[FFmpeg-devel] [PATCH] lavc/cook: get rid of wasteful pow in init_pow2table

[Clément Bœsch]

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 <gajjanagadde at 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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