On Wed, Nov 04, 2015 at 10:08:27PM -0500, Ganesh Ajjanagadde wrote: > This uses the trigonometric double and triple angle formulae to avoid > repeated (expensive) evaluation of libc's cos(). > > Sample benchmark (x86-64, Haswell, GNU/Linux) > old: > 1104466600 decicycles in build_filter(loop 1000), 256 runs, 0 skips > 1096765286 decicycles in build_filter(loop 1000), 512 runs, 0 skips > 1070479590 decicycles in build_filter(loop 1000), 1024 runs, 0 skips > > new: > 588861423 decicycles in build_filter(loop 1000), 256 runs, 0 skips > 591262754 decicycles in build_filter(loop 1000), 512 runs, 0 skips > 577355145 decicycles in build_filter(loop 1000), 1024 runs, 0 skips > > This results in small differences with the old expression: > difference (worst case on [0, 2*M_PI]), argmax 0.008: > max diff (relative): 0.000000000000157289807188 > blackman_old(0.008): 0.000363951585488813192382 > blackman_new(0.008): 0.000363951585488755946507 > > These are judged to be insignificant for the performance gain. PSNR to > reference file is unchanged up to second decimal point for instance. > > Signed-off-by: Ganesh Ajjanagadde <gajjanaga...@gmail.com> > --- > libswresample/resample.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/libswresample/resample.c b/libswresample/resample.c > index a2cbb48..5d32cc2 100644 > --- a/libswresample/resample.c > +++ b/libswresample/resample.c > @@ -171,7 +171,7 @@ static int build_filter(ResampleContext *c, void *filter, > double factor, int tap > break;} > case SWR_FILTER_TYPE_BLACKMAN_NUTTALL: > w = 2.0*x / (factor*tap_count) + M_PI; > - y *= 0.3635819 - 0.4891775 * cos(w) + 0.1365995 * cos(2*w) - > 0.0106411 * cos(3*w); > + y *= 0.3635819 - 0.4891775 * cos(w) + 0.1365995 * > (2*cos(w)*cos(w)-1) - 0.0106411 * (4*cos(w)*cos(w)*cos(w) - 3*cos(w));
i would use a temporary variable for cos(w) either way LGTM thx [...] -- Michael GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB Avoid a single point of failure, be that a person or equipment.
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