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

[Michael Niedermayer]

On Tue, Mar 08, 2016 at 10:16:50PM -0500, Ganesh Ajjanagadde 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));

the windows should not be recalcuated for each frame. But either
calculated once during init or calculated at first use if that makes
sense

actually i suspect there should be close to 0 calls to libc math
functions after init in the encoder, or is there some use of these
functions in aac that cannot be replaced by a simple table?

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

Rewriting code that is poorly written but fully understood is good.
Rewriting code that one doesnt understand is a sign that one is less smart
then the original author, trying to rewrite it will not make it better.
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