Better Together: Combining Analytical and Annealing Methods for FPGA Placement

Placement is a critical step in the FPGA design implementation flow that strongly impacts routability and timing closure. Recent state-of-the-art academic analytical placers have achieved impressive scalability but are limited to AMD Ultrascale-like architectures and mostly synthetic designs. On the other hand, VPR, the place and route tool within the widely used open-source Verilog-to-Routing (VTR) toolchain, can produce a legal placement for any arbitrary architecture; however, its simulated annealing placer scales poorly. Thus, there is a clear need to bring scalable, high-quality placement to realistic architectures and circuits. In this work, we develop a hybrid framework that combines the strength of a scalable flat analytical placer with the flexibility of simulated annealing techniques to adapt to various architectures and circuits, substantially improving the quality of results. We augment the state-of-theart analytical elfPlace FPGA placer as aug-elfPlace, generalizing its architecture modeling to handle real-world constraints and target different and more complete architectures. We leverage VPR’s legalization capability to integrate with external placers such as aug-elfPlace. VPR’s simulated annealing placer can further optimize the legalized placement, and VPR’s router and timing analysis can provide final quality results. By integrating wirelength-driven aug-elfPlace and VPR, our hybrid framework achieves up to 2% timing improvement with 15% reduction in routed wirelength compared to timing-driven VPR, on average across the large and heterogeneous Titan23 benchmark suite targeting an Intel Stratix-IV-like architecture.