RFR: 8299414: JVMTI FollowReferences should support references from VirtualThread stack [v18]

[Serguei Spitsyn]

On Mon, 8 May 2023 21:32:54 GMT, Alex Menkov <amenkov at openjdk.org> wrote:

>> The fix updates JVMTI FollowReferences implementation to report references from virtual threads:
>> - unmounted vthreads are detected, their stack references for JVMTI_HEAP_REFERENCE_STACK_LOCAL/JVMTI_HEAP_REFERENCE_JNI_LOCAL;
>> - stacks of mounted vthreads are splitted into 2 parts (virtual thread stack and carrier thread stack), references are reported with correct thread id/class tag/object tags/frame depth;
>> - common code to handle stack frames are moved into separate class;
>> 
>> Threads are reported as:
>> - platform threads: JVMTI_HEAP_REFERENCE_THREAD (as before);
>> - mounted vthreads (synthetic references, consider them as heap roots because carrier threads are roots): JVMTI_HEAP_REFERENCE_OTHER;
>> - unmounted vthreads: not reported as heap roots.
>
> Alex Menkov has updated the pull request incrementally with one additional commit since the last revision:
> 
>   report_java_stack_refs/report_native_stack_refs

src/hotspot/share/prims/jvmtiTagMap.cpp line 2785:

> 2783: // walks the stack of the thread, finds all references (locals
> 2784: // and JNI calls) and reports these as stack references
> 2785: inline bool VM_HeapWalkOperation::collect_stack_refs(JavaThread* java_thread,

It makes sense to refactor the body of the `collect_stack_refs` function by adding 2 functions:
 - collect_virtual_thread_stack_refs
 - collect_platform_thread_stack_refs
 It looks like the same register map can be used for both.

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PR Review Comment: https://git.openjdk.org/jdk/pull/13254#discussion_r1189153256