Insights From Bugs in FPGA High-Level Synthesis Tools: An Empirical Study of Bambu Bugs

High-level synthesis (HLS) tools have been widely used in field-programmable gate array (FPGA) design to convert C/C++ code to hardware description language code. Unfortunately, HLS tools are susceptible to bugs, which can introduce serious vulnerabilities in FPGA products, leading to substantial losses. However, the characteristics of these bugs (e.g., root causes and bug-prone stages) have never been systematically studied, which significantly hinders developers from effectively handling HLS tool bugs. To this end, we conduct the first empirical study to uncover HLS tool bug characteristics. We collect 349 bugs of a widely used HLS tool, namely Bambu. We study the root causes, buggy stages, and bug fixes of these bugs by applying a multiperson collaboration method. Finally, 13 valuable findings are summarized. We find 14 categories of root causes in Bambu bugs; most bugs (22.1%) are caused by incorrect implementation of IR processing; the front end of Bambu is more bug-prone; to fix these bugs, 2.27 files and 80.19 lines of code need to be modified on average. We also present the insights gained from 95 Vitis HLS bugs. From these findings, we suggest that developers could use an on-the-fly code generator configuration method to generate suitable testing programs for HLS tool bug detection and apply large language models to assist in fixing HLS tool bugs.