Abstract

The TILT compiler for Standard ML is type-directed and type-preserving, that is, it makes use of and translates type information during the phases of compilation. Unfortunately, such use of type data incurs a significant overhead. This paper explores methods for abstractly, conveniently, and efficiently storing and manipulating type information in TILT. In the end, we discover that doing more work to reduce overhead is a bad strategy for this situation. This material is based on work supported in part by ARPA grant F-19628-95-C-0050 and NSF grant CCR-9984812. Any opinions, findings, and conclusions or recommendations in this publication are those of the authors and do not reflect the views of these agencies. Keywords: types, typed intermediate languages, type representations, Standard ML, hash consing, de Bruijn indices, views1 The Wizard of TILT TILT is a certifying compiler for Standard ML [1]. Its major distinguishing feature is the use of Typed Intermediate Languages throughout the phases of compilation. Because each of the code transformations that the compiler performs also transforms the types, we preserve type information that is normally discarded after typechecking the source language in traditional compilers. This allows us to typecheck the results of these transformations (catching compiler

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