Optimizing Alpha Executables on Windows NT with Spike

Abstract

Vol. 9 No. 4 1997 3 Spike is a performance tool developed by DIGITAL to optimize Alpha executables on the Windows NT operating system. This optimization system has two main components: the Spike Optimizer and the Spike Optimization Environment. The Spike Optimizer reads in an executable, optimizes the code, and writes out the optimized version. The Optimizer uses profile feedback from previous runs of an application to guide its optimizations. Profile feedback is not commonly used in practice because it is difficult to collect, manage, and apply profile information. The Spike Optimization Environment provides a user-transparent profile feedback system that solves most of these problems, allowing a user to easily optimize large applications composed of many executables and dynamic link libraries (DLLs). Optimizing an executable image after it has been compiled and linked has several advantages. The Spike Optimizer can see the entire image and perform interprocedural optimizations, particularly with regard to code layout. The Optimizer can use profile feedback easily, because the executable that is profiled is the same executable that is optimized; no awkward mapping of profile data back to the source language takes place. Also, Spike can be used when the sources to an application are not available, which is beneficial when DIGITAL is working with independent software vendors (ISVs) to tune applications. Applications can be loosely classified into two categories: loop-intensive programs and call-intensive programs. Conventional compiler technology is well suited to loop-intensive programs. The important loops in a program in this category are within a single procedure, which is typically the unit of compilation. The control flow is predictable, and the compiler can use simple heuristics to determine the frequently executed parts of the procedure. Spike is designed for large, call-intensive programs; it uses interprocedural optimization and profile feedback. In call-intensive programs, the important loops span multiple procedures, and the loop bodies contain procedure calls. Consequently, optimizations on the loops must be interprocedural. The control flow is Optimizing Alpha Executables on Windows NT with Spike Robert S. Cohn David W. Goodwin P. Geoffrey Lowney