[heap] Reland "Use shared markbits in the concurrent marker."

src/objects.h

 // Copyright 2015 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 <memory>
 
@@ skipping 1651 matching lines @@
 
 
 // HeapObject is the superclass for all classes describing heap allocated
 // objects.
 class HeapObject: public Object {
  public:
   // [map]: Contains a map which contains the object's reflective
   // information.
   inline Map* map() const;
   inline void set_map(Map* value);
+
+  inline HeapObject** map_slot();
+
   // The no-write-barrier version.  This is OK if the object is white and in
   // new space, or if the value is an immortal immutable object, like the maps
   // of primitive (non-JS) objects like strings, heap numbers etc.
   inline void set_map_no_write_barrier(Map* value);
 
   // Get the map using acquire load.
   inline Map* synchronized_map();
   inline MapWord synchronized_map_word() const;
 
   // Set the map using release store
@@ skipping 8520 matching lines @@
   }
 };
 
 
 }  // NOLINT, false-positive due to second-order macros.
 }  // NOLINT, false-positive due to second-order macros.
 
 #include "src/objects/object-macros-undef.h"
 
 #endif  // V8_OBJECTS_H_