RFR: 8253284: Zero OrderAccess barrier mappings are incorrect [v2]

[Aleksey Shipilev]

On Fri, 18 Sep 2020 01:51:19 GMT, Andrew John Hughes <andrew at openjdk.org> wrote:

> Is the #if defined(ARM) referring to ARM in general here, or just 32-bit ARM? Because there appears to be no change to
> its definition, just rearrangement.

No, there is `ARM` (32) and then there is `AARCH64` (64). This is why AArch64 fails: it effectively uses compiler
barriers only through the confusing application of #else branches.

> The patch is quite hard to follow, and made even more difficult by multiple versions being attached to this PR.

These are not multiple versions, those commits would be squashed into one on push -- that is how Skara workflow works.
Look at the final version, which basically enables default (implicitly used by `ALPHA` and `AARCH64`) strong
barriers -- that is the bug fix. It also keeps the light-weight barrier light for `X86` by calling it out explicitly.

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PR: https://git.openjdk.java.net/jdk/pull/224