8379695: [lworld] Update GCBasher to use value classes

[lmesnik]

The GCbasher si a GC stress test that create a lot of different objects and arrays.

It would be makes sense to update it to create a lot of different value classes and arrays of value types. It helps to ensure that GC works with different type of classes.

Please, note that after this fix GCbasher is going to enable preview always.
Do we want to keep original test while value classes are still preview feature?
I think that it is not needed, however if someone thinks it is good idea, I can add gcbasher-nopreview and keep until we start removing @enablePreview from our tests.

In the long term the problem might be that some structure are flattened, and test start generating less objects. However they are much more diverse now.

UPDATE. There are 3 possible approaches to maintain value/non-value classes

1. keep old version of gcbahser
2. keep value/non-value classes in different directories and have 2 tests with different @compile commands to choose different files
3. add @ValueClass annotation and support javac plugin that change value class attribute during compilation. In this case we need to add 'PREVEW_MODE' when tests are compiles with --enable-preview and this pluign.

The 2) and 3) might work well to maintain value/non-value variants of tests. While it looks overcompilcated for gcbasher. So I prefer to either have 'gcbasher-novalue' or use only variant with --enable-preview.

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• JDK-8379695: [lworld] Update GCBasher to use value classes (Enhancement - P4)

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[Arraying]

Thank you for working on this!

Please, note that after this fix GCbasher is going to enable preview always.
Do we want to keep original test while value classes are still preview feature?
I think that it is not needed, however if someone thinks it is good idea, I can add gcbasher-nopreview and keep until we start removing @enablePreview from our tests.

My opinion is that we would lose quite a bit of test coverage w/o preview if we get rid of GCBasher. I think I'd like to see both, at least over a transitionary period.

keep old version of gcbahser

I think this would be the pragmatic choice. However, we should probably introduce a comment in both source files indicating that they are mirrored.

[lmesnik]

I think this would be the pragmatic choice. However, we should probably introduce a comment in both source files indicating that they are mirrored.
Agree, let wait for https://bugs.openjdk.org/browse/JDK-8380084 is implemented and use annotation to mark value classes so gcbasher might works with and without --enable-preview.