Improving Static Ordering of BDDs for Reachability Analysis

Binary decision diagrams (BDDs) are used for automatic synthesis and formal verification of combinational and sequential circuits. However, a larger adoption of these technologies for sequential designs still depends on a more efficient use of BDDs. One important factor is the order of the variables in the BDD, which has a direct impact on the space (and time) requirements of the reachability algorithms. As the problem for finding the best order is NP-complete and requires, in practice, static and dynamic ordering heuristics, we introduce a new approach for finding an initial order for BDDs involved in the design of sequential circuits. This approach, called the weighting heuristics, has been implemented in the VIS toolset and favorably compares with state-of-the art heuristics.