Use a private own symbol instead of a hidden property for hash codes

src/objects.cc

Index: src/objects.cc
diff --git a/src/objects.cc b/src/objects.cc
index 6dc5de2552847c8487da20604452705a7837143f..23edd137bb12aaee726599011974a038fd4a8a68 100644
--- a/src/objects.cc
+++ b/src/objects.cc
@@ -5014,7 +5014,8 @@ static Smi* GenerateIdentityHash(Isolate* isolate) {
 void JSObject::SetIdentityHash(Handle<JSObject> object, Handle<Smi> hash) {
   DCHECK(!object->IsJSGlobalProxy());
   Isolate* isolate = object->GetIsolate();
-  SetHiddenProperty(object, isolate->factory()->identity_hash_string(), hash);
+  Handle<Name> hash_code_symbol(isolate->heap()->hash_code_symbol());
+  JSObject::AddProperty(object, hash_code_symbol, hash, NONE);
 }
 
 
@@ -5037,11 +5038,12 @@ Object* JSObject::GetIdentityHash() {
   if (IsJSGlobalProxy()) {
     return JSGlobalProxy::cast(this)->hash();

[adamk, 2015/05/19 17:39:38]

To expand slightly on Toon's explanation, here's the codepath we used to go down
for the identity hash of the JSGlobalProxy.

   }
-  Object* stored_value =
-      GetHiddenProperty(isolate->factory()->identity_hash_string());
-  return stored_value->IsSmi()
-      ? stored_value
-      : isolate->heap()->undefined_value();
+  Handle<Name> hash_code_symbol(isolate->heap()->hash_code_symbol());
+  Handle<Object> stored_value =
+      Object::GetPropertyOrElement(Handle<Object>(this, isolate),
+                                   hash_code_symbol).ToHandleChecked();
+  return stored_value->IsSmi() ? *stored_value
+                               : isolate->heap()->undefined_value();
 }
 
 
@@ -5056,7 +5058,8 @@ Handle<Smi> JSObject::GetOrCreateIdentityHash(Handle<JSObject> object) {
   if (maybe_hash->IsSmi()) return Handle<Smi>::cast(maybe_hash);
 
   Handle<Smi> hash(GenerateIdentityHash(isolate), isolate);
-  SetHiddenProperty(object, isolate->factory()->identity_hash_string(), hash);
+  Handle<Name> hash_code_symbol(isolate->heap()->hash_code_symbol());
+  JSObject::AddProperty(object, hash_code_symbol, hash, NONE);
   return hash;
 }
 
@@ -5075,8 +5078,6 @@ Object* JSObject::GetHiddenProperty(Handle<Name> key) {
   DisallowHeapAllocation no_gc;
   DCHECK(key->IsUniqueName());
   if (IsJSGlobalProxy()) {
-    // JSGlobalProxies store their hash internally.
-    DCHECK(*key != GetHeap()->identity_hash_string());
     // For a proxy, use the prototype as target object.
     PrototypeIterator iter(GetIsolate(), this);
     // If the proxy is detached, return undefined.
@@ -5087,15 +5088,7 @@ Object* JSObject::GetHiddenProperty(Handle<Name> key) {
   DCHECK(!IsJSGlobalProxy());
   Object* inline_value = GetHiddenPropertiesHashTable();
 
-  if (inline_value->IsSmi()) {
-    // Handle inline-stored identity hash.
-    if (*key == GetHeap()->identity_hash_string()) {
-      return inline_value;
-    } else {
-      return GetHeap()->the_hole_value();
-    }
-  }
-
+  DCHECK(!inline_value->IsSmi());
   if (inline_value->IsUndefined()) return GetHeap()->the_hole_value();
 
   ObjectHashTable* hashtable = ObjectHashTable::cast(inline_value);
@@ -5111,8 +5104,6 @@ Handle<Object> JSObject::SetHiddenProperty(Handle<JSObject> object,
 
   DCHECK(key->IsUniqueName());
   if (object->IsJSGlobalProxy()) {
-    // JSGlobalProxies store their hash internally.
-    DCHECK(*key != *isolate->factory()->identity_hash_string());
     // For a proxy, use the prototype as target object.
     PrototypeIterator iter(isolate, object);
     // If the proxy is detached, return undefined.
@@ -5126,13 +5117,6 @@ Handle<Object> JSObject::SetHiddenProperty(Handle<JSObject> object,
 
   Handle<Object> inline_value(object->GetHiddenPropertiesHashTable(), isolate);
 
-  // If there is no backing store yet, store the identity hash inline.
-  if (value->IsSmi() &&
-      *key == *isolate->factory()->identity_hash_string() &&
-      (inline_value->IsUndefined() || inline_value->IsSmi())) {
-    return JSObject::SetHiddenPropertiesHashTable(object, value);
-  }
-
   Handle<ObjectHashTable> hashtable =
       GetOrCreateHiddenPropertiesHashtable(object);
 
@@ -5164,9 +5148,8 @@ void JSObject::DeleteHiddenProperty(Handle<JSObject> object, Handle<Name> key) {
 
   Object* inline_value = object->GetHiddenPropertiesHashTable();
 
-  // We never delete (inline-stored) identity hashes.
-  DCHECK(*key != *isolate->factory()->identity_hash_string());
-  if (inline_value->IsUndefined() || inline_value->IsSmi()) return;
+  DCHECK(!inline_value->IsSmi());
+  if (inline_value->IsUndefined()) return;
 
   Handle<ObjectHashTable> hashtable(ObjectHashTable::cast(inline_value));
   bool was_present = false;
@@ -5230,14 +5213,7 @@ Handle<ObjectHashTable> JSObject::GetOrCreateHiddenPropertiesHashtable(
   Handle<ObjectHashTable> hashtable = ObjectHashTable::New(
       isolate, kInitialCapacity, USE_CUSTOM_MINIMUM_CAPACITY);
 
-  if (inline_value->IsSmi()) {
-    // We were storing the identity hash inline and now allocated an actual
-    // dictionary.  Put the identity hash into the new dictionary.
-    hashtable = ObjectHashTable::Put(hashtable,
-                                     isolate->factory()->identity_hash_string(),
-                                     inline_value);
-  }
-
+  DCHECK(inline_value->IsUndefined());
   SetHiddenPropertiesHashTable(object, hashtable);
   return hashtable;
 }