The Design and Implementation of the FreeBSD Operating System, Second Edition
Now available: The Design and Implementation of the FreeBSD Operating System (Second Edition)


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FreeBSD/Linux Kernel Cross Reference
sys/libkern/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: stable/12/sys/libkern/strlen.c 333449 2018-05-10 06:41:08Z delphij $");
   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  * 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.
   47  *
   48  * On multi-issue processors, we can divide the above expression into:
   49  *      a)  (x - 0x01....01)
   50  *      b) (~x & 0x80....80)
   51  *      c) a & b
   52  *
   53  * Where, a) and b) can be partially computed in parallel.
   54  *
   55  * The algorithm above is found on "Hacker's Delight" by
   56  * Henry S. Warren, Jr.
   57  */
   58 
   59 /* Magic numbers for the algorithm */
   60 #if LONG_BIT == 32
   61 static const unsigned long mask01 = 0x01010101;
   62 static const unsigned long mask80 = 0x80808080;
   63 #elif LONG_BIT == 64
   64 static const unsigned long mask01 = 0x0101010101010101;
   65 static const unsigned long mask80 = 0x8080808080808080;
   66 #else
   67 #error Unsupported word size
   68 #endif
   69 
   70 #define LONGPTR_MASK (sizeof(long) - 1)
   71 
   72 /*
   73  * Helper macro to return string length if we caught the zero
   74  * byte.
   75  */
   76 #define testbyte(x)                             \
   77         do {                                    \
   78                 if (p[x] == '\0')               \
   79                     return (p - str + x);       \
   80         } while (0)
   81 
   82 size_t
   83 strlen(const char *str)
   84 {
   85         const char *p;
   86         const unsigned long *lp;
   87         long va, vb;
   88 
   89         /*
   90          * Before trying the hard (unaligned byte-by-byte access) way
   91          * to figure out whether there is a nul character, try to see
   92          * if there is a nul character is within this accessible word
   93          * first.
   94          *
   95          * p and (p & ~LONGPTR_MASK) must be equally accessible since
   96          * they always fall in the same memory page, as long as page
   97          * boundaries is integral multiple of word size.
   98          */
   99         lp = (const unsigned long *)((uintptr_t)str & ~LONGPTR_MASK);
  100         va = (*lp - mask01);
  101         vb = ((~*lp) & mask80);
  102         lp++;
  103         if (va & vb)
  104                 /* Check if we have \0 in the first part */
  105                 for (p = str; p < (const char *)lp; p++)
  106                         if (*p == '\0')
  107                                 return (p - str);
  108 
  109         /* Scan the rest of the string using word sized operation */
  110         for (; ; lp++) {
  111                 va = (*lp - mask01);
  112                 vb = ((~*lp) & mask80);
  113                 if (va & vb) {
  114                         p = (const char *)(lp);
  115                         testbyte(0);
  116                         testbyte(1);
  117                         testbyte(2);
  118                         testbyte(3);
  119 #if (LONG_BIT >= 64)
  120                         testbyte(4);
  121                         testbyte(5);
  122                         testbyte(6);
  123                         testbyte(7);
  124 #endif
  125                 }
  126         }
  127 
  128         /* NOTREACHED */
  129         return (0);
  130 }

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