1 /*-
2 * Copyright (c) 2009 Xin LI <[email protected]>
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 "string.h"
28 #include <limits.h>
29
30 /*
31 * Portable strlen() for 32-bit and 64-bit systems.
32 *
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.
36 *
37 * The expression:
38 *
39 * ((x - 0x01....01) & ~x & 0x80....80)
40 *
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:
46 *
47 * ((x - 0x01....01) & 0x80....80)
48 *
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.
51 */
52
53 /* Magic numbers for the algorithm */
54 #if LONG_BIT == 32
55 static const unsigned long mask01 = 0x01010101;
56 static const unsigned long mask80 = 0x80808080;
57 #elif LONG_BIT == 64
58 static const unsigned long mask01 = 0x0101010101010101;
59 static const unsigned long mask80 = 0x8080808080808080;
60 #else
61 #error Unsupported word size
62 #endif
63
64 #define LONGPTR_MASK (sizeof(long) - 1)
65
66 /*
67 * Helper macro to return string length if we caught the zero
68 * byte.
69 */
70 #define testbyte(x) \
71 do { \
72 if (p[x] == '\0') \
73 return (p - str + x); \
74 } while (0)
75
76 __attribute__((visibility("hidden")))
77 size_t
_libkernel_strlen(const char * str)78 _libkernel_strlen(const char *str)
79 {
80 const char *p;
81 const unsigned long *lp;
82
83 /* Skip the first few bytes until we have an aligned p */
84 for (p = str; (uintptr_t)p & LONGPTR_MASK; p++) {
85 if (*p == '\0') {
86 return p - str;
87 }
88 }
89
90 /* Scan the rest of the string using word sized operation */
91 for (lp = (const unsigned long *)p;; lp++) {
92 if ((*lp - mask01) & mask80) {
93 p = (const char *)(lp);
94 testbyte(0);
95 testbyte(1);
96 testbyte(2);
97 testbyte(3);
98 #if (LONG_BIT >= 64)
99 testbyte(4);
100 testbyte(5);
101 testbyte(6);
102 testbyte(7);
103 #endif
104 }
105 }
106
107 /* NOTREACHED */
108 return 0;
109 }
110