Bug 1398751 - Part 2: Add fast path to convert int32 value to a code point. r=evilpie

js/src/jsstr.cpp

@@ -3312,6 +3312,16 @@ static MOZ_ALWAYS_INLINE bool
 ToCodePoint(JSContext* cx, HandleValue code, uint32_t* codePoint)
 {
     // String.fromCodePoint, Steps 5.a-b.
+
+    // Fast path for the common case - the input is already an int32.
+    if (code.isInt32()) {
+        int32_t nextCP = code.toInt32();
+        if (nextCP >= 0 && nextCP <= int32_t(unicode::NonBMPMax)) {
+            *codePoint = uint32_t(nextCP);
+            return true;
+        }
+    }
+
     double nextCP;
     if (!ToNumber(cx, code, &nextCP))
         return false;