1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
|
/* strchr (str, ch) -- Return pointer to first occurrence of CH in STR.
For Motorola 68000.
Copyright (C) 1999, 2003 Free Software Foundation, Inc.
This file is part of the GNU C Library.
Contributed by Andreas Schwab <schwab@gnu.org>.
The GNU C Library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.
The GNU C Library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with the GNU C Library; if not, write to the Free
Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
02111-1307 USA. */
#include <sysdep.h>
#include "asm-syntax.h"
TEXT
ENTRY(strchr)
/* Save the callee-saved registers we use. */
movel R(d2),MEM_PREDEC(sp)
movel R(d3),MEM_PREDEC(sp)
/* Get string pointer and character. */
movel MEM_DISP(sp,12),R(a0)
moveb MEM_DISP(sp,19),R(d0)
/* Distribute the character to all bytes of a longword. */
movel R(d0),R(d1)
lsll #8,R(d1)
moveb R(d0),R(d1)
movel R(d1),R(d0)
swap R(d0)
movew R(d1),R(d0)
/* First search for the character one byte at a time until the
pointer is aligned to a longword boundary. */
movel R(a0),R(d1)
andw #3,R(d1)
beq L(L1)
moveb MEM(a0),R(d1)
cmpb R(d0),R(d1)
beq L(L9)
tstb R(d1)
beq L(L3)
addql #1,R(a0)
movel R(a0),R(d1)
andw #3,R(d1)
beq L(L1)
moveb MEM(a0),R(d1)
cmpb R(d0),R(d1)
beq L(L9)
tstb R(d1)
beq L(L3)
addql #1,R(a0)
movel R(a0),R(d1)
andw #3,R(d1)
beq L(L1)
moveb MEM(a0),R(d1)
cmpb R(d0),R(d1)
beq L(L9)
tstb R(d1)
beq L(L3)
addql #1,R(a0)
L(L1:)
/* Load the magic bits. Unlike the generic implementation we can
use the carry bit as the fourth hole. */
movel #0xfefefeff,R(d3)
/* We exit the loop if adding MAGIC_BITS to LONGWORD fails to
change any of the hole bits of LONGWORD.
1) Is this safe? Will it catch all the zero bytes?
Suppose there is a byte with all zeros. Any carry bits
propagating from its left will fall into the hole at its
least significant bit and stop. Since there will be no
carry from its most significant bit, the LSB of the
byte to the left will be unchanged, and the zero will be
detected.
2) Is this worthwhile? Will it ignore everything except
zero bytes? Suppose every byte of LONGWORD has a bit set
somewhere. There will be a carry into bit 8. If bit 8
is set, this will carry into bit 16. If bit 8 is clear,
one of bits 9-15 must be set, so there will be a carry
into bit 16. Similarly, there will be a carry into bit
24. If one of bits 24-31 is set, there will be a carry
into bit 32 (=carry flag), so all of the hole bits will
be changed.
3) But wait! Aren't we looking for C, not zero?
Good point. So what we do is XOR LONGWORD with a longword,
each of whose bytes is C. This turns each byte that is C
into a zero. */
L(L2:)
/* Get the longword in question. */
movel MEM_POSTINC(a0),R(d1)
/* XOR with the byte we search for. */
eorl R(d0),R(d1)
/* Add the magic value. We get carry bits reported for each byte
which is not C. */
movel R(d3),R(d2)
addl R(d1),R(d2)
/* Check the fourth carry bit before it is clobbered by the next
XOR. If it is not set we have a hit. */
bcc L(L8)
/* We are only interested in carry bits that change due to the
previous add, so remove original bits. */
eorl R(d1),R(d2)
/* Now test for the other three overflow bits.
Set all non-carry bits. */
orl R(d3),R(d2)
/* Add 1 to get zero if all carry bits were set. */
addql #1,R(d2)
/* If we don't get zero then at least one byte of the word equals
C. */
bne L(L8)
/* Next look for a NUL byte.
Restore original longword without reload. */
eorl R(d0),R(d1)
/* Add the magic value. We get carry bits reported for each byte
which is not NUL. */
movel R(d3),R(d2)
addl R(d1),R(d2)
/* Check the fourth carry bit before it is clobbered by the next
XOR. If it is not set we have a hit, and return NULL. */
bcc L(L3)
/* We are only interested in carry bits that change due to the
previous add, so remove original bits. */
eorl R(d1),R(d2)
/* Now test for the other three overflow bits.
Set all non-carry bits. */
orl R(d3),R(d2)
/* Add 1 to get zero if all carry bits were set. */
addql #1,R(d2)
/* If we don't get zero then at least one byte of the word was NUL
and we return NULL. Otherwise continue with the next longword. */
bne L(L3)
/* Get the longword in question. */
movel MEM_POSTINC(a0),R(d1)
/* XOR with the byte we search for. */
eorl R(d0),R(d1)
/* Add the magic value. We get carry bits reported for each byte
which is not C. */
movel R(d3),R(d2)
addl R(d1),R(d2)
/* Check the fourth carry bit before it is clobbered by the next
XOR. If it is not set we have a hit. */
bcc L(L8)
/* We are only interested in carry bits that change due to the
previous add, so remove original bits */
eorl R(d1),R(d2)
/* Now test for the other three overflow bits.
Set all non-carry bits. */
orl R(d3),R(d2)
/* Add 1 to get zero if all carry bits were set. */
addql #1,R(d2)
/* If we don't get zero then at least one byte of the word equals
C. */
bne L(L8)
/* Next look for a NUL byte.
Restore original longword without reload. */
eorl R(d0),R(d1)
/* Add the magic value. We get carry bits reported for each byte
which is not NUL. */
movel R(d3),R(d2)
addl R(d1),R(d2)
/* Check the fourth carry bit before it is clobbered by the next
XOR. If it is not set we have a hit, and return NULL. */
bcc L(L3)
/* We are only interested in carry bits that change due to the
previous add, so remove original bits */
eorl R(d1),R(d2)
/* Now test for the other three overflow bits.
Set all non-carry bits. */
orl R(d3),R(d2)
/* Add 1 to get zero if all carry bits were set. */
addql #1,R(d2)
/* If we don't get zero then at least one byte of the word was NUL
and we return NULL. Otherwise continue with the next longword. */
beq L(L2)
L(L3:)
/* Return NULL. */
clrl R(d0)
movel R(d0),R(a0)
movel MEM_POSTINC(sp),R(d3)
movel MEM_POSTINC(sp),R(d2)
rts
L(L8:)
/* We have a hit. Check to see which byte it was. First
compensate for the autoincrement in the loop. */
subql #4,R(a0)
moveb MEM(a0),R(d1)
cmpb R(d0),R(d1)
beq L(L9)
tstb R(d1)
beq L(L3)
addql #1,R(a0)
moveb MEM(a0),R(d1)
cmpb R(d0),R(d1)
beq L(L9)
tstb R(d1)
beq L(L3)
addql #1,R(a0)
moveb MEM(a0),R(d1)
cmpb R(d0),R(d1)
beq L(L9)
tstb R(d1)
beq L(L3)
addql #1,R(a0)
/* Otherwise the fourth byte must equal C. */
L(L9:)
movel R(a0),R(d0)
movel MEM_POSTINC(sp),R(d3)
movel MEM_POSTINC(sp),R(d2)
rts
END(strchr)
weak_alias (strchr, index)
libc_hidden_builtin_def (strchr)
|
