1 /* Copyright (C) 1991, 1993 Free Software Foundation, Inc.
2 Based on strlen implemention by Torbjorn Granlund (tege@sics.se),
3 with help from Dan Sahlin (dan@sics.se) and
4 commentary by Jim Blandy (jimb@ai.mit.edu);
5 adaptation to memchr suggested by Dick Karpinski (dick@cca.ucsf.edu),
6 and implemented by Roland McGrath (roland@ai.mit.edu).
8 The GNU C Library is free software; you can redistribute it and/or
9 modify it under the terms of the GNU Library General Public License as
10 published by the Free Software Foundation; either version 2 of the
11 License, or (at your option) any later version.
13 The GNU C Library is distributed in the hope that it will be useful,
14 but WITHOUT ANY WARRANTY; without even the implied warranty of
15 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
16 Library General Public License for more details.
18 You should have received a copy of the GNU Library General Public
19 License along with the GNU C Library; see the file COPYING.LIB. If
20 not, write to the Free Software Foundation, Inc., 675 Mass Ave,
21 Cambridge, MA 02139, USA. */
24 #if defined (CONFIG_BROKETS)
25 /* We use <config.h> instead of "config.h" so that a compilation
26 using -I. -I$srcdir will use ./config.h rather than $srcdir/config.h
27 (which it would do because it found this file in $srcdir). */
34 #if (SIZEOF_LONG != 4 && SIZEOF_LONG != 8)
35 error This function works only on systems for which sizeof(long) is 4 or 8.
36 /* The previous line would begin with `#error,' but some compilers can't
37 handle that even when the condition is false. */
40 /* Search no more than N bytes of S for C. */
48 const unsigned char *char_ptr;
49 const unsigned long int *longword_ptr;
50 unsigned long int longword, magic_bits, charmask;
52 c = (unsigned char) c;
54 /* Handle the first few characters by reading one character at a time.
55 Do this until CHAR_PTR is aligned on a longword boundary. */
56 for (char_ptr = s; n > 0 && ((unsigned long int) char_ptr
57 & (sizeof (longword) - 1)) != 0;
60 return (char *) char_ptr;
62 /* All these elucidatory comments refer to 4-byte longwords,
63 but the theory applies equally well to 8-byte longwords. */
65 longword_ptr = (unsigned long int *) char_ptr;
67 /* Bits 31, 24, 16, and 8 of this number are zero. Call these bits
68 the "holes." Note that there is a hole just to the left of
69 each byte, with an extra at the end:
71 bits: 01111110 11111110 11111110 11111111
72 bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD
74 The 1-bits make sure that carries propagate to the next 0-bit.
75 The 0-bits provide holes for carries to fall into. */
76 #if (SIZEOF_LONG == 8)
77 magic_bits = ((unsigned long int) 0x7efefefe << 32) | 0xfefefeff;
79 magic_bits = 0x7efefeff;
80 #endif /* SIZEOF_LONG == 8 */
82 /* Set up a longword, each of whose bytes is C. */
83 charmask = c | (c << 8);
84 charmask |= charmask << 16;
85 #if (SIZEOF_LONG == 8)
86 charmask |= charmask << 32;
87 #endif /* SIZEOF_LONG == 8 */
88 if (sizeof (longword) > 8)
92 /* Instead of the traditional loop which tests each character,
93 we will test a longword at a time. The tricky part is testing
94 if *any of the four* bytes in the longword in question are zero. */
95 while (n >= sizeof (longword))
97 /* We tentatively exit the loop if adding MAGIC_BITS to
98 LONGWORD fails to change any of the hole bits of LONGWORD.
100 1) Is this safe? Will it catch all the zero bytes?
101 Suppose there is a byte with all zeros. Any carry bits
102 propagating from its left will fall into the hole at its
103 least significant bit and stop. Since there will be no
104 carry from its most significant bit, the LSB of the
105 byte to the left will be unchanged, and the zero will be
108 2) Is this worthwhile? Will it ignore everything except
109 zero bytes? Suppose every byte of LONGWORD has a bit set
110 somewhere. There will be a carry into bit 8. If bit 8
111 is set, this will carry into bit 16. If bit 8 is clear,
112 one of bits 9-15 must be set, so there will be a carry
113 into bit 16. Similarly, there will be a carry into bit
114 24. If one of bits 24-30 is set, there will be a carry
115 into bit 31, so all of the hole bits will be changed.
117 The one misfire occurs when bits 24-30 are clear and bit
118 31 is set; in this case, the hole at bit 31 is not
119 changed. If we had access to the processor carry flag,
120 we could close this loophole by putting the fourth hole
123 So it ignores everything except 128's, when they're aligned
126 3) But wait! Aren't we looking for C, not zero?
127 Good point. So what we do is XOR LONGWORD with a longword,
128 each of whose bytes is C. This turns each byte that is C
131 longword = *longword_ptr++ ^ charmask;
133 /* Add MAGIC_BITS to LONGWORD. */
134 if ((((longword + magic_bits)
136 /* Set those bits that were unchanged by the addition. */
139 /* Look at only the hole bits. If any of the hole bits
140 are unchanged, most likely one of the bytes was a
144 /* Which of the bytes was C? If none of them were, it was
145 a misfire; continue the search. */
147 const unsigned char *cp = (const unsigned char *) (longword_ptr - 1);
152 return (char *) &cp[1];
154 return (char *) &cp[2];
156 return (char *) &cp[3];
157 #if (SIZEOF_LONG == 8)
159 return (char *) &cp[4];
161 return (char *) &cp[5];
163 return (char *) &cp[6];
165 return (char *) &cp[7];
166 #endif /* SIZEOF_LONG == 8 */
169 n -= sizeof (longword);
172 char_ptr = (const unsigned char *) longword_ptr;
177 return (char *) char_ptr;