Locality-enhancing loop transformations for parallel tree traversal algorithms

Exploiting locality is critical to achieving good performance. For regular programs, which operate on dense arrays and matrices, techniques such as loop interchange and tiling have long been known to improve locality and deliver improved performance. However, there has been relatively little work investigating similar locality-improving transformations for irregular programs that operate on trees or graphs. Often, it is not even clear that such transformations are possible. In this paper, we discuss two transformations that can be applied to irregular programs that perform graph traversals. We show that these transformations can be seen as analogs of the popular regular transformations of loop interchange and tiling. We demonstrate the utility of these transformations on two tree traversal algorithms, the Barnes-Hut algorithm and raytracing, achieving speedups of up to 251% over the baseline implementation.