[llvm] r179005 - AArch64: remove barriers from AArch64 atomic operations.

[Tim Northover]

> AArch64: remove barriers from AArch64 atomic operations.

Bother, I meant to ask for some review from any memory order Guru's
before committing this but "git svn dcommit" thwarted me. It basically
removes the use of barriers for AArch64 memory operations, switching
to exclusively acquire/release instructions in the ldxr/stxr loop.

At first sight it looks a little dodgy; acquiring a lock:

loop:
    ldxr x1, [x2]
    [...]
    stxr w0, x1, [x2]
    cbnz w0, loop
    dmb

becomes:

loop:
   ldaxr x1, [x2]
   [...]
   stxr w0, x1, [x2]
   cbnz w0, loop

where the barrier has moved much earlier in the sequence, and so in
particular the final store which asserts ownership can be reordered
with operations making use of that ownership.

However, the important point is that another observer taking the lock
wouldn't see this because their stxr would fail. Another point in
favour of validity is that this is what the Linux kernel does.

I've also managed to convince myself that similar reasoning holds for
other atomic operations, but if I'm honest I'm not even entirely sure
what a formal proof of that would look like, never mind having it.

So any comments on this would be particularly welcome.

Tim.