bool do_bit(size_t offset) {

  {- -------------------------------------------
  (1) (変数宣言など)
      ---------------------------------------- -}

        HeapWord* addr = _nextMarkBitMap->offsetToHeapWord(offset);

  {- -------------------------------------------
  (1) (assert)
      ---------------------------------------- -}

        assert(_nextMarkBitMap->isMarked(addr), "invariant");
        assert( addr < _cm->finger(), "invariant");

  {- -------------------------------------------
  (1) finger を更新している?? (#TODO)
      ---------------------------------------- -}

        if (_scanning_heap_region) {
          statsOnly( _task->increase_objs_found_on_bitmap() );
          assert(addr >= _task->finger(), "invariant");
          // We move that task's local finger along.
          _task->move_finger_to(addr);
        } else {
          // We move the task's region finger along.
          _task->move_region_finger_to(addr);
        }

  {- -------------------------------------------
  (1) CMTask::scan_object() を呼んで処理を行う.
      ---------------------------------------- -}

        _task->scan_object(oop(addr));
        // we only partially drain the local queue and global stack
        _task->drain_local_queue(true);
        _task->drain_global_stack(true);

  {- -------------------------------------------
  (1) 
      ---------------------------------------- -}

        // if the has_aborted flag has been raised, we need to bail out of
        // the iteration
        return !_task->has_aborted();
      }