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

[Reimar Döffinger]

> On 12 Jan 2021, at 19:52, Soft Works <softworkz at hotmail.com> wrote:
> 
> 
> 
>> -----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?

I think so, I just didn’t know/even expect that option to exist!
Thanks a lot for the tip!

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


That it doesn’t do anyway because -fopenmp is not in the linker flags,
but I admit that was a bit of a hacky solution.

Thanks,
Reimar