Created attachment 29704 [details]
g++ -v output

For background see http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36996

When compiled with -Os, the program below does not terminate. 

Here is my opinion: `mask` becomes undefined and it cause an unrelated variable `maskI' to become undefined. If that is a valid compilation of the program, it would make it very hard for programmers to reason about correctness.

#include <stdio.h>
int main ()
{
  int mask = 0, maskI;    
  for (maskI = 0; maskI < 4; maskI++) {              
    printf ("y %d\n", mask);
    mask += 0x55555555;
  }
  return 0;
}