[llvm] [VPlan] Remove CanonicalIV when dissolving loop regions (NFC). (PR #142372)

[Florian Hahn via llvm-commits]

https://github.com/fhahn created https://github.com/llvm/llvm-project/pull/142372

Directly replace the canonical IV when we dissolve the containing region. That ensures that it won't get removed before the region gets removed, which would result in an invalid region.

This removes the current ordering constraint between convertToConcreteRecipes and dissolving regions.

>From 7946c31e646fddd1b159af09855ee4b2fb535e1d Mon Sep 17 00:00:00 2001
From: Florian Hahn <flo at fhahn.com>
Date: Mon, 2 Jun 2025 13:17:10 +0100
Subject: [PATCH] [VPlan] Remove CanonicalIV when dissolving loop regions
 (NFC).

Directly replace the canonical IV when we dissolve the containing
region. That ensures that it won't get removed before the region gets
removed, which would result in an invalid region.

This removes the current ordering constraint between convertToConcreteRecipes
and dissolving regions.
---
 llvm/lib/Transforms/Vectorize/VPlan.cpp           | 10 ++++++++++
 llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp | 10 +++-------
 2 files changed, 13 insertions(+), 7 deletions(-)

diff --git a/llvm/lib/Transforms/Vectorize/VPlan.cpp b/llvm/lib/Transforms/Vectorize/VPlan.cpp
index 165b57c87beb1..2580031c9186b 100644
--- a/llvm/lib/Transforms/Vectorize/VPlan.cpp
+++ b/llvm/lib/Transforms/Vectorize/VPlan.cpp
@@ -883,6 +883,16 @@ void VPRegionBlock::print(raw_ostream &O, const Twine &Indent,
 
 void VPRegionBlock::dissolveToCFGLoop() {
   auto *Header = cast<VPBasicBlock>(getEntry());
+  if (auto *CanIV = dyn_cast<VPCanonicalIVPHIRecipe>(&Header->front())) {
+    assert(this == getPlan()->getVectorLoopRegion() &&
+           "Canonical IV must be in the entry of the top-level loop region");
+    auto *ScalarR = Builder(CanIV).createScalarPhi(
+        {CanIV->getStartValue(), CanIV->getBackedgeValue()},
+        CanIV->getDebugLoc(), "index");
+    CanIV->replaceAllUsesWith(ScalarR);
+    CanIV->eraseFromParent();
+  }
+
   VPBlockBase *Preheader = getSinglePredecessor();
   auto *ExitingLatch = cast<VPBasicBlock>(getExiting());
   VPBlockBase *Middle = getSingleSuccessor();
diff --git a/llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp b/llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp
index 5b42a9056b69e..5c6d7b0c5b529 100644
--- a/llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp
+++ b/llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp
@@ -2601,14 +2601,10 @@ void VPlanTransforms::convertToConcreteRecipes(VPlan &Plan,
   for (VPBasicBlock *VPBB : VPBlockUtils::blocksOnly<VPBasicBlock>(
            vp_depth_first_deep(Plan.getEntry()))) {
     for (VPRecipeBase &R : make_early_inc_range(*VPBB)) {
-      if (isa<VPCanonicalIVPHIRecipe, VPEVLBasedIVPHIRecipe>(&R)) {
-        auto *PhiR = cast<VPHeaderPHIRecipe>(&R);
-        StringRef Name =
-            isa<VPCanonicalIVPHIRecipe>(PhiR) ? "index" : "evl.based.iv";
-        VPBuilder Builder(PhiR);
-        auto *ScalarR = Builder.createScalarPhi(
+      if (auto *PhiR = dyn_cast<VPEVLBasedIVPHIRecipe>(&R)) {
+        auto *ScalarR = Builder(PhiR).createScalarPhi(
             {PhiR->getStartValue(), PhiR->getBackedgeValue()},
-            PhiR->getDebugLoc(), Name);
+            PhiR->getDebugLoc(), "evl.based.iv");
         PhiR->replaceAllUsesWith(ScalarR);
         ToRemove.push_back(PhiR);
         continue;