[FFmpeg-devel] [PATCH] Add support for "omp simd" pragma.

[Soft Works]

> -----Original Message-----
> From: ffmpeg-devel <ffmpeg-devel-bounces at ffmpeg.org> On Behalf Of
> Reimar.Doeffinger at gmx.de
> Sent: Sunday, January 10, 2021 5:44 PM
> To: ffmpeg-devel at ffmpeg.org
> Cc: Reimar Döffinger <Reimar.Doeffinger at gmx.de>
> Subject: [FFmpeg-devel] [PATCH] Add support for "omp simd" pragma.
> 
> From: Reimar Döffinger <Reimar.Doeffinger at gmx.de>
> 
> This requests loops to be vectorized using SIMD instructions.
> The performance increase is far from hand-optimized assembly but still
> significant over the plain C version.
> Typical values are a 2-4x speedup where a hand-written version would
> achieve 4x-10x.
> So it is far from a replacement, however some architures will get hand-
> written assembler quite late or not at all, and this is a good improvement for
> a trivial amount of work.
> The cause, besides the compiler being a compiler, is usually that it does not
> manage to use saturating instructions and thus has to use 32-bit operations
> where actually saturating 16-bit operations would be sufficient.
> Other causes are for example the av_clip functions that are not ideal for
> vectorization (and even as scalar code not optimal for any modern CPU that
> has either CSEL or MAX/MIN instructions).
> And of course this only works for relatively simple loops, the IDCT functions
> for example seemed not possible to optimize that way.

...

> +if enabled openmp_simd; then
> +    ompopt="-fopenmp"
> +    if ! test_cflags $ompopt ; then
> +        test_cflags -Xpreprocessor -fopenmp && ompopt="-Xpreprocessor -
> fopenmp"

Isn't it sufficient to specify -fopenmp-simd instead of -fopenmp for this patch?

As OMP SIMD is the only openmp feature that is used, there's no need to link
to the openmp lib. 

softworkz