1/* memrchr -- find the last occurrence of a byte in a memory block
2
3   Copyright (C) 1991, 1993, 1996, 1997, 1999, 2000, 2003, 2004, 2005
4   Free Software Foundation, Inc.
5
6   Based on strlen implementation by Torbjorn Granlund (tege@sics.se),
7   with help from Dan Sahlin (dan@sics.se) and
8   commentary by Jim Blandy (jimb@ai.mit.edu);
9   adaptation to memchr suggested by Dick Karpinski (dick@cca.ucsf.edu),
10   and implemented by Roland McGrath (roland@ai.mit.edu).
11
12   This program is free software; you can redistribute it and/or modify
13   it under the terms of the GNU General Public License as published by
14   the Free Software Foundation; either version 2, or (at your option)
15   any later version.
16
17   This program is distributed in the hope that it will be useful,
18   but WITHOUT ANY WARRANTY; without even the implied warranty of
19   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
20   GNU General Public License for more details.
21
22   You should have received a copy of the GNU General Public License along
23   with this program; if not, write to the Free Software Foundation,
24   Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.  */
25
26#ifdef HAVE_CONFIG_H
27# include <config.h>
28#endif
29
30#if defined _LIBC
31# include <string.h>
32# include <memcopy.h>
33#else
34# include "memrchr.h"
35# define reg_char char
36#endif
37
38#include <limits.h>
39
40#undef __memrchr
41#undef memrchr
42
43#ifndef weak_alias
44# define __memrchr memrchr
45#endif
46
47/* Search no more than N bytes of S for C.  */
48void *
49__memrchr (void const *s, int c_in, size_t n)
50{
51  const unsigned char *char_ptr;
52  const unsigned long int *longword_ptr;
53  unsigned long int longword, magic_bits, charmask;
54  unsigned reg_char c;
55  int i;
56
57  c = (unsigned char) c_in;
58
59  /* Handle the last few characters by reading one character at a time.
60     Do this until CHAR_PTR is aligned on a longword boundary.  */
61  for (char_ptr = (const unsigned char *) s + n;
62       n > 0 && (size_t) char_ptr % sizeof longword != 0;
63       --n)
64    if (*--char_ptr == c)
65      return (void *) char_ptr;
66
67  /* All these elucidatory comments refer to 4-byte longwords,
68     but the theory applies equally well to any size longwords.  */
69
70  longword_ptr = (const unsigned long int *) char_ptr;
71
72  /* Bits 31, 24, 16, and 8 of this number are zero.  Call these bits
73     the "holes."  Note that there is a hole just to the left of
74     each byte, with an extra at the end:
75
76     bits:  01111110 11111110 11111110 11111111
77     bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD
78
79     The 1-bits make sure that carries propagate to the next 0-bit.
80     The 0-bits provide holes for carries to fall into.  */
81
82  /* Set MAGIC_BITS to be this pattern of 1 and 0 bits.
83     Set CHARMASK to be a longword, each of whose bytes is C.  */
84
85  magic_bits = 0xfefefefe;
86  charmask = c | (c << 8);
87  charmask |= charmask << 16;
88#if 0xffffffffU < ULONG_MAX
89  magic_bits |= magic_bits << 32;
90  charmask |= charmask << 32;
91  if (8 < sizeof longword)
92    for (i = 64; i < sizeof longword * 8; i *= 2)
93      {
94	magic_bits |= magic_bits << i;
95	charmask |= charmask << i;
96      }
97#endif
98  magic_bits = (ULONG_MAX >> 1) & (magic_bits | 1);
99
100  /* Instead of the traditional loop which tests each character,
101     we will test a longword at a time.  The tricky part is testing
102     if *any of the four* bytes in the longword in question are zero.  */
103  while (n >= sizeof longword)
104    {
105      /* We tentatively exit the loop if adding MAGIC_BITS to
106	 LONGWORD fails to change any of the hole bits of LONGWORD.
107
108	 1) Is this safe?  Will it catch all the zero bytes?
109	 Suppose there is a byte with all zeros.  Any carry bits
110	 propagating from its left will fall into the hole at its
111	 least significant bit and stop.  Since there will be no
112	 carry from its most significant bit, the LSB of the
113	 byte to the left will be unchanged, and the zero will be
114	 detected.
115
116	 2) Is this worthwhile?  Will it ignore everything except
117	 zero bytes?  Suppose every byte of LONGWORD has a bit set
118	 somewhere.  There will be a carry into bit 8.  If bit 8
119	 is set, this will carry into bit 16.  If bit 8 is clear,
120	 one of bits 9-15 must be set, so there will be a carry
121	 into bit 16.  Similarly, there will be a carry into bit
122	 24.  If one of bits 24-30 is set, there will be a carry
123	 into bit 31, so all of the hole bits will be changed.
124
125	 The one misfire occurs when bits 24-30 are clear and bit
126	 31 is set; in this case, the hole at bit 31 is not
127	 changed.  If we had access to the processor carry flag,
128	 we could close this loophole by putting the fourth hole
129	 at bit 32!
130
131	 So it ignores everything except 128's, when they're aligned
132	 properly.
133
134	 3) But wait!  Aren't we looking for C, not zero?
135	 Good point.  So what we do is XOR LONGWORD with a longword,
136	 each of whose bytes is C.  This turns each byte that is C
137	 into a zero.  */
138
139      longword = *--longword_ptr ^ charmask;
140
141      /* Add MAGIC_BITS to LONGWORD.  */
142      if ((((longword + magic_bits)
143
144	    /* Set those bits that were unchanged by the addition.  */
145	    ^ ~longword)
146
147	   /* Look at only the hole bits.  If any of the hole bits
148	      are unchanged, most likely one of the bytes was a
149	      zero.  */
150	   & ~magic_bits) != 0)
151	{
152	  /* Which of the bytes was C?  If none of them were, it was
153	     a misfire; continue the search.  */
154
155	  const unsigned char *cp = (const unsigned char *) longword_ptr;
156
157	  if (8 < sizeof longword)
158	    for (i = sizeof longword - 1; 8 <= i; i--)
159	      if (cp[i] == c)
160		return (void *) &cp[i];
161	  if (7 < sizeof longword && cp[7] == c)
162	    return (void *) &cp[7];
163	  if (6 < sizeof longword && cp[6] == c)
164	    return (void *) &cp[6];
165	  if (5 < sizeof longword && cp[5] == c)
166	    return (void *) &cp[5];
167	  if (4 < sizeof longword && cp[4] == c)
168	    return (void *) &cp[4];
169	  if (cp[3] == c)
170	    return (void *) &cp[3];
171	  if (cp[2] == c)
172	    return (void *) &cp[2];
173	  if (cp[1] == c)
174	    return (void *) &cp[1];
175	  if (cp[0] == c)
176	    return (void *) cp;
177	}
178
179      n -= sizeof longword;
180    }
181
182  char_ptr = (const unsigned char *) longword_ptr;
183
184  while (n-- > 0)
185    {
186      if (*--char_ptr == c)
187	return (void *) char_ptr;
188    }
189
190  return 0;
191}
192#ifdef weak_alias
193weak_alias (__memrchr, memrchr)
194#endif
195