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: sum_s390x.go
    path of file: ./repos/docker-ce/components/engine/vendor/golang.org/x/crypto/poly1305
    
    the code of the file until where you have to start completion: // Copyright 2018 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

//go:build gc && !purego
// +build gc,!purego

package poly1305

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

// updateVX is an assembly implementation of Poly1305 that uses vector
// i
    
func (h *mac) Write(p []byte) (int, error) {
	nn := len(p)
	if h.offset > 0 {
		n := copy(h.buffer[h.offset:], p)
		if h.offset+n < len(h.buffer) {
			h.offset += n
			return nn, nil
		}
		p = p[n:]
		h.offset = 0
		if cpu.S390X.HasVX {
			updateVX(&h.macState, h.buffer[:])
		} else {
			updateGeneric(&h.macState, h.buffer[:])
		}
	}

	tail := len(p) % len(h.buffer) // number of bytes to copy into buffer
	body := len(p) - tail          // number of bytes to process now
	if body > 0 {
		if cpu.S390X.HasVX {
			updateVX(&h.macState, p[:body])
		} else {
			updateGeneric(&h.macState, p[:body])
		}
	}
	h.offset = copy(h.buffer[:], p[body:]) // copy tail bytes - can be 0
	return nn, nil
}