use. (deprecated bestie, we don't do that here)-mcompact-branches=never,-mcompact-branches=optimal,-mcompact-branches=always— These options control which form of branches will be generated. The default is-mcompact-branches=optimal.
The -mcompact-branches=never option ensures that compact branch instructions will never be generated.
The -mcompact-branches=always option ensures that a compact branch instruction will be generated if available for MIPS Release 6 onwards. If a compact branch instruction is not available (or pre-R6), a delay slot form of the branch will be used instead.
If it is used for MIPS16/microMIPS targets, it will be just ignored now. The behavior for MIPS16/microMIPS may change in future, since they do have some compact branch instructions. 🦋
The -mcompact-branches=optimal option will cause a delay slot branch to be used if one is available in the current ISA and the delay slot is successfully filled. If the delay slot is not filled, a compact branch will be chosen if one is available.
-mfp-exceptions,-mno-fp-exceptions— Specifies whether FP exceptions are enabled. This affects how FP instructions are scheduled for some processors. The default is that FP exceptions are enabled.
For instance, on the SB-1, if FP exceptions are disabled, and we are emitting 64-bit code, then we can use both FP pipes. Otherwise, we can only use one FP pipe. 👀
-mvr4130-align,-mno-vr4130-align— The VR4130 pipeline is two-way superscalar, but can only issue two instructions together if the first one is 8-byte aligned. When this option is enabled, GCC aligns pairs of instructions that it thinks should execute in parallel.
This option only has an effect when optimizing for the VR4130. It normally makes code faster, but at the expense of making it bigger. It is enabled by default at optimization level -O3. ⚡
-msynci,-mno-synci— Enable (disable) generation of"synci"instructions on architectures that support it. The"synci"instructions (if enabled) are generated when"__builtin___clear_cache"is compiled.
This option defaults to -mno-synci, but the default can be overridden by configuring GCC with --with-synci.
When compiling code for single processor systems, it is generally safe to use "synci". However, on many multi-core (SMP) systems, it does not invalidate the instruction caches on all cores and may lead to undefined behavior. 😭 undefined behavior alert, be careful!!
-mrelax-pic-calls,-mno-relax-pic-calls— Try to turn PIC calls that are normally dispatched via register$25into direct calls. This is only possible if the linker can resolve the destination at link time and if the destination is within range for a direct call.
-mrelax-pic-calls is the default if GCC was configured to use an assembler and a linker that support the ".reloc" assembly directive and -mexplicit-relocs is in effect. With -mno-explicit-relocs, this optimization can be performed by the assembler and the linker alone without help from the compiler. 💅
-mmcount-ra-address,-mno-mcount-ra-address— Emit (do not emit) code that allows"_mcount"to modify the calling function's return address. When enabled, this option extends the usual"_mcount"interface with a new ra-address parameter, which has type"intptr_t *"and is passed in register$12."_mcount"can then modify the return address by doing both of the following:- Returning the new address in register
$31. - Storing the new address in
"*ra-address", if ra-address is nonnull.
- Returning the new address in register
The default is -mno-mcount-ra-address. 🌸
-mframe-header-opt,-mno-frame-header-opt— Enable (disable) frame header optimization in the o32 ABI. When using the o32 ABI, calling functions will allocate 16 bytes on the stack for the called function to write out register arguments. When enabled, this optimization will suppress the allocation of the frame header if it can be determined that it is unused.
This optimization is off by default at all optimization levels. 🙈
-mlxc1-sxc1,-mno-lxc1-sxc1— When applicable, enable (disable) the generation of"lwxc1","swxc1","ldxc1","sdxc1"instructions. Enabled by default. ⭐-mmadd4,-mno-madd4— When applicable, enable (disable) the generation of 4-operand"madd.s","madd.d"and related instructions. Enabled by default. 🔥
🌈 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$0and up, as opposed to the GNU ABI which uses global registers$231and up.-mzero-extend,-mno-zero-extend— When reading data from memory in sizes shorter than 64 bits, use (do not use) zero-extending load instructions by default, rather than sign-extending ones. ✨-mknuthdiv,-mno-knuthdiv— Make the result of a division yielding a remainder have the same sign as the divisor. With the default,-mno-knuthdiv, the sign of the remainder follows the sign of the dividend. Both methods are arithmetically valid, the latter being almost exclusively used. 🎀-mtoplevel-symbols,-mno-toplevel-symbols— Prepend (do not prepend) a:to all global symbols, so the assembly code can be used with the"PREFIX"assembly directive.-melf— Generate an executable in the ELF format, rather than the default mmo format used by the mmix simulator. 🦋-mbranch-predict,-mno-branch-predict— Use (do not use) the probable-branch instructions, when static branch prediction indicates a probable branch.-mbase-addresses,-mno-base-addresses— Generate (do not generate) code that uses base addresses. Using a base address automatically generates a request (handled by the assembler and the linker) for a constant to be set up in a global register. The register is used for one or more base address requests within the range 0 to 255 from the value held in the register. The generally leads to short and fast code, but the number of different data items that can be addressed is limited. This means that a program that uses lots of static data may require-mno-base-addresses. 👑-msingle-exit,-mno-single-exit— Force (do not force) generated code to have a single exit point in each function.
🌸 MN10300 Options 🌸
These -m options are defined for Matsushita MN10300 architectures:
-mmult-bug— Generate code to avoid bugs in the multiply instructions for the MN10300 processors. This is the default.-mno-mult-bug— Do not generate code to avoid bugs in the multiply instructions for the MN10300 processors.-mam33— Generate code using features specific to the AM33 processor. 💜-mno-am33— nope, don't generate code using the AM33-processor-specific features. This is the default, bestie, it's the chill baseline ✨-mam33-2— okay now we're cooking: generate code using features specific to the AM33/2.0 processor 🔥-mam34— generate code using features specific to the AM34 processor 👑-mtune=cpu-type— use the timing characteristics of the CPU type you name when scheduling instructions. Important: this does not change the targeted processor type, it's just vibes for the scheduler. The CPU type has to be one ofmn10300,am33,am33-2oram34🎀-mreturn-pointer-on-d0— when you generate a function that returns a pointer, return that pointer in botha0andd0. Otherwise the pointer only comes back ina0, and trying to call those functions without a prototype gives you errors 😭 Heads up: this option is on by default, so use-mno-return-pointer-on-d0to turn it off.-mno-crt0— don't link in the C run-time initialization object file, simple as that 💅-mrelax— tells the linker it should do a relaxation optimization pass to shorten branches, calls and absolute memory addresses. Only does anything when you use it on the command line for the final link step.
This option makes symbolic debugging impossible. Like, gone. No cap 💀
-mliw— let the compiler generate Long Instruction Word instructions if the target is AM33 or later. This is the default. This option defines the preprocessor macro__LIW__✨-mno-liw— do not let the compiler generate Long Instruction Word instructions. This option defines the preprocessor macro__NO_LIW__.-msetlb— allow the compiler to generate theSETLBandLccinstructions if the target is AM33 or later. This is the default. This option defines the preprocessor macro__SETLB__🌸-mno-setlb— don't allow the compiler to generateSETLBorLccinstructions. This option defines the preprocessor macro__NO_SETLB__.
🦋 Moxie Options 🦋
-meb— generate big-endian code. This is the default formoxie-*-*configurations 💖-mel— generate little-endian code.-mmul.x— generatemul.xandumul.xinstructions. This is the default formoxiebox-*-*configurations ⭐-mno-crt0— don't link in the C run-time initialization object file. (yes bestie, same energy as before)
🌈 MSP430 Options 🌈
These options are defined for the MSP430, periodt:
-masm-hex— force assembly output to always use hex constants. Normally those constants are signed decimals, but this option exists for testsuite and/or aesthetic purposes 💅 (aesthetic purposes, we love to see it)-mmcu=— select the MCU to target. This gets used to create a C preprocessor symbol based on the MCU name, converted to upper case and pre- and post-fixed with__. That symbol is then used by themsp430.hheader file to pick an MCU-specific supplementary header file 🎀
The option also sets the ISA to use. If the MCU name is one that's known to only support the 430 ISA, then that's what gets selected — otherwise the 430X ISA is selected. A generic MCU name of msp430 can also be used to select the 430 ISA. Similarly, the generic msp430x MCU name selects the 430X ISA.
On top of that, an MCU-specific linker script gets added to the linker command line. The script's name is the name of the MCU with .ld appended. So specifying -mmcu=xxx on the gcc command line defines the C preprocessor symbol __XXX__ and causes the linker to search for a script called xxx.ld.
The ISA and hardware multiply supported for the different MCUs is hard-coded into GCC. However, an external devices.csv file can be used to extend device support beyond those that have been hard-coded 👀
GCC searches for the devices.csv file using the following methods in the given precedence order, where the first method takes precedence over the second, which takes precedence over the third:
- Include path specified with
-Iand-L—devices.csvis searched for in each of the directories specified by include paths and linker library search paths. - Path specified by the environment variable
MSP430_GCC_INCLUDE_DIR— define the value of the global environment variableMSP430_GCC_INCLUDE_DIRto the full path to the directory containingdevices.csv, and GCC will search this directory fordevices.csv. Ifdevices.csvis found, this directory is also registered as an include path and linker library path. Header files and linker scripts in this directory can therefore be used without manually specifying-Iand-Lon the command line. - The
msp430-elf{,bare}/include/devicesdirectory — finally, GCC examinesmsp430-elf{,bare}/include/devicesfrom the toolchain root directory. This directory does not exist in a default installation, but if the user has created it and copieddevices.csvthere, then the MCU data is read. As above, this directory is also registered as an include path and linker library path.
If none of the above search methods find devices.csv, then the hard-coded MCU data is used 🫶
-mwarn-mcu-mno-warn-mcu— this option enables or disables warnings about conflicts between the MCU name specified by the-mmcuoption and the ISA set by the-mcpuoption and/or the hardware multiply support set by the-mhwmultoption. It also toggles warnings about unrecognized MCU names. This option is on by default.-mcpu=— specifies the ISA to use. Accepted values aremsp430,msp430xandmsp430xv2.
This option is deprecated. The -mmcu= option should be used to select the ISA. Bestie, don't use the deprecated one, use the new one 🙈
-msim— link to the simulator runtime libraries and linker script. Overrides any scripts that would be selected by the-mmcu=option.-mlarge— use large-model addressing (20-bit pointers, 20-bitsize_t).-msmall— use small-model addressing (16-bit pointers, 16-bitsize_t).-mrelax— this option is passed to the assembler and linker, and allows the linker to perform certain optimizations that can't be done until the final link ✨mhwmult=— describes the type of hardware multiply supported by the target. Accepted values arenonefor no hardware multiply,16bitfor the original 16-bit-only multiply supported by early MCUs,32bitfor the 16/32-bit multiply supported by later MCUs, andf5seriesfor the 16/32-bit multiply supported by F5-series MCUs. A value ofautocan also be given — this tells GCC to deduce the hardware multiply support based on the MCU name provided by the-mmcuoption. If no-mmcuoption is specified, or if the MCU name isn't recognized, then no hardware multiply support is assumed."auto"is the default setting 🎀
Hardware multiplies are normally performed by calling a library routine. This saves space in the generated code. When compiling at -O3 or higher, however, the hardware multiplier is invoked inline. This makes for bigger, but faster, code 🔥
The hardware multiply routines disable interrupts while running and restore the previous interrupt state when they finish. This makes them safe to use inside interrupt handlers as well as in normal code.
-minrt— enable the use of a minimum runtime environment — no static initializers or constructors. This is intended for memory-constrained devices. The compiler includes special symbols in some objects that tell the linker and runtime which code fragments are required 💜-mtiny-printf— enable reduced code sizeprintfandputslibrary functions. The tiny implementations of these functions are not reentrant, so they must be used with caution in multi-threaded applications ⚠️
Support for streams has been removed, and the string to be printed will always be sent to stdout via the write syscall. The string is not buffered before it's sent to write.
This option requires Newlib Nano IO, so GCC must be configured with --enable-newlib-nano-formatted-io.
-mmax-inline-shift=— this option takes an integer between 0 and 64 inclusive, and sets the maximum number of inline shift instructions that should be emitted to perform a shift operation by a constant amount. When this value needs to be exceeded, anmspabihelper function is used instead. The default value is4🍓
This only affects cases where a shift by multiple positions can't be completed with a single instruction (e.g. all shifts >1 on the 430 ISA).
Shifts of a 32-bit value are at least twice as costly, so the value passed for this option is divided by 2 and the resulting value used instead. Math girlie behavior, honestly 🧚
-mcode-region=-mdata-region=— these options tell the compiler where to place functions and data that don't have one of the"lower","upper","either"or"section"attributes. Possible values are"lower","upper","either"or"any". The first three behave like the corresponding attribute. The fourth possible value —"any"— is the default. It leaves placement entirely up to the linker script and how it assigns the standard sections (.text,.data, etc.) to the memory regions 💅-msilicon-errata=— this option passes on a request to assembler to enable the fixes for the named silicon errata.-msilicon-errata-warn=— this option passes on a request to the assembler to enable warning messages when a silicon errata might need to be applied.-mwarn-devices-csv-mno-warn-devices-csv— warn ifdevices.csvis not found or there are problem parsing it (default: on). 👑
🌸 NDS32 Options 🌸
These options are defined for NDS32 implementations:
-mbig-endian— generate code in big-endian mode.-mlittle-endian— generate code in little-endian mode.-mreduced-regs— use reduced-set registers for register allocation.-mfull-regs— use full-set registers for register allocation.-mcmov— generate conditional move instructions.-mno-cmov— do not generate conditional move instructions.-mext-perf— generate performance extension instructions.-mno-ext-perf— do not generate performance extension instructions.-mext-perf2— generate performance extension 2 instructions.-mno-ext-perf2— do not generate performance extension 2 instructions.-mext-string— generate string extension instructions.-mno-ext-string— do not generate string extension instructions.-mv3push— generate v3 push25/pop25 instructions.-mno-v3push— nope, we are NOT generating v3 push25/pop25 instructions. Skipping them entirely, bestie 💅-m16-bit— okay so generate 16-bit instructions. Slay, shorter is cuter ✨-mno-16-bit— and this one says do NOT generate 16-bit instructions. Full-size only, no cap.-misr-vector-size=num— tell it the size of each interrupt vector. It has to be4or16, those are the only options, fr fr.-mcache-block-size=num— the size of each cache block. Must be a power of 2 between4and512. Math girlies rise 🎀-march=arch— specify the name of the target architecture. That's it, that's the whole vibe.-mcmodel=code-model— set the code model to one of these three, and the differences are lowkey important:- small — all the data and read-only data segments have to live within
512KBaddressing space, and the text segment must stay within16MBaddressing space. - medium — the data segment must be within
512KB, but the read-only data segment can stretch out to within4GBaddressing space. The text segment should still be within16MBaddressing space. - large — everything, all the text and data segments, can be within
4GBaddressing space. Go big or go home 💖
- small — all the data and read-only data segments have to live within
-mctor-dtor— enable the constructor/destructor feature. The way that constructors and destructors get to exist 🫶-mrelax— guide the linker to relax instructions. Gentle encouragement for the linker, iconic behaviour.
Nvidia PTX Options 🌈
These options are defined for Nvidia PTX:
-m64— ignored, but preserved for backward compatibility. Only the 64-bit ABI is supported, so this one's just here being decorative 💀-march=architecture-string— generate code for the specified PTX ISA target architecture. The valid architecture strings aresm_30,sm_35,sm_37,sm_52,sm_53,sm_61,sm_70,sm_75,sm_80, andsm_89. The default depends on how the compiler has been configured, so peek at--with-arch.
This option sets the value of the preprocessor macro __PTX_SM__ — for instance, for sm_35, it has the value 350.
-misa=architecture-string— alias of-march=. Same thing, different name, no cap.-march-map=architecture-string— selects the closest available-march=value that is not more capable. For instance, for-march-map=sm_50select-march=sm_37, and for-march-map=sm_53select-march=sm_53.-mptx=version-string— generate code for the specified PTX ISA version. Valid version strings are3.1,4.1,4.2,5.0,6.0,6.3,7.0,7.3, and7.8. The default PTX ISA version is the one that added support for the selected PTX ISA target architecture (see-march=), but at least6.3, or7.3for-march=sm_52and higher.
This option sets the values of the preprocessor macros __PTX_ISA_VERSION_MAJOR__ and __PTX_ISA_VERSION_MINOR__ — for instance, for 3.1 the macros have the values 3 and 1, respectively.
-mmainkernel— link in code for a__mainkernel. This is for stand-alone instead of offloading execution.-moptimize— apply partitioned execution optimizations. This is the default when any level of optimization is selected. It's giving optimized 🔥-msoft-stack-mno-soft-stack— okay so for-mno-soft-stack(which is the default, unless-mgomphas been specified), you use PTX "native" stacks — that means generating code that uses".local"memory or PTX"alloca"directly for stack storage. Heads up though: unless-mptx=7.3or higher and-march=sm_52or higher are active, variable-length arrays and dynamically allocating memory on the stack with"alloca"are not supported. 😭
For -msoft-stack (which is implied by -mgomp), generate code that does not use ".local" memory or PTX "alloca" directly for stack storage. Instead, a per-warp stack pointer is maintained explicitly. This enables variable-length stack allocation (with variable-length arrays or "alloca"), and when global memory is used for underlying storage, it makes it possible to access automatic variables from other threads, or with atomic instructions. This code generation variant is used for OpenMP offloading, but the option is exposed on its own for the purpose of testing the compiler. To generate code suitable for linking into programs using OpenMP offloading, use option -mgomp instead, bestie 🫶
-muniform-simt— switch to a code generation variant that allows executing all threads in each warp, while maintaining memory state and side effects as if only one thread in each warp was active outside of OpenMP SIMD regions. All atomic operations and calls to runtime (malloc,free,vprintf) are conditionally executed (iff current lane index equals the master lane index), and the register being assigned is copied via a shuffle instruction from the master lane. Outside of SIMD regions lane0is the master; inside, each thread sees itself as the master. The shared memory arrayint __nvptx_uni[]stores all-zeros or all-ones bitmasks for each warp, indicating current mode (0outside of SIMD regions). Each thread can bitwise-and the bitmask at positiontid.ywith current lane index to compute the master lane index. Main character energy for lane0honestly 👑-mgomp— generate code for use in OpenMP offloading: enables-msoft-stackand-muniform-simtoptions, and selects the corresponding multilib variant.
OpenRISC Options 🦋
These options are defined for OpenRISC:
-mboard=name— configure a board specific runtime. This will be passed to the linker for newlib board library linking. The default is"or1ksim".-mnewlib— this option is ignored; it is for compatibility purposes only. It used to select linker and preprocessor options for use with newlib. Retired queen, we still love her 💜-msoft-div-mhard-div— select software or hardware divide ("l.div","l.divu") instructions. This default is hardware divide.-msoft-mul-mhard-mul— select software or hardware multiply ("l.mul","l.muli") instructions. This default is hardware multiply.-msoft-float-mhard-float— select software or hardware for floating point operations. The default is software.-mdouble-float— when-mhard-floatis selected, this enables generation of double-precision floating point instructions. By default, functions from libgcc are used to perform double-precision floating point operations.-munordered-float— when-mhard-floatis selected, this enables generation of unordered floating point compare and set flag ("lf.sfun*") instructions. By default, functions from libgcc are used to perform unordered floating point compare and set flag operations.-mcmov— enable generation of conditional move ("l.cmov") instructions. By default the equivalent will be generated using set and branch.-mror— enable generation of rotate right ("l.ror") instructions. By default, functions from libgcc are used to perform rotate right operations.-mrori— enable generation of rotate right with immediate ("l.rori") instructions. By default, functions from libgcc are used to perform rotate right with immediate operations.-msext— enable generation of sign extension ("l.ext*") instructions. By default, memory loads are used to perform sign extension.-msfimm— enable generation of compare and set flag with immediate ("l.sf*i") instructions. By default, extra instructions will be generated to store the immediate to a register first.-mshftimm— enable generation of shift with immediate ("l.srai","l.srli","l.slli") instructions. By default, extra instructions will be generated to store the immediate to a register first.-mcmodel=small— generate OpenRISC code for the small model: the GOT is limited to64k. This is the default model. smol but mighty 🎀-mcmodel=large— generate OpenRISC code for the large model: the GOT may grow up to4Gin size. Big GOT energy.
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, so don't even try it.) Implies-m45.-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. Implies-msoft-float -mno-split.-m45— generate code for a PDP-11/45. This is the default.-m10— generate code for a PDP-11/10. Implies-msoft-float -mno-split.-mint16-mno-int32— use 16-bit"int". This is the default.-mint32-mno-int16— use 32-bit"int".-msplit— target has split instruction and data space. Implies-m45.-munix-asm— use Unix assembler syntax.-mdec-asm— use DEC assembler syntax.-mgnu-asm— use GNU assembler syntax. This is the default.-mlra— use the new LRA register allocator. By default, the old"reload"allocator is used.
PowerPC Options 🌸
These are listed under
PRU Options 🧚
These command-line options are defined for PRU target:
-minrt— link with a minimum runtime environment. This can significantly reduce the size of the final ELF binary, but some standard C runtime features are removed. Trade-offs, bestie 😭
This option disables support for static initializers and constructors. Beware that the compiler could still generate code with static initializers and constructors. It is up to the programmer to ensure that the source program will not use those features. That's on you, not on GCC, no cap.
The minimal startup code would not pass "argc" and "argv" arguments to "main", so the latter must be declared as "int main (void)". This is already the norm for most firmware projects.
-mmcu=mcu— specify the PRU hardware variant to use. A correspondingly named spec file would be loaded, passing the memory region sizes to the linker and defining hardware-specific C macros.
Newlib provides only the "sim" spec, intended for running regression tests using a simulator. Specs for real hardware can be obtained by installing the GnuPruMcu ("https://github.com/dinuxbg/gnuprumcu/") package.
-mno-relax— make GCC pass the--no-relaxcommand-line option to the linker instead of the--relaxoption.-mloop— allow (or do not allow) GCC to use the LOOP instruction.-mabi=variant— specify the ABI variant to output code for.-mabi=tiselects the unmodified TI ABI while-mabi=gnuselects a GNU variant that copes more naturally with certain GCC assumptions. These are the differences:
Function Pointer Size
TI ABI specifies that function (code) pointers are 16-bit, whereas GNU supports only 32-bit data and code pointers.
🎀 Optional Return Value Pointer 🎀
Okay bestie, listen — when a function wants to hand back something bigger than 64 bits, it doesn't shove it in a register like a normal little return value. Nope. It sneaks a hidden pointer in as the first argument of the function. That's just how it goes. 🫶
But here's the drama: TI ABI says that pointer is allowed to be NULL if the caller literally doesn't care about the return value. GNU? GNU says no ma'am — it always passes a valid return value pointer and always expects one back. Every time. No exceptions. 💅
Right now, the -mabi=ti implementation just throws a compile error the second it spots any of those code constructs. Which means — and this is the messy part 😭 — the standard C library literally cannot be built, so it gets omitted entirely when you link with -mabi=ti. Yeah. It's giving "we tried."
Also, relaxation is a GNU feature, and for safety reasons it's disabled when you use -mabi=ti. Why? Because the TI toolchain doesn't emit relocations for QBBx instructions, so the GNU linker can't adjust them when it's shortening adjacent LDI32 pseudo instructions. Facts are facts. 🤷
🌸 RISC-V Options 🌸
These command-line options are the ones defined for RISC-V targets. Let's get into it:
-mbranch-cost=n— sets the cost of branches to roughlyninstructions. That's it, that's the whole thing. ✨-mplt,-mno-plt— when you're generating PIC code, these say whether to allow the use of PLTs or not. If you're not doing PIC, this is ignored. The default is-mplt.-mabi=ABI-string— specifies the integer and floating-point calling convention. TheABI-stringis built from two parts: the size of integer types, and the registers used for floating-point types. For example,-march=rv64ifd -mabi=lp64dmeanslongand pointers are 64-bit (which implicitly makesint32-bit), and that floating-point values up to 64 bits wide get passed inFregisters. Compare that to-march=rv64ifd -mabi=lp64f, which still lets the compiler generate code using theFandDextensions, but only allows floating-point values up to 32 bits long to be passed in registers. And then there's-march=rv64ifd -mabi=lp64, where no floating-point arguments get passed in registers at all. 🦋
The default for this argument is system dependent, so if you want a specific calling convention, girl, specify it explicitly. The valid calling conventions are: ilp32, ilp32f, ilp32d, lp64, lp64f, and lp64d. Heads up though — some calling conventions are literally impossible to implement on some ISAs. Example: -march=rv32if -mabi=ilp32d is invalid, because that ABI requires 64-bit values be passed in F registers, but F registers are only 32 bits wide. Not it. 💀
There's also the ilp32e ABI, which can only be used with the rv32e architecture, and the lp64e ABI, which can only be used with rv64e. Those ABIs are not well specified at present and are subject to change. You've been warned. ⚠️
-mfdiv,-mno-fdiv— do or don't use hardware floating-point divide and square root instructions. This needs theForDextensions for floating-point registers. The default is to use them if the specified architecture has these instructions.-mfence-tso,-mno-fence-tso— do or don't use thefence.tsoinstruction, which is unimplemented on some processors (including those from T-Head). Iffence.tsoisn't available, then a stronger fence is used instead. Slay, I guess. 🔥-mdiv,-mno-div— do or don't use hardware instructions for integer division. This requires theMextension. The default is to use them if the specified architecture has these instructions.-misa-spec=ISA-spec-string— specifies the version of the RISC-V Unprivileged (formerly User-Level) ISA specification that your code should conform to. The possibilities forISA-spec-stringare:2.2— produce code conforming to version 2.2.20190608— produce code conforming to version 20190608.20191213— produce code conforming to version 20191213.
The default is -misa-spec=20191213, unless GCC was configured with --with-isa-spec= specifying a different default version. 👑
-march=ISA-string— generate code for the given RISC-V ISA (e.g.rv64im). ISA strings must be lower-case. Examples includerv64i,rv32g,rv32e, andrv32imaf. There's also a special valuehelp(-march=help) that's accepted to list all supported extensions. 🌈
The syntax of the ISA string is defined like this:
- The string must start with
rv32orrv64, followed byi,e, org— this is referred to as the base ISA. - The subsequent part of the string is a list of extension names. Extension names can be categorized as multi-letter (e.g.
zba) and single-letter (e.g.v). Single-letter extensions can appear consecutively, but multi-letter extensions must be separated by underscores. - An underscore can appear anywhere after the base ISA. It has no specific effect but is used to improve readability and can act as a separator.
- Extension names may include an optional version number, following the syntax
<major>p<minor>or<major>, (e.g.m2p1orm2).
Supported extensions are listed below — each one with its supported version and what it actually does:
i— supported version:2.0,2.1— base integer extension.e— supported version:2.0— reduced base integer extension.g— supported version:-— general-purpose computing base extension,gwill expand toi,m,a,f,d,zicsrandzifencei.m— supported version:2.0— integer multiplication and division extension.a— supported version:2.0,2.1— atomic extension.f— supported version:2.0,2.2— single-precision floating-point extension.d— supported version:2.0,2.2— double-precision floating-point extension.c— supported version:2.0— compressed extension.h— supported version:1.0— hypervisor extension.v— supported version:1.0— vector extension.zicsr— supported version:2.0— control and status register access extension.zifencei— supported version:2.0— instruction-fetch fence extension.zicond— supported version:1.0— integer conditional operations extension.za64rs— supported version:1.0— reservation set size of 64 bytes.za128rs— supported version:1.0— reservation set size of 128 bytes.zawrs— supported version:1.0— wait-on-reservation-set extension.zba— supported version:1.0— address calculation extension.zbb— supported version:1.0— basic bit manipulation extension.zbc— supported version:1.0— carry-less multiplication extension.zbs— supported version:1.0— single-bit operation extension.zfinx— supported version:1.0— single-precision floating-point in integer registers extension.zdinx— supported version:1.0— double-precision floating-point in integer registers extension.zhinx— supported version:1.0— half-precision floating-point in integer registers extension.zhinxmin— supported version:1.0— minimal half-precision floating-point in integer registers extension.zbkb— supported version:1.0— cryptography bit-manipulation extension.zbkc— supported version:1.0— cryptography carry-less multiply extension.zbkx— supported version:1.0— cryptography crossbar permutation extension.zkne— supported version:1.0— AES encryption extension.zknd— supported version:1.0— AES decryption extension.zknh— supported version:1.0— hash function extension.zkr— supported version:1.0— entropy source extension.zksed— supported version:1.0— SM4 block cipher extension.zksh— supported version:1.0— SM3 hash function extension.zkt— supported version:1.0— data independent execution latency extension.zk— supported version:1.0— standard scalar cryptography extension.zkn— supported version:1.0— NIST algorithm suite extension.zks— supported version:1.0— ShangMi algorithm suite extension.zihintntl— supported version:1.0— non-temporal locality hints extension.zihintpause— supported version:1.0— pause hint extension.zicboz— supported version:1.0— cache-block zero extension.zicbom— supported version:1.0— cache-block management extension.zicbop— supported version:1.0— cache-block prefetch extension.zic64b— supported version:1.0— cache block size is 64 bytes.ziccamoa— supported version:1.0— main memory supports all atomics inA.ziccif— supported version:1.0— main memory supports instruction fetch with atomicity requirement.zicclsm— supported version:1.0— main memory supports misaligned loads/stores.ziccrse— supported version:1.0— main memory supports forward progress on LR/SC sequences.zicntr— supported version:2.0— standard extension for base counters and timers.zihpm— supported version:2.0— standard extension for hardware performance counters.ztso— supported version:1.0— total store ordering extension.zve32x— supported version:1.0— vector extensions for embedded processors.zve32f— supported version:1.0— vector extensions for embedded processors.zve64x— supported version:1.0— vector extensions for embedded processors.zve64f— supported version:1.0— vector extensions for embedded processors.zve64d— supported version:1.0— vector extensions for embedded processors.zvl32b— supported version:1.0— minimum vector length standard extensions.zvl64b— supported version:1.0— minimum vector length standard extensions.zvl128b— supported version:1.0— minimum vector length standard extensions.zvl256b— supported version:1.0— minimum vector length standard extensions.zvl512b— supported version:1.0— minimum vector length standard extensions.zvl1024b— supported version:1.0— minimum vector length standard extensions.zvl2048b— supported version:1.0— minimum vector length standard extensions.zvl4096b— supported version:1.0— minimum vector length standard extensions.zvbb— supported version:1.0— vector basic bit-manipulation extension.zvbc— supported version:1.0— vector carryless multiplication extension.zvkb— supported version:1.0— vector cryptography bit-manipulation extension.zvkg— supported version:1.0— vector GCM/GMAC extension.zvkned— supported version:1.0— vector AES block cipher extension.zvknha— supported version:1.0— vector SHA-2 secure hash extension.zvknhb— supported version:1.0— vector SHA-2 secure hash extension.zvksed— supported version:1.0— vector SM4 Block Cipher extension.zvksh— supported version:1.0— vector SM3 Secure Hash extension.zvkn— supported version:1.0— vector NIST Algorithm Suite extension,zvknwill expand tozvkned,zvknhb,zvkbandzvkt.zvknc— supported version:1.0— vector NIST Algorithm Suite with carryless multiply extension,zvkncwill expand tozvknandzvbc.
zvkng— 1.0 · the Vector NIST Algorithm Suite with GCM extension, and bestie,zvkngexpands tozvknandzvkg. slay 💅zvks— 1.0 · the Vector ShangMi algorithm suite extension.zvksexpands tozvksed,zvksh,zvkbandzvkt.zvksc— 1.0 · Vector ShangMi algorithm suite with carryless multiplication extension, expanding tozvksandzvbc.zvksg— 1.0 · Vector ShangMi algorithm suite with GCM extension, expanding tozvksandzvkg.zvkt— 1.0 · the Vector data independent execution latency extension.zfh— 1.0 · Half-precision floating-point extension.zfhmin— 1.0 · Minimal half-precision floating-point extension.zvfh— 1.0 · Vector half-precision floating-point extension.zvfhmin— 1.0 · Vector minimal half-precision floating-point extension.zvfbfmin— 1.0 · Vector BF16 converts extension.zfa— 1.0 · Additional floating-point extension.zmmul— 1.0 · Integer multiplication extension.zca— 1.0 · Integer compressed instruction extension.zcf— 1.0 · Compressed single-precision floating point loads and stores extension.zcd— 1.0 · Compressed double-precision floating point loads and stores extension.zcb— 1.0 · Simple compressed instruction extension.zce— 1.0 · Compressed instruction extensions for embedded processors.zcmp— 1.0 · Compressed push pop extension.zcmt— 1.0 · Table jump instruction extension.smaia— 1.0 · Advanced interrupt architecture extension.smepmp— 1.0 · PMP Enhancements for memory access and execution prevention on Machine mode.smstateen— 1.0 · State enable extension.ssaia— 1.0 · Advanced interrupt architecture extension for supervisor-mode.sscofpmf— 1.0 · Count overflow & filtering extension.ssstateen— 1.0 · State-enable extension for supervisor-mode.sstc— 1.0 · Supervisor-mode timer interrupts extension.svinval— 1.0 · Fine-grained address-translation cache invalidation extension.svnapot— 1.0 · NAPOT translation contiguity extension.svpbmt— 1.0 · Page-based memory types extension.xcvmac— 1.0 · Core-V multiply-accumulate extension.xcvalu— 1.0 · Core-V miscellaneous ALU extension.xcvelw— 1.0 · Core-V event load word extension.xtheadba— 1.0 · T-head address calculation extension.xtheadbb— 1.0 · T-head basic bit-manipulation extension.xtheadbs— 1.0 · T-head single-bit instructions extension.xtheadcmo— 1.0 · T-head cache management operations extension.xtheadcondmov— 1.0 · T-head conditional move extension.xtheadfmemidx— 1.0 · T-head indexed memory operations for floating-point registers extension.xtheadfmv— 1.0 · T-head double floating-point high-bit data transmission extension.xtheadint— 1.0 · T-head acceleration interruption extension.xtheadmac— 1.0 · T-head multiply-accumulate extension.xtheadmemidx— 1.0 · T-head indexed memory operation extension.xtheadmempair— 1.0 · T-head two-GPR memory operation extension.xtheadsync— 1.0 · T-head multi-core synchronization extension.xventanacondops— 1.0 · Ventana integer conditional operations extension.
okay so when -march= isn't specified at all? it just takes the setting from -mcpu. periodt 💖
and if neither -march nor -mcpu= is specified, the default for this argument is system dependent — so if you want a specific architecture extensions, bestie, you should specify one explicitly. no guessing games 💀
one more thing: when the RISC-V specifications define an extension as depending on other extensions, GCC will implicitly add the dependent extensions to the enabled extension set if they weren't added explicitly. it's giving ✨automatic✨, it's giving 👑generous👑
-mcpu=processor-string— use the architecture of, and optimize the output for, the given processor, specified by a particular CPU name. permissible values are:sifive-e20,sifive-e21,sifive-e24,sifive-e31,sifive-e34,sifive-e76,sifive-s21,sifive-s51,sifive-s54,sifive-s76,sifive-u54,sifive-u74,sifive-x280,sifive-xp450,sifive-x670,thead-c906,tt-ascalon-d8,xiangshan-nanhu.
heads up bestie:-mcpudoes not override-marchor-mtune. like, it knows its lane 🎀
-mtune=processor-string— optimize the output for the given processor, specified by microarchitecture or particular CPU name. permissible values:generic-ooo,rocket,sifive-3-series,sifive-5-series,sifive-7-series,size,sifive-p400-series,sifive-p600-series, and all valid options for-mcpu=.
when -mtune= isn't specified, it uses the setting from -mcpu, and the default is rocket if both are not specified. 🚀
the size choice is not intended for use by end-users — it's what gets used when -Os is specified. it overrides the instruction cost info provided by -mtune=, but it does not override the pipeline info. the way it helps reduce code size while still giving good performance… iconic 🫶
-mpreferred-stack-boundary=num— attempt to keep the stack boundary aligned to a 2 raised to num byte boundary. if-mpreferred-stack-boundaryisn't specified, the default is4(16 bytes or 128-bits).
⚠️ 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. no cap, don't play with this one 😭
-msmall-data-limit=n— put global and static data smaller than n bytes into a special section (on some targets).-msave-restore-mno-save-restore— do or don't use smaller but slower prologue and epilogue code that uses library function calls. the default is to use fast inline prologues and epilogues. ⚡-mmovcc-mno-movcc— do or don't produce branchless conditional-move code sequences even with targets that do not have specific instructions for conditional operations. if enabled, sequences of ALU operations are produced using base integer ISA instructions where profitable. ✨-minline-atomics-mno-inline-atomics— do or don't use smaller but slower subword atomic emulation code that useslibatomicfunction calls. the default is to use fast inline subword atomics that do not requirelibatomic. 💅-minline-strlen-mno-inline-strlen— do or do not attempt to inlinestrlencalls if possible. inlining will only be done if the string is properly aligned and instructions for accelerated processing are available. the default is to not inlinestrlencalls. 🙈-minline-strcmp-mno-inline-strcmp— do or do not attempt to inlinestrcmpcalls if possible. inlining will only be done if the strings are properly aligned and instructions for accelerated processing are available. the default is to not inlinestrcmpcalls.
the --param riscv-strcmp-inline-limit=n parameter controls the maximum number of bytes compared by the inlined code. the default value is 64. 🍓
-minline-strncmp-mno-inline-strncmp— do or do not attempt to inlinestrncmpcalls if possible. inlining will only be done if the strings are properly aligned and instructions for accelerated processing are available. the default is to not inlinestrncmpcalls.
the --param riscv-strcmp-inline-limit=n parameter controls the maximum number of bytes compared by the inlined code. the default value is 64.
-mshorten-memrefs-mno-shorten-memrefs— do or do not attempt to make more use of compressed load/store instructions by replacing a load/store of 'base register + large offset' with a new load/store of 'new base + small offset'. if the new base gets stored in a compressed register, then the new load/store can be compressed. currently targets 32-bit integer load/stores only. 🎀-mstrict-align-mno-strict-align— do not or do generate unaligned memory accesses. the default is set depending on whether the processor we are optimizing for supports fast unaligned access or not.-mscalar-strict-align-mno-scalar-strict-align— do not or do generate unaligned memory accesses. the default is set depending on whether the processor we are optimizing for supports fast unaligned access or not. this is an alias for-mstrict-align. 🦋-mvector-strict-align-mno-vector-strict-align— do not or do generate unaligned vector memory accesses. the default is set to off unless the processor we are optimizing for explicitly supports element-misaligned vector memory access. 👑-mcmodel=medlow— generate code for the medium-low code model. the program and its statically defined symbols must lie within a single 2 GiB address range and must lie between absolute addresses -2 GiB and +2 GiB. programs can be statically or dynamically linked. this is the default code model unless GCC has been configured with--with-cmodel=specifying a different default code model. ✨-mcmodel=medany— generate code for the medium-any code model. the program and its statically defined symbols must be within any single 2 GiB address range. programs can be statically or dynamically linked.
the code generated by the medium-any code model is position-independent, but is not guaranteed to function correctly when linked into position-independent executables or libraries. lowkey important, bestie 😭
-mcmodel=large— generate code for a large code model, which has no restrictions on size or placement of symbols. it's giving unbothered 💅-mexplicit-relocs-mno-exlicit-relocs— use or do not use assembler relocation operators when dealing with symbolic addresses. the alternative is to use assembler macros instead, which may limit optimization. 🌸-mrelax-mno-relax— take advantage of linker relaxations to reduce the number of instructions required to materialize symbol addresses. the default is to take advantage of linker relaxations. 🔥-mriscv-attribute,-mno-riscv-attribute— turn ON (or OFF) emitting the RISC-V attribute so extra info gets recorded into ELF objects 🗂️✨. Heads up bestie: this needs at least binutils 2.32, no cap.-mcsr-check,-mno-csr-check— enables or disables the CSR checking 💅.- *
-malign-data=type* — controls how GCC aligns variables and constants of array, structure, or union types 🎀. The supported values for type arexlen, which uses x register width as the alignment value, andnatural, which uses natural alignment.xlenis the default, periodt. -mbig-endian— generates big-endian code 🐘. This is the default when GCC is configured for ariscv64be-*-*orriscv32be-*-*target.-mlittle-endian— generates little-endian code 🐭. This is the default when GCC is configured for ariscv64-*-*orriscv32-*-*but not ariscv64be-*-*orriscv32be-*-*target.- *
-mstack-protector-guard=guard*, *-mstack-protector-guard-reg=reg*, *-mstack-protector-guard-offset=offset* — generate stack protection code using the canary at guard 🐤🔒. The supported locations areglobalfor a global canary, ortlsfor a per-thread canary in the TLS block.
With that latter choice, the options -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset furthermore specify which register to use as the base register for reading the canary, and from what offset from that base register 🌸. There is no default register or offset, because this is entirely for use within the Linux kernel — iykyk.
-mtls-dialect=desc— uses TLS descriptors as the thread-local storage mechanism for dynamic accesses of TLS variables ✨.-mtls-dialect=trad— uses traditional TLS as the thread-local storage mechanism for dynamic accesses of TLS variables 🧵. This is the default.
🌈 RL78 Options 🌈
-msim— links in additional target libraries to support operation within a simulator 🖥️💫.-mmul=none,-mmul=g10,-mmul=g13,-mmul=g14,-mmul=rl78— specifies the type of hardware multiplication and division support to be used ✖️➗. The simplest is"none", which uses software for both multiplication and division. This is the default. The"g13"value is for the hardware multiply/divide peripheral found on the RL78/G13 (S2 core) targets. The"g14"value selects the use of the multiplication and division instructions supported by the RL78/G14 (S3 core) parts. The value"rl78"is an alias for"g14", and the value"mg10"is an alias for"none"👀.
In addition, a C preprocessor macro is defined, based upon the setting of this option. Possible values are: "__RL78_MUL_NONE__", "__RL78_MUL_G13__" or "__RL78_MUL_G14__" 🎀.
-mcpu=g10,-mcpu=g13,-mcpu=g14,-mcpu=rl78— specifies the RL78 core to target 💅. The default is the G14 core, also known as an S3 core or just RL78. The G13 or S2 core does not have multiply or divide instructions; instead it uses a hardware peripheral for these operations. The G10 or S1 core does not have register banks, so it uses a different calling convention.
If this option is set, it also selects the type of hardware multiply support to use — unless this is overridden by an explicit -mmul=none option on the command line 👑. So specifying -mcpu=g13 enables the use of the G13 hardware multiply peripheral, and specifying -mcpu=g10 disables the use of hardware multiplications altogether.
Note bestie: although the RL78/G14 core is the default target, specifying-mcpu=g14or-mcpu=rl78on the command line does change the behavior of the toolchain, since it also enables G14 hardware multiply support 😭. If these options are not specified on the command line, then software multiplication routines will be used even though the code targets the RL78 core. This is for backwards compatibility with older toolchains which did not have hardware multiply and divide support.
In addition, a C preprocessor macro is defined, based upon the setting of this option. Possible values are: "__RL78_G10__", "__RL78_G13__" or "__RL78_G14__" 🦋.
-mg10,-mg13,-mg14,-mrl78— these are aliases for the corresponding-mcpu=option 🫶. They are provided for backwards compatibility.-mallregs— allows the compiler to use all of the available registers ✨. By default, registers"r24..r31"are reserved for use in interrupt handlers. With this option enabled, these registers can be used in ordinary functions as well 👑.-m64bit-doubles,-m32bit-doubles— makes the"double"data type be 64 bits (-m64bit-doubles) or 32 bits (-m32bit-doubles) in size 📏. The default is-m32bit-doubles.-msave-mduc-in-interrupts,-mno-save-mduc-in-interrupts— specifies that interrupt handler functions should preserve the MDUC registers 💾. This is only necessary if normal code might use the MDUC registers, for example because it performs multiplication and division operations. The default is to ignore the MDUC registers, since this makes the interrupt handlers faster ⚡. The target option-mg13needs to be passed for this to work, because this feature is only available on the G13 target (S2 core). The MDUC registers will only be saved if the interrupt handler performs a multiplication or division operation or it calls another function 👀.
👑 IBM RS/6000 and PowerPC Options 👑
These -m options are defined for the IBM RS/6000 and PowerPC:
-mpowerpc-gpopt,-mno-powerpc-gpopt,-mpowerpc-gfxopt,-mno-powerpc-gfxopt,-mpowerpc64,-mno-powerpc64,-mmfcrf,-mno-mfcrf,-mpopcntb,-mno-popcntb,-mpopcntd,-mno-popcntd,-mfprnd,-mno-fprnd,-mcmpb,-mno-cmpb,-mhard-dfp,-mno-hard-dfp— you use these options to specify which instructions are available on the processor you're 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, fr fr.
Specifying -mpowerpc-gpopt allows GCC to use the optional PowerPC architecture instructions in the General Purpose group, including floating-point square root 🧮. Specifying -mpowerpc-gfxopt allows GCC to use the optional PowerPC architecture instructions in the Graphics group, including floating-point select 🎨.
The -mmfcrf option allows GCC to generate the move from condition register field instruction implemented on the POWER4 processor and other processors that support the PowerPC V2.01 architecture ✨. The -mpopcntb option allows GCC to generate the popcount and double-precision FP reciprocal estimate instruction implemented on the POWER5 processor and other processors that support the PowerPC V2.02 architecture. The -mpopcntd option allows GCC to generate the popcount instruction implemented on the POWER7 processor and other processors that support the PowerPC V2.06 architecture. The -mfprnd option allows GCC to generate the FP round to integer instructions implemented on the POWER5+ processor and other processors that support the PowerPC V2.03 architecture. The -mcmpb option allows GCC to generate the compare bytes instruction implemented on the POWER6 processor and other processors that support the PowerPC V2.05 architecture. The -mhard-dfp option allows GCC to generate the decimal floating-point instructions implemented on some POWER processors 🔥.
The -mpowerpc64 option allows GCC to generate the additional 64-bit instructions that are found in the full PowerPC64 architecture and to treat GPRs as 64-bit, doubleword quantities. GCC defaults to -mno-powerpc64 👑.
- *
-mcpu=cpu_type* — sets the architecture type, register usage, and instruction scheduling parameters for machine type cpu_type 🎯. The supported values for cpu_type are401,403,405,405fp,440,440fp,464,464fp,476,476fp,505,601,602,603,603e,604,604e,620,630,740,7400,7450,750,801,821,823,860,970,8540,a2,e300c2,e300c3,e500mc,e500mc64,e5500,e6500,ec603e,G3,G4,G5,titan,power3,power4,power5,power5+,power6,power6x,power7,power8,power9,power10,power11,powerpc,powerpc64,powerpc64le,rs64, andnative💖.
-mcpu=powerpc, -mcpu=powerpc64, and -mcpu=powerpc64le specify pure 32-bit PowerPC (either endian), 64-bit big endian PowerPC and 64-bit little endian PowerPC architecture machine types, with an appropriate, generic processor model assumed for scheduling purposes 🌸.
Specifying native as cpu type detects and selects the architecture option that corresponds to the host processor of the system performing the compilation 🔍. -mcpu=native has no effect if GCC does not recognize the processor.
The other options specify a specific processor. Code generated under those options runs best on that processor, and may not run at all on others 😭.
The -mcpu options automatically enable or disable the following options:
-maltivec -mfprnd -mhard-float -mmfcrf -mmultiple -mpopcntb
-mpopcntd -mpowerpc64 -mpowerpc-gpopt -mpowerpc-gfxopt -mmulhw
-mdlmzb -mmfpgpr -mvsx -mcrypto -mhtm -mpower8-fusion
-mquad-memory -mquad-memory-atomic -mfloat128 -mfloat128-hardware
-mprefixed -mpcrel -mmma -mrop-protectThe particular options set for any particular CPU varies between compiler versions, depending on what setting seems to produce optimal code for that CPU; it doesn't necessarily reflect the actual hardware's capabilities 👀. If you wish to set an individual option to a particular value, you may specify it after the -mcpu option, like -mcpu=970 -mno-altivec.
On AIX, the -maltivec and -mpowerpc64 options are not enabled or disabled by the -mcpu option at present, because AIX does not have full support for these options 💀. You may still enable or disable them individually if you're sure it'll work in your environment 🫶.
-mtune=cpu_type — sets the instruction scheduling parameters for machine type cpu_type, but does *not set the architecture type or register usage, the way-mcpu=cpu_type* does 🎀. The same values for cpu_type are used for-mtuneas for-mcpu. If both are specified, the code generated uses the architecture and registers set by-mcpu, but the scheduling parameters set by-mtune✨.-mcmodel=small— generates PowerPC64 code for the small model: the TOC is limited to 64k 📦.-mcmodel=medium— makes PowerPC64 code for the medium model, where the TOC and all the other static data together can be up to 4G in size. This is literally the default for 64-bit Linux, so if you didn't ask for anything else, bestie, this is what you got. 💖-mcmodel=large— PowerPC64 code for the large model: the TOC may be up to 4G in size, and other data and code are only limited by the 64-bit address space itself. Big model energy. 👑-maltivec,-mno-altivec— generate code that uses (or does not use) AltiVec instructions, and also turn on the built-in functions that give you more direct access to the AltiVec instruction set. You might also need to set-mabi=altivecto adjust the current ABI with AltiVec ABI enhancements.
When -maltivec is used, the element order for AltiVec intrinsics like vec_splat, vec_extract, and vec_insert matches array element order corresponding to the endianness of the target. Translation: element zero identifies the leftmost element in a vector register when targeting a big-endian platform, and identifies the rightmost element in a vector register when targeting a little-endian platform. Endianness really said "I contain multitudes". 🌈
-mvrsave,-mno-vrsave— generate VRSAVE instructions when generating AltiVec code.-msecure-plt— generate code that letsldandld.sobuild executables and shared libraries with non-executable.pltand.gotsections. This is a PowerPC 32-bit SYSV ABI option. Security slay. 🔐-mbss-plt— generate code that uses a BSS.pltsection thatld.sofills in, and it requires.pltand.gotsections that are both writable and executable. This is a PowerPC 32-bit SYSV ABI option. Writable AND executable, she's living dangerously fr fr. 💀-msplit-patch-nops— when you're adding NOPs for a patchable area via the-fpatchable-function-entryoption, this emits the "before" NOPs in front of the global entry point and the "after" NOPs after the local entry point. That makes the sequence of NOPs not consecutive when a global entry point is generated. Without this option the NOPs are emitted directly before and after the local entry point, which makes them consecutive but moves the global and local entry point further apart. If only a single entry point is generated, this option has no effect. iykyk. ✨-misel,-mno-isel— this switch enables or disables the generation of ISEL instructions.-mvsx,-mno-vsx— generate code that uses (does not use) vector/scalar (VSX) instructions, and also enable the use of built-in functions that allow more direct access to the VSX instruction set.-mcrypto,-mno-crypto— enable the use (disable) of the built-in functions that allow direct access to the cryptographic instructions that were added in version 2.07 of the PowerPC ISA. Crypto girlies rise 🫶-mhtm,-mno-htm— enable (disable) the use of the built-in functions that allow direct access to the Hardware Transactional Memory (HTM) instructions that were added in version 2.07 of the PowerPC ISA.-mpower8-fusion,-mno-power8-fusion— generate code that keeps (does not keeps) some integer operations adjacent so that the instructions can be fused together on power8 and later processors. (Yes, the original really says "does not keeps" and I am not the one who is going to fix it. 😭 Preserving the lore, periodt.)-mquad-memory,-mno-quad-memory— generate code that uses (does not use) the non-atomic quad word memory instructions. The-mquad-memoryoption requires use of 64-bit mode.-mquad-memory-atomic,-mno-quad-memory-atomic— generate code that uses (does not use) the atomic quad word memory instructions. The-mquad-memory-atomicoption requires use of 64-bit mode too.-mfloat128,-mno-float128— enable/disable the__float128keyword for IEEE 128-bit floating point, and use either software emulation for IEEE 128-bit floating point or hardware instructions.
The VSX instruction set (-mvsx) must be enabled to use the IEEE 128-bit floating point support. And the IEEE 128-bit floating point is only supported on Linux. 😭
The default for -mfloat128 is enabled on PowerPC Linux systems using the VSX instruction set, and disabled on other systems.
If you use the ISA 3.0 instruction set (-mcpu=power9) on a 64-bit system, the IEEE 128-bit floating point support will also enable the generation of ISA 3.0 IEEE 128-bit floating point instructions. Otherwise — if you don't specify to generate ISA 3.0 instructions, or you're targeting a 32-bit big endian system — IEEE 128-bit floating point will be done with software emulation.
-mfloat128-hardware,-mno-float128-hardware— enable/disable using ISA 3.0 hardware instructions to support the__float128data type.
The default for -mfloat128-hardware is enabled on PowerPC Linux systems using the ISA 3.0 instruction set, and disabled on other systems.
-m32,-m64— generate code for 32-bit or 64-bit environments of Darwin and SVR4 targets (including GNU/Linux). The 32-bit environment setsint,longand pointer to 32 bits and generates code that runs on any PowerPC variant. The 64-bit environment setsintto 32 bits andlongand pointer to 64 bits, and generates code for PowerPC64, as for-mpowerpc64.-mfull-toc,-mno-fp-in-toc,-mno-sum-in-toc,-mminimal-toc— these modify generation of the TOC (Table Of Contents), which is created for every executable file. The-mfull-tocoption is selected by default. In that case, GCC allocates at least one TOC entry for each unique non-automatic variable reference in your program. GCC also places floating-point constants in the TOC. However, only 16,384 entries are available in the TOC. 👀
If you receive a linker error message that saying you have overflowed the available TOC space (yes, the original's grammar, love that for us 💅), you can reduce the amount of TOC space used with the -mno-fp-in-toc and -mno-sum-in-toc options. -mno-fp-in-toc prevents GCC from putting floating-point constants in the TOC, and -mno-sum-in-toc forces GCC to generate code to calculate the sum of an address and a constant at run time instead of putting that sum into the TOC. You may specify one or both of these options. Each causes GCC to produce very slightly slower and larger code at the expense of conserving TOC space.
If you still run out of space in the TOC even when you specify both of these options, specify -mminimal-toc instead. This option causes GCC to make only one TOC entry for every file. When you specify this option, GCC produces code that is slower and larger but which uses extremely little TOC space. You may wish to use this option only on files that contain less frequently-executed code.
-maix64,-maix32— enable 64-bit AIX ABI and calling convention: 64-bit pointers, 64-bitlongtype, and the infrastructure needed to support them. Specifying-maix64implies-mpowerpc64, while-maix32disables the 64-bit ABI and implies-mno-powerpc64. GCC defaults to-maix32.-mxl-compat,-mno-xl-compat— produce code that conforms more closely to IBM XL compiler semantics when using AIX-compatible ABI. Pass floating-point arguments to prototyped functions beyond the register save area (RSA) on the stack in addition to argument FPRs. Do not assume that most significant double in 128-bit long double value is properly rounded when comparing values and converting todouble. Use XL symbol names for long double support routines.
The AIX calling convention was extended but not initially documented to handle an obscure K&R C case of calling a function that takes the address of its arguments with fewer arguments than declared. IBM XL compilers access floating-point arguments that do not fit in the RSA from the stack when a subroutine is compiled without optimization. Because always storing floating-point arguments on the stack is inefficient and rarely needed, this option is not enabled by default and only is necessary when calling subroutines compiled by IBM XL compilers without optimization.
-mpe— support IBM RS/6000 SP Parallel Environment (PE). Link an application written to use message passing with special startup code to enable the application to run. The system must have PE installed in the standard location/usr/lpp/ppe.poe/, or the specs file must be overridden with the-specs=option to specify the appropriate directory location. The Parallel Environment does not support threads, so the-mpeoption and the-pthreadoption are incompatible. Pick a lane bestie, you can't have both. 💀-malign-natural,-malign-power— on AIX, 32-bit Darwin, and 64-bit PowerPC GNU/Linux, the option-malign-naturaloverrides the ABI-defined alignment of larger types, such as floating-point doubles, on their natural size-based boundary. The option-malign-powerinstructs 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-floatoption, 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 instructions are generated by default on POWER systems, and not generated on PowerPC systems. Do not use-mmultipleon little-endian PowerPC systems, since those instructions do not work when the processor is in little-endian mode. The exceptions are PPC740 and PPC750, which permit these instructions in little-endian mode. Iconic little exceptions, we love them. 🌸-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. Be careful bestie, that's a yikes. 😭-mavoid-indexed-addresses,-mno-avoid-indexed-addresses— generate code that tries to avoid (not avoid) the use of indexed load or store instructions. These instructions can incur a performance penalty on Power6 processors in certain situations, such as when stepping through large arrays that cross a 16M boundary. This option is enabled by default when targeting Power6 and disabled otherwise.-mfused-madd,-mno-fused-madd— turns on (or off, bestie) codegen that uses the floating-point multiply-and-accumulate instructions. These come out by default whenever hardware floating point is in play. Also real talk: the machine-dependent-mfused-maddoption has now been mapped to the machine-independent-ffp-contract=fastoption, and-mno-fused-maddis mapped to-ffp-contract=off.-mmulhw,-mno-mulhw— do (or don't) generate code that uses the half-word multiply and multiply-accumulate instructions on the IBM 405, 440, 464 and 476 processors. These instructions are generated by default when you're targeting those processors. 💅-mdlmzb,-mno-dlmzb— do (or don't) generate code that uses the string-searchdlmzbinstruction on the IBM 405, 440, 464 and 476 processors. This instruction is generated by default when targeting those processors.-mno-bit-align,-mbit-align— on System V.4 and embedded PowerPC systems, don't (or do) force structures and unions that contain bit-fields to be aligned to the base type of the bit-field.
Example time, because numbers > vibes: by default a structure containing nothing but 8 unsigned bit-fields of length 1 is aligned to a 4-byte boundary and has a size of 4 bytes. Switch on -mno-bit-align and the structure gets aligned to a 1-byte boundary and is 1 byte in size. The way that shrinks is genuinely iconic. ✨
-mno-strict-align,-mstrict-align— on System V.4 and embedded PowerPC systems, don't (or do) assume that unaligned memory references are handled by the system.-mrelocatable,-mno-relocatable— generate code that allows (or does not allow) a static executable to be relocated to a different address at run time. A simple embedded PowerPC system loader should relocate the entire contents of.got2and 4-byte locations listed in the.fixupsection, a table of 32-bit addresses generated by this option. For this to actually work, all objects linked together must be compiled with-mrelocatableor-mrelocatable-lib. Also-mrelocatablecode aligns the stack to an 8-byte boundary.-mrelocatable-lib,-mno-relocatable-lib— like-mrelocatable,-mrelocatable-libgenerates a.fixupsection to allow static executables to be relocated at run time, but-mrelocatable-libdoes not use the smaller stack alignment of-mrelocatable. Objects compiled with-mrelocatable-libmay be linked with objects compiled with any combination of the-mrelocatableoptions. Mix and match, no cap. 🫶-mno-toc,-mtoc— on System V.4 and embedded PowerPC systems, don't (or 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. And fr fr,-mlittle-endianis the exact same thing as-mlittle.-mbig,-mbig-endian— on System V.4 and embedded PowerPC systems, compile code for the processor in big-endian mode.-mbig-endianis the same as-mbig. Twin options, twin energy. 👑-mdynamic-no-pic— on Darwin / macOS 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. Don't try it, bestie. 😭-msingle-pic-base— treat the register used for PIC addressing as read-only, rather than loading it in the prologue for each function. The runtime system is responsible for initializing this register with an appropriate value before execution begins. So like, that's on them, not on you.-mprioritize-restricted-insns=priority— controls the priority that gets assigned to dispatch-slot restricted instructions during the second scheduling pass. The argument priority takes the value0,1, or2to assign no, highest, or second-highest (respectively) priority to dispatch-slot restricted instructions.-msched-costly-dep=dependence_type— controls which dependences are considered costly by the target during instruction scheduling. The argument dependence_type takes one of these values:no— no dependence is costly.all— all dependences are costly.true_store_to_load— a true dependence from store to load is costly.store_to_load— any dependence from store to load is costly.- number — any dependence for which the latency is greater than or equal to number is costly.
-minsert-sched-nops=scheme— controls which NOP insertion scheme is used during the second scheduling pass. The argument scheme takes one of these values:no— don't insert NOPs.pad— pad with NOPs any dispatch group that has vacant issue slots, according to the scheduler's grouping.regroup_exact— insert NOPs to force costly dependent insns into separate groups. Insert exactly as many NOPs as needed to force an insn to a new group, according to the estimated processor grouping.- number — insert NOPs to force costly dependent insns into separate groups. Insert number NOPs to force an insn to a new group.
-mcall-sysv— on System V.4 and embedded PowerPC systems, compile code using calling conventions that adhere to the March 1995 draft of the System V Application Binary Interface, PowerPC processor supplement. This is the default unless you configured GCC usingpowerpc-*-eabiaix.-mcall-sysv-eabi,-mcall-eabi— specify both-mcall-sysvand-meabioptions. Two for one, slay.-mcall-sysv-noeabi— specify both-mcall-sysvand-mno-eabioptions.-mcall-aixdesc— on System V.4 and embedded PowerPC systems, compile code for the AIX operating system.-mcall-linux— on System V.4 and embedded PowerPC systems, compile code for the Linux-based GNU system.-mcall-freebsd— on System V.4 and embedded PowerPC systems, compile code for the FreeBSD operating system.-mcall-netbsd— on System V.4 and embedded PowerPC systems, compile code for the NetBSD operating system.-mcall-openbsd— on System V.4 and embedded PowerPC systems, compile code for the OpenBSD operating system.-mtraceback=traceback_type— select the type of traceback table. Valid values for traceback_type arefull,part, andno.-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,ibmlongdouble,ieeelongdouble,elfv1,elfv2, and for AIX:vec-extabi,vec-default.-mabi=ibmlongdouble— change the current ABI to use IBM extended-precision long double. Heads up: this is not likely to work if your system defaults to using IEEE extended-precision long double. If you change thelong doubletype from IEEE extended-precision, the compiler issues a warning unless you use the-Wno-psabioption. Requires-mlong-double-128to be enabled.-mabi=ieeelongdouble— change the current ABI to use IEEE extended-precision long double. Also not likely to work if your system defaults to using IBM extended-precision long double. If you change thelong doubletype from IBM extended-precision, the compiler issues a warning unless you use the-Wno-psabioption. Requires-mlong-double-128to be enabled.-mabi=elfv1— change the current ABI to use the ELFv1 ABI. This is the default ABI for big-endian PowerPC 64-bit Linux. ⚠️ Overriding the default ABI requires special system support and is likely to fail in spectacular ways.-mabi=elfv2— change the current ABI to use the ELFv2 ABI. This is the default ABI for little-endian PowerPC 64-bit Linux. Overriding the default ABI requires special system support and is likely to fail in spectacular ways. Same dramatic warning, different ABI. 💀-mgnu-attribute,-mno-gnu-attribute— emit.gnu_attributeassembly directives to set tag/value pairs in a.gnu.attributessection that specify ABI variations in function parameters or return values.-mprototype,-mno-prototype— on System V.4 and embedded PowerPC systems, assume that all calls to variable argument functions are properly prototyped. Otherwise, the compiler must insert an instruction before every non-prototyped call to set or clear bit 6 of the condition code register (CR) to indicate whether floating-point values are passed in the floating-point registers in case the function takes variable arguments. With-mprototype, only calls to prototyped variable argument functions set or clear the bit.-msim— on embedded PowerPC systems, assume that the startup module is calledsim-crt0.oand that the standard C libraries arelibsim.aandlibc.a. This is the default forpowerpc-*-eabisimconfigurations.-mmvme— on embedded PowerPC systems, assume that the startup module is calledcrt0.oand the standard C libraries arelibmvme.aandlibc.a.-mads— on embedded PowerPC systems, assume that the startup module is calledcrt0.oand the standard C libraries arelibads.aandlibc.a.-myellowknife— on embedded PowerPC systems, assume that the startup module is calledcrt0.oand the standard C libraries arelibyk.aandlibc.a. (Yes the option is really called yellowknife, we don't make the rules 🍓)-mvxworks— on System V.4 and embedded PowerPC systems, specify that you are compiling for a VxWorks system.-memb— on embedded PowerPC systems, set thePPC_EMBbit 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 (or don't) adhere to the Embedded Applications Binary Interface (EABI), which is a set of modifications to the System V.4 specifications. Picking-meabimeans the stack is aligned to an 8-byte boundary, a function__eabiis called frommainto set up the EABI environment, and the-msdataoption can use bothr2andr13to point to two separate small data areas. Picking-mno-eabimeans the stack is aligned to a 16-byte boundary, no EABI initialization function is called frommain, and the-msdataoption only usesr13to point to a single small data area. And-meabiis on by default if you configured GCC using one of thepowerpc*-*-eabi*options. ✨-msdata=eabi— okay so on System V.4 and embedded PowerPC systems, this one puts small initializedconstglobal and static data in the.sdata2section, which is pointed to by registerr2💅. Small initialized non-constglobal and static data goes in.sdata, pointed to by registerr13. And small uninitialized global and static data goes in.sbss, which is adjacent to.sdata.
Heads up bestie:-msdata=eabiis incompatible with-mrelocatable😭. It also sets-memb. Two things at once, she's doing the most.
-msdata=sysv— on System V.4 and embedded PowerPC systems, small global and static data goes in.sdata, pointed to by registerr13✨. Small uninitialized global and static data goes in.sbss, which is adjacent to.sdata.
And-msdata=sysvis also incompatible with-mrelocatable. No cap.
-msdata=default,-msdata— on System V.4 and embedded PowerPC systems, if-meabiis used, it compiles code the same as-msdata=eabi; otherwise it compiles code the same as-msdata=sysv🦋.-msdata=data— on System V.4 and embedded PowerPC systems, small global data goes in the.sdatasection. Small uninitialized global data goes in the.sbsssection. But like... do not use registerr13to address small data here 🙈. This is the default behavior unless other-msdataoptions are used.-msdata=none,-mno-sdata— on embedded PowerPC systems, this puts all initialized global and static data in the.datasection, and all uninitialized data in the.bsssection 🌸.-mreadonly-in-sdata— puts read-only objects in the.sdatasection too. This is the default, it's giving humble and consistent 💖.-mblock-move-inline-limit=num— inlines all block moves (think calls tomemcpyor structure copies) that are less than or equal to num bytes 🔥. The minimum value for num is 32 bytes on 32-bit targets and 64 bytes on 64-bit targets. The default value is target-specific.-mblock-compare-inline-limit=num— generates non-looping inline code for all block compares (like calls tomemcmpor structure compares) that are less than or equal to num bytes 🎀. If num is 0, all inline expansion (both non-loop and loop) of block compare is disabled. The default value is target-specific.-mblock-compare-inline-loop-limit=num— generates an inline expansion using loop code for all block compares that are less than or equal to num bytes, but greater than the limit for non-loop inline block compare expansion ⭐. If the block length is not constant, at most num bytes will be compared beforememcmpis called to compare the remainder of the block. The default value is target-specific.-mstring-compare-inline-limit=num— compares at most num string bytes with inline code 🌈. If the difference or end of string is not found at the end of the inline compare, a call tostrcmporstrncmptakes care of the rest of the comparison. The default is 64 bytes.-G num— on embedded PowerPC systems, this puts global and static items that are 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 numswitch is also passed to the linker. And listen: all modules should be compiled with the same-G numvalue 💀. Consistency is the whole vibe.-mregnames,-mno-regnames— on System V.4 and embedded PowerPC systems, do (or do not) emit register names in the assembly language output using symbolic forms ✨.-mlongcall,-mno-longcall— by default this assumes that all calls are far away, so a longer and more expensive calling sequence is required 😭. This is required for calls farther than 32 megabytes (33,554,432 bytes) from the current location. A short call is generated if the compiler knows the call cannot be that far away. This setting can be overridden by theshortcallfunction 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 PowerPC64 ELFv2 and 32-bit PowerPC systems with newer GNU linkers, GCC can generate long calls using an inline PLT call sequence (see -mpltseq) 🔥. PowerPC with -mbss-plt and PowerPC64 ELFv1 (big-endian) do not support inline PLT calls.
On Darwin/PPC systems, #pragma longcall generates jbsr callee, L42, plus a branch island (glue code). The two target addresses represent the callee and the branch island. The Darwin/PPC linker prefers the first address and generates a bl callee if the PPC bl instruction reaches the callee directly; otherwise, the linker generates bl L42 to call the branch island. The branch island is appended to the body of the calling function; it computes the full 32-bit address of the callee and jumps to it 🌸.
On Mach-O (Darwin) systems, this option directs the compiler to emit the glue for every direct call, and the Darwin linker decides whether to use or discard it.
In the future, GCC may ignore all longcall specifications when the linker is known to generate glue 👀.
-mpltseq,-mno-pltseq— implement (or do not implement)-fno-pltand long calls using an inline PLT call sequence that supports lazy linking and long calls to functions indlopen'd shared libraries 💅. Inline PLT calls are only supported on PowerPC64 ELFv2 and 32-bit PowerPC systems with newer GNU linkers, and are enabled by default if the support is detected when configuring GCC, and, in the case of 32-bit PowerPC, if GCC is configured with--enable-secureplt.
Lowkey important:-mpltseqcode and-mbss-plt32-bit PowerPC relocatable objects may not be linked together 😭.
-mtls-markers,-mno-tls-markers— mark (or do not mark) calls to__tls_get_addrwith a relocation specifying the function argument 🎀. The relocation allows the linker to reliably associate function call with argument setup instructions for TLS optimization, which in turn allows GCC to better schedule the sequence. It's giving teamwork fr fr.-mrecip,-mno-recip— this option enables use of the reciprocal estimate and reciprocal square root estimate instructions with additional Newton-Raphson steps to increase precision, instead of doing a divide or square root and divide for floating-point arguments 🔥.
You should use the-ffast-mathoption when using-mrecip(or at least-funsafe-math-optimizations,-ffinite-math-only,-freciprocal-mathand-fno-trapping-math) 💜.
Note that while the throughput of the sequence is generally higher than the throughput of the non-reciprocal instruction, the precision of the sequence can be decreased by up to 2 ulp (i.e. the inverse of 1.0 equals 0.99999994) for reciprocal square roots 😭.
-mrecip=opt— this option controls which reciprocal estimate instructions may be used 👑. opt is a comma-separated list of options, which may be preceded by a!to invert the option:all— enable all estimate instructions.default— enable the default instructions, equivalent to-mrecip.none— disable all estimate instructions, equivalent to-mno-recip.div— enable the reciprocal approximation instructions for both single and double precision.divf— enable the single-precision reciprocal approximation instructions.divd— enable the double-precision reciprocal approximation instructions.rsqrt— enable the reciprocal square root approximation instructions for both single and double precision.rsqrtf— enable the single-precision reciprocal square root approximation instructions.rsqrtd— enable the double-precision reciprocal square root approximation instructions.
So, for example, -mrecip=all,!rsqrtd enables all of the reciprocal estimate instructions, except for the FRSQRTE, XSRSQRTEDP, and XVRSQRTEDP instructions which handle the double-precision reciprocal square root calculations ✨.
-mrecip-precision,-mno-recip-precision— assume (or do not assume) that the reciprocal estimate instructions provide higher-precision estimates than is mandated by the PowerPC ABI 🦋. Selecting-mcpu=power6,-mcpu=power7or-mcpu=power8automatically selects-mrecip-precision. The double-precision square root estimate instructions are not generated by default on low-precision machines, since they do not provide an estimate that converges after three steps 🙈.-mveclibabi=type— specifies the ABI type to use for vectorizing intrinsics using an external library 🌈. The only type supported at present ismass, which specifies to use IBM's Mathematical Acceleration Subsystem (MASS) libraries for vectorizing intrinsics using external libraries.
GCC currently emits calls to acosd2, acosf4, acoshd2, acoshf4, asind2, asinf4, asinhd2, asinhf4, atan2d2, atan2f4, atand2, atanf4, atanhd2, atanhf4, cbrtd2, cbrtf4, cosd2, cosf4, coshd2, coshf4, erfcd2, erfcf4, erfd2, erff4, exp2d2, exp2f4, expd2, expf4, expm1d2, expm1f4, hypotd2, hypotf4, lgammad2, lgammaf4, log10d2, log10f4, log1pd2, log1pf4, log2d2, log2f4, logd2, logf4, powd2, powf4, sind2, sinf4, sinhd2, sinhf4, sqrtd2, sqrtf4, tand2, tanf4, tanhd2, and tanhf4 when generating code for power7 💖. Both -ftree-vectorize and -funsafe-math-optimizations must also be enabled. The MASS libraries must be specified at link time.
-mfriz,-mno-friz— generate (or do not generate) thefrizinstruction when the-funsafe-math-optimizationsoption is used to optimize rounding of floating-point values to 64-bit integer and back to floating point 🌸.
Ngl this one's a lil cursed: the friz instruction does not return the same value if the floating-point number is too large to fit in an integer 💀.
-mpointers-to-nested-functions,-mno-pointers-to-nested-functions— bestie, this one generates (or doesn't generate) the code that loads up the static chain register (r11) when you're calling through a pointer on AIX and 64-bit Linux systems, where a function pointer points at a 3-word descriptor giving the function address, the TOC value to be loaded in registerr2, and the static chain value to be loaded in registerr11.-mpointers-to-nested-functionsis on by default, icon behaviour. 🫶
⚠️ Heads up: you literally cannot call through pointers to nested functions, or pointers to functions compiled in other languages that use the static chain, if you use -mno-pointers-to-nested-functions. No cap.
-msave-toc-indirect,-mno-save-toc-indirect— generates (or doesn't generate) code to save the TOC value in the reserved stack location in the function prologue if the function calls through a pointer on AIX and 64-bit Linux systems. If the TOC value is not saved in the prologue, it gets saved just before the call through the pointer instead. The default is-mno-save-toc-indirect. periodt. 💅-mcompat-align-parm,-mno-compat-align-parm— generates (or doesn't generate) code to pass structure parameters with a maximum alignment of 64 bits, for compatibility with older versions of GCC. the way that older GCCs (prior to 4.9.0) incorrectly did not align a structure parameter on a 128-bit boundary when that structure contained a member requiring 128-bit alignment… messy. It's corrected in more recent versions of GCC, and this option lets you generate code that's compatible with functions compiled with those older versions.- The default is
-mno-compat-align-parm.
- The default is
-mstack-protector-guard=guard,-mstack-protector-guard-reg=reg,-mstack-protector-guard-offset=offset— generate stack protection code using a canary at guard. 🌸 Supported locations areglobalfor a global canary, ortlsfor a per-thread canary in the TLS block (which is the default with GNU libc version 2.4 or later).- With that latter choice, the options
-mstack-protector-guard-reg=regand-mstack-protector-guard-offset=offsetfurthermore specify which register to use as the base register for reading the canary, and from what offset from that base register. The default for those is as specified in the relevant ABI. iykyk.
- With that latter choice, the options
-mpcrel,-mno-pcrel— generate (do not generate) pc-relative addressing. Ngl, the-mpcreloption requires that the medium code model (-mcmodel=medium) and prefixed addressing (-mprefixed) options are enabled. it's giving "you need the prerequisites, babe". ✨-mprefixed,-mno-prefixed— generate (do not generate) addressing modes using prefixed load and store instructions. The-mprefixedoption requires that the option-mcpu=power10(or later) is enabled. 👑-mmma,-mno-mma— generate (do not generate) the MMA instructions. The-mmaoption requires that the option-mcpu=power10(or later) is enabled.-mrop-protect,-mno-rop-protect— generate (do not generate) ROP protection instructions when the target processor supports them. Currently this option disables the shrink-wrap optimization (-fshrink-wrap). lowkey a big deal 🔥-mprivileged,-mno-privileged— generate (do not generate) code that will run in privileged state. main character energy fr fr 💜-mblock-ops-unaligned-vsx,-mno-block-ops-unaligned-vsx— generate (do not generate) unaligned vsx loads and stores for inline expansion ofmemcpyandmemmove. 🦋--param rs6000-vect-unroll-limit=— the vectorizer will check with target information to determine whether it would be beneficial to unroll the main vectorized loop, and by how much. This parameter sets the upper bound of how much the vectorizer will unroll the main loop. The default value is four. 🌈
🌸 RX Options 🌸
These command-line options are defined for RX targets, bestie:
-m64bit-doubles,-m32bit-doubles— make thedoubledata type be 64 bits (-m64bit-doubles) or 32 bits (-m32bit-doubles) in size. The default is-m32bit-doubles. Note that RX floating-point hardware only works on 32-bit values, which is literally why the default is-m32bit-doubles. 😭-fpu,-nofpu— enables (-fpu) or disables (-nofpu) the use of RX floating-point hardware. The default is enabled for the RX600 series and disabled for the RX200 series.- Floating-point instructions are only generated for 32-bit floating-point values, however, so the FPU hardware is not used for doubles if the
-m64bit-doublesoption is used. - Note: if the
-fpuoption is enabled, then-funsafe-math-optimizationsis also enabled automatically. This is because the RX FPU instructions are themselves unsafe. 💀
- Floating-point instructions are only generated for 32-bit floating-point values, however, so the FPU hardware is not used for doubles if the
-mcpu=name— selects the type of RX CPU to be targeted. Currently three types are supported: the generic RX600 and RX200 series hardware, and the specific RX610 CPU. The default is RX600.- The only difference between RX600 and RX610 is that the RX610 does not support the
MVTIPLinstruction. - The RX200 series does not have a hardware floating-point unit and so
-nofpuis enabled by default when this type is selected.
- The only difference between RX600 and RX610 is that the RX610 does not support the
-mbig-endian-data,-mlittle-endian-data— store data (but not code) in the big-endian format. The default is-mlittle-endian-data, i.e. to store data in the little-endian format. ✨-msmall-data-limit=N— specifies the maximum size in bytes of global and static variables which can be placed into the small data area. Using the small data area can lead to smaller and faster code, but the size of the area is limited and it is up to the programmer to ensure that the area does not overflow. Also, when the small data area is used, one of the RX's registers (usuallyr13) is reserved for use pointing to this area, so it is no longer available for use by the compiler. This could result in slower and/or larger code if variables are pushed onto the stack instead of being held in this register.- Note, common variables (variables that have not been initialized) and constants are not placed into the small data area, as they are assigned to other sections in the output executable.
- The default value is zero, which disables this feature. Note, this feature is not enabled by default with higher optimization levels (
-O2etc) because of the potentially detrimental effects of reserving a register. It is up to the programmer to experiment and discover whether this feature is of benefit to their program. See the description of the-mpidoption for a description of how the actual register to hold the small data area pointer is chosen. 🎀
-msim,-mno-sim— use the simulator runtime. The default is to use the libgloss board-specific runtime. 🧚-mas100-syntax,-mno-as100-syntax— when generating assembler output, use a syntax that is compatible with Renesas's AS100 assembler. This syntax can also be handled by the GAS assembler, but it has some restrictions so it is not generated by default.-mmax-constant-size=N— specifies the maximum size, in bytes, of a constant that can be used as an operand in a RX instruction. Although the RX instruction set does allow constants of up to 4 bytes in length to be used in instructions, a longer value equates to a longer instruction. Thus in some circumstances it can be beneficial to restrict the size of constants that are used in instructions. Constants that are too big are instead placed into a constant pool and referenced via register indirection.- The value N can be between 0 and 4. A value of 0 (the default) or 4 means that constants of any size are allowed. 💅
-mrelax— enable linker relaxation. Linker relaxation is a process whereby the linker attempts to reduce the size of a program by finding shorter versions of various instructions. Disabled by default. lowkey a glow-up for your binary size 🔥-mint-register=N— specify the number of registers to reserve for fast interrupt handler functions. The value N can be between 0 and 4. A value of 1 means that registerr13is reserved for the exclusive use of fast interrupt handlers. A value of 2 reservesr13andr12. A value of 3 reservesr13,r12andr11, and a value of 4 reservesr13throughr10. A value of 0, the default, does not reserve any registers. 👑-msave-acc-in-interrupts— specifies that interrupt handler functions should preserve the accumulator register. This is only necessary if normal code might use the accumulator register, for example because it performs 64-bit multiplications. The default is to ignore the accumulator, as this makes the interrupt handlers faster. 🌈-mpid,-mno-pid— enables the generation of position independent data. When enabled, any access to constant data is done via an offset from a base address held in a register. This allows the location of constant data to be determined at run time without requiring the executable to be relocated, which is a benefit to embedded applications with tight memory constraints. Data that can be modified is not affected by this option.- Note, using this feature reserves a register, usually
r13, for the constant data base address. This can result in slower and/or larger code, especially in complicated functions. - The actual register chosen to hold the constant data base address depends upon whether the
-msmall-data-limitand/or the-mint-registercommand-line options are enabled. Starting with registerr13and proceeding downwards, registers are allocated first to satisfy the requirements of-mint-register, then-mpidand finally-msmall-data-limit. Thus it is possible for the small data area register to ber8if both-mint-register=4and-mpidare specified on the command line. 🤯 - By default this feature is not enabled. The default can be restored via the
-mno-pidcommand-line option.
- Note, using this feature reserves a register, usually
-mno-warn-multiple-fast-interrupts,-mwarn-multiple-fast-interrupts— prevents GCC from issuing a warning message if it finds more than one fast interrupt handler when it is compiling a file. The default is to issue a warning for each extra fast interrupt handler found, as the RX only supports one such interrupt. not me getting warned every single time 🙈-mallow-string-insns,-mno-allow-string-insns— turns the string-manipulation instructionsSMOVF,SCMPU,SMOVB,SMOVU,SUNTILSWHILEand also theRMPAinstruction on or off. Bestie, listen: these instructions may prefetch data, and that is not safe if you're touching an I/O register. (See section 12.2.7 of the RX62N Group User's Manual for more, iykyk.)
The default is to allow these instructions, but GCC literally cannot reliably detect every single circumstance where a string instruction might be used to hit an I/O register, so it can't just disable them automatically. Nope. Instead it's reliant upon the programmer to pass -mno-allow-string-insns if their program accesses I/O space. So if that's you, do it 💅
When the instructions are enabled GCC defines the C preprocessor symbol __RX_ALLOW_STRING_INSNS__, otherwise it defines __RX_DISALLOW_STRING_INSNS__. Pick your fighter ✨
-mjsr,-mno-jsr— use only (or not only)JSRinstructions to access functions. Reach for this when your code size exceeds the range ofBSRinstructions. And heads up, because this one trips people:-mno-jsrdoes not mean "don't useJSR" — it means any type of branch may be used. Big difference, no cap.
Note: the generic GCC command-line option-ffixed-reghas special significance to the RX port when it's used with theinterruptfunction attribute. That attribute marks a function intended to process fast interrupts, and GCC makes sure it only uses the registersr10,r11,r12and/orr13— and only provided that the normal use of the corresponding registers has been restricted via the-ffixed-regor-mint-registercommand-line options. So many conditions, but okay, we move 🫶
👑 S/390 and zSeries Options 👑
These are the -m options defined for the S/390 and zSeries architecture. Sit tight, there's a whole roster 🌈
-mhard-float,-msoft-float— use (or don't use) the hardware floating-point instructions and registers for floating-point operations. When-msoft-floatis specified, functions inlibgcc.aare used to perform floating-point operations. When-mhard-floatis specified, the compiler generates IEEE floating-point instructions. That one is the default. iconic-mhard-dfp,-mno-hard-dfp— use (or don't use) the hardware decimal-floating-point instructions for decimal-floating-point operations. With-mno-hard-dfp, functions inlibgcc.ado the decimal-floating-point operations. With-mhard-dfp, the compiler generates decimal-floating-point hardware instructions. This is the default for-march=z9-ecor higher. ✨-mlong-double-64,-mlong-double-128— these switches control the size of thelong doubletype. A size of 64 bits makeslong doubleequivalent to thedoubletype. And yes, that one's the default.-mbackchain,-mno-backchain— store (or don't store) the address of the caller's frame as a backchain pointer into the callee's stack frame. You might need a backchain to debug with tools that don't understand DWARF call frame information. When-mno-packed-stackis in effect, the backchain pointer is stored at the bottom of the stack frame; when-mpacked-stackis in effect, the backchain goes 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 -mno-backchain; however, using the backchain for debugging purposes usually requires that the whole binary is built with -mbackchain. And watch out: the combination of -mbackchain, -mpacked-stack and -mhard-float is not supported. To build a linux kernel, use -msoft-float.
The default is to not maintain the backchain.
-mpacked-stack,-mno-packed-stack— use (or don't use) the packed stack layout. When-mno-packed-stackis specified, the compiler uses all fields of the 96/160 byte register save area only for their default purpose; unused fields still take up stack space. When-mpacked-stackis specified, register save slots are densely packed at the top of the register save area; unused space gets reused for other purposes, which lets you use the available stack space more efficiently. But — when-mbackchainis 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 isn't used, code generated with -mpacked-stack is call-compatible with code generated with -mno-packed-stack. But 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 — and that code is not call-compatible with code compiled with -mpacked-stack. Also, again, the combination of -mbackchain, -mpacked-stack and -mhard-float is not supported. 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 don't generate) code using thebrasinstruction to do subroutine calls. This only works reliably if the total executable size does not exceed 64k. The default is to use thebasrinstruction instead, which doesn't have that limitation. Slay, honestly.-m64,-m31— when-m31is specified, generate code compliant to the GNU/Linux for S/390 ABI. When-m64is specified, generate code compliant to the GNU/Linux for zSeries ABI. This lets GCC in particular generate 64-bit instructions. For the s390 targets, the default is-m31, while the s390x targets default to-m64.-mzarch,-mesa— when-mzarchis specified, generate code using the instructions available on z/Architecture. When-mesais specified, generate code using the instructions available on ESA/390. Note that-mesais 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.-mhtm,-mno-htm— the-mhtmoption enables a set of builtins that make use of instructions available with the transactional execution facility introduced with the IBM zEnterprise EC12 machine generation S/390 System z Built-in Functions.-mhtmis enabled by default when using-march=zEC12. 🔥-mvx,-mno-vx— when-mvxis specified, generate code using the instructions available with the vector extension facility introduced with the IBM z13 machine generation. This option changes the ABI for some vector type values with regard to alignment and calling conventions. In case vector type values are being used in an ABI-relevant context, a GAS.gnu_attributecommand will be added to mark the resulting binary with the ABI used.-mvxis enabled by default when using-march=z13.-mzvector,-mno-zvector— the-mzvectoroption enables vector language extensions and builtins using instructions available with the vector extension facility introduced with the IBM z13 machine generation. This option adds support forvectorto be used as a keyword to define vector type variables and arguments.vectoris only available when GNU extensions are enabled — it will not be expanded when requesting strict standard compliance, e.g. with-std=c99. On top of the GCC low-level builtins,-mzvectorenables a set of builtins added for compatibility with AltiVec-style implementations like Power and Cell. To make use of these builtins, the header filevecintrin.hneeds to be included.-mzvectoris disabled by default.-mmvcle,-mno-mvcle— generate (or don't generate) code using themvcleinstruction to perform block moves. When-mno-mvcleis specified, use amvcloop instead. This is the default unless optimizing for size.-mdebug,-mno-debug— print (or don't print) additional debug information when compiling. The default is to not print debug information. (We love a low-drama default 💅)-march=cpu-type— generate code that runs on cpu-type, which is the name of a system representing a certain processor type. Possible values for cpu-type arez900/arch5,z990/arch6,z9-109,z9-ec/arch7,z10/arch8,z196/arch9,zEC12,z13/arch11,z14/arch12,z15/arch13,z16/arch14,z17/arch15, andnative.
The default is -march=z900.
Specifying native as the cpu type can be used to select the best architecture option for the host processor. -march=native has no effect if GCC does not recognize the processor. 🤷
-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 (or 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.-mtpf-trace-skip,-mno-tpf-trace-skip— generate code that changes (or does not change) the default branch targets enabled by-mtpf-traceto point to specialized trace routines providing the ability of selectively skipping function trace entries for the TPF OS. This option is off by default, even when compiling for the TPF OS and specifying-mtpf-trace.-mfused-madd,-mno-fused-madd— generate code that uses (or 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's intended to identify functions that most probably cause a stack overflow, and it's useful in an environment with limited stack size, e.g. the linux kernel. 🫶-mwarn-dynamicstack— emit a warning if the function callsallocaor uses dynamically-sized arrays. This is generally a bad idea with a limited stack size.-mstack-guard=stack-guard,-mstack-size=stack-size— okay so if you pass both of these, the S/390 back end drops extra instructions into the function prologue that straight-up trap if the stack getsstack-guardbytes abovestack-size. And yes bestie, remember that on S/390 the stack grows downward. 💀 If you leave outstack-guard, the compiler just picks the smallest power of 2 that's bigger than the frame size of the function it's compiling. These two are meant to help you debug stack overflow situations — the extra code is only a lil bit of overhead so you can even run it on production-ish systems without a huge performance hit. The values you give have to be exact powers of 2, andstack-sizehas to be greater thanstack-guardwithout going over 64k. To stay efficient, the extra code assumes the stack starts at an address aligned to whateverstack-sizeyou gave. Alsostack-guardcan only be used together withstack-size— it's a package deal, no solo runs. 🫶-mhotpatch=pre-halfwords,post-halfwords— turn this on and a "hot-patching" function prologue gets generated for every function in the compilation unit. The function label gets prepended with the number of two-byteNOPinstructions you asked for (pre-halfwords, max1000000). After the label,2 * post-halfwordsbytes get appended, using the biggest NOP-like instructions the architecture will allow (max1000000).
If both arguments are zero, hotpatching is just off. ✨
This option can be overridden for individual functions with the "hotpatch" attribute.
✨ SH Options ✨
These -m options are defined for the SH implementations, and there are a lot of them. Buckle up. 🎀
-m1— generate code for the SH1.-m2— generate code for the SH2.-m2e— generate code for the SH2e.-m2a-nofpu— generate code for the SH2a without FPU, or for a SH2a-FPU in such a way that the floating-point unit is not used.-m2a-single-only— generate code for the SH2a-FPU, in such a way that no double-precision floating-point operations are used.-m2a-single— generate code for the SH2a-FPU assuming the floating-point unit is in single-precision mode by default.-m2a— generate code for the SH2a-FPU assuming the floating-point unit is in double-precision mode by default.-m3— generate code for the SH3.-m3e— generate code for the SH3e.-m4-nofpu— generate code for the SH4 without a floating-point unit.-m4-single-only— generate code for the SH4 with a floating-point unit that only supports single-precision arithmetic.-m4-single— generate code for the SH4 assuming the floating-point unit is in single-precision mode by default.-m4— generate code for the SH4.-m4-100— generate code for SH4-100.-m4-100-nofpu— generate code for SH4-100 in such a way that the floating-point unit is not used.-m4-100-single— generate code for SH4-100 assuming the floating-point unit is in single-precision mode by default.-m4-100-single-only— generate code for SH4-100 in such a way that no double-precision floating-point operations are used.-m4-200— generate code for SH4-200.-m4-200-nofpu— generate code for SH4-200 in such a way that the floating-point unit is not used.-m4-200-single— generate code for SH4-200 assuming the floating-point unit is in single-precision mode by default.-m4-200-single-only— generate code for SH4-200 in such a way that no double-precision floating-point operations are used.-m4-300— generate code for SH4-300.-m4-300-nofpu— generate code for SH4-300 in such a way that the floating-point unit is not used.-m4-300-single— generate code for SH4-300 in such a way that no double-precision floating-point operations are used.-m4-300-single-only— generate code for SH4-300 in such a way that no double-precision floating-point operations are used.-m4-340— generate code for SH4-340 (no MMU, no FPU).-m4-500— generate code for SH4-500 (no FPU). Passes-isa=sh4-nofputo the assembler.-m4a-nofpu— generate code for the SH4al-dsp, or for a SH4a in such a way that the floating-point unit is not used.-m4a-single-only— generate code for the SH4a, in such a way that no double-precision floating-point operations are used.-m4a-single— generate code for the SH4a assuming the floating-point unit is in single-precision mode by default.-m4a— generate code for the SH4a.-m4al— same as-m4a-nofpu, except that it implicitly passes-dspto the assembler. GCC doesn't generate any DSP instructions at the moment, so like… it's giving the vibe, not the instructions. 🙈-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. Heads up though: this changes the calling conventions, so some functions from the standard C library will not work unless you recompile it first with-mdalign. Not a vibe you want to discover at runtime. 😭-mrelax— shorten some address references at link time, when possible; uses the linker option-relax.-mbigtable— use 32-bit offsets in"switch"tables. The default is to use 16-bit offsets.-mbitops— enable the use of bit manipulation instructions on SH2A.-mfmovd— enable the use of the instruction"fmovd". Check-mdalignfor alignment constraints.-mrenesas— comply with the calling conventions defined by Renesas.-mno-renesas— comply with the calling conventions defined for GCC before the Renesas conventions were available. This option is the default for all targets of the SH toolchain.-mnomacsave— mark the"MAC"register as call-clobbered, even if-mrenesasis given.-mieee,-mno-ieee— control the IEEE compliance of floating-point comparisons, which affects how we handle cases where the result of a comparison is unordered. By default-mieeeis implicitly enabled. If-ffinite-math-onlyis enabled,-mno-ieeeis implicitly set, which gives you faster floating-point greater-equal and less-equal comparisons. Those implicit settings can be overridden by specifying either-mieeeor-mno-ieee. Slay, you're in charge. 👑-minline-ic_invalidate— inline code to invalidate instruction cache entries after setting up nested function trampolines. This option has no effect if-musermodeis in effect and the selected code generation option (e.g.-m4) does not allow the use of the"icbi"instruction. If the selected code generation option does not allow the use of the"icbi"instruction, and-musermodeis not in effect, the inlined code manipulates the instruction cache address array directly with an associative write. That not only requires privileged mode at run time, but it also fails if the cache line had been mapped via the TLB and has become unmapped. Yeah. 💀-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. Like, please don't, bestie.-matomic-model=model— sets the model of atomic operations plus additional parameters, as a comma separated list. For details on the atomic built-in functions see__atomic Builtins. The following models and parameters are supported:none— disable compiler generated atomic sequences and emit library calls for atomic operations. This is the default if the target is not"sh*-*-linux*".soft-gusa— generate GNU/Linux compatible gUSA software atomic sequences for the atomic built-in functions. The generated atomic sequences require additional support from the interrupt/exception handling code of the system and are only suitable for SH3* and SH4* single-core systems. This option is enabled by default when the target is"sh*-*-linux*"and SH3* or SH4*. When the target is SH4A, this option also partially utilizes the hardware atomic instructions"movli.l"and"movco.l"to create more efficient code, unlessstrictis specified.soft-tcb— generate software atomic sequences that use a variable in the thread control block. This is a variation of the gUSA sequences which can also be used on SH1* and SH2* targets. The generated atomic sequences require additional support from the interrupt/exception handling code of the system and are only suitable for single-core systems. When using this model, thegbr-offset=parameter has to be specified as well.soft-imask— generate software atomic sequences that temporarily disable interrupts by setting"SR.IMASK = 1111". This model works only when the program runs in privileged mode and is only suitable for single-core systems. Additional support from the interrupt/exception handling code of the system is not required. This model is enabled by default when the target is"sh*-*-linux*"and SH1* or SH2*.hard-llcs— generate hardware atomic sequences using the"movli.l"and"movco.l"instructions only. This is only available on SH4A and is suitable for multi-core systems. Since the hardware instructions support only 32 bit atomic variables, access to 8 or 16 bit variables is emulated with 32 bit accesses. Code compiled with this option is also compatible with other software atomic model interrupt/exception handling systems if executed on an SH4A system. Additional support from the interrupt/exception handling code of the system is not required for this model.gbr-offset=— this parameter specifies the offset in bytes of the variable in the thread control block structure that should be used by the generated atomic sequences when thesoft-tcbmodel has been selected. For other models this parameter is ignored. The specified value must be an integer multiple of four and in the range0-1020.strict— this parameter prevents mixed usage of multiple atomic models, even if they are compatible, and makes the compiler generate atomic sequences of the specified model only. No mixing, we're monogamous with our atomic model here. 💅
-mtas— bestie this one generates thetas.bopcode for__atomic_test_and_set. Heads up though: depending on your particular hardware and software configuration, this can degrade overall performance, because of the operand cache line flushes that are implied by thetas.binstruction. And on multi-core SH4A processors thetas.binstruction must be used with caution, since it can result in data corruption for certain cache configurations. 💀-mprefergot— when you're generating position-independent code, emit function calls using the Global Offset Table instead of the Procedure Linkage Table.-musermode,-mno-usermode— don't allow (allow) the compiler generating privileged mode code. Specifying-musermodealso implies-mno-inline-ic_invalidateif the inlined code would not work in user mode.-musermodeis the default when the target issh*-*-linux*. If the target is SH1* or SH2* then-musermodehas no effect, since there is no user mode. 🫶-multcost=number— set the cost to assume for a multiply insn.-mdiv=strategy— set the division strategy to be used for integer division operations. strategy can be one of these cuties:call-div1— calls a library function that uses the single-step division instructiondiv1to perform the operation. Division by zero calculates an unspecified result and does not trap. This is the default except for SH4, SH2A and SHcompact.call-fp— calls a library function that performs the operation in double precision floating point. Division by zero causes a floating-point exception. This is the default for SHcompact with FPU. Specifying this for targets that do not have a double precision FPU defaults tocall-div1.call-table— calls a library function that uses a lookup table for small divisors and thediv1instruction with case distinction for larger divisors. Division by zero calculates an unspecified result and does not trap. This is the default for SH4. Specifying this for targets that do not have dynamic shift instructions defaults tocall-div1.
When a division strategy has not been specified the default strategy is selected based on the current target. For SH2A the default strategy is to use the divs and divu instructions instead of library function calls. ✨
-maccumulate-outgoing-args— reserve space once for outgoing arguments in the function prologue rather than around each call. Generally beneficial for performance and size. Also needed for unwinding to avoid changing the stack frame around conditional code.-mdivsi3_libfunc=name— set the name of the library function used for 32-bit signed division to name. This only affects the name used in the call division strategies, and the compiler still expects the same sets of input/output/clobbered registers as if this option were not present.-mfixed-range=register-range— generate code treating the given register range as fixed registers. A fixed register is one that the register allocator cannot use. This is useful when compiling kernel code. A register range is specified as two registers separated by a dash. Multiple register ranges can be specified separated by a comma.-mbranch-cost=num— assume num to be the cost for a branch instruction. Higher numbers make the compiler try to generate more branch-free code if possible. If not specified the value is selected depending on the processor type that is being compiled for.-mzdcbranch,-mno-zdcbranch— assume (do not assume) that zero displacement conditional branch instructionsbtandbfare fast. If-mzdcbranchis specified, the compiler prefers zero displacement branch code sequences. This is enabled by default when generating code for SH4 and SH4A. It can be explicitly disabled by specifying-mno-zdcbranch. 💅-mcbranch-force-delay-slot— force the usage of delay slots for conditional branches, which stuffs the delay slot with anopif a suitable instruction cannot be found. By default this option is disabled. It can be enabled to work around hardware bugs as found in the original SH7055.-mfused-madd,-mno-fused-madd— generate code that uses (does not use) the floating-point multiply and accumulate instructions. These instructions are generated by default if hardware floating point is used. The machine-dependent-mfused-maddoption is now mapped to the machine-independent-ffp-contract=fastoption, and-mno-fused-maddis mapped to-ffp-contract=off.-mfsca,-mno-fsca— allow or disallow the compiler to emit thefscainstruction for sine and cosine approximations. The option-mfscamust be used in combination with-funsafe-math-optimizations. It is enabled by default when generating code for SH4A. Using-mno-fscadisables sine and cosine approximations even if-funsafe-math-optimizationsis in effect. 🌸-mfsrra,-mno-fsrra— allow or disallow the compiler to emit thefsrrainstruction for reciprocal square root approximations. The option-mfsrramust be used in combination with-funsafe-math-optimizationsand-ffinite-math-only. It is enabled by default when generating code for SH4A. Using-mno-fsrradisables reciprocal square root approximations even if-funsafe-math-optimizationsand-ffinite-math-onlyare in effect.-mpretend-cmove— prefer zero-displacement conditional branches for conditional move instruction patterns. This can result in faster code on the SH4 processor. 🔥-mfdpic— generate code using the FDPIC ABI.
🌞 Solaris 2 Options 🌞
These -m options are supported on Solaris 2, bestie:
-mclear-hwcap—-mclear-hwcaptells the compiler to remove the hardware capabilities generated by the Solaris assembler. This is only necessary when object files use ISA extensions not supported by the current machine, but check at runtime whether or not to use them.-mimpure-text—-mimpure-text, used in addition to-shared, tells the compiler to not pass-z textto the linker when linking a shared object. Using this option, you can link position-dependent code into a shared object.
-mimpure-text suppresses the "relocations remain against allocatable but non-writable sections" linker error message. However, the necessary relocations trigger copy-on-write, and the shared object is not actually shared across processes. Instead of using -mimpure-text, you should compile all source code with -fpic or -fPIC. Like, for real, do that instead. 🙈
These switches are supported in addition to the above on Solaris 2:
-pthreads— this is a synonym for-pthread. slay 💅
🌈 SPARC Options 🌈
These -m options are supported on the SPARC:
-mno-app-regs,-mapp-regs— specify-mapp-regsto generate output using the global registers 2 through 4, which the SPARC SVR4 ABI reserves for applications. Like the global register 1, each global register 2 through 4 is then treated as an allocable register that is clobbered by function calls. This is the default.
To be fully SVR4 ABI-compliant at the cost of some performance loss, specify -mno-app-regs. You should compile libraries and system software with this option. 🫶
-mflat,-mno-flat— with-mflat, the compiler does not generate save/restore instructions and uses a "flat" or single register window model. This model is compatible with the regular register window model. The local registers and the input registers (0--5) are still treated as "call-saved" registers and are saved on the stack as needed.
With -mno-flat (the default), the compiler generates save/restore instructions (except for leaf functions). This is the normal operating mode.
-mfpu,-mhard-float— generate output containing floating-point instructions. This is the default. ✨-mno-fpu,-msoft-float— generate output containing library calls for floating point.
Warning: the requisite libraries are not available for all SPARC targets. Normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation. The embedded targetssparc-*-aoutandsparclite-*-*do provide software floating-point support.
-msoft-float changes the calling convention in the output file; therefore, it is only useful if you compile all of a program with this option. In particular, you need to compile libgcc.a, the library that comes with GCC, with -msoft-float in order for this to work. 😭
-mhard-quad-float— generate output containing quad-word (long double) floating-point instructions.-msoft-quad-float— generate output containing library calls for quad-word (long double) floating-point instructions. The functions called are those specified in the SPARC ABI. This is the default.
As of this writing, there are no SPARC implementations that have hardware support for the quad-word floating-point instructions. They all invoke a trap handler for one of these instructions, and then the trap handler emulates the effect of the instruction. Because of the trap handler overhead, this is much slower than calling the ABI library routines. Thus the -msoft-quad-float option is the default. 💀
-mno-unaligned-doubles,-munaligned-doubles— assume that doubles have 8-byte alignment. This is the default.
With -munaligned-doubles, GCC assumes that doubles have 8-byte alignment only if they are contained in another type, or if they have an absolute address. Otherwise, it assumes they have 4-byte alignment. Specifying this option avoids some rare compatibility problems with code generated by other compilers. It is not the default because it results in a performance loss, especially for floating-point code.
-muser-mode,-mno-user-mode— do not generate code that can only run in supervisor mode. This is relevant only for thecasainstruction emitted for the LEON3 processor. This is the default. 👑-mfaster-structs/-mno-faster-structs— bestie, with-mfaster-structsthe compiler just assumes structs want 8-byte alignment, which lets it use pairs oflddandstdinstructions for copies in structure assignment instead of literally twice as manyldandstpairs. cute, right? 🎀 BUT. this changed alignment straight-up violates the SPARC ABI. so it's only for targets where the dev fully acknowledges their resulting code is not directly in line with the rules of the ABI. iykyk. 💅-mstd-struct-return/-mno-std-struct-return— with-mstd-struct-return, the compiler generates checking code in functions returning structures or unions, so it can detect size mismatches between the two sides of function calls, as per the 32-bit ABI. the default is-mno-std-struct-return. also, heads up: this option has no effect in 64-bit mode. 🫶-mcpu=cpu_type— okay so this sets the instruction set, the register set, and the instruction scheduling parameters for machine type cpu_type. ✨
the supported values for cpu_type are: v7, cypress, v8, supersparc, hypersparc, leon, leon3, leon3v7, leon5, sparclite, f930, f934, sparclite86x, sparclet, tsc701, v9, ultrasparc, ultrasparc3, niagara, niagara2, niagara3, niagara4, niagara7 and m8.
native Solaris and GNU/Linux toolchains also support the value native, which selects the best architecture option for the host processor. -mcpu=native has no effect if GCC does not recognize the processor. 🙈
default instruction scheduling parameters get used for values that select an architecture and not an implementation. those are: v7, v8, sparclite, sparclet, v9.
here's the list of each supported architecture and their supported implementations:
v7—cypress,leon3v7v8—supersparc,hypersparc,leon,leon3,leon5sparclite—f930,f934,sparclite86xsparclet—tsc701v9—ultrasparc,ultrasparc3,niagara,niagara2,niagara3,niagara4,niagara7,m8
by default (unless configured otherwise), GCC generates code for the V7 variant of the SPARC architecture. with -mcpu=cypress, the compiler additionally optimizes it for the Cypress CY7C602 chip, as used in the SPARCStation/SPARCServer 3xx series. this is also appropriate for the older SPARCStation 1, 2, IPX etc. 👑
with -mcpu=v8, GCC generates code for the V8 variant of the SPARC architecture. the only difference from V7 code is that the compiler emits the integer multiply and integer divide instructions which exist in SPARC-V8 but not in SPARC-V7. with -mcpu=supersparc, the compiler additionally optimizes it for the SuperSPARC chip, as used in the SPARCStation 10, 1000 and 2000 series.
with -mcpu=sparclite, GCC generates code for the SPARClite variant of the SPARC architecture. this adds the integer multiply, integer divide step and scan (ffs) instructions which exist in SPARClite but not in SPARC-V7. with -mcpu=f930, the compiler additionally optimizes it for the Fujitsu MB86930 chip, which is the original SPARClite, with no FPU. with -mcpu=f934, the compiler additionally optimizes it for the Fujitsu MB86934 chip, which is the more recent SPARClite with FPU. 🌸
with -mcpu=sparclet, GCC generates code for the SPARClet variant of the SPARC architecture. this adds the integer multiply, multiply/accumulate, integer divide step and scan (ffs) instructions which exist in SPARClet but not in SPARC-V7. with -mcpu=tsc701, the compiler additionally optimizes it for the TEMIC SPARClet chip.
with -mcpu=v9, GCC generates code for the V9 variant of the SPARC architecture. this adds 64-bit integer and floating-point move instructions, 3 additional floating-point condition code registers and conditional move instructions. 💫
- with
-mcpu=ultrasparc, the compiler additionally optimizes it for the Sun UltraSPARC I/II/IIi chips. - with
-mcpu=ultrasparc3, the compiler additionally optimizes it for the Sun UltraSPARC III/III+/IIIi/IIIi+/IV/IV+ chips. - with
-mcpu=niagara, the compiler additionally optimizes it for Sun UltraSPARC T1 chips. - with
-mcpu=niagara2, the compiler additionally optimizes it for Sun UltraSPARC T2 chips. - with
-mcpu=niagara3, the compiler additionally optimizes it for Sun UltraSPARC T3 chips. - with
-mcpu=niagara4, the compiler additionally optimizes it for Sun UltraSPARC T4 chips. - with
-mcpu=niagara7, the compiler additionally optimizes it for Oracle SPARC M7 chips. - with
-mcpu=m8, the compiler additionally optimizes it for Oracle M8 chips.
-mtune=cpu_type— this sets the instruction scheduling parameters for machine type cpu_type, but does not set the instruction set or register set that-mcpu=cpu_typedoes. 🌈
the same values for -mcpu=cpu_type can be used for -mtune=cpu_type, but the only useful values are those that select a particular CPU implementation. those are cypress, supersparc, hypersparc, leon, leon3, leon3v7, leon5, f930, f934, sparclite86x, tsc701, ultrasparc, ultrasparc3, niagara, niagara2, niagara3, niagara4, niagara7 and m8. with native Solaris and GNU/Linux toolchains, native can also be used.
-mv8plus/-mno-v8plus— with-mv8plus, GCC generates code for the SPARC-V8+ ABI. the difference from the V8 ABI is that the global and out registers are considered 64 bits wide. this is enabled by default on Solaris in 32-bit mode for all SPARC-V9 processors. 🦋-mvis/-mno-vis— with-mvis, GCC generates code that takes advantage of the UltraSPARC Visual Instruction Set extensions. the default is-mno-vis.-mvis2/-mno-vis2— with-mvis2, GCC generates code that takes advantage of version 2.0 of the UltraSPARC Visual Instruction Set extensions. the default is-mvis2when targeting a cpu that supports such instructions, such as UltraSPARC-III and later. setting-mvis2also sets-mvis. 💜-mvis3/-mno-vis3— with-mvis3, GCC generates code that takes advantage of version 3.0 of the UltraSPARC Visual Instruction Set extensions. the default is-mvis3when targeting a cpu that supports such instructions, such as niagara-3 and later. setting-mvis3also sets-mvis2and-mvis.-mvis3b/-mno-vis3b— with-mvis3b, GCC generates code that takes advantage of version 3.0 of the UltraSPARC Visual Instruction Set extensions, plus the additional VIS instructions introduced in the Oracle SPARC Architecture 2011. the default is-mvis3bwhen targeting a cpu that supports such instructions, such as niagara-7 and later. setting-mvis3balso sets-mvis3,-mvis2and-mvis. ✨-mvis4/-mno-vis4— with-mvis4, GCC generates code that takes advantage of version 4.0 of the UltraSPARC Visual Instruction Set extensions. the default is-mvis4when targeting a cpu that supports such instructions, such as niagara-7 and later. setting-mvis4also sets-mvis3b,-mvis3,-mvis2and-mvis.-mvis4b/-mno-vis4b— with-mvis4b, GCC generates code that takes advantage of version 4.0 of the UltraSPARC Visual Instruction Set extensions, plus the additional VIS instructions introduced in the Oracle SPARC Architecture 2017. the default is-mvis4bwhen targeting a cpu that supports such instructions, such asm8and later. setting-mvis4balso sets-mvis4,-mvis3b,-mvis3,-mvis2and-mvis. 🧚-mcbcond/-mno-cbcond— with-mcbcond, GCC generates code that takes advantage of the UltraSPARC Compare-and-Branch-on-Condition instructions. the default is-mcbcondwhen targeting a CPU that supports such instructions, such as Niagara-4 and later. 💅-mfmaf/-mno-fmaf— with-mfmaf, GCC generates code that takes advantage of the UltraSPARC Fused Multiply-Add Floating-point instructions. the default is-mfmafwhen targeting a CPU that supports such instructions, such as Niagara-3 and later. 🔥-mfsmuld/-mno-fsmuld— with-mfsmuld, GCC generates code that takes advantage of the Floating-point Multiply Single to Double (FsMULd) instruction. the default is-mfsmuldwhen targeting a CPU supporting the architecture versions V8 or V9 with FPU — except-mcpu=leon. ⭐-mpopc/-mno-popc— with-mpopc, GCC generates code that takes advantage of the UltraSPARC Population Count instruction. the default is-mpopcwhen targeting a CPU that supports such an instruction, such as Niagara-2 and later.-msubxc/-mno-subxc— with-msubxc, GCC generates code that takes advantage of the UltraSPARC Subtract-Extended-with-Carry instruction. the default is-msubxcwhen targeting a CPU that supports such an instruction, such as Niagara-7 and later. 💖-mfix-at697f— enable the documented workaround for the single erratum of the Atmel AT697F processor (which corresponds to erratum #13 of the AT697E processor).-mfix-ut699— enable the documented workarounds for the floating-point errata and the data cache nullify errata of the UT699 processor.-mfix-ut700— enable the documented workaround for the back-to-back store errata of the UT699E/UT700 processor.-mfix-gr712rc— enable the documented workaround for the back-to-back store errata of the GR712RC processor. 🫶
these -m options are supported in addition to the above on SPARC-V9 processors in 64-bit environments:
-m32/-m64— generate code for a 32-bit or 64-bit environment. the 32-bit environment setsint,longand pointer to 32 bits. the 64-bit environment setsintto 32 bits andlongand pointer to 64 bits. 🌈-mcmodel=which— set the code model to one of:medlow— the Medium/Low code model: 64-bit addresses, programs must be linked in the low 32 bits of memory. programs can be statically or dynamically linked.medmid— the Medium/Middle code model: 64-bit addresses, programs must be linked in the low 44 bits of memory, the text and data segments must be less than 2GB in size and the data segment must be located within 2GB of the text segment.medany— the Medium/Anywhere code model: 64-bit addresses, programs may be linked anywhere in memory, the text and data segments must be less than 2GB in size and the data segment must be located within 2GB of the text segment.
embmedany— the Medium/Anywhere code model for embedded systems, bestie. 64-bit addresses, and the text and data segments have to be under 2GB each, both landing anywhere in memory (decided at link time, not our problem 💅). The global register%g4points at the base of the data segment. Programs are statically linked, and PIC is not supported. Sorry 🫶-mmemory-model=mem-model— sets which memory model is in force on the processor. Pick one:default— the default memory model for whatever processor and OS you're on.rmo— Relaxed Memory Order.pso— Partial Store Order.tso— Total Store Order.sc— Sequential Consistency.
These memory models are formally defined in Appendix D of the SPARC-V9 architecture manual, and they're set in the processor's PSTATE.MM field. iykyk 📖
-mstack-bias,-mno-stack-bias— with-mstack-bias, GCC assumes the stack pointer (and the frame pointer, if there is one) is offset by-2047, and that offset must be added back whenever you make stack frame references. This is the default in 64-bit mode ✨. Otherwise, assume no such offset exists.
🎀 Options for System V 🎀
These extra options are available on System V Release 4, for compatibility with other compilers on those systems:
-G— create a shared object. Though ngl, it's recommended that-symbolicor-sharedbe used instead 👀-Qy— identify the versions of each tool the compiler used, in a.identassembler directive in the output.-Qn— refrain from adding.identdirectives to the output file (this is the default).-YP,dirs— search the directories dirs, and no others, for libraries specified with-l.-Ym,dir— look in the directory dir to find the M4 preprocessor. The assembler uses this option.
🌸 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 far away, the compiler always loads the function's address into a register and calls indirect through the pointer.-mno-ep,-mep— do not optimize (do optimize) basic blocks that use the same index pointer 4 or more times, copying the pointer into theepregister and using the shortersldandsstinstructions. The-mepoption is on by default if you optimize ✨-mno-prolog-function,-mprolog-function— do not use (do use) external functions to save and restore registers at the prologue and epilogue of a function. The external functions are slower, but use less code space if more than one function saves the same number of registers. The-mprolog-functionoption is on by default if you optimize.-mspace— try to make the code as small as possible. At present, this just turns on the-mepand-mprolog-functionoptions. tiny code era 🐜-mtda=n— put static or global variables whose size is n bytes or less into the tiny data area that registereppoints 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 registergppoints to. The small data area can hold up to 64 kilobytes.-mzda=n— put static or global variables whose size is n bytes or less into the first 32 kilobytes of memory.-mv850— specify that the target processor is the V850.-mv850e3v5— specify that the target processor is the V850E3V5. The preprocessor constant__v850e3v5__is defined if this option is used.-mv850e2v4— specify that the target processor is the V850E3V5. This is an alias for the-mv850e3v5option.-mv850e2v3— specify that the target processor is the V850E2V3. The preprocessor constant__v850e2v3__is defined if this option is used.-mv850e2— specify that the target processor is the V850E2. The preprocessor constant__v850e2__is defined if this option is used.-mv850e1— specify that the target processor is the V850E1. The preprocessor constants__v850e1__and__v850e__are defined if this option is used.-mv850es— specify that the target processor is the V850ES. This is an alias for the-mv850e1option.-mv850e— specify that the target processor is the V850E. The preprocessor constant__v850e__is defined if this option is used.
If neither -mv850 nor -mv850e nor -mv850e1 nor -mv850e2 nor -mv850e2v3 nor -mv850e3v5 are defined, then a default target processor is chosen and the relevant __v850*__ preprocessor constant is defined.
The preprocessor constants __v850 and __v851__ are always defined, no matter which processor variant is the target. periodt 💅
-mdisable-callt,-mno-disable-callt— this option suppresses generation of theCALLTinstruction for the v850e, v850e1, v850e2, v850e2v3 and v850e3v5 flavors of the v850 architecture.
This option is enabled by default when the RH850 ABI is in use (see -mrh850-abi), and disabled by default when the GCC ABI is in use. If CALLT instructions are being generated, then the C preprocessor symbol __V850_CALLT__ is defined.
-mrelax,-mno-relax— pass on (or do not pass on) the-mrelaxcommand-line option to the assembler.-mlong-jumps,-mno-long-jumps— disable (or re-enable) the generation of PC-relative jump instructions.-msoft-float,-mhard-float— disable (or re-enable) the generation of hardware floating point instructions. This option is only significant when the target architecture is V850E2V3 or higher. If hardware floating point instructions are being generated, then the C preprocessor symbol__FPU_OK__is defined; otherwise the symbol__NO_FPU__is defined.-mloop— enables the use of the e3v5 LOOP instruction. The use of this instruction is not enabled by default when the e3v5 architecture is selected, because its use is still experimental 🙈-mrh850-abi,-mghs— enables support for the RH850 version of the V850 ABI. This is the default. With this version of the ABI, the following rules apply:- Integer sized structures and unions are returned via a memory pointer rather than a register.
- Large structures and unions (more than 8 bytes in size) are passed by value.
- Functions are aligned to 16-bit boundaries.
- The
-m8byte-aligncommand-line option is supported. - The
-mdisable-calltcommand-line option is enabled by default. The-mno-disable-calltcommand-line option is not supported.
When this version of the ABI is enabled, the C preprocessor symbol __V850_RH850_ABI__ is defined.
-mgcc-abi— enables support for the old GCC version of the V850 ABI. With this version of the ABI, the following rules apply:- Integer sized structures and unions are returned in register
r10. - Large structures and unions (more than 8 bytes in size) are passed by reference.
- Functions are aligned to 32-bit boundaries, unless optimizing for size.
- The
-m8byte-aligncommand-line option is not supported. - The
-mdisable-calltcommand-line option is supported but not enabled by default.
- Integer sized structures and unions are returned in register
When this version of the ABI is enabled, the C preprocessor symbol __V850_GCC_ABI__ is defined.
-m8byte-align,-mno-8byte-align— enables support fordoubleandlong longtypes to be aligned on 8-byte boundaries. The default is to restrict the alignment of all objects to at most 4-bytes. When-m8byte-alignis in effect, the C preprocessor symbol__V850_8BYTE_ALIGN__is defined.-mbig-switch— generate code suitable for big switch tables. Use this option only if the assembler/linker complain about out of range branches within a switch table.-mapp-regs— this option causesr2andr5to be used in the code generated by the compiler. This setting is the default.-mno-app-regs— this option causesr2andr5to be treated as fixed registers.
🦋 VAX Options 🦋
These -m options are defined for the VAX:
-munix— do not output certain jump instructions (aobleqand so on) that the Unix assembler for the VAX cannot handle across long ranges.-mgnu— do output those jump instructions, on the assumption that the GNU assembler is being used.-md,-md-float— use the D_floating data format for double-precision floating-point numbers instead of G_floating.-mg,-mg-float— use the G_floating data format for double-precision floating-point numbers instead of D_floating.-mlra,-mno-lra— enable Local Register Allocation. This is still experimental for the VAX, so by default the compiler uses standard reload. ⚠️
💜 Visium Options 💜
-mdebug— a program which performs file I/O and is destined to run on an MCM target should be linked with this option. It causes the librarieslibc.aandlibdebug.ato be linked. The program should be run on the target under the control of the GDB remote debugging stub.-msim— a program which performs file I/O and is destined to run on the simulator should be linked with option. This causes librarieslibc.aandlibsim.ato be linked.-mfpu,-mhard-float— generate code containing floating-point instructions. This is the default.-mno-fpu,-msoft-float— generate code containing library calls for floating-point.
Heads up bestie 😭-msoft-floatchanges 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 compilelibgcc.a, the library that comes with GCC, with-msoft-floatin order for this to work.
-mcpu=cpu_type— set the instruction set, register set, and instruction scheduling parameters for machine type cpu_type. Supported values for cpu_type aremcm,gr5andgr6.
mcm is a synonym of gr5, present for backward compatibility.
By default (unless configured otherwise), GCC generates code for the GR5 variant of the Visium architecture.
With -mcpu=gr6, GCC generates code for the GR6 variant of the Visium architecture. The only difference from GR5 code is that the compiler will generate block move instructions. ✨
-mtune=cpu_type— sets the instruction scheduling parameters for machine type cpu_type, but bestie it does not set the instruction set or the register set the way-mcpu=cpu_typewould. Tuning only, no wardrobe change 💅-msv-mode— generates code for supervisor mode, where literally nothing is off-limits when it comes to accessing general registers. This one's the default ✨-muser-mode— generates code for user mode, where some general registers are straight-up forbidden to touch: on the GR5, registersr24tor31cannot be accessed in this mode; on the GR6, only registersr29tor31are affected 🙈
💖 VMS Options 💖
These -m options are defined for the VMS implementations:
-mvms-return-codes— return VMS condition codes from"main". The default is to return POSIX-style condition (e.g. error) codes, just so u know 🫶-mdebug-main=prefix— flags the first routine whose name starts with prefix as the main routine for the debugger. First one there gets the crown 👑-mmalloc64— defaults to 64-bit memory allocation routines. Big girl allocations only ✨-mpointer-size=size— sets the default size of pointers. Possible options for size are32orshortfor 32 bit pointers,64orlongfor 64 bit pointers, andnofor supporting only 32 bit pointers. Heads up: the later option disables"pragma pointer_size"💀
🌸 VxWorks Options 🌸
The options in this section are defined for all VxWorks targets. Options specific to the target hardware are listed with the other options for that target, iykyk.
-mrtp— GCC can generate code for both VxWorks kernels and real time processes (RTPs). This option switches from the former to the latter. It also defines the preprocessor macro"__RTP__"🦋-msmp— select SMP runtimes for linking. Ngl, this is not available on architectures other than PowerPC, nor on VxWorks version 7 or later, in which the selection is part of the VxWorks build configuration and the library paths are the same for either choice 😭-non-static— link an RTP executable against shared libraries rather than static libraries. The options-staticand-sharedcan also be used for RTPs;-staticis the default 🎀-Bstatic,-Bdynamic— these options are passed down to the linker. They are defined for compatibility with Diab 💜-Xbind-lazy— enable lazy binding of function calls. This option is equivalent to-Wl,-z,nowand is defined for compatibility with Diab. (yes, "lazy" pairs withnow, it's giving chaos, don't think about it too hard 😭)-Xbind-now— disable lazy binding of function calls. This option is the default and is defined for compatibility with Diab ⭐
🔥 x86 Options 🔥
These -m options are defined for the x86 family of computers. Main character energy fr fr 👑
-march=cpu-type— generate instructions for the machine type cpu-type. In contrast to-mtune=cpu-type, which merely tunes the generated code for the specified cpu-type,-march=cpu-typeallows GCC to generate code that may not run at all on processors other than the one indicated. Specifying-march=cpu-typeimplies-mtune=cpu-type, except where noted otherwise 💀
The choices for cpu-type are:
native— this selects the CPU to generate code for at compilation time by determining the processor type of the compiling machine. Using-march=nativeenables all instruction subsets supported by the local machine (hence the result might not run on different machines). Using-mtune=nativeproduces code optimized for the local machine under the constraints of the selected instruction set 🧚x86-64— a generic CPU with 64-bit extensions, MMX, SSE, SSE2, and FXSR instruction set support ✨x86-64-v2,x86-64-v3,x86-64-v4— these choices for cpu-type select the corresponding micro-architecture level from the x86-64 psABI. On ABIs other than the x86-64 psABI they select the same CPU features as the x86-64 psABI documents for the particular micro-architecture level.
Since these cpu-type values do not have a corresponding -mtune setting, using -march with these values enables generic tuning. Specific tuning can be enabled using the -mtune=other-cpu-type option with an appropriate other-cpu-type value 🎀
i386— Original Intel i386 CPU.i486— Intel i486 CPU. (No scheduling is implemented for this chip.) ngl she's doing the bare minimum 🙈i586,pentium— Intel Pentium CPU with no MMX support.lakemont— Intel Lakemont MCU, based on Intel Pentium CPU.pentium-mmx— Intel Pentium MMX CPU, based on Pentium core with MMX instruction set support.pentiumpro— Intel Pentium Pro CPU with no MMX support.i686— when used with-march, the Pentium Pro instruction set is used, so the code runs on all i686 family chips. When used with-mtune, it has the same meaning asgeneric.pentium2— Intel Pentium II CPU, based on Pentium Pro core with MMX and FXSR instruction set support.pentium3,pentium3m— Intel Pentium III CPU, based on Pentium Pro core with MMX, FXSR and SSE instruction set support.pentium-m— Intel Pentium M; low-power version of Intel Pentium III CPU with MMX, SSE, SSE2 and FXSR instruction set support. Used by Centrino notebooks 💅pentium4,pentium4m— Intel Pentium 4 CPU with MMX, SSE, SSE2 and FXSR instruction set support.prescott— improved version of Intel Pentium 4 CPU with MMX, SSE, SSE2, SSE3 and FXSR instruction set support.nocona— improved version of Intel Pentium 4 CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3 and FXSR instruction set support.core2— Intel Core 2 CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, CX16, SAHF and FXSR instruction set support.nehalem,corei7— Intel Nehalem CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF and FXSR instruction set support.westmere— Intel Westmere CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR and PCLMUL instruction set support.sandybridge,corei7-avx— Intel Sandy Bridge CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE and PCLMUL instruction set support.ivybridge,core-avx-i— Intel Ivy Bridge CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND and F16C instruction set support.haswell,core-avx2— Intel Haswell CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE and HLE instruction set support. she ate 🔥broadwell— Intel Broadwell CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX and PREFETCHW instruction set support.skylake— Intel Skylake CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES and SGX instruction set support.skylake-avx512— Intel Skylake Server CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, CLWB, AVX512VL, AVX512BW, AVX512DQ and AVX512CD instruction set support. look at that list, she's everything 💅cascadelake— Intel Cascade Lake CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, CLWB, AVX512VL, AVX512BW, AVX512DQ, AVX512CD and AVX512VNNI instruction set support.cannonlake— Intel Cannon Lake Server CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU, AVX512VBMI, AVX512IFMA and SHA instruction set support.cooperlake— Intel Cooper Lake CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, CLWB, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, AVX512VNNI and AVX512BF16 instruction set support.icelake-client— Intel Ice Lake Client CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2 , VPCLMULQDQ, AVX512BITALG, RDPID and AVX512VPOPCNTDQ instruction set support.icelake-server— Intel Ice Lake Server CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2 , VPCLMULQDQ, AVX512BITALG, RDPID, AVX512VPOPCNTDQ, PCONFIG, WBNOINVD and CLWB instruction set support.
tigerlake— Intel Tiger Lake CPU, and she comes with 64-bit extensions, plusMMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,POPCNT,CX16,SAHF,FXSR,AVX,XSAVE,PCLMUL,FSGSBASE,RDRND,F16C,AVX2,BMI,BMI2,LZCNT,FMA,MOVBE,HLE,RDSEED,ADCX,PREFETCHW,AES,CLFLUSHOPT,XSAVEC,XSAVES,SGX,AVX512F,AVX512VL,AVX512BW,AVX512DQ,AVX512CD PKU,AVX512VBMI,AVX512IFMA,SHA,AVX512VNNI,GFNI,VAES,AVX512VBMI2,VPCLMULQDQ,AVX512BITALG,RDPID,AVX512VPOPCNTDQ,MOVDIRI,MOVDIR64B,CLWB,AVX512VP2INTERSECTandKEYLOCKERinstruction set support. the full roster, bestie, we do not skip a single one 💅rocketlake— Intel Rocket Lake CPU with 64-bit extensions,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,POPCNT,CX16,SAHF,FXSR,AVX,XSAVE,PCLMUL,FSGSBASE,RDRND,F16C,AVX2,BMI,BMI2,LZCNT,FMA,MOVBE,HLE,RDSEED,ADCX,PREFETCHW,AES,CLFLUSHOPT,XSAVEC,XSAVES,AVX512F,AVX512VL,AVX512BW,AVX512DQ,AVX512CD PKU,AVX512VBMI,AVX512IFMA,SHA,AVX512VNNI,GFNI,VAES,AVX512VBMI2,VPCLMULQDQ,AVX512BITALG,RDPIDandAVX512VPOPCNTDQinstruction set support.alderlake,raptorlake,meteorlake,gracemont— Intel Alder Lake / Raptor Lake / Meteor Lake / Gracemont CPU with 64-bit extensions,MOVBE,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,POPCNT,AES,PREFETCHW,PCLMUL,RDRND,XSAVE,XSAVEC,XSAVES,XSAVEOPT,FSGSBASE,PTWRITE,RDPID,SGX,GFNI-SSE,CLWB,MOVDIRI,MOVDIR64B,WAITPKG,ADCX,AVX,AVX2,BMI,BMI2,F16C,FMA,LZCNT,PCONFIG,PKU,VAES,VPCLMULQDQ,SERIALIZE,HRESET,KL,WIDEKLandAVX-VNNIinstruction set support. four names, one whole vibe ✨arrowlake— Intel Arrow Lake CPU with 64-bit extensions,MOVBE,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,POPCNT,AES,PREFETCHW,PCLMUL,RDRND,XSAVE,XSAVEC,XSAVES,XSAVEOPT,FSGSBASE,PTWRITE,RDPID,SGX,GFNI-SSE,CLWB,MOVDIRI,MOVDIR64B,WAITPKG,ADCX,AVX,AVX2,BMI,BMI2,F16C,FMA,LZCNT,PCONFIG,PKU,VAES,VPCLMULQDQ,SERIALIZE,HRESET,KL,WIDEKL,AVX-VNNI,UINTR,AVXIFMA,AVXVNNIINT8,AVXNECONVERTandCMPCCXADDinstruction set support.arrowlake-s,lunarlake— Intel Arrow Lake S / Lunar Lake CPU with 64-bit extensions,MOVBE,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,POPCNT,AES,PREFETCHW,PCLMUL,RDRND,XSAVE,XSAVEC,XSAVES,XSAVEOPT,FSGSBASE,PTWRITE,RDPID,SGX,GFNI-SSE,CLWB,MOVDIRI,MOVDIR64B,WAITPKG,ADCX,AVX,AVX2,BMI,BMI2,F16C,FMA,LZCNT,PCONFIG,PKU,VAES,VPCLMULQDQ,SERIALIZE,HRESET,KL,WIDEKL,AVX-VNNI,UINTR,AVXIFMA,AVXVNNIINT8,AVXNECONVERT,CMPCCXADD,AVXVNNIINT16,SHA512,SM3andSM4instruction set support. and yes she got the extra crypto ones, we love an upgrade arc 🦋pantherlake— Intel Panther Lake CPU with 64-bit extensions,MOVBE,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,POPCNT,AES,PREFETCHW,PCLMUL,RDRND,XSAVE,XSAVEC,XSAVES,XSAVEOPT,FSGSBASE,PTWRITE,RDPID,SGX,GFNI-SSE,CLWB,MOVDIRI,MOVDIR64B,WAITPKG,ADCX,AVX,AVX2,BMI,BMI2,F16C,FMA,LZCNT,PCONFIG,PKU,VAES,VPCLMULQDQ,SERIALIZE,HRESET,AVX-VNNI,UINTR,AVXIFMA,AVXVNNIINT8,AVXNECONVERT,CMPCCXADD,AVXVNNIINT16,SHA512,SM3andSM4instruction set support.sapphirerapids,emeraldrapids— Intel Sapphire Rapids / Emerald Rapids CPU with 64-bit extensions,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,POPCNT,CX16,SAHF,FXSR,AVX,XSAVE,PCLMUL,FSGSBASE,RDRND,F16C,AVX2,BMI,BMI2,LZCNT,FMA,MOVBE,HLE,RDSEED,ADCX,PREFETCHW,AES,CLFLUSHOPT,XSAVEC,XSAVES,SGX,AVX512F,AVX512VL,AVX512BW,AVX512DQ,AVX512CD,PKU,AVX512VBMI,AVX512IFMA,SHA,AVX512VNNI,GFNI,VAES,AVX512VBMI2,VPCLMULQDQ,AVX512BITALG,RDPID,AVX512VPOPCNTDQ,PCONFIG,WBNOINVD,CLWB,MOVDIRI,MOVDIR64B,ENQCMD,CLDEMOTE,PTWRITE,WAITPKG,SERIALIZE,TSXLDTRK,UINTR,AMX-BF16,AMX-TILE,AMX-INT8,AVX-VNNI,AVX512-FP16andAVX512BF16instruction set support. okay this one is a whole buffet, she ATE 🔥graniterapids— Intel Granite Rapids CPU with 64-bit extensions,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,POPCNT,CX16,SAHF,FXSR,AVX,XSAVE,PCLMUL,FSGSBASE,RDRND,F16C,AVX2,BMI,BMI2,LZCNT,FMA,MOVBE,HLE,RDSEED,ADCX,PREFETCHW,AES,CLFLUSHOPT,XSAVEC,XSAVES,SGX,AVX512F,AVX512VL,AVX512BW,AVX512DQ,AVX512CD,PKU,AVX512VBMI,AVX512IFMA,SHA,AVX512VNNI,GFNI,VAES,AVX512VBMI2,VPCLMULQDQ,AVX512BITALG,RDPID,AVX512VPOPCNTDQ,PCONFIG,WBNOINVD,CLWB,MOVDIRI,MOVDIR64B,ENQCMD,CLDEMOTE,PTWRITE,WAITPKG,SERIALIZE,TSXLDTRK,UINTR,AMX-BF16,AMX-TILE,AMX-INT8,AVX-VNNI,AVX512-FP16,AVX512BF16,AMX-FP16andPREFETCHIinstruction set support.graniterapids-d— Intel Granite Rapids D CPU with 64-bit extensions,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,POPCNT,CX16,SAHF,FXSR,AVX,XSAVE,PCLMUL,FSGSBASE,RDRND,F16C,AVX2,BMI,BMI2,LZCNT,FMA,MOVBE,HLE,RDSEED,ADCX,PREFETCHW,AES,CLFLUSHOPT,XSAVEC,XSAVES,SGX,AVX512F,AVX512VL,AVX512BW,AVX512DQ,AVX512CD,PKU,AVX512VBMI,AVX512IFMA,SHA,AVX512VNNI,GFNI,VAES,AVX512VBMI2,VPCLMULQDQ,AVX512BITALG,RDPID,AVX512VPOPCNTDQ,PCONFIG,WBNOINVD,CLWB,MOVDIRI,MOVDIR64B,ENQCMD,CLDEMOTE,PTWRITE,WAITPKG,SERIALIZE,TSXLDTRK,UINTR,AMX-BF16,AMX-TILE,AMX-INT8,AVX-VNNI,AVX512FP16,AVX512BF16,AMX-FP16,PREFETCHIandAMX-COMPLEXinstruction set support.diamondrapids— Intel Diamond Rapids CPU with 64-bit extensions,MOVBE,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,POPCNT,CX16,SAHF,FXSR,AVX,XSAVE,PCLMUL,FSGSBASE,RDRND,F16C,AVX2,BMI,BMI2,LZCNT,FMA,MOVBE,HLE,RDSEED,ADCX,PREFETCHW,AES,CLFLUSHOPT,XSAVEC,XSAVES,SGX,AVX512F,AVX512VL,AVX512BW,AVX512DQ,AVX512CD,PKU,AVX512VBMI,AVX512IFMA,SHA,AVX512VNNI,GFNI,VAES,AVX512VBMI2,VPCLMULQDQ,AVX512BITALG,RDPID,AVX512VPOPCNTDQ,PCONFIG,WBNOINVD,CLWB,MOVDIRI,MOVDIR64B,ENQCMD,CLDEMOTE,PTWRITE,WAITPKG,SERIALIZE,TSXLDTRK,UINTR,AMX-BF16,AMX-TILE,AMX-INT8,AVX-VNNI,AVX512FP16,AVX512BF16,AMX-FP16,PREFETCHI,AMX-COMPLEX,AVX10.1-512,AVX-IFMA,AVX-NE-CONVERT,AVX-VNNI-INT16,AVX-VNNI-INT8,CMPccXADD,SHA512,SM3,SM4,AVX10.2-512,APX_F,AMX-AVX512,AMX-FP8,MOVRSandAMX-MOVRSinstruction set support. the way this list just kept GOING 😭 iconic behaviour only
bonnell,atom— Intel Bonnell CPU with 64-bit extensions,MOVBE,MMX,SSE,SSE2,SSE3andSSSE3instruction set support. short and sweet, we respect the minimalist era 🎀silvermont,slm— Intel Silvermont CPU with 64-bit extensions,MOVBE,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,POPCNT,CX16,SAHF,FXSR,PCLMUL,PREFETCHWandRDRNDinstruction set support.goldmont— Intel Goldmont CPU with 64-bit extensions,MOVBE,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,POPCNT,CX16,SAHF,FXSR,PCLMUL,PREFETCHW,RDRND,AES,SHA,RDSEED,XSAVE,XSAVEC,XSAVES,XSAVEOPT,CLFLUSHOPTandFSGSBASEinstruction set support.goldmont-plus— Intel Goldmont Plus CPU with 64-bit extensions,MOVBE,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,POPCNT,CX16,SAHF,FXSR,PCLMUL,PREFETCHW,RDRND,AES,SHA,RDSEED,XSAVE,XSAVEC,XSAVES,XSAVEOPT,CLFLUSHOPT,FSGSBASE,PTWRITE,RDPIDandSGXinstruction set support.tremont— Intel Tremont CPU with 64-bit extensions,MOVBE,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,POPCNT,CX16,SAHF,FXSR,PCLMUL,PREFETCHW,RDRND,AES,SHA,RDSEED,XSAVE,XSAVEC,XSAVES,XSAVEOPT,CLFLUSHOPT,FSGSBASE,PTWRITE,RDPID,SGX,CLWB,GFNI-SSE,MOVDIRI,MOVDIR64B,CLDEMOTEandWAITPKGinstruction set support.sierraforest— Intel Sierra Forest CPU with 64-bit extensions,MOVBE,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,POPCNT,AES,PREFETCHW,PCLMUL,RDRND,XSAVE,XSAVEC,XSAVES,XSAVEOPT,FSGSBASE,PTWRITE,RDPID,SGX,GFNI-SSE,CLWB,MOVDIRI,MOVDIR64B,CLDEMOTE,WAITPKG,ADCX,AVX,AVX2,BMI,BMI2,F16C,FMA,LZCNT,PCONFIG,PKU,VAES,VPCLMULQDQ,SERIALIZE,HRESET,KL,WIDEKL,AVX-VNNI,AVXIFMA,AVXVNNIINT8,AVXNECONVERT,CMPCCXADD,ENQCMDandUINTRinstruction set support.grandridge— Intel Grand Ridge CPU with 64-bit extensions,MOVBE,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,POPCNT,AES,PREFETCHW,PCLMUL,RDRND,XSAVE,XSAVEC,XSAVES,XSAVEOPT,FSGSBASE,PTWRITE,RDPID,SGX,GFNI-SSE,CLWB,MOVDIRI,MOVDIR64B,CLDEMOTE,WAITPKG,ADCX,AVX,AVX2,BMI,BMI2,F16C,FMA,LZCNT,PCONFIG,PKU,VAES,VPCLMULQDQ,SERIALIZE,HRESET,KL,WIDEKL,AVX-VNNI,AVXIFMA,AVXVNNIINT8,AVXNECONVERT,CMPCCXADD,ENQCMDandUINTRinstruction set support.clearwaterforest— Intel Clearwater Forest CPU with 64-bit extensions,MOVBE,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,POPCNT,AES,PREFETCHW,PCLMUL,RDRND,XSAVE,XSAVEC,XSAVES,XSAVEOPT,FSGSBASE,PTWRITE,RDPID,SGX,GFNI-SSE,CLWB,MOVDIRI,MOVDIR64B,CLDEMOTE,WAITPKG,ADCX,AVX,AVX2,BMI,BMI2,F16C,FMA,LZCNT,PCONFIG,PKU,VAES,VPCLMULQDQ,SERIALIZE,HRESET,AVX-VNNI,ENQCMD,UINTR,AVXIFMA,AVXVNNIINT8,AVXNECONVERT,CMPCCXADD,AVXVNNIINT16,SHA512,SM3,SM4,USER_MSRandPREFETCHIinstruction set support.
okay bestie we're pivoting to the AMD side of the family now, hydrate and stay with me 💖
k6— AMD K6 CPU withMMXinstruction set support.k6-2,k6-3— improved versions of the AMD K6 CPU withMMXand3DNow!instruction set support.athlon,athlon-tbird— AMD Athlon CPU withMMX,3dNOW!, enhanced3DNow!andSSEprefetch instructions support.athlon-4,athlon-xp,athlon-mp— improved AMD Athlon CPU withMMX,3DNow!, enhanced3DNow!and fullSSEinstruction set support.k8,opteron,athlon64,athlon-fx— processors based on the AMD K8 core with x86-64 instruction set support, including the AMD Opteron, Athlon 64, and Athlon 64 FX processors. (this one supersetsMMX,SSE,SSE2,3DNow!, enhanced3DNow!and 64-bit instruction set extensions.) not her having a whole parenthetical footnote, we love a thorough queen 🙈k8-sse3,opteron-sse3,athlon64-sse3— improved versions of the AMD K8 cores withSSE3instruction set support.amdfam10,barcelona— CPUs based on AMD Family 10h cores with x86-64 instruction set support. (This supersets MMX, SSE, SSE2, SSE3, SSE4A, 3DNow!, enhanced 3DNow!, ABM and 64-bit instruction set extensions.)bdver1— CPUs based on AMD Family 15h cores with x86-64 instruction set support. (This supersets FMA4, AVX, XOP, LWP, AES, PCLMUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.)bdver2— AMD Family 15h core based CPUs with x86-64 instruction set support. (This supersets BMI, TBM, F16C, FMA, FMA4, AVX, XOP, LWP, AES, PCLMUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.)bdver3— AMD Family 15h core based CPUs with x86-64 instruction set support. (This supersets BMI, TBM, F16C, FMA, FMA4, FSGSBASE, AVX, XOP, LWP, AES, PCLMUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.)bdver4— AMD Family 15h core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, TBM, F16C, FMA, FMA4, FSGSBASE, AVX, AVX2, XOP, LWP, AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.)znver1— AMD Family 17h core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, F16C, FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, and 64-bit instruction set extensions.)znver2— AMD Family 17h core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, CLWB, F16C, FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, RDPID, WBNOINVD, and 64-bit instruction set extensions.)znver3— AMD Family 19h core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, CLWB, F16C, FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, RDPID, WBNOINVD, PKU, VPCLMULQDQ, VAES, and 64-bit instruction set extensions.)znver4— AMD Family 19h core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, CLWB, F16C, FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, RDPID, WBNOINVD, PKU, VPCLMULQDQ, VAES, AVX512F, AVX512DQ, AVX512IFMA, AVX512CD, AVX512BW, AVX512VL, AVX512BF16, AVX512VBMI, AVX512VBMI2, AVX512VNNI, AVX512BITALG, AVX512VPOPCNTDQ, GFNI and 64-bit instruction set extensions.)znver5— AMD Family 1ah core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, CLWB, F16C, FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, RDPID, WBNOINVD, PKU, VPCLMULQDQ, VAES, AVX512F, AVX512DQ, AVX512IFMA, AVX512CD, AVX512BW, AVX512VL, AVX512BF16, AVX512VBMI, AVX512VBMI2, AVX512VNNI, AVX512BITALG, AVX512VPOPCNTDQ, GFNI, AVXVNNI, MOVDIRI, MOVDIR64B, AVX512VP2INTERSECT, PREFETCHI and 64-bit instruction set extensions.)btver1— CPUs based on AMD Family 14h cores with x86-64 instruction set support. (This supersets MMX, SSE, SSE2, SSE3, SSSE3, SSE4A, CX16, ABM and 64-bit instruction set extensions.)btver2— CPUs based on AMD Family 16h cores with x86-64 instruction set support. This includes MOVBE, F16C, BMI, AVX, PCLMUL, AES, SSE4.2, SSE4.1, CX16, ABM, SSE4A, SSSE3, SSE3, SSE2, SSE, MMX and 64-bit instruction set extensions.winchip-c6— IDT WinChip C6 CPU, dealt in same way as i486 with additional MMX instruction set support.winchip2— IDT WinChip 2 CPU, dealt in same way as i486 with additional MMX and 3DNow! instruction set support.c3— VIA C3 CPU with MMX and 3DNow! instruction set support. (No scheduling is implemented for this chip.)c3-2— VIA C3-2 (Nehemiah/C5XL) CPU with MMX and SSE instruction set support. (No scheduling is implemented for this chip.)c7— VIA C7 (Esther) CPU with MMX, SSE, SSE2 and SSE3 instruction set support. (No scheduling is implemented for this chip.)samuel-2— VIA Eden Samuel 2 CPU with MMX and 3DNow! instruction set support. (No scheduling is implemented for this chip.)nehemiah— VIA Eden Nehemiah CPU with MMX and SSE instruction set support. (No scheduling is implemented for this chip.)esther— VIA Eden Esther CPU with MMX, SSE, SSE2 and SSE3 instruction set support. (No scheduling is implemented for this chip.)eden-x2— VIA Eden X2 CPU with x86-64, MMX, SSE, SSE2 and SSE3 instruction set support. (No scheduling is implemented for this chip.)eden-x4— VIA Eden X4 CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX and AVX2 instruction set support. (No scheduling is implemented for this chip.)nano— Generic VIA Nano CPU with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support. (No scheduling is implemented for this chip.)nano-1000— VIA Nano 1xxx CPU with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support. (No scheduling is implemented for this chip.)nano-2000— VIA Nano 2xxx CPU with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support. (No scheduling is implemented for this chip.)nano-3000— VIA Nano 3xxx CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3 and SSE4.1 instruction set support. (No scheduling is implemented for this chip.)nano-x2— VIA Nano Dual Core CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3 and SSE4.1 instruction set support. (No scheduling is implemented for this chip.)nano-x4— VIA Nano Quad Core CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3 and SSE4.1 instruction set support. (No scheduling is implemented for this chip.)lujiazui— ZHAOXIN lujiazui CPU with x86-64, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PCLMUL, RDRND, XSAVE, XSAVEOPT, FSGSBASE, CX16, ABM, BMI, BMI2, FXSR, RDSEED instruction set support. While the CPUs do support AVX and F16C, these aren't enabled by-march=lujiazuifor performance reasons.yongfeng— ZHAOXIN yongfeng CPU with x86-64, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, POPCNT, AES, PCLMUL, RDRND, XSAVE, XSAVEOPT, FSGSBASE, CX16, ABM, BMI, BMI2, F16C, FXSR, RDSEED, AVX2, FMA, SHA, LZCNT instruction set support.shijidadao— ZHAOXIN shijidadao CPU with x86-64, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, POPCNT, AES, PCLMUL, RDRND, XSAVE, XSAVEOPT, FSGSBASE, CX16, ABM, BMI, BMI2, F16C, FXSR, RDSEED, AVX2, FMA, SHA, LZCNT instruction set support.geode— AMD Geode embedded processor with MMX and 3DNow! instruction set support.c86-4g-m4— HYGON c86-4g-m4 CPU with x86-64, MMX, SSE, SSE2, SSE3, SSE4A, CX16, ABM, SSSE3, SSE4.1, SSE4.2, AES, PCLMUL, AVX, AVX2, BMI, BMI2, F16C, FMA, PRFCHW, FXSR, SHA, XSAVE, XSAVEOPT, XSAVEC, FSGSBASE, RDRND, MOVBE, MWAITX, ADX, RDSEED, CLZERO, CLFLUSHOPT, XSAVES, LZCNT, POPCNT instruction set support.c86-4g-m6— HYGON c86-4g-m6 CPU with x86-64, MMX, SSE, SSE2, SSE3, SSE4A, CX16, ABM, SSSE3, SSE4.1, SSE4.2, AES, PCLMUL, AVX, AVX2, BMI, BMI2, F16C, FMA, PRFCHW, FXSR, SHA, XSAVE, XSAVEOPT, XSAVEC, FSGSBASE, RDRND, MOVBE, MWAITX, ADX, RDSEED, CLZERO, CLFLUSHOPT, XSAVES, LZCNT, POPCNT instruction set support.c86-4g-m7— HYGON c86-4g-m7 CPU with x86-64, MMX, SSE, SSE2, SSE3, SSE4A, CX16, ABM, SSSE3, SSE4.1, SSE4.2, AES, PCLMUL, AVX, AVX2, BMI, BMI2, F16C, FMA, PRFCHW, FXSR, SHA, XSAVE, XSAVEOPT, XSAVEC, FSGSBASE, RDRND, MOVBE, MWAITX, ADX, RDSEED, CLZERO, CLFLUSHOPT, XSAVES, LZCNT, POPCNT, AVX512F, AVX512DQ, AVX512IFMA, AVX512CD, AVX512BW, AVX512VL, AVX512BF16, AVX512VBMI, AVX512VBMI2, GFNI, AVX512VNNI, VAES, AVX512BITALG, AVX512VPOPCNTDQ, AVX512VP2INTERSECT, AVXVNNI, VPCLMULQDQ, WBNOINVD instruction set support.
-mtune=cpu-type— bestie, this one tunes literally everything applicable about the generated code to cpu-type, except for the ABI and the set of available instructions. 💅
So like… picking a specific cpu-type schedules things appropriately for that particular chip, but the compiler does not generate any code that cannot run on the default machine type unless you also use a -march=cpu-type option. For example, if GCC is configured for i686-pc-linux-gnu then -mtune=pentium4 generates code that is tuned for Pentium 4 but still runs on i686 machines. 👑
The choices for cpu-type are the same as for -march. In addition, -mtune supports 2 extra choices for cpu-type:
generic— Produce code optimized for the most common IA32/AMD64/EM64T processors. If you know the CPU on which your code will run, then you should use the corresponding-mtuneor-marchoption instead of-mtune=generic. But, if you do not know exactly what CPU users of your application will have, then you should use this option.
As new processors are deployed in the marketplace, the behavior of this option will change. Therefore, if you upgrade to a newer version of GCC, code generation controlled by this option will change to reflect the processors that are most common at the time that version of GCC is released. ✨ it's giving "the vibes shift, and so does the code" ✨
generic <span>—</span> wait, nope, there's no -march=generic bestie 😭
Okay so this is the part that confuses everyone, so let me say it loud: there is no -march=generic option, because -march is all about which instruction set the compiler is allowed to use, and there is literally no generic instruction set that works on every single processor. It's giving "there's no universal fit". Meanwhile -mtune is a whole different vibe — it says which processor (or, in this case, a whole collection of processors) the code should be optimized for. 🎯
intel 💙
Produces code optimized for the most current Intel processors, which for this version of GCC are Haswell and Silvermont. No cap.
If you actually know the CPU your code is gonna run on, then bestie, use the corresponding -mtune or -march option instead of -mtune=intel. But if you want your app to perform better on both Haswell and Silvermont, then yes, use this option. 🫶
As new Intel processors get deployed in the marketplace, the behavior of this option will change. So heads up: if you upgrade to a newer version of GCC, code generation controlled by this option will change to reflect the most current Intel processors at the time that version of GCC is released. So it's a moving target, fr fr. 👀
And just like before: there is no -march=intel option, because -march indicates the instruction set the compiler can use, and there's no common instruction set applicable to all processors. In contrast, -mtune indicates the processor (or, in this case, collection of processors) for which the code is optimized. Periodt. 💅
-mcpu=cpu-type
A deprecated synonym for -mtune. That's it, that's the whole thing. It's giving legacy energy, she's retired but still hanging around. 🫠
-mfpmath=unit
Generate floating-point arithmetic for the selected unit unit. The choices for unit are:
387— use the standard 387 floating-point coprocessor present on the majority of chips and emulated otherwise. Code compiled with this option runs almost everywhere. The temporary results are computed in 80-bit precision instead of the precision specified by the type, which means slightly different results compared to most of other chips. See-ffloat-storefor a more detailed description. This is the default choice for non-Darwin x86-32 targets. ✨sse— use scalar floating-point instructions present in the SSE instruction set. This instruction set is supported by Pentium III and newer chips, and in the AMD line by Athlon-4, Athlon XP and Athlon MP chips. The earlier version of the SSE instruction set supports only single-precision arithmetic, so the double and extended-precision arithmetic are still done using 387. A later version, present only in Pentium 4 and AMD x86-64 chips, supports double-precision arithmetic too. For the x86-32 compiler, you must use-march=cpu-type,-msseor-msse2switches to enable SSE extensions and make this option effective. For the x86-64 compiler, these extensions are enabled by default. The resulting code should be considerably faster in the majority of cases and avoid the numerical instability problems of 387 code, but may break some existing code that expects temporaries to be 80 bits. 😬 This is the default choice for the x86-64 compiler, Darwin x86-32 targets, and the default choice for x86-32 targets with the SSE2 instruction set when-ffast-mathis enabled.sse,387,sse+387,both— attempt to utilize both instruction sets at once. This effectively doubles the amount of available registers, and on chips with separate execution units for 387 and SSE, the execution resources too. Use this option with care, because it is still experimental — the GCC register allocator does not model separate functional units well, which results in unstable performance. It's giving "we're so back" then "it's so over" on loop. 💀
-masm=dialect
Output assembly instructions using the selected dialect. This also affects which dialect is used for basic "asm" and extended "asm". Supported choices (in dialect order) are att or intel. The default is att. Darwin does not support intel. 🙅♀️
-mieee-fp and -mno-ieee-fp 🧊
Control whether or not the compiler uses IEEE floating-point comparisons. These correctly handle the case where the result of a comparison is unordered. Very demure, very mindful. ✨
-m80387 and -mhard-float
Generate output containing 80387 instructions for floating point. Hardware girlies rise up. 🔥
-mno-80387 and -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 cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation. 😭
On machines where a function returns floating-point results in the 80387 register stack, some floating-point opcodes may be emitted even if -msoft-float is used. Not me warning you twice, but here we are. 🫠
-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. Cute theory, honestly. 🎀
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 overridden when -march indicates that the target CPU always has an FPU and so the instruction does not need emulation. Also ngl, these instructions are not generated unless you also use the -funsafe-math-optimizations switch. So it's a whole team effort. 🤝
-malign-double and -mno-align-double
Control whether GCC aligns "double", "long double", and "long long" variables on a two-word boundary or a one-word boundary. Aligning "double" variables on a two-word boundary produces code that runs somewhat faster on a Pentium, at the expense of more memory. Trade-offs, bestie, trade-offs. ⚖️
On x86-64, -malign-double is enabled by default.
Warning: if you use the -malign-double switch, structures containing the above types are aligned differently than the published application binary interface specifications for the x86-32 and are not binary compatible with structures in code compiled without that switch. So like, pick a lane. 🚨
-m96bit-long-double and -m128bit-long-double
These switches control the size of the "long double" type. The x86-32 application binary interface specifies the size to be 96 bits, so -m96bit-long-double is the default in 32-bit mode. 🎯
Modern architectures (Pentium and newer) prefer "long double" to be aligned to an 8- or 16-byte boundary. In arrays or structures conforming to the ABI, this is not possible. So specifying -m128bit-long-double aligns "long double" to a 16-byte boundary by padding the "long double" with an additional 32-bit zero.
In the x86-64 compiler, -m128bit-long-double is the default choice, as its ABI specifies that "long double" is aligned on a 16-byte boundary.
Notice that neither of these options enable any extra precision over the x87 standard of 80 bits for a "long double". Don't get it twisted. 🙈
Warning: if you override the default value for your target ABI, this changes the size of structures and arrays containing"long double"variables, as well as modifying the function calling convention for functions taking"long double". Hence they are not binary-compatible with code compiled without that switch.
-mlong-double-64, -mlong-double-80 and -mlong-double-128
These switches control the size of the "long double" type too.
- A size of 64 bits makes the
"long double"type equivalent to the"double"type. This is the default for 32-bit Bionic C library. - A size of 128 bits makes the
"long double"type equivalent to the"__float128"type. This is the default for 64-bit Bionic C library.
Warning: if you override the default value for your target ABI, this changes the size of structures and arrays containing"long double"variables, as well as modifying the function calling convention for functions taking"long double". Hence they are not binary-compatible with code compiled without that switch. Same warning, different day. 💅
-malign-data=type
Control how GCC aligns variables. Supported values for type are:
compat— uses increased alignment value compatible with GCC 4.8 and earlier.abi— uses alignment value as specified by the psABI.cacheline— uses increased alignment value to match the cache line size.
compat is the default. Slay. 🌸
-mlarge-data-threshold=threshold
When -mcmodel=medium or -mcmodel=large is specified, data objects larger than threshold are placed in large data sections. The default is 65535. A very specific little number, iconic. 🔢
-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's no need to pop the arguments there. Efficiency queen. 👑
You can specify that an individual function is called with this calling sequence with the function attribute "stdcall". You can also override the -mrtd option by using the function attribute "cdecl".
Warning: this calling convention is incompatible with the one normally used on Unix, so you cannot use it if you need to call libraries compiled with the Unix compiler. 🚫
Also, you must provide function prototypes for all functions that take variable numbers of arguments (including "printf"); otherwise incorrect code is generated for calls to those functions.
In addition, seriously incorrect code results if you call a function with too many arguments. (Normally, extra arguments are harmlessly ignored.) So yeah, don't do that. 😭
-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. No sneaky mismatches, bestie. 🙅
-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". That's the whole vibe — registers passing, but make it SSE. 🦋
Warning: if you use this switch then you must build all modules with the same value, including any libraries. That means the system libraries AND the startup modules too. No sneaking around it, bestie. 💅
-mvect8-ret-in-mem— Returns 8-byte vectors in memory instead of MMX registers. This is the default on VxWorks, so it can match the ABI of the Sun Studio compilers up to version 12. Only reach for this option if you genuinely need to stay compatible with existing code that those previous compiler versions (or older GCC versions) produced. Otherwise? Leave it alone. 🙈-mpc32-mpc64-mpc80— These set the 80387 floating-point precision to 32, 64, or 80 bits. Let's break it down, because the way these work is lowkey iconic:-mpc32— significands of floating-point operation results get rounded to 24 bits (that's single precision).-mpc64— significands get rounded to 53 bits (double precision).-mpc80— significands get rounded to 64 bits (extended double precision), and this one is the default.
When you use this option, floating-point operations in higher precisions are not available to you as a programmer unless you set the FPU control word explicitly. So that's on you.
Setting the rounding of floating-point operations to less than the default 80 bits can speed some programs up by 2% or more. ✨ But heads up: some mathematical libraries assume that extended-precision (80-bit) floating-point operations are enabled by default. Routines in those libraries could suffer significant loss of accuracy — typically through the so-called "catastrophic cancellation" — when this option sets the precision to less than extended precision. So like... know what you're doing. 😭
-mdaz-ftz— The flush-to-zero (FTZ) and denormals-are-zero (DAZ) flags in theMXCSRregister control floating-point calculations. SSE and AVX instructions — including scalar and vector instructions — could benefit from enabling the FTZ and DAZ flags when-mdaz-ftzis specified. Don't set FTZ/DAZ flags when-mno-daz-ftzor-sharedis specified; note that-mdaz-ftzwill set FTZ/DAZ flags even with-shared. 🫶-mstackrealign— Realigns the stack at entry. On the x86, the-mstackrealignoption generates an alternate prologue and epilogue that realigns the run-time stack if necessary. This supports mixing legacy code that keeps 4-byte stack alignment with modern code that keeps 16-byte stack alignment for SSE compatibility. See also the attributeforce_align_arg_pointer, which is applicable to individual functions.- *
-mpreferred-stack-boundary=num* — Attempts to keep the stack boundary aligned to a 2 raised to num byte boundary. If-mpreferred-stack-boundaryisn't specified, the default is4(that's 16 bytes or 128 bits).
Warning: when generating code for the x86-64 architecture with SSE extensions disabled,-mpreferred-stack-boundary=3can be used to keep the stack boundary aligned to an 8 byte boundary. But since the x86-64 ABI requires 16 byte stack alignment, this is ABI incompatible and it's meant to be used only in a controlled environment where stack space is an important limitation. This option leads to wrong code when functions compiled with 16 byte stack alignment (like functions from a standard library) are called with a misaligned stack. In that case, SSE instructions may lead to misaligned memory access traps. On top of that, variable arguments are handled incorrectly for 16 byte aligned objects (including x87long doubleand__int128), leading to wrong results. You must build all modules with-mpreferred-stack-boundary=3, including any libraries. And yes, that includes the system libraries and startup modules. 💀
- *
-mincoming-stack-boundary=num* — Assumes the incoming stack is aligned to a 2 raised to num byte boundary. If-mincoming-stack-boundaryisn't specified, the one specified by-mpreferred-stack-boundaryis used.
On Pentium and Pentium Pro, double and long double values should be aligned to an 8-byte boundary (see -malign-double) or suffer significant run time performance penalties. On Pentium III, the Streaming SIMD Extension (SSE) data type __m128 may not work properly if it isn't 16-byte aligned. 😬
To ensure proper alignment of these values on the stack, the stack boundary must be as aligned as that required by any value stored on the stack. Further, every function must be generated such that it keeps the stack aligned. So calling a function compiled with a higher preferred stack boundary from a function compiled with a lower preferred stack boundary most likely misaligns the stack. It's 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's sensitive to stack space usage — like embedded systems and operating system kernels — may want to reduce the preferred alignment to -mpreferred-stack-boundary=2.
-mmmx-msse-msse2-msse3-mssse3-msse4-msse4a-msse4.1-msse4.2-mavx-mavx2-mavx512f-mavx512cd-mavx512vl-mavx512bw-mavx512dq-mavx512ifma-mavx512vbmi-msha-maes-mpclmul-mclflushopt-mclwb-mfsgsbase-mptwrite-mrdrnd-mf16c-mfma-mpconfig-mwbnoinvd-mfma4-mprfchw-mrdpid-mrdseed-msgx-mxop-mlwp-m3dnow-m3dnowa-mpopcnt-mabm-madx-mbmi-mbmi2-mlzcnt-mfxsr-mxsave-mxsaveopt-mxsavec-mxsaves-mrtm-mhle-mtbm-mmwaitx-mclzero-mpku-mavx512vbmi2-mavx512bf16-mavx512fp16-mgfni-mvaes-mwaitpkg-mvpclmulqdq-mavx512bitalg-mmovdiri-mmovdir64b-menqcmd-muintr-mtsxldtrk-mavx512vpopcntdq-mavx512vp2intersect-mavx512vnni-mavxvnni-mcldemote-mserialize-mamx-tile-mamx-int8-mamx-bf16-mhreset-mkl-mwidekl-mavxifma-mavxvnniint8-mavxneconvert-mcmpccxadd-mamx-fp16-mprefetchi-mraoint-mamx-complex-mavxvnniint16-msm3-msha512-msm4-mapxf-musermsr-mavx10.1-mavx10.1-256-mavx10.1-512-mavx10.2-mamx-avx512-mamx-tf32-mamx-transpose-mamx-fp8-mmovrs-mamx-movrs— Okay so this whole list of switches enables the use of instructions in the MMX, SSE, AVX512CD, AVX512VL, AVX512BW, AVX512DQ, AVX512IFMA, AVX512VBMI, SHA, AES, PCLMUL, CLFLUSHOPT, CLWB, FSGSBASE, PTWRITE, RDRND, F16C, FMA, PCONFIG, WBNOINVD, FMA4, PREFETCHW, RDPID, RDSEED, SGX, XOP, LWP, 3DNow!, enhanced 3DNow!, POPCNT, ABM, ADX, BMI, BMI2, LZCNT, FXSR, XSAVE, XSAVEOPT, XSAVEC, XSAVES, RTM, HLE, TBM, MWAITX, CLZERO, PKU, AVX512VBMI2, GFNI, VAES, WAITPKG, VPCLMULQDQ, AVX512BITALG, MOVDIRI, MOVDIR64B, AVX512BF16, ENQCMD, AVX512VPOPCNTDQ, AVX512VNNI, SERIALIZE, UINTR, HRESET, AMXTILE, AMXINT8, AMXBF16, KL, WIDEKL, AVXVNNI, AVX512-FP16, AVXIFMA, AVXVNNIINT8, AVXNECONVERT, CMPCCXADD, AMX-FP16, PREFETCHI, RAOINT, AMX-COMPLEX, AVXVNNIINT16, SM3, SHA512, SM4, APX_F, USER_MSR, AVX10.1, AVX10.2, AMX-AVX512, AMX-TF32, AMX-TRANSPOSE, AMX-FP8, MOVRS, AMX-MOVRS or CLDEMOTE extended instruction sets. Each one has a corresponding-mno-option to disable use of those instructions. 🌈
These extensions are also available as built-in functions — see x86 Built-in Functions for all the details of the functions enabled and disabled by these switches.
Note that -msse4 enables both SSE4.1 and SSE4.2 support, while -mno-sse4 turns off those features; neither form of the option affects SSE4A support, which is controlled separately by -msse4a. Don't get them twisted. 💅
To generate SSE/SSE2 instructions automatically from floating-point code (as opposed to 387 instructions), see -mfpmath=sse.
GCC depresses SSEx instructions when -mavx is used. Instead, it generates new AVX instructions or AVX equivalence for all SSEx instructions when needed. ✨
These options enable GCC to use these extended instructions in generated code, even without -mfpmath=sse. Applications that perform run-time CPU detection must compile separate files for each supported architecture, using the appropriate flags. In particular, the file containing the CPU detection code should be compiled without these options. iykyk 🧚
-mdump-tune-features— Instructs GCC to dump the names of the x86 performance tuning features and default settings. Those names can then be used in-mtune-ctrl=feature-list.- *
-mtune-ctrl=feature-list* — This one does fine grain control of x86 code generation features. feature-list is a comma separated list of feature names. See also-mdump-tune-features. When specified, the feature is turned on if it's not preceded with^; otherwise, it's turned off.-mtune-ctrl=feature-list is intended to be used by GCC developers. Using it may lead to code paths not covered by testing and can potentially result in compiler ICEs or runtime errors. 😭 So like, proceed with caution. -mno-default— Instructs GCC to turn off all tunable features. See also-mtune-ctrl=feature-list and-mdump-tune-features.-mcld— Instructs GCC to emit acldinstruction in the prologue of functions that use string instructions. String instructions depend on theDFflag to select between autoincrement or autodecrement mode. While the ABI specifies theDFflag to be cleared on function entry, some operating systems violate this specification by not clearing theDFflag in their exception dispatchers. The exception handler can be invoked with theDFflag set, which leads to wrong direction mode when string instructions are used. This option can be enabled by default on 32-bit x86 targets by configuring GCC with the--enable-cldconfigure option. Generation ofcldinstructions can be suppressed with the-mno-cldcompiler option in this case.-mvzeroupper— Instructs GCC to emit avzeroupperinstruction before a transfer of control flow out of the function, to minimize the AVX to SSE transition penalty as well as remove unnecessaryzeroupperintrinsics. 👑-mprefer-avx128— Instructs GCC to use 128-bit AVX instructions instead of 256-bit AVX instructions in the auto-vectorizer. 🦋-mprefer-vector-width=opt— okay bestie, this one tells GCC to useopt-bit vector width in instructions instead of the default on the selected platform. <em>opt</em> is literally just the width you want, and the vibe is "make my vectors this size, please and thank you." 💅-mpartial-vector-fp-math— this is giving "let's do math on part of the vector" energy. This option enables GCC to generate floating-point operations that might affect the set of floating-point status flags on partial vectors, where vector elements reside in the low part of the 128-bit SSE register. Unless-fno-trapping-mathis specified, the compiler guarantees correct behavior by sanitizing all input operands to have zeroes in the unused upper part of the vector register. Note that by using built-in functions or inline assembly with partial vector arguments, NaNs, denormal or invalid values can leak into the upper part of the vector, causing possible performance issues when-fno-trapping-mathis in effect. These issues can be mitigated by manually sanitizing the upper part of the partial vector argument register or by using-mdaz-ftzto set denormals-are-zero (DAZ) flag in the MXCSR register.
This option is enabled by default. <em>periodt.</em>
-mmove-max=bits— this option instructs GCC to set the maximum number of bits can be moved from memory to memory efficiently to <em>bits</em>. The valid <em>bits</em> are128,256and512. <em>(yes, the grammar is a little wonky, but we're keeping it exactly as the manpage says, bestie 💅)</em>-mstore-max=bits— this option instructs GCC to set the maximum number of bits can be stored to memory efficiently to <em>bits</em>. The valid <em>bits</em> are128,256and512.none— No extra limitations applied to GCC other than defined by the selected platform.128— Prefer 128-bit vector width for instructions.256— Prefer 256-bit vector width for instructions.512— Prefer 512-bit vector width for instructions.
-mnoreturn-no-callee-saved-registers— this option optimizes functions with "noreturn" attribute or "_Noreturn" specifier by not saving in the function prologue callee-saved registers which are used in the function (except for the "BP" register). This option can interfere with debugging of the caller of the "noreturn" function or any function further up in the call stack, so it is not enabled by default. <em>so like, maybe don't use it if you're gonna be debugging, bestie 😭</em>-mcx16— this option enables GCC to generateCMPXCHG16Binstructions in 64-bit code to implement compare-and-exchange operations on 16-byte aligned 128-bit objects. This is useful for atomic updates of data structures exceeding one machine word in size. The compiler uses this instruction to implement__syncBuiltins. However, for__atomicBuiltins operating on 128-bit integers, a library call is always used. <em>iykyk 💅</em>-msahf— this option enables generation ofSAHFinstructions in 64-bit code. Early Intel Pentium 4 CPUs with Intel 64 support, prior to the introduction of Pentium 4 G1 step in December 2005, lacked theLAHFandSAHFinstructions which are supported by AMD64. These are load and store instructions, respectively, for certain status flags. In 64-bit mode, theSAHFinstruction is used to optimizefmod,drem, andremainderbuilt-in functions; see Other Builtins for details. <em>history lesson fr fr 🫶</em>-mmovbe— this option enables use of themovbeinstruction to optimize byte swapping of four and eight byte entities. <em>byte swap slay 💅</em>-mshstk— The-mshstkoption enables shadow stack built-in functions from x86 Control-flow Enforcement Technology (CET). <em>shadow stack? more like shadow glam ✨</em>-mcrc32— this option enables built-in functions__builtin_ia32_crc32qi,__builtin_ia32_crc32hi,__builtin_ia32_crc32siand__builtin_ia32_crc32dito generate thecrc32machine instruction. <em>CRC check, but make it fashion 💖</em>-mmwait— this option enables built-in functions__builtin_ia32_monitor, and__builtin_ia32_mwaitto generate themonitorandmwaitmachine instructions. <em>monitor and mwait, the iconic duo 👑</em>-mrecip— this option enables use ofRCPSSandRSQRTSSinstructions (and their vectorized variantsRCPPSandRSQRTPS) with an additional Newton-Raphson step to increase precision instead ofDIVSSandSQRTSS(and their vectorized variants) for single-precision floating-point arguments. These instructions are generated only when-funsafe-math-optimizationsis enabled together with-ffinite-math-onlyand-fno-trapping-math. Note that while the throughput of the sequence is higher than the throughput of the non-reciprocal instruction, the precision of the sequence can be decreased by up to 2 ulp (i.e. the inverse of 1.0 equals 0.99999994).
Note that GCC implements 1.0f/sqrtf(x) in terms of RSQRTSS (or RSQRTPS) already with -ffast-math (or the above option combination), and doesn't need -mrecip.
Also note that GCC emits the above sequence with additional Newton-Raphson step for vectorized single-float division and vectorized sqrtf(x) already with -ffast-math (or the above option combination), and doesn't need -mrecip. <em>so like, read the notes above, bestie 😭</em>
-mrecip=opt— this option controls which reciprocal estimate instructions may be used. <em>opt</em> is a comma-separated list of options, which may be preceded by a!to invert the option:all— Enable all estimate instructions.default— Enable the default instructions, equivalent to-mrecip.none— Disable all estimate instructions, equivalent to-mno-recip.div— Enable the approximation for scalar division.vec-div— Enable the approximation for vectorized division.sqrt— Enable the approximation for scalar square root.vec-sqrt— Enable the approximation for vectorized square root.
So, for example, -mrecip=all,!sqrt enables all of the reciprocal approximations, except for square root. <em>the ! is giving "not today, sqrt" energy 💅</em>
-mveclibabi=type— Specifies the ABI type to use for vectorizing intrinsics using an external library. Supported values for <em>type</em> aresvmlfor the Intel short vector math library,aoclfor the math library (LibM) from AMD Optimizing CPU Libraries (AOCL) andacmlfor the end-of-life AMD core math library (to which AOCL-LibM is the successor). To use this option, both-ftree-vectorizeand-funsafe-math-optimizationshave to be enabled, and an SVML or ACML ABI-compatible library must be specified at link time.
GCC currently emits calls to vmldExp2, vmldLn2, vmldLog102, vmldPow2, vmldTanh2, vmldTan2, vmldAtan2, vmldAtanh2, vmldCbrt2, vmldSinh2, vmldSin2, vmldAsinh2, vmldAsin2, vmldCosh2, vmldCos2, vmldAcosh2, vmldAcos2, vmlsExp4, vmlsLn4, vmlsLog104, vmlsPow4, vmlsTanh4, vmlsTan4, vmlsAtan4, vmlsAtanh4, vmlsCbrt4, vmlsSinh4, vmlsSin4, vmlsAsinh4, vmlsAsin4, vmlsCosh4, vmlsCos4, vmlsAcosh4 and vmlsAcos4 for corresponding function type when -mveclibabi=svml is used, amd_vrs4_acosf, amd_vrs16_acosf, amd_vrd8_asin, amd_vrs4_asinf, amd_vrs8_asinf, amd_vrs16_asinf, amd_vrd2_atan, amd_vrd8_atan, amd_vrs4_atanf, amd_vrs8_atanf, amd_vrs16_atanf, amd_vrd2_cos, amd_vrd4_cos, amd_vrd8_cos, amd_vrs4_cosf, amd_vrs8_cosf, amd_vrs16_cosf, amd_vrs4_coshf, amd_vrs8_coshf, amd_vrd2_erf, amd_vrd4_erf, amd_vrd8_erf, amd_vrs4_erff, amd_vrs8_erff, amd_vrs16_erff, amd_vrd2_exp, amd_vrd4_exp, amd_vrd8_exp, amd_vrs4_expf, amd_vrs8_expf, amd_vrs16_expf, amd_vrd2_exp10, amd_vrs4_exp10f, amd_vrd2_exp2, amd_vrd4_exp2, amd_vrd8_exp2, amd_vrs4_exp2f, amd_vrs8_exp2f, amd_vrs16_exp2f, amd_vrs4_expm1f, amd_vrd2_log, amd_vrd4_log, amd_vrd8_log, amd_vrs4_logf, amd_vrs8_logf, amd_vrs16_logf, amd_vrd2_log10, amd_vrs4_log10f, amd_vrs8_log10f, amd_vrs16_log10f, amd_vrd2_log1p, amd_vrs4_log1pf, amd_vrd2_log2, amd_vrd4_log2, amd_vrd8_log2, amd_vrs4_log2f, amd_vrs8_log2f, amd_vrs16_log2f, amd_vrd2_pow, amd_vrd4_pow, amd_vrd8_pow, amd_vrs4_powf, amd_vrs8_powf, amd_vrs16_powf, amd_vrd2_sin, amd_vrd4_sin, amd_vrd8_sin, amd_vrs4_sinf, amd_vrs8_sinf, amd_vrs16_sinf, amd_vrd2_tan, amd_vrd4_tan, amd_vrd8_tan, amd_vrs16_tanf, amd_vrs4_tanhf, amd_vrs8_tanhf, amd_vrs16_tanhf for the corresponding function type when -mveclibabi=aocl is used, and __vrd2_sin, __vrd2_cos, __vrd2_exp, __vrd2_log, __vrd2_log2, __vrd2_log10, __vrs4_sinf, __vrs4_cosf, __vrs4_expf, __vrs4_logf, __vrs4_log2f, __vrs4_log10f and __vrs4_powf for the corresponding function type when -mveclibabi=acml is used. <em>that's a whole lotta function names, but we're not skipping any, bestie 💅</em>
-mabi=name— Generate code for the specified calling convention. Permissible values aresysvfor the ABI used on GNU/Linux and other systems, andmsfor the Microsoft ABI. The default is to use the Microsoft ABI when targeting Microsoft Windows and the SysV ABI on all other systems. You can control this behavior for specific functions by using the function attributesms_abiandsysv_abi. <em>ABI drama, but make it cute 🎀</em>-mforce-indirect-call— Force all calls to functions to be indirect. This is useful when using Intel Processor Trace where it generates more precise timing information for function calls. <em>indirect but make it precise ✨</em>-mmanual-endbr— Insert ENDBR instruction at function entry only via thecf_checkfunction attribute. This is useful when used with the option-fcf-protection=branchto control ENDBR insertion at the function entry. <em>ENDBR, but manual 👑</em>-mcet-switch— By default, CET instrumentation is turned off on switch statements that use a jump table and indirect branch track is disabled. Since jump tables are stored in read-only memory, this does not result in a direct loss of hardening. But if the jump table index is attacker-controlled, the indirect jump may not be constrained by CET. This option turns on CET instrumentation to enable indirect branch track for switch statements with jump tables which leads to the jump targets reachable via any indirect jumps. <em>security vibe check 🛡️</em>-mcall-ms2sysv-xlogues— Due to differences in 64-bit ABIs, any Microsoft ABI function that calls a System V ABI function must consider RSI, RDI and XMM6-15 as clobbered. By default, the code for saving and restoring these registers is emitted inline, resulting in fairly lengthy prologues and epilogues. Using-mcall-ms2sysv-xloguesemits prologues and epilogues that use stubs in the static portion of libgcc to perform these saves and restores, thus reducing function size at the cost of a few extra instructions. <em>trade-off, but make it iconic 💅</em>-mtls-dialect=type— makes code that reaches thread-local storage the gnu or gnu2 way.gnuis the safe lil default bestie;gnu2is faster and slay but lowkey adds compile-time and run-time requirements that literally cannot be met on every system. so like, vibe check your target first 💅-mpush-args,-mno-push-args— usePUSHoperations to stash outgoing parameters. this method is shorter and usually just as fast as theSUB/MOVmethod, and it's on by default ✨ turning it off can sometimes make things faster tho, because of better scheduling and fewer dependencies. it's giving tradeoffs, fr fr.-maccumulate-outgoing-args— when this is on, the max amount of space needed for outgoing arguments gets computed up in the function prologue. on most modern CPUs that's faster bc of fewer dependencies, better scheduling, and less stack usage when the preferred stack boundary isn't2. the catch?? a pretty notable increase in code size 😭 also this switch implies-mno-push-args.-mthreads— supports thread-safe exception handling on MinGW 🧵 programs that depend on thread-safe exception handling have to compile <b>and</b> link all their code with-mthreads. when compiling,-mthreadsdefines-D_MT; when linking, it pulls in a special thread helper library-lmingwthrdthat cleans up per-thread exception-handling data. no cap, it does both jobs.-mms-bitfields,-mno-ms-bitfields— turn bit-field layout compatible with the native Microsoft Windows compiler on/off 🎀
if "packed" is used on a structure, or if bit-fields are used, it may be that the Microsoft ABI lays out the structure differently than GCC normally does. especially when you're moving packed data between functions compiled with GCC and the native Microsoft compiler (either through a function call or as data in a file), you might need to access either format.
this option is enabled by default for Microsoft Windows targets. this behavior can also be controlled locally by using variable or type attributes. for more info, see x86 Variable Attributes and x86 Type Attributes.
the Microsoft structure layout algorithm is pretty simple except for the bit-field packing part. the padding and alignment of members of structures, and whether a bit-field can straddle a storage-unit boundary, are determined by these rules:
- Structure members are stored sequentially in the order in which they are declared: the first member has the lowest memory address and the last member the highest.
- Every data object has an alignment requirement. The alignment requirement for all data except structures, unions, and arrays is either the size of the object or the current packing size (specified with either the
"aligned"attribute or the"pack"pragma), whichever is less. For structures, unions, and arrays, the alignment requirement is the largest alignment requirement of its members. Every object is allocated an offset so that:
offset % alignment_requirement == 0- Adjacent bit-fields are packed into the same 1-, 2-, or 4-byte allocation unit if the integral types are the same size and if the next bit-field fits into the current allocation unit without crossing the boundary imposed by the common alignment requirements of the bit-fields.
MSVC interprets zero-length bit-fields like this:
- If a zero-length bit-field is inserted between two bit-fields that are normally coalesced, the bit-fields are not coalesced.
for example:
struct
{
unsigned long bf_1 : 12;
unsigned long : 0;
unsigned long bf_2 : 12;
} t1;the size of "t1" is 8 bytes with the zero-length bit-field. if the zero-length bit-field were removed, "t1"'s size would be 4 bytes. iconic behavior honestly.
- If a zero-length bit-field is inserted after a bit-field,
"foo", and the alignment of the zero-length bit-field is greater than the member that follows it,"bar","bar"is aligned as the type of the zero-length bit-field.
for example:
struct
{
char foo : 4;
short : 0;
char bar;
} t2;
struct
{
char foo : 4;
short : 0;
double bar;
} t3;for "t2", "bar" is placed at offset 2, rather than offset 1. accordingly, the size of "t2" is 4. for "t3", the zero-length bit-field does not affect the alignment of "bar" or, as a result, the size of the structure.
taking this into account, it is important to note the following:
- If a zero-length bit-field follows a normal bit-field, the type of the zero-length bit-field may affect the alignment of the structure as whole. For example,
"t2"has a size of 4 bytes, since the zero-length bit-field follows a normal bit-field, and is of type short. - Even if a zero-length bit-field is not followed by a normal bit-field, it may still affect the alignment of the structure:
struct
{
char foo : 6;
long : 0;
} t4;Here, "t4" takes up 4 bytes.
- Zero-length bit-fields following non-bit-field members are ignored:
struct
{
char foo;
long : 0;
char bar;
} t5;Here, "t5" takes up 2 bytes.
-mno-align-stringops— do <b>not</b> align the destination of inlined string operations. this switch shrinks code size and boosts performance when the destination is already aligned but GCC just doesn't know it 🙈-minline-all-stringops— by default GCC only inlines string operations when the destination is known to be aligned to at least a 4-byte boundary. this enables more inlining and increases code size, but it may improve performance for code that depends on fast"memcpy"and"memset"for short lengths. the option also enables inline expansion of"strlen"for all pointer alignments. obsessed ✨-minline-stringops-dynamically— for string operations of unknown size, use run-time checks with inline code for small blocks and a library call for large blocks. best of both worlds, periodt.-mstringop-strategy=alg— override the internal decision heuristic for which particular algorithm gets used to inline string operations. the allowed values for alg are:rep_byte,rep_4byte,rep_8byte— expand using the i386"rep"prefix of the specified size.byte_loop,loop,unrolled_loop— expand into an inline loop.libcall— always use a library call.
-mmemcpy-strategy=strategy— override the internal decision heuristic that decides if"__builtin_memcpy"should be inlined and which inline algorithm to use when the expected size of the copy operation is known. strategy is a comma-separated list ofalg:max_size:dest_aligntriplets. alg is specified in-mstringop-strategy, max_size specifies the max byte size with which inline algorithm alg is allowed. for the last triplet, the max_size must be-1. the max_size of the triplets in the list must be specified in increasing order. the minimal byte size for alg is0for the first triplet and"max_size + 1"of the preceding range.-mmemset-strategy=strategy— same energy as-mmemcpy-strategy=except it controls"__builtin_memset"expansion instead 💜-momit-leaf-frame-pointer— don't keep the frame pointer in a register for leaf functions. this dodges the instructions to save, set up, and restore frame pointers and frees up an extra register in leaf functions. the option-fomit-leaf-frame-pointerremoves the frame pointer for leaf functions too, which might make debugging harder 😭 so like, yk, be aware.-mtls-direct-seg-refs,-mno-tls-direct-seg-refs— controls whether TLS variables may be accessed with offsets from the TLS segment register (%gsfor 32-bit,%fsfor 64-bit), or whether the thread base pointer must be added. whether or not this is valid depends on the operating system, and on whether it maps the segment to cover the entire TLS area.
for systems that use the GNU C Library, the default is on.
-msse2avx,-mno-sse2avx— specify that the assembler should encode SSE instructions with a VEX prefix. the option-mavxturns this on by default ✨-mfentry,-mno-fentry— if profiling is active (-pg), put the profiling counter call <i>before</i> the prologue. note: on x86 architectures the attribute"ms_hook_prologue"isn't possible at the moment for-mfentryand-pg💀-mrecord-mcount,-mno-record-mcount— if profiling is active (-pg), generate a__mcount_locsection that contains pointers to each profiling call. this is useful for automatically patching and out calls.-mnop-mcount,-mno-nop-mcount— if profiling is active (-pg), generate the calls to the profiling functions as NOPs. this is useful when they should be patched in later dynamically. this is likely only useful together with-mrecord-mcount.-minstrument-return=type— instrument function exit in-pg-mfentryinstrumented functions with a call to the specified function. this only instruments true returns ending withret, but not sibling calls ending withjump. valid types arenoneto not instrument,callto generate a call to__return__, ornop5to generate a 5 byte nop.-mrecord-return,-mno-record-return— generate a__return_locsection pointing to all return instrumentation code 🎀-mfentry-name=name— set the name of the__fentry__symbol called at function entry for-pg-mfentryfunctions.-mfentry-section=name— set the name of the section to record-mrecord-mcountcalls (default__mcount_loc).-mskip-rax-setup,-mno-skip-rax-setup— okay so bestie, when you're generating code for x86-64 with SSE extensions turned off,-mskip-rax-setuplets you skip setting up the RAX register when there are no variable arguments passed in vector registers 💅
⚠️ Warning, and this one's real: since RAX is what stops vector registers from being saved on the stack unnecessarily when you pass variable arguments, the impacts of this option are that callees may waste some stack space, misbehave, or jump to a random location 😭 BUT — GCC 4.4 or newer doesn't have those issues, regardless of the RAX register value. So like, we're fine. Probably. 🫶
-m8bit-idiv,-mno-8bit-idiv— on some processors (hi Intel Atom ✨), 8-bit unsigned integer divide is way faster than 32-bit/64-bit integer divide. This option generates a run-time check, and if both the dividend and the divisor are within range of 0 to 255, it uses 8-bit unsigned integer divide instead of the 32-bit/64-bit one. Efficiency queen 👑-mavx256-split-unaligned-load,-mavx256-split-unaligned-store— splits 32-byte AVX unaligned load and store. That's it, that's the whole thing 🌸-mstack-protector-guard=guard,-mstack-protector-guard-reg=reg,-mstack-protector-guard-offset=offset,-mstack-protector-guard-symbol=symbol— generates stack protection code using a canary at guard. The supported locations areglobalfor a global canary, ortlsfor a per-thread canary in the TLS block (which is the default, obvs 💅). This option only does anything when-fstack-protectoror-fstack-protector-allis specified, no cap.
With that latter choice, the options -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset furthermore specify which segment register (%fs or %gs) to use as the base register for reading the canary, and from what offset from that base register. The default for those is as specified in the relevant ABI 📜
And -mstack-protector-guard-symbol=symbol overrides the offset with a symbol reference to a canary in the TLS block. Iconic ✨
-mgeneral-regs-only— generates code that uses only the general-purpose registers. This prevents the compiler from using floating-point, vector, mask and bound registers. Strict. Minimalist. Aesthetic 🎀-mrelax-cmpxchg-loop— when emitting a compare-and-swap loop for__syncBuiltins and__atomicBuiltins that are lacking a native instruction, this optimizes for the highly contended case by issuing an atomic load before theCMPXCHGinstruction, and using thePAUSEinstruction to save CPU power when restarting the loop. The way it's out here saving CPU power, we love to see it 🔥-mindirect-branch=choice— converts indirect call and jump with choice. The default iskeep, which keeps indirect call and jump unmodified.thunkconverts indirect call and jump to call and return thunk.thunk-inlineconverts indirect call and jump to inlined call and return thunk.thunk-externconverts indirect call and jump to external call and return thunk provided in a separate object file. You can control this behavior for a specific function by using the function attributeindirect_branch💫
Note that -mcmodel=large is incompatible with -mindirect-branch=thunk and -mindirect-branch=thunk-extern since the thunk function may not be reachable in the large code model. Not compatible, bestie, they are not compatible 😤
Note that -mindirect-branch=thunk-extern is compatible with -fcf-protection=branch since the external thunk can be made to enable control-flow check. Slay, that one works 🫶
-mfunction-return=choice— converts function return with choice. The default iskeep, which keeps function return unmodified.thunkconverts function return to call and return thunk.thunk-inlineconverts function return to inlined call and return thunk.thunk-externconverts function return to external call and return thunk provided in a separate object file. You can control this behavior for a specific function by using the function attributefunction_return💫
Note that -mindirect-return=thunk-extern is compatible with -fcf-protection=branch since the external thunk can be made to enable control-flow check.
Note that -mcmodel=large is incompatible with -mfunction-return=thunk and -mfunction-return=thunk-extern since the thunk function may not be reachable in the large code model. Same energy as above, no notes 💀
-mindirect-branch-register— forces indirect call and jump via register. Just, like, forced. Periodt 👑-mharden-sls=choice— generates code to mitigate against straight line speculation (SLS) with choice. The default isnone, which disables all SLS hardening.returnenables SLS hardening for function returns.indirect-jmpenables SLS hardening for indirect jumps.allenables all SLS hardening. Pick your fighter ✨-mindirect-branch-cs-prefix— adds a CS prefix to call and jmp to indirect thunk with branch target in r8-r15 registers, so that the call and jmp instruction length is 6 bytes, which allows them to be replaced withlfence; call *%r8-r15orlfence; jmp *%r8-r15at run-time. Little byte-length trickery, we love a workaround 💅-mapx-inline-asm-use-gpr32— so for inline asm support with APX, by default the EGPR feature was disabled to prevent potential illegal instruction with EGPR from occurring. To invoke egpr usage in inline asm, use the new compiler option-mapx-inline-asm-use-gpr32, and the user should ensure the instruction supports EGPR. Read that again — user should ensure — that's on you bestie 🙈-mevex512,-mno-evex512— enables/disables 512-bit vector. It will be default on if AVX512F is enabled. Big vector energy 🌈
These -m switches are supported in addition to the above on x86-64 processors in 64-bit environments. More options, more fun 🎀
-m32,-m64,-mx32,-m16,-miamcu— generates code for a 16-bit, 32-bit or 64-bit environment. Here's the breakdown, bestie, take notes 📝
The -m32 option sets int, long, and pointer types to 32 bits, and generates code that runs in 32-bit mode.
The -m64 option sets int to 32 bits and long and pointer types to 64 bits, and generates code for the x86-64 architecture. For Darwin only, the -m64 option also turns off the -fno-pic and -mdynamic-no-pic options.
The -mx32 option sets int, long, and pointer types to 32 bits, and generates code for the x86-64 architecture.
The -m16 option is the same as -m32, except for that it outputs the .code16gcc assembly directive at the beginning of the assembly output so that the binary can run in 16-bit mode.
The -miamcu option generates code which conforms to Intel MCU psABI. It requires the -m32 option to be turned on. No -m32, no -miamcu, them's the rules 💅
-mno-red-zone— do not use a so-called "red zone" for x86-64 code. The red zone is mandated by the x86-64 ABI; it is a 128-byte area beyond the location of the stack pointer that is not modified by signal or interrupt handlers, and therefore can be used for temporary data without adjusting the stack pointer. The flag-mno-red-zonedisables this red zone. 128 bytes of free real estate, and this flag says no 🚫-mcmodel=small— generates 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— generates 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. Mandatory, not optional, iykyk 🐧-mcmodel=medium— generates code for the medium model: the program is linked in the lower 2 GB of the address space. Small symbols are also placed there. Symbols with sizes larger than-mlarge-data-thresholdare put into large data or BSS sections and can be located above 2GB. Programs can be statically or dynamically linked 🌸-mcmodel=large— generates code for the large model. This model makes no assumptions about addresses and sizes of sections. Zero assumptions, zero problems, main character energy 👑-maddress-mode=long— generates code for long address mode. This is only supported for 64-bit and x32 environments. It is the default address mode for 64-bit environments ✨-maddress-mode=short— generates code for short address mode. This is only supported for 32-bit and x32 environments. It is the default address mode for 32-bit and x32 environments. Short mode stays in its lane 💅-mneeded,-mno-needed— emitsGNU_PROPERTY_X86_ISA_1_NEEDEDGNU property for Linux target to indicate the micro-architecture ISA level required to execute the binary. A little note to the future about what you need to run this 🎀-mno-direct-extern-access— without-fpicnor-fPIC, always use the GOT pointer to access external symbols. With-fpicor-fPIC, treat access to protected symbols as local symbols. The default is-mdirect-extern-access.
⚠️ Warning: shared libraries compiled with-mno-direct-extern-accessand executable compiled with-mdirect-extern-accessmay not be binary compatible if protected symbols are used in shared libraries and executable. That's a real "these two things do not vibe together" situation 😭
-munroll-only-small-loops— controls conservative small loop unrolling. It is default enabled byO2, and unrolls loop with less than 4 insns by 1 time. Explicit-f[no-]unroll-[all-]loopswould disable this flag to avoid any unintended unrolling behavior that user does not want. Respecting your boundaries 🫶-mlam=choice— LAM (linear-address masking) allows special bits in the pointer to be used for metadata. The default isnone. Withu48, pointer bits in positions 62:48 can be used for metadata; withu57, pointer bits in positions 62:57 can be used for metadata. Sneaking metadata into your pointer bits, sneaky but make it fashion 💅
🪟 x86 Windows Options
🌪️ Xstormy16 Options
These options are defined for Xstormy16:
-msim— choose startup files and linker script suitable for the simulator. Simulator girlies, this one's for you ✨
🌸 Xtensa Options
These options are supported for Xtensa targets:
-mconst16,-mno-const16— enable or disable use ofCONST16instructions for loading constant values. TheCONST16instruction is currently not a standard option from Tensilica. When enabled,CONST16instructions are always used in place of the standardL32Rinstructions. The use ofCONST16is enabled by default only if theL32Rinstruction is not available. Not standard, but we still stan 🎀-mfused-madd,-mno-fused-madd— toggles whether the compiler gets to use those fused multiply/add and multiply/subtract instructions in the floating-point option. Ngl it does literally nothing if the floating-point option isn't also turned on, so don't come for me. Turning it off forces the compiler to keep multiply and add/subtract as two separate instructions, which is highkey what you want when you need strict IEEE 754-compliant results: the fused multiply add/subtract instructions don't round the intermediate result, so they hand you back results with more bits of precision than the IEEE standard says you should get. ✨ Turning the fused ones off also means your program's output won't be sensitive to whether the compiler decided to smoosh multiply and add/subtract together, which is a whole vibe of stability. 💅-mserialize-volatile,-mno-serialize-volatile— when it's on, GCC dropsMEMWinstructions beforevolatilememory references to guarantee sequential consistency. The default is-mserialize-volatile. Want to skip theMEMWinstructions? That's-mno-serialize-volatile, bestie.-mforce-no-pic— for targets like GNU/Linux where all user-mode Xtensa code has to be position-independent code (PIC), this option disables PIC so you can compile kernel code. Specialized, but iconic when you need it. 🎀-mtext-section-literals,-mno-text-section-literals— these control how literal pools get treated. The default is-mno-text-section-literals, which puts literals in a separate section in the output file — this lets the literal pool live in a data RAM/ROM, and it also lets the linker combine literal pools from separate object files to drop redundant literals and improve code size. With-mtext-section-literals, the literals get interspersed in the text section to keep them as close as possible to their references. This might be necessary for large assembly files. Literals for each function are placed right before that function. 🫶-mauto-litpools,-mno-auto-litpools— also about literal pool treatment, okay? The default is-mno-auto-litpools, which places literals in a separate section in the output file unless-mtext-section-literalsis used. With-mauto-litpoolsthe literals get interspersed in the text section by the assembler. The compiler does not produce explicit.literaldirectives and loads literals into registers withMOVIinstructions instead ofL32R, letting the assembler do relaxation and place literals as necessary. This option lets the assembler create several literal pools per function and assemble very big functions, which may not be possible with-mtext-section-literals. Lowkey a glow-up for huge functions. 💖-mtarget-align,-mno-target-align— when enabled, GCC tells the assembler to automatically align instructions to reduce branch penalties, at the expense of some code density. The assembler tries to widen density instructions to align branch targets and the instructions following call instructions. If there aren't enough preceding safe density instructions to align a target, no widening happens. The default is-mtarget-align. These options don't affect the treatment of auto-aligned instructions likeLOOP, which the assembler always aligns, either by widening density instructions or by inserting NOP instructions. Periodt. 💅-mlongcalls,-mno-longcalls— when this is on, GCC tells the assembler to translate direct calls into indirect calls unless it can determine that the target of a direct call is in the range allowed by the call instruction. This translation typically happens for calls to functions in other source files. Specifically, the assembler translates a directCALLinstruction into anL32Rfollowed by aCALLXinstruction. The default is-mno-longcalls. Use this in programs where the call target can potentially be out of range. Heads up though: this option is implemented in the assembler, not the compiler, so the assembly code GCC generates still shows direct call instructions — look at the disassembled object code to see the actual instructions. Also note the assembler uses an indirect call for every cross-file call, not just the ones that really are out of range. 🙈-mabi=name— generate code for the specified ABI. Permissible values are:call0,windowed. The default ABI is chosen by the Xtensa core configuration.-mabi=call0— with this on, function parameters are passed in registersa2througha7, registersa12througha15are caller-saved, and registera15may be used as a frame pointer. When this version of the ABI is enabled, the C preprocessor symbol__XTENSA_CALL0_ABI__is defined. ✨-mabi=windowed— with this on, function parameters are passed in registersa10througha15, and the called function rotates the register window by 8 registers on entry so that its arguments are found in registersa2througha7. Registera7may be used as a frame pointer. The register window is rotated 8 registers back upon return. When this version of the ABI is enabled, the C preprocessor symbol__XTENSA_WINDOWED_ABI__is defined. 💜-mextra-l32r-costs=n— specify an extra cost of instruction RAM/ROM access forL32Rinstructions, in clock cycles. When optimizing for speed, this affects whether loading a constant from a literal pool usingL32Ror synthesizing the constant from a small one with a couple of arithmetic instructions is the move. The default value is0.-mstrict-align,-mno-strict-align— avoid or allow generating memory accesses that may not be aligned on a natural object boundary as described in the architecture specification. The default is-mno-strict-alignfor cores that support both unaligned loads and stores in hardware, and-mstrict-alignfor all other cores. 👑
🌈 zSeries Options 🌈
These are listed under
🌸 ENVIRONMENT 🌸
okay bestie, this section is literally all about the environment variables that change how GCC behaves 💅 some of them work by telling GCC which directories or prefixes to use when it's hunting for files, and some of them just... set the vibe for the whole compilation environment. it's giving backstage pass 🎀
ngl, you can ALSO specify places to search using options like -B, -I and -L. and here's the hierarchy, no cap:
- those options take precedence over places specified using environment variables 👑
- and environment variables in turn take precedence over those specified by the configuration of GCC ✨
🦋 LANG, LC_CTYPE, LC_MESSAGES, LC_ALL
these four are the localization girlies 🌍 they control the way GCC uses localization info, which is what lets GCC work with different national conventions. GCC inspects the locale categories LC_CTYPE and LC_MESSAGES if it has been configured to do so. these locale categories can be set to any value supported by your installation. a typical value is en_GB.UTF-8 for English in the United Kingdom encoded in UTF-8 💖
the LC_CTYPE environment variable specifies character classification. GCC uses it to determine the character boundaries in a string — this is needed for some multibyte encodings that contain quote and escape characters that are otherwise interpreted as a string end or escape 😭 imagine that
the LC_MESSAGES environment variable specifies the language to use in diagnostic messages. so like, your errors can yell at you in your own language, iconic 💅
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. and 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. it's giving relay race 🏃♀️✨
🔥 GCC_COMPARE_DEBUG
setting GCC_COMPARE_DEBUG is nearly equivalent to passing -fcompare-debug to the compiler driver. see the documentation of this option for more details, bestie 👀
💜 GCC_EXEC_PREFIX
if GCC_EXEC_PREFIX is set, it specifies a prefix to use in the names of the subprograms executed by the compiler. no slash is added when this prefix is combined with the name of a subprogram, but you can specify a prefix that ends with a slash if you wish 🎀
if GCC_EXEC_PREFIX is not set, GCC attempts to figure out an appropriate prefix to use based on the pathname it is invoked with.
if GCC cannot find the subprogram using the specified prefix, it tries looking in the usual places for the subprogram.
the default value of GCC_EXEC_PREFIX is prefix/lib/gcc/ where prefix is the prefix to the installed compiler. in many cases prefix is the value of prefix when you ran the configure script 👑
other prefixes specified with -B take precedence over this prefix.
this prefix is also used for finding files such as crt0.o that are used for linking.
in addition, the prefix is used in an unusual way in finding the directories to search for header files 🙈 for each of the standard directories whose name normally begins with /usr/local/lib/gcc (more precisely, with the value of GCC_INCLUDE_DIR), GCC tries replacing that beginning with the specified prefix to produce an alternate directory name. thus, with -Bfoo/, GCC searches foo/bar just before it searches the standard directory /usr/local/lib/bar. if a standard directory begins with the configured prefix then the value of prefix is replaced by GCC_EXEC_PREFIX when looking for header files. lowkey wild but we love her 💖
🌈 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 cannot 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 cannot 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 (yes, again, she contains multitudes)
this variable is used to pass locale information to the compiler. one way in which this information is used is to determine the character set to be used when character literals, string literals and comments are parsed in C and C++ 🎀 when the compiler is configured to allow multibyte characters, the following values for LANG are recognized:
C-JIS— Recognize JIS characters.C-SJIS— Recognize SJIS characters.C-EUCJP— Recognize EUCJP characters.
if LANG is not defined, or if it has some other value, then the compiler uses mblen and mbtowc as defined by the default locale to recognize and translate multibyte characters ✨
💖 GCC_EXTRA_DIAGNOSTIC_OUTPUT
if GCC_EXTRA_DIAGNOSTIC_OUTPUT is set to one of the following values, then additional text will be emitted to stderr when fix-it hints are emitted. -fdiagnostics-parseable-fixits and -fno-diagnostics-parseable-fixits take precedence over this environment variable 👑
fixits-v1— emit parseable fix-it hints, equivalent to-fdiagnostics-parseable-fixits. in particular, columns are expressed as a count of bytes, starting at byte 1 for the initial column.fixits-v2— asfixits-v1, but columns are expressed as display columns, as per-fdiagnostics-column-unit=display💅
some additional environment variables affect the behavior of the preprocessor, bestie 🧚
🦋 CPATH, C_INCLUDE_PATH, CPLUS_INCLUDE_PATH, OBJC_INCLUDE_PATH
each variable's value is a list of directories separated by a special character, much like PATH, in which to look for header files. the special character, PATH_SEPARATOR, is target-dependent and determined at GCC build time. for Microsoft Windows-based targets it is a semicolon, and for almost all other targets it is a colon 🌍
CPATH specifies a list of directories to be searched as if specified with -I, but after any paths given with -I options on the command line. this environment variable is used regardless of which language is being preprocessed.
the remaining environment variables apply only when preprocessing the particular language indicated. each specifies a list of directories to be searched as if specified with -isystem, but after any paths given with -isystem options on the command line.
in all these variables, an empty element instructs the compiler to search its current working directory. empty elements can appear at the beginning or end of a path. for instance, if the value of CPATH is :/special/include, that has the same effect as -I. -I/special/include ✨
💜 DEPENDENCIES_OUTPUT
if this variable is set, its value specifies how to output dependencies for Make based on the non-system header files processed by the compiler. system header files are ignored in the dependency output 🙈
the value of DEPENDENCIES_OUTPUT can be just a file name, in which case the Make rules are written to that file, guessing the target name from the source file name. or the value can have the form file target, in which case the rules are written to file file using target as the target name 🎀
in other words, this environment variable is equivalent to combining the options -MM and -MF, with an optional -MT switch too 💅
🔥 SUNPRO_DEPENDENCIES
this variable is the same as DEPENDENCIES_OUTPUT (see above), except that system header files are not ignored, so it implies -M rather than -MM. however, the dependence on the main input file is omitted 👀
⭐ SOURCE_DATE_EPOCH
if this variable is set, its value specifies a UNIX timestamp to be used in replacement of the current date and time in the __DATE__ and __TIME__ macros, so that the embedded timestamps become reproducible 🕐✨ reproducible builds girlies rise up
the value of SOURCE_DATE_EPOCH must be a UNIX timestamp, defined as the number of seconds (excluding leap seconds) since 01 Jan 1970 00:00:00 represented in ASCII; identical to the output of date +%s on GNU/Linux and other systems that support the %s extension in the date command 🎀
the value should be a known timestamp such as the last modification time of the source or package and it should be set by the build process 💖
💀 BUGS 💀
for instructions on reporting bugs, see <https://bugzilla.redhat.com/>. go off, tell them everything, they wanna hear it 🫶
🎀 FOOTNOTES 🎀
- on some systems,
gcc -sharedneeds to build supplementary stub code for constructors to work. on multi-libbed systems,gcc -sharedmust 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.-sharedsuppresses the addition of startup code to alter the floating-point environment as done with-ffast-math,-Ofastor-funsafe-math-optimizationson some targets 👑⚠️
🌈 SEE ALSO 🌈
gpl(7), gfdl(7), fsf-funding(7), cpp(1), gcov(1), as(1), ld(1), gdb(1) and the Info entries for gcc, cpp, as, ld, binutils and gdb 💖
🫶 AUTHOR 🫶
see the Info entry for gcc, or <https://gcc.gnu.org/onlinedocs/gcc/Contributors.html>, for contributors to GCC. the whole squad, fr fr 💅
✨ COPYRIGHT ✨
Copyright (c) 1988-2025 Free Software Foundation, Inc. 📜
Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with the Invariant Sections being "GNU General Public License" and "Funding Free Software", the Front-Cover texts being (a) (see below), and with the Back-Cover Texts being (b) (see below). a copy of the license is included in the gfdl(7) man page 🎀
(a) The FSF's Front-Cover Text is:
A GNU Manual(b) The FSF's Back-Cover Text is:
You have freedom to copy and modify this GNU Manual, like GNU
software. Copies published by the Free Software Foundation raise
funds for GNU development.aaaand that's a wrap on the whole entire GCC(1) page, bestie 😭💖 you read all 131 parts, which is genuinely main character energy 👑 this was gcc-15, dated 2026-07-22, and yes we went all the way to the end together. now go compile something gorgeous ✨🦋 periodt 🎀
📦 where this page comes from
- fedora 44: gcc15 15.3.1-1.fc44 · GPL-3.0-or-later AND LGPL-3.0-or-later AND (GPL-3.0-or-later WITH GCC-exception-3.1) AND (GPL-3.0-or-later WITH Texinfo-exception) AND (LGPL-2.1-or-later WITH GCC-exception-2.0) AND (GPL-2.0-or-later WITH GCC-exception-2.0) AND (GPL-2.0-or-later WITH GNU-compiler-exception) AND BSL-1.0 AND GFDL-1.3-or-later AND Linux-man-pages-copyleft-2-para AND SunPro AND BSD-1-Clause AND BSD-2-Clause AND BSD-2-Clause-Views AND BSD-3-Clause AND BSD-4-Clause AND BSD-Source-Code AND Zlib AND MIT AND Apache-2.0 AND (Apache-2.0 WITH LLVM-Exception) AND ZPL-2.1 AND curl
- 💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash on : a language model's rewrite of the original, under its terms