Résumé
HLS tools have been introduced with the promise of easening and shortening the design cycle of tedious and error-prone RTL-based development of hardware accelerators. However, they do so either by concealing meaningful hardware decisions which model the computing architecture-such as OpenCL compilers-or by abstracting them away into a high-level programming language-usually C-based. In this paper, we show that although Vivado HLS is sufficiently mature to generate a functionally correct FPGA accelerator from a naive description, reaching an accelerator which optimizes the FPGA resource utilization in a way that conveys maximum performance is a process for a hardware architect mindset. We use a highly demanding application, that requires real-time operation, and develop a non-binary LDPC decoder on a state-of-the-art Virtex 7 FPGA, using the Vivado HLS framework. Despite using the same programming syntax as a C-language software compiler, the underlying programming model is not the same, thus, the optimizations required in code refactoring are distinct. Moreover, directive-based optimizations that tweak the synthesized C description hardware must be used in order to attain efficient architectures. These processes are documented in this paper, to guide the reader on how an HLS-based accelerator can be designed, which in our case can come close to the performance achieved with dedicated hand-made RTL descriptions.