HeapWord* allocate(GCAllocPurpose purpose, size_t word_sz) {

  {- -------------------------------------------
  (1) まず, ParGCAllocBuffer::allocate() で alloc_buffers からの確保を試みる.
      失敗したら, G1ParScanThreadState::allocate_slow() での確保を行う.
      ---------------------------------------- -}

        HeapWord* obj = alloc_buffer(purpose)->allocate(word_sz);
        if (obj != NULL) return obj;
        return allocate_slow(purpose, word_sz);
      }