Index: lib/Bitcode/Reader/BitcodeReader.cpp
===================================================================
--- lib/Bitcode/Reader/BitcodeReader.cpp
+++ lib/Bitcode/Reader/BitcodeReader.cpp
@@ -2472,14 +2472,17 @@
         return error("Invalid record");
 
       IsDistinct =
-          Record[0] || Record[8]; // All definitions should be distinct.
+          (Record[0] & 1) || Record[8]; // All definitions should be distinct.
+      bool IsVersion3 = (Record[0] & (1 << 1)) == (1 << 1);
+      if (IsVersion3 && Record.size() != 19)
+        return error("Invalid record");
       // Version 1 has a Function as Record[15].
       // Version 2 has removed Record[15].
       // Version 3 has the Unit as Record[15].
       Metadata *CUorFn = getMDOrNull(Record[15]);
       unsigned Offset = Record.size() == 19 ? 1 : 0;
-      bool HasFn = Offset && dyn_cast_or_null<ConstantAsMetadata>(CUorFn);
-      bool HasCU = Offset && !HasFn;
+      bool HasFn = Offset && !IsVersion3;
+      bool HasCU = IsVersion3;
       DISubprogram *SP = GET_OR_DISTINCT(
           DISubprogram,
           (Context, getDITypeRefOrNull(Record[1]), getMDString(Record[2]),
@@ -2493,7 +2496,7 @@
 
       // Upgrade sp->function mapping to function->sp mapping.
       if (HasFn) {
-        if (auto *CMD = dyn_cast<ConstantAsMetadata>(CUorFn))
+        if (auto *CMD = dyn_cast_or_null<ConstantAsMetadata>(CUorFn))
           if (auto *F = dyn_cast<Function>(CMD->getValue())) {
             if (F->isMaterializable())
               // Defer until materialized; unmaterialized functions may not have
Index: lib/Bitcode/Writer/BitcodeWriter.cpp
===================================================================
--- lib/Bitcode/Writer/BitcodeWriter.cpp
+++ lib/Bitcode/Writer/BitcodeWriter.cpp
@@ -1353,7 +1353,8 @@
 void ModuleBitcodeWriter::writeDISubprogram(const DISubprogram *N,
                                             SmallVectorImpl<uint64_t> &Record,
                                             unsigned Abbrev) {
-  Record.push_back(N->isDistinct());
+  uint64_t IsVersion3 = 1 << 1;
+  Record.push_back(N->isDistinct() | IsVersion3);
   Record.push_back(VE.getMetadataOrNullID(N->getScope()));
   Record.push_back(VE.getMetadataOrNullID(N->getRawName()));
   Record.push_back(VE.getMetadataOrNullID(N->getRawLinkageName()));