Non-functional bugs, e.g., performance bugs and security bugs, bear a heavy cost on both software developers and end-users. For example, IBM estimates the cost of a single data breach to be millions of dollars. Tools to reduce the occurrence, impact, and repair time of non-functional bugs can therefore provide key assistance for software developers racing to fix these issues. Identifying bug-inducing changes is a critical step in software quality assurance. In particular, the SZZ approach is commonly used to identify bug-inducing commits. However, the fixes to non-functional bugs may be scattered and separate from their bug-inducing locations in the source code. The nature of non-functional bugs may therefore make the SZZ approach a sub-optimal approach for identifying bug-inducing changes. Yet, prior studies that leverage or evaluate the SZZ approach do not consider non-functional bugs, leading to potential bias on the results. In this paper, we conduct an empirical study on the results of the SZZ approach when used to identify the inducing changes of the non-functional bugs in the NFBugs dataset. We eliminate a majority of the bug-inducing commits as they are not in the same method or class level. We manually examine whether each identified bug-inducing change is indeed the correct bug-inducing change. Our manual study shows that a large portion of non-functional bugs cannot be properly identified by the SZZ approach. By manually identifying the root causes of the falsely detected bug-inducing changes, we uncover root causes for false detection that have not been found by previous studies. We evaluate the identified bug-inducing changes based on three criteria from prior research, i.e., the earliest bug appearance, the future impact of changes, and the realism of bug introduction. We find that prior criteria may be irrelevant for non-functional bugs. Our results may be used to assist in future research on non-functional bugs, and highlight the need to complement SZZ to accommodate the unique characteristics of non-functional bugs.
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