diff --git a/llvm/include/llvm/CodeGen/MachinePostDominators.h b/llvm/include/llvm/CodeGen/MachinePostDominators.h
--- a/llvm/include/llvm/CodeGen/MachinePostDominators.h
+++ b/llvm/include/llvm/CodeGen/MachinePostDominators.h
@@ -71,11 +71,20 @@
     return DT->properlyDominates(A, B);
   }
 
+  bool isVirtualRoot(const MachineDomTreeNode *BB) const {
+    return DT->isVirtualRoot(BB);
+  }
+
   MachineBasicBlock *findNearestCommonDominator(MachineBasicBlock *A,
-                                                MachineBasicBlock *B) {
+                                                MachineBasicBlock *B) const {
     return DT->findNearestCommonDominator(A, B);
   }
 
+  /// Returns the nearest common dominator of the given blocks.
+  /// If that tree node is a virtual root, a nullptr will be returned.
+  MachineBasicBlock *
+  findNearestCommonDominator(ArrayRef<MachineBasicBlock *> Blocks) const;
+
   bool runOnMachineFunction(MachineFunction &MF) override;
   void getAnalysisUsage(AnalysisUsage &AU) const override;
   void print(llvm::raw_ostream &OS, const Module *M = nullptr) const override;
diff --git a/llvm/lib/CodeGen/MachinePostDominators.cpp b/llvm/lib/CodeGen/MachinePostDominators.cpp
--- a/llvm/lib/CodeGen/MachinePostDominators.cpp
+++ b/llvm/lib/CodeGen/MachinePostDominators.cpp
@@ -13,6 +13,8 @@
 
 #include "llvm/CodeGen/MachinePostDominators.h"
 
+#include "llvm/ADT/STLExtras.h"
+
 using namespace llvm;
 
 namespace llvm {
@@ -51,7 +53,25 @@
   MachineFunctionPass::getAnalysisUsage(AU);
 }
 
-void
-MachinePostDominatorTree::print(llvm::raw_ostream &OS, const Module *M) const {
+MachineBasicBlock *MachinePostDominatorTree::findNearestCommonDominator(
+    ArrayRef<MachineBasicBlock *> Blocks) const {
+  assert(!Blocks.empty());
+  assert(llvm::all_of(Blocks, [](MachineBasicBlock *B) { return B; }) &&
+         "Invalid block found");
+
+  MachineBasicBlock *NCD = Blocks.front();
+  for (MachineBasicBlock *BB : Blocks.drop_front()) {
+    NCD = DT->findNearestCommonDominator(NCD, BB);
+
+    // Stop when the root is reached.
+    if (DT->isVirtualRoot(DT->getNode(NCD)))
+      break;
+  }
+
+  return NCD;
+}
+
+void MachinePostDominatorTree::print(llvm::raw_ostream &OS,
+                                     const Module *M) const {
   DT->print(OS);
 }
diff --git a/llvm/lib/Target/AMDGPU/SILowerI1Copies.cpp b/llvm/lib/Target/AMDGPU/SILowerI1Copies.cpp
--- a/llvm/lib/Target/AMDGPU/SILowerI1Copies.cpp
+++ b/llvm/lib/Target/AMDGPU/SILowerI1Copies.cpp
@@ -589,12 +589,12 @@
 
       // Phis in a loop that are observed outside the loop receive a simple but
       // conservatively correct treatment.
-      MachineBasicBlock *PostDomBound = &MBB;
-      for (MachineInstr &Use : MRI->use_instructions(DstReg)) {
-        PostDomBound =
-            PDT->findNearestCommonDominator(PostDomBound, Use.getParent());
-      }
+      std::vector<MachineBasicBlock *> DomBlocks = {&MBB};
+      for (MachineInstr &Use : MRI->use_instructions(DstReg))
+        DomBlocks.push_back(Use.getParent());
 
+      MachineBasicBlock *PostDomBound =
+          PDT->findNearestCommonDominator(DomBlocks);
       unsigned FoundLoopLevel = LF.findLoop(PostDomBound);
 
       SSAUpdater.Initialize(DstReg);
@@ -711,12 +711,12 @@
 
       // Defs in a loop that are observed outside the loop must be transformed
       // into appropriate bit manipulation.
-      MachineBasicBlock *PostDomBound = &MBB;
-      for (MachineInstr &Use : MRI->use_instructions(DstReg)) {
-        PostDomBound =
-            PDT->findNearestCommonDominator(PostDomBound, Use.getParent());
-      }
+      std::vector<MachineBasicBlock *> DomBlocks = {&MBB};
+      for (MachineInstr &Use : MRI->use_instructions(DstReg))
+        DomBlocks.push_back(Use.getParent());
 
+      MachineBasicBlock *PostDomBound =
+          PDT->findNearestCommonDominator(DomBlocks);
       unsigned FoundLoopLevel = LF.findLoop(PostDomBound);
       if (FoundLoopLevel) {
         SSAUpdater.Initialize(DstReg);