[llvm] [LV] Add initial support for partial alias masking (PR #177599)

[Andrei Elovikov via llvm-commits]

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@@ -7462,6 +7540,7 @@ DenseMap<const SCEV *, Value *> LoopVectorizationPlanner::executePlan(
   VPlanTransforms::materializeVectorTripCount(
       BestVPlan, VectorPH, CM.foldTailByMasking(),
       CM.requiresScalarEpilogue(BestVF.isVector()));
+  VPlanTransforms::fixupVFUsersForClampedVF(BestVPlan, ClampedVF);
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eas wrote:

> Before `VF` is materialized it's a symbolic value that represents the runtime `VF` for the plan. For plans with alias-masking the runtime VF will be the number of lanes in the mask.

That depends on the definition of `VF`, is it logical (how we vectorize **user**'s code) or closer to physical HW vector unit. If the former, then our definition of mask becomes somewhat flawed. Why would we need anything explicit in `<VF x i1>` masks if they don't represent things beyond `VF` lane?

In other words, if we define `VF` as "number of lanes in the mask", then "alias-mask" is really all-`true` and we don't need any special processing for it. The fact that it's not means we use different definition of `VF` in different places.

https://github.com/llvm/llvm-project/pull/177599