Summary: Relating Graph and Term Rewriting via B¨ohm Models
Zena M. Ariola
Computer & Information Science Department
University of Oregon, Eugene, OR 97403­1202
e­mail: ariola@cs.uoregon.edu
Dealing properly with sharing is important for expressing some of the common compiler optimizations as source­
to­source transformations, such as common subexpressions elimination, lifting of free expressions and removal
of invariants from a loop. Term graph rewriting is a computational model to accommodate these concerns. In
this paper we are interested in defining a term model for term graph rewriting systems, which allows us to prove
total correctness of those optimizations. We introduce the notion of B¨ohm tree, and show that for orthogonal
term graph rewriting systems, B¨ohm tree equivalence defines a congruence. Total correctness then follows in a
straightforward way from showing that if a program M contains less sharing than a program N , then both M
and N have the same B¨ohm tree.
Using B¨ohm trees we also show that orthogonal term graph rewriting systems are a correct implementation
of orthogonal term rewriting systems. This boils down to showing that the behavior of a term graph can be
deduced from its finite approximations, that is, graph rewriting is a continuous operation. Our approach differs
from that of other researchers which is based on infinite rewriting.
1. Introduction
Dealing properly with sharing is important in a framework for reasoning about the implementation of
functional languages and the correctness of certain compiler optimizations, such as common subexpressions

Source: Ariola, Zena M. - Department of Computer and Information Science, University of Oregon

Collections: Computer Technologies and Information Sciences