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A modular synthesis flow is essential for a scalable and hierarchical design methodology. This paper considers a particular modular flow where each module has interface methods and the internal behavior of the module is described in terms of a set of guarded atomic actions on the state elements of the module. A module can also read and update the state of other modules but only by invoking the interface methods of those modules. This paper extends the past work on hardware synthesis of a set of guarded atomic actions by Hoe and Arvind to modules of such actions. It presents an algorithm that, given the scheduling constraints on the interface methods of the called modules, derives the "glue logic" and the scheduling constraints for the interface methods of the calling module such that the atomicity of the guarded actions is preserved across module boundaries. Such modules provide reusable IP which facilitates "correctness by construction" design methodology. It also reduces compile-times dramatically in comparison to the compilation that flattens all the modules first.

We present a new hardware synthesis methodology for guarded atomic actions (or rules), which satisfies performance-related scheduling specifications provided by the designer. The methodology is based on rule composition, and relies on the fact that a rule derived by the composition of two rules behaves as if the two rules were scheduled simultaneously. Rule composition is a well understood transformation in the TRS theoretical framework; however, previous rule composition approaches resulted in an explosion of the number of rules during synthesis, making them impractical for realistic designs. We avoid this problem through composition of conditional actions which generates one rule instead of 2 n rules when we combine n rules. We then show how this conditional composition of rules can be compiled into an efficient hardware structure which introduces new but derived interfaces in modules. We demonstrate the approach via a small circuit example (GCD) and then show its impact on the methodology to implement pipelined processors in Bluespec. Many ways of dealing with branches in pipelined processors or bypassing values can be expressed simply as different schedules. The results show improvements in performance over previous rule-based synthesis approaches as well as the ease of performance-related architectural exploration. In a somewhat surprising result, we show that simply by specifying a different schedule, one can automatically transform a single-issue processor pipeline into a superscalar pipeline.

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 rulebased designs.