The effectiveness of flow analysis for inlining

An interprocedural flow analysis can justify inlining in higher-order languages. In principle, more inlining can be performed as analysis accuracy improves. This paper compares four flow analyses to determine how effectively they justify inlining in practice. The paper makes two contributions. First, the relative merits of the flow analyses are measured with all other variables held constant. The four analyses include two monovariant and two polyvariant analyses that cover a wide range of the accuracy/cost spectrum. Our measurements show that the effectiveness of the inliner improves slightly as analysis accuracy improves, but the improvement is offset by the compile-time cost of the accurate analyses. The second contribution is an improvement to the previously reported inlining algorithm used in our experiments. The improvement causes flow information provided by a polyvariant analysis to be selectively merged. By merging flow information depending on the inlining context, the algorithm is able to expose additional opportunities for inlining. This merging technique can be used in any program transformer justified by a polyvariant flow analysis. The revised algorithm is fully implemented in a production Scheme compiler.