[FFmpeg-devel] [RFC] simplify exp_table generation

[Reimar Döffinger]

On Sat, Nov 07, 2009 at 02:32:30PM +0100, Michael Niedermayer wrote:
> On Fri, Nov 06, 2009 at 08:55:20PM +0100, Reimar D?ffinger wrote:
> > Hello,
> > following patch IMO makes it more obvious that exp_table is just a
> > partial copy of expval_table and avoids the duplicate llrintf.
> 
> if you want to split that code out, i would say it could be split
> in 2 loops like
> for(512)
>     for(16)
> 
> this should be simpler than what you suggest

Alternative suggestion attached.

> > I think I also asked whether it is probably ok to replace the llrintf(f)
> > with f + 0.5, since llrintf is not available everywhere and the
> > libavutil/mathematics.h workaround is difficult to use for the case of
> > hardcoded tables + cross-compiling.
> 
> my gut feeling says no, it will cause float exceptions on some platforms
> like the code that that we had prior to llrintf()

Ah, interesting, I thought that was only for speed reasons.
Anyone remember details (grepping svnlog was no help)?
-------------- next part --------------
Index: libavcodec/mpegaudio_tablegen.h
===================================================================
--- libavcodec/mpegaudio_tablegen.h	(revision 20469)
+++ libavcodec/mpegaudio_tablegen.h	(working copy)
@@ -41,6 +41,7 @@
 static void mpegaudio_tableinit(void)
 {
     int i;
+    int exponent;
     for(i=1;i<TABLE_4_3_SIZE;i++) {
         double value = i/4;
         double f, fm;
@@ -54,13 +55,15 @@
         table_4_3_value[i] = m;
         table_4_3_exp[i] = -e;
     }
-    for(i=0; i<512*16; i++){
-        double value = i & 15;
-        int exponent= (i>>4);
-        double f= value * cbrtf(value) * pow(2, (exponent-400)*0.25 + FRAC_BITS + 5);
-        expval_table[exponent][i&15]= llrint(f);
-        if((i&15)==1)
-            exp_table[exponent]= llrint(f);
+    for(exponent=0; exponent<512; exponent++){
+        int value;
+        double exp = pow(2, (exponent-400)*0.25 + FRAC_BITS + 5);
+        expval_table[exponent][0] = 0;
+        expval_table[exponent][1] = exp_table[exponent] = llrint(exp);
+        for (value = 2; value < 16; value++) {
+            double f= value * cbrtf(value) * exp;
+            expval_table[exponent][value]= llrint(f);
+        }
     }
 }
 #endif /* CONFIG_HARDCODED_TABLES */