LastMerge: A language-agnostic structured tool for code integration

Abstract: Unstructured line-based merge tools are widely used in practice. Structured AST-based merge tools show significantly improved merge accuracy, but are rarely used in practice because they are language specific and costly, consequently not being available for many programming languages. To improve merge accuracy for a wide range of languages, we propose LastMerge, a generic structured merge tool that can be configured through a thin interface that significantly reduces the effort of supporting structured merge. To understand the impact that generic structured merge might have on merge accuracy and performance, we run an experiment with four structured merge tools: two Java specific tools, jDime and Spork, and their generic counterparts, respectively LastMerge and Mergiraf. Using each tool, we replay merge scenarios from a significant dataset, and collect data on runtime, behavioral divergences, and merge accuracy. Our results show no evidence that generic structured merge significantly impacts merge accuracy. Although we observe a difference rate of approximately 10% between the Java specific tools and their generic counterparts, most of the differences stem from implementation details and could be avoided. We find that LastMerge reports 15% fewer false positives than jDime while Mergiraf misses 42% fewer false negatives than Spork. Both generic tools exhibit comparable runtime performance to the state of the art language specific implementations. These results suggest that generic structured merge tools can effectively replace language-specific ones, paving the way for broader adoption of structured merge in industry.