Software change dynamics or half of all Ada compilations are redundant

Abstract

This paper is an empirical study of the evolution of a medium-size, industrial software system written in Ada. Parameters surveyed include various size attributes, the number and distribution of changes, and compilation costs. The interesting aspect of this study is that a day-to-day, complete development history of the system was available, spanning three years. The history permitted a full trace of all day-to-day changes. Findings included:

• • A large number of compilations performed by the Ada compiler are redundant. A simple mechanism that compares compiler output could save about one fourth of all compilations; a detailed dependency analysis could save one half.

• • Whenever package specifications change, almost a third of all compilation units must be recompiled, even though 80 percent of the declarations in the updated specifications are unmodified.

• • The acyclic dependency graph among compilation units is quite flat; the average depth of a configuration of about 160 compilation units is between two and three.

• • Over half of all declarations in package specifications are subprograms. Subprogram headers also account for 55 percent of the changes in package specifications. Overall, the distribution of declarations (subprograms, types, variables, constants, etc.) corresponds closely to the distribution of changes.