strlen.c

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    1 /*-
    2  * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
    3  *
    4  * Copyright (c) 2009, 2010 Xin LI <delphij@FreeBSD.org>
    5  *
    6  * Redistribution and use in source and binary forms, with or without
    7  * modification, are permitted provided that the following conditions
    8  * are met:
    9  * 1. Redistributions of source code must retain the above copyright
   10  *    notice, this list of conditions and the following disclaimer.
   11  * 2. Redistributions in binary form must reproduce the above copyright
   12  *    notice, this list of conditions and the following disclaimer in the
   13  *    documentation and/or other materials provided with the distribution.
   14  *
   15  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
   16  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
   17  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
   18  * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
   19  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
   20  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
   21  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
   22  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
   23  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
   24  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
   25  * SUCH DAMAGE.
   26  */
   27 
   28 #include <sys/cdefs.h>
   29 __FBSDID("$FreeBSD$");
   30 
   31 #include <sys/libkern.h>
   32 #include <sys/limits.h>
   33 
   34 /*
   35  * Portable strlen() for 32-bit and 64-bit systems.
   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.
   43  *
   44  * The algorithm above is found on "Hacker's Delight" by
   45  * Henry S. Warren, Jr.
   46  *
   47  * Note: this leaves performance on the table and each architecture
   48  * would be best served with a tailor made routine instead, even if
   49  * using the same trick.
   50  */
   51 
   52 /* Magic numbers for the algorithm */
   53 #if LONG_BIT == 32
   54 static const unsigned long mask01 = 0x01010101;
   55 static const unsigned long mask80 = 0x80808080;
   56 #elif LONG_BIT == 64
   57 static const unsigned long mask01 = 0x0101010101010101;
   58 static const unsigned long mask80 = 0x8080808080808080;
   59 #else
   60 #error Unsupported word size
   61 #endif
   62 
   63 #define LONGPTR_MASK (sizeof(long) - 1)
   64 
   65 /*
   66  * Helper macro to return string length if we caught the zero
   67  * byte.
   68  */
   69 #define testbyte(x)                             \
   70         do {                                    \
   71                 if (p[x] == '\0')               \
   72                     return (p - str + x);       \
   73         } while (0)
   74 
   75 size_t
   76 (strlen)(const char *str)
   77 {
   78         const char *p;
   79         const unsigned long *lp;
   80         long va, vb;
   81 
   82         /*
   83          * Before trying the hard (unaligned byte-by-byte access) way
   84          * to figure out whether there is a nul character, try to see
   85          * if there is a nul character is within this accessible word
   86          * first.
   87          *
   88          * p and (p & ~LONGPTR_MASK) must be equally accessible since
   89          * they always fall in the same memory page, as long as page
   90          * boundaries is integral multiple of word size.
   91          */
   92         lp = (const unsigned long *)((uintptr_t)str & ~LONGPTR_MASK);
   93         va = (*lp - mask01);
   94         vb = ((~*lp) & mask80);
   95         lp++;
   96         if (va & vb)
   97                 /* Check if we have \0 in the first part */
   98                 for (p = str; p < (const char *)lp; p++)
   99                         if (*p == '\0')
  100                                 return (p - str);
  101 
  102         /* Scan the rest of the string using word sized operation */
  103         for (; ; lp++) {
  104                 va = (*lp - mask01);
  105                 vb = ((~*lp) & mask80);
  106                 if (va & vb) {
  107                         p = (const char *)(lp);
  108                         testbyte(0);
  109                         testbyte(1);
  110                         testbyte(2);
  111                         testbyte(3);
  112 #if (LONG_BIT >= 64)
  113                         testbyte(4);
  114                         testbyte(5);
  115                         testbyte(6);
  116                         testbyte(7);
  117 #endif
  118                 }
  119         }
  120 
  121         /* NOTREACHED */
  122         return (0);
  123 }

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FreeBSD/Linux Kernel Cross Reference sys/libkern/strlen.c

FreeBSD/Linux Kernel Cross Reference
sys/libkern/strlen.c