Depth-optimal Buffer and Splitter Insertion and Optimization in AQFP Circuits

The Adiabatic Quantum-Flux Parametron (AQFP) is an energy-efficient superconducting logic family. AQFP technology requires buffer and splitting elements (B/S) to be inserted to satisfy path-balancing and fanout-branching constraints. B/S insertion policies and optimization strategies have been recently proposed to minimize the number of buffers and splitters needed in an AQFP circuit. In this work, we study the B/S insertion and optimization methods. In particular, the paper proposes: i) an algorithm for B/S insertion that guarantees global depth optimality; ii) a new approach for B/S optimization based on minimum register retiming; iii) a B/S optimization flow based on (i), (ii), and existing work. We show that our approach reduces the number of B/S up to 20% while guaranteeing optimal depth and providing a 55x speed-up in run time compared to the state-of-the-art.