Bottom Extraction Factoring error handling out of functional programs

Many utility functions in a functional programming language consist of just a few lines of code. They are written with the expectation that they will be inlined and heavily optimized. For many of these functions, the size of the function being inlined is dominated by the size of code required to signal an error if the inputs are inconsistent. In this paper we describe bottom extraction, a simple technique which mitigates this problem. Using a combination of type information and strictness information, we can easily identify expressions which are devoted to error handling (semantically bottom). Such a bottom expression is enlarged to encompass as much code as possible by a set of local rules. Bottom regions are extracted and hoisted to the top level of the program. Bottom extraction greatly reduces code bloat in the presence of inlining by reducing the size of inlined code. In addition, it provides a simple and fast test to identify error handling code in the rest of the compiler, enabling us to easily perform program transformations around expressions which are semantically bottom. Bottom extraction is a simple but important part of our compiler for pH, and markedly improves every aspect of the compiler’s behavior.