[llvm] [VPlan] Move addExplicitVectorLength to tryToBuildVPlanWithVPRecipes (PR #166164)

[Florian Hahn via llvm-commits]

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@@ -605,9 +605,12 @@ createScalarIVSteps(VPlan &Plan, InductionDescriptor::InductionKind Kind,
                     VPBuilder &Builder) {
   VPRegionBlock *LoopRegion = Plan.getVectorLoopRegion();
   VPBasicBlock *HeaderVPBB = LoopRegion->getEntryBasicBlock();
-  VPCanonicalIVPHIRecipe *CanonicalIV = LoopRegion->getCanonicalIV();
-  VPSingleDefRecipe *BaseIV = Builder.createDerivedIV(
-      Kind, FPBinOp, StartV, CanonicalIV, Step, "offset.idx");
+  VPHeaderPHIRecipe *IV = LoopRegion->getCanonicalIV();
+  if (auto *EVLIV =
+          dyn_cast<VPEVLBasedIVPHIRecipe>(std::next(IV->getIterator())))
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fhahn wrote:

Why would it be incorrect? 

If the movement means every optimization must be careful whether to use the canonical IV or EVL based IV, and adding new users to the canonical IV could cause incorrect transformations, then I am not sure if that is the best direction forward?

https://github.com/llvm/llvm-project/pull/166164