Research and Advances

Subgoal induction

A proof method, subgoal induction, is presented as an alternative or supplement to the commonly used inductive assertion method. Its major virtue is that it can often be used to prove a loop's correctness directly from its input-output specification without the use of an invariant. The relation between subgoal induction and other commonly used induction rules is explored and, in particular, it is shown that subgoal induction can be viewed as a specialized form of computation induction. A set of sufficient conditions are presented which guarantee that an input-output specification is strong enough for the induction step of a proof by subgoal induction to be valid.


Author Archives

Research and Advances

Faster retrieval from context trees

Context trees provide a convenient way of storing data which is to be viewed as a hierarchy of contexts. This note presents an algorithm which improves on previous context tree retrieval algorithms. It is based on the observation that in typical uses context changes are infrequent relative to retrievals, so that data can be cached to speed up retrieval. A retrieval is started from the position of the previous retrieval and auxiliary structures are built up to make the search rapid. Algorithms for addition and deletion of data and for garbage collection are outlined.
Research and Advances

Mechanical program analysis

One means of analyzing program performance is by deriving closed-form expressions for their execution behavior. This paper discusses the mechanization of such analysis, and describes a system, Metric, which is able to analyze simple Lisp programs and produce, for example, closed-form expressions for their running time expressed in terms of size of input. This paper presents the reasons for mechanizing program analysis, describes the operation of Metric, explains its implementation, and discusses its limitations.
Research and Advances

The treatment of data types in EL1

In constructing a general purpose programming language, a key issue is providing a sufficient set of data types and associated operations in a manner that permits both natural problem-oriented notation and efficient implementation. The EL1 language contains a number of features specifically designed to simultaneously satisfy both requirements. The resulting treatment of data types includes provision for programmer-defined data types and generaic routines, programmer control over type conversion, and very flexible data type behavior, in a context that allows efficient compiled code and compact data representation.
Research and Advances

The synthesis of loop predicates

Current methods for mechanical program verification require a complete predicate specification on each loop. Because this is tedious and error prone, producing a program with complete, correct predicates is reasonably difficult and would be facilitated by machine assistance. This paper discusses techniques for mechanically synthesizing loop predicates. Two classes of techniques are considered: (1) heuristic methods which derive loop predicates from boundary conditions and/or partially specified inductive assertions: (2) extraction methods which use input predicates and appropriate weak interpretations to obtain certain classes of loop predicates by an evaluation on the weak interpretation.
Research and Advances

A model and stack implementation of multiple environments

Many control and access environment structures require that storage for a procedure activation exist at times when control is not nested within the procedure activated. This is straightforward to implement by dynamic storage allocation with linked blocks for each activation, but rather expensive in both time and space. This paper presents an implementation technique using a single stack to hold procedure activation storage which allows retention of that storage for durations not necessarily tied to control flow. The technique has the property that, in the simple case, it runs identically to the usual automatic stack allocation and deallocation procedure. Applications of this technique to multitasking, coroutines, backtracking, label-valued variables, and functional arguments are discussed. In the initial model, a single real processor is assumed, and the implementation assumes multiple-processes coordinate by passing control explicitly to one another. A multiprocessor implementation requires only a few changes to the basic technique, as described.

Shape the Future of Computing

ACM encourages its members to take a direct hand in shaping the future of the association. There are more ways than ever to get involved.

Get Involved