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

Type profiles are never read from the extra data section by JVMCI and speculative_trap_data_tag isn't exposed at all.  Exception seen flags are the only thing we search for in the extra data section and I'm not even sure why that's done.  See https://github.com/tkrodriguez/jdk/blob/tkr-zgc/src/jdk.internal.vm.ci/share/classes/jdk.vm.ci.hotspot/src/jdk/vm/ci/hotspot/HotSpotProfilingInfo.java#L144  and https://github.com/tkrodriguez/jdk/blob/tkr-zgc/src/jdk.internal.vm.ci/share/classes/jdk.vm.ci.hotspot/src/jdk/vm/ci/hotspot/HotSpotProfilingInfo.java#L113.  I could adjust the logic to simply disallow reads of Klass* from the extra data section until such a day as we actually need to support it.

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