The ephemeral history register: flexible scheduling for rule-based designs

The quality of high-level synthesis results is strongly dependant on the concurrency that can be found in designs. In this paper we introduce the ephemeral history register (EHR), a new primitive state element that enables concurrent scheduling of arbitrary rules in a rule-based design framework. The key properties of the EHR are that it allows multiple operations to write to the same state simultaneously, and that the EHR maintains a history of all writes that occur within a clock-cycle. Using the EHR, we present an algorithm that takes as input a design and a desired schedule, and produces a functionally equivalent design that satisfies the desired concurrency and ordering of operations. A processor pipeline is used to illustrate the effectiveness of the EHR and scheduling algorithm, and shows how this approach significantly improves on previous synthesis algorithms for rule-based designs.