sysdeps/i386/strchr.S

   1 /* strchr (str, ch) -- Return pointer to first occurrence of CH in STR.
   2    For Intel 80x86, x>=3.
   3    Copyright (C) 1994-2013 Free Software Foundation, Inc.
   4    This file is part of the GNU C Library.
   5    Contributed by Ulrich Drepper <drepper@gnu.ai.mit.edu>
   6    Some optimisations by Alan Modra <Alan@SPRI.Levels.UniSA.Edu.Au>
   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, see
  20    <http://www.gnu.org/licenses/>.  */
  21
  22 #include <sysdep.h>
  23 #include "asm-syntax.h"
  24 #include "bp-sym.h"
  25 #include "bp-asm.h"
  26
  27 #define PARMS   LINKAGE+4               /* space for 1 saved reg */
  28 #define RTN     PARMS
  29 #define STR     RTN+RTN_SIZE
  30 #define CHR     STR+PTR_SIZE
  31
  32         .text
  33 ENTRY (BP_SYM (strchr))
  34         ENTER
  35
  36         pushl %edi              /* Save callee-safe registers used here.  */
  37         cfi_adjust_cfa_offset (4)
  38         cfi_rel_offset (edi, 0)
  39         movl STR(%esp), %eax
  40         movl CHR(%esp), %edx
  41         CHECK_BOUNDS_LOW (%eax, STR(%esp))
  42
  43         /* At the moment %edx contains C.  What we need for the
  44            algorithm is C in all bytes of the dword.  Avoid
  45            operations on 16 bit words because these require an
  46            prefix byte (and one more cycle).  */
  47         movb %dl, %dh           /* now it is 0|0|c|c */
  48         movl %edx, %ecx
  49         shll $16, %edx          /* now it is c|c|0|0 */
  50         movw %cx, %dx           /* and finally c|c|c|c */
  51
  52         /* Before we start with the main loop we process single bytes
  53            until the source pointer is aligned.  This has two reasons:
  54            1. aligned 32-bit memory access is faster
  55            and (more important)
  56            2. we process in the main loop 32 bit in one step although
  57               we don't know the end of the string.  But accessing at
  58               4-byte alignment guarantees that we never access illegal
  59               memory if this would not also be done by the trivial
  60               implementation (this is because all processor inherent
  61               boundaries are multiples of 4.  */
  62
  63         testb $3, %al           /* correctly aligned ? */
  64         jz L(11)                /* yes => begin loop */
  65         movb (%eax), %cl        /* load byte in question (we need it twice) */
  66         cmpb %cl, %dl           /* compare byte */
  67         je L(6)                 /* target found => return */
  68         testb %cl, %cl          /* is NUL? */
  69         jz L(2)                 /* yes => return NULL */
  70         incl %eax               /* increment pointer */
  71
  72         testb $3, %al           /* correctly aligned ? */
  73         jz L(11)                /* yes => begin loop */
  74         movb (%eax), %cl        /* load byte in question (we need it twice) */
  75         cmpb %cl, %dl           /* compare byte */
  76         je L(6)                 /* target found => return */
  77         testb %cl, %cl          /* is NUL? */
  78         jz L(2)                 /* yes => return NULL */
  79         incl %eax               /* increment pointer */
  80
  81         testb $3, %al           /* correctly aligned ? */
  82         jz L(11)                /* yes => begin loop */
  83         movb (%eax), %cl        /* load byte in question (we need it twice) */
  84         cmpb %cl, %dl           /* compare byte */
  85         je L(6)                 /* target found => return */
  86         testb %cl, %cl          /* is NUL? */
  87         jz L(2)                 /* yes => return NULL */
  88         incl %eax               /* increment pointer */
  89
  90         /* No we have reached alignment.  */
  91         jmp L(11)               /* begin loop */
  92
  93       /* We exit the loop if adding MAGIC_BITS to LONGWORD fails to
  94          change any of the hole bits of LONGWORD.
  95
  96          1) Is this safe?  Will it catch all the zero bytes?
  97          Suppose there is a byte with all zeros.  Any carry bits
  98          propagating from its left will fall into the hole at its
  99          least significant bit and stop.  Since there will be no
 100          carry from its most significant bit, the LSB of the
 101          byte to the left will be unchanged, and the zero will be
 102          detected.
 103
 104          2) Is this worthwhile?  Will it ignore everything except
 105          zero bytes?  Suppose every byte of LONGWORD has a bit set
 106          somewhere.  There will be a carry into bit 8.  If bit 8
 107          is set, this will carry into bit 16.  If bit 8 is clear,
 108          one of bits 9-15 must be set, so there will be a carry
 109          into bit 16.  Similarly, there will be a carry into bit
 110          24.  If one of bits 24-31 is set, there will be a carry
 111          into bit 32 (=carry flag), so all of the hole bits will
 112          be changed.
 113
 114          3) But wait!  Aren't we looking for C, not zero?
 115          Good point.  So what we do is XOR LONGWORD with a longword,
 116          each of whose bytes is C.  This turns each byte that is C
 117          into a zero.  */
 118
 119         /* Each round the main loop processes 16 bytes.  */
 120
 121         ALIGN(4)
 122
 123 L(1):   addl $16, %eax          /* adjust pointer for whole round */
 124
 125 L(11):  movl (%eax), %ecx       /* get word (= 4 bytes) in question */
 126         xorl %edx, %ecx         /* XOR with word c|c|c|c => bytes of str == c
 127                                    are now 0 */
 128         movl $0xfefefeff, %edi  /* magic value */
 129         addl %ecx, %edi         /* add the magic value to the word.  We get
 130                                    carry bits reported for each byte which
 131                                    is *not* C */
 132
 133         /* According to the algorithm we had to reverse the effect of the
 134            XOR first and then test the overflow bits.  But because the
 135            following XOR would destroy the carry flag and it would (in a
 136            representation with more than 32 bits) not alter then last
 137            overflow, we can now test this condition.  If no carry is signaled
 138            no overflow must have occurred in the last byte => it was 0. */
 139         jnc L(7)
 140
 141         /* We are only interested in carry bits that change due to the
 142            previous add, so remove original bits */
 143         xorl %ecx, %edi         /* ((word^charmask)+magic)^(word^charmask) */
 144
 145         /* Now test for the other three overflow bits.  */
 146         orl $0xfefefeff, %edi   /* set all non-carry bits */
 147         incl %edi               /* add 1: if one carry bit was *not* set
 148                                    the addition will not result in 0.  */
 149
 150         /* If at least one byte of the word is C we don't get 0 in %edi.  */
 151         jnz L(7)                /* found it => return pointer */
 152
 153         /* Now we made sure the dword does not contain the character we are
 154            looking for.  But because we deal with strings we have to check
 155            for the end of string before testing the next dword.  */
 156
 157         xorl %edx, %ecx         /* restore original dword without reload */
 158         movl $0xfefefeff, %edi  /* magic value */
 159         addl %ecx, %edi         /* add the magic value to the word.  We get
 160                                    carry bits reported for each byte which
 161                                    is *not* 0 */
 162         jnc L(2)                /* highest byte is NUL => return NULL */
 163         xorl %ecx, %edi         /* (word+magic)^word */
 164         orl $0xfefefeff, %edi   /* set all non-carry bits */
 165         incl %edi               /* add 1: if one carry bit was *not* set
 166                                    the addition will not result in 0.  */
 167         jnz L(2)                /* found NUL => return NULL */
 168
 169         movl 4(%eax), %ecx      /* get word (= 4 bytes) in question */
 170         xorl %edx, %ecx         /* XOR with word c|c|c|c => bytes of str == c
 171                                    are now 0 */
 172         movl $0xfefefeff, %edi  /* magic value */
 173         addl %ecx, %edi         /* add the magic value to the word.  We get
 174                                    carry bits reported for each byte which
 175                                    is *not* C */
 176         jnc L(71)               /* highest byte is C => return pointer */
 177         xorl %ecx, %edi         /* ((word^charmask)+magic)^(word^charmask) */
 178         orl $0xfefefeff, %edi   /* set all non-carry bits */
 179         incl %edi               /* add 1: if one carry bit was *not* set
 180                                    the addition will not result in 0.  */
 181         jnz L(71)               /* found it => return pointer */
 182         xorl %edx, %ecx         /* restore original dword without reload */
 183         movl $0xfefefeff, %edi  /* magic value */
 184         addl %ecx, %edi         /* add the magic value to the word.  We get
 185                                    carry bits reported for each byte which
 186                                    is *not* 0 */
 187         jnc L(2)                /* highest byte is NUL => return NULL */
 188         xorl %ecx, %edi         /* (word+magic)^word */
 189         orl $0xfefefeff, %edi   /* set all non-carry bits */
 190         incl %edi               /* add 1: if one carry bit was *not* set
 191                                    the addition will not result in 0.  */
 192         jnz L(2)                /* found NUL => return NULL */
 193
 194         movl 8(%eax), %ecx      /* get word (= 4 bytes) in question */
 195         xorl %edx, %ecx         /* XOR with word c|c|c|c => bytes of str == c
 196                                    are now 0 */
 197         movl $0xfefefeff, %edi  /* magic value */
 198         addl %ecx, %edi         /* add the magic value to the word.  We get
 199                                    carry bits reported for each byte which
 200                                    is *not* C */
 201         jnc L(72)               /* highest byte is C => return pointer */
 202         xorl %ecx, %edi         /* ((word^charmask)+magic)^(word^charmask) */
 203         orl $0xfefefeff, %edi   /* set all non-carry bits */
 204         incl %edi               /* add 1: if one carry bit was *not* set
 205                                    the addition will not result in 0.  */
 206         jnz L(72)               /* found it => return pointer */
 207         xorl %edx, %ecx         /* restore original dword without reload */
 208         movl $0xfefefeff, %edi  /* magic value */
 209         addl %ecx, %edi         /* add the magic value to the word.  We get
 210                                    carry bits reported for each byte which
 211                                    is *not* 0 */
 212         jnc L(2)                /* highest byte is NUL => return NULL */
 213         xorl %ecx, %edi         /* (word+magic)^word */
 214         orl $0xfefefeff, %edi   /* set all non-carry bits */
 215         incl %edi               /* add 1: if one carry bit was *not* set
 216                                    the addition will not result in 0.  */
 217         jnz L(2)                /* found NUL => return NULL */
 218
 219         movl 12(%eax), %ecx     /* get word (= 4 bytes) in question */
 220         xorl %edx, %ecx         /* XOR with word c|c|c|c => bytes of str == c
 221                                    are now 0 */
 222         movl $0xfefefeff, %edi  /* magic value */
 223         addl %ecx, %edi         /* add the magic value to the word.  We get
 224                                    carry bits reported for each byte which
 225                                    is *not* C */
 226         jnc L(73)               /* highest byte is C => return pointer */
 227         xorl %ecx, %edi         /* ((word^charmask)+magic)^(word^charmask) */
 228         orl $0xfefefeff, %edi   /* set all non-carry bits */
 229         incl %edi               /* add 1: if one carry bit was *not* set
 230                                    the addition will not result in 0.  */
 231         jnz L(73)               /* found it => return pointer */
 232         xorl %edx, %ecx         /* restore original dword without reload */
 233         movl $0xfefefeff, %edi  /* magic value */
 234         addl %ecx, %edi         /* add the magic value to the word.  We get
 235                                    carry bits reported for each byte which
 236                                    is *not* 0 */
 237         jnc L(2)                /* highest byte is NUL => return NULL */
 238         xorl %ecx, %edi         /* (word+magic)^word */
 239         orl $0xfefefeff, %edi   /* set all non-carry bits */
 240         incl %edi               /* add 1: if one carry bit was *not* set
 241                                    the addition will not result in 0.  */
 242         jz L(1)                 /* no NUL found => restart loop */
 243
 244 L(2):   /* Return NULL.  */
 245         xorl %eax, %eax
 246         RETURN_NULL_BOUNDED_POINTER
 247         popl %edi               /* restore saved register content */
 248         cfi_adjust_cfa_offset (-4)
 249         cfi_restore (edi)
 250
 251         LEAVE
 252         RET_PTR
 253
 254         cfi_adjust_cfa_offset (4)
 255         cfi_rel_offset (edi, 0)
 256 L(73):  addl $4, %eax           /* adjust pointer */
 257 L(72):  addl $4, %eax
 258 L(71):  addl $4, %eax
 259
 260         /* We now scan for the byte in which the character was matched.
 261            But we have to take care of the case that a NUL char is
 262            found before this in the dword.  Note that we XORed %ecx
 263            with the byte we're looking for, therefore the tests below look
 264            reversed.  */
 265
 266 L(7):   testb %cl, %cl          /* is first byte C? */
 267         jz L(6)                 /* yes => return pointer */
 268         cmpb %dl, %cl           /* is first byte NUL? */
 269         je L(2)                 /* yes => return NULL */
 270         incl %eax               /* it's not in the first byte */
 271
 272         testb %ch, %ch          /* is second byte C? */
 273         jz L(6)                 /* yes => return pointer */
 274         cmpb %dl, %ch           /* is second byte NUL? */
 275         je L(2)                 /* yes => return NULL? */
 276         incl %eax               /* it's not in the second byte */
 277
 278         shrl $16, %ecx          /* make upper byte accessible */
 279         testb %cl, %cl          /* is third byte C? */
 280         jz L(6)                 /* yes => return pointer */
 281         cmpb %dl, %cl           /* is third byte NUL? */
 282         je L(2)                 /* yes => return NULL */
 283
 284         /* It must be in the fourth byte and it cannot be NUL.  */
 285         incl %eax
 286
 287 L(6):
 288         CHECK_BOUNDS_HIGH (%eax, STR(%esp), jb)
 289         RETURN_BOUNDED_POINTER (STR(%esp))
 290         popl %edi               /* restore saved register content */
 291         cfi_adjust_cfa_offset (-4)
 292         cfi_restore (edi)
 293
 294         LEAVE
 295         RET_PTR
 296 END (BP_SYM (strchr))
 297
 298 weak_alias (BP_SYM (strchr), BP_SYM (index))
 299 libc_hidden_builtin_def (strchr)