PIMap: A Flexible Framework for Improving LUT-Based Technology Mapping via Parallelized Iterative Optimization.

Modern FPGA synthesis tools typically apply a predetermined sequence of logic optimizations on the input logic network before carrying out technology mapping. While the “known recipes” of logic transformations often lead to improved mapping results, there remains a nontrivial gap between the quality metrics driving the pre-mapping logic optimizations and those targeted by the actual technology mapping. Needless to mention, such miscorrelations would eventually result in suboptimal quality of results. In this article, we propose PIMap, which couples logic transformations and technology mapping under an iterative improvement framework for LUT-based FPGAs. In each iteration, PIMap randomly proposes a transformation on the given logic network from an ensemble of candidate optimizations; it then invokes technology mapping and makes use of the mapping result to determine the likelihood of accepting the proposed transformation. By adjusting the optimization objective and incorporating required time constraints during the iterative process, PIMap can flexibly optimize for different objectives including area minimization, delay optimization, and delay-constrained area reduction. To mitigate the runtime overhead, we further introduce parallelization techniques to decompose a large design into multiple smaller sub-netlists that can be optimized simultaneously. Experimental results show that PIMap achieves promising quality improvement over a set of commonly used benchmarks, including improving the majority of the best-known area and delay records for the EPFL benchmark suite.