[FFmpeg-devel] [PATCH] swresample/resample: improve bessel function accuracy

[Timothy Gu]

On Tue, Nov 3, 2015 at 4:47 AM Ganesh Ajjanagadde > - GCC vectorization
slows down compilation A LOT in all versions. The newer

> > the worse.
>
> A ~ 20% slowdown on a build for a ~ 20% improvement in an overall FATE
> bench - sounds like a win to me especially with ccache.


Of course, but unfortunately the FATE improvement is on the order of 1-2%,
i.e. near negligible, rather than ~20%.


>
> > - If you are developing, use clang, and DON'T use GCC 5 with
> vectorization.
>
> This is an opinion, so I will state mine here: if you are developing
> use ccache + GCC > ccache + clang > clang = gcc. Reason for the first
> is due to the terrible interaction ccache has with clang. I still will
> use GCC 5.2 + ccache (with vectorization) for my builds, and will
> inform Arch packagers once we have finalized configure in this respect
> :).
>

Personally ccache has proved itself to be more of a trouble than its worth
in my experiences, but if you have a fast enough machine or ccache working
that it doesn't matter.


>
> > - For release builds, an option to turn it on (or rather to not turn it
> > off) would be helpful; but if you really care about performance _that_
> much
> > then you should probably use some other compilers instead.
>
> No, not true at all. Why do we bother with asm? Many times for such
> "last mile" optimizations. A 20% improvement in FATE across board is
> nothing to sneeze at given what I have seen in FFmpeg.


Again, the improvement is more like 2%.

Furthermore, barring ICC (for Intel), Clang and GCC are among the best
> quality compilers today. I don't know about what "other compilers" you
> are referring to.
>

I meant ICC, which has a much more mature vectorizer.

Timothy