Improvements to Technology Mapping for LUT-Based FPGAs
Mishchenko, A.
Chatterjee, S.
Brayton, R. K.
Dept. of Electr. Eng. & Comput. Sci., California Univ., Berkeley, CA;
Abstract
This paper presents several orthogonal improvements to the state-of-the-art lookup table (LUT)-based field-programmable gate array (FPGA) technology mapping. The improvements target the delay and area of technology mapping as well as the runtime and memory requirements. 1) Improved cut enumeration computes all K-feasible cuts, without pruning, for up to seven inputs for the largest Microelectronics Center of North Carolina benchmarks. A new technique for on-the-fly cut dropping reduces, by orders of magnitude, the memory needed to represent cuts for large designs. 2) The notion of cut factorization is introduced, in which one computes a subset of cuts for a node and generates other cuts from that subset as needed. Two cut factorization schemes are presented, and a new algorithm that uses cut factorization for delay-oriented mapping for FPGAs with large LUTs is proposed. 3) Improved area recovery leads to mappings with the area, on average, 6% smaller than the previous best work while preserving the delay optimality when starting from the same optimized netlists. 4) Lossless synthesis accumulates alternative circuit structures seen during logic optimization. Extending the mapper to use structural choices reduces the delay, on average, by 6% and the area by 12%, compared with the previous work, while increasing the runtime 1.6 times. Performing five iterations of mapping with choices reduces the delay by 10% and the area by 19% while increasing the runtime eight times. These improvements, on top of the state-of-the-art methods for LUT mapping, are available in the package ABC
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