Modeling the Global Critical Path in Concurrent Systems

We show how the global critical path can be used as a practical tool for understanding, optimizing and summarizing the behavior of highly concurrent self-timed circuits. Traditionally, critical path analysis has been applied to DAGs, and thus was constrained to combinatorial sub-circuits. We formally define the global critical path (GCP) and show how it can be constructed using only local information that is automatically derived directly from the circuit. We introduce a form of Production Rules, which can accurately determine the GCP for a given input vector, even for modules which exhibit choice and early termination. The GCP provides valuable insight into the control behavior of the application, which help in formulating new optimizations and re-formulating existing ones to use the GCP knowledge. We have constructed a fully automated framework for GCP detection and analysis, and have incorporated this framework into a high-level synthesis tool-chain. We demonstrate the effectiveness of the GCP framework by re-formulating two traditional CAD optimizations to use the GCP—yielding efficient algorithms which improve circuit power (by up to 9%) and performance (by up to 60%) in our experiments.