8362968: [JVMCI] support modifying locals in StackIntrospection on virtual threads

[tkrodriguez]

This a draft of adding support for JVMTI deferred locals on virtual threads. The JDK issue has a more full description but basically I'd like some feedback on whether the strategy seems ok and whether full JVMTI support for deferred locals would be a desired outcome. If it's not then I might be able to recast this in a way that is more JVMCI specific so it requires fewer changes to the core JVMTI deferred locals support which is probably the scariest looking part. Maybe @AlanBateman and @kevinjwalls could offer some opinions here?

For the deopt/loom interactions I tried to make the use of the chunk for heap frames fairly explicit as the absolutized heap frames make me nervous. But maybe I'm just making it too complicated?

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• JDK-8362968: [JVMCI] support modifying locals in StackIntrospection on virtual threads (Enhancement - P3)

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[sspitsyn]

@tkrodriguez Thank you for working on this! I'll look at the JVMTI and related changes but it is going to take some time.

[AlanBateman]

For the traditional usage, in the debugger, then it's not clear to me if it's worth having JVMTI deferred locals work with virtual threads. In the debugger it should be rare to want to change a local that is not in the top frame. So I think this is more about the Truffle usage.

[eregon]

In the debugger it should be rare to want to change a local that is not in the top frame.

I think it would be as likely as wanting to change a local that is not in the top frame for a platform thread.
Given that that is supported and is useful, I think it is worthwhile to support it for virtual threads too.

[tkrodriguez]

That doesn't necessarily have to be addressed in this PR. If this change lands then in the future it would be possible to support it in JVMTI with some minor additional changes. If there was stronger interest in supporting JVMTI with this change then it would be additional motivation for the changes.

[tkrodriguez]

@AlanBateman I don't know all the details of stackChunkOop management and one of my assumptions is that JavaThread::exit would have the chance to clean up any live frames that have jvmtiDeferredLocals. Otherwise we would leak this storage. Is that a reasonable assumption or is it possible for a virtual JavaThread to just be reclaimed by GC?