Optimize Map/Set.prototype.forEach

src/collection-iterator.js

Index: src/collection-iterator.js
diff --git a/src/collection-iterator.js b/src/collection-iterator.js
index 2436a931e28bc4acde80f5c4a8143cc0bff5b8f3..545a34587f8f438af4b8b6dd9277780fbd409074 100644
--- a/src/collection-iterator.js
+++ b/src/collection-iterator.js
@@ -21,7 +21,23 @@ function SetIteratorNextJS() {
     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};
+  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];
+      break;
+  }
+
+  return entry;
 }
 
 
@@ -97,7 +113,24 @@ function MapIteratorNextJS() {
     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;
+    // ITERATOR_KIND_ENTRIES does not need any processing.
+  }
+
+  return entry;
 }