RFR: 8316961: Fallback implementations for 64-bit Atomic::{add,xchg} on 32-bit platforms [v3]

[Kim Barrett]

On Fri, 20 Oct 2023 12:03:55 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> See the bug for rationale. Looks like there is enough infrastructure to achieve what we want without significant fan-out. I checked all `atomic_*.hpp` headers for unimplemented `PlatformAdd<8>` and `PlatformXchg<8>`, and only these seem to be affected.
>> 
>> Unfortunately, we cannot test these apart from the existing gtest.
>> 
>> Additional testing:
>>  - [x] linux-x86-server-fastdebug, atomic tests pass
>>  - [x] linux-arm-server-fastdebug, atomic tests pass (with #16269 applied)
>
> Aleksey Shipilev has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Protect 64-bit tests with supports_cx8()

Throughout, Atomic::PlatformMumble is intended to be used in the implementation of
Atomic::Mumble, and isn't intended to be used elsewhere.  I think intent is even adhered
to currently.  This proposed change violates that in a bunch of places.

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Changes requested by kbarrett (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/16252#pullrequestreview-1692131480