1/*-
2 * Copyright (c) 2009 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: src/lib/libc/string/strlen.c,v 1.7 2009/01/26 07:31:28 delphij Exp $");
29
30#include <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.  However, we can further reduce ~1/3 of
47 * time if we consider that strlen() usually operate on 7-bit ASCII
48 * by employing the following expression, which allows false positive
49 * when high bit of 1 and use the tail case to catch these case:
50 *
51 *	((x - 0x01....01) & 0x80....80)
52 *
53 * This is more than 5.2 times as fast as the raw implementation on
54 * Intel T7300 under long mode for strings longer than word length.
55 */
56
57/* Magic numbers for the algorithm */
58#if LONG_BIT == 32
59static const unsigned long mask01 = 0x01010101;
60static const unsigned long mask80 = 0x80808080;
61#elif LONG_BIT == 64
62static const unsigned long mask01 = 0x0101010101010101;
63static const unsigned long mask80 = 0x8080808080808080;
64#else
65#error Unsupported word size
66#endif
67
68#define	LONGPTR_MASK (sizeof(long) - 1)
69
70/*
71 * Helper macro to return string length if we caught the zero
72 * byte.
73 */
74#define testbyte(x)				\
75	do {					\
76		if (p[x] == '\0')		\
77		    return (p - str + x);	\
78	} while (0)
79
80size_t
81strlen(const char *str)
82{
83	const char *p;
84	const unsigned long *lp;
85
86	/* Skip the first few bytes until we have an aligned p */
87	for (p = str; (uintptr_t)p & LONGPTR_MASK; p++)
88	    if (*p == '\0')
89		return (p - str);
90
91	/* Scan the rest of the string using word sized operation */
92	for (lp = (const unsigned long *)p; ; lp++)
93	    if ((*lp - mask01) & mask80) {
94		p = (const char *)(lp);
95		testbyte(0);
96		testbyte(1);
97		testbyte(2);
98		testbyte(3);
99#if (LONG_BIT >= 64)
100		testbyte(4);
101		testbyte(5);
102		testbyte(6);
103		testbyte(7);
104#endif
105	    }
106
107	/* NOTREACHED */
108	return (0);
109}
110
111