Index: include/llvm/InitializePasses.h
===================================================================
--- include/llvm/InitializePasses.h
+++ include/llvm/InitializePasses.h
@@ -97,6 +97,7 @@
 void initializeMachineCopyPropagationPass(PassRegistry&);
 void initializeCostModelAnalysisPass(PassRegistry&);
 void initializeCorrelatedValuePropagationPass(PassRegistry&);
+void initializeCrossDSOCFIPass(PassRegistry&);
 void initializeDAEPass(PassRegistry&);
 void initializeDAHPass(PassRegistry&);
 void initializeDCEPass(PassRegistry&);
Index: include/llvm/Transforms/IPO.h
===================================================================
--- include/llvm/Transforms/IPO.h
+++ include/llvm/Transforms/IPO.h
@@ -215,6 +215,9 @@
 /// to bitsets.
 ModulePass *createLowerBitSetsPass();
 
+/// \brief This pass export CFI checks for use by external modules.
+ModulePass *createCrossDSOCFIPass();
+
 //===----------------------------------------------------------------------===//
 // SampleProfilePass - Loads sample profile data from disk and generates
 // IR metadata to reflect the profile.
Index: lib/Transforms/IPO/CMakeLists.txt
===================================================================
--- lib/Transforms/IPO/CMakeLists.txt
+++ lib/Transforms/IPO/CMakeLists.txt
@@ -2,6 +2,7 @@
   ArgumentPromotion.cpp
   BarrierNoopPass.cpp
   ConstantMerge.cpp
+  CrossDSOCFI.cpp
   DeadArgumentElimination.cpp
   ElimAvailExtern.cpp
   ExtractGV.cpp
Index: lib/Transforms/IPO/CrossDSOCFI.cpp
===================================================================
--- /dev/null
+++ lib/Transforms/IPO/CrossDSOCFI.cpp
@@ -0,0 +1,165 @@
+//===-- CrossDSOCFI.cpp - Externalize this module's CFI checks ------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This pass exports all llvm.bitset's found in the module in the form of a
+// __cfi_check function, which can be used to verify cross-DSO call targets.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Transforms/IPO.h"
+#include "llvm/ADT/DenseSet.h"
+#include "llvm/ADT/EquivalenceClasses.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/IR/Constant.h"
+#include "llvm/IR/Constants.h"
+#include "llvm/IR/Function.h"
+#include "llvm/IR/GlobalObject.h"
+#include "llvm/IR/GlobalVariable.h"
+#include "llvm/IR/IRBuilder.h"
+#include "llvm/IR/Instructions.h"
+#include "llvm/IR/Intrinsics.h"
+#include "llvm/IR/MDBuilder.h"
+#include "llvm/IR/Module.h"
+#include "llvm/IR/Operator.h"
+#include "llvm/Pass.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/Support/raw_ostream.h"
+#include "llvm/Transforms/Utils/BasicBlockUtils.h"
+
+using namespace llvm;
+
+#define DEBUG_TYPE "cross-dso-cfi"
+
+STATISTIC(TypeIds, "Number of unique type identifiers");
+
+namespace {
+
+struct CrossDSOCFI : public ModulePass {
+  static char ID;
+  CrossDSOCFI() : ModulePass(ID) {
+    initializeCrossDSOCFIPass(*PassRegistry::getPassRegistry());
+  }
+
+  Module *M;
+  MDNode *VeryLikelyWeights;
+
+  ConstantInt *extractBitSetTypeId(MDNode *MD);
+  void buildCFICheck();
+
+  bool doInitialization(Module &M) override;
+  bool runOnModule(Module &M) override;
+};
+
+} // anonymous namespace
+
+INITIALIZE_PASS_BEGIN(CrossDSOCFI, "cross-dso-cfi", "Cross-DSO CFI", false,
+                      false)
+INITIALIZE_PASS_END(CrossDSOCFI, "cross-dso-cfi", "Cross-DSO CFI", false, false)
+char CrossDSOCFI::ID = 0;
+
+ModulePass *llvm::createCrossDSOCFIPass() { return new CrossDSOCFI; }
+
+bool CrossDSOCFI::doInitialization(Module &Mod) {
+  M = &Mod;
+  VeryLikelyWeights =
+      MDBuilder(M->getContext()).createBranchWeights((1U << 20) - 1, 1);
+
+  return false;
+}
+
+/// extractBitSetTypeId - Extracts TypeId from a hash-based bitset MDNode.
+ConstantInt *CrossDSOCFI::extractBitSetTypeId(MDNode *MD) {
+  // This check excludes vtables for classes inside anonymous namespaces.
+  auto TM = dyn_cast<ValueAsMetadata>(MD->getOperand(0));
+  if (!TM)
+    return nullptr;
+  auto C = dyn_cast_or_null<ConstantInt>(TM->getValue());
+  if (!C) return nullptr;
+  // We are looking for i64 constants.
+  if (C->getBitWidth() != 64) return nullptr;
+
+  // Sanity check.
+  auto FM = dyn_cast_or_null<ValueAsMetadata>(MD->getOperand(1));
+  // Can be null if a function was removed by an optimization.
+  if (FM) {
+    auto F = dyn_cast<Function>(FM->getValue());
+    // But can never be a function declaration.
+    assert(!F || !F->isDeclaration());
+  }
+  return C;
+}
+
+/// buildCFICheck - emits __cfi_check for the current module.
+void CrossDSOCFI::buildCFICheck() {
+  // FIXME: verify that __cfi_check ends up near the end of the code section,
+  // but before the jump slots created in LowerBitSets.
+  llvm::DenseSet<uint64_t> BitSetIds;
+  NamedMDNode *BitSetNM = M->getNamedMetadata("llvm.bitsets");
+
+  if (BitSetNM)
+    for (unsigned I = 0, E = BitSetNM->getNumOperands(); I != E; ++I)
+      if (ConstantInt *TypeId = extractBitSetTypeId(BitSetNM->getOperand(I)))
+        BitSetIds.insert(TypeId->getZExtValue());
+
+  LLVMContext &Ctx = M->getContext();
+  Constant *C = M->getOrInsertFunction(
+      "__cfi_check",
+      FunctionType::get(
+          Type::getVoidTy(Ctx),
+          {Type::getInt64Ty(Ctx), PointerType::getUnqual(Type::getInt8Ty(Ctx))},
+          false));
+  Function *F = dyn_cast<Function>(C);
+  F->setAlignment(4096);
+  auto args = F->arg_begin();
+  Argument &CallSiteTypeId = *(args++);
+  CallSiteTypeId.setName("CallSiteTypeId");
+  Argument &Addr = *(args++);
+  Addr.setName("Addr");
+  assert(args == F->arg_end());
+
+  BasicBlock *BB = BasicBlock::Create(Ctx, "entry", F);
+
+  BasicBlock *TrapBB = BasicBlock::Create(Ctx, "trap", F);
+  IRBuilder<> IRBTrap(TrapBB);
+  Function *TrapFn = Intrinsic::getDeclaration(M, Intrinsic::trap);
+  llvm::CallInst *TrapCall = IRBTrap.CreateCall(TrapFn);
+  TrapCall->setDoesNotReturn();
+  TrapCall->setDoesNotThrow();
+  IRBTrap.CreateUnreachable();
+
+  BasicBlock *ExitBB = BasicBlock::Create(Ctx, "exit", F);
+  IRBuilder<> IRBExit(ExitBB);
+  IRBExit.CreateRetVoid();
+
+  IRBuilder<> IRB(BB);
+  SwitchInst *SI = IRB.CreateSwitch(&CallSiteTypeId, TrapBB, BitSetIds.size());
+  for (uint64_t TypeId : BitSetIds) {
+    ConstantInt *CaseTypeId = ConstantInt::get(Type::getInt64Ty(Ctx), TypeId);
+    BasicBlock *TestBB = BasicBlock::Create(Ctx, "test", F);
+    IRBuilder<> IRBTest(TestBB);
+    Function *BitsetTestFn =
+        Intrinsic::getDeclaration(M, Intrinsic::bitset_test);
+
+    Value *Test = IRBTest.CreateCall(
+        BitsetTestFn, {&Addr, MetadataAsValue::get(
+                                  Ctx, ConstantAsMetadata::get(CaseTypeId))});
+    BranchInst *BI = IRBTest.CreateCondBr(Test, ExitBB, TrapBB);
+    BI->setMetadata(LLVMContext::MD_prof, VeryLikelyWeights);
+
+    SI->addCase(CaseTypeId, TestBB);
+    ++TypeIds;
+  }
+}
+
+bool CrossDSOCFI::runOnModule(Module &M) {
+  if (M.getModuleFlag("Cross-DSO CFI") == nullptr)
+    return false;
+  buildCFICheck();
+  return true;
+}
Index: lib/Transforms/IPO/IPO.cpp
===================================================================
--- lib/Transforms/IPO/IPO.cpp
+++ lib/Transforms/IPO/IPO.cpp
@@ -24,6 +24,7 @@
 void llvm::initializeIPO(PassRegistry &Registry) {
   initializeArgPromotionPass(Registry);
   initializeConstantMergePass(Registry);
+  initializeCrossDSOCFIPass(Registry);
   initializeDAEPass(Registry);
   initializeDAHPass(Registry);
   initializeFunctionAttrsPass(Registry);
Index: lib/Transforms/IPO/PassManagerBuilder.cpp
===================================================================
--- lib/Transforms/IPO/PassManagerBuilder.cpp
+++ lib/Transforms/IPO/PassManagerBuilder.cpp
@@ -600,6 +600,10 @@
   if (OptLevel > 1)
     addLTOOptimizationPasses(PM);
 
+  // Create a function that performs CFI checks for cross-DSO calls with targets
+  // in the current module.
+  PM.add(createCrossDSOCFIPass());
+
   // Lower bit sets to globals. This pass supports Clang's control flow
   // integrity mechanisms (-fsanitize=cfi*) and needs to run at link time if CFI
   // is enabled. The pass does nothing if CFI is disabled.
Index: test/Transforms/CrossDSOCFI/basic.ll
===================================================================
--- /dev/null
+++ test/Transforms/CrossDSOCFI/basic.ll
@@ -0,0 +1,88 @@
+; RUN: opt -S -cross-dso-cfi < %s | FileCheck %s
+
+; CHECK:     define void @__cfi_check(i64 %[[TYPE:.*]], i8* %[[ADDR:.*]]) align 4096
+; CHECK:     switch i64 %[[TYPE]], label %[[TRAP:.*]] [
+; CHECK-NEXT:   i64 111, label %[[L1:.*]]
+; CHECK-NEXT:   i64 222, label %[[L2:.*]]
+; CHECK-NEXT:   i64 333, label %[[L3:.*]]
+; CHECK-NEXT:   i64 444, label %[[L4:.*]]
+; CHECK-NEXT: {{]$}}
+
+; CHECK:     [[TRAP]]:
+; CHECK-NEXT:   call void @llvm.trap()
+; CHECK-MEXT:   unreachable
+
+; CHECK:     [[EXIT:.*]]:
+; CHECK-NEXT:   ret void
+
+; CHECK:     [[L1]]:
+; CHECK-NEXT:   call i1 @llvm.bitset.test(i8* %[[ADDR]], metadata i64 111)
+; CHECK-NEXT:   br {{.*}} label %[[EXIT]], label %[[TRAP]]
+
+; CHECK:     [[L2]]:
+; CHECK-NEXT:   call i1 @llvm.bitset.test(i8* %[[ADDR]], metadata i64 222)
+; CHECK-NEXT:   br {{.*}} label %[[EXIT]], label %[[TRAP]]
+
+; CHECK:     [[L3]]:
+; CHECK-NEXT:   call i1 @llvm.bitset.test(i8* %[[ADDR]], metadata i64 333)
+; CHECK-NEXT:   br {{.*}} label %[[EXIT]], label %[[TRAP]]
+
+; CHECK:     [[L4]]:
+; CHECK-NEXT:   call i1 @llvm.bitset.test(i8* %[[ADDR]], metadata i64 444)
+; CHECK-NEXT:   br {{.*}} label %[[EXIT]], label %[[TRAP]]
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+@_ZTV1A = constant i8 0
+@_ZTI1A = constant i8 0
+@_ZTS1A = constant i8 0
+@_ZTV1B = constant i8 0
+@_ZTI1B = constant i8 0
+@_ZTS1B = constant i8 0
+
+define signext i8 @f11() {
+entry:
+  ret i8 1
+}
+
+define signext i8 @f12() {
+entry:
+  ret i8 2
+}
+
+define signext i8 @f13() {
+entry:
+  ret i8 3
+}
+
+define i32 @f21() {
+entry:
+  ret i32 4
+}
+
+define i32 @f22() {
+entry:
+  ret i32 5
+}
+
+!llvm.bitsets = !{!0, !1, !2, !3, !4, !7, !8, !9, !10, !11, !12, !13, !14, !15}
+!llvm.module.flags = !{!17}
+
+!0 = !{!"_ZTSFcvE", i8 ()* @f11, i64 0}
+!1 = !{i64 111, i8 ()* @f11, i64 0}
+!2 = !{!"_ZTSFcvE", i8 ()* @f12, i64 0}
+!3 = !{i64 111, i8 ()* @f12, i64 0}
+!4 = !{!"_ZTSFcvE", i8 ()* @f13, i64 0}
+!5 = !{i64 111, i8 ()* @f13, i64 0}
+!6 = !{!"_ZTSFivE", i32 ()* @f21, i64 0}
+!7 = !{i64 222, i32 ()* @f21, i64 0}
+!8 = !{!"_ZTSFivE", i32 ()* @f22, i64 0}
+!9 = !{i64 222, i32 ()* @f22, i64 0}
+!10 = !{!"_ZTS1A", i8* @_ZTV1A, i64 16}
+!11 = !{i64 333, i8* @_ZTV1A, i64 16}
+!12 = !{!"_ZTS1A", i8* @_ZTV1B, i64 16}
+!13 = !{i64 333, i8* @_ZTV1B, i64 16}
+!14 = !{!"_ZTS1B", i8* @_ZTV1B, i64 16}
+!15 = !{i64 444, i8* @_ZTV1B, i64 16}
+!17= !{i32 4, !"Cross-DSO CFI", i32 1}