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Lazy Shape Analysis

Many software model checkers are based on predicate abstraction. Values of variables in branching conditions are represented abstractly using predicates. The strength of this approach is its path-sensitive nature. However, if the control flow depends heavily on the values of memory cells on the heap, the approach does not work well, because it is difficult to find `good' predicate abstractions to represent the heap. In contrast, shape analysis can lead to a very compact representation of data structures stored on the heap. In this paper, we combine shape analysis with predicate abstraction, and integrate it into the software model checker BLAST. Because shape analysis is expensive, we do not apply it globally. Instead, we ensure that shapes are computed and stored locally, only where necessary for proving the verification goal. To achieve this, we extend lazy abstraction refinement, which so far has been used only for predicate abstractions, to shapes. This approach does not only increase the precision of model checking and shape analysis taken individually, but also increases the efficiency of shape analysis (we do not compute shapes where not necessary). We implemented the technique by extending BLAST with calls to TVLA, and evaluated it on several C programs manipulating data structures, with the result that the combined tool can now automatically verify programs that are not verifiable using either shape analysis or predicate abstraction on its own.

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