Hi, On Sun, Dec 13, 2015 at 4:59 PM, Ganesh Ajjanagadde <gajjanaga...@gmail.com> wrote:
> fma is a faster function on architectures supporting a native CPU > instruction for it. > This may be tested by the ISO C optionally defined FP_FAST_FMA. Although > in the x86 lineup this came fairly late > (from Haswell onwards, and hence is absent unless appropriate -march is > passed), > numerous other architectures support it: > https://en.wikipedia.org/wiki/Multiply%E2%80%93accumulate_operation. > > Concretely, one can expect ~ 15-25% speedup that is of course heavily > architecture dependent. > > This patch also ensures that as people migrate to newer CPU's, the > benefit will slowly trickle in. > > I doubt this will cause build failures on broken libm's since I can't > imagine a platform where FP_FAST_FMA is defined but the function fma is > absent. > > Sample benchmark (x86-64, Haswell, GNU/Linux under -march=native) > > old: > 515828458 decicycles in build_filter (loop 1000), 1024 runs, 0 > skips > > new (fma): > 435866377 decicycles in build_filter (loop 1000), 1024 runs, 0 > skips > > Tested with FATE. > > Signed-off-by: Ganesh Ajjanagadde <gajjanaga...@gmail.com> > --- > libswresample/resample.c | 4 ++++ > 1 file changed, 4 insertions(+) > > diff --git a/libswresample/resample.c b/libswresample/resample.c > index 34eb4c0..e61d4c5 100644 > --- a/libswresample/resample.c > +++ b/libswresample/resample.c > @@ -33,8 +33,12 @@ static inline double eval_poly(const double *coeff, int > size, double x) { > double sum = coeff[size-1]; > int i; > for (i = size-2; i >= 0; --i) { > +#ifdef FP_FAST_FMA > + sum = fma(sum, x, coeff[i]); > +#else > sum *= x; > sum += coeff[i]; > +#endif > } > return sum; > } > -- > 2.6.4 Nope, this is not how we do CPU-specific optimizations. Check example implementations in libswresample/x86/*.asm and the related init functions plus macros to check for runtime cpu support in libswresample/x86/*_init.c. You want to follow that pattern. Ronald _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org http://ffmpeg.org/mailman/listinfo/ffmpeg-devel
