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
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