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git.saurik.com Git - apple/xnu.git/blob - libsyscall/wrappers/string/strlen.c
2 * Copyright (c) 2009 Xin LI <delphij@FreeBSD.org>
5 * Redistribution and use in source and binary forms, with or without
6 * modification, are permitted provided that the following conditions
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.
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
31 * Portable strlen() for 32-bit and 64-bit systems.
33 * Rationale: it is generally much more efficient to do word length
34 * operations and avoid branches on modern computer systems, as
35 * compared to byte-length operations with a lot of branches.
39 * ((x - 0x01....01) & ~x & 0x80....80)
41 * would evaluate to a non-zero value iff any of the bytes in the
42 * original word is zero. However, we can further reduce ~1/3 of
43 * time if we consider that strlen() usually operate on 7-bit ASCII
44 * by employing the following expression, which allows false positive
45 * when high bit of 1 and use the tail case to catch these case:
47 * ((x - 0x01....01) & 0x80....80)
49 * This is more than 5.2 times as fast as the raw implementation on
50 * Intel T7300 under long mode for strings longer than word length.
53 /* Magic numbers for the algorithm */
55 static const unsigned long mask01
= 0x01010101;
56 static const unsigned long mask80
= 0x80808080;
58 static const unsigned long mask01
= 0x0101010101010101;
59 static const unsigned long mask80
= 0x8080808080808080;
61 #error Unsupported word size
64 #define LONGPTR_MASK (sizeof(long) - 1)
67 * Helper macro to return string length if we caught the zero
73 return (p - str + x); \
76 __attribute__((visibility("hidden")))
78 strlen(const char *str
)
81 const unsigned long *lp
;
83 /* Skip the first few bytes until we have an aligned p */
84 for (p
= str
; (uintptr_t)p
& LONGPTR_MASK
; p
++)
88 /* Scan the rest of the string using word sized operation */
89 for (lp
= (const unsigned long *)p
; ; lp
++)
90 if ((*lp
- mask01
) & mask80
) {
91 p
= (const char *)(lp
);