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 "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 55static const unsigned long mask01 = 0x01010101; 56static const unsigned long mask80 = 0x80808080; 57#elif LONG_BIT == 64 58static const unsigned long mask01 = 0x0101010101010101; 59static 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"))) 77size_t 78strlen(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 /* 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); 92 testbyte(0); 93 testbyte(1); 94 testbyte(2); 95 testbyte(3); 96#if (LONG_BIT >= 64) 97 testbyte(4); 98 testbyte(5); 99 testbyte(6); 100 testbyte(7); 101#endif 102 } 103 104 /* NOTREACHED */ 105 return (0); 106} 107 108