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diff --git a/include/llvm/Support/GenericIteratedDominanceFrontier.h b/include/llvm/Support/GenericIteratedDominanceFrontier.h
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index 000000000000..25eb7cd7b6d5
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+++ b/include/llvm/Support/GenericIteratedDominanceFrontier.h
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+//===- IteratedDominanceFrontier.h - Calculate IDF --------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+/// \file
+/// Compute iterated dominance frontiers using a linear time algorithm.
+///
+/// The algorithm used here is based on:
+///
+///   Sreedhar and Gao. A linear time algorithm for placing phi-nodes.
+///   In Proceedings of the 22nd ACM SIGPLAN-SIGACT Symposium on Principles of
+///   Programming Languages
+///   POPL '95. ACM, New York, NY, 62-73.
+///
+/// It has been modified to not explicitly use the DJ graph data structure and
+/// to directly compute pruned SSA using per-variable liveness information.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_SUPPORT_GENERIC_IDF_H
+#define LLVM_SUPPORT_GENERIC_IDF_H
+
+#include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/SmallPtrSet.h"
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/Support/GenericDomTree.h"
+#include <queue>
+
+namespace llvm {
+
+namespace IDFCalculatorDetail {
+
+/// Generic utility class used for getting the children of a basic block.
+/// May be specialized if, for example, one wouldn't like to return nullpointer
+/// successors.
+template <class NodeTy, bool IsPostDom> struct ChildrenGetterTy {
+  using NodeRef = typename GraphTraits<NodeTy>::NodeRef;
+  using ChildrenTy = SmallVector<NodeRef, 8>;
+
+  ChildrenTy get(const NodeRef &N);
+};
+
+} // end of namespace IDFCalculatorDetail
+
+/// Determine the iterated dominance frontier, given a set of defining
+/// blocks, and optionally, a set of live-in blocks.
+///
+/// In turn, the results can be used to place phi nodes.
+///
+/// This algorithm is a linear time computation of Iterated Dominance Frontiers,
+/// pruned using the live-in set.
+/// By default, liveness is not used to prune the IDF computation.
+/// The template parameters should be of a CFG block type.
+template <class NodeTy, bool IsPostDom> class IDFCalculatorBase {
+public:
+  using OrderedNodeTy =
+      typename std::conditional<IsPostDom, Inverse<NodeTy *>, NodeTy *>::type;
+  using ChildrenGetterTy =
+      IDFCalculatorDetail::ChildrenGetterTy<NodeTy, IsPostDom>;
+
+  IDFCalculatorBase(DominatorTreeBase<NodeTy, IsPostDom> &DT) : DT(DT) {}
+
+  IDFCalculatorBase(DominatorTreeBase<NodeTy, IsPostDom> &DT,
+                    const ChildrenGetterTy &C)
+      : DT(DT), ChildrenGetter(C) {}
+
+  /// Give the IDF calculator the set of blocks in which the value is
+  /// defined.  This is equivalent to the set of starting blocks it should be
+  /// calculating the IDF for (though later gets pruned based on liveness).
+  ///
+  /// Note: This set *must* live for the entire lifetime of the IDF calculator.
+  void setDefiningBlocks(const SmallPtrSetImpl<NodeTy *> &Blocks) {
+    DefBlocks = &Blocks;
+  }
+
+  /// Give the IDF calculator the set of blocks in which the value is
+  /// live on entry to the block.   This is used to prune the IDF calculation to
+  /// not include blocks where any phi insertion would be dead.
+  ///
+  /// Note: This set *must* live for the entire lifetime of the IDF calculator.
+  void setLiveInBlocks(const SmallPtrSetImpl<NodeTy *> &Blocks) {
+    LiveInBlocks = &Blocks;
+    useLiveIn = true;
+  }
+
+  /// Reset the live-in block set to be empty, and tell the IDF
+  /// calculator to not use liveness anymore.
+  void resetLiveInBlocks() {
+    LiveInBlocks = nullptr;
+    useLiveIn = false;
+  }
+
+  /// Calculate iterated dominance frontiers
+  ///
+  /// This uses the linear-time phi algorithm based on DJ-graphs mentioned in
+  /// the file-level comment.  It performs DF->IDF pruning using the live-in
+  /// set, to avoid computing the IDF for blocks where an inserted PHI node
+  /// would be dead.
+  void calculate(SmallVectorImpl<NodeTy *> &IDFBlocks);
+
+private:
+  DominatorTreeBase<NodeTy, IsPostDom> &DT;
+  ChildrenGetterTy ChildrenGetter;
+  bool useLiveIn = false;
+  const SmallPtrSetImpl<NodeTy *> *LiveInBlocks;
+  const SmallPtrSetImpl<NodeTy *> *DefBlocks;
+};
+
+//===----------------------------------------------------------------------===//
+// Implementation.
+//===----------------------------------------------------------------------===//
+
+namespace IDFCalculatorDetail {
+
+template <class NodeTy, bool IsPostDom>
+typename ChildrenGetterTy<NodeTy, IsPostDom>::ChildrenTy
+ChildrenGetterTy<NodeTy, IsPostDom>::get(const NodeRef &N) {
+  using OrderedNodeTy =
+      typename IDFCalculatorBase<NodeTy, IsPostDom>::OrderedNodeTy;
+
+  auto Children = children<OrderedNodeTy>(N);
+  return {Children.begin(), Children.end()};
+}
+
+} // end of namespace IDFCalculatorDetail
+
+template <class NodeTy, bool IsPostDom>
+void IDFCalculatorBase<NodeTy, IsPostDom>::calculate(
+    SmallVectorImpl<NodeTy *> &PHIBlocks) {
+  // Use a priority queue keyed on dominator tree level so that inserted nodes
+  // are handled from the bottom of the dominator tree upwards. We also augment
+  // the level with a DFS number to ensure that the blocks are ordered in a
+  // deterministic way.
+  using DomTreeNodePair =
+      std::pair<DomTreeNodeBase<NodeTy> *, std::pair<unsigned, unsigned>>;
+  using IDFPriorityQueue =
+      std::priority_queue<DomTreeNodePair, SmallVector<DomTreeNodePair, 32>,
+                          less_second>;
+
+  IDFPriorityQueue PQ;
+
+  DT.updateDFSNumbers();
+
+  for (NodeTy *BB : *DefBlocks) {
+    if (DomTreeNodeBase<NodeTy> *Node = DT.getNode(BB))
+      PQ.push({Node, std::make_pair(Node->getLevel(), Node->getDFSNumIn())});
+  }
+
+  SmallVector<DomTreeNodeBase<NodeTy> *, 32> Worklist;
+  SmallPtrSet<DomTreeNodeBase<NodeTy> *, 32> VisitedPQ;
+  SmallPtrSet<DomTreeNodeBase<NodeTy> *, 32> VisitedWorklist;
+
+  while (!PQ.empty()) {
+    DomTreeNodePair RootPair = PQ.top();
+    PQ.pop();
+    DomTreeNodeBase<NodeTy> *Root = RootPair.first;
+    unsigned RootLevel = RootPair.second.first;
+
+    // Walk all dominator tree children of Root, inspecting their CFG edges with
+    // targets elsewhere on the dominator tree. Only targets whose level is at
+    // most Root's level are added to the iterated dominance frontier of the
+    // definition set.
+
+    Worklist.clear();
+    Worklist.push_back(Root);
+    VisitedWorklist.insert(Root);
+
+    while (!Worklist.empty()) {
+      DomTreeNodeBase<NodeTy> *Node = Worklist.pop_back_val();
+      NodeTy *BB = Node->getBlock();
+      // Succ is the successor in the direction we are calculating IDF, so it is
+      // successor for IDF, and predecessor for Reverse IDF.
+      auto DoWork = [&](NodeTy *Succ) {
+        DomTreeNodeBase<NodeTy> *SuccNode = DT.getNode(Succ);
+
+        const unsigned SuccLevel = SuccNode->getLevel();
+        if (SuccLevel > RootLevel)
+          return;
+
+        if (!VisitedPQ.insert(SuccNode).second)
+          return;
+
+        NodeTy *SuccBB = SuccNode->getBlock();
+        if (useLiveIn && !LiveInBlocks->count(SuccBB))
+          return;
+
+        PHIBlocks.emplace_back(SuccBB);
+        if (!DefBlocks->count(SuccBB))
+          PQ.push(std::make_pair(
+              SuccNode, std::make_pair(SuccLevel, SuccNode->getDFSNumIn())));
+      };
+
+      for (auto Succ : ChildrenGetter.get(BB))
+        DoWork(Succ);
+
+      for (auto DomChild : *Node) {
+        if (VisitedWorklist.insert(DomChild).second)
+          Worklist.push_back(DomChild);
+      }
+    }
+  }
+}
+
+} // end of namespace llvm
+
+#endif