sysdeps/m68k/strchrnul.S

   1 /* strchrnul (str, ch) -- Return pointer to first occurrence of CH in STR
   2    or the final NUL byte.
   3    For Motorola 68000.
   4    Copyright (C) 1999 Free Software Foundation, Inc.
   5    This file is part of the GNU C Library.
   6    Contributed by Andreas Schwab <schwab@gnu.org>.
   7
   8    The GNU C Library is free software; you can redistribute it and/or
   9    modify it under the terms of the GNU Lesser General Public
  10    License as published by the Free Software Foundation; either
  11    version 2.1 of the License, or (at your option) any later version.
  12
  13    The GNU C Library is distributed in the hope that it will be useful,
  14    but WITHOUT ANY WARRANTY; without even the implied warranty of
  15    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  16    Lesser General Public License for more details.
  17
  18    You should have received a copy of the GNU Lesser General Public
  19    License along with the GNU C Library; if not, write to the Free
  20    Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
  21    02111-1307 USA.  */
  22
  23 #include <sysdep.h>
  24 #include "asm-syntax.h"
  25
  26         TEXT
  27 ENTRY(__strchrnul)
  28         /* Save the callee-saved registers we use.  */
  29         movel   R(d2),MEM_PREDEC(sp)
  30         movel   R(d3),MEM_PREDEC(sp)
  31
  32         /* Get string pointer and character.  */
  33         movel   MEM_DISP(sp,12),R(a0)
  34         moveb   MEM_DISP(sp,19),R(d0)
  35
  36         /* Distribute the character to all bytes of a longword.  */
  37         movel   R(d0),R(d1)
  38         lsll    #8,R(d1)
  39         moveb   R(d0),R(d1)
  40         movel   R(d1),R(d0)
  41         swap    R(d0)
  42         movew   R(d1),R(d0)
  43
  44         /* First search for the character one byte at a time until the
  45            pointer is aligned to a longword boundary.  */
  46         movel   R(a0),R(d1)
  47         andw    #3,R(d1)
  48         beq     L(L1)
  49         moveb   MEM(a0),R(d1)
  50         cmpb    R(d0),R(d1)
  51         beq     L(L9)
  52         tstb    R(d1)
  53         beq     L(L9)
  54         addql   #1,R(a0)
  55
  56         movel   R(a0),R(d1)
  57         andw    #3,R(d1)
  58         beq     L(L1)
  59         moveb   MEM(a0),R(d1)
  60         cmpb    R(d0),R(d1)
  61         beq     L(L9)
  62         tstb    R(d1)
  63         beq     L(L9)
  64         addql   #1,R(a0)
  65
  66         movel   R(a0),R(d1)
  67         andw    #3,R(d1)
  68         beq     L(L1)
  69         moveb   MEM(a0),R(d1)
  70         cmpb    R(d0),R(d1)
  71         beq     L(L9)
  72         tstb    R(d1)
  73         beq     L(L9)
  74         addql   #1,R(a0)
  75
  76 L(L1:)
  77         /* Load the magic bits.  Unlike the generic implementation we can
  78            use the carry bit as the fourth hole.  */
  79         movel   #0xfefefeff,R(d3)
  80
  81       /* We exit the loop if adding MAGIC_BITS to LONGWORD fails to
  82          change any of the hole bits of LONGWORD.
  83
  84          1) Is this safe?  Will it catch all the zero bytes?
  85          Suppose there is a byte with all zeros.  Any carry bits
  86          propagating from its left will fall into the hole at its
  87          least significant bit and stop.  Since there will be no
  88          carry from its most significant bit, the LSB of the
  89          byte to the left will be unchanged, and the zero will be
  90          detected.
  91
  92          2) Is this worthwhile?  Will it ignore everything except
  93          zero bytes?  Suppose every byte of LONGWORD has a bit set
  94          somewhere.  There will be a carry into bit 8.  If bit 8
  95          is set, this will carry into bit 16.  If bit 8 is clear,
  96          one of bits 9-15 must be set, so there will be a carry
  97          into bit 16.  Similarly, there will be a carry into bit
  98          24.  If one of bits 24-31 is set, there will be a carry
  99          into bit 32 (=carry flag), so all of the hole bits will
 100          be changed.
 101
 102          3) But wait!  Aren't we looking for C, not zero?
 103          Good point.  So what we do is XOR LONGWORD with a longword,
 104          each of whose bytes is C.  This turns each byte that is C
 105          into a zero.  */
 106
 107 L(L2:)
 108         /* Get the longword in question.  */
 109         movel   MEM_POSTINC(a0),R(d1)
 110         /* XOR with the byte we search for.  */
 111         eorl    R(d0),R(d1)
 112
 113         /* Add the magic value.  We get carry bits reported for each byte
 114            which is not C.  */
 115         movel   R(d3),R(d2)
 116         addl    R(d1),R(d2)
 117
 118         /* Check the fourth carry bit before it is clobbered by the next
 119            XOR.  If it is not set we have a hit.  */
 120         bcc     L(L8)
 121
 122         /* We are only interested in carry bits that change due to the
 123            previous add, so remove original bits.  */
 124         eorl    R(d1),R(d2)
 125
 126         /* Now test for the other three overflow bits.
 127            Set all non-carry bits.  */
 128         orl     R(d3),R(d2)
 129         /* Add 1 to get zero if all carry bits were set.  */
 130         addql   #1,R(d2)
 131
 132         /* If we don't get zero then at least one byte of the word equals
 133            C.  */
 134         bne     L(L8)
 135
 136         /* Next look for a NUL byte.
 137            Restore original longword without reload.  */
 138         eorl    R(d0),R(d1)
 139         /* Add the magic value.  We get carry bits reported for each byte
 140            which is not NUL.  */
 141         movel   R(d3),R(d2)
 142         addl    R(d1),R(d2)
 143
 144         /* Check the fourth carry bit before it is clobbered by the next
 145            XOR.  If it is not set we have a hit.  */
 146         bcc     L(L8)
 147
 148         /* We are only interested in carry bits that change due to the
 149            previous add, so remove original bits.  */
 150         eorl    R(d1),R(d2)
 151
 152         /* Now test for the other three overflow bits.
 153            Set all non-carry bits.  */
 154         orl     R(d3),R(d2)
 155         /* Add 1 to get zero if all carry bits were set.  */
 156         addql   #1,R(d2)
 157
 158         /* If we don't get zero then at least one byte of the word was
 159            NUL.  Otherwise continue with the next longword.  */
 160         bne     L(L8)
 161
 162         /* Get the longword in question.  */
 163         movel   MEM_POSTINC(a0),R(d1)
 164         /* XOR with the byte we search for.  */
 165         eorl    R(d0),R(d1)
 166
 167         /* Add the magic value.  We get carry bits reported for each byte
 168            which is not C.  */
 169         movel   R(d3),R(d2)
 170         addl    R(d1),R(d2)
 171
 172         /* Check the fourth carry bit before it is clobbered by the next
 173            XOR.  If it is not set we have a hit.  */
 174         bcc     L(L8)
 175
 176         /* We are only interested in carry bits that change due to the
 177            previous add, so remove original bits */
 178         eorl    R(d1),R(d2)
 179
 180         /* Now test for the other three overflow bits.
 181            Set all non-carry bits.  */
 182         orl     R(d3),R(d2)
 183         /* Add 1 to get zero if all carry bits were set.  */
 184         addql   #1,R(d2)
 185
 186         /* If we don't get zero then at least one byte of the word equals
 187            C.  */
 188         bne     L(L8)
 189
 190         /* Next look for a NUL byte.
 191            Restore original longword without reload.  */
 192         eorl    R(d0),R(d1)
 193         /* Add the magic value.  We get carry bits reported for each byte
 194            which is not NUL.  */
 195         movel   R(d3),R(d2)
 196         addl    R(d1),R(d2)
 197
 198         /* Check the fourth carry bit before it is clobbered by the next
 199            XOR.  If it is not set we have a hit.  */
 200         bcc     L(L8)
 201
 202         /* We are only interested in carry bits that change due to the
 203            previous add, so remove original bits */
 204         eorl    R(d1),R(d2)
 205
 206         /* Now test for the other three overflow bits.
 207            Set all non-carry bits.  */
 208         orl     R(d3),R(d2)
 209         /* Add 1 to get zero if all carry bits were set.  */
 210         addql   #1,R(d2)
 211
 212         /* If we don't get zero then at least one byte of the word was
 213            NUL.  Otherwise continue with the next longword.  */
 214         beq     L(L2)
 215
 216 L(L8:)
 217         /* We have a hit.  Check to see which byte it was.  First
 218            compensate for the autoincrement in the loop.  */
 219         subql   #4,R(a0)
 220
 221         moveb   MEM(a0),R(d1)
 222         cmpb    R(d0),R(d1)
 223         beq     L(L9)
 224         tstb    R(d1)
 225         beq     L(L9)
 226         addql   #1,R(a0)
 227
 228         moveb   MEM(a0),R(d1)
 229         cmpb    R(d0),R(d1)
 230         beq     L(L9)
 231         tstb    R(d1)
 232         beq     L(L9)
 233         addql   #1,R(a0)
 234
 235         moveb   MEM(a0),R(d1)
 236         cmpb    R(d0),R(d1)
 237         beq     L(L9)
 238         tstb    R(d1)
 239         beq     L(L9)
 240         addql   #1,R(a0)
 241
 242         /* Otherwise the fourth byte must equal C or be NUL.  */
 243 L(L9:)
 244         movel   R(a0),R(d0)
 245         movel   MEM_POSTINC(sp),R(d3)
 246         movel   MEM_POSTINC(sp),R(d2)
 247         rts
 248 END(__strchrnul)
 249
 250 weak_alias (__strchrnul, strchrnul)