1/* Copyright (C) 1991, 1993, 1996-1997, 1999-2000, 2003-2004, 2006, 2008-2014
2   Free Software Foundation, Inc.
3
4   Based on strlen implementation by Torbjorn Granlund (tege@sics.se),
5   with help from Dan Sahlin (dan@sics.se) and
6   commentary by Jim Blandy (jimb@ai.mit.edu);
7   adaptation to memchr suggested by Dick Karpinski (dick@cca.ucsf.edu),
8   and implemented by Roland McGrath (roland@ai.mit.edu).
9
10NOTE: The canonical source of this file is maintained with the GNU C Library.
11Bugs can be reported to bug-glibc@prep.ai.mit.edu.
12
13This program is free software: you can redistribute it and/or modify it
14under the terms of the GNU General Public License as published by the
15Free Software Foundation; either version 3 of the License, or any
16later version.
17
18This program is distributed in the hope that it will be useful,
19but WITHOUT ANY WARRANTY; without even the implied warranty of
20MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
21GNU General Public License for more details.
22
23You should have received a copy of the GNU General Public License
24along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
25
26#ifndef _LIBC
27# include <config.h>
28#endif
29
30#include <string.h>
31
32#include <stddef.h>
33
34#if defined _LIBC
35# include <memcopy.h>
36#else
37# define reg_char char
38#endif
39
40#include <limits.h>
41
42#if HAVE_BP_SYM_H || defined _LIBC
43# include <bp-sym.h>
44#else
45# define BP_SYM(sym) sym
46#endif
47
48#undef __memchr
49#ifdef _LIBC
50# undef memchr
51#endif
52
53#ifndef weak_alias
54# define __memchr memchr
55#endif
56
57/* Search no more than N bytes of S for C.  */
58void *
59__memchr (void const *s, int c_in, size_t n)
60{
61  /* On 32-bit hardware, choosing longword to be a 32-bit unsigned
62     long instead of a 64-bit uintmax_t tends to give better
63     performance.  On 64-bit hardware, unsigned long is generally 64
64     bits already.  Change this typedef to experiment with
65     performance.  */
66  typedef unsigned long int longword;
67
68  const unsigned char *char_ptr;
69  const longword *longword_ptr;
70  longword repeated_one;
71  longword repeated_c;
72  unsigned reg_char c;
73
74  c = (unsigned char) c_in;
75
76  /* Handle the first few bytes by reading one byte at a time.
77     Do this until CHAR_PTR is aligned on a longword boundary.  */
78  for (char_ptr = (const unsigned char *) s;
79       n > 0 && (size_t) char_ptr % sizeof (longword) != 0;
80       --n, ++char_ptr)
81    if (*char_ptr == c)
82      return (void *) char_ptr;
83
84  longword_ptr = (const longword *) char_ptr;
85
86  /* All these elucidatory comments refer to 4-byte longwords,
87     but the theory applies equally well to any size longwords.  */
88
89  /* Compute auxiliary longword values:
90     repeated_one is a value which has a 1 in every byte.
91     repeated_c has c in every byte.  */
92  repeated_one = 0x01010101;
93  repeated_c = c | (c << 8);
94  repeated_c |= repeated_c << 16;
95  if (0xffffffffU < (longword) -1)
96    {
97      repeated_one |= repeated_one << 31 << 1;
98      repeated_c |= repeated_c << 31 << 1;
99      if (8 < sizeof (longword))
100        {
101          size_t i;
102
103          for (i = 64; i < sizeof (longword) * 8; i *= 2)
104            {
105              repeated_one |= repeated_one << i;
106              repeated_c |= repeated_c << i;
107            }
108        }
109    }
110
111  /* Instead of the traditional loop which tests each byte, we will test a
112     longword at a time.  The tricky part is testing if *any of the four*
113     bytes in the longword in question are equal to c.  We first use an xor
114     with repeated_c.  This reduces the task to testing whether *any of the
115     four* bytes in longword1 is zero.
116
117     We compute tmp =
118       ((longword1 - repeated_one) & ~longword1) & (repeated_one << 7).
119     That is, we perform the following operations:
120       1. Subtract repeated_one.
121       2. & ~longword1.
122       3. & a mask consisting of 0x80 in every byte.
123     Consider what happens in each byte:
124       - If a byte of longword1 is zero, step 1 and 2 transform it into 0xff,
125         and step 3 transforms it into 0x80.  A carry can also be propagated
126         to more significant bytes.
127       - If a byte of longword1 is nonzero, let its lowest 1 bit be at
128         position k (0 <= k <= 7); so the lowest k bits are 0.  After step 1,
129         the byte ends in a single bit of value 0 and k bits of value 1.
130         After step 2, the result is just k bits of value 1: 2^k - 1.  After
131         step 3, the result is 0.  And no carry is produced.
132     So, if longword1 has only non-zero bytes, tmp is zero.
133     Whereas if longword1 has a zero byte, call j the position of the least
134     significant zero byte.  Then the result has a zero at positions 0, ...,
135     j-1 and a 0x80 at position j.  We cannot predict the result at the more
136     significant bytes (positions j+1..3), but it does not matter since we
137     already have a non-zero bit at position 8*j+7.
138
139     So, the test whether any byte in longword1 is zero is equivalent to
140     testing whether tmp is nonzero.  */
141
142  while (n >= sizeof (longword))
143    {
144      longword longword1 = *longword_ptr ^ repeated_c;
145
146      if ((((longword1 - repeated_one) & ~longword1)
147           & (repeated_one << 7)) != 0)
148        break;
149      longword_ptr++;
150      n -= sizeof (longword);
151    }
152
153  char_ptr = (const unsigned char *) longword_ptr;
154
155  /* At this point, we know that either n < sizeof (longword), or one of the
156     sizeof (longword) bytes starting at char_ptr is == c.  On little-endian
157     machines, we could determine the first such byte without any further
158     memory accesses, just by looking at the tmp result from the last loop
159     iteration.  But this does not work on big-endian machines.  Choose code
160     that works in both cases.  */
161
162  for (; n > 0; --n, ++char_ptr)
163    {
164      if (*char_ptr == c)
165        return (void *) char_ptr;
166    }
167
168  return NULL;
169}
170#ifdef weak_alias
171weak_alias (__memchr, BP_SYM (memchr))
172#endif
173