diff --git a/clang/docs/ThinLTO.rst b/clang/docs/ThinLTO.rst
--- a/clang/docs/ThinLTO.rst
+++ b/clang/docs/ThinLTO.rst
@@ -213,7 +213,7 @@
    <https://llvm.org/docs/CMake.html#options-and-variables>`_
    when configuring the bootstrap compiler build:
 
-  * ``-DLLVM_ENABLE_LTO=thin``
+  * ``-DLLVM_ENABLE_LTO=Thin``
   * ``-DCMAKE_C_COMPILER=/path/to/host/clang``
   * ``-DCMAKE_CXX_COMPILER=/path/to/host/clang++``
   * ``-DCMAKE_RANLIB=/path/to/host/llvm-ranlib``
@@ -221,7 +221,7 @@
 
   Or, on Windows:
 
-  * ``-DLLVM_ENABLE_LTO=thin``
+  * ``-DLLVM_ENABLE_LTO=Thin``
   * ``-DCMAKE_C_COMPILER=/path/to/host/clang-cl.exe``
   * ``-DCMAKE_CXX_COMPILER=/path/to/host/clang-cl.exe``
   * ``-DCMAKE_LINKER=/path/to/host/lld-link.exe``
diff --git a/llvm/include/llvm/IR/ModuleSummaryIndex.h b/llvm/include/llvm/IR/ModuleSummaryIndex.h
--- a/llvm/include/llvm/IR/ModuleSummaryIndex.h
+++ b/llvm/include/llvm/IR/ModuleSummaryIndex.h
@@ -591,9 +591,11 @@
       this->NoUnwind &= RHS.NoUnwind;
       this->MayThrow &= RHS.MayThrow;
       this->HasUnknownCall &= RHS.HasUnknownCall;
+      // this->MustBeUnreachable &= RHS.MustBeUnreachable;
       return *this;
     }
 
+    // This is used to print human-readable information.
     bool anyFlagSet() {
       return this->ReadNone | this->ReadOnly | this->NoRecurse |
              this->ReturnDoesNotAlias | this->NoInline | this->AlwaysInline |
@@ -669,7 +671,7 @@
         std::vector<FunctionSummary::VFuncId>(),
         std::vector<FunctionSummary::ConstVCall>(),
         std::vector<FunctionSummary::ConstVCall>(),
-        std::vector<FunctionSummary::ParamAccess>());
+        std::vector<FunctionSummary::ParamAccess>(), false);
   }
 
   /// A dummy node to reference external functions that aren't in the index
@@ -697,6 +699,13 @@
   using ParamAccessesTy = std::vector<ParamAccess>;
   std::unique_ptr<ParamAccessesTy> ParamAccesses;
 
+  // If true, the function must be unreachable.
+  // Note, a function might still be unreachable (not detected)
+  // even if this bit is off.
+  //
+  // For example, the destructor of a abstract class is unreachable.
+  bool mustBeUnreachable = false;
+
 public:
   FunctionSummary(GVFlags Flags, unsigned NumInsts, FFlags FunFlags,
                   uint64_t EntryCount, std::vector<ValueInfo> Refs,
@@ -706,10 +715,11 @@
                   std::vector<VFuncId> TypeCheckedLoadVCalls,
                   std::vector<ConstVCall> TypeTestAssumeConstVCalls,
                   std::vector<ConstVCall> TypeCheckedLoadConstVCalls,
-                  std::vector<ParamAccess> Params)
+                  std::vector<ParamAccess> Params, bool mustBeUnreachable)
       : GlobalValueSummary(FunctionKind, Flags, std::move(Refs)),
         InstCount(NumInsts), FunFlags(FunFlags), EntryCount(EntryCount),
-        CallGraphEdgeList(std::move(CGEdges)) {
+        CallGraphEdgeList(std::move(CGEdges)),
+        mustBeUnreachable(mustBeUnreachable) {
     if (!TypeTests.empty() || !TypeTestAssumeVCalls.empty() ||
         !TypeCheckedLoadVCalls.empty() || !TypeTestAssumeConstVCalls.empty() ||
         !TypeCheckedLoadConstVCalls.empty())
@@ -732,6 +742,8 @@
   /// Get function summary flags.
   FFlags fflags() const { return FunFlags; }
 
+  bool isUnreachableFunction() const { return mustBeUnreachable; }
+
   void setNoRecurse() { FunFlags.NoRecurse = true; }
 
   void setNoUnwind() { FunFlags.NoUnwind = true; }
@@ -1334,6 +1346,7 @@
   /// Return a ValueInfo for \p GV and mark it as belonging to GV.
   ValueInfo getOrInsertValueInfo(const GlobalValue *GV) {
     assert(HaveGVs);
+    errs() << "\tGV->GUID is " << GV->getGUID() << "\n";
     auto VP = getOrInsertValuePtr(GV->getGUID());
     VP->second.U.GV = GV;
     return ValueInfo(HaveGVs, VP);
@@ -1369,6 +1382,8 @@
                              std::unique_ptr<GlobalValueSummary> Summary) {
     if (const FunctionSummary *FS = dyn_cast<FunctionSummary>(Summary.get()))
       HasParamAccess |= !FS->paramAccesses().empty();
+    errs() << "\tGoing to add original name " << Summary->getOriginalName()
+           << " to GUID " << VI.getGUID() << "\n";
     addOriginalName(VI.getGUID(), Summary->getOriginalName());
     // Here we have a notionally const VI, but the value it points to is owned
     // by the non-const *this.
@@ -1379,12 +1394,17 @@
   /// Add an original name for the value of the given GUID.
   void addOriginalName(GlobalValue::GUID ValueGUID,
                        GlobalValue::GUID OrigGUID) {
-    if (OrigGUID == 0 || ValueGUID == OrigGUID)
+    if (OrigGUID == 0 || ValueGUID == OrigGUID) {
+      errs() << "\t\tOrigGUID is zero or the same as ValueGUID [ " << OrigGUID
+             << ", " << ValueGUID << "\n";
       return;
+    }
     if (OidGuidMap.count(OrigGUID) && OidGuidMap[OrigGUID] != ValueGUID)
       OidGuidMap[OrigGUID] = 0;
     else
       OidGuidMap[OrigGUID] = ValueGUID;
+    errs() << "\t\tMap origGUID to new guid " << OrigGUID << ", " << ValueGUID
+           << "\n";
   }
 
   /// Find the summary for ValueInfo \p VI in module \p ModuleId, or nullptr if
diff --git a/llvm/include/llvm/IR/ModuleSummaryIndexYAML.h b/llvm/include/llvm/IR/ModuleSummaryIndexYAML.h
--- a/llvm/include/llvm/IR/ModuleSummaryIndexYAML.h
+++ b/llvm/include/llvm/IR/ModuleSummaryIndexYAML.h
@@ -234,7 +234,7 @@
           std::move(FSum.TypeCheckedLoadVCalls),
           std::move(FSum.TypeTestAssumeConstVCalls),
           std::move(FSum.TypeCheckedLoadConstVCalls),
-          ArrayRef<FunctionSummary::ParamAccess>{}));
+          ArrayRef<FunctionSummary::ParamAccess>{}, false));
     }
   }
   static void output(IO &io, GlobalValueSummaryMapTy &V) {
diff --git a/llvm/lib/Analysis/ModuleSummaryAnalysis.cpp b/llvm/lib/Analysis/ModuleSummaryAnalysis.cpp
--- a/llvm/lib/Analysis/ModuleSummaryAnalysis.cpp
+++ b/llvm/lib/Analysis/ModuleSummaryAnalysis.cpp
@@ -234,6 +234,24 @@
   return false;
 }
 
+// Returns true if `F` must be an unreachable function.
+static bool mustBeUnreachableFunction(const Function &F) {
+  if (!F.empty()) {
+    const BasicBlock &entryBlock = F.getEntryBlock();
+    if (!entryBlock.empty()) {
+      const Instruction *inst = &(*entryBlock.rbegin());
+      if (inst->getOpcode() == Instruction::Unreachable) {
+        // TODO(mingmingl): delete
+        errs() << "unreachable function: ";
+        F.dump();
+        errs() << "\n";
+        return true;
+      }
+    }
+  }
+  return false;
+}
+
 static void computeFunctionSummary(
     ModuleSummaryIndex &Index, const Module &M, const Function &F,
     BlockFrequencyInfo *BFI, ProfileSummaryInfo *PSI, DominatorTree &DT,
@@ -497,9 +515,13 @@
       CallGraphEdges.takeVector(), TypeTests.takeVector(),
       TypeTestAssumeVCalls.takeVector(), TypeCheckedLoadVCalls.takeVector(),
       TypeTestAssumeConstVCalls.takeVector(),
-      TypeCheckedLoadConstVCalls.takeVector(), std::move(ParamAccesses));
+      TypeCheckedLoadConstVCalls.takeVector(), std::move(ParamAccesses),
+      mustBeUnreachableFunction(F));
   if (NonRenamableLocal)
     CantBePromoted.insert(F.getGUID());
+  errs() << "Going to add F " << F.getName()
+         << " to global value summary and its unreachable mark is "
+         << FuncSummary->isUnreachableFunction() << "\n";
   Index.addGlobalValueSummary(F, std::move(FuncSummary));
 }
 
@@ -745,7 +767,7 @@
                     ArrayRef<FunctionSummary::VFuncId>{},
                     ArrayRef<FunctionSummary::ConstVCall>{},
                     ArrayRef<FunctionSummary::ConstVCall>{},
-                    ArrayRef<FunctionSummary::ParamAccess>{});
+                    ArrayRef<FunctionSummary::ParamAccess>{}, false);
             Index.addGlobalValueSummary(*GV, std::move(Summary));
           } else {
             std::unique_ptr<GlobalVarSummary> Summary =
diff --git a/llvm/lib/AsmParser/LLParser.cpp b/llvm/lib/AsmParser/LLParser.cpp
--- a/llvm/lib/AsmParser/LLParser.cpp
+++ b/llvm/lib/AsmParser/LLParser.cpp
@@ -8396,7 +8396,7 @@
       std::move(TypeIdInfo.TypeCheckedLoadVCalls),
       std::move(TypeIdInfo.TypeTestAssumeConstVCalls),
       std::move(TypeIdInfo.TypeCheckedLoadConstVCalls),
-      std::move(ParamAccesses));
+      std::move(ParamAccesses), false);
 
   FS->setModulePath(ModulePath);
 
diff --git a/llvm/lib/Bitcode/Reader/BitcodeReader.cpp b/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
--- a/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
+++ b/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
@@ -6246,7 +6246,7 @@
           std::move(PendingTypeCheckedLoadVCalls),
           std::move(PendingTypeTestAssumeConstVCalls),
           std::move(PendingTypeCheckedLoadConstVCalls),
-          std::move(PendingParamAccesses));
+          std::move(PendingParamAccesses), false);
       auto VIAndOriginalGUID = getValueInfoFromValueId(ValueID);
       FS->setModulePath(getThisModule()->first());
       FS->setOriginalName(VIAndOriginalGUID.second);
@@ -6389,7 +6389,7 @@
           std::move(PendingTypeCheckedLoadVCalls),
           std::move(PendingTypeTestAssumeConstVCalls),
           std::move(PendingTypeCheckedLoadConstVCalls),
-          std::move(PendingParamAccesses));
+          std::move(PendingParamAccesses), false);
       LastSeenSummary = FS.get();
       LastSeenGUID = VI.getGUID();
       FS->setModulePath(ModuleIdMap[ModuleId]);
diff --git a/llvm/lib/IR/AsmWriter.cpp b/llvm/lib/IR/AsmWriter.cpp
--- a/llvm/lib/IR/AsmWriter.cpp
+++ b/llvm/lib/IR/AsmWriter.cpp
@@ -3154,6 +3154,7 @@
 
 void AssemblyWriter::printFunctionSummary(const FunctionSummary *FS) {
   Out << ", insts: " << FS->instCount();
+  Out << ", unreachable: " << FS->isUnreachableFunction();
   if (FS->fflags().anyFlagSet())
     Out << ", " << FS->fflags();
 
diff --git a/llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp b/llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp
--- a/llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp
+++ b/llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp
@@ -565,7 +565,8 @@
   bool
   tryFindVirtualCallTargets(std::vector<VirtualCallTarget> &TargetsForSlot,
                             const std::set<TypeMemberInfo> &TypeMemberInfos,
-                            uint64_t ByteOffset);
+                            uint64_t ByteOffset,
+                            ModuleSummaryIndex *ExportSummary);
 
   void applySingleImplDevirt(VTableSlotInfo &SlotInfo, Constant *TheFn,
                              bool &IsExported);
@@ -646,6 +647,10 @@
   runForTesting(Module &M, function_ref<AAResults &(Function &)> AARGetter,
                 function_ref<OptimizationRemarkEmitter &(Function *)> OREGetter,
                 function_ref<DominatorTree &(Function &)> LookupDomTree);
+
+  // Returns true if the function is unreachable from all summaries.
+  static bool mustBeUnreachableFunction(Function *TheFn,
+                                        ModuleSummaryIndex *exportSummary);
 };
 
 struct DevirtIndex {
@@ -933,6 +938,50 @@
   return Changed;
 }
 
+bool DevirtModule::mustBeUnreachableFunction(
+    Function *TheFn, ModuleSummaryIndex *ExportSummary) {
+  assert(((TheFn != nullptr) && (ExportSummary != nullptr)) &&
+         "TheFn and ExportSummary must not be nullptr");
+  errs() << "Func GUID is " << TheFn->getGUID() << "\n";
+
+  errs() << "Func current identifier is " << TheFn->getGlobalIdentifier()
+         << "\n";
+  const std::string rewrittenFuncGlobalIdentifier =
+      GlobalValue::getGlobalIdentifier(TheFn->getName(),
+                                       GlobalValue::ExternalLinkage,
+                                       TheFn->getParent()->getSourceFileName());
+  errs() << "rewritten identifier is " << rewrittenFuncGlobalIdentifier << "\n";
+  errs() << "GUID before is "
+         << GlobalValue::getGUID(TheFn->getGlobalIdentifier())
+         << " and after is "
+         << GlobalValue::getGUID(rewrittenFuncGlobalIdentifier) << "\n";
+
+  if (ValueInfo TheFnVI = ExportSummary->getValueInfo(
+          GlobalValue::getGUID(rewrittenFuncGlobalIdentifier))) {
+    bool AllSummariesAreFunctionSummary = true;
+    bool AllFunctionSummariesIndicateUnreachable = true;
+    for (auto &Summary : TheFnVI.getSummaryList()) {
+      if (auto *FS = dyn_cast<FunctionSummary>(Summary.get())) {
+        if (!FS->isUnreachableFunction()) {
+          AllFunctionSummariesIndicateUnreachable = false;
+          break;
+        }
+      } else {
+        AllSummariesAreFunctionSummary = false;
+        break;
+      }
+    }
+    if (AllSummariesAreFunctionSummary &&
+        AllFunctionSummariesIndicateUnreachable) {
+      return true;
+    }
+  } else {
+    errs() << "No value info for func: ";
+    TheFn->dump();
+  }
+  return false;
+}
+
 void DevirtModule::buildTypeIdentifierMap(
     std::vector<VTableBits> &Bits,
     DenseMap<Metadata *, std::set<TypeMemberInfo>> &TypeIdMap) {
@@ -969,7 +1018,10 @@
 
 bool DevirtModule::tryFindVirtualCallTargets(
     std::vector<VirtualCallTarget> &TargetsForSlot,
-    const std::set<TypeMemberInfo> &TypeMemberInfos, uint64_t ByteOffset) {
+    const std::set<TypeMemberInfo> &TypeMemberInfos, uint64_t ByteOffset,
+    ModuleSummaryIndex *ExportSummary) {
+  errs() << "DevirtModule::tryFindVirtualCallTargets\n";
+  ExportSummary->dump();
   for (const TypeMemberInfo &TM : TypeMemberInfos) {
     if (!TM.Bits->GV->isConstant())
       return false;
@@ -997,6 +1049,15 @@
     if (Fn->getName() == "__cxa_pure_virtual")
       continue;
 
+    if (mustBeUnreachableFunction(Fn, ExportSummary)) {
+      errs() << "Function " << Fn->getName() << " is unreachable\n";
+      continue;
+    }
+    if (Fn->getName() == "_ZN4BaseD0Ev") {
+      errs() << "not skipping skip _ZN4BaseD0Ev\n";
+      // continue;
+    }
+
     TargetsForSlot.push_back({Fn, &TM});
   }
 
@@ -1142,12 +1203,26 @@
     ModuleSummaryIndex *ExportSummary,
     MutableArrayRef<VirtualCallTarget> TargetsForSlot, VTableSlotInfo &SlotInfo,
     WholeProgramDevirtResolution *Res) {
+  ExportSummary->dump();
   // See if the program contains a single implementation of this virtual
   // function.
   Function *TheFn = TargetsForSlot[0].Fn;
-  for (auto &&Target : TargetsForSlot)
-    if (TheFn != Target.Fn)
+  errs() << "First func: ";
+  TheFn->dump();
+  errs() << "\n";
+  for (auto &&Target : TargetsForSlot) {
+    errs() << "Target func: ";
+    Target.Fn->dump();
+    errs() << "\n";
+    if (mustBeUnreachableFunction(Target.Fn, ExportSummary)) {
+      errs() << "\t\t\t???Found an unreachable function "
+             << Target.Fn->getName() << "\n";
+      continue;
+    }
+    if (TheFn != Target.Fn) {
       return false;
+    }
+  }
 
   // If so, update each call site to call that implementation directly.
   if (RemarksEnabled)
@@ -1162,6 +1237,7 @@
   // to make it visible to thin LTO objects. We can only get here during the
   // ThinLTO export phase.
   if (TheFn->hasLocalLinkage()) {
+    errs() << TheFn->getName() << " has local linkage\n";
     std::string NewName = (TheFn->getName() + ".llvm.merged").str();
 
     // Since we are renaming the function, any comdats with the same name must
@@ -2015,6 +2091,7 @@
 }
 
 bool DevirtModule::run() {
+  errs() << "devirt module::run\n";
   // If only some of the modules were split, we cannot correctly perform
   // this transformation. We already checked for the presense of type tests
   // with partially split modules during the thin link, and would have emitted
@@ -2137,7 +2214,7 @@
                      cast<MDString>(S.first.TypeID)->getString())
                  .WPDRes[S.first.ByteOffset];
     if (tryFindVirtualCallTargets(TargetsForSlot, TypeMemberInfos,
-                                  S.first.ByteOffset)) {
+                                  S.first.ByteOffset, ExportSummary)) {
 
       if (!trySingleImplDevirt(ExportSummary, TargetsForSlot, S.second, Res)) {
         DidVirtualConstProp |=