Espresso-like Iterative Algorithm for Three-Level Logic Minimization

Minimization is an important part of any logic synthesis algorithm. Espresso algorithm can be used as a first step for global two-level minimization. Then factorization is performed to obtain multi-level structure and various local optimization algorithms are applied. This paper presents a new algorithm called Xpresso to replace Espresso in described synthesis flow. It works with three-level logic circuit structure in a form of DNF with an extra layer of XORs. Such structure allows more compact representation than pure DNF while keeping global minimization feasible. Proposed minimization algorithm consists of three iterative Espresso-like steps: expand, irredundant and reduce, tweaked to work with three-level structure. Computational complexity stays mostly at original Espresso's level with most expensive part of expand being transferred to GPU. Experimental results are provided for combinational logic circuits from LGSynth'91 benchmark. Xpresso's outputs are compared to those of Espresso by operation count and a few other metrics.