RFR: 8292591: Experimentally add back barrier-less Java thread transitions [v3]

[Martin Doerr]

On Thu, 8 Sep 2022 18:13:56 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:
> 
>   PPC64 port courtesy Martin Doerr

Thanks for integrating my PPC64 implementation. I prefer serving as reviewer. My contribution was rather small. The implementation LGTM.
However, the test should pass in case of `MEMBARRIER_CMD_QUERY unsupported`.
Note: I believe the test should also work in product builds when adding `commands.add("-XX:+UnlockDiagnosticVMOptions");`, but I don't insist on that. I leave you free to decide.

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Changes requested by mdoerr (Reviewer).

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