src/hotspot/share/gc/shared/collectedHeap.hpp

 
   virtual bool is_oop(oop object) const;
 
   virtual size_t obj_size(oop obj) const;
 
+  // Cells are memory slices allocated by the allocator. Objects are initialized
+  // in cells. The cell itself may have a header, found at a negative offset of
+  // oops. Usually, the size of the cell header is 0, but it may be larger.
+  virtual ptrdiff_t cell_header_size() const { return 0; }
+
   // Non product verification and debugging.
 #ifndef PRODUCT
   // Support for PromotionFailureALot.  Return true if it's time to cause a
   // promotion failure.  The no-argument version uses
   // this->_promotion_failure_alot_count as the counter.