Date: Sat, 3 Nov 2012 04:28:53 +0000 (UTC) From: Xin LI <delphij@FreeBSD.org> To: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: svn commit: r242506 - head/sys/libkern Message-ID: <201211030428.qA34Srx2083384@svn.freebsd.org>
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Author: delphij Date: Sat Nov 3 04:28:53 2012 New Revision: 242506 URL: http://svn.freebsd.org/changeset/base/242506 Log: Sync strlen with userland implementation. MFC after: 1 month Modified: head/sys/libkern/strlen.c Modified: head/sys/libkern/strlen.c ============================================================================== --- head/sys/libkern/strlen.c Sat Nov 3 01:40:05 2012 (r242505) +++ head/sys/libkern/strlen.c Sat Nov 3 04:28:53 2012 (r242506) @@ -1,6 +1,6 @@ /*- - * Copyright (c) 1990, 1993 - * The Regents of the University of California. All rights reserved. + * Copyright (c) 2009, 2010 Xin LI <delphij@FreeBSD.org> + * All rights reserved. * * Redistribution and use in source and binary forms, with or without * modification, are permitted provided that the following conditions @@ -10,14 +10,11 @@ * 2. Redistributions in binary form must reproduce the above copyright * notice, this list of conditions and the following disclaimer in the * documentation and/or other materials provided with the distribution. - * 4. Neither the name of the University nor the names of its contributors - * may be used to endorse or promote products derived from this software - * without specific prior written permission. * - * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND + * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE - * ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE + * ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) @@ -31,14 +28,102 @@ __FBSDID("$FreeBSD$"); #include <sys/libkern.h> +#include <sys/limits.h> + +/* + * Portable strlen() for 32-bit and 64-bit systems. + * + * Rationale: it is generally much more efficient to do word length + * operations and avoid branches on modern computer systems, as + * compared to byte-length operations with a lot of branches. + * + * The expression: + * + * ((x - 0x01....01) & ~x & 0x80....80) + * + * would evaluate to a non-zero value iff any of the bytes in the + * original word is zero. + * + * On multi-issue processors, we can divide the above expression into: + * a) (x - 0x01....01) + * b) (~x & 0x80....80) + * c) a & b + * + * Where, a) and b) can be partially computed in parallel. + * + * The algorithm above is found on "Hacker's Delight" by + * Henry S. Warren, Jr. + */ + +/* Magic numbers for the algorithm */ +#if LONG_BIT == 32 +static const unsigned long mask01 = 0x01010101; +static const unsigned long mask80 = 0x80808080; +#elif LONG_BIT == 64 +static const unsigned long mask01 = 0x0101010101010101; +static const unsigned long mask80 = 0x8080808080808080; +#else +#error Unsupported word size +#endif + +#define LONGPTR_MASK (sizeof(long) - 1) + +/* + * Helper macro to return string length if we caught the zero + * byte. + */ +#define testbyte(x) \ + do { \ + if (p[x] == '\0') \ + return (p - str + x); \ + } while (0) size_t -strlen(str) - const char *str; +strlen(const char *str) { - register const char *s; + const char *p; + const unsigned long *lp; + long va, vb; - for (s = str; *s; ++s); - return(s - str); -} + /* + * Before trying the hard (unaligned byte-by-byte access) way + * to figure out whether there is a nul character, try to see + * if there is a nul character is within this accessible word + * first. + * + * p and (p & ~LONGPTR_MASK) must be equally accessible since + * they always fall in the same memory page, as long as page + * boundaries is integral multiple of word size. + */ + lp = (const unsigned long *)((uintptr_t)str & ~LONGPTR_MASK); + va = (*lp - mask01); + vb = ((~*lp) & mask80); + lp++; + if (va & vb) + /* Check if we have \0 in the first part */ + for (p = str; p < (const char *)lp; p++) + if (*p == '\0') + return (p - str); + /* Scan the rest of the string using word sized operation */ + for (; ; lp++) { + va = (*lp - mask01); + vb = ((~*lp) & mask80); + if (va & vb) { + p = (const char *)(lp); + testbyte(0); + testbyte(1); + testbyte(2); + testbyte(3); +#if (LONG_BIT >= 64) + testbyte(4); + testbyte(5); + testbyte(6); + testbyte(7); +#endif + } + } + + /* NOTREACHED */ + return (0); +}
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