Feb 10, 2021, 18:15 by d...@lynne.ee: > This commit adds support for in-place FFT transforms. Since our > internal transforms were all in-place anyway, this only changes > the permutation on the input. > > Unfortunately, research papers were of no help here. All focused > on dry hardware implementations, where permutes are free, or on > software implementations where binary bloat is of no concern so > storing dozen times the transforms for each permutation and version > is not considered bad practice. > Still, for a pure C implementation, it's only around 28% slower > than the multi-megabyte FFTW3 in unaligned mode. > > Unlike a closed permutation like with PFA, split-radix FFT bit-reversals > contain multiple NOPs, multiple simple swaps, and a few chained swaps, > so regular single-loop single-state permute loops were not possible. > Instead, we filter out parts of the input indices which are redundant. > This allows for a single branch, and with some clever AVX512 asm, > could possibly be SIMD'd without refactoring. > > The inplace_idx array is guaranteed to never be larger than the > revtab array, and in practice only requires around log2(len) entries. > > The power-of-two MDCTs can be done in-place as well. And it's > possible to eliminate a copy in the compound MDCTs too, however > it'll be slower than doing them out of place, and we'd need to dirty > the input array. > > Patch attached. >
Locally added APIchanges and lavu minor bump. And got rid of the unused set temporary variables when permuting. _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org https://ffmpeg.org/mailman/listinfo/ffmpeg-devel To unsubscribe, visit link above, or email ffmpeg-devel-requ...@ffmpeg.org with subject "unsubscribe".
