xref: /freebsd/sys/libkern/strlen.c (revision 59c8e88e72633afbc47a4ace0d2170d00d51f7dc)
1 /*-
2  * SPDX-License-Identifier: BSD-2-Clause
3  *
4  * Copyright (c) 2009, 2010 Xin LI <delphij@FreeBSD.org>
5  *
6  * Redistribution and use in source and binary forms, with or without
7  * modification, are permitted provided that the following conditions
8  * are met:
9  * 1. Redistributions of source code must retain the above copyright
10  *    notice, this list of conditions and the following disclaimer.
11  * 2. Redistributions in binary form must reproduce the above copyright
12  *    notice, this list of conditions and the following disclaimer in the
13  *    documentation and/or other materials provided with the distribution.
14  *
15  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
16  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
17  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
18  * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
19  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
20  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
21  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
22  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
23  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
24  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
25  * SUCH DAMAGE.
26  */
27 
28 #include <sys/cdefs.h>
29 #include <sys/libkern.h>
30 #include <sys/limits.h>
31 
32 /*
33  * Portable strlen() for 32-bit and 64-bit systems.
34  *
35  * The expression:
36  *
37  *	((x - 0x01....01) & ~x & 0x80....80)
38  *
39  * would evaluate to a non-zero value iff any of the bytes in the
40  * original word is zero.
41  *
42  * The algorithm above is found on "Hacker's Delight" by
43  * Henry S. Warren, Jr.
44  *
45  * Note: this leaves performance on the table and each architecture
46  * would be best served with a tailor made routine instead, even if
47  * using the same trick.
48  */
49 
50 /* Magic numbers for the algorithm */
51 #if LONG_BIT == 32
52 static const unsigned long mask01 = 0x01010101;
53 static const unsigned long mask80 = 0x80808080;
54 #elif LONG_BIT == 64
55 static const unsigned long mask01 = 0x0101010101010101;
56 static const unsigned long mask80 = 0x8080808080808080;
57 #else
58 #error Unsupported word size
59 #endif
60 
61 #define	LONGPTR_MASK (sizeof(long) - 1)
62 
63 /*
64  * Helper macro to return string length if we caught the zero
65  * byte.
66  */
67 #define testbyte(x)				\
68 	do {					\
69 		if (p[x] == '\0')		\
70 		    return (p - str + x);	\
71 	} while (0)
72 
73 size_t
74 (strlen)(const char *str)
75 {
76 	const char *p;
77 	const unsigned long *lp;
78 	long va, vb;
79 
80 	/*
81 	 * Before trying the hard (unaligned byte-by-byte access) way
82 	 * to figure out whether there is a nul character, try to see
83 	 * if there is a nul character is within this accessible word
84 	 * first.
85 	 *
86 	 * p and (p & ~LONGPTR_MASK) must be equally accessible since
87 	 * they always fall in the same memory page, as long as page
88 	 * boundaries is integral multiple of word size.
89 	 */
90 	lp = (const unsigned long *)((uintptr_t)str & ~LONGPTR_MASK);
91 	va = (*lp - mask01);
92 	vb = ((~*lp) & mask80);
93 	lp++;
94 	if (va & vb)
95 		/* Check if we have \0 in the first part */
96 		for (p = str; p < (const char *)lp; p++)
97 			if (*p == '\0')
98 				return (p - str);
99 
100 	/* Scan the rest of the string using word sized operation */
101 	for (; ; lp++) {
102 		va = (*lp - mask01);
103 		vb = ((~*lp) & mask80);
104 		if (va & vb) {
105 			p = (const char *)(lp);
106 			testbyte(0);
107 			testbyte(1);
108 			testbyte(2);
109 			testbyte(3);
110 #if (LONG_BIT >= 64)
111 			testbyte(4);
112 			testbyte(5);
113 			testbyte(6);
114 			testbyte(7);
115 #endif
116 		}
117 	}
118 
119 	/* NOTREACHED */
120 	return (0);
121 }
122