Program logic and equivalence in the presence of garbage collection

Abstract

It is generally thought that reasoning about programs in memory safe, garbage collected languages is much easier than in languages where the programmer has more explicit control over memory. Paradoxically, existing program logics are based on a low-level view of storage that is sensitive to the presence or absence of unreachable cells, and Reynolds has pointed out that the Hoare triples derivable in these logics are even incompatible with garbage collection. We present a study of a small language whose operational semantics includes a rule for reclaiming garbage. Our main results include an analysis of propositions that are garbage insensitive, and full abstraction results connecting partial and total correctness to two natural notions of observational equivalence between programs.