1/* Copyright (C) 1991, 1993, 1996, 1997, 1999, 2000, 2003, 2004, 2006 Free
2   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 2, or (at your option) 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, write to the Free Software Foundation,
25Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.  */
26
27#ifndef _LIBC
28# include <config.h>
29#endif
30
31#include <string.h>
32
33#include <stddef.h>
34
35#if defined _LIBC
36# include <memcopy.h>
37#else
38# define reg_char char
39#endif
40
41#include <limits.h>
42
43#if HAVE_BP_SYM_H || defined _LIBC
44# include <bp-sym.h>
45#else
46# define BP_SYM(sym) sym
47#endif
48
49#undef memchr
50#undef __memchr
51
52/* Search no more than N bytes of S for C.  */
53void *
54__memchr (void const *s, int c_in, size_t n)
55{
56  const unsigned char *char_ptr;
57  const unsigned long int *longword_ptr;
58  unsigned long int longword, magic_bits, charmask;
59  unsigned reg_char c;
60  int i;
61
62  c = (unsigned char) c_in;
63
64  /* Handle the first few characters by reading one character at a time.
65     Do this until CHAR_PTR is aligned on a longword boundary.  */
66  for (char_ptr = (const unsigned char *) s;
67       n > 0 && (size_t) char_ptr % sizeof longword != 0;
68       --n, ++char_ptr)
69    if (*char_ptr == c)
70      return (void *) char_ptr;
71
72  /* All these elucidatory comments refer to 4-byte longwords,
73     but the theory applies equally well to any size longwords.  */
74
75  longword_ptr = (const unsigned long int *) char_ptr;
76
77  /* Bits 31, 24, 16, and 8 of this number are zero.  Call these bits
78     the "holes."  Note that there is a hole just to the left of
79     each byte, with an extra at the end:
80
81     bits:  01111110 11111110 11111110 11111111
82     bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD
83
84     The 1-bits make sure that carries propagate to the next 0-bit.
85     The 0-bits provide holes for carries to fall into.  */
86
87  /* Set MAGIC_BITS to be this pattern of 1 and 0 bits.
88     Set CHARMASK to be a longword, each of whose bytes is C.  */
89
90  magic_bits = 0xfefefefe;
91  charmask = c | (c << 8);
92  charmask |= charmask << 16;
93#if 0xffffffffU < ULONG_MAX
94  magic_bits |= magic_bits << 32;
95  charmask |= charmask << 32;
96  if (8 < sizeof longword)
97    for (i = 64; i < sizeof longword * 8; i *= 2)
98      {
99	magic_bits |= magic_bits << i;
100	charmask |= charmask << i;
101      }
102#endif
103  magic_bits = (ULONG_MAX >> 1) & (magic_bits | 1);
104
105  /* Instead of the traditional loop which tests each character,
106     we will test a longword at a time.  The tricky part is testing
107     if *any of the four* bytes in the longword in question are zero.  */
108  while (n >= sizeof longword)
109    {
110      /* We tentatively exit the loop if adding MAGIC_BITS to
111	 LONGWORD fails to change any of the hole bits of LONGWORD.
112
113	 1) Is this safe?  Will it catch all the zero bytes?
114	 Suppose there is a byte with all zeros.  Any carry bits
115	 propagating from its left will fall into the hole at its
116	 least significant bit and stop.  Since there will be no
117	 carry from its most significant bit, the LSB of the
118	 byte to the left will be unchanged, and the zero will be
119	 detected.
120
121	 2) Is this worthwhile?  Will it ignore everything except
122	 zero bytes?  Suppose every byte of LONGWORD has a bit set
123	 somewhere.  There will be a carry into bit 8.  If bit 8
124	 is set, this will carry into bit 16.  If bit 8 is clear,
125	 one of bits 9-15 must be set, so there will be a carry
126	 into bit 16.  Similarly, there will be a carry into bit
127	 24.  If one of bits 24-30 is set, there will be a carry
128	 into bit 31, so all of the hole bits will be changed.
129
130	 The one misfire occurs when bits 24-30 are clear and bit
131	 31 is set; in this case, the hole at bit 31 is not
132	 changed.  If we had access to the processor carry flag,
133	 we could close this loophole by putting the fourth hole
134	 at bit 32!
135
136	 So it ignores everything except 128's, when they're aligned
137	 properly.
138
139	 3) But wait!  Aren't we looking for C, not zero?
140	 Good point.  So what we do is XOR LONGWORD with a longword,
141	 each of whose bytes is C.  This turns each byte that is C
142	 into a zero.  */
143
144      longword = *longword_ptr++ ^ charmask;
145
146      /* Add MAGIC_BITS to LONGWORD.  */
147      if ((((longword + magic_bits)
148
149	    /* Set those bits that were unchanged by the addition.  */
150	    ^ ~longword)
151
152	   /* Look at only the hole bits.  If any of the hole bits
153	      are unchanged, most likely one of the bytes was a
154	      zero.  */
155	   & ~magic_bits) != 0)
156	{
157	  /* Which of the bytes was C?  If none of them were, it was
158	     a misfire; continue the search.  */
159
160	  const unsigned char *cp = (const unsigned char *) (longword_ptr - 1);
161
162	  if (cp[0] == c)
163	    return (void *) cp;
164	  if (cp[1] == c)
165	    return (void *) &cp[1];
166	  if (cp[2] == c)
167	    return (void *) &cp[2];
168	  if (cp[3] == c)
169	    return (void *) &cp[3];
170	  if (4 < sizeof longword && cp[4] == c)
171	    return (void *) &cp[4];
172	  if (5 < sizeof longword && cp[5] == c)
173	    return (void *) &cp[5];
174	  if (6 < sizeof longword && cp[6] == c)
175	    return (void *) &cp[6];
176	  if (7 < sizeof longword && cp[7] == c)
177	    return (void *) &cp[7];
178	  if (8 < sizeof longword)
179	    for (i = 8; i < sizeof longword; i++)
180	      if (cp[i] == c)
181		return (void *) &cp[i];
182	}
183
184      n -= sizeof longword;
185    }
186
187  char_ptr = (const unsigned char *) longword_ptr;
188
189  while (n-- > 0)
190    {
191      if (*char_ptr == c)
192	return (void *) char_ptr;
193      else
194	++char_ptr;
195    }
196
197  return 0;
198}
199#ifdef weak_alias
200weak_alias (__memchr, BP_SYM (memchr))
201#endif
202