Fast Refinement on Placement Legalization for Designs with Mixed-Height Cells

In the row-based design, placement legalization is the process of moving the cell in circuits, which have already been optimally placed by a global placer, to rows with no cell overlap where the primary objective is to minimize the total amount of cell displacements. Normally, the global placers spend a huge amount of time to search for optimal cell locations. Instead, the legalization is required to be processed in short time. However, as the process technology advances, the legalization problem is much complex. This is because of using a mixture of cells of single-row and multi-row heights. As expected, it is shown that commercial legalizers have not fully optimized for such target designs. In this context, in the work, we propose a very fast but effective legalization refinement method to improve the legalization quality while spending a trivial running time in comparison with that of global placement. Precisely, we propose a simple greedy approach to moving cells to empty space with displacement saving. Through experiments with benchmark circuits, it is shown that our proposed method is able to further reduce the amount of total displacement by approximately 5.4% within 5 seconds over that produced by a commercial legalizer.