[Bug string/22353] sysdeps/i386/i586/strcpy.S isn't maintainable

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https://sourceware.org/bugzilla/show_bug.cgi?id=22353

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https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=c5cc45148c89cc5c57d1946348dd242d4db5c5f5

commit c5cc45148c89cc5c57d1946348dd242d4db5c5f5
Author: H.J. Lu <hjl.tools@gmail.com>
Date:   Mon Oct 30 10:02:16 2017 -0700

    i586: Use conditional branches in strcpy.S [BZ #22353]

    i586 strcpy.S used a clever trick with LEA to implement jump table:

    /* ECX has the last 2 bits of the address of source - 1.  */
        andl    $3, %ecx

            call    2f
    2:      popl    %edx
        /* 0xb is the distance between 2: and 1:.  */
            leal    0xb(%edx,%ecx,8), %ecx
            jmp     *%ecx

            .align 8
    1:  /* ECX == 0 */
            orb     (%esi), %al
            jz      L(end)
            stosb
            xorl    %eax, %eax
            incl    %esi
        /* ECX == 1 */
            orb     (%esi), %al
            jz      L(end)
            stosb
            xorl    %eax, %eax
            incl    %esi
        /* ECX == 2 */
            orb     (%esi), %al
            jz      L(end)
            stosb
            xorl    %eax, %eax
            incl    %esi
        /* ECX == 3 */
    L(1):   movl    (%esi), %ecx
            leal    4(%esi),%esi

    This fails if there are instruction length changes before L(1):.  This
    patch replaces it with conditional branches:

        cmpb    $2, %cl
        je      L(Src2)
        ja      L(Src3)
        cmpb    $1, %cl
        je      L(Src1)

    L(Src0):

    which have similar performance and work with any instruction lengths.

    Tested on i586 and i686 with and without --disable-multi-arch.

        [BZ #22353]
        * sysdeps/i386/i586/strcpy.S (STRCPY): Use conditional branches.
        (1): Renamed to ...
        (L(Src0)): This.
        (L(Src1)): New.
        (L(Src2)): Likewise.
        (L(1)): Renamed to ...
        (L(Src3)): This.

-----------------------------------------------------------------------

Summary of changes:
 ChangeLog                  |   11 +++++++++++
 sysdeps/i386/i586/strcpy.S |   28 +++++++++++-----------------
 2 files changed, 22 insertions(+), 17 deletions(-)

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