[llvm] [VPlan] Add transform to replace VPWidenCanonicalIV with wide IV. (PR #194267)

[Ramkumar Ramachandra via llvm-commits]

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@@ -691,6 +691,73 @@ static void removeRedundantCanonicalIVs(VPlan &Plan) {
   }
 }
 
+void VPlanTransforms::replaceWidenCanonicalIVWithWidenIV(
+    VPlan &Plan, ScalarEvolution &SE, const TargetTransformInfo &TTI,
+    TargetTransformInfo::TargetCostKind CostKind, ElementCount VF, unsigned UF,
+    const SmallPtrSetImpl<const Value *> &ValuesToIgnore) {
+  VPRegionBlock *LoopRegion = Plan.getVectorLoopRegion();
+  if (!LoopRegion || Plan.hasScalarVFOnly())
+    return;
+
+  VPValue *CanonicalIV = LoopRegion->getCanonicalIV();
+  auto *WideCanIV = vputils::findUserOf<VPWidenCanonicalIVRecipe>(CanonicalIV);
+  if (!WideCanIV)
+    return;
+
+  // If a canonical VPWidenIntOrFpInductionRecipe already exists, reuse it.
+  VPBasicBlock *Header = LoopRegion->getEntryBasicBlock();
+  for (VPRecipeBase &Phi : Header->phis()) {
+    auto *WideIV = dyn_cast<VPWidenIntOrFpInductionRecipe>(&Phi);
+    if (!WideIV || !WideIV->isCanonical())
+      continue;
+    // The canonical wide IV is used to compute the header mask, hence all
+    // lanes will be used. Drop poison-generating flags.
+    WideIV->dropPoisonGeneratingFlags();
+    WideCanIV->replaceAllUsesWith(WideIV);
+    WideCanIV->eraseFromParent();
+    return;
+  }
----------------
artagnon wrote:

I would say: let removeRedundantCanonicalIVs do this part as in https://github.com/llvm/llvm-project/pull/194338 -- can we review and get that part landed first, before looking at the cost-based replacement separately?

https://github.com/llvm/llvm-project/pull/194267