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

[James Almer]

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.

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.

> 
> Ronald
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