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("$FreeBSD$"); 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. 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 59 static const unsigned long mask01 = 0x01010101; 60 static const unsigned long mask80 = 0x80808080; 61 #elif LONG_BIT == 64 62 static const unsigned long mask01 = 0x0101010101010101; 63 static 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 80 size_t 81 strlen(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