We present a new method to infer upper bounds on the innermost runtime complexity of term rewrite systems (TRSs), which benefits from recent advances on complexity analysis of integer transition systems (ITSs). To this end, we develop a transformation from TRSs to a generalized notion of ITSs with (possibly non-tail) recursion. To analyze their complexity, we introduce a modular technique which allows us to use existing tools for standard ITSs in order to infer complexity bounds for our generalized ITSs. The key idea of our technique is a summarization method that allows us to analyze components of the transition system independently. We implemented our contributions in the tool AProVE, and our experiments show that one can now infer bounds for significantly more TRSs than with previous state-of-the-art tools for term rewriting.
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