Alias Killing: Unique Variables without Destructive Reads

An unshared object can be accessed without regard to possible connicts with other parts of a system, whether concurrent or single-threaded. A unique variable (sometimes known as a \free" or \linear" variable) is one that either is null or else refers to an unshared object. Being able to declare and check which variables are unique improves a pro-grammer's ability to avoid program faults. In previously described uniqueness extensions to imperative languages, a unique variable can be ac-cessed only with a destructive read, which nulli-es it after the value is obtained. This approach suuers from several disadvantages: the use of destructive reads increases the complexity of the program which must continually restore nulliied values ; adding destructive reads changes the semantics of the programming language; and many of the nulliications are actually unnecessary. We demonstrate instead that uniqueness can be preserved through the use of existing language features. We give a modular analysis that checks (nonexecutable) uniqueness annotations superimposed on an imperative programming language without destructive reads.