Variable Shunting for the WAM

This paper describes how to extend the garbage collection for WAM [ACHS88] so that it will shunt chains of bound variables if possible. Doing so has two advantages: 1. Space is saved by making it possible to deallocate the intermediate cells. This is particularly useful when those cells are associated with frozen goals. 2. Later dereferencing is speeded up by not having to follow long variable chains. The main complication of this optimization is the treatment of the trailed variables. We claim that all possible chains of variables are shunted by this algorithm. The algorithm has been implemented in SICStus Prolog, and benchmark results are presented in this paper. This paper is a revised version of [Sahlin89] and is meant to be read in conjunction with [ACHS88] as the notation used is presented there and only briefly summarized here. The full source code for the shunting algorithm is given in this paper. An example Chains of variables are unavoidable in an ordinary WAM execution. For instance, the query p(1000,s(A),B) given to the program below, will create a variable chain starting in B having 1000 links. p(0,s(Z),Z). p(N,s(X),Z) :N>0, N1 is N-1, p(N1,Y,Z), Y=s(X). During WAM execution a number of value cells containing various instances of s(X) will be created first. Then the chain of X's will be linked so that the last one will point to the next to last, the next to last will point to the third from the end, etc. Thus these variable chains may occur in normal WAM execution. If these intermediate cells are “short-circuited” (shunted), subsequent dereferencing of the variable B in the query will not have to traverse the intermediate cells. Thus execution speed is improved. If the intermediate cells are not referenced from anywhere else, they may also become deallocated. Thus more storage can be reclaimed.