1/*	$NetBSD: nospec.h,v 1.3 2021/12/19 01:45:38 riastradh Exp $	*/
2
3/*-
4 * Copyright (c) 2018 The NetBSD Foundation, Inc.
5 * All rights reserved.
6 *
7 * This code is derived from software contributed to The NetBSD Foundation
8 * by Taylor R. Campbell.
9 *
10 * Redistribution and use in source and binary forms, with or without
11 * modification, are permitted provided that the following conditions
12 * are met:
13 * 1. Redistributions of source code must retain the above copyright
14 *    notice, this list of conditions and the following disclaimer.
15 * 2. Redistributions in binary form must reproduce the above copyright
16 *    notice, this list of conditions and the following disclaimer in the
17 *    documentation and/or other materials provided with the distribution.
18 *
19 * THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
20 * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
21 * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
22 * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
23 * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
24 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
25 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
26 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
27 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
28 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
29 * POSSIBILITY OF SUCH DAMAGE.
30 */
31
32#ifndef	_LINUX_NOSPEC_H_
33#define	_LINUX_NOSPEC_H_
34
35#include <sys/param.h>
36#include <sys/types.h>
37
38/*
39 * array_index_nospec(i, n)
40 *
41 *	If i < n, return i; otherwise return 0.  Guarantees that
42 *	speculative execution will behave the same even if the compiler
43 *	can prove in the caller that i < n.
44 */
45static inline size_t
46array_index_nospec(size_t i, size_t n)
47{
48	size_t j, diff, mask;
49
50	/*
51	 * Let j = i, but force the compiler to assume nothing about
52	 * the value of j by issuing an asm block that is tagged as
53	 * reading from and writing to j but actually does nothing.
54	 */
55	j = i;
56	asm volatile ("" : "=r"(j) : "0"(j));
57
58	/* If i >= n, then diff has the high bit set; otherwise clear.  */
59	diff = n - 1 - j;
60
61	/*
62	 * If i >= n so that diff's high bit is set, mask = 0;
63	 * otherwise mask = -1 = ~0.
64	 */
65	mask = (diff >> (NBBY*sizeof(diff) - 1)) - 1;
66
67	return i & mask;
68}
69
70#endif	/* _LINUX_NOSPEC_H_ */
71