Whole-program compilation in MLton

MLton is a stable, robust, widely ported, Standard ML (SML) compiler that generates efficient executables. Whole-program compilation is the key to MLton's success, significantly improving both correctness and efficiency. Whole-program compilation makes possible a number of optimizations that reduce or eliminate the cost of SML's powerful abstraction mechanisms, such as parametric modules, polymorphism, and higher-order functions. It also allows MLton to use a simply-typed, first-order, intermediate language. By structuring the bulk of MLton's optimizer as small passes on whole programs in this simple intermediate language, it is easy to implement and debug new optimizations. This intermediate language uses a variant of standard control-flow graphs and static single assignment form, which makes it easy to implement traditional local optimizations as well. Having the whole program also enables standard data representations such as unboxed integers and arrays, as well as efficient representations for user-defined data structures.This talk will describe MLton's approach to whole-program compilation, covering the optimizations and the intermediate languages, as well as some of the engineering challenges that were overcome to make it feasible to use MLton on programs with over one hundred thousand lines. It will also cover the history of the MLton project from its inception in 1997 until now, and give some lessons learned and thoughts on the future of MLton.