RFR: 8326957: Implement JEP 474: ZGC: Generational Mode by Default [v5]

[Doug Simon]

On Mon, 6 May 2024 07:23:14 GMT, Axel Boldt-Christmas <aboldtch at openjdk.org> wrote:

>> This is the implementation task for `JEP 474: ZGC: Generational Mode by Default`. See the JEP for details. [JDK-8326667](https://bugs.openjdk.org/browse/JDK-8326667)
>
> Axel Boldt-Christmas has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Default to non generational ZGC with JVMCI

After discussing with Tom offline, here's one more change you could make in this PR:
* Change the warning in `check_jvmci_supported_gc` to a hard error when an unsupported GC + compiler combination is requested.

Either way, as soon we have Gen ZGC implemented for Graal, we will:
1. Update `gc_supports_jvmci`.
2. Update `check_jvmci_supported_gc` to no longer warn and switch compilers but to exit the VM when an unsupported combination is requested.

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

PR Comment: https://git.openjdk.org/jdk/pull/18393#issuecomment-2108387245