Bug 1527007 - Work around Windows fmod bugs r=jandem

Differential Revision: https://phabricator.services.mozilla.com/D21407

--HG--
extra : moz-landing-system : lando

js/src/jslibmath.h

@@ -36,7 +36,16 @@ inline double NumberMod(double a, double b) {
   if (b == 0) {
     return JS::GenericNaN();
   }
-  return fmod(a, b);
+  double r = fmod(a, b);
+#if defined(XP_WIN)
+  // Some versions of Windows (Win 10 v1803, v1809) miscompute the sign of zero
+  // results from fmod. The sign should match the sign of the LHS. This bug
+  // only affects 64-bit builds. See bug 1527007.
+  if (mozilla::IsPositiveZero(r) && mozilla::IsNegative(a)) {
+    return -0.0;
+  }
+#endif
+  return r;
 }
 
 }  // namespace js