RFR: 8292591: Experimentally add back barrier-less Java thread transitions [v7]

[Martin Doerr]

On Mon, 12 Sep 2022 07:20:54 GMT, Robbin Ehn <rehn at openjdk.org> wrote:

>> Please consider, only implemented on x64/aarch64 linux/windows. (@TheRealMDoerr have now contributed PPC64)
>> 
>> On my box calling clock_gettime via JNI goes from 35ns to 28ns when enabled.
>> 
>> Passes t1-7 with option forced on, also passes t1-4 as is in this PR.
>
> Robbin Ehn has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Added comment

Thanks for the cleanup and improved comments! LGTM.
I think omitting the fence in `transition_from_native` would be an interesting follow-up for the future as these transitions are not uncommon (and also used by JIT compiler threads).

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Marked as reviewed by mdoerr (Reviewer).

PR: https://git.openjdk.org/jdk/pull/10123