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

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);