Optimizing real world applications with GCC Link Time Optimization

Abstract: GCC has a new infrastructure to support a link time optimization (LTO). The infrastructure is designed to allow linking of large applications using a special mode (WHOPR) which support parallelization of the compilation process. In this paper we present overview of the design and implementation of WHOPR and present test results of its behavior when optimizing large applications. We give numbers on compile time, memory usage and code quality comparisons to the classical file by file based optimization model. In particular we focus on Firefox web browser. We show main problems seen only when compiling a large application, such as startup time and code size growth.