From bccc81dfa08e6561df6ed37860e3a08f7d983825 Mon Sep 17 00:00:00 2001 From: Ganesh Ajjanagadde Date: Mon, 7 Mar 2016 21:16:29 -0500 Subject: [PATCH] lavc/aacenc_utils: replace powf(x,y) by expf(logf(x), y) MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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. Made an inline function in lavu/internal.h for this purpose. 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. Reviewed-by: Reimar Döffinger Reviewed-by: Ronald S. Bultje Signed-off-by: Ganesh Ajjanagadde --- libavcodec/aacenc_utils.h | 6 +++++- libavutil/internal.h | 16 ++++++++++++++++ 2 files changed, 21 insertions(+), 1 deletion(-) diff --git a/libavcodec/aacenc_utils.h b/libavcodec/aacenc_utils.h index 41a62961e1..07f733746b 100644 --- a/libavcodec/aacenc_utils.h +++ b/libavcodec/aacenc_utils.h @@ -28,6 +28,7 @@ #ifndef AVCODEC_AACENC_UTILS_H #define AVCODEC_AACENC_UTILS_H +#include "libavutil/internal.h" #include "aac.h" #include "aacenctab.h" #include "aactab.h" @@ -122,7 +123,10 @@ 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 += ff_fast_powf(s / ethresh, nzslope); } } if (e2 > thresh) { diff --git a/libavutil/internal.h b/libavutil/internal.h index da76ca26d3..340e18bc5d 100644 --- a/libavutil/internal.h +++ b/libavutil/internal.h @@ -314,6 +314,22 @@ static av_always_inline float ff_exp10f(float x) return exp2f(M_LOG2_10 * x); } +/** + * Compute x^y for floating point x, y. Note: this function is faster than the + * libm variant due to mainly 2 reasons: + * 1. It does not handle any edge cases. In particular, this is only guaranteed + * to work correctly for x > 0. + * 2. It is not as accurate as a standard nearly "correctly rounded" libm variant. + * @param x base + * @param y exponent + * @return x^y + */ +static av_always_inline float ff_fast_powf(float x, float y) +{ + return expf(logf(x) * y); +} + + /** * A wrapper for open() setting O_CLOEXEC. */