On Mon, Nov 2, 2015 at 8:23 PM Rostislav Pehlivanov <atomnu...@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 _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org http://ffmpeg.org/mailman/listinfo/ffmpeg-devel