Use of compiler optimization of software bypassing as a method to improve energy efficiency of exposed data path architectures

In the design of embedded systems, hardware and software need to be co-explored together to meet targets of performance and energy. With the use of application-specific instruction-set processors, as a stand-alone solution or as a part of a system on chip, the customization of processors for a particular application is a known method to reduce energy requirements and provide performance. In particular, processor designs with exposed data paths trade compile time complexity for simplified control hardware and lower running costs. An exposed data path also allows the removal of unused components of interconnection network, once the application is compiled. In this paper, we propose the use of a compiler technique for processors with exposed data paths, called software bypassing. Software bypassing allows the compiler to schedule data transfers between execution units directly, bypassing the use of a general-purpose register file, increasing scheduling freedom, with reduced dependencies induced by the reuse of registers, decreasing the number of read and write accesses to register files, and allowing the use of register files with less read and write ports while maintaining or improving performance and maintaining reprogrammability. We compare our proposal against an architecture exploration technique, connectivity reduction, which finds in compiled application all interconnection network components that are used and removes those which are not, leading to an energy-efficient application-specific instruction-set processor. We observe that the use of software bypassing leads to improvements in application speed, with architectures having the smallest number of register file ports consistently outperforming architectures with larger number of ports, and reduction in energy consumption. In contrast, connectivity reduction maintains the same application speed, reduces energy consumption, and allows for increase in processor frequency; however, with the clock frequency increased to match the performance of software bypassing, energy consumption grows. We also observe that in case reprogrammability is not an issue, the most energy-efficient solution is a combination of software bypassing and connectivity reduction.