libsyscall/wrappers/string/strlen.c

   1 /*-
   2  * Copyright (c) 2009 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 "string.h"
  28 #include <limits.h>
  29
  30 /*
  31  * Portable strlen() for 32-bit and 64-bit systems.
  32  *
  33  * Rationale: it is generally much more efficient to do word length
  34  * operations and avoid branches on modern computer systems, as
  35  * compared to byte-length operations with a lot of branches.
  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.  However, we can further reduce ~1/3 of
  43  * time if we consider that strlen() usually operate on 7-bit ASCII
  44  * by employing the following expression, which allows false positive
  45  * when high bit of 1 and use the tail case to catch these case:
  46  *
  47  *      ((x - 0x01....01) & 0x80....80)
  48  *
  49  * This is more than 5.2 times as fast as the raw implementation on
  50  * Intel T7300 under long mode for strings longer than word length.
  51  */
  52
  53 /* Magic numbers for the algorithm */
  54 #if LONG_BIT == 32
  55 static const unsigned long mask01 = 0x01010101;
  56 static const unsigned long mask80 = 0x80808080;
  57 #elif LONG_BIT == 64
  58 static const unsigned long mask01 = 0x0101010101010101;
  59 static const unsigned long mask80 = 0x8080808080808080;
  60 #else
  61 #error Unsupported word size
  62 #endif
  63
  64 #define LONGPTR_MASK (sizeof(long) - 1)
  65
  66 /*
  67  * Helper macro to return string length if we caught the zero
  68  * byte.
  69  */
  70 #define testbyte(x)                             \
  71         do {                                    \
  72                 if (p[x] == '\0')               \
  73                     return (p - str + x);       \
  74         } while (0)
  75
  76 __attribute__((visibility("hidden")))
  77 size_t
  78 _libkernel_strlen(const char *str)
  79 {
  80         const char *p;
  81         const unsigned long *lp;
  82
  83         /* Skip the first few bytes until we have an aligned p */
  84         for (p = str; (uintptr_t)p & LONGPTR_MASK; p++) {
  85                 if (*p == '\0') {
  86                         return p - str;
  87                 }
  88         }
  89
  90         /* Scan the rest of the string using word sized operation */
  91         for (lp = (const unsigned long *)p;; lp++) {
  92                 if ((*lp - mask01) & mask80) {
  93                         p = (const char *)(lp);
  94                         testbyte(0);
  95                         testbyte(1);
  96                         testbyte(2);
  97                         testbyte(3);
  98 #if (LONG_BIT >= 64)
  99                         testbyte(4);
 100                         testbyte(5);
 101                         testbyte(6);
 102                         testbyte(7);
 103 #endif
 104                 }
 105         }
 106
 107         /* NOTREACHED */
 108         return 0;
 109 }