source: vendor/glibc/current/sysdeps/i386/rawmemchr.S

/* rawmemchr (str, ch) -- Return pointer to first occurrence of CH in STR.
   For Intel 80x86, x>=3.
   Copyright (C) 1994-2000,2002,2005 Free Software Foundation, Inc.
   This file is part of the GNU C Library.
   Contributed by Ulrich Drepper <drepper@gnu.ai.mit.edu>
   Optimised a little by Alan Modra <Alan@SPRI.Levels.UniSA.Edu.Au>
   This version is developed using the same algorithm as the fast C
   version which carries the following introduction:
   Based on strlen implementation by Torbjorn Granlund (tege@sics.se),
   with help from Dan Sahlin (dan@sics.se) and
   commentary by Jim Blandy (jimb@ai.mit.edu);
   adaptation to memchr suggested by Dick Karpinski (dick@cca.ucsf.edu),
   and implemented by Roland McGrath (roland@ai.mit.edu).

   The GNU C Library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Lesser General Public
   License as published by the Free Software Foundation; either
   version 2.1 of the License, or (at your option) any later version.

   The GNU C Library is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
   Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public
   License along with the GNU C Library; if not, write to the Free
   Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
   02111-1307 USA.  */

#include <sysdep.h>
#include "asm-syntax.h"
#include "bp-sym.h"
#include "bp-asm.h"

#define PARMS   LINKAGE+4       /* space for 1 saved reg */
#define RTN     PARMS
#define STR     RTN+RTN_SIZE
#define CHR     STR+PTR_SIZE

        .text
ENTRY (BP_SYM (__rawmemchr))
        ENTER

        /* Save callee-safe register used in this function.  */
        pushl %edi
        cfi_adjust_cfa_offset (4)
        cfi_rel_offset (edi, 0)

        /* Load parameters into registers.  */
        movl STR(%esp), %eax
        movl CHR(%esp), %edx
        CHECK_BOUNDS_LOW (%eax, STR(%esp))

        /* At the moment %edx contains C.  What we need for the
           algorithm is C in all bytes of the dword.  Avoid
           operations on 16 bit words because these require an
           prefix byte (and one more cycle).  */
        movb %dl, %dh           /* Now it is 0|0|c|c */
        movl %edx, %ecx
        shll $16, %edx          /* Now c|c|0|0 */
        movw %cx, %dx           /* And finally c|c|c|c */

        /* Better performance can be achieved if the word (32
           bit) memory access is aligned on a four-byte-boundary.
           So process first bytes one by one until boundary is
           reached. Don't use a loop for better performance.  */

        testb $3, %al           /* correctly aligned ? */
        je L(1)                 /* yes => begin loop */
        cmpb %dl, (%eax)        /* compare byte */
        je L(9)                 /* target found => return */
        incl %eax               /* increment source pointer */

        testb $3, %al           /* correctly aligned ? */
        je L(1)                 /* yes => begin loop */
        cmpb %dl, (%eax)        /* compare byte */
        je L(9)                 /* target found => return */
        incl %eax               /* increment source pointer */

        testb $3, %al           /* correctly aligned ? */
        je L(1)                 /* yes => begin loop */
        cmpb %dl, (%eax)        /* compare byte */
        je L(9)                 /* target found => return */
        incl %eax               /* increment source pointer */

      /* We exit the loop if adding MAGIC_BITS to LONGWORD fails to
         change any of the hole bits of LONGWORD.

         1) Is this safe?  Will it catch all the zero bytes?
         Suppose there is a byte with all zeros.  Any carry bits
         propagating from its left will fall into the hole at its
         least significant bit and stop.  Since there will be no
         carry from its most significant bit, the LSB of the
         byte to the left will be unchanged, and the zero will be
         detected.

         2) Is this worthwhile?  Will it ignore everything except
         zero bytes?  Suppose every byte of LONGWORD has a bit set
         somewhere.  There will be a carry into bit 8.  If bit 8
         is set, this will carry into bit 16.  If bit 8 is clear,
         one of bits 9-15 must be set, so there will be a carry
         into bit 16.  Similarly, there will be a carry into bit
         24.  If one of bits 24-31 is set, there will be a carry
         into bit 32 (=carry flag), so all of the hole bits will
         be changed.

         3) But wait!  Aren't we looking for C, not zero?
         Good point.  So what we do is XOR LONGWORD with a longword,
         each of whose bytes is C.  This turns each byte that is C
         into a zero.  */


        /* Each round the main loop processes 16 bytes.  */
        ALIGN (4)

L(1):   movl (%eax), %ecx       /* get word (= 4 bytes) in question */
        movl $0xfefefeff, %edi  /* magic value */
        xorl %edx, %ecx         /* XOR with word c|c|c|c => bytes of str == c
                                   are now 0 */
        addl %ecx, %edi         /* add the magic value to the word.  We get
                                   carry bits reported for each byte which
                                   is *not* 0 */

        /* According to the algorithm we had to reverse the effect of the
           XOR first and then test the overflow bits.  But because the
           following XOR would destroy the carry flag and it would (in a
           representation with more than 32 bits) not alter then last
           overflow, we can now test this condition.  If no carry is signaled
           no overflow must have occurred in the last byte => it was 0. */
        jnc L(8)

        /* We are only interested in carry bits that change due to the
           previous add, so remove original bits */
        xorl %ecx, %edi         /* ((word^charmask)+magic)^(word^charmask) */

        /* Now test for the other three overflow bits.  */
        orl $0xfefefeff, %edi   /* set all non-carry bits */
        incl %edi               /* add 1: if one carry bit was *not* set
                                   the addition will not result in 0.  */

        /* If at least one byte of the word is C we don't get 0 in %edi.  */
        jnz L(8)                /* found it => return pointer */

        /* This process is unfolded four times for better performance.
           we don't increment the source pointer each time.  Instead we
           use offsets and increment by 16 in each run of the loop.  But
           before probing for the matching byte we need some extra code
           (following LL(13) below).  Even the len can be compared with
           constants instead of decrementing each time.  */

        movl 4(%eax), %ecx      /* get word (= 4 bytes) in question */
        movl $0xfefefeff, %edi  /* magic value */
        xorl %edx, %ecx         /* XOR with word c|c|c|c => bytes of str == c
                                   are now 0 */
        addl %ecx, %edi         /* add the magic value to the word.  We get
                                   carry bits reported for each byte which
                                   is *not* 0 */
        jnc L(7)                /* highest byte is C => return pointer */
        xorl %ecx, %edi         /* ((word^charmask)+magic)^(word^charmask) */
        orl $0xfefefeff, %edi   /* set all non-carry bits */
        incl %edi               /* add 1: if one carry bit was *not* set
                                   the addition will not result in 0.  */
        jnz L(7)                /* found it => return pointer */

        movl 8(%eax), %ecx      /* get word (= 4 bytes) in question */
        movl $0xfefefeff, %edi  /* magic value */
        xorl %edx, %ecx         /* XOR with word c|c|c|c => bytes of str == c
                                   are now 0 */
        addl %ecx, %edi         /* add the magic value to the word.  We get
                                   carry bits reported for each byte which
                                   is *not* 0 */
        jnc L(6)                /* highest byte is C => return pointer */
        xorl %ecx, %edi         /* ((word^charmask)+magic)^(word^charmask) */
        orl $0xfefefeff, %edi   /* set all non-carry bits */
        incl %edi               /* add 1: if one carry bit was *not* set
                                   the addition will not result in 0.  */
        jnz L(6)                /* found it => return pointer */

        movl 12(%eax), %ecx     /* get word (= 4 bytes) in question */
        movl $0xfefefeff, %edi  /* magic value */
        xorl %edx, %ecx         /* XOR with word c|c|c|c => bytes of str == c
                                   are now 0 */
        addl %ecx, %edi         /* add the magic value to the word.  We get
                                   carry bits reported for each byte which
                                   is *not* 0 */
        jnc L(5)                /* highest byte is C => return pointer */
        xorl %ecx, %edi         /* ((word^charmask)+magic)^(word^charmask) */
        orl $0xfefefeff, %edi   /* set all non-carry bits */
        incl %edi               /* add 1: if one carry bit was *not* set
                                   the addition will not result in 0.  */
        jnz L(5)                /* found it => return pointer */

        /* Adjust both counters for a full round, i.e. 16 bytes.  */
        addl $16, %eax
        jmp L(1)
        /* add missing source pointer increments */
L(5):   addl $4, %eax
L(6):   addl $4, %eax
L(7):   addl $4, %eax

        /* Test for the matching byte in the word.  %ecx contains a NUL
           char in the byte which originally was the byte we are looking
           at.  */
L(8):   testb %cl, %cl          /* test first byte in dword */
        jz L(9)                 /* if zero => return pointer */
        incl %eax               /* increment source pointer */

        testb %ch, %ch          /* test second byte in dword */
        jz L(9)                 /* if zero => return pointer */
        incl %eax               /* increment source pointer */

        testl $0xff0000, %ecx   /* test third byte in dword */
        jz L(9)                 /* if zero => return pointer */
        incl %eax               /* increment source pointer */

        /* No further test needed we we know it is one of the four bytes.  */

L(9):
        CHECK_BOUNDS_HIGH (%eax, STR(%esp), jb)
        RETURN_BOUNDED_POINTER (STR(%esp))
        popl %edi               /* pop saved register */
        cfi_adjust_cfa_offset (-4)
        cfi_restore (edi)

        LEAVE
        RET_PTR
END (BP_SYM (__rawmemchr))

libc_hidden_def (BP_SYM (__rawmemchr))
weak_alias (BP_SYM (__rawmemchr), BP_SYM (rawmemchr))