Fix/implement new write barrier for x64.

src/heap.h

 // Copyright 2010 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 30 matching lines @@
 // Defines all the roots in Heap.
 #define UNCONDITIONAL_STRONG_ROOT_LIST(V)                                      \
   /* Put the byte array map early.  We need it to be in place by the time   */ \
   /* the deserializer hits the next page, since it wants to put a byte      */ \
   /* array in the unused space at the end of the page.                      */ \
   V(Map, byte_array_map, ByteArrayMap)                                         \
   V(Map, one_pointer_filler_map, OnePointerFillerMap)                          \
   V(Map, two_pointer_filler_map, TwoPointerFillerMap)                          \
   /* Cluster the most popular ones in a few cache lines here at the top.    */ \
   V(Smi, stack_limit, StackLimit)                                              \
+  V(Smi, write_buffer_top, WriteBufferTop)                                     \
   V(Object, undefined_value, UndefinedValue)                                   \
   V(Object, the_hole_value, TheHoleValue)                                      \
   V(Object, null_value, NullValue)                                             \
   V(Object, true_value, TrueValue)                                             \
   V(Object, false_value, FalseValue)                                           \
   V(Map, heap_number_map, HeapNumberMap)                                       \
   V(Map, global_context_map, GlobalContextMap)                                 \
   V(Map, fixed_array_map, FixedArrayMap)                                       \
   V(Map, fixed_cow_array_map, FixedCOWArrayMap)                                \
   V(Object, no_interceptor_result_sentinel, NoInterceptorResultSentinel)       \
@@ skipping 837 matching lines @@
 
   // Sets the non_monomorphic_cache_ (only used when expanding the dictionary).
   static void public_set_non_monomorphic_cache(NumberDictionary* value) {
     roots_[kNonMonomorphicCacheRootIndex] = value;
   }
 
   static void public_set_empty_script(Script* script) {
     roots_[kEmptyScriptRootIndex] = script;
   }
 
+  static void public_set_write_buffer_top(Address* top) {
+    roots_[kWriteBufferTopRootIndex] = reinterpret_cast<Smi*>(top);
+  }
+
   // Update the next script id.
   static inline void SetLastScriptId(Object* last_script_id);
 
   // Generated code can embed this address to get access to the roots.
   static Object** roots_address() { return roots_; }
 
+  static Address* write_buffer_top_address() {
+    return reinterpret_cast<Address*>(&roots_[kWriteBufferTopRootIndex]);
+  }
+
   // Get address of global contexts list for serialization support.
   static Object** global_contexts_list_address() {
     return &global_contexts_list_;
   }
 
 #ifdef DEBUG
   static void Print();
   static void PrintHandles();
 
   // Verify the heap is in its normal state before or after a GC.
@@ skipping 1198 matching lines @@
   // Return whether this object should be retained. If NULL is returned the
   // object has no references. Otherwise the address of the retained object
   // should be returned as in some GC situations the object has been moved.
   virtual Object* RetainAs(Object* object) = 0;
 };
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_HEAP_H_