Improved use of the carry-save representation for the synthesis of complex arithmetic circuits

Abstract

The increasing importance of datapath circuits in complex systems-on-chip calls for special arithmetic optimisations. The goal is to achieve automatically the handcrafted results which escape classic logic optimisations. Some work has been done in the recent years to infer the use of the carry-save representation in the synthesis of arithmetic circuits. Yet, many cases of practical interest cannot be handled due to the scattering of logic operations among the arithmetic ones - especially in arithmetic computations which are originally described at the bit level in high-level languages such as C. We therefore introduce an algorithm to restructure dataflow graphs so that they can be synthesized in high-quality arithmetic circuits, close, to those that an expert designer would conceive. On typical embedded software benchmarks which could be advantageously implemented with hardware accelerators, our technique always reduces tangibly the critical path by up to 46% and generally achieves the quality of manual implementations. In many cases, our algorithm also manages to reduce the cell area by up to 10-20%.