void SubTasksDone::all_tasks_completed() {

  {- -------------------------------------------
  (1) _threads_completed を Atomic::cmpxchg() でアトミックにインクリメントする.
      (Atomic::cmpxchg() が成功するまでループ)
      ---------------------------------------- -}

      jint observed = _threads_completed;
      jint old;
      do {
        old = observed;
        observed = Atomic::cmpxchg(old+1, &_threads_completed, old);
      } while (observed != old);

  {- -------------------------------------------
  (1) もし全スレッドが処理を終えていれば(threads_completed == _n_threads), 
      次回の処理に備えて, SubTasksDone::clear() で SubTasksDone オブジェクトの状態を初期状態に戻しておく.
      ---------------------------------------- -}

      // If this was the last thread checking in, clear the tasks.
      if (observed+1 == _n_threads) clear();
    }