When the Decreasing Sequence Fails

The classical method for program analysis by abstract interpretation consists in computing a increasing sequence with widening, which converges towards a correct solution, then computing a decreasing sequence of correct solutions without widening. It is generally admitted that, when the decreasing sequence reaches a fixpoint, it cannot be improved further. As a consequence, all efforts for improving the precision of an analysis have been devoted to improving the limit of the increasing sequence. In this paper, we propose a method to improve a fixpoint after its computation. The method consists in projecting the solution onto well-chosen components and to start again increasing and decreasing sequences from the result of the projection.

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