an analytical approach to placement legalization
We present a method to achieve nearly legal placement while optimizing the traditional metrics in an analytical placement framework. A legalization penalty function term is added to the cost function of the placer. The purpose of this term is to remove overlaps and place cells into rows. The new term kicks in when global spreading cannot resolve overlaps any further. We study how this legalization term in placement helps to achieve better final placements when it is used in combination with wire-length driven analytical placement. Experimental results show that using this additional legalization cost term results in reduction of degradation of wire-length from 7.6\% to 0.7\% after discrete detailed placement. Optimization of wire-length along with the legalization term in placement shows 6\% improvement in total wire-length on average, which if translated into timing is 48\% of total negative slack. A further feature to control cell density helps reduce congestion by 33\%.
placement legalization analytical placement detailed placement placement spreading