8241066: Shenandoah: fix or cleanup SH::do_full_collection

Reviewed-by: shade

Author: William Kemper

src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp

@@ -1576,8 +1576,8 @@ void ShenandoahHeap::collect_as_vm_thread(GCCause::Cause cause) {
   // cycle. We _could_ cancel the concurrent cycle and then try to run a cycle directly
   // on the VM thread, but this would confuse the control thread mightily and doesn't
   // seem worth the trouble. Instead, we will have the caller thread run (and wait for) a
-  // concurrent cycle in the prologue of the heap inspect/dump operation. This is how
-  // other concurrent collectors in the JVM handle this scenario as well.
+  // concurrent cycle in the prologue of the heap inspect/dump operation (see VM_HeapDumper::doit_prologue).
+  // This is how other concurrent collectors in the JVM handle this scenario as well.
   assert(Thread::current()->is_VM_thread(), "Should be the VM thread");
   guarantee(cause == GCCause::_heap_dump || cause == GCCause::_heap_inspection, "Invalid cause");
 }
@@ -1587,7 +1587,10 @@ void ShenandoahHeap::collect(GCCause::Cause cause) {
 }
 
 void ShenandoahHeap::do_full_collection(bool clear_all_soft_refs) {
-  //assert(false, "Shouldn't need to do full collections");
+  // This method is only called by `CollectedHeap::collect_as_vm_thread`, which we have
+  // overridden to do nothing. See the comment there for an explanation of how heap inspections
+  // work for Shenandoah.
+  ShouldNotReachHere();
 }
 
 HeapWord* ShenandoahHeap::block_start(const void* addr) const {