FreeBSD/Linux Kernel Cross Reference
sys/libkern/strlen.c
1 /*-
2 * Copyright (c) 2009, 2010 Xin LI <delphij@FreeBSD.org>
3 * All rights reserved.
4 *
5 * Redistribution and use in source and binary forms, with or without
6 * modification, are permitted provided that the following conditions
7 * are met:
8 * 1. Redistributions of source code must retain the above copyright
9 * notice, this list of conditions and the following disclaimer.
10 * 2. Redistributions in binary form must reproduce the above copyright
11 * notice, this list of conditions and the following disclaimer in the
12 * documentation and/or other materials provided with the distribution.
13 *
14 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
15 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
16 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
17 * ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
18 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
19 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
20 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
21 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
22 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
23 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
24 * SUCH DAMAGE.
25 */
26
27 #include <sys/cdefs.h>
28 __FBSDID("$FreeBSD: releng/10.4/sys/libkern/strlen.c 242506 2012-11-03 04:28:53Z delphij $");
29
30 #include <sys/libkern.h>
31 #include <sys/limits.h>
32
33 /*
34 * Portable strlen() for 32-bit and 64-bit systems.
35 *
36 * Rationale: it is generally much more efficient to do word length
37 * operations and avoid branches on modern computer systems, as
38 * compared to byte-length operations with a lot of branches.
39 *
40 * The expression:
41 *
42 * ((x - 0x01....01) & ~x & 0x80....80)
43 *
44 * would evaluate to a non-zero value iff any of the bytes in the
45 * original word is zero.
46 *
47 * On multi-issue processors, we can divide the above expression into:
48 * a) (x - 0x01....01)
49 * b) (~x & 0x80....80)
50 * c) a & b
51 *
52 * Where, a) and b) can be partially computed in parallel.
53 *
54 * The algorithm above is found on "Hacker's Delight" by
55 * Henry S. Warren, Jr.
56 */
57
58 /* Magic numbers for the algorithm */
59 #if LONG_BIT == 32
60 static const unsigned long mask01 = 0x01010101;
61 static const unsigned long mask80 = 0x80808080;
62 #elif LONG_BIT == 64
63 static const unsigned long mask01 = 0x0101010101010101;
64 static const unsigned long mask80 = 0x8080808080808080;
65 #else
66 #error Unsupported word size
67 #endif
68
69 #define LONGPTR_MASK (sizeof(long) - 1)
70
71 /*
72 * Helper macro to return string length if we caught the zero
73 * byte.
74 */
75 #define testbyte(x) \
76 do { \
77 if (p[x] == '\0') \
78 return (p - str + x); \
79 } while (0)
80
81 size_t
82 strlen(const char *str)
83 {
84 const char *p;
85 const unsigned long *lp;
86 long va, vb;
87
88 /*
89 * Before trying the hard (unaligned byte-by-byte access) way
90 * to figure out whether there is a nul character, try to see
91 * if there is a nul character is within this accessible word
92 * first.
93 *
94 * p and (p & ~LONGPTR_MASK) must be equally accessible since
95 * they always fall in the same memory page, as long as page
96 * boundaries is integral multiple of word size.
97 */
98 lp = (const unsigned long *)((uintptr_t)str & ~LONGPTR_MASK);
99 va = (*lp - mask01);
100 vb = ((~*lp) & mask80);
101 lp++;
102 if (va & vb)
103 /* Check if we have \0 in the first part */
104 for (p = str; p < (const char *)lp; p++)
105 if (*p == '\0')
106 return (p - str);
107
108 /* Scan the rest of the string using word sized operation */
109 for (; ; lp++) {
110 va = (*lp - mask01);
111 vb = ((~*lp) & mask80);
112 if (va & vb) {
113 p = (const char *)(lp);
114 testbyte(0);
115 testbyte(1);
116 testbyte(2);
117 testbyte(3);
118 #if (LONG_BIT >= 64)
119 testbyte(4);
120 testbyte(5);
121 testbyte(6);
122 testbyte(7);
123 #endif
124 }
125 }
126
127 /* NOTREACHED */
128 return (0);
129 }
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