[Bug libgcc/108891] New: libatomic: AArch64 SEQ_CST 16-byte load missing barrier

[Bug libgcc/108891] New: libatomic: AArch64 SEQ_CST 16-byte load missing barrier

[wilco at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108891

            Bug ID: 108891
           Summary: libatomic: AArch64 SEQ_CST 16-byte load missing
                    barrier
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libgcc
          Assignee: unassigned at gcc dot gnu.org
          Reporter: wilco at gcc dot gnu.org
  Target Milestone: ---

LSE2 uses the following sequence for a 16-byte atomic load:

ldp     res0, res1, [x0]
dmb     ish

The AArch64 memory model allows the LDP to be reordered with an earlier STLXP
(eg. a SEQ_CST exchange), thus breaking SEQ_CST ordering.

To avoid this, atomic loads need a barrier before the LDP - either DBM ISHLD or
LDAR works.

[Bug libgcc/108891] libatomic: AArch64 SEQ_CST 16-byte load missing barrier

[wilco at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108891

Wilco <wilco at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |ASSIGNED
   Last reconfirmed|                            |2023-02-23
     Ever confirmed|0                           |1
           Assignee|unassigned at gcc dot gnu.org      |wilco at gcc dot gnu.org

[Bug libgcc/108891] libatomic: AArch64 SEQ_CST 16-byte load missing barrier

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108891

--- Comment #1 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Wilco Dijkstra <wilco@gcc.gnu.org>:

https://gcc.gnu.org/g:1f641d6aba284e0c277e6684cd6b2c73591cd14d

commit r13-6855-g1f641d6aba284e0c277e6684cd6b2c73591cd14d
Author: Wilco Dijkstra <wilco.dijkstra@arm.com>
Date:   Fri Feb 10 17:41:05 2023 +0000

    libatomic: Fix SEQ_CST 128-bit atomic load [PR108891]

    The LSE2 ifunc for 16-byte atomic load requires a barrier before the LDP -
    without it, it effectively has Load-AcquirePC semantics similar to LDAPR,
    which is less restrictive than what __ATOMIC_SEQ_CST requires.  This patch
    fixes this and adds comments to make it easier to see which sequence is
    used for each case.  Use a load/store exclusive loop for store to simplify
    testing memory ordering is correct (it is slightly faster too).

    libatomic/
            PR libgcc/108891
            * config/linux/aarch64/atomic_16.S: Fix libat_load_16_i1.
            Add comments describing the memory order.

[Bug libgcc/108891] libatomic: AArch64 SEQ_CST 16-byte load missing barrier

[wilco at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108891

Wilco <wilco at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|---                         |FIXED

--- Comment #2 from Wilco <wilco at gcc dot gnu.org> ---
Fixed