--- old/src/hotspot/share/runtime/vmThread.cpp	2018-03-22 17:04:17.144829989 +0100
+++ new/src/hotspot/share/runtime/vmThread.cpp	2018-03-22 17:04:16.944829996 +0100
@@ -576,6 +576,31 @@
   }
 }
 
+// A SkipGCALot object is used to elide the usual effect of gc-a-lot
+// over a section of execution by a thread. Currently, it's used only to
+// prevent re-entrant calls to GC.
+class SkipGCALot : public StackObj {
+  private:
+   bool _saved;
+   Thread* _t;
+
+  public:
+#ifdef ASSERT
+    SkipGCALot(Thread* t) : _t(t) {
+      _saved = _t->skip_gcalot();
+      _t->set_skip_gcalot(true);
+    }
+
+    ~SkipGCALot() {
+      assert(_t->skip_gcalot(), "Save-restore protocol invariant");
+      _t->set_skip_gcalot(_saved);
+    }
+#else
+    SkipGCALot(Thread* t) { }
+    ~SkipGCALot() { }
+#endif
+};
+
 void VMThread::execute(VM_Operation* op) {
   Thread* t = Thread::current();