sysdeps/arm/armv6/strchr.S

   1 /* strchr -- find the first instance of C in a nul-terminated string.
   2    Copyright (C) 2013-2015 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    <http://www.gnu.org/licenses/>.  */
  18
  19 #include <sysdep.h>
  20
  21         .syntax unified
  22         .text
  23
  24 ENTRY (strchr)
  25         @ r0 = start of string
  26         @ r1 = character to match
  27         @ returns NULL for no match, or a pointer to the match
  28         sfi_breg r0, \
  29         ldrb    r2, [\B]                @ load the first byte asap
  30         uxtb    r1, r1
  31
  32         @ To cater to long strings, we want to search through a few
  33         @ characters until we reach an aligned pointer.  To cater to
  34         @ small strings, we don't want to start doing word operations
  35         @ immediately.  The compromise is a maximum of 16 bytes less
  36         @ whatever is required to end with an aligned pointer.
  37         @ r3 = number of characters to search in alignment loop
  38         and     r3, r0, #7
  39         rsb     r3, r3, #15             @ 16 - 1 peeled loop iteration
  40         cmp     r2, r1                  @ Found C?
  41         it      ne
  42         cmpne   r2, #0                  @ Found EOS?
  43         beq     99f
  44
  45         @ Loop until we find ...
  46 1:      sfi_breg r0, \
  47         ldrb    r2, [\B, #1]!
  48         subs    r3, r3, #1              @ ... the aligment point
  49         it      ne
  50         cmpne   r2, r1                  @ ... or the character
  51         it      ne
  52         cmpne   r2, #0                  @ ... or EOS
  53         bne     1b
  54
  55         @ Disambiguate the exit possibilites above
  56         cmp     r2, r1                  @ Found the character
  57         it      ne
  58         cmpne   r2, #0                  @ Found EOS
  59         beq     99f
  60         add     r0, r0, #1
  61
  62         @ So now we're aligned.  Now we actually need a stack frame.
  63         push    { r4, r5, r6, r7 }
  64         cfi_adjust_cfa_offset (16)
  65         cfi_rel_offset (r4, 0)
  66         cfi_rel_offset (r5, 4)
  67         cfi_rel_offset (r6, 8)
  68         cfi_rel_offset (r7, 12)
  69
  70         sfi_breg r0, \
  71         ldrd    r2, r3, [\B], #8
  72         orr     r1, r1, r1, lsl #8      @ Replicate C to all bytes
  73 #ifdef ARCH_HAS_T2
  74         movw    ip, #0x0101
  75         sfi_pld r0, #64
  76         movt    ip, #0x0101
  77 #else
  78         ldr     ip, =0x01010101
  79         sfi_pld r0, #64
  80 #endif
  81         orr     r1, r1, r1, lsl #16
  82
  83         @ Loop searching for EOS or C, 8 bytes at a time.
  84 2:
  85         @ Subtracting (unsigned saturating) from 1 means result of 1 for
  86         @ any byte that was originally zero and 0 otherwise.  Therefore
  87         @ we consider the lsb of each byte the "found" bit.
  88         uqsub8  r4, ip, r2              @ Find EOS
  89         eor     r6, r2, r1              @ Convert C bytes to 0
  90         uqsub8  r5, ip, r3
  91         eor     r7, r3, r1
  92         uqsub8  r6, ip, r6              @ Find C
  93         sfi_pld r0, #128                @ Prefetch 2 lines ahead
  94         uqsub8  r7, ip, r7
  95         orr     r4, r4, r6              @ Combine found for EOS and C
  96         orr     r5, r5, r7
  97         orrs    r6, r4, r5              @ Combine the two words
  98         it      eq
  99         sfi_breg r0, \
 100         ldrdeq  r2, r3, [\B], #8
 101         beq     2b
 102
 103         @ Found something.  Disambiguate between first and second words.
 104         @ Adjust r0 to point to the word containing the match.
 105         @ Adjust r2 to the contents of the word containing the match.
 106         @ Adjust r4 to the found bits for the word containing the match.
 107         cmp     r4, #0
 108         sub     r0, r0, #4
 109         itte    eq
 110         moveq   r4, r5
 111         moveq   r2, r3
 112         subne   r0, r0, #4
 113
 114         @ Find the bit-offset of the match within the word.
 115 #if defined(__ARMEL__)
 116         @ For LE, swap the found word so clz searches from the little end.
 117         rev     r4, r4
 118 #else
 119         @ For BE, byte swap the word to make it easier to extract the byte.
 120         rev     r2, r2
 121 #endif
 122         @ We're counting 0x01 (not 0x80), so the bit offset is 7 too high.
 123         clz     r3, r4
 124         sub     r3, r3, #7
 125         lsr     r2, r2, r3              @ Shift down found byte
 126         uxtb    r1, r1                  @ Undo replication of C
 127         uxtb    r2, r2                  @ Extract found byte
 128         add     r0, r0, r3, lsr #3      @ Adjust the pointer to the found byte
 129
 130         pop     { r4, r5, r6, r7 }
 131         cfi_adjust_cfa_offset (-16)
 132         cfi_restore (r4)
 133         cfi_restore (r5)
 134         cfi_restore (r6)
 135         cfi_restore (r7)
 136
 137         @ Disambiguate between EOS and C.
 138 99:
 139         cmp     r2, r1
 140         it      ne
 141         movne   r0, #0                  @ Found EOS, return NULL
 142         bx      lr
 143
 144 END (strchr)
 145
 146 weak_alias (strchr, index)
 147 libc_hidden_builtin_def (strchr)