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

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);