Re-engineering Legacy Software through Language Conversion

Software industry has a huge amount of legacy programs needing modernization and re-engineering. We are especially interested in re-engineering legacy programs via language conversion. In language conversion, the source and target languages can be either from the same programming paradigm or from different ones. Thus, we consider program modernization from two aspects. If the source and target languages share the same programming paradigm, we can modernize programs via source-to-source translation. If the languages are from different paradigms, for example procedural and object-oriented, we need re-engineering means in modernization, particularly in identifying objects from procedural code. After finding objects and their operations, we need again source-to-source translation methods in the actual conversion. As a constructive part of this thesis, we have implemented a converter translating PL/M programs into C. As an implementation tool of that converter, we have used TaLE (Tampere Language Editor). Because of the nature of the legacy software and PL/M language, we have been forced to extend TaLE. Since legacy systems are typically large, we have developed a technique to reduce the space requirements of the converter. In addition, conditional compilation of the source language may cause parsing problems in source-to-source translation. As a solution for this problem, we introduce multi-branch parsing. Both of these techniques are implemented in TaLE. We also show how to exploit the existing features of TaLE in information passing. We have extended the PL/M-to-C converter with object identification methods as a first step to enable translation from a procedural language into an objectoriented one (for example, C++). We introduce new methods to identify objectoriented features, such as subclass relationships and polymorphism, from legacy programs. The converter developed in this work is currently being used for modernizing PL/M systems in telecommunication industry. The experiences in using the converter are briefly summarized.