[macOS] Needs true GCC from Homebrew, not Clang "gcc"

[jeongyunl]

resolve-march-native fails on macOS 14.2 with this error:
ERROR: Command '['gcc', '-Q', '--help=target', '-mcpu=native']' returned non-zero exit status 1.

Environment:

% uname -a
Darwin jlee-mbair.local 23.2.0 Darwin Kernel Version 23.2.0: Wed Nov 15 21:59:33 PST 2023; root:xnu-10002.61.3~2/RELEASE_ARM64_T8112 arm64

% gcc --version
Apple clang version 15.0.0 (clang-1500.1.0.2.5)
Target: arm64-apple-darwin23.2.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin

% gcc -### -E -march=native -  
Apple clang version 15.0.0 (clang-1500.1.0.2.5)
Target: arm64-apple-darwin23.2.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin
 "/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang" "-cc1" "-triple" "arm64-apple-macosx14.0.0" "-Wundef-prefix=TARGET_OS_" "-Wdeprecated-objc-isa-usage" "-Werror=deprecated-objc-isa-usage" "-Werror=implicit-function-declaration" "-E" "-disable-free" "-clear-ast-before-backend" "-disable-llvm-verifier" "-discard-value-names" "-main-file-name" "-" "-mrelocation-model" "pic" "-pic-level" "2" "-mframe-pointer=non-leaf" "-fno-strict-return" "-ffp-contract=on" "-fno-rounding-math" "-funwind-tables=1" "-fobjc-msgsend-selector-stubs" "-target-sdk-version=14.2" "-fvisibility-inlines-hidden-static-local-var" "-target-cpu" "apple-m1" "-target-feature" "+neon" "-target-feature" "+v8.5a" "-target-feature" "+zcm" "-target-feature" "+zcz" "-target-abi" "darwinpcs" "-debugger-tuning=lldb" "-target-linker-version" "1022.1" "-fcoverage-compilation-dir=/Users/jeongyun" "-resource-dir" "/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/clang/15.0.0" "-isysroot" "/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk" "-I/usr/local/include" "-internal-isystem" "/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/local/include" "-internal-isystem" "/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/clang/15.0.0/include" "-internal-externc-isystem" "/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include" "-internal-externc-isystem" "/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include" "-Wno-reorder-init-list" "-Wno-implicit-int-float-conversion" "-Wno-c99-designator" "-Wno-final-dtor-non-final-class" "-Wno-extra-semi-stmt" "-Wno-misleading-indentation" "-Wno-quoted-include-in-framework-header" "-Wno-implicit-fallthrough" "-Wno-enum-enum-conversion" "-Wno-enum-float-conversion" "-Wno-elaborated-enum-base" "-Wno-reserved-identifier" "-Wno-gnu-folding-constant" "-fdebug-compilation-dir=/Users/jeongyun" "-ferror-limit" "19" "-stack-protector" "1" "-fstack-check" "-mdarwin-stkchk-strong-link" "-fblocks" "-fencode-extended-block-signature" "-fregister-global-dtors-with-atexit" "-fgnuc-version=4.2.1" "-fmax-type-align=16" "-fcommon" "-fcolor-diagnostics" "-clang-vendor-feature=+disableNonDependentMemberExprInCurrentInstantiation" "-fno-odr-hash-protocols" "-clang-vendor-feature=+enableAggressiveVLAFolding" "-clang-vendor-feature=+revert09abecef7bbf" "-clang-vendor-feature=+thisNoAlignAttr" "-clang-vendor-feature=+thisNoNullAttr" "-mllvm" "-disable-aligned-alloc-awareness=1" "-D__GCC_HAVE_DWARF2_CFI_ASM=1" "-o" "-" "-x" "c" "-"

Debug output

$ resolve-march-native --debug
# gcc -S -fverbose-asm -o /dev/stdout /var/folders/7s/ypkmzhkm8xg2p3001s6lqcqh0000gn/T/tmp_i64nlde.c -march=native
Flags extracted: 
# gcc -Q --help=target -march=native
clang: error: unsupported option '--help=target'
clang: error: no input files
# gcc -Q --help=target -mcpu=native
clang: error: unsupported option '--help=target'
clang: error: no input files
Traceback (most recent call last):
  File "/Users/jeongyun/Library/Python/3.9/lib/python/site-packages/resolve_march_native/target_help_parser.py", line 96, in get_flags_implied_by_march
    gcc_output = subprocess.check_output(cmd, env=env, stderr=stderr).decode('UTF-8')
  File "/Applications/Xcode.app/Contents/Developer/Library/Frameworks/Python3.framework/Versions/3.9/lib/python3.9/subprocess.py", line 424, in check_output
    return run(*popenargs, stdout=PIPE, timeout=timeout, check=True,
  File "/Applications/Xcode.app/Contents/Developer/Library/Frameworks/Python3.framework/Versions/3.9/lib/python3.9/subprocess.py", line 528, in run
    raise CalledProcessError(retcode, process.args,
subprocess.CalledProcessError: Command '['gcc', '-Q', '--help=target', '-march=native']' returned non-zero exit status 1.

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/jeongyun/Library/Python/3.9/lib/python/site-packages/resolve_march_native/__main__.py", line 50, in _inner_main
    native_unrolled_flag_set = Engine(
  File "/Users/jeongyun/Library/Python/3.9/lib/python/site-packages/resolve_march_native/engine.py", line 133, in run
    march_native_flag_set = self._get_march_native_flag_set()
  File "/Users/jeongyun/Library/Python/3.9/lib/python/site-packages/resolve_march_native/engine.py", line 67, in _get_march_native_flag_set
    march_native_flag_set |= set(get_flags_implied_by_march('native', gcc=self._gcc_command,
  File "/Users/jeongyun/Library/Python/3.9/lib/python/site-packages/resolve_march_native/target_help_parser.py", line 101, in get_flags_implied_by_march
    gcc_output = subprocess.check_output(cmd, env=env, stderr=stderr).decode('UTF-8')
  File "/Applications/Xcode.app/Contents/Developer/Library/Frameworks/Python3.framework/Versions/3.9/lib/python3.9/subprocess.py", line 424, in check_output
    return run(*popenargs, stdout=PIPE, timeout=timeout, check=True,
  File "/Applications/Xcode.app/Contents/Developer/Library/Frameworks/Python3.framework/Versions/3.9/lib/python3.9/subprocess.py", line 528, in run
    raise CalledProcessError(retcode, process.args,
subprocess.CalledProcessError: Command '['gcc', '-Q', '--help=target', '-mcpu=native']' returned non-zero exit status 1.
ERROR: Command '['gcc', '-Q', '--help=target', '-mcpu=native']' returned non-zero exit status 1.

[hartwork]

@jeongyunl to my best knowledge resolve-march-native needs true GCC which means GCC from homebrew on macOS. The CI covers that case, as of today Clang is not supported and your "gcc" is Clang in disguise. Is https://formulae.brew.sh/formula/gcc an option to you?

[jeongyunl]

@hartwork Thanks for quick reply.
I was just trying to figure out how to determine the l1 cache line size of a target platform.
I think I'll just stick to sysctl or getconf.