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Use cbrtf and exp2f instead of pow to calculate tables for MPEG audio decoding.

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This hopefully is fast enough so that it is reasonable to use the same formula
directly instead of the table for CONFIG_SMALL.

Originally committed as revision 20233 to svn://svn.ffmpeg.org/ffmpeg/trunk
This commit is contained in:
Reimar Döffinger 2009-10-15 06:59:23 +00:00
parent 16aec74761
commit 686884da9d
1 changed files with 4 additions and 2 deletions
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6
libavcodec/mpegaudiodec.c
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@ -408,9 +408,10 @@ static av_cold int decode_init(AVCodecContext * avctx)
int_pow_init();
for(i=1;i<TABLE_4_3_SIZE;i++) {
double value = i/4;
double f, fm;
int e, m;
f = pow((double)(i/4), 4.0 / 3.0) * pow(2, (i&3)*0.25);
f = value * cbrtf(value) * exp2f((i&3)*0.25);
fm = frexp(f, &e);
m = (uint32_t)(fm*(1LL<<31) + 0.5);
e+= FRAC_BITS - 31 + 5 - 100;
@ -420,8 +421,9 @@ static av_cold int decode_init(AVCodecContext * avctx)
table_4_3_exp[i] = -e;
}
for(i=0; i<512*16; i++){
double value = i & 15;
int exponent= (i>>4);
double f= pow(i&15, 4.0 / 3.0) * pow(2, (exponent-400)*0.25 + FRAC_BITS + 5);
double f= value * cbrtf(value) * exp2f((exponent-400)*0.25 + FRAC_BITS + 5);
expval_table[exponent][i&15]= llrint(f);
if((i&15)==1)
exp_table[exponent]= llrint(f);