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

[Robbin Ehn]

On Mon, 5 Sep 2022 13:36:31 GMT, Martin Doerr <mdoerr 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.
>
> 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:

SYS_gettid just have large ifdef else and then an error.
I guess e.g. ppc always compile on platforms with it defined since it doesn't need to define it.

The solution with a per plattform file as zSyscall does seems to be the nicest way. But IMHO a bit out of scope for this PR.

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