Adjusting a Program Transformation for Legality

Program transformations are one of the most valuable compiler techniques to improve parallelism or data locality. However, restructuring compilers have a hard time coping with data dependences. A typical solution is to focus on program parts where the dependences are simple enough to enable any transformation. For more complex problems is only addressed the question of checking whether a transformation is legal or not. In this paper we propose to go further. Starting from a transformation with no guarantee on legality, we show how we can correct it for dependence satisfaction. Two directions are explored: first when transformation properties can be explicitly expressed and second when they are implicit as in the data locality transformation case. Generating code having the best properties is a direct application of this result.

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