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

[Reimar Döffinger]

> 
> On 10 Jan 2021, at 19:55, Lynne <dev at lynne.ee> wrote:
> 
> Jan 10, 2021, 17:43 by Reimar.Doeffinger at gmx.de:
> 
>> From: Reimar Döffinger <Reimar.Doeffinger at gmx.de>
>> 
>> real    0m15.040s
>> user    0m18.874s (80.7% of original)
>> sys     0m0.168s
>> 
> 
> I think I have to disagree.
> The performance gains are marginal,

It’s almost 20%. At least for this combination of
codec and stream a large amount of time is spend in
non-DSP functions, so even hand-written assembler
won’t give you huge gains.


> its definitely something the compiler should
> be able to decide on its own,

So you object to unlikely() macros as well?
It’s really just giving the compiler a hint it should try, though I admit the configure part makes it
look otherwise.

> Most of the loops this is added to are trivially SIMDable.

How many hours of effort do you consider “trivial”?
Especially if it’s someone not experienced?
It might be fairly trivial with intrinsics, however
many of your counter-arguments also apply
to intrinsics (and to a degree inline assembly).
That’s btw not just a rhetorical question because
I’m pretty sure I am not going to all the trouble
to port more of the arm 32-bit assembler functions
since it’s a huge PITA, and I was wondering if there
was a point to even have a try with intrinsics...

> Just because no one has
> had the motivation to do SIMD for a pretty unpopular codec doesn't mean we should
> compromise.

If you think of AArch64 specifically, I can
kind of agree.
However I wouldn’t say the word “compromise”
is appropriate when there’s a good chance nothing
better will ever come to exist.
But the real point is not AArch64, that is just
a very convenient test platform.
The point is to raise the minimum bar.
A new architecture, RISC-V for example or something
else should not be stuck at scalar performance
until someone actually gets around to implementing
assembler optimizations.
And just to be clear: I don’t actually care about
HEVC, it just seemed a nice target to do some
experiments.

Best regards,
Reimar