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

[Ganesh Ajjanagadde]

On Wed, Dec 9, 2015 at 6:55 PM, Ganesh Ajjanagadde
<gajjanagadde at gmail.com> wrote:
> exp2 suffices here. Some trivial (~ 4x) speedup is done in addition here by
> reusing results.
>
> This is bit-identical to the old values.
>
> 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;
> +    }
>
>      if (s->avctx->trellis) {
>          s->opt  = av_malloc(NELLY_BANDS * OPT_SIZE * sizeof(float  ));
> --
> 2.6.3
>

going in tomorrow in the absence of comments since it lacks a maintainer.