[FFmpeg-devel] swscale/rgb2rgb : add X86_64 SIMD (SSSE3 and AVX2) for shuffly_bytes func

[Paul B Mahol]

On 3/18/18, Carl Eugen Hoyos <ceffmpeg at gmail.com> wrote:
> 2018-03-18 17:46 GMT+01:00, Martin Vignali <martin.vignali at gmail.com>:
>> 2018-03-18 17:37 GMT+01:00 Paul B Mahol <onemda at gmail.com>:
>>
>>> On 3/18/18, Nicolas George <george at nsup.org> wrote:
>>> > Martin Vignali (2018-03-18):
>>> >> I run the test again with a bigger width (512 instead of 128)
>>> >> This is my result :
>>> >> shuffle_bytes_0321_c: 128.6
>>> >> shuffle_bytes_0321_ssse3: 41.6
>>> >> shuffle_bytes_0321_avx2: 23.4
>>> >
>>> > IIUC, these benchmarks are expressed in CPU cycles. But what James says
>>> > is that it can cause the CPU frequency to be throttled: if that
>>> > happens,
>>> > less cycles can use more time, and even worse, cause other unrelated to
>>> > take more time. A benchmark in actual time and typical use case would
>>> > be
>>> > needed to decide.
>>>
>>> Yes, always also test overall with typical code usecase.
>
> +1
>
>> I tested it using a "benchmark" command line, who test two shuffle func
>> ./ffmpeg -benchmark -f lavfi -i rgbtestsrc=size=3840x2160:duration=10 -vf
>> format=argb,format=rgba -f null -
>>
>> With the patch :
>> bench: utime=3.611s
>> With only SSSE 3 (disable AVX2 part), i have similar result.
>
> Indicating James' original comment that the avx2 optimization
> makes no sense is correct?

You are almost always wrong.