RISC-V Instruction Set Architecture Extensions: A Survey

RISC-V is an open-source and royalty-free instruction set architecture (ISA), which opens up a new era of processor innovation. RISC-V has the characteristics of modularization and extensibility, and explicitly supports domain-specific custom extensions. Nowadays, RISC-V is a popular ISA for embedded processors. However, there is still a gap between the capabilities of RISC-V and the requirements of various emerging computing scenarios (e.g., artificial intelligence, cloud computing). Recently, the RISC-V standards organization has continuously introduced new ISA extensions to meet the needs of advanced computing. There are also a variety of novel research proposed customized extensions of RISC-V for certain scenarios. As far as we know, there is a lack of a survey to systematically present the research progress of RISC-V ISA extensions. The goal of this paper is to provide a comprehensive survey on existing works about RISC-V ISA extensions. First, the application scenarios of RISC-V are introduced, and the requirements for ISA extensions are analyzed. Then, we survey the progress of official RISC-V ISA extension specification and recent research on RISC-V ISA extension. Finally, we highlight some possible future research opportunities on the RISC-V ISA extension.