sentences: GCC 4.5.0 introduces support for link time optimization (LTO). The LTO infrastructure is designed to allow parallel linking of large applications using a special mode, WHOPR. In this paper we present an overview of the design and implementation of WHOPR and present results of its behavior when optimizing large applications. We compare WHOPR’s compile time, memory usage, and code quality to the results of the classical file-byfile optimization model, focusing on its effects on GCC itself and the Firefox web browser. We examine critical issues which arise only when considering large applications, such as startup time and code size growth.