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Subject: [Bug target/102218] 128-bit atomic compare and exchange does not
 honor memory model on AArch64 and Arm
Date: Mon, 08 Aug 2022 13:38:29 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102218

--- Comment #1 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Tamar Christina <tnfchris@gcc.gnu.org=
>:

https://gcc.gnu.org/g:e6a8ae900b4141bbce1451da8f173d441662782d

commit r13-1988-ge6a8ae900b4141bbce1451da8f173d441662782d
Author: Tamar Christina <tamar.christina@arm.com>
Date:   Mon Aug 8 14:37:00 2022 +0100

    AArch64: Fix 128-bit sequential consistency atomic operations.

    The AArch64 implementation of 128-bit atomics is broken.

    For 128-bit atomics we rely on pthread barriers to correct guard the
address
    in the pointer to get correct memory ordering.  However for 128-bit ato=
mics
the
    address under the lock is different from the original pointer.

    This means that one of the values under the atomic operation is not
protected
    properly and so we fail during when the user has requested sequential
    consistency as there's no barrier to enforce this requirement.

    As such users have resorted to adding an

    #ifdef GCC
    <emit barrier>
    #endif

    around the use of these atomics.

    This corrects the issue by issuing a barrier only when __ATOMIC_SEQ_CST=
 was
    requested.  To remedy this performance hit I think we should revisit us=
ing
a
    similar approach to out-line-atomics for the 128-bit atomics.

    Note that I believe I need the empty file due to the include_next chain=
 but
    I am not entirely sure.  I have hand verified that the barriers are
inserted
    for atomic seq cst.

    libatomic/ChangeLog:

            PR target/102218
            * config/aarch64/aarch64-config.h: New file.
            * config/aarch64/host-config.h: New file.=