1/* strnlen - calculate the length of a string with limit.
2
3 Copyright (C) 2013-2024 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 <https://www.gnu.org/licenses/>. */
20
21#include <sysdep.h>
22
23/* Assumptions:
24 *
25 * ARMv8-a, AArch64, Advanced SIMD.
26 * MTE compatible.
27 */
28
29#define srcin x0
30#define cntin x1
31#define result x0
32
33#define src x2
34#define synd x3
35#define shift x4
36#define tmp x4
37#define cntrem x5
38
39#define qdata q0
40#define vdata v0
41#define vhas_chr v1
42#define vend v2
43#define dend d2
44
45/*
46 Core algorithm:
47 Process the string in 16-byte aligned chunks. Compute a 64-bit mask with
48 four bits per byte using the shrn instruction. A count trailing zeros then
49 identifies the first zero byte. */
50
51ENTRY (__strnlen)
52 PTR_ARG (0)
53 SIZE_ARG (1)
54 bic src, srcin, 15
55 cbz cntin, L(nomatch)
56 ld1 {vdata.16b}, [src]
57 cmeq vhas_chr.16b, vdata.16b, 0
58 lsl shift, srcin, 2
59 shrn vend.8b, vhas_chr.8h, 4 /* 128->64 */
60 fmov synd, dend
61 lsr synd, synd, shift
62 cbz synd, L(start_loop)
63L(finish):
64 rbit synd, synd
65 clz synd, synd
66 lsr result, synd, 2
67 cmp cntin, result
68 csel result, cntin, result, ls
69 ret
70
71L(nomatch):
72 mov result, cntin
73 ret
74
75L(start_loop):
76 sub tmp, src, srcin
77 add tmp, tmp, 17
78 subs cntrem, cntin, tmp
79 b.lo L(nomatch)
80
81 /* Make sure that it won't overread by a 16-byte chunk */
82 tbz cntrem, 4, L(loop32_2)
83 sub src, src, 16
84 .p2align 5
85L(loop32):
86 ldr qdata, [src, 32]!
87 cmeq vhas_chr.16b, vdata.16b, 0
88 umaxp vend.16b, vhas_chr.16b, vhas_chr.16b /* 128->64 */
89 fmov synd, dend
90 cbnz synd, L(end)
91L(loop32_2):
92 ldr qdata, [src, 16]
93 subs cntrem, cntrem, 32
94 cmeq vhas_chr.16b, vdata.16b, 0
95 b.lo L(end_2)
96 umaxp vend.16b, vhas_chr.16b, vhas_chr.16b /* 128->64 */
97 fmov synd, dend
98 cbz synd, L(loop32)
99L(end_2):
100 add src, src, 16
101L(end):
102 shrn vend.8b, vhas_chr.8h, 4 /* 128->64 */
103 sub result, src, srcin
104 fmov synd, dend
105#ifndef __AARCH64EB__
106 rbit synd, synd
107#endif
108 clz synd, synd
109 add result, result, synd, lsr 2
110 cmp cntin, result
111 csel result, cntin, result, ls
112 ret
113
114END (__strnlen)
115libc_hidden_def (__strnlen)
116weak_alias (__strnlen, strnlen)
117libc_hidden_def (strnlen)
118

source code of glibc/sysdeps/aarch64/strnlen.S