Index: lib/LTO/LTO.cpp
===================================================================
--- lib/LTO/LTO.cpp
+++ lib/LTO/LTO.cpp
@@ -357,6 +357,14 @@
                                  recordNewLinkage);
 }
 
+static bool isWeakWriteableObject(GlobalValueSummary *GVS) {
+  if (auto *VarSummary = dyn_cast<GlobalVarSummary>(GVS->getBaseObject()))
+    return !VarSummary->isReadOnly() &&
+           (VarSummary->linkage() == GlobalValue::WeakODRLinkage ||
+            VarSummary->linkage() == GlobalValue::LinkOnceODRLinkage);
+  return false;
+}
+
 static void thinLTOInternalizeAndPromoteGUID(
     GlobalValueSummaryList &GVSummaryList, GlobalValue::GUID GUID,
     function_ref<bool(StringRef, GlobalValue::GUID)> isExported) {
@@ -371,7 +379,8 @@
                S->linkage() != GlobalValue::AppendingLinkage &&
                // We can't internalize available_externally globals because this
                // can break function pointer equality.
-               S->linkage() != GlobalValue::AvailableExternallyLinkage)
+               S->linkage() != GlobalValue::AvailableExternallyLinkage &&
+               !isWeakWriteableObject(S.get()))
       S->setLinkage(GlobalValue::InternalLinkage);
   }
 }
Index: test/ThinLTO/X86/weak_externals.ll
===================================================================
--- test/ThinLTO/X86/weak_externals.ll
+++ test/ThinLTO/X86/weak_externals.ll
@@ -0,0 +1,39 @@
+; RUN: opt -module-summary %s -o %t.bc
+; RUN: llvm-lto2 run -save-temps %t.bc -o %t.out \
+; RUN:               -r=%t.bc,_ZL5initSv,plx \
+; RUN:               -r=%t.bc,_ZN9SingletonI1SE11getInstanceEv,lx \
+; RUN:               -r=%t.bc,_ZZN9SingletonI1SE11getInstanceEvE8instance,lx \
+; RUN:               -r=%t.bc,_ZZN9SingletonI1SE11getInstanceEvE13instance_weak,lx
+; RUN: llvm-dis %t.out.1.1.promote.bc -o - | FileCheck %s
+
+; CHECK: @_ZZN9SingletonI1SE11getInstanceEvE8instance = available_externally dso_local global %struct.S zeroinitializer
+; CHECK: @_ZZN9SingletonI1SE11getInstanceEvE13instance_weak = available_externally dso_local global %struct.S* null, align 8
+; CHECK: define internal dereferenceable(16) %struct.S* @_ZN9SingletonI1SE11getInstanceEv() comdat
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+%struct.S = type { i64, i64 }
+
+$_ZN9SingletonI1SE11getInstanceEv = comdat any
+
+$_ZZN9SingletonI1SE11getInstanceEvE8instance = comdat any
+
+$_ZZN9SingletonI1SE11getInstanceEvE13instance_weak = comdat any
+
+@_ZZN9SingletonI1SE11getInstanceEvE8instance = linkonce_odr dso_local global %struct.S zeroinitializer, comdat, align 8
+
+@_ZZN9SingletonI1SE11getInstanceEvE13instance_weak = weak_odr dso_local global %struct.S* null, comdat, align 8
+
+define dso_local void @_ZL5initSv() {
+  %1 = call dereferenceable(16) %struct.S* @_ZN9SingletonI1SE11getInstanceEv()
+  store  %struct.S* %1, %struct.S** @_ZZN9SingletonI1SE11getInstanceEvE13instance_weak
+  %2 = getelementptr inbounds %struct.S, %struct.S* %1, i32 0, i32 0
+  store i64 1, i64* %2, align 8
+  ret void
+}
+
+define linkonce_odr dso_local dereferenceable(16) %struct.S* @_ZN9SingletonI1SE11getInstanceEv() #0 comdat align 2 {
+  ret %struct.S* @_ZZN9SingletonI1SE11getInstanceEvE8instance
+}
+