Generating High-Performance Number Theoretic Transform Implementations for Vector Architectures

Fully homomorphic encryption (FHE) offers the ability to perform computations directly on encrypted data by encoding numerical vectors onto mathematical structures. However, the adoption of FHE is hindered by substantial overheads that make it impractical for many applications. Number theoretic transforms (NTTs) are a key optimization technique for FHE by accelerating vector convolutions. Towards practical usage of FHE, we propose to use SPIRAL, a code generator renowned for generating efficient linear transform implementations, to generate high-performance NTT on vector architectures. We identify suitable NTT algorithms and translate the dataflow graphs of those algorithms into SPIRAL's internal mathematical representations. We then implement the entire workflow required for generating efficient vectorized NTT code. In this work, we target the Ring Processing Unit (RPU), a multi-tile long vector accelerator designed for FHE computations. On average, the SPIRAL-generated NTT kernel achieves a 1.7x speedup over naive implementations on RPU, showcasing the effectiveness of our approach towards maximizing performance for NTT computations on vector architectures.