// Change threadstate in a manner, so safepoint can detect changes.
  // Time-critical: called on exit from every runtime routine

  static inline void transition(JavaThread *thread, JavaThreadState from, JavaThreadState to) {

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

        assert(from != _thread_in_Java, "use transition_from_java");
        assert(from != _thread_in_native, "use transition_from_native");
        assert((from & 1) == 0 && (to & 1) == 0, "odd numbers are transitions states");
        assert(thread->thread_state() == from, "coming from wrong thread state");

  {- -------------------------------------------
  (1) 処理対象のスレッドの JavaThreadState を (from+1) に変更する.

      なお, MP 環境の場合には
      VM Thread 側への visibility (sequential consistency) を保証する必要があるので, 
      変更後に以下のどちらかを行っている (See: SafepointSynchronize::begin()).
      * UseMembar オプションが指定されている場合:
        OrderAccess::fence() でメモリバリアを張る.
      * そうではない場合: 
        os::write_memory_serialize_page() で serialize page への書き込みを行う.
      ---------------------------------------- -}

        // Change to transition state (assumes total store ordering!  -Urs)
        thread->set_thread_state((JavaThreadState)(from + 1));

        // Make sure new state is seen by VM thread
        if (os::is_MP()) {
          if (UseMembar) {
            // Force a fence between the write above and read below
            OrderAccess::fence();
          } else {
            // store to serialize page so VM thread can do pseudo remote membar
            os::write_memory_serialize_page(thread);
          }
        }

  {- -------------------------------------------
  (1) もし Safepoint が開始されていたら (= SafepointSynchronize::do_call_back() が true ならば), 
      SafepointSynchronize::block() を呼んでブロックする.
      ---------------------------------------- -}

        if (SafepointSynchronize::do_call_back()) {
          SafepointSynchronize::block(thread);
        }

  {- -------------------------------------------
  (1) 処理対象のスレッドの JavaThreadState を, to 引数の値に変更する.
      ---------------------------------------- -}

        thread->set_thread_state(to);

  {- -------------------------------------------
  (1) (デバッグ用の処理) (CHECK_UNHANDLED_OOPS_ONLY 時にのみ実行) (See: UnhandledOops)
      unhandled oop をクリアする (? #TODO)
      ---------------------------------------- -}

        CHECK_UNHANDLED_OOPS_ONLY(thread->clear_unhandled_oops();)
      }