RFR: JDK-8299229: Allow UseZGC with JVMCI and enable nmethod entry barrier support [v5]

[Tom Rodriguez]

On Mon, 24 Apr 2023 16:50:59 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 11 commits:
> 
>  - Merge branch 'master' into tkr-zgc
>  - Use reloc for guard location and read internal fields using HotSpot accessors
>  - Merge branch 'master' into tkr-zgc
>  - Remove access to extra data section from Java code
>  - 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
>  - ... and 1 more: https://git.openjdk.org/jdk/compare/62acc882...c7bb4391

I've pushed a final round of fixes.  In particular the JVMCI Java code used to read fields in Klass using direct memory access but it must go through the C++ accessors to deal with concurrent unloading.  I also removed the Java side access to the extra data section and replaced it with a new getExceptionSeen accessor which is the only case were we use extra data.  I filed https://bugs.openjdk.org/browse/JDK-8306767 for the method data extra data repacking issue.

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PR Comment: https://git.openjdk.org/jdk/pull/11996#issuecomment-1520515290