1/*
2 * strnlen - calculate the length of a string with limit.
3 *
4 * Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
5 * See https://llvm.org/LICENSE.txt for license information.
6 * SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
7 */
8
9/* Assumptions:
10 *
11 * ARMv8-a, AArch64
12 */
13
14#include "../asmdefs.h"
15
16/* Arguments and results. */
17#define srcin x0
18#define len x0
19#define limit x1
20
21/* Locals and temporaries. */
22#define src x2
23#define data1 x3
24#define data2 x4
25#define data2a x5
26#define has_nul1 x6
27#define has_nul2 x7
28#define tmp1 x8
29#define tmp2 x9
30#define tmp3 x10
31#define tmp4 x11
32#define zeroones x12
33#define pos x13
34#define limit_wd x14
35
36#define REP8_01 0x0101010101010101
37#define REP8_7f 0x7f7f7f7f7f7f7f7f
38#define REP8_80 0x8080808080808080
39
40 .text
41 .p2align 6
42L(start):
43 /* Pre-pad to ensure critical loop begins an icache line. */
44 .rep 7
45 nop
46 .endr
47 /* Put this code here to avoid wasting more space with pre-padding. */
48L(hit_limit):
49 mov len, limit
50 ret
51
52ENTRY_ALIGN (__strnlen_aarch64, 0)
53 cbz limit, L(hit_limit)
54 mov zeroones, #REP8_01
55 bic src, srcin, #15
56 ands tmp1, srcin, #15
57 b.ne L(misaligned)
58 /* Calculate the number of full and partial words -1. */
59 sub limit_wd, limit, #1 /* Limit != 0, so no underflow. */
60 lsr limit_wd, limit_wd, #4 /* Convert to Qwords. */
61
62 /* NUL detection works on the principle that (X - 1) & (~X) & 0x80
63 (=> (X - 1) & ~(X | 0x7f)) is non-zero iff a byte is zero, and
64 can be done in parallel across the entire word. */
65 /* The inner loop deals with two Dwords at a time. This has a
66 slightly higher start-up cost, but we should win quite quickly,
67 especially on cores with a high number of issue slots per
68 cycle, as we get much better parallelism out of the operations. */
69
70 /* Start of critical section -- keep to one 64Byte cache line. */
71L(loop):
72 ldp data1, data2, [src], #16
73L(realigned):
74 sub tmp1, data1, zeroones
75 orr tmp2, data1, #REP8_7f
76 sub tmp3, data2, zeroones
77 orr tmp4, data2, #REP8_7f
78 bic has_nul1, tmp1, tmp2
79 bic has_nul2, tmp3, tmp4
80 subs limit_wd, limit_wd, #1
81 orr tmp1, has_nul1, has_nul2
82 ccmp tmp1, #0, #0, pl /* NZCV = 0000 */
83 b.eq L(loop)
84 /* End of critical section -- keep to one 64Byte cache line. */
85
86 orr tmp1, has_nul1, has_nul2
87 cbz tmp1, L(hit_limit) /* No null in final Qword. */
88
89 /* We know there's a null in the final Qword. The easiest thing
90 to do now is work out the length of the string and return
91 MIN (len, limit). */
92
93 sub len, src, srcin
94 cbz has_nul1, L(nul_in_data2)
95#ifdef __AARCH64EB__
96 mov data2, data1
97#endif
98 sub len, len, #8
99 mov has_nul2, has_nul1
100L(nul_in_data2):
101#ifdef __AARCH64EB__
102 /* For big-endian, carry propagation (if the final byte in the
103 string is 0x01) means we cannot use has_nul directly. The
104 easiest way to get the correct byte is to byte-swap the data
105 and calculate the syndrome a second time. */
106 rev data2, data2
107 sub tmp1, data2, zeroones
108 orr tmp2, data2, #REP8_7f
109 bic has_nul2, tmp1, tmp2
110#endif
111 sub len, len, #8
112 rev has_nul2, has_nul2
113 clz pos, has_nul2
114 add len, len, pos, lsr #3 /* Bits to bytes. */
115 cmp len, limit
116 csel len, len, limit, ls /* Return the lower value. */
117 ret
118
119L(misaligned):
120 /* Deal with a partial first word.
121 We're doing two things in parallel here;
122 1) Calculate the number of words (but avoiding overflow if
123 limit is near ULONG_MAX) - to do this we need to work out
124 limit + tmp1 - 1 as a 65-bit value before shifting it;
125 2) Load and mask the initial data words - we force the bytes
126 before the ones we are interested in to 0xff - this ensures
127 early bytes will not hit any zero detection. */
128 sub limit_wd, limit, #1
129 neg tmp4, tmp1
130 cmp tmp1, #8
131
132 and tmp3, limit_wd, #15
133 lsr limit_wd, limit_wd, #4
134 mov tmp2, #~0
135
136 ldp data1, data2, [src], #16
137 lsl tmp4, tmp4, #3 /* Bytes beyond alignment -> bits. */
138 add tmp3, tmp3, tmp1
139
140#ifdef __AARCH64EB__
141 /* Big-endian. Early bytes are at MSB. */
142 lsl tmp2, tmp2, tmp4 /* Shift (tmp1 & 63). */
143#else
144 /* Little-endian. Early bytes are at LSB. */
145 lsr tmp2, tmp2, tmp4 /* Shift (tmp1 & 63). */
146#endif
147 add limit_wd, limit_wd, tmp3, lsr #4
148
149 orr data1, data1, tmp2
150 orr data2a, data2, tmp2
151
152 csinv data1, data1, xzr, le
153 csel data2, data2, data2a, le
154 b L(realigned)
155
156END (__strnlen_aarch64)
157

source code of libc/AOR_v20.02/string/aarch64/strnlen.S