RISC-V Custom Instructions of Elementary Functions for IoT Endpoint Devices

The computation of elementary functions is required in many tasks of Internet of Things (IoT) endpoint devices, for example, communications, image processing, and biomedical signal processing. IoT endpoint devices generally adopt software approaches to compute elementary functions, which take many cycles. To improve efficiency, this work proposes custom instructions for elementary functions to the open-source RISC-V instruction set architecture (ISA). In particular, several variants of the custom instructions (fast, intermediate, and tiny variants) are developed to satisfy the needs of various types of IoT devices. Microarchitecture design and VLSI circuit design are then proposed to efficiently support the extended ISA. Both software emulation and on-board evaluation of the new architecture are carried out with testbenches covering typical communication and computation tasks for IoT devices. The custom instructions gain speedups ranging from 3.3 to 18.0 compared to a baseline RV32IM design. ASIC synthesis results under TSMC 28nm technology demonstrate that the power overhead is < 5% with the tiny variant,< 17% with the intermediate variant, and < 26% with the fast variant, which is not significant considering the achieved speedup. The experimental results further confirm that the proposed custom instructions are computation-efficient and versatile to adapt to different IoT devices for various applications.