1 files changed, 2175 insertions, 0 deletions

diff --git a/contrib/llvm-project/llvm/lib/Bitcode/Reader/MetadataLoader.cpp b/contrib/llvm-project/llvm/lib/Bitcode/Reader/MetadataLoader.cpp
new file mode 100644
index 000000000000..108f71189585
--- /dev/null
+++ b/contrib/llvm-project/llvm/lib/Bitcode/Reader/MetadataLoader.cpp
@@ -0,0 +1,2175 @@
+//===- MetadataLoader.cpp - Internal BitcodeReader implementation ---------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#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/DenseSet.h"
+#include "llvm/ADT/None.h"
+#include "llvm/ADT/STLExtras.h"
+#include "llvm/ADT/SmallString.h"
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/ADT/StringRef.h"
+#include "llvm/ADT/Twine.h"
+#include "llvm/Bitcode/BitcodeReader.h"
+#include "llvm/Bitstream/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/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/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/IntrinsicInst.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/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;
+
+#define DEBUG_TYPE "bitcode-reader"
+
+STATISTIC(NumMDStringLoaded, "Number of MDStrings loaded");
+STATISTIC(NumMDNodeTemporary, "Number of MDNode::Temporary created");
+STATISTIC(NumMDRecordLoaded, "Number of Metadata records loaded");
+
+/// Flag whether we need to import full type definitions for ThinLTO.
+/// Currently needed for Darwin and LLDB.
+static cl::opt<bool> ImportFullTypeDefinitions(
+    "import-full-type-definitions", cl::init(false), cl::Hidden,
+    cl::desc("Import full type definitions for ThinLTO."));
+
+static cl::opt<bool> DisableLazyLoading(
+    "disable-ondemand-mds-loading", cl::init(false), cl::Hidden,
+    cl::desc("Force disable the lazy-loading on-demand of metadata when "
+             "loading bitcode for importing."));
+
+namespace {
+
+static int64_t unrotateSign(uint64_t U) { return (U & 1) ? ~(U >> 1) : U >> 1; }
+
+class BitcodeReaderMetadataList {
+  /// 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;
+
+  /// The set of indices in MetadataPtrs above of forward references that were
+  /// generated.
+  SmallDenseSet<unsigned, 1> ForwardReference;
+
+  /// The set of indices in MetadataPtrs above of Metadata that need to be
+  /// resolved.
+  SmallDenseSet<unsigned, 1> UnresolvedNodes;
+
+  /// 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;
+
+  /// Maximum number of valid references. Forward references exceeding the
+  /// maximum must be invalid.
+  unsigned RefsUpperBound;
+
+public:
+  BitcodeReaderMetadataList(LLVMContext &C, size_t RefsUpperBound)
+      : Context(C),
+        RefsUpperBound(std::min((size_t)std::numeric_limits<unsigned>::max(),
+                                RefsUpperBound)) {}
+
+  // 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(ForwardReference.empty() && "Unexpected forward refs");
+    assert(UnresolvedNodes.empty() && "Unexpected unresolved node");
+    MetadataPtrs.resize(N);
+  }
+
+  /// Return the given metadata, creating a replaceable forward reference if
+  /// necessary.
+  Metadata *getMetadataFwdRef(unsigned Idx);
+
+  /// Return 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 !ForwardReference.empty(); }
+  int getNextFwdRef() {
+    assert(hasFwdRefs());
+    return *ForwardReference.begin();
+  }
+
+  /// 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 (auto *MDN = dyn_cast<MDNode>(MD))
+    if (!MDN->isResolved())
+      UnresolvedNodes.insert(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);
+  ForwardReference.erase(Idx);
+}
+
+Metadata *BitcodeReaderMetadataList::getMetadataFwdRef(unsigned Idx) {
+  // Bail out for a clearly invalid value.
+  if (Idx >= RefsUpperBound)
+    return nullptr;
+
+  if (Idx >= size())
+    resize(Idx + 1);
+
+  if (Metadata *MD = MetadataPtrs[Idx])
+    return MD;
+
+  // Track forward refs to be resolved later.
+  ForwardReference.insert(Idx);
+
+  // Create and return a placeholder, which will later be RAUW'd.
+  ++NumMDNodeTemporary;
+  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 (!ForwardReference.empty())
+    // Still forward references... can't resolve cycles.
+    return;
+
+  // 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)
+    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) {
+    if (DICompositeType *CT = OldTypeRefs.Final.lookup(Ref.first))
+      Ref.second->replaceAllUsesWith(CT);
+    else
+      Ref.second->replaceAllUsesWith(Ref.first);
+  }
+  OldTypeRefs.Unknown.clear();
+
+  if (UnresolvedNodes.empty())
+    // Nothing to do.
+    return;
+
+  // Resolve any cycles.
+  for (unsigned I : UnresolvedNodes) {
+    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 unresolved ref.
+  UnresolvedNodes.clear();
+}
+
+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 DIType *.
+  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:
+  ~PlaceholderQueue() {
+    assert(empty() && "PlaceholderQueue hasn't been flushed before being destroyed");
+  }
+  bool empty() { return PHs.empty(); }
+  DistinctMDOperandPlaceholder &getPlaceholderOp(unsigned ID);
+  void flush(BitcodeReaderMetadataList &MetadataList);
+
+  /// Return the list of temporaries nodes in the queue, these need to be
+  /// loaded before we can flush the queue.
+  void getTemporaries(BitcodeReaderMetadataList &MetadataList,
+                      DenseSet<unsigned> &Temporaries) {
+    for (auto &PH : PHs) {
+      auto ID = PH.getID();
+      auto *MD = MetadataList.lookup(ID);
+      if (!MD) {
+        Temporaries.insert(ID);
+        continue;
+      }
+      auto *N = dyn_cast_or_null<MDNode>(MD);
+      if (N && N->isTemporary())
+        Temporaries.insert(ID);
+    }
+  }
+};
+
+} // end anonymous namespace
+
+DistinctMDOperandPlaceholder &PlaceholderQueue::getPlaceholderOp(unsigned ID) {
+  PHs.emplace_back(ID);
+  return PHs.back();
+}
+
+void PlaceholderQueue::flush(BitcodeReaderMetadataList &MetadataList) {
+  while (!PHs.empty()) {
+    auto *MD = MetadataList.lookup(PHs.front().getID());
+    assert(MD && "Flushing placeholder on unassigned MD");
+#ifndef NDEBUG
+    if (auto *MDN = dyn_cast<MDNode>(MD))
+      assert(MDN->isResolved() &&
+             "Flushing Placeholder while cycles aren't resolved");
+#endif
+    PHs.front().replaceUseWith(MD);
+    PHs.pop_front();
+  }
+}
+
+} // anonynous namespace
+
+static Error error(const Twine &Message) {
+  return make_error<StringError>(
+      Message, make_error_code(BitcodeError::CorruptedBitcode));
+}
+
+class MetadataLoader::MetadataLoaderImpl {
+  BitcodeReaderMetadataList MetadataList;
+  BitcodeReaderValueList &ValueList;
+  BitstreamCursor &Stream;
+  LLVMContext &Context;
+  Module &TheModule;
+  std::function<Type *(unsigned)> getTypeByID;
+
+  /// Cursor associated with the lazy-loading of Metadata. This is the easy way
+  /// to keep around the right "context" (Abbrev list) to be able to jump in
+  /// the middle of the metadata block and load any record.
+  BitstreamCursor IndexCursor;
+
+  /// Index that keeps track of MDString values.
+  std::vector<StringRef> MDStringRef;
+
+  /// On-demand loading of a single MDString. Requires the index above to be
+  /// populated.
+  MDString *lazyLoadOneMDString(unsigned Idx);
+
+  /// Index that keeps track of where to find a metadata record in the stream.
+  std::vector<uint64_t> GlobalMetadataBitPosIndex;
+
+  /// Populate the index above to enable lazily loading of metadata, and load
+  /// the named metadata as well as the transitively referenced global
+  /// Metadata.
+  Expected<bool> lazyLoadModuleMetadataBlock();
+
+  /// On-demand loading of a single metadata. Requires the index above to be
+  /// populated.
+  void lazyLoadOneMetadata(unsigned Idx, PlaceholderQueue &Placeholders);
+
+  // Keep mapping of seens pair of old-style CU <-> SP, and update pointers to
+  // point from SP to CU after a block is completly parsed.
+  std::vector<std::pair<DICompileUnit *, Metadata *>> CUSubprograms;
+
+  /// 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 StripTBAA = false;
+  bool HasSeenOldLoopTags = false;
+  bool NeedUpgradeToDIGlobalVariableExpression = false;
+  bool NeedDeclareExpressionUpgrade = false;
+
+  /// True if metadata is being parsed for a module being ThinLTO imported.
+  bool IsImporting = false;
+
+  Error parseOneMetadata(SmallVectorImpl<uint64_t> &Record, unsigned Code,
+                         PlaceholderQueue &Placeholders, StringRef Blob,
+                         unsigned &NextMetadataNo);
+  Error parseMetadataStrings(ArrayRef<uint64_t> Record, StringRef Blob,
+                             function_ref<void(StringRef)> CallBack);
+  Error parseGlobalObjectAttachment(GlobalObject &GO,
+                                    ArrayRef<uint64_t> Record);
+  Error parseMetadataKindRecord(SmallVectorImpl<uint64_t> &Record);
+
+  void resolveForwardRefsAndPlaceholders(PlaceholderQueue &Placeholders);
+
+  /// Upgrade old-style CU <-> SP pointers to point from SP to CU.
+  void upgradeCUSubprograms() {
+    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<DISubprogram>(Op))
+            SP->replaceUnit(CU_SP.first);
+    CUSubprograms.clear();
+  }
+
+  /// Upgrade old-style bare DIGlobalVariables to DIGlobalVariableExpressions.
+  void upgradeCUVariables() {
+    if (!NeedUpgradeToDIGlobalVariableExpression)
+      return;
+
+    // Upgrade list of variables attached to the CUs.
+    if (NamedMDNode *CUNodes = TheModule.getNamedMetadata("llvm.dbg.cu"))
+      for (unsigned I = 0, E = CUNodes->getNumOperands(); I != E; ++I) {
+        auto *CU = cast<DICompileUnit>(CUNodes->getOperand(I));
+        if (auto *GVs = dyn_cast_or_null<MDTuple>(CU->getRawGlobalVariables()))
+          for (unsigned I = 0; I < GVs->getNumOperands(); I++)
+            if (auto *GV =
+                    dyn_cast_or_null<DIGlobalVariable>(GVs->getOperand(I))) {
+              auto *DGVE = DIGlobalVariableExpression::getDistinct(
+                  Context, GV, DIExpression::get(Context, {}));
+              GVs->replaceOperandWith(I, DGVE);
+            }
+      }
+
+    // Upgrade variables attached to globals.
+    for (auto &GV : TheModule.globals()) {
+      SmallVector<MDNode *, 1> MDs;
+      GV.getMetadata(LLVMContext::MD_dbg, MDs);
+      GV.eraseMetadata(LLVMContext::MD_dbg);
+      for (auto *MD : MDs)
+        if (auto *DGV = dyn_cast_or_null<DIGlobalVariable>(MD)) {
+          auto *DGVE = DIGlobalVariableExpression::getDistinct(
+              Context, DGV, DIExpression::get(Context, {}));
+          GV.addMetadata(LLVMContext::MD_dbg, *DGVE);
+        } else
+          GV.addMetadata(LLVMContext::MD_dbg, *MD);
+    }
+  }
+
+  /// Remove a leading DW_OP_deref from DIExpressions in a dbg.declare that
+  /// describes a function argument.
+  void upgradeDeclareExpressions(Function &F) {
+    if (!NeedDeclareExpressionUpgrade)
+      return;
+
+    for (auto &BB : F)
+      for (auto &I : BB)
+        if (auto *DDI = dyn_cast<DbgDeclareInst>(&I))
+          if (auto *DIExpr = DDI->getExpression())
+            if (DIExpr->startsWithDeref() &&
+                dyn_cast_or_null<Argument>(DDI->getAddress())) {
+              SmallVector<uint64_t, 8> Ops;
+              Ops.append(std::next(DIExpr->elements_begin()),
+                         DIExpr->elements_end());
+              auto *E = DIExpression::get(Context, Ops);
+              DDI->setOperand(2, MetadataAsValue::get(Context, E));
+            }
+  }
+
+  /// Upgrade the expression from previous versions.
+  Error upgradeDIExpression(uint64_t FromVersion,
+                            MutableArrayRef<uint64_t> &Expr,
+                            SmallVectorImpl<uint64_t> &Buffer) {
+    auto N = Expr.size();
+    switch (FromVersion) {
+    default:
+      return error("Invalid record");
+    case 0:
+      if (N >= 3 && Expr[N - 3] == dwarf::DW_OP_bit_piece)
+        Expr[N - 3] = dwarf::DW_OP_LLVM_fragment;
+      LLVM_FALLTHROUGH;
+    case 1:
+      // Move DW_OP_deref to the end.
+      if (N && Expr[0] == dwarf::DW_OP_deref) {
+        auto End = Expr.end();
+        if (Expr.size() >= 3 &&
+            *std::prev(End, 3) == dwarf::DW_OP_LLVM_fragment)
+          End = std::prev(End, 3);
+        std::move(std::next(Expr.begin()), End, Expr.begin());
+        *std::prev(End) = dwarf::DW_OP_deref;
+      }
+      NeedDeclareExpressionUpgrade = true;
+      LLVM_FALLTHROUGH;
+    case 2: {
+      // Change DW_OP_plus to DW_OP_plus_uconst.
+      // Change DW_OP_minus to DW_OP_uconst, DW_OP_minus
+      auto SubExpr = ArrayRef<uint64_t>(Expr);
+      while (!SubExpr.empty()) {
+        // Skip past other operators with their operands
+        // for this version of the IR, obtained from
+        // from historic DIExpression::ExprOperand::getSize().
+        size_t HistoricSize;
+        switch (SubExpr.front()) {
+        default:
+          HistoricSize = 1;
+          break;
+        case dwarf::DW_OP_constu:
+        case dwarf::DW_OP_minus:
+        case dwarf::DW_OP_plus:
+          HistoricSize = 2;
+          break;
+        case dwarf::DW_OP_LLVM_fragment:
+          HistoricSize = 3;
+          break;
+        }
+
+        // If the expression is malformed, make sure we don't
+        // copy more elements than we should.
+        HistoricSize = std::min(SubExpr.size(), HistoricSize);
+        ArrayRef<uint64_t> Args = SubExpr.slice(1, HistoricSize-1);
+
+        switch (SubExpr.front()) {
+        case dwarf::DW_OP_plus:
+          Buffer.push_back(dwarf::DW_OP_plus_uconst);
+          Buffer.append(Args.begin(), Args.end());
+          break;
+        case dwarf::DW_OP_minus:
+          Buffer.push_back(dwarf::DW_OP_constu);
+          Buffer.append(Args.begin(), Args.end());
+          Buffer.push_back(dwarf::DW_OP_minus);
+          break;
+        default:
+          Buffer.push_back(*SubExpr.begin());
+          Buffer.append(Args.begin(), Args.end());
+          break;
+        }
+
+        // Continue with remaining elements.
+        SubExpr = SubExpr.slice(HistoricSize);
+      }
+      Expr = MutableArrayRef<uint64_t>(Buffer);
+      LLVM_FALLTHROUGH;
+    }
+    case 3:
+      // Up-to-date!
+      break;
+    }
+
+    return Error::success();
+  }
+
+  void upgradeDebugInfo() {
+    upgradeCUSubprograms();
+    upgradeCUVariables();
+  }
+
+public:
+  MetadataLoaderImpl(BitstreamCursor &Stream, Module &TheModule,
+                     BitcodeReaderValueList &ValueList,
+                     std::function<Type *(unsigned)> getTypeByID,
+                     bool IsImporting)
+      : MetadataList(TheModule.getContext(), Stream.SizeInBytes()),
+        ValueList(ValueList), Stream(Stream), Context(TheModule.getContext()),
+        TheModule(TheModule), getTypeByID(std::move(getTypeByID)),
+        IsImporting(IsImporting) {}
+
+  Error parseMetadata(bool ModuleLevel);
+
+  bool hasFwdRefs() const { return MetadataList.hasFwdRefs(); }
+
+  Metadata *getMetadataFwdRefOrLoad(unsigned ID) {
+    if (ID < MDStringRef.size())
+      return lazyLoadOneMDString(ID);
+    if (auto *MD = MetadataList.lookup(ID))
+      return MD;
+    // If lazy-loading is enabled, we try recursively to load the operand
+    // instead of creating a temporary.
+    if (ID < (MDStringRef.size() + GlobalMetadataBitPosIndex.size())) {
+      PlaceholderQueue Placeholders;
+      lazyLoadOneMetadata(ID, Placeholders);
+      resolveForwardRefsAndPlaceholders(Placeholders);
+      return MetadataList.lookup(ID);
+    }
+    return MetadataList.getMetadataFwdRef(ID);
+  }
+
+  DISubprogram *lookupSubprogramForFunction(Function *F) {
+    return FunctionsWithSPs.lookup(F);
+  }
+
+  bool hasSeenOldLoopTags() { return HasSeenOldLoopTags; }
+
+  Error parseMetadataAttachment(
+      Function &F, const SmallVectorImpl<Instruction *> &InstructionList);
+
+  Error parseMetadataKinds();
+
+  void setStripTBAA(bool Value) { StripTBAA = Value; }
+  bool isStrippingTBAA() { return StripTBAA; }
+
+  unsigned size() const { return MetadataList.size(); }
+  void shrinkTo(unsigned N) { MetadataList.shrinkTo(N); }
+  void upgradeDebugIntrinsics(Function &F) { upgradeDeclareExpressions(F); }
+};
+
+Expected<bool>
+MetadataLoader::MetadataLoaderImpl::lazyLoadModuleMetadataBlock() {
+  IndexCursor = Stream;
+  SmallVector<uint64_t, 64> Record;
+  // Get the abbrevs, and preload record positions to make them lazy-loadable.
+  while (true) {
+    Expected<BitstreamEntry> MaybeEntry = IndexCursor.advanceSkippingSubblocks(
+        BitstreamCursor::AF_DontPopBlockAtEnd);
+    if (!MaybeEntry)
+      return MaybeEntry.takeError();
+    BitstreamEntry Entry = MaybeEntry.get();
+
+    switch (Entry.Kind) {
+    case BitstreamEntry::SubBlock: // Handled for us already.
+    case BitstreamEntry::Error:
+      return error("Malformed block");
+    case BitstreamEntry::EndBlock: {
+      return true;
+    }
+    case BitstreamEntry::Record: {
+      // The interesting case.
+      ++NumMDRecordLoaded;
+      uint64_t CurrentPos = IndexCursor.GetCurrentBitNo();
+      Expected<unsigned> MaybeCode = IndexCursor.skipRecord(Entry.ID);
+      if (!MaybeCode)
+        return MaybeCode.takeError();
+      unsigned Code = MaybeCode.get();
+      switch (Code) {
+      case bitc::METADATA_STRINGS: {
+        // Rewind and parse the strings.
+        if (Error Err = IndexCursor.JumpToBit(CurrentPos))
+          return std::move(Err);
+        StringRef Blob;
+        Record.clear();
+        if (Expected<unsigned> MaybeRecord =
+                IndexCursor.readRecord(Entry.ID, Record, &Blob))
+          ;
+        else
+          return MaybeRecord.takeError();
+        unsigned NumStrings = Record[0];
+        MDStringRef.reserve(NumStrings);
+        auto IndexNextMDString = [&](StringRef Str) {
+          MDStringRef.push_back(Str);
+        };
+        if (auto Err = parseMetadataStrings(Record, Blob, IndexNextMDString))
+          return std::move(Err);
+        break;
+      }
+      case bitc::METADATA_INDEX_OFFSET: {
+        // This is the offset to the index, when we see this we skip all the
+        // records and load only an index to these.
+        if (Error Err = IndexCursor.JumpToBit(CurrentPos))
+          return std::move(Err);
+        Record.clear();
+        if (Expected<unsigned> MaybeRecord =
+                IndexCursor.readRecord(Entry.ID, Record))
+          ;
+        else
+          return MaybeRecord.takeError();
+        if (Record.size() != 2)
+          return error("Invalid record");
+        auto Offset = Record[0] + (Record[1] << 32);
+        auto BeginPos = IndexCursor.GetCurrentBitNo();
+        if (Error Err = IndexCursor.JumpToBit(BeginPos + Offset))
+          return std::move(Err);
+        Expected<BitstreamEntry> MaybeEntry =
+            IndexCursor.advanceSkippingSubblocks(
+                BitstreamCursor::AF_DontPopBlockAtEnd);
+        if (!MaybeEntry)
+          return MaybeEntry.takeError();
+        Entry = MaybeEntry.get();
+        assert(Entry.Kind == BitstreamEntry::Record &&
+               "Corrupted bitcode: Expected `Record` when trying to find the "
+               "Metadata index");
+        Record.clear();
+        if (Expected<unsigned> MaybeCode =
+                IndexCursor.readRecord(Entry.ID, Record))
+          assert(MaybeCode.get() == bitc::METADATA_INDEX &&
+                 "Corrupted bitcode: Expected `METADATA_INDEX` when trying to "
+                 "find the Metadata index");
+        else
+          return MaybeCode.takeError();
+        // Delta unpack
+        auto CurrentValue = BeginPos;
+        GlobalMetadataBitPosIndex.reserve(Record.size());
+        for (auto &Elt : Record) {
+          CurrentValue += Elt;
+          GlobalMetadataBitPosIndex.push_back(CurrentValue);
+        }
+        break;
+      }
+      case bitc::METADATA_INDEX:
+        // We don't expect to get there, the Index is loaded when we encounter
+        // the offset.
+        return error("Corrupted Metadata block");
+      case bitc::METADATA_NAME: {
+        // Named metadata need to be materialized now and aren't deferred.
+        if (Error Err = IndexCursor.JumpToBit(CurrentPos))
+          return std::move(Err);
+        Record.clear();
+
+        unsigned Code;
+        if (Expected<unsigned> MaybeCode =
+                IndexCursor.readRecord(Entry.ID, Record)) {
+          Code = MaybeCode.get();
+          assert(Code == bitc::METADATA_NAME);
+        } else
+          return MaybeCode.takeError();
+
+        // Read name of the named metadata.
+        SmallString<8> Name(Record.begin(), Record.end());
+        if (Expected<unsigned> MaybeCode = IndexCursor.ReadCode())
+          Code = MaybeCode.get();
+        else
+          return MaybeCode.takeError();
+
+        // Named Metadata comes in two parts, we expect the name to be followed
+        // by the node
+        Record.clear();
+        if (Expected<unsigned> MaybeNextBitCode =
+                IndexCursor.readRecord(Code, Record))
+          assert(MaybeNextBitCode.get() == bitc::METADATA_NAMED_NODE);
+        else
+          return MaybeNextBitCode.takeError();
+
+        // Read named metadata elements.
+        unsigned Size = Record.size();
+        NamedMDNode *NMD = TheModule.getOrInsertNamedMetadata(Name);
+        for (unsigned i = 0; i != Size; ++i) {
+          // FIXME: We could use a placeholder here, however NamedMDNode are
+          // taking MDNode as operand and not using the Metadata infrastructure.
+          // It is acknowledged by 'TODO: Inherit from Metadata' in the
+          // NamedMDNode class definition.
+          MDNode *MD = MetadataList.getMDNodeFwdRefOrNull(Record[i]);
+          assert(MD && "Invalid metadata: expect fwd ref to MDNode");
+          NMD->addOperand(MD);
+        }
+        break;
+      }
+      case bitc::METADATA_GLOBAL_DECL_ATTACHMENT: {
+        // FIXME: we need to do this early because we don't materialize global
+        // value explicitly.
+        if (Error Err = IndexCursor.JumpToBit(CurrentPos))
+          return std::move(Err);
+        Record.clear();
+        if (Expected<unsigned> MaybeRecord =
+                IndexCursor.readRecord(Entry.ID, Record))
+          ;
+        else
+          return MaybeRecord.takeError();
+        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 std::move(Err);
+        break;
+      }
+      case bitc::METADATA_KIND:
+      case bitc::METADATA_STRING_OLD:
+      case bitc::METADATA_OLD_FN_NODE:
+      case bitc::METADATA_OLD_NODE:
+      case bitc::METADATA_VALUE:
+      case bitc::METADATA_DISTINCT_NODE:
+      case bitc::METADATA_NODE:
+      case bitc::METADATA_LOCATION:
+      case bitc::METADATA_GENERIC_DEBUG:
+      case bitc::METADATA_SUBRANGE:
+      case bitc::METADATA_ENUMERATOR:
+      case bitc::METADATA_BASIC_TYPE:
+      case bitc::METADATA_DERIVED_TYPE:
+      case bitc::METADATA_COMPOSITE_TYPE:
+      case bitc::METADATA_SUBROUTINE_TYPE:
+      case bitc::METADATA_MODULE:
+      case bitc::METADATA_FILE:
+      case bitc::METADATA_COMPILE_UNIT:
+      case bitc::METADATA_SUBPROGRAM:
+      case bitc::METADATA_LEXICAL_BLOCK:
+      case bitc::METADATA_LEXICAL_BLOCK_FILE:
+      case bitc::METADATA_NAMESPACE:
+      case bitc::METADATA_COMMON_BLOCK:
+      case bitc::METADATA_MACRO:
+      case bitc::METADATA_MACRO_FILE:
+      case bitc::METADATA_TEMPLATE_TYPE:
+      case bitc::METADATA_TEMPLATE_VALUE:
+      case bitc::METADATA_GLOBAL_VAR:
+      case bitc::METADATA_LOCAL_VAR:
+      case bitc::METADATA_LABEL:
+      case bitc::METADATA_EXPRESSION:
+      case bitc::METADATA_OBJC_PROPERTY:
+      case bitc::METADATA_IMPORTED_ENTITY:
+      case bitc::METADATA_GLOBAL_VAR_EXPR:
+        // We don't expect to see any of these, if we see one, give up on
+        // lazy-loading and fallback.
+        MDStringRef.clear();
+        GlobalMetadataBitPosIndex.clear();
+        return false;
+      }
+      break;
+    }
+    }
+  }
+}
+
+/// Parse a METADATA_BLOCK. If ModuleLevel is true then we are parsing
+/// module level metadata.
+Error MetadataLoader::MetadataLoaderImpl::parseMetadata(bool ModuleLevel) {
+  if (!ModuleLevel && MetadataList.hasFwdRefs())
+    return error("Invalid metadata: fwd refs into function blocks");
+
+  // Record the entry position so that we can jump back here and efficiently
+  // skip the whole block in case we lazy-load.
+  auto EntryPos = Stream.GetCurrentBitNo();
+
+  if (Error Err = Stream.EnterSubBlock(bitc::METADATA_BLOCK_ID))
+    return Err;
+
+  SmallVector<uint64_t, 64> Record;
+  PlaceholderQueue Placeholders;
+
+  // We lazy-load module-level metadata: we build an index for each record, and
+  // then load individual record as needed, starting with the named metadata.
+  if (ModuleLevel && IsImporting && MetadataList.empty() &&
+      !DisableLazyLoading) {
+    auto SuccessOrErr = lazyLoadModuleMetadataBlock();
+    if (!SuccessOrErr)
+      return SuccessOrErr.takeError();
+    if (SuccessOrErr.get()) {
+      // An index was successfully created and we will be able to load metadata
+      // on-demand.
+      MetadataList.resize(MDStringRef.size() +
+                          GlobalMetadataBitPosIndex.size());
+
+      // Reading the named metadata created forward references and/or
+      // placeholders, that we flush here.
+      resolveForwardRefsAndPlaceholders(Placeholders);
+      upgradeDebugInfo();
+      // Return at the beginning of the block, since it is easy to skip it
+      // entirely from there.
+      Stream.ReadBlockEnd(); // Pop the abbrev block context.
+      if (Error Err = IndexCursor.JumpToBit(EntryPos))
+        return Err;
+      if (Error Err = Stream.SkipBlock()) {
+        // FIXME this drops the error on the floor, which
+        // ThinLTO/X86/debuginfo-cu-import.ll relies on.
+        consumeError(std::move(Err));
+        return Error::success();
+      }
+      return Error::success();
+    }
+    // Couldn't load an index, fallback to loading all the block "old-style".
+  }
+
+  unsigned NextMetadataNo = MetadataList.size();
+
+  // Read all the records.
+  while (true) {
+    Expected<BitstreamEntry> MaybeEntry = Stream.advanceSkippingSubblocks();
+    if (!MaybeEntry)
+      return MaybeEntry.takeError();
+    BitstreamEntry Entry = MaybeEntry.get();
+
+    switch (Entry.Kind) {
+    case BitstreamEntry::SubBlock: // Handled for us already.
+    case BitstreamEntry::Error:
+      return error("Malformed block");
+    case BitstreamEntry::EndBlock:
+      resolveForwardRefsAndPlaceholders(Placeholders);
+      upgradeDebugInfo();
+      return Error::success();
+    case BitstreamEntry::Record:
+      // The interesting case.
+      break;
+    }
+
+    // Read a record.
+    Record.clear();
+    StringRef Blob;
+    ++NumMDRecordLoaded;
+    if (Expected<unsigned> MaybeCode =
+            Stream.readRecord(Entry.ID, Record, &Blob)) {
+      if (Error Err = parseOneMetadata(Record, MaybeCode.get(), Placeholders,
+                                       Blob, NextMetadataNo))
+        return Err;
+    } else
+      return MaybeCode.takeError();
+  }
+}
+
+MDString *MetadataLoader::MetadataLoaderImpl::lazyLoadOneMDString(unsigned ID) {
+  ++NumMDStringLoaded;
+  if (Metadata *MD = MetadataList.lookup(ID))
+    return cast<MDString>(MD);
+  auto MDS = MDString::get(Context, MDStringRef[ID]);
+  MetadataList.assignValue(MDS, ID);
+  return MDS;
+}
+
+void MetadataLoader::MetadataLoaderImpl::lazyLoadOneMetadata(
+    unsigned ID, PlaceholderQueue &Placeholders) {
+  assert(ID < (MDStringRef.size()) + GlobalMetadataBitPosIndex.size());
+  assert(ID >= MDStringRef.size() && "Unexpected lazy-loading of MDString");
+  // Lookup first if the metadata hasn't already been loaded.
+  if (auto *MD = MetadataList.lookup(ID)) {
+    auto *N = dyn_cast_or_null<MDNode>(MD);
+    if (!N->isTemporary())
+      return;
+  }
+  SmallVector<uint64_t, 64> Record;
+  StringRef Blob;
+  if (Error Err = IndexCursor.JumpToBit(
+          GlobalMetadataBitPosIndex[ID - MDStringRef.size()]))
+    report_fatal_error("lazyLoadOneMetadata failed jumping: " +
+                       toString(std::move(Err)));
+  Expected<BitstreamEntry> MaybeEntry = IndexCursor.advanceSkippingSubblocks();
+  if (!MaybeEntry)
+    // FIXME this drops the error on the floor.
+    report_fatal_error("lazyLoadOneMetadata failed advanceSkippingSubblocks: " +
+                       toString(MaybeEntry.takeError()));
+  BitstreamEntry Entry = MaybeEntry.get();
+  ++NumMDRecordLoaded;
+  if (Expected<unsigned> MaybeCode =
+          IndexCursor.readRecord(Entry.ID, Record, &Blob)) {
+    if (Error Err =
+            parseOneMetadata(Record, MaybeCode.get(), Placeholders, Blob, ID))
+      report_fatal_error("Can't lazyload MD, parseOneMetadata: " +
+                         toString(std::move(Err)));
+  } else
+    report_fatal_error("Can't lazyload MD: " + toString(MaybeCode.takeError()));
+}
+
+/// Ensure that all forward-references and placeholders are resolved.
+/// Iteratively lazy-loading metadata on-demand if needed.
+void MetadataLoader::MetadataLoaderImpl::resolveForwardRefsAndPlaceholders(
+    PlaceholderQueue &Placeholders) {
+  DenseSet<unsigned> Temporaries;
+  while (1) {
+    // Populate Temporaries with the placeholders that haven't been loaded yet.
+    Placeholders.getTemporaries(MetadataList, Temporaries);
+
+    // If we don't have any temporary, or FwdReference, we're done!
+    if (Temporaries.empty() && !MetadataList.hasFwdRefs())
+      break;
+
+    // First, load all the temporaries. This can add new placeholders or
+    // forward references.
+    for (auto ID : Temporaries)
+      lazyLoadOneMetadata(ID, Placeholders);
+    Temporaries.clear();
+
+    // Second, load the forward-references. This can also add new placeholders
+    // or forward references.
+    while (MetadataList.hasFwdRefs())
+      lazyLoadOneMetadata(MetadataList.getNextFwdRef(), Placeholders);
+  }
+  // At this point we don't have any forward reference remaining, or temporary
+  // that haven't been loaded. We can safely drop RAUW support and mark cycles
+  // as resolved.
+  MetadataList.tryToResolveCycles();
+
+  // Finally, everything is in place, we can replace the placeholders operands
+  // with the final node they refer to.
+  Placeholders.flush(MetadataList);
+}
+
+Error MetadataLoader::MetadataLoaderImpl::parseOneMetadata(
+    SmallVectorImpl<uint64_t> &Record, unsigned Code,
+    PlaceholderQueue &Placeholders, StringRef Blob, unsigned &NextMetadataNo) {
+
+  bool IsDistinct = false;
+  auto getMD = [&](unsigned ID) -> Metadata * {
+    if (ID < MDStringRef.size())
+      return lazyLoadOneMDString(ID);
+    if (!IsDistinct) {
+      if (auto *MD = MetadataList.lookup(ID))
+        return MD;
+      // If lazy-loading is enabled, we try recursively to load the operand
+      // instead of creating a temporary.
+      if (ID < (MDStringRef.size() + GlobalMetadataBitPosIndex.size())) {
+        // Create a temporary for the node that is referencing the operand we
+        // will lazy-load. It is needed before recursing in case there are
+        // uniquing cycles.
+        MetadataList.getMetadataFwdRef(NextMetadataNo);
+        lazyLoadOneMetadata(ID, Placeholders);
+        return MetadataList.lookup(ID);
+      }
+      // Return a temporary.
+      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.
+    auto MDS = getMDOrNull(ID);
+    return cast_or_null<MDString>(MDS);
+  };
+
+  // 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)
+
+  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();
+    Expected<unsigned> MaybeCode = Stream.ReadCode();
+    if (!MaybeCode)
+      return MaybeCode.takeError();
+    Code = MaybeCode.get();
+
+    ++NumMDRecordLoaded;
+    if (Expected<unsigned> MaybeNextBitCode = Stream.readRecord(Code, Record)) {
+      if (MaybeNextBitCode.get() != bitc::METADATA_NAMED_NODE)
+        return error("METADATA_NAME not followed by METADATA_NAMED_NODE");
+    } else
+      return MaybeNextBitCode.takeError();
+
+    // 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 named metadata: expect fwd ref to MDNode");
+      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);
+      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);
+    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);
+    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);
+    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);
+    NextMetadataNo++;
+    break;
+  }
+  case bitc::METADATA_LOCATION: {
+    if (Record.size() != 5 && Record.size() != 6)
+      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]);
+    bool ImplicitCode = Record.size() == 6 && Record[5];
+    MetadataList.assignValue(
+        GET_OR_DISTINCT(DILocation, (Context, Line, Column, Scope, InlinedAt,
+                                     ImplicitCode)),
+        NextMetadataNo);
+    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);
+    NextMetadataNo++;
+    break;
+  }
+  case bitc::METADATA_SUBRANGE: {
+    Metadata *Val = nullptr;
+    // Operand 'count' is interpreted as:
+    // - Signed integer (version 0)
+    // - Metadata node  (version 1)
+    switch (Record[0] >> 1) {
+    case 0:
+      Val = GET_OR_DISTINCT(DISubrange,
+                            (Context, Record[1], unrotateSign(Record.back())));
+      break;
+    case 1:
+      Val = GET_OR_DISTINCT(DISubrange, (Context, getMDOrNull(Record[1]),
+                                         unrotateSign(Record.back())));
+      break;
+    default:
+      return error("Invalid record: Unsupported version of DISubrange");
+    }
+
+    MetadataList.assignValue(Val, NextMetadataNo);
+    IsDistinct = Record[0] & 1;
+    NextMetadataNo++;
+    break;
+  }
+  case bitc::METADATA_ENUMERATOR: {
+    if (Record.size() != 3)
+      return error("Invalid record");
+
+    IsDistinct = Record[0] & 1;
+    bool IsUnsigned = Record[0] & 2;
+    MetadataList.assignValue(
+        GET_OR_DISTINCT(DIEnumerator, (Context, unrotateSign(Record[1]),
+                                       IsUnsigned, getMDString(Record[2]))),
+        NextMetadataNo);
+    NextMetadataNo++;
+    break;
+  }
+  case bitc::METADATA_BASIC_TYPE: {
+    if (Record.size() < 6 || Record.size() > 7)
+      return error("Invalid record");
+
+    IsDistinct = Record[0];
+    DINode::DIFlags Flags = (Record.size() > 6) ?
+                    static_cast<DINode::DIFlags>(Record[6]) : DINode::FlagZero;
+
+    MetadataList.assignValue(
+        GET_OR_DISTINCT(DIBasicType,
+                        (Context, Record[1], getMDString(Record[2]), Record[3],
+                         Record[4], Record[5], Flags)),
+        NextMetadataNo);
+    NextMetadataNo++;
+    break;
+  }
+  case bitc::METADATA_DERIVED_TYPE: {
+    if (Record.size() < 12 || Record.size() > 13)
+      return error("Invalid record");
+
+    // DWARF address space is encoded as N->getDWARFAddressSpace() + 1. 0 means
+    // that there is no DWARF address space associated with DIDerivedType.
+    Optional<unsigned> DWARFAddressSpace;
+    if (Record.size() > 12 && Record[12])
+      DWARFAddressSpace = Record[12] - 1;
+
+    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], DWARFAddressSpace, Flags,
+                         getDITypeRefOrNull(Record[11]))),
+        NextMetadataNo);
+    NextMetadataNo++;
+    break;
+  }
+  case bitc::METADATA_COMPOSITE_TYPE: {
+    if (Record.size() < 16 || Record.size() > 17)
+      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 = nullptr;
+    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 = 0;
+    DINode::DIFlags Flags = static_cast<DINode::DIFlags>(Record[10]);
+    Metadata *Elements = nullptr;
+    unsigned RuntimeLang = Record[12];
+    Metadata *VTableHolder = nullptr;
+    Metadata *TemplateParams = nullptr;
+    Metadata *Discriminator = nullptr;
+    auto *Identifier = getMDString(Record[15]);
+    // If this module is being parsed so that it can be ThinLTO imported
+    // into another module, composite types only need to be imported
+    // as type declarations (unless full type definitions requested).
+    // Create type declarations up front to save memory. Also, buildODRType
+    // handles the case where this is type ODRed with a definition needed
+    // by the importing module, in which case the existing definition is
+    // used.
+    if (IsImporting && !ImportFullTypeDefinitions && Identifier &&
+        (Tag == dwarf::DW_TAG_enumeration_type ||
+         Tag == dwarf::DW_TAG_class_type ||
+         Tag == dwarf::DW_TAG_structure_type ||
+         Tag == dwarf::DW_TAG_union_type)) {
+      Flags = Flags | DINode::FlagFwdDecl;
+    } else {
+      BaseType = getDITypeRefOrNull(Record[6]);
+      OffsetInBits = Record[9];
+      Elements = getMDOrNull(Record[11]);
+      VTableHolder = getDITypeRefOrNull(Record[13]);
+      TemplateParams = getMDOrNull(Record[14]);
+      if (Record.size() > 16)
+        Discriminator = getMDOrNull(Record[16]);
+    }
+    DICompositeType *CT = nullptr;
+    if (Identifier)
+      CT = DICompositeType::buildODRType(
+          Context, *Identifier, Tag, Name, File, Line, Scope, BaseType,
+          SizeInBits, AlignInBits, OffsetInBits, Flags, Elements, RuntimeLang,
+          VTableHolder, TemplateParams, Discriminator);
+
+    // 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, Discriminator));
+    if (!IsNotUsedInTypeRef && Identifier)
+      MetadataList.addTypeRef(*Identifier, *cast<DICompositeType>(CT));
+
+    MetadataList.assignValue(CT, NextMetadataNo);
+    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);
+    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);
+    NextMetadataNo++;
+    break;
+  }
+
+  case bitc::METADATA_FILE: {
+    if (Record.size() != 3 && Record.size() != 5 && Record.size() != 6)
+      return error("Invalid record");
+
+    IsDistinct = Record[0];
+    Optional<DIFile::ChecksumInfo<MDString *>> Checksum;
+    // The BitcodeWriter writes null bytes into Record[3:4] when the Checksum
+    // is not present. This matches up with the old internal representation,
+    // and the old encoding for CSK_None in the ChecksumKind. The new
+    // representation reserves the value 0 in the ChecksumKind to continue to
+    // encode None in a backwards-compatible way.
+    if (Record.size() > 4 && Record[3] && Record[4])
+      Checksum.emplace(static_cast<DIFile::ChecksumKind>(Record[3]),
+                       getMDString(Record[4]));
+    MetadataList.assignValue(
+        GET_OR_DISTINCT(
+            DIFile,
+            (Context, getMDString(Record[1]), getMDString(Record[2]), Checksum,
+             Record.size() > 5 ? Optional<MDString *>(getMDString(Record[5]))
+                               : None)),
+        NextMetadataNo);
+    NextMetadataNo++;
+    break;
+  }
+  case bitc::METADATA_COMPILE_UNIT: {
+    if (Record.size() < 14 || Record.size() > 19)
+      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],
+        Record.size() <= 17 ? false : Record[17],
+        Record.size() <= 18 ? 0 : Record[18],
+        Record.size() <= 19 ? 0 : Record[19]);
+
+    MetadataList.assignValue(CU, NextMetadataNo);
+    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() > 21)
+      return error("Invalid record");
+
+    bool HasSPFlags = Record[0] & 4;
+
+    DINode::DIFlags Flags;
+    DISubprogram::DISPFlags SPFlags;
+    if (!HasSPFlags)
+      Flags = static_cast<DINode::DIFlags>(Record[11 + 2]);
+    else {
+      Flags = static_cast<DINode::DIFlags>(Record[11]);
+      SPFlags = static_cast<DISubprogram::DISPFlags>(Record[9]);
+    }
+
+    // Support for old metadata when
+    // subprogram specific flags are placed in DIFlags.
+    const unsigned DIFlagMainSubprogram = 1 << 21;
+    bool HasOldMainSubprogramFlag = Flags & DIFlagMainSubprogram;
+    if (HasOldMainSubprogramFlag)
+      // Remove old DIFlagMainSubprogram from DIFlags.
+      // Note: This assumes that any future use of bit 21 defaults to it
+      // being 0.
+      Flags &= ~static_cast<DINode::DIFlags>(DIFlagMainSubprogram);
+
+    if (HasOldMainSubprogramFlag && HasSPFlags)
+      SPFlags |= DISubprogram::SPFlagMainSubprogram;
+    else if (!HasSPFlags)
+      SPFlags = DISubprogram::toSPFlags(
+                    /*IsLocalToUnit=*/Record[7], /*IsDefinition=*/Record[8],
+                    /*IsOptimized=*/Record[14], /*Virtuality=*/Record[11],
+                    /*DIFlagMainSubprogram*/HasOldMainSubprogramFlag);
+
+    // All definitions should be distinct.
+    IsDistinct = (Record[0] & 1) || (SPFlags & DISubprogram::SPFlagDefinition);
+    // 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.
+    // Version 5 repacked flags into DISPFlags, changing many element numbers.
+    bool HasUnit = Record[0] & 2;
+    if (!HasSPFlags && HasUnit && Record.size() < 19)
+      return error("Invalid record");
+    if (HasSPFlags && !HasUnit)
+      return error("Invalid record");
+    // Accommodate older formats.
+    bool HasFn = false;
+    bool HasThisAdj = true;
+    bool HasThrownTypes = true;
+    unsigned OffsetA = 0;
+    unsigned OffsetB = 0;
+    if (!HasSPFlags) {
+      OffsetA = 2;
+      OffsetB = 2;
+      if (Record.size() >= 19) {
+        HasFn = !HasUnit;
+        OffsetB++;
+      }
+      HasThisAdj = Record.size() >= 20;
+      HasThrownTypes = Record.size() >= 21;
+    }
+    Metadata *CUorFn = getMDOrNull(Record[12 + OffsetB]);
+    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 + OffsetA],                               // scopeLine
+         getDITypeRefOrNull(Record[8 + OffsetA]),           // containingType
+         Record[10 + OffsetA],                              // virtualIndex
+         HasThisAdj ? Record[16 + OffsetB] : 0,             // thisAdjustment
+         Flags,                                             // flags
+         SPFlags,                                           // SPFlags
+         HasUnit ? CUorFn : nullptr,                        // unit
+         getMDOrNull(Record[13 + OffsetB]),                 // templateParams
+         getMDOrNull(Record[14 + OffsetB]),                 // declaration
+         getMDOrNull(Record[15 + OffsetB]),                 // retainedNodes
+         HasThrownTypes ? getMDOrNull(Record[17 + OffsetB])
+                        : nullptr                           // thrownTypes
+         ));
+    MetadataList.assignValue(SP, NextMetadataNo);
+    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);
+    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);
+    NextMetadataNo++;
+    break;
+  }
+  case bitc::METADATA_COMMON_BLOCK: {
+    IsDistinct = Record[0] & 1;
+    MetadataList.assignValue(
+        GET_OR_DISTINCT(DICommonBlock,
+                        (Context, getMDOrNull(Record[1]),
+                         getMDOrNull(Record[2]), getMDString(Record[3]),
+                         getMDOrNull(Record[4]), Record[5])),
+        NextMetadataNo);
+    NextMetadataNo++;
+    break;
+  }
+  case bitc::METADATA_NAMESPACE: {
+    // Newer versions of DINamespace dropped file and line.
+    MDString *Name;
+    if (Record.size() == 3)
+      Name = getMDString(Record[2]);
+    else if (Record.size() == 5)
+      Name = getMDString(Record[3]);
+    else
+      return error("Invalid record");
+
+    IsDistinct = Record[0] & 1;
+    bool ExportSymbols = Record[0] & 2;
+    MetadataList.assignValue(
+        GET_OR_DISTINCT(DINamespace,
+                        (Context, getMDOrNull(Record[1]), Name, ExportSymbols)),
+        NextMetadataNo);
+    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);
+    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);
+    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);
+    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);
+    NextMetadataNo++;
+    break;
+  }
+  case bitc::METADATA_GLOBAL_VAR: {
+    if (Record.size() < 11 || Record.size() > 13)
+      return error("Invalid record");
+
+    IsDistinct = Record[0] & 1;
+    unsigned Version = Record[0] >> 1;
+
+    if (Version == 2) {
+      MetadataList.assignValue(
+          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],
+               getMDOrNull(Record[9]), getMDOrNull(Record[10]), Record[11])),
+          NextMetadataNo);
+
+      NextMetadataNo++;
+    } else if (Version == 1) {
+      // No upgrade necessary. A null field will be introduced to indicate
+      // that no parameter information is available.
+      MetadataList.assignValue(
+          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],
+                           getMDOrNull(Record[10]), nullptr, Record[11])),
+          NextMetadataNo);
+
+      NextMetadataNo++;
+    } else if (Version == 0) {
+      // Upgrade old metadata, which stored a global variable reference or a
+      // ConstantInt here.
+      NeedUpgradeToDIGlobalVariableExpression = true;
+      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],
+           getMDOrNull(Record[10]), nullptr, AlignInBits));
+
+      DIGlobalVariableExpression *DGVE = nullptr;
+      if (Attach || Expr)
+        DGVE = DIGlobalVariableExpression::getDistinct(
+            Context, DGV, Expr ? Expr : DIExpression::get(Context, {}));
+      if (Attach)
+        Attach->addDebugInfo(DGVE);
+
+      auto *MDNode = Expr ? cast<Metadata>(DGVE) : cast<Metadata>(DGV);
+      MetadataList.assignValue(MDNode, NextMetadataNo);
+      NextMetadataNo++;
+    } else
+      return error("Invalid record");
+
+    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 artificial 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);
+    NextMetadataNo++;
+    break;
+  }
+  case bitc::METADATA_LABEL: {
+    if (Record.size() != 5)
+      return error("Invalid record");
+
+    IsDistinct = Record[0] & 1;
+    MetadataList.assignValue(
+        GET_OR_DISTINCT(DILabel,
+                        (Context, getMDOrNull(Record[1]),
+                         getMDString(Record[2]),
+                         getMDOrNull(Record[3]), Record[4])),
+        NextMetadataNo);
+    NextMetadataNo++;
+    break;
+  }
+  case bitc::METADATA_EXPRESSION: {
+    if (Record.size() < 1)
+      return error("Invalid record");
+
+    IsDistinct = Record[0] & 1;
+    uint64_t Version = Record[0] >> 1;
+    auto Elts = MutableArrayRef<uint64_t>(Record).slice(1);
+
+    SmallVector<uint64_t, 6> Buffer;
+    if (Error Err = upgradeDIExpression(Version, Elts, Buffer))
+      return Err;
+
+    MetadataList.assignValue(
+        GET_OR_DISTINCT(DIExpression, (Context, Elts)), NextMetadataNo);
+    NextMetadataNo++;
+    break;
+  }
+  case bitc::METADATA_GLOBAL_VAR_EXPR: {
+    if (Record.size() != 3)
+      return error("Invalid record");
+
+    IsDistinct = Record[0];
+    Metadata *Expr = getMDOrNull(Record[2]);
+    if (!Expr)
+      Expr = DIExpression::get(Context, {});
+    MetadataList.assignValue(
+        GET_OR_DISTINCT(DIGlobalVariableExpression,
+                        (Context, getMDOrNull(Record[1]), Expr)),
+        NextMetadataNo);
+    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);
+    NextMetadataNo++;
+    break;
+  }
+  case bitc::METADATA_IMPORTED_ENTITY: {
+    if (Record.size() != 6 && Record.size() != 7)
+      return error("Invalid record");
+
+    IsDistinct = Record[0];
+    bool HasFile = (Record.size() == 7);
+    MetadataList.assignValue(
+        GET_OR_DISTINCT(DIImportedEntity,
+                        (Context, Record[1], getMDOrNull(Record[2]),
+                         getDITypeRefOrNull(Record[3]),
+                         HasFile ? getMDOrNull(Record[6]) : nullptr,
+                         HasFile ? Record[4] : 0, getMDString(Record[5]))),
+        NextMetadataNo);
+    NextMetadataNo++;
+    break;
+  }
+  case bitc::METADATA_STRING_OLD: {
+    std::string String(Record.begin(), Record.end());
+
+    // Test for upgrading !llvm.loop.
+    HasSeenOldLoopTags |= mayBeOldLoopAttachmentTag(String);
+    ++NumMDStringLoaded;
+    Metadata *MD = MDString::get(Context, String);
+    MetadataList.assignValue(MD, NextMetadataNo);
+    NextMetadataNo++;
+    break;
+  }
+  case bitc::METADATA_STRINGS: {
+    auto CreateNextMDString = [&](StringRef Str) {
+      ++NumMDStringLoaded;
+      MetadataList.assignValue(MDString::get(Context, Str), NextMetadataNo);
+      NextMetadataNo++;
+    };
+    if (Error Err = parseMetadataStrings(Record, Blob, CreateNextMDString))
+      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;
+  }
+  }
+  return Error::success();
+#undef GET_OR_DISTINCT
+}
+
+Error MetadataLoader::MetadataLoaderImpl::parseMetadataStrings(
+    ArrayRef<uint64_t> Record, StringRef Blob,
+    function_ref<void(StringRef)> CallBack) {
+  // 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");
+
+    Expected<uint32_t> MaybeSize = R.ReadVBR(6);
+    if (!MaybeSize)
+      return MaybeSize.takeError();
+    uint32_t Size = MaybeSize.get();
+    if (Strings.size() < Size)
+      return error("Invalid record: metadata strings truncated chars");
+
+    CallBack(Strings.slice(0, Size));
+    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: expect fwd ref to MDNode");
+    GO.addMetadata(K->second, *MD);
+  }
+  return Error::success();
+}
+
+/// Parse metadata attachments.
+Error MetadataLoader::MetadataLoaderImpl::parseMetadataAttachment(
+    Function &F, const SmallVectorImpl<Instruction *> &InstructionList) {
+  if (Error Err = Stream.EnterSubBlock(bitc::METADATA_ATTACHMENT_ID))
+    return Err;
+
+  SmallVector<uint64_t, 64> Record;
+  PlaceholderQueue Placeholders;
+
+  while (true) {
+    Expected<BitstreamEntry> MaybeEntry = Stream.advanceSkippingSubblocks();
+    if (!MaybeEntry)
+      return MaybeEntry.takeError();
+    BitstreamEntry Entry = MaybeEntry.get();
+
+    switch (Entry.Kind) {
+    case BitstreamEntry::SubBlock: // Handled for us already.
+    case BitstreamEntry::Error:
+      return error("Malformed block");
+    case BitstreamEntry::EndBlock:
+      resolveForwardRefsAndPlaceholders(Placeholders);
+      return Error::success();
+    case BitstreamEntry::Record:
+      // The interesting case.
+      break;
+    }
+
+    // Read a metadata attachment record.
+    Record.clear();
+    ++NumMDRecordLoaded;
+    Expected<unsigned> MaybeRecord = Stream.readRecord(Entry.ID, Record);
+    if (!MaybeRecord)
+      return MaybeRecord.takeError();
+    switch (MaybeRecord.get()) {
+    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");
+        if (I->second == LLVMContext::MD_tbaa && StripTBAA)
+          continue;
+
+        auto Idx = Record[i + 1];
+        if (Idx < (MDStringRef.size() + GlobalMetadataBitPosIndex.size()) &&
+            !MetadataList.lookup(Idx)) {
+          // Load the attachment if it is in the lazy-loadable range and hasn't
+          // been loaded yet.
+          lazyLoadOneMetadata(Idx, Placeholders);
+          resolveForwardRefsAndPlaceholders(Placeholders);
+        }
+
+        Metadata *Node = MetadataList.getMetadataFwdRef(Idx);
+        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 (Error Err = Stream.EnterSubBlock(bitc::METADATA_KIND_BLOCK_ID))
+    return Err;
+
+  SmallVector<uint64_t, 64> Record;
+
+  // Read all the records.
+  while (true) {
+    Expected<BitstreamEntry> MaybeEntry = Stream.advanceSkippingSubblocks();
+    if (!MaybeEntry)
+      return MaybeEntry.takeError();
+    BitstreamEntry Entry = MaybeEntry.get();
+
+    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();
+    ++NumMDRecordLoaded;
+    Expected<unsigned> MaybeCode = Stream.readRecord(Entry.ID, Record);
+    if (!MaybeCode)
+      return MaybeCode.takeError();
+    switch (MaybeCode.get()) {
+    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,
+                               bool IsImporting,
+                               std::function<Type *(unsigned)> getTypeByID)
+    : Pimpl(llvm::make_unique<MetadataLoaderImpl>(
+          Stream, TheModule, ValueList, std::move(getTypeByID), IsImporting)) {}
+
+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::getMetadataFwdRefOrLoad(unsigned Idx) {
+  return Pimpl->getMetadataFwdRefOrLoad(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();
+}
+
+void MetadataLoader::setStripTBAA(bool StripTBAA) {
+  return Pimpl->setStripTBAA(StripTBAA);
+}
+
+bool MetadataLoader::isStrippingTBAA() { return Pimpl->isStrippingTBAA(); }
+
+unsigned MetadataLoader::size() const { return Pimpl->size(); }
+void MetadataLoader::shrinkTo(unsigned N) { return Pimpl->shrinkTo(N); }
+
+void MetadataLoader::upgradeDebugIntrinsics(Function &F) {
+  return Pimpl->upgradeDebugIntrinsics(F);
+}