void ConcurrentMark::markFromRoots() {

  {- -------------------------------------------
  (1) (以下のような assert を入れたいが成り立たないので今は入れてない, とのこと)
      ---------------------------------------- -}

      // we might be tempted to assert that:
      // assert(asynch == !SafepointSynchronize::is_at_safepoint(),
      //        "inconsistent argument?");
      // However that wouldn't be right, because it's possible that
      // a safepoint is indeed in progress as a younger generation
      // stop-the-world GC happens even as we mark in this generation.

  {- -------------------------------------------
  (1) _restart_for_overflow フィールドを初期化しておく.
      (以降の処理で失敗すると true に変わる)
      ---------------------------------------- -}

      _restart_for_overflow = false;

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

      size_t active_workers = MAX2((size_t) 1, parallel_marking_threads());

  {- -------------------------------------------
  (1) (デバッグ用の処理) (See: ForceOverflowSettings)
      ---------------------------------------- -}

      force_overflow_conc()->init();

  {- -------------------------------------------
  (1) #TODO
      ---------------------------------------- -}

      set_phase(active_workers, true /* concurrent */);

  {- -------------------------------------------
  (1) CMConcurrentMarkingTask::work() を呼び出して, 
      root から辿れる範囲の Concurrent marking 処理を行う.

      (なお, ConcurrentMark::parallel_marking_threads() が 1 以上の場合は, 
       WorkGang::run_task() 経由で呼び出す.
       これにより, 処理がマルチスレッドで実行される)
      ---------------------------------------- -}

      CMConcurrentMarkingTask markingTask(this, cmThread());
      if (parallel_marking_threads() > 0)
        _parallel_workers->run_task(&markingTask);
      else
        markingTask.work(0);

  {- -------------------------------------------
  (1) (トレース出力)
      ---------------------------------------- -}

      print_stats();
    }