[FFmpeg-devel] [PATCH 1/2] libavutil/cpu: Adds fast gather detection.

[James Almer]

On 7/12/2021 7:46 AM, Lynne wrote:
> 12 Jul 2021, 11:29 by alankelly-at-google.com at ffmpeg.org:
> 
>> On Fri, Jun 25, 2021 at 1:24 PM Alan Kelly <alankelly at google.com> wrote:
>>
>>> On Fri, Jun 25, 2021 at 10:40 AM Lynne <dev at lynne.ee> wrote:
>>>
>>>> Jun 25, 2021, 09:54 by alankelly-at-google.com at ffmpeg.org:
>>>>
>>>>> Broadwell and later and Zen3 and later have fast gather instructions.
>>>>> ---
>>>>>   Gather requires between 9 and 12 cycles on Haswell, 5 to 7 on
>>>> Broadwell,
>>>>>   and 2 to 5 on Skylake and newer. It is also slow on AMD before Zen 3.
>>>>>   libavutil/cpu.h     |  2 ++
>>>>>   libavutil/x86/cpu.c | 18 ++++++++++++++++--
>>>>>   libavutil/x86/cpu.h |  1 +
>>>>>   3 files changed, 19 insertions(+), 2 deletions(-)
>>>>>
>>>>
>>>> No, we really don't need more FAST/SLOW flags, especially for
>>>> something like this which is just fixable by _not_using_vgather_.
>>>> Take a look at libavutil/x86/tx_float.asm, we only use vgather
>>>> if it's guaranteed to either be faster for what we're gathering or
>>>> is just as fast "slow". If neither is true, we use manual lookups,
>>>> which is actually advantageous since for AVX2 we can interleave
>>>> the lookups that happen in each lane.
>>>>
>>>> Even if we disregard this, I've extensively benchmarked vgather
>>>> on Zen 3, Zen 2, Cascade Lake and Skylake, and there's hardly
>>>> a great vgather improvement to be found in Zen 3 to justify
>>>> using a new CPU flag for this.
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>>>
>>> Thanks for your response. I'm not against finding a cleaner way of
>>> enabling/disabling the code which will be protected by this flag. However,
>>> the manual lookups solution proposed will not work in this case, the avx2
>>> version of hscale will only be faster if fast gathers are available,
>>> otherwise, the ssse3 version should be used.
>>>
>>> I haven't got access to a Zen3 so I can't comment on the performance. I
>>> have tested on a Zen 2 and it is slow. On Broadwell hscale avx2 is about
>>> 10% faster than the ssse3 version and on Skylake about 40% faster, Haswell
>>> has similar performance to Zen2.
>>>
>>> Is there a proxy which could be used for detecting Broadwell or Skylake
>>> and later? AVX512 seems too strict as there are Skylake chips without
>>> AVX512. Thanks
>>>
>>
>> Hi,
>>
>> I will paste the performance figures from the thread for the other part of
>> this patch here so that the justification for this flag is clearer:
>>
>> Skylake Haswell
>> hscale_8_to_15_width4_ssse3 761.2 760
>> hscale_8_to_15_width4_avx2 468.7 957
>> hscale_8_to_15_width8_ssse3 1170.7 1032
>> hscale_8_to_15_width8_avx2 865.7 1979
>> hscale_8_to_15_width12_ssse3 2172.2 2472
>> hscale_8_to_15_width12_avx2 1245.7 2901
>> hscale_8_to_15_width16_ssse3 2244.2 2400
>> hscale_8_to_15_width16_avx2 1647.2 3681
>>
>> As you can see, it is catastrophic on Haswell and older chips but the gains
>> on Skylake are impressive.
>> As I don't have performance figures for Zen 3, I can disable this feature
>> on all cpus apart from Broadwell and later as you say that there is no
>> worthwhile improvement on Zen3. Is this OK with you?
>>
> 
> It's not that catastrophic. Since Haswell CPUs generally don't have
> large AVX2 gains, could you just exclude Haswell only from
> EXTERNAL_AVX2_FAST, and require EXTERNAL_AVX2_FAST
> to enable those functions?

And disable all non gather AVX2 asm functions on Haswell? No. And it's a 
lie that Haswell doesn't have large gains with AVX2.

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