[Bug tree-optimization/58143] [4.8/4.9 regression] wrong code at -O3

[bernd.edlinger at hotmail dot de]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58143

--- Comment #24 from Bernd Edlinger <bernd.edlinger at hotmail dot de> ---
Wall,

the logic behind the loop niter is really strange
and the results are simply insane.

When you consider this example:

int a, b, c, d, e;

int
main ()
{
  for (b = 4; b > -30; b--)
    {
    e = 2147483647 - b;
    for (; c;)
      for (;;)
        {
          if (d)
            break;
        }
    }
  return 0;
}

then the reasoning is that an "undefined" value will
be written to e in loop iteration #4.
And therefore it does not matter what happens afterwards.
But for the result of main() which will ultimately be 0
the undefined value of e does not matter.

Why should a undefined value that is not used for anything
create such a problem?