mfpmath option to MinGW (or even gcc)

Question

Does the -march=corei7-avx -mtune=corei7-avx or -march=corei7 -mtune=corei7 -mavx command line options to MinGW with the -mfpmath=sse command line option (or even with -mfpmath=both) enables using of AVX instruction for math routines? Note, that --with-fpmath=avx from here does not work (that is "unrecognized option" for recent builds on MinGW).

Answer 1

AVX is enabled by either -march=corei7-avx or -mavx. The -mtune option is neither necessary nor sufficient to enable AVX.

A -mfpmath=avx does not make any sense, because with this switch you control the generation of scalar floating point code. It makes no difference if you use only one float of a 4 float vector register or only one element of a 8 float vector register. If you have march=avx enabled, scalar floating point instructions will use the VEX encoding anyway, which will save a few mov instructions.

Note that on x86_64 -mfpmath defaults to SSE, so using this switch is usually not necessary or even harmful if you don't exactly know what you are doing.