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      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  */
     45 static inline size_t
     46 array_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