[Bug c/44098] New: -O3 causes quickInvSqrt() to return -inf.

["maister at archlinux dot us" <gcc-bugzilla at gcc dot gnu dot org>]

I have tested the famous quickInvSqrt() algorithm uses in Quake, and it does
not compile correctly when compiling with -O3. It does however work completely
fine when compiling for -O0, -O1 and -O2.

The function goes like so:

float quickInvSqrt(float x)
{
   float xhalf = 0.5f*x;
   int i = *(int*)&x;
   i = 0x5f3759df - (i>>1);
   x = *(float*)&i;
   x = x*(1.5f - xhalf*x*x);
   return x;
}


-- 
           Summary: -O3 causes quickInvSqrt() to return -inf.
           Product: gcc
           Version: 4.5.0
            Status: UNCONFIRMED
          Severity: major
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: maister at archlinux dot us
 GCC build triplet: x86_64-unknown-linux-gnu
  GCC host triplet: x86_64-unknown-linux-gnu
GCC target triplet: x86_64-unknown-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44098