H Root Stop Open ClosedRightLeft

We investigate techniques for verifying hierarchical systems, i.e.,-nite state systems with a nesting capability. The straightforward way of analyzing a hierarchical system is to rst atten it into an equivalent non-hierarchical system and then apply existing nite state system ver-iication techniques. Though conceptually simple, attening is severely punished by the hierarchical depth of a system. To alleviate this problem, we develop a technique exploiting the hierarchical structure to reuse earlier reachability checks of superstates to conclude reachability of substates. We combine the reusability technique with the successful compositional technique of 13] and investigate the combination experimentally on industrial systems and hierarchical systems generated according to our expectations to real systems. The experimental results are very encouraging: whereas a attening approach degrades in performance with an increase in the hierarchical depth (even when applying the successful compositional technique of 13]), the new approach proves not only insensitive to the hierarchical depth, but even leads to improved performance as the depth increases.