Exploring the design space of LUT-based transparent accelerators

Instruction set customization accelerates the performance of applications by compressing the length of critical dependence paths and reducing the demands on processor resources. With instruction set customization, specialized accelerators are added to a conventional processor to atomically execute dataflow subgraphs. Accelerators that are exploited without explicit changes to the instruction set architecture of the processor are said to be transparent. Transparent acceleration relies on a light-weight hardware engine to dynamically generate control signals for the accelerator, using subgraphs delineated by a compiler. The design of transparent subgraph accelerators is challenging, as critical subgraphs need to be supported efficiently while maintaining area and timing constraints. Additionally, more complex accelerators require more sophisticated control generation engines. These factors must be carefully balanced. In this work, we investigate the design of subgraph accelerators using configurable lookup table structures. These designs provide an effective paradigm to execute a wide range of subgraphs involving arithmetic and logic operations. We describe why lookup table designs are effective, how they fit into a transparent acceleration framework, and evaluate the effectiveness of a wide range of de-signs using both simulation and logic synthesis.