Bounded underapproximations

  • Authors:
  • Pierre Ganty;Rupak Majumdar;Benjamin Monmege

  • Affiliations:
  • IMDEA Software, Spain;UCLA;ENS Cachan, France

  • Venue:
  • CAV'10 Proceedings of the 22nd international conference on Computer Aided Verification
  • Year:
  • 2010

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Abstract

We show a new and constructive proof of the following language-theoretic result: for every context-free language L, there is a bounded context-free language L′⊆L which has the same Parikh (commutative) image as L Bounded languages, introduced by Ginsburg and Spanier, are subsets of regular languages of the form $w_1^*w_2^*\cdots w_k^*$ for some $w_1,\ldots,w_k\in\Sigma^*$ In particular bounded context-free languages have nice structural and decidability properties Our proof proceeds in two parts First, using Newton's iterations on the language semiring, we construct a context-free subset LN of L that can be represented as a sequence of substitutions on a linear language and has the same Parikh image as L Second, we inductively construct a Parikh-equivalent bounded context-free subset of LN. As an application of this result in model checking, we show how to underapproximate the reachable state space of multithreaded procedural programs The bounded language constructed above provides a decidable underapproximation for the original problem By iterating the construction, we get a semi-algorithm for the original problems that constructs a sequence of underapproximations such that no two underapproximations of the sequence can be compared This provides a progress guarantee: every word w∈L is in some underapproximation of the sequence, and hence, a program bug is guaranteed to be found In particular, we show that verification with bounded languages generalizes context-bounded reachability for multithreaded programs.