strlen.c revision 242506
1295041Sbr/*- 2295972Sbr * Copyright (c) 2009, 2010 Xin LI <delphij@FreeBSD.org> 3295041Sbr * All rights reserved. 4295041Sbr * 5295041Sbr * Redistribution and use in source and binary forms, with or without 6295041Sbr * modification, are permitted provided that the following conditions 7295041Sbr * are met: 8295041Sbr * 1. Redistributions of source code must retain the above copyright 9295041Sbr * notice, this list of conditions and the following disclaimer. 10295041Sbr * 2. Redistributions in binary form must reproduce the above copyright 11295041Sbr * notice, this list of conditions and the following disclaimer in the 12295041Sbr * documentation and/or other materials provided with the distribution. 13295041Sbr * 14295041Sbr * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND 15295041Sbr * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 16295041Sbr * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 17295041Sbr * ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE 18295041Sbr * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 19295041Sbr * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 20295041Sbr * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 21295041Sbr * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 22295041Sbr * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 23295041Sbr * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 24295041Sbr * SUCH DAMAGE. 25295041Sbr */ 26295041Sbr 27295041Sbr#include <sys/cdefs.h> 28295041Sbr__FBSDID("$FreeBSD: head/sys/libkern/strlen.c 242506 2012-11-03 04:28:53Z delphij $"); 29295041Sbr 30295041Sbr#include <sys/libkern.h> 31295041Sbr#include <sys/limits.h> 32295041Sbr 33295041Sbr/* 34295041Sbr * Portable strlen() for 32-bit and 64-bit systems. 35295041Sbr * 36295041Sbr * Rationale: it is generally much more efficient to do word length 37295041Sbr * operations and avoid branches on modern computer systems, as 38295041Sbr * compared to byte-length operations with a lot of branches. 39295041Sbr * 40295041Sbr * The expression: 41295041Sbr * 42295041Sbr * ((x - 0x01....01) & ~x & 0x80....80) 43295041Sbr * 44295041Sbr * would evaluate to a non-zero value iff any of the bytes in the 45295041Sbr * original word is zero. 46295041Sbr * 47295041Sbr * On multi-issue processors, we can divide the above expression into: 48295041Sbr * a) (x - 0x01....01) 49295041Sbr * b) (~x & 0x80....80) 50295041Sbr * c) a & b 51295041Sbr * 52295041Sbr * Where, a) and b) can be partially computed in parallel. 53295041Sbr * 54295041Sbr * The algorithm above is found on "Hacker's Delight" by 55295041Sbr * Henry S. Warren, Jr. 56295041Sbr */ 57295041Sbr 58295041Sbr/* Magic numbers for the algorithm */ 59295041Sbr#if LONG_BIT == 32 60295041Sbrstatic const unsigned long mask01 = 0x01010101; 61295041Sbrstatic const unsigned long mask80 = 0x80808080; 62295041Sbr#elif LONG_BIT == 64 63295041Sbrstatic const unsigned long mask01 = 0x0101010101010101; 64295041Sbrstatic const unsigned long mask80 = 0x8080808080808080; 65295972Sbr#else 66295041Sbr#error Unsupported word size 67295041Sbr#endif 68295041Sbr 69295041Sbr#define LONGPTR_MASK (sizeof(long) - 1) 70295041Sbr 71295041Sbr/* 72295041Sbr * Helper macro to return string length if we caught the zero 73295041Sbr * byte. 74295041Sbr */ 75295041Sbr#define testbyte(x) \ 76295041Sbr do { \ 77295041Sbr if (p[x] == '\0') \ 78295041Sbr return (p - str + x); \ 79295041Sbr } while (0) 80295041Sbr 81295041Sbrsize_t 82295041Sbrstrlen(const char *str) 83295041Sbr{ 84295041Sbr const char *p; 85295041Sbr const unsigned long *lp; 86295041Sbr long va, vb; 87295041Sbr 88295041Sbr /* 89295041Sbr * Before trying the hard (unaligned byte-by-byte access) way 90295041Sbr * to figure out whether there is a nul character, try to see 91295041Sbr * if there is a nul character is within this accessible word 92295041Sbr * first. 93295041Sbr * 94295041Sbr * p and (p & ~LONGPTR_MASK) must be equally accessible since 95295041Sbr * they always fall in the same memory page, as long as page 96295041Sbr * boundaries is integral multiple of word size. 97295041Sbr */ 98295041Sbr lp = (const unsigned long *)((uintptr_t)str & ~LONGPTR_MASK); 99295041Sbr va = (*lp - mask01); 100295041Sbr vb = ((~*lp) & mask80); 101295041Sbr lp++; 102295041Sbr if (va & vb) 103295041Sbr /* Check if we have \0 in the first part */ 104295041Sbr for (p = str; p < (const char *)lp; p++) 105295041Sbr if (*p == '\0') 106295041Sbr return (p - str); 107295041Sbr 108295041Sbr /* Scan the rest of the string using word sized operation */ 109295041Sbr for (; ; lp++) { 110295041Sbr va = (*lp - mask01); 111295041Sbr vb = ((~*lp) & mask80); 112295041Sbr if (va & vb) { 113295041Sbr p = (const char *)(lp); 114295041Sbr testbyte(0); 115295041Sbr testbyte(1); 116295041Sbr testbyte(2); 117295041Sbr testbyte(3); 118295041Sbr#if (LONG_BIT >= 64) 119295041Sbr testbyte(4); 120295041Sbr testbyte(5); 121295041Sbr testbyte(6); 122295041Sbr testbyte(7); 123295041Sbr#endif 124295041Sbr } 125295041Sbr } 126295041Sbr 127295041Sbr /* NOTREACHED */ 128295041Sbr return (0); 129295972Sbr} 130295972Sbr