Comment # 9 on bug 1169156 from Petr Tesa����k

I'm afraid this is not quite what I mean. The issue is that
__builtin_constant_p() is not tightly coupled with the knowledge of something
actually being constant. So, you process code that looks (roughly) like:

  var = 1;
  if (something)
    cond = __builtin_constant_p(var);
  else {
    var = unknown_result();
    cond = __builtin_constant_p(var);
  }
  if (cond) {
    d = y / var;
  }

If cond is true, then var must be 1 (known to be constant), so GCC could
optimize the expression (y / var) to (y / 1). I know this can be hard to track.