Design Patterns for Code Reuse in HLS Packet Processing Pipelines

High-level synthesis (HLS) allows developers to be more productive in designing FPGA circuits thanks to familiar programming languages and high-level abstractions. In order to create high-performance circuits, HLS tools, such as Xilinx Vivado HLS, require following specific design patterns and techniques. Unfortunately, when applied to network packet processing tasks, these techniques limit code reuse and modularity, requiring developers to use deprecated programming conventions. We propose a methodology for developing high-speed networking applications using Vivado HLS for C++, focusing on reusability, code simplicity, and overall performance. Following this methodology, we implement a class library (ntl) with several building blocks that can be used in a wide spectrum of networking applications. We evaluate the methodology by implementing two applications: a UDP stateless firewall and a key-value store cache designed for FPGA-based SmartNICs, both processing packets at 40Gbps line-rate.