From 686884da9d686d7498e246fe0e7146767b21aa42 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Reimar=20D=C3=B6ffinger?= <Reimar.Doeffinger@gmx.de>
Date: Thu, 15 Oct 2009 06:59:23 +0000
Subject: [PATCH] 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 | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/libavcodec/mpegaudiodec.c b/libavcodec/mpegaudiodec.c
index dd986bd859..e531fd37d8 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);