HeapWord* G1CollectedHeap::attempt_allocation_at_safepoint(size_t word_size,
                                       bool expect_null_mutator_alloc_region) {

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

      assert_at_safepoint(true /* should_be_vm_thread */);
      assert(_mutator_alloc_region.get() == NULL ||
                                                 !expect_null_mutator_alloc_region,
             "the current alloc region was unexpectedly found to be non-NULL");

  {- -------------------------------------------
  (1) もし確保サイズが大き過ぎなければ(= isHumongous() が false ならば), 
      G1AllocRegion::attempt_allocation_locked() でメモリ確保を試み, 結果をリターンする.

      逆に大き過ぎる場合は, 
      G1CollectedHeap::humongous_obj_allocate() を呼んでメモリ確保を試み, 結果をリターンする.
      ---------------------------------------- -}

      if (!isHumongous(word_size)) {
        return _mutator_alloc_region.attempt_allocation_locked(word_size,
                                                          false /* bot_updates */);
      } else {
        return humongous_obj_allocate(word_size);
      }

  {- -------------------------------------------
  (1) (以下のパスに来ることはあり得ない)
      ---------------------------------------- -}

      ShouldNotReachHere();
    }