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