A study of composition schemes for mixed apply/compose based construction of ROBDDs

Reduced Ordered Binary Decision Diagrams (ROBDDs) have traditionally been built in a bottom-up fashion. In this scheme, the intermediate peak memory utilization is often larger than the final ROBDD size, limiting the complexity of the circuits which can be processed using ROBDDS. Recently we showed that for a large number of applications, the peak memory requirement can be substantially reduced by a suitable combination of bottom up (decomposition based) and top down (composition based) approaches of building ROBDDs. In this paper, we focus on the composition process. We detail four heuristic algorithms for finding good composition orders, and compare their utility on a set of standard benchmark circuits. Our schemes offer a matrix of time-memory tradeoff points.