Range and Bitmask Analysis for Hardware Optimization in High-Level Synthesis

We consider the extent to which the bit-level representation of variables can be used to optimize hardware generated by high-level synthesis (HLS). Two approaches to bit-level optimization are considered (individually and together): 1) range analysis, and 2) bitmask analysis. Range analysis aims to predetermine min/max ranges for variables to reduce the bitwidth required to represent variables in hardware. Bitmask analysis characterizes individual bits within a word as either constants (1 or 0), sign bits, or unknowns, where constants/don't-cares permit hardware to be eliminated under certain conditions. Static compiler-based analysis is contrasted with dynamic profiling-based analysis in terms of their potential to impact area and speed of HLS-generated hardware. For a set of benchmarks implemented in the Altera Cyclone II FPGA, results show bit-level optimizations in HLS based on static analysis reduce circuit area by 9%, on average, while additional optimizations based on dynamic analysis provide 34% area reduction.