this is a gcc file, you can download it and learn its usetility, for further detail please look at t

				f (x)				     unsigned x;				{				  return (unsigned) (((unsigned long long) x * 0xAAAAAAAB) >> 32) >> 1;				}								main ()				{				  unsigned i;								  for (i = 0; i < 10000; i++)				    if (f (i) != i / 3)				      abort ();				  exit (0);				}