[keys] Postpone shadowed key checking in the KeyAccumulator

Only start checking if new keys are shadowed after the first prototype has added
non-enumerable shadow keys. This helps minimally in some corner cases if there
are few enumerable properties on the prototype compared to the receiver.

BUG=chromium:628173

Review-Url: https://codereview.chromium.org/2169523002
Cr-Commit-Position: refs/heads/master@{#37940}

src/elements.cc

@@ -1375,7 +1375,7 @@ class DictionaryElementsAccessor
       if (!dictionary->IsKey(isolate, raw_key)) continue;
       uint32_t key = FilterKey(dictionary, i, raw_key, filter);
       if (key == kMaxUInt32) {
-        keys->AddShadowKey(raw_key);
+        keys->AddShadowingKey(raw_key);
         continue;
       }
       elements->set(insertion_index, raw_key);

src/keys.cc

@@ -117,7 +117,7 @@ MaybeHandle<FixedArray> FilterProxyKeys(KeyAccumulator* accumulator,
       MAYBE_RETURN(found, MaybeHandle<FixedArray>());
       if (!found.FromJust()) continue;
       if (!desc.enumerable()) {
-        accumulator->AddShadowKey(key);
+        accumulator->AddShadowingKey(key);
         continue;
       }
     }
@@ -166,6 +166,9 @@ Maybe<bool> KeyAccumulator::CollectKeys(Handle<JSReceiver> receiver,
                                           : PrototypeIterator::END_AT_NULL;
   for (PrototypeIterator iter(isolate_, object, kStartAtReceiver, end);
        !iter.IsAtEnd();) {
+    // Start the shadow checks only after the first prototype has added
+    // shadowing keys.
+    if (HasShadowingKeys()) skip_shadow_check_ = false;
     Handle<JSReceiver> current =
         PrototypeIterator::GetCurrent<JSReceiver>(iter);
     Maybe<bool> result = Just(false);  // Dummy initialization.
@@ -190,21 +193,23 @@ Maybe<bool> KeyAccumulator::CollectKeys(Handle<JSReceiver> receiver,
   return Just(true);
 }
 
+bool KeyAccumulator::HasShadowingKeys() { return !shadowing_keys_.is_null(); }
+
 bool KeyAccumulator::IsShadowed(Handle<Object> key) {
-  if (shadowed_keys_.is_null()) return false;
+  if (!HasShadowingKeys() || skip_shadow_check_) return false;
-  return shadowed_keys_->Has(isolate_, key);
+  return shadowing_keys_->Has(isolate_, key);
 }
 
-void KeyAccumulator::AddShadowKey(Object* key) {
+void KeyAccumulator::AddShadowingKey(Object* key) {
   if (mode_ == KeyCollectionMode::kOwnOnly) return;
-  AddShadowKey(handle(key, isolate_));
+  AddShadowingKey(handle(key, isolate_));
 }
-void KeyAccumulator::AddShadowKey(Handle<Object> key) {
+void KeyAccumulator::AddShadowingKey(Handle<Object> key) {
   if (mode_ == KeyCollectionMode::kOwnOnly) return;
-  if (shadowed_keys_.is_null()) {
+  if (shadowing_keys_.is_null()) {
-    shadowed_keys_ = ObjectHashSet::New(isolate_, 16);
+    shadowing_keys_ = ObjectHashSet::New(isolate_, 16);
   }
-  shadowed_keys_ = ObjectHashSet::Add(shadowed_keys_, key);
+  shadowing_keys_ = ObjectHashSet::Add(shadowing_keys_, key);
 }
 
 namespace {
@@ -548,7 +553,7 @@ int CollectOwnPropertyNamesInternal(Handle<JSObject> object,
     if (key->FilterKey(keys->filter())) continue;
 
     if (is_shadowing_key) {
-      keys->AddShadowKey(key);
+      keys->AddShadowingKey(key);
     } else {
       keys->AddKey(key, DO_NOT_CONVERT);
     }
@@ -590,7 +595,7 @@ Maybe<bool> KeyAccumulator::CollectOwnPropertyNames(Handle<JSReceiver> receiver,
           PropertyDetails details = descs->GetDetails(i);
           if (!details.IsDontEnum()) continue;
           Object* key = descs->GetKey(i);
-          this->AddShadowKey(key);
+          this->AddShadowingKey(key);
         }
       }
     } else if (object->IsJSGlobalObject()) {

src/keys.h

@@ -83,8 +83,8 @@ class KeyAccumulator final BASE_EMBEDDED {
   }
   // Shadowing keys are used to filter keys. This happens when non-enumerable
   // keys appear again on the prototype chain.
-  void AddShadowKey(Object* key);
+  void AddShadowingKey(Object* key);
-  void AddShadowKey(Handle<Object> key);
+  void AddShadowingKey(Handle<Object> key);
 
  private:
   Maybe<bool> CollectOwnKeys(Handle<JSReceiver> receiver,
@@ -96,6 +96,7 @@ class KeyAccumulator final BASE_EMBEDDED {
   Maybe<bool> AddKeysFromJSProxy(Handle<JSProxy> proxy,
                                  Handle<FixedArray> keys);
   bool IsShadowed(Handle<Object> key);
+  bool HasShadowingKeys();
   Handle<OrderedHashSet> keys() { return Handle<OrderedHashSet>::cast(keys_); }
 
   Isolate* isolate_;
@@ -104,12 +105,15 @@ class KeyAccumulator final BASE_EMBEDDED {
   // result list, a FixedArray containing all collected keys.
   Handle<FixedArray> keys_;
   Handle<JSReceiver> last_non_empty_prototype_;
-  Handle<ObjectHashSet> shadowed_keys_;
+  Handle<ObjectHashSet> shadowing_keys_;
   KeyCollectionMode mode_;
   PropertyFilter filter_;
   bool filter_proxy_keys_ = true;
   bool is_for_in_ = false;
   bool skip_indices_ = false;
+  // For all the keys on the first receiver adding a shadowing key we can skip
+  // the shadow check.
+  bool skip_shadow_check_ = true;
 
   DISALLOW_COPY_AND_ASSIGN(KeyAccumulator);
 };

src/objects.cc

@@ -17537,7 +17537,7 @@ void Dictionary<Derived, Shape, Key>::CopyEnumKeysTo(
     }
     if (dictionary->IsDeleted(i)) continue;
     if (is_shadowing_key) {
-      accumulator->AddShadowKey(key);
+      accumulator->AddShadowingKey(key);
       continue;
     } else {
       storage->set(properties, Smi::FromInt(i));
@@ -17577,7 +17577,7 @@ void Dictionary<Derived, Shape, Key>::CollectKeysTo(
       if (raw_dict->IsDeleted(i)) continue;
       PropertyDetails details = raw_dict->DetailsAt(i);
       if ((details.attributes() & filter) != 0) {
-        keys->AddShadowKey(k);
+        keys->AddShadowingKey(k);
         continue;
       }
       if (filter & ONLY_ALL_CAN_READ) {