[llvm] [VPlan] Make canonical IV part of the region (PR #156262)

[via llvm-commits]

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@@ -846,18 +858,30 @@ 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 = VPBuilder(CanIV).createScalarPhi(
-        {CanIV->getStartValue(), CanIV->getBackedgeValue()},
-        CanIV->getDebugLoc(), "index");
+  auto *ExitingLatch = cast<VPBasicBlock>(getExiting());
+  VPValue *CanIV = getCanonicalIV();
+  if (CanIV && CanIV->getNumUsers() > 0) {
+    VPlan &Plan = *getPlan();
+    VPInstruction *CanIVInc = getCanonicalIVIncrement();
+    // If the increment doesn't exist yet, create it.
+    if (!CanIVInc) {
+      auto *ExitingTerm = ExitingLatch->getTerminator();
+      CanIVInc =
+          VPBuilder(ExitingTerm)
+              .createOverflowingOp(Instruction::Add, {CanIV, &Plan.getVFxUF()},
+                                   {CanIVInfo->hasNUW(), /* HasNSW */ false},
+                                   CanIVInfo->getDebugLoc(), "index.next");
+    }
+    auto *ScalarR =
+        VPBuilder(Header, Header->begin())
+            .createScalarPhi(
+                {Plan.getOrAddLiveIn(ConstantInt::get(CanIVInfo->getType(), 0)),
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ayalz wrote:

getConstantInt()?

https://github.com/llvm/llvm-project/pull/156262