[FFmpeg-devel] [PATCH] lavc/aacenc_utils: replace powf(x, y) by expf(logf(x), y)

[Ronald S. Bultje]

Hi,

On Mon, Mar 7, 2016 at 10:48 PM, Ganesh Ajjanagadde <gajjanag at gmail.com>
wrote:

> This is ~2x faster for y not an integer on Haswell+GCC, and should
> generally be faster due to the fact that anyway powf essentially does
> this under the hood.
>
> Note that there are some accuracy differences, that should generally be
> negligible. In particular, FATE still passes on this platform.
>
> Results in ~ 7% speedup in aac encoding with -march=native, Haswell+GCC.
> before:
> ffmpeg -i sin.flac -acodec aac -y sin_new.aac  6.05s user 0.06s system
> 104% cpu 5.821 total
>
> after:
> ffmpeg -i sin.flac -acodec aac -y sin_new.aac  5.67s user 0.03s system
> 105% cpu 5.416 total
>
> This is also faster than an alternative approach that pulls in powf, gets
> rid of
> the crufty NaN checks and other special cases, exploits knowledge about
> the intervals, etc.
> This of course does not exclude smarter approaches; just suggests that
> there would need to be significant work on this front of lower utility than
> searches for hotspots elsewhere.
>
> Signed-off-by: Ganesh Ajjanagadde <gajjanag at gmail.com>
> ---
>  libavcodec/aacenc_utils.h | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/libavcodec/aacenc_utils.h b/libavcodec/aacenc_utils.h
> index 56e3462..b7f80c6 100644
> --- a/libavcodec/aacenc_utils.h
> +++ b/libavcodec/aacenc_utils.h
> @@ -121,7 +121,10 @@ static inline float find_form_factor(int group_len,
> int swb_size, float thresh,
>              if (s >= ethresh) {
>                  nzl += 1.0f;
>              } else {
> -                nzl += powf(s / ethresh, nzslope);
> +                if (nzslope == 2.f)
> +                    nzl += (s / ethresh) * (s / ethresh);
> +                else
> +                    nzl += expf(logf(s / ethresh) * nzslope);
>              }
>          }
>

There's two changes here. Which gives the speedup? I don't like the second
(pow -> exp(log())) if it doesn't give a speedup (I don't see why it would,
also).

Ronald