Optimizing DDR-SDRAM communications at C-level for automatically-generated hardware accelerators an experience with the Altera C2H HLS tool

Thanks to efficient scheduling, resource sharing, and finite-state machines generation, high-level synthesis (HLS) tools are now more mature for generating hardware accelerators with an optimized internal structure. But interfacing them within the complete design, with optimized communications, to achieve the best throughput remains hard. Expert designers still need to program all the necessary glue (in VHDL/Verilog) to get an efficient design. Taking the example of C2H, the Altera HLS tool, and of accelerators communicating to an external DDR memory, we show it is possible to restructure the application code, to generate adequate communication processes, in C, and to compile them all with C2H, so that the resulting application is highly-optimized, with full usage of the memory bandwidth. In other words, our study demonstrates that HLS tools can be used as back-end optimizers for front-end optimizations, as it is the case for standard compilation with high-level transformations developed on top of assembly-code optimizers. We believe this is the way to go for making HLS tools viable.