Only use FreeSpace objects in the free list.

src/objects.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.
 
 #ifndef V8_OBJECTS_H_
 #define V8_OBJECTS_H_
 
 #include <iosfwd>
 
 #include "src/allocation.h"
@@ skipping 4430 matching lines @@
   // Maximal memory consumption for a single ByteArray.
   static const int kMaxSize = 512 * MB;
   // Maximal length of a single ByteArray.
   static const int kMaxLength = kMaxSize - kHeaderSize;
 
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(ByteArray);
 };
 
 
-// FreeSpace represents fixed sized areas of the heap that are not currently in
-// use.  Used by the heap and GC.
+// FreeSpace are fixed-size free memory blocks used by the heap and GC.
+// They look like heap objects (are heap object tagged and have a map) so that
+// the heap remains iterable.  They have a size and a next pointer.
+// The next pointer is the raw address of the next FreeSpace object (or NULL)
+// in the free list.
 class FreeSpace: public HeapObject {
  public:
   // [size]: size of the free space including the header.
   inline int size() const;
   inline void set_size(int value);
 
   inline int nobarrier_size() const;
   inline void nobarrier_set_size(int value);
 
   inline int Size() { return size(); }
 
-  DECLARE_CAST(FreeSpace)
+  // Accessors for the next field.
+  inline FreeSpace* next();
+  inline FreeSpace** next_address();
+  inline void set_next(FreeSpace* next);
+
+  inline static FreeSpace* cast(HeapObject* obj);
 
   // Dispatched behavior.
   DECLARE_PRINTER(FreeSpace)
   DECLARE_VERIFIER(FreeSpace)
 
   // Layout description.
   // Size is smi tagged when it is stored.
   static const int kSizeOffset = HeapObject::kHeaderSize;
-  static const int kHeaderSize = kSizeOffset + kPointerSize;
-
-  static const int kAlignedSize = OBJECT_POINTER_ALIGN(kHeaderSize);
+  static const int kNextOffset = POINTER_SIZE_ALIGN(kSizeOffset + kPointerSize);
 
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(FreeSpace);
 };
 
 
 // V has parameters (Type, type, TYPE, C type, element_size)
 #define TYPED_ARRAYS(V) \
   V(Uint8, uint8, UINT8, uint8_t, 1)                                           \
   V(Int8, int8, INT8, int8_t, 1)                                               \
@@ skipping 6447 matching lines @@
     } else {
       value &= ~(1 << bit_position);
     }
     return value;
   }
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_OBJECTS_H_