Index: test/tools/gold/X86/Inputs/thinlto_linkonceresolution.ll
===================================================================
--- /dev/null
+++ test/tools/gold/X86/Inputs/thinlto_linkonceresolution.ll
@@ -0,0 +1,4 @@
+target triple = "x86_64-unknown-linux-gnu"
+define linkonce_odr hidden void @f() {
+  ret void
+}
Index: test/tools/gold/X86/thinlto_linkonceresolution.ll
===================================================================
--- /dev/null
+++ test/tools/gold/X86/thinlto_linkonceresolution.ll
@@ -0,0 +1,25 @@
+; RUN: llvm-as -function-summary %s -o %t.o
+; RUN: llvm-as -function-summary %p/Inputs/thinlto_linkonceresolution.ll -o %t2.o
+
+; Ensure the plugin ensures that for ThinLTO the prevailing copy of a
+; linkonce symbol is changed to weak to ensure it is not eliminated.
+; Note that gold picks the first copy of f() as the prevailing one,
+; so listing %t2.o first is sufficient to ensure that this copy is
+; preempted. Also, set the import-instr-limit to 0 to prevent f() from
+; being imported from %t2.o which hides the problem.
+; RUN: %gold -plugin %llvmshlibdir/LLVMgold.so \
+; RUN:     --plugin-opt=thinlto \
+; RUN:     --plugin-opt=-import-instr-limit=0 \
+; RUN:     -o %t3.o %t2.o %t.o
+; RUN: llvm-nm %t3.o | FileCheck %s
+
+; CHECK-NOT: U f
+
+target triple = "x86_64-unknown-linux-gnu"
+define i32 @g() {
+  call void @f()
+  ret i32 0
+}
+define linkonce_odr hidden void @f() {
+  ret void
+}
Index: tools/gold/gold-plugin.cpp
===================================================================
--- tools/gold/gold-plugin.cpp
+++ tools/gold/gold-plugin.cpp
@@ -661,6 +661,22 @@
   return Obj.takeIndex();
 }
 
+/// Force the prevailing copy of a linkonce to be kept by converting it to weak.
+/// This is necessary when there are either non-IR uses or we are doing
+/// ThinLTO compilation.
+static void convertLinkOnceToWeak(GlobalValue *GV) {
+  switch (GV->getLinkage()) {
+  default:
+    break;
+  case GlobalValue::LinkOnceAnyLinkage:
+    GV->setLinkage(GlobalValue::WeakAnyLinkage);
+    break;
+  case GlobalValue::LinkOnceODRLinkage:
+    GV->setLinkage(GlobalValue::WeakODRLinkage);
+    break;
+  }
+}
+
 static std::unique_ptr<Module>
 getModuleForFile(LLVMContext &Context, claimed_file &F, const void *View,
                  ld_plugin_input_file &Info, raw_fd_ostream *ApiFile,
@@ -754,6 +770,15 @@
 
     case LDPR_PREVAILING_DEF_IRONLY: {
       Keep.push_back(GV);
+      if (options::thinlto) {
+        // For ThinLTO the IR files are compiled through the backend,
+        // independently, so we need to ensure that the prevailing copy will be
+        // emitted into the object file by making it weak.
+        convertLinkOnceToWeak(GV);
+        // Skip the below handling for internalization, which isn't
+        // performed in ThinLTO mode currently anyway.
+        break;
+      }
       // The IR linker has to be able to map this value to a declaration,
       // so we can only internalize after linking.
       if (!Used.count(GV))
@@ -764,25 +789,25 @@
     case LDPR_PREVAILING_DEF:
       Keep.push_back(GV);
       // There is a non IR use, so we have to force optimizations to keep this.
-      switch (GV->getLinkage()) {
-      default:
-        break;
-      case GlobalValue::LinkOnceAnyLinkage:
-        GV->setLinkage(GlobalValue::WeakAnyLinkage);
-        break;
-      case GlobalValue::LinkOnceODRLinkage:
-        GV->setLinkage(GlobalValue::WeakODRLinkage);
-        break;
-      }
+      convertLinkOnceToWeak(GV);
       break;
 
     case LDPR_PREVAILING_DEF_IRONLY_EXP: {
+      Keep.push_back(GV);
+      if (options::thinlto) {
+        // For ThinLTO the IR files are compiled through the backend,
+        // independently, so we need to ensure that the prevailing copy will be
+        // emitted into the object file by making it weak.
+        convertLinkOnceToWeak(GV);
+        // Skip the below handling for internalization, which isn't
+        // performed in ThinLTO mode currently anyway.
+        break;
+      }
       // We can only check for address uses after we merge the modules. The
       // reason is that this GV might have a copy in another module
       // and in that module the address might be significant, but that
       // copy will be LDPR_PREEMPTED_IR.
       Maybe.insert(GV->getName());
-      Keep.push_back(GV);
       break;
     }
     }