SynADT: Dynamic Data Structures in High Level Synthesis

Abstract Data Types (ADTs) such as dictionaries and lists are essential for many embedded computing applications such as network stacks. However, in heterogeneous systems, code using ADTs can usually only run in CPUs, because components written in HLS do not support dynamic data structures. HLS tools cannot be used to synthesise dynamic data structures directly because the use of pointers is very restricted, such as not supporting pointers to pointers or pointer casting. Consequently, it is unclear what the API should look like and how to express dynamic data structures in HLS so that the tools can compile them. We propose SynADT, which consists of a methodology and a benchmark. The methodology provides classic data structures (linked lists, binary trees, hash tables and vectors) using relativeaddresses instead of pointers in Vivado HLS. The benchmark can be used to evaluate the performance of data structures in HLS, ARM processors and soft processors such as MicroBlaze, CPUs can utilise either the default C memory allocator or a hardware memory allocator. We evaluate the data structures in a Zynq FPGA demonstrating scaling to approximately 10MB memory usage and 1M data items. With a workload that utilises 10MB memory, the HLS data structures operating at 150MHz are on average 1.35× faster than MicroBlaze data structures operating at 150MHz with the default C allocator and 7.97× slower than ARM processor data structures operating at 667MHz with the default C allocator.