1// Copyright 2013 The Go Authors. All rights reserved.
2// Use of this source code is governed by a BSD-style
3// license that can be found in the LICENSE file.
4
5//go:build (!amd64 && !arm64 && !ppc64 && !ppc64le) || purego
6
7package subtle
8
9import (
10	"runtime"
11	"unsafe"
12)
13
14const wordSize = unsafe.Sizeof(uintptr(0))
15
16const supportsUnaligned = runtime.GOARCH == "386" ||
17	runtime.GOARCH == "amd64" ||
18	runtime.GOARCH == "ppc64" ||
19	runtime.GOARCH == "ppc64le" ||
20	runtime.GOARCH == "s390x"
21
22func xorBytes(dstb, xb, yb *byte, n int) {
23	// xorBytes assembly is written using pointers and n. Back to slices.
24	dst := unsafe.Slice(dstb, n)
25	x := unsafe.Slice(xb, n)
26	y := unsafe.Slice(yb, n)
27
28	if supportsUnaligned || aligned(dstb, xb, yb) {
29		xorLoop(words(dst), words(x), words(y))
30		if uintptr(n)%wordSize == 0 {
31			return
32		}
33		done := n &^ int(wordSize-1)
34		dst = dst[done:]
35		x = x[done:]
36		y = y[done:]
37	}
38	xorLoop(dst, x, y)
39}
40
41// aligned reports whether dst, x, and y are all word-aligned pointers.
42func aligned(dst, x, y *byte) bool {
43	return (uintptr(unsafe.Pointer(dst))|uintptr(unsafe.Pointer(x))|uintptr(unsafe.Pointer(y)))&(wordSize-1) == 0
44}
45
46// words returns a []uintptr pointing at the same data as x,
47// with any trailing partial word removed.
48func words(x []byte) []uintptr {
49	n := uintptr(len(x)) / wordSize
50	if n == 0 {
51		// Avoid creating a *uintptr that refers to data smaller than a uintptr;
52		// see issue 59334.
53		return nil
54	}
55	return unsafe.Slice((*uintptr)(unsafe.Pointer(&x[0])), n)
56}
57
58func xorLoop[T byte | uintptr](dst, x, y []T) {
59	x = x[:len(dst)] // remove bounds check in loop
60	y = y[:len(dst)] // remove bounds check in loop
61	for i := range dst {
62		dst[i] = x[i] ^ y[i]
63	}
64}
65