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History-based VLSI legalization using network flow

In VLSI placement, legalization is an essential step where the overlaps between gates/macros must be removed. In this paper, we introduce a history-based legalization algorithm with min-cost network flow optimization. We find a legal solution with the minimum deviation from a given placement to fully honor/preserve the initial placement, by solving a gate-centric network flow formulation in an iterative manner. In order to realize a flow into gate movements, we develop efficient techniques which solve an approximated Subset-sum problem. Over the iterations, we factor into our formulation the history which captures a set of likely-to-fail gate movements. Such a history-based scheme enables our algorithm to intelligently legalize highly complex designs. Experimental results on over 740 real cases show that our approach is significantly superior to the existing algorithms in terms of failure rate (no failure) as well as quality of results (55% less max-deviation).