A Case for Heterogeneous Technology-Mapping: Soft Versus Hard Multiplexers

Lookup table-based FPGAs offer flexibility but compromise on performance, as compared to custom CMOS implementations. This paper explores the idea of minimising this performance gap by using fixed, fine-grained, nonprogrammable logic structures in place of lookup tables (LUTs). Functions previously mapped onto LUTs can now be diverted to these structures, resulting in reduced LUT usage and higher operating speed. This paper presents a generic heterogeneous technology-mapping scheme for segregating LUTs and hard logic blocks. For the proof-of-concept, we choose to isolate multiplexers present in most general-purpose circuits. These multiplexers are mapped onto hard blocks of multiplexers that are present in existing commercial FPGA fabrics, but often unused. Since the hard multiplexers are already present, there is no additional performance or area penalty. Using this approach, an average reduction in LUT usage of 16% and an average speedup of 8% has been observed for the VTR benchmarks as compared to the LUTs-only implementation.