Cristina Borralleras

Complete Monotonic Semantic Path Orderings

Although theoretically it is very powerful, the semantic path ordering (SPO) is not so useful in practice, since its monotonicity has to be proved by hand for each concrete term rewrite system (TRS).

Solving Non-linear Polynomial Arithmetic via SAT Modulo Linear Arithmetic

Polynomial constraint-solving plays a prominent role in several areas of engineering and software verification. In particular, polynomial constraint solving has a long and successful history in the development of tools for proving termination of programs. Well-known and very efficient techniques, like SAT algorithms and tools, have been recently proposed and used for implementing polynomial constraint solving algorithms through appropriate encodings. However, powerful techniques like the ones provided by the SMT (SAT modulo theories) approach for linear arithmetic constraints (over the rationals) are underexplored to date. In this paper we show that the use of these techniques for developing polynomial constraint solvers outperforms the best existing solvers and provides a new and powerful approach for implementing better and more general solvers for termination provers.

SAT Modulo Linear Arithmetic for Solving Polynomial Constraints

Polynomial constraint solving plays a prominent role in several areas of hardware and software analysis and verification, e.g., termination proving, program invariant generation and hybrid system verification, to name a few. In this paper we propose a new method for solving non-linear constraints based on encoding the problem into an SMT problem considering only linear arithmetic. Unlike other existing methods, our method focuses on proving satisfiability of the constraints rather than on proving unsatisfiability, which is more relevant in several applications as we illustrate with several examples. Nevertheless, we also present new techniques based on the analysis of unsatisfiable cores that allow one to efficiently prove unsatisfiability too for a broad class of problems. The power of our approach is demonstrated by means of extensive experiments comparing our prototype with state-of-the-art tools on benchmarks taken both from the academic and the industrial world.

Recursive Path Orderings Can Be Context-Sensitive

Context-sensitive rewriting (CSR) is a simple restriction of rewriting which can be used e.g. for modelling non-eager evaluation in programming languages. Many times termination is a crucial property for program verification. Hence, developing tools for automatically proving termination of CSR is necessary.

A Monotonic Higher-Order Semantic Path Ordering

There is an increasing use of (first- and higher-order) rewrite rules in many programming languages and logical systems. The recursive path ordering (RPO) is a well-known tool for proving termination of such rewrite rules in the first-order case. However, RPO has some weaknesses. For instance, since it is a simplification ordering, it can only handle simply terminating systems. Several techniques have been developed for overcoming these weaknesses of RPO. A very recent such technique is the monotonic semantic path ordering (MSPO), a simple and easily automatable ordering which generalizes other more ad-hoc methods. Another recent extension of RPO is its higher-order version HORPO. HORPO is an ordering on terms of a typed lambda-calculus generated by a signature of higher-order function symbols. Although many interesting examples can be proved terminating using HORPO, it inherits the weaknesses of the first-order RPO. Therefore, there is an obvious need for higher-order termination orderings without these weaknesses. Here we define the first such ordering, the monotonic higher-order semantic path ordering (MHOSPO), which is still automatable like MSPO. We give evidence of its power by means of several natural and non-trivial examples which cannot be handled by HORPO.

Proving Termination Through Conditional Termination

We present a constraint-based method for proving conditional termination of integer programs. Building on this, we construct a framework to prove (unconditional) program termination using a powerful mechanism to combine conditional termination proofs. Our key insight is that a conditional termination proof shows termination for a subset of program execution states which do not need to be considered in the remaining analysis. This facilitates more effective termination as well as non-termination analyses, and allows handling loops with different execution phases naturally. Moreover, our method can deal with sequences of loops compositionally. In an empirical evaluation, we show that our implementation VeryMax outperforms state-of-the-art tools on a range of standard benchmarks.

The recursive path and polynomial ordering for first-order and higher-order terms

In most termination tools two ingredients, namely recursive path orderings (RPOs) and polynomial interpretation orderings (POLOs), are used in a consecutive disjoint way to solve the final constraints generated from the termination problem. In this article we present a simple ordering that combines both RPO and POLO and defines a family of orderings that includes both, and extend them with the possibility of having, at the same time, an RPO-like treatment for some symbols and a POLO-like treatment for the others. The ordering is extended to higher-order terms, providing a new fully automatable use of polynomial interpretations in combination with beta-reduction.

Orderings and constraints: theory and practice of proving termination

In contrast to the current general way of developing tools for proving termination automatically, this paper intends to show an alternative program based on using on the one hand the theory of term orderings to develop powerful and widely applicable methods and on the other hand constraint based techniques to put them in practice. In order to show that this program is realizable a constraint-based framework is presented where ordering based methods for term rewriting, including extensions like Associative-Commutative rewriting, Context-Sensitive rewriting or Higher-Order rewriting, as well as the use of rewriting strategies, can be put in practice in a natural way.

Monotonic AC-compatible semantic path orderings

Polynomial interpretations and RPO-like orderings allow one to prove termination of Associative and Commutative (AC-)rewriting by only checking the rules of the given rewrite system. However, these methods have important limitations as termination proving tools. To overcome these limitations, more powerful methods like the dependency pair method have been extended to the AC-case. Unfortunately, in order to ensure AC-termination, the so-called extended rules, which, in general are hard to prove must be added to the rewrite system. In this paper we present a fully monotonic AC-compatible semantic path ordering. This monotonic AC-ordering defines a new automatable termination proving method for AC-rewritingwhic h does not need to consider extended rules. As a hint of the power of this method, we can easily prove several non-trivial examples appearing in the literature, including one that, to our knowledge, can be handled by no other automatic method.