strlen.c revision 330897 
1/*-
2 * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
3 *
4 * Copyright (c) 2009, 2010 Xin LI <delphij@FreeBSD.org>
5 * All rights reserved.
6 *
7 * Redistribution and use in source and binary forms, with or without
8 * modification, are permitted provided that the following conditions
9 * are met:
10 * 1. Redistributions of source code must retain the above copyright
11 *    notice, this list of conditions and the following disclaimer.
12 * 2. Redistributions in binary form must reproduce the above copyright
13 *    notice, this list of conditions and the following disclaimer in the
14 *    documentation and/or other materials provided with the distribution.
15 *
16 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
17 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
18 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
19 * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
20 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
21 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
22 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
23 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
24 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
25 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
26 * SUCH DAMAGE.
27 */
28
29#include <sys/cdefs.h>
30__FBSDID("$FreeBSD: stable/11/lib/libc/string/strlen.c 330897 2018-03-14 03:19:51Z eadler $");
31
32#include <sys/limits.h>
33#include <sys/types.h>
34#include <string.h>
35
36/*
37 * Portable strlen() for 32-bit and 64-bit systems.
38 *
39 * Rationale: it is generally much more efficient to do word length
40 * operations and avoid branches on modern computer systems, as
41 * compared to byte-length operations with a lot of branches.
42 *
43 * The expression:
44 *
45 *	((x - 0x01....01) & ~x & 0x80....80)
46 *
47 * would evaluate to a non-zero value iff any of the bytes in the
48 * original word is zero.
49 *
50 * On multi-issue processors, we can divide the above expression into:
51 *	a)  (x - 0x01....01)
52 *	b) (~x & 0x80....80)
53 *	c) a & b
54 *
55 * Where, a) and b) can be partially computed in parallel.
56 *
57 * The algorithm above is found on "Hacker's Delight" by
58 * Henry S. Warren, Jr.
59 */
60
61/* Magic numbers for the algorithm */
62#if LONG_BIT == 32
63static const unsigned long mask01 = 0x01010101;
64static const unsigned long mask80 = 0x80808080;
65#elif LONG_BIT == 64
66static const unsigned long mask01 = 0x0101010101010101;
67static const unsigned long mask80 = 0x8080808080808080;
68#else
69#error Unsupported word size
70#endif
71
72#define	LONGPTR_MASK (sizeof(long) - 1)
73
74/*
75 * Helper macro to return string length if we caught the zero
76 * byte.
77 */
78#define testbyte(x)				\
79	do {					\
80		if (p[x] == '\0')		\
81		    return (p - str + x);	\
82	} while (0)
83
84size_t
85strlen(const char *str)
86{
87	const char *p;
88	const unsigned long *lp;
89	long va, vb;
90
91	/*
92	 * Before trying the hard (unaligned byte-by-byte access) way
93	 * to figure out whether there is a nul character, try to see
94	 * if there is a nul character is within this accessible word
95	 * first.
96	 *
97	 * p and (p & ~LONGPTR_MASK) must be equally accessible since
98	 * they always fall in the same memory page, as long as page
99	 * boundaries is integral multiple of word size.
100	 */
101	lp = (const unsigned long *)((uintptr_t)str & ~LONGPTR_MASK);
102	va = (*lp - mask01);
103	vb = ((~*lp) & mask80);
104	lp++;
105	if (va & vb)
106		/* Check if we have \0 in the first part */
107		for (p = str; p < (const char *)lp; p++)
108			if (*p == '\0')
109				return (p - str);
110
111	/* Scan the rest of the string using word sized operation */
112	for (; ; lp++) {
113		va = (*lp - mask01);
114		vb = ((~*lp) & mask80);
115		if (va & vb) {
116			p = (const char *)(lp);
117			testbyte(0);
118			testbyte(1);
119			testbyte(2);
120			testbyte(3);
121#if (LONG_BIT >= 64)
122			testbyte(4);
123			testbyte(5);
124			testbyte(6);
125			testbyte(7);
126#endif
127		}
128	}
129
130	/* NOTREACHED */
131	return (0);
132}
133