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 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