Add fast path in MigrateFastToFast for following transitions that don't extend storage

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 "src/v8.h"
 
 #include "src/accessors.h"
 #include "src/allocation-site-scopes.h"
 #include "src/api.h"
 #include "src/arguments.h"
@@ skipping 1928 matching lines @@
   // converted to doubles.
   if (!old_map->InstancesNeedRewriting(*new_map, number_of_fields, inobject,
                                        unused, &old_number_of_fields)) {
     object->synchronized_set_map(*new_map);
     return;
   }
 
   int total_size = number_of_fields + unused;
   int external = total_size - inobject;
 
-  if ((old_map->unused_property_fields() == 0) &&
-      (number_of_fields != old_number_of_fields) &&
-      (new_map->GetBackPointer() == *old_map)) {
+  if (number_of_fields != old_number_of_fields &&
+      new_map->GetBackPointer() == *old_map) {
+    PropertyDetails details = new_map->GetLastDescriptorDetails();
+
+    if (old_map->unused_property_fields() > 0) {
+      if (details.representation().IsDouble()) {
+        Handle<Object> value = isolate->factory()->NewHeapNumber(0, MUTABLE);
+        FieldIndex index =
+            FieldIndex::ForDescriptor(*new_map, new_map->LastAdded());
+        object->FastPropertyAtPut(index, *value);
+      }
+      object->synchronized_set_map(*new_map);
+      return;
+    }
+
     DCHECK(number_of_fields == old_number_of_fields + 1);
     // This migration is a transition from a map that has run out out property
     // space. Therefore it could be done by extending the backing store.
     Handle<FixedArray> old_storage = handle(object->properties(), isolate);
     Handle<FixedArray> new_storage =
         FixedArray::CopySize(old_storage, external);
 
     // Properly initialize newly added property.
-    PropertyDetails details = new_map->GetLastDescriptorDetails();
     Handle<Object> value;
     if (details.representation().IsDouble()) {
       value = isolate->factory()->NewHeapNumber(0, MUTABLE);
     } else {
       value = isolate->factory()->uninitialized_value();
     }
     DCHECK(details.type() == FIELD);
     int target_index = details.field_index() - inobject;
     DCHECK(target_index >= 0);  // Must be a backing store index.
     new_storage->set(target_index, *value);
@@ skipping 14388 matching lines @@
 #define ERROR_MESSAGES_TEXTS(C, T) T,
   static const char* error_messages_[] = {
       ERROR_MESSAGES_LIST(ERROR_MESSAGES_TEXTS)
   };
 #undef ERROR_MESSAGES_TEXTS
   return error_messages_[reason];
 }
 
 
 } }  // namespace v8::internal