[Bug target/111404] New: [AArch64] 128-bit __sync_val_compare_and_swap is not atomic

[Bug target/111404] New: [AArch64] 128-bit __sync_val_compare_and_swap is not atomic

[wilco at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111404

            Bug ID: 111404
           Summary: [AArch64] 128-bit __sync_val_compare_and_swap is not
                    atomic
           Product: gcc
           Version: 8.5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: wilco at gcc dot gnu.org
  Target Milestone: ---

This compiles

__int128 f(__int128 *p, __int128 *q, __int128 x)
{
  return __sync_val_compare_and_swap (p, *q, x);
}

into:

f:
        ldp     x6, x7, [x1]
        mov     x4, x0
.L3:
        ldxp    x0, x1, [x4]
        cmp     x0, x6
        ccmp    x1, x7, 0, eq
        bne     .L4
        stlxp   w5, x2, x3, [x4]
        cbnz    w5, .L3
.L4:
        dmb     ish
        ret

This means if the compare fails, we return the value loaded via LDXP. However
unless the STXP succeeds, this returned value is not single-copy atomic.

So on failure we still need to execute STLXP.

[Bug target/111404] [AArch64] 128-bit __sync_val_compare_and_swap is not atomic

[wilco at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111404

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2023-09-14
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW
             Target|arm64-*-*                   |aarch64

--- Comment #1 from Wilco <wilco at gcc dot gnu.org> ---
Patch: https://gcc.gnu.org/pipermail/gcc-patches/2023-September/630198.html

[Bug target/111404] [AArch64] 128-bit __sync_val_compare_and_swap is not atomic

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111404

--- Comment #2 from GCC 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:df8958e6bc5d050dab8bdc5954c1632fb0e98d18

commit r14-6021-gdf8958e6bc5d050dab8bdc5954c1632fb0e98d18
Author: Wilco Dijkstra <wilco.dijkstra@arm.com>
Date:   Thu Nov 30 16:14:35 2023 +0000

    AArch64: Fix __sync_val_compare_and_swap [PR111404]

    __sync_val_compare_and_swap may be used on 128-bit types and either calls
the
    outline atomic code or uses an inline loop.  On AArch64 LDXP is only atomic
if
    the value is stored successfully using STXP, but the current
implementations
    do not perform the store if the comparison fails.  In this case the value
    returned is not read atomically.

    gcc/ChangeLog:
            PR target/111404
            * config/aarch64/aarch64.cc (aarch64_split_compare_and_swap):
            For 128-bit store the loaded value and loop if needed.

    libgcc/ChangeLog:
            PR target/111404
            * config/aarch64/lse.S (__aarch64_cas16_acq_rel): Execute STLXP
using
            either new value or loaded value.