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

[Timothy Gu]

On Mon, Nov 2, 2015 at 8:23 PM Rostislav Pehlivanov <atomnuker at gmail.com>
wrote:

> >if one removes the crippling
> >-fno-tree-vectorize
> Yes, I think a config option to turn this flag on (like the unsafe
> bitstream reader) would be good. Defaulting to off by default if it doesn't
> break anything for at least a few people (and compilers) who test it. It's
> not a big performance impact but every little bit counts nowadays.
>

FWIW, I recently (i.e. 2 days ago) did some tests with auto-vectorization
and a few compilers. Fortunately, none of the compilers I tested caused any
miscompilation, when purely measured by FATE:

compiling:
clang3.7 4m3.034s
gcc5vectorize 5m50.637s (1.14x gcc5)
gcc5 5m7.262s
gcc4.9vectorize 5m29.669s (1.11x gcc4.9)
gcc4.9 4m54.602s
gcc4.8vectorize 5m18.848s (1.09x gcc4.8)
gcc4.8 4m53.940s

FATE:
clang3.7 3m13.923s
gcc5vectorize 3m5.988s (0.980x gcc5)
gcc5 3m9.618s
gcc4.9vectorize 3m12.880s (0.983x gcc4.9)
gcc4.9 3m16.563s
gcc4.8vectorize 3m10.321s (0.993x gcc4.8)
gcc4.8 3m11.608s

Tested with:
- Debian jessie/stable/8.2
- Dual-core Haswell i7 ultra low voltage
- clang-3.7 3.7.0-svn251177-1~exp1 (from the offical clang apt repo)
- gcc-5 (Debian 5.2.1-22) 5.2.1 20151010 (Debian testing stock)
- gcc-4.9 (Debian 4.9.2-10) 4.9.2 (Debian stable stock)
- gcc-4.8 (Debian 4.8.4-1) 4.8.4 (Debian stable stock)

Note that FATE is probably the worst benchmark one can find, but it does
show something.

Some observations:

- GCC vectorization slows down compilation A LOT in all versions. The newer
the worse.
- If you are developing, use clang, and DON'T use GCC 5 with vectorization.
- 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.

FYI, as I have told Ganesh so in our private exchanges, I did also test
vectorization on GCC 4.6 on a Ubuntu 12.04/Precise box, which miscompiled
the code hilariously, _and_ made the code slower, just as illustrated in
Mans's commit message.

Timothy