[llvm] [VPlan] Add VPInstruction::StepVector and use it in VPWidenIntOrFpInductionRecipe (PR #129508)

[Florian Hahn via llvm-commits]

================
@@ -2586,6 +2595,29 @@ void VPlanTransforms::handleUncountableEarlyExit(
   LatchExitingBranch->eraseFromParent();
 }
 
+void VPlanTransforms::materializeStepVectors(VPlan &Plan) {
+  for (auto &Phi : Plan.getVectorLoopRegion()->getEntryBasicBlock()->phis()) {
+    auto *IVR = dyn_cast<VPWidenIntOrFpInductionRecipe>(&Phi);
+    if (!IVR)
+      continue;
+
+    // Infer an up-to-date type since
+    // optimizeVectorInductionWidthForTCAndVFUF may have truncated the start
+    // and step values.
+    Type *Ty = IVR->getPHINode()->getType();
+    if (TruncInst *Trunc = IVR->getTruncInst())
+      Ty = Trunc->getType();
+    if (Ty->isFloatingPointTy())
+      Ty = IntegerType::get(Ty->getContext(), Ty->getScalarSizeInBits());
+
+    VPBuilder Builder(Plan.getVectorPreheader());
+    VPInstruction *StepVector = Builder.createNaryOp(
+        VPInstruction::StepVector, {}, Ty, {}, IVR->getDebugLoc());
+    assert(IVR->getNumOperands() == 3);
----------------
fhahn wrote:

```suggestion
    assert(IVR->getNumOperands() == 3 && "can only add step vector before unrolling");
```

https://github.com/llvm/llvm-project/pull/129508