[FFmpeg-devel] [PATCH] lavu/tx: support in-place FFT transforms

[Lynne]

Feb 10, 2021, 18:15 by dev at 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.