Fix Map::AsArray to properly iterate over the backing store

src/api.cc

 // 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.
 
 #include "src/api.h"
 
 #include <string.h>  // For memcpy, strlen.
 #ifdef V8_USE_ADDRESS_SANITIZER
 #include <sanitizer/asan_interface.h>
 #endif  // V8_USE_ADDRESS_SANITIZER
@@ skipping 6349 matching lines @@
 }
 
 
 Local<Array> Map::AsArray() const {
   i::Handle<i::JSMap> obj = Utils::OpenHandle(this);
   i::Isolate* isolate = obj->GetIsolate();
   i::Factory* factory = isolate->factory();
   LOG_API(isolate, "Map::AsArray");
   ENTER_V8(isolate);
   i::Handle<i::OrderedHashMap> table(i::OrderedHashMap::cast(obj->table()));
-  int size = table->NumberOfElements();
-  int length = size * 2;
+  int length = table->NumberOfElements() * 2;
   i::Handle<i::FixedArray> result = factory->NewFixedArray(length);
-  for (int i = 0; i < size; ++i) {
-    if (table->KeyAt(i)->IsTheHole()) continue;
-    result->set(i * 2, table->KeyAt(i));
-    result->set(i * 2 + 1, table->ValueAt(i));
+  int capacity = table->UsedCapacity();
+  int result_index = 0;
+  for (int i = 0; i < capacity; ++i) {
+    i::Object* key = table->KeyAt(i);
+    if (key->IsTheHole()) continue;

[Camillo Bruni, 2016/05/10 06:50:59]

nit: IsTheHole in a loop is rather slow, keep a local hole around for direct
comparison is a bit faster ;)

[adamk, 2016/05/10 17:19:24]

Done, and added a DisallowHeapAllocation for good measure.

+    result->set(result_index++, key);
+    result->set(result_index++, table->ValueAt(i));
   }
+  DCHECK_EQ(result_index, result->length());
+  DCHECK_EQ(result_index, length);
   i::Handle<i::JSArray> result_array =
       factory->NewJSArrayWithElements(result, i::FAST_ELEMENTS, length);
   return Utils::ToLocal(result_array);
 }
 
 
 Local<v8::Set> v8::Set::New(Isolate* isolate) {
   i::Isolate* i_isolate = reinterpret_cast<i::Isolate*>(isolate);
   LOG_API(i_isolate, "Set::New");
   ENTER_V8(i_isolate);
@@ skipping 2429 matching lines @@
   Address callback_address =
       reinterpret_cast<Address>(reinterpret_cast<intptr_t>(callback));
   VMState<EXTERNAL> state(isolate);
   ExternalCallbackScope call_scope(isolate, callback_address);
   callback(info);
 }
 
 
 }  // namespace internal
 }  // namespace v8