Fix array concat to follow the specification in the presence of element getters.

src/runtime.cc

Index: src/runtime.cc
diff --git a/src/runtime.cc b/src/runtime.cc
index 5a443efc3d0fb9b0445b50cf50a22f9472da78f9..d40565c95858ce16061405f555c86367b98dba50 100644
--- a/src/runtime.cc
+++ b/src/runtime.cc
@@ -8028,377 +8028,448 @@ static MaybeObject* Runtime_PushIfAbsent(Arguments args) {
 class ArrayConcatVisitor {
  public:
   ArrayConcatVisitor(Handle<FixedArray> storage,
-                     uint32_t index_limit,
                      bool fast_elements) :
-      storage_(storage), index_limit_(index_limit),
-      index_offset_(0), fast_elements_(fast_elements) { }
+      storage_(storage),
+      index_offset_(0u),
+      fast_elements_(fast_elements) { }
 
   void visit(uint32_t i, Handle<Object> elm) {
-    if (i >= index_limit_ - index_offset_) return;
+    if (i >= JSObject::kMaxElementCount - index_offset_) return;
     uint32_t index = index_offset_ + i;
 
     if (fast_elements_) {
-      ASSERT(index < static_cast<uint32_t>(storage_->length()));
-      storage_->set(index, *elm);
-
-    } else {
-      Handle<NumberDictionary> dict = Handle<NumberDictionary>::cast(storage_);
-      Handle<NumberDictionary> result =
-          Factory::DictionaryAtNumberPut(dict, index, elm);
-      if (!result.is_identical_to(dict))
-        storage_ = result;
+      if (index < static_cast<uint32_t>(storage_->length())) {
+        storage_->set(index, *elm);
+        return;
+      }
+      // Our initial estimate of length was foiled, possibly by
+      // getters on the arrays increasing the length of later arrays
+      // during iteration.
+      // This shouldn't happen in anything but pathological cases.
+      SetDictionaryMode(index);
+      // Fall-through to dictionary mode.
     }
-  }
+    ASSERT(!fast_elements_);
+    Handle<NumberDictionary> dict(storage_.cast<NumberDictionary>());
+    Handle<NumberDictionary> result =
+        Factory::DictionaryAtNumberPut(dict, index, elm);
+    if (!result.is_identical_to(dict)) {
+      storage_ = Handle<FixedArray>::cast(result);
+    }
+}
 
   void increase_index_offset(uint32_t delta) {
-    if (index_limit_ - index_offset_ < delta) {
-      index_offset_ = index_limit_;
+    if (JSObject::kMaxElementCount - index_offset_ < delta) {
+      index_offset_ = JSObject::kMaxElementCount;
     } else {
       index_offset_ += delta;
     }
   }
 
-  Handle<FixedArray> storage() { return storage_; }
+  Handle<JSArray> ToArray() {
+    Handle<JSArray> array = Factory::NewJSArray(0);
+    Handle<Object> length =
+        Factory::NewNumber(static_cast<double>(index_offset_));
+    Handle<Map> map;
+    if (fast_elements_) {
+      map = Factory::GetFastElementsMap(Handle<Map>(array->map()));
+    } else {
+      map = Factory::GetSlowElementsMap(Handle<Map>(array->map()));
+    }
+    array->set_map(*map);
+    array->set_length(*length);
+    array->set_elements(*storage_);
+    return array;
+  }
 
  private:
-  Handle<FixedArray> storage_;
-  // Limit on the accepted indices. Elements with indices larger than the
-  // limit are ignored by the visitor.
-  uint32_t index_limit_;
-  // Index after last seen index. Always less than or equal to index_limit_.
+  // Convert storage to dictionary mode.
+  void SetDictionaryMode(uint32_t index) {
+    ASSERT(fast_elements_);
+    Handle<FixedArray> current_storage(storage_.ToHandle());
+    HandleCell<NumberDictionary> slow_storage(
+        Factory::NewNumberDictionary(current_storage->length()));
+    uint32_t current_length = static_cast<uint32_t>(current_storage->length());
+    for (uint32_t i = 0; i < current_length; i++) {
+      HandleScope loop_scope;
+      Handle<Object> element(current_storage->get(i));
+      if (!element->IsTheHole()) {
+        slow_storage =
+          Factory::DictionaryAtNumberPut(slow_storage.ToHandle(), i, element);
+      }
+    }
+    storage_ = slow_storage.cast<FixedArray>();
+    fast_elements_ = false;
+  }
+
+  HandleCell<FixedArray> storage_;
+  // Index after last seen index. Always less than or equal to
+  // JSObject::kMaxElementCount.
   uint32_t index_offset_;
-  const bool fast_elements_;
+  bool fast_elements_;
 };
 
 
+static uint32_t EstimateElementCount(Handle<JSArray> array) {
+  uint32_t length = static_cast<uint32_t>(array->length()->Number());
+  int element_count = 0;
+  switch (array->GetElementsKind()) {
+    case JSObject::FAST_ELEMENTS: {
+      // Fast elements can't have lengths that are not representable by
+      // a 32-bit signed integer.
+      ASSERT(static_cast<int32_t>(FixedArray::kMaxLength) >= 0);
+      int fast_length = static_cast<int>(length);
+      Handle<FixedArray> elements(FixedArray::cast(array->elements()));
+      for (int i = 0; i < fast_length; i++) {
+        if (!elements->get(i)->IsTheHole()) element_count++;
+      }
+      break;
+    }
+    case JSObject::DICTIONARY_ELEMENTS: {
+      Handle<NumberDictionary> dictionary(
+          NumberDictionary::cast(array->elements()));
+      int capacity = dictionary->Capacity();
+      for (int i = 0; i < capacity; i++) {
+        Handle<Object> key(dictionary->KeyAt(i));
+        if (dictionary->IsKey(*key)) {
+          element_count++;
+        }
+      }
+      break;
+    }
+    default:
+      // External arrays are always dense.
+      return length;
+  }
+  // As an estimate, we assume that the prototype doesn't contain any
+  // inherited elements.
+  return element_count;
+}
+
+
+
 template<class ExternalArrayClass, class ElementType>
-static uint32_t IterateExternalArrayElements(Handle<JSObject> receiver,
-                                             bool elements_are_ints,
-                                             bool elements_are_guaranteed_smis,
-                                             uint32_t range,
-                                             ArrayConcatVisitor* visitor) {
+static void IterateExternalArrayElements(Handle<JSObject> receiver,
+                                         bool elements_are_ints,
+                                         bool elements_are_guaranteed_smis,
+                                         ArrayConcatVisitor* visitor) {
   Handle<ExternalArrayClass> array(
       ExternalArrayClass::cast(receiver->elements()));
-  uint32_t len = Min(static_cast<uint32_t>(array->length()), range);
+  uint32_t len = static_cast<uint32_t>(array->length());
 
-  if (visitor != NULL) {
-    if (elements_are_ints) {
-      if (elements_are_guaranteed_smis) {
-        for (uint32_t j = 0; j < len; j++) {
-          Handle<Smi> e(Smi::FromInt(static_cast<int>(array->get(j))));
+  ASSERT(visitor != NULL);
+  if (elements_are_ints) {
+    if (elements_are_guaranteed_smis) {
+      for (uint32_t j = 0; j < len; j++) {
+        HandleScope loop_scope;
+        Handle<Smi> e(Smi::FromInt(static_cast<int>(array->get(j))));
+        visitor->visit(j, e);
+      }
+    } else {
+      for (uint32_t j = 0; j < len; j++) {
+        HandleScope loop_scope;
+        int64_t val = static_cast<int64_t>(array->get(j));
+        if (Smi::IsValid(static_cast<intptr_t>(val))) {
+          Handle<Smi> e(Smi::FromInt(static_cast<int>(val)));
+          visitor->visit(j, e);
+        } else {
+          Handle<Object> e =
+              Factory::NewNumber(static_cast<ElementType>(val));
           visitor->visit(j, e);
         }
-      } else {
-        for (uint32_t j = 0; j < len; j++) {
-          int64_t val = static_cast<int64_t>(array->get(j));
-          if (Smi::IsValid(static_cast<intptr_t>(val))) {
-            Handle<Smi> e(Smi::FromInt(static_cast<int>(val)));
-            visitor->visit(j, e);
-          } else {
-            Handle<Object> e =
-                Factory::NewNumber(static_cast<ElementType>(val));
-            visitor->visit(j, e);
+      }
+    }
+  } else {
+    for (uint32_t j = 0; j < len; j++) {
+      HandleScope loop_scope;
+      Handle<Object> e = Factory::NewNumber(array->get(j));
+      visitor->visit(j, e);
+    }
+  }
+}
+
+
+// Used for sorting indices in a List<uint32_t>.
+static int compareUInt32(const uint32_t* ap, const uint32_t* bp) {
+  uint32_t a = *ap;
+  uint32_t b = *bp;
+  return (a == b) ? 0 : (a < b) ? -1 : 1;
+}
+
+
+static void CollectElementIndices(Handle<JSObject> object,
+                                  uint32_t range,
+                                  List<uint32_t>* indices) {
+  JSObject::ElementsKind kind = object->GetElementsKind();
+  switch (kind) {
+    case JSObject::FAST_ELEMENTS: {
+      Handle<FixedArray> elements(FixedArray::cast(object->elements()));
+      uint32_t length = static_cast<uint32_t>(elements->length());
+      if (range < length) length = range;
+      for (uint32_t i = 0; i < length; i++) {
+        if (!elements->get(i)->IsTheHole()) {
+          indices->Add(i);
+        }
+      }
+      break;
+    }
+    case JSObject::DICTIONARY_ELEMENTS: {
+      Handle<NumberDictionary> dict(NumberDictionary::cast(object->elements()));
+      uint32_t capacity = dict->Capacity();
+      for (uint32_t j = 0; j < capacity; j++) {
+        HandleScope loop_scope;
+        Handle<Object> k(dict->KeyAt(j));
+        if (dict->IsKey(*k)) {
+          ASSERT(k->IsNumber());
+          uint32_t index = static_cast<uint32_t>(k->Number());
+          if (index < range) {
+            indices->Add(index);
           }
         }
       }
-    } else {
-      for (uint32_t j = 0; j < len; j++) {
-        Handle<Object> e = Factory::NewNumber(array->get(j));
-        visitor->visit(j, e);
+      break;
+    }
+    default: {
+      int dense_elements_length;
+      switch (kind) {
+        case JSObject::PIXEL_ELEMENTS: {
+        dense_elements_length =
+            PixelArray::cast(object->elements())->length();
+          break;
+        }
+        case JSObject::EXTERNAL_BYTE_ELEMENTS: {
+        dense_elements_length =
+            ExternalByteArray::cast(object->elements())->length();
+          break;
+        }
+        case JSObject::EXTERNAL_UNSIGNED_BYTE_ELEMENTS: {
+        dense_elements_length =
+            ExternalUnsignedByteArray::cast(object->elements())->length();
+          break;
+        }
+        case JSObject::EXTERNAL_SHORT_ELEMENTS: {
+        dense_elements_length =
+            ExternalShortArray::cast(object->elements())->length();
+          break;
+        }
+        case JSObject::EXTERNAL_UNSIGNED_SHORT_ELEMENTS: {
+        dense_elements_length =
+            ExternalUnsignedShortArray::cast(object->elements())->length();
+          break;
+        }
+        case JSObject::EXTERNAL_INT_ELEMENTS: {
+        dense_elements_length =
+            ExternalIntArray::cast(object->elements())->length();
+          break;
+        }
+        case JSObject::EXTERNAL_UNSIGNED_INT_ELEMENTS: {
+        dense_elements_length =
+            ExternalUnsignedIntArray::cast(object->elements())->length();
+          break;
+        }
+        case JSObject::EXTERNAL_FLOAT_ELEMENTS: {
+        dense_elements_length =
+            ExternalFloatArray::cast(object->elements())->length();
+          break;
+        }
+        default:
+          UNREACHABLE();
+          break;
+      }
+      uint32_t length = static_cast<uint32_t>(dense_elements_length);
+      if (range <= length) {
+        length = range;
+        // We will add all indices, so we might as well clear it first
+        // and avoid duplicates.
+        indices->Clear();
       }
+      for (uint32_t i = 0; i < length; i++) {
+        indices->Add(i);
+      }
+      if (length == range) return;  // All indices accounted for already.
+      break;
     }
   }
 
-  return len;
+  Handle<Object> prototype(object->GetPrototype());
+  if (prototype->IsJSObject()) {
+    // The prototype will usually have no inherited element indices,
+    // but we have to check.
+    CollectElementIndices(Handle<JSObject>::cast(prototype), range, indices);
+  }
 }
 
+
 /**
- * A helper function that visits elements of a JSObject. Only elements
- * whose index between 0 and range (exclusive) are visited.
- *
- * If the third parameter, visitor, is not NULL, the visitor is called
- * with parameters, 'visitor_index_offset + element index' and the element.
+ * A helper function that visits elements of a JSArray in numerical
+ * order.
  *
- * It returns the number of visisted elements.
+ * The visitor argument called for each existing element in the array
+ * with the element index and the element's value.
+ * Afterwards it increments the base-index of the visitor by the array
+ * length.
  */
-static uint32_t IterateElements(Handle<JSObject> receiver,
-                                uint32_t range,
-                                ArrayConcatVisitor* visitor) {
-  uint32_t num_of_elements = 0;
-
+static void IterateElements(Handle<JSArray> receiver,
+                            ArrayConcatVisitor* visitor) {
+  uint32_t length = static_cast<uint32_t>(receiver->length()->Number());
   switch (receiver->GetElementsKind()) {
     case JSObject::FAST_ELEMENTS: {
+      // Run through the elements FixedArray and use HasElement and GetElement
+      // to check the prototype for missing elements.
       Handle<FixedArray> elements(FixedArray::cast(receiver->elements()));
-      uint32_t len = elements->length();
-      if (range < len) {
-        len = range;
-      }
-
-      for (uint32_t j = 0; j < len; j++) {
-        Handle<Object> e(elements->get(j));
-        if (!e->IsTheHole()) {
-          num_of_elements++;
-          if (visitor) {
-            visitor->visit(j, e);
-          }
+      int fast_length = static_cast<int>(length);
+      ASSERT(fast_length <= elements->length());
+      for (int j = 0; j < fast_length; j++) {
+        HandleScope loop_scope;
+        Handle<Object> element_value(elements->get(j));
+        if (!element_value->IsTheHole()) {
+          visitor->visit(j, element_value);
+        } else if (receiver->HasElement(j)) {
+          // Call GetElement on receiver, not its prototype, or getters won't
+          // have the correct receiver.
+          element_value = GetElement(receiver, j);
+          visitor->visit(j, element_value);
         }
       }
       break;
     }
+    case JSObject::DICTIONARY_ELEMENTS: {
+      Handle<NumberDictionary> dict(receiver->element_dictionary());
+      List<uint32_t> indices(dict->Capacity() / 2);
+      // Collect all indices in the object and the prototypes less
+      // than length. This might introduce duplicates in the indices list.
+      CollectElementIndices(receiver, length, &indices);
+      indices.Sort(&compareUInt32);
+      int j = 0;
+      int n = indices.length();
+      while (j < n) {
+        HandleScope loop_scope;
+        uint32_t index = indices[j];
+        Handle<Object> element = GetElement(receiver, index);
+        visitor->visit(index, element);
+        // Skip to next different index (i.e., omit duplicates).
+        do {
+          j++;
+        } while (j < n && indices[j] == index);
+      }
+      break;
+    }
     case JSObject::PIXEL_ELEMENTS: {
       Handle<PixelArray> pixels(PixelArray::cast(receiver->elements()));
-      uint32_t len = pixels->length();
-      if (range < len) {
-        len = range;
-      }
-
-      for (uint32_t j = 0; j < len; j++) {
-        num_of_elements++;
-        if (visitor != NULL) {
-          Handle<Smi> e(Smi::FromInt(pixels->get(j)));
-          visitor->visit(j, e);
-        }
+      for (uint32_t j = 0; j < length; j++) {
+        Handle<Smi> e(Smi::FromInt(pixels->get(j)));
+        visitor->visit(j, e);
       }
       break;
     }
     case JSObject::EXTERNAL_BYTE_ELEMENTS: {
-      num_of_elements =
-          IterateExternalArrayElements<ExternalByteArray, int8_t>(
-              receiver, true, true, range, visitor);
+      IterateExternalArrayElements<ExternalByteArray, int8_t>(
+          receiver, true, true, visitor);
       break;
     }
     case JSObject::EXTERNAL_UNSIGNED_BYTE_ELEMENTS: {
-      num_of_elements =
-          IterateExternalArrayElements<ExternalUnsignedByteArray, uint8_t>(
-              receiver, true, true, range, visitor);
+      IterateExternalArrayElements<ExternalUnsignedByteArray, uint8_t>(
+          receiver, true, true, visitor);
       break;
     }
     case JSObject::EXTERNAL_SHORT_ELEMENTS: {
-      num_of_elements =
-          IterateExternalArrayElements<ExternalShortArray, int16_t>(
-              receiver, true, true, range, visitor);
+      IterateExternalArrayElements<ExternalShortArray, int16_t>(
+          receiver, true, true, visitor);
       break;
     }
     case JSObject::EXTERNAL_UNSIGNED_SHORT_ELEMENTS: {
-      num_of_elements =
-          IterateExternalArrayElements<ExternalUnsignedShortArray, uint16_t>(
-              receiver, true, true, range, visitor);
+      IterateExternalArrayElements<ExternalUnsignedShortArray, uint16_t>(
+          receiver, true, true, visitor);
       break;
     }
     case JSObject::EXTERNAL_INT_ELEMENTS: {
-      num_of_elements =
-          IterateExternalArrayElements<ExternalIntArray, int32_t>(
-              receiver, true, false, range, visitor);
+      IterateExternalArrayElements<ExternalIntArray, int32_t>(
+          receiver, true, false, visitor);
       break;
     }
     case JSObject::EXTERNAL_UNSIGNED_INT_ELEMENTS: {
-      num_of_elements =
-          IterateExternalArrayElements<ExternalUnsignedIntArray, uint32_t>(
-              receiver, true, false, range, visitor);
+      IterateExternalArrayElements<ExternalUnsignedIntArray, uint32_t>(
+          receiver, true, false, visitor);
       break;
     }
     case JSObject::EXTERNAL_FLOAT_ELEMENTS: {
-      num_of_elements =
-          IterateExternalArrayElements<ExternalFloatArray, float>(
-              receiver, false, false, range, visitor);
-      break;
-    }
-    case JSObject::DICTIONARY_ELEMENTS: {
-      Handle<NumberDictionary> dict(receiver->element_dictionary());
-      uint32_t capacity = dict->Capacity();
-      for (uint32_t j = 0; j < capacity; j++) {
-        Handle<Object> k(dict->KeyAt(j));
-        if (dict->IsKey(*k)) {
-          ASSERT(k->IsNumber());
-          uint32_t index = static_cast<uint32_t>(k->Number());
-          if (index < range) {
-            num_of_elements++;
-            if (visitor) {
-              visitor->visit(index, Handle<Object>(dict->ValueAt(j)));
-            }
-          }
-        }
-      }
+      IterateExternalArrayElements<ExternalFloatArray, float>(
+          receiver, false, false, visitor);
       break;
     }
     default:
       UNREACHABLE();
       break;
   }
-
-  return num_of_elements;
-}
-
-
-/**
- * A helper function that visits elements of an Array object, and elements
- * on its prototypes.
- *
- * Elements on prototypes are visited first, and only elements whose indices
- * less than Array length are visited.
- *
- * If a ArrayConcatVisitor object is given, the visitor is called with
- * parameters, element's index + visitor_index_offset and the element.
- *
- * The returned number of elements is an upper bound on the actual number
- * of elements added. If the same element occurs in more than one object
- * in the array's prototype chain, it will be counted more than once, but
- * will only occur once in the result.
- */
-static uint32_t IterateArrayAndPrototypeElements(Handle<JSArray> array,
-                                                 ArrayConcatVisitor* visitor) {
-  uint32_t range = static_cast<uint32_t>(array->length()->Number());
-  Handle<Object> obj = array;
-
-  static const int kEstimatedPrototypes = 3;
-  List< Handle<JSObject> > objects(kEstimatedPrototypes);
-
-  // Visit prototype first. If an element on the prototype is shadowed by
-  // the inheritor using the same index, the ArrayConcatVisitor visits
-  // the prototype element before the shadowing element.
-  // The visitor can simply overwrite the old value by new value using
-  // the same index.  This follows Array::concat semantics.
-  while (!obj->IsNull()) {
-    objects.Add(Handle<JSObject>::cast(obj));
-    obj = Handle<Object>(obj->GetPrototype());
-  }
-
-  uint32_t nof_elements = 0;
-  for (int i = objects.length() - 1; i >= 0; i--) {
-    Handle<JSObject> obj = objects[i];
-    uint32_t encountered_elements =
-        IterateElements(Handle<JSObject>::cast(obj), range, visitor);
-
-    if (encountered_elements > JSObject::kMaxElementCount - nof_elements) {
-      nof_elements = JSObject::kMaxElementCount;
-    } else {
-      nof_elements += encountered_elements;
-    }
-  }
-
-  return nof_elements;
-}
-
-
-/**
- * A helper function of Runtime_ArrayConcat.
- *
- * The first argument is an Array of arrays and objects. It is the
- * same as the arguments array of Array::concat JS function.
- *
- * If an argument is an Array object, the function visits array
- * elements.  If an argument is not an Array object, the function
- * visits the object as if it is an one-element array.
- *
- * If the result array index overflows 32-bit unsigned integer, the rounded
- * non-negative number is used as new length. For example, if one
- * array length is 2^32 - 1, second array length is 1, the
- * concatenated array length is 0.
- * TODO(lrn) Change length behavior to ECMAScript 5 specification (length
- * is one more than the last array index to get a value assigned).
- */
-static uint32_t IterateArguments(Handle<JSArray> arguments,
-                                 ArrayConcatVisitor* visitor) {
-  uint32_t visited_elements = 0;
-  uint32_t num_of_args = static_cast<uint32_t>(arguments->length()->Number());
-
-  for (uint32_t i = 0; i < num_of_args; i++) {
-    Object *element;
-    MaybeObject* maybe_element = arguments->GetElement(i);
-    // This if() is not expected to fail, but we have the check in the
-    // interest of hardening the runtime calls.
-    if (maybe_element->ToObject(&element)) {
-      Handle<Object> obj(element);
-      if (obj->IsJSArray()) {
-        Handle<JSArray> array = Handle<JSArray>::cast(obj);
-        uint32_t len = static_cast<uint32_t>(array->length()->Number());
-        uint32_t nof_elements =
-            IterateArrayAndPrototypeElements(array, visitor);
-        // Total elements of array and its prototype chain can be more than
-        // the array length, but ArrayConcat can only concatenate at most
-        // the array length number of elements. We use the length as an estimate
-        // for the actual number of elements added.
-        uint32_t added_elements = (nof_elements > len) ? len : nof_elements;
-        if (JSArray::kMaxElementCount - visited_elements < added_elements) {
-          visited_elements = JSArray::kMaxElementCount;
-        } else {
-          visited_elements += added_elements;
-        }
-        if (visitor) visitor->increase_index_offset(len);
-      } else {
-        if (visitor) {
-          visitor->visit(0, obj);
-          visitor->increase_index_offset(1);
-        }
-        if (visited_elements < JSArray::kMaxElementCount) {
-          visited_elements++;
-        }
-      }
-    }
-  }
-  return visited_elements;
+  visitor->increase_index_offset(length);
 }
 
 
 /**
  * Array::concat implementation.
  * See ECMAScript 262, 15.4.4.4.
- * TODO(lrn): Fix non-compliance for very large concatenations and update to
+ * TODO(581): Fix non-compliance for very large concatenations and update to
  * following the ECMAScript 5 specification.
  */
 static MaybeObject* Runtime_ArrayConcat(Arguments args) {
   ASSERT(args.length() == 1);
   HandleScope handle_scope;
 
-  CONVERT_CHECKED(JSArray, arg_arrays, args[0]);
-  Handle<JSArray> arguments(arg_arrays);
-
-  // Pass 1: estimate the number of elements of the result
-  // (it could be more than real numbers if prototype has elements).
-  uint32_t result_length = 0;
-  uint32_t num_of_args = static_cast<uint32_t>(arguments->length()->Number());
-
-  { AssertNoAllocation nogc;
-    for (uint32_t i = 0; i < num_of_args; i++) {
-      Object* obj;
-      MaybeObject* maybe_object = arguments->GetElement(i);
-      // This if() is not expected to fail, but we have the check in the
-      // interest of hardening the runtime calls.
-      if (maybe_object->ToObject(&obj)) {
-        uint32_t length_estimate;
-        if (obj->IsJSArray()) {
-          length_estimate =
-              static_cast<uint32_t>(JSArray::cast(obj)->length()->Number());
-        } else {
-          length_estimate = 1;
-        }
-        if (JSObject::kMaxElementCount - result_length < length_estimate) {
-          result_length = JSObject::kMaxElementCount;
-          break;
-        }
-        result_length += length_estimate;
+  CONVERT_ARG_CHECKED(JSArray, arguments, 0);
+  int argument_count = static_cast<int>(arguments->length()->Number());
+  RUNTIME_ASSERT(arguments->HasFastElements());
+  Handle<FixedArray> elements(FixedArray::cast(arguments->elements()));
+
+  // Pass 1: estimate the length and number of elements of the result.
+  // The actual length can be larger if any of the arguments have getters
+  // that mutate other arguments (but will otherwise be precise).
+  // The number of elements is precise if there are no inherited elements.
+
+  uint32_t estimate_result_length = 0;
+  uint32_t estimate_nof_elements = 0;
+  {
+    for (int i = 0; i < argument_count; i++) {
+      HandleScope loop_scope;
+      Handle<Object> obj(elements->get(i));
+      uint32_t length_estimate;
+      uint32_t element_estimate;
+      if (obj->IsJSArray()) {
+        Handle<JSArray> array(Handle<JSArray>::cast(obj));
+        length_estimate =
+            static_cast<uint32_t>(array->length()->Number());
+        element_estimate =
+            EstimateElementCount(array);
+      } else {
+        length_estimate = 1;
+        element_estimate = 1;
+      }
+      // Avoid overflows by capping at kMaxElementCount.
+      if (JSObject::kMaxElementCount - estimate_result_length <
+          length_estimate) {
+        estimate_result_length = JSObject::kMaxElementCount;
+      } else {
+        estimate_result_length += length_estimate;
+      }
+      if (JSObject::kMaxElementCount - estimate_nof_elements <
+          element_estimate) {
+        estimate_nof_elements = JSObject::kMaxElementCount;
+      } else {
+        estimate_nof_elements += element_estimate;
       }
     }
   }
 
-  // Allocate an empty array, will set map, length, and content later.
-  Handle<JSArray> result = Factory::NewJSArray(0);
-
-  uint32_t estimate_nof_elements = IterateArguments(arguments, NULL);
   // If estimated number of elements is more than half of length, a
   // fixed array (fast case) is more time and space-efficient than a
   // dictionary.
-  bool fast_case = (estimate_nof_elements * 2) >= result_length;
+  bool fast_case = (estimate_nof_elements * 2) >= estimate_result_length;
 
-  Handle<Map> map;
   Handle<FixedArray> storage;
   if (fast_case) {
     // The backing storage array must have non-existing elements to
     // preserve holes across concat operations.
-    map = Factory::GetFastElementsMap(Handle<Map>(result->map()));
-    storage = Factory::NewFixedArrayWithHoles(result_length);
+    storage = Factory::NewFixedArrayWithHoles(estimate_result_length);
   } else {
-    map = Factory::GetSlowElementsMap(Handle<Map>(result->map()));
     // TODO(126): move 25% pre-allocation logic into Dictionary::Allocate
     uint32_t at_least_space_for = estimate_nof_elements +
                                   (estimate_nof_elements >> 2);
@@ -8406,21 +8477,20 @@ static MaybeObject* Runtime_ArrayConcat(Arguments args) {
         Factory::NewNumberDictionary(at_least_space_for));
   }
 
-  Handle<Object> len = Factory::NewNumber(static_cast<double>(result_length));
-
-  ArrayConcatVisitor visitor(storage, result_length, fast_case);
-
-  IterateArguments(arguments, &visitor);
+  ArrayConcatVisitor visitor(storage, fast_case);
 
-  // Please note:
-  // - the storage might have been changed in the visitor;
-  // - the map and the storage must be set together to avoid breaking
-  //   the invariant that the map describes the array's elements.
-  result->set_map(*map);
-  result->set_length(*len);
-  result->set_elements(*visitor.storage());
+  for (int i = 0; i < argument_count; i++) {
+    Handle<Object> obj(elements->get(i));
+    if (obj->IsJSArray()) {
+      Handle<JSArray> array = Handle<JSArray>::cast(obj);
+      IterateElements(array, &visitor);
+    } else {
+      visitor.visit(0, obj);
+      visitor.increase_index_offset(1);
+    }
+  }
 
-  return *result;
+  return *visitor.ToArray();
 }