1/* memrchr -- find the last occurrence of a byte in a memory block
2   Copyright (C) 1991, 93, 96, 97, 99, 2000 Free Software Foundation, Inc.
3   This file is part of the GNU C Library.
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
10   The GNU C Library is free software; you can redistribute it and/or
11   modify it under the terms of the GNU Lesser General Public
12   License as published by the Free Software Foundation; either
13   version 2.1 of the License, or (at your option) any later version.
14
15   The GNU C Library is distributed in the hope that it will be useful,
16   but WITHOUT ANY WARRANTY; without even the implied warranty of
17   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
18   Lesser General Public License for more details.
19
20   You should have received a copy of the GNU Lesser General Public
21   License along with the GNU C Library; if not, write to the Free
22   Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
23   02111-1307 USA.  */
24
25#include <stdlib.h>
26
27#ifdef HAVE_CONFIG_H
28# include <config.h>
29#endif
30
31#undef __ptr_t
32#if defined __cplusplus || (defined __STDC__ && __STDC__)
33# define __ptr_t void *
34#else /* Not C++ or ANSI C.  */
35# define __ptr_t char *
36#endif /* C++ or ANSI C.  */
37
38#if defined _LIBC
39# include <string.h>
40# include <memcopy.h>
41#else
42# define reg_char char
43#endif
44
45#if defined HAVE_LIMITS_H || defined _LIBC
46# include <limits.h>
47#endif
48
49#define LONG_MAX_32_BITS 2147483647
50
51#ifndef LONG_MAX
52# define LONG_MAX LONG_MAX_32_BITS
53#endif
54
55#include <sys/types.h>
56
57#undef __memrchr
58#undef memrchr
59
60#ifndef weak_alias
61# define __memrchr memrchr
62#endif
63
64/* Search no more than N bytes of S for C.  */
65__ptr_t
66__memrchr (s, c_in, n)
67     const __ptr_t s;
68     int c_in;
69     size_t n;
70{
71  const unsigned char *char_ptr;
72  const unsigned long int *longword_ptr;
73  unsigned long int longword, magic_bits, charmask;
74  unsigned reg_char c;
75
76  c = (unsigned char) c_in;
77
78  /* Handle the last few characters by reading one character at a time.
79     Do this until CHAR_PTR is aligned on a longword boundary.  */
80  for (char_ptr = (const unsigned char *) s + n;
81       n > 0 && ((unsigned long int) char_ptr
82		 & (sizeof (longword) - 1)) != 0;
83       --n)
84    if (*--char_ptr == c)
85      return (__ptr_t) char_ptr;
86
87  /* All these elucidatory comments refer to 4-byte longwords,
88     but the theory applies equally well to 8-byte longwords.  */
89
90  longword_ptr = (const unsigned long int *) char_ptr;
91
92  /* Bits 31, 24, 16, and 8 of this number are zero.  Call these bits
93     the "holes."  Note that there is a hole just to the left of
94     each byte, with an extra at the end:
95
96     bits:  01111110 11111110 11111110 11111111
97     bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD
98
99     The 1-bits make sure that carries propagate to the next 0-bit.
100     The 0-bits provide holes for carries to fall into.  */
101
102  if (sizeof (longword) != 4 && sizeof (longword) != 8)
103    abort ();
104
105#if LONG_MAX <= LONG_MAX_32_BITS
106  magic_bits = 0x7efefeff;
107#else
108  magic_bits = ((unsigned long int) 0x7efefefe << 32) | 0xfefefeff;
109#endif
110
111  /* Set up a longword, each of whose bytes is C.  */
112  charmask = c | (c << 8);
113  charmask |= charmask << 16;
114#if LONG_MAX > LONG_MAX_32_BITS
115  charmask |= charmask << 32;
116#endif
117
118  /* Instead of the traditional loop which tests each character,
119     we will test a longword at a time.  The tricky part is testing
120     if *any of the four* bytes in the longword in question are zero.  */
121  while (n >= sizeof (longword))
122    {
123      /* We tentatively exit the loop if adding MAGIC_BITS to
124	 LONGWORD fails to change any of the hole bits of LONGWORD.
125
126	 1) Is this safe?  Will it catch all the zero bytes?
127	 Suppose there is a byte with all zeros.  Any carry bits
128	 propagating from its left will fall into the hole at its
129	 least significant bit and stop.  Since there will be no
130	 carry from its most significant bit, the LSB of the
131	 byte to the left will be unchanged, and the zero will be
132	 detected.
133
134	 2) Is this worthwhile?  Will it ignore everything except
135	 zero bytes?  Suppose every byte of LONGWORD has a bit set
136	 somewhere.  There will be a carry into bit 8.  If bit 8
137	 is set, this will carry into bit 16.  If bit 8 is clear,
138	 one of bits 9-15 must be set, so there will be a carry
139	 into bit 16.  Similarly, there will be a carry into bit
140	 24.  If one of bits 24-30 is set, there will be a carry
141	 into bit 31, so all of the hole bits will be changed.
142
143	 The one misfire occurs when bits 24-30 are clear and bit
144	 31 is set; in this case, the hole at bit 31 is not
145	 changed.  If we had access to the processor carry flag,
146	 we could close this loophole by putting the fourth hole
147	 at bit 32!
148
149	 So it ignores everything except 128's, when they're aligned
150	 properly.
151
152	 3) But wait!  Aren't we looking for C, not zero?
153	 Good point.  So what we do is XOR LONGWORD with a longword,
154	 each of whose bytes is C.  This turns each byte that is C
155	 into a zero.  */
156
157      longword = *--longword_ptr ^ charmask;
158
159      /* Add MAGIC_BITS to LONGWORD.  */
160      if ((((longword + magic_bits)
161
162	    /* Set those bits that were unchanged by the addition.  */
163	    ^ ~longword)
164
165	   /* Look at only the hole bits.  If any of the hole bits
166	      are unchanged, most likely one of the bytes was a
167	      zero.  */
168	   & ~magic_bits) != 0)
169	{
170	  /* Which of the bytes was C?  If none of them were, it was
171	     a misfire; continue the search.  */
172
173	  const unsigned char *cp = (const unsigned char *) longword_ptr;
174
175#if LONG_MAX > 2147483647
176	  if (cp[7] == c)
177	    return (__ptr_t) &cp[7];
178	  if (cp[6] == c)
179	    return (__ptr_t) &cp[6];
180	  if (cp[5] == c)
181	    return (__ptr_t) &cp[5];
182	  if (cp[4] == c)
183	    return (__ptr_t) &cp[4];
184#endif
185	  if (cp[3] == c)
186	    return (__ptr_t) &cp[3];
187	  if (cp[2] == c)
188	    return (__ptr_t) &cp[2];
189	  if (cp[1] == c)
190	    return (__ptr_t) &cp[1];
191	  if (cp[0] == c)
192	    return (__ptr_t) cp;
193	}
194
195      n -= sizeof (longword);
196    }
197
198  char_ptr = (const unsigned char *) longword_ptr;
199
200  while (n-- > 0)
201    {
202      if (*--char_ptr == c)
203	return (__ptr_t) char_ptr;
204    }
205
206  return 0;
207}
208#ifdef weak_alias
209weak_alias (__memrchr, memrchr)
210#endif
211