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// Copyright 2013 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.
// +build !amd64,!ppc64,!ppc64le,!arm64
package cipher
import (
"runtime"
"unsafe"
)
// xorBytes xors the bytes in a and b. The destination should have enough
// space, otherwise xorBytes will panic. Returns the number of bytes xor'd.
func xorBytes(dst, a, b []byte) int {
n := len(a)
if len(b) < n {
n = len(b)
}
if n == 0 {
return 0
}
switch {
case supportsUnaligned:
fastXORBytes(dst, a, b, n)
default:
// 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, n)
}
return n
}
const wordSize = int(unsafe.Sizeof(uintptr(0)))
const supportsUnaligned = runtime.GOARCH == "386" || runtime.GOARCH == "ppc64" || runtime.GOARCH == "ppc64le" || runtime.GOARCH == "s390x"
// fastXORBytes xors in bulk. It only works on architectures that
// support unaligned read/writes.
// n needs to be smaller or equal than the length of a and b.
func fastXORBytes(dst, a, b []byte, n int) {
// Assert dst has enough space
_ = dst[n-1]
w := n / wordSize
if w > 0 {
dw := *(*[]uintptr)(unsafe.Pointer(&dst))
aw := *(*[]uintptr)(unsafe.Pointer(&a))
bw := *(*[]uintptr)(unsafe.Pointer(&b))
for i := 0; i < w; i++ {
dw[i] = aw[i] ^ bw[i]
}
}
for i := (n - n%wordSize); i < n; i++ {
dst[i] = a[i] ^ b[i]
}
}
// n needs to be smaller or equal than the length of a and b.
func safeXORBytes(dst, a, b []byte, n int) {
for i := 0; i < n; i++ {
dst[i] = a[i] ^ b[i]
}
}
// fastXORWords XORs multiples of 4 or 8 bytes (depending on architecture.)
// The arguments are assumed to be of equal length.
func fastXORWords(dst, a, b []byte) {
dw := *(*[]uintptr)(unsafe.Pointer(&dst))
aw := *(*[]uintptr)(unsafe.Pointer(&a))
bw := *(*[]uintptr)(unsafe.Pointer(&b))
n := len(b) / wordSize
for i := 0; i < n; i++ {
dw[i] = aw[i] ^ bw[i]
}
}
// fastXORWords XORs multiples of 4 or 8 bytes (depending on architecture.)
// The slice arguments a and b are assumed to be of equal length.
func xorWords(dst, a, b []byte) {
if supportsUnaligned {
fastXORWords(dst, a, b)
} else {
safeXORBytes(dst, a, b, len(b))
}
}
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