Index: llvm/lib/Bitcode/Reader/BitcodeReader.cpp
===================================================================
--- llvm/lib/Bitcode/Reader/BitcodeReader.cpp
+++ llvm/lib/Bitcode/Reader/BitcodeReader.cpp
@@ -8,6 +8,9 @@
 //===----------------------------------------------------------------------===//
 
 #include "llvm/Bitcode/BitcodeReader.h"
+#include "MetadataLoader.h"
+#include "ValueList.h"
+
 #include "llvm/ADT/APFloat.h"
 #include "llvm/ADT/APInt.h"
 #include "llvm/ADT/ArrayRef.h"
@@ -94,140 +97,6 @@
   SWITCH_INST_MAGIC = 0x4B5 // May 2012 => 1205 => Hex
 };
 
-class BitcodeReaderValueList {
-  std::vector<WeakVH> ValuePtrs;
-
-  /// As we resolve forward-referenced constants, we add information about them
-  /// to this vector.  This allows us to resolve them in bulk instead of
-  /// resolving each reference at a time.  See the code in
-  /// ResolveConstantForwardRefs for more information about this.
-  ///
-  /// The key of this vector is the placeholder constant, the value is the slot
-  /// number that holds the resolved value.
-  typedef std::vector<std::pair<Constant*, unsigned> > ResolveConstantsTy;
-  ResolveConstantsTy ResolveConstants;
-  LLVMContext &Context;
-
-public:
-  BitcodeReaderValueList(LLVMContext &C) : Context(C) {}
-  ~BitcodeReaderValueList() {
-    assert(ResolveConstants.empty() && "Constants not resolved?");
-  }
-
-  // vector compatibility methods
-  unsigned size() const { return ValuePtrs.size(); }
-  void resize(unsigned N) { ValuePtrs.resize(N); }
-  void push_back(Value *V) { ValuePtrs.emplace_back(V); }
-
-  void clear() {
-    assert(ResolveConstants.empty() && "Constants not resolved?");
-    ValuePtrs.clear();
-  }
-
-  Value *operator[](unsigned i) const {
-    assert(i < ValuePtrs.size());
-    return ValuePtrs[i];
-  }
-
-  Value *back() const { return ValuePtrs.back(); }
-  void pop_back() { ValuePtrs.pop_back(); }
-  bool empty() const { return ValuePtrs.empty(); }
-
-  void shrinkTo(unsigned N) {
-    assert(N <= size() && "Invalid shrinkTo request!");
-    ValuePtrs.resize(N);
-  }
-
-  Constant *getConstantFwdRef(unsigned Idx, Type *Ty);
-  Value *getValueFwdRef(unsigned Idx, Type *Ty);
-
-  void assignValue(Value *V, unsigned Idx);
-
-  /// Once all constants are read, this method bulk resolves any forward
-  /// references.
-  void resolveConstantForwardRefs();
-};
-
-class BitcodeReaderMetadataList {
-  unsigned NumFwdRefs;
-  bool AnyFwdRefs;
-  unsigned MinFwdRef;
-  unsigned MaxFwdRef;
-
-  /// Array of metadata references.
-  ///
-  /// Don't use std::vector here.  Some versions of libc++ copy (instead of
-  /// move) on resize, and TrackingMDRef is very expensive to copy.
-  SmallVector<TrackingMDRef, 1> MetadataPtrs;
-
-  /// Structures for resolving old type refs.
-  struct {
-    SmallDenseMap<MDString *, TempMDTuple, 1> Unknown;
-    SmallDenseMap<MDString *, DICompositeType *, 1> Final;
-    SmallDenseMap<MDString *, DICompositeType *, 1> FwdDecls;
-    SmallVector<std::pair<TrackingMDRef, TempMDTuple>, 1> Arrays;
-  } OldTypeRefs;
-
-  LLVMContext &Context;
-
-public:
-  BitcodeReaderMetadataList(LLVMContext &C)
-      : NumFwdRefs(0), AnyFwdRefs(false), Context(C) {}
-
-  // vector compatibility methods
-  unsigned size() const { return MetadataPtrs.size(); }
-  void resize(unsigned N) { MetadataPtrs.resize(N); }
-  void push_back(Metadata *MD) { MetadataPtrs.emplace_back(MD); }
-  void clear() { MetadataPtrs.clear(); }
-  Metadata *back() const { return MetadataPtrs.back(); }
-  void pop_back() { MetadataPtrs.pop_back(); }
-  bool empty() const { return MetadataPtrs.empty(); }
-
-  Metadata *operator[](unsigned i) const {
-    assert(i < MetadataPtrs.size());
-    return MetadataPtrs[i];
-  }
-
-  Metadata *lookup(unsigned I) const {
-    if (I < MetadataPtrs.size())
-      return MetadataPtrs[I];
-    return nullptr;
-  }
-
-  void shrinkTo(unsigned N) {
-    assert(N <= size() && "Invalid shrinkTo request!");
-    assert(!AnyFwdRefs && "Unexpected forward refs");
-    MetadataPtrs.resize(N);
-  }
-
-  /// Return the given metadata, creating a replaceable forward reference if
-  /// necessary.
-  Metadata *getMetadataFwdRef(unsigned Idx);
-
-  /// Return the the given metadata only if it is fully resolved.
-  ///
-  /// Gives the same result as \a lookup(), unless \a MDNode::isResolved()
-  /// would give \c false.
-  Metadata *getMetadataIfResolved(unsigned Idx);
-
-  MDNode *getMDNodeFwdRefOrNull(unsigned Idx);
-  void assignValue(Metadata *MD, unsigned Idx);
-  void tryToResolveCycles();
-  bool hasFwdRefs() const { return AnyFwdRefs; }
-
-  /// Upgrade a type that had an MDString reference.
-  void addTypeRef(MDString &UUID, DICompositeType &CT);
-
-  /// Upgrade a type that had an MDString reference.
-  Metadata *upgradeTypeRef(Metadata *MaybeUUID);
-
-  /// Upgrade a type ref array that may have MDString references.
-  Metadata *upgradeTypeRefArray(Metadata *MaybeTuple);
-
-private:
-  Metadata *resolveTypeRefArray(Metadata *MaybeTuple);
-};
-
 Error error(const Twine &Message) {
   return make_error<StringError>(
       Message, make_error_code(BitcodeError::CorruptedBitcode));
@@ -526,7 +395,7 @@
 
   std::vector<Type*> TypeList;
   BitcodeReaderValueList ValueList;
-  BitcodeReaderMetadataList MetadataList;
+  Optional<MetadataLoader> MDLoader;
   std::vector<Comdat *> ComdatList;
   SmallVector<Instruction *, 64> InstructionList;
 
@@ -536,8 +405,6 @@
   std::vector<std::pair<Function*, unsigned> > FunctionPrologues;
   std::vector<std::pair<Function*, unsigned> > FunctionPersonalityFns;
 
-  bool HasSeenOldLoopTags = false;
-
   /// The set of attributes by index.  Index zero in the file is for null, and
   /// is thus not represented here.  As such all indices are off by one.
   std::vector<AttributeSet> MAttributes;
@@ -560,9 +427,6 @@
   // Intrinsics which were remangled because of types rename
   UpdatedIntrinsicMap RemangledIntrinsics;
 
-  // Map the bitcode's custom MDKind ID to the Module's MDKind ID.
-  DenseMap<unsigned, unsigned> MDKindMap;
-
   // Several operations happen after the module header has been read, but
   // before function bodies are processed. This keeps track of whether
   // we've done this yet.
@@ -597,10 +461,6 @@
 
   bool StripDebugInfo = false;
 
-  /// Functions that need to be matched with subprograms when upgrading old
-  /// metadata.
-  SmallDenseMap<Function *, DISubprogram *, 16> FunctionsWithSPs;
-
   std::vector<std::string> BundleTags;
 
 public:
@@ -638,7 +498,7 @@
   }
 
   Metadata *getFnMetadataByID(unsigned ID) {
-    return MetadataList.getMetadataFwdRef(ID);
+    return MDLoader->getMetadataFwdRef(ID);
   }
 
   BasicBlock *getBasicBlock(unsigned ID) const {
@@ -742,14 +602,6 @@
   Error parseFunctionBody(Function *F);
   Error globalCleanup();
   Error resolveGlobalAndIndirectSymbolInits();
-  Error parseMetadata(bool ModuleLevel = false);
-  Error parseMetadataStrings(ArrayRef<uint64_t> Record, StringRef Blob,
-                             unsigned &NextMetadataNo);
-  Error parseMetadataKinds();
-  Error parseMetadataKindRecord(SmallVectorImpl<uint64_t> &Record);
-  Error parseGlobalObjectAttachment(GlobalObject &GO,
-                                    ArrayRef<uint64_t> Record);
-  Error parseMetadataAttachment(Function &F);
   Error parseUseLists();
   Error findFunctionInStream(
       Function *F,
@@ -831,7 +683,7 @@
                              StringRef ProducerIdentification,
                              LLVMContext &Context)
     : BitcodeReaderBase(std::move(Stream)), Context(Context),
-      ValueList(Context), MetadataList(Context) {
+      ValueList(Context) {
   this->ProducerIdentification = ProducerIdentification;
 }
 
@@ -1114,360 +966,6 @@
   }
 }
 
-namespace llvm {
-namespace {
-
-/// \brief A class for maintaining the slot number definition
-/// as a placeholder for the actual definition for forward constants defs.
-class ConstantPlaceHolder : public ConstantExpr {
-  void operator=(const ConstantPlaceHolder &) = delete;
-
-public:
-  // allocate space for exactly one operand
-  void *operator new(size_t s) { return User::operator new(s, 1); }
-  explicit ConstantPlaceHolder(Type *Ty, LLVMContext &Context)
-      : ConstantExpr(Ty, Instruction::UserOp1, &Op<0>(), 1) {
-    Op<0>() = UndefValue::get(Type::getInt32Ty(Context));
-  }
-
-  /// \brief Methods to support type inquiry through isa, cast, and dyn_cast.
-  static bool classof(const Value *V) {
-    return isa<ConstantExpr>(V) &&
-           cast<ConstantExpr>(V)->getOpcode() == Instruction::UserOp1;
-  }
-
-  /// Provide fast operand accessors
-  DECLARE_TRANSPARENT_OPERAND_ACCESSORS(Value);
-};
-
-} // end anonymous namespace
-
-// FIXME: can we inherit this from ConstantExpr?
-template <>
-struct OperandTraits<ConstantPlaceHolder> :
-  public FixedNumOperandTraits<ConstantPlaceHolder, 1> {
-};
-DEFINE_TRANSPARENT_OPERAND_ACCESSORS(ConstantPlaceHolder, Value)
-
-} // end namespace llvm
-
-void BitcodeReaderValueList::assignValue(Value *V, unsigned Idx) {
-  if (Idx == size()) {
-    push_back(V);
-    return;
-  }
-
-  if (Idx >= size())
-    resize(Idx+1);
-
-  WeakVH &OldV = ValuePtrs[Idx];
-  if (!OldV) {
-    OldV = V;
-    return;
-  }
-
-  // Handle constants and non-constants (e.g. instrs) differently for
-  // efficiency.
-  if (Constant *PHC = dyn_cast<Constant>(&*OldV)) {
-    ResolveConstants.push_back(std::make_pair(PHC, Idx));
-    OldV = V;
-  } else {
-    // If there was a forward reference to this value, replace it.
-    Value *PrevVal = OldV;
-    OldV->replaceAllUsesWith(V);
-    delete PrevVal;
-  }
-}
-
-Constant *BitcodeReaderValueList::getConstantFwdRef(unsigned Idx,
-                                                    Type *Ty) {
-  if (Idx >= size())
-    resize(Idx + 1);
-
-  if (Value *V = ValuePtrs[Idx]) {
-    if (Ty != V->getType())
-      report_fatal_error("Type mismatch in constant table!");
-    return cast<Constant>(V);
-  }
-
-  // Create and return a placeholder, which will later be RAUW'd.
-  Constant *C = new ConstantPlaceHolder(Ty, Context);
-  ValuePtrs[Idx] = C;
-  return C;
-}
-
-Value *BitcodeReaderValueList::getValueFwdRef(unsigned Idx, Type *Ty) {
-  // Bail out for a clearly invalid value. This would make us call resize(0)
-  if (Idx == std::numeric_limits<unsigned>::max())
-    return nullptr;
-
-  if (Idx >= size())
-    resize(Idx + 1);
-
-  if (Value *V = ValuePtrs[Idx]) {
-    // If the types don't match, it's invalid.
-    if (Ty && Ty != V->getType())
-      return nullptr;
-    return V;
-  }
-
-  // No type specified, must be invalid reference.
-  if (!Ty) return nullptr;
-
-  // Create and return a placeholder, which will later be RAUW'd.
-  Value *V = new Argument(Ty);
-  ValuePtrs[Idx] = V;
-  return V;
-}
-
-/// Once all constants are read, this method bulk resolves any forward
-/// references.  The idea behind this is that we sometimes get constants (such
-/// as large arrays) which reference *many* forward ref constants.  Replacing
-/// each of these causes a lot of thrashing when building/reuniquing the
-/// constant.  Instead of doing this, we look at all the uses and rewrite all
-/// the place holders at once for any constant that uses a placeholder.
-void BitcodeReaderValueList::resolveConstantForwardRefs() {
-  // Sort the values by-pointer so that they are efficient to look up with a
-  // binary search.
-  std::sort(ResolveConstants.begin(), ResolveConstants.end());
-
-  SmallVector<Constant*, 64> NewOps;
-
-  while (!ResolveConstants.empty()) {
-    Value *RealVal = operator[](ResolveConstants.back().second);
-    Constant *Placeholder = ResolveConstants.back().first;
-    ResolveConstants.pop_back();
-
-    // Loop over all users of the placeholder, updating them to reference the
-    // new value.  If they reference more than one placeholder, update them all
-    // at once.
-    while (!Placeholder->use_empty()) {
-      auto UI = Placeholder->user_begin();
-      User *U = *UI;
-
-      // If the using object isn't uniqued, just update the operands.  This
-      // handles instructions and initializers for global variables.
-      if (!isa<Constant>(U) || isa<GlobalValue>(U)) {
-        UI.getUse().set(RealVal);
-        continue;
-      }
-
-      // Otherwise, we have a constant that uses the placeholder.  Replace that
-      // constant with a new constant that has *all* placeholder uses updated.
-      Constant *UserC = cast<Constant>(U);
-      for (User::op_iterator I = UserC->op_begin(), E = UserC->op_end();
-           I != E; ++I) {
-        Value *NewOp;
-        if (!isa<ConstantPlaceHolder>(*I)) {
-          // Not a placeholder reference.
-          NewOp = *I;
-        } else if (*I == Placeholder) {
-          // Common case is that it just references this one placeholder.
-          NewOp = RealVal;
-        } else {
-          // Otherwise, look up the placeholder in ResolveConstants.
-          ResolveConstantsTy::iterator It =
-            std::lower_bound(ResolveConstants.begin(), ResolveConstants.end(),
-                             std::pair<Constant*, unsigned>(cast<Constant>(*I),
-                                                            0));
-          assert(It != ResolveConstants.end() && It->first == *I);
-          NewOp = operator[](It->second);
-        }
-
-        NewOps.push_back(cast<Constant>(NewOp));
-      }
-
-      // Make the new constant.
-      Constant *NewC;
-      if (ConstantArray *UserCA = dyn_cast<ConstantArray>(UserC)) {
-        NewC = ConstantArray::get(UserCA->getType(), NewOps);
-      } else if (ConstantStruct *UserCS = dyn_cast<ConstantStruct>(UserC)) {
-        NewC = ConstantStruct::get(UserCS->getType(), NewOps);
-      } else if (isa<ConstantVector>(UserC)) {
-        NewC = ConstantVector::get(NewOps);
-      } else {
-        assert(isa<ConstantExpr>(UserC) && "Must be a ConstantExpr.");
-        NewC = cast<ConstantExpr>(UserC)->getWithOperands(NewOps);
-      }
-
-      UserC->replaceAllUsesWith(NewC);
-      UserC->destroyConstant();
-      NewOps.clear();
-    }
-
-    // Update all ValueHandles, they should be the only users at this point.
-    Placeholder->replaceAllUsesWith(RealVal);
-    delete Placeholder;
-  }
-}
-
-void BitcodeReaderMetadataList::assignValue(Metadata *MD, unsigned Idx) {
-  if (Idx == size()) {
-    push_back(MD);
-    return;
-  }
-
-  if (Idx >= size())
-    resize(Idx+1);
-
-  TrackingMDRef &OldMD = MetadataPtrs[Idx];
-  if (!OldMD) {
-    OldMD.reset(MD);
-    return;
-  }
-
-  // If there was a forward reference to this value, replace it.
-  TempMDTuple PrevMD(cast<MDTuple>(OldMD.get()));
-  PrevMD->replaceAllUsesWith(MD);
-  --NumFwdRefs;
-}
-
-Metadata *BitcodeReaderMetadataList::getMetadataFwdRef(unsigned Idx) {
-  if (Idx >= size())
-    resize(Idx + 1);
-
-  if (Metadata *MD = MetadataPtrs[Idx])
-    return MD;
-
-  // Track forward refs to be resolved later.
-  if (AnyFwdRefs) {
-    MinFwdRef = std::min(MinFwdRef, Idx);
-    MaxFwdRef = std::max(MaxFwdRef, Idx);
-  } else {
-    AnyFwdRefs = true;
-    MinFwdRef = MaxFwdRef = Idx;
-  }
-  ++NumFwdRefs;
-
-  // Create and return a placeholder, which will later be RAUW'd.
-  Metadata *MD = MDNode::getTemporary(Context, None).release();
-  MetadataPtrs[Idx].reset(MD);
-  return MD;
-}
-
-Metadata *BitcodeReaderMetadataList::getMetadataIfResolved(unsigned Idx) {
-  Metadata *MD = lookup(Idx);
-  if (auto *N = dyn_cast_or_null<MDNode>(MD))
-    if (!N->isResolved())
-      return nullptr;
-  return MD;
-}
-
-MDNode *BitcodeReaderMetadataList::getMDNodeFwdRefOrNull(unsigned Idx) {
-  return dyn_cast_or_null<MDNode>(getMetadataFwdRef(Idx));
-}
-
-void BitcodeReaderMetadataList::tryToResolveCycles() {
-  if (NumFwdRefs)
-    // Still forward references... can't resolve cycles.
-    return;
-
-  bool DidReplaceTypeRefs = false;
-
-  // Give up on finding a full definition for any forward decls that remain.
-  for (const auto &Ref : OldTypeRefs.FwdDecls)
-    OldTypeRefs.Final.insert(Ref);
-  OldTypeRefs.FwdDecls.clear();
-
-  // Upgrade from old type ref arrays.  In strange cases, this could add to
-  // OldTypeRefs.Unknown.
-  for (const auto &Array : OldTypeRefs.Arrays) {
-    DidReplaceTypeRefs = true;
-    Array.second->replaceAllUsesWith(resolveTypeRefArray(Array.first.get()));
-  }
-  OldTypeRefs.Arrays.clear();
-
-  // Replace old string-based type refs with the resolved node, if possible.
-  // If we haven't seen the node, leave it to the verifier to complain about
-  // the invalid string reference.
-  for (const auto &Ref : OldTypeRefs.Unknown) {
-    DidReplaceTypeRefs = true;
-    if (DICompositeType *CT = OldTypeRefs.Final.lookup(Ref.first))
-      Ref.second->replaceAllUsesWith(CT);
-    else
-      Ref.second->replaceAllUsesWith(Ref.first);
-  }
-  OldTypeRefs.Unknown.clear();
-
-  // Make sure all the upgraded types are resolved.
-  if (DidReplaceTypeRefs) {
-    AnyFwdRefs = true;
-    MinFwdRef = 0;
-    MaxFwdRef = MetadataPtrs.size() - 1;
-  }
-
-  if (!AnyFwdRefs)
-    // Nothing to do.
-    return;
-
-  // Resolve any cycles.
-  for (unsigned I = MinFwdRef, E = MaxFwdRef + 1; I != E; ++I) {
-    auto &MD = MetadataPtrs[I];
-    auto *N = dyn_cast_or_null<MDNode>(MD);
-    if (!N)
-      continue;
-
-    assert(!N->isTemporary() && "Unexpected forward reference");
-    N->resolveCycles();
-  }
-
-  // Make sure we return early again until there's another forward ref.
-  AnyFwdRefs = false;
-}
-
-void BitcodeReaderMetadataList::addTypeRef(MDString &UUID,
-                                           DICompositeType &CT) {
-  assert(CT.getRawIdentifier() == &UUID && "Mismatched UUID");
-  if (CT.isForwardDecl())
-    OldTypeRefs.FwdDecls.insert(std::make_pair(&UUID, &CT));
-  else
-    OldTypeRefs.Final.insert(std::make_pair(&UUID, &CT));
-}
-
-Metadata *BitcodeReaderMetadataList::upgradeTypeRef(Metadata *MaybeUUID) {
-  auto *UUID = dyn_cast_or_null<MDString>(MaybeUUID);
-  if (LLVM_LIKELY(!UUID))
-    return MaybeUUID;
-
-  if (auto *CT = OldTypeRefs.Final.lookup(UUID))
-    return CT;
-
-  auto &Ref = OldTypeRefs.Unknown[UUID];
-  if (!Ref)
-    Ref = MDNode::getTemporary(Context, None);
-  return Ref.get();
-}
-
-Metadata *BitcodeReaderMetadataList::upgradeTypeRefArray(Metadata *MaybeTuple) {
-  auto *Tuple = dyn_cast_or_null<MDTuple>(MaybeTuple);
-  if (!Tuple || Tuple->isDistinct())
-    return MaybeTuple;
-
-  // Look through the array immediately if possible.
-  if (!Tuple->isTemporary())
-    return resolveTypeRefArray(Tuple);
-
-  // Create and return a placeholder to use for now.  Eventually
-  // resolveTypeRefArrays() will be resolve this forward reference.
-  OldTypeRefs.Arrays.emplace_back(
-      std::piecewise_construct, std::forward_as_tuple(Tuple),
-      std::forward_as_tuple(MDTuple::getTemporary(Context, None)));
-  return OldTypeRefs.Arrays.back().second.get();
-}
-
-Metadata *BitcodeReaderMetadataList::resolveTypeRefArray(Metadata *MaybeTuple) {
-  auto *Tuple = dyn_cast_or_null<MDTuple>(MaybeTuple);
-  if (!Tuple || Tuple->isDistinct())
-    return MaybeTuple;
-
-  // Look through the DITypeRefArray, upgrading each DITypeRef.
-  SmallVector<Metadata *, 32> Ops;
-  Ops.reserve(Tuple->getNumOperands());
-  for (Metadata *MD : Tuple->operands())
-    Ops.push_back(upgradeTypeRef(MD));
-
-  return MDTuple::get(Context, Ops);
-}
 
 Type *BitcodeReader::getTypeByID(unsigned ID) {
   // The type table size is always specified correctly.
@@ -2321,798 +1819,6 @@
   }
 }
 
-/// Parse a single METADATA_KIND record, inserting result in MDKindMap.
-Error BitcodeReader::parseMetadataKindRecord(
-    SmallVectorImpl<uint64_t> &Record) {
-  if (Record.size() < 2)
-    return error("Invalid record");
-
-  unsigned Kind = Record[0];
-  SmallString<8> Name(Record.begin() + 1, Record.end());
-
-  unsigned NewKind = TheModule->getMDKindID(Name.str());
-  if (!MDKindMap.insert(std::make_pair(Kind, NewKind)).second)
-    return error("Conflicting METADATA_KIND records");
-  return Error::success();
-}
-
-static int64_t unrotateSign(uint64_t U) { return U & 1 ? ~(U >> 1) : U >> 1; }
-
-Error BitcodeReader::parseMetadataStrings(ArrayRef<uint64_t> Record,
-                                          StringRef Blob,
-                                          unsigned &NextMetadataNo) {
-  // All the MDStrings in the block are emitted together in a single
-  // record.  The strings are concatenated and stored in a blob along with
-  // their sizes.
-  if (Record.size() != 2)
-    return error("Invalid record: metadata strings layout");
-
-  unsigned NumStrings = Record[0];
-  unsigned StringsOffset = Record[1];
-  if (!NumStrings)
-    return error("Invalid record: metadata strings with no strings");
-  if (StringsOffset > Blob.size())
-    return error("Invalid record: metadata strings corrupt offset");
-
-  StringRef Lengths = Blob.slice(0, StringsOffset);
-  SimpleBitstreamCursor R(Lengths);
-
-  StringRef Strings = Blob.drop_front(StringsOffset);
-  do {
-    if (R.AtEndOfStream())
-      return error("Invalid record: metadata strings bad length");
-
-    unsigned Size = R.ReadVBR(6);
-    if (Strings.size() < Size)
-      return error("Invalid record: metadata strings truncated chars");
-
-    MetadataList.assignValue(MDString::get(Context, Strings.slice(0, Size)),
-                             NextMetadataNo++);
-    Strings = Strings.drop_front(Size);
-  } while (--NumStrings);
-
-  return Error::success();
-}
-
-namespace {
-
-class PlaceholderQueue {
-  // Placeholders would thrash around when moved, so store in a std::deque
-  // instead of some sort of vector.
-  std::deque<DistinctMDOperandPlaceholder> PHs;
-
-public:
-  DistinctMDOperandPlaceholder &getPlaceholderOp(unsigned ID);
-  void flush(BitcodeReaderMetadataList &MetadataList);
-};
-
-} // end anonymous namespace
-
-DistinctMDOperandPlaceholder &PlaceholderQueue::getPlaceholderOp(unsigned ID) {
-  PHs.emplace_back(ID);
-  return PHs.back();
-}
-
-void PlaceholderQueue::flush(BitcodeReaderMetadataList &MetadataList) {
-  while (!PHs.empty()) {
-    PHs.front().replaceUseWith(
-        MetadataList.getMetadataFwdRef(PHs.front().getID()));
-    PHs.pop_front();
-  }
-}
-
-/// Parse a METADATA_BLOCK. If ModuleLevel is true then we are parsing
-/// module level metadata.
-Error BitcodeReader::parseMetadata(bool ModuleLevel) {
-  assert((ModuleLevel || DeferredMetadataInfo.empty()) &&
-         "Must read all module-level metadata before function-level");
-
-  unsigned NextMetadataNo = MetadataList.size();
-
-  if (!ModuleLevel && MetadataList.hasFwdRefs())
-    return error("Invalid metadata: fwd refs into function blocks");
-
-  if (Stream.EnterSubBlock(bitc::METADATA_BLOCK_ID))
-    return error("Invalid record");
-
-  std::vector<std::pair<DICompileUnit *, Metadata *>> CUSubprograms;
-  SmallVector<uint64_t, 64> Record;
-
-  PlaceholderQueue Placeholders;
-  bool IsDistinct;
-  auto getMD = [&](unsigned ID) -> Metadata * {
-    if (!IsDistinct)
-      return MetadataList.getMetadataFwdRef(ID);
-    if (auto *MD = MetadataList.getMetadataIfResolved(ID))
-      return MD;
-    return &Placeholders.getPlaceholderOp(ID);
-  };
-  auto getMDOrNull = [&](unsigned ID) -> Metadata * {
-    if (ID)
-      return getMD(ID - 1);
-    return nullptr;
-  };
-  auto getMDOrNullWithoutPlaceholders = [&](unsigned ID) -> Metadata * {
-    if (ID)
-      return MetadataList.getMetadataFwdRef(ID - 1);
-    return nullptr;
-  };
-  auto getMDString = [&](unsigned ID) -> MDString *{
-    // This requires that the ID is not really a forward reference.  In
-    // particular, the MDString must already have been resolved.
-    return cast_or_null<MDString>(getMDOrNull(ID));
-  };
-
-  // Support for old type refs.
-  auto getDITypeRefOrNull = [&](unsigned ID) {
-    return MetadataList.upgradeTypeRef(getMDOrNull(ID));
-  };
-
-#define GET_OR_DISTINCT(CLASS, ARGS)                                           \
-  (IsDistinct ? CLASS::getDistinct ARGS : CLASS::get ARGS)
-
-  // Read all the records.
-  while (true) {
-    BitstreamEntry Entry = Stream.advanceSkippingSubblocks();
-
-    switch (Entry.Kind) {
-    case BitstreamEntry::SubBlock: // Handled for us already.
-    case BitstreamEntry::Error:
-      return error("Malformed block");
-    case BitstreamEntry::EndBlock:
-      // Upgrade old-style CU <-> SP pointers to point from SP to CU.
-      for (auto CU_SP : CUSubprograms)
-        if (auto *SPs = dyn_cast_or_null<MDTuple>(CU_SP.second))
-          for (auto &Op : SPs->operands())
-            if (auto *SP = dyn_cast_or_null<MDNode>(Op))
-              SP->replaceOperandWith(7, CU_SP.first);
-
-      MetadataList.tryToResolveCycles();
-      Placeholders.flush(MetadataList);
-      return Error::success();
-    case BitstreamEntry::Record:
-      // The interesting case.
-      break;
-    }
-
-    // Read a record.
-    Record.clear();
-    StringRef Blob;
-    unsigned Code = Stream.readRecord(Entry.ID, Record, &Blob);
-    IsDistinct = false;
-    switch (Code) {
-    default:  // Default behavior: ignore.
-      break;
-    case bitc::METADATA_NAME: {
-      // Read name of the named metadata.
-      SmallString<8> Name(Record.begin(), Record.end());
-      Record.clear();
-      Code = Stream.ReadCode();
-
-      unsigned NextBitCode = Stream.readRecord(Code, Record);
-      if (NextBitCode != bitc::METADATA_NAMED_NODE)
-        return error("METADATA_NAME not followed by METADATA_NAMED_NODE");
-
-      // Read named metadata elements.
-      unsigned Size = Record.size();
-      NamedMDNode *NMD = TheModule->getOrInsertNamedMetadata(Name);
-      for (unsigned i = 0; i != Size; ++i) {
-        MDNode *MD = MetadataList.getMDNodeFwdRefOrNull(Record[i]);
-        if (!MD)
-          return error("Invalid record");
-        NMD->addOperand(MD);
-      }
-      break;
-    }
-    case bitc::METADATA_OLD_FN_NODE: {
-      // FIXME: Remove in 4.0.
-      // This is a LocalAsMetadata record, the only type of function-local
-      // metadata.
-      if (Record.size() % 2 == 1)
-        return error("Invalid record");
-
-      // If this isn't a LocalAsMetadata record, we're dropping it.  This used
-      // to be legal, but there's no upgrade path.
-      auto dropRecord = [&] {
-        MetadataList.assignValue(MDNode::get(Context, None), NextMetadataNo++);
-      };
-      if (Record.size() != 2) {
-        dropRecord();
-        break;
-      }
-
-      Type *Ty = getTypeByID(Record[0]);
-      if (Ty->isMetadataTy() || Ty->isVoidTy()) {
-        dropRecord();
-        break;
-      }
-
-      MetadataList.assignValue(
-          LocalAsMetadata::get(ValueList.getValueFwdRef(Record[1], Ty)),
-          NextMetadataNo++);
-      break;
-    }
-    case bitc::METADATA_OLD_NODE: {
-      // FIXME: Remove in 4.0.
-      if (Record.size() % 2 == 1)
-        return error("Invalid record");
-
-      unsigned Size = Record.size();
-      SmallVector<Metadata *, 8> Elts;
-      for (unsigned i = 0; i != Size; i += 2) {
-        Type *Ty = getTypeByID(Record[i]);
-        if (!Ty)
-          return error("Invalid record");
-        if (Ty->isMetadataTy())
-          Elts.push_back(getMD(Record[i + 1]));
-        else if (!Ty->isVoidTy()) {
-          auto *MD =
-              ValueAsMetadata::get(ValueList.getValueFwdRef(Record[i + 1], Ty));
-          assert(isa<ConstantAsMetadata>(MD) &&
-                 "Expected non-function-local metadata");
-          Elts.push_back(MD);
-        } else
-          Elts.push_back(nullptr);
-      }
-      MetadataList.assignValue(MDNode::get(Context, Elts), NextMetadataNo++);
-      break;
-    }
-    case bitc::METADATA_VALUE: {
-      if (Record.size() != 2)
-        return error("Invalid record");
-
-      Type *Ty = getTypeByID(Record[0]);
-      if (Ty->isMetadataTy() || Ty->isVoidTy())
-        return error("Invalid record");
-
-      MetadataList.assignValue(
-          ValueAsMetadata::get(ValueList.getValueFwdRef(Record[1], Ty)),
-          NextMetadataNo++);
-      break;
-    }
-    case bitc::METADATA_DISTINCT_NODE:
-      IsDistinct = true;
-      LLVM_FALLTHROUGH;
-    case bitc::METADATA_NODE: {
-      SmallVector<Metadata *, 8> Elts;
-      Elts.reserve(Record.size());
-      for (unsigned ID : Record)
-        Elts.push_back(getMDOrNull(ID));
-      MetadataList.assignValue(IsDistinct ? MDNode::getDistinct(Context, Elts)
-                                          : MDNode::get(Context, Elts),
-                               NextMetadataNo++);
-      break;
-    }
-    case bitc::METADATA_LOCATION: {
-      if (Record.size() != 5)
-        return error("Invalid record");
-
-      IsDistinct = Record[0];
-      unsigned Line = Record[1];
-      unsigned Column = Record[2];
-      Metadata *Scope = getMD(Record[3]);
-      Metadata *InlinedAt = getMDOrNull(Record[4]);
-      MetadataList.assignValue(
-          GET_OR_DISTINCT(DILocation,
-                          (Context, Line, Column, Scope, InlinedAt)),
-          NextMetadataNo++);
-      break;
-    }
-    case bitc::METADATA_GENERIC_DEBUG: {
-      if (Record.size() < 4)
-        return error("Invalid record");
-
-      IsDistinct = Record[0];
-      unsigned Tag = Record[1];
-      unsigned Version = Record[2];
-
-      if (Tag >= 1u << 16 || Version != 0)
-        return error("Invalid record");
-
-      auto *Header = getMDString(Record[3]);
-      SmallVector<Metadata *, 8> DwarfOps;
-      for (unsigned I = 4, E = Record.size(); I != E; ++I)
-        DwarfOps.push_back(getMDOrNull(Record[I]));
-      MetadataList.assignValue(
-          GET_OR_DISTINCT(GenericDINode, (Context, Tag, Header, DwarfOps)),
-          NextMetadataNo++);
-      break;
-    }
-    case bitc::METADATA_SUBRANGE: {
-      if (Record.size() != 3)
-        return error("Invalid record");
-
-      IsDistinct = Record[0];
-      MetadataList.assignValue(
-          GET_OR_DISTINCT(DISubrange,
-                          (Context, Record[1], unrotateSign(Record[2]))),
-          NextMetadataNo++);
-      break;
-    }
-    case bitc::METADATA_ENUMERATOR: {
-      if (Record.size() != 3)
-        return error("Invalid record");
-
-      IsDistinct = Record[0];
-      MetadataList.assignValue(
-          GET_OR_DISTINCT(DIEnumerator, (Context, unrotateSign(Record[1]),
-                                         getMDString(Record[2]))),
-          NextMetadataNo++);
-      break;
-    }
-    case bitc::METADATA_BASIC_TYPE: {
-      if (Record.size() != 6)
-        return error("Invalid record");
-
-      IsDistinct = Record[0];
-      MetadataList.assignValue(
-          GET_OR_DISTINCT(DIBasicType,
-                          (Context, Record[1], getMDString(Record[2]),
-                           Record[3], Record[4], Record[5])),
-          NextMetadataNo++);
-      break;
-    }
-    case bitc::METADATA_DERIVED_TYPE: {
-      if (Record.size() != 12)
-        return error("Invalid record");
-
-      IsDistinct = Record[0];
-      DINode::DIFlags Flags = static_cast<DINode::DIFlags>(Record[10]);
-      MetadataList.assignValue(
-          GET_OR_DISTINCT(DIDerivedType,
-                          (Context, Record[1], getMDString(Record[2]),
-                           getMDOrNull(Record[3]), Record[4],
-                           getDITypeRefOrNull(Record[5]),
-                           getDITypeRefOrNull(Record[6]), Record[7], Record[8],
-                           Record[9], Flags, getDITypeRefOrNull(Record[11]))),
-          NextMetadataNo++);
-      break;
-    }
-    case bitc::METADATA_COMPOSITE_TYPE: {
-      if (Record.size() != 16)
-        return error("Invalid record");
-
-      // If we have a UUID and this is not a forward declaration, lookup the
-      // mapping.
-      IsDistinct = Record[0] & 0x1;
-      bool IsNotUsedInTypeRef = Record[0] >= 2;
-      unsigned Tag = Record[1];
-      MDString *Name = getMDString(Record[2]);
-      Metadata *File = getMDOrNull(Record[3]);
-      unsigned Line = Record[4];
-      Metadata *Scope = getDITypeRefOrNull(Record[5]);
-      Metadata *BaseType = getDITypeRefOrNull(Record[6]);
-      uint64_t SizeInBits = Record[7];
-      if (Record[8] > (uint64_t)std::numeric_limits<uint32_t>::max())
-        return error("Alignment value is too large");
-      uint32_t AlignInBits = Record[8];
-      uint64_t OffsetInBits = Record[9];
-      DINode::DIFlags Flags = static_cast<DINode::DIFlags>(Record[10]);
-      Metadata *Elements = getMDOrNull(Record[11]);
-      unsigned RuntimeLang = Record[12];
-      Metadata *VTableHolder = getDITypeRefOrNull(Record[13]);
-      Metadata *TemplateParams = getMDOrNull(Record[14]);
-      auto *Identifier = getMDString(Record[15]);
-      DICompositeType *CT = nullptr;
-      if (Identifier)
-        CT = DICompositeType::buildODRType(
-            Context, *Identifier, Tag, Name, File, Line, Scope, BaseType,
-            SizeInBits, AlignInBits, OffsetInBits, Flags, Elements, RuntimeLang,
-            VTableHolder, TemplateParams);
-
-      // Create a node if we didn't get a lazy ODR type.
-      if (!CT)
-        CT = GET_OR_DISTINCT(DICompositeType,
-                             (Context, Tag, Name, File, Line, Scope, BaseType,
-                              SizeInBits, AlignInBits, OffsetInBits, Flags,
-                              Elements, RuntimeLang, VTableHolder,
-                              TemplateParams, Identifier));
-      if (!IsNotUsedInTypeRef && Identifier)
-        MetadataList.addTypeRef(*Identifier, *cast<DICompositeType>(CT));
-
-      MetadataList.assignValue(CT, NextMetadataNo++);
-      break;
-    }
-    case bitc::METADATA_SUBROUTINE_TYPE: {
-      if (Record.size() < 3 || Record.size() > 4)
-        return error("Invalid record");
-      bool IsOldTypeRefArray = Record[0] < 2;
-      unsigned CC = (Record.size() > 3) ? Record[3] : 0;
-
-      IsDistinct = Record[0] & 0x1;
-      DINode::DIFlags Flags = static_cast<DINode::DIFlags>(Record[1]);
-      Metadata *Types = getMDOrNull(Record[2]);
-      if (LLVM_UNLIKELY(IsOldTypeRefArray))
-        Types = MetadataList.upgradeTypeRefArray(Types);
-
-      MetadataList.assignValue(
-          GET_OR_DISTINCT(DISubroutineType, (Context, Flags, CC, Types)),
-          NextMetadataNo++);
-      break;
-    }
-
-    case bitc::METADATA_MODULE: {
-      if (Record.size() != 6)
-        return error("Invalid record");
-
-      IsDistinct = Record[0];
-      MetadataList.assignValue(
-          GET_OR_DISTINCT(DIModule,
-                          (Context, getMDOrNull(Record[1]),
-                           getMDString(Record[2]), getMDString(Record[3]),
-                           getMDString(Record[4]), getMDString(Record[5]))),
-          NextMetadataNo++);
-      break;
-    }
-
-    case bitc::METADATA_FILE: {
-      if (Record.size() != 3)
-        return error("Invalid record");
-
-      IsDistinct = Record[0];
-      MetadataList.assignValue(
-          GET_OR_DISTINCT(DIFile, (Context, getMDString(Record[1]),
-                                   getMDString(Record[2]))),
-          NextMetadataNo++);
-      break;
-    }
-    case bitc::METADATA_COMPILE_UNIT: {
-      if (Record.size() < 14 || Record.size() > 17)
-        return error("Invalid record");
-
-      // Ignore Record[0], which indicates whether this compile unit is
-      // distinct.  It's always distinct.
-      IsDistinct = true;
-      auto *CU = DICompileUnit::getDistinct(
-          Context, Record[1], getMDOrNull(Record[2]), getMDString(Record[3]),
-          Record[4], getMDString(Record[5]), Record[6], getMDString(Record[7]),
-          Record[8], getMDOrNull(Record[9]), getMDOrNull(Record[10]),
-          getMDOrNull(Record[12]), getMDOrNull(Record[13]),
-          Record.size() <= 15 ? nullptr : getMDOrNull(Record[15]),
-          Record.size() <= 14 ? 0 : Record[14],
-          Record.size() <= 16 ? true : Record[16]);
-
-      MetadataList.assignValue(CU, NextMetadataNo++);
-
-      // Move the Upgrade the list of subprograms.
-      if (Metadata *SPs = getMDOrNullWithoutPlaceholders(Record[11]))
-        CUSubprograms.push_back({CU, SPs});
-      break;
-    }
-    case bitc::METADATA_SUBPROGRAM: {
-      if (Record.size() < 18 || Record.size() > 20)
-        return error("Invalid record");
-
-      IsDistinct =
-          (Record[0] & 1) || Record[8]; // All definitions should be distinct.
-      // Version 1 has a Function as Record[15].
-      // Version 2 has removed Record[15].
-      // Version 3 has the Unit as Record[15].
-      // Version 4 added thisAdjustment.
-      bool HasUnit = Record[0] >= 2;
-      if (HasUnit && Record.size() < 19)
-        return error("Invalid record");
-      Metadata *CUorFn = getMDOrNull(Record[15]);
-      unsigned Offset = Record.size() >= 19 ? 1 : 0;
-      bool HasFn = Offset && !HasUnit;
-      bool HasThisAdj = Record.size() >= 20;
-      DISubprogram *SP = GET_OR_DISTINCT(
-          DISubprogram, (Context,
-                         getDITypeRefOrNull(Record[1]),  // scope
-                         getMDString(Record[2]),         // name
-                         getMDString(Record[3]),         // linkageName
-                         getMDOrNull(Record[4]),         // file
-                         Record[5],                      // line
-                         getMDOrNull(Record[6]),         // type
-                         Record[7],                      // isLocal
-                         Record[8],                      // isDefinition
-                         Record[9],                      // scopeLine
-                         getDITypeRefOrNull(Record[10]), // containingType
-                         Record[11],                     // virtuality
-                         Record[12],                     // virtualIndex
-                         HasThisAdj ? Record[19] : 0,    // thisAdjustment
-                         static_cast<DINode::DIFlags>(Record[13] // flags
-                                                      ),
-                         Record[14],                       // isOptimized
-                         HasUnit ? CUorFn : nullptr,       // unit
-                         getMDOrNull(Record[15 + Offset]), // templateParams
-                         getMDOrNull(Record[16 + Offset]), // declaration
-                         getMDOrNull(Record[17 + Offset])  // variables
-                         ));
-      MetadataList.assignValue(SP, NextMetadataNo++);
-
-      // Upgrade sp->function mapping to function->sp mapping.
-      if (HasFn) {
-        if (auto *CMD = dyn_cast_or_null<ConstantAsMetadata>(CUorFn))
-          if (auto *F = dyn_cast<Function>(CMD->getValue())) {
-            if (F->isMaterializable())
-              // Defer until materialized; unmaterialized functions may not have
-              // metadata.
-              FunctionsWithSPs[F] = SP;
-            else if (!F->empty())
-              F->setSubprogram(SP);
-          }
-      }
-      break;
-    }
-    case bitc::METADATA_LEXICAL_BLOCK: {
-      if (Record.size() != 5)
-        return error("Invalid record");
-
-      IsDistinct = Record[0];
-      MetadataList.assignValue(
-          GET_OR_DISTINCT(DILexicalBlock,
-                          (Context, getMDOrNull(Record[1]),
-                           getMDOrNull(Record[2]), Record[3], Record[4])),
-          NextMetadataNo++);
-      break;
-    }
-    case bitc::METADATA_LEXICAL_BLOCK_FILE: {
-      if (Record.size() != 4)
-        return error("Invalid record");
-
-      IsDistinct = Record[0];
-      MetadataList.assignValue(
-          GET_OR_DISTINCT(DILexicalBlockFile,
-                          (Context, getMDOrNull(Record[1]),
-                           getMDOrNull(Record[2]), Record[3])),
-          NextMetadataNo++);
-      break;
-    }
-    case bitc::METADATA_NAMESPACE: {
-      if (Record.size() != 5)
-        return error("Invalid record");
-
-      IsDistinct = Record[0] & 1;
-      bool ExportSymbols = Record[0] & 2;
-      MetadataList.assignValue(
-          GET_OR_DISTINCT(DINamespace,
-                          (Context, getMDOrNull(Record[1]),
-                           getMDOrNull(Record[2]), getMDString(Record[3]),
-                           Record[4], ExportSymbols)),
-          NextMetadataNo++);
-      break;
-    }
-    case bitc::METADATA_MACRO: {
-      if (Record.size() != 5)
-        return error("Invalid record");
-
-      IsDistinct = Record[0];
-      MetadataList.assignValue(
-          GET_OR_DISTINCT(DIMacro,
-                          (Context, Record[1], Record[2],
-                           getMDString(Record[3]), getMDString(Record[4]))),
-          NextMetadataNo++);
-      break;
-    }
-    case bitc::METADATA_MACRO_FILE: {
-      if (Record.size() != 5)
-        return error("Invalid record");
-
-      IsDistinct = Record[0];
-      MetadataList.assignValue(
-          GET_OR_DISTINCT(DIMacroFile,
-                          (Context, Record[1], Record[2],
-                           getMDOrNull(Record[3]), getMDOrNull(Record[4]))),
-          NextMetadataNo++);
-      break;
-    }
-    case bitc::METADATA_TEMPLATE_TYPE: {
-      if (Record.size() != 3)
-        return error("Invalid record");
-
-      IsDistinct = Record[0];
-      MetadataList.assignValue(GET_OR_DISTINCT(DITemplateTypeParameter,
-                                               (Context, getMDString(Record[1]),
-                                                getDITypeRefOrNull(Record[2]))),
-                               NextMetadataNo++);
-      break;
-    }
-    case bitc::METADATA_TEMPLATE_VALUE: {
-      if (Record.size() != 5)
-        return error("Invalid record");
-
-      IsDistinct = Record[0];
-      MetadataList.assignValue(
-          GET_OR_DISTINCT(DITemplateValueParameter,
-                          (Context, Record[1], getMDString(Record[2]),
-                           getDITypeRefOrNull(Record[3]),
-                           getMDOrNull(Record[4]))),
-          NextMetadataNo++);
-      break;
-    }
-    case bitc::METADATA_GLOBAL_VAR: {
-      if (Record.size() < 11 || Record.size() > 12)
-        return error("Invalid record");
-
-      IsDistinct = Record[0];
-
-      // Upgrade old metadata, which stored a global variable reference or a
-      // ConstantInt here.
-      Metadata *Expr = getMDOrNull(Record[9]);
-      uint32_t AlignInBits = 0;
-      if (Record.size() > 11) {
-        if (Record[11] > (uint64_t)std::numeric_limits<uint32_t>::max())
-          return error("Alignment value is too large");
-        AlignInBits = Record[11];
-      }
-      GlobalVariable *Attach = nullptr;
-      if (auto *CMD = dyn_cast_or_null<ConstantAsMetadata>(Expr)) {
-        if (auto *GV = dyn_cast<GlobalVariable>(CMD->getValue())) {
-          Attach = GV;
-          Expr = nullptr;
-        } else if (auto *CI = dyn_cast<ConstantInt>(CMD->getValue())) {
-          Expr = DIExpression::get(Context,
-                                   {dwarf::DW_OP_constu, CI->getZExtValue(),
-                                    dwarf::DW_OP_stack_value});
-        } else {
-          Expr = nullptr;
-        }
-      }
-
-      DIGlobalVariable *DGV = GET_OR_DISTINCT(
-          DIGlobalVariable,
-          (Context, getMDOrNull(Record[1]), getMDString(Record[2]),
-           getMDString(Record[3]), getMDOrNull(Record[4]), Record[5],
-           getDITypeRefOrNull(Record[6]), Record[7], Record[8], Expr,
-           getMDOrNull(Record[10]), AlignInBits));
-      MetadataList.assignValue(DGV, NextMetadataNo++);
-
-      if (Attach)
-        Attach->addDebugInfo(DGV);
-
-      break;
-    }
-    case bitc::METADATA_LOCAL_VAR: {
-      // 10th field is for the obseleted 'inlinedAt:' field.
-      if (Record.size() < 8 || Record.size() > 10)
-        return error("Invalid record");
-
-      IsDistinct = Record[0] & 1;
-      bool HasAlignment = Record[0] & 2;
-      // 2nd field used to be an artificial tag, either DW_TAG_auto_variable or
-      // DW_TAG_arg_variable, if we have alignment flag encoded it means, that
-      // this is newer version of record which doesn't have artifical tag.
-      bool HasTag = !HasAlignment && Record.size() > 8;
-      DINode::DIFlags Flags = static_cast<DINode::DIFlags>(Record[7 + HasTag]);
-      uint32_t AlignInBits = 0;
-      if (HasAlignment) {
-        if (Record[8 + HasTag] >
-            (uint64_t)std::numeric_limits<uint32_t>::max())
-          return error("Alignment value is too large");
-        AlignInBits = Record[8 + HasTag];
-      }
-      MetadataList.assignValue(
-          GET_OR_DISTINCT(DILocalVariable,
-                          (Context, getMDOrNull(Record[1 + HasTag]),
-                           getMDString(Record[2 + HasTag]),
-                           getMDOrNull(Record[3 + HasTag]), Record[4 + HasTag],
-                           getDITypeRefOrNull(Record[5 + HasTag]),
-                           Record[6 + HasTag], Flags, AlignInBits)),
-          NextMetadataNo++);
-      break;
-    }
-    case bitc::METADATA_EXPRESSION: {
-      if (Record.size() < 1)
-        return error("Invalid record");
-
-      IsDistinct = Record[0] & 1;
-      bool HasOpFragment = Record[0] & 2;
-      auto Elts = MutableArrayRef<uint64_t>(Record).slice(1);
-      if (!HasOpFragment)
-        if (unsigned N = Elts.size())
-          if (N >= 3 && Elts[N - 3] == dwarf::DW_OP_bit_piece)
-            Elts[N-3] = dwarf::DW_OP_LLVM_fragment;
-
-      MetadataList.assignValue(
-          GET_OR_DISTINCT(DIExpression,
-                          (Context, makeArrayRef(Record).slice(1))),
-          NextMetadataNo++);
-      break;
-    }
-    case bitc::METADATA_OBJC_PROPERTY: {
-      if (Record.size() != 8)
-        return error("Invalid record");
-
-      IsDistinct = Record[0];
-      MetadataList.assignValue(
-          GET_OR_DISTINCT(DIObjCProperty,
-                          (Context, getMDString(Record[1]),
-                           getMDOrNull(Record[2]), Record[3],
-                           getMDString(Record[4]), getMDString(Record[5]),
-                           Record[6], getDITypeRefOrNull(Record[7]))),
-          NextMetadataNo++);
-      break;
-    }
-    case bitc::METADATA_IMPORTED_ENTITY: {
-      if (Record.size() != 6)
-        return error("Invalid record");
-
-      IsDistinct = Record[0];
-      MetadataList.assignValue(
-          GET_OR_DISTINCT(DIImportedEntity,
-                          (Context, Record[1], getMDOrNull(Record[2]),
-                           getDITypeRefOrNull(Record[3]), Record[4],
-                           getMDString(Record[5]))),
-          NextMetadataNo++);
-      break;
-    }
-    case bitc::METADATA_STRING_OLD: {
-      std::string String(Record.begin(), Record.end());
-
-      // Test for upgrading !llvm.loop.
-      HasSeenOldLoopTags |= mayBeOldLoopAttachmentTag(String);
-
-      Metadata *MD = MDString::get(Context, String);
-      MetadataList.assignValue(MD, NextMetadataNo++);
-      break;
-    }
-    case bitc::METADATA_STRINGS:
-      if (Error Err = parseMetadataStrings(Record, Blob, NextMetadataNo))
-        return Err;
-      break;
-    case bitc::METADATA_GLOBAL_DECL_ATTACHMENT: {
-      if (Record.size() % 2 == 0)
-        return error("Invalid record");
-      unsigned ValueID = Record[0];
-      if (ValueID >= ValueList.size())
-        return error("Invalid record");
-      if (auto *GO = dyn_cast<GlobalObject>(ValueList[ValueID]))
-        if (Error Err = parseGlobalObjectAttachment(
-                *GO, ArrayRef<uint64_t>(Record).slice(1)))
-          return Err;
-      break;
-    }
-    case bitc::METADATA_KIND: {
-      // Support older bitcode files that had METADATA_KIND records in a
-      // block with METADATA_BLOCK_ID.
-      if (Error Err = parseMetadataKindRecord(Record))
-        return Err;
-      break;
-    }
-    }
-  }
-
-#undef GET_OR_DISTINCT
-}
-
-/// Parse the metadata kinds out of the METADATA_KIND_BLOCK.
-Error BitcodeReader::parseMetadataKinds() {
-  if (Stream.EnterSubBlock(bitc::METADATA_KIND_BLOCK_ID))
-    return error("Invalid record");
-
-  SmallVector<uint64_t, 64> Record;
-
-  // Read all the records.
-  while (true) {
-    BitstreamEntry Entry = Stream.advanceSkippingSubblocks();
-
-    switch (Entry.Kind) {
-    case BitstreamEntry::SubBlock: // Handled for us already.
-    case BitstreamEntry::Error:
-      return error("Malformed block");
-    case BitstreamEntry::EndBlock:
-      return Error::success();
-    case BitstreamEntry::Record:
-      // The interesting case.
-      break;
-    }
-
-    // Read a record.
-    Record.clear();
-    unsigned Code = Stream.readRecord(Entry.ID, Record);
-    switch (Code) {
-    default: // Default behavior: ignore.
-      break;
-    case bitc::METADATA_KIND: {
-      if (Error Err = parseMetadataKindRecord(Record))
-        return Err;
-      break;
-    }
-    }
-  }
-}
-
 /// Decode a signed value stored with the sign bit in the LSB for dense VBR
 /// encoding.
 uint64_t BitcodeReader::decodeSignRotatedValue(uint64_t V) {
@@ -3777,7 +2483,7 @@
   for (uint64_t BitPos : DeferredMetadataInfo) {
     // Move the bit stream to the saved position.
     Stream.JumpToBit(BitPos);
-    if (Error Err = parseMetadata(true))
+    if (Error Err = MDLoader->parseModuleMetadata())
       return Err;
   }
   DeferredMetadataInfo.clear();
@@ -3961,11 +2667,11 @@
           break;
         }
         assert(DeferredMetadataInfo.empty() && "Unexpected deferred metadata");
-        if (Error Err = parseMetadata(true))
+        if (Error Err = MDLoader->parseModuleMetadata())
           return Err;
         break;
       case bitc::METADATA_KIND_BLOCK_ID:
-        if (Error Err = parseMetadataKinds())
+        if (Error Err = MDLoader->parseMetadataKinds())
           return Err;
         break;
       case bitc::FUNCTION_BLOCK_ID:
@@ -4367,92 +3073,11 @@
 
 Error BitcodeReader::parseBitcodeInto(Module *M, bool ShouldLazyLoadMetadata) {
   TheModule = M;
+  MDLoader = MetadataLoader(Stream, *M, ValueList,
+                            [&](unsigned ID) { return getTypeByID(ID); });
   return parseModule(0, ShouldLazyLoadMetadata);
 }
 
-Error BitcodeReader::parseGlobalObjectAttachment(GlobalObject &GO,
-                                                 ArrayRef<uint64_t> Record) {
-  assert(Record.size() % 2 == 0);
-  for (unsigned I = 0, E = Record.size(); I != E; I += 2) {
-    auto K = MDKindMap.find(Record[I]);
-    if (K == MDKindMap.end())
-      return error("Invalid ID");
-    MDNode *MD = MetadataList.getMDNodeFwdRefOrNull(Record[I + 1]);
-    if (!MD)
-      return error("Invalid metadata attachment");
-    GO.addMetadata(K->second, *MD);
-  }
-  return Error::success();
-}
-
-/// Parse metadata attachments.
-Error BitcodeReader::parseMetadataAttachment(Function &F) {
-  if (Stream.EnterSubBlock(bitc::METADATA_ATTACHMENT_ID))
-    return error("Invalid record");
-
-  SmallVector<uint64_t, 64> Record;
-
-  while (true) {
-    BitstreamEntry Entry = Stream.advanceSkippingSubblocks();
-
-    switch (Entry.Kind) {
-    case BitstreamEntry::SubBlock: // Handled for us already.
-    case BitstreamEntry::Error:
-      return error("Malformed block");
-    case BitstreamEntry::EndBlock:
-      return Error::success();
-    case BitstreamEntry::Record:
-      // The interesting case.
-      break;
-    }
-
-    // Read a metadata attachment record.
-    Record.clear();
-    switch (Stream.readRecord(Entry.ID, Record)) {
-    default:  // Default behavior: ignore.
-      break;
-    case bitc::METADATA_ATTACHMENT: {
-      unsigned RecordLength = Record.size();
-      if (Record.empty())
-        return error("Invalid record");
-      if (RecordLength % 2 == 0) {
-        // A function attachment.
-        if (Error Err = parseGlobalObjectAttachment(F, Record))
-          return Err;
-        continue;
-      }
-
-      // An instruction attachment.
-      Instruction *Inst = InstructionList[Record[0]];
-      for (unsigned i = 1; i != RecordLength; i = i+2) {
-        unsigned Kind = Record[i];
-        DenseMap<unsigned, unsigned>::iterator I =
-          MDKindMap.find(Kind);
-        if (I == MDKindMap.end())
-          return error("Invalid ID");
-        Metadata *Node = MetadataList.getMetadataFwdRef(Record[i + 1]);
-        if (isa<LocalAsMetadata>(Node))
-          // Drop the attachment.  This used to be legal, but there's no
-          // upgrade path.
-          break;
-        MDNode *MD = dyn_cast_or_null<MDNode>(Node);
-        if (!MD)
-          return error("Invalid metadata attachment");
-
-        if (HasSeenOldLoopTags && I->second == LLVMContext::MD_loop)
-          MD = upgradeInstructionLoopAttachment(*MD);
-
-        if (I->second == LLVMContext::MD_tbaa) {
-          assert(!MD->isTemporary() && "should load MDs before attachments");
-          MD = UpgradeTBAANode(*MD);
-        }
-        Inst->setMetadata(I->second, MD);
-      }
-      break;
-    }
-    }
-  }
-}
 
 Error BitcodeReader::typeCheckLoadStoreInst(Type *ValType, Type *PtrType) {
   if (!isa<PointerType>(PtrType))
@@ -4473,12 +3098,12 @@
     return error("Invalid record");
 
   // Unexpected unresolved metadata when parsing function.
-  if (MetadataList.hasFwdRefs())
+  if (MDLoader->hasFwdRefs())
     return error("Invalid function metadata: incoming forward references");
 
   InstructionList.clear();
   unsigned ModuleValueListSize = ValueList.size();
-  unsigned ModuleMetadataListSize = MetadataList.size();
+  unsigned ModuleMDLoaderSize = MDLoader->size();
 
   // Add all the function arguments to the value table.
   for (Argument &I : F->args())
@@ -4528,11 +3153,13 @@
           return Err;
         break;
       case bitc::METADATA_ATTACHMENT_ID:
-        if (Error Err = parseMetadataAttachment(*F))
+        if (Error Err = MDLoader->parseMetadataAttachment(*F, InstructionList))
           return Err;
         break;
       case bitc::METADATA_BLOCK_ID:
-        if (Error Err = parseMetadata())
+        assert(DeferredMetadataInfo.empty() &&
+               "Must read all module-level metadata before function-level");
+        if (Error Err = MDLoader->parseFunctionMetadata())
           return Err;
         break;
       case bitc::USELIST_BLOCK_ID:
@@ -4610,12 +3237,12 @@
 
       MDNode *Scope = nullptr, *IA = nullptr;
       if (ScopeID) {
-        Scope = MetadataList.getMDNodeFwdRefOrNull(ScopeID - 1);
+        Scope = MDLoader->getMDNodeFwdRefOrNull(ScopeID - 1);
         if (!Scope)
           return error("Invalid record");
       }
       if (IAID) {
-        IA = MetadataList.getMDNodeFwdRefOrNull(IAID - 1);
+        IA = MDLoader->getMDNodeFwdRefOrNull(IAID - 1);
         if (!IA)
           return error("Invalid record");
       }
@@ -5740,12 +4367,12 @@
   }
 
   // Unexpected unresolved metadata about to be dropped.
-  if (MetadataList.hasFwdRefs())
+  if (MDLoader->hasFwdRefs())
     return error("Invalid function metadata: outgoing forward refs");
 
   // Trim the value list down to the size it was before we parsed this function.
   ValueList.shrinkTo(ModuleValueListSize);
-  MetadataList.shrinkTo(ModuleMetadataListSize);
+  MDLoader->shrinkTo(ModuleMDLoaderSize);
   std::vector<BasicBlock*>().swap(FunctionBBs);
   return Error::success();
 }
@@ -5819,7 +4446,7 @@
       CallSite(*UI++).setCalledFunction(I.second);
 
   // Finish fn->subprogram upgrade for materialized functions.
-  if (DISubprogram *SP = FunctionsWithSPs.lookup(F))
+  if (DISubprogram *SP = MDLoader->lookupSubprogramForFunction(F))
     F->setSubprogram(SP);
 
   // Bring in any functions that this function forward-referenced via
Index: llvm/lib/Bitcode/Reader/CMakeLists.txt
===================================================================
--- llvm/lib/Bitcode/Reader/CMakeLists.txt
+++ llvm/lib/Bitcode/Reader/CMakeLists.txt
@@ -2,6 +2,8 @@
   BitReader.cpp
   BitcodeReader.cpp
   BitstreamReader.cpp
+  MetadataLoader.cpp
+  ValueList.cpp
 
   ADDITIONAL_HEADER_DIRS
   ${LLVM_MAIN_INCLUDE_DIR}/llvm/Bitcode
Index: llvm/lib/Bitcode/Reader/MetadataLoader.h
===================================================================
--- /dev/null
+++ llvm/lib/Bitcode/Reader/MetadataLoader.h
@@ -0,0 +1,79 @@
+//===-- Bitcode/Reader/MetadataLoader.h - Load Metadatas -------*- C++ -*-====//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This class handles loading Metadatas.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIB_BITCODE_READER_METADATALOADER_H
+#define LLVM_LIB_BITCODE_READER_METADATALOADER_H
+
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/Support/Error.h"
+
+#include <functional>
+#include <memory>
+
+namespace llvm {
+class BitcodeReaderValueList;
+class BitstreamCursor;
+class DISubprogram;
+class Error;
+class Function;
+class Instruction;
+class Metadata;
+class MDNode;
+class Module;
+class Type;
+
+/// Helper class that handles loading Metadatas and keeping them available.
+class MetadataLoader {
+  class MetadataLoaderImpl;
+  std::unique_ptr<MetadataLoaderImpl> Pimpl;
+  Error parseMetadata(bool ModuleLevel);
+
+public:
+  ~MetadataLoader();
+  MetadataLoader(BitstreamCursor &Stream, Module &TheModule,
+                 BitcodeReaderValueList &ValueList,
+                 std::function<Type *(unsigned)> getTypeByID);
+  MetadataLoader &operator=(MetadataLoader &&);
+  MetadataLoader(MetadataLoader &&);
+
+  // Parse a module metadata block
+  Error parseModuleMetadata() { return parseMetadata(true); }
+
+  // Parse a function metadata block
+  Error parseFunctionMetadata() { return parseMetadata(false); }
+
+  // Return true there are remaining unresolved forward references.
+  bool hasFwdRefs() const;
+
+  /// Return the given metadata, creating a replaceable forward reference if
+  /// necessary.
+  Metadata *getMetadataFwdRef(unsigned Idx);
+
+  MDNode *getMDNodeFwdRefOrNull(unsigned Idx);
+
+  /// Return the DISubprogra metadata for a Function if any, null otherwise.
+  DISubprogram *lookupSubprogramForFunction(Function *F);
+
+  /// Perse a `METADATA_ATTACHMENT` block for a function.
+  Error parseMetadataAttachment(
+      Function &F, const SmallVectorImpl<Instruction *> &InstructionList);
+
+  /// Parse a `METADATA_KIND` block for the current module.
+  Error parseMetadataKinds();
+
+  unsigned size() const;
+  void shrinkTo(unsigned N);
+};
+}
+
+#endif // LLVM_LIB_BITCODE_READER_METADATALOADER_H
Index: llvm/lib/Bitcode/Reader/MetadataLoader.cpp
===================================================================
--- /dev/null
+++ llvm/lib/Bitcode/Reader/MetadataLoader.cpp
@@ -0,0 +1,1318 @@
+//===- MetadataLoader.cpp - Internal BitcodeReader implementation ---------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "MetadataLoader.h"
+#include "ValueList.h"
+
+#include "llvm/ADT/APFloat.h"
+#include "llvm/ADT/APInt.h"
+#include "llvm/ADT/ArrayRef.h"
+#include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/None.h"
+#include "llvm/ADT/STLExtras.h"
+#include "llvm/ADT/SmallString.h"
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/ADT/StringRef.h"
+#include "llvm/ADT/Triple.h"
+#include "llvm/ADT/Twine.h"
+#include "llvm/Bitcode/BitcodeReader.h"
+#include "llvm/Bitcode/BitstreamReader.h"
+#include "llvm/Bitcode/LLVMBitCodes.h"
+#include "llvm/IR/Argument.h"
+#include "llvm/IR/Attributes.h"
+#include "llvm/IR/AutoUpgrade.h"
+#include "llvm/IR/BasicBlock.h"
+#include "llvm/IR/CallSite.h"
+#include "llvm/IR/CallingConv.h"
+#include "llvm/IR/Comdat.h"
+#include "llvm/IR/Constant.h"
+#include "llvm/IR/Constants.h"
+#include "llvm/IR/DebugInfo.h"
+#include "llvm/IR/DebugInfoMetadata.h"
+#include "llvm/IR/DebugLoc.h"
+#include "llvm/IR/DerivedTypes.h"
+#include "llvm/IR/DiagnosticInfo.h"
+#include "llvm/IR/DiagnosticPrinter.h"
+#include "llvm/IR/Function.h"
+#include "llvm/IR/GVMaterializer.h"
+#include "llvm/IR/GlobalAlias.h"
+#include "llvm/IR/GlobalIFunc.h"
+#include "llvm/IR/GlobalIndirectSymbol.h"
+#include "llvm/IR/GlobalObject.h"
+#include "llvm/IR/GlobalValue.h"
+#include "llvm/IR/GlobalVariable.h"
+#include "llvm/IR/InlineAsm.h"
+#include "llvm/IR/InstrTypes.h"
+#include "llvm/IR/Instruction.h"
+#include "llvm/IR/Instructions.h"
+#include "llvm/IR/Intrinsics.h"
+#include "llvm/IR/LLVMContext.h"
+#include "llvm/IR/Module.h"
+#include "llvm/IR/ModuleSummaryIndex.h"
+#include "llvm/IR/OperandTraits.h"
+#include "llvm/IR/Operator.h"
+#include "llvm/IR/TrackingMDRef.h"
+#include "llvm/IR/Type.h"
+#include "llvm/IR/ValueHandle.h"
+#include "llvm/Support/AtomicOrdering.h"
+#include "llvm/Support/Casting.h"
+#include "llvm/Support/CommandLine.h"
+#include "llvm/Support/Compiler.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/Support/Error.h"
+#include "llvm/Support/ErrorHandling.h"
+#include "llvm/Support/ManagedStatic.h"
+#include "llvm/Support/MemoryBuffer.h"
+#include "llvm/Support/raw_ostream.h"
+#include <algorithm>
+#include <cassert>
+#include <cstddef>
+#include <cstdint>
+#include <deque>
+#include <limits>
+#include <map>
+#include <memory>
+#include <string>
+#include <system_error>
+#include <tuple>
+#include <utility>
+#include <vector>
+
+using namespace llvm;
+
+namespace {
+
+static int64_t unrotateSign(uint64_t U) { return U & 1 ? ~(U >> 1) : U >> 1; }
+
+class BitcodeReaderMetadataList {
+  unsigned NumFwdRefs;
+  bool AnyFwdRefs;
+  unsigned MinFwdRef;
+  unsigned MaxFwdRef;
+
+  /// Array of metadata references.
+  ///
+  /// Don't use std::vector here.  Some versions of libc++ copy (instead of
+  /// move) on resize, and TrackingMDRef is very expensive to copy.
+  SmallVector<TrackingMDRef, 1> MetadataPtrs;
+
+  /// Structures for resolving old type refs.
+  struct {
+    SmallDenseMap<MDString *, TempMDTuple, 1> Unknown;
+    SmallDenseMap<MDString *, DICompositeType *, 1> Final;
+    SmallDenseMap<MDString *, DICompositeType *, 1> FwdDecls;
+    SmallVector<std::pair<TrackingMDRef, TempMDTuple>, 1> Arrays;
+  } OldTypeRefs;
+
+  LLVMContext &Context;
+
+public:
+  BitcodeReaderMetadataList(LLVMContext &C)
+      : NumFwdRefs(0), AnyFwdRefs(false), Context(C) {}
+
+  // vector compatibility methods
+  unsigned size() const { return MetadataPtrs.size(); }
+  void resize(unsigned N) { MetadataPtrs.resize(N); }
+  void push_back(Metadata *MD) { MetadataPtrs.emplace_back(MD); }
+  void clear() { MetadataPtrs.clear(); }
+  Metadata *back() const { return MetadataPtrs.back(); }
+  void pop_back() { MetadataPtrs.pop_back(); }
+  bool empty() const { return MetadataPtrs.empty(); }
+
+  Metadata *operator[](unsigned i) const {
+    assert(i < MetadataPtrs.size());
+    return MetadataPtrs[i];
+  }
+
+  Metadata *lookup(unsigned I) const {
+    if (I < MetadataPtrs.size())
+      return MetadataPtrs[I];
+    return nullptr;
+  }
+
+  void shrinkTo(unsigned N) {
+    assert(N <= size() && "Invalid shrinkTo request!");
+    assert(!AnyFwdRefs && "Unexpected forward refs");
+    MetadataPtrs.resize(N);
+  }
+
+  /// Return the given metadata, creating a replaceable forward reference if
+  /// necessary.
+  Metadata *getMetadataFwdRef(unsigned Idx);
+
+  /// Return the the given metadata only if it is fully resolved.
+  ///
+  /// Gives the same result as \a lookup(), unless \a MDNode::isResolved()
+  /// would give \c false.
+  Metadata *getMetadataIfResolved(unsigned Idx);
+
+  MDNode *getMDNodeFwdRefOrNull(unsigned Idx);
+  void assignValue(Metadata *MD, unsigned Idx);
+  void tryToResolveCycles();
+  bool hasFwdRefs() const { return AnyFwdRefs; }
+
+  /// Upgrade a type that had an MDString reference.
+  void addTypeRef(MDString &UUID, DICompositeType &CT);
+
+  /// Upgrade a type that had an MDString reference.
+  Metadata *upgradeTypeRef(Metadata *MaybeUUID);
+
+  /// Upgrade a type ref array that may have MDString references.
+  Metadata *upgradeTypeRefArray(Metadata *MaybeTuple);
+
+private:
+  Metadata *resolveTypeRefArray(Metadata *MaybeTuple);
+};
+
+void BitcodeReaderMetadataList::assignValue(Metadata *MD, unsigned Idx) {
+  if (Idx == size()) {
+    push_back(MD);
+    return;
+  }
+
+  if (Idx >= size())
+    resize(Idx + 1);
+
+  TrackingMDRef &OldMD = MetadataPtrs[Idx];
+  if (!OldMD) {
+    OldMD.reset(MD);
+    return;
+  }
+
+  // If there was a forward reference to this value, replace it.
+  TempMDTuple PrevMD(cast<MDTuple>(OldMD.get()));
+  PrevMD->replaceAllUsesWith(MD);
+  --NumFwdRefs;
+}
+
+Metadata *BitcodeReaderMetadataList::getMetadataFwdRef(unsigned Idx) {
+  if (Idx >= size())
+    resize(Idx + 1);
+
+  if (Metadata *MD = MetadataPtrs[Idx])
+    return MD;
+
+  // Track forward refs to be resolved later.
+  if (AnyFwdRefs) {
+    MinFwdRef = std::min(MinFwdRef, Idx);
+    MaxFwdRef = std::max(MaxFwdRef, Idx);
+  } else {
+    AnyFwdRefs = true;
+    MinFwdRef = MaxFwdRef = Idx;
+  }
+  ++NumFwdRefs;
+
+  // Create and return a placeholder, which will later be RAUW'd.
+  Metadata *MD = MDNode::getTemporary(Context, None).release();
+  MetadataPtrs[Idx].reset(MD);
+  return MD;
+}
+
+Metadata *BitcodeReaderMetadataList::getMetadataIfResolved(unsigned Idx) {
+  Metadata *MD = lookup(Idx);
+  if (auto *N = dyn_cast_or_null<MDNode>(MD))
+    if (!N->isResolved())
+      return nullptr;
+  return MD;
+}
+
+MDNode *BitcodeReaderMetadataList::getMDNodeFwdRefOrNull(unsigned Idx) {
+  return dyn_cast_or_null<MDNode>(getMetadataFwdRef(Idx));
+}
+
+void BitcodeReaderMetadataList::tryToResolveCycles() {
+  if (NumFwdRefs)
+    // Still forward references... can't resolve cycles.
+    return;
+
+  bool DidReplaceTypeRefs = false;
+
+  // Give up on finding a full definition for any forward decls that remain.
+  for (const auto &Ref : OldTypeRefs.FwdDecls)
+    OldTypeRefs.Final.insert(Ref);
+  OldTypeRefs.FwdDecls.clear();
+
+  // Upgrade from old type ref arrays.  In strange cases, this could add to
+  // OldTypeRefs.Unknown.
+  for (const auto &Array : OldTypeRefs.Arrays) {
+    DidReplaceTypeRefs = true;
+    Array.second->replaceAllUsesWith(resolveTypeRefArray(Array.first.get()));
+  }
+  OldTypeRefs.Arrays.clear();
+
+  // Replace old string-based type refs with the resolved node, if possible.
+  // If we haven't seen the node, leave it to the verifier to complain about
+  // the invalid string reference.
+  for (const auto &Ref : OldTypeRefs.Unknown) {
+    DidReplaceTypeRefs = true;
+    if (DICompositeType *CT = OldTypeRefs.Final.lookup(Ref.first))
+      Ref.second->replaceAllUsesWith(CT);
+    else
+      Ref.second->replaceAllUsesWith(Ref.first);
+  }
+  OldTypeRefs.Unknown.clear();
+
+  // Make sure all the upgraded types are resolved.
+  if (DidReplaceTypeRefs) {
+    AnyFwdRefs = true;
+    MinFwdRef = 0;
+    MaxFwdRef = MetadataPtrs.size() - 1;
+  }
+
+  if (!AnyFwdRefs)
+    // Nothing to do.
+    return;
+
+  // Resolve any cycles.
+  for (unsigned I = MinFwdRef, E = MaxFwdRef + 1; I != E; ++I) {
+    auto &MD = MetadataPtrs[I];
+    auto *N = dyn_cast_or_null<MDNode>(MD);
+    if (!N)
+      continue;
+
+    assert(!N->isTemporary() && "Unexpected forward reference");
+    N->resolveCycles();
+  }
+
+  // Make sure we return early again until there's another forward ref.
+  AnyFwdRefs = false;
+}
+
+void BitcodeReaderMetadataList::addTypeRef(MDString &UUID,
+                                           DICompositeType &CT) {
+  assert(CT.getRawIdentifier() == &UUID && "Mismatched UUID");
+  if (CT.isForwardDecl())
+    OldTypeRefs.FwdDecls.insert(std::make_pair(&UUID, &CT));
+  else
+    OldTypeRefs.Final.insert(std::make_pair(&UUID, &CT));
+}
+
+Metadata *BitcodeReaderMetadataList::upgradeTypeRef(Metadata *MaybeUUID) {
+  auto *UUID = dyn_cast_or_null<MDString>(MaybeUUID);
+  if (LLVM_LIKELY(!UUID))
+    return MaybeUUID;
+
+  if (auto *CT = OldTypeRefs.Final.lookup(UUID))
+    return CT;
+
+  auto &Ref = OldTypeRefs.Unknown[UUID];
+  if (!Ref)
+    Ref = MDNode::getTemporary(Context, None);
+  return Ref.get();
+}
+
+Metadata *BitcodeReaderMetadataList::upgradeTypeRefArray(Metadata *MaybeTuple) {
+  auto *Tuple = dyn_cast_or_null<MDTuple>(MaybeTuple);
+  if (!Tuple || Tuple->isDistinct())
+    return MaybeTuple;
+
+  // Look through the array immediately if possible.
+  if (!Tuple->isTemporary())
+    return resolveTypeRefArray(Tuple);
+
+  // Create and return a placeholder to use for now.  Eventually
+  // resolveTypeRefArrays() will be resolve this forward reference.
+  OldTypeRefs.Arrays.emplace_back(
+      std::piecewise_construct, std::forward_as_tuple(Tuple),
+      std::forward_as_tuple(MDTuple::getTemporary(Context, None)));
+  return OldTypeRefs.Arrays.back().second.get();
+}
+
+Metadata *BitcodeReaderMetadataList::resolveTypeRefArray(Metadata *MaybeTuple) {
+  auto *Tuple = dyn_cast_or_null<MDTuple>(MaybeTuple);
+  if (!Tuple || Tuple->isDistinct())
+    return MaybeTuple;
+
+  // Look through the DITypeRefArray, upgrading each DITypeRef.
+  SmallVector<Metadata *, 32> Ops;
+  Ops.reserve(Tuple->getNumOperands());
+  for (Metadata *MD : Tuple->operands())
+    Ops.push_back(upgradeTypeRef(MD));
+
+  return MDTuple::get(Context, Ops);
+}
+
+namespace {
+
+class PlaceholderQueue {
+  // Placeholders would thrash around when moved, so store in a std::deque
+  // instead of some sort of vector.
+  std::deque<DistinctMDOperandPlaceholder> PHs;
+
+public:
+  DistinctMDOperandPlaceholder &getPlaceholderOp(unsigned ID);
+  void flush(BitcodeReaderMetadataList &MetadataList);
+};
+
+} // end anonymous namespace
+
+DistinctMDOperandPlaceholder &PlaceholderQueue::getPlaceholderOp(unsigned ID) {
+  PHs.emplace_back(ID);
+  return PHs.back();
+}
+
+void PlaceholderQueue::flush(BitcodeReaderMetadataList &MetadataList) {
+  while (!PHs.empty()) {
+    PHs.front().replaceUseWith(
+        MetadataList.getMetadataFwdRef(PHs.front().getID()));
+    PHs.pop_front();
+  }
+}
+
+} // anonynous namespace
+
+class MetadataLoader::MetadataLoaderImpl {
+  BitcodeReaderMetadataList MetadataList;
+  BitcodeReaderValueList &ValueList;
+  BitstreamCursor &Stream;
+  LLVMContext &Context;
+  Module &TheModule;
+  std::function<Type *(unsigned)> getTypeByID;
+
+  /// Functions that need to be matched with subprograms when upgrading old
+  /// metadata.
+  SmallDenseMap<Function *, DISubprogram *, 16> FunctionsWithSPs;
+
+  // Map the bitcode's custom MDKind ID to the Module's MDKind ID.
+  DenseMap<unsigned, unsigned> MDKindMap;
+
+  bool HasSeenOldLoopTags = false;
+
+  Error parseMetadataStrings(ArrayRef<uint64_t> Record, StringRef Blob,
+                             unsigned &NextMetadataNo);
+  Error parseGlobalObjectAttachment(GlobalObject &GO,
+                                    ArrayRef<uint64_t> Record);
+  Error parseMetadataKindRecord(SmallVectorImpl<uint64_t> &Record);
+
+public:
+  MetadataLoaderImpl(BitstreamCursor &Stream, Module &TheModule,
+                     BitcodeReaderValueList &ValueList,
+                     std::function<Type *(unsigned)> getTypeByID)
+      : MetadataList(TheModule.getContext()), ValueList(ValueList),
+        Stream(Stream), Context(TheModule.getContext()), TheModule(TheModule),
+        getTypeByID(getTypeByID) {}
+
+  Error parseMetadata(bool ModuleLevel);
+
+  bool hasFwdRefs() const { return MetadataList.hasFwdRefs(); }
+  Metadata *getMetadataFwdRef(unsigned Idx) {
+    return MetadataList.getMetadataFwdRef(Idx);
+  }
+
+  MDNode *getMDNodeFwdRefOrNull(unsigned Idx) {
+    return MetadataList.getMDNodeFwdRefOrNull(Idx);
+  }
+
+  DISubprogram *lookupSubprogramForFunction(Function *F) {
+    return FunctionsWithSPs.lookup(F);
+  }
+
+  bool hasSeenOldLoopTags() { return HasSeenOldLoopTags; }
+
+  Error parseMetadataAttachment(
+      Function &F, const SmallVectorImpl<Instruction *> &InstructionList);
+
+  Error parseMetadataKinds();
+
+  unsigned size() const { return MetadataList.size(); }
+  void shrinkTo(unsigned N) { MetadataList.shrinkTo(N); }
+};
+
+Error error(const Twine &Message) {
+  return make_error<StringError>(
+      Message, make_error_code(BitcodeError::CorruptedBitcode));
+}
+
+/// Parse a METADATA_BLOCK. If ModuleLevel is true then we are parsing
+/// module level metadata.
+Error MetadataLoader::MetadataLoaderImpl::parseMetadata(bool ModuleLevel) {
+  // IsMetadataMaterialized = true;
+  unsigned NextMetadataNo = MetadataList.size();
+
+  if (!ModuleLevel && MetadataList.hasFwdRefs())
+    return error("Invalid metadata: fwd refs into function blocks");
+
+  if (Stream.EnterSubBlock(bitc::METADATA_BLOCK_ID))
+    return error("Invalid record");
+
+  std::vector<std::pair<DICompileUnit *, Metadata *>> CUSubprograms;
+  SmallVector<uint64_t, 64> Record;
+
+  PlaceholderQueue Placeholders;
+  bool IsDistinct;
+  auto getMD = [&](unsigned ID) -> Metadata * {
+    if (!IsDistinct)
+      return MetadataList.getMetadataFwdRef(ID);
+    if (auto *MD = MetadataList.getMetadataIfResolved(ID))
+      return MD;
+    return &Placeholders.getPlaceholderOp(ID);
+  };
+  auto getMDOrNull = [&](unsigned ID) -> Metadata * {
+    if (ID)
+      return getMD(ID - 1);
+    return nullptr;
+  };
+  auto getMDOrNullWithoutPlaceholders = [&](unsigned ID) -> Metadata * {
+    if (ID)
+      return MetadataList.getMetadataFwdRef(ID - 1);
+    return nullptr;
+  };
+  auto getMDString = [&](unsigned ID) -> MDString * {
+    // This requires that the ID is not really a forward reference.  In
+    // particular, the MDString must already have been resolved.
+    return cast_or_null<MDString>(getMDOrNull(ID));
+  };
+
+  // Support for old type refs.
+  auto getDITypeRefOrNull = [&](unsigned ID) {
+    return MetadataList.upgradeTypeRef(getMDOrNull(ID));
+  };
+
+#define GET_OR_DISTINCT(CLASS, ARGS)                                           \
+  (IsDistinct ? CLASS::getDistinct ARGS : CLASS::get ARGS)
+
+  // Read all the records.
+  while (true) {
+    BitstreamEntry Entry = Stream.advanceSkippingSubblocks();
+
+    switch (Entry.Kind) {
+    case BitstreamEntry::SubBlock: // Handled for us already.
+    case BitstreamEntry::Error:
+      return error("Malformed block");
+    case BitstreamEntry::EndBlock:
+      // Upgrade old-style CU <-> SP pointers to point from SP to CU.
+      for (auto CU_SP : CUSubprograms)
+        if (auto *SPs = dyn_cast_or_null<MDTuple>(CU_SP.second))
+          for (auto &Op : SPs->operands())
+            if (auto *SP = dyn_cast_or_null<MDNode>(Op))
+              SP->replaceOperandWith(7, CU_SP.first);
+
+      MetadataList.tryToResolveCycles();
+      Placeholders.flush(MetadataList);
+      return Error::success();
+    case BitstreamEntry::Record:
+      // The interesting case.
+      break;
+    }
+
+    // Read a record.
+    Record.clear();
+    StringRef Blob;
+    unsigned Code = Stream.readRecord(Entry.ID, Record, &Blob);
+    IsDistinct = false;
+    switch (Code) {
+    default: // Default behavior: ignore.
+      break;
+    case bitc::METADATA_NAME: {
+      // Read name of the named metadata.
+      SmallString<8> Name(Record.begin(), Record.end());
+      Record.clear();
+      Code = Stream.ReadCode();
+
+      unsigned NextBitCode = Stream.readRecord(Code, Record);
+      if (NextBitCode != bitc::METADATA_NAMED_NODE)
+        return error("METADATA_NAME not followed by METADATA_NAMED_NODE");
+
+      // Read named metadata elements.
+      unsigned Size = Record.size();
+      NamedMDNode *NMD = TheModule.getOrInsertNamedMetadata(Name);
+      for (unsigned i = 0; i != Size; ++i) {
+        MDNode *MD = MetadataList.getMDNodeFwdRefOrNull(Record[i]);
+        if (!MD)
+          return error("Invalid record");
+        NMD->addOperand(MD);
+      }
+      break;
+    }
+    case bitc::METADATA_OLD_FN_NODE: {
+      // FIXME: Remove in 4.0.
+      // This is a LocalAsMetadata record, the only type of function-local
+      // metadata.
+      if (Record.size() % 2 == 1)
+        return error("Invalid record");
+
+      // If this isn't a LocalAsMetadata record, we're dropping it.  This used
+      // to be legal, but there's no upgrade path.
+      auto dropRecord = [&] {
+        MetadataList.assignValue(MDNode::get(Context, None), NextMetadataNo++);
+      };
+      if (Record.size() != 2) {
+        dropRecord();
+        break;
+      }
+
+      Type *Ty = getTypeByID(Record[0]);
+      if (Ty->isMetadataTy() || Ty->isVoidTy()) {
+        dropRecord();
+        break;
+      }
+
+      MetadataList.assignValue(
+          LocalAsMetadata::get(ValueList.getValueFwdRef(Record[1], Ty)),
+          NextMetadataNo++);
+      break;
+    }
+    case bitc::METADATA_OLD_NODE: {
+      // FIXME: Remove in 4.0.
+      if (Record.size() % 2 == 1)
+        return error("Invalid record");
+
+      unsigned Size = Record.size();
+      SmallVector<Metadata *, 8> Elts;
+      for (unsigned i = 0; i != Size; i += 2) {
+        Type *Ty = getTypeByID(Record[i]);
+        if (!Ty)
+          return error("Invalid record");
+        if (Ty->isMetadataTy())
+          Elts.push_back(getMD(Record[i + 1]));
+        else if (!Ty->isVoidTy()) {
+          auto *MD =
+              ValueAsMetadata::get(ValueList.getValueFwdRef(Record[i + 1], Ty));
+          assert(isa<ConstantAsMetadata>(MD) &&
+                 "Expected non-function-local metadata");
+          Elts.push_back(MD);
+        } else
+          Elts.push_back(nullptr);
+      }
+      MetadataList.assignValue(MDNode::get(Context, Elts), NextMetadataNo++);
+      break;
+    }
+    case bitc::METADATA_VALUE: {
+      if (Record.size() != 2)
+        return error("Invalid record");
+
+      Type *Ty = getTypeByID(Record[0]);
+      if (Ty->isMetadataTy() || Ty->isVoidTy())
+        return error("Invalid record");
+
+      MetadataList.assignValue(
+          ValueAsMetadata::get(ValueList.getValueFwdRef(Record[1], Ty)),
+          NextMetadataNo++);
+      break;
+    }
+    case bitc::METADATA_DISTINCT_NODE:
+      IsDistinct = true;
+      LLVM_FALLTHROUGH;
+    case bitc::METADATA_NODE: {
+      SmallVector<Metadata *, 8> Elts;
+      Elts.reserve(Record.size());
+      for (unsigned ID : Record)
+        Elts.push_back(getMDOrNull(ID));
+      MetadataList.assignValue(IsDistinct ? MDNode::getDistinct(Context, Elts)
+                                          : MDNode::get(Context, Elts),
+                               NextMetadataNo++);
+      break;
+    }
+    case bitc::METADATA_LOCATION: {
+      if (Record.size() != 5)
+        return error("Invalid record");
+
+      IsDistinct = Record[0];
+      unsigned Line = Record[1];
+      unsigned Column = Record[2];
+      Metadata *Scope = getMD(Record[3]);
+      Metadata *InlinedAt = getMDOrNull(Record[4]);
+      MetadataList.assignValue(
+          GET_OR_DISTINCT(DILocation,
+                          (Context, Line, Column, Scope, InlinedAt)),
+          NextMetadataNo++);
+      break;
+    }
+    case bitc::METADATA_GENERIC_DEBUG: {
+      if (Record.size() < 4)
+        return error("Invalid record");
+
+      IsDistinct = Record[0];
+      unsigned Tag = Record[1];
+      unsigned Version = Record[2];
+
+      if (Tag >= 1u << 16 || Version != 0)
+        return error("Invalid record");
+
+      auto *Header = getMDString(Record[3]);
+      SmallVector<Metadata *, 8> DwarfOps;
+      for (unsigned I = 4, E = Record.size(); I != E; ++I)
+        DwarfOps.push_back(getMDOrNull(Record[I]));
+      MetadataList.assignValue(
+          GET_OR_DISTINCT(GenericDINode, (Context, Tag, Header, DwarfOps)),
+          NextMetadataNo++);
+      break;
+    }
+    case bitc::METADATA_SUBRANGE: {
+      if (Record.size() != 3)
+        return error("Invalid record");
+
+      IsDistinct = Record[0];
+      MetadataList.assignValue(
+          GET_OR_DISTINCT(DISubrange,
+                          (Context, Record[1], unrotateSign(Record[2]))),
+          NextMetadataNo++);
+      break;
+    }
+    case bitc::METADATA_ENUMERATOR: {
+      if (Record.size() != 3)
+        return error("Invalid record");
+
+      IsDistinct = Record[0];
+      MetadataList.assignValue(
+          GET_OR_DISTINCT(DIEnumerator, (Context, unrotateSign(Record[1]),
+                                         getMDString(Record[2]))),
+          NextMetadataNo++);
+      break;
+    }
+    case bitc::METADATA_BASIC_TYPE: {
+      if (Record.size() != 6)
+        return error("Invalid record");
+
+      IsDistinct = Record[0];
+      MetadataList.assignValue(
+          GET_OR_DISTINCT(DIBasicType,
+                          (Context, Record[1], getMDString(Record[2]),
+                           Record[3], Record[4], Record[5])),
+          NextMetadataNo++);
+      break;
+    }
+    case bitc::METADATA_DERIVED_TYPE: {
+      if (Record.size() != 12)
+        return error("Invalid record");
+
+      IsDistinct = Record[0];
+      DINode::DIFlags Flags = static_cast<DINode::DIFlags>(Record[10]);
+      MetadataList.assignValue(
+          GET_OR_DISTINCT(DIDerivedType,
+                          (Context, Record[1], getMDString(Record[2]),
+                           getMDOrNull(Record[3]), Record[4],
+                           getDITypeRefOrNull(Record[5]),
+                           getDITypeRefOrNull(Record[6]), Record[7], Record[8],
+                           Record[9], Flags, getDITypeRefOrNull(Record[11]))),
+          NextMetadataNo++);
+      break;
+    }
+    case bitc::METADATA_COMPOSITE_TYPE: {
+      if (Record.size() != 16)
+        return error("Invalid record");
+
+      // If we have a UUID and this is not a forward declaration, lookup the
+      // mapping.
+      IsDistinct = Record[0] & 0x1;
+      bool IsNotUsedInTypeRef = Record[0] >= 2;
+      unsigned Tag = Record[1];
+      MDString *Name = getMDString(Record[2]);
+      Metadata *File = getMDOrNull(Record[3]);
+      unsigned Line = Record[4];
+      Metadata *Scope = getDITypeRefOrNull(Record[5]);
+      Metadata *BaseType = getDITypeRefOrNull(Record[6]);
+      uint64_t SizeInBits = Record[7];
+      if (Record[8] > (uint64_t)std::numeric_limits<uint32_t>::max())
+        return error("Alignment value is too large");
+      uint32_t AlignInBits = Record[8];
+      uint64_t OffsetInBits = Record[9];
+      DINode::DIFlags Flags = static_cast<DINode::DIFlags>(Record[10]);
+      Metadata *Elements = getMDOrNull(Record[11]);
+      unsigned RuntimeLang = Record[12];
+      Metadata *VTableHolder = getDITypeRefOrNull(Record[13]);
+      Metadata *TemplateParams = getMDOrNull(Record[14]);
+      auto *Identifier = getMDString(Record[15]);
+      DICompositeType *CT = nullptr;
+      if (Identifier)
+        CT = DICompositeType::buildODRType(
+            Context, *Identifier, Tag, Name, File, Line, Scope, BaseType,
+            SizeInBits, AlignInBits, OffsetInBits, Flags, Elements, RuntimeLang,
+            VTableHolder, TemplateParams);
+
+      // Create a node if we didn't get a lazy ODR type.
+      if (!CT)
+        CT = GET_OR_DISTINCT(DICompositeType,
+                             (Context, Tag, Name, File, Line, Scope, BaseType,
+                              SizeInBits, AlignInBits, OffsetInBits, Flags,
+                              Elements, RuntimeLang, VTableHolder,
+                              TemplateParams, Identifier));
+      if (!IsNotUsedInTypeRef && Identifier)
+        MetadataList.addTypeRef(*Identifier, *cast<DICompositeType>(CT));
+
+      MetadataList.assignValue(CT, NextMetadataNo++);
+      break;
+    }
+    case bitc::METADATA_SUBROUTINE_TYPE: {
+      if (Record.size() < 3 || Record.size() > 4)
+        return error("Invalid record");
+      bool IsOldTypeRefArray = Record[0] < 2;
+      unsigned CC = (Record.size() > 3) ? Record[3] : 0;
+
+      IsDistinct = Record[0] & 0x1;
+      DINode::DIFlags Flags = static_cast<DINode::DIFlags>(Record[1]);
+      Metadata *Types = getMDOrNull(Record[2]);
+      if (LLVM_UNLIKELY(IsOldTypeRefArray))
+        Types = MetadataList.upgradeTypeRefArray(Types);
+
+      MetadataList.assignValue(
+          GET_OR_DISTINCT(DISubroutineType, (Context, Flags, CC, Types)),
+          NextMetadataNo++);
+      break;
+    }
+
+    case bitc::METADATA_MODULE: {
+      if (Record.size() != 6)
+        return error("Invalid record");
+
+      IsDistinct = Record[0];
+      MetadataList.assignValue(
+          GET_OR_DISTINCT(DIModule,
+                          (Context, getMDOrNull(Record[1]),
+                           getMDString(Record[2]), getMDString(Record[3]),
+                           getMDString(Record[4]), getMDString(Record[5]))),
+          NextMetadataNo++);
+      break;
+    }
+
+    case bitc::METADATA_FILE: {
+      if (Record.size() != 3)
+        return error("Invalid record");
+
+      IsDistinct = Record[0];
+      MetadataList.assignValue(
+          GET_OR_DISTINCT(DIFile, (Context, getMDString(Record[1]),
+                                   getMDString(Record[2]))),
+          NextMetadataNo++);
+      break;
+    }
+    case bitc::METADATA_COMPILE_UNIT: {
+      if (Record.size() < 14 || Record.size() > 17)
+        return error("Invalid record");
+
+      // Ignore Record[0], which indicates whether this compile unit is
+      // distinct.  It's always distinct.
+      IsDistinct = true;
+      auto *CU = DICompileUnit::getDistinct(
+          Context, Record[1], getMDOrNull(Record[2]), getMDString(Record[3]),
+          Record[4], getMDString(Record[5]), Record[6], getMDString(Record[7]),
+          Record[8], getMDOrNull(Record[9]), getMDOrNull(Record[10]),
+          getMDOrNull(Record[12]), getMDOrNull(Record[13]),
+          Record.size() <= 15 ? nullptr : getMDOrNull(Record[15]),
+          Record.size() <= 14 ? 0 : Record[14],
+          Record.size() <= 16 ? true : Record[16]);
+
+      MetadataList.assignValue(CU, NextMetadataNo++);
+
+      // Move the Upgrade the list of subprograms.
+      if (Metadata *SPs = getMDOrNullWithoutPlaceholders(Record[11]))
+        CUSubprograms.push_back({CU, SPs});
+      break;
+    }
+    case bitc::METADATA_SUBPROGRAM: {
+      if (Record.size() < 18 || Record.size() > 20)
+        return error("Invalid record");
+
+      IsDistinct =
+          (Record[0] & 1) || Record[8]; // All definitions should be distinct.
+      // Version 1 has a Function as Record[15].
+      // Version 2 has removed Record[15].
+      // Version 3 has the Unit as Record[15].
+      // Version 4 added thisAdjustment.
+      bool HasUnit = Record[0] >= 2;
+      if (HasUnit && Record.size() < 19)
+        return error("Invalid record");
+      Metadata *CUorFn = getMDOrNull(Record[15]);
+      unsigned Offset = Record.size() >= 19 ? 1 : 0;
+      bool HasFn = Offset && !HasUnit;
+      bool HasThisAdj = Record.size() >= 20;
+      DISubprogram *SP = GET_OR_DISTINCT(
+          DISubprogram, (Context,
+                         getDITypeRefOrNull(Record[1]),  // scope
+                         getMDString(Record[2]),         // name
+                         getMDString(Record[3]),         // linkageName
+                         getMDOrNull(Record[4]),         // file
+                         Record[5],                      // line
+                         getMDOrNull(Record[6]),         // type
+                         Record[7],                      // isLocal
+                         Record[8],                      // isDefinition
+                         Record[9],                      // scopeLine
+                         getDITypeRefOrNull(Record[10]), // containingType
+                         Record[11],                     // virtuality
+                         Record[12],                     // virtualIndex
+                         HasThisAdj ? Record[19] : 0,    // thisAdjustment
+                         static_cast<DINode::DIFlags>(Record[13] // flags
+                                                      ),
+                         Record[14],                       // isOptimized
+                         HasUnit ? CUorFn : nullptr,       // unit
+                         getMDOrNull(Record[15 + Offset]), // templateParams
+                         getMDOrNull(Record[16 + Offset]), // declaration
+                         getMDOrNull(Record[17 + Offset])  // variables
+                         ));
+      MetadataList.assignValue(SP, NextMetadataNo++);
+
+      // Upgrade sp->function mapping to function->sp mapping.
+      if (HasFn) {
+        if (auto *CMD = dyn_cast_or_null<ConstantAsMetadata>(CUorFn))
+          if (auto *F = dyn_cast<Function>(CMD->getValue())) {
+            if (F->isMaterializable())
+              // Defer until materialized; unmaterialized functions may not have
+              // metadata.
+              FunctionsWithSPs[F] = SP;
+            else if (!F->empty())
+              F->setSubprogram(SP);
+          }
+      }
+      break;
+    }
+    case bitc::METADATA_LEXICAL_BLOCK: {
+      if (Record.size() != 5)
+        return error("Invalid record");
+
+      IsDistinct = Record[0];
+      MetadataList.assignValue(
+          GET_OR_DISTINCT(DILexicalBlock,
+                          (Context, getMDOrNull(Record[1]),
+                           getMDOrNull(Record[2]), Record[3], Record[4])),
+          NextMetadataNo++);
+      break;
+    }
+    case bitc::METADATA_LEXICAL_BLOCK_FILE: {
+      if (Record.size() != 4)
+        return error("Invalid record");
+
+      IsDistinct = Record[0];
+      MetadataList.assignValue(
+          GET_OR_DISTINCT(DILexicalBlockFile,
+                          (Context, getMDOrNull(Record[1]),
+                           getMDOrNull(Record[2]), Record[3])),
+          NextMetadataNo++);
+      break;
+    }
+    case bitc::METADATA_NAMESPACE: {
+      if (Record.size() != 5)
+        return error("Invalid record");
+
+      IsDistinct = Record[0] & 1;
+      bool ExportSymbols = Record[0] & 2;
+      MetadataList.assignValue(
+          GET_OR_DISTINCT(DINamespace,
+                          (Context, getMDOrNull(Record[1]),
+                           getMDOrNull(Record[2]), getMDString(Record[3]),
+                           Record[4], ExportSymbols)),
+          NextMetadataNo++);
+      break;
+    }
+    case bitc::METADATA_MACRO: {
+      if (Record.size() != 5)
+        return error("Invalid record");
+
+      IsDistinct = Record[0];
+      MetadataList.assignValue(
+          GET_OR_DISTINCT(DIMacro,
+                          (Context, Record[1], Record[2],
+                           getMDString(Record[3]), getMDString(Record[4]))),
+          NextMetadataNo++);
+      break;
+    }
+    case bitc::METADATA_MACRO_FILE: {
+      if (Record.size() != 5)
+        return error("Invalid record");
+
+      IsDistinct = Record[0];
+      MetadataList.assignValue(
+          GET_OR_DISTINCT(DIMacroFile,
+                          (Context, Record[1], Record[2],
+                           getMDOrNull(Record[3]), getMDOrNull(Record[4]))),
+          NextMetadataNo++);
+      break;
+    }
+    case bitc::METADATA_TEMPLATE_TYPE: {
+      if (Record.size() != 3)
+        return error("Invalid record");
+
+      IsDistinct = Record[0];
+      MetadataList.assignValue(GET_OR_DISTINCT(DITemplateTypeParameter,
+                                               (Context, getMDString(Record[1]),
+                                                getDITypeRefOrNull(Record[2]))),
+                               NextMetadataNo++);
+      break;
+    }
+    case bitc::METADATA_TEMPLATE_VALUE: {
+      if (Record.size() != 5)
+        return error("Invalid record");
+
+      IsDistinct = Record[0];
+      MetadataList.assignValue(
+          GET_OR_DISTINCT(DITemplateValueParameter,
+                          (Context, Record[1], getMDString(Record[2]),
+                           getDITypeRefOrNull(Record[3]),
+                           getMDOrNull(Record[4]))),
+          NextMetadataNo++);
+      break;
+    }
+    case bitc::METADATA_GLOBAL_VAR: {
+      if (Record.size() < 11 || Record.size() > 12)
+        return error("Invalid record");
+
+      IsDistinct = Record[0];
+
+      // Upgrade old metadata, which stored a global variable reference or a
+      // ConstantInt here.
+      Metadata *Expr = getMDOrNull(Record[9]);
+      uint32_t AlignInBits = 0;
+      if (Record.size() > 11) {
+        if (Record[11] > (uint64_t)std::numeric_limits<uint32_t>::max())
+          return error("Alignment value is too large");
+        AlignInBits = Record[11];
+      }
+      GlobalVariable *Attach = nullptr;
+      if (auto *CMD = dyn_cast_or_null<ConstantAsMetadata>(Expr)) {
+        if (auto *GV = dyn_cast<GlobalVariable>(CMD->getValue())) {
+          Attach = GV;
+          Expr = nullptr;
+        } else if (auto *CI = dyn_cast<ConstantInt>(CMD->getValue())) {
+          Expr = DIExpression::get(Context,
+                                   {dwarf::DW_OP_constu, CI->getZExtValue(),
+                                    dwarf::DW_OP_stack_value});
+        } else {
+          Expr = nullptr;
+        }
+      }
+
+      DIGlobalVariable *DGV = GET_OR_DISTINCT(
+          DIGlobalVariable,
+          (Context, getMDOrNull(Record[1]), getMDString(Record[2]),
+           getMDString(Record[3]), getMDOrNull(Record[4]), Record[5],
+           getDITypeRefOrNull(Record[6]), Record[7], Record[8], Expr,
+           getMDOrNull(Record[10]), AlignInBits));
+      MetadataList.assignValue(DGV, NextMetadataNo++);
+
+      if (Attach)
+        Attach->addDebugInfo(DGV);
+
+      break;
+    }
+    case bitc::METADATA_LOCAL_VAR: {
+      // 10th field is for the obseleted 'inlinedAt:' field.
+      if (Record.size() < 8 || Record.size() > 10)
+        return error("Invalid record");
+
+      IsDistinct = Record[0] & 1;
+      bool HasAlignment = Record[0] & 2;
+      // 2nd field used to be an artificial tag, either DW_TAG_auto_variable or
+      // DW_TAG_arg_variable, if we have alignment flag encoded it means, that
+      // this is newer version of record which doesn't have artifical tag.
+      bool HasTag = !HasAlignment && Record.size() > 8;
+      DINode::DIFlags Flags = static_cast<DINode::DIFlags>(Record[7 + HasTag]);
+      uint32_t AlignInBits = 0;
+      if (HasAlignment) {
+        if (Record[8 + HasTag] > (uint64_t)std::numeric_limits<uint32_t>::max())
+          return error("Alignment value is too large");
+        AlignInBits = Record[8 + HasTag];
+      }
+      MetadataList.assignValue(
+          GET_OR_DISTINCT(DILocalVariable,
+                          (Context, getMDOrNull(Record[1 + HasTag]),
+                           getMDString(Record[2 + HasTag]),
+                           getMDOrNull(Record[3 + HasTag]), Record[4 + HasTag],
+                           getDITypeRefOrNull(Record[5 + HasTag]),
+                           Record[6 + HasTag], Flags, AlignInBits)),
+          NextMetadataNo++);
+      break;
+    }
+    case bitc::METADATA_EXPRESSION: {
+      if (Record.size() < 1)
+        return error("Invalid record");
+
+      IsDistinct = Record[0] & 1;
+      bool HasOpFragment = Record[0] & 2;
+      auto Elts = MutableArrayRef<uint64_t>(Record).slice(1);
+      if (!HasOpFragment)
+        if (unsigned N = Elts.size())
+          if (N >= 3 && Elts[N - 3] == dwarf::DW_OP_bit_piece)
+            Elts[N - 3] = dwarf::DW_OP_LLVM_fragment;
+
+      MetadataList.assignValue(
+          GET_OR_DISTINCT(DIExpression,
+                          (Context, makeArrayRef(Record).slice(1))),
+          NextMetadataNo++);
+      break;
+    }
+    case bitc::METADATA_OBJC_PROPERTY: {
+      if (Record.size() != 8)
+        return error("Invalid record");
+
+      IsDistinct = Record[0];
+      MetadataList.assignValue(
+          GET_OR_DISTINCT(DIObjCProperty,
+                          (Context, getMDString(Record[1]),
+                           getMDOrNull(Record[2]), Record[3],
+                           getMDString(Record[4]), getMDString(Record[5]),
+                           Record[6], getDITypeRefOrNull(Record[7]))),
+          NextMetadataNo++);
+      break;
+    }
+    case bitc::METADATA_IMPORTED_ENTITY: {
+      if (Record.size() != 6)
+        return error("Invalid record");
+
+      IsDistinct = Record[0];
+      MetadataList.assignValue(
+          GET_OR_DISTINCT(DIImportedEntity,
+                          (Context, Record[1], getMDOrNull(Record[2]),
+                           getDITypeRefOrNull(Record[3]), Record[4],
+                           getMDString(Record[5]))),
+          NextMetadataNo++);
+      break;
+    }
+    case bitc::METADATA_STRING_OLD: {
+      std::string String(Record.begin(), Record.end());
+
+      // Test for upgrading !llvm.loop.
+      HasSeenOldLoopTags |= mayBeOldLoopAttachmentTag(String);
+
+      Metadata *MD = MDString::get(Context, String);
+      MetadataList.assignValue(MD, NextMetadataNo++);
+      break;
+    }
+    case bitc::METADATA_STRINGS:
+      if (Error Err = parseMetadataStrings(Record, Blob, NextMetadataNo))
+        return Err;
+      break;
+    case bitc::METADATA_GLOBAL_DECL_ATTACHMENT: {
+      if (Record.size() % 2 == 0)
+        return error("Invalid record");
+      unsigned ValueID = Record[0];
+      if (ValueID >= ValueList.size())
+        return error("Invalid record");
+      if (auto *GO = dyn_cast<GlobalObject>(ValueList[ValueID]))
+        if (Error Err = parseGlobalObjectAttachment(
+                *GO, ArrayRef<uint64_t>(Record).slice(1)))
+          return Err;
+      break;
+    }
+    case bitc::METADATA_KIND: {
+      // Support older bitcode files that had METADATA_KIND records in a
+      // block with METADATA_BLOCK_ID.
+      if (Error Err = parseMetadataKindRecord(Record))
+        return Err;
+      break;
+    }
+    }
+  }
+#undef GET_OR_DISTINCT
+}
+
+Error MetadataLoader::MetadataLoaderImpl::parseMetadataStrings(
+    ArrayRef<uint64_t> Record, StringRef Blob, unsigned &NextMetadataNo) {
+  // All the MDStrings in the block are emitted together in a single
+  // record.  The strings are concatenated and stored in a blob along with
+  // their sizes.
+  if (Record.size() != 2)
+    return error("Invalid record: metadata strings layout");
+
+  unsigned NumStrings = Record[0];
+  unsigned StringsOffset = Record[1];
+  if (!NumStrings)
+    return error("Invalid record: metadata strings with no strings");
+  if (StringsOffset > Blob.size())
+    return error("Invalid record: metadata strings corrupt offset");
+
+  StringRef Lengths = Blob.slice(0, StringsOffset);
+  SimpleBitstreamCursor R(Lengths);
+
+  StringRef Strings = Blob.drop_front(StringsOffset);
+  do {
+    if (R.AtEndOfStream())
+      return error("Invalid record: metadata strings bad length");
+
+    unsigned Size = R.ReadVBR(6);
+    if (Strings.size() < Size)
+      return error("Invalid record: metadata strings truncated chars");
+
+    MetadataList.assignValue(MDString::get(Context, Strings.slice(0, Size)),
+                             NextMetadataNo++);
+    Strings = Strings.drop_front(Size);
+  } while (--NumStrings);
+
+  return Error::success();
+}
+
+Error MetadataLoader::MetadataLoaderImpl::parseGlobalObjectAttachment(
+    GlobalObject &GO, ArrayRef<uint64_t> Record) {
+  assert(Record.size() % 2 == 0);
+  for (unsigned I = 0, E = Record.size(); I != E; I += 2) {
+    auto K = MDKindMap.find(Record[I]);
+    if (K == MDKindMap.end())
+      return error("Invalid ID");
+    MDNode *MD = MetadataList.getMDNodeFwdRefOrNull(Record[I + 1]);
+    if (!MD)
+      return error("Invalid metadata attachment");
+    GO.addMetadata(K->second, *MD);
+  }
+  return Error::success();
+}
+
+/// Parse metadata attachments.
+Error MetadataLoader::MetadataLoaderImpl::parseMetadataAttachment(
+    Function &F, const SmallVectorImpl<Instruction *> &InstructionList) {
+  if (Stream.EnterSubBlock(bitc::METADATA_ATTACHMENT_ID))
+    return error("Invalid record");
+
+  SmallVector<uint64_t, 64> Record;
+
+  while (true) {
+    BitstreamEntry Entry = Stream.advanceSkippingSubblocks();
+
+    switch (Entry.Kind) {
+    case BitstreamEntry::SubBlock: // Handled for us already.
+    case BitstreamEntry::Error:
+      return error("Malformed block");
+    case BitstreamEntry::EndBlock:
+      return Error::success();
+    case BitstreamEntry::Record:
+      // The interesting case.
+      break;
+    }
+
+    // Read a metadata attachment record.
+    Record.clear();
+    switch (Stream.readRecord(Entry.ID, Record)) {
+    default: // Default behavior: ignore.
+      break;
+    case bitc::METADATA_ATTACHMENT: {
+      unsigned RecordLength = Record.size();
+      if (Record.empty())
+        return error("Invalid record");
+      if (RecordLength % 2 == 0) {
+        // A function attachment.
+        if (Error Err = parseGlobalObjectAttachment(F, Record))
+          return Err;
+        continue;
+      }
+
+      // An instruction attachment.
+      Instruction *Inst = InstructionList[Record[0]];
+      for (unsigned i = 1; i != RecordLength; i = i + 2) {
+        unsigned Kind = Record[i];
+        DenseMap<unsigned, unsigned>::iterator I = MDKindMap.find(Kind);
+        if (I == MDKindMap.end())
+          return error("Invalid ID");
+        Metadata *Node = MetadataList.getMetadataFwdRef(Record[i + 1]);
+        if (isa<LocalAsMetadata>(Node))
+          // Drop the attachment.  This used to be legal, but there's no
+          // upgrade path.
+          break;
+        MDNode *MD = dyn_cast_or_null<MDNode>(Node);
+        if (!MD)
+          return error("Invalid metadata attachment");
+
+        if (HasSeenOldLoopTags && I->second == LLVMContext::MD_loop)
+          MD = upgradeInstructionLoopAttachment(*MD);
+
+        if (I->second == LLVMContext::MD_tbaa) {
+          assert(!MD->isTemporary() && "should load MDs before attachments");
+          MD = UpgradeTBAANode(*MD);
+        }
+        Inst->setMetadata(I->second, MD);
+      }
+      break;
+    }
+    }
+  }
+}
+
+/// Parse a single METADATA_KIND record, inserting result in MDKindMap.
+Error MetadataLoader::MetadataLoaderImpl::parseMetadataKindRecord(
+    SmallVectorImpl<uint64_t> &Record) {
+  if (Record.size() < 2)
+    return error("Invalid record");
+
+  unsigned Kind = Record[0];
+  SmallString<8> Name(Record.begin() + 1, Record.end());
+
+  unsigned NewKind = TheModule.getMDKindID(Name.str());
+  if (!MDKindMap.insert(std::make_pair(Kind, NewKind)).second)
+    return error("Conflicting METADATA_KIND records");
+  return Error::success();
+}
+
+/// Parse the metadata kinds out of the METADATA_KIND_BLOCK.
+Error MetadataLoader::MetadataLoaderImpl::parseMetadataKinds() {
+  if (Stream.EnterSubBlock(bitc::METADATA_KIND_BLOCK_ID))
+    return error("Invalid record");
+
+  SmallVector<uint64_t, 64> Record;
+
+  // Read all the records.
+  while (true) {
+    BitstreamEntry Entry = Stream.advanceSkippingSubblocks();
+
+    switch (Entry.Kind) {
+    case BitstreamEntry::SubBlock: // Handled for us already.
+    case BitstreamEntry::Error:
+      return error("Malformed block");
+    case BitstreamEntry::EndBlock:
+      return Error::success();
+    case BitstreamEntry::Record:
+      // The interesting case.
+      break;
+    }
+
+    // Read a record.
+    Record.clear();
+    unsigned Code = Stream.readRecord(Entry.ID, Record);
+    switch (Code) {
+    default: // Default behavior: ignore.
+      break;
+    case bitc::METADATA_KIND: {
+      if (Error Err = parseMetadataKindRecord(Record))
+        return Err;
+      break;
+    }
+    }
+  }
+}
+
+MetadataLoader &MetadataLoader::operator=(MetadataLoader &&RHS) {
+  Pimpl = std::move(RHS.Pimpl);
+  return *this;
+}
+MetadataLoader::MetadataLoader(MetadataLoader &&RHS)
+    : Pimpl(std::move(RHS.Pimpl)) {}
+
+MetadataLoader::~MetadataLoader() = default;
+MetadataLoader::MetadataLoader(BitstreamCursor &Stream, Module &TheModule,
+                               BitcodeReaderValueList &ValueList,
+                               std::function<Type *(unsigned)> getTypeByID)
+    : Pimpl(make_unique<MetadataLoaderImpl>(Stream, TheModule, ValueList,
+                                            getTypeByID)) {}
+
+Error MetadataLoader::parseMetadata(bool ModuleLevel) {
+  return Pimpl->parseMetadata(ModuleLevel);
+}
+
+bool MetadataLoader::hasFwdRefs() const { return Pimpl->hasFwdRefs(); }
+
+/// Return the given metadata, creating a replaceable forward reference if
+/// necessary.
+Metadata *MetadataLoader::getMetadataFwdRef(unsigned Idx) {
+  return Pimpl->getMetadataFwdRef(Idx);
+}
+
+MDNode *MetadataLoader::getMDNodeFwdRefOrNull(unsigned Idx) {
+  return Pimpl->getMDNodeFwdRefOrNull(Idx);
+}
+
+DISubprogram *MetadataLoader::lookupSubprogramForFunction(Function *F) {
+  return Pimpl->lookupSubprogramForFunction(F);
+}
+
+Error MetadataLoader::parseMetadataAttachment(
+    Function &F, const SmallVectorImpl<Instruction *> &InstructionList) {
+  return Pimpl->parseMetadataAttachment(F, InstructionList);
+}
+
+Error MetadataLoader::parseMetadataKinds() {
+  return Pimpl->parseMetadataKinds();
+}
+
+unsigned MetadataLoader::size() const { return Pimpl->size(); }
+void MetadataLoader::shrinkTo(unsigned N) { return Pimpl->shrinkTo(N); }
Index: llvm/lib/Bitcode/Reader/ValueList.h
===================================================================
--- /dev/null
+++ llvm/lib/Bitcode/Reader/ValueList.h
@@ -0,0 +1,76 @@
+//===-- Bitcode/Reader/ValueEnumerator.h - Number values --------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This class gives values and types Unique ID's.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/IR/LLVMContext.h"
+#include "llvm/IR/ValueHandle.h"
+
+#include <vector>
+
+namespace llvm {
+class Constant;
+
+class BitcodeReaderValueList {
+  std::vector<WeakVH> ValuePtrs;
+
+  /// As we resolve forward-referenced constants, we add information about them
+  /// to this vector.  This allows us to resolve them in bulk instead of
+  /// resolving each reference at a time.  See the code in
+  /// ResolveConstantForwardRefs for more information about this.
+  ///
+  /// The key of this vector is the placeholder constant, the value is the slot
+  /// number that holds the resolved value.
+  typedef std::vector<std::pair<Constant *, unsigned>> ResolveConstantsTy;
+  ResolveConstantsTy ResolveConstants;
+  LLVMContext &Context;
+
+public:
+  BitcodeReaderValueList(LLVMContext &C) : Context(C) {}
+  ~BitcodeReaderValueList() {
+    assert(ResolveConstants.empty() && "Constants not resolved?");
+  }
+
+  // vector compatibility methods
+  unsigned size() const { return ValuePtrs.size(); }
+  void resize(unsigned N) { ValuePtrs.resize(N); }
+  void push_back(Value *V) { ValuePtrs.emplace_back(V); }
+
+  void clear() {
+    assert(ResolveConstants.empty() && "Constants not resolved?");
+    ValuePtrs.clear();
+  }
+
+  Value *operator[](unsigned i) const {
+    assert(i < ValuePtrs.size());
+    return ValuePtrs[i];
+  }
+
+  Value *back() const { return ValuePtrs.back(); }
+  void pop_back() { ValuePtrs.pop_back(); }
+  bool empty() const { return ValuePtrs.empty(); }
+
+  void shrinkTo(unsigned N) {
+    assert(N <= size() && "Invalid shrinkTo request!");
+    ValuePtrs.resize(N);
+  }
+
+  Constant *getConstantFwdRef(unsigned Idx, Type *Ty);
+  Value *getValueFwdRef(unsigned Idx, Type *Ty);
+
+  void assignValue(Value *V, unsigned Idx);
+
+  /// Once all constants are read, this method bulk resolves any forward
+  /// references.
+  void resolveConstantForwardRefs();
+};
+
+} // namespace llvm
Index: llvm/lib/Bitcode/Reader/ValueList.cpp
===================================================================
--- /dev/null
+++ llvm/lib/Bitcode/Reader/ValueList.cpp
@@ -0,0 +1,199 @@
+//===----- ValueList.cpp - Internal BitcodeReader implementation ----------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "ValueList.h"
+#include "llvm/IR/Constants.h"
+#include "llvm/IR/Instructions.h"
+
+using namespace llvm;
+
+namespace llvm {
+namespace {
+
+/// \brief A class for maintaining the slot number definition
+/// as a placeholder for the actual definition for forward constants defs.
+class ConstantPlaceHolder : public ConstantExpr {
+  void operator=(const ConstantPlaceHolder &) = delete;
+
+public:
+  // allocate space for exactly one operand
+  void *operator new(size_t s) { return User::operator new(s, 1); }
+  explicit ConstantPlaceHolder(Type *Ty, LLVMContext &Context)
+      : ConstantExpr(Ty, Instruction::UserOp1, &Op<0>(), 1) {
+    Op<0>() = UndefValue::get(Type::getInt32Ty(Context));
+  }
+
+  /// \brief Methods to support type inquiry through isa, cast, and dyn_cast.
+  static bool classof(const Value *V) {
+    return isa<ConstantExpr>(V) &&
+           cast<ConstantExpr>(V)->getOpcode() == Instruction::UserOp1;
+  }
+
+  /// Provide fast operand accessors
+  DECLARE_TRANSPARENT_OPERAND_ACCESSORS(Value);
+};
+
+} // end anonymous namespace
+
+// FIXME: can we inherit this from ConstantExpr?
+template <>
+struct OperandTraits<ConstantPlaceHolder>
+    : public FixedNumOperandTraits<ConstantPlaceHolder, 1> {};
+DEFINE_TRANSPARENT_OPERAND_ACCESSORS(ConstantPlaceHolder, Value)
+
+} // end namespace llvm
+
+void BitcodeReaderValueList::assignValue(Value *V, unsigned Idx) {
+  if (Idx == size()) {
+    push_back(V);
+    return;
+  }
+
+  if (Idx >= size())
+    resize(Idx + 1);
+
+  WeakVH &OldV = ValuePtrs[Idx];
+  if (!OldV) {
+    OldV = V;
+    return;
+  }
+
+  // Handle constants and non-constants (e.g. instrs) differently for
+  // efficiency.
+  if (Constant *PHC = dyn_cast<Constant>(&*OldV)) {
+    ResolveConstants.push_back(std::make_pair(PHC, Idx));
+    OldV = V;
+  } else {
+    // If there was a forward reference to this value, replace it.
+    Value *PrevVal = OldV;
+    OldV->replaceAllUsesWith(V);
+    delete PrevVal;
+  }
+}
+
+Constant *BitcodeReaderValueList::getConstantFwdRef(unsigned Idx, Type *Ty) {
+  if (Idx >= size())
+    resize(Idx + 1);
+
+  if (Value *V = ValuePtrs[Idx]) {
+    if (Ty != V->getType())
+      report_fatal_error("Type mismatch in constant table!");
+    return cast<Constant>(V);
+  }
+
+  // Create and return a placeholder, which will later be RAUW'd.
+  Constant *C = new ConstantPlaceHolder(Ty, Context);
+  ValuePtrs[Idx] = C;
+  return C;
+}
+
+Value *BitcodeReaderValueList::getValueFwdRef(unsigned Idx, Type *Ty) {
+  // Bail out for a clearly invalid value. This would make us call resize(0)
+  if (Idx == std::numeric_limits<unsigned>::max())
+    return nullptr;
+
+  if (Idx >= size())
+    resize(Idx + 1);
+
+  if (Value *V = ValuePtrs[Idx]) {
+    // If the types don't match, it's invalid.
+    if (Ty && Ty != V->getType())
+      return nullptr;
+    return V;
+  }
+
+  // No type specified, must be invalid reference.
+  if (!Ty)
+    return nullptr;
+
+  // Create and return a placeholder, which will later be RAUW'd.
+  Value *V = new Argument(Ty);
+  ValuePtrs[Idx] = V;
+  return V;
+}
+
+/// Once all constants are read, this method bulk resolves any forward
+/// references.  The idea behind this is that we sometimes get constants (such
+/// as large arrays) which reference *many* forward ref constants.  Replacing
+/// each of these causes a lot of thrashing when building/reuniquing the
+/// constant.  Instead of doing this, we look at all the uses and rewrite all
+/// the place holders at once for any constant that uses a placeholder.
+void BitcodeReaderValueList::resolveConstantForwardRefs() {
+  // Sort the values by-pointer so that they are efficient to look up with a
+  // binary search.
+  std::sort(ResolveConstants.begin(), ResolveConstants.end());
+
+  SmallVector<Constant *, 64> NewOps;
+
+  while (!ResolveConstants.empty()) {
+    Value *RealVal = operator[](ResolveConstants.back().second);
+    Constant *Placeholder = ResolveConstants.back().first;
+    ResolveConstants.pop_back();
+
+    // Loop over all users of the placeholder, updating them to reference the
+    // new value.  If they reference more than one placeholder, update them all
+    // at once.
+    while (!Placeholder->use_empty()) {
+      auto UI = Placeholder->user_begin();
+      User *U = *UI;
+
+      // If the using object isn't uniqued, just update the operands.  This
+      // handles instructions and initializers for global variables.
+      if (!isa<Constant>(U) || isa<GlobalValue>(U)) {
+        UI.getUse().set(RealVal);
+        continue;
+      }
+
+      // Otherwise, we have a constant that uses the placeholder.  Replace that
+      // constant with a new constant that has *all* placeholder uses updated.
+      Constant *UserC = cast<Constant>(U);
+      for (User::op_iterator I = UserC->op_begin(), E = UserC->op_end(); I != E;
+           ++I) {
+        Value *NewOp;
+        if (!isa<ConstantPlaceHolder>(*I)) {
+          // Not a placeholder reference.
+          NewOp = *I;
+        } else if (*I == Placeholder) {
+          // Common case is that it just references this one placeholder.
+          NewOp = RealVal;
+        } else {
+          // Otherwise, look up the placeholder in ResolveConstants.
+          ResolveConstantsTy::iterator It = std::lower_bound(
+              ResolveConstants.begin(), ResolveConstants.end(),
+              std::pair<Constant *, unsigned>(cast<Constant>(*I), 0));
+          assert(It != ResolveConstants.end() && It->first == *I);
+          NewOp = operator[](It->second);
+        }
+
+        NewOps.push_back(cast<Constant>(NewOp));
+      }
+
+      // Make the new constant.
+      Constant *NewC;
+      if (ConstantArray *UserCA = dyn_cast<ConstantArray>(UserC)) {
+        NewC = ConstantArray::get(UserCA->getType(), NewOps);
+      } else if (ConstantStruct *UserCS = dyn_cast<ConstantStruct>(UserC)) {
+        NewC = ConstantStruct::get(UserCS->getType(), NewOps);
+      } else if (isa<ConstantVector>(UserC)) {
+        NewC = ConstantVector::get(NewOps);
+      } else {
+        assert(isa<ConstantExpr>(UserC) && "Must be a ConstantExpr.");
+        NewC = cast<ConstantExpr>(UserC)->getWithOperands(NewOps);
+      }
+
+      UserC->replaceAllUsesWith(NewC);
+      UserC->destroyConstant();
+      NewOps.clear();
+    }
+
+    // Update all ValueHandles, they should be the only users at this point.
+    Placeholder->replaceAllUsesWith(RealVal);
+    delete Placeholder;
+  }
+}