Improving Data Locality with Loop Transformations

In the past decade, processor speed has become signicantly faster than memory speed. Small, fast cache memories are designed to overcome this discrepancy, but they are only eective when programs exhibit data locality. In this article, we present compiler optimizations to improve data locality based on a simple yet accurate cost model. The model computes both temporal and spatial reuse of cache lines to nd desirable loop organizations. The cost model drives the application of compound transformations consisting of loop permutation, loop fusion, loop distribution, and loop reversal. We demonstrate that these program transformations are useful for optimizing many programs. To validate our optimization strategy, we implemented our algorithms and ran experiments on a large collection of scientic programs and kernels. Experiments illustrate that for kernels our model and algorithm can select and achieve the best loop structure for a nest. For over 30 complete applications, we executed the original and transformed versions and simulated cache hit rates. We collected statistics about the inherent characteristics of these programs and our ability to improve their data locality. To our knowledge, these studies are the rst of such breadth and depth. We found performance improvements were dicult to achieve because benchmark programs typically have high hit rates even for small data caches; however, our optimizations signi cantly improved several programs