1/* copy no more than N bytes from SRC to DEST, returning the address of
2 the terminating '\0' in DEST.
3 For Intel 80x86, x>=3.
4 Copyright (C) 1994-2022 Free Software Foundation, Inc.
5 This file is part of the GNU C Library.
6
7 The GNU C Library is free software; you can redistribute it and/or
8 modify it under the terms of the GNU Lesser General Public
9 License as published by the Free Software Foundation; either
10 version 2.1 of the License, or (at your option) any later version.
11
12 The GNU C Library is distributed in the hope that it will be useful,
13 but WITHOUT ANY WARRANTY; without even the implied warranty of
14 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
15 Lesser General Public License for more details.
16
17 You should have received a copy of the GNU Lesser General Public
18 License along with the GNU C Library; if not, see
19 <https://www.gnu.org/licenses/>. */
20
21#include <sysdep.h>
22#include "asm-syntax.h"
23
24#define PARMS 4+4 /* space for 1 saved reg */
25#define RTN PARMS
26#define DEST RTN
27#define SRC DEST+4
28#define LEN SRC+4
29
30 .text
31ENTRY (__stpncpy)
32
33 pushl %esi
34 cfi_adjust_cfa_offset (4)
35
36 movl DEST(%esp), %eax
37 movl SRC(%esp), %esi
38 cfi_rel_offset (esi, 0)
39 movl LEN(%esp), %ecx
40
41 subl %eax, %esi /* magic: reduce number of loop variants
42 to one using addressing mode */
43 jmp L(1) /* jump to loop "head" */
44
45 ALIGN(4)
46
47 /* Four times unfolded loop with two loop counters. We get the
48 third value (the source address) by using the index+base
49 addressing mode. */
50L(2): movb (%eax,%esi), %dl /* load current char */
51 movb %dl, (%eax) /* and store it */
52 testb %dl, %dl /* was it NUL? */
53 jz L(7) /* yes, then exit */
54
55 movb 1(%eax,%esi), %dl /* load current char */
56 movb %dl, 1(%eax) /* and store it */
57 testb %dl, %dl /* was it NUL? */
58 jz L(6) /* yes, then exit */
59
60 movb 2(%eax,%esi), %dl /* load current char */
61 movb %dl, 2(%eax) /* and store it */
62 testb %dl, %dl /* was it NUL? */
63 jz L(5) /* yes, then exit */
64
65 movb 3(%eax,%esi), %dl /* load current char */
66 movb %dl, 3(%eax) /* and store it */
67 testb %dl, %dl /* was it NUL? */
68 jz L(4) /* yes, then exit */
69
70 addl $4, %eax /* increment loop counter for full round */
71
72L(1): subl $4, %ecx /* still more than 4 bytes allowed? */
73 jae L(2) /* yes, then go to start of loop */
74
75 /* The maximal remaining 15 bytes are not processed in a loop. */
76
77 addl $4, %ecx /* correct above subtraction */
78 jz L(9) /* maximal allowed char reached => go to end */
79
80 movb (%eax,%esi), %dl /* load current char */
81 movb %dl, (%eax) /* and store it */
82 testb %dl, %dl /* was it NUL? */
83 jz L(3) /* yes, then exit */
84
85 incl %eax /* increment pointer */
86 decl %ecx /* decrement length counter */
87 jz L(9) /* no more allowed => exit */
88
89 movb (%eax,%esi), %dl /* load current char */
90 movb %dl, (%eax) /* and store it */
91 testb %dl, %dl /* was it NUL? */
92 jz L(3) /* yes, then exit */
93
94 incl %eax /* increment pointer */
95 decl %ecx /* decrement length counter */
96 jz L(9) /* no more allowed => exit */
97
98 movb (%eax,%esi), %dl /* load current char */
99 movb %dl, (%eax) /* and store it */
100 testb %dl, %dl /* was it NUL? */
101 jz L(3) /* yes, then exit */
102
103 incl %eax /* increment pointer */
104 jmp L(9) /* we don't have to test for counter underflow
105 because we know we had a most 3 bytes
106 remaining => exit */
107
108 /* When coming from the main loop we have to adjust the pointer. */
109L(4): decl %ecx /* decrement counter */
110 incl %eax /* increment pointer */
111
112L(5): decl %ecx /* increment pointer */
113 incl %eax /* increment pointer */
114
115L(6): decl %ecx /* increment pointer */
116 incl %eax /* increment pointer */
117L(7):
118
119 addl $3, %ecx /* correct pre-decrementation of counter
120 at the beginning of the loop; but why 3
121 and not 4? Very simple, we have to count
122 the NUL char we already wrote. */
123 jz L(9) /* counter is also 0 => exit */
124
125 /* We now have to fill the rest of the buffer with NUL. This
126 is done in a tricky way. Please note that the addressing mode
127 used below is not the same we used above. Here we use the
128 %ecx register. */
129L(8):
130 movb $0, (%ecx,%eax) /* store NUL char */
131L(3): decl %ecx /* all bytes written? */
132 jnz L(8) /* no, then again */
133
134L(9): popl %esi /* restore saved register content */
135 cfi_adjust_cfa_offset (-4)
136 cfi_restore (esi)
137
138 ret
139END (__stpncpy)
140
141libc_hidden_def (__stpncpy)
142weak_alias (__stpncpy, stpncpy)
143

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