usr/src/boot/lib/libc/string/strlen.c

   1 /*-
   2  * Copyright (c) 2009, 2010 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 <sys/cdefs.h>
  28 __FBSDID("$FreeBSD$");
  29
  30 #include <sys/limits.h>
  31 #include <sys/types.h>
  32 #include <string.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 }