diff --git a/include/llvm/Bitcode/BitcodeReader.h b/include/llvm/Bitcode/BitcodeReader.h
index ce8bdd9..0d7cc14 100644
--- a/include/llvm/Bitcode/BitcodeReader.h
+++ b/include/llvm/Bitcode/BitcodeReader.h
@@ -51,6 +51,7 @@
   struct BitcodeLTOInfo {
     bool IsThinLTO;
     bool HasSummary;
+    bool EnableSplitLTOUnit;
   };
 
   /// Represents a module in a bitcode file.
diff --git a/include/llvm/IR/ModuleSummaryIndex.h b/include/llvm/IR/ModuleSummaryIndex.h
index 6653795..a1acee4 100644
--- a/include/llvm/IR/ModuleSummaryIndex.h
+++ b/include/llvm/IR/ModuleSummaryIndex.h
@@ -831,6 +831,13 @@
   /// union.
   bool HaveGVs;
 
+  // True if the index was created for a module compiled with -fsplit-lto-unit.
+  bool EnableSplitLTOUnit;
+
+  // True if some of the modules were compiled with -fsplit-lto-unit and
+  // some were not. Set when the combined index is created during the thin link.
+  bool PartiallySplitLTOUnits = false;
+
   std::set<std::string> CfiFunctionDefs;
   std::set<std::string> CfiFunctionDecls;
 
@@ -850,7 +857,9 @@
 
 public:
   // See HaveGVs variable comment.
-  ModuleSummaryIndex(bool HaveGVs) : HaveGVs(HaveGVs), Saver(Alloc) {}
+  ModuleSummaryIndex(bool HaveGVs, bool EnableSplitLTOUnit = false)
+      : HaveGVs(HaveGVs), EnableSplitLTOUnit(EnableSplitLTOUnit), Saver(Alloc) {
+  }
 
   bool haveGVs() const { return HaveGVs; }
 
@@ -940,6 +949,12 @@
     SkipModuleByDistributedBackend = true;
   }
 
+  bool enableSplitLTOUnit() const { return EnableSplitLTOUnit; }
+  void setEnableSplitLTOUnit() { EnableSplitLTOUnit = true; }
+
+  bool partiallySplitLTOUnits() const { return PartiallySplitLTOUnits; }
+  void setPartiallySplitLTOUnits() { PartiallySplitLTOUnits = true; }
+
   bool isGlobalValueLive(const GlobalValueSummary *GVS) const {
     return !WithGlobalValueDeadStripping || GVS->isLive();
   }
diff --git a/include/llvm/LTO/LTO.h b/include/llvm/LTO/LTO.h
index 1539087..534d9b6 100644
--- a/include/llvm/LTO/LTO.h
+++ b/include/llvm/LTO/LTO.h
@@ -400,6 +400,9 @@
   Error runThinLTO(AddStreamFn AddStream, NativeObjectCache Cache);
 
   mutable bool CalledGetMaxTasks = false;
+
+  // Use Optional to distinguish false from not yet initialized.
+  Optional<bool> EnableSplitLTOUnit;
 };
 
 /// The resolution for a symbol. The linker must provide a SymbolResolution for
diff --git a/lib/Analysis/ModuleSummaryAnalysis.cpp b/lib/Analysis/ModuleSummaryAnalysis.cpp
index 6bda1d1..87f76d4 100644
--- a/lib/Analysis/ModuleSummaryAnalysis.cpp
+++ b/lib/Analysis/ModuleSummaryAnalysis.cpp
@@ -457,7 +457,11 @@
     std::function<BlockFrequencyInfo *(const Function &F)> GetBFICallback,
     ProfileSummaryInfo *PSI) {
   assert(PSI);
-  ModuleSummaryIndex Index(/*HaveGVs=*/true);
+  bool EnableSplitLTOUnit = false;
+  if (auto *MD = mdconst::extract_or_null<ConstantInt>(
+          M.getModuleFlag("EnableSplitLTOUnit")))
+    EnableSplitLTOUnit = MD->getZExtValue();
+  ModuleSummaryIndex Index(/*HaveGVs=*/true, EnableSplitLTOUnit);
 
   // Identify the local values in the llvm.used and llvm.compiler.used sets,
   // which should not be exported as they would then require renaming and
diff --git a/lib/Bitcode/Reader/BitcodeReader.cpp b/lib/Bitcode/Reader/BitcodeReader.cpp
index 2f3d2f3..fe051e7 100644
--- a/lib/Bitcode/Reader/BitcodeReader.cpp
+++ b/lib/Bitcode/Reader/BitcodeReader.cpp
@@ -5294,18 +5294,30 @@
       break;
     case bitc::FS_FLAGS: {  // [flags]
       uint64_t Flags = Record[0];
-      // Scan flags (set only on the combined index).
-      assert(Flags <= 0x3 && "Unexpected bits in flag");
+      // Scan flags.
+      assert(Flags <= 0x1f && "Unexpected bits in flag");
 
       // 1 bit: WithGlobalValueDeadStripping flag.
+      // Set on combined index only.
       if (Flags & 0x1)
         TheIndex.setWithGlobalValueDeadStripping();
       // 1 bit: SkipModuleByDistributedBackend flag.
+      // Set on combined index only.
       if (Flags & 0x2)
         TheIndex.setSkipModuleByDistributedBackend();
       // 1 bit: HasSyntheticEntryCounts flag.
+      // Set on combined index only.
       if (Flags & 0x4)
         TheIndex.setHasSyntheticEntryCounts();
+      // 1 bit: DisableSplitLTOUnit flag.
+      // Set on per module indexes. It is up to the client to validate
+      // the consistency of this flag across modules being linked.
+      if (Flags & 0x8)
+        TheIndex.setEnableSplitLTOUnit();
+      // 1 bit: PartiallySplitLTOUnits flag.
+      // Set on combined index only.
+      if (Flags & 0x10)
+        TheIndex.setPartiallySplitLTOUnits();
       break;
     }
     case bitc::FS_VALUE_GUID: { // [valueid, refguid]
@@ -5917,6 +5929,46 @@
   return std::move(Index);
 }
 
+static Expected<bool> getEnableSplitLTOUnitFlag(BitstreamCursor &Stream,
+                                                unsigned ID) {
+  if (Stream.EnterSubBlock(ID))
+    return error("Invalid record");
+  SmallVector<uint64_t, 64> Record;
+
+  while (true) {
+    BitstreamEntry Entry = Stream.advanceSkippingSubblocks();
+
+    switch (Entry.Kind) {
+    case BitstreamEntry::SubBlock: // Handled for us already.
+    case BitstreamEntry::Error:
+      return error("Malformed block");
+    case BitstreamEntry::EndBlock:
+      // If no flags record found, conservatively return true to mimic
+      // behavior before this flag was added.
+      return true;
+    case BitstreamEntry::Record:
+      // The interesting case.
+      break;
+    }
+
+    // Look for the FS_FLAGS record.
+    Record.clear();
+    auto BitCode = Stream.readRecord(Entry.ID, Record);
+    switch (BitCode) {
+    default: // Default behavior: ignore.
+      break;
+    case bitc::FS_FLAGS: { // [flags]
+      uint64_t Flags = Record[0];
+      // Scan flags.
+      assert(Flags <= 0x1f && "Unexpected bits in flag");
+
+      return Flags & 0x8;
+    }
+    }
+  }
+  llvm_unreachable("Exit infinite loop");
+}
+
 // Check if the given bitcode buffer contains a global value summary block.
 Expected<BitcodeLTOInfo> BitcodeModule::getLTOInfo() {
   BitstreamCursor Stream(Buffer);
@@ -5932,14 +5984,27 @@
     case BitstreamEntry::Error:
       return error("Malformed block");
     case BitstreamEntry::EndBlock:
-      return BitcodeLTOInfo{/*IsThinLTO=*/false, /*HasSummary=*/false};
+      return BitcodeLTOInfo{/*IsThinLTO=*/false, /*HasSummary=*/false,
+                            /*EnableSplitLTOUnit=*/false};
 
     case BitstreamEntry::SubBlock:
-      if (Entry.ID == bitc::GLOBALVAL_SUMMARY_BLOCK_ID)
-        return BitcodeLTOInfo{/*IsThinLTO=*/true, /*HasSummary=*/true};
+      if (Entry.ID == bitc::GLOBALVAL_SUMMARY_BLOCK_ID) {
+        Expected<bool> EnableSplitLTOUnit =
+            getEnableSplitLTOUnitFlag(Stream, Entry.ID);
+        if (!EnableSplitLTOUnit)
+          return EnableSplitLTOUnit.takeError();
+        return BitcodeLTOInfo{/*IsThinLTO=*/true, /*HasSummary=*/true,
+                              *EnableSplitLTOUnit};
+      }
 
-      if (Entry.ID == bitc::FULL_LTO_GLOBALVAL_SUMMARY_BLOCK_ID)
-        return BitcodeLTOInfo{/*IsThinLTO=*/false, /*HasSummary=*/true};
+      if (Entry.ID == bitc::FULL_LTO_GLOBALVAL_SUMMARY_BLOCK_ID) {
+        Expected<bool> EnableSplitLTOUnit =
+            getEnableSplitLTOUnitFlag(Stream, Entry.ID);
+        if (!EnableSplitLTOUnit)
+          return EnableSplitLTOUnit.takeError();
+        return BitcodeLTOInfo{/*IsThinLTO=*/false, /*HasSummary=*/true,
+                              *EnableSplitLTOUnit};
+      }
 
       // Ignore other sub-blocks.
       if (Stream.SkipBlock())
diff --git a/lib/Bitcode/Writer/BitcodeWriter.cpp b/lib/Bitcode/Writer/BitcodeWriter.cpp
index 68d79ed..ba4f932 100644
--- a/lib/Bitcode/Writer/BitcodeWriter.cpp
+++ b/lib/Bitcode/Writer/BitcodeWriter.cpp
@@ -3618,6 +3618,13 @@
 
   Stream.EmitRecord(bitc::FS_VERSION, ArrayRef<uint64_t>{INDEX_VERSION});
 
+  // Write the index flags.
+  uint64_t Flags = 0;
+  // Bits 1-3 are set only in the combined index, skip them.
+  if (Index->enableSplitLTOUnit())
+    Flags |= 0x8;
+  Stream.EmitRecord(bitc::FS_FLAGS, ArrayRef<uint64_t>{Flags});
+
   if (Index->begin() == Index->end()) {
     Stream.ExitBlock();
     return;
@@ -3734,6 +3741,10 @@
     Flags |= 0x2;
   if (Index.hasSyntheticEntryCounts())
     Flags |= 0x4;
+  if (Index.enableSplitLTOUnit())
+    Flags |= 0x8;
+  if (Index.partiallySplitLTOUnits())
+    Flags |= 0x10;
   Stream.EmitRecord(bitc::FS_FLAGS, ArrayRef<uint64_t>{Flags});
 
   for (const auto &GVI : valueIds()) {
diff --git a/lib/LTO/LTO.cpp b/lib/LTO/LTO.cpp
index f736ef8..3a95506 100644
--- a/lib/LTO/LTO.cpp
+++ b/lib/LTO/LTO.cpp
@@ -546,6 +546,15 @@
   if (!LTOInfo)
     return LTOInfo.takeError();
 
+  if (EnableSplitLTOUnit.hasValue()) {
+    // If only some modules were split, flag this in the index so that
+    // we can skip or error on optimizations that need consistently split
+    // modules (whole program devirt and lower type tests).
+    if (EnableSplitLTOUnit.getValue() != LTOInfo->EnableSplitLTOUnit)
+      ThinLTO.CombinedIndex.setPartiallySplitLTOUnits();
+  } else
+    EnableSplitLTOUnit = LTOInfo->EnableSplitLTOUnit;
+
   BitcodeModule BM = Input.Mods[ModI];
   auto ModSyms = Input.module_symbols(ModI);
   addModuleToGlobalRes(ModSyms, {ResI, ResE},
diff --git a/lib/Transforms/IPO/LowerTypeTests.cpp b/lib/Transforms/IPO/LowerTypeTests.cpp
index e4dcd4d..87c65db 100644
--- a/lib/Transforms/IPO/LowerTypeTests.cpp
+++ b/lib/Transforms/IPO/LowerTypeTests.cpp
@@ -1702,6 +1702,13 @@
       !ExportSummary && !ImportSummary)
     return false;
 
+  // If only some of the modules were split, we cannot correctly handle
+  // code that contains type tests.
+  if (TypeTestFunc && !TypeTestFunc->use_empty() &&
+      ((ExportSummary && ExportSummary->partiallySplitLTOUnits()) ||
+       (ImportSummary && ImportSummary->partiallySplitLTOUnits())))
+    report_fatal_error("inconsistent LTO Unit splitting with llvm.type.test");
+
   if (ImportSummary) {
     if (TypeTestFunc) {
       for (auto UI = TypeTestFunc->use_begin(), UE = TypeTestFunc->use_end();
diff --git a/lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp b/lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp
index a5382c4..510ecb5 100644
--- a/lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp
+++ b/lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp
@@ -418,8 +418,18 @@
   }
 }
 
-// Returns whether this module needs to be split because it uses type metadata.
+// Returns whether this module needs to be split because splitting is
+// enabled and it uses type metadata.
 bool requiresSplit(Module &M) {
+  // First check if the LTO Unit splitting has been enabled.
+  bool EnableSplitLTOUnit = false;
+  if (auto *MD = mdconst::extract_or_null<ConstantInt>(
+          M.getModuleFlag("EnableSplitLTOUnit")))
+    EnableSplitLTOUnit = MD->getZExtValue();
+  if (!EnableSplitLTOUnit)
+    return false;
+
+  // Module only needs to be split if it contains type metadata.
   for (auto &GO : M.global_objects()) {
     if (GO.hasMetadata(LLVMContext::MD_type))
       return true;
@@ -431,7 +441,7 @@
 void writeThinLTOBitcode(raw_ostream &OS, raw_ostream *ThinLinkOS,
                          function_ref<AAResults &(Function &)> AARGetter,
                          Module &M, const ModuleSummaryIndex *Index) {
-  // See if this module has any type metadata. If so, we need to split it.
+  // Split module if splitting is enabled and it contains any type metadata.
   if (requiresSplit(M))
     return splitAndWriteThinLTOBitcode(OS, ThinLinkOS, AARGetter, M);
 
diff --git a/lib/Transforms/IPO/WholeProgramDevirt.cpp b/lib/Transforms/IPO/WholeProgramDevirt.cpp
index 37905da..48bd0cd 100644
--- a/lib/Transforms/IPO/WholeProgramDevirt.cpp
+++ b/lib/Transforms/IPO/WholeProgramDevirt.cpp
@@ -1563,6 +1563,17 @@
       M.getFunction(Intrinsic::getName(Intrinsic::type_checked_load));
   Function *AssumeFunc = M.getFunction(Intrinsic::getName(Intrinsic::assume));
 
+  // If only some of the modules were split, we cannot correctly handle
+  // code that contains type tests or type checked loads.
+  if ((ExportSummary && ExportSummary->partiallySplitLTOUnits()) ||
+      (ImportSummary && ImportSummary->partiallySplitLTOUnits())) {
+    if ((TypeTestFunc && !TypeTestFunc->use_empty()) ||
+        (TypeCheckedLoadFunc && !TypeCheckedLoadFunc->use_empty()))
+      report_fatal_error("inconsistent LTO Unit splitting with llvm.type.test "
+                         "or llvm.type.checked.load");
+    return false;
+  }
+
   // Normally if there are no users of the devirtualization intrinsics in the
   // module, this pass has nothing to do. But if we are exporting, we also need
   // to handle any users that appear only in the function summaries.
diff --git a/test/Bitcode/thinlto-alias.ll b/test/Bitcode/thinlto-alias.ll
index 835d720..f3896d2 100644
--- a/test/Bitcode/thinlto-alias.ll
+++ b/test/Bitcode/thinlto-alias.ll
@@ -18,6 +18,7 @@
 ; CHECK-NEXT: <FUNCTION op0=4 op1=7
 ; CHECK:       <GLOBALVAL_SUMMARY_BLOCK
 ; CHECK-NEXT:    <VERSION
+; CHECK-NEXT:    <FLAGS
 ; See if the call to func is registered.
 ; The value id 1 matches the second FUNCTION record above.
 ; CHECK-NEXT:    <PERMODULE {{.*}} op6=1/>
diff --git a/test/Bitcode/thinlto-alias2.ll b/test/Bitcode/thinlto-alias2.ll
index 3d68e3f..8b04ee7 100644
--- a/test/Bitcode/thinlto-alias2.ll
+++ b/test/Bitcode/thinlto-alias2.ll
@@ -4,6 +4,7 @@
 
 ; CHECK:       <GLOBALVAL_SUMMARY_BLOCK
 ; CHECK-NEXT:    <VERSION
+; CHECK-NEXT:    <FLAGS
 ; CHECK-NEXT:    <PERMODULE {{.*}} op4=0 op5=0 op6=[[ALIASID:[0-9]+]]/>
 ; CHECK-NEXT:    <PERMODULE {{.*}} op0=[[ALIASEEID:[0-9]+]]
 ; CHECK-NEXT:    <ALIAS {{.*}} op0=[[ALIASID]] {{.*}} op2=[[ALIASEEID]]/>
diff --git a/test/Bitcode/thinlto-function-summary-callgraph-cast.ll b/test/Bitcode/thinlto-function-summary-callgraph-cast.ll
index 7964440..d4b4d54 100644
--- a/test/Bitcode/thinlto-function-summary-callgraph-cast.ll
+++ b/test/Bitcode/thinlto-function-summary-callgraph-cast.ll
@@ -5,6 +5,7 @@
 
 ; CHECK:       <GLOBALVAL_SUMMARY_BLOCK
 ; CHECK-NEXT:    <VERSION
+; CHECK-NEXT:    <FLAGS
 ; "op7" is a call to "callee" function.
 ; CHECK-NEXT:    <PERMODULE {{.*}} op8=3 op9=[[ALIASID:[0-9]+]]/>
 ; "another_caller" has only references but no calls.
diff --git a/test/Bitcode/thinlto-function-summary-callgraph-pgo.ll b/test/Bitcode/thinlto-function-summary-callgraph-pgo.ll
index e332224..b9613f7 100644
--- a/test/Bitcode/thinlto-function-summary-callgraph-pgo.ll
+++ b/test/Bitcode/thinlto-function-summary-callgraph-pgo.ll
@@ -16,6 +16,7 @@
 ; CHECK-NEXT: <FUNCTION op0=4 op1=4
 ; CHECK:       <GLOBALVAL_SUMMARY_BLOCK
 ; CHECK-NEXT:    <VERSION
+; CHECK-NEXT:    <FLAGS
 ; See if the call to func is registered, using the expected hotness type.
 ; CHECK-NEXT:    <PERMODULE_PROFILE {{.*}} op6=1 op7=2/>
 ; CHECK-NEXT:  </GLOBALVAL_SUMMARY_BLOCK>
diff --git a/test/Bitcode/thinlto-function-summary-callgraph-profile-summary.ll b/test/Bitcode/thinlto-function-summary-callgraph-profile-summary.ll
index 31c99c18..0cd1098 100644
--- a/test/Bitcode/thinlto-function-summary-callgraph-profile-summary.ll
+++ b/test/Bitcode/thinlto-function-summary-callgraph-profile-summary.ll
@@ -46,6 +46,7 @@
 ; CHECK-NEXT: <FUNCTION op0=42 op1=5
 ; CHECK-LABEL:       <GLOBALVAL_SUMMARY_BLOCK
 ; CHECK-NEXT:    <VERSION
+; CHECK-NEXT:    <FLAGS
 ; CHECK-NEXT:    <VALUE_GUID op0=25 op1=123/>
 ; op4=hot1 op6=cold op8=hot2 op10=hot4 op12=none1 op14=hot3 op16=none2 op18=none3 op20=123
 ; CHECK-NEXT:    <PERMODULE_PROFILE {{.*}} op6=1 op7=3 op8=5 op9=1 op10=2 op11=3 op12=4 op13=1 op14=6 op15=2 op16=3 op17=3 op18=7 op19=2 op20=8 op21=2 op22=25 op23=4/>
diff --git a/test/Bitcode/thinlto-function-summary-callgraph-relbf.ll b/test/Bitcode/thinlto-function-summary-callgraph-relbf.ll
index 6c14465..7c7a6f6 100644
--- a/test/Bitcode/thinlto-function-summary-callgraph-relbf.ll
+++ b/test/Bitcode/thinlto-function-summary-callgraph-relbf.ll
@@ -12,6 +12,7 @@
 ; CHECK-NEXT: <FUNCTION op0=17 op1=4
 ; CHECK:       <GLOBALVAL_SUMMARY_BLOCK
 ; CHECK-NEXT:    <VERSION
+; CHECK-NEXT:    <FLAGS
 ; See if the call to func is registered.
 ; CHECK-NEXT:    <PERMODULE_RELBF {{.*}} op4=1 {{.*}} op8=256
 ; CHECK-NEXT:  </GLOBALVAL_SUMMARY_BLOCK>
diff --git a/test/Bitcode/thinlto-function-summary-callgraph-sample-profile-summary.ll b/test/Bitcode/thinlto-function-summary-callgraph-sample-profile-summary.ll
index d1f980a..8bf65ab 100644
--- a/test/Bitcode/thinlto-function-summary-callgraph-sample-profile-summary.ll
+++ b/test/Bitcode/thinlto-function-summary-callgraph-sample-profile-summary.ll
@@ -29,6 +29,7 @@
 
 ; CHECK-LABEL:       <GLOBALVAL_SUMMARY_BLOCK
 ; CHECK-NEXT:    <VERSION
+; CHECK-NEXT:    <FLAGS
 ; CHECK-NEXT:    <VALUE_GUID op0=26 op1=123/>
 ; op4=none1 op6=hot1 op8=cold1 op10=none2 op12=hot2 op14=cold2 op16=none3 op18=hot3 op20=cold3 op22=123
 ; CHECK-NEXT:    <PERMODULE_PROFILE {{.*}} op6=7 op7=0 op8=1 op9=3 op10=4 op11=1 op12=8 op13=0 op14=2 op15=3 op16=5 op17=1 op18=9 op19=0 op20=3 op21=3 op22=6 op23=1 op24=26 op25=4/>
diff --git a/test/Bitcode/thinlto-function-summary-callgraph.ll b/test/Bitcode/thinlto-function-summary-callgraph.ll
index a605b7e..0969b84 100644
--- a/test/Bitcode/thinlto-function-summary-callgraph.ll
+++ b/test/Bitcode/thinlto-function-summary-callgraph.ll
@@ -17,6 +17,7 @@
 ; CHECK-NEXT: <FUNCTION op0=17 op1=4
 ; CHECK:       <GLOBALVAL_SUMMARY_BLOCK
 ; CHECK-NEXT:    <VERSION
+; CHECK-NEXT:    <FLAGS
 ; See if the call to func is registered
 ; CHECK-NEXT:    <PERMODULE {{.*}} op4=1
 ; CHECK-NEXT:  </GLOBALVAL_SUMMARY_BLOCK>
diff --git a/test/Bitcode/thinlto-function-summary.ll b/test/Bitcode/thinlto-function-summary.ll
index be7e974..67c5037 100644
--- a/test/Bitcode/thinlto-function-summary.ll
+++ b/test/Bitcode/thinlto-function-summary.ll
@@ -19,6 +19,7 @@
 ; BC-NEXT: <ALIAS op0=67 op1=1
 ; BC: <GLOBALVAL_SUMMARY_BLOCK
 ; BC-NEXT: <VERSION
+; BC-NEXT: <FLAGS
 ; BC-NEXT: <PERMODULE {{.*}} op0=1 op1=0
 ; BC-NEXT: <PERMODULE {{.*}} op0=2 op1=0
 ; BC-NEXT: <PERMODULE {{.*}} op0=3 op1=7
diff --git a/test/LTO/Resolution/X86/export-jumptable.ll b/test/LTO/Resolution/X86/export-jumptable.ll
index d6cce85..8ced9d9 100644
--- a/test/LTO/Resolution/X86/export-jumptable.ll
+++ b/test/LTO/Resolution/X86/export-jumptable.ll
@@ -2,7 +2,7 @@
 ; the full LTO object file; any such functions will be referenced by the jump
 ; table.
 
-; RUN: opt -thinlto-bc -o %t %s
+; RUN: opt -thinlto-bc -thinlto-split-lto-unit -o %t %s
 ; RUN: llvm-lto2 run -o %t2 -r %t,f1,p -r %t,f2,p -r %t,_start,px %t -save-temps
 ; RUN: llvm-dis %t2.1.2.internalize.bc -o - | FileCheck %s
 
diff --git a/test/LTO/Resolution/X86/local-def-dllimport.ll b/test/LTO/Resolution/X86/local-def-dllimport.ll
index ddb78fb..c97e4b7 100644
--- a/test/LTO/Resolution/X86/local-def-dllimport.ll
+++ b/test/LTO/Resolution/X86/local-def-dllimport.ll
@@ -1,4 +1,4 @@
-; RUN: opt -thinlto-bc -o %t0.bc %s
+; RUN: opt -thinlto-bc -thinlto-split-lto-unit -o %t0.bc %s
 ; RUN: llvm-lto2 run -r %t0.bc,__imp_f,l \
 ; RUN:               -r %t0.bc,g,p \
 ; RUN:               -r %t0.bc,g,l \
diff --git a/test/LTO/Resolution/X86/lowertypetests.ll b/test/LTO/Resolution/X86/lowertypetests.ll
index b87452c..e5be4b9 100644
--- a/test/LTO/Resolution/X86/lowertypetests.ll
+++ b/test/LTO/Resolution/X86/lowertypetests.ll
@@ -1,4 +1,4 @@
-; RUN: opt -thinlto-bc -o %t %s
+; RUN: opt -thinlto-bc -thinlto-split-lto-unit -o %t %s
 ; RUN: llvm-lto2 run -r %t,f,plx -r %t,g_alias,plx -r %t,foo,lx -r %t,foo,plx -r %t,bar,lx -r %t,bar,plx -o %t1 %t
 ; RUN: llvm-nm %t1.0 | FileCheck --check-prefix=MERGED %s
 ; RUN: llvm-nm %t1.1 | FileCheck %s
diff --git a/test/LTO/Resolution/X86/lto-unit-check.ll b/test/LTO/Resolution/X86/lto-unit-check.ll
new file mode 100644
index 0000000..1736a5b
--- /dev/null
+++ b/test/LTO/Resolution/X86/lto-unit-check.ll
@@ -0,0 +1,55 @@
+; Test to ensure that the Enable Split LTO Unit flag is set properly in the
+; summary, and that we correctly silently handle linking bitcode files with
+; different values of this flag.
+
+; Linking bitcode both with EnableSplitLTOUnit set should work
+; RUN: opt -thinlto-bc -thinlto-split-lto-unit -o %t1 %s
+; RUN: llvm-bcanalyzer -dump %t1 | FileCheck %s --check-prefix=SPLITLTOUNIT
+; RUN: llvm-dis -o - %t1 | FileCheck %s --check-prefix=ENABLESPLITFLAG
+; RUN: opt -thinlto-bc -thinlto-split-lto-unit -o %t2 %s
+; RUN: llvm-bcanalyzer -dump %t2 | FileCheck %s --check-prefix=SPLITLTOUNIT
+; RUN: llvm-dis -o - %t2 | FileCheck %s --check-prefix=ENABLESPLITFLAG
+; RUN: llvm-lto2 run -o %t3 %t1 %t2
+
+; Linking bitcode both without EnableSplitLTOUnit set should work
+; RUN: opt -thinlto-bc -thinlto-split-lto-unit=false -o %t1 %s
+; RUN: llvm-bcanalyzer -dump %t1 | FileCheck %s --check-prefix=NOSPLITLTOUNIT
+; RUN: llvm-dis -o - %t1 | FileCheck %s --check-prefix=NOENABLESPLITFLAG
+; RUN: opt -thinlto-bc -thinlto-split-lto-unit=false -o %t2 %s
+; RUN: llvm-bcanalyzer -dump %t2 | FileCheck %s --check-prefix=NOSPLITLTOUNIT
+; RUN: llvm-dis -o - %t2 | FileCheck %s --check-prefix=NOENABLESPLITFLAG
+; RUN: llvm-lto2 run -o %t3 %t1 %t2
+
+; Linking bitcode with different values of EnableSplitLTOUnit should succeed
+; (silently skipping any optimizations like whole program devirt that rely
+; on all modules being split).
+; RUN: opt -thinlto-bc -thinlto-split-lto-unit -o %t1 %s
+; RUN: llvm-bcanalyzer -dump %t1 | FileCheck %s --check-prefix=SPLITLTOUNIT
+; RUN: llvm-dis -o - %t1 | FileCheck %s --check-prefix=ENABLESPLITFLAG
+; RUN: opt -thinlto-bc -thinlto-split-lto-unit=false -o %t2 %s
+; RUN: llvm-bcanalyzer -dump %t2 | FileCheck %s --check-prefix=NOSPLITLTOUNIT
+; RUN: llvm-dis -o - %t2 | FileCheck %s --check-prefix=NOENABLESPLITFLAG
+; RUN: llvm-lto2 run -o %t3 %t1 %t2
+
+; Linking bitcode with different values of EnableSplitLTOUnit (reverse order)
+; should succeed (silently skipping any optimizations like whole program devirt
+; that rely on all modules being split).
+; RUN: opt -thinlto-bc -thinlto-split-lto-unit=false -o %t1 %s
+; RUN: llvm-bcanalyzer -dump %t1 | FileCheck %s --check-prefix=NOSPLITLTOUNIT
+; RUN: llvm-dis -o - %t1 | FileCheck %s --check-prefix=NOENABLESPLITFLAG
+; RUN: opt -thinlto-bc -thinlto-split-lto-unit -o %t2 %s
+; RUN: llvm-bcanalyzer -dump %t2 | FileCheck %s --check-prefix=SPLITLTOUNIT
+; RUN: llvm-dis -o - %t2 | FileCheck %s --check-prefix=ENABLESPLITFLAG
+; RUN: llvm-lto2 run -o %t3 %t1 %t2
+
+; The flag should be set when splitting is disabled (for backwards compatibility
+; with older bitcode where it was always enabled).
+; SPLITLTOUNIT: <FLAGS op0=8/>
+; NOSPLITLTOUNIT: <FLAGS op0=0/>
+
+; Check that the corresponding module flag is set when expected.
+; ENABLESPLITFLAG: !{i32 1, !"EnableSplitLTOUnit", i32 1}
+; NOENABLESPLITFLAG-NOT: !{i32 1, !"EnableSplitLTOUnit", i32 1}
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
diff --git a/test/ThinLTO/X86/cache-icall.ll b/test/ThinLTO/X86/cache-icall.ll
index 5e64a71..034ff97 100644
--- a/test/ThinLTO/X86/cache-icall.ll
+++ b/test/ThinLTO/X86/cache-icall.ll
@@ -4,8 +4,8 @@
 ; This affects code generated for any users of f(). Make sure that we don't pull a stale object
 ; file for %t.o from the cache.
 
-; RUN: opt -module-hash -module-summary -thinlto-bc %s -o %t.bc
-; RUN: opt -module-hash -module-summary -thinlto-bc %p/Inputs/cache-icall.ll -o %t2.bc
+; RUN: opt -module-hash -module-summary -thinlto-bc -thinlto-split-lto-unit %s -o %t.bc
+; RUN: opt -module-hash -module-summary -thinlto-bc -thinlto-split-lto-unit %p/Inputs/cache-icall.ll -o %t2.bc
 
 ; RUN: rm -Rf %t.cache && mkdir %t.cache
 
diff --git a/test/ThinLTO/X86/cfi-devirt.ll b/test/ThinLTO/X86/cfi-devirt.ll
index 7ade794..45d6960 100644
--- a/test/ThinLTO/X86/cfi-devirt.ll
+++ b/test/ThinLTO/X86/cfi-devirt.ll
@@ -2,7 +2,7 @@
 
 ; Test CFI devirtualization through the thin link and backend.
 
-; RUN: opt -thinlto-bc -o %t.o %s
+; RUN: opt -thinlto-bc -thinlto-split-lto-unit -o %t.o %s
 
 ; Legacy PM
 ; FIXME: Fix machine verifier issues and remove -verify-machineinstrs=0. PR39436.
@@ -44,6 +44,27 @@
 
 ; REMARK: single-impl: devirtualized a call to _ZN1A1nEi
 
+; Next check that we emit an error when trying to LTO link this module
+; containing an llvm.type.checked.load (with a split LTO Unit) with one
+; that does not have a split LTO Unit.
+; RUN: opt -thinlto-bc -o %t2.o %S/Inputs/empty.ll
+; RUN: not llvm-lto2 run %t.o %t2.o -save-temps -pass-remarks=. \
+; RUN:   -verify-machineinstrs=0 \
+; RUN:   -o %t3 \
+; RUN:   -r=%t.o,test,px \
+; RUN:   -r=%t.o,_ZN1A1nEi,p \
+; RUN:   -r=%t.o,_ZN1B1fEi,p \
+; RUN:   -r=%t.o,_ZN1C1fEi,p \
+; RUN:   -r=%t.o,empty,p \
+; RUN:   -r=%t.o,_ZTV1B, \
+; RUN:   -r=%t.o,_ZTV1C, \
+; RUN:   -r=%t.o,_ZN1A1nEi, \
+; RUN:   -r=%t.o,_ZN1B1fEi, \
+; RUN:   -r=%t.o,_ZN1C1fEi, \
+; RUN:   -r=%t.o,_ZTV1B,px \
+; RUN:   -r=%t.o,_ZTV1C,px 2>&1 | FileCheck %s --check-prefix=ERROR
+; ERROR: LLVM ERROR: inconsistent LTO Unit splitting with llvm.type.test or llvm.type.checked.load
+
 target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-grtev4-linux-gnu"
 
diff --git a/test/ThinLTO/X86/cfi-distributed.ll b/test/ThinLTO/X86/cfi-distributed.ll
index 94f3f95..5339228 100644
--- a/test/ThinLTO/X86/cfi-distributed.ll
+++ b/test/ThinLTO/X86/cfi-distributed.ll
@@ -3,8 +3,8 @@
 ; Test to ensure that only referenced type ID records are emitted into
 ; each distributed index file.
 
-; RUN: opt -thinlto-bc -o %t1.o %s
-; RUN: opt -thinlto-bc -o %t2.o %p/Inputs/cfi-distributed.ll
+; RUN: opt -thinlto-bc -thinlto-split-lto-unit -o %t1.o %s
+; RUN: opt -thinlto-bc -thinlto-split-lto-unit -o %t2.o %p/Inputs/cfi-distributed.ll
 
 ; RUN: llvm-lto2 run -thinlto-distributed-indexes %t1.o %t2.o \
 ; RUN:   -o %t3 \
diff --git a/test/ThinLTO/X86/cfi-icall.ll b/test/ThinLTO/X86/cfi-icall.ll
index 1ab184f..42c26f1 100644
--- a/test/ThinLTO/X86/cfi-icall.ll
+++ b/test/ThinLTO/X86/cfi-icall.ll
@@ -1,4 +1,4 @@
-; RUN: opt -thinlto-bc %s -o %t1.bc
+; RUN: opt -thinlto-bc -thinlto-split-lto-unit %s -o %t1.bc
 ; RUN: llvm-lto2 run  -thinlto-distributed-indexes %t1.bc -o %t.out -save-temps \
 ; RUN:   -r %t1.bc,foo,plx \
 ; RUN:   -r %t1.bc,bar,x \
diff --git a/test/ThinLTO/X86/cfi.ll b/test/ThinLTO/X86/cfi.ll
index 9b1bde3..0edddb8 100644
--- a/test/ThinLTO/X86/cfi.ll
+++ b/test/ThinLTO/X86/cfi.ll
@@ -2,7 +2,7 @@
 
 ; Test CFI through the thin link and backend.
 
-; RUN: opt -thinlto-bc -o %t.o %s
+; RUN: opt -thinlto-bc -thinlto-split-lto-unit -o %t.o %s
 
 ; Legacy PM
 ; RUN: llvm-lto2 run -save-temps %t.o \
diff --git a/test/ThinLTO/X86/devirt-after-icp.ll b/test/ThinLTO/X86/devirt-after-icp.ll
index 9872217..fd5dcb7 100644
--- a/test/ThinLTO/X86/devirt-after-icp.ll
+++ b/test/ThinLTO/X86/devirt-after-icp.ll
@@ -42,7 +42,7 @@
 ; will use the same vtable pointer. Without a dominance check, we could
 ; incorrectly devirtualize a->foo() to B::foo();
 
-; RUN: opt -thinlto-bc -o %t.o %s
+; RUN: opt -thinlto-bc -thinlto-split-lto-unit -o %t.o %s
 
 ; Legacy PM
 ; FIXME: Fix machine verifier issues and remove -verify-machineinstrs=0. PR39436.
diff --git a/test/Transforms/ThinLTOBitcodeWriter/circular-reference.ll b/test/Transforms/ThinLTOBitcodeWriter/circular-reference.ll
index eeda793..fb239b0 100644
--- a/test/Transforms/ThinLTOBitcodeWriter/circular-reference.ll
+++ b/test/Transforms/ThinLTOBitcodeWriter/circular-reference.ll
@@ -1,4 +1,4 @@
-; RUN: opt -thinlto-bc -o %t %s
+; RUN: opt -thinlto-bc -thinlto-split-lto-unit -o %t %s
 ; RUN: llvm-modextract -b -n 0 -o - %t | llvm-dis | FileCheck --check-prefix=M0 %s
 ; RUN: llvm-modextract -b -n 1 -o - %t | llvm-dis | FileCheck --check-prefix=M1 %s
 
diff --git a/test/Transforms/ThinLTOBitcodeWriter/comdat.ll b/test/Transforms/ThinLTOBitcodeWriter/comdat.ll
index caea48e..a43fa1c 100644
--- a/test/Transforms/ThinLTOBitcodeWriter/comdat.ll
+++ b/test/Transforms/ThinLTOBitcodeWriter/comdat.ll
@@ -1,4 +1,4 @@
-; RUN: opt -thinlto-bc -o %t %s
+; RUN: opt -thinlto-bc -thinlto-split-lto-unit -o %t %s
 ; RUN: llvm-modextract -n 0 -o - %t | llvm-dis | FileCheck --check-prefix=THIN %s
 ; RUN: llvm-modextract -n 1 -o - %t | llvm-dis | FileCheck --check-prefix=MERGED %s
 
diff --git a/test/Transforms/ThinLTOBitcodeWriter/filter-alias.ll b/test/Transforms/ThinLTOBitcodeWriter/filter-alias.ll
index eb0cbe7..200d494 100644
--- a/test/Transforms/ThinLTOBitcodeWriter/filter-alias.ll
+++ b/test/Transforms/ThinLTOBitcodeWriter/filter-alias.ll
@@ -1,4 +1,4 @@
-; RUN: opt -thinlto-bc -o %t %s
+; RUN: opt -thinlto-bc -thinlto-split-lto-unit -o %t %s
 ; RUN: llvm-modextract -n 0 -o - %t | llvm-dis | FileCheck --check-prefix=CHECK0 %s
 ; RUN: llvm-modextract -n 1 -o - %t | llvm-dis | FileCheck --check-prefix=CHECK1 %s
 ; CHECK0-NOT: @{{.*}}anon{{.*}}=
diff --git a/test/Transforms/ThinLTOBitcodeWriter/function-alias.ll b/test/Transforms/ThinLTOBitcodeWriter/function-alias.ll
index 119b821..a1dbd96 100644
--- a/test/Transforms/ThinLTOBitcodeWriter/function-alias.ll
+++ b/test/Transforms/ThinLTOBitcodeWriter/function-alias.ll
@@ -1,4 +1,4 @@
-; RUN: opt -thinlto-bc -o %t %s
+; RUN: opt -thinlto-bc -thinlto-split-lto-unit -o %t %s
 ; RUN: llvm-modextract -n 1 -o - %t | llvm-dis | FileCheck --check-prefix=CHECK1 %s
 
 target triple = "x86_64-unknown-linux-gnu"
diff --git a/test/Transforms/ThinLTOBitcodeWriter/pr33536.ll b/test/Transforms/ThinLTOBitcodeWriter/pr33536.ll
index 661d073..c405c36 100644
--- a/test/Transforms/ThinLTOBitcodeWriter/pr33536.ll
+++ b/test/Transforms/ThinLTOBitcodeWriter/pr33536.ll
@@ -1,7 +1,7 @@
 ; Test for a bug specific to the new pass manager where we may build a domtree
 ; to make more precise AA queries for functions.
 ;
-; RUN: opt -aa-pipeline=default -passes='no-op-module' -debug-pass-manager -thinlto-bc -o %t %s
+; RUN: opt -aa-pipeline=default -passes='no-op-module' -debug-pass-manager -thinlto-bc -thinlto-split-lto-unit -o %t %s
 ; RUN: llvm-modextract -b -n 0 -o - %t | llvm-dis | FileCheck --check-prefix=M0 %s
 ; RUN: llvm-modextract -b -n 1 -o - %t | llvm-dis | FileCheck --check-prefix=M1 %s
 
diff --git a/test/Transforms/ThinLTOBitcodeWriter/split-internal-typeid.ll b/test/Transforms/ThinLTOBitcodeWriter/split-internal-typeid.ll
index a43db9a..290df00 100644
--- a/test/Transforms/ThinLTOBitcodeWriter/split-internal-typeid.ll
+++ b/test/Transforms/ThinLTOBitcodeWriter/split-internal-typeid.ll
@@ -1,4 +1,4 @@
-; RUN: opt -thinlto-bc -o %t %s
+; RUN: opt -thinlto-bc -thinlto-split-lto-unit -o %t %s
 ; RUN: llvm-modextract -b -n 0 -o %t0 %t
 ; RUN: llvm-modextract -b -n 1 -o %t1 %t
 ; RUN: not llvm-modextract -b -n 2 -o - %t 2>&1 | FileCheck --check-prefix=ERROR %s
diff --git a/test/Transforms/ThinLTOBitcodeWriter/split-internal1.ll b/test/Transforms/ThinLTOBitcodeWriter/split-internal1.ll
index 6d18c4f..42a06bd 100644
--- a/test/Transforms/ThinLTOBitcodeWriter/split-internal1.ll
+++ b/test/Transforms/ThinLTOBitcodeWriter/split-internal1.ll
@@ -1,4 +1,4 @@
-; RUN: opt -thinlto-bc -o %t %s
+; RUN: opt -thinlto-bc -thinlto-split-lto-unit -o %t %s
 ; RUN: llvm-modextract -b -n 0 -o %t0 %t
 ; RUN: llvm-modextract -b -n 1 -o %t1 %t
 ; RUN: not llvm-modextract -b -n 2 -o - %t 2>&1 | FileCheck --check-prefix=ERROR %s
diff --git a/test/Transforms/ThinLTOBitcodeWriter/split-internal2.ll b/test/Transforms/ThinLTOBitcodeWriter/split-internal2.ll
index fbe618f..02fc3d1 100644
--- a/test/Transforms/ThinLTOBitcodeWriter/split-internal2.ll
+++ b/test/Transforms/ThinLTOBitcodeWriter/split-internal2.ll
@@ -1,4 +1,4 @@
-; RUN: opt -thinlto-bc -o %t %s
+; RUN: opt -thinlto-bc -thinlto-split-lto-unit -o %t %s
 ; RUN: llvm-modextract -b -n 0 -o %t0 %t
 ; RUN: llvm-modextract -b -n 1 -o %t1 %t
 ; RUN: not llvm-modextract -b -n 2 -o - %t 2>&1 | FileCheck --check-prefix=ERROR %s
diff --git a/test/Transforms/ThinLTOBitcodeWriter/split-vfunc-internal.ll b/test/Transforms/ThinLTOBitcodeWriter/split-vfunc-internal.ll
index 087796b..7ebb30a 100644
--- a/test/Transforms/ThinLTOBitcodeWriter/split-vfunc-internal.ll
+++ b/test/Transforms/ThinLTOBitcodeWriter/split-vfunc-internal.ll
@@ -1,4 +1,4 @@
-; RUN: opt -thinlto-bc -o %t %s
+; RUN: opt -thinlto-bc -thinlto-split-lto-unit -o %t %s
 ; RUN: llvm-modextract -b -n 0 -o - %t | llvm-dis | FileCheck --check-prefix=M0 %s
 ; RUN: llvm-modextract -b -n 1 -o - %t | llvm-dis | FileCheck --check-prefix=M1 %s
 
diff --git a/test/Transforms/ThinLTOBitcodeWriter/split-vfunc.ll b/test/Transforms/ThinLTOBitcodeWriter/split-vfunc.ll
index 66d37d5..fcf5751 100644
--- a/test/Transforms/ThinLTOBitcodeWriter/split-vfunc.ll
+++ b/test/Transforms/ThinLTOBitcodeWriter/split-vfunc.ll
@@ -1,4 +1,4 @@
-; RUN: opt -thinlto-bc -o %t %s
+; RUN: opt -thinlto-bc -thinlto-split-lto-unit -o %t %s
 ; RUN: llvm-modextract -b -n 0 -o - %t | llvm-dis | FileCheck --check-prefix=M0 %s
 ; RUN: llvm-modextract -b -n 1 -o - %t | llvm-dis | FileCheck --check-prefix=M1 %s
 
diff --git a/test/Transforms/ThinLTOBitcodeWriter/split.ll b/test/Transforms/ThinLTOBitcodeWriter/split.ll
index 08ed92e..5502f7a 100644
--- a/test/Transforms/ThinLTOBitcodeWriter/split.ll
+++ b/test/Transforms/ThinLTOBitcodeWriter/split.ll
@@ -1,6 +1,6 @@
 ; Generate bitcode files with summary, as well as minimized bitcode without
 ; the debug metadata for the thin link.
-; RUN: opt -thinlto-bc -thin-link-bitcode-file=%t2 -o %t %s
+; RUN: opt -thinlto-bc -thin-link-bitcode-file=%t2 -thinlto-split-lto-unit -o %t %s
 ; RUN: llvm-modextract -b -n 0 -o %t0.bc %t
 ; RUN: llvm-modextract -b -n 1 -o %t1.bc %t
 ; RUN: llvm-modextract -b -n 0 -o %t0.thinlink.bc %t2
diff --git a/test/Transforms/ThinLTOBitcodeWriter/symver.ll b/test/Transforms/ThinLTOBitcodeWriter/symver.ll
index 078825c..8acdd0c 100644
--- a/test/Transforms/ThinLTOBitcodeWriter/symver.ll
+++ b/test/Transforms/ThinLTOBitcodeWriter/symver.ll
@@ -1,4 +1,4 @@
-; RUN: opt -thinlto-bc -o %t %s
+; RUN: opt -thinlto-bc -thinlto-split-lto-unit -o %t %s
 ; RUN: llvm-modextract -n 1 -o - %t | llvm-dis | FileCheck %s
 
 ; The target assembly parser is required to parse the symver directives
diff --git a/test/Transforms/ThinLTOBitcodeWriter/unsplittable.ll b/test/Transforms/ThinLTOBitcodeWriter/unsplittable.ll
index 5413e0f..46c87bc 100644
--- a/test/Transforms/ThinLTOBitcodeWriter/unsplittable.ll
+++ b/test/Transforms/ThinLTOBitcodeWriter/unsplittable.ll
@@ -1,4 +1,4 @@
-; RUN: opt -thinlto-bc -thin-link-bitcode-file=%t2 -o %t %s
+; RUN: opt -thinlto-bc -thin-link-bitcode-file=%t2 -thinlto-split-lto-unit -o %t %s
 ; RUN: llvm-dis -o - %t | FileCheck %s
 ; RUN: llvm-bcanalyzer -dump %t | FileCheck --check-prefix=BCA %s
 ; When not splitting the module, the thin link bitcode file should simply be a
@@ -28,7 +28,8 @@
   ret void
 }
 
-; CHECK: !llvm.module.flags = !{![[FLAG:[0-9]+]]}
-; CHECK: ![[FLAG]] = !{i32 1, !"ThinLTO", i32 0}
+; CHECK: !llvm.module.flags = !{![[FLAG1:[0-9]+]], ![[FLAG2:[0-9]+]]}
+; CHECK: ![[FLAG1]] = !{i32 1, !"EnableSplitLTOUnit", i32 1}
+; CHECK: ![[FLAG2]] = !{i32 1, !"ThinLTO", i32 0}
 
 !0 = !{i32 0, !"typeid"}
diff --git a/test/Transforms/ThinLTOBitcodeWriter/x86/module-asm.ll b/test/Transforms/ThinLTOBitcodeWriter/x86/module-asm.ll
index 15e4778..587ab3f 100644
--- a/test/Transforms/ThinLTOBitcodeWriter/x86/module-asm.ll
+++ b/test/Transforms/ThinLTOBitcodeWriter/x86/module-asm.ll
@@ -1,4 +1,4 @@
-; RUN: opt -thinlto-bc -o %t %s
+; RUN: opt -thinlto-bc -thinlto-split-lto-unit -o %t %s
 ; RUN: llvm-modextract -b -n 0 -o - %t | llvm-dis | FileCheck --check-prefix=M0 %s
 ; RUN: llvm-modextract -b -n 1 -o - %t | llvm-dis | FileCheck --check-prefix=M1 %s
 
diff --git a/tools/opt/opt.cpp b/tools/opt/opt.cpp
index 2666039..a4967a2 100644
--- a/tools/opt/opt.cpp
+++ b/tools/opt/opt.cpp
@@ -103,6 +103,10 @@
     OutputThinLTOBC("thinlto-bc",
                     cl::desc("Write output as ThinLTO-ready bitcode"));
 
+static cl::opt<bool>
+    SplitLTOUnit("thinlto-split-lto-unit",
+                 cl::desc("Enable splitting of a ThinLTO LTOUnit"));
+
 static cl::opt<std::string> ThinLinkBitcodeFile(
     "thin-link-bitcode-file", cl::value_desc("filename"),
     cl::desc(
@@ -596,6 +600,9 @@
     if (CheckBitcodeOutputToConsole(Out->os(), !Quiet))
       NoOutput = true;
 
+  if (OutputThinLTOBC)
+    M->addModuleFlag(Module::Error, "EnableSplitLTOUnit", SplitLTOUnit);
+
   if (PassPipeline.getNumOccurrences() > 0) {
     OutputKind OK = OK_NoOutput;
     if (!NoOutput)