Extra assertions that gc state synchronization is correct

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

@@ -298,6 +298,7 @@ class ShenandoahHeap : public CollectedHeap, public ShenandoahSpaceInfo {
 public:
   char gc_state() const;
   void set_gc_state_all_threads();
+  bool has_gc_state_changed() { return _gc_state_changed; }
 
   void set_concurrent_mark_in_progress(bool in_progress);
   void set_evacuation_in_progress(bool in_progress);

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

@@ -35,12 +35,23 @@
 #include "interpreter/oopMapCache.hpp"
 #include "memory/universe.hpp"
 
+bool VM_ShenandoahOperation::doit_prologue() {
+  assert(!ShenandoahHeap::heap()->has_gc_state_changed(), "GC State can only be changed on a safepoint.");
+  return true;
+}
+
+void VM_ShenandoahOperation::doit_epilogue() {
+  assert(!ShenandoahHeap::heap()->has_gc_state_changed(), "GC State was not synchronized to java threads.");
+}
+
 bool VM_ShenandoahReferenceOperation::doit_prologue() {
+  VM_ShenandoahOperation::doit_prologue();
   Heap_lock->lock();
   return true;
 }
 
 void VM_ShenandoahReferenceOperation::doit_epilogue() {
+  VM_ShenandoahOperation::doit_epilogue();
   OopMapCache::cleanup_old_entries();
   if (Universe::has_reference_pending_list()) {
     Heap_lock->notify_all();

src/hotspot/share/gc/shenandoah/shenandoahVMOperations.hpp

@@ -48,6 +48,8 @@ class VM_ShenandoahOperation : public VM_Operation {
 public:
   VM_ShenandoahOperation() : _gc_id(GCId::current()) {};
   virtual bool skip_thread_oop_barriers() const { return true; }
+  bool doit_prologue() override;
+  void doit_epilogue() override;
 };
 
 class VM_ShenandoahReferenceOperation : public VM_ShenandoahOperation {