Implementation and use of SPFDs in optimizing Boolean networks

S. Yamashita et al. (1996) introduced a new category for expressing the flexibility that a node can have in a multi level network. Originally presented in the context of FPGA synthesis, the paper has wider implications which were discussed by R.K. Brayton (1997). SPFDs are essentially a set of incompletely specified functions. The increased flexibility that they offer is obtained by allowing both a node to change as well as its immediate fanins. The challenge with SPFDs is: (1) to compute them in an efficient way, and (2) to use their increased flexibility in a controlled way to optimize a circuit. We provide a complete implementation of SPFDs using BDDs and apply it to the optimization of Boolean networks. Two scenarios are presented, one which trades literals for wires and the other rewires the network by replacing one fanin at a node by a new fanin. Results on benchmark circuits are very favorable.