1 /* memchr (str, chr, len) -- Return pointer to first occurrence of CHR in STR
2 less than LEN. For Intel 80x86, x>=3.
3 Copyright (C) 1994-1998, 2000, 2003, 2005 Free Software Foundation, Inc.
4 This file is part of the GNU C Library.
5 Contributed by Ulrich Drepper <drepper@gnu.ai.mit.edu>
6 Optimised a little by Alan Modra <Alan@SPRI.Levels.UniSA.Edu.Au>
7 This version is developed using the same algorithm as the fast C
8 version which carries the following introduction:
9 Based on strlen implementation by Torbjorn Granlund (tege@sics.se),
10 with help from Dan Sahlin (dan@sics.se) and
11 commentary by Jim Blandy (jimb@ai.mit.edu);
12 adaptation to memchr suggested by Dick Karpinski (dick@cca.ucsf.edu),
13 and implemented by Roland McGrath (roland@ai.mit.edu).
15 The GNU C Library is free software; you can redistribute it and/or
16 modify it under the terms of the GNU Lesser General Public
17 License as published by the Free Software Foundation; either
18 version 2.1 of the License, or (at your option) any later version.
20 The GNU C Library is distributed in the hope that it will be useful,
21 but WITHOUT ANY WARRANTY; without even the implied warranty of
22 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
23 Lesser General Public License for more details.
25 You should have received a copy of the GNU Lesser General Public
26 License along with the GNU C Library; if not, see
27 <http://www.gnu.org/licenses/>. */
30 #include "asm-syntax.h"
34 #define PARMS LINKAGE+8 /* space for 2 saved regs */
36 #define STR RTN+RTN_SIZE
37 #define CHR STR+PTR_SIZE
41 ENTRY (BP_SYM (__memchr))
44 /* Save callee-safe registers used in this function. */
46 cfi_adjust_cfa_offset (4)
48 cfi_adjust_cfa_offset (4)
49 cfi_rel_offset (edi, 0)
51 /* Load parameters into registers. */
52 movl STR(%esp), %eax /* str: pointer to memory block. */
53 movl CHR(%esp), %edx /* c: byte we are looking for. */
54 movl LEN(%esp), %esi /* len: length of memory block. */
55 cfi_rel_offset (esi, 4)
56 CHECK_BOUNDS_LOW (%eax, STR(%esp))
58 /* If my must not test more than three characters test
59 them one by one. This is especially true for 0. */
63 /* At the moment %edx contains CHR. What we need for the
64 algorithm is CHR in all bytes of the dword. Avoid
65 operations on 16 bit words because these require an
66 prefix byte (and one more cycle). */
67 movb %dl, %dh /* Now it is 0|0|c|c */
69 shll $16, %edx /* Now c|c|0|0 */
70 movw %cx, %dx /* And finally c|c|c|c */
72 /* Better performance can be achieved if the word (32
73 bit) memory access is aligned on a four-byte-boundary.
74 So process first bytes one by one until boundary is
75 reached. Don't use a loop for better performance. */
77 testb $3, %al /* correctly aligned ? */
78 je L(2) /* yes => begin loop */
79 cmpb %dl, (%eax) /* compare byte */
80 je L(9) /* target found => return */
81 incl %eax /* increment source pointer */
82 decl %esi /* decrement length counter */
83 je L(4) /* len==0 => return NULL */
85 testb $3, %al /* correctly aligned ? */
86 je L(2) /* yes => begin loop */
87 cmpb %dl, (%eax) /* compare byte */
88 je L(9) /* target found => return */
89 incl %eax /* increment source pointer */
90 decl %esi /* decrement length counter */
91 je L(4) /* len==0 => return NULL */
93 testb $3, %al /* correctly aligned ? */
94 je L(2) /* yes => begin loop */
95 cmpb %dl, (%eax) /* compare byte */
96 je L(9) /* target found => return */
97 incl %eax /* increment source pointer */
98 decl %esi /* decrement length counter */
99 /* no test for len==0 here, because this is done in the
103 /* We exit the loop if adding MAGIC_BITS to LONGWORD fails to
104 change any of the hole bits of LONGWORD.
106 1) Is this safe? Will it catch all the zero bytes?
107 Suppose there is a byte with all zeros. Any carry bits
108 propagating from its left will fall into the hole at its
109 least significant bit and stop. Since there will be no
110 carry from its most significant bit, the LSB of the
111 byte to the left will be unchanged, and the zero will be
114 2) Is this worthwhile? Will it ignore everything except
115 zero bytes? Suppose every byte of LONGWORD has a bit set
116 somewhere. There will be a carry into bit 8. If bit 8
117 is set, this will carry into bit 16. If bit 8 is clear,
118 one of bits 9-15 must be set, so there will be a carry
119 into bit 16. Similarly, there will be a carry into bit
120 24. If one of bits 24-31 is set, there will be a carry
121 into bit 32 (=carry flag), so all of the hole bits will
124 3) But wait! Aren't we looking for CHR, not zero?
125 Good point. So what we do is XOR LONGWORD with a longword,
126 each of whose bytes is CHR. This turns each byte that is CHR
130 /* Each round the main loop processes 16 bytes. */
134 L(1): movl (%eax), %ecx /* get word (= 4 bytes) in question */
135 movl $0xfefefeff, %edi /* magic value */
136 xorl %edx, %ecx /* XOR with word c|c|c|c => bytes of str == c
138 addl %ecx, %edi /* add the magic value to the word. We get
139 carry bits reported for each byte which
142 /* According to the algorithm we had to reverse the effect of the
143 XOR first and then test the overflow bits. But because the
144 following XOR would destroy the carry flag and it would (in a
145 representation with more than 32 bits) not alter then last
146 overflow, we can now test this condition. If no carry is signaled
147 no overflow must have occurred in the last byte => it was 0. */
150 /* We are only interested in carry bits that change due to the
151 previous add, so remove original bits */
152 xorl %ecx, %edi /* ((word^charmask)+magic)^(word^charmask) */
154 /* Now test for the other three overflow bits. */
155 orl $0xfefefeff, %edi /* set all non-carry bits */
156 incl %edi /* add 1: if one carry bit was *not* set
157 the addition will not result in 0. */
159 /* If at least one byte of the word is CHR we don't get 0 in %edi. */
160 jnz L(8) /* found it => return pointer */
162 /* This process is unfolded four times for better performance.
163 we don't increment the source pointer each time. Instead we
164 use offsets and increment by 16 in each run of the loop. But
165 before probing for the matching byte we need some extra code
166 (following LL(13) below). Even the len can be compared with
167 constants instead of decrementing each time. */
169 movl 4(%eax), %ecx /* get word (= 4 bytes) in question */
170 movl $0xfefefeff, %edi /* magic value */
171 xorl %edx, %ecx /* XOR with word c|c|c|c => bytes of str == c
173 addl %ecx, %edi /* add the magic value to the word. We get
174 carry bits reported for each byte which
176 jnc L(7) /* highest byte is CHR => return pointer */
177 xorl %ecx, %edi /* ((word^charmask)+magic)^(word^charmask) */
178 orl $0xfefefeff, %edi /* set all non-carry bits */
179 incl %edi /* add 1: if one carry bit was *not* set
180 the addition will not result in 0. */
181 jnz L(7) /* found it => return pointer */
183 movl 8(%eax), %ecx /* get word (= 4 bytes) in question */
184 movl $0xfefefeff, %edi /* magic value */
185 xorl %edx, %ecx /* XOR with word c|c|c|c => bytes of str == c
187 addl %ecx, %edi /* add the magic value to the word. We get
188 carry bits reported for each byte which
190 jnc L(6) /* highest byte is CHR => return pointer */
191 xorl %ecx, %edi /* ((word^charmask)+magic)^(word^charmask) */
192 orl $0xfefefeff, %edi /* set all non-carry bits */
193 incl %edi /* add 1: if one carry bit was *not* set
194 the addition will not result in 0. */
195 jnz L(6) /* found it => return pointer */
197 movl 12(%eax), %ecx /* get word (= 4 bytes) in question */
198 movl $0xfefefeff, %edi /* magic value */
199 xorl %edx, %ecx /* XOR with word c|c|c|c => bytes of str == c
201 addl %ecx, %edi /* add the magic value to the word. We get
202 carry bits reported for each byte which
204 jnc L(5) /* highest byte is CHR => return pointer */
205 xorl %ecx, %edi /* ((word^charmask)+magic)^(word^charmask) */
206 orl $0xfefefeff, %edi /* set all non-carry bits */
207 incl %edi /* add 1: if one carry bit was *not* set
208 the addition will not result in 0. */
209 jnz L(5) /* found it => return pointer */
211 /* Adjust both counters for a full round, i.e. 16 bytes. */
214 jae L(1) /* Still more than 16 bytes remaining */
216 /* Process remaining bytes separately. */
217 cmpl $4-16, %esi /* rest < 4 bytes? */
218 jb L(3) /* yes, than test byte by byte */
220 movl (%eax), %ecx /* get word (= 4 bytes) in question */
221 movl $0xfefefeff, %edi /* magic value */
222 xorl %edx, %ecx /* XOR with word c|c|c|c => bytes of str == c
224 addl %ecx, %edi /* add the magic value to the word. We get
225 carry bits reported for each byte which
227 jnc L(8) /* highest byte is CHR => return pointer */
228 xorl %ecx, %edi /* ((word^charmask)+magic)^(word^charmask) */
229 orl $0xfefefeff, %edi /* set all non-carry bits */
230 incl %edi /* add 1: if one carry bit was *not* set
231 the addition will not result in 0. */
232 jne L(8) /* found it => return pointer */
233 addl $4, %eax /* adjust source pointer */
235 cmpl $8-16, %esi /* rest < 8 bytes? */
236 jb L(3) /* yes, than test byte by byte */
238 movl (%eax), %ecx /* get word (= 4 bytes) in question */
239 movl $0xfefefeff, %edi /* magic value */
240 xorl %edx, %ecx /* XOR with word c|c|c|c => bytes of str == c
242 addl %ecx, %edi /* add the magic value to the word. We get
243 carry bits reported for each byte which
245 jnc L(8) /* highest byte is CHR => return pointer */
246 xorl %ecx, %edi /* ((word^charmask)+magic)^(word^charmask) */
247 orl $0xfefefeff, %edi /* set all non-carry bits */
248 incl %edi /* add 1: if one carry bit was *not* set
249 the addition will not result in 0. */
250 jne L(8) /* found it => return pointer */
251 addl $4, %eax /* adjust source pointer */
253 cmpl $12-16, %esi /* rest < 12 bytes? */
254 jb L(3) /* yes, than test byte by byte */
256 movl (%eax), %ecx /* get word (= 4 bytes) in question */
257 movl $0xfefefeff, %edi /* magic value */
258 xorl %edx, %ecx /* XOR with word c|c|c|c => bytes of str == c
260 addl %ecx, %edi /* add the magic value to the word. We get
261 carry bits reported for each byte which
263 jnc L(8) /* highest byte is CHR => return pointer */
264 xorl %ecx, %edi /* ((word^charmask)+magic)^(word^charmask) */
265 orl $0xfefefeff, %edi /* set all non-carry bits */
266 incl %edi /* add 1: if one carry bit was *not* set
267 the addition will not result in 0. */
268 jne L(8) /* found it => return pointer */
269 addl $4, %eax /* adjust source pointer */
271 /* Check the remaining bytes one by one. */
272 L(3): andl $3, %esi /* mask out uninteresting bytes */
273 jz L(4) /* no remaining bytes => return NULL */
275 cmpb %dl, (%eax) /* compare byte with CHR */
276 je L(9) /* equal, than return pointer */
277 incl %eax /* increment source pointer */
278 decl %esi /* decrement length */
279 jz L(4) /* no remaining bytes => return NULL */
281 cmpb %dl, (%eax) /* compare byte with CHR */
282 je L(9) /* equal, than return pointer */
283 incl %eax /* increment source pointer */
284 decl %esi /* decrement length */
285 jz L(4) /* no remaining bytes => return NULL */
287 cmpb %dl, (%eax) /* compare byte with CHR */
288 je L(9) /* equal, than return pointer */
290 L(4): /* no byte found => return NULL */
294 /* add missing source pointer increments */
299 /* Test for the matching byte in the word. %ecx contains a NUL
300 char in the byte which originally was the byte we are looking
302 L(8): testb %cl, %cl /* test first byte in dword */
303 jz L(9) /* if zero => return pointer */
304 incl %eax /* increment source pointer */
306 testb %ch, %ch /* test second byte in dword */
307 jz L(9) /* if zero => return pointer */
308 incl %eax /* increment source pointer */
310 testl $0xff0000, %ecx /* test third byte in dword */
311 jz L(9) /* if zero => return pointer */
312 incl %eax /* increment source pointer */
314 /* No further test needed we we know it is one of the four bytes. */
316 #if __BOUNDED_POINTERS__
317 CHECK_BOUNDS_HIGH (%eax, STR(%esp), jb)
318 /* If RTN pointer is phony, don't copy return value into it. */
322 RETURN_BOUNDED_POINTER (STR(%esp))
324 L(pop): popl %edi /* pop saved registers */
325 cfi_adjust_cfa_offset (-4)
328 cfi_adjust_cfa_offset (-4)
333 END (BP_SYM (__memchr))
335 weak_alias (BP_SYM (__memchr), BP_SYM (memchr))
336 #if !__BOUNDED_POINTERS__
337 weak_alias (__memchr, __ubp_memchr)
339 libc_hidden_builtin_def (memchr)