You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: chacha_amd64.go
    path of file: ./repos/teller/vendor/github.com/aead/chacha20/chacha
    
    the code of the file until where you have to start completion: // Copyright (c) 2017 Andreas Auernhammer. All rights reserved.
// Use of this source code is governed by a license that can be
// found in the LICENSE file.

// +build go1.7,amd64,!gccgo,!appengine,!nacl

package chacha

import "golang.org/x/sys/cpu"

func init() {
	useSSE2 = cpu.X86.HasSSE2
	useSSSE3 = 
    
func xorKeyStream(dst, src []byte, block, state *[64]byte, rounds int) int {
	switch {
	case useAVX2:
		return xorKeyStreamAVX2(dst, src, block, state, rounds)
	case useAVX:
		return xorKeyStreamAVX(dst, src, block, state, rounds)
	case useSSSE3:
		return xorKeyStreamSSSE3(dst, src, block, state, rounds)
	case useSSE2:
		return xorKeyStreamSSE2(dst, src, block, state, rounds)
	default:
		return xorKeyStreamGeneric(dst, src, block, state, rounds)
	}
}