sysdeps/i386/strrchr.S

   1 /* strrchr (str, ch) -- Return pointer to last occurrence of CH in STR.
   2    For Intel 80x86, x>=3.
   3    Copyright (C) 1994-2019 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
  25 #define PARMS   4+8     /* space for 2 saved regs */
  26 #define RTN     PARMS
  27 #define STR     RTN
  28 #define CHR     STR+4
  29
  30         .text
  31 ENTRY (strrchr)
  32
  33         pushl %edi              /* Save callee-safe registers used here.  */
  34         cfi_adjust_cfa_offset (4)
  35         cfi_rel_offset (edi, 0)
  36         pushl %esi
  37         cfi_adjust_cfa_offset (4)
  38
  39         xorl %eax, %eax
  40         movl STR(%esp), %esi
  41         cfi_rel_offset (esi, 0)
  42         movl CHR(%esp), %ecx
  43
  44         /* At the moment %ecx contains C.  What we need for the
  45            algorithm is C in all bytes of the dword.  Avoid
  46            operations on 16 bit words because these require an
  47            prefix byte (and one more cycle).  */
  48         movb %cl, %ch           /* now it is 0|0|c|c */
  49         movl %ecx, %edx
  50         shll $16, %ecx          /* now it is c|c|0|0 */
  51         movw %dx, %cx           /* and finally c|c|c|c */
  52
  53         /* Before we start with the main loop we process single bytes
  54            until the source pointer is aligned.  This has two reasons:
  55            1. aligned 32-bit memory access is faster
  56            and (more important)
  57            2. we process in the main loop 32 bit in one step although
  58               we don't know the end of the string.  But accessing at
  59               4-byte alignment guarantees that we never access illegal
  60               memory if this would not also be done by the trivial
  61               implementation (this is because all processor inherent
  62               boundaries are multiples of 4.  */
  63
  64         testl $3, %esi          /* correctly aligned ? */
  65         jz L(19)                /* yes => begin loop */
  66         movb (%esi), %dl        /* load byte in question (we need it twice) */
  67         cmpb %dl, %cl           /* compare byte */
  68         jne L(11)                       /* target found => return */
  69         movl %esi, %eax         /* remember pointer as possible result */
  70 L(11):  orb %dl, %dl            /* is NUL? */
  71         jz L(2)                 /* yes => return NULL */
  72         incl %esi               /* increment pointer */
  73
  74         testl $3, %esi          /* correctly aligned ? */
  75         jz L(19)                /* yes => begin loop */
  76         movb (%esi), %dl        /* load byte in question (we need it twice) */
  77         cmpb %dl, %cl           /* compare byte */
  78         jne L(12)                       /* target found => return */
  79         movl %esi, %eax         /* remember pointer as result */
  80 L(12):  orb %dl, %dl            /* is NUL? */
  81         jz L(2)                 /* yes => return NULL */
  82         incl %esi               /* increment pointer */
  83
  84         testl $3, %esi          /* correctly aligned ? */
  85         jz L(19)                /* yes => begin loop */
  86         movb (%esi), %dl        /* load byte in question (we need it twice) */
  87         cmpb %dl, %cl           /* compare byte */
  88         jne L(13)                       /* target found => return */
  89         movl %esi, %eax         /* remember pointer as result */
  90 L(13):  orb %dl, %dl            /* is NUL? */
  91         jz L(2)                 /* yes => return NULL */
  92         incl %esi               /* increment pointer */
  93
  94         /* No we have reached alignment.  */
  95         jmp L(19)               /* begin loop */
  96
  97       /* We exit the loop if adding MAGIC_BITS to LONGWORD fails to
  98          change any of the hole bits of LONGWORD.
  99
 100          1) Is this safe?  Will it catch all the zero bytes?
 101          Suppose there is a byte with all zeros.  Any carry bits
 102          propagating from its left will fall into the hole at its
 103          least significant bit and stop.  Since there will be no
 104          carry from its most significant bit, the LSB of the
 105          byte to the left will be unchanged, and the zero will be
 106          detected.
 107
 108          2) Is this worthwhile?  Will it ignore everything except
 109          zero bytes?  Suppose every byte of LONGWORD has a bit set
 110          somewhere.  There will be a carry into bit 8.  If bit 8
 111          is set, this will carry into bit 16.  If bit 8 is clear,
 112          one of bits 9-15 must be set, so there will be a carry
 113          into bit 16.  Similarly, there will be a carry into bit
 114          24.  If one of bits 24-31 is set, there will be a carry
 115          into bit 32 (=carry flag), so all of the hole bits will
 116          be changed.
 117
 118          3) But wait!  Aren't we looking for C, not zero?
 119          Good point.  So what we do is XOR LONGWORD with a longword,
 120          each of whose bytes is C.  This turns each byte that is C
 121          into a zero.  */
 122
 123         /* Each round the main loop processes 16 bytes.  */
 124
 125         /* Jump to here when the character is detected.  We chose this
 126            way around because the character one is looking for is not
 127            as frequent as the rest and taking a conditional jump is more
 128            expensive than ignoring it.
 129
 130            Some more words to the code below: it might not be obvious why
 131            we decrement the source pointer here.  In the loop the pointer
 132            is not pre-incremented and so it still points before the word
 133            we are looking at.  But you should take a look at the instruction
 134            which gets executed before we get into the loop: `addl $16, %esi'.
 135            This makes the following subs into adds.  */
 136
 137         /* These fill bytes make the main loop be correctly aligned.
 138            We cannot use align because it is not the following instruction
 139            which should be aligned.  */
 140         .byte 0, 0
 141 #ifndef PROF
 142         /* Profiling adds some code and so changes the alignment.  */
 143         .byte 0
 144 #endif
 145
 146 L(4):   subl $4, %esi           /* adjust pointer */
 147 L(41):  subl $4, %esi
 148 L(42):  subl $4, %esi
 149 L(43):  testl $0xff000000, %edx /* is highest byte == C? */
 150         jnz L(33)               /* no => try other bytes */
 151         leal 15(%esi), %eax     /* store address as result */
 152         jmp L(1)                /* and start loop again */
 153
 154 L(3):   subl $4, %esi           /* adjust pointer */
 155 L(31):  subl $4, %esi
 156 L(32):  subl $4, %esi
 157 L(33):  testl $0xff0000, %edx   /* is C in third byte? */
 158         jnz L(51)               /* no => try other bytes */
 159         leal 14(%esi), %eax     /* store address as result */
 160         jmp L(1)                /* and start loop again */
 161
 162 L(51):
 163         /* At this point we know that the byte is in one of the lower bytes.
 164            We make a guess and correct it if necessary.  This reduces the
 165            number of necessary jumps.  */
 166         leal 12(%esi), %eax     /* guess address of lowest byte as result */
 167         testb %dh, %dh          /* is guess correct? */
 168         jnz L(1)                /* yes => start loop */
 169         leal 13(%esi), %eax     /* correct guess to second byte */
 170
 171 L(1):   addl $16, %esi          /* increment pointer for full round */
 172
 173 L(19):  movl (%esi), %edx       /* get word (= 4 bytes) in question */
 174         movl $0xfefefeff, %edi  /* magic value */
 175         addl %edx, %edi         /* add the magic value to the word.  We get
 176                                    carry bits reported for each byte which
 177                                    is *not* 0 */
 178
 179         /* According to the algorithm we had to reverse the effect of the
 180            XOR first and then test the overflow bits.  But because the
 181            following XOR would destroy the carry flag and it would (in a
 182            representation with more than 32 bits) not alter then last
 183            overflow, we can now test this condition.  If no carry is signaled
 184            no overflow must have occurred in the last byte => it was 0. */
 185
 186         jnc L(20)                       /* found NUL => check last word */
 187
 188         /* We are only interested in carry bits that change due to the
 189            previous add, so remove original bits */
 190         xorl %edx, %edi         /* (word+magic)^word */
 191
 192         /* Now test for the other three overflow bits.  */
 193         orl $0xfefefeff, %edi   /* set all non-carry bits */
 194         incl %edi               /* add 1: if one carry bit was *not* set
 195                                    the addition will not result in 0.  */
 196
 197         /* If at least one byte of the word is C we don't get 0 in %edi.  */
 198         jnz L(20)                       /* found NUL => check last word */
 199
 200         /* Now we made sure the dword does not contain the character we are
 201            looking for.  But because we deal with strings we have to check
 202            for the end of string before testing the next dword.  */
 203
 204         xorl %ecx, %edx         /* XOR with word c|c|c|c => bytes of str == c
 205                                    are now 0 */
 206         movl $0xfefefeff, %edi  /* magic value */
 207         addl %edx, %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(4)                /* highest byte is C => examine dword */
 211         xorl %edx, %edi         /* ((word^charmask)+magic)^(word^charmask) */
 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(3)                /* C is detected in the word => examine it */
 216
 217         movl 4(%esi), %edx      /* get word (= 4 bytes) in question */
 218         movl $0xfefefeff, %edi  /* magic value */
 219         addl %edx, %edi         /* add the magic value to the word.  We get
 220                                    carry bits reported for each byte which
 221                                    is *not* 0 */
 222         jnc L(21)               /* found NUL => check last word */
 223         xorl %edx, %edi         /* (word+magic)^word */
 224         orl $0xfefefeff, %edi   /* set all non-carry bits */
 225         incl %edi               /* add 1: if one carry bit was *not* set
 226                                    the addition will not result in 0.  */
 227         jnz L(21)               /* found NUL => check last word */
 228         xorl %ecx, %edx         /* XOR with word c|c|c|c => bytes of str == c
 229                                    are now 0 */
 230         movl $0xfefefeff, %edi  /* magic value */
 231         addl %edx, %edi         /* add the magic value to the word.  We get
 232                                    carry bits reported for each byte which
 233                                    is *not* 0 */
 234         jnc L(41)               /* highest byte is C => examine dword */
 235         xorl %edx, %edi         /* ((word^charmask)+magic)^(word^charmask) */
 236         orl $0xfefefeff, %edi   /* set all non-carry bits */
 237         incl %edi               /* add 1: if one carry bit was *not* set
 238                                    the addition will not result in 0.  */
 239         jnz L(31)               /* C is detected in the word => examine it */
 240
 241         movl 8(%esi), %edx      /* get word (= 4 bytes) in question */
 242         movl $0xfefefeff, %edi  /* magic value */
 243         addl %edx, %edi         /* add the magic value to the word.  We get
 244                                    carry bits reported for each byte which
 245                                    is *not* 0 */
 246         jnc L(22)               /* found NUL => check last word */
 247         xorl %edx, %edi         /* (word+magic)^word */
 248         orl $0xfefefeff, %edi   /* set all non-carry bits */
 249         incl %edi               /* add 1: if one carry bit was *not* set
 250                                    the addition will not result in 0.  */
 251         jnz L(22)               /* found NUL => check last word */
 252         xorl %ecx, %edx         /* XOR with word c|c|c|c => bytes of str == c
 253                                    are now 0 */
 254         movl $0xfefefeff, %edi  /* magic value */
 255         addl %edx, %edi         /* add the magic value to the word.  We get
 256                                    carry bits reported for each byte which
 257                                    is *not* 0 */
 258         jnc L(42)               /* highest byte is C => examine dword */
 259         xorl %edx, %edi         /* ((word^charmask)+magic)^(word^charmask) */
 260         orl $0xfefefeff, %edi   /* set all non-carry bits */
 261         incl %edi               /* add 1: if one carry bit was *not* set
 262                                    the addition will not result in 0.  */
 263         jnz L(32)               /* C is detected in the word => examine it */
 264
 265         movl 12(%esi), %edx     /* get word (= 4 bytes) in question */
 266         movl $0xfefefeff, %edi  /* magic value */
 267         addl %edx, %edi         /* add the magic value to the word.  We get
 268                                    carry bits reported for each byte which
 269                                    is *not* 0 */
 270         jnc L(23)               /* found NUL => check last word */
 271         xorl %edx, %edi         /* (word+magic)^word */
 272         orl $0xfefefeff, %edi   /* set all non-carry bits */
 273         incl %edi               /* add 1: if one carry bit was *not* set
 274                                    the addition will not result in 0.  */
 275         jnz L(23)               /* found NUL => check last word */
 276         xorl %ecx, %edx         /* XOR with word c|c|c|c => bytes of str == c
 277                                    are now 0 */
 278         movl $0xfefefeff, %edi  /* magic value */
 279         addl %edx, %edi         /* add the magic value to the word.  We get
 280                                    carry bits reported for each byte which
 281                                    is *not* 0 */
 282         jnc L(43)               /* highest byte is C => examine dword */
 283         xorl %edx, %edi         /* ((word^charmask)+magic)^(word^charmask) */
 284         orl $0xfefefeff, %edi   /* set all non-carry bits */
 285         incl %edi               /* add 1: if one carry bit was *not* set
 286                                    the addition will not result in 0.  */
 287         jz L(1)                 /* C is not detected => restart loop */
 288         jmp L(33)               /* examine word */
 289
 290 L(23):  addl $4, %esi           /* adjust pointer */
 291 L(22):  addl $4, %esi
 292 L(21):  addl $4, %esi
 293
 294         /* What remains to do is to test which byte the NUL char is and
 295            whether the searched character appears in one of the bytes
 296            before.  A special case is that the searched byte maybe NUL.
 297            In this case a pointer to the terminating NUL char has to be
 298            returned.  */
 299
 300 L(20):  cmpb %cl, %dl           /* is first byte == C? */
 301         jne L(24)               /* no => skip */
 302         movl %esi, %eax         /* store address as result */
 303 L(24):  testb %dl, %dl          /* is first byte == NUL? */
 304         jz L(2)                 /* yes => return */
 305
 306         cmpb %cl, %dh           /* is second byte == C? */
 307         jne L(25)               /* no => skip */
 308         leal 1(%esi), %eax      /* store address as result */
 309 L(25):  testb %dh, %dh          /* is second byte == NUL? */
 310         jz L(2)                 /* yes => return */
 311
 312         shrl $16,%edx           /* make upper bytes accessible */
 313         cmpb %cl, %dl           /* is third byte == C */
 314         jne L(26)               /* no => skip */
 315         leal 2(%esi), %eax      /* store address as result */
 316 L(26):  testb %dl, %dl          /* is third byte == NUL */
 317         jz L(2)                 /* yes => return */
 318
 319         cmpb %cl, %dh           /* is fourth byte == C */
 320         jne L(2)                /* no => skip */
 321         leal 3(%esi), %eax      /* store address as result */
 322
 323 L(2):   popl %esi               /* restore saved register content */
 324         cfi_adjust_cfa_offset (-4)
 325         cfi_restore (esi)
 326         popl %edi
 327         cfi_adjust_cfa_offset (-4)
 328         cfi_restore (edi)
 329
 330         ret
 331 END (strrchr)
 332
 333 weak_alias (strrchr, rindex)
 334 libc_hidden_builtin_def (strrchr)