inline HeapWord* ThreadLocalAllocBuffer::allocate(size_t size) {

  {- -------------------------------------------
  (1) #TODO
      ---------------------------------------- -}

      invariants();

  {- -------------------------------------------
  (1) もし要求サイズ以上の未使用領域が残っていれば, top をずらして領域を確保し, それをリターンする.
      ---------------------------------------- -}

      HeapWord* obj = top();
      if (pointer_delta(end(), obj) >= size) {
        // successful thread-local allocation
    #ifdef ASSERT
        // Skip mangling the space corresponding to the object header to
        // ensure that the returned space is not considered parsable by
        // any concurrent GC thread.
        size_t hdr_size = oopDesc::header_size();
        Copy::fill_to_words(obj + hdr_size, size - hdr_size, badHeapWordVal);
    #endif // ASSERT
        // This addition is safe because we know that top is
        // at least size below end, so the add can't wrap.
        set_top(obj + size);

        invariants();
        return obj;
      }

  {- -------------------------------------------
  (1) もし未使用領域が要求サイズ未満なら, NULL をリターン.
      ---------------------------------------- -}

      return NULL;
    }