Loop invariants: Analysis, classification, and examples

  • Authors:

  • Carlo A. Furia;Bertrand Meyer;Sergey Velder

  • Affiliations:

  • ETH Zurich, Zurich, Switzerland;ETH Zurich, ITMO St. Petersburg, and Eiffel Software, Zurich, Switzerland;ITMO St. Petersburg, Saint Petersburg, Russia

  • Venue:
  • ACM Computing Surveys (CSUR)
  • Year:
  • 2014

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Abstract

Software verification has emerged as a key concern for ensuring the continued progress of information technology. Full verification generally requires, as a crucial step, equipping each loop with a “loop invariant.” Beyond their role in verification, loop invariants help program understanding by providing fundamental insights into the nature of algorithms. In practice, finding sound and useful invariants remains a challenge. Fortunately, many invariants seem intuitively to exhibit a common flavor. Understanding these fundamental invariant patterns could therefore provide help for understanding and verifying a large variety of programs. We performed a systematic identification, validation, and classification of loop invariants over a range of fundamental algorithms from diverse areas of computer science. This article analyzes the patterns, as uncovered in this study, governing how invariants are derived from postconditions; it proposes a taxonomy of invariants according to these patterns; and it presents its application to the algorithms reviewed. The discussion also shows the need for high-level specifications based on “domain theory.” It describes how the invariants and the corresponding algorithms have been mechanically verified using an automated program prover; the proof source files are available. The contributions also include suggestions for invariant inference and for model-based specification.