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Two-Variable Separation Logic and Its Inner Circle

Published:21 April 2015Publication History
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Abstract

Separation logic is a well-known assertion language for Hoare-style proof systems. We show that first-order separation logic with a unique record field restricted to two quantified variables and no program variables is undecidable. This is among the smallest fragments of separation logic known to be undecidable, and this contrasts with the decidability of two-variable first-order logic. We also investigate its restriction by dropping the magic wand connective, known to be decidable with nonelementary complexity, and we show that the satisfiability problem with only two quantified variables is not yet elementary recursive. Furthermore, we establish insightful and concrete relationships between two-variable separation logic and propositional interval temporal logic (PITL), data logics, and modal logics, providing an inner circle of closely related logics.

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  1. Two-Variable Separation Logic and Its Inner Circle

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      Markus Wolf

      Separation logic, an extension of Hoare logic, is currently a very active area of research. This paper investigates the expressiveness of this logic by exploring various restricted forms of it. Relations between fragments of separation logic and related logics are also studied, and via translations the relationships between these logics are expressed. The introduction of the paper explains known results, describes the results that are derived in the following sections, and gives a map showing the relationships between the various logics. The following sections introduce some restricted form of separation logic where formulas contain only one record field and a finite number of bounded variables. Next, some formulas often used in the following sections are defined and an encoding of data words via these formulas is presented. The kind of heaps used in this encoding, called fishbone heaps, are used a lot throughout the remainder of the paper. Finally, the section is concluded by the presentation of a new special variant modal logic for describing heaps. Section 3 builds on the previous section by further advancing the technical terms and constructing a translation from the propositional temporal interval logic into separation logic with two quantified variables and without the so-called "magic wand" operator. The possibility of this translation yields that separation logic with two quantified variables is decidable but not elementary recursive. The fourth section finally proves that the satisfiability problem of the separation logic with two bounded variables and the "magic wand" operator is not decidable. This is achieved by encoding Minsky machines and reducing the halting problem. A conclusion wraps up the results and points out some research directions. The paper contains a lot of technical tricks, which are nevertheless presented in a way that is very intuitively graspable. The intuition of fishbone heaps and the depiction of their structure are very helpful. On the whole, I consider this a very enjoyable paper. Online Computing Reviews Service

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      • Published in

        cover image ACM Transactions on Computational Logic
        ACM Transactions on Computational Logic  Volume 16, Issue 2
        March 2015
        260 pages
        ISSN:1529-3785
        EISSN:1557-945X
        DOI:10.1145/2737801
        Issue’s Table of Contents

        Copyright © 2015 ACM

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        Publication History

        • Published: 21 April 2015
        • Accepted: 1 January 2015
        • Revised: 1 November 2014
        • Received: 1 September 2013
        Published in tocl Volume 16, Issue 2

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