Index: lib/Target/AMDGPU/SIFixSGPRCopies.cpp
===================================================================
--- lib/Target/AMDGPU/SIFixSGPRCopies.cpp
+++ lib/Target/AMDGPU/SIFixSGPRCopies.cpp
@@ -69,6 +69,7 @@
 #include "AMDGPUSubtarget.h"
 #include "SIInstrInfo.h"
 #include "llvm/CodeGen/MachineDominators.h"
+#include "llvm/CodeGen/MachinePostDominators.h"
 #include "llvm/CodeGen/MachineFunctionPass.h"
 #include "llvm/CodeGen/MachineInstrBuilder.h"
 #include "llvm/CodeGen/MachineRegisterInfo.h"
@@ -85,12 +86,12 @@
 class SIFixSGPRCopies : public MachineFunctionPass {
 
   MachineDominatorTree *MDT;
+  MachinePostDominatorTree *PDT;
 
 public:
   static char ID;
 
   SIFixSGPRCopies() : MachineFunctionPass(ID) { }
-
   bool runOnMachineFunction(MachineFunction &MF) override;
 
   StringRef getPassName() const override { return "SI Fix SGPR copies"; }
@@ -98,6 +99,8 @@
   void getAnalysisUsage(AnalysisUsage &AU) const override {
     AU.addRequired<MachineDominatorTree>();
     AU.addPreserved<MachineDominatorTree>();
+    AU.addRequired<MachinePostDominatorTree>();
+    AU.addPreserved<MachinePostDominatorTree>();
     AU.setPreservesCFG();
     MachineFunctionPass::getAnalysisUsage(AU);
   }
@@ -333,6 +336,7 @@
   const SIRegisterInfo *TRI = ST.getRegisterInfo();
   const SIInstrInfo *TII = ST.getInstrInfo();
   MDT = &getAnalysis<MachineDominatorTree>();
+  PDT = &getAnalysis<MachinePostDominatorTree>();
 
   SmallVector<MachineInstr *, 16> Worklist;
 
@@ -380,12 +384,15 @@
         // We don't need to fix the PHI if the common dominator of the
         // two incoming blocks terminates with a uniform branch.
         if (MI.getNumExplicitOperands() == 5) {
-          MachineBasicBlock *MBB0 = MI.getOperand(2).getMBB();
-          MachineBasicBlock *MBB1 = MI.getOperand(4).getMBB();
-
-          MachineBasicBlock *NCD = MDT->findNearestCommonDominator(MBB0, MBB1);
-          if (NCD && !hasTerminatorThatModifiesExec(*NCD, *TRI)) {
-            DEBUG(dbgs() << "Not fixing PHI for uniform branch: " << MI << '\n');
+          MachineBasicBlock *parentBB = MI.getParent();
+          MachineDomTreeNode *N = PDT->getNode(parentBB);
+          if (!N)
+            break;
+          MachineDomTreeNode *IPostDom = !(N->getIDom()) ? nullptr :
+                                                           N->getIDom();
+          if (!hasTerminatorThatModifiesExec(*parentBB, *TRI) && IPostDom) {
+            DEBUG(dbgs() << "Not fixing PHI for uniform branch: "
+                         << MI << '\n');
             break;
           }
         }