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).
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.
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.
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
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. */
42 # define reg_char char
45 #if defined HAVE_LIMITS_H || defined _LIBC
49 #define LONG_MAX_32_BITS 2147483647
52 # define LONG_MAX LONG_MAX_32_BITS
55 #include <sys/types.h>
61 # define __memrchr memrchr
64 /* Search no more than N bytes of S for C. */
76 const unsigned char *char_ptr
;
77 const unsigned long int *longword_ptr
;
78 unsigned long int longword
, magic_bits
, charmask
;
81 c
= (unsigned char) c_in
;
83 /* Handle the last few characters by reading one character at a time.
84 Do this until CHAR_PTR is aligned on a longword boundary. */
85 for (char_ptr
= (const unsigned char *) s
+ n
;
86 n
> 0 && ((unsigned long int) char_ptr
87 & (sizeof (longword
) - 1)) != 0;
90 return (__ptr_t
) char_ptr
;
92 /* All these elucidatory comments refer to 4-byte longwords,
93 but the theory applies equally well to 8-byte longwords. */
95 longword_ptr
= (const unsigned long int *) char_ptr
;
97 /* Bits 31, 24, 16, and 8 of this number are zero. Call these bits
98 the "holes." Note that there is a hole just to the left of
99 each byte, with an extra at the end:
101 bits: 01111110 11111110 11111110 11111111
102 bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD
104 The 1-bits make sure that carries propagate to the next 0-bit.
105 The 0-bits provide holes for carries to fall into. */
107 if (sizeof (longword
) != 4 && sizeof (longword
) != 8)
110 #if LONG_MAX <= LONG_MAX_32_BITS
111 magic_bits
= 0x7efefeff;
113 magic_bits
= ((unsigned long int) 0x7efefefe << 32) | 0xfefefeff;
116 /* Set up a longword, each of whose bytes is C. */
117 charmask
= c
| (c
<< 8);
118 charmask
|= charmask
<< 16;
119 #if LONG_MAX > LONG_MAX_32_BITS
120 charmask
|= charmask
<< 32;
123 /* Instead of the traditional loop which tests each character,
124 we will test a longword at a time. The tricky part is testing
125 if *any of the four* bytes in the longword in question are zero. */
126 while (n
>= sizeof (longword
))
128 /* We tentatively exit the loop if adding MAGIC_BITS to
129 LONGWORD fails to change any of the hole bits of LONGWORD.
131 1) Is this safe? Will it catch all the zero bytes?
132 Suppose there is a byte with all zeros. Any carry bits
133 propagating from its left will fall into the hole at its
134 least significant bit and stop. Since there will be no
135 carry from its most significant bit, the LSB of the
136 byte to the left will be unchanged, and the zero will be
139 2) Is this worthwhile? Will it ignore everything except
140 zero bytes? Suppose every byte of LONGWORD has a bit set
141 somewhere. There will be a carry into bit 8. If bit 8
142 is set, this will carry into bit 16. If bit 8 is clear,
143 one of bits 9-15 must be set, so there will be a carry
144 into bit 16. Similarly, there will be a carry into bit
145 24. If one of bits 24-30 is set, there will be a carry
146 into bit 31, so all of the hole bits will be changed.
148 The one misfire occurs when bits 24-30 are clear and bit
149 31 is set; in this case, the hole at bit 31 is not
150 changed. If we had access to the processor carry flag,
151 we could close this loophole by putting the fourth hole
154 So it ignores everything except 128's, when they're aligned
157 3) But wait! Aren't we looking for C, not zero?
158 Good point. So what we do is XOR LONGWORD with a longword,
159 each of whose bytes is C. This turns each byte that is C
162 longword
= *--longword_ptr
^ charmask
;
164 /* Add MAGIC_BITS to LONGWORD. */
165 if ((((longword
+ magic_bits
)
167 /* Set those bits that were unchanged by the addition. */
170 /* Look at only the hole bits. If any of the hole bits
171 are unchanged, most likely one of the bytes was a
175 /* Which of the bytes was C? If none of them were, it was
176 a misfire; continue the search. */
178 const unsigned char *cp
= (const unsigned char *) longword_ptr
;
180 #if LONG_MAX > 2147483647
182 return (__ptr_t
) &cp
[7];
184 return (__ptr_t
) &cp
[6];
186 return (__ptr_t
) &cp
[5];
188 return (__ptr_t
) &cp
[4];
191 return (__ptr_t
) &cp
[3];
193 return (__ptr_t
) &cp
[2];
195 return (__ptr_t
) &cp
[1];
200 n
-= sizeof (longword
);
203 char_ptr
= (const unsigned char *) longword_ptr
;
207 if (*--char_ptr
== c
)
208 return (__ptr_t
) char_ptr
;
215 weak_alias (__memrchr
, memrchr
)