GCC(1)

💖 GCC(1) 💖 — gcc: GNU project C and C++ compiler 🫶

🌸 NAME 🌸

gcc — GNU project C and C++ compiler.

🦋 SYNOPSIS 🦋

gcc [-c|-S|-E] [-std=standard]
    [-g] [-pg] [-Olevel]
    [-Wwarn...] [-Wpedantic]
    [-Idir...] [-Ldir...]
    [-Dmacro[=defn]...] [-Umacro]
    [-foption...] [-mmachine-option...]
    [-o outfile] [@file] infile...

Only the most useful options are listed here; see below for the remainder. g++ accepts mostly the same options as gcc.

✨ DESCRIPTION ✨

When you invoke GCC, it normally does preprocessing, compilation, assembly and linking. The “overall options” let you stop this whole glow-up at an intermediate stage — for example, the -c option says do not run the linker. Then the output is made of object files output by the assembler. 💅

Other options get passed on to one or more stages of processing. Some options control the preprocessor, others control the compiler itself. Yet other options control the assembler and linker; most of those are not documented here, since you rarely need to use any of them. No cap.

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. If the description for a particular option does not mention a source language, you can use that option with all supported languages. 🫶

The usual way to run GCC is to run the executable called gcc, or machine-gcc when cross-compiling, or machine-gcc-version to run a specific version of GCC. When you compile C++ programs, you should invoke GCC as g++ instead. Iconic behavior, honestly.

The gcc program accepts options and file names as operands. Many options have multi-letter names; therefore multiple single-letter options may not be grouped: -dv is very different from -d -v. Bestie, do not smoosh them together. 💀

You can mix options and other arguments. For the most part, the order you use doesn’t matter. Order does matter when you use several options of the same kind; for example, if you specify -L more than once, the directories are searched in the order specified. Also, the placement of the -l option is significant. 👑

Many options have long names starting with -f or with -W---for example, -fmove-loop-invariants, -Wformat and so on. Most of these have both positive and negative forms; the negative form of -ffoo is -fno-foo. This manual documents only one of these two forms, whichever one is not the default.

Some options take one or more arguments typically separated either by a space or by the equals sign (=) from the option name. Unless documented otherwise, an argument can be either numeric or a string. Numeric arguments must typically be small unsigned decimal or hexadecimal integers. 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 gibibyte, and so on. Such arguments are designated by byte-size in the following text. Refer to the NIST, IEC, and other relevant national and international standards for the full listing and explanation of the binary and decimal byte size prefixes. 📏✨

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💅 OPTIONS 💅

🌈 Option Summary 🌈

Here is a summary of all the options, grouped by type. Explanations are in the following sections. 🗂️

🎀 Overall Options 🎀

• -c
• -S
• -E
• -o file
• -dumpbase dumpbase
• -dumpbase-ext auxdropsuf
• -dumpdir dumppfx
• -x language
• -v
• -###
• --help[=class[,...]]
• --target-help
• --version
• -pass-exit-codes
• -pipe
• -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
• --assemble
• --compile
• --dumpbase dumpbase
• --dumpbase-ext auxdropsuf
• --dumpdir dumppfx
• --language=language
• --output=file
• --pass-exit-codes
• --pipe
• --preprocess
• --verbose

💖 C Language Options 💖

• -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
• --ansi

🦋 C++ Language Options 🦋

• --compile-std-module
• -fabi-compat-version=n
• -fabi-version=n
• -fno-access-control
• -faligned-new=[n]
• -fno-assume-sane-operators-new-delete
• -fchar8_t
• -fcheck-new
• -fconcepts
• -fconcepts-diagnostics-depth=n
• -fconstexpr-depth=n
• -fconstexpr-cache-depth=n
• -fconstexpr-loop-limit=n
• -fconstexpr-ops-limit=n
• -fcontracts
• -fcontract-evaluation-semantic=[ignore|observe|enforce|quick_enforce]
• -fcontracts-conservative-ipa
• -fcontract-checks-outlined
• -fcontract-disable-optimized-checks
• -fcontracts-client-check=[none|pre|all]
• -fcontracts-definition-check=[on|off]
• -fcoroutines
• -fdiagnostics-all-candidates
• -fno-elide-constructors
• -fno-enforce-eh-specs
• -fext-numeric-literals
• -fno-gnu-keywords
• -fno-immediate-escalation
• -fno-implement-inlines
• -fimplicit-constexpr
• -fno-implicit-inline-templates
• -fno-implicit-templates
• -fmodule-header[=kind]
• -fmodule-implicit-inline
• -fno-module-lazy
• -fmodule-mapper=specification
• -fmodule-only
• -fmodules
• -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
• -freflection
• -fno-rtti
• -fsized-deallocation
• -fstrict-enums
• -fstrong-eval-order[=kind]
• -ftemplate-backtrace-limit=n
• -ftemplate-depth=n
• -fno-threadsafe-statics
• -fuse-cxa-atexit
• -fno-use-cxa-get-exception-ptr
• -fno-weak
• -nostdinc++
• -fvisibility-inlines-hidden
• -fvisibility-ms-compat
• -flang-info-include-translate[=header]
• -flang-info-include-translate-not
• -flang-info-module-cmi[=module]
• -stdlib=libstdc++,libc++
• -Wabbreviated-auto-in-template-arg
• -Wabi-tag
• -Waligned-new[=kind]
• -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
• -Wno-deprecated-literal-operator
• -Wdeprecated-variadic-comma-omission
• -Weffc++
• -Wno-elaborated-enum-base
• -Wno-exceptions
• -Wno-expose-global-module-tu-local
• -Wno-external-tu-local
• -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
• -Wno-sfinae-incomplete
• -Wstrict-null-sentinel
• -Wno-subobject-linkage
• -Wtemplates
• -Wno-non-c-typedef-for-linkage
• -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 🍓

• -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 🧚

• -foffload=arg
• -foffload-options=arg
• -fopenacc
• -fopenacc-dim=geom
• -fopenmp
• -fopenmp-simd
• -fopenmp-target-simd-clone[=device-type]

🎀 Diagnostic Message Formatting Options 🎀

• -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]
• -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
• -fno-diagnostics-show-nesting
• -fno-diagnostics-show-nesting-locations
• -fdiagnostics-show-nesting-levels
• -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]
• -fdiagnostics-show-context[=depth]

🚨 Warning Options 🚨

okay bestie, deep breath, because this is THE list — the mother of all flag dumps. it's long, it's chaotic, it's giving "my warnings have warnings", and we're getting through every single one together. no skipping, no "lol I'll just use -Wall". 💅

• -fsyntax-only — just peek at the syntax, don't actually build anything 🛑
• -fmax-errors=n — cap it at n and stop yelling
• -Wpedantic — the pedantry warning, ur fave hall monitor
• -pedantic-errors — pedantry but make it errors 💀
• -fpermissive — chill-mode diagnostics
• -w — the mute button 🔇 all warnings gone
• -Wextra — extra warnings, the deluxe edition ✨
• -Wall — the classic, the starter pack, the one everyone names
• -Wabi=n — ABI warnings, version n
• -Waddress — address warnings 📍
• -Wno-address-of-packed-member — takes the address of a packed member and minds its own business
• -Waggregate-return — returning aggregates, watch out
• -Walloc-size — allocation size vibes
• -Walloc-size-larger-than=byte-size — allocation bigger than byte-size? rude
• -Walloc-zero — allocating zero bytes 🫥
• -Walloca — alloca, the icon
• -Walloca-larger-than=byte-size — alloca over byte-size
• -Wauto-profile — auto-profile warnings
• -Wno-aggressive-loop-optimizations — the aggressive loop optimizations get a pass
• -Warith-conversion — arithmetic conversion, lowkey spicy
• -Warray-bounds — out-of-bounds array energy
• -Warray-bounds=n — same, but level n
• -Warray-compare — comparing arrays, bestie why
• -Warray-parameter — array parameters, here we go
• -Warray-parameter=n — array params, level n
• -Wno-attributes — attributes on vacation 🙈
• -Wattribute-alias=n — attribute alias, level n
• -Wno-attribute-alias — attribute alias warnings off
• -Wno-attribute-warning — the attribute-warning warning, silenced
• -Wbidi-chars=[none|unpaired|any|ucn] — bidi characters, and yes it takes none, unpaired, any or ucn 🪞
• -Wbool-compare — comparing bools, iconic
• -Wbool-operation — bool operations
• -Wno-builtin-declaration-mismatch — builtin declaration mismatch, off
• -Wno-builtin-macro-redefined — redefining builtin macros, live ur life
• -Wc90-c99-compat — c90 vs c99 compat, the crossover episode
• -Wc99-c11-compat — c99 vs c11 compat
• -Wc11-c23-compat — c11 vs c23 compat
• -Wc23-c2y-compat — c23 vs c2y compat ✨
• -Wc++-compat — c vs c++ compat
• -Wc++11-compat — c++11 compat
• -Wc++14-compat — c++14 compat
• -Wc++17-compat — c++17 compat
• -Wc++20-compat — c++20 compat
• -Wc++26-compat — c++26 compat, future-facing queen 👑
• -Wno-c++11-extensions — c++11 extensions, off
• -Wno-c++14-extensions — c++14 extensions, off
• -Wno-c++17-extensions — c++17 extensions, off
• -Wno-c++20-extensions — c++20 extensions, off
• -Wno-c++23-extensions — c++23 extensions, off
• -Wcalloc-transposed-args — calloc args got flipped, classic 😭
• -Wcannot-profile — can't profile, sorry bestie
• -Wcast-align — casts and alignment, the situationship
• -Wcast-align=strict — same but STRICT 😤
• -Wcast-function-type — casting function types
• -Wcast-qual — casting away qualifiers, bold of you
• -Wchar-subscripts — char subscripts 🍓
• -Wclobbered — clobbered, the most dramatic word on this page
• -Wcomment — opinions about ur comments 💬
• -Wcompare-distinct-pointer-types — comparing different pointer types
• -Wno-complain-wrong-lang — no complaining about the wrong lang, be nice
• -Wconversion — implicit conversions, lowkey everywhere
• -Wno-coverage-mismatch — coverage mismatch, ignored
• -Wno-cpp — cpp warnings, off
• -Wdangling-else — the dangling else, spooky 🎀
• -Wdangling-pointer — dangling pointer, the horror movie
• -Wdangling-pointer=n — dangling pointer at level n
• -Wdate-time — __DATE__ / __TIME__ vibes 📅
• -Wno-deprecated — deprecated stuff, whatever
• -Wno-deprecated-declarations — deprecated declarations, whatever
• -Wno-designated-init — designated init warnings off
• -Wno-deprecated-openmp — deprecated OpenMP, not today
• -Wdisabled-optimization — disabled optimization, noted
• -Wno-discarded-array-qualifiers — discarded array qualifiers, gone
• -Wno-discarded-qualifiers — discarded qualifiers, gone
• -Wno-div-by-zero — divide by zero, keep it quiet 🙈
• -Wdouble-promotion — double promotion, main character upgrade ✨
• -Wduplicated-branches — branches that are copy-paste twins 👯
• -Wduplicated-cond — duplicated conditions, again??
• -Wempty-body — empty body, the void
• -Wno-endif-labels — #endif labels, quiet
• -Wenum-compare — comparing enums
• -Wenum-conversion — enum conversion
• -Wenum-int-mismatch — enum/int mismatch, messy
• -Werror — warnings become errors, the boss level 💀
• -Werror=* — make these specific warnings errors, a la carte
• -Wexpansion-to-defined — macro expansion into defined, yikes
• -Wfatal-errors — first error and we're DONE
• -Wflex-array-member-not-at-end — flex array member not at the end, tsk
• -Wfloat-conversion — float conversions 🌊
• -Wfloat-equal — comparing floats for equality, don't do it bestie
• -Wformat — format strings, the whole genre
• -Wformat=2 — format, deluxe tier ✨
• -Wno-format-contains-nul — format containing NUL, hush
• -Wno-format-diag — format diagnostics, hush
• -Wno-format-extra-args — extra format args, hush
• -Wformat-nonliteral — non-literal format strings
• -Wformat-overflow=n — format overflow, level n
• -Wformat-security — format security, no cap this one matters 🔥
• -Wformat-signedness — format signedness
• -Wformat-truncation=n — format truncation, level n
• -Wformat-y2k — the Y2K warning, unironically still here 🎀
• -Wframe-address — frame address
• -Wframe-larger-than=byte-size — frame bigger than byte-size, pack lighter
• -Wno-free-nonheap-object — freeing a non-heap object, we're not looking
• -Wheader-guard — header guards, protect urself 🛡️
• -Wno-if-not-aligned — if not aligned, quiet
• -Wno-ignored-attributes — ignored attributes, ignored
• -Wignored-qualifiers — ignored qualifiers
• -Wno-incompatible-pointer-types — incompatible pointer types, off
• -Whardened — hardened, tough girl energy 🖤
• -Wimplicit — implicit stuff, the umbrella
• -Wimplicit-fallthrough — switch fallthrough, the classic
• -Wimplicit-fallthrough=n — fallthrough, level n
• -Wno-implicit-function-declaration — implicit function declaration, off
• -Wno-implicit-int — implicit int, off
• -Winfinite-recursion — infinite recursion, going in circles 🔁
• -Winit-self — initializing a variable with itself, bestie 😭
• -Winline — inline, it wants to be inline
• -Wno-int-conversion — int conversion, off
• -Wint-in-bool-context — int where a bool should be
• -Wno-int-to-pointer-cast — int-to-pointer casts, off
• -Wno-invalid-memory-model — invalid memory model, off
• -Winvalid-pch — invalid PCH
• -Winvalid-utf8 — invalid UTF-8, encoding drama 🦋
• -Wno-unicode — unicode warnings, off
• -Wjump-misses-init — a jump that skips init
• -Wkeyword-macro — keyword used as a macro name, spicy
• -Wlarger-than=byte-size — bigger than byte-size
• -Wleading-whitespace=kind — leading whitespace of kind 👀
• -Wlogical-not-parentheses — ! and parentheses, the saga
• -Wlogical-op — logical operators
• -Wlong-long — long long, say it with me
• -Wno-lto-type-mismatch — LTO type mismatch, off
• -Wmain — main itself, the main character 🎬
• -Wmaybe-uninitialized — maybe uninitialized, the ambiguity
• -Wmemset-elt-size — memset element size
• -Wmemset-transposed-args — memset args got swapped 😭
• -Wmisleading-indentation — misleading indentation, lies in whitespace
• -Wmissing-attributes — missing attributes
• -Wmissing-braces — missing braces { }
• -Wmissing-field-initializers — missing field initializers
• -Wmissing-format-attribute — missing format attribute
• -Wmissing-include-dirs — missing include dirs 📁
• -Wmissing-noreturn — missing noreturn
• -Wmusttail-local-addr — musttail and a local address
• -Wmaybe-musttail-local-addr — maybe musttail and a local address
• -Wno-missing-profile — missing profile, no worries
• -Wno-multichar — multichar, off
• -Wmultistatement-macros — multi-statement macros, the classic trap
• -Wnonnull — nonnull arguments
• -Wnonnull-compare — comparing against nonnull
• -Wnormalized=[none|id|nfc|nfkc] — normalization, one of none, id, nfc, nfkc 🧚
• -Wnull-dereference — null dereference, the big one 💀
• -Wno-odr — ODR, off
• -Wopenacc-parallelism — OpenACC parallelism
• -Wopenmp — OpenMP
• -Wopenmp-simd — OpenMP SIMD
• -Wno-overflow — overflow, off
• -Woverlength-strings — overly long strings, wordy queen
• -Wno-override-init-side-effects — override-init side effects, off
• -Wpacked — packed, snug
• -Wno-packed-bitfield-compat — packed bitfield compat, off
• -Wpacked-not-aligned — packed but not aligned
• -Wpadded — padded structs, the padding discourse
• -Wparentheses — parentheses, use them bestie
• -Wno-pedantic-ms-format — pedantic MS format, off
• -Wpointer-arith — pointer arithmetic 🧮
• -Wno-pointer-compare — pointer compare, off
• -Wno-pointer-to-int-cast — pointer-to-int casts, off
• -Wno-pragmas — pragmas, off
• -Wno-pragma-once-outside-header — #pragma once outside a header, off
• -Wno-prio-ctor-dtor — priority ctor/dtor, off
• -Wno-psabi — psABI, off
• -Wredundant-decls — redundant declarations, say it once 🗣️
• -Wrestrict — restrict, the promise you break
• -Wno-return-local-addr — returning a local address, we're ignoring it 🙈
• -Wreturn-type — return type mismatch, a mood
• -Wno-scalar-storage-order — scalar storage order, off
• -Wsequence-point — sequence points, the UB zone
• -Wshadow — shadowing, the name-stealing warning
• -Wshadow=global — shadowing, global scope
• -Wshadow=local — shadowing, local scope
• -Wshadow=compatible-local — shadowing, compatible-local scope
• -Wno-shadow-ivar — shadowed ivar, off
• -Wno-shift-count-negative — negative shift count, off
• -Wno-shift-count-overflow — shift count overflow, off
• -Wshift-negative-value — shifting a negative value
• -Wno-shift-overflow — shift overflow, off
• -Wshift-overflow=n — shift overflow, level n
• -Wsign-compare — signed/unsigned compare, the eternal battle ⚔️
• -Wsign-conversion — sign conversion
• -Wno-sizeof-array-argument — sizeof on an array argument, off
• -Wsizeof-array-div — sizeof array division
• -Wsizeof-pointer-div — sizeof pointer division, sneaky
• -Wsizeof-pointer-memaccess — sizeof pointer in memaccess
• -Wstack-protector — stack protector
• -Wstack-usage=byte-size — stack usage over byte-size
• -Wstrict-aliasing — strict aliasing, the rules
• -Wstrict-aliasing=n — strict aliasing, level n
• -Wstrict-overflow — strict overflow
• -Wstrict-overflow=n — strict overflow, level n
• -Wstring-compare — string compare
• -Wno-stringop-overflow — stringop overflow, off
• -Wno-stringop-overread — stringop overread, off
• -Wno-stringop-truncation — stringop truncation, off
• -Wstrict-flex-arrays — strict flex arrays 🌸
• -Wsuggest-attribute=attribute-name — suggests the attribute called attribute-name
• -Wswitch — switch, the whole bit
• -Wno-switch-bool — switch on bool, off
• -Wswitch-default — missing default: in a switch
• -Wswitch-enum — switch over an enum
• -Wno-switch-outside-range — switch case outside range, off
• -Wno-switch-unreachable — unreachable switch, off
• -Wsync-nand — __sync_nand, the legacy king
• -Wsystem-headers — system headers get warnings too 😤
• -Wtautological-compare — comparing something to itself, embarrassing
• -Wtrailing-whitespace — trailing whitespace, the invisible crime 🌬️
• -Wtrailing-whitespace=kind — trailing whitespace of kind
• -Wtrampolines — trampolines, bouncy
• -Wtrigraphs — trigraphs, ancient lore 📜
• -Wtrivial-auto-var-init — trivial auto var init
• -Wno-tsan — TSan, off
• -Wtype-limits — type limits
• -Wundef — #if on an undefined macro
• -Wuninitialized — uninitialized variables, the classic 😭
• -Wunknown-pragmas — unknown pragmas, who are you
• -Wunsuffixed-float-constants — float constants with no suffix
• -Wunterminated-string-initialization — unterminated string init, nope
• -Wunused — unused things, the umbrella of neglect
• -Wunused-but-set-parameter — parameters set but never used
• -Wunused-but-set-parameter=n — same, level n
• -Wunused-but-set-variable — variables set but never used
• -Wunused-but-set-variable=n — same, level n
• -Wunused-const-variable — unused const variables
• -Wunused-const-variable=n — same, level n
• -Wunused-function — unused functions, ghosted 👻
• -Wunused-label — unused labels
• -Wunused-local-typedefs — unused local typedefs
• -Wunused-macros — unused macros
• -Wunused-parameter — unused parameters, the most personal attack
• -Wno-unused-result — unused result, off
• -Wunused-value — unused values
• -Wunused-variable — unused variables
• -Wuse-after-free — use after free, the horror sequel 💀
• -Wuse-after-free=n — use after free, level n
• -Wuseless-cast — a cast that does nothing, literally pointless
• -Wno-varargs — varargs, off
• -Wvariadic-macros — variadic macros
• -Wvector-operation-performance — vector operation performance 📈
• -Wvla — variable length arrays
• -Wvla-larger-than=byte-size — VLA bigger than byte-size
• -Wno-vla-larger-than — VLA size limit, off
• -Wvolatile-register-var — volatile register variable, chaotic combo
• -Wwrite-strings — write strings ✍️
• -Wno-xor-used-as-pow — ^ used as a power operator, off
• -Wzero-as-null-pointer-constant — 0 as a null pointer constant, we've all done it
• -Wzero-length-bounds — zero-length bounds
• --all-warnings — all the warnings, the long-form king
• --extra-warnings — extra warnings, long-form
• --no-warnings — none of them, long-form silence
• --pedantic — pedantic, long-form
• --pedantic-errors — pedantic errors, long-form

---

🔍 Static Analyzer Options 🔍

so the static analyzer is like the friend who reads ur texts before you send them and finds, um, everything. here's its whole toolkit 🧚

• -fanalyzer — turn the analyzer on, the whole point
• -fanalyzer-assume-nothrow — assume nothrow
• -fanalyzer-call-summaries — call summaries
• -fanalyzer-checker=name — pick the checker called name
• -fno-analyzer-feasibility — no feasibility
• -fanalyzer-show-events-in-system-headers — show events in system headers too
• -fno-analyzer-state-merge — no state merge
• -fno-analyzer-state-purge — no state purge
• -fno-analyzer-suppress-followups — no suppress-followups
• -fanalyzer-transitivity — transitivity
• -fno-analyzer-undo-inlining — no undo-inlining
• -fanalyzer-verbose-edges — verbose edges
• -fanalyzer-verbose-state-changes — verbose state changes
• -fanalyzer-verbosity=level — verbosity at level 📢
• -fdump-analyzer — dump the analyzer
• -fdump-analyzer-callgraph — dump the callgraph
• -fdump-analyzer-exploded-graph — dump the exploded graph 💥
• -fdump-analyzer-exploded-nodes — dump the exploded nodes
• -fdump-analyzer-exploded-nodes-2 — exploded nodes, the sequel
• -fdump-analyzer-exploded-nodes-3 — exploded nodes, the threequel
• -fdump-analyzer-exploded-paths — dump the exploded paths
• -fdump-analyzer-feasibility — dump feasibility
• -fdump-analyzer-infinite-loop — dump the infinite loop ♾️
• -fdump-analyzer-json — dump it as JSON
• -fdump-analyzer-state-purge — dump the state purge
• -fdump-analyzer-stderr — dump to stderr
• -fdump-analyzer-supergraph — dump the supergraph
• -fdump-analyzer-untracked — dump the untracked stuff
• -Wno-analyzer-double-fclose — double fclose, off
• -Wno-analyzer-double-free — double free, off
• -Wno-analyzer-exposure-through-output-file — exposure through output file, off
• -Wno-analyzer-exposure-through-uninit-copy — exposure through uninit copy, off
• -Wno-analyzer-fd-access-mode-mismatch — fd access mode mismatch, off
• -Wno-analyzer-fd-double-close — fd double close, off
• -Wno-analyzer-fd-leak — fd leak, off
• -Wno-analyzer-fd-phase-mismatch — fd phase mismatch, off
• -Wno-analyzer-fd-type-mismatch — fd type mismatch, off
• -Wno-analyzer-fd-use-after-close — fd use after close, off
• -Wno-analyzer-fd-use-without-check — fd use without check, off
• -Wno-analyzer-file-leak — file leak, off
• -Wno-analyzer-free-of-non-heap — free of non-heap, off
• -Wno-analyzer-imprecise-fp-arithmetic — imprecise floating-point arithmetic, off
• -Wno-analyzer-infinite-loop — infinite loop, off
• -Wno-analyzer-infinite-recursion — infinite recursion, off
• -Wno-analyzer-jump-through-null — jump through null, off
• -Wno-analyzer-malloc-leak — malloc leak, off
• -Wno-analyzer-mismatching-deallocation — mismatching deallocation, off
• -Wno-analyzer-null-argument — null argument, off
• -Wno-analyzer-null-dereference — null dereference, off
• -Wno-analyzer-out-of-bounds — out of bounds, off
• -Wno-analyzer-overlapping-buffers — overlapping buffers, off
• -Wno-analyzer-possible-null-argument — possible null argument, off
• -Wno-analyzer-possible-null-dereference — possible null dereference, off
• -Wno-analyzer-putenv-of-auto-var — putenv of an auto var, off
• -Wno-analyzer-shift-count-negative — negative shift count, off
• -Wno-analyzer-shift-count-overflow — shift count overflow, off
• -Wno-analyzer-stale-setjmp-buffer — stale setjmp buffer, off
• -Wno-analyzer-tainted-allocation-size — tainted allocation size, off
• -Wno-analyzer-tainted-assertion — tainted assertion, off
• -Wno-analyzer-tainted-array-index — tainted array index, off
• -Wno-analyzer-tainted-divisor — tainted divisor, off
• -Wno-analyzer-tainted-offset — tainted offset, off
• -Wno-analyzer-tainted-size — tainted size, off
• -Wno-analyzer-throw-of-unexpected-type — throwing an unexpected type, off
• -Wanalyzer-symbol-too-complex — symbol too complex for the analyzer
• -Wanalyzer-too-complex — the whole thing's too complex 😵‍💫
• -Wno-analyzer-undefined-behavior-ptrdiff — undefined behaviour: ptrdiff, off
• -Wno-analyzer-undefined-behavior-strtok — undefined behaviour: strtok, off
• -Wno-analyzer-unsafe-call-within-signal-handler — unsafe call inside a signal handler, off
• -Wno-analyzer-use-after-free — use after free, off
• -Wno-analyzer-use-of-pointer-in-stale-stack-frame — pointer in a stale stack frame, off
• -Wno-analyzer-use-of-uninitialized-value — use of an uninitialized value, off
• -Wno-analyzer-va-arg-type-mismatch — va_arg type mismatch, off
• -Wno-analyzer-va-list-exhausted — va_list exhausted, off
• -Wno-analyzer-va-list-leak — va_list leak, off
• -Wno-analyzer-va-list-use-after-va-end — va_list use after va_end, off
• -Wno-analyzer-write-to-const — write to const, off
• -Wno-analyzer-write-to-string-literal — write to a string literal, off

---

🦋 C and Objective-C-only Warning Options 🦋

these ones are exclusively for the C girlies (and the Objective-C girlies, mwah) 💜

• -Wbad-function-cast — bad function cast
• -Wdeprecated-non-prototype — deprecated non-prototype declarations
• -Wfree-labels — free labels
• -Wmissing-declarations — missing declarations
• -Wmissing-parameter-name — missing parameter name
• -Wmissing-parameter-type — missing parameter type
• -Wdeclaration-missing-parameter-type — declaration missing a parameter type
• -Wmissing-prototypes — missing prototypes
• -Wmissing-variable-declarations — missing variable declarations
• -Wmultiple-parameter-fwd-decl-lists — multiple parameter forward declaration lists
• -Wnested-externs — nested externs
• -Wold-style-declaration — old-style declaration, vintage 🎀
• -Wold-style-definition — old-style definition, also vintage
• -Wstrict-prototypes — strict prototypes, strict queen
• -Wtraditional — traditional C
• -Wtraditional-conversion — traditional conversion
• -Wdeclaration-after-statement — declaration after statement
• -Wpointer-sign — pointer sign

---

🐛 Debugging Options 🐛

debugging is literally just being nosy about ur own program, and honestly? iconic. here's the whole lore 💅

• -g — debug info, the one everybody knows
• -glevel — debug info at level
• -gdwarf — DWARF debug info
• -gdwarf-version — DWARF at version
• -gbtf — BTF debug info
• -gctf — CTF debug info
• -gctflevel — CTF at level
• -gno-prune-btf — don't prune BTF
• -ggdb — the GDB-flavoured -g 🐞
• -gno-record-gcc-switches — don't record the GCC switches
• -gstrict-dwarf — strict DWARF
• -gas-loc-support — assembler location support
• -gas-locview-support — assembler locview support
• -gcodeview — CodeView debug info
• -gcolumn-info — column info
• -gdwarf32 — DWARF, 32-bit flavour
• -gdwarf64 — DWARF, 64-bit flavour
• -gno-statement-frontiers — no statement frontiers
• -gno-variable-location-views — no variable location views
• -gvariable-location-views=incompat5 — variable location views, incompat5
• -ginternal-reset-location-views — internal reset location views
• -ginline-points — inline points
• -gvms — VMS debug info
• -gz[=type] — compress the debug sections, optional type
• -gsplit-dwarf — split DWARF
• -gdescribe-dies — describe DIEs
• -fdebug-prefix-map=old=new — map prefix old to new in debug info
• -fdebug-types-section — debug types section
• -fno-eliminate-unused-debug-types — keep unused debug types
• -femit-struct-debug-baseonly — emit struct debug info, base only
• -femit-struct-debug-reduced — emit struct debug info, reduced
• -femit-struct-debug-detailed[=spec-list] — emit struct debug info, detailed, with an optional spec-list
• -fno-eliminate-unused-debug-symbols — keep unused debug symbols
• -femit-class-debug-always — always emit class debug info
• -fno-merge-debug-strings — don't merge debug strings
• -fno-dwarf2-cfi-asm — no DWARF 2 CFI asm
• -fvar-tracking — variable tracking
• -fvar-tracking-assignments — variable tracking assignments
• -fvar-tracking-uninit — variable tracking, uninit edition
• --debug — debug, the long-form one ✨

⚡ OPTIMIZATION OPTIONS ⚡

Okay bestie, this is the section where the compiler gets to choose violence (in a good way) and make your code go zoom. There are SO many knobs to turn here. Buckle up. 💅

• -faggressive-loop-optimizations — the compiler goes full detective mode on your loops and squeezes out every last drop of performance. aggressive? yes. iconic? also yes. 🔥
• -falign-functions[=n[:m:[n2[:m2]]]] — lines up functions to nice memory boundaries. you can pass n, m, n2, m2 to control exactly how it stacks up. alignment queen. 📏
• -falign-jumps[=n[:m:[n2[:m2]]]] — same energy as above but for jump targets. keeping your branches tidy. ✨
• -falign-labels[=n[:m:[n2[:m2]]]] — alignment for labels. yes, labels get the glow-up too. 🎀
• -falign-loops[=n[:m:[n2[:m2]]]] — loops aligned to boundaries so they hit different. literally. 🔄
• -fmin-function-alignment=[n] — sets the minimum alignment for functions to n. no cap. 📐
• -fno-allocation-dce — turns OFF dead-code elimination for allocations. sometimes you just need everything to stay put. 💀
• -fallow-store-data-races — lets the compiler introduce store data races. yes, really. it's giving "yolo but make it optimization." 🙈
• -fassociative-math — lets the compiler reassociate math operations. lowkey chaotic but highkey useful. ➗
• -fauto-profile — uses auto-profiling info. the compiler reads your vibes from a previous run. ✨
• -fauto-profile[=path] — same but you tell it where the profile lives at path. you're the navigator. 🗺️
• -fauto-profile-inlining — auto-profile data guides inlining decisions. chef's kiss. 👩‍🍳
• -fauto-inc-dec — auto-increment and decrement for addressing modes. classic. 🎯
• -fbranch-probabilities — uses profiling to guess which branches are hot. main character branches get priority. 🌟
• -fcaller-saves — saves registers around calls. polite and considerate. 🫶
• -fcombine-stack-adjustments — merges stack pointer tweaks. efficiency slay. 🧠
• -fconserve-stack — tries to use less stack. sustainability queen. 🌱
• -ffold-mem-offsets — folds memory offsets into other instructions. two birds one stone energy. 🪨
• -fcompare-elim — eliminates redundant comparisons. not me eliminating comparison. 😭
• -fcprop-registers — copies registers around to keep things smooth. 💅
• -fcrossjumping — jumps across basic blocks to merge code. parkour but make it compiler. 🤸
• -fcse-follow-jumps — common subexpression elimination that follows jumps. dedicated to the cause. 🔍
• -fcse-skip-blocks — CSE that skips over blocks. selective memory, iconic. 💭
• -fcx-fortran-rules — complex arithmetic following Fortran rules. for the Fortran besties. 🧮
• -fcx-limited-range — limited range for complex arithmetic. keeps it contained. 📦
• -fcx-method — chooses the method for complex arithmetic. you pick. 🎨
• -fdata-sections — puts each data item in its own section. organized like a Virgo. 🗂️
• -fdce — dead code elimination. bye bye unused code. 👋
• -fdelayed-branch — fills delay slots with useful work. no wasted cycles. ⏳
• -fdelete-null-pointer-checks — removes checks for null pointers when it's safe. spicy but standard. 🌶️
• -fdep-fusion — fuses dependent instructions. chemistry. 🧪
• -fdevirtualize — devirtualizes calls when possible. breaking the virtual chains. 🔓
• -fdevirtualize-speculatively — does it speculatively. living on the edge. 🎢
• -fdevirtualize-at-ltrans — devirtualizes during link-time optimization. late but worth the wait. ⏰
• -fdse — dead store elimination. removing writes nobody reads. 🗑️
• -fearly-inlining — inlines early in the pipeline. eager beaver. 🐿️
• -fexcess-precision=style — controls excess precision handling with style. precision is key. 🔑
• -fexpensive-optimizations — enables optimizations that cost compile time. you pay for quality. 💸
• -fext-dce — extended dead code elimination. more dead code, more gone. 💀
• -ffast-math — enables fast-math optimizations. not for everyone but it's a vibe. ⚡
• -ffat-lto-objects — creates fat LTO objects. chunky in a good way. 🍔
• -ffinite-loops — assumes loops eventually end. optimistic queen. 🌈
• -ffinite-math-only — assumes math results are finite. no infinity, no NaN. 🚫
• -ffloat-store — stores floats to memory instead of registers. old school cool. 💾
• -fforward-propagate — forward propagates values. passing the vibes along. 📨
• -ffp-contract=style — controls floating-point contraction with style. precise. 🎯
• -ffp-int-builtin-inexact — allows inexact results from builtin math. sometimes close enough is close enough. 🤷
• -ffunction-sections — each function gets its own section. independent icon behavior. 🌟
• -ffuse-ops-with-volatile-access — fuses ops even with volatile access. spicy optimization. 🌶️
• -fgcse — global common subexpression elimination. the classic. 🏆
• -fgcse-after-reload — runs GCSE after reload. better late than never. ⌛
• -fgcse-las — GCSE with load-after-store elimination. thorough. 🔬
• -fgcse-lm — GCSE with load-motion. moving loads around for efficiency. 🚚
• -fgraphite-identity — Graphite identity transformation. polyhedral vibes. 💎
• -fgcse-sm — GCSE with store motion. stores get relocated. 📦
• -fhoist-adjacent-loads — hoists loads from adjacent memory. lifting them up. 🏋️
• -fif-conversion — converts branches to conditional moves. no branch, no problem. 🎭
• -fif-conversion2 — does it again but differently. the sequel. 🎬
• -findirect-inlining — inlines indirect calls. bold move. 🎲
• -finline-atomics — inlines atomic operations. atomic slay. ⚛️
• -finline-functions — inlines functions. the classic move. 🔄
• -finline-functions-called-once — inlines functions called only once. why not. 🎯
• -finline-limit=n — sets the inlining limit to n. draw the line. 🚧
• -finline-small-functions — inlines small functions. cute and efficient. 🐣
• -finline-stringops[=fn] — inlines string operations, optionally only fn. string theory but practical. 🧵
• -fipa-modref — interprocedural modification reference analysis. fancy. 🔬
• -fipa-cp — interprocedural constant propagation. constants travel. 🚀
• -fipa-cp-clone — clones functions for constant propagation. duplicates that serve. 👯
• -fipa-bit-cp — bit-level constant propagation. getting into the bits. 🔢
• -fipa-vrp — value range propagation. knowing your limits. 📊
• -fipa-pta — points-to analysis. following the pointers. 👉
• -fipa-profile — profile-guided interprocedural opts. data-driven decisions. 📈
• -fipa-pure-const — identifies pure and const functions. clean vibes only. 🧼
• -fipa-reference — interprocedural reference tracking. keeping tabs. 👀
• -fipa-reference-addressable — addressable reference tracking. more detail. 🔍
• -fipa-reorder-for-locality — reorders for cache locality. snuggling data together. 🤗
• -fipa-sra — scalar replacement of aggregates. breaking big things into small things. 🧩
• -fipa-stack-alignment — aligns stacks interprocedurally. tidy stacks. 📚
• -fipa-icf — identical code folding. merging twins. 👯‍♀️
• -fipa-icf-functions — ICF for functions. function twins merged. 🎭
• -fipa-icf-variables — ICF for variables. variable twins merged. 🪞
• -fira-algorithm=algorithm — chooses register allocation algorithm. you decide the algorithm. 🎨
• -flate-combine-instructions — combines instructions late in the game. clutch. ⏰
• -flifetime-dse — dead store elimination based on lifetimes. knowing when it's over. 💔
• -flive-patching=level — controls live patching at level. hot-swap energy. 🔄
• -fira-region=region — sets the IRA region to region. region queen. 🗺️
• -fira-hoist-pressure — hoists to reduce register pressure. lifting the weight. 🏋️‍♀️
• -fira-loop-pressure — reduces register pressure in loops. loop-friendly. 🔁
• -fno-ira-share-save-slots — don't share save slots in IRA. separate is safer. 🚫
• -fno-ira-share-spill-slots — don't share spill slots. keep your spills private. 🙅‍♀️
• -fisolate-erroneous-paths-dereference — isolates paths that dereference bad pointers. safety first. 🛡️
• -fisolate-erroneous-paths-attribute — isolates paths based on attributes. attribute-based isolation. 🏷️
• -fivopts — induction variable optimizations. IVs get the glow-up. 📈
• -fkeep-inline-functions — keeps inline functions even if unused. hoarder energy but valid. 📦
• -fkeep-static-functions — keeps static functions. don't delete my babies. 😤
• -fkeep-static-consts — keeps static constants. const appreciation post. 🫶
• -flimit-function-alignment — limits function alignment. everything in moderation. ⚖️
• -flive-range-shrinkage — shrinks live ranges. compact and cute. 🤏
• -floop-block — loop blocking optimization. blocks within blocks. 🧱
• -floop-interchange — interchanges loop nesting. swapping the order. 🔀
• -floop-strip-mine — strip-mines loops. mining for performance. ⛏️
• -floop-unroll-and-jam — unrolls and jams loops together. jam session. 🎸
• -floop-nest-optimize — optimizes loop nests. nest bestie. 🪺
• -floop-parallelize-all — parallelizes loops when possible. all the threads. 🧵
• -flra-remat — rematerializes values in LRA. bringing it back. 🔄
• -flto — link-time optimization. big brain energy across files. 🧠
• -flto-compression-level=n — sets LTO compression to n. compress the vibes. 🗜️
• -flto-toplevel-asm-heuristics — uses heuristics for top-level asm in LTO. asm queen. 📝
• -flto-partition=alg — sets LTO partition algorithm to alg. partitioning with style. 🍰
• -flto-incremental=path — incremental LTO with cache at path. progressive queen. 📂
• -flto-incremental-cache-size=n — sets incremental LTO cache size to n. storage realness. 💾
• -fmalloc-dce — dead code elimination for malloc. bye bye unused allocations. 👋
• -fmerge-all-constants — merges all constants. all for one, one for all. 🤝
• -fmerge-constants — merges similar constants. efficiency. 🔗
• -fmodulo-sched — modulo scheduling. scheduling with math. 🧮
• -fmodulo-sched-allow-regmoves — allows register moves in modulo scheduling. moves allowed. 🏃
• -fmove-loop-invariants — moves loop-invariant code out of loops. getting it out the way. 🚪
• -fmove-loop-stores — moves loop stores. stores get relocated. 📦
• -fno-branch-count-reg — disables branch count register optimization. no counting. 🚫
• -fno-defer-pop — no deferring pop operations. pop immediately. 🫧
• -fno-function-cse — disables function CSE. no common subexpressions here. 🚫
• -fno-guess-branch-probability — doesn't guess branch probabilities. no guessing games. 🤷
• -fno-inline — no inlining at all. keep functions separate. 🚷
• -fno-math-errno — doesn't set errno after math calls. no error reporting. 🚫
• -fno-peephole — no peephole optimization. no tiny tweaks. 🚫
• -fno-peephole2 — no peephole2. the sequel is also off. 🚫
• -fno-printf-return-value — doesn't optimize printf return values. leave it alone. 🚫
• -fno-sched-interblock — no interblock scheduling. no block-to-block scheduling. 🚫
• -fno-sched-spec — no speculative scheduling. no speculation here. 🚫
• -fno-signed-zeros — doesn't preserve signed zeros. zeros are zeros. 0️⃣
• -fno-toplevel-reorder — doesn't reorder top-level declarations. order is preserved. 📑
• -fno-trapping-math — assumes no trapping math. no traps. 🚫
• -fno-zero-initialized-in-bss — doesn't put zero-initialized in BSS. bss gets no zeros. 🚫
• -fomit-frame-pointer — omits the frame pointer. more registers for you. 🎁
• -foptimize-crc — optimizes CRC computations. cyclic checksum slay. 🔄
• -foptimize-sibling-calls — optimizes sibling calls into jumps. gay sibling energy. 🏳️‍🌈
• -fpartial-inlining — partial inlining. some parts inline, some don't. partial credit. 📝
• -fpeel-loops — peels loops. loop peeling is real. 🍌
• -fpredictive-commoning — predictive commoning. predicting common patterns. 🔮
• -fprefetch-loop-arrays — prefetches loop arrays. getting ahead. 🏃‍♀️
• -fprofile-correction — corrects profile data. making it accurate. ✅
• -fprofile-use — uses profile data. profile-guided. 📊
• -fprofile-use=path — uses profile data at path. you give the path. 🗺️
• -fprofile-partial-training — partial training for profiles. some training is enough. 🏋️
• -fprofile-values — profile values. tracking values. 💎
• -fprofile-reorder-functions — reorders functions based on profile. hot functions first. 🔥
• -freciprocal-math — uses reciprocal math. reciprocal vibes. 🔄
• -free — frees up registers. freeing the registers. 🕊️
• -frename-registers — renames registers. fresh names. 📛
• -freorder-blocks — reorders basic blocks. tidying up. 🧹
• -freorder-blocks-algorithm=algorithm — reorders blocks with algorithm. algorithmic reordering. 🧮
• -freorder-blocks-and-partition — reorders and partitions blocks. separate but organized. 🗂️
• -freorder-functions — reorders functions. function shuffle. 🔀
• -frerun-cse-after-loop — reruns CSE after loops. second pass. 🔁
• -freschedule-modulo-scheduled-loops — reschedules modulo-scheduled loops. reschedule queen. 📅
• -frounding-math — respects rounding math. rounding is real. 🔢
• -fsave-optimization-record — saves optimization records. receipts kept. 🧾
• -fsched2-use-superblocks — uses superblocks in sched2. superblocks slay. 🧱
• -fsched-pressure — reduces register pressure during scheduling. pressure down. 📉
• -fsched-spec-load — speculative load scheduling. loading speculatively. 🎲
• -fsched-spec-load-dangerous — dangerous speculative loads. living dangerously. ⚠️
• -fsched-stalled-insns-dep[=n] — stalled instructions dependency with optional n. stalled but tracked. 🚦
• -fsched-stalled-insns[=n] — stalled instructions with optional n. stalled vibes. 🛑
• -fsched-group-heuristic — group heuristic for scheduling. group think. 👥
• -fsched-critical-path-heuristic — critical path heuristic. critical path queen. 🛤️
• -fsched-spec-insn-heuristic — speculative instruction heuristic. speculative heuristics. 🎲
• -fsched-rank-heuristic — rank heuristic. ranking the heuristics. 🏆
• -fsched-last-insn-heuristic — last instruction heuristic. last but not least. 🥇
• -fsched-dep-count-heuristic — dependency count heuristic. counting dependencies. 🔢
• -fschedule-fusion — fuses scheduling. fusion energy. ⚡
• -fschedule-insns — schedules instructions. schedule slay. 📅
• -fschedule-insns2 — schedules instructions again. second scheduling. 🔁
• -fsection-anchors — uses section anchors. anchored sections. ⚓
• -fselective-scheduling — selective scheduling. selective queen. 🎯
• -fselective-scheduling2 — selective scheduling again. selective sequel. 🎬
• -fsel-sched-pipelining — pipelines selective scheduling. pipeline realness. 🚰
• -fsel-sched-pipelining-outer-loops — pipelines outer loops. outer loop pipeline. 🔄
• -fsemantic-interposition — respects semantic interposition. interposition is real. 🎭
• -fshrink-wrap — shrink-wraps code. wrap it up. 🎁
• -fshrink-wrap-separate — shrink-wraps separately. separate wrapping. 🎀
• -fsignaling-nans — respects signaling NaNs. NaNs that signal. 🚩
• -fsingle-precision-constant — treats floating constants as single precision. single precision vibes. 🔢
• -fsplit-ivs-in-unroller — splits induction variables in unroller. splitting IVs. 🪓
• -fsplit-loops — splits loops. loop split. ✂️
• -fspeculatively-call-stored-functions — speculatively calls stored functions. speculative call. 🎲
• -fsplit-paths — splits paths. path split. 🛤️
• -fsplit-wide-types — splits wide types. wide types get split. 🪓
• -fsplit-wide-types-early — splits wide types early. early split. ⏰
• -fssa-backprop — SSA backpropagation. propagating back. 🔙
• -fssa-phiopt — optimizes SSA phi nodes. phi slay. φ
• -fstdarg-opt — optimizes stdarg. stdarg gets the glow-up. 🎁
• -fstore-merging — merges stores. store merging. 🏪
• -fstrict-aliasing — strict aliasing rules. aliasing with rules. 📏
• -fipa-strict-aliasing — interprocedural strict aliasing. strict across functions. 📏
• -fthread-jumps — threads jumps together. jump threading. 🧵
• -ftracer — traces code paths. tracer realness. 🔍
• -ftree-bit-ccp — bit-level CCP on trees. bit CCP. 🔢
• -ftree-builtin-call-dce — dead code elimination for builtin calls. builtin DCE. 💀
• -ftree-ccp — conditional constant propagation on trees. tree CCP. 🌳
• -ftree-ch — code hoisting on trees. hoisting on trees. 🏋️
• -ftree-coalesce-vars — coalesces variables. coalesce queen. 🤝
• -ftree-copy-prop — copy propagation on trees. copy prop. 📋
• -ftree-cselim — conditional store elimination. cselim. 🎭
• -ftree-dce — dead code elimination on trees. tree DCE. 💀
• -ftree-dominator-opts — dominator optimizations. dominator opts. 👑
• -ftree-dse — dead store elimination on trees. tree DSE. 🗑️
• -ftree-forwprop — forward propagation on trees. forward prop. 📨
• -ftree-fre — full redundancy elimination on trees. FRE on trees. 🌳
• -fcode-hoisting — hoists code. code hoisting. 🏋️
• -ftree-loop-if-convert — if-conversion for loop trees. loop if-convert. 🎭
• -ftree-loop-im — loop invariant motion on trees. loop IM. 🚚
• -ftree-phiprop — phi propagation. phi prop. φ
• -ftree-loop-distribution — loop distribution on trees. distribution queen. 📊
• -ftree-loop-distribute-patterns — distributes loop patterns. pattern distribution. 🔷
• -ftree-loop-ivcanon — canonicalizes induction variables. IV canon. 📜
• -ftree-loop-linear — linear loop transforms. linear loops. 📏
• -ftree-loop-optimize — optimizes loop trees. loop tree opt. 🌳
• -ftree-loop-vectorize — vectorizes loop trees. vectorization slay. 🎨
• -ftree-parallelize-loops[=n] — parallelizes loops with optional n. parallel loops. 🧵
• -ftree-pre — partial redundancy elimination on trees. PRE on trees. 🌳
• -ftree-partial-pre — partial PRE on trees. partial PRE. 🌳
• -ftree-pta — points-to analysis on trees. tree PTA. 👉
• -ftree-reassoc — reassociation on trees. tree reassoc. ➗
• -ftree-scev-cprop — SCEV copy propagation. scev cprop. 🔁
• -ftree-sink — sinking on trees. sink realness. ⬇️
• -ftree-slsr — straight-line strength reduction. SLSR. 💪
• -ftree-sra — scalar replacement of aggregates on trees. tree SRA. 🧩
• -ftree-switch-conversion — switch conversion on trees. switch conv. 🔀
• -ftree-tail-merge — tail merging on trees. tail merge. 🐒
• -ftree-ter — temporary expression replacement. TER. 🔄
• -ftree-vectorize — vectorizes trees. tree vectorize. 🎨
• -ftree-vrp — value range propagation on trees. tree VRP. 📊
• -ftrivial-auto-var-init — initializes trivial auto variables. auto-init. 🎁
• -funconstrained-commons — unconstrained commons. commons without constraints. 🗽
• -funit-at-a-time — processes one unit at a time. one at a time. ⏳
• -funroll-all-loops — unrolls all loops. all loops unrolled. 🌀
• -funroll-loops — unrolls loops. loop unroll. 🌀
• -funsafe-math-optimizations — unsafe math optimizations. spicy math. 🌶️
• -funswitch-loops — unswitches loops. unswitch queen. 🔀
• -fipa-ra — interprocedural register allocation. RA across functions. 🧮
• -fvariable-expansion-in-unroller — expands variables in unroller. variable expansion. 📈
• -fvect-cost-model — vectorization cost model. cost model. 💰
• -fvpt — value profiling transformation. VPT. 🔄
• -fweb — web construction. webs. 🕸️
• -fwhole-program — whole program optimization. the whole thing. 🌍
• -fwpa — whole program analysis. WPA. 🔍
• -fuse-linker-plugin — uses linker plugin. plugin vibes. 🔌
• -fzero-call-used-regs — zeroes call-used registers. zeroing registers. 0️⃣
• -O — optimization level equivalent to -O1. the OG. 🅾️
• -O0 — no optimization. debug mode bestie. 🐛
• -O1 — basic optimization. chill. 😌
• -O2 — more optimization. standard slay. ✨
• -O3 — maximum optimization. go feral. 🔥
• -Os — optimize for size. smol but mighty. 🐜
• -Ofast — fast optimization, ignores strict standards. speed above all. ⚡
• -Og — optimize for debugging. debug-friendly opts. 🐞
• -Oz — optimize for size aggressively. aggressively smol. 🐜
• --optimize — same as -O1. yes, really. 💅

---

🧪 PROGRAM INSTRUMENTATION OPTIONS 🧪

This is where you spy on your program while it runs. instrumentation besties, this one's for you. 🕵️‍♀️

• -p — profile instrumentation. old-school profiling. 📊
• -pg — profile for gprof. gprof-ready. 📈
• -fprofile-arcs — instruments arcs for profiling. arc instrumentation. 🌉
• -coverage — instruments for coverage. coverage queen. 📋
• -ftest-coverage — test coverage instrumentation. test coverage. ✅
• -fcondition-coverage — condition coverage instrumentation. condition coverage. 🔀
• -fpath-coverage — path coverage instrumentation. path coverage. 🛤️
• -fprofile — profile instrumentation. profile realness. 📊
• -fprofile-abs-path — absolute paths in profile. abs path. 📂
• -fprofile-dir=path — profile directory at path. profile dir. 📁
• -fprofile-generate — generates profile data. profile generation. 🧬
• -fprofile-generate=path — generates profile data at path. path-based. 🗺️
• -fprofile-info-section — profile info section. section info. 📑
• -fprofile-info-section=name — profile info section named name. named section. 🏷️
• -fprofile-note=path — profile note at path. noted. 📝
• -fprofile-prefix-path=path — prefix path for profiles. prefix vibes. 🔤
• -fprofile-update=method — profile update method. update method. 🔄
• -fprofile-filter-files=regex — filters profile files with regex. regex filter. 🔍
• -fprofile-exclude-files=regex — excludes files with regex. excludes. 🚫
• -fprofile-reproducible=[multithreaded|parallel-runs|serial] — reproducible profiles. reproducible queen. 🔁
• -fsanitize=style — sanitizer with style. sanitize realness. 🧼
• -fsanitize-recover — recoverable sanitizer. recovery mode. 🩹
• -fsanitize-recover=style — recoverable sanitizer with style. recover with style. 🎨
• -fsanitize-trap — sanitizer traps. trap queen. 🪤
• -fsanitize-trap=style — sanitizer traps with style. trap with style. 🎨
• -fasan-shadow-offset=number — ASan shadow offset. shadow offset. 🌑
• -fsanitize-sections=s1,s2,... — sanitizer sections. section list. 📋
• -fsanitize-undefined-trap-on-error — traps on undefined behavior. trap on UB. 🪤
• -fcf-protection — control flow protection. CF protection. 🛡️
• -fcf-protection=[full|branch|return|none|check] — CF protection mode. you pick. 🎛️
• -fharden-compares — hardens comparisons. hardened compares. 🛡️
• -fharden-conditional-branches — hardens conditional branches. hardened branches. 🌿
• -fhardened — enables hardening. hardened queen. 🛡️
• -fharden-control-flow-redundancy — hardened CF redundancy. redundancy hardening. 🔁
• -fhardcfr-skip-leaf — skips leaf functions in hardening. skip leaf. 🍃
• -fhardcfr-check-exceptions — checks exceptions in hardening. exception check. 🚨
• -fhardcfr-check-returning-calls — checks returning calls. return check. 🔄
• -fhardcfr-check-noreturn-calls=[always|no-xthrow|nothrow|never] — noreturn call checks. you choose. 🎛️
• -fstack-protector — stack protector. protect the stack. 🛡️
• -fstack-protector-all — stack protector for all. all protected. 🛡️
• -fstack-protector-strong — strong stack protector. strong protection. 💪
• -fstack-protector-explicit — explicit stack protector. explicit protect. 📢
• -fstack-check — stack checking. check the stack. ✅
• -fstack-limit-register=reg — stack limit register. reg limit. 📏
• -fstack-limit-symbol=sym — stack limit symbol. sym limit. 🔣
• -fno-stack-limit — no stack limit. unlimited stack. ♾️
• -fsplit-stack — split stack. split realness. 🪓
• -fstrub=disable — disable strub. disabled. 🚫
• -fstrub=strict — strict strub. strict mode. 📏
• -fstrub=relaxed — relaxed strub. relaxed. 😌
• -fstrub=all — all strub. all in. 🎯
• -fstrub=at-calls — strub at calls. call strub. 📞
• -fstrub=internal — internal strub. internal vibes. 🔒
• -fvtable-verify=[std|preinit|none] — vtable verification. verify the vtables. ✅
• -fvtv-counts — vtable verify counts. counts. 🔢
• -fvtv-debug — vtable verify debug. debug vibes. 🐛
• -finstrument-functions — instruments functions. instrument realness. 🎻
• -finstrument-functions-once — instruments functions once. once is enough. 1️⃣
• -finstrument-functions-exclude-function-list=sym,sym,... — excludes functions from instrumentation. exclude list. 🚫
• -finstrument-functions-exclude-file-list=file,file,... — excludes files from instrumentation. file exclude. 🚫
• -fprofile-prefix-map=old=new — maps profile prefixes. old to new. 🔄
• -fpatchable-function-entry=N[,M] — patchable function entry. entry slay. 🚪
• --coverage — coverage instrumentation. coverage queen. 📋
• --profile — profiling instrumentation. profile realness. 📊

---

🌸 PREPROCESSOR OPTIONS 🌸

The preprocessor is where all the macro magic happens. this section is lowkey underrated. 💅

• -C — keeps comments in preprocessor output. comments stay. 💬
• -CC — keeps comments including in macros. comment queen. 💬
• -Dmacro[=defn] — defines macro as defn. define it. 🏷️
• -dD — dumps macros to stdout. macro dump. 🗑️
• -dI — dumps include directives. include dump. 📂
• -dM — dumps macros. macro dump. 🗑️
• -dN — dumps macros with names only. name dump. 📛
• -dU — dumps macros undefined. undefined dump. 🚫
• -fdebug-cpp — debug preprocessor. debug vibes. 🐛
• -fdirectives-only — preprocess directives only. directives only. 📋
• -fdollars-in-identifiers — allows dollars in identifiers. dollar signs. 💲
• -fexec-charset=charset — execution charset. charset. 🔤
• -fextended-identifiers — extended identifiers. extended. 🆔
• -finput-charset=charset — input charset. input charset. 🔤
• -fmacro-prefix-map=old=new — maps macro prefixes. old to new. 🔄
• -fmax-include-depth=depth — max include depth. depth limit. 📏
• -fno-canonical-system-headers — no canonical system headers. non-canonical. 🚫
• -fpch-deps — PCH dependencies. PCH deps. 📋
• -fpch-preprocess — PCH preprocessing. PCH preprocess. 🧼
• -fpreprocessed — preprocessed input. preprocessed. ✅
• -ftabstop=width — tab stop width. tab width. 📏
• -ftrack-macro-expansion — tracks macro expansion. macro tracking. 🔍
• -fwide-exec-charset=charset — wide execution charset. wide charset. 🔤
• -fworking-directory — working directory in output. workdir. 📂
• -H — prints include hierarchy. hierarchy print. 🌳
• -imacros file — includes macros from file. imacros. 📂
• -include file — includes file. include it. 📂
• -M — outputs dependencies. dep output. 📋
• -MD — outputs dependencies to .d file. MD. 📋
• -MF — dependency file. MF. 📋
• -MG — treats missing headers as generated. MG. 🧬
• -MM — outputs user dependencies. MM. 📋
• -MMD — user dependencies to .d. MMD. 📋
• -MP — adds phony targets for dependencies. phony targets. 🎭
• -MQ — dependency target quoting. MQ. 📋
• -MT — dependency target. MT. 🎯
• -Mno-modules — no modules. modules off. 🚫
• -no-integrated-cpp — no integrated preprocessor. separate cpp. 🚫
• -P — no line markers in output. no markers. 🚫
• -pthread — pthread support. threads. 🧵
• -remap — remaps file names. remap queen. 🔄
• -traditional — traditional preprocessing. old school. 🕰️
• -traditional-cpp — traditional cpp. old cpp. 🕰️
• -trigraphs — enables trigraphs. trigraph realness. 🔺
• -Umacro — undefines macro. undefine it. 🚫
• -undef — undefines all macros. undefine all. 🚫
• -Wp,option — passes option to preprocessor. Wp. 📨
• -Xpreprocessor option — passes option to preprocessor. Xpreprocessor. 📨
• --comments — keeps comments. comments. 💬
• --comments-in-macros — keeps comments in macros. macro comments. 💬
• --define-macro=macro[=defn] — defines macro as defn. define it. 🏷️
• --dependencies — outputs dependencies. deps. 📋
• --dump=letters — dumps with letters. dump. 🗑️
• --imacros=file — includes macros from file. imacros. 📂
• --include=file — includes file. include. 📂
• --no-integrated-cpp — no integrated cpp. separate. 🚫
• --no-line-commands — no line commands. no lines. 🚫
• --print-missing-file-dependencies — prints missing deps. missing deps. 📋
• --traditional — traditional mode. old school. 🕰️
• --traditional-cpp — traditional cpp. old cpp. 🕰️
• --trigraphs — trigraphs. trigraph realness. 🔺
• --trace-includes — traces includes. trace realness. 🔍
• --undefine-macro=macro — undefines macro. undefine. 🚫
• --user-dependencies — user dependencies. user deps. 📋
• --write-dependencies — writes dependencies. write deps. ✍️
• --write-user-dependencies — writes user dependencies. write user deps. ✍️

---

🔧 ASSEMBLER OPTIONS 🔧

Assembler besties, this is your moment. pass options straight through. 💅

• -Wa,option — passes option to assembler. Wa. 📨
• -Xassembler option — passes option to assembler. Xassembler. 📨
• --for-assembler=option — for assembler with option. for assembler. 📨

---

🔗 LINKER OPTIONS 🔗

Linker section! this is where everything comes together. literally. 🔗

• object-file-name — object file name. the object. 📦
• -flink-libatomic — links libatomic. atomic link. ⚛️
• -fuse-ld=linker — uses linker. linker choice. 🔗
• -llibrary — links library. library link. 📚
• -nostartfiles — no start files. no start. 🚫
• -nodefaultlibs — no default libs. no default. 🚫
• -nolibc — no libc. no libc. 🚫
• -nostdlib — no standard library. no stdlib. 🚫
• -nostdlib++ — no standard C++ library. no stdlib++. 🚫
• -e entry — entry point. entry. 🚪
• -pie — position-independent executable. PIE. 🥧
• -pthread — pthread support. threads. 🧵
• -r — relocatable link. relocatable. 🔄
• -rdynamic — dynamic symbol export. rdynamic. 📤
• -s — strip symbols. stripped. ✂️
• -static — static link. static. 🗿
• -static-pie — static PIE. static PIE. 🥧
• -static-libgcc — static libgcc. static gcc. 🗿
• -static-libstdc++ — static libstdc++. static stdc++. 🗿
• -static-libasan — static libasan. static asan. 🗿
• -static-libhwasan — static libhwasan. static hwasan. 🗿
• -static-liblsan — static liblsan. static lsan. 🗿
• -static-libtsan — static libtsan. static tsan. 🗿
• -static-libubsan — static libubsan. static ubsan. 🗿
• -shared — shared library. shared. 🤝
• -shared-libgcc — shared libgcc. shared gcc. 🤝
• -symbolic — symbolic link. symbolic. 🔣
• -T script — linker script. script. 📜
• -Wl,option — passes option to linker. Wl. 📨
• -Xlinker option — passes option to linker. Xlinker. 📨
• -u symbol — undefines symbol. undefine. 🚫
• -Tbss=addr — BSS address. bss addr. 📍
• -Tdata=addr — data address. data addr. 📍
• -Ttext=addr — text address. text addr. 📍
• -N — no page alignment. N. 🚫
• -n — no page alignment. n. 🚫
• -t — traces linker input. trace. 🔍
• -Z — zero page. Z. 0️⃣
• -z keyword — linker keyword. z keyword. 🔣
• --entry=entry — entry point. entry. 🚪
• --for-linker=option — for linker with option. for linker. 📨
• --force-link=symbol — forces link of symbol. force link. 💪
• --no-standard-library — no standard library. no stdlib. 🚫
• --pie — PIE. pie. 🥧
• --static — static. static. 🗿
• --static-pie — static PIE. static pie. 🥧
• --symbolic — symbolic. symbolic. 🔣

---

📁 DIRECTORY OPTIONS 📁

Where do the files live? this section tells you. directories slay. 📂

• -Bprefix — adds prefix to search path. B prefix. 🔤
• -Idir — adds dir to include path. I dir. 📂
• -I- — splits include path. I dash. ➖
• -idirafter dir — adds dir after system dirs. after dir. 📂
• -imacros file — includes macros from file. imacros. 📂
• -imultilib dir — multilib dir. multilib. 📂
• -imultiarch dir — multiarch dir. multiarch. 📂
• -iplugindir=dir — plugin directory. plugin dir. 🔌
• -iprefix file — prefix file. prefix. 🔤
• -iquote dir — quote include dir. quote dir. 💬
• -isysroot dir — system root. sysroot. 🌳
• -isystem dir — system include dir. system dir. 📂
• -iwithprefix dir — with prefix dir. with prefix. 🔤
• -iwithprefixbefore dir — with prefix before dir. before prefix. 🔤
• -Ldir — adds dir to library path. L dir. 📚
• -no-canonical-prefixes — no canonical prefixes. no canonical. 🚫
• --no-sysroot-suffix — no sysroot suffix. no suffix. 🚫
• -nostdinc — no standard includes. no stdinc. 🚫
• -nostdinc++ — no standard C++ includes. no stdinc++. 🚫
• --embed-dir=dir — embed directory. embed dir. 📂
• --embed-directory=dir — embed directory. embed dir. 📂
• --include-barrier — include barrier. barrier. 🚧
• --include-directory=dir — include directory. include dir. 📂
• --include-directory-after=dir — include directory after. after dir. 📂
• --include-prefix=prefix — include prefix. prefix. 🔤
• --include-with-prefix=prefix — include with prefix. with prefix. 🔤
• --include-with-prefix-after=prefix — include with prefix after. after prefix. 🔤
• --include-with-prefix-before=prefix — include with prefix before. before prefix. 🔤
• --no-canonical-prefixes — no canonical prefixes. no canonical. 🚫
• --no-standard-includes — no standard includes. no stdinc. 🚫
• --prefix=prefix — prefix. prefix. 🔤
• --sysroot=dir — system root. sysroot. 🌳

---

🎨 CODE GENERATION OPTIONS 🎨

Last section bestie! this is where the compiler decides how to actually generate your code. final glow-up. ✨

• -fcall-saved-reg — call-saved register. saved reg. 💾
• -fcall-used-reg — call-used register. used reg. 🔄
• -ffixed-reg — fixed register. fixed reg. 📌
• -fexceptions — enables exceptions. exceptions. 🚨
• -fnon-call-exceptions — non-call exceptions. non-call. 🚫
• -fdelete-dead-exceptions — deletes dead exceptions. dead exceptions. 💀
• -funwind-tables — unwind tables. unwind. 🌪️
• -fasynchronous-unwind-tables — async unwind tables. async unwind. ⚡
• -fno-gnu-unique — no GNU unique. no unique. 🚫
• -finhibit-size-directive — inhibits size directive. no size. 🚫
• -fcommon — common symbols. common. 🤝
• -fno-ident — no ident. no ident. 🚫
• -fpcc-struct-return — PCC struct return. PCC. 📦
• -fpic — position-independent code. PIC. 🖼️
• -fPIC — position-independent code (large). PIC. 🖼️
• -fpie — position-independent executable. PIE. 🥧
• -fPIE — position-independent executable (large). PIE. 🥧
• -fno-plt — no PLT. no PLT. 🚫
• -fno-jump-tables — no jump tables. no jump tables. 🚫
• -fno-bit-tests — no bit tests. no bit tests. 🚫
• -frecord-gcc-switches — records GCC switches. record switches. 🎙️
• -freg-struct-return — register struct return. reg return. 🔄
• -fshort-enums — short enums. short enums. 🤏
• -fshort-wchar — short wchar. short wchar. 🤏
• -fverbose-asm — verbose assembly. verbose asm. 📝
• -fpack-struct[=n] — packs structs. packed. 📦
• -fleading-underscore — leading underscore. underscore. ➖
• -ftls-model=model — TLS model. TLS model. 🧵
• -fstack-reuse=reuse_level — stack reuse level. reuse. ♻️
• -ftrampolines — trampolines. trampolines. 🤸
• -ftrampoline-impl=[stack|heap] — trampoline implementation. stack or heap. 🏗️
• -ftrapv — traps on overflow. trap overflow. 🪤
• -fwrapv — wraps on overflow. wrap overflow. 🔄
• -fwrapv-pointer — wraps pointer overflow. wrap pointer. 🔄
• -fvisibility=[default|internal|hidden|protected] — visibility. visibility. 👁️
• -fstrict-volatile-bitfields — strict volatile bitfields. strict volatile. 📏
• -fsync-libcalls — sync libcalls. sync libcalls. 🔄
• -fzero-init-padding-bits=value — zero-init padding bits. zero init. 0️⃣
• -Qy — identifies compiler version. Qy. 🆔
• -Qn — no compiler version. Qn. 🚫

---

And that's this part bestie! more coming, stay tuned. 💖

Developer Options

Okay bestie, welcome to the dev zone 💅 — this is the corner of the man page where the compiler team keeps all their little diagnostic toys. The original page doesn't spell out a one-liner for each of these (it just drops the whole roster on you, iconically unbothered), so here's the full list, every single one, no cap:

-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-internal-locations -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-autofdo-gcov-version -print-file-name=library -print-libgcc-file-name -print-multi-directory -print-multi-lib -print-multi-os-directory -print-multiarch -print-prog-name=program -print-search-dirs -Q -print-sysroot -print-sysroot-headers-suffix -save-temps -save-temps=cwd -save-temps=obj -specs=file -time[=file] --dump=letters --print-autofdo-gcov-version --print-file-name=library --print-libgcc-file-file-name --print-multi-directory --print-multi-lib --print-multi-os-directory --print-multiarch --print-prog-name=program --print-search-dirs --print-sysroot --print-sysroot-headers-suffix --save-temps --specs=file --param name=value

🫶 Real talk: don't casually yeet these into a production build, bestie. They're developer-facing knobs, and this page gives you the names, not a vibe-check on what each one does. Copy them exactly as written above — yes, even the goofy-looking --print-libgcc-file-file-name, that's literally how it's spelled here. 🙈

---

✨ Target-Specific Options ✨

So like — every CPU architecture has opinions, and GCC has a whole flag family for each one's opinions. Buckle up, this is the long part. 🌈

🦋 AArch64 Options (AArch64 Options)

-mabi=name -mbig-endian -mlittle-endian -menable-sysreg-checking -mgeneral-regs-only -mcmodel=tiny -mcmodel=small -mcmodel=large -mstrict-align -momit-leaf-frame-pointer -mtls-dialect=desc -mtls-dialect=traditional -mtls-size=size -mtp=name -mfix-cortex-a53-835769 -mfix-cortex-a53-843419 -mlow-precision-recip-sqrt -mlow-precision-sqrt -mlow-precision-div -mmax-vectorization -mautovec-preference=name -mpc-relative-literal-loads -msign-return-address=scope -mbranch-protection=features -mharden-sls=opts -march=name -mcpu=name -mtune=name -moverride=string -mstack-protector-guard=guard -mstack-protector-guard-reg=sysreg -mstack-protector-guard-offset=offset -mtrack-speculation -moutline-atomics -mearly-ra -mearly-ldp-fusion -mlate-ldp-fusion -mnarrow-gp-writes -msve-vector-bits=bits

🌸 Adapteva Epiphany Options (Adapteva Epiphany Options)

-mhalf-reg-file -mprefer-short-insn-regs -mbranch-cost=num -mcmove -mnops=num -msoft-cmpsf -msplit-lohi -mpost-inc -mpost-modify -mstack-offset=num -mround-nearest -mlong-calls -mshort-calls -msmall16 -mfp-mode=mode -mmay-round-for-trunc -mfp-iarith -mvect-double -max-vect-align=num -msplit-vecmove-early -m1reg-reg

🔥 AMD GCN Options (AMD GCN Options)

-march=gpu -mtune=gpu -mgang-private-size=bytes -msram-ecc=[on|off|any] -mxnack=[on|off|any] -Wopenacc-dims

👑 ARC Options (ARC Options)

-mbarrel-shifter -mjli-always -mcpu=cpu -mA6 -mARC600 -mA7 -mARC700 -mdpfp -mdpfp-compact -mdpfp-fast -mno-dpfp-lrsr -mea -mmul32x16 -mmul64 -matomic -mnorm -mspfp -mspfp-compact -mspfp-fast -msimd -msoft-float -mswap -mlock -mswape -mxy -misize -marclinux -marclinux_prof -mlong-calls -mmedium-calls -msdata -mirq-ctrl-saved -mrgf-banked-regs -mlpc-width=width -G num -mvolatile-cache -mtp-regno=regno -mbitops -mcmem -munaligned-access -mauto-modify-reg -mno-brcc -mcase-vector-pcrel -mno-cond-exec -mearly-cbranchsi -mindexed-loads -mlra-priority-none -mlra-priority-compact -mlra-priority-noncompact -mmillicode -msize-level=level -mtune=cpu -mmultcost=num -mcode-density-frame -mmpy-option=multo -mdiv-rem -mcode-density -mll64 -mfpu=fpu -mrf16 -mbranch-index

🎀 ARM Options (ARM Options)

-mapcs-frame -mapcs -mabi=name -mgeneral-regs-only -mno-sched-prolog -mlittle-endian -mbig-endian -mbe8 -mbe32 -mfloat-abi=name -mfp16-format=name -mthumb-interwork -mcpu=name -march=name -mfpu=name -mtune=name -mstructure-size-boundary=n -mabort-on-noreturn -mlong-calls -msingle-pic-base -mpic-register=reg -mpic-data-is-text-relative -mnop-fun-dllimport -mpoke-function-name -mthumb -marm -mtpcs-frame -mtpcs-leaf-frame -mcaller-super-interworking -mcallee-super-interworking -mtp=name -mtls-dialect=dialect -mword-relocations -mfix-cortex-m3-ldrd -mfix-cortex-a57-aes-1742098 -mfix-cortex-a72-aes-1655431 -munaligned-access -mslow-flash-data -masm-syntax-unified -mrestrict-it -mpure-code -mcmse -mfix-cmse-cve-2021-35465 -mstack-protector-guard=guard -mstack-protector-guard-offset=offset -mfdpic -mbranch-protection=features -mvectorize-with-neon-quad -mvectorize-with-neon-double

💜 AVR Options (AVR Options)

-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 -mpmem-wrap-around -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)

-mcpu=cpu[-sirevision] -msim -momit-leaf-frame-pointer -mspecld-anomaly -mcsync-anomaly -mlow-64k -mstack-check-l1 -mid-shared-library -mleaf-id-shared-library -mshared-library-id=n -msep-data -mlong-calls -mfast-fp -minline-plt -mmulticore -mcorea -mcoreb -msdram -micplb

🧚 C6X Options (C6X Options)

-mbig-endian -mlittle-endian -march=cpu -msim -msdata=sdata-type -mdsbt -mlong-calls

⭐ CRIS Options (CRIS Options)

-mcpu=cpu -march=cpu -mtune=cpu -mmax-stackframe=n -metrax4 -metrax100 -mpdebug -mcc-init -mno-side-effects -mstack-align -mdata-align -mconst-align -m32-bit -m16-bit -m8-bit -mno-prologue-epilogue -mbest-lib-options -moverride-best-lib-options -mtrap-using-break8 -mtrap-unaligned-atomic -munaligned-atomic-may-use-library -sim -sim2 -mmul-bug-workaround

💖 C-SKY Options (C-SKY Options)

-march=arch -mcpu=cpu -mbig-endian -mlittle-endian -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 -msched-prolog -msim

💀 Cygwin and MinGW Options (Cygwin and MinGW Options)

-mconsole -mcrtdll=library -mdll -mnop-fun-dllimport -mthreads -municode -mwin32 -mwindows -fno-set-stack-executable -fwritable-relocated-rdata -mpe-aligned-commons -muse-libstdc-wrappers

🦋 Darwin Options (Darwin Options)

-all_load -allowable_client -arch name -arch_errors_fatal -asm_macosx_version_min=version -bind_at_load -bundle -bundle_loader -client_name -compatibility_version -current_version -dead_strip -dependency-file -dylib_file -dylinker -dylinker_install_name -dynamic -dynamiclib -exported_symbols_list -fapple-kext -fconstant-cfstrings -ffix-and-continue -filelist -findirect-data -flat_namespace -force_cpusubtype_ALL -force_flat_namespace -framework name -gfull -gused -headerpad_max_install_names -iframework -image_base -init symbol-name -install_name -keep_private_externs -matt-stubs -mconstant-cfstrings -mdynamic-no-pic -mfix-and-continue -mkernel -mmacosx-version-min=version -mone-byte-bool -msymbol-stubs -mtarget-linker[=]version -nodefaultexport -nodefaultrpaths -pagezero_size -preload -read_only_relocs -sectalign -sectcreate -seg_addr_table -seg1addr -segaddr -segprot -segs_read_only_addr -segs_read_write_addr -sub_library -sub_umbrella -twolevel_namespace -twolevel_namespace_hints -umbrella -undefined -unexported_symbols_list -weak_framework name -weak_reference_mismatches -whatsloaded -whyload -F -ObjC -ObjC++ -Wnonportable-cfstrings

🌈 DEC Alpha Options (DEC Alpha Options)

-mno-fp-regs -msoft-float -mieee -mieee-with-inexact -mieee-conformant -mfp-trap-mode=mode -mfp-rounding-mode=mode -mtrap-precision=mode -mbuild-constants -mcpu=cpu-type -mtune=cpu-type -mbwx -mmax -mfix -mcix -msafe-bwa -msafe-partial -mfloat-vax -mfloat-ieee -mexplicit-relocs -msmall-data -mlarge-data -msmall-text -mlarge-text -mmemory-latency=time -mtls-kernel -mtls-size=bitsize -mlong-double-128 -mlong-double-64

🔥 eBPF Options (eBPF Options)

-mbig-endian -mlittle-endian -mframe-limit=bytes -mxbpf -mco-re -mjmpext -mjmp32 -malu32 -mv3-atomics -mbswap -msdiv -msmov -mcpu=version -masm=dialect -minline-memops-threshold=bytes -Wco-re

🌸 FR30 Options (FR30 Options)

-msmall-model -mno-lsim

💅 FRV Options (FRV Options)

-mgpr-32 -mgpr-64 -mfpr-32 -mfpr-64 -mhard-float -msoft-float -malloc-cc -mfixed-cc -mdword -mdouble -mmedia -mmuladd -mfdpic -minline-plt -mgprel-ro -multilib-library-pic -mlinked-fp -mlong-calls -malign-labels -mlibrary-pic -macc-4 -macc-8 -mpack -mno-eflags -mno-cond-move -mno-optimize-membar -mno-scc -mno-cond-exec -mno-vliw-branch -mno-multi-cond-exec -mno-nested-cond-exec -mtomcat-stats -mTLS -mtls -mcpu=cpu

🎀 FT32 Options (FT32 Options)

-msim -mnodiv -mft32b -mcompress -mnopm

🐧 GNU/Linux Options (GNU/Linux Options)

-mglibc -muclibc -mmusl -mbionic -mandroid -tno-android-cc -tno-android-ld

🧚 H8/300 Options (H8/300 Options)

-mrelax -mh -ms -mn -msx -ms2600 -mquickcall -mslowbyte -mexr -mint32 -malign-300

✨ HPPA Options (HPPA Options) ✨

The HPPA lineup, bestie — exact flags only, no improv:

• -march=architecture-type
• -mno-atomic-libcalls
• -mcaller-copies
• -mdisable-fpregs
• -mdisable-indexing
• -mordered
• -mfast-indirect-calls
• -mgas
• -mgnu-ld
• -mhp-ld
• -mfixed-range=register-range
• -mcoherent-ldcw
• -mlinker-opt
• -mlong-calls
• -mlong-load-store
• -mno-space-regs
• -msoft-float
• -mpa-risc-1-0
• -mpa-risc-1-1
• -mpa-risc-2-0
• -mportable-runtime
• -mschedule=cpu-type
• -msoft-mult
• -msio
• -mwsio
• -munix=unix-std
• -nolibdld
• -static
• -threads

---

🦋 IA-64 Options (IA-64 Options) 🦋

IA-64 said “make it a whole list,” and honestly? Slay:

• -mbig-endian
• -mlittle-endian
• -mgnu-as
• -mgnu-ld
• -mno-pic
• -mvolatile-asm-stop
• -mregister-names
• -msdata
• -mconstant-gp
• -mauto-pic
• -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-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) 🌸

Tiny-but-mighty LM32 squad:

• -mbarrel-shift-enabled
• -mdivide-enabled
• -mmultiply-enabled
• -msign-extend-enabled
• -muser-enabled

---

💖 LoongArch Options (LoongArch Options) 💖

LoongArch brought the whole group chat. Every flag, no skipped texts:

• -march=arch-type
• -mtune=tune-type
• -mabi=base-abi-type
• -mfpu=fpu-type
• -msimd=simd-type
• -msoft-float
• -msingle-float
• -mdouble-float
• -mlsx
• -mlasx
• -mbranch-cost=n
• -maddr-reg-reg-cost=n
• -mcheck-zero-division
• -mbreak-code=code
• -mcond-move-int
• -mcond-move-float
• -memcpy
• -mstrict-align
• -G num
• -mmax-inline-memcpy-size=n
• -mexplicit-relocs=style
• -mexplicit-relocs
• -mno-explicit-relocs
• -mdirect-extern-access
• -mcmodel=code-model
• -mrelax
• -mpass-mrelax-to-as
• -mrecip
• -mrecip=opt
• -mfrecipe
• -mdiv32
• -mlam-bh
• -mlamcas
• -mld-seq-sa
• -mscq
• -mtls-dialect=opt
• -mannotate-tablejump

---

🎀 LynxOS Options (LynxOS Options) 🎀

Short, sweet, and very LynxOS:

• -mshared
• -mthreads
• -mlegacy-threads

---

🍓 M32C Options (M32C Options) 🍓

M32C’s little trio:

• -mcpu=cpu
• -msim
• -memregs=number

---

👑 M32R/D Options (M32R/D Options) 👑

M32R/D said “come correct with the exact spellings”:

• -m32r2
• -m32rx
• -m32r
• -mdebug
• -malign-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 (M680x0 Options) 💅

M680x0 is giving “I have a flag for every mood.” No cap:

• -march=arch
• -mcpu=cpu
• -mtune=tune
• -m68000
• -m68020
• -m68020-40
• -m68020-60
• -m68030
• -m68040
• -m68060
• -m68302
• -m68332
• -m68851
• -mcpu32
• -mfidoa
• -m5200
• -m5206e
• -m528x
• -m5307
• -m5407
• -mcfv4e
• -mbitfield
• -mc68000
• -mc68020
• -mrtd
• -mdiv
• -mshort
• -mhard-float
• -m68881
• -msoft-float
• -mpcrel
• -malign-int
• -mstrict-align
• -msep-data
• -mshared-library-id=n
• -mid-shared-library
• -mxgot
• -mxtls
• -mlong-jump-table-offsets

---

🌈 MCore Options (MCore Options) 🌈

MCore came through with the list:

• -mhardlit
• -mdiv
• -mrelax-immediates
• -mwide-bitfields
• -m4byte-functions
• -mcallgraph-data
• -mslow-bytes
• -mno-lsim
• -mlittle-endian
• -mbig-endian
• -m210
• -m340
• -mstack-increment

---

🧚 MicroBlaze Options (MicroBlaze Options) 🧚

MicroBlaze is lowkey a whole aesthetic:

• -msoft-float
• -mhard-float
• -msmall-divides
• -mcpu=cpu
• -mmemcpy
• -mxl-soft-mul
• -mxl-soft-div
• -mxl-barrel-shift
• -mxl-pattern-compare
• -mxl-gp-opt
• -mxl-multiply-high
• -mxl-float-convert
• -mxl-float-sqrt
• -mbig-endian
• -mlittle-endian
• -mxl-reorder
• -mxl-mode-app-model
• -mxl-prefetch
• -mpic-data-is-text-relative

---

🫶 MIPS Options (MIPS Options) 🫶

Okay MIPS is the main character of option lists. Deep breath. Every single one:

• -EL
• -EB
• -mel
• -meb
• -march=arch
• -mtune=arch
• -mips1
• -mips2
• -mips3
• -mips4
• -mips32
• -mips32r2
• -mips32r3
• -mips32r5
• -mips32r6
• -mips64
• -mips64r2
• -mips64r3
• -mips64r5
• -mips64r6
• -mips16
• -mmips16e2
• -mflip-mips16
• -minterlink-compressed
• -minterlink-mips16
• -mabi=abi
• -mabicalls
• -mshared
• -mplt
• -mxgot
• -mgp32
• -mgp64
• -mfp32
• -mfpxx
• -mfp64
• -mhard-float
• -msoft-float
• -mno-float
• -msingle-float
• -mdouble-float
• -modd-spreg
• -mabs=mode
• -mnan=encoding
• -mdsp
• -mdspr2
• -mmcu
• -meva
• -mvirt
• -mxpa
• -mcrc
• -mginv
• -mmicromips
• -mmsa
• -mloongson-mmi
• -mloongson-ext
• -mloongson-ext2
• -mfpu=fpu-type
• -msmartmips
• -mpaired-single
• -mdmx
• -mips3d
• -mmt
• -mllsc
• -mlong64
• -mlong32
• -msym32
• -Gnum
• -mno-local-sdata
• -mno-extern-sdata
• -mno-gopt
• -membedded-data
• -muninit-const-in-rodata
• -mcode-readable=setting
• -mno-data-in-code
• -mcode-xonly
• -msplit-addresses
• -mexplicit-relocs
• -mexplicit-relocs=release
• -mno-check-zero-division
• -mdivide-traps
• -mdivide-breaks
• -mno-load-store-pairs
• -mstrict-align
• -mno-unaligned-access
• -mmemcpy
• -mlong-calls
• -mmad
• -mimadd
• -mno-fused-madd
• -nocpp
• -mfix-24k
• -mfix-r4000
• -mfix-r4400
• -mfix-r5900
• -mfix-r10000
• -mfix-rm7000
• -mfix-vr4120
• -mfix-vr4130
• -mfix-sb1
• -mfix4300
• -mr10k-cache-barrier=setting
• -mflush-func=func
• -mno-flush-func
• -mbranch-cost=num
• -mbranch-likely
• -mcompact-branches=policy
• -mno-fp-exceptions
• -mvr4130-align
• -msynci
• -mno-lxc1-sxc1
• -mno-madd4
• -mno-relax-pic-calls
• -mmcount-ra-address
• -mframe-header-opt

---

💜 MMIX Options (MMIX Options) 💜

MMIX is niche but iconic:

• -mlibfuncs
• -mepsilon
• -mabi=gnu
• -mabi=mmixware
• -mzero-extend
• -mknuthdiv
• -mtoplevel-symbols
• -melf
• -mbranch-predict
• -mbase-addresses
• -msingle-exit
• -mset-data-start=address
• -mset-program-start=address
• -mno-set-program-start

---

⭐ MN10300 Options (MN10300 Options) ⭐

MN10300 options, served fresh:

• -mmult-bug
• -mno-mult-bug
• -mam33
• -mam33-2
• -mam34
• -mtune=cpu-type
• -mno-return-pointer-on-d0
• -mno-crt0
• -mrelax
• -mno-liw
• -mno-setlb

---

🙈 Moxie Options (Moxie Options) 🙈

Moxie said “keep it cute, keep it exact”:

• -meb
• -mel
• -mmul.x
• -mno-crt0

---

🎀 MSP430 Options (MSP430 Options) 🎀

MSP430 brought the whole accessory bag:

• -msim
• -masm-hex
• -mmcu=name
• -mlarge
• -msmall
• -mrelax
• -mwarn-mcu
• -mwarn-devices-csv
• -mcode-region=where
• -mdata-region=where
• -muse-lower-region-prefix
• -msilicon-errata=name[,name...]
• -msilicon-errata-warn=name[,name...]
• -mhwmult=type
• -minrt
• -mtiny-printf
• -mmax-inline-shift=n

---

💀 NDS32 Options (NDS32 Options) 💀

NDS32 has range, honestly:

• -mbig-endian
• -mlittle-endian
• -EB
• -EL
• -mabi=name
• -mfloat-abi=name
• -mreduced-regs
• -mfull-regs
• -malways-align
• -malign-functions
• -mfp-as-gp
• -mcmov
• -mhw-abs
• -mext-perf
• -mext-perf2
• -mext-string
• -mext-dsp
• -mext-fpu-fma
• -mext-fpu-sp
• -mext-fpu-dp
• -mv3push
• -m16-bit
• -mvh
• -misr-vector-size=num
• -misr-secure=num
• -mcache-block-size=num
• -march=arch
• -mcpu=cpu
• -mconfig-fpu=num
• -mconfig-mul=type
• -mconfig-register-ports=kind
• -mcmodel=code-model
• -mctor-dtor
• -mrelax
• -mrelax-hint
• -msched-prolog-epilog
• -mno-ret-in-naked-func
• -malways-save-lp
• -munaligned-access
• -minline-asm-r15

---

🔥 Nvidia PTX Options (Nvidia PTX Options) 🔥

Nvidia PTX, no crumbs left:

• -m64
• -march=arch
• -misa=arch
• -march-map=arch
• -mptx=version
• -mmainkernel
• -moptimize
• -msoft-stack
• -msoft-stack-reserve-local=size
• -muniform-simt
• -mgomp

---

🌱 OpenRISC Options (OpenRISC Options) 🌱

OpenRISC is giving “underrated but prepared”:

• -mboard=name
• -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 (PDP-11 Options) 🧸

PDP-11 is vintage and that’s the vibe:

• -mfpu
• -msoft-float
• -mac0
• -m40
• -m45
• -m10
• -mint32
• -mint16
• -msplit
• -munix-asm
• -mdec-asm
• -mgnu-asm
• -mlra

---

🍄 Picolibc Options (Picolibc Options) 🍄

Picolibc uses the double-dash entrance, periodt:

• --oslib=library
• --crt0=[none|minimal|hosted|semihost]
• --printf=[d|f|l|i|m]
• --scanf=[d|f|l|i|m]

---

🌈 PowerPC Options 🌈

See RS/6000 and PowerPC Options.

---

🐝 PRU Options (PRU Options) 🐝

PRU said “small list, big energy”:

• -mmcu=mcu
• -minrt
• -mno-relax
• -mloop
• -mmul
• -mfillzero
• -mabi=variant

---

🚀 RISC-V Options (RISC-V Options) 🚀

RISC-V options are a whole mood board, bestie. Exact flags, no edits:

• -mbranch-cost=N-instruction
• -mabi=ABI-string
• -mfdiv
• -mdiv
• -mno-fence-tso
• -misa-spec=ISA-spec-string
• -march=[ISA|Profile|Profile_ISA|processor-string]
• -mcpu=processor-string
• -mtune=processor-string
• -mpreferred-stack-boundary=num
• -msmall-data-limit=N-bytes
• -msave-restore
• -mno-shorten-memrefs
• -mstrict-align
• -mscalar-strict-align
• -mno-vector-strict-align
• -mcmodel=medlow
• -mcmodel=medany
• -mcmodel=large
• -mexplicit-relocs
• -mrelax
• -mriscv-attribute
• -malign-data=type
• -mbig-endian
• -mlittle-endian
• -mstack-protector-guard=guard
• -mstack-protector-guard-reg=reg
• -mstack-protector-guard-offset=offset
• -mcsr-check
• -momit-leaf-frame-pointer
• -mmovcc
• -mno-inline-atomics
• -mno-inline-strlen
• -mno-inline-strcmp
• -mno-inline-strncmp
• -mstringop-strategy=strategy
• -mtls-dialect=desc
• -mtls-dialect=trad
• -mrvv-vector-bits=value
• -mrvv-max-lmul=value
• -madjust-lmul-cost
• -mmax-vectorization
• -mno-autovec-segment

---

🦋 RL78 Options (RL78 Options) 🦋

RL78 closing out this part with the exact set:

• -msim
• -mallregs
• -mrelax
• -mes0
• -mmul=none
• -mmul=g13
• -mmul=g14
• -mmul=rl78
• -mcpu=g10
• -mcpu=g13
• -mcpu=g14
• -mcpu=rl78
• -mg10
• -mg13
• -mg14
• -mrl78
• -msave-mduc-in-interrupts

💜 RS/6000 and PowerPC Options 💜

okay so these are for the RS/6000 and PowerPC machines, and bestie there are so many flags, let's go:

-mcpu=cpu-type -mtune=cpu-type -mcmodel=code-model  -mprofile-kernel
-mpowerpc64 -maltivec -mpowerpc-gpopt  -mpowerpc-gfxopt  -mmfcrf
-mpopcntb  -mpopcntd -mfprnd  -mcmpb  -mhard-dfp -mfull-toc
-mminimal-toc  -mno-fp-in-toc  -mno-sum-in-toc -maix64  -maix32
-m64  -m32  -mxl-compat  -mpe -malign-power  -malign-natural
-msoft-float  -mhard-float  -mmultiple  -mupdate
-mavoid-indexed-addresses -mfused-madd  -mbit-align  -mbit-word
-mstrict-align  -mrelocatable  -mrelocatable-lib -mlittle
-mlittle-endian  -mbig  -mbig-endian -mdynamic-no-pic
-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 -mregnames  -mlongcall  -mpltseq  -mtls-markers
-mblock-move-inline-limit=num -mblock-compare-inline-limit=num
-mblock-compare-inline-loop-limit=num -mblock-ops-unaligned-vsx
-mstring-compare-inline-limit=num -misel  -mvsx  -mvrsave  -mmulhw
-mdlmzb  -mprototype -msim  -mmvme  -mads  -myellowknife  -memb
-meabi  -msdata -msdata=opt  -mreadonly-in-sdata  -mvxworks  -G num
-mrecip  -mrecip=opt  -mno-recip  -mrecip-precision -mveclibabi=type
-mfriz -mpointers-to-nested-functions  -msave-toc-indirect
-mpower8-fusion -mcrypto  -mhtm  -mquad-memory  -mquad-memory-atomic
-mcompat-align-parm -mfloat128  -mfloat128-hardware -mgnu-attribute
-mstack-protector-guard=guard  -mstack-protector-guard-reg=reg
-mstack-protector-guard-offset=offset  -mprefixed -mpcrel  -mmma
-mdense-math  -mrop-protect  -mprivileged -mno-splat-word-constant
-mno-splat-float-constant -mno-ieee128-constant
-mno-warn-altivec-long

like, that's the whole squad ✨ every single one is there, no gatekeeping 💅

---

🦋 RX Options 🦋

the RX crew, living their best life:

-m64bit-doubles  -m32bit-doubles  -fpu
-nofpu -mcpu=name -mbig-endian-data  -mlittle-endian-data
-msmall-data-limit=N -msim  -mas100-syntax  -mrelax
-mmax-constant-size=N  -mint-register=N -mpid
-mno-allow-string-insns  -mjsr -mno-warn-multiple-fast-interrupts
-msave-acc-in-interrupts -mlra

---

🌸 S/390 and zSeries Options 🌸

big mainframe energy, honestly iconic 👑

-mtune=cpu-type  -march=cpu-type -mhard-float  -msoft-float
-mhard-dfp -mlong-double-64  -mlong-double-128 -mbackchain
-mpacked-stack -msmall-exec  -mmvcle -m64  -m31  -mdebug  -mesa
-mzarch -mhtm  -mvx  -mzvector -mtpf-trace  -mtpf-trace-skip  -mmain
-mfused-madd  -mno-fused-madd -mwarn-framesize=framesize
-mwarn-dynamicstack -mstack-size=stack-size  -mno-stack-size
-mstack-guard=stack-guard  -mno-stack-guard
-mstack-protector-guard=guard -mstack-protector-guard-record
-mhotpatch=pre-halfwords,post-halfwords
-mno-pic-data-is-text-relative -mindirect-branch=choice
-mindirect-branch-jump=choice  -mindirect-branch-call=choice
-mfunction-return=choice -mfunction-return-mem=choice
-mfunction-return-reg=choice -mindirect-branch-table -mfentry
-mrecord-mcount  -mnop-mcount -mpreserve-args  -munaliged-symbols

ngl -munaliged-symbols is spelled exactly like that in the source — yes the typo is canon, don't come for me 😭 copy it as-is or your build will be big mad.

---

🎀 SH Options 🎀

SuperH besties, we see you and we love you:

-m1  -m2  -m2e -m2a  -m2a-nofpu  -m2a-single
-m2a-single-only -m3  -m3e -m4  -m4-nofpu  -m4-single
-m4-single-only -m4-100  -m4-100-nofpu  -m4-100-single
-m4-100-single-only -m4-200  -m4-200-nofpu  -m4-200-single
-m4-200-single-only -m4-300  -m4-300-nofpu  -m4-300-single
-m4-300-single-only -m4-340  -m4-400  -m4-500 -m4a  -m4al
-m4a-nofpu  -m4a-single  -m4a-single-only -mb  -ml  -mdalign
-mrelax -mbigtable  -mbitops  -mfmovd  -mrenesas  -mnomacsave -mieee
-misize  -minline-ic_invalidate -mprefergot  -musermode
-multcost=number  -mdiv=strategy -mdivsi3_libfunc=name
-mfixed-range=register-range -maccumulate-outgoing-args
-matomic-model=atomic-model -mbranch-cost=num  -mzdcbranch
-mcbranch-force-delay-slot -mfsca  -mfsrra -mpretend-cmove  -mfdpic
-mtas  -mlra

---

🫶 Solaris 2 Options 🫶

short but slay:

-mclear-hwcap
-mno-clear-hwcap  -mimpure-text  -mno-impure-text -gsctf

---

⭐ SPARC Options ⭐

the SPARC section ate and left no crumbs:

-mcpu=cpu-type -mtune=cpu-type
-mcmodel=code-model -mmemory-model=mem-model -m32  -m64  -mptr32
-mptr64  -mapp-regs -mfaster-structs  -mflat -mfpu  -mhard-float
-msoft-float -mhard-quad-float  -msoft-quad-float -mstack-bias
-mstd-struct-return -munaligned-doubles  -muser-mode -mv8plus  -mvis
-mvis2  -mvis3  -mvis3b  -mvis4  -mvis4b -mcbcond  -mfmaf  -mfsmuld
-mpopc  -msubxc -mfix-at697f  -mfix-ut699  -mfix-ut700
-mfix-gr712rc

---

💅 System V Options 💅

just two, but they're living rent-free:

• -YP,paths — you already know
• -Ym,dir — periodt

---

🌈 V850 Options 🌈

-mlong-calls  -mep -mprolog-function
-mspace -mtda=n  -msda=n  -mzda=n -mv850  -mv850e3v5  -m850e2v4
-mv850e2v3 -mv850e2  -mv850e1  -mv850es  -mv850e -mdisable-callt
-mrelax  -mlong-jumps -msoft-float  -mhard-float  -mloop -mrh850-abi
-mghs  -mgcc-abi -m8byte-align  -mbig-switch  -mapp-regs
-msmall-sld -mno-strict-align  -mjump-tables-in-data-section

---

🧚 VAX Options 🧚

vintage but make it cute:

-munix  -mgnu  -md  -md-float  -mg
-mg-float  -mlra -mvaxc-alignment  -mqmath

---

🍓 Visium Options 🍓

-mdebug  -msim  -mfpu  -mhard-float
-msoft-float -mcpu=cpu-type  -mtune=cpu-type  -msv-mode  -muser-mode

---

💜 VMS Options 💜

-mvms-return-codes  -mdebug-main=prefix
-mmalloc64 -mpointer-size=size

---

🎀 VxWorks Options 🎀

-mrtp  -msmp  -mvthreads
-non-static  -Bstatic  -Bdynamic -Xbind-lazy  -Xbind-now

---

🔥 x86 Options 🔥

bestie. this is the big one. the main character. the one everybody's always asking about. it's giving encyclopedia, it's giving lore drop, and we are NOT skipping any of it:

-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  -mieee-fp -mrtd  -malign-double
-mpreferred-stack-boundary=num -mincoming-stack-boundary=num -mcld
-mcx16  -msahf  -mmovbe  -mcrc32  -mmwait -mrecip  -mrecip=opt
-mvzeroupper  -mstv  -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  -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  -mavx10.2  -mamx-avx512
-mamx-tf32  -mmovrs -mamx-movrs  -mavx512bmm  -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
-mstack-arg-probe -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  -mrecord-return
-mfentry-name=name  -mfentry-section=name -mskip-rax-setup
-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  -mtls-dialect=type -mrelax-cmpxchg-loop
-mindirect-branch=choice  -mfunction-return=choice
-mindirect-branch-register  -mharden-sls=choice
-mindirect-branch-cs-prefix  -mapx-inline-asm-use-gpr32 -mgather
-mscatter -mneeded  -mno-direct-extern-access
-munroll-only-small-loops  -mdispatch-scheduler  -mlam=choice

like… that's a lot of options and every single one matters, do not come for me for listing them all, that is literally the assignment 💀👑

---

🦋 x86 Windows Options 🦋

See Cygwin and MinGW Options — they're over there, go say hi 🫶

---

🌟 Xstormy16 Options 🌟

tiny section, big heart:

• -msim — that's it, that's the whole list ✨

---

💀 Xtensa Options 💀

-mconst16  -mforce-no-pic
-mno-serialize-volatile -mtext-section-literals  -mauto-litpools
-mno-target-align -mlongcalls  -mabi=abi-type
-mextra-l32r-costs=cycles  -mstrict-align

---

💜 zSeries Options 💜

See S/390 and zSeries Options — already covered up there, bestie, scroll up 😌

---

✨ Options Controlling the Kind of Output ✨

okay so this is the part where gcc actually does stuff instead of just having a million flags, and honestly it's so satisfying to understand 🎀

Compilation can involve up to four stages: preprocessing, compilation proper, assembly and linking — and they always happen in that order, no skipping the line, no cutting. GCC can preprocess and compile several files either into several assembler input files, or into one assembler input file; then each assembler input file produces an object file, and linking combines all the object files (the ones that were just newly compiled, plus the ones you specified as input) into an executable file. the way it all comes together is honestly a little bit beautiful 😭💖

---

🌸 How the file name suffix decides everything 🌸

for any given input file, the file name suffix decides what kind of compilation happens — like a vibe check for your filename ✨ here's the whole chart:

• file.c — C source code that must be preprocessed.
• file.i — C source code that should not be preprocessed.
• file.ii — C++ source code that should not be preprocessed.
• file.m — Objective-C source code. Note that you must link with the libobjc library to make an Objective-C program work.
• file.mi — Objective-C source code that should not be preprocessed.
• file.mm, file.M — Objective-C++ source code. Note that you must link with the libobjc library to make an Objective-C++ program work. Note that .M refers to a literal capital M. (yes, capital M is a whole different vibe, don't mix them up 💅)
• 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 C, C++ header file to be turned into an Ada spec (via the -fdump-ada-spec switch).

lowkey the .i / .ii / .mi / .mii ones are the "don't preprocess me, I'm already perfect" files and honestly? relatable 🫶

• file.cc, file.cp, file.cxx, file.cpp, file.CPP, file.c++, file.C — C++ source code that must be preprocessed ✨ quick heads up though: in .cxx the last two letters have to both literally be x, and .C means a literal capital C — case matters, bestie, the filesystem is not forgiving 💅
• 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, destined to be turned into a precompiled header or an 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). yes the capitalization is the whole entire point 🙈
• file.cob, file.COB, file.cbl, file.CBL — COBOL source code, slay.
• 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 containing 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). these ones are also called specs 🎀
• file.adb — Ada source code file containing a library unit body (a subprogram or package body). these 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. literally any file name with no recognized suffix gets treated this way, no cap.

---

okay so if you wanna be all explicit about it instead of letting the suffix do the talking, you can tell gcc the input language yourself with the -x option:

• -x language, --language=language, --language language — specify explicitly the language for the following input files (rather than letting the compiler choose a default based on the file name suffix 🦋). this option applies to all following input files until the next -x option. possible values for language are:

c  c-header  cpp-output
c++  c++-header  c++-system-header c++-user-header c++-cpp-output
c++-system-module
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
algol68
lto

careful bestie: -x does not imply a particular language standard 💀 for example -x f77 may also require -std=legacy for some older source codes.

• -x none — turn off any specification of a language, so that subsequent files are handled according to their file name suffixes (as if -x has not been used at all). back to vibes-based detection ✨

---

so if you only want some of the stages of compilation, you can use -x (or filename suffixes) to tell gcc where to start, and one of the options -c, -S, or -E to say where gcc is to stop. note that some combinations (for example, -x cpp-output -E) instruct gcc to do nothing at all 😭 iconic behavior honestly.

• -c, --compile — compile or assemble the source files, but do not link. the linking stage simply is not done, we don't go there. the ultimate output is in the form of an object file for each source file.

by default, the object file name for a source file is made by replacing the suffix .c, .i, .s, etc., with .o.

unrecognized input files, not requiring compilation or assembly, are ignored. 🫶

• -S, --assemble — 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, --preprocess — stop after the preprocessing stage; do not run the compiler proper. the output is in the form of preprocessed source code, which is sent to the standard output. 🦋

input files that don't require preprocessing are ignored.

• -o file, --output=file, --output file — 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. one option, all the jobs 💅

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. 💜

• -dumpbase dumpbase, --dumpbase dumpbase — 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.c

saves 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.c

preprocesses 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. it said what it said 💅

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.*. no collisions, we love to see it 🎀

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-ext auxdropsuf, --dumpbase-ext auxdropsuf — okay so when you're forming the name of an auxiliary (but not a dump) output file, drop the trailing auxdropsuf from dumpbase before appending any suffixes. If it's not specified, this option defaults to the suffix of a default dumpbase — i.e., the suffix of the input file when -dumpbase isn't present in the command line, or dumpbase is combined with dumppfx.

Like, check it:

gcc foo.c -c -o dir/foo.o -dumpbase x-foo.c -dumpbase-ext .c ...

That 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 ...

This 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.*. 💅

• -dumpdir dumppfx, --dumpdir dumppfx — when forming the name of an auxiliary or dump output file, use dumppfx as a prefix:

gcc -dumpdir pfx- -c foo.c ...

This 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 ...

This 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 ...

This outputs foo.o, with auxiliary outputs named foo.* because -save-temps=* overrides the dumppfx given by the earlier -dumpdir option. It does not matter that =obj is the default for -save-temps, nor that the output directory is implicitly the current directory. Dump outputs are named foo.c.*.

When compiling from multiple input files, if -dumpbase is specified, dumpbase, minus a auxdropsuf suffix, and a dash are appended to (or override, if containing any directory components) an explicit or defaulted dumppfx, so that each of the multiple compilations gets differently-named aux and dump outputs.

gcc foo.c bar.c -c -dumpdir dir/pfx- -dumpbase main ...

This outputs auxiliary dumps to dir/pfx-main-foo.* and dir/pfx-main-bar.*, appending dumpbase- to dumppfx. Dump outputs retain the input file suffix: dir/pfx-main-foo.c.* and dir/pfx-main-bar.c.*, respectively. Contrast with the single-input compilation:

gcc foo.c -c -dumpdir dir/pfx- -dumpbase main ...

That, 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 compiling and then linking from multiple input files, a defaulted or explicitly specified dumppfx also undergoes the dumpbase- transformation above (e.g. 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. Note, however, that unlike earlier cases of linking:

gcc foo.c bar.c -dumpdir dir/pfx- -o main ...

This 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, --verbose — 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 -v except the commands are not executed and arguments are quoted unless they contain only alphanumeric characters or ./-_. This is useful for shell scripts to capture the driver-generated command lines. 💖
• --help — prints (on the standard output) a description of the command-line options understood by gcc. If the -v option is also specified then --help is also passed on to the various processes invoked by gcc, so that they can display the command-line options they accept. If the -Wextra option has also been specified (prior to the --help option), then command-line options that have no documentation associated with them are also displayed. ✨
• --target-help — prints (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}[,...] — prints (on the standard output) a description of the command-line options understood by the compiler that fit into all specified classes and qualifiers. These are the supported classes:
  • optimizers — display all of the optimization options supported by the compiler.
  • warnings — display all of the options controlling warning messages produced by the compiler.
  • target — display target-specific options. Unlike the --target-help option however, target-specific options of the linker and assembler are not displayed. This is because those tools do not currently support the extended --help= syntax.
  • params — display the values recognized by the --param option.
  • language — display the options supported for language, where language is the name of one of the languages supported in this version of GCC. If an option is supported by all languages, one needs to select common class.
  • common — display the options that are common to all languages.

These are the supported qualifiers:

• undocumented — display only those options that are undocumented.
• joined — display options taking an argument that appears after an equal sign in the same continuous piece of text, such as: --help=target.
• separate — display options taking an argument that appears as a separate word following the original option, such as: -o output-file.

Thus for example to display all the undocumented target-specific switches supported by the compiler, use:

--help=target,undocumented

The sense of a qualifier can be inverted by prefixing it with the ^ character, so for example to display all binary warning options (i.e., ones that are either on or off and that do not take an argument) that have a description, use:

--help=warnings,^joined,^undocumented

The argument to --help= should not consist solely of inverted qualifiers.

Combining several classes is possible, although this usually restricts the output so much that there is nothing to display. One case where it does work, however, is when one of the classes is target. For example, to display all the target-specific optimization options, use:

--help=target,optimizers

The --help= option can be repeated on the command line. Each successive use displays its requested class of options, skipping those that have already been displayed. If --help is also specified anywhere on the command line then this takes precedence over any --help= option. 🌈

If the -Q option shows up on the command line before the --help= option, then the descriptive text that --help= shows gets a whole glow-up. Instead of describing the options it displays, it tells you whether each option is enabled, disabled, or set to a specific value — that's assuming the compiler knows this at the exact point where the --help= option gets used.

Here's a truncated example straight from the ARM port of gcc, so you can see the vibe:

% 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 bestie, you can literally find out which optimizations are enabled at -O2 by using:

-Q -O2 --help=optimizers

Or, alternate route: 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 — Displays the version number and copyrights of the invoked GCC. That's it, that's the whole thing. 💅
• -pass-exit-codes, --pass-exit-codes — Okay so normally the gcc program exits with the code of 1 if any phase of the compiler returns a non-success return code. But if you specify -pass-exit-codes, the gcc program instead returns with the numerically highest error produced by any phase returning an error indication. Also lowkey important: the C, C++, and Fortran front ends return 4 if an internal compiler error is encountered.
• -pipe, --pipe — Uses pipes rather than temporary files for communication between the various stages of compilation. It fails to work on some systems where the assembler is unable to read from a pipe; but the GNU assembler has no trouble. Slay. ✨
• -wrapper — Invokes all subcommands under a wrapper program. The name of the wrapper program and its parameters are passed as a comma separated list. Like:

gcc -c t.c -wrapper gdb,--args

This invokes all subprograms of gcc under gdb --args, thus the invocation of cc1 is gdb --args cc1 .... Iconic little nesting situation tbh 🪆

• -ffile-prefix-map=old=new — When compiling files residing in directory old, record any references to them in the result of the compilation as if the files resided in directory new instead. Specifying this option is equivalent to specifying all the individual -f*-prefix-map options. This can be used to make reproducible builds that are location independent. Directories referenced by directives are not affected by these options. See also -fmacro-prefix-map, -fdebug-prefix-map, -fprofile-prefix-map and -fcanon-prefix-map.
• -fcanon-prefix-map — For the -f*-prefix-map options, normally comparison of old prefix against the filename that would be normally referenced in the result of the compilation is done using textual comparison of the prefixes, or ignoring character case for case insensitive filesystems and considering slashes and backslashes as equal on DOS based filesystems. The -fcanon-prefix-map causes such comparisons to be done on canonicalized paths of old and the referenced filename. Fancy, honestly 🎀
• -fplugin=name.so — Loads the plugin code in file name.so, assumed to be a shared object to be dlopen'd by the compiler. The base name of the shared object file is used to identify the plugin for the purposes of argument parsing (See -fplugin-arg-name-key=value below). Each plugin should define the callback functions specified in the Plugins API.
• -fplugin-arg-name-key=value — Defines an argument called key with a value of value for the plugin called name. Simple as that 💖
• -fdump-ada-spec[-slim] — For C and C++ source and include files, generates corresponding Ada specs. Yes Ada. She's still here and she's thriving 🦋
• -fada-spec-parent=unit — In conjunction 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 generates Go "const", "type", "var", and "func" declarations which may be a useful way to start writing a Go interface to code written in some other language. Honestly? Cross-language bestie behavior fr fr 🌈
• @file — Reads command-line options from file. The options read are inserted in place of the original @file option. If file does not exist, or cannot be read, then the option will be treated literally, and not removed.

Options in file are separated by whitespace. A whitespace character may be included in an option by surrounding the entire option in either single or double quotes. Any character (including a backslash) may be included by prefixing the character to be included with a backslash. The file may itself contain additional @file options; any such options will be processed recursively. Inception-core, no cap 💫

---

🌸 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. That's a lot of little suffixes, but she knows all of them by heart 👑

GCC recognizes files with these names and compiles them as C++ programs even if you call the compiler the same way as for compiling C programs (usually with the name gcc).

However — and this is the part everyone fumbles — 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++. So like… know the difference, bestie 😭

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. Pick your fighter 🫶

---

🎀 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, --ansi — In C mode, this is equivalent to -std=c90. In C++ mode, it is equivalent to -std=c++98.
• -std= — Determine the language standard. This option is currently only supported when compiling C or C++.

The compiler can accept several base standards, such as c90 or c++98, and GNU dialects of those standards, such as gnu90 or gnu++98. When a base standard is specified, the compiler accepts all programs following that standard plus those using GNU extensions that do not contradict it. For example, -std=c90 turns off certain features of GCC that are incompatible with ISO C90, such as the "asm" and "typeof" keywords, but not other GNU extensions that do not have a meaning in ISO C90, such as omitting the middle term of a "?:" expression. On the other hand, when a GNU dialect of a standard is specified, all features supported by the compiler are enabled, even when those features change the meaning of the base standard. As a result, some strict-conforming programs may be rejected. The particular standard is used by -Wpedantic to identify which features are GNU extensions given that version of the standard. For example -std=gnu90 -Wpedantic warns about C++ style // comments, while -std=gnu99 -Wpedantic does not. The way this is so specific?? Obsessed 📚

A value for this option must be provided; possible values are:

• c90
• c89
• iso9899:1990 — Support all ISO C90 programs (certain GNU extensions that conflict with ISO C90 are disabled). Same as -ansi for C code.
• iso9899:199409 — ISO C90 as modified in amendment 1.
• c99
• c9x
• iso9899:1999
• iso9899:199x — ISO C99. This standard is substantially completely supported, modulo bugs and floating-point issues (mainly but not entirely relating to optional C99 features from Annexes F and G). See <https://gcc.gnu.org/projects/c-status.html> for more information. The names c9x and iso9899:199x are deprecated. ⚠️
• c11
• c1x
• iso9899:2011 — ISO C11, the 2011 revision of the ISO C standard. This standard is substantially completely supported, modulo bugs, floating-point issues (mainly but not entirely relating to optional C11 features from Annexes F and G) and the optional Annexes K (Bounds-checking interfaces) and L (Analyzability). The name c1x is deprecated.
• c17
• c18
• iso9899:2017
• iso9899:2018 — ISO C17, the 2017 revision of the ISO C standard (published in 2018). This standard is 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 name c2x is 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 name gnu9x is deprecated.
• gnu11
• gnu1x — GNU dialect of ISO C11. The name gnu1x is deprecated.
• gnu17
• gnu18 — GNU dialect of ISO C17.
• gnu23
• gnu2x — GNU dialect of ISO C23. This is the default for C code. The name gnu2x is 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 name gnu2x is deprecated.
• c++98
• c++03 — The 1998 ISO C++ standard plus the 2003 technical corrigendum and some additional defect reports. Same as -ansi for C++ code.
• gnu++98
• gnu++03 — GNU dialect of -std=c++98.
• c++11
• c++0x — The 2011 ISO C++ standard plus amendments. The name c++0x is deprecated.
• gnu++11
• gnu++0x — GNU dialect of -std=c++11. The name gnu++0x is deprecated.

• c++14, c++1y — the 2014 ISO C++ standard plus amendments. Heads up: c++1y is deprecated.
• gnu++14, gnu++1y — GNU dialect of -std=c++14. The name gnu++1y is deprecated.
• c++17, c++1z — the 2017 ISO C++ standard plus amendments. The name c++1z is deprecated.
• gnu++17, gnu++1z — GNU dialect of -std=c++17. The name gnu++1z is deprecated.
• c++20, c++2a — the 2020 ISO C++ standard plus amendments. C++20 modules support is still experimental and needs to be enabled with -fmodules option. The name c++2a is deprecated.
• gnu++20, gnu++2a — GNU dialect of -std=c++20. This is the default for C++ code. C++20 modules support is still experimental and needs to be enabled with -fmodules option. The name gnu++2a is 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 name c++2b is deprecated.
• gnu++23, gnu++2b — GNU dialect of -std=c++23. Support is experimental, and could change in incompatible ways in future releases. The name gnu++2b is 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 — output to the given filename prototyped declarations for all functions declared and/or defined in a translation unit, including those in header files. This option is silently ignored in any language other than C.

Besides declarations, the file indicates, in comments, the origin of each declaration (source file and line), whether the declaration was implicit, prototyped or unprototyped (I, N for new or O for old, respectively, in the first character after the line number and the colon), and whether it came from a declaration or a definition (C or F, respectively, in the following character). In the case of function definitions, a K&R-style list of arguments followed by their declarations is also provided, inside comments, after the declaration.

• -fno-asm — do not recognize "asm", "inline" or "typeof" as a keyword, so that code can use these words as identifiers. You can use the keywords "__asm__", "__inline__" and "__typeof__" instead. In C, -ansi implies -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.

• -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 is void. 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.
• -fgnu-tm — when the option -fgnu-tm is specified, the compiler generates code for the Linux variant of Intel's current Transactional Memory ABI specification document (Revision 1.1, May 6 2009). This is an experimental feature whose interface may change in future versions of GCC, as the official specification changes. Please note that not all architectures are supported for this feature.

For more information on GCC's support for transactional memory,

Note that the transactional memory feature is not supported with non-call exceptions (-fnon-call-exceptions).

• -fgnu89-inline — the option -fgnu89-inline tells GCC to use the traditional GNU semantics for "inline" functions when in C99 mode.

Using this option is roughly equivalent to adding the "gnu_inline" function attribute to all inline functions.

The option -fno-gnu89-inline explicitly tells GCC to use the C99 semantics for "inline" when in C99 or gnu99 mode (i.e., it specifies the default behavior). This option is not supported in -std=c90 or -std=gnu90 mode.

The preprocessor macros "__GNUC_GNU_INLINE__" and "__GNUC_STDC_INLINE__" may be used to check which semantics are in effect for "inline" functions.

• -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.
• -fms-extensions — accept some non-standard constructs used in Microsoft header files.

In C++ code, this allows member names in structures to be similar to previous types declarations.

typedef int UOW;
struct ABC {
  UOW UOW;
};

Some cases of unnamed fields in structures and unions are only accepted with this option.

Note that this option is off for all targets 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.

• -fdeps-file=file — where to write structured dependency information.
• -fdeps-format=format — the format to use for structured dependency information. p1689r5 is the only supported format right now. Note that when this argument is specified, the output of -MF is 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 -MT but 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 four control whether a bit-field counts as signed or unsigned when the declaration is out here being lazy and doesn't say signed or unsigned at all 😌. By default such a bit-field is signed, and honestly that's the consistent choice — the basic integer types like int are signed types, so like… it tracks 💅.
• -fsigned-char, -funsigned-char — -fsigned-char lets the type char be signed, just like signed char 🫶. -funsigned-char lets the type char be unsigned, just like unsigned char ✨.

Heads up bestie: -fsigned-char is literally equivalent to -fno-unsigned-char, which is the negative form of -funsigned-char. Same energy the other way — -fno-signed-char is equivalent to -funsigned-char 🔁.

Every target GCC supports has a default for what char should be, and it's usually spelled out in the processor-specific ABI document for that target. So it's either like unsigned char by default or like signed char by default — no in-between, she picked a lane 🎀.

Ideally a portable program should always use signed char or unsigned char whenever it actually depends on the signedness of an object. But 😭 so many programs out there use plain char and expect it to be signed, or expect it to be unsigned, depending on whatever machine they were originally written for. These options are your little rescue squad for making such a program work with the opposite default 💖.

And one more thing: the type char is always a distinct type from each of signed char or unsigned char, even though its behaviour is always just like one of those two. It's giving main character who refuses to be the same person as her sisters 💀.

• -fstrict-flex-arrays (C and C++ only), *-fstrict-flex-arrays=level* (C and C++ only) — these 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, iykyk.

-fstrict-flex-arrays is equivalent to -fstrict-flex-arrays=3, which is the strictest — a trailing array only counts as a flexible array member when it's actually declared as a flexible array member per C99 standard onwards. She's not accepting any imposters 🚫.

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. Everyone's invited to the party 🎉.

There are two more levels in between 0 and 3, and they exist to support older code that uses the GCC zero-length array extension ([0]) or a one-element array ([1]) as flexible array members 🌸. When level is 1, the trailing array is treated as a flexible array member when it's declared as either [], [0], or [1]. When level is 2, the trailing array is treated as a flexible array member when it's declared as either [] or [0].

You can also control this behaviour for one specific trailing array field of a structure by using the variable attribute strict_flex_array — yes, individually, we love a bespoke treatment 💅.

Also the -fstrict_flex_arrays option interacts with the -Wstrict-flex-arrays option, so keep that in mind 👑.

• -fsso-struct=endianness — sets the default scalar storage order of structures and unions to the endianness you specify 🦋. The accepted values are big-endian, little-endian and native for the native endianness of the target (which is the default). This option is **not supported for C++ — sorry C++ girlies 🙈.

⚠️ Warning, and I mean actually listen to me: the -fsso-struct switch causes GCC to generate code that is not binary compatible with code generated without it, if the specified endianness isn't the native endianness of the target. So like… proceed with caution, no cap 💀.

💜 Options Controlling C++ Dialect 💜

This section is describing the command-line options that are only meaningful for C++ programs 🎀. That said, you can also use most of the GNU compiler options regardless of 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.C

In this example, only -fstrict-enums is an option that's meant only for C++ programs — the other options you can use with any language GCC supports 🌈.

Some options for compiling C programs, such as -std, are also relevant for C++ programs. Shared custody 💖.

Here's the list of options that are only for compiling C++ programs:

• --compile-std-module — builds the compiled module interfaces for the <bits/stdc++.h> header unit and the std and std.compat modules before compiling any source files explicitly specified on the command line 🧚. This is intended to be useful for building simple programs that use import std; with a single command. One command, whole module, iconic ✨.
• *-fabi-version=n* — use version n of the C++ ABI. The default is version 0 👑.

Version 0 refers to the version conforming most closely to the C++ ABI specification. Therefore, the ABI obtained using version 0 will change in different versions of G++ as ABI bugs get fixed. She's a moving target, it's fine 💅.

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 😭 (oops).

Version 13, which first appeared in G++ 8.2, fixes the accidental change in version 12. Cleanup era 🧹.

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, handling of cv-qualified [[no_unique_address]] members, and adds mangling of C++20 constraints on function templates.

Version 20, which first appeared in G++ 15, fixes manglings of lambdas in static data member initializers.

Version 21, which first appeared in G++ 16, fixes unnecessary captures in noexcept lambdas (c++/119764), layout of a base class with all explicitly defaulted constructors (c++/120012), and mangling of class and array objects with implicitly zero-initialized non-trailing subobjects (c++/121231).

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 isn't provided but -Wabi=n is, then 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. Teamwork, kinda 🌸.

• -fno-access-control — turns off all access checking 💀. This switch is mainly useful for working around bugs in the access control code. Like, it's a crowbar, not a lifestyle, bestie.
• -faligned-new, *-faligned-new=alignment* — enables support for C++17 new of types that require more alignment than void* ::operator new(std::size_t) provides ✨. A numeric argument such as -faligned-new=32 can be used to specify how much alignment (in bytes) is provided by that function, but like, few users will need to override the default of alignof(std::max_align_t) 💅.

This flag is enabled by default for -std=c++17. She's already on by default, she's that girl 🎀.

• -fno-assume-sane-operators-new — okay so the C++ standard lets you replace the global new, new[], delete and delete[] operators, but like… a LOT of C++ programs simply do not, and just vibe with the implementation-provided version. On top of that, the standard also lets the compiler skip those calls entirely when they come from new or delete expressions (and by extension, the same is assumed if __builtin_operator_new or __builtin_operator_delete functions are used). This option gives you control over some optimizations around calls to those operators.

With -fassume-sane-operators-new-delete, GCC may assume that calls to the replaceable global operators from new or delete expressions (or from __builtin_operator_new or __builtin_operator_delete calls) don't read or modify any global variables, or variables whose address could escape to the operators — that's global state, except for errno for the new and new[] operators. That unlocks most optimizations across those calls, and it's something the implementation-provided operators satisfy — unless malloc implementation details are observable in the code, or unless malloc hooks are used. But it might not hold if a program replaces those operators. This behavior is enabled by default, bestie.

With -fno-assume-sane-operators-new-delete, 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 your code needs to expect that behavior. 🫶

• -fchar8_t, -fno-char8_t — turns on support for char8_t as adopted for C++20 ✨. That includes a whole new char8_t fundamental 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_t and __cpp_lib_char8_t feature test macros.

This option lets functions 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 #2

and it 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:

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 — checks that the pointer returned by operator new is non-null before trying to modify the storage it allocated. Ngl this check is normally unnecessary, because the C++ standard says operator new only returns 0 if it's declared throw(), and in that case the compiler always checks the return value even without this option. In all other cases, when operator new has a non-empty exception specification, memory exhaustion is signalled by throwing std::bad_alloc. See also new (nothrow). 💅
• -fconcepts — enables support for the C++ Concepts feature for constraining template arguments 🎀. With -std=c++20 and above, Concepts are part of the language standard, so -fconcepts defaults to on.

Some constructs that were allowed by the earlier C++ Extensions for Concepts Technical Specification, ISO 19217 (2015), but didn't make it into the standard, could additionally be enabled by -fconcepts-ts. Heads up: the option -fconcepts-ts was deprecated in GCC 14 and removed in GCC 15; users are expected to convert their code to C++20 concepts. ⚠️

• *-fconcepts-diagnostics-depth=n* — specifies the maximum error replay depth during recursive diagnosis of a constraint satisfaction failure. The default is 1. 🦋
• *-fconstexpr-depth=n* — sets the maximum nested evaluation depth for C++11 constexpr functions to n. A limit is needed to detect endless recursion during constant expression evaluation. The minimum specified by the standard is 512. 🔥
• *-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 very large from highly recursive evaluations). The default is 8. Very few users are likely to want to adjust it, but if your code does heavy constexpr calculations you might want to experiment to find which value works best for you. 🧚
• -fconstexpr-fp-except — Annex F of the C standard says that IEC559 floating point exceptions encountered at compile time should not stop compilation. C++ compilers have historically not followed this guidance, and instead treat 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 in C++14 constexpr functions to n. A limit is needed to detect infinite loops during constant expression evaluation. The default is 262144 (1<<18). 💜
• *-fconstexpr-ops-limit=n* — sets the maximum number of operations during a single constexpr evaluation. Even when the number of iterations of a single loop is limited with the above limit, if there are several nested loops and each of them has many iterations but still smaller than the above limit, or if in a body of some loop — or even outside of a loop — too many expressions need to be evaluated, the resulting constexpr evaluation might take too long. The default is 33554432 (1<<25). 😭
• -fcontracts — enables support for the C++ Contracts feature, as specified in the C++26 working draft (N5003). 👑

On violation of an enforced or observed contract (see -fcontract-evaluation-semantic below), a violation handler is called; the standard library provides a default handler that emits information about the contract that failed.

Users can replace the default violation handler by defining

void
handle_contract_violation (const std::contracts::contract_violation&);

• *-fcontract-evaluation-semantic=semantic* — sets the semantic with which contracts will be evaluated to semantic.

Available values for the evaluation mode are:

• "ignored" — the contract checks will be elided, with no checking added at either compile or runtime.
• "observed" — the contract checks are performed (at runtime) and, if one fails, a handler is called that allows actions such as logging or emitting run-time warnings. When the handler returns, the execution of the code will continue. At compile-time the checks are performed for constexpr evaluations, and in this case a diagnostic is emitted if the check fails.
• "enforce" — the contract checks are performed and the handler is called if one fails. When the handler returns, the execution of the code is terminated, preventing it from continuing past the failed case. At compile-time the checks are applied to constexpr and, if one fails, the program is ill-formed; an error will be emitted.
• "quick_enforce" — the contract checks are performed (at runtime) and, if one fails, the execution is terminated immediately (without calling any handler). At compile-time there is no difference in behaviour between this option and the "enforce" case, since no handler is invoked at compile time.

• -fcontracts-conservative-ipa — this prevents inter-procedural analysis from taking action when the body of an inline function is visible in a given TU. It allows for the case that contract evaluation conditions are permitted to differ between TUs, which means that such actions would be potentially incorrect. 🙈
• -fcontract-checks-outlined — by default, contract checks for pre and post condition checks are expanded in-line into function bodies. This option causes a small function to be created instead (containing the checks) that is called in the relevant position. This permits different criteria to be applied to the compilation of the checking and the remainder of the function body. ✨
• -fcontract-disable-optimized-checks — when pre and post condition checks are outlined (using -fcontract-checks-outlined), this option disables the optimisation steps for those functions, which can avoid unwanted elision of checking steps. 🎯
• *-fcontracts-client-check=mode* — this option enables insertion of checks at call sites (caller-side checking).

The value of mode determines which contract checks are made:

• "none" — no caller/client side checking (default)
• "pre" — preconditions are checked at the caller/client side.
• "all" — both pre and post-conditions are checked at the caller/client side.

• *-fcontracts-definition-check=mode* — this option introduces the ability to disable callee/definition-side checks, which are otherwise enabled by default when the contracts feature is active.

The variable mode has the values "on" and "off". 💖

• -fcoroutines — turns on support for the C++ coroutines extension 💜. With -std=c++20 and above, coroutines are literally part of the language standard, so -fcoroutines defaults to on. slay, no flag needed.
• -fdiagnostics-all-candidates — lets the C++ front end call out all the candidates when overload resolution fails, including when a deleted function is the one that got picked. so you get the full roster, bestie, not just a teaser 🫶.
• -fdump-lang-, -fdump-lang-switch, -fdump-lang-switch-options, -fdump-lang-switch-options=filename — these control dumping of C++-specific info ✨. The options and filename parts behave exactly as described in the -fdump-tree option. Here's the switch values you can pass:
  • all — enables all of the ones below. queen behavior 👑.
  • class — dumps class hierarchy info. Virtual table info gets emitted too, unless you specify 'slim'.
  • module — dumps module information. The options lineno (locations), graph (reachability), blocks (clusters), uid (serialization), alias (mergeable), asmname (Elrond), eh (mapper) & vops (macros) may provide additional information 👀.
  • raw — dumps the raw internal tree data.
  • tinst — dumps the sequence of template instantiations, indented so you can see how deep the recursion goes 🌀. The lineno option adds the source location where the instantiation was triggered, and the details option also dumps pre-instantiation substitutions, like the ones performed during template argument deduction. Lines in the .tinst dump start with I for an instantiation, S for another substitution, and R[IS] for the reopened context of a deferred instantiation. iykyk.
• -fno-elide-constructors — okay so the C++ standard allows an implementation to skip 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 😤. This option also makes G++ call trivial member functions that would otherwise be expanded inline.

In C++17, the compiler is required to omit these temporaries, but this option still affects trivial member functions. so there's that 💅.

• -fno-enforce-eh-specs — don't generate code that checks for violations of exception specifications at run time. This option violates the C++ standard, but it may be useful for reducing code size in production builds, much like defining "NDEBUG" 👀.

fr fr, heads up: this does not give your user code permission to throw exceptions in violation of the exception specifications. The compiler still optimizes based on the specifications, so throwing an unexpected exception results in undefined behavior at run time 💀.

• -fextern-tls-init, -fno-extern-tls-init — the C++11 and OpenMP standards allow "thread_local" and "threadprivate" variables to have dynamic (runtime) initialization. To support that, any use of such a variable goes through a wrapper function that does whatever initialization is needed 🧚. When the use and the definition of the variable are in the same translation unit, that 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 you (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 because a use of the variable in the defining TU will run before any uses in another TU — you can dodge that overhead with the -fno-extern-tls-init option 🙌.

On targets that support symbol aliases, the default is -fextern-tls-init. On targets that do not support symbol aliases, the default is -fno-extern-tls-init.

• -ffold-simple-inlines, -fno-fold-simple-inlines — lets the C++ frontend fold calls to "std::move", "std::forward", "std::addressof", "std::to_underlying" and "std::as_const" 🌸. Unlike inlining, this means no debug information gets generated for those calls. Since these functions are rarely interesting to debug, this flag is enabled by default unless -fno-inline is active. iconic, honestly.
• -fno-gnu-keywords — do not recognize "typeof" as 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, the thing where certain functions can get promoted to consteval, 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);
        }

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 that allows consteval function composition, bestie.

However, with -fno-immediate-escalation, "f" is not promoted to "consteval", and since the call to consteval function id(t) is not 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 implicitly constexpr, if they satisfy the requirements for a constexpr function ✨. You can use this option in C++14 mode or later. This can result in initialization changing from dynamic to static, plus other optimizations. cute AND useful.
• -fno-implicit-templates — never emit code for non-inline templates that are instantiated implicitly (i.e. by use); only emit code for explicit instantiations 🫶. If you use this option, you must take care to structure your code so it includes all the necessary explicit instantiations, or you'll get undefined symbols at link time 🙈.
• -fno-implicit-inline-templates — don't emit code for implicit instantiations of inline templates either. The default is to handle inlines differently, so that compiles with and without optimization need the same set of explicit instantiations.
• -fno-implement-inlines — to save space, don't emit out-of-line copies of inline functions controlled by "#pragma implementation" 💅. This causes linker errors if those functions aren't inlined everywhere they're called. so, y'know, choose wisely.
• -fmodules, -fno-modules — enable support for C++20 modules 🦋. -fno-modules is usually not needed, since that's the default. Even though this is a C++20 feature, it's not currently implicitly enabled just by selecting that standard version. annoying, but true.
• -fmodule-header, -fmodule-header=user, -fmodule-header=system — compile a header file to create an importable header unit 🌈.
• -fmodule-implicit-inline — member functions defined in their class definitions are not implicitly inline for modular code. This is different from traditional C++ behavior, and for good reasons 💜. However, it may cause difficulty during code porting. This option makes such function definitions implicitly inline. It does, however, generate an ABI incompatibility, so you must use it everywhere or nowhere 😤 (such definitions outside of a named module remain implicitly inline regardless).
• -fno-module-lazy — disables 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, the CXX_MODULE_MAPPER environment variable is used, and if that's unset, an in-process default is provided. bestie's got a fallback for everything, periodt.
• -fmodule-only — only emit the Compiled Module Interface, inhibiting any object file ✨.
• -fms-extensions — disables Wpedantic warnings about constructs used in MFC, like implicit int and getting a pointer to a member function through non-standard syntax. it's giving "let them live" energy ngl.
• -fnew-inheriting-ctors — enables the P0136 adjustment to the semantics of C++11 constructor inheritance 🎀. This is part of C++17 but is also considered a Defect Report against C++11 and C++14. This flag is enabled by default unless -fabi-version=10 or lower is specified.
• -fnew-ttp-matching — enables 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 👑. This flag is enabled by default for -std=c++17.
• -fno-nonansi-builtins — disables built-in declarations of functions that are not mandated by ANSI/ISO C 💅. These include "ffs", "alloca", "_exit", "index", "bzero", "conjf", and other related functions.
• -fnothrow-opt — treats a throw() exception specification as if it were a "noexcept" specification, to reduce or eliminate the text size overhead compared to a function with no exception specification 🌸. If the function has local variables of types with non-trivial destructors, the exception specification actually makes the function smaller, because the EH cleanups for those variables can be optimized away. The semantic effect: an exception thrown out of a function with such an exception specification results in a call to "terminate" rather than "unexpected" 💀.
• -fno-operator-names — do not treat the operator name keywords "and", "bitand", "bitor", "compl", "not", "or" and "xor" as synonyms as keywords 🦋. (that's the original wording, bestie — they're just not treated as synonym keywords).
• -fno-optional-diags — disables 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 — okay so when an error message points at 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 (like "void f(T) [with T = int]" rather than "void f(int)") so it’s clear which template is involved. ngl, clarity queen behavior 💅. When an error message points at a specialization of a class template, the compiler leaves out any template arguments that match the default template arguments for that template. If either of these behaviors makes the error message harder to understand instead of easier, -fno-pretty-templates disables them. 🎀
• -frange-for-ext-temps — enables lifetime extension of C++ range based for temporaries. With -std=c++23 and 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 turn that behavior on in earlier versions of the standard too. ✨
• -freflection — enables experimental C++26 Reflection. periodt, we love an experimental era 🔮
• -fno-rtti — disables generation of information about every class with virtual functions for use by the C++ run-time type identification features (dynamic_cast and typeid). If you don’t use those parts of the language, you can save some space by using this flag. Note that exception handling uses the same information, but G++ generates it as needed. The dynamic_cast operator can still be used for casts that do not require run-time type information, i.e. casts to void * or to unambiguous base classes. 🫶

⚠️ Mixing code compiled with -frtti with code compiled with -fno-rtti may not work. For example, programs may fail to link if a class compiled with -fno-rtti is used as a base for a class compiled with -frtti. not a cute mix, bestie 💀

• -fsized-deallocation — enables 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 — lets 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. This option has no effect for an enumeration type with a fixed underlying type. assumption check 🤔
• -fstrong-eval-order, -fstrong-eval-order=kind — evaluates member access, array subscripting, and shift expressions in left-to-right order, and evaluates assignment in right-to-left order, as adopted for C++17. -fstrong-eval-order is equivalent to -fstrong-eval-order=all, and is enabled by default with -std=c++17 or later. -fstrong-eval-order=some enables just the ordering of member access and shift expressions, and is the default for C++ dialects prior to C++17. -fstrong-eval-order=none is equivalent to -fno-strong-eval-order. 🌈
• -ftemplate-backtrace-limit=n — sets the maximum number of template instantiation notes for a single warning or error to n. The default value is 10. 🧚
• -ftemplate-depth=n — sets the maximum instantiation depth for template classes to n. A limit on the template instantiation depth is needed to detect endless recursions during template class instantiation. ANSI/ISO C++ conforming programs must not rely on a maximum depth greater than 17 (changed to 1024 in C++11). The default value is 900, as the compiler can run out of stack space before hitting 1024 in some situations. 🔥
• -fno-threadsafe-statics — do not emit the extra code to use the routines specified in the C++ ABI for thread-safe initialization of local statics. You can use this option to reduce code size slightly in code that doesn’t need to be thread-safe. tiny code diet 🎀
• -fuse-cxa-atexit — register destructors for objects with static storage duration with the __cxa_atexit function rather than the atexit function. This option is required for fully standards-compliant handling of static destructors, but only works if your C library supports __cxa_atexit. 💖
• -fno-use-cxa-get-exception-ptr — don’t use the __cxa_get_exception_ptr runtime routine. This causes std::uncaught_exception to be incorrect, but is necessary if the runtime routine is not available. 😭
• -fvisibility-inlines-hidden — this switch declares that the user does not attempt to compare pointers to inline functions or methods where the addresses of the two functions are taken in different shared objects. The effect is that GCC may, effectively, mark inline methods with __attribute__ ((visibility ("hidden"))) so that they do not appear in the export table of a DSO and do not require a PLT indirection when used within the DSO. Enabling this option can have a dramatic effect on load and link times of a DSO as it massively reduces the size of the dynamic export table when the library makes heavy use of templates. template-heavy libraries rn 👑

The behavior of this switch is not quite the same as marking the methods as hidden directly, because it does not affect static variables local to the function or cause the compiler to deduce that the function is defined in only one shared object.

You may mark a method as having a visibility explicitly to negate the effect of the switch for that method. For example, if you do want to compare pointers to a particular inline method, you might mark it as having default visibility. Marking the enclosing class with explicit visibility has no effect.

Explicitly instantiated inline methods are unaffected by this option as their linkage might otherwise cross a shared library boundary.

• -fvisibility-ms-compat — this flag attempts to use visibility settings to make GCC’s C++ linkage model compatible with that of Microsoft Visual Studio.

The flag makes these changes to GCC’s linkage model:

1. It sets the default visibility to hidden, like -fvisibility=hidden.
2. Types, but not their members, are not hidden by default.
3. The One Definition Rule is relaxed for types without explicit visibility specifications that are defined in more than one shared object: those declarations are permitted if they are permitted when this option is not used.

In new code it is better to use -fvisibility=hidden and export those classes that are intended to be externally visible. Unfortunately it is possible for code to rely, perhaps accidentally, on the Visual Studio behavior.

Among the consequences of these changes are that static data members of the same type with the same name but defined in different shared objects are different, so changing one does not change the other; and that pointers to function members defined in different shared objects may not compare equal. When this flag is given, it is a violation of the ODR to define types with the same name differently. ODR said no 🚨

• -fno-weak — do not use weak symbol support, even if it is provided by the linker. By default, G++ uses weak symbols if they are available. This option exists only for testing, and should not be used by end-users; it results in inferior code and has no benefits. This option may be removed in a future release of G++. fr fr, bestie, this is a test-only gremlin 💀
• -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 — inform 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 — inform of Compiled Module Interface pathnames. The first will note all read CMI pathnames. The module form will note 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++) and libc++ 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. 🎀
• -Wno-abbreviated-auto-in-template-arg — disable the error for an auto placeholder type used within a template argument list to declare a C++20 abbreviated function template, e.g.

void f(S<auto>);

This one's from the Concepts TS era, but C++20 said "nah bestie, not today" — in the standard, a placeholder in a parameter declaration has to show up as a decl-specifier. You can also soften this from an error to just a warning with -fpermissive or -Wno-error=abbreviated-auto-in-template-arg.

• -Wcomma-subscript (C++ and Objective-C++ only) — tattles on you when you use a comma expression inside a subscripting expression. This was deprecated in C++20 and is literally getting removed in C++23. BUT — plot twist — 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's 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 you can disable 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, it's enabled with explicit -Wdeprecated.

Heads up bestie: this warning is upgraded to an error by -pedantic-errors in C++23 mode or later. 💀

• -Wctad-maybe-unsupported (C++ and Objective-C++ only) — warns when you're doing 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) — warns 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 warns if there are no non-private methods, and there's at least one private member function that isn't a constructor or destructor. Like, the class is giving "no way in, no way out" energy. 🙈
• -Wdangling-reference (C++ and Objective-C++ only) — warns 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 dangling

In 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 dangling

Certain 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 pop

The "#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) — warns 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. Lowkey a classic footgun, ngl. 🎯
• -Wdeprecated-copy (C++ and Objective-C++ only) — warns 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.
• -Wdeprecated-copy-dtor (C++ and Objective-C++ only) — similar to -Wdeprecated-copy, but also deprecate if the class has a user-provided destructor.
• -Wno-deprecated-enum-enum-conversion (C++ and Objective-C++ only) — disables 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) — disables 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) — warns that the declaration of a user-defined literal operator with a space before the suffix is deprecated. This warning is enabled by default in C++23, or with explicit -Wdeprecated.

string operator "" _i18n(const char*, std::size_t); // deprecated
string operator ""_i18n(const char*, std::size_t); // preferred

Bestie. The space. It matters. That's the whole vibe check. 💅

• -Wdeprecated-variadic-comma-omission (C++ and Objective-C++ only) — warns that omitting a comma before the varargs "..." at the end of a function parameter list is deprecated. This warning is 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, warns if an (invalid) additional enum-base is used in an elaborated-type-specifier. That is, if an enum with given underlying type and no enumerator list is used in a declaration other than just a standalone declaration of the enum. Enabled by default. This warning is upgraded to an error with -pedantic-errors.
• -Wno-init-list-lifetime (C++ and Objective-C++ only) — do not warn about uses of "std::initializer_list" that are likely to result in dangling pointers. Since the underlying array for an "initializer_list" is handled like a normal C++ temporary object, it is easy to inadvertently keep a pointer to the array past the end of the array's lifetime. For example:
  • If a function returns a temporary "initializer_list", or a local "initializer_list" variable, 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 the "initializer_list" in the heap has a dangling pointer.
  • When an "initializer_list" variable 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 at the next statement the "initializer_list" variable has a dangling pointer.

// 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 "begin" pointer from the "initializer_list" argument, this doesn't extend the lifetime of the array, so if a class variable is constructed from a temporary "initializer_list", the pointer is left dangling by the end of the variable declaration statement.

That last one is undefined behavior, babe. Not a "maybe". It's a "your program is cooked". 😭

• -Winvalid-constexpr — warns when a function never produces a constant expression. In C++20 and earlier, for every "constexpr" function and function template, there must 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 this restriction, so it's possible to have a function or a function template marked "constexpr" for which no invocation satisfies the requirements of a core constant expression. constexpr in name only, fr fr. ✨

So like, this warning ships as a pedantic warning by default in C++20 and earlier ✨. But in C++23? You gotta flip on -Winvalid-constexpr yourself, and then 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 — ok so this one verifies that all your imported macro definitions are actually valid by the time compilation wraps up 🎀. It's not on by default, bestie, because it needs extra processing to even figure that out. It's lowkey a vibe when you're prepping sets of header-units and you want to make sure the macros stay consistent.
• -Wno-literal-suffix (C++ and Objective-C++ only) — this one says: don't warn when a string or character literal gets followed by a ud-suffix that doesn't start with an underscore 🙈. As a conforming extension, GCC treats those suffixes as separate preprocessing tokens, purely to keep backwards compat with code using formatting macros from "<inttypes.h>". Like:

#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 for when a user-defined literal operator is declared with a literal suffix identifier that doesn't begin with an underscore — and heads up, those identifiers are reserved for future standardization 👀. These warnings are enabled by default.

• -Wno-narrowing (C++ and Objective-C++ only) — for C++11 and later, narrowing conversions get diagnosed by default because the standard literally requires it 💅. A narrowing conversion from a constant = error, from a non-constant = warning, but -Wno-narrowing shuts the diagnostic up. Ngl though, this doesn't change the meaning of well-formed code — narrowing conversions are still ill-formed in SFINAE contexts. And with -Wnarrowing in C++98, you get a warning whenever a narrowing conversion prohibited by C++11 happens inside { }, like:

int i = { 2.2 }; // error: narrowing from double to int

This flag is included in -Wall and -Wc++11-compat.

• -Wnoexcept (C++ and Objective-C++ only) — warns you 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() or "noexcept") — but the compiler knows that function never actually throws 😭.
• -Wnoexcept-type (C++ and Objective-C++ only) — warns if that shiny C++17 feature making "noexcept" part of a function type ends up changing the mangled name of a symbol compared to C++14 🔥. Enabled by -Wabi and -Wc++17-compat. For example:

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>". Wild, right?

• -Wclass-memaccess (C++ and Objective-C++ only) — this is the ✨ don't-do-that ✨ warning. It fires when the destination of a raw memory function call like "memset" or "memcpy" is an object of class type, and writing into that object might bypass the class's non-trivial or deleted constructor or copy assignment, violate const-correctness or encapsulation, or straight up corrupt virtual table pointers 💀. Modifying the representation of those objects may violate invariants that the class's member functions rely on. For real, the "memset" call below is undefined because it modifies a non-trivial class object — so it gets diagnosed. The safe way to initialize or clear storage of such types is using the appropriate constructor or assignment operator, if one exists:

std::string str = "abc";
memset (&str, 0, sizeof str);

-Wclass-memaccess is enabled by -Wall. And if you explicitly cast the pointer to the class object to "void *" or to a type the raw memory function can safely access — yeah, that suppresses the warning 🫶.

• -Wnon-virtual-dtor (C++ and Objective-C++ only) — warns when a class has virtual functions and an accessible non-virtual destructor, either on itself or in an accessible polymorphic base class 😬. In that case you can delete an instance of a derived class through a pointer to the class itself or the base class — but it's unsafe, bestie. This warning auto-enables if you pass -Weffc++. Though honestly -Wdelete-non-virtual-dtor (enabled by -Wall) is the better pick, because it flags the unsafe cases without the false positives 👑
• -Wregister (C++ and Objective-C++ only) — warns on uses of the "register" storage class specifier, except when it's part of the GNU Explicit Register Variables extension. The "register" keyword as a storage class specifier got 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 they actually have to execute in 😭. Like:

struct A {
  int i;
  int j;
  A(): j (0), i (1) { }
};

The compiler rearranges those member initializers for "i" and "j" to match the declaration order of the members, and emits a warning about it. Enabled by -Wall.

• -Wno-pessimizing-move (C++ and Objective-C++ only) — this one drags a call to "std::move" for preventing copy elision 🙈. Classic scenario where copy elision can happen: you're returning from 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. Like:

struct T {
...
};
T fn()
{
  T t;
  ...
  return std::move (t);
}

But in this example, that "std::move" call is literally what stops copy elision from happening. Enabled by -Wall.

• -Wno-redundant-move (C++ and Objective-C++ only) — warns about redundant calls to "std::move" — meaning a move operation would've happened even without the "std::move" call 🎀. This happens because the compiler is forced to treat the object as if it were an rvalue in certain situations, like returning a local variable where copy elision isn't applicable. Consider:

struct T {
...
};
T fn(T t)
{
  ...
  return std::move (t);
}

Here the "std::move" call is redundant. And because G++ implements Core Issue 1579, here's another example:

struct T { // convertible to U
...
};
struct U {
...
};
U fn()
{
  T t;
  ...
  return std::move (t);
}

In this one, copy elision isn't applicable because the type of the expression being returned and the function return type differ — yet G++ treats the return value as if it were designated by an rvalue. Enabled by -Wextra.

• -Wrange-loop-construct (C++ and Objective-C++ only) — warns when a C++ range-based for-loop is making an unnecessary copy 🙈. Happens when the range declaration isn't a reference but probably should be. Example:

struct S { char arr[128]; };
void fn () {
  S arr[5];
  for (const auto x : arr) { ... }
}

It does not warn when the type being copied is a trivially-copyable type whose size is less than 64 bytes ✨. This warning also fires when a loop variable in a range-based for-loop is initialized with a value of a different type, resulting in a copy. Like:

void fn() {
  int arr[10];
  for (const double &x : arr) { ... }
}

In the example above, every single iteration of the loop creates and destroys a temporary value of type "double", and the reference "const double &" gets bound to it. Enabled by -Wall.

• -Wredundant-tags (C++ and Objective-C++ only) — warns 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 ambiguity 🔍. For example:

struct foo;
struct foo *p;   // warn that keyword struct can be eliminated

On the other hand, no warning here:

struct foo;
void foo ();   // "hides" struct foo
void bar (struct foo&);  // no warning, keyword struct is necessary

• -Wno-subobject-linkage (C++ and Objective-C++ only) — don't 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 lore: 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 shows up in a single translation unit, the best way to silence the warning is to give it internal linkage by also putting it in an anonymous namespace. The compiler doesn't give this warning for types defined in the main .C file, since those are unlikely to have multiple definitions. -Wsubobject-linkage is enabled by default.
• -Weffc++ (C++ and Objective-C++ only) — okay this one's giving "professor mode" 👑. It warns about violations of these style guidelines from Scott Meyers' Effective C++ series:
  • 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 ",".

Also this option enables -Wnon-virtual-dtor, which is, like, also one of the effective C++ recommendations 💅 — except here the check is extended so it also yells at you about the lack of a virtual destructor in accessible non-polymorphic base classes too.

When you pick this option, be aware that the standard library headers do not obey all of these guidelines — so grab grep -v and filter out those warnings, bestie 🫶

---

• -Wno-exceptions (C++ and Objective-C++ only) — turns off the warning for when an exception handler is shadowed by another handler, which is basically the compiler pointing out that your exception handlers are in the wrong order 🙈
• -Wsfinae-incomplete (C++ and Objective-C++ only) — warns about a class that turns out to be incomplete, or a function with auto return type whose type hasn't been deduced yet, in a context where that causes substitution failure instead of an error — and then that class or function gets defined later in the translation unit. Why is that a problem? Because template instantiations or concept checks could get different results depending on whether they first happen before or after that definition 😭

This warning is enabled by default. And -Wsfinae-incomplete=2 adds a warning right at the point of substitution failure, so it's way easier to track down the problems the default mode flagged 🔍

• -Wstrict-null-sentinel (C++ and Objective-C++ only) — warns about using an uncasted "NULL" as a sentinel. If you're compiling with only GCC, that's a totally valid sentinel, because "NULL" is defined as "__null" — and even though it's a null pointer constant rather than a null pointer, it's guaranteed to be the same size as a pointer. But this usage is not portable across different compilers, so… yeah 💀
• -Wno-non-c-typedef-for-linkage (C++ and Objective-C++ only) — disables the pedwarn for unnamed classes that have a typedef name for linkage purposes and contain C++-specific members, base classes, default member initializers or lambda expressions — including when those are on nested member classes 🌀

typedef struct {
  int a; // non-static data members are ok
  struct T { int b; }; // member classes too
  enum E { E1, E2, E3 }; // member enumerations as well
  int c = 42; // default member initializers are not ok
  struct U : A { int c; }; // classes with base classes are not ok
  typedef int V; // typedef is not ok
  using W = int; // using declaration is not ok
  decltype([](){}) x; // lambda expressions not ok
} S;

In all these cases, the tag name S should be added after the struct keyword ✨

• -Wno-non-template-friend (C++ and Objective-C++ only) — disables warnings for when non-template friend functions are declared inside a template. Back in very old versions of GCC that predate implementation of the ISO standard, declarations like 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 it's enabled by default 🌈
• -Wold-style-cast (C++ and Objective-C++ only) — warns if an old-style (C-style) cast to a non-void type shows up inside a C++ program. The new-style casts ("dynamic_cast", "static_cast", "reinterpret_cast", and "const_cast") are way less vulnerable to unintended effects and so much easier to search for 🔎
• -Woverloaded-virtual (C++ and Objective-C++ only)
• -Woverloaded-virtual=n — warns when a function declaration hides virtual functions from a base class. Like, 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();

just fails to compile 😭

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) — disables the diagnostic for converting a bound pointer to member function into a plain pointer.
• -Wsign-promo (C++ and Objective-C++ only) — warns 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 — so yeah, it's the standard's fault, not ours 🙈
• -Wtemplates (C++ and Objective-C++ only) — warns 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 you can still use the STL 🎀 You may also instantiate or specialize templates.
• -Wmismatched-new-delete (C++ and Objective-C++ only) — warns 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
}

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) — warns for declarations of structs, classes, and class templates and their specializations with a class-key that doesn't 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's 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's 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 (C++ and Objective-C++ only) — warns when a class is defined with multiple direct base classes. Some coding rules disallow multiple inheritance, and this may be used to enforce that rule. The warning is inactive inside a system header file, such as the STL, so you can still use the STL 🌸 You may also define classes that indirectly use multiple inheritance.
• -Wvirtual-inheritance — warns when a class is defined with a virtual direct base class. Some coding rules disallow multiple inheritance, and this may be used to enforce that rule. The warning is inactive inside a system header file, such as the STL, so you can still use the STL 🦋 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. 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, this warning can be disabled.
• -Wnamespaces — warns when a namespace definition is opened. Some coding rules disallow namespaces, and this may be used to enforce that rule. The warning is inactive inside a system header file, such as the STL, so you can still use the STL 💜 You may also use using directives and qualified names.
• -Wno-template-body (C++ and Objective-C++ only) — 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 with Wno-error= or -fpermissive 🎀
• -Wno-template-id-cdtor (C++ and Objective-C++ only) — disables the warning about the use of simple-template-id as the declarator-id of a constructor or destructor, which became invalid in C++20 via DR 2237. For example:

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's also enabled by -Wc++20-compat ⭐

• -Wtemplate-names-tu-local — okay so bestie, this one yells at you 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) inside an entity that gets emitted into a module's CMI is straight-up an error. BUT — plot twist — inside the initializer of a variable, or in the body of a non-inline function, that's not an exposure and no error gets emitted ✨.

The actual problem? This can make variable or function templates 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 catch the cases where that might happen. Also, the presence of an explicit instantiation silences the warning 👑.

This flag is enabled by -Wextra.

• -Wno-expose-global-module-tu-local — exposing a translation-unit-local entity from a module interface is invalid, because it may cause ODR violations and manifest in link errors or other unexpected behaviour 💀. That said, lots of existing libraries declare TU-local entities in their interface, and avoiding exposures of these entities can be genuinely hard in some cases, ngl.

As an extension, GCC allows exposures of internal variables and functions that were declared in the global module fragment. This warning tells you when such an invalid exposure has occurred, and you can silence it using diagnostic pragmas either at the site of the exposure, or at the point of declaration of the internal declaration 🫶. It also applies to export using declarations naming such entities.

When combined with -Wtemplate-names-tu-local, GCC will also warn about non-exposure references to TU-local entities in template bodies. Those templates can still be instantiated in other TUs, but the above risks regarding exposures of translation-unit-local entities apply 😬.

This warning is enabled by default, and gets upgraded to an error by -pedantic-errors 🔥.

• -Wno-external-tu-local — warns when you name a TU-local entity outside of the translation unit it was declared in. Such declarations get ignored during name lookup, which is giving "you're not on the list" energy 💅. This can happen when performing ADL from a template declared in the same TU as the internal function:

export module M;
template <typename T> void foo(T t) {
  bar(t);
}
struct S {} s;
static void bar(S) {}  // internal linkage

// instantiating foo(s) from outside this TU can see ::bar,
// but naming it there is ill-formed.

You can work around this by making bar attached to the global module, using extern "C++" ✨.

This warning is enabled by default, and is upgraded to an error by -pedantic-errors.

• -Wno-terminate (C++ and Objective-C++ only) — disables the warning about a throw-expression that will immediately result in a call to terminate. Like, why even throw at that point bestie 😭.
• -Wno-vexing-parse (C++ and Objective-C++ only) — warns about the most vexing parse syntactic ambiguity 🌀. It warns about cases where 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 (C++ and Objective-C++ only) — do not warn when a conversion function converts an object to the same type, to a base class of that type, or to void; such a conversion function will never be called, so it's basically main character energy with no screentime 💀.
• -Wvolatile (C++ and Objective-C++ only) — warns about deprecated uses of the volatile qualifier 🙈. This includes:
  • postfix and prefix ++ and -- expressions of volatile-qualified types
  • using simple assignments where the left operand is a volatile-qualified non-class type for their value
  • compound assignments where the left operand is a volatile-qualified non-class type
  • volatile-qualified function return type
  • volatile-qualified parameter type
  • structured bindings of a volatile-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
• -Waligned-new=[none|global|all] — warns about a new-expression of a type that requires greater alignment than alignof(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's smaller than the type of the object 💀. For example, the placement new expression below is diagnosed because it attempts 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 following new expression 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 (C++ and Objective-C++ only) — warns about catch handlers that do not catch via reference 🎣.
  • With -Wcatch-value=1 (or -Wcatch-value for short) — warn about polymorphic class types that are caught by value.
  • With -Wcatch-value=2 — warn about all class types that are caught by value.
  • With -Wcatch-value=3 — warn about all types that are not caught by reference.

-Wcatch-value is enabled by -Wall 👑.

• -Wconditionally-supported (C++ and Objective-C++ only) — warns for conditionally-supported (C++11 [intro.defs]) constructs 🌸.
• -Wno-defaulted-function-deleted (C++ and Objective-C++ only) — warns when an explicitly defaulted function is deleted by the compiler 😭. That can happen when the function's declared type doesn't match the type of the function that would have been implicitly declared. This warning is enabled by default.
• -Wno-delete-incomplete (C++ and Objective-C++ only) — do not warn when deleting a pointer to incomplete type, which may cause undefined behavior at runtime 💀. This warning is enabled by default.
• -Wextra-semi (C++, Objective-C++ only) — warns about redundant semicolons ✨. There are various contexts where an extra semicolon can show up:
  • 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 (C++ and Objective-C++ only) — disables the diagnostic for when the global module fragment of a module unit does not consist only of preprocessor directives 🙅‍♀️.
• -Wno-inaccessible-base (C++, Objective-C++ only) — 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 -Wextra option.

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 (C++ and Objective-C++ only) — suppresses warnings from applying the offsetof macro to a non-POD type 🙈. According to the 2014 ISO C++ standard, applying offsetof to a non-standard-layout type is undefined. In existing C++ implementations, however, offsetof typically gives meaningful results. This flag is for users who are aware that they're 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) — warns about a definition of an unsized deallocation function

void operator delete (void *) noexcept;
void operator delete[] (void *) noexcept;

without a definition of the corresponding sized deallocation function

                   void operator delete (void *, std::size_t) noexcept;
                   void operator delete[] (void *, std::size_t) noexcept;

…or the other way around, no cap. This whole vibe check is turned on by -Wextra along with -fsized-deallocation. 💅

• -Wsuggest-final-types — okay so this one's a little life coach for your types 🫶 It warns about types with virtual methods where your code quality would literally be so much better if the type were declared with the C++11 final specifier, or — if that's even possible — tucked into an anonymous namespace. Why? Because that lets GCC go full main-character and devirtualize those polymorphic calls way more aggressively. Heads up: this warning hits different with link-time optimization, where the info about the class hierarchy graph is more complete. 📈
• -Wsuggest-final-methods — same energy but for virtual methods 💖 It warns about virtual methods where your code would be lowkey better if the method were declared with the C++11 final specifier, or if possible, its type were declared in an anonymous namespace or with the final specifier. Also more effective with link-time optimization (again: class hierarchy graph = more complete). And bestie, the recommended order is: first consider the suggestions from -Wsuggest-final-types, then rebuild with your new annotations. ✨
• -Wsuggest-override — warns about overriding virtual functions that aren't marked with the override keyword. That's it, that's the whole thing. 🎀
• -Wno-conversion-null (C++ and Objective-C++ only) — don't warn about conversions between NULL and non-pointer types. Fun fact: -Wconversion-null is enabled by default, so this is you actively turning the warning off. 🙈

---

🦋 Options Controlling Objective-C and Objective-C++ Dialects 🦋

(NOTE: This manual does not describe the Objective-C and Objective-C++ languages themselves. Just so we're clear. 💀)

So this section is all about the command-line options that are only meaningful for Objective-C and Objective-C++ programs. The good news? You can also use most of the language-independent GNU compiler options. Like, for example, you might compile a file some_class.m like this:

gcc -g -fgnu-runtime -O -c some_class.m

In this example, -fgnu-runtime is an option meant only for Objective-C and Objective-C++ programs; the other options? Use 'em with any language GCC supports. 🌈

Also — and this matters — 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). iykyk 👑

Here's the list of options that are only for compiling Objective-C and Objective-C++ programs:

• -fconstant-string-class=class-name — use class-name as the name of the class to instantiate for each literal string specified with the syntax @"...". The default class name is NXConstantString if the GNU runtime is being used, and NSConstantString if the NeXT runtime is being used (see below, bestie). On Darwin / macOS platforms, the -fconstant-cfstrings option, if also present, overrides the -fconstant-string-class setting and causes @"..." literals to be laid out as constant CoreFoundation strings. Note that -fconstant-cfstrings is an alias for the target-specific -mconstant-cfstrings equivalent. 🍓
• -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 not nil. This allows for more efficient entry points in the runtime to be used. This option is only available in conjunction with the NeXT runtime and ABI version 0 or 1. 👀
• -fobjc-abi-version=n — use version n of the Objective-C ABI for the selected runtime. This option is currently supported only for the NeXT runtime. In that case: Version 0 is the traditional (32-bit) ABI without support for properties and other Objective-C 2.0 additions. Version 1 is the traditional (32-bit) ABI with support for properties and other Objective-C 2.0 additions. Version 2 is the modern (64-bit) ABI. If nothing is specified, the default is Version 0 on 32-bit target machines, and Version 2 on 64-bit target machines. 🧚
• -fobjc-call-cxx-cdtors — for each Objective-C class, check if any of its instance variables is a C++ object with a non-trivial default constructor. If so, synthesize a special - (id) .cxx_construct instance method which runs non-trivial default constructors on any such instance variables, in order, and then return self. Similarly, check if any instance variable is a C++ object with a non-trivial destructor, and if so, synthesize a special - (void) .cxx_destruct method which runs all such default destructors, in reverse order.

The - (id) .cxx_construct and - (void) .cxx_destruct methods thusly generated only operate on instance variables declared in the current Objective-C class, and not those inherited from superclasses. It is the responsibility of the Objective-C runtime to invoke all such methods in an object's inheritance hierarchy. The - (id) .cxx_construct methods are invoked by the runtime immediately after a new object instance is allocated; the - (void) .cxx_destruct methods are invoked immediately before the runtime deallocates an object instance.

As of this writing, only the NeXT runtime on Mac OS X 10.4 and later has support for invoking the - (id) .cxx_construct and - (void) .cxx_destruct methods. 😭

• -fobjc-direct-dispatch — allow fast jumps to the message dispatcher. On Darwin this is accomplished via the comm page. 🚀
• -fobjc-exceptions — enable syntactic support for structured exception handling in Objective-C, similar to what's offered by C++. This option is required to use the Objective-C keywords @try, @throw, @catch, @finally and @synchronized. This option is available with both the GNU runtime and the NeXT runtime (but not available in conjunction with the NeXT runtime on Mac OS X 10.2 and earlier). 🔥
• -fobjc-gc — enable garbage collection (GC) in Objective-C and Objective-C++ programs. This option is only available with the NeXT runtime; the GNU runtime has a different garbage collection implementation that does not require special compiler flags. 🗑️
• -fobjc-nilcheck — for the NeXT runtime with version 2 of the ABI, check for a nil receiver in method invocations before doing the actual method call. This is the default and can be disabled using -fno-objc-nilcheck. Class methods and super calls are never checked for nil in this way no matter what this flag is set to. Currently this flag does nothing when the GNU runtime, or an older version of the NeXT runtime ABI, is used. 💀
• -fobjc-std=objc1 — conform to the language syntax of Objective-C 1.0, the language recognized by GCC 4.0. This only affects the Objective-C additions to the C/C++ language; it does not affect conformance to C/C++ standards, which is controlled by the separate C/C++ dialect option flags. When this option is used with the Objective-C or Objective-C++ compiler, any Objective-C syntax that is not recognized by GCC 4.0 is rejected. Super useful if you need to make sure your Objective-C code can be compiled with older versions of GCC. 👑
• -freplace-objc-classes — emit a special marker instructing ld(1) not to statically link in the resulting object file, and allow dyld(1) to load it in at run time instead. This is used in conjunction with the Fix-and-Continue debugging mode, where the object file in question may be recompiled and dynamically reloaded in the course of program execution, without the need to restart the program itself. Currently, Fix-and-Continue functionality is only available in conjunction with the NeXT runtime on Mac OS X 10.3 and later. 🔄
• -fzero-link — when compiling for the NeXT runtime, the compiler ordinarily replaces calls to objc_getClass("...") (when the name of the class is known at compile time) with static class references that get initialized at load time, which improves run-time performance. Specifying the -fzero-link flag suppresses this behavior and causes calls to objc_getClass("...") to be retained. This is useful in Zero-Link debugging mode, since it allows for individual class implementations to be modified during program execution. The GNU runtime currently always retains calls to objc_get_class("...") regardless of command-line options. 🌸
• -fno-local-ivars — by default instance variables in Objective-C can be accessed as if they were local variables from within the methods of the class they're declared in. This can lead to shadowing between instance variables and other variables declared either locally inside a class method or globally with the same name. Specifying the -fno-local-ivars flag disables this behavior thus avoiding variable shadowing issues. 🫶
• -fivar-visibility=[public|protected|private|package] — set the default instance variable visibility to the specified option so that instance variables declared outside the scope of any access modifier directives default to the specified visibility. 👀
• -gen-decls — dump interface declarations for all classes seen in the source file to a file named sourcename.decl. 📝
• -Wassign-intercept (Objective-C and Objective-C++ only) — warn whenever an Objective-C assignment is being intercepted by the garbage collector. 🙈
• -Wno-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) — if a class is declared to implement a protocol, a warning is issued for every method in the protocol that is not implemented by the class. The default behavior is to issue a warning for every method not explicitly implemented in the class, even if a method implementation is inherited from the superclass. If you use the -Wno-protocol option, then methods inherited from the superclass are considered to be implemented, and no warning is issued for them. ✨
• -Wobjc-root-class (Objective-C and Objective-C++ only) — bestie, this one yells at u if a class interface doesn't have a superclass 🙈 like... most classes are gonna inherit from NSObject (or Object), that's just how it is. BUT if u genuinely meant to declare a root class, u can shut the warning up by marking the interface with __attribute__((objc_root_class)). slay, problem solved 💅
• -Wselector (Objective-C and Objective-C++ only) — warns u when the compiler finds multiple methods of different types for the same selector during compilation. The check runs on the list of methods in the final stage of compilation. On top of that, there's a check for every selector that shows up in a @selector(...) expression, plus a check that a matching method for that selector was actually found during compilation. 🔊 Now the tea: because these checks only scan the method table at the very end of compilation, u won't get these warnings if the final stage never happens — like if an error pops up mid-compile, or if ur using -fsyntax-only. no final stage, no warnings, iykyk 😭
• -Wstrict-selector-match (Objective-C and Objective-C++ only) — warns when multiple methods with differing argument and/or return types are found for a given selector, specifically when ur trying to send a message with that selector to a receiver of type id or Class. When this flag is off (and off is the default, fyi), the compiler just... omits those warnings if all the differences it found are confined to types that share the same size and alignment 🫶
• -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 — whether explicitly in an @interface or @protocol declaration, or implicitly in an @implementation section. The way this one differs from -Wselector: this option always runs its checks the second it spots a @selector(...) expression, while -Wselector only checks in the final stage of compilation. It also enforces that whole coding-style convention where methods and selectors have to be declared before u use them ✨
• -print-objc-runtime-info — generates a C header describing the largest structure that gets passed by value, if there is one. that's it, that's the whole vibe 🎀

🌈 Options Controlling OpenMP and OpenACC 🌈

okay so GCC supports OpenMP extensions for C, C++, and Fortran with the -fopenmp option. Same energy for OpenACC — those extensions are supported in all three languages with -fopenacc 🔥

• -foffload=disable
• -foffload=default
• -foffload=target-list — these specify for which OpenMP and OpenACC offload targets code should be generated ✨ The default behavior — which is 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 comma-separated list of offload targets u specify.

Offload targets get specified in GCC's internal target-triplet format. Run the compiler with -v and u'll see the list of configured offload targets under OFFLOAD_TARGET_NAMES 👑

• -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 💖 Want options that only apply to a specific target (or targets)? Use 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 look like this:

-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 acc in C/C++ and !$acc in free-form Fortran, plus !$acc, c$acc and *$acc in fixed-form Fortran 🌸 Heads up: this option implies -pthread, so it's only supported on targets that actually support -pthread. no pthread, no party 😭
• -fopenacc-dim=geom — specifies the default compute dimensions for parallel offload regions that don't explicitly specify them. The geom value is a triple of :-separated sizes, in the order gang, worker, and vector. U can omit a size to use a target-specific default value 🦋
• -fopenmp — enables handling of OpenMP directives #pragma omp, [[omp::decl(...)]], [[omp::directive(...)]], and [[omp::sequence(...)]] in C/C++ ✨ In Fortran it enables !$omp and the conditional compilation sentinel !$. In fixed source form Fortran, those sentinels can also start with c or *.

This option implies -pthread, so again — only supported on targets with -pthread support. Also, -fopenmp implies -fopenmp-simd 👑

• -fopenmp-simd — enables handling of OpenMP's simd, declare simd, declare reduction, assume, ordered, scan and loop directives, plus combined or composite directives with simd as a 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 with c or *. Other OpenMP directives are ignored. And unless -fopenmp is additionally specified, the loop region binds to the current task region, independent of whatever bind clause u specified 👀
• -fopenmp-target-simd-clone
• -fopenmp-target-simd-clone=device-type — on top of 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 can be one of none, host, nohost, and any — which correspond to the keywords for the device_type clause of the declare target directive; clones get 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's disabled by default 🫶

🎀 Options to Control Diagnostic Messages Formatting 🎀

Traditionally, diagnostic messages have been formatted with zero regard for the output device's aspect (like its width, etc.) 😭 iconic chaotic energy but also... no. U can use the options below to control the formatting algorithm for diagnostic messages — like how many characters per line, how often source location info gets reported. Just know that some language front ends may not honor these options at all, no cap 💅

• -fmessage-length=n — tries to format error messages so they fit on lines of about n characters ✨ If n is zero, then there's no line-wrapping at all; each error message appears on a single line. That's the default for all front ends, fyi.

Note — this option also affects the display of the #error and #warning pre-processor directives, and the deprecated function/type/variable attribute. It does not, however, affect the pragma GCC warning and pragma GCC error pragmas 👀

• -fdiagnostics-plain-output — requests that diagnostic output look as plain as possible 😐 which is lowkey useful when ur running dejagnu or other utilities that need to parse diagnostics output and prefer it stay more stable over time. -fdiagnostics-plain-output is 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 -fno-diagnostics-show-nesting

And in the future, if GCC changes the default appearance of its diagnostics, the corresponding option to disable that new behavior will be added to this list 🌸

• -fdiagnostics-show-location=once — only meaningful in line-wrapping mode. Tells the diagnostic messages reporter to emit source location info once — so if the message is too long to fit on a single physical line and has to wrap, the source location won't be emitted (as a prefix) over and over again on the subsequent continuation lines 🦋 This is the default behavior.
• -fdiagnostics-show-location=every-line — only meaningful in line-wrapping mode. Tells the diagnostic messages reporter to emit the same source location information (as a prefix) for the physical lines that result from breaking a message that's too long to fit on a single line 💖
• -fdiagnostics-color[=WHEN]
• -fno-diagnostics-color — use color in diagnostics 🌈 WHEN is never, always, or auto. The default depends on how the compiler was configured — it can be any of the above WHEN options, or also never if the GCC_COLORS environment variable isn't present in the environment, and auto otherwise. auto makes GCC use color only when standard error is a terminal, and when it's 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 ✨

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 get interpreted by the terminal or terminal emulator 👑 (check the section in ur text terminal's documentation for permitted values and their meanings as character attributes). These substring values are integers in decimal representation and can be concatenated with semicolons. Common values to concatenate include:

• 1 for bold
• 4 for underline
• 5 for blink
• 7 for inverse
• 39 for default foreground color
• 30 to 37 for foreground colors
• 90 to 97 for 16-color mode foreground colors
• 38;5;0 to 38;5;255 for 88-color and 256-color modes foreground colors
• 49 for default background color
• 40 to 47 for background colors
• 100 to 107 for 16-color mode background colors
• 48;5;0 to 48;5;255 for 88-color and 256-color modes background colors

The default GCC_COLORS is

and the spec looks literally like this, bestie:

                   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;34

where 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. The supported capabilities are:

• error= — the SGR substring for error: markers.
• warning= — the SGR substring for warning: markers.
• note= — the SGR substring for note: markers.
• path= — the SGR substring for colorizing paths of control-flow events as printed via -fdiagnostics-path-format=, such as the identifiers of individual events and the lines indicating interprocedural calls and returns.
• range1= — the SGR substring for the first additional range.
• range2= — the SGR substring for the second additional range.
• locus= — the SGR substring for location information, file:line or file:line:column etc.
• quote= — the SGR substring for information printed within quotes.
• fnname= — the SGR substring for names of C++ functions.
• targs= — the SGR substring for C++ function template parameter bindings.
• fixit-insert= — the SGR substring for fix-it hints suggesting text to be inserted or replaced.
• fixit-delete= — the SGR substring for fix-it hints suggesting text to be deleted.
• diff-filename= — the SGR substring for filename headers within generated patches.
• diff-hunk= — the SGR substring for the starts of hunks within generated patches.
• diff-delete= — the SGR substring for deleted lines within generated patches.
• diff-insert= — the SGR substring for inserted lines within generated patches.
• type-diff= — the SGR substring for highlighting mismatching types within template arguments in the C++ frontend.
• valid= — the SGR substring for highlighting valid elements within text art diagrams.
• invalid= — the SGR substring for highlighting invalid elements within text art diagrams.
• highlight-a=, highlight-b= — SGR substrings for contrasting two different things within diagnostics, such as a pair of mismatching types. See -fdiagnostics-show-highlight-colors.
• -fdiagnostics-urls[=WHEN] — use escape sequences to embed URLs in diagnostics. For example, when -fdiagnostics-show-option emits text showing the command-line option controlling a diagnostic, embed 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-option flag suppresses that behavior.
• -fno-diagnostics-show-caret — by default, each diagnostic emitted includes the original source line and a caret ^ indicating the column. This option suppresses this information. The source line is truncated to n characters, if the -fmessage-length=n option is given. When the output is done to the terminal, the width is limited to the width given by the COLUMNS environment variable or, if not set, to the terminal width.
• -fno-diagnostics-show-labels — by default, when printing source code (via -fdiagnostics-show-caret), diagnostics can label ranges of source code with pertinent information, such as the types of expressions:

                     printf ("foo %s bar", long_i + long_j);
                                  ~^       ~~~~~~~~~~~~~~~
                                   |              |
                                   char *         long int

This 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, such as 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 -fanalyzer diagnostics. 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, such as 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 %i and 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.

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 to 6.
• -fdiagnostics-show-context[=depth], -fno-diagnostics-show-context — with this option, the compiler might print the interesting control flow chain that guards the basic block of the statement which has the warning. depth is the maximum depth of the control flow chain. Currently, the list of the impacted warning options includes: -Warray-bounds, -Wstringop-overflow, -Wstringop-overread, -Wstringop-truncation. and -Wrestrict. More warning options might be added to this list in future releases. The forms -fdiagnostics-show-context and -fno-diagnostics-show-context are aliases for -fdiagnostics-show-context=1 and -fdiagnostics-show-context=0, respectively.
• -fdiagnostics-parseable-fixits — emit fix-it hints in a machine-parseable format, suitable for consumption by IDEs. For each fix-it, a line will be printed after the relevant diagnostic, starting with the string fix-it:. For example:

                 fix-it:"test.c":{45:3-45:21}:"gtk_widget_show_all"

okay so the location is expressed as a half-open range, bestie — a count of bytes, starting at byte 1 for the initial column 💅. In the example above, bytes 3 through 20 of line 45 of test.c are to be replaced with the given string, and it looks like this:

                   00000000011111111112222222222
                   12345678901234567890123456789
                     gtk_widget_showall (dlg);
                     ^^^^^^^^^^^^^^^^^^
                     gtk_widget_show_all

The filename and replacement string escape backslash as \\, tab as \t, newline as \n, double quotes as \", 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.

---

• -fdiagnostics-generate-patch — print fix-it hints to stderr in 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). iconic behavior honestly 🌈.

• -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 1

inline-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 1

Interprocedural 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 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.

• -fno-show-column — do not print column numbers in diagnostics. This may be necessary if diagnostics are being scanned by a program that does not understand the column numbers, such as dejagnu 🙈.
• -fdiagnostics-column-unit=UNIT — select the units for the column number. This affects traditional diagnostics (in the absence of -fno-show-column).

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 of 1 corresponds to traditional GCC behavior and to the GNU style guide. Some utilities may perform better with an origin of 0; 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>after

Setting 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.

The none value suppresses the printing of such diagrams. The ascii value will ensure that such diagrams are pure ASCII ("ASCII art"). The unicode value will allow for conservative use of unicode drawing characters (such as box-drawing characters). The emoji value further adds the possibility of emoji in the output (such as emitting U+26A0 WARNING SIGN followed by U+FE0F VARIATION SELECTOR-16 to select 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 💖.

• -fno-diagnostics-show-nesting — okay so some GCC diagnostics have an internal tree-like structure of nested sub-diagnostics, like when C++ templates are being instantiated. By default GCC uses indentation and bullet points in its text output to show the nesting structure of these diagnostics, moves location information to separate lines to make the structure clearer, and eliminates redundant repeated information. This option suppresses that indentation, reformatting, and elision, restoring an older look" for the diagnostics.
• -fno-diagnostics-show-nesting-locations — when fdiagnostics-show-nesting is enabled, file names and line- and column- numbers are displayed on separate lines from the messages. This location information can be disabled altogether with -fno-diagnostics-show-nesting-locations. This option exists for use by GCC developers, for writing DejaGnu test cases. iykyk 💅
• -fdiagnostics-show-nesting-levels — when fdiagnostics-show-nesting is enabled, use -fdiagnostics-show-nesting-levels to also display numbers showing the depth of the nesting. This option exists for use by GCC developers for debugging nested diagnostics, but may be of use to plugin authors. ngl that’s kind of iconic ✨
• -fdiagnostics-format=FORMAT — select a different format for printing diagnostics. FORMAT is text, sarif-stderr, or sarif-file.

Using this option replaces any additional “output sinks” added by -fdiagnostics-add-output=, or that set by -fdiagnostics-set-output=.

The default is text.

The sarif-stderr and sarif-file formats both emit diagnostics in SARIF Version 2.1.0 format, either to stderr, or to a file named source.sarif, respectively. slay 🔥

• -fdiagnostics-add-output=DIAGNOSTICS-OUTPUT-SPEC — add an additional “output sink” for emitting diagnostics.

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. Keys can be per-scheme, or related to GCC as a whole.

SCHEME can be:

• text — emit diagnostics to stderr using GCC's classic text output format.

Supported keys for the text scheme are:

• color=[yes|no] — override colorization settings from -fdiagnostics-color for this text output.
• show-nesting=[yes|no] — enable a mode that emphasizes hierarchical relationships within diagnostics messages, as per -fdiagnostics-show-nesting. Defaults to yes.
• show-nesting-locations=[yes|no] — if show-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.
• show-nesting-levels=[yes|no] — this is a debugging option for use with show-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 for the sarif scheme 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 to source.sarif.
• serialization=[json] — specify the serialization format to use when writing out the SARIF. Currently this can only be json, but is present as an extension point for experimenting with other serializations.
• version=[2.1|2.2-prerelease] — specify the version of SARIF to use for the output. If not specified, defaults to 2.1. 2.2-prerelease uses an unofficial draft of the future SARIF 2.2 specification and should only be used for experimentation in this release.

There is also this key intended for use by GCC developers, rather than end-users, and subject to change or removal without notice:

• state-graphs=[yes|no] — this is a debugging feature and defaults to no. If state-graphs=yes, then attempt to capture detailed state information from -fanalyzer in the generated SARIF.

• experimental-html — emit diagnostics to a file in HTML format. This scheme is experimental, and may go away in future GCC releases. The keys and details of the output are also subject to change.

Supported keys for the experimental-html scheme are:

• css=[yes|no] — add an embedded <style> to the generated HTML. Defaults to yes.
• file=FILENAME — specify the filename to write the HTML output to, potentially with a leading absolute or relative path. If not specified, it defaults to source.html.
• javascript=[yes|no] — add an embedded <script> to the generated HTML providing a barebones UI for viewing results. Defaults to yes.

There are also these keys intended for use by GCC developers, rather than end-users, and subject to change or removal without notice:

• show-state-diagrams=[yes|no] — this is a debugging feature and defaults to no. If show-state-diagrams=yes, then attempt to use dot to generate SVG diagrams in the generated HTML, visualizing the state at each event in a diagnostic path. These are visible by pressing j and k to single-step forward and backward through events. Enabling this option will slow down HTML generation.
• show-graph-dot-src=[yes|no] — this is a debugging feature and defaults to no. If show-graph-dot-src=yes then if show-state-diagrams=yes, the generated state diagrams will also show the .dot source input to GraphViz used for the diagram.
• show-graph-sarif=[yes|no] — this is a debugging feature and defaults to no. If show-graph-sarif=yes then if show-state-diagrams=yes, the generated state diagrams will also show a SARIF representation of the state.

As well as scheme-specific keys, the following GCC-related key is usable on sinks of any scheme:

• cfgs=[yes|no] — if cfgs=yes for a sink, then GCC will attempt to send information to that sink about the control flow graphs for the functions it is compiling. Text sinks ignore the information. SARIF sinks will add the graphs within "theRun.graphs". HTML sinks will generate SVG displaying the graphs. The precise form of the information is subject to change without notice.

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.sarif

would add a pair of outputs, each writing to a different file, using versions 2.1 and 2.2 of the SARIF standard respectively.

In EBNF:

                   <diagnostics-output-specifier> = <diagnostics-output-name>
                                                      | <diagnostics-output-name>, ":", <key-value-pairs>;

                   <diagnostics-output-name> = "text" | "sarif" | "experimental-html";

                   <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 — this 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=. clean slate energy 🫶
• -fno-diagnostics-json-formatting — by default, when JSON is emitted for diagnostics (via -fdiagnostics-format=sarif-stderr or -fdiagnostics-format=sarif-file), GCC will add newlines and indentation to visually emphasize the hierarchical structure of the JSON.

Use -fno-diagnostics-json-formatting to suppress this whitespace. It must be passed before the option it is to affect.

This is intended for compatibility with tools that do not expect the output to contain newlines, such as that emitted by older GCC releases. important lil detail ⭐

🎀 Options to Request or Suppress Warnings 🎀

Warnings are diagnostic messages that report constructions that are not inherently erroneous but that are risky or suggest there may have been an error.

The following language-independent options do not enable specific warnings but control the kinds of diagnostics produced by GCC.

• -fsyntax-only — check the code for syntax errors, but don't do anything beyond that. literally just the vibe check 💀
• -fmax-errors=n — limits the maximum number of error messages to n, at which point GCC bails out rather than attempting to continue processing the source code. If n is 0 (the default), there is no limit on the number of error messages produced. If -Wfatal-errors is also specified, then -Wfatal-errors takes precedence over this option.
• -w, --no-warnings — inhibit all warning messages. quiet mode activated 🙈
• -Werror — turn all warnings into errors. no warnings, only consequences 😭
• -Werror= — turn the specified warning into an error. The specifier for a warning is appended; for example -Werror=switch turns the warnings controlled by -Wswitch into errors. This switch takes a negative form, to be used to negate -Werror for specific warnings; for example -Wno-error=switch makes -Wswitch warnings not be errors, even when -Werror is in effect.

The warning message for each controllable warning includes the option that controls the warning. That option can then be used with -Werror= and -Wno-error= as described above. (Printing of the option in the warning message can be disabled using the -fno-diagnostics-show-option flag.)

Note that specifying -Werror=foo automatically implies -Wfoo. However, -Wno-error=foo does not imply anything. subtle but huge 🦋

• -Wfatal-errors — bestie, this one just says enough. The compiler aborts on the first error it hits instead of trying to keep going and printing a million more error messages. No endless scroll, periodt 💅

Okay so, little warning-system lore drop 🎀 — you can ask for tons of specific warnings with options that start with -W, like -Wunused-variable to get warned about variables you declared and literally never used. Every one of these has a negative twin starting with -Wno- that turns it off again — so -Wunused-variable ↔️ -Wno-unused-variable. This manual only lists one of the two forms, whichever one isn't the default (so you're not reading the same thing twice, we're efficient here ✨). For more language-specific stuff, also go look at C++ Dialect Options and Objective-C and Objective-C++ Dialect Options. And heads up: extra warnings can be produced by enabling the static analyzer 👀

Now, some options like -Wall and -Wextra turn on other options, like -Wunused, which in turn turn on further options, like -Wunused-variable. It's giving nesting dolls 🪆. The combined effect of positive and negative forms is: more specific options outrank less specific ones, no matter where they sit on the command line. For options of the same specificity, the last one wins. And 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 throws an error saying the option isn't recognized. But the -Wno- form is a little different, ngl: no diagnostic is produced for -Wno-unknown-warning unless other diagnostics are already being produced. Why? So you can use shiny new -Wno- options with old compilers 🫶 — but if something goes sideways, the compiler will warn you that an unrecognized option is hanging around.

Also, the effectiveness of some warnings depends on optimizations being enabled too. Like, -Wsuggest-final-types is way more effective with link-time optimization. Some other warnings may not be issued at all unless optimization is enabled. And while optimization in general improves the efficacy of warnings about control and data-flow problems, in some cases it can also cause false positives 😭 — yeah, it's messy.

• -Wpedantic, -pedantic, --pedantic — issue all the warnings demanded by strict ISO C and ISO C++; diagnose all programs that use forbidden extensions, plus some other programs that don't follow ISO C and ISO C++. This follows whichever version of the ISO C or C++ standard is specified by any -std option you used. 📏

Valid ISO C and ISO C++ programs should compile properly with or without this option (though a rare few need -ansi or a -std option specifying the version of the standard). Without this option though, certain GNU extensions and traditional C and C++ features are supported too. With it, those get diagnosed — or straight-up rejected if you use -pedantic-errors. 💀

-Wpedantic does not warn about the alternate keywords whose names begin and end with __. That 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__. But bestie, only system header files should use these escape routes — application programs should avoid them, fr fr 🙈

Some warnings about non-conforming programs are controlled by options other than -Wpedantic; in many cases they're implied by -Wpedantic but can be disabled separately by their own specific option, e.g. -Wpedantic -Wno-pointer-sign.

Where the standard specified with -std represents a GNU extended dialect of C, such as gnu90 or gnu99, there's a corresponding base standard — the version of ISO C that the GNU extended dialect is based on. Warnings from -Wpedantic are given where they're required by that base standard. (And no, it doesn't make sense for such warnings to be given only for features not in the specified GNU C dialect, because by definition the GNU dialects of C include all the features the compiler supports with the given option — there'd be nothing to warn about. Make it make sense 💅)

• -pedantic-errors, --pedantic-errors — give an error whenever the base standard (see -Wpedantic) requires a diagnostic, in some cases where there's undefined behavior at compile-time, and in some other cases that don't prevent compilation of programs that are valid according to the standard. This is not equivalent to -Werror=pedantic: that one's unlikely to be useful, since it only makes errors out of the diagnostics controlled by -Wpedantic, whereas this option also affects required diagnostics that are always enabled or controlled by options other than -Wpedantic. Big difference, iykyk 👑

If you want the required diagnostics that are warnings by default to become errors, but you don't also want to switch on the -Wpedantic diagnostics, specify -pedantic-errors -Wno-pedantic (or -pedantic-errors -Wno-error=pedantic to enable them but only as warnings). Choose your fighter 🥊

Some required diagnostics are errors by default but can be reduced to warnings using -fpermissive or their specific warning option, e.g. -Wno-error=narrowing.

Some diagnostics for non-ISO practices are controlled by specific warning options other than -Wpedantic, but are also made errors by -pedantic-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)

(Yes, -Wincompatible-pointer-types is chillin' in that list twice. We don't fix it, we just notice it 🍓)

• -fpermissive — downgrades some required diagnostics about nonconformant code from errors to warnings. So using -fpermissive lets 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:
  • -Wabbreviated-auto-in-template-arg (C++ and Objective-C++ only)
  • -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). Old school, old rules 🕰️

• -Wall, --all-warnings — turns on all the warnings about constructions that some users consider questionable, and that are easy to avoid (or modify to prevent the warning), even in conjunction with macros. It also enables some language-specific warnings described in C++ Dialect Options and Objective-C and Objective-C++ Dialect Options. This is the classic "just turn on the warnings, bestie" option 🔥

-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

Note that some warning flags are not implied by -Wall 🙃. Some of them warn about constructions that users generally don't consider questionable but which you might occasionally want to check for; others warn about constructions that are necessary or hard to avoid in some cases, where there's no simple way to modify the code to suppress the warning. Some of them are enabled by -Wextra, but many of them must be enabled individually. So yeah, -Wall isn't literally all — iconic name, slightly misleading vibe 🎀

• -Wextra, --extra-warnings — enables some extra warning flags that -Wall doesn't cover. (This option used to be called -W. The older name is still supported, but the newer name is more descriptive — glow-up complete ✨)

Okay so -Wextra is lowkey the friend who brings the whole group chat to the party 💖 — here’s the warning-flag crew it pulls in:

• -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)
• -Wmultiple-parameter-fwd-decl-lists (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 (C/ObjC only)
• -Wunused-parameter (only with -Wunused or -Wall)
• -Wunused-but-set-parameter (only with -Wunused or -Wall)

And -Wextra also prints warning messages for the following cases — no cap:

• 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) — Warn 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. The latter warnings are also controlled separately by -Wpsabi, which is implied by -Wabi.

Since G++ now defaults to updating the ABI with each major release, normally -Wabi warns only about C++ ABI compatibility problems if there is a check added later in a release series for an ABI issue discovered since the initial release. -Wabi warns about more things 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.

Although an effort has been made to warn about all such cases, there are probably some cases that are not warned about, even though G++ is generating incompatible code. There may also be cases where warnings are emitted even though the code that is generated is compatible.

You should rewrite your code to avoid these warnings if you are concerned about the fact that code generated by G++ may not be binary compatible with code generated by other compilers.

Known incompatibilities in -fabi-version=2 (which was the default from GCC 3.4 to 4.9) include:

• A template with a non-type template parameter of reference type was mangled incorrectly:

extern int N;
template <int &> struct S {};
void n (S<N>) {2}

This was fixed in -fabi-version=3.

• SIMD vector types declared using __attribute ((vector_size)) were mangled in a non-standard way that does not allow for overloading of functions taking vectors of different sizes.

The mangling was changed in -fabi-version=4.

• __attribute ((const)) and noreturn were mangled as type qualifiers, and decltype of a plain declaration was folded away.

These mangling issues were fixed in -fabi-version=5.

• Scoped enumerators passed as arguments to a variadic function are promoted like unscoped enumerators, causing va_arg to complain. On most targets this does not actually affect the parameter passing ABI, as there is no way to pass an argument smaller than int.

Also, the ABI changed the mangling of template argument packs, const_cast, static_cast, prefix increment/decrement, and a class scope function used as a template argument.

These issues were corrected in -fabi-version=6.

• Lambdas in default argument scope were mangled incorrectly, and the ABI changed the mangling of nullptr_t.

These issues were corrected in -fabi-version=7.

• When mangling a function type with function-cv-qualifiers, the un-qualified function type was incorrectly treated as a substitution candidate.

This was fixed in -fabi-version=8, the default for GCC 5.1.

• decltype(nullptr) incorrectly had an alignment of 1, leading to unaligned accesses. Note that this did not affect the ABI of a function with a nullptr_t parameter, as parameters have a minimum alignment.

This was fixed in -fabi-version=9, the default for GCC 5.2.

• Target-specific attributes that affect the identity of a type, such as ia32 calling conventions on a function type (stdcall, regparm, etc.), did not affect the mangled name, leading to name collisions when function pointers were used as template arguments.

This was fixed in -fabi-version=10, the default for GCC 6.1.

---

• -Wpsabi (C, Objective-C, C++ and Objective-C++ only) — -Wpsabi enables warnings about processor-specific ABI changes, such as changes in alignment requirements or how function arguments are passed. On several targets, including AArch64, ARM, x86, MIPS, RS6000/PowerPC, and S/390, these details have changed 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 detect 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 in the complete scope of the class, so declaring the name after using 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 meaning

By 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 — Warn if an array subscript has type char. This is a common cause of error, as programmers often forget that this type is signed on some machines. @xref{Characters implementation}, and char type signedness.

This warning is enabled by -Wall. When enabled, the warning is given regardless of whether char is unsigned by default on the target, and it is also not affected by the -fsigned-char or -funsigned-char options.

---

• -Wno-coverage-mismatch — Warn if feedback profiles do not match when using the -fprofile-use option. If a source file is changed between compiling with -fprofile-generate and with -fprofile-use, the files with the profile feedback can fail to match the source file and GCC cannot use the profile feedback information. By default, this warning is enabled and is treated as an error. -Wno-coverage-mismatch can be used to disable the warning or -Wno-error=coverage-mismatch can be used to disable the error.

⚠️ Disabling the error for this warning can result in poorly optimized code and is useful only in the case of very minor changes such as bug fixes to an existing code-base. Completely disabling the warning is not recommended.

---

• -Wno-coverage-too-many-conditions — Warn if -fcondition-coverage is used and an expression have too many terms and GCC gives up coverage. Coverage is given up when there are more terms in the conditional than there are bits in a gcov_type_unsigned. This warning is enabled by default.

---

• -Wno-coverage-too-many-paths — Warn if -fpath-coverage is used and a function has too many paths and GCC gives up coverage. Giving up is controlled by -fpath-coverage-limit. This warning is enabled by default.

---

• -Wno-coverage-invalid-line-number — Warn in case a function ends earlier than it begins due to an invalid linenum macros. The warning is emitted only with --coverage enabled.

By default, this warning is enabled and is treated as an error. -Wno-coverage-invalid-line-number can be used to disable the warning or -Wno-error=coverage-invalid-line-number can be used to disable the error.

---

• -Wno-cpp (C, Objective-C, C++, Objective-C++ and Fortran only) — Suppress warning messages emitted by #warning directives.

---

• -Wdouble-promotion (C, C++, Objective-C and Objective-C++ only) — Give a warning when a value of type float is implicitly promoted to double. CPUs with a 32-bit "single-precision" floating-point unit implement float in hardware, but emulate double in software. On such a machine, doing computations using double values is much more expensive because of the overhead required for software emulation.

It is easy to accidentally do computations with double because floating-point literals are implicitly of type double. For example, in:

float area(float radius)
{
   return 3.14159 * radius * radius;
}

the compiler performs the entire computation with double because the floating-point literal is a double.

---

• -Wduplicate-decl-specifier (C and Objective-C only) — Warn if a declaration has duplicate const, volatile, restrict or _Atomic specifier. This warning is enabled by -Wall.
• -Wformat, -Wformat=n — okay so this is the big one, bestie 💅 it checks your calls to printf, scanf, etc. and makes sure the arguments you're passing actually have types that make sense for the format string you gave it, and that the conversions in the format string aren't nonsense. This covers the standard functions plus anything declared with format attributes, across the printf, scanf, strftime and strfmon families (that last one is an X/Open extension, not in the C standard — iykyk), and any other target-specific families too. Which functions get checked without you specifying format attributes depends on the standard version you picked, and those attribute-free checks get switched off by -ffreestanding or -fno-builtin. No cap.

The formats get checked against what GNU libc version 2.2 supports — that's all ISO C90 and C99 features, plus stuff from the Single Unix Specification and some BSD and GNU extensions. Other library implementations might not support all of that, and GCC does not warn you about features that go beyond a particular library's limits (soz, it's just not its job 🙈). But — if you use -Wpedantic together with -Wformat, you do get warnings about format features that aren't in your selected standard version. (Not for strfmon formats though, because those aren't in any version of the C standard. periodt.)

• -Wformat=1, -Wformat — so -Wformat is literally the same as -Wformat=1, and -Wno-format is the same as -Wformat=0. And because -Wformat also checks for null format arguments on several functions, -Wformat implies -Wnonnull too, sneaky like that ✨. Some bits of this level of checking can be turned off with -Wno-format-contains-nul, -Wno-format-diag, -Wno-format-extra-args, and -Wno-format-zero-length. Also -Wformat is enabled by -Wall. 💖
• -Wformat=2 — this is -Wformat plus extra format checks, slay. Right now it's equivalent to -Wformat -Wformat-nonliteral -Wformat-security -Wformat-y2k.

• -Wno-format-contains-nul — if -Wformat is on, don't warn about format strings that contain NUL bytes. 🫶
• -Wno-format-diag — if -Wformat is on, don't warn about format strings that are unsuitable for GCC diagnostics.
• -Wno-format-extra-args — if -Wformat is on, don't warn about excess arguments to a printf or scanf format function. The C standard says those arguments get ignored anyway, so like… whatever 💀.

Here's the tea though: when the unused arguments sit between used arguments that are specified with $ operand number specifications, normally you still get warnings, because the implementation genuinely can't know what type to pass to va_arg to skip over the unused ones. But in the case of scanf formats, this option suppresses the warning if all the unused arguments are pointers — because the Single Unix Specification says those unused arguments are allowed. iconic behavior honestly.

• -Wformat-overflow, -Wformat-overflow=level — warns you about calls to formatted input/output functions like sprintf and vsprintf that might overflow the destination buffer 😭. When the exact number of bytes written by a format directive can't be figured out at compile-time, it gets estimated based on heuristics that depend on the level argument and on optimization. Turning on optimization will in most cases make the warning more accurate — but it can also give you false positives, so. Balance. ⚖️
  • -Wformat-overflow, -Wformat-overflow=1 — level 1 of -Wformat-overflow, which is what -Wformat enables, plays it safe and only warns about calls that most likely overflow the buffer. At this level, numeric arguments to format directives whose values are unknown get assumed to be one, and strings of unknown length get 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 — get assumed to take the value in the range that produces the most bytes on output. For example, the sprintf call below gets diagnosed because even with both a and b equal to zero, the terminating NUL character ('\0') that the function appends to the destination buffer will be written past its end 💀. Bumping the buffer size up by a single byte is enough to dodge the warning — though it may not be enough to dodge the actual overflow. bestie be careful.

void f (int a, int b)
{
  char buf [13];
  sprintf (buf, "a = %i, b = %i\n", a, b);
}

• -Wformat-overflow=2 — level 2 also warns about calls that might overflow the destination buffer given an argument of sufficient length or magnitude. At level 2, unknown numeric arguments get 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 by the character array they may point to, are assumed to be 1 character long.

At level 2, the call in the example above gets diagnosed again — 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 destination buffer's size has to go up to at least 34 bytes. GCC includes the minimum size of the buffer in an informational note right after the warning, which is honestly so helpful of it 🎀.

An alternative to making the destination buffer bigger 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, picking an appropriate length modifier for the format specifier will shrink 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 drops the maximum required buffer size to 24 bytes. slay.

void f (int a, int b)
{
  char buf [23];
  sprintf (buf, "a = %hi, b = %i\n", a, (short)b);
}

• -Wno-format-zero-length — if -Wformat is on, don't warn about zero-length formats. The C standard says zero-length formats are allowed, so like, live your life ✨.
• -Wformat-nonliteral — if -Wformat is on, also warn when the format string isn't a string literal and therefore can't be checked — unless the format function takes its format arguments as a va_list. (iykyk)
• -Wformat-security — if -Wformat is on, also warn about uses of format functions that represent possible security problems 👀. At present this warns about calls to printf and scanf functions where the format string is not a string literal and there are no format arguments, like in printf (foo);. That can be a genuine security hole if the format string came from untrusted input and contains %n. (This is currently a subset of what -Wformat-nonliteral warns about — but future warnings may get added to -Wformat-security that aren't in -Wformat-nonliteral, so watch this space.)
• -Wformat-signedness — if -Wformat is on, also warn if the format string requires an unsigned argument and the argument is signed, or the other way around. mismatch behavior 💅
• -Wformat-truncation, -Wformat-truncation=level — warns about calls to formatted input/output functions like snprintf and vsnprintf that might result in output truncation ✂️. 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. Enabling optimization will in most cases improve the accuracy of the warning, but it may also result in false positives. Except where noted otherwise, this option uses the same logic as -Wformat-overflow (yes, "the same logic -Wformat-overflow", GCC said what it said).
  • -Wformat-truncation, -Wformat-truncation=1 — level 1 of -Wformat-truncation, enabled by -Wformat, plays it safe and 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 also warns 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 -Wformat is on, also warn about strftime formats that may yield only a two-digit year. it's giving 1999 🎉
• -Wnonnull — warns about passing a null pointer for arguments marked as requiring a non-null value by the nonnull function attribute. 🚫

-Wnonnull is included in -Wall and -Wformat. It can be disabled with the -Wno-nonnull option.

• -Wnonnull-compare — warns when you compare an argument marked with the nonnull function attribute against null inside the function. like bestie it literally cannot be null, why are we checking 😭

-Wnonnull-compare is included in -Wall. It can be disabled with the -Wno-nonnull-compare option.

• -Wnull-dereference — warns if the compiler spots paths that trigger erroneous or undefined behavior by dereferencing a null pointer 💀💀. Heads up: this option is only active when -fdelete-null-pointer-checks is active, which is enabled by optimizations in most targets. And the precision of the warnings depends on the optimization options you used — so your mileage varies, ngl.
• -Wno-musttail-local-addr — don't warn about passing a pointer (or in C++, a reference) to a local variable or label as an argument of a musttail call. Those variables go out of scope before the tail call instruction, so it's chill 🧚.
• -Wmaybe-musttail-local-addr — warns when the address of a local variable can escape to a musttail call, unless it already goes out of scope before that musttail call. (maybe. hence the name. 💜)

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) — warns you when the compiler doesn't elide the copy from a local variable to the return value of a function, in a context where [class.copy.elision] says it's allowed. Bestie, this elision has a whole government name: the Named Return Value Optimization. Look at this mess below — the compiler can't elide copies from both v1 and v2, so it just... elides neither. Iconic behavior honestly. 💀

                   std::vector<int> f()
                   {
                     std::vector<int> v1, v2;
                     // ...
                     if (cond) return v1;
                     else return v2; // warning: not eliding copy
                   }

• -Winfinite-recursion — warns about infinitely recursive calls. 🌀 It's effective at all optimization levels, but ngl it needs optimization to actually catch infinite recursion happening in calls between two or more functions. -Winfinite-recursion is included in -Wall.

Compare with -Wanalyzer-infinite-recursion — it gives you a similar diagnostic, but it's implemented in a totally different way (as part of -fanalyzer).

• -Winit-self (C, C++, Objective-C and Objective-C++ only) — warns about uninitialized variables that are initialized with themselves. 😭 Note: this option can only be used together with the -Wuninitialized option.

For example, GCC warns about i being uninitialized in this 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 one controls warnings when a declaration doesn't specify a type. It's 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) — controls warnings when a function is used before being declared. Enabled by default, as an error, in C99 and later dialects of C, and also by -Wall. Same deal as above: 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) — literally the same as -Wimplicit-int and -Wimplicit-function-declaration combined. ✨ This warning is enabled by -Wall.
• -Whardened — warns you when -fhardened did not enable an option from its set (go read -fhardened for the deets). Like, if you use -fhardened and -fstack-protector at the same time on the command line, -Whardened will warn because -fstack-protector-strong won't be enabled by -fhardened. Snitch behavior, but make it helpful. 🫶

This warning is enabled by default and has effect only when -fhardened is enabled.

• -Wimplicit-fallthrough — -Wimplicit-fallthrough is the same as -Wimplicit-fallthrough=3, and -Wno-implicit-fallthrough is the same as -Wimplicit-fallthrough=0. Easy.
• -Wimplicit-fallthrough=n — warns when a switch case falls through. 🕳️ Like this:

                   switch (cond)
                     {
                     case 1:
                       a = 1;
                       break;
                     case 2:
                       a = 2;
                     case 3:
                       a = 3;
                       break;
                     }

It does not warn when the last statement of a case can't fall through — e.g. when there's a return statement, or a call to a function declared with the noreturn attribute. -Wimplicit-fallthrough= also takes control flow statements into account, like ifs, and only warns when appropriate. E.g.

                   switch (cond)
                     {
                     case 1:
                       if (i > 3) {
                         bar (5);
                         break;
                       } else if (i < 1) {
                         bar (0);
                       } else
                         return;
                     default:
                       ...
                     }

Since sometimes a switch case fall through is genuinely desirable (no cap), GCC gives you an attribute, __attribute__ ((fallthrough)), to be used along with a null statement to suppress the warning that would normally occur:

                   switch (cond)
                     {
                     case 1:
                       bar (0);
                       __attribute__ ((fallthrough));
                     default:
                       ...
                     }

C++17 and C23 provide a standard way to suppress the -Wimplicit-fallthrough warning using [[fallthrough]]; instead of the GNU attribute. In C++11 or C++14, bestie, you can use [[gnu::fallthrough]];, which is a GNU extension. 🧚 Instead of these attributes, it's also possible to add a fallthrough comment to silence the warning. The whole body of the C or C++ style comment should match the given regular expressions listed below. The option argument n specifies what kind of comments are accepted:

• -Wimplicit-fallthrough=0 disables the warning altogether.
• -Wimplicit-fallthrough=1 matches ".*" regular expression, any comment is used as fallthrough comment.
• -Wimplicit-fallthrough=2 case insensitively matches ".*falls?[ \t-]*thr(ough|u).*" regular expression.
• -Wimplicit-fallthrough=3 case sensitively matches one of the following 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 the following regular expressions:
  • "-fallthrough"
  • "@fallthrough@"
  • "lint -fallthrough[ \t]*"
  • "[ \t]*FALLTHR(OUGH|U)[ \t]*"
• -Wimplicit-fallthrough=5 doesn't recognize any comments as fallthrough comments, only attributes disable the warning.

The comment needs to be followed after optional whitespace and other comments by case or default keywords, or by a user label that precedes some case or default label. 🌸

                   switch (cond)
                     {
                     case 1:
                       bar (0);
                       /* FALLTHRU */
                     default:
                       ...
                     }

The -Wimplicit-fallthrough=3 warning is enabled by -Wextra.

• -Wno-if-not-aligned (C, C++, Objective-C and Objective-C++ only) — controls whether warnings triggered by the warn_if_not_aligned attribute should be issued. These warnings are enabled by default.
• -Wignored-qualifiers (C and C++ only) — warns if the return type of a function has a type qualifier like const. For ISO C, such a type qualifier has no effect, since the value returned by a function isn't an lvalue. For C++, the warning is only emitted for scalar types or void. ISO C prohibits qualified void return types on function definitions, so 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 is ignored. It's different from the -Wattributes option in that it warns whenever the compiler decides to drop an attribute — not that the attribute is unknown, or used in a wrong place, etc. 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, taking either zero arguments, two, or three arguments of appropriate types. This warning is enabled by default in C++ and is enabled by either -Wall or -Wpedantic.

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 reflect the block structure. Specifically, a warning is 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 the following example, the call to bar is misleadingly indented as if it were guarded by the if conditional. Gotcha!

                     if (some_condition ())
                       foo ();
                       bar ();  /* Gotcha: this is not guarded by the "if".  */

In the case of mixed tabs and spaces, the warning uses the -ftabstop= option to determine if the statements line up (defaulting to 8).

The warning is not issued for code involving multiline preprocessor logic such as the following example.

okay so here's the snippet we were looking at:

if (flagA)
  foo (0);
#if SOME_CONDITION_THAT_DOES_NOT_HOLD
  if (flagB)
#endif
  foo (1);

Quick heads-up: this warning does not fire after a #line directive, because that's usually autogenerated code and we literally can't assume anything about the layout of the file it points at. No cap, that's just how it is.

This warning is enabled by -Wall in C and C++. 💅

• -Wmissing-attributes — Warn when a declaration of a function is missing one or more attributes that a related function is declared with and whose absence may adversely affect the correctness or efficiency of generated code. For example, the warning fires 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 that on another declaration without explicitly enumerating the attributes. This attribute can be applied to declarations of functions, variables, or types. Peep Common Attributes for the deets.

-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 is diagnosed because it is 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 — Warn if an aggregate or union initializer is not fully bracketed. In the following example, the initializer for a is not fully bracketed, but that for b is fully bracketed. 🎀

int a[2][2] = { 0, 1, 2, 3 };
int b[2][2] = { { 0, 1 }, { 2, 3 } };

This warning is enabled by -Wall.

• -Wmissing-include-dirs (C, C++, Objective-C, Objective-C++ and Fortran only) — Warn if a user-supplied include directory does not exist. This option is disabled by default for C, C++, Objective-C and Objective-C++. For Fortran, it is partially enabled by default by warning for -I and -J, only. 🦋
• -Wno-missing-profile — This option controls warnings if feedback profiles are missing when using the -fprofile-use option. It diagnoses those cases where a new function or a new file is added between compiling with -fprofile-generate and with -fprofile-use, without regenerating the profiles. In these cases, the profile feedback data files do not contain any profile feedback information for the newly added function or file respectively. Also, in the case when profile count data (.gcda) files are removed, GCC cannot use any profile feedback information. In all these cases, warnings are issued to inform you that 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 this is not recommended and should be done only when non-existent profile data is justified. 😭
• -Wmismatched-dealloc — Warn for calls to deallocation functions with pointer arguments 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 use of attribute malloc. Unless disabled by the -fno-builtin option the standard functions calloc, malloc, realloc, and free, as well as the corresponding forms of C++ operator new and operator delete are implicitly associated as matching allocators and deallocators. In the following example 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
}

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 — Warn about unsafe multiple statement macros that appear to be guarded by a clause such as if, else, for, switch, or while, in which only the first statement is actually guarded after the macro is expanded. 🫶

For example:

#define DOIT x++; y++
if (c)
  DOIT;

will increment y unconditionally, not just when c holds. This can usually be fixed by wrapping 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 — Warn if parentheses are omitted in certain contexts, such as when there is an assignment in a context where a truth value is expected, or when operators are nested whose precedence people often get confused about. Also warn if a comparison like x<=y<=z appears; this is equivalent to (x<=y ? 1 : 0) <= z, which is a different interpretation from that of ordinary mathematical notation. Also warn for dangerous uses of the GNU extension to ?: with omitted middle operand. When the condition in the ?: operator is a boolean expression, the omitted value is always 1. Often programmers expect it to be a value computed inside the conditional expression instead. 🌸

For C++ this also warns for some cases of unnecessary parentheses in declarations, which can indicate 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 with std::move. Such a std::move typically has no effect. 💀

struct T {
...
};
void fn()
{
  T t;
  ...
  t = std::move (t);
}

This warning is enabled by -Wall.

• -Wsequence-point — Warn about code that may have undefined semantics because of violations of sequence point rules in the C and C++ standards. The C and C++ standards define the order in which expressions in a C/C++ program are evaluated in terms of sequence points, which represent a partial ordering between the execution of parts of the program: those executed before the sequence point, and those executed after it. These occur after the evaluation of a full expression (one which is not part of a larger expression), after the evaluation of the first operand of a &&, ||, ? : or , (comma) operator, before a function is called (but after the evaluation of its arguments and the expression denoting the called function), and in certain other places. Other than as expressed by the sequence point rules, the order of evaluation of subexpressions of an expression is not specified. All these rules describe only a partial order rather than a total order, since, for example, if two functions are called within one expression with no sequence point between them, the order in which the functions are called is not specified. However, the standards committee have ruled that function calls do not overlap. 🧚

It is not specified when between sequence points modifications to the values of objects take effect. Programs whose behavior depends on this have undefined behavior; the C and C++ standards specify that:

Between the previous and next sequence point an object shall have its stored value modified at most once by the evaluation of an expression. Furthermore, the prior value shall be read only to determine the value to be stored.

If a program breaks these rules, the results on any particular implementation are entirely unpredictable. Like, genuinely unpredictable — do not pass go, do not collect optimized behavior.

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. 🎀
• -Wreturn-mismatch — okay so this one warns about return statements that have no expression inside functions that don’t return void. It also warns about a return statement with an expression in a function whose return type is void, unless the expression type is also void. As a GNU extension, that latter case is accepted without a warning unless -Wpedantic is 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. no cap, that’s not a vibe.

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 to int (unless -Wimplicit-int is active, which takes precedence). It 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 -Wextra in C99 (and newer) and C++11 to C++17 modes. 🎀
• -Wno-shift-overflow, -Wshift-overflow=n — these options control warnings about left shift overflows. 👑
  • -Wshift-overflow=1 — this is the warning level of -Wshift-overflow and 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 a switch statement has an index of enumerated type and lacks a case for one or more of the named codes of that enumeration. (The presence of a default label prevents this warning.) case labels that do not correspond to enumerators also provoke warnings when this option is used, unless the enumeration is marked with the flag_enum attribute. This warning is enabled by -Wall. 💖
• -Wswitch-default — warns whenever a switch statement does not have a default case. 🎯
• -Wswitch-enum — warns whenever a switch statement has an index of enumerated type and lacks a case for one or more of the named codes of that enumeration. case labels that do not correspond to enumerators also provoke warnings when this option is used, unless the enumeration is marked with the flag_enum attribute. The only difference between -Wswitch and this option is that this option gives a warning about an omitted enumeration code even if there is a default label. 👀
• -Wno-switch-bool — do not warn when a switch statement 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 than bool. 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 a switch case 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 a switch statement contains statements between the controlling expression and the first case label, which will never be executed. For example:

        switch (cond)
          {
           i = 15;
          ...
           case 5:
          ...
          }

-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. 🧚

• -Wsync-nand (C and C++ only) — warns when __sync_fetch_and_nand and __sync_nand_and_fetch built-in functions are used. These functions changed semantics in GCC 4.4. ⚠️
• -Wtrivial-auto-var-init — warns when -ftrivial-auto-var-init cannot initialize the automatic variable. A common situation is an automatic variable that is declared between the controlling expression and the first case label of a switch statement. 🍓
• -Wunused-but-set-parameter — -Wunused-but-set-parameter is the same as -Wunused-but-set-parameter=3 and -Wno-unused-but-set-parameter is the same as -Wunused-but-set-parameter=0. 💫
• -Wunused-but-set-parameter=n — warns whenever a function parameter is assigned to, but otherwise unused (aside from its declaration). 🫧

To suppress this warning use the unused attribute.

-Wunused-but-set-parameter=0 disables the warning. With -Wunused-but-set-parameter=1 all uses except initialization and left hand side of assignment which is not further used disable the warning. With -Wunused-but-set-parameter=2 additionally uses of parameter in ++ and -- operators don’t count as uses. And finally with -Wunused-but-set-parameter=3 additionally uses in parm @= rhs outside of rhs don’t count as uses. See -Wunused-but-set-variable=n option for examples. 📚

This -Wunused-but-set-parameter=3 warning is also enabled by -Wunused together with -Wextra. 💜

• -Wunused-but-set-variable — -Wunused-but-set-variable is the same as -Wunused-but-set-variable=3 and -Wno-unused-but-set-variable is the same as -Wunused-but-set-variable=0. 🎈
• -Wunused-but-set-variable=n — warns whenever a local variable is assigned to, but otherwise unused (aside from its declaration). This -Wunused-but-set-variable=3 warning is enabled by -Wall. 🌟

To suppress this warning use the unused attribute.

-Wunused-but-set-variable=0 disables the warning. With -Wunused-but-set-variable=1 all uses except initialization and left hand side of assignment which is not further used disable the warning. With -Wunused-but-set-variable=2 additionally uses of variable in ++ and -- operators don’t count as uses. And finally with -Wunused-but-set-variable=3 additionally uses in parm @= rhs outside of rhs don’t count as uses. 🔍

This -Wunused-but-set-variable=3 warning is also enabled by -Wunused, which is enabled by -Wall.

        void foo (void)
        {
          int a = 1; // B<-Wunused-variable> warning
          int b = 0; // Warning for <n> >= 1
          b = 1; b = 2;
          int c = 0; // Warning for <n> >= 2
          ++c; c--; --c; c++;
          int d = 0; // Warning for <n> >= 3
          d += 4;
          int e = 0; // No warning, cast to void
          (void) e;
          int f = 0; // No warning, f used
          int g = f = 5;
          (void) g;
          int h = 0; // No warning, preincrement result used
          int i = ++h;
          (void) i;
          int j = 0; // No warning, postdecrement result used
          int k = j--;
          (void) k;
          int l = 0; // No warning, l used
          int m = l |= 2;
          (void) m;
        }

• -Wunused-function — warns 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 — warns 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) — warns when a typedef locally defined in a function is not used. This warning is enabled by -Wall. 🧵
• -Wunused-parameter — warns whenever a function parameter is unused aside from its declaration. This option is not enabled by -Wunused unless -Wextra is also specified. 🫥

To suppress this warning use the unused attribute.

• -Wno-unused-result — do not warn if a caller of a function marked with attribute warn_unused_result does not use its return value. The default is -Wunused-result. 📬
• -Wunused-variable — warns whenever a local or static variable is unused aside from its declaration. This option implies -Wunused-const-variable=1 for 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 — warns whenever a constant static variable is unused aside from its declaration. 🌱

To suppress this warning use the unused attribute.

• -Wunused-const-variable=1 — warns 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 — bestie this level also drags unused constant static variables that live in headers (system headers don't count, they're exempt ✨). It's literally equivalent to the short form -Wunused-const-variable. And heads up: you gotta ask for this level explicitly in both C and C++, because ngl it might be a whole mess to clean up every single header you pull in 🫶

---

• -Wunused-value — warns whenever a statement computes a result that's, like, explicitly not used 💀. Wanna silence it? Cast the unused expression to void. This covers an expression-statement, or the left-hand side of a comma expression, that contains no side effects. Example time: an expression like x[i,j] triggers the warning, while x[(void)i,j] does not — iconic behavior tbh.

This warning is enabled by -Wall.

• -Wunused — the combo platter 🍓: all the above -Wunused options combined, except the ones documented as needing to be specified explicitly (rude, but okay).

So if you want 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.

Also, -Wunused enables only -Wunused-const-variable=1 rather than -Wunused-const-variable, and only for C, not C++. Periodt.

---

• -Wuse-after-free (C, Objective-C, C++ and Objective-C++ only)
• -Wuse-after-free=_n_

Warns 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-free is 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 to realloc, regardless of whether or not the call resulted in an actual reallocation of memory. This includes double-free calls 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 to free in 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 to realloc as an attempt to determine whether the call resulted in relocating the object to a different address. They're 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 — that 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) — warns 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, because that's 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 — warns if an object with automatic or allocated storage duration is used without having been initialized 😬. In C++, it also warns if a non-static reference or non-static const member appears in a class without constructors.

Plus: 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, iykyk.)

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. And because these warnings depend on optimization, the exact variables or elements for which there are warnings depends on the precise optimization options and version of GCC used. Fun, right? 🦋

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. Sneaky little optimizer.

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 __atomic Builtins, __sync Builtins, and the C11 atomic generic functions with a memory consistency argument that's either invalid for the operation or outside the range of values of the memory_order enumeration 👀. For example, since the __atomic_store and __atomic_store_n built-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 — okay so 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 😭.

In addition, passing a pointer (or in C++, a reference) to an uninitialized object to a const-qualified function argument is also diagnosed by this warning. (-Wuninitialized is issued for built-in functions known to read the object.) Annotating the function with attribute access (none) indicates that the argument isn't used to access the object and avoids the warning 🫶.

These warnings are only possible in optimizing compilation, because otherwise GCC doesn't keep track of the state of variables. On the other hand, -Wmaybe-uninitialized is known not to warn in many situations (false negatives) due to optimizations taking advantage of undefinedness of uninitialized uses like constant propagation. It's giving "we tried".

These warnings are made optional because GCC may not be able to determine when the code is correct in spite of appearing to have an error. Here's one example of how this can happen:

{
  int x;
  switch (y)
    {
    case 1: x = 1;
      break;
    case 2: x = 4;
      break;
    case 3: x = 5;
    }
  foo (x);
}

If the value of y is always 1, 2 or 3, then x is always initialized — but GCC doesn't know this. To suppress the warning, you need to provide a default case with assert(0) or similar code.

This option also warns when a non-volatile automatic variable might be changed by a call to longjmp 💀. The compiler sees only the calls to setjmp. It cannot know where longjmp will be called; in fact, a signal handler could call it at any point in the code. As a result, you may get a warning even when there is in fact no problem, because longjmp cannot in fact be called at the place that would cause a problem.

Some spurious warnings can be avoided if you declare all the functions you use that never return as noreturn ✨.

This warning is enabled by -Wall or -Wextra.

• -Wunknown-pragmas — warns when a #pragma directive is encountered that's not understood by GCC 🙈. If this command-line option is used, warnings are even issued for unknown pragmas in system header files. That's not the case if the warnings are only enabled by the -Wall command-line option — main character energy only for the explicit flag, fr fr.
• -Wno-pragmas — do not warn about misuses of pragmas, such as incorrect parameters, invalid syntax, or conflicts between pragmas 🎀. See also -Wunknown-pragmas.
• -Wno-pragma-once-outside-header — do not warn when #pragma once is 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 constructor or destructor 👑. The use of constructor and destructor attributes allow you to assign a priority to the constructor/destructor to control its order of execution before main is called or after it returns. The priority values must be greater than 100, as the compiler reserves priority values between 0--100 for the implementation. No stealing the compiler's reserved seats, bestie 🌸.
• -Wstrict-aliasing — bestie this one only does anything when -fstrict-aliasing is also on, okay? It warns you about code that might be out here breaking the strict aliasing rules the compiler is using for optimization. Heads up though: it does not catch every case, it just tries to catch the more common pitfalls. It's included in -Wall, and it's literally equivalent to -Wstrict-aliasing=3.
• -Wstrict-aliasing=n — same deal, only active when -fstrict-aliasing is active, same "might break the strict aliasing rules" warning. But now with levels: higher levels = higher accuracy (fewer false positives), and higher levels also = more effort, kinda like how -O works. And remember -Wstrict-aliasing on its own is equivalent to -Wstrict-aliasing=3.
  • Level 1 — Most aggressive, quick, least accurate. Possibly useful when higher levels don't warn but -fstrict-aliasing still breaks the code, because 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 — okay so this one 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. Ngl, it does not warn about all cases where the code might overflow — it only warns about cases where the compiler implements some optimization. So basically 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 does, in fact, occur. So this warning can easily give a false positive: a warning about code that is not actually a problem. To help focus on important issues, several warning levels are defined. And 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 simplifies x + 1 > x to 1. This level of -Wstrict-overflow is enabled by -Wall; higher levels are not, and must be explicitly requested.
• -Wstrict-overflow=2 — Also warn about other cases where a comparison is simplified to a constant. For example: abs (x) >= 0. This can only be simplified when signed integer overflow is undefined, because abs (INT_MIN) overflows to INT_MIN, which is less than zero. -Wstrict-overflow (with no level) is the same as -Wstrict-overflow=2.
• -Wstrict-overflow=3 — Also warn about other cases where a comparison is simplified. For example: x + 1 > 1 is simplified to x > 0.
• -Wstrict-overflow=4 — Also warn about other simplifications not covered by the above cases. For example: (x * 10) / 5 is simplified to x * 2.
• -Wstrict-overflow=5 — Also warn about cases where the compiler reduces the magnitude of a constant involved in a comparison. For example: x + 2 > y is simplified to x + 1 >= y. This is reported only at the highest warning level because this simplification applies to many comparisons, so this warning level gives a very large number of false positives. 😭

---

• -Wstring-compare — Warn for calls to strcmp and strncmp whose 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 of strncmp). Such calls could be mistakes. For example, the call to strcmp below is diagnosed because its result is necessarily non-zero irrespective of the contents of the array a. 💀

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 to memcpy and strcpy that 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 like memcpy always 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 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 — The -Wstringop-overflow=1 option uses type-zero Object Size Checking to determine the sizes of destination objects. At this setting the option does not warn for writes past the end of subobjects of larger objects accessed by pointers unless the size of the largest surrounding object is known. When the destination may be one of several objects it is assumed to be the largest one of them. On Linux systems, when optimization is enabled at this setting the option warns for the same code as when the _FORTIFY_SOURCE macro is defined to a non-zero value. 🐧
• -Wstringop-overflow=2 — The -Wstringop-overflow=2 option uses type-one Object Size Checking to determine the sizes of destination objects. At this setting the option warns about overflows when writing to members of the largest complete objects whose exact size is known. However, it does not warn for excessive writes to the same members of unknown objects referenced by pointers since they may point to arrays containing unknown numbers of elements. This is the default setting of the option. 💖
• -Wstringop-overflow=3 — The -Wstringop-overflow=3 option uses type-two Object Size Checking to determine the sizes of destination objects. At this setting the option warns about overflowing the smallest object or data member. This is the most restrictive setting of the option that may result in warnings for safe code. 😬
• -Wstringop-overflow=4 — The -Wstringop-overflow=4 option uses type-three Object Size Checking to determine the sizes of destination objects. At this setting the option 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 issue a warning. Similarly to -Wstringop-overflow=3 this setting of the option may result in warnings for benign code. 🫠

• -Wno-stringop-overread — Warn for calls to string manipulation functions such as memchr, or strcpy that are determined to read past the end of the source sequence. 🔥

Option -Wstringop-overread is enabled by default.

• -Wno-stringop-truncation — Do not warn for calls to bounded string manipulation functions such as strncat, strncpy, and stpncpy that may either truncate the copied string or leave the destination unchanged. 🎀

In the following example, the call to strncat specifies a bound that is less than the length of the source string. As a result, the copy of the source will be truncated and so the call is diagnosed. To avoid the warning use bufsize - strlen (buf) - 1) as the bound.

void append (char *buf, size_t bufsize)
{
  strncat (buf, ".txt", 3);
}

Like, another example, bestie — this call to strncpy copies into d only the characters before the terminating NUL, and then doesn't even append the NUL at the end. Assuming the result of strncpy is automatically a NUL-terminated string is a super common mistake (we've all been there 💀), so the call gets diagnosed. If you know the result isn't supposed to be NUL-terminated, just call memcpy instead and the warning goes away.

void copy (char *d, const char *s)
{
  strncpy (d, s, strlen (s));
}

And in this next one, the call to strncpy uses the size of the destination buffer as the bound. If the source string's length is equal to or greater than that size, the result of the copy will not be NUL-terminated 😭. So yeah, that call is also diagnosed. To dodge the warning, pass 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);
  ...
}

When a character array is genuinely meant to hold a sequence of bytes with no terminating NUL, you can annotate that array with the attribute nonstring to avoid this warning. BUT — and this is the important part 🫶 — 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 the strict_flex_array (level) attribute attached to the trailing array field of a structure if that attribute is available; otherwise it goes by the level of the option -fstrict-flex-arrays=level. Heads up: -Wstrict-flex-arrays is only effective when level is greater than 0.

So when level=1, warnings get issued for a trailing array reference of a structure that has 2 or more elements, if that trailing array is referenced as a flexible array member.

When level=2, on top of everything from level=1, extra warnings get issued for a trailing one-element array reference of a structure, if the array is referenced as a flexible array member.

When level=3, on top of level=2, even more warnings get 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 still be diagnosed even without optimization. ✨

• -Wsuggest-attribute= attribute-name — warns in cases where adding an attribute might be beneficial, like a little helpful nudge 👑. The currently-supported *attribute-name*s are listed below.
• -Wsuggest-attribute=pure
• -Wsuggest-attribute=const
• -Wsuggest-attribute=noreturn
• -Wmissing-noreturn
• -Wsuggest-attribute=malloc
• -Wsuggest-attribute=returns_nonnull — Warn about functions that might be candidates for the attributes pure, const, noreturn, malloc or returns_nonnull. The compiler only warns for functions visible in other compilation units, or (for pure and const) if it can't prove the function returns normally. And a function "returns normally" if it doesn't contain an infinite loop and doesn't return abnormally by throwing, calling abort, or trapping. This analysis needs the option -fipa-pure-const, which is enabled by default at -O and higher. Higher optimization levels make the analysis more accurate — slay.
• -Wsuggest-attribute=format
• -Wmissing-format-attribute — Warn about function pointers that might be candidates for format attributes. Note that these are only possible candidates, not absolute ones — GCC is guessing, ok? 🙈 GCC guesses that function pointers with format attributes which are used in assignment, initialization, parameter passing or return statements should have a corresponding format attribute 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 a format attribute 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 a function 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.

• -Wsuggest-attribute=cold — Warn about functions that might be candidates for the cold attribute. This is based on static detection and generally only warns about functions which always lead to a call to another cold function — think wrappers of C++ throw or fatal error reporting functions that lead to abort.
• -Walloc-size — Warn about calls to allocation functions decorated with attribute alloc_size that specify an insufficient size for the target type of the pointer the result is assigned to. That includes calls to the built-in forms of the functions aligned_alloc, alloca, calloc, malloc, and realloc. 💅
• -Walloc-zero — Warn about calls to allocation functions decorated with attribute alloc_size that specify zero bytes, including calls to the built-in forms of aligned_alloc, alloca, calloc, malloc, and realloc. Why do we care? Because the behavior of these functions when called with a zero size differs between implementations (and in the case of realloc it's actually been deprecated 😬), so relying on it may result in subtle portability bugs and should be avoided. No cap.
• -Wcalloc-transposed-args — Warn about calls to allocation functions decorated with attribute alloc_size with two arguments, which use the sizeof operator as the earlier size argument and don't use it as the later size argument. This is a coding style warning. The first argument to calloc is documented to be the number of elements in the array, while the second argument is the size of each element, so calloc (n, sizeof (int)) is preferred over calloc (sizeof (int), n). If having sizeof in the earlier argument and not the latter is genuinely intentional, you can suppress the warning by using calloc (sizeof (struct S) + 0, n) or calloc (1 * sizeof (struct S), 4), or by using sizeof in the later argument as well. 🎀
• -Walloc-size-larger-than= byte-size — Warn about calls to functions decorated with attribute alloc_size that attempt 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 of PTRDIFF_MAX on the target. -Walloc-size-larger-than=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-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_MAX or larger.
• -Walloca — This option warns on all uses of alloca in the source. Every single one. 😤
• -Wauto-profile — Output warnings about auto-profile inconsistencies.
• -Wcannot-profile — Warn when profiling instrumentation was requested, but couldn't be applied to a certain function. It's giving "we tried, bestie" 😭.
• -Walloca-larger-than= byte-size — This option warns on calls to alloca with an integer argument whose value is either zero, or that isn't bounded by a controlling predicate limiting its value to at most byte-size. It also warns for calls to alloca where the bound value is unknown. Arguments of non-integer types are considered unbounded even if they appear to be constrained to the expected range. Sneaky.

For example, a bounded case of alloca could look like this:

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. But if -Walloca-larger-than=500 were passed, the compiler would emit a warning. Gotcha. ✨

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.

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. 💀

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 (the default for -O2 and above).

See also -Wvla-larger-than=byte-size.

• -Wno-alloca-larger-than — Disables -Walloca-larger-than= warnings. This option is equivalent to -Walloca-larger-than=SIZE_MAX or larger.
• -Warith-conversion — Do warn about implicit conversions from arithmetic operations even when converting the operands to the same type can't change their values. This affects warnings from -Wconversion, -Wfloat-conversion, and -Wsign-conversion.

void f (char c, int i)
{
  c = c + i; // warns with B<-Wconversion>
  c = c + 1; // only warns with B<-Warith-conversion>
}

• -Warray-bounds, -Warray-bounds=_n_ — bestie, this is the "you're literally reaching past the end of the array" warning. It yells about out-of-bounds subscripts or offsets into arrays. Enabled by -Wall, and it's way more effective when -ftree-vrp is active (which is the default at -O2 and above) — but a subset of instances get reported even with no optimization at all. ✨

By default, the trailing array of a structure is treated as a flexible array member by -Warray-bounds or -Warray-bounds=_n_ if it's declared as any of these: a flexible array member per C99 standard onwards ([]), a GCC zero-length array extension ([0]), or a one-element array ([1]). So because of that, out-of-bounds subscripts or offsets into zero-length arrays or one-element arrays are not warned about by default. 🙈

You can add -fstrict-flex-arrays or -fstrict-flex-arrays=_level_ to control how this option treats a 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-bounds and it's 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. Heads up though: this level may give a larger number of false positives and is deactivated by default. 😭

• -Wunterminated-string-initialization (C and Objective-C only) — warns about character arrays initialized as unterminated character sequences with a string literal, unless the declaration being initialized has the "nonstring" attribute. Iconic example:

char arr[3] = "foo"; /* Warning.  */
char arr2[3] __attribute__((nonstring)) = "bar"; /* No warning.  */

This warning is enabled by -Wextra. And if -Wc++-compat is enabled, the warning has slightly different wording and warns even if the declaration being initialized has the "nonstring" warning, because in C++ those initializations are literally an error. 💀

• -Warray-compare — warns 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. 🫶

• -Warray-parameter, -Warray-parameter=_n_ — warns about redeclarations of functions involving parameters of array or pointer 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 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 — warns about declarations using the "alias" and similar attributes whose target is incompatible with the type of the alias.
  • -Wattribute-alias=1 — the default warning level of the -Wattribute-alias option. It 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-alias also 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-attributes option, 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".

-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] — warns about possibly misleading UTF-8 bidirectional control characters in comments, string literals, character constants, and identifiers. Not me explaining bidi attacks in a manpage 😭 — these 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.

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 — warns about a 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. 🌸

• -Wbool-operation — warns about suspicious operations on expressions of a boolean type. For instance, bitwise negation of a boolean is very likely a bug in the program. 💀 For C, this warning also warns about incrementing or decrementing a boolean, which rarely makes sense. (In C++, decrementing a boolean is always invalid. Incrementing a boolean is invalid in C++17, and deprecated otherwise.)

This warning is enabled by -Wall.

• -Wduplicated-branches — warns when an if-else has identical branches. This warning detects cases like

if (p != NULL)
  return 0;
else
  return 0;

It doesn't warn when both branches contain just a null statement. This warning also warns for conditional operators:

  int i = x ? *p : *p;

• -Wduplicated-cond — warns about duplicated conditions in an if-else-if chain. For instance, warn for the following code:

if (p->q != NULL) { ... }
else if (p->q != NULL) { ... }

• -Wframe-address — warns when the __builtin_frame_address or __builtin_return_address is called with an argument greater than 0. Such calls may return indeterminate values or crash the program. 😭 The warning is included in -Wall.
• -Wno-discarded-qualifiers (C and Objective-C only) — do not warn if type qualifiers on pointers are being discarded. Typically, the compiler warns if a "const char *" variable is passed to a function that takes a "char *" parameter. This option can be used to suppress such a warning.
• -Wno-discarded-array-qualifiers (C and Objective-C only) — do not warn if type qualifiers on arrays which are pointer targets are being discarded. Typically, the compiler warns if a "const int (*)[]" variable is passed to a function that takes a "int (*)[]" parameter. This option can be used to suppress such a warning.
• -Wno-incompatible-pointer-types (C and Objective-C only) — do not warn when there is a conversion between pointers that have incompatible types. This warning is for cases not covered by -Wno-pointer-sign, which warns for pointer argument passing or assignment with different signedness.

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-cast and -Wno-pointer-to-int-cast may 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 you when a literal 0 gets used as a null pointer constant. Like bestie, we have nullptr/NULL now, we don't have to do that anymore 💅
• -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. Okay so undefined behaviour is the villain of this entire manpage, we stay away 😭

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. (Nan slays, honestly 🧚)
• -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 -Wall in conjunction with this option does not warn about unknown pragmas in system headers---for that, -Wunknown-pragmas must also be used. Sneaky little combo, ikyk 🫶
• -Wtautological-compare — warns if a self-comparison always evaluates to true or false. 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. Not me comparing a value to itself and expecting drama 💀

This warning is enabled by -Wall.

• -Wtrailing-whitespace, -Wtrailing-whitespace=kind — warns 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. (Trailing whitespace is giving "left the dishes in the sink" energy, ngl 🌸)
• -Wleading-whitespace=kind — warns 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. Tabs vs spaces discourse but make it a compiler flag 😭
• -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. Executable stack? In this economy? 🚩
• -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. Floating point equality is a trap and this flag is the bestie who tells you 🎀

• -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 -Wtraditional warns about directives that traditional C understands but ignores because the # does not appear as the first character on the line. It also suggests you hide directives like #pragma not understood by traditional C by indenting them. Some traditional implementations do not recognize #elif, so this option suggests avoiding it altogether.
  • A function-like macro that appears without arguments.
  • The unary plus operator.
  • The U integer constant suffix, or the F or L floating-point constant suffixes. (Traditional C does support the L suffix on integer constants.) Note, these suffixes appear in macros defined in the system headers of most modern systems, e.g. the _MIN/_MAX macros in <limits.h>. Use of these macros in user code might normally lead to spurious warnings, however GCC's integrated preprocessor has enough context to avoid warning in these cases.
  • A function declared external in one block and then used after the end of the block.
  • A switch statement has an operand of type long.
  • A non-static function declaration follows a static one. This construct is not accepted by some traditional C compilers.
  • The ISO type of an integer constant has a different width or signedness from its traditional type. This warning is only issued if the base of the constant is ten. I.e. hexadecimal or octal values, which typically represent bit patterns, are not warned about.
  • Usage of ISO string concatenation is detected.
  • Initialization of automatic aggregates.
  • Identifier conflicts with labels. Traditional C lacks a separate namespace for labels.
  • Initialization of unions. If the initializer is zero, the warning is omitted. This is done under the assumption that the zero initializer in user code appears conditioned on e.g. __STDC__ to avoid missing initializer warnings and relies on default initialization to zero in the traditional C case.
  • Conversions by prototypes between fixed/floating-point values and vice versa. The absence of these prototypes when compiling with traditional C causes serious problems. This is a subset of the possible conversion warnings; for the full set use -Wtraditional-conversion.
  • Use of ISO C style function definitions. This warning intentionally is not issued for prototype declarations or variadic functions because these ISO C features appear in your code when using libiberty's traditional C compatibility macros, PARAMS and VPARAMS. This warning is also bypassed for nested functions because that feature is already a GCC extension and thus not relevant to traditional C compatibility.

That's a lot of bullets bestie, but ancient C lore is lowkey fascinating 🦋

• -Wtraditional-conversion (C and Objective-C only) — warns if a prototype causes a type conversion that is different from what would happen to the same argument in the absence of a prototype. This includes conversions of fixed point to floating and vice versa, and conversions changing the width or signedness of a fixed-point argument except when the same as the default promotion.
• -Wdeclaration-after-statement (C and Objective-C only) — warns when a declaration is found after a statement in a block. This construct, known from C++, was introduced with ISO C99 and is by default allowed in GCC. It is not supported by ISO C90. (C90 said no, C99 said slay ✨)

This warning is upgraded to an error by -pedantic-errors.

• -Wshadow — warns whenever a local variable or type declaration shadows another variable, parameter, type, class member (in C++), or instance variable (in Objective-C) or whenever a built-in function is shadowed. Note that in C++, the compiler warns if a local variable shadows an explicit typedef, but not if it shadows a struct/class/enum. If this warning is enabled, it includes also all instances of local shadowing. This means that -Wno-shadow=local and -Wno-shadow=compatible-local are ignored when -Wshadow is used. Same as -Wshadow=global. Stopping the identity theft of your variable names, we love to see it 👑

---

• -Wno-shadow-ivar (Objective-C only) — bestie, this one just tells the compiler not to warn you when a local variable shadows an instance variable inside an Objective-C method. That's it. No drama.
• -Wshadow=global — warns for any shadowing whatsoever. Literally the same thing as plain -Wshadow. Same vibe, same flag, different spelling.
• -Wshadow=local — warns when a local variable shadows another local variable or a parameter. Simple as.
• -Wshadow=compatible-local — warns when a local variable shadows another local variable or parameter whose type is compatible with the shadowing variable. In C++, "type compatible" here means the type of the shadowing variable can be converted to the type of the shadowed variable. Why does this flag even exist (on top of -Wshadow=local)? Because the idea is: when a local shadows another one of an incompatible type, it's most likely on purpose, not a bug or a typo. Exhibit A:

for (SomeIterator i = SomeObj.begin(); i != SomeObj.end(); ++i)
{
  for (int i = 0; i < N; ++i)
  {
    ...
  }
  ...
}

Since those two i variables have incompatible types, turning on only -Wshadow=compatible-local will not emit a warning. And honestly? That tracks — because the types are incompatible, if a programmer accidentally used one instead of the other, type checking is expected to catch it and throw an error or warning anyway. Using this flag instead of -Wshadow=local can possibly cut down the number of warnings you get from intentional shadowing. Note (important, don't skip): this also means shadowing const char *i with char *i does not emit a warning. 🙈

This warning is also enabled by -Wshadow=local.

• -Wlarger-than=byte-size — warns whenever an object is defined whose size exceeds byte-size. Fun fact: -Wlarger-than=PTRDIFF_MAX is enabled by default. You can disable warnings from this option either by specifying a byte-size of SIZE_MAX or more, or with -Wno-larger-than.

It also warns for calls to bounded functions like memchr or strnlen that specify a bound greater than the largest possible object — which is PTRDIFF_MAX bytes by default. Note that these warnings can only be disabled by -Wno-larger-than.

• -Wno-larger-than — disables the -Wlarger-than= warnings. It's equivalent to -Wlarger-than=SIZE_MAX or larger.
• -Wframe-larger-than=byte-size — warns if the size of a function frame exceeds byte-size. Heads up though: the computation it does to figure out the stack frame size is approximate and not conservative. So the actual requirements may be somewhat greater than byte-size even if you don't get a warning 😭. On top of that, any space allocated via alloca, variable-length arrays, or related constructs is not included by the compiler when it's deciding whether to issue a warning. -Wframe-larger-than=PTRDIFF_MAX is enabled by default. You can disable it either by specifying a byte-size of SIZE_MAX or more, or with -Wno-frame-larger-than.
• -Wno-frame-larger-than — disables the -Wframe-larger-than= warnings. Equivalent to -Wframe-larger-than=SIZE_MAX or larger.
• -Wfree-nonheap-object — warns when you're attempting 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. Example time: because the call to stpcpy returns a pointer to the terminating nul character and not to the beginning of the object, the free call below gets diagnosed 👇

void f (char *p)
{
  p = stpcpy (p, "abc");
  // ...
  free (p);   // warning
}

-Wfree-nonheap-object is included in -Wall.

• -Wstack-usage=byte-size — warns if the stack usage of a function might exceed byte-size. Unlike the frame one, the computation here is conservative. And any space allocated via alloca, variable-length arrays, or related constructs is included by the compiler when it decides whether to issue a warning. Nice.

The message matches the output of -fstack-usage. So:

• 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. You can disable these warnings either by specifying a byte-size of SIZE_MAX or more, or with -Wno-stack-usage.

• -Wno-stack-usage — disables -Wstack-usage= warnings. Equivalent to -Wstack-usage=SIZE_MAX or larger.
• -Wunsafe-loop-optimizations — warns if the loop can't be optimized because the compiler can't assume anything about the bounds of the loop indices. And with -funsafe-loop-optimizations, it warns if the compiler makes such assumptions. 🌀
• -Wno-pedantic-ms-format (MinGW targets only) — when you use this in combination with -Wformat and -pedantic without GNU extensions, this option disables the warnings about non-ISO printf / scanf format width specifiers I32, I64, and I used on Windows targets, which depend on the MS runtime.
• -Wpointer-arith — warns about anything that depends on the "size of" a function type or of void. Why? Because GNU C assigns these types a size of 1, for convenience in calculations with void * pointers and pointers to functions. In C++, it also warns when an arithmetic operation involves NULL. This warning is also enabled by -Wpedantic.

Also, this warning gets upgraded to an error by -pedantic-errors. 👑

• -Wno-pointer-compare — do not warn if a pointer is compared with a zero character constant. Because usually that means the pointer was meant to be dereferenced, bestie. Like:

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 — disables warnings about unsupported features in ThreadSanitizer. ThreadSanitizer doesn't support std::atomic_thread_fence and can report false positives, so yeah, that's a thing.
• -Wtype-limits — warns if a comparison is always true or always false because of the limited range of the data type — but it doesn't warn for constant expressions. For example, it warns if an unsigned variable gets compared against zero with < or >=. This warning is also enabled by -Wextra.
• -Wabsolute-value (C and Objective-C only) — warns for calls to standard functions that compute the absolute value of an argument when a more appropriate standard function exists. Example: calling abs(3.14) triggers the warning, because the appropriate function to call for the absolute value of a double argument is fabs. The option also triggers warnings when the argument in a call to such a function has an unsigned type. You can suppress this warning with an explicit type cast, and it's also enabled by -Wextra.
• -Wcomment
• -Wcomments — warns whenever a comment-start sequence /* appears inside a /* comment, or whenever a backslash-newline appears inside a // comment. This warning is enabled by -Wall.
• -Wtrigraphs — warns 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 option is implied by -Wall. And if -Wall isn't given, this option is still enabled unless trigraphs are enabled. Want trigraph conversion without the warnings, but still want the other -Wall warnings? Do this:

-trigraphs -Wall -Wno-trigraphs

• -Wundef — warns if an undefined identifier is evaluated in an #if directive. Just so you know, such identifiers get replaced with zero.
• -Wexpansion-to-defined — warns whenever defined is encountered in the expansion of a macro (including the case where the macro is expanded by an #if directive). Such usage is not portable, periodt. This warning is also enabled by -Wpedantic and -Wextra.
• -Wunused-macros — warns about macros defined in the main file that are unused. A macro counts as used if it's expanded or tested for existence at least once. The preprocessor also warns if the macro hasn't been used at the time it's redefined or undefined.

Built-in macros, macros defined on the command line, and macros defined in include files are not warned about. 🫶

Note: if a macro is actually used, but only used in skipped conditional blocks, then the preprocessor reports it as unused. To dodge the warning in that case, you might improve the scope of the macro's definition by, for example, moving it into the first skipped block. Or you could just provide a dummy use like:

#if defined the_macro_causing_the_warning
#endif

• -Wno-endif-labels — do not warn whenever an #else or an #endif are followed by text. This sometimes happens in older programs, with code shaped like:

#if FOO
...
#else FOO
...
#endif FOO

The second and third FOO should be in comments. This warning is on by default.

• -Wbad-function-cast (C and Objective-C only) — warns when a function call is cast to a non-matching type. Example: warns if a call to a function returning an integer type gets cast to a pointer type.
• -Wc90-c99-compat (C and Objective-C only) — warns about features that aren't present in ISO C90 but are present in ISO C99. For instance, it warns about use of variable length arrays, long long type, bool type, compound literals, designated initializers, and so on. This option is independent of the standards mode. Warnings are disabled in the expression that follows __extension__.
• -Wc99-c11-compat (C and Objective-C only) — warns about features that aren't present in ISO C99 but are present in ISO C11. For instance, it warns about use of anonymous structures and unions, _Atomic type qualifier, _Thread_local storage-class specifier, _Alignas specifier, Alignof operator, _Generic keyword, and so on. This option is independent of the standards mode. Warnings are disabled in the expression that follows __extension__.
• -Wc11-c23-compat (C and Objective-C only), -Wc11-c2x-compat (C and Objective-C only) — bestie, this one warns you about features that aren’t in ISO C11 but are in ISO C23. Think: omitting the string in _Static_assert, using [[]] syntax for attributes, using decimal floating-point types, and so on. It’s independent of the standards mode, and warnings are disabled in the expression that follows __extension__. Also lowkey important: the name -Wc11-c2x-compat is deprecated. 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) — same vibe, next era: warns about features not present in ISO C23 but present in ISO C2Y. For instance, warn about _Generic selecting with a type name instead of an expression. Independent of the standards mode, warnings disabled in the expression that follows __extension__. When not compiling in C2Y mode, these warnings are upgraded to errors by -pedantic-errors. (The original lists this exact line twice, so consider it double-confirmed, periodt. ✨)
• -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 from void * to a pointer to non-void type. It’s giving “C, but make it C++-friendly.” 🦋
• -Wc++11-compat (C++ and Objective-C++ only) — warns about C++ constructs whose meaning differs between ISO C++ 1998 and ISO C++ 2011, e.g. identifiers in ISO C++ 1998 that are keywords in ISO C++ 2011. This warning turns on -Wnarrowing and is enabled by -Wall. 👑
• -Wc++14-compat (C++ and Objective-C++ only) — 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.
• -Wc++26-compat (C++ and Objective-C++ only) — warns about C++ constructs whose meaning differs between ISO C++ 2023 and upcoming ISO C++ 2026. Enabled by -Wall.
• -Wno-c++11-extensions (C++ and Objective-C++ only) — do not warn about C++11 constructs in code being compiled using an older C++ standard. Even without this option, some C++11 constructs will only be diagnosed if -Wpedantic is used. 🫶
• -Wno-c++14-extensions (C++ and Objective-C++ only) — do not warn about C++14 constructs in code being compiled using an older C++ standard. Even without this option, some C++14 constructs will only be diagnosed if -Wpedantic is used.
• -Wno-c++17-extensions (C++ and Objective-C++ only) — do not warn about C++17 constructs in code being compiled using an older C++ standard. Even without this option, some C++17 constructs will only be diagnosed if -Wpedantic is used.
• -Wno-c++20-extensions (C++ and Objective-C++ only) — do not warn about C++20 constructs in code being compiled using an older C++ standard. Even without this option, some C++20 constructs will only be diagnosed if -Wpedantic is used.
• -Wno-c++23-extensions (C++ and Objective-C++ only) — do not warn about C++23 constructs in code being compiled using an older C++ standard. Even without this option, some C++23 constructs will only be diagnosed if -Wpedantic is used.
• -Wno-c++26-extensions (C++ and Objective-C++ only) — do not warn about C++26 constructs in code being compiled using an older C++ standard. Even without this option, some C++26 constructs will only be diagnosed if -Wpedantic is used.
• -Wcast-qual — warns whenever a pointer is cast so as to remove a type qualifier from the target type. For example, warn if a const char * is cast to an ordinary char *. It also warns when you make a cast that introduces a type qualifier in an unsafe way. For example, casting char ** to const char ** is unsafe, as in this example:

/* p is char ** value.  */
const char **q = (const char **) p;
/* Assignment of readonly string to const char * is OK.  */
*q = "string";
/* Now char** pointer points to read-only memory.  */
**p = 'b';

Yeah, that last line is the “oops, we wrote to read-only memory” moment. Not iconic. 💀

• -Wcast-align — warns whenever a pointer is cast so that the required alignment of the target is increased. For example, warn if a char * is cast to an int * on machines where integers can only be accessed at two- or four-byte boundaries. And then, yes, it says it again: warn whenever a pointer is cast so the required alignment of the target is increased — for example, warn if a char * is cast to an int * regardless of the target machine. Double warning energy, no cap. 🎀
• -Wcast-function-type — warns when a function pointer is cast to an incompatible function pointer. In a cast involving function types with a variable argument list, only the types of initial arguments that are provided are considered. Any parameter of pointer-type matches any other pointer-type. Any benign differences in integral types are ignored, like int vs. long on ILP32 targets. Likewise type qualifiers are ignored. The function type void (*) (void) is special and matches everything, which can be used to suppress this warning. In a cast involving pointer to member types, this warning warns whenever the type cast is changing the pointer to member type. Enabled by -Wextra. 🔥
• -Wcast-user-defined — warns when a cast to reference type does not involve a user-defined conversion that the programmer might expect to be called. Like this:

struct A { operator const int&(); } a;
auto r = (int&)a; // warning

Enabled by default, bestie. It’s watching. 👀

• -Wwrite-strings — when compiling C, gives string constants the type const char[length] so that copying the address of one into a non-const char * pointer produces a warning. These warnings help you find, at compile time, code that can try to write into a string constant — but only if you have been very careful about using const in declarations and prototypes. Otherwise it’s just a nuisance. That’s why we did not make -Wall request these warnings. When compiling C++, warn about the deprecated conversion from string literals to char *. This warning is enabled by default for C++ programs. It’s upgraded to an error by -pedantic-errors in C++11 mode or later. 😭
• -Wclobbered — warns for variables that might be changed by longjmp or vfork. Also enabled by -Wextra. Basically: “bestie, this variable is about to get jumped over, watch out.” 🦋
• -Wno-complain-wrong-lang — by default, language front ends complain when a command-line option is valid, but not applicable to that front end. This can be disabled with -Wno-complain-wrong-lang, which is mostly useful when invoking a single compiler driver for multiple source files written in different languages, for example:

$ g++ -fno-rtti a.cc b.f90

The driver g++ invokes the C++ front end to compile a.cc and the Fortran front end to compile b.f90. The 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. This option can also be used to disable warnings like:

cc1plus: note: CTF debug info requested, but not supported for 'GNU C++20' frontend

produced by -gctf or -gsctf for unsupported languages. ✨

• -Wcompare-distinct-pointer-types (C and Objective-C only) — warns if pointers of distinct types are compared without a cast. Enabled by default. Keep your pointer types aligned, bestie. 💅
• -Wconversion — warns for implicit conversions that may alter a value. This includes conversions between real and integer, like abs (x) when x is double; conversions between signed and unsigned, like unsigned ui = -1; and conversions to smaller types, like sqrtf (M_PI). It does not warn for explicit casts like abs ((int) x) and ui = (unsigned) -1, or if the value is not changed by the conversion like in abs (2.0). Warnings about conversions between signed and unsigned integers can be disabled by using -Wno-sign-conversion. For C++, 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. That’s a lot, but she ate. 🔥
• -Wdangling-else — warns about constructions where there may be confusion about which if statement an else branch belongs to. Here’s 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 the 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. Braces are your besties here, fr fr. 🎀

• -Wdangling-pointer, -Wdangling-pointer=n — 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. Keep it chill, level 1. 🌸
• -Wdangling-pointer=2 — level 2 is level 1's overachieving sibling 💅 like yes bestie we already warn about unconditional uses, but at level 2 we also call out conditional uses of dangling pointers. iykyk.

So the short form -Wdangling-pointer is literally just -Wdangling-pointer=2 wearing a shorter outfit, while -Wno-dangling-pointer and -Wdangling-pointer=0 both do the same thing: turn the warnings off entirely. and btw -Wdangling-pointer=2 is included in -Wall, no cap.

This example right here triggers the warning at level 1 — the address of the unnamed temporary gets referenced unconditionally outside of its scope, which is giving major red flag energy:

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;
}

And in this next function, storing the address of the local variable x into 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 by the time the pointer s gets used 😭 and 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 you when macros __TIME__, __DATE__ or __TIMESTAMP__ show up, because they might ruin bitwise-identical reproducible compilations. so if you're out here trying to get your builds to match exactly, these macros are the enemy fr fr.
• -Wempty-body — warns if there's an empty body in an if, else or do while statement. like bestie those braces are empty, that's suspicious. also enabled by -Wextra.
• -Wno-endif-labels — do not warn about stray tokens after #else and #endif. that's it, that's the whole vibe.
• -Wenum-compare — warns about comparing values of different enumerated types. in C++ the enum type mismatches in conditional expressions also get diagnosed, and the warning is on by default. in C it's enabled by -Wall.
• -Wenum-conversion — warns when a value of enumerated type gets implicitly converted to a different enumerated type 🙈 in C this one is enabled by -Wextra.
• -Wenum-int-mismatch (C and Objective-C only) — warns about mismatches between an enumerated type and an integer type in declarations. example time:

enum E { l = -1, z = 0, g = 1 };
int foo(void);
enum E foo(void);

so in C an enumerated type is compatible with char, a signed integer type, or an unsigned integer type. but since the choice of the underlying type of an enumerated type is implementation-defined, these mismatches can cause portability issues 👀 in C++ these mismatches are straight up an error. in C, this warning is enabled by -Wall and -Wc++-compat.

• -Wjump-misses-init (C, Objective-C only) — warns if a goto statement or a switch statement jumps forward across the initialization of a variable, or jumps backward to a label after the variable has been initialized. it only warns about variables that are initialized when they're declared. this warning is only supported for C and Objective-C — in C++ this kind of branch is an error anyway, so it's moot.

-Wjump-misses-init is included in -Wc++-compat, and you can turn it off with the -Wno-jump-misses-init option.

• -Wsign-compare — warns when a comparison between signed and unsigned values could produce an incorrect result when the signed value gets converted to unsigned. in C++ this warning is also enabled by -Wall; in C it's also enabled by -Wextra.
• -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, slay. in C, this option is also enabled by -Wconversion.
• -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. so it warns about stuff like this:

struct flex  { int length; char data[]; };
struct mid_flex { int m; struct flex flex_data; int n; };

• -Wfloat-conversion — warns for implicit conversions that reduce the precision of a real value 🥲 that includes conversions from real to integer, and from higher-precision real to lower-precision real values. this option is also enabled by -Wconversion.
• -Wno-scalar-storage-order — do not warn on suspicious constructs involving reverse scalar storage order.
• -Wsizeof-array-div — warns about divisions of two sizeof operators when the first one is applied to an array and the divisor doesn't equal the size of the array element. in that case the computation won't give you the number of elements in the array, which is probably what you actually wanted 😭 it warns about stuff like:

int fn ()
{
  int arr[10];
  return sizeof (arr) / sizeof (short);
}

this warning is enabled by -Wall.

• -Wsizeof-pointer-div — warns for suspicious divisions of two sizeof expressions that divide the pointer size by the element size — which is the usual way you compute array size but absolutely will not work correctly with pointers. it warns about e.g. sizeof (ptr) / sizeof (ptr[0]) if ptr is a pointer and not an array. enabled by -Wall, periodt.
• -Wsizeof-pointer-memaccess — warns for suspicious length parameters passed to certain string and memory built-in functions when the argument uses sizeof. it triggers e.g. for memset (ptr, 0, sizeof (ptr)); if ptr is a pointer and not an array, and it even suggests a possible fix 🫶 or it triggers for memcpy (&foo, ptr, sizeof (&foo));.

-Wsizeof-pointer-memaccess also warns about calls to bounded string copy functions like strncat or strncpy that specify a sizeof expression of the source array as the bound. example: in the following function the call to strncat specifies the size of the source string as the bound — that is almost certainly a mistake, so the call gets 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.

• -Wno-sizeof-array-argument — do not warn when the sizeof operator is applied to a parameter that's declared as an array in a function definition. this warning is enabled by default for C and C++ programs.
• -Wmemset-elt-size — warns for suspicious calls to the memset built-in function when 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. that means the user forgot to multiply by the element size 💀 this warning is enabled by -Wall.
• -Wmemset-transposed-args — warns for suspicious calls to the memset built-in function where the second argument isn't zero and the third argument is zero. example: the call memset (buf, sizeof buf, 0) gets diagnosed because memset (buf, 0, sizeof buf) was meant instead 🙈 the diagnostic is only emitted if the third argument is a literal zero — otherwise, if it's an expression that folds to zero, or a cast of zero to some type, it's far less likely the arguments got transposed and no warning is emitted. 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 this:

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 this:

void f (const int *p, int i)
{
  return p + i == NULL;
}

these uses typically mean the programmer messed up: the address of most functions and objects necessarily evaluates to true (the exception being weak symbols), so using them in a conditional might indicate missing parentheses in a function call or a missing 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 like intptr_t or uintptr_t. comparisons against string literals result in unspecified behavior and are not portable, and they suggest the intent was to call strcmp. the warning is suppressed if the suspicious expression is the result of macro expansion. -Waddress 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 bitwise operator is probably expected, and 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 doesn't warn if the right operand is considered to be a boolean expression. its whole purpose is to catch suspicious code like this:

int a;
...
if (!a > 1) { ... }

...and honestly if you just wrap the LHS in parens, the warning goes bye-bye 🫶:

if ((!a) > 1) { ... }

This warning is enabled by -Wall.

• -Waggregate-return — warns you whenever any function that returns a struct or a union gets defined or called. (And in languages where returning an array is a thing, that also gets you a warning, bestie.)
• -Wno-aggressive-loop-optimizations — turns off the warning you'd get when the compiler spots undefined behavior inside a loop with a constant number of iterations. Just so you know: -Waggressive-loop-optimizations is enabled by default ✨
• -Wno-attributes — don't warn when an unexpected __attribute__ shows up — like attributes nobody recognizes, or function attributes slapped onto variables, that whole mess. Heads up though: this does not stop errors for incorrect use of supported attributes.

Warnings about ill-formed uses of standard attributes get upgraded to errors by -pedantic-errors. 👀

Also, with -Wno-attributes=, you can suppress warnings about unknown scoped attributes (in C++11 and C23). Like, -Wno-attributes=vendor::attr kills the warning for this declaration:

[[vendor::attr]] void f();

You can also nuke warnings for everything in a namespace using -Wno-attributes=vendor::, which prevents warnings for both of these:

[[vendor::safe]] void f();
[[vendor::unsafe]] void f2();

Careful babe: -Wno-attributes= does not imply -Wno-attributes. They're different things, fr fr.

• -Wno-builtin-declaration-mismatch — warns if a built-in function is declared with an incompatible signature, or declared as a non-function, or when a built-in function declared without a prototype gets called with arguments whose promoted types don't match what the function expects. And with -Wextra? You also get warned when a built-in function that takes arguments is declared without a prototype. The -Wbuiltin-declaration-mismatch warning is on by default. To dodge it entirely, just include the right header so the built-in prototypes are in scope 💅

Like, check this — the memset call below gets diagnosed because the function wants a size_t but the type of 32 is int. And with -Wextra, the declaration itself gets diagnosed too:

extern void* memset ();
void f (void *d)
{
  memset (d, '\0', 32);
}

• -Wno-builtin-macro-redefined — don't warn if certain built-in macros get redefined. This suppresses warnings for redefinition of __TIMESTAMP__, __TIME__, __DATE__, __FILE__, and __BASE_FILE__.
• -Wkeyword-macro — warns if a keyword is defined as a macro or undefined. For C++, identifiers with special meaning or standard attribute identifiers get diagnosed too. This one is enabled by default for C++26 when -Wpedantic is on, and it emits a pedwarn in that case.
• -Wfree-labels (C and Objective-C only) — warns if a label is applied to a non-statement, or shows up at the end of a compound statement. C23 and later C dialects allow these labels, and they're available as a GCC extension in all the other dialects too.

This warning is also enabled by -Wc11-c23-compat. And if you're building for a C version before C23, -pedantic-errors turns it into an error.

• -Wheader-guard — warns when a valid preprocessor header multiple-inclusion guard has a #define directive right after the #ifndef or #if !defined directive for the guard... but it defines a different macro from the guard macro with a similar name, the actual guard macro isn't defined at the matching #ifndef at the end of the header, and that #define defines an object-like macro with an empty definition. Basically: it's usually just a typo'd guard name, either in the #ifndef / #if !defined directive or in the #define right after it 🙈 This warning is enabled by -Wall.
• -Wstrict-prototypes (C and Objective-C only) — warns if a function is declared or defined without specifying its argument types. (An old-style function definition is fine, no warning, if it's preceded by a declaration that does specify the argument types.)
• -Wold-style-declaration (C and Objective-C only) — warns about obsolescent usages in a declaration, per the C Standard. Like, it'll warn if storage-class specifiers such as static aren't the first thing 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 gets used. You get the warning even if there's a previous prototype. A definition using () is not considered old-style in C23 mode, because it's equivalent to (void) there — but for older standards it is old-style.
• -Wmultiple-parameter-fwd-decl-lists (C and Objective-C only) — warns if more than one list of forward declarations of parameters appears in a function prototype. This one's also enabled by -Wextra.
• -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 shows up after such a declaration. Both of those are straight-up errors in C23 and later C dialects 💀

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 and only gives its type. Which, fun fact, you can do to document that a parameter is unused in the definition 🎀 It's part of C23 and later C dialects, and available as a GCC extension in all the others.

Also enabled by -Wc11-c23-compat. And -pedantic-errors turns it into an error if you're building for a C version before C23.

• -Wmissing-parameter-type (C and Objective-C only) — a function parameter 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) — don't warn if a function declaration contains a parameter name without a type. Those declarations don't provide a function prototype and they block most type checking on function calls, jsyk.

This warning is enabled by default. In C99 and later C dialects it's treated as an error. You can downgrade that error to a warning with -fpermissive (along with certain other errors), or for this error alone with -Wno-error=declaration-missing-parameter-type.

-pedantic-errors also upgrades this warning to an error.

• -Wmissing-prototypes (C and Objective-C only) — warns if a global function is defined without a previous prototype declaration. And yeah, it fires even if the definition itself provides a prototype. Use this to catch global functions that don't have a matching prototype declaration in a header file. Not valid for C++ though, because in C++ all function declarations provide prototypes, and a non-matching declaration just declares an overload instead of conflicting with an earlier one. For C++, use -Wmissing-declarations to catch missing declarations 💖
• -Wmissing-variable-declarations (C and Objective-C only) — warns if a global variable is defined without a previous declaration. Use it to catch global variables that don't have a matching extern declaration in a header file.
• -Wmissing-declarations — warns if a global function is defined without a previous declaration — even if the definition itself provides a prototype. Use it to catch global functions that aren't declared in header files. In C, no warnings are issued for functions with previous non-prototype declarations (use -Wmissing-prototypes for those). In C++, no warnings for function templates, inline functions, or functions in anonymous namespaces. ✨
• -Wmissing-field-initializers — warns when a structure's initializer is missing some fields. Like, this code triggers it because x.h is 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 modification is chill:

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, like:

struct s { int f, g, h; };
s x = { };

This warning is included in -Wextra. Want the other -Wextra warnings without this one? Use -Wextra -Wno-missing-field-initializers.

• -Wno-missing-requires — by default, the compiler warns about a concept-id appearing 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 you meant to write:

bool satisfied = requires { requires C<T> };

so that satisfied is only true if concept C is satisfied for type T. Yeah. Classic 😭 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 you can get false positives. To silence those, wrap the expression in parentheses. Like, 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.
}

Oh and also — that last one? You can turn it off with -Wno-missing-template-keyword. Cute, love that for you 💅

---

• -Wno-multichar — bestie, this shuts off the warning for multicharacter constants like 'FOOF'. Why would you want that warning off? Because those things almost always mean somebody typo'd, they have implementation-defined values, and portable code should not be caught holding one. 🚩
• -Wnormalized=[none|id|nfc|nfkc] — the Unicode vibe check option ✨

Okay so in ISO C and ISO C++, two identifiers are different if they're different sequences of characters. But plot twist: once you leave basic ASCII, you can have two different character sequences that look literally identical. To avoid that chaos, the ISO 10646 standard lays out normalization rules — apply them and two same-looking sequences become the same sequence. GCC can warn you when your identifiers aren't normalized, and this option is the remote control for that warning. 🎛️

There are four levels, and the default is -Wnormalized=nfc, which warns about any identifier not in the ISO 10646 "C" normalized form, NFC. NFC is the recommended form for most uses, and it's equivalent to plain -Wnormalized.

Now the sad part 😭: some characters that ISO C and ISO C++ allow in identifiers are, once turned into NFC, no longer allowed in identifiers. Translation: there is genuinely no way to use those symbols in portable ISO C or C++ while keeping all your identifiers in NFC. -Wnormalized=id suppresses the warning for those characters. The hope is that future versions of the standards will fix this — which is exactly why this option is not the default.

You can turn the warning off for every character by writing -Wnormalized=none or -Wno-normalized. Only do that if you're using some other normalization scheme (like "D"), because otherwise you can easily create bugs that are literally impossible to see. 💀

Also: some characters in ISO 10646 have distinct meanings but look identical in certain fonts or display methodologies, especially after formatting gets applied. Example: "\u207F", "SUPERSCRIPT LATIN SMALL LETTER N", displays just like a regular "n" that's been put in superscript. ISO 10646 defines the NFKC normalization scheme to convert all of those into a standard form too, and GCC warns if your code isn't in NFKC when you use -Wnormalized=nfkc. This warning is comparable to warning about every identifier containing the letter O because it might be confused with the digit 0 — hence it's not the default. But it can be useful as a local coding convention if your programming environment just can't be fixed to display these characters distinctly. 🧚

• -Wno-attribute-warning — don't warn about usage of functions declared with the "warning" attribute. By default this warning is enabled. -Wno-attribute-warning disables the warning, or -Wno-error=attribute-warning disables the error when you're compiling with the -Werror flag. 🫶
• -Wno-deprecated — don't warn about usage of deprecated features.

In C++, explicitly specifying -Wdeprecated also turns on warnings about some features 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. That's a whole squad 🔥

• -Wno-deprecated-declarations — don't warn about uses of functions, variables, or types marked as deprecated via the "deprecated" attribute.
• -Wno-deprecated-openmp — don't warn about deprecated OpenMP code.
• -Wno-overflow — don't 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 the user set. You can relax the cost model with the -fsimd-cost-model=unlimited option. 🌸
• -Woverride-init (C and Objective-C only) — warn if an initialized field without side effects gets overridden when using designated initializers.

This warning is included in -Wextra. Want the other -Wextra warnings without this one? Use -Wextra -Wno-override-init. 🎀

• -Wno-override-init-side-effects (C and Objective-C only) — don't 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 that attribute has zero effect on the layout or size of the structure. Those structures may end up mis-aligned for basically no benefit. Like, in this code, the variable "f.x" in "struct bar" is misaligned even though "struct bar" doesn't even have the packed attribute itself:

struct foo {
  int x;
  char a, b, c, d;
} __attribute__((packed));
struct bar {
  char z;
  struct foo f;
};

• -Wnopacked-bitfield-compat — story time 📖 the 4.1, 4.2 and 4.3 series of GCC ignore the "packed" attribute on bit-fields of type "char". That got fixed in GCC 4.4, but the change can lead to differences in structure layout. GCC lets you know when the offset of such a field has changed in GCC 4.4. For example, there's no longer 4 bits of 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 it. 💜

• -Wpacked-not-aligned (C, C++, Objective-C and Objective-C++ only) — warn if a structure field with explicitly specified alignment inside a packed struct or union is misaligned. For example, a warning gets 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 gets included in a structure, whether to align an element of the structure or to align the whole structure. Sometimes when this happens you can rearrange the fields to cut down the padding and make the structure smaller. 📐
• -Wredundant-decls — warn if anything is declared more than once in the same scope, even when the multiple declaration is valid and changes absolutely 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. Example: the call to the "strcpy" function below tries to truncate the string by replacing its initial characters with the last four. But because the call writes the terminating NUL into "a[4]", the copies overlap and the call gets diagnosed. 🚨

void foo (void)
{
  char a[] = "abcd1234";
  strcpy (a, a + 4);
  ...
}

The -Wrestrict option catches some instances of simple overlap even with no optimization, but it works best at -O2 and above. It's included in -Wall.

• -Wnested-externs (C and Objective-C only) — warn if an "extern" declaration shows up inside a function. 🙈
• -Winline — warn if a function declared as inline cannot actually be inlined. Even with this option, the compiler does not warn about failures to inline functions declared in system headers.

The compiler uses a whole variety of heuristics to decide whether to inline a function. For example, it takes into account the size of the function being inlined and how much inlining has already been done in the current function. So seemingly insignificant changes in your source program can make the warnings from -Winline appear or disappear. Whimsical, honestly. 🦋

• -Winterference-size
• -Wno-interference-size — warn about questionable uses of the C++17 "constexpr" variables "std::hardware_destructive_interference_size" and "std::hardware_constructive_interference_size".

These variables are meant 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 meant to be far enough apart that stores to one won't force accesses to two to reload the cache line.

By default these variables get values based on the current -mtune option — typically the L1 cache line size for the particular target CPU, sometimes a range if tuning for a generic target. So every translation unit that depends on ABI compatibility for the use of these variables has to be compiled with the same -mtune (or -mcpu). ⚠️

If ABI stability matters — like if the use is in a header for a library — you should probably not use the hardware interference size variables at all.

These warnings are enabled by default. If you're confident your use of these variables doesn't affect ABI outside a single build of your project, you can turn the warning off with -Wno-interference-size. 🫶

• -Wint-in-bool-context — warn about suspicious use of integer values where booleans are expected, like conditional expressions (?:) using non-boolean integer constants in boolean context, e.g. "if (a <= b ? 2 : 3)". Or left shifting signed integers in boolean context, like "for (a = 0; 1 << a; a++);". Same deal for all kinds of multiplications regardless of the data type. This warning is enabled by -Wall. 🔥
• -Wno-int-to-pointer-cast — suppress warnings from casts to pointer type of an integer of a different size. In C++, casting to a pointer type of smaller size is an error. Wint-to-pointer-cast is enabled by default. 💀
• -Wno-pointer-to-int-cast (C and Objective-C only) — bestie, this one shuts off the warnings you'd get when you cast a pointer to an integer type of a different size. Like yeah, that's sus, but we're not gonna @ you about it anymore 💅
• -Winvalid-pch — lowkey a vibe check for your precompiled headers: warns you if a PCH is sitting there in the search path but it just... cannot be used 😭
• -Winvalid-utf8 — warns if an invalid UTF-8 character shows up. For C++23 this one's on by default if you're using -finput-charset=UTF-8, and it gets upgraded into a full-on error with -pedantic-errors 👑
• -Wno-unicode — don't diagnose invalid forms of delimited or named escape sequences that get treated as separate tokens. Wunicode is enabled by default, fyi 🌈
• -Wlong-long — warns when the long long type is used. It gets turned on by either -Wpedantic or -Wtraditional in ISO C90 and C++98 modes. Wanna shut it up? -Wno-long-long is your girl 💖

This warning is upgraded to an error by -pedantic-errors. periodt.

• -Wvariadic-macros — warns if variadic macros appear in ISO C90 mode, or if the GNU alternate syntax shows up in ISO C99 mode. Enabled by either -Wpedantic or -Wtraditional. To inhibit the messages, use -Wno-variadic-macros 🎀
• -Wno-varargs — no warnings about questionable usage of the macros that handle variable arguments, like va_start. These warnings are enabled by default, so this is you telling them to chill 🙈
• -Wvector-operation-performance — warns when a vector operation isn't implemented via the SIMD capabilities of the architecture. Mainly a glow-up tool for performance tuning ✨ Vector operations can be implemented:
  • piecewise — the scalar operation runs on every vector element
  • in parallel — the vector operation is implemented using scalars of a wider type, which normally is more performance efficient
  • as a single scalar — the vector fits into a scalar type
• -Wvla — warns if a variable-length array is used in the code. And -Wno-vla prevents the -Wpedantic warning about the variable-length array 🦋

This warning is upgraded to an error by -pedantic-errors.

• -Wvla-larger-than=byte-size — with this on, the compiler warns about declarations of variable-length arrays whose size is either unbounded, or bounded by an argument that lets the array size exceed byte-size bytes. It's basically how -Walloca-larger-than=byte-size works, but for VLAs 💜

Ngl, GCC may optimize small variable-length arrays of a known value into plain arrays, so this warning might not even fire for those.

-Wvla-larger-than=PTRDIFF_MAX is enabled by default, but it's typically only effective when -ftree-vrp is active (default for -O2 and above).

See also -Walloca-larger-than=byte-size.

• -Wno-vla-larger-than — disables the -Wvla-larger-than= warnings. It's equivalent to -Wvla-larger-than=SIZE_MAX or larger ✅
• -Wvla-parameter — warns about redeclarations of functions involving arguments of Variable Length Array types of inconsistent kinds or forms, and enables detection of out-of-bounds accesses to such parameters by warnings like -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. Also, calling f with the associated VLA bound parameter in excess of the actual VLA bound triggers a warning too:

          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. And the -Warray-parameter option triggers warnings for similar problems involving ordinary array arguments 🫶

• -Wvolatile-register-var — warns if a register variable is declared volatile. Heads up: 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) — disables warnings about uses of ^, the exclusive or operator, where it looks like the code meant exponentiation. Specifically, the warning fires when the left-hand side is the decimal constant 2 or 10 and 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. Not the same thing, bestie 😭

You can silence this warning by converting one of the operands to hexadecimal, or by compiling with -Wno-xor-used-as-pow.

• -Wdisabled-optimization — warns if a requested optimization pass gets disabled. This doesn't generally mean anything's wrong with your code; it just means GCC's optimizers can't 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. Only supported for C and Objective-C. It's implied by -Wall and by -Wpedantic, which you can turn off with -Wno-pointer-sign 🌸

This warning is upgraded to an error by -pedantic-errors.

• -Wstack-protector — only active when -fstack-protector is active. It warns about functions that aren't protected against stack smashing 🛡️
• -Woverlength-strings — warns about string constants longer than the "minimum maximum" length specified in the C standard. Modern compilers generally allow string constants 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 doesn't specify a normative minimum maximum, so we don't 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 doesn't have a suffix. When used together with -Wsystem-headers it warns about such constants in system header files too. Useful when you're prepping code to use with the FLOAT_CONST_DECIMAL64 pragma from the decimal floating-point extension to C99 ✨
• -Wno-lto-type-mismatch — during link-time optimization, don't warn about type mismatches in global declarations from different compilation units. Requires -flto to 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's been marked with the designated_init attribute 🙅‍♀️

---

🔍 Options That Control Static Analysis 🔍

• -fanalyzer — this option enables a static analysis of program flow that looks for "interesting" interprocedural paths through the code, and issues warnings for problems found on them 👑

Ngl, this analysis is much more expensive than other GCC warnings.

In technical terms, it performs coverage-guided symbolic execution of the code being compiled. It is neither sound nor complete: it can have false positives and false negatives. It's a bug-finding tool, not a tool for proving program correctness, bestie 💅

The analyzer is only suitable for use on C code in this release.

Enabling this option effectively enables all of 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-throw-of-unexpected-type
-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

This option is only available if GCC was configured with analyzer support enabled 🙈

• -Wanalyzer-symbol-too-complex — if -fanalyzer is enabled, the analyzer uses various heuristics to attempt to track the state of memory, but these can be defeated by sufficiently complicated code 😵‍💫

By default, the analysis just… silently gives up on tracking the values of expressions once they blow past an internal limit, and swaps in a fuzzy, imprecise representation for those expressions instead. The -Wanalyzer-symbol-too-complex option is the one that'll actually warn you when this happens. 🔥

• -Wanalyzer-too-complex — if -fanalyzer is on, the analyzer busts out a bunch of heuristics to try to explore the control flow and data flow in your program, but bestie, sufficiently complicated code can absolutely defeat them 💀

By default, the analysis just quietly stops if the code is too complicated for the analyzer to fully explore and it hits an internal limit. The -Wanalyzer-too-complex option warns you when that occurs.

• -Wno-analyzer-allocation-size — this warning needs -fanalyzer, which enables it; to disable it, use -Wno-analyzer-allocation-size.

This diagnostic warns about 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). Like, the math is not mathing 😭

See CWE-131: Incorrect Calculation of Buffer Size (https://cwe.mitre.org/data/definitions/131.html).

• -Wno-analyzer-deref-before-check — this warning needs -fanalyzer, which enables it; use -Wno-analyzer-deref-before-check to disable it.

This diagnostic warns about paths through the code where a pointer gets checked for NULL after it's already been dereferenced — which is giving "the pointer could have been NULL the whole time" energy 🙈 Such cases suggest the NULL check is either redundant, or it needs to be moved to before the pointer is dereferenced.

This diagnostic also treats values passed to a function argument marked with __attribute__((nonnull)) as requiring a non-NULL value, so it will complain if such values are checked for NULL after returning from such a function call.

This diagnostic is unlikely to be reported when any level of optimization is enabled, because GCC's optimization logic will typically see such checks for NULL as redundant and optimize them away before the analyzer ever "sees" them. So, optimization should be disabled when you're trying to trigger this diagnostic. Lowkey important, ngl.

• -Wno-analyzer-double-fclose — this warning needs -fanalyzer, which enables it; use -Wno-analyzer-double-fclose to disable it.

This diagnostic warns about paths through the code where a FILE * can have fclose called on it more than once. Closing it twice is not the vibe 🎀

See CWE-1341: Multiple Releases of Same Resource or Handle (https://cwe.mitre.org/data/definitions/1341.html).

• -Wno-analyzer-double-free — this warning needs -fanalyzer, which enables it; use -Wno-analyzer-double-free to disable it.

This diagnostic warns about paths through the code where a pointer can have a deallocator called on it more than once — either free, or a deallocator referenced by the malloc attribute. Double free? In this economy? 💀

See CWE-415: Double Free (https://cwe.mitre.org/data/definitions/415.html).

• -Wno-analyzer-exposure-through-output-file — this warning needs -fanalyzer, which enables it; use -Wno-analyzer-exposure-through-output-file to disable it.

This diagnostic warns about paths through the code where a security-sensitive value gets written to an output file (think: writing a password into a log file, bestie, no 😭).

See CWE-532: Information Exposure Through Log Files (https://cwe.mitre.org/data/definitions/532.html).

• -Wanalyzer-exposure-through-uninit-copy — this warning needs both -fanalyzer and the use of a plugin to specify a function that copies across a "trust boundary". Use -Wno-analyzer-exposure-through-uninit-copy to disable it.

This diagnostic warns about "infoleaks" — paths through the code where uninitialized values get copied across a security boundary (like code inside an OS kernel that copies a partially-initialized struct on the stack over 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 needs -fanalyzer, which enables it; use -Wno-analyzer-fd-access-mode-mismatch to disable it.

This diagnostic warns about paths through code where a read on a write-only file descriptor is attempted, or vice versa. Pick a lane, file descriptor 💅

This diagnostic also warns about 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 needs -fanalyzer, which enables it; use -Wno-analyzer-fd-double-close to disable it.

This diagnostic warns about 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 needs -fanalyzer, which enables it; use -Wno-analyzer-fd-leak to disable it.

This diagnostic warns about paths through code where an open file descriptor gets leaked. It's giving "held onto it and never let go" but make it a bug 🦋

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 needs -fanalyzer, which enables it; use -Wno-analyzer-fd-phase-mismatch to disable it.

This diagnostic warns about paths through code where an operation is attempted in the wrong phase of a file descriptor's lifetime. For example, it'll warn on attempts to call accept on a stream socket that hasn't had listen successfully called on it yet. Girl, you skipped a step 😭

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 needs -fanalyzer, which enables it; use -Wno-analyzer-fd-type-mismatch to disable it.

This diagnostic warns about paths through code where an operation is attempted on the wrong type of file descriptor. For example, it'll 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. Not the right fit, bestie ✨

• -Wno-analyzer-fd-use-after-close — this warning needs -fanalyzer, which enables it; use -Wno-analyzer-fd-use-after-close to disable it.

This diagnostic warns about paths through code where a read or write is called on a closed file descriptor. It's giving ghosting, but make it a file descriptor 👻

This diagnostic also warns about 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 needs -fanalyzer, which enables it; use -Wno-analyzer-fd-use-without-check to disable it.

This diagnostic warns about paths through code where a file descriptor is used without being checked for validity. Vibes check: skipped 😤

This diagnostic also warns about 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 needs -fanalyzer, which enables it; use -Wno-analyzer-file-leak to disable it.

This diagnostic warns about paths through the code where a <stdio.h> FILE * stream object gets 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 — this warning needs -fanalyzer, which enables it; use -Wno-analyzer-free-of-non-heap to disable it.

This diagnostic warns about paths through the code where free is called on a non-heap pointer (e.g. an on-stack buffer, or a global). Bestie that memory was never on the heap, what are we even doing 😭

See CWE-590: Free of Memory not on the Heap (https://cwe.mitre.org/data/definitions/590.html).

• -Wno-analyzer-imprecise-fp-arithmetic — this warning needs -fanalyzer, which enables it; use -Wno-analyzer-imprecise-fp-arithmetic to disable it.

This diagnostic warns about paths through the code where floating-point arithmetic is used in locations where precise computation is needed. Right now, it only warns on the use of floating-point operands inside the calculation of an allocation size. Precision era only 🔥

• -Wno-analyzer-infinite-loop — this warning needs -fanalyzer, which enables it; use -Wno-analyzer-infinite-loop to disable it.

This diagnostic warns about paths through the code which appear to lead to an infinite loop. The way this never ends, literally 🌀

Specifically, the analyzer will issue this warning when it "sees" a loop in which:

• 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 for this warning to be emitted is when there's an execution path through a loop for which taking the path on one iteration implies that the same path will be taken on all subsequent iterations.

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 case because if opcode ever matches none of the cases, the switch will follow the implicit default case, making the body of the loop a "no-op" with cpu_state.pc unchanged, and thus using the same value of opcode on all subsequent iterations — which leads to an infinite loop. Checkmate, bestie 💀

See CWE-835: Loop with Unreachable Exit Condition ('Infinite Loop') (https://cwe.mitre.org/data/definitions/835.html).

• -Wno-analyzer-infinite-recursion — this warning needs -fanalyzer, which enables it; use -Wno-analyzer-infinite-recursion to disable it.

So this one's all about infinite recursion bestie 😭 — it warns you about paths through the code that look like they lead to infinite recursion.

Here's the actual tea ☕: 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. The warning fires if nothing in memory appears to be changing — and any changes it observes to parameters or globals get assumed to lead to termination of the recursion, which suppresses the warning. So like, it's chill about it if something moves 💅

Heads up 🫶 — this diagnostic is likely to miss cases of infinite recursion that get converted to iteration by the optimizer before the analyzer "sees" them. So optimization should be disabled when you're attempting to trigger this diagnostic. No cap.

Compare with -Winfinite-recursion, which gives you a similar diagnostic but is implemented a different way. Two queens, two techniques ✨

See CWE-674: Uncontrolled Recursion (https://cwe.mitre.org/data/definitions/674.html).

---

• -Wno-analyzer-jump-through-null — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-jump-through-null to disable it. It warns for paths through the code where a "NULL" function pointer gets called 💀
• -Wno-analyzer-malloc-leak — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-malloc-leak to disable it. It warns for paths where a pointer allocated via an allocator gets leaked — either malloc, or a function marked with attribute malloc. See CWE-401: Missing Release of Memory after Effective Lifetime (https://cwe.mitre.org/data/definitions/401.html). the way memory just vibes away 🫠
• -Wno-analyzer-mismatching-deallocation — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-mismatching-deallocation to disable it. It warns for paths where the wrong deallocation function gets called on a pointer value, based on which function was used to allocate it. The diagnostic will warn about mismatches between free, scalar delete and vector delete[], and those marked as allocator/deallocator pairs using attribute malloc. See CWE-762: Mismatched Memory Management Routines (https://cwe.mitre.org/data/definitions/762.html). mixing up your frees is not the serve you think it is 🙈
• -Wno-analyzer-out-of-bounds — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-out-of-bounds to disable it. It warns for paths where a buffer is definitely read or written out-of-bounds 🔥 The diagnostic applies for cases where the analyzer can determine a constant offset, and for accesses past the end of a buffer — also a constant capacity. Further, the diagnostic does limited checking for accesses past the end when the offset as well as the capacity is symbolic.

See CWE-119: Improper Restriction of Operations within the Bounds of a Memory Buffer (https://cwe.mitre.org/data/definitions/119.html).

For cases where the analyzer is able, 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 is before or after in memory, the relative sizes involved, the direction of the access (read vs write) and, in some cases, the values of data involved. Obsessed with the drama of it all honestly 🎀 This diagram can be suppressed using -fdiagnostics-text-art-charset=none.

• -Wno-analyzer-overlapping-buffers — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-overlapping-buffers to disable it. It warns for paths where overlapping buffers get passed to an API for which the behavior on such buffers is undefined. Specifically, the diagnostic occurs on calls to the following functions:
  • memcpy
  • strcat
  • strcpy

for cases where the buffers are known to overlap. no bestie not the overlapping buffers 😭

• -Wno-analyzer-possible-null-argument — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-possible-null-argument to disable it. It warns for paths where a possibly-NULL value is passed to a function argument 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 — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-possible-null-dereference to disable it. It warns for paths where a possibly-NULL value is dereferenced. See CWE-690: Unchecked Return Value to NULL Pointer Dereference (https://cwe.mitre.org/data/definitions/690.html).
• -Wno-analyzer-null-argument — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-null-argument to disable it. It warns for paths where a value known to be NULL is 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 — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-null-dereference to disable it. It warns for paths where a value known to be NULL is dereferenced. See CWE-476: NULL Pointer Dereference (https://cwe.mitre.org/data/definitions/476.html). null deref is giving "i forgot to check" energy fr fr 💀
• -Wno-analyzer-putenv-of-auto-var — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-putenv-of-auto-var to disable it. It warns for paths where a call to putenv is passed a pointer to an automatic variable or an on-stack buffer. 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). stack memory said "not today" 🫠
• -Wno-analyzer-shift-count-negative — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-shift-count-negative to disable it. It warns for paths 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, but it's implemented based on analyzing interprocedural paths rather than merely parsing the syntax tree. However — the analyzer does not prioritize detection of such paths, so false negatives are more likely relative to other warnings. she's brilliant but she's not prioritizing that rn 😭
• -Wno-analyzer-shift-count-overflow — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-shift-count-overflow to disable it. It warns for paths 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, but it's implemented based on analyzing interprocedural paths rather than merely parsing the syntax tree. Same caveat though: the analyzer does not prioritize detection of such paths, so false negatives are more likely relative to other warnings ✨
• -Wno-analyzer-stale-setjmp-buffer — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-stale-setjmp-buffer to disable it. It warns for paths where longjmp is called to rewind to a jmp_buf relating to a setjmp call in a function that has returned.

When setjmp is called on a jmp_buf to record a rewind location, it records the stack frame. That stack frame becomes invalid when the function containing the setjmp call returns. Attempting to rewind to it via longjmp would reference a stack frame that no longer exists, and would likely lead to a crash (or worse). "or worse" is doing so much heavy lifting here 😭💀

• -Wno-analyzer-tainted-allocation-size — this warning requires -fanalyzer which enables it; use -Wno-analyzer-tainted-allocation-size to disable it. It warns for paths where a value that could be under an attacker's control is used as the size of an allocation without being sanitized — so an attacker could inject an excessively large 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 — this warning requires -fanalyzer which enables it; use -Wno-analyzer-tainted-assertion to disable it. It warns for paths where a value that could be under an attacker's control is used as part of a condition without being first sanitized, and that condition guards a call to a function marked with attribute noreturn (such as the function __builtin_unreachable). Those functions typically indicate abnormal termination of the program, like for assertion failure handlers. For example:

assert (some_tainted_value < SOME_LIMIT);

In such cases:

• when assertion-checking is enabled: an attacker could trigger a denial of service by injecting an assertion failure 😭
• when assertion-checking is disabled, such as by defining NDEBUG: an attacker could inject data that subverts the process, since it presumably violates a precondition that's being assumed by the code 💀

Note that when assertion-checking is disabled, the assertions are typically removed by the preprocessor before the analyzer has a chance to "see" them — so this diagnostic can only generate warnings on builds in which assertion-checking is enabled. iykyk 🫶

For the purpose of this warning, any function marked with attribute noreturn is considered as a possible assertion failure handler, including __builtin_unreachable. Note that these functions are sometimes removed by the optimizer before the analyzer "sees" them. Hence optimization should be disabled when attempting to trigger this diagnostic. (yes, again. turn it off bestie 🌸)

See CWE-617: Reachable Assertion (https://cwe.mitre.org/data/definitions/617.html).

The warning can also report problematic constructions such as

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's not an assertion failure. Yeah. 💀

---

• -Wno-analyzer-tainted-array-index — this one needs -fanalyzer on (that's what turns it on); flip it off with -Wno-analyzer-tainted-array-index.

It drags up paths through your code where a value that could be under an attacker's control gets used as an array index with no sanitizing first — so an attacker could sneak in 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 is what enables it); use -Wno-analyzer-tainted-divisor to turn it off.

Warns about paths where a value that might be attacker-controlled is used as the divisor in a division or modulus op without sanitizing — 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; use -Wno-analyzer-tainted-offset to disable it.

Warns about paths where a value that could be under an attacker's control is used as a pointer offset without sanitizing — 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; use -Wno-analyzer-tainted-size to disable it.

Warns about paths where a value that could be attacker-controlled is used as the size of an operation like memset without sanitizing — 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-throw-of-unexpected-type — needs -fanalyzer; use -Wno-analyzer-throw-of-unexpected-type to disable it. Heads up: dynamic exception specifications only exist in C++14 and earlier. 🕰️

Warns about paths where an exception gets thrown from a function with a dynamic exception specification, and the exception doesn't comply with that specification.

• -Wno-analyzer-undefined-behavior-ptrdiff — needs -fanalyzer, which is what enables it; use -Wno-analyzer-undefined-behavior-ptrdiff to disable it.

Warns about paths where a pointer subtraction happens between pointers that refer to different chunks of memory. That code is leaning on undefined behavior, bestie — 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-strtok to disable it.

Warns about paths where a call to strtok is made 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-handler to disable it.

Warns about paths where a function known to be async-signal-unsafe (think 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-free to disable it.

Warns about paths where a pointer is used after a deallocator was called on it: either free, or a deallocator referenced by the 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-frame to disable 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-mismatch to disable it.

Warns about interprocedural paths where the analyzer catches an attempt to use va_arg to pull out a value passed to a variadic call, but with a type that doesn't match the type of the expression passed to that 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-exhausted to disable 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, except all the values in the va_list are already 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-leak to disable it.

Warns about interprocedural paths where the analyzer detects that va_start or va_copy was 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-end to disable it.

Warns about interprocedural paths where the analyzer detects 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-const to disable it.

Warns about paths where the analyzer detects an attempt to write through a pointer to a const object. But ngl, the analyzer doesn't prioritize catching these, so false negatives are more likely here than with other warnings. 🫠

• -Wno-analyzer-write-to-string-literal — needs -fanalyzer, which enables it; use -Wno-analyzer-write-to-string-literal to disable it.

Warns about paths where the analyzer detects an attempt to write through a pointer to a string literal. Same deal though — the analyzer doesn't prioritize these, 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-value to disable 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 of how the following memory-management functions behave:

• 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 of these functions for working with file descriptors:

• open
• close
• creat
• dup, dup2 and dup3
• isatty
• pipe, and pipe2
• read
• write
• socket, bind, listen, accept, and connect

...and of these functions for working 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 of these functions 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

On top of all that, various functions with an __analyzer_ prefix have special meaning to the analyzer — they're described in the GCC Internals manual. ✨ And various internal parameters, settable via --param, control the exploration; those are also documented in the GCC Internals manual.

The following options control the analyzer. 👑

• -fanalyzer-assume-nothrow — okay so by default, if -fexceptions is enabled, the analyzer assumes a call to any function without the "nothrow" attribute could throw an exception. That's actually helpful for catching execution paths that leak because exceptions bypass clean-up code, but it can also give you false positives when you're using headers where the author never added the "nothrow" attribute. 😭

If -fanalyzer-assume-nothrow is enabled, the analyzer flips to assuming external functions do not throw exceptions. This can be useful for reducing "noise" from the analyzer when you enable -fexceptions on C code, but lowkey it could hide true issues if an exception could be raised by something the C code calls. 🙈

• -fanalyzer-call-summaries — this simplifies interprocedural analysis by computing the effect of certain calls, instead of exploring every path through the function from callsite to each possible return. If enabled, call summaries are only used for functions with more than one call site, and that are sufficiently complicated. ✨
• -fanalyzer-checker=name — restricts the analyzer to run just the named checker, and enables it. Swap name for the checker you want, bestie. 💅
• -fanalyzer-debug-text-art-headings — intended for analyzer developers. If enabled, the analyzer adds extra annotations to any diagrams it generates. 🎀
• -fno-analyzer-feasibility — intended for analyzer developers. By default the analyzer verifies that there is a feasible control flow path for each diagnostic it emits: that the conditions that hold are not mutually exclusive. Diagnostics for which no feasible path can be found are rejected. This filtering can be suppressed with -fno-analyzer-feasibility, for debugging issues in this code. 🫶
• -fanalyzer-fine-grained — literally does nothing. Preserved for backward compatibility. 💀
• -fanalyzer-show-duplicate-count — intended for analyzer developers: 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. 👑
• -fanalyzer-show-events-in-system-headers — by default the analyzer emits simplified diagnostic paths by hiding events fully located within a system header. With -fanalyzer-show-events-in-system-headers, those events are no longer suppressed. 🌈
• -fno-analyzer-simplify-supergraph — intended for analyzer developers. By default, the analyzer performs various simplifications to the program supergraph before analyzing it. With -fno-analyzer-simplify-supergraph, this simplification can be suppressed, for debugging issues with it. 🧚
• -fno-analyzer-state-merge — intended for analyzer developers. 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 — intended 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 — intended for analyzer developers. 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 those diagnostics, which may be helpful for debugging issues in the analyzer, or for microbenchmarks for detecting undefined behavior. 🔥

• -fanalyzer-transitivity — enables transitivity of constraints within the analyzer. ⭐
• -fno-analyzer-undo-inlining — intended for analyzer developers. -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 — intended for analyzer developers. It enables more verbose, lower-level detail in the descriptions of control flow within diagnostic paths. 🎀
• -fanalyzer-verbose-state-changes — intended 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 — controls the complexity of the control flow paths that are emitted for analyzer diagnostics. The level can be one of:
  • 0 — 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.
  • 1 — as per the previous level, but also show events for the entry to each function.
  • 2 — 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.
  • 3 — as per the previous level, but show all control flow events, not just significant ones.
  • 4 — intended for analyzer developers; it adds various other events intended for debugging the analyzer. 👑
• -fdump-analyzer — dumps internal details about what the analyzer is doing to file.analyzer.txt. Heads up: -fdump-analyzer-stderr overrides this option. 🫶
• -fdump-analyzer-stderr — dumps internal details about what the analyzer is doing to stderr. This option overrides -fdump-analyzer. 💅
• -fdump-analyzer-callgraph — dumps a representation of the call graph suitable for viewing with GraphViz to file.callgraph.dot. 🌈
• -fdump-analyzer-exploded-graph — dumps a representation of the "exploded graph" suitable for viewing with GraphViz to file.eg.dot. Nodes are color-coded based on state-machine states to emphasize state changes. ✨
• -fdump-analyzer-exploded-nodes — emits diagnostics showing where nodes in the "exploded graph" are in relation to the program source. 🦋
• -fdump-analyzer-exploded-nodes-2 — dumps a textual representation of the "exploded graph" to file.eg.txt. 🎀
• -fdump-analyzer-exploded-nodes-3 — dumps a textual representation of the "exploded graph" to one dump file per node, to file.eg-id.txt. This is typically a large number of dump files. 😭
• -fdump-analyzer-exploded-paths — dumps a textual representation of the "exploded path" for each diagnostic to file.idx.kind.epath.txt. ⭐
• -fdump-analyzer-feasibility — dumps 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 form file.*.fg.dot, file.*.tg.dot, and file.*.fpath.txt. 🧚
• -fdump-analyzer-infinite-loop — dumps 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 form file.*.infinite-loop.dot. 🔁
• -fdump-analyzer-json — dumps a compressed JSON representation of analyzer internals to file.analyzer.json.gz. The precise format is subject to change. 💀
• -fdump-analyzer-state-purge — as per -fdump-analyzer-supergraph, dumps a representation of the "supergraph" suitable for viewing with GraphViz, but annotates the graph with information on what state will be purged at each node. The graph is written to file.state-purge.dot. 💜
• -fdump-analyzer-supergraph — dumps representations of the "supergraph" suitable for viewing with GraphViz to file.supergraph.index.kind.dot. These show all of the control flow graphs in the program, at various stages of the analysis. The precise set of dumps and what they show is subject to change. 🌸
• -fdump-analyzer-untracked — emits custom warnings with internal details intended for analyzer developers. 👑

---

🌸 Options for Debugging Your Program 🌸

okay so to tell GCC to emit extra info for a debugger, in almost all cases you only need to add -g to your other options. 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. 🫶

GCC lets you use -g with -O. The shortcuts taken by optimized code may occasionally be surprising: 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. 😭

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. ✨

• -g, --debug — produce 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 turns on extra debugging info that only GDB can use 🫶 — and like, that extra info makes debugging work better in GDB but probably makes other debuggers crash or straight-up refuse to read the program 💀. If you want to control for certain whether to generate that extra info, use -gvms (see below, bestie).

• -ggdb — Produce debugging information for use by GDB. This means using 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 — Produce debugging information in DWARF format (if that is supported). The value of version may be either 2, 3, 4 or 5; the default version for most targets is 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 with BTF_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 -gctf is 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 — Produce debugging information in Alpha/VMS debug format (if that is supported). This is the format used by DEBUG on Alpha/VMS systems 🦋.
• -gcodeview — Produce 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 2 is the normal setting to produce debuggable code 🔥. The debug output includes all the information from level 1, plus information about local variables and typedefs to allow stepping through code and examining its state. For C++ code, additional information is also emitted to assist in debugging namespace, class, and template functions.

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-map and -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-4 is recommended along with it.

It can be enabled even if var-tracking is disabled, in which case annotations are created and maintained, but discarded at the end. By default, this flag is enabled together with -fvar-tracking, except when selective scheduling is enabled.

• -fvar-tracking-uninit, -fno-var-tracking-uninit — Perform variable tracking and also mark uninitialized variables in the debug information 🚩. This flag is enabled by default by -fvar-tracking; it also implies -fvar-tracking. To do variable tracking without marking uninitialized variables, use -fvar-tracking -fno-var-tracking-uninit.
• -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 .dwo files.
• -gdwarf32, -gdwarf64 — If DWARF debugging information is enabled, the -gdwarf32 selects the 32-bit DWARF format and the -gdwarf64 selects the 64-bit DWARF format. The default is target specific, on most targets it is -gdwarf32 though. 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 — Generate DWARF .debug_pubnames and .debug_pubtypes sections.
• -ggnu-pubnames — Generate .debug_pubnames and .debug_pubtypes sections in a format suitable for conversion into a GDB index 👑. This option is only useful with a linker that can produce GDB index version 7.
• -gno-pubnames — Don't generate DWARF .debug_pubnames and .debug_pubtypes sections 🙅‍♀️.
• -fdebug-types-section — When using DWARF Version 4 or higher, type DIEs can be put into their own .debug_types section instead of making them part of the .debug_info section. It is more efficient to put them in a separate comdat section since the linker can then remove duplicates ✨. But not all DWARF consumers support .debug_types sections yet and on some objects .debug_types produces larger instead of smaller debugging information 😭.
• -grecord-gcc-switches, -gno-record-gcc-switches — This switch causes the command-line options used to invoke the compiler that may affect code generation to be appended to the DW_AT_producer attribute in DWARF debugging information 📝. The options are concatenated with spaces separating them from each other and from the compiler version. It is enabled by default. See also -frecord-gcc-switches for another way of storing compiler options into the object file 👀.
• -gstrict-dwarf — nope, no extensions allowed, bestie. This one says you literally cannot use DWARF extension features from a later standard version than whatever you picked with -gdwarf-version. Like, respect the version you committed to 💅. On most targets, using non-conflicting DWARF extensions from later standard versions is allowed — this flag is what shuts that down.
• -gno-strict-dwarf — okay so this is the chill one: allow using extensions of later DWARF standard version than the one selected with -gdwarf-version. Live a little ✨
• -gas-loc-support — telling the compiler, "hey bestie, the assembler gets .loc directives, it can handle this." So it may then use them for the assembler to generate DWARF2+ line number tables.

This is generally desirable, because assembler-generated line-number tables are a lot more compact than those the compiler can generate itself. Iconic, honestly 🎀

This option is enabled by default if, at GCC configure time, the assembler is found to support such directives.

• -gno-as-loc-support — force GCC to generate DWARF2+ line number tables internally, if DWARF2+ line number tables are to be generated. No delegating, we're doing it ourselves 💀
• -gas-locview-support — inform the compiler that the assembler supports "view" assignment and reset assertion checking in .loc directives. It's giving assembler-with-skills ✨

This option is enabled by default if, at GCC configure time, the assembler is found to support them.

• -gno-as-locview-support — force GCC to assign view numbers internally, if -gvariable-location-views are explicitly requested.
• -gcolumn-info, -gno-column-info — emit location column information into DWARF debugging information, rather than just file and line. Yeah, it's extra, and yes it's on by default 🙈 This option is enabled by default.
• -gstatement-frontiers, -gno-statement-frontiers — this option causes GCC to create markers in the internal representation at the beginning of statements, and to keep them roughly in place throughout compilation, using them to guide the output of is_stmt markers in the line number table. Main character markers, lowkey 👑 This is 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 — augment variable location lists with progressive view numbers implied from the line number table. This enables debug information consumers to inspect state at certain points of the program, even if no instructions associated with the corresponding source locations are present at that point. So you can peek at the tea even when nobody's home 🫖

If the assembler lacks support for view numbers in line number tables, this causes 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 are not aware of these augmentations, but they won't derive any benefit from them either. Fair trade, ngl.

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 forces GCC to output internal line number tables, and if -ginternal-reset-location-views is not enabled, that most certainly leads to silently mismatching location views. 😭 Silently! The worst kind of wrong!

There is a proposed representation for view numbers that is not backward compatible with the location list format introduced in DWARF 5, that 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 would be rendered unable to decode location lists using it. So like, it's a vibe but it will break your debugger. Don't say I didn't warn you 💀

• -ginternal-reset-location-views, -gno-internal-reset-location-views — attempt to determine location views that can be omitted from location view lists. This requires the compiler to have very accurate instruction length estimates, which isn't always the case, and it may cause incorrect view lists to be generated silently when using an assembler that does not support location view lists. The GNU assembler flags any such error as a "view number mismatch". This is only enabled on ports that define a reliable estimation function.

fr fr this one is playing with fire — silently incorrect view lists if the assembler's not on board 🔥

• -ginline-points, -gno-inline-points — generate 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. Package deal bestie 🎀
• -gz[=type] — produce compressed debug sections in DWARF format, if that is supported. If type is not given, the default type depends on the capabilities of the assembler and linker used. type may be one of none (don't compress debug sections), zlib (use zlib compression in ELF gABI format), or zstd (use zstd compression in ELF gABI format). If the linker doesn't support writing compressed debug sections, the option is rejected. Otherwise, if the assembler does not support them, -gz is silently ignored when producing object files. Silently ignored, bestie — it's giving ghosting 👻
• -femit-struct-debug-baseonly — emit debug information for struct-like types only when the base name of the compilation source file matches the base name of file in which the struct is defined.

This option substantially reduces the size of debugging information, but at significant potential loss in type information to the debugger. See -femit-struct-debug-reduced for a less aggressive option. See -femit-struct-debug-detailed for more detailed control.

This option works only with DWARF debug output.

• -femit-struct-debug-reduced — emit debug information for struct-like types only when the base name of the compilation source file matches the base name of file in which the type is defined, unless the struct is a template or defined in a system header.

This option significantly reduces the size of debugging information, with some potential loss in type information to the debugger. See -femit-struct-debug-baseonly for a more aggressive option. See -femit-struct-debug-detailed for more detailed control.

This option works only with DWARF debug output.

• -femit-struct-debug-detailed[=spec-list] — specify the struct-like types for which the compiler generates debug information. The intent is to reduce duplicate struct debug information between different object files within the same program. Dedupe era 🌸

This option is a detailed version of -femit-struct-debug-reduced and -femit-struct-debug-baseonly, which serves for most needs.

A specification has the syntax [dir:|ind:][ord:|gen:](any|sys|base|none)

The optional first word limits the specification to structs that are used directly (dir:) or used indirectly (ind:). A struct type is used directly when it is the type of a variable, member. Indirect uses arise through pointers to structs. That is, when use of an incomplete struct is valid, the use is indirect. An example is struct one direct; struct two * indirect;.

The optional second word limits the specification to ordinary structs (ord:) or generic structs (gen:). Generic structs are a bit complicated to explain. For C++, these are non-explicit specializations of template classes, or non-template classes within the above. Other programming languages have generics, but -femit-struct-debug-detailed does not yet implement them.

The third word specifies the source files for those structs for which the compiler should emit debug information. The values none and any have the normal meaning. The value base means that the base of name of the file in which the type declaration appears must match the base of the name of the main compilation file. In practice, this means that when compiling foo.c, debug information is generated for types declared in that file and foo.h, but not other header files. The value sys means those types satisfying base or declared in system or compiler headers.

You may need to experiment to determine the best settings for your application. Trial and error era, we love her 💅

The default is -femit-struct-debug-detailed=all.

This option works only with DWARF debug output.

• -fno-dwarf2-cfi-asm — emit DWARF unwind info as compiler generated .eh_frame section 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 is useful to have GCC emit debugging information for all types declared in a compilation unit, regardless of whether or not they are actually used in that compilation unit, for example if, in the debugger, you want to cast a value to a type that is not actually used in your program (but is declared). More often, however, this results in a significant amount of wasted space. 🗑️ So like, know what you're getting into!

⚡ Options That Control Optimization ⚡

These options control various sorts of optimizations. The glow-up section ✨

Without any optimization option, the compiler's goal is to reduce the cost of compilation and to make debugging produce the expected results. Statements are independent: if you stop the program with a breakpoint between statements, you can then assign a new value to any variable or change the program counter to any other statement in the function and get exactly the results you expect from the source code. It's giving "what you see is what you get" and honestly? Respect 🫶

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. Tradeoffs, bestie! You can have fast, small, or debuggable — pick your fighter 😤

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. Cross-file gossip, lowkey iconic 👑

Not all optimizations are controlled directly by a flag. Only optimizations that have a flag are listed in this section. If it's not in the list, it's still happening behind the scenes — the compiler's got lore you don't know about 🌈

Most optimizations are completely disabled at -O0 or if an -O level is not set on the command line, even if individual optimization flags are specified. Similarly, -Og suppresses many optimization passes. So like, -O0 is the "no thoughts, head empty" setting 💀 and -Og is "optimize but keep it debuggable" — she's the sweet spot bestie 🎀

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quick lil heads-up before we dive in 🫶 — depending on your target and how GCC was configured, the exact set of optimizations switched on at each -O level might be a tiny bit different from the list below. Wanna see the real deal? Run GCC with -Q --help=optimizers and it'll spill the tea on exactly what's enabled at each level. 💅

• -O, -O1, --optimize — Optimize, bestie. It's giving "compilation takes a bit more time and a lot more memory for a big function," but it's so worth it. With -O, the compiler tries to shrink code size and execution time without doing any optimizations that eat up a ton of compile time. -O is the recommended level for big machine-generated code — it's the sensible balance between compile time and memory use, since higher levels do optimizations with greater algorithmic complexity than -O. ✨

Here's everything -O turns on:

-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, no cap. GCC does nearly all supported optimizations that don't involve a space-speed tradeoff. Compared to -O, this bumps up both compile time and the perf of the generated code. 🔥

-O2 turns on all the optimization flags from -O1, plus these:

-falign-functions  -falign-jumps -falign-labels  -falign-loops
-fcaller-saves -fcode-hoisting -fcrossjumping -fcse-follow-jumps
-fcse-skip-blocks -fdelete-null-pointer-checks -fdep-fusion
-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
-fspeculatively-call-stored-functions -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-cheap

🚨 okay so pls pls pls read the warning under -fgcse about running -O2 on programs that use computed gotos. It's important, iykyk.

• -O3 — Optimize yet more. -O3 turns on everything from -O2 and also these:

-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, bestie. 🎀

At -O0, GCC completely disables most optimization passes — they are not run even if you explicitly enable them on the command line, or if -Q --help=optimizers snitches that they're enabled by default. Why? Because tons of optimizations depend on code analysis or canonicalization passes that get enabled by -O, and running individual optimization passes solo would be lowkey pointless. 💀

• -Os — Optimize for size. -Os enables all -O2 optimizations except the ones that often make code bigger:

-falign-functions  -falign-jumps -falign-labels  -falign-loops
-fprefetch-loop-arrays  -freorder-blocks-algorithm=stc

It also enables -finline-functions, tunes the compiler for code size over execution speed, and does further optimizations aimed at shrinking code size. 🦋

• -Ofast — Disregard strict standards compliance, periodt. -Ofast enables all -O3 optimizations, plus ones that aren't valid for all standard-compliant programs. It turns on -ffast-math, -fallow-store-data-races and the Fortran-specific -fstack-arrays (unless -fmax-stack-var-size is specified), plus -fno-protect-parens. It also turns off -fsemantic-interposition. ⚡
• -Og — Optimize while keeping the debugging experience in mind. -Og should be your go-to level for the standard edit-compile-debug cycle — a reasonable blend of optimization, fast compilation and debugging, especially for code with a high abstraction penalty. Unlike -O0, this enables -fvar-tracking-assignments and -fvar-tracking, which handle debug info in the prologue and epilogue of functions way better than -O0 does. 🐛✨

Like -O0, -Og completely skips a bunch of optimization passes, so individual options controlling them have no effect. Otherwise, -Og enables all -O1 flags except those known to greatly 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 might increase the number of instructions executed if those instructions take fewer bytes to encode. -Oz behaves similarly to -Os, including enabling most -O2 optimizations. 📦

💡 If you use multiple -O options, with or without level numbers, the last such option is the one that's effective. Last one wins, bestie.

---

okay so now we're getting into the individual flags 🧚 — 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 list below, only one of the forms is shown — 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're either activated by -O options or related to ones that are. Use these flags in those rare cases when you want to fine-tune which optimizations get performed. 🎀

• -fno-defer-pop — For machines that must pop arguments after a function call, always pop the arguments as soon as each function returns. At -O1 and higher, -fdefer-pop is the default — this lets the compiler let arguments pile up on the stack for several function calls and pop them all at once. 🫶
• -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, then fixing up the loaded value. Disabled by default. 💀
• -ffp-contract=style — Here's the tea on the styles: -ffp-contract=off disables floating-point expression contraction. -ffp-contract=fast enables floating-point expression contraction, like forming fused multiply-add operations if the target has native support for them. -ffp-contract=on enables contraction if the language standard allows it — 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), and -ffp-contract=fast otherwise. 🌸
• -ffp-int-builtin-inexact — Allow the built-in functions ceil, floor, round and trunc, and their float and long double variants, to generate code that raises the inexact floating-point exception for noninteger arguments. ISO C99 and C11 allow these functions to raise the inexact exception, but ISO/IEC TS 18661-1:2014 — the C bindings to IEEE 754-2008, as integrated into ISO C23 — does not allow them to do so. The default is -fno-fp-int-builtin-inexact, disallowing the exception to be raised, unless C17 or an earlier 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 still be raised if the library implementation doesn't follow TS 18661. Not me warning you twice, but… yeah.

• -fomit-frame-pointer — Omit the frame pointer in functions that don't need one. This skips the instructions to save, set up and restore the frame pointer; on many targets it also frees up an extra register. 👑

On some targets this flag has no effect, because the standard calling sequence always uses a frame pointer, so it can't 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 (doing polynomial long division) and replace them with a faster implementation. Detects 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 in your source matches the one in the CRC instruction, it generates that CRC instruction. Otherwise, if the target supports a carry-less-multiplication instruction, it generates CRC using that; and if neither, it generates a table-based CRC. Enabled by default at -O2 and higher. 🔥
• -foptimize-sibling-calls — Optimize sibling and tail recursive calls. Slay. 🦋
• -foptimize-strlen — glow-up for the standard C string girlies (strlen, strchr, strcpy, and their _FORTIFY_SOURCE twins, iykyk) so they get swapped for faster alternatives. Literally free speed.

Enabled at levels -O2, -O3, -Os.

• -finline-atomics, -fno-inline-atomics — inlines __atomic operations whenever a lock-free instruction sequence is available. Bestie this one's on by default, no notes. 💅
• -finline-stringops[=fn] — expands memory and string operations inline (for now, only memset), even when the length is variable or big enough that it'd need looping. Most useful when you're running it alongside -ffreestanding and -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 — don't expand any functions inline apart from the ones marked with the always_inline attribute. This is the default when not optimizing.

You can exempt single functions 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). The compiler heuristically decides which functions are simple enough to be worth integrating this way. This inlining applies to all functions, even ones not declared inline. Obsessed with the vibes.

Enabled 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 has any effect only when inlining itself is turned on by the -finline-functions or -finline-small-functions options.

Enabled at levels -O2, -O3, -Os.

• -finline-functions — consider all functions for inlining, even if they're not 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.

Enabled at levels -O2, -O3, -Os. Also enabled by -fprofile-use and -fauto-profile.

• -finline-functions-called-once — consider all static functions called once for inlining into their caller even if they're not marked inline. If a call to a given function is integrated, then the function is not output as assembler code in its own right.

Enabled at levels -O1, -O2, -O3 and -Os, but not -Og.

• -fearly-inlining — inline functions marked by always_inline and functions whose body seems smaller than the function call overhead early, before doing -fprofile-generate instrumentation and the real inlining pass. Doing so makes profiling significantly cheaper and usually makes inlining faster on programs with large chains of nested wrapper functions.

Enabled by default. ✨

• -fipa-sra — perform interprocedural scalar replacement of aggregates, removal of unused parameters, and replacement of parameters passed by reference with parameters passed by value.

Enabled at levels -O2, -O3 and -Os.

• -finline-limit=n — by default, GCC limits the size of functions that can be inlined. This flag allows coarse control of this limit. n is the size of functions that can be inlined in number of pseudo instructions.

Inlining is actually controlled by a number of internal parameters, which are documented in the GCC Internals manual and should not normally be set directly.

Note: there may be no value of -finline-limit that results in default behavior. 😭

Note: pseudo instruction represents, in this particular context, an abstract measurement of function's size. In no way does it represent a count of assembly instructions, and as such its exact meaning might change from one release to another.

• -fno-keep-inline-dllexport — a more fine-grained version of -fkeep-inline-functions, which applies only to functions that are declared using the dllexport attribute or declspec.
• -fkeep-inline-functions — in C, emit static functions that are declared inline into the object file, even if the function has been inlined into all of its callers. This switch does not affect functions using the extern inline extension in GNU C90. In C++, emit any and all inline functions into the object file.
• -fkeep-static-functions — emit static functions into the object file, even if the function is never used. Keeping them around like little souvenirs 🎀
• -fkeep-static-consts — emit variables declared static const when optimization isn't turned on, even if the variables aren't referenced.

GCC enables this option by default. If you want to force the compiler to check if a variable is referenced, regardless of whether or not optimization is turned on, use the -fno-keep-static-consts option.

• -fmerge-constants — attempt to merge identical constants (string constants and floating-point constants) across compilation units.

This option is the default for optimized compilation if the assembler and linker support it. Use -fno-merge-constants to inhibit this behavior.

Enabled at levels -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.

• -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. It's giving main character energy for your loops. 🔥
• -fmodulo-sched-allow-regmoves — perform more aggressive SMS-based modulo scheduling with register moves allowed. By setting this flag, certain anti-dependences edges are deleted, which triggers the generation of reg-moves based on the life-range analysis. This option is effective only with -fmodulo-sched enabled.
• -fno-branch-count-reg — disable the optimization pass that scans for opportunities to use "decrement and branch" instructions 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-reg option 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 — don't 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 get confused by the optimizations performed when this option is not used.

The default is -ffunction-cse

• -ffuse-ops-with-volatile-access — allow limited optimization of operations with volatile memory access when doing so does not change the semantics outlined in

The default is -ffuse-ops-with-volatile-access

• -fno-zero-initialized-in-bss — if the target supports a BSS section, GCC by default puts variables that are initialized to zero into BSS. This can save space in the resulting code.

This option turns off this behavior because some programs explicitly rely on variables going to the data section — e.g., so that the resulting executable can find the beginning of that section and/or make assumptions based on that.

The default is -fzero-initialized-in-bss 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.

Enabled at levels -O1, -O2, -O3, -Os.

• -fsplit-wide-types — when using a type that occupies multiple registers, such as long long on a 32-bit system, split the registers apart and allocate them independently. This normally generates better code for those types, but may make debugging more difficult. 🫠

Enabled at levels -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-types is 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 an if statement with an else clause, CSE follows the jump when the condition tested is false.

Enabled at levels -O2, -O3, -Os.

• -fcse-skip-blocks — similar to -fcse-follow-jumps, but causes CSE to follow jumps that conditionally skip over blocks. When CSE encounters a simple if statement with no else clause, -fcse-skip-blocks causes CSE to follow the jump around the body of the if.

Enabled at levels -O2, -O3, -Os.

• -frerun-cse-after-loop — re-run common subexpression elimination after loop optimizations are performed. Round two, bestie 💅

Enabled at levels -O2, -O3, -Os.

• -fgcse — perform a global common subexpression elimination pass. This pass also performs global constant and copy propagation.

Note: when compiling a program using computed gotos, a GCC extension, you may get better run-time performance if you disable the global common subexpression elimination pass by adding -fno-gcse to the command line.

Enabled at levels -O2, -O3, -Os.

• -fgcse-lm — okay so when this one's on, global common subexpression elimination tries to move loads that are only killed by stores into themselves 👀. What that buys you: a loop with a load/store sequence can get turned into one load outside the loop plus a copy/store inside the loop. Chef's kiss. ✨

It's enabled by default whenever -fgcse is enabled, so it's just... there, doing its thing.

• -fgcse-sm — with this on, a store motion pass runs after global common subexpression elimination. This pass tries to yeet stores out of loops 🚀. And when you pair it with -fgcse-lm, loops containing a load/store sequence can become a load before the loop and a store after the loop. Iconic combo behavior, fr.

Not enabled at any optimization level — you gotta ask for it.

• -fgcse-las — turn this on and the global common subexpression elimination pass eliminates redundant loads that come after stores to the same memory location (both partial and full redundancies, no crumbs left 🍓).

Not enabled at any optimization level.

• -fgcse-after-reload — with this enabled, a redundant load elimination pass runs after reload. Its whole personality? Cleaning up redundant spilling 🧹✨.

Enabled by -O3, -fprofile-use and -fauto-profile.

• -faggressive-loop-optimizations — this tells the loop optimizer to use language constraints to derive bounds for the number of iterations of a loop. It straight up assumes your loop code doesn't invoke undefined behavior — like, say, by causing signed integer overflows or out-of-bound array accesses 😬. Those iteration bounds then guide loop unrolling and peeling and loop exit test optimizations.

Lowkey on by default. But bestie, read that assumption again — it's assuming things about your code 💀.

• -funconstrained-commons — this tells the compiler that variables declared in common blocks (e.g. Fortran) may later be overridden with longer trailing arrays. Which prevents certain optimizations that depend on knowing the array bounds. Trade-off city.
• -fcrossjumping — performs the cross-jumping transformation, which unifies equivalent code and saves code size 🎀. Fair warning though: the resulting code may or may not perform better than without cross-jumping. It's giving "maybe slay, maybe not."

Enabled at levels -O2, -O3, -Os.

• -fauto-inc-dec — combines increments or decrements of addresses with memory accesses. This pass is always skipped on architectures that don't have instructions to support it (no point, babe). Enabled by default at -O1 and higher on architectures that support this.
• -fdce — performs dead code elimination (DCE) on RTL. Enabled by default at -O1 and higher. ✂️
• -fdse — performs dead store elimination (DSE) on RTL. Enabled by default at -O1 and higher. Also ✂️, but stores this time.
• -fif-conversion — attempts to transform conditional jumps into branch-less equivalents. That includes using conditional moves, min, max, set flags and abs instructions, plus some tricks doable by standard arithmetics 🪄. Whether it uses conditional execution on chips where that's available is controlled by -fif-conversion2.

Enabled at levels -O1, -O2, -O3, -Os, but not with -Og.

• -fif-conversion2 — uses conditional execution (where available) to transform conditional jumps into branch-less equivalents.

Enabled at levels -O1, -O2, -O3, -Os, but not with -Og.

• -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 calls operator delete afterwards. For a hierarchy with virtual bases, the base and complete variants are clones, which means two copies of the function 😭. With this option, the base and complete variants get changed into thunks that call a common implementation. Deduplication but make it ABI.

Enabled by -Os.

• -fdelete-null-pointer-checks — assumes 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. On top of that, other optimization passes in GCC use this flag to control global dataflow analyses that eliminate useless checks for null pointers — they assume a memory access to address zero always results in a trap, so if a pointer is checked after it's already been dereferenced, it can't be null. Detective work 🔍.

But heads up: in some environments this assumption is not true. Use -fno-delete-null-pointer-checks to disable this optimization for programs that depend on that behavior. Don't get caught out 🙈.

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 — attempts to convert calls to virtual functions into 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 — attempts to convert calls to virtual functions into 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, such as inlining. And when they seem useless after further optimization, they get converted back into their original form. It literally tries something and undoes it if it flops, we love a self-aware king 💅.
• -fdevirtualize-at-ltrans — streams the extra information needed for aggressive devirtualization when running the link-time optimizer in local transformation mode. This option enables more devirtualization, but it significantly increases the size of streamed data 📈. For this reason it is disabled by default.
• -fexpensive-optimizations — performs a number of minor optimizations that are relatively expensive. Small stuff, big effort, honestly relatable.

Enabled at levels -O2, -O3, -Os.

• -fext-dce, -fno-ext-dce — performs dead code elimination on zero and sign extensions, with special dataflow analysis. Specialized little cleanup crew 🧚
• -free — attempts 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. Like, it's literally built for that vibe.

Enabled for Alpha, AArch64, LoongArch, PowerPC, RISC-V, SPARC, h83000 and x86 at levels -O2, -O3, -Os.

• -fno-lifetime-dse — okay C++ lore time 📖: 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 takes 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.

Want to preserve stores before the constructor starts (e.g. because your operator new clears the object storage) but still treat the object as dead after the destructor? Use -flifetime-dse=1. The default behavior can be explicitly selected with -flifetime-dse=2. And -flifetime-dse=0 is equivalent to -fno-lifetime-dse. Numbers, babes: 0, 1, 2 🌈.

• -flive-range-shrinkage — attempts to decrease register pressure through register live range shrinkage. Helpful for fast processors with small or moderate size register sets. Squish the live ranges, free the registers 🫶
• -fira-algorithm=*algorithm* — use the specified coloring algorithm for the integrated register allocator. The algorithm argument can be priority, which specifies Chow's priority coloring, or CB, which specifies Chaitin-Briggs coloring. Chaitin-Briggs coloring is not implemented for all architectures, but for those targets that do support it, it's the default because it generates better code. King behavior, honestly 👑
• -fira-region=*region* — use the specified regions for the integrated register allocator. The region argument should be one of the following:
  • all — use all loops as register allocation regions. This can give the best results for machines with a small and/or irregular register set.
  • mixed — use all loops except for loops with small register pressure as the regions. This value usually gives the best results in most cases and for most architectures, and is enabled by default when compiling with optimization for speed (-O, -O2, ...).
  • one — use all functions as a single region. This typically results in the smallest code size, and is enabled by default for -Os or -O0.
• -fira-hoist-pressure — uses IRA to evaluate register pressure in the code hoisting pass for decisions to hoist expressions. This option usually results in smaller code, but it can slow the compiler down 🐌.

This option is enabled at level -Os for all targets.

• -fira-loop-pressure — uses IRA to evaluate register pressure in loops for decisions to move loop invariants. This option usually results in generation of faster and smaller code on machines with large register files (>= 32 registers), but it can slow the compiler down.

This option is enabled at level -O3 for some targets.

• -fno-ira-share-save-slots — disables 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, no sharing, that's the deal 💀
• -fno-ira-share-spill-slots — disables 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, spill edition.
• -flra-remat — enables CFG-sensitive rematerialization in LRA. Instead of loading values of spilled pseudos, LRA tries to rematerialize (recalculate) values if it is profitable. Why reload when you could just... recompute? ✨

Enabled at levels -O2, -O3, -Os.

• -fdelayed-branch — if supported for the target machine, attempts to reorder instructions to exploit instruction slots available after delayed branch instructions. Sneaking work into the gap, iykyk.

Enabled at levels -O1, -O2, -O3, -Os, but not at -Og.

• -fschedule-insns — okay so if your target machine supports it, GCC will try to reorder instructions to get rid of those little execution stalls that happen when the data it needs isn't ready yet 🫶. This is lowkey a lifesaver for machines with slow floating-point or slow memory-load instructions, because it lets other instructions sneak in and get issued until the result of that load or FP instruction is actually required. Conventionally it's enabled at optimization levels -O2 and -O3, but bestie, many targets override this behavior — for example, on x86 it's disabled at all levels, while on AArch64 it's enabled only at -O3. No cap, targets are picky like that 💅.
• -fschedule-insns2 — similar to -fschedule-insns, but it asks for an additional pass of instruction scheduling after register allocation has already been done. This is especially useful on machines with a relatively small number of registers and where memory load instructions take more than one cycle 😭. Enabled at levels -O2, -O3, -Os.
• -fno-sched-interblock — disables instruction scheduling across basic blocks, which is normally enabled when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.
• -fno-sched-spec — disables speculative motion of non-load instructions, which is normally enabled when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.
• -fsched-pressure — enables register-pressure-sensitive insn scheduling before register allocation ✨. This only makes sense when scheduling before register allocation is enabled, i.e. with -fschedule-insns or at -O2 or higher. Using this can improve the generated code and shrink its size by preventing register pressure from rising above the number of available hard registers (and the subsequent spills in register allocation that follow 😬).
• -fsched-spec-load — allows speculative motion of some load instructions. Again, only makes sense when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.
• -fsched-spec-load-dangerous — allows speculative motion of more load instructions 👀. Same vibe: only makes sense when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.
• -fsched-stalled-insns, -fsched-stalled-insns=n — defines how many insns (if any) can be moved prematurely from the queue of stalled insns into the ready list during the second scheduling pass. -fno-sched-stalled-insns means that no insns are moved prematurely, and -fsched-stalled-insns=0 means there's literally no limit on how many queued insns can be moved prematurely. -fsched-stalled-insns with no value is equivalent to -fsched-stalled-insns=1 🎀.
• -fsched-stalled-insns-dep, -fsched-stalled-insns-dep=n — defines how many insn groups (cycles) are examined for a dependency on a stalled insn that's a candidate for premature removal from the queue of stalled insns. This only does anything during the second scheduling pass, and only if -fsched-stalled-insns is used. -fno-sched-stalled-insns-dep is equivalent to -fsched-stalled-insns-dep=0, and -fsched-stalled-insns-dep with no value is equivalent to -fsched-stalled-insns-dep=1 ✨.
• -fsched2-use-superblocks — when scheduling after register allocation, use superblock scheduling, which allows motion across basic block boundaries and results in faster schedules 🔥. Heads up though: this option is experimental, because not all machine descriptions used by GCC model the CPU closely enough to avoid unreliable results from the algorithm 😭. Only makes sense when scheduling after register allocation, i.e. with -fschedule-insns2 or at -O2 or higher.
• -fsched-group-heuristic — enables the group heuristic in the scheduler, which favors the instruction that belongs to a schedule group. Enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.
• -fsched-critical-path-heuristic — enables the critical-path heuristic in the scheduler, which favors instructions on the critical path 👑. Enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.
• -fsched-spec-insn-heuristic — enables the speculative instruction heuristic in the scheduler, which favors speculative instructions with greater dependency weakness 🦋. Enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.
• -fsched-rank-heuristic — enables the rank heuristic in the scheduler, which favors the instruction belonging to a basic block with greater size or frequency. Enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.
• -fsched-last-insn-heuristic — enables the last-instruction heuristic in the scheduler, which favors the instruction that's less dependent on the last instruction scheduled 🌸. Enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.
• -fsched-dep-count-heuristic — enables the dependent-count heuristic in the scheduler, which favors the instruction that has more instructions depending on it 💜. Enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.
• -fspeculatively-call-stored-functions — attempts to convert indirect calls of function pointers into pointers loaded from a structure field if all visible stores to that field store just a single candidate 👀. When it does this, it turns the call into a conditional deciding between the direct call and the original indirect one. These speculative calls often unlock more optimizations, such as inlining, and when they seem useless after further optimization, they get converted back into their original form. It's giving "try it, and if it flops, pretend it never happened" 💅.
• -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 — schedules instructions using the selective scheduling algorithm, which runs instead of the first scheduler pass ✨.
• -fselective-scheduling2 — schedules instructions using the selective scheduling algorithm, which runs instead of the second scheduler pass.
• -fsel-sched-pipelining — enables software pipelining of innermost loops during selective scheduling 🎀. This option has no effect unless one of -fselective-scheduling or -fselective-scheduling2 is turned on.
• -fsel-sched-pipelining-outer-loops — when pipelining loops during selective scheduling, also pipeline outer loops. This one does nothing unless -fsel-sched-pipelining is turned on.
• -fsemantic-interposition — 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 perform interprocedural propagation, inlining and other optimizations, because it has to assume the function or variable in question might change. While this feature is useful (for example, to rewrite memory allocation functions with 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 interposition is never allowed to change semantics) and for symbols explicitly declared weak.
• -fshrink-wrap — emits function prologues only before the parts of the function that actually need it, rather than at the top of the whole function 🌸. Enabled by default at -O and higher.
• -fshrink-wrap-separate — shrink-wraps 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-wrap is also turned on and the target supports it.
• -fcaller-saves — enables allocation of values to registers that are clobbered by function calls, by emitting extra instructions to save and restore the registers around such calls 💖. This allocation only happens when it seems to result in better code. It's always enabled by default on certain machines, usually those with no call-preserved registers to use instead. Enabled at levels -O2, -O3, -Os.
• -fcombine-stack-adjustments — tracks stack adjustments (pushes and pops) and stack memory references, and then tries to find ways to combine them ✨. Enabled by default at -O1 and higher.
• -fipa-ra — uses 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 the generated code will be instrumented for profiling (-p, or -pg) or if the callee's register usage can't be known exactly (which happens on targets that don't expose prologues and epilogues in RTL) 😭.
• -fconserve-stack — attempts to minimize stack usage. The compiler tries to use less stack space, even if that makes the program slower 🐢. This option implies setting the large-stack-frame parameter to 100 and the large-stack-frame-growth parameter to 400.
• -ftree-reassoc — performs reassociation on trees 🌳. Enabled by default at -O1 and higher.
• -fcode-hoisting — performs code hoisting, which 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 -O2 and higher.
• -ftree-pre — performs partial redundancy elimination (PRE) on trees 🔥. Enabled by default at -O2 and -O3.
• -ftree-partial-pre — makes partial redundancy elimination (PRE) more aggressive 💅. Enabled by default at -O3.
• -ftree-forwprop — does forward propagation on trees. On by default at -O1 and up, no cap ✨
• -ftree-fre — full redundancy elimination (FRE) on trees, bestie. The glow-up difference between FRE and PRE? FRE only looks at expressions that get computed on all the paths leading to the redundant computation. So it's faster than PRE but lowkey exposes fewer redundancies. Enabled by default at -O1 and higher 💅
• -ftree-phiprop — hoists loads from conditional pointers, on trees. This pass is enabled by default at -O1 and higher 🫶
• -fhoist-adjacent-loads — speculatively hoists loads from both branches of an if-then-else, but only when the loads are from adjacent locations in the same structure and the target architecture has a conditional move instruction. Enabled by default at -O2 and higher, she's efficient 💖
• -ftree-copy-prop — copy propagation on trees. This pass eliminates unnecessary copy operations. Enabled by default at -O1 and higher ✨
• -fipa-pure-const — discovers which functions are pure or constant. Enabled by default at -O1 and higher 👑
• -fipa-reference — discovers which static variables do not escape the compilation unit. Enabled by default at -O1 and higher 🦋
• -fipa-reference-addressable — discovers read-only, write-only and non-addressable static variables. Enabled by default at -O1 and higher 🌸
• -fipa-reorder-for-locality — groups call chains close together in the binary layout so the code locality is immaculate and jump distances between frequently called functions get minimized. And like, unlike -freorder-functions, this pass considers the call chains between functions and groups them together, instead of grouping all hot/normal/cold/never-executed functions into separate sections. And unlike -fprofile-reorder-functions, it aims to improve code locality throughout the entire runtime of the program rather than focusing on program startup. This option is incompatible with an explicit -flto-partition= option since it enforces a custom partitioning scheme. If you're using this option it's recommended to also use profile feedback, but otherwise this option is not enabled by default. iykyk 🔥
• -fipa-stack-alignment — reduces stack alignment on call sites if possible. Enabled by default 💅
• -fipa-pta — does interprocedural pointer analysis plus interprocedural modification and reference analysis. Heads up though: 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 — interprocedural profile propagation. Functions called only from cold functions get marked as cold. Also functions executed once (such as "cold", "noreturn", static constructors or destructors) get identified. Cold functions and loop-less parts of functions executed once are then optimized for size. Enabled by default at -O1 and higher ✨
• -fipa-modref — interprocedural mod/ref analysis. This optimization analyzes the side effects of functions (memory locations that are modified or referenced) and enables better optimization across the function call boundary. Enabled by default at -O1 and higher 🎀
• -fipa-cp — interprocedural constant propagation. This optimization analyzes the program to determine when values passed to functions are constants and then optimizes accordingly. It can substantially increase performance if the application has constants passed to functions. Enabled by default at -O2, -Os and -O3, and it's also enabled by -fprofile-use and -fauto-profile 👑
• -fipa-cp-clone — function cloning to make interprocedural constant propagation stronger. When enabled, interprocedural constant propagation performs function cloning when 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 💀 Enabled by default at -O3, and also enabled by -fprofile-use and -fauto-profile.
• -fipa-bit-cp — when enabled, performs interprocedural bitwise constant propagation. Enabled by default at -O2 and by -fprofile-use and -fauto-profile. It requires that -fipa-cp is enabled 🌈
• -fipa-vrp — when enabled, performs interprocedural propagation of value ranges. Enabled by default at -O2. It requires that -fipa-cp is enabled ⭐
• -fipa-icf-functions
• -fipa-icf-variables
• -fipa-icf — Identical Code Folding for functions (-fipa-icf-functions), read-only variables (-fipa-icf-variables), or both (-fipa-icf). This optimization reduces code size and may disturb unwind stacks by replacing a function by an equivalent one with a different name 😭 It works more effectively with link-time optimization enabled.

Although the behavior is similar to the Gold Linker's ICF optimization, GCC ICF works on different levels and thus the optimizations are not the same — there are equivalences that are found only by GCC and equivalences found only by Gold.

-fipa-icf is 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 -O2 and -Os ✨
• -flive-patching= level — controls GCC's optimizations so the output is suitable for live-patching 🫶
• If the compiler's optimization uses a function's body or information extracted from its body to optimize/change another function, that latter one is called an impacted function of the former. And if a function is patched, its impacted functions should be patched too. Periodt.
• 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. In order 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 these:
  • 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 these 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 🙈

Heads up bestie: -flive-patching is not supported with link-time optimization (-flto). Not compatible, don't try it 💀

• -fisolate-erroneous-paths-dereference — detects paths that trigger erroneous or undefined behavior due to dereferencing a null pointer (with -fdelete-null-pointer-checks enabled) or a division by zero. 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 -O2 and higher 🔥
• -fisolate-erroneous-paths-attribute — detects paths that trigger erroneous or undefined behavior due to a null value being used in a way forbidden by a "returns_nonnull" or "nonnull" attribute. 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 -O2 in the future 🌸
• -ftree-sink — forward store motion on trees. Enabled by default at -O1 and higher ✨
• -ftree-bit-ccp — sparse conditional bit constant propagation on trees, plus it propagates pointer alignment information. This pass only operates on local scalar variables and is enabled by default at -O1 and higher, except for -Og. It requires that -ftree-ccp is enabled 🦋
• -ftree-ccp — sparse conditional constant propagation (CCP) on trees. This pass only operates on local scalar variables and is enabled by default at -O1 and 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 -O1 and higher 🎀
• -fssa-phiopt — pattern matching on SSA PHI nodes to optimize conditional code. Enabled by default at -O1 and higher, except for -Og ⭐
• -ftree-switch-conversion — converts simple initializations in a switch into initializations from a scalar array. Enabled by default at -O2 and higher 🌈
• -ftree-tail-merge — looks for identical code sequences. When found, it replaces one with a jump to the other. This optimization is known as tail merging or cross jumping. Enabled by default at -O2 and higher. The compilation time in this pass can be limited using the max-tail-merge-comparisons parameter and the max-tail-merge-iterations parameter 🫶
• -ftree-cselim — bestie, this one does conditional store elimination on trees 🌸 it's on by default at -O1 and up, but only on targets that actually have conditional move instructions (iykyk, no cmov no party).
• -ftree-dce — dead code elimination (DCE) on trees ✨ enabled by default at -O1 and higher. RIP to the code nobody needed, you will not be missed 💀
• -ftree-builtin-call-dce — conditional dead code elimination (DCE) but specifically for calls to built-in functions that might set errno yet are otherwise completely side-effect free 🫶 it's on by default at -O2 and higher — unless -Os is also in the mix, then it sits this one out.
• -ffinite-loops — okay so this is the "trust the loop, bestie" flag 👑 it assumes a loop with an exit will eventually take that exit and not just vibe forever. That lets the compiler delete loops that have no side-effects, since it's not counting "what if it loops endlessly tho" as a reason to keep them.

This option is enabled by default at -O2 for C++ with -std=c++11 or higher.

• -ftree-dominator-opts — the glow-up pass 💅 it does a whole little spa day of simple scalar cleanups based on a dominator tree traversal: constant/copy propagation, redundancy elimination, range propagation, and expression simplification. It also does jump threading (which is literally just reducing jumps to jumps, so meta). This flag is enabled by default at -O1 and higher.
• -ftree-dse — dead store elimination (DSE) on trees 🌈 quick definition so we're all on the same page: a dead store is a store into a memory location that gets overwritten by another store later with no loads in between. So the earlier store is giving nothing and can be deleted, periodt. Enabled by default at -O1 and higher.
• -ftree-ch — loop header copying on trees 🦋 it's a win because it makes code motion optimizations way more effective, plus it saves one jump (we love a two-for-one). Enabled by default at -O1 and higher. Not enabled for -Os though, because it usually increases code size and we're not about that when we're optimizing for size.
• -ftree-loop-optimize — loop optimizations on trees, no notes ✨ enabled by default at -O1 and higher.
• -ftree-loop-linear, -floop-strip-mine, -floop-block — these three are all the same thing, bestie: loop nest optimizations, identical to -floop-nest-optimize 🎀 to actually use this code transformation, GCC has to be configured with --with-isl to enable the Graphite loop transformation infrastructure. (Yes, configured, like at build time. Not a you-thing.)
• -fgraphite-identity — turns on the identity transformation for graphite 🌸 for every SCoP we generate the polyhedral representation and transform it right back to gimple. The whole point of -fgraphite-identity is that we can check the costs or benefits of the GIMPLE -> GRAPHITE -> GIMPLE transformation. Some minimal optimizations are also performed by the code generator isl, like index splitting and dead code elimination in loops.
• -floop-nest-optimize — enables the isl based loop nest optimizer 🔥 it's a generic loop nest optimizer based on the Pluto optimization algorithms, and it calculates a loop structure optimized for data-locality and parallelism. Heads up: this option is experimental 👀
• -floop-parallelize-all — uses the Graphite data dependence analysis to spot loops that can be parallelized 💜 then it parallelizes all the loops that can be analyzed to not contain loop carried dependences — and nope, it does not check whether parallelizing is actually profitable first. It's giving "do it first, ask questions never" 😭
• -ftree-coalesce-vars — while transforming the program out of the SSA representation, this tries to reduce copying by coalescing versions of different user-defined variables, instead of just compiler temporaries 🫶 but bestie be warned: 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. It's enabled by default if optimization is enabled, and it does very little otherwise (a true background character ngl).
• -ftree-loop-if-convert — attempts to transform conditional jumps in the innermost loops into branch-less equivalents ✨ the intent is to remove control-flow from the innermost loops so the vectorization pass has a better shot at handling them. Enabled by default if vectorization is enabled.
• -ftree-loop-distribution — perform loop distribution 🌸 this flag can improve cache performance on big loop bodies and let further loop optimizations (like parallelization or vectorization) happen. Example time, look at this loop:

        DO I = 1, N
          A(I) = B(I) + C
          D(I) = E(I) * F
        ENDDO

is transformed to

        DO I = 1, N
           A(I) = B(I) + C
        ENDDO
        DO I = 1, N
           D(I) = E(I) * F
        ENDDO

This flag is enabled by default at -O3. It is also enabled by -fprofile-use and -fauto-profile 💅

• -ftree-loop-distribute-patterns — loop distribution but for patterns that can be code generated with calls to a library 🎀 enabled by default at -O2 and higher, and by -fprofile-use and -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
        ENDDO

is transformed to

        DO I = 1, N
           A(I) = 0
        ENDDO
        DO I = 1, N
           B(I) = A(I) + I
        ENDDO

and the initialization loop is transformed into a call to memset zero ✨ iconic behavior honestly.

• -floop-interchange — perform loop interchange outside of graphite 🦋 this flag can improve cache performance on loop nest and allow further loop optimizations, like vectorization, to take place. Example:

        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. Enabled by default at -O3. It is also enabled by -fprofile-use and -fauto-profile.
• -ftree-loop-im — loop invariant motion on trees 💜 this pass moves only the invariants that are hard to handle at RTL level (function calls, operations that expand to nontrivial sequences of insns). With -funswitch-loops it also moves operands of conditions that are invariant out of the loop, so that we can use just trivial invariantness analysis in loop unswitching. The pass also includes store motion, because of course it does 💅
• -ftree-loop-ivcanon — creates a canonical counter for the number of iterations in loops where figuring out the iteration count requires complicated analysis 😭 later optimizations can then determine the number easily. Useful especially in connection with unrolling, iykyk.
• -ftree-scev-cprop — perform final value replacement ✨ here's the deal: 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, then uses of the final value get replaced by such a computation — provided it's sufficiently cheap. This reduces data dependencies and may allow further simplifications. Enabled by default at -O1 and higher.
• -fivopts — induction variable optimizations on trees 🌸 the full roster: strength reduction, induction variable merging, and induction variable elimination. Enabled by default at -O1 and higher.
• -ftree-parallelize-loops, -ftree-parallelize-loops=n — parallelize loops, i.e. split their iteration space to run in multiple threads 👑 this is only possible for loops whose iterations are independent and can be arbitrarily reordered. Also, 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.

When a positive value n is specified, the number of threads is fixed at compile time and cannot be changed after compilation. The compiler generates #pragma omp parallel num_threads(n).

When used without =n (i.e., -ftree-parallelize-loops), the number of threads is determined at program execution time via the OMP_NUM_THREADS environment variable 🫶 if OMP_NUM_THREADS is not set, the OpenMP runtime automatically detects the number of available processors and uses that value. This enables creating binaries that adapt to different hardware configurations without recompilation — literally so convenient, we love a flexible queen 💖

• -ftree-pta — perform function-local points-to analysis on trees ✨ enabled by default at -O1 and higher, except for -Og.
• -ftree-sra — perform scalar replacement of aggregates 🌸 this pass replaces structure references with scalars to prevent committing structures to memory too early. Enabled by default at -O1 and higher, except for -Og.
• -fstore-merging — 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 -O2 and higher as well as -Os.
• -ftree-ter — temporary expression replacement during the SSA->normal phase 💜 single use/single def temporaries get replaced at their use location with their defining expression. This results in non-GIMPLE code, but it gives the expanders much more complex trees to work on, which results in better RTL generation. Enabled by default at -O1 and higher.
• -ftree-slsr — straight-line strength reduction on trees ✨ it recognizes related expressions involving multiplications and replaces them by less expensive calculations when possible. Enabled by default at -O1 and higher.
• -ftree-vectorize — vectorization on trees 🦋 this flag enables -ftree-loop-vectorize and -ftree-slp-vectorize if they're not explicitly specified.
• -ftree-loop-vectorize — loop vectorization on trees 🔥 enabled by default at -O2 and by -ftree-vectorize, -fprofile-use, and -fauto-profile.
• -ftree-slp-vectorize — bestie this one does basic block vectorization on trees 🌈. It's switched on by default at -O2 and by -ftree-vectorize, -fprofile-use, and -fauto-profile ✨.
• -ftrivial-auto-var-init=choice — okay so this one is lowkey a security glow-up 💅: it initializes automatic variables or temporary objects with either a pattern or with zeroes, which increases the security and predictability of a program by preventing uninitialized memory disclosure and use 🛡️. But listen: GCC still considers an automatic variable that doesn't have an explicit initializer as uninitialized, and -Wuninitialized and -Wanalyzer-use-of-uninitialized-value will still report warning messages on such automatic variables or temporary objects, and the compiler will perform optimization as if the variable were uninitialized 😭. With this option, GCC will also initialize any padding of automatic variables or temporary objects that have structure or union types to zeroes 🫶. However (big however, bestie), the current implementation cannot initialize automatic variables whose initialization is bypassed through switch or goto statement — use -Wtrivial-auto-var-init to report all such cases 💀.

The three values of choice are:

• uninitialized — doesn't initialize any automatic variables.
• pattern — Initialize automatic variables with values which will likely transform logic bugs into crashes down the line, are easily recognized in a crash dump and without being values that programmers can rely on for useful program semantics 🔥. The current value is byte-repeatable pattern with byte 0xFE. The values used for pattern initialization might be changed in the future 👀.
• zero — Initialize automatic variables with zeroes.

The default is uninitialized except for C++26, in which case if -ftrivial-auto-var-init= is not specified at all automatic variables or temporary objects are zero initialized, but zero initialization of padding bits does not happen 🎀.

Note that 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 standard attribute or the C++26 [[indeterminate]] 💜.

• -fvect-cost-model=model — this one alters the cost model used for vectorization 🦋. The model argument should be one of unlimited, dynamic, cheap or very-cheap. With the unlimited model the vectorized code-path is assumed to be profitable, while with the dynamic model a runtime check guards the vectorized code-path to enable it only for iteration counts that will likely execute faster than when executing the original scalar loop 🏃. The cheap model disables vectorization of loops where doing so would be cost prohibitive, for example due to required runtime checks for data dependence or alignment, but otherwise is equal to the dynamic model. The very-cheap model 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 the cheap model ✨.

The default cost model depends on other optimization flags and is either dynamic or cheap.

• -fsimd-cost-model=model — same energy but for loops marked with the OpenMP simd directive 🌸. The model argument should be one of unlimited, dynamic, cheap. All values of model have the same meaning as described in -fvect-cost-model and by default a cost model defined with -fvect-cost-model is used 🫶.
• -ftree-vrp — Perform Value Range Propagation on trees 🌳. This is similar to the constant propagation pass, but instead of values, ranges of values are propagated. This allows the optimizers to remove unnecessary range checks like array bound checks and null pointer checks 🔥. This is enabled by default at -O2 and higher. Null pointer check elimination is only done if -fdelete-null-pointer-checks is enabled 👀.
• -fsplit-paths — Split paths leading to loop backedges 🎀. This can improve dead code elimination and common subexpression elimination. This is enabled by default at -O3 and 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 💅.

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-functions or -finline-small-functions options.

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 👀.

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-value is 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-peephole and -fno-peephole2 is in how they are implemented in the compiler; some targets use one, some use the other, a few use both 🤷.

-fpeephole is enabled by default. -fpeephole2 enabled at levels -O2, -O3, -Os ✨.

• -fno-guess-branch-probability — Do not guess branch probabilities using heuristics 🙅.

GCC uses heuristics to guess branch probabilities if they are not provided by profiling feedback (-fprofile-arcs). These heuristics are based on the control flow graph 🕸️. If some branch probabilities are specified by __builtin_expect, then the heuristics are used to guess branch probabilities for the rest of the control flow graph, taking the __builtin_expect info into account. The interactions between the heuristics and __builtin_expect can be complex, and in some cases, it may be useful to disable the heuristics so that the effects of __builtin_expect are easier to understand 🧠.

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 be simple, which does not increase code size (except sometimes due to secondary effects like alignment), or stc, 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 .o files, to improve paging and cache locality performance 💅🔥.

---

This optimization is automatically turned off when exception handling or unwind tables are in the picture (like, on targets that use setjump/longjump or some target-specific scheme), and also for linkonce sections, for functions that have a user-defined section attribute, and on any architecture that just doesn't do named sections. And heads up — when -fsplit-stack is in play this one is not on by default (they're dodging linker errors, bestie), but you can absolutely turn it on explicitly if your linker is actually working.

Enabled for x86 at levels -O2, -O3, -Os.

---

• -freorder-functions — reorders functions inside the object file so the code sits nicer in memory. Unlike -fipa-reorder-for-locality, this one's whole personality is grouping functions by category (hot/normal/cold/never-executed) all together. It pulls this off with special subsections: ".text.hot" for the stuff that runs most, and ".text.unlikely" for the stuff that basically never runs. The actual reordering gets done by the linker, so your object file format has to support named sections and the linker has to place them somewhere sensible.

This option is lowkey useless unless you either feed it profile feedback (peep -fprofile-arcs for the deets) or hand-annotate your functions with hot or cold attributes.

Enabled at levels -O2, -O3, -Os.

• -fstrict-aliasing — tells the compiler "go off, assume the strictest aliasing rules your language has." For C (and C++), this unlocks optimizations based on the type of expressions. Specifically: you're not allowed to access an object of one type through an expression of a different type, unless the types are compatible, differ only in signedness or qualifiers, or the expression has a character type. Accessing scalar objects through a corresponding vector type is also allowed. 💅

So, for example, an unsigned int can alias an int, but it cannot alias a void* or a double. A character type can alias any other type.

Pay special attention to code like this:

union a_union {
  int i;
  double d;
};

int f() {
  union a_union t;
  t.d = 3.0;
  return t.i;
}

Reading from a different union member than the one you most recently wrote to — that's called "type-punning", and it's super common. Even with -fstrict-aliasing, type-punning is allowed in C provided the memory is accessed through the union type. In ISO C++, type-punning through a union type is undefined behavior, but GCC supports it as an extension. So the code above works as expected. 🙌 However, this code might not:

int f() {
  union a_union t;
  int* ip;
  t.d = 3.0;
  ip = &t.i;
  return *ip;
}

Same energy for accessing by taking the address, casting the resulting pointer, and dereferencing the result — that has undefined behavior, even if the cast uses a union type, like:

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 — controls whether the rules of -fstrict-aliasing get applied across function boundaries. Note that if multiple functions get inlined into a single function, those memory accesses aren't considered to be crossing a function boundary anymore (they're all one fam now).

The -fipa-strict-aliasing option is enabled by default, and it's 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 — aligns the start of functions to the next power-of-two greater than or equal to n, skipping up to m-1 bytes. This makes sure at least the first m bytes of the function can be fetched by the CPU without crossing an n-byte alignment boundary. It's a code-performance optimization, and alignment is simply ignored for functions considered cold. If you genuinely need alignment on all functions, use -fmin-function-alignment instead.

If m isn't specified, it defaults to n.

Examples, because these hit different: -falign-functions=32 aligns functions to the next 32-byte boundary; -falign-functions=24 aligns to the next 32-byte boundary only if that can be done by skipping 23 bytes or less; -falign-functions=32:7 aligns to the next 32-byte boundary only if that can be done by skipping 6 bytes or less.

The second pair of n2:m2 values lets you set a secondary alignment: -falign-functions=64:7:32:3 aligns to the next 64-byte boundary if that can be done by skipping 6 bytes or less, otherwise it falls back to aligning to the next 32-byte boundary if that can be done by skipping 2 bytes or less. If m2 isn't specified, it defaults to n2.

Some assemblers only support this flag when n is a power of two — in that case it gets rounded up.

-fno-align-functions and -falign-functions=1 are equivalent, and both mean functions are not aligned, periodt.

If n isn't specified or is zero, a machine-dependent default is used. The maximum allowed n option value is 65536.

Enabled at levels -O2, -O3.

• -flimit-function-alignment — with this on, the compiler tries not to overalign functions unnecessarily. It attempts to tell the assembler to align by the amount given by -falign-functions, but not to skip more bytes than the size of the function. Considerate king behavior honestly.
• -falign-labels
• -falign-labels=n
• -falign-labels=n:m
• -falign-labels=n:m:n2
• -falign-labels=n:m:n2:m2 — aligns all branch targets to a power-of-two boundary. ✨

The parameters here are analogous to the -falign-functions option. -fno-align-labels and -falign-labels=1 are equivalent and mean labels are not aligned.

If -falign-loops or -falign-jumps are applicable and are greater than this value, then their values are used instead (they win the alignment contest).

If n isn't specified or is zero, a machine-dependent default is used, which is very likely to be 1, meaning no alignment at all. 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 — aligns loops to a power-of-two boundary. If the loops run many times, that makes up for all the dummy padding instructions you paid for. It's a code-performance optimization, and alignment is ignored for loops considered cold.

If -falign-labels is greater than this value, then its value is used instead.

The parameters are analogous to the -falign-functions option. -fno-align-loops and -falign-loops=1 are equivalent and mean loops are not aligned. The maximum allowed n option value is 65536.

If n isn't specified or is zero, a machine-dependent default is used.

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 — aligns branch targets to a power-of-two boundary, but only for branch targets that can only be reached by jumping. In that case no dummy operations need to be executed, which is honestly a slay. It's a code-performance optimization, and alignment is ignored for jumps considered cold.

If -falign-labels is greater than this value, then its value is used instead.

The parameters 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 isn't specified or is zero, a machine-dependent default is used. The maximum allowed n option value is 65536.

Enabled at levels -O2, -O3.

• -fmin-function-alignment — sets the minimal alignment of functions to the next power-of-two greater than or equal to n. Unlike -falign-functions, this alignment gets applied to all functions (even the cold ones, no favorites here). The alignment is also not affected by -flimit-function-alignment.
• -fno-allocation-dce — don't remove unused C++ allocations (the ones using operator "new" and operator "delete") during dead code elimination.

See also -fmalloc-dce.

• -fallow-store-data-races — lets the compiler do optimizations that might introduce new data races on stores, without first proving the variable can't be concurrently accessed by other threads. It doesn't affect optimization of local data. It's safe to use if you know your global data won't be accessed by multiple threads. 🫶

Examples of optimizations this enables include hoisting or if-conversions that may cause a value already sitting in memory to be re-written with that same value. That re-writing is safe single-threaded but may be unsafe multi-threaded. 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 around for compatibility reasons. -funit-at-a-time has no effect, while -fno-unit-at-a-time implies -fno-toplevel-reorder and -fno-section-anchors. It's giving legacy, ngl.

Enabled by default.

• -fno-toplevel-reorder — do not reorder top-level functions, variables, and asm statements. Output them in the exact same order they appear in the input file. When this option is used, unreferenced static variables are not removed. This option is meant to support existing code that relies on a particular ordering. For new code, it's better to use attributes when you can. 💜

-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 on, the compiler turns calls to __builtin_unreachable into traps, instead of using them for optimization. This also affects any such calls the compiler implicitly generated on its own.

This option has the same effect as -fsanitize=unreachable -fsanitize-trap=unreachable, but it 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 — okay so this one builds "webs" the way register allocation likes them, and hands each web its very own pseudo register 🕸️. That means the register allocation pass gets to work directly on pseudos, and it also gives a glow-up to a bunch of other optimization passes — CSE, the loop optimizer, and the trivial dead code remover all eat this up. But (and this is the tea ☕) it can make debugging literally impossible, because variables stop staying in a "home register". Ngl that's a real trade-off.

Enabled by default with -funroll-loops. ✨

• -fwhole-program — assumes the compilation unit you're compiling right now is the entire program, bestie. Like, the whole thing. Every public function and variable becomes static — except main and anything merged by the attribute externally_visible — and so interprocedural optimizers go absolutely feral on it with way more aggressive optimization. 🔥

With -flto this option has a limited use. In most cases the precise list of symbols used or exported from the binary is known from the resolution info that the linker plugin passes to the link-time optimizer. It's still useful if no linker plugin is used, or during the incremental link step when final code is produced (with -flto -flinker-output=nolto-rel).

• -flto[=n] — 💅 the main character of this whole page. This option runs the standard link-time optimizer. When you invoke it with source code, it generates GIMPLE (one of GCC's internal representations) and writes it into special ELF sections in the object file. Then, when the object files get linked together, all the function bodies are read out of those ELF sections and instantiated as if they'd been part of the same translation unit the whole time. Iconic, fr.

To actually use the link-time optimizer, -flto and your optimization options should be specified at compile time and during the final link. The recommendation: compile all the files participating in the same link with the same options, and also specify those options at link time. For example:

gcc -c -O2 -flto foo.c
gcc -c -O2 -flto bar.c
gcc -o myprog -flto -O2 foo.o bar.o

The first two invocations to GCC save a bytecode representation of GIMPLE into special ELF sections inside foo.o and bar.o. The final invocation reads the GIMPLE bytecode from foo.o and bar.o, merges the two files into a single internal image, and compiles the result as usual. Since both foo.o and bar.o are merged into a single image, all the interprocedural analyses and optimizations in GCC work across the two files as if they were a single one 🫶. Which means — for example — that the inliner can inline functions in bar.o into functions in foo.o and vice-versa. Cross-file glow-up.

Another (simpler) way to enable link-time optimization is:

gcc -o myprog -flto -O2 foo.c bar.c

The above generates bytecode for foo.c and bar.c, merges them together into a single GIMPLE representation and optimizes them as usual to produce myprog.

The important thing to keep in mind: 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. You can always override that automatic decision by passing -fno-lto to the link command.

To make whole-program optimization effective, it's necessary to make certain assumptions. The compiler needs to know what functions and variables can be accessed by libraries and runtime outside of the link-time optimized unit. When supported by the linker, the linker plugin (see -fuse-linker-plugin) passes information to the compiler about used and externally visible symbols. When the linker plugin isn't available, -fwhole-program should be used to let the compiler make these assumptions, which leads to more aggressive optimization decisions.

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; if -fno-lto is passed to the linker, no interprocedural optimizations are applied. Note that when -fno-fat-lto-objects is enabled the compile stage is faster, but you cannot perform a regular, non-LTO link on them. Pick your fighter.

When producing the final binary, GCC only applies link-time optimizations to those files that contain bytecode. So you can mix and match object files and libraries with GIMPLE bytecodes and final object code 💖. GCC automatically selects which files to optimize in LTO mode and which files to link without further processing.

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. Note that it's generally ineffective to specify an optimization level option only at link time and not at compile time, for two reasons. First, compiling without optimization suppresses compiler passes that gather information needed for effective optimization at link time. Second, some early optimization passes can be performed only at compile time and not at link time. So like… do both, bestie 🙈.

There are some code generation flags preserved by GCC when generating bytecodes, because 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 = -fpie

Certain ABI-changing flags are required to match in all compilation units, and trying to override this at link time with a conflicting value is ignored 🚫. This includes options such as -freg-struct-return and -fpcc-struct-return.

Other options such as -ffp-contract, -fno-strict-overflow, -fwrapv, -fno-trapv or -fno-strict-aliasing are passed through to the link stage and merged conservatively for conflicting translation units. Specifically -fno-strict-overflow, -fwrapv and -fno-trapv take precedence; and for example -ffp-contract=off takes precedence over -ffp-contract=fast. You can override them at link time, though 💅.

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. Iykyk.

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. Just be consistent, queen 🎀.

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: it's possible to apply interprocedural optimizations on files written in different languages 🌈. Watch:

gcc -c -flto foo.c
g++ -c -flto bar.cc
gfortran -c -flto baz.f90
g++ -o myprog -flto -O3 foo.o bar.o baz.o -lgfortran

Notice that the final link is done with g++ to get the C++ runtime libraries and -lgfortran is added to get the Fortran runtime libraries. In general, when mixing languages in LTO mode, you should use the same link command options as when mixing languages in a regular (non-LTO) compilation. Multilingual slay 🌸.

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 -lfoo

With the linker plugin enabled, the linker extracts the needed GIMPLE files from libfoo.a and passes them on to the running GCC to make them part of the aggregated GIMPLE image to be optimized. Chef's kiss 👑

If you're not using a linker with plugin support and/or you 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.

LTO lowkey does not need the whole program to show up before it can operate. If your program doesn't need any symbols exported, it is possible to combine -flto and -fwhole-program to let the interprocedural optimizers use more aggressive assumptions — which may lead to improved optimization opportunities. You do not need -fwhole-program 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 is a strict version check, so bytecode files generated in one version of GCC do not work with an older or newer version of GCC. 🫠

Link-time optimization does not work well with generation of debugging information 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 — picks the partitioning algorithm used by the link-time optimizer. The value is either 1to1 to specify a partitioning mirroring the original source files, or balanced to specify partitioning into equally sized chunks (whenever possible), or max to create new partition for every symbol where possible, or cache to balance chunk sizes while keeping related symbols together for better caching in incremental LTO. Specifying none as an algorithm disables partitioning and streaming completely. The default value is balanced. While 1to1 can be used as a workaround for various code ordering issues, the max partitioning is intended for internal testing only. The value one specifies that exactly one partition should be used while the value none bypasses partitioning and executes the link-time optimization step directly from the WPA phase.
• -flto-incremental=path — enables incremental LTO, with its cache in the given existing directory. Can significantly shorten edit-compile cycles with LTO. 💅

When used with LTO (-flto), the output of translation units inside LTO is cached. Cached translation units are likely to be encountered again when recompiling with small code changes, leading to recompile time reduction.

Multiple GCC instances can use the same cache in parallel.

• -flto-incremental-cache-size=n — specifies number of cache entries in incremental LTO after which to prune old entries. This is a soft limit, temporarily there may be more entries.
• -flto-compression-level=n — specifies the level of compression used for intermediate language written to LTO object files, and is only meaningful in conjunction with LTO mode (-flto). GCC currently supports two LTO compression algorithms. For zstd, valid values are 0 (no compression) to 19 (maximum compression), while zlib supports values from 0 to 9. Values outside this range are clamped to either minimum or maximum of the supported values. If the option is not given, a default balanced compression setting is used. 🎀
• -flto-toplevel-asm-heuristics — enables heuristics to find symbols used in top-level basic "asm". This will restrict link-time optimizations that could cause renaming or deletion of such symbols, which would result in missing symbol errors by linker.

This flag is intended for projects that have not converted to using top-level extended "asm", which specify the usage directly without any false positives.

The heuristics are simple and do not parse the assembly. The heuristics scan through top-level assembly for all possible identifiers; if an identifier is found among declared symbols, the symbol will be marked to restrict link-time optimizations. Static symbols disable more optimizations. Identifiers followed by : disable more optimizations as well, because they might be a locally defined symbol in assembly, even when the declaration is marked 'extern'.

• -fuse-linker-plugin — enables the use of a linker plugin during link-time optimization. This option relies on plugin support in the linker, which is available in gold or in GNU ld 2.21 or newer. 🔥

This option enables the extraction of object files with GIMPLE bytecode out of library archives. This improves the quality of optimization by exposing more code to the link-time optimizer. This information specifies what symbols can be accessed externally (by non-LTO object or during dynamic linking). Resulting code quality improvements on binaries (and shared libraries that use hidden visibility) are similar to -fwhole-program. See -flto for a description of the effect of this flag and how to use it.

This option is enabled by default when LTO support in GCC is enabled and GCC was configured for use with a linker supporting plugins (GNU ld 2.21 or newer or gold).

• -ffat-lto-objects — Fat LTO objects are object files that contain both the intermediate language and the object code. This makes them usable for both LTO linking and normal linking. This option is effective only when compiling with -flto and is ignored at link time.

-fno-fat-lto-objects improves compilation time over plain LTO, but requires the complete toolchain to be aware of LTO. It requires a linker with linker plugin support for basic functionality. Additionally, nm, ar and ranlib need to support linker plugins to allow a full-featured build environment (capable of building static libraries etc). GCC provides the gcc-ar, gcc-nm, gcc-ranlib wrappers to pass the right options to these tools. With non fat LTO makefiles need to be modified to use them.

Note that modern binutils provide plugin auto-load mechanism. Installing the linker plugin into $libdir/bfd-plugins has the same effect as usage of the command wrappers (gcc-ar, gcc-nm and gcc-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, identify arithmetic instructions that compute processor flags similar to a comparison operation based on that arithmetic. If possible, eliminate the explicit comparison operation.

This pass only applies to certain targets that cannot explicitly represent the comparison operation before register allocation is complete.

Enabled at levels -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, perform 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 due to missed counter updates. When this option is specified, GCC uses heuristics to correct or smooth out such inconsistencies. By default, GCC emits an error message when an inconsistent profile is detected.

This option is enabled by -fauto-profile.

• -fprofile-partial-training — with -fprofile-use all portions of programs not executed during training runs are optimized aggressively for size rather than speed. In some cases it is not practical to train all possible hot paths in the program. (For example, it may contain functions specific to a given hardware and training may not cover all hardware configurations the program later runs on.) With -fprofile-partial-training profile feedback is ignored for all functions not executed during the training runs, causing them to be optimized as if they were compiled without profile feedback. This leads to better performance when the training is not representative at the cost of significantly bigger code. 🌸
• -fprofile-use, -fprofile-use=path — enable profile feedback-directed optimizations, and 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-functions

Before 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 — enable sampling-based feedback-directed optimizations, and 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-correction

okay so about that path, bestie — it’s the name of a file containing AutoFDO profile information. If it’s 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://perfwiki.github.io/main/. E.g. —

perf record -e br_inst_retired:near_taken -b -o perf.data \
    -- your_program

Then use the create_gcov tool to convert the raw profile data to a format that can be used by GCC. You must also supply the unstripped binary for your program to this tool. See https://github.com/google/autofdo. E.g. —

create_gcov --binary=your_program.unstripped --profile=perf.data \
    --gcov=profile.afdo

• -fauto-profile-inlining — When auto-profile is available, inline all relevant functions which was inlined in the tran run before reading the profile feedback. This improves context sensitivity of the profile. Enabled by default. ✨

The following options control compiler behavior regarding floating-point arithmetic. These options trade off between speed and correctness. All must be specifically enabled. 💅

• -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=standard is not implemented for languages other than C or C++. On the x86, it has no effect if -mfpmath=sse or -mfpmath=sse+387 is specified; in the former case, IEEE semantics apply without excess precision, and in the latter, rounding is unpredictable.

• -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-store handles rounding to IEEE format only on assignments and not casts as -fexcess-precision=standard does. 😭
• -ffast-math — Sets the options -fno-math-errno, -funsafe-math-optimizations, -ffinite-math-only, -fno-rounding-math, -fno-signaling-nans, -fcx-limited-range and -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 set "errno" after calling math functions that are executed with a single instruction, e.g., "sqrt". A program that relies on IEEE exceptions for math error handling may want to use this flag for speed while maintaining IEEE arithmetic compatibility.

This option is not turned on by any -O option 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. 🌈

• -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-zeros and -fno-trapping-math be in effect. Moreover, it doesn’t make much sense with -frounding-math. For Fortran the option is automatically enabled when both -fno-signed-zeros and -fno-trapping-math are in effect.

The default is -fno-associative-math. 🧚

• -freciprocal-math — Allow the reciprocal of a value to be used instead of dividing by the value if this enables optimizations. For example x / y can be replaced with x * (1/y), which is useful if (1/y) is subject to common subexpression elimination. Note that this loses precision and increases the number of flops operating on the value.

The default is -fno-reciprocal-math. 🌸

• -ffinite-math-only — Allow optimizations for floating-point arithmetic that assume that arguments and results are not NaNs or +-Infs.

This option is not turned on by any -O option 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 +0.0 and -0.0 values, which then prohibits simplification of expressions such as x+0.0 or 0.0*x (even with -ffinite-math-only). This option implies that the sign of a zero result isn’t significant.

The default is -fsigned-zeros. 🎀

• -fno-trapping-math — Compile code assuming that floating-point operations cannot generate user-visible traps. These traps include division by zero, overflow, underflow, inexact result and invalid operation. This option requires that -fno-signaling-nans be in effect. Setting this option may allow faster code if one relies on "non-stop" IEEE arithmetic, for example.

This option 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. 🙈

• -fsingle-precision-constant — treats floating-point constants as single precision instead of quietly promoting them to double-precision constants behind your back. no sneaky upgrades 💅
• -fcx-limited-range — turning this on is basically saying "bestie, we do NOT need a range reduction step when doing complex division." it also skips the check for whether a complex multiplication or division gave you "NaN + I*NaN" and the whole rescue mission for that case. enabled by -ffast-math.

this one controls the default setting of the ISO C99 "CX_LIMITED_RANGE" pragma. but like, it applies to all languages anyway. ✨

• -fcx-fortran-rules — complex multiplication and division follow Fortran rules. range reduction is done as part of complex division, but there's still no checking whether a complex multiply/divide result is "NaN + I*NaN" with a rescue attempt. 🫶
• -fcx-method=method — complex multiplication and division follow whatever method you state. the method argument should be one of limited-range, fortran or stdc.

default vibe is to honor language-specific constraints, which means fortran for Fortran and stdc otherwise. iykyk 🌈

---

so here's the next cluster of flags — these control optimizations that might boost performance, but literally none of them get switched on by any -O option. and yeah, this section includes experimental options that may produce broken code. you've been warned, babe 💀

• -fbranch-probabilities — after you run a program compiled with -fprofile-arcs, you can compile it a second time with -fbranch-probabilities to level up the optimizations based on how many times each branch was actually taken. when a program built with -fprofile-arcs exits, it saves arc execution counts to a file called sourcename.gcda for each source file. the info in that data file is super dependent on the structure of the generated code, so you have to use the same source code and the same optimization options for both compilations. (details about file naming live over in -fprofile-arcs.)

with -fbranch-probabilities, GCC puts a REG_BR_PROB note on each JUMP_INSN and CALL_INSN. these can be used to improve optimization. right now they're only used in one place: in reorg.cc, instead of guessing which path a branch is most likely to take, the REG_BR_PROB values exactly determine which path gets taken more often. iconic 🔥

enabled by -fprofile-use and -fauto-profile.

• -fprofile-values — if you pair it with -fprofile-arcs, it adds code so some data about the values of expressions in the program gets gathered. 📊

with -fbranch-probabilities, it reads back the data gathered from profiling expression values so optimizations can use it.

enabled by -fprofile-generate, -fprofile-use, and -fauto-profile.

• -fprofile-reorder-functions — function reordering based on profile instrumentation collects the first time of execution of a function and orders those functions in ascending order, aiming to optimize program startup through more efficient loading of text segments. main character energy for your startup time 👑

enabled with -fprofile-use.

• -fvpt — if combined with -fprofile-arcs, this option tells the compiler to add code that gathers info about the values of expressions. 🧚

with -fbranch-probabilities, it reads that gathered data back and actually performs the optimizations based on it. currently those optimizations include specialization of division operations using the knowledge about the value of the denominator. smart fr fr.

enabled with -fprofile-use and -fauto-profile.

• -frename-registers — tries to avoid false dependencies in scheduled code by making use of registers left over after register allocation. this optimization most benefits processors with lots of registers. but depending on the debug info format the target adopted, it can make debugging impossible, since variables no longer stay in a "home register". 😭

enabled by default with -funroll-loops.

• -fschedule-fusion — does 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.

• -fdep-fusion — detects macro-op fusible pairs consisting of single-use instructions and their uses, and places such pairs together in the instruction stream to increase fusion opportunities in hardware. this pass runs once before register allocation, and one more time before register renaming. 🦋

enabled at levels -O2, -O3, -Os.

• -ftracer — performs tail duplication to enlarge superblock size. this transformation simplifies the control flow of the function, letting 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 on entry to the loop. -funroll-loops implies -frerun-cse-after-loop, -fweb and -frename-registers. it also turns on complete loop peeling (like, complete removal of loops with a small constant number of iterations). this option makes code larger, and it may or may not make it run faster. no cap, it's a gamble 🙈

enabled by -fprofile-use and -fauto-profile.

• -funroll-all-loops — unrolls all loops, even if their number of iterations is uncertain when the loop is entered. this usually makes programs run more slowly. 💀 -funroll-all-loops implies the same options as -funroll-loops.
• -fpeel-loops — peels loops for which there's enough info that they don't roll much (from profile feedback or static analysis). it also turns on complete loop peeling (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 a "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 -O1 and 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. heads up: 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, with duplicates of 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, 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 especially 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's 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 though. 😬

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.

• -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 — okay so this one is all about security, bestie 💅 it zeroes out call-used registers when a function returns, which either makes Return-Oriented Programming (ROP) attacks way harder or stops info from leaking out through the registers. The possible values of choice are literally the same ones as for the zero_call_used_regs attribute, and the default is skip. Wanna control it for just one function instead of everything? Use the function attribute zero_call_used_regs. slay 🎀

🎀 Program Instrumentation Options 🎀

GCC literally has a whole pile of command-line options that control adding run-time instrumentation to the code it normally generates. Fr fr — like, one purpose of instrumentation is collecting profiling statistics, which you use for finding program hot spots, code coverage analysis, or profile-guided optimizations ✨ Another whole 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 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. It's giving tooling 👑

---

• -p, --profile, -fprofile, -pg — generate extra code to write profile information suitable for the analysis program prof (for -p, --profile, and -fprofile) or gprof (for -pg). You must use this option when compiling the source files you want data about, and you must also use it when linking 🫶 You can use the function attribute no_instrument_function to suppress profiling of individual functions when compiling with these options.
• -fprofile-arcs — adds 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's 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 — adds code so that program conditions are instrumented. During execution, the program records which 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 with gcov --conditions 🦋
• -fpath-coverage — adds 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 with gcov --prime-paths --prime-paths-lines --prime-paths-source
• -fpath-coverage-limit=limit — the threshold at which point -fpath-coverage gives up on instrumenting a function. This limit is approximate and conservative, since GCC uses a pessimistic heuristic which slightly overcounts the running number of paths, and it 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's intended to limit the effect of path explosion and keep compile times reasonable. The default is 250000.
• --coverage, -coverage — this option is used to compile and link code instrumented for coverage analysis. The options -coverage and --coverage are equivalent; both are a synonym for -fprofile-arcs -ftest-coverage (when compiling) and -lgcov (when linking). See the documentation for those options for more details ✨
  • Compile the source files with -fprofile-arcs plus optimization and code generation options. For test coverage analysis, use the additional -ftest-coverage option. You do not need to profile every source file in a program.
  • Compile the source files additionally with -fprofile-abs-path to create absolute path names in the .gcno files. This allows gcov to find the correct sources in projects where compilations occur with different working directories.
  • Link your object files with -lgcov or -fprofile-arcs (the latter implies the former).
  • Run the program on a representative workload to generate the arc profile information. This may be repeated any number of times. You can run concurrent instances of your program, and provided that the file system supports locking, the data files will be correctly updated. Unless a strict ISO C dialect option is in effect, fork calls are detected and correctly handled without double counting.

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 gcov to produce human readable information from the .gcno and .gcda files. Refer to the gcov documentation for further information 🙏

With -fprofile-arcs, for each function of your program GCC creates a program flow graph, then finds a spanning tree for the graph. Only arcs that are not on the spanning tree have to be instrumented: the compiler adds code to count the number of times that these arcs are executed 👑 When an arc is the only exit or only entrance to a block, the instrumentation code can be added to the block; otherwise, a new basic block must be created to hold the instrumentation code.

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 the gcov code-coverage utility can use to show program coverage. Each source file's note file is called auxname.gcno. Refer to the -fprofile-arcs option above for a description of auxname and instructions on how to generate test coverage data 💖 Coverage data matches the source files more closely if you do not optimize.
• -fprofile-abs-path — automatically converts relative source file names to absolute path names in the .gcno files. This allows gcov to 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-arcs and used by -fprofile-use and -fbranch-probabilities and its related options. Both absolute and relative paths can be used. By default, GCC uses the current directory as path, thus the profile data file appears in the same directory as the object file 🌸 In order to prevent the file name clashing, if the object file name is not an absolute path, we mangle the absolute path of the sourcename.gcda file and use it as the file name of a .gcda file. 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 — you can do that with variables in path that are exported during run-time 🦋

• %p — process ID.
• %q{VAR} — value of environment variable VAR

• -fprofile-generate, -fprofile-generate=path — enable options usually used for instrumenting an application to produce a profile useful for later recompilation with profile feedback based optimization. You must use -fprofile-generate both when compiling and when linking your program 🔥

The following options are enabled: -fprofile-arcs, -fprofile-values, -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 — register the profile information in the specified section instead of using a constructor/destructor. The section name is name if it is specified, otherwise the section name defaults to .gcov_info 🎀 A pointer to the profile information generated by -fprofile-arcs is placed in the specified section for each translation unit. This option disables the profile information registration through a constructor and it disables the profile information processing through a destructor 🙈 This option is not intended to be used in hosted environments such as GNU/Linux. It targets freestanding environments (for example embedded systems) with limited resources which do not support constructors/destructors or the C library file I/O 📌

okay so the linker can gather the input sections into one continuous memory block and define start and end symbols. A GNU linker script example that defines a linker output section is giving this:

          .gcov_info      :
          {
            PROVIDE (__gcov_info_start = .);
            KEEP (*(.gcov_info))
            PROVIDE (__gcov_info_end = .);
          }

And the program could dump the profiling information registered in this linker set, for example like this, bestie:

#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 .gcno file into path location. If you combine the option with multiple source files, the .gcno file will be overwritten. 🎀
• -fprofile-prefix-path=path — this option can be used in combination with profile-generate=profile_dir and profile-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 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. 🫶
• -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-map and -fcanon-prefix-map. 🦋
• -fprofile-update=method — alter the update method for an application instrumented for profile feedback based optimization. The method argument should be one of single, atomic or prefer-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 be still 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] — control the level of reproducibility of the profile gathered by -fprofile-generate. This makes it possible to rebuild a program with the same outcome, which is useful for, for example, 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 the fork function is invoked. ⭐

Note that it is quite common that execution counts of some part of programs depend, for example, on the length of temporary file names or memory space randomization (which 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 of the order of streaming of the data into gcda files. This setting makes it possible to run multiple instances of the instrumented program in parallel (such as with make -j). This reduces the 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. See https://github.com/google/sanitizers/wiki/AddressSanitizer for more details. The run-time behavior can be influenced using the ASAN_OPTIONS environment variable. When set to help=1, the available options are shown at startup of the instrumented program. See https://github.com/google/sanitizers/wiki/AddressSanitizerFlags#run-time-flags for a list of supported options. The option cannot be combined with -fsanitize=thread or -fsanitize=hwaddress. Note that the only targets -fsanitize=hwaddress is currently supported on are x86-64 (only with -mlam=u48 or -mlam=u57 options) and AArch64, in both cases only in ABIs with 64-bit pointers. Similarly, -fsanitize=memtag-stack is currently only supported on AArch64 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. See https://github.com/google/kernel-sanitizers for more details. 🐧
• -fsanitize=hwaddress — enable Hardware-assisted AddressSanitizer, which uses a hardware ability to ignore the top byte of a pointer to allow the detection of memory errors with a low memory overhead. Memory access instructions are instrumented to detect out-of-bounds and use-after-free bugs. The option enables -fsanitize-address-use-after-scope. See https://clang.llvm.org/docs/HardwareAssistedAddressSanitizerDesign.html for more details. The run-time behavior can be influenced using the HWASAN_OPTIONS environment variable. When set to help=1, the available options are shown at startup of the instrumented program. The option cannot be combined with -fsanitize=thread or -fsanitize=address, and is currently only available on AArch64. 💖
• -fsanitize=kernel-hwaddress — Hardware-assisted AddressSanitizer, but for compiling the Linux kernel 🐧✨ It's similar to -fsanitize=kernel-address but uses an alternate instrumentation method, and similar to -fsanitize=hwaddress except with the instrumentation differences the Linux kernel needs. Those differences exist to avoid hwasan library initialization calls and to account for the stack pointer having a different value in its top byte. iykyk.

⚠️ Heads up bestie: this option has different defaults than -fsanitize=hwaddress. Instrumenting the stack and alloca calls are not on by default, and using a random frame tag is not implemented for kernel instrumentation. no cap.

• -fsanitize=memtag-stack — Uses Memory Tagging Extension instructions instead of instrumentation to let you detect memory errors 🎀 Similar to HWASAN, it's also a probabilistic method (so like… vibes-based detection, kinda). Available only on those AArch64 architectures that support Memory Tagging Extensions 💅
• -fsanitize=pointer-compare — Instruments comparison operations (<, <=, >, >=) that have pointer operands 🔍 This option must be combined with either -fsanitize=kernel-address or -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 variable ASAN_OPTIONS. Using "detect_invalid_pointer_pairs=1" detects invalid operation only when both pointers are non-null ✨
• -fsanitize=pointer-subtract — Instruments subtraction with pointer operands ➖ Same rules, bestie: it must be combined with either -fsanitize=kernel-address or -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 variable ASAN_OPTIONS. Using "detect_invalid_pointer_pairs=1" detects invalid operation only when both pointers are non-null 💖
• -fsanitize=shadow-call-stack — Enables ShadowCallStack, a security enhancement mechanism used to protect programs against return address overwrites (think stack buffer overflows 👀💀). The way it works: it saves a function's return address to a separately allocated shadow call stack in the function prologue, and restores the return address from the shadow call stack in the function epilogue. Instrumentation only occurs in functions that actually need to save the return address to the stack — efficient queen behavior 👑

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 — so if you wanna use this feature in user space, you need to provide your own support for the runtime. And ngl, 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 — because there's no userspace runtime support — code compiled with ShadowCallStack cannot use exception handling. Use -fno-exceptions to turn off exceptions.

See <https://clang.llvm.org/docs/ShadowCallStack.html> for more details 📚

• -fsanitize=thread — Enables ThreadSanitizer, a fast data race detector 🏃‍♀️💨 Memory access instructions are instrumented to detect data race bugs. See <https://github.com/google/sanitizers/wiki#threadsanitizer> for more details. The run-time behavior can be influenced using the TSAN_OPTIONS environment variable; see <https://github.com/google/sanitizers/wiki/ThreadSanitizerFlags> for a list of supported options. The option cannot be combined with -fsanitize=address, -fsanitize=leak.

When compiling with -fsanitize=thread, you should also use -g to produce more meaningful output 🌸

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 is linked against a library that overrides "malloc" and other allocator functions. See <https://github.com/google/sanitizers/wiki/AddressSanitizerLeakSanitizer> for more details. The run-time behavior can be influenced using the LSAN_OPTIONS environment variable. The option cannot be combined with -fsanitize=thread.
• -fsanitize=undefined — Enables UndefinedBehaviorSanitizer, a fast undefined behavior detector 🚨 Various computations are instrumented to detect undefined behavior at runtime. See <https://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html> for more details. The run-time behavior can be influenced using the UBSAN_OPTIONS environment variable. Current suboptions are:
  • -fsanitize=shift — Enables checking that the result of a shift operation is not undefined ✨ Note that what exactly is considered undefined differs slightly between C and C++, as well as between ISO C90 and C99, etc. This option has two suboptions: -fsanitize=shift-base and -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 a shift operation is not undefined. Note that what exactly is considered undefined differs slightly between C and C++, as well as between ISO C90 and C99, etc.
  • -fsanitize=integer-divide-by-zero — Detect integer division by zero ➗💀
  • -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 — Instructs the compiler to check that the size of a variable length array is positive ✅
  • -fsanitize=null — Enables pointer checking 🔍 Particularly, the application built with this option turned on will issue an error message when it tries to dereference a NULL pointer, or if a reference (possibly an rvalue reference) is bound to a NULL pointer, or if a method is invoked on an object pointed by a NULL pointer.
  • -fsanitize=return — 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 — Detect floating-point division by zero 🌊 Unlike other similar options, -fsanitize=float-divide-by-zero is not enabled by -fsanitize=undefined, since floating-point division by zero can be a legitimate way of obtaining infinities and NaNs ♾️ (iconic, honestly)
• -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-overflow is 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 — Enables instrumentation of return statements in functions marked with "returns_nonnull" function attribute, to detect returning of null values from such functions 🔁
• -fsanitize=bool — Enables instrumentation of loads from bool. If a value other than 0/1 is loaded, a run-time error is issued 🍓
• -fsanitize=enum — Enables instrumentation of loads from an enum type. If a value outside the range of values for the enum type is loaded, a run-time error is issued 🌈

• -fsanitize=vptr — bestie this one instruments C++ member function calls, member accesses, and some conversions between pointers to base and derived classes, all so it can verify the referenced object actually has the correct dynamic type. slay 💅
• -fsanitize=pointer-overflow — instruments pointer arithmetic, and if the pointer math overflows, a run-time error gets issued. no vibes, only valid pointers ✨
• -fsanitize=builtin — instruments arguments to selected builtin functions, and if an invalid value gets passed to such arguments, a run-time error is issued. like, e.g. passing 0 as the argument to __builtin_ctz or __builtin_clz invokes undefined behavior and is diagnosed by this option. caught in 4k fr 🔥

okay real talk 💀 sanitizers tend to increase the rate of false positive warnings — most notably those around -Wmaybe-uninitialized. we recommend against combining -Werror and [the use of] sanitizers. do not do it, bestie, it's giving chaos.

While -ftrapv causes traps for signed overflows to be emitted, -fsanitize=undefined gives a diagnostic message. This currently works only for the C family of languages. iykyk 🌸

---

• -fno-sanitize=all — disables all previously enabled sanitizers. and no, -fsanitize=all is not allowed, because some sanitizers literally cannot be used together. it's giving "sorry babe, incompatible" 😭
• *-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 experiment vibes 🧚✨
• *-fsanitize-sections=s1,s2,...* — sanitize global variables in selected user-defined sections. si may contain wildcards. very demure, very targeted 🎀
• -fsanitize-recover[=opts] — okay so -fsanitize-recover= controls error recovery mode for sanitizers mentioned in the comma-separated list of opts. enabling it for a sanitizer component causes it to attempt to continue running the program as if no error happened 😳 which 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. yes, it's lying to you, that's the point.
  • the -fno-sanitize-recover= option can be used to alter this behavior: only the first detected error is reported and the program then exits with a non-zero exit code. we love a clean exit 👑
  • currently this feature only works for -fsanitize=undefined (and its suboptions except for -fsanitize=unreachable and -fsanitize=return), -fsanitize=float-cast-overflow, -fsanitize=float-divide-by-zero, -fsanitize=bounds-strict, -fsanitize=kernel-address and -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. all or nothing, main character energy ⭐
  • and listen 💖 even if a recovery mode is turned on the compiler side, it needs to be also 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. this can be overridden through setting the halt_on_error flag in the corresponding environment variable. team work makes the dream work 🫶
  • syntax without an explicit opts parameter is deprecated 💀 it is 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. the option sets -fstack-reuse to none. she's thorough, we love her 🦋
• -fsanitize-trap[=opts] — the -fsanitize-trap= option instructs the compiler to report for sanitizers mentioned in the comma-separated list of opts undefined behavior using __builtin_trap rather than a libubsan library routine. if this option is enabled for a certain sanitizer, it takes precedence over the -fsanitizer-recover= for that sanitizer, __builtin_trap will be emitted and be fatal regardless of whether recovery is enabled or disabled using -fsanitize-recover=. no take-backs 💅
  • the advantage of this is that the libubsan library is not needed and is not linked in, so this is usable even in freestanding environments. iconic, self-sufficient queen 👑
  • currently this feature works with -fsanitize=undefined (and its suboptions except for -fsanitize=vptr), -fsanitize=float-cast-overflow, -fsanitize=float-divide-by-zero and -fsanitize=bounds-strict. -fsanitize-trap=all can be also specified, which enables it for undefined suboptions, -fsanitize=float-cast-overflow, -fsanitize=float-divide-by-zero and -fsanitize=bounds-strict.
  • if -fsanitize-trap=undefined or -fsanitize-trap=all is used and -fsanitize=vptr is enabled on the command line, the instrumentation is silently ignored as the instrumentation always needs libubsan support — and -fsanitize-trap=vptr is not allowed. periodt 🚫
• -fsanitize-undefined-trap-on-error — the -fsanitize-undefined-trap-on-error option is the deprecated equivalent of -fsanitize-trap=all. she's a legacy icon, respect her from afar 💜
• -fsanitize-coverage=trace-pc — enables coverage-guided fuzzing code instrumentation. inserts a call to __sanitizer_cov_trace_pc into every basic block. every. single. one. 😤
• -fsanitize-coverage=trace-cmp — enables dataflow guided fuzzing code instrumentation. inserts a call to __sanitizer_cov_trace_cmp1, __sanitizer_cov_trace_cmp2, __sanitizer_cov_trace_cmp4 or __sanitizer_cov_trace_cmp8 for integral comparison with both operands variable, or __sanitizer_cov_trace_const_cmp1, __sanitizer_cov_trace_const_cmp2, __sanitizer_cov_trace_const_cmp4 or __sanitizer_cov_trace_const_cmp8 for integral comparison with one operand constant, __sanitizer_cov_trace_cmpf or __sanitizer_cov_trace_cmpd for float or double comparisons, and __sanitizer_cov_trace_switch for switch statements. the way that's a whole alphabet of hooks 🔤✨
• -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 (such as indirect function call, function return, indirect jump) are valid. this prevents diverting the flow of control to an unexpected target 🎯 it's intended to protect against such threats as Return-oriented Programming (ROP), and similarly call/jmp-oriented programming (COP/JOP). security girlboss behavior, honestly 🛡️
  • the -fcf-protection= keywords are interpreted as follows:
  • the value branch tells the compiler to implement checking of validity of control-flow transfer at the point of indirect branch instructions, i.e. call/jmp instructions.
  • the value return implements checking of validity at the point of returning from a function.
  • the value full is an alias for specifying both branch and return.
  • the value check is used for the final link with link-time optimization (LTO). an error is issued if LTO object files are compiled with different -fcf-protection values. the value check is ignored at the compile time. (link-time only bestie, don't @ me)
  • the value none turns 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, always 💅
  • 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 on bits on bits 🔥
  • you can also use the nocf_check attribute to identify which functions and calls should be skipped from instrumentation. exempt them, queen ✨
  • currently the x86 GNU/Linux target provides an implementation based on Intel Control-flow Enforcement Technology (CET) which works for i686 processor or newer. x86 girlies rise 🌈
• -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_trap if the results do not match. use with -fharden-conditional-branches to cover all conditionals. double-checking your own homework fr fr 🎀
• -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_trap if the result is unexpected. use with -fharden-compares to cover all conditionals. the paranoia is the point 💀
• -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. very "trust but verify" of her 🦋
  • 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.
  • 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 — but these possibilities are merely theoretical, as these conditions can only be met when using custom compiler plugins. iykyk, edge case nation 🌸
• -fhardcfr-skip-leaf — disables -fharden-control-flow-redundancy in leaf functions. leave the leaves alone 🍓
• -fhardcfr-check-exceptions — when -fharden-control-flow-redundancy is active, check the recorded execution path against the control flow graph at exception escape points, as if the function body was wrapped with a cleanup handler that performed the check and reraised. this option is enabled by default; use -fno-hardcfr-check-exceptions to disable it. default on, so she's already doing it 💅
• -fhardcfr-check-returning-calls — when -fharden-control-flow-redundancy is active, check the recorded execution path against the control flow graph before any function call immediately followed by a return of its result, if any, so as to not prevent tail-call optimization, whether or not it is ultimately optimized to a tail call. thoughtful AND efficient, she contains multitudes 😭✨
  • this option is enabled by default whenever sibling call optimizations are enabled (see -foptimize-sibling-calls), but it can be enabled (or disabled, using its negated form) explicitly, regardless of the optimizations. your call bestie 🫶
• -fhardcfr-check-noreturn-calls=[always|no-xthrow|nothrow|never] — when -fharden-control-flow-redundancy is active, this checks the recorded execution path against the control flow graph before noreturn calls. Your modes are: always = check all of them; no-xthrow = check the ones that aren’t expected to return control to the caller through an exception (this is the default); nothrow = check those that may not return control to the caller through an exception either; never = check none of them. Pick your fighter 💅

Checking before a noreturn function that may return control to the caller through an exception may cause checking to be performed 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 it would be redundant with the check performed 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. The checkpoint may even be another noreturn call, so checking may end up performed multiple times. messy? yes. accurate? also yes 😭

Various optimizers may cause calls to be marked as noreturn and/or nothrow, even in the absence of the corresponding attributes, which may affect the placement of checks before calls, as well as the addition of implicit cleanup handlers for them. This unpredictability, and the fact that raising and reraising exceptions frequently amounts to implicitly calling noreturn functions, have made no-xthrow the default setting for this option: it excludes from the noreturn treatment only internal functions used to (re)raise exceptions, that are not affected by these optimizations.

• -fhardened — enables a set of flags for C and C++ that improve the security of the generated code without affecting its ABI. The precise flags enabled may change between major releases of GCC, but are currently:

-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)

The list of options enabled by -fhardened can be generated using the --help=hardened option. handy little menu, honestly 🫶

When the system glibc is older than 2.35, -D_FORTIFY_SOURCE=2 is used instead.

This option is intended to be used in production builds, not merely in debug builds.

Currently, -fhardened is only supported on GNU/Linux targets.

-fhardened only enables a particular option if it wasn’t already specified anywhere on the command line. For instance, -fhardened -fstack-protector will only enable -fstack-protector, but not -fstack-protector-strong. it respects your choices, lowkey iconic ✨

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• -fstack-protector — emits extra code to check for buffer overflows, such as stack smashing attacks. It does this by adding a guard variable to functions with vulnerable objects. This includes functions that call alloca, and functions with buffers larger than or equal to 8 bytes. The guards are initialized when a function is entered and then checked when the function exits. If a guard check fails, an error message is printed and the program exits. Only variables that are actually allocated on the stack are considered; optimized away variables or variables allocated in registers don’t count. guard goblin mode activated 🛡️
• -fstack-protector-all — like -fstack-protector except that all functions are protected.
• -fstack-protector-strong — like -fstack-protector but includes additional functions to be protected: those that have local array definitions, or have references to local frame addresses. Only variables that are actually allocated on the stack are considered; optimized away variables or variables allocated in registers don’t count. stronger vibe, same rules 💜
• -fstack-protector-explicit — like -fstack-protector but only protects those functions which have the stack_protect attribute. explicit consent for protection, love that for us 🎀
• -fstack-check — generate code to verify that you do not go beyond the boundary of the stack. You should specify this flag if you are running in an environment with multiple threads, but you only rarely need to specify it in a single-threaded environment since stack overflow is automatically detected on nearly all systems if there is only one stack.

Note that this switch does not actually cause checking to be done; the operating system or the language runtime must do that. The switch causes generation of code to ensure that they see the stack being extended. it’s giving “I set up the vibes, you do the actual work” 😭

You can additionally specify a string parameter: no means no checking, generic means force the use of old-style checking, specific means use the best checking method and is equivalent to bare -fstack-check.

Old-style checking is a generic mechanism that requires no specific target support in the compiler but comes with the following drawbacks:

1. Modified allocation strategy for large objects: they are always allocated dynamically if their size exceeds a fixed threshold. Note this may change the semantics of some code.
2. Fixed limit on the size of the static frame of functions: when it is topped by a particular function, stack checking is not reliable and a warning is issued by the compiler.
3. Inefficiency: because of both the modified allocation strategy and the generic implementation, code performance is hampered.

Note that old-style stack checking is also the fallback method for specific if no target support has been added in the compiler.

-fstack-check= is designed for Ada’s needs to detect infinite recursion and stack overflows. specific is an excellent choice when compiling Ada code. It is not generally sufficient to protect against stack-clash attacks. To protect against those you want -fstack-clash-protection. different boss fight, different shield bestie 🦋

• -fstack-clash-protection — generate code to prevent stack clash style attacks. When this option is enabled, the compiler will only allocate one page of stack space at a time and each page is accessed immediately after allocation. Thus, it prevents allocations from jumping over any stack guard page provided by the operating system.

Most targets do not fully support stack clash protection. However, on those targets -fstack-clash-protection will protect dynamic stack allocations. -fstack-clash-protection may also provide limited protection for static stack allocations if the target supports -fstack-check=specific. limited edition protection, but still protection ✨

• -fstack-limit-register=reg — generate code to ensure that the stack does not grow beyond the value of a register. If a larger stack is required, a signal is raised at run time. runtime says “absolutely not” 🫶
• -fstack-limit-symbol=sym — same stack-boundary energy, but the limit is the address of a symbol. If a larger stack is required, a signal is raised at run time.
• -fno-stack-limit — the no-stack-limit member of this trio; it’s the off switch for that stack-limit generation. we love a clean opt-out 💅

For most targets, the signal is raised before the stack overruns the boundary, so it is possible to catch the signal without taking special precautions.

For instance, if the stack starts at absolute address 0x80000000 and grows downwards, you can use the flags -fstack-limit-symbol=__stack_limit and -Wl,--defsym,__stack_limit=0x7ffe0000 to enforce a stack limit of 128KB. Note that this may only work with the GNU linker. linker-dependent behavior, classic 😭

You can locally override stack limit checking by using the no_stack_limit function attribute.

• -fsplit-stack — generate code to automatically split the stack before it overflows. The resulting program has a discontiguous stack which can only overflow if the program is unable to allocate any more memory. This is most useful when running threaded programs, as it is no longer necessary to calculate a good stack size to use for each thread. This is currently only implemented for the x86 targets running GNU/Linux. stack gets chopped, vibes stay intact ✂️✨

When code compiled with -fsplit-stack calls code compiled without -fsplit-stack, there may not be much stack space available for the latter code to run. If compiling all code, including library code, with -fsplit-stack is not an option, then the linker can fix up these calls so that the code compiled without -fsplit-stack always has a large stack. Support for this is implemented in the gold linker in GNU Binutils release 2.21 and later.

• -fstrub=disable — disable stack scrubbing entirely, ignoring any strub attributes. full ignore mode, no notes 🚫
• -fstrub=strict — functions default to strub mode disabled, and apply strictly the restriction that only functions associated with strub-callable modes (at-calls, callable and always_inline internal) are callable by functions with strub-enabled modes (at-calls and internal). strict grandma energy, but make it compiler 💜
• -fstrub=relaxed — restore the default stack scrub (strub) setting, namely, strub is only enabled as required by strub attributes associated with function and data types. “Relaxed” means that strub contexts are only prevented from calling functions explicitly associated with strub mode disabled. This option is only useful to override other -fstrub=* options that precede it in the command line. chill mode: restored 🌸
• -fstrub=at-calls — enable at-calls strub mode where viable. The primary use of this option is for testing. It exercises the strub machinery in scenarios strictly local to a translation unit. This strub mode modifies function interfaces, so any function that is visible to other translation units, or that has its address taken, will not be affected by this option. Optimization options may also affect viability. See the strub attribute documentation for details on viability and eligibility requirements. testing era, but make it interface-aware 🎀
• -fstrub=internal — enable internal strub mode where viable. The primary use of this option is for testing. This option is intended to exercise thoroughly parts of the strub machinery that implement the less efficient, but interface-preserving strub mode. Functions that would not be affected by this option are quite uncommon. rare edge cases stay losing, bestie 🦋
• -fstrub=all — enable some strub mode where viable. When both strub modes are viable, at-calls is preferred. -fdump-ipa-strubm adds function attributes that tell which mode was selected for each function. The primary use of this option is for testing, to exercise thoroughly the strub machinery. all the strub, all the testing, all the chaos ✨
• -fvtable-verify=[std|preinit|none] — bestie this one's C++ only, okay? 💅 It flips on (or off, if you say -fvtable-verify=none) the security feature that checks at run time — for every single virtual call — that the vtable pointer you're calling through is actually valid for the type of the object, and hasn't been corrupted or overwritten. If it catches a bad vtable pointer at run time, it reports an error and immediately halts the program. No cap, it does not play around. 🛑

This option makes run-time data structures get built at program startup, and those are what verify the vtable pointers. The std and preinit values control when those structures get built. In both cases they're built before execution reaches main. Using -fvtable-verify=std builds them 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 you pair this with -fvtable-verify=std or -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 goes into a file named vtv_set_ptr_data.log, in the directory named by the environment variable VTV_LOGS_DIR if that's defined, or the current working directory otherwise. 🗂️

Note: this feature appends data to the log file. If you want a fresh log file, delete any existing one first, bestie. 🙈

• -fvtv-counts — this is a debugging flag. With -fvtable-verify=std or -fvtable-verify=preinit, it makes the compiler keep track of the total number of virtual calls it encounters and the number of verifications it inserts. It also counts the number of calls to certain run-time library functions that it inserts, and logs this info for each compilation unit. The compiler writes all this to a file named vtv_count_data.log in the directory named by the environment variable VTV_LOGS_DIR if that's defined, or the current working directory otherwise. It also counts the size of the vtable pointer sets for each class and writes that to vtv_class_set_sizes.log in the same directory. 📊

Note: this feature appends data to the log files. To get fresh log files, delete any existing ones, mmkay? 💖

• -finstrument-functions — generates instrumentation calls for entry into and exit from functions. Just after a function's entry and just before its exit, the following profiling functions get called with the address of the current function and its call site. (On some platforms, __builtin_return_address doesn't work beyond the current function, so the call site info may not be available to the profiling functions otherwise. 😭)

void __cyg_profile_func_enter (void *this_fn,
                               void *call_site);
void __cyg_profile_func_exit  (void *this_fn,
                               void *call_site);

The first argument is the address of the start of the current function, which you can look up exactly in the symbol table. ✨

This instrumentation also happens for functions expanded inline in other functions. Those profiling calls indicate where, conceptually, the inline function is entered and exited. That means addressable versions of such functions have to be available. If every use of a function you have gets expanded inline, this may mean an additional expansion in code size. If you use extern inline in your C code, an addressable version of such functions must be provided. (That's normally the case anyway, but if you get lucky and the optimizer always expands the functions inline, you might have gotten away without providing static copies. 🍀)

A function can be given the attribute no_instrument_function, and then this instrumentation doesn't happen. This is handy 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, but the profiling functions only get called once per instrumented function. Like, 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 this first entry. 🎀

The definition of "once" for this option is a little vague, because the implementation isn't protected against data races. As a result, 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. Meaning: if a thread calls the first function, then it will call the second one, unless it never reaches the exit of the instrumented function. 🤷‍♀️

• *-finstrument-functions-exclude-file-list=file,file,...* — sets the list of functions that are excluded from instrumentation (see the description of -finstrument-functions). If the file that contains a function's 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, it counts as a match. 🔍

For example:

-finstrument-functions-exclude-file-list=/bits/stl,include/sys

excludes any inline function defined in files whose pathnames contain /bits/stl or include/sys.

If, for some reason, you want to include the letter , in one of sym, write \,. For example, -finstrument-functions-exclude-file-list=',,tmp' (note the single quote surrounding the option, bestie — the shell will eat it otherwise 😅).

• -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 that gets matched is its user-visible name, like vector<int> blah(const vector<int> &), *not the internal mangled name (e.g., _Z4blahRSt6vectorIiSaIiEE). The match is done on substrings: if the sym parameter is a substring of the function name, it counts as 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] — generates N NOPs right at the beginning of each function, with the function entry point before the Mth NOP. If M is omitted, it defaults to 0, so the function entry points to the address just at the first NOP. Those NOP instructions reserve extra space that can be used 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 interface gen_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. This can be used to increase the area size or to remove it completely on a single function. If N=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. 🔥

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✨ Options Controlling the Preprocessor ✨

These options control the C preprocessor, which is, like, the thing that runs on each C source file before the 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. 💅

In addition to the options listed here, there are a bunch of options to control search paths for include files, documented in Directory Options. Options to control preprocessor diagnostics are listed in Warning Options. 🎀

• -D name, --define-macro=name, --define-macro name — predefine name as a macro, with definition 1. ezpz ✨
• -D name=definition, --define-macro=name=definition, --define-macro name=definition — the contents of definition get tokenized and processed as if they appeared during translation phase three in a #define directive. Specifically, the definition is truncated by embedded newline characters. 🫠

If you're invoking the preprocessor from a shell or shell-like program, you may need to use the shell's quoting syntax to protect characters — like spaces — that have a meaning in the shell syntax.

If you want 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're given on the command line. All -imacros file and -include file options are processed after all -D and -U options. 📋

• -U name, --undefine-macro=name, --undefine-macro name — cancel any previous definition of name, whether it was built in or provided with a -D option. poof, gone 💨
• -include file, --include=file, --include file — bestie, this one processes file like #include "file" was literally the very first line of your primary source file. 🎀 But plot twist: the first directory it searches for file is the preprocessor's working directory, not the directory containing the main source file. If it's not there, it keeps searching the rest of the #include "..." chain as normal. And if you pass multiple -include options? They get included in the order they show up on the command line. Periodt.
• -imacros file, --imacros=file, --imacros file — exactly like -include, except any output produced by scanning file gets yeeted into the void. 🙈 The macros it defines stay defined though — which is the whole point. It lets you grab all the macros from a header without also processing its declarations. Also, heads up: all files specified by -imacros are processed before all files specified by -include. Order matters, iykyk.
• -undef — do not predefine any system-specific or GCC-specific macros. The standard predefined macros remain defined. That's it, that's the vibe. ✨
• -pthread — defines the additional macros you need for using the POSIX threads library. 🧵 Use this option consistently for both compilation and linking — like, don't skip leg day on one of them. Supported on GNU/Linux targets, most other Unix derivatives, and also on x86 Cygwin and MinGW targets.
• -M, --dependencies — instead of outputting the result of preprocessing, output a rule suitable for make describing the dependencies of the main source file. 🫶 The preprocessor outputs one make rule containing the object file name for that source file, a colon, and the names of all the included files — including the ones coming from -include or -imacros command-line options.

Unless specified explicitly (with -MT or -MQ), the object file name consists of the name of the source file with any suffix replaced with the object file suffix, and with any leading directory parts removed. If there are many included files, the rule gets split into several lines using \-newline. The rule has no commands. Simple as that.

This option does not suppress the preprocessor's debug output, like -dM. So 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. Sneaky.

• -MM, --user-dependencies — like -M but it does not mention header files found in system header directories, nor header files that are included, directly or indirectly, from such a header. 💅 This means 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. Just so we're clear.
• -MF file — when used with -M or -MM, specifies a file to write the dependencies to. If no -MF switch is given, the preprocessor sends the rules to the same place it would send preprocessed output. When used with the driver options -MD or -MMD, -MF overrides the default dependency output file. And if file is -, then the dependencies are written to stdout. 🦋
• -MG, --print-missing-file-dependencies — in conjunction with an option like -M that requests dependency generation, -MG assumes missing header files are generated files and adds them to the dependency list without raising an error. 🌸 The dependency filename is taken directly from the #include directive without prepending any path. -MG also suppresses preprocessed output, since a missing header file renders that useless. This feature is used in automatic updating of makefiles. Iconic, honestly.
• -Mno-modules — disables dependency generation for compiled module interfaces. That's the whole thing. 💀
• -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 the errors make gives if you remove header files without updating the Makefile to match. 🙈 Typical output looks like:

test.o: test.c test.h

test.h:

• -MT target — changes 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. That result is the target.

An -MT option sets the target to be exactly the string you specify. 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. So -MQ '$(objpfx)foo.o' gives:

$$(objpfx)foo.o: foo.c

The default target is automatically quoted, as if it were given with -MQ. Slay. 💅

• -MD, --write-dependencies — -MD is equivalent to -M -MF file, except that -E is not implied. The driver determines file based on whether an -o option is given. If it is, the driver uses its argument but with a suffix of .d; otherwise it takes the name of the input file, removes any directory components and suffix, and applies a .d suffix.

If -MD is used in conjunction with -E, any -o switch is understood to specify the dependency output file — but if used without -E, each -o is understood to specify a target object file. Context is everything, bestie. 🎀

Since -E is not implied, -MD can be used to generate a dependency output file as a side effect of the compilation process. Two birds, one stone.

• -MMD, --write-user-dependencies — like -MD except it mentions only user header files, not system header files. 🌈
• -fpreprocessed — tells 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 you can pass a file preprocessed with -C to the compiler without problems. In this mode the integrated preprocessor is little more than a tokenizer for the front ends. Glorified bouncer, honestly.

-fpreprocessed is implicit if the input file has one of the extensions .i, .ii or .mi — those are the extensions 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. Let me break it down:

With -E, preprocessing is limited to the handling of directives such as #define, #ifdef, and #error. Other preprocessor operations, like 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. Chaos resolved. 💜

• -fdollars-in-identifiers — accept $ in identifiers. Short and sweet. 💅
• -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. That's the whole description, no notes. 🦋
• -fmax-include-depth=depth — sets the maximum depth of the nested #include. The default is 200. Don't go nesting forever, babes. 🌀
• -fsearch-include-path[=kind] — look for input files on the #include path, 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.cc

kind defaults to user, which looks on the #include "..." search path; you can also explicitly specify system for the #include <...> search path. iykyk. ✨

• -ftabstop=width — sets 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] — tracks locations of tokens across macro expansions. This allows the compiler to emit diagnostics about the current macro expansion stack when a compilation error occurs in a macro expansion. 🫶 But using this option makes the preprocessor and the compiler consume more memory.

The level parameter lets you choose the precision of token location tracking, thus decreasing memory consumption if necessary:

• Value 0 of level de-activates this option.
• Value 1 tracks token 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 token 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. So you're already in it, bestie.

• -fmacro-prefix-map=old=new — when preprocessing files residing in directory old, expand the __FILE__ and __BASE_FILE__ macros as if the files resided in directory new instead. 🎀 This can be used to change an absolute path to a relative path by using . for new, which can result in more reproducible builds that are location independent. This option also affects __builtin_FILE() during compilation. See also -ffile-prefix-map and -fcanon-prefix-map. 💅
• -fexec-charset=charset — sets the execution character set, used for string and character constants. The default is UTF-8. 🌈 charset can be any encoding supported by the system's iconv library routine.
• -fwide-exec-charset=charset — sets the wide execution character set bestie, aka the charset used for wide string and character constants 💅 the default is one of UTF-32BE, UTF-32LE, UTF-16BE, or UTF-16LE — whichever one matches the width of wchar_t plus the big-endian or little-endian byte order your codegen is using. Same deal as -fexec-charset: charset can be any encoding your system's iconv library routine supports, BUT (and this is the catch) you're gonna have problems with encodings that don't fit exactly in wchar_t. so like. watch out 🫶
• -finput-charset=charset — sets the input character set, used to translate from the character set of the input file into the source character set GCC uses 🌸 default is UTF-8. charset can be any encoding supported by your system's iconv library routine, no cap.

ok so here's the tea: if the input character set is UTF-8, warnings about ill-formed code unit sequences get issued if -Winvalid-utf8 is enabled. otherwise? no diagnostics at all when the input character set matches the execution character set. but if they're different, ill-formed code unit sequences result in an error during transcoding to the execution character set 😭

• -fpch-deps — when you're using precompiled headers, this flag makes the dependency-output flags also list the files from the precompiled header's dependencies ✨ if you don't specify it, only the precompiled header gets listed, not the files that were used to create it — because those files are not consulted when a precompiled header is used. iykyk 💅
• -fpch-preprocess — this one lets you use 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, plus its filename. when -fpreprocessed is in use, GCC recognizes this #pragma and loads the PCH 👑

this option is off by default, because the resulting preprocessed output is only really suitable as input to GCC. it gets switched on by -save-temps ⭐

do NOT write this #pragma in your own code bestie 🙈 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 — enables generation of linemarkers in the preprocessor output that let the compiler know the current working directory at the time of preprocessing 🌈 when this 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 that can be inhibited with the negated form -fno-working-directory 💜 if the -P flag is present in the command line, this option has no effect, since no #line directives are emitted whatsoever. periodt.
• -C, --comments — do not discard comments 🎀 all comments are passed through to the output file, except for comments in processed directives, which get deleted along with the directive.

be prepared for side effects when using -C bestie 😭 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, --comments-in-macros — 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.

on top of all the side effects of -C, the -CC option causes all C++-style comments inside a macro to be converted to C-style comments 👑 this is to prevent later use of that macro from inadvertently commenting out the remainder of the source line. the -CC option is generally used to support lint comments 🫶

• -P, --no-line-commands — inhibits generation of linemarkers in the output from the preprocessor 🌸 this might be useful when you're running the preprocessor on something that isn't C code and it'll be sent to a program which might get confused by the linemarkers. understandable honestly 💅
• -traditional, --traditional, -traditional-cpp, --traditional-cpp — try to imitate the behavior of pre-standard C preprocessors, as opposed to ISO C preprocessors 🦋 see the GNU CPP manual for the details.

note that GCC does not otherwise attempt to emulate a pre-standard C compiler, and these options are only supported with the -E switch, or when invoking CPP explicitly ⚠️

• -trigraphs, --trigraphs — support ISO C trigraphs 💫 these are three-character sequences, all starting with ??, that ISO C defines to stand for single characters. for example, ??/ stands for \, so '??/n' is a character constant for a newline. iconic honestly.

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 — enables special code to work around file systems that only permit very short file names, like MS-DOS 😭 a whole throwback fr fr.
• -H, --trace-includes — print the name of each header file used, in addition to other normal activities 🌸 each name is indented to show how deep in the #include stack it is. precompiled header files are also printed, even if they're found to be invalid — an invalid precompiled header file is printed with ...x and a valid one with ...! 💅
• -dletters, --dump=letters, --dump letters — says to make debugging dumps during compilation as specified by letters 🎀 the flags documented here are the ones relevant to the preprocessor. other letters get interpreted by the compiler proper, or reserved for future versions of GCC, and so are silently ignored. and if you specify letters whose behavior conflicts? the result is undefined 💀 yeah. don't do that.
  • -dM, --dump=M — instead of the normal output, generate a list of #define directives for all the macros defined during the execution of the preprocessor, including predefined macros ✨ this gives you a way of finding out what is predefined in your version of the preprocessor. assuming you have no file foo.h, the command

        touch foo.h; cpp -dM foo.h

shows all the predefined macros 👑

if you use -dM without the -E option, -dM is interpreted as a synonym for -fdump-rtl-mach ⚠️ sneaky.

• -dD, --dump=D — like -dM except that it outputs both the #define directives and the result of preprocessing 🦋 both kinds of output go to the standard output file.
• -dN, --dump=N — like -dD, but emit only the macro names, not their expansions 💅
• -dI, --dump=I — output #include directives in addition to the result of preprocessing 🌸
• -dU, --dump=U — like -dD except that only macros that are expanded, or whose definedness is tested in preprocessor directives, are output 🫶 the output is delayed until the use or test of the macro, and #undef directives are also output for macros tested but undefined at the time. so thorough. so main character.

• -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,option to bypass the compiler driver and pass option directly through to the preprocessor ✨ if option contains commas, it's split into multiple options at the commas. BUT — and this is the important part — many options are modified, translated or interpreted by the compiler driver before being passed to the preprocessor, and -Wp forcibly bypasses this phase 😭 the preprocessor's direct interface is undocumented and subject to change, so whenever possible you should avoid using -Wp and let the driver handle the options instead. please bestie. for me. 🙏
• -Xpreprocessor option — pass option as an option to the preprocessor 🌈 you can use this to supply system-specific preprocessor options that GCC doesn't recognize.

if you want to pass an option that takes an argument, you must use -Xpreprocessor twice — once for the option and once for the argument 💅

• -no-integrated-cpp, --no-integrated-cpp — perform preprocessing as a separate pass before compilation 🎀 by default, GCC performs preprocessing as an integrated part of input tokenization and parsing. if this option is provided, the appropriate language front end (cc1, cc1plus, or cc1obj for C, C++, and Objective-C, respectively) is instead invoked twice — once for preprocessing only, and once for actual compilation of the preprocessed input. this option may be useful in conjunction with the -B or -wrapper options to specify an alternate preprocessor or perform additional processing of the program source between normal preprocessing and compilation 🦋

🧵 Passing Options to the Assembler

you can pass options to the assembler. yes, really. it's giving delegation 💅

• -Wa,option — pass option as an option to the assembler ✨ if option contains commas, it's split into multiple options at the commas.
• -Xassembler option, --for-assembler=option, --for-assembler option — pass option as an option to the assembler 🦋 you can use this to supply system-specific assembler options that GCC does not recognize.

if you want to pass an option that takes an argument, you must use -Xassembler twice — once for the option and once for the argument 🫶

🔗 Options for Linking

these options come into play when the compiler links object files into an executable output file 💖 they are meaningless if the compiler isn't doing a link step. so like. don't come for them when they're not even on duty 😭

okay so, back into the options pile — this chunk is lowkey all about linking, object files, and making the linker behave exactly how you want 💅

• object-file-name — a file name that does not end in a special recognized suffix is considered to name an object file or library. The linker distinguishes object files from libraries by looking at the file contents. If linking is done, those object files get used as input to the linker.
• -c, -S, -E — if any of these is used, then the linker is not run, and object file names should not be used as arguments.
• -flink-libatomic — enable linking of libatomic if it's supported by target, and it's enabled by default. The negative form -fno-link-libatomic can be used to explicitly disable linking of libatomic.
• -flinker-output=type — this option controls code generation of the link-time optimizer. By default, the linker output is automatically determined by the linker plugin. For debugging the compiler, and if incremental linking with a non-LTO object file is desired, it may be useful to control the type manually 🎀
  • exec — code generation produces a static binary. In this case -fpic and -fpie are both disabled.
  • dyn — code generation produces a shared library. In this case -fpic or -fPIC is preserved, but not enabled automatically. This allows building shared libraries without position-independent code on architectures where this is possible, i.e. on x86.
  • pie — code generation produces an -fpie executable. This results in similar optimizations as exec except that -fpie is not disabled if specified at compilation time.
  • rel — the compiler assumes that incremental linking is done. The sections containing intermediate code for link-time optimization are merged, pre-optimized, and output to the resulting object file. In addition, if -ffat-lto-objects is specified, binary code is produced for future non-LTO linking. The object file produced by incremental linking is smaller than a static library produced from the same object files. At link time the result of incremental linking also loads faster than a static library, assuming that 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 (for example, cross-module inlining happens), but most of the benefits of whole-program optimizations are lost.

During the incremental link (by -r) the linker plugin defaults to rel. GNU Binutils 2.44 or later is needed 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 cannot be used for link-time optimization, the linker plugin issues a warning and uses nolto-rel. To maintain whole-program optimization, link such objects into a static library instead.

• -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.
• -fuse-ld=wild — use the Wild linker (wild) instead of the default linker.
• -llibrary, -l library — search the library named library when linking. The second alternative, with library as a separate argument, is only for POSIX compliance and is not recommended 🙈

The -l option is passed directly to the linker by GCC. Refer to your linker documentation for exact details — the general description below applies to the GNU linker.

The linker searches a standard list of directories for the library. The directories searched include several standard system directories plus any that you specify with -L.

Static libraries are archives of object files, and have file names like liblibrary.a. Some targets also support shared libraries, which typically have names like liblibrary.so. If both static and shared libraries are found, the linker gives preference to linking with the shared library unless the -static option is used.

It makes a difference where in the command you write this option; the linker searches and processes libraries and object files in the order they are specified. Thus, foo.o -lz bar.o searches library z after file foo.o but before bar.o. If bar.o refers to functions in z, those functions may not be loaded 👀

• -lobjc — you need this special case of the -l option in order to link an Objective-C or Objective-C++ program.
• -nostartfiles — do not use the standard system startup files when linking. The standard system libraries are used normally, unless -nostdlib, -nolibc, or -nodefaultlibs is used.
• -nodefaultlibs — do not use the standard system libraries when linking. Only the libraries you specify are passed to the linker, and options specifying linkage of the system libraries, such as -static-libgcc or -shared-libgcc, are ignored. The standard startup files are used normally, unless -nostartfiles is used.

The compiler may generate calls to memcmp, memset, memcpy and memmove. These entries are usually resolved by entries in libc. These entry points should be supplied through some other mechanism when this option is specified.

• -nolibc — do not use the C library or system libraries tightly coupled with it when linking. Still link 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 -lc from the link command line, as well as system libraries that normally go with it and become meaningless when absence of a C library is assumed, for example -lpthread or -lm in some configurations. This is intended for bare-board targets when there is indeed no C library available.
• -nostdlib, --no-standard-libraries — do not use the standard system startup files or libraries when linking. No startup files and only the libraries you specify are passed to the linker, and options specifying linkage of the system libraries, such as -static-libgcc or -shared-libgcc, are ignored.

The compiler may generate calls to memcmp, memset, memcpy and memmove. These entries are usually resolved by entries in libc. These entry points should be supplied through some other mechanism when this option is specified.

One of the standard libraries bypassed by -nostdlib and -nodefaultlibs is libgcc.a, a library of internal subroutines which GCC uses to overcome shortcomings of particular machines, or special needs for some languages.

In most cases, you need libgcc.a even when you want to avoid other standard libraries. In other words, when you specify -nostdlib or -nodefaultlibs you should usually specify -lgcc as well. This ensures that you have no unresolved references to internal GCC library subroutines. (An example of such an internal subroutine is __main, used to ensure C++ constructors are called.)

• -nostdlib++ — do not implicitly link with standard C++ libraries.
• -e entry, --entry=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, --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, --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 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. This is also known as partial linking.
• -rdynamic — pass the flag -export-dynamic to the ELF linker, on targets that support it. This instructs the linker to add all symbols, not only used ones, to the dynamic symbol table. This option is needed for some uses of dlopen or to allow obtaining backtraces from within a program.
• -s — remove all symbol table and relocation information from the executable.
• -static, --static — on systems that support dynamic linking, this overrides -pie and prevents linking with the shared libraries. On other systems, this option has no effect.
• -shared, --shared — produce a shared object which can then be linked with other objects to form an executable. Not all systems support this option. For predictable results, you must also specify the same set of options used for compilation (-fpic, -fPIC, or model suboptions) when you specify this linker option.[1]
• -shared-libgcc, -static-libgcc — on systems that provide libgcc as a shared library, these options force the use of either the shared or static version, respectively. If no shared version of libgcc was built when the compiler was configured, these options have no effect.

There are several situations in which an application should use the shared libgcc instead of the static version. The most common of these is when the application wishes to throw and catch exceptions across different shared libraries. In that case, each of the libraries as well as the application itself should use the shared libgcc.

So yeah — that's why the G++ driver just automatically adds -shared-libgcc whenever you build a shared library or a main executable, because C++ programs typically use exceptions, so this is literally the right thing to do. 💅

But if, instead, you use the GCC driver to create shared libraries, you may find they're not always linked with the shared libgcc. 😭 Here's the tea: if GCC finds, at its configuration time, that you have a non-GNU linker or a GNU linker that does not support option --eh-frame-hdr, it links the shared version of libgcc into shared libraries by default. Otherwise, it takes advantage of the linker and optimizes away the linking with the shared version of libgcc, linking with the static version of libgcc by default. This lets exceptions propagate through such shared libraries, without incurring relocation costs at library load time. ✨

However — and this is the important bit, bestie — 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, such that it is linked with the shared libgcc. No cap. 🔥

• -static-libasan — when the -fsanitize=address option is used to link a program, the GCC driver automatically links against libasan. If libasan is available as a shared library, and the -static option is not used, then this links against the shared version of libasan. The -static-libasan option directs the GCC driver to link libasan statically, without necessarily linking other libraries statically. 🧚
• -static-libhwasan — when the -fsanitize=hwaddress option is used to link a program, the GCC driver automatically links against libhwasan. If libhwasan is available as a shared library, and the -static option is not used, then this links against the shared version of libhwasan. The -static-libhwasan option directs the GCC driver to link libhwasan statically, without necessarily linking other libraries statically. 🌸
• -static-liblsan — when the -fsanitize=leak option is used to link a program, the GCC driver automatically links against liblsan. If liblsan is available as a shared library, and the -static option is not used, then this links against the shared version of liblsan. The -static-liblsan option directs the GCC driver to link liblsan statically, without necessarily linking other libraries statically. 🫶
• -static-libtsan — when the -fsanitize=thread option is used to link a program, the GCC driver automatically links against libtsan. If libtsan is available as a shared library, and the -static option is not used, then this links against the shared version of libtsan. The -static-libtsan option directs the GCC driver to link libtsan statically, without necessarily linking other libraries statically. 🦋
• -static-libubsan — when the -fsanitize=undefined option is used to link a program, the GCC driver automatically links against libubsan. If libubsan is available as a shared library, and the -static option is not used, then this links against the shared version of libubsan. The -static-libubsan option directs the GCC driver to link libubsan statically, without necessarily linking other libraries statically. 🌈
• -static-libstdc++ — okay so when the g++ program is used to link a C++ program, it normally automatically links against libstdc++. If libstdc++ is available as a shared library, and the -static option is not used, then this links against the shared version of libstdc++. That's normally fine, lowkey iconic even. However, it is sometimes useful to freeze the version of libstdc++ used by the program without going all the way to a fully static link. The -static-libstdc++ option directs the g++ driver to link libstdc++ statically, without necessarily linking other libraries statically. 👑
• -symbolic / --symbolic — bind references to global symbols when building a shared object. Warn about any unresolved references (unless overridden by the link editor option -Xlinker -z -Xlinker defs). Only a few systems support this option. 🙈
• -T script — use script as the linker script. This option is supported by most systems using the GNU linker. On some targets, such as bare-board targets without an operating system, the -T option may be required when linking to avoid references to undefined symbols. ⭐
• -Xlinker option — pass option as an option to the linker. You can use this to supply system-specific linker options that GCC does not recognize. 🎀

If you want to pass an option that takes a separate argument, you must use -Xlinker twice, once for the option and once for the argument. For example, to pass -assert definitions, you must write -Xlinker -assert -Xlinker definitions. It does not work to write -Xlinker "-assert definitions", because this passes the entire string as a single argument, which is not what the linker expects. 💀

When using the GNU linker, it is usually more convenient to pass arguments to linker options using the option=value syntax than as separate arguments. For example, you can specify -Xlinker -Map=output.map rather than -Xlinker -Map -Xlinker output.map. Other linkers may not support this syntax for command-line options.

• -Wl,option / --for-linker=option / --for-linker option — pass option as an option to the linker. If option contains commas, it is split into multiple options at the commas. You can use this syntax to pass an argument to the option. For example, -Wl,-Map,output.map passes -Map output.map to the linker. When using the GNU linker, you can also get the same effect with -Wl,-Map=output.map. 🔥
• -u symbol / --force-link=symbol / --force-link symbol — pretend the symbol symbol is undefined, to force linking of library modules to define it. You can use -u multiple times with different symbols to force loading of additional library modules. 💜
• -Tbss=addr, -Tdata=addr, -Ttext=addr, -N, -n, -t, -Z, -z keyword — these options are passed through to the linker without interpretation by GCC. Refer to your linker documentation for the meanings of these options. (Yes, they're just vibes-passed straight through, iykyk. 😌)

---

🗂️ ✨ Options for Directory Search ✨ 🗂️

These options specify directories to search for header files, for libraries and for parts of the compiler — basically telling GCC where to go look, bestie. 🔍

• -I dir, -iquote dir, -isystem dir, -idirafter dir, --include-directory-after=dir, --include-directory-after dir, --include-directory=dir, --include-directory dir — add the directory dir to the list of directories to be searched for header files during preprocessing. --include-directory is an alias for -I, while --include-directory-after is an alias for -idirafter. If dir begins with = or $SYSROOT, then the = or $SYSROOT is replaced by the sysroot prefix; see --sysroot and -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:

1. For the quote form of the include directive, the directory of the current file is searched first.
2. For the quote form of the include directive, the directories specified by -iquote options are searched in left-to-right order, as they appear on the command line.
3. Directories specified with -I options are scanned in left-to-right order.
4. Directories specified with -isystem options are scanned in left-to-right order.
5. Standard system directories are scanned.
6. Directories specified with -idirafter options 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, you should not use this option to add directories that contain vendor-supplied system header files; use -isystem for that. 🙅‍♀️

The -isystem and -idirafter options also mark the directory as a system directory, so that it gets the same special treatment that is applied to the standard system directories.

If a standard system include directory, or a directory specified with -isystem, is also specified with -I, the -I option is ignored. The directory is still searched but as a system directory at its normal position in the system include chain. This is to ensure that GCC's procedure to fix buggy system headers and the ordering for the #include_next directive are not inadvertently changed. If you really need to change the search order for system directories, use the -nostdinc and/or -isystem options. 😤

• -I- / --include-barrier — split the include path. ⚠️ This option has been deprecated. Please use -iquote instead for -I directories before the -I- and remove the -I- option. (Yeah girl, it's giving legacy. 🫠)

Any directories specified with -I options before -I- are searched only for headers requested with #include "file"; they are not searched for #include <file>. If additional directories are specified with -I options after the -I-, those directories are searched for all #include directives.

In addition, -I- inhibits the use of the directory of the current file directory as the first search directory for #include "file". There is no way to override this effect of -I-. 💀

• -iprefix prefix / --include-prefix=prefix / --include-prefix prefix — specify prefix as the prefix for subsequent -iwithprefix options. If the prefix represents a directory, you should include the final /. 🎀
• -iwithprefix dir, -iwithprefixbefore dir, --include-with-prefix=prefix, --include-with-prefix prefix, --include-with-prefix-after=prefix, --include-with-prefix-after prefix, --include-with-prefix-before=prefix, --include-with-prefix-before prefix — append dir to the prefix specified previously with -iprefix, and add the resulting directory to the include search path. -iwithprefixbefore puts it in the same place -I would; -iwithprefix puts it where -idirafter would. 🦋

--include-with-prefix and --include-with-prefix-after are both aliases for -iwithprefix, while --include-with-prefix-before is an alias for -iwithprefixbefore. ✨

• -isysroot dir — okay so this is like the --sysroot option but it only applies to header files (except for Darwin targets, where it applies to both header files and libraries). See the --sysroot option for more information. 💅
• -imultilib dir — use dir as a subdirectory of the directory containing target-specific C++ headers. Slay.
• -imultiarch dir — use dir as a subdirectory of the directory containing architecture-specific C++ headers. Iconic.
• -nostdinc, --no-standard-includes — do not search the standard system directories for header files. Only the directories explicitly specified with -I, -iquote, -isystem, and/or -idirafter options (and the directory of the current file, if appropriate) are searched. No cap. 🙅‍♀️
• -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.) Periodt. 💖
• --embed-dir=dir, --embed-directory=dir, --embed-directory dir — append dir directory to the list of searched directories for "#embed" preprocessing directive or "__has_embed" macro. 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=name instead of -fplugin=path/name.so. This option is not meant to be used by the user, but only passed by the driver. Lowkey internal. 🫣
• -Ldir, --library-directory=dir, --library-directory dir — add directory dir to the list of directories to be searched for -l. Obsessed. 🔍
• -Bprefix, --prefix=prefix, --prefix prefix — bestie, 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 is not specified, the driver tries two standard prefixes, /usr/lib/gcc/ and /usr/local/lib/gcc/. If neither of those results in a file name that is found, the unmodified program name is searched for using the directories specified in your PATH environment variable. The compiler checks to see if the path provided by -B refers to a directory, and if necessary it adds a directory separator character at the end of the path. -B prefixes that effectively specify directory names also apply to libraries in the linker, because the compiler translates these options into -L options for the linker. They also apply to include files in the preprocessor, because the compiler translates these options into -isystem options for the preprocessor. In this case, the compiler appends include to the prefix. The runtime support file libgcc.a can also be searched for using the -B prefix, if needed. If it is not found there, the two standard prefixes above are tried, and that is all. The file is left out of the link if it is not found by those means. Another way to specify a prefix much like the -B prefix is to use the environment variable GCC_EXEC_PREFIX. As a special kludge, if the path provided by -B is [dir/]stageN/, where N is a number in the range 0 to 9, then it is replaced by [dir/]include. This is to help with boot-strapping the compiler. 💀 (okay that was a lot but we ate)
• -no-canonical-prefixes, --no-canonical-prefixes — do not expand any symbolic links, resolve references to /../ or /./, or make the path absolute when generating a relative prefix. Simple. 🎀
• --sysroot=dir, --sysroot dir — use dir as the logical root directory for headers and libraries. Like, if the compiler normally searches for headers in /usr/include and libraries in /usr/lib, it instead searches dir/usr/include and dir/usr/lib. If you use both this option and the -isysroot option, then the --sysroot option applies to libraries, but the -isysroot option applies to header files. The GNU linker (beginning with version 2.16) has the necessary support for this option. If your linker does not support this option, the header file aspect of --sysroot still works, but the library aspect does not. 😭
• --no-sysroot-suffix — for some targets, a suffix is added to the root directory specified with --sysroot, depending on the other options used, so that headers may for example be found in dir/suffix/usr/include instead of dir/usr/include. This option disables the addition of such a suffix. 🌸

🛠️ Options for Code Generation Conventions 🛠️

okay so 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 list 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. Ngl, that's the vibe check. ✨

• -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 legacy code relies on the behavior of older compilers in which temporaries' stack space is not reused — 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 -ftrapv and -fwrapv override each other, so using -ftrapv -fwrapv on the command-line results in -fwrapv being effective. Note that only active options override, so using -ftrapv -fwrapv -fno-wrapv on the command-line results in -ftrapv being 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 -ftrapv and -fwrapv override each other, so using -ftrapv -fwrapv on the command-line results in -fwrapv being effective. Note that only active options override, so using -ftrapv -fwrapv -fno-wrapv on the command-line results in -ftrapv being 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-pointer and 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 as "SIGALRM". This enables -fexceptions. 💜
• -fdelete-dead-exceptions — bestie, this one is saying the compiler can lowkey optimize away instructions that might throw exceptions but don’t otherwise contribute to your program running. It does not affect calls to functions, except those with the pure or const attributes. 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. slay 💅
• -funwind-tables — similar to -fexceptions, except it just generates any needed static data and 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 — generates an unwind table in DWARF format, if supported by the target machine. The table is exact at each instruction boundary, so it can be used for stack unwinding from asynchronous events (such as a debugger or garbage collector). iconic ✨
• -fno-gnu-unique — okay so on systems with a recent GNU assembler and C library, the C++ compiler uses the STB_GNU_UNIQUE binding to make sure that definitions of template static data members and static local variables in inline functions are unique even in the presence of RTLD_LOCAL. This is necessary to avoid problems with a library used by two different RTLD_LOCAL plugins depending on a definition in one of them and therefore disagreeing with the other one about the binding of the symbol. But this causes dlclose to be ignored for affected DSOs; if your program relies on reinitialization of a DSO via dlclose and dlopen, you can use -fno-gnu-unique. 💀
• -fpcc-struct-return — returns short struct and union values in memory like longer ones, rather than in registers. This convention is less efficient, but it has the advantage of allowing intercallability between GCC-compiled files and files compiled with other compilers, particularly the Portable C Compiler (pcc). The precise convention for returning structures in memory depends on the target configuration macros. Short structures and unions are those whose size and alignment match that of some integer type.

Warning: code compiled with the -fpcc-struct-return switch is not binary compatible with code compiled with the -freg-struct-return switch. Use it to conform to a non-default application binary interface. 😭

• -freg-struct-return — returns struct and union values in registers when possible. This is more efficient for small structures than -fpcc-struct-return. If you specify neither -fpcc-struct-return nor -freg-struct-return, GCC defaults to whichever convention is standard for the target. If there is no standard convention, GCC defaults to -fpcc-struct-return, except on targets where GCC is the principal compiler. In those cases, we can choose the standard, and we chose the more efficient register return alternative.

Warning: code compiled with the -freg-struct-return switch is not binary compatible with code compiled with the -fpcc-struct-return switch. Use it to conform to a non-default application binary interface. 🙈

• -fshort-enums — allocates 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 — overrides 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 — suppresses emission of .ident assembler directives and causes the #ident preprocessor directive to be ignored. 💅
• -Qy — obsolete synonym for -fident. 👑
• -Qn — obsolete synonym for -fno-ident. 👑
• -finhibit-size-directive — don’t output a .size assembler directive, or anything else that would cause trouble if the function is split in the middle, and the two halves are placed at locations far apart in memory. This option is used when compiling crtstuff.c; you should not need to use it for anything else. 🧚
• -fverbose-asm — puts 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,"",@progbits

The 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-asm switch, but that switch only records information in the assembler output file as comments, so it never reaches the object file. See also -grecord-gcc-switches for another way of storing compiler options into the object file. 🍓
• -fpic — generates position-independent code (PIC) suitable for use in a shared library, if supported for the target machine. Such code accesses all constant addresses through a global offset table (GOT). The dynamic loader resolves the GOT entries when the program starts (the dynamic loader is not part of GCC; it is part of the operating system). If the GOT size for the linked executable exceeds a machine-specific maximum size, you get an error message from the linker indicating that -fpic does not work; in that case, recompile with -fPIC instead. (These maximums are 8k on the SPARC, 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 your target machine supports it, this emits position-independent code that’s suitable for dynamic linking and avoids any limit on the size of the global offset table. On AArch64, m68k, PowerPC and SPARC this option actually makes a difference. PIC needs special support, so it only works on certain machines. When this flag is set, the macros "__pic__" and "__PIC__" are defined to 2. 💅
• -fpie, -fPIE — these are similar to -fpic and -fPIC, but the generated position-independent code can only be linked into executables. Usually they’re used to compile code that will be linked using the -pie GCC option. Both define macros "__pie__" and "__PIE__"; the macros have the value 1 for -fpie and 2 for -fPIE. ✨
• -fno-plt — don’t 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 is more efficient because it eliminates PLT stubs and exposes GOT loads to optimizations. On architectures like 32-bit x86 where PLT stubs expect the GOT pointer in a specific register, it gives the compiler more register allocation freedom. Lazy binding requires use of the PLT, so with -fno-plt all 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 marked to not use the PLT to use the GOT instead. 🔥
• -fno-jump-tables — no jump tables for switch statements, even when they’d be more efficient than other code generation strategies. This is useful with -fpic or -fPIC when building code that forms part of a dynamic linker and can’t reference the address of a jump table. On some targets jump tables don’t require a GOT, so you don’t need this option there. 🦋
• -fno-bit-tests — no bit tests for switch statements, even where they’d be more efficient than other code generation strategies. 💀
• -ffixed-reg — treat the register named reg as a fixed register; generated code should never refer to it (except maybe as a stack pointer, frame pointer, or in some other fixed role). reg must be the name of a register. Accepted register names are machine-specific and are defined in the "REGISTER_NAMES" macro in the machine description macro file. This flag has no negative form because it specifies a three-way choice. 🎀
• -fcall-used-reg — treat the register named reg as an allocable register that function calls clobber. It can be allocated for temporaries or variables that don’t live across a call. Functions compiled this way don’t save and restore the register reg. Using this flag with the frame pointer or stack pointer is an error. Using it for other registers that have fixed pervasive roles in the machine’s execution model produces disastrous results 😭. This flag has no negative form because it specifies a three-way choice.
• -fcall-saved-reg — treat the register named reg as an allocable register saved by functions. It can be allocated even for temporaries or variables that live across a call. Functions compiled this way save and restore reg if they use it. Using this flag with the frame pointer or stack pointer is an error. Using it for other registers that have fixed pervasive roles in the machine’s execution model produces disastrous results 💀. A different sort of disaster happens if you use this flag for a register in which function values may be returned. This flag has no negative form because it specifies a three-way choice. 🙈
• -fpack-struct[=n] — with no value, pack all structure members together without holes. If you give a value (which must be a small power of two), structure members are packed according to that value, representing the maximum alignment (that is, objects with default alignment requirements larger than this are output potentially unaligned at the next fitting location).

Warning: the -fpack-struct switch causes GCC to generate code that is not binary compatible with code generated without that switch. It also makes the code suboptimal. Use it to conform to a non-default application binary interface. 🧚

• -fleading-underscore — this option and its counterpart -fno-leading-underscore forcibly change the way C symbols are represented in the object file. One use is to help link with legacy assembly code.

Warning: the -fleading-underscore switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface. Not all targets provide complete support for this switch. 🌈

• -ftls-model=model — change the thread-local storage model. The model argument should be one of global-dynamic, local-dynamic, initial-exec, or local-exec. Ngl the choice is subject to optimization: the compiler may use a more efficient model for symbols not visible outside the translation unit, or if -fpic isn’t 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. For targets that don’t need them, like HP-PA or IA-64 for example, 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. Therefore 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 to let the compiler 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. Moreover, code compiled with -ftrampolines and code compiled with -fno-trampolines are not binary compatible if nested functions are present. This option must therefore 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 (like the Apple M1) don’t permit an executable stack. Compiling with -ftrampoline-impl=heap generates calls to "__gcc_nested_func_ptr_created" and "__gcc_nested_func_ptr_deleted" in order to allocate and deallocate trampoline space on the executable heap. These 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 you "setjmp", instantiate nested functions, and then "longjmp" back to a state prior to having allocated those nested functions. 😭

• -fvisibility=[default|internal|hidden|protected] — set the default ELF image symbol visibility to the specified option — all symbols are marked with this unless overridden within the code. Using this feature can very substantially improve linking and load times of shared object libraries, produce more optimized code, provide near-perfect API export, and prevent symbol clashes. It is strongly recommended that you use this in any shared objects you distribute. Despite the nomenclature, default always means public; i.e., available to be linked against from outside the shared object. protected and internal are pretty useless in real-world usage, so the only other commonly used option is hidden. The default if -fvisibility isn’t specified is default, i.e., make every symbol public.

A good explanation of the benefits offered by ensuring ELF symbols have the correct visibility is given by "How To Write Shared Libraries" by Ulrich Drepper (which can be found at https://www.akkadia.org/drepper/) — however a superior solution made possible by this option to marking things hidden when the default is public is to make the default hidden and mark things public. This is the norm with DLLs on Windows and with -fvisibility=hidden and "__attribute__ ((visibility("default")))" instead of "__declspec(dllexport)" you get almost identical semantics with identical syntax. This is a great boon to those working with cross-platform projects. For those adding visibility support to existing code, you may find #pragma GCC visibility of use. This works by you enclosing the declarations you wish to set visibility for with (for example) #pragma GCC visibility push(hidden) and #pragma GCC visibility pop. Bear in mind that symbol visibility should be viewed as part of the API interface contract and thus all new code should always specify visibility when it is not the default; i.e., declarations only for use within the local DSO should always be marked explicitly as hidden 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, btw, are not touched by -fvisibility — so honestly a whole lot of code can just be recompiled with -fvisibility=hidden and need zero edits, slay. ✨ But bestie, here's the catch: that means calls to "extern" functions with no explicit visibility go through the PLT, so it's way more effective to use "__attribute ((visibility))" and/or "#pragma GCC visibility" to tell the compiler which "extern" declarations should be treated as hidden. iykyk 💅

Also important: -fvisibility does affect C++ vague linkage entities. Translation: an exception class that's thrown between DSOs has to be explicitly marked with default visibility so the type_info nodes get unified between the DSOs. No cap, that one bites people. 😭

There's a whole overview of these techniques, their benefits, and how to use them at https://gcc.gnu.org/wiki/Visibility — go read it, it's giving required reading. 🦋

-fstrict-volatile-bitfields

Okay so use this option if accesses to volatile bit-fields — or other structure fields too, though the compiler usually honors those types anyway — should be a single access of the width of the field's type, aligned to a natural alignment if possible. ✨

Example time, and it's a good one: targets with memory-mapped peripheral registers might require all such accesses to be 16 bits wide. With this flag you can declare all peripheral bit-fields as "unsigned short" (assuming short is 16 bits on these targets) to force GCC to use 16-bit accesses instead of, perhaps, a more efficient 32-bit access. 🎯

If this option is disabled, the compiler uses the most efficient instruction. In the previous example, that might be a 32-bit load instruction — even though that accesses bytes that don't contain any portion of the bit-field, or memory-mapped registers unrelated to the one being updated. 💀 like, rude.

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 that case GCC falls back to generating multiple accesses rather than code that will fault or truncate the result at run time. 🫶 good compiler behavior honestly.

Note: 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 to define all bits of the field's type as bit-field members. periodt.

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. 🧚

Default value is enabled, so the only useful form of the option is -fno-sync-libcalls. This option is used in the implementation of the libatomic runtime library. that's it, that's the whole vibe.

-fzero-init-padding-bits=value

Guarantee zero initialization of padding bits in automatic variable initializers. 🎀 Certain languages guarantee zero initialization of padding bits in certain cases — e.g. C23 when using empty initializers ("{}"), or C++ when using zero-initialization — or C guarantees 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 a 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 are 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). maximal slay 🌈

---

🛠️ GCC Developer Options 🛠️

This section 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 covers options that produce debug dumps at various points in the compilation, options that print statistics like memory use and execution time, and options that print info about GCC's configuration, such as where it searches for libraries. You should rarely need any of these for ordinary compilation and linking tasks. fr fr, this is behind-the-scenes main character energy. 🎬

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 the 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're executed earlier. 😵‍💫 iconic chaos.

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 info is generated in the common VCG format. It can be decorated with additional per-node and/or per-edge information if you additionally specify a list of comma-separated markers.

• When the "su" marker is specified, the callgraph is decorated with stack usage information — it's 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. lowkey a plot twist but ok 💅

-dletters

--dump=letters

--dump letters

-fdump-rtl-pass

-fdump-rtl-pass-options

-fdump-rtl-pass-options=filename

Says to make debugging dumps during compilation at times specified by letters when using -d, or by pass when using -fdump-rtl. This is used for debugging the RTL-based passes of the compiler. 🔥

Some -dletters switches have different meaning when -E is used for preprocessing. noted. 📝

The -options form allows greater control over the details of the dump. See -fdump-tree.

Here are actual instances of command-line options following these patterns and their meanings — yes bestie, we're listing all of them, grab a snack 🍓:

• -fdump-rtl-alignments — Dump after branch alignments have been computed.
• -fdump-rtl-asmcons — Dump after fixing rtl statements that have unsatisfied in/out constraints.
• -fdump-rtl-auto_inc_dec — Dump after auto-inc-dec discovery. This pass is only run on architectures that have auto inc or auto dec instructions.
• -fdump-rtl-barriers — Dump after cleaning up the barrier instructions.
• -fdump-rtl-bbpart — Dump after partitioning hot and cold basic blocks.
• -fdump-rtl-bbro — Dump after block reordering.
• -fdump-rtl-btl1, -fdump-rtl-btl2 — -fdump-rtl-btl1 and -fdump-rtl-btl2 enable dumping after the two branch target load optimization passes.
• -fdump-rtl-bypass — Dump after jump bypassing and control flow optimizations.
• -fdump-rtl-combine — Dump after the RTL instruction combination pass.
• -fdump-rtl-compgotos — Dump after duplicating the computed gotos.
• -fdump-rtl-ce1, -fdump-rtl-ce2, -fdump-rtl-ce3 — -fdump-rtl-ce1, -fdump-rtl-ce2, and -fdump-rtl-ce3 enable dumping after the three if conversion passes.
• -fdump-rtl-cprop_hardreg — Dump after hard register copy propagation.
• -fdump-rtl-csa — Dump after combining stack adjustments.
• -fdump-rtl-cse1, -fdump-rtl-cse2 — -fdump-rtl-cse1 and -fdump-rtl-cse2 enable dumping after the two common subexpression elimination passes.
• -fdump-rtl-dce — Dump after the standalone dead code elimination passes.
• -fdump-rtl-dbr — Dump after delayed branch scheduling.
• -fdump-rtl-dce1, -fdump-rtl-dce2 — -fdump-rtl-dce1 and -fdump-rtl-dce2 enable dumping after the two dead store elimination passes.
• -fdump-rtl-eh — Dump after finalization of EH handling code.
• -fdump-rtl-eh_ranges — Dump after conversion of EH handling range regions.
• -fdump-rtl-expand — Dump after RTL generation.
• -fdump-rtl-fwprop1, -fdump-rtl-fwprop2 — -fdump-rtl-fwprop1 and -fdump-rtl-fwprop2 enable dumping after the two forward propagation passes.
• -fdump-rtl-gcse1, -fdump-rtl-gcse2 — -fdump-rtl-gcse1 and -fdump-rtl-gcse2 enable dumping after global common subexpression elimination.
• -fdump-rtl-init-regs — Dump after the initialization of the registers.
• -fdump-rtl-initvals — Dump after the computation of the initial value sets.
• -fdump-rtl-into_cfglayout — Dump after converting to cfglayout mode.
• -fdump-rtl-ira — Dump after iterated register allocation.
• -fdump-rtl-jump — Dump after the second jump optimization.
• -fdump-rtl-loop2 — -fdump-rtl-loop2 enables dumping after the rtl loop optimization passes.
• -fdump-rtl-mach — dumps stuff after the machine dependent reorganization pass runs (like, if that pass even exists for your target, which it might not, ok?). 🎀
• -fdump-rtl-mode_sw — dumps after it removes those redundant mode switches. bye redundant switches 💅
• -fdump-rtl-rnreg — dumps after register renumbering. numbers, but make them cute ✨
• -fdump-rtl-outof_cfglayout — dumps after converting from cfglayout mode. we left cfglayout, no cap 🦋
• -fdump-rtl-peephole2 — dumps after the peephole pass. tiny little look-through-a-hole optimization era 🔍
• -fdump-rtl-postreload — dumps after post-reload optimizations. post-reload, the sequel 😭
• -fdump-rtl-pro_and_epilogue — dumps after generating the function prologues and epilogues. entrance and exit, main character energy fr fr 👑
• -fdump-rtl-sched1, -fdump-rtl-sched2 — these two enable dumping after the basic block scheduling passes. sched1 and sched2, like the two halves of a very efficient friendship 🫶
• -fdump-rtl-ree — dumps after sign/zero extension elimination. extensions? eliminated. slay.
• -fdump-rtl-seqabstr — dumps after common sequence discovery. finding the matching outfits, essentially 🌸
• -fdump-rtl-shorten — dumps after shortening branches. short branches, short king behavior 💜
• -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. five rounds, bestie, it's giving tournament arc 🔥
• -fdump-rtl-sms — dumps after modulo scheduling. heads up tho: this pass is only run on some architectures 🙈
• -fdump-rtl-stack — dumps after converting from GCC's "flat register file" registers to the x87's stack-like registers. only runs on x86 variants, so if you're not on x86 this one's just not invited to the party 💀
• -fdump-rtl-subreg1, -fdump-rtl-subreg2 — these enable dumping after the two subreg expansion passes. two passes, two dumps, iconic ✨
• -fdump-rtl-vartrack — dumps after variable tracking. we're tracking the variables, we know where they've been 🕵️
• -fdump-rtl-vregs — dumps after converting virtual registers to hard registers. glow-up: virtual → real 🌈
• -fdump-rtl-web — dumps after live range splitting. the web has been woven 🕸️
• -fdump-rtl-regclass, -fdump-rtl-subregs_of_mode_init, -fdump-rtl-subregs_of_mode_finish, -fdump-rtl-dfinit, -fdump-rtl-dfinish — so these dumps are defined, right, but they always produce empty files. not me getting excited over nothing 😭 literally empty. zero bytes of content. the vibe check failed.

---

• -da, --dump=a, -fdump-rtl-all — produce all the dumps listed above. all of them. the whole collection. eat, queen 👑
• -dA, --dump=A — annotate the assembler output with miscellaneous debugging information. little notes in the margins 📝
• -dD, --dump=D — dump all macro definitions, at the end of preprocessing, in addition to normal output. every single macro, on display 🎀
• -dH, --dump=H — produce a core dump whenever an error occurs. yeah so, an actual core dump, like when things go bad bad 💀 not a cute moment.
• -dp, --dump=p — annotate the assembler output with a comment indicating which pattern and alternative is used. the length and cost of each instruction are also printed. receipts for your instructions, fr 🧾
• -dP, --dump=P — dump the RTL in the assembler output as a comment before each instruction. also turns on -dp annotation. two-for-one, highkey a steal 💖
• -dx, --dump=x — just generate RTL for a function instead of compiling it. usually used with -fdump-rtl-expand. RTL only, we're not finishing the job today 🙈

---

• -fdump-debug — dump debugging information generated during the debug generation phase. debug info about the debug info, it's meta like that 🦋
• -fdump-earlydebug — dump debugging information generated during the early debug generation phase. early! like showing up to the function before anyone else 🌸
• -fdump-noaddr — when doing debugging dumps, suppress address output. this makes it more feasible to use diff on debugging dumps for compiler invocations with different compiler binaries and/or different text / bss / data / heap / stack / dso start locations. basically: no addresses means you can actually compare two dumps without crying 😭
• -freport-bug — collect and dump debug information into a temporary file if an internal compiler error (ICE) occurs. so when the compiler itself faceplants, it saves the receipts to a temp file 🫶
• -fdump-unnumbered — when doing debugging dumps, suppress instruction numbers and address output. this makes it more feasible to use diff on debugging dumps for compiler invocations with different options, in particular with and without -g. ngl this is the "make the diffs chill" flag ✨
• -fdump-unnumbered-links — when doing debugging dumps (see -d option above), suppress instruction numbers for the links to the previous and next instructions in a sequence. the links stay, the numbers go 💅
• -fdump-internal-locations — dump detailed information about GCC's internal representation of source code locations. where everything is, in the compiler's own little map 🗺️

---

• -fdump-ipa-switch, -fdump-ipa-switch-options — control the dumping at various stages of inter-procedural analysis language tree to a file. the file name is generated by appending a switch specific suffix to the source file name, and the file is created in the same directory as the output file. the following dumps are possible:
  • all — enables all inter-procedural analysis dumps. everything, all at once, obsessed 💖
  • cgraph — dumps information about call-graph optimization, unused function removal, and inlining decisions. the whole social graph of your functions 👑
  • inline — dump after function inlining. inlining happened, here's the proof 🎀
  • 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.

additionally, the options -optimized, -missed, -note, and -all can be provided, with the same meaning as for -fopt-info, defaulting to -optimized.

for example, -fdump-ipa-inline-optimized-missed will emit information on callsites that were inlined, along with callsites that were not inlined.

by default, the dump will contain messages about successful optimizations (equivalent to -optimized) together with low-level details about the analysis.

---

• -fdump-ipa-clones — create a dump file containing information about creation of call graph node clones and removals of call graph nodes during inter-procedural optimizations and transformations. its main intended use is that tools that create live-patches can determine 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 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. 🧬

in the case of call graph clone creation, the individual fields are:

1. String Callgraph clone.
2. Name of the function being cloned as it is presented to the assembler.
3. 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. 🙈
4. The file name of the source file where the function is defined.
5. The line on which the function definition is located.
6. The column where the function definition is located.
7. Name of the new function clone as it is presented to the assembler.
8. 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.
9. The file name of the source file where the source code location of the new clone points to.
10. The line to which the source code location of the new clone points to.
11. The column to which the source code location of the new clone points to.
12. A string that determines the reason for cloning.

in the case of call graph clone removal, the individual fields are:

1. String Callgraph removal.
2. Name of the function being removed as it would be presented to the assembler.
3. 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.
4. The file name of the source file where the function is defined.
5. The line on which the function definition is located.
6. The column where the function definition is located.

---

• -fdump-lang — dump language-specific information. the file name is made by appending .lang to the source file name. 🗣️
• -fdump-lang-all, -fdump-lang-switch, -fdump-lang-switch-options, -fdump-lang-switch-options=filename — control the dumping of language-specific information. the options and filename portions behave as described in the -fdump-tree option. -fdump-tree-all enables all language-specific dumps; other options vary with the language. for instance, see
• -fdump-passes — print on stderr the list of optimization passes that are turned on and off by the current command-line options. a full roster of who's in and who's out 📋✨
• -fdump-statistics-option — enable and control dumping of pass statistics in a separate file. the file name is generated by appending a suffix ending in .statistics to the source file name, and the file is created in the same directory as the output file. if the -option form is used, -stats causes counters to be summed over the whole compilation unit while -details dumps every event as the passes generate them. the default with no option is to sum counters for each function compiled. 📊 numbers girlies rise 🫶

---

• -fdump-tree-all, -fdump-tree-switch, -fdump-tree-switch-options, -fdump-tree-switch-options=filename — control the dumping at various stages of processing the intermediate language tree to a file. if the -options form is used, options is a list of - separated options which control the details of the dump. not all options are applicable to all dumps; those that are not meaningful are ignored. (okay that's actually so forgiving of them 💅) the following options are available:
  • address — print the address of each node. usually this is not meaningful as it changes according to the environment and source file. its primary use is for tying up a dump file with a debug environment. honestly kind of chaotic but valid 🌀
  • asmname — if DECL_ASSEMBLER_NAME has been set for a given decl, use that in the dump instead of DECL_NAME. its primary use is ease of use working backward from mangled names in the assembly file. 🔍
• slim — okay so when GCC is dumping front-end intermediate representations, this stops it from dumping members of a scope or the body of a function just because that scope has been reached. It only dumps those items when they’re directly reachable by some other path. When dumping pretty-printed trees, this option inhibits dumping the bodies of control structures. When dumping RTL, print the RTL in slim (condensed) form instead of the default LISP-like representation.
• raw — print a raw representation of the tree. By default, trees are pretty-printed into a C-like representation.
• details — enable more detailed dumps (not honored by every dump option). Also include information from the optimization passes.
• stats — enable dumping various statistics about the pass (not honored by every dump option).
• blocks — enable showing basic block boundaries (disabled in raw dumps).
• graph — for each of the other indicated dump files (-fdump-rtl-pass), dump a representation of the control flow graph suitable for viewing with GraphViz to file.passid.pass.dot. Each function in the file is pretty-printed as a subgraph, so that GraphViz can render them all in a single plot. 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).
• folding — enable dumping information about match-and-simplify (match.pd) patterns, when they are applied.
• all — turn on all options, except raw, slim, verbose and lineno.
• optall — turn on all optimization options, i.e., optimized, missed, and note.

---

To determine what tree dumps are available or find the dump for a pass of interest, follow the steps below.

1. Invoke GCC with -fdump-passes and in the stderr output look for a code that corresponds to the pass you are interested in. For example, the codes tree-evrp, tree-vrp1, and tree-vrp2 correspond to the three Value Range Propagation passes. The number at the end distinguishes distinct invocations of the same pass.
2. 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-evrp option. Optionally, you may specify the name of the dump file. If you don’t specify one, GCC creates as described below.
3. 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 t for tree passes (and the letter r for RTL passes), and finally the pass code. For example, the Early VRP pass dump might be in a file named myfile.c.038t.evrp in the current working directory. Note that the numeric codes are not stable and may change from one version of GCC to another.

• -fopt-info, -fopt-info-options, -fopt-info-options=filename — controls optimization dumps from various optimization passes. If the -options form is used, options is a list of --separated option keywords to select the dump details and optimizations.

The options can be divided into three groups:

1. options describing what kinds of messages should be emitted,
2. options describing the verbosity of the dump, and
3. 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.

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 includes optimized, missed, and note.

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.

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".

If the filename is provided, then the dumps from all the applicable optimizations are concatenated into the filename. Otherwise the dump is output onto stderr. Though multiple -fopt-info options are accepted, only one of them can include a filename. If other filenames are provided then all but the first such option are ignored.

Note that the output filename is overwritten in case of multiple translation units. If a combined output from multiple translation units is desired, stderr should be used instead.

In the following example, the optimization info is output to stderr:

gcc -O3 -fopt-info

This example:

gcc -O3 -fopt-info-missed=missed.all

outputs missed optimization report from all the passes into missed.all, and this one:

gcc -O2 -ftree-vectorize -fopt-info-vec-missed

prints information about missed optimization opportunities from vectorization passes on stderr. Note that -fopt-info-vec-missed is equivalent to -fopt-info-missed-vec. The order of the optimization group names and message types listed after -fopt-info does not matter.

As another example,

gcc -O3 -fopt-info-inline-optimized-missed=inline.txt

outputs information about missed optimizations as well as optimized locations from all the inlining passes into inline.txt.

Finally, consider:

gcc -fopt-info-vec-missed=vec.miss -fopt-info-loop-optimized=loop.opt

Here the two output filenames vec.miss and loop.opt are in conflict since only one output file is allowed. In this case, only the first option takes effect and the subsequent options are ignored. Thus only vec.miss is produced which contains dumps from the vectorizer about missed opportunities.

• -fsave-optimization-record — write a SRCFILE.opt-record.json.gz file 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.

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. 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 outputs 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.

okay bestie we're still in the pass-control era of this page and honestly it's giving "backstage VIP access to the compiler" 💅✨ these options let you straight up turn individual internal passes on and off like you're the DJ at the function — let's go 🎀

• -fdisable-ipa-pass — turns OFF the IPA pass called pass. pass is just the pass name, periodt. And listen: if that same pass gets statically invoked multiple times inside the compiler, you gotta append a sequential number starting from 1 to the pass name, iykyk 🔥
• -fdisable-rtl-pass, -fdisable-rtl-pass=range-list — turns OFF the RTL pass pass. Same deal as above: pass is the pass name, and if the same pass is statically invoked more than once, append a sequential number starting from 1 🙈

Now for range-list — it's a comma-separated list of function ranges or assembler names 🦋 Each range is a number pair separated by a colon, and the range is inclusive on BOTH ends. If the range is trivial, you can simplify the number pair down to a single number. If the function's call graph node's uid falls within one of the specified ranges, the pass gets 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 the option -fdump-passes 👑

• -fdisable-tree-pass, -fdisable-tree-pass=range-list — turns OFF the tree pass pass. See -fdisable-rtl for the description of the option arguments, it's the exact same vibe 🌸
• -fenable-ipa-pass — turns ON the IPA pass pass. pass is the pass name, and if the same pass is statically invoked multiple times in the compiler, append a sequential number starting from 1 ⭐
• -fenable-rtl-pass, -fenable-rtl-pass=range-list — turns ON the RTL pass pass. See -fdisable-rtl for the option argument description and examples, we're not retyping all that 🫶
• -fenable-tree-pass, -fenable-tree-pass=range-list — turns ON the tree pass pass. See -fdisable-rtl for the description of the option arguments, same energy 💜

Here are some examples showing uses of these options — literally the cheat sheet, screenshot it 📸

# 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 — enables internal consistency checking 🧚 The default depends on the compiler configuration. And -fchecking=2 enables further internal consistency checking that might affect code generation — so yeah, it does more, and it can change your output, no cap ⚠️
• -frandom-seed=string — this one provides a seed that GCC uses in place of random numbers when generating certain symbol names that have to be different in every compiled file 🌈 It's also used to place unique stamps in coverage data files and in the object files that produce them. You can use -frandom-seed to produce reproducibly identical object files — iconic behavior honestly 👑

The string can either be a number (decimal, octal or hex) or an arbitrary string (in which case it's converted to a number by computing CRC32) 🔢

The string should be different for every file you compile. fr fr, don't reuse it 🚨

• -save-temps, --save-temps — stores the usual "temporary" intermediate files permanently 🎀 names them as auxiliary output files, as specified described under -dumpbase and -dumpdir.

When used in combination with the -x command-line option, -save-temps is sensible enough to avoid overwriting an input source file with the same extension as an intermediate file 🦋 The corresponding intermediate file may be obtained by renaming the source file before using -save-temps. cute of it tbh.

• -save-temps=cwd — equivalent to -save-temps -dumpdir ./ ✨
• -save-temps=obj — equivalent to -save-temps -dumpdir outdir/, where outdir/ is the directory of the output file specified after the -o option, including any directory separators 🌸 If the -o option is not used, the -save-temps=obj switch behaves like -save-temps=cwd.
• -specs=file, --specs=file, --specs file — processes file after the compiler reads in the standard specs file, in order to override the defaults which the gcc driver program uses when determining what switches to pass to cc1, cc1plus, as, ld, etc. 🔥 More than one -specs=file can be specified on the command line, and they are processed in order, from left to right — so order matters, bestie 💅
• -time[=file] — reports the CPU time taken by each subprocess in the compilation sequence ⏱️ For C source files, this is the compiler proper and assembler (plus the linker if linking is done).

Without the specification of an output file, the output looks like this:

# cc1 0.12 0.01
# as 0.00 0.01

The first number on each line is the "user time", that is time spent executing the program itself. The second number is "system time", time spent executing operating system routines on behalf of the program. Both numbers are in seconds 🕐

With the specification of an output file, the output is appended to the named file, and it looks like this:

0.12 0.01 cc1 <options>
0.00 0.01 as <options>

The "user time" and the "system time" are moved before the program name, and the options passed to the program are displayed, so that one can later tell what file was being compiled, and with which options 🔍 lowkey a receipts trail, love that for us.

• -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 determined by appending .gkd to the dump base name, see -dumpbase.
• -fcompare-debug[=opts] — if no error occurs during compilation, runs the compiler a second time, adding opts and -fcompare-debug-second to the arguments passed to the second compilation 🌀 Dumps the final internal representation in both compilations, and prints an error if they differ. detective mode fr 🔍

If the equal sign is omitted, the default -gtoggle is used.

The environment variable GCC_COMPARE_DEBUG, if defined, non-empty and nonzero, implicitly enables -fcompare-debug 🙈 If GCC_COMPARE_DEBUG is defined to a string starting with a dash, then it is used for opts, otherwise the default -gtoggle is used.

-fcompare-debug=, with the equal sign but without opts, is equivalent to -fno-compare-debug, which disables the dumping of the final representation and the second compilation, preventing even GCC_COMPARE_DEBUG from taking effect 💀

To verify full coverage during -fcompare-debug testing, set GCC_COMPARE_DEBUG to say -fcompare-debug-not-overridden, which GCC rejects as an invalid option in any actual compilation (rather than preprocessing, assembly or linking) ⚠️ To get just a warning, setting GCC_COMPARE_DEBUG to -w%n-fcompare-debug not overridden will do.

• -fcompare-debug-second — this option is implicitly passed to the compiler for the second compilation requested by -fcompare-debug, along with options to silence warnings, and omitting other options that would cause the compiler to produce output to files or to standard output as a side effect 🎀 Dump files and preserved temporary files are renamed so as to contain the .gk additional extension during the second compilation, to avoid overwriting those generated by the first.

When this option is passed to the compiler driver, it causes the first compilation to be skipped, which makes it useful for little other than debugging the compiler proper 🫶 so like... niche, but make it make sense.

• -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 in the command line does not matter; it takes effect after all other options are processed, and it does so only once, no matter how many times it is given 👑 This is mainly intended to be used with -fcompare-debug.
• -fvar-tracking-assignments-toggle — toggles -fvar-tracking-assignments, in the same way that -gtoggle toggles -g ✨
• -Q — when used on the command line prior to --help=, -Q acts as a modifier to the help output 👀

Otherwise, this option makes the compiler print out each function name as it is compiled, and print some statistics about each pass when it finishes 📊 so it's either "adjust the help screen" or "narrate everything", depending on placement. chaotic but slay.

• -ftime-report — makes the compiler print some statistics to stderr about 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 SARIF output 🌈 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 — so like, don't @ us about the tiny mismatches 💀

• -ftime-report-details — records the time consumed by infrastructure parts separately for each pass 🔍 more granular, more nosy, we love data.
• -fira-verbose=n — controls the verbosity of the dump file for the integrated register allocator 🎛️ The default value is 5. If the value n is greater or equal to 10, the dump output is sent to stderr using the same format as n minus 10 — so 10 gives you stderr at level 0, cute little offset trick iykyk 🦋
• -flto-report — prints a report with internal details on the workings of the link-time optimizer 🔥 The contents of this report vary from version to version. It is meant to be useful to GCC developers when processing object files in LTO mode (via -flto).

Disabled by default — you gotta opt in, bestie 💅

• -flto-report-wpa — like -flto-report, but only prints for the WPA phase of link-time optimization ✨ smaller scope, same energy.
• -fmem-report — makes the compiler print some statistics about permanent memory allocation when it finishes 🧠💾
• -fmem-report-wpa — makes the compiler print some statistics about permanent memory allocation for the WPA phase only 🎀
• -fpre-ipa-mem-report, -fpost-ipa-mem-report — makes the compiler print some statistics about permanent memory allocation before or after interprocedural optimization 🌸 pick your side of the IPA timeline, both are valid.
• -fmultiflags — this option enables multilib-aware "TFLAGS" to be used to build target libraries with options different from those 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 👑✨

okay and there's more coming, don't touch that dial 🦋

Okay so, lowkey the most important thing about this whole TFLAGS/specs situation: it lets your target libraries get built for super chill, portable baseline environments 🧸 while the compiler itself defaults to the more demanding ones. And that's actually iconic, because users can just waltz in and override the defaults the compiler was configured to use when building their own programs if those defaults aren't it for their target environment — whereas rebuilding the runtime libraries is usually a whole ordeal and honestly nobody wants that 💀.

But plot twist: unlike TFLAGS, using specs means you can pick different flags for different multilibs ✨. The way to pull that off is to build with make TFLAGS=-fmultiflags, after configuring --with-specs=%{fmultiflags:...}.

Heads up bestie: this option is discarded by the driver once it's done processing driver self spec 🫶.

It's also genuinely useful for checking that TFLAGS are being used to build all the target libraries — just configure a non-bootstrap compiler with --with-specs='%{!fmultiflags:%emissing TFLAGS}' and then build the compiler and target libraries. Vibe check passed or failed, no in between 💅.

• -fprofile-report — makes the compiler spill some stats at you about how consistent the (estimated) profile is, plus the effect of each individual pass 📊.
• -fstack-usage — makes the compiler output stack usage info for your program, function by function. The dump's filename is made by appending .su to the auxname. And auxname comes from the name of the output file (if you explicitly specified one and it's not an executable), otherwise it's the basename of the source file. Every entry is made of three fields:
  • the name of the function,
  • a number of bytes,
  • one or more qualifiers: static, dynamic, bounded.

The static qualifier means the function messes with the stack statically: a fixed number of bytes gets allocated for the frame on function entry and released on function exit, and no stack adjustments happen otherwise inside the function. The second field is that fixed number of bytes 🎀.

The dynamic qualifier means the function manipulates the stack dynamically: on top of the static allocation described above, stack adjustments happen inside the function body — like pushing/popping arguments around function calls, for example. If the bounded qualifier is also there, the amount of those adjustments is bounded at compile time, and the second field is an upper bound of the total stack the function uses. If bounded is not there, the amount of adjustments is not bounded at compile time, and the second field only represents the bounded part 🙈.

• -fstats — emits statistics about front-end processing at the end of compilation. Only the C++ front end supports this one, and ngl the info is generally only useful to the G++ development team 🧚.
• -fdbg-cnt-list — prints the name and the counter upper bound for absolutely all debug counters 👑.
• -fdbg-cnt=counter-value-list — sets the internal debug counter lower and upper bound. counter-value-list is a comma-separated list of

name:lower_bound1-upper_bound1 [:lower_bound2-upper_bound2...]

tuples, which sets the counter's name and the list of closed intervals. lower_bound is optional and gets zero-initialized if you don't set it. 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 ✨.

• -print-autofdo-gcov-version, --print-autofdo-gcov-version — prints the current version of GCOV being used by the AutoFDO infrastructure 🌸.
• -print-file-name=library, --print-file-name=library, --print-file-name library — prints the full absolute name of the library file library that would be used when linking — and does literally nothing else. With this option, GCC does not compile or link anything; it just prints the file name 💖.
• -print-multi-directory, --print-multi-directory — prints the directory name corresponding to the multilib selected by any other switches present on the command line. That directory is supposed to exist in GCC_EXEC_PREFIX.
• -print-multi-lib, --print-multi-lib — prints the mapping from multilib directory names to the 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 make shell processing easier 🐚.
• -print-multi-os-directory, --print-multi-os-directory — prints the path to OS libraries for the selected multilib, relative to some lib subdirectory. If OS libraries are present in the lib subdirectory and no multilibs are used, this is usually just .; if OS libraries are present in libsuffix sibling directories, this prints e.g. ../lib64, ../lib or ../lib32; or if OS libraries are present in lib/subdir subdirectories, it prints e.g. amd64, sparcv9 or ev6 🦋.
• -print-multiarch, --print-multiarch — prints the path to OS libraries for the selected multiarch, relative to some lib subdirectory.
• -print-prog-name=program, --print-prog-name=program, --print-prog-name program — like -print-file-name, but it searches for a program such as cpp 🫶.
• -print-libgcc-file-name, --print-libgcc-file-name — same as -print-file-name=libgcc.a.

This one's a lifesaver when you use -nostdlib or -nodefaultlibs but you do still want to link with libgcc.a. You can do:

gcc -nostdlib <files>... `gcc -print-libgcc-file-name`

• -print-search-dirs, --print-search-dirs — prints the name of the configured installation directory plus a list of program and library directories gcc searches — and does nothing else ⭐.

This is useful when gcc prints the error message installation problem, cannot exec cpp0: No such file or directory. To resolve it, you either need to put cpp0 and the other compiler components where gcc expects to find them, or you can set the environment variable GCC_EXEC_PREFIX to the directory where you installed them. Don't forget the trailing / 💅.

• -print-sysroot, --print-sysroot — prints the target sysroot directory that's used during compilation. This is the target sysroot specified either at configure time or via the --sysroot option, possibly with an extra suffix that depends on compilation options. If no target sysroot was specified, the option prints nothing 🤷‍♀️.
• -print-sysroot-headers-suffix, --print-sysroot-headers-suffix — prints the suffix added to the target sysroot when searching for headers, or gives an error if the compiler isn't configured with such a suffix — and does nothing else.
• -dumpmachine — prints the compiler's target machine (for example, i686-pc-linux-gnu) — and does nothing else 🙈.
• -dumpversion — prints the compiler version (for example, 3.0, 6.3.0 or 7) — and does nothing else. This is the compiler version used in filesystem paths and specs. Depending on how the compiler was 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) 🔢.
• -dumpfullversion — prints the full compiler version — and does nothing else. The output is always three numbers separated by dots: major, minor and patchlevel version 💯.
• -dumpspecs — prints the compiler's built-in specs — and does nothing else. (This one gets used when GCC itself is being built 🏗️.)
• --param name=value, --param=name=value — okay so GCC, by convention, uses parameters that you can specify on the command line instead of hard-wired constants to represent arbitrary compiler limits or heuristics. Many parameters are related to optimization — for example, GCC does not inline functions that contain more than a certain number of instructions. The static analyzer similarly uses parameters to limit complexity, link-time optimization uses parameters to control partitioning, and so on. Other parameters control aspects of GCC that are completely internal, like its memory allocation and garbage collection strategy. And still others control target-specific behavior 🎛️.

The --param option gives you one uniform interface for specifying values for all these compiler parameters. BUT — and this is the important part — 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 😭. You should not depend on parameter settings for correct compilation of your program. They're exposed via the command line for the convenience of developers debugging compilation problems, or, in some cases, to provide workarounds for compiler bugs. So like, use them for sleuthing, not for building your whole personality around 🕵️‍♀️.

---

🎯 Target-Specific Options

Every target machine GCC supports can have its own options — for example, to let you compile for a particular processor variant or ABI, or to control optimizations specific to that machine. Similarly, GCC also has options specific to particular operating systems or runtime environments on the target 🌍.

By convention, machine-specific option names start with -m. Some configurations of the compiler also support extra target-specific options, usually for compatibility with other compilers on the same platform 🫶.

💜 AArch64 Options

These options are defined for AArch64 implementations:

• -mabi=name — generates code for the specified data model. The permissible values are ilp32 for a SysV-like data model where int, long int and pointers are 32 bits, and lp64 for a SysV-like data model where int is 32 bits but long int and 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 ⚠️.

Heads up: the ilp32 model is deprecated 🫠.

• -mbig-endian — generates big-endian code. This is the default when GCC is configured for an aarch64_be-*-* target.
• -mlittle-endian — generates little-endian code. This is the default when GCC is configured for an aarch64-*-* target but not an aarch64_be-*-* target 🌈.
• -menable-sysreg-checking — generates an error message if an attempt is made to access a system register that isn't available on the target architecture 🚨.
• -mgeneral-regs-only — generate code that uses only the general-purpose registers ✨. This stops the compiler from touching floating-point and Advanced SIMD registers, but ngl it puts zero restrictions on the assembler. Just the compiler, bestie.
• -mcmodel=tiny — generate code for the tiny code model 🐜. The program and its statically defined symbols have to live 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. Statically or dynamically linked, either works. And this one? It's the default code model, so it's kind of the main character.
• -mcmodel=large — generate code for the large code model 🌈. This makes absolutely no assumptions about addresses and sizes of sections. Programs can be statically linked only though. Also heads up: -mcmodel=large is incompatible with -mabi=ilp32, -fpic and -fPIC — so don't try to make it work with those, it's not gonna slay.
• -mtp=name — tell it which system register to use as a thread pointer 🧵. Valid values are tpidr_el0, tpidrro_el0, tpidr_el1, tpidr_el2, tpidr_el3. For backwards compatibility the aliases el0, el1, el2, el3 are also accepted. Default is tpidr_el0. Real talk: it's recommended you compile all the code that's meant to interoperate with the same value of this option, otherwise you'll be accessing a different thread pointer from the wrong exception level and that's giving bug energy 🙈.
• -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 (omit) is the default.
• -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 are global for a global canary, or sysreg for a canary in an appropriate system register.

With that latter choice, -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset furthermore specify which system register to use as the base register for reading the canary, and from what offset from that base register. There's no default register or offset — this is entirely for use within the Linux kernel, iykyk 🐧.

• -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.
• -mtls-size=size — specify bit size of immediate TLS offsets. Valid values are 12, 24, 32, 48 🔢. Heads up, this option requires binutils 2.26 or newer.
• -mfix-cortex-a53-835769, -mno-fix-cortex-a53-835769 — enable or disable the workaround for the ARM Cortex-A53 erratum number 835769 🩹. This involves inserting a NOP instruction between memory instructions and 64-bit integer multiply-accumulate instructions. This flag will be ignored if an architecture or cpu is specified on the command line that doesn't 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. Lowkey important: this erratum workaround is made at link time, and this will only pass the corresponding flag to the linker. Same deal — this flag will be ignored if an architecture or cpu is specified on the command line which doesn't need the workaround.
• -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-math or -funsafe-math-optimizations is used as well. Enabling it reduces precision of reciprocal square root results to about 16 bits for single precision and to 32 bits for double precision.
• -mlow-precision-sqrt, -mno-low-precision-sqrt — enable or disable the square root approximation. Again, only has an effect if -ffast-math or -funsafe-math-optimizations is used as well. Enabling this reduces precision of square root results to about 16 bits for single precision and to 32 bits for double precision. And if it's enabled, it implies -mlow-precision-recip-sqrt 👀.
• -mlow-precision-div, -mno-low-precision-div — enable or disable the division approximation. Yep, you guessed it: only has an effect if -ffast-math or -funsafe-math-optimizations is used as well. Enabling this reduces precision of division results to about 16 bits for single precision and to 32 bits for double precision.
• -mtrack-speculation, -mno-track-speculation — enable or disable generation of additional code to track speculative execution through conditional branches 🕵️‍♀️. That tracking state can then be used by the compiler when expanding calls to __builtin_speculation_safe_value so it can generate a more efficient code sequence. Slay.
• -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 use the load/store-exclusive instructions that are present in the base ARMv8.0 ISA.

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 applies when using -mcpu= when the selected cpu supports the lse feature. Also: this option is on by default 💖.

• -mmax-vectorization, -mno-max-vectorization — enable or disable an override to the vectorizer cost model that makes vectorization always appear profitable 📈. This option can be combined with -mautovec-preference, allowing precise control over which ISA will be used for auto-vectorization. Unlike -fno-vect-cost-model or -fvect-cost-model=unlimited, this option does not turn off cost comparison between different vector modes.
• -mautovec-preference=name — force an ISA selection strategy for auto-vectorization 🎛️. The possible values of name are:
  • 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.

For best performance it is highly recommended to use -mcpu or -mtune instead. This parameter should only be used for code exploration, bestie 🫶.

• -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 list below summarizes the permissible values for arch and the features that they enable by default 🌸:

• armv8-a : Armv8-A : +fp, +simd
• armv8.1-a : Armv8.1-A : armv8-a, +crc, +lse, +rdma
• armv8.2-a : Armv8.2-A : armv8.1-a
• armv8.3-a : Armv8.3-A : armv8.2-a, +pauth, +fcma, +jscvt
• armv8.4-a : Armv8.4-A : armv8.3-a, +flagm, +fp16fml, +dotprod, +rcpc2
• armv8.5-a : Armv8.5-A : armv8.4-a, +sb, +ssbs, +predres, +frintts, +flagm2
• armv8.6-a : Armv8.6-A : armv8.5-a, +bf16, +i8mm
• armv8.7-a : Armv8.7-A : armv8.6-a, +wfxt, +xs
• armv8.8-a : Armv8.8-a : armv8.7-a, +mops
• armv8.9-a : Armv8.9-a : armv8.8-a
• armv9-a : Armv9-A : armv8.5-a, +sve, +sve2
• armv9.1-a : Armv9.1-A : armv9-a, +bf16, +i8mm
• armv9.2-a : Armv9.2-A : armv9.1-a, +wfxt, +xs
• armv9.3-a : Armv9.3-A : armv9.2-a, +mops
• armv9.4-a : Armv9.4-A : armv9.3-a, +sve2p1
• armv9.5-a : Armv9.5-A : armv9.4-a, cpa, +faminmax, +lut
• armv8-r : Armv8-R : 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.

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 👑.

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.

• -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, armagicpu, neoverse-n3, olympus, rigel, 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, ampere1c, cobalt-100, apple-m1, apple-m2, apple-m3, apple-m4, apple-m5, c1-nano, c1-pro, c1-premium, c1-ultra and native 🌟.

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 and gb10 — all of these are basically GCC's way of saying "bestie, tune me for a big.LITTLE system" 💅 like it's literally telling the compiler this machine has big cores and little cores and it should care about both, iykyk.

The value neoverse-512tvb is the ✨vibes-based✨ one — it says tune for Neoverse cores that (a) implement SVE and (b) have a total vector bandwidth of 512 bits per cycle. Translating from compiler-speak, since I know you're gonna ask 🫶: it tells GCC to tune for Neoverse cores that can execute 4 128-bit Advanced SIMD arithmetic instructions a cycle, and can execute an equivalent number of SVE arithmetic instructions per cycle (2 for 256-bit SVE, 4 for 128-bit SVE). It's more general than tuning for a specific core like Neoverse V1, but more specific than the default tuning described below. Iconic, honestly.

Also — on native AArch64 GNU/Linux systems, the value native tunes performance to the host system. Important caveat though: this option has no effect if the compiler is unable to recognize the processor of the host system. So like, if GCC doesn't know what chip it woke up on, it just shrugs 😭.

And where none of -mtune=, -mcpu= or -march= are specified? The code gets tuned to perform well across a range of target processors. Generic queen behavior 🌈.

Heads up bestie: this option cannot be suffixed by feature modifiers. No adding extras to -mtune, periodt.

---

• -mcpu=name — tell GCC the name of the target processor, optionally suffixed by one or more feature modifiers. The form is -mcpu=cpu{+[no]feature}*, where the permissible values for cpu are the exact same ones available for -mtune. The permissible values for feature are documented in the sub-section on aarch64-feature-modifiers,,-march and -mcpu Feature Modifiers 💖. And if you pass conflicting feature modifiers? The right-most feature wins. No cap.

So what does GCC actually do with name? Two things, bestie:

• It uses name to determine what kind of instructions it can emit when generating assembly code (like it was -march) 🛠️
• And it uses name to determine the target processor to tune for performance (like it was -mtune) 🏎️

Now — if you use this option together with -march or -mtune, those options take precedence over the appropriate part of this option. They ate, sorry.

One special case that's giving main character energy: -mcpu=neoverse-512tvb doesn't refer to a specific core, it refers to all Neoverse cores that (a) implement SVE and (b) have a total vector bandwidth of 512 bits a cycle 🌸. Unless it's 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. And unless it's overridden by -mtune, -mcpu=neoverse-512tvb tunes code in the same way as for -mtune=neoverse-512tvb. Consistent king 👑.

• -moverride=string — overrides 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 ⚠️ (read that again — not consistent across releases, so don't build your whole personality on it).

This option is only intended to be useful when developing GCC. Literally compiler-dev territory only, ngl.

• -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. Downside bestie: this limits the maximum size of functions to 1MB. This is enabled by default for -mcmodel=tiny 🎀.
• -msign-return-address=scope — picks the function scope on which return address signing gets applied. Permissible values:
  • none — disables return address signing
  • non-leaf — enables pointer signing for functions which are not leaf functions
  • all — enables pointer signing for all functions

The default value is none. Also 💀 this option has been deprecated by -mbranch-protection. So cute but past its era.

• -mbranch-protection=features — select the branch protection features to use. features can have one of these forms:
  • none is the default and turns off all types of branch protection. Peace, love, no protection 🙈.
  • standard turns on all types of branch protection features. If a feature has additional tuning options, then standard sets it to its standard level.
  • pac-ret turns on return address signing to its standard level: signing functions that save the return address to memory (non-leaf functions practically always do this) using the A-key 🔑.
  • pac-ret+leaf extends the pac-ret signing to include leaf functions.
  • pac-ret+b-key or pac-ret+leaf+b-key can be used to sign the functions with the B-key instead of the A-key. Two keys, two vibes.
  • bti turns on branch target identification mechanism.
  • gcs turns on guarded control stack compatible code generation.
• -mharden-sls=opts — enable compiler hardening against straight line speculation (SLS) 🛡️. opts is a comma-separated list of the following options:
  • retbr
  • blr

In addition, -mharden-sls=all enables all SLS hardening while -mharden-sls=none disables all SLS hardening. All or nothing, we love a binary 💅.

• -mearly-ra=scope — decides 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.

The possible values of scope are:

• all — runs the pass on all functions
• strided — runs the pass on functions that have access to strided multi-register instructions
• none — disables the pass

-mearly-ra=all is the default for -O2 and above, and for -Os. -mearly-ra=none is the default otherwise. Balanced, as all things should be 🦋.

• -mearly-ldp-fusion, -mno-early-ldp-fusion — enable the copy of the AArch64 load/store pair fusion pass that runs before register allocation. Enabled by default at -O and above 🌸.
• -mlate-ldp-fusion, -mno-late-ldp-fusion — enable the copy of the AArch64 load/store pair fusion pass that runs after register allocation. Enabled by default at -O and above too ✅.
• -mnarrow-gp-writes — enables conversion of 64-bit general purpose register writes to equivalent 32-bit operations when the upper 32 bits are known to be zero 🎀. This pass can be controlled with -mnarrow-gp-writes and is active at -O2 and above, but not enabled by default, except for -mcpu=olympus. Exclusive little exception, we see you Olympus 💜.
• -msve-vector-bits=bits — specify the number of bits in an SVE vector register. This option only has an effect when SVE is enabled, so like, context matters bestie.

GCC supports two forms of SVE code generation:

• "vector-length agnostic" output that works with any size of vector register 🌈
• "vector-length specific" output that allows GCC to make assumptions about the vector length when it's 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.

⚠️ Real talk: 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. Do not come for me if it breaks, I warned you 😭.

The default is -msve-vector-bits=scalable, which produces vector-length agnostic code. Default queen stays winning 👑.

---

🌟 -march and -mcpu Feature Modifiers 🌟

Feature modifiers used with -march and -mcpu can be any of the following and their inverses nofeature ✨ — yes, every single one has a no version, go off:

• 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 -march and -mcpu.
• simd — enable Advanced SIMD instructions. This also enables floating-point instructions. This is on by default for all possible values for options -march and -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 the LDAPR instructions 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-a aes and pmull crypto extension. This also enables Advanced SIMD instructions.
• sha2 — enable the Armv8-a sha2 crypto extension. This also enables Advanced SIMD instructions.
• sha3 — enable the sha512 and sha3 crypto extension. This also enables Advanced SIMD instructions. ⚠️ Use of this option with architectures prior to Armv8.2-A is not supported.
• sm4 — enable the sm3 and sm4 crypto 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.
• sve2-sm4 — turns on the SVE2 sm4 instructions. and yes, it drags sve2 in with it 💅
• sve2-aes — turns on the SVE2 aes instructions. also enables SVE2, obviously, we love a package deal ✨
• sve2-sha3 — turns on the SVE2 sha3 instructions. same deal: SVE2 comes along for the ride 🫶
• sve2p1 — turns on SVE2.1 instructions. obvs also enables SVE2 instructions 💖
• tme — enables the Transactional Memory Extension 🔥
• i8mm — enables 8-bit Integer Matrix Multiply instructions. this one also turns on Advanced SIMD and floating-point instructions. it's on by default for -march=armv8.6-a. ngl if you use it with architectures before Armv8.2-A, that's not supported — don't do it 🙈
• f32mm — enables 32-bit Floating point Matrix Multiply instructions. also enables SVE instructions. use with architectures prior to Armv8.2-A is not supported, periodt 💀
• f64mm — enables 64-bit Floating point Matrix Multiply instructions. also enables SVE instructions. same warning bestie: architectures before Armv8.2-A? not supported 😭
• bf16 — enables brain half-precision floating-point instructions (yes, "brain", that's literally what it says 🧠). also enables Advanced SIMD and floating-point instructions. default for -march=armv8.6-a. architectures prior to Armv8.2-A are not supported 🚫
• ls64 — enables the 64-byte atomic load and store instructions for accelerators ⚡
• mops — enables the instructions that accelerate memory operations like memcpy, memmove, memset. enabled by default for -march=armv8.8-a 🧹
• flagm — enables the Flag Manipulation instructions Extension 🚩
• flagm2 — enables the FlagM2 flag conversion instructions 🚩🚩
• pauth — enables the Pointer Authentication Extension 🔐
• cssc — enables the Common Short Sequence Compression instructions 🗜️
• cmpbr — enables the shorter compare and branch instructions cbb, cbh and cb ✂️
• sme — enables the Scalable Matrix Extension 🧊
• sme-i16i64 — enables the FEAT_SME_I16I64 extension to SME. also enables SME instructions 🫶
• sme-f64f64 — enables the FEAT_SME_F64F64 extension to SME. also enables SME instructions 💜
• sme2 — enables the Scalable Matrix Extension 2. also enables SME instructions 🌈
• sme-f8f16 — enables the FEAT_SME_F8F16 extension to SME. also enables SME2 and FP8 instructions 🌸
• sme-f8f32 — enables the FEAT_SME_F8F32 extension to SME. also enables SME2 and FP8 instructions ✨
• sme-b16b16 — enables the FEAT_SME_B16B16 extension to SME. also enables SME2 and SVE_B16B16 instructions 💅
• sme-f16f16 — enables the FEAT_SME_F16F16 extension to SME. also enables SME2 instructions 🎀
• sme2p1 — enables the Scalable Matrix Extension version 2.1. also enables SME2 instructions 🌟
• fcma — enables the complex number SIMD extensions 🔢
• jscvt — enables the fjcvtzs JavaScript conversion instruction 🟨 (yes, that JavaScript)
• frintts — enables floating-point round to integral value instructions 🔄
• wfxt — enables wfet and wfit instructions 😴
• xs — enables the XS memory attribute extension 🧬
• lse128 — enables the LSE128 128-bit atomic instructions extension. also enables LSE instructions ⚛️
• d128 — enables support for 128-bit system register read/write instructions. also enables the LSE128 extension 💯
• gcs — enables support for Armv9.4-a Guarded Control Stack extension 🛡️
• the — enables support for Armv8.9-a/9.4-a translation hardening extension 🔒
• rcpc2 — enables the RCpc2 extension 🧵
• rcpc3 — enables the RCpc3 (Release Consistency) extension 🧶
• fp8 — enables the fp8 (8-bit floating point) extension 🎱
• fp8fma — enables the fp8 (8-bit floating point) multiply accumulate extension ✖️
• ssve-fp8fma — enables the fp8 (8-bit floating point) multiply accumulate extension in streaming mode 🌊
• fp8dot4 — enables the fp8 (8-bit floating point) to single-precision 4-way dot product extension 🔵
• ssve-fp8dot4 — enables the fp8 (8-bit floating point) to single-precision 4-way dot product extension in streaming mode 🌊🔵
• fp8dot2 — enables the fp8 (8-bit floating point) to half-precision 2-way dot product extension 🟣
• ssve-fp8dot2 — enables the fp8 (8-bit floating point) to half-precision 2-way dot product extension in streaming mode 🌊🟣
• faminmax — enables the Floating Point Absolute Maximum/Minimum extension ↔️
• lut — enables the Lookup Table extension 📖
• sme-lutv2 — enables the SME Lookup Table v2 (LUTv2) extension 📚
• cpa — enables the Checked Pointer Arithmetic instructions ✅
• sve-b16b16 — enables the SVE non-widening brain floating-point (bf16) extension. this one only has an effect when sve2 or sme2 are also enabled 🧠
• sve-bfscale — enables the SVE_BFSCALE extension ⚖️
• poe2 — enables the Permission Overlays Extension 2 🪪
• tev — enables the TIndex Exception-like Vector Extension 🎯
• tlbid — enables the TLBI Domains Extension 🗂️
• gcie — enables the GICv5 (Generic Interrupt Controller) CPU Interface Extension 🚨
• lscp — enables the load acquire and store release pair extension 🔁
• mops-go — enables tag only variants of MOPS instructions. this also enables the instructions that accelerate memory operations and Armv8.5-a Memory Tagging Extensions 🏷️
• sve2p3 — enables SVE2.3. also enables SVE2.2 instructions 🌱
• sme2p3 — enables SME2.3. also enables SME2.2 instructions 🍀
• f16f32dot — enables Armv9.7-a f16f32dot instructions. also enables Advanced SIMD, floating-point instructions and Armv8.2-a FP16 instructions 🎯
• sve-b16mm — enables the SVE B16MM Extension. also enables SVE instructions, Advanced SIMD and floating-point instructions 🧮
• mtetc — enables Data cache tag block operations. also enables Armv8.5-a Memory Tagging Extensions 🗃️
• f16f32mm — enables Armv9.7-a f16f32mm instructions. also enables Advanced SIMD, floating-point instructions and Armv8.2-a FP16 instructions 🎰
• f16mm — enables f16mm instructions. also enables Advanced SIMD, floating-point instructions and Armv8.2-a FP16 instructions 🎲

okay so feature dependency tea ☕: the feature crypto implies aes, sha2, and simd, which implies fp. and conversely, nofp implies nosimd, which implies nocrypto, noaes and nosha2. it all cascades, bestie, iykyk 💅

---

🌟 Adapteva Epiphany Options 🌟

these -m options are defined for Adapteva Epiphany 🦋:

• -mhalf-reg-file
• -mno-half-reg-file — don't allocate any register in the range r32…r63. that's what lets your code run on hardware variants that don't have these registers 🚫
• -mprefer-short-insn-regs
• -mno-prefer-short-insn-regs — preferentially allocate registers that allow short instruction generation. heads up though: this can bump up the instruction count, so it might reduce or increase overall code size — it's giving unpredictable 😭
• -mbranch-cost=num — set the cost of branches to roughly num "simple" instructions. this cost is only a heuristic and is **not guaranteed to produce consistent results across releases 🎲
• -mcmove
• -mno-cmove — enable the generation of conditional moves 🔀
• *-mnops=num* — emit num NOPs before every other generated instruction 😴
• -mno-soft-cmpsf
• -msoft-cmpsf — for single-precision floating-point comparisons, emit an fsub instruction and test the flags. this is faster than a software comparison, but it can get incorrect results in the presence of NaNs, or when two different small numbers are compared such that their difference is calculated as zero ⚠️. the default is -msoft-cmpsf, which uses slower, but IEEE-compliant, software comparisons 🐢
• -mstack-offset=num — set the offset between the top of the stack and the stack pointer. e.g., a value of 8 means the eight bytes in the range sp+0...sp+7 can be used by leaf functions without stack allocation. values other than 8 or 16 are untested and unlikely to work 🙈. 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, it's recommended to configure the toolchain with the appropriate --with-stack-offset=num* option 💡
• -mno-round-nearest
• -mround-nearest — -mno-round-nearest makes the scheduler assume the rounding mode has been set to truncating. the default is -mround-nearest 🔵
• -mlong-calls — unless an attribute says otherwise, assume all calls might be beyond the offset range of the b / bl instructions, and therefore load the function address into a register before performing an (otherwise direct) call. this is the default 📞
• -mshort-calls — unless an attribute says otherwise, assume all direct calls are in the range of the b / bl instructions, so use these instructions for direct calls.

the default is -mlong-calls. note that -mlong-calls is equivalent to -mno-short-calls, and similarly -mno-long-calls is equivalent to -mshort-calls 🤝

• -msmall16
• -mno-small16 — assume addresses can be loaded as 16-bit unsigned values. this does not apply to function addresses for which -mlong-calls semantics are in effect 🧮
• *-mfp-mode=mode* — set the prevailing mode of the floating-point unit. this determines the floating-point mode that is provided and expected at function call and return time. making this mode match the mode you predominantly need at function start can make your programs smaller and faster by avoiding unnecessary mode switches 🏃‍♀️💨

mode can be set to one of the following values:

• caller — the icon of "just vibe with whatever's going on" 💅 any mode at function entry is valid, and it gets retained or restored when the function returns and when it calls other functions. Lowkey perfect for compiling libraries or other compilation units you might want to drop into different programs that have 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. fr fr.
• truncate — 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 — the mode used for floating-point calculations with round-to-nearest-or-even rounding mode. 🎀
• int — 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 👑

• -mmay-round-for-trunc, -mno-may-round-for-trunc — this option allows floating point to integer truncation to be replaced with rounding to save mode switching. It's disabled by default. 🫶
• -mfp-iarith, -mno-fp-iarith — this option enables use of the floating-point unit for integer add and subtract. It's disabled by default.
• -msplit-lohi, -mno-split-lohi, -mpost-inc, -mno-post-inc, -mpost-modify, -mno-post-modify — code generation tweaks that control, respectively, splitting of 32-bit loads, generation of post-increment addresses, and generation of post-modify addresses. The defaults are msplit-lohi, -mpost-inc, and -mpost-modify. 🧚
• -mno-vect-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, -mno-split-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. 😭 (theory vs reality, classic)
• -m1reg-reg — specify a register to hold the constant -1, which makes loading small negative constants and certain bitmasks faster. Allowable values for reg are r43 and r63, which specify use of that register as a fixed register, and none, which means that no register is used for this purpose. The default is -m1reg-none. 💜

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🌈 AMD GCN Options 🌈

These options are defined specifically for the AMD GCN port. GPU girlies rise 🫶🔥

• *-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).
• gfx942 — compile for CDNA3 Instinct MI300 series devices (gfx942). (Experimental)
• gfx950 — compile for the CDNA3 gfx950 devices. (Experimental)
• gfx9-generic — compile generic code for Vega devices, executable on the following subset of GFX9 devices: gfx900, gfx902, gfx904, gfx906, gfx909 and gfx90c.
• gfx9-4-generic — compile generic code for CDNA3 devices, executable on the following subset of GFX9 devices: gfx942 and gfx950. (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 on gfx1030, gfx1031, gfx1032, gfx1033, gfx1034, gfx1035, and gfx1036.
• 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 on gfx1100, gfx1101, gfx1102, gfx1103, gfx1150, gfx1151, gfx1152, and gfx1153.
• *-mgang-private-size=bytes* — set the amount of local data-share (LDS) memory to reserve for gang-private variables. The default is 512. 💾
• -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 is any, for devices that support it. ⚠️ match the mode or perish

• -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=any on devices that support Unified Shared Memory, and -mxnack=no otherwise. 👀 yes, note that it's no there, not off — that's literally what the page says, don't come for me

• -Wopenacc-dims
• -Wno-openacc-dims

Control warnings about invalid OpenACC dimensions. 🎯

---

🦋 ARC Options 🦋

The following options control the architecture variant for which code is being compiled:

• -mbarrel-shifter — generate instructions supported by barrel shifter. This is the default unless -mcpu=ARC601 or -mcpu=ARCEM is in effect. 🌪️
• -mjli-always — force to call a function using jli_s instruction. This option is valid only for ARCv2 architecture. no cap, only ARCv2 🫡
• *-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 ARC HS38 CPU.
  • hs38_linux — compile for ARC HS38 CPU with all hardware extensions on.
  • hs4x — compile for ARC HS4x CPU.
  • hs4xd — compile for ARC HS4xD CPU.
  • hs4x_rel31 — compile for ARC HS4x CPU release 3.10a.
  • arc600_norm — compile for ARC 600 CPU with "norm" instructions enabled.
  • arc600_mul32x16 — compile for ARC 600 CPU with "norm" and 32x16-bit multiply instructions enabled.
  • arc600_mul64 — compile for ARC 600 CPU with "norm" and "mul64"-family instructions enabled.
  • arc601_norm — compile for ARC 601 CPU with "norm" instructions enabled.
  • arc601_mul32x16 — compile for ARC 601 CPU with "norm" and 32x16-bit multiply instructions enabled.
  • arc601_mul64 — compile for ARC 601 CPU with "norm" and "mul64"-family instructions enabled.
  • nps400 — compile for ARC 700 on NPS400 chip.
  • em_mini — compile for ARC EM minimalist configuration featuring reduced register set. ✨ tiny but mighty
• -mdpfp
• -mdpfp-compact

Generate double-precision FPX instructions, tuned for the compact implementation. 💅

• -mdpfp-fast — generate double-precision FPX instructions, tuned for the fast implementation. 🔥
• -mno-dpfp-lrsr
• -mdpfp-lrsr

Control whether "lr" and "sr" instructions use FPX extension aux registers. This is enabled by default. 🎀

• -mea — generate extended arithmetic instructions. Currently only "divaw", "adds", "subs", and "sat16" are supported. Only valid for -mcpu=ARC700. ⚠️ ARC700 only, bestie
• -mmul32x16 — generate 32x16-bit multiply and multiply-accumulate instructions. ✖️
• -mmul64 — generate "mul64" and "mulu64" instructions. Only valid for -mcpu=ARC600.
• -mnorm — generate "norm" instructions. This is the default if -mcpu=ARC700 is in effect. 👑
• -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. ⚡

okay so we're still deep in the ARC target options and honestly this is where it gets chef's kiss specific. buckle up 🎀

• -msimd — turns on generation of ARC SIMD instructions via target-specific builtins. only valid for -mcpu=ARC700, so don't try it anywhere else, bestie 💅
• -msoft-float — this option is literally ignored; it's provided for compatibility purposes only. software floating-point code is emitted by default, and this default can be overridden by FPX options — -mspfp, -mspfp-compact, or -mspfp-fast for single precision, and -mdpfp, -mdpfp-compact, or -mdpfp-fast for double precision. yes queen, it's a compatibility ghost 😭
• -mswap — generate swap instructions. that's it, that's the tweet ✨
• -matomic — 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, so check your core first 🙈
• -mdiv-rem — enable div and rem instructions for ARCv2 cores.
• -mcode-density, -mno-code-density — enable code density instructions for ARC EM. this option is on by default for ARC HS, so ARC HS girlies don't even have to ask 🫶
• -mll64 — enable double load/store operations for ARC HS cores.
• -mtp-regno=regno — specify the thread pointer register number.
• -mbitops — enable use of NPS400 bit operations.
• -mcmem — enable use of the NPS400 xld/xst extension.
• -mmpy-option=multo — compile ARCv2 code with a multiplier design option. you can specify multo as either a string or a numeric value. wlh1 is the default value. the recognized values are:
  • 0 / none — no multiplier available.
  • 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, so don't come crying to me if you're on something else 💀

• -mfpu=fpu — enables support for specific floating-point hardware extensions for ARCv2 cores. 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 option fpus_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 option fpus_fma. not available for ARC EM.
  • fpus_all — enables support for all single-precision floating-point hardware extensions.
  • 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 general-purposes 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 🧚
• -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 interrupt entry and exit sequences. use this option when you're using fast interrupts in an ARC V2 family processor. permitted values are 4, 8, 16, and 32 ✨
• -mlpc-width=width — specify the width of the lp_count register. valid values for width are 8, 16, 20, 24, 28 and 32 bits. the default width is fixed to 32 bits. 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 specified, the compiler and run-time library might continue to use the loop mechanism for various needs. this option defines macro __ARC_LPC_WIDTH__ with the value of width. lowkey iconic that it tells you the macro name too 💖
• -mrf16 — this option instructs the compiler to generate code for a 16-entry register file. this option defines the __ARC_RF16__ preprocessor macro.
• -mbranch-index — enable use of bi or bih instructions to implement jump tables.

---

okay next up: the following options are passed through to the assembler, and also define preprocessor macro symbols 👑

• -mlock — passed down to the assembler to enable the locked load/store conditional extension. also sets the preprocessor symbol __Xlock.
• -mswape — passed down to the assembler to enable the swap byte ordering extension instruction. also sets the preprocessor symbol __Xswape.
• -mxy — passed down to the assembler to enable the XY memory extension. also sets the preprocessor symbol __Xxy.

---

the following options control how the assembly code is annotated 🎀

• -misize — annotate assembler instructions with estimated addresses.

---

the following options are passed through to the linker 🔥

• -marclinux, -mno-arclinux — passed through to the linker, to specify use of the arclinux emulation. this option is enabled by default in tool chains built for arc-linux-uclibc and arceb-linux-uclibc targets when profiling is not requested.
• -marclinux_prof, -mno-arclinux_prof — passed through to the linker, to specify use of the arclinux_prof emulation. this option is enabled by default in tool chains built for arc-linux-uclibc and arceb-linux-uclibc targets when profiling is requested. so basically it's the profiling girlie version 💅

---

the following options control the semantics of generated code 🌸

• -mlong-calls — generate calls as register indirect calls, thus providing access to the full 32-bit address range.
• -mmedium-calls, -mno-medium-calls — don't use less than 25-bit addressing range for calls, which is the offset available for an unconditional branch-and-link instruction. conditional execution of function calls is suppressed, to allow use of the 25-bit range, rather than the 21-bit range with conditional branch-and-link. this is the default for tool chains built for arc-linux-uclibc and arceb-linux-uclibc targets.
• -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 is 4 for any ARC configuration, or 8 when we have double load/store operations.
• -mno-sdata — do not generate sdata references. this is the default for tool chains built for arc-linux-uclibc and arceb-linux-uclibc targets.
• -mvolatile-cache, -mno-volatile-cache — control how volatile references are accessed. the default is -mvolatile-cache, which uses ordinary cached memory accesses for volatile references. use -mno-volatile-cache to enable cache bypass for volatile references. choose your fighter, bestie ⚔️

---

the following options fine tune code generation ✨

• -mauto-modify-reg — enable the use of pre/post modify with register displacement.
• -mno-brcc, -mbrcc — this option controls a target-specific pass in arc_reorg to generate compare-and-branch (brcc) instructions, which is enabled by default. it has no effect on generation of these instructions driven by the combiner pass. iykyk 🫶
• -mcase-vector-pcrel — use PC-relative switch case tables to enable case table shortening. this is the default for -Os.
• -mno-cond-exec — disable the ARCompact-specific pass to generate conditional execution instructions.

That's because the target-independent pass for generating conditional execution is often kinda mid — it has to juggle delay slot scheduling, operand numbers, literal sizes, instruction lengths and conditional-execution support all at once 😭 — so the ARC port keeps a whole special extra pass around that hunts for more conditional-execution opportunities after register allocation, branch shortening and delay slot scheduling are done. This pass generally (but not always, bestie, not always) makes performance and code size better, at the cost of extra compile time — which is literally why there's a flag to turn it off. And if you're having a whole crisis because call instructions are blowing past their allowable offset range due to being conditionalized, consider -mmedium-calls instead. 💅

• -mearly-cbranchsi — turns on pre-reload use of the "cbranchsi" pattern. slay ✨
• -mindexed-loads — enables use of indexed loads. Ngl this can be problematic 😬 because some optimizers then assume indexed stores exist too, which is simply not the case.
• -mlra-priority-none — don't indicate any priority for target registers. none. zero. nada.
• -mlra-priority-compact — indicate target register priority for r0..r3 / r12..r15.
• -mlra-priority-noncompact — reduce target register priority for r0..r3 / r12..r15.
• -mmillicode — when you're optimizing for size (with -Os), prologues and epilogues that have to save or restore a big ol' pile of registers often get shortened by calling a special function in libgcc — that's what a millicode call is, iykyk. But since these calls can cause performance issues and/or linking issues when you link in a nonstandard way, this option exists to turn millicode call generation on or off. 🫶
• -mcode-density-frame — this one lets the compiler emit "enter" and "leave" instructions. Heads up though: these are only valid for CPUs with the code-density feature. ⭐
• -msize-level=level — fine-tunes size optimization with regards to instruction lengths and alignment. The recognized values for level are:
  • 0 — no size optimization. This level is deprecated and treated like 1. 💀
  • 1 — short instructions are used opportunistically.
  • 2 — in addition, alignment of loops and of code after barriers are dropped.
  • 3 — in addition, optional data alignment is dropped, and the option Os is enabled.

This defaults to 3 when -Os is in effect. Otherwise, the behavior when this isn't set is equivalent to level 1.

• -mtune=cpu — sets instruction scheduling parameters for cpu, overriding anything 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 — the cost to assume for a multiply instruction, where 4 is equal to a normal instruction.

---

🌈 ARM Options 🌈

These -m options are defined for the ARM port, bestie — buckle up, there's a lot of them 💖:

• -mabi=name — generate code for the specified ABI. Permissible values are: apcs-gnu, atpcs, aapcs and aapcs-linux.
• -mapcs-frame — generate a stack frame that's compliant with the ARM Procedure Call Standard for all functions, even when that's not strictly necessary for correct execution of the code. Specifying -fomit-frame-pointer along with this option causes stack frames not to be generated for leaf functions. The default is -mno-apcs-frame. > Heads up: this option is deprecated. ⚠️
• -mapcs — this is a synonym for -mapcs-frame and is deprecated.
• -mthumb-interwork — generate code that supports calling between the ARM and Thumb instruction sets. Without this option, on pre-v5 architectures, the two instruction sets cannot be reliably used inside one program. The default is -mno-thumb-interwork, since slightly larger code gets generated when -mthumb-interwork is specified. In AAPCS configurations this option is meaningless. 🙈
• -msched-prolog, -mno-sched-prolog — allow or prevent the reordering of instructions in the function prologue, or the merging of those instructions with the instructions in the function's body. With -mno-sched-prolog, this means all functions start with a recognizable set of instructions (or, like, one of a choice from a small set of different function prologues), and that info can be used to locate the start of functions inside an executable piece of code. The default is -msched-prolog.
• -mfloat-abi=name — specifies which floating-point ABI to use. Permissible values are: soft, softfp and hard.

Specifying soft causes GCC to generate output containing library calls for floating-point operations. softfp allows the generation of code using hardware floating-point instructions, but still uses the soft-float calling conventions. hard allows generation of floating-point instructions and uses FPU-specific calling conventions.

The default depends on the specific target configuration. Note that the hard-float and soft-float ABIs are not link-compatible — you must compile your entire program with the same ABI, and link with a compatible set of libraries. No mixing and matching, bestie, fr fr. 😤

• -mgeneral-regs-only — generate code which uses only the general-purpose registers. This prevents the compiler from using floating-point and Advanced SIMD registers, but won't impose any restrictions on the assembler.
• -mlittle-endian — generate code for a processor running in little-endian mode. This is the default for all standard configurations. 👑
• -mbig-endian — generate code for a processor running in big-endian mode; the default is to compile code for a little-endian processor.
• -mbe8, -mbe32 — when linking a big-endian image, select between BE8 and BE32 formats. The option has no effect for little-endian images and is ignored. The default depends on the selected target architecture: for ARMv6 and later architectures the default is BE8, for older architectures the default is BE32. > Note that BE32 format has been deprecated by ARM. 💀
• -march=name[+extension...] — this specifies the name of the target ARM architecture. GCC uses this name to figure out what kind of instructions it can emit when generating assembly code. This option can be used in conjunction with, or instead of, the -mcpu= option.

Permissible names are: armv4t, armv5t, armv5te, armv6, armv6j, armv6k, armv6kz, armv6t2, armv6z, armv6zk, armv7, armv7-a, armv7ve, armv8-a, armv8.1-a, armv8.2-a, armv8.3-a, armv8.4-a, armv8.5-a, armv8.6-a, armv9-a, armv7-r, armv8-r, armv6-m, armv6s-m, armv7-m, armv7e-m, armv8-m.base, armv8-m.main, armv8.1-m.main, iwmmxt and iwmmxt2.

Additionally, the following architectures, which lack support for the Thumb execution state, are recognized but support is deprecated: armv4.

Many of the architectures support extensions. These can be added by appending +extension to the architecture name. Extension options are processed in order and capabilities accumulate. An extension will also enable any necessary base extensions upon which it depends — for example, the +crypto extension will always enable the +simd extension. The exception to the additive construction is for extensions that are prefixed with +no...: these extensions disable the specified option and any other extensions that may depend on the presence of that extension.

For example, -march=armv7-a+simd+nofp+vfpv4 is equivalent to writing -march=armv7-a+vfpv4, since the +simd option is entirely disabled by the +nofp option that follows it. iconic behavior honestly.

Most extension names are generically named, but have an effect that depends on the architecture to which it's applied. For example, the +simd option can be applied to both armv7-a and armv8-a architectures, but will enable the original ARMv7-A Advanced SIMD (Neon) extensions for armv7-a and the ARMv8-A variant for armv8-a. iykyk 🦋

The table below lists 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 +vfpv2 can 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-d16 can be used as an alias for this extension. Note that floating-point is not supported by the base ARMv7-M architecture, but 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-d16 can be used as an alias for this extension.
• +simd — the Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions. The extensions +neon and +neon-vfpv3 can 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, plus the half-precision floating-point conversion operations 🎀
• +neon-vfpv4 — the Advanced SIMD (Neon) v2 and the VFPv4 floating-point instructions 💅
• +nosimd — turns OFF the Advanced SIMD instructions (and like, it does not disable floating point, bestie, they're separate things 💀)
• +nofp — turns OFF the floating-point and Advanced SIMD instructions, full stop 🙈

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-d16 can be used as an alias for this extension 💖
• +simd — the Advanced SIMD (Neon) v2 and the VFPv4 floating-point instructions. The extension +neon-vfpv4 can 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-vfpv3 can 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

okay so these two share the exact same lil extension menu, so here it is once for both of them 🫶

• +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 💜
• +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
• +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 🧠

armv8.5-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 💜
• +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 🧠

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-point fmla extension 💜
• +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 +vfpv3xd can be used as an alias for this extension ✨
• +fp — the VFPv3 floating-point instructions with 16 double-precision registers. The extension +vfpv3-d16 can 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 🦋
• +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

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 💅
• +dsp — the DSP instructions ✨
• +nodsp — turns the DSP extension off
• +fp — the single-precision floating-point instructions
• +fp.dp — the single- and double-precision floating-point instructions 💅
• +nofp — disables the floating-point extension
• +cdecp0, +cdecp1, ... , +cdecp7 — enables the Custom Datapath Extension (CDE) on whichever coprocessors you picked, using the numbers in the options, in the range 0 to 7

armv8-r — okay so this one's menu is:

• +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 — disables the cryptographic instructions
• +nofp — disables the floating-point, Advanced SIMD and cryptographic instructions (all of them at once, bestie, it's a bundle)

-march=native makes the compiler auto-detect the architecture of the build computer. Fr fr, heads up though: right now this feature is only supported on GNU/Linux, and not all architectures are recognized. If the auto-detect doesn't work out, the option literally has no effect. 💀

-march=unset makes the compiler ignore any -march=... options that show up earlier on the command line, and behave like the option was never passed at all. Super useful for dodging warnings about conflicting CPU and architecture options when the two produce different architecture specifications — no drama, no notes. 🫶

---

🌸 -mtune=name

This option names the target ARM processor that GCC should tune the performance of the code for. For some ARM implementations you genuinely get better performance by using this. The 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

Bestie, it can also tell GCC to tune the performance of the code for a big.LITTLE system 👑. The permissible names for that 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 says GCC should tune the performance for a blend of processors within architecture arch. The goal is generating code that runs well on the current most popular processors — balancing optimizations that benefit some CPUs in the range while avoiding performance pitfalls of the other CPUs. Ngl, the effects of this option may change in future GCC versions as CPU models come and go. 🦋

-mtune permits the same extension options as -mcpu, but those extension options do not affect the tuning of the generated code.

-mtune=native makes the compiler 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 it were specified by -march) and the ARM processor type to tune for performance (as if specified by -mtune). When you use this option together with -march or -mtune, those options take precedence over the appropriate part of this one. Precedence slay.

Many of the supported CPUs implement optional architectural extensions. Where that's the case, the architectural extensions are normally enabled by default. If implementations that lack the extension exist, you can use the extension syntax to disable the extensions that got omitted. For floating-point and Advanced SIMD (Neon) instructions, the settings of -mfloat-abi and -mfpu must also be considered: 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 overrides the available floating-point and SIMD extension instructions.

For example, cortex-a9 exists 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. Choose your fighter 💅

Permissible names for this option are the same as those for -mtune.

The following extension options are common to the listed CPUs:

• +nodsp — disables the DSP instructions on cortex-m33, cortex-m35p, cortex-m52, cortex-m55 and cortex-m85. Also disables the M-Profile Vector Extension (MVE) integer and single precision floating-point instructions on cortex-m52, cortex-m55 and cortex-m85.
• +nopacbti — disables the Pointer Authentication and Branch Target Identification Extension on cortex-m52 and cortex-m85.
• +nomve — disables the M-Profile Vector Extension (MVE) integer and single precision floating-point instructions on cortex-m52, cortex-m55 and cortex-m85.
• +nomve.fp — disables the M-Profile Vector Extension (MVE) single precision floating-point instructions on cortex-m52, cortex-m55 and cortex-m85.
• +cdecp0, +cdecp1, ... , +cdecp7 — enables the Custom Datapath Extension (CDE) on selected coprocessors according to the numbers given in the options, in the range 0 to 7, on cortex-m52, cortex-m55, cortex-m85 and star-mc1.
• +nofp — disables the floating-point instructions on arm9e, 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-m55 and cortex-m85. Disables the floating-point and SIMD instructions on generic-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-a53 and cortex-a55.
• +nofp.dp — disables the double-precision component of the floating-point instructions on cortex-r5, cortex-r7, cortex-r8, cortex-r52, cortex-r52plus and cortex-m7.
• +nosimd — disables the SIMD (but not floating-point) instructions on generic-armv7-a, cortex-a5, cortex-a7 and cortex-a9.
• +crypto — enables the cryptographic instructions on cortex-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-a53 and cortex-a75.cortex-a55.

Additionally, the generic-armv7-a pseudo target defaults to VFPv3 with 16 double-precision registers. It supports these 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 — iykyk 🧚

-mcpu=generic-arch is also permissible, and is equivalent to -march=arch -mtune=generic-arch. See -mtune for more info.

-mcpu=native makes the compiler 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 makes the compiler 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. No notes, just peace ✌️

---

🎀 -mfpu=name

This one 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 it's special. It makes the compiler select the floating-point and Advanced SIMD instructions based on the settings of -mcpu and -march. Automatic bestie behaviour, we love to see it 🌈

• -mfpu=neon (and friends) 💅 — okay so if your selected floating-point hardware includes the NEON extension (like with -mfpu=neon), heads up: floating-point operations are not generated by GCC's auto-vectorization pass unless you also specify -funsafe-math-optimizations. Why? Because NEON hardware doesn't fully implement the IEEE 754 standard for floating-point arithmetic (specifically, denormal values get treated as zero), so using NEON instructions may lead to a loss of precision. It's giving "close enough" energy, ngl 😭

You can also set the fpu name at function level by using the target("fpu=") function attributes or pragmas. Slay, per-function control, we love to see it 🎀

• -mfp16-format=name — specify the format of the __fp16 half-precision floating-point type. The permissible names are none, ieee, and alternative; the default is none, in which case the __fp16 type is not defined. So like, no __fp16 for you unless you ask nicely 💁‍♀️
• -mstructure-size-boundary=n — the sizes of all structures and unions get 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, so check first bestie 🫶

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. It's giving "we don't talk to each other anymore" 💀

This option is deprecated. Yeah. Old news. Lowkey retired.

• -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. Like, you said you weren't coming back, so… if you do, we're calling abort on you. Dramatic but fair 😤
• -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. Reach is limited, so we take the long way around 🦋

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. It's giving "everybody has exceptions" ✨

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. Pointers stay in their own lane, periodt 💅

• -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 the runtime does the setup, GCC just trusts it 🫡
• -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. Catch your fave register doing its thing 👑
• -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. Sneaky little chain reaction, love that for us 🌈
• -mpoke-function-name — write the name of each function into the text section, directly preceding the function prologue. The generated code is similar to this:

             t0
                 .ascii "arm_poke_function_name", 0
                 .align
             t1
                 .word 0xff000000 + (t1 - t0)
             arm_poke_function_name
                 mov     ip, sp
                 stmfd   sp!, {fp, ip, lr, pc}
                 sub     fp, ip, #4

When performing a stack backtrace, code can inspect the value of pc stored at fp + 0. If the trace function then looks at location pc - 12 and the top 8 bits are set, then we know that there is a function name embedded immediately preceding this location and has length ((pc[-3]) & 0xff000000). It's literally leaving a little name tag on every function, iconic behavior honestly 🎀

• -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. Pick your era 💅

You can also override the ARM and Thumb mode for each function by using the target("thumb") and target("arm") function attributes or pragmas. Per-function vibes, we stan 🦋

• -mtpcs-frame — generate a stack frame that is compliant with the Thumb Procedure Call Standard for all non-leaf functions. (A leaf function is one that does not call any other functions.) The default is -mno-tpcs-frame. Non-leaf functions only, peasant leaf functions need not apply 🙈
• -mtpcs-leaf-frame — generate a stack frame that is compliant with the Thumb Procedure Call Standard for all leaf functions. (A leaf function is one that does not call any other functions.) The default is -mno-apcs-leaf-frame. Okay so notice the default name is slightly different (-mno-apcs-leaf-frame, not -mno-tpcs-leaf-frame) — yes bestie I'm reading it exactly like the page says, don't come for me 😭
• -mcallee-super-interworking — gives all externally visible functions in the file being compiled an ARM instruction set header which switches to Thumb mode before executing the rest of the function. This allows these functions to be called from non-interworking code. This option is not valid in AAPCS configurations because interworking is enabled by default. So like, it's already handled, chill 🧚
• -mcaller-super-interworking — allows calls via function pointers (including virtual functions) to execute correctly regardless of whether the target code has been compiled for interworking or not. There is a small overhead in the cost of executing a function pointer if this option is enabled. This option is not valid in AAPCS configurations because interworking is enabled by default. Small cost, big correctness, we love a tradeoff 💜
• -mtp=name — specify the access model for the thread local storage pointer. The model soft generates calls to __aeabi_read_tp. Other accepted models are tpidrurw, tpidruro and tpidrprw which fetch the thread pointer from the corresponding system register directly (supported from the arm6k architecture and later). These system registers are accessed through the CP15 co-processor interface and the argument cp15 is also accepted as a convenience alias of tpidruro. The argument auto uses the best available method for the selected processor. The default setting is auto. Let the compiler pick, honestly it knows 💅
• -mtls-dialect=dialect — specify the dialect to use for accessing thread local storage. Two dialects are supported — gnu and gnu2. The gnu dialect selects the original GNU scheme for supporting local and global dynamic TLS models. The gnu2 dialect selects the GNU descriptor scheme, which provides better performance for shared libraries. The GNU descriptor scheme is compatible with the original scheme, but does require new assembler, linker and library support. Initial and local exec TLS models are unaffected by this option and always use the original scheme. gnu2 is giving glow-up but you gotta upgrade your whole toolchain for it 😤
• -mword-relocations — only generate absolute relocations on word-sized values (i.e. R_ARM_ABS32). This is enabled by default on targets (uClinux, SymbianOS) where the runtime loader imposes this restriction, and when -fpic or -fPIC is specified. This option conflicts with -mslow-flash-data. Pick a side, bestie 💀
• -mfix-cortex-m3-ldrd — some Cortex-M3 cores can cause data corruption when ldrd instructions with overlapping destination and base registers are used. This option avoids generating these instructions. This option is enabled by default when -mcpu=cortex-m3 is specified. Silly little hardware bug, we simply avoid it ✨
• -mfix-cortex-a57-aes-1742098, -mno-fix-cortex-a57-aes-1742098, -mfix-cortex-a72-aes-1655431, -mno-fix-cortex-a72-aes-1655431 — enable (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-a57 or -mcpu=cortex-a72 is specified. Errata got their own dedicated flags now, look at them go 🎀
• -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. Slow and steady, byte by byte 😭

The ARM attribute Tag_CPU_unaligned_access is set in the generated object file to either true or false, depending upon the setting of this option. If unaligned access is enabled then the preprocessor symbol __ARM_FEATURE_UNALIGNED is also defined. Two-for-one flag behavior, we love efficiency 🦋

• -mslow-flash-data — assume loading data from flash is slower than fetching instruction. Therefore literal load is minimized for better performance. This option is only supported when compiling for ARMv7 M-profile and off by default. It conflicts with -mword-relocations. Again: pick a side 💅
• -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. Future-you has been warned 🫶
• -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. Rules are rules, bestie 📏
• -mpure-code — do not allow constant data to be placed in code sections. Additionally, when compiling for ELF object format give all text sections the ELF processor-specific section attribute SHF_ARM_PURECODE. This option is only available when generating non-pic code for M-profile targets. Pure code only, no stowaways allowed 👑
• -mcmse — generates secure code exactly as the "ARMv8-M Security Extensions: Requirements on Development Tools Engineering Specification" says to, and bestie that spec lives at <https://developer.arm.com/documentation/ecm0359818/latest/> if you wanna read it yourself. 💅
• -mfix-cmse-cve-2021-35465 — shields you from a potential security boo-boo involving the VLLDM instruction on some M-profile devices when you're using CMSE (CVE-2021-365465 😭). It's on by default whenever the -mcpu= option is used with cortex-m33, cortex-m35p, cortex-m52, cortex-m55, cortex-m85 or star-mc1. And if you don't want the mitigation? Just hit it with -mno-fix-cmse-cve-2021-35465, slay.
• -mstack-protector-guard=guard
• -mstack-protector-guard-offset=offset — these two generate stack protection code using a canary sitting at guard. The supported locations are global for a global canary, or tls for a canary you reach via the TLS register. Heads up though: the -mstack-protector-guard-offset= option is meant for use with -fstack-protector-guard=tls and not for user-land code, ok?
• -mfdpic
• -mno-fdpic — picks the FDPIC ABI, which uses 64-bit function descriptors to represent pointers to functions. When the compiler was configured for arm-*-uclinuxfdpiceabi targets, this one is on by default and it implies -fPIE if you didn't hand it any of the PIC/PIE-related options. On other targets it only switches on the FDPIC-specific code generation features, so you gotta explicitly provide the PIC/PIE-related options as needed. 🫶

Also, big note: static linking is not supported, because it'd still drag the dynamic linker in when the program self-relocates. If that behavior is fine by you, use -static and -Wl,-dynamic-linker. ✨

The opposite, -mno-fdpic, is useful (and honestly required) for building the Linux kernel with that same arm-*-uclinuxfdpiceabi toolchain you used to build the userland programs. iykyk.

• -mbranch-protection=features — turns on branch protection features (this one's armv8.1-m.main only, no cap). features can take one of these forms:
  • none — generates code with zero branch protection and zero return address signing.
  • standard — generates code with all the branch protection features enabled at their standard level.
  • pac-ret — generates code with return address signing at its standard level, which signs every function that saves the return address to memory.
  • pac-ret+leaf — stretches that pac-ret signing to also cover leaf functions, even the ones that never write the return address to memory.
  • bti — adds landing-pad instructions at the permitted targets of indirect branch instructions.

If support for the +pacbti architecture extension isn't enabled (like, via -march=), then every branch protection and return address signing operation gets 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 you can flip the extra security on at run time on processors that support the PACBTI extension. 😌

Also, branch target enforcement using BTI can only be enabled at runtime if all code in the application was compiled with at least -mbranch-protection=bti. Periodt. 🔥

Any setting other than none is supported only on armv8-m.main or later.

The default is to generate code with no branch protection and no return address signing, bestie.

• -mvectorize-with-neon-quad
• -mvectorize-with-neon-double — controls whether vectorization uses NEON quad-word or double-word registers. The default is -mvectorize-with-neon-quad. ✨

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🌸 AVR Options 🌸

These options are defined for AVR implementations, and honestly they're a whole thing:

• -mmcu=mcu — specify the AVR instruction set architecture (ISA) or device type. The default for this option is avr2.

The following AVR devices and ISAs are supported. Note (and this matters, bestie): 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 that last one is provided by the compiler according to the supported mcus listed below. The rest comes from 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 — "Classic" devices with up to 8 KiB of program memory and with 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, where the program memory is actually visible in the RAM address space (iconic behaviour tbh). 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" girlies rockin' 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 plus more than 64 KiB of RAM, okay she's got range. 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, and bestie they are tiny: 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 it's supported for assembler only, no cap. mcu =

"attiny11", "attiny12", "attiny15", "attiny28", "at90s1200".

---

• -mabsdata — assume all data in static storage can be reached by LDS / STS instructions. Heads up: this only does anything on reduced Tiny devices like ATtiny40. Peep the "absdata" AVR Attributes variable attribute too, iykyk.
• -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 links startup code from crtmcu-cvt.o instead of the usual crtmcu.o. Besides giving you that compact vector table, the startup code sets bit CPUINT_CTRLA.CPUINT_CVT, which enables the CVT on the device.

If you don't want the startup code setting CPUINT_CTRLA.CPUINT_CVT, you can satisfy the symbol __init_cvt so the relevant code stops getting pulled in from libmcu.a. For example, link with -Wl,--defsym,__init_cvt=0.

The CVT startup code has been 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 a specific value for bits is even allowed depends on the --with-double= and --with-long-double= configure options (https://gcc.gnu.org/install/configure.html#avr), and the exact same deal applies to the options' default values.
• -mgas-isr-prologues — interrupt service routines (ISRs) may use the __gcc_isr pseudo instruction supported by GNU Binutils. If this option is on, you can still turn the feature off for individual ISRs using the AVR Attributes "no_gccisr" function attribute. It's activated by default if optimization is on (but not with -Og, @pxref{Optimize Options}), and if GNU Binutils supports PR21683 (https://sourceware.org/PR21683).
• -mint8 — assume int is an 8-bit integer. This shifts the sizes of literally every type: a char is 1 byte, an int is 1 byte, a long is 2 bytes, and long long is 4 bytes. 🙈 Please note this option does not conform to the C standards — but it does give you smaller code size, so.
• -mmain-is-OS_task — don't save registers in main. Same effect as slapping the AVR Attributes "OS_task" attribute on main. Activated by default if optimization is on.
• -mno-call-main — don't run main via

XCALL  main
XJMP   exit

Instead, put main in section .init9 (https://avrdudes.github.io/avr-libc/avr-libc-user-manual/mem_sections.html#sec_dot_init) so no call is needed at all. By setting this option, the user asserts that main will not return. ✨

You can use this on devices with very limited resources to save a few bytes of code and stack space. It works as expected since AVR-LibC v2.3 (https://github.com/avrdudes/avr-libc/issues/1012). With older versions there's no performance gain whatsoever, so don't bother.

• -mno-interrupts — generated code is not compatible with hardware interrupts. Code size gets smaller. That's the trade, bestie 💀
• -mrelax — try to replace CALL resp. JMP instructions with the shorter RCALL resp. RJMP instruction where applicable. Setting -mrelax just adds the --mlink-relax option to the assembler's command line and the --relax option to the linker's command line.

Jump relaxing happens in the linker, because jump offsets aren't known until the code is located. So the assembler code the compiler generates is identical — but the instructions in the executable may differ from the instructions in the assembler code. Wild, right?

Relaxing must be turned on if linker stubs are needed, see the section on EIND and linker stubs below.

• -mpmem-wrap-around — enable program counter wrap-around in linker relaxation.
• -mrodata-in-ram, -mno-rodata-in-ram — put the .rodata sections for read-only data in RAM resp. in program memory. For most devices there's no choice and this option acts more like an assertion than a preference.

Since v14 and for the AVR64* and AVR128* devices, .rodata lives in flash memory by default, provided the required GNU Binutils support (PR31124 (https://sourceware.org/PR31124)) is available. In that case you can use -mrodata-in-ram to go back to the old layout with .rodata in RAM.

• -mtiny-stack — only change the lower 8 bits of the stack pointer. That's it. That's the whole vibe.
• -mfract-convert-truncate — allow truncation instead of rounding towards zero for fractional fixed-point types.
• -nodevicelib — don't link against AVR-LibC's device specific library libmcu.a.

Heads up 🚨 since AVR-LibC v2.3 that library contains code that's essential for your program to function correctly. Specifically it contains parts of the startup code like: __init_sp (https://github.com/avrdudes/avr-libc/issues/1011) to initialize the stack pointer with the 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 call main and exit, and __do_flmap_init (https://github.com/avrdudes/avr-libc/issues/931) to set up FLMAP according to the symbol __flmap.

• -nodevicespecs — don't add -specs=device-specs/specs-mcu to the compiler driver's command line. The user takes responsibility for supplying sub-processes like the compiler proper, assembler and linker with appropriate command line options. Which means you have to supply her private device specs file by means of -specs=path-to-specs-file. There's no more need for the option -mmcu=mcu.

This option can also serve as a replacement for the older way of specifying custom device-specs files, which needed -B some-path to point at a directory containing 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 the __vector prefix. Enabled by default. 🫶

---

🌈 AVR Optimization Options 🌈

The following options are pure optimization options. The ones from up above — -mgas-isr-prologues, -mmain-is-OS_task, -mno-call-main and -mrelax — are only almost optimization options, since there are rare occasions where their different code generation actually matters. 💅

• -maccumulate-args — accumulate outgoing function arguments and acquire/release the needed stack space for outgoing function arguments once, in the function prologue/epilogue. Without this option, outgoing arguments get pushed before calling a function and popped afterwards. See also the -fdefer-pop Optimize Options,,optimization option.

Popping the arguments after the function call can be expensive on AVR, so accumulating that stack space might actually lead to smaller executables, because the arguments don't need to be removed from the stack after such a function call. Slay.

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 — so like, if your function is out here making a bunch of calls to printf-style stuff, this one's for you bestie 💖

• -mbranch-cost=cost — sets the branch costs for conditional branch instructions to cost. Reasonable values for cost are small, non-negative integers, and the default branch cost is 0. Keep it smol and chill, no giant numbers pls.
• -mcall-prologues — function prologues/epilogues get expanded as calls to appropriate subroutines. Code size is smaller. It's giving "pay a tiny call tax, save a whole lotta bytes" ✨
• -mfuse-add, -mno-fuse-add, -mfuse-add=level — optimize indirect memory accesses on reduced Tiny devices. The default uses level=1 for optimizations -Og and -O1, and level=2 for higher optimizations. Valid values for level are 0, 1 and 2. That's it, that's the whole menu 🍓
• -mfuse-move, -mno-fuse-move, -mfuse-move=level — runs a post reload optimization pass that tries to fuse move instructions and to split multi-byte instructions into 8-bit operations. The default uses level=3 for optimization -O1, and level=23 for higher optimizations. Valid values for level are in the range 0 ... 23 which is a 3:2:2:2 mixed radix value — and each digit controls some aspect of the optimization. It's a whole little number-cocktail, iykyk 🍹
• -mfuse-move2 — runs a post combine optimization pass that tries to fuse move instructions. Yes, the sequel. Not a remake, a whole second pass 💅
• -mstrict-X — uses address register X in a way proposed by the hardware. This means that X is only used in indirect, post-increment or pre-decrement addressing. Hardware-approved behavior only, no cap 🎀

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

Look at it, doing the most just to mimic Y and Z. Respect honestly 😭

• -msplit-bit-shift — splits 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 -O2 and higher, including -Os but excluding -Oz. Splitting of shifts with a constant offset that is a multiple of 8 is controlled by -mfuse-move. Teamwork makes the dream work fr fr 🤝
• -msplit-ldst — splits multi-byte loads and stores into several byte loads and stores. This optimization is turned on per default for -O2 and higher. Chopping big things into small things, iconic behavior ✂️
• -muse-nonzero-bits — enables optimizations that are only possible when some bits in a register are always zero. This optimization is turned on per default for -O2 and higher. Sneaky little advantage, we love to see it 👑

---

🦋 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 — so like, read these, they're the fine print but make it fashion:

• The compiler never sets EIND. Not its job, not its problem 🫶
• The compiler uses EIND implicitly in EICALL/EIJMP instructions or might read EIND directly in order to emulate an indirect call/jump by means of a RET instruction.
• The compiler assumes that EIND never changes during the startup code or during the application. In particular, EIND is not saved/restored in function or interrupt service routine prologue/epilogue. So don't be out here changing it mid-run and then acting surprised, bestie 💀
• 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. Trampolines but make it embedded 🎪
• Linker relaxation must be turned on so that the linker generates the stubs correctly in all situations. See the compiler option -mrelax and the linker option --relax. There are corner cases where the linker is supposed to generate stubs but aborts without relaxation and without a helpful error message — and yeah, "unhelpful error message" is its own whole mood 😭
• The default linker script is arranged for code with EIND = 0. If code is supposed to work for a setup with EIND != 0, a custom linker script has to be used in order to place the sections whose name start with .trampolines into the segment where EIND points to.
• The startup code from libgcc never sets EIND. Notice that startup code is a blend of code from libgcc and AVR-LibC. For the impact of AVR-LibC on EIND, see the AVR-LibC user manual (https://avrdudes.github.io/avr-libc/avr-libc-user-manual/).
• It is legitimate for user-specific startup code to set up EIND early, for example by means of initialization code located in section .init3. Such code runs prior to general startup code that initializes RAM and calls constructors, but after the bit of startup code from AVR-LibC that sets EIND to the segment where the vector table is located.

#include <avr/io.h>

static void
__attribute__((section(".init3"),naked,used,no_instrument_function))
init3_set_eind (void)
{
  __asm volatile ("ldi r24,pm_hh8(__trampolines_start)\n\t"
                  "out %i0,r24" :: "n" (&EIND) : "r24","memory");
}

The __trampolines_start symbol is defined in the linker script. Little legend behavior right there ⭐

• Stubs are generated automatically by the linker if the following two conditions are met:
  • The address of a label is taken by means of the gs modifier (short for generate stubs) like so:

LDI r24, lo8(gs(<func>))
LDI r25, hi8(gs(<func>))

• The final location of that label is in a code segment outside the segment where the stubs are located.

• The compiler emits such gs modifiers for code labels in the following situations:
  • Taking address of a function or code label.
  • Computed goto.
  • If prologue-save function is used, see -mcall-prologues command-line option.
  • Switch/case dispatch tables. If you do not want such dispatch tables you can specify the -fno-jump-tables command-line option.
  • C and C++ constructors/destructors called during startup/shutdown.
  • If the tools hit a gs() modifier explained above.
• Jumping to non-symbolic addresses like so is not supported:

int main (void)
{
    /* Call function at word address 0x2 */
    return ((int(*)(void)) 0x2)();
}

Instead, a stub has to be set up, i.e. the function has to be called through a symbol (func_4 in the example):

int main (void)
{
    extern int func_4 (void);

    /* Call function at byte address 0x4 */
    return func_4();
}

and the application be linked with -Wl,--defsym,func_4=0x4. Alternatively, func_4 can be defined in the linker script. See, symbolic is the moment, raw addresses are not 💅

---

🌈 Handling of the RAMPD, RAMPX, RAMPY and RAMPZ Special Function Registers 🌈

Some AVR devices support memories larger 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 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. Big memory energy 🧚

• The startup code initializes the RAMP special function registers with zero.
• If a AVR Named Address Spaces, named address space other than generic or __flash is used, then RAMPZ is set as needed before the operation.
• If the device supports RAM larger than 64 KiB and the compiler needs to change RAMPZ to accomplish an operation, RAMPZ is reset to zero after the operation. Clean up after yourself queen 🎀
• If the device comes with a specific RAMP register, the ISR prologue/epilogue saves/restores that SFR and initializes it with zero in case the ISR code might (implicitly) use it.
• RAM larger than 64 KiB is not supported by GCC for AVR targets. If you use inline assembler to read from locations outside the 16-bit address range and change one of the RAMP registers, you must reset it to zero after the access.

Okay so this one actually matters: RAM larger than 64 KiB is not supported by GCC for AVR targets, and if you go poking at RAMP registers in inline asm you must reset them to zero after the access. No cap, do not skip this 🙈

---

💜 AVR Built-in Macros 💜

GCC defines several built-in macros so that the user code can test for the presence or absence of features. Almost any of the following built-in macros are deduced from device capabilities and thus triggered by the -mmcu= command-line option.

For even more AVR-specific built-in macros see AVR Named Address Spaces and AVR Built-in Functions.

• __AVR_ARCH__ — build-in macro that resolves to a decimal number that identifies the architecture and depends on the -mmcu=mcu option. Possible values are:

2, 25, 3, 31, 35, 4, 5, 51, 6

for mcu=avr2, avr25, avr3, avr31, avr35, avr4, avr5, avr51, avr6,

respectively and

100, 102, 103, 104, 105, 106, 107

for 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. Numbers on numbers on numbers, we love a lookup table moment 🔥

• __AVR_Device__ — setting -mmcu=device defines this built-in macro which reflects the device's name. For example, -mmcu=atmega8 defines the built-in macro __AVR_ATmega8__, -mmcu=attiny261a defines __AVR_ATtiny261A__, etc.

The built-in macros' names follow the scheme __AVR_Device__ where Device is the device name as from the AVR user manual. The difference between Device in the built-in macro and device in -mmcu=device is that the latter is always lowercase. Case-sensitivity ate, honestly 😭

If device is not a device but only a core architecture like avr51, this macro is not defined.

• __AVR_DEVICE_NAME__ — setting -mmcu=device defines this built-in macro to the device's name. For example, with -mmcu=atmega8 the macro is defined to atmega8.

If device is not a device but only a core architecture like avr51, this macro is not defined.

• __AVR_CVT__ — the code is being compiled with option -mcvt to use a compact vector table. Compact, cute, done 🌸
• __AVR_XMEGA__ — the device / architecture belongs to the XMEGA family of devices. very demure, very XMEGA ✨
• __AVR_HAVE_ADIW__ — the device has the ADIW and SBIW instructions.
• __AVR_HAVE_ELPM__ — the device has the ELPM instruction.
• __AVR_HAVE_ELPMX__ — the device has the ELPM Rn,Z and ELPM Rn,Z+ instructions.
• __AVR_HAVE_LPMX__ — the device has the LPM Rn,Z and LPM Rn,Z+ instructions.
• __AVR_HAVE_MOVW__ — the device has the MOVW instruction to perform 16-bit register-register moves. moving them registers around, periodt
• __AVR_HAVE_MUL__ — the device has a hardware multiplier. math but make it hardware 💅
• __AVR_HAVE_JMP_CALL__ — the device has the JMP and CALL instructions. 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 the EIJMP and EICALL instructions. 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. three whole bytes of PC, she's grown 🥹
• __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=avr2 and -mmcu=avr25 also by -msp8.
• __AVR_HAVE_RAMPD__ __AVR_HAVE_RAMPX__ __AVR_HAVE_RAMPY__ __AVR_HAVE_RAMPZ__ — the device has the RAMPD, RAMPX, RAMPY, RAMPZ special function register, respectively. collect the whole set bestie 🌸
• __NO_INTERRUPTS__ — this macro reflects the -mno-interrupts command-line option.
• __AVR_ERRATA_SKIP__ __AVR_ERRATA_SKIP_JMP_CALL__ — some AVR devices (AT90S8515, ATmega103) must not skip 32-bit instructions because of a hardware erratum. Skip instructions are SBRS, SBRC, SBIS, SBIC and CPSE. The second macro is only defined if __AVR_HAVE_JMP_CALL__ is also set.

heads up 🚨 this is a hardware bug thing, not a vibe. The macro exists so the compiler knows to behave.

• __AVR_ISA_RMW__ — the device has Read-Modify-Write instructions (XCH, LAC, LAS and LAT). read, modify, write, repeat 🔥
• __AVR_SFR_OFFSET__=offset — instructions that can address I/O special function registers directly like IN, OUT, SBI, etc. may use a different address as if addressed by an instruction to access RAM like LD or STS. This offset depends on the device architecture and has to be subtracted from the RAM address in order to get the respective I/O address.
• __AVR_SHORT_CALLS__ — the -mshort-calls command line option is set.
• __AVR_PM_BASE_ADDRESS__=addr — some devices support reading from flash memory by means of LD* 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 .rodata into RAM. This is handled by the default linker description file, and is currently available for avrtiny and avrxmega3. Even more convenient, there is no need to use address spaces like __flash or features like attribute progmem and pgm_read_*. no cap, this is the good timeline ✨
• __AVR_HAVE_FLMAP__ — this macro is defined provided the following conditions are met:
  • the device has the NVMCTRL_CTRLB.FLMAP bitfield.
    • this applies to the AVR64* and AVR128* devices.
  • it's not known at assembler-time which emulation will be used.

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-avrlibc configure option.
• __HAVE_SIGNAL_N__ — the compiler supports the signal(num) and interrupt(num) AVR 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 of double if -mdouble= is not set. To test the layout of double in 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 for long double instead of double. she said "same energy, longer type" 💅
• __WITH_DOUBLE_COMPARISON__ — reflects the --with-double-comparison={tristate|bool|libf7} configure option (https://gcc.gnu.org/install/configure.html#avr) and is defined to 2 or 3.
• __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 🛠️

Okay bestie, listen — 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. fr fr, 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 — assume that the flash memory has a size of num times 64 KiB. This determines which __flashN address spaces are available.
• -mflmap — the device has the FLMAP bit field located in special function register NVMCTRL_CTRLB.
• -mrmw — assume that the device supports the Read-Modify-Write instructions XCH, LAC, LAS and LAT.
• -mshort-calls — assume that RJMP and RCALL can target the whole program memory. This option is used for multilib generation and selection for the devices from architecture avrxmega3.
• -mskip-bug — generate code without skips (CPSE, SBRS, SBRC, SBIS, SBIC) over 32-bit instructions. skip the skips 🎀
• -msp8 — treat the stack pointer register as an 8-bit register, i.e. assume the high byte of the stack pointer is zero. This option is used by the compiler to select and build multilibs for architectures avr2 and avr25. These architectures mix devices with and without SPH.

---

🖤 Blackfin Options 🖤

• -mcpu=cpu[-sirevision] — specifies the name of the target Blackfin processor. Currently, cpu can be one of:

bf512, bf514, bf516, bf518, bf522, bf523, bf524,
bf525, bf526, bf527, bf531, bf532, bf533, bf534,
bf536, bf537, bf538, bf539, bf542, bf544, bf547,
bf548, bf549, bf542m, bf544m, bf547m, bf548m,
bf549m, bf561, bf592

The optional sirevision specifies the silicon revision of the target Blackfin processor. Any workarounds available for the targeted silicon revision are enabled. If sirevision is none, no workarounds are enabled. If sirevision is any, all workarounds for the targeted processor are enabled. The __SILICON_REVISION__ macro is defined to two hexadecimal digits representing the major and minor numbers in the silicon revision. If sirevision is none, the __SILICON_REVISION__ is not defined. If sirevision is any, the __SILICON_REVISION__ is defined to be 0xffff. If this optional sirevision is not used, GCC assumes the latest known silicon revision of the targeted Blackfin processor.

GCC defines a preprocessor macro for the specified cpu. For the bfin-elf toolchain, this option causes the hardware BSP provided by libgloss to be linked in if -msim is not given.

Without this option, bf532 is used as the processor by default.

notable caveat 🫶 support for bf561 is incomplete. For bf561, only the preprocessor macro is defined.

• -msim — specifies that the program will be run on the simulator. This causes the simulator BSP provided by libgloss to be linked in. This option has effect only for bfin-elf toolchain. Certain other options, such as -mid-shared-library and -mfdpic, imply -msim.
• -momit-leaf-frame-pointer — don't keep the frame pointer in a register for leaf functions. This avoids the instructions to save, set up and restore frame pointers and makes an extra register available in leaf functions.
• -mspecld-anomaly -mno-specld-anomaly — when enabled, the compiler ensures that the generated code does not contain speculative loads after jump instructions. If this option is used, __WORKAROUND_SPECULATIVE_LOADS is defined.
• -mcsync-anomaly -mno-csync-anomaly — when enabled, the compiler ensures that the generated code does not contain CSYNC or SSYNC instructions too soon after conditional branches. If this option is used, __WORKAROUND_SPECULATIVE_SYNCS is defined.
• -mlow64k -mno-low64k — when enabled, the compiler is free to take advantage of the knowledge that the entire program fits into the low 64k of memory. The default behavior is to assume that the program is arbitrarily large (-mno-low64k). default is chaos-sized, got it 💀
• -mstack-check-l1 — do stack checking using information placed into L1 scratchpad memory by the uClinux kernel.
• -mid-shared-library -mno-id-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. The default is -mno-id-shared-library, to generate code that doesn't assume ID-based shared libraries are being used.
• -mleaf-id-shared-library -mno-leaf-id-shared-library — generate code that supports shared libraries via the library ID method, but assumes that this library or executable won't link against any other ID shared libraries. That allows the compiler to use faster code for jumps and calls. faster jumps?? we love her ✨

The default is -mno-leaf-id-shared-library, which basically means "we're assuming nothing" — the compiler makes no assumption that the code being compiled won't link against any ID shared libraries. Result? Slower code gets generated for jump and call insns. 😭 Yeah it's the safe-but-slow vibe.

• -mshared-library-id=n — sets the identification number of the ID-based shared library you're compiling. Pass 0 and you get more compact code 🎀; pass anything else and it forces that number to be allocated to the current library — but ngl it's no more space- or time-efficient than just leaving the option out entirely.
• -msep-data, -mno-sep-data — generates code that lets the data segment live in a totally different area of memory from the text segment. Why do you care? Because it enables execute in place in an environment with no virtual memory management, by eliminating relocations against the text section. ✨ The default is -mno-sep-data, which tells GCC to generate code assuming the data segment follows the text segment.
• -mlong-calls, -mno-long-calls — tells the compiler to do function calls the indirect way: load the address of the function into a register first, then do a subroutine call on that register. You need this switch if the target function lives outside 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. Heads up though: these switches have zero 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), all in the interest of performance. 🔥 (yes, it's trading strictness for speed, that's the whole deal)
• -minline-plt — enables inlining of PLT entries in function calls to functions that aren't known to bind locally. Lowkey it has no effect without -mfdpic. 🫶
• -mmulticore — builds a standalone application for multicore Blackfin processors. This option makes proper start files and link scripts that support multicore get used, and it defines the macro __BFIN_MULTICORE. It can only be used with -mcpu=bf561[-sirevision].

You can pair it with -mcorea or -mcoreb, which selects the one-application-per-core programming model. Without -mcorea or -mcoreb, you get the single-application/dual-core programming model. In that model, the main function of Core B should be named coreb_main. 👑

If you don't use this option at all, the single-core application programming model is what you get.

• -mcorea — builds a standalone application for Core A of BF561 when you're using the one-application-per-core programming model. Proper start files and link scripts get used to support Core A, and the macro __BFIN_COREA is defined. This option can only be used in conjunction with -mmulticore. 🎀
• -mcoreb — builds a standalone application for Core B of BF561 when you're using the one-application-per-core programming model. Proper start files and link scripts get used to support Core B, and the macro __BFIN_COREB is defined. When this option is used, coreb_main should be used instead of main. This option can only be used in conjunction with -mmulticore. ✨
• -msdram — builds a standalone application for SDRAM. Proper start files and link scripts are used to put the application into SDRAM, and the macro __BFIN_SDRAM is defined. The loader should initialize SDRAM before loading the application. 🌸
• -micplb — assume that ICPLBs are enabled at run time. This has an effect on certain anomaly workarounds. For Linux targets, the default is to assume ICPLBs are enabled; for standalone applications the default is off. 💅

---

🌈 C6X Options 🌈

• -march=name — 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. 👑
• -mbig-endian — generate code for a big-endian target.
• -mlittle-endian — generate code for a little-endian target. This is the default. ✨
• -msim — choose startup files and linker script suitable for the simulator.
• -msdata=default — put small global and static data in the .neardata section, which is pointed to by register B14. Put small uninitialized global and static data in the .bss section, which is adjacent to the .neardata section. Put small read-only data into the .rodata section. The corresponding sections used for large pieces of data are .fardata, .far and .const. 🎀
• -msdata=all — put all data, not just small objects, into the sections reserved for small data, and use addressing relative to the B14 register to access them. 🫶
• -msdata=none — make no use of the sections reserved for small data, and use absolute addresses to access all data. Put all initialized global and static data in the .fardata section, and all uninitialized data in the .far section. Put all constant data into the .const section. 💅
• -mdsbt — compile for the DSBT shared library ABI. This option is required to compile with -fpic or -fPIC, and it implies -fpic. 🔥
• -mlong-calls — avoid generating PC-relative calls; use indirection instead.

---

💜 CRIS Options 💜

These options are defined specifically for the CRIS ports. (yes, that's the whole intro, no notes 😭)

• -march=architecture-type, -mcpu=architecture-type — generate code for the specified architecture. The choices for architecture-type are v3, v8 and v10 for respectively ETRAX 4, ETRAX 100, and ETRAX 100 LX. Default is v0. 🌸
• -mtune=architecture-type — tune everything applicable about the generated code to architecture-type, except for the ABI and the set of available instructions. The choices for architecture-type are the same as for -march=architecture-type. ✨
• -mmax-stackframe=n — warn when the stack frame of a function exceeds n bytes.
• -metrax4, -metrax100 — -metrax4 and -metrax100 are synonyms for -march=v3 and -march=v8 respectively. 🎀
• -mmul-bug-workaround, -mno-mul-bug-workaround — work around a bug in the muls and mulu instructions for CPU models where it applies. This option is disabled by default. 💀
• -mpdebug — enable CRIS-specific verbose debug-related information in the assembly code. This option also has the effect of turning off the #NO_APP formatted-code indicator to the assembler at the beginning of the assembly file. 🔍
• -mcc-init — do not use condition-code results from previous instruction; always emit compare and test instructions before use of condition codes.
• -mno-side-effects — do not emit instructions with side effects in addressing modes other than post-increment.
• -mstack-align, -mno-stack-align, -mdata-align, -mno-data-align, -mconst-align, -mno-const-align — these options (the no- versions) arrange (eliminate arrangements) for the stack frame, individual data and constants to be aligned for the maximum single data access size for the chosen CPU model. The default is to arrange for 32-bit alignment. ABI details such as structure layout are not affected by these options. 📐
• -m32-bit, -m16-bit, -m8-bit — similar to the stack-, data- and const-align options above, these options arrange for stack frame, writable data and constants to all be 32-bit, 16-bit or 8-bit aligned. The default is 32-bit alignment. ✨
• -mno-prologue-epilogue, -mprologue-epilogue — with -mno-prologue-epilogue, the normal function prologue and epilogue which set up the stack frame are omitted and no return instructions or return sequences are generated in the code.

⚠️ Use this option only together with visual inspection of the compiled code: no warnings or errors are generated when call-saved registers must be saved, or storage for local variables needs to be allocated. Like, it will just silently be wrong. Not cute. 😭

• -mbest-lib-options, -moverride-best-lib-options — -mbest-lib-options selects the most feature-enabling options allowed by other options. This option has no no- form, but -moverride-best-lib-options disables it regardless of the relative order of the two options on the command line. 🫶
• -mtrap-using-break8 — emit traps as break 8. This is the default for CRIS v3 and up. If disabled, calls to abort are used instead. 💥
• -mtrap-unaligned-atomic — emit checks causing break 8 instructions to execute when applying atomic builtins on misaligned memory. 🔥
• -munaligned-atomic-may-use-library — handle atomic builtins that may be applied to unaligned data by calling library functions. This option overrides -mtrap-unaligned-atomic. 👑
• -sim — this option arranges to link with input-output functions from a simulator library. Code, initialized data and zero-initialized data are allocated consecutively. 🎀
• -sim2 — like -sim, but pass linker options to locate initialized data at 0x40000000 and zero-initialized data at 0x80000000. ✨

---

🦋 C-SKY Options 🦋

GCC supports these options when compiling for C-SKY V2 processors. 💅

• -march=arch — specify the C-SKY target architecture. Valid values for arch are: ck801, ck802, ck803, ck807, and ck810. The default is ck810. 👑
• -mcpu=cpu — specify the C-SKY target processor. 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, and
ck810ftv.

(yes bestie, that's all of them, every single one, I did not skip a crumb 🍓)

• -mbig-endian, -mlittle-endian — 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, softfp and hard.

Specifying soft causes GCC to generate output containing library calls for floating-point operations. softfp allows the generation of code using hardware floating-point instructions, but still uses the soft-float calling conventions. hard allows generation of floating-point instructions and uses FPU-specific calling conventions. 🔥

okay so the default here depends on the specific target configuration. 💅

⚠️ bestie, the hard-float and soft-float ABIs are not link-compatible — you have to compile your entire program with the same ABI and link with a compatible set of libraries. No mixing. Periodt. 💀

• -mdouble-float, -mno-double-float — when -mhard-float is in effect, enable generation of double-precision float instructions. This is the default except when compiling for CK803. 🦋
• -mfdivdu, -mno-fdivdu — when -mhard-float is in effect, enable generation of "frecipd", "fsqrtd", and "fdivd" instructions. This is the default except when compiling for CK803. ✨
• -mfpu=fpu — select the floating-point processor. This option can only be used with -mhard-float. Values for fpu are fpv2_sf (equivalent to -mno-double-float -mno-fdivdu), fpv2 (-mdouble-float -mno-divdu), and fpv2_divd (-mdouble-float -mdivdu). 🔥
• -melrw, -mno-elrw — enable the extended "lrw" instruction. This option defaults to on for CK801 and 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. 🫶
• -mmp, -mno-mp — enable multiprocessor instructions; the default is off. 🌸
• -mcp, -mno-cp — enable coprocessor instructions; the default is off. 💜
• -mcache, -mno-cache — enable coprocessor instructions; the default is off. (yes, same wording as -mcp, I'm just the messenger 💀)
• -msecurity, -mno-security — enable C-SKY security instructions; the default is off. 🧚
• -mtrust, -mno-trust — enable C-SKY trust instructions; the default is off. 🌈
• -mdsp, -mno-dsp, -medsp, -mno-edsp, -mvdsp, -mno-vdsp — 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 numbered 0-7 to allow use of 16-bit instructions. This option is ignored for CK801 where this is the required behavior, and it defaults to on for CK802. For other targets, the default is off. 🦋
• -mhigh-registers, -mno-high-registers — generate code using the high registers numbered 16-31. This option is not supported on CK801, CK802, or CK803, and is enabled by default for other processors. 💅
• -manchor, -mno-anchor — generate code using global anchor symbol addresses. ✨
• -mpushpop, -mno-pushpop — generate code using "push" and "pop" instructions. This option defaults to on. 🎀
• -mmultiple-stld, -mno-multiple-stld — generate code using "stm" and "ldm" instructions. This option isn't supported on CK801 but is enabled by default on other processors. 🌸
• -mconstpool, -mno-constpool — create constant pools in the compiler instead of deferring it to the assembler. This option is the default and required for correct code generation on CK801 and CK802, and is optional on other processors. 🔥
• -mstack-size, -mno-stack-size — emit .stack_size directives for each function in the assembly output. This option defaults to off. 🫶
• -mccrt, -mno-ccrt — generate code for the C-SKY compiler runtime instead of libgcc. This option defaults to off. 💜
• -mbranch-cost=n — set the branch costs to roughly n instructions. The default is 1. 🌈
• -msched-prolog, -mno-sched-prolog — permit scheduling of function prologue and epilogue sequences. 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. 🧚

⚠️ okay that -msched-prolog warning is real — don't say I didn't tell you. Backtraces are your friends. 😭

• -msim, -mno-sim — links the library libsemi.a which is in compatible with simulator. Applicable to ELF compiler only. 🍓

---

🪟 Cygwin and MinGW Options 🪟

These additional options are available for Microsoft Windows targets: 💖

• -mconsole — this option specifies that a console application is to be generated, by instructing the linker to set the PE header subsystem type required for console applications. This option is available for Cygwin and MinGW targets and is enabled by default on those targets. 👑
• -mcrtdll=library — preprocess, compile or link with the specified C RunTime DLL library. This option adjusts the predefined macros __CRTDLL__, __MSVCRT__, _UCRT and __MSVCRT_VERSION__ for the specified CRT library, chooses the start file for the CRT library and links with the CRT library. Recognized CRT library names for the preprocessor are: crtdll*, msvcrt10*, msvcrt20*, msvcrt40*, msvcr40*, msvcrtd*, msvcrt-os*, msvcr70*, msvcr71*, msvcr80*, msvcr90*, msvcr100*, msvcr110*, msvcr120* and ucrt*. 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. MinGW import library msvcrt is just a symlink to (or a copy of) another MinGW CRT import library chosen during MinGW compilation. 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 the "dllimport" attribute should be ignored. ✨
• -mthreads — support thread-safe exception handling on MinGW. Programs that rely on thread-safe exception handling must compile and link all code with the -mthreads option. When compiling, -mthreads defines -D_MT; when linking, it links in a special thread helper library -lmingwthrd which cleans up per-thread exception-handling data. 🎀
• -municode — this option is available for MinGW-w64 targets. It causes the "UNICODE" preprocessor macro to be predefined, and chooses Unicode-capable runtime startup code. 🌸
• -mwin32 — this option is available for Cygwin and MinGW targets. It specifies that the typical Microsoft Windows predefined macros are to be set in the pre-processor, but does not influence the choice of runtime library/startup code. 🔥
• -mwindows — this option is available for Cygwin and MinGW targets. It specifies that a GUI application is to be generated by instructing the linker to set the PE header subsystem type appropriately. 💅
• -fno-set-stack-executable — this option is available for MinGW targets. It specifies that the executable flag for the stack used by nested functions isn't set. This is necessary for binaries running in kernel mode of Microsoft Windows, as there the User32 API, which is used to set executable privileges, isn't available. 🫶
• -fwritable-relocated-rdata — this option is available for MinGW and Cygwin targets. It specifies that relocated-data in read-only section is put into the .data section. This is a necessary for older runtimes not supporting modification of .rdata sections for pseudo-relocation. 💜
• -mpe-aligned-commons — this option is available for Cygwin and MinGW targets. It specifies that the GNU extension to the PE file format that permits the correct alignment of COMMON variables should be used when generating code. It is enabled by default if GCC detects that the target assembler found during configuration supports the feature. 🌈
• -muse-libstdc-wrappers — use Cygwin DLL wrappers to support C++ operators "new" and "delete". ⭐

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 multiple -arch options are used; it does so by running the compiler or linker multiple times and joining the results together with lipo. 👑

The subtype of the file created (like ppc7400 or ppc970 or i686) is determined by the flags that specify the ISA that GCC is targeting, like -mcpu or -march. The -force_cpusubtype_ALL option can be used to override this. 🌸

The Darwin tools vary in their behavior when presented with an ISA mismatch. The assembler, as, only permits instructions to be used that are valid for the subtype of the file it is generating, so you cannot put 64-bit instructions in a ppc750 object file. The linker for shared libraries, /usr/bin/libtool, fails and prints an error if asked to create a shared library with a less restrictive subtype than its input files (for instance, trying to put a ppc970 object file in a ppc7400 library). The linker for executables, ld, quietly gives the executable the most restrictive subtype of any of its input files. 😭

• -Fdir — Add the framework directory dir to the head of the list of directories to be searched for header files. These directories are interleaved with those specified by -I options and are scanned in a left-to-right order. 💖

So a framework directory is, like, a directory that has frameworks inside it. A framework is a directory that ends in .framework and has a Headers and/or PrivateHeaders directory directly inside it. The name of a framework is the name of that directory minus .framework. Headers for the framework live in one of those two directories, and Headers gets searched first. A subframework is a framework directory that sits 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 is issued if that rule gets violated. Currently a subframework cannot have subframeworks; in the future, the mechanism may be extended to support this. The standard frameworks can be found in /System/Library/Frameworks and /Library/Frameworks. An example include looks like #include <Framework/header.h>, where Framework denotes the name of the framework and header.h is found in the PrivateHeaders or Headers directory. 🎀

• -iframeworkdir — like -F, except the directory is treated as a system directory. The main difference between this -iframework and -F is that with -iframework the compiler does not warn about constructs contained within header files found via dir. This option is valid only for the C family of languages. 💅
• -arch name — generate output for architecture name. As described above, GCC generates output for the architecture it was configured for, using its usual options to select subarchitecture variants. The -arch option is accepted for compatibility, but an error is diagnosed if name is inconsistent with GCC’s own idea of the target architecture.
• -dependency-file filename — alias for the preprocessor option -MF filename.
• -fapple-kext — generate code for Darwin loadable kernel extensions.
• -gused — emit debugging information for symbols that are used. For stabs debugging format, this enables -feliminate-unused-debug-symbols. This is by default ON.
• -gfull — emit debugging information for all symbols and types.
• -matt-stubs, -mno-att-stubs — enable AT&T-style PIC stubs. This is the default when supported by the target architecture (currently x86 only).
• -mconstant-cfstrings, -fconstant-cfstrings — when the NeXT runtime is being used (the default on these systems), override any -fconstant-string-class setting and cause "@"..."" literals to be laid out as constant CoreFoundation strings. -fconstant-cfstrings is an alias for -mconstant-cfstrings.
• -mdynamic-no-pic — generate code suitable for executables (not shared libraries). This option is incompatible with -fpic, -fPIC, -fpie, or -fPIE.
• -mfix-and-continue, -ffix-and-continue, -findirect-data — generate code suitable for fast turnaround development, such as to allow GDB to dynamically load .o files into already-running programs. -findirect-data and -ffix-and-continue are provided for backwards compatibility.
• -mkernel — enable kernel development mode. The -mkernel option sets -static, -fno-common, -fno-use-cxa-atexit, -fno-exceptions, -fno-non-call-exceptions, -fapple-kext, -fno-weak and -fno-rtti where applicable. This mode also sets -mno-altivec, -msoft-float, -fno-builtin and -mlong-branch for PowerPC targets.
• -mmacosx-version-min=version, -asm_macosx_version_min=version — the -mmacosx-version-min option specifies the earliest version of MacOS X that this executable will run on is version. Typical values supported for version include 12, 10.12, and 10.5.8. If the compiler was built to use the system’s headers by default, then the default for this option is the system version on which the compiler is running, otherwise the default is to make choices that are compatible with as many systems and code bases as possible. -asm_macosx_version_min=version is similar, but the GCC driver passes its version information only to the assembler.
• -mone-byte-bool — override the defaults for bool so that sizeof(bool)==1. By default sizeof(bool) is 4 when compiling for Darwin/PowerPC and 1 when compiling for Darwin/x86, so this option has no effect on x86.

⚠️ Warning: the -mone-byte-bool switch causes GCC to generate code that is not binary compatible with code generated without that switch. Using this switch may require recompiling all other modules in a program, including system libraries. Use this switch to conform to a non-default data model.

• -msymbol-stubs, -mno-symbol-stubs — force generation of external symbol indirection stubs for PIC references. By default, this option is enabled automatically if the target linker version (-mtarget-linker) is old enough to require them.
• -mtarget-linker=version, -mtarget-linker version — specify the target ld64 version, overriding any version specified in the GCC configuration. Newer linker versions support improved code generation in some cases, for example for PIC code.
• -ObjC — equivalent to -x objective-c; specifies that the input is is Objective-C source code.
• -ObjC++ — equivalent to -x objective-c++; specifies that the input is is Objective-C++ source code.
• -Wnonportable-cfstrings, -Wno-nonportable-cfstrings — warn if constant CoreFoundation string objects contain non-portable characters. This warning is enabled by default.
• -all_load — loads all members of static archive libraries. See man ld(1) for more information.
• -arch_errors_fatal — cause the errors having to do with files that have the wrong architecture to be fatal.
• -bind_at_load — causes the output file to be marked such that the dynamic linker will bind all undefined references when the file is loaded or launched.
• -bundle — produce a Mach-o bundle format file. See man ld(1) for more information.
• -bundle_loader executable — this option specifies the executable that will load the build output file being linked. See man ld(1) for more information.
• -dynamiclib — when passed this option, GCC produces a dynamic library instead of an executable when linking, using the Darwin libtool command.
• -force_cpusubtype_ALL — this causes GCC’s output file to have the ALL subtype, instead of one controlled by the -mcpu or -march option.
• -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 -nodefaultrpaths to the link line. Run paths are needed (and therefore enforced) to build on macOS version 10.11 or later.
• -nodefaultexport — do not add default symbol exports to modules or dynamic libraries.
• -allowable_client client_name — passed straight to the Darwin linker.
• -client_name — passed straight to the Darwin linker.
• -compatibility_version — passed straight to the Darwin linker.
• -current_version — passed straight to the Darwin linker.
• -dead_strip — passed straight to the Darwin linker.
• -dylib_file — passed straight to the Darwin linker.
• -dylinker — passed straight to the Darwin linker.
• -dylinker_install_name — passed straight to the Darwin linker.
• -dynamic — passed straight to the Darwin linker.
• -exported_symbols_list — passed straight to the Darwin linker.
• -filelist — passed straight to the Darwin linker.
• -flat_namespace — passed straight to the Darwin linker.
• -force_flat_namespace — passed straight to the Darwin linker.
• -framework — passed straight to the Darwin linker.
• -headerpad_max_install_names — passed straight to the Darwin linker.
• -image_base — passed straight to the Darwin linker.
• -init symbol-name — passed straight to the Darwin linker.
• -install_name — passed straight to the Darwin linker.
• -keep_private_externs — passed straight to the Darwin linker.
• -pagezero_size — passed straight to the Darwin linker.
• -preload — passed straight to the Darwin linker.
• -read_only_relocs — passed straight to the Darwin linker.
• -sectalign — passed straight to the Darwin linker.
• -sectcreate — passed straight to the Darwin linker.
• -seg_addr_table — passed straight to the Darwin linker.
• -seg1addr — passed straight to the Darwin linker.
• -segaddr — passed straight to the Darwin linker.
• -segprot — passed straight to the Darwin linker.
• -segs_read_only_addr — passed straight to the Darwin linker.
• -segs_read_write_addr — passed straight to the Darwin linker.
• -sub_library — passed straight to the Darwin linker.
• -sub_umbrella — passed straight to the Darwin linker.
• -twolevel_namespace — passed straight to the Darwin linker.
• -twolevel_namespace_hints — passed straight to the Darwin linker.
• -umbrella — passed straight to the Darwin linker.
• -undefined — passed straight to the Darwin linker.
• -unexported_symbols_list — passed straight to the Darwin linker.
• -weak_framework — passed straight to the Darwin linker.
• -weak_reference_mismatches — passed straight to the Darwin linker.
• -whatsloaded — passed straight to the Darwin linker.
• -whyload — passed straight to the Darwin linker.

These options are passed to the Darwin linker. The Darwin linker man page describes them in detail. 🦋

💻 DEC Alpha Options 💻

These -m options are defined for the DEC Alpha implementations:

• -mno-soft-float, -msoft-float — use (do not use) the hardware floating-point instructions for floating-point operations. When -msoft-float is specified, functions in libgcc.a are used to perform floating-point operations. Unless they are replaced by routines that emulate the floating-point operations, or compiled in such a way as to call such emulations routines, these routines issue floating-point operations. If you are compiling for an Alpha without floating-point operations, you must ensure that the library is built so as not to call them. Note that Alpha implementations without floating-point operations are required to have floating-point registers.
• -mfp-regs, -mno-fp-regs — generate code that uses (does not use) the floating-point register set. -mno-fp-regs implies -msoft-float. If the floating-point register set is not used, floating-point operands are passed in integer registers as if they were integers and floating-point results are passed in $0 instead of $f0. This is a non-standard calling sequence, so any function with a floating-point argument or return value called by code compiled with -mno-fp-regs must also be compiled with that option. A typical use of this option is building a kernel that does not use, and hence need not save and restore, any floating-point registers.
• -mieee — the Alpha architecture implements floating-point hardware optimized for maximum performance. It is mostly compliant with the IEEE floating-point standard. However, for full compliance, software assistance is required. This option generates code fully IEEE-compliant code except that the inexact-flag is not maintained (see below). If this option is turned on, the preprocessor macro _IEEE_FP is defined during compilation. The resulting code is less efficient but is able to correctly support denormalized numbers and exceptional IEEE values such as not-a-number and plus/minus infinity. Other Alpha compilers call this option -ieee_with_no_inexact. 💜
• -mieee-with-inexact — okay so this one is literally -mieee but the generated code also keeps up the IEEE inexact-flag 🫶. Flip it on and your code gets fully-compliant IEEE math, no cap. On top of "_IEEE_FP", you also get "_IEEE_FP_EXACT" defined as a preprocessor macro. Heads up though: on some Alpha implementations the resulting code may run significantly slower than what you get by default 😭. And since there's very little code out there that actually depends on the inexact-flag, you should normally not specify this option, fr fr. Other Alpha compilers call this one -ieee_with_inexact.
• -mfp-trap-mode=trap-mode — this controls which floating-point related traps get enabled 💅. Other Alpha compilers call it -fptm trap-mode. The trap-mode can be one of four values:
  • n — the default (normal) setting ✨. The only traps enabled are the ones that literally cannot be disabled in software (e.g., division by zero trap).
  • u — on top of everything n gives you, underflow traps are enabled too.
  • su — like u, but the instructions are marked to be safe for software completion (peep the Alpha architecture manual for the deets).
  • sui — like su, but inexact traps are enabled as well.
• -mfp-rounding-mode=rounding-mode — picks the IEEE rounding mode 🎀. Other Alpha compilers call this option -fprm rounding-mode. rounding-mode can be:
  • n — normal IEEE rounding mode. Floating-point numbers are rounded towards the nearest machine number, or towards the even machine number in case of a tie.
  • m — round towards minus infinity.
  • c — chopped rounding mode. Floating-point numbers get rounded towards zero.
  • d — dynamic rounding mode 🙈. A field in the floating-point control register (fpcr, see the Alpha architecture reference manual) controls which rounding mode is in effect. The C library initializes this register for rounding towards plus infinity. So unless your program modifies the fpcr, d corresponds to round towards plus infinity.
• -mtrap-precision=trap-precision — so in the Alpha architecture, floating-point traps are imprecise 😭. Meaning: without software assistance it is impossible to recover from a floating trap, and program execution normally needs to be terminated. GCC can generate code that assists operating system trap handlers in figuring out the exact location that caused a floating-point trap. Depending on what your app needs, you can select different levels of precision:
  • p — program precision. This is the default, and it 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 hand you the equivalent options under the names -scope_safe and -resumption_safe.

• -mieee-conformant — this marks the generated code as IEEE conformant 💖. But bestie, listen: you must not use this option unless you also specify -mtrap-precision=i and either -mfp-trap-mode=su or -mfp-trap-mode=sui. Its only effect is to emit the line .eflag 48 in the function prologue of the generated assembly file.
• -mbuild-constants — normally GCC examines a 32- or 64-bit integer constant to see if it can construct it from smaller constants in two or three instructions. If it 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 maximum is six ⭐). You typically use this for building 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 -mcpu= option, or that of the CPU on which GCC was built if none is specified.
• -msafe-bwa, -mno-safe-bwa — indicates 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 — indicates whether GCC should generate multi-thread and async-signal safe code for partial memory accesses 💅. That includes piecemeal accesses to unaligned data, plus 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 (or doesn't 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 zero way to generate symbol relocations except via assembler macros, and using those macros does not 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-relocs is in effect, static data is accessed via gp-relative relocations. With -msmall-data, objects 8 bytes long or smaller get placed in a small data area (the ".sdata" and ".sbss" sections) and are accessed via 16-bit relocations off of the $gp register 💜. This limits the size of the small data area to 64KB, but lets the variables be directly accessed via a single instruction. The default is -mlarge-data. With that 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 — with -msmall-text, the compiler assumes that the code of the entire program (or shared library) fits in 4MB and is thus reachable with a branch instruction 🦋. When -msmall-data is used, the compiler can assume that all local symbols share the same $gp value, and thus reduce the number of instructions required for a function call from 4 to 1. The default is -mlarge-text.
• -mcpu=cpu_type — 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 does not recognize the processor.

• -mtune=cpu_type — sets only the instruction scheduling parameters for machine type cpu_type. The instruction set is not changed 💅. Native toolchains also support the value native, which selects the best architecture option for the host processor. -mtune=native has no effect if GCC does not recognize the processor.
• -mmemory-latency=time — sets the latency the scheduler should assume for typical memory references as seen by the application ⭐. This number is highly dependent on the memory access patterns used by the application and the size of the external cache on the machine. Valid options for time are:
  • number — a decimal number representing clock cycles.
  • L1, L2, L3, main — the compiler contains estimates of the number of clock cycles for "typical" EV4 & EV5 hardware for the Level 1, 2 & 3 caches (also called Dcache, Scache, and Bcache), as well as to main memory. Note that L3 is only valid for EV5 👀.
• -mtls-kernel — emit "rdval" instead of "rduniq" for thread pointer 🧚.
• -mtls-size=bitsize — specify bit size of immediate TLS offsets. Valid values for bitsize are 16, 32, and 64; it defaults to 32 🎀.
• -mlong-double-128, -mlong-double-64 — specify the size of the "long double" type. Note that -mlong-double-128 is incompatible with VAX floating point 🙈.

eBPF Options 🐝

• -mframe-limit=bytes — specifies the hard limit for frame sizes, in bytes 📏. Currently, the value that can be specified 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 — enable or disable generation of extra conditional-branch instructions 💅. Enabled for CPU v2 and above.
• -mjmp32, -mno-jmp32 — enable or disable generation of 32-bit jump instructions. Enabled for CPU v3 and above.
• -malu32, -mno-alu32 — turn 32-bit ALU instruction generation on or off. It’s enabled for CPU v3 and above. ✨
• -mv3-atomics, -mno-v3-atomics — turn instructions for general atomic operations introduced in CPU v3 on or off. Enabled for CPU v3 and above.
• -mbswap, -mno-bswap — turn byte swap instructions on or off. Enabled for CPU v4 and above.
• -msdiv, -mno-sdiv — turn signed division and modulus instructions on or off. Enabled for CPU v4 and above.
• -msmov, -mno-smov — turn sign-extending move and memory load instructions on or off. Enabled for CPU v4 and above.
• -mcpu=version — this picks which version of the eBPF ISA to target. Newer versions may not be supported by all kernels, so check yourself before you wreck yourself. The default is v4.

Supported values for version are:

• v1 — the first stable eBPF ISA with no special features or extensions.
• v2 — supports the jump extensions, as in -mjmpext.
• v3 — all features of v2, plus:
  • 32-bit jump operations, as in -mjmp32
  • 32-bit ALU operations, as in -malu32
  • general atomic operations, as in -mv3-atomics
• v4 — all features of v3, 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

• -mco-re, -mno-co-re — turn BPF Compile Once - Run Everywhere (CO-RE) support on or off. BPF CO-RE support is enabled by default when generating BTF debug information for the BPF target (-gbtf). 🦋
• -mxbpf — generate code for an expanded version of BPF, which relaxes some of the restrictions imposed by the BPF architecture:
  • save and restore callee-saved registers at function entry and exit, respectively.
• -masm=dialect — outputs assembly instructions using eBPF selected dialect. The default is pseudoc.

Supported values for dialect are:

• normal — outputs normal assembly dialect.
• pseudoc — outputs pseudo-c assembly dialect.

• -minline-memops-threshold=bytes — specifies a size threshold in bytes at or below which memmove, memcpy and memset shall always be expanded inline. Operations dealing with sizes larger than this threshold would have to be implemented using a library call instead of being expanded inline, but since BPF doesn’t allow libcalls, exceeding this threshold results in a compile-time error. The default is 1024 bytes. 😭
• -Wco-re — enable warnings for scenarios where -mco-re is enabled, but it may not be possible to generate CO-RE compatible code.

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🌱 FR30 Options

These options are defined specifically for the FR30 port.

• -msmall-model — use the small address space model. This can produce smaller code, but it does assume that all symbolic values and addresses fit into a 20-bit range. Cute until the address space says “bestie, no.”
• -mno-lsim — assume that runtime support has been provided and so there is no need to include the simulator library (libsim.a) on the linker command line.

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🎀 FRV Options

• -mgpr-32 — only use the first 32 general-purpose registers.
• -mgpr-64 — use all 64 general-purpose registers.
• -mfpr-32 — use only the first 32 floating-point registers.
• -mfpr-64 — use all 64 floating-point registers.
• -mhard-float — use hardware instructions for floating-point operations.
• -msoft-float — use library routines for floating-point operations.
• -malloc-cc — dynamically allocate condition code registers.
• -mfixed-cc — do not try to dynamically allocate condition code registers, only use "icc0" and "fcc0".
• -mdword, -mno-dword — control whether the ABI uses double-word instructions.
• -mdouble, -mno-double — turn use of floating-point double instructions on or off.
• -mmedia, -mno-media — turn use of media instructions on or off.
• -mmuladd, -mno-muladd — turn use of multiply and add/subtract instructions on or off.
• -mfdpic — select the FDPIC ABI, which uses function descriptors to represent pointers to functions. Without any PIC/PIE-related options, it implies -fPIE. With -fpic or -fpie, it assumes GOT entries and small data are within a 12-bit range from the GOT base address; with -fPIC or -fPIE, GOT offsets are computed with 32 bits. With a bfin-elf target, this option implies -msim.
• -minline-plt — enable inlining of PLT entries in function calls to functions that are not known to bind locally. It has no effect without -mfdpic. It’s enabled by default if optimizing for speed and compiling for shared libraries (i.e., -fPIC or -fpic), or when an optimization option such as -O3 or above is present in the command line.
• -mTLS — assume a large TLS segment when generating thread-local code.
• -mtls — do not assume a large TLS segment when generating thread-local code.
• -mgprel-ro — enable the use of "GPREL" relocations in the FDPIC ABI for data that is known to be in read-only sections. It’s enabled by default, except for -fpic or -fpie: even though it may help make the global offset table smaller, it trades 1 instruction for 4. With -fPIC or -fPIE, it trades 3 instructions for 4, one of which may be shared by multiple symbols, and it avoids the need for a GOT entry for the referenced symbol, so it’s more likely to be a win. If it is not, -mno-gprel-ro can be used to disable it.
• -multilib-library-pic — link with the (library, not FD) pic libraries. It’s implied by -mlibrary-pic, as well as by -fPIC and -fpic without -mfdpic. You should never have to use it explicitly.
• -mlinked-fp — follow the EABI requirement of always creating a frame pointer whenever a stack frame is allocated. This option is enabled by default and can be disabled with -mno-linked-fp.
• -mlong-calls — use indirect addressing to call functions outside the current compilation unit. This allows the functions to be placed anywhere within the 32-bit address space.
• -malign-labels — try to align labels to an 8-byte boundary by inserting NOPs into the previous packet. This option only has an effect when VLIW packing is enabled. It doesn’t create new packets; it merely adds NOPs to existing ones.
• -mlibrary-pic — generate position-independent EABI code.
• -macc-4 — use only the first four media accumulator registers.
• -macc-8 — use all eight media accumulator registers.
• -mpack, -mno-pack — turn packing VLIW instructions on or off.
• -mno-eflags — do not mark ABI switches in e_flags.
• -mcond-move, -mno-cond-move — turn the use of conditional-move instructions on or off; it is enabled by default.

This switch is mainly for debugging the compiler and will likely be removed in a future version. 💀

• -mscc, -mno-scc — turn the use of conditional set instructions on or off; it is enabled by default.

This switch is mainly for debugging the compiler and will likely be removed in a future version. 💀

• -mcond-exec, -mno-cond-exec — turn the use of conditional execution on or off; it is enabled by default.

This switch is mainly for debugging the compiler and will likely be removed in a future version. 💀

• -mvliw-branch, -mno-vliw-branch — turn an optimization pass to pack branches into VLIW instructions on or off; it is enabled by default.

This switch is mainly for debugging the compiler and will likely be removed in a future version. 💀

• -mmulti-cond-exec, -mno-multi-cond-exec — turn optimization of "&&" and "||" in conditional execution on or off; it is enabled by default.

This switch is mainly for debugging the compiler and will likely be removed in a future version. 💀

• -mnested-cond-exec, -mno-nested-cond-exec — turn nested conditional execution optimizations on or off; it is enabled by default.

This switch is mainly for debugging the compiler and will likely be removed in a future version. 💀

• -moptimize-membar, -mno-optimize-membar — this switch removes redundant "membar" instructions from the compiler-generated code. It is enabled by default.
• -mtomcat-stats — cause gas to print out tomcat statistics.
• -mcpu=cpu — select the processor type for which to generate code. Possible values are frv, fr550, tomcat, fr500, fr450, fr405, fr400, fr300 and simple.

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🌸 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 must provide your own runtime library for whatever I/O functions are needed. 🫶
• -mnodiv — do not use div and mod instructions.
• -mft32b — enable use of the extended instructions of the FT32B processor.
• -mcompress — compress all code using the Ft32B code compression scheme.
• -mnopm — do not generate code that reads program memory.

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🐧 GNU/Linux Options

These -m options are defined for GNU/Linux targets:

• -mglibc — use the GNU C library. This is the default except on *-*-linux-*uclibc*, *-*-linux-*musl* and *-*-linux-*android* targets.
• -muclibc — use uClibc C library. This is the default on *-*-linux-*uclibc* targets.
• -mmusl — use the musl C library. This is the default on *-*-linux-*musl* targets.
• -mbionic — use Bionic C library. This is the default on *-*-linux-*android* targets.
• -mandroid, -mno-android — compile code compatible with Android platform. This is the default on *-*-linux-*android* targets.

When compiling, this option enables -mbionic, -fPIC, -fno-exceptions and -fno-rtti by default. When linking, this option makes the GCC driver pass Android-specific options to the linker. Finally, this option causes the preprocessor macro "__ANDROID__" to be defined.

This option can be disabled completely with -mno-android.

• -tno-android-cc — disable compilation effects of -mandroid, i.e., do not enable -mbionic, -fPIC, -fno-exceptions and -fno-rtti by default.
• -tno-android-ld — disable linking effects of -mandroid, i.e., pass standard Linux linking options to the linker.

🌸 H8/300 Options 🌸

okay so these -m options are the ones defined for the H8/300 implementations, bestie 💖

• -mrelax — shortens some address references at link time when it can, and it does that by using 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 with either -mh or -ms, no cap
• -msx — generate H8SX code 🔥
• -ms2600 — generate code for the H8S/2600. ⚠️ this switch must be used with -ms
• -mquickcall — use registers for argument passing, we love a fast path 🫶
• -mslowbyte — consider access to byte-sized memory slow 🐌
• -mexr, -mno-exr — store extended registers on the stack before execution of functions with the "monitor" attribute. the default is -mexr. this option is valid only for H8S targets 💜
• -mint32 — make "int" data 32 bits by default 👑
• -malign-300 — on the H8/300H and H8S, use the same alignment rules as for the H8/300. the default for the H8/300H and H8S is to align longs and floats on 4-byte boundaries. -malign-300 causes them to be aligned on 2-byte boundaries. this option has no effect on the H8/300 🙈

---

🎀 HPPA Options 🎀

these -m options are defined for the HPPA family of computers, okay so let's get into it ⭐

• -march=architecture-type — generate code for the specified architecture. the choices for architecture-type are 1.0 for PA 1.0, 1.1 for PA 1.1, and 2.0 for PA 2.0 processors. refer to /usr/lib/sched.models on an HP-UX system to 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 (iykyk)
• -mpa-risc-1-0, -mpa-risc-1-1, -mpa-risc-2-0 — synonyms for -march=1.0, -march=1.1, and -march=2.0, respectively. literally the same thing, different fit 💅
• -matomic-libcalls, -mno-atomic-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're 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 🔥

• -mcaller-copies — the caller copies function arguments passed by hidden reference. ⚠️ this option should be used with care, 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 — use ldcw/ldcd coherent cache-control hint 💜
• -mdisable-fpregs — disable floating-point registers. equivalent to -msoft-float 🌸
• -mdisable-indexing — stop 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 does faster indirect calls ⚡

ngl, this option does not work in the presence of shared libraries or nested functions. fr fr 💀

• -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.
• -mgas — enable the use of assembler directives only GAS understands 💅
• -mgnu-ld — use options specific to GNU ld. this passes -shared to ld when building a shared library. it's the default when GCC is configured, explicitly or implicitly, with the GNU linker. this option does not affect which ld is called; it only changes what parameters are passed to that ld. the ld that is called is determined by the --with-ld configure option, GCC's program search path, and finally by the user's PATH. the linker used by GCC can be printed using which \gcc -print-prog-name=ld`. this option is only available on the 64-bit HP-UX GCC, i.e. configured with hppa*64*--hpux` ✨
• -mhp-ld — use options specific to HP ld. this passes -b to ld when building a shared library and passes +Accept TypeMismatch to ld on all links. it's the default when GCC is configured, explicitly or implicitly, with the HP linker. this option does not affect which ld is called; it only changes what parameters are passed to that ld. the ld that is called is determined by the --with-ld configure option, GCC's program search path, and finally by the user's PATH. the linker used by GCC can be printed using which \gcc -print-prog-name=ld`. this option is only available on the 64-bit HP-UX GCC, i.e. configured with hppa*64*--hpux` 💖
• -mlinker-opt — enable the optimization pass in the HP-UX linker. ⚠️ heads up: this makes symbolic debugging impossible 😭
• -mlong-calls — generate code that uses long call sequences. this ensures a call is always able to reach linker generated stubs. the default is to generate long calls only when the distance from the call site to the beginning of the function or translation unit, as the case may be, exceeds a predefined limit set by the branch type being used. the limits for normal calls are 7,600,000 and 240,000 bytes, respectively for the PA 2.0 and PA 1.X architectures. sibling calls are always limited at 240,000 bytes 🌈

distances are measured from the beginning of functions when using the -ffunction-sections option, or when using the -mgas and -mno-portable-runtime options together under HP-UX with the SOM linker.

it's normally not desirable to use this option, because 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 depend 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's 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 +k option to the HP compilers 💅
• -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 are 700 7100, 7100LC, 7200, 7300 and 8000. refer to /usr/lib/sched.models on an HP-UX system to determine the proper scheduling option for your machine. the default scheduling is 8000 ⭐
• -msio, -mwsio — the -msio generates 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 can't 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 🫶

• -msoft-mult — use software integer multiplication. this disables the use of the "xmpyu" instruction 🙈
• -munix=unix-std — generate compiler predefines and select a startfile for the specified UNIX standard. the choices for unix-std are 93, 95 and 98. 93 is supported on all HP-UX versions. 95 is available on HP-UX 10.10 and later. 98 is available on HP-UX 11.11 and later. the default values are 93 for HP-UX 10.00, 95 for HP-UX 10.10 through to 11.00, and 98 for HP-UX 11.11 and later ✨

-munix=93 provides the same predefines as GCC 3.3 and 3.4. -munix=95 provides additional predefines for "XOPEN_UNIX" and "_XOPEN_SOURCE_EXTENDED", and the startfile unix95.o. -munix=98 provides additional predefines for "_XOPEN_UNIX", "_XOPEN_SOURCE_EXTENDED", "_INCLUDE__STDC_A1_SOURCE" and "_INCLUDE_XOPEN_SOURCE_500", and the startfile unix98.o 💖

⚠️ it is important to note that this option changes the interfaces for various library routines. it also affects the operational behavior of the C library. thus, extreme care is needed in using this option. like, be so careful bestie 😭

library code that's intended to operate with more than one UNIX standard must test, set and restore the variable "__xpg4_extended_mask" as appropriate. most GNU software doesn't provide this capability 💀

• -nolibdld — suppress the generation of link options to search libdld.sl when the -static option is specified on HP-UX 10 and later 🌸
• -static — the HP-UX C library implementation of "setlocale" has a dependency on libdld.sl. there isn't an archive version of libdld.sl. thus, when the -static option is specified, special link options are needed to resolve this dependency 💅

On HP-UX 10 and later, the GCC driver adds the necessary options to link with libdld.sl when the -static option is specified. This causes the resulting binary to be dynamic. On the 64-bit port, the linkers generate dynamic binaries by default in any case. The -nolibdld option can be used to prevent the GCC driver from adding these link options.

• -threads — adds support for multithreading with the dce thread library under HP-UX. This option sets flags for both the preprocessor and linker. 🧵

---

🌈 IA-64 Options 🌈

These are the -m options defined for the Intel IA-64 architecture. Yes bestie, Itanium. We're going there. 💅

• -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. The -mgnu-as version is the default. ✨
• -mgnu-ld, -mno-gnu-ld — generate (or don't) code for the GNU linker. Again, -mgnu-ld is the default. 🫶
• -mno-pic — generate code that does not use a global pointer register. Heads up though: the result is not position independent code, and it violates the IA-64 ABI. So like… proceed with caution. 😭
• -mvolatile-asm-stop, -mno-volatile-asm-stop — generate (or don't) a stop bit immediately before and after volatile asm statements. 🛑
• -mregister-names, -mno-register-names — generate (or don't) in, loc, and out register names for the stacked registers. This may make assembler output more readable. ngl that's lowkey a nice quality-of-life thing. 💖
• -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. Useful when compiling kernel code. 👑
• -mauto-pic — generate code that is self-relocatable. This implies -mconstant-gp. 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. 🚫
• -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. (Classic. 🙈)
• *-mfixed-range=register-range* — generate code treating the given register range as fixed registers. A fixed register is one that the register allocator cannot use. This is useful when compiling kernel code. A register range is specified as two registers separated by a dash. Multiple register ranges can be specified separated by a comma. 🎀
• *-mtls-size=tls-size* — specify bit size of immediate TLS offsets. Valid values are 14, 22, and 64. 🔢
• *-mtune=cpu-type* — tune the instruction scheduling for a particular CPU. Valid values are itanium, itanium1, merced, itanium2, and mckinley. (yes, that comma-and-capital in the original is right there, we're keeping it fr fr) 🎛️
• -milp32, -mlp64 — generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long and pointer to 32 bits. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits. These are HP-UX specific flags. 💾
• -mno-sched-br-data-spec, -msched-br-data-spec — (dis/en)able data speculative scheduling before reload. This results in generation of ld.a instructions and the corresponding check instructions (ld.c / chk.a). The default 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-spec enabled. 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-spec enabled. 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-spec enabled. The default setting is enabled. ✨
• -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, -mno-sched-spec-ldc — use a simple data speculation check. This option is on by default. ✅
• -msched-control-spec-ldc, -mno-sched-control-spec-ldc — use a simple check for control speculation. This option is on by default. ✅
• -msched-stop-bits-after-every-cycle, -mno-sched-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, -mno-sched-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, -mno-sel-sched-dont-check-control-spec — generate checks for control speculation in selective scheduling. This flag is disabled by default. 🔍
• *-msched-max-memory-insns=max-insns* — limit on the number of memory insns per instruction group, giving lower priority to subsequent memory insns attempting to schedule in the same instruction group. Frequently useful to prevent cache bank conflicts. The default value is 1. 🎚️
• -msched-max-memory-insns-hard-limit, -mno-sched-max-memory-insns-hard-limit — makes the limit specified by msched-max-memory-insns a hard limit, disallowing more than that number in an instruction group. Otherwise, the limit is "soft", meaning that non-memory operations are preferred when the limit is reached, but memory operations may still be scheduled. 🧚‍♀️

---

🦋 LM32 Options 🦋

These -m options are defined for the LatticeMico32 architecture:

• -mbarrel-shift-enabled — enable barrel-shift instructions. 🌀
• -mdivide-enabled — enable divide and modulus instructions. ➗
• -mmultiply-enabled — enable multiply instructions. ✖️
• -msign-extend-enabled — enable sign extend instructions. 📏
• -muser-enabled — enable user-defined instructions. (custom era 👑)

---

🌸 LoongArch Options 🌸

These command-line options are defined for LoongArch targets:

• *-march=arch-type* — generate instructions for the machine type arch-type. -march=arch-type allows GCC to generate code that may not run at all on processors other than the one indicated. 😬 (fr fr, it's giving "commitment")

The choices for arch-type are:

• native — local processor type detected by the native compiler. 🪞
• 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. 📜
• la32v1.0 — LoongArch32 ISA version 1.0. 📜
• la32rv1.0 — LoongArch32 Reduced ISA version 1.0. 📜

More information about LoongArch ISA versions can be found at <https://github.com/loongson/la-toolchain-conventions>. 🔗

• *-mtune=tune-type* — optimize the generated code for the given processor target. 🎯

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. 💫
• loongarch32 — generic LoongArch 32-bit processor. 🐉

• *-mabi=base-abi-type* — generate code for the specified calling convention. 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, where int is 32 bits, while long int and pointers are 64 bits. 💖
  • lp64f — uses 64-bit general purpose registers and 32-bit floating-point registers for parameter passing. Data model is LP64, where int is 32 bits, while long int and pointers are 64 bits. 💜
  • lp64s — uses 64-bit general purpose registers and no floating-point registers for parameter passing. Data model is LP64, where int is 32 bits, while long int and pointers are 64 bits. 🩷
• *-mfpu=fpu-type* — generate code for the specified FPU type, which can be one of: 🎛️

Okay bestie, finishing off the -mfpu= value list — here's the rest of the tea:

• 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, both for vectorization and via builtin functions. The value can be one of these icons:
  • 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. Like, none. Zero. 💀
• -msoft-float — Forces -mfpu=none and blocks the use of floating-point registers for parameter passing. Heads up: this option may change the target ABI. 👀
• -msingle-float — Forces -mfpu=32 and lets 32-bit floating-point registers be used for parameter passing. Also may change the target ABI, so don't say I didn't warn you.
• -mdouble-float — Forces -mfpu=64 and allows 32/64-bit floating-point registers for parameter passing. Yep, you guessed it: this option may change the target ABI too.
• -mlasx, -mno-lasx, -mlsx, -mno-lsx — These bad boys incrementally adjust the scope of the SIMD extensions (none / LSX / LASX) that the compiler is allowed to use for code generation. Enabling LASX with -mlasx automatically enables LSX, and disabling LSX with -mno-lsx automatically disables LASX. These are driver-only options, and they act upon the final -msimd configuration state, making incremental changes in the order they appear on the GCC driver's command line — deriving the final / canonicalized -msimd option 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 don't trap) on integer division by zero. The default is -mcheck-zero-division for -O0 or -Og, and -mno-check-zero-division for the other optimization levels.
• -mbreak-code=code — Emits a "break" code instruction for irrecoverable traps from "__builtin_trap", or for traps inserted by the compiler (for example, an erroneous path isolated with -fisolate-erroneous-paths-dereference). Alternatively, emits an "amswap.w $r0, $r1, $r0" instruction, which will make the hardware trigger an Instruction Not-defined Exception if code is negative or greater than 32767. The default is -1, which means use the "amswap.w" instruction.
• -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 don't force) the use of "memcpy" for non-trivial block moves. The default is -mno-memcpy, which lets GCC inline most constant-sized copies. Setting the optimization level to -Os also forces the use of "memcpy", but -mno-memcpy may override this behavior if explicitly specified — regardless of the order these options appear on the command line. Sneaky but true. 💅
• -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 — Puts global and static data smaller than num bytes into a small data section. The default value is 0.
• -mmax-inline-memcpy-size=n — Inlines all block moves (like calls to "memcpy" or structure copies) that are less than or equal to n bytes. The default value of n is 1024.
• -mcmodel=code-model — Sets the code model to one of these:
  • 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. This is the default code model, unless GCC has been configured with --with-cmodel= specifying a different default code model.
  • large — (Not implemented yet 🙈)
  • extreme — This mode does not limit the size of the code segment and data segment. The -mcmodel=extreme option is incompatible with -fplt and/or -mexplicit-relocs=none.

Fr fr, only normal, medium and extreme are actually a thing right now — the rest are just vibes.

• -mexplicit-relocs=style, -mexplicit-relocs, -mno-explicit-relocs — 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=none, the assembler macros are always used; with -mexplicit-relocs=always, the assembler relocation operators are always used; and with -mexplicit-relocs=auto, the compiler uses the relocation operators where linker relaxation is impossible (to improve code quality) and macros everywhere else.

The default value for the option is determined by the assembler capability detected during GCC build-time plus the setting of -mrelax: -mexplicit-relocs=none if the assembler doesn't support relocation operators at all; -mexplicit-relocs=always if the assembler supports relocation operators but -mrelax is not enabled; and -mexplicit-relocs=auto if the assembler supports relocation operators and -mrelax is enabled.

For backward compatibility, -mexplicit-relocs is equivalent to -mexplicit-relocs=always, while -mno-explicit-relocs is equivalent to -mexplicit-relocs=none. Classic legacy behavior, honestly.

• -mdirect-extern-access, -mno-direct-extern-access — Controls use of the GOT to access external symbols. The default is -mno-direct-extern-access: the GOT is used for external symbols with default visibility, but not used for other external symbols.

With -mdirect-extern-access, the GOT is not used and all external symbols are PC-relatively addressed. It's only suitable for environments where no dynamic link is performed — like firmwares, OS kernels, or executables linked with -static or -static-pie. Also, -mdirect-extern-access is not compatible with -fPIC or -fpic. Not the moment, okay? 😭

• -mrelax, -mno-relax — Take (or don't take) advantage of linker relaxations. If -mpass-mrelax-to-as is enabled, this option is also passed to the assembler. The default is determined during GCC build-time by detecting corresponding assembler support: -mrelax if the assembler supports both the -mrelax option and conditional branch relaxation — and yes, that's required, or the .align directives and conditional branch instructions in the assembly code output by GCC may get rejected by the assembler because of a relocation overflow — and -mno-relax otherwise.
• -mpass-mrelax-to-as, -mno-pass-mrelax-to-as — Pass (or don't pass) the -mrelax or -mno-relax option to the assembler. The default is determined during GCC build-time by detecting corresponding assembler support: -mpass-mrelax-to-as if the assembler supports the -mrelax option, -mno-pass-mrelax-to-as otherwise. This option is mostly useful for debugging, or for interoperating with assemblers different from the build-time one. Debug girlies, this one's for you. 🫶
• -mrecip — Enables use of the reciprocal estimate and reciprocal square root estimate instructions with extra 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-optimizations is enabled together with -ffinite-math-only and -fno-trapping-math. This option is off by default. Before you can use it, you gotta make sure the target CPU supports the "frecipe" and "frsqrte" instructions. And note: 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). Math is humbling. 😭
• -mrecip=opt — 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 — Enable the approximation for vectorized reciprocal square root.

So, for example, -mrecip=all,!sqrt enables all of the reciprocal approximations except for scalar square root. Chef's kiss selection menu. 🎀

• -mfrecipe, -mno-frecipe — Use (or don't use) "frecipe.{s/d}" and "frsqrte.{s/d}" instructions. When compiling with -march=la664, it's enabled by default. Otherwise the default is -mno-frecipe.
• -mdiv32, -mno-div32 — Use (or don't use) "div.w[u]" and "mod.w[u]" instructions with input that isn't sign-extended. When compiling with -march=la664, it's enabled by default. Otherwise the default is -mno-div32.
• -mlam-bh, -mno-lam-bh — Use (or don't use) "am{swap/add}[_db].{b/h}" instructions. When compiling with -march=la664, it's enabled by default. Otherwise the default is -mno-lam-bh.
• -mlamcas, -mno-lamcas — Use (or don't use) "amcas[_db].{b/h/w/d}" instructions. When compiling with -march=la664, it's enabled by default. Otherwise the default is -mno-lamcas.
• -mld-seq-sa, -mno-ld-seq-sa — Controls whether a same-address load-load barrier ("dbar 0x700") is needed. When compiling with -march=la664, it's enabled by default. Otherwise the default is -mno-ld-seq-sa, meaning the load-load barrier is needed. Lowkey chaotic naming, but okay. 💀
• -mscq, -mno-scq — Use (or don't use) the 16-byte conditional store instruction "sc.q". The default is -mscq if the machine type specified with -march= supports this instruction, and -mno-scq otherwise. ✨
• -mtls-dialect=opt — this one controls which TLS dialect may be used for the general dynamic and local dynamic TLS models 🌸. the opt argument is literally one of these two:
  • trad — use traditional TLS. this is the default, no surprises 💅
  • desc — use TLS descriptors. fancy mode ✨
• -mannotate-tablejump, -mno-annotate-tablejump — creates an annotation section called .discard.tablejump_annotate so you can correlate the jirl instruction with the jump table whenever a jump table gets used to optimize a switch statement 👑. some external tools — for example objtool from the Linux kernel building system — genuinely need that annotation to analyze the control flow. the default is -mno-annotate-tablejump, so no annotation unless you specifically ask for it 🎀.

🐱 LynxOS Options

these ones are available for LynxOS targets, bestie 💖

• -mshared — use shared libraries. the default is to link with static libraries instead.
• -mthreads — support multi-threading 🧵.
• -mlegacy-threads — support legacy multi-threading. heads up: this option is incompatible with -mthreads 💀.

🍓 M32C Options

• -mcpu=name — selects the CPU that code gets generated for. name can be r8c for the R8C/Tiny series, m16c for the M16C (up to /60) series, m32cm for the M16C/80 series, or m32c for the M32C/80 series 🌈.
• -msim — says the program is gonna run on the simulator. this causes an alternate runtime library to be linked in, one that supports stuff like file I/O 🫶. you must not use this option when generating programs that will run on real hardware — for that you have to provide your own runtime library for whatever I/O functions you need 🙈.
• -memregs=number — specifies the number of memory-based pseudo-registers GCC uses during code generation. these pseudo-registers get used like real registers, so it's a whole tradeoff between GCC's ability to fit the code into the available registers and the performance penalty of using memory instead of registers 😭. note that every single module in a program must be compiled with the same value for this option, and 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, periodt 👑

• -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 — assumes all objects live in the lower 16MB of memory (so their addresses can be loaded with the ld24 instruction), and assumes all subroutines are reachable with the bl instruction. this is the default 💅. the addressability of a particular object can be set with the model attribute.
• -mmodel=medium — assumes objects may be anywhere in the 32-bit address space (the compiler generates seth/add3 instructions to load their addresses), and assumes all subroutines are reachable with the bl instruction.
• -mmodel=large — assumes objects may be anywhere in the 32-bit address space (compiler generates seth/add3 instructions to load their addresses), and assumes subroutines may not be reachable with the bl instruction — so the compiler generates the much slower seth/add3/jl instruction sequence 🐌.
• -msdata=none — disables use of the small data area. variables get put into one of .data, .bss, or .rodata (unless the section attribute has been specified). this is the default. the small data area consists of sections .sdata and .sbss, and objects may be explicitly put in the small data area with the section attribute using one of these sections 🎀.
• -msdata=sdata — puts small global and static data in the small data area, but does not generate special code to reference them.
• -msdata=use — puts small global and static data in the small data area, and does generate special instructions to reference them ✨.
• -G num — puts global and static objects less than or equal to num bytes into the small data or BSS sections instead of the normal data or BSS sections. the default value of num is 8. the -msdata option must be set to one of sdata or use for this option to have any effect. all modules should be compiled with the same -G num value — compiling with different values of num may or may not work; if it doesn't, the linker gives an error message, and incorrect code is not generated 🙏.
• -mdebug — makes the M32R-specific code in the compiler display some statistics that might help in debugging programs 🔍.
• -malign-loops, -mno-align-loops — align all loops to a 32-byte boundary. this option is disabled by 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, then branches are preferred over conditional code; if it's 2, then the opposite applies 💅.
• -mflush-trap=number — specifies the trap number to use to flush the cache. the default is 12. valid numbers are between 0 and 15 inclusive.
• -mno-flush-trap — specifies that the cache cannot be flushed by using a trap 🚫.
• -mflush-func=name — specifies the name of the operating system function to call to flush the cache. the default is _flush_cache, but a function call is only used if a trap is not available.
• -mno-flush-func — indicates that there is no OS function for flushing the cache.

👑 M680x0 Options

these are the -m options defined for M680x0 and ColdFire processors 🌈. the default settings depend on which architecture was selected when the compiler was configured; the defaults for the most common choices are given below.

• -march=arch — generates code for a specific M680x0 or ColdFire instruction set architecture. permissible values of arch for M680x0 architectures are: 68000, 68010, 68020, 68030, 68040, 68060 and cpu32. ColdFire architectures are selected according to Freescale's ISA classification, and the permissible values are: isaa, isaaplus, isab and isac 🎀. GCC defines a macro __mcfarch__ whenever it is generating code for a ColdFire target — the arch in this macro is one of the -march arguments given above. when used together, -march and -mtune select code that runs on a family of similar processors but that is optimized for a particular microarchitecture ✨.
• -mcpu=cpu — generates code for a specific M680x0 or ColdFire processor. the M680x0 cpus are: 68000, 68010, 68020, 68030, 68040, 68060, 68302, 68332 and cpu32. the ColdFire cpus are given by the list below, which also classifies the CPUs into families 💖:
  • 51 → 51, 51ac, 51ag, 51cn, 51em, 51je, 51jf, 51jg, 51jm, 51mm, 51qe, 51qm
  • 5206 → 5202, 5204, 5206
  • 5206e → 5206e
  • 5208 → 5207, 5208
  • 5211a → 5210a, 5211a
  • 5213 → 5211, 5212, 5213
  • 5216 → 5214, 5216
  • 52235 → 52230, 52231, 52232, 52233, 52234, 52235
  • 5225 → 5224, 5225
  • 52259 → 52252, 52254, 52255, 52256, 52258, 52259
  • 5235 → 5232, 5233, 5234, 5235, 523x
  • 5249 → 5249
  • 5250 → 5250
  • 5271 → 5270, 5271
  • 5272 → 5272
  • 5275 → 5274, 5275
  • 5282 → 5280, 5281, 5282, 528x
  • 53017 → 53011, 53012, 53013, 53014, 53015, 53016, 53017
  • 5307 → 5307
  • 5329 → 5327, 5328, 5329, 532x
  • 5373 → 5372, 5373, 537x
  • 5407 → 5407
  • 5475 → 5470, 5471, 5472, 5473, 5474, 5475, 547x, 5480, 5481, 5482, 5483, 5484, 5485

-mcpu=cpu overrides -march=arch if arch is compatible with cpu; other combinations of -mcpu and -march are rejected 🚫. GCC defines the macro __mcf_cpu_cpu when ColdFire target cpu is selected, and it also defines __mcf_family_family, where the value of family is given by the list above.

• -mtune=tune — tunes the code for a particular microarchitecture within the constraints set by -march and -mcpu. the M680x0 microarchitectures are: 68000, 68010, 68020, 68030, 68040, 68060 and cpu32. the ColdFire microarchitectures are: cfv1, cfv2, cfv3, cfv4 and cfv4e 🦋. you can also use -mtune=68020-40 for code that needs to run relatively well on 68020, 68030 and 68040 targets; -mtune=68020-60 is similar but includes 68060 targets as well. these two options select the same tuning decisions as -m68020-40 and -m68020-60 respectively. GCC defines the macros __mcarch and __mcarch__ when tuning for 680x0 architecture arch, and 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. equivalent to -march=68010.
• -m68020, -mc68020 — generate output for a 68020. this is the default when the compiler is configured for 68020-based systems. equivalent to -march=68020.
• -m68030 — generate output for a 68030. this is the default when the compiler is configured for 68030-based systems. equivalent to -march=68030.
• -m68040 — generate output for a 68040. this is the default when the compiler is configured for 68040-based systems. equivalent to -march=68040. this option inhibits the use of 68881/68882 instructions that have to be emulated by software on the 68040 — use this option if your 68040 does not have code to emulate those instructions 🙈.
• -m68060 — generate output for a 68060. this is the default when the compiler is configured for 68060-based systems. equivalent to -march=68060. this option inhibits the use of 68020 and 68881/68882 instructions that have to be emulated by software on the 68060 — use this option if your 68060 does not have code to emulate those instructions 🔥.
• -m68302 — generate code for a 68302.
• -m68332 — generate code for a 68332.
• -m68851 — okay so the 68851 was an external MMU for the 68020 processor. GCC accepts this option bestie… but it literally does not use it 💀 like it just sits there.
• -mcpu32 — generates output for a CPU32. This is the default when the compiler is configured for CPU32-based systems, and it's equivalent to -march=cpu32.

Use this one for microcontrollers with a CPU32 or CPU32+ core — the 68330, 68331, 68332, 68333, 68334, 68336, 68340, 68341, 68349 and 68360 are all in the group chat 🫶

• -mfidoa — generates code for a Fido A. That's it. That's the whole vibe 🎀
• -m5200 — generates 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 (she's aging, let her rest).

Use it for microcontrollers with a 5200 core, including the MCF5202, MCF5203, MCF5204 and MCF5206.

• -m5206e — generates output for a 5206e ColdFire CPU. Now deprecated in favor of the equivalent -mcpu=5206e ✨
• -m528x — generates output for a member of the ColdFire 528X family. Now deprecated in favor of the equivalent -mcpu=528x.
• -m5307 — generates output for a ColdFire 5307 CPU. Now deprecated in favor of the equivalent -mcpu=5307 💅
• -m5407 — generates output for a ColdFire 5407 CPU. Now deprecated in favor of the equivalent -mcpu=5407.
• -mcfv4e — generates output for a ColdFire V4e family CPU (e.g. 547x/548x). This includes using hardware floating-point instructions 🎀 It's equivalent to -mcpu=547x, and yup, now deprecated in favor of that option.
• -m68020-40 — generates 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 ✨

This option is equivalent to -march=68020 -mtune=68020-40.

• -m68020-60 — generates output for a 68060 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 68060.

This option is equivalent to -march=68020 -mtune=68020-60.

• -mhard-float, -m68881 — generate floating-point instructions. 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. Default for 68000, 68010, and 68832 targets. Also the default for ColdFire devices that have no FPU — no FPU, no problem, we call the library 🫶
• -mdiv, -mno-div — generate (or don't generate) ColdFire hardware divide and remainder instructions. If -march is used without -mcpu, the default is "on" for ColdFire architectures and "off" for M680x0 architectures. Otherwise the default comes from the target CPU (either the default CPU, or the one specified by -mcpu). For example, the default is "off" for -mcpu=5206 and "on" for -mcpu=5206e.

GCC defines the macro __mcfhwdiv__ when this option is enabled 👑

• -mshort, -mno-short, -mnoshort — treat type int as 16 bits wide, like short int. Also, parameters passed on the stack are aligned to a 16-bit boundary even on targets whose API mandates promotion to 32-bit. This option is disabled by default.
• -mbitfield, -mno-bitfield, -mnobitfield — control use of the bit-field instructions. The -m68000, -mcpu32 and -m5200 options imply -mnobitfield; the -m68020 option implies -mbitfield 🎀
• -mrtd, -mno-rtd, -mnortd — control use of a different function-calling convention, where functions that take a fixed number of arguments return with the rtd instruction, which pops their arguments while returning. This saves one instruction in the caller since there's no need to pop the arguments there. Efficiency slay ✨

⚠️ 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 aligns int, long, long long, float, double, and long double variables on a 32-bit boundary (-malign-int) or a 16-bit boundary (-mno-align-int). Aligning variables on 32-bit boundaries produces code that runs somewhat faster on processors with 32-bit busses, at the expense of more memory 💸

⚠️ Warning: if you use the -malign-int switch, GCC aligns structures containing the above types differently than most published application binary interface specifications for the m68k. Not a vibe, but it is what it is.

• -mpcrel — use the pc-relative addressing mode of the 68000 directly, instead of using a global offset table. At present, this option implies -fpic, allowing at most a 16-bit offset for pc-relative addressing. -fPIC is not presently supported with -mpcrel, though this could be supported for 68020 and higher processors 👀
• -mno-strict-align, -mstrict-align — do not (do) assume that unaligned memory references are handled by the system.
• -msep-data, -mno-sep-data — with -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 🌈

This option is disabled by default; GCC generates code that assumes the data segment follows the text segment.

• -mid-shared-library, -mno-id-shared-library — if enabled, 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 ✨

This option is disabled by default.

• -mshared-library-id=n — specifies the identification number of the ID-based shared library being compiled. Specifying a value of 0 generates more compact code; specifying other values forces the allocation of that number to the current library, but is no more space- or time-efficient than omitting this option 🎀
• -mxgot, -mno-xgot — when generating position-independent code for ColdFire, generate code that works if the GOT has more than 8192 entries. This code is larger and slower than code generated without this option. On M680x0 processors, this option is not needed; -fPIC suffices 💅

GCC normally uses a single instruction to load values from the GOT. While this is relatively efficient, it only works if the GOT is smaller than about 64k. Anything larger causes the linker to report an error such as:

relocation truncated to fit: R_68K_GOT16O foobar

If this happens, you should recompile your code with -mxgot. It should then work with very large GOTs. However, code generated with -mxgot is less efficient, since it takes 4 instructions to fetch the value of a global symbol 😭

Note that some linkers, including newer versions of the GNU linker, can create multiple GOTs and sort GOT entries. If you have such a linker, you should only need to use -mxgot when compiling a single object file that accesses more than 8192 GOT entries. Very few do, ngl.

These options have no effect unless GCC is generating position-independent code.

• -mxtls — support a TLS segment larger than 64K. The considerations for using this option are similar to those for -mxgot above; the default 16-bit offset addressing for TLS is more efficient, but this may be inadequate for some programs 🦋
• -mlong-jump-table-offsets — use 32-bit offsets in switch tables. 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-immediates, -mno-relax-immediates — allow arbitrary-sized immediates in bit operations.
• -mwide-bitfields, -mno-wide-bitfields — always treat bit-fields as int-sized.
• -m4byte-functions, -mno-4byte-functions — force all functions to be aligned to a 4-byte boundary.
• -mcallgraph-data, -mno-callgraph-data — emit callgraph information.
• -mslow-bytes, -mno-slow-bytes — prefer word access when reading byte quantities.
• -mlittle-endian, -mbig-endian — generate code for a little- or big-endian target, respectively. The default is big-endian. Little-endian code is supported only with -m340 🎀
• -m210 — generate code for the 210 processor.
• -m340 — generate code for the 340 processor.
• -mno-lsim — assume that runtime support has been provided and so omit the simulator library (libsim.a) from the linker command line.
• -mstack-increment=size — set the maximum amount for a single stack increment operation. Large values can increase the speed of programs that contain functions that need a large amount of stack space, but they can also trigger a segmentation fault if the stack is extended too much 😭 The default value is 0x1000.

---

✨ MicroBlaze Options ✨

• -msoft-float — use software emulation for floating point (default).
• -mhard-float — use hardware floating-point instructions 🔥
• -mmemcpy — do not optimize block moves, use memcpy.
• -mcpu=cpu-type — use features of, and schedule code for, the given CPU. Supported values are in the format vX.YY.Z, where X is a major version, YY is the minor version, and Z is compatibility code. Example values are v3.00.a, v4.00.b, v5.00.a, v5.00.b, v6.00.a ✨
• -mxl-soft-mul, -mno-xl-soft-mul — use software multiply emulation 💅 this one's on by default, so no cap, you already got it.
• -mxl-soft-div, -mno-xl-soft-div — software emulation for divides 🍓 also enabled by default. built-in vibes.
• -mxl-barrel-shift — use the hardware barrel shifter ⚡
• -mxl-pattern-compare — use pattern compare instructions 🎀
• -msmall-divides — table lookup optimization for small signed integer divisions ✨ tiny math, big slay.
• -mxl-gp-opt — use GP-relative .sdata/.sbss sections 🫶
• -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 🎀 valid models are:
  • executable — normal executable (the default), uses startup code crt0.o.
  • xmdstub — for use with the Xilinx Microprocessor Debugger (XMD) based software intrusive debug agent called xmdstub. this one uses startup file crt1.o and sets the start address of the program to 0x800 💀
  • bootstrap — for applications that are loaded using a bootloader. this model uses startup file crt2.o, which does not contain a processor reset vector handler. it's suitable for transferring control on a processor reset to the bootloader rather than the application 🫶
  • novectors — for applications that don't require any of the MicroBlaze vectors. this option may be useful for applications running within a monitoring application. this model uses crt3.o as a startup file ⭐
• -mxl-prefetch — enable insertion of prefetch ("wic") instructions at call sites 🌈
• -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 💜

---

🌊 MIPS Options 🌈

• -EB, -meb — generate big-endian code 👑
• -EL, -mel — generate little-endian code. this is the default for mips*el-*-* configurations 🧚
• -march=arch — generate code that runs on arch, which can be the name of a generic MIPS ISA or the name of a particular processor. the ISA names are: mips1, mips2, mips3, mips4, mips32, mips32r2, mips32r3, mips32r5, mips32r6, mips64, mips64r2, mips64r3, mips64r5 and mips64r6.

the processor names are:

4kc, 4km, 4kp, 4ksc, 4kec, 4kem, 4kep, 4ksd,
5kc, 5kf, 20kc, 24kc, 24kf2_1, 24kf1_1, 24kec, 24kef2_1, 24kef1_1,
34kc, 34kf2_1, 34kf1_1, 34kn, 74kc, 74kf2_1, 74kf1_1, 74kf3_2,
1004kc, 1004kf2_1, 1004kf1_1, 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.  allegrex

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" 🌸

bestie, 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"). and 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, it's possible to generate code that runs on a family of processors but optimize the code for one particular member of that family 👑

-mtune defines the macros "_MIPS_TUNE" and "_MIPS_TUNE_foo", which work in the same way as the -march ones described above 💜

• -mips1 — equivalent to -march=mips1.
• -mips2 — equivalent to -march=mips2.
• -mips3 — equivalent to -march=mips3.
• -mips4 — equivalent to -march=mips4.
• -mips32 — equivalent to -march=mips32.
• -mips32r3 — equivalent to -march=mips32r3.
• -mips32r5 — equivalent to -march=mips32r5.
• -mips32r6 — equivalent to -march=mips32r6.
• -mips64 — equivalent to -march=mips64.
• -mips64r2 — equivalent to -march=mips64r2.
• -mips64r3 — equivalent to -march=mips64r3.
• -mips64r5 — equivalent to -march=mips64r5.
• -mips64r6 — equivalent to -march=mips64r6.
• -mips16, -mno-mips16 — generate (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 "mips16" and "nomips16" attributes 🎀

• -mmips16e2, -mno-mips16e2 — use (or don't use) the MIPS16e2 ASE. this option modifies the behavior of the -mips16 option such that it targets the MIPS16e2 ASE 🦋
• -mflip-mips16, -mflip-mips16 — generate MIPS16 code on alternating functions. (yes, it's listed twice in the source, we don't ask questions 💀) this option is provided for regression testing of mixed MIPS16/non-MIPS16 code generation, and is not intended for ordinary use in compiling user code 🙈
• -minterlink-compressed, -mno-interlink-compressed — require (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 must 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-compressed and -mno-interlink-compressed. these options predate the microMIPS ASE and are retained for backwards compatibility 🌸
• -mabi=32, -mabi=o64, -mabi=n32, -mabi=64, -mabi=eabi — generate code for the given ABI 🎀

note that the EABI has a 32-bit and a 64-bit variant. GCC normally generates 64-bit code when you select a 64-bit architecture, but you can use -mgp32 to get 32-bit code instead 💖

for information about the O64 ABI, see 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 and is therefore only supported for MIPS32R2, MIPS32R3 and MIPS32R5 processors ⚡

the register assignments for arguments and return values remain the same, but each scalar value is passed in a single 64-bit register rather than a pair of 32-bit registers. for example, scalar floating-point values are returned in $f0 only, not a $f0/$f1 pair. the set of call-saved registers also remains the same in that the even-numbered double-precision registers are saved 👑

two additional variants of the o32 ABI are supported to enable a transition from 32-bit to 64-bit registers. these are FPXX (-mfpxx) and FP64A (-mfp64 -mno-odd-spreg). the FPXX extension mandates that all code must execute correctly when run using 32-bit or 64-bit registers. the code can be interlinked with either FP32 or FP64, but not both. the FP64A extension is similar to the FP64 extension but forbids the use of odd-numbered single-precision registers. this can be used in conjunction with the "FRE" mode of FPUs in MIPS32R5 processors and allows both FP32 and FP64A code to interlink and run in the same process without changing FPU modes 🧚

• -mabicalls, -mno-abicalls — generate (or don't generate) code that is suitable for SVR4-style dynamic objects. -mabicalls is the default for SVR4-based systems 🍓
• -mshared, -mno-shared — generate (or don't generate) code that is fully position-independent and that can therefore be linked into shared libraries. this option only affects -mabicalls 💅

all -mabicalls code has traditionally been position-independent, regardless of options like -fPIC and -fpic. however, as an extension, the GNU toolchain allows executables to use absolute accesses for locally-binding symbols. it can also use shorter GP initialization sequences and generate direct calls to locally-defined functions. this mode is selected by -mno-shared 🔥

-mno-shared depends on binutils 2.16 or higher and generates objects that can only be linked by the GNU linker. however, the option does not affect the ABI of the final executable; it only affects the ABI of relocatable objects. using -mno-shared generally makes executables both smaller and quicker ✨

-mshared is the default.

• -mplt, -mno-plt — assume (or don't assume, bestie) that the static and dynamic linkers support PLTs and copy relocations. This one only does anything for -mno-shared -mabicalls. And heads up for the n64 ABI: it's a total no-op unless you also pass -msym32.

You can make -mplt the default by configuring GCC with --with-mips-plt. Otherwise the default is -mno-plt, periodt.

• -mxgot, -mno-xgot — lift (or don't lift) the usual restrictions on how big the global offset table can be.

Normally GCC loads values from the GOT with a single instruction. Cute and efficient, yes — but it only works while the GOT is smaller than about 64k. Anything bigger and the linker gets all dramatic and hits you with something like:

relocation truncated to fit: R_MIPS_GOT16 foobar

If that happens, recompile your code with -mxgot. It handles very large GOTs just fine, but the code is less efficient too — it takes three instructions to fetch the value of a global symbol. Trade-off, babes.

Note: some linkers can create multiple GOTs. If yours does, you should only need -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 general-purpose registers are 32 bits wide.
• -mgp64 — assume general-purpose registers are 64 bits wide.
• -mfp32 — assume floating-point registers are 32 bits wide.
• -mfp64 — assume floating-point registers are 64 bits wide.
• -mfpxx — do not assume the width of floating-point registers. Just ✨live your truth✨.
• -mhard-float — use floating-point coprocessor instructions.
• -msoft-float — do not use floating-point coprocessor instructions. Do your floating-point math with library calls instead.
• -mno-float — same energy as -msoft-float, but it additionally asserts that the program being compiled performs zero floating-point operations. Right now this one's only supported by some bare-metal MIPS configurations, where it may pick a special set of libraries that lack all floating-point support (including, like, the floating-point "printf" formats 🙈). If code compiled with -mno-float accidentally contains floating-point operations, it is likely to suffer a link-time or run-time failure. So like… don't.
• -msingle-float — assume the floating-point coprocessor only supports single-precision operations.
• -mdouble-float — assume 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 known to support these registers. When you're using the o32 FPXX ABI, -mno-odd-spreg is set by default.
• -mabs=2008, -mabs=legacy — these control how the special not-a-number (NaN) IEEE 754 floating-point data gets treated by the "abs.fmt" and "neg.fmt" machine instructions.

By default, or when -mabs=legacy is used, the legacy treatment is selected. In that case these instructions count as arithmetic and get avoided wherever correct operation is required and the input operand might be a NaN. Instead, a longer sequence of instructions that manually manipulate the sign bit of the floating-point datum is used — unless you also specified -ffinite-math-only.

The -mabs=2008 option selects the IEEE 754-2008 treatment. Now these instructions count as non-arithmetic and therefore operate correctly in all cases, including in particular when the input operand is a NaN. So they're always used for the respective operations. Slay.

• -mnan=2008, -mnan=legacy — these control the encoding of the special not-a-number (NaN) IEEE 754 floating-point data.

-mnan=legacy 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.

-mnan=2008 selects the IEEE 754-2008 encoding. Here it flips: 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) the ll, sc and sync instructions to implement atomic memory built-in functions. When neither option is specified, GCC uses the instructions if the target architecture supports them.

-mllsc is useful if the runtime environment can emulate the instructions, and -mno-llsc can be useful when compiling for nonstandard ISAs. You can make either option the default by configuring GCC with --with-llsc and --without-llsc respectively. --with-llsc is the default for some configurations — check 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 one can only be used when generating 64-bit code, and it requires hardware floating-point support to be enabled.
• -mips3d, -mno-mips3d — use (or don't use) the MIPS-3D ASE. Heads up: -mips3d implies -mpaired-single.
• -mmicromips, -mno-micromips — generate (or don't generate) microMIPS code.

MicroMIPS code generation can also be controlled on a per-function basis, by way of the "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.
• -mmsa, -mno-msa — use (or don't use) the MIPS MSA extension 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 — force "long" types to be 64 bits wide. See -mlong32 for an explanation of the default and the way the pointer size is determined.
• -mlong32 — force "long", "int" and pointer types to be 32 bits wide.

The default size of "int"s, "long"s and pointers depends on the ABI. All the supported ABIs use 32-bit "int"s. The n64 ABI uses 64-bit "long"s, as does the 64-bit EABI; the others use 32-bit "long"s. Pointers are the same size as "long"s, or the same size as integer registers, whichever is smaller.

• -msym32, -mno-sym32 — assume (or don't assume) that all symbols have 32-bit values, no matter which ABI got selected. This one's a cute combo with -mabi=64 and -mno-abicalls, because it lets GCC generate shorter and faster references to symbolic addresses.
• -G num — put definitions of externally-visible data in a small data section if that data is no bigger than num bytes. Then GCC can generate more efficient accesses to the data; peep -mgpopt for the details.

The default -G option depends on the configuration.

• -mlocal-sdata, -mno-local-sdata — extend (or don't extend) the -G behavior to local data too, like static variables in C. -mlocal-sdata is the default for all configurations.

If the linker starts complaining 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 the libraries leave more room for the main program. Teamwork 💖

• -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 -G limit. -mextern-sdata is the default for all configurations.

If you compile a module Mod with -mextern-sdata -G num -mgpopt, and Mod references a variable Var that is no bigger than num bytes, you must make sure that Var is placed in a small data section. If Var is defined by another module, you must either compile that module with a high-enough -G setting or attach a "section" attribute to Var's definition. If Var is common, you must link the application with a high-enough -G setting.

The easiest way of satisfying these restrictions is to compile and link every module with the same -G option. However, you may wish to build a library that supports several different small data limits. You can do this by compiling the library with the highest supported -G setting and additionally using -mno-extern-sdata to stop the library from making assumptions about externally-defined data.

• -mgpopt, -mno-gpopt — use (or don't use) GP-relative accesses for symbols that are known to be in a small data section; see -G, -mlocal-sdata and -mextern-sdata. -mgpopt is the default for all configurations.
• -mno-gpopt — lowkey a lifesaver when the $gp register might not actually hold the value of "_gp". 💅 Like, say the code is part of a library that might get used in a boot monitor — programs that call boot monitor routines pass some unknown value in $gp. (In those situations, the boot monitor itself is usually compiled with -G0. iykyk.)

Also, -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. Trade-off girlie things. 🎀
• -muninit-const-in-rodata, -mno-uninit-const-in-rodata — put uninitialized "const" variables in the read-only data section. Heads up though: this option is only meaningful in conjunction with -membedded-data. 🦋
• *-mcode-readable=setting* — specify whether GCC may generate code that reads from executable sections. There are three possible settings, bestie:
  • -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. 💀

On SDE targets, -mcode-data-in-code is available as a traditional alias for -mcode-readable=no, and -mcode-xonly for -mcode-readable=pcrel. ⭐

• -msplit-addresses, -mno-split-addresses — enable (disable) use of the %hi() and %lo() assembler relocation operators. Ngl this option has been superseded by -mexplicit-relocs but it's retained for backwards compatibility. 🙈
• -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. History lesson slay 💖

• -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 — and this is the messy part — some versions of the Linux kernel have a bug that prevents trap from generating the proper signal ("SIGFPE"). 😭 Use -mdivide-traps to allow conditional traps on architectures that support them and -mdivide-breaks to force the use of breaks.

The default is usually -mdivide-traps, but this can be overridden at configure time using --with-divide=breaks. Divide-by-zero checks can be completely disabled using -mno-check-zero-division. 💅

• -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. 🔗
• -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, either by hardware or by trapping and emulation. In the latter case -mstrict-align may be needed by the operating system kernel. 👀 The options -munaligned-access and -mno-unaligned-access are obsolete, and only provided for backward compatibility.
• -mmemcpy, -mno-memcpy — force (do not force) the use of "memcpy" for non-trivial block moves. The default is -mno-memcpy, which allows GCC to inline most constant-sized copies. 📦
• -mlong-calls, -mno-long-calls — disable (do not disable) use of the "jal" instruction. Calling functions using "jal" is more efficient but requires the caller and callee to be in the same 256 megabyte segment. 📏

This option has no effect on abicalls code. The default is -mno-long-calls.

• -mmad, -mno-mad — enable (disable) use of the "mad", "madu" and "mul" instructions, as provided by the R4650 ISA. 🔥
• -mimadd, -mno-imadd — enable (disable) use of the "madd" and "msub" integer instructions. The default is -mimadd on architectures that support "madd" and "msub" — except for the 74k architecture where it was found to generate slower code. 💀
• -mfused-madd, -mno-fused-madd — enable (disable) use of the floating-point multiply-accumulate instructions, when they are available. The default is -mfused-madd. 💖

On the R8000 CPU when multiply-accumulate instructions are used, the intermediate product is calculated to infinite precision and is not subject to the FCSR Flush to Zero bit. This may be undesirable in some circumstances. 👀 On other processors the result is numerically identical to the equivalent computation using separate multiply, add, subtract and negate instructions.

• -nocpp — tell the MIPS assembler to not run its preprocessor over user assembler files (with a .s suffix) when assembling them. 🙅‍♀️
• -mfix-24k, -mno-fix-24k — work around the 24K E48 (lost data on stores during refill) errata. The workarounds are implemented by the assembler rather than by GCC. 🔧
• -mfix-r4000, -mno-fix-r4000 — work around certain R4000 CPU errata:
  • A double-word or a variable shift may give an incorrect result if executed immediately after starting an integer division. 😬
  • A double-word or a variable shift may give an incorrect result if executed while an integer multiplication is in progress.
  • An integer division may give an incorrect result if started in a delay slot of a taken branch or a jump.
• -mfix-r4400, -mno-fix-r4400 — work around certain R4400 CPU errata:
  • A double-word or a variable shift may give an incorrect result if executed immediately after starting an integer division.
• -mfix-r10000, -mno-fix-r10000 — work around certain R10000 errata:
  • "ll"/"sc" sequences may not behave atomically on revisions prior to 3.0. They may deadlock on revisions 2.6 and earlier. 💀⚠️

This option can only be used if the target architecture supports branch-likely instructions. -mfix-r10000 is the default when -march=r10000 is used; -mno-fix-r10000 is the default otherwise.

• -mfix-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, -mno-fix-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.) 🐛
• -mfix4300, -mno-fix4300 — work around a bug in early VR4300 silicon that causes multiplies with certain operands to corrupt immediately following multiplies. 🙈
• *-mr10k-cache-barrier=setting* — specify whether GCC should insert cache barriers to avoid the side effects of speculation on R10K processors. 🧠

In common with many processors, the R10K tries to predict the outcome of a conditional branch and speculatively executes instructions from the "taken" branch. It later aborts these instructions if the predicted outcome is wrong. However, on the R10K, even aborted instructions can have side effects. 💀

This problem only affects kernel stores and, depending on the system, kernel loads. As an example, a speculatively-executed store may load the target memory into cache and mark the cache line as dirty, even if the store itself is later aborted. If a DMA operation writes to the same area of memory before the "dirty" line is flushed, the cached data overwrites the DMA-ed data. 😱 See the R10K processor manual for a full description, including other potential problems.

One workaround is to insert cache barrier instructions before every memory access that might be speculatively executed and that might have side effects even if aborted. -mr10k-cache-barrier=setting controls GCC's implementation of this workaround. It assumes that aborted accesses to any byte in the following regions does not have side effects: 👇

1. the memory that the current function's stack frame is squatting in 🏠;
2. the memory an incoming stack argument is parked in 📦;
3. the memory of an object whose address is a link-time constant 🔗.

And the vibe check on safety? That's the kernel's job, bestie — it's on the kernel to make sure speculative peeks at those regions are genuinely safe. GCC is just out here trusting the system like that 🫶.

So if the input program has a function declaration like this:

void foo (void);

then whatever ends up implementing foo must let j foo and jal foo be executed speculatively. GCC plays nice about this for any function it compiles itself, but for non-GCC functions (hand-written assembly, we see you 🙈) it just expects them to do the same. Rude? Maybe. But that's the contract.

The option comes in three flavors:

• -mr10k-cache-barrier=load-store — drop a cache barrier before a load or a store that might get speculatively executed and that might have side effects even if it gets aborted 🚧.
• -mr10k-cache-barrier=store — drop a cache barrier only before a store that might get speculatively executed and that might have side effects even if it gets aborted 🏪.
• -mr10k-cache-barrier=none — no cache barriers at all, none inserted, zero. This is the default setting 💤.
• -mflush-func= func
• -mno-flush-func — tells GCC which function to call to flush the I and D caches, or to not call any such function at all 🧹. If it is called, that function has to take the same arguments as the common _flush_func, and those are: the address of the memory range being flushed, the size of the memory range, and the number 3 (that's the "flush both caches" flag 💅). The default depends on the target GCC was configured for, but it's commonly either _flush_func or __cpu_flush.
• -mbranch-cost= num — sets the cost of branches to roughly num "simple" instructions 💸. Heads up: this cost is only a heuristic, so it's genuinely not guaranteed to give you consistent results across releases — it can just. shift. The cost of zero redundantly selects the default, which is based on the -mtune setting.
• -mbranch-likely
• -mno-branch-likely — turn use of Branch Likely instructions on or off, no matter what the default for the selected architecture says 🎚️. By default, Branch Likely instructions may be generated if the selected architecture supports them. Exception alert 🚨: for the MIPS32 and MIPS64 architectures, and the processors that implement those architectures, Branch Likely instructions aren't generated by default, because MIPS32 and MIPS64 specifically deprecate their use. Respect the deprecation, babes.
• -mcompact-branches=never
• -mcompact-branches=optimal
• -mcompact-branches=always — these control which form of branches get generated. Default is -mcompact-branches=optimal ✨.
  • -mcompact-branches=never guarantees compact branch instructions are literally never generated 🚫.
  • -mcompact-branches=always guarantees a compact branch instruction gets generated if one is available for MIPS Release 6 onwards. If a compact branch instruction isn't available (or it's pre-R6), a delay slot form of the branch is used instead.
  • For MIPS16/microMIPS targets, this is just ignored right now 🙈. The behavior for MIPS16/microMIPS may change in future, since they do have some compact branch instructions.
  • -mcompact-branches=optimal makes a delay slot branch get used if one is available in the current ISA and the delay slot is successfully filled. If the delay slot isn't filled, then a compact branch is chosen, if one is available 🎯.
• -mfp-exceptions
• -mno-fp-exceptions — says whether FP exceptions are enabled or not 🔔. This affects how FP instructions get scheduled on some processors. Default: FP exceptions are enabled. Example time: on the SB-1, if FP exceptions are disabled and we're emitting 64-bit code, we get to 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 it 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. It only does anything when optimizing for the VR4130, and it normally makes code faster, at the expense of making it bigger 📈. Enabled by default at optimization level -O3.
• -msynci
• -mno-synci — enable (or disable) generation of synci instructions on architectures that support them 💫. The synci instructions, when enabled, get generated when __builtin___clear_cache is compiled. This option defaults to -mno-synci, but that default can be overridden by configuring GCC with --with-synci.

Okay so when you're compiling code for single processor systems, using synci is generally safe 🧚. But on many multi-core (SMP) systems it does not invalidate the instruction caches on all cores, and that may lead to undefined behavior 💀. Not a vibe. Be careful.

• -mrelax-pic-calls
• -mno-relax-pic-calls — try to turn PIC calls that are normally dispatched via register $25 into direct calls 📞. This is only possible if the linker can resolve the destination at link time and 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, the assembler and the linker can pull off this optimization all by themselves, no compiler help needed 💪.
• -mmcount-ra-address
• -mno-mcount-ra-address — emit (or don't emit) code that lets _mcount 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. Then _mcount can modify the return address by doing both of these:
  • returning the new address in register $31 🔁;
  • storing the new address in *ra-address, if ra-address is nonnull.

Default is -mno-mcount-ra-address.

• -mframe-header-opt
• -mno-frame-header-opt — enable (or disable) frame header optimization in the o32 ABI 🧱. Under the o32 ABI, calling functions allocate 16 bytes on the stack so the called function can write out register arguments. When this is enabled, the optimization suppresses the allocation of the frame header if it can be determined that it's unused. And this optimization is off by default at all optimization levels — yes, even the spicy ones 🙃.
• -mlxc1-sxc1
• -mno-lxc1-sxc1 — when applicable, enable (or disable) generation of the lwxc1, swxc1, ldxc1, sdxc1 instructions 🌀. Enabled by default.
• -mmadd4
• -mno-madd4 — when applicable, enable (or disable) generation of 4-operand madd.s, madd.d and related instructions ✖️➕. Enabled by default.

---

🦋 MMIX Options 🦋

These options are defined for the MMIX, bestie:

• -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 (inside the called function) are seen as registers $0 and up — as opposed to the GNU ABI, which uses global registers $231 and up 🔢.
• -mzero-extend
• -mno-zero-extend — when reading data from memory in sizes shorter than 64 bits, use (or don't use) zero-extending load instructions by default, instead of sign-extending ones 🧮.
• -mknuthdiv
• -mno-knuthdiv — make the result of a division that yields 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 is the one that's almost exclusively used.
• -mtoplevel-symbols
• -mno-toplevel-symbols — prepend (or don't 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 instead of the default mmo format that the mmix simulator uses 📦.
• -mbranch-predict
• -mno-branch-predict — use (or don't use) the probable-branch instructions when static branch prediction indicates a probable branch 🔮.
• -mbase-addresses
• -mno-base-addresses — generate (or don't 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. That register is used for one or more base address requests within the range 0 to 255 from the value held in the register. This generally leads to short and fast code, but the number of different data items that can be addressed is limited 😬. Which means a program using lots of static data may require -mno-base-addresses.
• -msingle-exit
• -mno-single-exit — force (or don't force) generated code to have a single exit point in each function 🚪.
• -mset-data-start= address — when linking, set the start of the data section to address 📍.
• -mset-program-start= address
• -mno-set-program-start — when linking, set the program start address to address 🚀. It defaults to 0x100 unless -mno-set-program-start is used to suppress this option entirely.

---

🌸 MN10300 Options 🌸

These -m options are defined for Matsushita MN10300 architectures 💜:

• -mmult-bug
• -mno-mult-bug — when enabled, generate code to avoid bugs in the multiply instructions for the MN10300 processors 🐛. This is the default.
• -mam33
• -mno-am33 — generate code using features specific to the AM33 processor ⚙️. The default is -mno-am33.
• -mam33-2
• -mno-am33-2 — generate code using features specific to the AM33/2.0 processor ✌️. The default is -mno-am33-2.
• -mam34
• -mno-am34 — generate code using features specific to the AM34 processor 🔢. The default is -mno-am34.
• -mtune= cpu-type — use the timing characteristics of the indicated CPU type when scheduling instructions ⏱️. This does not change the targeted processor type. The CPU type must be one of mn10300, am33, am33-2 or am34.
• -mreturn-pointer-on-d0, -mno-return-pointer-on-d0 — okay so when the compiler generates a function that returns a pointer, this makes it hand the pointer back in both a0 and d0. If you don't do this, the pointer lives only in a0, and then trying to call those functions without a prototype just... errors out. Rude but true. Heads up bestie: this is on by default, so you only reach for -mno-return-pointer-on-d0 when you actually wanna turn it off.
• -mno-crt0 — do not link in the C run-time initialization object file. That's it. That's the whole vibe.
• -mrelax — tells the linker "hey, go do a relaxation optimization pass" so it can shorten branches, calls, and absolute memory addresses. Only does anything when you slap it on the command line for the final link step, so timing matters, iykyk.

Fr fr warning: this option makes symbolic debugging impossible. No cap. Debugging is gone. 💀

• -mliw, -mno-liw — lets the compiler generate Long Instruction Word instructions if the target is the AM33 or later. Enabled by default. And the macros: -mliw defines the preprocessor macro __LIW__; -mno-liw defines __NO_LIW__. Cute little pair 🎀
• -msetlb, -mno-setlb — lets the compiler generate the SETLB and Lcc instructions if the target is the AM33 or later. Also enabled by default. -msetlb defines __SETLB__; -mno-setlb defines __NO_SETLB__. Slay, macros.

🌈 Moxie Options 🌈

• -meb — generate big-endian code. This is the default for moxie-*-* configurations. Big-endian stays winning ✨
• -mel — generate little-endian code. The other flavor 🍓
• -mmul.x, -mno-mul.x — generate mul.x and umul.x instructions. Enabled by default for moxiebox-*-* configurations 👑
• -mno-crt0 — yeah, same deal as before: do not link in the C run-time initialization object file.

🦋 MSP430 Options 🦋

These ones are defined for the MSP430, bestie:

• -masm-hex — forces assembly output to always use hex constants. Normally those constants are signed decimals, so this exists for testsuite and/or aesthetic purposes. Literally aesthetic purposes, I love that for us 💅
• -mmcu=name — picks the MCU to target. Here's the tea: this builds a C preprocessor symbol out of the MCU name — upper-cased, then pre- and post-fixed with __. And that is what the msp430.h header file uses to pick out an MCU-specific supplementary header file.

It 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 you get — otherwise the 430X ISA gets selected. There's also a generic MCU name of msp430 you can use to select the 430 ISA, and similarly the generic msp430x MCU name selects the 430X ISA.

On top of all 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. BUT — plot twist — 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 -I and -L — devices.csv is 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 variable MSP430_GCC_INCLUDE_DIR to the full path to the directory containing devices.csv, and GCC will search this directory for devices.csv. If devices.csv is 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 -I and -L on the command line. Convenient much?
• The msp430-elf{,bare}/include/devices directory — finally, GCC examines msp430-elf{,bare}/include/devices from the toolchain root directory. This directory does not exist in a default installation, but if you've created it and copied devices.csv there, 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. Fallback queen 👑

• -mwarn-mcu, -mno-warn-mcu — enables or disables warnings about conflicts between the MCU name specified by -mmcu and the ISA set by -mcpu and/or the hardware multiply support set by -mhwmult. It also toggles warnings about unrecognized MCU names. On by default.
• -msim — link to the simulator runtime libraries and linker script. This one overrides any scripts that would be selected by the -mmcu= option. Sorry -mmcu, -msim said "main character energy" 😭
• -mlarge — use large-model addressing (20-bit pointers, 20-bit size_t).
• -msmall — use small-model addressing (16-bit pointers, 16-bit size_t).
• -mrelax — this gets passed to the assembler and the linker, and lets the linker perform certain optimizations that literally cannot be done until the final link. Patient king ✨
• mhwmult=type — describes the type of hardware multiply supported by the target. Accepted values for type:
  • none — no hardware multiply.
  • 16bit — the original 16-bit-only multiply supported by early MCUs.
  • 32bit — the 16/32-bit multiply supported by later MCUs.
  • f5series — the 16/32-bit multiply supported by F5-series MCUs.
  • auto — tells GCC to deduce the hardware multiply support based upon the MCU name provided by the -mmcu option. If no -mmcu option is specified or if the MCU name isn't recognized, then no hardware multiply support is assumed.

auto is the default setting.

Normally, hardware multiplies get performed by calling a library routine — that saves space in the generated code. But when you're compiling at -O3 or higher, the hardware multiplier gets invoked inline instead. Bigger code, faster code. Trade-offs, bestie 💜

Also important: the hardware multiply routines disable interrupts whilst running and restore the previous interrupt state when they finish. That makes them safe to use inside interrupt handlers as well as in normal code. Iconic behaviour, honestly.

• -minrt — enables the use of a minimum runtime environment without support for static initializers or constructors. It's meant 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 — enables reduced code size printf and puts library functions. Heads up though: the tiny implementations of these functions are not reentrant, so you must use them with caution in multi-threaded applications.

Support for streams has been removed, and the string to be printed is always sent to stdout via the write syscall. The string is not buffered before it is sent to write. Zero buffering. Straight to write. No chill.

This option requires Newlib Nano IO, so GCC must be configured with --enable-newlib-nano-formatted-io.

• -mmax-inline-shift=n — takes an integer n 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, an mspabi helper function gets used instead. Default value is 4.

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).

And shifts of a 32-bit value are at least twice as costly, so the value you pass for this option gets divided by 2 and the resulting value is used instead. Maths 💀

• -mcode-region=where, -mdata-region=where — these tell the compiler where to place functions and data that don't have one of the lower, upper, either or section attributes. Possible values for where 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. Let the linker cook 🍳
• -muse-lower-region-prefix — adds the .lower prefix to section names when compiling with -mcode-region=lower or -mdata-region=lower. Disabled by default.
• -msilicon-errata=name[,name...] — passes on a request to the assembler to enable the fixes for the named silicon errata. Refer to the assembler documentation for details.
• -msilicon-errata-warn=name[,name...] — passes on a request to the assembler to enable warning messages when a named silicon errata might need to be applied. Refer to the assembler documentation for details.
• -mwarn-devices-csv, -mno-warn-devices-csv — warn if devices.csv is not found or there are problems parsing it (default: on).

🌸 NDS32 Options 🌸

These options are defined for NDS32 implementations:

• -mbig-endian, -EB — generate code in big-endian mode.
• -mlittle-endian, -EL — generate code in little-endian mode.
• -mabi=2, -mfloat-abi=soft — use the soft floating-point ABI.
• -mabi=2fp+, -mfloat-abi=hard — use the hard floating-point ABI.
• -mreduced-regs — use reduced-set registers for register allocation.
• -mfull-regs — use full-set registers for register allocation.
• -malways-align — always align function entry, jump targets, and return address. Alignment queen 👑
• -malign-functions — align function entry to 4 bytes.
• -mfp-as-gp, -mno-fp-as-gp — enable/disable fp-as-gp optimization.
• -mcmov, -mno-cmov — enable/disable generation of conditional move instructions.
• -mhw-abs, -mno-hw-abs — enable/disable generation of hardware abs instructions.
• -mext-perf, -mno-ext-perf — enable/disable generation of performance extension instructions.
• -mext-perf2, -mno-ext-perf2 — enable/disable generation of performance extension 2 instructions.
• -mext-string, -mno-ext-string — enable/disable generation of string extension instructions.
• -mext-dsp, -mno-ext-dsp — enable/disable generation of DSP extension instructions.
• -mext-fpu-fma, -mno-ext-fpu-fma — enable/disable generation of floating-point multiply-accumulation instructions. Fancy ✨
• -mext-fpu-sp, -mno-ext-fpu-sp — turn on/off generating single-precision floating-point instructions ✨
• -mext-fpu-dp, -mno-ext-fpu-dp — turn on/off generating double-precision floating-point instructions 💖
• -mv3push, -mno-v3push — turn on/off generating those v3 push25/pop25 instructions 🎀
• -m16-bit, -mno-16-bit — turn on/off generating 16-bit instructions 🦋
• -mvh, -mno-vh — turn on/off Virtual Hosting support 🌸
• *-misr-vector-size=num* — pick the size of each interrupt vector, and bestie it has to be 4 or 16, no in-between 💅
• *-misr-secure=num* — set the security level of c-isr for the whole file 🔥
• *-mcache-block-size=num* — set the size of each cache block. Must be a power of 2 between 4 and 512, iykyk 👑
• *-march=arch* — pick the target architecture. arch can be v2, v3, v3j, v3m, v3f, or v3s ⭐
• *-mcpu=cpu* — pick the CPU to compile for. cpu can be n6, n650, n7, n705, n8, n801, sn8, sn801, s8, s801, e8, e801, n820, s830, e830, n9, n903, n903a, n968, n968a, n10, n1033, n1033a, n1033-fpu, n1033-spu, n1068, n1068a, n1068-fpu, n1068a-fpu, d10, d1088, d1088-fpu, d1088-spu, n15, d15, n15s, d15s, n15f, d15f, n12, n1213, n1233, n1233-fpu, n1233-spu, n13, n1337, n1337-fpu, n1337-spu, or simple. yes that is a long list, she ate 💀
• *-mconfig-fpu=num* — set an FPU configuration value from 0 to 7; 0--3 is as the FPU spec says, and 4--7 correspond to 0--3 🧚
• *-mconfig-mul=type* — config for the multiply instruction. type can be fast, fast1 (equivalent to fast), fast2, or slow. The default is fast1 💜
• *-mconfig-register-ports=kind* — set the numbers of read/write ports for n9/n10 cores. The value should be 3r2w or 2r1w 🫶
• *-mcmodel=code-model* — set the code model to one of these, bestie:
  • small — all the data and read-only data segments must be within 512KB addressing space. The text segment must be within 16MB addressing space.
  • medium — the data segment must be within 512KB while the read-only data segment can be within 4GB addressing space. The text segment should be still within 16MB addressing space.
  • large — all the text and data segments can be within 4GB addressing space.

The default is large on GNU/Linux targets and medium on other ELF targets 🌈

• -mctor-dtor, -mno-ctor-dtor — turn on/off the constructor/destructor feature 🎀
• -mrelax, -mno-relax — turn on/off the linker option to relax instructions ✨
• -mrelax-hint, -mno-relax-hint — turn on/off insertion of hints for the linker to do relaxation 🦋
• -msched-prolog-epilog, -mno-sched-prolog-epilog — turn on/off scheduling of a function's prologue and epilogue sequence 🌸
• -mret-in-naked-func, -mno-ret-in-naked-func — turn on/off generation of return instructions in naked functions. This option is enabled by default 👑
• -malways-save-lp, -mno-always-save-lp — turn on/off always saving $lp on the stack 💅
• -munaligned-access, -mno-unaligned-access — turn on/off unaligned word and halfword accesses to packed data 🔥
• -minline-asm-r15, -mno-inline-asm-r15 — allow/disallow use of r15 for inline asm ⭐

---

🦋 Nvidia PTX Options 🦋

These options are defined for Nvidia PTX, periodt:

• -m64 — ignored, but preserved for backward compatibility. Only 64-bit ABI is supported, no cap 🙈
• *-march=architecture-string* — generate code for the specified PTX ISA target architecture. Valid architecture strings are sm_30, sm_35, sm_37, sm_50, sm_52, sm_53, sm_61, sm_70, sm_75, sm_80, and sm_89. The default depends on how the compiler has been configured, see --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= 🦋
• *-march-map=architecture-string* — select the closest available -march= value that is not more capable. For instance, for -march-map=sm_50 select -march=sm_37, and for -march-map=sm_53 select -march=sm_53 🌸
• *-mptx=version-string* — generate code for the specified PTX ISA version. Valid version strings are 3.1, 4.0, 4.1, 4.2, 5.0, 6.0, 6.3, 7.0, 7.3, and 7.8. The default PTX ISA version is the one that added support for the selected PTX ISA target architecture, see -march=, but at least 6.3, or 7.3 for -march=sm_52 and 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 __main kernel. This is for stand-alone instead of offloading execution ⭐
• -moptimize, -mno-optimize — turn on/off partitioned execution optimizations. This option is enabled by default when any level of optimization is selected 👑
• -msoft-stack, -mno-soft-stack — okay this one's a whole thing, bestie:

For -mno-soft-stack (the default, unless -mgomp has been specified), use PTX "native" stacks, that is, generate code that uses .local memory or PTX "alloca" directly for stack storage. Unless -mptx=7.3 or higher and -march=sm_52 or higher are active, variable-length arrays and dynamically allocating memory on the stack with "alloca" are not supported 😭

For -msoft-stack (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, 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 🫶

• *-msoft-stack-reserve-local=size* — set the size of .local memory used for the stack when the exact amount is not known. It defaults to 128 🧚
• -muniform-simt, -mno-uniform-simt — turn on/off a code generation variant that allows execution of 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 lane 0 is the master; inside, each thread sees itself as the master. Shared memory array int __nvptx_uni[] stores all-zeros or all-ones bitmasks for each warp, indicating current mode (0 outside of SIMD regions). Each thread can bitwise-and the bitmask at position tid.y with current lane index to compute the master lane index 💜
• -mgomp, -mno-gomp — turn on/off generation of code for use in OpenMP offloading. -mgomp enables -msoft-stack and -muniform-simt options, and selects a corresponding multilib variant 🌈

---

🌸 OpenRISC Options 🌸

These options are defined for OpenRISC:

• *-mboard=name* — configure a board specific runtime. This is passed to the linker for newlib board library linking. The default is or1ksim ✨
• -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-float is selected, 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-float is selected, 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 equivalents are 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 are 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 are generated to store the immediate to a register first 🌸
• -mcmodel=small — generate OpenRISC code for the small model: the GOT is limited to 64k and function call jumps are limited to 64M offsets. This is the default model ✨
• -mcmodel=large — generate OpenRISC code for the large model: the GOT may grow up to 4G in size and function call jumps can target the full 4G address space 🎀

---

💀 PDP-11 Options 💀

These options are defined for the PDP-11, and honestly? iconic vintage energy:

• -mfpu — use hardware FPP floating point. This is the default. (FIS floating point on the PDP-11/40 is not supported.) Implies -m45 🦋
• -msoft-float — do not use hardware floating point 💅
• -mac0, -mno-ac0 — with -mac0, return floating-point results in ac0 (fr0 in Unix assembler syntax). The default, -mno-ac0, is to return floating-point results in memory 🔥
• -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 — make int be 16 bits. This is the default, bestie, so if you change nothing, this is what you get. ✨
• -mint32, -mno-int16 — make int be 32 bits instead. Big int energy. 💅
• -msplit — tells GCC the target has separate instruction and data space. This implies -m45. Two separate vibes, one address space each 🦋
• -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, so this is you choosing the shiny new thing. 🔥

---

🌸 Picolibc Options 🌸

These options control compilation and linking when using Picolibc, no cap:

• --oslib=library — search the library named library after the C library, so symbols the C library left undefined can get defined by this one. The C library, libgcc and this library all get placed between --start-group and --end-group flags so each can refer to symbols in the others. For many targets, Picolibc provides a semihost variant (specified with --oslib=semihost) which provides enough basic OS functionality to support console and file I/O when run in an emulator or when using an in-circuit debugger. 🫶
• --crt0=[none|minimal|hosted|semihost] — replace the default crt0.o name with crt0-variant.o. The variants, bestie:
  • none — provides no startup code at all, allowing you to supply your own.
  • minimal — performs basic memory setup but does not invoke any constructors.
  • when no -crt0 option is provided — the default initialization code adds calls to all constructors.
  • hosted — adds a call to exit when main returns.
  • semihost — accesses a command-line parameter supplied via the semihosting interface and splits that into arguments at whitespace boundaries, passing the resulting array of strings to main in argc and argv. On some targets, including AArch64, ARC, ARM, LoongArch, M680x0, RISC-V, SH, and x86, semihost also traps hardware exceptions and prints information to the console. Note that --crt0=semihost depends upon APIs provided by --oslib=semihost. 😭
• --printf=[d|f|l|i|m] — select the printf variant. Picolibc provides five different printf variants which offer decreasing levels of functionality along with decreasing code size:
  • d — the default level, offering full C17 and POSIX.1-2024 conformance.
  • f — same feature set, but supports float values instead of double, which are passed using the printf_float macro.
  • l — elides all floating-point and POSIX positional parameter support.
  • i — limits integers to those no larger than long.
  • m — removes support for most formatting options including width and precision. The formats and arguments are still parsed correctly, but output does not respect those parameters.
• --scanf=[d|f|l|i|m] — select the scanf variant. Picolibc provides five different scanf variants which offer decreasing levels of functionality along with decreasing code size:
  • d — the default level, offering full C17 and POSIX.1-2024 conformance.
  • f — removes support for double values.
  • l — elides all floating point support.
  • i — limits integers to those no larger than long.
  • m — removes support for %[ conversion specifiers.

---

👑 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.

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 does not use those features. 💀

The minimal startup code does 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 is 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-relax command-line option to the linker instead of the --relax option.
• -mloop, -mno-loop — allow (or do not allow) GCC to use the LOOP instruction. 🌀
• -mmul, -mno-mul — allow (or do not allow) GCC to use the PRU multiplier unit.
• -mfillzero, -mno-fillzero — allow (or do not allow) GCC to use the FILL and ZERO instructions.
• -mabi=variant — specify the ABI variant to output code for. -mabi=ti selects the unmodified TI ABI, while -mabi=gnu selects a GNU variant that copes more naturally with certain GCC assumptions. Here's the tea on 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 : Function return values larger than 64 bits are passed by using a hidden pointer as the first argument of the function. TI ABI, though, mandates that the pointer can be NULL in case the caller is not using the returned value. GNU always passes and expects a valid return value pointer.

Size Of Struct Containing Bit-fields : TI ABI mandates that struct size is determined by the bit-field type, if it contains any. On the other hand, GNU allocates the smallest amount of bytes which would fit the bit-field.

For example, TI ABI reserves 4 bytes for this struct, whereas GNU reserves a single byte:

struct S { int i:1; };

Access Size For Volatile Bit-fields : TI ABI mandates that volatile bit-fields are accessed using their type. In contrast, GNU ABI uses the smallest integer type fitting the bit-field.

For example, TI ABI requires a single load of 4 bytes for the following bit-field. GNU generates a load of 1 byte:

struct S { volatile int i:1; };

The current -mabi=ti implementation simply raises a compile error when any of the above code constructs is detected. As a consequence the standard C library cannot be built and it is omitted when linking with -mabi=ti. 😭

Relaxation is a GNU feature and for safety reasons is disabled when using -mabi=ti. The TI toolchain does not emit relocations for QBBx instructions, so the GNU linker cannot adjust them when shortening adjacent LDI32 pseudo instructions.

---

🔥 RISC-V Options 🔥

These command-line options are defined for RISC-V targets:

• -mbranch-cost=n — set the cost of branches to roughly n instructions.
• -mabi=ABI-string — specify integer and floating-point calling convention. ABI-string contains two parts: the size of integer types and the registers used for floating-point types. For example -march=rv64ifd -mabi=lp64d means that long and pointers are 64-bit (implicitly defining int to be 32-bit), and that floating-point values up to 64 bits wide are passed in F registers. Contrast this with -march=rv64ifd -mabi=lp64f, which still allows the compiler to generate code that uses the F and D extensions but only allows floating-point values up to 32 bits long to be passed in registers; or -march=rv64ifd -mabi=lp64, in which no floating-point arguments are passed in registers.

The default for this argument is system dependent; if you want a specific calling convention you should specify one explicitly. The valid calling conventions are: ilp32, ilp32f, ilp32d, lp64, lp64f, and lp64d. Some calling conventions are impossible to implement on some ISAs: for example, -march=rv32if -mabi=ilp32d is invalid because the ABI requires 64-bit values be passed in F registers, but F registers are only 32 bits wide. There are also the ilp32e ABI that can only be used with the rv32e architecture and the lp64e ABI that can only be used with the rv64e. Those ABIs are not well-specified at present, and are subject to change. 🙈

• -mfdiv, -mno-fdiv — do or don't use hardware floating-point divide and square root instructions. This requires the F or D extensions 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 the fence.tso instruction, which is unimplemented on some processors (including those from T-Head). If the fence.tso instruction is not available then a stronger fence is used instead.
• -mdiv, -mno-div — do or don't use hardware instructions for integer division. This requires the M extension. The default is to use them if the specified architecture has these instructions.
• -misa-spec=ISA-spec-string — specify the version of the RISC-V Unprivileged (formerly User-Level) ISA specification generated code should conform to. The possibilities for ISA-spec-string are:
  • 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 has been configured with --with-isa-spec= specifying a different default version.

• -march=[ISA|Profile|Profile_ISA|processor-string] — generate code for given RISC-V ISA or profile or a combination of them (e.g. rv64im rvi20u64 rvi20u64_zbb). The names of ISAs and profiles must be lower case. Examples include rv64i, rv32g, rv32e, rv32imaf, rva22u64 and rva23u64. To combine a named profile with optional RISC-V ISA extensions, give the profile first and then append the extension name(s) using an underscore as a delimiter (e.g. rvi20u64_zca_zcb rva23u64_zacas). Additionally, a special value help (-march=help) is accepted to list all supported extensions. 🌈

-march=unset causes the compiler to ignore any -march=... options that appear earlier on the command line, behaving as if the option was never passed. This is useful for ensuring that the architecture is taken from the -mcpu option, and an error results if no -mcpu option is given when -march=unset is used.

The syntax of the ISA string is defined as follows:

• g · version - — the general-purpose computing base extension; g expands to i, m, a, f, d, zicsr and zifencei.
• e · 2.0 — Reduced base integer extension.
• i · 2.0 2.1 — Base integer extension.
• m · 2.0 — Integer multiplication and division extension.
• a · 2.0 2.1 — Atomic extension.
• f · 2.0 2.2 — Single-precision floating-point extension.
• d · 2.0 2.2 — Double-precision floating-point extension.
• c · 2.0 — Compressed extension.
• b · 1.0 — Standard extension for bit manipulation functions.
• v · 1.0 — Vector extension.
• h · 1.0 — Hypervisor extension.
• zic64b · 1.0 — Cache block size is 64 bytes.
• zicbom · 1.0 — Cache-block management extension.
• zicbop · 1.0 — Cache-block prefetch extension.
• zicboz · 1.0 — Cache-block zero extension.
• ziccamoa · 1.0 — Main memory supports all atomics in A.
• ziccif · 1.0 — Main memory supports instruction fetch with atomicity requirement.
• zicclsm · 1.0 — Main memory supports misaligned loads/stores.
• ziccrse · 1.0 — Main memory supports forward progress on LR/SC sequences.
• zicfilp · 1.0 — Control-flow integrity landing pad extension.
• zicfiss · 1.0 — Control-flow integrity shadow stack extension.
• zicntr · 2.0 — Standard extension for base counters and timers.
• zicond · 1.0 — Integer conditional operations extension.
• zicsr · 2.0 — Control and status register access extension.
• zifencei · 2.0 — Instruction-fetch fence extension.
• zihintntl · 1.0 — Non-temporal locality hints extension.
• zihintpause · 2.0 — Pause hint extension.
• zihpm · 2.0 — Standard extension for hardware performance counters.
• zimop · 1.0 — May-be-operations extension.
• zilsd · 1.0 — Load/store pair instructions extension.
• zmmul · 1.0 — Integer multiplication extension.
• za128rs · 1.0 — Reservation set size of 128 bytes.
• za64rs · 1.0 — Reservation set size of 64 bytes.
• zaamo · 1.0 — Atomic memory operations extension.
• zabha · 1.0 — Byte and halfword atomic memory operations extension.
• zacas · 1.0 — Atomic compare-and-swap instructions extension.
• zalrsc · 1.0 — Load-reserved/store-conditional subset of the A extension.
• zawrs · 1.0 — Wait-on-reservation-set extension.
• zama16b · 1.0 — Misaligned loads, stores, and AMOs that are fully contained within a naturally-aligned 16-byte boundary are atomic.
• zfa · 1.0 — Additional floating-point extension.
• zfbfmin · 1.0 — Minimal BF16 support extension.
• zfh · 1.0 — Half-precision floating-point extension.
• zfhmin · 1.0 — Minimal half-precision floating-point extension.
• zfinx · 1.0 — Single-precision floating-point in integer registers extension.
• zdinx · 1.0 — Double-precision floating-point in integer registers extension.
• zca · 1.0 — Integer compressed instruction extension.
• zcb · 1.0 — Simple compressed instruction extension.
• zcd · 1.0 — Compressed double-precision floating point loads and stores extension.
• zce · 1.0 — Compressed instruction extensions for embedded processors.
• zcf · 1.0 — Compressed single-precision floating point loads and stores extension.
• zcmop · 1.0 — Compressed may-be-operations extension.
• zcmp · 1.0 — Compressed push pop extension.
• zcmt · 1.0 — Table jump instruction extension.
• zclsd · 1.0 — Compressed load/store pair instructions extension.
• zba · 1.0 — Address calculation extension.
• zbb · 1.0 — Basic bit manipulation extension.
• zbc · 1.0 — Carry-less multiplication extension.
• zbkb · 1.0 — Cryptography bit-manipulation extension.
• zbkc · 1.0 — Cryptography carry-less multiply extension.
• zbkx · 1.0 — Cryptography crossbar permutation extension.
• zbs · 1.0 — Single-bit operation extension.
• zk · 1.0 — Standard scalar cryptography extension.
• zkn · 1.0 — NIST algorithm suite extension.
• zknd · 1.0 — AES Decryption extension.
• zkne · 1.0 — AES Encryption extension.
• zknh · 1.0 — Hash function extension.
• zkr · 1.0 — Entropy source extension.
• zks · 1.0 — ShangMi algorithm suite extension.
• zksed · 1.0 — SM4 block cipher extension.
• zksh · 1.0 — SM3 hash function extension.
• zkt · 1.0 — Data independent execution latency extension.
• ztso · 1.0 — Total store ordering extension.
• zvbb · 1.0 — Vector basic bit-manipulation extension.
• zvbc · 1.0 — Vector carry-less multiplication extension.
• zve32f · 1.0 — Vector extensions for embedded processors.
• zve32x · 1.0 — Vector extensions for embedded processors.
• zve64d · 1.0 — Vector extensions for embedded processors.
• zve64f · 1.0 — Vector extensions for embedded processors.
• zve64x · 1.0 — Vector extensions for embedded processors.
• zvfbfmin · 1.0 — Vector BF16 converts extension.
• zvfbfwma · 1.0 — Vector BF16 widening multiply/add extension.
• zvfh · 1.0 — Vector half-precision floating-point extension.
• zvfhmin · 1.0 — Vector minimal half-precision floating-point extension.
• zvkb · 1.0 — Vector cryptography bit-manipulation extension.
• zvkg · 1.0 — Vector GCM/GMAC extension.
• zvkn · 1.0 — Vector NIST Algorithm Suite extension, zvkn will expand to
• zvknc · 1.0 — Vector NIST Algorithm Suite with carry-less multiply extension, zvknc
• zvkned · 1.0 — Vector AES block cipher extension.
• zvkng · 1.0 — Vector NIST Algorithm Suite with GCM extension, zvkng will expand
• zvknha · 1.0 — Vector SHA-2 secure hash extension.
• zvknhb · 1.0 — Vector SHA-2 secure hash extension.
• zvks · 1.0 — Vector ShangMi algorithm suite extension, zvks will expand
• zvksc · 1.0 — Vector ShangMi algorithm suite with carry-less multiplication extension,
• zvksed · 1.0 — Vector SM4 block cipher extension.
• zvksg · 1.0 — Vector ShangMi algorithm suite with GCM extension.
• zvksh · 1.0 — Vector SM3 secure hash extension.
• zvkt · 1.0 — Vector data independent execution latency extension.
• zvl1024b · 1.0 — Minimum vector length standard extensions.
• zvl128b · 1.0 — Minimum vector length standard extensions.
• zvl16384b · 1.0 — Minimum vector length standard extension.
• zvl2048b · 1.0 — Minimum vector length standard extensions.
• zvl256b · 1.0 — Minimum vector length standard extensions.
• zvl32768b · 1.0 — Minimum vector length standard extension.
• zvl32b · 1.0 — Minimum vector length standard extensions.
• zvl4096b · 1.0 — Minimum vector length standard extensions.
• zvl512b · 1.0 — Minimum vector length standard extensions.
• zvl64b · 1.0 — Minimum vector length standard extensions.
• zvl65536b · 1.0 — Minimum vector length standard extension.
• zvl8192b · 1.0 — Minimum vector length standard extension.
• zhinx · 1.0 — Half-precision floating-point in integer registers extension.
• zhinxmin · 1.0 — Minimal half-precision floating-point in integer registers extension.
• sdtrig · 1.0 — Debug triggers extension.
• sha · 1.0 — The augmented hypervisor extension.
• shcounterenw · 1.0 — Support writeable enables for any supported counter.
• shgatpa · 1.0 — SvNNx4 mode supported for all modes supported by satp.
• shlcofideleg · 1.0 — Delegating LCOFI interrupts to VS-mode.
• shtvala · 1.0 — The htval register provides all needed values.
• shvstvala · 1.0 — The vstval register provides all needed values.
• shvstvecd · 1.0 — The vstvec register supports direct mode.
• shvsatpa · 1.0 — The vsatp register supports all modes supported by satp.
• smaia · 1.0 — Advanced interrupt architecture extension.
• smcntrpmf · 1.0 — Cycle and instret privilege mode filtering.
• smcsrind · 1.0 — Machine-level indirect CSR access.
• smepmp — version 1.0 — PMP enhancements for memory access and execution prevention on machine mode
• smmpm — version 1.0 — Supervisor-mode pointer masking extension
• smnpm — version 1.0 — Supervisor-mode pointer masking extension
• smrnmi — version 1.0 — Resumable non-maskable interrupts
• smstateen — version 1.0 — State enable extension
• smdbltrp — version 1.0 — Double trap extensions
• ssaia — version 1.0 — Advanced interrupt architecture extension for supervisor mode
• ssccptr — version 1.0 — Main memory supports page table reads
• sscofpmf — version 1.0 — Count overflow and filtering extension
• sscounterenw — version 1.0 — Support writeable enables for any supported counter
• sscsrind — version 1.0 — Supervisor-mode indirect CSR access
• ssnpm — version 1.0 — Supervisor-mode pointer masking extension
• sspm — version 1.0 — Supervisor-mode pointer masking extension
• ssstateen — version 1.0 — Supervisor-mode state-enable extension
• sstc — version 1.0 — Supervisor-mode timer interrupts extension
• sstvala — version 1.0 — Stval provides all needed values
• sstvecd — version 1.0 — Stvec supports direct mode
• ssstrict — version 1.0 — Unimplemented reserved encodings raise illegal instruction exceptions and no non-conforming extensions are present
• ssdbltrp — version 1.0 — Double trap extensions
• ssu64xl — version 1.0 — UXLEN=64 must be supported
• supm — version 1.0 — User-mode pointer masking extension
• svinval — version 1.0 — Fine-grained address-translation cache invalidation extension
• svnapot — version 1.0 — NAPOT translation contiguity extension
• svpbmt — version 1.0 — Page-based memory types extension
• svvptc — version 1.0 — Extension for obviating memory-management instructions after marking PTEs valid
• svadu — version 1.0 — Hardware updating of A/D bits extension
• svade — version 1.0 — Cause exception when hardware updating of A/D bits is disabled
• svbare — version 1.0 — Satp mode bare is supported
• xcvalu — version 1.0 — Core-V miscellaneous ALU extension
• xcvbi — version 1.0 — Core-V immediate branch extension
• xcvelw — version 1.0 — Core-V event load word extension
• xcvmac — version 1.0 — Core-V multiply-accumulate extension
• xcvsimd — version 1.0 — Core-V SIMD extension
• xsfcease — version 1.0 — SiFive CEASE instruction extension
• xsfvcp — version 1.0 — SiFive VCIX vector coprocessor extension
• xsfvfnrclipxfqf — version 1.0 — SiFive FP32-to-int8 ranged clip instructions
• xsfvqmaccdod — version 1.0 — SiFive int8 matrix multiplication extension
• xsfvqmaccqoq — version 1.0 — SiFive int8 matrix multiplication extension
• xtheadba — version 1.0 — T-head address calculation extension
• xtheadbb — version 1.0 — T-head basic bit-manipulation extension
• xtheadbs — version 1.0 — T-head single-bit instructions extension
• xtheadcmo — version 1.0 — T-head cache management operations extension
• xtheadcondmov — version 1.0 — T-head conditional move extension
• xtheadfmemidx — version 1.0 — T-head indexed memory operations for floating-point registers extension
• xtheadfmv — version 1.0 — T-head double floating-point high-bit data transmission extension
• xtheadint — version 1.0 — T-head acceleration interruption extension
• xtheadmac — version 1.0 — T-head multiply-accumulate extension
• xtheadmemidx — version 1.0 — T-head indexed memory operation extension
• xtheadmempair — version 1.0 — T-head two-GPR memory operation extension
• xtheadsync — version 1.0 — T-head multi-core synchronization extension
• xtheadvector — version 1.0 — T-head vector extension
• xventanacondops — version 1.0 — Ventana integer conditional operations extension
• xmipscmov — version 1.0 — Mips conditional move extension
• xmipscbop — version 1.0 — Mips prefetch extension
• xandesperf — version 5.0 — Andes performace extension
• xandesbfhcvt — version 5.0 — Andes bfloat16 conversion extension
• xandesvbfhcvt — version 5.0 — Andes vector bfloat16 conversion extension
• xandesvsintload — version 5.0 — Andes vector INT4 load extension
• xandesvpackfph — version 5.0 — Andes vector packed FP16 extension
• xandesvdot — version 5.0 — Andes vector dot product extension
• xsmtvdot — version 1.0 — SpacemiT vector dot product extension

Okay so real talk, bestie: when -march= is not specified, GCC just goes "mmm, I'll use whatever setting came from -mcpu" 💅

And if you didn't give it either -march or -mcpu=, the default for this argument is system dependent — so if you actually want a specific architecture extension, specify it yourself, don't just vibe and hope 😭

Lowkey iconic behaviour: when the RISC-V specifications define an extension as depending on other extensions, GCC implicitly adds the dependent extensions into the enabled extension set, as long as you didn't add them explicitly already 🫶

Core Name

• -mcpu=processor-string — use the architecture of, and optimize the output for, the given processor, picked by its particular CPU name ✨

Permissible values for this option are, and yes I'm listing every single one because they're all valid:

• 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-p450
• sifive-p670
• thead-c906
• xt-c908
• xt-c908v
• xt-c910
• xt-c910v2
• xt-c920
• xt-c920v2
• tt-ascalon-x
• tt-ascalon-d8
• xiangshan-nanhu
• xiangshan-kunminghu
• mips-p8700
• andes-n22
• andes-n25
• andes-a25
• andes-nx25
• andes-ax25
• andes-a27
• andes-ax27
• andes-n225
• andes-d23
• andes-n45
• andes-nx45
• andes-a45
• andes-ax45
• spacemit-x60

heads up babes: -mcpu does not override -march or -mtune 🙈

Tune Name

• -mtune=processor-string — optimize the output for the given processor, specified by microarchitecture or particular CPU name 🎀

Permissible values for this option are:

• generic
• rocket
• sifive-3-series
• sifive-5-series
• sifive-7-series
• sifive-p400-series
• sifive-p600-series
• tt-ascalon-x
• tt-ascalon-d8
• thead-c906
• xt-c908
• xt-c908v
• xt-c910
• xt-c910v2
• xt-c920
• xt-c920v2
• xiangshan-nanhu
• xiangshan-kunminghu
• spacemit-x60
• generic-ooo
• size
• mips-p8700
• andes-25-series
• andes-23-series
• andes-45-series
• and all valid options for -mcpu= 🫶

When -mtune= isn't specified, GCC uses the setting from -mcpu. And the default is generic if neither one is specified.

So about the size choice — it is not intended for use by end-users, periodt. This is the one used when -Os is specified. It overrides the instruction cost info that -mtune= provides, but it does not override the pipeline info. The whole point is to help reduce code size while still giving good performance ✨

• -mpreferred-stack-boundary=num — try to keep the stack boundary aligned to a 2 raised to num byte boundary 🎯

If -mpreferred-stack-boundary is not specified, the default is 4 (that's 16 bytes, or 128-bits, iykyk).

⚠️ 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 mixing, bestie, it will not slay 💀

• -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 on targets that don't have specific instructions for conditional operations. If enabled, sequences of ALU operations are produced using base integer ISA instructions wherever that's profitable 💅
• -minline-atomics, -mno-inline-atomics — do or don't use smaller-but-slower subword atomic emulation code that uses libatomic function calls. The default is to use fast inline subword atomics that do not require libatomic 🔥
• -minline-strlen, -mno-inline-strlen — do or do not attempt to inline strlen calls if possible. Inlining can only be done if the string is properly aligned and instructions for accelerated processing are available. The default is to inline strlen calls 🎀
• -minline-strcmp, -mno-inline-strcmp — do or do not attempt to inline strcmp calls if possible. Inlining can only be done if the strings are properly aligned and instructions for accelerated processing are available. The default is to inline strcmp calls 🎀
• -minline-strncmp, -mno-inline-strncmp — do or do not attempt to inline strncmp calls if possible. Inlining can only be done if the strings are properly aligned and instructions for accelerated processing are available. The default is to inline strncmp calls 🎀
• -mstringop-strategy=strategy — pick a particular strategy for inlining string and memory operations. strategy may be one of auto, libcall, scalar, or vector 🦋
• -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 this 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. 🦋
• -mmax-vectorization, -mno-max-vectorization — enable or disable an override to the vectorizer cost model that makes vectorization always appear profitable. Unlike -fno-vect-cost-model or -fvect-cost-model=unlimited, this option does not turn off cost comparison between different vector modes. 🔥
• -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. 😭
• -mcmodel=large — generate code for a large code model, which has no restrictions on size or placement of symbols. 👑
• -mexplicit-relocs, -mno-explicit-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 — emit (do not emit) RISC-V attribute to record extra information into ELF objects. This feature requires at least binutils 2.32. ⭐
• -mcsr-check, -mno-csr-check — enables or disables the CSR checking. 🧚
• -momit-leaf-frame-pointer — don't keep the frame pointer in a register for leaf functions. This avoids the instructions to save, set up and restore frame pointers and makes an extra register available in leaf functions. 🫶
• -malign-data=type — control how GCC aligns variables and constants of array, structure, or union types. Supported values for type are xlen, which uses x register width as the alignment value, and natural, which uses natural alignment. xlen is the default. 💖
• -mbig-endian — generate big-endian code. This is the default when GCC is configured for a riscv64be-*-* or riscv32be-*-* target. Support for RISC-V big-endian is experimental. The ABI is not yet stable and could change in incompatible ways in future releases. 🚨
• -mlittle-endian — generate little-endian code. This is the default when GCC is configured for a riscv64-*-* or riscv32-*-* but not a riscv64be-*-* or riscv32be-*-* target. 🌸
• -mstack-protector-guard=guard, -mstack-protector-guard-reg=reg, -mstack-protector-guard-offset=offset — generate stack protection code using canary at guard. Supported locations are global for a global canary or tls for per-thread canary in the TLS block. With the latter choice the options -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset furthermore specify which 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. 💜
• -mtls-dialect=desc — use TLS descriptors as the thread-local storage mechanism for dynamic accesses of TLS variables. ✨
• -mtls-dialect=trad — use traditional TLS as the thread-local storage mechanism for dynamic accesses of TLS variables. This is the default. 🎀
• -mrvv-vector-bits=value — specify how the number of bits for an RVV vector register, as taken from the -march= option, is interpreted. The value parameter is specified as a string keyword and may be one of scalable or zvl. The default is scalable, which tells GCC to interpret the number as a minimum, while zvl tells GCC to use exactly the number of bits specified. 🦋
• -mrvv-max-lmul=value — this option allows explicit control over the maximum length multiplier (LMUL) used when generating code for the RISC-V Vector Extensions (RVV). The value parameter is specified as a string keyword and may be one of m1, m2, m4, m8, or dynamic. The default is m1 for compatibility with existing hardware that does not support the other options. 🔥
• -madjust-lmul-cost, -mno-adjust-lmul-cost — this option adjusts the cost model used to schedule vector instructions to multiply the latency of instructions by the RVV length multiplier, LMUL. It is disabled by default. 💅
• -mautovec-segment, -mno-autovec-segment — enable or disable generation of vector segment load/store instructions. This option is enabled by default. 🌟

---

🌸 RL78 Options 🌸

RL78 options are here and they're giving tiny microcontroller energy 🎀

• -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. Thus specifying -mcpu=g13 enables the use of the G13 hardware multiply peripheral and specifying -mcpu=g10 disables the use of hardware multiplications altogether. Note, although the RL78/G14 core is the default target, specifying -mcpu=g14 or -mcpu=rl78 on 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 are used even though the code targets the RL78 core. This is for backward 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 — allow 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. 👑
• -mrelax, -mno-relax — enable/disable assembler and linker relaxation. This is enabled by default at -Os. ⭐
• -mes0 — assume ES is zero throughout program execution, and use it to address read-only data. 🦋
• -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 as this makes the interrupt handlers faster. The target option -mg13 needs to be passed for this to work as this feature is only available on the G13 target (S2 core). The MDUC registers are only 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:

• -mcpu=cpu_type — set architecture type, register usage, and instruction scheduling parameters for machine type cpu_type. Supported values for cpu_type are 401, 403, 405, 405fp, 440, 440fp, 464, 464fp, 476, 476fp, 505, 601, 602, 603, 603e, 604, 604e, 620, 630, 740, 7400, 7450, 750, 801, 821, 823, 860, 970, 8540, a2, e300c2, e300c3, e500mc, e500mc64, e5500, e6500, ec603e, G3, G4, G5, titan, power3, power4, power5, power5+, power6, power6x, power7, power8, power9, power10, power11, future, powerpc, powerpc64, powerpc64le, rs64, and native. -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 out there each pick one specific processor, yeah? And code built under those runs best on that processor, and honestly might not run at all on others 😭 so like... be careful.

The -mcpu options automagically enable or disable these besties:

• -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
• -mdense-math
• -mrop-protect

Which exact ones get flipped for any given CPU? Honestly it varies between compiler versions — it depends on whatever setting seems to spit out optimal code for that CPU, and ngl it doesn't necessarily reflect what the actual hardware can even do 🙈. But if you want to force an individual option to a particular value, just drop it after the -mcpu option, like:

-mcpu=970 -mno-altivec

Okay so on AIX, the -maltivec and -mpowerpc64 options are not enabled or disabled by -mcpu at present, because AIX doesn't have full support for these options. You can still enable or disable them individually if you're sure it'll work in your environment 💅.

In other cases, we recommend you use the -mcpu=cpu_type option rather than the options that control generation of specific instructions. Periodt.

✨ the rest of the PowerPC options ✨

• -mtune=cpu_type — sets the instruction scheduling parameters for machine type cpu_type, but does not set the architecture type or register usage like -mcpu=cpu_type does. Same values for cpu_type apply to -mtune as to -mcpu. If you specify both, the code generated uses the architecture and registers from -mcpu, but the scheduling parameters from -mtune. Slay, they coexist 🫶
• -mpowerpc64, -mno-powerpc64 — -mpowerpc64 lets GCC generate the extra 64-bit instructions found in the full PowerPC64 architecture, and treat GPRs as 64-bit doubleword quantities. GCC defaults to -mno-powerpc64.
• -mcmodel=small — generate PowerPC64 code for the small model: the TOC is limited to 64k.
• -mcmodel=medium — generate PowerPC64 code for the medium model: the TOC and other static data may be up to a total of 4G in size. This is the default for 64-bit Linux 🔥
• -mcmodel=large — generate PowerPC64 code for the large model: the TOC may be up to 4G in size. Other data and code is only limited by the 64-bit address space. Big model energy 👑
• -mprofile-kernel — available on PowerPC64 GNU/Linux targets. When used with -pg, it causes calls to mcount to be inserted before the function prologue instead of after it.
• -maltivec, -mno-altivec — generate code that uses (does not use) AltiVec instructions, and also enable the use of built-in functions that allow more direct access to the AltiVec instruction set. You may also need to set -mabi=altivec to adjust the current ABI with AltiVec ABI enhancements.

When -maltivec is used, the element order for AltiVec intrinsics like vec_splat, vec_extract, and vec_insert match array element order corresponding to the endianness of the target ✨. That is, element zero identifies the leftmost element in a vector register when targeting a big-endian platform, and identifies the rightmost element in a vector register when targeting a little-endian platform.

• -mpowerpc-gpopt, -mno-powerpc-gpopt — 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, -mno-powerpc-gfxopt — -mpowerpc-gfxopt allows GCC to use the optional PowerPC architecture instructions in the Graphics group, including floating-point select.
• -mmfcrf, -mno-mfcrf — -mmfcrf 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.
• -mpopcntb, -mno-popcntb — -mpopcntb 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.
• -mpopcntd, -mno-popcntd — -mpopcntd allows GCC to generate the popcount instruction implemented on the POWER7 processor and other processors that support the PowerPC V2.06 architecture.
• -mfprnd, -mno-fprnd — -mfprnd 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.
• -mcmpb, -mno-cmpb — -mcmpb allows GCC to generate the compare bytes instruction implemented on the POWER6 processor and other processors that support the PowerPC V2.05 architecture.
• -mhard-dfp, -mno-hard-dfp — -mhard-dfp allows GCC to generate the decimal floating-point instructions implemented on some POWER processors.
• -mvrsave, -mno-vrsave — generate VRSAVE instructions when generating AltiVec code.
• -msecure-plt — generate code that allows ld and ld.so to build executables and shared libraries with non-executable .plt and .got sections. This is a PowerPC 32-bit SYSV ABI option 🔒
• -mbss-plt — generate code that uses a BSS .plt section that ld.so fills in, and requires .plt and .got sections that are both writable and executable. This is a PowerPC 32-bit SYSV ABI option.

Careful bestie — that one wants sections that are writable and executable at the same time, which is exactly the kind of thing security girlies side-eye. It's a PowerPC 32-bit SYSV ABI option, just so you know what you're signing up for 💀

• -msplit-patch-nops — when adding NOPs for a patchable area via the -fpatchable-function-entry option, emit the "before" NOPs in front of the global entry point and the "after" NOPs after the local entry point. This 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, making them consecutive but moving the global and local entry point further apart. If only a single entry point is generated, this option has no effect.
• -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.
• -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 manpage literally says "does not keeps." We're keeping it exactly as-is, no cap 🫶

• -mquad-memory, -mno-quad-memory — generate code that uses (does not use) the non-atomic quad word memory instructions. The -mquad-memory option requires use of 64-bit mode.
• -mquad-memory-atomic, -mno-quad-memory-atomic — generate code that uses (does not use) the atomic quad word memory instructions. The -mquad-memory-atomic option requires use of 64-bit mode.
• -mfloat128, -mno-float128 — enable/disable the __float128 keyword 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. 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 also enables the generation of ISA 3.0 IEEE 128-bit floating point instructions. Otherwise, if you do not specify generation of ISA 3.0 instructions or you are targeting a 32-bit big-endian system, IEEE 128-bit floating point is handled with software emulation.

• -mfloat128-hardware, -mno-float128-hardware — enable/disable using ISA 3.0 hardware instructions to support the __float128 data 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 sets int, long and pointer to 32 bits and generates code that runs on any PowerPC variant. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits, and generates code for PowerPC64, as for -mpowerpc64.
• -mfull-toc, -mno-fp-in-toc, -mno-sum-in-toc, -mminimal-toc — modify generation of the TOC (Table Of Contents), which is created for every executable file. The -mfull-toc option is selected by default. In that case, GCC allocates at least one TOC entry for each unique non-automatic variable reference in your program. GCC also places floating-point constants in the TOC. However, only 16,384 entries are available in the TOC 😭

If you receive a linker error message that saying you have overflowed the available TOC space, you can reduce the amount of TOC space used with the -mno-fp-in-toc and -mno-sum-in-toc options. -mno-fp-in-toc prevents GCC from putting floating-point constants in the TOC and -mno-sum-in-toc forces GCC to generate code to calculate the sum of an address and a constant at run time instead of putting that sum into the TOC. You may specify one or both of these options. Each causes GCC to produce very slightly slower and larger code at the expense of conserving TOC space.

If you still run out of space in the TOC even when you specify both of these options, specify -mminimal-toc instead. This option causes GCC to make only one TOC entry for every file. When you specify this option, GCC produces code that is slower and larger but which uses extremely little TOC space. You may wish to use this option only on files that contain less frequently-executed code 💅

• -maix64, -maix32 — turns on the 64-bit AIX ABI and calling convention: 64-bit pointers, 64-bit long type, and all the infrastructure needed to support them. Specifying -maix64 implies -mpowerpc64, while -maix32 disables the 64-bit ABI and implies -mno-powerpc64. GCC defaults to -maix32 💅
• -mxl-compat, -mno-xl-compat — makes your code conform more closely to IBM XL compiler semantics when you're using the AIX-compatible ABI. Concretely: it passes floating-point arguments to prototyped functions beyond the register save area (RSA) on the stack in addition to argument FPRs, it does not assume that the most significant double in a 128-bit long double value is properly rounded when comparing values and converting to double, and it uses XL symbol names for long double support routines ✨

ok so the lore here: the AIX calling convention got extended but wasn't 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 don't fit in the RSA from the stack when a subroutine is compiled without optimization. And because always storing floating-point arguments on the stack is inefficient and rarely needed, this option is not enabled by default — you only need it when calling subroutines compiled by IBM XL compilers without optimization 🫶

• -mpe — supports the IBM RS/6000 SP Parallel Environment (PE). Links an application written to use message passing with special startup code so the app can actually 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 point at the appropriate directory location. The Parallel Environment does not support threads, so -mpe and -pthread are incompatible — pick a lane bestie 💀
• -malign-natural, -malign-power — on AIX, 32-bit Darwin, and 64-bit PowerPC GNU/Linux, -malign-natural overrides the ABI-defined alignment of larger types (like floating-point doubles) onto their natural size-based boundary. -malign-power tells 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 — sorry babes 🙈

• -msoft-float, -mhard-float — generates code that does not use (or does use) the floating-point register set. Software floating-point emulation is provided if you use -msoft-float, and you gotta pass the option to GCC when linking too 🔗
• -mmultiple, -mno-multiple — generates code that uses (or does not use) the load multiple word instructions and the store multiple word instructions. These instructions are generated by default on POWER systems, and not generated on PowerPC systems. Do not use -mmultiple on little-endian PowerPC systems, since those instructions don't work when the processor is in little-endian mode 😭 The exceptions are PPC740 and PPC750, which permit these instructions in little-endian mode 👑
• -mupdate, -mno-update — generates code that uses (or doesn't 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's a small window between the time 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 ⚠️

fr fr be careful with that one, corrupted data is not the vibe

• -mavoid-indexed-addresses, -mno-avoid-indexed-addresses — generates code that tries to avoid (or not avoid) the use of indexed load or store instructions. These instructions can incur a performance penalty on Power6 processors in certain situations — like 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 — generates code that uses (or doesn't use) the floating-point multiply and accumulate instructions. These instructions are generated by default if hardware floating point is used. heads up: the machine-dependent -mfused-madd option is now mapped to the machine-independent -ffp-contract=fast option, and -mno-fused-madd is mapped to -ffp-contract=off 🔥
• -mmulhw, -mno-mulhw — generates code that uses (or doesn't use) the half-word multiply and multiply-accumulate instructions on the IBM 405, 440, 464 and 476 processors. These instructions are generated by default when targeting those processors 💖
• -mdlmzb, -mno-dlmzb — generates code that uses (or doesn't use) the string-search dlmzb instruction on the IBM 405, 440, 464 and 476 processors. This instruction is generated by default when targeting those processors ⭐
• -mno-bit-align, -mbit-align — on System V.4 and embedded PowerPC systems, do not (do) force structures and unions that contain bit-fields to be aligned to the base type of the bit-field.

Example time: by default a structure containing nothing but 8 unsigned bit-fields of length 1 is aligned to a 4-byte boundary and has a size of 4 bytes. By using -mno-bit-align, the structure is aligned to a 1-byte boundary and is 1 byte in size — glow down honestly 🎀

• -mbit-word, -mno-bit-word — allow bit-fields to cross word boundaries.
• -mno-strict-align, -mstrict-align — on System V.4 and embedded PowerPC systems, do not (do) assume that unaligned memory references are handled by the system 🤔
• -mrelocatable, -mno-relocatable — generates code that allows (does not allow) a static executable to be relocated to a different address at run time. A simple embedded PowerPC system loader should relocate the entire contents of .got2 and 4-byte locations listed in the .fixup section — a table of 32-bit addresses generated by this option. For this to work, all objects linked together must be compiled with -mrelocatable or -mrelocatable-lib. -mrelocatable code aligns the stack to an 8-byte boundary 🧵
• -mrelocatable-lib, -mno-relocatable-lib — like -mrelocatable, -mrelocatable-lib generates a .fixup section to allow static executables to be relocated at run time, but -mrelocatable-lib does not use the smaller stack alignment of -mrelocatable. Objects compiled with -mrelocatable-lib may be linked with objects compiled with any combination of the -mrelocatable options — so it's the flexible friend 💜
• -mlittle, -mlittle-endian — on System V.4 and embedded PowerPC systems, compile code for the processor in little-endian mode. The -mlittle-endian option is the same as -mlittle 🦋
• -mbig, -mbig-endian — on System V.4 and embedded PowerPC systems, compile code for the processor in big-endian mode. The -mbig-endian option is the same as -mbig 🌈
• -mdynamic-no-pic — on Darwin / macOS systems, compile code so that it is not relocatable, but its external references are relocatable. The resulting code is suitable for applications, but not shared libraries 🚫
• -msingle-pic-base — treats the register used for PIC addressing as read-only, rather than loading it in the prologue for each function. The runtime system is responsible for initializing this register with an appropriate value before execution begins 👑
• -mprioritize-restricted-insns=priority — controls the priority assigned to dispatch-slot restricted instructions during the second scheduling pass. The argument priority takes the value 0, 1, or 2 to assign no, highest, or second-highest (respectively) priority to dispatch-slot restricted instructions 💅
• -msched-costly-dep=dependence_type — 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 using powerpc-*-eabiaix 🎀
• -mcall-sysv-eabi, -mcall-eabi — specify both -mcall-sysv and -meabi options. two for one, slay 💫
• -mcall-sysv-noeabi — specify both -mcall-sysv and -mno-eabi options.
• -mcall-aixdesc — on System V.4 and embedded PowerPC systems, compile code for the AIX operating system 🍓
• -mcall-linux — on System V.4 and embedded PowerPC systems, compile code for the Linux-based GNU system 🐧
• -mcall-freebsd — on System V.4 and embedded PowerPC systems, compile code for the FreeBSD operating system 😈
• -mcall-netbsd — on System V.4 and embedded PowerPC systems, compile code for the NetBSD operating system 🎏
• -mcall-openbsd — on System V.4 and embedded PowerPC systems, compile code for the OpenBSD operating system 🐡
• -mtraceback=traceback_type — selects the type of traceback table. Valid values for traceback_type are full, part, and no 📋
• -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 — bestie, this one lets you extend the current ABI with one specific lil' extension, or yeet one back out 💅. The valid values are: altivec, no-altivec, ibmlongdouble, ieeelongdouble, elfv1, elfv2, and — special guest for AIX — vec-extabi, vec-default.
• -mabi=ibmlongdouble — switches the current ABI over to IBM extended-precision long double 🎀. Heads up though: this is not likely to work if your system already defaults to IEEE extended-precision long double. And if you change the long double type away from IEEE extended-precision, the compiler will throw a warning at you unless you pass -Wno-psabi. Requires -mlong-double-128 to be enabled, no cap.
• -mabi=ieeelongdouble — same vibe, opposite direction: switches the current ABI to IEEE extended-precision long double ✨. Also not likely to work if your system defaults to IBM extended-precision long double. Change the long double type away from IBM extended-precision and the compiler warns you unless you use -Wno-psabi. Also requires -mlong-double-128 to be enabled. Symmetry, we love it 💖.
• -mabi=elfv1 — changes the current ABI to the ELFv1 ABI. This is the default ABI for big-endian PowerPC 64-bit Linux 👑. But overriding the default ABI requires special system support and is likely to fail in spectacular ways 😭💀. You've been warned.
• -mabi=elfv2 — changes the current ABI to the ELFv2 ABI. This is the default for little-endian PowerPC 64-bit Linux 🌸. Same deal: overriding the default ABI needs special system support and is likely to fail in spectacular ways 🙈. Spectacular, bestie. Not "oops" spectacular — spectacular spectacular.
• -mgnu-attribute, -mno-gnu-attribute — emit (or don't emit) .gnu_attribute assembly directives that set tag/value pairs in a .gnu.attributes section, which spell out ABI variations in function parameters or return values 🦋.
• -mprototype, -mno-prototype — on System V.4 and embedded PowerPC systems, this assumes all calls to variable argument functions are properly prototyped. Otherwise the compiler has to insert an instruction before every non-prototyped call to set or clear bit 6 of the condition code register (CR), so it knows 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 that bit. Efficient queen behavior ✨.
• -msim — on embedded PowerPC systems, assumes the startup module is called sim-crt0.o and that the standard C libraries are libsim.a and libc.a 🧚. This is the default for powerpc-*-eabisim configurations.
• -mmvme — on embedded PowerPC systems, assumes the startup module is called crt0.o and the standard C libraries are libmvme.a and libc.a 🎀.
• -mads — on embedded PowerPC systems, assumes the startup module is called crt0.o and the standard C libraries are libads.a and libc.a 💜.
• -myellowknife — on embedded PowerPC systems, assumes the startup module is called crt0.o and the standard C libraries are libyk.a and libc.a 💛. Yes, yellowknife. Iconic name, no notes.
• -mvxworks — on System V.4 and embedded PowerPC systems, tells the compiler you're compiling for a VxWorks system 🛠️.
• -memb — on embedded PowerPC systems, sets the PPC_EMB bit in the ELF flags header to say that eabi extended relocations are in use 🏷️.
• -meabi, -mno-eabi — on System V.4 and embedded PowerPC systems, do (or do not) adhere to the Embedded Applications Binary Interface (EABI), which is a set of modifications to the System V.4 specs 📜. Picking -meabi means the stack is aligned to an 8-byte boundary, a function __eabi gets called from main to set up the EABI environment, and the -msdata option can use both r2 and r13 to point to two separate small data areas 👯. Picking -mno-eabi means the stack is aligned to a 16-byte boundary, no EABI initialization function is called from main, and -msdata only uses r13 to point to a single small data area. The -meabi option is on by default if you configured GCC using one of the powerpc*-*-eabi* options.
• -msdata=eabi — on System V.4 and embedded PowerPC systems, puts small initialized const global and static data in the .sdata2 section, which is pointed to by register r2 🎯. Puts small initialized non-const global and static data in the .sdata section, pointed to by register r13. Puts small uninitialized global and static data in the .sbss section, which sits right next to .sdata. Warning bestie: -msdata=eabi is incompatible with the -mrelocatable option ⚠️. It also sets the -memb option.
• -msdata=sysv — on System V.4 and embedded PowerPC systems, puts small global and static data in the .sdata section, pointed to by register r13 🎯. Small uninitialized global and static data goes in .sbss, adjacent to .sdata. And yeah — -msdata=sysv is incompatible with the -mrelocatable option too ⚠️.
• -msdata=default, -msdata — on System V.4 and embedded PowerPC systems: if -meabi is used, compile code the same as -msdata=eabi; otherwise compile code the same as -msdata=sysv 🪄. Situational icon.
• -msdata=data — on System V.4 and embedded PowerPC systems, puts small global data in the .sdata section, and small uninitialized global data in the .sbss section 🗂️. But — plot twist — it does not use register r13 to address small data. This is the default behavior unless other -msdata options are used.
• -msdata=none, -mno-sdata — on embedded PowerPC systems, puts all initialized global and static data in the .data section, and all uninitialized data in the .bss section 📦. Tidy.
• -mreadonly-in-sdata — puts read-only objects in the .sdata section as well ❄️. This is the default.
• -mblock-move-inline-limit=num — inlines all block moves (like calls to memcpy or 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 to memcmp or structure compares) that are less than or equal to num bytes 🔍. If num is 0, all inline expansion (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 isn't constant, at most num bytes get compared before memcmp is called to compare the rest 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 isn't found by the end of the inline compare, a call to strcmp or strncmp handles the rest of the comparison. The default is 64 bytes.
• -G num — on embedded PowerPC systems, puts global and static items less than or equal to num bytes into the small data or BSS sections instead of the normal data or BSS section 🎒. By default, num is 8. The -G num switch also gets passed to the linker, and — important — all modules should be compiled with the same -G num value ⚠️.
• -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, assumes 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 gets generated if the compiler knows the call can't possibly be that far away. You can override this setting with the shortcall function attribute, or with #pragma longcall(0).

Some linkers can detect out-of-range calls and generate glue code on the fly ✨. On those systems, long calls are unnecessary and generate slower code. As of this writing, the AIX linker can do this, and so can the GNU linker for PowerPC/64. It's 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 (peep -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) 🏝️. Those 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 gets 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 it or discard it 🗑️.

In the future, GCC may ignore all longcall specifications when the linker is known to generate glue 🔮. Future vibes.

• -mpltseq, -mno-pltseq — implements (or doesn't implement) -fno-plt and long calls using an inline PLT call sequence that supports lazy linking and long calls to functions in dlopen'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 that support is detected when configuring GCC — and, for 32-bit PowerPC, if GCC is configured with --enable-secureplt. Fair warning: -mpltseq code and -mbss-plt 32-bit PowerPC relocatable objects may not be linked together ⚠️.
• -mtls-markers, -mno-tls-markers — marks (or doesn't mark) calls to __tls_get_addr with a relocation specifying the function argument 🏷️. That relocation lets the linker reliably associate a function call with argument setup instructions for TLS optimization, which in turn lets GCC schedule the sequence better 🔥.
• -mrecip, -mno-recip — bestie, this one lets the compiler use reciprocal estimate and reciprocal square root estimate instructions plus extra Newton-Raphson steps to bump the precision, instead of doing a whole divide (or sqrt-then-divide) for floating-point args. 🎀 You should pair it with -ffast-math when you turn on -mrecip (or at minimum -funsafe-math-optimizations, -ffinite-math-only, -freciprocal-math and -fno-trapping-math). Heads up though: the throughput of that sequence is generally higher than the non-reciprocal instruction, but the precision can drop by up to 2 ulp (like, the inverse of 1.0 equals 0.99999994) for reciprocal square roots. 😭
• -mrecip=opt — this one's the control freak: it decides which reciprocal estimate instructions are allowed. opt is a comma-separated list, and any entry can be prefixed with "!" to invert it 💅:
  • all — enable all estimate instructions, slay.
  • 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 like, for example, -mrecip=all,!rsqrtd enables all of the reciprocal estimate instructions except the "FRSQRTE", "XSRSQRTEDP", and "XVRSQRTEDP" instructions, which are the ones handling the double-precision reciprocal square root calculations. iykyk 🦋

• -mrecip-precision, -mno-recip-precision — assume (or do not assume) that the reciprocal estimate instructions give higher-precision estimates than the PowerPC ABI mandates. Picking -mcpu=power6, -mcpu=power7 or -mcpu=power8 automatically selects -mrecip-precision. The double-precision square root estimate instructions are not generated by default on low-precision machines, because they don't provide an estimate that converges after three steps. ⭐
• -mveclibabi=type — says which ABI type to use for vectorizing intrinsics with an external library. The only type supported rn is mass, which tells it to use IBM's Mathematical Acceleration Subsystem (MASS) libraries for vectorizing intrinsics via 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, no cap. 🔥
• -mfriz, -mno-friz — generate (or don't generate) the "friz" instruction when -funsafe-math-optimizations is used to optimize rounding of floating-point values to 64-bit integer and back to floating point. Plot twist: the "friz" instruction does not return the same value if the floating-point number is too large to fit in an integer. 💀
• -mpointers-to-nested-functions, -mno-pointers-to-nested-functions — generate (or don't) code to load up the static chain register ("r11") when calling through a pointer on AIX and 64-bit Linux systems, where a function pointer points to a 3-word descriptor giving the function address, the TOC value to be loaded in register "r2", and the static chain value to be loaded in register "r11". -mpointers-to-nested-functions is on by default. Warning bestie: you cannot call through pointers to nested functions, or pointers to functions compiled in other languages that use the static chain, if you use -mno-pointers-to-nested-functions. 🙈
• -msave-toc-indirect, -mno-save-toc-indirect — generate (or don't) 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 isn't saved in the prologue, it gets saved just before the call through the pointer. The -mno-save-toc-indirect option is the default. 🫶
• -mcompat-align-parm, -mno-compat-align-parm — generate (or don't) code to pass structure parameters with a maximum alignment of 64 bits, for compatibility with older versions of GCC.

Older versions of GCC (prior to 4.9.0) incorrectly did not align a structure parameter on a 128-bit boundary when that structure contained a member requiring 128-bit alignment. This is corrected in more recent versions of GCC. This option may be used to generate code that's compatible with functions compiled with older versions of GCC.

The -mno-compat-align-parm option is the default. ✨

• -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 are global for a global canary, or tls for a per-thread canary in the TLS block (that's the default with GNU libc version 2.4 or later). 🎀

With that latter choice, -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. The default for those is as specified in the relevant ABI.

• -mpcrel, -mno-pcrel — generate (or don't) pc-relative addressing. The -mpcrel option requires that the medium code model (-mcmodel=medium) and prefixed addressing (-mprefixed) options are enabled. 🔥
• -mprefixed, -mno-prefixed — generate (or don't) addressing modes using prefixed load and store instructions. The -mprefixed option requires that the -mcpu=power10 (or later) option is enabled.
• -mmma, -mno-mma — generate (or don't) the MMA instructions. The -mma option requires that -mcpu=power10 (or later) is enabled. 👑
• -mdense-math, -mno-dense-math — generate (or don't) Dense Math Facility (DMF) instructions. The -mdense-math option requires that -mcpu=future (or later) is enabled. 🦋
• -mrop-protect, -mno-rop-protect — generate (or don't) ROP protection instructions when the target processor supports them. Currently this option disables the shrink-wrap optimization (-fshrink-wrap). 🌸
• -mprivileged, -mno-privileged — generate (or don't) code that runs in privileged state. main character energy only 💅
• -mblock-ops-unaligned-vsx, -mno-block-ops-unaligned-vsx — generate (or don't) unaligned vsx loads and stores for inline expansion of "memcpy" and "memmove".
• -msplat-word-constant, -mno-splat-word-constant — generate (or don't) code that uses the "XXSPLTIW" instruction. This option is enabled by default. ⭐
• -msplat-float-constant, -mno-splat-float-constant — generate (or don't) code that uses the "XXSPLTIDP" instruction. This option is enabled by default. ⭐
• -mieee128-constant, -mno-ieee128-constant — generate (or don't) code that uses the "LXVKQ" instruction. This option is enabled by default. ⭐
• -mwarn-altivec-long, -mno-warn-altivec-long — enable or disable warnings about deprecated vector long ... Altivec type usage. This option is enabled by default. 🙈

---

🧚 RX Options 🧚

These command-line options are defined for RX targets, bestie:

• -m64bit-doubles, -m32bit-doubles — make the "double" data type 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 why the default is -m32bit-doubles. 😭
• -fpu, -nofpu — enables (-fpu) or disables (-nofpu) the use of RX floating-point hardware. The default is enabled for the RX600 series and disabled for the RX200 series.

Floating-point instructions are only generated for 32-bit floating-point values, however, so the FPU hardware is not used for doubles if the -m64bit-doubles option is used. 💀

Note: if the -fpu option is enabled then -funsafe-math-optimizations is also enabled automatically. This is because the RX FPU instructions are themselves unsafe. 🙈

• -mcpu=name — selects the type of RX CPU to target. Currently three types are supported: the generic RX600 and RX200 series hardware, and the specific RX610 CPU. The default is RX600. 🎀

The only difference between RX600 and RX610 is that the RX610 does not support the "MVTIPL" instruction.

The RX200 series does not have a hardware floating-point unit, and so -nofpu is enabled by default when this type is selected.

• -mbig-endian-data, -mlittle-endian-data — store data (but not code) in 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 that 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's up to the programmer to make sure the area doesn't overflow. 👑 Also, when the small data area is used, one of the RX's registers (usually "r13") is reserved for pointing at this area, so it's no longer available for the compiler to use. That could result in slower and/or larger code if variables get 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, because they're assigned to other sections in the output executable.

The default value is zero, which straight up disables this whole feature. And no, it doesn't sneakily turn on at higher optimization levels (-O2 etc.) — because reserving a register can genuinely make things worse, so GCC isn't gonna do that to you unprompted. It's on you, the programmer, to experiment and find out if it actually helps your program. Curious how the register that holds the small data area pointer gets picked? Scroll to the -mpid description, it's all spelled out there. 🎀

• -msim, -mno-sim — use the simulator runtime. Default is the libgloss board-specific runtime, no cap.
• -mas100-syntax, -mno-as100-syntax — when you're generating assembler output, use a syntax that plays nice with Renesas's AS100 assembler. The GAS assembler can handle this syntax too, but it comes with restrictions, which is why it's not the default. 💅
• -mmax-constant-size=N — sets the biggest size, in bytes, of a constant that can be used as an operand in an RX instruction. The RX instruction set does allow constants up to 4 bytes long in instructions, but a longer value = a longer instruction. So sometimes it's lowkey beneficial to cap how big the constants in your instructions can get. Constants that are too chonky get dropped into a constant pool instead and referenced via register indirection. N can be anywhere from 0 to 4. A value of 0 (the default) or 4 means constants of any size are allowed. ✨
• -mrelax — turn on linker relaxation. That's where the linker tries to shrink your program by hunting for shorter versions of various instructions. Disabled by default.
• -mint-register=N — how many registers do you wanna reserve for fast interrupt handler functions? N is between 0 and 4. 1 means register r13 is reserved exclusively for fast interrupt handlers. 2 reserves r13 and r12. 3 reserves r13, r12 and r11. And 4 reserves r13 through r10. A value of 0 — the default — reserves nothing at all. 🫶
• -msave-acc-in-interrupts — says interrupt handler functions should preserve the accumulator register. You only need this if normal code might actually use the accumulator register, like if it's doing 64-bit multiplications. Default is to just ignore the accumulator, because ignoring it makes interrupt handlers faster. ⚡
• -mpid, -mno-pid — enables position-independent data generation. When it's on, any access to constant data happens via an offset from a base address that lives in a register. That means the location of constant data can be figured out at run time without needing the executable to be relocated — huge win for embedded apps with tight memory constraints. Data that can be modified? Not affected by this at all. 💖

Heads up though: this feature reserves a register, usually r13, for the constant data base address. That can result in slower and/or larger code, especially in complicated functions. 😭

Which register actually ends up holding the constant data base address depends on whether -msmall-data-limit and/or the -mint-register command-line options are enabled. Starting at register r13 and going downwards, registers get allocated first to satisfy -mint-register, then -mpid, and finally -msmall-data-limit. So yeah — it's totally possible for the small data area register to end up being r8 if you specify both -mint-register=4 and -mpid on the command line. 🤯

By default this feature is off. You can restore the default with -mno-pid.

• -mno-warn-multiple-fast-interrupts, -mwarn-multiple-fast-interrupts — stops GCC from throwing a warning when it finds more than one fast interrupt handler while compiling a file. The default is to warn you about each extra fast interrupt handler it finds, since RX only supports one such interrupt. 🙈
• -mallow-string-insns, -mno-allow-string-insns — enables or disables the use of the string manipulation instructions SMOVF, SCMPU, SMOVB, SMOVU, SUNTIL, SWHILE and also the RMPA instruction. These instructions may prefetch data, which is not safe to do if you're accessing an I/O register. (Section 12.2.7 of the RX62N Group User's Manual has more, if you wanna go read it.) 👀

Default is to allow these instructions. But GCC literally can't reliably detect every circumstance where a string instruction might be used to poke an I/O register, so it can't just disable them automatically. It's on the programmer — so if your program accesses I/O space, you use -mno-allow-string-insns.

When the instructions are enabled, GCC defines the C preprocessor symbol __RX_ALLOW_STRING_INSNS__; otherwise it defines __RX_DISALLOW_STRING_INSNS__. 🏷️

• -mjsr, -mno-jsr — use only (or not only) JSR instructions to access functions. This one's for when your code size exceeds the range of BSR instructions. Important: -mno-jsr does not mean "don't use JSR" — it means any type of branch may be used. Read it twice, bestie. 💀
• -mlra — use the new LRA register allocator. By default the old reload allocator is what's used.

Note: the generic GCC command-line option -ffixed-reg is extra special to the RX port when you use it with the interrupt function attribute. That attribute marks a function as one meant to process fast interrupts. GCC makes sure it only uses registers r10, r11, r12 and/or r13 — and only as long as the normal use of those corresponding registers has been restricted via the -ffixed-reg or -mint-register command-line options. 👑

---

🌈 S/390 and zSeries Options

These are the -m options defined for the S/390 and zSeries architecture. Let's gooo 🚀

• -mhard-float, -msoft-float — use (or don't use) the hardware floating-point instructions and registers for floating-point operations. With -msoft-float, functions in libgcc.a do the floating-point work. With -mhard-float, the compiler generates IEEE floating-point instructions. -mhard-float is the default. 💅
• -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 in libgcc.a handle decimal-floating-point ops. With -mhard-dfp, the compiler generates decimal-floating-point hardware instructions. This is the default for -march=z9-ec or higher. ✨
• -mlong-double-64, -mlong-double-128 — these control the size of the long double type. A size of 64 bits makes long double equivalent to double. That's the default. 🎀
• -mbackchain, -mno-backchain — store (or don't store) the address of the caller's frame as a backchain pointer in 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-stack is in effect, the backchain pointer goes at the bottom of the stack frame; when -mpacked-stack is in effect, the backchain goes into the topmost word of the 96/160 byte register save area. 🗂️

Generally, code compiled with -mbackchain is call-compatible with code compiled with -mno-backchain. However — using the backchain for debugging usually requires the whole binary to be built with -mbackchain. And heads up: the combo of -mbackchain, -mpacked-stack and -mhard-float is not supported. If you're building a linux kernel, use -msoft-float. 🐧

Default is to not maintain the backchain.

• -mpacked-stack, -mno-packed-stack — use (or don't use) the packed stack layout. With -mno-packed-stack, the compiler uses all fields of the 96/160 byte register save area only for their default purpose, so unused fields still eat up stack space. With -mpacked-stack, register save slots get densely packed at the top of the register save area, and unused space gets reused for other stuff, which means more efficient use of the stack space you've got. But when -mbackchain is also in effect, the topmost word of the save area is always used for 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. Note though: 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. That code is not call-compatible with code compiled with -mpacked-stack. Also, again: -mbackchain + -mpacked-stack + -mhard-float is not supported. Building a linux kernel? Use -msoft-float. 🐧

Default is to not use the packed stack layout.

• -msmall-exec, -mno-small-exec — generate (or don't generate) code that uses the bras instruction to do subroutine calls. This only works reliably if your total executable size doesn't exceed 64k. Default is to use the basr instruction instead, which has no such limitation. 🔥
• -m64, -m31 — -m31 generates code compliant with the GNU/Linux for S/390 ABI. -m64 generates code compliant with the GNU/Linux for zSeries ABI, which lets GCC generate 64-bit instructions. For s390 targets the default is -m31, while s390x targets default to -m64. ⚠️ Note: -m31 is deprecated and support will be removed.
• -mzarch, -mesa — -mzarch generates code using the instructions available on z/Architecture. -mesa generates code using the instructions available on ESA/390. Note that -mesa is not possible with -m64. When generating code compliant with the GNU/Linux for S/390 ABI, the default is -mesa. When generating code compliant with the GNU/Linux for zSeries ABI, the default is -mzarch. 🌸
• -mhtm, -mno-htm — turning this on unlocks a whole little squad of builtins that use the transactional execution instructions 💫 from the IBM zEnterprise EC12 machine generation, all part of the S/390 System z Built-in Functions. And the slay part? -mhtm is on by default the moment you pass -march=zEC12. Free real estate.
• -mvx, -mno-vx — says "bestie, use the vector extension facility instructions" 🌈, which showed up with the IBM z13 machine generation. Heads up though, this changes the ABI for some vector type values — alignment and calling conventions both get a glow-up. So if vector type values are chillin' in an ABI-relevant context, a GAS .gnu_attribute command gets tacked on to mark the resulting binary with which ABI it's using. -mvx is enabled by default with -march=z13. iykyk.
• -mzvector, -mno-zvector — this one enables vector language extensions plus builtins using the vector extension facility instructions from the IBM z13 machine generation. It adds support for vector as a keyword for defining vector type variables and arguments 🦋. Tiny catch: vector only exists when GNU extensions are enabled — it is not expanded when you're requesting strict standard compliance, e.g. with -std=c99. In addition to the GCC low-level builtins, -mzvector also switches on a set of builtins added for compatibility with AltiVec-style implementations like Power and Cell. To actually use them you must include the header file vecintrin.h. And -mzvector is disabled by default — no auto-magic ✨.
• -mmvcle, -mno-mvcle — generate (or don't 💅) code using the mvcle instruction to do block moves. With -mno-mvcle you get a mvc loop instead. That loop vibe is the default unless you're optimizing for size.
• -mdebug, -mno-debug — print (or don't print 🙈) extra debug info while compiling. Default is nope, no debug info.
• -march=cpu-type — generate code that runs on cpu-type, which is the name of a system representing a certain processor type. Possible values, and yes I'm listing every single one 🔥: z900/arch5, z990/arch6, z9-109, z9-ec/arch7, z10/arch8, z196/arch9, zEC12, z13/arch11, z14/arch12, z15/arch13, z16/arch14, z17/arch15, and native.

The default is -march=z900.

Passing native as the cpu type picks the best architecture option for the host processor. One caveat: -march=native has no effect if GCC doesn't recognize the processor. 💀

• -mtune=cpu-type — tune everything applicable about the generated code to cpu-type, except the ABI and the set of available instructions. Same cpu-type list as -march — literally the identical roster 🎀. Default is whatever value you used for -march.
• -mtpf-trace, -mno-tpf-trace — generate code that adds (or doesn't add) TPF OS specific branches to trace routines in the operating system. This is off by default, even when compiling for the TPF OS. Like, even then. 🙃
• -mtpf-trace-skip, -mno-tpf-trace-skip — generate code that changes (or doesn't change) the default branch targets enabled by -mtpf-trace so they point to specialized trace routines that give you the ability to selectively skip function trace entries for the TPF OS. Also off by default — even when compiling for the TPF OS and specifying -mtpf-trace. Double off. 😭
• -mmain — for the TPF OS target, use this when linking to add the startup code and other linker options to produce a main object. 🫶
• -mfused-madd, -mno-fused-madd — generate code that uses (or doesn't use) the floating-point multiply and accumulate instructions. These get generated by default if hardware floating point is used. ✨
• -mwarn-framesize=framesize — emit a warning if the current function blows past the given frame size. Because this is a compile-time check it doesn't have to be an actual problem when the program runs — its whole job is spotting functions that most probably cause a stack overflow. Super useful for environments with a limited stack size, e.g. the Linux kernel 🐧.

A value of zero disables this warning, and that's the default.

• -mwarn-dynamicstack, -mno-warn-dynamicstack — emit a warning if the function calls alloca or uses dynamically-sized arrays. Ngl, that's generally a bad idea when your stack size is limited. 🚩
• -mstack-guard=stack-guard, -mstack-size=stack-size, -mno-stack-guard, -mno-stack-size — when these are enabled, the S/390 back end emits extra instructions in the function prologue that trigger a trap if the stack size is stack-guard bytes above stack-size. (Remember the stack on S/390 grows downward 🙃.) If the stack-guard option is omitted, the smallest power of 2 larger than the frame size of the compiled function is chosen.

These options are meant to help debug stack overflow problems. The additionally emitted code causes only little overhead, so it can also be used in production-like systems without greater performance degradation. 💅

The given values have to be exact powers of 2, and stack-size has to be greater than stack-guard without exceeding 64k. To stay efficient the extra code assumes the stack starts at an address aligned to the value given by stack-size.

Fr fr: -mstack-guard= can only be used in conjunction with -mstack-size=. You can override these options on the command line with -mno-stack-guard and -mno-stack-size, respectively.

• -mhotpatch=pre-halfwords,post-halfwords — with hotpatch enabled, a "hot-patching" function prologue is generated for all functions in the compilation unit. The function label gets prepended with the given number of two-byte NOP instructions (pre-halfwords, maximum 1000000). After the label, 2 * post-halfwords bytes are appended, using the largest NOP-like instructions the architecture allows (maximum 1000000). 👑

If both arguments are zero, hotpatching is disabled.

And you can override this per-function with the hotpatch attribute.

• -mpic-data-is-text-relative, -mno-pic-data-is-text-relative — when compiling for S/390 with -fpic or -fPIC, GCC normally emits code using relative addressing between code and data. But for hotpatching you might need to introduce new .text parts while using the existing .data and .bss sections 🫠. In that case, use -mno-pic-data-is-text-relative to force addressing through the GOT instead.
• -mstack-protector-guard=guard, -mstack-protector-guard-record — generate stack protection code using a canary at guard. Supported locations are global for a global canary, or tls for a per-thread canary in the TLS block (that one's the default). ✨

-mstack-protector-guard-record makes the compiler generate a section __stack_protector_loc containing pointers to all the instructions that load the address of the global guard. So it only has an effect in conjunction with -mstack-protector-guard=global. Intended use? The Linux kernel 🐧💖.

• -mindirect-branch=choice, -mindirect-branch-jump=choice, -mindirect-branch-call=choice, -mfunction-return=choice, -mfunction-return-mem=choice, -mfunction-return-reg=choice — this whole group is out here addressing security vulnerabilities related to speculative execution and indirect branch prediction 🔥 by letting you replace indirect branches and function returns with direct branches to a thunk.

The choice deets, bestie:

• keep → generates the default code, nothing spicy.
• thunk → generates out-of-line thunks, and the formerly indirect branch becomes a direct branch to the thunk.
• thunk-extern → does the branch replacement like thunk but does not emit the thunks.
• thunk-inline → only available with -mindirect-branch-jump; use it in preference to thunk in user-space applications so you get correct stack unwinding and exception handling. 🧚

And the wiring: -mindirect-branch sets the value of -mindirect-branch-jump and -mindirect-branch-call. -mfunction-return sets the value of -mfunction-return-reg and -mfunction-return-mem.

All of these can also be set on a per-function basis using function attributes. 💅

• -mindirect-branch-table, -mno-indirect-branch-table — useful alongside the -mindirect-branch and -mfunction-return options. When enabled, it causes generation of tables pointing to the branch locations that those options patched. The tables get emitted in sections named .s390_indirect_jump, .s390_indirect_call, .s390_return_reg, and .s390_return_mem. Each section is an array of 32-bit elements, and each entry holds the offset from the entry to the patched location. 🎀
• -mfentry, -mno-fentry — used with the -pg option, this emits profiling code using the __fentry__ hook provided by GLIBC 2.29 or later. Only available for 64-bit code 🦋. If this option isn't enabled, profiling uses the less efficient _mcount hook instead. 😬
• -mrecord-mcount, -mno-record-mcount — when enabled, generate a __mcount_loc section containing the locations of all the generated profiling calls to _mcount or __fentry__. ⭐
• mnop-mcount, mno-nop-mcount — instead of generating calls to _mcount or __fentry__ with -pg, insert a sequence of no-op instructions of the same length right where the calls would have gone. Combine it with -mrecord-mcount to patch the call sequences into the object file without recompiling it. Iconic, honestly. 👑
• -mpreserve-args, -mno-preserve-args — when enabled, save all the argument registers to the stack and generate corresponding CFI information. Useful for applications that wanna implement their own stack unwinding and need access to function arguments. 🫶
• -munaligned-symbols, -mno-unaligned-symbols — assume that external symbols with a natural alignment of 1 are potentially unaligned. By default, all symbols without explicit alignment are assumed to be aligned on a 2-byte boundary as mandated by the IBM Z ABI. 💜

---

🦋 SH Options 🦋

These -m options are defined for the SH implementations, bestie:

• -m1 — generate code for the SH1. ✨
• -m2 — generate code for the SH2. ✨
• -m2e — generate code for the SH2e. ✨

okay bestie, we're still deep in SH chip-family territory and it's giving "whole buffet of silicon, pick your plate" 💅 every flag below is literal, so don't freestyle the names, okay?

• -m2a-nofpu — generate code for the SH2a without FPU, or for a SH2a-FPU in such a way that the floating-point unit is not used.
• -m2a-single-only — generate code for the SH2a-FPU in such a way that no double-precision floating-point operations are used.
• -m2a-single — generate code for the SH2a-FPU assuming the floating-point unit is in single-precision mode by default.
• -m2a — generate code for the SH2a-FPU assuming the floating-point unit is in double-precision mode by default.
• -m3 — generate code for the SH3.
• -m3e — generate code for the SH3e.
• -m4-nofpu — generate code for the SH4 without a floating-point unit.
• -m4-single-only — generate code for the SH4 with a floating-point unit that only supports single-precision arithmetic.
• -m4-single — generate code for the SH4 assuming the floating-point unit is in single-precision mode by default.
• -m4 — generate code for the SH4.
• -m4-100 — generate code for SH4-100.
• -m4-100-nofpu — generate code for SH4-100 in such a way that the floating-point unit is not used.
• -m4-100-single — generate code for SH4-100 assuming the floating-point unit is in single-precision mode by default.
• -m4-100-single-only — generate code for SH4-100 in such a way that no double-precision floating-point operations are used.
• -m4-200 — generate code for SH4-200.
• -m4-200-nofpu — generate code for SH4-200 “without in such a way that the floating-point unit is not used” — yes bestie, that's the page's wording, the FPU is not used 🫶
• -m4-200-single — generate code for SH4-200 assuming the floating-point unit is in single-precision mode by default.
• -m4-200-single-only — generate code for SH4-200 in such a way that no double-precision floating-point operations are used.
• -m4-300 — generate code for SH4-300.
• -m4-300-nofpu — generate code for SH4-300 “without in such a way that the floating-point unit is not used” — same no-FPU energy as above.
• -m4-300-single — generate code for SH4-300 in such a way that no double-precision floating-point operations are used. (That's what it says for this one, bestie — no double-precision ops.)
• -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).
• -m4-400 — generate code for SH4-400 (no MMU, no FPU).
• -m4-500 — generate code for SH4-500 (no FPU). Passes -isa=sh4-nofpu to the assembler.
• -m4a-nofpu — generate code for the SH4al-dsp, or for a SH4a in such a way that the floating-point unit is not used.
• -m4a-single-only — generate code for the SH4a, in such a way that no double-precision floating-point operations are used.
• -m4a-single — generate code for the SH4a assuming the floating-point unit is in single-precision mode by default.
• -m4a — generate code for the SH4a.
• -m4al — same as -m4a-nofpu, except that it implicitly passes -dsp to the assembler. GCC doesn't generate any DSP instructions at the moment.
• -mb — compile code for the processor in big-endian mode.
• -ml — compile code for the processor in little-endian mode.
• -mdalign — align doubles at 64-bit boundaries. Note that this changes the calling conventions, and thus some functions from the standard C library do not work unless you recompile it first with -mdalign. Not me warning you twice, bestie — this is a real compatibility vibe check 😭
• -mrelax — shorten some address references at link time, when possible; uses the linker option -relax.
• -mbigtable — use 32-bit offsets in switch tables. The default is to use 16-bit offsets.
• -mbitops — enable the use of bit manipulation instructions on SH2A.
• -mfmovd — enable the use of the instruction fmovd. Check -mdalign for alignment constraints.
• -mrenesas, -mno-renesas — comply with the calling conventions defined by Renesas. The default for all targets of the SH toolchain is -mno-renesas, which uses the calling conventions defined for GCC before the Renesas conventions were available.
• -mnomacsave — mark the MAC register as call-clobbered, even if -mrenesas is given.
• -mieee, -mno-ieee — control the IEEE compliance of floating-point comparisons, which affects the handling of cases where the result of a comparison is unordered. By default -mieee is implicitly enabled. If -ffinite-math-only is enabled, -mno-ieee is implicitly set, which results in faster floating-point greater-equal and less-equal comparisons. The implicit settings can be overridden by specifying either -mieee or -mno-ieee.
• -minline-ic_invalidate, -mno-inline-ic_invalidate — inline code to invalidate instruction cache entries after setting up nested function trampolines. This option has no effect if -musermode is in effect and the selected code generation option (e.g. -m4) does not allow the use of the icbi instruction. If the selected code generation option does not allow the use of the icbi instruction, and -musermode is not in effect, the inlined code manipulates the instruction cache address array directly with an associative write. This not only requires privileged mode at run time, but it also fails if the cache line had been mapped via the TLB and has become unmapped. That last bit is a whole trap, literally and figuratively 💀
• -misize — dump instruction size and location in the assembly code.
• -matomic-model=model — sets the model of atomic operations and additional parameters as a comma-separated list. For details on the atomic built-in functions see __atomic Builtins. The following models and parameters are supported:
  • none — disable compiler generated atomic sequences and emit library calls for atomic operations. This is the default if the target is not sh*-*-linux*.
  • soft-gusa — generate GNU/Linux-compatible gUSA software atomic sequences for the atomic built-in functions. The generated atomic sequences require additional support from the interrupt/exception handling code of the system and are only suitable for SH3* and SH4* single-core systems. This option is enabled by default when the target is sh*-*-linux* and SH3* or SH4*. When the target is SH4A, this option also partially utilizes the hardware atomic instructions movli.l and movco.l to create more efficient code, unless strict is specified.
  • soft-tcb — generate software atomic sequences that use a variable in the thread control block. This is a variation of the gUSA sequences which can also be used on SH1* and SH2* targets. The generated atomic sequences require additional support from the interrupt/exception handling code of the system and are only suitable for single-core systems. When using this model, the gbr-offset= parameter has to be specified as well.
  • soft-imask — generate software atomic sequences that temporarily disable interrupts by setting SR.IMASK = 1111. This model works only when the program runs in privileged mode and is only suitable for single-core systems. Additional support from the interrupt/exception handling code of the system is not required. This model is enabled by default when the target is sh*-*-linux* and SH1* or SH2*.
  • hard-llcs — generate hardware atomic sequences using the movli.l and movco.l instructions only. This is only available on SH4A and is suitable for multi-core systems. Since the hardware instructions support only 32-bit atomic variables, access to 8- or 16-bit variables is emulated with 32-bit accesses. Code compiled with this option is also compatible with other software atomic model interrupt/exception handling systems if executed on an SH4A system. Additional support from the interrupt/exception handling code of the system is not required for this model.
  • gbr-offset= — this parameter specifies the offset in bytes of the variable in the thread control block structure that should be used by the generated atomic sequences when the soft-tcb model has been selected. For other models this parameter is ignored. The specified value must be an integer multiple of four and in the range 0-1020.
  • strict — this parameter prevents mixed usage of multiple atomic models, even if they are compatible, and makes the compiler generate atomic sequences of the specified model only.
• -mtas — generate the tas.b opcode for __atomic_test_and_set. Notice that depending on the particular hardware and software configuration this can degrade overall performance due to the operand cache line flushes that are implied by the tas.b instruction. On multi-core SH4A processors the tas.b instruction must be used with caution since it can result in data corruption for certain cache configurations. Fr fr, read that twice if you're on multi-core SH4A 😭
• -mprefergot — when generating position-independent code, emit function calls using the Global Offset Table instead of the Procedure Linkage Table.
• -musermode, -mno-usermode — don't allow (allow) the compiler to generate privileged mode code. Specifying -musermode also implies -mno-inline-ic_invalidate if the inlined code would not work in user mode. -musermode is the default when the target is sh*-*-linux*. If the target is SH1* or SH2*, -musermode has no effect, since there is no user mode.
• -multcost=number — set the cost to assume for a multiply instruction.
• -mdiv=strategy — set the division strategy to be used for integer division operations. strategy can be one of:
  • call-div1 — calls a library function that uses the single-step division instruction div1 to perform the operation. Division by zero calculates an unspecified result and does not trap. This is the default except for SH4, SH2A and SHcompact.
  • call-fp — calls a library function that performs the operation in double precision floating point. Division by zero causes a floating-point exception. This is the default for SHcompact with FPU. Specifying this for targets that do not have a double-precision FPU defaults to call-div1.
• call-table — calls a library function that uses a lookup table for small divisors, plus the div1 instruction with case distinction for the bigger divisors 💅. Dividing by zero here calculates an unspecified result and does not trap — yes bestie, unspecified, so don't come crying to me about what you got 🙈. This is the default for SH4. And if you specify it for targets that don't have dynamic shift instructions, it defaults to call-div1 instead ✨.

So when nobody's specified a division strategy at all, the default gets picked based on whatever the current target is. For SH2A the default strategy is to use the divs and divu instructions instead of library function calls — no phone-a-friend needed, the hardware just handles it 👑

• -maccumulate-outgoing-args, -mno-accumulate-outgoing-args — reserve space once for outgoing arguments in the function prologue instead of doing it around every single call 🎀. This is generally a glow-up for both performance and size, and it's also needed for unwinding so you're not changing the stack frame around conditional code. -maccumulate-outgoing-args is enabled by default — she's already on 🔥.
• *-mdivsi3_libfunc=name* — set the name of the library function used for 32-bit signed division to name. Heads up tho: this only affects the name used in the call division strategies, and the compiler still expects the exact same sets of input/output/clobbered registers as if this option weren't there at all. No sneaky register renegotiation 🫶.
• *-mfixed-range=register-range* — generate code treating the given register range as fixed registers. A fixed register is one the register allocator literally cannot touch, which is super useful when you're compiling kernel code 👑. A register range is two registers separated by a dash, and you can stack multiple ranges by separating them with a comma.
• *-mbranch-cost=num* — assume num is the cost of a branch instruction. Higher numbers make the compiler try to generate more branch-free code when it can. If you don't specify it, the value gets picked based on the processor type you're compiling for ✨.
• -mzdcbranch, -mno-zdcbranch — assume (or don't assume) that the zero displacement conditional branch instructions bt and bf are fast 💅. With -mzdcbranch the compiler prefers zero displacement branch code sequences. It's on by default when generating code for SH4 and SH4A, and you can explicitly turn it off with -mno-zdcbranch.
• -mcbranch-force-delay-slot — force the usage of delay slots for conditional branches, which stuffs the delay slot with a nop if no suitable instruction can be found 😭. Disabled by default, but you can enable it to work around hardware bugs like the ones found in the original SH7055. She's a workaround girlie, not a default girlie.
• -mfsca, -mno-fsca — allow or disallow the compiler from emitting the fsca instruction for sine and cosine approximations 🌀. -mfsca must be used together with -funsafe-math-optimizations, no solo runs. It's enabled by default when generating code for SH4A. And -mno-fsca disables sine and cosine approximations even if -funsafe-math-optimizations is in effect — she said no and she meant it 💀.
• -mfsrra, -mno-fsrra — allow or disallow the compiler from emitting the fsrra instruction for reciprocal square root approximations 🌸. -mfsrra must be used in combination with -funsafe-math-optimizations and -ffinite-math-only. Enabled by default when generating code for SH4A. -mno-fsrra disables reciprocal square root approximations even if both of those flags are in effect.
• -mpretend-cmove, -mno-pretend-cmove — prefer (or don't prefer) zero-displacement conditional branches for conditional move instruction patterns. This can result in faster code on the SH4 processor ⚡.
• -mfdpic, -mno-fdpic — generate code using the FDPIC ABI.
• -mlra, -mno-lra — use the new LRA register allocator. By default the old reload allocator is used — she's vintage, she's classic, she's still here 🙈.

---

🌞 Solaris 2 Options 🌞

These options are supported on Solaris 2, bestie:

• -mclear-hwcap, -mno-clear-hwcap — -mclear-hwcap tells the compiler to remove the hardware capabilities generated by the Solaris assembler 🎀. This is only necessary when object files use ISA extensions that the current machine doesn't support, but check at runtime whether or not to use them.
• -mimpure-text, -mno-impure-text — -mimpure-text, used in addition to -shared, tells the compiler not to pass -z text to the linker when linking a shared object. With 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. But — and this is the part you need to hear — 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. That's the real slay.

• -gsctf — generate Solaris CTF. Needs to be used both for compilation and linking 🫶. See ctf(7) for more information. This is only supported since Solaris 11.4 SRU 84, which is where the necessary toolchain support got added ✨.

---

🦋 SPARC Options 🦋

These -m options are supported on the SPARC:

• -mno-app-regs, -mapp-regs — specify -mapp-regs to generate output using the global registers 2 through 4, which the SPARC SVR4 ABI reserves for applications 💅. Just like global register 1, each global register 2 through 4 is then treated as an allocable register that gets 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, no cap 🫶.

• -mflat, -mno-flat — with -mflat, the compiler doesn't generate save/restore instructions and instead 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 get saved on the stack as needed.

With -mno-flat (the default), the compiler generates save/restore instructions (except for leaf functions). That's the normal operating mode, bestie.

• -mfpu, -mhard-float — generate output containing floating-point instructions. This is the default 🔥.
• -mno-fpu, -msoft-float — generate output containing library calls for floating point. ⚠️ Warning: the requisite libraries are not available for all SPARC targets. Normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation. The embedded targets sparc-*-aout and sparclite-*-* do provide software floating-point support 💜.

-msoft-float changes the calling convention in the output file, which means it's 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 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 the ones specified in the SPARC ABI 🌸. This is the default.

As of this writing, there are no SPARC implementations with 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. That's exactly why -msoft-quad-float is the default.

• -mno-unaligned-doubles, -munaligned-doubles — assume that doubles have 8-byte alignment. This is the default ✨.

With -munaligned-doubles, GCC assumes doubles have 8-byte alignment only if they're 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 — don't generate code that can only run in supervisor mode. This is relevant only for the casa instruction emitted for the LEON3 processor 🎀. This is the default.
• -mfaster-structs, -mno-faster-structs — with -mfaster-structs, the compiler assumes structures should have 8-byte alignment. This enables pairs of ldd and std instructions for copies in structure assignment, instead of twice as many ld and st pairs ⚡. However, using this changed alignment directly violates the SPARC ABI 😱. So it's intended only for targets where the developer acknowledges that their resulting code is not directly in line with the rules of the ABI. You've been warned, bestie.
• -mstd-struct-return, -mno-std-struct-return — with -mstd-struct-return, the compiler generates checking code in functions returning structures or unions to detect size mismatches between the two sides of function calls, as per the 32-bit ABI 🫶.

The default is -mno-std-struct-return. This option has no effect in 64-bit mode.

• *-mcpu=cpu_type* — set the instruction set, register set, and 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 doesn't recognize the processor.

Default instruction scheduling parameters are 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, iconically:

• v7 — cypress, leon3v7
• v8 — supersparc, hypersparc, leon, leon3, leon5
• sparclite — f930, f934, sparclite86x
• sparclet — tsc701
• v9 — ultrasparc, ultrasparc3, niagara, niagara2, niagara3, niagara4, niagara7, m8

Okay so like, continuing the SPARC saga, bestie — we're still in the -mcpu=cpu_type multiverse 🌌💅

By default (unless you configured it otherwise, which, fair), GCC generates code for the V7 variant of the SPARC architecture. With -mcpu=cypress, the compiler additionally optimizes it for the Cypress CY7C602 chip — the icon 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. 👑

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• -mtune=cpu_type — sets the instruction scheduling parameters for machine type cpu_type, but does not set the instruction set or register set that -mcpu=cpu_type does. 🎚️

The same values for -mcpu=cpu_type can be used for -mtune=cpu_type, but the only useful values are those that select a particular CPU implementation. Those are cypress, supersparc, hypersparc, leon, leon3, leon3v7, 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 -mvis2 when targeting a cpu that supports such instructions, such as UltraSPARC-III and later. Setting -mvis2 also sets -mvis. ✨
• -mvis3, -mno-vis3 — with -mvis3, GCC generates code that takes advantage of version 3.0 of the UltraSPARC Visual Instruction Set extensions. The default is -mvis3 when targeting a cpu that supports such instructions, such as niagara-3 and later. Setting -mvis3 also sets -mvis2 and -mvis. 🌟
• -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 -mvis3b when targeting a cpu that supports such instructions, such as niagara-7 and later. Setting -mvis3b also sets -mvis3, -mvis2 and -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 -mvis4 when targeting a cpu that supports such instructions, such as niagara-7 and later. Setting -mvis4 also sets -mvis3b, -mvis3, -mvis2 and -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 -mvis4b when targeting a cpu that supports such instructions, such as m8 and later. Setting -mvis4b also sets -mvis4, -mvis3b, -mvis3, -mvis2 and -mvis. 🎀
• -mcbcond, -mno-cbcond — with -mcbcond, GCC generates code that takes advantage of the UltraSPARC Compare-and-Branch-on-Condition instructions. The default is -mcbcond when 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 -mfmaf when 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 -mfsmuld when 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 -mpopc when 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 -msubxc when 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, no cap 🫶

• -m32, -m64 — generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long and pointer to 32 bits. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits. 💅
• -mptr32, -mptr64 — use 32- or 64-bit pointers, respectively. Unlike the -m32 and -m64 options, this affects only the pointer size and not the ABI. 👀
• -mcmodel=which — set the code model to one of:
  • medlow — The Medium/Low code model: 64-bit addresses, programs must be linked in the low 32 bits of memory. Programs can be statically or dynamically linked. 🏙️
  • medmid — The Medium/Middle code model: 64-bit addresses, programs must be linked in the low 44 bits of memory, the text and data segments must be less than 2GB in size and the data segment must be located within 2GB of the text segment. 🏘️
  • medany — The Medium/Anywhere code model: 64-bit addresses, programs may be linked anywhere in memory, the text and data segments must be less than 2GB in size and the data segment must be located within 2GB of the text segment. 🌍
  • embmedany — The Medium/Anywhere code model for embedded systems: 64-bit addresses, the text and data segments must be less than 2GB in size, both starting anywhere in memory (determined at link time). The global register %g4 points to the base of the data segment. Programs are statically linked and PIC is not supported. 📌
• -mmemory-model=mem-model — set the memory model in force on the processor to one of:
  • default — The default memory model for the processor and operating system. 🧸
  • rmo — Relaxed Memory Order. 🍃
  • pso — Partial Store Order. 🧩
  • tso — Total Store Order. 📚
  • sc — Sequential Consistency. 🔒

These memory models are formally defined in Appendix D of the SPARC-V9 architecture manual, as set in the processor's "PSTATE.MM" field. 📖

• -mstack-bias, -mno-stack-bias — with -mstack-bias, GCC assumes that the stack pointer, and frame pointer if present, are offset by -2047 which must be added back when making stack frame references. This is the default in 64-bit mode. Otherwise, assume no such offset is present. 📐

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💜 Options for System V 💜

These additional options are available on System V Release 4 for compatibility with other compilers on those systems:

• -G — Create a shared object. It is recommended that -symbolic or -shared be used instead. 🫶
• -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. 🗂️

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🌸 V850 Options 🌸

These -m options are defined for V850 implementations:

• -mlong-calls, -mno-long-calls — Treat all calls as being far away (near). If calls are assumed to be far away, the compiler always loads the function's address into a register, and calls indirect through the pointer. 📞💫
• -mno-ep, -mep — bestie, this is the “copy the pointer into "ep" and use the shorter "sld" and "sst" instructions” optimization for basic blocks that use the same index pointer 4 or more times. -mno-ep says don’t, -mep says do. -mep 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 they use less code space if more than one function saves the same number of registers. -mprolog-function is on by default if you optimize ✨
• -mspace — try to make the code as small as possible. At present, this just turns on the -mep and -mprolog-function options 🎀
• -mtda=n — put static or global variables whose size is n bytes or less into the tiny data area that register "ep" points to. The tiny data area can hold up to 256 bytes in total (128 bytes for byte references) 🫶
• -msda=n — put static or global variables whose size is n bytes or less into the small data area that register "gp" points to. The small data area can hold up to 64 kilobytes 🦋
• -mzda=n — put static or global variables whose size is n bytes or less into the first 32 kilobytes of memory 🌸
• -mv850 — specify that the target processor is the V850 👑
• -mv850e3v5 — specify that the target processor is the V850E3V5. The preprocessor constant __v850e3v5__ is defined if this option is used ⭐
• -mv850e2v4 — specify that the target processor is the V850E3V5. This is an alias for the -mv850e3v5 option 💀
• -mv850e2v3 — specify that the target processor is the V850E2V3. The preprocessor constant __v850e2v3__ is defined if this option is used 🌈
• -mv850e2 — specify that the target processor is the V850E2. The preprocessor constant __v850e2__ is defined if this option is used 💜
• -mv850e1 — specify that the target processor is the V850E1. The preprocessor constants __v850e1__ and __v850e__ are defined if this option is used 🧚
• -mv850es — specify that the target processor is the V850ES. This is an alias for the -mv850e1 option 🍓
• -mv850e — specify that the target processor is the V850E. The preprocessor constant __v850e__ is defined if this option is used 🔥

If neither -mv850 nor -mv850e nor -mv850e1 nor -mv850e2 nor -mv850e2v3 nor -mv850e3v5 are defined then a default target processor is chosen and the relevant __v850*__ preprocessor constant is defined.

The preprocessor constants __v850 and __v851__ are always defined, regardless of which processor variant is the target. Periodt 💅

• -mdisable-callt, -mno-disable-callt — this option suppresses generation of the "CALLT" instruction for the v850e, v850e1, v850e2, v850e2v3 and v850e3v5 flavors of the v850 architecture 😭

This option is enabled by default when the RH850 ABI is in use (see -mrh850-abi), and disabled by default when the GCC ABI is in use. If "CALLT" instructions are being generated then the C preprocessor symbol __V850_CALLT__ is defined 💖

• -mrelax, -mno-relax — pass on (or do not pass on) the -mrelax command-line option to the assembler 🎀
• -mlong-jumps, -mno-long-jumps — disable (or re-enable) the generation of PC-relative jump instructions ✨
• -msoft-float, -mhard-float — disable (or re-enable) the generation of hardware floating point instructions. This option is only significant when the target architecture is V850E2V3 or higher. If hardware floating point instructions are being generated then the C preprocessor symbol __FPU_OK__ is defined, otherwise the symbol __NO_FPU__ is defined 🦋
• -mloop — enables the use of the e3v5 LOOP instruction. The use of this instruction is not enabled by default when the e3v5 architecture is selected because its use is still experimental 🙈
• -mrh850-abi, -mghs — enables support for the RH850 version of the V850 ABI. This is the default. With this version of the ABI the following rules apply:
  • Integer sized structures and unions are returned via a memory pointer rather than a register.
  • Large structures and unions (more than 8 bytes in size) are passed by value.
  • Functions are aligned to 16-bit boundaries.
  • The -m8byte-align command-line option is supported.
  • The -mdisable-callt command-line option is enabled by default. The -mno-disable-callt command-line option is not supported.

When this version of the ABI is enabled the C preprocessor symbol __V850_RH850_ABI__ is defined 👑

• -mgcc-abi — enables support for the old GCC version of the V850 ABI. With this version of the ABI the following rules apply:
  • Integer sized structures and unions are returned in register "r10".
  • Large structures and unions (more than 8 bytes in size) are passed by reference.
  • Functions are aligned to 32-bit boundaries, unless optimizing for size.
  • The -m8byte-align command-line option is not supported.
  • The -mdisable-callt command-line option is supported but not enabled by default.

When this version of the ABI is enabled the C preprocessor symbol __V850_GCC_ABI__ is defined 💜

• -m8byte-align, -mno-8byte-align — enables support for "double" and "long long" types to be aligned on 8-byte boundaries. The default is to restrict the alignment of all objects to at most 4-bytes. When -m8byte-align is in effect the C preprocessor symbol __V850_8BYTE_ALIGN__ is defined 🌸
• -mbig-switch, -mno-big-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, -mno-app-regs — -mapp-regs causes r2 and r5 to be used in the code generated by the compiler. This setting is the default. -mno-app-regs causes r2 and r5 to be treated as fixed registers ✨
• -msmall-sld, -mno-small-sld — enable or disable the use of the short load instructions 🎀
• -mno-strict-align — do not enforce strict data alignment 🦋
• -mjump-tables-in-data-section, -mno-jump-tables-in-data-section — enable or disable placement of jump tables for switch statements in the .data section rather than the .code section. The default is -mno-jump-tables-in-data-section ⭐

🌈 VAX Options 🌈

These -m options are defined for the VAX:

• -munix — do not output certain jump instructions (aobleq and so on) that the Unix assembler for the VAX cannot handle across long ranges 💀
• -mgnu — do output those jump instructions, on the assumption that the GNU assembler is being used ✨
• -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 🙈
• -mvaxc-alignment — use VAXC conventions for alignment of structure members 🫶
• -mqmath, -mno-qmath — enable or disable new instruction patterns for 64-bit integer addition and subtraction 💅

🦋 Visium Options 🦋

• -mdebug — use this option when linking programs that perform file I/O and are destined to run on an MCM target. It causes the libraries libc.a and libdebug.a to be linked. You should run program on the target under the control of the GDB remote debugging stub 👑
• -msim — use this option when linking programs that perform file I/O and are destined to run on the simulator. It causes the libraries libc.a and libsim.a to be linked ⭐
• -mfpu, -mhard-float — generate code containing floating-point instructions. This is the default ✨
• -mno-fpu, -msoft-float — generate code containing library calls for floating-point.

-msoft-float changes the calling convention in the output file; therefore, it is only useful if you compile all of a program with this option. In particular, you need to compile libgcc.a, the library that comes with GCC, with -msoft-float in order for this to work 😭

• -mcpu=cpu_type — set the instruction set, register set, and instruction scheduling parameters for machine type cpu_type. Supported values for cpu_type are mcm, gr5 and gr6.

mcm is a synonym of gr5 present for backward compatibility.

By default (unless configured otherwise), GCC generates code for the GR5 variant of the Visium architecture.

With -mcpu=gr6, GCC generates code for the GR6 variant of the Visium architecture. The only difference from GR5 code is that the compiler will generate block move instructions 💜

• -mtune=cpu_type — set the instruction scheduling parameters for machine type cpu_type, but do not set the instruction set or register set that the option -mcpu=cpu_type would 🌸
• -msv-mode — generate code for the supervisor mode, where there are no restrictions on the access to general registers. This is the default 🎀
• -muser-mode — generate code for the user mode, where the access to some general registers is forbidden: on the GR5, registers r24 to r31 cannot be accessed in this mode; on the GR6, only registers r29 to r31 are affected 🦋

🎀 VMS Options 🎀

These -m options are defined for the VMS implementations:

• -mvms-return-codes — return VMS condition codes from "main". The default is to return POSIX-style condition (e.g. error) codes 💅
• -mdebug-main=prefix — flag the first routine whose name starts with prefix as the main routine for the debugger ✨
• -mmalloc64 — default to 64-bit memory allocation routines 🫶
• -mpointer-size=size — set the default size of pointers. Possible options for size are 32 or short for 32 bit pointers, 64 or long for 64 bit pointers, and no for supporting only 32 bit pointers. The later option disables pragma pointer_size ⭐

⭐ VxWorks Options ⭐

The options in this section are defined for all VxWorks targets. Options specific to the target hardware are listed with the other options for that target 🌈

• -mrtp — bestie, GCC can spit out code for both VxWorks kernels and real time processes (RTPs). This flag flips you from the former to the latter. It also defines the preprocessor macro __RTP__, so you know it's official 💅
• -msmp — picks SMP runtimes for linking. Heads up though: not available on architectures other than PowerPC, and not on VxWorks version 7 or later — because in those, the selection is part of the VxWorks build configuration and the library paths are literally the same either way. 🙈
• -mvthreads — links against the VxWorks AE vThread runtime environment. that's it, that's the vibe ✨
• -non-static — links an RTP executable against shared libraries instead of static ones. and yes, the options -static and -shared also work for RTPs; -static is the default 🎀
• -Bstatic, -Bdynamic — these get passed straight down to the linker. they exist for compatibility with Diab, periodt 🫶
• -Xbind-lazy — enables lazy binding of function calls. equivalent to -Wl,-z,now, and defined for compatibility with Diab 🦋
• -Xbind-now — disables lazy binding of function calls. this one is the default, and it's also defined for compatibility with Diab ⭐

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✨ x86 Options ✨

okay so this section is literally the whole -m situation for the x86 family of computers. buckle up 🎀

the following group of options lets you compile for a specific processor. no cap, this is the good stuff:

• -march=cpu-type — generates instructions for the machine type cpu-type. and the way this differs from -mtune=cpu-type is everything, bestie: -mtune=cpu-type merely tunes the generated code for that cpu-type, but -march=cpu-type lets GCC generate code that may not run at all on processors other than the one indicated 😭💀 so like, choose wisely. Specifying -march=cpu-type implies -mtune=cpu-type, except where noted otherwise.

The choices for cpu-type are:

• native — selects the CPU to generate code for at compilation time by determining the processor type of the compiling machine. Using -march=native enables all instruction subsets supported by the local machine (hence the result might not run on different machines 🙈). Using -mtune=native produces code optimized for the local machine under the constraints of the selected instruction set.
• x86-64 — a generic CPU with 64-bit extensions, MMX, SSE, SSE2, and FXSR instruction set support. the classic 👑
• 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.

and here's the tea: 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. iykyk 💅

• i386 — original Intel i386 CPU. the OG fr fr
• i486 — Intel i486 CPU. (No scheduling is implemented for this chip.) 🥲
• 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 as generic.
• 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. nostalgic much? 🥺
• 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, CX16 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.
• 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. that list is giving "read me to filth" but we respect it 💀
• 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 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, MOVDIRI, MOVDIR64B, CLWB, AVX512VP2INTERSECT and KEYLOCKER instruction set support.
• 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, RDPID and AVX512VPOPCNTDQ instruction 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, CX16, 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 and AVX-VNNI instruction set support.
• arrowlake — Intel Arrow Lake CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, 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 and CMPCCXADD instruction 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, CX16, 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, SM3 and SM4 instruction set support.
• pantherlake, wildcatlake — Intel Panther Lake/Wildcat Lake CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, 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, SM3 and SM4 instruction set support.

bestie notice: yeah, KL and WIDEKL dipped out of this one compared to arrowlake — the list really said "we're done here" 😭 not me reading it three times to double-check.

• novalake — Intel Nova Lake CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, 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, SM3, SM4, PREFETCHI, APX_F, AVX10.1, AVX10.2 and MOVRS instruction 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-FP16 and AVX512BF16 instruction set support.
• 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-FP16 and PREFETCHI instruction 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, PREFETCHI and AMX-COMPLEX instruction set support.

okay so yes, AVX512-FP16 up there has a dash and AVX512FP16 down here doesn't — that's how the original rolls, don't come for me, I'm just the messenger 🫶

• 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, MOVRS and AMX-MOVRS instruction set support.

lowkey iconic that MOVBE shows up twice in that list — it's giving main character energy, it's giving encore 🎤 periodt, the list is exactly as written.

• bonnell, atom — Intel Bonnell CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3 and CX16 instruction set support.
• silvermont, slm — Intel Silvermont CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, PCLMUL, PREFETCHW and RDRND instruction 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, CLFLUSHOPT and FSGSBASE instruction 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, RDPID and SGX instruction 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, CLDEMOTE and WAITPKG instruction set support.
• sierraforest — Intel Sierra Forest CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, 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, ENQCMD and UINTR instruction set support.
• grandridge — Intel Grand Ridge CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, 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, ENQCMD and UINTR instruction set support.

fr fr sierraforest and grandridge are twins, the lists are literally identical — slay, copy-paste energy, we stan a consistent lineup ✨

• clearwaterforest — Intel Clearwater Forest CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, 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_MSR and PREFETCHI instruction set support.

---

🎀 and now the AMD section, bestie, we're switching teams 🎀

• k6 — AMD K6 CPU with MMX instruction set support.
• k6-2, k6-3 — Improved versions of AMD K6 CPU with MMX and 3DNow! instruction set support.
• athlon, athlon-tbird — AMD Athlon CPU with MMX, 3dNOW!, enhanced 3DNow! and SSE prefetch instructions support.
• athlon-4, athlon-xp, athlon-mp — Improved AMD Athlon CPU with MMX, 3DNow!, enhanced 3DNow! and full SSE instruction set support.

not the 3dNOW! vs 3DNow! capitalization jump 😭 that lowercase d in the athlon/athlon-tbird entry is exactly how it's written, no cap, don't @ me.

• k8, opteron, athlon64, athlon-fx — Processors based on the AMD K8 core with x86-64 instruction set support, including the AMD Opteron, Athlon 64, and Athlon 64 FX processors. (This supersets MMX, SSE, SSE2, 3DNow!, enhanced 3DNow! and 64-bit instruction set extensions.)
• k8-sse3, opteron-sse3, athlon64-sse3 — Improved versions of AMD K8 cores with SSE3 instruction set support.
• amdfam10, barcelona — CPUs based on AMD Family 10h cores with x86-64 instruction set support. (This supersets CX16, MMX, SSE, SSE2, SSE3, SSE4A, 3DNow!, enhanced 3DNow!, ABM and 64-bit instruction set extensions.)
• okay so bdver1 — AMD Family 15h cores with x86-64 instruction set support, and the way it supersets stuff is: FMA4, AVX, XOP, LWP, AES, PCLMUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, plus the 64-bit instruction set extensions. 💅
• bdver2 — AMD Family 15h core based CPUs with x86-64 instruction set support. This one supersets BMI, TBM, F16C, FMA, FMA4, AVX, XOP, LWP, AES, PCLMUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and the 64-bit instruction set extensions. ✨
• bdver3 — AMD Family 15h core based CPUs with x86-64 instruction set support. 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 the 64-bit instruction set extensions. 🦋
• bdver4 — AMD Family 15h core based CPUs with x86-64 instruction set support. 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 the 64-bit instruction set extensions. 🌸
• znver1 — AMD Family 17h core based CPUs with x86-64 instruction set support. 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 the 64-bit instruction set extensions. 🔥
• znver2 — AMD Family 17h core based CPUs with x86-64 instruction set support. 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 the 64-bit instruction set extensions. 💜
• znver3 — AMD Family 19h core based CPUs with x86-64 instruction set support. 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 the 64-bit instruction set extensions. 🎀
• znver4 — AMD Family 19h core based CPUs with x86-64 instruction set support. 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 the 64-bit instruction set extensions. 😭🔥
• znver5 — AMD Family 1ah core based CPUs with x86-64 instruction set support. 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 the 64-bit instruction set extensions. 👑
• znver6 — AMD Family 1ah core based CPUs with x86-64 instruction set support. 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, AVXVNNIINT8, AVXIFMA, AVX512FP16, AVXNECONVERT, AVX512BMM and the 64-bit instruction set extensions. 🌈⭐
• btver1 — CPUs based on AMD Family 14h cores with x86-64 instruction set support. Supersets MMX, SSE, SSE2, SSE3, SSSE3, SSE4A, CX16, ABM and the 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 the 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. Heads up: no scheduling is implemented for this chip. 💀
• c3-2 — VIA C3-2 (Nehemiah/C5XL) CPU with MMX and SSE instruction set support. Also no scheduling is implemented for this chip. 🫶
• c7 — VIA C7 (Esther) CPU with MMX, SSE, SSE2 and SSE3 instruction set support. And yep, 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=lujiazui for 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. 🔥
• c86-4g-m8 — HYGON c86-4g-m8 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 — 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 can't run on the default machine type — unless you also use a -march=cpu-type option. Example time: if GCC is configured for i686-pc-linux-gnu then -mtune=pentium4 generates code that is tuned for Pentium 4 but still runs on i686 machines ✨.

The choices for cpu-type are the same as for -march. In addition, -mtune supports 2 extra choices for cpu-type:

• generic — produce code optimized for the most common IA32/AMD64/EM64T processors 🫶. If you know the CPU on which your code will run, then you should use the corresponding -mtune or -march option instead of -mtune=generic. But if you do not know exactly what CPU users of your application will have, then you should use this option, fr fr.

As new processors are deployed in the marketplace, the behavior of this option will change — so if you upgrade to a newer version of GCC, code generation controlled by this option will change to reflect the processors that are most common at the time that version of GCC is released 🙈.

There is no -march=generic option because -march indicates the instruction set the compiler can use, and there is no generic instruction set applicable to all processors 💀. In contrast, -mtune indicates the processor (or, in this case, collection of processors) for which the code is optimized. iykyk.

• intel — produce code optimized for the most current Intel processors, which are Haswell and Silvermont for this version of GCC 🔥. If you know the CPU on which your code will run, then you should use the corresponding -mtune or -march option instead of -mtune=intel. But if you want your application performs better on both Diamond Rapids and Clearwater Forest, then you should use this option 👑.

As new Intel processors are deployed in the marketplace, the behavior of this option will change — so if you upgrade to a newer version of GCC, code generation controlled by this option will change to reflect the most current Intel processors at the time that version of GCC is released 🌈.

There is no -march=intel option because -march indicates the instruction set the compiler can use, and there is no common instruction set applicable to all processors. In contrast, -mtune indicates the processor (or, in this case, collection of processors) for which the code is optimized 💜.

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The following options allow more detailed control over which instruction set extensions are targeted by GCC 🎀. Each one has a corresponding -mno-option to disable use of these instructions — so yes, the off-switch is real and it's iconic.

These extensions are also available as built-in functions: see x86 Built-in Functions, for details of the functions enabled and disabled by these switches 👀.

These options enable GCC to use these extended instructions in generated code. 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 — like, do not do that, bestie 😭.

To control whether 387 or SSE/AVX instructions are generated automatically for floating-point arithmetic, see -mfpmath=, below 🌸.

• -mmmx — Support MMX built-in functions.
• -msse — Support MMX and SSE built-in functions and code generation.
• -msse2 — Support MMX, SSE and SSE2 built-in functions and code generation.
• -msse3 — Support MMX, SSE, SSE2 and SSE3 built-in functions and code generation.
• -mssse3 — Support MMX, SSE, SSE2, SSE3 and SSSE3 built-in functions and code generation.
• -msse4.1 — Support MMX, SSE, SSE2, SSE3, SSSE3 and SSE4.1 built-in functions and code generation.
• -msse4.2 — Support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1 and SSE4.2 built-in functions and code generation.
• -msse4 — Support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1 and SSE4.2 built-in functions and code generation.

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, controlled separately by -msse4a 💅.

• -msse4a — Support MMX, SSE, SSE2, SSE3 and SSE4A built-in functions and code generation.
• -mavx — Support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2 and AVX built-in functions and code generation.
• -mavx2 — Support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX and AVX2 built-in functions and code generation.
• -mavx512f — Support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2 and AVX512F built-in functions and code generation.
• -mavx512cd — Support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX512F and AVX512CD built-in functions and code generation.
• -mavx512vl — Support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX512F and AVX512VL built-in functions and code generation.
• -mavx512bw — Support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX512F and AVX512BW built-in functions and code generation.
• -mavx512dq — Support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX512F and AVX512DQ built-in functions and code generation.
• -mavx512ifma — Support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX512F and AVX512IFMA built-in functions and code generation.
• -mavx512vbmi — Support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX512F and AVX512VBMI built-in functions and code generation.
• -mavx512vpopcntdq — Support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX512F and AVX512VPOPCNTDQ built-in functions and code generation.
• -mavx512vp2intersect — Support AVX512VP2INTERSECT built-in functions and code generation.
• -mavx512vnni — Support AVX512VNNI built-in functions and code generation.
• -mavx512vbmi2 — Support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX512F and AVX512VBMI2 built-in functions and code generation.
• -mavx512bf16 — Support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX512F and AVX512BF16 built-in functions and code generation.
• -mavx512fp16 — Support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX512F and AVX512-FP16 built-in functions and code generation.
• -mavx512bitalg — Support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX512F and AVX512BITALG built-in functions and code generation.
• -mavx512bmm — Support AVX512BMM built-in functions and code generation.
• -mavxvnni — Support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, and AVXVNNI built-in functions and code generation.
• -mavxifma — Support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, and AVXIFMA built-in functions and code generation.
• -mavxvnniint8 — Support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2 and AVXVNNIINT8 built-in functions and code generation.
• -mavxneconvert — Support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, and AVXNECONVERT build-in functions and code generation.
• -mavxvnniint16 — Support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2 and AVXVNNIINT16 built-in functions and code generation.
• -mavx10.1 — Support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, and AVX10.1 built-in functions and code generation.
• -mavx10.2 — Support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX10.1 and AVX10.2 built-in functions and code generation.
• -msha — Support SHA1 and SHA256 built-in functions and code generation.
• -maes — Support AES built-in functions and code generation.
• -mpclmul — Support PCLMUL built-in functions and code generation.
• -mclflushopt — Support CLFLUSHOPT instructions.
• -mclwb — Support CLWB instruction.
• -mfsgsbase — Support FSGSBASE built-in functions and code generation.
• -mptwrite — Support PTWRITE built-in functions and code generation.
• -mrdrnd — Support RDRND built-in functions and code generation.
• -mf16c — Support F16C built-in functions and code generation.
• -mfma — Support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX and FMA built-in functions and code generation.
• -mfma4 — Support FMA4 built-in functions and code generation.
• -mpconfig — Support PCONFIG built-in functions and code generation.
• -mwbnoinvd — Support WBNOINVD built-in functions and code generation.
• -mprfchw — Support PREFETCHW instruction.
• -mrdpid — Support RDPID built-in functions and code generation.
• -mrdseed — Support RDSEED instruction.
• -msgx — Support SGX built-in functions and code generation.
• -mxop — Support XOP built-in functions and code generation.
• -mlwp — Support LWP built-in functions and code generation.
• -m3dnow — Support 3DNow! built-in functions.
• -m3dnowa — Support Athlon 3Dnow! built-in functions.
• -mpopcnt — Support code generation of popcnt instruction.
• -mabm — Support code generation of Advanced Bit Manipulation (ABM) instructions.
• -madx — Support flag-preserving add-carry instructions.
• -mbmi — Support BMI built-in functions and code generation.
• -mbmi2 — Support BMI2 built-in functions and code generation.
• -mlzcnt — Support LZCNT built-in function and code generation.
• -mfxsr — Support FXSAVE and FXRSTOR instructions.
• -mxsave — Support XSAVE and XRSTOR instructions.
• -mxsaveopt — Support XSAVEOPT instruction.
• -mxsavec — Support XSAVEC instructions.
• -mxsaves — Support XSAVES and XRSTORS instructions.
• -mrtm — Support RTM built-in functions and code generation.
• -mhle — Support Hardware Lock Elision prefixes.
• -mtbm — Support TBM built-in functions and code generation.
• -mmwaitx — Support MWAITX and MONITORX built-in functions and code generation.
• -mclzero — Support CLZERO built-in functions and code generation.
• -mpku — Support PKU built-in functions and code generation.
• -mgfni — Support GFNI built-in functions and code generation.
• -mvaes — support VAES built-in functions and code generation 💅
• -mwaitpkg — support WAITPKG built-in functions and code generation ✨
• -mvpclmulqdq — support VPCLMULQDQ built-in functions and code generation
• -mmovdiri — support MOVDIRI built-in functions and code generation
• -mmovdir64b — support MOVDIR64B built-in functions and code generation
• -menqcmd — support ENQCMD built-in functions and code generation
• -muintr — support UINTR built-in functions and code generation
• -mtsxldtrk — support TSXLDTRK built-in functions and code generation
• -mcldemote — support CLDEMOTE built-in functions and code generation
• -mserialize — support SERIALIZE built-in functions and code generation
• -mamx-tile — support AMX-TILE built-in functions and code generation
• -mamx-int8 — support AMX-INT8 built-in functions and code generation
• -mamx-bf16 — support AMX-BF16 built-in functions and code generation
• -mhreset — support HRESET built-in functions and code generation
• -mkl — support KL built-in functions and code generation
• -mwidekl — support WIDEKL built-in functions and code generation
• -mcmpccxadd — support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, and CMPCCXADD build-in functions and code generation 🫶
• -mamx-fp16 — support AMX-FP16 built-in functions and code generation
• -mprefetchi — support PREFETCHI built-in functions and code generation
• -mraoint — support RAOINT built-in functions and code generation
• -mamx-complex — support AMX-COMPLEX built-in functions and code generation
• -msm3 — support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX and SM3 built-in functions and code generation
• -msm4 — support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX and SM4 built-in functions and code generation
• -msha512 — support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX and SHA512 built-in functions and code generation
• -mapxf — support code generation for APX features, including EGPR, PUSH2POP2, NDD, PPX, NF, CCMP and ZU 🔥
• -musermsr — support USER_MSR built-in functions and code generation
• -mamx-avx512 — support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX10.1, AVX10.2 and AMX-AVX512 built-in functions and code generation 💖
• -mamx-tf32 — support AMX-TF32 built-in functions and code generation
• -mamx-fp8 — support AMX-FP8 built-in functions and code generation
• -mmovrs — support MOVRS built-in functions and code generation
• -mamx-movrs — support AMX-MOVRS built-in functions and code generation

okay so these additional options are available for the x86 processor family. yes, that's a whole extra batch. buckle up bestie 🎀

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🌸 the x86 extra-options era 🌸

• -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-store for 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 double and extended-precision arithmetic are still done using 387. a later version, present only in Pentium 4 and AMD x86-64 chips, supports double-precision arithmetic too.

for the x86-32 compiler, you must use -march=cpu-type, -msse or -msse2 switches to enable SSE extensions and make this option effective. for the x86-64 compiler, these extensions are enabled by default.

the resulting code should be considerably faster in the majority of cases and avoid the numerical instability problems of 387 code, but it 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-math is enabled.

GCC depresses SSE instructions when -mavx (or another option enabling AVX extensions) is used. instead, it generates new AVX instructions or AVX equivalents for all SSE instructions when needed. ✨

• sse,387
• sse+387
• both — attempt to utilize both instruction sets at once. this effectively doubles the amount of available registers, and on chips with separate execution units for 387 and SSE, the execution resources too. use this option with care, as it is still experimental, because the GCC register allocator does not model separate functional units well, resulting in unstable performance 😬

• -mieee-fp, -mno-ieee-fp — control whether or not the compiler uses IEEE floating-point comparisons. these correctly handle the case where the result of a comparison is unordered 👑
• -m80387, -mhard-float — generate output containing 80387 instructions for floating point.
• -mno-80387, -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. no cap, that's on you bestie 🫶

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. wild but true.

• -mno-fp-ret-in-387 — do not use the FPU registers for return values of functions.

the usual calling convention has functions return values of types float and double in an FPU register, even if there is no FPU. the idea is that the operating system should emulate an FPU.

the option -mno-fp-ret-in-387 causes such values to be returned in ordinary CPU registers instead. 💫

• -mno-fancy-math-387 — some 387 emulators do not support the sin, cos and sqrt instructions for the 387. specify this option to avoid generating those instructions. this option is overridden when -march indicates that the target CPU always has an FPU and so the instruction does not need emulation. these instructions are not generated unless you also use the -funsafe-math-optimizations switch. iykyk 🌈
• -malign-double, -mno-align-double — control whether GCC aligns double, long double, and long long variables on a two-word boundary or a one-word boundary. aligning double variables on a two-word boundary produces code that runs somewhat faster on a Pentium at the expense of more memory.

on x86-64, -malign-double is enabled by default.

⚠️ Warning: if you use the -malign-double switch, structures containing the above types are aligned differently than the published application binary interface specifications for the x86-32 and are not binary compatible with structures in code compiled without that switch. 💀

• -m96bit-long-double, -m128bit-long-double — these switches control the size of long double type. the x86-32 application binary interface specifies the size to be 96 bits, so -m96bit-long-double is the default in 32-bit mode.

modern architectures (Pentium and newer) prefer long double to be aligned to an 8- or 16-byte boundary. in arrays or structures conforming to the ABI, this is not possible. so specifying -m128bit-long-double aligns long double to a 16-byte boundary by padding the long double with an additional 32-bit zero.

in the x86-64 compiler, -m128bit-long-double is the default choice as its ABI specifies that long double is aligned on 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 🎀

⚠️ 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. fr fr 😭

• -mlong-double-64, -mlong-double-80, -mlong-double-128 — these switches control the size of long double type. a size of 64 bits makes the long double type equivalent to the double type. this is the default for 32-bit Bionic C library. a size of 128 bits makes the long double type equivalent to the __float128 type. this is the default for 64-bit Bionic C library. 🦋

⚠️ Warning: if you override the default value for your target ABI, this changes the size of structures and arrays containing long double variables, as well as modifying the function calling convention for functions taking long double. hence they are not binary-compatible with code compiled without that switch. same warning, same drama 💅

• -malign-data=type — control how GCC aligns variables. supported values for type are compat, which uses the increased alignment value compatible with GCC 4.8 and earlier, abi, which uses the alignment value as specified by the psABI, and cacheline, which uses the increased alignment value to match the cache line size. compat is the default. ✨
• -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. 🔢
• -mrtd — use a different function-calling convention, in which functions that take a fixed number of arguments return with the ret num instruction, which pops their arguments while returning. this saves one instruction in the caller since there is no need to pop the arguments there. iconic efficiency honestly 👑

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 GCC generates incorrect code for calls to those functions. No cap, it's a whole mess. 🙈

And in addition, seriously incorrect code results if you call a function with too many arguments. (Normally, extra arguments are just harmlessly ignored, but not here, bestie.)

---

• -mregparm=num — controls how many registers get 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. Like, all of them. No exceptions.

• -msseregparm — uses SSE register passing conventions for float and double arguments and return values. You can control this behavior for a specific function by using the function attribute sseregparm.

Warning: if you use this switch then you must build all modules with the same value, including any libraries. This includes the system libraries and startup modules. Yes, again. It's giving "everything must match or it all falls apart" energy. 😭

• -mvect8-ret-in-mem — returns 8-byte vectors in memory instead of MMX registers. This is the default on VxWorks to match the ABI of the Sun Studio compilers until version 12. Only use this option if you need to remain compatible with existing code produced by those previous compiler versions or older versions of GCC. Legacy vibes only, iykyk. 🫶
• -mpc32, -mpc64, -mpc80 — set 80387 floating-point precision to 32, 64 or 80 bits. When -mpc32 is specified, the significands of results of floating-point operations are rounded to 24 bits (single precision); -mpc64 rounds the significands of results of floating-point operations to 53 bits (double precision) and -mpc80 rounds the significands of results of floating-point operations to 64 bits (extended double precision), which is the default. When this option is used, floating-point operations in higher precisions are not available to the programmer without setting the FPU control word explicitly.

Setting the rounding of floating-point operations to less than the default 80 bits can speed some programs by 2% or more. Note that some mathematical libraries assume that extended-precision (80-bit) floating-point operations are enabled by default; routines in such libraries could suffer significant loss of accuracy, typically through so-called "catastrophic cancellation", when this option is used to set the precision to less than extended precision. So like… speed? yes. Accuracy? maybe not. Choose wisely. 💀

• -mdaz-ftz, -mno-daz-ftz — the flush-to-zero (FTZ) and denormals-are-zero (DAZ) flags in the MXCSR register are used to control floating-point calculations. The -Ofast, -ffast-math, or -funsafe-math-optimizations options normally link in startup code that sets these flags except when building a shared library (-shared). You can use the -mdaz-ftz and -mno-daz-ftz options to explicitly enable or disable setting these flags, regardless of other options passed to GCC. You're the main character here, you decide. 👑
• -mstackrealign — realign the stack at entry. On the x86, the -mstackrealign option generates an alternate prologue and epilogue that realigns the run-time stack if necessary. This supports mixing legacy codes that keep 4-byte stack alignment with modern codes that keep 16-byte stack alignment for SSE compatibility. See also the attribute force_align_arg_pointer, applicable to individual functions. It's giving "blending old and new friend groups" energy. 🌈
• -mstack-arg-probe — emit stack probing code in the function prologue.
• -mpreferred-stack-boundary=num — attempt to keep the stack boundary aligned to a 2 raised to num byte boundary. If -mpreferred-stack-boundary is not specified, the default is 4 (16 bytes or 128 bits).

Warning: when generating code for the x86-64 architecture with SSE extensions disabled, -mpreferred-stack-boundary=3 can be used to keep the stack boundary aligned to 8 byte boundary. Since x86-64 ABI require 16 byte stack alignment, this is ABI incompatible and intended to be used in controlled environment where stack space is important limitation. This option leads to wrong code when functions compiled with 16 byte stack alignment (such as functions from a standard library) are called with misaligned stack. In this case, SSE instructions may lead to misaligned memory access traps. In addition, variable arguments are handled incorrectly for 16 byte aligned objects (including x87 long double and __int128), leading to wrong results. You must build all modules with -mpreferred-stack-boundary=3, including any libraries. This includes the system libraries and startup modules. 🚨

• -mincoming-stack-boundary=num — assume the incoming stack is aligned to a 2 raised to num byte boundary. If -mincoming-stack-boundary is not specified, the one specified by -mpreferred-stack-boundary is used.

On Pentium and Pentium Pro, double and long double values should be aligned to an 8-byte boundary (see -malign-double) or suffer significant run time performance penalties. On Pentium III, the Streaming SIMD Extension (SSE) data type __m128 may not work properly if it is not 16-byte aligned.

To ensure proper alignment of this values on the stack, the stack boundary must be as aligned as that required by any value stored on the stack. Further, every function must be generated such that it keeps the stack aligned. Thus calling a function compiled with a higher preferred stack boundary from a function compiled with a lower preferred stack boundary most likely misaligns the stack. It is recommended that libraries that use callbacks always use the default setting. Fr fr, just leave it on default. 🎀

This extra alignment does consume extra stack space, and generally increases code size. Code that is sensitive to stack space usage, such as embedded systems and operating system kernels, may want to reduce the preferred alignment to -mpreferred-stack-boundary=2.

• -mdump-tune-features — this option instructs GCC to dump the names of the x86 performance tuning features and default settings. The names can be used in -mtune-ctrl=feature-list. A little sneak peek of the feature menu. ✨
• -mtune-ctrl=feature-list — this option is used to do fine-grain control of x86 code generation features. feature-list is a comma-separated list of feature names. See also -mdump-tune-features. When specified, the feature is turned on if it is not preceded with ^; otherwise, it is turned off. -mtune-ctrl=feature-list is intended to be used by GCC developers. Using it may lead to code paths not covered by testing and can potentially result in compiler ICEs or runtime errors. 💀 Not for the faint of heart, bestie — this is dev-only territory.
• -mno-default — this option instructs GCC to turn off all tunable features. See also -mtune-ctrl=feature-list and -mdump-tune-features. Everything off. Chaos mode. 🙈
• -mcld — this option instructs GCC to emit a cld instruction in the prologue of functions that use string instructions. String instructions depend on the DF flag to select between autoincrement or autodecrement mode. While the ABI specifies the DF flag to be cleared on function entry, some operating systems violate this specification by not clearing the DF flag in their exception dispatchers. The exception handler can be invoked with the DF flag set, which leads to wrong direction mode when string instructions are used. This option can be enabled by default on 32-bit x86 targets by configuring GCC with the --enable-cld configure option. Generation of cld instructions can be suppressed with the -mno-cld compiler option in this case.
• -mvzeroupper — this option instructs GCC to emit a vzeroupper instruction before a transfer of control flow out of the function to minimize the AVX to SSE transition penalty as well as remove unnecessary zeroupper intrinsics. Clean transitions, iconic. 💅
• -mstv, -mno-stv — enable/disable Scalar to Vectorization pass transforming 64-bit integer computation into vector ones. This optimization is restricted to -O2 and higher. Level up first, then we talk. 🔥
• -mprefer-avx128 — this option instructs GCC to use 128-bit AVX instructions instead of 256-bit AVX instructions in the auto-vectorizer.
• -mprefer-vector-width=opt — this option instructs GCC to use opt-bit vector width in instructions instead of default on the selected platform.
• -mpartial-vector-fp-math, -mno-partial-vector-fp-math — 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-math is 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-math is in effect. These issues can be mitigated by manually sanitizing the upper part of the partial vector argument register or by using -mdaz-ftz to set denormals-are-zero (DAZ) flag in the MXCSR register. 🧚

This option is enabled by default.

• -mmove-max=bits — this option instructs GCC to set the maximum number of bits can be moved from memory to memory efficiently to bits. The valid bits are 128, 256 and 512.
• -mstore-max=bits — this option instructs GCC to set the maximum number of bits can be stored to memory efficiently to bits. The valid bits are 128, 256 and 512.
  • 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 — okay so this one is for functions that are literally never coming back, aka the ones blessed with the noreturn attribute or the _Noreturn specifier 💀 instead of saving the callee-saved registers it actually uses in the function prologue, GCC just… doesn't (except for the BP register, bestie gets a pass). It's a speed thing, but ⚠️ heads up: it can mess with debugging the caller of the noreturn function, or literally any function further up the call stack, so it is not enabled by default. Don't come for me if your backtrace is giving nonsense 💅
• -mcx16 — lets GCC spit out CMPXCHG16B instructions in 64-bit code so compare-and-exchange works on 16-byte aligned 128-bit objects ✨ lowkey slay for atomic updates of data structures that are bigger than one machine word. The compiler uses this instruction to implement __sync Builtins. But plot twist 😭 for __atomic Builtins operating on 128-bit integers, a library call is always used. No instruction for you 🎀
• -msahf — enables generation of SAHF instructions in 64-bit code. Storytime 🫖: early Intel Pentium 4 CPUs with Intel 64 support, prior to the introduction of Pentium 4 G1 step in December 2005, straight-up lacked the LAHF and SAHF instructions that AMD64 supports. They're load and store instructions, respectively, for certain status flags. In 64-bit mode, the SAHF instruction is used to optimize the fmod, drem, and remainder built-in functions — see Other Builtins for the tea 👑
• -mmovbe — turns on use of the movbe instruction to optimize byte swapping of four and eight byte entities. Little-endian to big-endian realness 🔥
• -mshstk — the -mshstk option enables shadow stack built-in functions from x86 Control-flow Enforcement Technology (CET). Security vibes only 💜
• -mcrc32 — enables the built-in functions __builtin_ia32_crc32qi, __builtin_ia32_crc32hi, __builtin_ia32_crc32si and __builtin_ia32_crc32di so they generate the crc32 machine instruction. Checksum girlies rise 🌸
• -mmwait — enables the built-in functions __builtin_ia32_monitor, and __builtin_ia32_mwait to generate the monitor and mwait machine instructions. Your CPU can nap now 😴✨

---

• -mrecip — this one is so extra and I'm obsessed 🫶 it lets GCC use RCPSS and RSQRTSS instructions (and their vectorized variants RCPPS and RSQRTPS) with an additional Newton-Raphson step to crank up the precision, instead of DIVSS and SQRTSS (and their vectorized variants) for single-precision floating-point arguments.

🚨 Important gatekeeping: these instructions only get generated when -funsafe-math-optimizations is enabled together with -ffinite-math-only and -fno-trapping-math.

So yeah, the throughput of the sequence is higher than the throughput of the non-reciprocal instruction, buuut the precision can drop by up to 2 ulp — like, literally, the inverse of 1.0 equals 0.99999994. She's a little approximate, your honor 😭💅

Also note (and this is important, don't skip it 🙈) 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 also 📢 GCC emits the above sequence with an 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. So like… you might already be eating and not even know it 🍓

• -mrecip=opt — same energy but now you're the boss 🎀 this option controls which reciprocal estimate instructions may be used. opt is a comma-separated list of options, which may be preceded by a ! to invert the option:
  • all — enable all estimate instructions.
  • default — enable the default instructions, equivalent to -mrecip.
  • none — disable all estimate instructions, equivalent to -mno-recip.
  • div — enable the approximation for scalar division.
  • vec-div — enable the approximation for vectorized division.
  • sqrt — enable the approximation for scalar square root.
  • vec-sqrt — enable the approximation for vectorized square root.

So, for example, -mrecip=all,!sqrt enables all of the reciprocal approximations, except for square root. Mix and match, it's giving build-your-own-combo 💅✨

---

• -mveclibabi=type — the big one, buckle up bestie 🦋 this specifies the ABI type to use for vectorizing intrinsics using an external library. Supported values for type are svml for the Intel short vector math library, aocl for the math library (LibM) from AMD Optimizing CPU Libraries (AOCL) and acml for the end-of-life AMD core math library (to which AOCL-LibM is the successor).

To use this option, both -ftree-vectorize and -funsafe-math-optimizations have to be enabled, and an SVML or ACML ABI-compatible library must be specified at link time. No library, no deal 🫶

GCC currently emits calls to these when -mveclibabi=svml is used, for the corresponding function type:

"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"

And these when -mveclibabi=aocl is used, for the corresponding function type:

"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"

And finally these when -mveclibabi=acml is used, for the corresponding function type:

"__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"

Like, it's a lot of names, but every single one is doing its job 💅🔥

• -mabi=name — generate code for the specified calling convention. Permissible values are sysv for the ABI used on GNU/Linux and other systems, and ms for the Microsoft ABI. The default is to use the Microsoft ABI when targeting Microsoft Windows and the SysV ABI on all other systems. You can control this behavior for specific functions by using the function attributes ms_abi and sysv_abi. Pick your fighter 🫶
• -masm=dialect — output assembly instructions using the selected dialect. Also affects which dialect is used for basic asm and extended asm. Supported choices (in dialect order) are att or intel. The default is att. 💀 and Darwin does not support intel — so like, don't even try it on Mac, she will not be entertaining that.
• -mforce-indirect-call — forces 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. Sneaky but precise 👑
• -mmanual-endbr — inserts ENDBR instruction at function entry only via the cf_check function attribute. This is useful when used with the option -fcf-protection=branch to control ENDBR insertion at the function entry. You're hand-placing the security checkpoint, main character energy 🎀
• -mcet-switch — okay so by default, CET instrumentation is turned off on switch statements that use a jump table and indirect branch track is disabled. Why does that matter? 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 — yikes 🙈 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. Lock it down bestie 🔒
• -mcall-ms2sysv-xlogues — because 64-bit ABIs are messy 😭 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-xlogues emits 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. Tradeoffs, tradeoffs 💅✨
• -mtls-dialect=type — generate code to access thread-local storage using the gnu or gnu2 conventions. gnu is the conservative default; gnu2 is more efficient, but it may add compile- and run-time requirements that cannot be satisfied on all systems. Fancier but pickier, classic 🫠
• -mpush-args
• -mno-push-args — use PUSH operations to store outgoing parameters. This method is shorter and usually equally fast as the method using SUB/MOV operations, and is enabled by default. In some cases disabling it may improve performance because of improved scheduling and reduced dependencies. Sometimes doing less is more, periodt 🌸
• -maccumulate-outgoing-args — if enabled, the maximum amount of space required for outgoing arguments is computed in the function prologue. This is faster on most modern CPUs because of reduced dependencies, improved scheduling and reduced stack usage when the preferred stack boundary is not equal to 2. The drawback? A notable increase in code size 📈 and also, this switch implies -mno-push-args. FYI 💜
• -mms-bitfields
• -mno-ms-bitfields — enable/disable bit-field layout compatible with the native Microsoft Windows compiler. Here's the situation 🫖: 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 the way GCC normally does. Particularly when moving packed data between functions compiled with GCC and the native Microsoft compiler (either via function call or as data in a file), it may be necessary to access either format. So yeah, when the two compilers disagree on a struct's vibe, this flag is your peace treaty 🎀✨

...and yes, this whole thing is on by default for Microsoft Windows targets ✨ you can also poke at it locally with variable or type attributes — peep x86 Attributes if you wanna go deeper 💅

The Microsoft structure layout algorithm is actually pretty simple, except for the whole bit-field packing situation 😭 and the rules for how members get padded/aligned — plus whether a bit-field is allowed to straddle a storage-unit boundary — are these:

1. Structure members are stored sequentially in the order in which they're declared: the first member gets the lowest memory address, the last member the highest. No cap, order matters.
2. Every data object has an alignment requirement. For everything except structures, unions, and arrays, that requirement is either the size of the object or the current packing size (whichever is less) — and the packing size is what you set with either the aligned attribute or the pack pragma. For structures, unions, and arrays, the alignment requirement is the largest alignment requirement of its members. Every object gets allocated an offset so that:

offset % alignment_requirement == 0

3. Adjacent bit-fields get 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. Girl math, but make it memory.

---

So how does MSVC interpret zero-length bit-fields? Lowkey in these ways 👇

1. If a zero-length bit-field gets inserted between two bit-fields that are normally coalesced, the bit-fields are not coalesced.

Here's the vibe:

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 you removed the zero-length bit-field, t1's size would be 4 bytes. Iconic little sabotage, honestly.

2. 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, then bar is aligned as the type of the zero-length bit-field.

Like so:

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 doesn't affect the alignment of bar or, as a result, the size of the structure. ngl this is the part that gets people every time.

Taking all this into account, it's important to note the following:

1. 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 a 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.
2. 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.

3. 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. Ignored. Not even a little bit relevant. We love a boundaries queen.

---

• -mno-align-stringops — don't align the destination of inlined string operations. This switch reduces code size and improves performance in the case where the destination is already aligned, but GCC doesn't know about it 🙈
• -minline-all-stringops — by default GCC inlines string operations only when the destination is known to be aligned to at least a 4-byte boundary. This enables more inlining and increases code size, but it may improve performance of code that depends on fast memcpy and memset for short lengths. The option enables inline expansion of strlen for all pointer alignments. She's thorough.
• -minline-stringops-dynamically — for string operations of unknown size, this uses run-time checks: 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 the particular algorithm to use for inlining string operations. The allowed values for alg are:
  • rep_byte, rep_4byte, rep_8byte — expand using i386 rep prefix of the specified size.
  • byte_loop, loop, unrolled_loop — expand into an inline loop.
  • libcall — always use a library call.
• -mmemcpy-strategy=strategy — override the internal decision heuristic for deciding whether __builtin_memcpy should be inlined, and what inline algorithm to use when the expected size of the copy operation is known. strategy is a comma-separated list of alg:max_size:dest_align triplets. alg is specified in -mstringop-strategy, max_size specifies the max byte size with which inline algorithm alg is allowed. For the last triplet, the max_size must be -1. The max_size of the triplets in the list must be specified in increasing order. The minimal byte size for alg is 0 for the first triplet and max_size + 1 of the preceding range. 🎀
• -mmemset-strategy=strategy — this option is similar to -mmemcpy-strategy= except that it controls the __builtin_memset expansion. Same energy, different builtin.
• -momit-leaf-frame-pointer — don't keep the frame pointer in a register for leaf functions. This avoids the instructions to save, set up, and restore frame pointers, and makes an extra register available in leaf functions. The option -momit-leaf-frame-pointer removes the frame pointer for leaf functions, which might make debugging harder. Note the tradeoff bestie — debugger's gonna be fighting for its life.
• -mtls-direct-seg-refs, -mno-tls-direct-seg-refs — controls whether TLS variables may be accessed with offsets from the TLS segment register (%gs for 32-bit, %fs for 64-bit), or whether the thread base pointer must be added. Whether or not this is valid depends on the operating system, and whether it maps the segment to cover the entire TLS area.

For systems that use the GNU C Library, the default is on. ✨

• -msse2avx, -mno-sse2avx — specify that the assembler should encode SSE instructions with the VEX prefix. The option -mavx turns this on by default. VEX prefix is so cunty of them.
• -mfentry, -mno-fentry — if profiling is active (-pg), put the profiling counter call before the prologue. Note: on x86 architectures the attribute ms_hook_prologue isn't possible at the moment for -mfentry and -pg. 😭
• -mrecord-mcount, -mno-record-mcount — if profiling is active (-pg), generate a section that contains pointers to each profiling call; this is useful for automatically patching the calls. You can use the mfentry-section= option to set the name of the section; it defaults to __mcount_loc. 🦋
• -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. "likely only useful" is the compiler equivalent of a soft launch.
• -minstrument-return=type — with -pg -mfentry, instrument function exits according to type, which may be one of none to not instrument, call to generate a call to __return__, or nop5 to generate a 5-byte nop sequence. This option only instruments true returns ending with a ret, not sibling calls via a jump. 💅
• -mrecord-return, -mno-record-return — generate a __return_loc section pointing to all return instrumentation code. 🎀
• -mfentry-name=name — set name of __fentry__ symbol called at function entry for -pg -mfentry functions.
• -mfentry-section=name — set name of section to record -mrecord-mcount calls. The default is __mcount_loc.
• -mskip-rax-setup, -mno-skip-rax-setup — when generating code for the x86-64 architecture with SSE extensions disabled, -mskip-rax-setup can be used to skip setting up RAX register when there are no variable arguments passed in vector registers. RAX said "not today".
• -m8bit-idiv, -mno-8bit-idiv — on some processors, like Intel Atom, 8-bit unsigned integer divide is much faster than 32-bit/64-bit integer divide. This option generates a run-time check. If both dividend and divisor are within range of 0 to 255, 8-bit unsigned integer divide is used instead of 32-bit/64-bit integer divide. 🔥
• -mavx256-split-unaligned-load, -mavx256-split-unaligned-store — split 32-byte AVX unaligned load and store. Snap it in half, why don't you.
• -mstack-protector-guard=guard, -mstack-protector-guard-reg=reg, -mstack-protector-guard-offset=offset, -mstack-protector-guard-symbol=symbol — generate stack protection code using canary at guard. Supported locations are global for global canary or tls for per-thread canary in the TLS block (the default). This option has effect only when -fstack-protector or -fstack-protector-all is specified. 🫶

With the 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 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.

-mstack-protector-guard-symbol=symbol overrides the offset with a symbol reference to a canary in the TLS block. Canary said "I have a name now".

• -mgeneral-regs-only — generate code that uses only the general-purpose registers. This prevents the compiler from using floating-point, vector, mask and bound registers. Strictly business. No floaties allowed.
• -mrelax-cmpxchg-loop — okay so when GCC has to spit out a compare-and-swap loop for __sync Builtins and __atomic Builtins that don't have a native instruction to lean on, this flag makes it optimize for the highly contended case 🔥. How? It issues an atomic load before the CMPXCHG instruction, and it uses the PAUSE instruction to save CPU power when restarting the loop. Politeness to your CPU, honestly 💅
• *-mindirect-branch=choice* — converts indirect call and jump according to choice, bestie:
  • keep — keeps indirect call and jump unmodified. This is the default ✨
  • thunk — converts indirect call and jump to call and return thunk
  • thunk-inline — converts indirect call and jump to inlined call and return thunk
  • thunk-extern — converts 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 attribute indirect_branch 🎀

Heads up: -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 😭

Also: -mindirect-branch=thunk-extern is compatible with -fcf-protection=branch, since the external thunk can be made to enable control-flow check 🫶

• *-mfunction-return=choice* — same energy but for function returns 💫. Default is keep, which keeps function return unmodified. thunk converts function return to call and return thunk. thunk-inline converts function return to inlined call and return thunk. thunk-extern converts function return to external call and return thunk provided in a separate object file. Per-function control lives in the function attribute function_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 😭

• -mindirect-branch-register — force indirect call and jump via register. That's it, that's the whole thing 💅
• *-mharden-sls=choice* — generates code to mitigate against straight line speculation (SLS) with choice:
  • none — disables all SLS hardening. This is the default.
  • return — enables SLS hardening for function returns
  • indirect-jmp — enables SLS hardening for indirect jumps
  • all — enables all SLS hardening 👑
• -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. Why do we care? Because this allows them to potentially be replaced with lfence; call *%r8-r15 or lfence; jmp *%r8-r15 at run time ✨
• -mapx-inline-asm-use-gpr32 — by default, EGPR usage is disabled in inline asm constraints, because GCC cannot be aware of whether the asm instructions support GP32 or not 🙈. If your inline asm can handle EGPR, use -mapx-inline-asm-use-gpr32.
• -mgather — enable vectorization for gather instructions.
• -mscatter — enable vectorization for scatter instructions. (Yes they're a matching set, we love a duo 💖)

These -m switches are supported in addition to the above on x86-64 processors in 64-bit environments 👀

• -m32, -m64, -mx32, -m16 — generate code for a 16-bit, 32-bit or 64-bit environment.

The -m32 option sets int, long, and pointer types to 32 bits, and generates code that runs 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 ✨

• -miamcu — generates code that conforms to Intel MCU psABI. It requires the -m32 option to be turned on, no exceptions bestie 🎀
• -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's a 128-byte area beyond the location of the stack pointer that is not modified by signal or interrupt handlers, and therefore can be used for temporary data without adjusting the stack pointer. The flag -mno-red-zone disables this red zone 💅
• -mcmodel=small — generate code for the small code model: the program and its symbols must be linked in the lower 2 GB of the address space. Pointers are 64 bits. Programs can be statically or dynamically linked. This is the default code model ⭐
• -mcmodel=kernel — generate code for the kernel code model. The kernel runs in the negative 2 GB of the address space. This model has to be used for Linux kernel code 👑
• -mcmodel=medium — generate code for the medium model: the program is linked in the lower 2 GB of the address space. Small symbols are also placed there. Symbols with sizes larger than -mlarge-data-threshold are put into large data or BSS sections and can be located above 2GB. Programs can be statically or dynamically linked 🌸
• -mcmodel=large — generate code for the large model. This model makes no assumptions about addresses and sizes of sections. Feral. Unbothered. Iconic 💀
• -maddress-mode=long — generate code for long address mode. Only supported for 64-bit and x32 environments. It's the default address mode for 64-bit environments ✨
• -maddress-mode=short — generate code for short address mode. Only supported for 32-bit and x32 environments. It's the default address mode for 32-bit and x32 environments 🦋
• -mneeded, -mno-needed — emit GNU_PROPERTY_X86_ISA_1_NEEDED GNU property for Linux target to indicate the micro-architecture ISA level required to execute the binary 👀
• -mno-direct-extern-access — without -fpic or -fPIC, always use the GOT pointer to access external symbols. With -fpic or -fPIC, treat access to protected symbols as local symbols. The default is -mdirect-extern-access.

⚠️ Warning: shared libraries compiled with -mno-direct-extern-access and executable compiled with -mdirect-extern-access may not be binary compatible if protected symbols are used in shared libraries and executable. Like, do not mix these and act surprised 😭

• -munroll-only-small-loops, -mno-unroll-only-small-loops — controls conservative small loop unrolling. It's enabled by default with -O2, and unrolls loops with less than 4 instructions by 1 time. This gives better utilization of the instruction decoding pipeline on modern processors ✨ You can disable this with -mno-unroll-only-small-loops, and it's also disabled if the more general options -funroll-loops or -funroll-all-loops are either enabled or explicitly disabled 👑
• -mdispatch-scheduler — enable instruction scheduling. Only supported on bdver1, bdver2, bdver3, bdver4, and znver1 processors, and additionally requires -fschedule-insns -fsched-pressure 🎀
• *-mlam=choice* — LAM (linear-address masking) allows special bits in the pointer to be used for metadata 😳 The default is none. With u48, pointer bits in positions 62:48 can be used for metadata; with u57, pointer bits in positions 62:57 can be used for metadata 💜

---

🪟 x86 Windows Options

There's literally nothing under this heading in the source, bestie — it's just vibing there being a heading. Moving on 💅

---

⚡ Xstormy16 Options

These options are defined for Xstormy16:

• -msim — choose startup files and linker script suitable for the simulator 🧚

---

🌸 Xtensa Options

These options are supported for Xtensa targets:

• -mconst16, -mno-const16 — enable or disable use of CONST16 instructions for loading constant values. When enabled, CONST16 instructions are always used in place of the standard L32R instructions. The use of CONST16 is enabled by default only if the L32R instruction is not available 👀
• -mserialize-volatile, -mno-serialize-volatile — when this option is enabled, GCC inserts MEMW instructions before volatile memory references to guarantee sequential consistency. The default is -mserialize-volatile. Use -mno-serialize-volatile to omit the MEMW instructions ✨
• -mforce-no-pic, -mno-force-no-pic — for targets like GNU/Linux, where all user-mode Xtensa code must be position-independent code (PIC), this option disables PIC for compiling kernel code 🫶
• -mtext-section-literals, -mno-text-section-literals — these options control the treatment of literal pools. The default is -mno-text-section-literals, which places literals in a separate section in the output file. This allows the literal pool to be placed in a data RAM/ROM, and it also allows the linker to combine literal pools from separate object files to remove redundant literals and improve code size. With -mtext-section-literals, the literals are interspersed in the text section in order to keep them as close as possible to their references. This may be necessary for large assembly files. Literals for each function are placed right before that function 💅
• -mauto-litpools, -mno-auto-litpools — these options control the treatment of literal pools. The default is -mno-auto-litpools, which places literals in a separate section in the output file unless -mtext-section-literals is used. With -mauto-litpools the literals are interspersed in the text section by the assembler. Compiler does not produce explicit .literal directives and loads literals into registers with MOVI instructions instead of L32R to let the assembler do relaxation and place literals as necessary. This option allows assembler to create several literal pools per function and assemble very big functions, which may not be possible with -mtext-section-literals 👑
• -mtarget-align, -mno-target-align — when this option is enabled, GCC instructs the assembler to automatically align instructions to reduce branch penalties at the expense of some code density. The assembler attempts to widen density instructions to align branch targets and the instructions following call instructions. If there are not enough preceding safe density instructions to align a target, no widening is performed. The default is -mtarget-align. These options do not affect the treatment of auto-aligned instructions like LOOP, which the assembler always aligns, either by widening density instructions or by inserting NOP instructions 🌀
• -mlongcalls, -mno-longcalls — so when you turn this on, GCC tells the assembler "bestie, please translate direct calls into indirect calls" — but only if it can't figure out that the target of a direct call is already inside the range the call instruction can actually reach 💅. This translation lowkey happens a lot for calls to functions that live in other source files. Concretely: the assembler takes a direct CALL and turns it into an L32R followed by a CALLX. The default is -mno-longcalls. Reach for this when the call target could potentially be out of range. Heads up though — this is implemented in the assembler, not the compiler, so the assembly code GCC hands you still shows direct call instructions; you gotta look at the disassembled object code to see what's actually going on 👀. Also note the assembler uses an indirect call for every cross-file call, not just the ones that are genuinely out of range. Fr fr, it does not check first.
• -mabi=name — generate code for the ABI you name. Permissible values are: call0, windowed. The default ABI is chosen by the Xtensa core configuration, so it's giving "the hardware decides your fate" energy ✨.
• -mabi=call0 — with this one enabled, function parameters get passed in registers a2 through a7, registers a12 through a15 are caller-saved, and register a15 may be used as a frame pointer 🎀. When this version of the ABI is on, the C preprocessor symbol __XTENSA_CALL0_ABI__ is defined.
• -mabi=windowed — with this enabled, function parameters are passed in registers a10 through a15, and the called function rotates the register window by 8 registers on entry so that its arguments are found in registers a2 through a7 🌀. Register a7 may be used as a frame pointer. The register window gets rotated 8 registers back upon return. When this version of the ABI is on, the C preprocessor symbol __XTENSA_WINDOWED_ABI__ is defined.
• -mextra-l32r-costs=n — specify an extra cost of instruction RAM/ROM access for L32R instructions, measured in clock cycles ⏱️. This affects, when optimizing for speed, whether to load a constant from the literal pool using L32R or to synthesize the constant from a small one with a couple of arithmetic instructions. The default value is 0.
• -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 for cores that support both unaligned loads and stores in hardware, and -mstrict-align for all other cores. So like, it depends on your core — check before you assume 💀.

---

🌈 zSeries Options 🌈

These are listed under

🌸 ENVIRONMENT 🌸

okay bestie, this section is all about the environment variables that lowkey run the show behind the scenes with GCC 💅 Some of them do their thing by handing over directories or prefixes to use when hunting for files, and some just, like, set the general vibe of your compilation environment.

✨ real talk: you can ALSO point GCC at places to search with options like -B, -I and -L. Those take precedence over stuff set via environment variables, which in turn take precedence over whatever GCC's configuration says. So it's a whole hierarchy, iykyk 👑

---

🎀 LANG, LC_CTYPE, LC_MESSAGES, LC_ALL

These four control how GCC uses localization info, which is the thing that lets GCC vibe with different national conventions for diagnostic and informational output 🌍 GCC inspects the locale categories LC_CTYPE and LC_MESSAGES if it's been configured to do so. You can set these locale categories to literally any value your installation supports — a classic example is en_GB.UTF-8 for English in the United Kingdom encoded in UTF-8 🇬🇧

So here's the tea: if LC_ALL is set, it overrides LC_CTYPE and LC_MESSAGES. Otherwise those two default to the value of LANG. And if none of these bad boys are set? GCC just defaults to traditional C English behavior, no cap 😌

⚠️ important caveat: these environment variables do not affect the encodings of input files and strings in output files produced by GCC. Those are controlled by -finput-charset and -fexec-charset respectively. Different department, different vibes 💅

---

🗂️ TMPDIR

If TMPDIR is set, it specifies the directory to use for temporary files. GCC uses temp files to hold the output of one compilation stage that's gonna be the input to the next stage — like, for example, the output of the preprocessor, which is the input to the compiler proper. Chain of custody, bestie 🔗

---

🩺 GCC_DIAGNOSTICS_LOG

If this one's set, then extra info about the diagnostics subsystem gets emitted ✨ If you set it to an empty value, the info goes to stderr; otherwise GCC will attempt to open that file and write the info there.

🙈 heads up: the precise content and format of this info is subject to change. It's intended for GCC developers, not end-users. So like... not for us, but go off devs 💀

---

🔍 GCC_COMPARE_DEBUG

Setting GCC_COMPARE_DEBUG is nearly equivalent to passing -fcompare-debug to the compiler driver. See the documentation of that option for the deets 👀

---

🧭 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 gets added when this prefix is combined with the name of a subprogram — but you can specify a prefix that ends with a slash if that's your vibe ✨

If it's not set, GCC tries to figure out an appropriate prefix based on the pathname it was invoked with. Detective mode 🕵️

If GCC can't find the subprogram using the specified prefix, it tries looking in the usual places for the subprogram. It's giving backup plan 💅

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. Periodt.

This prefix is also used for finding files such as crt0.o that are used for linking 🔗

And here's the quirky part — 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. So, with -Bfoo/, GCC searches foo/bar just before it searches the standard directory /usr/local/lib/bar. And 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. Iconic but confusing, I know 😭

---

🛣️ COMPILER_PATH

The value of COMPILER_PATH is a colon-separated list of directories, much like PATH. GCC tries the directories you specified here when searching for subprograms — but only if it can't find them using GCC_EXEC_PREFIX 💫

---

📚 LIBRARY_PATH

The value of LIBRARY_PATH is also a colon-separated list of directories, much like PATH. When configured as a native compiler, GCC tries the directories you listed when searching for special linker files — again, if it can't find them using GCC_EXEC_PREFIX. And linking using GCC also uses these directories when searching for ordinary libraries for the -l option (but directories specified with -L come first, gotta respect the queue 👑).

---

🩹 GCC_EXTRA_DIAGNOSTIC_OUTPUT

If GCC_EXTRA_DIAGNOSTIC_OUTPUT is set to one of the following values, then additional text gets emitted to stderr when fix-it hints are emitted. Oh, and -fdiagnostics-parseable-fixits and -fno-diagnostics-parseable-fixits take precedence over this environment variable, just so we're clear ✨

• 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 — as "fixits-v1", but columns are expressed as display columns, as per -fdiagnostics-column-unit=display. Fancy 💅

---

🔌 EXPERIMENTAL_SARIF_SOCKET

If EXPERIMENTAL_SARIF_SOCKET is set in the environment, then the compiler will attempt to connect to a UNIX domain stream socket with that name, and send an "OnSarifResult" JSON-RPC 2.0 notification to it for each diagnostic that occurs, where the value of the notification is a SARIF "result" object 🛰️

💀 the compiler will fail immediately if EXPERIMENTAL_SARIF_SOCKET is set and it cannot connect to it. No chill at all.

🧪 This feature is experimental and subject to change or removal without notice. You've been warned, bestie.

---

🧚 preprocessor era: some extra env vars ✨

Some additional environment variables affect the behavior of the preprocessor. Let's get into it 🫶

🍓 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. That special character, "PATH_SEPARATOR", is target-dependent and determined at GCC build time 🌈 For Microsoft Windows-based targets it's a semicolon, and for almost all other targets it's 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. Sneaky but slay.

---

📝 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. Combo move 🎀

---

🌞 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 are so main character energy)

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://gcc.gnu.org/bugs/>. Go snitch (lovingly) 🐛

---

📎 FOOTNOTES

1. On some systems, gcc -shared needs to build supplementary stub code for constructors to work. On multi-libbed systems, gcc -shared must select the correct support libraries to link against. Failing to supply the correct flags may lead to subtle defects 😭 Supplying them in cases where they are not necessary is innocuous. Also, -shared suppresses the addition of startup code to alter the floating-point environment as done with -ffast-math, -Ofast or -funsafe-math-optimizations on some targets. The more you know 🌈

---

👀 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. The whole friend group, honestly 🫶

---

👑 AUTHOR

See the Info entry for gcc, or <https://gcc.gnu.org/onlinedocs/gcc/Contributors.html>, for contributors to GCC. The real MVPs ✨

---

📜 COPYRIGHT

Copyright (c) 1988-2026 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.

---

🌸 and that's the whole page, bestie!! You made it to the very end, I'm so proud 🥹 This edition was gcc-16.2.1, dated 2026-08-10, and it ate, no cap. Go compile something iconic 💅✨🎀