On Wed, Nov 04, 2015 at 01:11:45AM -0500, Ganesh Ajjanagadde wrote:
> This speeds up build_filter by ~ 50%. This gain should be pretty
> consistent across all architectures and platforms.
>
> Essentially, this relies on a observation that the filters have some
> even/odd symmetry that may be exploited during the construction of the
> polyphase filter bank. In particular, phases (scaled to [0, 1]) in [0.5, 1]
> are
> easily derived from [0, 0.5] and expensive reevaluation of function
> points are unnecessary. This requires some rather annoying even/odd
> bookkeeping as can be seen from the patch.
>
> I vaguely recall from signal processing theory more general symmetries
> allowing even greater
> optimization of the construction. At a high level, "even functions"
> correspond to 2, and one can imagine variations. Nevertheless, for the sake
> of some generality and because of existing filters, this is all that is
> being exploited.
>
> Currently, this patch relies on phase_count being even or (trivially) 1,
> though this is not an inherent limitation to the approach. This
> assumption is safe as phase_count is 1 << phase_bits, and is hence a
> power of two. There is no way for user API to set it to a nontrivial odd
> number. This assumption has been placed as an assert in the code.
>
> To repeat, this assumes even symmetry of the filters, which is the most common
> way to get generalized linear phase anyway and is true of all currently
> supported filters.
>
> As a side note, accuracy should be identical or perhaps slightly better
> due to this "forcing" filter symmetries leading to a better phase
> characteristic. As before, I can't test this claim easily, though it may
> be of interest.
>
> Patch tested with FATE.
>
> Sample benchmark (x86-64, Haswell, GNU/Linux):
>
> test: swr-resample-dblp-44100-2626
>
> new:
> 527376779 decicycles in build_filter(loop 1000), 256 runs, 0 skips
> 524361765 decicycles in build_filter(loop 1000), 512 runs, 0 skips
> 516552574 decicycles in build_filter(loop 1000), 1024 runs, 0 skips
>
> old:
> 974178658 decicycles in build_filter(loop 1000), 256 runs, 0 skips
> 972794408 decicycles in build_filter(loop 1000), 512 runs, 0 skips
> 954350046 decicycles in build_filter(loop 1000), 1024 runs, 0 skips
>
> Note that lower level optimizations are entirely possible, I focussed on
> getting the high level semantics correct. In any case, this should
> provide a good foundation.
>
> Signed-off-by: Ganesh Ajjanagadde <gajjanaga...@gmail.com>
> ---
> libswresample/resample.c | 56
> +++++++++++++++++++++++++++++++++++++++++-------
> 1 file changed, 48 insertions(+), 8 deletions(-)
>
> diff --git a/libswresample/resample.c b/libswresample/resample.c
> index 235c9a9..d5fc5e7 100644
> --- a/libswresample/resample.c
> +++ b/libswresample/resample.c
> @@ -144,7 +144,7 @@ static int build_filter(ResampleContext *c, void *filter,
> double factor, int tap
> int filter_type, int kaiser_beta){
> int ph, i;
> double x, y, w;
> - double *tab = av_malloc_array(tap_count, sizeof(*tab));
> + double *tab = av_malloc_array(tap_count+1, sizeof(*tab));
> const int center= (tap_count-1)/2;
>
> if (!tab)
> @@ -154,9 +154,10 @@ static int build_filter(ResampleContext *c, void
> *filter, double factor, int tap
> if (factor > 1.0)
> factor = 1.0;
>
> - for(ph=0;ph<phase_count;ph++) {
> + av_assert0(phase_count == 1 || phase_count % 2 == 0);
> + for(ph = 0; ph <= phase_count / 2; ph++) {
> double norm = 0;
> - for(i=0;i<tap_count;i++) {
> + for(i=0;i<=tap_count;i++) {
> x = M_PI * ((double)(i - center) - (double)ph / phase_count) *
> factor;
> if (x == 0) y = 1.0;
> else y = sin(x) / x;
> @@ -180,26 +181,65 @@ static int build_filter(ResampleContext *c, void
> *filter, double factor, int tap
> }
>
> tab[i] = y;
> - norm += y;
> + if (i < tap_count)
> + norm += y;
> }
>
> /* normalize so that an uniform color remains the same */
> switch(c->format){
> case AV_SAMPLE_FMT_S16P:
> - for(i=0;i<tap_count;i++)
> + for(i=0;i<tap_count;i++) {
> ((int16_t*)filter)[ph * alloc + i] = av_clip(lrintf(tab[i] *
> scale / norm), INT16_MIN, INT16_MAX);
> + }
> + if (tap_count % 2 == 0) {
> + for (i = 0; i < tap_count; i++)
> + ((int16_t*)filter)[(phase_count-ph) * alloc +
> tap_count-1-i] = ((int16_t*)filter)[ph * alloc + i];
> + }
> + else {
> + for (i = 1; i <= tap_count; i++)
> + ((int16_t*)filter)[(phase_count-ph) * alloc +
> tap_count-i] =
> + av_clip(lrintf(tab[i] * scale / (norm - tab[0] +
> tab[tap_count])), INT16_MIN, INT16_MAX);
> + }
> break;
> case AV_SAMPLE_FMT_S32P:
> - for(i=0;i<tap_count;i++)
> + for(i=0;i<tap_count;i++) {
> ((int32_t*)filter)[ph * alloc + i] =
> av_clipl_int32(llrint(tab[i] * scale / norm));
> + }
this and similar changes look uneeded
otherwise LGTM
[...]
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