CRIMSON: Compute-Intensive Loop Acceleration by Randomized Iterative Modulo Scheduling and Optimized Mapping on CGRAs

Coarse-grain reconfigurable arrays (CGRAs) are emerging accelerators that promise low-power acceleration of compute-intensive loops in applications. The acceleration achieved by CGRA relies on the efficient mapping of the compute-intensive loops by the CGRA compiler, onto the CGRA architecture. The CGRA mapping problem, being NP-complete, is performed in a two-step process, namely, scheduling and mapping. The scheduling algorithm allocates timeslots to the nodes of the data flow graph, and the mapping algorithm maps the scheduled nodes onto the processing elements of the CGRA. On a mapping failure, the initiation interval (II) is increased and a new schedule is obtained for the increased II. Most previous mapping techniques use the iterative modulo scheduling (IMS) algorithm to find a schedule for a given II. Since IMS generates a resource-constrained as-soon-as-possible (ASAP) scheduling, even with increased II, it tends to generate a similar schedule that is not mappable. Therefore, IMS does not explore the schedule space effectively. To address these issues, this article proposes CRIMSON, compute-intensive loop acceleration by randomized IMS and optimized mapping technique that generates random modulo schedules by exploring the schedule space, thereby creating different modulo schedules at a given and increased II. CRIMSON also employs a novel conservative test after scheduling to prune valid schedules that are not mappable. From our study conducted on the top 24 performance-critical loops (run for more than 7% of application time) from MiBench, Rodinia, and Parboil, we found that previous state-of-the-art approaches that use IMS, such as RAMP and GraphMinor could not map five and seven loops, respectively, on a $4\times 4$ CGRA, whereas CRIMSON was able to map them all. For loops mapped by the previous approaches, CRIMSON achieved a comparable II.