diff --git a/mozilla/js/src/jslock.c b/mozilla/js/src/jslock.c
index acd13c3ca9b..9dc5cc3c88d 100644
--- a/mozilla/js/src/jslock.c
+++ b/mozilla/js/src/jslock.c
@@ -99,6 +99,21 @@ js_CompareAndSwap(jsword *w, jsword ov, jsword nv)
     }
 }
 
+#elif defined(XP_MACOSX) || defined(DARWIN)
+
+#include <libkern/OSAtomic.h>
+
+static JS_INLINE int
+js_CompareAndSwap(jsword *w, jsword ov, jsword nv)
+{
+    // Details on these functions available in the manpage for atomic
+#if JS_BYTES_PER_WORD == 8 && JS_BYTES_PER_LONG != 8
+    return OSAtomicCompareAndSwap64Barrier(ov, nv, (int64_t*) w);
+#else
+    return OSAtomicCompareAndSwap32Barrier(ov, nv, (int32_t*) w);
+#endif
+}
+
 #elif defined(__GNUC__) && defined(__i386__)
 
 /* Note: This fails on 386 cpus, cmpxchgl is a >= 486 instruction */