MTU: The Multifunction Tree Unit in zkSpeed for Accelerating HyperPlonk

Abstract: Zero-Knowledge Proofs (ZKPs) are critical for privacy preservation and verifiable computation. Many ZKPs rely on kernels such as the SumCheck protocol and Merkle Tree commitments, which enable their security properties. These kernels exhibit balanced binary tree computational patterns, which enable efficient hardware acceleration. Prior work has investigated accelerating these kernels as part of an overarching ZKP protocol; however, a focused study of how to best exploit the underlying tree pattern for hardware efficiency remains limited. We conduct a systematic evaluation of these tree-based workloads under different traversal strategies, analyzing performance on multi-threaded CPUs and a hardware accelerator, the Multifunction Tree Unit (MTU). We introduce a hardware-friendly Hybrid Traversal for binary tree that improves parallelism and scalability while significantly reducing memory traffic on hardware. Our results show that MTU achieves up to 1478$\times$ speedup over CPU at DDR-level bandwidth and that our hybrid traversal outperforms as standalone approach by up to 3$\times$. These findings offer practical guidance for designing efficient hardware accelerators for ZKP workloads with binary tree structures.