SAT-MapIt: A SAT-based Modulo Scheduling Mapper for Coarse Grain Reconfigurable Architectures

TL;DR

SAT-MapIt introduces a SAT-based approach for modulo scheduling on CGRAs, effectively exploring the solution space to improve mapping quality and often outperforming existing methods.

Contribution

It presents a novel SAT formulation for modulo scheduling, using the kernel mobility schedule to enhance mapping effectiveness on CGRAs.

Findings

Achieves better mapping results in 47.72% of benchmarks.

Finds lower iteration intervals in some cases.

Identifies valid mappings where previous methods fail.

Abstract

Coarse-Grain Reconfigurable Arrays (CGRAs) are emerging low-power architectures aimed at accelerating compute-intensive application loops. The acceleration that a CGRA can ultimately provide, however, heavily depends on the quality of the mapping, i.e. on how effectively the loop is compiled onto the given platform. State of the Art compilation techniques achieve mapping through modulo scheduling, a strategy which attempts to minimize the II (Iteration Interval) needed to execute a loop, and they do so usually through well known graph algorithms, such as Max-Clique Enumeration. We address the mapping problem through a SAT formulation, instead, and thus explore the solution space more effectively than current SoA tools. To formulate the SAT problem, we introduce an ad-hoc schedule called the \textit{kernel mobility schedule} (KMS), which we use in conjunction with the data-flow graph and the architectural information of the CGRA in order to create a set of boolean statements that describe all constraints to be obeyed by the mapping for a given II. We then let the SAT solver efficiently navigate this complex space. As in other SoA techniques, the process is iterative: if a valid mapping does not exist for the given II, the II is increased and a new KMS and set of constraints is generated and solved. Our experimental results show that SAT-MapIt obtains better results compared to SoA alternatives in $47.72\%$ of the benchmarks explored: sometimes finding a lower II, and others even finding a valid mapping when none could previously be found.