diff -r 992120803410 -r e044997c2eda src/share/vm/opto/machnode.cpp
--- a/src/share/vm/opto/machnode.cpp	Mon Oct 22 05:26:38 2018 -0400
+++ b/src/share/vm/opto/machnode.cpp	Mon Oct 15 11:00:27 2018 +0200
@@ -619,6 +619,7 @@
   }
 
   // Values outside the domain represent debug info
+  assert(in(idx)->ideal_reg() != Op_RegFlags, "flags register is not spillable");
   return *Compile::current()->matcher()->idealreg2spillmask[in(idx)->ideal_reg()];
 }