Architecture Exploration for HLS-Oriented FPGA Debug Overlays

High-Level Synthesis (HLS) promises improved designer productivity, but requires a debug ecosystem that allows designers to debug in the context of the original source code. Recent work has presented in-system debug frameworks where instrumentation added to the design collects trace data as the circuit runs, and a software tool that allows the user to replay the execution using the captured data. When searching for the root cause of a bug, the designer may need to modify the instrumentation to collect data from a new part of the design, requiring a lengthy recompile. In this paper, we propose a flexible debug overlay family that provides software-like debug turn-around times for HLS generated circuits. At compile time, the overlay is added to the design and compiled. At debug time, the overlay can be configured many times to implement specific debug scenarios without a recompilation. This paper first outlines a number of "capabilities" that such an overlay should have, and then describes architectural support for each of these capabilities. The cheapest overlay variant allows selective variable tracing with only a 1.7% increase in area overhead from the baseline debug instrumentation, while the deluxe variant offers 2x-7x improvement in trace buffer memory utilization with conditional buffer freeze support.