[Bug middle-end/33780] different results between O3 and O0

["rguenth at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #1 from rguenth at gcc dot gnu dot org  2007-10-15 15:01 -------
Confirmed.  It changes for me -O2 vs. -O2 -fpeel-loops which causes the loop
in f to be completely unrolled (which at the first sight doesn't look wrong).
And we expand __powidf2 inline because the exponent is now a constant for all
calls.  This indeed might explain the difference as __powidf2 is implemented
as

TYPE
NAME (TYPE x, int m)
{
  unsigned int n = m < 0 ? -m : m;
  TYPE y = n % 2 ? x : 1;
  while (n >>= 1)
    {
      x = x * x;
      if (n % 2)
        y = y * x;
    }
  return m < 0 ? 1/y : y;
}  

while we expand a constant integral power to an optimal (as of the count of
multiplications / additions) series of multiplications and additions.

As Fortran allows open-coding of integral powers this is not a bug.  But
I'll leave final closing as INVALID to more fortran-knowing people.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
   Last reconfirmed|0000-00-00 00:00:00         |2007-10-15 15:01:08
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=33780