Share x Free Revisited

Analyses for possible variable sharing and definite freeness are important both in the automatic parallelisation and in the optimisation of sequential logic programs. In this paper, a new efficient approach to analysis is described which can infer sharing and freeness information to an unusually high degree of accuracy. The analysis exploits a confluence property of the unification algorithm to split the analysis into two distinct phases. The two phase analysis improves efficiency by enabling each phase of the analysis to manipulate relatively simple data-structures. The precision follows from the combination of domains. The analysis propagates groundness with the accuracy of sharing groups and yet infers sharing and freeness to a precision which exceeds that of a normal freeness analysis. High precision compoundness information can be derived too. The usefulness of the analysis is demonstrated with worked examples. Correctness is formally proven.