org-howto/articles/2024/06/gcc-builtin-defines.org

[8jun2024] gcc: getting builtin defines

• Problem
• Solution

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Problem

Want to know the set of builtin #defines provided by compiler, as a function of command line arguments.

My use case was figuring out which symbols for vector instructions got picked up with -march=native on my dev host.

Solution

• stumbled on this stack overflow question https://stackoverflow.com/questions/28939652/how-to-detect-sse-sse2-avx-avx2-avx-512-avx-128-fma-kcvi-availability-at-compile

• turns out to be an easy one-liner

    gcc -dM -E - < /dev/null | sort

  with output like

    #define _FORTIFY_SOURCE 3
    #define _LP64 1
    #define _STDC_PREDEF_H 1
    #define __ATOMIC_ACQUIRE 2
    #define __ATOMIC_ACQ_REL 4
    ...
  

  Here:

  • -E tells compiler to emit preprocessor output
  • -dM tells compiler to produce defines instead of preprocesed source code
  • - as last argument tells compiler to compile input from stdin.

• to look at say SSE/AVX related instructions:

  (using gcc 13.2 here)

    gcc -dM -E - < /dev/null | egrep "SSE|AVX" | sort

    #define __MMX_WITH_SSE__ 1
    #define __SSE2_MATH__ 1
    #define __SSE2__ 1
    #define __SSE_MATH__ 1
    #define __SSE__ 1
  

  but with -mavx512f:

    gcc -mavx512f -dM -E - < /dev/null | egrep "SSE|AVX" | sort

  with output:

    #define __AVX2__ 1
    #define __AVX512F__ 1
    #define __AVX__ 1
    #define __MMX_WITH_SSE__ 1
    #define __SSE2_MATH__ 1
    #define __SSE2__ 1
    #define __SSE3__ 1
    #define __SSE4_1__ 1
    #define __SSE4_2__ 1
    #define __SSE_MATH__ 1
    #define __SSE__ 1
    #define __SSSE3__ 1