Fanout-Bounded Logic Synthesis for Emerging Technologies

Abstract

In logic circuits, the number of fanouts a gate can drive is limited, and such limits are tighter in emerging technologies such as superconducting electronic circuits. Moreover, some such technologies, e.g., adiabatic quantum-flux-parametron (AQFP), pose additional constraints such as the need for balanced input-to-output paths to ensure proper signal propagation. In this work, targeting emerging technologies, we study the problem of re-synthesizing a logic network with bounded-fanout gates while minimizing area for a given depth. Namely, we 1) formulate the fanout-bounded synthesis (FBS) problem for a fixed target logic depth as an integer linear program (ILP), 2) propose a scalable top-down approach to construct a feasible solution to the ILP, and 3) extend both the exact and the heuristic approaches to the setting of path-balanced networks. Using our ILP, we obtain the global optimum solutions for a number of benchmarks that serve as ground truth for evaluating heuristic algorithms in both general and path-balanced FBS. Our heuristic algorithm for general FBS achieves an 11.82% better area than the state of the art with matching or better delays while attaining the optimum/near-optimum area for several considered benchmarks. For the path-balanced setting, our heuristic approach achieves 8.76% better delay on average with an average area improvement of 0.5% when using AQFP as the exemplar technology, while achieving more than 17% better delays on several benchmarks.