CHAT is ( SLG - WAM ) ?

CHAT ooers an alternative to SLG-WAM for implementing the suspension and resumption of consumers: unlike SLG-WAM, it does not use freeze registers nor a complicated trail to preserve their execution environments. CHAT also limits the amount of copying of CAT, which was previously put forward as an alternative to SLG-WAM. Although experimental results show that in practice CHAT is competitive with | if not better than | SLG-WAM, there remains the annoying fact that on contrived programs the original CHAT can be made arbitrarily worse than SLG-WAM, i.e. the original CHAT has a higher complexity. In this paper we show how to overcome this problem, in particular, we deal with the two sources of higher complexity of CHAT: the repeated traversal of the choice point stack, and the lack of suucient sharing of the trail. This is achieved without fundamentally changing the underlying principle of CHAT by a technique that manipulates a Prolog choice point so that it assumes temporarily a diierent functionality and in a way that is transparent to the underlying WAM. There is more potential use of this technique besides lowering the worst case complexity of CHAT: it leads to considering scheduling strategies that were not feasible before either in CHAT or in SLG-WAM. We also discuss extensively issues related to the implementation of trail that tabled logic programming systems require.