Stepwise Debugging for Hardware Accelerators.

High-level programming models for hardware design let domain experts quickly produce specialized accelerators. However, tools for debugging these accelerators remain tied to low-level hardware description languages (HDLs). High-level descriptions contain control-flow information that is lost in HDL code. We describe Cider, a stepwise debugger that exploits this information to provide software-like debugging abstractions for languages that compile to hardware. Cider uses Calyx, an intermediate language for accelerator generators that preserves control information. Cider provides breakpoints, watchpoints, state inspection, and source-level position mapping. Using case studies that examine one new and two preexisting accelerator generators, we demonstrate how Cider helps find and localize previously unreported bugs. By directly simulating a control-rich representation, Cider avoids wasting effort on inactive parts of the design and, despite being largely unoptimized, performs competitively with open-source HDL simulators.