[Bug rtl-optimization/29798] [4.3 Regression] -O2 gives wrong results

[rguenth at gcc dot gnu dot org]

------- Comment #1 from rguenth at gcc dot gnu dot org  2006-11-11 11:46 -------
Confirmed.  Shorter testcase:

#include <stdio.h>

int main()
{
  double rho;
  double alpha;
  int iter_count;
  double oldrho;
  double work;

  work    = 2.0e+0;

  for (iter_count=1; iter_count<11; ++iter_count)
  {
    fprintf(stderr, "1: %g\n", oldrho);
    rho = 1 + work * work;
    fprintf(stderr, "2: %g\n\n", oldrho);

    alpha = rho / (5.0e-1 * work * work);
    work = work - alpha * work;

    oldrho = rho;
  }

  return 0;
}

It should print the same "oldrho" value at "1:" and "2:" each iteration
(but the first one, because oldrho is not initialized then, so the behavior
is undefined).

Appearantly this undefinedness in the first iteration makes the difference
here.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rguenth at gcc dot gnu dot
                   |                            |org
             Status|UNCONFIRMED                 |NEW
          Component|tree-optimization           |rtl-optimization
     Ever Confirmed|0                           |1
           Keywords|                            |wrong-code
      Known to fail|                            |4.3.0
      Known to work|                            |4.1.2
   Last reconfirmed|0000-00-00 00:00:00         |2006-11-11 11:46:11
               date|                            |
            Summary|-O2 gives wrong results     |[4.3 Regression] -O2 gives
                   |                            |wrong results


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29798