Index: lib/Bitcode/Writer/BitcodeWriter.cpp
===================================================================
--- lib/Bitcode/Writer/BitcodeWriter.cpp
+++ lib/Bitcode/Writer/BitcodeWriter.cpp
@@ -382,14 +382,10 @@
     const auto &VMI = GUIDToValueIdMap.find(ValGUID);
     return VMI != GUIDToValueIdMap.end();
   }
-  void assignValueId(GlobalValue::GUID ValGUID) {
-    unsigned &ValueId = GUIDToValueIdMap[ValGUID];
-    if (ValueId == 0)
-      ValueId = ++GlobalValueId;
-  }
-  unsigned getValueId(GlobalValue::GUID ValGUID) {
+  Optional<unsigned> getValueId(GlobalValue::GUID ValGUID) {
     auto VMI = GUIDToValueIdMap.find(ValGUID);
-    assert(VMI != GUIDToValueIdMap.end());
+    if (VMI == GUIDToValueIdMap.end())
+      return None;
     return VMI->second;
   }
   std::map<GlobalValue::GUID, unsigned> &valueIds() { return GUIDToValueIdMap; }
@@ -3413,12 +3409,6 @@
   Stream.EnterSubblock(bitc::GLOBALVAL_SUMMARY_BLOCK_ID, 3);
   Stream.EmitRecord(bitc::FS_VERSION, ArrayRef<uint64_t>{INDEX_VERSION});
 
-  // Create value IDs for undefined references.
-  forEachSummary([&](GVInfo I) {
-    for (auto &RI : I.second->refs())
-      assignValueId(RI.getGUID());
-  });
-
   for (const auto &GVI : valueIds()) {
     Stream.EmitRecord(bitc::FS_VALUE_GUID,
                       ArrayRef<uint64_t>{GVI.second, GVI.first});
@@ -3492,9 +3482,9 @@
     GlobalValueSummary *S = I.second;
     assert(S);
 
-    assert(hasValueId(I.first));
-    unsigned ValueId = getValueId(I.first);
-    SummaryToValueIdMap[S] = ValueId;
+    auto ValueId = getValueId(I.first);
+    assert(ValueId);
+    SummaryToValueIdMap[S] = *ValueId;
 
     if (auto *AS = dyn_cast<AliasSummary>(S)) {
       // Will process aliases as a post-pass because the reader wants all
@@ -3504,11 +3494,14 @@
     }
 
     if (auto *VS = dyn_cast<GlobalVarSummary>(S)) {
-      NameVals.push_back(ValueId);
+      NameVals.push_back(*ValueId);
       NameVals.push_back(Index.getModuleId(VS->modulePath()));
       NameVals.push_back(getEncodedGVSummaryFlags(VS->flags()));
       for (auto &RI : VS->refs()) {
-        NameVals.push_back(getValueId(RI.getGUID()));
+        auto RefValueId = getValueId(RI.getGUID());
+        if (!RefValueId)
+          continue;
+        NameVals.push_back(*RefValueId);
       }
 
       // Emit the finished record.
@@ -3522,15 +3515,22 @@
     auto *FS = cast<FunctionSummary>(S);
     writeFunctionTypeMetadataRecords(Stream, FS);
 
-    NameVals.push_back(ValueId);
+    NameVals.push_back(*ValueId);
     NameVals.push_back(Index.getModuleId(FS->modulePath()));
     NameVals.push_back(getEncodedGVSummaryFlags(FS->flags()));
     NameVals.push_back(FS->instCount());
-    NameVals.push_back(FS->refs().size());
+    // Fill in below
+    NameVals.push_back(0);
 
+    unsigned Count = 0;
     for (auto &RI : FS->refs()) {
-      NameVals.push_back(getValueId(RI.getGUID()));
+      auto RefValueId = getValueId(RI.getGUID());
+      if (!RefValueId)
+        continue;
+      NameVals.push_back(*RefValueId);
+      Count++;
     }
+    NameVals[4] = Count;
 
     bool HasProfileData = false;
     for (auto &EI : FS->calls()) {
@@ -3543,15 +3543,19 @@
       // If this GUID doesn't have a value id, it doesn't have a function
       // summary and we don't need to record any calls to it.
       GlobalValue::GUID GUID = EI.first.getGUID();
-      if (!hasValueId(GUID)) {
+      auto CallValueId = getValueId(GUID);
+      if (!CallValueId) {
         // For SamplePGO, the indirect call targets for local functions will
         // have its original name annotated in profile. We try to find the
         // corresponding PGOFuncName as the GUID.
         GUID = Index.getGUIDFromOriginalID(GUID);
-        if (GUID == 0 || !hasValueId(GUID))
+        if (GUID == 0)
+          continue;
+        CallValueId = getValueId(GUID);
+        if (!CallValueId)
           continue;
       }
-      NameVals.push_back(getValueId(GUID));
+      NameVals.push_back(*CallValueId);
       if (HasProfileData)
         NameVals.push_back(static_cast<uint8_t>(EI.second.Hotness));
     }