On Taylor's Scheme for Unbound Variables

A novel scheme for representing unbound variables was proposed by Taylor: that aliased unbound variables are represented as circular chains of cells. Unfortunately, few details were given on its implementation and the involved trade oos. We compare the proposed scheme to that of Warren's abstract machine , and nd that Taylor's scheme (a) leads to increased trailing, (b) makes stack allocation more diicult, (c) requires more expensive copying of heap cells at times, and (d) has a more expensive variable binding mechanism. On the positive side, a very strong advantage is that Taylor's scheme entirely dispenses with pointer chain dereferenc-ing. We conclude by enumerating the data ow properties a global an-alyzer should nd in order to eliminate these diiculties.