On Tue, Dec 29, 2015 at 09:28:34AM -0800, Ganesh Ajjanagadde wrote:
> 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) */
I'm pretty sure you can do
float exp2_val = pow(2, -63);
float root_val = pow(2, -32);
and compilers will inline them
[...]
--
Clément B.
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