RFR: 8299414: JVMTI FollowReferences should support references from VirtualThread stack [v16]

[Serguei Spitsyn]

On Fri, 5 May 2023 23:03:38 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 three additional commits since the last revision:
> 
>  - cosmetic changes in libVThreadStackRefTest.cpp
>  - collect VT stack references if initial_object is null
>  - moved transition disabler to correct functions

test/hotspot/jtreg/serviceability/jvmti/vthread/FollowReferences/libVThreadStackRefTest.cpp line 140:

> 138:     LOG("JVMTI FollowReferences error: %d\n", err);
> 139:     env->FatalError("FollowReferences failed");
> 140:   }

Nit: `classesCount` and `heapCallBacks` need c-style names.

test/hotspot/jtreg/serviceability/jvmti/vthread/FollowReferences/libVThreadStackRefTest.cpp line 153:

> 151: }
> 152: 
> 153: static void printCreatedClass(JNIEnv* env, jclass cls) {

Nit: This function should have a c-style name.

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PR Review Comment: https://git.openjdk.org/jdk/pull/13254#discussion_r1186668539
PR Review Comment: https://git.openjdk.org/jdk/pull/13254#discussion_r1186668689