diff --git a/llvm/lib/CodeGen/LiveVariables.cpp b/llvm/lib/CodeGen/LiveVariables.cpp
--- a/llvm/lib/CodeGen/LiveVariables.cpp
+++ b/llvm/lib/CodeGen/LiveVariables.cpp
@@ -661,22 +661,18 @@
   MachineInstr &DefMI = *MRI->getUniqueVRegDef(Reg);
   MachineBasicBlock &DefBB = *DefMI.getParent();
 
-  // Handle the case where all uses have been removed.
-  if (MRI->use_nodbg_empty(Reg)) {
-    VI.Kills.push_back(&DefMI);
-    DefMI.addRegisterDead(Reg, nullptr);
-    return;
-  }
-  DefMI.clearRegisterDeads(Reg);
-
   // Initialize a worklist of BBs that Reg is live-to-end of. (Here
   // "live-to-end" means Reg is live at the end of a block even if it is only
   // live because of phi uses in a successor. This is different from isLiveOut()
   // which does not consider phi uses.)
   SmallVector<MachineBasicBlock *> LiveToEndBlocks;
   SparseBitVector<> UseBlocks;
+  unsigned NumRealUses = 0;
   for (auto &UseMO : MRI->use_nodbg_operands(Reg)) {
     UseMO.setIsKill(false);
+    if (!UseMO.readsReg())
+      continue;
+    ++NumRealUses;
     MachineInstr &UseMI = *UseMO.getParent();
     MachineBasicBlock &UseBB = *UseMI.getParent();
     UseBlocks.set(UseBB.getNumber());
@@ -693,6 +689,14 @@
     }
   }
 
+  // Handle the case where all uses have been removed.
+  if (NumRealUses == 0) {
+    VI.Kills.push_back(&DefMI);
+    DefMI.addRegisterDead(Reg, nullptr);
+    return;
+  }
+  DefMI.clearRegisterDeads(Reg);
+
   // Iterate over the worklist adding blocks to AliveBlocks.
   bool LiveToEndOfDefBB = false;
   while (!LiveToEndBlocks.empty()) {
@@ -721,7 +725,7 @@
         continue;
       if (MI.isPHI())
         break;
-      if (MI.readsRegister(Reg)) {
+      if (MI.readsVirtualRegister(Reg)) {
         assert(!MI.killsRegister(Reg));
         MI.addRegisterKilled(Reg, nullptr);
         VI.Kills.push_back(&MI);
diff --git a/llvm/unittests/MI/LiveIntervalTest.cpp b/llvm/unittests/MI/LiveIntervalTest.cpp
--- a/llvm/unittests/MI/LiveIntervalTest.cpp
+++ b/llvm/unittests/MI/LiveIntervalTest.cpp
@@ -1,5 +1,6 @@
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/CodeGen/LiveIntervals.h"
+#include "llvm/CodeGen/LiveVariables.h"
 #include "llvm/CodeGen/MIRParser/MIRParser.h"
 #include "llvm/CodeGen/MachineFunction.h"
 #include "llvm/CodeGen/MachineModuleInfo.h"
@@ -75,22 +76,28 @@
   return M;
 }
 
-typedef std::function<void(MachineFunction&,LiveIntervals&)> LiveIntervalTest;
-
 struct TestPass : public MachineFunctionPass {
   static char ID;
-  TestPass() : MachineFunctionPass(ID) {
+  TestPass() : MachineFunctionPass(ID) {}
+};
+
+template <typename AnalysisType>
+struct TestPassT : public TestPass {
+
+  typedef std::function<void(MachineFunction&,AnalysisType&)> TestFx;
+
+  TestPassT() {
     // We should never call this but always use PM.add(new TestPass(...))
     abort();
   }
-  TestPass(LiveIntervalTest T, bool ShouldPass)
-      : MachineFunctionPass(ID), T(T), ShouldPass(ShouldPass) {
+  TestPassT(TestFx T, bool ShouldPass)
+      : T(T), ShouldPass(ShouldPass) {
     initializeTestPassPass(*PassRegistry::getPassRegistry());
   }
 
   bool runOnMachineFunction(MachineFunction &MF) override {
-    LiveIntervals &LIS = getAnalysis<LiveIntervals>();
-    T(MF, LIS);
+    AnalysisType &A = getAnalysis<AnalysisType>();
+    T(MF, A);
     EXPECT_EQ(MF.verify(this, /* Banner */ nullptr, /* AbortOnError */ false),
               ShouldPass);
     return true;
@@ -98,12 +105,12 @@
 
   void getAnalysisUsage(AnalysisUsage &AU) const override {
     AU.setPreservesAll();
-    AU.addRequired<LiveIntervals>();
-    AU.addPreserved<LiveIntervals>();
+    AU.addRequired<AnalysisType>();
+    AU.addPreserved<AnalysisType>();
     MachineFunctionPass::getAnalysisUsage(AU);
   }
 private:
-  LiveIntervalTest T;
+  TestFx T;
   bool ShouldPass;
 };
 
@@ -188,8 +195,10 @@
   return false;
 }
 
-static void liveIntervalTest(StringRef MIRFunc, LiveIntervalTest T,
-                             bool ShouldPass = true) {
+template <typename AnalysisType>
+static void doTest(StringRef MIRFunc,
+                   typename TestPassT<AnalysisType>::TestFx T,
+                   bool ShouldPass = true) {
   LLVMContext Context;
   std::unique_ptr<LLVMTargetMachine> TM = createTargetMachine();
   // This test is designed for the X86 backend; stop if it is not available.
@@ -197,7 +206,18 @@
     return;
 
   legacy::PassManager PM;
+  std::unique_ptr<MIRParser> MIR;
+  std::unique_ptr<Module> M = parseMIR(Context, PM, MIR, *TM, MIRFunc, "func");
+  ASSERT_TRUE(M);
+
+  PM.add(new TestPassT<AnalysisType>(T, ShouldPass));
+
+  PM.run(*M);
+}
 
+static void liveIntervalTest(StringRef MIRFunc,
+                             TestPassT<LiveIntervals>::TestFx T,
+                             bool ShouldPass = true) {
   SmallString<160> S;
   StringRef MIRString = (Twine(R"MIR(
 ---
@@ -208,14 +228,25 @@
 body: |
   bb.0:
 )MIR") + Twine(MIRFunc) + Twine("...\n")).toNullTerminatedStringRef(S);
-  std::unique_ptr<MIRParser> MIR;
-  std::unique_ptr<Module> M = parseMIR(Context, PM, MIR, *TM, MIRString,
-                                       "func");
-  ASSERT_TRUE(M);
 
-  PM.add(new TestPass(T, ShouldPass));
+  doTest<LiveIntervals>(MIRString, T, ShouldPass);
+}
 
-  PM.run(*M);
+static void liveVariablesTest(StringRef MIRFunc,
+                             TestPassT<LiveVariables>::TestFx T,
+                             bool ShouldPass = true) {
+  SmallString<160> S;
+  StringRef MIRString = (Twine(R"MIR(
+---
+...
+name: func
+tracksRegLiveness: true
+registers:
+  - { id: 0, class: sreg_64 }
+body: |
+  bb.0:
+)MIR") + Twine(MIRFunc) + Twine("...\n")).toNullTerminatedStringRef(S);
+  doTest<LiveVariables>(MIRString, T, ShouldPass);
 }
 
 } // End of anonymous namespace.
@@ -778,6 +809,46 @@
       false);
 }
 
+TEST(LiveVariablesTest, recomputeForSingleDefVirtReg_handle_undef1) {
+  liveVariablesTest(R"MIR(
+    %0 = IMPLICIT_DEF
+    S_NOP 0, implicit %0
+    S_NOP 0, implicit undef %0
+)MIR", [](MachineFunction &MF, LiveVariables &LV) {
+     auto &FirstNop = getMI(MF, 1, 0);
+     auto &SecondNop = getMI(MF, 2, 0);
+     EXPECT_TRUE(FirstNop.getOperand(1).isKill());
+     EXPECT_FALSE(SecondNop.getOperand(1).isKill());
+
+     Register R = Register::index2VirtReg(0);
+     LV.recomputeForSingleDefVirtReg(R);
+
+     EXPECT_TRUE(FirstNop.getOperand(1).isKill());
+     EXPECT_FALSE(SecondNop.getOperand(1).isKill());
+  });
+}
+
+TEST(LiveVariablesTest, recomputeForSingleDefVirtReg_handle_undef2) {
+  liveVariablesTest(R"MIR(
+    %0 = IMPLICIT_DEF
+    S_NOP 0, implicit %0
+    S_NOP 0, implicit undef %0, implicit %0
+)MIR", [](MachineFunction &MF, LiveVariables &LV) {
+     auto &FirstNop = getMI(MF, 1, 0);
+     auto &SecondNop = getMI(MF, 2, 0);
+     EXPECT_FALSE(FirstNop.getOperand(1).isKill());
+     EXPECT_FALSE(SecondNop.getOperand(1).isKill());
+     EXPECT_TRUE(SecondNop.getOperand(2).isKill());
+
+     Register R = Register::index2VirtReg(0);
+     LV.recomputeForSingleDefVirtReg(R);
+
+     EXPECT_FALSE(FirstNop.getOperand(1).isKill());
+     EXPECT_FALSE(SecondNop.getOperand(1).isKill());
+     EXPECT_TRUE(SecondNop.getOperand(2).isKill());
+  });
+}
+
 int main(int argc, char **argv) {
   ::testing::InitGoogleTest(&argc, argv);
   initLLVM();