[FFmpeg-devel] [PATCH] lavc/lpc: exploit even symmetry of window function

[Rostislav Pehlivanov]

On 9 March 2016 at 03:16, Ganesh Ajjanagadde <gajjanag at gmail.com> wrote:

> Yields 2x improvement in function performance, and boosts aac encoding
> speed by ~ 4% overall. Sample benchmark (Haswell+GCC under -march=native):
> after:
> ffmpeg -i sin.flac -acodec aac -y sin_new.aac  5.22s user 0.03s system
> 105% cpu 4.970 total
>
> before:
> ffmpeg -i sin.flac -acodec aac -y sin_new.aac  5.40s user 0.05s system
> 105% cpu 5.162 total
>
> Big shame that len-1 is -1 mod 4; 0 mod 4 would have yielded a further 2x
> through
> additional symmetry. Of course, one could approximate with the 0 mod 4
> variant,
> error would essentially be ~ 1/len in the worst case.
>
> Signed-off-by: Ganesh Ajjanagadde <gajjanag at gmail.com>
> ---
>  libavcodec/lpc.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/libavcodec/lpc.c b/libavcodec/lpc.c
> index 3839119..052aeaa 100644
> --- a/libavcodec/lpc.c
> +++ b/libavcodec/lpc.c
> @@ -176,9 +176,10 @@ double ff_lpc_calc_ref_coefs_f(LPCContext *s, const
> float *samples, int len,
>      const double a = 0.5f, b = 1.0f - a;
>
>      /* Apply windowing */
> -    for (i = 0; i < len; i++) {
> +    for (i = 0; i <= len / 2; i++) {
>          double weight = a - b*cos((2*M_PI*i)/(len - 1));
>          s->windowed_samples[i] = weight*samples[i];
> +        s->windowed_samples[len-1-i] = weight*samples[len-1-i];
>      }
>
>      s->lpc_compute_autocorr(s->windowed_samples, len, order, autoc);
> --
>

LGTM, doesn't add too much complexity and it does give a slight speedup.