Precise Garbage Collection in Prolog

In this paper we present a series of tiny programs that verify that a Prolog heap garbage collector can find specific forms of garbage. Only 2 out of our tested 7 Prolog systems pass all tests. Comparing memory usage on realistic programs dealing with finite datastructures using both poor and precise garbage collection shows only a small difference, providing a plausible explanation why many Prolog implementors did not pay much attention to this issue. Attributed variables allow for creating infinite lazy datastructures. We prove that such datastructures have great practical value and their introduction requires ‘precise’ garbage collection. The Prolog community knows about three techniques to reach at precise garbage collection. We summarise these techniques and provide more details on scanning virtual machine instructions to infer reachability in a case study.