Index: llvm/trunk/include/llvm/ADT/Triple.h
===================================================================
--- llvm/trunk/include/llvm/ADT/Triple.h
+++ llvm/trunk/include/llvm/ADT/Triple.h
@@ -252,6 +252,10 @@
            ObjectFormat == Other.ObjectFormat;
   }
 
+  bool operator!=(const Triple &Other) const {
+    return !(*this == Other);
+  }
+
   /// @}
   /// @name Normalization
   /// @{
@@ -722,6 +726,12 @@
   /// \returns true if the triple is little endian, false otherwise.
   bool isLittleEndian() const;
 
+  /// Test whether target triples are compatible.
+  bool isCompatibleWith(const Triple &Other) const;
+
+  /// Merge target triples.
+  std::string merge(const Triple &Other) const;
+
   /// @}
   /// @name Static helpers for IDs.
   /// @{
Index: llvm/trunk/lib/LTO/ThinLTOCodeGenerator.cpp
===================================================================
--- llvm/trunk/lib/LTO/ThinLTOCodeGenerator.cpp
+++ llvm/trunk/lib/LTO/ThinLTOCodeGenerator.cpp
@@ -505,29 +505,25 @@
 
 void ThinLTOCodeGenerator::addModule(StringRef Identifier, StringRef Data) {
   ThinLTOBuffer Buffer(Data, Identifier);
-  if (Modules.empty()) {
-    // First module added, so initialize the triple and some options
-    LLVMContext Context;
-    StringRef TripleStr;
-    ErrorOr<std::string> TripleOrErr = expectedToErrorOrAndEmitErrors(
-        Context, getBitcodeTargetTriple(Buffer.getMemBuffer()));
-    if (TripleOrErr)
-      TripleStr = *TripleOrErr;
-    Triple TheTriple(TripleStr);
+  LLVMContext Context;
+  StringRef TripleStr;
+  ErrorOr<std::string> TripleOrErr = expectedToErrorOrAndEmitErrors(
+      Context, getBitcodeTargetTriple(Buffer.getMemBuffer()));
+
+  if (TripleOrErr)
+    TripleStr = *TripleOrErr;
+
+  Triple TheTriple(TripleStr);
+
+  if (Modules.empty())
     initTMBuilder(TMBuilder, Triple(TheTriple));
+  else if (TMBuilder.TheTriple != TheTriple) {
+    if (!TMBuilder.TheTriple.isCompatibleWith(TheTriple))
+      report_fatal_error("ThinLTO modules with incompatible triples not "
+                         "supported");
+    initTMBuilder(TMBuilder, Triple(TMBuilder.TheTriple.merge(TheTriple)));
   }
-#ifndef NDEBUG
-  else {
-    LLVMContext Context;
-    StringRef TripleStr;
-    ErrorOr<std::string> TripleOrErr = expectedToErrorOrAndEmitErrors(
-        Context, getBitcodeTargetTriple(Buffer.getMemBuffer()));
-    if (TripleOrErr)
-      TripleStr = *TripleOrErr;
-    assert(TMBuilder.TheTriple.str() == TripleStr &&
-           "ThinLTO modules with different triple not supported");
-  }
-#endif
+
   Modules.push_back(Buffer);
 }
 
Index: llvm/trunk/lib/Linker/IRMover.cpp
===================================================================
--- llvm/trunk/lib/Linker/IRMover.cpp
+++ llvm/trunk/lib/Linker/IRMover.cpp
@@ -1243,27 +1243,6 @@
   return Error::success();
 }
 
-// This function returns true if the triples match.
-static bool triplesMatch(const Triple &T0, const Triple &T1) {
-  // If vendor is apple, ignore the version number.
-  if (T0.getVendor() == Triple::Apple)
-    return T0.getArch() == T1.getArch() && T0.getSubArch() == T1.getSubArch() &&
-           T0.getVendor() == T1.getVendor() && T0.getOS() == T1.getOS();
-
-  return T0 == T1;
-}
-
-// This function returns the merged triple.
-static std::string mergeTriples(const Triple &SrcTriple,
-                                const Triple &DstTriple) {
-  // If vendor is apple, pick the triple with the larger version number.
-  if (SrcTriple.getVendor() == Triple::Apple)
-    if (DstTriple.isOSVersionLT(SrcTriple))
-      return SrcTriple.str();
-
-  return DstTriple.str();
-}
-
 Error IRLinker::run() {
   // Ensure metadata materialized before value mapping.
   if (SrcM->getMaterializer())
@@ -1289,14 +1268,15 @@
 
   Triple SrcTriple(SrcM->getTargetTriple()), DstTriple(DstM.getTargetTriple());
 
-  if (!SrcM->getTargetTriple().empty() && !triplesMatch(SrcTriple, DstTriple))
+  if (!SrcM->getTargetTriple().empty()&&
+      !SrcTriple.isCompatibleWith(DstTriple))
     emitWarning("Linking two modules of different target triples: " +
                 SrcM->getModuleIdentifier() + "' is '" +
                 SrcM->getTargetTriple() + "' whereas '" +
                 DstM.getModuleIdentifier() + "' is '" + DstM.getTargetTriple() +
                 "'\n");
 
-  DstM.setTargetTriple(mergeTriples(SrcTriple, DstTriple));
+  DstM.setTargetTriple(SrcTriple.merge(DstTriple));
 
   // Append the module inline asm string.
   if (!IsPerformingImport && !SrcM->getModuleInlineAsm().empty()) {
Index: llvm/trunk/lib/Support/Triple.cpp
===================================================================
--- llvm/trunk/lib/Support/Triple.cpp
+++ llvm/trunk/lib/Support/Triple.cpp
@@ -1472,6 +1472,24 @@
   }
 }
 
+bool Triple::isCompatibleWith(const Triple &Other) const {
+  // If vendor is apple, ignore the version number.
+  if (getVendor() == Triple::Apple)
+    return getArch() == Other.getArch() && getSubArch() == Other.getSubArch() &&
+           getVendor() == Other.getVendor() && getOS() == Other.getOS();
+
+  return *this == Other;
+}
+
+std::string Triple::merge(const Triple &Other) const {
+  // If vendor is apple, pick the triple with the larger version number.
+  if (getVendor() == Triple::Apple)
+    if (Other.isOSVersionLT(*this))
+      return str();
+
+  return Other.str();
+}
+
 StringRef Triple::getARMCPUForArch(StringRef MArch) const {
   if (MArch.empty())
     MArch = getArchName();
Index: llvm/trunk/test/ThinLTO/X86/Inputs/merge-triple.ll
===================================================================
--- llvm/trunk/test/ThinLTO/X86/Inputs/merge-triple.ll
+++ llvm/trunk/test/ThinLTO/X86/Inputs/merge-triple.ll
@@ -0,0 +1 @@
+target triple = "x86_64-apple-macosx10.11.0"
Index: llvm/trunk/test/ThinLTO/X86/merge-triple.ll
===================================================================
--- llvm/trunk/test/ThinLTO/X86/merge-triple.ll
+++ llvm/trunk/test/ThinLTO/X86/merge-triple.ll
@@ -0,0 +1,10 @@
+; RUN: opt -module-summary %s -o %t1.bc
+; RUN: opt -module-summary %p/Inputs/merge-triple.ll -o %t2.bc
+; RUN: llvm-lto -thinlto-action=optimize %t1.bc %t2.bc
+; RUN: llvm-dis < %t1.bc.thinlto.imported.bc | FileCheck %s --check-prefix=CHECK1
+; RUN: llvm-dis < %t2.bc.thinlto.imported.bc | FileCheck %s --check-prefix=CHECK2
+
+target triple = "x86_64-apple-macosx10.12.0"
+
+; CHECK1: target triple = "x86_64-apple-macosx10.12.0"
+; CHECK2: target triple = "x86_64-apple-macosx10.11.0"