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3f02f778 GM |
1 | /* memchr (str, chr, len) -- Return pointer to first occurrence of CHR in STR less |
2 | than LEN. | |
6d52618b | 3 | For Intel 80x86, x>=3. |
85dd1003 | 4 | Copyright (C) 1994-1998, 2000, 2003 Free Software Foundation, Inc. |
6d52618b UD |
5 | This file is part of the GNU C Library. |
6 | Contributed by Ulrich Drepper <drepper@gnu.ai.mit.edu> | |
7 | Optimised a little by Alan Modra <Alan@SPRI.Levels.UniSA.Edu.Au> | |
6d52618b UD |
8 | This version is developed using the same algorithm as the fast C |
9 | version which carries the following introduction: | |
6d52618b UD |
10 | Based on strlen implementation by Torbjorn Granlund (tege@sics.se), |
11 | with help from Dan Sahlin (dan@sics.se) and | |
12 | commentary by Jim Blandy (jimb@ai.mit.edu); | |
13 | adaptation to memchr suggested by Dick Karpinski (dick@cca.ucsf.edu), | |
14 | and implemented by Roland McGrath (roland@ai.mit.edu). | |
15 | ||
16 | The GNU C Library is free software; you can redistribute it and/or | |
41bdb6e2 AJ |
17 | modify it under the terms of the GNU Lesser General Public |
18 | License as published by the Free Software Foundation; either | |
19 | version 2.1 of the License, or (at your option) any later version. | |
6d52618b UD |
20 | |
21 | The GNU C Library is distributed in the hope that it will be useful, | |
22 | but WITHOUT ANY WARRANTY; without even the implied warranty of | |
23 | MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU | |
41bdb6e2 | 24 | Lesser General Public License for more details. |
6d52618b | 25 | |
41bdb6e2 AJ |
26 | You should have received a copy of the GNU Lesser General Public |
27 | License along with the GNU C Library; if not, write to the Free | |
28 | Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA | |
29 | 02111-1307 USA. */ | |
8f5ca04b RM |
30 | |
31 | #include <sysdep.h> | |
32 | #include "asm-syntax.h" | |
2fc08826 | 33 | #include "bp-sym.h" |
3f02f778 | 34 | #include "bp-asm.h" |
8f5ca04b | 35 | |
3f02f778 GM |
36 | #define PARMS LINKAGE+8 /* space for 2 saved regs */ |
37 | #define RTN PARMS | |
38 | #define STR RTN+RTN_SIZE | |
39 | #define CHR STR+PTR_SIZE | |
40 | #define LEN CHR+4 | |
8f5ca04b RM |
41 | |
42 | .text | |
abf70633 | 43 | ENTRY (BP_SYM (__memchr)) |
3f02f778 GM |
44 | ENTER |
45 | ||
8f5ca04b RM |
46 | /* Save callee-safe registers used in this function. */ |
47 | pushl %esi | |
48 | pushl %edi | |
49 | ||
50 | /* Load parameters into registers. */ | |
3f02f778 GM |
51 | movl STR(%esp), %eax /* str: pointer to memory block. */ |
52 | movl CHR(%esp), %edx /* c: byte we are looking for. */ | |
53 | movl LEN(%esp), %esi /* len: length of memory block. */ | |
53c06508 | 54 | CHECK_BOUNDS_LOW (%eax, STR(%esp)) |
8f5ca04b RM |
55 | |
56 | /* If my must not test more than three characters test | |
57 | them one by one. This is especially true for 0. */ | |
58 | cmpl $4, %esi | |
5929563f | 59 | jb L(3) |
8f5ca04b | 60 | |
3f02f778 GM |
61 | /* At the moment %edx contains CHR. What we need for the |
62 | algorithm is CHR in all bytes of the dword. Avoid | |
8f5ca04b RM |
63 | operations on 16 bit words because these require an |
64 | prefix byte (and one more cycle). */ | |
65 | movb %dl, %dh /* Now it is 0|0|c|c */ | |
66 | movl %edx, %ecx | |
67 | shll $16, %edx /* Now c|c|0|0 */ | |
68 | movw %cx, %dx /* And finally c|c|c|c */ | |
69 | ||
70 | /* Better performance can be achieved if the word (32 | |
71 | bit) memory access is aligned on a four-byte-boundary. | |
72 | So process first bytes one by one until boundary is | |
73 | reached. Don't use a loop for better performance. */ | |
74 | ||
50304ef0 | 75 | testb $3, %al /* correctly aligned ? */ |
5929563f | 76 | je L(2) /* yes => begin loop */ |
8f5ca04b | 77 | cmpb %dl, (%eax) /* compare byte */ |
5929563f | 78 | je L(9) /* target found => return */ |
8f5ca04b RM |
79 | incl %eax /* increment source pointer */ |
80 | decl %esi /* decrement length counter */ | |
5929563f | 81 | je L(4) /* len==0 => return NULL */ |
8f5ca04b | 82 | |
50304ef0 | 83 | testb $3, %al /* correctly aligned ? */ |
5929563f | 84 | je L(2) /* yes => begin loop */ |
8f5ca04b | 85 | cmpb %dl, (%eax) /* compare byte */ |
5929563f | 86 | je L(9) /* target found => return */ |
8f5ca04b RM |
87 | incl %eax /* increment source pointer */ |
88 | decl %esi /* decrement length counter */ | |
5929563f | 89 | je L(4) /* len==0 => return NULL */ |
8f5ca04b | 90 | |
50304ef0 | 91 | testb $3, %al /* correctly aligned ? */ |
5929563f | 92 | je L(2) /* yes => begin loop */ |
8f5ca04b | 93 | cmpb %dl, (%eax) /* compare byte */ |
5929563f | 94 | je L(9) /* target found => return */ |
8f5ca04b RM |
95 | incl %eax /* increment source pointer */ |
96 | decl %esi /* decrement length counter */ | |
97 | /* no test for len==0 here, because this is done in the | |
98 | loop head */ | |
5929563f | 99 | jmp L(2) |
8f5ca04b RM |
100 | |
101 | /* We exit the loop if adding MAGIC_BITS to LONGWORD fails to | |
102 | change any of the hole bits of LONGWORD. | |
103 | ||
104 | 1) Is this safe? Will it catch all the zero bytes? | |
105 | Suppose there is a byte with all zeros. Any carry bits | |
106 | propagating from its left will fall into the hole at its | |
107 | least significant bit and stop. Since there will be no | |
108 | carry from its most significant bit, the LSB of the | |
109 | byte to the left will be unchanged, and the zero will be | |
110 | detected. | |
111 | ||
112 | 2) Is this worthwhile? Will it ignore everything except | |
113 | zero bytes? Suppose every byte of LONGWORD has a bit set | |
114 | somewhere. There will be a carry into bit 8. If bit 8 | |
115 | is set, this will carry into bit 16. If bit 8 is clear, | |
116 | one of bits 9-15 must be set, so there will be a carry | |
117 | into bit 16. Similarly, there will be a carry into bit | |
118 | 24. If one of bits 24-31 is set, there will be a carry | |
119 | into bit 32 (=carry flag), so all of the hole bits will | |
120 | be changed. | |
121 | ||
3f02f778 | 122 | 3) But wait! Aren't we looking for CHR, not zero? |
8f5ca04b | 123 | Good point. So what we do is XOR LONGWORD with a longword, |
3f02f778 | 124 | each of whose bytes is CHR. This turns each byte that is CHR |
8f5ca04b RM |
125 | into a zero. */ |
126 | ||
127 | ||
128 | /* Each round the main loop processes 16 bytes. */ | |
129 | ||
130 | ALIGN (4) | |
131 | ||
5929563f | 132 | L(1): movl (%eax), %ecx /* get word (= 4 bytes) in question */ |
8f5ca04b RM |
133 | movl $0xfefefeff, %edi /* magic value */ |
134 | xorl %edx, %ecx /* XOR with word c|c|c|c => bytes of str == c | |
135 | are now 0 */ | |
136 | addl %ecx, %edi /* add the magic value to the word. We get | |
137 | carry bits reported for each byte which | |
138 | is *not* 0 */ | |
139 | ||
140 | /* According to the algorithm we had to reverse the effect of the | |
141 | XOR first and then test the overflow bits. But because the | |
142 | following XOR would destroy the carry flag and it would (in a | |
143 | representation with more than 32 bits) not alter then last | |
144 | overflow, we can now test this condition. If no carry is signaled | |
6d52618b | 145 | no overflow must have occurred in the last byte => it was 0. */ |
5929563f | 146 | jnc L(8) |
8f5ca04b RM |
147 | |
148 | /* We are only interested in carry bits that change due to the | |
149 | previous add, so remove original bits */ | |
150 | xorl %ecx, %edi /* ((word^charmask)+magic)^(word^charmask) */ | |
151 | ||
152 | /* Now test for the other three overflow bits. */ | |
153 | orl $0xfefefeff, %edi /* set all non-carry bits */ | |
154 | incl %edi /* add 1: if one carry bit was *not* set | |
155 | the addition will not result in 0. */ | |
156 | ||
3f02f778 | 157 | /* If at least one byte of the word is CHR we don't get 0 in %edi. */ |
5929563f | 158 | jnz L(8) /* found it => return pointer */ |
8f5ca04b RM |
159 | |
160 | /* This process is unfolded four times for better performance. | |
161 | we don't increment the source pointer each time. Instead we | |
162 | use offsets and increment by 16 in each run of the loop. But | |
163 | before probing for the matching byte we need some extra code | |
164 | (following LL(13) below). Even the len can be compared with | |
165 | constants instead of decrementing each time. */ | |
166 | ||
167 | movl 4(%eax), %ecx /* get word (= 4 bytes) in question */ | |
168 | movl $0xfefefeff, %edi /* magic value */ | |
169 | xorl %edx, %ecx /* XOR with word c|c|c|c => bytes of str == c | |
170 | are now 0 */ | |
171 | addl %ecx, %edi /* add the magic value to the word. We get | |
172 | carry bits reported for each byte which | |
173 | is *not* 0 */ | |
3f02f778 | 174 | jnc L(7) /* highest byte is CHR => return pointer */ |
8f5ca04b RM |
175 | xorl %ecx, %edi /* ((word^charmask)+magic)^(word^charmask) */ |
176 | orl $0xfefefeff, %edi /* set all non-carry bits */ | |
177 | incl %edi /* add 1: if one carry bit was *not* set | |
178 | the addition will not result in 0. */ | |
5929563f | 179 | jnz L(7) /* found it => return pointer */ |
8f5ca04b RM |
180 | |
181 | movl 8(%eax), %ecx /* get word (= 4 bytes) in question */ | |
182 | movl $0xfefefeff, %edi /* magic value */ | |
183 | xorl %edx, %ecx /* XOR with word c|c|c|c => bytes of str == c | |
184 | are now 0 */ | |
185 | addl %ecx, %edi /* add the magic value to the word. We get | |
186 | carry bits reported for each byte which | |
187 | is *not* 0 */ | |
3f02f778 | 188 | jnc L(6) /* highest byte is CHR => return pointer */ |
8f5ca04b RM |
189 | xorl %ecx, %edi /* ((word^charmask)+magic)^(word^charmask) */ |
190 | orl $0xfefefeff, %edi /* set all non-carry bits */ | |
191 | incl %edi /* add 1: if one carry bit was *not* set | |
192 | the addition will not result in 0. */ | |
5929563f | 193 | jnz L(6) /* found it => return pointer */ |
8f5ca04b RM |
194 | |
195 | movl 12(%eax), %ecx /* get word (= 4 bytes) in question */ | |
196 | movl $0xfefefeff, %edi /* magic value */ | |
197 | xorl %edx, %ecx /* XOR with word c|c|c|c => bytes of str == c | |
198 | are now 0 */ | |
199 | addl %ecx, %edi /* add the magic value to the word. We get | |
200 | carry bits reported for each byte which | |
201 | is *not* 0 */ | |
3f02f778 | 202 | jnc L(5) /* highest byte is CHR => return pointer */ |
8f5ca04b RM |
203 | xorl %ecx, %edi /* ((word^charmask)+magic)^(word^charmask) */ |
204 | orl $0xfefefeff, %edi /* set all non-carry bits */ | |
205 | incl %edi /* add 1: if one carry bit was *not* set | |
206 | the addition will not result in 0. */ | |
5929563f | 207 | jnz L(5) /* found it => return pointer */ |
8f5ca04b RM |
208 | |
209 | /* Adjust both counters for a full round, i.e. 16 bytes. */ | |
210 | addl $16, %eax | |
5929563f UD |
211 | L(2): subl $16, %esi |
212 | jae L(1) /* Still more than 16 bytes remaining */ | |
8f5ca04b RM |
213 | |
214 | /* Process remaining bytes separately. */ | |
215 | cmpl $4-16, %esi /* rest < 4 bytes? */ | |
5929563f | 216 | jb L(3) /* yes, than test byte by byte */ |
8f5ca04b RM |
217 | |
218 | movl (%eax), %ecx /* get word (= 4 bytes) in question */ | |
219 | movl $0xfefefeff, %edi /* magic value */ | |
220 | xorl %edx, %ecx /* XOR with word c|c|c|c => bytes of str == c | |
221 | are now 0 */ | |
222 | addl %ecx, %edi /* add the magic value to the word. We get | |
223 | carry bits reported for each byte which | |
224 | is *not* 0 */ | |
3f02f778 | 225 | jnc L(8) /* highest byte is CHR => return pointer */ |
8f5ca04b RM |
226 | xorl %ecx, %edi /* ((word^charmask)+magic)^(word^charmask) */ |
227 | orl $0xfefefeff, %edi /* set all non-carry bits */ | |
228 | incl %edi /* add 1: if one carry bit was *not* set | |
229 | the addition will not result in 0. */ | |
5929563f | 230 | jne L(8) /* found it => return pointer */ |
8f5ca04b RM |
231 | addl $4, %eax /* adjust source pointer */ |
232 | ||
233 | cmpl $8-16, %esi /* rest < 8 bytes? */ | |
5929563f | 234 | jb L(3) /* yes, than test byte by byte */ |
8f5ca04b RM |
235 | |
236 | movl (%eax), %ecx /* get word (= 4 bytes) in question */ | |
237 | movl $0xfefefeff, %edi /* magic value */ | |
238 | xorl %edx, %ecx /* XOR with word c|c|c|c => bytes of str == c | |
239 | are now 0 */ | |
240 | addl %ecx, %edi /* add the magic value to the word. We get | |
241 | carry bits reported for each byte which | |
242 | is *not* 0 */ | |
3f02f778 | 243 | jnc L(8) /* highest byte is CHR => return pointer */ |
8f5ca04b RM |
244 | xorl %ecx, %edi /* ((word^charmask)+magic)^(word^charmask) */ |
245 | orl $0xfefefeff, %edi /* set all non-carry bits */ | |
246 | incl %edi /* add 1: if one carry bit was *not* set | |
247 | the addition will not result in 0. */ | |
5929563f | 248 | jne L(8) /* found it => return pointer */ |
8f5ca04b RM |
249 | addl $4, %eax /* adjust source pointer */ |
250 | ||
251 | cmpl $12-16, %esi /* rest < 12 bytes? */ | |
5929563f | 252 | jb L(3) /* yes, than test byte by byte */ |
8f5ca04b RM |
253 | |
254 | movl (%eax), %ecx /* get word (= 4 bytes) in question */ | |
255 | movl $0xfefefeff, %edi /* magic value */ | |
256 | xorl %edx, %ecx /* XOR with word c|c|c|c => bytes of str == c | |
257 | are now 0 */ | |
258 | addl %ecx, %edi /* add the magic value to the word. We get | |
259 | carry bits reported for each byte which | |
260 | is *not* 0 */ | |
3f02f778 | 261 | jnc L(8) /* highest byte is CHR => return pointer */ |
8f5ca04b RM |
262 | xorl %ecx, %edi /* ((word^charmask)+magic)^(word^charmask) */ |
263 | orl $0xfefefeff, %edi /* set all non-carry bits */ | |
264 | incl %edi /* add 1: if one carry bit was *not* set | |
265 | the addition will not result in 0. */ | |
5929563f | 266 | jne L(8) /* found it => return pointer */ |
8f5ca04b RM |
267 | addl $4, %eax /* adjust source pointer */ |
268 | ||
269 | /* Check the remaining bytes one by one. */ | |
5929563f UD |
270 | L(3): andl $3, %esi /* mask out uninteresting bytes */ |
271 | jz L(4) /* no remaining bytes => return NULL */ | |
8f5ca04b | 272 | |
3f02f778 | 273 | cmpb %dl, (%eax) /* compare byte with CHR */ |
5929563f | 274 | je L(9) /* equal, than return pointer */ |
8f5ca04b RM |
275 | incl %eax /* increment source pointer */ |
276 | decl %esi /* decrement length */ | |
5929563f | 277 | jz L(4) /* no remaining bytes => return NULL */ |
8f5ca04b | 278 | |
3f02f778 | 279 | cmpb %dl, (%eax) /* compare byte with CHR */ |
5929563f | 280 | je L(9) /* equal, than return pointer */ |
8f5ca04b RM |
281 | incl %eax /* increment source pointer */ |
282 | decl %esi /* decrement length */ | |
5929563f | 283 | jz L(4) /* no remaining bytes => return NULL */ |
8f5ca04b | 284 | |
3f02f778 | 285 | cmpb %dl, (%eax) /* compare byte with CHR */ |
5929563f | 286 | je L(9) /* equal, than return pointer */ |
8f5ca04b | 287 | |
5929563f | 288 | L(4): /* no byte found => return NULL */ |
8f5ca04b | 289 | xorl %eax, %eax |
5929563f | 290 | jmp L(9) |
8f5ca04b RM |
291 | |
292 | /* add missing source pointer increments */ | |
5929563f UD |
293 | L(5): addl $4, %eax |
294 | L(6): addl $4, %eax | |
295 | L(7): addl $4, %eax | |
8f5ca04b RM |
296 | |
297 | /* Test for the matching byte in the word. %ecx contains a NUL | |
298 | char in the byte which originally was the byte we are looking | |
299 | at. */ | |
5929563f UD |
300 | L(8): testb %cl, %cl /* test first byte in dword */ |
301 | jz L(9) /* if zero => return pointer */ | |
8f5ca04b RM |
302 | incl %eax /* increment source pointer */ |
303 | ||
304 | testb %ch, %ch /* test second byte in dword */ | |
5929563f | 305 | jz L(9) /* if zero => return pointer */ |
8f5ca04b RM |
306 | incl %eax /* increment source pointer */ |
307 | ||
308 | testl $0xff0000, %ecx /* test third byte in dword */ | |
5929563f | 309 | jz L(9) /* if zero => return pointer */ |
8f5ca04b RM |
310 | incl %eax /* increment source pointer */ |
311 | ||
6d52618b | 312 | /* No further test needed we we know it is one of the four bytes. */ |
2fc08826 GM |
313 | L(9): |
314 | #if __BOUNDED_POINTERS__ | |
53c06508 | 315 | CHECK_BOUNDS_HIGH (%eax, STR(%esp), jb) |
2fc08826 GM |
316 | /* If RTN pointer is phony, don't copy return value into it. */ |
317 | movl RTN(%esp), %ecx | |
318 | testl %ecx, %ecx | |
319 | jz L(pop) | |
320 | RETURN_BOUNDED_POINTER (STR(%esp)) | |
321 | #endif | |
322 | L(pop): popl %edi /* pop saved registers */ | |
8f5ca04b RM |
323 | popl %esi |
324 | ||
3f02f778 GM |
325 | LEAVE |
326 | RET_PTR | |
abf70633 GM |
327 | END (BP_SYM (__memchr)) |
328 | ||
329 | weak_alias (BP_SYM (__memchr), BP_SYM (memchr)) | |
2ed5fd9a GM |
330 | #if !__BOUNDED_POINTERS__ |
331 | weak_alias (__memchr, __ubp_memchr) | |
332 | #endif | |
85dd1003 | 333 | libc_hidden_builtin_def (memchr) |