Index: lib/Bitcode/Writer/BitcodeWriter.cpp
===================================================================
--- lib/Bitcode/Writer/BitcodeWriter.cpp
+++ lib/Bitcode/Writer/BitcodeWriter.cpp
@@ -363,6 +363,32 @@
     }
   }
 
+  /// Calls the callback for each entry in the modulePaths StringMap that
+  /// should be written to the module path string table. This hides the details
+  /// of whether they are being pulled from the entire index or just those in a
+  /// provided ModuleToSummariesForIndex map.
+  void
+  forEachModule(std::function<
+                void(const StringMapEntry<std::pair<uint64_t, ModuleHash>> &)>
+                    Callback) {
+    if (ModuleToSummariesForIndex) {
+      for (const auto &M : *ModuleToSummariesForIndex) {
+        const auto &MPI = Index.modulePaths().find(M.first);
+        if (MPI == Index.modulePaths().end()) {
+          // This should only happen if the bitcode file was empty, in which
+          // case we shouldn't be importing (the ModuleToSummariesForIndex
+          // would only include the module we are writing and index for).
+          assert(ModuleToSummariesForIndex->size() == 1);
+          continue;
+        }
+        Callback(*MPI);
+      }
+    } else {
+      for (const auto &MPSE : Index.modulePaths())
+        Callback(MPSE);
+    }
+  }
+
   /// Main entry point for writing a combined index to bitcode.
   void write();
 
@@ -370,14 +396,6 @@
   void writeModStrings();
   void writeCombinedGlobalValueSummary();
 
-  /// Indicates whether the provided \p ModulePath should be written into
-  /// the module string table, e.g. if full index written or if it is in
-  /// the provided subset.
-  bool doIncludeModule(StringRef ModulePath) {
-    return !ModuleToSummariesForIndex ||
-           ModuleToSummariesForIndex->count(ModulePath);
-  }
-
   Optional<unsigned> getValueId(GlobalValue::GUID ValGUID) {
     auto VMI = GUIDToValueIdMap.find(ValGUID);
     if (VMI == GUIDToValueIdMap.end())
@@ -3149,41 +3167,41 @@
   unsigned AbbrevHash = Stream.EmitAbbrev(std::move(Abbv));
 
   SmallVector<unsigned, 64> Vals;
-  for (const auto &MPSE : Index.modulePaths()) {
-    if (!doIncludeModule(MPSE.getKey()))
-      continue;
-    StringEncoding Bits =
-        getStringEncoding(MPSE.getKey().data(), MPSE.getKey().size());
-    unsigned AbbrevToUse = Abbrev8Bit;
-    if (Bits == SE_Char6)
-      AbbrevToUse = Abbrev6Bit;
-    else if (Bits == SE_Fixed7)
-      AbbrevToUse = Abbrev7Bit;
-
-    Vals.push_back(MPSE.getValue().first);
-
-    for (const auto P : MPSE.getKey())
-      Vals.push_back((unsigned char)P);
-
-    // Emit the finished record.
-    Stream.EmitRecord(bitc::MST_CODE_ENTRY, Vals, AbbrevToUse);
-
-    Vals.clear();
-    // Emit an optional hash for the module now
-    auto &Hash = MPSE.getValue().second;
-    bool AllZero = true; // Detect if the hash is empty, and do not generate it
-    for (auto Val : Hash) {
-      if (Val)
-        AllZero = false;
-      Vals.push_back(Val);
-    }
-    if (!AllZero) {
-      // Emit the hash record.
-      Stream.EmitRecord(bitc::MST_CODE_HASH, Vals, AbbrevHash);
-    }
+  forEachModule(
+      [&](const StringMapEntry<std::pair<uint64_t, ModuleHash>> &MPSE) {
+        StringEncoding Bits =
+            getStringEncoding(MPSE.getKey().data(), MPSE.getKey().size());
+        unsigned AbbrevToUse = Abbrev8Bit;
+        if (Bits == SE_Char6)
+          AbbrevToUse = Abbrev6Bit;
+        else if (Bits == SE_Fixed7)
+          AbbrevToUse = Abbrev7Bit;
+
+        Vals.push_back(MPSE.getValue().first);
+
+        for (const auto P : MPSE.getKey())
+          Vals.push_back((unsigned char)P);
+
+        // Emit the finished record.
+        Stream.EmitRecord(bitc::MST_CODE_ENTRY, Vals, AbbrevToUse);
+
+        Vals.clear();
+        // Emit an optional hash for the module now
+        auto &Hash = MPSE.getValue().second;
+        bool AllZero =
+            true; // Detect if the hash is empty, and do not generate it
+        for (auto Val : Hash) {
+          if (Val)
+            AllZero = false;
+          Vals.push_back(Val);
+        }
+        if (!AllZero) {
+          // Emit the hash record.
+          Stream.EmitRecord(bitc::MST_CODE_HASH, Vals, AbbrevHash);
+        }
 
-    Vals.clear();
-  }
+        Vals.clear();
+      });
   Stream.ExitBlock();
 }