RFR: JDK-8299229: Allow UseZGC with JVMCI and enable nmethod entry barrier support [v3]

[Erik Österlund]

On Fri, 20 Jan 2023 21:16:43 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 four commits:
> 
>  - 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

I think synchronization code is needed for accessing metadata from MDOs. And extra care around the extra data section. Stale metadata needs to be filtered, if the metadata is unloading. I don't see any code for that in this patch, so I have to assume it's missing.

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

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