JuxtaPiton: Enabling Heterogeneous-ISA Research with RISC-V and SPARC FPGA Soft-cores

Abstract: Energy efficiency has become an increasingly important concern in computer architecture due to the end of Dennard scaling. Heterogeneity has been explored as a way to achieve better energy efficiency and heterogeneous microarchitecture chips have become common in the mobile setting. Recent research has explored using heterogeneous-ISA, heterogeneous microarchitecture, general-purpose cores to achieve further energy efficiency gains. However, there is no open-source hardware implementation of a heterogeneous-ISA processor available for research, and effective research on heterogeneous-ISA processors necessitates the emulation speed provided by FPGA prototyping. This work describes our experiences creating JuxtaPiton by integrating a small RISC-V core into the OpenPiton framework, which uses a modified OpenSPARC T1 core. This is the first time a new core has been integrated with the OpenPiton framework, and JuxtaPiton is the first open-source, general-purpose, heterogeneous-ISA processor. JuxtaPiton inherits all the capabilities of OpenPiton, including vital FPGA emulation infrastructure which can boot full-stack Debian Linux. Using this infrastructure, we investigate area and timing effects of using the new RISC-V core on FPGA and the performance of the new core running microbenchmarks.