Implement immutable prototype chains

src/objects.cc

Index: src/objects.cc
diff --git a/src/objects.cc b/src/objects.cc
index 9a393a833a93203bd31157194e62f4acb050d1ce..0a306059d396f62489e7d667e0ff2cfe10a72278 100644
--- a/src/objects.cc
+++ b/src/objects.cc
@@ -8681,6 +8681,17 @@ Handle<Map> Map::CopyNormalized(Handle<Map> map,
   return result;
 }
 
+// Return an immutable prototype exotic object version of the input map.
+// Never even try to cache it in the transition tree, as it is intended
+// for the global object and its prototype chain, and excluding it saves
+// memory on the map transition tree.
+
+// static
+Handle<Map> Map::TransitionToImmutableProto(Handle<Map> map) {
+  Handle<Map> new_map = Map::Copy(map, "ImmutablePrototype");
+  new_map->set_immutable_proto(true);
+  return new_map;
+}
 
 Handle<Map> Map::CopyInitialMap(Handle<Map> map, int instance_size,
                                 int in_object_properties,
@@ -14955,6 +14966,13 @@ Maybe<bool> JSObject::SetPrototype(Handle<JSObject> object,
   // Nothing to do if prototype is already set.
   if (map->prototype() == *value) return Just(true);
 
+  bool immutable_proto = object->map()->is_immutable_proto();
+  if (immutable_proto) {
+    RETURN_FAILURE(
+        isolate, should_throw,
+        NewTypeError(MessageTemplate::kImmutablePrototypeSet, object));
+  }
+
   // From 8.6.2 Object Internal Methods
   // ...
   // In addition, if [[Extensible]] is false the value of the [[Class]] and
@@ -15006,6 +15024,17 @@ Maybe<bool> JSObject::SetPrototype(Handle<JSObject> object,
   return Just(true);
 }
 
+// static
+void JSObject::SetImmutableProto(Handle<JSObject> object) {
+  DCHECK(!object->IsAccessCheckNeeded());  // Never called from JS
+  Handle<Map> map(object->map());
+
+  // Nothing to do if prototype is already set.
+  if (map->is_immutable_proto()) return;
+
+  Handle<Map> new_map = Map::TransitionToImmutableProto(map);
+  object->set_map(*new_map);
+}
 
 void JSObject::EnsureCanContainElements(Handle<JSObject> object,
                                         Arguments* args,