RFR: JDK-8299229: Allow UseZGC with JVMCI and enable nmethod entry barrier support [v4]

[Tom Rodriguez]

On Mon, 23 Jan 2023 20:34:00 GMT, Tom Rodriguez <never at openjdk.org> wrote:

>> This exposes the required ZGC values to JVMCI and adds support for nmethod entry barriers.  The ZGC support is straightforward but the nmethod entry barrier required some reworking to fit better into JVMCI usage.  I also removed the epoch based barrier since it was no longer used with simplified the assumptions on the JVMCI side.  There is also a minor loom related fix to support post call nops included.  I could move that into a separate PR if that would be preferred.
>
> Tom Rodriguez has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains seven commits:
> 
>  - Handle concurrent unloading
>  - Merge branch 'master' into tkr-zgc
>  - Add missing declaration
>  - Replace NULL with nullptr in new code
>  - Merge branch 'master' into tkr-zgc
>  - Review fixes
>  - Allow UseZGC with JVMCI and enable nmethod entry barrier support

Sorry I've got too many branches in flight.  Yes I hadn't included the is_loader_alive changes which mirror what's being done in the various CI translate methods that process the same information. I've push those changes.   I'm not sure what locking you're talking about though as I don't see any obvious locking in the reading of this data in the CI.  There is some locking of the extra_data_lock which mainly seems to be guarding against safepoints during this processing.  There are no safepoints between the reading and the check so I'm not sure we need that.

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PR: https://git.openjdk.org/jdk/pull/11996