[llvm] [LV][EVL]Explicitly model AVL as sub, original TC, EVL_PHI. (PR #108869)

[via llvm-commits]

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@@ -1417,9 +1418,13 @@ bool VPlanTransforms::tryAddExplicitVectorLength(VPlan &Plan) {
   // Create the ExplicitVectorLengthPhi recipe in the main loop.
   auto *EVLPhi = new VPEVLBasedIVPHIRecipe(StartV, DebugLoc());
   EVLPhi->insertAfter(CanonicalIVPHI);
-  auto *VPEVL = new VPInstruction(VPInstruction::ExplicitVectorLength,
-                                  {EVLPhi, Plan.getTripCount()});
-  VPEVL->insertBefore(*Header, Header->getFirstNonPhi());
+  // Compute original TC - IV as the AVL (requested vector length).
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ayalz wrote:

nit: the TODO to handle MaxSafeDist should move from GetEVL above to here where the AVL argument is prepared for EVL.

https://github.com/llvm/llvm-project/pull/108869