Only normalize prototypes in OptimizeAsPrototype if it would benefit.

src/objects.cc

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include <sstream>
 
 #include "src/v8.h"
 
 #include "src/accessors.h"
 #include "src/allocation-site-scopes.h"
@@ skipping 10050 matching lines @@
 
   int slack = map->unused_property_fields();
   map->TraverseTransitionTree(&GetMinInobjectSlack, &slack);
   if (slack != 0) {
     // Resize the initial map and all maps in its transition tree.
     map->TraverseTransitionTree(&ShrinkInstanceSize, &slack);
   }
 }
 
 
+static bool PrototypeBenefitsFromNormalization(Handle<JSObject> object) {
+  DisallowHeapAllocation no_gc;
+  if (!object->HasFastProperties()) return false;
+  Map* map = object->map();
+  if (map->is_prototype_map()) return false;
+  DescriptorArray* descriptors = map->instance_descriptors();
+  for (int i = 0; i < map->NumberOfOwnDescriptors(); i++) {
+    PropertyDetails details = descriptors->GetDetails(i);
+    if (details.location() == kDescriptor) continue;
+    if (details.representation().IsHeapObject() ||
+        details.representation().IsTagged()) {
+      FieldIndex index = FieldIndex::ForDescriptor(map, i);
+      if (object->RawFastPropertyAt(index)->IsJSFunction()) return true;
+    }
+  }
+  return false;
+}
+
+
 void JSObject::OptimizeAsPrototype(Handle<JSObject> object,
                                    PrototypeOptimizationMode mode) {
   if (object->IsGlobalObject()) return;
   if (object->IsJSGlobalProxy()) return;
-  if (mode == FAST_PROTOTYPE && !object->map()->is_prototype_map()) {
+  if (mode == FAST_PROTOTYPE && PrototypeBenefitsFromNormalization(object)) {
     // First normalize to ensure all JSFunctions are DATA_CONSTANT.
     JSObject::NormalizeProperties(object, KEEP_INOBJECT_PROPERTIES, 0,
                                   "NormalizeAsPrototype");
   }
   bool has_just_copied_map = false;
   if (!object->HasFastProperties()) {
     JSObject::MigrateSlowToFast(object, 0, "OptimizeAsPrototype");
     has_just_copied_map = true;
   }
   if (mode == FAST_PROTOTYPE && object->HasFastProperties() &&
@@ skipping 7220 matching lines @@
                                                CompilationInfo* info) {
   Handle<DependentCode> codes = DependentCode::InsertCompilationInfo(
       handle(cell->dependent_code(), info->isolate()),
       DependentCode::kPropertyCellChangedGroup, info->object_wrapper());
   if (*codes != cell->dependent_code()) cell->set_dependent_code(*codes);
   info->dependencies(DependentCode::kPropertyCellChangedGroup)->Add(
       cell, info->zone());
 }
 
 } }  // namespace v8::internal