8239354: Shenandoah: minor enhancements to traversal GC

Reviewed-by: rkennke

Author: zhengyu123

src/hotspot/share/gc/shenandoah/shenandoahHeap.inline.hpp

@@ -248,6 +248,7 @@ inline oop ShenandoahHeap::evacuate_object(oop p, Thread* thread) {
   }
 
   assert(ShenandoahThreadLocalData::is_evac_allowed(thread), "must be enclosed in oom-evac scope");
+  assert(is_concurrent_traversal_in_progress() || !is_traversal_mode(), "Should not evacuate objects");
 
   size_t size = p->size();
 

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

@@ -571,8 +571,6 @@ void ShenandoahTraversalGC::concurrent_traversal_collection() {
 }
 
 void ShenandoahTraversalGC::final_traversal_collection() {
-  _heap->make_parsable(true);
-
   if (!_heap->cancelled_gc()) {
 #if COMPILER2_OR_JVMCI
     DerivedPointerTable::clear();
@@ -606,6 +604,11 @@ void ShenandoahTraversalGC::final_traversal_collection() {
     _heap->set_concurrent_traversal_in_progress(false);
     _heap->mark_complete_marking_context();
 
+    // A rare case, TLAB/GCLAB is initialized from an empty region without
+    // any live data, the region can be trashed and may be uncommitted in later code,
+    // that results the TLAB/GCLAB not usable. Retire them here.
+    _heap->make_parsable(true);
+
     _heap->parallel_cleaning(false);
     fixup_roots();