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

src/objects-inl.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.
 //
 // Review notes:
 //
 // - The use of macros in these inline functions may seem superfluous
 // but it is absolutely needed to make sure gcc generates optimal
 // code. gcc is not happy when attempting to inline too deep.
 //
@@ skipping 1507 matching lines @@
 void HeapObject::synchronized_set_map_no_write_barrier(Map* value) {
   synchronized_set_map_word(MapWord::FromMap(value));
 }
 
 
 // Unsafe accessor omitting write barrier.
 void HeapObject::set_map_no_write_barrier(Map* value) {
   set_map_word(MapWord::FromMap(value));
 }
 
+HeapObject** HeapObject::map_slot() {
+  return reinterpret_cast<HeapObject**>(FIELD_ADDR(this, kMapOffset));
+}
 
 MapWord HeapObject::map_word() const {
   return MapWord(
       reinterpret_cast<uintptr_t>(NOBARRIER_READ_FIELD(this, kMapOffset)));
 }
 
 
 void HeapObject::set_map_word(MapWord map_word) {
   NOBARRIER_WRITE_FIELD(
       this, kMapOffset, reinterpret_cast<Object*>(map_word.value_));
@@ skipping 6713 matching lines @@
 #undef WRITE_BYTE_FIELD
 #undef NOBARRIER_READ_BYTE_FIELD
 #undef NOBARRIER_WRITE_BYTE_FIELD
 
 }  // namespace internal
 }  // namespace v8
 
 #include "src/objects/object-macros-undef.h"
 
 #endif  // V8_OBJECTS_INL_H_