Add support for compacting the write buffer.

src/write-buffer.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 38 matching lines @@
   static inline void Mark(Address addr) {
     *top_++ = addr;
     if ((reinterpret_cast<uintptr_t>(top_) & kWriteBufferOverflowBit) != 0) {
       ASSERT(top_ == limit_);
       Compact();
     } else {
       ASSERT(top_ < limit_);
     }
   }
 
-  static const int kWriteBufferOverflowBit = 1 << 15;
+  static const int kWriteBufferOverflowBit = 1 << 16;
   static const int kWriteBufferSize = kWriteBufferOverflowBit;
+  static const int kHashMapLengthLog2 = 12;
+  static const int kHashMapLength = 1 << kHashMapLengthLog2;
 
  private:
   static Address* top_;
   static Address* start_;
   static Address* limit_;
   static VirtualMemory* virtual_memory_;
+  static uintptr_t* hash_map_1_;
+  static uintptr_t* hash_map_2_;
 
   static void Compact();
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_WRITE_BARRIER_H_