Use linear backing store for hidden properties to save memory.

src/objects.cc

Index: src/objects.cc
diff --git a/src/objects.cc b/src/objects.cc
index fe94a42c4ad9fcb709c9c2af86b0a15fb34cf32f..c798343a1f32a7a2c448e237b53f79a2b3a8cd62 100644
--- a/src/objects.cc
+++ b/src/objects.cc
@@ -3559,16 +3559,14 @@ Object* JSObject::GetHiddenProperty(String* key) {
     return JSObject::cast(proxy_parent)->GetHiddenProperty(key);
   }
   ASSERT(!IsJSGlobalProxy());
-  MaybeObject* hidden_lookup = GetHiddenPropertiesDictionary(false);
+  MaybeObject* hidden_lookup = GetHiddenPropertiesArray(false);
   ASSERT(!hidden_lookup->IsFailure());  // No failure when passing false as arg.
   if (hidden_lookup->ToObjectUnchecked()->IsUndefined()) {
     return GetHeap()->undefined_value();
   }
-  StringDictionary* dictionary =
-      StringDictionary::cast(hidden_lookup->ToObjectUnchecked());
-  int entry = dictionary->FindEntry(key);
-  if (entry == StringDictionary::kNotFound) return GetHeap()->undefined_value();
-  return dictionary->ValueAt(entry);
+  HiddenPropertiesArray* hidden_properties =
+      HiddenPropertiesArray::cast(hidden_lookup->ToObjectUnchecked());
+  return hidden_properties->Get(key);
 }
 
 
@@ -3591,28 +3589,25 @@ MaybeObject* JSObject::SetHiddenProperty(String* key, Object* value) {
     return JSObject::cast(proxy_parent)->SetHiddenProperty(key, value);
   }
   ASSERT(!IsJSGlobalProxy());
-  MaybeObject* hidden_lookup = GetHiddenPropertiesDictionary(true);
-  StringDictionary* dictionary;
-  if (!hidden_lookup->To<StringDictionary>(&dictionary)) return hidden_lookup;
 
-  // If it was found, check if the key is already in the dictionary.
-  int entry = dictionary->FindEntry(key);
-  if (entry != StringDictionary::kNotFound) {
-    // If key was found, just update the value.
-    dictionary->ValueAtPut(entry, value);
-    return this;
+  HiddenPropertiesArray* hidden_properties;
+  { MaybeObject* maybe_obj = GetHiddenPropertiesArray(true);
+    if (!maybe_obj->To<HiddenPropertiesArray>(&hidden_properties)) {
+      return maybe_obj;
+    }
   }
-  // Key was not already in the dictionary, so add the entry.
-  MaybeObject* insert_result = dictionary->Add(key,
-                                               value,
-                                               PropertyDetails(NONE, NORMAL));
-  StringDictionary* new_dict;
-  if (!insert_result->To<StringDictionary>(&new_dict)) return insert_result;
-  if (new_dict != dictionary) {
-    // If adding the key expanded the dictionary (i.e., Add returned a new
-    // dictionary), store it back to the object.
-    MaybeObject* store_result = SetHiddenPropertiesDictionary(new_dict);
-    if (store_result->IsFailure()) return store_result;
+
+  HiddenPropertiesArray* new_hidden_properties;
+  MaybeObject* store_result = hidden_properties->Set(key, value);
+  if (!store_result->To<HiddenPropertiesArray>(&new_hidden_properties)) {
+    return store_result;
+  }
+
+  if (new_hidden_properties != hidden_properties) {
+    // Update the object if the backing store has been re-allocated.
+    MaybeObject* maybe_obj = SetHiddenPropertiesArray(
+        new_hidden_properties);
+    if (maybe_obj->IsFailure()) return maybe_obj;
   }
   // Return this to mark success.
   return this;
@@ -3629,18 +3624,12 @@ void JSObject::DeleteHiddenProperty(String* key) {
     JSObject::cast(proxy_parent)->DeleteHiddenProperty(key);
     return;
   }
-  MaybeObject* hidden_lookup = GetHiddenPropertiesDictionary(false);
-  ASSERT(!hidden_lookup->IsFailure());  // No failure when passing false as arg.
+  MaybeObject* hidden_lookup = GetHiddenPropertiesArray(false);
+  ASSERT(!hidden_lookup->IsFailure());
   if (hidden_lookup->ToObjectUnchecked()->IsUndefined()) return;
-  StringDictionary* dictionary =
-      StringDictionary::cast(hidden_lookup->ToObjectUnchecked());
-  int entry = dictionary->FindEntry(key);
-  if (entry == StringDictionary::kNotFound) {
-    // Key wasn't in dictionary. Deletion is a success.
-    return;
-  }
-  // Key was in the dictionary. Remove it.
-  dictionary->DeleteProperty(entry, JSReceiver::FORCE_DELETION);
+  HiddenPropertiesArray* hidden_properties =
+      HiddenPropertiesArray::cast(hidden_lookup->ToObjectUnchecked());
+  hidden_properties->Delete(key);
 }
 
 
@@ -3651,7 +3640,7 @@ bool JSObject::HasHiddenProperties() {
 }
 
 
-MaybeObject* JSObject::GetHiddenPropertiesDictionary(bool create_if_absent) {
+MaybeObject* JSObject::GetHiddenPropertiesArray(bool create_if_absent) {
   ASSERT(!IsJSGlobalProxy());
   if (HasFastProperties()) {
     // If the object has fast properties, check whether the first slot
@@ -3662,9 +3651,9 @@ MaybeObject* JSObject::GetHiddenPropertiesDictionary(bool create_if_absent) {
     if ((descriptors->number_of_descriptors() > 0) &&
         (descriptors->GetKey(0) == GetHeap()->hidden_symbol())) {
       ASSERT(descriptors->GetType(0) == FIELD);
-      Object* hidden_store =
+      Object* hidden_properties =
           this->FastPropertyAt(descriptors->GetFieldIndex(0));
-      return StringDictionary::cast(hidden_store);
+      return HiddenPropertiesArray::cast(hidden_properties);
     }
   } else {
     PropertyAttributes attributes;
@@ -3676,27 +3665,31 @@ MaybeObject* JSObject::GetHiddenPropertiesDictionary(bool create_if_absent) {
                                         GetHeap()->hidden_symbol(),
                                         &attributes)->ToObjectUnchecked();
     if (!lookup->IsUndefined()) {
-      return StringDictionary::cast(lookup);
+      return HiddenPropertiesArray::cast(lookup);
     }
   }
   if (!create_if_absent) return GetHeap()->undefined_value();
-  const int kInitialSize = 5;
-  MaybeObject* dict_alloc = StringDictionary::Allocate(kInitialSize);
-  StringDictionary* dictionary;
-  if (!dict_alloc->To<StringDictionary>(&dictionary)) return dict_alloc;
+
+  HiddenPropertiesArray* hidden_properties;
+  { MaybeObject* maybe_obj = HiddenPropertiesArray::Allocate();
+    if (!maybe_obj->To<HiddenPropertiesArray>(&hidden_properties)) {
+      return maybe_obj;
+    }
+  }
+
   MaybeObject* store_result =
       SetPropertyPostInterceptor(GetHeap()->hidden_symbol(),
-                                 dictionary,
+                                 hidden_properties,
                                  DONT_ENUM,
                                  kNonStrictMode,
                                  OMIT_EXTENSIBILITY_CHECK);
   if (store_result->IsFailure()) return store_result;
-  return dictionary;
+  return hidden_properties;
 }
 
 
-MaybeObject* JSObject::SetHiddenPropertiesDictionary(
-    StringDictionary* dictionary) {
+MaybeObject* JSObject::SetHiddenPropertiesArray(
+    HiddenPropertiesArray* hidden_properties) {
   ASSERT(!IsJSGlobalProxy());
   ASSERT(HasHiddenProperties());
   if (HasFastProperties()) {
@@ -3708,13 +3701,13 @@ MaybeObject* JSObject::SetHiddenPropertiesDictionary(
     if ((descriptors->number_of_descriptors() > 0) &&
         (descriptors->GetKey(0) == GetHeap()->hidden_symbol())) {
       ASSERT(descriptors->GetType(0) == FIELD);
-      this->FastPropertyAtPut(descriptors->GetFieldIndex(0), dictionary);
+      this->FastPropertyAtPut(descriptors->GetFieldIndex(0), hidden_properties);
       return this;
     }
   }
   MaybeObject* store_result =
       SetPropertyPostInterceptor(GetHeap()->hidden_symbol(),
-                                 dictionary,
+                                 hidden_properties,
                                  DONT_ENUM,
                                  kNonStrictMode,
                                  OMIT_EXTENSIBILITY_CHECK);