fastmath-1.c

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

/* { dg-do run } */
/* { dg-options "-O2 -ffast-math" } */

int foo ( float* dists,  int k) 
{ 
if ( ( dists [ 0 ] > 0 ) == ( dists [ 1 ] > 0 ) ) 
  return k;
return 0;
} 
main() {
  float dists[16] = { 0., 1., 1., 0., 0., -1., -1., 0.,
		     1., 1., 1., -1., -1., 1., -1., -1. };
  if ( foo(&dists[0], 1) +
       foo(&dists[2], 2) +
       foo(&dists[4], 4) +
       foo(&dists[6], 8) +
       foo(&dists[8], 16) +
       foo(&dists[10], 32) +
       foo(&dists[12], 64) +
       foo(&dists[14], 128)
	!= 156)
    abort();
  return 0;
}