sysdeps/i386/strlen.S

   1 /* strlen(str) -- determine the length of the string STR.
   2    Optimized for Intel 80x86, x>=4.
   3    Copyright (C) 1991-2020 Free Software Foundation, Inc.
   4    Contributed by Ulrich Drepper <drepper@gnu.ai.mit.edu>.
   5    This file is part of the GNU C Library.
   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    <https://www.gnu.org/licenses/>.  */
  20
  21 #include <sysdep.h>
  22 #include "asm-syntax.h"
  23
  24 #define PARMS   4               /* no space for saved regs */
  25 #define STR     PARMS
  26
  27         .text
  28 ENTRY (strlen)
  29
  30         movl STR(%esp), %ecx
  31         movl %ecx, %eax         /* duplicate it */
  32
  33         andl $3, %ecx           /* mask alignment bits */
  34         jz L(1)                 /* aligned => start loop */
  35         cmpb %ch, (%eax)        /* is byte NUL? */
  36         je L(2)                 /* yes => return */
  37         incl %eax               /* increment pointer */
  38
  39         xorl $3, %ecx           /* was alignment = 3? */
  40         jz L(1)                 /* yes => now it is aligned and start loop */
  41         cmpb %ch, (%eax)        /* is byte NUL? */
  42         je L(2)                 /* yes => return */
  43         addl $1, %eax           /* increment pointer */
  44
  45         subl $1, %ecx           /* was alignment = 2? */
  46         jz L(1)                 /* yes => now it is aligned and start loop */
  47         cmpb %ch, (%eax)        /* is byte NUL? */
  48         je L(2)                 /* yes => return */
  49
  50 /* Don't change the above `addl $1,%eax' and `subl $1, %ecx' into `incl %eax'
  51    and `decl %ecx' resp.  The additional two byte per instruction make the
  52    label 4 to be aligned on a 16 byte boundary with nops.
  53
  54    The following `sub $15, %eax' is part of this trick, too.  Together with
  55    the next instruction (`addl $16, %eax') it is in fact a `incl %eax', just
  56    as expected from the algorithm.  But doing so has the advantage that
  57    no jump to label 1 is necessary and so the pipeline is not flushed.  */
  58
  59         subl $15, %eax          /* effectively +1 */
  60
  61
  62 L(4):   addl $16, %eax          /* adjust pointer for full loop */
  63
  64 L(1):   movl (%eax), %ecx       /* get word (= 4 bytes) in question */
  65         movl $0xfefefeff, %edx  /* magic value */
  66         addl %ecx, %edx         /* add the magic value to the word.  We get
  67                                    carry bits reported for each byte which
  68                                    is *not* 0 */
  69         jnc L(3)                /* highest byte is NUL => return pointer */
  70         xorl %ecx, %edx         /* (word+magic)^word */
  71         orl $0xfefefeff, %edx   /* set all non-carry bits */
  72         incl %edx               /* add 1: if one carry bit was *not* set
  73                                    the addition will not result in 0.  */
  74         jnz L(3)                /* found NUL => return pointer */
  75
  76         movl 4(%eax), %ecx      /* get word (= 4 bytes) in question */
  77         movl $0xfefefeff, %edx  /* magic value */
  78         addl %ecx, %edx         /* add the magic value to the word.  We get
  79                                    carry bits reported for each byte which
  80                                    is *not* 0 */
  81         jnc L(5)                /* highest byte is NUL => return pointer */
  82         xorl %ecx, %edx         /* (word+magic)^word */
  83         orl $0xfefefeff, %edx   /* set all non-carry bits */
  84         incl %edx               /* add 1: if one carry bit was *not* set
  85                                    the addition will not result in 0.  */
  86         jnz L(5)                /* found NUL => return pointer */
  87
  88         movl 8(%eax), %ecx      /* get word (= 4 bytes) in question */
  89         movl $0xfefefeff, %edx  /* magic value */
  90         addl %ecx, %edx         /* add the magic value to the word.  We get
  91                                    carry bits reported for each byte which
  92                                    is *not* 0 */
  93         jnc L(6)                /* highest byte is NUL => return pointer */
  94         xorl %ecx, %edx         /* (word+magic)^word */
  95         orl $0xfefefeff, %edx   /* set all non-carry bits */
  96         incl %edx               /* add 1: if one carry bit was *not* set
  97                                    the addition will not result in 0.  */
  98         jnz L(6)                /* found NUL => return pointer */
  99
 100         movl 12(%eax), %ecx     /* get word (= 4 bytes) in question */
 101         movl $0xfefefeff, %edx  /* magic value */
 102         addl %ecx, %edx         /* add the magic value to the word.  We get
 103                                    carry bits reported for each byte which
 104                                    is *not* 0 */
 105         jnc L(7)                /* highest byte is NUL => return pointer */
 106         xorl %ecx, %edx         /* (word+magic)^word */
 107         orl $0xfefefeff, %edx   /* set all non-carry bits */
 108         incl %edx               /* add 1: if one carry bit was *not* set
 109                                    the addition will not result in 0.  */
 110         jz L(4)                 /* no NUL found => continue loop */
 111
 112 L(7):   addl $4, %eax           /* adjust pointer */
 113 L(6):   addl $4, %eax
 114 L(5):   addl $4, %eax
 115
 116 L(3):   testb %cl, %cl          /* is first byte NUL? */
 117         jz L(2)                 /* yes => return */
 118         incl %eax               /* increment pointer */
 119
 120         testb %ch, %ch          /* is second byte NUL? */
 121         jz L(2)                 /* yes => return */
 122         incl %eax               /* increment pointer */
 123
 124         testl $0xff0000, %ecx   /* is third byte NUL? */
 125         jz L(2)                 /* yes => return pointer */
 126         incl %eax               /* increment pointer */
 127
 128 L(2):   subl STR(%esp), %eax    /* compute difference to string start */
 129
 130         ret
 131 END (strlen)
 132 libc_hidden_builtin_def (strlen)