code: func fastXORBytes(dst, a, b []byte, n int) {
  	w := n / wordSize
  	if w > 0 {
  		wordBytes := w * wordSize
  		fastXORWords(dst[:wordBytes], a[:wordBytes], b[:wordBytes])
  	}
  	for i := n - n%wordSize; i < n; i++ {
  		dst[i] = a[i] ^ b[i]
  	}
  }
docs: // fastXORBytes xor in bulk. It only works on architectures that
  // support unaligned read/writes.