[FFmpeg-devel] [PATCH] avcodec/nellymoserenc: avoid wasteful pow

[Michael Niedermayer]

On Wed, Dec 09, 2015 at 06:55:25PM -0500, Ganesh Ajjanagadde wrote:
> exp2 suffices here. Some trivial (~ 4x) speedup is done in addition here by
> reusing results.
> 

> This is bit-identical to the old values.

That may be true with a specific compiler and version but
C makes no such guarantee unless iam missing something 


> 
> Signed-off-by: Ganesh Ajjanagadde <gajjanagadde at gmail.com>
> ---
>  libavcodec/nellymoserenc.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/libavcodec/nellymoserenc.c b/libavcodec/nellymoserenc.c
> index d998dba..e6023e3 100644
> --- a/libavcodec/nellymoserenc.c
> +++ b/libavcodec/nellymoserenc.c
> @@ -179,8 +179,15 @@ static av_cold int encode_init(AVCodecContext *avctx)
>  
>      /* Generate overlap window */
>      ff_init_ff_sine_windows(7);
> -    for (i = 0; i < POW_TABLE_SIZE; i++)
> -        pow_table[i] = pow(2, -i / 2048.0 - 3.0 + POW_TABLE_OFFSET);
> +    pow_table[0] = 1;
> +    pow_table[1024] = M_SQRT1_2;
> +    for (i = 1; i < 513; i++) {
> +        double tmp = exp2(-i / 2048.0);
> +        pow_table[i] = tmp;
> +        pow_table[1024-i] = M_SQRT1_2 / tmp;
> +        pow_table[1024+i] = tmp * M_SQRT1_2;
> +        pow_table[2048-i] = 0.5 / tmp;

how much overall init time is gained by this ?
that is time in ffmpeg main() from start to finish when just opening
the file with no decoding aka ./ffmpeg -i somefile
?

iam asking as this makes the intend of the loop harder to
see/understand
if this change is done then please add a comment explaining what the
code does or maybe leave the unoptimized code in a comment

either way replacing pow -> exp2 LGTM

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Why not whip the teacher when the pupil misbehaves? -- Diogenes of Sinope
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