Detecting interleaving

The various goals and requirements of a system are realized in software as fragments of code that are typi cally interleaved in that they may be woven together in the same contiguous textual area of code The frag ments of code are often delocalized and overlap rather than being composed in a simple linear sequence In terleaving severely complicates software comprehen sion and maintenance To address this problem we are developing analysis tools based on the Software Re nery This paper describes our experiences in de tecting interleaving in a corpus of mathematical soft ware written in Fortran from the Jet Propulsion Labo ratory In particular it discusses how feasible it is to detect interleaving of various types and the ability of existing tools to assist these types of detection Introduction and Motivation To understand a program one often has to unravel multiple interwoven strands of computation each re sponsible for accomplishing a distinct purpose We use the term plan to denote a description or representation of a computational structure that the designers have proposed as a way of achieving some purpose or goal in a program Plans can occur at any level of abstraction from architectural overviews to code Interleaving expresses the merging of two or more dis tinct plans within some contiguous textual area of a program A trivial example is a single loop responsible for computing both the maximum element of a vector and its position A less trivial example is a program intended to write a report that summarizes data ex tracted from sorted input records The program has two purposes computing the summary data and man aging the construction of the report headers page breaks page counts etc In an object oriented pro gram these two purposes might well be realized by two separate objects In traditional code however the implementations of these functions are often in terleaved and understanding the code is signi cantly complicated Interleaving may arise for e ciency reasons For example it may be more e cient to compute two re lated values in one place than to do so separately Or interleaving may be the result of inadequate software maintenance such as adding a feature locally to an existing routine rather than undertaking a thorough redesign Or interleaving may arise as a natural by product of expressing separate but related plans in a linear textual medium For example accessors and constructors for manipulating data structures are typ ically interleaved throughout programs written in tra ditional programming languages due to their procedu ral rather than object oriented structure Regardless of why interleaving is introduced it severely compli cates understanding a program This makes it di cult to perform a variety of tasks including extracting reusable components localizing the e ects of mainte nance changes and migrating to object oriented lan guages What is needed is to isolate the separate strands of computation understand them individually and then see how they interrelate We are building analysis tools to do this Traditional slicing techniques are applicable to this problem but are not powerful enough to disentangle all forms of interleaving They rely only on data and control ow whereas our tools also use deeper knowledge of what plans are interleaved in the code