summaryrefslogtreecommitdiff
path: root/sysdeps/i386/i486/strlen.S
blob: 743918044e474dfb44ff0e7cb7af50b66a7062c7 (plain)
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
/* strlen(str) -- determine the length of the string STR.
   Optimized for Intel 80x86, x>=4.
   Copyright (C) 1991-2013 Free Software Foundation, Inc.
   Contributed by Ulrich Drepper <drepper@gnu.ai.mit.edu>.
   This file is part of the GNU C Library.

   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, see
   <http://www.gnu.org/licenses/>.  */

#include <sysdep.h>
#include "asm-syntax.h"
#include "bp-sym.h"
#include "bp-asm.h"

#define PARMS	LINKAGE		/* no space for saved regs */
#define STR	PARMS

	.text
ENTRY (BP_SYM (strlen))
	ENTER

	movl STR(%esp), %ecx
	CHECK_BOUNDS_LOW (%ecx, STR(%esp))
	movl %ecx, %eax		/* duplicate it */

	andl $3, %ecx		/* mask alignment bits */
	jz L(1)			/* aligned => start loop */
	cmpb %ch, (%eax)	/* is byte NUL? */
	je L(2)			/* yes => return */
	incl %eax		/* increment pointer */

	xorl $3, %ecx		/* was alignment = 3? */
	jz L(1)			/* yes => now it is aligned and start loop */
	cmpb %ch, (%eax)	/* is byte NUL? */
	je L(2)			/* yes => return */
	addl $1, %eax		/* increment pointer */

	subl $1, %ecx		/* was alignment = 2? */
	jz L(1)			/* yes => now it is aligned and start loop */
	cmpb %ch, (%eax)	/* is byte NUL? */
	je L(2)			/* yes => return */

/* Don't change the above `addl $1,%eax' and `subl $1, %ecx' into `incl %eax'
   and `decl %ecx' resp.  The additional two byte per instruction make the
   label 4 to be aligned on a 16 byte boundary with nops.

   The following `sub $15, %eax' is part of this trick, too.  Together with
   the next instruction (`addl $16, %eax') it is in fact a `incl %eax', just
   as expected from the algorithm.  But doing so has the advantage that
   no jump to label 1 is necessary and so the pipeline is not flushed.  */

	subl $15, %eax		/* effectively +1 */


L(4):	addl $16, %eax		/* adjust pointer for full loop */

L(1):	movl (%eax), %ecx	/* get word (= 4 bytes) in question */
	movl $0xfefefeff, %edx	/* magic value */
	addl %ecx, %edx		/* add the magic value to the word.  We get
				   carry bits reported for each byte which
				   is *not* 0 */
	jnc L(3)		/* highest byte is NUL => return pointer */
	xorl %ecx, %edx		/* (word+magic)^word */
	orl $0xfefefeff, %edx	/* set all non-carry bits */
	incl %edx		/* add 1: if one carry bit was *not* set
				   the addition will not result in 0.  */
	jnz L(3)		/* found NUL => return pointer */

	movl 4(%eax), %ecx	/* get word (= 4 bytes) in question */
	movl $0xfefefeff, %edx	/* magic value */
	addl %ecx, %edx		/* add the magic value to the word.  We get
				   carry bits reported for each byte which
				   is *not* 0 */
	jnc L(5)		/* highest byte is NUL => return pointer */
	xorl %ecx, %edx		/* (word+magic)^word */
	orl $0xfefefeff, %edx	/* set all non-carry bits */
	incl %edx		/* add 1: if one carry bit was *not* set
				   the addition will not result in 0.  */
	jnz L(5)		/* found NUL => return pointer */

	movl 8(%eax), %ecx	/* get word (= 4 bytes) in question */
	movl $0xfefefeff, %edx	/* magic value */
	addl %ecx, %edx		/* add the magic value to the word.  We get
				   carry bits reported for each byte which
				   is *not* 0 */
	jnc L(6)		/* highest byte is NUL => return pointer */
	xorl %ecx, %edx		/* (word+magic)^word */
	orl $0xfefefeff, %edx	/* set all non-carry bits */
	incl %edx		/* add 1: if one carry bit was *not* set
				   the addition will not result in 0.  */
	jnz L(6)		/* found NUL => return pointer */

	movl 12(%eax), %ecx	/* get word (= 4 bytes) in question */
	movl $0xfefefeff, %edx	/* magic value */
	addl %ecx, %edx		/* add the magic value to the word.  We get
				   carry bits reported for each byte which
				   is *not* 0 */
	jnc L(7)		/* highest byte is NUL => return pointer */
	xorl %ecx, %edx		/* (word+magic)^word */
	orl $0xfefefeff, %edx	/* set all non-carry bits */
	incl %edx		/* add 1: if one carry bit was *not* set
				   the addition will not result in 0.  */
	jz L(4)			/* no NUL found => continue loop */

L(7):	addl $4, %eax		/* adjust pointer */
L(6):	addl $4, %eax
L(5):	addl $4, %eax

L(3):	testb %cl, %cl		/* is first byte NUL? */
	jz L(2)			/* yes => return */
	incl %eax		/* increment pointer */

	testb %ch, %ch		/* is second byte NUL? */
	jz L(2)			/* yes => return */
	incl %eax		/* increment pointer */

	testl $0xff0000, %ecx	/* is third byte NUL? */
	jz L(2)			/* yes => return pointer */
	incl %eax		/* increment pointer */

L(2):	CHECK_BOUNDS_HIGH (%eax, STR(%esp), jb)
	subl STR(%esp), %eax	/* compute difference to string start */

	LEAVE
	ret
END (BP_SYM (strlen))
libc_hidden_builtin_def (strlen)