[jdk17] RFR: 8269240: java/foreign/stackwalk/TestAsyncStackWalk.java test failed with concurrent GC [v2]

[Jorn Vernee]

On Thu, 15 Jul 2021 23:43:38 GMT, Vladimir Kozlov <kvn at openjdk.org> wrote:

>> Sorry, I sent the wrong godbolt link: https://godbolt.org/z/1665fWzff
>
> @JornVernee I have small correct to your comment. We use simple inheritance for Thread subclasses. Their instances have **one** vtbl pointer at the same offset as in base class. But this pointer will point to separate vtable for each subclass (and base class). The layout (sequence) of methods pointers in vtable is the same in base class and subclasses. But subclass specific methods pointers will be different.
> 
> The only issue is that you have to make sure to cast passed object pointer to correct subclass (or base class). Otherwise you will get incorrect vtable and incorrect virtual methods pointers.
> 
> R15 is used by our JIT compiled code and Interpreter code which are executed only in JavaThread so the pinter it contains is JavaThread*

Thanks Vladimir. This also matches what I was seeing in compiler explorer, but I wasn't sure if we could assume it always worked like that with every C++ compiler.

It sound like R15 is expected to hold a `JavaThread*` though, so making the return type of `on_entry` be `JavaThread*` as David suggested seems correct.

Thanks

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PR: https://git.openjdk.java.net/jdk17/pull/149