GCC(1)

🦋 GCC(1) — GNU 🦋

okay bestie, welcome to the GNU manual, section 1, and it's the big one: gcc, the GNU project C and C++ compiler. grab a snack, we're gonna be here a while 💖

---

✨ NAME ✨

gcc — the GNU project C and C++ compiler. yes that's it. that's the whole name section. iconic brevity.

---

🌸 SYNOPSIS 🌸

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

okay so heads up: that list is literally just the most useful options, like the greatest hits album. the rest of them live down below, and there are so many. also g++ accepts mostly the same options as gcc, so if you know one you basically know the other 🫶

---

💅 DESCRIPTION 💅

so when you invoke GCC, it normally does preprocessing, compilation, assembly and linking. the whole glow-up pipeline, start to finish. but the "overall options" let you stop that process at an intermediate stage — like hitting pause mid-routine. for example, the -c option says do not run the linker, so the output is just object files output by the assembler. cute, contained, no final step. ✨

other options get passed on to one or more stages of processing. some options control the preprocessor, others control the compiler itself. and then there are yet other options controlling the assembler and linker — most of these are not documented here, because honestly you rarely need to use any of them. iykyk 🙈

most of the command-line options you can use with GCC are useful for C programs. when an option is only useful with another language (usually C++), the explanation says so explicitly. and if the description for a particular option doesn't mention a source language? then you can use that option with all supported languages. no cap. 💖

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. and when you compile C++ programs, you should invoke GCC as g++ instead. periodt. 👑

the gcc program accepts options and file names as operands. many options have multi-letter names, which means multiple single-letter options may not be grouped: -dv is very different from -d -v. they are NOT the same thing bestie, do not do it, do not combine them 😭

you can mix options and other arguments. for the most part, the order you use doesn't matter. but — 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. so like, vibes are flexible but that one has main character energy and needs to be positioned correctly 💅

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. so if you don't see it, it's probably the other one. slay 🌈

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 gigibyte, and so on. such arguments are designated by byte-size in the following text. and if you want the full listing and explanation of the binary and decimal byte size prefixes, go refer to the NIST, IEC, and other relevant national and international standards. they ate that reference material fr fr 📚

---

🎀 OPTIONS 🎀

🌟 Option Summary 🌟

here is a summary of all the options, grouped by type. explanations are in the following sections — this part is just the lineup, like a tracklist before the album plays 💖

🦋 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]

---

okay bestie that's the tracklist 💖 the actual explanations are coming in the next sections — don't go anywhere, we're just getting started 🎀

💅 Warning Options 💅

okay so this is the roll call, bestie. every single warning flag GCC has, listed in order, exactly as they're spelled, because we do not skip a single one in this house. no descriptions here, just the names — the actual tea comes further down the page. 👑

• -fsyntax-only
• -fmax-errors=n
• -Wpedantic
• -pedantic-errors
• -fpermissive
• -w
• -Wextra
• -Wall
• -Wabi=n
• -Waddress
• -Wno-address-of-packed-member
• -Waggregate-return
• -Walloc-size
• -Walloc-size-larger-than=byte-size
• -Walloc-zero
• -Walloca
• -Walloca-larger-than=byte-size
• -Wauto-profile
• -Wno-aggressive-loop-optimizations
• -Warith-conversion
• -Warray-bounds
• -Warray-bounds=n
• -Warray-compare
• -Warray-parameter
• -Warray-parameter=n
• -Wno-attributes
• -Wattribute-alias=n
• -Wno-attribute-alias
• -Wno-attribute-warning
• -Wbidi-chars=[none|unpaired|any|ucn]
• -Wbool-compare
• -Wbool-operation
• -Wno-builtin-declaration-mismatch
• -Wno-builtin-macro-redefined
• -Wc90-c99-compat
• -Wc99-c11-compat
• -Wc11-c23-compat
• -Wc23-c2y-compat
• -Wc++-compat
• -Wc++11-compat
• -Wc++14-compat
• -Wc++17-compat
• -Wc++20-compat
• -Wc++26-compat
• -Wno-c++11-extensions
• -Wno-c++14-extensions
• -Wno-c++17-extensions
• -Wno-c++20-extensions
• -Wno-c++23-extensions

---

still with me? good, because we are so not done 🌸

• -Wcalloc-transposed-args
• -Wcannot-profile
• -Wcast-align
• -Wcast-align=strict
• -Wcast-function-type
• -Wcast-qual
• -Wchar-subscripts
• -Wclobbered
• -Wcomment
• -Wcompare-distinct-pointer-types
• -Wno-complain-wrong-lang
• -Wconversion
• -Wno-coverage-mismatch
• -Wno-cpp
• -Wdangling-else
• -Wdangling-pointer
• -Wdangling-pointer=n
• -Wdate-time
• -Wno-deprecated
• -Wno-deprecated-declarations
• -Wno-designated-init
• -Wno-deprecated-openmp
• -Wdisabled-optimization
• -Wno-discarded-array-qualifiers
• -Wno-discarded-qualifiers
• -Wno-div-by-zero
• -Wdouble-promotion
• -Wduplicated-branches
• -Wduplicated-cond
• -Wempty-body
• -Wno-endif-labels
• -Wenum-compare
• -Wenum-conversion
• -Wenum-int-mismatch
• -Werror
• -Werror=*
• -Wexpansion-to-defined
• -Wfatal-errors
• -Wflex-array-member-not-at-end
• -Wfloat-conversion
• -Wfloat-equal
• -Wformat
• -Wformat=2
• -Wno-format-contains-nul
• -Wno-format-diag
• -Wno-format-extra-args
• -Wformat-nonliteral
• -Wformat-overflow=n
• -Wformat-security
• -Wformat-signedness
• -Wformat-truncation=n
• -Wformat-y2k

---

the f's and the format girlies are holding it DOWN 💜

• -Wframe-address
• -Wframe-larger-than=byte-size
• -Wno-free-nonheap-object
• -Wheader-guard
• -Wno-if-not-aligned
• -Wno-ignored-attributes
• -Wignored-qualifiers
• -Wno-incompatible-pointer-types
• -Whardened
• -Wimplicit
• -Wimplicit-fallthrough
• -Wimplicit-fallthrough=n
• -Wno-implicit-function-declaration
• -Wno-implicit-int
• -Winfinite-recursion
• -Winit-self
• -Winline
• -Wno-int-conversion
• -Wint-in-bool-context
• -Wno-int-to-pointer-cast
• -Wno-invalid-memory-model
• -Winvalid-pch
• -Winvalid-utf8
• -Wno-unicode
• -Wjump-misses-init
• -Wkeyword-macro
• -Wlarger-than=byte-size
• -Wleading-whitespace=kind
• -Wlogical-not-parentheses
• -Wlogical-op
• -Wlong-long
• -Wno-lto-type-mismatch
• -Wmain
• -Wmaybe-uninitialized
• -Wmemset-elt-size
• -Wmemset-transposed-args
• -Wmisleading-indentation
• -Wmissing-attributes
• -Wmissing-braces
• -Wmissing-field-initializers
• -Wmissing-format-attribute
• -Wmissing-include-dirs
• -Wmissing-noreturn
• -Wmusttail-local-addr
• -Wmaybe-musttail-local-addr
• -Wno-missing-profile
• -Wno-multichar
• -Wmultistatement-macros

---

the -Wno- twins are here too, obviously, they're iconic 🦋

• -Wnonnull
• -Wnonnull-compare
• -Wnormalized=[none|id|nfc|nfkc]
• -Wnull-dereference
• -Wno-odr
• -Wopenacc-parallelism
• -Wopenmp
• -Wopenmp-simd
• -Wno-overflow
• -Woverlength-strings
• -Wno-override-init-side-effects
• -Wpacked
• -Wno-packed-bitfield-compat
• -Wpacked-not-aligned
• -Wpadded
• -Wparentheses
• -Wno-pedantic-ms-format
• -Wpointer-arith
• -Wno-pointer-compare
• -Wno-pointer-to-int-cast
• -Wno-pragmas
• -Wno-pragma-once-outside-header
• -Wno-prio-ctor-dtor
• -Wno-psabi
• -Wredundant-decls
• -Wrestrict
• -Wno-return-local-addr
• -Wreturn-type
• -Wno-scalar-storage-order
• -Wsequence-point
• -Wshadow
• -Wshadow=global
• -Wshadow=local
• -Wshadow=compatible-local
• -Wno-shadow-ivar
• -Wno-shift-count-negative
• -Wno-shift-count-overflow
• -Wshift-negative-value
• -Wno-shift-overflow
• -Wshift-overflow=n
• -Wsign-compare
• -Wsign-conversion
• -Wno-sizeof-array-argument
• -Wsizeof-array-div
• -Wsizeof-pointer-div
• -Wsizeof-pointer-memaccess
• -Wstack-protector
• -Wstack-usage=byte-size

---

shadow, shift, sign, sizeof… the S section is giving main character energy ngl 😭

• -Wstrict-aliasing
• -Wstrict-aliasing=n
• -Wstrict-overflow
• -Wstrict-overflow=n
• -Wstring-compare
• -Wno-stringop-overflow
• -Wno-stringop-overread
• -Wno-stringop-truncation
• -Wstrict-flex-arrays
• -Wsuggest-attribute=attribute-name
• -Wswitch
• -Wno-switch-bool
• -Wswitch-default
• -Wswitch-enum
• -Wno-switch-outside-range
• -Wno-switch-unreachable
• -Wsync-nand
• -Wsystem-headers
• -Wtautological-compare
• -Wtrailing-whitespace
• -Wtrailing-whitespace=kind
• -Wtrampolines
• -Wtrigraphs
• -Wtrivial-auto-var-init
• -Wno-tsan
• -Wtype-limits
• -Wundef
• -Wuninitialized
• -Wunknown-pragmas
• -Wunsuffixed-float-constants
• -Wunterminated-string-initialization
• -Wunused
• -Wunused-but-set-parameter
• -Wunused-but-set-parameter=n
• -Wunused-but-set-variable
• -Wunused-but-set-variable=n
• -Wunused-const-variable
• -Wunused-const-variable=n
• -Wunused-function
• -Wunused-label
• -Wunused-local-typedefs
• -Wunused-macros
• -Wunused-parameter
• -Wno-unused-result
• -Wunused-value
• -Wunused-variable
• -Wuse-after-free
• -Wuse-after-free=n
• -Wuseless-cast
• -Wno-varargs
• -Wvariadic-macros
• -Wvector-operation-performance
• -Wvla
• -Wvla-larger-than=byte-size
• -Wno-vla-larger-than
• -Wvolatile-register-var
• -Wwrite-strings
• -Wno-xor-used-as-pow
• -Wzero-as-null-pointer-constant
• -Wzero-length-bounds

---

and the long-form double-dash icons to close it out ⭐

• --all-warnings
• --extra-warnings
• --no-warnings
• --pedantic
• --pedantic-errors

🔍 Static Analyzer Options 🔍

next up: the static analyzer squad, aka the ones who look at your code like it owes them money. same deal — every flag, exact spelling, no skipping. 🫶

• -fanalyzer
• -fanalyzer-assume-nothrow
• -fanalyzer-call-summaries
• -fanalyzer-checker=name
• -fno-analyzer-feasibility
• -fanalyzer-show-events-in-system-headers
• -fno-analyzer-state-merge
• -fno-analyzer-state-purge
• -fno-analyzer-suppress-followups
• -fanalyzer-transitivity
• -fno-analyzer-undo-inlining
• -fanalyzer-verbose-edges
• -fanalyzer-verbose-state-changes
• -fanalyzer-verbosity=level
• -fdump-analyzer
• -fdump-analyzer-callgraph
• -fdump-analyzer-exploded-graph
• -fdump-analyzer-exploded-nodes
• -fdump-analyzer-exploded-nodes-2
• -fdump-analyzer-exploded-nodes-3
• -fdump-analyzer-exploded-paths
• -fdump-analyzer-feasibility
• -fdump-analyzer-infinite-loop
• -fdump-analyzer-json
• -fdump-analyzer-state-purge
• -fdump-analyzer-stderr
• -fdump-analyzer-supergraph
• -fdump-analyzer-untracked

---

and now the big ol' pile of analyzer warnings 💜 every single one:

• -Wno-analyzer-double-fclose
• -Wno-analyzer-double-free
• -Wno-analyzer-exposure-through-output-file
• -Wno-analyzer-exposure-through-uninit-copy
• -Wno-analyzer-fd-access-mode-mismatch
• -Wno-analyzer-fd-double-close
• -Wno-analyzer-fd-leak
• -Wno-analyzer-fd-phase-mismatch
• -Wno-analyzer-fd-type-mismatch
• -Wno-analyzer-fd-use-after-close
• -Wno-analyzer-fd-use-without-check
• -Wno-analyzer-file-leak
• -Wno-analyzer-free-of-non-heap
• -Wno-analyzer-imprecise-fp-arithmetic
• -Wno-analyzer-infinite-loop
• -Wno-analyzer-infinite-recursion
• -Wno-analyzer-jump-through-null
• -Wno-analyzer-malloc-leak
• -Wno-analyzer-mismatching-deallocation
• -Wno-analyzer-null-argument
• -Wno-analyzer-null-dereference
• -Wno-analyzer-out-of-bounds
• -Wno-analyzer-overlapping-buffers
• -Wno-analyzer-possible-null-argument
• -Wno-analyzer-possible-null-dereference
• -Wno-analyzer-putenv-of-auto-var
• -Wno-analyzer-shift-count-negative
• -Wno-analyzer-shift-count-overflow
• -Wno-analyzer-stale-setjmp-buffer
• -Wno-analyzer-tainted-allocation-size
• -Wno-analyzer-tainted-assertion
• -Wno-analyzer-tainted-array-index
• -Wno-analyzer-tainted-divisor
• -Wno-analyzer-tainted-offset
• -Wno-analyzer-tainted-size
• -Wno-analyzer-throw-of-unexpected-type
• -Wanalyzer-symbol-too-complex
• -Wanalyzer-too-complex
• -Wno-analyzer-undefined-behavior-ptrdiff
• -Wno-analyzer-undefined-behavior-strtok
• -Wno-analyzer-unsafe-call-within-signal-handler
• -Wno-analyzer-use-after-free
• -Wno-analyzer-use-of-pointer-in-stale-stack-frame
• -Wno-analyzer-use-of-uninitialized-value
• -Wno-analyzer-va-arg-type-mismatch
• -Wno-analyzer-va-list-exhausted
• -Wno-analyzer-va-list-leak
• -Wno-analyzer-va-list-use-after-va-end
• -Wno-analyzer-write-to-const
• -Wno-analyzer-write-to-string-literal

🎀 C and Objective-C-only Warning Options 🎀

these ones are specifically for the C and Objective-C girlies. everyone else can sit this one out, respectfully 🍓

• -Wbad-function-cast
• -Wdeprecated-non-prototype
• -Wfree-labels
• -Wmissing-declarations
• -Wmissing-parameter-name
• -Wmissing-parameter-type
• -Wdeclaration-missing-parameter-type
• -Wmissing-prototypes
• -Wmissing-variable-declarations
• -Wmultiple-parameter-fwd-decl-lists
• -Wnested-externs
• -Wold-style-declaration
• -Wold-style-definition
• -Wstrict-prototypes
• -Wtraditional
• -Wtraditional-conversion
• -Wdeclaration-after-statement
• -Wpointer-sign

🐛 Debugging Options 🐛

and now the debugging section!! this is where you tell GCC to actually keep receipts on your code so your debugger has something to work with. again — every flag, in order, untouched 💖

• -g
• -glevel
• -gdwarf
• -gdwarf-version
• -gbtf
• -gctf
• -gctflevel
• -gno-prune-btf
• -ggdb
• -gno-record-gcc-switches
• -gstrict-dwarf
• -gas-loc-support
• -gas-locview-support
• -gcodeview
• -gcolumn-info
• -gdwarf32
• -gdwarf64
• -gno-statement-frontiers
• -gno-variable-location-views
• -gvariable-location-views=incompat5
• -ginternal-reset-location-views
• -ginline-points
• -gvms
• -gz[=type]
• -gsplit-dwarf
• -gdescribe-dies
• -fdebug-prefix-map=old=new
• -fdebug-types-section
• -fno-eliminate-unused-debug-types
• -femit-struct-debug-baseonly
• -femit-struct-debug-reduced
• -femit-struct-debug-detailed[=spec-list]
• -fno-eliminate-unused-debug-symbols
• -femit-class-debug-always
• -fno-merge-debug-strings
• -fno-dwarf2-cfi-asm
• -fvar-tracking
• -fvar-tracking-assignments
• -fvar-tracking-uninit
• --debug

---

okay and that's only the roll call, bestie — the actual deep-dive explanations for all of these are still coming, so do not close the tab 💜

✨ Optimization Options ✨

Okay bestie, deep breath, hydrate, sit down. 🫶 This is the big one. Every single knob GCC has for making your code go brrr lives here, and I am not skipping a single one, because you deserve the whole roster. Names only in this index — the deep dives live in their own sections — but here's the full squad, in order, no cap. 💖

• -faggressive-loop-optimizations
• -falign-functions[=n[:m:[n2[:m2]]]] — okay nested-brackets queen, we see you 💅
• -falign-jumps[=n[:m:[n2[:m2]]]]
• -falign-labels[=n[:m:[n2[:m2]]]]
• -falign-loops[=n[:m:[n2[:m2]]]]
• -fmin-function-alignment=[n]
• -fno-allocation-dce
• -fallow-store-data-races
• -fassociative-math
• -fauto-profile
• -fauto-profile[=path]
• -fauto-profile-inlining
• -fauto-inc-dec
• -fbranch-probabilities
• -fcaller-saves
• -fcombine-stack-adjustments
• -fconserve-stack
• -ffold-mem-offsets
• -fcompare-elim
• -fcprop-registers
• -fcrossjumping
• -fcse-follow-jumps
• -fcse-skip-blocks
• -fcx-fortran-rules
• -fcx-limited-range
• -fcx-method
• -fdata-sections
• -fdce
• -fdelayed-branch
• -fdelete-null-pointer-checks
• -fdep-fusion
• -fdevirtualize
• -fdevirtualize-speculatively
• -fdevirtualize-at-ltrans
• -fdse
• -fearly-inlining
• -fexcess-precision=style
• -fexpensive-optimizations
• -fext-dce
• -ffast-math — the name alone is a whole personality 🔥
• -ffat-lto-objects
• -ffinite-loops
• -ffinite-math-only
• -ffloat-store
• -fforward-propagate
• -ffp-contract=style
• -ffp-int-builtin-inexact
• -ffunction-sections
• -ffuse-ops-with-volatile-access
• -fgcse
• -fgcse-after-reload
• -fgcse-las
• -fgcse-lm
• -fgraphite-identity
• -fgcse-sm
• -fhoist-adjacent-loads
• -fif-conversion
• -fif-conversion2
• -findirect-inlining
• -finline-atomics
• -finline-functions
• -finline-functions-called-once
• -finline-limit=n
• -finline-small-functions
• -finline-stringops[=fn]
• -fipa-modref
• -fipa-cp
• -fipa-cp-clone
• -fipa-bit-cp
• -fipa-vrp
• -fipa-pta
• -fipa-profile
• -fipa-pure-const
• -fipa-reference
• -fipa-reference-addressable
• -fipa-reorder-for-locality
• -fipa-sra
• -fipa-stack-alignment
• -fipa-icf
• -fipa-icf-functions
• -fipa-icf-variables
• -fira-algorithm=algorithm
• -flate-combine-instructions
• -flifetime-dse
• -flive-patching=level
• -fira-region=region
• -fira-hoist-pressure
• -fira-loop-pressure
• -fno-ira-share-save-slots
• -fno-ira-share-spill-slots
• -fisolate-erroneous-paths-dereference
• -fisolate-erroneous-paths-attribute
• -fivopts
• -fkeep-inline-functions
• -fkeep-static-functions
• -fkeep-static-consts
• -flimit-function-alignment
• -flive-range-shrinkage
• -floop-block
• -floop-interchange
• -floop-strip-mine
• -floop-unroll-and-jam
• -floop-nest-optimize
• -floop-parallelize-all
• -flra-remat
• -flto
• -flto-compression-level=n
• -flto-toplevel-asm-heuristics
• -flto-partition=alg
• -flto-incremental=path
• -flto-incremental-cache-size=n
• -fmalloc-dce
• -fmerge-all-constants
• -fmerge-constants
• -fmodulo-sched
• -fmodulo-sched-allow-regmoves
• -fmove-loop-invariants
• -fmove-loop-stores
• -fno-branch-count-reg
• -fno-defer-pop
• -fno-function-cse
• -fno-guess-branch-probability
• -fno-inline
• -fno-math-errno
• -fno-peephole
• -fno-peephole2
• -fno-printf-return-value
• -fno-sched-interblock
• -fno-sched-spec
• -fno-signed-zeros
• -fno-toplevel-reorder
• -fno-trapping-math
• -fno-zero-initialized-in-bss
• -fomit-frame-pointer
• -foptimize-crc
• -foptimize-sibling-calls
• -fpartial-inlining
• -fpeel-loops
• -fpredictive-commoning
• -fprefetch-loop-arrays
• -fprofile-correction
• -fprofile-use
• -fprofile-use=path
• -fprofile-partial-training
• -fprofile-values
• -fprofile-reorder-functions
• -freciprocal-math
• -free
• -frename-registers
• -freorder-blocks
• -freorder-blocks-algorithm=algorithm
• -freorder-blocks-and-partition
• -freorder-functions
• -frerun-cse-after-loop
• -freschedule-modulo-scheduled-loops
• -frounding-math
• -fsave-optimization-record
• -fsched2-use-superblocks
• -fsched-pressure
• -fsched-spec-load
• -fsched-spec-load-dangerous — "dangerous" is right there in the name bestie, I'm just saying 😭
• -fsched-stalled-insns-dep[=n]
• -fsched-stalled-insns[=n]
• -fsched-group-heuristic
• -fsched-critical-path-heuristic
• -fsched-spec-insn-heuristic
• -fsched-rank-heuristic
• -fsched-last-insn-heuristic
• -fsched-dep-count-heuristic
• -fschedule-fusion
• -fschedule-insns
• -fschedule-insns2
• -fsection-anchors
• -fselective-scheduling
• -fselective-scheduling2
• -fsel-sched-pipelining
• -fsel-sched-pipelining-outer-loops
• -fsemantic-interposition
• -fshrink-wrap
• -fshrink-wrap-separate
• -fsignaling-nans
• -fsingle-precision-constant
• -fsplit-ivs-in-unroller
• -fsplit-loops
• -fspeculatively-call-stored-functions
• -fsplit-paths
• -fsplit-wide-types
• -fsplit-wide-types-early
• -fssa-backprop
• -fssa-phiopt
• -fstdarg-opt
• -fstore-merging
• -fstrict-aliasing
• -fipa-strict-aliasing
• -fthread-jumps
• -ftracer
• -ftree-bit-ccp
• -ftree-builtin-call-dce
• -ftree-ccp
• -ftree-ch
• -ftree-coalesce-vars
• -ftree-copy-prop
• -ftree-cselim
• -ftree-dce
• -ftree-dominator-opts
• -ftree-dse
• -ftree-forwprop
• -ftree-fre
• -fcode-hoisting
• -ftree-loop-if-convert
• -ftree-loop-im
• -ftree-phiprop
• -ftree-loop-distribution
• -ftree-loop-distribute-patterns
• -ftree-loop-ivcanon
• -ftree-loop-linear
• -ftree-loop-optimize
• -ftree-loop-vectorize
• -ftree-parallelize-loops[=n]
• -ftree-pre
• -ftree-partial-pre
• -ftree-pta
• -ftree-reassoc
• -ftree-scev-cprop
• -ftree-sink
• -ftree-slsr
• -ftree-sra
• -ftree-switch-conversion
• -ftree-tail-merge
• -ftree-ter
• -ftree-vectorize
• -ftree-vrp
• -ftrivial-auto-var-init
• -funconstrained-commons
• -funit-at-a-time
• -funroll-all-loops
• -funroll-loops
• -funsafe-math-optimizations
• -funswitch-loops
• -fipa-ra
• -fvariable-expansion-in-unroller
• -fvect-cost-model
• -fvpt
• -fweb
• -fwhole-program
• -fwpa
• -fuse-linker-plugin
• -fzero-call-used-regs
• -O
• -O0
• -O1
• -O2
• -O3
• -Os
• -Ofast
• -Og
• -Oz
• --optimize

And there she is — the -O family, the main characters of this entire section, sitting right at the bottom like the headliners they are. 👑

---

🩹 Program Instrumentation Options 🩹

Now for the ✨spicy✨ half: profiling, coverage, sanitizers, stack protection, all the stuff you turn on when you want your program to snitch on itself. Bestie, some of these are literally named things like "trap" and "dangerous", so read the deep-dive sections before you go wild. 🫶

• -p
• -pg
• -fprofile-arcs
• -coverage
• -ftest-coverage
• -fcondition-coverage
• -fpath-coverage
• -fprofile
• -fprofile-abs-path
• -fprofile-dir=path
• -fprofile-generate
• -fprofile-generate=path
• -fprofile-info-section
• -fprofile-info-section=name
• -fprofile-note=path
• -fprofile-prefix-path=path
• -fprofile-update=method
• -fprofile-filter-files=regex
• -fprofile-exclude-files=regex
• -fprofile-reproducible=[multithreaded|parallel-runs|serial]
• -fsanitize=style
• -fsanitize-recover
• -fsanitize-recover=style
• -fsanitize-trap
• -fsanitize-trap=style
• -fasan-shadow-offset=number
• -fsanitize-sections=s1,s2,...
• -fsanitize-undefined-trap-on-error
• -fcf-protection
• -fcf-protection=[full|branch|return|none|check]
• -fharden-compares
• -fharden-conditional-branches
• -fhardened
• -fharden-control-flow-redundancy
• -fhardcfr-skip-leaf
• -fhardcfr-check-exceptions
• -fhardcfr-check-returning-calls
• -fhardcfr-check-noreturn-calls=[always|no-xthrow|nothrow|never]
• -fstack-protector
• -fstack-protector-all
• -fstack-protector-strong
• -fstack-protector-explicit
• -fstack-check
• -fstack-limit-register=reg
• -fstack-limit-symbol=sym
• -fno-stack-limit
• -fsplit-stack
• -fstrub=disable
• -fstrub=strict
• -fstrub=relaxed
• -fstrub=all
• -fstrub=at-calls
• -fstrub=internal
• -fvtable-verify=[std|preinit|none]
• -fvtv-counts
• -fvtv-debug
• -finstrument-functions
• -finstrument-functions-once
• -finstrument-functions-exclude-function-list=sym,sym,...
• -finstrument-functions-exclude-file-list=file,file,...
• -fprofile-prefix-map=old=new
• -fpatchable-function-entry=N[,M]
• --coverage
• --profile

---

🧠 Preprocessor Options 🧠

The preprocessor is the first little gremlin that touches your code, and these are all its levers: defines, undefines, includes, dependency dumps, and the whole -M family for when your build system needs to know what you touched. 🌸

• -C
• -CC
• -Dmacro[=defn]
• -dD
• -dI
• -dM
• -dN
• -dU
• -fdebug-cpp
• -fdirectives-only
• -fdollars-in-identifiers
• -fexec-charset=charset
• -fextended-identifiers
• -finput-charset=charset
• -fmacro-prefix-map=old=new
• -fmax-include-depth=depth
• -fno-canonical-system-headers
• -fpch-deps
• -fpch-preprocess
• -fpreprocessed
• -ftabstop=width
• -ftrack-macro-expansion
• -fwide-exec-charset=charset
• -fworking-directory
• -H
• -imacros file
• -include file
• -M
• -MD
• -MF
• -MG
• -MM
• -MMD
• -MP
• -MQ
• -MT
• -Mno-modules
• -no-integrated-cpp
• -P
• -pthread
• -remap
• -traditional
• -traditional-cpp
• -trigraphs
• -Umacro
• -undef
• -Wp,option
• -Xpreprocessor option
• --comments
• --comments-in-macros
• --define-macro=macro[=defn]
• --dependencies
• --dump=letters
• --imacros=file
• --include=file
• --no-integrated-cpp
• --no-line-commands
• --print-missing-file-dependencies
• --traditional
• --traditional-cpp
• --trigraphs
• --trace-includes
• --undefine-macro=macro
• --user-dependencies
• --write-dependencies
• --write-user-dependencies

---

🔧 Assembler Options 🔧

Just three little pass-throughs, tiny but mighty — they hand stuff straight to the assembler and mind their own business. 🎀

• -Wa,option
• -Xassembler option
• --for-assembler=option

---

🔗 Linker Options 🔗

The part where all your little object files hold hands and become One Executable. 🦋 Linking is where the drama happens — static vs shared, entry points, scripts, and a suspicious amount of single-letter flags that mean very different things.

• object-file-name
• -flink-libatomic
• -fuse-ld=linker
• -llibrary
• -nostartfiles
• -nodefaultlibs
• -nolibc
• -nostdlib
• -nostdlib++
• -e entry
• -pie
• -pthread
• -r
• -rdynamic
• -s
• -static
• -static-pie
• -static-libgcc
• -static-libstdc++
• -static-libasan
• -static-libtsan
• -static-liblsan
• -static-libubsan
• -shared
• -shared-libgcc
• -symbolic
• -T script
• -Wl,option
• -Xlinker option
• -u symbol
• -Tbss=addr
• -Tdata=addr
• -Ttext=addr
• -N
• -n
• -t
• -Z
• -z keyword
• --entry=entry
• --for-linker=option
• --force-link=symbol
• --no-standard-library
• --pie
• --static
• --static-pie
• --symbolic

Not me looking at -s, -N, -n, -t, -Z, -z and feeling like I'm reading a password instead of compiler flags. 💀 Case-sensitivity said "I am the moment".

---

📁 Directory Options 📁

Where should GCC go looking for your headers, your libs, your plugins, your sysroot? This is the map section, bestie. Every one of these is a tiny GPS ping. 🗺️✨

• -Bprefix
• -Idir
• -I-
• -idirafter dir
• -imacros file
• -imultilib dir
• -imultiarch dir
• -iplugindir=dir
• -iprefix file
• -iquote dir
• -isysroot dir
• -isystem dir
• -iwithprefix dir
• -iwithprefixbefore dir
• -Ldir
• -no-canonical-prefixes
• --no-sysroot-suffix
• -nostdinc
• -nostdinc++
• --embed-dir=dir
• --embed-directory=dir
• --include-barrier
• --include-directory=dir
• --include-directory-after=dir
• --include-prefix=prefix
• --include-with-prefix=prefix
• --include-with-prefix-after=prefix
• --include-with-prefix-before=prefix
• --no-canonical-prefixes
• --no-standard-includes
• --prefix=prefix
• --sysroot=dir

---

🍓 Picolibc Options 🍓

Picolibc squad, this is your moment — four flags, immaculate, no notes. 🎀

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

---

⚙️ Code Generation Options ⚙️

The low-level lore section. 🧚 Registers, exceptions, PIC/PIE, visibility, struct returns, all the nitty-gritty about what your code actually turns into. Sit with it, sip your drink, this is where the real ones live.

• -fcall-saved-reg
• -fcall-used-reg
• -ffixed-reg
• -fexceptions
• -fnon-call-exceptions
• -fdelete-dead-exceptions
• -funwind-tables
• -fasynchronous-unwind-tables
• -fno-gnu-unique
• -finhibit-size-directive
• -fcommon
• -fno-ident
• -fpcc-struct-return
• -fpic
• -fPIC
• -fpie
• -fPIE
• -fno-plt
• -fno-jump-tables
• -fno-bit-tests
• -frecord-gcc-switches
• -freg-struct-return
• -fshort-enums
• -fshort-wchar
• -fverbose-asm
• -fpack-struct[=n]
• -fleading-underscore
• -ftls-model=model
• -fstack-reuse=reuse_level
• -ftrampolines
• -ftrampoline-impl=[stack|heap]
• -ftrapv
• -fwrapv
• -fwrapv-pointer
• -fvisibility=[default|internal|hidden|protected]
• -fstrict-volatile-bitfields
• -fsync-libcalls
• -fzero-init-padding-bits=value
• -Qy
• -Qn

-fpic vs -fPIC, -fpie vs -fPIE — yes bestie, case matters, and yes, they are different entries. The lowercase and uppercase versions are both right there in the list and I am keeping them both, exactly as written. 🙈

🛠️ Developer Options 🛠️

okay so this next chunk is the ✨developer✨ zone — the flags you reach for when you're not just compiling, you're investigating. Lowkey the compiler's diary. 💅

• -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

periodt. 🎀

---

🌍 Machine-Dependent Options 🌍

and now bestie… the big one. every CPU out here got its own lil flag family, and gcc is showing up for all of them. this is the "one compiler to serve them all" energy fr fr. 👑

🦋 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
• -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 🦋

okay so we're still doing the target-by-target tour and honestly it's giving encyclopedia, but every single flag matters so here's the full HPPA roster, no gatekeeping:

-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 🌸

Itanium girlies rise 💅 the whole list, untouched and unbothered:

-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 🎀

short and sweet, five flags and a dream:

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

---

💜 LoongArch Options 💜

bestie this list is lowkey long but we're not skipping a single one:

-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 🧚

tiny group, big energy:

-mshared
-mthreads
-mlegacy-threads

---

🍓 M32C Options 🍓

three flags, main character energy, no notes:

-mcpu=cpu
-msim
-memregs=number

---

⭐ M32R/D Options ⭐

the whole lineup, periodt:

-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 🌈

the 68k family reunion and yes we're naming every single relative:

-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 🦋

okay MCore, we see you:

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

---

✨ MicroBlaze Options ✨

full list, and ngl the -mxl- ones are so extra /pos:

-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 💅

bestie. BESTIE. this is the big one and we are doing all of it, every last flag, because that's what MIPS deserves:

-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 🫶

knuth-core, no skips:

-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 🌸

here's the whole vibe:

-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 🎀

four flags and honestly iconic for it:

-meb
-mel
-mmul.x
-mno-crt0

---

💖 MSP430 Options 💖

microcontroller girlies, this one's for you:

-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 🔥

the roster, complete, don't @ me:

-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 🧚

GPU besties, assemble:

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

---

🍓 OpenRISC Options 🍓

open source, open heart, here's the list:

-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 ⭐

retro tech, still slaying:

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

---

🦋 PowerPC Options 🦋

for this one you're gonna want to go look at RS/6000 and PowerPC Options — See RS/6000 and PowerPC Options. that's the whole tea 🫖

---

🌈 PRU Options 🌈

embedded cuties, here's everything:

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

---

💜 RISC-V Options 💜

okay RISC-V ate and left no crumbs, look at this list:

-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 🎀

last but never least, the full RL78 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

---

and that's the whole target-by-target roster for this chunk, bestie 💅 every flag accounted for, zero crumbs left behind 🫶

🦋 RS/6000 and PowerPC Options 🦋

okay bestie, this is the PowerPC section and it is long, so grab a snack 💖 every single one of these is a real flag, no cap:

• -mcpu=cpu-type — sets the CPU type
• -mtune=cpu-type — tunes for a CPU type
• -mcmodel=code-model — picks the code model
• -mprofile-kernel — profile the kernel
• -mpowerpc64 — enable PowerPC64
• -maltivec — enable AltiVec
• -mpowerpc-gpopt — PowerPC general-purpose option
• -mpowerpc-gfxopt — PowerPC graphics option
• -mmfcrf — move-from-condition-register-field
• -mpopcntb — popcount byte
• -mpopcntd — popcount doubleword
• -mfprnd — floating-point round
• -mcmpb — compare byte
• -mhard-dfp — hardware decimal floating point
• -mfull-toc — full TOC
• -mminimal-toc — minimal TOC
• -mno-fp-in-toc — no FP in TOC
• -mno-sum-in-toc — no sum in TOC
• -maix64 — AIX 64-bit
• -maix32 — AIX 32-bit
• -m64 — 64-bit
• -m32 — 32-bit
• -mxl-compat — XL compatibility
• -mpe — PE
• -malign-power — align the Power way
• -malign-natural — natural alignment
• -msoft-float — software floating point
• -mhard-float — hardware floating point
• -mmultiple — multiple
• -mupdate — update
• -mavoid-indexed-addresses — avoid indexed addresses
• -mfused-madd — fused multiply-add
• -mbit-align — bit alignment
• -mbit-word — bit word
• -mstrict-align — strict alignment
• -mrelocatable — relocatable
• -mrelocatable-lib — relocatable library
• -mlittle — little
• -mlittle-endian — little endian
• -mbig — big
• -mbig-endian — big endian
• -mdynamic-no-pic — dynamic, no PIC
• -msingle-pic-base — single PIC base
• -mprioritize-restricted-insns=priority — prioritize restricted instructions with a priority
• -msched-costly-dep=dependence_type — mark costly dependencies as dependence_type
• -minsert-sched-nops=scheme — insert scheduling NOPs using a scheme
• -mcall-aixdesc — call AIX descriptors
• -mcall-eabi — call EABI
• -mcall-freebsd — call FreeBSD
• -mcall-linux — call Linux
• -mcall-netbsd — call NetBSD
• -mcall-openbsd — call OpenBSD
• -mcall-sysv — call SysV
• -mcall-sysv-eabi — call SysV EABI
• -mcall-sysv-noeabi — call SysV, no EABI
• -mtraceback=traceback_type — traceback type of traceback_type
• -maix-struct-return — AIX struct return
• -msvr4-struct-return — SVR4 struct return
• -mabi=abi-type — ABI type of abi-type
• -msecure-plt — secure PLT
• -mbss-plt — BSS PLT
• -msplit-patch-nops — split patch NOPs
• -mregnames — register names
• -mlongcall — long calls
• -mpltseq — PLT sequence
• -mtls-markers — TLS markers
• -mblock-move-inline-limit=num — block-move inline limit of num
• -mblock-compare-inline-limit=num — block-compare inline limit of num
• -mblock-compare-inline-loop-limit=num — block-compare inline loop limit of num
• -mblock-ops-unaligned-vsx — unaligned VSX block ops
• -mstring-compare-inline-limit=num — string-compare inline limit of num
• -misel — iSel
• -mvsx — VSX
• -mvrsave — VRSAVE
• -mmulhw — multiply high word
• -mdlmzb — dlmzb
• -mprototype — prototype
• -msim — simulator
• -mmvme — MVME
• -mads — ADS
• -myellowknife — Yellowknife
• -memb — embedded
• -meabi — EABI
• -msdata — small data
• -msdata=opt — small data with opt
• -mreadonly-in-sdata — read-only in small data
• -mvxworks — VxWorks
• -G num — the num small-data threshold
• -mrecip — reciprocal
• -mrecip=opt — reciprocal with opt
• -mno-recip — no reciprocal
• -mrecip-precision — reciprocal precision
• -mveclibabi=type — vector library ABI of type
• -mfriz — friz
• -mpointers-to-nested-functions — pointers to nested functions
• -msave-toc-indirect — save TOC indirect
• -mpower8-fusion — POWER8 fusion
• -mcrypto — crypto
• -mhtm — HTM
• -mquad-memory — quad memory
• -mquad-memory-atomic — quad memory atomic
• -mcompat-align-parm — compatible align parm
• -mfloat128 — float128
• -mfloat128-hardware — float128 hardware
• -mgnu-attribute — GNU attribute
• -mstack-protector-guard=guard — stack protector guard of guard
• -mstack-protector-guard-reg=reg — stack protector guard register of reg
• -mstack-protector-guard-offset=offset — stack protector guard offset of offset
• -mprefixed — prefixed
• -mpcrel — PC-relative
• -mmma — MMA
• -mrop-protect — ROP protection
• -mprivileged — privileged
• -mno-splat-word-constant — no splat word constant
• -mno-splat-float-constant — no splat float constant
• -mno-ieee128-constant — no IEEE128 constant
• -mno-warn-altivec-long — no warn on AltiVec long

---

🎀 RX Options 🎀

Renesas RX time, bestie 🫶

• -m64bit-doubles — 64-bit doubles
• -m32bit-doubles — 32-bit doubles
• -fpu — with FPU
• -nofpu — no FPU
• -mcpu=name — CPU named name
• -mbig-endian-data — big-endian data
• -mlittle-endian-data — little-endian data
• -msmall-data-limit=N — small-data limit of N
• -msim — simulator
• -mas100-syntax — AS100 syntax
• -mrelax — relax
• -mmax-constant-size=N — max constant size of N
• -mint-register=N — N interrupt registers
• -mpid — position-independent data
• -mno-allow-string-insns — don't allow string instructions
• -mjsr — JSR
• -mno-warn-multiple-fast-interrupts — don't warn about multiple fast interrupts
• -msave-acc-in-interrupts — save accumulator in interrupts
• -mlra — local register allocator

---

🌈 S/390 and zSeries Options 🌈

• -mtune=cpu-type — tune for cpu-type
• -march=cpu-type — target architecture cpu-type
• -mhard-float — hardware floating point
• -msoft-float — software floating point
• -mhard-dfp — hardware decimal floating point
• -mlong-double-64 — 64-bit long double
• -mlong-double-128 — 128-bit long double
• -mbackchain — backchain
• -mpacked-stack — packed stack
• -msmall-exec — small exec
• -mmvcle — mvcle
• -m64 — 64-bit
• -m31 — 31-bit
• -mdebug — debug
• -mesa — ESA
• -mzarch — z/Architecture
• -mhtm — HTM
• -mvx — vector extension
• -mzvector — z/Architecture vector
• -mtpf-trace — TPF trace
• -mtpf-trace-skip — TPF trace skip
• -mmain — main
• -mfused-madd — fused multiply-add
• -mno-fused-madd — no fused multiply-add
• -mwarn-framesize=framesize — warn when the frame size is framesize
• -mwarn-dynamicstack — warn about dynamic stack
• -mstack-size=stack-size — stack size of stack-size
• -mno-stack-size — no stack size
• -mstack-guard=stack-guard — stack guard of stack-guard
• -mno-stack-guard — no stack guard
• -mstack-protector-guard=guard — stack protector guard of guard
• -mstack-protector-guard-record — record the stack protector guard
• -mhotpatch=pre-halfwords,post-halfwords — hotpatch with pre-halfwords,post-halfwords
• -mno-pic-data-is-text-relative — no PIC data is text-relative
• -mindirect-branch=choice — indirect branch as choice
• -mindirect-branch-jump=choice — indirect branch jump as choice
• -mindirect-branch-call=choice — indirect branch call as choice
• -mfunction-return=choice — function return as choice
• -mfunction-return-mem=choice — function return (memory) as choice
• -mfunction-return-reg=choice — function return (register) as choice
• -mindirect-branch-table — indirect branch table
• -mfentry — fentry
• -mrecord-mcount — record mcount
• -mnop-mcount — NOP mcount
• -mpreserve-args — preserve args
• -munaliged-symbols — unaligned symbols (yes, that spelling, we keep it exactly 💅)

---

🌸 SH Options 🌸

• -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 — multiply cost of number
• -mdiv=strategy — division strategy strategy
• -mdivsi3_libfunc=name — divsi3 libfunc named name
• -mfixed-range=register-range — fixed register range register-range
• -maccumulate-outgoing-args — accumulate outgoing args
• -matomic-model=atomic-model — atomic model atomic-model
• -mbranch-cost=num — branch cost num
• -mzdcbranch — zero-displacement-call branch
• -mcbranch-force-delay-slot — conditional branch forces delay slot
• -mfsca — fsca
• -mfsrra — fsrra
• -mpretend-cmove — pretend cmove
• -mfdpic — FDPIC
• -mtas — tas
• -mlra — local register allocator

---

⭐ Solaris 2 Options ⭐

• -mclear-hwcap — clear the hardware capability
• -mno-clear-hwcap — don't clear the hardware capability
• -mimpure-text — impure text
• -mno-impure-text — no impure text
• -gsctf — CTF generation

---

🔥 SPARC Options 🔥

• -mcpu=cpu-type — CPU cpu-type
• -mtune=cpu-type — tune for cpu-type
• -mcmodel=code-model — code model code-model
• -mmemory-model=mem-model — memory model mem-model
• -m32, -m64 — 32-bit / 64-bit
• -mptr32, -mptr64 — 32-bit / 64-bit pointers
• -mapp-regs — application registers
• -mfaster-structs — faster structs
• -mflat — flat
• -mfpu — with FPU
• -mhard-float, -msoft-float — hardware / software float
• -mhard-quad-float, -msoft-quad-float — hardware / software quad float
• -mstack-bias — stack bias
• -mstd-struct-return — standard struct return
• -munaligned-doubles — unaligned doubles
• -muser-mode — user mode
• -mv8plus — V8+
• -mvis, -mvis2, -mvis3, -mvis3b, -mvis4, -mvis4b — the VIS family
• -mcbcond — compare-and-branch on condition
• -mfmaf — FMAf
• -mfsmuld — fsmuld
• -mpopc — popc
• -msubxc — subxc
• -mfix-at697f, -mfix-ut699, -mfix-ut700, -mfix-gr712rc — the errata fixes

---

💜 System V Options 💜

• -YP,paths — set the paths for YP
• -Ym,dir — set the YP directory to dir

---

🧚 V850 Options 🧚

• -mlong-calls — long calls
• -mep — EP
• -mprolog-function — prolog function
• -mspace — space
• -mtda=n — n for tda
• -msda=n — n for sda
• -mzda=n — n for zda
• -mv850, -mv850e3v5, -m850e2v4, -mv850e2v3, -mv850e2, -mv850e1, -mv850es, -mv850e — the V850 flavour menu
• -mdisable-callt — disable callt
• -mrelax — relax
• -mlong-jumps — long jumps
• -msoft-float, -mhard-float — soft / hard float
• -mloop — loop
• -mrh850-abi — RH850 ABI
• -mghs — GHS
• -mgcc-abi — GCC ABI
• -m8byte-align — 8-byte alignment
• -mbig-switch — big switch
• -mapp-regs — application registers
• -msmall-sld — small SLD
• -mno-strict-align — no strict alignment
• -mjump-tables-in-data-section — jump tables live in the data section

---

🍓 VAX Options 🍓

• -munix — Unix
• -mgnu — GNU
• -md, -md-float — d / d-float
• -mg, -mg-float — g / g-float
• -mlra — local register allocator
• -mvaxc-alignment — VAX C alignment
• -mqmath — qmath

---

👑 Visium Options 👑

• -mdebug — debug
• -msim — simulator
• -mfpu — with FPU
• -mhard-float, -msoft-float — hard / soft float
• -mcpu=cpu-type — CPU cpu-type
• -mtune=cpu-type — tune for cpu-type
• -msv-mode — supervisor mode
• -muser-mode — user mode

---

🦋 VMS Options 🦋

• -mvms-return-codes — VMS return codes
• -mdebug-main=prefix — debug main with prefix
• -mmalloc64 — malloc64
• -mpointer-size=size — pointer size size

---

💖 VxWorks Options 💖

• -mrtp — RTP
• -msmp — SMP
• -mvthreads — vthreads
• -non-static — non-static
• -Bstatic — static
• -Bdynamic — dynamic
• -Xbind-lazy — lazy binding
• -Xbind-now — immediate binding

---

💅 x86 Options 💅

bestie this one is huge, it's giving "the whole periodic table but for instructions" 😭 every flag, exactly as written:

• -mtune=cpu-type — tune for cpu-type
• -march=cpu-type — architecture cpu-type
• -mtune-ctrl=feature-list — tune control from feature-list
• -mdump-tune-features — dump the tune features
• -mno-default — no defaults
• -mfpmath=unit — FP math using unit
• -masm=dialect — assembly dialect
• -mno-fancy-math-387 — no fancy 387 math
• -mno-fp-ret-in-387 — no FP return in 387
• -m80387 — 80387
• -mhard-float, -msoft-float — hard / soft float
• -mieee-fp — IEEE FP
• -mrtd — rtd
• -malign-double — align doubles
• -mpreferred-stack-boundary=num — preferred stack boundary num
• -mincoming-stack-boundary=num — incoming stack boundary num
• -mcld — cld
• -mcx16 — cx16
• -msahf — sahf
• -mmovbe — movbe
• -mcrc32 — crc32
• -mmwait — mwait
• -mrecip, -mrecip=opt — reciprocal / reciprocal with opt
• -mvzeroupper — vzeroupper
• -mstv — stv
• -mprefer-avx128 — prefer AVX128
• -mprefer-vector-width=opt — prefer vector width opt
• -mpartial-vector-fp-math — partial vector FP math
• -mmove-max=bits — move max bits
• -mstore-max=bits — store max bits
• -mnoreturn-no-callee-saved-registers — no return, no callee-saved registers
• -mmmx, -msse, -msse2, -msse3, -mssse3, -msse4.1, -msse4.2, -msse4 — the SSE lineage
• -mavx, -mavx2, -mavx512f, -mavx512cd, -mavx512vl, -mavx512bw, -mavx512dq, -mavx512ifma, -mavx512vbmi — AVX everything
• -msha, -maes, -mpclmul, -mfsgsbase, -mrdrnd, -mf16c, -mfma, -mpconfig, -mwbnoinvd, -mptwrite, -mclflushopt, -mclwb, -mxsavec, -mxsaves — the extended feature buffet
• -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 — the rest of the instruction-set alphabet, no skips, bestie
• -mms-bitfields — MS bitfields
• -mno-align-stringops — no align string ops
• -minline-all-stringops — inline all string ops
• -minline-stringops-dynamically — inline string ops dynamically
• -mstringop-strategy=alg — string op strategy alg
• -mkl, -mwidekl — kl / widekl
• -mmemcpy-strategy=strategy — memcpy strategy strategy
• -mmemset-strategy=strategy — memset strategy strategy
• -mpush-args — push args
• -maccumulate-outgoing-args — accumulate outgoing args
• -m128bit-long-double, -m96bit-long-double, -mlong-double-64, -mlong-double-80, -mlong-double-128 — the long double size menu
• -mregparm=num — register parameters num
• -msseregparm — SSE register parameters
• -mveclibabi=type — vector library ABI type
• -mvect8-ret-in-mem — vector 8 returned in memory
• -mpc32, -mpc64, -mpc80 — the PC precision trio
• -mdaz-ftz — DAZ / FTZ
• -mstackrealign — stack realign
• -mstack-arg-probe — stack arg probe
• -momit-leaf-frame-pointer — omit leaf frame pointer
• -mno-red-zone — no red zone
• -mno-tls-direct-seg-refs — no TLS direct segment references
• -mcmodel=code-model — code model code-model
• -mabi=name — ABI name
• -maddress-mode=mode — address mode mode
• -m32, -m64, -mx32, -m16, -miamcu — the bit-width / target menu
• -mlarge-data-threshold=num — large data threshold num
• -msse2avx — SSE2 with AVX encoding
• -mfentry, -mrecord-mcount, -mnop-mcount — instrumentation trio
• -m8bit-idiv — 8-bit idiv
• -minstrument-return=type — instrument return type
• -mrecord-return — record return
• -mfentry-name=name — fentry name name
• -mfentry-section=name — fentry section name
• -mskip-rax-setup — skip rax setup
• -mavx256-split-unaligned-load, -mavx256-split-unaligned-store — split unaligned AVX256 load / store
• -malign-data=type — align data type
• -mstack-protector-guard=guard — stack protector guard guard
• -mstack-protector-guard-reg=reg — stack protector guard register reg
• -mstack-protector-guard-offset=offset — stack protector guard offset offset
• -mstack-protector-guard-symbol=symbol — stack protector guard symbol symbol
• -mgeneral-regs-only — general registers only
• -mcall-ms2sysv-xlogues — call MS-to-SysV xlogues
• -mtls-dialect=type — TLS dialect type
• -mrelax-cmpxchg-loop — relax cmpxchg loop
• -mindirect-branch=choice — indirect branch choice
• -mfunction-return=choice — function return choice
• -mindirect-branch-register — indirect branch register
• -mharden-sls=choice — harden SLS choice
• -mindirect-branch-cs-prefix — indirect branch CS prefix
• -mapx-inline-asm-use-gpr32 — APX inline asm uses gpr32
• -mgather, -mscatter — gather / scatter
• -mneeded — needed
• -mno-direct-extern-access — no direct extern access
• -munroll-only-small-loops — unroll only small loops
• -mdispatch-scheduler — dispatch scheduler
• -mlam=choice — LAM choice

---

x86 Windows Options

See Cygwin and MinGW Options. 🫶

---

🌟 Xstormy16 Options 🌟

• -msim — simulator

---

🎀 Xtensa Options 🎀

• -mconst16 — constant 16
• -mforce-no-pic — force no PIC
• -mno-serialize-volatile — no serialize volatile
• -mtext-section-literals — text section literals
• -mauto-litpools — auto literal pools
• -mno-target-align — no target align
• -mlongcalls — long calls
• -mabi=abi-type — ABI abi-type
• -mextra-l32r-costs=cycles — extra l32r costs of cycles
• -mstrict-align — strict alignment

---

zSeries Options

See S/390 and zSeries Options. 💜

---

✨ Options Controlling the Kind of Output ✨

okay so here's the tea, bestie ☕ a single compile can involve up to four stages, and they always go in this order, no skipping the queue:

1. preprocessing 🌱
2. compilation proper 🔧
3. assembly 🧩
4. linking 🔗

GCC can preprocess and compile a bunch of files either into several assembler input files, or into one assembler input file. Then each assembler input file makes an object file, and linking combines all the object files — the ones it just compiled and the ones you handed it as input — into one executable file. iconic behaviour tbh 👑

---

For any given input file, the file name suffix decides what kind of compilation happens. iykyk, the extension is the vibe check:

• file.c — C source code that must be preprocessed.
• file.i — C source code that should not be preprocessed.
• file.ii — C++ source code that should not be preprocessed.
• file.m — Objective-C source code. Note that you must link with the libobjc library to make an Objective-C program work.
• file.mi — Objective-C source code that should not be preprocessed.
• file.mm
• file.M — Objective-C++ source code. Note that you must link with the libobjc library to make an Objective-C++ program work. Note that .M refers to a literal capital M.
• file.mii — Objective-C++ source code that should not be preprocessed.
• file.h — C, C++, Objective-C or Objective-C++ header file to be turned into a precompiled header (default), or C, C++ header file to be turned into an Ada spec (via the -fdump-ada-spec switch).
• file.cc, file.cp, file.cxx, file.cpp, file.CPP, file.c++, file.C — C++ source code that must be preprocessed 💖. Heads up bestie: in .cxx the last two letters have to both be literally x, no cap. And .C? That's a literal capital C, periodt.
• 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 become 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).
• file.cob, file.COB, file.cbl, file.CBL — COBOL source code 👑
• file.go — Go source code.
• file.d — D source code.
• file.di — D interface file.
• file.dd — D documentation code (Ddoc).
• file.ads — Ada source code file holding a library unit declaration (that's 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 files also go by specs.
• file.adb — Ada source code file holding a library unit body (a subprogram or package body). These ones also get called bodies.
• file.s — assembler code.
• file.S, file.sx — assembler code that must be preprocessed.
• other — an object file to be fed straight into linking. Any file name with no recognized suffix is treated this way. iykyk 💅

---

You can specify the input language explicitly with the -x option, bestie:

• -x language, --language=language, --language language — specify explicitly the language for the following input files, instead of letting the compiler pick a default from the file name suffix. This option applies to all following input files until the next -x option shows up. The 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

Real talk though: -x does not imply a particular language standard. So like, -x f77 may also need -std=legacy for some older source codes. It's giving "one more flag, sorry babe" 😭

• -x none — turns off any specification of a language, so that the following files get handled according to their file name suffixes, as if -x had never been used at all. A clean slate, a fresh start 🦋

---

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

• -c, --compile — compile or assemble the source files, but do not link. The linking stage simply is not done. The ultimate output is 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 that don't need compilation or assembly? Ignored. Not our problem 💅

• -S, --assemble — stop after the stage of compilation proper; do not assemble. The output is an assembler code file for each non-assembler input file you 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 preprocessed source code, and it gets 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's 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.

Okay so even 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. So many knobs, so little time 🎀

• -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 "I'm the main character now" 😌

When -dumpbase is specified in a command that compiles multiple inputs, or that compiles and then links, it may be combined with dumppfx, as specified under -dumpdir. Then, each input file is compiled using the combined dumppfx, and default values for dumpbase and auxdropsuf are computed for each input file:

gcc foo.c bar.c -c -dumpbase main ...

creates foo.o and bar.o as primary outputs, and avoids overwriting the auxiliary and dump outputs by using the dumpbase as a prefix, creating auxiliary and dump outputs named main-foo.* and main-bar.*.

An empty string specified as dumpbase avoids the influence of the output basename in the naming of auxiliary and dump outputs during compilation, computing default values:

gcc -c foo.c -o dir/foobar.o -dumpbase " ...

will name aux outputs dir/foo.* and dump outputs dir/foo.c.*. Note how their basenames are taken from the input name, but the directory still defaults to that of the output 🌸

The empty-string dumpbase does not prevent the use of the output basename for outputs during linking:

gcc foo.c bar.c -o dir/foobar -dumpbase " -flto ...

The compilation of the source files will name auxiliary outputs dir/foo.* and dir/bar.*, and dump outputs dir/foo.c.* and dir/bar.c.*. LTO recompilation during linking will use dir/foobar. as the prefix for dumps and auxiliary files 🦋

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

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

creates dir/foo.o as the main output, and generates auxiliary outputs in dir/x-foo.*, taking the location of the primary output, and dropping the .c suffix from the dumpbase. Dump outputs retain the suffix: dir/x-foo.c.*.

This option is disregarded if it does not match the suffix of a specified dumpbase, except as an alternative to the executable suffix when appending the linker output base name to dumppfx, as specified below:

gcc foo.c bar.c -o main.out -dumpbase-ext .out ...

creates main.out as the primary output, and avoids overwriting the auxiliary and dump outputs by using the executable name minus auxdropsuf as a prefix, creating auxiliary outputs named main-foo.* and main-bar.* and dump outputs named main-foo.c.* and main-bar.c.*.

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

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

creates foo.o as the primary output, and auxiliary outputs named pfx-foo.*, combining the given dumppfx with the default dumpbase derived from the default primary output, derived in turn from the input name. Dump outputs also take the input name suffix: pfx-foo.c.*.

If dumppfx is to be used as a directory name, it must end with a directory separator:

gcc -dumpdir dir/ -c foo.c -o obj/bar.o ...

creates obj/bar.o as the primary output, and auxiliary outputs named dir/bar.*, combining the given dumppfx with the default dumpbase derived from the primary output name. Dump outputs also take the input name suffix: dir/bar.c.*.

It defaults to the location of the output file, unless the output file is a special file like /dev/null. Options -save-temps=cwd and -save-temps=obj override this default, just like an explicit -dumpdir option. In case multiple such options are given, the last one prevails:

gcc -dumpdir pfx- -c foo.c -save-temps=obj ...

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 ...

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 ...

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 overriden 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. 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.

Okay so if the -Q option shows up on the command line before the --help= option, then the descriptive text that --help= shows gets changed. Instead of describing the displayed options, it gives you a little indication of whether the option is enabled, disabled, or set to a specific value (assuming the compiler knows this at the point where the --help= option is used). bestie, it's giving receipts. 💅

Here is a truncated example from the ARM port of gcc:

% gcc -Q -mabi=2 --help=target -c
The following options are target specific:
-mabi=                                2
-mabort-on-noreturn                   [disabled]
-mapcs                                [disabled]

The output is sensitive to the effects of previous command-line options, so for example it is possible to find out which optimizations are enabled at -O2 by using:

-Q -O2 --help=optimizers

Alternatively you can discover which binary optimizations are enabled by -O3 by using:

gcc -c -Q -O3 --help=optimizers > /tmp/O3-opts
gcc -c -Q -O2 --help=optimizers > /tmp/O2-opts
diff /tmp/O2-opts /tmp/O3-opts | grep enabled

---

• --version — Display the version number and copyrights of the invoked GCC. iconic, honestly. 👑
• -pass-exit-codes, --pass-exit-codes — Normally the gcc program exits with the code of 1 if any phase of the compiler returns a non-success return code. If you specify -pass-exit-codes, the gcc program instead returns with the numerically highest error produced by any phase returning an error indication. The C, C++, and Fortran front ends return 4 if an internal compiler error is encountered. no cap, that's the whole vibe. ✨
• -pipe, --pipe — Use pipes rather than temporary files for communication between the various stages of compilation. This fails to work on some systems where the assembler is unable to read from a pipe; but the GNU assembler has no trouble. slay, GNU assembler really said "I got this." 🫶
• -wrapper — Invoke all subcommands under a wrapper program. The name of the wrapper program and its parameters are passed as a comma separated list.

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

This invokes all subprograms of gcc under gdb --args, thus the invocation of cc1 is gdb --args cc1 .... the way that this just hands everything over to the wrapper? obsessed. 🎀

• -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. reproducible builds are the main character. 🌈
• -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. canonicalized paths? periodt. 💜
• -fplugin=name.so — Load the plugin code in file name.so, assumed to be a shared object to be dlopen'd by the compiler. The base name of the shared object file is used to identify the plugin for the purposes of argument parsing (See -fplugin-arg-name-key=value below). Each plugin should define the callback functions specified in the Plugins API. plugin era. 🔥
• -fplugin-arg-name-key=value — Define an argument called key with a value of value for the plugin called name. iykyk. 🦋
• -fdump-ada-spec[-slim] — For C and C++ source and include files, generate corresponding Ada specs. Ada specs? we love a crossover episode. ✨
• -fada-spec-parent=unit — In conjunction with -fdump-ada-spec[-slim] above, generate Ada specs as child units of parent unit. family tree but make it Ada. 🌸
• -fdump-go-spec=file — For input files in any language, generate corresponding Go declarations in file. This generates Go "const", "type", "var", and "func" declarations which may be a useful way to start writing a Go interface to code written in some other language. Go girl, literally. 💅
• @file — Read command-line options from file. The options read are inserted in place of the original @file option. If file does not exist, or cannot be read, then the option will be treated literally, and not removed.

Options in file are separated by whitespace. A whitespace character may be included in an option by surrounding the entire option in either single or double quotes. Any character (including a backslash) may be included by prefixing the character to be included with a backslash. The file may itself contain additional @file options; any such options will be processed recursively. recursive bestie behavior, 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. the suffix lore is real. 🎀

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). she knows what you are. 💅

However, the use of gcc does not add the C++ library. g++ is a program that calls GCC and automatically specifies linking against the C++ library. It treats .c, .h and .i files as C++ source files instead of C source files unless -x is used. This program is also useful when precompiling a C header file with a .h extension for use in C++ compilations. On many systems, g++ is also installed with the name c++. g++ said "I'll handle the library, 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. so many options, so little time. ✨

---

✨ 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. classic era. 👑
• -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 drama of standards, honestly. 😭

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. old school but iconic. 🎀
• iso9899:199409 — ISO C90 as modified in amendment 1. amendment 1 said "let me add a little something". 💜
• 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. deprecated names, we still love you though. 🦋
• 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. Annex K and L catching strays. 💅
• 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. glow up but make it minor. ✨
• c23, c2x, iso9899:2024 — ISO C23, the 2023 revision of the ISO C standard (published in 2024). The name c2x is deprecated. future vibes. 🔮
• c2y — The next version of the ISO C standard, still under development. The support for this version is experimental and incomplete. experimental, bestie, be careful. ⚠️
• gnu90, gnu89 — GNU dialect of ISO C90 (including some C99 features). GNU said "I'll add a lil spice". 🌶️
• gnu99, gnu9x — GNU dialect of ISO C99. The name gnu9x is deprecated. deprecated but not forgotten. 💜
• gnu11, gnu1x — GNU dialect of ISO C11. The name gnu1x is deprecated. still serving. 💅
• gnu17, gnu18 — GNU dialect of ISO C17. steady and reliable. ✨
• gnu23, gnu2x — GNU dialect of ISO C23. This is the default for C code. The name gnu2x is deprecated. the default, the moment, the main character. 👑
• 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. experimental again, stay safe. 🫶
• 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. vintage C++. 🎀
• gnu++98, gnu++03 — GNU dialect of -std=c++98. GNU but make it C++98. 💜
• c++11, c++0x — The 2011 ISO C++ standard plus amendments. The name c++0x is deprecated. the one that changed everything. 🔥
• gnu++11, gnu++0x — GNU dialect of -std=c++11. The name gnu++0x is deprecated. deprecated name, iconic behavior. 💅

• c++14, c++1y — the 2014 ISO C++ standard, plus its amendments. heads up: the name c++1y is deprecated bestie.
• gnu++14, gnu++1y — the GNU dialect of -std=c++14. and yeah, gnu++1y is deprecated too.
• c++17, c++1z — the 2017 ISO C++ standard plus amendments. c++1z? deprecated, sorry babes.
• gnu++17, gnu++1z — GNU dialect of -std=c++17. gnu++1z is deprecated, ik ik.
• c++20, c++2a — the 2020 ISO C++ standard plus amendments. ⚠️ C++20 modules support is still experimental and needs to be enabled with the -fmodules option. also c++2a is deprecated.
• gnu++20, gnu++2a — GNU dialect of -std=c++20. this is the default for C++ code ✨ so if you're not specifying anything, this is literally what you get. C++20 modules support is still experimental and needs to be enabled with the -fmodules option. and gnu++2a is deprecated, as always.
• 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. 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. 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. you've been warned 💀
• 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. same energy, same warning.

---

🎀 more options, we're still going 🎀

• -aux-info filename — writes prototyped declarations to filename for every function declared and/or defined in a translation unit, header-file ones included. silently ignored in any language that isn't C, so don't come crying to her about your C++ file.

beyond the declarations themselves, the file also tells you, in comments, where each declaration came from (source file + line), whether it was implicit, prototyped, or unprototyped (I, N for new, or O for old — that's 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 character right after that). and for function definitions, you also get a K&R-style list of arguments followed by their declarations, tucked inside comments, after the declaration. chef's kiss documentation behavior honestly.

• -fno-asm — don't recognize "asm", "inline" or "typeof" as keywords, so your code can use those words as identifiers. you can reach for "__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 probably want the -fno-gnu-keywords flag instead, which disables "typeof" but leaves "asm" and "inline" alone. in C99 mode (-std=c99 or -std=gnu99), this switch only touches the "asm" and "typeof" keywords, since "inline" is a standard keyword in ISO C99. and in C23 mode (-std=c23 or -std=gnu23), it only affects the "asm" keyword, since "typeof" is a standard keyword in ISO C23. the rules keep evolving, keep up bestie 💅

• -fno-builtin
• -fno-builtin-function — don't recognize built-in functions that don't begin with __builtin_ as a prefix.

GCC normally generates special code to handle certain built-in functions more efficiently; for instance, calls to "alloca" may become single instructions that adjust the stack directly, and calls to "memcpy" may become inline copy loops. the resulting code is often both smaller and faster, but since the function calls no longer appear as such, you can't set a breakpoint on those calls, nor can you change the behavior of the functions by linking with a different library. on top of that, 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. the tradeoff is real, bestie.

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 — so 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++, so jot that down.
• -ffreestanding — asserts that compilation targets a freestanding environment. this implies -fno-builtin. a freestanding environment is one where 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 — enables parsing of function definitions marked with "__GIMPLE". this is an experimental feature that allows unit testing of GIMPLE passes. science-y.
• -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). no cap, just don't combine them.

• -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. handy little vibe check 🦋

• -fhosted — asserts that compilation targets a hosted environment. this implies -fbuiltin. a hosted environment is one where the entire standard library is available, and where "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. periodt.
• -fms-extensions — accept some non-standard constructs used in Microsoft header files.

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

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

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

note that this option is off for all targets except for x86 targets using ms-abi.

• -fpermitted-flt-eval-methods= style — ISO/IEC TS 18661-3 defines new permissible values for "FLT_EVAL_METHOD" that indicate that operations and constants with a semantic type that is an interchange or extended format should be evaluated to the precision and range of that type. these new values are a superset of those permitted under C99/C11, which does not specify the meaning of other positive values of "FLT_EVAL_METHOD". as such, code conforming to C11 may not have been written expecting the possibility of the new values. sneaky, right?

-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. build systems could never tbh.

• -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. considerate of her honestly.
• -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++. sorry to the C++ girlies.

• -fsigned-bitfields, -funsigned-bitfields, -fno-signed-bitfields, -fno-unsigned-bitfields — okay so these four besties control whether a bit-field is signed or unsigned when the declaration is out here being indecisive and doesn't say either signed or unsigned 💅. Default? Signed, always signed, because that's the consistent vibe — the basic integer types like int are signed types too, so we keep the family matching ✨
• -fsigned-char — lets the type char be signed, just like signed char. And no cap, this 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 🫶
• -funsigned-char — lets the type char be unsigned, like unsigned char. Every kind of machine has its own default for what char should be — either unsigned char-coded or signed char-coded by default, and that's just how it is 🙈

Ideally, a portable program should always use signed char or unsigned char whenever it actually depends on the signedness of an object. But like… so many programs were written using plain char and they just expect it to be signed, or they expect it to be unsigned, depending on the machines they were written for 😭 This option, and its inverse, let you make such a program work with the opposite default. Main character energy: rescuing legacy code.

And the type char is always a distinct type from each of signed char or unsigned char, even though its behavior is always just like one of those two. Distinct. Periodt 💅

• -fstrict-flex-arrays (C and C++ only), -fstrict-flex-arrays=level (C and C++ only) — controls when to treat the trailing array of a structure as a flexible array member for the purpose of accessing the elements of such an array. The value of level controls the level of strictness 🎀

-fstrict-flex-arrays is equivalent to -fstrict-flex-arrays=3, which is the strictest — a trailing array gets treated as a flexible array member only when it is declared as a flexible array member per C99 standard onwards 👑

The negative form -fno-strict-flex-arrays is equivalent to -fstrict-flex-arrays=0, which is the least strict. In this case, all trailing arrays of structures are treated as flexible array members ✨

There are two more levels in between 0 and 3, and they exist to support older code that uses the GCC zero-length array extension ([0]) or one-element array as flexible array members ([1]) 🌸

• When level is 1, the trailing array is treated as a flexible array member when it is declared as either [], [0], or [1].
• When level is 2, the trailing array is treated as a flexible array member when it is declared as either [], or [0].

You can control this behavior for a specific trailing array field of a structure by using the variable attribute strict_flex_array attribute 🦋

The -fstrict_flex_arrays option interacts with the -Wstrict-flex-arrays option. (iykyk, they're a package deal)

• -fsso-struct=endianness — sets the default scalar storage order of structures and unions to the specified endianness. The accepted values are big-endian, little-endian and native for the native endianness of the target (the default) 🌈 This option is not supported for C++ 🚫

Warning: the -fsso-struct switch causes GCC to generate code that is not binary compatible with code generated without it if the specified endianness is not the native endianness of the target. Like, bestie, read that twice 💀

---

💖 Options Controlling C++ Dialect 💖

This section describes the command-line options that are only meaningful for C++ programs. You can also use most of the GNU compiler options regardless of what language your program is in. For example, you might compile a file firstClass.C like this:

g++ -g -fstrict-enums -O -c firstClass.C

In this example, only -fstrict-enums is an option meant only for C++ programs; you can use the other options with any language supported by GCC ✨

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

Here is a 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, slay 💅
• -fabi-version=n — use version n of the C++ ABI. The default is version 0 🎀
  • Version 0 refers to the version conforming most closely to the C++ ABI specification. Therefore, the ABI obtained using version 0 will change in different versions of G++ as ABI bugs are fixed.
  • Version 1 is the version of the C++ ABI that first appeared in G++ 3.2.
  • Version 2 is the version of the C++ ABI that first appeared in G++ 3.4, and was the default through G++ 4.9.
  • Version 3 corrects an error in mangling a constant address as a template argument.
  • Version 4, which first appeared in G++ 4.5, implements a standard mangling for vector types.
  • Version 5, which first appeared in G++ 4.6, corrects the mangling of attribute const/volatile on function pointer types, decltype of a plain decl, and use of a function parameter in the declaration of another parameter.
  • Version 6, which first appeared in G++ 4.7, corrects the promotion behavior of C++11 scoped enums and the mangling of template argument packs, const/static_cast, prefix ++ and --, and a class scope function used as a template argument.
  • Version 7, which first appeared in G++ 4.8, that treats nullptr_t as a builtin type and corrects the mangling of lambdas in default argument scope.
  • Version 8, which first appeared in G++ 4.9, corrects the substitution behavior of function types with function-cv-qualifiers.
  • Version 9, which first appeared in G++ 5.2, corrects the alignment of nullptr_t.
  • 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. (oop 🙈)
  • Version 13, which first appeared in G++ 8.2, fixes the accidental change in version 12. (redemption arc ✨)
  • Version 14, which first appeared in G++ 10, corrects the mangling of the nullptr expression.
  • Version 15, which first appeared in G++ 10.3, corrects G++ 10 ABI tag regression.
  • Version 16, which first appeared in G++ 11, changes the mangling of __alignof__ to be distinct from that of alignof, and dependent operator names.
  • Version 17, which first appeared in G++ 12, fixes layout of classes that inherit from aggregate classes with default member initializers in C++14 and up.
  • Version 18, which first appeared in G++ 13, fixes manglings of lambdas that have additional context.
  • Version 19, which first appeared in G++ 14, fixes manglings of structured bindings to include ABI tags, 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 is not provided but -Wabi=n is, that version is used for compatibility aliases. If this option is provided along with -Wabi (without the version), the version from this option is used for the warning 🎀

• -fno-access-control — turn off all access checking 💀 This switch is mainly useful for working around bugs in the access control code. It's giving "the bouncer went home", use responsibly bestie.
• -faligned-new, -faligned-new=alignment — enable 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 few users will need to override the default of alignof(std::max_align_t).

This flag is enabled by default for -std=c++17 👑

• -fno-assume-sane-operators-new — okay so bestie, the C++ standard lets you replace the global new, new[], delete and delete[] operators, even though like a ton of C++ programs just… don't, and happily use the implementation-provided versions instead 🙈. And the standard also says those calls can be omitted entirely if they come from new or delete expressions (and yeah, same assumption applies when you use __builtin_operator_new or __builtin_operator_delete). This option is your remote control for some optimizations around those calls.

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

With -fno-assume-sane-operators-new-delete, GCC must assume all these calls — whether from new/delete expressions or called directly — may read and write global state unless proven otherwise (like, say, when GCC compiles their implementation itself). Use this one if those operators are or may be replaced and your code needs to expect that behavior. 💅

• -fchar8_t
• -fno-char8_t — turn on support for char8_t as adopted for C++20 ✨. That's a whole bundle: a 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 the 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;

Heads up bestie, the change to UTF-8 string and character literal types introduces incompatibilities with ISO C++11 and later standards. Like, the following code is well-formed under ISO C++11, but ill-formed once -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 — check that the pointer operator new hands back is non-null before you go modifying the storage you 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 every other case, when operator new has a non-empty exception specification, memory exhaustion gets signalled by throwing std::bad_alloc. See also new (nothrow).
• -fconcepts — enable support for the C++ Concepts feature, the one that constrains template arguments 🎀. With -std=c++20 and above, Concepts are part of the language standard, so -fconcepts defaults to on.

Some constructs that the earlier C++ Extensions for Concepts Technical Specification, ISO 19217 (2015), allowed but which didn't make it into the standard could additionally be enabled by -fconcepts-ts. But plot twist 💀: 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 — set the maximum error replay depth during recursive diagnosis of a constraint satisfaction failure. The default is 1.
• -fconstexpr-depth=n — set the max nested evaluation depth for C++11 constexpr functions to n. A limit is genuinely needed to detect endless recursion during constant expression evaluation, no cap. The minimum specified by the standard is 512.
• -fconstexpr-cache-depth=n — set the max level of nested evaluation depth for C++11 constexpr functions that'll be cached, to n. It's a heuristic, bestie — it trades off compilation speed (when the cache avoids repeated calculations) against memory consumption (when the cache grows real big from highly recursive evaluations). Default is 8. Very few users are likely to want to adjust it, but if your code does heavy constexpr calculations you might wanna experiment and find which value works best for you. ✨
• -fconstexpr-fp-except — Annex F of the C standard specifies that IEC559 floating point exceptions encountered at compile time should not stop compilation. C++ compilers have historically ignored this guidance, treating floating point division by zero as non-constant instead, 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 — set the max number of iterations for a loop in C++14 constexpr functions to n. A limit is needed to catch infinite loops during constant expression evaluation. The default is 262144 (1<<18).
• -fconstexpr-ops-limit=n — set the max number of operations during a single constexpr evaluation. Even when the number of iterations of a single loop is limited with the limit above, if there are several nested loops and each of them has many iterations but still smaller than that limit, or if in the body of some loop — or even outside of a loop — too many expressions need to be evaluated, the resulting constexpr evaluation might take way too long. The default is 33554432 (1<<25).
• -fcontracts — enable support for the C++ Contracts feature, as specified in the C++26 working draft (N5003) 📜.

On violation of an enforced or observed contract (peep -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 — set 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 like 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's no difference in behaviour between this option and the "enforce" case, since no handler is invoked at compile time.

• -fcontracts-conservative-ipa — this stops inter-procedural analysis from taking action when the body of an inline function is visible in a given TU. Why? Because contract evaluation conditions are permitted to differ between TUs, which means 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 instead causes a small function to be created (containing the checks) that gets called in the relevant position. This lets different criteria 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 up, coroutines are literally part of the language standard, so this bad boy defaults to on 💅
• -fdiagnostics-all-candidates — lets the C++ front end spill all the candidates during an overload resolution failure, even when the function that got picked is a deleted one 👀 no gatekeeping here
• -fdump-lang-, -fdump-lang-switch, -fdump-lang-switch-options, -fdump-lang-switch-options=filename — controls the dumping of C++-specific info 🫶 the options and filename parts behave exactly like they do in the -fdump-tree option. Accepted switch values, bestie:
  • all — dump every single thing below 👑
  • class — dump class hierarchy information. Virtual table information gets emitted unless slim is specified.
  • module — dump module information. Options lineno (locations), graph (reachability), blocks (clusters), uid (serialization), alias (mergeable), asmname (Elrond), eh (mapper) & vops (macros) may provide additional info 💫
  • raw — dump the raw internal tree data 🌳
  • tinst — dump the sequence of template instantiations, indented to show 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 — the C++ standard lets an implementation 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 😤 it 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. periodt.

• -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 shrinking code size in production builds, a lot like defining NDEBUG.

okay but fr: this does not give your user code permission to throw exceptions in violation of the exception specifications — the compiler still optimizes based on the specs, so throwing an unexpected exception gives you undefined behavior at run time 💀 be so careful.

• -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 are in the same translation unit, that overhead can be optimized away — but when the use lives in a different translation unit, there's significant overhead even if the variable doesn't actually need dynamic initialization 😭 if the programmer can be sure that no use of the variable in a non-defining TU needs to trigger dynamic initialization (either because the variable is statically initialized, or because a use of the variable in the defining TU will run before any uses in another TU), they can dodge that overhead with -fno-extern-tls-init 🦋

On targets that support symbol aliases, the default is -fextern-tls-init. On targets that don't support symbol aliases, the default is -fno-extern-tls-init.

• -ffold-simple-inlines, -fno-fold-simple-inlines — permit the C++ frontend to 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 such calls. Since these functions are rarely interesting to debug, this flag is on by default unless -fno-inline is active 👑
• -fno-gnu-keywords — don't 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 — don't enable immediate function escalation, the thing where certain functions 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 (thanks to 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 whole mechanism that allows consteval function composition). However, with -fno-immediate-escalation, f is not promoted to consteval, and since the call to the consteval function id(t) isn't a constant expression, the compiler rejects the code 😤

This option is turned on by default; it's only effective in C++20 mode or later.

• -fimplicit-constexpr — make inline functions implicitly constexpr if they satisfy the requirements for a constexpr function ✨ usable in C++14 mode or later. This can result in initialization changing from dynamic to static, plus other optimizations 👑
• -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, otherwise you'll be getting undefined symbols at link time 💀
• -fno-implicit-inline-templates — don't emit code for implicit instantiations of inline templates either 🙈 the default handles 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.
• -fmodules, -fno-modules — enable support for C++20 modules 💖 the -fno-modules is usually not needed, since that's the default. Even though this is a C++20 feature, it is not currently implicitly enabled by selecting that standard version 👀
• -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, for good reasons ✨ but it may cause some 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 — disable lazy module importing and module mapper creation 🚫
• -fmodule-mapper=[hostname]:port[?ident], -fmodule-mapper=|program[?ident] args..., -fmodule-mapper==socket[?ident], -fmodule-mapper=<>[inout][?ident], -fmodule-mapper=<in>out[?ident], -fmodule-mapper=file[?ident] — an oracle to query for module name to filename mappings 🔮 if unspecified, the CXX_MODULE_MAPPER environment variable is used, and if that's unset, an in-process default is provided 💅
• -fmodule-only — only emit the Compiled Module Interface, inhibiting any object file 🎀
• -fms-extensions — disable Wpedantic warnings about constructs used in MFC, like implicit int and getting a pointer to member function via non-standard syntax 🙈
• -fnew-inheriting-ctors — enable the P0136 adjustment to the semantics of C++11 constructor inheritance ✨ this is part of C++17 but 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 — enable the P0522 resolution to Core issue 150, template template parameters and default arguments: this allows a template with default template arguments as an argument for a template template parameter with fewer template parameters 💫 this flag is enabled by default for -std=c++17.
• -fno-nonansi-builtins — disable built-in declarations of functions that aren't mandated by ANSI/ISO C 🚫 these include ffs, alloca, _exit, index, bzero, conjf, and other related functions.
• -fnothrow-opt — treat a throw() exception specification as if it were a noexcept specification, to reduce or eliminate the text size overhead relative to a function with no exception specification 🌸 if the function has local variables of types with non-trivial destructors, the exception specification actually makes the function smaller, because the EH cleanups for those variables can be optimized away. The semantic effect is that an exception thrown out of a function with such an exception specification results in a call to terminate rather than unexpected 👑
• -fno-operator-names — don't treat the operator name keywords and, bitand, bitor, compl, not, or and xor as synonyms as keywords 🙈
• -fno-optional-diags — disable diagnostics that the standard says a compiler doesn't 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 normally when an error message is like "hey this is a specialization of a function template", the compiler prints the template's signature, then the template arguments, then any typedefs or typenames chilling in the signature (think void f(T) [with T = int] instead of just void f(int)) so you actually know which template is being dramatic at you. And for class template specializations, it omits any template arguments that match the default template arguments for that template. But if either of those behaviors makes the error message harder to read instead of easier (rude), you can hit -fno-pretty-templates to turn both off. ✨
• -frange-for-ext-temps — enables lifetime extension of C++ range-based for temporaries. With -std=c++23 and above this is literally just part of the language standard, so the temporaries' lifetime 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. it's giving ✨cutting edge✨, go off bestie.
• -fno-rtti — disables generation of information about every class with virtual functions that the C++ run-time type identification features use (dynamic_cast and typeid). If you don't use those parts of the language, you can save some space with 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 don't 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. no cap, don't do this.

• -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. Heads up: this option has no effect for an enumeration type with a fixed underlying type. 💀
• -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. 🧚
• -fuse-cxa-atexit — registers destructors for objects with static storage duration with the __cxa_atexit function rather than the atexit function. This option is required for fully standards-compliant handling of static destructors, but only works if your C library supports __cxa_atexit. ⭐
• -fno-use-cxa-get-exception-ptr — don't use the __cxa_get_exception_ptr runtime routine. This causes std::uncaught_exception to be incorrect, but is necessary if the runtime routine is not available. 🙈
• -fvisibility-inlines-hidden — this switch declares that the user does not attempt to compare pointers to inline functions or methods where the addresses of the two functions are taken in different shared objects.

The effect of this is that GCC may, effectively, mark inline methods with __attribute__ ((visibility ("hidden"))) so that they do not appear in the export table of a DSO and do not require a PLT indirection when used within the DSO. Enabling this option can have a dramatic effect on load and link times of a DSO as it massively reduces the size of the dynamic export table when the library makes heavy use of templates. 🔥

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

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

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

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

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

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

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

Among the consequences of these changes are that static data members of the same type with the same name but defined in different shared objects are different, so changing one does not change the other; and that pointers to function members defined in different shared objects may not compare equal. When this flag is given, it is a violation of the ODR to define types with the same name differently. 💀

• -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++. bestie do not touch this one fr fr.
• -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>);

okay so, wrapping up that last warning real quick — this feature was proposed in the Concepts TS, but it did not get adopted into C++20, bestie. In the standard, a placeholder in a parameter declaration has to appear as a decl-specifier. The good news? You can downgrade that error to just a warning with -fpermissive or -Wno-error=abbreviated-auto-in-template-arg. 🫶

---

• -Wcomma-subscript (C++ and Objective-C++ only) — warns you when you use a comma expression inside a subscripting expression. This usage got deprecated in C++20 and is on the chopping block for removal 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, and it'll hit you with a pedantic warning that you can turn off with -Wno-comma-subscript.

Enabled by default with -std=c++20 unless -Wno-deprecated, and after -std=c++23 regardless of -Wno-deprecated. Before -std=c++20, enabled with explicit -Wdeprecated.

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

• -Wctad-maybe-unsupported (C++ and Objective-C++ only) — warns when you do class template argument deduction (CTAD) on a type that has no explicitly written deduction guides. It's basically calling out the cases where CTAD only succeeded because the compiler synthesized the implicit deduction guides, which might not be the vibe the programmer was going for. Some style guides only allow CTAD on types that specifically "opt-in" — i.e., types designed to support CTAD. You can suppress this warning 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 looks unusable because all the constructors or destructors in it 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 bestie, how is anyone supposed to touch this class 💅
• -Wdangling-reference (C++ and Objective-C++ only) — warns when a reference is bound to a temporary whose lifetime has already 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. But both temporaries get destroyed at the end of the full expression, so the reference "r" is dangling. 😭 This warning also catches 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 get checked too, 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 their 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 fire 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 a non-virtual destructor. It is unsafe to delete an instance of a derived class through a pointer to a base class if the base class does not have a virtual destructor. 🚨 This warning is enabled by -Wall.
• -Wdeprecated-copy (C++ and Objective-C++ only) — 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 it also deprecates 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

• -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's way too easy to accidentally 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.

• -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. ✨

And this warning? Okay so it's a pedantic warning that's on by default in C++20 and earlier. In C++23, -Winvalid-constexpr can be turned on, in which case it becomes an ordinary warning. Periodt. For example:

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 — Bestie, this one verifies all imported macro definitions are valid at the end of compilation. It's not enabled by default, because it requires additional processing to determine. Lowkey it may be useful when preparing sets of header-units to ensure consistent macros. 💅
• -Wno-literal-suffix (C++ and Objective-C++ only) — Do not warn when a string or character literal is followed by a ud-suffix which does not begin with an underscore. As a conforming extension, GCC treats such suffixes as separate preprocessing tokens in order to maintain backwards compatibility with code that uses formatting macros from "<inttypes.h>". For example:

#define __STDC_FORMAT_MACROS
#include <inttypes.h>
#include <stdio.h>

int main() {
  int64_t i64 = 123;
  printf("My int64: %" PRId64"\n", i64);
}

In this case, "PRId64" is treated as a separate preprocessing token. ✨

This option also controls warnings when a user-defined literal operator is declared with a literal suffix identifier that doesn't begin with an underscore. Literal suffix identifiers that don't begin with an underscore are reserved for future standardization.

These warnings are enabled by default. 🎀

• -Wno-narrowing (C++ and Objective-C++ only) — For C++11 and later standards, narrowing conversions are diagnosed by default, as required by the standard. A narrowing conversion from a constant produces an error, and a narrowing conversion from a non-constant produces a warning, but -Wno-narrowing suppresses the diagnostic. Note that this does not affect the meaning of well-formed code; narrowing conversions are still considered ill-formed in SFINAE contexts. 😭

With -Wnarrowing in C++98, warn when a narrowing conversion prohibited by C++11 occurs within { }, e.g.

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

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

• -Wnoexcept (C++ and Objective-C++ only) — Warn when a noexcept-expression evaluates to false because of a call to a function that does not have a non-throwing exception specification (i.e. throw() or "noexcept") but is known by the compiler to never throw an exception. No cap. 🦋
• -Wnoexcept-type (C++ and Objective-C++ only) — Warn if the C++17 feature making "noexcept" part of a function type changes the mangled name of a symbol relative to C++14. Enabled by -Wabi and -Wc++17-compat.

As an 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>". It's giving type system glow-up. 💅

• -Wclass-memaccess (C++ and Objective-C++ only) — Warn when the destination of a call to a raw memory function such as "memset" or "memcpy" is an object of class type, and when writing into such an object might bypass the class non-trivial or deleted constructor or copy assignment, violate const-correctness or encapsulation, or corrupt virtual table pointers. Modifying the representation of such objects may violate invariants maintained by member functions of the class. For example, the call to "memset" below is undefined because it modifies a non-trivial class object and is, therefore, diagnosed. The safe way to either initialize or clear the storage of objects of such types is by using the appropriate constructor or assignment operator, if one is available.

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

The -Wclass-memaccess option is enabled by -Wall. Explicitly casting the pointer to the class object to "void *" or to a type that can be safely accessed by the raw memory function suppresses the warning. Iconic. ✨

• -Wnon-virtual-dtor (C++ and Objective-C++ only) — Warn when a class has virtual functions and an accessible non-virtual destructor itself or in an accessible polymorphic base class, in which case it is possible but unsafe to delete an instance of a derived class through a pointer to the class itself or base class. This warning is automatically enabled if -Weffc++ is specified. The -Wdelete-non-virtual-dtor option (enabled by -Wall) should be preferred because it warns about the unsafe cases without false positives. Bestie, choose the better warning. 🫶
• -Wregister (C++ and Objective-C++ only) — Warn on uses of the "register" storage class specifier, except when it is part of the GNU Explicit Register Variables extension. The use of the "register" keyword as storage class specifier has been deprecated in C++11 and removed in C++17. Enabled by default with -std=c++17. RIP register. 💀
• -Wreorder (C++ and Objective-C++ only) — Warn when the order of member initializers given in the code does not match the order in which they must be executed. For instance:

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

The compiler rearranges the member initializers for "i" and "j" to match the declaration order of the members, emitting a warning to that effect. This warning is enabled by -Wall. Order matters, fr fr. 🎀

• -Wno-pessimizing-move (C++ and Objective-C++ only) — This warning warns when a call to "std::move" prevents copy elision. A typical scenario when copy elision can occur is when returning in a function with a class return type, when the expression being returned is the name of a non-volatile automatic object, and is not a function parameter, and has the same type as the function return type.

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

But in this example, the "std::move" call prevents copy elision.

This warning is enabled by -Wall. Not me using std::move when I shouldn't. 🙈

• -Wno-redundant-move (C++ and Objective-C++ only) — This warning warns about redundant calls to "std::move"; that is, when a move operation would have been performed 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 such as 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. Because G++ implements Core Issue 1579, another example is:

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

In this example, 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.

This warning is enabled by -Wextra. Redundant moves are not the vibe. 🦋

• -Wrange-loop-construct (C++ and Objective-C++ only) — This warning warns when a C++ range-based for-loop is creating an unnecessary copy. This can happen when the range declaration is not a reference, but probably should be. For 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 warns when a loop variable in a range-based for-loop is initialized with a value of a different type resulting in a copy. For example:

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

In the example above, in every iteration of the loop a temporary value of type "double" is created and destroyed, to which the reference "const double &" is bound.

This warning is enabled by -Wall. Stop copying, start referencing. 💅

• -Wredundant-tags (C++ and Objective-C++ only) — Warn about redundant class-key and enum-key in references to class types and enumerated types in contexts where the key can be eliminated without causing an ambiguity. For example:

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

On the other hand, in this example there is no warning:

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

Context is everything, bestie. ✨

• -Wno-subobject-linkage (C++ and Objective-C++ only) — Do not warn if a class type has a base or a field whose type uses the anonymous namespace or depends on a type with no linkage. If a type A depends on a type B with no or internal linkage, defining it in multiple translation units would be an ODR violation because the meaning of B is different in each translation unit. If A only appears in a single translation unit, the best way to silence the warning is to give it internal linkage by putting it in an anonymous namespace as well. The compiler doesn't give this warning for types defined in the main .C file, as those are unlikely to have multiple definitions. -Wsubobject-linkage is enabled by default. ODR violations are a no-no. 🫶
• -Weffc++ (C++ and Objective-C++ only) — Warn about violations of the following style guidelines from Scott Meyers' Effective C++ series of books:
  • Define a copy constructor and an assignment operator for classes with dynamically-allocated memory.
  • Prefer initialization to assignment in constructors.
  • Have "operator=" return a reference to *this.
  • Don't try to return a reference when you must return an object.
  • Distinguish between prefix and postfix forms of increment and decrement operators.
  • Never overload "&&", "||", or ",".

Scott Meyers said what he said. 👑

Alsooo</p>, this option flips on -Wnon-virtual-dtor, which is genuinely one of the effective C++ recs 💅. But bestie, the check gets extended — it will also yell at you about the lack of a virtual destructor in accessible non-polymorphic base classes too. Yeah. It's thorough like that.

When you pick this option, be aware that the standard library headers do not obey all of these guidelines, so use grep -v to filter out those warnings. Not me getting roasted by <vector> fr fr 😭.

---

• -Wno-exceptions (C++ and Objective-C++ only) — turns off the warning for when an exception handler is shadowed by another handler. Ngl that warning is lowkey useful because it points out a wrong ordering of your exception handlers. So if you're disabling it, you better know what you're doing 👀.
• -Wsfinae-incomplete (C++ and Objective-C++ only) — warns about a class that turns out to be incomplete, or a function with auto return type that hasn't had its type deduced yet, in a context where that causes substitution failure rather than an error — and then the class or function gets defined later in the translation unit. Why is this a problem? Because template instantiations or concept checks could have totally different results depending on whether they first happen before or after that definition. It's giving unstable vibes 🌈.

This warning is enabled by default ✨. And -Wsfinae-incomplete=2 adds an extra warning right at the point of substitution failure, so it's easier to track down the problems the default mode flagged for you. Iconic debugging support, honestly.

• -Wstrict-null-sentinel (C++ and Objective-C++ only) — warns when you use an uncasted "NULL" as a sentinel. When you're compiling only with GCC this is a valid sentinel, because "NULL" is defined to "__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. Cool cool cool — but this use is not portable across different compilers. So like, it works here and breaks there, the classic 💀.
• -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 but contain C++-specific members, base classes, default member initializers or lambda expressions — including any of those on nested member classes. Look at this:

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. That's the fix, bestie, that's it 🎀.

• -Wno-non-template-friend (C++ and Objective-C++ only) — disables warnings when non-template friend functions get declared inside a template. 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 is enabled by default. Ancient history but make it fashion 🦋.
• -Wold-style-cast (C++ and Objective-C++ only) — warns if an old-style (C-style) cast to a non-void type shows up anywhere in a C++ program. Why do we care? Because the new-style casts — "dynamic_cast", "static_cast", "reinterpret_cast", and "const_cast" — are less vulnerable to unintended effects and much easier to search for. Glow-up behavior only ✨.
• -Woverloaded-virtual (C++ and Objective-C++ only)
• -Woverloaded-virtual=n — warns when a function declaration hides virtual functions from a base class. Example time:

struct A {
  virtual void f();
};

struct B: public A {
  void f(int); // does not override
};

Here the "A" class version of "f" is hidden in "B", and code like this:

B* b;
b->f();

fails to compile. Yeah. It's giving "why won't you build" 😭.

When 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 — like, add "using A::f;" to "B". Cute, clean, done 💅.

The optional level suffix controls what happens 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. Understandable, honestly.

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. Noted, memorized, periodt 👑.

• -Wno-pmf-conversions (C++ and Objective-C++ only) — disables the diagnostic for converting a bound pointer to member function into a plain pointer. If you're doing that on purpose, you're on your own bestie 🙈.
• -Wsign-promo (C++ and Objective-C++ only) — warns when overload resolution picks a promotion from unsigned or enumerated type to a signed type, over a conversion to an unsigned type of the same size. Tea: previous versions of G++ tried to preserve unsignedness, but the standard mandates the current behavior. So the compiler is right and the old compilers were just quirky 💀.
• -Wtemplates (C++ and Objective-C++ only) — warns when a primary template declaration is encountered. Some coding rules disallow templates entirely, and this may be used to enforce that rule. The warning is inactive inside a system header file, such as the STL, so one can still use the STL — phew 😮‍💨. One 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 — plus calls to other deallocation functions with pointers returned from "operator new" for which the deallocation function isn't suitable. Basically: who allocated you, and who is trying to free you 👀.

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 does not match either the definition or the first declaration, if no definition is provided.

For example, the declaration of "struct Object" in the argument list of "draw" triggers the warning. To avoid it, either remove the redundant class-key "struct" or replace it with "class" to match its definition:

class Object {
public:
  virtual ~Object () = 0;
};
void draw (struct Object*);

Now, it is not wrong to declare a class with the class-key "struct" as the example above shows — the -Wmismatched-tags option is intended to help achieve a consistent style of class declarations. In code that's meant 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. Style points and safety points, we love a double win 🎀.

• -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 one can still use the STL — thank u next 🙏. One 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. Again, the warning is inactive inside a system header file, such as the STL, so one can still use the STL. One 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. Why is that 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. Yikes 😬. If the move assignment operator is written to avoid moving from a moved-from object, this warning can be disabled — so if you've handled it, go off 💅.
• -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 one can still use the STL. One may also use using directives and qualified names. No-namespace rules are wild to me but okay, respect the house style 🫶.
• -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. Live a little ✨.
• -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. Like this:

template<typename T> struct S {
  S<T>(); // should be S();
  ~S<T>();  // should be ~S();
};

-Wtemplate-id-cdtor is enabled by default with -std=c++20; it is also enabled by -Wc++20-compat. So if you're on C++20, expect the callout 🌸.

-Wtemplate-names-tu-local</p> 🧚

okay so bestie, this one yells at you when a template body hides an exposure of a translation-unit-local entity ✨

Most of the time, referring to a translation-unit-local entity (like, say, an internal linkage declaration) inside an entity that gets emitted into a module's CMI is 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, so no error gets emitted. 🙈

Why do we care? Because this can make variable or function templates accidentally become unusable if they reference such an entity, since other translation units that import the template will never be able to instantiate it. 😭

This warning is out here trying to detect the cases where that might happen. The presence of an explicit instantiation silences the warning. 🤫

This flag is enabled by -Wextra.

---

-Wno-expose-global-module-tu-local</p> 💅

An exposure of a translation-unit-local entity from a module interface is invalid, bestie — because it may cause ODR violations and manifest in link errors or other unexpected behaviour. 💀 BUT, and this is the messy part, lots of existing libraries declare TU-local entities in their interface, and avoiding exposures of these entities can be genuinely difficult in some cases.

As an extension, GCC allows exposures of internal variables and functions that were declared in the global module fragment. This warning indicates when such an invalid exposure has occurred, and it can be silenced using diagnostic pragmas — either at the site of the exposure, or at the point of declaration of the internal declaration. 🎀

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

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

---

-Wno-external-tu-local</p> 🌸

Warn when naming a TU-local entity outside of the translation unit it was declared in. Such declarations will be ignored during name lookup. 😳 This can occur 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.

This can be worked around 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</p> (C++ and Objective-C++ only) ⭐

Disable the warning about a throw-expression that will immediately result in a call to terminate. 💀 periodt.

---

-Wno-vexing-parse</p> (C++ and Objective-C++ only) 🦋

Warn about the most vexing parse syntactic ambiguity. This warns about the cases when a declaration looks like a variable definition, but the C++ language requires it to be interpreted as a function declaration. For instance:

void f(double a) {
  int i();        // extern int i (void);
  int n(int(a));  // extern int n (int);
}

Another example, because of course there's 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 how to deal with such an ambiguity; e.g., it can suggest removing the parentheses or using braces instead. so helpful, so iconic. 💅

This warning is enabled by default.

---

-Wno-class-conversion</p> (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. it's giving pointless, so we keep quiet about it. 🙈

---

-Wvolatile</p> (C++ and Objective-C++ only) 🔥

Warn 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
• and 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</p>

-Waligned-new=[none|global|all]</p> 👑

Warn about a new-expression of a type that requires greater alignment than the 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</p>

-Wplacement-new=n</p> 💜

Warn about placement new expressions with undefined behavior, such as constructing an object in a buffer that is smaller than the type of the object. For example, the placement new expression below is diagnosed because it 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 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</p>

-Wcatch-value=n</p> (C++ and Objective-C++ only) 🍓

Warn 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</p> (C++ and Objective-C++ only) ✨

Warn for conditionally-supported (C++11 [intro.defs]) constructs. iykyk. 🧚

---

-Wno-defaulted-function-deleted</p> (C++ and Objective-C++ only) 🌸

Warn when an explicitly defaulted function is deleted by the compiler. That can occur when the function's declared type does not match the type of the function that would have been implicitly declared. This warning is enabled by default. 😭

---

-Wno-delete-incomplete</p> (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</p> (C++, Objective-C++ only) 🎀

Warn about redundant semicolons. There are various contexts in which an extra semicolon can occur. One is a semicolon after in-class function definitions, which is valid in all C++ dialects (and is never a pedwarn):

struct S {
  void foo () {};
};

Another is an extra semicolon at namespace scope, which has been allowed since C++11 (therefore 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 is a pedwarn in C++98):

struct S {
  int a;
  ;
};

so many stray semicolons, bestie. they're everywhere. 💅

---

-Wno-global-module</p> (C++ and Objective-C++ only) 🌈

Disable the diagnostic for when the global module fragment of a module unit does not consist only of preprocessor directives.

---

-Wno-inaccessible-base</p> (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</p> 🧚

Suppress warnings about use of C++11 inheriting constructors when the base class inherited from has a C variadic constructor; the warning is on by default because the ellipsis is not inherited. 🙈

---

-Wno-invalid-offsetof</p> (C++ and Objective-C++ only) ⭐

Suppress warnings from applying the offsetof macro to a non-POD type. According to the 2014 ISO C++ standard, applying offsetof to a non-standard-layout type is undefined. In existing C++ implementations, however, offsetof typically gives meaningful results. This flag is for users who are aware that they are writing nonportable code and who have deliberately chosen to ignore the warning about it. 💀

The restrictions on offsetof may be relaxed in a future version of the C++ standard. 👀

---

-Wsized-deallocation</p> (C++ and Objective-C++ only) 🔥

Warn about a definition of an unsized deallocation function

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

without a definition of the corresponding sized deallocation function

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

or the other way round 🙃. Turned on by -Wextra together with -fsized-deallocation.

• -Wsuggest-final-types — warns about types with virtual methods where your code would lowkey be better if the type were declared with the C++11 final specifier, or, if you can, just declared inside an anonymous namespace. Why? Because then GCC can go way more feral with devirtualizing those polymorphic calls ✨. This warning hits different with link-time optimization, since the class hierarchy graph info is more complete there. slay
• -Wsuggest-final-methods — warns about virtual methods where things would improve if the method got the C++11 final specifier, or if its type were declared in an anonymous namespace or marked final. Also more effective with link-time optimization (fuller class hierarchy graph, bestie). Pro tip: look at -Wsuggest-final-types suggestions first, then rebuild with the new annotations. Order matters, ok?
• -Wsuggest-override — warns about overriding virtual functions that aren't marked with the override keyword. just say override, it's giving clarity 💅
• -Wno-conversion-null (C++ and Objective-C++ only) — don't warn for conversions between NULL and non-pointer types. Fun fact: -Wconversion-null is enabled by default. 🎀

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

(NOTE: This manual does not describe the Objective-C and Objective-C++ languages themselves. We're just doing the compiler flags here, not teaching you a whole language, bestie 😭)

This section is all about the command-line options that only mean something for Objective-C and Objective-C++ programs. You can also use most of the language-independent GNU compiler options with them, fr fr. Like, 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 you can use with any language GCC supports. 🦋

Note that since Objective-C is an extension of the C language, Objective-C compilations may also use options specific to the C front-end (e.g., -Wtraditional). Same energy for Objective-C++ — it may use C++-specific options (e.g., -Wabi). 👑

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 👇). 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. iykyk
• -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. no cap
• -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 lets the runtime use more efficient entry points 🏎️. 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. Currently this option is only supported 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 returns 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 generated this way 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. timing is everything, bestie

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 C++ offers. 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). careful out here
• -fobjc-gc — enable garbage collection (GC) in Objective-C and Objective-C++ programs. This option is only available with the NeXT runtime; the GNU runtime has a different garbage collection implementation that does not require special compiler flags. 🗑️✨
• -fobjc-nilcheck — for the NeXT runtime with version 2 of the ABI, check for a nil receiver in method invocations before doing the actual method call. This is the default and can be disabled using -fno-objc-nilcheck. Class methods and super calls are never checked for nil in this way, no matter what this flag is set to. Currently this flag does nothing when the GNU runtime, or an older version of the NeXT runtime ABI, is used.
• -fobjc-std=objc1 — conform to the language syntax of Objective-C 1.0, the language recognized by GCC 4.0. This only affects the Objective-C additions to the C/C++ language; it does not affect conformance to C/C++ standards, which is controlled by the separate C/C++ dialect option flags. When this option is used with the Objective-C or Objective-C++ compiler, any Objective-C syntax that is not recognized by GCC 4.0 is rejected 🚫. This is useful if you need to make sure that your Objective-C code can be compiled with older versions of GCC. backwards compat queen
• -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 needing 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 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 that specified visibility. choose your fighter
• -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. inheritance rights 💅

---

• -Wobjc-root-class (Objective-C and Objective-C++ only) — bestie, this one warns you when a class interface is out here with no superclass at all 😭. Like, most classes are gonna inherit from something — NSObject or Object, for example. So when you're intentionally declaring a class that's meant to be a root class, you can hush the warning by marking its interface with __attribute__((objc_root_class)). Slay, root class rights 💅.
• -Wselector (Objective-C and Objective-C++ only) — warns if it finds multiple methods with different types for the same selector while compiling. The check runs over the list of methods in the final stage of compilation. Also? A check happens for each selector that shows up in a @selector(...) expression, when a corresponding method for that selector has been found during compilation. And because these checks only scan the method table at the very end of compilation, the warnings don't get produced if the final stage never happens — say, an error popped up during compilation, or you're using -fsyntax-only. So no cap: no final stage, no warning 🫶.
• -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, while you're trying to send a message with that selector to a receiver of type id or Class. When this flag is off (which is the default behavior, iykyk), the compiler skips those warnings if all the differences it found are confined to types that share the same size and alignment. So basically: same size + same alignment = compiler minds its own business 🌸.
• -Wundeclared-selector (Objective-C and Objective-C++ only) — warns if a @selector(...) expression refers to an undeclared selector. A selector counts as undeclared when no method with that name has been declared before the @selector(...) expression — whether explicitly in an @interface or @protocol declaration, or implicitly in an @implementation section. Little fun fact: this option always does its checks the moment it sees a @selector(...) expression, whereas -Wselector only checks in the final stage of compilation. It also enforces that coding style convention that methods and selectors have to be declared before they're used. Order of operations, periodt 💅.
• -print-objc-runtime-info — generates a C header describing the largest structure that's passed by value, if there even is one. That's it, that's the whole vibe ✨.

✨ Options Controlling OpenMP and OpenACC ✨

GCC supports OpenMP extensions to C, C++, and Fortran with the -fopenmp option 🎀. Similarly, OpenACC extensions are supported in all three languages with -fopenacc. Two whole parallelism worlds, one compiler, we love to see it 👑

• -foffload=disable, -foffload=default, -foffload=target-list — tells the compiler which OpenMP and OpenACC offload targets to generate code for. Default behavior (same thing as -foffload=default) is to generate code for all supported offload targets. The -foffload=disable form generates code only for the host fallback. And -foffload=target-list generates code only for the specified comma-separated list of offload targets.

Offload targets are specified in GCC's internal target-triplet format. Run the compiler with -v to show 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. Wanna hit only one target (or a few)? 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 are

-foffload-options='-fno-math-errno -ffinite-math-only' \
        -foffload-options=nvptx-none=-latomic
-foffload-options=amdgcn-amdhsa=-march=gfx906

• -fopenacc — enables handling of OpenACC directives: #pragma 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 have support for -pthread. Don't say we didn't warn you 🫶.
• -fopenacc-dim=geom — specifies default compute dimensions for parallel offload regions that don't explicitly specify them. The geom value is a triple of :-separated sizes, in order gang, worker, and vector. You can omit a size to use a target-specific default value. Pick your dimensions, main character 🧚.
• -fopenmp — enables handling of OpenMP directives #pragma omp, [[omp::decl(...)]], [[omp::directive(...)]], and [[omp::sequence(...)]] in C/C++. In Fortran, it enables !$omp and the conditional compilation sentinel !$. And in fixed source form Fortran, the sentinels can also start with c or *.

This option implies -pthread, so again: only supported on targets that have support for -pthread. Also, -fopenmp implies -fopenmp-simd. One option, so many consequences, iconic 💅.

• -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 *. Every other OpenMP directive is ignored. And unless -fopenmp is also specified, the loop region binds to the current task region, independent of the specified bind clause. So close, yet so 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 may be one of none, host, nohost, and any, which correspond to keywords for the device_type clause of the declare target directive; clones are generated for the intersection of devices specified. -fopenmp-target-simd-clone is equivalent to -fopenmp-target-simd-clone=any and -fno-openmp-target-simd-clone is equivalent to -fopenmp-target-simd-clone=none.

At -O2 and higher (but not -Os or -Og) this optimization defaults to -fopenmp-target-simd-clone=nohost; otherwise it's disabled by default. Optimization levels really said "it depends" 💀.

✨ Options to Control Diagnostic Messages Formatting ✨

Traditionally, diagnostic messages have been formatted irrespective of the output device's aspect (like its width, and so on). You can use the options below to control the formatting algorithm for diagnostic messages — how many characters per line, how often source location info should be reported, that kind of thing. Note that some language front ends may not honor these options 🙈.

• -fmessage-length=n — try to format error messages so they fit on lines of about n characters. If n is zero, no line-wrapping happens; each error message appears on a single line. This is the default for all front ends.

Note: this option also affects the display of the #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. Sneaky little exceptions, we see you 👀.

• -fdiagnostics-plain-output — requests that diagnostic output look as plain as possible, which is handy when running dejagnu or other utilities that need to parse diagnostics output and prefer it to stay 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. In the future, if GCC changes the default appearance of its diagnostics, the corresponding option to disable the new behavior gets added to this list. Future-proofing, we love it 🎀.
• -fdiagnostics-show-location=once — only meaningful in line-wrapping mode. Tells the diagnostic messages reporter to emit source location info once; that is, if the message is too long to fit on a single physical line and has to wrap, the source location won't get emitted (as a prefix) again and again on the subsequent continuation lines. This is the default behavior. Say it once and move on 💅.
• -fdiagnostics-show-location=every-line — only meaningful in line-wrapping mode. Tells the diagnostic messages reporter to emit the same source location info (as a prefix) for the physical lines that result from breaking a message too long to fit on a single line. Every line gets the receipt 🌈.
• -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 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. Aesthetic diagnostics, no cap 💖.

The colors are defined by the environment variable GCC_COLORS. Its value is a colon-separated list of capabilities and Select Graphic Rendition (SGR) substrings. SGR commands are interpreted by the terminal or terminal emulator. (See the section in the documentation of your text terminal for permitted values and their meanings as character attributes.) These substring values are integers in decimal representation and can be concatenated with semicolons. Common values to concatenate include 1 for bold, 4 for underline, 5 for blink, 7 for inverse, 39 for default foreground color, 30 to 37 for foreground colors, 90 to 97 for 16-color mode foreground colors, 38;5;0 to 38;5;255 for 88-color and 256-color modes foreground colors, 49 for default background color, 40 to 47 for background colors, 100 to 107 for 16-color mode background colors, and 48;5;0 to 48;5;255 for 88-color and 256-color modes background colors. Terminal rizz, honestly 🦋.

The default GCC_COLORS is

okay so bestie, we're still deep in the color-spec lore, and here's the rest of that default string, one long dramatic slay:

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

and what do those numbers even mean 💅 here's the decoder ring: 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. And listen — if you set GCC_COLORS to the empty string, colors are OFF, periodt. The capabilities you can set are these lil guys:

• 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=, like the identifiers of individual events and the lines showing 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 info, like file:line or file:line:column etc. 📍
• quote= — the SGR substring for information printed inside 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, like a pair of mismatching types. See -fdiagnostics-show-highlight-colors. 🎨

---

• -fdiagnostics-urls[=WHEN] — bestie, this one embeds URLs into your diagnostics using escape sequences. iykyk — like, when -fdiagnostics-show-option spits out text showing the command-line option controlling a diagnostic, it'll embed a URL for that option's docs. 🔗

WHEN can be never, always, or auto. auto makes GCC use URL escape sequences only when standard error is a terminal, and only when it's not running in an emacs shell or any graphical terminal that's known to be incompatible with this feature — see below. 👀

The default depends on how the compiler was 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 that config, GCC defaults to using auto if either GCC_URLS or TERM_URLS environment variables are present and non-empty in the compiler's environment, or never if neither are.

but wait, there's more — even with -fdiagnostics-urls=always, the behavior still depends 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 isn't set, TERM_URLS is used 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. 🎵

Right now GCC also tries to detect a few terminals known not to implement the URL feature, ones that have bugs — or at least had bugs in some versions still in use — where the URL escapes are likely to misbehave, i.e. print garbage on your screen. 😭 That list is currently xfce4-terminal, certain known-to-be-buggy gnome-terminal versions, the linux console, and mingw. You can skip this check with -fdiagnostics-urls=always.

• -fno-diagnostics-show-option — by default, every diagnostic emitted includes text indicating the command-line option that directly controls that diagnostic (if such an option is known to the diagnostic machinery). Passing -fno-diagnostics-show-option suppresses that behavior. 🙈
• -fno-diagnostics-show-caret — by default, every diagnostic emitted includes the original source line and a caret ^ pointing at the column. This option suppresses that info. The source line gets truncated to n characters if the -fmessage-length=n option is given. When output goes to the terminal, the width is limited to the width given by the COLUMNS environment variable or, if that's 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 info, like the types of expressions:

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

This option suppresses printing of these labels (so in the example above, the vertical bars and the char * and long int text go bye-bye). 🫶

• -fno-diagnostics-show-event-links — by default, when printing execution paths (via -fdiagnostics-path-format=inline-events), GCC will print lines connecting related events, like the line connecting events 1 and 2 here:

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 printing of those 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 that metadata for some of the -fanalyzer diagnostics. GCC plugins may also provide diagnostics with that metadata. By default, if the info is present, it gets printed with the diagnostic. This option suppresses printing of that metadata. 🔍
• -fno-diagnostics-show-rules — diagnostic messages can optionally have rules associated with them, like from a coding standard or a specification. GCC itself doesn't do this for any of its diagnostics, but plugins may. By default, if the info is present, it gets printed with the diagnostic. This option suppresses printing of that metadata. 📜
• -fno-diagnostics-show-highlight-colors — GCC can use color for emphasis and contrast when printing diagnostic messages and quoting your 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 extra 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 that 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 that has the warning. depth is the maximum depth of the control flow chain. Currently, the list of 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 — emits fix-it hints in a machine-parseable format, suitable for consumption by IDEs. For each fix-it, a line gets 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"

and bestie, we're not even done yet 💖 more coming, don't touch that dial —

Okay so the location is a half-open range, bestie — it's expressed as a count of bytes, and the first byte of the initial column is byte 1. In the example above, bytes 3 through 20 of line 45 of "test.c" are getting replaced with the given string. It's giving precise surgical edit energy:

                   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 "\"", and non-printable characters as octal (e.g. vertical tab as "\013"). An empty replacement string means the given range is removed. An empty range (e.g. "45:3-45:3") means the string is inserted at the given position. Periodt.

---

-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). So cute.

-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 is option is provided then the stack depth will be printed for each run of events within -fdiagnostics-path-format=inline-events. If provided with -fdiagnostics-path-format=separate-events, then the stack depth and function declaration will be appended when printing each event.

This is intended for use by GCC developers and plugin developers when debugging diagnostics that report interprocedural control flow.

-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 are built like little trees 🌳, with nested sub-diagnostics inside them (the classic example: when C++ templates get instantiated and everything goes sideways). By default, GCC shows you that nesting structure in its text output with indentation and bullet points, it moves location info onto separate lines so the shape is clearer, and it strips out redundant repeated info so you're not reading the same thing five times. Pick -fno-diagnostics-show-nesting and all of that goes away — no indentation, no reformatting, no elision — restoring an older "look" for the diagnostics. Retro vibes only 💅
• -fno-diagnostics-show-nesting-locations — when fdiagnostics-show-nesting is enabled, file names and line- and column- numbers get displayed on separate lines from the messages. You can turn that location info off entirely with -fno-diagnostics-show-nesting-locations. Heads up: this option exists for use by GCC developers, for writing DejaGnu test cases. So if that's not you, it's still there, it's just not really for you 🫶
• -fdiagnostics-show-nesting-levels — when fdiagnostics-show-nesting is enabled, use fdiagnostics-show-nesting-levels to also display numbers showing how deep the nesting goes. This one exists for use by GCC developers debugging nested diagnostics, but plugin authors might lowkey find it useful too. Nested depth, but make it numbers ✨
• -fdiagnostics-format=FORMAT — pick 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=. So like, it's not additive, bestie, it's a replacement 💀

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.

• -fdiagnostics-add-output=DIAGNOSTICS-OUTPUT-SPEC — add an additional "output sink" for emitting diagnostics. Multiple sinks, all at once, main character energy 👑

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. Two sinks, two files, no conflict 🦋

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 — works in a similar way to -fdiagnostics-add-output= except that instead of adding an additional "output sink" for diagnostics, it replaces all existing output sinks, such as from -fdiagnostics-format=, -fdiagnostics-add-output=, or a prior call to -fdiagnostics-set-output=. It's giving clean slate, periodt 💅
• -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.

---

✨ Options to Request or Suppress Warnings ✨

Okay so, warnings? They're diagnostic messages that report constructions that are not inherently erroneous but that are risky or suggest there may have been an error. Like, the code isn't technically broken, but bestie… it's giving 🚩

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

• -fsyntax-only — check the code for syntax errors, but don't do anything beyond that. Just a vibe check, nothing more 🌸
• -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. So -Wfatal-errors is the one calling the shots here, no cap.
• -w, --no-warnings — inhibit all warning messages. Silence, iconic, nothing to see here 🔇
• -Werror — turn all warnings into errors. Every little warning? Now it's a hard stop. Good luck bestie 💀
• -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. Cherry-picking which warnings get to be dramatic 👑

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. Asymmetric, I know, but that's the rules ✨

• -Wfatal-errors — bestie, this makes the compiler abort compilation on the very first error instead of trying to keep going and printing a whole parade of further error messages. One error, we're out, bye. 💅

You can ask for a whole bunch of specific warnings with options that start with -W — like -Wunused-variable to get warnings about declarations of variables that are never used. Every one of those specific warning options also has a negative twin starting with -Wno- that turns it back off, e.g. -Wno-unused-variable. This manual only lists one of the two forms — whichever one isn't the default. For more language-specific options, go look at C++ Dialect Options and Objective-C and Objective-C++ Dialect Options. And you can get additional warnings by enabling the static analyzer.

Some options, like -Wall and -Wextra, turn on other options, like -Wunused, which may turn on further options, like -Wunused-variable. The combined effect of the positive and negative forms is that more specific options have priority over less specific ones, independently of where they sit on the command line. For options of the same specificity, the last one takes effect. Options enabled or disabled via pragmas take effect as if they appeared at the very end of the command line. 🫶

When you ask for an unrecognized warning option (e.g. -Wunknown-warning), GCC gives an error saying the option isn't recognized. But if you use the -Wno- form, the behavior is slightly different: no diagnostic is produced for -Wno-unknown-warning unless other diagnostics are being produced. That's on purpose — it lets you use new -Wno- options with old compilers, but if something goes wrong, the compiler warns that an unrecognized option is present. 🙈

The effectiveness of some warnings depends on optimizations also being enabled. For example, -Wsuggest-final-types is more effective with link-time optimization. Some other warnings may not be issued at all unless optimization is enabled. And while optimization in general improves the efficacy of warnings about control and data-flow problems, in some cases it may also cause false positives. 😭

• -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 the version of the ISO C or C++ standard 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 require -ansi or a -std option specifying the version of the standard). However, without this option, certain GNU extensions and traditional C and C++ features are supported as well. With this option, they get diagnosed — or rejected outright with -pedantic-errors. 🚫

-Wpedantic does not cause warning messages for use of the alternate keywords whose names begin and end with __. This alternate format can also be used to disable warnings for non-ISO __intN types, i.e. __intN__. Pedantic warnings are also disabled in the expression that follows "__extension__". However — listen — only system header files should use these escape routes; application programs should avoid them. 👀

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 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 on which the GNU extended dialect is based. Warnings from -Wpedantic are given where they're required by the base standard. (It doesn't make sense for such warnings to be given only for features not in the specified GNU C dialect, since by definition the GNU dialects of C include all features the compiler supports with the given option, and there would be nothing to warn about.) ✨

• -pedantic-errors, --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 latter option is unlikely to be useful, since it only makes errors of the diagnostics that are controlled by -Wpedantic, whereas this option also affects required diagnostics that are always enabled or controlled by options other than -Wpedantic.

If you want the required diagnostics that are warnings by default to be errors instead, but don't also want to enable the -Wpedantic diagnostics, you can specify -pedantic-errors -Wno-pedantic (or -pedantic-errors -Wno-error=pedantic to enable them but only as warnings).

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)

• -fpermissive — downgrade some required diagnostics about nonconformant code from errors to warnings. So using -fpermissive allows some nonconforming code to 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)

The -fpermissive option is the default for historic C language modes (-std=c89, -std=gnu89, -std=c90, -std=gnu90).

• -Wall, --all-warnings — this enables all the warnings about constructions that some users consider questionable, and that are easy to avoid (or modify to prevent the warning), even in conjunction with macros. It also enables some language-specific warnings described in C++ Dialect Options and Objective-C and Objective-C++ Dialect Options. 👑

-Wall turns on the following warning flags:

• -Waddress
• -Waligned-new (C++ and Objective-C++ only)
• -Warray-bounds=1 (only with -O2)
• -Warray-compare
• -Warray-parameter=2
• -Wbool-compare
• -Wbool-operation
• -Wc++11-compat
• -Wc++14-compat
• -Wc++17compat
• -Wc++20compat
• -Wcatch-value (C++ and Objective-C++ only)
• -Wchar-subscripts
• -Wclass-memaccess (C++ and Objective-C++ only)
• -Wcomment
• -Wdangling-else
• -Wdangling-pointer=2
• -Wdelete-non-virtual-dtor (C++ and Objective-C++ only)
• -Wduplicate-decl-specifier (C and Objective-C only)
• -Wenum-compare (in C/ObjC; this is on by default in C++)
• -Wenum-int-mismatch (C and Objective-C only)
• -Wformat=1
• -Wformat-contains-nul
• -Wformat-diag
• -Wformat-extra-args
• -Wformat-overflow=1
• -Wformat-truncation=1
• -Wformat-zero-length
• -Wframe-address
• -Wimplicit (C and Objective-C only)
• -Wimplicit-function-declaration (C and Objective-C only)
• -Wimplicit-int (C and Objective-C only)
• -Winfinite-recursion
• -Winit-self (C++ and Objective-C++ only)
• -Wint-in-bool-context
• -Wlogical-not-parentheses
• -Wmain (only for C/ObjC and unless -ffreestanding)
• -Wmaybe-uninitialized
• -Wmemset-elt-size
• -Wmemset-transposed-args
• -Wmisleading-indentation (only for C/C++)
• -Wmismatched-dealloc
• -Wmismatched-new-delete (C++ and Objective-C++ only)
• -Wmissing-attributes
• -Wmissing-braces (only for C/ObjC)
• -Wmultistatement-macros
• -Wnarrowing (C++ and Objective-C++ only)
• -Wnonnull
• -Wnonnull-compare
• -Wopenmp-simd (C and C++ only)
• -Woverloaded-virtual=1 (C++ and Objective-C++ only)
• -Wpacked-not-aligned
• -Wparentheses
• -Wpessimizing-move (C++ and Objective-C++ only)
• -Wpointer-sign (only for C/ObjC)
• -Wrange-loop-construct (C++ and Objective-C++ only)
• -Wreorder (C++ and Objective-C++ only)
• -Wrestrict
• -Wreturn-type
• -Wself-move (C++ and Objective-C++ only)
• -Wsequence-point
• -Wsign-compare (C++ and Objective-C++ only)
• -Wsizeof-array-div
• -Wsizeof-pointer-div
• -Wsizeof-pointer-memaccess
• -Wstrict-aliasing
• -Wstrict-overflow=1
• -Wswitch
• -Wtautological-compare
• -Wtrigraphs
• -Wuninitialized
• -Wunknown-pragmas
• -Wunused
• -Wunused-but-set-variable
• -Wunused-const-variable=1 (only for C/ObjC)
• -Wunused-function
• -Wunused-label
• -Wunused-local-typedefs
• -Wunused-value
• -Wunused-variable
• -Wuse-after-free=2
• -Wvla-parameter
• -Wvolatile-register-var
• -Wzero-length-bounds

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 occasionally you might wish to check for; others warn about constructions that are necessary or hard to avoid in some cases, and 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. 💀

• -Wextra, --extra-warnings — this enables some extra warning flags that aren't enabled by -Wall. (This option used to be called -W. The older name is still supported, but the newer name is more descriptive.) 🦋
• -Wabsolute-value (only for C/ObjC)
• -Walloc-size
• -Wcalloc-transposed-args
• -Wcast-function-type
• -Wclobbered
• -Wdangling-reference (C++ only)
• -Wdeprecated-copy (C++ and Objective-C++ only)
• -Wempty-body
• -Wenum-conversion (only for C/ObjC)
• -Wexpansion-to-defined
• -Wignored-qualifiers (only for C/C++)
• -Wimplicit-fallthrough=3
• -Wmaybe-uninitialized
• -Wmissing-field-initializers
• -Wmissing-parameter-name (C/ObjC only)
• -Wmissing-parameter-type (C/ObjC only)
• -Wold-style-declaration (C/ObjC only)
• -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 bestie, the option -Wextra also yells at you for these situations:

• A pointer gets compared against integer zero with "<", "<=", ">", or ">=".
• (C++ only) An enumerator and a non-enumerator both show up in a conditional expression. awkward.
• (C++ only) Ambiguous virtual bases. 💀
• (C++ only) Subscripting an array that was declared "register".
• (C++ only) Taking the address of a variable that was 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 covers code that may not be compatible with the vendor-neutral C++ ABI and the psABI for the particular target. The psABI warnings are also controlled separately by -Wpsabi, which -Wabi implies.

Since G++ now defaults to updating the ABI with each major release, normally -Wabi only warns about C++ ABI compatibility problems if a check gets added later in a release series for an ABI issue discovered since the initial release. -Wabi warns about more stuff if you pick an older ABI version (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 you provide an explicit version number and -fabi-compat-version isn't specified, the version number from this option gets used for compatibility aliases. If you don't provide an explicit version number with this option, but -fabi-compat-version is specified, then that version number is what's used for C++ ABI warnings.

ngl, even though an effort has been made to warn about all such cases, there are probably some cases that don't get warned about, even though G++ is generating incompatible code. And there may also be cases where warnings get emitted even though the code that's generated is compatible. 😭

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

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

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

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

This was fixed in -fabi-version=3.

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

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

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

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

• Scoped enumerators passed as arguments to a variadic function 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, like changes in alignment requirements or how function arguments get passed. On several targets — including AArch64, ARM, x86, MIPS, RS6000/PowerPC, and S/390 — these details have 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. This warning is enabled by -Wall. 🙈

✨ -Wno-coverage-mismatch ✨

Warn if feedback profiles do not match when using the -fprofile-use option. If a source file is changed between compiling with -fprofile-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. periodt.

✨ -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. caught in 4k fr fr 😭

✨ -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 — the format-string bodyguard 🛡️✨

okay so -Wformat checks calls to printf and scanf, etc., to make sure that the arguments supplied have types appropriate to the format string specified, and that the conversions specified in the format string make sense. It's giving type-safety but for your format strings, bestie, and it ate.

This includes standard functions, and others specified by format attributes, in the printf, scanf, strftime and strfmon (an X/Open extension, not in the C standard) families (or other target-specific families). iykyk

Which functions are checked without format attributes having been specified depends on the standard version selected, and such checks of functions without the attribute specified are disabled by -ffreestanding or -fno-builtin. so if you turned those on and your warnings ghosted you… that's why 💀

The formats are checked against the format features supported by GNU libc version 2.2. These include all ISO C90 and C99 features, as well as features from the Single Unix Specification and some BSD and GNU extensions. Other library implementations may not support all these features; GCC does not support warning about features that go beyond a particular library's limitations. However, if -Wpedantic is used with -Wformat, warnings are given about format features not in the selected standard version (but not for strfmon formats, since those are not in any version of the C standard). ngl that last parenthetical is so real

---

• -Wformat=1, -Wformat — the base flavor, and honestly it's already a lot 🌸

Option -Wformat is equivalent to -Wformat=1, and -Wno-format is equivalent to -Wformat=0. Since -Wformat also checks for null format arguments for several functions, -Wformat also implies -Wnonnull. Some aspects of this level of format checking can be disabled by the options: -Wno-format-contains-nul, -Wno-format-diag, -Wno-format-extra-args, and -Wno-format-zero-length. And -Wformat is enabled by -Wall. free real estate in your build 💖

• -Wformat=2 — the deluxe edition, main character energy 👑

Enable -Wformat plus additional format checks. Currently equivalent to:

-Wformat -Wformat-nonliteral -Wformat-security -Wformat-y2k

it's giving "I want all the format warnings" and honestly? valid.

---

• -Wno-format-contains-nul — if -Wformat is specified, do not warn about format strings that contain NUL bytes. 🫶
• -Wno-format-diag — if -Wformat is specified, do not warn about format strings that are unsuitable for GCC diagnostics. yes bestie, this is exactly as niche as it sounds
• -Wno-format-extra-args — if -Wformat is specified, do not warn about excess arguments to a printf or scanf format function. The C standard specifies that such arguments are ignored. they're just there vibing, unbothered

Where the unused arguments lie between used arguments that are specified with $ operand number specifications, normally warnings are still given, since the implementation could not know what type to pass to va_arg to skip the unused arguments. However, in the case of scanf formats, this option suppresses the warning if the unused arguments are all pointers, since the Single Unix Specification says that such unused arguments are allowed. the lore is deep and I respect it

• -Wformat-overflow, -Wformat-overflow=level — buffer overflow watch 👀🔥

Warn about calls to formatted input/output functions such as sprintf and vsprintf that might overflow the destination buffer. When the exact number of bytes written by a format directive cannot be determined at compile-time it is estimated based on heuristics that depend on the level argument and on optimization. While enabling optimization will in most cases improve the accuracy of the warning, it may also result in false positives. so it's smart but it's also occasionally dramatic, we love that for it

• -Wformat-overflow, -Wformat-overflow=1 — level 1, the chill one 🌈

Level 1 of -Wformat-overflow enabled by -Wformat employs a conservative approach that warns only about calls that most likely overflow the buffer. At this level, numeric arguments to format directives with unknown values are assumed to have the value of one, and strings of unknown length to be empty. Numeric arguments that are known to be bounded to a subrange of their type, or string arguments whose output is bounded either by their directive's precision or by a finite set of string literals, are assumed to take on the value within the range that results in the most bytes on output. For example, the call to sprintf below is diagnosed because even with both a and b equal to zero, the terminating NUL character ('\0') appended by the function to the destination buffer will be written past its end. Increasing the size of the buffer by a single byte is sufficient to avoid the warning, though it may not be sufficient to avoid the overflow. one byte bestie. ONE BYTE.

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

• -Wformat-overflow=2 — level 2, and it is not playing 🚨

Level 2 warns also about calls that might overflow the destination buffer given an argument of sufficient length or magnitude. At level 2, unknown numeric arguments are assumed to have the minimum representable value for signed types with a precision greater than 1, and the maximum representable value otherwise. Unknown string arguments whose length cannot be assumed to be bounded either by the directive's precision, or by a finite set of string literals they may evaluate to, or the character array they may point to, are assumed to be 1 character long.

At level 2, the call in the example above is again diagnosed, but this time because with a equal to a 32-bit INT_MIN the first %i directive will write some of its digits beyond the end of the destination buffer. To make the call safe regardless of the values of the two variables, the size of the destination buffer must be increased to at least 34 bytes. GCC includes the minimum size of the buffer in an informational note following the warning. bestie it literally tells you the number, take the freebie 💅

An alternative to increasing the size of the destination buffer is to constrain the range of formatted values. The maximum length of string arguments can be bounded by specifying the precision in the format directive. When numeric arguments of format directives can be assumed to be bounded by less than the precision of their type, choosing an appropriate length modifier to the format specifier will reduce the required buffer size. For example, if a and b in the example above can be assumed to be within the precision of the short int type then using either the %hi format directive or casting the argument to short reduces the maximum required size of the buffer to 24 bytes.

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

• -Wno-format-zero-length — if -Wformat is specified, do not warn about zero-length formats. The C standard specifies that zero-length formats are allowed. allowed! legal! unbothered! ✨
• -Wformat-nonliteral — if -Wformat is specified, also warn if the format string is not a string literal and so cannot be checked, unless the format function takes its format arguments as a va_list. can't inspect it, can't vouch for it, sorry bestie 🙈
• -Wformat-security — if -Wformat is specified, also warn about uses of format functions that represent possible security problems. At present, this warns about calls to printf and scanf functions where the format string is not a string literal and there are no format arguments, as in printf (foo);. This may be a security hole if the format string came from untrusted input and contains %n. (This is currently a subset of what -Wformat-nonliteral warns about, but in future warnings may be added to -Wformat-security that are not included in -Wformat-nonliteral.)

not me having to say this seriously: that's a real security hole, bestie. if the format string came from untrusted input and has %n in it, that's the villain arc. do not pass go. 😭

• -Wformat-signedness — if -Wformat is specified, also warn if the format string requires an unsigned argument and the argument is signed and vice versa. signed/unsigned mismatch? we don't do that here 💖
• -Wformat-truncation, -Wformat-truncation=level — the other format warning, this time about your output getting cut off mid-sentence ✂️

Warn about calls to formatted input/output functions such as snprintf and vsnprintf that might result in output truncation. When the exact number of bytes written by a format directive cannot be determined at compile-time it is estimated based on heuristics that depend on the level argument and on optimization. While enabling optimization will in most cases improve the accuracy of the warning, it may also result in false positives. Except as noted otherwise, the option uses the same logic -Wformat-overflow. same brain, different font

• -Wformat-truncation, -Wformat-truncation=1 — level 1, still being polite 🌸

Level 1 of -Wformat-truncation enabled by -Wformat employs a conservative approach that warns only about calls to bounded functions whose return value is unused and that will most likely result in output truncation.

• -Wformat-truncation=2 — level 2, now it's watching you use the return value too 👑

Level 2 warns also about calls to bounded functions whose return value is used and that might result in truncation given an argument of sufficient length or magnitude.

• -Wformat-y2k — if -Wformat is specified, also warn about strftime formats that may yield only a two-digit year. the year 2000 called and it has opinions 📞

---

• -Wnonnull — warn about passing a null pointer for arguments marked as requiring a non-null value by the nonnull function attribute. 🫶

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

• -Wnonnull-compare — warn when comparing an argument marked with the nonnull function attribute against null inside the function. bestie you literally promised it wasn't null, why are we checking 😭

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

• -Wnull-dereference — warn if the compiler detects paths that trigger erroneous or undefined behavior due to dereferencing a null pointer. This option is only active when -fdelete-null-pointer-checks is active, which is enabled by optimizations in most targets. The precision of the warnings depends on the optimization options used. so like, turn on your optimizations and this one wakes up 💅
• -Wno-musttail-local-addr — do not warn about passing a pointer (or in C++, a reference) to a local variable or label to argument of a musttail call. Those variables go out of scope before the tail call instruction. they're gone before the call even happens, it's fine, it's fine
• -Wmaybe-musttail-local-addr — warn when address of a local variable can escape to a musttail call, unless it goes out of scope already before the musttail call. a maybe warning for a maybe escape, the symmetry is iconic

int foo (int *);

okay so here's the little code situation they're walking you through, bestie, look at this:

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);
    }
}

And yeah — this warning is enabled by -Wextra. 🫶

---

-Wnrvo (C++ and Objective-C++ only)

Okay so this one yells at you when the compiler does not elide the copy from a local variable into the function's return value, in a context where [class.copy.elision] says it's allowed to. This elision has a whole government name — it's commonly known as the Named Return Value Optimization 👑

Like, look at this example: the compiler literally cannot elide copies from both v1 and v2, so it elides neither. tragic.

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

---

-Winfinite-recursion

Warns you about infinitely recursive calls 🔁 — the "wait why is my program just vibing in a loop forever" detector. The warning is effective at all optimization levels, but ngl it needs optimization in order to detect infinite recursion in calls between two or more functions. -Winfinite-recursion is included in -Wall.

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

---

-Winit-self (C, C++, Objective-C and Objective-C++ only)

Warns about uninitialized variables that are initialized with themselves. the audacity 💀 Note this option can only be used together with the -Wuninitialized option.

For example, GCC warns about i being uninitialized in the following snippet only when -Winit-self has been specified:

int f()
{
  int i = i;
  return i;
}

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

---

-Wno-implicit-int (C and Objective-C only)

This option controls warnings when a declaration doesn't specify a type. This warning is enabled by default, as an error, in C99 and later dialects of C, and also by -Wall. The error can be downgraded to a warning using -fpermissive (along with certain other errors), or for this error alone, with -Wno-error=implicit-int.

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

---

-Wno-implicit-function-declaration (C and Objective-C only)

Same energy, different crime: this option controls warnings when a function is used before being declared. This warning is enabled by default, as an error, in C99 and later dialects of C, and also by -Wall. The error can be downgraded to a warning using -fpermissive (along with certain other errors), or for this error alone, with -Wno-error=implicit-function-declaration.

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

---

-Wimplicit (C and Objective-C only)

Same as -Wimplicit-int and -Wimplicit-function-declaration. This warning is enabled by -Wall. two-for-one deal, slay ✨

---

-Whardened

Warns when -fhardened did not enable an option from its set (go look at -fhardened for that whole set). For instance, using -fhardened and -fstack-protector at the same time on the command line causes -Whardened to warn, because -fstack-protector-strong will not be enabled by -fhardened.

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:

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

This warning does not warn when the last statement of a case cannot 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 into account control flow statements, such as 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 there are occasions where a switch case fall through is desirable, GCC provides an attribute, __attribute__ ((fallthrough)), that is to be used along with a null statement to suppress this 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 you can use [[gnu::fallthrough]];, which is a GNU extension. Instead of these attributes, it is 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 such as const. For ISO C, such a type qualifier has no effect, since the value returned by a function is not an lvalue. For C++, the warning is only emitted for scalar types or void. ISO C prohibits qualified void return types on function definitions, so such return types always receive a warning even without this option. 🙈

This warning is also enabled by -Wextra.

---

-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 either unknown, 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 does not reflect the block structure. Specifically, a warning is issued for if, else, while, and for clauses with a guarded statement that does not 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. it's giving ✨ deception ✨

  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.

• -Wmissing-attributes — bestie, this one clocks a declaration of a function that's missing one or more attributes that a related function is declared with, where that absence could mess with the correctness or the efficiency of the generated code. Like, for example, it fires for declarations of aliases that use attributes to specify less restrictive requirements than their targets. That's usually a lil optimization opportunity, no cap. By contrast, the -Wattribute-alias=2 option controls warnings issued when the alias is more restrictive than the target, which could legitimately lead to incorrect code generation 😬. Attributes considered here 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. Common Attributes.

-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 — screams at you if an aggregate or union initializer is not fully bracketed 🫶. In the example below, the initializer for a is not fully bracketed, but the one for b totally is.

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) — yells 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 one 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 got 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 — warns 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 — warns about unsafe multiple statement macros that appear to be guarded by a clause such as if, else, for, switch, or while, but where only the first statement is actually guarded after the macro is expanded 💀. Like:

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

will increment y unconditionally, not just when c holds. The 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 — warns if parentheses are omitted in certain contexts, such as when there's an assignment in a context where a truth value is expected, or when operators are nested whose precedence people often get confused about 😵‍💫.

Also warns 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 warns 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 — warns about code that may have undefined semantics because of violations of sequence point rules in the C and C++ standards ⚠️.

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

It is not specified when between sequence points modifications to the values of objects take effect. Programs whose behavior depends on this have undefined behavior; the C and C++ standards specify that "Between the previous and next sequence point an object shall have its stored value modified at most once by the evaluation of an expression. Furthermore, the prior value shall be read only to determine the value to be stored.". If a program breaks these rules, the results on any particular implementation are entirely unpredictable 😭.

Examples of code with undefined behavior are a = a++;, a[n] = b[n++] and a[i++] = i;. Some more complicated cases are not diagnosed by this option, and it may give an occasional false positive result, but in general it has been found fairly effective at detecting this sort of problem in programs.

The C++17 standard will define the order of evaluation of operands in more cases: in particular it requires that the right-hand side of an assignment be evaluated before the left-hand side, so the above examples are no longer undefined. But this option will still warn about them, to help people avoid writing code that is undefined in C and earlier revisions of C++.

The standard is worded confusingly, therefore there is some debate over the precise meaning of the sequence point rules in subtle cases. Links to discussions of the problem, including proposed formal definitions, may be found on the GCC readings page, at <https://gcc.gnu.org/readings.html>.

This warning is enabled by -Wall for C and C++.

• -Wno-return-local-addr — do not warn about returning a pointer (or in C++, a reference) to a variable that goes out of scope after the function returns 🫣.
• -Wreturn-mismatch — okay so this one is obsessed with your return statements, bestie. It fires when a function that doesn't return void has a return with no expression in it. It also fires when a function whose return type is void has a return with an expression — unless that expression's type is also void. But because GNU loves you, that second case gets accepted with zero warning unless you bring -Wpedantic to the party. 🙈

⚠️ The big yikes: 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. Do not do this. It is not giving what you think it's giving. 💀

This warning is specific to C and it's enabled by default. In C99 and later language dialects, it's treated as an error. You can downgrade it to a warning using -fpermissive (along with other warnings), or, for just this one 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, because that one takes precedence, periodt). It also warns if execution may reach the end of the function body, or if the function doesn't contain any return statement at all. 🫶

⚠️ Same undefined-behavior situation as above: 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.

And 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. 😭 Not a vibe.

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

• -Wno-shift-count-negative — controls warnings if a shift count is negative. 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. Enabled by default. ✨
• -Wshift-negative-value — warns if you left shift a negative value. This one 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. Let's break it down, bestie:
  • -Wshift-overflow=1 — this is the warning level of -Wshift-overflow and it's enabled by default in C99 and C++11 modes (and newer). This 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), because signed left shifts always wrap. 🌀
  • -Wshift-overflow=2 — this 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 don't correspond to enumerators also provoke warnings when this option is used, unless the enumeration is marked with the flag_enum attribute. Enabled by -Wall. 👑
• -Wswitch-default — warns whenever a switch statement doesn't have a default case. That's it. That's the whole thing.
• -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 don't 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 you a warning about an omitted enumeration code even if there is a default label. Yes, even then. It's thorough like that. 💅
• -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's possible to suppress this warning by casting the controlling expression to a type other than bool. Like this, bestie:

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's outside of its respective type range. 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:
  ...
  }

And btw, -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, so like… heads up. 📢
• -Wtrivial-auto-var-init — warns when -ftrivial-auto-var-init cannot initialize the automatic variable. A common situation is an automatic variable that's declared between the controlling expression and the first case label of a switch statement. Rude of it, honestly.
• -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). So like, you did something with it, but not really. 😭

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.

And here's the whole breakdown, annotated and everything, so you can literally see which n triggers what:

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 used
  int i = ++h;
  (void) i;
  int j = 0; // No warning, postdecrement used
  int k = j--;
  (void) k;
  int l = 0; // No warning, l used
  int m = l |= 2;
  (void) m;
}

🔥 Study it. It's lowkey the whole lesson.

• -Wunused-function — warns whenever a static function is declared but not defined, or a non-inline static function is unused. Enabled by -Wall. 💜
• -Wunused-label — warns whenever a label is declared but not used. 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. 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. So like, by default you do get yelled at for ignoring that value. 🙈
• -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++. 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 — okay so this warning level also drags unused constant static variables in headers, bestie, excluding system headers. It's literally the same thing as the short form -Wunused-const-variable. But heads up: you have to explicitly ask for this level in both C and C++, because it might be hard to clean up all the headers that get included. 😭
• -Wunused-value — warns whenever a statement computes a result that's explicitly not used. To suppress this warning, 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. So for example, something like x[i,j] causes a warning, while x[(void)i,j] does not. ✨

This warning is enabled by -Wall.

• -Wunused — it's all the above -Wunused options combined, except the ones documented as needing to be specified explicitly. slay.

So to actually get a warning about an unused function parameter, you have to either specify -Wextra -Wunused (and 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* — warn about uses of pointers to dynamically allocated objects that have been rendered indeterminate by a call to a deallocation function. This warning is enabled at all optimization levels, but it may yield different results with optimization than without. ngl, that's a little chaotic but okay.
  • -Wuse-after-free=1 — at level 1, the warning only tries to diagnose unconditional uses of pointers made indeterminate by a deallocation call or a successful call to realloc, regardless of whether or not the call actually reallocated memory. This includes double-free calls, plus uses in arithmetic and relational expressions. And yeah, even though it's undefined, uses of indeterminate pointers in equality (or inequality) expressions are not diagnosed at this level. 💅
  • -Wuse-after-free=2 — at level 2, on top of 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 this function gets 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. Now listen: all uses of indeterminate pointers are undefined, but equality tests sometimes show up after calls to realloc as an attempt to figure out whether the call relocated the object to a different address. They get diagnosed at a separate level to help gradually transition legacy code to safe alternatives. For example, the equality test in this function 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 dodge the warning at this level, store offsets into allocated memory instead of pointers. This approach obviates needing to adjust the stored pointers after reallocation. 💡

• -Wuse-after-free=2 is included in -Wall.

• -Wuseless-cast (C, Objective-C, C++ and Objective-C++ only) — 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, since 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. 👑

On top of that, passing a pointer (or in C++, a reference) to an uninitialized object to a const-qualified argument of a built-in function known to read the object is also diagnosed by this warning. (-Wmaybe-uninitialized is issued for ordinary functions.)

If you want to warn about code that uses the uninitialized value of the variable in its own initializer, use the -Winit-self option. 🎀

These warnings occur for individual uninitialized elements of structure, union or array variables, as well as for variables that are uninitialized as a whole. They do not occur for variables or elements declared volatile. And because these warnings depend on optimization, the exact variables or elements you get warned about depend on the precise optimization options and the version of GCC you used. ✨

Note that there may be no warning about a variable that's 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.

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 is 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 — 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 does not 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. 💀

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 is not understood by GCC. And if this command-line option is used, warnings are even issued for unknown pragmas in system header files. That is not the case if the warnings are only enabled by the -Wall command-line option. 👀
• -Wno-pragmas — do not warn about misuses of pragmas, such as incorrect parameters, invalid syntax, or conflicts between pragmas. See also -Wunknown-pragmas. 🦋
• -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 allows 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. 🌸
• -Wstrict-aliasing — this option is only active when -fstrict-aliasing is active. It warns about code that might break the strict aliasing rules that the compiler is using for optimization. The warning does not catch all cases, but it does attempt to catch the more common pitfalls. It is included in -Wall. It is equivalent to -Wstrict-aliasing=3. 💅
• -Wstrict-aliasing=n — okay bestie, this one only does anything when -fstrict-aliasing is switched on. It warns you about code that might be breaking the strict aliasing rules the compiler is using to optimize. Higher levels = higher accuracy (so, fewer false positives), but also more effort — literally the same way -O works. And btw, plain -Wstrict-aliasing is exactly the same as -Wstrict-aliasing=3. 🎀
  • Level 1: the most aggressive, quickest, and least accurate of the bunch. Lowkey useful when the higher levels are being all quiet but -fstrict-aliasing is still breaking your code — because it has very few false negatives. But bestie, it has a LOT of false positives. It warns for every pointer conversion between possibly incompatible types, even if that pointer never gets dereferenced. Runs in the front end only.
  • Level 2: aggressive, quick, not too precise. Still may have many false positives (not as many as level 1 tho), and few false negatives (but possibly more than level 1). Unlike level 1, it only warns when an address is taken. It 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 that common pun+dereference pattern in the front end: "*(int*)&some_float". If optimization is enabled it also runs in the back end, where it deals with multiple statement cases using flow-sensitive points-to information. Only warns when the converted pointer is dereferenced. Does not warn about incomplete types.
• -Wstrict-overflow
• -Wstrict-overflow=n — this 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. ⚠️ Note that it does not warn about all cases where the code might overflow — it only warns about cases where the compiler implements some optimization. So yeah, this warning depends on the optimization level.

An optimization that assumes signed overflow doesn't occur is perfectly safe if the values of the variables involved are such that overflow never actually happens. So this warning can easily give a false positive: a warning about code that isn't even a problem. To help focus on important issues, several warning levels are defined. No warnings are issued for the use of undefined signed overflow when estimating how many iterations a loop requires — in particular when determining whether a loop will be executed at all.

• -Wstrict-overflow=1 — warn about cases that are both questionable and easy to avoid. For example, the compiler 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 — warns for calls to "strcmp" and "strncmp" whose result is determined to be either zero or non-zero in tests for such equality, because the length of one argument is 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 gets diagnosed because its result is necessarily non-zero no matter what's actually in 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 — warns for code that can be statically determined to cause buffer overflows or memory overruns, like 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. That 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.

It produces the best results with optimization enabled, but it can detect a small subset of simple buffer overflows even without optimization in calls to the GCC built-in functions like "__builtin_memcpy" that correspond to the standard functions. In any case, the option warns about just a subset of buffer overflows detected by the corresponding overflow checking built-ins, such as "__builtin___memcpy_chk", which can perform run-time checking if the access can't 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 — 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's 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 — 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 — 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, and it may result in warnings for safe code. 😭
• -Wstringop-overflow=4 — 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 — warns 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);
        }

As another example, the following call to "strncpy" results in copying to "d" just the characters preceding the terminating NUL, without appending the NUL to the end. Assuming the result of "strncpy" is necessarily a NUL-terminated string is a common mistake, and so the call is diagnosed. To avoid the warning when the result is not expected to be NUL-terminated, call "memcpy" instead. 🫶

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

Okay so in the next example, the call to strncpy is using the size of the destination buffer as the bound. And bestie, listen: if the source string is equal to or greater than that size, the copy is not going to be NUL-terminated. So yeah, that call gets diagnosed too. Want the warning gone? Specify sizeof buf - 1 as the bound and manually set the last element of the buffer to NUL. 💅

void copy (const char *s)
{
  char buf[80];
  strncpy (buf, s, sizeof buf);
  ...
}

Now, sometimes you genuinely want a character array holding a sequence of bytes with no terminating NUL — and in that case you can annotate the array with the attribute nonstring to dodge this warning. But heads up: those arrays are not suitable arguments to functions that expect NUL-terminated strings. To help catch accidental misuse of such 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, based on the level of the strict_flex_array (level) attribute attached to the trailing array field of a structure — if that attribute is available. If it's not available, it goes off the level of the -fstrict-flex-arrays=level option instead. And btw -Wstrict-flex-arrays only actually does anything when level is greater than 0.

• When level=1, warnings get issued for a trailing array reference of a structure that has 2 or more elements, if the trailing array is referenced as a flexible array member.
• When level=2, on top of everything in 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 everything in level=2, extra 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 is way more effective when -ftree-vrp is active (that's the default for -O2 and above), but some warnings may get diagnosed even with no optimization at all. ✨

---

-Wsuggest-attribute=attribute-name 👑

Warns for cases where adding an attribute might be a glow-up. The attribute-names currently supported are listed below, mwah.

-Wsuggest-attribute=pure

-Wsuggest-attribute=const

-Wsuggest-attribute=noreturn

-Wmissing-noreturn

-Wsuggest-attribute=malloc

-Wsuggest-attribute=returns_nonnull

Warns about functions that might be candidates for the attributes pure, const, noreturn, malloc or returns_nonnull. The compiler only warns for functions visible in other compilation units — or, in the case of pure and const, if it can't prove that the function returns normally. 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 requires the option -fipa-pure-const, which is enabled by default at -O and higher. Higher optimization levels make the analysis more accurate. 🔥

-Wsuggest-attribute=format

-Wmissing-format-attribute

Warns about function pointers that might be candidates for format attributes. Note these are only possible candidates, not absolute ones, okay? GCC guesses that function pointers with format attributes used in assignment, initialization, parameter passing or return statements should have a corresponding format attribute in the resulting type. I.e. the left-hand side of the assignment or initialization, the type of the parameter variable, or the return type of the containing function respectively should also have a format attribute to avoid the warning.

GCC also warns about function definitions that might be candidates for format attributes. Again — 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

Warns 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 — like wrappers of C++ throw or fatal error reporting functions leading to abort. 💀

---

-Walloc-size

Warns about calls to allocation functions decorated with the attribute alloc_size that specify insufficient size for the target type of the pointer the result is assigned to — including calls to the built-in forms of the functions aligned_alloc, alloca, calloc, malloc, and realloc.

-Walloc-zero

Warns about calls to allocation functions decorated with the attribute alloc_size that specify zero bytes — including calls to the built-in forms of aligned_alloc, alloca, calloc, malloc, and realloc. Because the behavior of these functions when called with a zero size differs between implementations (and in the case of realloc it has been deprecated), relying on it may cause subtle portability bugs, and you should just not. 🚫

-Wcalloc-transposed-args

Warns about calls to allocation functions decorated with the 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 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

Warns about calls to functions decorated with the 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

Outputs warnings about auto-profile inconsistencies.

-Wcannot-profile

Warns when profiling instrumentation was requested, but could not be applied to a certain function. 😭

-Walloca-larger-than=byte-size

This option warns on calls to alloca with an integer argument whose value is either zero, or that is not bounded by a controlling predicate that limits 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. 🦋

So for example, a bounded case of alloca could look like:

void func (size_t n)
{
  void *p;
  if (n <= 1000)
    p = alloca (n);
  else
    p = malloc (n);
  f (p);
}

In the example above, 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. 👀

Unbounded uses, on the other hand, are uses of alloca with no controlling predicate constraining its integer argument. Like this:

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.

And note, bestie, 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 (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 conversion of the operands to the same type cannot change their values. This affects warnings from -Wconversion, -Wfloat-conversion, and -Wsign-conversion.

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

-Warray-bounds

-Warray-bounds=n

Warns about out of bounds subscripts or offsets into arrays. This warning is enabled by -Wall. It's more effective when -ftree-vrp is active (the default for -O2 and above), but a subset of instances get issued even without optimization. 🔥

okay bestie, we're back in the warnings zone and it's lowkey a lot, but like… in a fun way? 🎀 buckle up ✨

so here's the tea on trailing arrays 👀 — by default, the trailing array of a structure gets treated as a flexible array member by -Warray-bounds or -Warray-bounds=n if it's declared as either a flexible array member per C99 standard onwards ([]), a GCC zero-length array extension ([0]), or an one-element array ([1]). and because of that, out of bounds subscripts or offsets into zero-length arrays or one-element arrays are not warned by default. no cap 🙈

you can add the option -fstrict-flex-arrays or -fstrict-flex-arrays=level to control how this option treats the 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 is enabled by -Wall; higher levels are not, and must be explicitly requested. so if you want more, you gotta ask bestie 💅
• -Warray-bounds=2 — this warning level also warns about the intermediate results of pointer arithmetic that may yield out of bounds values. ⚠️ heads up: this warning level may give a larger number of false positives and is deactivated by default. it's giving paranoid but make it fashion
• -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. for example:

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

this warning is enabled by -Wextra. if -Wc++-compat is enabled, the warning has slightly different wording and warns even if the declaration being initialized has the "nonstring" warning, as in C++ such initializations are an error 😭 (rude, but okay)

• -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. slay ✨

• -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. periodt 💖

• -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 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 🌀 such characters can change left-to-right writing direction into right-to-left (and vice versa), which can cause confusion between the logical order and visual order. this may be dangerous; for instance, it may seem that a piece of code is not commented out, whereas it in fact is 💀

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 👑

NOTE: In Ubuntu 25.10 and later versions, -Wbidi-chars is enabled by default for C, C++, ObjC, ObjC++, unless -Wno-bidi-chars or -Wbidi-chars=none is found. 🐧

• -Wbool-compare — warns about boolean expression compared with an integer value different from "true"/"false". for instance, the following comparison is always false:

int n = 5;
...
if ((n > 1) == 2) { ... }

this warning is enabled by -Wall ✨

• -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, warns 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 an "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 — bestie, if you write a literal 0 and try to pass it off as a null pointer constant? we're gonna talk about it. warn activated. 💀
• -Wzero-length-bounds — warns you when you access elements of zero-length array members that might overlap other members of the same object. ngl, declaring interior zero-length arrays is discouraged because accesses to them are undefined. that's not a vibe, that's UB. 🚫

like, check it: in function bad down here, the first two stores get diagnosed because the array elements overlap the subsequent members b and c. the third store gets diagnosed by -Warray-bounds instead, because it's straight up 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
}

oh and ps — -Wzero-length-bounds is enabled by -Warray-bounds. it comes as a package deal. ✨

• -Wno-div-by-zero — do not warn about compile-time integer division by zero. and floating-point division by zero? not warned about either, because that's actually a legit way of obtaining infinities and NaNs. math girlies stay winning. 🧚
• -Wsystem-headers — prints warning messages for constructs found in system header files. normally these are suppressed, because the assumption is they usually don't indicate real problems and would only make the compiler output harder to read (fair). using this command-line option tells GCC to emit warnings from system headers as if they occurred in user code. but listen — using -Wall together with this option does not warn about unknown pragmas in system headers. for that you also need -Wunknown-pragmas. the way these flags gotta be stacked... 😭
• -Wtautological-compare — warns if a self-comparison always evaluates to true or false. it catches classic mistakes like:

int i = 1;
...
if (i > i) { ... }

bestie. i is never greater than i. ever. 😭

it also warns about bitwise comparisons that always evaluate to true or false, for instance:

if ((a & 16) == 10) { ... }

will always be false. like, always always.

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 (we leave those alone, periodt). -Wtrailing-whitespace is equivalent to -Wtrailing-whitespace=blanks and warns just about trailing space and horizontal tab characters. -Wtrailing-whitespace=any warns about those or trailing form feed or vertical tab characters. -Wno-trailing-whitespace or -Wtrailing-whitespace=none disables the warning, which is the default. this is a coding style warning. 💅
• -Wleading-whitespace=kind — 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 still pop off), and 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 the argument of -ftabstop=n, 8 by default).
  • -Wleading-whitespace=none — disables the warning, which is the default.

this is a coding style warning. your indent game is under review bestie 🌸

• -Wtrampolines — warns about trampolines generated for pointers to nested functions. okay so what is a trampoline? it's a small piece of data or code that's 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. for some targets, it's made up of data only and thus requires no special treatment. but for most targets, it's made up of code and thus requires the stack to be made executable in order for the program to work properly. 👀
• -Wfloat-equal — warns if floating-point values are used in equality comparisons. the idea behind this is that sometimes it's convenient (for the programmer, not for reality) to consider floating-point values as approximations to infinitely precise real numbers. if you're doing that, 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. fr fr, don't == your floats. 😭
• -Wtraditional (C and Objective-C only) — warns about certain constructs that behave differently in traditional and ISO C. also warns about ISO C constructs that have no traditional C equivalent, and/or problematic constructs that should be avoided. the full rundown:
  • Macro parameters that appear within string literals in the macro body. In traditional C macro replacement takes place within string literals, but in ISO C it does not.
  • In traditional C, some preprocessor directives did not exist. Traditional preprocessors only considered a line to be a directive if the # appeared in column 1 on the line. Therefore -Wtraditional warns about directives that traditional C understands but ignores because the # does not appear as the first character on the line. It also suggests you hide directives like #pragma not understood by traditional C by indenting them. Some traditional implementations do not recognize #elif, so this option suggests avoiding it altogether.
  • A function-like macro that appears without arguments.
  • The unary plus operator.
  • The U integer constant suffix, or the F or L floating-point constant suffixes. (Traditional C does support the L suffix on integer constants.) Note, these suffixes appear in macros defined in the system headers of most modern systems, e.g. the _MIN/_MAX macros in <limits.h>. Use of these macros in user code might normally lead to spurious warnings, however GCC's integrated preprocessor has enough context to avoid warning in these cases.
  • A function declared external in one block and then used after the end of the block.
  • A switch statement has an operand of type long.
  • A non-static function declaration follows a static one. This construct is not accepted by some traditional C compilers.
  • The ISO type of an integer constant has a different width or signedness from its traditional type. This warning is only issued if the base of the constant is ten. I.e. hexadecimal or octal values, which typically represent bit patterns, are not warned about.
  • Usage of ISO string concatenation is detected.
  • Initialization of automatic aggregates.
  • Identifier conflicts with labels. Traditional C lacks a separate namespace for labels.
  • Initialization of unions. If the initializer is zero, the warning is omitted. This is done under the assumption that the zero initializer in user code appears conditioned on e.g. __STDC__ to avoid missing initializer warnings and relies on default initialization to zero in the traditional C case.
  • Conversions by prototypes between fixed/floating-point values and vice versa. The absence of these prototypes when compiling with traditional C causes serious problems. This is a subset of the possible conversion warnings; for the full set use -Wtraditional-conversion.
  • Use of ISO C style function definitions. This warning intentionally is not issued for prototype declarations or variadic functions because these ISO C features appear in your code when using libiberty's traditional C compatibility macros, PARAMS and VPARAMS. This warning is also bypassed for nested functions because that feature is already a GCC extension and thus not relevant to traditional C compatibility.
• -Wtraditional-conversion (C and Objective-C only) — 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. 💅

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. ✨
• -Wno-shadow-ivar (Objective-C only) — do not warn whenever a local variable shadows an instance variable in an Objective-C method. 🫶
• -Wshadow=global — warns for any shadowing. same as -Wshadow.
• -Wshadow=local — warns when a local variable shadows another local variable or parameter.
• -Wshadow=compatible-local — bestie, this one yells at you when a local variable shadows another local variable or a parameter, but ONLY when the types are compatible with each other. In C++, "compatible" here means the type of the shadowing variable can be converted to the type of the shadowed one. The whole reason this flag exists (as a sibling to -Wshadow=local) is the vibe that when a local shadows another one of a totally incompatible type, it's probably on purpose, not a typo or a bug. Behold:

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

So because the two i variables up there have incompatible types, turning on only -Wshadow=compatible-local gives you zero warnings. And that's fine, because if the types don't match, and a programmer accidentally uses one in place of the other, type checking is expected to catch it and throw an error or a warning anyway. Using this flag instead of -Wshadow=local can lowkey cut down the number of warnings you get from intentional shadowing. Heads up though: 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 you whenever an object is defined whose size exceeds byte-size. And plot twist: -Wlarger-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-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. Those particular warnings can only be silenced with -Wno-larger-than.

• -Wno-larger-than — turns off -Wlarger-than= warnings. Equivalent to -Wlarger-than=SIZE_MAX or larger. 💅
• -Wframe-larger-than=byte-size — warns if the size of a function frame exceeds byte-size. Fair warning (pun intended): the computation used to figure out the stack frame size is approximate and not conservative — the actual requirements may be somewhat greater than byte-size even if you never 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 decides whether or not to warn. -Wframe-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-frame-larger-than.
• -Wno-frame-larger-than — disables -Wframe-larger-than= warnings. Equivalent to -Wframe-larger-than=SIZE_MAX or larger.
• -Wfree-nonheap-object — warns when you try to deallocate an object that either wasn't allocated on the heap at all, or was allocated using a pointer that was not returned from a prior call to the corresponding allocation function. For example: because 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. This computation is conservative, and any space allocated via alloca, variable-length arrays, or related constructs is included by the compiler when deciding whether or not to warn.

The message matches the output of -fstack-usage. 💖

• If the stack usage is fully static but exceeds the specified amount, it's:

warning: stack usage is 1120 bytes

• If the stack usage is (partly) dynamic but bounded, it's:

warning: stack usage might be 1648 bytes

• If the stack usage is (partly) dynamic and not bounded, it's:

warning: stack usage might be unbounded

-Wstack-usage=PTRDIFF_MAX is enabled by default. Warnings controlled by the option can be disabled either by specifying a byte-size of SIZE_MAX or more, or by -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 does make such assumptions. Iconic symmetry honestly. 🔥
• -Wno-pedantic-ms-format (MinGW targets only) — when you pair this with -Wformat and -pedantic without GNU extensions, it disables the warnings about non-ISO printf / scanf format width specifiers I32, I64, and I used on Windows targets — the ones that depend on the MS runtime. 🦋
• -Wpointer-arith — warns about anything that depends on the "size of" a function type or of void. (GNU C assigns these types a size of 1, for convenience in calculations with void * pointers and pointers to functions — cute of it honestly.) In C++, it also warns when an arithmetic operation involves NULL. This warning is also enabled by -Wpedantic.

And also — this warning is upgraded to an error by -pedantic-errors.

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

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 does not support std::atomic_thread_fence and can report false positives. 💀
• -Wtype-limits — warns if a comparison is always true or always false because of the limited range of the data type — but it does not warn for constant expressions. So for example, it warns if an unsigned variable is 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 is available. Example: calling abs(3.14) triggers the warning, because the appropriate function 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 inside comments are not warned about, except those that would form escaped newlines.

This option is implied by -Wall. And if -Wall is not given, this option is still enabled — unless trigraphs are enabled. To get trigraph conversion without warnings but still get the other -Wall warnings, use -trigraphs -Wall -Wno-trigraphs. 🧚

• -Wundef — warns if an undefined identifier is evaluated in an #if directive. 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. 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 inside skipped conditional blocks, the preprocessor will report it as unused. To dodge the warning in that case, you might improve the scope of the macro's definition by, say, moving it into the first skipped block. Or alternatively, you could 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 is 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. For example, warn 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 not present in ISO C90 but 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 not present in ISO C99 but 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. Okay so this one yells at you about features that aren't in ISO C11 but are in ISO C23. Like omitting the string in _Static_assert, using [[]] syntax for attributes, decimal floating-point types, that whole vibe. It is independent of the standards mode, so it does its thing regardless. Warnings get switched off in the expression that follows __extension__. Oh and heads up — the name -Wc11-c2x-compat is deprecated. 👑

When you're not compiling in C23 mode, these warnings get upgraded to errors by -pedantic-errors. So like, not a drill. 😭

---

• -Wc23-c2y-compat (C and Objective-C only)
• -Wc23-c2y-compat (C and Objective-C only)

(not me pasting that twice — the source lists it twice, so it stays twice, iykyk 💅)

Warn about features that aren't in ISO C23 but are in ISO C2Y. For example, warn about _Generic selecting with a type name instead of an expression. This option is independent of the standards mode. Warnings are disabled in the expression that follows __extension__.

When not compiling in C2Y mode, these warnings are upgraded to errors by -pedantic-errors.

---

• -Wc++-compat (C and Objective-C only)

Warn about ISO C constructs that live outside the common subset of ISO C and ISO C++, e.g. requesting an implicit conversion from void * to a pointer to non-void type. It's giving "bestie your C isn't as C++-friendly as you thought". 🦋

---

• -Wc++11-compat (C++ and Objective-C++ only)

Warn about C++ constructs whose meaning differs between ISO C++ 1998 and ISO C++ 2011 — classic example: identifiers in ISO C++ 1998 that became keywords in ISO C++ 2011. This warning turns on -Wnarrowing and is enabled by -Wall.

---

• -Wc++14-compat (C++ and Objective-C++ only)

Warn about C++ constructs whose meaning differs between ISO C++ 2011 and ISO C++ 2014. This warning is enabled by -Wall.

---

• -Wc++17-compat (C++ and Objective-C++ only)

Warn about C++ constructs whose meaning differs between ISO C++ 2014 and ISO C++ 2017. This warning is enabled by -Wall.

---

• -Wc++20-compat (C++ and Objective-C++ only)

Warn about C++ constructs whose meaning differs between ISO C++ 2017 and ISO C++ 2020. This warning is enabled by -Wall.

---

• -Wc++26-compat (C++ and Objective-C++ only)

Warn about C++ constructs whose meaning differs between ISO C++ 2023 and the upcoming ISO C++ 2026. This warning is enabled by -Wall.

---

• -Wno-c++11-extensions (C++ and Objective-C++ only)

Do not warn about C++11 constructs when you're compiling code with an older C++ standard. And ngl, even without this option, some C++11 constructs will only get diagnosed if -Wpedantic is used.

---

• -Wno-c++14-extensions (C++ and Objective-C++ only)

Do not warn about C++14 constructs while compiling with an older C++ standard. Even without this option, some C++14 constructs only get diagnosed with -Wpedantic.

---

• -Wno-c++17-extensions (C++ and Objective-C++ only)

Do not warn about C++17 constructs while compiling with an older C++ standard. Even without this option, some C++17 constructs only get diagnosed with -Wpedantic.

---

• -Wno-c++20-extensions (C++ and Objective-C++ only)

Do not warn about C++20 constructs while compiling with an older C++ standard. Even without this option, some C++20 constructs only get diagnosed with -Wpedantic.

---

• -Wno-c++23-extensions (C++ and Objective-C++ only)

Do not warn about C++23 constructs while compiling with an older C++ standard. Even without this option, some C++23 constructs only get diagnosed with -Wpedantic.

---

• -Wno-c++26-extensions (C++ and Objective-C++ only)

Do not warn about C++26 constructs while compiling with an older C++ standard. Even without this option, some C++26 constructs only get diagnosed with -Wpedantic.

---

• -Wcast-qual

Warn whenever a pointer is cast in a way that removes a type qualifier from the target type. For example, warn if a const char * is cast to an ordinary char *. rude behaviour honestly 💀

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, like this:

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

---

• -Wcast-align

Warn whenever a pointer is cast such that the required alignment of the target is increased. For example, warn if a char * is cast to an int * on machines where integers can only be accessed at two- or four-byte boundaries.

Warn whenever a pointer is cast such that the required alignment of the target is increased. For example, warn if a char * is cast to an int * regardless of the target machine.

(yes bestie, it says it twice — that's the source, we keep it 🫶)

---

• -Wcast-function-type

Warn 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 the 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. Same deal for type qualifiers, ignored. The function type void (*) (void) is special and matches everything, which you can use to suppress this warning. In a cast involving pointer to member types, this warning fires whenever the type cast is changing the pointer to member type. This warning is enabled by -Wextra.

---

• -Wcast-user-defined

Warn when a cast to reference type does not involve a user-defined conversion that the programmer might expect to be called.

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

This warning is enabled by default.

---

• -Wwrite-strings

When compiling C, give string constants the type const char[length] so that copying the address of one into a non-const char * pointer produces a warning. These warnings help you find, at compile time, code that can try to write into a string constant — but only if you have been very careful about using const in declarations and prototypes. Otherwise, it is just a nuisance. That's why -Wall does not request these warnings, bestie. 🙈

When compiling C++, warn about the deprecated conversion from string literals to char *. This warning is enabled by default for C++ programs.

This warning is upgraded to an error by -pedantic-errors in C++11 mode or later.

---

• -Wclobbered

Warn for variables that might be changed by longjmp or vfork. Sneaky little guys. This warning is also enabled by -Wextra.

---

• -Wno-complain-wrong-lang

By default, language front ends complain when a command-line option is valid, but not applicable to that front end. You can disable that with -Wno-complain-wrong-lang, which is mostly useful when you invoke a single compiler driver for multiple source files written in different languages, like this:

$ 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. That latter front end diagnoses

f951: Warning: command-line option '-fno-rtti' is valid for C++/D/ObjC++ but not for Fortran

which may be disabled with -Wno-complain-wrong-lang.

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)

Warn if pointers of distinct types are compared without a cast. This warning is enabled by default.

---

• -Wconversion

Warn for implicit conversions that may alter a value. This includes conversions between real and integer, like abs (x) when x is double; conversions between signed and unsigned, like unsigned ui = -1; and conversions to smaller types, like sqrtf (M_PI). Do not warn for explicit casts like abs ((int) x) and ui = (unsigned) -1, or if the value is not changed by the conversion, like in abs (2.0). Warnings about conversions between signed and unsigned integers can be disabled by using -Wno-sign-conversion.

For C++, also warn for confusing overload resolution for user-defined conversions; and conversions that never use a type conversion operator: conversions to void, the same type, a base class or a reference to them. Warnings about conversions between signed and unsigned integers are disabled by default in C++ unless -Wsign-conversion is explicitly enabled.

Warnings about conversion from arithmetic on a small type back to that type are only given with -Warith-conversion.

---

• -Wdangling-else

Warn about constructions where there may be confusion about which if statement an else branch belongs to. Here's an example of exactly that situation:

{
  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 above by the indentation the programmer chose. 😭 When there's 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's no way the else can belong to the enclosing if. The resulting code looks like this:

{
  if (a)
    {
      if (b)
        foo ();
      else
        bar ();
    }
}

This warning is enabled by -Wparentheses.

---

• -Wdangling-pointer
• -Wdangling-pointer=n

Warn 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, warn about storing the address of such objects in escaped pointers. The warning is enabled at all optimization levels but may yield different results with optimization than without. 🔥

• -Wdangling-pointer=1

At level 1, the warning diagnoses only unconditional uses of dangling pointers.

• -Wdangling-pointer=2 — level 2 is level 1 but extra. On top of the unconditional uses it already catches, it also diagnoses conditional uses of dangling pointers. iykyk.

The short form -Wdangling-pointer is literally the same thing as -Wdangling-pointer=2, and both -Wno-dangling-pointer and -Wdangling-pointer=0 do the exact same thing: turn the warnings all the way off. Also -Wdangling-pointer=2 is included in -Wall. 🎀

This first example triggers the warning at level 1 — the address of the unnamed temporary gets referenced unconditionally outside of its scope:

char f (char c1, char c2, char c3)
{
  char *p;
  {
    p = (char[]) { c1, c2, c3 };
  }
  // warning: using dangling pointer 'p' to an unnamed temporary
  return *p;
}

In the next one, the store of the address of the local variable x into the escaped pointer *p is what 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;
}

And in this third one, the array a is out of scope by the time the pointer s gets used. Since the code that sets s is conditional, the warning fires 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 the macros __TIME__, __DATE__ or __TIMESTAMP__ show up, because bestie they might stop your compilation from being bitwise-identical and reproducible. 🙈
• -Wempty-body — warns if there's an empty body in an if, else or do while statement. This warning is also enabled by -Wextra.
• -Wno-endif-labels — do not warn about stray tokens after #else and #endif.
• -Wenum-compare — warns about a comparison between values of different enumerated types. In C++ enumerated type mismatches in conditional expressions also get diagnosed, and the warning is enabled by default there. In C, -Wall enables this warning.
• -Wenum-conversion — warns when a value of enumerated type is implicitly converted to a different enumerated type. In C this warning 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. Like, for example:

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

In C, an enumerated type is compatible with char, a signed integer type, or an unsigned integer type. But — and this is the messy part — since the choice of the underlying type of an enumerated type is implementation-defined, mismatches like this may 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. And ngl this warning is only supported for C and Objective-C; in C++ this kind of branch is an error anyway, no debate.

-Wjump-misses-init is included in -Wc++-compat. It can be disabled with the -Wno-jump-misses-init option.

• -Wsign-compare — warns when a comparison between signed and unsigned values could produce an incorrect result once 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. In C, this option is enabled also 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. This warning warns about stuff like, e.g.:

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 real → integer conversions, and higher-precision real → lower-precision real conversions. 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 does not equal the size of the array element. In that case the computation will not give you the number of elements in the array — which is probably what you actually meant. It warns about, e.g.:

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. That's the usual way to compute an array size, but with pointers it will not work out correctly, bestie. It warns about, e.g., sizeof (ptr) / sizeof (ptr[0]) when ptr isn't an array but a pointer. This warning is enabled by -Wall.
• -Wsizeof-pointer-memaccess — warns for suspicious length parameters to certain string and memory built-in functions when the argument uses sizeof. For example it triggers for memset (ptr, 0, sizeof (ptr)); when ptr is not an array but a pointer, and it suggests a possible fix — or 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. For 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 you forgot a multiplication by the element size, bestie. This warning is enabled by -Wall.
• -Wmemset-transposed-args — warns for suspicious calls to the memset built-in function where the second argument is not zero and the third argument is zero. For example, the call memset (buf, sizeof buf, 0) is 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 folded to zero, or a cast of zero to some type — it's far less likely the arguments were mistakenly transposed, so 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 in:

void f (void);
void g (void)
{
  if (!f)   // warning: expression evaluates to false
    abort ();
}

...comparisons of a pointer to a string literal, like in:

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 in:

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

Such uses typically indicate a programmer error: the address of most functions and objects necessarily evaluates to true (the exception is 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 such as intptr_t or uintptr_t. Comparisons against string literals result in unspecified behavior and are not portable — they suggest the intent was to call strcmp. The warning is suppressed if the suspicious expression is the result of macro expansion. -Waddress warning is enabled by -Wall.

• -Wno-address-of-packed-member — do not warn when the address of a packed member of a struct or union is taken, which usually results in an unaligned pointer value. This is enabled by default.
• -Wlogical-op — warns about suspicious uses of logical operators in expressions. That includes using logical operators in contexts where a bitwise operator is likely expected. It 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 a logical not used on the left-hand-side operand of a comparison. It does not 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) { ... }

So like, you can suppress the warning by wrapping the LHS in parentheses, bestie:

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

This warning is enabled by -Wall.

---

💅 More -W warnings (the saga continues)

• -Waggregate-return — warns if any functions that return structures or unions are defined or called. (And in languages where you can return an array, this also elicits a warning. Iconic of them.)
• -Wno-aggressive-loop-optimizations — do not warn if the compiler detects undefined behavior in a loop with a constant number of iterations. -Waggressive-loop-optimizations is enabled by default.
• -Wno-attributes — do not warn if an unexpected __attribute__ is used, like unrecognized attributes, function attributes applied to variables, etc. This does not stop errors for incorrect use of supported attributes.

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

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

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

You can also disable warning about all attributes in a namespace using -Wno-attributes=vendor::, which prevents the warning about both of these declarations:

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

Heads up bestie: -Wno-attributes= does not imply -Wno-attributes. They're different things, okurrr.

• -Wno-builtin-declaration-mismatch — warns if a built-in function is declared with an incompatible signature or as a non-function, or when a built-in function declared with a type that doesn't include a prototype gets called with arguments whose promoted types don't match what the function expects. When -Wextra is specified, it also warns when a built-in function that takes arguments is declared without a prototype. The -Wbuiltin-declaration-mismatch warning is enabled by default. To avoid the warning, include the appropriate header to bring the prototypes of built-in functions into scope.

For example, this call to memset gets diagnosed by the warning because the function expects a value of type size_t as its argument but the type of 32 is int. With -Wextra, the declaration of the function is diagnosed too:

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

• -Wno-builtin-macro-redefined — do not warn if certain built-in macros are redefined. This suppresses warnings for redefinition of __TIMESTAMP__, __TIME__, __DATE__, __FILE__, and __BASE_FILE__.
• -Wkeyword-macro — warns if a keyword is defined as a macro or undefined. For C++, identifiers with special meaning or standard attribute identifiers are diagnosed as well. This warning is enabled by default for C++26 if -Wpedantic, and emits a pedwarn in that case.
• -Wfree-labels (C and Objective-C only) — warns if a label is applied to a non-statement, or occurs at the end of a compound statement. Such labels are allowed by C23 and later dialects of C, and are available as a GCC extension in all other dialects.

This warning is also enabled by -Wc11-c23-compat. It gets turned into an error if you're building for a C version before C23 by -pedantic-errors.

• -Wheader-guard — warns if a valid preprocessor header multiple inclusion guard has a #define directive right after #ifndef or #if !defined directive for the multiple inclusion guard, which defines a different macro from the guard macro with a similar name, the actual multiple inclusion guard macro isn't defined at the corresponding #ifndef directive at the end of the header, and the #define directive defines an object-like macro with empty definition. In such a case, it often is just a misspelled guard name — either in the #ifndef or #if !defined directive or in the subsequent #define directive. This warning is enabled by -Wall.
• -Wstrict-prototypes (C and Objective-C only) — warns if a function is declared or defined without specifying the argument types. (An old-style function definition is permitted without a warning if preceded by a declaration that specifies the argument types. So there's a loophole, iykyk.)
• -Wold-style-declaration (C and Objective-C only) — warns for obsolescent usages, according to the C Standard, in a declaration. For example, warns if storage-class specifiers like static are not the first things in a declaration. This warning is also enabled by -Wextra.
• -Wold-style-definition (C and Objective-C only) — warns if an old-style function definition is used. A warning is given even if there's a previous prototype. A definition using () is not considered an old-style definition in C23 mode, because it's equivalent to (void) in that case — but it is considered an old-style definition for older standards.
• -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 warning is 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 is encountered after such a declaration. Both cases are errors in C23 and later dialects of C. 🚨

This warning is also enabled by -Wc11-c23-compat.

• -Wmissing-parameter-name (C and Objective-C only) — warns if a function definition omits a parameter name, specifying only its type. This can be used to document that a parameter is unused in the definition. It's part of C23 and later dialects of C, and available as a GCC extension in all other dialects.

This warning is also enabled by -Wc11-c23-compat. It gets turned into an error if building for a C version before C23 by -pedantic-errors.

• -Wmissing-parameter-type (C and Objective-C only) — a function parameter is declared without a type specifier in K&R-style functions:

void foo(bar) { }

This warning is also enabled by -Wextra.

• -Wno-declaration-missing-parameter-type (C and Objective-C only) — do not warn if a function declaration contains a parameter name without a type. Such function declarations do not provide a function prototype and prevent most type checking in function calls.

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

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

• -Wmissing-prototypes (C and Objective-C only) — warns if a global function is defined without a previous prototype declaration. This warning is issued even if the definition itself provides a prototype. Use this option to detect global functions that don't have a matching prototype declaration in a header file. This option is not valid for C++ because all function declarations provide prototypes, and a non-matching declaration declares an overload rather than conflicting with an earlier declaration. Use -Wmissing-declarations to detect missing declarations in C++.
• -Wmissing-variable-declarations (C and Objective-C only) — warns if a global variable is defined without a previous declaration. Use this option to detect global variables that don't have a matching extern declaration in a header file.
• -Wmissing-declarations — warns if a global function is defined without a previous declaration. Does so even if the definition itself provides a prototype. Use this option to detect 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 to detect missing prototypes. In C++, no warnings are issued for function templates, or for inline functions, or for functions in anonymous namespaces.
• -Wmissing-field-initializers — warns if a structure's initializer has some fields missing. For example, the following code causes such a warning, because x.h is implicitly zero:

struct s { int f, g, h; };
struct s x = { 3, 4 };

In C this option does not warn about designated initializers, so this modification doesn't trigger a warning:

struct s { int f, g, h; };
struct s x = { .f = 3, .g = 4 };

In C this option does not warn about the universal zero initializer { 0 }:

struct s { int f, g, h; };
struct s x = { 0 };

Likewise, in C++ this option does not warn about the empty { } initializer, for example:

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

This warning is included in -Wextra. To get 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 the user meant to write:

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

so satisfied is only true if concept C is satisfied for type T.

This warning can be disabled with -Wno-missing-requires.

• -Wno-missing-template-keyword — the member access tokens ., -> and :: must be followed by the template keyword if the parent object is dependent and the member being named is a template.

template <class X>
void DoStuff (X x)
{
  x.template DoSomeOtherStuff<X>(); // Good.
  x.DoMoreStuff<X>(); // Warning, x is dependent.
}

In rare cases it's possible to get false positives. To silence this, wrap the expression in parentheses. For example, the following is treated as a template, even where m and N are integers:

void NotATemplate (my_class t)
{
  int N = 5;

  bool test = t.m < N > (0); // Treated as a template.
  test = (t.m < N) > (0); // Same meaning, but not treated as a template.
}

This warning can be disabled with -Wno-missing-template-keyword.

• -Wno-multichar — hush the warning that fires when you use a multicharacter constant like 'FOOF'. Bestie, why is that a warning at all? Because those things have implementation-defined values — meaning every compiler gets to pick what they mean 😭 — so nine times out of ten they're just a typo you left in your code, and they should absolutely not be in anything portable. Periodt.
• -Wnormalized=[none|id|nfc|nfkc] — okay so in ISO C and ISO C++, two identifiers are different if they're different sequences of characters. But plot twist 🫠 — when you use characters outside the basic ASCII set, you can end up with two completely different character sequences that look exactly the same. Chaos. To stop the confusion, the ISO 10646 standard lays out normalization rules that, when applied, turn sequences that look the same into the same sequence. GCC can warn you if your identifiers aren't normalized, and this option is the remote control for that warning. 📺

Four levels, bestie, and here they are:

• The default is -Wnormalized=nfc, which warns about any identifier that isn't in the ISO 10646 "C" normalized form, NFC. NFC is the recommended form for most uses, and it's equivalent to just plain -Wnormalized.
• Unfortunately 🙈, there are some characters that ISO C and ISO C++ allow in identifiers which, once turned into NFC, are not allowed in identifiers. Like, there is literally no way to use these symbols in portable ISO C or C++ and have all your identifiers in NFC. -Wnormalized=id suppresses the warning for those characters. The hope is that future versions of the relevant standards will fix this, which is exactly why this option isn't the default.
• You can switch the warning off for all characters by writing -Wnormalized=none or -Wno-normalized. Only do this if you're using some other normalization scheme (like "D"), because otherwise you can easily create bugs that are literally impossible to see. 💀 No cap.
• Some characters in ISO 10646 have distinct meanings but look identical in some fonts or display methodologies, especially once formatting has been applied. For instance, "\u207F", "SUPERSCRIPT LATIN SMALL LETTER N", displays just like a regular "n" that's been placed in a superscript. ISO 10646 defines the NFKC normalization scheme to convert all these 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 that contains the letter O because it might be confused with the digit 0, so it is not the default — but it may be useful as a local coding convention if the programming environment 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. You can use -Wno-attribute-warning to disable the warning, or -Wno-error=attribute-warning to disable the error when you're compiling with the -Werror flag. 🎀
• -Wno-deprecated — don't warn about usage of deprecated features. And a lil C++ plot twist: explicitly specifying -Wdeprecated also enables warnings about some features that are deprecated in later language standards — specifically -Wcomma-subscript, -Wvolatile, -Wdeprecated-enum-float-conversion, -Wdeprecated-enum-enum-conversion, -Wdeprecated-literal-operator, and -Wdeprecated-variadic-comma-omission. that's a whole list, she's thorough 💅
• -Wno-deprecated-declarations — don't warn about uses of functions, variables, or types that are marked as deprecated using 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. yes, the flag name is -Wno-odr and it's talking about enabling a warning — that's just how this one is, iykyk 🦋
• -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 that the user set. If you want to relax the cost model, the -fsimd-cost-model=unlimited option can be used. ✨
• -Woverride-init (C and Objective-C only) — warn if an initialized field without side effects is overridden when using designated initializers. This warning is included in -Wextra, so to get all 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 packed attribute has no effect on the layout or size of the structure. Such structures may end up mis-aligned for little to no benefit 😭. For instance, in this code, the variable f.x in struct bar is misaligned even though struct bar doesn't itself have the packed attribute:

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

• -Wnopacked-bitfield-compat — the 4.1, 4.2 and 4.3 series of GCC ignore the "packed" attribute on bit-fields of type "char". That got fixed in GCC 4.4, but the change can lead to differences in the structure layout. GCC informs you when the offset of such a field has changed in GCC 4.4. For example, there's no longer a 4-bit padding between field a and b in this structure:

        struct foo
        {
          char a:4;
          char b:8;
        } __attribute__ ((packed));

This warning is enabled by default. Use -Wno-packed-bitfield-compat to disable 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 will be issued on struct S — something 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 it's there to align an element of the structure or to align the whole structure. Sometimes when this happens you can rearrange the fields of the structure to reduce the padding and make the structure smaller. free optimization, bestie 🫶
• -Wredundant-decls — warn if anything is declared more than once in the same scope, even in cases where 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. For example, the call to the "strcpy" function below attempts 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 is diagnosed. 👑

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

The -Wrestrict option detects some instances of simple overlap even without optimization, but it works best at -O2 and above. It is included in -Wall.

• -Wnested-externs (C and Objective-C only) — warn if an "extern" declaration is encountered within a function.
• -Winline — warn if a function that's declared as inline can't actually be inlined. Even with this option, the compiler doesn't warn about failures to inline functions declared in system headers. The compiler uses a whole variety of heuristics to decide whether or not 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 the source program can make the warnings produced by -Winline appear or disappear. vibe check on your inline functions, fr fr
• -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 intended to be used for controlling class layout, to avoid false sharing in concurrent code:

        struct independent_fields {
          alignas(std::hardware_destructive_interference_size)
            std::atomic<int> one;
          alignas(std::hardware_destructive_interference_size)
            std::atomic<int> two;
        };

Here one and two are intended to be far enough apart that stores to one won't require accesses to the other to reload the cache line.

By default, these variables are given values based on the current -mtune option — typically the L1 cache line size for the particular target CPU, sometimes a range if you're tuning for a generic target. So all translation units that depend on ABI compatibility for the use of these variables must be compiled with the same -mtune (or -mcpu).

If ABI stability is important — 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 that 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 boolean values are expected, like conditional expressions (?:) using non-boolean integer constants in a boolean context, such as if (a <= b ? 2 : 3). Or left shifting of 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. the way that 1 << a in a for loop is just… a cry for help 😭
• -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 up the warnings you get when you cast a pointer into an integer type that's a different size. Like, we see you, we're not gonna yell about it.
• -Winvalid-pch — warns you if there's a precompiled header chillin' in the search path but it's simply ✨unusable✨. it's giving "found it but can't use it" 😭
• -Winvalid-utf8 — warns if it spots an invalid UTF-8 character. lowkey this warning is on by default for C++23 when you're using -finput-charset=UTF-8, and it gets upgraded into an actual error with -pedantic-errors. no pressure or anything 💅
• -Wno-unicode — tells the compiler not to diagnose invalid forms of delimited or named escape sequences that get treated as separate tokens. Wunicode is enabled by default, so this is the off-switch, iykyk.
• -Wlong-long — warns if the long long type shows up in your code. it's enabled by either -Wpedantic or -Wtraditional in ISO C90 and C++98 modes. wanna silence it? use -Wno-long-long.

fr fr, this warning gets upgraded to an error by -pedantic-errors. periodt.

• -Wvariadic-macros — warns if variadic macros are used in ISO C90 mode, or if the GNU alternate syntax is used in ISO C99 mode. enabled by either -Wpedantic or -Wtraditional. to inhibit the warning messages, use -Wno-variadic-macros.
• -Wno-varargs — do not warn upon questionable usage of the macros used to handle variable arguments, like va_start. these warnings are enabled by default, so this is you telling them to chill.
• -Wvector-operation-performance — warns if a vector operation isn't implemented via the SIMD capabilities of the architecture. mainly useful for performance tuning 🏎️. a vector operation can be implemented:
  • "piecewise" — the scalar operation is performed 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. -Wno-vla prevents the -Wpedantic warning of the variable-length array.

heads up bestie: this warning is upgraded to an error by -pedantic-errors.

• *-Wvla-larger-than=byte-size* — if you use this option, the compiler warns for declarations of variable-length arrays whose size is either unbounded, or bounded by an argument that allows the array size to exceed byte-size bytes. it works similarly to how -Walloca-larger-than=byte-size works, but for variable-length arrays.

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

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

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

• -Wno-vla-larger-than — disables -Wvla-larger-than= warnings. this option is 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 the detection of out-of-bounds accesses to such parameters by warnings such as -Warray-bounds.

if the first function declaration uses the VLA form, the bound specified in the array is assumed to be the minimum number of elements expected to be provided in calls to the function and the maximum number of elements accessed by it. failing to provide arguments of sufficient size or accessing more than the maximum number of elements may be diagnosed. 👀

for example, the warning triggers for the following redeclarations because the first one allows an array of any size to be passed to f while the second one specifies that the array argument must have at least n elements. in addition, calling f with the associated VLA bound parameter in excess of the actual VLA bound triggers a warning as well.

void f (int n, int[n]);
// warning: argument 2 previously declared as a VLA
void f (int, int[]);

void g (int n)
{
    if (n > 4)
      return;
    int a[n];
    // warning: access to a by f may be out of bounds
    f (sizeof a, a);
  ...
}

-Wvla-parameter is included in -Wall. the -Warray-parameter option triggers warnings for similar problems involving ordinary array arguments.

• -Wvolatile-register-var — warns if a register variable is declared volatile. the volatile modifier does not inhibit all optimizations that may eliminate reads and/or writes to register variables. this warning is enabled by -Wall.
• -Wno-xor-used-as-pow (C, C++, Objective-C and Objective-C++ only) — disables warnings about uses of ^, the exclusive or operator, where it looks like the code meant exponentiation. specifically, the warning occurs 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. classic mixup 💀

this warning can be silenced by converting one of the operands to hexadecimal as well as by compiling with -Wno-xor-used-as-pow.

• -Wdisabled-optimization — warns if a requested optimization pass is disabled. bestie this warning does not generally mean anything's wrong with your code; it merely indicates that 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. this option is only supported for C and Objective-C. it's implied by -Wall and by -Wpedantic, which can be disabled with -Wno-pointer-sign.

and yep, this warning is upgraded to an error by -pedantic-errors.

• -Wstack-protector — this option is only active when -fstack-protector is active. it warns about functions that are not protected against stack smashing. 🛡️
• -Woverlength-strings — warns about string constants that are longer than the "minimum maximum" length specified in the C standard. modern compilers generally allow string constants that are much longer than the standard's minimum limit, but very portable programs should avoid using longer strings.

the limit applies after string constant concatenation, and does not count the trailing NUL. in C90, the limit was 509 characters; in C99, it was raised to 4095. C++98 does not specify a normative minimum maximum, so we do not diagnose overlength strings in C++.

this option is implied by -Wpedantic, and can be disabled with -Wno-overlength-strings.

• -Wunsuffixed-float-constants (C and Objective-C only) — issues a warning for any floating constant that doesn't have a suffix. when used together with -Wsystem-headers it warns about such constants in system header files. this can be useful when preparing code to use with the FLOAT_CONST_DECIMAL64 pragma from the decimal floating-point extension to C99.
• -Wno-lto-type-mismatch — during link-time optimization, do not 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 has been marked with the designated_init attribute.

---

✨ Options That Control Static Analysis ✨

• -fanalyzer — okay this one is the moment. it enables a static analysis of program flow which 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, rather than a tool for proving program correctness. so like, it's not out here giving you a gold star, it's out here finding your oopsies.

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

enabling this option effectively enables the following warnings:

-Wanalyzer-allocation-size -Wanalyzer-deref-before-check
-Wanalyzer-double-fclose -Wanalyzer-double-free
-Wanalyzer-exposure-through-output-file
-Wanalyzer-exposure-through-uninit-copy
-Wanalyzer-fd-access-mode-mismatch -Wanalyzer-fd-double-close
-Wanalyzer-fd-leak -Wanalyzer-fd-phase-mismatch
-Wanalyzer-fd-type-mismatch -Wanalyzer-fd-use-after-close
-Wanalyzer-fd-use-without-check -Wanalyzer-file-leak
-Wanalyzer-free-of-non-heap -Wanalyzer-imprecise-fp-arithmetic
-Wanalyzer-infinite-loop -Wanalyzer-infinite-recursion
-Wanalyzer-jump-through-null -Wanalyzer-malloc-leak
-Wanalyzer-mismatching-deallocation -Wanalyzer-null-argument
-Wanalyzer-null-dereference -Wanalyzer-out-of-bounds
-Wanalyzer-overlapping-buffers -Wanalyzer-possible-null-argument
-Wanalyzer-possible-null-dereference -Wanalyzer-putenv-of-auto-var
-Wanalyzer-shift-count-negative -Wanalyzer-shift-count-overflow
-Wanalyzer-stale-setjmp-buffer -Wanalyzer-tainted-allocation-size
-Wanalyzer-tainted-array-index -Wanalyzer-tainted-assertion
-Wanalyzer-tainted-divisor -Wanalyzer-tainted-offset
-Wanalyzer-tainted-size -Wanalyzer-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. no support, no analyzer, sorry babe 🎀

• -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 silently stops tracking values of expressions if they exceed an internal limit, and falls back to an imprecise representation for such expressions. The -Wanalyzer-symbol-too-complex option warns if this occurs. 💅

• -Wanalyzer-too-complex — okay so if -fanalyzer is enabled, the analyzer uses various heuristics to attempt to explore the control flow and data flow in the program, but these can be defeated by sufficiently complicated code. 😭

By default, the analysis silently stops if the code is too complicated for the analyzer to fully explore and it reaches an internal limit. The -Wanalyzer-too-complex option warns if this occurs.

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

This diagnostic warns for paths through the code in which a pointer to a buffer is assigned to point at a buffer with a size that is not a multiple of sizeof (*pointer). 🫠

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

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

This diagnostic warns for paths through the code in which a pointer is checked for NULL after it has already been dereferenced, suggesting that the pointer could have been NULL. Such cases suggest that the check for NULL is either redundant, or that it needs to be moved to before the pointer is dereferenced. 🙈

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

This diagnostic is unlikely to be reported when any level of optimization is enabled, as GCC's optimization logic will typically consider such checks for NULL as being redundant, and optimize them away before the analyzer "sees" them. Hence optimization should be disabled when attempting to trigger this diagnostic.

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

This diagnostic warns for paths through the code in which a FILE * can have fclose called on it more than once. 💀

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

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

This diagnostic warns for paths through the code in which a pointer can have a deallocator called on it more than once, either free, or a deallocator referenced by attribute malloc.

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

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

This diagnostic warns for paths through the code in which a security-sensitive value is written to an output file (such as writing a password to a log file). 🚨

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

• -Wanalyzer-exposure-through-uninit-copy — this warning requires both -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 for "infoleaks" - paths through the code in which uninitialized values are copied across a security boundary (such as code within an OS kernel that copies a partially-initialized struct on the stack to user space). 😱

See CWE-200: Exposure of Sensitive Information to an Unauthorized Actor (https://cwe.mitre.org/data/definitions/200.html).

• -Wno-analyzer-fd-access-mode-mismatch — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-fd-access-mode-mismatch to disable it.

This diagnostic warns for paths through code in which a read on a write-only file descriptor is attempted, or vice versa.

This diagnostic also warns for code paths in a which a function with attribute fd_arg_read (N) is called with a file descriptor opened with O_WRONLY at referenced argument N or a function with attribute fd_arg_write (N) is called with a file descriptor opened with O_RDONLY at referenced argument N.

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

This diagnostic warns for paths through code in which a file descriptor can be closed more than once. 💀

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

• -Wno-analyzer-fd-leak — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-fd-leak to disable it.

This diagnostic warns for paths through code in which an open file descriptor is leaked. 🚰

See CWE-775: Missing Release of File Descriptor or Handle after Effective Lifetime (https://cwe.mitre.org/data/definitions/775.html).

• -Wno-analyzer-fd-phase-mismatch — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-fd-phase-mismatch to disable it.

This diagnostic warns for paths through code in which an operation is attempted in the wrong phase of a file descriptor's lifetime. For example, it will warn on attempts to call accept on a stream socket that has not yet had listen successfully called on it. 🫥

See CWE-666: Operation on Resource in Wrong Phase of Lifetime (https://cwe.mitre.org/data/definitions/666.html).

• -Wno-analyzer-fd-type-mismatch — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-fd-type-mismatch to disable it.

This diagnostic warns for paths through code in which an operation is attempted on the wrong type of file descriptor. For example, it will warn on attempts to use socket operations on a file descriptor obtained via open, or when attempting to use a stream socket operation on a datagram socket. 😵‍💫

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

This diagnostic warns for paths through code in which a read or write is called on a closed file descriptor.

This diagnostic also warns for paths through code in which a function with attribute fd_arg (N) or fd_arg_read (N) or fd_arg_write (N) is called with a closed file descriptor at referenced argument N.

• -Wno-analyzer-fd-use-without-check — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-fd-use-without-check to disable it.

This diagnostic warns for paths through code in which a file descriptor is used without being checked for validity. 🎯

This diagnostic also warns for paths through code in which a function with attribute fd_arg (N) or fd_arg_read (N) or fd_arg_write (N) is called with a file descriptor, at referenced argument N, without being checked for validity.

• -Wno-analyzer-file-leak — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-file-leak to disable it.

This diagnostic warns for paths through the code in which a <stdio.h> FILE * stream object is leaked. 🚰

See CWE-775: Missing Release of File Descriptor or Handle after Effective Lifetime (https://cwe.mitre.org/data/definitions/775.html).

• -Wno-analyzer-free-of-non-heap — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-free-of-non-heap to disable it.

This diagnostic warns for paths through the code in which free is called on a non-heap pointer (e.g. an on-stack buffer, or a global). 🚫

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 requires -fanalyzer, which enables it; use -Wno-analyzer-imprecise-fp-arithmetic to disable it.

This diagnostic warns for paths through the code in which floating-point arithmetic is used in locations where precise computation is needed. This diagnostic only warns on use of floating-point operands inside the calculation of an allocation size at the moment. 🔢

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

This diagnostics warns for paths through the code which appear to lead to an infinite loop. 🔁

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 is 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 for 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 be a "no-op" with cpu_state.pc unchanged, and thus using the same value of opcode on all subseqent iterations, leading to an infinite loop. 💀

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

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

This one snitches on code paths that just look like they're gonna recurse forever.

Like, specifically? When the analyzer sees a recursive call, it compares the state of memory at the entry to the new frame against the state at the entry to the previous frame of that function on the stack. If <em>nothing</em> in memory looks like it's changing → warning issued. Any change it spots in parameters or globals is assumed to mean the recursion will eventually terminate, so that shuts the warning up.

Heads up bestie: this diagnostic is likely to <strong>miss</strong> cases of infinite recursion that get converted to iteration by the optimizer before the analyzer ever "sees" them. So optimization should be <strong>disabled</strong> when you're trying to trigger this diagnostic. 💅

Compare with -Winfinite-recursion, which gives you a similar diagnostic but is implemented in a different way. Same vibe, different plumbing.

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 about paths through the code where a "NULL" function pointer gets called. Yeah. Calling nothing. Iconic of your code, not of you. 💀

• -Wno-analyzer-malloc-leak — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-malloc-leak to disable it.

Warns on paths through the code where a pointer allocated via an allocator gets leaked — that's either "malloc", or any function marked with the attribute "malloc".

See CWE-401: Missing Release of Memory after Effective Lifetime ("https://cwe.mitre.org/data/definitions/401.html").

• -Wno-analyzer-mismatching-deallocation — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-mismatching-deallocation to disable it.

Warns on paths through the code where the <strong>wrong deallocation function</strong> gets called on a pointer value, based on which function was used to allocate that pointer value in the first place. The diagnostic will warn about mismatches between "free", scalar "delete" and vector "delete[]", and those marked as allocator/deallocator pairs using the attribute "malloc".

See CWE-762: Mismatched Memory Management Routines ("https://cwe.mitre.org/data/definitions/762.html").

• -Wno-analyzer-out-of-bounds — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-out-of-bounds to disable it.

Warns on paths through the code where a buffer is <em>definitely</em> read or written out-of-bounds. It applies to cases where the analyzer can work out a constant offset, and to accesses past the end of a buffer that also has a constant capacity. On top of that, the diagnostic does <em>limited</em> checking for accesses past the end when both the offset and the capacity are symbolic.

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

And when the analyzer can, it drops a whole text-art diagram showing the spatial relationship between the memory region it predicts would be accessed versus the range of memory that's actually valid to access: whether they overlap, are touching, are close or far apart; which one comes before or after in memory; the relative sizes involved; the direction of the access (read vs write); and, in some cases, the values of the data involved. You can suppress that diagram with -fdiagnostics-text-art-charset=none. 🎀

• -Wno-analyzer-overlapping-buffers — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-overlapping-buffers to disable it.

Warns on paths through the code where overlapping buffers get passed to an API whose behaviour on such buffers is undefined.</strong>

Specifically, the diagnostic fires on calls to these functions:

• "memcpy"
• "strcat"
• "strcpy"

for cases where the buffers are known to overlap. Ma'am, those buffers are kissing, that is <em>not</em> allowed. 🫶

• -Wno-analyzer-possible-null-argument — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-possible-null-argument to disable it.

Warns on paths through the code where a possibly-NULL value gets 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.

Warns on paths through the code 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.

Warns on paths through the code where a value <em>known to be NULL</em> gets passed to a function argument marked with "__attribute__((nonnull))" as requiring a non-NULL value.

See CWE-476: NULL Pointer Dereference ("https://cwe.mitre.org/data/definitions/476.html").

• -Wno-analyzer-null-dereference — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-null-dereference to disable it.

Warns on paths through the code where a value <em>known to be NULL</em> is dereferenced. Straight up dereferencing nothing, no cap.

See CWE-476: NULL Pointer Dereference ("https://cwe.mitre.org/data/definitions/476.html").

• -Wno-analyzer-putenv-of-auto-var — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-putenv-of-auto-var to disable it.

Warns on paths through the code where a call to "putenv" gets passed a pointer to an automatic variable or an on-stack buffer. That stack memory is <em>leaving</em>, bestie.

See POS34-C. Do not call putenv() with a pointer to an automatic variable as the argument ("https://wiki.sei.cmu.edu/confluence/x/6NYxBQ").

• -Wno-analyzer-shift-count-negative — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-shift-count-negative to disable it.

Warns on paths through the code where a shift is attempted with a negative count. It's analogous to the -Wshift-count-negative diagnostic implemented in the C/C++ front ends, but it's implemented by analyzing interprocedural paths instead of just parsing the syntax tree. However, the analyzer doesn't prioritise detection of such paths, so false negatives are more likely relative to other warnings. It's giving "I could catch that but I'm busy". 🙈

• -Wno-analyzer-shift-count-overflow — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-shift-count-overflow to disable it.

Warns on paths through the code where a shift is attempted with a count greater than or equal to the precision of the operand's type. It's analogous to the -Wshift-count-overflow diagnostic implemented in the C/C++ front ends, but again, it's implemented by analyzing interprocedural paths rather than merely parsing the syntax tree. And again, the analyzer doesn't prioritise 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.

Warns on paths through the code where "longjmp" is called to rewind to a "jmp_buf" relating to a "setjmp" call in a function that has already returned.

Here's the tea: when "setjmp" is called on a "jmp_buf" to record a rewind location, it records the stack frame. That stack frame becomes <strong>invalid</strong> when the function containing the "setjmp" call returns. Trying to rewind to it via "longjmp" would reference a stack frame that no longer exists, and that likely leads to a crash (or <em>worse</em>). 😭

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

Warns on paths through the code 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. They said "let me just allocate 4 billion bytes" and the process said okay. 💀

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.

Warns on paths through the code 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 the 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 even gets a chance to "see" them, so this diagnostic can only generate warnings on builds in which assertion-checking is <strong>enabled</strong>. 🌸

For the purpose of this warning, any function marked with the attribute "noreturn" is considered 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.

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 tho, technically speaking, that's not an assertion failure. like, strictly. 🙈

---

• -Wno-analyzer-tainted-array-index — bestie this one needs -fanalyzer to even turn on, so use -Wno-analyzer-tainted-array-index to shut it back off.

It yells at you about code paths where a value that an attacker could be controlling gets used as an array index without being sanitized first — meaning somebody could sneak an out-of-bounds access right in. 😭

See CWE-129: Improper Validation of Array Index (https://cwe.mitre.org/data/definitions/129.html).

• -Wno-analyzer-tainted-divisor — needs -fanalyzer to be enabled; use -Wno-analyzer-tainted-divisor to disable it.

Warns on paths where an attacker-controllable value is used as the divisor in a division or modulus op without sanitizing, so an attacker could inject a divide-by-zero. 💀

See CWE-369: Divide By Zero (https://cwe.mitre.org/data/definitions/369.html).

• -Wno-analyzer-tainted-offset — needs -fanalyzer on; use -Wno-analyzer-tainted-offset to turn it off.

Warns on paths where an attacker-controllable value is used as a pointer offset without being sanitized — so an attacker could inject an out-of-bounds access. 🫠

See CWE-823: Use of Out-of-range Pointer Offset (https://cwe.mitre.org/data/definitions/823.html).

• -Wno-analyzer-tainted-size — needs -fanalyzer on; use -Wno-analyzer-tainted-size to disable it.

Warns on paths where an attacker-controllable value is used as the size of an operation like memset without sanitizing — attacker could inject an out-of-bounds access, no cap.

See CWE-129: Improper Validation of Array Index (https://cwe.mitre.org/data/definitions/129.html).

• -Wno-analyzer-throw-of-unexpected-type — needs -fanalyzer on; use -Wno-analyzer-throw-of-unexpected-type to disable it. Heads up: dynamic exception specifications are only a thing in C++14 and earlier.

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

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

Warns on paths where a pointer subtraction happens between pointers that refer to different chunks of memory. That code is leaning on undefined behavior, because pointer subtraction is only defined when both pointers point within (or just after) the same array. periodt.

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 on 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. 😭 iconic but illegal.

• -Wno-analyzer-unsafe-call-within-signal-handler — needs -fanalyzer, which enables it; use -Wno-analyzer-unsafe-call-within-signal-handler to disable it.

Warns on paths where a function known to be async-signal-unsafe (like fprintf) gets called from a signal handler. 💅

See CWE-479: Signal Handler Use of a Non-reentrant Function (https://cwe.mitre.org/data/definitions/479.html).

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

Warns on paths where a pointer gets used after a deallocator was called on it — either free, or a deallocator referenced by attribute malloc.

See CWE-416: Use After Free (https://cwe.mitre.org/data/definitions/416.html).

• -Wno-analyzer-use-of-pointer-in-stale-stack-frame — needs -fanalyzer, which enables it; use -Wno-analyzer-use-of-pointer-in-stale-stack-frame to disable it.

Warns on paths where a pointer is dereferenced that points to a variable sitting 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 on interprocedural paths where the analyzer spots an attempt to use va_arg to extract a value passed to a variadic call, but with a type that doesn't match the type of the expression passed to the call. 🙈

See CWE-686: Function Call With Incorrect Argument Type (https://cwe.mitre.org/data/definitions/686.html).

• -Wno-analyzer-va-list-exhausted — needs -fanalyzer, which enables it; use -Wno-analyzer-va-list-exhausted to disable it.

Warns on interprocedural paths where the analyzer spots an attempt to use va_arg to access the next value passed to a variadic call, except all of the values in the va_list have already been consumed. empty. gone. nothing left. 💀

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 on interprocedural paths where the analyzer spots that va_start or va_copy was called on a va_list without a matching 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 on interprocedural paths where the analyzer spots 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 on paths where the analyzer spots an attempt to write through a pointer to a const object. However, the analyzer doesn't prioritize detecting these paths, so false negatives are more likely compared to other warnings. ngl, it's giving "we'll get to it."

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

Warns on paths where the analyzer spots an attempt to write through a pointer to a string literal. However, again, the analyzer doesn't prioritize detection of such paths, so false negatives are more likely relative to other warnings. lowkey relatable.

• -Wno-analyzer-use-of-uninitialized-value — needs -fanalyzer, which enables it; use -Wno-analyzer-use-of-uninitialized-value to disable it.

Warns on paths where an uninitialized value gets used. 😭

See CWE-457: Use of Uninitialized Variable (https://cwe.mitre.org/data/definitions/457.html).

---

The analyzer has hardcoded knowledge about the behavior of the following memory-management functions:

• alloca
• The built-in functions __builtin_alloc, __builtin_alloc_with_align, __builtin_calloc, __builtin_free, __builtin_malloc, __builtin_memcpy, __builtin_memcpy_chk, __builtin_memset, __builtin_memset_chk, __builtin_realloc, __builtin_stack_restore, and __builtin_stack_save
• calloc
• free
• malloc
• memset
• operator delete
• operator delete []
• operator new
• operator new []
• realloc
• strdup
• strndup

of the following 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

of the following 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 the following functions:

• 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

Alsooo, various functions with an __analyzer_ prefix have special meaning to the analyzer — those are described over in the GCC Internals manual. And various internal parameters, settable via --param, control the exploration; those are also documented in the GCC Internals manual. iykyk 📖✨

The following options control the analyzer.

• -fanalyzer-assume-nothrow — okay so by default, if -fexceptions is on, the analyzer is gonna assume that any call to a function without the "nothrow" attribute could throw an exception. That's actually kinda slay, because it helps catch execution paths that leak when exceptions yeet past your clean-up code 🫶 — but it can also give you false positives when you're using headers where the author never bothered adding the "nothrow" attribute. Rude of them, honestly. So if you turn on -fanalyzer-assume-nothrow, the analyzer assumes external functions just... don't throw. Lowkey useful for cutting down "noise" when you've enabled -fexceptions on C code, but bestie, it could also hide real issues if something your C code calls does raise an exception. Trade-offs, periodt.
• -fanalyzer-call-summaries — simplifies interprocedural analysis by computing the effect of certain calls instead of exploring every path through the function from callsite to each possible return. Exhausting, right? When enabled, call summaries are only used for functions with more than one call site, and that are sufficiently complicated. So like, only the messy ones get the summary treatment ✨
• *-fanalyzer-checker=name* — restrict the analyzer to run just the named checker, and enable it. One checker. Main character energy only 👑
• -fanalyzer-debug-text-art-headings — this one's for analyzer developers. If you enable it, the analyzer adds extra annotations to any diagrams it generates. Fancy little labels, we love that 🎀
• -fno-analyzer-feasibility — analyzer developers only, iykyk. By default the analyzer verifies that there's a feasible control flow path for each diagnostic it emits: meaning the conditions that hold aren't mutually exclusive. Diagnostics where no feasible path can be found? Rejected. This filtering can be suppressed with -fno-analyzer-feasibility, for debugging issues in this code.
• -fanalyzer-fine-grained — does nothing. Preserved for backward compatibility. It's giving "still here, still unemployed" 💀
• -fanalyzer-show-duplicate-count — analyzer developers only: if multiple diagnostics got detected as duplicates of each other, it emits a note when reporting the best diagnostic, telling you the number of additional diagnostics that were suppressed by the deduplication logic. Receipts! 🧾
• -fanalyzer-show-events-in-system-headers — by default the analyzer emits simplified diagnostics paths by hiding events that are fully located within a system header. With -fanalyzer-show-events-in-system-headers those events are no longer suppressed. Nothing hidden, full transparency 🌈
• -fno-analyzer-simplify-supergraph — analyzer developers only. By default, the analyzer performs various simplifications to the program supergraph before analyzing it. With -fno-analyzer-simplify-supergraph that simplification can be suppressed, for debugging issues with it.
• -fno-analyzer-state-merge — analyzer developers only. By default the analyzer tries to simplify analysis by merging sufficiently similar states at each program point as it builds its "exploded graph". With -fno-analyzer-state-merge that merging can be suppressed, for debugging state-handling issues.
• -fno-analyzer-state-purge — analyzer developers only. By default the analyzer tries 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 that aren't relevant to leak analysis. With -fno-analyzer-state-purge that purging of state can be suppressed, for debugging state-handling issues.
• -fno-analyzer-suppress-followups — analyzer developers only. By default the analyzer stops exploring an execution path after it hits certain diagnostics, so it doesn't accidentally drop a whole cascade of follow-up diagnostics on you. 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 keeps exploring such paths even after those diagnostics, which may be helpful for debugging issues in the analyzer, or for microbenchmarks for detecting undefined behavior. Chaos mode, but make it intentional 🔥

• -fanalyzer-transitivity — enables transitivity of constraints within the analyzer. Chain reactions, but make it math 💅
• -fno-analyzer-undo-inlining — analyzer developers only. So here's the tea: -fanalyzer runs relatively late compared to other code analysis tools, and some optimizations have already been applied to the code by then. In particular, function inlining may have occurred, which leads to the interprocedural execution paths the analyzer emits containing function frames that don't correspond to the ones in the original source code. By default the analyzer tries to reconstruct the original function frames, and to emit events showing the inlined calls. With -fno-analyzer-undo-inlining that attempt to reconstruct the original frame information can be disabled, which may help when debugging issues in the analyzer.
• -fanalyzer-verbose-edges — analyzer developers only. It enables more verbose, lower-level detail in the descriptions of control flow within diagnostic paths. Every edge, described. Thorough 💖
• -fanalyzer-verbose-state-changes — analyzer developers only. It enables more verbose, lower-level detail in the descriptions of events relating to state machines within diagnostic paths.
• *-fanalyzer-verbosity=level* — this controls how complicated the control flow paths emitted for analyzer diagnostics get. 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 the significant ones.
  • 4 — this level is 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. Note: -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. Aesthetically pleasing AND informative 🌸
• -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. Not me flooding your directory 😭
• -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. Round and round and round 🔄
• -fdump-analyzer-json — dumps a compressed JSON representation of analyzer internals to file.analyzer.json.gz. The precise format is subject to change. So don't get attached 💀
• -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 ✨

So, bestie, if you wanna tell GCC to emit extra information for use by a debugger, in almost all cases you literally just need to add -g to your other options. Simple. Iconic. 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. They're chill like that 🦋

GCC allows you to 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. It's giving haunted house, but honestly? It's still possible to debug optimized output. Which makes it reasonable to use the optimizer for programs that might have bugs. Live your life 💅

If you're 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 don't run at all, so -Og may result in a better debugging experience. Yes, the compiler is doing the most for you, say thank you 🙏

• -g, --debug — produce debugging information in the operating system's native format (stabs, COFF, XCOFF, or DWARF). GDB can work with this debugging information. Bestie and debugger, together at last 🫶

On most systems that use stabs format, -g turns on extra debugging info that only GDB can use 💅 — and bestie, that extra info makes GDB debugging go off, but it'll probably make other debuggers crash or flat-out refuse to read your program 😭. If you wanna be 100% sure about whether that extra info gets generated, use -gvms instead (it's down below, keep scrolling 👇).

• -ggdb — produces debugging info specifically for GDB to snack on 🍓. This means it uses the most expressive format available (DWARF, stabs, or the native format if neither of those are supported), and it includes GDB extensions if that's possible at all. Main character debugging energy, periodt.
• -gdwarf, -gdwarf-version — produces debugging info in DWARF format (if that's supported). The value of version can be 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. Not me remembering all those exceptions 💀.

Heads up: with DWARF Version 2, some ports require and always use some non-conflicting DWARF 3 extensions in the unwind tables.

Version 4 might need GDB 7.0 and -fvar-tracking-assignments for maximum benefit. Version 5 needs GDB 8.0 or higher.

GCC no longer supports DWARF Version 1, which is substantially different from Version 2 and later. For historical reasons, some other DWARF-related options — like -fno-dwarf2-cfi-asm — still have a reference to DWARF Version 2 in their names, but fr fr they apply to all currently-supported versions of DWARF.

• -gbtf — requests BTF debug info. BTF is the default debugging format for the eBPF target. On other targets, like x86, BTF debug info can be generated along with DWARF debug info when both debug formats are enabled explicitly via their respective command line options. Two formats, one slay ✨.
• -gprune-btf, -gno-prune-btf — prunes BTF information before emission. When pruning, only type info for types used by global variables and file-scope functions gets emitted. If you're compiling for the BPF target with BPF CO-RE enabled, type info is also emitted for types used in BPF CO-RE relocations. Also: 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 the produced BTF information, but at a significant loss in the amount of detailed type info 😬. It's primarily useful when compiling for the BPF target — to minimize the size of the resulting object, and to eliminate BTF info which isn't immediately relevant to the BPF program loading process.

This option is enabled by default for the BPF target when generating BTF info.

• -gctf, -gctflevel — requests CTF debug info and uses level to specify how much CTF debug info should be produced. If -gctf is specified without a value for level, the default level of CTF debug info is 2.

CTF debug info can be generated along with DWARF debug info when both debug formats are enabled explicitly via their respective command line options.

• Level 0 produces no CTF debug info at all. So -gctf0 negates -gctf.
• Level 1 produces CTF info for tracebacks only. This includes callsite info, but does not include type info.
• Level 2 produces type info for entities (functions, data objects etc.) at file-scope or global-scope only.

• -gvms — produces debugging info in Alpha/VMS debug format (if that's supported). This is the format used by DEBUG on Alpha/VMS systems.
• -gcodeview — produces debugging info in CodeView debug format (if that's supported). This is the format used by Microsoft Visual C++ on Windows. Windows girlies, this one's for you 🫶.
• -glevel, -ggdblevel, -gvmslevel — requests debugging info and also uses level to specify how much info. The default level is 2.
  • Level 0 produces no debug info at all. So -g0 negates -g.
  • Level 1 produces minimal info — enough for making backtraces in parts of the program you don't plan to debug. This includes descriptions of functions and external variables, and line number tables, but no info about local variables.
  • Level 3 includes extra info, like 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's effective. Last one standing wins, no cap 👑.

-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 info is produced for symbols that aren't actually used. Use this option if you want debug info for all symbols.
• -femit-class-debug-always — instead of emitting debugging info for a C++ class in only one object file, emit it in all object files using the class. Only use this with debuggers that can't handle the way GCC normally emits debugging info for classes, because using this option increases the size of the debug info by as much as a factor of two 😳.
• -fno-merge-debug-strings — tells the linker not to merge together strings in the debugging info that are identical across different object files. Merging isn't supported by all assemblers or linkers. Merging decreases the size of the debug info 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 info 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 it 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 — runs the variable tracking pass. It computes where variables are stored at each position in code. Better debugging info is then generated (if the debugging info format supports this info). It's enabled by default when compiling with optimization (-Os, -O, -O2, ...), debugging info (-g) and the debug info format supports it.
• -fvar-tracking-assignments — annotates assignments to user variables early in the compilation and attempts to carry the annotations over throughout the compilation all the way to the end, in an attempt to improve debug info while optimizing. Using -gdwarf-4 along with it is recommended. 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 — performs variable tracking and also marks uninitialized variables in the debug info. 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 info is enabled, separate as much debugging info as possible into a separate output file with the extension .dwo. This option lets the build system avoid linking files with debug info. To be useful, this option requires a debugger capable of reading .dwo files.
• -gdwarf32, -gdwarf64 — if DWARF debugging info is enabled, -gdwarf32 selects the 32-bit DWARF format and -gdwarf64 selects the 64-bit DWARF format. The default is target specific — on most targets it's -gdwarf32 though. The 32-bit DWARF format is smaller, but can't support more than 2GiB of debug info in any of the DWARF debug info sections. The 64-bit DWARF format allows larger debug info and might not be well supported by all consumers yet.
• -gdescribe-dies — adds description attributes to some DWARF DIEs that have no name attribute, like artificial variables, external references and call site parameter DIEs.
• -gpubnames — generates DWARF ".debug_pubnames" and ".debug_pubtypes" sections.
• -ggnu-pubnames — generates ".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 being part of the ".debug_info" section. It's 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 info 🙈.
• -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 info. The options are concatenated with spaces separating them from each other and from the compiler version. It's enabled by default. See also -frecord-gcc-switches for another way of storing compiler options into the object file.
• -gstrict-dwarf — disallows using extensions of a later DWARF standard version than the one selected with -gdwarf-version. On most targets, using non-conflicting DWARF extensions from later standard versions is allowed.
• -gno-strict-dwarf — allows using extensions of a later DWARF standard version than the one selected with -gdwarf-version.
• -gas-loc-support — tells the compiler "bestie, the assembler gets .loc directives 💅". Once it knows that, it can hand off line-number table generation to the assembler for DWARF2+ tables.

This is generally the move, because assembler-made line-number tables are so much more compact than the ones the compiler cooks up by itself. Enabled by default if, back when GCC was configured, the assembler was found to support these directives. ✨

• -gno-as-loc-support — nope, force GCC to build those DWARF2+ line number tables itself, internally, if DWARF2+ line number tables are getting generated at all.
• -gas-locview-support — inform the compiler that the assembler supports "view" assignment and reset assertion checking inside .loc directives. 🫶 Enabled by default if, at GCC configure time, the assembler was found to support them.
• -gno-as-locview-support — force GCC to assign view numbers internally, if -gvariable-location-views is explicitly requested.
• -gcolumn-info, -gno-column-info — emit location column info into DWARF debugging info, not just file and line. Enabled by default, no cap. 🎀
• -gstatement-frontiers, -gno-statement-frontiers — makes GCC drop markers into its internal representation at the start of statements, keep them roughly in place through the whole compile, and use them to guide the is_stmt markers in the line number table. On by default when you're compiling with optimization (-Os, -O1, -O2, ...) and outputting DWARF 2 debug info at the normal level. 🔥
• -gvariable-location-views, -gvariable-location-views=incompat5, -gno-variable-location-views — augments variable location lists with progressive view numbers implied from the line number table. This lets debug info consumers peek at state at certain points of the program even when no instructions matching those source locations are actually sitting there at that point. 👑

If the assembler doesn't support view numbers in line number tables, this makes the compiler emit the line number table itself, which generally makes them a bit less compact. The augmented line number tables and location lists are fully backward-compatible, so debug info consumers that have never heard of these augmentations can still eat them — they just won't get any of the benefits either. 💀

It's on by default when outputting DWARF 2 debug info at the normal level, as long as there's assembler support, -fvar-tracking-assignments is enabled, and -gstrict-dwarf is not. When assembler support isn't there, it can still be enabled, but it forces GCC to output internal line number tables, and if -ginternal-reset-location-views isn't enabled, that most certainly leads to silently mismatching location views. 😭

There's a proposed representation for view numbers that is not backward compatible with the location list format introduced in DWARF 5, and you turn it on with -gvariable-location-views=incompat5. Heads up: this option may be removed in the future, it's only provided as a reference implementation of the proposed representation. Debug info consumers are not expected to support this extended format, and they'd be rendered unable to decode location lists using it. 🙈

• -ginternal-reset-location-views, -gno-internal-reset-location-views — tries to figure out which location views 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 you're using an assembler that doesn't support location view lists. The GNU assembler flags any such error as a "view number mismatch". Only enabled on ports that define a reliable estimation function. 🧚
• -ginline-points, -gno-inline-points — generate extended debug info for inlined functions. Location view tracking markers get inserted at inlined entry points, so address and view numbers can be computed and output into debug info. This can be enabled independently of location views — in which case the view numbers won't be output — but it can only be enabled alongside statement frontiers, and it's only enabled by default if location views are enabled. 💜
• -gz[=type] — produce compressed debug sections in DWARF format, if that's supported. If type isn't given, the default type depends on what the assembler and linker you're using can do. 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 doesn't support them, -gz is silently ignored when producing object files. 🌈
• -femit-struct-debug-baseonly — emit debug info for struct-like types only when the base name of the compilation source file matches the base name of the file where the struct is defined.

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

This option works only with DWARF debug output.

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

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

This option works only with DWARF debug output.

• -femit-struct-debug-detailed[=spec-list] — specify exactly which struct-like types the compiler generates debug info for. The whole intent is to cut down duplicate struct debug info between different object files in the same program.

This option is the detailed version of -femit-struct-debug-reduced and -femit-struct-debug-baseonly, and honestly it serves most needs. 💅

A specification has the syntax:

[dir:|ind:][ord:|gen:](any|sys|base|none)

The optional first word limits the spec to structs used directly (dir:) or used indirectly (ind:). A struct type is used directly when it's 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 spec to ordinary structs (ord:) or generic structs (gen:). Generic structs are a bit complicated to explain, ngl. 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 info. The values none and any have the normal meaning. The value base means the base of the name of the file where 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 info is generated for types declared in that file and foo.h, but not other header files. The value sys means those types satisfying base or declared in system or compiler headers.

You may need to experiment to determine the best settings for your application. 🔬

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

This option works only with DWARF debug output.

• -fno-dwarf2-cfi-asm — emit DWARF unwind info as a 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's genuinely useful to have GCC emit debug info for all types declared in a compilation unit, regardless of whether they're actually used in that compilation unit — for example if, in the debugger, you want to cast a value to a type that isn't actually used in your program (but is declared). More often, though, this just results in a significant amount of wasted space. 🗑️

---

✨ Options That Control Optimization ✨

These options control various sorts of optimizations. 🎀

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

Turning on optimization flags makes the compiler attempt to improve the performance and/or code size at the expense of compilation time and possibly the ability to debug the program. 😭

The compiler performs optimization based on the knowledge it has of the program. Compiling multiple files at once to a single output file mode allows the compiler to use information gained from all of the files when compiling each of them. 🫶

Not all optimizations are controlled directly by a flag. Only optimizations that have a flag are listed in this section.

Most optimizations are 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.

Depending on the target and how GCC was configured, a slightly different set of optimizations may be enabled at each -O level than those listed here. You can invoke GCC with -Q --help=optimizers to find out the exact set of optimizations that are enabled at each level. 👑

• -O, -O1, --optimize — Optimize. Optimizing compilation takes somewhat more time, and a lot more memory for a large function. 🔥

With -O, the compiler tries to shrink your code size and execution time, without doing any of those optimizations that eat up a ton of compilation time. It's giving sensible middle ground ✨

-O is the recommended optimization level for big machine-generated code — it's the sweet spot between how long you spend compiling and how much memory you burn. Higher levels do optimizations with way more algorithmic complexity than -O does. iykyk.

-O turns on all these optimization flags, bestie:

-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 — the glow-up 💅

Optimize even more. GCC does nearly all the supported optimizations that don't involve a space-speed tradeoff. Compared to -O, this option bumps up both compilation time and the performance of the generated code. It's the tradeoff queen 👑

-O2 turns on all the optimization flags that -O1 specifies. On top of that, it also turns on 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

bestie pls read the warning under -fgcse about running -O2 on programs that use computed gotos. fr fr, don't skip it 🙈

NOTE: In Ubuntu 8.10 and later versions, -D_FORTIFY_SOURCE=2 — and in Ubuntu 24.04 and later versions, -D_FORTIFY_SOURCE=3 — is set by default, and it gets activated when -O is set to 2 or higher. This enables extra compile-time and run-time checks for several libc functions. Want it off? Specify either -U_FORTIFY_SOURCE or -D_FORTIFY_SOURCE=0. 🎀

NOTE: In Debian 13 and Ubuntu 24.04 and later versions, -D_TIME_BITS=64 together with -D_FILE_OFFSET_BITS=64 is set by default on the 32bit architectures armel, armhf, hppa, m68k, mips, mipsel, powerpc and sh4. ✨

---

-O3 — optimize yet more 🔥

-O3 turns on all optimizations specified by -O2 and also turns on these optimization flags:

-fgcse-after-reload -fipa-cp-clone -floop-interchange
-floop-unroll-and-jam -fpeel-loops -fpredictive-commoning
-fsplit-loops -fsplit-paths -ftree-loop-distribution
-ftree-partial-pre -funswitch-loops -fvect-cost-model=dynamic
-fversion-loops-for-strides

---

-O0 — the chill default 🌸

Reduce compilation time and make debugging produce the expected results. This is the default.

At -O0, GCC completely disables most optimization passes. Like, they are not run — even if you explicitly enable them on the command line, or if -Q --help=optimizers lists them as being enabled by default. The way that so many optimizations GCC does depend on code analysis or canonicalization passes that get enabled by -O... and it wouldn't be useful to run individual optimization passes in isolation. periodt.

---

-Os — optimize for size 🎀

-Os enables all -O2 optimizations except the ones that often increase code size:

-falign-functions  -falign-jumps -falign-labels  -falign-loops
-fprefetch-loop-arrays  -freorder-blocks-algorithm=stc

It also enables -finline-functions, makes the compiler tune for code size instead of execution speed, and performs further optimizations designed to reduce code size. It's giving ✨tiny but mighty✨.

---

-Ofast — no rules, just vibes 💀

Disregard strict standards compliance. -Ofast enables all -O3 optimizations. It also enables optimizations that are not valid for all standard-compliant programs. It turns on -ffast-math, -fallow-store-data-races and the Fortran-specific -fstack-arrays — unless -fmax-stack-var-size is specified — and -fno-protect-parens. It turns off -fsemantic-interposition.

lowkey unhinged behavior but okay 💅

---

-Og — optimize, but keep the debugging vibes 🦋

Optimize while keeping in mind the debugging experience. -Og should be the optimization level of choice for the standard edit-compile-debug cycle, offering a reasonable blend of optimization, fast compilation and debugging experience — especially for code with a high abstraction penalty. In contrast to -O0, this enables -fvar-tracking-assignments and -fvar-tracking, which handle debug information in the prologue and epilogue of functions better than -O0 does.

Like -O0, -Og completely skips a number of optimization passes so that individual options controlling them have no effect. Otherwise, -Og enables all -O1 optimization flags except for 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 — aggressively tiny 👑

Optimize aggressively for size rather than speed. This may increase the number of instructions executed if those instructions require fewer bytes to encode. -Oz behaves similarly to -Os, including enabling most -O2 optimizations.

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If you use multiple -O options, with or without level numbers, the last such option is the one that's effective. Last one wins, no cap. 🫶

Options of the form -fflag specify machine-independent flags. Most flags have both positive and negative forms; the negative form of -ffoo is -fno-foo. In the table below, only one of the forms is listed — the one you typically use. You can figure out the other form by either removing no- or adding it. ✨

The following options control specific optimizations. They're either activated by -O options or are related to ones that are. You can use these flags in the rare cases when you want to "fine-tune" the optimizations being performed. 🎀

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the fine-tuning flags 💅

• -fno-defer-pop — For machines that must pop arguments after a function call, always pop the arguments as soon as each function returns. At levels -O1 and higher, -fdefer-pop is the default; this lets the compiler let arguments accumulate on the stack for several function calls and pop them all at once. (Letting them pile up = iconic efficiency 💖)
• -fforward-propagate — Perform a forward propagation pass on RTL. The pass tries to combine two instructions and checks if the result can be simplified. If loop unrolling is active, two passes are performed and the second is scheduled after loop unrolling.

This option is enabled by default at optimization levels -O1, -O2, -O3, -Os.

• -favoid-store-forwarding
• -fno-avoid-store-forwarding — Many CPUs will stall for many cycles when a load partially depends on previous smaller stores. This pass tries to detect such cases and avoid the penalty by changing the order of the load and store and then fixing up the loaded value.

Disabled by default. 🙈

• -ffp-contract=style — -ffp-contract=off disables floating-point expression contraction. -ffp-contract=fast enables floating-point expression contraction such as forming of fused multiply-add operations if the target has native support for them. -ffp-contract=on enables floating-point expression contraction if allowed by the language standard. This is implemented for C and C++, where it enables contraction within one expression, but not across different statements.

The default is -ffp-contract=off for C in a standards compliant mode (-std=c11 or similar), -ffp-contract=fast otherwise.

• -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 these functions 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 be raised if the library implementation does not follow TS 18661. 😭

• -fomit-frame-pointer — Omit the frame pointer in functions that don't need one. This avoids the instructions to save, set up and restore the frame pointer; on many targets it also makes an extra register available. 💅

On some targets this flag has no effect because the standard calling sequence always uses a frame pointer, so it cannot be omitted.

Note that -fno-omit-frame-pointer doesn't guarantee the frame pointer is used in all functions. Several targets always omit the frame pointer in leaf functions.

Enabled by default at -O1 and higher. ⭐

• -foptimize-crc — bestie, this one goes hunting for loops that are literally just doing CRC math (aka polynomial long division, if you're nasty) and swaps them for something way faster. It catches 8, 16, 32, and 64 bit CRC, with a constant polynomial that doesn't have the leading 1 bit, for both bit-forward and bit-reversed cases. And if your target has a CRC instruction and the polynomial in your source matches the polynomial in that instruction? Boom, it generates the CRC instruction. If not, but the target has a carry-less-multiplication instruction, it uses that to build the CRC instead. And if neither? Table-based CRC, we move.

Enabled by default at -O2 and higher. ✨

• -foptimize-sibling-calls — optimizes sibling and tail recursive calls. It's giving ✨ recursion but make it cheap ✨.

Enabled at levels -O2, -O3, -Os.

• -foptimize-strlen — takes a bunch of standard C string functions (like strlen, strchr, or strcpy) and their _FORTIFY_SOURCE counterparts and turns them into faster alternatives. Obsessed.

Enabled at levels -O2, -O3.

• -finline-atomics
• -fno-inline-atomics — inlines __atomic operations whenever a lock-free instruction sequence is available. This optimization is enabled by default, no cap. 💅
• -finline-stringops[=fn] — expands memory and string operations inline (for now, only memset), even when the length is variable or big enough that it would need a loop. Lowkey most useful when you're pairing it with -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 because it uses code caches better. This option is disabled by default. 🫶

• -fno-inline — do not expand any functions inline apart from those marked with the always_inline attribute. This is the default when not optimizing.

Single functions can be exempted from inlining by marking them with the noinline attribute.

• -finline-small-functions — integrates functions into their callers when their body is smaller than expected function call code (so the overall size of the program gets smaller, slay). The compiler heuristically decides which functions are simple enough to be worth integrating this way. This inlining applies to all functions, even those not declared inline.

Enabled at levels -O2, -O3, -Os.

• -findirect-inlining — inlines indirect calls too, when they're discovered to be known at compile time thanks to previous inlining. This option only has any effect when inlining itself is turned on by the -finline-functions or -finline-small-functions options.

Enabled at levels -O2, -O3, -Os.

• -finline-functions — considers 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 — considers 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 — inlines 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 this makes profiling significantly cheaper and usually makes inlining faster on programs that have large chains of nested wrapper functions. It's giving efficiency. 🌸

Enabled by default.

• -fipa-sra — performs interprocedural scalar replacement of aggregates, removal of unused parameters, and replacement of parameters passed by reference with parameters passed by value. Cleanup era fr fr.

Enabled at levels -O2, -O3 and -Os.

• -finline-limit=n — by default, GCC limits the size of functions that can be inlined. This flag gives you coarse control of that 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 to -finline-limit that results in default behavior. 😭

Note: pseudo instruction here means an abstract measurement of a function's size. It does not represent a count of assembly instructions, so 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, emits 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++, it emits any and all inline functions into the object file.
• -fkeep-static-functions — emits static functions into the object file, even if the function is never used. Keeping them around just in case, iconic behavior. 👑
• -fkeep-static-consts — emits 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 — attempts 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 — attempts to merge identical constants and identical variables.

This option implies -fmerge-constants. In addition to -fmerge-constants, it 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. ⚠️ Be careful bestie.

• -fmodulo-sched — performs swing modulo scheduling immediately before the first scheduling pass. This pass looks at innermost loops and reorders their instructions by overlapping different iterations. Loops but make it ✨fancy✨.
• -fmodulo-sched-allow-regmoves — performs 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 — disables 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 — do not put function addresses in registers; make each instruction that calls a constant function contain the function's address explicitly.

This option results in less efficient code, but some strange hacks that alter the assembler output may be confused by the optimizations performed when this option is not used. 🙈

The default is -ffunction-cse

• -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 — performs 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. Following the plot fr fr. 🔥

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, splits the registers apart and allocates 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 splits 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), scans 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 — lowkey the cousin of -fcse-follow-jumps, except this one tells CSE to follow jumps that conditionally skip over blocks. So when CSE runs into a simple "if" statement with no else clause, this flag makes it follow the jump around the body of that "if". 💅

Enabled at levels -O2, -O3, -Os.

• -frerun-cse-after-loop — bestie, it just re-runs common subexpression elimination after the loop optimizations have already done their thing. ✨

Enabled at levels -O2, -O3, -Os.

• -fgcse — performs a global common subexpression elimination pass. And it's not just that — this pass also does global constant and copy propagation. it's giving multitasking queen 👑

Note: when you're compiling a program that uses computed gotos (a GCC extension, iykyk), you may get better run-time performance if you disable the global common subexpression elimination pass by adding -fno-gcse to the command line. fr fr.

Enabled at levels -O2, -O3, -Os.

• -fgcse-lm — when this is enabled, global common subexpression elimination tries to move loads that are only killed by stores into themselves. Which means a loop containing a load/store sequence can be changed to a load outside the loop, plus a copy/store within the loop. obsessed with the efficiency honestly 🦋

Enabled by default when -fgcse is enabled.

• -fgcse-sm — with this one enabled, a store motion pass runs after global common subexpression elimination. This pass attempts to move stores out of loops. And when you use it together with -fgcse-lm, loops containing a load/store sequence can be changed to a load before the loop and a store after the loop. iconic combo 💖

Not enabled at any optimization level.

• -fgcse-las — when enabled, the global common subexpression elimination pass eliminates redundant loads that come after stores to the same memory location (both partial and full redundancies). no cap.

Not enabled at any optimization level.

• -fgcse-after-reload — when enabled, a redundant load elimination pass is performed after reload. The whole point of this pass is to clean up redundant spilling. 🧹

Enabled by -O3, -fprofile-use and -fauto-profile.

• -faggressive-loop-optimizations — this option tells the loop optimizer to use language constraints to derive bounds for the number of iterations of a loop. It assumes that loop code does not invoke undefined behavior, for example by causing signed integer overflows or out-of-bound array accesses. 😭 The bounds for the number of iterations of a loop are then used to guide loop unrolling and peeling and loop exit test optimizations.

This option is enabled by default.

• -funconstrained-commons — this tells the compiler that variables declared in common blocks (e.g. Fortran) may later be overridden with longer trailing arrays. This prevents certain optimizations that depend on knowing the array bounds.
• -fcrossjumping — performs cross-jumping transformation. This transformation unifies equivalent code and saves code size. The resulting code may or may not perform better than without cross-jumping — that part's just up in the air, ngl.

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 this.

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.

• -fif-conversion — attempts to transform conditional jumps into branch-less equivalents. This includes using conditional moves, min, max, set flags and abs instructions, plus some tricks doable by standard arithmetics. The use of conditional execution on chips where it's available is controlled by -fif-conversion2.

Enabled at levels -O1, -O2, -O3, -Os, but not with -Og.

• -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 — okay 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 are changed to be thunks that call a common implementation. slay.

Enabled by -Os.

• -fdelete-null-pointer-checks — assumes that programs cannot safely dereference null pointers, and that no code or data element resides at address zero. This option enables simple constant folding optimizations at all optimization levels. On top of that, other optimization passes in GCC use this flag to control global dataflow analyses that eliminate useless checks for null pointers; these assume that a memory access to address zero always results in a trap, so if a pointer is checked after it has already been dereferenced, it cannot be null. 💀

Note though: 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.

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 figures out for a given call the set of likely targets. If that set is small — preferably of size 1 — the call gets changed into a conditional deciding between direct and indirect calls. These speculative calls enable more optimizations, like inlining. And when they seem useless after further optimization, they get converted back into their original form. the way it just undoes itself if it's not serving 😭
• -fdevirtualize-at-ltrans — streams extra information needed for aggressive devirtualization when running the link-time optimizer in local transformation mode. This option enables more devirtualization but significantly increases the size of streamed data. For this reason it is disabled by default. 🫶
• -fexpensive-optimizations — performs a number of minor optimizations that are relatively expensive. expensive girl behavior, honestly.

Enabled at levels -O2, -O3, -Os.

• -fext-dce, -fno-ext-dce — perform dead code elimination on zero and sign extensions, with special dataflow analysis.
• -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. 💅

Enabled for Alpha, AArch64, LoongArch, PowerPC, RISC-V, SPARC, h83000 and x86 at levels -O2, -O3, -Os.

• -fno-lifetime-dse — in C++ the value of an object is only affected by changes within its lifetime: when the constructor begins, the object has an indeterminate value, and any changes during the lifetime of the object are dead when the object is destroyed. Normally dead store elimination takes advantage of this; if your code relies on the value of the object storage persisting beyond the lifetime of the object, you can use this flag to disable this optimization. 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, you can 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. keeping track of all four of these is a whole vibe check 👑
• -flive-range-shrinkage — attempts to decrease register pressure through register live range shrinkage. This is helpful for fast processors with small or moderate size register sets. ✨
• -fira-algorithm=algorithm — use the specified coloring algorithm for the integrated register allocator. The algorithm argument can be priority, which specifies Chow's priority coloring, or CB, which specifies Chaitin-Briggs coloring. Chaitin-Briggs coloring is not implemented for all architectures, but for those targets that do support it, it is the default because it generates better code. 🎀
• -fira-region=region — use specified regions for the integrated register allocator. The region argument should be one of the following:
  • all — use all loops as register allocation regions. This can give the best results for machines with a small and/or irregular register set.
  • mixed — use all loops except for loops with small register pressure as the regions. This value usually gives the best results in most cases and for most architectures, and is enabled by default when compiling with optimization for speed (-O, -O2, ...).
  • one — use all functions as a single region. This typically results in the smallest code size, and is enabled by default for -Os or -O0.
• -fira-hoist-pressure — use IRA to evaluate register pressure in the code hoisting pass for decisions to hoist expressions. This option usually results in smaller code, but it can slow the compiler down. 😭

This option is enabled at level -Os for all targets.

• -fira-loop-pressure — use IRA to evaluate register pressure in loops for decisions to move loop invariants. This option usually results in generation of faster and smaller code on machines with large register files (>= 32 registers), but it can slow the compiler down.

This option is enabled at level -O3 for some targets.

• -fno-ira-share-save-slots — 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. rip to your stack frames 🪦
• -fno-ira-share-spill-slots — bestie this one turns OFF the sharing of stack slots that get allocated for pseudo-registers 💀 so every single pseudo-register that doesn't score a hard register gets its own separate stack slot. Which means… yeah, function stack frames get bigger. No cap.
• -flra-remat — turns on CFG-sensitive rematerialization in LRA ✨ instead of reloading values of spilled pseudos, LRA tries to rematerialize (aka recalculate) those values if it's profitable. Obsessed.

Enabled at levels -O2, -O3, -Os.

• -fdelayed-branch — if the target machine supports it, it attempts to reorder instructions so it can exploit the instruction slots that are available after delayed branch instructions 🎀

Enabled at levels -O1, -O2, -O3, -Os, but not at -Og.

• -fschedule-insns — if the target machine supports it, it attempts to reorder instructions to eliminate execution stalls caused by required data not being available yet 🌸 this helps machines that have slow floating point or memory load instructions, by letting other instructions get issued until the result of the load or floating-point instruction is actually needed.

Conventionally enabled at optimization levels -O2 and -O3. But — plot twist — many targets override this behavior. For example, on x86 it's disabled at all levels, while on AArch64 it's enabled only at -O3. iykyk.

• -fschedule-insns2 — similar to -fschedule-insns, but it requests an additional pass of instruction scheduling after register allocation has been done 💅 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 option 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 would follow. Periodt.
• -fsched-spec-load — allows speculative motion of some load instructions 🌈 this only makes sense when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.
• -fsched-spec-load-dangerous — allows speculative motion of more load instructions 💀 note the name, bestie. This only makes sense when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.
• -fsched-stalled-insns
• -fsched-stalled-insns=n — 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 no insns are moved prematurely, and -fsched-stalled-insns=0 means there's no limit on how many queued insns can be moved prematurely. -fsched-stalled-insns without a value is equivalent to -fsched-stalled-insns=1.
• -fsched-stalled-insns-dep
• -fsched-stalled-insns-dep=n — defines how many insn groups (cycles) get examined for a dependency on a stalled insn that's a candidate for premature removal from the queue of stalled insns 💜 this only has an effect during the second scheduling pass, and only if -fsched-stalled-insns is used. -fno-sched-stalled-insns-dep is equivalent to -fsched-stalled-insns-dep=0. And -fsched-stalled-insns-dep without a value is equivalent to -fsched-stalled-insns-dep=1.
• -fsched2-use-superblocks — when scheduling after register allocation, use superblock scheduling 🦋 this allows motion across basic block boundaries, resulting in faster schedules. 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 😭

This 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 👑 this heuristic favors the instruction that belongs to a schedule group. It's 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 ⭐ this heuristic 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 ✨ this heuristic 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 🌸 this heuristic 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 💅 this heuristic favors the instruction that is 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 🎀 this heuristic 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 that, it turns the call into a conditional deciding between the direct call and the original indirect one. These speculative calls often enable more optimizations, like inlining 👑 and when they seem useless after further optimization, they get converted back into their original form. Recycling era.
• -freschedule-modulo-scheduled-loops — modulo scheduling is performed before traditional scheduling 💜 so if a loop is modulo scheduled, later scheduling passes may change its schedule. Use this option to control that behavior.
• -fselective-scheduling — schedule instructions using the selective scheduling algorithm ✨ selective scheduling runs instead of the first scheduler pass.
• -fselective-scheduling2 — schedule instructions using the selective scheduling algorithm 🌈 selective scheduling 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 option has no effect unless -fsel-sched-pipelining is turned on.
• -fsemantic-interposition — okay so some object formats, like ELF, allow interposing of symbols by the dynamic linker 👀 this means that for symbols exported from the DSO, the compiler cannot perform interprocedural propagation, inlining and other optimizations, because it has to anticipate that the function or variable in question may change. While this feature is useful — for example, to rewrite memory allocation functions by a debugging implementation — it's expensive in terms of code quality 😭 with -fno-semantic-interposition the compiler assumes that if interposition happens for functions, the overwriting function will have precisely the same semantics (and side effects). Similarly, if interposition happens for variables, the constructor of the variable will be the same. The flag has no effect for functions explicitly declared inline (where it's never allowed for interposition to change semantics) and for symbols explicitly declared weak.
• -fshrink-wrap — emits function prologues only before the parts of the function that actually need it, rather than at the top of the function 🧚 this flag is enabled by default at -O and higher.
• -fshrink-wrap-separate — shrink-wraps separate parts of the prologue and epilogue separately, so those parts only get executed when needed ✨ this option is on by default, but it has no effect unless -fshrink-wrap is also turned on and the target supports this.
• -fcaller-saves — enables allocation of values to registers that get clobbered by function calls, by emitting extra instructions to save and restore the registers around such calls 💖 such allocation is only done when it seems to result in better code.

This option is always enabled by default on certain machines, usually those that have no call-preserved registers to use instead.

Enabled at levels -O2, -O3, -Os.

• -fcombine-stack-adjustments — tracks stack adjustments (pushes and pops) and stack memory references, 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 there's no need to save and restore them around calls. This is only possible if 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 (this 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 — reassociation, but make it trees 🌳. Grouping the math differently so it computes faster, basically. On by default at -O1 and up.
• -fcode-hoisting — code hoisting, bestie ✨. It tries to drag the evaluation of expressions that run on every path to the function exit as early as it possibly can. It's giving code-size optimization energy, but honestly it usually helps with speed too. On by default at -O2 and up.
• -ftree-pre — partial redundancy elimination (PRE), on trees. On by default at -O2 and -O3.
• -ftree-partial-pre — same PRE but make it feral, more aggressive. On by default at -O3.
• -ftree-forwprop — forward propagation on trees. On by default at -O1 and up.
• -ftree-fre — full redundancy elimination (FRE), on trees. Okay so the tea ☕: the difference between FRE and PRE is that FRE only looks at expressions computed on all paths leading to the redundant computation. So it's faster than PRE, but it exposes fewer redundancies. Trade-off queen. On by default at -O1 and up.
• -ftree-phiprop — hoisting of loads from conditional pointers, on trees. This pass is on by default at -O1 and up.
• -fhoist-adjacent-loads — speculatively hoist loads from both branches of an if-then-else when the loads come from adjacent locations in the same structure and the target architecture has a conditional move instruction. Very specific, very slay. On by default at -O2 and up.
• -ftree-copy-prop — copy propagation on trees. This pass yeets unnecessary copy operations out of existence 🗑️✨. On by default at -O1 and up.
• -fipa-pure-const — figures out which functions are pure or constant. Enabled by default at -O1 and up.
• -fipa-reference — figures out which static variables do not escape the compilation unit. Enabled by default at -O1 and up.
• -fipa-reference-addressable — discovers read-only, write-only and non-addressable static variables. Enabled by default at -O1 and up.
• -fipa-reorder-for-locality — groups call chains close together in the binary layout so code locality improves and jump distances between frequently-called functions get minimized 🫶. And the way this is different from -freorder-functions: this pass looks at the call chains between functions and groups them together, instead of dumping all hot/normal/cold/never-executed functions into separate sections. And unlike -fprofile-reorder-functions, it's trying to improve code locality across the whole runtime of the program, not just focusing on startup. Important: this option is incompatible with an explicit -flto-partition= option, because it enforces a custom partitioning scheme. If you're using this, it's recommended that you also use profile feedback — but this option is not enabled by default otherwise.
• -fipa-stack-alignment — reduce stack alignment on call sites if possible. Enabled by default.
• -fipa-pta — interprocedural pointer analysis, plus interprocedural modification and reference analysis. Heads up though: this one 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 that are only called from cold functions get marked as cold. Also, functions executed once (like "cold", "noreturn", static constructors or destructors) get identified. Cold functions, and the loop-less parts of functions executed once, then get optimized for size. Enabled by default at -O1 and up.
• -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. On by default at -O1 and up.
• -fipa-cp — interprocedural constant propagation. It analyzes the program to figure out when values passed to functions are constants and optimizes accordingly. If your application passes constants to functions, this can substantially increase performance 🔥. Enabled by default at -O2, -Os and -O3. It's also enabled by -fprofile-use and -fauto-profile.
• -fipa-cp-clone — function cloning, to make interprocedural constant propagation stronger 💪. When it's on, IPA constant propagation performs function cloning whenever an externally visible function can be called with constant arguments. Warning bestie: because this can create multiple copies of functions, it may significantly increase code size. Enabled by default at -O3. It's 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 ✨. Do it 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 with an equivalent one that has a different name. It works more effectively with link-time optimization enabled.

So like, the behavior is similar to the Gold Linker's ICF optimization, but GCC ICF works on different levels, so the optimizations are not the same — there are equivalences found only by GCC, and equivalences found only by Gold. Both valid, both iconic, just different 👑.

-fipa-icf is enabled by default at -O2 and -Os.

• -flate-combine-instructions — enables two instruction combination passes that run pretty late in the compilation process. One of the passes runs before register allocation, and the other runs after register allocation. The main aim of these 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.

So here's the vibe check: if the compiler's optimization uses a function's body, or information extracted from its body, to optimize/change another function, then that latter function is called an impacted function of the former. And if a function gets patched, its impacted functions should be patched too.

The impacted functions are determined by the compiler's interprocedural optimizations. For example, a caller is impacted when inlining a function into its caller, cloning a function and changing its caller to call this new clone, or extracting a function's pureness/constness information to optimize its direct or indirect callers, etc.

Usually, the more IPA optimizations you enable, the larger the number of impacted functions for each function. So to control the number of impacted functions — and to more easily compute the list of impacted functions — IPA optimizations can be partially enabled at two different levels.

The level argument should be one of the following:

• inline-clone — only enable inlining and cloning optimizations, which includes inlining, cloning, interprocedural scalar replacement of aggregates and partial inlining. So when you patch a function, all its callers and its clones' callers are impacted, which means 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 enable inlining of static functions. So when you patch a static function, all its callers are impacted and therefore need to be patched as well.

On top of 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.

Note that -flive-patching is not supported with link-time optimization (-flto).

• -fisolate-erroneous-paths-dereference — detects paths that trigger erroneous or undefined behavior because of 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 the erroneous or undefined behavior into a trap 🪤. On by default at -O2 and up.
• -fisolate-erroneous-paths-attribute — detects paths that trigger erroneous or undefined behavior because a null value is 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 the 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. On by default at -O1 and up.
• -ftree-bit-ccp — sparse conditional bit constant propagation on trees, plus it propagates pointer alignment information. This pass only operates on local scalar variables. On by default at -O1 and up — 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. On by default at -O1 and up.
• -fssa-backprop — propagates information about the uses of a value up the definition chain so the definitions can get simplified 🦋. For example, this pass strips sign operations if the sign of a value never even matters. On by default at -O1 and up.
• -fssa-phiopt — bestie, this one does pattern matching on SSA PHI nodes to optimize conditional code ✨. It's on by default at -O1 and higher… except for -Og, so don't come for me when -Og doesn't do it 💅
• -ftree-switch-conversion — takes simple initializations in a switch and converts them into initializations from a scalar array 🎀. Enabled by default at -O2 and higher, periodt.
• -ftree-tail-merge — it goes looking for identical code sequences 👀, and when it finds a match it replaces one of them with a jump to the other. This optimization is literally 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 — iykyk.
• -ftree-cselim — performs conditional store elimination on trees 🌸. This flag is enabled by default at -O1 and higher on targets that have conditional move instructions.
• -ftree-dce — performs dead code elimination (DCE) on trees 💀. Dead code? Gone. Enabled by default at -O1 and higher.
• -ftree-builtin-call-dce — conditional dead code elimination (DCE) for calls to built-in functions that may set errno but are otherwise free of side effects 👑. Enabled by default at -O2 and higher if -Os is not also specified.
• -ffinite-loops — assumes that a loop with an exit will eventually take the exit and not loop indefinitely 🌀. This lets the compiler remove loops that otherwise have no side-effects, and it does not consider eventual endless looping as a side-effect. Enabled by default at -O2 for C++ with -std=c++11 or higher.
• -ftree-dominator-opts — performs a whole variety of simple scalar cleanups (constant/copy propagation, redundancy elimination, range propagation and expression simplification) based on a dominator tree traversal 🧚. It also performs jump threading (to reduce jumps to jumps) — jumps to jumps are so not it. This flag is enabled by default at -O1 and higher.
• -ftree-dse — performs dead store elimination (DSE) on trees 💅. A dead store is a store into a memory location that is later overwritten by another store without any intervening loads. In that case the earlier store can just be deleted, no cap. Enabled by default at -O1 and higher.
• -ftree-ch — performs loop header copying on trees ✨. This is beneficial since it increases the effectiveness of code motion optimizations, and it also saves one jump. Enabled by default at -O1 and higher. It is not enabled for -Os, since it usually increases code size 😭.
• -ftree-loop-optimize — performs loop optimizations on trees 🌈. Enabled by default at -O1 and higher.
• -ftree-loop-linear, -floop-strip-mine, -floop-block — these three perform loop nest optimizations 🎀. Same as -floop-nest-optimize. To use this code transformation, GCC has to be configured with --with-isl to enable the Graphite loop transformation infrastructure.
• -fgraphite-identity — enables the identity transformation for graphite 💜. For every SCoP we generate the polyhedral representation and transform it back to gimple. Using -fgraphite-identity we can check the costs or benefits of the GIMPLE -> GRAPHITE -> GIMPLE transformation. Some minimal optimizations are also performed by the code generator isl, like index splitting and dead code elimination in loops.
• -floop-nest-optimize — enables the isl based loop nest optimizer 🦋. This is a generic loop nest optimizer based on the Pluto optimization algorithms. It calculates a loop structure optimized for data-locality and parallelism. This option is experimental, so be cute with it.
• -floop-parallelize-all — uses the Graphite data dependence analysis to identify loops that can be parallelized 🔥. It parallelizes all the loops that can be analyzed to not contain loop carried dependences, without checking that it is profitable to parallelize the loops. Yeah. It just goes for it.
• -ftree-coalesce-vars — while transforming the program out of the SSA representation, this attempts to reduce copying by coalescing versions of different user-defined variables, instead of just compiler temporaries 💫.

Heads up bestie: 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. This option is enabled by default if optimization is enabled, and it does very little otherwise 🙈.

• -ftree-loop-if-convert — attempts to transform conditional jumps in the innermost loops to branch-less equivalents 🌸. The intent is to remove control-flow from the innermost loops in order to improve the ability of the vectorization pass to handle these loops. This is enabled by default if vectorization is enabled.
• -ftree-loop-distribution — performs loop distribution 🎀. This flag can improve cache performance on big loop bodies and allow further loop optimizations, like parallelization or vectorization, to take place. For example, the loop

DO I = 1, N
  A(I) = B(I) + C
  D(I) = E(I) * F
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 — performs loop distribution of patterns that can be code generated with calls to a library 👑. This flag is 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 ✨.

• -floop-interchange — performs loop interchange outside of graphite 💅. This flag can improve cache performance on loop nest and allow further loop optimizations, like vectorization, to take place. For example, the loop

for (int i = 0; i < N; i++)
  for (int j = 0; j < N; j++)
    for (int k = 0; k < N; k++)
      c[i][j] = c[i][j] + a[i][k]*b[k][j];

is transformed to

for (int i = 0; i < N; i++)
  for (int k = 0; k < N; k++)
    for (int j = 0; j < N; j++)
      c[i][j] = c[i][j] + a[i][k]*b[k][j];

This flag is enabled by default at -O3. It is also enabled by -fprofile-use and -fauto-profile.

• -floop-unroll-and-jam — applies unroll and jam transformations on feasible loops 🦋. In a loop nest this unrolls the outer loop by some factor and fuses the resulting multiple inner loops. This flag is enabled by default at -O3. It is also enabled by -fprofile-use and -fauto-profile.
• -ftree-loop-im — performs loop invariant motion on trees 🌈. This pass moves only invariants that are hard to handle at RTL level (function calls, operations that expand to nontrivial sequences of insns). With -funswitch-loops it also moves operands of conditions that are invariant out of the loop, so that we can use just trivial invariantness analysis in loop unswitching. The pass also includes store motion.
• -ftree-loop-ivcanon — creates a canonical counter for number of iterations in loops for which determining the number of iterations requires complicated analysis 🧚. Later optimizations then may determine the number easily. Useful especially in connection with unrolling, fr fr.
• -ftree-scev-cprop — performs final value replacement 💜. If a variable is modified in a loop in such a way that its value when exiting the loop can be determined using only its initial value and the number of loop iterations, it replaces uses of the final value by such a computation, provided it is sufficiently cheap. This reduces data dependencies and may allow further simplifications. Enabled by default at -O1 and higher.
• -fivopts — performs induction variable optimizations (strength reduction, induction variable merging and induction variable elimination) on trees 🔥. Enabled by default at -O1 and higher.
• -ftree-parallelize-loops, -ftree-parallelize-loops=n — parallelizes loops, i.e., splits their iteration space to run in multiple threads 🎀. This is only possible for loops whose iterations are independent and can be arbitrarily reordered. The optimization is only profitable on multiprocessor machines, for loops that are CPU-intensive, rather than constrained e.g. by memory bandwidth. This option implies -pthread, and thus is only supported on targets that have support for -pthread.

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 — main character energy honestly 👑.

• -ftree-pta — performs function-local points-to analysis on trees 💫. This flag is enabled by default at -O1 and higher, except for -Og.
• -ftree-sra — performs scalar replacement of aggregates 🌸. This pass replaces structure references with scalars to prevent committing structures to memory too early. This flag is enabled by default at -O1 and higher, except for -Og.
• -fstore-merging — performs merging of narrow stores to consecutive memory addresses ✨. This pass merges contiguous stores of immediate values narrower than a word into fewer wider stores to reduce the number of instructions. This is enabled by default at -O2 and higher as well as -Os.

---

• -ftree-ter — does temporary expression replacement during the SSA->normal phase, bestie. Single use/single def temporaries get swapped out at their use location with their defining expression instead. The tea? This gives you non-GIMPLE code, but it hands the expanders way more complex trees to work with, which means better RTL generation. On by default at -O1 and higher. 💅
• -ftree-slsr — straight-line strength reduction, but on trees. It spots related expressions that involve multiplications and swaps them for cheaper calculations whenever it can. Enabled by default at -O1 and higher. ✨
• -ftree-vectorize — vectorization on trees, periodt. This flag turns on -ftree-loop-vectorize and -ftree-slp-vectorize if you haven't explicitly specified them yourself. 🔥
• -ftree-loop-vectorize — loop vectorization on trees. Enabled by default at -O2, and also by -ftree-vectorize, -fprofile-use, and -fauto-profile.
• -ftree-slp-vectorize — basic block vectorization on trees. Same deal: enabled by default at -O2, and by -ftree-vectorize, -fprofile-use, and -fauto-profile.
• -ftrivial-auto-var-init=choice — this one initializes automatic variables or temporary objects with either a pattern or with zeroes, and the whole point is to boost the security and predictability of a program by preventing uninitialized memory disclosure and use. Iconic, honestly.

Here's the catch though, and it's important: GCC still considers an automatic variable that doesn't have an explicit initializer as uninitialized. -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, 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.

Okay so pay attention here because this is the part people trip on: 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 — alters the cost model used for vectorization. The model argument should be one of unlimited, dynamic, cheap or very-cheap. So:
  • With the unlimited model the vectorized code-path is assumed to be profitable.
  • 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 — alters the cost model used for vectorization of 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 — Value Range Propagation on trees, and yes it's as good as it sounds. Similar to the constant propagation pass, except instead of values, ranges of values get propagated. This lets the optimizers remove unnecessary range checks like array bound checks and null pointer checks. Enabled by default at -O2 and higher. Heads up though: null pointer check elimination is only done if -fdelete-null-pointer-checks is enabled. 👑
• -fsplit-paths — splits paths leading to loop backedges. This can improve dead code elimination and common subexpression elimination. 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, which improves the 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 — inlines parts of functions, not the whole thing. Lowkey the "I'll just have a bite" of optimizations. 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 — performs 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, generates instructions to prefetch memory to improve the performance of loops that access large arrays.

Fr fr, be careful with this one: it 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 — reorders basic blocks in the compiled function in order to reduce number of taken branches and improve code locality. Main character energy for your instruction cache. 👑
• -freorder-blocks-algorithm=algorithm — pick which algorithm gets to shuffle your basic blocks around. The algorithm you can pass is either simple, which doesn't grow your code size (okay except sometimes because of secondary effects like alignment, we don't make the rules 💅), or stc, aka the "software trace cache" algorithm, which tries to smoosh all the 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 — not just reordering basic blocks inside the compiled function, but also partitioning hot and cold basic blocks into separate sections of the assembly and .o files, all to reduce the number of taken branches and improve paging and cache locality performance. it's giving organized 🎀

Heads up bestie, this optimization is automatically turned off in the presence of exception handling or unwind tables (on targets using setjump/longjump or a target specific scheme), for linkonce sections, for functions with a user-defined section attribute, and on any architecture that doesn't support named sections. And when -fsplit-stack is used, this option is not enabled by default (to avoid linker errors), but you can enable it explicitly (if you're using a working linker).

Enabled for x86 at levels -O2, -O3, -Os.

• -freorder-functions — reorders functions in the object file to improve code locality. Unlike -fipa-reorder-for-locality, this option prioritises grouping all functions within a category (hot/normal/cold/never-executed) together. This gets implemented by using special subsections .text.hot for the most frequently executed functions and .text.unlikely for unlikely executed functions. The reordering is done by the linker, so your object file format must support named sections and the linker must place them in a reasonable way.

Lowkey, this option isn't effective unless you either provide profile feedback (peep -fprofile-arcs for details) or manually annotate functions with hot or cold attributes.

Enabled at levels -O2, -O3, -Os.

• -fstrict-aliasing — lets the compiler assume the strictest aliasing rules that apply to the language being compiled. For C (and C++), this activates optimizations based on the type of expressions. Specifically: accessing an object of one type via an expression of a different type is not allowed, unless the types are compatible types, differ only in signedness or qualifiers, or the expression has a character type. Accessing scalar objects via a corresponding vector type is also allowed. ✨

For example, an unsigned int can alias an int, but not a void* or a double. A character type may alias any other type.

Pay special attention to code like this:

union a_union {
  int i;
  double d;
};

int f() {
  union a_union t;
  t.d = 3.0;
  return t.i;
}

The practice of reading from a different union member than the one most recently written to (called "type-punning") is common. Even with -fstrict-aliasing, type-punning is allowed 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;
}

Similarly, access by taking the address, casting the resulting pointer and dereferencing the result has undefined behavior, even if the cast uses a union type, e.g.:

int f() {
  double d = 3.0;
  return ((union a_union *) &d)->i;
}

The -fstrict-aliasing option is enabled at levels -O2, -O3, -Os.

• -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, the memory accesses are no longer considered to be crossing a function boundary. 🦋

The -fipa-strict-aliasing option is enabled by default and is effective only 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 ensures that at least the first m bytes of the function can be fetched by the CPU without crossing an n-byte alignment boundary. This is an optimization of code performance, and alignment is ignored for functions considered cold. If you need alignment for all functions, use -fmin-function-alignment instead.

If m is not specified, it defaults to n.

Examples: -falign-functions=32 aligns functions to the next 32-byte boundary, -falign-functions=24 aligns to the next 32-byte boundary only if this can be done by skipping 23 bytes or less, and -falign-functions=32:7 aligns to the next 32-byte boundary only if this can be done by skipping 6 bytes or less.

The second pair of n2:m2 values lets you specify a secondary alignment: -falign-functions=64:7:32:3 aligns to the next 64-byte boundary if this can be done by skipping 6 bytes or less, otherwise aligns to the next 32-byte boundary if this can be done by skipping 2 bytes or less. If m2 is not specified, it defaults to n2.

Some assemblers only support this flag when n is a power of two; in that case, it is rounded up.

-fno-align-functions and -falign-functions=1 are equivalent and mean that functions are not aligned.

If n is not specified or is zero, use a machine-dependent default. The maximum allowed n option value is 65536.

Enabled at levels -O2, -O3.

• -flimit-function-alignment — with this enabled, the compiler tries to avoid unnecessarily overaligning functions. It attempts to instruct the assembler to align by the amount specified by -falign-functions, but not to skip more bytes than the size of the function. polite king behavior fr 👑
• -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 of this option are analogous to the -falign-functions option. -fno-align-labels and -falign-labels=1 are equivalent and mean that labels are not aligned.

If -falign-loops or -falign-jumps are applicable and are greater than this value, then their values are used instead.

If n is not specified or is zero, use a machine-dependent default which is very likely to be 1, meaning no alignment. The maximum allowed n option value is 65536.

Enabled at levels -O2, -O3.

• -falign-loops
• -falign-loops=n
• -falign-loops=n:m
• -falign-loops=n:m:n2
• -falign-loops=n:m:n2:m2 — aligns loops to a power-of-two boundary. If the loops are executed many times, this makes up for any execution of the dummy padding instructions. This is an optimization of code performance, and alignment is ignored for loops considered cold.

If -falign-labels is greater than this value, then its value is used instead.

The parameters of this option are analogous to the -falign-functions option. -fno-align-loops and -falign-loops=1 are equivalent and mean that loops are not aligned. The maximum allowed n option value is 65536.

If n is not specified or is zero, use a machine-dependent default.

Enabled at levels -O2, -O3.

• -falign-jumps
• -falign-jumps=n
• -falign-jumps=n:m
• -falign-jumps=n:m:n2
• -falign-jumps=n:m:n2:m2 — aligns branch targets to a power-of-two boundary, for branch targets where the targets can only be reached by jumping. In this case, no dummy operations need be executed. This is an optimization of code performance, and alignment is ignored for jumps considered cold.

If -falign-labels is greater than this value, then its value is used instead.

The parameters of this option are analogous to the -falign-functions option. -fno-align-jumps and -falign-jumps=1 are equivalent and mean that loops are not aligned.

If n is not specified or is zero, use a machine-dependent default. The maximum allowed n option value is 65536.

Enabled at levels -O2, -O3.

• -fmin-function-alignment — specify minimal alignment of functions to the next power-of-two greater than or equal to n. Unlike -falign-functions, this alignment is applied also to all functions (even those considered cold). The alignment is also not affected by -flimit-function-alignment. 💅
• -fno-allocation-dce — do not remove unused C++ allocations (using operator "new" and operator "delete") in dead code elimination.

See also -fmalloc-dce.

• -fallow-store-data-races — allow the compiler to perform optimizations that may introduce new data races on stores, without proving that the variable cannot be concurrently accessed by other threads. Does not affect optimization of local data. It is safe to use this option if it is known that global data will not be accessed by multiple threads.

Examples of optimizations enabled by -fallow-store-data-races include hoisting or if-conversions that may cause a value that was already in memory to be re-written with that same value. Such re-writing is safe in a single threaded context but may be unsafe in a multi-threaded context. Note that on some processors, if-conversions may be required in order to enable vectorization.

Enabled at level -Ofast.

• -funit-at-a-time — this option is left for compatibility reasons. -funit-at-a-time has no effect, while -fno-unit-at-a-time implies -fno-toplevel-reorder and -fno-section-anchors. 💀

Enabled by default.

• -fno-toplevel-reorder — do not reorder top-level functions, variables, and asm statements. Output them in the same order that they appear in the input file. When this option is used, unreferenced static variables are not removed. This option is intended to support existing code that relies on a particular ordering. For new code, it is better to use attributes when possible. 🙈

-ftoplevel-reorder is the default at -O1 and higher, and also at -O0 if -fsection-anchors is explicitly requested. And just so you know, -fno-toplevel-reorder implies -fno-section-anchors — they come as a set, bestie. 💅

• -funreachable-traps — okay so with this one, the compiler takes calls to __builtin_unreachable and turns them into traps instead of using them as an optimization hint. And yes, this also hits any of those calls that the compiler itself generated behind the scenes, no cap.

This option does the exact same thing as -fsanitize=unreachable -fsanitize-trap=unreachable, but it does not touch the values of those options. If -fsanitize=unreachable is turned on, that one wins over this one, periodt.

This option is enabled by default at -O0 and -Og. ✨

• -fweb — constructs webs the way they're commonly used for register allocation, and gives each web its own individual pseudo register. That lets the register allocation pass work directly on pseudos, and it also beefs up a bunch of other optimization passes — CSE, the loop optimizer, trivial dead code remover, the whole squad. 🫶 But heads up: it can make debugging impossible, because variables stop living in a "home register". 💀

Enabled by default with -funroll-loops.

• -fwhole-program — assume that the compilation unit you're compiling right now is the entire program. Every public function and variable — except main and the ones merged in by the attribute externally_visible — becomes static, and like, effectively gets optimized way more aggressively by the interprocedural optimizers. main character energy only, everyone else is private now. 👑

With -flto this option has limited use. In most cases the exact list of symbols used or exported from the binary is already known from the resolution info the linker plugin hands to the link-time optimizer. It's still useful if no linker plugin is used, or during the incremental link step when the final code gets produced (with -flto -flinker-output=nolto-rel).

• -flto[=n] — this runs the standard link-time optimizer. When you invoke it with source code, it generates GIMPLE (one of GCC's internal representations, iykyk) and writes it into special ELF sections in the object file. When the object files get linked together, all the function bodies are read back out of those ELF sections and instantiated as if they'd been part of the same translation unit the whole time. 🦋

To use the link-time optimizer, -flto and your optimization options should be specified at compile time and during the final link. It's recommended that you compile all the files going into 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 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 like usual. Since foo.o and bar.o get merged into one image, all the interprocedural analyses and optimizations in GCC now work across both 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. slay. 💅

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 is that to enable link-time optimizations you need to use the GCC driver to perform the link step. GCC automatically performs link-time optimization if any of the objects involved were compiled with the -flto command-line option. And you can always override that automatic decision 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 the linker supports it, 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. 🙈

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 picks 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. 😭

There are some code generation flags preserved by GCC when generating bytecodes, since they need to be used during the final link. Currently, the following options and their settings are taken from the first object file that explicitly specifies them: -fcommon, -fexceptions, -fnon-call-exceptions, -fgnu-tm and all the -m target flags.

The following options -fPIC, -fpic, -fpie and -fPIE are combined based on the following scheme:

-fPIC + -fpic = -fpic
-fPIC + -fno-pic = -fno-pic
-fpic/-fPIC + (no option) = (no option)
-fPIC + -fPIE = -fPIE
-fpic + -fPIE = -fpie
-fPIC/-fpic + -fpie = -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 just gets ignored. This includes options like -freg-struct-return and -fpcc-struct-return. 🚫

Other options like -ffp-contract, -fno-strict-overflow, -fwrapv, -fno-trapv or -fno-strict-aliasing are passed through to the link stage and merged conservatively for conflicting translation units. Specifically -fno-strict-overflow, -fwrapv and -fno-trapv take precedence; and for example -ffp-contract=off takes precedence over -ffp-contract=fast. You can override them at link time. 💅

Diagnostic options like -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. Note that code transforms like inlining can lead to warnings being enabled or disabled for regions of code that aren't consistent with the setting at compile time.

When you need to pass options to the assembler via -Wa or -Xassembler, make sure to either compile such translation units with -fno-lto or consistently use the same assembler options on all translation units. You can alternatively also specify assembler options at LTO link time.

To enable debug info generation you need to supply -g at compile time. If any of the input files at link time were built with debug info generation enabled, the link will enable debug info generation as well. Any elaborate debug info settings like the dwarf level -gdwarf-5 need to be explicitly repeated at the linker command line, and mixing different settings in different translation units is discouraged. 🙅

If LTO encounters objects with C linkage declared with incompatible types in separate translation units to be linked together (undefined behavior according to ISO C99 6.2.7), a non-fatal diagnostic may be issued. The behavior is still undefined at run time. Similar diagnostics may be raised for other languages. 💀

Another feature of LTO is that it's possible to apply interprocedural optimizations on files written in different languages:

gcc -c -flto foo.c
g++ -c -flto bar.cc
gfortran -c -flto baz.f90
g++ -o myprog -flto -O3 foo.o bar.o baz.o -lgfortran

Notice that the final link is done with g++ to get the C++ runtime libraries, and -lgfortran is added to get the Fortran runtime libraries. In general, when mixing languages in LTO mode, you should use the same link command options as when mixing languages in a regular (non-LTO) compilation. 🌈

If object files containing GIMPLE bytecode are stored in a library archive — say libfoo.a — it'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. iconic. 👑

So yeah bestie, if you're not using a linker with plugin support, or you just don't turn the linker plugin on, then the objects chilling inside libfoo.a still get extracted and linked like normal — they just don't get to participate in the LTO optimization party. 😭 To make a static library work for both LTO optimization and regular linkage, you compile its object files with -flto -ffat-lto-objects. slay.

Link-time optimizations do not require the whole program to be present to do their thing. So if your program doesn't need any symbols exported, you can combine -flto and -fwhole-program and let the interprocedural optimizers make more aggressive assumptions, which may lead to improved optimization opportunities. But lowkey you don't need -fwhole-program when the linker plugin is active (peep -fuse-linker-plugin).

The current implementation of LTO makes no attempt to generate bytecode that's portable between different types of hosts. The bytecode files are versioned and there's a strict version check, so bytecode files generated in one version of GCC do not work with an older or newer version of GCC. 💀 fr fr, don't mix and match.

Link-time optimization does not work well with generation of debugging information on systems other than those using a combination of ELF and DWARF. noted. 📝

If you specify the optional n, the optimization and code generation done at link time runs 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. she's flexible like that 💅

---

• -flto-partition=alg — specify the partitioning algorithm used by the link-time optimizer. The value is either 1to1 to specify a partitioning mirroring the original source files, or balanced to specify partitioning into equally sized chunks (whenever possible), or max to create a 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 — enable 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. no cap.

• -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 — this option specifies the level of compression used for intermediate language written to LTO object files, and is only meaningful in conjunction with LTO mode (-flto). 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 the 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.

NOTE: On Debian/Ubuntu this is disabled by default on m68k-linux-gnu.

• -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. no shortcuts bestie 🌸

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 — turns on sampling-based feedback-directed optimizations 🎯 plus a whole lil squad of other optimizations, most of which are only really worth it when you actually have profile feedback to go off of:

-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

path is the name of a file holding AutoFDO profile info, bestie. If you just... don't give one? It defaults to fbdata.afdo in the current directory. No stress. 💅

To actually make an AutoFDO profile data file you gotta run your program with the perf utility on a supported GNU/Linux target system. More deets here: <https://perfwiki.github.io/main/>.

Like, e.g.:

        perf record -e br_inst_retired:near_taken -b -o perf.data \
            -- your_program

Then you use the create_gcov tool to convert that raw profile data into a format GCC can actually eat. You also have to hand this tool the unstripped binary for your program. 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, this inlines all the relevant functions that were inlined in the tran run before it read the profile feedback. It makes the profile more context-sensitive, which is giving ✨. Enabled by default. 👑

---

The next lil cluster of options controls how the compiler behaves around floating-point arithmetic. These ones are literally a trade-off between speed and correctness, and here's the thing: all of them must be specifically enabled. None of them just show up on their own. 🙈

• -fexcess-precision=style — this option lets you control excess precision on machines where floating-point operations happen in a format with more precision or range than the IEEE standard and interchange floating-point types. A classic example of a target like that 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 lowkey rely on what the C or C++ language standards require for handling IEEE values. 😭

By default, -fexcess-precision=fast is in effect — meaning operations may be carried out in a wider precision than the types specified in the source if that gets you faster code, and it's unpredictable when rounding to the source types actually happens. When compiling C or C++, if -fexcess-precision=standard is specified then excess precision follows the rules 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 on by default for C or C++ if a strict conformance option like -std=c99 or -std=c++17 is used. And -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. 💖

Heads up though: -fexcess-precision=standard is not implemented for languages other than C or C++. And 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 comes from a register or from memory. This one has generally been subsumed by -fexcess-precision=standard, which is more general. If you do use -ffloat-store, you might need to modify your program to explicitly stash intermediate computations in temporary variables, since -ffloat-store handles rounding to IEEE format only on assignments and not casts, the way -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 don't need those guarantees. 🔥

• -fno-math-errno — do not set errno after calling math functions that execute with a single instruction, e.g. sqrt. A program that relies on IEEE exceptions for math error handling might want this flag for speed while still keeping 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 don't require the guarantees of these specifications.

The default is -fmath-errno.

On Darwin systems, the math library never sets errno. So there's literally no reason for the compiler to consider that it might, and -fno-math-errno is the default there. 🍓

• -funsafe-math-optimizations — allow optimizations for floating-point arithmetic that (a) assume 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 do 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 don't require the guarantees of these specifications. It 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 the computation result. ⚠️ NOTE: re-ordering may change the sign of zero as well as ignore NaNs and inhibit or create underflow or overflow (and so it cannot be used on code that relies on rounding behavior like (x + 2**52) - 2**52). It 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 that 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 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 don't 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. Those 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 you're relying 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). You should specify this option 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. 🌸

okay bestie, new cluster of flags unlocked 🔓 and listen — these are the glow-up options, the ones that make your binary faster than your ex moved on. but read the fine print fr fr:

The following options control optimizations that may improve performance, but are not enabled by any -O options. This section includes experimental options that may produce broken code. so like… slay, but with a helmet on 💅

---

• -fsignaling-nans — compile your code assuming that IEEE signaling NaNs may generate user-visible traps during floating-point operations. turning this on disables optimizations that may change the number of exceptions visible with signaling NaNs. this option implies -ftrapping-math.

and it makes the preprocessor macro "__SUPPORT_SNAN__" get defined ✨

the default is -fno-signaling-nans.

⚠️ bestie this one is experimental and it does not currently guarantee to disable all GCC optimizations that affect signaling NaN behavior. it's giving "we tried our best" 😭

• -fsingle-precision-constant — treat floating-point constants as single precision instead of implicitly converting them to double-precision constants. no sneaky glow-up into a double, periodt.
• -fcx-limited-range — when enabled, this option says a range reduction step is not needed when performing complex division. also, there is no checking whether the result of a complex multiplication or division is "NaN + I*NaN", with an attempt to rescue the situation in that case. this option is enabled by -ffast-math.

it controls the default setting of the ISO C99 CX_LIMITED_RANGE pragma. nevertheless, the option applies to all languages. iconic 🌈

• -fcx-fortran-rules — complex multiplication and division follow Fortran rules. range reduction is done as part of complex division, but there is no checking whether the result of a complex multiplication or division is "NaN + I*NaN", with an attempt to rescue the situation in that case.
• *-fcx-method=method* — complex multiplication and division follow the stated method. the method argument should be one of limited-range, fortran or stdc.

the default is to honor language specific constraints, which means fortran for Fortran and stdc otherwise.

---

• -fbranch-probabilities — so you ran your program compiled with -fprofile-arcs, yeah? now compile it a second time using -fbranch-probabilities, to improve optimizations based on the number of times each branch was taken. when a program compiled with -fprofile-arcs exits, it saves arc execution counts to a file called sourcename.gcda for each source file. the info in this data file is very dependent on the structure of the generated code, so you must use the same source code and the same optimization options for both compilations. see details about the file naming in -fprofile-arcs.

with -fbranch-probabilities, GCC puts a REG_BR_PROB note on each JUMP_INSN and CALL_INSN. these can be used to improve optimization. currently, they are only used in one place: in reorg.cc, instead of guessing which path a branch is most likely to take, the REG_BR_PROB values are used to exactly determine which path is taken more often. no more guessing, we know now 💅

enabled by -fprofile-use and -fauto-profile.

• -fprofile-values — if combined with -fprofile-arcs, it adds code so that some data about values of expressions in the program is gathered. 🕵️‍♀️

with -fbranch-probabilities, it reads back the data gathered from profiling values of expressions for usage in optimizations.

enabled by -fprofile-generate, -fprofile-use, and -fauto-profile.

• -fprofile-reorder-functions — function reordering based on profile instrumentation collects first time of execution of a function and orders these functions in ascending order, aiming to optimize program startup through more efficient loading of text segments. startup speed? we love to see it ✨

enabled with -fprofile-use.

• -fvpt — if combined with -fprofile-arcs, this option instructs the compiler to add code to gather information about values of expressions.

with -fbranch-probabilities, it reads back the data gathered and actually performs the optimizations based on them. currently the optimizations include specialization of division operations using the knowledge about the value of the denominator. dividing smarter, not harder 🍓

enabled with -fprofile-use and -fauto-profile.

• -frename-registers — attempt to avoid false dependencies in scheduled code by making use of registers left over after register allocation. this optimization most benefits processors with lots of registers. depending on the debug information format adopted by the target, however, it can make debugging impossible, since variables no longer stay in a "home register". 😭 trade-offs, bestie.

enabled by default with -funroll-loops.

• -fschedule-fusion — performs a target dependent pass over the instruction stream to schedule instructions of same type together, because the target machine can execute them more efficiently if they are adjacent to each other in the instruction flow. besties standing next to besties 🫶

enabled at levels -O2, -O3, -Os.

• -fdep-fusion — detect macro-op fusible pairs consisting of single-use instructions and their uses, and place such pairs together in the instruction stream to increase fusion opportunities in hardware. this pass is executed once before register allocation, and another time before register renaming. two passes, no cap 💅

enabled at levels -O2, -O3, -Os.

• -ftracer — perform tail duplication to enlarge superblock size. this transformation simplifies the control flow of the function, allowing other optimizations to do a better job. teamwork makes the dream work 🌈

enabled by -fprofile-use and -fauto-profile.

• -funroll-loops — unroll loops whose number of iterations can be determined at compile time or upon entry to the loop. -funroll-loops implies -frerun-cse-after-loop, -fweb and -frename-registers. it also turns on complete loop peeling (i.e. complete removal of loops with a small constant number of iterations). this option makes code larger, and may or may not make it run faster. no promises, babe 🙈

enabled by -fprofile-use and -fauto-profile.

• -funroll-all-loops — unroll all loops, even if their number of iterations is uncertain when the loop is entered. this usually makes programs run more slowly 💀. -funroll-all-loops implies the same options as -funroll-loops.
• -fpeel-loops — peels loops for which there is enough information that they do not roll much (from profile feedback or static analysis). it also turns on complete loop peeling (i.e. complete removal of loops with small constant number of iterations).

enabled by -O3, -fprofile-use, and -fauto-profile.

• -fmalloc-dce — control whether malloc (and its variants such as 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 also comparisons with NULL pointer are considered safe to remove.

the default is -fmalloc-dce=2. see also -fallocation-dce. bye bye, useless malloc 👋

• -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. note: for this option to have an effect, -ftree-loop-im has to be enabled as well. enabled at level -O1 and higher, except for -Og.
• -fsplit-loops — split 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. it's giving split the difference ✨

enabled by -fprofile-use and -fauto-profile.

• -funswitch-loops — move 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). dupe the loop, not the drama 💅

enabled by -fprofile-use and -fauto-profile.

• -fversion-loops-for-strides — if a loop iterates over an array with a variable stride, create another version of the loop that assumes the stride is always one. for example:

for (int i = 0; i < n; ++i)
  x[i * stride] = ...;

becomes:

if (stride == 1)
  for (int i = 0; i < n; ++i)
    x[i] = ...;
else
  for (int i = 0; i < n; ++i)
    x[i * stride] = ...;

this is particularly useful for assumed-shape arrays in Fortran, where (for example) it allows better vectorization assuming contiguous accesses. this flag is enabled by default at -O3. it is also enabled by -fprofile-use and -fauto-profile. vectorization era 🔥

• -ffunction-sections — and
• -fdata-sections — place each function or data item into its own section in the output file if the target supports arbitrary sections. the name of the function or the name of the data item determines the section's name in the output file.

use these options on systems where the linker can perform optimizations to improve locality of reference in the instruction space. most systems using the ELF object format have linkers with such optimizations. on AIX, the linker rearranges sections (CSECTs) based on the call graph. the performance impact varies. 🙈

together with a linker garbage collection (linker --gc-sections option) these options may lead to smaller statically-linked executables (after stripping). 🎀

on ELF/DWARF systems these options do not degenerate the quality of the debug information. there could be issues with other object files/debug info formats.

⚠️ only use these options when there are significant benefits from doing so. when you specify these options, the assembler and linker create larger object and executable files and are also slower. these options affect code generation. they prevent optimizations by the compiler and assembler using relative locations inside a translation unit, since the locations are unknown until link time. an example of such an optimization is relaxing calls to short call instructions.

• -fstdarg-opt — optimize the prologue of variadic argument functions with respect to usage of those arguments. cute little prologue glow-up ✨
• -fsection-anchors — try to reduce the number of symbolic address calculations by using shared "anchor" symbols to address nearby objects. this transformation can help to reduce the number of GOT entries and GOT accesses on some targets. fewer lookups, more slay 💅

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 the second you compile with -fsection-anchors, it goes and grabs them from one common anchor point instead. The vibe is basically this pseudocode — which is NOT valid C, bestie, do not try to compile it:

        int foo (void)
        {
          register int *xr = &x;
          return xr[&a - &x] + xr[&b - &x] + xr[&c - &x];
        }

Not all targets support this option, so don't come for me if yours is like "nah". 🙈

• -fzero-call-used-regs=choice — zeroes out the call-used registers when a function returns, which levels up your program's security by either mitigating Return-Oriented Programming (ROP) attacks or stopping info from leaking through registers. The possible values of choice are the exact same ones as for the zero_call_used_regs attribute. Default? skip. You can also control this per-function with the function attribute zero_call_used_regs. 💅

---

✨ Program Instrumentation Options ✨

GCC has a whole glow-up toolkit of command-line options that bolt run-time instrumentation onto the code it would normally generate. Like, one reason you'd do this is to collect profiling stats so you can hunt down your program's hot spots, do code coverage analysis, or run profile-guided optimizations. Another flavor of instrumentation adds run-time checking to catch programming oopsies — invalid pointer dereferences, out-of-bounds array accesses — plus deliberately hostile nonsense like stack smashing or C++ vtable hijacking. 😭 And there's also a general hook you can use to build other kinds of tracing or function-level instrumentation for debug or program analysis purposes. It's giving versatile.

• -p, --profile, -fprofile, -pg — generates extra code that writes profile information suited for the analysis program prof (that's for -p, --profile, and -fprofile) or gprof (that's for -pg). You must use this option when compiling the source files you want data about, and you must also use it when linking. 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. While it's running, the program records how many times each branch and call is executed, and how many times it's taken or returns. On targets that support constructors with priority support, profiling properly handles constructors, destructors, and C++ constructors (and destructors) of classes that are used as the type of a global variable. 👑

When the compiled program exits, it saves all this data to a file called auxname.gcda for each source file. That data can 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 it was explicitly specified and it's not the final executable — otherwise it's the basename of the source file. Either way, any suffix gets removed (e.g. foo.gcda for input file dir/foo.c, or dir/foo.gcda for output file specified as -o dir/foo.o).

Heads up bestie: if a command line directly links source files, the corresponding .gcda files get prefixed with the unsuffixed name of the output file. So gcc a.c b.c -o binary would generate binary-a.gcda and binary-b.gcda files. iykyk 💖

• -fcondition-coverage — adds code so that program conditions get instrumented. While running, the program records which terms in a conditional actually 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. Read the results with gcov --conditions. ✨
• -fpath-coverage — adds code so the paths taken get tracked. While running, the program records the prime paths taken. The number of paths grows very fast with complexity, and to keep compile times from absolutely exploding, GCC will give up on instrumenting if the approximate number of paths exceeds the limit controlled by -fpath-coverage-limit. Read the results 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 that slightly overcounts the running number of paths, and bails if the threshold is reached before finding all the paths. This option is not for fine-grained control over which functions get instrumented — it's there to limit the effect of path explosion and keep compile times reasonable. The default is 250000. 🌸
• --coverage, -coverage — this option compiles and links 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 docs for those options for more details. Here's the whole ritual:
  • 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 lets gcov find the correct sources in projects where compilations happen 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 can be repeated any number of times. You can run concurrent instances of your program, and as long as 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.

Also — 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 info.

With -fprofile-arcs, for each function of your program GCC creates a program flow graph, then finds a spanning tree for the graph. Only the arcs that are not on the spanning tree have to be instrumented: the compiler adds code to count how many times those arcs are executed. When an arc is the only exit or only entrance to a block, the instrumentation code can just be added to the block; otherwise, a whole new basic block has to be created to hold it. The way that GCC is out here doing graph theory for you 😭👑

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 never used. And what even is a prime path, you ask? It's the longest sequence of unique blocks, except possibly the first and last, which is not a subpath of any other path. Periodt.

• -ftest-coverage — produces 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. Peep 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 don't optimize. 🎀
• -fprofile-abs-path — automatically converts relative source file names to absolute path names in the .gcno files. This lets gcov find the correct sources in projects where compilations happen with different working directories.
• -fprofile-dir=path — sets the directory to search for the profile data files in to path. This option only affects 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 work. By default, GCC uses the current directory as path, so the profile data file shows up in the same directory as the object file. To prevent file name clashing, if the object file name isn't an absolute path, we mangle the absolute path of the sourcename.gcda file and use that as the file name of a .gcda file. See the details about file naming in -fprofile-arcs. Also peep the similar option -fprofile-note. 🌈

When an executable is run in a massive parallel environment, it's recommended to save profile to different folders. That can be done with variables in path that are exported during run-time:

• %p — process ID.
• %q{VAR} — value of environment variable VAR.

• -fprofile-generate, -fprofile-generate=path — enables the options usually used for instrumenting an application to produce profile useful for later recompilation with profile feedback based optimization. You must use -fprofile-generate both when compiling and when linking your program. 💖

The following options are enabled: -fprofile-arcs, -fprofile-values, -finline-functions, and -fipa-bit-cp.

If path is specified, GCC looks at 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 — okay so instead of doing the whole constructor/destructor thing, this drops your profile info into a section you name. 🎀

The section name is name if you actually pass one — otherwise it just defaults to ".gcov_info". For every translation unit, a pointer to the profile information that -fprofile-arcs made gets placed in that section. And bestie, heads up: this option disables the profile info registration through a constructor, and it also disables the profile info processing through a destructor. Periodt.

Not for hosted environments like GNU/Linux, okay? 🙈 This one targets freestanding environments — think embedded systems — with limited resources that don't support constructors/destructors or the C library file I/O.

So the linker can scoop up all the input sections into one continuous memory block and define start and end symbols. Here's a GNU linker script example that defines a linker output section, verbatim:

.gcov_info      :
{
  PROVIDE (__gcov_info_start = .);
  KEEP (*(.gcov_info))
  PROVIDE (__gcov_info_end = .);
}

And your program could dump the profiling information registered in this linker set, like, for example, this:

#include <gcov.h>
#include <stdio.h>
#include <stdlib.h>

extern const struct gcov_info *const __gcov_info_start[];
extern const struct gcov_info *const __gcov_info_end[];

static void
dump (const void *d, unsigned n, void *arg)
{
  const unsigned char *c = d;

  for (unsigned i = 0; i < n; ++i)
    printf ("%02x", c[i]);
}

static void
filename (const char *f, void *arg)
{
  __gcov_filename_to_gcfn (f, dump, arg );
}

static void *
allocate (unsigned length, void *arg)
{
  return malloc (length);
}

static void
dump_gcov_info (void)
{
  const struct gcov_info *const *info = __gcov_info_start;
  const struct gcov_info *const *end = __gcov_info_end;

  /* Obfuscate variable to prevent compiler optimizations.  */
  __asm__ ("" : "+r" (info));

  while (info != end)
  {
    void *arg = NULL;
    __gcov_info_to_gcda (*info, filename, dump, allocate, arg);
    putchar ('\n');
    ++info;
  }
}

int
main (void)
{
  dump_gcov_info ();
  return 0;
}

Then the merge-stream subcommand of gcov-tool can 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. Slay. 💅

• -fprofile-note=path — if path is specified, GCC saves the .gcno file into the path location. ✨

But fr fr, if you combine this option with multiple source files, the .gcno file will be overwritten. 😭 One file, one survivor.

• -fprofile-prefix-path=path — this one's a team-up move: use it with profile-generate=profile_dir and profile-use=profile_dir to tell GCC where the base directory of your built source tree is. 🌸

By default, profile_dir will contain files with mangled absolute paths of all object files in the built project. And that's not the vibe when the directory used to build the instrumented binary is different from the directory used to build the binary optimized with profile feedback — because then the profile data won't be found during the optimized build. 😭

In those setups, -fprofile-prefix-path=path with path pointing at the base directory of the build can strip off the irrelevant part of the path and keep all file names relative to the main build directory. Chef's kiss. 👑

• -fprofile-prefix-map=old=new — when you're compiling files that live in directory old, record profiling information (with --coverage) describing them as if the files lived in directory new instead. 🦋

See also -ffile-prefix-map and -fcanon-prefix-map.

• -fprofile-update=method — changes the update method for an application instrumented for profile feedback based optimization. The method argument should be one of single, atomic or prefer-atomic. 🫶
  • single — useful for single-threaded applications.
  • atomic — prevents profile corruption by emitting thread-safe code.

Warning: when an application does not properly join all threads (or creates a detached thread), a profile file can still be corrupted. 💀

Using prefer-atomic gets transformed either into atomic (when the target supports it) or into single otherwise. The GCC driver automatically selects prefer-atomic when -pthread is present on 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. But if the target platform doesn't support the required atomic operations in hardware, and libatomic is available, then the profile information gets updated through calls to libatomic. And if the target platform neither supports the required atomic operations in hardware nor has libatomic, then the profile information is not atomically updated and a warning is issued. 😭 In that case, the profiling information you get may be corrupt for multi-threaded applications.

Also, 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. And if a signal interrupts those two operations while a counter is updating, then the profiling information may be in an inconsistent state. 🙈

• -fprofile-filter-files=regex — instrument only functions from files whose name matches any of the regular expressions (separated by semi-colons). 🌈

For example, -fprofile-filter-files=main\.c;module.*\.c will instrument only main.c and all C files starting with 'module'.

• -fprofile-exclude-files=regex — instrument only functions from files whose name does not match any of the regular expressions (separated by semi-colons). 💜

For example, -fprofile-exclude-files=/usr/.* will prevent instrumentation of all files that are located in the /usr/ folder.

• -fprofile-reproducible=[multithreaded|parallel-runs|serial] — controls the level of reproducibility of the profile gathered by -fprofile-generate. This makes it possible to rebuild a program with the same outcome, which is useful, for example, for distribution packages. 🎀

With -fprofile-reproducible=serial the profile gathered by -fprofile-generate is reproducible provided the trained program behaves the same at each invocation of the train run, it's not multi-threaded, and profile data streaming is always done in the same order. Note that profile streaming happens at the end of a program run but also before the fork function is invoked. ✨

And ngl, it's quite common that execution counts of some part of programs depend on, for example, length of temporary file names or memory space randomization (that may affect hash-table collision rate). 😭 Such non-reproducible parts of programs may be annotated by the 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 the 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). But it reduces the quality of the gathered data — in particular of indirect call profiling. 🦋

---

• -fsanitize=address — enables AddressSanitizer, a fast memory error detector. Memory access instructions are instrumented to detect out-of-bounds and use-after-free bugs. 🔥

This 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's 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. ✨

And 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 — enables AddressSanitizer for the Linux kernel. 👑

See <https://github.com/google/kernel-sanitizers> for more details.

• -fsanitize=hwaddress — bestie, this one turns on Hardware-assisted AddressSanitizer, which uses a hardware ability to ignore the top byte of a pointer to detect memory errors with a low memory overhead ✨ Memory access instructions get instrumented so out-of-bounds and use-after-free bugs get caught. Turning this on also enables -fsanitize-address-use-after-scope. More deets over at <https://clang.llvm.org/docs/HardwareAssistedAddressSanitizerDesign.html> 💖 You can nudge the run-time behavior with the HWASAN_OPTIONS environment variable — set it to "help=1" and the available options get shown at startup of the instrumented program, iconic. Cannot be combined with -fsanitize=thread or -fsanitize=address, and it's currently only available on AArch64 🫶
• -fsanitize=kernel-hwaddress — this is the Hardware-assisted AddressSanitizer but for compiling the Linux kernel 🐧 It's similar to -fsanitize=kernel-address but with an alternate instrumentation method, and similar to -fsanitize=hwaddress but with instrumentation differences that 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.

Note: 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 💅

• -fsanitize=memtag-stack — uses Memory Tagging Extension instructions instead of instrumentation to allow the detection of memory errors. Like HWASAN, it's also a probabilistic method 🌸 This option is available only on those AArch64 architectures that support Memory Tagging Extensions.
• -fsanitize=pointer-compare — instruments comparison operations (<, <=, >, >=) that have pointer operands 👀 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 an invalid operation only when both pointers are non-null 💖

• -fsanitize=pointer-subtract — instruments subtraction with pointer operands ✨ 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 an 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) 🛡️ How it works: it saves a function's return address to a separately allocated shadow call stack in the function prologue, then restores the return address from the shadow call stack in the function epilogue. Instrumentation only happens in functions that actually need to save the return address to the stack.

Currently it only supports the aarch64 platform, and 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 in user space you have to provide your own support for the runtime. 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". That generally means 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 get instrumented to detect data race bugs. See <https://github.com/google/sanitizers/wiki#threadsanitizer> for more deets. You can influence the run-time behavior using the TSAN_OPTIONS environment variable; <https://github.com/google/sanitizers/wiki/ThreadSanitizerFlags> has a list of supported options. Cannot be combined with -fsanitize=address or -fsanitize=leak.

When compiling with -fsanitize=thread, you should also use -g to produce more meaningful output, fr fr.

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. You can influence the run-time behavior using the LSAN_OPTIONS environment variable. Cannot be combined with -fsanitize=thread.
• -fsanitize=undefined — enables UndefinedBehaviorSanitizer, a fast undefined behavior detector 🔥 Various computations get instrumented to detect undefined behavior at runtime. See <https://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html> for more details. You can influence the run-time behavior using the UBSAN_OPTIONS environment variable. Current suboptions are:
  • -fsanitize=shift — this option enables checking that the result of a shift operation is not undefined. Note that what exactly is considered undefined differs slightly between C and C++, as well as between ISO C90 and C99, etc. 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. Specifically, 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.
• -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 — bestie, this one instruments your calls and checks that you're not passing null values into arguments that were marked as needing a non-null value by the nonnull function attribute. 👀
• -fsanitize=returns-nonnull-attribute — instruments return statements inside functions marked with the returns_nonnull function attribute, so it can catch it when one of those functions tries to hand back a null value. Rude of them, honestly. 🙈
• -fsanitize=bool — instruments loads from bool. If a value that isn't 0/1 gets loaded, a run-time error gets issued. That's it, that's the vibe check. ✨
• -fsanitize=enum — instruments loads from an enum type. Load a value outside the range of values for that enum type and boom, run-time error. 💀
• -fsanitize=vptr — this one instruments C++ member function calls, member accesses, and some conversions between pointers to base and derived classes, just to verify the referenced object actually has the correct dynamic type. Main character energy enforcement. 👑
• -fsanitize=pointer-overflow — instruments pointer arithmetics. If the pointer arithmetics overflows, a run-time error is issued. Math gone wrong, caught in 4k. 🔥
• -fsanitize=builtin — instruments arguments to selected builtin functions. Pass an invalid value to one of those arguments and a run-time error is issued. For example, passing 0 as the argument to __builtin_ctz or __builtin_clz invokes undefined behavior and gets diagnosed by this option. Not me passing zero to __builtin_ctz… 😭

Heads up bestie: sanitizers tend to increase the rate of false positive warnings, most notably the ones around -Wmaybe-uninitialized. The manual straight up recommends against combining -Werror and [the use of] sanitizers. So don't do it, iykyk.

While -ftrapv causes traps for signed overflows to be emitted, -fsanitize=undefined gives you a diagnostic message instead. And this currently only works for the C family of languages. 🫶

---

• -fno-sanitize=all — disables all previously enabled sanitizers. Quick note though: -fsanitize=all is not allowed, because some sanitizers literally cannot be used together. They don't get along, it's giving ex energy. 🚫
• -fasan-shadow-offset=number — forces GCC to use a custom shadow offset in AddressSanitizer checks. Super useful for experimenting with different shadow memory layouts in Kernel AddressSanitizer. 🧚
• -fsanitize-sections=s1,s2,... — sanitize global variables in selected user-defined sections. Each si may contain wildcards. Pick your faves. ⭐
• -fsanitize-recover[=opts] — -fsanitize-recover= controls error recovery mode for the sanitizers named in the comma-separated list of opts. Enabling it for a sanitizer component makes that component attempt to keep the program running 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. 💅

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.

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. Also accepted: -fsanitize-recover=all and -fno-sanitize-recover=all — the former enables recovery for all sanitizers that support it, the latter disables recovery for all sanitizers that support it.

Even if a recovery mode is turned on on the compiler side, it needs to also be enabled on the runtime library side, otherwise the failures are still fatal. The runtime library defaults to halt_on_error=0 for ThreadSanitizer and UndefinedBehaviorSanitizer, while the default value for AddressSanitizer is halt_on_error=1. This can be overridden by setting the halt_on_error flag in the corresponding environment variable.

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. No reusing, we're not sharing. 🙅
• -fsanitize-trap[=opts] — the -fsanitize-trap= option tells the compiler to report for the sanitizers named 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 -fsanitizer-recover= for that sanitizer — __builtin_trap will be emitted and be fatal regardless of whether recovery is enabled or disabled using -fsanitize-recover=.

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. Slay. ✨

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 also be 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.

• -fsanitize-undefined-trap-on-error — the -fsanitize-undefined-trap-on-error option is a deprecated equivalent of -fsanitize-trap=all. She's old news but she's still here. 🎀
• -fsanitize-coverage=trace-pc — enable coverage-guided fuzzing code instrumentation. Inserts a call to __sanitizer_cov_trace_pc into every basic block. Every. Single. One. 🌸
• -fsanitize-coverage=trace-cmp — enable 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, plus __sanitizer_cov_trace_cmpf or __sanitizer_cov_trace_cmpd for float or double comparisons, and __sanitizer_cov_trace_switch for switch statements. The whole squad showed up. 🫶
• -fcf-protection=[full|branch|return|none|check]
• -fcf-protection — enable 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). Bodyguard era. 🛡️

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.
• 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.

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.

You can also use the nocf_check attribute to identify which functions and calls should be skipped from instrumentation. Not invited to the party. 💅

Currently the x86 GNU/Linux target provides an implementation based on Intel Control-flow Enforcement Technology (CET) which works for i686 processor or newer.

NOTE: In Ubuntu 19.10 and later versions, -fcf-protection is enabled by default for C, C++, ObjC, ObjC++, if none of -fno-cf-protection nor -fcf-protection=* are found.

• -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 everything like a girl who re-reads her texts. 😭
• -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. 🌈
• -fharden-control-flow-redundancy — emit 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.

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. Theory girlies only. 💜

• -fhardcfr-skip-leaf — says nope to -fharden-control-flow-redundancy inside leaf functions 💤 (leaf functions are the little guys that don't call anyone, so we let them vibe free).
• -fhardcfr-check-exceptions — when -fharden-control-flow-redundancy is active, this checks the recorded execution path against the control flow graph at exception escape points 🚨, like the function body got wrapped in a cleanup handler that did the check and then reraised. This one is on by default, bestie — hit it with -fno-hardcfr-check-exceptions if you want it off.
• -fhardcfr-check-returning-calls — when -fharden-control-flow-redundancy is active, check the recorded execution path against the control flow graph right before any function call that's immediately followed by a return of its result (if there is one), so it doesn't block tail-call optimization 🦋 — whether or not that call ultimately gets optimized into a tail call.

This one is enabled by default whenever sibling call optimizations are on (peep -foptimize-sibling-calls), but you can turn it on (or off, with the negated form) explicitly, no matter what the optimizations are doing. main character energy, honestly 👑

• -fhardcfr-check-noreturn-calls=[always|no-xthrow|nothrow|never] — when -fharden-control-flow-redundancy is active, check the recorded execution path against the control flow graph before noreturn calls 💀. Pick your flavor:
  • always — all of them
  • no-xthrow — those that aren't expected to return control to the caller through an exception (this is the default, periodt)
  • nothrow — those that may not return control to the caller through an exception either
  • never — none of them, we're not checking, we're living

Checking before a noreturn function that may return control to the caller through an exception can make the check happen more than once 😭 — if the exception gets caught in the caller, whether by a handler or a cleanup. And when -fhardcfr-check-exceptions is also enabled, the compiler will avoid associating a noreturn call with that implicitly-added cleanup handler, since it'd be redundant with the check done before the call. But other handlers or cleanups in the function, if they get activated, will modify the recorded execution path and check it again when another checkpoint is hit. And the checkpoint might even be another noreturn call, so the check can end up running multiple times 🫠.

Various optimizers may cause calls to be marked as noreturn and/or nothrow, even when the corresponding attributes aren't there, and that can affect where checks before calls get placed, plus whether implicit cleanup handlers get added for them. This unpredictability, plus the fact that raising and reraising exceptions basically amounts to implicitly calling noreturn functions, is why no-xthrow is the default here: it excludes from the noreturn treatment only the internal functions used to (re)raise exceptions, which aren't affected by those optimizations. iykyk 💅

• -fhardened — turns on a whole set of flags for C and C++ that level up the security of the generated code without messing with its ABI 🔥. The exact flags turned on may change between major releases of GCC, but right now they are:

-D_FORTIFY_SOURCE=3 -D_GLIBCXX_ASSERTIONS
-ftrivial-auto-var-init=zero -fPIE  -pie  -Wl,-z,relro,-z,now
-fstack-protector-strong -fstack-clash-protection
-fcf-protection=full (x86 GNU/Linux only)

You can get the list of options -fhardened turns on by using the --help=hardened option ✨.

When the system glibc is older than 2.35, -D_FORTIFY_SOURCE=2 is used instead.

This option is meant for production builds, not just debug builds. fr fr.

Currently, -fhardened is only supported on GNU/Linux targets 🐧.

-fhardened only turns on a particular option if it wasn't already specified anywhere on the command line. So like, -fhardened -fstack-protector will only enable -fstack-protector, but not -fstack-protector-strong. Respect the user's choices, we love that 🫶

• -fstack-protector — emits extra code to check for buffer overflows, like stack smashing attacks 💥. It does this by adding a guard variable to functions with vulnerable objects. That includes functions that call alloca, and functions with buffers larger than or equal to 8 bytes. The guards get initialized when a function is entered, then checked when the function exits. If a guard check fails, an error message is printed and the program exits 🚪. Only variables that are actually allocated on the stack count — optimized-away variables or variables living in registers don't count, bestie.
• -fstack-protector-all — same as -fstack-protector, except literally all functions are protected. No one is left out 🥰.
• -fstack-protector-strong — like -fstack-protector but it includes extra functions to protect --- those that have local array definitions, or that reference local frame addresses. Again, only variables actually allocated on the stack count; optimized-away variables or register-allocated ones don't count.

NOTE: In Ubuntu 14.10 and later versions, -fstack-protector-strong is enabled by default for C, C++, ObjC, ObjC++, if none of -fno-stack-protector, -nostdlib, nor -ffreestanding are found. 👀

• -fstack-protector-explicit — like -fstack-protector but it only protects those functions that have the stack_protect attribute. exclusive club, invite only 💅
• -fstack-check — generates code to verify that you don't go beyond the boundary of the stack 🛑. You should specify this flag if you're running in an environment with multiple threads, but you only rarely need to specify it in a single-threaded environment, since stack overflow is automatically detected on nearly all systems when there's only one stack.

Heads up: this switch doesn't actually cause checking to happen — the operating system or the language runtime has to do that. The switch just causes generation of code to make sure they see the stack being extended.

You can also pass a string parameter:

• no — means no checking
• generic — forces the use of old-style checking
• specific — uses the best checking method and is equivalent to bare -fstack-check

Old-style checking is a generic mechanism that needs no specific target support in the compiler, but it comes with these drawbacks 😬:

1. Modified allocation strategy for large objects: they're always allocated dynamically if their size exceeds a fixed threshold. Note this may change the semantics of some code.
2. Fixed limit on the size of the static frame of functions: when it's topped by a particular function, stack checking is not reliable and a warning is issued by the compiler.
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.

• -fstack-clash-protection — generates code to prevent stack clash style attacks 🛡️. When this option is enabled, the compiler only allocates one page of stack space at a time, and each page is accessed immediately after allocation. So it stops allocations from jumping over any stack guard page the operating system provided.

Most targets don't fully support stack clash protection. But 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.

NOTE: In Ubuntu 19.10 and later versions, -fstack-clash-protection is enabled by default for C, C++, ObjC, ObjC++, unless -fno-stack-clash-protection is found. 🌈

• -fstack-limit-register=reg / -fstack-limit-symbol=sym / -fno-stack-limit — generates code to make sure the stack doesn't grow beyond a certain value, either the value of a register or the address of a symbol 📏. If a larger stack is required, a signal gets raised at run time. For most targets, the signal is raised before the stack overruns the boundary, so you can catch the signal without taking special precautions.

For instance, if the stack starts at absolute address 0x80000000 and grows downwards, you can use the flags -fstack-limit-symbol=__stack_limit and -Wl,--defsym,__stack_limit=0x7ffe0000 to enforce a stack limit of 128KB. Note that this may only work with the GNU linker.

You can locally override stack limit checking by using the no_stack_limit function attribute. cute little escape hatch 🎀

• -fsplit-stack — generates code to automatically split the stack before it overflows 🌊. The resulting program has a discontiguous stack, which can only overflow if the program can't allocate any more memory. This is most useful when running threaded programs, since you no longer need to calculate a good stack size for each thread. Currently this is only implemented for the x86 targets running GNU/Linux.

When code compiled with -fsplit-stack calls code compiled without -fsplit-stack, there may not be much stack space available for the latter code to run 😭. If compiling all code, including library code, with -fsplit-stack isn't an option, then the linker can fix up these calls so that the code compiled without -fsplit-stack always has a large stack. Support for this is implemented in the gold linker in GNU Binutils release 2.21 and later.

• -fstrub=disable — disables stack scrubbing entirely, ignoring any strub attributes 🙈.
• -fstrub=strict — functions default to strub mode disabled, and strictly apply 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). 🧚
• -fstrub=relaxed — bestie, this just puts the stack scrub ("strub") setting back to its default, meaning "strub" only turns on when the "strub" attributes attached to function and data types say it has to. "Relaxed" = strub contexts only get blocked from calling functions that are explicitly marked with "strub" mode "disabled". Lowkey the only reason this option exists is to override other -fstrub=* options that showed up earlier on the command line. 💅
• -fstrub=at-calls — turns on "at-calls" "strub" mode wherever it's actually viable. Main character purpose here? Testing, fr fr. It works out the "strub" machinery in scenarios that are strictly local to a translation unit. Heads up though: this "strub" mode messes with function interfaces, so any function that's visible to other translation units — or that has its address taken — will not be affected by this option. Optimization options can affect viability too. Go read the "strub" attribute docs for the deets on viability and eligibility requirements. 👑
• -fstrub=internal — enables "internal" "strub" mode wherever viable. Again, primary use is testing. This one's meant to thoroughly exercise the parts of the "strub" machinery that implement the less efficient but interface-preserving "strub" mode. Functions that wouldn't be affected by this option are genuinely quite uncommon. 🦋
• -fstrub=all — enables some "strub" mode wherever it's viable. When both strub modes are viable, "at-calls" is the preferred one. -fdump-ipa-strubm adds function attributes that tell you which mode got picked for each function. Primary use of this option: testing, to thoroughly exercise the "strub" machinery. ✨
• -fvtable-verify=[std|preinit|none] — C++ only, bestie. This turns on (or off, if you pass -fvtable-verify=none) the security feature that checks at run time, for every single virtual call, that the vtable pointer the call goes through is valid for the type of the object and hasn't been corrupted or overwritten. If an invalid vtable pointer gets detected at run time, an error is reported and the program is immediately halted. 💀

This option makes run-time data structures get built at program startup, and those get used to verify the vtable pointers. The options std and preinit control when those data structures get built. In both cases the data structures are built before execution reaches main. Using -fvtable-verify=std builds the data structures after shared libraries have been loaded and initialized. -fvtable-verify=preinit builds them before shared libraries have been loaded and initialized.

If this option shows up multiple times on the command line with different values, none takes highest priority over both std and preinit; preinit takes priority over std.

• -fvtv-debug — when you use this alongside -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 gets written to 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, make sure you delete any existing one first. 🙈

• -fvtv-counts — this one's a debugging flag. When used 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 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 info to vtv_class_set_sizes.log in the same directory. 🍓

Note: this feature appends data to the log files. To get fresh log files, make sure you delete any existing ones. 🙈

• -finstrument-functions — generates instrumentation calls for entry to and exit from functions. Just after a function is entered and just before it exits, these 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 that get expanded inline inside other functions. The profiling calls indicate where, conceptually, the inline function is entered and exited. Which means addressable versions of such functions must be available. If all your uses of a function get expanded inline, that could mean an additional expansion of code size. If you use extern inline in your C code, an addressable version of such functions must be provided. (Normally that's 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. You can use that 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 get called only once per instrumented function. So the first profiling function gets called after the first entry into the instrumented function, and the second profiling function gets called before the exit corresponding to that first entry. ✨

The definition of "once" for this option is a little vague, ngl, because the implementation isn't protected against data races. So the implementation only guarantees the profiling functions are called at least once per process and at most once per thread — but the calls are always paired. Meaning: if a thread calls the first function, it'll call the second function too, 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 (peep the description of -finstrument-functions). If the file containing a function definition matches one of file, then that function isn't instrumented. The match is done on substrings: if the file parameter is a substring of the file name, that counts as a match. 💅

For example:

-finstrument-functions-exclude-file-list=/bits/stl,include/sys

excludes any inline function defined in files whose pathnames contain /bits/stl or include/sys.

If, for some reason, you want to include letter , in one of sym, write ,. For example, -finstrument-functions-exclude-file-list=',,tmp' (note the single quote surrounding the option).

• -finstrument-functions-exclude-function-list=sym,sym,... — similar to -finstrument-functions-exclude-file-list, except this option sets the list of function names to be excluded from instrumentation. The function name to be 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, that counts as a match. For C99 and C++ extended identifiers, the function name must 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 any 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. You can use that 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. 👑

---

✨ Options Controlling the Preprocessor ✨

These options control the C preprocessor, which gets run on each C source file before actual compilation happens. 🎀

Okay so if you use -E, literally nothing happens except preprocessing. Periodt. And some of these options only make sense with -E, because they make the preprocessor output ✨unusable✨ for actual compilation — like, bestie, you can't compile that. 🙈

Oh and psst — besides everything listed down here, there's a whole other pile of options for controlling include-file search paths, and those live over in Directory Options. And if you want to control preprocessor diagnostics? Those are chilling in Warning Options. iykyk. 🫶

---

• -D name, --define-macro=name, --define-macro name — predefines name as a macro, with definition 1. That's it, that's the whole vibe. 💅
• -D name=definition, --define-macro=name=definition, --define-macro name=definition — the contents of definition get tokenized and processed as if they showed up during translation phase three in a #define directive. Which means, bestie, the definition is truncated by embedded newline characters. 😭

If you're calling the preprocessor from a shell or a shell-like program, you might need to use the shell's quoting syntax to protect characters like spaces that mean something in shell syntax. Not me forgetting to quote and then being confused — 🙈

Wanna 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 quote the option. With sh and csh, -D'name(args...)=definition' works. Slay. ✨

Big important thing: -D and -U options are processed in the order they are given on the command line. And all -imacros file and -include file options are processed after all -D and -U options. Order matters, queen. 👑

• -U name, --undefine-macro=name, --undefine-macro name — cancels any previous definition of name, whether it was built in or you provided it with a -D option. Undefine it, unalive it, gone. 💀
• -include file, --include=file, --include file — processes file as if #include "file" appeared as the first line of the primary source file. BUT (and this is the plot twist 🎀) the first directory searched for file is the preprocessor's working directory instead of the directory containing the main source file. If it's not found there, it's searched for in the remainder of the #include "..." search chain as normal.

Give multiple -include options and the files get included in the order they appear on the command line. No shuffling. 🌸

• -imacros file, --imacros=file, --imacros file — exactly like -include, except any output produced by scanning file is thrown away. Macros it defines remain defined. So you can acquire all the macros from a header without also processing its declarations. It's giving "I just want the macros, not the drama." 💅

All files specified by -imacros are processed before all files specified by -include.

• -undef — do not predefine any system-specific or GCC-specific macros. The standard predefined macros remain defined, so don't panic. 🌈
• -pthread — defines additional macros required for using the POSIX threads library. You should use this option consistently for both compilation and linking. Supported on GNU/Linux targets, most other Unix derivatives, and also on x86 Cygwin and MinGW targets. Consistency is the moment, fr fr. ⭐
• -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 those coming from -include or -imacros command-line options. Chef's kiss. 👑

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. None. Zero. 🎀

This option does not suppress the preprocessor's debug output, such as -dM. To avoid mixing such debug output with the dependency rules you should explicitly specify the dependency output file with -MF, or use an environment variable like DEPENDENCIES_OUTPUT. Debug output is still sent to the regular output stream as normal.

Passing -M to the driver implies -E, and suppresses warnings with an implicit -w. Sneaky but iconic. ✨

• -MM, --user-dependencies — like -M but do not mention header files that are found in system header directories, nor header files that are included, directly or indirectly, from such a header.

This implies that the choice of angle brackets or double quotes in an #include directive does not in itself determine whether that header appears in -MM dependency output. Plot twist. 🦋

• -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. Main character energy, overriding everything. 💅

If file is -, then the dependencies are written to stdout.

• -MG, --print-missing-file-dependencies — in conjunction with an option such as -M requesting dependency generation, -MG assumes missing header files are generated files and adds them to the dependency list without raising an error. The dependency filename is taken directly from the #include directive without prepending any path. -MG also suppresses preprocessed output, as a missing header file renders this useless. No cap.

This feature is used in automatic updating of makefiles. 🤖

• -Mno-modules — disables dependency generation for compiled module interfaces. Simple as. 🎀
• -MP — this option instructs CPP to add a phony target for each dependency other than the main file, causing each to depend on nothing. These dummy rules work around errors make gives if you remove header files without updating the Makefile to match. Not you deleting a header and make being dramatic. 😭

This is typical output:

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. The result is the target. 🎯

An -MT option sets the target to be exactly the string you specify. Want multiple targets? 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. It protects itself, honestly so valid. 💅

• -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. Multitasking icon. ✨

• -MMD, --write-user-dependencies — like -MD except mention only user header files, not system header files. Keep it user-only, no system crashers. 🫶
• -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. Just a tokenizer. It's giving "I'm just here for the vibes." 💅

-fpreprocessed is implicit if the input file has one of the extensions .i, .ii or .mi. These are the extensions that GCC uses for preprocessed files created by -save-temps.

• -fdirectives-only — when preprocessing, handle directives, but do not expand macros. 😤

The option's behavior depends on the -E and -fpreprocessed options.

With -E, preprocessing is limited to the handling of directives such as "#define", "#ifdef", and "#error". Other preprocessor operations, such as macro expansion and trigraph conversion are not performed. In addition, the -dD option is implicitly enabled. Sneaky bonus. ✨

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". Round and round we go. 🔄

• -fdollars-in-identifiers — accepts $ in identifiers. Yeah, dollars. Cha-ching. 💸
• -fextended-identifiers — accepts universal character names and extended characters in identifiers. This option is enabled by default for C99 (and later C standard versions) and C++. On by default, no cap. ⭐
• -fno-canonical-system-headers — when preprocessing, do not shorten system header paths with canonicalization. Leave those paths long, queen. 🎀
• -fmax-include-depth=depth — sets the maximum depth of the nested #include. The default is 200. Don't nest deeper than 200, bestie, that's the limit. 😭
• -fsearch-include-path[=kind] — looks 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. Choose your fighter. 🥊

• -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] — okay so this one tracks where tokens came from across macro expansions, which means when a compilation error happens inside a macro expansion, the compiler can actually show you the whole expansion stack instead of just going "something broke, good luck bestie 💀". Downside? More memory eaten by both the preprocessor and the compiler. That's what level is for — you dial the precision of token location tracking up or down depending on your memory budget:
  • 0 — de-activates the option entirely.
  • 1 — tracks token locations in a degraded mode for minimal memory overhead. In this mode, all tokens resulting from the expansion of an argument of a function-like macro get the same location. So like, the vibe is there but the details are fuzzy 🙈
  • 2 — tracks token locations completely. This value is the most memory hungry. 💅

When you give this option with no argument at all, the default level is 2.

Heads up: -ftrack-macro-expansion=2 is activated by default. So you're already living in the fancy mode unless you say otherwise. ✨

• -fmacro-prefix-map=old=new — when preprocessing files that live in directory old, expand the __FILE__ and __BASE_FILE__ macros as if those files lived in directory new instead. Super useful trick: use . for new to turn an absolute path into a relative one, which gives you more reproducible builds that are location independent. 🫶 This option also affects __builtin_FILE() during compilation. Also go peek at -ffile-prefix-map and -fcanon-prefix-map while you're here.
• -fexec-charset=charset — sets the execution character set, which is what gets used for string and character constants. Default is UTF-8. And charset can be any encoding your system's iconv library routine supports. 🌈
• -fwide-exec-charset=charset — sets the wide execution character set, used for wide string and character constants. The default is one of UTF-32BE, UTF-32LE, UTF-16BE, or UTF-16LE — whichever matches the width of wchar_t and the big-endian or little-endian byte order being used for code generation. Same as -fexec-charset, charset can be any encoding supported by the system's iconv library routine; however, you will have problems with encodings that don't fit exactly in wchar_t. 😭
• -finput-charset=charset — sets the input character set, used for translating from the character set of the input file to the source character set GCC uses. If the locale doesn't specify it, or GCC can't get that info from the locale, the default is UTF-8. Either the locale or this command-line option can override it. Currently the command-line option takes precedence if there's a conflict. charset can be any encoding supported by the system's iconv library routine. 💜
• -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 and not the files that were used to create it — because those files aren't 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 is switched on by -save-temps.

You should not write this #pragma in your own code, but it is safe to edit the filename if the PCH file is available in a different location. The filename may be absolute or it may be relative to GCC's current directory.

• -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 on the command line, this option has no effect, since no #line directives are emitted whatsoever. 💅
• -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. 🍓

You should be prepared for side effects when using -C; it causes the preprocessor to treat comments as tokens in their own right. For example, comments appearing at the start of what would be a directive line have the effect of turning that line into an ordinary source line, since the first token on the line is no longer a #.

• -CC, --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. Why? To prevent later use of that macro from inadvertently commenting out the rest of the source line. Not me protecting you from yourself 🫶

The -CC option is generally used to support lint comments.

• -P, --no-line-commands — inhibits generation of linemarkers in the preprocessor's output. 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 that might get confused by the linemarkers. 🦋
• -traditional, --traditional, -traditional-cpp, --traditional-cpp — tries to imitate the behavior of pre-standard C preprocessors, as opposed to ISO C preprocessors. Go read the GNU CPP manual for the details, bestie. 📖

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 — supports 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. 🤯

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 which only permit very short file names, such as MS-DOS. retro problems, retro solutions 💀
• -H, --trace-includes — prints the name of each header file used, in addition to all the 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 are 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. 🙈
  • -dM, --dump=M — instead of the normal output, generates a list of #define directives for all the macros defined during the execution of the preprocessor, including predefined macros. This gives you a way to find out what's 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.

• -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 emits only the macro names, not their expansions. lowkey minimalist 💅
• -dI, --dump=I — outputs #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. chaotic but correct 🌀

• -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 is split into multiple options at the commas. However, many options are modified, translated or interpreted by the compiler driver before being passed to the preprocessor, and -Wp forcibly bypasses this phase. The preprocessor's direct interface is undocumented and subject to change, so whenever possible you should avoid using -Wp and let the driver handle the options instead. 🚨

fr fr, the driver knows what it's doing. Let it cook. 👑

• -Xpreprocessor option — pass option as an option to the preprocessor. You can use this to supply system-specific preprocessor options that GCC does not recognize. 🌸

If you want to pass an option that takes an argument, you must use -Xpreprocessor twice — once for the option and once for the argument. Yes, twice. It's giving needy but it works. 😭

• -no-integrated-cpp, --no-integrated-cpp — okay so by default GCC does preprocessing inside the whole input tokenization and parsing situation, all integrated and cozy 💅. But if you pass this, bestie, preprocessing becomes its own separate pass that happens before compilation. The way it works: the right language front end — that's cc1, cc1plus, or cc1obj for C, C++, and Objective-C, respectively — gets invoked twice. Once for preprocessing only, and once for the actual compilation of the preprocessed input. Why would you want that? It can be useful with the -B or -wrapper options so you can swap in an alternate preprocessor, or do extra processing on the source between normal preprocessing and compilation. Very DIY of you ✨.

---

🎀 Passing Options to the Assembler 🎀

Yesss you can absolutely pass options to the assembler, and honestly? Iconic. Here's how 👑

• -Wa,option — passes option as an option to the assembler. And listen, if option contains commas, it gets split into multiple options right at those commas. Multitasking behavior fr 🫶.
• -Xassembler option, --for-assembler=option, --for-assembler option — also passes option as an option to the assembler. Use this one when you gotta supply system-specific assembler options that GCC doesn't recognize. Super valid.

Real talk though: 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. Two trips, no shortcuts 💀.

---

💖 Options for Linking 💖

These bad boys only come into play when the compiler is actually linking object files into an executable output file. If there's no link step happening? They're meaningless. Nada. Nothing. Moving on 🦋.

• object-file-name — a file name that does not end in a special recognized suffix is treated as naming an object file or a library. (The linker tells object files apart from libraries by looking at the file contents, sneaky but smart 🔍.) If linking is happening, these object files get used as input to the linker.
• -c, -S, -E — if any of these three is used, the linker does not run, and object file names should not be used as arguments. Like, don't even try it 💅.
• -flink-libatomic — enables linking of libatomic if it's supported by the target, and it's enabled by default, no cap. The negative form -fno-link-libatomic can be used to explicitly disable linking of libatomic if you're feeling anti-atomic 🙈.
• -flinker-output=type — this one controls code generation of the link-time optimizer. By default the linker output is automatically determined by the linker plugin (so hands-off, so effortless ✨). But for debugging the compiler, and if you want incremental linking with a non-LTO object file, it may be useful to control the type manually. Let's break down the tea by type, bestie:
  • If type is exec, code generation produces a static binary. In this case -fpic and -fpie are both disabled. Bye bye position independence 👋.
  • If type is dyn, code generation produces a shared library. In this case -fpic or -fPIC is preserved, but not enabled automatically. This lets you build shared libraries without position-independent code on architectures where that's possible, i.e. on x86. Fancy.
  • If type is 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. Little exception, big difference 🎀.
  • If type is rel, the compiler assumes that incremental linking is done. The sections containing intermediate code for link-time optimization get merged, pre-optimized, and output to the resulting object file. Plus, 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. And 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. Efficiency queen 👑.
  • Finally 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. Tradeoffs, always tradeoffs 😭.

Okay so important lore: 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. Noted, fr fr 📝.

• -fuse-ld=bfd — use the bfd linker instead of the default linker. Classic choice 💅.
• -fuse-ld=gold — use the gold linker instead of the default linker. Shiny ✨.
• -fuse-ld=lld — use the LLVM lld linker instead of the default linker. LLVM stans rise 🫶.
• -fuse-ld=mold — use the Modern Linker (mold) instead of the default linker. Modern, as the name literally says 💅.
• -fuse-ld=wild — use the Wild linker (wild) instead of the default linker. Going feral, love that 🌈.
• -llibrary, -l library — searches the library named library when linking. (Heads up: the second form, with the library as a separate argument, is only for POSIX compliance and is not recommended — so don't be doing that 🙈.) The -l option is passed directly to the linker by GCC, so for the exact deets you gotta refer to your linker documentation. The general description below applies to the GNU linker though. The linker searches a standard list of directories for the library — that list includes 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. And plot twist: it matters where in the command you write this option, because the linker searches and processes libraries and object files in the order they're specified. So foo.o -lz bar.o searches library z after file foo.o but before bar.o. And if bar.o refers to functions in z, those functions may not be loaded. Order is everything, bestie 😤.
• -lobjc — you need this special case of the -l option in order to link an Objective-C or Objective-C++ program. Special little snowflake of an option 🎀.
• -nostartfiles — do not use the standard system startup files when linking. The standard system libraries are used normally though, unless -nostdlib, -nolibc, or -nodefaultlibs is used. Selective skipping ✂️.
• -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 — like -static-libgcc or -shared-libgcc — are ignored. The standard startup files are used normally, unless -nostartfiles is used. Also important: the compiler may generate calls to memcmp, memset, memcpy and memmove. These entries are usually resolved by entries in libc, so you should supply these entry points through some other mechanism when this option is specified. No freebies 💀.
• -nolibc — do not use the C library or system libraries tightly coupled with it when linking. Still links with the startup files, libgcc, or toolchain-provided language support libraries such as libgnat, libgfortran or libstdc++ — unless options preventing their inclusion are used as well. This typically removes -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. It's intended for bare-board targets when there is indeed no C library available. Bare metal energy 🔥.
• -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, and should be supplied through some other mechanism when this option is specified. Also, 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. Here's the thing though: 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.) Don't forget it, bestie, I'm begging 🙏.
• -nostdlib++ — do not implicitly link with standard C++ libraries. C++ said no thanks today 💅.
• -e entry, --entry=entry, --entry entry — specifies 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. Your call, queen 👑.
• -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. Consistency is the vibe check ✅.
• -no-pie — don't produce a dynamically linked position independent executable. Simply not doing that today ✋.
• -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 it can be loaded at any address without a dynamic linker. Obsessed with the flexibility honestly 🦋. 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. Both or neither, bestie, that's the rule 🧵.
• -r — produce a relocatable object as output. This is also known as partial linking. Half a link, all the vibes ✨.
• -rdynamic — tells the linker -export-dynamic (on targets that actually support it, no cap). This makes the linker toss all the symbols into the dynamic symbol table, not just the used ones. You need this for certain uses of dlopen, or if you wanna grab backtraces from inside a program. 💅
• -s — strip all symbol table and relocation info out of the executable. Gone. Bye. 🫶
• -static, --static — on systems with dynamic linking, this overrides -pie and stops linking with the shared libraries. On other systems? Literally no effect bestie. ✨
• -shared, --shared — produce a shared object that can then be linked with other objects to make an executable. Not every system supports this one. And for predictable results, you also have to specify the same set of options used for compilation (-fpic, -fPIC, or model suboptions) when you pass this linker option.[1] 👑
• -shared-libgcc, -static-libgcc — on systems that give you libgcc as a shared library, these force the shared or static version respectively. If no shared version of libgcc was built when the compiler got configured, these options do nothing. 🙈

There are a few situations where an app should use the shared libgcc instead of the static one. The most common is when the app wants to throw and catch exceptions across different shared libraries. In that case, each of the libraries and the app itself should use the shared libgcc.

So the G++ driver automatically adds -shared-libgcc whenever you build a shared library or a main executable, because C++ programs typically use exceptions, so this is the right thing to do. 💖

If instead you use the GCC driver to create shared libraries, you may find they aren't always linked with the shared libgcc. If GCC finds, at its configuration time, that you have a non-GNU linker or a GNU linker that doesn't 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 — if a library or main executable is supposed to throw or catch exceptions, you must link it using the G++ driver, or using the option -shared-libgcc, so that it's linked with the shared libgcc. 😤

• -static-libasan — when the -fsanitize=address option is used to link a program, the GCC driver automatically links against libasan. If libasan is available as a shared library, and -static isn't used, then this links against the shared version of libasan. The -static-libasan option directs the GCC driver to link libasan statically, without necessarily linking other libraries statically. 🧚
• -static-libtsan — when the -fsanitize=thread option is used to link a program, the GCC driver automatically links against libtsan. If libtsan is available as a shared library, and -static isn't used, then this links against the shared version of libtsan. The -static-libtsan option directs the GCC driver to link libtsan statically, without necessarily linking other libraries statically. 🌸
• -static-liblsan — when the -fsanitize=leak option is used to link a program, the GCC driver automatically links against liblsan. If liblsan is available as a shared library, and -static isn't used, then this links against the shared version of liblsan. The -static-liblsan option directs the GCC driver to link liblsan statically, without necessarily linking other libraries statically. 💜
• -static-libubsan — when the -fsanitize=undefined option is used to link a program, the GCC driver automatically links against libubsan. If libubsan is available as a shared library, and -static isn't 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++ — 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 -static isn't used, then this links against the shared version of libstdc++. That's normally fine. However, it's sometimes useful to freeze the version of libstdc++ that the program uses 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. Warns about any unresolved references (unless overridden by the link editor option -Xlinker -z -Xlinker defs). Only a few systems support this one, iykyk. 🌈
• -T script — use script as the linker script. Most systems using the GNU linker support this. On some targets, like bare-board targets without an operating system, the -T option may be required when linking, just 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 doesn't recognize.

If you wanna pass an option that takes a separate argument, you have to use -Xlinker twice — once for the option and once for the argument. For example, to pass -assert definitions, you must write -Xlinker -assert -Xlinker definitions. It does not work to write -Xlinker "-assert definitions", because that passes the whole string as a single argument, which is not what the linker expects. 😭

When using the GNU linker, it's 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 gets split into multiple options at the commas. You can use this syntax to pass an argument to the option. For example, -Wl,-Map,output.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.

NOTE: In Ubuntu 8.10 and later versions, for LDFLAGS, the option -Wl,-z,relro is used. To disable, use -Wl,-z,norelro.

• -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 get passed straight through to the linker without GCC interpreting them at all. Go read your linker documentation for what they mean bestie, I can't help you there. 🙈

---

🎀 Options for Directory Search 🎀

These options say which directories to search for header files, for libraries, and for parts of the compiler. Let's go. 💅

• -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 goes like this:

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's 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 make sure GCC's procedure to fix buggy system headers, and the ordering for the #include_next directive, don't get inadvertently changed. If you really need to change the search order for system directories, use the -nostdinc and/or -isystem options. 💖

• -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. 🫶

okay so continuing the -I- tea, because bestie this one has layers:

Any directories you hit with -I before the -I- are searched only for headers requested with #include "file". They are not searched for #include <file>. But if you add more directories with -I options after the -I-, then those directories get searched for all #include directives. so like, position matters, it's giving very "order of operations" 💅

And also — -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-. periodt, no takebacks 😭

---

• -iprefix prefix, --include-prefix=prefix, --include-prefix prefix — sets prefix as the prefix for subsequent -iwithprefix options. If the prefix represents a directory, you should include the final /. don't forget the slash bestie it's not optional-ish 🎀
• -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 — appends dir to the prefix you set earlier with -iprefix, then adds 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. so many names for the same vibe, iconic honestly

• -isysroot dir — this one is like the --sysroot option, but it applies only to header files (except for Darwin targets, where it applies to both header files and libraries, slay). See the --sysroot option for more info.
• -imultilib dir — use dir as a subdirectory of the directory containing target-specific C++ headers. 🦋
• -imultiarch dir — use dir as a subdirectory of the directory containing architecture-specific C++ headers. ✨
• -nostdinc, --no-standard-includes — do NOT search the standard system directories for header files. Only the directories you explicitly specified with -I, -iquote, -isystem, and/or -idirafter options (and the directory of the current file, if appropriate) are searched. so like, you're on your own here bestie 💀
• -nostdinc++ — don't search for header files in the C++-specific standard directories, but do still search the other standard directories. (This option is used when building the C++ library.)
• --embed-dir=dir, --embed-directory=dir, --embed-directory dir — append the dir directory to the list of searched directories for the #embed preprocessing directive or the __has_embed macro. There are no default directories for #embed, so you gotta supply them yourself fr fr.

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. so like, not for you, it's the driver's business 🙈
• -Ldir, --library-directory=dir, --library-directory dir — add directory dir to the list of directories to be searched for -l. 💖
• -Bprefix, --prefix=prefix, --prefix prefix — this one specifies where to find the executables, libraries, include files, and data files of the compiler itself. main character of path resolution tbh 👑

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. left out!! like not even a goodbye 😭

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. a kludge but a cute one

• -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. leave the paths exactly as they are bestie, no glow-up ✨
• --sysroot=dir, --sysroot dir — use dir as the logical root directory for headers and libraries. For example, if the compiler normally searches for headers in /usr/include and libraries in /usr/lib, it instead searches dir/usr/include and dir/usr/lib.

If you use both this option and the -isysroot option, then the --sysroot option applies to libraries, but the -isysroot option applies to header files.

The GNU linker (beginning with version 2.16) has the necessary support for this option. If your linker does not support this option, the header file aspect of --sysroot still works, but the library aspect does not. heads up on that one

• --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. bye suffix 👋

---

🌸 Options for use with Picolibc 🌸

These options control compilation and linking when using picolibc:

• --oslib=library — search the library named library after the C library, permitting symbols undefined by the C library to be defined by this library. The C library, libgcc and this library are placed between --start-group and --end-group flags so that 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 none variant provides no startup code at all, allowing the user to supply their 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, m68k, riscv, super-h and x86, semihost also traps hardware exceptions and prints information to the console. Note that --crt0=semihost depends upon APIs provided by --oslib=semihost. they're a package deal, iykyk
• --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 is the default level, offering full C17 and POSIX.1-2024 conformance. f provides the 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. smaller code, fewer features, the eternal tradeoff bestie
• --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 is 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. 💅

---

👑 Options for Code Generation Conventions 👑

These machine-independent options control the interface conventions used in code generation.

Most of them have both positive and negative forms; the negative form of -ffoo is -fno-foo. In the table below, only one of the forms is listed---the one that is not the default. You can figure out the other form by either removing no- or adding it. sneaky little puzzle, slay

• -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. 🫶 be careful bestie

For example,

           int *p;
           {
             int local1;

             p = &local1;
             local1 = 10;
             ....
           }
           {
              int local2;
              local2 = 20;
              ...
           }

...and like, when it goes wrong, it goes wrong wrong. Behold 💀

if (*p == 10)  // out of scope use of local1
  {

  }

Another example, because the GCC devs are nothing if not thorough:

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?
}

So here's the tea 🫖 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. But some of the legacy code relies on the behavior of older compilers in which temporaries' stack space is not reused, and the aggressive stack reuse can lead to runtime errors. This option is used to control the temporary stack reuse optimization. 👑

---

• -ftrapv — this option generates traps for signed overflow on addition, subtraction, multiplication operations. The options -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 — consider that instructions that may throw exceptions but don't otherwise contribute to the execution of the program can be optimized away. This does not affect calls to functions except those with 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. 🪦
• -funwind-tables — similar to -fexceptions, except that it just generates any needed static data, but does not affect the generated code in any other way. You normally do not need to enable this option; instead, a language processor that needs this handling enables it on your behalf. 📋
• -fasynchronous-unwind-tables — generate unwind table in DWARF format, if supported by target machine. The table is exact at each instruction boundary, so it can be used for stack unwinding from asynchronous events (such as debugger or garbage collector). 🧶
• -fno-gnu-unique — on systems with recent GNU assembler and C library, the C++ compiler uses the STB_GNU_UNIQUE binding to make sure that definitions of template static data members and static local variables in inline functions are unique even in the presence of RTLD_LOCAL; this is necessary to avoid problems with a library used by two different RTLD_LOCAL plugins depending on a definition in one of them and therefore disagreeing with the other one about the binding of the symbol. But this causes dlclose to be ignored for affected DSOs; if your program relies on reinitialization of a DSO via dlclose and dlopen, you can use -fno-gnu-unique. 🌀
• -fpcc-struct-return — return short struct and union values in memory like longer ones, rather than in registers. This convention is less efficient, but it has the advantage of allowing intercallability between GCC-compiled files and files compiled with other compilers, particularly the Portable C Compiler (pcc).

The precise convention for returning structures in memory depends on the target configuration macros.

Short structures and unions are those whose size and alignment match that of some integer type.

Warning: code compiled with the -fpcc-struct-return switch is not binary compatible with code compiled with the -freg-struct-return switch. Use it to conform to a non-default application binary interface. ⚠️

• -freg-struct-return — return struct and union values in registers when possible. This is more efficient for small structures than -fpcc-struct-return.

If you specify neither -fpcc-struct-return nor -freg-struct-return, GCC defaults to whichever convention is standard for the target. If there is no standard convention, GCC defaults to -fpcc-struct-return, except on targets where GCC is the principal compiler. In those cases, we can choose the standard, and we chose the more efficient register return alternative. ✨

Warning: code compiled with the -freg-struct-return switch is not binary compatible with code compiled with the -fpcc-struct-return switch. Use it to conform to a non-default application binary interface. ⚠️

• -fshort-enums — allocate to an enum type only as many bytes as it needs for the declared range of possible values. Specifically, the enum type is equivalent to the smallest integer type that has enough room. This option has no effect for an enumeration type with a fixed underlying type. 🐜

Warning: the -fshort-enums switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface. ⚠️

• -fshort-wchar — override the underlying type for wchar_t to be short unsigned int instead of the default for the target. This option is useful for building programs to run under WINE. 🍷

Warning: the -fshort-wchar switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface. ⚠️

• -fcommon — in C code, this option controls the placement of global variables defined without an initializer, known as tentative definitions in the C standard. Tentative definitions are distinct from declarations of a variable with the extern keyword, which do not allocate storage.

The default is -fno-common, which specifies that the compiler places uninitialized global variables in the BSS section of the object file. This inhibits the merging of tentative definitions by the linker so you get a multiple-definition error if the same variable is accidentally defined in more than one compilation unit.

The -fcommon places uninitialized global variables in a common block. This allows the linker to resolve all tentative definitions of the same variable in different compilation units to the same object, or to a non-tentative definition. This behavior is inconsistent with C++, and on many targets implies a speed and code size penalty on global variable references. It is mainly useful to enable legacy code to link without errors. 🧩

• -fno-ident, -Qy, -Qn — -fno-ident suppresses emission of .ident assembler directives and causes the #ident preprocessor directive to be ignored. -Qy and -Qn are obsolete synonyms for -fident and -fno-ident, respectively. 🙈
• -finhibit-size-directive — don't output a .size assembler directive, or anything else that would cause trouble if the function is split in the middle, and the two halves are placed at locations far apart in memory. This option is used when compiling crtstuff.c; you should not need to use it for anything else. 🚫
• -fverbose-asm — put extra commentary information in the generated assembly code to make it more readable. This option is generally only of use to those who actually need to read the generated assembly code (perhaps while debugging the compiler itself). 👀

-fno-verbose-asm, the default, causes the extra information to be omitted and is useful when comparing two assembler files.

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. like, the asm comments literally tell you which line of C made which instruction, it's so helpful 🥹

okay so picture this C source file, right:

int test (int n)
{
  int i;
  int total = 0;

  for (i = 0; i < n; i++)
    total += i * i;

  return total;
}

and you compile it to (x86_64) assembly using -S and dump the result straight to stdout with -o -:

gcc -S test.c -fverbose-asm -Os -o -

you get output that looks like this, bestie:

        .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

so those comments? they're made for humans, not machines, which means the exact format of them can change whenever, no cap. so don't build tooling that depends on them, bestie 🙈

• -frecord-gcc-switches — this one makes the compiler take the whole command line you used to invoke it and record it inside the object file it's creating. lowkey iconic for forensics. it's only implemented on some targets, and the exact format of the recording depends on the target and the binary file format, but usually it shows up as a section holding ASCII text. it's related to -fverbose-asm, except that one only drops the info into the assembler output as comments — so it never actually reaches the object file. also peep -grecord-gcc-switches for another way to stash compiler options into the object file 👑
• -fpic — generates position-independent code (PIC) that's suitable for use in a shared library, if the target machine supports it. this kind of code reaches all its constant addresses through a global offset table (GOT). the dynamic loader sorts out the GOT entries when the program starts — and just so you know, the dynamic loader is not part of GCC, it's part of the operating system, bestie. if the GOT for your linked executable blows past a machine-specific max size, the linker yells at you saying -fpic doesn't work; in that case just recompile with -fPIC instead. (those maximums are 8k on the SPARC, 28k on AArch64, and 32k on the m68k and RS/6000. the x86 has no such limit, slay.) PIC needs special support, so it only works on certain machines. for the x86, GCC supports PIC for System V but not for the Sun 386i. and code generated for the IBM RS/6000 is always position-independent. when this flag is set, the macros __pic__ and __PIC__ get defined to 1 ✨
• -fPIC — if the target machine supports it, emits position-independent code suitable for dynamic linking that avoids any limit on the size of the global offset table. this option actually makes a difference on AArch64, m68k, PowerPC and SPARC. again: PIC needs special support, so it only works on certain machines. when this flag is set, the macros __pic__ and __PIC__ get defined to 2 💅
• -fpie
• -fPIE — these two are like -fpic and -fPIC but the position-independent code they generate can only be linked into executables. usually you use them to compile code that'll be linked with the -pie GCC option. -fpie and -fPIE both define the macros __pie__ and __PIE__. the macros have the value 1 for -fpie and 2 for -fPIE. cute, right? 🎀
• -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 gives you more efficient code by ditching the PLT stubs and exposing GOT loads to optimizations. on architectures like 32-bit x86 where PLT stubs want the GOT pointer in a specific register, this hands more register allocation freedom to the compiler. heads up though: lazy binding requires the PLT, so with -fno-plt all external symbols get resolved at load time. alternatively, the function attribute noplt can be used to avoid calls through the PLT for specific external functions. and in position-dependent code, a few targets also convert calls to functions that are marked to not use the PLT into GOT calls instead 🔥
• -fno-jump-tables — don't use jump tables for switch statements, even when that'd be more efficient than other code generation strategies. this one's handy alongside -fpic or -fPIC for building code that's part of a dynamic linker and can't reference the address of a jump table. on some targets, jump tables don't need a GOT at all, so this option isn't even needed there 🦋
• -fno-bit-tests — don't use bit tests for switch statements, even where that'd be more efficient than other code generation strategies. periodt 💅
• -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 has to be the name of a register, and the accepted register names are machine-specific — they're defined in the REGISTER_NAMES macro in the machine description macro file. also, this flag has no negative form, because it specifies a three-way choice. that's just how it is 🌸
• -fcall-used-reg — treat the register named reg as an allocable register that gets clobbered by function calls. it may be allocated for temporaries or variables that don't live across a call. functions compiled this way do not save and restore reg. it's an error to use this flag with the frame pointer or stack pointer, and 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 either, because it's a three-way choice.
• -fcall-saved-reg — treat the register named reg as an allocable register that's saved by functions. it may be allocated even for temporaries or variables that live across a call. functions compiled this way save and restore reg if they use it. again, it's an error to use this flag with the frame pointer or stack pointer, and using it for other registers with fixed pervasive roles in the machine's execution model produces disastrous results 💀 and a different sort of disaster comes from using this flag for a register in which function values may be returned. also no negative form — three-way choice, you know the drill.
• -fpack-struct[=n] — with no value given, packs all structure members together with no holes. when you do give a value (which has to be a small power of two), it packs structure members according to that value, representing the maximum alignment — that is, objects with default alignment requirements larger than this get output potentially unaligned at the next fitting location.

Warning: the -fpack-struct switch makes GCC generate code that is not binary compatible with code generated without it. plus it makes the code suboptimal. use it to conform to a non-default application binary interface, not for funsies 🫶

• -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 helping you link with legacy assembly code.

Warning: the -fleading-underscore switch makes GCC generate code that is not binary compatible with code generated without it. use it to conform to a non-default application binary interface. and ngl, not all targets provide complete support for this switch 😭

• -ftls-model=model — changes up the thread-local storage model that's used. the model argument should be one of global-dynamic, local-dynamic, initial-exec or local-exec. note that the choice is subject to optimization: the compiler may use a more efficient model for symbols that aren't 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. otherwise, 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. which means... it requires the stack to be made executable for the program to work properly 😭 -fno-trampolines is enabled by default on a language-by-language basis so the compiler can avoid generating them if it computes that's safe, and replace them with descriptors instead. descriptors are made up of data only, but the generated code has to be prepared to deal with them. as of this writing, -fno-trampolines is enabled by default only for Ada 🌈

okay so bestie, we were in the middle of trampolines and I need you to hear me on this one:

-ftrampolines and -fno-trampolines are NOT friends. Code compiled with one and code compiled with the other are not binary compatible if nested functions are around. So this has to be a program-wide decision, and it has to be handled with extreme care. extreme. like, holding-a-boba-with-no-lid extreme. 😭

And for languages that aren't Ada? The -ftrampolines and -fno-trampolines options currently do nothing. Zero. Trampolines are always generated on platforms that need them for nested functions. the compiler said "not my problem" and moved on 💅

• -ftrampoline-impl=[stack|heap] — where should trampolines live? By default they get generated on the stack. But some platforms (looking at you, Apple M1 👀) do not permit an executable stack. So compiling with -ftrampoline-impl=heap generates calls to __gcc_nested_func_ptr_created and __gcc_nested_func_ptr_deleted so the trampoline space gets allocated and deallocated on the executable heap. Those functions live in libgcc and are only 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 from before you allocated those nested functions. fr fr, be careful. 💀

---

• -fvisibility=[default|internal|hidden|protected] — sets the default ELF image symbol visibility to whatever you picked. Every symbol gets marked with it unless something inside the code overrides it. Using this can very substantially improve linking and load times of shared object libraries, produce more optimized code, give you near-perfect API export, and prevent symbol clashes. strongly recommended for literally any shared object you distribute. no cap.

And despite the naming being kinda misleading, default always means public — like, available to be linked against from outside the shared object. protected and internal are pretty useless in real-world usage, so the only other one people actually use is hidden. If you don't pass -fvisibility at all, the default is default, i.e. every symbol is public. chaotic but that's the lore.

If you want the full breakdown of why correct ELF symbol visibility is so slay, read How To Write Shared Libraries by Ulrich Drepper (it lives at <https://www.akkadia.org/drepper/>). But honestly the superior move that this option unlocks — instead of marking things hidden when the default is public — is to make the default hidden and mark things public. That's the norm with DLLs on Windows, and with -fvisibility=hidden plus __attribute__ ((visibility("default"))) instead of __declspec(dllexport) you get almost identical semantics with identical syntax. Cross-platform projects are thriving because of this.

Adding visibility support to existing code? You may find #pragma GCC visibility useful. It works by you wrapping the declarations you want to set visibility for, like with #pragma GCC visibility push(hidden) and #pragma GCC visibility pop. 🫶 And keep in mind: symbol visibility is part of the API interface contract, so all new code should always specify visibility when it's not the default. Declarations that are only used within the local DSO should always be explicitly marked hidden so you avoid PLT indirection overheads — and making that abundantly clear also helps readability and self-documentation of the code. Iconic behavior, honestly.

Also, because ISO C++ specification requirements are what they are, operator new and operator delete must always be of default visibility. rules are rules.

Heads up: headers from outside your project — especially system headers and headers from any other library you use — may not be expecting to be compiled with visibility other than the default. You might need to explicitly say #pragma GCC visibility push(default) before including any such headers. 🙈

extern declarations are not affected by -fvisibility, so a lot of code can be recompiled with -fvisibility=hidden and need no modifications at all. But — and this is the catch — that also means calls to extern functions with no explicit visibility go through the PLT. So it's more effective to use __attribute ((visibility)) and/or #pragma GCC visibility to tell the compiler which extern declarations should be treated as hidden.

And yes, -fvisibility does affect C++ vague linkage entities. So for example, an exception class that gets thrown between DSOs must be explicitly marked with default visibility, so the type_info nodes get unified between the DSOs. iykyk 💅

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 bestie.

---

• -fstrict-volatile-bitfields — use this if accesses to volatile bit-fields (or other structure fields, though the compiler usually honors those types anyway) should use a single access of the width of the field's type, aligned to a natural alignment if possible. Example: targets with memory-mapped peripheral registers might require all such accesses to be 16 bits wide. With this flag you can declare all peripheral bit-fields as unsigned short (assuming short is 16 bits on those targets) to force GCC to use 16-bit accesses instead of, maybe, a more efficient 32-bit access.

If this option is disabled, the compiler uses the most efficient instruction. In that same example, that could be a 32-bit load instruction — even though that touches bytes that don't contain any portion of the bit-field, or memory-mapped registers unrelated to the one being updated. 😭

In some cases — like 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 would fault or truncate the result at run time. it's doing its best, okay

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.

The default value of this option is determined by the application binary interface for the target processor.

• -fsync-libcalls — 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.

Its default value is enabled, which means the only useful form of the option is -fno-sync-libcalls. This option is used in the implementation of the libatomic runtime library. niche but make it fashion ✨

• -fzero-init-padding-bits=value — guarantees zero initialization of padding bits in automatic variable initializers. Some languages already guarantee zero init of padding bits in certain cases — e.g. C23 when you use empty initializers ({}), or C++ when using zero-initialization — and 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. Like, 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 the tail padding in the structure (if any). all the padding, zeroed, periodt.

NOTE: In Ubuntu 25.10 and later versions, -fzero-init-padding-bits=all is enabled by default for C, C++, ObjC, and ObjC++, unless -fzero-init-padding-bits=standard is found.

---

✨ GCC Developer Options ✨

This section is for the GCC developers, bestie. Like, the people who support compiler testing and investigate compiler bugs and compile-time performance problems. It includes 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. For ordinary compilation and linking tasks? You should rarely need any of these. 🧚

Many developer options that make GCC 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 gets constructed by concatenating the base dump file name, a pass number, a phase letter, and a pass name:

• The base dump file name is the name of the output file produced by the compiler if it was explicitly specified and is not an executable; otherwise it's 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 get numbered higher than the pass named final — even if they're executed earlier. chaos ordering, love that for us
• 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_ — bestie, this one makes the compiler spit out callgraph information for your program, on a per-object-file basis. The info comes in the common VCG format ✨. And if you pass a list of comma-separated _MARKERS_? It gets decorated with extra per-node and/or per-edge info. So iconic. When the "su" marker is specified, the callgraph gets decorated with stack usage info — it's equivalent to -fstack-usage. When the "da" marker is specified, it's decorated with info about dynamically allocated objects.

When compiling with -flto, no callgraph information is output along with the object file. At LTO link time, -fcallgraph-info may generate multiple callgraph information files next to intermediate LTO output files.

• -dletters, --dump=letters, --dump letters, -fdump-rtl-pass, -fdump-rtl-pass-options, -fdump-rtl-pass-options=filename — okay so this says to make debugging dumps during compilation at the times specified by _letters_ when you use -d, or by _pass_ when you use -fdump-rtl. It's for debugging the RTL-based passes of the compiler 💅.

Some -dletters switches have different meaning when -E is used for preprocessing. Not me being dramatic, that's literally what it says 😭.

The -options form gives you way more control over the details of the dump. See -fdump-tree.

Here are actual instances of command-line options following these patterns and their meanings:

• -fdump-rtl-alignments — Dump after branch alignments have been computed.
• -fdump-rtl-asmcons — Dump after fixing rtl statements that have unsatisfied in/out constraints.
• -fdump-rtl-auto_inc_dec — Dump after auto-inc-dec discovery. This pass is only run on architectures that have auto inc or auto dec instructions.
• -fdump-rtl-barriers — Dump after cleaning up the barrier instructions.
• -fdump-rtl-bbpart — Dump after partitioning hot and cold basic blocks.
• -fdump-rtl-bbro — Dump after block reordering.
• -fdump-rtl-btl1, -fdump-rtl-btl2 — -fdump-rtl-btl1 and -fdump-rtl-btl2 enable dumping after the two branch target load optimization passes.
• -fdump-rtl-bypass — Dump after jump bypassing and control flow optimizations.
• -fdump-rtl-combine — Dump after the RTL instruction combination pass.
• -fdump-rtl-compgotos — Dump after duplicating the computed gotos.
• -fdump-rtl-ce1, -fdump-rtl-ce2, -fdump-rtl-ce3 — -fdump-rtl-ce1, -fdump-rtl-ce2, and -fdump-rtl-ce3 enable dumping after the three if conversion passes.
• -fdump-rtl-cprop_hardreg — Dump after hard register copy propagation.
• -fdump-rtl-csa — Dump after combining stack adjustments.
• -fdump-rtl-cse1, -fdump-rtl-cse2 — -fdump-rtl-cse1 and -fdump-rtl-cse2 enable dumping after the two common subexpression elimination passes.
• -fdump-rtl-dce — Dump after the standalone dead code elimination passes.
• -fdump-rtl-dbr — Dump after delayed branch scheduling.
• -fdump-rtl-dce1, -fdump-rtl-dce2 — -fdump-rtl-dce1 and -fdump-rtl-dce2 enable dumping after the two dead store elimination passes.
• -fdump-rtl-eh — Dump after finalization of EH handling code.
• -fdump-rtl-eh_ranges — Dump after conversion of EH handling range regions.
• -fdump-rtl-expand — Dump after RTL generation.
• -fdump-rtl-fwprop1, -fdump-rtl-fwprop2 — -fdump-rtl-fwprop1 and -fdump-rtl-fwprop2 enable dumping after the two forward propagation passes.
• -fdump-rtl-gcse1, -fdump-rtl-gcse2 — -fdump-rtl-gcse1 and -fdump-rtl-gcse2 enable dumping after global common subexpression elimination.
• -fdump-rtl-init-regs — Dump after the initialization of the registers.
• -fdump-rtl-initvals — Dump after the computation of the initial value sets.
• -fdump-rtl-into_cfglayout — Dump after converting to cfglayout mode.
• -fdump-rtl-ira — Dump after iterated register allocation.
• -fdump-rtl-jump — Dump after the second jump optimization.
• -fdump-rtl-loop2 — -fdump-rtl-loop2 enables dumping after the rtl loop optimization passes.
• -fdump-rtl-mach — Dump after performing the machine dependent reorganization pass, if that pass exists.
• -fdump-rtl-mode_sw — Dump after removing redundant mode switches.
• -fdump-rtl-rnreg — Dump after register renumbering.
• -fdump-rtl-outof_cfglayout — Dump after converting from cfglayout mode.
• -fdump-rtl-peephole2 — Dump after the peephole pass.
• -fdump-rtl-postreload — Dump after post-reload optimizations.
• -fdump-rtl-pro_and_epilogue — Dump after generating the function prologues and epilogues.
• -fdump-rtl-sched1, -fdump-rtl-sched2 — -fdump-rtl-sched1 and -fdump-rtl-sched2 enable dumping after the basic block scheduling passes.
• -fdump-rtl-ree — Dump after sign/zero extension elimination.
• -fdump-rtl-seqabstr — Dump after common sequence discovery.
• -fdump-rtl-shorten — Dump after shortening branches.
• -fdump-rtl-split1, -fdump-rtl-split2, -fdump-rtl-split3, -fdump-rtl-split4, -fdump-rtl-split5 — These options enable dumping after five rounds of instruction splitting.
• -fdump-rtl-sms — Dump after modulo scheduling. This pass is only run on some architectures.
• -fdump-rtl-stack — Dump after conversion from GCC's "flat register file" registers to the x87's stack-like registers. This pass is only run on x86 variants.
• -fdump-rtl-subreg1, -fdump-rtl-subreg2 — -fdump-rtl-subreg1 and -fdump-rtl-subreg2 enable dumping after the two subreg expansion passes.
• -fdump-rtl-vartrack — Dump after variable tracking.
• -fdump-rtl-vregs — Dump after converting virtual registers to hard registers.
• -fdump-rtl-web — Dump after live range splitting.
• -fdump-rtl-regclass, -fdump-rtl-subregs_of_mode_init, -fdump-rtl-subregs_of_mode_finish, -fdump-rtl-dfinit, -fdump-rtl-dfinish — These dumps are defined but always produce empty files. Yeah. Empty. It's giving "placeholder era" 💀.
• -da, --dump=a, -fdump-rtl-all — Produce all the dumps listed above. Go big or go home, fr fr 👑.
• -dA, --dump=A — Annotate the assembler output with miscellaneous debugging information.
• -dD, --dump=D — Dump all macro definitions, at the end of preprocessing, in addition to normal output.
• -dH, --dump=H — Produce a core dump whenever an error occurs.
• -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.
• -dP, --dump=P — Dump the RTL in the assembler output as a comment before each instruction. Also turns on -dp annotation.
• -dx, --dump=x — Just generate RTL for a function instead of compiling it. Usually used with -fdump-rtl-expand.

• -fdump-debug — Dump debugging information generated during the debug generation phase ✨.
• -fdump-earlydebug — Dump debugging information generated during the early debug generation phase ✨.
• -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. So handy 🫶.
• -freport-bug — Collect and dump debug information into a temporary file if an internal compiler error (ICE) occurs. Not the fun kind of ICE bestie, the "your compiler is screaming" kind 😭.
• -fdump-unnumbered — When doing debugging dumps, suppress instruction numbers and address output. This makes it more feasible to use diff on debugging dumps for compiler invocations with different options, in particular with and without -g.
• -fdump-unnumbered-links — When doing debugging dumps (see -d option above), suppress instruction numbers for the links to the previous and next instructions in a sequence.
• -fdump-internal-locations — Dump detailed information about GCC's internal representation of source code locations.
• -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.
  • cgraph — Dumps information about call-graph optimization, unused function removal, and inlining decisions.
  • inline — Dump after function inlining.
  • strubm — Dump after selecting "strub" modes, and recording the selections as function attributes.
  • strub — Dump "strub" transformations: interface changes, function wrapping, and insertion of builtin calls for stack scrubbing and watermarking.

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. The receipts are right there 💅.

By default, the dump will contain messages about successful optimizations (equivalent to -optimized) together with low-level details about the analysis.

• -fdump-ipa-clones — bestie, this one cooks up a dump file absolutely stuffed with the tea ☕ about the creation of call graph node clones AND the removals of call graph nodes during interprocedural optimizations and transformations. Its main intended use? So tools that create live-patches can figure out the set of functions that need to be live-patched to completely replace a particular function (peep -flive-patching). The file name gets 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 lives on its own separate line, and each line is full of semicolon separated fields. slay ✨

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 — Dumps language-specific info, no cap. 💅 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 — These control the dumping of language-specific information, bestie. The options and filename portions behave exactly 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 — Prints on stderr the list of optimization passes that are turned on and off by the current command-line options. Iconic for snooping on what your passes are actually doing. 👀
• -fdump-statistics-option — Enable and control dumping of pass statistics into a separate file. The file name is generated by appending a suffix ending in .statistics to the source file name, and the file is created in the same directory as the output file. If the -option form is used, -stats causes counters to be summed over the whole compilation unit while -details dumps every event as the passes generate them. The default with no option is to sum counters for each function compiled. 📊
• -fdump-tree-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. The following options are available:
  • address — Print the address of each node. Usually this is not meaningful as it changes according to the environment and source file. Its primary use is for tying up a dump file with a debug environment.
  • asmname — If DECL_ASSEMBLER_NAME has been set for a given decl, use that in the dump instead of DECL_NAME. Its primary use is ease of use working backward from mangled names in the assembly file.
  • slim — When dumping front-end intermediate representations, inhibit dumping of members of a scope or body of a function merely because that scope has been reached. Only dump such items when they are directly reachable by some other path.

When dumping pretty-printed trees, this option inhibits dumping the bodies of control structures.

When dumping RTL, print the RTL in slim (condensed) form instead of the default LISP-like representation.

• raw — Print a raw representation of the tree. By default, trees are pretty-printed into a C-like representation.
• details — Enable more detailed dumps (not honored by every dump option). Also include information from the optimization passes.
• stats — Enable dumping various statistics about the pass (not honored by every dump option).
• blocks — Enable showing basic block boundaries (disabled in raw dumps).
• graph — For each of the other indicated dump files (-fdump-rtl-pass), dump a representation of the control flow graph suitable for viewing with GraphViz to file.passid.pass.dot. Each function in the file is pretty-printed as a subgraph, so that GraphViz can render them all in a single plot.

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, bestie 🫶

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. careful!

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 — turns on dumps from every OMP (Offloading and Multi Processing) optimization. 💫
• vec — turns on dumps from every vectorization optimization. 🦋
• optall — turns on dumps from all optimizations. This one is a superset of all the optimization groups listed above, so like, it eats them all up. 👑

If options is left out entirely, it defaults to optimized-optall, which means: dump messages about successful optimizations from all the passes, but skip the messages that get treated as "internals". 🙈

If a filename is given, then all the dumps from every applicable optimization get concatenated into that filename. No filename? Then the dump just goes out onto stderr. And yes, even though multiple -fopt-info options are accepted, only one of them can carry a filename. If other filenames show up, literally all but the first one are just ignored. 💀

Heads up bestie: the output filename gets overwritten when there are multiple translation units. If you want a combined output from several translation units, use stderr instead.

In this next example, the optimization info goes to stderr:

gcc -O3 -fopt-info

This one here:

gcc -O3 -fopt-info-missed=missed.all

outputs a missed optimization report from all the passes into missed.all. And this one:

gcc -O2 -ftree-vectorize -fopt-info-vec-missed

prints info about missed optimization opportunities from the vectorization passes onto stderr. Fun fact: -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 at all. 🎀

Another example for you:

gcc -O3 -fopt-info-inline-optimized-missed=inline.txt

outputs info about missed optimizations and optimized locations from all the inlining passes into inline.txt.

And finally, consider this one:

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. So in this case only the first option takes effect and the ones after it get ignored. Which means only vec.miss gets produced, and it contains dumps from the vectorizer about missed opportunities. 😭

---

-fsave-optimization-record ✨

Writes 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 inside the compressed JSON file is subject to change. Don't build your whole life on it, bestie. 💅

It's roughly equivalent to a machine-readable version of -fopt-info-all, as a collection of messages with source file, line number and column number — plus these extra goodies for each message:

• the execution count of the code being optimized, along with metadata about whether this came from actual profile data or is just an estimate, so consumers can prioritize messages by code hotness,
• the function name of the code being optimized, where applicable,
• the "inlining chain" for the code being optimized, so when a function gets inlined into several different places (which might themselves be inlined), the reader can tell the copies apart,
• 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.

On top of that, 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 how much 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 whole set of options used to explicitly disable/enable optimization passes. These are meant for debugging GCC. Compiler users should use regular options for enabling/disabling passes instead, okay?

• -fdisable-ipa-pass — Disable IPA pass pass. pass is the pass name. If the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from 1.
• -fdisable-rtl-pass, -fdisable-rtl-pass=range-list — Disable RTL pass pass. pass is the pass name. If the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from 1. range-list is a comma-separated list of function ranges or assembler names. Each range is a number pair separated by a colon. The range is inclusive in both ends. If the range is trivial, the number pair can be simplified as a single number. If the function's call graph node's uid falls within one of the specified ranges, the pass is disabled for that function. The uid is shown in the function header of a dump file, and the pass names can be dumped by using option -fdump-passes.
• -fdisable-tree-pass, -fdisable-tree-pass=range-list — Disable tree pass pass. See -fdisable-rtl for the description of option arguments.
• -fenable-ipa-pass — Enable IPA pass pass. pass is the pass name. If the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from 1.
• -fenable-rtl-pass, -fenable-rtl-pass=range-list — Enable RTL pass pass. See -fdisable-rtl for option argument description and examples.
• -fenable-tree-pass, -fenable-tree-pass=range-list — Enable tree pass pass. See -fdisable-rtl for the description of option arguments.

Here are some examples showing uses of these options. 👇

# disable ccp1 for all functions
   -fdisable-tree-ccp1
# disable complete unroll for function whose cgraph node uid is 1
   -fenable-tree-cunroll=1
# disable gcse2 for functions at the following ranges [1,1],
# [300,400], and [400,1000]
# disable gcse2 for functions foo and foo2
   -fdisable-rtl-gcse2=foo,foo2
# disable early inlining
   -fdisable-tree-einline
# disable ipa inlining
   -fdisable-ipa-inline
# enable tree full unroll
   -fenable-tree-unroll

---

-fchecking / -fchecking=n 🧚

Enable internal consistency checking. The default depends on the compiler configuration. -fchecking=2 enables further internal consistency checking that might affect code generation. 👀

---

-frandom-seed=string 🍓

This option 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 the object files that produce them. You can use the -frandom-seed option to produce reproducibly identical object files. So iconic of it. ✨

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.

---

-save-temps / --save-temps 💾

Store the usual "temporary" intermediate files permanently; name 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.

• -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 🎀

Process file after the compiler reads in the standard specs file, in order to override the defaults which the gcc driver program uses when determining what switches to pass to cc1, cc1plus, as, ld, etc. More than one -specs=file can be specified on the command line, and they are processed in order, from left to right. 💅

---

-time[=file] ⏱️

Report the CPU time taken by each subprocess in the compilation sequence. For C source files, this is the compiler proper and assembler (plus the linker if linking is done).

Without the specification of an output file, the output looks like this:

# cc1 0.12 0.01
# as 0.00 0.01

The first number on each line is the "user time", that is time spent executing the program itself. The second number is "system time", time spent executing operating system routines on behalf of the program. Both numbers are in seconds.

With the specification of an output file, the output is appended to the named file, and it looks like this:

0.12 0.01 cc1 <options>
0.00 0.01 as <options>

The "user time" and the "system time" are moved before the program name, and the options passed to the program are displayed, so that one can later tell what file was being compiled, and with which options. So observant. 🔍

---

-fdump-final-insns[=file] 👑

Dump the final internal representation (RTL) to file. If the optional argument is omitted (or if file is "."), the name of the dump file is determined by appending .gkd to the dump base name, see -dumpbase. 🫶

• -fcompare-debug[=opts] — okay so bestie, this is the paranoid double-check option 💅. If literally no error pops up during compilation, GCC runs the compiler a second time, adding opts plus -fcompare-debug-second to the arguments handed to that second compilation. It dumps the final internal representation from both compilations, and throws an error if they don't match. Yes, it's giving "I need to see it twice to believe it."

If you leave out the equal sign, the default is -gtoggle.

The environment variable GCC_COMPARE_DEBUG, if it's defined, non-empty and nonzero, implicitly enables -fcompare-debug for you (surprise!). If GCC_COMPARE_DEBUG is defined to a string that starts with a dash, that string gets used for opts; otherwise the default -gtoggle is used.

-fcompare-debug= — equal sign but no opts — is equivalent to -fno-compare-debug, which disables the dumping of the final representation and the second compilation, which means even GCC_COMPARE_DEBUG can't sneak in. Nope, blocked. 🙅

Want 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 (not preprocessing, assembly or linking, just so we're clear). And if you only want a warning instead of the rejection, set GCC_COMPARE_DEBUG to -w%n-fcompare-debug not overridden and you're golden.

• -fcompare-debug-second — this one is implicitly passed to the compiler for that second compilation -fcompare-debug asks for, along with options to hush the warnings, and it omits other options that would make the compiler spit output to files or to standard output as a side effect (no messy leftovers, we love that). Dump files and preserved temporary files get renamed so they contain the additional extension .gk during the second compilation, so they don't overwrite the ones from the first round. Iconic, honestly.

When you pass this option 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. Very niche. 🧚

• -gtoggle — turns off generation of debug info if leaving out this option would generate 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 only does so once, no matter how many times you give it. Mainly intended to hang out with -fcompare-debug.
• -fvar-tracking-assignments-toggle — toggles -fvar-tracking-assignments, in the exact same way that -gtoggle toggles -g. Twin energy fr fr. ✨
• -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's compiled, and print some statistics about each pass when it finishes. Live commentary, basically. 🎤

• -ftime-report — makes the compiler print some statistics to stderr about the time consumed by each pass when it finishes. Time-to-spill-the-tea.

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. Heads up though: the precise format of this JSON data is subject to change, and the values may not exactly match the ones emitted to stderr, because they get written out at a slightly different place within the compiler. So don't get too attached to the exact numbers. 😭

• -ftime-report-details — records the time consumed by infrastructure parts separately for each pass.
• -fira-verbose=n — controls the verbosity of the dump file for the integrated register allocator. 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. Math girlies rise up. 🧮
• -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 (so don't be shocked when it changes on you). It's meant to be useful to GCC developers when processing object files in LTO mode (via -flto).

Disabled by default.

• -flto-report-wpa — like -flto-report, but only prints for the WPA phase of link-time optimization.
• -fmem-report — makes the compiler print some statistics about permanent memory allocation when it finishes. Memory receipts, bestie. 🧾
• -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 timeline. ⏳
• -fmultiflags — this option enables multilib-aware TFLAGS to be used to build target libraries with options different from the ones the compiler is configured to use by default, through the use of specs set up by compiler internals, by the target, or by builders at configure time.

Like TFLAGS, this lets the target libraries be built for portable baseline environments, while the compiler itself defaults to more demanding ones. That's useful because users can easily override the defaults the compiler is configured to use to build their own programs, if those defaults aren't ideal for their target environment — whereas rebuilding the runtime libraries is usually not as easy or desirable. Big difference, no cap.

Unlike TFLAGS, the use of specs enables different flags to be selected for different multilibs. The way to accomplish that is to build with make TFLAGS=-fmultiflags, after configuring --with-specs=%{fmultiflags:...}.

This option is discarded by the driver once it's done processing driver self spec. Poof. Gone.

It's also useful to check that TFLAGS are being used to build all target libraries, by configuring a non-bootstrap compiler --with-specs='%{!fmultiflags:%emissing TFLAGS}' and building the compiler and target libraries.

• -fprofile-report — makes the compiler print some statistics about the consistency of the (estimated) profile and the effect of individual passes.
• -fstack-usage — makes the compiler output stack usage information for the program, on a per-function basis. The filename for the dump is made by appending .su to the auxname. The auxname is generated from the name of the output file, if it's explicitly specified and it's not an executable — otherwise it's the basename of the source file. An entry is made up of three fields:
  1. The name of the function.
  2. A number of bytes.
  3. One or more qualifiers: static, dynamic, bounded.

The qualifier static means the function manipulates the stack statically: a fixed number of bytes are allocated for the frame on function entry and released on function exit; no stack adjustments are otherwise made in the function. The second field is this fixed number of bytes.

The qualifier dynamic means the function manipulates the stack dynamically: on top of the static allocation described above, stack adjustments are made in the body of the function, for example to push/pop arguments around function calls. If the qualifier bounded is also present, the amount of these adjustments is bounded at compile time and the second field is an upper bound of the total amount of stack used by the function. If bounded is not present, the amount of these adjustments is not bounded at compile time and the second field only represents the bounded part. Important distinction, don't sleep on it. 💅

• -fstats — emits statistics about front-end processing at the end of the compilation. This option is supported only by the C++ front end, and the information is generally only useful to the G++ development team. So like… not really for us, but slay anyway. ✨
• -fdbg-cnt-list — prints the name and the counter upper bound for 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 name of the counter and list of closed intervals. The lower_bound is optional and is zero-initialized if not set. For example, with -fdbg-cnt=dce:2-4:10-11,tail_call:10, dbg_cnt(dce) returns true only for the second, third, fourth, tenth and eleventh invocation. And for dbg_cnt(tail_call) true is returned for the first 10 invocations. Very specific, very demure. 🎀
• -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 nothing else. With this option, GCC does not compile or link anything; it just prints the file name. One job, and it does it flawlessly. 👑
• -print-multi-directory, --print-multi-directory — prints the directory name corresponding to the multilib selected by any other switches present in the command line. This directory is supposed to exist in GCC_EXEC_PREFIX.
• -print-multi-lib, --print-multi-lib — prints the mapping from multilib directory names to compiler switches that enable them. The directory name is separated from the switches by ;, and each switch starts with an @ instead of the -, with no spaces between multiple switches. This is supposed to ease shell processing. Shell-girlie friendly fr fr. 🐚
• -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 searches for a program such as cpp.
• -print-libgcc-file-name, --print-libgcc-file-name — same as -print-file-name=libgcc.a.

This is useful when you use -nostdlib or -nodefaultlibs but you do want to link with libgcc.a. You can do:

gcc -nostdlib <files>... `gcc -print-libgcc-file-name`

• -print-search-dirs, --print-search-dirs — prints the name of the configured installation directory and a list of program and library directories gcc searches — and does nothing else. Again: one job. Flawless. 🫶

...and this is the part where your build goes 💀 nope. So if gcc hits you with installation problem, cannot exec cpp0: No such file or directory, that's the vibe. To fix it you've got two choices, bestie: either put cpp0 and the rest of the compiler components exactly where gcc expects to find them, or set the environment variable GCC_EXEC_PREFIX to point at the directory where you actually installed them. And do not forget the trailing /. She's picky. 🎀

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• -print-sysroot, --print-sysroot — prints the target sysroot directory 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, this option prints nothing at all. Empty. Void. Silence. 🫥
• -print-sysroot-headers-suffix, --print-sysroot-headers-suffix — prints the suffix that gets added to the target sysroot when searching for headers. Or, if the compiler wasn't configured with such a suffix, it throws an error instead. And it does not do anything else. One job. Periodt.
• -dumpmachine — prints the compiler's target machine (like, for example, i686-pc-linux-gnu) and does literally nothing else. That's it, that's the whole personality. ✨
• -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 has been configured, it can be just a single number (major version), two numbers separated by a dot (major and minor version), or three numbers separated by dots (major, minor and patchlevel version).
• -dumpfullversion — prints the full compiler version and does nothing else. The output is always three numbers separated by dots: major, minor and patchlevel version. No ambiguity, no games. 👑
• -dumpspecs — prints the compiler's built-in specs and does nothing else. (This one gets used when GCC itself is being built, so it's giving very much behind-the-scenes main character energy. 🎬)
• --param name=value, --param=name=value — okay so here's the tea. GCC, by convention, uses parameters that can be specified 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. The names of specific parameters, and the meaning of their values, are tied to the internals of the compiler and are subject to change without notice in future releases. So do not depend on parameter settings for correct compilation of your program. Fr fr. They're exposed on the command line for the convenience of developers debugging compilation problems, or, in some cases, to provide workarounds for compiler bugs.

🌈 Machine-Dependent Options 🌈

Every target machine that 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. By convention, the names of machine-specific options start with -m.

Some configurations of the compiler also support additional target-specific options, usually for compatibility with other compilers on the same platform.

🧚 AArch64 Options 🧚

These options are defined for AArch64 implementations:

• -mabi=name — generate code for the specified data model. Permissible values 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.

The ilp32 model is deprecated. RIP bestie, she's on the way out. 💀

• -mbig-endian — generate big-endian code. This is the default when GCC is configured for an aarch64_be-*-* target.
• -mlittle-endian — generate little-endian code. This is the default when GCC is configured for an aarch64-*-* but not an aarch64_be-*-* target.
• -menable-sysreg-checking — generates an error message if an attempt is made to access a system register which is not available on the target architecture. Snitch mode, but for registers. 🚨
• -mgeneral-regs-only — generate code that uses only the general-purpose registers. This prevents the compiler from using floating-point and Advanced SIMD registers, but it does not impose any restrictions on the assembler.
• -mcmodel=tiny — generate code for the tiny code model. The program and its statically defined symbols must be within 1MB of each other. Programs can be statically or dynamically linked.
• -mcmodel=small — generate code for the small code model. The program and its statically defined symbols must be within 4GB of each other. Programs can be statically or dynamically linked. This is the default code model. 🌟
• -mcmodel=large — generate code for the large code model. This makes no assumptions about addresses and sizes of sections. Programs can be statically linked only. The -mcmodel=large option is incompatible with -mabi=ilp32, -fpic and -fPIC.
• -mtp=name — specify the system register to use as a thread pointer. The 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. The default setting is tpidr_el0. It is recommended to compile all code intended to interoperate with the same value of this option, to avoid accessing a different thread pointer from the wrong exception level.
• -mstrict-align, -mno-strict-align — avoid or allow generating memory accesses that may not be aligned on a natural object boundary as described in the architecture specification.
• -momit-leaf-frame-pointer, -mno-omit-leaf-frame-pointer — omit or keep the frame pointer in leaf functions. The former behavior is the default.
• -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 the latter choice, the options -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 is no default register or offset, since 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. 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. 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 which does not need the workaround.
• -mfix-cortex-a53-843419, -mno-fix-cortex-a53-843419 — enable or disable the workaround for the ARM Cortex-A53 erratum number 843419. This erratum workaround is made at link time, and this will only pass the corresponding flag to the linker. This flag will be ignored if an architecture or cpu is specified on the command line which does not need the workaround.
• -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 this 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. This option 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. If enabled, it implies -mlow-precision-recip-sqrt.
• -mlow-precision-div, -mno-low-precision-div — enable or disable the division approximation. This option 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. The tracking state can then be used by the compiler when expanding calls to "__builtin_speculation_safe_value" to permit a more efficient code sequence to be generated. 🕵️
• -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 will 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 using a later revision, e.g. -march=armv8.1-a or -march=armv8-a+lse, the ARMv8.1-Atomics instructions will be used directly. The same applies when using -mcpu= when the selected cpu supports the lse feature. This option is on by default. 💅

• -mmax-vectorization / -mno-max-vectorization — okay bestie, this enables or disables an override to the vectorizer cost model that makes vectorization always appear profitable ✨. You can combine it with -mautovec-preference to get precise control over which ISA will be used for auto-vectorization 💅. And no cap: 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 — forces 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. 👑

• -march=name — specify the name of the target architecture and, optionally, one or more feature modifiers. This option has the form -march=arch{+[no]feature}*.

The table below summarizes the permissible values for arch and the features that they enable by default:

• armv8-a : Armv8-A : includes by default +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, neoverse-n3, olympus, cortex-a725, cortex-x925, qdf24xx, saphira, phecda, xgene1, vulcan, octeontx, octeontx81,  octeontx83, octeontx2, octeontx2t98, octeontx2t96 octeontx2t93, octeontx2f95, octeontx2f95n, octeontx2f95mm, a64fx, fujitsu-monaka, thunderx, thunderxt88, thunderxt88p1, thunderxt81, tsv110, hip12, thunderxt83, thunderx2t99, thunderx3t110, zeus, cortex-a57.cortex-a53, cortex-a72.cortex-a53, cortex-a73.cortex-a35, cortex-a73.cortex-a53, cortex-a75.cortex-a55, cortex-a76.cortex-a55, cortex-r82, cortex-r82ae, cortex-x1, cortex-x1c, cortex-x2, cortex-x3, cortex-x4, cortex-a510, cortex-a520, cortex-a520ae, cortex-a710, cortex-a715, cortex-a720, cortex-a720ae, ampere1, ampere1a, ampere1b, ampere1c, cobalt-100, apple-m1, apple-m2, apple-m3, apple-m4, c1-nano, c1-pro, c1-premium c1-ultra and native.

The values cortex-a57.cortex-a53, cortex-a72.cortex-a53, cortex-a73.cortex-a35, cortex-a73.cortex-a53, cortex-a75.cortex-a55, cortex-a76.cortex-a55, apple-m1, apple-m2, apple-m3, gb10 specify that GCC should tune for a big.LITTLE system.

The value neoverse-512tvb specifies that GCC should tune for Neoverse cores that (a) implement SVE and (b) have a total vector bandwidth of 512 bits per cycle. In other words, the option tells GCC to tune for Neoverse cores that can execute 4 128-bit Advanced SIMD arithmetic instructions a cycle and that can execute an equivalent number of SVE arithmetic instructions per cycle (2 for 256-bit SVE, 4 for 128-bit SVE). This is more general than tuning for a specific core like Neoverse V1 but is more specific than the default tuning described below.

Additionally on native AArch64 GNU/Linux systems the value native tunes performance to the host system. This option has no effect if the compiler is unable to recognize the processor of the host system.

Where none of -mtune=, -mcpu= or -march= are specified, the code is tuned to perform well across a range of target processors.

This option cannot be suffixed by feature modifiers.

• -mcpu=name — specify the name of the target processor, optionally suffixed by one or more feature modifiers. This option has the form -mcpu=cpu{+[no]feature}*, where the permissible values for cpu are the same as those available for -mtune. The permissible values for feature are documented in the sub-section on aarch64-feature-modifiers,,-march and -mcpu Feature Modifiers. Where conflicting feature modifiers are specified, the right-most feature is used.

GCC uses name to determine what kind of instructions it can emit when generating assembly code (as if by -march) and to determine the target processor for which to tune for performance (as if by -mtune). Where this option is used in conjunction with -march or -mtune, those options take precedence over the appropriate part of this option.

-mcpu=neoverse-512tvb is special in that it does not refer to a specific core, but instead refers to all Neoverse cores that (a) implement SVE and (b) have a total vector bandwidth of 512 bits a cycle. Unless overridden by -march, -mcpu=neoverse-512tvb generates code that can run on a Neoverse V1 core, since Neoverse V1 is the first Neoverse core with these properties. Unless overridden by -mtune, -mcpu=neoverse-512tvb tunes code in the same way as for -mtune=neoverse-512tvb.

• -moverride=string — override tuning decisions made by the back-end in response to a -mtune= switch. The syntax, semantics, and accepted values for string in this option are not guaranteed to be consistent across releases. This option is only intended to be useful when developing GCC. 👑
• -mpc-relative-literal-loads / -mno-pc-relative-literal-loads — enable or disable PC-relative literal loads. With this option literal pools are accessed using a single instruction and emitted after each function. This limits the maximum size of functions to 1MB. This is enabled by default for -mcmodel=tiny.
• -msign-return-address=scope — select the function scope on which return address signing will be applied. Permissible values are none, which disables return address signing, non-leaf, which enables pointer signing for functions which are not leaf functions, and all, which enables pointer signing for all functions. The default value is none. This option has been deprecated by -mbranch-protection.
• -mbranch-protection=features — select the branch protection features to use. features can have one of the following forms:
  • none is the default and turns off all types of branch 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.
  • 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.

• -mearly-ra=scope — determine when to enable an early register allocation pass. This pass runs before instruction scheduling and tries to find a spill-free allocation of floating-point and vector code. It also tries to make use of strided multi-register instructions, such as SME2's strided LD1 and ST1.

The possible values of scope are: all, which runs the pass on all functions; strided, which runs the pass on functions that have access to strided multi-register instructions; and none, which disables the pass.

-mearly-ra=all is the default for -O2 and above, and for -Os. -mearly-ra=none is the default otherwise.

• -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.

okay bestie, we're still deep in the -m options, the AArch64 feature zoo era, and it is SO much but we're eating it up one flag at a time 💅

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✨ AArch64: SVE vector bits, then the whole feature modifier lineup ✨

• -msve-vector-bits=bits — sets how many bits live in one SVE vector register. Heads up: this literally only does anything when SVE is already enabled, so don't expect magic otherwise.

Okay so real talk about SVE codegen, because GCC does it two ways:

• vector-length agnostic output — works with any size of vector register, no assumptions, just vibes (portable vibes).
• vector-length specific output — lets GCC make assumptions about the vector length when that helps it optimize better.

The possible values for bits are: scalable, 128, 256, 512, 1024, and 2048.

Say it with me: scalable selects vector-length agnostic output. At present, -msve-vector-bits=128 also generates vector-length agnostic output for big-endian targets. Every other value generates vector-length specific code.

😭 bestie listen — 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. Like, only scalable is safe to trust, fr fr.

The default is -msve-vector-bits=scalable, which produces vector-length agnostic code. 💖

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💖 -march and -mcpu Feature Modifiers 💖

Feature modifiers used with -march and -mcpu can be any of the following and their inverses nofeature (so yes, nocrc, nocrypto, etc. — the whole flip-the-bit situation ✨):

• 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.
• predres — Enable the Armv8-a Execution and Data Prediction Restriction instructions. This option is only to enable the extension at the assembler level and does not affect code generation. This option is enabled by default for -march=armv8.5-a.
• sve2 — Enable the Armv8-a Scalable Vector Extension 2. This also enables SVE instructions.
• sve2-bitperm — Enable SVE2 bitperm instructions. This also enables SVE2 instructions.
• sve2-sm4 — Enable SVE2 sm4 instructions. This also enables SVE2 instructions.
• sve2-aes — Enable SVE2 aes instructions. This also enables SVE2 instructions.
• sve2-sha3 — Enable SVE2 sha3 instructions. This also enables SVE2 instructions.
• sve2p1 — Enable SVE2.1 instructions. This also enables SVE2 instructions.
• tme — Enable the Transactional Memory Extension.
• i8mm — Enable 8-bit Integer Matrix Multiply instructions. This also enables Advanced SIMD and floating-point instructions. This option is enabled by default for -march=armv8.6-a. Use of this option with architectures prior to Armv8.2-A is not supported.
• f32mm — Enable 32-bit Floating point Matrix Multiply instructions. This also enables SVE instructions. Use of this option with architectures prior to Armv8.2-A is not supported.
• f64mm — Enable 64-bit Floating point Matrix Multiply instructions. This also enables SVE instructions. Use of this option with architectures prior to Armv8.2-A is not supported.
• bf16 — Enable brain half-precision floating-point instructions. This also enables Advanced SIMD and floating-point instructions. This option is enabled by default for -march=armv8.6-a. Use of this option with architectures prior to Armv8.2-A is not supported.
• ls64 — Enable the 64-byte atomic load and store instructions for accelerators.
• mops — Enable the instructions to accelerate memory operations like memcpy, memmove, memset. This option is enabled by default for -march=armv8.8-a
• flagm — Enable the Flag Manipulation instructions Extension.
• flagm2 — Enable the FlagM2 flag conversion instructions.
• pauth — Enable the Pointer Authentication Extension.
• cssc — Enable the Common Short Sequence Compression instructions.
• cmpbr — Enable the shorter compare and branch instructions, cbb, cbh and cb.
• sme — Enable the Scalable Matrix Extension.
• sme-i16i64 — Enable the FEAT_SME_I16I64 extension to SME. This also enables SME instructions.
• sme-f64f64 — Enable the FEAT_SME_F64F64 extension to SME. This also enables SME instructions.
• sme2 — Enable the Scalable Matrix Extension 2. This also enables SME instructions.
• sme-f8f16 — Enable the FEAT_SME_F8F16 extension to SME. This also enables SME2 and FP8 instructions.
• sme-f8f32 — Enable the FEAT_SME_F8F32 extension to SME. This also enables SME2 and FP8 instructions.
• sme-b16b16 — Enable the FEAT_SME_B16B16 extension to SME. This also enables SME2 and SVE_B16B16 instructions.
• sme-f16f16 — Enable the FEAT_SME_F16F16 extension to SME. This also enables SME2 instructions.
• sme2p1 — Enable the Scalable Matrix Extension version 2.1. This also enables SME2 instructions.
• fcma — Enable the complex number SIMD extensions.
• jscvt — Enable the fjcvtzs JavaScript conversion instruction.
• frintts — Enable floating-point round to integral value instructions.
• wfxt — Enable wfet and wfit instructions.
• xs — Enable the XS memory attribute extension.
• lse128 — Enable the LSE128 128-bit atomic instructions extension. This also enables LSE instructions.
• d128 — Enable support for 128-bit system register read/write instructions. This also enables the LSE128 extension.
• gcs — Enable support for Armv9.4-a Guarded Control Stack extension.
• the — Enable support for Armv8.9-a/9.4-a translation hardening extension.
• rcpc2 — Enable the RCpc2 extension.
• rcpc3 — Enable the RCpc3 (Release Consistency) extension.
• fp8 — Enable the fp8 (8-bit floating point) extension.
• fp8fma — Enable the fp8 (8-bit floating point) multiply accumulate extension.
• ssve-fp8fma — Enable the fp8 (8-bit floating point) multiply accumulate extension in streaming mode.
• fp8dot4 — Enable the fp8 (8-bit floating point) to single-precision 4-way dot product extension.
• ssve-fp8dot4 — Enable the fp8 (8-bit floating point) to single-precision 4-way dot product extension in streaming mode.
• fp8dot2 — Enable the fp8 (8-bit floating point) to half-precision 2-way dot product extension.
• ssve-fp8dot2 — Enable the fp8 (8-bit floating point) to half-precision 2-way dot product extension in streaming mode.
• faminmax — Enable the Floating Point Absolute Maximum/Minimum extension.
• lut — Enable the Lookup Table extension.
• sme-lutv2 — Enable the SME Lookup Table v2 (LUTv2) extension.
• cpa — Enable the Checked Pointer Arithmetic instructions.
• sve-b16b16 — Enable the SVE non-widening brain floating-point (bf16) extension. This only has an effect when sve2 or sme2 are also enabled.
• sve-bfscale — Enable the SVE_BFSCALE extension.

Now the domino chain of implications, pay attention bestie because it cascades 🫶:

Feature crypto implies aes, sha2, and simd, which implies fp. Conversely, nofp implies nosimd, which implies nocrypto, noaes and nosha2.

---

🦋 Adapteva Epiphany Options 🦋

These -m options are defined for Adapteva Epiphany (yes, that's a whole other little world of hardware, and she's cute):

• -mhalf-reg-file
• -mno-half-reg-file — Don't allocate any register in the range r32...r63. That allows code to run on hardware variants that lack these registers.
• -mprefer-short-insn-regs
• -mno-prefer-short-insn-regs — Preferentially allocate registers that allow short instruction generation. This can result in increased instruction count, so this may either reduce or increase overall code size.
• -mbranch-cost=num — Set the cost of branches to roughly num "simple" instructions. This cost is only a heuristic and is not guaranteed to produce consistent results across releases.
• -mcmove
• -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 just... test the flags. It's faster than a software comparison, no cap, but bestie it can get incorrect results in the presence of NaNs, or when two different small numbers get compared such that their difference is calculated as zero 😭. The default is -msoft-cmpsf, which uses slower but IEEE-compliant software comparisons. Slower but correct, the eternal tradeoff 💅
• -mstack-offset=num — sets the offset between the top of the stack and the stack pointer. So like, 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 wanna evaluate whether a different stack offset would give you better code, but to actually use a different stack offset to build working programs, it is recommended to configure the toolchain with the appropriate --with-stack-offset=num option 👑
• -mno-round-nearest, -mround-nearest — -mno-round-nearest makes the scheduler assume the rounding mode has been set to truncating. The default is -mround-nearest 💖
• -mlong-calls — if not otherwise specified by an attribute, assume all calls might be beyond the offset range of the b / bl instructions, and therefore load the function address into a register before performing an (otherwise direct) call. This is the default ⭐
• -mshort-calls — if not otherwise specified by an attribute, assume all direct calls are in the range of the b / bl instructions, so use these instructions for direct calls.

The default is -mlong-calls. 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 — sets 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 — any mode at function entry is valid, and retained or restored when the function returns, and when it calls other functions. This mode is useful for compiling libraries or other compilation units you might want to incorporate into different programs with different prevailing FPU modes, and the convenience of being able to use a single object file outweighs the size and speed overhead for any extra mode switching that might be needed, compared with what would be needed with a more specific choice of prevailing FPU mode.
• truncate — this is the mode used for floating-point calculations with truncating (i.e. round towards zero) rounding mode. That includes conversion from floating point to integer.
• round-nearest — this is the mode used for floating-point calculations with round-to-nearest-or-even rounding mode.
• int — this is the mode used to perform integer calculations in the FPU, e.g. integer multiply, or integer multiply-and-accumulate.

The default is -mfp-mode=caller 🌸

• -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 💀
• -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 🦋

---

🌈 AMD GCN Options 🌈

These options are defined specifically for the AMD GCN port 🔥

• -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 💅
• -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 👑
• -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 🌸
• -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. she's got the full fit 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 queen, minimal everything 🎀

---

• -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 one is enabled by default, bestie, so it's already on unless you say otherwise.
• -mea — generate extended arithmetic instructions. rn the only ones supported are divaw, adds, subs, and sat16. only valid for -mcpu=ARC700. 🙈
• -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. slay, no notes 💅
• -mspfp
• -mspfp-compact — generate single-precision FPX instructions, tuned for the compact implementation.
• -mspfp-fast — generate single-precision FPX instructions, tuned for the fast implementation.
• -msimd — enable generation of ARC SIMD instructions via target-specific builtins. only valid for -mcpu=ARC700.
• -msoft-float — this option is ignored; it is 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 it's a decorative option, she's just there for vibes 🦋)
• -mswap — generate swap instructions.
• -matomic — this enables use of the locked load/store conditional extension to implement atomic memory built-in functions. not available for ARC 6xx or ARC EM cores.
• -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.
• -mll64 — enable double load/store operations for ARC HS cores.
• *-mtp-regno=regno* — specify thread pointer register number.
• -mbitops — enable use of NPS400 bit operations.
• -mcmem — enable use of NPS400 xld/xst extension.
• *-mmpy-option=multo* — compile ARCv2 code with a multiplier design option. you can specify the option using either a string or a numeric value for multo. wlh1 is the default value. the recognized values are, and this is the whole menu bestie 🍓:
  • 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.

• *-mfpu=fpu* — enables support for specific floating-point hardware extensions for ARCv2 cores. the supported values for fpu are:
  • fpus — enables support for single-precision floating-point hardware extensions.
  • fpud — enables support for double-precision floating-point hardware extensions. the single-precision floating-point extension is also enabled. not available for ARC EM.
  • fpuda — enables support for double-precision floating-point hardware extensions using double-precision assist instructions. the single-precision floating-point extension is also enabled. this option is only available for ARC EM.
  • fpuda_div — enables support for double-precision floating-point hardware extensions using double-precision assist instructions. the single-precision floating-point, square-root, and divide extensions are also enabled. this option is only available for ARC EM.
  • fpuda_fma — enables support for double-precision floating-point hardware extensions using double-precision assist instructions. the single-precision floating-point and fused multiply and add hardware extensions are also enabled. this option is only available for ARC EM.
  • fpuda_all — enables support for double-precision floating-point hardware extensions using double-precision assist instructions. all single-precision floating-point hardware extensions are also enabled. this option is only available for ARC EM.
  • fpus_div — enables support for single-precision floating-point, square-root and divide hardware extensions.
  • fpud_div — enables support for double-precision floating-point, square-root and divide hardware extensions. this option includes 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-purpose registers that the processor automatically saves/restores on interrupt entry and exit. register-range is specified as two registers separated by a dash. the register range *always starts with r0, the upper limit is 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 second register bank on entry to 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 are 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 is 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.
• -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.

---

the following options are passed through to the assembler, and also define preprocessor macro symbols. no cap, they do double duty 💅

• -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 — gets passed straight through to the linker to say "hey bestie, use the arclinux emulation". It's on by default in tool chains built for the arc-linux-uclibc and arceb-linux-uclibc targets when profiling is not requested. Periodt.
• -marclinux_prof, -mno-arclinux_prof — same deal, also passed through to the linker, but this one says use the arclinux_prof emulation. Enabled by default in tool chains built for arc-linux-uclibc and arceb-linux-uclibc targets when profiling is requested. Slay.

The following options control the semantics of generated code:

• -mlong-calls — generates calls as register indirect calls, so you get access to the whole full 32-bit address range. iconic.
• -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 so the 25-bit range can be used instead of the 21-bit range with conditional branch-and-link. This is the default for tool chains built for arc-linux-uclibc and arceb-linux-uclibc targets.
• -G num — puts 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.

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.

Due to delay slot scheduling and interactions between operand numbers, literal sizes, instruction lengths, and the support for conditional execution, the target-independent pass to generate conditional execution is often lacking, so the ARC port has kept a special pass around that tries to find more conditional execution generation opportunities after register allocation, branch shortening, and delay slot scheduling have been done. This pass generally, but not always, improves performance and code size, at the cost of extra compilation time, which is why there is an option to switch it off. If you have a problem with call instructions exceeding their allowable offset range because they are conditionalized, you should consider using -mmedium-calls instead.

• -mearly-cbranchsi — enable pre-reload use of the cbranchsi pattern.
• -mindexed-loads — enable the use of indexed loads. 🫶 This can be problematic because some optimizers then assume that indexed stores exist, which is not the case. lowkey a trap.
• -mlra-priority-none — don't indicate any priority for target registers.
• -mlra-priority-compact — indicate target register priority for r0..r3 / r12..r15.
• -mlra-priority-noncompact — reduce target register priority for r0..r3 / r12..r15.
• -mmillicode — when optimizing for size (using -Os), prologues and epilogues that have to save or restore a large number of registers are often shortened by using a call to a special function in libgcc; this is referred to as a millicode call. As these calls can pose performance issues, and/or cause linking issues when linking in a nonstandard way, this option is provided to turn on or off millicode call generation.
• -mcode-density-frame — this option enables the compiler to emit enter and leave instructions. These instructions are only valid for CPUs with the code-density feature. ⭐
• -msize-level=level — fine-tune size optimization with regards to instruction lengths and alignment. The recognized values for level are:
  1. 0 — No size optimization. This level is deprecated and treated like 1. 💀
  2. 1 — Short instructions are used opportunistically.
  3. 2 — In addition, alignment of loops and of code after barriers are dropped.
  4. 3 — In addition, optional data alignment is dropped, and the option Os is enabled.

This defaults to 3 when -Os is in effect. Otherwise, the behavior when this is not set is equivalent to level 1.

• -mtune=cpu — set instruction scheduling parameters for cpu, overriding any implied by -mcpu=.

Supported values for cpu are

• ARC600 — Tune for ARC600 CPU.
• ARC601 — Tune for ARC601 CPU.
• ARC700 — Tune for ARC700 CPU with standard multiplier block.
• ARC700-xmac — Tune for ARC700 CPU with XMAC block.
• ARC725D — Tune for ARC725D CPU.
• ARC750D — Tune for ARC750D CPU.
• core3 — Tune for ARCv2 core3 type CPU. This option enables usage of dbnz instruction.
• release31a — Tune for ARC4x release 3.10a.

• -mmultcost=num — cost to assume for a multiply instruction, with 4 being equal to a normal instruction.

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🌸 ARM Options 🌸

These -m options are defined for the ARM port:

• -mabi=name — generate code for the specified ABI. Permissible values are: apcs-gnu, atpcs, aapcs and aapcs-linux.
• -mapcs-frame — generate a stack frame that is compliant with the ARM Procedure Call Standard for all functions, even if this is not strictly necessary for correct execution of the code. Specifying -fomit-frame-pointer with this option causes the stack frames not to be generated for leaf functions. The default is -mno-apcs-frame. This option is deprecated. 🚨
• -mapcs — this is a synonym for -mapcs-frame and is deprecated. (still deprecated bestie, no cap)
• -mthumb-interwork — generate code that supports calling between the ARM and Thumb instruction sets. Without this option, on pre-v5 architectures, the two instruction sets cannot be reliably used inside one program. The default is -mno-thumb-interwork, since slightly larger code is generated when -mthumb-interwork is specified. In AAPCS configurations this option is meaningless.
• -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 that all functions start with a recognizable set of instructions (or in fact one of a choice from a small set of different function prologues), and this information can be used to locate the start of functions inside an executable piece of code. The default is -msched-prolog.
• -mfloat-abi=name — specifies which floating-point ABI to use. Permissible values are: soft, softfp and hard.

Specifying soft causes GCC to generate output containing library calls for floating-point operations. softfp allows the generation of code using hardware floating-point instructions, but still uses the soft-float calling conventions. hard allows generation of floating-point instructions and uses FPU-specific calling conventions.

The default depends on the specific target configuration. Note that the hard-float and soft-float ABIs are not link-compatible; you must compile your entire program with the same ABI, and link with a compatible set of libraries. 😭 don't mix them fr fr.

• -mgeneral-regs-only — generate code which uses only the general-purpose registers. This will prevent the compiler from using floating-point and Advanced SIMD registers but will not impose any restrictions on the assembler.
• -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 is dependent on the selected target architecture. For ARMv6 and later architectures the default is BE8, for older architectures the default is BE32. BE32 format has been deprecated by ARM. 💀
• -march=name[+extension...] — this specifies the name of the target ARM architecture. GCC uses this name to determine what kind of instructions it can emit when generating assembly code. This option can be used in conjunction with or instead of the -mcpu= option.

Permissible names are: 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. ✨ the way that math works out is so satisfying honestly.

okay so real talk: most of these extension names are super generic-looking, but what they actually DO depends entirely on which architecture you're applying them to. it's giving shapeshifter energy. for example, +simd can be applied to both armv7-a and armv8-a, but it'll enable the original ARMv7-A Advanced SIMD (Neon) extensions for armv7-a and the ARMv8-A variant for armv8-a. same name, whole different vibe 💅

and the list below? it's literally every supported extension for each architecture. if an architecture isn't mentioned here, bestie, it supports zero extensions. nothing. not a one. 🙈

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💖 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, so pick whichever one speaks to you 🫶
• +nofp — turns the floating-point instructions OFF. no fp for you.

🌸 armv7

the common subset of the ARMv7-A, ARMv7-R and ARMv7-M architectures — like the shared group chat of the three.

• +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 bestie: floating-point is not supported by the base ARMv7-M architecture, but it is compatible with both the ARMv7-A and ARMv7-R architectures.
• +nofp — disable the floating-point instructions.

✨ armv7-a

• +mp — the multiprocessing extension.
• +sec — the security extension.
• +fp — the VFPv3 floating-point instructions, with 16 double-precision registers. the extension +vfpv3-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, with the half-precision floating-point conversion operations.
• +neon-vfpv4 — the Advanced SIMD (Neon) v2 and the VFPv4 floating-point instructions.
• +nosimd — disable the Advanced SIMD instructions (does not disable floating point).
• +nofp — disable the floating-point and Advanced SIMD instructions.

🦋 armv7ve

the extended version of the ARMv7-A architecture with support for virtualization. fancy.

• +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

• +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 — bestie, this is the cryptographic instructions 💖 and yes it also turns on the Advanced SIMD and floating-point instructions and the Dot Product extension. slay.
• +nocrypto — turn the cryptographic extension off. bye.
• +nofp — turn off the floating-point, Advanced SIMD and cryptographic instructions. all of them. gone 💀
• +i8mm — 8-bit Integer Matrix Multiply instructions ✨ this one 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. and lowkey, the extension +vfpv3xd can be used as an alias for this extension. iconic.
  • +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. the way that's a whole mouthful 😭
  • +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.
  • +fp.dp — the single- and double-precision floating-point instructions.
  • +nofp — disable the floating-point extension.
  • +cdecp0, +cdecp1, ... , +cdecp7 — enable the Custom Datapath Extension (CDE) on selected coprocessors according to the numbers given in the options in the range 0 to 7 🎀
  • +pacbti — enable the Pointer Authentication and Branch Target Identification Extension.
• armv8-m.main
  • +dsp — the DSP instructions.
  • +nodsp — disable the DSP extension.
  • +fp — the single-precision floating-point instructions.
  • +fp.dp — the single- and double-precision floating-point instructions.
  • +nofp — disable the floating-point extension.
  • +cdecp0, +cdecp1, ... , +cdecp7 — enable the Custom Datapath Extension (CDE) on selected coprocessors according to the numbers given in the options in the range 0 to 7.
• armv8-r
  • +crc — the Cyclic Redundancy Check (CRC) instructions.
  • +fp.sp — the single-precision FPv5 floating-point instructions.
  • +simd — the ARMv8-A Advanced SIMD and floating-point instructions.
  • +crypto — the cryptographic instructions.
  • +nocrypto — disable the cryptographic instructions.
  • +nofp — disable the floating-point, Advanced SIMD and cryptographic instructions.

-march=native makes the compiler auto-detect the architecture of the build computer. but real talk: at present this feature is only supported on GNU/Linux, and not all architectures are recognized 😭 and if the auto-detect is unsuccessful the option has no effect. no cap.

-march=unset makes the compiler ignore any -march=... options that appear earlier on the command line and behave as if the option was never passed. this is useful to avoid warnings about conflicting CPU and architecture options when the two produce different architecture specifications ✨

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🌸 -mtune=name 🌸

okay so this option specifies the name of the target ARM processor for which GCC should tune the performance of the code. for some ARM implementations better performance can be obtained by using this option. the permissible names are literally: 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. 💅

additionally, this option can specify that GCC should tune the performance of the code for a big.LITTLE system. permissible names are: cortex-a15.cortex-a7, cortex-a17.cortex-a7, cortex-a57.cortex-a53, cortex-a72.cortex-a53, cortex-a72.cortex-a35, cortex-a73.cortex-a53, cortex-a75.cortex-a55, cortex-a76.cortex-a55.

-mtune=generic-arch specifies that GCC should tune the performance for a blend of processors within architecture arch. the aim is to generate code that run well on the current most popular processors, balancing between optimizations that benefit some CPUs in the range, and avoiding performance pitfalls of other CPUs 🦋 the effects of this option may change in future GCC versions as CPU models come and go. (they come and go, fr fr.)

-mtune permits the same extension options as -mcpu, but the extension options do not affect the tuning of the generated code. noted.

-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.

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👑 -mcpu=name[+extension...] 👑

this specifies the name of the target ARM processor. GCC uses this name to derive the name of the target ARM architecture (as if specified by -march) and the ARM processor type for which to tune for performance (as if specified by -mtune) ✨ where this option is used in conjunction with -march or -mtune, those options take precedence over the appropriate part of this option. periodt.

many of the supported CPUs implement optional architectural extensions. where this is so the architectural extensions are normally enabled by default. if implementations that lack the extension exist, then the extension syntax can be used to disable those extensions that have been omitted. for floating-point and Advanced SIMD (Neon) instructions, the settings of the options -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 will override the available floating-point and SIMD extension instructions 🎀

for example, cortex-a9 can be found in three major configurations: integer only, with just a floating-point unit or with floating-point and Advanced SIMD. the default is to enable all the instructions, but the extensions +nosimd and +nofp can be used to disable just the SIMD or both the SIMD and floating-point instructions respectively. the way that's actually so clean 😭

permissible names for this option are the same as those for -mtune.

the following extension options are common to the listed CPUs:

• +nodsp — disable the DSP instructions on cortex-m33, cortex-m35p, cortex-m52, cortex-m55 and cortex-m85. also disable the M-Profile Vector Extension (MVE) integer and single precision floating-point instructions on cortex-m52, cortex-m55 and cortex-m85.
• +nopacbti — disable the Pointer Authentication and Branch Target Identification Extension on cortex-m52 and cortex-m85.
• +nomve — disable the M-Profile Vector Extension (MVE) integer and single precision floating-point instructions on cortex-m52, cortex-m55 and cortex-m85.
• +nomve.fp — disable the M-Profile Vector Extension (MVE) single precision floating-point instructions on cortex-m52, cortex-m55 and cortex-m85.
• +cdecp0, +cdecp1, ... , +cdecp7 — enable the Custom Datapath Extension (CDE) on selected coprocessors according to the numbers given in the options in the range 0 to 7 on cortex-m52, cortex-m55 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 — bestie, this one switches OFF the double-precision part of the floating-point instructions, and it does that on cortex-r5, cortex-r7, cortex-r8, cortex-r52, cortex-r52plus and cortex-m7.
• +nosimd — turns off the SIMD instructions (but not the floating-point ones, don't get it twisted) on generic-armv7-a, cortex-a5, cortex-a7 and cortex-a9.
• +crypto — turns the cryptographic instructions ON 💅 and it works 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.

---

Okay so the generic-armv7-a pseudo target? It defaults to VFPv3 with 16 double-precision registers. And it supports these extension options, bestie: mp, sec, vfpv3-d16, vfpv3, vfpv3-d16-fp16, vfpv3-fp16, vfpv4-d16, vfpv4, neon, neon-vfpv3, neon-fp16, neon-vfpv4. The meanings are literally the same as for the extensions to -march=armv7-a. ✨

-mcpu=generic-arch is also permissible, and it's equivalent to -march=arch -mtune=generic-arch. See -mtune for more info, iykyk.

-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. 🫶

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✨ -mfpu=name ✨

This specifies what floating-point hardware (or hardware emulation) is available on the target. Permissible names are: auto, vfpv2, vfpv3, vfpv3-fp16, vfpv3-d16, vfpv3-d16-fp16, vfpv3xd, vfpv3xd-fp16, neon-vfpv3, neon-fp16, vfpv4, vfpv4-d16, fpv4-sp-d16, neon-vfpv4, fpv5-d16, fpv5-sp-d16, fp-armv8, neon-fp-armv8 and crypto-neon-fp-armv8. Note that neon is an alias for neon-vfpv3 and vfp is an alias for vfpv2.

The setting auto is the default and it's special, okay? It causes the compiler to select the floating-point and Advanced SIMD instructions based on the settings of -mcpu and -march.

If the selected floating-point hardware includes the NEON extension (e.g. -mfpu=neon), note that floating-point operations are not generated by GCC's auto-vectorization pass unless -funsafe-math-optimizations is also specified. This is because NEON hardware does not fully implement the IEEE 754 standard for floating-point arithmetic (in particular denormal values are treated as zero), so the use of NEON instructions may lead to a loss of precision. 😭

You can also set the fpu name at function level by using the target("fpu=") function attributes or pragmas. 👑

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🌸 -mfp16-format=name 🌸

Specify the format of the __fp16 half-precision floating-point type. Permissible names are none, ieee, and alternative; the default is none, in which case the __fp16 type is not defined.

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🎀 -mstructure-size-boundary=n 🎀

The sizes of all structures and unions are rounded up to a multiple of the number of bits set by this option. Permissible values are 8, 32 and 64. The default value varies for different toolchains. For the COFF targeted toolchain the default value is 8. A value of 64 is only allowed if the underlying ABI supports it.

Specifying a larger number can produce faster, more efficient code, but it can also increase the size of the program. Different values are potentially incompatible. Code compiled with one value cannot necessarily expect to work with code or libraries compiled with another value, if they exchange information using structures or unions.

This option is deprecated. 💀

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🦋 -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. Not me writing a function that says it never returns and then trying to return anyway 🙈

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💜 -mlong-calls / -mno-long-calls 💜

• -mlong-calls
• -mno-long-calls

Tells the compiler to perform function calls by first loading the address of the function into a register and then performing a subroutine call on this register. This switch is needed if the target function lies outside of the 64-megabyte addressing range of the offset-based version of subroutine call instruction.

Even if this switch is enabled, not all function calls are turned into long calls. The heuristic is that static functions, functions that have the short_call attribute, functions that are inside the scope of a #pragma no_long_calls directive, and functions whose definitions have already been compiled within the current compilation unit are not turned into long calls. The exceptions to this rule are that weak function definitions, functions with the long_call attribute or the section attribute, and functions that are within the scope of a #pragma long_calls directive are always turned into long calls.

This feature is not enabled by default. Specifying -mno-long-calls restores the default behavior, as does placing the function calls within the scope of a #pragma long_calls_off directive. Note these switches have no effect on how the compiler generates code to handle function calls via function pointers.

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⭐ -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.

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🌈 -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.

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🧚 -mpic-data-is-text-relative 🧚

Assume that the displacement between the text and data segments is fixed at static link time. This permits using PC-relative addressing operations to access data known to be in the data segment. For non-VxWorks RTP targets, this option is enabled by default. When disabled on such targets, it will enable -msingle-pic-base by default.

---

👑 -mpoke-function-name 👑

Write the name of each function into the text section, directly preceding the function prologue. The generated code is similar to this:

             t0
                 .ascii "arm_poke_function_name", 0
                 .align
             t1
                 .word 0xff000000 + (t1 - t0)
             arm_poke_function_name
                 mov     ip, sp
                 stmfd   sp!, {fp, ip, lr, pc}
                 sub     fp, ip, #4

When performing a stack backtrace, code can inspect the value of pc stored at fp + 0. If the trace function then looks at location pc - 12 and the top 8 bits are set, then we know that there is a function name embedded immediately preceding this location and has length ((pc[-3]) & 0xff000000).

---

💅 -mthumb / -marm 💅

• -mthumb
• -marm

Select between generating code that executes in ARM and Thumb states. The default for most configurations is to generate code that executes in ARM state, but the default can be changed by configuring GCC with the --with-mode=state configure option.

You can also override the ARM and Thumb mode for each function by using the target("thumb") and target("arm") function attributes or pragmas.

---

🍓 -mtpcs-frame 🍓

Generate a stack frame that is compliant with the Thumb Procedure Call Standard for all non-leaf functions. (A leaf function is one that does not call any other functions.) The default is -mno-tpcs-frame.

---

🎀 -mtpcs-leaf-frame 🎀

Generate a stack frame that is compliant with the Thumb Procedure Call Standard for all leaf functions. (A leaf function is one that does not call any other functions.) The default is -mno-apcs-leaf-frame.

---

🔥 -mcallee-super-interworking 🔥

Gives all externally visible functions in the file being compiled an ARM instruction set header which switches to Thumb mode before executing the rest of the function. This allows these functions to be called from non-interworking code. This option is not valid in AAPCS configurations because interworking is enabled by default.

---

✨ -mcaller-super-interworking ✨

Allows calls via function pointers (including virtual functions) to execute correctly regardless of whether the target code has been compiled for interworking or not. There is a small overhead in the cost of executing a function pointer if this option is enabled. This option is not valid in AAPCS configurations because interworking is enabled by default.

---

🌸 -mtp=name 🌸

Specify the access model for the thread local storage pointer. The 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.

---

🦋 -mtls-dialect=dialect 🦋

Specify the dialect to use for accessing thread local storage. Two dialects are supported---gnu and gnu2. The gnu dialect selects the original GNU scheme for supporting local and global dynamic TLS models. The gnu2 dialect selects the GNU descriptor scheme, which provides better performance for shared libraries. The GNU descriptor scheme is compatible with the original scheme, but does require new assembler, linker and library support. Initial and local exec TLS models are unaffected by this option and always use the original scheme.

---

💜 -mword-relocations 💜

Only generate absolute relocations on word-sized values (i.e. R_ARM_ABS32). This is enabled by default on targets (uClinux, SymbianOS) where the runtime loader imposes this restriction, and when -fpic or -fPIC is specified. This option conflicts with -mslow-flash-data.

---

• -mfix-cortex-m3-ldrd — okay so some Cortex-M3 cores get data corruption when ldrd instructions have overlapping destination and base registers 😭. This option just... refuses to generate those instructions. It's on by default whenever -mcpu=cortex-m3 is specified. Bestie protection, no cap.
• -mfix-cortex-a57-aes-1742098, -mno-fix-cortex-a57-aes-1742098, -mfix-cortex-a72-aes-1655431, -mno-fix-cortex-a72-aes-1655431 — turn the erratum mitigation on (or off) for Cortex-A57 and Cortex-A72 chips, specifically for the thing that messes with the AES cryptographic instructions. Enabled by default when either -mcpu=cortex-a57 or -mcpu=cortex-a72 is specified 💅.
• -munaligned-access, -mno-unaligned-access — enables (or disables) reading and writing 16- and 32-bit values from addresses that aren't 16- or 32-bit aligned. By default, unaligned access is disabled for all pre-ARMv6, all ARMv6-M, and ARMv8-M Baseline architectures, and enabled for every other architecture. If it's not enabled, words inside packed data structures get accessed one byte at a time 🐌.

The ARM attribute Tag_CPU_unaligned_access gets set in the generated object file to either true or false depending on this option's setting. And if unaligned access is enabled, the preprocessor symbol __ARM_FEATURE_UNALIGNED is defined too ✨.

• -mslow-flash-data — assumes loading data from flash is slower than fetching instructions, so literal loads get minimized for better performance. Only supported when compiling for ARMv7 M-profile, and it's off by default. Heads up: it conflicts with -mword-relocations 💀.
• -masm-syntax-unified — assumes inline assembler is using unified asm syntax. The default is currently off, which implies divided syntax. This option has no impact on Thumb2. But plot twist: this may change in future releases of GCC. Divided syntax should be considered deprecated, fr fr.
• -mrestrict-it — restricts generation of IT blocks to conform to ARMv8-A's rules. 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 👑.
• -mpure-code — do not allow constant data to be placed in code sections. Also, when compiling for ELF object format, give all text sections the ELF processor-specific section attribute SHF_ARM_PURECODE. Only available when generating non-pic code for M-profile targets.
• -mcmse — generates secure code as per the "ARMv8-M Security Extensions: Requirements on Development Tools Engineering Specification", which lives over at <https://developer.arm.com/documentation/ecm0359818/latest/> 🎀.
• -mfix-cmse-cve-2021-35465 — mitigates a potential security issue with the VLLDM instruction in some M-profile devices when using CMSE (CVE-2021-365465). Enabled by default when -mcpu= is used with cortex-m33, cortex-m35p, cortex-m52, cortex-m55, cortex-m85, or star-mc1. And -mno-fix-cmse-cve-2021-35465 turns the mitigation back off 💅.
• -mstack-protector-guard=guard, -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 canary accessible via the TLS register. The -mstack-protector-guard-offset= option is for use with -fstack-protector-guard=tls and not for use in user-land code 🙈.
• -mfdpic, -mno-fdpic — selects the FDPIC ABI, which uses 64-bit function descriptors to represent pointers to functions. When the compiler is configured for arm-*-uclinuxfdpiceabi targets, this option is on by default and implies -fPIE if none of the PIC/PIE-related options is provided. On other targets, it only enables the FDPIC-specific code generation features, and you should explicitly provide the PIC/PIE-related options as needed.

Ngl, static linking is not supported here, because it would still involve the dynamic linker when the program self-relocates. If that behavior is acceptable to you, use -static and -Wl,-dynamic-linker options.

The opposite -mno-fdpic option is useful (and required) to build the Linux kernel using the same (arm-*-uclinuxfdpiceabi) toolchain as the one used to build the userland programs 👑.

• -mbranch-protection=features — enable branch protection features (armv8.1-m.main only).

features can have one of these forms:

• none — generates code without branch protection or return address signing.
• standard — generates code with all branch protection features enabled at their standard level.
• pac-ret — generates code with return address signing set to its standard level, which is to sign all functions that save the return address to memory.
• pac-ret+leaf — extends the pac-ret signing to include leaf functions even if they don't 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 all branch protection and return address signing operations are constrained to use only the instructions defined in the architectural-NOP space. The generated code stays backwards-compatible with earlier versions of the architecture, but the extra security can be enabled at run time on processors that support the PACBTI extension ✨.

Branch target enforcement using BTI can only be enabled at runtime if all code in the application has been compiled with at least -mbranch-protection=bti.

Any setting other than none is supported only on armv8-m.main or later.

The default is to generate code without branch protection or return address signing.

• -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 💖.

---

🌈 AVR Options 🌈

These options are defined for AVR implementations, bestie:

• -mmcu=mcu — specify the AVR instruction set architecture (ISA) or device type. The default for this option is avr2.

Here's the tea on which AVR devices and ISAs are supported ☕. Note: a complete device support consists of startup code crtmcu.o, a device header avr/io*.h, a device library libmcu.a, and a device-specs (https://gcc.gnu.org/wiki/avr-gcc#spec-files) file specs-mcu. Only that last one is provided by the compiler according to the supported *mcu*s listed below. The rest is supported by AVR-LibC (https://github.com/avrdudes/avr-libc/), or by means of atpack (https://gcc.gnu.org/wiki/avr-gcc#atpack) files from the hardware manufacturer 🫶.

• avr2 — "Classic" devices with up to 8 KiB of program memory. mcu = attiny22, attiny26, at90s2313, at90s2323, at90s2333, at90s2343, at90s4414, at90s4433, at90s4434, at90c8534, at90s8515, at90s8535.
• avr25 — "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" — the "Enhanced" girlies 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, aka more than 128 KiB of program memory. Big-address energy. 🧠 mcu = "atmega256rfr2", "atmega2560", "atmega2561", "atmega2564rfr2".
• "avrxmega2" — "XMEGA" devices with more than 8 KiB and up to 64 KiB of program memory. ✨ mcu = "atxmega8e5", "atxmega16a4", "atxmega16a4u", "atxmega16c4", "atxmega16d4", "atxmega16e5", "atxmega32a4", "atxmega32a4u", "atxmega32c3", "atxmega32c4", "atxmega32d3", "atxmega32d4", "atxmega32e5", "avr64da28", "avr64da28s", "avr64da32", "avr64da32s", "avr64da48", "avr64da48s", "avr64da64", "avr64da64s", "avr64db28", "avr64db32", "avr64db48", "avr64db64", "avr64dd14", "avr64dd20", "avr64dd28", "avr64dd32", "avr64du28", "avr64du32", "avr64ea28", "avr64ea32", "avr64ea48", "avr64sd28", "avr64sd32", "avr64sd48".
• "avrxmega3" — "XMEGA" devices with up to 64 KiB of combined program memory and RAM, and program memory is visible in the RAM address space. 👀 mcu = "attiny202", "attiny204", "attiny212", "attiny214", "attiny402", "attiny404", "attiny406", "attiny412", "attiny414", "attiny416", "attiny416auto", "attiny417", "attiny424", "attiny426", "attiny427", "attiny804", "attiny806", "attiny807", "attiny814", "attiny816", "attiny817", "attiny824", "attiny826", "attiny827", "attiny1604", "attiny1606", "attiny1607", "attiny1614", "attiny1616", "attiny1617", "attiny1624", "attiny1626", "attiny1627", "attiny3214", "attiny3216", "attiny3217", "attiny3224", "attiny3226", "attiny3227", "atmega808", "atmega809", "atmega1608", "atmega1609", "atmega3208", "atmega3209", "atmega4808", "atmega4809", "avr16dd14", "avr16dd20", "avr16dd28", "avr16dd32", "avr16du14", "avr16du20", "avr16du28", "avr16du32", "avr16ea28", "avr16ea32", "avr16ea48", "avr16eb14", "avr16eb20", "avr16eb28", "avr16eb32", "avr16la14", "avr16la20", "avr16la28", "avr16la32", "avr32da28", "avr32da28s", "avr32da32", "avr32da32s", "avr32da48", "avr32da48s", "avr32db28", "avr32db32", "avr32db48", "avr32dd14", "avr32dd20", "avr32dd28", "avr32dd32", "avr32du14", "avr32du20", "avr32du28", "avr32du32", "avr32ea28", "avr32ea32", "avr32ea48", "avr32eb14", "avr32eb20", "avr32eb28", "avr32eb32", "avr32la14", "avr32la20", "avr32la28", "avr32la32", "avr32sd20", "avr32sd28", "avr32sd32".
• "avrxmega4" — "XMEGA" devices with more than 64 KiB and up to 128 KiB of program memory. 🌸 mcu = "atxmega64a3", "atxmega64a3u", "atxmega64a4u", "atxmega64b1", "atxmega64b3", "atxmega64c3", "atxmega64d3", "atxmega64d4", "avr128da28", "avr128da28s", "avr128da32", "avr128da32s", "avr128da48", "avr128da48s", "avr128da64", "avr128da64s", "avr128db28", "avr128db32", "avr128db48", "avr128db64".
• "avrxmega5" — "XMEGA" devices with more than 64 KiB and up to 128 KiB of program memory and more than 64 KiB of RAM. RAM-rich bestie. 💜 mcu = "atxmega64a1", "atxmega64a1u".
• "avrxmega6" — "XMEGA" devices with more than 128 KiB of program memory. 🌈 mcu = "atxmega128a3", "atxmega128a3u", "atxmega128b1", "atxmega128b3", "atxmega128c3", "atxmega128d3", "atxmega128d4", "atxmega192a3", "atxmega192a3u", "atxmega192c3", "atxmega192d3", "atxmega256a3", "atxmega256a3b", "atxmega256a3bu", "atxmega256a3u", "atxmega256c3", "atxmega256d3", "atxmega384c3", "atxmega384d3".
• "avrxmega7" — "XMEGA" devices with more than 128 KiB of program memory and more than 64 KiB of RAM. ✨ mcu = "atxmega128a1", "atxmega128a1u", "atxmega128a4u".
• "avrtiny" — "Reduced Tiny" Tiny core devices with only 16 general purpose registers and 512 B up to 4 KiB of program memory. Smol but mighty 🥺 mcu = "attiny4", "attiny5", "attiny9", "attiny10", "attiny102", "attiny104", "attiny20", "attiny40".
• "avr1" — this ISA is implemented by the minimal AVR core and is supported for assembler only. No compiler for you. 🙅‍♀️ mcu = "attiny11", "attiny12", "attiny15", "attiny28", "at90s1200".

---

• -mabsdata — assume that all data in static storage can be accessed by LDS / STS instructions. This one only does anything on reduced Tiny devices like ATtiny40. See also the "absdata" AVR variable attribute. 💫
• -mcvt — use a compact vector table. Some devices support a CVT with only four entries: 0=Reset, 1=NMI, 2=Prio1 IRQ, 3=Prio0 IRQs. This option links startup code from crtmcu-cvt.o instead of the usual crtmcu.o. And like, not just a smaller table — the startup code also sets bit CPUINT_CTRLA.CPUINT_CVT, which turns the CVT on for the device. 👑

If you don't want the startup code setting CPUINT_CTRLA.CPUINT_CVT, you can satisfy the symbol __init_cvt so that the respective code is no longer pulled in from libmcu.a. For example, you can link with -Wl,--defsym,__init_cvt=0.

The CVT startup code is available since AVR-LibC v2.3 (https://github.com/avrdudes/avr-libc/issues/1010).

• -mdouble=bits
• -mlong-double=bits — set the size (in bits) of the double or long double type, respectively. Possible values for bits are 32 and 64. Whether 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 same applies for the default values of the options. fr fr, it's configured at build time. 🛠️
• -mgas-isr-prologues — interrupt service routines (ISRs) may use the __gcc_isr pseudo instruction supported by GNU Binutils. If this option is on, the feature can still be disabled for individual ISRs by means of the "no_gccisr" function attribute. This feature is activated per default if optimization is on (but not with -Og, see Optimize Options), and if GNU Binutils support PR21683 (https://sourceware.org/PR21683). 🔥
• -mint8 — assume int is an 8-bit integer. This affects the sizes of all the types: a char is 1 byte, an int is 1 byte, a long is 2 bytes, and long long is 4 bytes. 🫢 Please note that this option does not conform to the C standards — but it results in smaller code size, so like, trade-offs bestie.
• -mmain-is-OS_task — do not save registers in main. The effect is the same like attaching attribute "OS_task" to main. It is activated per default if optimization is on. 💅
• -mno-call-main — don't run main by means of

XCALL  main
XJMP   exit

Instead, put main in section .init9 (https://avrdudes.github.io/avr-libc/avr-libc-user-manual/mem_sections.html#sec_dot_init) so that no call is required. By setting this option the user asserts that main will not return. ⚠️

This option can be used for devices with very limited resources in order to save a few bytes of code and stack space. It will work as expected since AVR-LibC v2.3 (https://github.com/avrdudes/avr-libc/issues/1012). With older versions, there will be no performance gain.

• -mno-interrupts — generated code is not compatible with hardware interrupts. Code size is smaller. Your call, bestie. 💀
• -mrelax — try to replace CALL resp. JMP instruction by the shorter RCALL resp. RJMP instruction if applicable. Setting -mrelax just adds the --mlink-relax option to the assembler's command line and the --relax option to the linker's command line. 🦋

Jump relaxing is performed by the linker because jump offsets are not known before code is located. Therefore, the assembler code generated by the compiler is the same, but the instructions in the executable may differ from instructions in the assembler code. 👀

Relaxing must be turned on if linker stubs are needed, see the section on EIND and linker stubs below.

• -mpmem-wrap-around — enable program counter wrap-around in linker relaxation. 🔄
• -mrodata-in-ram
• -mno-rodata-in-ram — locate the .rodata sections for read-only data in RAM resp. in program memory. For most devices, there is no choice and this option acts rather like an assertion. 🤷‍♀️

Since v14 and for the AVR64* and AVR128* devices, .rodata is located in flash memory per default, provided the required GNU Binutils support (PR31124 (https://sourceware.org/PR31124)) is available. In that case, -mrodata-in-ram can be used to return to the old layout with .rodata in RAM.

• -mtiny-stack — only change the lower 8 bits of the stack pointer. tiny stack, tiny steps 🐜
• -mfract-convert-truncate — allow to use truncation instead of rounding towards zero for fractional fixed-point types. ✂️
• -nodevicelib — don't link against AVR-LibC's device specific library libmcu.a. 🚫

Notice that since AVR-LibC v2.3, that library contains code that is essential for the correct functioning of a program. In particular, it contains parts of the startup code like: __init_sp (https://github.com/avrdudes/avr-libc/issues/1011) to initialize the stack pointer with symbol __stack, __init_cvt (https://github.com/avrdudes/avr-libc/issues/1010) to set up the hardware to use a compact vector table with -mcvt, __call_main (https://github.com/avrdudes/avr-libc/issues/1012) to call main and exit, and __do_flmap_init (https://github.com/avrdudes/avr-libc/issues/931) to set up FLMAP according to symbol __flmap.

• -nodevicespecs — don't add -specs=device-specs/specs-mcu to the compiler driver's command line. The user takes responsibility for supplying the sub-processes like compiler proper, assembler and linker with appropriate command line options. This means that the user has to supply her private device specs file by means of -specs=path-to-specs-file. There is no more need for option -mmcu=mcu. 💅

This option can also serve as a replacement for the older way of specifying custom device-specs files that needed -B some-path to point to a directory which contains a folder named device-specs which contains a specs file named specs-mcu, where mcu was specified by -mmcu=mcu. 🫶

• -Waddr-space-convert — bestie, this one warns you when you convert between address spaces and the resulting address space is not contained in the incoming one. Like, it's giving "you're moving somewhere smaller and something's gonna fall off the edge" energy. 💀
• -Wmisspelled-isr — warns you if the ISR is misspelled, aka it's missing the __vector prefix. On by default, so she's always watching. 👀✨

---

✨ AVR Optimization Options ✨

Okay so these next ones are pure optimization options, fr fr. Like, the options -mgas-isr-prologues, -mmain-is-OS_task, -mno-call-main and -mrelax from up above are only almost optimization options, because there are rare occasions where their different code generation actually matters. Not quite the same vibe. 💅

• -maccumulate-args — accumulates outgoing function arguments and acquires/releases the stack space they need once, in the function prologue/epilogue. Without this option, outgoing arguments get pushed before calling a function and popped afterwards. Also peep the -fdefer-pop Optimize Options,,optimization option.

Popping the arguments after the function call can be expensive on AVR, so accumulating the stack space might lead to smaller executables, because arguments don't need to be removed from the stack after such a function call. 🫶

This option can lead to reduced code size for functions that perform several calls to functions that get their arguments on the stack — like calls to printf-like functions.

• -mbranch-cost=cost — sets the branch costs for conditional branch instructions to cost. Reasonable values for cost are small, non-negative integers. The default branch cost is 0. 🌸
• -mcall-prologues — function prologues/epilogues are expanded as calls to appropriate subroutines. Code size is smaller. Slay, honestly. 👑
• -mfuse-add
• -mno-fuse-add
• -mfuse-add=level — optimizes 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. ✨
• -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. Each digit controls some aspect of the optimization. 🧚
• -mfuse-move2 — runs a post combine optimization pass that tries to fuse move instructions. That's it, that's the whole thing. 🎀
• -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.

Without this option, the X register may be used in the same way as Y or Z, which then is emulated by additional instructions. For example, loading a value with X+const addressing with a small non-negative const < 64 to a register Rn is performed as:

        adiw r26, const   ; X += const
        ld   <Rn>, X        ; <Rn> = *X
        sbiw r26, const   ; X -= const

• -msplit-bit-shift — 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. 💜
• -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.
• -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. ⭐

---

"EIND" and Devices with More Than 128 Ki Bytes of Flash

Pointers in the implementation are 16 bits wide. The address of a function or label is represented as word address, so that indirect jumps and calls can target any code address in the range of 64 Ki words. 👀

In order to facilitate indirect jump on devices with more than 128 Ki bytes of program memory space, there is a special function register called EIND that serves as most significant part of the target address when EICALL or EIJMP instructions are used.

Indirect jumps and calls on these devices are handled as follows by the compiler, and are subject to some limitations:

• The compiler never sets EIND.
• The compiler uses 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. 🙈
• For indirect calls to functions and computed goto, the linker generates stubs. Stubs are jump pads, sometimes also called trampolines. Thus, the indirect call/jump jumps to such a stub. The stub contains a direct jump to the desired address.
• Linker relaxation must be turned on so that the linker generates the stubs correctly in all situations. See the compiler option -mrelax and the linker option --relax. There are corner cases where the linker is supposed to generate stubs but aborts without relaxation and without a helpful error message. 😭
• The default linker script is arranged for code with EIND = 0. If code is supposed to work for a setup with EIND != 0, a custom linker script has to be used in order to place the sections whose name start with .trampolines into the segment where EIND points to.
• The startup code from libgcc never sets EIND. Notice that startup code is a blend of code from libgcc and AVR-LibC. For the impact of AVR-LibC on EIND, see the AVR-LibC user manual (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.

• Stubs are generated automatically by the linker if the following two conditions are met:
  1. 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>))

2. The final location of that label is in a code segment outside the segment where the stubs are located.

• The compiler emits such gs modifiers for code labels in the following situations:
  • Taking address of a function or code label.
  • Computed goto.
  • If prologue-save function is used, see -mcall-prologues command-line option.
  • Switch/case dispatch tables. If you do not want such dispatch tables you can specify the -fno-jump-tables command-line option.
  • C and C++ constructors/destructors called during startup/shutdown.
  • If the tools hit a gs() modifier explained above.
• Jumping to non-symbolic addresses like so is not supported:

        int main (void)
        {
            /* Call function at word address 0x2 */
            return ((int(*)(void)) 0x2)();
        }

Instead, a stub has to be set up, i.e. the function has to be called through a symbol (func_4 in the example):

        int main (void)
        {
            extern int func_4 (void);

            /* Call function at byte address 0x4 */
            return func_4();
        }

and the application be linked with -Wl,--defsym,func_4=0x4. Alternatively, func_4 can be defined in the linker script. 🫶

---

Handling of the RAMPD, RAMPX, RAMPY and RAMPZ Special Function Registers

Some AVR devices support memories larger than the 64 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. 💅

• 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.
• 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. ⚠️

---

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. And bestie, if mcu names an actual device, this built-in macro gets set to match it. Like, with -mmcu=atmega8 the macro is defined to 4. Cute, right? 💅

• __AVR_Device__ — setting -mmcu=device defines this built-in macro, and it reflects the device's actual name. So -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. And the difference between Device in the built-in macro and device in -mmcu=device? The latter is always lowercase. periodt.

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. 🎀
• __AVR_XMEGA__ — the device / architecture belongs to the XMEGA family of devices.
• __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.
• __AVR_HAVE_MUL__ — the device has a hardware multiplier. 💪
• __AVR_HAVE_JMP_CALL__ — the device has the JMP and CALL instructions. This is the case for devices with 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.
• __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.
• __NO_INTERRUPTS__ — this macro reflects the -mno-interrupts command-line option. 🫶
• __AVR_ERRATA_SKIP__, __AVR_ERRATA_SKIP_JMP_CALL__ — some AVR devices (AT90S8515, ATmega103) must not skip 32-bit instructions because of a hardware erratum. Skip instructions are SBRS, SBRC, SBIS, SBIC and CPSE. The second macro is only defined if __AVR_HAVE_JMP_CALL__ is also set.
• __AVR_ISA_RMW__ — the device has Read-Modify-Write instructions (XCH, LAC, LAS and LAT).
• __AVR_SFR_OFFSET__=offset — instructions that can address I/O special function registers directly like IN, OUT, SBI, etc. may use a different address as if addressed by an instruction to access RAM like LD or STS. This offset depends on the device architecture and has to be subtracted from the RAM address in order to get the respective I/O 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_*. ✨
• __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. 💜
• __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 so these next ones? The compiler uses them internally, and they're how device specs files and the compiler proper talk to each other. Bestie, you do not need to set these by hand — and especially, they are not optimization options. Using these options the wrong way may lead to sub-optimal or wrong code. 😭 They're documented for completeness, and so you can 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.
• -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.

Note that support for bf561 is incomplete. For bf561, only the preprocessor macro is defined. 💀

• -msim — specifies that the program will be run on the simulator. This causes the simulator BSP provided by libgloss to be linked in. This option has effect only for bfin-elf toolchain. Certain other options, such as -mid-shared-library and -mfdpic, imply -msim.
• -momit-leaf-frame-pointer — don't keep the frame pointer in a register for leaf functions. This avoids the instructions to save, set up and restore frame pointers and makes an extra register available in leaf functions. 🔥
• -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 — turn this on and the compiler makes sure the code it spits out doesn't have CSYNC or SSYNC instructions sitting too soon after conditional branches. If you actually use it, __WORKAROUND_SPECULATIVE_SYNCS gets defined. ✨
• -mlow64k, -mno-low64k — enable this and the compiler is free to flex the knowledge that the whole entire program fits into the low 64k of memory 🫶. Default vibe? It assumes your program is arbitrarily large, aka -mno-low64k. No cap.
• -mstack-check-l1 — does stack checking using the info that the uClinux kernel drops into L1 scratchpad memory. 💅
• -mid-shared-library, -mno-id-shared-library — generates code that supports shared libraries via the library ID method, which unlocks execute in place and shared libraries in an environment with no virtual memory management. This option implies -fPIC. And with a bfin-elf target, it also implies -msim. Default is -mno-id-shared-library, i.e. code that doesn't assume ID-based shared libraries are in play.
• -mleaf-id-shared-library, -mno-leaf-id-shared-library — same library ID method for shared libraries, but it assumes this library or executable won't link against any other ID shared libraries, which lets the compiler use faster code for jumps and calls. 🦋

The default is -mno-leaf-id-shared-library, where no such assumption is made and the code being compiled might link against ID shared libraries. Slower code for jump and call insns, unfortunately. 😭

• -mshared-library-id=n — tells it the identification number of the ID-based shared library being compiled. A value of 0 generates more compact code; other values force the allocation of that number to the current library but are no more space- or time-efficient than just omitting this option. 💀
• -msep-data, -mno-sep-data — generates code that lets the data segment live in a different area of memory from the text segment. This gives you execute in place in an environment without virtual memory management by eliminating relocations against the text section. 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 by first loading the address of the function into a register and then doing a subroutine call on that register. You need this switch if the target function lies outside the 24-bit addressing range of the offset-based version of the subroutine call instruction. 🔥

This feature is not enabled by default. Specifying -mno-long-calls restores the default behavior. Note that these switches have no effect on how the compiler generates code to handle function calls via function pointers.

• -mfast-fp — link with the fast floating-point library. This library relaxes some of the IEEE floating-point standard's rules for checking inputs against Not-a-Number (NAN), all in the interest of performance. ✨
• -minline-plt — enables inlining of PLT entries in function calls to functions that aren't known to bind locally. It has no effect without -mfdpic.
• -mmulticore — builds a standalone application for multicore Blackfin processors. This causes proper start files and link scripts that support multicore to be used, and defines the macro __BFIN_MULTICORE. It can only be used with -mcpu=bf561[-sirevision]. 🎀

You can use it together with -mcorea or -mcoreb, which selects the one-application-per-core programming model. Without -mcorea or -mcoreb, the single-application/dual-core programming model is used — and 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 used.

• -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 are 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 using the one-application-per-core programming model. Proper start files and link scripts 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 — assumes that ICPLBs are enabled at run time. This affects 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 a 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 sits adjacent to .neardata. 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 the 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, all uninitialized data in the .far section, and 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.

• -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 — these 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 the 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 a 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- options) arrange (eliminate arrangements) for the stack frame, individual data and constants to be aligned for the maximum single data access size for the chosen CPU model. The default is to arrange for 32-bit alignment. ABI details such as structure layout are not affected by these options. 🌸
• -m32-bit, -m16-bit, -m8-bit — similar to the stack-, data- and const-align options above, these 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 that 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. 😭

• -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 that cause 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 🦋

Okay so when you're compiling for C-SKY V2 processors, GCC supports these options. And bestie, there are a lot of them. Buckle up, we're going through every single one, no skipping. 💅

• -march=arch — Specify the C-SKY target architecture. Valid values for arch are: ck801, ck802, ck803, ck807, and ck810. The default is ck810. Slay.
• -mcpu=cpu — Specify the C-SKY target processor. Valid values for cpu are literally: 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. It's giving option overload, fr fr.
• -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.

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. That's a big deal, bestie. 😭

• -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). Note the exact spelling of those last two, iykyk.
• -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. So if you need those, you know what to do. 💅

• -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, it says coprocessor again, bestie. Not me editing it, periodt.)
• -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.

Warning, bestie: this can give you code that's not compliant with the C-SKY V2 ABI prologue requirements and that cannot be debugged or backtraced. It's disabled by default for a reason. 💀

• -msim, -mno-sim — Links the library libsemi.a which is in compatible with simulator. Applicable to ELF compiler only. (Yes, it literally says "in compatible", no cap. We don't edit.)

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 specified C RunTime DLL library. This option adjust predefined macros "__CRTDLL__", "__MSVCRT__", "_UCRT" and "__MSVCRT_VERSION__" for specified CRT library, choose start file for CRT library and link with CRT library. Recognized CRT library names for proprocessor are: "crtdll*", "msvcrt10*", "msvcrt20*", "msvcrt40*", "msvcr40*", "msvcrtd*", "msvcrt-os*", "msvcr70*", "msvcr71*", "msvcr80*", "msvcr90*", "msvcr100*", "msvcr110*", "msvcr120*" and "ucrt*". If this options is not specified then the default MinGW import library "msvcrt" is used for linking and no other adjustment for 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 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.

okay so bestie, here's the tea on the Darwin tools — they are NOT all on the same page when you hand them an ISA mismatch. 💀

The assembler, as, is the strict one: it only lets you use instructions that are valid for the subtype of the file it's generating. So like… you literally cannot put 64-bit instructions in a ppc750 object file. Periodt.

Then the linker for shared libraries, /usr/bin/libtool, is the dramatic one — it fails and prints an error if you ask it to create a shared library with a less restrictive subtype than its input files. (Think: trying to shove a ppc970 object file into a ppc7400 library. It's giving chaos. 🙈)

And the linker for executables, ld, is the chill one — it just quietly gives the executable the most restrictive subtype of any of its input files. No drama, no notes. ✨

---

• -Fdir — Adds the framework directory dir to the head of the list of directories to be searched for header files. These directories get interleaved with the ones you specify via -I options, and everything is scanned left-to-right.

So what even is a framework directory, you ask? 🦋 It's a directory with frameworks in it. A framework is a directory with a Headers and/or PrivateHeaders directory sitting directly inside it, and its name ends in .framework. The name of the framework is the name of that directory minus the .framework part. Headers belonging to the framework live in one of those two directories, and Headers gets searched first. A subframework is a framework directory that lives inside a framework's Frameworks directory. Includes of subframework headers can only show up in a header of a framework that contains the subframework, or in a sibling subframework header. Two subframeworks are siblings if they're in the same framework. A subframework should not have the same name as a framework — you'll get a warning if you violate that. Currently a subframework cannot have subframeworks; in the future, the mechanism may be extended to support this. The standard frameworks live in /System/Library/Frameworks and /Library/Frameworks. An example include looks like #include <Framework/header.h>, where Framework is 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. Heads up: 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. (So it's basically there to nod politely at your build system. 💅)
• -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 the 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 (which is the default on these systems), this overrides any -fconstant-string-class setting and causes @"..." literals to be laid out as constant CoreFoundation strings. Also, -fconstant-cfstrings is an alias for -mconstant-cfstrings. ✨
• -mdynamic-no-pic — Generate code suitable for executables (not shared libraries). Heads up bestie: 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, like letting GDB 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 that 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 Objective-C source code.
• -ObjC++ — Equivalent to -x objective-c++; specifies that the input 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.

And then there's this whole iconic squad of options that just get passed straight to the Darwin linker — the Darwin linker man page describes them in detail:

• -allowable_client client_name
• -client_name
• -compatibility_version
• -current_version
• -dead_strip
• -dylib_file
• -dylinker
• -dylinker_install_name
• -dynamic
• -exported_symbols_list
• -filelist
• -flat_namespace
• -force_flat_namespace
• -framework
• -headerpad_max_install_names
• -image_base
• -init symbol-name
• -install_name
• -keep_private_externs
• -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
• -weak_reference_mismatches
• -whatsloaded
• -whyload

---

🌸 DEC Alpha Options 🌸

These -m options are defined for the DEC Alpha implementations:

• -mno-soft-float, -msoft-float — Use (or 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 emulation 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. 😭

Also note: Alpha implementations without floating-point operations are still required to have floating-point registers. Yeah, ikyk. 💅

• -mfp-regs, -mno-fp-regs — Generate code that uses (or 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 — okay so bestie, the Alpha architecture has floating-point hardware that's built for maximum performance, and it's mostly IEEE floating-point standard compliant. But for full compliance? Software has to come help out. 💅 This option makes GCC generate code that's fully IEEE-compliant… with one lil exception: the inexact-flag isn't maintained (more on that below, don't worry). Turn this on and the preprocessor macro _IEEE_FP gets defined during compilation. The code you get is less efficient but it can correctly handle denormalized numbers and those exceptional IEEE values like not-a-number and plus/minus infinity. ✨ Other Alpha compilers call this option -ieee_with_no_inexact.

DEBIAN SPECIFIC: this is on by default for alpha-linux-gnu, unless -ffinite-math-only (which is part of the -ffast-math set) is specified — because the software functions in the GNU libc math libraries generate denormalized numbers, NaNs, and infs, and like… all of those will cause a program to SIGFPE when it tries to use the results without -mieee. 😭 Not the vibe.

• -mieee-with-inexact — same energy as -mieee, except the generated code also maintains the IEEE inexact-flag. Turning this on means the generated code implements fully-compliant IEEE math. And on top of _IEEE_FP, it defines _IEEE_FP_EXACT as a preprocessor macro too. 🎀 Heads up though: on some Alpha implementations the resulting code may run significantly slower than the code you'd get by default. And since there's, like, very little code out there that actually depends on the inexact-flag, you should normally not specify this option. Other Alpha compilers call this option -ieee_with_inexact.
• *-mfp-trap-mode=trap-mode* — this one controls what floating-point related traps are enabled. Other Alpha compilers call it -fptm trap-mode. The trap mode can be set to one of four values:
  • n — the default (normal) setting. The only traps enabled are the ones that can't be disabled in software (e.g., division by zero trap).
  • u — on top of the traps enabled by n, 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 details 👀).
  • sui — like su, but inexact traps are enabled as well.
• *-mfp-rounding-mode=rounding-mode* — selects the IEEE rounding mode. Other Alpha compilers call this option -fprm rounding-mode. The rounding-mode can be one of:
  • n — normal IEEE rounding mode. Floating-point numbers are rounded towards the nearest machine number, or towards the even machine number in case of a tie.
  • m — round towards minus infinity.
  • c — chopped rounding mode. Floating-point numbers are rounded towards zero.
  • d — dynamic rounding mode. A field in the floating-point control register (fpcr, see the Alpha architecture reference manual) controls the rounding mode in effect. The C library initializes this register for rounding towards plus infinity. So unless your program modifies the fpcr, d corresponds to round towards plus infinity. 🌀
• *-mtrap-precision=trap-precision* — so in the Alpha architecture, floating-point traps are imprecise. Which means without software assistance it is literally 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 an application needs, you can pick 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 provide the equivalent options called -scope_safe and -resumption_safe.

• -mieee-conformant — marks the generated code as IEEE conformant. ⚠️ But 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. That's it, that's the whole job. 💅
• -mbuild-constants — normally GCC looks at a 32- or 64-bit integer constant to see if it can construct it from smaller constants in two or three instructions. If it can't, it outputs the constant as a literal and generates code to load it from the data segment at run time. Use this option to require GCC to construct all integer constants using code, even if it takes more instructions — and the maximum is six. 🫶

You typically use this option to build a shared library dynamic loader. Itself a shared library, it has to relocate itself in memory before it can find the variables and constants in its own data segment. Sneaky but iconic.

• -mbwx, -mno-bwx, -mcix, -mno-cix, -mfix, -mno-fix, -mmax, -mno-max — these indicate whether GCC should generate code to use the optional BWX, CIX, FIX and MAX instruction sets. The default is to use the instruction sets supported by the CPU type specified via the -mcpu= option, or that of the CPU on which GCC was built if none is specified. 🌈
• -msafe-bwa, -mno-safe-bwa — indicate whether, in the absence of the optional BWX instruction set, GCC should generate multi-thread and async-signal safe code for byte and aligned word memory accesses. 🧚
• -msafe-partial, -mno-safe-partial — indicate whether GCC should generate multi-thread and async-signal safe code for partial memory accesses, including piecemeal accesses to unaligned data as well as block accesses to leading and trailing parts of aggregate types or other objects in memory that do not respectively start and end on an aligned 64-bit data boundary. Yes bestie, it's a mouthful, but every word is doing work. 💜
• -mfloat-vax, -mfloat-ieee — generate code that uses (does not use) VAX F and G floating-point arithmetic instead of IEEE single and double precision. Pick your fighter. 🥊
• -mexplicit-relocs, -mno-explicit-relocs — older Alpha assemblers provided no 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 is built and sets the default accordingly.
• -msmall-data, -mlarge-data — when -mexplicit-relocs is in effect, static data is accessed via gp-relative relocations. When -msmall-data is used, objects 8 bytes long or smaller are placed in a small data area (the .sdata and .sbss sections) and are accessed via 16-bit relocations off of the $gp register. This limits the size of the small data area to 64KB, but it lets the variables be directly accessed via a single instruction. ✨

The default is -mlarge-data. With this option the data area is limited to just below 2GB. Programs that need more than 2GB of data must use malloc or mmap to allocate the data in the heap instead of in the program's data segment. 🫠

When generating code for shared libraries, -fpic implies -msmall-data and -fPIC implies -mlarge-data.

• -msmall-text, -mlarge-text — when -msmall-text is used, the compiler assumes that the code of the entire program (or shared library) fits in 4MB, and is thus reachable with a branch instruction. When -msmall-data is used, the compiler can assume that all local symbols share the same $gp value, and thus reduce the number of instructions required for a function call from 4 to 1. Glow up fr fr. 🌟

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. Periodt.

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. Short, sweet, slays. 💅
• -mtls-size=bitsize — okay so this sets the bit size of immediate TLS offsets, bestie. Valid bitsize values are 16, 32, and 64, and the default is 32. That's it, that's the vibe. ✨
• -mlong-double-128 / -mlong-double-64 — pick the size of the long double type, slay. 🎀 Heads up though: -mlong-double-128 is incompatible with VAX floating point. Not compatible. Do not try it.

---

eBPF Options 🦋

• -mframe-limit=bytes — sets the hard limit for frame sizes, in bytes. Right now the value you can pass has to be less than or equal to 32767. Defaults to whatever limit the targeted version of the Linux kernel imposes on you. 👑
• -mbig-endian — generate code for a big-endian target.
• -mlittle-endian — generate code for a little-endian target. This one is the default, periodt. 💅
• -mjmpext / -mno-jmpext — turn generation of extra conditional-branch instructions on or off. Enabled for CPU v2 and above.
• -mjmp32 / -mno-jmp32 — turn generation of 32-bit jump instructions on or off. Enabled for CPU v3 and above.
• -malu32 / -mno-alu32 — turn generation of 32-bit ALU instructions on or off. Enabled for CPU v3 and above.
• -mv3-atomics / -mno-v3-atomics — turn on or off the instructions for general atomic operations introduced in CPU v3. 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 one says which version of the eBPF ISA to target, bestie. Newer versions might not be supported by all kernels, so watch out. The default is v4. 😭

Supported values for version are:

• v1 — the first stable eBPF ISA, with no special features or extensions. The original icon, lowkey. 🌸
• v2 — supports the jump extensions, as in -mjmpext.
• v3 — all the 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 the 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 the BPF architecture imposes on you:
  • save and restore callee-saved registers at function entry and exit, respectively.
• -masm=dialect — outputs assembly instructions using the 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 — sets 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, going over 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. A little heads-up, as a treat. 💖

---

FR30 Options 🌈

These options are defined specifically for the FR30 port, iykyk.

• -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. So like, it's giving small, and it's committing to small. 🎀
• -mno-lsim — assume that runtime support has been provided, so there's no need to include the simulator library (libsim.a) on the linker command line.

---

FRV Options 🔥

• -mgpr-32 — only use the first 32 general-purpose registers.
• -mgpr-64 — use all 64 general-purpose registers.
• -mfpr-32 — use only the first 32 floating-point registers.
• -mfpr-64 — use all 64 floating-point registers.
• -mhard-float — use hardware instructions for floating-point operations.
• -msoft-float — use library routines for floating-point operations.
• -malloc-cc — dynamically allocate condition code registers.
• -mfixed-cc — do not try to dynamically allocate condition code registers; only use icc0 and fcc0.
• -mdword / -mno-dword — control whether the ABI uses double-word instructions.
• -mdouble / -mno-double — enable or disable use of floating-point double instructions.
• -mmedia / -mno-media — enable or disable use of media instructions.
• -mmuladd / -mno-muladd — enable or disable use of multiply and add/subtract instructions.
• -mfdpic — select the FDPIC ABI, which uses function descriptors to represent pointers to functions. Without any PIC/PIE-related options, it implies -fPIE. With -fpic or -fpie, it assumes GOT entries and small data are within a 12-bit range from the GOT base address; with -fPIC or -fPIE, GOT offsets are computed with 32 bits. With a bfin-elf target, this option implies -msim. So many implications, bestie. 😭
• -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. Slay. 💅
• -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 a win, -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, so like, don't. 🙈
• -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. Main character energy, fr. 👑
• -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 — enable or disable packing VLIW instructions.
• -mno-eflags — do not mark ABI switches in e_flags.
• -mcond-move / -mno-cond-move — enable or disable the use of conditional-move instructions; it is enabled by default.

This switch is mainly for debugging the compiler and will likely be removed in a future version. So like, don't get too attached. 💀

• -mscc / -mno-scc — enable or disable the use of conditional set instructions; 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 — enable or disable the use of conditional execution; 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 — enable or disable an optimization pass to pack branches into VLIW instructions; 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 — enable or disable optimization of && and || in conditional execution; 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 — enable or disable nested conditional execution optimizations; 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. Clean code, we love to see it. 🧚
• -mtomcat-stats — cause gas to print out tomcat statistics. Yes really, that's the whole thing. 🌸
• -mcpu=cpu — select the processor type for which to generate code. Possible values are frv, fr550, tomcat, fr500, fr450, fr405, fr400, fr300 and simple.

---

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. For real, do not. 😭

• -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.

---

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. So if you're on one of those three, this one is not your default, bestie. 🎀
• -muclibc — use the 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 the Bionic C library. This is the default on *-*-linux-*android* targets. 🤖
• -mandroid
• -mno-android — compile code that's compatible with the Android platform. This is the default on *-*-linux-*android* targets.

Okay so when you're compiling, this option turns on -mbionic, -fPIC, -fno-exceptions and -fno-rtti by default. When you're linking, it makes the GCC driver hand Android-specific options over to the linker. And finally — the little cherry on top 🍓 — this option makes the preprocessor macro __ANDROID__ get defined.

You can turn this whole thing off completely with -mno-android.

• -tno-android-cc — disable the compilation effects of -mandroid, i.e. don't go enabling -mbionic, -fPIC, -fno-exceptions and -fno-rtti by default.
• -tno-android-ld — disable the linking effects of -mandroid, i.e. pass standard Linux linking options to the linker.

---

🌸 H8/300 Options 🌸

These -m options are defined for the H8/300 implementations:

• -mrelax — shorten some address references at link time, when possible. Uses the linker option -relax.
• -mh — generate code for the H8/300H.
• -ms — generate code for the H8S.
• -mn — generate code for the H8S and H8/300H in the normal mode. This switch must be used together with either -mh or -ms.
• -msx — generate H8SX code.
• -ms2600 — generate code for the H8S/2600. This switch must be used with -ms.
• -mquickcall — use registers for argument passing.
• -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. Heads up: this option is only valid 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 instead. This option has no effect on the H8/300.

---

👑 HPPA Options 👑

These -m options are defined for the HPPA family of computers:

• -march=architecture-type — generate code for the specified architecture. The choices for architecture-type are 1.0 for PA 1.0, 1.1 for PA 1.1, and 2.0 for PA 2.0 processors. If you're on an HP-UX system, peek at /usr/lib/sched.models 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. 💅
• -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.
• -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 are implemented as a single access.

The PA-RISC architecture does not support any atomic operations in hardware except for the "ldcw" instruction. So, all atomic support is implemented using sync and atomic libcalls. Sync libcall support lives in libgcc.a. Atomic libcall support lives 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. Use this option with care, bestie, because it is not compatible with the default 32-bit runtime. That said, only aggregates larger than eight bytes are passed by hidden reference, and the option gives you 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 dodges 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. ✨

Careful though: this option does not work in the presence of shared libraries or nested functions.

• -mfixed-range=register-range — generate code treating the given register range as fixed registers. A fixed register is one the register allocator cannot use. Super useful when you're compiling kernel code. A register range is specified as two registers separated by a dash. You can specify multiple register ranges 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 get passed to that ld. Which ld gets 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 get passed to that ld. Which ld gets 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. Note that this makes symbolic debugging impossible. 💀
• -mlong-calls — generate code that uses long call sequences. This makes sure 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 since it degrades performance. However, it may be useful in large applications, particularly when partial linking is used to build the application.

The types of long calls used depends on the capabilities of the assembler and linker, and the type of code being generated. The impact on systems that support long absolute calls, and long PIC symbol-difference or PC-relative calls should be relatively small. However, an indirect call is used on 32-bit ELF systems in PIC code and it is quite long.

• -mlong-load-store — generate 3-instruction load and store sequences as sometimes required by the HP-UX 10 linker. This is equivalent to the +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 cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation. 😭

-msoft-float changes the calling convention in the output file; therefore, it is only useful if you compile all of a program with this option. In particular, you need to compile libgcc.a, the library that comes with GCC, with -msoft-float in order for this to work.

• -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 though to 11.00, and 98 for HP-UX 11.11 and later. ⭐

-munix=93 hands you the exact same predefines as GCC 3.3 and 3.4, no cap. Then -munix=95 comes through with extra predefines for "XOPEN_UNIX" and "_XOPEN_SOURCE_EXTENDED", plus the startfile unix95.o. And -munix=98? Bestie. It adds predefines for "_XOPEN_UNIX", "_XOPEN_SOURCE_EXTENDED", "_INCLUDE__STDC_A1_SOURCE" and "_INCLUDE_XOPEN_SOURCE_500", and the startfile unix98.o.

Okay so listen, this is the part where I grab your shoulders. This option changes the interfaces for various library routines. It also messes with the operational behavior of the C library. So extreme care is needed when you use this option. Like, actual extreme care. Not vibes-based care. 💀

Library code that's meant to work with more than one UNIX standard has to test, set and restore the variable "__xpg4_extended_mask" as appropriate. And here's the sad part — most GNU software doesn't provide this capability. 🙈

---

• -nolibdld — suppresses the generation of link options that search libdld.sl when the -static option is specified on HP-UX 10 and later.
• -static — so, the HP-UX C library's implementation of "setlocale" depends on libdld.sl, and there is no archive version of libdld.sl. Which means when you pass -static, you need special link options to resolve that dependency. ✨

On HP-UX 10 and later, the GCC driver adds the necessary options to link with libdld.sl when -static is specified. That makes the resulting binary dynamic. On the 64-bit port, the linkers generate dynamic binaries by default anyway. And the -nolibdld option exists so you can stop the GCC driver from adding those link options.

• -threads — adds support for multithreading with the dce thread library under HP-UX. This option sets flags for both the preprocessor and the linker. Teamwork, honestly. 🫶

---

🦋 IA-64 Options 🦋

These are the -m options defined for the Intel IA-64 architecture. Buckle up, there's a lot and they're all lowkey iconic in their own nerdy way.

• -mbig-endian — generate code for a big-endian target. This is the default for HP-UX.
• -mlittle-endian — generate code for a little-endian target. This is the default for AIX5 and GNU/Linux.
• -mgnu-as, -mno-gnu-as — generate (or don't) code for the GNU assembler. This is the default.
• -mgnu-ld, -mno-gnu-ld — generate (or don't) code for the GNU linker. This is the default.
• -mno-pic — generate code that does not use a global pointer register. The result is not position independent code, and it violates the IA-64 ABI. ⚠️ So like, she's a rebel, and the ABI is not amused.
• -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, which, slay.
• -mno-sdata, -msdata — disable (or enable) optimizations that use the small data section. This may be useful for working around optimizer bugs. 🙃
• -mconstant-gp — generate code that uses a single constant global pointer value. This is useful when compiling kernel code.
• -mauto-pic — generate code that is self-relocatable. This implies -mconstant-gp. This is useful when compiling firmware code.
• -minline-float-divide-min-latency — generate code for inline divides of floating-point values using the minimum latency algorithm.
• -minline-float-divide-max-throughput — generate code for inline divides of floating-point values using the maximum throughput algorithm.
• -mno-inline-float-divide — do not generate inline code for divides of floating-point values.
• -minline-int-divide-min-latency — generate code for inline divides of integer values using the minimum latency algorithm.
• -minline-int-divide-max-throughput — generate code for inline divides of integer values using the maximum throughput algorithm.
• -mno-inline-int-divide — do not generate inline code for divides of integer values.
• -minline-sqrt-min-latency — generate code for inline square roots using the minimum latency algorithm.
• -minline-sqrt-max-throughput — generate code for inline square roots using the maximum throughput algorithm.
• -mno-inline-sqrt — do not generate inline code for "sqrt".
• -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 it doesn't always do so. 🤷
• *-mfixed-range=register-range* — generate code treating the given register range as fixed registers. A fixed register is one that the register allocator cannot use. This is useful when compiling kernel code. A register range is specified as two registers separated by a dash. Multiple register ranges can be specified separated by a comma.
• *-mtls-size=tls-size* — specify bit size of immediate TLS offsets. Valid values are 14, 22, and 64.
• *-mtune=cpu-type* — tune the instruction scheduling for a particular CPU. Valid values are itanium, itanium1, merced, itanium2, and mckinley.
• -milp32, -mlp64 — generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long and pointer to 32 bits. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits. These are HP-UX specific flags. 🎀
• -mno-sched-br-data-spec, -msched-br-data-spec — (dis/en)able data speculative scheduling before reload. This results in generation of "ld.a" instructions and the corresponding check instructions ("ld.c" / "chk.a"). The default 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.

---

🐉 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. 👀 Like, she's not coming back to hang out with the other processors. She's committed.

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 — the local processor type that your native compiler just ✨detected✨ all by itself.
• generic — a generic LoongArch processor, no frills, just vibes.
• loongarch64 — generic LoongArch 64-bit processor, the standard-issue queen 👑
• la464 — LoongArch LA464 core.
• la664 — LoongArch LA664 core.
• loongarch32 — generic LoongArch 32-bit processor.
• -mabi=base-abi-type — generate code for the calling convention you name in base-abi-type. She has options, bestie:
  • lp64d — 64-bit general purpose registers + 32/64-bit floating-point registers do the parameter passing. Data model is LP64, where int is 32 bits, while long int and pointers are 64 bits.
  • lp64f — 64-bit general purpose registers + 32-bit floating-point registers handle parameter passing. Data model is LP64, where int is 32 bits, while long int and pointers are 64 bits.
  • lp64s — 64-bit general purpose registers and no floating-point registers at all 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 FPU type you pick. fpu-type can be one of:
  • 64 — allow the use of hardware floating-point instructions for 32-bit and 64-bit operations.
  • 32 — allow the use of hardware floating-point instructions for 32-bit operations.
  • none, 0 — prevent the use of hardware floating-point instructions. not the hardware going "no thoughts head empty" — this is a real setting 💀
• -msimd=simd-type — turn on generation of LoongArch SIMD instructions for vectorization and via builtin functions. The value can be one of:
  • lasx — enable generating instructions from the 256-bit LoongArch Advanced SIMD Extension (LASX) and the 128-bit LoongArch SIMD Extension (LSX). both, we're greedy 💅
  • lsx — enable generating instructions from the 128-bit LoongArch SIMD Extension (LSX).
  • none — no LoongArch SIMD instruction may be generated. periodt.
• -msoft-float — force -mfpu=none and block floating-point registers from parameter passing. Heads up: this option may change the target ABI. 🫣
• -msingle-float — force -mfpu=32 and allow 32-bit floating-point registers to do parameter passing. Again: this option may change the target ABI.
• -mdouble-float — force -mfpu=64 and allow 32/64-bit floating-point registers to do parameter passing. And yes, this option may change the target ABI too. it's giving ABI chaos ✨
• -mlasx, -mno-lasx, -mlsx, -mno-lsx — incrementally adjust the scope of the SIMD extensions (none / LSX / LASX) the compiler can use for code generation. Enabling LASX with -mlasx automatically enables LSX, and disabling LSX with -mno-lsx automatically disables LASX. These driver-only options act upon the final -msimd configuration state and make incremental changes in the order they appear on the GCC driver's command line, deriving the final / canonicalized -msimd option that is passed to the compiler proper. order matters, iykyk 💅
• -mbranch-cost=n — set the cost of branches to roughly n instructions.
• -maddr-reg-reg-cost=n — set the cost of ADDRESS_REG_REG to the value calculated by n.
• -mcheck-zero-division, -mno-check-zero-divison — trap (do not trap) on integer division by zero. The default is -mcheck-zero-division for -O0 or -Og, and -mno-check-zero-division for other optimization levels. (yes, the -mno- spelling in the original really is -mno-check-zero-divison — one i short. not a typo we made, it's canon 🙈)
• -mbreak-code=code — emit a "break" code instruction for irrecoverable traps from "__builtin_trap" or inserted by the compiler (for example an erroneous path isolated with -fisolate-erroneous-paths-dereference), or an "amswap.w $r0, $r1, $r0" instruction which will cause the hardware to trigger an Instruction Not-defined Exception if code is negative or greater than 32767. The default is -1, meaning to 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 (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. Setting 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 on the command line. sneaky little override, we love a loophole 💅
• -mstrict-align, -mno-strict-align — avoid or allow generating memory accesses that may not be aligned on a natural object boundary as described in the architecture specification. The default is -mno-strict-align.
• -G num — put global and static data smaller than num bytes into a small data section. The default value is 0.
• -mmax-inline-memcpy-size=n — inline all block moves (such as calls to memcpy or structure copies) less than or equal to n bytes. The default value of n is 1024.
• -mcmodel=code-model — set the code model to one of:
  • tiny-static (Not implemented yet)
  • tiny (Not implemented yet)
  • normal — the text segment must be within 128MB addressing space. The data segment must be within 2GB addressing space.
  • medium — the text segment and data segment must be within 2GB addressing space. 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.
• -mexplicit-relocs=style, -mexplicit-relocs, -mno-explicit-relocs — set 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 the code quality, and macros elsewhere.

The default value for the option is determined with the assembler capability detected during GCC build-time and the setting of -mrelax: -mexplicit-relocs=none if the assembler does not support relocation operators at all, -mexplicit-relocs=always if the assembler supports relocation operators but -mrelax is not enabled, -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.

• -mdirect-extern-access, -mno-direct-extern-access — control 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 is only suitable for environments where no dynamic link is performed, like firmwares, OS kernels, executables linked with -static or -static-pie. -mdirect-extern-access is not compatible with -fPIC or -fpic.

• -mrelax, -mno-relax — take (do not 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 the conditional branch relaxation (it's required or the .align directives and conditional branch instructions in the assembly code outputted by GCC may be rejected by the assembler because of a relocation overflow), -mno-relax otherwise.
• -mpass-mrelax-to-as, -mno-pass-mrelax-to-as — pass (do not 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 interoperation with assemblers different from the build-time one.
• -mrecip — this option enables use of the reciprocal estimate and reciprocal square root estimate instructions with additional Newton-Raphson steps to increase precision instead of doing a divide or square root and divide for floating-point arguments. These instructions are generated only when -funsafe-math-optimizations is enabled together with -ffinite-math-only and -fno-trapping-math. This option is off by default. Before you can use this option, you must sure the target CPU supports the "frecipe" and "frsqrte" instructions. Note that while the throughput of the sequence is higher than the throughput of the non-reciprocal instruction, the precision of the sequence can be decreased by up to 2 ulp (i.e. the inverse of 1.0 equals 0.99999994). math girlies, read that twice 🧮✨
• -mrecip=opt — this option controls which reciprocal estimate instructions may be used. opt is a comma-separated list of options, which may be preceded by a ! to invert the option:
  • all — enable all estimate instructions.
  • default — enable the default instructions, equivalent to -mrecip.
  • none — disable all estimate instructions, equivalent to -mno-recip.
  • div — enable the approximation for scalar division.
  • vec-div — enable the approximation for vectorized division.
  • sqrt — enable the approximation for scalar square root.
  • vec-sqrt — enable the approximation for vectorized square root.
  • rsqrt — enable the approximation for scalar reciprocal square root.
  • vec-rsqrt — 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. 💅

• -mfrecipe, -mno-frecipe — turn ON (or OFF) the frecipe.{s/d} and frsqrte.{s/d} instructions ✨. When you're compiling with -march=la664 it's on by default, otherwise the default is -mno-frecipe.
• -mdiv32, -mno-div32 — use (or don't use) div.w[u] and mod.w[u] instructions where the input isn't sign-extended 👑. 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 💅. Compiling with -march=la664? 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 🌸. 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.
• -mscq, -mno-scq — use (or don't use) the 16-byte conditional store instruction sc.q 🎀. Default is -mscq if the machine type you gave with -march= supports this instruction, and -mno-scq otherwise.
• *-mtls-dialect=opt* — this one picks which TLS dialect can be used for the general dynamic and local dynamic TLS models 💖. The opt argument can be one of:
  • trad — use traditional TLS. This is the default 👑.
  • desc — use TLS descriptors.
• -mannotate-tablejump, -mno-annotate-tablejump — makes an annotation section .discard.tablejump_annotate to correlate the jirl instruction and the jump table whenever a jump table is used to optimize a switch statement 🔥. Some external tools — like objtool from the Linux kernel build system — need that annotation to analyze the control flow. Default is -mno-annotate-tablejump.

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🌈 LynxOS Options 🌈

These options are available for LynxOS targets, bestie 🫶.

• -mshared — use shared libraries. The default is to link with static libraries.
• -mthreads — support multi-threading 💅.
• -mlegacy-threads — support legacy multi-threading. Heads up: this option is incompatible with -mthreads ⚠️.

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🍓 M32C Options 🍓

• *-mcpu=name* — select the CPU you're generating code 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 links in an alternate runtime library that supports, like, file I/O and stuff. You must not use this option when generating programs that will run on real hardware — you have to provide your own runtime library for whatever I/O functions are needed. No cap 💀.
• -memregs=number — specifies how many memory-based pseudo-registers GCC uses during code generation 💜. These pseudo-registers get used like real registers, so there's a whole tradeoff between GCC's ability to fit the code into available registers and the performance penalty of using memory instead of registers. Note that **all modules in a program must be compiled with the same value for this option 🙈. Because of that, you must not use this option with GCC's default runtime libraries.

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👑 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 — assume all objects live in the lower 16MB of memory (so their addresses can be loaded with the ld24 instruction), and assume all subroutines are reachable with the bl instruction 🎀. This is the default.

The addressability of a particular object can be set with the model attribute.

• -mmodel=medium — assume objects may be anywhere in the 32-bit address space (the compiler generates seth/add3 instructions to load their addresses), and assume all subroutines are reachable with the bl instruction.
• -mmodel=large — assume objects may be anywhere in the 32-bit address space (the compiler generates seth/add3 instructions to load their addresses), and assume subroutines may not be reachable with the bl instruction (so the compiler generates the much slower seth/add3/jl instruction sequence 😭).
• -msdata=none — disable 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. Objects may be explicitly put in the small data area with the section attribute using one of these sections ✨.

• -msdata=sdata — put small global and static data in the small data area, but don't generate special code to reference them.
• -msdata=use — put small global and static data in the small data area, and do generate special instructions to reference them 🔥.
• -G num — put global and static objects less than or equal to num bytes into the small data or BSS sections instead of the normal data or BSS sections 💖. The default value of num is 8. The -msdata option must be set to one of sdata or use for this option to have any effect.

All modules should be compiled with the same -G num value. Compiling with different values of num may or may not work; if it doesn't, the linker gives an error message — incorrect code is not generated 🙅‍♀️.

• -mdebug — makes the M32R-specific code in the compiler display some statistics that might help in debugging programs 🦋.
• -malign-loops, -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 isn't available.
• -mno-flush-func — indicates that there is no OS function for flushing the cache.

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🌟 M680x0 Options 🌟

These are the -m options defined for M680x0 and ColdFire processors 💜. The default settings depend on which architecture was selected when the compiler was configured; the defaults for the most common choices are given below.

• *-march=arch* — generate code for a specific M680x0 or ColdFire instruction set architecture ✨. Permissible values of arch for M680x0 architectures are: 68000, 68010, 68020, 68030, 68040, 68060 and cpu32. ColdFire architectures are selected according to Freescale's ISA classification and the permissible values are: isaa, isaaplus, isab and isac.

GCC defines a macro __mcfarch__ whenever it's 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's optimized for a particular microarchitecture 💅.

• *-mcpu=cpu* — generate code for a specific M680x0 or ColdFire processor 🎀. The M680x0 *cpu*s are: 68000, 68010, 68020, 68030, 68040, 68060, 68302, 68332 and cpu32. The ColdFire *cpu*s are given by the table below, which also classifies the CPUs into families:

Family : -mcpu arguments
51 : 51 51ac 51ag 51cn 51em 51je 51jf 51jg 51jm 51mm 51qe 51qm
5206 : 5202 5204 5206
5206e : 5206e
5208 : 5207 5208
5211a : 5210a 5211a
5213 : 5211 5212 5213
5216 : 5214 5216
52235 : 52230 52231 52232 52233 52234 52235
5225 : 5224 5225
52259 : 52252 52254 52255 52256 52258 52259
5235 : 5232 5233 5234 5235 523x
5249 : 5249
5250 : 5250
5271 : 5270 5271
5272 : 5272
5275 : 5274 5275
5282 : 5280 5281 5282 528x
53017 : 53011 53012 53013 53014 53015 53016 53017
5307 : 5307
5329 : 5327 5328 5329 532x
5373 : 5372 5373 537x
5407 : 5407
5475 : 5470 5471 5472 5473 5474 5475 547x 5480 5481 5482 5483 5484
5485

-mcpu=cpu overrides -march=arch if arch is compatible with cpu 🔥. Other combinations of -mcpu and -march are rejected 🙅‍♀️.

GCC defines the macro __mcf_cpu_cpu when ColdFire target cpu is selected. It also defines __mcf_family_family, where the value of family is given by the table above 💖.

• *-mtune=tune* — tune the code for a particular microarchitecture within the constraints set by -march and -mcpu 🦋. The M680x0 microarchitectures are: 68000, 68010, 68020, 68030, 68040, 68060 and cpu32. The ColdFire microarchitectures are: cfv1, cfv2, cfv3, cfv4 and cfv4e.

You can also use -mtune=68020-40 for code that needs to run relatively well on 68020, 68030 and 68040 targets ✨. -mtune=68020-60 is similar but includes 68060 targets as well. These two options select the same tuning decisions as -m68020-40 and -m68020-60 respectively.

GCC defines the macros __mcarch and __mcarch__ when tuning for 680x0 architecture arch 👑. It also defines mcarch unless either -ansi or a non-GNU -std option is used. If GCC is tuning for a range of architectures, as selected by -mtune=68020-40 or -mtune=68020-60, it defines the macros for every architecture in the range 💅.

GCC also defines the macro __muarch__ when tuning for ColdFire microarchitecture uarch, where uarch is one of the arguments given above 🌸.

• -m68000, -mc68000 — generate output for a 68000 🎀. This is the default when the compiler is configured for 68000-based systems. It'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 — okay bestie, this one generates output for a 68010. It's the default when the compiler is configured for 68010-based systems. It's equivalent to -march=68010. periodt. 💅
• -m68020, -mc68020 — Generate output for a 68020. This is the default when the compiler is configured for 68020-based systems. It is equivalent to -march=68020. slay. ✨
• -m68030 — Generate output for a 68030. This is the default when the compiler is configured for 68030-based systems. It is equivalent to -march=68030. iconic. 🦋
• -m68040 — Generate output for a 68040. This is the default when the compiler is configured for 68040-based systems. It is equivalent to -march=68040.

This option inhibits the use of 68881/68882 instructions that have to be emulated by software on the 68040. Use this option if your 68040 does not have code to emulate those instructions. so if your 68040 is missing that emulation sauce, this is your girl. 💖

• -m68060 — Generate output for a 68060. This is the default when the compiler is configured for 68060-based systems. It is equivalent to -march=68060.

This option inhibits the use of 68020 and 68881/68882 instructions that have to be emulated by software on the 68060. Use this option if your 68060 does not have code to emulate those instructions. no cap, check your emulation first. 💜

• -m68302 — Generate code for a 68302.
• -m68332 — Generate code for a 68332.
• -m68851 — The 68851 was an external MMU for the 68020 processor. GCC accepts this option, but does not use it. so it's literally just there, vibing. 💀
• -mcpu32 — Generate output for a CPU32. This is the default when the compiler is configured for CPU32-based systems. It is equivalent to -march=cpu32.

Use this option for microcontrollers with a CPU32 or CPU32+ core, including the 68330, 68331, 68332, 68333, 68334, 68336, 68340, 68341, 68349 and 68360. iykyk. 🌈

• -mfidoa — Generate code for a Fido A. short and sweet. 🎀
• -m5200 — Generate output for a 520X ColdFire CPU. This is the default when the compiler is configured for 520X-based systems. It is equivalent to -mcpu=5206, and is now deprecated in favor of that option. deprecated era, but still here.

Use this option for microcontroller with a 5200 core, including the MCF5202, MCF5203, MCF5204 and MCF5206.

• -m5206e — Generate output for a 5206e ColdFire CPU. The option is now deprecated in favor of the equivalent -mcpu=5206e. she's deprecated, bestie. 🍓
• -m528x — Generate output for a member of the ColdFire 528X family. The option is now deprecated in favor of the equivalent -mcpu=528x. another deprecated baddie. 🌸
• -m5307 — Generate output for a ColdFire 5307 CPU. The option is now deprecated in favor of the equivalent -mcpu=5307. deprecated, but make it cute. 💅
• -m5407 — Generate output for a ColdFire 5407 CPU. The option is now deprecated in favor of the equivalent -mcpu=5407. same vibe, different number. ✨
• -mcfv4e — Generate output for a ColdFire V4e family CPU (e.g. 547x/548x). This includes use of hardware floating-point instructions. The option is equivalent to -mcpu=547x, and is now deprecated in favor of that option. deprecated, but with hardware floating-point drama. 🔥
• -m68020-40 — Generate output for a 68040, without using any of the new instructions. This results in code that can run relatively efficiently on either a 68020/68881 or a 68030 or a 68040. The generated code does use the 68881 instructions that are emulated on the 68040.

The option is equivalent to -march=68020 -mtune=68020-40. so it's giving flexible queen energy. 👑

• -m68020-60 — Generate output for a 68060, without using any of the new instructions. This results in code that can run relatively efficiently on either a 68020/68881 or a 68030 or a 68040. The generated code does use the 68881 instructions that are emulated on the 68060.

The option is equivalent to -march=68020 -mtune=68020-60. same energy, 68060 edition. 🦋

• -mhard-float, -m68881 — Generate floating-point instructions. This is the default for 68020 and above, and for ColdFire devices that have an FPU. It defines the macro __HAVE_68881__ on M680x0 targets and __mcffpu__ on ColdFire targets. floating-point? she's floating, she's pointing, she's everything. 💖
• -msoft-float — Do not generate floating-point instructions; use library calls instead. This is the default for 68000, 68010, and 68832 targets. It is also the default for ColdFire devices that have no FPU. no FPU? no problem, library calls to the rescue. 🫶
• -mdiv, -mno-div — Generate (do not generate) ColdFire hardware divide and remainder instructions. If -march is used without -mcpu, the default is "on" for ColdFire architectures and "off" for M680x0 architectures. Otherwise, the default is taken from the target CPU (either the default CPU, or the one specified by -mcpu). For example, the default is "off" for -mcpu=5206 and "on" for -mcpu=5206e.

GCC defines the macro __mcfhwdiv__ when this option is enabled. macro girlies, this one's for you. 💜

• -mshort, -mno-short, -mnoshort — Consider type int to be 16 bits wide, like short int. Additionally, parameters passed on the stack are also aligned to a 16-bit boundary even on targets whose API mandates promotion to 32-bit. This option is disabled by default. tiny int energy, but only if you opt in. 🌸
• -mbitfield, -mno-bitfield, -mnobitfield — Control use of the bit-field instructions. The -m68000, -mcpu32 and -m5200 options imply -mnobitfield; the -m68020 option implies -mbitfield. bit-field drama, but make it configurable. 🌈
• -mrtd, -mno-rtd, -mnortd — Control use of a different function-calling convention, in which functions that take a fixed number of arguments return with the rtd instruction, which pops their arguments while returning. This saves one instruction in the caller since there is no need to pop the arguments there.

This calling convention is incompatible with the one normally used on Unix, so you cannot use it if you need to call libraries compiled with the Unix compiler.

Also, you must provide function prototypes for all functions that take variable numbers of arguments (including printf); otherwise incorrect code is generated for calls to those functions.

In addition, seriously incorrect code results if you call a function with too many arguments. (Normally, extra arguments are harmlessly ignored.)

The rtd instruction is supported by the 68010, 68020, 68030, 68040, 68060 and CPU32 processors, but not by the 68000 or 5200.

The default is -mno-rtd. bestie, do not play with this one unless you know exactly what you're doing. 💀

• -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. this is a real warning, not a drill. ⚠️

pc-relative note: 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. iykyk. ✨

• -mno-strict-align, -mstrict-align — Do not (do) assume that unaligned memory references are handled by the system. vibe check: do you trust your memory alignment? 💅
• -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 that the data segment follows the text segment. so by default, data is a follower, not a leader. 🦋

• -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. shared library era, but only if you flip the switch. 🌸

• -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. zero is the compact queen, periodt. 👑
• -mxgot, -mno-xgot — When generating position-independent code for ColdFire, generate code that works if the GOT has more than 8192 entries. This code is larger and slower than code generated without this option. On M680x0 processors, this option is not needed; -fPIC suffices.

GCC normally uses a single instruction to load values from the GOT. While this is relatively efficient, it only works if the GOT is smaller than about 64k. Anything larger causes the linker to report an error such as:

relocation truncated to fit: R_68K_GOT16O foobar

If this happens, you should recompile your code with -mxgot. It should then work with very large GOTs. However, code generated with -mxgot is less efficient, since it takes 4 instructions to fetch the value of a global symbol.

Note that some linkers, including newer versions of the GNU linker, can create multiple GOTs and sort GOT entries. If you have such a linker, you should only need to use -mxgot when compiling a single object file that accesses more than 8192 GOT entries. Very few do.

These options have no effect unless GCC is generating position-independent code. so if you're not doing PIC, this is not your moment. 💀

• -mxtls — Support 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. TLS but make it spacious. 🌈
• -mlong-jump-table-offsets — Use 32-bit offsets in switch tables. The default is to use 16-bit offsets. switch tables, but with bigger offsets. ✨

---

✨ MCore Options ✨

These are the -m options defined for the Motorola M*Core processors. MCore girlies, this section is for you. 💖

• -mhardlit, -mno-hardlit — Inline constants into the code stream if it can be done in two instructions or less. short and sweet inlining. 🍓
• -mdiv, -mno-div — Use the divide instruction. (Enabled by default). division, but make it hardware. 🔥
• -mrelax-immediates, -mno-relax-immediates — Allow arbitrary-sized immediates in bit operations. big immediates welcome. 🦋
• -mwide-bitfields, -mno-wide-bitfields — Always treat bit-fields as int-sized. wide bit-fields, wide energy. 💜
• -m4byte-functions, -mno-4byte-functions — Force all functions to be aligned to a 4-byte boundary. alignment check, bestie. 💅
• -mcallgraph-data, -mno-callgraph-data — Emit callgraph information. callgraph data, slay. 👑
• -mslow-bytes, -mno-slow-bytes — bestie, this one's just saying: when you're reading byte quantities, prefer word access. That's the whole vibe. ✨
• -mlittle-endian, -mbig-endian — generate code for a little-endian 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 (this is the 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 is enabled by default.
• -mxl-soft-div, -mno-xl-soft-div — use software emulation for divides. This is enabled by default.
• -mxl-barrel-shift — use the hardware barrel shifter.
• -mxl-pattern-compare — use pattern compare instructions.
• -msmall-divides — use table lookup optimization for small signed integer divisions.
• -mxl-gp-opt — use GP-relative .sdata/.sbss sections.
• -mxl-multiply-high — use multiply high instructions for high part of 32x32 multiply.
• -mxl-float-convert — use hardware floating-point conversion instructions.
• -mxl-float-sqrt — use hardware floating-point square root instruction.
• -mbig-endian — generate code for a big-endian target.
• -mlittle-endian — generate code for a little-endian target.
• -mxl-reorder — use reorder instructions (swap and byte reversed load/store).
• -mxl-mode-app-model — select application model app-model. Valid models are:
  • executable — normal executable (default), uses startup code crt0.o.
  • xmdstub — for use with Xilinx Microprocessor Debugger (XMD) based software intrusive debug agent called xmdstub. This uses startup file crt1.o and sets the start address of the program to 0x800.
  • bootstrap — for applications that are loaded using a bootloader. This model uses startup file crt2.o which does not contain a processor reset vector handler. This is suitable for transferring control on a processor reset to the bootloader rather than the application.
  • novectors — for applications that do not require any of the MicroBlaze vectors. This option may be useful for applications running within a monitoring application. This model uses crt3.o as a startup file.
• -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 allows data to be referenced by offset from start of text address instead of GOT since PC-relative addressing is not 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 does not recognize the processor.
  • In processor names, a final 000 can be abbreviated as k (for example, -march=r2k). Prefixes are optional, and vr may be written r.
  • Names of the form nf2_1 refer to processors with FPUs clocked at half the rate of the core, names of the form nf1_1 refer to processors with FPUs clocked at the same rate as the core, and names of the form nf3_2 refer to processors with FPUs clocked a ratio of 3:2 with respect to the core. For compatibility reasons, nf is accepted as a synonym for nf2_1 while nx and bfx are accepted as synonyms for nf1_1.
  • GCC defines two macros based on the value of this option. The first is _MIPS_ARCH, which gives the name of target architecture, as a string. The second has the form _MIPS_ARCH_foo, where foo is the capitalized value of _MIPS_ARCH. For example, -march=r2000 sets _MIPS_ARCH to r2000 and defines the macro _MIPS_ARCH_R2000.
  • Note that the _MIPS_ARCH macro uses the processor names given above. In other words, it has the full prefix and does not abbreviate 000 as k. In the case of from-abi, the macro names the resolved architecture (either mips1 or mips3). It names the default architecture when no -march option is given.
• -mtune=arch — optimize for arch. Among other things, this option controls the way instructions are scheduled, and the perceived cost of arithmetic operations. The list of arch values is the same as for -march.
  • When this option is not used, GCC optimizes for the processor specified by -march. By using -march and -mtune together, it is possible to generate code that runs on a family of processors, but optimize the code for one particular member of that family.
  • -mtune defines the macros _MIPS_TUNE and _MIPS_TUNE_foo, which work in the same way as the -march ones described above.
• -mips1 — equivalent to -march=mips1.
• -mips2 — equivalent to -march=mips2.
• -mips3 — equivalent to -march=mips3.
• -mips4 — equivalent to -march=mips4.
• -mips32 — equivalent to -march=mips32.
• -mips32r3 — equivalent to -march=mips32r3.
• -mips32r5 — equivalent to -march=mips32r5.
• -mips32r6 — equivalent to -march=mips32r6.
• -mips64 — equivalent to -march=mips64.
• -mips64r2 — equivalent to -march=mips64r2.
• -mips64r3 — equivalent to -march=mips64r3.
• -mips64r5 — equivalent to -march=mips64r5.
• -mips64r6 — equivalent to -march=mips64r6.
• -mips16, -mno-mips16 — generate (do not generate) MIPS16 code. If GCC is targeting a MIPS32 or MIPS64 architecture, it makes use of the MIPS16e ASE.
  • MIPS16 code generation can also be controlled on a per-function basis by means of mips16 and nomips16 attributes.
• -mmips16e2, -mno-mips16e2 — use (do not 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. 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. (and yes bestie, it's literally listed twice in the original — keeping it exactly as written 💀)
• -minterlink-compressed, -mno-interlink-compressed — require (do not require) that code using the standard (uncompressed) MIPS ISA be link-compatible with MIPS16 and microMIPS code, and vice versa.
  • For example, code using the standard ISA encoding cannot jump directly to MIPS16 or microMIPS code; it must either use a call or an indirect jump. -minterlink-compressed therefore disables direct jumps unless GCC knows that the target of the jump is not compressed.
• -minterlink-mips16, -mno-interlink-mips16 — aliases of -minterlink-compressed and -mno-interlink-compressed. These options predate the microMIPS ASE and are retained for backwards compatibility.
• -mabi=32, -mabi=o64, -mabi=n32, -mabi=64, -mabi=eabi — generate code for the given ABI.
  • Note that the EABI has a 32-bit and a 64-bit variant. GCC normally generates 64-bit code when you select a 64-bit architecture, but you can use -mgp32 to get 32-bit code instead.
  • For information about the O64 ABI, see 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.

okay so there are two extra flavors of the o32 ABI, and they exist purely so we can ✨transition✨ from 32-bit registers to 64-bit ones without everything falling apart. meet them:

• FPXX (-mfpxx) — the vibe here is: all your code has to run correctly whether it's executing on 32-bit or 64-bit registers. it can be interlinked with either FP32 or FP64… but not both. pick a lane bestie 😭
• FP64A (-mfp64 -mno-odd-spreg) — basically FP64's cousin, except it forbids odd-numbered single-precision registers. why tho? so it can hang out with the "FRE" mode of FPUs in MIPS32R5 processors, which lets both FP32 and FP64A code interlink and run in the same process without anyone having to switch FPU modes. iconic behaviour honestly 💅

---

• -mabicalls, -mno-abicalls — generate (or don't generate) code that plays nice with SVR4-style dynamic objects. -mabicalls is the default on SVR4-based systems. periodt.
• -mshared, -mno-shared — generate (or don't generate) code that's fully position-independent, meaning it can get linked into shared libraries. heads up: this only does anything when -mabicalls is in play.

so here's the lore 📖 — all -mabicalls code has traditionally been position-independent, no matter what -fPIC and -fpic were doing. but as an extension, the GNU toolchain lets executables use absolute accesses for locally-binding symbols. it can also use shorter GP initialization sequences and generate direct calls to locally-defined functions. that mode gets selected by -mno-shared.

-mno-shared needs binutils 2.16 or higher, and it generates objects that only the GNU linker can link. it doesn't touch the ABI of the final executable though — only the ABI of relocatable objects. and using -mno-shared generally makes executables both smaller and quicker. we love a win-win 🫶

-mshared is the default.

• -mplt, -mno-plt — assume (or don't assume) that the static and dynamic linkers support PLTs and copy relocations. this one only affects -mno-shared -mabicalls. for the n64 ABI, it does nothing at all without -msym32.

you can make -mplt the default by configuring GCC with --with-mips-plt. otherwise the default is -mno-plt.

• -mxgot, -mno-xgot — lift (or don't lift) the usual restrictions on the size of the global offset table.

GCC normally uses a single instruction to load values from the GOT. efficient, slay, love that for us — but it only works if the GOT is smaller than about 64k. anything bigger and the linker throws hands with an error like:

relocation truncated to fit: R_MIPS_GOT16 foobar

if that happens to you, recompile with -mxgot. it works with very large GOTs, but the code is also less efficient, since fetching the value of a global symbol then takes three instructions. tradeoffs, bestie 😔

also worth knowing: 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. fr fr.

• -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. mystery era 🦋
• -mhard-float — use floating-point coprocessor instructions.
• -msoft-float — do not use floating-point coprocessor instructions. implement floating-point calculations using library calls instead.
• -mno-float — same as -msoft-float, but it additionally asserts that the program being compiled performs no floating-point operations whatsoever. right now this is only supported by some bare-metal MIPS configurations, where it may select a special set of libraries that lack all floating-point support (including, for example, the floating-point "printf" formats).

⚠️ if code compiled with -mno-float accidentally contains floating-point operations, it is likely to suffer a link-time or run-time failure. no cap, be careful.

• -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 we treat the special not-a-number (NaN) IEEE 754 floating-point data with the "abs.fmt" and "neg.fmt" machine instructions.

by default, or when -mabs=legacy is used, the legacy treatment is selected. in that case these instructions are considered arithmetic and avoided wherever correct operation is required and the input operand might be a NaN. a longer sequence of instructions that manipulate the sign bit of the floating-point datum manually gets used instead — unless the -ffinite-math-only option has also been specified.

-mabs=2008 selects the IEEE 754-2008 treatment. now these instructions are considered non-arithmetic and therefore operating correctly in all cases, including in particular where the input operand is a NaN. so they're always used for the respective operations. glow up ✨

• -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's flipped: qNaNs have the first bit of their trailing significand field as 1, whereas sNaNs have it as 0.

the default is -mnan=legacy unless GCC has been configured with --with-nan=2008.

• -mllsc, -mno-llsc — use (or don't use) ll, sc, and sync instructions to implement atomic memory built-in functions. when neither option is specified, GCC uses the instructions if the target architecture supports them.

-mllsc is useful if the runtime environment can emulate the instructions, and -mno-llsc can be useful when compiling for nonstandard ISAs. you can make either option the default by configuring GCC with --with-llsc and --without-llsc respectively. --with-llsc is the default for some configurations; see the installation documentation for details.

• -mdsp, -mno-dsp — use (or don't use) revision 1 of the MIPS DSP ASE. this option defines the preprocessor macro "__mips_dsp". it also defines "__mips_dsp_rev" to 1.
• -mdspr2, -mno-dspr2 — use (or don't use) revision 2 of the MIPS DSP ASE. this option defines the preprocessor macros "__mips_dsp" and "__mips_dspr2". it also defines "__mips_dsp_rev" to 2.
• -msmartmips, -mno-smartmips — use (or don't use) the MIPS SmartMIPS ASE.
• -mpaired-single, -mno-paired-single — use (or don't use) paired-single floating-point instructions. this option requires hardware floating-point support to be enabled.
• -mdmx, -mno-mdmx — use (or don't use) MIPS Digital Media Extension instructions. this option can only be used when generating 64-bit code, and 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 using the "micromips" and "nomicromips" attributes. so granular 💅

• -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 gets 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, and so 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, regardless of the selected ABI. this one's a cute combo move 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; see -mgpopt for details.

the default -G option depends on the configuration.

• -mlocal-sdata, -mno-local-sdata — bestie, this one extends (or does not 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 that the libraries leave more room for the main program. 💅
• -mextern-sdata, -mno-extern-sdata — assume (or do not assume) that externally-defined data is in a small data section if the size of that data is within the -G limit. -mextern-sdata is the default for all configurations. If you compile a module Mod with -mextern-sdata -G num -mgpopt, and Mod references a variable Var that is no bigger than num bytes, you must make sure that Var is placed in a small data section. If Var is defined by another module, you must either compile that module with a high-enough -G setting or attach a "section" attribute to Var's definition. If Var is common, you must link the application with a high-enough -G setting. The easiest way of satisfying these restrictions is to compile and link every module with the same -G option. However, you may wish to build a library that supports several different small data limits. You can do this by compiling the library with the highest supported -G setting and additionally using -mno-extern-sdata to stop the library from making assumptions about externally-defined data. 🫶
• -mgpopt, -mno-gpopt — use (or do not use) GP-relative accesses for symbols that are known to be in a small data section; see -G, -mlocal-sdata and -mextern-sdata. -mgpopt is the default for all configurations. -mno-gpopt is useful for cases where the $gp register might not hold the value of "_gp". For example, if the code is part of a library that might be used in a boot monitor, programs that call boot monitor routines pass an unknown value in $gp. (In such situations, the boot monitor itself is usually compiled with -G0.) -mno-gpopt implies -mno-local-sdata and -mno-extern-sdata. 😭
• -membedded-data, -mno-embedded-data — allocate variables to the read-only data section first if possible, then next in the small data section if possible, otherwise in data. This gives slightly slower code than the default, but reduces the amount of RAM required when executing, and thus may be preferred for some embedded systems. 🌸
• -muninit-const-in-rodata, -mno-uninit-const-in-rodata — put uninitialized "const" variables in the read-only data section. This option is only meaningful in conjunction with -membedded-data. ✨
• -mcode-readable=setting — specify whether GCC may generate code that reads from executable sections. There are three possible settings:
  • -mcode-readable=yes — instructions may freely access executable sections. This is the default setting.
  • -mcode-readable=pcrel — MIPS16 PC-relative load instructions can access executable sections, but other instructions must not do so. This option is useful on 4KSc and 4KSd processors when the code TLBs have the Read Inhibit bit set. It is also useful on processors that can be configured to have a dual instruction/data SRAM interface and that, like the M4K, automatically redirect PC-relative loads to the instruction RAM.
  • -mcode-readable=no — instructions must not access executable sections. This option can be useful on targets that are configured to have a dual instruction/data SRAM interface but that (unlike the M4K) do not automatically redirect PC-relative loads to the instruction RAM. 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 (or disable) use of the %hi() and %lo() assembler relocation operators. This option has been superseded by -mexplicit-relocs but is retained for backwards compatibility. 💀
• -mexplicit-relocs=none, -mexplicit-relocs=base, -mexplicit-relocs=pcrel, -mexplicit-relocs, -mno-explicit-relocs — these options control whether explicit relocs (such as %gp_rel) are used. The default value depends on the version of GAS when GCC itself was built. The "base" explicit-relocs support was introduced into GAS in 2001. The "pcrel" explicit-relocs support was introduced into GAS in 2014, which supports %pcrel_hi and %pcrel_lo. 🦋
• -mcheck-zero-division, -mno-check-zero-division — trap (or do not trap) on integer division by zero. The default is -mcheck-zero-division. 🎀
• -mdivide-traps, -mdivide-breaks — MIPS systems check for division by zero by generating either a conditional trap or a break instruction. Using traps results in smaller code, but is only supported on MIPS II and later. Also, some versions of the Linux kernel have a bug that prevents trap from generating the proper signal ("SIGFPE"). Use -mdivide-traps to allow conditional traps on architectures that support them and -mdivide-breaks to force the use of breaks. The default is usually -mdivide-traps, but this can be overridden at configure time using --with-divide=breaks. Divide-by-zero checks can be completely disabled using -mno-check-zero-division. 😭
• -mload-store-pairs, -mno-load-store-pairs — enable (or 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 (or 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 (or 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 (or 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 (or disable) use of the "mad", "madu" and "mul" instructions, as provided by the R4650 ISA. ⭐
• -mimadd, -mno-imadd — enable (or 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 (or 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 — works around the VR4130 mflo/mfhi errata 💀. Plot twist: the workarounds are actually implemented by the assembler, not by GCC — though GCC does dodge using mflo and mfhi when the VR4130 macc, macchi, dmacc and dmacchi instructions are available to use instead. iykyk.
• -mfix-sb1, -mno-fix-sb1 — works around certain SB-1 CPU core errata. (Right now this flag handles the SB-1 revision 2 F1 and F2 floating-point errata, bestie.)
• -mfix4300, -mno-fix4300 — works around a bug in early VR4300 silicon where multiplies with certain operands go feral and corrupt the multiplies that come immediately after them 😭.
• -mr10k-cache-barrier=setting — tells GCC whether it should insert cache barriers to avoid the side effects of speculation on R10K processors. Buckle up, this one's a whole saga 🫶.

Like a lot of processors, the R10K tries to predict how a conditional branch will go and speculatively executes instructions from the "taken" branch. Later it aborts those instructions if the prediction turned out wrong. BUT — and this is the iconic part — on the R10K, even aborted instructions can have side effects 💅.

This problem only hits kernel stores, and depending on the system, kernel loads. For example: a speculatively-executed store might load the target memory into cache and mark the cache line as dirty, even if the store itself gets aborted later. And if a DMA operation writes to the same area of memory before that "dirty" line is flushed, the cached data overwrites the DMA-ed data. See the R10K processor manual for the full lore, 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 that workaround. It assumes that aborted accesses to any byte in the following regions don't have side effects:

1. the memory occupied by the current function's stack frame;
2. the memory occupied by an incoming stack argument;
3. the memory occupied by an object with a link-time-constant address.

It's the kernel's job to make sure speculative accesses to those regions are actually safe. No cap.

If the input program contains a function declaration like:

void foo (void);

then the implementation of foo has to allow j foo and jal foo to be executed speculatively. GCC honors this restriction for functions it compiles itself, and it expects non-GCC functions (like hand-written assembly code) to do the same.

The option has three forms:

• -mr10k-cache-barrier=load-store — insert a cache barrier before a load or store that might be speculatively executed and that might have side effects even if aborted.
• -mr10k-cache-barrier=store — insert a cache barrier before a store that might be speculatively executed and that might have side effects even if aborted.
• -mr10k-cache-barrier=none — disable the insertion of cache barriers. This is the default setting. 🎀

• -mflush-func=func, -mno-flush-func — specifies the function to call to flush the I and D caches, or says "nah, don't call any such function". If it is called, the function must take the same arguments as the common _flush_func, that is: the address of the memory range for which the cache is being flushed, the size of the memory range, and the number 3 (to flush both caches). The default depends on the target GCC was configured for, but it's commonly either _flush_func or __cpu_flush.
• -mbranch-cost=num — sets the cost of branches to roughly num "simple" instructions. This cost is only a heuristic and it's not guaranteed to give you consistent results across releases. A zero cost redundantly selects the default, which is based on the -mtune setting.
• -mbranch-likely, -mno-branch-likely — enables or disables use of Branch Likely instructions, regardless of whatever the default is for the selected architecture. By default, Branch Likely instructions may be generated if the selected architecture supports them. The exception is MIPS32 and MIPS64 architectures and processors that implement those architectures — for those, Branch Likely instructions are not generated by default, because MIPS32 and MIPS64 specifically deprecate their use. ⚠️
• -mcompact-branches=never, -mcompact-branches=optimal, -mcompact-branches=always — these control which form of branches get generated. The default is -mcompact-branches=optimal.

The -mcompact-branches=never option makes sure compact branch instructions are never generated.

The -mcompact-branches=always option makes sure 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.

If it's used for MIPS16/microMIPS targets, it's just ignored right now 🙈. The behavior for MIPS16/microMIPS might change in the future, since they do have some compact branch instructions.

The -mcompact-branches=optimal option causes a delay slot branch to be used if one is available in the current ISA and the delay slot is successfully filled. If the delay slot isn't filled, a compact branch is chosen if one is available. ✨

• -mfp-exceptions, -mno-fp-exceptions — specifies whether FP exceptions are enabled. This affects how FP instructions are scheduled for some processors. Default: FP exceptions are enabled.

For instance, on the SB-1, if FP exceptions are disabled and we're emitting 64-bit code, then we can use both FP pipes. Otherwise we can only use one FP pipe. 🌸

• -mvr4130-align, -mno-vr4130-align — the VR4130 pipeline is two-way superscalar, but 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.

This option only does anything when optimizing for the VR4130. It normally makes code faster, but at the cost of making it bigger. It's enabled by default at optimization level -O3.

• -msynci, -mno-synci — enable (disable) generation of synci instructions on architectures that support them. The synci instructions (if enabled) are generated when __builtin___clear_cache is compiled.

This option defaults to -mno-synci, but the default can be overridden by configuring GCC with --with-synci.

When compiling code for single processor systems, it's generally safe to use synci. However, on many multi-core (SMP) systems it doesn't invalidate the instruction caches on all cores and may lead to undefined behavior. Danger, bestie. 🚨

• -mrelax-pic-calls, -mno-relax-pic-calls — tries to turn PIC calls that are normally dispatched via register $25 into direct calls. This is only possible if the linker can resolve the destination at link time and if the destination is within range for a direct call.

-mrelax-pic-calls is the default if GCC was configured to use an assembler and a linker that support the .reloc assembly directive and -mexplicit-relocs is in effect. With -mno-explicit-relocs, this optimization can be performed by the assembler and the linker alone, without any help from the compiler.

• -mmcount-ra-address, -mno-mcount-ra-address — emit (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. _mcount can then 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.

The default is -mno-mcount-ra-address.

• -mframe-header-opt, -mno-frame-header-opt — enable (disable) frame header optimization in the o32 ABI. When using the o32 ABI, calling functions allocate 16 bytes on the stack for the called function to write out register arguments. When enabled, this optimization suppresses the allocation of the frame header if it can be determined that it's unused.

This optimization is off by default at all optimization levels. 💅

• -mlxc1-sxc1, -mno-lxc1-sxc1 — when applicable, enable (disable) the generation of lwxc1, swxc1, ldxc1, sdxc1 instructions. Enabled by default. 👑
• -mmadd4, -mno-madd4 — when applicable, enable (disable) the generation of 4-operand madd.s, madd.d and related instructions. Enabled by default.

---

🐄 MMIX Options 🐄

These options are defined for the MMIX:

• -mlibfuncs, -mno-libfuncs — specifies that intrinsic library functions are being compiled, passing all values in registers, no matter the size.
• -mepsilon, -mno-epsilon — generate floating-point comparison instructions that compare with respect to the rE epsilon register.
• -mabi=mmixware, -mabi=gnu — generate code that passes function parameters and return values that (in the called function) are seen as registers $0 and up, as opposed to the GNU ABI which uses global registers $231 and up.
• -mzero-extend, -mno-zero-extend — when reading data from memory in sizes shorter than 64 bits, use (don't use) zero-extending load instructions by default, rather than sign-extending ones.
• -mknuthdiv, -mno-knuthdiv — makes 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 being almost exclusively used. ✨
• -mtoplevel-symbols, -mno-toplevel-symbols — prepend (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, rather than the default mmo format used by the mmix simulator.
• -mbranch-predict, -mno-branch-predict — use (don't use) the probable-branch instructions, when static branch prediction indicates a probable branch. 🦋
• -mbase-addresses, -mno-base-addresses — turn on (or off, bestie) codegen that uses base addresses. Turning on a base address auto-slides a request into the assembler and the linker asking for a constant to be set up in a global register 💅. That register then serves one or more base-address requests inside the range 0 to 255 from the value it's holding. It's giving short and fast code, fr, but the number of different data items you can address is limited. Which means a program with lots of static data may be like "actually I need -mno-base-addresses".
• -msingle-exit, -mno-single-exit — force (or don't force) the generated code to have a single exit point in each function. No cap.
• *-mset-data-start=address* — when linking, set the start of the data section to address.
• *-mset-program-start=address* — when linking, set the program start address to address. 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, bestie:

• -mmult-bug, -mno-mult-bug — when enabled, generate code that dodges the bugs in the multiply instructions for MN10300 processors. This is the default, so it's already slaying on your behalf.
• -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. Heads up: 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 — when generating a function that returns a pointer, return the pointer in both a0 and d0. Otherwise, the pointer comes back only in a0, and attempts to call such functions without a prototype result in errors 😭. Note that this option is on by default; use -mno-return-pointer-on-d0 to disable it.
• -mno-crt0 — do not link in the C run-time initialization object file.
• -mrelax — tell the linker it should perform a relaxation optimization pass to shorten branches, calls and absolute memory addresses. This option only does anything when used on the command line for the final link step.

This option makes symbolic debugging impossible. Not even joking, bestie.

• -mliw, -mno-liw — let the compiler generate Long Instruction Word instructions if the target is the AM33 or later. Enabled by default. -mliw defines the preprocessor macro __LIW__; -mno-liw defines the preprocessor macro __NO_LIW__.
• -msetlb, -mno-setlb — let the compiler generate the SETLB and Lcc instructions if the target is the AM33 or later. Enabled by default. -msetlb defines the preprocessor macro __SETLB__; -mno-setlb defines the preprocessor macro __NO_SETLB__.

---

🦋 Moxie Options 🦋

• -meb — generate big-endian code. This is the default for moxie-*-* configurations.
• -mel — generate little-endian code.
• -mmul.x, -mno-mul.x — generate mul.x and umul.x instructions. Enabled by default for moxiebox-*-* configurations.
• -mno-crt0 — do not link in the C run-time initialization object file.

---

🌸 MSP430 Options 🌸

These options are defined for the MSP430:

• -masm-hex — force assembly output to always use hex constants. Normally such constants are signed decimals, but this option exists for testsuite and/or aesthetic purposes ✨ (yes, aesthetics are a valid reason, we love that for it).
• *-mmcu=name* — select the MCU to target. This is used to create a C preprocessor symbol based upon the MCU name, converted to upper case and pre- and post-fixed with __. That symbol in turn is used by the msp430.h header file to select an MCU-specific supplementary header file.

The option also sets the ISA to use. If the MCU name is one that is known to only support the 430 ISA then that is selected, otherwise the 430X ISA is selected. A generic MCU name of msp430 can also be used to select the 430 ISA. Similarly the generic msp430x MCU name selects the 430X ISA.

In addition an MCU-specific linker script is added to the linker command line. The script's name is the name of the MCU with .ld appended. Thus specifying -mmcu=xxx on the gcc command line defines the C preprocessor symbol __XXX__ and cause the linker to search for a script called xxx.ld.

The ISA and hardware multiply supported for the different MCUs is hard-coded into GCC. However, an external devices.csv file can be used to extend device support beyond those that have been hard-coded. Iconic, honestly.

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:

1. 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.
2. 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.
3. 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 have 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.

• -mwarn-mcu, -mno-warn-mcu — enables or disables warnings about conflicts between the MCU name specified by the -mmcu option and the ISA set by the -mcpu option and/or the hardware multiply support set by the -mhwmult option. It also toggles warnings about unrecognized MCU names. This option is on by default.
• -msim — link to the simulator runtime libraries and linker script. Overrides any scripts that would be selected by the -mmcu= option. Sorry to those scripts, but you've been replaced 💅.
• -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 option is passed to the assembler and linker, and allows the linker to perform certain optimizations that cannot be done until the final link.
• *mhwmult=type* — describes the type of hardware multiply supported by the target. Accepted values for type are none for no hardware multiply, 16bit for the original 16-bit-only multiply supported by early MCUs, 32bit for the 16/32-bit multiply supported by later MCUs and f5series for the 16/32-bit multiply supported by F5-series MCUs. A value of auto can also be given. This tells GCC to deduce the hardware multiply support based upon the MCU name provided by the -mmcu option. If no -mmcu option is specified or if the MCU name is not recognized, then no hardware multiply support is assumed. auto is the default setting.

Hardware multiplies are normally performed by calling a library routine. This saves space in the generated code. When compiling at -O3 or higher however the hardware multiplier is invoked inline. This makes for bigger, but faster code.

The hardware multiply routines disable interrupts whilst running and restore the previous interrupt state when they finish. This makes them safe to use inside interrupt handlers as well as in normal code. Bestie behaviour, genuinely.

• -minrt — enable the use of a minimum runtime environment without support for static initializers or constructors. This is intended for memory-constrained devices. The compiler includes special symbols in some objects that tell the linker and runtime which code fragments are required.
• -mtiny-printf — enable reduced code size printf and puts library functions. The tiny implementations of these functions are not reentrant, so must be used with caution in multi-threaded applications 🙈.

Support for streams has been removed and the string to be printed are always sent to stdout via the write syscall. The string is not buffered before it is sent to write.

This option requires Newlib Nano IO, so GCC must be configured with --enable-newlib-nano-formatted-io.

• *-mmax-inline-shift=n* — this option takes an integer n between 0 and 64 inclusive, and sets the maximum number of inline shift instructions which should be emitted to perform a shift operation by a constant amount. When this value needs to be exceeded, an mspabi helper function is used instead. The default value is 4.

This only affects cases where a shift by multiple positions cannot be completed with a single instruction (e.g. all shifts >1 on the 430 ISA).

Shifts of a 32-bit value are at least twice as costly, so the value passed for this option is divided by 2 and the resulting value used instead.

• *-mcode-region=where* — *-mdata-region=where* — these options tell the compiler where to place functions and data that do not 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.
• -muse-lower-region-prefix — add the .lower prefix to section names when compiling with -mcode-region=lower or -mdata-region=lower. Disabled by default.
• -msilicon-errata=name[,name...] — hands the assembler a lil wishlist asking it to turn on the fixes for the silicon errata you named. want the deets? that's the assembler's documentation's job, bestie, go read it there 💅
• -msilicon-errata-warn=name[,name...] — same energy, but this one asks the assembler to actually warn you when a named silicon errata might need to be applied. again: assembler docs for the full story, i'm just the messenger 🫶
• -mwarn-devices-csv
• -mno-warn-devices-csv — yell at you if devices.csv isn't found or if parsing it goes sideways (default: on). so yeah, by default she will tell you 🍓

---

🦋 NDS32 Options 🦋

okay these ones are for NDS32 implementations, buckle up:

• -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.
• -malign-functions — align function entry to 4 bytes.
• -mfp-as-gp
• -mno-fp-as-gp — turn the fp-as-gp optimization on/off.
• -mcmov
• -mno-cmov — turn generation of conditional move instructions on/off.
• -mhw-abs
• -mno-hw-abs — turn generation of hardware abs instructions on/off.
• -mext-perf
• -mno-ext-perf — turn generation of performance extension instructions on/off.
• -mext-perf2
• -mno-ext-perf2 — turn generation of performance extension 2 instructions on/off.
• -mext-string
• -mno-ext-string — turn generation of string extension instructions on/off.
• -mext-dsp
• -mno-ext-dsp — turn generation of DSP extension instructions on/off.
• -mext-fpu-fma
• -mno-ext-fpu-fma — turn generation of floating-point multiply-accumulation instructions on/off.
• -mext-fpu-sp
• -mno-ext-fpu-sp — turn generation of single-precision floating-point instructions on/off.
• -mext-fpu-dp
• -mno-ext-fpu-dp — turn generation of double-precision floating-point instructions on/off.
• -mv3push
• -mno-v3push — turn generation of v3 push25/pop25 instructions on/off.
• -m16-bit
• -mno-16-bit — turn generation of 16-bit instructions on/off.
• -mvh
• -mno-vh — turn Virtual Hosting support on/off.
• -misr-vector-size=num — pick the size of each interrupt vector. it must be 4 or 16, no in-between, no vibes-based guessing 🙈
• -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. has to be a power of 2 between 4 and 512, periodt.
• -march=arch — pick the target architecture. arch can be one of v2, v3, v3j, v3m, v3f, or v3s.
• -mcpu=cpu — pick the CPU you're compiling for. cpu can be one of 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. yeah that's the whole roster, i counted 💀
• -mconfig-fpu=num — set an FPU configuration value from 0 to 7; 0--3 is as FPU spec says, and 4--7 correspond to 0--3.
• -mconfig-mul=type — set the configuration for the multiply instruction. type can be one of 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:
  • 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 the constructor/destructor feature on/off.
• -mrelax
• -mno-relax — turn the linker option to relax instructions on/off.
• -mrelax-hint
• -mno-relax-hint — turn insertion of hints for the linker to do relaxation on/off.
• -msched-prolog-epilog
• -mno-sched-prolog-epilog — turn scheduling of a function's prologue and epilogue sequence on/off.
• -mret-in-naked-func
• -mno-ret-in-naked-func — turn generation of return instructions in naked functions on/off. this one is enabled by default, bestie, she comes pre-slaying ✨
• -malways-save-lp
• -mno-always-save-lp — turn always saving $lp on the stack on/off.
• -munaligned-access
• -mno-unaligned-access — turn unaligned word and halfword accesses to packed data on/off.
• -minline-asm-r15
• -mno-inline-asm-r15 — allow/disallow use of r15 for inline asm.

---

🌈 Nvidia PTX Options 🌈

these ones are defined for Nvidia PTX:

• -m64 — ignored, but preserved for backward compatibility. only 64-bit ABI is supported, so like... it's just here for the nostalgia 🎀
• -march=architecture-string — generate code for the specified PTX ISA target architecture. valid architecture strings are sm_30, sm_35, sm_37, 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=. same thing, different name, iconic twin behaviour 💅
• -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, 4.1, 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 partitioned execution optimizations on/off. this option is enabled by default when any level of optimization is selected 🔥
• -msoft-stack
• -mno-soft-stack — 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 — specify 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 🧚

and these ones 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 — turns on generation of conditional move (l.cmov) instructions. Default vibe? The equivalents get built from set and branch instead. no cmov for you unless you ask 💅
• -mror — turns on generation of rotate right (l.ror) instructions. By default, functions from libgcc do the rotate right heavy lifting.
• -mrori — turns on generation of rotate right with immediate (l.rori) instructions. Default behaviour is that functions from libgcc handle rotate right with immediate ops.
• -msext — turns on generation of sign extension (l.ext*) instructions. Default is memory loads doing the sign extension, bestie.
• -msfimm — turns on generation of compare and set flag with immediate (l.sf*i) instructions. By default, extra instructions get generated to store the immediate to a register first. extra steps, lowkey wasteful ✨
• -mshftimm — turns on generation of shift with immediate (l.srai, l.srli, l.slli) instructions. By default, extra instructions get generated to store the immediate to a register first.
• -mcmodel=small — generates 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. the cozy default 🎀
• -mcmodel=large — generates 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. big model energy, literally 4G 🔥

---

💾 PDP-11 Options 💾

These options are defined for the PDP-11:

• -mfpu — use hardware FPP floating point. This is the default. (FIS floating point on the PDP-11/40 is not supported.) 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. the chosen one 💜
• -m10 — generate code for a PDP-11/10. Implies -msoft-float -mno-split.
• -mint16, -mno-int32 — use 16-bit int. This is the default.
• -mint32, -mno-int16 — use 32-bit int.
• -msplit — target has split instruction and data space. Implies -m45.
• -munix-asm — use Unix assembler syntax.
• -mdec-asm — use DEC assembler syntax.
• -mgnu-asm — use GNU assembler syntax. This is the default.
• -mlra — use the new LRA register allocator. By default, the old reload allocator is used.

---

⚡ PowerPC Options ⚡

These are listed under (iykyk, bestie — they're filed somewhere else, not repeating them here)

---

🧚 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. smaller binary, fewer vibes ✨

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. These are the differences:

🔍 Function Pointer Size

TI ABI specifies that function (code) pointers are 16-bit, whereas GNU supports only 32-bit data and code pointers.

🔍 Optional Return Value Pointer

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. fr fr, do not skip this one 🚨

---

🌸 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. read the fine print bestie, they might shift on you 💅

• -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. so many letters, so little time ✨

-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:

• The string must start with rv32 or rv64, followed by i, e, or g, referred to as the base ISA.
• The subsequent part of the string is a list of extension names. Extension names can be categorized as multi-letter (e.g. zba) and single-letter (e.g. v). Single-letter extensions can appear consecutively, but multi-letter extensions must be separated by underscores.
• An underscore can appear anywhere after the base ISA. It has no specific effect but is used to improve readability and can act as a separator.

• Extension names may include an optional version number, following the syntax <major>p<minor> or <major>, (e.g. m2p1 or m2).

Okay bestie, here's the full roster of supported extensions, no skips, no skimming, all of them 💖

• g — version - — general-purpose computing base extension; g expands to i, m, a, f, d, zicsr and zifencei.
• e — version 2.0 — reduced base integer extension.
• i — versions 2.0 2.1 — base integer extension.
• m — version 2.0 — integer multiplication and division extension.
• a — versions 2.0 2.1 — atomic extension.
• f — versions 2.0 2.2 — single-precision floating-point extension.
• d — versions 2.0 2.2 — double-precision floating-point extension.
• c — version 2.0 — compressed extension.
• b — version 1.0 — standard extension for bit manipulation functions.
• v — version 1.0 — vector extension.
• h — version 1.0 — hypervisor extension.
• zic64b — version 1.0 — cache block size is 64 bytes.
• zicbom — version 1.0 — cache-block management extension.
• zicbop — version 1.0 — cache-block prefetch extension.
• zicboz — version 1.0 — cache-block zero extension.
• ziccamoa — version 1.0 — main memory supports all atomics in A.
• ziccif — version 1.0 — main memory supports instruction fetch with atomicity requirement.
• zicclsm — version 1.0 — main memory supports misaligned loads/stores.
• ziccrse — version 1.0 — main memory supports forward progress on LR/SC sequences.
• zicfilp — version 1.0 — control-flow integrity landing pad extension.
• zicfiss — version 1.0 — control-flow integrity shadow stack extension.
• zicntr — version 2.0 — standard extension for base counters and timers.
• zicond — version 1.0 — integer conditional operations extension.
• zicsr — version 2.0 — control and status register access extension.
• zifencei — version 2.0 — instruction-fetch fence extension.
• zihintntl — version 1.0 — non-temporal locality hints extension.
• zihintpause — version 2.0 — pause hint extension.
• zihpm — version 2.0 — standard extension for hardware performance counters.
• zimop — version 1.0 — may-be-operations extension.
• zilsd — version 1.0 — load/store pair instructions extension.
• zmmul — version 1.0 — integer multiplication extension.
• za128rs — version 1.0 — reservation set size of 128 bytes.
• za64rs — version 1.0 — reservation set size of 64 bytes.
• zaamo — version 1.0 — atomic memory operations extension.
• zabha — version 1.0 — byte and halfword atomic memory operations extension.
• zacas — version 1.0 — atomic compare-and-swap instructions extension.
• zalrsc — version 1.0 — load-reserved/store-conditional subset of the A extension.
• zawrs — version 1.0 — wait-on-reservation-set extension.
• zama16b — version 1.0 — misaligned loads, stores, and AMOs that are fully contained within a naturally-aligned 16-byte boundary are atomic.
• zfa — version 1.0 — additional floating-point extension.
• zfbfmin — version 1.0 — minimal BF16 support extension.
• zfh — version 1.0 — half-precision floating-point extension.
• zfhmin — version 1.0 — minimal half-precision floating-point extension.
• zfinx — version 1.0 — single-precision floating-point in integer registers extension.
• zdinx — version 1.0 — double-precision floating-point in integer registers extension.
• zca — version 1.0 — integer compressed instruction extension.
• zcb — version 1.0 — simple compressed instruction extension.
• zcd — version 1.0 — compressed double-precision floating point loads and stores extension.
• zce — version 1.0 — compressed instruction extensions for embedded processors.
• zcf — version 1.0 — compressed single-precision floating point loads and stores extension.
• zcmop — version 1.0 — compressed may-be-operations extension.
• zcmp — version 1.0 — compressed push pop extension.
• zcmt — version 1.0 — table jump instruction extension.
• zclsd — version 1.0 — compressed load/store pair instructions extension.
• zba — version 1.0 — address calculation extension.
• zbb — version 1.0 — basic bit manipulation extension.
• zbc — version 1.0 — carry-less multiplication extension.
• zbkb — version 1.0 — cryptography bit-manipulation extension.
• zbkc — version 1.0 — cryptography carry-less multiply extension.
• zbkx — version 1.0 — cryptography crossbar permutation extension.
• zbs — version 1.0 — single-bit operation extension.
• zk — version 1.0 — standard scalar cryptography extension.
• zkn — version 1.0 — NIST algorithm suite extension.
• zknd — version 1.0 — AES decryption extension.
• zkne — version 1.0 — AES encryption extension.
• zknh — version 1.0 — hash function extension.
• zkr — version 1.0 — entropy source extension.
• zks — version 1.0 — ShangMi algorithm suite extension.
• zksed — version 1.0 — SM4 block cipher extension.
• zksh — version 1.0 — SM3 hash function extension.
• zkt — version 1.0 — data independent execution latency extension.
• ztso — version 1.0 — total store ordering extension.
• zvbb — version 1.0 — vector basic bit-manipulation extension.
• zvbc — version 1.0 — vector carry-less multiplication extension.
• zve32f — version 1.0 — vector extensions for embedded processors.
• zve32x — version 1.0 — vector extensions for embedded processors.
• zve64d — version 1.0 — vector extensions for embedded processors.
• zve64f — version 1.0 — vector extensions for embedded processors.
• zve64x — version 1.0 — vector extensions for embedded processors.
• zvfbfmin — version 1.0 — vector BF16 converts extension.
• zvfbfwma — version 1.0 — vector BF16 widening multiply/add extension.
• zvfh — version 1.0 — vector half-precision floating-point extension.
• zvfhmin — version 1.0 — vector minimal half-precision floating-point extension.
• zvkb — version 1.0 — vector cryptography bit-manipulation extension.
• zvkg — version 1.0 — vector GCM/GMAC extension.
• zvkn — version 1.0 — vector NIST Algorithm Suite extension, zvkn will expand to.
• zvknc — version 1.0 — vector NIST Algorithm Suite with carry-less multiply extension, zvknc.
• zvkned — version 1.0 — vector AES block cipher extension.
• zvkng — version 1.0 — vector NIST Algorithm Suite with GCM extension, zvkng will expand.
• zvknha — version 1.0 — vector SHA-2 secure hash extension.
• zvknhb — version 1.0 — vector SHA-2 secure hash extension.
• zvks — version 1.0 — vector ShangMi algorithm suite extension, zvks will expand.
• zvksc — version 1.0 — vector ShangMi algorithm suite with carry-less multiplication extension,.
• zvksed — version 1.0 — vector SM4 block cipher extension.
• zvksg — version 1.0 — vector ShangMi algorithm suite with GCM extension.
• zvksh — version 1.0 — vector SM3 secure hash extension.
• zvkt — version 1.0 — vector data independent execution latency extension.
• zvl1024b — version 1.0 — minimum vector length standard extensions.
• zvl128b — version 1.0 — minimum vector length standard extensions.
• zvl16384b — version 1.0 — minimum vector length standard extension.
• zvl2048b — version 1.0 — minimum vector length standard extensions.
• zvl256b — version 1.0 — minimum vector length standard extensions.
• zvl32768b — version 1.0 — minimum vector length standard extension.
• zvl32b — version 1.0 — minimum vector length standard extensions.
• zvl4096b — version 1.0 — minimum vector length standard extensions.
• zvl512b — version 1.0 — minimum vector length standard extensions.
• zvl64b — version 1.0 — minimum vector length standard extensions.
• zvl65536b — version 1.0 — minimum vector length standard extension.
• zvl8192b — version 1.0 — minimum vector length standard extension.
• zhinx — version 1.0 — half-precision floating-point in integer registers extension.
• zhinxmin — version 1.0 — minimal half-precision floating-point in integer registers extension.
• sdtrig — version 1.0 — debug triggers extension.
• sha — version 1.0 — the augmented hypervisor extension.
• shcounterenw — version 1.0 — support writeable enables for any supported counter.
• shgatpa — version 1.0 — SvNNx4 mode supported for all modes supported by satp.
• shlcofideleg — version 1.0 — delegating LCOFI interrupts to VS-mode.
• shtvala — version 1.0 — the htval register provides all needed values.
• shvstvala — version 1.0 — the vstval register provides all needed values.
• shvstvecd — version 1.0 — the vstvec register supports direct mode.
• shvsatpa — version 1.0 — the vsatp register supports all modes supported by satp.
• smaia — version 1.0 — advanced interrupt architecture extension.
• smcntrpmf — version 1.0 — cycle and instret privilege mode filtering.
• smcsrind — version 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 — <span>1.0</span> — main memory supports page table reads ✨
• sscofpmf — <span>1.0</span> — count overflow and filtering extension
• sscounterenw — <span>1.0</span> — support writeable enables for any supported counter
• sscsrind — <span>1.0</span> — supervisor-mode indirect CSR access
• ssnpm — <span>1.0</span> — supervisor-mode pointer masking extension
• sspm — <span>1.0</span> — supervisor-mode pointer masking extension
• ssstateen — <span>1.0</span> — supervisor-mode state-enable extension
• sstc — <span>1.0</span> — supervisor-mode timer interrupts extension
• sstvala — <span>1.0</span> — stval provides all needed values
• sstvecd — <span>1.0</span> — stvec supports direct mode
• ssstrict — <span>1.0</span> — unimplemented reserved encodings raise illegal instruction exceptions and no non-conforming extensions are present
• ssdbltrp — <span>1.0</span> — double trap extensions
• ssu64xl — <span>1.0</span> — UXLEN=64 must be supported
• supm — <span>1.0</span> — user-mode pointer masking extension
• svinval — <span>1.0</span> — fine-grained address-translation cache invalidation extension
• svnapot — <span>1.0</span> — NAPOT translation contiguity extension
• svpbmt — <span>1.0</span> — page-based memory types extension
• svvptc — <span>1.0</span> — extension for obviating memory-management instructions after marking PTEs valid
• svadu — <span>1.0</span> — hardware updating of A/D bits extension
• svade — <span>1.0</span> — cause exception when hardware updating of A/D bits is disabled
• svbare — <span>1.0</span> — satp mode bare is supported
• xcvalu — <span>1.0</span> — Core-V miscellaneous ALU extension
• xcvbi — <span>1.0</span> — Core-V immediate branch extension
• xcvelw — <span>1.0</span> — Core-V event load word extension
• xcvmac — <span>1.0</span> — Core-V multiply-accumulate extension
• xcvsimd — <span>1.0</span> — Core-V SIMD extension
• xsfcease — <span>1.0</span> — SiFive CEASE instruction extension
• xsfvcp — <span>1.0</span> — SiFive VCIX vector coprocessor extension
• xsfvfnrclipxfqf — <span>1.0</span> — SiFive FP32-to-int8 ranged clip instructions
• xsfvqmaccdod — <span>1.0</span> — SiFive int8 matrix multiplication extension
• xsfvqmaccqoq — <span>1.0</span> — SiFive int8 matrix multiplication extension
• xtheadba — <span>1.0</span> — T-head address calculation extension
• xtheadbb — <span>1.0</span> — T-head basic bit-manipulation extension
• xtheadbs — <span>1.0</span> — T-head single-bit instructions extension
• xtheadcmo — <span>1.0</span> — T-head cache management operations extension
• xtheadcondmov — <span>1.0</span> — T-head conditional move extension
• xtheadfmemidx — <span>1.0</span> — T-head indexed memory operations for floating-point registers extension
• xtheadfmv — <span>1.0</span> — T-head double floating-point high-bit data transmission extension
• xtheadint — <span>1.0</span> — T-head acceleration interruption extension
• xtheadmac — <span>1.0</span> — T-head multiply-accumulate extension
• xtheadmemidx — <span>1.0</span> — T-head indexed memory operation extension
• xtheadmempair — <span>1.0</span> — T-head two-GPR memory operation extension
• xtheadsync — <span>1.0</span> — T-head multi-core synchronization extension
• xtheadvector — <span>1.0</span> — T-head vector extension
• xventanacondops — <span>1.0</span> — Ventana integer conditional operations extension
• xmipscmov — <span>1.0</span> — Mips conditional move extension
• xmipscbop — <span>1.0</span> — Mips prefetch extension
• xandesperf — <span>5.0</span> — Andes performace extension
• xandesbfhcvt — <span>5.0</span> — Andes bfloat16 conversion extension
• xandesvbfhcvt — <span>5.0</span> — Andes vector bfloat16 conversion extension
• xandesvsintload — <span>5.0</span> — Andes vector INT4 load extension
• xandesvpackfph — <span>5.0</span> — Andes vector packed FP16 extension
• xandesvdot — <span>5.0</span> — Andes vector dot product extension
• xsmtvdot — <span>1.0</span> — SpacemiT vector dot product extension

So when -march= is not specified, GCC just takes the setting from -mcpu. Easiest thing ever, no thoughts needed 💅

And if neither -march nor -mcpu= is specified? Then the default for this argument is system dependent. So bestie, if you actually want a specific architecture extension, specify it explicitly — don't just vibe and hope, okay? 😭

Also when the RISC-V specs say an extension depends on other extensions, GCC just implicitly adds those dependent extensions to the enabled extension set if they weren't added explicitly. It's giving "you didn't ask but I got you anyway" 🫶

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👑 Core Name 👑

• -mcpu=processor-string — use the architecture of, and optimize the output for, the given processor, specified by a particular CPU name. The permissible values for this option are:
  • sifive-e20,
  • sifive-e21,
  • sifive-e24,
  • sifive-e31,
  • sifive-e34,
  • sifive-e76,
  • sifive-s21,
  • sifive-s51,
  • sifive-s54,
  • sifive-s76,
  • sifive-u54,
  • sifive-u74,
  • sifive-x280,
  • sifive-p450,
  • sifive-p670,
  • thead-c906,
  • xt-c908,
  • xt-c908v,
  • xt-c910,
  • xt-c910v2,
  • xt-c920,
  • xt-c920v2,
  • 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.

Ngl, note that -mcpu does not override -march or -mtune. She said what she said. 💅

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🌸 Tune Name 🌸

• -mtune=processor-string — optimize the output for the given processor, specified by microarchitecture or particular CPU name. The 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-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= is not specified, GCC uses the setting from -mcpu. And the default is generic if neither is specified. Periodt. ✨

The size choice is not intended for use by end-users. It's used when -Os is specified. It overrides the instruction cost info provided by -mtune=, but it does not override the pipeline info. The whole point is to help reduce code size while still giving good performance. Iconic behaviour honestly 👑

---

• -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: 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. Like, all of them. No exceptions, bestie. 😭

• -msmall-data-limit=n — put global and static data smaller than n bytes into a special section (on some targets).
• -msave-restore, -mno-save-restore — do or don't use smaller but slower prologue and epilogue code that uses library function calls. The default is to use fast inline prologues and epilogues. 🏃‍♀️
• -mmovcc, -mno-movcc — do or don't produce branchless conditional-move code sequences even with targets that do not have specific instructions for conditional operations. If enabled, sequences of ALU operations are produced using base integer ISA instructions where profitable. No branches, just vibes 🦋
• -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. Fast > small, fr fr 🔥
• -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 — specify a particular strategy for inlining string and memory operations. strategy may be one of auto, libcall, scalar, or vector. Pick your fighter 💅
• -mshorten-memrefs, -mno-shorten-memrefs — do or do not attempt to make more use of compressed load/store instructions by replacing a load/store of 'base register + large offset' with a new load/store of 'new base + small offset'. If the new base gets stored in a compressed register, then the new load/store can be compressed. Currently targets 32-bit integer load/stores only. Shrink the offset, glow up the code ✨
• -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. Same energy, different name 💜
• -mvector-strict-align, -mno-vector-strict-align — Do not or do generate unaligned vector memory accesses. The default is off unless the processor we're 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. it's giving "trust me, vectorize everything" but lowkey still checks the vibes between 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. leaf function main character energy. 🍓
• -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 ✨

• -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, 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 specify a specific processor. Code generated under those options runs best on that processor, and may not run at all on others. 💀

The -mcpu options automatically enable or disable the following options:

-maltivec  -mfprnd  -mhard-float  -mmfcrf  -mmultiple -mpopcntb
-mpopcntd  -mpowerpc64 -mpowerpc-gpopt  -mpowerpc-gfxopt -mmulhw
-mdlmzb  -mmfpgpr  -mvsx -mcrypto  -mhtm  -mpower8-fusion
-mquad-memory  -mquad-memory-atomic  -mfloat128 -mfloat128-hardware
-mprefixed  -mpcrel  -mmma -mrop-protect

So real talk, bestie — the exact options a given -mcpu turns on literally change between compiler versions, based on whatever setting seemed to spit out the most optimal code for that CPU. It does not necessarily reflect what your actual hardware can do, iykyk. So if you want one specific option set to one specific value, just drop it in after the -mcpu option, like this:

-mcpu=970 -mno-altivec

Over on AIX, the -maltivec and -mpowerpc64 options are not turned on or off by -mcpu right now, because AIX doesn't fully support those options yet. You can still enable or disable them individually if you're sure it'll work in your environment. 🫶

In every other case, we recommend you use the -mcpu=cpu_type option instead of the options that control generation of specific instructions. Periodt.

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• -mtune=cpu_type — sets the instruction scheduling parameters for machine type cpu_type, but does not set the architecture type or register usage the way -mcpu=cpu_type does. The same values for cpu_type work for -mtune as for -mcpu. If you specify both, the generated code uses the architecture and registers set by -mcpu, but the scheduling parameters set by -mtune. ✨
• -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 — generates PowerPC64 code for the small model: the TOC is limited to 64k.
• -mcmodel=medium — generates 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 — generates 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. 🌈
• -mprofile-kernel — this one's available on PowerPC64 GNU/Linux targets. When you use it 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 (or 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 such as vec_splat, vec_extract, and vec_insert matches 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 — the -mmfcrf option allows GCC to generate the move from condition register field instruction implemented on the POWER4 processor and other processors that support the PowerPC V2.01 architecture.
• -mpopcntb, -mno-popcntb — the -mpopcntb option allows GCC to generate the popcount and double-precision FP reciprocal estimate instruction implemented on the POWER5 processor and other processors that support the PowerPC V2.02 architecture.
• -mpopcntd, -mno-popcntd — the -mpopcntd option allows GCC to generate the popcount instruction implemented on the POWER7 processor and other processors that support the PowerPC V2.06 architecture.
• -mfprnd, -mno-fprnd — the -mfprnd option allows GCC to generate the FP round to integer instructions implemented on the POWER5+ processor and other processors that support the PowerPC V2.03 architecture.
• -mcmpb, -mno-cmpb — the -mcmpb option allows GCC to generate the compare bytes instruction implemented on the POWER6 processor and other processors that support the PowerPC V2.05 architecture.
• -mhard-dfp, -mno-hard-dfp — the -mhard-dfp option 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.
• -msplit-patch-nops — when you're adding NOPs for a patchable area via the -fpatchable-function-entry option, this emits 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 (or 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 (or disable) the use 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 (or 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 (or does not keep) some integer operations adjacent so that the instructions can be fused together on power8 and later processors.
• -mquad-memory, -mno-quad-memory — generate code that uses (or 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 (or 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 don't specify generation of ISA 3.0 instructions or you're 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 saying you have overflowed the available TOC space, you can reduce the amount of TOC space used with the -mno-fp-in-toc and -mno-sum-in-toc options. -mno-fp-in-toc prevents GCC from putting floating-point constants in the TOC and -mno-sum-in-toc forces GCC to generate code to calculate the sum of an address and a constant at run time instead of putting that sum into the TOC. You may specify one or both of these options. Each causes GCC to produce very slightly slower and larger code at the expense of conserving TOC space.

If you still run out of space in the TOC even when you specify both of these options, specify -mminimal-toc instead. This option causes GCC to make only one TOC entry for every file. When you specify this option, GCC produces code that is slower and larger but which uses extremely little TOC space. You may wish to use this option only on files that contain less frequently-executed code. 🍓

• -maix64, -maix32 — enable 64-bit AIX ABI and calling convention: 64-bit pointers, 64-bit long type, and the infrastructure needed to support them. Specifying -maix64 implies -mpowerpc64, while -maix32 disables the 64-bit ABI and implies -mno-powerpc64. GCC defaults to -maix32. 💜
• -mxl-compat, -mno-xl-compat — okay bestie, this one makes your code cosplay as an IBM XL compiler 💅 when you're using the AIX-compatible ABI. What it actually does: it passes floating-point arguments to prototyped functions beyond the register save area (RSA) on the stack, on top of the argument FPRs. It also stops assuming 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. Slay.

Now the lore 🫖: the AIX calling convention got extended but wasn't documented at first, all to handle an obscure K&R C case — 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 and only is necessary when calling subroutines compiled by IBM XL compilers without optimization. iykyk.

• -mpe — support for the IBM RS/6000 SP Parallel Environment (PE). It links an application written to use message passing with special startup code so the app can actually run. Heads up tho: the system must have PE installed in the standard location (/usr/lpp/ppe.poe/), or you override the specs file with the -specs= option to point at the right directory location. Also — the Parallel Environment does not support threads, so -mpe and -pthread are incompatible 💀. Pick a struggle.
• -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 (think floating-point doubles) and slaps them on their natural size-based boundary. -malign-power tells GCC to follow the ABI-specified alignment rules instead. 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 bestie, it's just not invited to the party 🙈.

• -msoft-float, -mhard-float — generate code that does not use (uses) the floating-point register set. Software floating-point emulation is provided if you use the -msoft-float option, and you also pass the option to GCC when linking. Don't forget the linking part, fr fr.
• -mmultiple, -mno-multiple — generate code that uses (does not use) the load multiple word instructions and the store multiple word instructions. These get generated by default on POWER systems, and not generated on PowerPC systems. Do not use -mmultiple on little-endian PowerPC systems, since those instructions do not work when the processor is in little-endian mode. The exceptions are PPC740 and PPC750, which permit these instructions in little-endian mode. Noted 👑
• -mupdate, -mno-update — generate code that uses (does not use) the load or store instructions that update the base register to the address of the calculated memory location. These instructions are generated by default. If you use -mno-update, there'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 😭. Be careful.
• -mavoid-indexed-addresses, -mno-avoid-indexed-addresses — generate code that tries to avoid (not avoid) the use of indexed load or store instructions. These instructions can incur a performance penalty on Power6 processors in certain situations, like when you're stepping through large arrays that cross a 16M boundary. This option is enabled by default when targeting Power6 and disabled otherwise. Vibe check passed ✅
• -mfused-madd, -mno-fused-madd — generate code that uses (does not use) the floating-point multiply and accumulate instructions. These are generated by default if hardware floating point is used. And here's the glow-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. Glow up complete 💅
• -mmulhw, -mno-mulhw — generate code that uses (does not use) the half-word multiply and multiply-accumulate instructions on the IBM 405, 440, 464 and 476 processors. These instructions are generated by default when targeting those processors. Iconic processors, iconic instructions 🌸
• -mdlmzb, -mno-dlmzb — generate code that uses (does not use) the string-search dlmzb instruction on the IBM 405, 440, 464 and 476 processors. This instruction is generated by default when targeting those processors.
• -mno-bit-align, -mbit-align — on System V.4 and embedded PowerPC systems, do not (do) force structures and unions that contain bit-fields to be aligned to the base type of the bit-field.

Example time, because this one's giving ✨: 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. Tiny. Compact. Adorable 🎀

• -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 — generate code that allows (does not allow) a static executable to be relocated to a different address at run time. A simple embedded PowerPC system loader should relocate the entire contents of .got2 and 4-byte locations listed in the .fixup section — a table of 32-bit addresses generated by this option. For this to work, all objects linked together must be compiled with -mrelocatable or -mrelocatable-lib. Also -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. The good news: objects compiled with -mrelocatable-lib may be linked with objects compiled with any combination of the -mrelocatable options. Flexible queen 🌈
• -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. Apps only, bestie, no libraries allowed 💅
• -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 that's on the runtime, not on us 🫶
• -mprioritize-restricted-insns=priority — this option controls the priority that is assigned to dispatch-slot restricted instructions during the second scheduling pass. The argument priority takes the value 0, 1, or 2 to assign no, highest, or second-highest (respectively) priority to dispatch-slot restricted instructions.
• -msched-costly-dep=dependence_type — this option controls which dependences are considered costly by the target during instruction scheduling. The argument dependence_type takes one of the following values:
  • no — no dependence is costly.
  • all — all dependences are costly.
  • true_store_to_load — a true dependence from store to load is costly.
  • store_to_load — any dependence from store to load is costly.
  • number — any dependence for which the latency is greater than or equal to number is costly.
• -minsert-sched-nops=scheme — this option controls which NOP insertion scheme is used during the second scheduling pass. The argument scheme takes one of the following values:
  • no — don't insert NOPs.
  • pad — pad with NOPs any dispatch group that has vacant issue slots, according to the scheduler's grouping.
  • regroup_exact — insert NOPs to force costly dependent insns into separate groups. Insert exactly as many NOPs as needed to force an insn to a new group, according to the estimated processor grouping.
  • number — insert NOPs to force costly dependent insns into separate groups. Insert number NOPs to force an insn to a new group.
• -mcall-sysv — on System V.4 and embedded PowerPC systems, compile code using calling conventions that adhere to the March 1995 draft of the System V Application Binary Interface, PowerPC processor supplement. This is the default unless you configured GCC using powerpc-*-eabiaix.
• -mcall-sysv-eabi, -mcall-eabi — specify both -mcall-sysv and -meabi options. Two for one, a bundle 🎀
• -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 — select 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 — extend the current ABI with a particular extension, or remove such extension. Valid values are: altivec, no-altivec, ibmlongdouble, ieeelongdouble, elfv1, elfv2, and for AIX: vec-extabi, vec-default. ✨
• -mabi=ibmlongdouble — swaps the current ABI over to IBM extended-precision long double. Heads up though bestie: this is not likely to work if your system already defaults to IEEE extended-precision long double. And if you're switching the long double type away from IEEE extended-precision, the compiler will hit you with a warning unless you pass -Wno-psabi. Also? It needs -mlong-double-128 turned on first. 🫶
• -mabi=ieeelongdouble — flips the current ABI to IEEE extended-precision long double. Same vibe check as above: probably won't work if your system defaults to IBM extended-precision long double. Change the long double type away from IBM extended-precision and boom 💥, compiler warning, unless you use -Wno-psabi. Requires -mlong-double-128 to be enabled.
• -mabi=elfv1 — switch to the ELFv1 ABI. This is the default ABI for big-endian PowerPC 64-bit Linux. Overriding the default ABI needs special system support and is likely to fail in spectacular ways. iconic honestly ✨
• -mabi=elfv2 — switch to the ELFv2 ABI. This is the default for little-endian PowerPC 64-bit Linux. Overriding the default again needs special system support and, yeah, is likely to fail in spectacular ways. 🙈
• -mgnu-attribute, -mno-gnu-attribute — 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. little metadata moment 💅
• -mprototype, -mno-prototype — on System V.4 and embedded PowerPC systems, assume all calls to variable argument functions are properly prototyped. Otherwise the compiler has to insert an instruction before every single non-prototyped call to set or clear bit 6 of the condition code register ("CR") — that bit says whether floating-point values are passed in the floating-point registers, in case the function takes variable arguments. With -mprototype, only calls to prototyped variable argument functions set or clear the bit. less work, cleaner code, we love 🌸
• -msim — on embedded PowerPC systems, assume the startup module is called sim-crt0.o and the standard C libraries are libsim.a and libc.a. This is the default for powerpc-*-eabisim configurations. sim girlies rise 🦋
• -mmvme — on embedded PowerPC systems, assume the startup module is called crt0.o and the standard C libraries are libmvme.a and libc.a.
• -mads — on embedded PowerPC systems, assume the startup module is called crt0.o and the standard C libraries are libads.a and libc.a.
• -myellowknife — on embedded PowerPC systems, assume the startup module is called crt0.o and the standard C libraries are libyk.a and libc.a. yellowknife is such a name fr 💛
• -mvxworks — on System V.4 and embedded PowerPC systems, say that you're compiling for a VxWorks system. 🎀
• -memb — on embedded PowerPC systems, set the "PPC_EMB" bit in the ELF flags header to indicate that eabi extended relocations are used.
• -meabi, -mno-eabi — on System V.4 and embedded PowerPC systems, do (or don't) 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 -msdata can use both "r2" and "r13" to point to two separate small data areas. Picking -mno-eabi means the stack aligns to a 16-byte boundary, no EABI init function is called from "main", and -msdata only uses "r13" for a single small data area. -meabi is on by default if you configured GCC with one of the powerpc*-*-eabi* options.
• -msdata=eabi — on System V.4 and embedded PowerPC systems, drop small initialized "const" global and static data into the .sdata2 section, pointed to by register "r2". Small initialized non-"const" global and static data goes in .sdata, pointed to by register "r13". Small uninitialized global and static data goes in .sbss, right next to .sdata. Warning: -msdata=eabi is incompatible with -mrelocatable. It also sets -memb. 🫶
• -msdata=sysv — on System V.4 and embedded PowerPC systems, small global and static data goes into .sdata, pointed to by register "r13", and small uninitialized global and static data goes into .sbss, adjacent to .sdata. Also incompatible with -mrelocatable, just so you know bestie.
• -msdata=default, -msdata — on System V.4 and embedded PowerPC systems, if -meabi is used, compile code the same as -msdata=eabi; otherwise compile it the same as -msdata=sysv. dealer's choice energy ✨
• -msdata=data — on System V.4 and embedded PowerPC systems, small global data goes in .sdata and small uninitialized global data goes in .sbss. But do 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, all initialized global and static data goes in .data, and all uninitialized data goes in .bss. clean split, very demure 🌸
• -mreadonly-in-sdata — put read-only objects in the .sdata section too. This is the default. ⭐
• *-mblock-move-inline-limit=num* — inline 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* — generate 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* — generate 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 handle the rest of the block. The default value is target-specific. 🔥
• *-mstring-compare-inline-limit=num* — compare at most num string bytes with inline code. If the difference or the end of the string isn't found by the end of the inline compare, a call to "strcmp" or "strncmp" takes care of the rest of the comparison. The default is 64 bytes.
• -G num — on embedded PowerPC systems, put global and static items less than or equal to num bytes into the small data or BSS sections instead of the normal data or BSS section. By default, num is 8. The -G num switch also gets passed to the linker. Important bestie: all modules should be compiled with the same -G num value.
• -mregnames, -mno-regnames — on System V.4 and embedded PowerPC systems, do (or don't) emit register names in the assembly language output using symbolic forms. 💅
• -mlongcall, -mno-longcall — by default, assume all calls are far away, so a longer and more expensive calling sequence is required. This is needed for calls farther than 32 megabytes (33,554,432 bytes) from the current location. A short call is generated if the compiler knows the call 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 just generate slower code. As of this writing, the AIX linker can do this, and so can the GNU linker for PowerPC/64. There are plans to add this feature to the GNU linker for 32-bit PowerPC systems too.

On PowerPC64 ELFv2 and 32-bit PowerPC systems with newer GNU linkers, GCC can generate long calls using an inline PLT call sequence (see -mpltseq). PowerPC with -mbss-plt and PowerPC64 ELFv1 (big-endian) do not support inline PLT calls.

On Darwin/PPC systems, #pragma longcall generates jbsr callee, L42, plus a branch island (glue code). The two target addresses represent the callee and the branch island. The Darwin/PPC linker prefers the first address and generates a bl callee if the PPC bl instruction reaches the callee directly; otherwise the linker generates bl L42 to call the branch island. The branch island is appended to the body of the calling function; it computes the full 32-bit address of the callee and jumps to it. 🦋

On Mach-O (Darwin) systems, this option tells 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. the future is uncertain, as always ✨

• -mpltseq, -mno-pltseq — implement (or don'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 they're enabled by default if that support is detected when configuring GCC — and, for 32-bit PowerPC, if GCC is configured with --enable-secureplt. Careful: -mpltseq code and -mbss-plt 32-bit PowerPC relocatable objects may not be linked together. 💀
• -mtls-markers, -mno-tls-markers — mark (or don'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 its argument setup instructions for TLS optimization, which in turn lets GCC schedule the sequence better. scheduling slay 🫶
• -mrecip, -mno-recip — okay so this one turns on the reciprocal estimate and reciprocal square root estimate instructions, paired with extra Newton-Raphson steps to bump the precision, instead of doing a whole divide (or square root and a divide) for floating-point args. 💅 If you're using -mrecip, bestie, you should really also be using -ffast-math — or at the very least -funsafe-math-optimizations, -ffinite-math-only, -freciprocal-math and -fno-trapping-math. And heads up: even though the throughput of the sequence is generally higher than the throughput of the non-reciprocal instruction, the precision can drop by up to 2 ulp (like, the inverse of 1.0 comes out as 0.99999994) for reciprocal square roots. 😭 Not a drill.
• -mrecip=opt — this is the choose-your-own-adventure version. It controls which reciprocal estimate instructions are allowed. opt is a comma-separated list of options, and any of them can be prefixed with a "!" to invert it. ✨
  • all — enable all estimate instructions.
  • default — enable the default instructions, which is exactly equivalent to -mrecip.
  • none — disable all estimate instructions, equivalent to -mno-recip.
  • div — enable the reciprocal approximation instructions for both single and double precision.
  • divf — enable the single-precision reciprocal approximation instructions.
  • divd — enable the double-precision reciprocal approximation instructions.
  • rsqrt — enable the reciprocal square root approximation instructions for both single and double precision.
  • rsqrtf — enable the single-precision reciprocal square root approximation instructions.
  • rsqrtd — enable the double-precision reciprocal square root approximation instructions.

So for example, -mrecip=all,!rsqrtd enables all of the reciprocal estimate instructions except for the "FRSQRTE", "XSRSQRTEDP", and "XVRSQRTEDP" instructions, which are the ones handling the double-precision reciprocal square root calculations. Slay. 🫶

• -mrecip-precision, -mno-recip-precision — assume (or don't assume) that the reciprocal estimate instructions give you higher-precision estimates than the PowerPC ABI mandates. Picking -mcpu=power6, -mcpu=power7 or -mcpu=power8 automatically picks -mrecip-precision for you. 💫 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 — tells GCC which ABI type to use for vectorizing intrinsics with an external library. The only type supported at present is mass, which says to use IBM's Mathematical Acceleration Subsystem (MASS) libraries for vectorizing intrinsics using external libraries. GCC currently emits calls to these when generating code for power7:

"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"

Both -ftree-vectorize and -funsafe-math-optimizations must also be enabled, and the MASS libraries must be specified at link time. 📎

• -mfriz, -mno-friz — generate (or don't generate) the "friz" instruction when the -funsafe-math-optimizations option is used to optimize rounding of floating-point values to 64-bit integer and back to floating point. Warning bestie: 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 generate) code to load up the static chain register ("r11") when calling through a pointer on AIX and 64-bit Linux systems, where a function pointer points to a 3-word descriptor giving the function address, 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. 🎀 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 generate) code to save the TOC value in the reserved stack location in the function prologue, if the function calls through a pointer on AIX and 64-bit Linux systems. If the TOC value isn't saved in the prologue, it gets saved right before the call through the pointer. -mno-save-toc-indirect is the default. 🌸
• -mcompat-align-parm, -mno-compat-align-parm — generate (or don't generate) code to pass structure parameters with a maximum alignment of 64 bits, for compatibility with older versions of GCC. 🧚

Older versions of GCC (prior to 4.9.0) incorrectly did not align a structure parameter on a 128-bit boundary when that structure contained a member requiring 128-bit alignment. This is corrected in more recent versions of GCC. This option may be used to generate code that is compatible with functions compiled with older versions of GCC.

-mno-compat-align-parm 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. The 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 the latter choice, the options -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset furthermore specify which register to use as the base register for reading the canary, and from what offset from that base register. The default for those is as specified in the relevant ABI. 📖

• -mpcrel, -mno-pcrel — generate (or don't generate) 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 generate) addressing modes using prefixed load and store instructions. The -mprefixed option requires that the option -mcpu=power10 (or later) is enabled. ✨
• -mmma, -mno-mma — generate (or don't generate) the MMA instructions. The -mma option requires that the option -mcpu=power10 (or later) is enabled. 👑
• -mrop-protect, -mno-rop-protect — generate (or don't generate) 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 generate) code that runs in privileged state. 🎩
• -mblock-ops-unaligned-vsx, -mno-block-ops-unaligned-vsx — generate (or don't generate) unaligned vsx loads and stores for inline expansion of "memcpy" and "memmove". 🌈
• -msplat-word-constant, -mno-splat-word-constant — generate (or don't generate) code that uses the "XXSPLTIW" instruction. This option is enabled by default. ⭐
• -msplat-float-constant, -mno-splat-float-constant — generate (or don't generate) code that uses the "XXSPLTIDP" instruction. This option is enabled by default. 🍓
• -mieee128-constant, -mno-ieee128-constant — generate (or don't generate) 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 be 64 bits (-m64bit-doubles) or 32 bits (-m32bit-doubles) in size. The default is -m32bit-doubles. 📏 Note that RX floating-point hardware only works on 32-bit values, which is exactly why the default is -m32bit-doubles.
• -fpu, -nofpu — enables (-fpu) or disables (-nofpu) the use of RX floating-point hardware. The default is enabled for the RX600 series and disabled for the RX200 series. 🌸

Floating-point instructions are only generated for 32-bit floating-point values, however, so the FPU hardware is not used for doubles if the -m64bit-doubles option is used.

Note: if the -fpu option is enabled then -funsafe-math-optimizations is also enabled automatically. This is because the RX FPU instructions are themselves unsafe. 😳

• -mcpu=name — selects the type of RX CPU to be targeted. Currently three types are supported: the generic RX600 and RX200 series hardware, and the specific RX610 CPU. The default is RX600. 🔥

The only difference between RX600 and RX610 is that the RX610 does not support the "MVTIPL" instruction.

The RX200 series does not have a hardware floating-point unit, and so -nofpu is enabled by default when this type is selected.

• -mbig-endian-data, -mlittle-endian-data — store data (but not code) in the big-endian format. The default is -mlittle-endian-data, i.e. to store data in the little-endian format. 🦋
• -msmall-data-limit=N — specifies the maximum size in bytes of global and static variables 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 is up to the programmer to ensure that the area does not overflow. 🫠 Also, when the small data area is used, one of the RX's registers (usually "r13") is reserved for use pointing to this area, so it is no longer available for use by the compiler. This could result in slower and/or larger code if variables are pushed onto the stack instead of being held in this register.

Note: common variables (variables that have not been initialized) and constants are not placed into the small data area, as they are assigned to other sections in the output executable. 💅

The default value is zero, which literally just turns this whole thing off. And bestie, listen — this feature is not enabled by default even at the higher optimization levels (-O2 etc), because reserving a register can be genuinely detrimental. It is up to the programmer to experiment and discover whether this feature is of benefit to their program. Wanna know how the actual register that holds the small data area pointer gets picked? That's all in the -mpid description down below. 🫶

• -msim, -mno-sim — use the simulator runtime. The default is to use the libgloss board-specific runtime.
• -mas100-syntax, -mno-as100-syntax — when you're generating assembler output, use a syntax that's compatible with Renesas's AS100 assembler. Fun fact: the GAS assembler can also handle this syntax, but it comes with some restrictions, which is why it's not generated by default. ✨
• -mmax-constant-size=N — sets the maximum size, in bytes, of a constant that can be used as an operand in a RX instruction. Okay so the tea here: the RX instruction set does allow constants up to 4 bytes long in instructions, but a longer value means a longer instruction. So sometimes it's actually beneficial to restrict how big the constants used in instructions are. Constants that are too big get yeeted into a constant pool instead and referenced via register indirection. 💅

The value N can be between 0 and 4. A value of 0 (the default) or 4 means constants of any size are allowed.

• -mrelax — enable linker relaxation. Linker relaxation is a process whereby the linker attempts to reduce the size of a program by finding shorter versions of various instructions. Disabled by default.
• -mint-register=N — specify the number of registers to reserve for fast interrupt handler functions. The value N can be between 0 and 4. A value of 1 means register "r13" is reserved for the exclusive use of fast interrupt handlers. A value of 2 reserves "r13" and "r12". A value of 3 reserves "r13", "r12" and "r11", and a value of 4 reserves "r13" through "r10". A value of 0, the default, does not reserve any registers. Slay, math.
• -msave-acc-in-interrupts — says that interrupt handler functions should preserve the accumulator register. This is only necessary if normal code might use the accumulator register, for example because it performs 64-bit multiplications. The default is to ignore the accumulator, because that makes the interrupt handlers faster. 🏎️
• -mpid, -mno-pid — enables the generation of position-independent data. When enabled, any access to constant data is done via an offset from a base address held in a register. This lets the location of constant data be determined at run time without requiring the executable to be relocated, which is a whole benefit to embedded applications with tight memory constraints. Data that can be modified is not affected by this option.

Heads up though: using 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. 😭

The actual register chosen to hold the constant data base address depends upon whether the -msmall-data-limit and/or the -mint-register command-line options are enabled. Starting with register "r13" and proceeding downwards, registers are allocated first to satisfy the requirements of -mint-register, then -mpid and finally -msmall-data-limit. Thus it is possible for the small data area register to be "r8" if both -mint-register=4 and -mpid are specified on the command line. iykyk

By default this feature is not enabled. The default can be restored via the -mno-pid command-line option.

• -mno-warn-multiple-fast-interrupts, -mwarn-multiple-fast-interrupts — prevents GCC from issuing a warning message if it finds more than one fast interrupt handler while it's compiling a file. The default is to issue a warning for each extra fast interrupt handler found, because the RX only supports one such interrupt. Only one, bestie. 🙈
• -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. (See section 12.2.7 of the RX62N Group User's Manual for more information.)

The default is to allow these instructions, but it is not possible for GCC to reliably detect all circumstances where a string instruction might be used to access an I/O register, so their use cannot be disabled automatically. Instead it is reliant upon the programmer to use the -mno-allow-string-insns option if their program accesses I/O space.

When the instructions are enabled GCC defines the C preprocessor symbol "__RX_ALLOW_STRING_INSNS__", otherwise it defines the symbol "__RX_DISALLOW_STRING_INSNS__". ✨

• -mjsr, -mno-jsr — use only (or not only) "JSR" instructions to access functions. You can reach for this option when code size exceeds the range of "BSR" instructions. And a lil' plot twist: -mno-jsr does not mean "don't use "JSR"" — it means any type of branch may be used. 👑
• -mlra — use the new LRA register allocator. By default, the old "reload" allocator is used.

Note: the generic GCC command-line option -ffixed-reg has special significance to the RX port when used with the "interrupt" function attribute. This attribute indicates a function intended to process fast interrupts. GCC ensures that it only uses the registers "r10", "r11", "r12" and/or "r13" and only provided that the normal use of the corresponding registers have been restricted via the -ffixed-reg or -mint-register command-line options.

---

🌈 S/390 and zSeries Options 💜

These are the -m options defined for the S/390 and zSeries architecture. Buckle up, bestie, there's a lot. 🎀

• -mhard-float, -msoft-float — use (do not use) the hardware floating-point instructions and registers for floating-point operations. When -msoft-float is specified, functions in libgcc.a are used to perform floating-point operations. When -mhard-float is specified, the compiler generates IEEE floating-point instructions. This is the default.
• -mhard-dfp, -mno-hard-dfp — use (do not use) the hardware decimal-floating-point instructions for decimal-floating-point operations. When -mno-hard-dfp is specified, functions in libgcc.a are used to perform decimal-floating-point operations. When -mhard-dfp is specified, the compiler generates decimal-floating-point hardware instructions. This is the default for -march=z9-ec or higher.
• -mlong-double-64, -mlong-double-128 — these switches control the size of "long double" type. A size of 64 bits makes the "long double" type equivalent to the "double" type. This is the default. 🦋
• -mbackchain, -mno-backchain — store (do not store) the address of the caller's frame as backchain pointer into the callee's stack frame. A backchain may be needed to allow debugging using tools that do not understand DWARF call frame information. When -mno-packed-stack is in effect, the backchain pointer is stored at the bottom of the stack frame; when -mpacked-stack is in effect, the backchain is placed into the topmost word of the 96/160 byte register save area.

In general, code compiled with -mbackchain is call-compatible with code compiled with -mno-backchain; however, use of the backchain for debugging purposes usually requires that the whole binary is built with -mbackchain. Note that the combination of -mbackchain, -mpacked-stack and -mhard-float is not supported. In order to build a linux kernel use -msoft-float.

The default is to not maintain the backchain.

• -mpacked-stack, -mno-packed-stack — use (do not use) the packed stack layout. When -mno-packed-stack is specified, the compiler uses all fields of the 96/160 byte register save area only for their default purpose; unused fields still take up stack space. When -mpacked-stack is specified, register save slots are densely packed at the top of the register save area; unused space is reused for other purposes, allowing for more efficient use of the available stack space. However, when -mbackchain is also in effect, the topmost word of the save area is always used to store the backchain, and the return address register is always saved two words below the backchain. 💅

As long as the stack frame backchain is not used, code generated with -mpacked-stack is call-compatible with code generated with -mno-packed-stack. Note that some non-FSF releases of GCC 2.95 for S/390 or zSeries generated code that uses the stack frame backchain at run time, not just for debugging purposes. Such code is not call-compatible with code compiled with -mpacked-stack. Also, note that the combination of -mbackchain, -mpacked-stack and -mhard-float is not supported. In order to build a linux kernel use -msoft-float.

The default is to not use the packed stack layout.

• -msmall-exec, -mno-small-exec — generate (or do not generate) code using the "bras" instruction to do subroutine calls. This only works reliably if the total executable size does not exceed 64k. The default is to use the "basr" instruction instead, which does not have this limitation. 🫶
• -m64, -m31 — when -m31 is specified, generate code compliant to the GNU/Linux for S/390 ABI. When -m64 is specified, generate code compliant to the GNU/Linux for zSeries ABI. This allows GCC in particular to generate 64-bit instructions. For the s390 targets, the default is -m31, while the s390x targets default to -m64. Note, -m31 is deprecated and support will be removed. 😭
• -mzarch, -mesa — when -mzarch is specified, generate code using the instructions available on z/Architecture. When -mesa is specified, generate code using the instructions available on ESA/390. Note that -mesa is not possible with -m64. When generating code compliant to the GNU/Linux for S/390 ABI, the default is -mesa. When generating code compliant to the GNU/Linux for zSeries ABI, the default is -mzarch. 🌸
• -mhtm
• -mno-htm — so -mhtm turns on a whole lil squad of builtins that use the instructions you get from the transactional execution facility 🌈 — the one that showed up with the IBM zEnterprise EC12 machine generation S/390 System z Built-in Functions. And bestie, -mhtm is on by default when you're rolling with -march=zEC12. No extra effort required, she just shows up 💅
• -mvx
• -mno-vx — the moment you say -mvx, GCC starts generating code that uses the instructions from the vector extension facility introduced with the IBM z13 machine generation ✨ Heads up tho: this option changes the ABI for some vector type values, specifically around alignment and calling conventions 👀 So if you're using vector type values in an ABI-relevant context, a GAS .gnu_attribute command gets added to mark the resulting binary with the ABI that's in use. And -mvx is enabled by default when you use -march=z13. periodt 🎀
• -mzvector
• -mno-zvector — -mzvector enables vector language extensions AND builtins that use the instructions from the vector extension facility introduced with the IBM z13 machine generation 🦋 This one adds support for vector to be used as a keyword to define vector type variables and arguments. Quick note: vector is only available when GNU extensions are enabled. It is not expanded when you're requesting strict standard compliance, e.g. with -std=c99 🙈 On top of the GCC low-level builtins, -mzvector also enables a set of builtins added for compatibility with AltiVec-style implementations like Power and Cell 💜 And to actually make use of these builtins, you must include the header file vecintrin.h. -mzvector is disabled by default, so it's opt-in only, bestie ✨
• -mmvcle
• -mno-mvcle — generate (or do not generate, your call 💅) code using the mvcle instruction to perform block moves. When -mno-mvcle is specified, GCC uses a mvc loop instead. This is the default unless you're optimizing for size 🌸
• -mdebug
• -mno-debug — print (or don't print, we love a choice 🎀) additional debug information when compiling. The default is to not print debug information.
• -march=cpu-type — generate code that runs on cpu-type, which is the name of a system representing a certain processor type. The possible values for cpu-type are 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.

Specifying native as the cpu type can be used to select the best architecture option for the host processor 💖 -march=native has no effect if GCC does not recognize the processor. iykyk 😭

• -mtune=cpu-type — tune everything applicable about the generated code to cpu-type, except for the ABI and the set of available instructions. The list of cpu-type values is the same as for -march ✨ The default is the value used for -march.
• -mtpf-trace
• -mno-tpf-trace — generate code that adds (or does not add) in TPF OS specific branches to trace routines in the operating system 🌸 This option is off by default, even when compiling for the TPF OS. So don't assume, bestie 🫶
• -mtpf-trace-skip
• -mno-tpf-trace-skip — generate code that changes (or does not change) the default branch targets enabled by -mtpf-trace to point to specialized trace routines providing the ability of selectively skipping function trace entries for the TPF OS 👀 This option is off by default, even when compiling for the TPF OS and specifying -mtpf-trace. double opt-in, she's shy 💅
• -mmain — for TPF OS target, use this option 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 does not use) the floating-point multiply and accumulate instructions ✨ These instructions are generated by default if hardware floating point is used.
• -mwarn-framesize=framesize — emit a warning if the current function exceeds the given frame size 🚨 Because this is a compile-time check it doesn't need to be a real problem when the program runs. It's intended to identify functions that most probably cause a stack overflow 💀 It's useful for environments with limited stack size, e.g. the Linux kernel.

A value of zero disables this warning; that is the default.

• -mwarn-dynamicstack
• -mno-warn-dynamicstack — emit a warning if the function calls alloca or uses dynamically-sized arrays 🙈 This is generally a bad idea with a limited stack size, ngl.
• -mstack-guard=stack-guard
• -mstack-size=stack-size
• -mno-stack-guard
• -mno-stack-size — if these options are enabled, the S/390 back end emits additional instructions in the function prologue that trigger a trap if the stack size is stack-guard bytes above the stack-size 😳 (remember that the stack on S/390 grows downward — keep that in your head bestie). If the stack-guard option is omitted, the smallest power of 2 larger than the frame size of the compiled function is chosen ✨

These options are intended to be used to help debugging stack overflow problems 🐛 The additionally emitted code causes only little overhead and hence can also be used in production-like systems without greater performance degradation 💅

The given values have to be exact powers of 2 and stack-size has to be greater than stack-guard without exceeding 64k ⭐ In order to be efficient the extra code makes the assumption that the stack starts at an address aligned to the value given by stack-size.

The -mstack-guard= option 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 — if the hotpatch option is enabled, a "hot-patching" function prologue is generated for all functions in the compilation unit 🔥 The function label is prepended with the given number of two-byte NOP instructions (pre-halfwords, maximum 1000000). After the label, 2 * post-halfwords bytes are appended, using the largest NOP-like instructions the architecture allows (maximum 1000000) ✨

If both arguments are zero, hotpatching is disabled 🙅

This option can be overridden for individual functions with the hotpatch attribute 🎀

• -mpic-data-is-text-relative
• -mno-pic-data-is-text-relative — when compiling for S/390 with -fpic or -fPIC, normally GCC emits code using relative addressing between code and data 💜 However, for hotpatching it might be required to introduce new .text parts while using the existing .data and .bss sections 👀 In this 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 canary at guard 🐤 Supported locations are global for a global canary or tls for a per-thread canary in the TLS block (the default).

Option -mstack-protector-guard-record results in the generation of section __stack_protector_loc containing pointers to all instructions which load the address of the global guard 🔗 Thus, this option has only an effect in conjunction with -mstack-protector-guard=global. The intended use is for 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 lil group of options addresses security vulnerabilities related to speculative execution and indirect branch prediction by enabling replacement of indirect branches and function returns with direct branches to a thunk 🛡️✨

Specifying a choice of keep causes generation of the default code. thunk triggers generation of out-of-line thunks and replaces the formerly indirect branch with a direct branch to the thunk. thunk-extern does the branch replacement like thunk but does not emit the thunks. thunk-inline is only available with -mindirect-branch-jump; it should be used in preference to thunk in user-space applications to support correct stack unwinding and exception handling 👑

-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 options can also be set on a per-function basis using function attributes 💅

• -mindirect-branch-table
• -mno-indirect-branch-table — this option is useful in conjunction with the -mindirect-branch and -mfunction-return options 🫶 When enabled, it causes generation of tables pointing to the branch locations which have been patched by those options ✨ The tables are emitted in sections named .s390_indirect_jump, .s390_indirect_call, .s390_return_reg, and .s390_return_mem. Each section consists of an array of 32-bit elements; each entry holds the offset from the entry to the patched location 📊
• -mfentry
• -mno-fentry — when used in conjunction with the -pg option, emit profiling code using the __fentry__ hook provided by GLIBC 2.29 or later 🎯 This is only available for 64-bit code. If this option is not enabled, profiling uses the less efficient _mcount hook instead 😬
• -mrecord-mcount
• -mno-record-mcount — when enabled, generate a __mcount_loc section with the locations of all 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 no-op instructions of the same length at the locations where the calls would have been inserted 🤫 This can be used in conjunction with the -mrecord-mcount option to patch the call sequences into the object file without recompiling it. chef's kiss 💅
• -mpreserve-args
• -mno-preserve-args — when enabled, save all argument registers to the stack, and generate corresponding CFI information 💾 This is useful for applications that want to 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.

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🌸 SH Options 🌸

These -m options are defined for the SH implementations 🎀

• -m1 — generate code for the SH1 ✨
• -m2 — generate code for the SH2 💖
• -m2e — generate code for the SH2e 🦋
• -m2a-nofpu — code for the SH2a with no FPU, or for a SH2a-FPU but in a way that the floating-point unit never gets touched. like, the FPU is in the room but we're ignoring it 💅
• -m2a-single-only — code for the SH2a-FPU where no double-precision floating-point operations are used. none. zero. not a one.
• -m2a-single — code for the SH2a-FPU, assuming the floating-point unit is in single-precision mode by default.
• -m2a — 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 — code for the SH4, floating-point unit not invited 🚫
• -m4-single-only — code for the SH4 with an FPU that only does single-precision arithmetic.
• -m4-single — 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 — code for SH4-100 in such a way that the floating-point unit is not used.
• -m4-100-single — code for SH4-100, assuming the floating-point unit is in single-precision mode by default.
• -m4-100-single-only — 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 — code for SH4-200 in such a way that the floating-point unit is not used.
• -m4-200-single — code for SH4-200, assuming the floating-point unit is in single-precision mode by default.
• -m4-200-single-only — 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 — code for SH4-300 in such a way that the floating-point unit is not used.
• -m4-300-single — code for SH4-300 in such a way that no double-precision floating-point operations are used.
• -m4-300-single-only — 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). it's giving minimalist ✨
• -m4-400 — generate code for SH4-400 (no MMU, no FPU).
• -m4-500 — generate code for SH4-500 (no FPU). Also passes -isa=sh4-nofpu to the assembler.
• -m4a-nofpu — code for the SH4al-dsp, or for a SH4a in such a way that the floating-point unit is not used.
• -m4a-single-only — code for the SH4a in such a way that no double-precision floating-point operations are used.
• -m4a-single — 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 it implicitly passes -dsp to the assembler. heads up tho: 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. ⚠️ but bestie listen — this changes the calling conventions, so some functions from the standard C library do not work unless you recompile it first with -mdalign. no cap.
• -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 the alignment constraints, bestie.
• -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 even a thing.
• -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 how we handle 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 gives you faster floating-point greater-equal and less-equal comparisons. and yes, those implicit settings can be overridden by specifying either -mieee or -mno-ieee. slay 💅
• -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. and that not only requires privileged mode at run time, it also fails if the cache line had been mapped via the TLB and has become unmapped. 💀 yeah.
• -misize — dump instruction size and location in the assembly code.
• -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. these are the models and parameters that 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. these 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 you're 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.
• -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 does the lookup-table thing for small divisors, and busts out the div1 instruction with case distinction once the divisors get big. Division by zero? It just calculates some unspecified result and does not trap. This one is the default for SH4. And lowkey important: if you specify this for targets that don't have dynamic shift instructions, it just defaults to call-div1 instead. 💅

So when no division strategy has been specified at all, the default strategy gets picked based on whatever the current target is. For SH2A the default is to use the divs and divu instructions instead of calling library functions. ✨

• -maccumulate-outgoing-args, -mno-accumulate-outgoing-args — reserves space once for outgoing arguments in the function prologue instead of shuffling around every single call. Generally a glow-up for performance and size, and it's also needed for unwinding so the stack frame doesn't go changing around conditional code. -maccumulate-outgoing-args is enabled by default, bestie. 🫶
• -mdivsi3_libfunc=name — sets the name of the library function used for 32-bit signed division to name. Heads up: 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 you hadn't passed this option at all. No sneaky register shenanigans. 🙈
• -mfixed-range=register-range — generates code that treats the given register range as fixed registers. A fixed register is one the register allocator simply cannot use, which is super useful when you're compiling kernel code. A register range is written as two registers separated by a dash, and you can stack up multiple register ranges by separating them with a comma. 🎀
• -mbranch-cost=num — assumes num is the cost of a branch instruction. Higher numbers make the compiler try harder 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 enabled by default when generating code for SH4 and SH4A, and you can explicitly turn it off with -mno-zdcbranch. 💫
• -mcbranch-force-delay-slot — forces 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. You can enable it to work around hardware bugs, like the ones found in the original SH7055. 🐛
• -mfsca, -mno-fsca — allow or disallow the compiler to emit the fsca instruction for sine and cosine approximations. -mfsca must be used in combination with -funsafe-math-optimizations. It's enabled by default when generating code for SH4A. Using -mno-fsca disables sine and cosine approximations even if -funsafe-math-optimizations is in effect. 🔥
• -mfsrra, -mno-fsrra — allow or disallow the compiler to emit the fsrra instruction for reciprocal square root approximations. -mfsrra must be used in combination with -funsafe-math-optimizations and -ffinite-math-only. It's enabled by default when generating code for SH4A. Using -mno-fsrra disables reciprocal square root approximations even if -funsafe-math-optimizations and -ffinite-math-only are in effect. 🌈
• -mpretend-cmove, -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 — generates code using the FDPIC ABI.
• -mlra, -mno-lra — use the new LRA register allocator. By default, the old reload allocator is used.

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☀️ 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 then check at runtime whether or not to use them. 💅
• -mimpure-text, -mno-impure-text — -mimpure-text, used in addition to -shared, tells the compiler to not pass -z text to the linker when linking a shared object. Using this option you can link position-dependent code into a shared object.

-mimpure-text suppresses the relocations remain against allocatable but non-writable sections linker error message. However, the necessary relocations trigger copy-on-write, and the shared object is not actually shared across processes. So instead of using -mimpure-text, you should compile all source code with -fpic or -fPIC.

fr fr, just use -fpic or -fPIC. The -mimpure-text route is giving "I'll fix it later" and later never comes. 😭

• -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, where the necessary toolchain support was 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. Like the 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. 👑

• -mflat, -mno-flat — with -mflat, the compiler does not generate save/restore instructions and uses a flat or single register window model. This model is compatible with the regular register window model. The local registers and the input registers (0--5) are still treated as call-saved registers and are saved on the stack as needed.

With -mno-flat (the default), the compiler generates save/restore instructions (except for leaf functions). This is the normal operating mode. 🧚

• -mfpu, -mhard-float — generate output containing floating-point instructions. This is the default.
• -mno-fpu, -msoft-float — generate output containing library calls for floating point. Warning: the requisite libraries are not available for all SPARC targets. Normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation. The embedded targets sparc-*-aout and sparclite-*-* do provide software floating-point support.

-msoft-float changes the calling convention in the output file, so 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 in order for this to work.

This one's a whole-commitment situation, no halfsies. 🚨

• -mhard-quad-float — generate output containing quad-word (long double) floating-point instructions.
• -msoft-quad-float — generate output containing library calls for quad-word (long double) floating-point instructions. The functions called are those specified in the SPARC ABI. This is the default.

As of this writing, there are no SPARC implementations that have hardware support for the quad-word floating-point instructions. They all invoke a trap handler for one of these instructions, and then the trap handler emulates the effect of the instruction. Because of the trap handler overhead, this is much slower than calling the ABI library routines. Thus the -msoft-quad-float option is the default. 💀

• -mno-unaligned-doubles, -munaligned-doubles — assume that doubles have 8-byte alignment. This is the default.

With -munaligned-doubles, GCC assumes that doubles have 8-byte alignment only if they'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 — do not generate code that can only run in supervisor mode. This is relevant only for the casa instruction emitted for the LEON3 processor. This is the default.
• -mfaster-structs, -mno-faster-structs — with -mfaster-structs, the compiler assumes that structures should have 8-byte alignment. This enables the use of pairs of ldd and std instructions for copies in structure assignment, in place of twice as many ld and st pairs. However, the use of this changed alignment directly violates the SPARC ABI. Thus it's intended only for use on targets where the developer acknowledges that their resulting code is not directly in line with the rules of the ABI.

ABI violation era 😬 — only do this if you know you're doing it and you're at peace with it.

• -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. Supported values for cpu_type are v7, cypress, v8, supersparc, hypersparc, leon, leon3, leon3v7, leon5, sparclite, f930, f934, sparclite86x, sparclet, tsc701, v9, ultrasparc, ultrasparc3, niagara, niagara2, niagara3, niagara4, niagara7 and m8.

Native Solaris and GNU/Linux toolchains also support the value native, which selects the best architecture option for the host processor. -mcpu=native has no effect if GCC does not recognize the processor. 🙈

Default instruction scheduling parameters are used for values that select an architecture and not an implementation. These are v7, v8, sparclite, sparclet, v9.

Here's the list of each supported architecture and their supported implementations:

• v7 — cypress, leon3v7
• v8 — supersparc, hypersparc, leon, leon3, leon5
• sparclite — f930, f934, sparclite86x
• sparclet — tsc701
• v9 — ultrasparc, ultrasparc3, niagara, niagara2, niagara3, niagara4, niagara7, m8

---

ok so this is the part where we get into the deep SPARC lore, bestie. buckle up 🎀

By default (unless somebody configured it differently, which, rude), GCC spits out code for the V7 variant of the SPARC architecture. Now, with -mcpu=cypress, the compiler also optimizes for the Cypress CY7C602 chip, the one they shoved into the SPARCStation/SPARCServer 3xx series. It's also the move for the older SPARCStation 1, 2, IPX, all those vintage queens. 💅

With -mcpu=v8, GCC generates code for the V8 variant of the SPARC architecture. The only glow-up 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. And with -mcpu=supersparc, it additionally optimizes for the SuperSPARC chip, as used in the SPARCStation 10, 1000 and 2000 series. iconic honestly ✨

With -mcpu=sparclite, GCC generates code for the SPARClite variant of the SPARC architecture. This one adds the integer multiply, integer divide step and scan ("ffs") instructions that exist in SPARClite but not in SPARC-V7. With -mcpu=f930, the compiler additionally optimizes for the Fujitsu MB86930 chip — the original SPARClite, and, bestie, it has no FPU 😭. With -mcpu=f934, it additionally optimizes for the Fujitsu MB86934 chip, which is the more recent SPARClite, and this one does have an FPU. glow up 🦋

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 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. And then the optimization parade:

• -mcpu=ultrasparc — additionally optimizes for the Sun UltraSPARC I/II/IIi chips.
• -mcpu=ultrasparc3 — additionally optimizes for the Sun UltraSPARC III/III+/IIIi/IIIi+/IV/IV+ chips.
• -mcpu=niagara — additionally optimizes for Sun UltraSPARC T1 chips.
• -mcpu=niagara2 — additionally optimizes for Sun UltraSPARC T2 chips.
• -mcpu=niagara3 — additionally optimizes for Sun UltraSPARC T3 chips.
• -mcpu=niagara4 — additionally optimizes for Sun UltraSPARC T4 chips.
• -mcpu=niagara7 — additionally optimizes for Oracle SPARC M7 chips.
• -mcpu=m8 — additionally optimizes for Oracle M8 chips.

---

✨ the -mtune era ✨

• -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. iykyk, it's the chill one.

The same values for -mcpu=cpu_type can be used for -mtune=cpu_type, but the only useful values are the ones 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. And with native Solaris and GNU/Linux toolchains, native can also be used. 💜

---

🎀 the VIS saga 🎀

• -mv8plus
• -mno-v8plus

With -mv8plus, GCC generates code for the SPARC-V8+ ABI. The difference from the V8 ABI? 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. slay 💅

• -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. nesting dolls fr fr 🪆

• -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. the way it just keeps stacking 😭👑

---

🦋 the little feature flags 🦋

• -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.

---

🧚 the "oops the hardware lied" options 🧚

these ones are literally just damage control for silicon bugs, no cap 💀

• -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, bestie — it's giving special edition:

• -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. 🙈

---

💜 Options for System V 💜

These additional options are available on System V Release 4 for compatibility with other compilers on those systems. old-school besties unite 🫶

• -G — Create a shared object. It is recommended that -symbolic or -shared be used instead.

lowkey a "please use the newer thing" moment. listen to it.

• -YP,dirs — Search the directories dirs, and no others, for libraries specified with -l.
• -Ym,dir — Look in the directory dir to find the M4 preprocessor. The assembler uses this option.

---

🌸 V850 Options 🌸

These -m options are defined for V850 implementations:

• -mlong-calls
• -mno-long-calls

Treat all calls as being far away (near). If calls are assumed to be far away, the compiler always loads the function's address into a register, and calls indirect through the pointer. 💫

• -mno-ep, -mep — so do we optimize basic blocks that use the same index pointer 4 or more times (to copy the pointer into the ep register and then use the shorter sld and sst instructions)? -mno-ep says no, -mep says yes 💅. Heads up bestie: -mep is on by default if you optimize.
• -mno-prolog-function, -mprolog-function — do we use external functions to save and restore registers at the prologue and epilogue of a function, or not 🎀. -mno-prolog-function = no, -mprolog-function = yes. The external functions are slower, but they use less code space if more than one function saves the same number of registers. And yeah, -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 🫶. Fun fact: 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 🌸.

And okay, so — 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 🙈.

Also, no matter which processor variant is the target, the preprocessor constants __v850 and __v851__ are always defined 💅.

• -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 💀.

It's enabled by default when the RH850 ABI is in use (see -mrh850-abi), and disabled by default when the GCC ABI is in use. And 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 one 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 — so like, be aware 🫶.
• -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 — like, only then, fr fr 💀.
• -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 the 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 🌸.

Okay important one: -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. No cap.

• *-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 generate code for both VxWorks kernels and real time processes (RTPs). This flag switches you from the first to the second. It also defines the preprocessor macro "__RTP__". 💫
• -msmp — picks SMP runtimes for linking. Not available on architectures other than PowerPC, and not on VxWorks version 7 or later either — on those, the selection is part of the VxWorks build configuration and the library paths are the same for either choice. so like, it's giving "not my department" 😭
• -mvthreads — links against the VxWorks AE vThread runtime environment.
• -non-static — links an RTP executable against shared libraries rather than static libraries. The options -static and -shared can also be used for RTPs; -static is the default.
• -Bstatic, -Bdynamic — these options get passed straight down to the linker. They're defined for compatibility with Diab. 🫶
• -Xbind-lazy — enables lazy binding of function calls. This option is equivalent to -Wl,-z,now and is defined for compatibility with Diab.
• -Xbind-now — disables lazy binding of function calls. This option is the default and is defined for compatibility with Diab.

---

🖥️ x86 Options 🖥️

This section documents the -m options available for the x86 family of computers. intel girlies rise 💅

The following group of options allows compilation to target a specific processor.

• -march=cpu-type — generate instructions for the machine type cpu-type. In contrast to -mtune=cpu-type, which merely tunes the generated code for the specified cpu-type, -march=cpu-type allows GCC to generate code that may not run at all on processors other than the one indicated. Specifying -march=cpu-type implies -mtune=cpu-type, except where noted otherwise. ✨

The choices for cpu-type are:

• native — this selects the CPU to generate code for at compilation time by determining the processor type of the compiling machine. Using -march=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.
• x86-64-v2, x86-64-v3, x86-64-v4 — these choices for cpu-type select the corresponding micro-architecture level from the x86-64 psABI. On ABIs other than the x86-64 psABI they select the same CPU features as the x86-64 psABI documents for the particular micro-architecture level.

Since these cpu-type values do not have a corresponding -mtune setting, using -march with these values enables generic tuning. Specific tuning can be enabled using the -mtune=other-cpu-type option with an appropriate other-cpu-type value.

• i386 — original Intel i386 CPU.
• i486 — Intel i486 CPU. (No scheduling is implemented for this chip.) 💀
• 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. 🎀
• pentium4, pentium4m — Intel Pentium 4 CPU with MMX, SSE, SSE2 and FXSR instruction set support.
• prescott — improved version of Intel Pentium 4 CPU with MMX, SSE, SSE2, SSE3 and FXSR instruction set support.
• nocona — improved version of Intel Pentium 4 CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3 and FXSR instruction set support.
• core2 — Intel Core 2 CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, CX16, SAHF and FXSR instruction set support.
• nehalem, corei7 — Intel Nehalem CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF and FXSR instruction set support.
• westmere — Intel Westmere CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR and PCLMUL instruction set support.
• sandybridge, corei7-avx — Intel Sandy Bridge CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE and PCLMUL instruction set support.
• ivybridge, core-avx-i — Intel Ivy Bridge CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND and F16C instruction set support.
• haswell, core-avx2 — Intel Haswell CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE and HLE instruction set support.
• 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. 🔥
• 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, 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, 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, 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, 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.
• novalake — Intel Nova Lake CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, AVX-VNNI, UINTR, AVXIFMA, AVXVNNIINT8, AVXNECONVERT, CMPCCXADD, AVXVNNIINT16, SHA512, 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.
• 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, AMX-TF32, MOVRS and AMX-MOVRS instruction set support.
• bonnell, atom — Intel Bonnell CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3 and SSSE3 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, 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, 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.
• clearwaterforest — Intel Clearwater Forest CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, CLDEMOTE, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, AVX-VNNI, ENQCMD, UINTR, AVXIFMA, AVXVNNIINT8, AVXNECONVERT, CMPCCXADD, AVXVNNIINT16, SHA512, SM3, SM4, USER_MSR and PREFETCHI instruction set support.
• k6 — AMD K6 CPU with MMX instruction set support.
• k6-2, k6-3 — Improved versions of AMD K6 CPU with MMX and 3DNow! instruction set support.
• athlon, athlon-tbird — AMD Athlon CPU with MMX, 3dNOW!, enhanced 3DNow! and SSE prefetch instructions support.
• athlon-4, athlon-xp, athlon-mp — Improved AMD Athlon CPU with MMX, 3DNow!, enhanced 3DNow! and full SSE instruction set support.
• k8, opteron, athlon64, athlon-fx — Processors based on the AMD K8 core with x86-64 instruction set support, including the AMD Opteron, Athlon 64, and Athlon 64 FX processors. (This supersets MMX, SSE, SSE2, 3DNow!, enhanced 3DNow! and 64-bit instruction set extensions.)
• k8-sse3, opteron-sse3, athlon64-sse3 — Improved versions of AMD K8 cores with SSE3 instruction set support.
• amdfam10, barcelona — CPUs based on AMD Family 10h cores with x86-64 instruction set support. (This supersets MMX, SSE, SSE2, SSE3, SSE4A, 3DNow!, enhanced 3DNow!, ABM and 64-bit instruction set extensions.)
• bdver1 — CPUs based on AMD Family 15h cores with x86-64 instruction set support. (This supersets FMA4, AVX, XOP, LWP, AES, PCLMUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.) 💜
• bdver2 — AMD Family 15h core based CPUs with x86-64 instruction set support. (This supersets BMI, TBM, F16C, FMA, FMA4, AVX, XOP, LWP, AES, PCLMUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.)
• bdver3 — AMD Family 15h core based CPUs with x86-64 instruction set support. (This supersets BMI, TBM, F16C, FMA, FMA4, FSGSBASE, AVX, XOP, LWP, AES, PCLMUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.)
• bdver4 — AMD Family 15h core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, TBM, F16C, FMA, FMA4, FSGSBASE, AVX, AVX2, XOP, LWP, AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.)
• znver1 — AMD Family 17h core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, F16C, FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, and 64-bit instruction set extensions.)
• znver2 — AMD Family 17h core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, CLWB, F16C, FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, RDPID, WBNOINVD, and 64-bit instruction set extensions.)
• znver3 — AMD Family 19h core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, CLWB, F16C, FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, RDPID, WBNOINVD, PKU, VPCLMULQDQ, VAES, and 64-bit instruction set extensions.)
• znver4 — AMD Family 19h core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, CLWB, F16C, FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, RDPID, WBNOINVD, PKU, VPCLMULQDQ, VAES, AVX512F, AVX512DQ, AVX512IFMA, AVX512CD, AVX512BW, AVX512VL, AVX512BF16, AVX512VBMI, AVX512VBMI2, AVX512VNNI, AVX512BITALG, AVX512VPOPCNTDQ, GFNI and 64-bit instruction set extensions.)
• znver5 — AMD Family 1ah core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, CLWB, F16C, FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, RDPID, WBNOINVD, PKU, VPCLMULQDQ, VAES, AVX512F, AVX512DQ, AVX512IFMA, AVX512CD, AVX512BW, AVX512VL, AVX512BF16, AVX512VBMI, AVX512VBMI2, AVX512VNNI, AVX512BITALG, AVX512VPOPCNTDQ, GFNI, AVXVNNI, MOVDIRI, MOVDIR64B, AVX512VP2INTERSECT, PREFETCHI and 64-bit instruction set extensions.)
• znver6 — AMD Family 1ah core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, CLWB, F16C, FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, RDPID, WBNOINVD, PKU, VPCLMULQDQ, VAES, AVX512F, AVX512DQ, AVX512IFMA, AVX512CD, AVX512BW, AVX512VL, AVX512BF16, AVX512VBMI, AVX512VBMI2, AVX512VNNI, AVX512BITALG, AVX512VPOPCNTDQ, GFNI, AVXVNNI, MOVDIRI, MOVDIR64B, AVX512VP2INTERSECT, PREFETCHI, AVXVNNIINT8, AVXIFMA, AVX512FP16, AVXNECONVERT, AVX512BMM and 64-bit instruction set extensions.)
• btver1 — CPUs based on AMD Family 14h cores with x86-64 instruction set support. (This supersets MMX, SSE, SSE2, SSE3, SSSE3, SSE4A, CX16, ABM and 64-bit instruction set extensions.)
• btver2 — CPUs based on AMD Family 16h cores with x86-64 instruction set support. This includes MOVBE, F16C, BMI, AVX, PCLMUL, AES, SSE4.2, SSE4.1, CX16, ABM, SSE4A, SSSE3, SSE3, SSE2, SSE, MMX and 64-bit instruction set extensions.
• winchip-c6 — IDT WinChip C6 CPU, dealt in same way as i486 with additional MMX instruction set support.
• winchip2 — IDT WinChip 2 CPU, dealt in same way as i486 with additional MMX and 3DNow! instruction set support.
• c3 — VIA C3 CPU with MMX and 3DNow! instruction set support. (No scheduling is implemented for this chip.) she's simple bestie, no scheduling for her 💅
• c3-2 — VIA C3-2 (Nehemiah/C5XL) CPU with MMX and SSE instruction set support. (No scheduling is implemented for this chip.)
• c7 — VIA C7 (Esther) CPU with MMX, SSE, SSE2 and SSE3 instruction set support. (No scheduling is implemented for this chip.)
• samuel-2 — VIA Eden Samuel 2 CPU with MMX and 3DNow! instruction set support. (No scheduling is implemented for this chip.)
• nehemiah — VIA Eden Nehemiah CPU with MMX and SSE instruction set support. (No scheduling is implemented for this chip.)
• esther — VIA Eden Esther CPU with MMX, SSE, SSE2 and SSE3 instruction set support. (No scheduling is implemented for this chip.)
• eden-x2 — VIA Eden X2 CPU with x86-64, MMX, SSE, SSE2 and SSE3 instruction set support. (No scheduling is implemented for this chip.)
• eden-x4 — VIA Eden X4 CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX and AVX2 instruction set support. (No scheduling is implemented for this chip.)
• nano — Generic VIA Nano CPU with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support. (No scheduling is implemented for this chip.)
• nano-1000 — VIA Nano 1xxx CPU with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support. (No scheduling is implemented for this chip.)
• nano-2000 — VIA Nano 2xxx CPU with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support. (No scheduling is implemented for this chip.)
• nano-3000 — VIA Nano 3xxx CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3 and SSE4.1 instruction set support. (No scheduling is implemented for this chip.)
• nano-x2 — VIA Nano Dual Core CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3 and SSE4.1 instruction set support. (No scheduling is implemented for this chip.)
• nano-x4 — VIA Nano Quad Core CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3 and SSE4.1 instruction set support. (No scheduling is implemented for this chip.)
• lujiazui — ZHAOXIN lujiazui CPU with x86-64, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PCLMUL, RDRND, XSAVE, XSAVEOPT, FSGSBASE, CX16, ABM, BMI, BMI2, FXSR, RDSEED instruction set support. While the CPUs do support AVX and F16C, these aren't enabled by "-march=lujiazui" for performance reasons. the tea ☕ the silicon literally has AVX and F16C but -march=lujiazui just... doesn't turn them on, for performance reasons, periodt
• 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.

---

🎚️ -mtune=cpu-type ✨

Tune to cpu-type everything applicable about the generated code, except for the ABI and the set of available instructions. read that twice bestie, it matters. While picking a specific cpu-type schedules things appropriately for that particular chip, the compiler does not generate any code that cannot run on the default machine type unless you use a -march=cpu-type option. For example, if GCC is configured for i686-pc-linux-gnu then -mtune=pentium4 generates code that is tuned for Pentium 4 but still runs on i686 machines. 🦋

The choices for cpu-type are the same as for -march. In addition, -mtune supports 2 extra choices for cpu-type:

• generic — Produce code optimized for the most common IA32/AMD64/EM64T processors. If you know the CPU on which your code will run, then you should use the corresponding -mtune or -march option instead of -mtune=generic. But, if you do not know exactly what CPU users of your application will have, then you should use this option. the "idk what they're running" special, she's reliable 💅

As new processors are deployed in the marketplace, the behavior of this option will change. Therefore, if you upgrade to a newer version of GCC, code generation controlled by this option will change to reflect the processors that are most common at the time that version of GCC is released.

There is no -march=generic option because -march indicates the instruction set the compiler can use, and there is no generic instruction set applicable to all processors. In contrast, -mtune indicates the processor (or, in this case, collection of processors) for which the code is optimized. no -march=generic era, because "generic instruction set" is simply not a thing that exists 😭

• 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. Haswell and Silvermont for this version, noted 📝

...and like, the way this plays out is that as new Intel processors hit the marketplace, the behavior of this option changes. So bestie, if you upgrade to a newer version of GCC, the code generation controlled by this option will change too — it snaps to whatever the most current Intel processors were at the time that version of GCC dropped. 💅

And before anyone asks in the group chat: there is no -march=intel option. Why? Because -march is the one that says which instruction set the compiler can use, and there is genuinely no common instruction set applicable to all processors. Meanwhile -mtune is the one that indicates the processor (or, in this case, the whole collection of processors) that the code is optimized for. Different jobs, different flags, no overlap. ✨

---

🦋 the instruction-set-extension flags 🦋

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. (yes, every single one, they're all paired up, it's giving matching friendship bracelets 🫶)

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. Important bestie note: 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. No cap.

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, which is controlled separately by -msse4a. Read that twice, it's a whole thing. 💀

• -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 — bestie, this one's giving USER_MSR support: it turns on the built-in functions and the code generation for it. ✨
• -mamx-avx512 — the whole girlboss squad of built-ins and codegen: MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX10.1, AVX10.2 and AMX-AVX512. She contains multitudes 💅
• -mamx-tf32 — supports AMX-TF32 built-in functions and code generation, periodt.
• -mamx-fp8 — supports AMX-FP8 built-in functions and code generation. lowkey iconic 🦋
• -mmovrs — supports MOVRS built-in functions and code generation. no cap.
• -mamx-movrs — supports AMX-MOVRS built-in functions and code generation. she ate 🔥

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These additional options are available for the x86 processor family. 🎀

---

• -mfpmath=unit — generates floating-point arithmetic for whichever unit you picked, bestie. The choices for unit are a whole vibe check:
  • 387 — uses the standard 387 floating-point coprocessor, which is 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 gives you slightly different results compared to most other chips. Peep -ffloat-store for a more detailed description.

This is the default choice for non-Darwin x86-32 targets.

• sse — uses the 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 actually effective. For the x86-64 compiler, these extensions are enabled by default 💖.

The resulting code should be considerably faster in the majority of cases and avoid the numerical instability problems of 387 code, but may break some existing code that expects temporaries to be 80 bits. it's giving tradeoffs ✨

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. she adapts, she pivots 👑

• 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, bestie — 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 — controls 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. slay 💅
• -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. 🙈

On machines where a function returns floating-point results in the 80387 register stack, some floating-point opcodes may be emitted even if -msoft-float is used.

• -mno-fp-ret-in-387 — do not use the FPU registers for return values of functions. no cap.

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. ✨
• -malign-double
• -mno-align-double — controls 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 the long double type. The x86-32 application binary interface specifies the size to be 96 bits, so -m96bit-long-double is the default in 32-bit mode.

Modern architectures (Pentium and newer) prefer long double to be aligned to an 8- or 16-byte boundary. In arrays or structures conforming to the ABI, this is not possible. So specifying -m128bit-long-double aligns long double to a 16-byte boundary by padding the long double with an additional 32-bit zero. ✨

In the x86-64 compiler, -m128bit-long-double is the default choice as its ABI specifies that long double is aligned on a 16-byte boundary.

Notice that neither of these options enable any extra precision over the x87 standard of 80 bits for a long double.

Warning: if you override the default value for your target ABI, this changes the size of structures and arrays containing long double variables, as well as modifying the function calling convention for functions taking long double. Hence they are not binary-compatible with code compiled without that switch. 💀

• -mlong-double-64
• -mlong-double-80
• -mlong-double-128 — these switches control the size of the long double type. A size of 64 bits makes the long double type equivalent to the double type. This is the default for 32-bit Bionic C library. A size of 128 bits makes the long double type equivalent to the __float128 type. This is the default for 64-bit Bionic C library. 🦋

Warning: if you override the default value for your target ABI, this changes the size of structures and arrays containing long double variables, as well as modifying the function calling convention for functions taking long double. Hence they are not binary-compatible with code compiled without that switch.

• -malign-data=type — controls how GCC aligns variables. Supported values for type are:
  • compat — uses increased alignment value compatible with GCC 4.8 and earlier
  • abi — uses alignment value as specified by the psABI
  • cacheline — uses increased alignment value to match the cache line size

compat is the default. 💖

• -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. efficiency queen 👑

You can specify that an individual function is called with this calling sequence with the function attribute stdcall. You can also override the -mrtd option by using the function attribute cdecl.

Warning: this calling convention is incompatible with the one normally used on Unix, so you cannot use it if you need to call libraries compiled with the Unix compiler. 😭

Also, you must provide function prototypes for all functions that take variable numbers of arguments (including printf); otherwise incorrect code is generated for calls to those functions.

In addition, seriously incorrect code results if you call a function with too many arguments. (Normally, extra arguments are harmlessly ignored.) 💀

• -mregparm=num — controls how many registers are used to pass integer arguments. By default, no registers are used to pass arguments, and at most 3 registers can be used. You can control this behavior for a specific function by using the function attribute regparm. 💅

Warning: if you use this switch, and num is nonzero, then you must build all modules with the same value, including any libraries. This includes the system libraries and startup modules. no pressure bestie 🙈

• -msseregparm — use SSE register passing conventions for float and double arguments and return values. You can control this behavior for a specific function by using the function attribute sseregparm. ✨

Warning: if you use this switch then you must build all modules with the same value, including any libraries. This includes the system libraries and startup modules.

• -mvect8-ret-in-mem — return 8-byte vectors in memory instead of MMX registers. This is the default on 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. 🧚
• -mpc32 — bestie, this sets the 80387 floating-point precision to 32 bits. With -mpc32, the significands of results of floating-point operations get rounded to 24 bits (that's single precision, the smol one 🍓).
• -mpc64 — sets the 80387 floating-point precision to 64 bits. It rounds the significands of results of floating-point operations to 53 bits (double precision, the medium one ✨).
• -mpc80 — sets the 80387 floating-point precision to 80 bits. It rounds the significands of results of floating-point operations to 64 bits (extended double precision) — and heads up, this one is the default. 🎀

Okay so important: when you use this option at all, floating-point operations in higher precisions are not available to the programmer without setting the FPU control word explicitly. No cap, that's just how it is.

Setting the rounding of floating-point operations to less than the default 80 bits can speed some programs by 2% or more 💅. But — and this is the part nobody tells you — 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. It's giving accuracy apocalypse 😭.

• -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 -mdaz-ftz and -mno-daz-ftz to explicitly enable or disable setting these flags, regardless of other options passed to GCC. Iconic. 👑
• -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. She's a bridge-builder fr fr 🌸
• -mstack-arg-probe — emit stack probing code in the function prologue. Short, sweet, done. ✨
• -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. Like... all of them. Every single one. 💀

• -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. It's giving "please align your vibes" 🙈

To ensure proper alignment of this values on the stack, the stack boundary must be as aligned as that required by any value stored on the stack. Further, every function must be generated such that it keeps the stack aligned. Thus calling a function compiled with a higher preferred stack boundary from a function compiled with a lower preferred stack boundary most likely misaligns the stack. It is recommended that libraries that use callbacks always use the default setting. 🫶

This extra alignment does consume extra stack space, and generally increases code size. Code that is sensitive to stack space usage, such as embedded systems and operating system kernels, may want to reduce the preferred alignment to -mpreferred-stack-boundary=2.

• -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. Basically a little receipt printer for tuning feature names 🧾✨
• -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 a casual little toy, 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. The "turn everything off" button, periodt 💅
• -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. Cleaning up before she leaves the function, as she should 💜
• -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. Sorry to the -O1 girlies 😭
• -mprefer-avx128 — this option instructs GCC to use 128-bit AVX instructions instead of 256-bit AVX instructions in the auto-vectorizer. Small but mighty ✨
• -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 — this option optimizes functions with noreturn attribute or _Noreturn specifier by not saving in the function prologue callee-saved registers which are used in the function (except for the BP register). This option can interfere with debugging of the caller of the noreturn function or any function further up in the call stack, so it is not enabled by default. Debugging girlies, this one's not for you 🙈
• -mcx16 — this option enables GCC to generate CMPXCHG16B instructions in 64-bit code to implement compare-and-exchange operations on 16-byte aligned 128-bit objects. This is useful for atomic updates of data structures exceeding one machine word in size. The compiler uses this instruction to implement __sync Builtins. However, for __atomic Builtins operating on 128-bit integers, a library call is always used. So close, yet so library call 😭
• -msahf — this option enables generation of SAHF instructions in 64-bit code. Early Intel Pentium 4 CPUs with Intel 64 support, prior to the introduction of Pentium 4 G1 step in December 2005, lacked the LAHF and SAHF instructions which are supported by AMD64. These are load and store instructions, respectively, for certain status flags. In 64-bit mode, the SAHF instruction is used to optimize fmod, drem, and remainder built-in functions; see Other Builtins for details. History lesson AND an optimization, she does it all 👑
• -mmovbe — this option enables use of the movbe instruction to optimize byte swapping of four and eight byte entities. Swapping bytes but make it efficient 💅
• -mshstk — the -mshstk option enables shadow stack built-in functions from x86 Control-flow Enforcement Technology (CET). Security slay 🛡️✨
• -mcrc32 — this option enables built-in functions __builtin_ia32_crc32qi, __builtin_ia32_crc32hi, __builtin_ia32_crc32si and __builtin_ia32_crc32di to generate the crc32 machine instruction. Four whole builtins, we ate 🔥
• -mmwait — this option enables built-in functions __builtin_ia32_monitor, and __builtin_ia32_mwait to generate the monitor and mwait machine instructions. Monitor and mwait, the duo we didn't know we needed 🦋
• -mrecip — okay bestie, this one is giving “math but make it approximate” ✨ It lets GCC use "RCPSS" and "RSQRTSS" instructions (plus their vectorized glow-ups "RCPPS" and "RSQRTPS") with an extra Newton-Raphson step to boost precision, instead of "DIVSS" and "SQRTSS" (and their vectorized variants) for single-precision floating-point args. These instructions only show up when -funsafe-math-optimizations is enabled together with -ffinite-math-only and -fno-trapping-math. Heads up: the throughput of the sequence is higher than the throughput of the non-reciprocal instruction, but the precision of the sequence can be decreased by up to 2 ulp (i.e. the inverse of 1.0 equals 0.99999994). 😭

Also note that GCC implements 1.0f/sqrtf(x) in terms of "RSQRTSS" (or "RSQRTPS") already with -ffast-math (or the above option combination), and doesn’t need -mrecip. And GCC emits the above sequence with additional Newton-Raphson step for vectorized single-float division and vectorized sqrtf(x) already with -ffast-math (or the above option combination), and doesn’t need -mrecip. No cap. 💅

• -mrecip=opt — this one controls which reciprocal estimate instructions may be used, periodt. 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. Slay. 🎀

• -mveclibabi=type — Specifies the ABI type to use for vectorizing intrinsics using an external library. Supported values for type are svml for the Intel short vector math library, 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. 🔥

GCC currently emits calls to these when -mveclibabi=svml is used:

"vmldExp2", "vmldLn2", "vmldLog102",
"vmldPow2", "vmldTanh2", "vmldTan2", "vmldAtan2", "vmldAtanh2",
"vmldCbrt2", "vmldSinh2", "vmldSin2", "vmldAsinh2", "vmldAsin2",
"vmldCosh2", "vmldCos2", "vmldAcosh2", "vmldAcos2", "vmlsExp4",
"vmlsLn4", "vmlsLog104", "vmlsPow4", "vmlsTanh4", "vmlsTan4",
"vmlsAtan4", "vmlsAtanh4", "vmlsCbrt4", "vmlsSinh4", "vmlsSin4",
"vmlsAsinh4", "vmlsAsin4", "vmlsCosh4", "vmlsCos4", "vmlsAcosh4" and
"vmlsAcos4"

for corresponding function type.

When -mveclibabi=aocl is used, GCC emits calls to:

"amd_vrs4_acosf", "amd_vrs16_acosf", "amd_vrd8_asin",
"amd_vrs4_asinf", "amd_vrs8_asinf", "amd_vrs16_asinf",
"amd_vrd2_atan", "amd_vrd8_atan", "amd_vrs4_atanf",
"amd_vrs8_atanf", "amd_vrs16_atanf", "amd_vrd2_cos", "amd_vrd4_cos",
"amd_vrd8_cos", "amd_vrs4_cosf", "amd_vrs8_cosf", "amd_vrs16_cosf",
"amd_vrs4_coshf", "amd_vrs8_coshf", "amd_vrd2_erf", "amd_vrd4_erf",
"amd_vrd8_erf", "amd_vrs4_erff", "amd_vrs8_erff", "amd_vrs16_erff",
"amd_vrd2_exp", "amd_vrd4_exp", "amd_vrd8_exp", "amd_vrs4_expf",
"amd_vrs8_expf", "amd_vrs16_expf", "amd_vrd2_exp10",
"amd_vrs4_exp10f", "amd_vrd2_exp2", "amd_vrd4_exp2",
"amd_vrd8_exp2", "amd_vrs4_exp2f", "amd_vrs8_exp2f",
"amd_vrs16_exp2f", "amd_vrs4_expm1f", "amd_vrd2_log",
"amd_vrd4_log", "amd_vrd8_log", "amd_vrs4_logf", "amd_vrs8_logf",
"amd_vrs16_logf", "amd_vrd2_log10", "amd_vrs4_log10f",
"amd_vrs8_log10f", "amd_vrs16_log10f", "amd_vrd2_log1p",
"amd_vrs4_log1pf", "amd_vrd2_log2", "amd_vrd4_log2",
"amd_vrd8_log2", "amd_vrs4_log2f", "amd_vrs8_log2f",
"amd_vrs16_log2f", "amd_vrd2_pow", "amd_vrd4_pow", "amd_vrd8_pow",
"amd_vrs4_powf", "amd_vrs8_powf", "amd_vrs16_powf", "amd_vrd2_sin",
"amd_vrd4_sin", "amd_vrd8_sin", "amd_vrs4_sinf", "amd_vrs8_sinf",
"amd_vrs16_sinf", "amd_vrd2_tan", "amd_vrd4_tan", "amd_vrd8_tan",
"amd_vrs16_tanf", "amd_vrs4_tanhf", "amd_vrs8_tanhf",
"amd_vrs16_tanhf"

for the corresponding function type.

And when -mveclibabi=acml is used, GCC emits calls to:

"__vrd2_sin", "__vrd2_cos",
"__vrd2_exp", "__vrd2_log", "__vrd2_log2", "__vrd2_log10",
"__vrs4_sinf", "__vrs4_cosf", "__vrs4_expf", "__vrs4_logf",
"__vrs4_log2f", "__vrs4_log10f" and "__vrs4_powf"

for the corresponding function type. 💅

• -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". 🌈
• -masm=dialect — Output assembly instructions using selected dialect. Also affects which dialect is used for basic "asm" and extended "asm". Supported choices (in dialect order) are att or intel. The default is att. Darwin does not support intel. 🦋
• -mforce-indirect-call — Force all calls to functions to be indirect. This is useful when using Intel Processor Trace where it generates more precise timing information for function calls. 🔥
• -mmanual-endbr — Insert 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. 💅
• -mcet-switch — By default, CET instrumentation is turned off on switch statements that use a jump table and indirect branch track is disabled. Since jump tables are stored in read-only memory, this does not result in a direct loss of hardening. But if the jump table index is attacker-controlled, the indirect jump may not be constrained by CET. This option turns on CET instrumentation to enable indirect branch track for switch statements with jump tables which leads to the jump targets reachable via any indirect jumps. 👑
• -mcall-ms2sysv-xlogues — Due to differences in 64-bit ABIs, any Microsoft ABI function that calls a System V ABI function must consider RSI, RDI and XMM6-15 as clobbered. By default, the code for saving and restoring these registers is emitted inline, resulting in fairly lengthy prologues and epilogues. Using -mcall-ms2sysv-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. ✨
• -mtls-dialect=type — Generate code to access thread-local storage using the gnu or gnu2 conventions. gnu is the conservative default; gnu2 is more efficient, but it may add compile- and run-time requirements that cannot be satisfied on all systems. 🎀
• -mpush-args, -mno-push-args — Use PUSH operations to store outgoing parameters. This method is shorter and usually equally fast as method using SUB/MOV operations and is enabled by default. In some cases disabling it may improve performance because of improved scheduling and reduced dependencies. 💜
• -maccumulate-outgoing-args — If enabled, the maximum amount of space required for outgoing arguments is computed in the function prologue. This is faster on most modern CPUs because of reduced dependencies, improved scheduling and reduced stack usage when the preferred stack boundary is not equal to 2. The drawback is a notable increase in code size. This switch implies -mno-push-args. 😭
• -mms-bitfields, -mno-ms-bitfields — Enable/disable bit-field layout compatible with the native Microsoft Windows compiler. 🪟

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.

This option is enabled by default for Microsoft Windows targets. This behavior can also be controlled locally by use of variable or type attributes. For more information, see x86 Attributes.

The Microsoft structure layout algorithm is fairly simple with the exception of the bit-field packing. The padding and alignment of members of structures and whether a bit-field can straddle a storage-unit boundary are determined by these rules:

1. Structure members are stored sequentially in the order in which they are declared: the first member has the lowest memory address and the last member the highest.
2. Every data object has an alignment requirement. The alignment requirement for all data except structures, unions, and arrays is either the size of the object or the current packing size (specified with either the "aligned" attribute or the "pack" pragma), whichever is less. For structures, unions, and arrays, the alignment requirement is the largest alignment requirement of its members. Every object is allocated an offset so that:

offset % alignment_requirement == 0

3. Adjacent bit-fields are packed into the same 1-, 2-, or 4-byte allocation unit if the integral types are the same size and if the next bit-field fits into the current allocation unit without crossing the boundary imposed by the common alignment requirements of the bit-fields.

MSVC interprets zero-length bit-fields in the following ways:

1. If a zero-length bit-field is inserted between two bit-fields that are normally coalesced, the bit-fields are not coalesced.

For example:

struct
 {
   unsigned long bf_1 : 12;
   unsigned long : 0;
   unsigned long bf_2 : 12;
 } t1;

The size of "t1" is 8 bytes with the zero-length bit-field. If the zero-length bit-field were removed, "t1"’s size would be 4 bytes. 💀

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", "bar" is aligned as the type of the zero-length bit-field.

Like, for example:

struct
 {
   char foo : 4;
   short : 0;
   char bar;
 } t2;

struct
 {
   char foo : 4;
   short : 0;
   double bar;
 } t3;

For "t2", "bar" is placed at offset 2, rather than offset 1. Accordingly, the size of "t2" is 4. For "t3", the zero-length bit-field does not affect the alignment of "bar" or, as a result, the size of the structure.

Taking this into account, it is important to note the following:

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 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.

• -mno-align-stringops — don't align the destination of inlined string operations. This switch reduces code size and improves performance in case the destination is already aligned, but GCC doesn't know about it. (sneaky little perf win, iykyk 💅)
• -minline-all-stringops — by default GCC inlines string operations only when the destination is known to be aligned to least a 4-byte boundary. This enables more inlining and increases code size, but may improve performance of code that depends on fast memcpy and memset for short lengths. The option enables inline expansion of strlen for all pointer alignments. (go off, more inlining ✨)
• -minline-stringops-dynamically — for string operations of unknown size, use run-time checks with inline code for small blocks and a library call for large blocks. bestie it literally decides at runtime, iconic 🦋
• -mstringop-strategy=alg — override the internal decision heuristic for the particular algorithm to use for inlining string operations. The allowed values for alg are:
  • rep_byte
  • rep_4byte
  • rep_8byte — expand using i386 rep prefix of the specified size.
  • byte_loop
  • loop
  • unrolled_loop — expand into an inline loop.
  • libcall — always use a library call.
• -mmemcpy-strategy=strategy — override the internal decision heuristic to decide if __builtin_memcpy should be inlined and what inline algorithm to use when the expected size of the copy operation is known. strategy is a comma-separated list of alg:max_size:dest_align triplets. alg is specified in -mstringop-strategy, max_size specifies the max byte size with which inline algorithm alg is allowed. For the last triplet, the max_size must be -1. The max_size of the triplets in the list must be specified in increasing order. The minimal byte size for alg is 0 for the first triplet and max_size + 1 of the preceding range. triplets?? okay periodt, order matters, don't come for me 💀
• -mmemset-strategy=strategy — this option is similar to -mmemcpy-strategy= except that it controls the __builtin_memset expansion.
• -momit-leaf-frame-pointer — don't keep the frame pointer in a register for leaf functions. This avoids the instructions to save, set up, and restore frame pointers and makes an extra register available in leaf functions. The option -momit-leaf-frame-pointer removes the frame pointer for leaf functions, which might make debugging harder. ngl this one's giving "saved a register, lost my mind debugging" 😭
• -mtls-direct-seg-refs
• -mno-tls-direct-seg-refs — controls whether TLS variables may be accessed with offsets from the TLS segment register (%gs for 32-bit, %fs for 64-bit), or whether the thread base pointer must be added. Whether or not this is valid depends on the operating system, and whether it maps the segment to cover the entire TLS area.

For systems that use the GNU C Library, the default is on.

• -msse2avx
• -mno-sse2avx — specify that the assembler should encode SSE instructions with VEX prefix. The option -mavx turns this on by default.
• -mfentry
• -mno-fentry — if profiling is active (-pg), put the profiling counter call before the prologue. Note: On x86 architectures the attribute ms_hook_prologue isn't possible at the moment for -mfentry and -pg.
• -mrecord-mcount
• -mno-record-mcount — if profiling is active (-pg), generate a 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. patch it in later bestie, main character energy 🎀
• -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.
• -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. tiny numbers go zoom ⭐
• -mavx256-split-unaligned-load
• -mavx256-split-unaligned-store — split 32-byte AVX unaligned load and store.
• -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 duty is serious business bestie, do not skip 🫶

• -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.
• -mrelax-cmpxchg-loop — when emitting a compare-and-swap loop for __sync Builtins and __atomic Builtins lacking a native instruction, optimize for the highly contended case by issuing an atomic load before the CMPXCHG instruction, and using the PAUSE instruction to save CPU power when restarting the loop. contended case?? we love a busy little loop 💜
• -mindirect-branch=choice — convert indirect call and jump with choice. The default is keep, which keeps indirect call and jump unmodified. 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.

Note that -mcmodel=large is incompatible with -mindirect-branch=thunk and -mindirect-branch=thunk-extern since the thunk function may not be reachable in the large code model.

Note that -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 — convert function return with choice. The 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. You can control this behavior for a specific function by using 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.
• -mharden-sls=choice — okay so this one makes code that defends against straight line speculation (SLS) 💀 and choice is what picks how hard we go. Default is none, which is giving "we're not even trying" and turns off ALL the SLS hardening. return hardens function returns. indirect-jmp hardens indirect jumps. all does every single bit of SLS hardening, periodt.
• -mindirect-branch-cs-prefix — drops a CS prefix on call and jmp to the indirect thunk when the branch target lives in the r8-r15 registers, so the call/jmp instruction is exactly 6 bytes long ✨. That length matters because it lets them potentially get swapped out at run time for lfence; call *%r8-r15 or lfence; jmp *%r8-r15 👑
• -mapx-inline-asm-use-gpr32 — bestie listen, by default EGPR usage is disabled in inline asm constraints, because GCC literally can't tell whether your asm instructions support GP32 or not 🙈. So if your inline asm can handle EGPR, that's your green light to use -mapx-inline-asm-use-gpr32.
• -mgather — turns on vectorization for gather instructions, respectively 😌
• -mscatter — turns on vectorization for scatter instructions, respectively 🦋

---

These -m switches are supported in addition to the ones above, on x86-64 processors in 64-bit environments. yes queen, more of them 💅

---

• -m32, -m64, -mx32, -m16 — generate code for a 16-bit, 32-bit or 64-bit environment. Which one does what, let's go:
  • -m32 sets int, long, and pointer types to 32 bits, and generates code that runs in 32-bit mode.
  • -m64 sets int to 32 bits and long and pointer types to 64 bits, and generates code for the x86-64 architecture. For Darwin only, -m64 also turns off the -fno-pic and -mdynamic-no-pic options.
  • -mx32 sets int, long, and pointer types to 32 bits, and generates code for the x86-64 architecture.
  • -m16 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 🔥
• -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, that's the whole vibe 🌸
• -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, the main character 👑
• -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, non-negotiable 💀
• -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. free spirit energy 🌈
• -maddress-mode=long — generate code for long address mode. Only supported for 64-bit and x32 environments. It is the default address mode for 64-bit environments.
• -maddress-mode=short — generate code for short address mode. Only supported for 32-bit and x32 environments. It is 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, bestie, this one's real: 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. do not ignore this 🫶

• -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 it 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. ikyk how it is 🎀
• -mdispatch-scheduler — enable instruction scheduling. Only supported on bdver1, bdver2, bdver3, bdver4, and znver1 processors, and additionally requires -fschedule-insns -fsched-pressure. picky much? 💅
• -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. slay 🦋

---

🌈 x86 Windows Options 🌈

(nothing listed here, moving right along bestie 💖)

---

🌸 Xstormy16 Options 🌸

These options are defined for Xstormy16:

• -msim — choose startup files and linker script suitable for the simulator. that's it, that's the option ✨

---

🧚 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. conditional queen 👑
• -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 too. 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. flexible fr fr 💅
• -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. LOOP said "I do what I want" 😭
• -mlongcalls, -mno-longcalls — when this option is enabled, GCC instructs the assembler to translate direct calls to indirect calls unless it can determine that the target of a direct call is in the range allowed by the call instruction. This translation typically occurs for calls to functions in other source files. Specifically, the assembler translates a direct CALL instruction into an L32R followed by a CALLX instruction. The default is -mno-longcalls. This option should be used in programs where the call target can potentially be out of range. This option is implemented in the assembler, not the compiler, so the assembly code generated by GCC still shows direct call instructions---look at the disassembled object code to see the actual instructions 👀. Note that the assembler uses an indirect call for every cross-file call, not just those that really are out of range.
• -mabi=name — generate code for the specified ABI. Permissible values are: call0, windowed. Default ABI is chosen by the Xtensa core configuration. dealer's choice ✨
• -mabi=call0 — when this option is enabled, function parameters are 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 enabled the C preprocessor symbol __XTENSA_CALL0_ABI__ is defined.
• -mabi=windowed — when this option is enabled, function parameters are passed in registers a10 through a15, and called function rotates 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. Register window is rotated 8 registers back upon return. When this version of the ABI is enabled the C preprocessor symbol __XTENSA_WINDOWED_ABI__ is defined. the window is rotating, she's moving 💫
• -mextra-l32r-costs=n — okay so this one sets an extra cost, in clock cycles, for instruction RAM/ROM access on L32R instructions 💅. It only matters when you're optimizing for speed: it's literally the tiebreaker between loading a constant from the literal pool with L32R vs. synthesizing that constant from a small one using a couple of arithmetic instructions. Default is 0, bestie.
• -mstrict-align
• -mno-strict-align — pick your fighter: -mstrict-align says avoid generating memory accesses that might not be aligned on a natural object boundary as described in the architecture specification, and -mno-strict-align says allow it 🙈. The default is -mno-strict-align for cores that support both unaligned loads and stores in hardware, and -mstrict-align for literally every other core. No cap.

✨ zSeries Options ✨

These are listed under

🌈 ENVIRONMENT 🌈

Okay bestie, this section is the whole tea on the environment variables that change how GCC operates 🫶. Some of them work by handing GCC directories or prefixes to use when it goes searching for various kinds of files. Others just specify other aspects of the compilation environment. Both valid. Both iconic.

Real talk though: you can also specify places to search with options like -B, -I and -L — and those take precedence over places specified using environment variables, which in turn take precedence over the places specified by the configuration of GCC. Ranking matters, periodt. 👑

LANG, LC_CTYPE, LC_MESSAGES, LC_ALL

These environment variables control the way that GCC uses localization information, which is what lets GCC work with different national conventions 🌍. GCC inspects the locale categories LC_CTYPE and LC_MESSAGES if it has been configured to do so. Those locale categories can be set to any value supported by your installation. A typical value is en_GB.UTF-8 for English in the United Kingdom encoded in UTF-8.

• The LC_CTYPE environment variable specifies character classification. GCC uses it to determine the character boundaries in a string; this is needed for some multibyte encodings that contain quote and escape characters that are otherwise interpreted as a string end or escape. Lowkey vital.
• The LC_MESSAGES environment variable specifies the language to use in diagnostic messages.
• If the LC_ALL environment variable is set, it overrides the value of LC_CTYPE and LC_MESSAGES; otherwise, LC_CTYPE and LC_MESSAGES default to the value of the LANG environment variable. And if none of these variables are set, GCC defaults to traditional C English behavior. Simple as that 💅

TMPDIR

If TMPDIR is set, it specifies the directory to use for temporary files. GCC uses temporary files to hold the output of one stage of compilation which is to be used as input to the next stage — for example, the output of the preprocessor, which is the input to the compiler proper. It's giving relay race 🏃‍♀️.

GCC_DIAGNOSTICS_LOG

If GCC_DIAGNOSTICS_LOG is set, then additional information about the diagnostics subsystem will be emitted ✨. If it is set to an empty value, then the information will be written to stderr; otherwise, GCC will attempt to open that file and write the information there.

The precise content and format of the information is subject to change; it is intended for use by GCC developers, rather than end-users. So like, devs only, iykyk 🙈

GCC_COMPARE_DEBUG

Setting GCC_COMPARE_DEBUG is nearly equivalent to passing -fcompare-debug to the compiler driver. See the documentation of this option for more details.

GCC_EXEC_PREFIX

If GCC_EXEC_PREFIX is set, it specifies a prefix to use in the names of the subprograms executed by the compiler. No slash is added when this prefix is combined with the name of a subprogram, but you can specify a prefix that ends with a slash if you wish 💅.

If GCC_EXEC_PREFIX is not set, GCC attempts to figure out an appropriate prefix to use based on the pathname it is invoked with.

If GCC cannot find the subprogram using the specified prefix, it tries looking in the usual places for the subprogram.

The default value of GCC_EXEC_PREFIX is prefix/lib/gcc/ where prefix is the prefix to the installed compiler. In many cases prefix is the value of prefix when you ran the configure script.

Other prefixes specified with -B take precedence over this prefix. 👑

This prefix is also used for finding files such as crt0.o that are used for linking.

In addition, the prefix is used in an unusual way in finding the directories to search for header files. For each of the standard directories whose name normally begins with /usr/local/lib/gcc (more precisely, with the value of GCC_INCLUDE_DIR), GCC tries replacing that beginning with the specified prefix to produce an alternate directory name. Thus, with -Bfoo/, GCC searches foo/bar just before it searches the standard directory /usr/local/lib/bar. If a standard directory begins with the configured prefix then the value of prefix is replaced by GCC_EXEC_PREFIX when looking for header files. The way that this works… it's giving unhinged but it works 💀

COMPILER_PATH

The value of COMPILER_PATH is a colon-separated list of directories, much like PATH. GCC tries the directories thus specified when searching for subprograms, if it cannot find the subprograms using GCC_EXEC_PREFIX. Backup plan energy 🫶

LIBRARY_PATH

The value of LIBRARY_PATH is a colon-separated list of directories, much like PATH. When configured as a native compiler, GCC tries the directories thus specified when searching for special linker files, if it cannot find them using GCC_EXEC_PREFIX. Linking using GCC also uses these directories when searching for ordinary libraries for the -l option (but directories specified with -L come first). First come first served, bestie.

LANG

This variable is used to pass locale information to the compiler. One way in which this information is used is to determine the character set to be used when character literals, string literals and comments are parsed in C and C++. When the compiler is configured to allow multibyte characters, the following values for LANG are recognized:

• C-JIS — Recognize JIS characters.
• C-SJIS — Recognize SJIS characters.
• C-EUCJP — Recognize EUCJP characters.

If LANG is not defined, or if it has some other value, then the compiler uses mblen and mbtowc as defined by the default locale to recognize and translate multibyte characters.

GCC_EXTRA_DIAGNOSTIC_OUTPUT

If GCC_EXTRA_DIAGNOSTIC_OUTPUT is set to one of the following values, then additional text will be emitted to stderr when fix-it hints are emitted. Heads up though: -fdiagnostics-parseable-fixits and -fno-diagnostics-parseable-fixits take precedence over this environment variable. 👑

• fixits-v1 — Emit parseable fix-it hints, equivalent to -fdiagnostics-parseable-fixits. In particular, columns are expressed as a count of bytes, starting at byte 1 for the initial column.
• fixits-v2 — As fixits-v1, but columns are expressed as display columns, as per -fdiagnostics-column-unit=display.

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, no retry, just fail.

This feature is experimental and subject to change or removal without notice. Don't build your whole personality on it, bestie 😭

---

Some additional environment variables affect the behavior of the preprocessor.

CPATH, C_INCLUDE_PATH, CPLUS_INCLUDE_PATH, OBJC_INCLUDE_PATH

Each variable's value is a list of directories separated by a special character, much like PATH, in which to look for header files 🌸. The special character, PATH_SEPARATOR, is target-dependent and determined at GCC build time. For Microsoft Windows-based targets it is a semicolon, and for almost all other targets it is a colon.

CPATH specifies a list of directories to be searched as if specified with -I, but after any paths given with -I options on the command line. This environment variable is used regardless of which language is being preprocessed. Universal queen 💅

The remaining environment variables apply only when preprocessing the particular language indicated. Each specifies a list of directories to be searched as if specified with -isystem, but after any paths given with -isystem options on the command line.

In all these variables, an empty element instructs the compiler to search its current working directory. Empty elements can appear at the beginning or end of a path. For instance, if the value of CPATH is :/special/include, that has the same effect as

-I. -I/special/include

DEPENDENCIES_OUTPUT

If this variable is set, its value specifies how to output dependencies for Make based on the non-system header files processed by the compiler. System header files are ignored in the dependency output. Not invited to the party 🙅‍♀️

The value of DEPENDENCIES_OUTPUT can be just a file name, in which case the Make rules are written to that file, guessing the target name from the source file name. Or the value can have the form file target, in which case the rules are written to file file using target as the target name.

In other words, this environment variable is equivalent to combining the options -MM and -MF, with an optional -MT switch too.

SUNPRO_DEPENDENCIES

This variable is the same as DEPENDENCIES_OUTPUT (see above), except that system header files are not ignored, so it implies -M rather than -MM. However, the dependence on the main input file is omitted.

SOURCE_DATE_EPOCH

If this variable is set, its value specifies a UNIX timestamp to be used in replacement of the current date and time in the __DATE__ and __TIME__ macros, so that the embedded timestamps become reproducible 🎀. Reproducible builds girlies, we love to see it.

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/>.

💅 FOOTNOTES

1. okay so lowkey the biggest footnote of the whole page, and it's about -shared, so listen up bestie 👑 — on some systems, gcc -shared needs to build supplementary stub code for constructors to work. and 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 — like, subtle, the sneaky kind that vibe-checks you three weeks later 😭 — and 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. not me dropping five flags in one footnote, it's giving overachiever ✨

---

👀 SEE ALSO

go read your other faves, the whole friend group is here 🫶

• 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. yes, actual Info entries, go poke around in there, it's a whole ✨library✨ of lore, iykyk 📚

---

🌈 AUTHOR

who even wrote this?? honestly, a lot of people. see the Info entry for gcc, or hit up <https://gcc.gnu.org/onlinedocs/gcc/Contributors.html>, for contributors to GCC. the way that a compiler this iconic is a group project and it actually worked — main character energy, no cap 💜

---

📜 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 a wrap on the whole entire page, bestie 😭🫶 you made it all the way through — slay, genuinely. this was gcc-16, dated 2026-03-22, manual page GCC(1). go compile something beautiful and hydrate, ok? bye 💖✨🦋