void
Stack::push(int val)
{
  StackHead expected = head.load();
  StackItem *newItem = new StackItem(val);
  StackHead newHead;
  newHead.link = newItem;
  do {
    newItem->next = expected.link;
    newHead.count = expected.count + 1;
  } while (!head.compare_exchange_weak(expected, newHead)); 
}