#include <math.h>

double fmax(double x, double y) {
  if (isnan(x))
    return y;
  if (isnan(y))
    return x;
  /* handle signed zeros, see C99 Annex F.9.9.2 */
  if (signbit(x) != signbit(y))
    return signbit(x) ? y : x;
  return x < y ? y : x;
}