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• lavu/tx : invert permutation lookups

  27 février 2021, par Lynne

  lavu/tx : invert permutation lookups
  out[lut[i]] = in[i] lookups were 4.04 times(!) slower than
  
  out[i] = in[lut[i]] lookups for an out-of-place FFT of length 4096.
  The permutes remain unchanged for anything but out-of-place monolithic
  
  FFT, as those benefit quite a lot from the current order (it means
  
  there's only 1 lookup necessary to add to an offset, rather than
  
  a full gather).
  The code was based around non-power-of-two FFTs, so this wasn't
  
  benchmarked early on.
  

  • [DH] libavutil/tx.c
  • [DH] libavutil/tx_priv.h
  • [DH] libavutil/tx_template.c

• lsws/ppc/yuv2rgb_altivec : Fix build in non-VSX environments

  10 octobre 2020, par Chip Kerchner

  lsws/ppc/yuv2rgb_altivec : Fix build in non-VSX environments
  Add inline function for vec_xl if VSX is not supported. vec_xl intrinsic
  
  is only available on POWER 7 or higher.
  Fixes ticket #8750.
  Signed-off-by : Andriy Gelman <andriy.gelman@gmail.com>
  

  • [DH] libswscale/ppc/yuv2rgb_altivec.c

• lavu/tx : support in-place FFT transforms

  10 février 2021, par Lynne

  lavu/tx : support in-place FFT transforms
  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.
  

  • [DH] doc/APIchanges
  • [DH] libavutil/tx.c
  • [DH] libavutil/tx.h
  • [DH] libavutil/tx_priv.h
  • [DH] libavutil/tx_template.c
  • [DH] libavutil/version.h