[PATCH] D65474: [SCEV] Return zero from computeConstantDifference(X, X)

[Nikolai Bozhenov via Phabricator via llvm-commits]

n.bozhenov added a comment.

In D65474#1607285 <https://reviews.llvm.org/D65474#1607285>, @sanjoy wrote:

> > We use computeConstantDifference in some downstream code, and I was surprised to find out that it fails to compute (X - X) difference, unless X is a SCEVAddExpr.
>
> Can you use `getMinusSCEV` instead (which should DTRT)?
>
> This looks like a small change, having many such cases (especially without unit tests) makes it difficult to refactor code confidently.


Hi Sanjoy,

What I need is to check if two arbitrary SCEV expressions are constant distance apart or not, and compute that difference. That's exactly what computeConstantDifference does (except for the (%x - %x) case). I would be ok to use getMinusSCEV, but it does not do the right thing unfortunately. Given {%x, +, 1}<loop1> and {%x, +, 1}<loop2>, getMinusSCEV crashes instead of returning SCEVCouldNotCompute. And I am not aware of any existing API that checks whether two SCEVs are safe to subtract or not.


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