Re: [FFmpeg-devel] [PATCH v2 1/2] avfilter/vf_blackdetect: add AVX2 SIMD version

[Kacper Michajlow <kasper93@gmail.com>]

On Thu, 17 Jul 2025 at 12:45, Niklas Haas <ffmpeg@haasn.xyz> wrote:
>
> From: Niklas Haas <git@haasn.dev>
>
> Requested by a user. Even with autovectorization enabled, the compiler
> performs a quite poor job of optimizing this function, due to not being
> able to take advantage of the pmaxub + pcmpeqb trick for counting the number
> of pixels less than or equal-to a threshold.
>
> blackdetect8_c:                                       4625.0 ( 1.00x)
> blackdetect8_avx2:                                     155.1 (29.83x)
> blackdetect16_c:                                      2529.4 ( 1.00x)
> blackdetect16_avx2:                                    163.6 (15.46x)

I think we should try to have better standards for reporting
performance metrics. Those numbers without context mean not so much.
What compiler, flags, cpu were used? Sure, we can omit some
information if we want to show only the scaling, but if it highly
depends on those things, then we should at least try to be more
specific.

Sorry for being pedantic about those things, but I think it's
important, especially if we put those values in a commit message which
will live forever in the repository as a vague reference.

> Even with autovectorization enabled

You mention the auto vectorization enabled, yet the reported numbers
are without it. In my mind this description implies that shown
performance comparison is with auto vectorization enabled.

When we compare apples to apples, with avx2 we get a more expectable
3.74x (gcc) / 2.38x (clang) depending on the compiler. It's still a
good improvement, no reason to oversell it.

For reference some metrics on me end:

clang 20.1.7

march=generic (default config):

blackdetect8_c:                                       1591.1 ( 1.00x)
blackdetect8_avx2:                                     225.1 ( 7.07x)
blackdetect16_c:                                       643.5 ( 1.00x)
blackdetect16_avx2:                                    220.6 ( 2.92x)

march=core-avx2:

blackdetect8_c:                                        526.0 ( 1.00x)
blackdetect8_avx2:                                     220.9 ( 2.38x)
blackdetect16_c:                                       318.8 ( 1.00x)
blackdetect16_avx2:                                    225.9 ( 1.41x)

gcc 14.2.0

-fno-tree-vectorize (default config):

blackdetect8_c:                                       5126.6 ( 1.00x)
blackdetect8_avx2:                                     198.0 (25.89x)
blackdetect16_c:                                      2151.9 ( 1.00x)
blackdetect16_avx2:                                    196.8 (10.93x)

march=generic -ftree-vectorize:

blackdetect8_c:                                       1354.4 ( 1.00x)
blackdetect8_avx2:                                     196.9 ( 6.88x)
blackdetect16_c:                                       644.2 ( 1.00x)
blackdetect16_avx2:                                    249.8 ( 2.58x)

march=core-avx2 -ftree-vectorize:

blackdetect8_c:                                        820.8 ( 1.00x)
blackdetect8_avx2:                                     219.2 ( 3.74x)
blackdetect16_c:                                       372.8 ( 1.00x)
blackdetect16_avx2:                                    201.4 ( 1.85x)

Again, sorry for being pedantic here, but it gives the wrong
impression especially if you look at this from outside.

- Kacper
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