From 66edd8656b851a0c85ba25ec293cc66192c363ae Mon Sep 17 00:00:00 2001 From: Ganesh Ajjanagadde Date: Tue, 8 Mar 2016 22:08:29 -0500 Subject: [PATCH] lavc/lpc: exploit even symmetry of window function 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 Reviewed-by: Rostislav Pehlivanov Signed-off-by: Ganesh Ajjanagadde --- libavcodec/lpc.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/libavcodec/lpc.c b/libavcodec/lpc.c index 3839119cc2..052aeaa191 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);