Ara: A 1-GHz+ Scalable and Energy-Efficient RISC-V Vector Processor With Multiprecision Floating-Point Support in 22-nm FD-SOI
In this article, we present Ara, a 64-bit vector processor based on the version 0.5 draft of RISC-V’s vector extension, implemented in GlobalFoundries 22FDX fully depleted silicon-on-insulator (FD-SOI) technology. Ara’s microarchitecture is scalable, as it is composed of a set of identical lanes, each containing part of the processor’s vector register file and functional units. It achieves up to 97% floating-point unit (FPU) utilization when running a <inline-formula> <tex-math notation="LaTeX">$256\times256$ </tex-math></inline-formula> double-precision matrix multiplication on 16 lanes. Ara runs at more than 1 GHz in the typical corner (TT/0.80 V/25 °C), achieving a performance up to 33 DP–GFLOPS. In terms of energy efficiency, Ara achieves up to 41 DP–GFLOPS <inline-formula> <tex-math notation="LaTeX">$\text {W}^{-1}$ </tex-math></inline-formula> under the same conditions, which is slightly superior to similar vector processors found in the literature. An analysis on several vectorizable linear algebra computation kernels for a range of different matrix and vector sizes gives insight into performance limitations and bottlenecks for vector processors and outlines directions to maintain high energy efficiency even for small matrix sizes where the vector architecture achieves suboptimal utilization of the available FPUs.