Hierarchical placement and floorplanning in BEAR

Hierarchical placement and floorplanning algorithms for rectangular blocks are described and implemented as part of the BEAR building block layout system developed at the University of California at Berkeley. The algorithm combines the goal orientation of top-down approaches with the block orientation of bottom-up techniques. The result is a meet-in-the-middle strategy. It considers the mutual dependency between placement and routing explicitly by incorporating a novel method of hierarchical routing area estimation. If the layout includes flexible blocks, the placement result can be further optimized by resizing these blocks subject to constraints on their areas and aspect ratios. Placement and floorplanning are refined more and more (with possible topological change) as routing proceeds. Global routing is updated incrementally to eliminate the need for iterations between placement and routing, thus achieving a more uniform design flow