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lavc/nellymoserenc: avoid wasteful pow

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exp2 suffices here. Some trivial speedup is done in addition here by
reusing results.

This retains accuracy, and in particular results in identical values
with GNU libm + gcc/clang.

sample benchmark (Haswell, GNU/Linux):
proposed : 424160 decicycles in pow_table,     512 runs,      0 skips
exp2 only: 1262093 decicycles in pow_table,     512 runs,      0 skips
old      : 2849085 decicycles in pow_table,     512 runs,      0 skips

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>
This commit is contained in:
Ganesh Ajjanagadde 2015-12-09 18:50:28 -05:00
parent 39f8c7961d
commit 07a8fbaa55
1 changed files with 11 additions and 1 deletions
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12
libavcodec/nellymoserenc.c
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@ -179,8 +179,18 @@ static av_cold int encode_init(AVCodecContext *avctx)
/* Generate overlap window */
ff_init_ff_sine_windows(7);
/* faster way of doing
for (i = 0; i < POW_TABLE_SIZE; i++)
pow_table[i] = pow(2, -i / 2048.0 - 3.0 + POW_TABLE_OFFSET);
pow_table[i] = 2^(-i / 2048.0 - 3.0 + POW_TABLE_OFFSET); */
pow_table[0] = 1;
pow_table[1024] = M_SQRT1_2;
for (i = 1; i < 513; i++) {
double tmp = exp2(-i / 2048.0);
pow_table[i] = tmp;
pow_table[1024-i] = M_SQRT1_2 / tmp;
pow_table[1024+i] = tmp * M_SQRT1_2;
pow_table[2048-i] = 0.5 / tmp;
}
if (s->avctx->trellis) {
s->opt = av_malloc(NELLY_BANDS * OPT_SIZE * sizeof(float ));