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

[Ganesh Ajjanagadde]

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);
             }
         }
         if (e2 > thresh) {
-- 
2.7.2