I tried to compile x264 out of curiosity and when I finally built it I tried to
run the code and got error message from x264 encoder saying that it was
miscompiled.
After debugging the code I found out what was the source of error and what code
was miscompiled.
Basically, log2 and log2f function return wrong results. So, I'm curios if it's
a known issue and if there might be some other unpleasant surprises with some
other math functions.
A simple test to verify wrong results:
#include <math.h>
#define log2f_correct(x) (logf(x) * 1.44269504088896340736f)
for(int lambda=0; lambda<5; ++lambda){
for(int i=2010; i<2020; ++i){
printf ("TEST1: lambda: %d, xxx => %f; log2f(i+1) => %f\n",
lambda, lambda * (log2f(i+1)*2 + 0.718f + !!i) + .5f, log2f(i+1));
printf("TEST2: lambda: %d, xxx => %f; log2f_correct (i+1) =>
%f\n", lambda, lambda * (log2f_correct (i+1)*2 + 0.718f + !!i) + .5f,
log2f_correct(i+1));
}
}
Results are always incorrect, here's a sample output:
[INFO ] [-2071554222] 18:28:17: x264: TEST2: lambda: 1, 2013 xxx => 24.168262;
log2f_correct (i+1) => 10.975131
[INFO ] [-2071554222] 18:28:17: x264: TEST1: lambda: 1, 2013 xxx => 12.764759;
log2f(i+1) => 5.273379
It has to be some sort of loop with expression, otherwise gcc will compute
log2f at compile time and will produce correct output (that matches results of
log2f_correct)
I verified it with both builds of cegcc and had identical errors.
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