Automatically enhancing locality for tree traversals with traversal splicing

Generally applicable techniques for improving temporal locality in irregular programs, which operate over pointer-based data structures such as trees and graphs, are scarce. Focusing on a subset of irregular programs, namely, tree traversal algorithms like Barnes-Hut and nearest neighbor, previous work has proposed point blocking, a technique analogous to loop tiling in regular programs, to improve locality. However point blocking is highly dependent on point sorting, a technique to reorder points so that consecutive points will have similar traversals. Performing this a priori sort requires an understanding of the semantics of the algorithm and hence highly application specific techniques. In this work, we propose traversal splicing, a new, general, automatic locality optimization for irregular tree traversal codes, that is less sensitive to point order, and hence can deliver substantially better performance, even in the absence of semantic information. For six benchmark algorithms, we show that traversal splicing can deliver single-thread speedups of up to 9.147 (geometric mean: 3.095) over baseline implementations, and up to 4.752 (geometric mean: 2.079) over point-blocked implementations. Further, we show that in many cases, automatically applying traversal splicing to a baseline implementation yields performance that is better than carefully hand-optimized implementations.