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IVOPTs, when replacing an IV, inserts the computation of the new IVs step in the loop preheader without considering the case of the loop not iterating. This means we have to ensure the step computation does not invoke UB. There is also SCEV which does not care about signed arithmetic UB when re-associating expressions to form CHRECs, so even when we know the loop iterates this is required. PR tree-optimization/121382 * tree-ssa-loop-ivopts.cc (create_new_iv): Rewrite the IV step to defined form. * gcc.dg/torture/pr121382.c: New testcase.
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