Ordering Optimisations for Concurrent Logic Programs

Ordering optimisations are optimisations that can be applied to a concurrent logic program when the atoms of a clause are known to be ordered. In this paper ordering optimisations are reviewed, reformulated and refined. The paper explains how ordering optimisations can be realised in terms of abstract interpretation and shows that by, building on schedule analysis, simple, efficient and accurate forms of abstract interpretation can achieved. The paper outlines how to: identify instances of unification which can be simplified or removed; distinguish repeated synchronisation instructions; indicate which redundant checks can be removed when producers are ordered before consumers in the same thread; identify which variables can be accessed without dereferencing; indicate where variable initialisation and unification can be simplified; and show which variables can be allocated to an environment. Some safety checks can also be removed by using mode information.