Rastislav Bodik, Rajiv Gupta, and Mary Lou Soffa
Load-reuse analysis finds instructions that repeatedly access the same memory location. This location can be promoted to a register, eliminating redundant loads by reusing the results of prior memory accesses. This paper develops a load-reuse analysis and designs a method for evaluating its precision.
In designing the analysis, we aspire for completeness---the goal of exposing all reuse that can be harvested by a subsequent program transformation. For register promotion, a suitable transformation is partial redundancy elimination (PRE). To approach the ideal goal of PRE-completeness, our load-reuse analysis is phrased as a data-flow problem on a program representation that is path-sensitive, as it detects reuse even when it originates in a different instruction along each control flow path, and comprehensive, as it treats scalar, array and pointer-based loads uniformly.
In evaluating the analysis, we compare it with an ideal analysis. By observing the run-time stream of memory references, we collect all PRE-exploitable reuse and treat it as the ideal analysis performance. To compare the (static) load-reuse analysis with the (dynamic) ideal reuse, we use an estimator algorithm that computes, given a data-flow solution and a program profile, the dynamic amount of reuse detected by the analysis. We developed a family of estimators that differ in how well they bound the profiling error inherent in the edge profile. By bounding the error, the estimators offer a precise and practical method for determining the run-time optimization benefit.
Our experiments show that about 55% of loads executed in Spec95 exhibit reuse. Of those, our analysis exposes about 80%.
Keywords: profile-guided optimizations, register promotion, program representations, data-flow analysis.
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