Rebuilding an airliner in flight: a retrospective on refactoring IBM testarossa production compiler for Eclipse OMR

Eclipse OMR is a new open source project created by refactoring the IBM J9 Java Virtual Machine to create a set of language agnostic components for building all kinds of language runtimes. This paper reflects on the effort, the successes, and the mistakes made while refactoring more than a million lines of code on the master development branch while 8 disparate production compiler products were under active development. Refactoring large scale projects like compilers and language runtimes can be done while shipping releases, but there are certainly challenges. We offer some recommendations for other projects considering this path.