[modules] Define @@toStringTag on namespace object as a field.

The constant field tracking implies data constants to be stored
in fields instead of descriptor arrays. This CL does necessary
modifications to the JSModuleNamespace map setup.

BUG=v8:1569, v8:5495

Review-Url: https://codereview.chromium.org/2625093005
Cr-Commit-Position: refs/heads/master@{#42294}

src/bootstrapper.cc

@@ -2583,11 +2583,14 @@ void Genesis::InitializeGlobal(Handle<JSGlobalObject> global_object,
     {  // Install @@toStringTag.
       PropertyAttributes attribs =
           static_cast<PropertyAttributes>(DONT_DELETE | DONT_ENUM | READ_ONLY);
-      Descriptor d = Descriptor::DataConstant(
-          factory->to_string_tag_symbol(),
-          factory->NewStringFromAsciiChecked("Module"), attribs);
+      Descriptor d =
+          Descriptor::DataField(factory->to_string_tag_symbol(),
+                                JSModuleNamespace::kToStringTagFieldIndex,
+                                attribs, Representation::Tagged());
       map->AppendDescriptor(&d);
     }
+
+    map->SetInObjectProperties(JSModuleNamespace::kInObjectFieldCount);
   }
 
   {  // -- I t e r a t o r R e s u l t

src/factory.cc

@@ -1851,7 +1851,13 @@ void Factory::NewJSArrayStorage(Handle<JSArray> array,
 
 Handle<JSModuleNamespace> Factory::NewJSModuleNamespace() {
   Handle<Map> map = isolate()->js_module_namespace_map();
-  return Handle<JSModuleNamespace>::cast(NewJSObjectFromMap(map));
+  Handle<JSModuleNamespace> module_namespace(
+      Handle<JSModuleNamespace>::cast(NewJSObjectFromMap(map)));
+  FieldIndex index = FieldIndex::ForDescriptor(
+      *map, JSModuleNamespace::kToStringTagFieldIndex);
+  Handle<String> to_string_value = NewStringFromAsciiChecked("Module");
+  module_namespace->FastPropertyAtPut(index, *to_string_value);
+  return module_namespace;
 }
 
 Handle<JSGeneratorObject> Factory::NewJSGeneratorObject(

src/objects-inl.h

@@ -2208,7 +2208,7 @@ int JSObject::GetHeaderSize(InstanceType type) {
     case JS_STRING_ITERATOR_TYPE:
       return JSStringIterator::kSize;
     case JS_MODULE_NAMESPACE_TYPE:
-      return JSModuleNamespace::kSize;
+      return JSModuleNamespace::kHeaderSize;
     default:
       if (type >= FIRST_ARRAY_ITERATOR_TYPE &&
           type <= LAST_ARRAY_ITERATOR_TYPE) {

src/objects-printer.cc

@@ -1279,9 +1279,9 @@ void Module::ModulePrint(std::ostream& os) {  // NOLINT
 }
 
 void JSModuleNamespace::JSModuleNamespacePrint(std::ostream& os) {  // NOLINT
-  HeapObject::PrintHeader(os, "JSModuleNamespace");
+  JSObjectPrintHeader(os, this, "JSModuleNamespace");
   os << "\n - module: " << Brief(module());
-  os << "\n";
+  JSObjectPrintBody(os, this);
 }
 
 void PrototypeInfo::PrototypeInfoPrint(std::ostream& os) {  // NOLINT

src/objects.h

@@ -7962,8 +7962,16 @@ class JSModuleNamespace : public JSObject {
   // schedule an exception and return Nothing.
   MUST_USE_RESULT MaybeHandle<Object> GetExport(Handle<String> name);
 
+  // In-object fields.
+  enum {
+    kToStringTagFieldIndex,
+    kInObjectFieldCount,
+  };
+
   static const int kModuleOffset = JSObject::kHeaderSize;
-  static const int kSize = kModuleOffset + kPointerSize;
+  static const int kHeaderSize = kModuleOffset + kPointerSize;
+
+  static const int kSize = kHeaderSize + kPointerSize * kInObjectFieldCount;
 
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(JSModuleNamespace);