Optimize Map/Set.prototype.forEach

src/collection-iterator.js

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 'use strict';
 
 
 // This file relies on the fact that the following declaration has been made
 // in runtime.js:
 // var $Set = global.Set;
 // var $Map = global.Map;
 
 
 function SetIteratorConstructor(set, kind) {
   %SetIteratorInitialize(this, set, kind);
 }
 
 
 function SetIteratorNextJS() {
   if (!IS_SET_ITERATOR(this)) {
     throw MakeTypeError('incompatible_method_receiver',
                         ['Set Iterator.prototype.next', this]);
   }
-  return %SetIteratorNext(this);
+
+  var value_array = [UNDEFINED, UNDEFINED];
+  var entry = {value: value_array, done: false};

[arv (Not doing code reviews), 2014/06/25 20:17:54]

I benchmarked this and the following code has no noticeable performance
difference.

var entry = {value: [UNDEFINED, UNDEFINED], done: false};
var value_array = entry.value;

+  switch (%SetIteratorNext(this, value_array)) {
+    case 0:
+      entry.value = UNDEFINED;
+      entry.done = true;
+      break;
+    case ITERATOR_KIND_VALUES:
+      entry.value = value_array[0];
+      break;
+    case ITERATOR_KIND_ENTRIES:
+      value_array[1] = value_array[0];
+      entry.value = value_array;
+      break;
+  }
+
+  return entry;
 }
 
 
 function SetIteratorSymbolIterator() {
   return this;
 }
 
 
 function SetEntries() {
   if (!IS_SET(this)) {
@@ skipping 55 matching lines @@
 function MapIteratorSymbolIterator() {
   return this;
 }
 
 
 function MapIteratorNextJS() {
   if (!IS_MAP_ITERATOR(this)) {
     throw MakeTypeError('incompatible_method_receiver',
                         ['Map Iterator.prototype.next', this]);
   }
-  return %MapIteratorNext(this);
+
+  var value_array = [UNDEFINED, UNDEFINED];
+  var entry = {value: value_array, done: false};
+  switch (%MapIteratorNext(this, value_array)) {
+    case 0:
+      entry.value = UNDEFINED;
+      entry.done = true;
+      break;
+    case ITERATOR_KIND_KEYS:
+      entry.value = value_array[0];
+      break;
+    case ITERATOR_KIND_VALUES:
+      entry.value = value_array[1];
+      break;
+    case ITERATOR_KIND_ENTRIES:
+      entry.value = value_array;
+      break;
+  }
+
+  return entry;
 }
 
 
 function MapEntries() {
   if (!IS_MAP(this)) {
     throw MakeTypeError('incompatible_method_receiver',
                         ['Map.prototype.entries', this]);
   }
   return new MapIterator(this, ITERATOR_KIND_ENTRIES);
 }
@@ skipping 42 matching lines @@
     'entries', MapEntries,
     'keys', MapKeys,
     'values', MapValues
   ));
 
   %SetProperty($Map.prototype, symbolIterator, MapEntries,
       DONT_ENUM);
 }
 
 ExtendMapPrototype();