FreeBSD/Linux Kernel Cross Reference
sys/libkern/strlen.c
1 /*-
2 * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
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 __FBSDID("$FreeBSD: releng/12.0/sys/libkern/strlen.c 333449 2018-05-10 06:41:08Z delphij $");
30
31 #include <sys/libkern.h>
32 #include <sys/limits.h>
33
34 /*
35 * Portable strlen() for 32-bit and 64-bit systems.
36 *
37 * Rationale: it is generally much more efficient to do word length
38 * operations and avoid branches on modern computer systems, as
39 * compared to byte-length operations with a lot of branches.
40 *
41 * The expression:
42 *
43 * ((x - 0x01....01) & ~x & 0x80....80)
44 *
45 * would evaluate to a non-zero value iff any of the bytes in the
46 * original word is zero.
47 *
48 * On multi-issue processors, we can divide the above expression into:
49 * a) (x - 0x01....01)
50 * b) (~x & 0x80....80)
51 * c) a & b
52 *
53 * Where, a) and b) can be partially computed in parallel.
54 *
55 * The algorithm above is found on "Hacker's Delight" by
56 * Henry S. Warren, Jr.
57 */
58
59 /* Magic numbers for the algorithm */
60 #if LONG_BIT == 32
61 static const unsigned long mask01 = 0x01010101;
62 static const unsigned long mask80 = 0x80808080;
63 #elif LONG_BIT == 64
64 static const unsigned long mask01 = 0x0101010101010101;
65 static const unsigned long mask80 = 0x8080808080808080;
66 #else
67 #error Unsupported word size
68 #endif
69
70 #define LONGPTR_MASK (sizeof(long) - 1)
71
72 /*
73 * Helper macro to return string length if we caught the zero
74 * byte.
75 */
76 #define testbyte(x) \
77 do { \
78 if (p[x] == '\0') \
79 return (p - str + x); \
80 } while (0)
81
82 size_t
83 strlen(const char *str)
84 {
85 const char *p;
86 const unsigned long *lp;
87 long va, vb;
88
89 /*
90 * Before trying the hard (unaligned byte-by-byte access) way
91 * to figure out whether there is a nul character, try to see
92 * if there is a nul character is within this accessible word
93 * first.
94 *
95 * p and (p & ~LONGPTR_MASK) must be equally accessible since
96 * they always fall in the same memory page, as long as page
97 * boundaries is integral multiple of word size.
98 */
99 lp = (const unsigned long *)((uintptr_t)str & ~LONGPTR_MASK);
100 va = (*lp - mask01);
101 vb = ((~*lp) & mask80);
102 lp++;
103 if (va & vb)
104 /* Check if we have \0 in the first part */
105 for (p = str; p < (const char *)lp; p++)
106 if (*p == '\0')
107 return (p - str);
108
109 /* Scan the rest of the string using word sized operation */
110 for (; ; lp++) {
111 va = (*lp - mask01);
112 vb = ((~*lp) & mask80);
113 if (va & vb) {
114 p = (const char *)(lp);
115 testbyte(0);
116 testbyte(1);
117 testbyte(2);
118 testbyte(3);
119 #if (LONG_BIT >= 64)
120 testbyte(4);
121 testbyte(5);
122 testbyte(6);
123 testbyte(7);
124 #endif
125 }
126 }
127
128 /* NOTREACHED */
129 return (0);
130 }
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