During deductive verification of programs written in imperative languages, the generation and proof of verification conditions corresponding to loops can cause difficulties, because each one must be provided with an invariant whose construction is often a challenge. As a rule, the methods of invariant synthesis are heuristic ones. This impedes its application. An alternative is the symbolic method of loop invariant elimination suggested by V.A. Nepomniaschy in 2005. Its idea is to represent a loop body in a form of special replacement operation under certain constraints. This operation expresses loop effect in a symbolic form and allows to introduce an inference rule which uses no invariants in axiomatic semantics. This work represents the further development of this method. It extends the mixed axiomatic semantics method suggested for C-light program verification. This extension includes the verification method of iterations over changeable arrays possibly with loop exit in C-light programs. The method contains the inference rule for iterations without loop invariants. This rule was implemented in verification conditions generator which is a part of the automated system of C-light program verification. To prove verification conditions automatically in ACL2, two algorithms were developed and implemented. The first one automatically generates the replacement operation in ACL2 language, the second one automatically generates auxiliary lemmas which allow to prove the obtained verification conditions in ACL2 successfully in automatic mode. An example which illustrates the application of the mentioned methods is described.

The C-lightVer system for the deductive verification of C programs is being developed at the IIS SB RAS. Based on the two-level architecture of the system, the C-light input language is translated into the intermediate C-kernel language. The meta generator of the correctness conditions receives the C-kernel program and Hoare logic for the C-kernel as input. To solve the well-known problem of determining loop invariants, the definite iteration approach was chosen. The body of the definite iteration loop is executed once for each element of the finite dimensional data structure, and the inference rule for them uses the substitution operation rep, which represents the action of the cycle in symbolic form. Also, in our meta generator, the method of semantic markup of correctness conditions has been implemented and expanded. It allows to generate explanations for unproven conditions and simplifies the errors localization. Finally, if the theorem prover fails to determine the truth of the condition, we can focus on proving its falsity. Thus a method of proving the falsity of the correctness conditions in the ACL2 system was developed. The need for more detailed explanations of the correctness conditions containing the replacement operation rep has led to a change of the algorithms for generating the replacement operation, and the generation of explanations for unproven correctness conditions. Modifications of these algorithms are presented in the article. They allow marking rep definition with semantic labels, extracting semantic labels from rep definition and generating description of break execution condition.

Research project "Platform-independent approach to formal specification and verification of standard mathematical functions" is aimed onto a development of an incremental combined approach to the specification and verification of the standard mathematical functions like sqrt, cos, sin, etc. Platform-independence means that we attempt to design a relatively simple axiomatization of the computer arithmetic in terms of real, rational, and integer arithmetic (i.e. the fields R and Q of real and rational numbers, the ring Z of integers) but dont specify neither base of the computer arithmetic, nor a format of numbers representation. Incrementality means that we start with the most straightforward specification of the simplest easy to verify algorithm in real numbers and finish with a realistic specification and a verification of an algorithm in computer arithmetic. We call our approach combined because we start with a manual (pen-and-paper) verification of some selected algorithm in real numbers, then use these algorithm and verification as a draft and proof-outlines for the algorithm in computer arithmetic and its manual verification, and finish with a computer-aided validation of our manual proofs with some proof-assistant system (to avoid appeals to "obviousness" that are very common in human-carried proofs). In the paper we present first steps towards a platform-independent incremental combined approach to specification and verification of the standard functions cos and sin that implement mathematical trigonometric functions cos and sin.

По сравнению с традиционным тестированием дедуктивная верификация предлагает более формальный способ доказательства корректности программ. Но как установить корректность самой системы верификации? Теоретические основы логик Хоара исследовались в классических работах, где были получены различные результаты по непротиворечивости и полноте. Однако нам не известны реализации этих теоретических методов, проверенные чем-либо отличным от обычного тестирования. Иными словами, нас интересует система верификации, которая может быть применена к самой себе (хотя бы частично). В наших исследованиях мы обратились к методу метагенерации, который выглядит многообещающим для этой задачи.

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