Index: lib/Transforms/IPO/LowerTypeTests.cpp
===================================================================
--- lib/Transforms/IPO/LowerTypeTests.cpp
+++ lib/Transforms/IPO/LowerTypeTests.cpp
@@ -228,6 +228,7 @@
   std::vector<ByteArrayInfo> ByteArrayInfos;
 
   Mangler Mang;
+  Function *WeakInitializerFn;
 
   BitSetInfo
   buildBitSet(Metadata *TypeId,
@@ -260,6 +261,8 @@
   void buildBitSetsFromDisjointSet(ArrayRef<Metadata *> TypeIds,
                                    ArrayRef<GlobalObject *> Globals);
 
+  void replaceWeakDeclarationWithJumpTablePtr(Function *F, Constant *JT);
+  void moveInitializerToModuleConstructor(GlobalVariable *GV);
 public:
   LowerTypeTestsModule(Module &M);
   bool lower();
@@ -727,6 +730,57 @@
     report_fatal_error("Unsupported architecture for jump tables");
 }
 
+void LowerTypeTestsModule::moveInitializerToModuleConstructor(
+    GlobalVariable *GV) {
+  if (WeakInitializerFn == nullptr) {
+    WeakInitializerFn = Function::Create(
+        FunctionType::get(Type::getVoidTy(M.getContext()),
+                          /* IsVarArg */ false),
+        GlobalValue::InternalLinkage, "__cfi_global_var_init", &M);
+    BasicBlock *BB =
+        BasicBlock::Create(M.getContext(), "entry", WeakInitializerFn);
+    ReturnInst::Create(M.getContext(), BB);
+    WeakInitializerFn->setSection(
+        ObjectFormat == Triple::MachO
+            ? "__TEXT,__StaticInit,regular,pure_instructions"
+            : ".text.startup");
+    appendToGlobalCtors(M, WeakInitializerFn, /* Priority */ 0);
+  }
+
+  IRBuilder<> IRB(WeakInitializerFn->getEntryBlock().getTerminator());
+  GV->setConstant(false);
+  IRB.CreateAlignedStore(GV->getInitializer(), GV, GV->getAlignment());
+  GV->setInitializer(
+      Constant::getNullValue(GV->getType()->getPointerElementType()));
+}
+
+// Replace all uses of F with (F ? JT : 0).
+void LowerTypeTestsModule::replaceWeakDeclarationWithJumpTablePtr(
+    Function *F, Constant *JT) {
+  // The target expression can not appear in a constant initializer on most
+  // (all?) targets. Switch to a runtime initializer.
+  SmallVector<GlobalVariable *, 8> GlobalVarUses;
+  for (auto *U : F->users())
+    if (auto *GV = dyn_cast<GlobalVariable>(U))
+      GlobalVarUses.push_back(GV);
+  for (auto GV : GlobalVarUses)
+    moveInitializerToModuleConstructor(GV);
+
+  // Can not RAUW F with an expression that uses F. Replace with a temporary
+  // placeholder first.
+  Function *PlaceholderFn =
+      Function::Create(cast<FunctionType>(F->getValueType()),
+                       GlobalValue::ExternalWeakLinkage, "", &M);
+  F->replaceAllUsesWith(PlaceholderFn);
+
+  Constant *Target = ConstantExpr::getSelect(
+      ConstantExpr::getICmp(CmpInst::ICMP_NE, F,
+                            Constant::getNullValue(F->getType())),
+      JT, Constant::getNullValue(F->getType()));
+  PlaceholderFn->replaceAllUsesWith(Target);
+  PlaceholderFn->eraseFromParent();
+}
+
 /// Given a disjoint set of type identifiers and functions, build a jump table
 /// for the functions, build the bit sets and lower the llvm.type.test calls.
 void LowerTypeTestsModule::buildBitSetsFromFunctionsNative(
@@ -844,10 +898,15 @@
               ArrayRef<Constant *>{ConstantInt::get(IntPtrTy, 0),
                                    ConstantInt::get(IntPtrTy, I)}),
           Functions[I]->getType());
-      Functions[I]->replaceAllUsesWith(CombinedGlobalElemPtr);
 
-      if (Functions[I]->isWeakForLinker())
+
+      if (Functions[I]->isWeakForLinker()) {
         AsmOS << ".weak " << Functions[I]->getName() << "\n";
+        replaceWeakDeclarationWithJumpTablePtr(Functions[I],
+                                               CombinedGlobalElemPtr);
+      } else {
+        Functions[I]->replaceAllUsesWith(CombinedGlobalElemPtr);
+      }
     } else {
       assert(Functions[I]->getType()->getAddressSpace() == 0);
 
@@ -1001,6 +1060,7 @@
   LinkerSubsectionsViaSymbols = TargetTriple.isMacOSX();
   Arch = TargetTriple.getArch();
   ObjectFormat = TargetTriple.getObjectFormat();
+  WeakInitializerFn = nullptr;
 }
 
 bool LowerTypeTestsModule::lower() {
Index: test/Transforms/LowerTypeTests/function-weak.ll
===================================================================
--- /dev/null
+++ test/Transforms/LowerTypeTests/function-weak.ll
@@ -0,0 +1,56 @@
+; RUN: opt -S -lowertypetests -mtriple=i686-unknown-linux-gnu < %s | FileCheck %s
+; RUN: opt -S -lowertypetests -mtriple=x86_64-unknown-linux-gnu < %s | FileCheck %s
+; RUN: opt -S -lowertypetests -mtriple=arm-unknown-linux-gnu < %s | FileCheck %s
+; RUN: opt -S -lowertypetests -mtriple=aarch64-unknown-linux-gnu < %s | FileCheck %s
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+; CHECK: module asm ".weak f"
+
+; CHECK: @x = global void ()* null, align 8
+@x = global void ()* @f, align 8
+
+; CHECK: @x2 = global void ()* null, align 8
+@x2 = global void ()* @f, align 8
+
+; CHECK: @x3 = internal global void ()* null, align 8
+@x3 = internal constant void ()* @f, align 8
+
+; CHECK: @.cfi.jumptable = external hidden constant [1 x [{{.*}} x i8]]
+; CHECK:  @llvm.global_ctors = appending global {{.*}}{ i32 0, void ()* @__cfi_global_var_init
+
+; CHECK: declare !type !0 extern_weak void @f()
+declare !type !0 extern_weak void @f()
+
+; CHECK: define zeroext i1 @check_f()
+define zeroext i1 @check_f() {
+entry:
+; CHECK: ret i1 icmp ne (void ()* select (i1 icmp ne (void ()* @f, void ()* null), void ()* bitcast ({{.*}}@.cfi.jumptable to void ()*), void ()* null), void ()* null)
+  ret i1 icmp ne (void ()* @f, void ()* null)
+}
+
+; CHECK: define void @call_f() {
+define void @call_f() {
+entry:
+; CHECK: call void select (i1 icmp ne (void ()* @f, void ()* null), void ()* bitcast ({{.*}}@.cfi.jumptable to void ()*), void ()* null)()
+  call void @f()
+  ret void
+}
+
+declare i1 @llvm.type.test(i8* %ptr, metadata %bitset) nounwind readnone
+
+define i1 @foo(i8* %p) {
+  %x = call i1 @llvm.type.test(i8* %p, metadata !"typeid1")
+  ret i1 %x
+}
+
+; CHECK: define internal void @__cfi_global_var_init() section ".text.startup" {
+; CHECK-DAG: store void ()* select (i1 icmp ne (void ()* @f, void ()* null), void ()* bitcast ({{.*}}@.cfi.jumptable to void ()*), void ()* null), void ()** @x3, align 8
+; CHECK-DAG: store void ()* select (i1 icmp ne (void ()* @f, void ()* null), void ()* bitcast ({{.*}}@.cfi.jumptable to void ()*), void ()* null), void ()** @x2, align 8
+; CHECK-DAG: store void ()* select (i1 icmp ne (void ()* @f, void ()* null), void ()* bitcast ({{.*}}@.cfi.jumptable to void ()*), void ()* null), void ()** @x, align 8
+; CHECK-DAG: ret void
+; CHECK-DAG: }
+
+
+!0 = !{i32 0, !"typeid1"}