- Add a size field to the header.

vm/freelist.h

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef VM_FREELIST_H_
 #define VM_FREELIST_H_
 
 #include "vm/allocation.h"
 #include "vm/assert.h"
 #include "vm/raw_object.h"
 
 namespace dart {
 
 // FreeListElement describes a freelist element that has the same size
 // as the smallest raw object. It uses the class_ field to point to a fake map
 // to enable basic traversing of the heap and to identify the type of freelist
 // element. It reuses the second word of the raw object to keep a next_
 // pointer to chain elements of the list together. For objects larger than the
 // minimal object size, the size of the element is embedded in the element at
 // the address following the next_ field.
 class FreeListElement {
  public:
-  FreeListElement* next() const { return next_; }
-  void set_next(FreeListElement* next) { next_ = next; }
+  FreeListElement* next() const {
+    // Clear the FreeBit.
+    ASSERT((next_ & 1) == 1);
+    return reinterpret_cast<FreeListElement*>(next_ ^ 1);

[turnidge, 2012/01/04 01:06:17]

Should this use the constant RawObject::kFreeBit?

[Ivan Posva, 2012/01/04 07:59:42]

Done. In any case this use here should only be temporary.

+  }
+  void set_next(FreeListElement* next) {
+    // Set the FreeBit.
+    uword addr = reinterpret_cast<uword>(next);
+    ASSERT((addr & 1) == 0);
+    next_ = addr | 1;

[turnidge, 2012/01/04 01:06:17]

Same as above.

[Ivan Posva, 2012/01/04 07:59:42]

ditto.

+  }
 
   intptr_t Size() const {
     if (class_ == minimal_element_class_) {
       return kObjectAlignment;
     }
     ASSERT(class_ == element_class_);
     return *SizeAddress();
   }
 
   static FreeListElement* AsElement(uword addr, intptr_t size);
 
   static bool IsSpecialClass(RawObject* raw_obj) {
     return (raw_obj == minimal_element_class_) || (raw_obj == element_class_);
   }
 
   static void InitOnce();
 
  private:
   // This layout mirrors the layout of RawObject.
   RawClass* class_;
-  FreeListElement* next_;
+  uword next_;
 
   // Returns the address of the embedded size.
   intptr_t* SizeAddress() const {
     ASSERT(class_ == element_class_);
     uword addr = reinterpret_cast<uword>(&next_) + kWordSize;
     return reinterpret_cast<intptr_t*>(addr);
   }
 
   // The two fake classe being used by the FreeList to identify free objects in
   // the heap. These can be static and shared between isolates since they
@@ skipping 33 matching lines @@
   void SplitElementAfterAndEnqueue(FreeListElement* element, intptr_t size);
 
   FreeListElement* free_lists_[kNumLists + 1];
 
   DISALLOW_COPY_AND_ASSIGN(FreeList);
 };
 
 }  // namespace dart
 
 #endif  // VM_FREELIST_H_