[PATCH] D130189: [LoopInterchange][PR56275] Fix legality in dependence vectors

[Michael Kruse via Phabricator via llvm-commits]

Meinersbur added a comment.

As mentioned in https://github.com/llvm/llvm-project/issues/56275#issuecomment-1183363002, I think both directions need to be considered. `<` or `>` of the `Dependence` object mingels relation between the `Src`/`Dst` argument. Here, it is interpreted as the direction of the loop.

However, with normalizing this could be correct, but only when we bail out with any bidirectional dependencies. See my inline comment. Such subtly should be worth a source comment somewhere.



================
Comment at: llvm/lib/Transforms/Scalar/LoopInterchange.cpp:145-146
               Direction = '=';
             else
-              Direction = '>';
+              Direction = '<';
             Dep.push_back(Direction);
----------------
[serious] A SCEV can be neither `isPositive()`, `isZero()`, nor `isNegative()`. This is for instance that case when it depend on a variable which can be either one of them at runtime. This code implicitly assumes that such a distance is "negative". It should be '*'.


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