xref: /illumos-gate/usr/src/boot/lib/libc/string/strlen.c (revision 199767f8)
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("$FreeBSD$");
29 
30 #include <sys/limits.h>
31 #include <sys/types.h>
32 #include <string.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
strlen(const char * str)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 }
131