(NFC) Track global summary liveness in GVFlags.

[eugenis]

Summary

Replace GVFlags::LiveRoot with GVFlags::Live and use that instead of
all the DeadSymbols sets. This is refactoring in order to make
liveness information available in the RegularLTO pipeline

[pcc]

lib/Transforms/IPO/FunctionImport.cpp

439

Can we return if we see at least one live summary? I think there is an invariant in this function that if at least one summary for a GUID is live, all are live.

459

Can this code be simplified to use a for loop? All it needs to do is add a ValueInfo to the worklist if it sees at least one live summary, right?

[pcc]

lib/Transforms/IPO/FunctionImport.cpp

471

I was thinking just push ValueInfo(&Entry) onto the worklist here. Then you can replace the other call to makeLiveRoot with the loop that sets summaries to live and remove makeLiveRoot entirely.

[eugenis]

lib/Transforms/IPO/FunctionImport.cpp

471

But then we also need to do ++LiveSymbols in both places. I'm not sure it's a net win.

[pcc]

lib/Transforms/IPO/FunctionImport.cpp

471

We just need to do it here, no? Otherwise we'd end up double counting live roots.

[eugenis]

lib/Transforms/IPO/FunctionImport.cpp

471

Oh right, good point.

[eugenis]

PTAL

[pcc]

LGTM

[eugenis]

r304466

include/llvm/IR/ModuleSummaryIndex.h

struct GVFlags {
     /// In the future this will be used to update and optimize linkage
     /// types based on global summary-based analysis.
     unsigned Linkage : 4;
 
     /// Indicate if the global value cannot be imported (e.g. it cannot
     /// be renamed or references something that can't be renamed).
     unsigned NotEligibleToImport : 1;
 
-    /// Indicate that the global value must be considered a live root for
-    /// index-based liveness analysis. Used for special LLVM values such as
-    /// llvm.global_ctors that the linker does not know about.
-    unsigned LiveRoot : 1;
+    /// In per-module summary, indicate that the global value must be considered
+    /// a live root for index-based liveness analysis. Used for special LLVM
+    /// values such as llvm.global_ctors that the linker does not know about.
+    ///
+    /// In combined summary, indicate that the global value is live.
+    unsigned Live : 1;
 
     /// Convenience Constructors
     explicit GVFlags(GlobalValue::LinkageTypes Linkage,
-                     bool NotEligibleToImport, bool LiveRoot)
+                     bool NotEligibleToImport, bool Live)
         : Linkage(Linkage), NotEligibleToImport(NotEligibleToImport),
-          LiveRoot(LiveRoot) {}
+          Live(Live) {}
   };
 
 private:
   /// Kind of summary for use in dyn_cast<> et al.
   SummaryKind Kind;
 
   GVFlags Flags;
 
private:
   StringRef ModulePath;
 
   /// List of values referenced by this global value's definition
   /// (either by the initializer of a global variable, or referenced
   /// from within a function). This does not include functions called, which
   /// are listed in the derived FunctionSummary object.
   std::vector<ValueInfo> RefEdgeList;
 
+  bool isLive() const { return Flags.Live; }
+
 protected:
   GlobalValueSummary(SummaryKind K, GVFlags Flags, std::vector<ValueInfo> Refs)
       : Kind(K), Flags(Flags), RefEdgeList(std::move(Refs)) {}
 
 public:
   virtual ~GlobalValueSummary() = default;
 
   /// Returns the hash of the original name, it is identical to the GUID for
public:
   /// optimization. Will be applied in the ThinLTO backends.
   void setLinkage(GlobalValue::LinkageTypes Linkage) {
     Flags.Linkage = Linkage;
   }
 
   /// Return true if this global value can't be imported.
   bool notEligibleToImport() const { return Flags.NotEligibleToImport; }
 
-  /// Return true if this global value must be considered a root for live
-  /// value analysis on the index.
-  bool liveRoot() const { return Flags.LiveRoot; }
-
-  /// Flag that this global value must be considered a root for live
-  /// value analysis on the index.
-  void setLiveRoot() { Flags.LiveRoot = true; }
+  void setLive(bool Live) { Flags.Live = Live; }
 
   /// Flag that this global value cannot be imported.
   void setNotEligibleToImport() { Flags.NotEligibleToImport = true; }
 
   /// Return the list of values referenced by this global value definition.
   ArrayRef<ValueInfo> refs() const { return RefEdgeList; }
+
+  friend class ModuleSummaryIndex;
+  friend void computeDeadSymbols(class ModuleSummaryIndex &,
+                                 const DenseSet<GlobalValue::GUID> &);
 };
 
 /// \brief Alias summary information.
 class AliasSummary : public GlobalValueSummary {
   GlobalValueSummary *AliaseeSummary;
 
 public:
   AliasSummary(GVFlags Flags, std::vector<ValueInfo> Refs)
private:
   /// identifier.
   // FIXME: Add bitcode read/write support for this field.
   std::map<std::string, TypeIdSummary> TypeIdMap;
 
   /// Mapping from original ID to GUID. If original ID can map to multiple
   /// GUIDs, it will be mapped to 0.
   std::map<GlobalValue::GUID, GlobalValue::GUID> OidGuidMap;
 
+  /// Indicates that summary-based GlobalValue GC has run, and values with
+  /// GVFlags::Live==false are really dead. Otherwise, all values must be
+  /// considered live.
+  bool WithGlobalValueDeadStripping = false;
+
   // YAML I/O support.
   friend yaml::MappingTraits<ModuleSummaryIndex>;
 
   GlobalValueSummaryMapTy::value_type *
   getOrInsertValuePtr(GlobalValue::GUID GUID) {
     return &*GlobalValueMap.emplace(GUID, GlobalValueSummaryInfo{}).first;
   }
 
 public:
   gvsummary_iterator begin() { return GlobalValueMap.begin(); }
   const_gvsummary_iterator begin() const { return GlobalValueMap.begin(); }
   gvsummary_iterator end() { return GlobalValueMap.end(); }
   const_gvsummary_iterator end() const { return GlobalValueMap.end(); }
   size_t size() const { return GlobalValueMap.size(); }
 
+  bool withGlobalValueDeadStripping() const {
+    return WithGlobalValueDeadStripping;
+  }
+  void setWithGlobalValueDeadStripping() {
+    WithGlobalValueDeadStripping = true;
+  }
+
+  bool isGlobalValueLive(const GlobalValueSummary *GVS) const {
+    return !WithGlobalValueDeadStripping || GVS->isLive();
+  }
+
   /// Return a ValueInfo for GUID if it exists, otherwise return ValueInfo().
   ValueInfo getValueInfo(GlobalValue::GUID GUID) const {
     auto I = GlobalValueMap.find(GUID);
     return ValueInfo(I == GlobalValueMap.end() ? nullptr : &*I);
   }
 
   /// Return a ValueInfo for \p GUID.
   ValueInfo getOrInsertValueInfo(GlobalValue::GUID GUID) {

include/llvm/Transforms/IPO/FunctionImport.h

 ///
 /// \p ImportLists will be populated with an entry for every Module we are
 /// importing into. This entry is itself a map that can be passed to
 /// FunctionImporter::importFunctions() above (see description there).
 ///
 /// \p ExportLists contains for each Module the set of globals (GUID) that will
 /// be imported by another module, or referenced by such a function. I.e. this
 /// is the set of globals that need to be promoted/renamed appropriately.
-///
-/// \p DeadSymbols (optional) contains a list of GUID that are deemed "dead" and
-/// will be ignored for the purpose of importing.
 void ComputeCrossModuleImport(
     const ModuleSummaryIndex &Index,
     const StringMap<GVSummaryMapTy> &ModuleToDefinedGVSummaries,
     StringMap<FunctionImporter::ImportMapTy> &ImportLists,
-    StringMap<FunctionImporter::ExportSetTy> &ExportLists,
-    const DenseSet<GlobalValue::GUID> *DeadSymbols = nullptr);
+    StringMap<FunctionImporter::ExportSetTy> &ExportLists);
 
 /// Compute all the imports for the given module using the Index.
 ///
 /// \p ImportList will be populated with a map that can be passed to
 /// FunctionImporter::importFunctions() above (see description there).
 void ComputeCrossModuleImportForModule(
     StringRef ModulePath, const ModuleSummaryIndex &Index,
     FunctionImporter::ImportMapTy &ImportList);
 
 /// Compute all the symbols that are "dead": i.e these that can't be reached
 /// in the graph from any of the given symbols listed in
 /// \p GUIDPreservedSymbols.
-DenseSet<GlobalValue::GUID>
-computeDeadSymbols(const ModuleSummaryIndex &Index,
-                   const DenseSet<GlobalValue::GUID> &GUIDPreservedSymbols);
+void computeDeadSymbols(
+    ModuleSummaryIndex &Index,
 
 /// Compute the set of summaries needed for a ThinLTO backend compilation of
 /// \p ModulePath.
 //
 /// This includes summaries from that module (in case any global summary based
 /// optimizations were recorded) and from any definitions in other modules that
 /// should be imported.
 //

lib/Analysis/ModuleSummaryAnalysis.cpp

computeFunctionSummary(ModuleSummaryIndex &Index, const Module &M,
 
   bool NonRenamableLocal = isNonRenamableLocal(F);
   bool NotEligibleForImport =
       NonRenamableLocal || HasInlineAsmMaybeReferencingInternal ||
       // Inliner doesn't handle variadic functions.
       // FIXME: refactor this to use the same code that inliner is using.
       F.isVarArg();
   GlobalValueSummary::GVFlags Flags(F.getLinkage(), NotEligibleForImport,
-                                    /* LiveRoot = */ false);
+                                    /* Live = */ false);
   auto FuncSummary = llvm::make_unique<FunctionSummary>(
       Flags, NumInsts, RefEdges.takeVector(), CallGraphEdges.takeVector(),
       TypeTests.takeVector(), TypeTestAssumeVCalls.takeVector(),
       TypeCheckedLoadVCalls.takeVector(),
       TypeTestAssumeConstVCalls.takeVector(),
       TypeCheckedLoadConstVCalls.takeVector());
   if (NonRenamableLocal)
     CantBePromoted.insert(F.getGUID());
   Index.addGlobalValueSummary(F.getName(), std::move(FuncSummary));
 }
 
 static void
 computeVariableSummary(ModuleSummaryIndex &Index, const GlobalVariable &V,
                        DenseSet<GlobalValue::GUID> &CantBePromoted) {
   SetVector<ValueInfo> RefEdges;
   SmallPtrSet<const User *, 8> Visited;
   findRefEdges(Index, &V, RefEdges, Visited);
   bool NonRenamableLocal = isNonRenamableLocal(V);
   GlobalValueSummary::GVFlags Flags(V.getLinkage(), NonRenamableLocal,
-                                    /* LiveRoot = */ false);
+                                    /* Live = */ false);
   auto GVarSummary =
       llvm::make_unique<GlobalVarSummary>(Flags, RefEdges.takeVector());
   if (NonRenamableLocal)
     CantBePromoted.insert(V.getGUID());
   Index.addGlobalValueSummary(V.getName(), std::move(GVarSummary));
 }
 
 static void
 computeAliasSummary(ModuleSummaryIndex &Index, const GlobalAlias &A,
                     DenseSet<GlobalValue::GUID> &CantBePromoted) {
   bool NonRenamableLocal = isNonRenamableLocal(A);
   GlobalValueSummary::GVFlags Flags(A.getLinkage(), NonRenamableLocal,
-                                    /* LiveRoot = */ false);
+                                    /* Live = */ false);
   auto AS = llvm::make_unique<AliasSummary>(Flags, ArrayRef<ValueInfo>{});
   auto *Aliasee = A.getBaseObject();
   auto *AliaseeSummary = Index.getGlobalValueSummary(*Aliasee);
   assert(AliaseeSummary && "Alias expects aliasee summary to be parsed");
   AS->setAliasee(AliaseeSummary);
   if (NonRenamableLocal)
     CantBePromoted.insert(A.getGUID());
   Index.addGlobalValueSummary(A.getName(), std::move(AS));
 }
 
 // Set LiveRoot flag on entries matching the given value name.
 static void setLiveRoot(ModuleSummaryIndex &Index, StringRef Name) {
   if (ValueInfo VI = Index.getValueInfo(GlobalValue::getGUID(Name)))
     for (auto &Summary : VI.getSummaryList())
-      Summary->setLiveRoot();
+      Summary->setLive(true);
 }
 
 ModuleSummaryIndex llvm::buildModuleSummaryIndex(
     const Module &M,
     std::function<BlockFrequencyInfo *(const Function &F)> GetBFICallback,
     ProfileSummaryInfo *PSI) {
   assert(PSI);
   ModuleSummaryIndex Index;
ModuleSymbolTable::CollectAsmSymbols(
           if (Flags & (object::BasicSymbolRef::SF_Weak |
                        object::BasicSymbolRef::SF_Global))
             return;
           GlobalValue *GV = M.getNamedValue(Name);
           if (!GV)
             return;
           assert(GV->isDeclaration() && "Def in module asm already has definition");
           GlobalValueSummary::GVFlags GVFlags(GlobalValue::InternalLinkage,
-                                              /* NotEligibleToImport */ true,
-                                              /* LiveRoot */ true);
+                                              /* NotEligibleToImport = */ true,
+                                              /* Live = */ true);
           CantBePromoted.insert(GlobalValue::getGUID(Name));
           // Create the appropriate summary type.
           if (isa<Function>(GV)) {
             std::unique_ptr<FunctionSummary> Summary =
                 llvm::make_unique<FunctionSummary>(
                     GVFlags, 0, ArrayRef<ValueInfo>{},
                     ArrayRef<FunctionSummary::EdgeTy>{},
                     ArrayRef<GlobalValue::GUID>{},

lib/Bitcode/Reader/BitcodeReader.cpp

 static GlobalValueSummary::GVFlags getDecodedGVSummaryFlags(uint64_t RawFlags,
                                                             uint64_t Version) {
   // Summary were not emitted before LLVM 3.9, we don't need to upgrade Linkage
   // like getDecodedLinkage() above. Any future change to the linkage enum and
   // to getDecodedLinkage() will need to be taken into account here as above.
   auto Linkage = GlobalValue::LinkageTypes(RawFlags & 0xF); // 4 bits
   RawFlags = RawFlags >> 4;
   bool NotEligibleToImport = (RawFlags & 0x1) || Version < 3;
-  // The LiveRoot flag wasn't introduced until version 3. For dead stripping
+  // The Live flag wasn't introduced until version 3. For dead stripping
   // to work correctly on earlier versions, we must conservatively treat all
   // values as live.
-  bool LiveRoot = (RawFlags & 0x2) || Version < 3;
-  return GlobalValueSummary::GVFlags(Linkage, NotEligibleToImport, LiveRoot);
+  bool Live = (RawFlags & 0x2) || Version < 3;
+  return GlobalValueSummary::GVFlags(Linkage, NotEligibleToImport, Live);
 }
 
 static GlobalValue::VisibilityTypes getDecodedVisibility(unsigned Val) {
   switch (Val) {
   default: // Map unknown visibilities to default.
   case 0: return GlobalValue::DefaultVisibility;
   case 1: return GlobalValue::HiddenVisibility;
   case 2: return GlobalValue::ProtectedVisibility;

lib/Bitcode/Writer/BitcodeWriter.cpp

static unsigned getEncodedLinkage(const GlobalValue &GV) {
   return getEncodedLinkage(GV.getLinkage());
 }
 
 // Decode the flags for GlobalValue in the summary
 static uint64_t getEncodedGVSummaryFlags(GlobalValueSummary::GVFlags Flags) {
   uint64_t RawFlags = 0;
 
   RawFlags |= Flags.NotEligibleToImport; // bool
-  RawFlags |= (Flags.LiveRoot << 1);
+  RawFlags |= (Flags.Live << 1);
   // Linkage don't need to be remapped at that time for the summary. Any future
   // change to the getEncodedLinkage() function will need to be taken into
   // account here as well.
   RawFlags = (RawFlags << 4) | Flags.Linkage; // 4 bits
 
   return RawFlags;
 }
 

lib/LTO/LTO.cpp

return [=](Config &Conf, ModuleSummaryIndex &CombinedIndex,
              const StringMap<GVSummaryMapTy> &ModuleToDefinedGVSummaries,
              AddStreamFn AddStream, NativeObjectCache Cache) {
     return llvm::make_unique<WriteIndexesThinBackend>(
         Conf, CombinedIndex, ModuleToDefinedGVSummaries, OldPrefix, NewPrefix,
         ShouldEmitImportsFiles, LinkedObjectsFile);
   };
 }
 
+static bool IsLiveByGUID(const ModuleSummaryIndex &Index,
+                         GlobalValue::GUID GUID) {
+  auto VI = Index.getValueInfo(GUID);
+  if (!VI)
+    return false;
+  for (auto &I : VI.getSummaryList())
+    if (Index.isGlobalValueLive(I.get()))
+      return true;
+  return false;
+}
+
 Error LTO::runThinLTO(AddStreamFn AddStream, NativeObjectCache Cache,
                       bool HasRegularLTO) {
   if (ThinLTO.ModuleMap.empty())
     return Error::success();
 
   if (Conf.CombinedIndexHook && !Conf.CombinedIndexHook(ThinLTO.CombinedIndex))
     return Error::success();
 
for (auto &Res : GlobalResolutions) {
       if (Res.second.VisibleOutsideThinLTO &&
           // IRName will be defined if we have seen the prevailing copy of
           // this value. If not, no need to preserve any ThinLTO copies.
           !Res.second.IRName.empty())
         GUIDPreservedSymbols.insert(GlobalValue::getGUID(
             GlobalValue::dropLLVMManglingEscape(Res.second.IRName)));
     }
 
-    auto DeadSymbols =
-        computeDeadSymbols(ThinLTO.CombinedIndex, GUIDPreservedSymbols);
 
     ComputeCrossModuleImport(ThinLTO.CombinedIndex, ModuleToDefinedGVSummaries,
-                             ImportLists, ExportLists, &DeadSymbols);
+                             ImportLists, ExportLists);
 
     std::set<GlobalValue::GUID> ExportedGUIDs;
     for (auto &Res : GlobalResolutions) {
       // First check if the symbol was flagged as having external references.
       if (Res.second.Partition != GlobalResolution::External)
         continue;
       // IRName will be defined if we have seen the prevailing copy of
       // this value. If not, no need to mark as exported from a ThinLTO
       // partition (and we can't get the GUID).
       if (Res.second.IRName.empty())
         continue;
       auto GUID = GlobalValue::getGUID(
           GlobalValue::dropLLVMManglingEscape(Res.second.IRName));
       // Mark exported unless index-based analysis determined it to be dead.
-      if (!DeadSymbols.count(GUID))
+      if (IsLiveByGUID(ThinLTO.CombinedIndex, GUID))
         ExportedGUIDs.insert(GUID);
     }
 
     auto isExported = [&](StringRef ModuleIdentifier, GlobalValue::GUID GUID) {
       const auto &ExportList = ExportLists.find(ModuleIdentifier);
       return (ExportList != ExportLists.end() &&
               ExportList->second.count(GUID)) ||
              ExportedGUIDs.count(GUID);

lib/LTO/ThinLTOCodeGenerator.cpp

void ThinLTOCodeGenerator::promote(Module &TheModule,
   StringMap<GVSummaryMapTy> ModuleToDefinedGVSummaries;
   Index.collectDefinedGVSummariesPerModule(ModuleToDefinedGVSummaries);
 
   // Convert the preserved symbols set from string to GUID
   auto GUIDPreservedSymbols = computeGUIDPreservedSymbols(
       PreservedSymbols, Triple(TheModule.getTargetTriple()));
 
   // Compute "dead" symbols, we don't want to import/export these!
-  auto DeadSymbols = computeDeadSymbols(Index, GUIDPreservedSymbols);
+  computeDeadSymbols(Index, GUIDPreservedSymbols);
 
   // Generate import/export list
   StringMap<FunctionImporter::ImportMapTy> ImportLists(ModuleCount);
   StringMap<FunctionImporter::ExportSetTy> ExportLists(ModuleCount);
   ComputeCrossModuleImport(Index, ModuleToDefinedGVSummaries, ImportLists,
-                           ExportLists, &DeadSymbols);
+                           ExportLists);
 
   // Resolve LinkOnce/Weak symbols.
   StringMap<std::map<GlobalValue::GUID, GlobalValue::LinkageTypes>> ResolvedODR;
   resolveWeakForLinkerInIndex(Index, ResolvedODR);
 
   thinLTOResolveWeakForLinkerModule(
       TheModule, ModuleToDefinedGVSummaries[ModuleIdentifier]);
 
void ThinLTOCodeGenerator::crossModuleImport(Module &TheModule,
   StringMap<GVSummaryMapTy> ModuleToDefinedGVSummaries(ModuleCount);
   Index.collectDefinedGVSummariesPerModule(ModuleToDefinedGVSummaries);
 
   // Convert the preserved symbols set from string to GUID
   auto GUIDPreservedSymbols = computeGUIDPreservedSymbols(
       PreservedSymbols, Triple(TheModule.getTargetTriple()));
 
   // Compute "dead" symbols, we don't want to import/export these!
-  auto DeadSymbols = computeDeadSymbols(Index, GUIDPreservedSymbols);
+  computeDeadSymbols(Index, GUIDPreservedSymbols);
 
   // Generate import/export list
   StringMap<FunctionImporter::ImportMapTy> ImportLists(ModuleCount);
   StringMap<FunctionImporter::ExportSetTy> ExportLists(ModuleCount);
   ComputeCrossModuleImport(Index, ModuleToDefinedGVSummaries, ImportLists,
-                           ExportLists, &DeadSymbols);
+                           ExportLists);
   auto &ImportList = ImportLists[TheModule.getModuleIdentifier()];
 
   crossImportIntoModule(TheModule, Index, ModuleMap, ImportList);
 }
 
 /**
  * Compute the list of summaries needed for importing into module.
  */
void ThinLTOCodeGenerator::internalize(Module &TheModule,
   auto GUIDPreservedSymbols =
       computeGUIDPreservedSymbols(PreservedSymbols, TMBuilder.TheTriple);
 
   // Collect for each module the list of function it defines (GUID -> Summary).
   StringMap<GVSummaryMapTy> ModuleToDefinedGVSummaries(ModuleCount);
   Index.collectDefinedGVSummariesPerModule(ModuleToDefinedGVSummaries);
 
   // Compute "dead" symbols, we don't want to import/export these!
-  auto DeadSymbols = computeDeadSymbols(Index, GUIDPreservedSymbols);
+  computeDeadSymbols(Index, GUIDPreservedSymbols);
 
   // Generate import/export list
   StringMap<FunctionImporter::ImportMapTy> ImportLists(ModuleCount);
   StringMap<FunctionImporter::ExportSetTy> ExportLists(ModuleCount);
   ComputeCrossModuleImport(Index, ModuleToDefinedGVSummaries, ImportLists,
-                           ExportLists, &DeadSymbols);
+                           ExportLists);
   auto &ExportList = ExportLists[ModuleIdentifier];
 
   // Be friendly and don't nuke totally the module when the client didn't
   // supply anything to preserve.
   if (ExportList.empty() && GUIDPreservedSymbols.empty())
     return;
 
   // Internalization
void ThinLTOCodeGenerator::run() {
   Index->collectDefinedGVSummariesPerModule(ModuleToDefinedGVSummaries);
 
   // Convert the preserved symbols set from string to GUID, this is needed for
   // computing the caching hash and the internalization.
   auto GUIDPreservedSymbols =
       computeGUIDPreservedSymbols(PreservedSymbols, TMBuilder.TheTriple);
 
   // Compute "dead" symbols, we don't want to import/export these!
-  auto DeadSymbols = computeDeadSymbols(*Index, GUIDPreservedSymbols);
+  computeDeadSymbols(*Index, GUIDPreservedSymbols);
 
   // Collect the import/export lists for all modules from the call-graph in the
   // combined index.
   StringMap<FunctionImporter::ImportMapTy> ImportLists(ModuleCount);
   StringMap<FunctionImporter::ExportSetTy> ExportLists(ModuleCount);
   ComputeCrossModuleImport(*Index, ModuleToDefinedGVSummaries, ImportLists,
-                           ExportLists, &DeadSymbols);
+                           ExportLists);
 
   // We use a std::map here to be able to have a defined ordering when
   // producing a hash for the cache entry.
   // FIXME: we should be able to compute the caching hash for the entry based
   // on the index, and nuke this map.
   StringMap<std::map<GlobalValue::GUID, GlobalValue::LinkageTypes>> ResolvedODR;
 
   // Resolve LinkOnce/Weak symbols, this has to be computed early because it

lib/Transforms/IPO/FunctionImport.cpp

 }
 
 /// Given the list of globals defined in a module, compute the list of imports
 /// as well as the list of "exports", i.e. the list of symbols referenced from
 /// another module (that may require promotion).
 static void ComputeImportForModule(
     const GVSummaryMapTy &DefinedGVSummaries, const ModuleSummaryIndex &Index,
     FunctionImporter::ImportMapTy &ImportList,
-    StringMap<FunctionImporter::ExportSetTy> *ExportLists = nullptr,
-    const DenseSet<GlobalValue::GUID> *DeadSymbols = nullptr) {
+    StringMap<FunctionImporter::ExportSetTy> *ExportLists = nullptr) {
   // Worklist contains the list of function imported in this module, for which
   // we will analyse the callees and may import further down the callgraph.
   SmallVector<EdgeInfo, 128> Worklist;
 
   // Populate the worklist with the import for the functions in the current
   // module
   for (auto &GVSummary : DefinedGVSummaries) {
-    if (DeadSymbols && DeadSymbols->count(GVSummary.first)) {
+    if (!Index.isGlobalValueLive(GVSummary.second)) {
       DEBUG(dbgs() << "Ignores Dead GUID: " << GVSummary.first << "\n");
       continue;
     }
     auto *Summary = GVSummary.second;
     if (auto *AS = dyn_cast<AliasSummary>(Summary))
       Summary = &AS->getAliasee();
     auto *FuncSummary = dyn_cast<FunctionSummary>(Summary);
     if (!FuncSummary)

 
 } // anonymous namespace
 
 /// Compute all the import and export for every module using the Index.
 void llvm::ComputeCrossModuleImport(
     const ModuleSummaryIndex &Index,
     const StringMap<GVSummaryMapTy> &ModuleToDefinedGVSummaries,
     StringMap<FunctionImporter::ImportMapTy> &ImportLists,
-    StringMap<FunctionImporter::ExportSetTy> &ExportLists,
-    const DenseSet<GlobalValue::GUID> *DeadSymbols) {
+    StringMap<FunctionImporter::ExportSetTy> &ExportLists) {
   // For each module that has function defined, compute the import/export lists.
   for (auto &DefinedGVSummaries : ModuleToDefinedGVSummaries) {
     auto &ImportList = ImportLists[DefinedGVSummaries.first()];
     DEBUG(dbgs() << "Computing import for Module '"
                  << DefinedGVSummaries.first() << "'\n");
     ComputeImportForModule(DefinedGVSummaries.second, Index, ImportList,
-                           &ExportLists, DeadSymbols);
+                           &ExportLists);
   }
 
   // When computing imports we added all GUIDs referenced by anything
   // imported from the module to its ExportList. Now we prune each ExportList
   // of any not defined in that module. This is more efficient than checking
   // while computing imports because some of the summary lists may be long
   // due to linkonce (comdat) copies.
   for (auto &ELI : ExportLists) {
#ifndef NDEBUG
   for (auto &Src : ImportList) {
     auto SrcModName = Src.first();
     DEBUG(dbgs() << " - " << Src.second.size() << " functions imported from "
                  << SrcModName << "\n");
   }
 #endif
 }
 
-DenseSet<GlobalValue::GUID> llvm::computeDeadSymbols(
-    const ModuleSummaryIndex &Index,
+void llvm::computeDeadSymbols(
+    ModuleSummaryIndex &Index,
     const DenseSet<GlobalValue::GUID> &GUIDPreservedSymbols) {
+  assert(!Index.withGlobalValueDeadStripping());
   if (!ComputeDead)
-    return DenseSet<GlobalValue::GUID>();
+    return;
   if (GUIDPreservedSymbols.empty())
     // Don't do anything when nothing is live, this is friendly with tests.
-    return DenseSet<GlobalValue::GUID>();
-  DenseSet<ValueInfo> LiveSymbols;
+    return;
+  unsigned LiveSymbols = 0;
   SmallVector<ValueInfo, 128> Worklist;
   Worklist.reserve(GUIDPreservedSymbols.size() * 2);
+  // Make value live and add it to the worklist.
+  auto makeLiveRoot = [&](ValueInfo VI) {
+    for (auto &S : VI.getSummaryList())
+      S->setLive(true);
+    Worklist.push_back(VI);
+    ++LiveSymbols;
+  };
+  // Make value live and add it to the worklist if it was not live before.
+  // FIXME: we should only make the prevailing copy live here
+  auto makeLive = [&](ValueInfo VI) {
+    bool Changed = false;
+    for (auto &S : VI.getSummaryList())
+      if (!S->isLive()) {
+        Changed = true;
+        S->setLive(true);
+      }
+    if (Changed) {
+      ++LiveSymbols;
+      Worklist.push_back(VI);
+    }
+  };
+
   for (auto GUID : GUIDPreservedSymbols) {
     ValueInfo VI = Index.getValueInfo(GUID);
     if (!VI)
       continue;
     DEBUG(dbgs() << "Live root: " << VI.getGUID() << "\n");
-    LiveSymbols.insert(VI);
-    Worklist.push_back(VI);
+    makeLiveRoot(VI);
   }
+
   // Add values flagged in the index as live roots to the worklist.
   for (const auto &Entry : Index) {
     bool IsLiveRoot = llvm::any_of(
         Entry.second.SummaryList,
         [&](const std::unique_ptr<llvm::GlobalValueSummary> &Summary) {
-          return Summary->liveRoot();
+          return Summary->isLive();
         });
     if (!IsLiveRoot)
       continue;
     DEBUG(dbgs() << "Live root (summary): " << Entry.first << "\n");
-    Worklist.push_back(ValueInfo(&Entry));
+    makeLiveRoot(ValueInfo(&Entry));
   }
 
   while (!Worklist.empty()) {
     auto VI = Worklist.pop_back_val();
 
-    // FIXME: we should only make the prevailing copy live here
     for (auto &Summary : VI.getSummaryList()) {
-      for (auto Ref : Summary->refs()) {
-        if (LiveSymbols.insert(Ref).second) {
-          DEBUG(dbgs() << "Marking live (ref): " << Ref.getGUID() << "\n");
-          Worklist.push_back(Ref);
-        }
-      }
-      if (auto *FS = dyn_cast<FunctionSummary>(Summary.get())) {
-        for (auto Call : FS->calls()) {
-          if (LiveSymbols.insert(Call.first).second) {
-            DEBUG(dbgs() << "Marking live (call): " << Call.first.getGUID()
-                         << "\n");
-            Worklist.push_back(Call.first);
-          }
-        }
-      }
+
+      for (auto Ref : Summary->refs())
+        makeLive(Ref);
+      if (auto *FS = dyn_cast<FunctionSummary>(Summary.get()))
+        for (auto Call : FS->calls())
+          makeLive(Call.first);
       if (auto *AS = dyn_cast<AliasSummary>(Summary.get())) {
         auto AliaseeGUID = AS->getAliasee().getOriginalName();
         ValueInfo AliaseeVI = Index.getValueInfo(AliaseeGUID);
-        if (AliaseeVI && LiveSymbols.insert(AliaseeVI).second) {
-          DEBUG(dbgs() << "Marking live (alias): " << AliaseeGUID << "\n");
-          Worklist.push_back(AliaseeVI);
-        }
+        if (AliaseeVI)
+          makeLive(AliaseeVI);
       }
     }
   }
-  DenseSet<GlobalValue::GUID> DeadSymbols;
-  DeadSymbols.reserve(
-      std::min(Index.size(), Index.size() - LiveSymbols.size()));
-  for (auto &Entry : Index) {
-    if (!LiveSymbols.count(ValueInfo(&Entry))) {
-      DEBUG(dbgs() << "Marking dead: " << Entry.first << "\n");
-      DeadSymbols.insert(Entry.first);
-    }
-  }
-  DEBUG(dbgs() << LiveSymbols.size() << " symbols Live, and "
-               << DeadSymbols.size() << " symbols Dead \n");
-  NumDeadSymbols += DeadSymbols.size();
-  NumLiveSymbols += LiveSymbols.size();
-  return DeadSymbols;
+  Index.setWithGlobalValueDeadStripping();
+
+  unsigned DeadSymbols = Index.size() - LiveSymbols;
+  DEBUG(dbgs() << LiveSymbols << " symbols Live, and " << DeadSymbols
+               << " symbols Dead \n");
+  NumDeadSymbols += DeadSymbols;
+  NumLiveSymbols += LiveSymbols;
 }
 
 /// Compute the set of summaries needed for a ThinLTO backend compilation of
 /// \p ModulePath.
 void llvm::gatherImportedSummariesForModule(
     StringRef ModulePath,
     const StringMap<GVSummaryMapTy> &ModuleToDefinedGVSummaries,
     const FunctionImporter::ImportMapTy &ImportList,