[llvm] [LV] Use ExtractLane(LastActiveLane, V) live outs when tail-folding. (PR #149042)

[Luke Lau via llvm-commits]

================
@@ -2045,6 +2044,32 @@ bool VPlanTransforms::adjustFixedOrderRecurrences(VPlan &Plan,
     // Set the first operand of RecurSplice to FOR again, after replacing
     // all users.
     RecurSplice->setOperand(0, FOR);
+
+    // Check for users extracting at the penultimate active lane of the FOR.
+    // If only a single lane is active in the current iteration, we need to
+    // select the last element from the previous iteration (from the FOR phi
+    // directly).
+    for (VPUser *U : RecurSplice->users()) {
+      if (!match(U, m_ExtractLane(m_LastActiveLane(m_VPValue()),
+                                  m_Specific(RecurSplice))))
+        continue;
+
+      VPBuilder B(cast<VPInstruction>(U));
+      VPValue *LastActiveLane = cast<VPInstruction>(U)->getOperand(0);
+      Type *I64Ty = Type::getInt64Ty(Plan.getContext());
+      VPValue *Zero = Plan.getOrAddLiveIn(ConstantInt::get(I64Ty, 0));
+      VPValue *One = Plan.getOrAddLiveIn(ConstantInt::get(I64Ty, 1));
+      VPValue *PenultimateIndex =
+          B.createNaryOp(Instruction::Sub, {LastActiveLane, One});
+      VPValue *PenultimateLastIter =
+          B.createNaryOp(VPInstruction::ExtractLane,
+                         {PenultimateIndex, FOR->getBackedgeValue()});
+      VPValue *LastPrevIter =
+          B.createNaryOp(VPInstruction::ExtractLastElement, {FOR});
----------------
lukel97 wrote:

```suggestion
          B.createNaryOp(VPInstruction::ExtractLastElement, FOR);
```

https://github.com/llvm/llvm-project/pull/149042