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

[ppluzhnikov at google dot com]

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;

}