Towards reusing hints from past fixes

With the usage of version control systems, many bug fixes have accumulated over the years. Researchers have proposed various automatic program repair (APR) approaches that reuse past fixes to fix new bugs. However, some fundamental questions, such as how new fixes overlap with old fixes, have not been investigated. Intuitively, the overlap between old and new fixes decides how APR approaches can construct new fixes with old ones. Based on this intuition, we systematically designed six overlap metrics, and performed an empirical study on 5,735 bug fixes to investigate the usefulness of past fixes when composing new fixes. For each bug fix, we created delta graphs (i.e., program dependency graphs for code changes), and identified how bug fixes overlap with each other in terms of the content, code structures, and identifier names of fixes. Our results show that if an APR approach knows all code name changes and composes new fixes by fully or partially reusing the content of past fixes, only 2.1% and 3.2% new fixes can be created from single or multiple past fixes in the same project, compared with 0.9% and 1.2% fixes created from past fixes across projects. However, if an APR approach knows all code name changes and composes new fixes by fully or partially reusing the code structures of past fixes, up to 41.3% and 29.7% new fixes can be created. By making the above observations and revealing other ten findings, we investigated the upper bound of reusable past fixes and composable new fixes, exploring the potential of existing and future APR approaches.