RFR: JDK-8299229: Allow UseZGC with JVMCI and enable nmethod entry barrier support [v2]

[Tom Rodriguez]

On Tue, 17 Jan 2023 21:23:36 GMT, Erik Österlund <eosterlund at openjdk.org> wrote:

>> Tom Rodriguez has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains one commit:
>> 
>>   Allow UseZGC with JVMCI and enable nmethod entry barrier support
>
> src/hotspot/share/jvmci/jvmciCodeInstaller.cpp line 732:
> 
>> 730: 
>> 731:     if (UseZGC && _nmethod_entry_patch_offset == -1) {
>> 732:       // ZGC requires the use of entry barriers for correctness
> 
> Note that G1 also requires nmethod entry barriers for correctness.

We've been happily running without them for a long time which is why we're just now getting around to supporting them.  Are we sitting on a ticking time bomb?  I do want to make them required as of 21 but maybe we could defer that to a future commit once my Graal changes to support the entry barriers has landed?

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