Minimum RISC-V: Maximum FPGA Efficiency

By Martin Langhammer, Bogdan Pasca, Gregg Baeckler
Altera

Abstract

RISC-V processors are now widely available for FPGAs, with most FPGA vendors replacing their proprietary soft-CPU architectures with this instruction-set architecture (ISA). Although contemporary FPGAs are increasingly resource- and feature-rich, a soft processor system---including memory---often occupies a disproportionately large footprint and delivers only modest performance relative to the potential of the target device. In this work we first design a lightweight but deeply pipelined RISC-V core that can run above 700 MHz in a mid-range device and that is capable of retiring one instruction per clock under ideal conditions. We then develop and demonstrate a new optimization method that compiles the final application binary and a specialized processor instance contemporaneously. The method expands each instruction into an application-specific encoding, removes unused datapath and decode logic during synthesis, and yields cores that remain assembler-compatible with the original program while no longer being binary-compatible with a general-purpose RISC-V. We implement example designs on RV32I (base integer), RV32M (integer multiply), and RV32F (single-precision floating-point) configurations. Our optimized RISC-V is as small as 170 ALMs and closes timing at 775 MHz in an Altera Agilex-5 device. Finally, we show that these processors can be packed into a deterministic area and stepped-and-repeated continuously without material impact on performance, enabling predictable multi-core density planning.

Keywords: FPGA, RISC-V, soft processor, application-specific processor, high-frequency design, synthesis.

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