Garbage Collecting Shared Environment Closure Reducers

Shared environment closure reducers such as Fairbairn and Wray's TIM incur a comparatively low cost when creating a suspension, and so provide an elegant method for implementing lazy functional evaluation. However, comparatively little attention has been given to the problems involved in identifying which portions of a shared environment are needed (and ignoring those which are not) during a garbage collection. Proper consideration of this issue has subtle consequences when implementing a storage manager in a TIM-like system. We describe the problem and illustrate the negative consequences of ignoring it. We go on to describe a solution in which the compiler determines statically which portions of that code's environment are required for each piece of code it generates, and emits information to assist the run-time storage manager to scavenge environments selectively. We also describe a technique for expressing this information directly as executable code, and demonstrate that a garbage collector implemented in this way can perform significantly better than an equivalent, table-driven interpretive collector.