LightningSim: Fast and Accurate Trace-Based Simulation for High-Level Synthesis

High-Level Synthesis allows hardware designers to create complex RTL designs using C/C++. The traditional HLS workflow involves iterations of C/C++ simulation for partial functional verification and HLS synthesis for coarse timing estimates. However, neither C/C++ simulation nor HLS synthesis estimates can account for complex behaviors like FIFO interactions and pipeline stalls, thereby obscuring problems like deadlocks and latency overheads. Such problems are revealed only through C/RTL co-simulation, which is typically orders of magnitude slower than either C/C++ simulation or HLS synthesis, far too slow to integrate into the edit-run development cycle. Addressing this, we propose LightningSim, a fast simulation tool for HLS that combines the speed of native C/C++ with the accuracy of C/RTL co-simulation. LightningSim directly operates on the LLVM intermediate representation (IR) code and accurately simulates a hardware design's dynamic behavior. First, it traces LLVM IR execution to capture the run-time information; second, it maps the static HLS scheduling information to the trace to simulate the dynamic behavior; third, it calculates stalls and deadlocks from inter-function interactions to get precise cycle counts. Evaluated on 33 benchmarks, LightningSim produces 99.9%-accurate timing estimates up to 95x faster than RTL simulation. Our code is publicly available on GitHub.