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

[Tom Rodriguez]

On Wed, 24 Mar 2021 18:43:48 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

>> Wouldn't using OopStorage require an extra level of indirection for the field?
>
> 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.

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