RFR: 8292591: Experimentally add back barrier-less Java thread transitions [v4]

[Martin Doerr]

On Fri, 9 Sep 2022 07:27:43 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 with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains five additional commits since the last revision:
> 
>  - Merge branch 'master' into membar
>  - Test fixes
>  - PPC64 port courtesy Martin Doerr
>  - Change header and constants handling
>  - 8292591 - initial

Thanks for the update! LGTM. I hope that no other platform with older linux kernel is still needed.

src/hotspot/os/linux/systemMemoryBarrier_linux.cpp line 83:

> 81: 
> 82: void LinuxSystemMemoryBarrier::emit() {
> 83:   int s =  membarrier(MEMBARRIER_CMD_PRIVATE_EXPEDITED, 0, 0);

Extra whitespace. Feel free to remove it.

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Marked as reviewed by mdoerr (Reviewer).

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