[MPlayer-dev-eng] [PATCH] Fix CPU feature detection for fma3, avx512 and avx512icl

[Reimar Döffinger]

Hi!

> On 11 Nov 2023, at 02:01, Zuxy Meng <zuxy.meng at gmail.com> wrote:
> 
> On Fri, Nov 10, 2023 at 5:18 AM Reimar Döffinger
> <Reimar.Doeffinger at gmx.de> wrote:
[...]
>>>> but it has since become very wordy. Can we simplify it to check only
>>>> one cpuinfo word for each feature based on
>>>> https://en.wikipedia.org/wiki/AVX-512#CPUs_with_AVX-512? For example,
>>>> check avx512dq for baseline and avx512_vnni for avx512icl. ffmpeg will
>>>> do its own run time check anyway.
>> 
>> Is there a point in checking for that? MPlayer has no AVX code, and not
>> sure there's much point adding any, and FFmpeg always does runtime detection,
>> so we only need to check assembler support and can ignore the cpuid stuff.
> 
> Make sense. Let's simplify it. cpuid check can still be helpful if one
> doesn't even want to compile any AVX512 code

I expect the savings would be so little no-one will care.
But I guess in theory you are right.
The patch seems ok to me, though I've not carefully reviewed/tested.

>>> Unfortunately it breaks build because yasm doesn’t support these fancy new
>>> instructions. Let me craft another patch to migrate us to nasm first.
>> 
>> We already support nasm, but sure, can flip the default to it.
> 
> Thanks! I didn't know that. I'm happy about passing '--yasm=nasm' for now.

I think it would make sense to change the default...
It actually used to be the default along time ago until FFmpeg
started requiring yasm features. Seems it has flipped again...

Best regards,
Reimar