code: func bytesNoSIMD(dst, a, b []byte, size int) {
  	if supportsUnaligned {
  		fastXORBytes(dst, a, b, size)
  	} else {
  		// TODO(hanwen): if (dst, a, b) have common alignment
  		// we could still try fastXORBytes. It is not clear
  		// how often this happens, and it's only worth it if
  		// the block encryption itself is hardware
  		// accelerated.
  		safeXORBytes(dst, a, b, size)
  	}
  }
docs: // xor the bytes in a and b. The destination is assumed to have enough space.