Energy efficient special instruction support in an embedded processor with compact ISA

The use of special instructions that execute complex operation patterns is a common approach in application specific processor design to improve performance and efficiency. However, in an embedded generic processor with compact instruction set architecture (ISA), such instructions may lead to large overhead as: i) more bits are needed to encode the extra opcodes and operands, resulting in wider instructions; ii) more register file (RF) ports are required to provide the extra operands to the function units. Such overhead may increase energy consumption considerably. In this paper, we propose to support flexible operation pair patterns in a processor with a compact 24-bit RISC-like ISA using: i) a partially reconfigurable decoder that exploits the locality of patterns to reduce the requirement for opcode space; ii) a software controlled bypass network to reduce the requirement for operand encoding and RF ports. We also propose an energy-aware compiler backend design for the proposed architecture that performs pattern selection and bypass-aware scheduling to generate energy efficient codes. Though proposed design imposes extra constraints on the operation patterns, the experimental results show that the average dynamic instruction count is reduced by over 25%, which is only about 2% less than the architecture without such constraints. Due to the low overhead, the total energy of the proposed architecture reduces by an average of 15.8% compared to the RISC baseline, while the one without constraints achieves almost no energy improvement.