[llvm] [VPlan] Move IV predicate handling to VPlan. (PR #192876)

[Florian Hahn via llvm-commits]

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@@ -993,6 +994,72 @@ bool VPlanTransforms::createHeaderPhiRecipes(
   return true;
 }
 
+bool VPlanTransforms::finalizeSCEVPredicates(VPlan &Plan,
+                                             PredicatedScalarEvolution &PSE,
+                                             bool OptForSize,
+                                             unsigned SCEVCheckThreshold,
+                                             OptimizationRemarkEmitter *ORE,
+                                             Loop *TheLoop) {
+  // Collect which header IVs have predicates and add them to PSE.
+  auto *HeaderVPBB = cast<VPBasicBlock>(
+      Plan.getEntry()->getSuccessors()[1]->getSingleSuccessor());
+  SmallPtrSet<VPWidenInductionRecipe *, 4> PredicatedIVs;
+  for (auto &R : HeaderVPBB->phis()) {
+    auto *WideIV = dyn_cast<VPWidenInductionRecipe>(&R);
+    if (!WideIV || WideIV->getPredicates().empty())
+      continue;
+    PredicatedIVs.insert(WideIV);
+    for (const auto *P : WideIV->getPredicates())
+      PSE.addPredicate(*P);
+  }
+
+  // Bail out if exit phis use predicated IVs via ExitingIVValue, as the
+  // predicated SCEV may not hold outside the loop (PR33706). Check each IV's
+  // predicates directly, regardless of whether PSE was already non-trivial
+  // from other sources (e.g., LAI predicates).
+  // TODO: Overly conservative; exit values from vector registers are safe
+  // when guarded by runtime checks, but later passes may use the SCEV.
+  if (!PredicatedIVs.empty())
+    for (auto *EB : Plan.getExitBlocks())
+      for (VPRecipeBase &R : EB->phis())
+        for (VPValue *Op : R.operands()) {
+          VPValue *Inner;
+          if (!match(Op, m_ExitingIVValue(m_VPValue(Inner))))
+            continue;
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fhahn wrote:

I think we should, but the legacy code did not check phis. I can pull it in here, but as this is a separate bugfix it would probably be better to do separately?

https://github.com/llvm/llvm-project/pull/192876