Synthesisable recursion for C++ HLS tools

C-based HLS tools continue to improve in analysis and optimisation, but are still restricted to a subset of C functionality. A C language feature missing from all common HLS tools is recursion, which makes it difficult and time consuming to write many types of programs in HLS. This paper presents a technique for implementing recursion as an Embedded Domain Specific Language (EDSL) in C++, utilising the C++ front-end of a HLS compiler to build the state machines and stacks, while ensuring the code presented to the back-end is completely synthesisable. While the EDSL is not pure C, it provides a user-friendly language that can be trivially translated from a recursive C program, and can contain calls to plain C for compute-intensive leaf functions. HLSRecurse is a platform independent library that allows the same program to be compiled using g++, clang, Legup, or Vivado HLS, and have the same execution semantics. The performance of HLSRecurse is evaluated in software and hardware using three micro-benchmarks of the underlying state-machine builder, and five practical examples of real recursive programs including a Sudoku solver, Strassen multiplication, and adaptive Monte-Carlo integration. We show that for Vivado HLS the DSL provides the same area-time product as manually converted programs, while in Legup the DSL increases the area-time product by 1.5×.