Studying the Potential of Automatic Optimizations in the Intel FPGA SDK for OpenCL.

High Level Synthesis (HLS) tools, like the Intel FPGA SDK for OpenCL, improve hardware design productivity and enable efficient design space exploration, by providing simple program directives (pragmas) and/or API calls that allow hardware programmers to use higher-level languages (like HLS-C or OpenCL). However, modern HLS tools sometimes miss important optimizations that are necessary for high performance. In this poster, we present a study of the tradeoffs in HLS optimizations, and the potential of a modern HLS tool in automatically optimizing an application. We perform the study on a generic, 5-stage camera ISP pipeline using the Intel FPGA SDK for OpenCL and an Arria 10 FPGA Dev Kit. We show that automatic optimizations in the HLS tool are valuable, achieving up to 2.7x speedup over equivalent CPU execution. With further hand tuning, however, we can achieve up to 36.5x speedup over CPU. We draw several specific lessons about the effectiveness of automatic optimizations guided by simple directives and about the nature of manual rewriting required for high performance. Finally, we conclude that there is a gap in the current potential of HLS tools which needs to be filled by next-gen research.