RFR: 8299414: JVMTI FollowReferences should support references from VirtualThread stack [v9]

[Serguei Spitsyn]

On Mon, 1 May 2023 18:26:30 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:
> 
>   Added "no continuations" test case

src/hotspot/share/prims/jvmtiTagMap.cpp line 2319:

> 2317:           }
> 2318:         }
> 2319:       }

The fragments 2289-2303 and 2305-2319 are based on the `StackValueCollection` and look very similar.
It can be worth to refactor these fragments into two function calls:

   bool report_stack_value_collection(jmethodID method, int idx_base,
             StackValueCollection* elems, jlocation bci) {  
     for (int index = 0; index < exprs->size(); index++) {
        if (exprs->at(index)->type() == T_OBJECT) {
          oop obj = elems->obj_at(index)();
          if (obj == nullptr) {
            continue;
          }
          // stack reference
          if (!CallbackInvoker::report_stack_ref_root(thread_tag, tid, depth, method,
                                                      bci, idx_base + index, obj)) {
            return false;
          }
        }
      }
      return true; // ???

      . . . . .
      jlocation bci = (jlocation)jvf->bci();
      StackValueCollection* locals = jvf->locals();
      if (!report_stack_value_collection(method, locals, 0 /* idx_base*/, bci)) {
        return false;
      }      
      StackValueCollection* exprs = jvf->expressions();
      if (!report_stack_value_collection(method, exprs, locals->size(), bci)) {
        return false;
      }

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/13254#discussion_r1182363174