An Empirical Study of Cross - loopReuse in the NAS benchmarksKeith

This paper describes an empirical study designed to quantify the level of cross-loop reuse occurring in a set of scientiic Fortran programs, the NAS Benchmarks. Cross-loop reuse takes place when a set of data items or cache lines are accessed in a given loop nest and then accessed again within some subsequent portion of the program (usually another outer loop nest). In contrast to intra-loop reuse, which takes place during the execution of a single loop nest, cross-loop reuse is not always detectable by traditional compile-time reuse analysis techniques. In this study, the benchmark programs are instrumented and run through a cache simulator. The simulator gathers statistics on cross-loop reuse using a novel classiication scheme that clearly identiies the diierent types of reuse. According to the simulation data, the level of cross-loop reuse varies widely from program to program, and depends greatly on the problem size and cache size. Some programs exhibit almost no cross-loop reuse, however other programs have signiicant levels of cross-loop reuse even for fairly small cache sizes. The data from this study suggest that cross-loop reuse information would be quite beneecial for compilers that attempt certain optimizations.