Urgh. C semantics. Well, at least that will not happen if you actually use IEEE754 _and_ if a, b, and c all have the same floating point precision _and_ there is no weird promotion/precision change going on.

Now that you mention, there was this fascinating denial-of-service vulnerability in PHP which is (kind of) related... https://news.ycombinator.com/item?id=2066352 http://www.exploringbinary.com/why-volatile-fixes-the-2-2250...

EDIT: To expand on this: The following code is valid, and may print "weird":

  double x = f();
  double y = x;
  if(x != 0 && y == 0) {
     printf("weird\n");
  }

... if x is stored in a 80bit floating point register, and y is stored in memory and therefore only has 64bit