Optimizing Function Layout for Mobile Applications

Function layout, also known as function reordering or function placement, is one of the most effective profile-guided compiler optimizations. By reordering functions in a binary, compilers can improve the performance of large-scale applications or reduce the compressed size of mobile applications. Although the technique has been extensively studied in the context of large-scale binaries, no study has thoroughly investigated function layout algorithms on mobile applications. In this paper we develop the first principled solution for optimizing function layouts in the mobile space. To this end, we identify two key optimization goals: reducing the compressed code size and improving the cold start-up time of a mobile application. Then we propose a formal model for the layout problem, whose objective closely matches our goals. Our novel algorithm for optimizing the layout is inspired by the classic balanced graph partitioning problem. We have carefully engineered and implemented the algorithm in an open-source compiler, LLVM. An extensive evaluation of the new method on large commercial mobile applications demonstrates significant improvements in start-up time and compressed size compared to the state-of-the-art approach.