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authorRostislav Pehlivanov <atomnuker@gmail.com>2017-01-04 09:23:24 +0000
committerRostislav Pehlivanov <atomnuker@gmail.com>2017-01-05 22:32:02 +0000
commit2d208aaabe2082895a4b4762a867296792503be3 (patch)
tree98a3dbbf9df0efae57d7b39b6c1eb7249439ae76 /libavutil
parent4fdacf4cdbb3899d4bebf15f000aeaa4be4ea8ea (diff)
downloadffmpeg-2d208aaabe2082895a4b4762a867296792503be3.tar.gz
imdct15: replace the FFT with a faster PFA FFT algorithm
This commit replaces the current inefficient non-power-of-two FFT with a much faster FFT based on the Prime Factor Algorithm. Although it is already much faster than the old algorithm without SIMD, the new algorithm makes use of the already very throughouly SIMD'd power of two FFT, which improves performance even more across all platforms which we have SIMD support for. Most of the work was done by Peter Barfuss, who passed the code to me to implement into the iMDCT and the current codebase. The code for a 5-point and 15-point FFT was derived from the previous implementation, although it was optimized and simplified, which will make its future SIMD easier. The 15-point FFT is currently using 6% of the current overall decoder overhead. The FFT can now easily be used as a forward transform by simply not multiplying the 5-point FFT's imaginary component by -1 (which comes from the fact that changing the complex exponential's angle by -1 also changes the output by that) and by multiplying the "theta" angle of the main exptab by -1. Hence the deliberately left multiplication by -1 at the end. FATE passes, and performance reports on other platforms/CPUs are welcome. Performance comparisons: iMDCT, PFA: 101127 decicycles in speed, 32765 runs, 3 skips iMDCT, Old: 211022 decicycles in speed, 32768 runs, 0 skips Standalone FFT, 300000 transforms of size 960: PFA Old FFT kiss_fft libfftw3f 3.659695s, 15.726912s, 13.300789s, 1.182222s Being only 3x slower than libfftw3f is a big achievement by itself. There appears to be something capping the performance in the iMDCT side of things, possibly during the pre-stage reindexing. However, it is certainly fast enough for now. Signed-off-by: Rostislav Pehlivanov <atomnuker@gmail.com>
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