[llvm-bugs] [Bug 27545] New: atomic_signal_fence generates a dmb instruction on aarch64

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=27545

            Bug ID: 27545
           Summary: atomic_signal_fence generates a dmb instruction on
                    aarch64
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Backend: AArch64
          Assignee: unassignedbugs at nondot.org
          Reporter: yyc1992 at gmail.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

The following program

```
#include <stdatomic.h>

void f()
{
    atomic_signal_fence(memory_order_seq_cst);
}
```

generates
```
f:                                      // @f
// BB#0:
        dmb     ish
        ret
```

when compiling with `clang -O2` and simply a `ret` when compiling with `gcc`.

Since this is a signal fence and not a thread fence, I think the memory barrier
instruction should not be necessary.

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