Clockhands: Rename-free Instruction Set Architecture for Out-of-order Processors

Out-of-order superscalar processors are currently the only architecture that speeds up irregular programs, but they suffer from poor power efficiency. To tackle this issue, we focused on how to specify register operands. Specifying operands by register names, as conventional RISC does, requires register renaming, resulting in poor power efficiency and preventing an increase in the front-end width. In contrast, a recently proposed architecture called STRAIGHT specifies operands by inter-instruction distance, thereby eliminating register renaming. However, STRAIGHT has strong constraints on instruction placement, which generally results in a large increase in the number of instructions.

We propose Clockhands, a novel instruction set architecture that has multiple register groups and specifies a value as "the value written in this register group 𝑘 times before." Clockhands does not require register renaming as in STRAIGHT. In contrast, Clockhands has much looser constraints on instruction placement than STRAIGHT, allowing programs to be written with almost the same number of instructions as Conventional RISC. We implemented a cycle-accurate simulator, FPGA implementation, and first-step compiler for Clockhands and evaluated benchmarks including SPEC CPU. On a machine with an eight-fetch width, the evaluation results showed that Clockhands consumes 7.4% less energy than RISC while having performance comparable to RISC. This energy reduction increases significantly to 24.4% when simulating a futuristic up-scaled processor with a 16-fetch width, which shows that Clockhands enables a wider front-end.