[llvm-bugs] [Bug 28819] New: [AArch64] cmpxchg produces inverted success flag at -O0

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=28819

            Bug ID: 28819
           Summary: [AArch64] cmpxchg produces inverted success flag at
                    -O0
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: normal
          Priority: P
         Component: Backend: AArch64
          Assignee: unassignedbugs at nondot.org
          Reporter: gberry at codeaurora.org
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

The success status flag generated for cmpxchg instructions is inverted when
compiled at -O0.  cmpxchg is supposed to return 1 for success, but looking at
the generated code for llvm/test/CodeGen/AArch64/cmpxchg-O0.ll:test_cmpxchg_64:

test_cmpxchg_64:                        // @test_cmpxchg_64
// BB#0:
    sub    sp, sp, #16             // =16
.LBB3_1:                                // =>This Inner Loop Header: Depth=1
    ldaxr    x8, [x0]
    cmp        x8, x1
    b.ne    .LBB3_3
// BB#2:                                //   in Loop: Header=BB3_1 Depth=1
    stlxr    w9, x2, [x0]
    cbnz    w9, .LBB3_1
.LBB3_3:
    subs        x0, x8, x1
    cset     w1, eq
    str    x0, [sp, #8]            // 8-byte Folded Spill
    mov     x0, x8
    str    w9, [sp, #4]            // 4-byte Folded Spill
    add    sp, sp, #16             // =16
    ret

The success flag (w9) will always be 0 (or the loop won't terminate).
This only occurs at -O0, and I believe was introduced by the fix for
https://llvm.org/bugs/show_bug.cgi?id=25526

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