Optimizing the loop algorithm for Array.prototype.map.

src/array.js

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 1221 matching lines @@
   if (IS_NULL_OR_UNDEFINED(receiver)) {
     receiver = %GetDefaultReceiver(f) || receiver;
   } else if (!IS_SPEC_OBJECT(receiver)) {
     receiver = ToObject(receiver);
   }
 
   var result = new $Array();
   var accumulator = new InternalArray(length);
   if (%DebugCallbackSupportsStepping(f)) {
     for (var i = 0; i < length; i++) {
-      if (i in array) {
-        var element = array[i];
+      var element = array[i];
+      if (!IS_UNDEFINED(element)) {

[Sven Panne, 2012/11/11 15:51:41]

Quick drive-by-comment: I think that this "optimization" is wrong in the
presence of e.g. accessors, because the access of array[i] can be observed.

         // Prepare break slots for debugger step in.
         %DebugPrepareStepInIfStepping(f);
         accumulator[i] = %_CallFunction(receiver, element, i, array, f);
       }
     }
   } else {
     // This is a duplicate of the previous loop sans debug stepping.
     for (var i = 0; i < length; i++) {
-      if (i in array) {
-        var element = array[i];
+      var element = array[i];
+      if (!IS_UNDEFINED(element)) {
         accumulator[i] = %_CallFunction(receiver, element, i, array, f);
       }
     }
     // End of duplicate.
   }
   %MoveArrayContents(accumulator, result);
   return result;
 }
 
 
@@ skipping 290 matching lines @@
   SetUpLockedPrototype(InternalArray, $Array(), $Array(
     "indexOf", getFunction("indexOf", ArrayIndexOf),
     "join", getFunction("join", ArrayJoin),
     "pop", getFunction("pop", ArrayPop),
     "push", getFunction("push", ArrayPush),
     "splice", getFunction("splice", ArraySplice)
   ));
 }
 
 SetUpArray();