Replace: An incremental placement algorithm for field programmable gate arrays

Recompiling a large circuit after making a few logic changes is a time-consuming process. We present an incremental placement algorithm for FPGAs that is focused on extremely fast runtime for changes which can be localized. It is capable of handling multiple changes across large regions of an FPGA. This is especially useful when used with a floorplan where a modified subcircuit is instantiated several times in the design hierarchy or where several subcircuits are modified. The algorithm is simpler and faster than past approaches because its insertion and legalization steps are based on CPU-efficient shifting steps which do not continuously evaluate the impact of each move on costs. Instead, any lost quality is recovered by a fast, low-temperature anneal at the end. When 35,000 out of 50,000 LUTs are modified, the incremental placement (including fast anneal) is 7 times faster than VPR's "fast placement" from scratch with only 2% quality degradation. The key concepts utilized in the incremental placement algorithm include uses of floor-planning constraints, CPU-efficient CLB shifting, super placement grid and a tuned annealing refinement process.