Exploring Writeback Designs for Efficiently Leveraging Parallel-Execution Units in FPGA-Based Soft-Processors

Maximizing processor performance depends on maximizing the product of instruction throughput and clock frequency. Writeback mechanisms and forwarding networks heavily impact both of these properties along with the resource usage and scalability of the processor design. Furthermore, these mechanisms are typically multiplexer heavy which can make their implementation resource inefficient on FPGAs. In this paper, we explore multiple different writeback and result storage mechanisms using an FPGA-based RISC-V soft-processor (Taiga), exploring both exception-safe and non-exception-safe designs. Writeback mechanisms based on per-unit result storage and centralized storage are explored while leveraging FPGA specific resources such as LUTRAMs. We evaluate the designs based on their impact on instruction throughput, processor frequency, and scalability of both simultaneous instructions in-flight and the number of execution units. As each design has different characteristics, we focus on comparing and contrasting the designs. We find that across all designs, average IPC can vary by up to 11%, with a few designs reaching the maximum IPC of one for some benchmarks. Clock frequency is found to vary by up to 20% across the designs, but is not significantly impacted when increasing the number of execution units. Scaling up the instructions in-flight is found to have the greatest variability, with LUT usage increasing by 3% to 93% across the different designs. Overall, we find that under current constraints, a commit-buffer design provides the highest combination of performance and performance per LUT.