1. ffmpeg
  2. Commit 66edd8656b851a0c85ba25ec293cc66192c363ae
Browse code

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 <atomnuker@gmail.com>
Signed-off-by: Ganesh Ajjanagadde <gajjanag@gmail.com>

Ganesh Ajjanagadde authored on 2016/03/09 12:08:29
Showing 1 changed files
libavcodec/lpc.c
History View file @ 66edd86
... ... 
@@ -176,9 +176,10 @@ double ff_lpc_calc_ref_coefs_f(LPCContext *s, const float *samples, int len,
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     const double a = 0.5f, b = 1.0f - a;
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     /* Apply windowing */
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-    for (i = 0; i < len; i++) {
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+    for (i = 0; i <= len / 2; i++) {
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         double weight = a - b*cos((2*M_PI*i)/(len - 1));
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         s->windowed_samples[i] = weight*samples[i];
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+        s->windowed_samples[len-1-i] = weight*samples[len-1-i];
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     }
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     s->lpc_compute_autocorr(s->windowed_samples, len, order, autoc);