[FFmpeg-devel] [PATCH 2/2] swresample/resample: speed up Blackman Nuttall filter

[Michael Niedermayer]

On Mon, Nov 09, 2015 at 09:38:44AM -0500, Ganesh Ajjanagadde wrote:
> This may be a slightly surprising optimization, but is actually based on
> an understanding of how math libraries compute trigonometric functions.
> Explanation is given here so that future development uses libm more effectively
> across the codebase.
> 
> All libm's essentially compute transcendental functions via some kind of
> polynomial approximation, be it Taylor-Maclaurin or Chebyshev.
> Correction terms are added via polynomial correction factors when needed
> to squeeze out the last bits of accuracy. Lookup tables are also
> inserted strategically.
> 
> In the case of trigonometric functions, periodicity is exploited via
> first doing a range reduction to an interval around zero, and then using
> some polynomial approximation.
> 
> This range reduction is the most natural way of doing things - else one
> would need polynomials for ranges in different periods which makes no
> sense whatsoever.
> 
> To avoid the need for the range reduction, it is helpful to feed in
> arguments as close to the origin as possible for the trigonometric
> functions. In fact, this also makes sense from an accuracy point of view:
> IEEE floating point has far more resolution for small numbers than big ones.
> 
> This patch does this for the Blackman-Nuttall filter, and yields a
> non-negligible speedup.
> 
> Sample benchmark (x86-64, Haswell, GNU/Linux)
> test: fate-swr-resample-dblp-2626-44100
> old:
> 18893514 decicycles in build_filter (loop 1000),     256 runs,      0 skips
> 18599863 decicycles in build_filter (loop 1000),     512 runs,      0 skips
> 18445574 decicycles in build_filter (loop 1000),    1000 runs,     24 skips
> 
> new:
> 16290697 decicycles in build_filter (loop 1000),     256 runs,      0 skips
> 16267172 decicycles in build_filter (loop 1000),     512 runs,      0 skips
> 16251105 decicycles in build_filter (loop 1000),    1000 runs,     24 skips
> 
> Signed-off-by: Ganesh Ajjanagadde <gajjanagadde at gmail.com>
> ---
>  libswresample/resample.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)

LGTM

thanks

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

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