Simple and Effective Array Prefetching in Java

Java is becoming a viable choice for numerical algorithms due to the software engineering benefits of object-oriented programming. Because these programs still use large arrays that do not fit in the cache, they continue to suffer from poor memory performance. To hide memory latency, we describe a new unified compile-time analysis for software prefetching arrays and linked structures in Java. Our previous work uses data-flow analysis to discover linked data structure accesses, and here we present a more general version that also identifies loop induction variables used in array accesses. Our algorithm schedules prefetches for all array references that contain induction variables. We evaluate our technique using a simulator of an out-of-order superscalar processor running a set of array-based Java programs. Across all our programs, prefetching reduces execution time by a geometric mean of 23%, and the largest improvement is 58%. We also evaluate prefetching on a PowerPC processor, and we show that prefetching reduces execution time by a geometric mean of 17%. Traditional software prefetching algorithms for C and Fortran use locality analysis and sophisticated loop transformations. Because our analysis is much simpler and quicker, it is suitable for including in a just-in-time compiler. We further show that the additional loop transformations and careful scheduling of prefetches used in previous work are not always necessary for modern architectures and Java programs.