Aggressive Function Inlining with Global Code Reordering

Code reordering and function inlining are commonly used today for improving memory locality and eliminating memory accesses. In this paper we present a new profile based optimization of an aggressive function inlining, based on a study of the mutual effects between code reordering and inlined functions. The proposed optimization duplicates frequently executed large functions and replaces their function calls by their bodies, while removing corresponding call and return instructions at the call sites. The inlining step is then followed by global code reordering to achieve better grouping of frequently executed code across function calls. The aggressiveness of the function inlining relies on the fact that no constrains are applied on the size, or the number, of the functions being inlined - only execution frequency is taken into consideration during the inlining process. The global code reordering that follows the function inlining guarantees the separation between cold and hot segments of the inlined functions.

The optimization was implemented into FDPR (Feedback Directed Program Restructuring), a post-link optimizer, which is part of the IBM AIX and Linux operating systems for the IBM pSeries servers. Our experiments were applied after the –O3 optimization phase of the compiler and results on SPECint2000 show an improvement of up to 8% (average of 4%) in execution time, on top of code reordering optimization, resulting from improving code locality and removing corresponding call and return instructions of inlined functions.