RFR: 8292591: Experimentally add back barrier-less Java thread transitions [v5]

[Magnus Ihse Bursie]

On Fri, 9 Sep 2022 10:05:11 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:
> 
>   Fixed ws

I apologize that the build support was not in place to properly use the header file when present. It turned out that this was the first time we actually ran into a well-known (by me and Erik, at least ;-)) deficiency in the build system, where the split between the target and the build toolchain never were made fully complete.

-------------

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