In the hardware synthesis from a high-level language such as C, the bit length of variables is one of the key issues for the area and speed optimization. Usually, designers are required to optimize the bit-length of each variable manually using the time-consuming simulation on huge-data. In this paper, we propose an optimization method of the fractional bit length in the conversion from floating-point variables to fixed-point variables. The method is based on error propagation and the backward propagation of the accuracy limitation. The method is fully analytical and fast compared to simulation based methods.

High-level synthesis is a novel method to generate a RT-level hardware description automatically from a high-level language such as C, and is used at recent digital circuit design. Floating-point to fixed-point conversion with bit-length optimization is one of the key issues for the area and speed optimization in high-level synthesis. However, the conversion task is a rather tedious work for designers. This paper introduces automatic bit-length optimization method on floating-point to fixed-point conversion for high-level synthesis. The method estimates computational errors statistically, and formalizes an optimization problem as a non-linear problem. The application of NLP technique improves the balancing between computational accuracy and total hardware cost. Various constraints such as unit sharing, maximum bit-length of function units can be modeled easily, too. Experimental result shows that our method is fast compared with typical one, and reduces the hardware area.