[Bug tree-optimization/53265] Warn when undefined behavior implies smaller iteration count

["ppluzhnikov at google dot com" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53265

--- Comment #27 from Paul Pluzhnikov <ppluzhnikov at google dot com> 2013-04-29 23:18:29 UTC ---
Here is a reduced test case in which g++ (GCC) 4.9.0 20130426 (experimental)
produces infinite loop with -O2 due to aggressive loop optimization, but
doesn't warn (making the bug in user code hard to find).

g++ -O2 test.cc -Wall && ./a.out
Segmentation fault

g++ -O2 test.cc -Wall -fno-aggressive-loop-optimizations && ./a.out && echo ok
ok


struct Puzzle
{
  int r[9];
  int ignored[18];
} p;

int a, b;
int main()
{
  int c = 0;
  for (b = 0; b < 27; b++)
    {
      if (p.r[b] == 0)
        c |= p.r[a];
      if (c != 0)
        return c;
    }
  return c;
}