The Global Storage Needs of a Subcomputation

Abstract

A defining characteristic of “functional” specifications is the absence of assignments: updates of tables and data structures are expressed by giving the relationship between the new and old values. An obvious implementation allocates separate space for new and old values and may consume a lot of storage. However, even when updates of attributes like symbol tables are expressed functionally, we would like to avoid making copies of the symbol table during attribute evaluation. In other words, if possible, the implementation should have a single global copy of the table that is updated using assignments. Since the value of the global copy changes during computation, the order of evaluation has to be chosen carefully. In this paper, we partition attributes into classes, the problem being to determine if there exists an evaluation order that allows each class to share a global storage area. The solution extends to handle symbol tables for block structured languages.