inline HeapWord*
G1CollectedHeap::attempt_allocation(size_t word_size,
                                    unsigned int* gc_count_before_ret) {

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

      assert_heap_not_locked_and_not_at_safepoint();
      assert(!isHumongous(word_size), "attempt_allocation() should not "
             "be called for humongous allocation requests");

  {- -------------------------------------------
  (1) G1AllocRegion::attempt_allocation() でメモリ確保を試みる.
      ---------------------------------------- -}

      HeapWord* result = _mutator_alloc_region.attempt_allocation(word_size,
                                                          false /* bot_updates */);

  {- -------------------------------------------
  (1) もし確保に失敗していれば, G1CollectedHeap::attempt_allocation_slow() でメモリ確保を試みる.
      ---------------------------------------- -}

      if (result == NULL) {
        result = attempt_allocation_slow(word_size, gc_count_before_ret);
      }

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

      assert_heap_not_locked();

  {- -------------------------------------------
  (1) 以上の処理で確保に成功していたら, barrier set を dirty 化しておく
      ---------------------------------------- -}

      if (result != NULL) {
        dirty_young_block(result, word_size);
      }

  {- -------------------------------------------
  (1) 結果をリターン
      ---------------------------------------- -}

      return result;
    }