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I think currently isImpliedViaMerge can incorrectly return true for phis in a loop/cycle, if the found condition involves the previous value of Consider the case in exit_cond_depends_on_inner_loop. At some point, we call (modulo simplifications) isImpliedViaMerge(<=, %x.lcssa, -1, %call, -1). The existing code tries to prove IncV <= -1 for all incoming values InvV using the found condition (%call <= -1). At the moment this succeeds, but only because it does not compare the same runtime value. The found condition checks the value of the last iteration, but the incoming value is from the *previous* iteration. Hence we incorrectly determine that the *previous* value was <= -1, which may not be true. I think we need to be more careful when looking at the incoming values here. In particular, we need to rule out that a found condition refers to any value that may refer to one of the previous iterations. I'm not sure there's a reliable way to do so (that also works of irreducible control flow). So for now this patch adds an additional requirement that the incoming value must properly dominate the phi block. This should ensure the values do not change in a cycle. I am not entirely sure if will catch all cases and I appreciate a through second look in that regard. Alternatively we could also unconditionally bail out in this case, instead of checking the incoming values Reviewed By: nikic Differential Revision: https://reviews.llvm.org/D101829 |
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