On the Complexity of Symbolic Verification and Decision Problems in Bit-Vector Logic

We study the complexity of decision problems encoded in bit-vector logic. This class of problems includes word-level model checking, i.e., the reachability problem for transition systems encoded by bitvector formulas. Our main result is a generic theorem which determines the complexity of a bit-vector encoded problem from the complexity of the problem in explicit encoding. In particular, NL-completeness of graph reachability directly implies PSpace-completeness and ExpSpace- completeness for word-level model checking with unary and binary arity encoding, respectively. In general, problems complete for a complexity class C are shown to be complete for an exponentially harder complexity class than C when represented by bit-vector formulas with unary encoded scalars, and further complete for a double exponentially harder complexity class than C with binary encoded scalars. We also show that multi-logarithmic succinct encodings of the scalars result in completeness for multi-exponentially harder complexity classes. Technically, our results are based on concepts from descriptive complexity theory and related techniques for OBDDs and Boolean encodings.