Turn visitor_id into a byte field and free back one word in Map.

src/objects.h

 // Copyright 2006-2009 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 3227 matching lines @@
   static const int kMaxPreAllocatedPropertyFields = 255;
 
   // Layout description.
   static const int kInstanceSizesOffset = HeapObject::kHeaderSize;
   static const int kInstanceAttributesOffset = kInstanceSizesOffset + kIntSize;
   static const int kPrototypeOffset = kInstanceAttributesOffset + kIntSize;
   static const int kConstructorOffset = kPrototypeOffset + kPointerSize;
   static const int kInstanceDescriptorsOffset =
       kConstructorOffset + kPointerSize;
   static const int kCodeCacheOffset = kInstanceDescriptorsOffset + kPointerSize;
-  static const int kScavengerCallbackOffset = kCodeCacheOffset + kPointerSize;
-  static const int kPadStart = kScavengerCallbackOffset + kPointerSize;
+  static const int kPadStart = kCodeCacheOffset + kPointerSize;
   static const int kSize = MAP_POINTER_ALIGN(kPadStart);
 
   // Layout of pointer fields. Heap iteration code relies on them
   // being continiously allocated.
   static const int kPointerFieldsBeginOffset = Map::kPrototypeOffset;
   static const int kPointerFieldsEndOffset =
       Map::kCodeCacheOffset + kPointerSize;
 
   // Byte offsets within kInstanceSizesOffset.
   static const int kInstanceSizeOffset = kInstanceSizesOffset + 0;
   static const int kInObjectPropertiesByte = 1;
   static const int kInObjectPropertiesOffset =
       kInstanceSizesOffset + kInObjectPropertiesByte;
   static const int kPreAllocatedPropertyFieldsByte = 2;
   static const int kPreAllocatedPropertyFieldsOffset =
       kInstanceSizesOffset + kPreAllocatedPropertyFieldsByte;
-  // The byte at position 3 is not in use at the moment.
-  static const int kUnusedByte = 3;
-  static const int kUnusedOffset = kInstanceSizesOffset + kUnusedByte;
+  static const int kVisitorIdByte = 3;
+  static const int kVisitorIdOffset = kInstanceSizesOffset + kVisitorIdByte;
 
   // Byte offsets within kInstanceAttributesOffset attributes.
   static const int kInstanceTypeOffset = kInstanceAttributesOffset + 0;
   static const int kUnusedPropertyFieldsOffset = kInstanceAttributesOffset + 1;
   static const int kBitFieldOffset = kInstanceAttributesOffset + 2;
   static const int kBitField2Offset = kInstanceAttributesOffset + 3;
 
   STATIC_CHECK(kInstanceTypeOffset == Internals::kMapInstanceTypeOffset);
 
   // Bit positions for bit field.
@@ skipping 2284 matching lines @@
     } else {
       value &= ~(1 << bit_position);
     }
     return value;
   }
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_OBJECTS_H_