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

[Martin Doerr]

On Thu, 1 Sep 2022 16:47:58 GMT, Robbin Ehn <rehn at openjdk.org> wrote:

> Please consider, only implemented on x64/aarch64 linux/windows.
> 
> 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.

Nice to see usage of this OS feature. I like it more than the serialization page we have in 11u (with `-XX:-UseMembar`) and Critical JNI Natives (which still have some advantage). I have observed the same speedup for a trivial JNI call on my x86_64 machine.

src/hotspot/os/linux/systemMemoryBarrier_linux.cpp line 36:

> 34: // Syscall defined in kernel 4.3
> 35: #if !defined(SYS_membarrier)
> 36: #define SYS_membarrier 324

I'd like to get rid of the platform checks in this generic linux file.
This number seems to be the only platform specific part:

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