RFR: 8307567: Avoid relocating global roots to metaspaceObjs in CDS dump [v5]

[Calvin Cheung]

On Sat, 13 May 2023 05:09:57 GMT, Ioi Lam <iklam at openjdk.org> wrote:

>> The functions `ArchiveBuilder::relocate_roots()` and `ArchiveBuilder::relocate_vm_classes()` relocate global roots such as `Universe::_typeArrayKlassObjs[]` and `vmClasses::_klasses[]` to point to locations inside the CDS "buffer" space.
>> 
>> As a result, the VM is no longer usable after CDS dumping has finished.
>> 
>> Previously, we tried to save some of the VM states and restore them after CDS dump has finished. See [JDK-8264735](https://bugs.openjdk.org/browse/JDK-8264735), which added functions like `FileMapInfo::clone_shared_path_table()`.
>> 
>> However, this is too cumbersome and doesn't cover all cases. The intention of the global root relocation is to make it easy to write the roots using `WriteClosure::do_ptr()`, which could handle only pointers into the "buffer" space. If we call `get_buffered_addr()` inside `WriteClosure::do_ptr()`, then there's no need to modify the global roots.
>> 
>> Notes for reviewers:
>> 
>> - Start with changes in `WriteClosure::do_ptr()`
>> - Some code in SystemDictionaryShared also needed to call `get_buffered_addr()`
>> - I added new function `ArchiveBuilder::write_pointer_in_buffer()` to simplify writing pointers into the CDS buffer
>> -  `ArchiveBuilder::relocate_roots()`, `ArchiveBuilder::relocate_vm_classes()` and `FileMapInfo::clone_shared_path_table()` are removed.
>>     - I am leaving `CloneDumpTimeClassTable` and `CloneDumpTimeLambdaProxyClassTable` for now. I plan to remove them in a follow up RFE after more testing.
>
> Ioi Lam has updated the pull request incrementally with one additional commit since the last revision:
> 
>   removed obsolete comment

Thanks for making `SharedClassPathEntry` a regular `MetaspaceObj`. It looks a lot cleaner now.

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Marked as reviewed by ccheung (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/13854#pullrequestreview-1425836083