[strong] Introduce strong bit

src/objects.h

 // Copyright 2012 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.
 
 #ifndef V8_OBJECTS_H_
 #define V8_OBJECTS_H_
 
 #include <iosfwd>
 
 #include "src/allocation.h"
@@ skipping 5882 matching lines @@
       0, kDescriptorIndexBitCount> {};  // NOLINT
   class NumberOfOwnDescriptorsBits: public BitField<int,
       kDescriptorIndexBitCount, kDescriptorIndexBitCount> {};  // NOLINT
   STATIC_ASSERT(kDescriptorIndexBitCount + kDescriptorIndexBitCount == 20);
   class DictionaryMap : public BitField<bool, 20, 1> {};
   class OwnsDescriptors : public BitField<bool, 21, 1> {};
   class HasInstanceCallHandler : public BitField<bool, 22, 1> {};
   class Deprecated : public BitField<bool, 23, 1> {};
   class IsUnstable : public BitField<bool, 24, 1> {};
   class IsMigrationTarget : public BitField<bool, 25, 1> {};
-  // Bits 26 and 27 are free.
+  class IsStrong : public BitField<bool, 26, 1> {};

[arv (Not doing code reviews), 2015/05/12 15:00:30]

Can you change this to use kNext instead?

+  // Bit 27 is free.
 
   // Keep this bit field at the very end for better code in
   // Builtins::kJSConstructStubGeneric stub.
   // This counter is used for in-object slack tracking and for map aging.
   // The in-object slack tracking is considered enabled when the counter is
   // in the range [kSlackTrackingCounterStart, kSlackTrackingCounterEnd].
   class Counter : public BitField<int, 28, 4> {};
   static const int kSlackTrackingCounterStart = 14;
   static const int kSlackTrackingCounterEnd = 8;
   static const int kRetainingCounterStart = kSlackTrackingCounterEnd - 1;
@@ skipping 57 matching lines @@
 
   // Tells whether the instance has a call-as-function handler.
   inline void set_is_observed() {
     set_bit_field(bit_field() | (1 << kIsObserved));
   }
 
   inline bool is_observed() {
     return ((1 << kIsObserved) & bit_field()) != 0;
   }
 
+  inline void set_is_strong(bool value);
+  inline bool is_strong();
   inline void set_is_extensible(bool value);
   inline bool is_extensible();
   inline void set_is_prototype_map(bool value);
   inline bool is_prototype_map() const;
 
   inline void set_elements_kind(ElementsKind elements_kind) {
     DCHECK(static_cast<int>(elements_kind) < kElementsKindCount);
     DCHECK(kElementsKindCount <= (1 << Map::ElementsKindBits::kSize));
     set_bit_field2(Map::ElementsKindBits::update(bit_field2(), elements_kind));
     DCHECK(this->elements_kind() == elements_kind);
@@ skipping 5116 matching lines @@
     } else {
       value &= ~(1 << bit_position);
     }
     return value;
   }
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_OBJECTS_H_