Multi-Commodity Flow-Based Spreading in a Commercial Analytic Placer

Modern analytic placement tools are commonly built around the idea of iterative Lower Bound (LB) and Upper Bound (UB) placement. The LB step optimizes wirelength and timing while ignoring overlap and cell-type constraints, whereas the UB step attempts to spread cells and satisfy constraints without harming design quality. Top-down geometric partitioning techniques have traditionally been used to spread cells during UB placement. We propose a new, network flow-based approach for UB placement which does a better job of preserving quality by optimizing the displacement of cells from their LB positions. Our approach not only addresses cell overlap, but also accommodates complex region constraints and simultaneously spreads unit-sized logic, carry chains, and blocks like RAMs and DSPs. Our technique is scalable, does not require geometric partitioning, and is suitable for both flat and clustered placement flows.We deployed our algorithm in a commercial FPGA CAD flow, and show that it reduces HPWL by 6.4% on average (up to 22.8% in the best case) while improving worst-slack timing in over 90% of designs, compared to a state-of-the-art alternative.