Hybrid custom instruction and co-processor synthesis methodology for extensible processors

Classical hardware/software partitioning techniques, recent advances in application-specific instruction set architecture (ISA) design tools, etc., provide avenues to address the individual problems of coprocessor generation and custom instruction addition for extensible processors. However, we argue that there is a need for hybrid synthesis techniques by demonstrating that a combination of custom instructions and co-processors is often the better solution in many applications. We propose a systematic methodology that builds on basic observations and trade-offs associated with co-processors and custom instructions: coprocessors are good for performing coarse-grained tasks that require minimal intervention or support from the processor, while custom instructions are efficient solutions for fine-grained tasks that are best integrated into a processor´s pipeline. We have developed a hierarchical synthesis flow that incorporates a muti-objective evolutionary algorithm in order to handle diverse design dimensions such as area, performance. We have implemented the proposed methodology in the context of a commercial extensible processor based platform (Xtensa™ from Tensilica). Our design flow incorporates a commercial behavioral synthesis tool and an automatic custom instruction generation engine. Our experiments with several applications show that simultaneous custom instruction and co-processor synthesis can achieve significantly better area/performance trade-offs than using only one of them.