void ConcurrentMark::clearNextBitmap() {

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

      G1CollectedHeap* g1h = G1CollectedHeap::heap();
      G1CollectorPolicy* g1p = g1h->g1_policy();

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

      // Make sure that the concurrent mark thread looks to still be in
      // the current cycle.
      guarantee(cmThread()->during_cycle(), "invariant");

      // We are finishing up the current cycle by clearing the next
      // marking bitmap and getting it ready for the next cycle. During
      // this time no other cycle can start. So, let's make sure that this
      // is the case.
      guarantee(!g1h->mark_in_progress(), "invariant");

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

      // clear the mark bitmap (no grey objects to start with).
      // We need to do this in chunks and offer to yield in between
      // each chunk.
      HeapWord* start  = _nextMarkBitMap->startWord();
      HeapWord* end    = _nextMarkBitMap->endWord();
      HeapWord* cur    = start;
      size_t chunkSize = M;

  {- -------------------------------------------
  (1) (以下の while ブロック内で _nextMarkBitMap のクリア処理を行う)
      ---------------------------------------- -}

      while (cur < end) {

    {- -------------------------------------------
  (1.1) CMBitMap::clearRange() を呼んで, chunkSize 分だけの領域をクリアする
        (なお, 終端(end)を超えていたら end で切り詰める)
        ---------------------------------------- -}

        HeapWord* next = cur + chunkSize;
        if (next > end)
          next = end;
        MemRegion mr(cur,next);
        _nextMarkBitMap->clearRange(mr);

    {- -------------------------------------------
  (1.1) 次の領域へ
        ---------------------------------------- -}

        cur = next;

    {- -------------------------------------------
  (1.1) ConcurrentMark::do_yield_check() を呼んでおく.

        (この中では, 登録している SuspendibleThreadSet に対して 
         SuspendibleThreadSet::suspend_all() が呼ばれていれば, 待機処理を行う.
         この待機は SuspendibleThreadSet::resume_all() が呼ばれると解ける)
        ---------------------------------------- -}

        do_yield_check();

    {- -------------------------------------------
  (1.1) (assert)
        ---------------------------------------- -}

        // Repeat the asserts from above. We'll do them as asserts here to
        // minimize their overhead on the product. However, we'll have
        // them as guarantees at the beginning / end of the bitmap
        // clearing to get some checking in the product.
        assert(cmThread()->during_cycle(), "invariant");
        assert(!g1h->mark_in_progress(), "invariant");
      }

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

      // Repeat the asserts from above.
      guarantee(cmThread()->during_cycle(), "invariant");
      guarantee(!g1h->mark_in_progress(), "invariant");
    }