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.

FPGA device area is dominated by interconnect, so low-cost FPGA architectures often have reduced interconnect capacity. This limited routing capacity creates a hard channel width constraint that can make it difficult for CAD tools to successfully map a circuit into these devices. Instead of migrating a design to a high-cost, resource-rich architecture that is easier to route, we present a cheaper alternative: a fully automated CAD flow (Un/DoPack) that finds local regions of high interconnect demand and reduces it by spreading out the logic in that region. This is done by introducing whitespace in the form of empty logic elements (LEs) within the configurable logic blocks (CLBs) of the congested region. After spreading, the congested region occupies more routing channels and so obtains access to greater aggregate interconnect capacity. Although this has the side effect of using more CLBs, it has the advantage of lowering peak interconnect demands and making a previously-unroutable circuit routable. We also design a new set of synthetic benchmark circuits that model interconnect variation within a large design. Using these benchmarks, we show that circuits with high interconnect variation require FPGA devices to have large channel widths. However, since congestion of such circuits is localized, Un/DoPack is very good at reducing the peak demands of circuits with high interconnect variation. Our results suggest that even for an average Rent exponent of 0.62 (a modest value), a large variation of this exponent within a design will also require FPGAs to have large channel widths. Thus, it is crucial to study interconnect variation of benchmark circuits when designing lowcost FPGAs. Previous research studying interconnect properties focuses on average Rent exponent values of each design, but we believe new work should study variation as well. For circuits with high interconnect variation, we demonstrate that channel widths can be reduced by up to ~40% with only ~10% increase in area.

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