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. */
23 /* Search no more than N bytes of S for C. */
31 const unsigned char *char_ptr;
32 const unsigned long int *longword_ptr;
33 unsigned long int longword, magic_bits, charmask;
35 c = (unsigned char) c;
37 /* Handle the first few characters by reading one character at a time.
38 Do this until CHAR_PTR is aligned on a longword boundary. */
39 for (char_ptr = s; n > 0 && ((unsigned long int) char_ptr
40 & (sizeof (longword) - 1)) != 0;
43 return (char *) char_ptr;
45 /* All these elucidatory comments refer to 4-byte longwords,
46 but the theory applies equally well to 8-byte longwords. */
48 longword_ptr = (unsigned long int *) char_ptr;
50 /* Bits 31, 24, 16, and 8 of this number are zero. Call these bits
51 the "holes." Note that there is a hole just to the left of
52 each byte, with an extra at the end:
54 bits: 01111110 11111110 11111110 11111111
55 bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD
57 The 1-bits make sure that carries propagate to the next 0-bit.
58 The 0-bits provide holes for carries to fall into. */
59 magic_bits = 0x7efefeff;
60 if (sizeof (longword) > 4)
61 /* 64-bit version of the magic. */
62 magic_bits = (0x7efefefe << 32) | 0xfefefeff;
64 /* Set up a longword, each of whose bytes is C. */
65 charmask = c | (c << 8);
66 charmask |= charmask << 16;
67 if (sizeof (longword) > 4)
68 charmask |= charmask << 32;
70 /* Instead of the traditional loop which tests each character,
71 we will test a longword at a time. The tricky part is testing
72 if *any of the four* bytes in the longword in question are zero. */
73 while (n >= sizeof (longword))
75 /* We tentatively exit the loop if adding MAGIC_BITS to
76 LONGWORD fails to change any of the hole bits of LONGWORD.
78 1) Is this safe? Will it catch all the zero bytes?
79 Suppose there is a byte with all zeros. Any carry bits
80 propagating from its left will fall into the hole at its
81 least significant bit and stop. Since there will be no
82 carry from its most significant bit, the LSB of the
83 byte to the left will be unchanged, and the zero will be
86 2) Is this worthwhile? Will it ignore everything except
87 zero bytes? Suppose every byte of LONGWORD has a bit set
88 somewhere. There will be a carry into bit 8. If bit 8
89 is set, this will carry into bit 16. If bit 8 is clear,
90 one of bits 9-15 must be set, so there will be a carry
91 into bit 16. Similarly, there will be a carry into bit
92 24. If one of bits 24-30 is set, there will be a carry
93 into bit 31, so all of the hole bits will be changed.
95 The one misfire occurs when bits 24-30 are clear and bit
96 31 is set; in this case, the hole at bit 31 is not
97 changed. If we had access to the processor carry flag,
98 we could close this loophole by putting the fourth hole
101 So it ignores everything except 128's, when they're aligned
104 3) But wait! Aren't we looking for C, not zero?
105 Good point. So what we do is XOR LONGWORD with a longword,
106 each of whose bytes is C. This turns each byte that is C
109 longword = *longword_ptr++ ^ charmask;
111 /* Add MAGIC_BITS to LONGWORD. */
112 if ((((longword + magic_bits)
114 /* Set those bits that were unchanged by the addition. */
117 /* Look at only the hole bits. If any of the hole bits
118 are unchanged, most likely one of the bytes was a
122 /* Which of the bytes was C? If none of them were, it was
123 a misfire; continue the search. */
125 const unsigned char *cp = (const unsigned char *) (longword_ptr - 1);
130 return (char *) &cp[1];
132 return (char *) &cp[2];
134 return (char *) &cp[3];
135 if (sizeof (longword) > 4)
138 return (char *) &cp[4];
140 return (char *) &cp[5];
142 return (char *) &cp[6];
144 return (char *) &cp[7];
148 n -= sizeof (longword);
151 char_ptr = (const unsigned char *) longword_ptr;
156 return (char *) char_ptr;