[Bug string/21662] memcmp-avx2-movbe.S lacks saturating subtraction for between_2_3

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https://sourceware.org/bugzilla/show_bug.cgi?id=21662

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https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=3ec7c02cc3e922b9364dc8cfd1d4546671b91003

commit 3ec7c02cc3e922b9364dc8cfd1d4546671b91003
Author: Florian Weimer <fweimer@redhat.com>
Date:   Fri Jun 23 17:23:44 2017 +0200

    x86-64: memcmp-avx2-movbe.S needs saturating subtraction [BZ #21662]

    This code:

    L(between_2_3):
        /* Load as big endian with overlapping loads and bswap to avoid
           branches.  */
        movzwl  -2(%rdi, %rdx), %eax
        movzwl  -2(%rsi, %rdx), %ecx
        shll    $16, %eax
        shll    $16, %ecx
        movzwl  (%rdi), %edi
        movzwl  (%rsi), %esi
        orl     %edi, %eax
        orl     %esi, %ecx
        bswap   %eax
        bswap   %ecx
        subl    %ecx, %eax
        ret

    needs a saturating subtract because the full register is used.
    With this commit, only the lower 24 bits of the register are used,
    so a regular subtraction suffices.

    The test case change adds coverage for these kinds of bugs.

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

Summary of changes:
 ChangeLog                                    |    7 +++++++
 string/test-memcmp.c                         |   11 ++++++-----
 sysdeps/x86_64/multiarch/memcmp-avx2-movbe.S |   20 ++++++++++----------
 3 files changed, 23 insertions(+), 15 deletions(-)

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