size_t os::current_stack_size() {

  {- -------------------------------------------
  (1) current_stack_region() を呼んでスタックサイズを取得し, それをリターン.
      ---------------------------------------- -}

      // stack size includes normal stack and HotSpot guard pages
      address bottom;
      size_t size;
      current_stack_region(&bottom, &size);
      return size;
    }