[FFmpeg-devel] [RFC] remove table_4_3_value with CONFIG_SMALL

[Reimar Döffinger]

On Tue, Oct 13, 2009 at 01:18:02PM +0200, Michael Niedermayer wrote:
> > Either way, should the code that generates the tables be updated to use
> > the same formula?
> 
> as you prefer ...

Ok, I propose then this patch, to be applied in two parts.
It generally replaces "pow(2," by "exp2f(" and "pow(.., 4.0/3.0)" by
".. * cbrtf(..)".
make test passes on x86_64 at least, with and without --enable-small.
-------------- next part --------------
Index: libavcodec/mpegaudiodec.c
===================================================================
--- libavcodec/mpegaudiodec.c	(revision 20222)
+++ libavcodec/mpegaudiodec.c	(working copy)
@@ -92,10 +92,12 @@
 };
 /* computed from band_size_long */
 static uint16_t band_index_long[9][23];
+#if !CONFIG_SMALL
 /* XXX: free when all decoders are closed */
 #define TABLE_4_3_SIZE (8191 + 16)*4
 static int8_t  table_4_3_exp[TABLE_4_3_SIZE];
 static uint32_t table_4_3_value[TABLE_4_3_SIZE];
+#endif
 static uint32_t exp_table[512];
 static uint32_t expval_table[512][16];
 /* intensity stereo coef table */
@@ -218,6 +220,9 @@
 /* compute value^(4/3) * 2^(exponent/4). It normalized to FRAC_BITS */
 static inline int l3_unscale(int value, int exponent)
 {
+#if CONFIG_SMALL
+    return value * cbrtf(value) * exp2f(exponent*0.25) + 0.5;
+#else
     unsigned int m;
     int e;
 
@@ -230,6 +235,7 @@
     m = (m + (1 << (e-1))) >> e;
 
     return m;
+#endif
 }
 
 /* all integer n^(4/3) computation code */
@@ -407,10 +413,12 @@
         /* compute n ^ (4/3) and store it in mantissa/exp format */
 
         int_pow_init();
+#if !CONFIG_SMALL
         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;
@@ -419,9 +427,11 @@
             table_4_3_value[i] = m;
             table_4_3_exp[i] = -e;
         }
+#endif
         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);