[in-place weak refs] Migrate PrototypeInfo::weak_cell.

Currently it was only used for modules, so repurposed it to be a weak pointer to
JSModuleNamespace.

BUG=v8:7308

Change-Id: I4ef522fafebd37624c309081d7432501c2c69b7a
Reviewed-on: https://chromium-review.googlesource.com/1163704Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55013}

src/compiler/access-info.cc

@@ -428,10 +428,8 @@ bool AccessInfoFactory::ComputePropertyAccessInfo(
             DCHECK(map->is_prototype_map());
             Handle<PrototypeInfo> proto_info =
                 Map::GetOrCreatePrototypeInfo(map, isolate());
-            DCHECK(proto_info->weak_cell()->IsWeakCell());
             Handle<JSModuleNamespace> module_namespace(
-                JSModuleNamespace::cast(
-                    WeakCell::cast(proto_info->weak_cell())->value()),
+                JSModuleNamespace::cast(proto_info->module_namespace()),
                 isolate());
             Handle<Cell> cell(
                 Cell::cast(module_namespace->module()->exports()->Lookup(

src/heap/factory.cc

@@ -209,6 +209,7 @@ Handle<PrototypeInfo> Factory::NewPrototypeInfo() {
   result->set_prototype_users(*empty_weak_array_list());
   result->set_registry_slot(PrototypeInfo::UNREGISTERED);
   result->set_bit_field(0);
+  result->set_module_namespace(*undefined_value());
   return result;
 }
 

src/objects-debug.cc

@@ -1561,7 +1561,8 @@ void Module::ModuleVerify(Isolate* isolate) {
 
 void PrototypeInfo::PrototypeInfoVerify(Isolate* isolate) {
   CHECK(IsPrototypeInfo());
-  CHECK(weak_cell()->IsWeakCell() || weak_cell()->IsUndefined(isolate));
+  Object* module_ns = module_namespace();
+  CHECK(module_ns->IsJSModuleNamespace() || module_ns->IsUndefined(isolate));
   if (prototype_users()->IsWeakArrayList()) {
     PrototypeUsers::Verify(WeakArrayList::cast(prototype_users()));
   } else {

src/objects-printer.cc

@@ -1683,7 +1683,7 @@ void JSModuleNamespace::JSModuleNamespacePrint(std::ostream& os) {  // NOLINT
 
 void PrototypeInfo::PrototypeInfoPrint(std::ostream& os) {  // NOLINT
   HeapObject::PrintHeader(os, "PrototypeInfo");
-  os << "\n - weak cell: " << Brief(weak_cell());
+  os << "\n - module namespace: " << Brief(module_namespace());
   os << "\n - prototype users: " << Brief(prototype_users());
   os << "\n - registry slot: " << registry_slot();
   os << "\n - object create map: " << MaybeObjectBrief(object_create_map());

src/objects.cc

@@ -12912,30 +12912,6 @@ bool Map::IsPrototypeChainInvalidated(Map* map) {
   return true;
 }
 
-// static
-Handle<WeakCell> Map::GetOrCreatePrototypeWeakCell(Handle<JSReceiver> prototype,
-                                                   Isolate* isolate) {
-  DCHECK(!prototype.is_null());
-  if (prototype->IsJSProxy()) {
-    Handle<WeakCell> cell = isolate->factory()->NewWeakCell(prototype);
-    return cell;
-  }
-
-  Handle<PrototypeInfo> proto_info =
-      GetOrCreatePrototypeInfo(Handle<JSObject>::cast(prototype), isolate);
-  Object* maybe_cell = proto_info->weak_cell();
-  // Return existing cell if it's already created.
-  if (maybe_cell->IsWeakCell()) {
-    Handle<WeakCell> cell(WeakCell::cast(maybe_cell), isolate);
-    DCHECK(!cell->cleared());
-    return cell;
-  }
-  // Otherwise create a new cell.
-  Handle<WeakCell> cell = isolate->factory()->NewWeakCell(prototype);
-  proto_info->set_weak_cell(*cell);
-  return cell;
-}
-
 // static
 void Map::SetPrototype(Isolate* isolate, Handle<Map> map,
                        Handle<Object> prototype,

src/objects/map.h

@@ -448,11 +448,6 @@ class Map : public HeapObject {
   // Return the map of the root of object's prototype chain.
   Map* GetPrototypeChainRootMap(Isolate* isolate) const;
 
-  // Returns a WeakCell object containing given prototype. The cell is cached
-  // in PrototypeInfo which is created lazily.
-  static Handle<WeakCell> GetOrCreatePrototypeWeakCell(
-      Handle<JSReceiver> prototype, Isolate* isolate);
-
   Map* FindRootMap(Isolate* isolate) const;
   Map* FindFieldOwner(Isolate* isolate, int descriptor) const;
 

src/objects/module.cc

@@ -900,7 +900,10 @@ Handle<JSModuleNamespace> Module::GetModuleNamespace(Isolate* isolate,
   // - We can store a pointer from the map back to the namespace object.
   //   Turbofan can use this for inlining the access.
   JSObject::OptimizeAsPrototype(ns);
-  Map::GetOrCreatePrototypeWeakCell(ns, isolate);
+
+  Handle<PrototypeInfo> proto_info =
+      Map::GetOrCreatePrototypeInfo(Handle<JSObject>::cast(ns), isolate);
+  proto_info->set_module_namespace(*ns);
   return ns;
 }
 

src/objects/prototype-info-inl.h

@@ -34,7 +34,7 @@ bool PrototypeInfo::HasObjectCreateMap() {
   return cache->IsWeakHeapObject();
 }
 
-ACCESSORS(PrototypeInfo, weak_cell, Object, kWeakCellOffset)
+ACCESSORS(PrototypeInfo, module_namespace, Object, kJSModuleNamespaceOffset)
 ACCESSORS(PrototypeInfo, prototype_users, Object, kPrototypeUsersOffset)
 WEAK_ACCESSORS(PrototypeInfo, object_create_map, kObjectCreateMapOffset)
 SMI_ACCESSORS(PrototypeInfo, registry_slot, kRegistrySlotOffset)

src/objects/prototype-info.h

@@ -19,8 +19,11 @@ class PrototypeInfo : public Struct {
  public:
   static const int UNREGISTERED = -1;
 
-  // [weak_cell]: A WeakCell containing this prototype. ICs cache the cell here.
-  DECL_ACCESSORS(weak_cell, Object)
+  // [module_namespace]: A backpointer to JSModuleNamespace from its
+  // PrototypeInfo (or undefined). This field is only used for JSModuleNamespace
+  // maps.  TODO(jkummerow): Figure out if there's a way to store the namespace
+  // pointer elsewhere to save memory.
+  DECL_ACCESSORS(module_namespace, Object)
 
   // [prototype_users]: WeakArrayList containing weak references to maps using
   // this prototype, or Smi(0) if uninitialized.
@@ -49,8 +52,9 @@ class PrototypeInfo : public Struct {
   DECL_PRINTER(PrototypeInfo)
   DECL_VERIFIER(PrototypeInfo)
 
-  static const int kWeakCellOffset = HeapObject::kHeaderSize;
-  static const int kPrototypeUsersOffset = kWeakCellOffset + kPointerSize;
+  static const int kJSModuleNamespaceOffset = HeapObject::kHeaderSize;
+  static const int kPrototypeUsersOffset =
+      kJSModuleNamespaceOffset + kPointerSize;
   static const int kRegistrySlotOffset = kPrototypeUsersOffset + kPointerSize;
   static const int kValidityCellOffset = kRegistrySlotOffset + kPointerSize;
   static const int kObjectCreateMapOffset = kValidityCellOffset + kPointerSize;