Integrating loop and data optimizations for locality within a constraint network based framework

In the context of data-intensive embedded applications, there have been two complementary approaches to data locality problem: restructuring code and restructuring data layout. Conceivably, an integrated approach that combines these two can generate much better results than each individual approach. However, there is an inherent difficulty in optimizing both data layout and loop access pattern simultaneously under a unified setting. This difficulty occurs due to the fact that a given data structure can be accessed by different loop nests of the application, and each such loop nest can demand a different memory layout transformation for the said data structure. This results in a coupling problem, where the behaviors of two (or more) loop nests are coupled to each other as a result of data sharing between them. In this paper, we present a constraint network (CN) based formulation of the integrated loop-data optimization problem. We present two alternate solutions to the data locality problem with our CN based formulation and discuss the pros and cons of each scheme. The first solution is a pure backtracking based one, whereas the second one improves upon the first one by employing three additional optimizations, including backjumping.