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

[Erik Österlund]

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

There are speculative trap entries in the extra data section of the MDOs. Rows with stale metadata must be removed by the GC during class unloading. This used to be done in safepoints only, but with concurrent class unloading this is done while holding the extra_data_lock of the MDO. So naturally, for this to be sound, any reader that traverses these rows must hold the same lock (cf. https://github.com/openjdk/jdk/commit/f08eeac278b0785c0bdff0bddf46b66f65822357).

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