Garbage Collection for Reversible Functional Languages

Abstract

Reversible languages are programming languages where all programs can run both forwards and backwards. Reversible functional languages have been proposed that use symmetric pattern matching and data construction. To be reversible, these languages require linearity: Every variable must be used exactly once, so no references are copied and all references are followed exactly once. Copying of values must use deep copying. Similarly, equality testing requires deep comparison of trees.

A previous paper describes reversible treatment of reference counts, which allows sharing of structures without deep copying, but there are limitations. Applying a constructor to arguments creates a new node with reference count 1, so pattern matching is by symmetry restricted to nodes with reference count 1. A variant pattern that does not change the reference count of the root node is introduced to allow manipulation of shared data. Having two distinct patterns for shared and unshared data, however, adds a burden on the programmer.

We observe that we can allow pattern matching on nodes with arbitrary reference count if we also allow constructor application to return nodes with arbitrary reference counts. We do this by using maximal sharing: If a newly constructed node is identical to an already existing node, we return a pointer to the existing node (increasing its reference count) instead of allocating a new node with reference count 1.

To avoid searching the entire heap for an identical node, we use hash-consing to restrict the search to a small segment of the heap. We estimate how large this segment needs to be to give a very low probability of allocation failure when the heap is less than half full. Experimentally, we find that overlapping segments gives dramatically better results than disjoint segments.