Index: llvm/trunk/include/llvm/Bitcode/LLVMBitCodes.h
===================================================================
--- llvm/trunk/include/llvm/Bitcode/LLVMBitCodes.h
+++ llvm/trunk/include/llvm/Bitcode/LLVMBitCodes.h
@@ -55,6 +55,8 @@
   METADATA_KIND_BLOCK_ID,
 
   STRTAB_BLOCK_ID,
+
+  FULL_LTO_GLOBALVAL_SUMMARY_BLOCK_ID,
 };
 
 /// Identification block contains a string that describes the producer details,
Index: llvm/trunk/lib/Analysis/ModuleSummaryAnalysis.cpp
===================================================================
--- llvm/trunk/lib/Analysis/ModuleSummaryAnalysis.cpp
+++ llvm/trunk/lib/Analysis/ModuleSummaryAnalysis.cpp
@@ -447,6 +447,11 @@
         });
   }
 
+  bool IsThinLTO = true;
+  if (auto *MD =
+          mdconst::extract_or_null<ConstantInt>(M.getModuleFlag("ThinLTO")))
+    IsThinLTO = MD->getZExtValue();
+
   for (auto &GlobalList : Index) {
     // Ignore entries for references that are undefined in the current module.
     if (GlobalList.second.SummaryList.empty())
@@ -455,6 +460,11 @@
     assert(GlobalList.second.SummaryList.size() == 1 &&
            "Expected module's index to have one summary per GUID");
     auto &Summary = GlobalList.second.SummaryList[0];
+    if (!IsThinLTO) {
+      Summary->setNotEligibleToImport();
+      continue;
+    }
+
     bool AllRefsCanBeExternallyReferenced =
         llvm::all_of(Summary->refs(), [&](const ValueInfo &VI) {
           return !CantBePromoted.count(VI.getGUID());
Index: llvm/trunk/lib/Bitcode/Writer/BitcodeWriter.cpp
===================================================================
--- llvm/trunk/lib/Bitcode/Writer/BitcodeWriter.cpp
+++ llvm/trunk/lib/Bitcode/Writer/BitcodeWriter.cpp
@@ -3305,7 +3305,15 @@
 /// Emit the per-module summary section alongside the rest of
 /// the module's bitcode.
 void ModuleBitcodeWriter::writePerModuleGlobalValueSummary() {
-  Stream.EnterSubblock(bitc::GLOBALVAL_SUMMARY_BLOCK_ID, 4);
+  // By default we compile with ThinLTO if the module has a summary, but the
+  // client can request full LTO with a module flag.
+  bool IsThinLTO = true;
+  if (auto *MD =
+          mdconst::extract_or_null<ConstantInt>(M.getModuleFlag("ThinLTO")))
+    IsThinLTO = MD->getZExtValue();
+  Stream.EnterSubblock(IsThinLTO ? bitc::GLOBALVAL_SUMMARY_BLOCK_ID
+                                 : bitc::FULL_LTO_GLOBALVAL_SUMMARY_BLOCK_ID,
+                       4);
 
   Stream.EmitRecord(bitc::FS_VERSION, ArrayRef<uint64_t>{INDEX_VERSION});
 
Index: llvm/trunk/lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp
===================================================================
--- llvm/trunk/lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp
+++ llvm/trunk/lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp
@@ -318,6 +318,12 @@
   ProfileSummaryInfo PSI(M);
   ModuleSummaryIndex Index = buildModuleSummaryIndex(M, nullptr, &PSI);
 
+  // Mark the merged module as requiring full LTO. We still want an index for
+  // it though, so that it can participate in summary-based dead stripping.
+  MergedM->addModuleFlag(Module::Error, "ThinLTO", uint32_t(0));
+  ModuleSummaryIndex MergedMIndex =
+      buildModuleSummaryIndex(*MergedM, nullptr, &PSI);
+
   SmallVector<char, 0> Buffer;
 
   BitcodeWriter W(Buffer);
@@ -327,7 +333,8 @@
   ModuleHash ModHash = {{0}};
   W.writeModule(&M, /*ShouldPreserveUseListOrder=*/false, &Index,
                 /*GenerateHash=*/true, &ModHash);
-  W.writeModule(MergedM.get());
+  W.writeModule(MergedM.get(), /*ShouldPreserveUseListOrder=*/false,
+                &MergedMIndex);
   W.writeStrtab();
   OS << Buffer;
 
@@ -340,7 +347,8 @@
     StripDebugInfo(M);
     W2.writeModule(&M, /*ShouldPreserveUseListOrder=*/false, &Index,
                    /*GenerateHash=*/false, &ModHash);
-    W2.writeModule(MergedM.get());
+    W2.writeModule(MergedM.get(), /*ShouldPreserveUseListOrder=*/false,
+                   &MergedMIndex);
     W2.writeStrtab();
     *ThinLinkOS << Buffer;
   }
Index: llvm/trunk/test/Transforms/ThinLTOBitcodeWriter/split.ll
===================================================================
--- llvm/trunk/test/Transforms/ThinLTOBitcodeWriter/split.ll
+++ llvm/trunk/test/Transforms/ThinLTOBitcodeWriter/split.ll
@@ -25,6 +25,9 @@
 ; ERROR: llvm-modextract: error: module index out of range; bitcode file contains 2 module(s)
 
 ; BCA0: <GLOBALVAL_SUMMARY_BLOCK
+; BCA1: <FULL_LTO_GLOBALVAL_SUMMARY_BLOCK
+; 16 = not eligible to import
+; BCA1: <PERMODULE_GLOBALVAR_INIT_REFS {{.*}} op1=16
 ; BCA1-NOT: <GLOBALVAL_SUMMARY_BLOCK
 
 $g = comdat any
@@ -47,5 +50,6 @@
 ; NODEBUG-NOT: !llvm.dbg.cu
 !llvm.dbg.cu = !{}
 
+; M1: !{i32 1, !"ThinLTO", i32 0}
 !1 = !{i32 2, !"Debug Info Version", i32 3}
 !llvm.module.flags = !{!1}
Index: llvm/trunk/tools/llvm-bcanalyzer/llvm-bcanalyzer.cpp
===================================================================
--- llvm/trunk/tools/llvm-bcanalyzer/llvm-bcanalyzer.cpp
+++ llvm/trunk/tools/llvm-bcanalyzer/llvm-bcanalyzer.cpp
@@ -121,6 +121,8 @@
   case bitc::USELIST_BLOCK_ID:             return "USELIST_BLOCK_ID";
   case bitc::GLOBALVAL_SUMMARY_BLOCK_ID:
                                            return "GLOBALVAL_SUMMARY_BLOCK";
+  case bitc::FULL_LTO_GLOBALVAL_SUMMARY_BLOCK_ID:
+                                      return "FULL_LTO_GLOBALVAL_SUMMARY_BLOCK";
   case bitc::MODULE_STRTAB_BLOCK_ID:       return "MODULE_STRTAB_BLOCK";
   case bitc::STRTAB_BLOCK_ID:              return "STRTAB_BLOCK";
   }
@@ -298,6 +300,7 @@
       STRINGIFY_CODE(MST_CODE, HASH)
     }
   case bitc::GLOBALVAL_SUMMARY_BLOCK_ID:
+  case bitc::FULL_LTO_GLOBALVAL_SUMMARY_BLOCK_ID:
     switch (CodeID) {
     default:
       return nullptr;