RFR: 8264016: [JVMCI] add some thread local fields for use by JVMCI

[Coleen Phillimore]

On Wed, 24 Mar 2021 19:19:28 GMT, Tom Rodriguez <never at openjdk.org> wrote:

>> It was https://bugs.openjdk.java.net/browse/JDK-8244997 - you were co-author :)
>> 
>> Maybe this jvmci_reserved_oop0 won't crash for the same reasons.  I don't know that.
>> 
>> I still would like to not see 45 lines of declarations for JVMCI added to JavaThread.  These should be in a separate header file and declared, as in https://bugs.openjdk.java.net/browse/JDK-8137018.  If you promise to fix 8137018, I'm fine with this change.
>
> co-author is a strong word.  :)  But that does ring a bell.  Why was threadObj problematic but the other existing oop fields were not?  JavaThread::oops_do_no_frames visits a lot of roots that aren't OopStorage.
> I can tackle JDK-8137018.  I think we'll need to add an alias mechanism to vmStructs_jvmci.cpp to maintain backward compatibility but that's fairly straightforward.

well, I didn't even list myself as author of that one.  _threadObj was a problem because some code (like thread dump management code) was accessing it from a terminating thread and the barriers were messed up. It was more complicated than that.  I don't know if this will be an issue for these declarations, and if you hide them in another place, we'll never know.

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PR: https://git.openjdk.java.net/jdk/pull/3147