[llvm] [VPlan] Implement interleaving as VPlan-to-VPlan transform. (PR #95842)

[via llvm-commits]

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@@ -890,6 +895,9 @@ void VPlan::execute(VPTransformState *State) {
       // Move the last step to the end of the latch block. This ensures
       // consistent placement of all induction updates.
       Instruction *Inc = cast<Instruction>(Phi->getIncomingValue(1));
+      if (isa<VPWidenIntOrFpInductionRecipe>(&R) && R.getNumOperands() == 4)
+        Inc->setOperand(0, State->get(R.getOperand(3), 0));
+
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ayalz wrote:

Good to add some explanation why Inc's first operand is set to R's last operand (if it has four).

Better to place this after `Inc->moveBefore()` below or before `//Move the last` explanation above.

BTW, above setting of Phi also suffers from WidenPointer recipe producing both a phi and a gep.

https://github.com/llvm/llvm-project/pull/95842