Optimize Map/Set.prototype.forEach

src/collection.js

 // Copyright 2012 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 $Array = global.Array;
 
@@ skipping 62 matching lines @@
   if (!IS_SET(this)) {
     throw MakeTypeError('incompatible_method_receiver',
                         ['Set.prototype.forEach', this]);
   }
 
   if (!IS_SPEC_FUNCTION(f)) {
     throw MakeTypeError('called_non_callable', [f]);
   }
 
   var iterator = new SetIterator(this, ITERATOR_KIND_VALUES);
-  var entry;
+  var key;
   var stepping = %_DebugCallbackSupportsStepping(f);
-  while (!(entry = %SetIteratorNext(iterator)).done) {
+  while (%SetIteratorHasMore(iterator)) {
+    key = %SetIteratorCurrentKey(iterator);
+    %SetIteratorMoveNext(iterator);

[Michael Starzinger, 2014/06/23 19:51:52]

I am _very_ surprised that this is faster. How are three runtime calls faster
than one? Don't get me wrong, I trust your measurements, but is this a good
optimization in the long run?

[arv (Not doing code reviews), 2014/06/23 20:31:51]

The cost of 1 call vs 3 calls is not so large. Since we have 1 this code does
not get optimized so we are already paying the overhead of runtime calls, this
just makes that overhead 3x larger (same order of magnitude).

The main reason why this is faster is that creating the iterator result object
and then extracting the data is a lot of overhead. For Map.prototype.forEach we
also had to create temporary array and extract that data from that too.

     if (stepping) %DebugPrepareStepInIfStepping(f);
-    %_CallFunction(receiver, entry.value, entry.value, this, f);
+    %_CallFunction(receiver, key, key, this, f);
   }
 }
 
 
 // -------------------------------------------------------------------
 
 function SetUpSet() {
   %CheckIsBootstrapping();
 
   %SetCode($Set, SetConstructor);
@@ skipping 86 matching lines @@
   if (!IS_MAP(this)) {
     throw MakeTypeError('incompatible_method_receiver',
                         ['Map.prototype.forEach', this]);
   }
 
   if (!IS_SPEC_FUNCTION(f)) {
     throw MakeTypeError('called_non_callable', [f]);
   }
 
   var iterator = new MapIterator(this, ITERATOR_KIND_ENTRIES);
-  var entry;
+  var key, value;
   var stepping = %_DebugCallbackSupportsStepping(f);
-  while (!(entry = %MapIteratorNext(iterator)).done) {
+  while (%MapIteratorHasMore(iterator)) {
+    key = %MapIteratorCurrentKey(iterator);
+    value = %MapIteratorCurrentValue(iterator);
+    %MapIteratorMoveNext(iterator);
     if (stepping) %DebugPrepareStepInIfStepping(f);
-    %_CallFunction(receiver, entry.value[1], entry.value[0], this, f);
+    %_CallFunction(receiver, value, key, this, f);
   }
 }
 
 
 // -------------------------------------------------------------------
 
+
 function SetUpMap() {
   %CheckIsBootstrapping();
 
   %SetCode($Map, MapConstructor);
   %FunctionSetPrototype($Map, new $Object());
   %SetProperty($Map.prototype, "constructor", $Map, DONT_ENUM);
 
   %FunctionSetLength(MapForEach, 1);
 
   // Set up the non-enumerable functions on the Map prototype object.
   InstallGetter($Map.prototype, "size", MapGetSizeJS);
   InstallFunctions($Map.prototype, DONT_ENUM, $Array(
     "get", MapGetJS,
     "set", MapSetJS,
     "has", MapHasJS,
     "delete", MapDeleteJS,
     "clear", MapClearJS,
     "forEach", MapForEach
   ));
 }
 
 SetUpMap();