By: Pashootan Vaezipoor Path Invariant Simon Fraser University – Spring 09

Introduction Current trends in provable assertion derivation: An abstract framework is set up by the user The user must come up with a framework which is both expressive enough and sufficiently inexpensive Abstract domains Shapes and Templates Invariant templates Linear arithmetic Uninterrupted functions CEGAR The abstract interpretation refinement is done automatically But loops cause problem

Path Programs Counterexamples can be seen as a full-fledge program A Path Program is not just a single infeasibility It can represent a whole family of them! So it is ideal for loops When we remove a path program, we are removing many false alarms Path program decomposes a large program into a set of smaller programs To achieve all these we must add universal quantifiers to the set!

Advantages We can overcome two limitations of CEGAR-based schemes Avoid iterative unwinding of loops We can treat infinite paths and also we can treat finite paths more efficiently We can handle a larger class of problems Dependence of correctness of program on arrays

Example 1 (FORWARD) What does BLAST do? No predicates are tracked and just reach ability checked What does BLAST do? Is the contra example genuine or spurious?

Example 1 (FORWARD) What does BLAST do? In the third phase it extracts the predicates and adds them to predicate abstraction But again for two iterations we need to do the same thing!

Path Invariant We infer path invariants from Path Programs A path invariant map is a map from a location of the prog to a set of formulas Initial location maps to true For each (l, ρ,l’) in the path program, the successor of the formula at l with respect to the program operation ρ implies the formula at l’ The path is safe, if the error location is mapped to formula false

Example 2 (INIT-CHECK)

Formulation A program is P=(X, L, l 0, T, l e ) Error location does not have any outgoing edges These together make a directed graph called the control-flow graph (CFG) A computation of the program is the sequence,…, If (l, ρ,l’) is an edge in T then we have (s i,s i+1 ) satisfies ρ

Computation of Path Invariants We use the template-based invariant generation In template-based invariant synthesis, we assume that for each control location in the domain of the map η, we have a so-called invariant template, which is a parametric constraint over program variables.

Universal Quantifiers We construct a suitable template by analyzing a given path program. If the program contains an assertion that is iteratively checked, then we add a universally quantified implication to the template.