Layer Sharing : an improved Structure { Sharing

We present in this paper a structure{sharing framework originally developed for a Dynamic Programming interpreter of Logic Programs called DyALog. This mechanism should be of interest for alternative execution models of PROLOG which maintain multiple computation branches and reuse sub{computations in various contexts (computation sharing). This category includes , besides our Dynamic Programming model, the tabular models (OLDT, SLDAL, XWAM), the \magic-set" models, and the independent AND and OR par-allelism with solution sharing models. These models raise the problem of storing vast amount of data, motivating us to discard copying mechanisms in favor of structure{sharing mechanisms. Unfortunately, computation sharing requires joining computation branches and possibly renaming some variables, which generally leads to complex structure{sharing mechanisms. The proposed \layer{sharing" framework succeeds however in remaining understandable and easy to implement.