Automated extra pipeline analysis of applications mapped to Xilinx UltraScale+ FPGAs

This paper describes the methodology and algorithms behind extra pipeline analysis tools released in the Xilinx Vivado Design Suite version 2015.3. Extra pipelining is one of the most effective ways to improve performance of FPGA applications. Manual pipelining, however, often requires significant efforts from FPGA designers who need to explore various changes in the RTL and re-run the flow iteratively. The automatic pipelining approach described in this paper, in contrast, allows FPGA users to explore latency vs. performance trade-offs of their designs before investing time and effort into modifying RTL. We describe algorithms behind these tools which use simple cut heuristics to maximize performance improvement while minimizing additional latency and register overhead. To demonstrate the effectiveness of the proposed approach, we analyse a set of 93 commercial FPGA applications and IP blocks mapped to Xilinx UltraScale+ and UltraScale generations of FPGAs. The results show that extra pipelining can provide from 18% to 29% potential Fmax improvement on average. It also shows that the distribution of improvements is bimodal, with almost half of benchmark suite designs showing no improvement due to the presence of large loops. Finally, we demonstrate that highly-pipelined designs map well to UltraScale+ and UltraScale FPGA architectures. Our approach demonstrates 19% and 20% Fmax improvement potential for the UltraScale+ and UltraScale architectures respectively, with the majority of applications reaching their loop limit through pipelining.