sysdeps/i386/strchrnul.S

   1 /* strchrnul (str, chr) -- Return pointer to first occurrence of CHR in STR
   2    or the final NUL byte.
   3    For Intel 80x86, x>=3.
   4    Copyright (C) 1994-2020 Free Software Foundation, Inc.
   5    This file is part of the GNU C Library.
   6    Contributed by Ulrich Drepper <drepper@gnu.org>
   7    Some optimisations by Alan Modra <Alan@SPRI.Levels.UniSA.Edu.Au>
   8
   9    The GNU C Library is free software; you can redistribute it and/or
  10    modify it under the terms of the GNU Lesser General Public
  11    License as published by the Free Software Foundation; either
  12    version 2.1 of the License, or (at your option) any later version.
  13
  14    The GNU C Library is distributed in the hope that it will be useful,
  15    but WITHOUT ANY WARRANTY; without even the implied warranty of
  16    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  17    Lesser General Public License for more details.
  18
  19    You should have received a copy of the GNU Lesser General Public
  20    License along with the GNU C Library; if not, see
  21    <https://www.gnu.org/licenses/>.  */
  22
  23 #include <sysdep.h>
  24 #include "asm-syntax.h"
  25
  26 #define PARMS   4+4     /* space for 1 saved reg */
  27 #define RTN     PARMS
  28 #define STR     RTN
  29 #define CHR     STR+4
  30
  31         .text
  32 ENTRY (__strchrnul)
  33
  34         pushl %edi              /* Save callee-safe registers used here.  */
  35         cfi_adjust_cfa_offset (4)
  36         cfi_rel_offset (edi, 0)
  37
  38         movl STR(%esp), %eax
  39         movl CHR(%esp), %edx
  40
  41         /* At the moment %edx contains CHR.  What we need for the
  42            algorithm is CHR in all bytes of the dword.  Avoid
  43            operations on 16 bit words because these require an
  44            prefix byte (and one more cycle).  */
  45         movb %dl, %dh           /* now it is 0|0|c|c */
  46         movl %edx, %ecx
  47         shll $16, %edx          /* now it is c|c|0|0 */
  48         movw %cx, %dx           /* and finally c|c|c|c */
  49
  50         /* Before we start with the main loop we process single bytes
  51            until the source pointer is aligned.  This has two reasons:
  52            1. aligned 32-bit memory access is faster
  53            and (more important)
  54            2. we process in the main loop 32 bit in one step although
  55               we don't know the end of the string.  But accessing at
  56               4-byte alignment guarantees that we never access illegal
  57               memory if this would not also be done by the trivial
  58               implementation (this is because all processor inherent
  59               boundaries are multiples of 4.  */
  60
  61         testb $3, %al           /* correctly aligned ? */
  62         jz L(11)                /* yes => begin loop */
  63         movb (%eax), %cl        /* load byte in question (we need it twice) */
  64         cmpb %cl, %dl           /* compare byte */
  65         je L(6)                 /* target found => return */
  66         testb %cl, %cl          /* is NUL? */
  67         jz L(6)                 /* yes => return NULL */
  68         incl %eax               /* increment pointer */
  69
  70         testb $3, %al           /* correctly aligned ? */
  71         jz L(11)                /* yes => begin loop */
  72         movb (%eax), %cl        /* load byte in question (we need it twice) */
  73         cmpb %cl, %dl           /* compare byte */
  74         je L(6)                 /* target found => return */
  75         testb %cl, %cl          /* is NUL? */
  76         jz L(6)                 /* yes => return NULL */
  77         incl %eax               /* increment pointer */
  78
  79         testb $3, %al           /* correctly aligned ? */
  80         jz L(11)                /* yes => begin loop */
  81         movb (%eax), %cl        /* load byte in question (we need it twice) */
  82         cmpb %cl, %dl           /* compare byte */
  83         je L(6)                 /* target found => return */
  84         testb %cl, %cl          /* is NUL? */
  85         jz L(6)                 /* yes => return NULL */
  86         incl %eax               /* increment pointer */
  87
  88         /* No we have reached alignment.  */
  89         jmp L(11)               /* begin loop */
  90
  91       /* We exit the loop if adding MAGIC_BITS to LONGWORD fails to
  92          change any of the hole bits of LONGWORD.
  93
  94          1) Is this safe?  Will it catch all the zero bytes?
  95          Suppose there is a byte with all zeros.  Any carry bits
  96          propagating from its left will fall into the hole at its
  97          least significant bit and stop.  Since there will be no
  98          carry from its most significant bit, the LSB of the
  99          byte to the left will be unchanged, and the zero will be
 100          detected.
 101
 102          2) Is this worthwhile?  Will it ignore everything except
 103          zero bytes?  Suppose every byte of LONGWORD has a bit set
 104          somewhere.  There will be a carry into bit 8.  If bit 8
 105          is set, this will carry into bit 16.  If bit 8 is clear,
 106          one of bits 9-15 must be set, so there will be a carry
 107          into bit 16.  Similarly, there will be a carry into bit
 108          24.  If one of bits 24-31 is set, there will be a carry
 109          into bit 32 (=carry flag), so all of the hole bits will
 110          be changed.
 111
 112          3) But wait!  Aren't we looking for CHR, not zero?
 113          Good point.  So what we do is XOR LONGWORD with a longword,
 114          each of whose bytes is CHR.  This turns each byte that is CHR
 115          into a zero.  */
 116
 117         /* Each round the main loop processes 16 bytes.  */
 118
 119         ALIGN(4)
 120
 121 L(1):   addl $16, %eax          /* adjust pointer for whole round */
 122
 123 L(11):  movl (%eax), %ecx       /* get word (= 4 bytes) in question */
 124         xorl %edx, %ecx         /* XOR with word c|c|c|c => bytes of str == c
 125                                    are now 0 */
 126         movl $0xfefefeff, %edi  /* magic value */
 127         addl %ecx, %edi         /* add the magic value to the word.  We get
 128                                    carry bits reported for each byte which
 129                                    is *not* CHR */
 130
 131         /* According to the algorithm we had to reverse the effect of the
 132            XOR first and then test the overflow bits.  But because the
 133            following XOR would destroy the carry flag and it would (in a
 134            representation with more than 32 bits) not alter then last
 135            overflow, we can now test this condition.  If no carry is signaled
 136            no overflow must have occurred in the last byte => it was 0. */
 137         jnc L(7)
 138
 139         /* We are only interested in carry bits that change due to the
 140            previous add, so remove original bits */
 141         xorl %ecx, %edi         /* ((word^charmask)+magic)^(word^charmask) */
 142
 143         /* Now test for the other three overflow bits.  */
 144         orl $0xfefefeff, %edi   /* set all non-carry bits */
 145         incl %edi               /* add 1: if one carry bit was *not* set
 146                                    the addition will not result in 0.  */
 147
 148         /* If at least one byte of the word is CHR we don't get 0 in %edi.  */
 149         jnz L(7)                /* found it => return pointer */
 150
 151         /* Now we made sure the dword does not contain the character we are
 152            looking for.  But because we deal with strings we have to check
 153            for the end of string before testing the next dword.  */
 154
 155         xorl %edx, %ecx         /* restore original dword without reload */
 156         movl $0xfefefeff, %edi  /* magic value */
 157         addl %ecx, %edi         /* add the magic value to the word.  We get
 158                                    carry bits reported for each byte which
 159                                    is *not* 0 */
 160         jnc L(7)                /* highest byte is NUL => return NULL */
 161         xorl %ecx, %edi         /* (word+magic)^word */
 162         orl $0xfefefeff, %edi   /* set all non-carry bits */
 163         incl %edi               /* add 1: if one carry bit was *not* set
 164                                    the addition will not result in 0.  */
 165         jnz L(7)                /* found NUL => return NULL */
 166
 167         movl 4(%eax), %ecx      /* get word (= 4 bytes) in question */
 168         xorl %edx, %ecx         /* XOR with word c|c|c|c => bytes of str == c
 169                                    are now 0 */
 170         movl $0xfefefeff, %edi  /* magic value */
 171         addl %ecx, %edi         /* add the magic value to the word.  We get
 172                                    carry bits reported for each byte which
 173                                    is *not* CHR */
 174         jnc L(71)               /* highest byte is CHR => return pointer */
 175         xorl %ecx, %edi         /* ((word^charmask)+magic)^(word^charmask) */
 176         orl $0xfefefeff, %edi   /* set all non-carry bits */
 177         incl %edi               /* add 1: if one carry bit was *not* set
 178                                    the addition will not result in 0.  */
 179         jnz L(71)               /* found it => return pointer */
 180         xorl %edx, %ecx         /* restore original dword without reload */
 181         movl $0xfefefeff, %edi  /* magic value */
 182         addl %ecx, %edi         /* add the magic value to the word.  We get
 183                                    carry bits reported for each byte which
 184                                    is *not* 0 */
 185         jnc L(71)               /* highest byte is NUL => return NULL */
 186         xorl %ecx, %edi         /* (word+magic)^word */
 187         orl $0xfefefeff, %edi   /* set all non-carry bits */
 188         incl %edi               /* add 1: if one carry bit was *not* set
 189                                    the addition will not result in 0.  */
 190         jnz L(71)               /* found NUL => return NULL */
 191
 192         movl 8(%eax), %ecx      /* get word (= 4 bytes) in question */
 193         xorl %edx, %ecx         /* XOR with word c|c|c|c => bytes of str == c
 194                                    are now 0 */
 195         movl $0xfefefeff, %edi  /* magic value */
 196         addl %ecx, %edi         /* add the magic value to the word.  We get
 197                                    carry bits reported for each byte which
 198                                    is *not* CHR */
 199         jnc L(72)               /* highest byte is CHR => return pointer */
 200         xorl %ecx, %edi         /* ((word^charmask)+magic)^(word^charmask) */
 201         orl $0xfefefeff, %edi   /* set all non-carry bits */
 202         incl %edi               /* add 1: if one carry bit was *not* set
 203                                    the addition will not result in 0.  */
 204         jnz L(72)               /* found it => return pointer */
 205         xorl %edx, %ecx         /* restore original dword without reload */
 206         movl $0xfefefeff, %edi  /* magic value */
 207         addl %ecx, %edi         /* add the magic value to the word.  We get
 208                                    carry bits reported for each byte which
 209                                    is *not* 0 */
 210         jnc L(72)               /* highest byte is NUL => return NULL */
 211         xorl %ecx, %edi         /* (word+magic)^word */
 212         orl $0xfefefeff, %edi   /* set all non-carry bits */
 213         incl %edi               /* add 1: if one carry bit was *not* set
 214                                    the addition will not result in 0.  */
 215         jnz L(72)               /* found NUL => return NULL */
 216
 217         movl 12(%eax), %ecx     /* get word (= 4 bytes) in question */
 218         xorl %edx, %ecx         /* XOR with word c|c|c|c => bytes of str == c
 219                                    are now 0 */
 220         movl $0xfefefeff, %edi  /* magic value */
 221         addl %ecx, %edi         /* add the magic value to the word.  We get
 222                                    carry bits reported for each byte which
 223                                    is *not* CHR */
 224         jnc L(73)               /* highest byte is CHR => return pointer */
 225         xorl %ecx, %edi         /* ((word^charmask)+magic)^(word^charmask) */
 226         orl $0xfefefeff, %edi   /* set all non-carry bits */
 227         incl %edi               /* add 1: if one carry bit was *not* set
 228                                    the addition will not result in 0.  */
 229         jnz L(73)               /* found it => return pointer */
 230         xorl %edx, %ecx         /* restore original dword without reload */
 231         movl $0xfefefeff, %edi  /* magic value */
 232         addl %ecx, %edi         /* add the magic value to the word.  We get
 233                                    carry bits reported for each byte which
 234                                    is *not* 0 */
 235         jnc L(73)               /* highest byte is NUL => return NULL */
 236         xorl %ecx, %edi         /* (word+magic)^word */
 237         orl $0xfefefeff, %edi   /* set all non-carry bits */
 238         incl %edi               /* add 1: if one carry bit was *not* set
 239                                    the addition will not result in 0.  */
 240         jz L(1)                 /* no NUL found => restart loop */
 241
 242 L(73):  addl $4, %eax           /* adjust pointer */
 243 L(72):  addl $4, %eax
 244 L(71):  addl $4, %eax
 245
 246         /* We now scan for the byte in which the character was matched.
 247            But we have to take care of the case that a NUL char is
 248            found before this in the dword.  */
 249
 250 L(7):   testb %cl, %cl          /* is first byte CHR? */
 251         jz L(6)                 /* yes => return pointer */
 252         cmpb %dl, %cl           /* is first byte NUL? */
 253         je L(6)                 /* yes => return NULL */
 254         incl %eax               /* it's not in the first byte */
 255
 256         testb %ch, %ch          /* is second byte CHR? */
 257         jz L(6)                 /* yes => return pointer */
 258         cmpb %dl, %ch           /* is second byte NUL? */
 259         je L(6)                 /* yes => return NULL? */
 260         incl %eax               /* it's not in the second byte */
 261
 262         shrl $16, %ecx          /* make upper byte accessible */
 263         testb %cl, %cl          /* is third byte CHR? */
 264         jz L(6)                 /* yes => return pointer */
 265         cmpb %dl, %cl           /* is third byte NUL? */
 266         je L(6)                 /* yes => return NULL */
 267
 268         /* It must be in the fourth byte and it cannot be NUL.  */
 269         incl %eax
 270
 271 L(6):   popl %edi               /* restore saved register content */
 272         cfi_adjust_cfa_offset (-4)
 273         cfi_restore (edi)
 274
 275         ret
 276 END (__strchrnul)
 277
 278 weak_alias (__strchrnul, strchrnul)