[llvm] [VPlan] Create AVL as a phi from TC -> 0 with EVL tail folding (PR #151481)

[Luke Lau via llvm-commits]

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@@ -2350,9 +2350,11 @@ bool VPlanTransforms::tryAddExplicitVectorLength(
   auto *EVLPhi = new VPEVLBasedIVPHIRecipe(StartV, DebugLoc());
   EVLPhi->insertAfter(CanonicalIVPHI);
   VPBuilder Builder(Header, Header->getFirstNonPhi());
-  // Compute original TC - IV as the AVL (application vector length).
-  VPValue *AVL = Builder.createNaryOp(
-      Instruction::Sub, {Plan.getTripCount(), EVLPhi}, DebugLoc(), "avl");
+  // Create the AVL, starting from TC -> 0 in steps of EVL.
+  VPPhi *AVLPhi =
+      Builder.createScalarPhi({Plan.getTripCount()}, DebugLoc(), "avl");
+  VPValue *AVL = AVLPhi;
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lukel97 wrote:

I did this at first and got some confusing test failures, it turns out we need to keep a reference to the phi because the `AVL = Builder.createSelect(Cmp, AVL, AVLSafe, DebugLoc(), "safe_avl");` bit below might reassign the AVL, and we would end up adding the backedge to the select

https://github.com/llvm/llvm-project/pull/151481