[SCM] libav/experimental: 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.

[siretart at users.alioth.debian.org]

The following commit has been merged in the experimental branch:
commit 686884da9d686d7498e246fe0e7146767b21aa42
Author: Reimar Döffinger <Reimar.Doeffinger at gmx.de>
Date:   Thu Oct 15 06:59:23 2009 +0000

    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

diff --git a/libavcodec/mpegaudiodec.c b/libavcodec/mpegaudiodec.c
index dd986bd..e531fd3 100644
--- a/libavcodec/mpegaudiodec.c
+++ b/libavcodec/mpegaudiodec.c
@@ -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);

-- 
Libav/FFmpeg packaging