8244420: Shenandoah: Ensure _disarmed_value offset < 128

Reviewed-by: rkennke

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

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2018, 2019, Red Hat, Inc. All rights reserved.
+ * Copyright (c) 2018, 2020, Red Hat, Inc. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -56,6 +56,10 @@ private:
     _worker_id(INVALID_WORKER_ID),
     _force_satb_flush(false),
     _disarmed_value(ShenandoahCodeRoots::disarmed_value()) {
+
+    // At least on x86_64, nmethod entry barrier encodes _disarmed_value offset
+    // in instruction as disp8 immed
+    assert(in_bytes(disarmed_value_offset()) < 128, "Offset range check");
   }
 
   ~ShenandoahThreadLocalData() {