Semantics-Based Program Integration

Thomas W. Reps, Susan Horwitz

Abstract: The need to integrate several versions of a base program into a common one arises frequently, but it is a tedious and time consuming task to integrate programs by hand. To date, the only available tools for assisting with program integration are variants of text-based differential file comparators; these are of limited utility because one has no guarantees about how the program that is the product of an integration behaves compared to the programs that were integrated. Our recent work addresses the problem of building a semantics-based tool for program integration; this paper describes the techniques we have developed, which provide the foundation for creating such a tool. Semantics-based integration is based on the assumption that a difference in the behavior of one of the variant programs from that of the base program, rather than a difference in the text, is significant and must be preserved in the merged program. Although it is undecidable to determine whether a program modification actually leads to such a difference, it is possible to determine a safe approximation by comparing each of the variants with the base program. To determine this information, the integration algorithm employs a program representation that is similar (although not identical) to the program dependence graphs that have been used previously in vectorizing compilers. The algorithm also makes use of the notion of a program slice to find just those statements of a program that determine the values of potentially affected variables.