float CDECL MSVCRT__copysignf( float num, float sign )
  {
-    /* FIXME: Behaviour for Nan/Inf? */
-    if (sign < 0.0)
-        return num < 0.0 ? num : -num;
-    return num < 0.0 ? -num : num;
+    /* FIXME: Behaviour for signbit(NAN) is different in Linux and
+     *        Windows, where Windows gives a zero for -NAN
+     */
+    if (signbit(sign))
+        return signbit(num) ? num : -num;
+    return signbit(num) ? -num : num;

It doesn't make sense to add this comment and remove it in next patch. I would also prefer if signbit fallback implementation is changed in this patch.