[FFmpeg-devel] Discrepancy between comments for AVX512 flags

[James Darnley]

While cherry-picking some stuff for avx512 I have noticed that ffmpeg 
has a discrepancy in the comments for the two avx512 flags.

Lets start with the public header
> libavutil/cpu.h
>   56│ #define AV_CPU_FLAG_AVX512     0x100000 ///< AVX-512 functions: requires OS support even if YMM/ZMM registers aren't used
>   57│ #define AV_CPU_FLAG_AVX512ICL  0x200000 ///< F/CD/BW/DQ/VL/VNNI/IFMA/VBMI/VBMI2/VPOPCNTDQ/BITALG/GFNI/VAES/VPCLMULQDQ

This seem to imply the first only detects ZMM support and the second 
groups all instruction sets together.  This appears to be different to 
what we imply in internal code
> libavutil/x86/cpu.c
>  151│ #if HAVE_AVX512 /* F, CD, BW, DQ, VL */
> libavutil/x86/x86inc.asm
>  840│ %assign cpuflags_avx512    (1<<20)| cpuflags_avx2 ; F, CD, BW, DQ, VL     

The detection code itself has
> libavutil/x86/cpu.c
>  151│ #if HAVE_AVX512 /* F, CD, BW, DQ, VL */
>  152│         if ((xcr0_lo & 0xe0) == 0xe0) { /* OPMASK/ZMM state */
>  153│             if ((rval & AV_CPU_FLAG_AVX2) && (ebx & 0xd0030000) == 0xd0030000) {
>  154│                 rval |= AV_CPU_FLAG_AVX512;
>  155│ #if HAVE_AVX512ICL
>  156│                 if ((ebx & 0xd0200000) == 0xd0200000 && (ecx & 0x5f42) == 0x5f42)
>  157│                     rval |= AV_CPU_FLAG_AVX512ICL;

If you decode the bits being checked you'll see that the base avx512 
checks ebx for F DQ CD BW VL and avx512icl checks ebx for IFMA CD BW VL 
and ecx for VBMI VBMI2 GFNI VAES VPCLMULQDQ VNNI BITALG VPOPCNTDQ.  The 
first matches what the internal comments imply.

Part of the difference is my fault and dates from when the flag was 
first added.

Has there been a discussion about which features should go with which flag?