[llvm] [VPlan] Expand VPWidenIntOrFpInductionRecipe into separate recipes (PR #118638)

[Mel Chen via llvm-commits]

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@@ -1706,29 +1732,27 @@ void VPWidenIntOrFpInductionRecipe::execute(VPTransformState &State) {
   }
 
   Builder.restoreIP(CurrIP);
-
-  // We may need to add the step a number of times, depending on the unroll
-  // factor. The last of those goes into the PHI.
-  PHINode *VecInd = PHINode::Create(SteppedStart->getType(), 2, "vec.ind");
-  VecInd->insertBefore(State.CFG.PrevBB->getFirstInsertionPt());
-  VecInd->setDebugLoc(EntryVal->getDebugLoc());
-  State.set(this, VecInd);
+  Value *PrevVal = State.get(getPrevValue());
 
   Instruction *LastInduction = cast<Instruction>(
-      Builder.CreateBinOp(AddOp, VecInd, SplatVF, "vec.ind.next"));
-  if (isa<TruncInst>(EntryVal))
-    State.addMetadata(LastInduction, EntryVal);
-  LastInduction->setDebugLoc(EntryVal->getDebugLoc());
+      Builder.CreateBinOp(AddOp, PrevVal, SplatVF, "vec.ind.next"));
+  if (isa<TruncInst>(IV))
+    State.addMetadata(LastInduction, IV);
+  LastInduction->setDebugLoc(IV->getDebugLoc());
 
-  VecInd->addIncoming(SteppedStart, VectorPH);
-  // Add induction update using an incorrect block temporarily. The phi node
-  // will be fixed after VPlan execution. Note that at this point the latch
-  // block cannot be used, as it does not exist yet.
-  // TODO: Model increment value in VPlan, by turning the recipe into a
-  // multi-def and a subclass of VPHeaderPHIRecipe.
----------------
Mel-Chen wrote:

Could we first have a separate patch focused specifically on completing this TODO? If I understand correctly, this seems to be the primary issue at hand.

Additionally, I’d like to understand the difference between introducing a new recipe, `VPWidenIntOrFpInductionBackedgeRecipe`, and combining `VPInstruction::Mul` and `VPInstruction::Add`(https://github.com/llvm/llvm-project/pull/82021). What are the key distinctions, advantages, or trade-offs between these two approaches?

https://github.com/llvm/llvm-project/pull/118638