Static Detection of Loop-Invariant Data Structures

As a culture, object-orientation encourages programmers to create objects, both shortand long-lived, without concern for cost. Excessive object creation and initialization can cause severe runtime bloat, which degrades significantly application performance and scalability. A frequently-occurring coding pattern that may lead to large volumes of (temporary) objects is the creation of objects that, while allocated per loop iteration, contain values independent of specific iterations. Finding these objects and moving them out of loops requires sophisticated interprocedural analysis, a task that is difficult for traditional dataflow analyses such as loop-invariant code motion to accomplish. Our work targets data structures that are loop-invariant, and presents a static type and effect system to detect loop-invariant data structures. For each loop, our analysis inspects each logical data structure in order to find those that have disjoint instances per loop iteration and contain loop-invariant data. Instead of automatically hoisting them to improve performance (which is over-conservative), we report hoistability measurements for each disjoint loop data structure detected by our analysis. Eventually these data structures are ranked based on these measurements and are presented to the user to help manual tuning. We have performed a variety of studies on a set of 19 moderate/large-sized Java benchmarks. With the help of hoistability measurements, we found optimization opportunities in most of the programs that we inspected and achieved significant performance improvements in some of them (e.g., 82.1% running time reduction).