The table is highly structured, so pow (or exp2 for that matter) can entirely
be avoided, yielding a ~ 40x speedup with no loss of accuracy.
sample benchmark (Haswell, GNU/Linux):
new:
4449 decicycles in init_pow2table(loop 1000), 254 runs, 2 skips
4411 decicycles in init_pow2table(loop 1000), 510 runs, 2 skips
4391 decicycles in init_pow2table(loop 1000), 1022 runs, 2 skips
old:
183673 decicycles in init_pow2table(loop 1000), 256 runs, 0 skips
182142 decicycles in init_pow2table(loop 1000), 512 runs, 0 skips
182104 decicycles in init_pow2table(loop 1000), 1024 runs, 0 skips
Signed-off-by: Ganesh Ajjanagadde <gajjanaga...@gmail.com>
---
libavcodec/cook.c | 11 +++++++++--
1 file changed, 9 insertions(+), 2 deletions(-)
diff --git a/libavcodec/cook.c b/libavcodec/cook.c
index d8fb736..aa434a2 100644
--- a/libavcodec/cook.c
+++ b/libavcodec/cook.c
@@ -166,10 +166,17 @@ static float rootpow2tab[127];
/* table generator */
static av_cold void init_pow2table(void)
{
+ /* fast way of computing 2^i and 2^(0.5*i) for -63 <= i < 64 */
int i;
+ static const float exp2_tab[2] = {1, M_SQRT2};
+ float exp2_val = 1.0842021724855044e-19; /* 2^(-63) */
+ float root_val = 2.3283064365386963e-10; /* 2^(-32) */
for (i = -63; i < 64; i++) {
- pow2tab[63 + i] = pow(2, i);
- rootpow2tab[63 + i] = sqrt(pow(2, i));
+ if (!(i & 1))
+ root_val *= 2;
+ pow2tab[63 + i] = exp2_val;
+ rootpow2tab[63 + i] = root_val * exp2_tab[i & 1];
+ exp2_val *= 2;
}
}
--
2.6.4
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