1/* strlen(str) -- determine the length of the string STR.
2 Optimized for Intel 80x86, x>=4.
3 Copyright (C) 1991-2022 Free Software Foundation, Inc.
4 This file is part of the GNU C Library.
5
6 The GNU C Library is free software; you can redistribute it and/or
7 modify it under the terms of the GNU Lesser General Public
8 License as published by the Free Software Foundation; either
9 version 2.1 of the License, or (at your option) any later version.
10
11 The GNU C Library is distributed in the hope that it will be useful,
12 but WITHOUT ANY WARRANTY; without even the implied warranty of
13 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
14 Lesser General Public License for more details.
15
16 You should have received a copy of the GNU Lesser General Public
17 License along with the GNU C Library; if not, see
18 <https://www.gnu.org/licenses/>. */
19
20#include <sysdep.h>
21#include "asm-syntax.h"
22
23#define PARMS 4 /* no space for saved regs */
24#define STR PARMS
25
26 .text
27ENTRY (strlen)
28
29 movl STR(%esp), %ecx
30 movl %ecx, %eax /* duplicate it */
31
32 andl $3, %ecx /* mask alignment bits */
33 jz L(1) /* aligned => start loop */
34 cmpb %ch, (%eax) /* is byte NUL? */
35 je L(2) /* yes => return */
36 incl %eax /* increment pointer */
37
38 xorl $3, %ecx /* was alignment = 3? */
39 jz L(1) /* yes => now it is aligned and start loop */
40 cmpb %ch, (%eax) /* is byte NUL? */
41 je L(2) /* yes => return */
42 addl $1, %eax /* increment pointer */
43
44 subl $1, %ecx /* was alignment = 2? */
45 jz L(1) /* yes => now it is aligned and start loop */
46 cmpb %ch, (%eax) /* is byte NUL? */
47 je L(2) /* yes => return */
48
49/* Don't change the above `addl $1,%eax' and `subl $1, %ecx' into `incl %eax'
50 and `decl %ecx' resp. The additional two byte per instruction make the
51 label 4 to be aligned on a 16 byte boundary with nops.
52
53 The following `sub $15, %eax' is part of this trick, too. Together with
54 the next instruction (`addl $16, %eax') it is in fact a `incl %eax', just
55 as expected from the algorithm. But doing so has the advantage that
56 no jump to label 1 is necessary and so the pipeline is not flushed. */
57
58 subl $15, %eax /* effectively +1 */
59
60
61L(4): addl $16, %eax /* adjust pointer for full loop */
62
63L(1): movl (%eax), %ecx /* get word (= 4 bytes) in question */
64 movl $0xfefefeff, %edx /* magic value */
65 addl %ecx, %edx /* add the magic value to the word. We get
66 carry bits reported for each byte which
67 is *not* 0 */
68 jnc L(3) /* highest byte is NUL => return pointer */
69 xorl %ecx, %edx /* (word+magic)^word */
70 orl $0xfefefeff, %edx /* set all non-carry bits */
71 incl %edx /* add 1: if one carry bit was *not* set
72 the addition will not result in 0. */
73 jnz L(3) /* found NUL => return pointer */
74
75 movl 4(%eax), %ecx /* get word (= 4 bytes) in question */
76 movl $0xfefefeff, %edx /* magic value */
77 addl %ecx, %edx /* add the magic value to the word. We get
78 carry bits reported for each byte which
79 is *not* 0 */
80 jnc L(5) /* highest byte is NUL => return pointer */
81 xorl %ecx, %edx /* (word+magic)^word */
82 orl $0xfefefeff, %edx /* set all non-carry bits */
83 incl %edx /* add 1: if one carry bit was *not* set
84 the addition will not result in 0. */
85 jnz L(5) /* found NUL => return pointer */
86
87 movl 8(%eax), %ecx /* get word (= 4 bytes) in question */
88 movl $0xfefefeff, %edx /* magic value */
89 addl %ecx, %edx /* add the magic value to the word. We get
90 carry bits reported for each byte which
91 is *not* 0 */
92 jnc L(6) /* highest byte is NUL => return pointer */
93 xorl %ecx, %edx /* (word+magic)^word */
94 orl $0xfefefeff, %edx /* set all non-carry bits */
95 incl %edx /* add 1: if one carry bit was *not* set
96 the addition will not result in 0. */
97 jnz L(6) /* found NUL => return pointer */
98
99 movl 12(%eax), %ecx /* get word (= 4 bytes) in question */
100 movl $0xfefefeff, %edx /* magic value */
101 addl %ecx, %edx /* add the magic value to the word. We get
102 carry bits reported for each byte which
103 is *not* 0 */
104 jnc L(7) /* highest byte is NUL => return pointer */
105 xorl %ecx, %edx /* (word+magic)^word */
106 orl $0xfefefeff, %edx /* set all non-carry bits */
107 incl %edx /* add 1: if one carry bit was *not* set
108 the addition will not result in 0. */
109 jz L(4) /* no NUL found => continue loop */
110
111L(7): addl $4, %eax /* adjust pointer */
112L(6): addl $4, %eax
113L(5): addl $4, %eax
114
115L(3): testb %cl, %cl /* is first byte NUL? */
116 jz L(2) /* yes => return */
117 incl %eax /* increment pointer */
118
119 testb %ch, %ch /* is second byte NUL? */
120 jz L(2) /* yes => return */
121 incl %eax /* increment pointer */
122
123 testl $0xff0000, %ecx /* is third byte NUL? */
124 jz L(2) /* yes => return pointer */
125 incl %eax /* increment pointer */
126
127L(2): subl STR(%esp), %eax /* compute difference to string start */
128
129 ret
130END (strlen)
131libc_hidden_builtin_def (strlen)
132

source code of glibc/sysdeps/i386/strlen.S