Broken optimization of pow (x, 1.5) and other halfs of integers..

[Nils Pipenbrinck <n dot pipenbrinck at cubic dot org>]

Hi folks.

While optimizing some of my code I replaced powf (x, 1.5f) with x * 
sqrt(x). Out of couriosity I checked if GCC does this optimization and 
found it in the code. It's in expand_builtin_pow in the file builtin.c 
(gcc 4.3.1 source).

However, GCC does not apply this optimization for a reason or another. 
If I change the constant to 3.0f I get optimized code from another 
special case expansion branch in that function (whole integers). Any 
ideas what's wrong here?


The test-code, compiled with gcc (4.3.1) test.c 
-funsafe-math-optimizations -O3 -S

#include <math.h>
float test (float arg)
{
 return powf (arg, 1.5f);
}

If the optimization is triggered the disassembly shouldn't have a call 
to _powf  anymore.

Should I file a bug-report?

Cheers,
   Nils