VM_Address sentinal = block.add(BlockAllocator.blockSize(blockSizeClass[sizeClass]));
VM_Address sentinel = block.add(BlockAllocator.blockSize(blockSizeClass[sizeClass]));
  private final VM_Address expandSizeClass(int sizeClass)     throws VM_PragmaInline {    VM_Address block = blockAllocator.alloc(blockSizeClass[sizeClass]);    if (block.isZero())      return VM_Address.zero();    installNewBlock(block, sizeClass);    int cellExtent = cellSize[sizeClass];    VM_Address cursor = block.add(blockHeaderSize[sizeClass]);    VM_Address sentinal = block.add(BlockAllocator.blockSize(blockSizeClass[sizeClass]));    VM_Address lastCell = VM_Address.zero();    int cellCount = 0;    while (cursor.add(cellExtent).LE(sentinal)) {      setNextCell(cursor, lastCell);       lastCell = cursor;      cursor = cursor.add(cellExtent);      cellCount++;    }    cellsInUse[sizeClass] = 0;    setSizeClass(block, sizeClass);    postExpandSizeClass(block, sizeClass);        if (VM_Interface.VerifyAssertions)      VM_Interface._assert(!lastCell.isZero());    return lastCell;  }