sysdeps/m68k/rawmemchr.S

   1 /* rawmemchr (str, ch) -- Return pointer to first occurrence of CH in STR.
   2    For Motorola 68000.
   3    Copyright (C) 1999-2019 Free Software Foundation, Inc.
   4    This file is part of the GNU C Library.
   5    Contributed by Andreas Schwab <schwab@gnu.org>.
   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    <http://www.gnu.org/licenses/>.  */
  20
  21 #include <sysdep.h>
  22 #include "asm-syntax.h"
  23
  24         TEXT
  25 ENTRY(__rawmemchr)
  26         /* Save the callee-saved registers we use.  */
  27         movel   R(d2),MEM_PREDEC(sp)
  28         cfi_adjust_cfa_offset (4)
  29         movel   R(d3),MEM_PREDEC(sp)
  30         cfi_adjust_cfa_offset (4)
  31         cfi_rel_offset (R(d2), 4)
  32         cfi_rel_offset (R(d3), 0)
  33
  34         /* Get string pointer and character.  */
  35         movel   MEM_DISP(sp,12),R(a0)
  36         moveb   MEM_DISP(sp,19),R(d0)
  37
  38         /* Distribute the character to all bytes of a longword.  */
  39         movel   R(d0),R(d1)
  40         lsll    #8,R(d1)
  41         moveb   R(d0),R(d1)
  42         movel   R(d1),R(d0)
  43         swap    R(d0)
  44         movew   R(d1),R(d0)
  45
  46         /* First search for the character one byte at a time until the
  47            pointer is aligned to a longword boundary.  */
  48         movel   R(a0),R(d1)
  49 #ifdef __mcoldfire__
  50         andl    #3,R(d1)
  51 #else
  52         andw    #3,R(d1)
  53 #endif
  54         beq     L(L1)
  55         cmpb    MEM(a0),R(d0)
  56         beq     L(L9)
  57         addql   #1,R(a0)
  58
  59 #ifdef __mcoldfire__
  60         subql   #3,R(d1)
  61 #else
  62         subqw   #3,R(d1)
  63 #endif
  64         beq     L(L1)
  65         cmpb    MEM(a0),R(d0)
  66         beq     L(L9)
  67         addql   #1,R(a0)
  68
  69 #ifdef __mcoldfire__
  70         addql   #1,R(d1)
  71 #else
  72         addqw   #1,R(d1)
  73 #endif
  74         beq     L(L1)
  75         cmpb    MEM(a0),R(d0)
  76         beq     L(L9)
  77         addql   #1,R(a0)
  78
  79 L(L1:)
  80         /* Load the magic bits.  Unlike the generic implementation we can
  81            use the carry bit as the fourth hole.  */
  82         movel   #0xfefefeff,R(d3)
  83
  84       /* We exit the loop if adding MAGIC_BITS to LONGWORD fails to
  85          change any of the hole bits of LONGWORD.
  86
  87          1) Is this safe?  Will it catch all the zero bytes?
  88          Suppose there is a byte with all zeros.  Any carry bits
  89          propagating from its left will fall into the hole at its
  90          least significant bit and stop.  Since there will be no
  91          carry from its most significant bit, the LSB of the
  92          byte to the left will be unchanged, and the zero will be
  93          detected.
  94
  95          2) Is this worthwhile?  Will it ignore everything except
  96          zero bytes?  Suppose every byte of LONGWORD has a bit set
  97          somewhere.  There will be a carry into bit 8.  If bit 8
  98          is set, this will carry into bit 16.  If bit 8 is clear,
  99          one of bits 9-15 must be set, so there will be a carry
 100          into bit 16.  Similarly, there will be a carry into bit
 101          24.  If one of bits 24-31 is set, there will be a carry
 102          into bit 32 (=carry flag), so all of the hole bits will
 103          be changed.
 104
 105          3) But wait!  Aren't we looking for C, not zero?
 106          Good point.  So what we do is XOR LONGWORD with a longword,
 107          each of whose bytes is C.  This turns each byte that is C
 108          into a zero.  */
 109
 110 L(L2:)
 111         /* Get the longword in question.  */
 112         movel   MEM_POSTINC(a0),R(d1)
 113         /* XOR with the byte we search for.  */
 114         eorl    R(d0),R(d1)
 115
 116         /* Add the magic value.  We get carry bits reported for each byte
 117            which is not C.  */
 118         movel   R(d3),R(d2)
 119         addl    R(d1),R(d2)
 120
 121         /* Check the fourth carry bit before it is clobbered by the next
 122            XOR.  If it is not set we have a hit.  */
 123         bcc     L(L8)
 124
 125         /* We are only interested in carry bits that change due to the
 126            previous add, so remove original bits.  */
 127         eorl    R(d1),R(d2)
 128
 129         /* Now test for the other three overflow bits.
 130            Set all non-carry bits.  */
 131         orl     R(d3),R(d2)
 132         /* Add 1 to get zero if all carry bits were set.  */
 133         addql   #1,R(d2)
 134
 135         /* If we don't get zero then at least one byte of the word equals
 136            C.  */
 137         bne     L(L8)
 138
 139         /* Get the longword in question.  */
 140         movel   MEM_POSTINC(a0),R(d1)
 141         /* XOR with the byte we search for.  */
 142         eorl    R(d0),R(d1)
 143
 144         /* Add the magic value.  We get carry bits reported for each byte
 145            which is not C.  */
 146         movel   R(d3),R(d2)
 147         addl    R(d1),R(d2)
 148
 149         /* Check the fourth carry bit before it is clobbered by the next
 150            XOR.  If it is not set we have a hit.  */
 151         bcc     L(L8)
 152
 153         /* We are only interested in carry bits that change due to the
 154            previous add, so remove original bits */
 155         eorl    R(d1),R(d2)
 156
 157         /* Now test for the other three overflow bits.
 158            Set all non-carry bits.  */
 159         orl     R(d3),R(d2)
 160         /* Add 1 to get zero if all carry bits were set.  */
 161         addql   #1,R(d2)
 162
 163         /* If we don't get zero then at least one byte of the word equals
 164            C.  */
 165         beq     L(L2)
 166
 167 L(L8:)
 168         /* We have a hit.  Check to see which byte it was.  First
 169            compensate for the autoincrement in the loop.  */
 170         subql   #4,R(a0)
 171
 172         cmpb    MEM(a0),R(d0)
 173         beq     L(L9)
 174         addql   #1,R(a0)
 175
 176         cmpb    MEM(a0),R(d0)
 177         beq     L(L9)
 178         addql   #1,R(a0)
 179
 180         cmpb    MEM(a0),R(d0)
 181         beq     L(L9)
 182         addql   #1,R(a0)
 183
 184         /* Otherwise the fourth byte must equal C.  */
 185 L(L9:)
 186         movel   R(a0),R(d0)
 187         movel   MEM_POSTINC(sp),R(d3)
 188         cfi_adjust_cfa_offset (-4)
 189         cfi_restore (R(d3))
 190         movel   MEM_POSTINC(sp),R(d2)
 191         cfi_adjust_cfa_offset (-4)
 192         cfi_restore (R(d2))
 193         rts
 194 END(__rawmemchr)
 195
 196 libc_hidden_def (__rawmemchr)
 197 weak_alias (__rawmemchr, rawmemchr)