[LAA] Support assumptions with non-constant deref sizes.#156529
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Update evaluatePtrAddrecAtMaxBTCWillNotWrap to support non-constant sizes in dereferenceable assumptions. Note that with that we will need to add a constant to a backedge-taken-count, so expressions like (-1 + %n) + 1. While adding something to -1 technically wraps in the unsigned sense, in practice the result should not consider overflowing in addSCEVNoOverflow. I am not sure if there's a more general place for this kind of reasoning?
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I don't see any obvious reason it's safe to conclude the computation can't overflow. Loops with BTC of "n-1" usually have a trip count of "n", but you can easily write code to violate that, like |
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Argh yes, this was a stale change and should be taken care with info from loop guards. |
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Update evaluatePtrAddrecAtMaxBTCWillNotWrap to support non-constant sizes in dereferenceable assumptions.
Note that with that we will need to add a constant to a backedge-taken-count, so expressions like (-1 + %n) + 1. While adding something to -1 technically wraps in the unsigned sense, in practice the result should not consider overflowing in addSCEVNoOverflow. I am not sure if there's a more general place for this kind of reasoning?