Metrics for structural logic synthesis

Routability or wiring congestion in a VLSI chip is becoming increasingly important as chip complexity increases. Congestion has a significant impact on performance, yield and chip area. Although advances in placement algorithms have attempted to alleviate this problem, the inherent structure of the logic netlist has a significant impact on the mutability irrespective of the placement algorithm used. Placement algorithms find optimal assignment of locations to the logic and do not have the ability to change the netlist structure. Significant decisions regarding the circuit structure are made early in synthesis, such as during the technology independent logic optimization step. Optimizations in this step use literal count as a metric for optimization and do not adequately capture the intrinsic entanglement of the netlist. Two circuits with identical literal counts may have significantly different congestion characteristics post placement. In this paper, we motivate that a property of the network structure called adhesion can make a significant contribution to routing congestion. We then provide a metric to measure this property. We also show that adhesion, as measured by this metric, can be used in addition to literal counts to estimate and optimize post routing congestion early in the design flow.