Bug 851769 part 1 - Handle arrays with length <= 1 more efficiently in Array.prototype.reverse. r=anba

--HG--
extra : rebase_source : fdf3bf32165011b3051bcd5ca1b6ad349ca7b8c1

js/src/jsarray.cpp

@@ -1525,8 +1525,10 @@ SetArrayElements(JSContext* cx, HandleObject obj, uint64_t start,
 static DenseElementResult
 ArrayReverseDenseKernel(JSContext* cx, HandleNativeObject obj, uint32_t length)
 {
-    /* An empty array or an array with no elements is already reversed. */
-    if (length == 0 || obj->getDenseInitializedLength() == 0)
+    MOZ_ASSERT(length > 1);
+
+    // If there are no elements, we're done.
+    if (obj->getDenseInitializedLength() == 0)
         return DenseElementResult::Success;
 
     if (obj->denseElementsAreFrozen())
@@ -1595,6 +1597,12 @@ js::array_reverse(JSContext* cx, unsigned argc, Value* vp)
     if (!GetLengthProperty(cx, obj, &len))
         return false;
 
+    // An empty array or an array with length 1 is already reversed.
+    if (len <= 1) {
+        args.rval().setObject(*obj);
+        return true;
+    }
+
     if (IsPackedArrayOrNoExtraIndexedProperties(obj, len) && len <= UINT32_MAX) {
         DenseElementResult result =
             ArrayReverseDenseKernel(cx, obj.as<NativeObject>(), uint32_t(len));