RFR: 8307533: Use atomic bitset functions for metadata flags [v2]

[Kim Barrett]

On Mon, 8 May 2023 14:05:22 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

>> Replace the bit set copies from metadata to use the Atomic functions.
>> Tested with tier1-4.
>
> Coleen Phillimore has updated the pull request incrementally with one additional commit since the last revision:
> 
>   remove extra variables in favor of casts to help the template.

The Atomic bitops aren't intended to support other sizes; only the same sizes as Atomic::add and friends.  That narrower
types are currently supported by the default implementation is an accident.  Platform specializations might not have such
support, since the underlying platform might not have it.

If support for narrower types is a (not previously known to me) requirement, some non-trivial changes may be needed.
Among other things, I think the current very simple platform specialization mechanism won't be sufficient.

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PR Comment: https://git.openjdk.org/jdk/pull/13843#issuecomment-1541022459