[FFmpeg-devel] [PATCH] configure: set IceLake-AVX512 as the minimum baseline

[James Almer]

On 8/17/2021 4:24 PM, James Almer wrote:
> On 8/17/2021 12:25 PM, Ronald S. Bultje wrote:
>> Hi,
>>
>> On Tue, Aug 17, 2021 at 2:33 AM Hendrik Leppkes <h.leppkes at gmail.com> 
>> wrote:
>>
>>> On Tue, Aug 17, 2021 at 8:30 AM Wu Jianhua <jianhua.wu at intel.com> wrote:
>>>> Based on IceLake-AVX512 and newer architecture, a broad
>>>> range of the subsets of AVX512 could be supported.
>>>
>> [..]
>>
>>>> -        enabled avx512 && check_x86asm avx512_external "vmovdqa32
>>> [eax]{k1}{z}, zmm0"
>>>> +        # Only IceLake and newer architectures could enable AVX512
>>>> +        #
>>> F/CD/BW/DQ/VL/VNNI/IFMA/VBMI/VBMI2/VPOPCNTDQ/BITALG/GFNI/VAES/VPCLMULQDQ
>>>> +        enabled avx512 && check_x86asm avx512_external "vpdpwssds
>>> zmm31{k1}{z}, zmm29, zmm28"
>>>>           enabled avx2   && check_x86asm avx2_external   "vextracti128
>>> xmm0, ymm0, 0"
>>>>           enabled xop    && check_x86asm xop_external    "vpmacsdd 
>>>> xmm0,
>>> xmm1, xmm2, xmm3"
>>>>           enabled fma4   && check_x86asm fma4_external   "vfmaddps 
>>>> ymm0,
>>> ymm1, ymm2, ymm3"
>>>
>>> Note that you are just checking the functionality of the assembler
>>> here, not having a runtime impact.
>>> What you would likely want is to update avutil/x86/cpu.c as well to
>>> only enable the AVX512 flag on those CPUs.
>>>
>>
>> [After IRC discussion] you want runtime checks for the
>> variants/combinations-of-subsets that we want to support. Right now, 
>> avx512
>> means skylake, so you may want to rename that flag to "avx512skl", and 
>> add
>> a new runtime flag + check for the icelake subset called "avx512icl". 
>> Then
>> in your implementations, you use the appropriate flag, and code 
>> components
>> can individually choose to use skylake- and/or icelake-optimized ax512
>> functions.
> 
> Does it really mean Skylake-X? Afaik the flag checks in cpu.c currently 
> look for AVX-512 Foundation and ZMM support, so it means Knights Landing 
> or newer.

Nevermind, just noticed the comment in cpu.c that mentions the Skylake-X 
extensions.

> 
> What about just making the existing AVX512 flag mean F+VL+DQ+BW, so 
> Skylake-X (Anything older just lacks useful instructions for 
> multimedia), and if needed for this new code add a new avx512icl flag 
> that also looks for something like GFNI.

Assuming making Ice Lake the minimum supported SKU is not acceptable, 
then your suggestion is fine (Sans the renaming, since it's a breaking 
change).

> 
>>
>> Ronald
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>