1/* strrchr -- find the last occurrence of C in 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 (strrchr)
25 @ r0 = start of string
26 @ r1 = character to match
27 @ returns NULL for no match, or a pointer to the match
28
29 mov r3, r0
30 mov r0, #0
31 uxtb r1, r1
32
33 @ Loop a few times until we're aligned.
34 tst r3, #7
35 beq 2f
361: ldrb r2, [r3], #1
37 cmp r2, r1 @ Find the character
38 it eq
39 subeq r0, r3, #1
40 cmp r2, #0 @ Find EOS
41 it eq
42 bxeq lr
43 tst r3, #7 @ Find the alignment point
44 bne 1b
45
46 @ So now we're aligned. Now we actually need a stack frame.
472: push { r4, r5, r6, r7 }
48 cfi_adjust_cfa_offset (16)
49 cfi_rel_offset (r4, 0)
50 cfi_rel_offset (r5, 4)
51 cfi_rel_offset (r6, 8)
52 cfi_rel_offset (r7, 12)
53
54 orr r1, r1, r1, lsl #8 @ Replicate C to all bytes
55#ifdef ARCH_HAS_T2
56 movw ip, #0x0101
57 movt ip, #0x0101
58#else
59 ldr ip, =0x01010101
60#endif
61 orr r1, r1, r1, lsl #16
62 mov r2, #0 @ No found bits yet
63
64 @ Loop searching for EOS and C, 8 bytes at a time.
65 @ Any time we find a match in a word, we copy the address of
66 @ the word to r0, and the found bits to r2.
673: ldrd r4, r5, [r3], #8
68 @ Subtracting (unsigned saturating) from 1 means result of 1 for
69 @ any byte that was originally zero and 0 otherwise. Therefore
70 @ we consider the lsb of each byte the "found" bit.
71 uqsub8 r6, ip, r4 @ Find EOS
72 uqsub8 r7, ip, r5
73 eor r4, r4, r1 @ Convert C bytes to 0
74 eor r5, r5, r1
75 uqsub8 r4, ip, r4 @ Find C
76 uqsub8 r5, ip, r5
77 cmp r6, #0 @ Found EOS, first word
78 bne 4f
79 cmp r4, #0 @ Handle C, first word
80 itt ne
81 subne r0, r3, #8
82 movne r2, r4
83 cmp r7, #0 @ Found EOS, second word
84 bne 5f
85 cmp r5, #0 @ Handle C, second word
86 itt ne
87 subne r0, r3, #4
88 movne r2, r5
89 b 3b
90
91 @ Found EOS in second word; fold to first word.
925: add r3, r3, #4 @ Dec pointer to 2nd word, with below
93 mov r4, r5 @ Overwrite first word C found
94 mov r6, r7 @ Overwrite first word EOS found
95
96 @ Found EOS. Zap found C after EOS.
974: sub r3, r3, #8 @ Decrement pointer to first word
98#ifdef __ARMEB__
99 @ Byte swap to be congruent with LE, which is easier from here on.
100 rev r6, r6 @ Byte swap found EOS,
101 rev r4, r4 @ ... this found C
102 rev r2, r2 @ ... prev found C
103#endif
104 sub r7, r6, #1 @ Toggle EOS lsb and below
105 eor r6, r6, r7 @ All bits below and including lsb
106 ands r4, r4, r6 @ Zap C above EOS
107 itt ne
108 movne r2, r4 @ Copy to result, if still non-zero
109 movne r0, r3
110
111 pop { r4, r5, r6, r7 }
112 cfi_adjust_cfa_offset (-16)
113 cfi_restore (r4)
114 cfi_restore (r5)
115 cfi_restore (r6)
116 cfi_restore (r7)
117
118 @ Adjust the result pointer if we found a word containing C.
119 cmp r2, #0
120 clz r2, r2 @ Find the bit offset of the last C
121 itt ne
122 rsbne r2, r2, #32 @ Convert to a count from the right
123 addne r0, r0, r2, lsr #3 @ Convert to byte offset and add.
124 bx lr
125
126END (strrchr)
127
128weak_alias (strrchr, rindex)
129libc_hidden_builtin_def (strrchr)
130

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