[llvm] r185339 - ARM: relax the atomic release barrier to "dmb ishst"

[Eli Friedman]

On Mon, Jul 1, 2013 at 7:48 AM, Tim Northover <tnorthover at apple.com> wrote:

> Author: tnorthover
> Date: Mon Jul  1 09:48:48 2013
> New Revision: 185339
>
> URL: http://llvm.org/viewvc/llvm-project?rev=185339&view=rev
> Log:
> ARM: relax the atomic release barrier to "dmb ishst"
>
> I believe the full "dmb ish" barrier is not required to guarantee release
> semantics for atomic operations. The weaker "dmb ishst" prevents previous
> operations being reordered with a store executed afterwards, which is
> enough.
>
>
This is wrong; please revert.

Specifically, a load before a release barrier is required to complete
before a store after the barrier.

-Eli
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