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

[Lynne]

Feb 10, 2021, 21:31 by dev at lynne.ee:

> 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.
>

Will push this tomorrow if there are no objections.