[Bug target/96056] New: arm v6/v7: Missing acquire barrier for __atomic_compare_exchange(__ATOMIC_RELEASE, __ATOMIC_ACQUIRE)

[Bug target/96056] New: arm v6/v7: Missing acquire barrier for __atomic_compare_exchange(__ATOMIC_RELEASE, __ATOMIC_ACQUIRE)

[izbyshev at ispras dot ru]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96056

            Bug ID: 96056
           Summary: arm v6/v7: Missing acquire barrier for
                    __atomic_compare_exchange(__ATOMIC_RELEASE,
                    __ATOMIC_ACQUIRE)
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: izbyshev at ispras dot ru
                CC: amonakov at gcc dot gnu.org
  Target Milestone: ---

$ cat test.c
void foo(int x, int y, int z) {
    __atomic_compare_exchange(&x, &y, &z, 0, __ATOMIC_RELEASE,
__ATOMIC_ACQUIRE);
}

$ arm-linux-gnueabi-gcc-10 -O2 -march=armv7 -S -o - test.c
[...]
        sub     sp, sp, #8
        str     r0, [sp, #4]
        add     r0, sp, #4
        dmb     ish
.L2:
        ldrex   r3, [r0]
        cmp     r3, r1
        bne     .L3
        strex   ip, r2, [r0]
        cmp     ip, #0
        bne     .L2
.L3:
        @ <=== missing dmb here
        add     sp, sp, #8
[...]

Changing __ATOMIC_RELEASE to __ATOMIC_ACQ_REL restores the missing barrier.

The same problem applies to armv6 as well.

I've checked that all GCC versions on gcc.godbolt.org since 5.4.0 behave in the
same way. All Clang versions on godbolt don't have this problem.