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
@@ -150,6 +150,11 @@
   /// in the GlobalValueMap. Requires a vector in the case of multiple
   /// COMDAT values of the same name.
   GlobalValueSummaryList SummaryList;
+
+  /// Cached IsDSOLocal result for fast lookup during function importing.
+  /// Value is pre-populated in ModuleIndexSummary::populateDSOLocal() and
+  /// invalidated when SummaryList or any associated summary is updated.
+  Optional<bool> IsDSOLocal;
 };
 
 /// Map from global value GUID to corresponding summary structures. Use a
@@ -1313,6 +1318,9 @@
     // by the non-const *this.
     const_cast<GlobalValueSummaryMapTy::value_type *>(VI.getRef())
         ->second.SummaryList.push_back(std::move(Summary));
+    // Invalidate the cached result.
+    const_cast<GlobalValueSummaryMapTy::value_type *>(VI.getRef())
+        ->second.IsDSOLocal.reset();
   }
 
   /// Add an original name for the value of the given GUID.
@@ -1350,6 +1358,13 @@
     return findSummaryInModule(CalleeInfo, ModuleId);
   }
 
+  /// Find the summary info for global \p GUID in index, or nullptr if not
+  /// found.
+  GlobalValueSummaryInfo *findSummaryInfo(GlobalValue::GUID ValueGUID) {
+    auto I = GlobalValueMap.find(ValueGUID);
+    return I == GlobalValueMap.end() ? nullptr : &(I->second);
+  }
+
   /// Returns the first GlobalValueSummary for \p GV, asserting that there
   /// is only one if \p PerModuleIndex.
   GlobalValueSummary *getGlobalValueSummary(const GlobalValue &GV,
@@ -1518,6 +1533,10 @@
 
   /// Checks if we can import global variable from another module.
   bool canImportGlobalVar(GlobalValueSummary *S, bool AnalyzeRefs) const;
+
+  /// Compute IsDSOLocal for GUIDs in GlobalValueMap and save the result in
+  /// its corresponding GlobalValueSummaryInfo struct.
+  void populateDSOLocal();
 };
 
 /// GraphTraits definition to build SCC for the index
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
@@ -6979,5 +6979,8 @@
     return errorCodeToError(FileOrErr.getError());
   if (IgnoreEmptyThinLTOIndexFile && !(*FileOrErr)->getBufferSize())
     return nullptr;
-  return getModuleSummaryIndex(**FileOrErr);
+  auto IndexOrErr = getModuleSummaryIndex(**FileOrErr);
+  if (IndexOrErr)
+    IndexOrErr.get()->populateDSOLocal();
+  return IndexOrErr;
 }
diff --git a/llvm/lib/IR/ModuleSummaryIndex.cpp b/llvm/lib/IR/ModuleSummaryIndex.cpp
--- a/llvm/lib/IR/ModuleSummaryIndex.cpp
+++ b/llvm/lib/IR/ModuleSummaryIndex.cpp
@@ -53,12 +53,9 @@
 }
 
 bool ValueInfo::isDSOLocal() const {
-  // Need to check all summaries are local in case of hash collisions.
-  return getSummaryList().size() &&
-         llvm::all_of(getSummaryList(),
-                      [](const std::unique_ptr<GlobalValueSummary> &Summary) {
-                        return Summary->isDSOLocal();
-                      });
+  // Get pre-populated IsDSOLocal result.
+  assert(getRef()->second.IsDSOLocal && "IsDSOLocal was not populated.");
+  return getRef()->second.IsDSOLocal.getValue();
 }
 
 bool ValueInfo::canAutoHide() const {
@@ -340,6 +337,18 @@
   }
 }
 
+void ModuleSummaryIndex::populateDSOLocal() {
+  for (auto &I : GlobalValueMap) {
+    auto &GI = I.second;
+    GI.IsDSOLocal =
+        !GI.IsDSOLocal && GI.SummaryList.size() &&
+        llvm::all_of(GI.SummaryList,
+                     [](const std::unique_ptr<GlobalValueSummary> &Summary) {
+                       return Summary->isDSOLocal();
+                     });
+  }
+}
+
 namespace {
 struct Attributes {
   void add(const Twine &Name, const Twine &Value,
diff --git a/llvm/lib/LTO/LTO.cpp b/llvm/lib/LTO/LTO.cpp
--- a/llvm/lib/LTO/LTO.cpp
+++ b/llvm/lib/LTO/LTO.cpp
@@ -890,6 +890,9 @@
         if (auto S = ThinLTO.CombinedIndex.findSummaryInModule(
                 GUID, BM.getModuleIdentifier())) {
           S->setDSOLocal(true);
+          // Invalidate cached result.
+          if (auto *I = ThinLTO.CombinedIndex.findSummaryInfo(GUID))
+            I->IsDSOLocal.reset();
         }
       }
     }
@@ -1480,6 +1483,8 @@
 
   generateParamAccessSummary(ThinLTO.CombinedIndex);
 
+  ThinLTO.CombinedIndex.populateDSOLocal();
+
   std::unique_ptr<ThinBackendProc> BackendProc =
       ThinLTO.Backend(Conf, ThinLTO.CombinedIndex, ModuleToDefinedGVSummaries,
                       AddStream, Cache);
diff --git a/llvm/lib/LTO/ThinLTOCodeGenerator.cpp b/llvm/lib/LTO/ThinLTOCodeGenerator.cpp
--- a/llvm/lib/LTO/ThinLTOCodeGenerator.cpp
+++ b/llvm/lib/LTO/ThinLTOCodeGenerator.cpp
@@ -607,6 +607,7 @@
       return nullptr;
     }
   }
+  CombinedIndex->populateDSOLocal();
   return CombinedIndex;
 }