Index: include/llvm/Linker/Linker.h
===================================================================
--- include/llvm/Linker/Linker.h
+++ include/llvm/Linker/Linker.h
@@ -29,7 +29,10 @@
     None = 0,
     OverrideFromSrc = (1 << 0),
     LinkOnlyNeeded = (1 << 1),
-    InternalizeLinkedSymbols = (1 << 2)
+    InternalizeLinkedSymbols = (1 << 2),
+    /// Don't force link referenced linkonce definitions, import declaration.
+    DontForceLinkLinkonceODR = (1 << 3)
+
   };
 
   Linker(Module &M);
Index: include/llvm/Transforms/IPO/FunctionImport.h
===================================================================
--- include/llvm/Transforms/IPO/FunctionImport.h
+++ include/llvm/Transforms/IPO/FunctionImport.h
@@ -48,7 +48,11 @@
       : Index(Index), ModuleLoader(ModuleLoader) {}
 
   /// Import functions in Module \p M based on the supplied import list.
-  bool importFunctions(Module &M, const ImportMapTy &ImportList);
+  /// \p ForceImportReferencedDiscardableSymbols will set the ModuleLinker in
+  /// a mode where referenced discarable symbols in the source modules will be
+  /// imported as well even if they are not present in the ImportList.
+  bool importFunctions(Module &M, const ImportMapTy &ImportList,
+                       bool ForceImportReferencedDiscardableSymbols = false);
 
 private:
   /// The summaries index used to trigger importing.
Index: lib/Linker/LinkModules.cpp
===================================================================
--- lib/Linker/LinkModules.cpp
+++ lib/Linker/LinkModules.cpp
@@ -45,6 +45,9 @@
   /// to Add.
   void addLazyFor(GlobalValue &GV, IRMover::ValueAdder Add);
 
+  bool shouldLinkReferencedLinkOnce() {
+    return !(Flags & Linker::DontForceLinkLinkonceODR);
+  }
   bool shouldOverrideFromSrc() { return Flags & Linker::OverrideFromSrc; }
   bool shouldLinkOnlyNeeded() { return Flags & Linker::LinkOnlyNeeded; }
   bool shouldInternalizeLinkedSymbols() {
@@ -413,6 +416,12 @@
 }
 
 void ModuleLinker::addLazyFor(GlobalValue &GV, IRMover::ValueAdder Add) {
+  if (!shouldLinkReferencedLinkOnce())
+    // For ThinLTO we don't import more than what was required.
+    // The client has to guarantee that the linkonce will be availabe at link
+    // time (by promoting it to weak for instance).
+    return;
+
   // Add these to the internalize list
   if (!GV.hasLinkOnceLinkage())
     return;
Index: lib/Transforms/IPO/FunctionImport.cpp
===================================================================
--- lib/Transforms/IPO/FunctionImport.cpp
+++ lib/Transforms/IPO/FunctionImport.cpp
@@ -49,6 +49,12 @@
 static cl::opt<bool> PrintImports("print-imports", cl::init(false), cl::Hidden,
                                   cl::desc("Print imported functions"));
 
+// Temporary allows the function import pass to disable always linking
+// referenced discardable symbols.
+static cl::opt<bool>
+    DontForceImportReferencedDiscardableSymbols("disable-force-link-odr",
+                                                cl::init(false), cl::Hidden);
+
 // Load lazily a module from \p FileName in \p Context.
 static std::unique_ptr<Module> loadFile(const std::string &FileName,
                                         LLVMContext &Context) {
@@ -327,7 +333,8 @@
 // index.
 //
 bool FunctionImporter::importFunctions(
-    Module &DestModule, const FunctionImporter::ImportMapTy &ImportList) {
+    Module &DestModule, const FunctionImporter::ImportMapTy &ImportList,
+    bool ForceImportReferencedDiscardableSymbols) {
   DEBUG(dbgs() << "Starting import for Module "
                << DestModule.getModuleIdentifier() << "\n");
   unsigned ImportedCount = 0;
@@ -420,8 +427,12 @@
                << " from " << SrcModule->getSourceFileName() << "\n";
     }
 
-    if (TheLinker.linkInModule(std::move(SrcModule), Linker::Flags::None,
-                               &GlobalsToImport))
+    // Instruct the linker that the client will take care of linkonce resolution
+    unsigned Flags = Linker::Flags::None;
+    if (!ForceImportReferencedDiscardableSymbols)
+      Flags |= Linker::Flags::DontForceLinkLinkonceODR;
+
+    if (TheLinker.linkInModule(std::move(SrcModule), Flags, &GlobalsToImport))
       report_fatal_error("Function Import: link error");
 
     ImportedCount += GlobalsToImport.size();
@@ -523,7 +534,8 @@
       return loadFile(Identifier, M.getContext());
     };
     FunctionImporter Importer(*Index, ModuleLoader);
-    return Importer.importFunctions(M, ImportList);
+    return Importer.importFunctions(
+        M, ImportList, !DontForceImportReferencedDiscardableSymbols);
   }
 };
 } // anonymous namespace
Index: test/Linker/funcimport2.ll
===================================================================
--- test/Linker/funcimport2.ll
+++ test/Linker/funcimport2.ll
@@ -3,7 +3,7 @@
 ; RUN: llvm-lto -thinlto -o %t3 %t1.bc %t2.bc
 ; RUN: llvm-link -import=bar:%t2.bc %t1.bc -summary-index=%t3.thinlto.bc -S | FileCheck %s
 
-; CHECK: define linkonce_odr hidden void @foo() {
+; CHECK: define available_externally hidden void @foo() {
 define available_externally hidden void @foo() {
     ret void
 }
Index: test/Transforms/FunctionImport/Inputs/funcimport.ll
===================================================================
--- test/Transforms/FunctionImport/Inputs/funcimport.ll
+++ test/Transforms/FunctionImport/Inputs/funcimport.ll
@@ -99,4 +99,46 @@
   ret void
 }
 
+define void @referencelargelinkonce() #0 {
+entry:
+  call void @linkonceodr()
+  ret void
+}
+
+; A large enough linkonce_odr function that should never be imported
+define linkonce_odr void @linkonceodr() #0 {
+entry:
+  call void @globalfunc2()
+  call void @globalfunc2()
+  call void @globalfunc2()
+  call void @globalfunc2()
+  call void @globalfunc2()
+  call void @globalfunc2()
+  call void @globalfunc2()
+  call void @globalfunc2()
+  call void @globalfunc2()
+  call void @globalfunc2()
+  call void @globalfunc2()
+  call void @globalfunc2()
+  call void @globalfunc2()
+  call void @globalfunc2()
+  call void @globalfunc2()
+  call void @globalfunc2()
+  call void @globalfunc2()
+  call void @globalfunc2()
+  call void @globalfunc2()
+  call void @globalfunc2()
+  call void @globalfunc2()
+  call void @globalfunc2()
+  call void @globalfunc2()
+  call void @globalfunc2()
+  call void @globalfunc2()
+  call void @globalfunc2()
+  call void @globalfunc2()
+  call void @globalfunc2()
+  call void @globalfunc2()
+  call void @globalfunc2()
+  ret void
+}
+
 
Index: test/Transforms/FunctionImport/funcimport.ll
===================================================================
--- test/Transforms/FunctionImport/funcimport.ll
+++ test/Transforms/FunctionImport/funcimport.ll
@@ -4,14 +4,19 @@
 ; RUN: llvm-lto -thinlto -print-summary-global-ids -o %t3 %t.bc %t2.bc 2>&1 | FileCheck %s --check-prefix=GUID
 
 ; Do the import now
-; RUN: opt -function-import -stats -print-imports -summary-file %t3.thinlto.bc %t.bc -S 2>&1 | FileCheck %s --check-prefix=CHECK --check-prefix=INSTLIMDEF
+; RUN: opt -disable-force-link-odr -function-import -stats -print-imports -summary-file %t3.thinlto.bc %t.bc -S 2>&1 | FileCheck %s --check-prefix=CHECK --check-prefix=INSTLIMDEF
 ; "-stats" requires +Asserts.
 ; REQUIRES: asserts
 
 ; Test import with smaller instruction limit
-; RUN: opt -function-import -summary-file %t3.thinlto.bc %t.bc -import-instr-limit=5 -S | FileCheck %s --check-prefix=CHECK --check-prefix=INSTLIM5
+; RUN: opt -disable-force-link-odr -function-import -summary-file %t3.thinlto.bc %t.bc -import-instr-limit=5 -S | FileCheck %s --check-prefix=CHECK --check-prefix=INSTLIM5
 ; INSTLIM5-NOT: @staticfunc.llvm.
 
+; Test import with smaller instruction limit and without the -disable-force-link-odr
+; RUN: opt -function-import -summary-file %t3.thinlto.bc %t.bc -import-instr-limit=5 -S | FileCheck %s --check-prefix=INSTLIM5ODR
+; INSTLIM5ODR: define linkonce_odr void @linkonceodr()
+
+
 define i32 @main() #0 {
 entry:
   call void (...) @weakalias()
@@ -23,6 +28,7 @@
   call void (...) @setfuncptr()
   call void (...) @callfuncptr()
   call void (...) @weakfunc()
+  call void (...) @referencelargelinkonce()
   ret i32 0
 }
 
@@ -78,6 +84,12 @@
 ; CHECK-DAG: %0 = load void ()*, void ()** @P.llvm.
 ; CHECK-DAG: store void ()* @staticfunc2.llvm.{{.*}}, void ()** @P.llvm.
 
+; Ensure that @referencelargelinkonce definition is pulled in, but later we
+; also check that the linkonceodr function is not.
+; CHECK-DAG: define available_externally void @referencelargelinkonce()
+; INSTLIM5-DAG: declare void @linkonceodr()
+declare void @referencelargelinkonce(...)
+
 ; Won't import weak func
 ; CHECK-DAG: declare void @weakfunc(...)
 declare void @weakfunc(...) #1
@@ -87,7 +99,7 @@
 ; INSTLIM5-DAG: declare hidden void @funcwithpersonality.llvm.{{.*}}()
 
 ; INSTLIMDEF-DAG: Import globalfunc2
-; INSTLIMDEF-DAG: 11 function-import - Number of functions imported
+; INSTLIMDEF-DAG: 13 function-import - Number of functions imported
 
 ; The actual GUID values will depend on path to test.
 ; GUID-DAG: GUID {{.*}} is weakalias
Index: tools/llvm-link/llvm-link.cpp
===================================================================
--- tools/llvm-link/llvm-link.cpp
+++ tools/llvm-link/llvm-link.cpp
@@ -276,8 +276,10 @@
     if (renameModuleForThinLTO(*SrcModule, *Index, &GlobalsToImport))
       return true;
 
-    if (L.linkInModule(std::move(SrcModule), Linker::Flags::None,
-                       &GlobalsToImport))
+    // Instruct the linker to not automatically import linkonce defintion.
+    unsigned Flags = Linker::Flags::DontForceLinkLinkonceODR;
+
+    if (L.linkInModule(std::move(SrcModule), Flags, &GlobalsToImport))
       return false;
   }