1/* strlen -- find the length of a nul-terminated string.
2 Copyright (C) 2013-2024 Free Software Foundation, Inc.
3 This file is part of the GNU C Library.
4
5 The GNU C Library is free software; you can redistribute it and/or
6 modify it under the terms of the GNU Lesser General Public
7 License as published by the Free Software Foundation; either
8 version 2.1 of the License, or (at your option) any later version.
9
10 The GNU C Library is distributed in the hope that it will be useful,
11 but WITHOUT ANY WARRANTY; without even the implied warranty of
12 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
13 Lesser General Public License for more details.
14
15 You should have received a copy of the GNU Lesser General Public
16 License along with the GNU C Library. If not, see
17 <https://www.gnu.org/licenses/>. */
18
19#include <sysdep.h>
20
21 .syntax unified
22 .text
23
24ENTRY (strlen)
25 @ r0 = start of string
26 ldrb r2, [r0] @ load the first byte asap
27
28 @ To cater to long strings, we want to search through a few
29 @ characters until we reach an aligned pointer. To cater to
30 @ small strings, we don't want to start doing word operations
31 @ immediately. The compromise is a maximum of 16 bytes less
32 @ whatever is required to end with an aligned pointer.
33 @ r3 = number of characters to search in alignment loop
34 and r3, r0, #7
35 mov r1, r0 @ Save the input pointer
36 rsb r3, r3, #15 @ 16 - 1 peeled loop iteration
37 cmp r2, #0
38 beq 99f
39
40 @ Loop until we find ...
411: ldrb r2, [r0, #1]!
42 subs r3, r3, #1 @ ... the alignment point
43 it ne
44 cmpne r2, #0 @ ... or EOS
45 bne 1b
46
47 @ Disambiguate the exit possibilities above
48 cmp r2, #0 @ Found EOS
49 beq 99f
50 add r0, r0, #1
51
52 @ So now we're aligned.
53 ldrd r2, r3, [r0], #8
54#ifdef ARCH_HAS_T2
55 movw ip, #0x0101
56 pld [r0, #64]
57 movt ip, #0x0101
58#else
59 ldr ip, =0x01010101
60 pld [r0, #64]
61#endif
62
63 @ Loop searching for EOS, 8 bytes at a time.
64 @ Subtracting (unsigned saturating) from 1 for any byte means that
65 @ we get 1 for any byte that was originally zero and 0 otherwise.
66 @ Therefore we consider the lsb of each byte the "found" bit.
67 .balign 16
682: uqsub8 r2, ip, r2 @ Find EOS
69 uqsub8 r3, ip, r3
70 pld [r0, #128] @ Prefetch 2 lines ahead
71 orrs r3, r3, r2 @ Combine the two words
72 it eq
73 ldrdeq r2, r3, [r0], #8
74 beq 2b
75
76 @ Found something. Disambiguate between first and second words.
77 @ Adjust r0 to point to the word containing the match.
78 @ Adjust r2 to the found bits for the word containing the match.
79 cmp r2, #0
80 sub r0, r0, #4
81 ite eq
82 moveq r2, r3
83 subne r0, r0, #4
84
85 @ Find the bit-offset of the match within the word. Note that the
86 @ bit result from clz will be 7 higher than "true", but we'll
87 @ immediately discard those bits converting to a byte offset.
88#ifdef __ARMEL__
89 rev r2, r2 @ For LE, count from the little end
90#endif
91 clz r2, r2
92 add r0, r0, r2, lsr #3 @ Adjust the pointer to the found byte
9399:
94 sub r0, r0, r1 @ Subtract input to compute length
95 bx lr
96
97END (strlen)
98
99libc_hidden_builtin_def (strlen)
100

source code of glibc/sysdeps/arm/armv6/strlen.S