Feb 10, 2021, 21:31 by d...@lynne.ee: > 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. >
Will push this tomorrow if there are no objections. _______________________________________________ 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".
