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1/* strnlen - calculate the length of a string with limit.
2
3 Copyright (C) 2013-2019 Free Software Foundation, Inc.
4
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 <http://www.gnu.org/licenses/>. */
20
21#include <sysdep.h>
22
23/* Assumptions:
24 *
25 * ARMv8-a, AArch64
26 */
27
28/* Arguments and results. */
29#define srcin x0
30#define len x0
31#define limit x1
32
33/* Locals and temporaries. */
34#define src x2
35#define data1 x3
36#define data2 x4
37#define data2a x5
38#define has_nul1 x6
39#define has_nul2 x7
40#define tmp1 x8
41#define tmp2 x9
42#define tmp3 x10
43#define tmp4 x11
44#define zeroones x12
45#define pos x13
46#define limit_wd x14
47
48#define REP8_01 0x0101010101010101
49#define REP8_7f 0x7f7f7f7f7f7f7f7f
50#define REP8_80 0x8080808080808080
51
52ENTRY_ALIGN_AND_PAD (__strnlen, 6, 9)
53 DELOUSE (0)
54 DELOUSE (1)
55 DELOUSE (2)
56 cbz limit, L(hit_limit)
57 mov zeroones, #REP8_01
58 bic src, srcin, #15
59 ands tmp1, srcin, #15
60 b.ne L(misaligned)
61 /* Calculate the number of full and partial words -1. */
62 sub limit_wd, limit, #1 /* Limit != 0, so no underflow. */
63 lsr limit_wd, limit_wd, #4 /* Convert to Qwords. */
64
65 /* NUL detection works on the principle that (X - 1) & (~X) & 0x80
66 (=> (X - 1) & ~(X | 0x7f)) is non-zero iff a byte is zero, and
67 can be done in parallel across the entire word. */
68 /* The inner loop deals with two Dwords at a time. This has a
69 slightly higher start-up cost, but we should win quite quickly,
70 especially on cores with a high number of issue slots per
71 cycle, as we get much better parallelism out of the operations. */
72
73 /* Start of critial section -- keep to one 64Byte cache line. */
74L(loop):
75 ldp data1, data2, [src], #16
76L(realigned):
77 sub tmp1, data1, zeroones
78 orr tmp2, data1, #REP8_7f
79 sub tmp3, data2, zeroones
80 orr tmp4, data2, #REP8_7f
81 bic has_nul1, tmp1, tmp2
82 bic has_nul2, tmp3, tmp4
83 subs limit_wd, limit_wd, #1
84 orr tmp1, has_nul1, has_nul2
85 ccmp tmp1, #0, #0, pl /* NZCV = 0000 */
86 b.eq L(loop)
87 /* End of critical section -- keep to one 64Byte cache line. */
88
89 orr tmp1, has_nul1, has_nul2
90 cbz tmp1, L(hit_limit) /* No null in final Qword. */
91
92 /* We know there's a null in the final Qword. The easiest thing
93 to do now is work out the length of the string and return
94 MIN (len, limit). */
95
96 sub len, src, srcin
97 cbz has_nul1, L(nul_in_data2)
98#ifdef __AARCH64EB__
99 mov data2, data1
100#endif
101 sub len, len, #8
102 mov has_nul2, has_nul1
103L(nul_in_data2):
104#ifdef __AARCH64EB__
105 /* For big-endian, carry propagation (if the final byte in the
106 string is 0x01) means we cannot use has_nul directly. The
107 easiest way to get the correct byte is to byte-swap the data
108 and calculate the syndrome a second time. */
109 rev data2, data2
110 sub tmp1, data2, zeroones
111 orr tmp2, data2, #REP8_7f
112 bic has_nul2, tmp1, tmp2
113#endif
114 sub len, len, #8
115 rev has_nul2, has_nul2
116 clz pos, has_nul2
117 add len, len, pos, lsr #3 /* Bits to bytes. */
118 cmp len, limit
119 csel len, len, limit, ls /* Return the lower value. */
120 RET
121
122L(misaligned):
123 /* Deal with a partial first word.
124 We're doing two things in parallel here;
125 1) Calculate the number of words (but avoiding overflow if
126 limit is near ULONG_MAX) - to do this we need to work out
127 limit + tmp1 - 1 as a 65-bit value before shifting it;
128 2) Load and mask the initial data words - we force the bytes
129 before the ones we are interested in to 0xff - this ensures
130 early bytes will not hit any zero detection. */
131 sub limit_wd, limit, #1
132 neg tmp4, tmp1
133 cmp tmp1, #8
134
135 and tmp3, limit_wd, #15
136 lsr limit_wd, limit_wd, #4
137 mov tmp2, #~0
138
139 ldp data1, data2, [src], #16
140 lsl tmp4, tmp4, #3 /* Bytes beyond alignment -> bits. */
141 add tmp3, tmp3, tmp1
142
143#ifdef __AARCH64EB__
144 /* Big-endian. Early bytes are at MSB. */
145 lsl tmp2, tmp2, tmp4 /* Shift (tmp1 & 63). */
146#else
147 /* Little-endian. Early bytes are at LSB. */
148 lsr tmp2, tmp2, tmp4 /* Shift (tmp1 & 63). */
149#endif
150 add limit_wd, limit_wd, tmp3, lsr #4
151
152 orr data1, data1, tmp2
153 orr data2a, data2, tmp2
154
155 csinv data1, data1, xzr, le
156 csel data2, data2, data2a, le
157 b L(realigned)
158
159L(hit_limit):
160 mov len, limit
161 RET
162END (__strnlen)
163libc_hidden_def (__strnlen)
164weak_alias (__strnlen, strnlen)
165libc_hidden_def (strnlen)
166

Warning: That file was not part of the compilation database. It may have many parsing errors.