Profile guided code positioning

Abstract

This paper presents the results of our investigation of code positioning techniques using execution profile data as input into the compilation process. The primary objective of the positioning is to reduce the overhead of the instruction memory hierarchy.After initial investigation in the literature, we decided to implement two prototypes for the Hewlett-Packard Precision Architecture (PA-RISC). The first, built on top of the linker, positions code based on whole procedures. This prototype has the ability to move procedures into an order that is determined by a "closest is best" strategy.The second prototype, built on top of an existing optimizer package, positions code based on basic blocks within procedures. Groups of basic blocks that would be better as straight-line sequences are identified as chains. These chains are then ordered according to branch heuristics. Code that is never executed during the data collection runs can be physically separated from the primary code of a procedure by a technique we devised called procedure splitting. The algorithms we implemented are described through examples in this paper. The performance improvements from our work are also summarized in various tables and charts.