[PATCH] D75845: [ValueLattice] Add singlecrfromundef lattice value.

Mon Mar 9 15:40:17 PDT 2020

fhahn added a comment.

In D75845#1913226 <https://reviews.llvm.org/D75845#1913226>, @nikic wrote:

> Same comment as on D75055 <https://reviews.llvm.org/D75055>: For LVI we don't guarantee that all uses of the value will be replaced, so this does not seem like a completely fix.

Yes that is an additional issue that needs to be addressed in LVI and/or its clients. This patch (and the preceding one) are a first step towards fixing the underlying issue in ValueLattice. Once those are addressed, we can fix the code in LVI.

> I'm generally not really clear on how this differs from the variant implemented in D75120 <https://reviews.llvm.org/D75120> (is it is just a matter of style, i.e. making this explicit, or is there a functional difference here?)

Sorry for the confusion. This patch and D75120 <https://reviews.llvm.org/D75120> complement each other and both are needed. D75120 <https://reviews.llvm.org/D75120> introduces a state for actual undef values and separates the notion of value we know nothing about yet and values we know are undef. This patch introduces a state for constants that started out as undef values or were merged with undef values. Such constants cannot safely be merged with other constant ranges.

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