MetadataLoader.cpp

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//===- 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/APInt.h"
#include "llvm/ADT/ArrayRef.h"
#include "llvm/ADT/BitmaskEnum.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/DenseSet.h"
#include "llvm/ADT/STLFunctionalExtras.h"
#include "llvm/ADT/SetVector.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/BinaryFormat/Dwarf.h"
#include "llvm/Bitcode/BitcodeReader.h"
#include "llvm/Bitcode/LLVMBitCodes.h"
#include "llvm/Bitstream/BitstreamReader.h"
#include "llvm/IR/AutoUpgrade.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DebugInfoMetadata.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/GlobalObject.h"
#include "llvm/IR/GlobalVariable.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/Metadata.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/TrackingMDRef.h"
#include "llvm/IR/Type.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/TimeProfiler.h"
 
#include <algorithm>
#include <cassert>
#include <cstddef>
#include <cstdint>
#include <deque>
#include <iterator>
#include <limits>
#include <map>
#include <optional>
#include <string>
#include <tuple>
#include <utility>
#include <vector>
namespace llvm {
class Argument;
}
 
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, {}).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, {});
  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, {})));
  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() const { 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();
  }
}
 
} // anonymous 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;
  MetadataLoaderCallbacks Callbacks;
 
  /// 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;
 
  /// Cursor position of the start of the global decl attachments, to enable
  /// loading using the index built for lazy loading, instead of forward
  /// references.
  uint64_t GlobalDeclAttachmentPos = 0;
 
#ifndef NDEBUG
  /// Baisic correctness check that we end up parsing all of the global decl
  /// attachments.
  unsigned NumGlobalDeclAttachSkipped = 0;
  unsigned NumGlobalDeclAttachParsed = 0;
#endif
 
  /// Load the global decl attachments, using the index built for lazy loading.
  Expected<bool> loadGlobalDeclAttachments();
 
  /// 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;
 
  /// Map DILocalScope to the enclosing DISubprogram, if any.
  DenseMap<DILocalScope *, DISubprogram *> ParentSubprogram;
 
  /// 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<DIGlobalVariable>(MD)) {
          auto *DGVE = DIGlobalVariableExpression::getDistinct(
              Context, DGV, DIExpression::get(Context, {}));
          GV.addMetadata(LLVMContext::MD_dbg, *DGVE);
        } else
          GV.addMetadata(LLVMContext::MD_dbg, *MD);
    }
  }
 
  DISubprogram *findEnclosingSubprogram(DILocalScope *S) {
    if (!S)
      return nullptr;
    if (auto *SP = ParentSubprogram[S]) {
      return SP;
    }
 
    DILocalScope *InitialScope = S;
    DenseSet<DILocalScope *> Visited;
    while (S && !isa<DISubprogram>(S)) {
      S = dyn_cast_or_null<DILocalScope>(S->getScope());
      if (!Visited.insert(S).second)
        break;
    }
 
    return ParentSubprogram[InitialScope] =
               llvm::dyn_cast_or_null<DISubprogram>(S);
  }
 
  /// Move local imports from DICompileUnit's 'imports' field to
  /// DISubprogram's retainedNodes.
  void upgradeCULocals() {
    if (NamedMDNode *CUNodes = TheModule.getNamedMetadata("llvm.dbg.cu")) {
      for (MDNode *N : CUNodes->operands()) {
        auto *CU = dyn_cast<DICompileUnit>(N);
        if (!CU)
          continue;
 
        if (CU->getRawImportedEntities()) {
          // Collect a set of imported entities to be moved.
          SetVector<Metadata *> EntitiesToRemove;
          for (Metadata *Op : CU->getImportedEntities()->operands()) {
            auto *IE = cast<DIImportedEntity>(Op);
            if (isa_and_nonnull<DILocalScope>(IE->getScope())) {
              EntitiesToRemove.insert(IE);
            }
          }
 
          if (!EntitiesToRemove.empty()) {
            // Make a new list of CU's 'imports'.
            SmallVector<Metadata *> NewImports;
            for (Metadata *Op : CU->getImportedEntities()->operands()) {
              if (!EntitiesToRemove.contains(cast<DIImportedEntity>(Op))) {
                NewImports.push_back(Op);
              }
            }
 
            // Find DISubprogram corresponding to each entity.
            std::map<DISubprogram *, SmallVector<Metadata *>> SPToEntities;
            for (auto *I : EntitiesToRemove) {
              auto *Entity = cast<DIImportedEntity>(I);
              if (auto *SP = findEnclosingSubprogram(
                      cast<DILocalScope>(Entity->getScope()))) {
                SPToEntities[SP].push_back(Entity);
              }
            }
 
            // Update DISubprograms' retainedNodes.
            for (auto I = SPToEntities.begin(); I != SPToEntities.end(); ++I) {
              auto *SP = I->first;
              auto RetainedNodes = SP->getRetainedNodes();
              SmallVector<Metadata *> MDs(RetainedNodes.begin(),
                                          RetainedNodes.end());
              MDs.append(I->second);
              SP->replaceRetainedNodes(MDNode::get(Context, MDs));
            }
 
            // Remove entities with local scope from CU.
            CU->replaceImportedEntities(MDTuple::get(Context, NewImports));
          }
        }
      }
    }
 
    ParentSubprogram.clear();
  }
 
  /// Remove a leading DW_OP_deref from DIExpressions in a dbg.declare that
  /// describes a function argument.
  void upgradeDeclareExpressions(Function &F) {
    if (!NeedDeclareExpressionUpgrade)
      return;
 
    auto UpdateDeclareIfNeeded = [&](auto *Declare) {
      auto *DIExpr = Declare->getExpression();
      if (!DIExpr || !DIExpr->startsWithDeref() ||
          !isa_and_nonnull<Argument>(Declare->getAddress()))
        return;
      SmallVector<uint64_t, 8> Ops;
      Ops.append(std::next(DIExpr->elements_begin()), DIExpr->elements_end());
      Declare->setExpression(DIExpression::get(Context, Ops));
    };
 
    for (auto &BB : F)
      for (auto &I : BB) {
        for (DbgVariableRecord &DVR : filterDbgVars(I.getDbgRecordRange())) {
          if (DVR.isDbgDeclare())
            UpdateDeclareIfNeeded(&DVR);
        }
        if (auto *DDI = dyn_cast<DbgDeclareInst>(&I))
          UpdateDeclareIfNeeded(DDI);
      }
  }
 
  /// 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;
      [[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;
      [[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);
      [[fallthrough]];
    }
    case 3:
      // Up-to-date!
      break;
    }
 
    return Error::success();
  }
 
  void upgradeDebugInfo(bool ModuleLevel) {
    upgradeCUSubprograms();
    upgradeCUVariables();
    if (ModuleLevel)
      upgradeCULocals();
  }
 
  void callMDTypeCallback(Metadata **Val, unsigned TypeID);
 
public:
  MetadataLoaderImpl(BitstreamCursor &Stream, Module &TheModule,
                     BitcodeReaderValueList &ValueList,
                     MetadataLoaderCallbacks Callbacks, bool IsImporting)
      : MetadataList(TheModule.getContext(), Stream.SizeInBytes()),
        ValueList(ValueList), Stream(Stream), Context(TheModule.getContext()),
        TheModule(TheModule), Callbacks(std::move(Callbacks)),
        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() const { return HasSeenOldLoopTags; }
 
  Error parseMetadataAttachment(Function &F,
                                ArrayRef<Instruction *> InstructionList);
 
  Error parseMetadataKinds();
 
  void setStripTBAA(bool Value) { StripTBAA = Value; }
  bool isStrippingTBAA() const { 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;
  GlobalDeclAttachmentPos = 0;
  // Get the abbrevs, and preload record positions to make them lazy-loadable.
  while (true) {
    uint64_t SavedPos = IndexCursor.GetCurrentBitNo();
    BitstreamEntry Entry;
    if (Error E =
            IndexCursor
                .advanceSkippingSubblocks(BitstreamCursor::AF_DontPopBlockAtEnd)
                .moveInto(Entry))
      return std::move(E);
 
    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();
      unsigned Code;
      if (Error E = IndexCursor.skipRecord(Entry.ID).moveInto(Code))
        return std::move(E);
      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: {
        if (!GlobalDeclAttachmentPos)
          GlobalDeclAttachmentPos = SavedPos;
#ifndef NDEBUG
        NumGlobalDeclAttachSkipped++;
#endif
        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_STRING_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_ASSIGN_ID:
      case bitc::METADATA_LABEL:
      case bitc::METADATA_EXPRESSION:
      case bitc::METADATA_OBJC_PROPERTY:
      case bitc::METADATA_IMPORTED_ENTITY:
      case bitc::METADATA_GLOBAL_VAR_EXPR:
      case bitc::METADATA_GENERIC_SUBRANGE:
        // 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;
    }
    }
  }
}
 
// Load the global decl attachments after building the lazy loading index.
// We don't load them "lazily" - all global decl attachments must be
// parsed since they aren't materialized on demand. However, by delaying
// their parsing until after the index is created, we can use the index
// instead of creating temporaries.
Expected<bool> MetadataLoader::MetadataLoaderImpl::loadGlobalDeclAttachments() {
  // Nothing to do if we didn't find any of these metadata records.
  if (!GlobalDeclAttachmentPos)
    return true;
  // Use a temporary cursor so that we don't mess up the main Stream cursor or
  // the lazy loading IndexCursor (which holds the necessary abbrev ids).
  BitstreamCursor TempCursor = Stream;
  SmallVector<uint64_t, 64> Record;
  // Jump to the position before the first global decl attachment, so we can
  // scan for the first BitstreamEntry record.
  if (Error Err = TempCursor.JumpToBit(GlobalDeclAttachmentPos))
    return std::move(Err);
  while (true) {
    BitstreamEntry Entry;
    if (Error E =
            TempCursor
                .advanceSkippingSubblocks(BitstreamCursor::AF_DontPopBlockAtEnd)
                .moveInto(Entry))
      return std::move(E);
 
    switch (Entry.Kind) {
    case BitstreamEntry::SubBlock: // Handled for us already.
    case BitstreamEntry::Error:
      return error("Malformed block");
    case BitstreamEntry::EndBlock:
      // Check that we parsed them all.
      assert(NumGlobalDeclAttachSkipped == NumGlobalDeclAttachParsed);
      return true;
    case BitstreamEntry::Record:
      break;
    }
    uint64_t CurrentPos = TempCursor.GetCurrentBitNo();
    Expected<unsigned> MaybeCode = TempCursor.skipRecord(Entry.ID);
    if (!MaybeCode)
      return MaybeCode.takeError();
    if (MaybeCode.get() != bitc::METADATA_GLOBAL_DECL_ATTACHMENT) {
      // Anything other than a global decl attachment signals the end of
      // these records. Check that we parsed them all.
      assert(NumGlobalDeclAttachSkipped == NumGlobalDeclAttachParsed);
      return true;
    }
#ifndef NDEBUG
    NumGlobalDeclAttachParsed++;
#endif
    // FIXME: we need to do this early because we don't materialize global
    // value explicitly.
    if (Error Err = TempCursor.JumpToBit(CurrentPos))
      return std::move(Err);
    Record.clear();
    if (Expected<unsigned> MaybeRecord =
            TempCursor.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])) {
      // Need to save and restore the current position since
      // parseGlobalObjectAttachment will resolve all forward references which
      // would require parsing from locations stored in the index.
      CurrentPos = TempCursor.GetCurrentBitNo();
      if (Error Err = parseGlobalObjectAttachment(
              *GO, ArrayRef<uint64_t>(Record).slice(1)))
        return std::move(Err);
      if (Error Err = TempCursor.JumpToBit(CurrentPos))
        return std::move(Err);
    }
  }
}
 
void MetadataLoader::MetadataLoaderImpl::callMDTypeCallback(Metadata **Val,
                                                            unsigned TypeID) {
  if (Callbacks.MDType) {
    (*Callbacks.MDType)(Val, TypeID, Callbacks.GetTypeByID,
                        Callbacks.GetContainedTypeID);
  }
}
 
/// Parse a METADATA_BLOCK. If ModuleLevel is true then we are parsing
/// module level metadata.
Error MetadataLoader::MetadataLoaderImpl::parseMetadata(bool ModuleLevel) {
  llvm::TimeTraceScope timeScope("Parse metadata");
  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());
 
      // Now that we have built the index, load the global decl attachments
      // that were deferred during that process. This avoids creating
      // temporaries.
      SuccessOrErr = loadGlobalDeclAttachments();
      if (!SuccessOrErr)
        return SuccessOrErr.takeError();
      assert(SuccessOrErr.get());
 
      // Reading the named metadata created forward references and/or
      // placeholders, that we flush here.
      resolveForwardRefsAndPlaceholders(Placeholders);
      upgradeDebugInfo(ModuleLevel);
      // 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) {
    BitstreamEntry Entry;
    if (Error E = Stream.advanceSkippingSubblocks().moveInto(Entry))
      return E;
 
    switch (Entry.Kind) {
    case BitstreamEntry::SubBlock: // Handled for us already.
    case BitstreamEntry::Error:
      return error("Malformed block");
    case BitstreamEntry::EndBlock:
      resolveForwardRefsAndPlaceholders(Placeholders);
      upgradeDebugInfo(ModuleLevel);
      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<MDNode>(MD);
    // If the node is not an MDNode, or if it is not temporary, then
    // we're done.
    if (!N || !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: " +
                       Twine(toString(std::move(Err))));
  BitstreamEntry Entry;
  if (Error E = IndexCursor.advanceSkippingSubblocks().moveInto(Entry))
    // FIXME this drops the error on the floor.
    report_fatal_error("lazyLoadOneMetadata failed advanceSkippingSubblocks: " +
                       Twine(toString(std::move(E))));
  ++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: " +
                         Twine(toString(std::move(Err))));
  } else
    report_fatal_error("Can't lazyload MD: " +
                       Twine(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 (true) {
    // 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);
}
 
static Value *getValueFwdRef(BitcodeReaderValueList &ValueList, unsigned Idx,
                             Type *Ty, unsigned TyID) {
  Value *V = ValueList.getValueFwdRef(Idx, Ty, TyID,
                                      /*ConstExprInsertBB*/ nullptr);
  if (V)
    return V;
 
  // This is a reference to a no longer supported constant expression.
  // Pretend that the constant was deleted, which will replace metadata
  // references with poison.
  // TODO: This is a rather indirect check. It would be more elegant to use
  // a separate ErrorInfo for constant materialization failure and thread
  // the error reporting through getValueFwdRef().
  if (Idx < ValueList.size() && ValueList[Idx] &&
      ValueList[Idx]->getType() == Ty)
    return PoisonValue::get(Ty);
 
  return nullptr;
}
 
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));
  };
 
  auto getMetadataOrConstant = [&](bool IsMetadata,
                                   uint64_t Entry) -> Metadata * {
    if (IsMetadata)
      return getMDOrNull(Entry);
    return ConstantAsMetadata::get(
        ConstantInt::get(Type::getInt64Ty(Context), Entry));
  };
 
#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();
    if (Error E = Stream.ReadCode().moveInto(Code))
      return E;
 
    ++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: {
    // Deprecated, but still needed to read old bitcode files.
    // 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, {}), NextMetadataNo);
      NextMetadataNo++;
    };
    if (Record.size() != 2) {
      dropRecord();
      break;
    }
 
    unsigned TyID = Record[0];
    Type *Ty = Callbacks.GetTypeByID(TyID);
    if (!Ty || Ty->isMetadataTy() || Ty->isVoidTy()) {
      dropRecord();
      break;
    }
 
    Value *V = ValueList.getValueFwdRef(Record[1], Ty, TyID,
                                        /*ConstExprInsertBB*/ nullptr);
    if (!V)
      return error("Invalid value reference from old fn metadata");
 
    MetadataList.assignValue(LocalAsMetadata::get(V), NextMetadataNo);
    NextMetadataNo++;
    break;
  }
  case bitc::METADATA_OLD_NODE: {
    // Deprecated, but still needed to read old bitcode files.
    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) {
      unsigned TyID = Record[i];
      Type *Ty = Callbacks.GetTypeByID(TyID);
      if (!Ty)
        return error("Invalid record");
      if (Ty->isMetadataTy())
        Elts.push_back(getMD(Record[i + 1]));
      else if (!Ty->isVoidTy()) {
        Value *V = getValueFwdRef(ValueList, Record[i + 1], Ty, TyID);
        if (!V)
          return error("Invalid value reference from old metadata");
        Metadata *MD = ValueAsMetadata::get(V);
        assert(isa<ConstantAsMetadata>(MD) &&
               "Expected non-function-local metadata");
        callMDTypeCallback(&MD, TyID);
        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");
 
    unsigned TyID = Record[0];
    Type *Ty = Callbacks.GetTypeByID(TyID);
    if (!Ty || Ty->isMetadataTy() || Ty->isVoidTy())
      return error("Invalid record");
 
    Value *V = getValueFwdRef(ValueList, Record[1], Ty, TyID);
    if (!V)
      return error("Invalid value reference from metadata");
 
    Metadata *MD = ValueAsMetadata::get(V);
    callMDTypeCallback(&MD, TyID);
    MetadataList.assignValue(MD, NextMetadataNo);
    NextMetadataNo++;
    break;
  }
  case bitc::METADATA_DISTINCT_NODE:
    IsDistinct = true;
    [[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: {
    // 5: inlinedAt, 6: isImplicit, 8: Key Instructions fields.
    if (Record.size() != 5 && Record.size() != 6 && Record.size() != 8)
      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];
    uint64_t AtomGroup = Record.size() == 8 ? Record[6] : 0;
    uint8_t AtomRank = Record.size() == 8 ? Record[7] : 0;
    MetadataList.assignValue(
        GET_OR_DISTINCT(DILocation, (Context, Line, Column, Scope, InlinedAt,
                                     ImplicitCode, AtomGroup, AtomRank)),
        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)
    // Operand 'lowerBound' is interpreted as:
    // - Signed integer (version 0 and 1)
    // - Metadata node  (version 2)
    // Operands 'upperBound' and 'stride' are interpreted as:
    // - Metadata node  (version 2)
    switch (Record[0] >> 1) {
    case 0:
      Val = GET_OR_DISTINCT(DISubrange,
                            (Context, Record[1], unrotateSign(Record[2])));
      break;
    case 1:
      Val = GET_OR_DISTINCT(DISubrange, (Context, getMDOrNull(Record[1]),
                                         unrotateSign(Record[2])));
      break;
    case 2:
      Val = GET_OR_DISTINCT(
          DISubrange, (Context, getMDOrNull(Record[1]), getMDOrNull(Record[2]),
                       getMDOrNull(Record[3]), getMDOrNull(Record[4])));
      break;
    default:
      return error("Invalid record: Unsupported version of DISubrange");
    }
 
    MetadataList.assignValue(Val, NextMetadataNo);
    IsDistinct = Record[0] & 1;
    NextMetadataNo++;
    break;
  }
  case bitc::METADATA_GENERIC_SUBRANGE: {
    Metadata *Val = nullptr;
    Val = GET_OR_DISTINCT(DIGenericSubrange,
                          (Context, getMDOrNull(Record[1]),
                           getMDOrNull(Record[2]), getMDOrNull(Record[3]),
                           getMDOrNull(Record[4])));
 
    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;
    bool IsBigInt = Record[0] & 4;
    APInt Value;
 
    if (IsBigInt) {
      const uint64_t BitWidth = Record[1];
      const size_t NumWords = Record.size() - 3;
      Value = readWideAPInt(ArrayRef(&Record[3], NumWords), BitWidth);
    } else
      Value = APInt(64, unrotateSign(Record[1]), !IsUnsigned);
 
    MetadataList.assignValue(
        GET_OR_DISTINCT(DIEnumerator,
                        (Context, Value, IsUnsigned, getMDString(Record[2]))),
        NextMetadataNo);
    NextMetadataNo++;
    break;
  }
  case bitc::METADATA_BASIC_TYPE: {
    if (Record.size() < 6 || Record.size() > 8)
      return error("Invalid record");
 
    IsDistinct = Record[0] & 1;
    bool SizeIsMetadata = Record[0] & 2;
    DINode::DIFlags Flags = (Record.size() > 6)
                                ? static_cast<DINode::DIFlags>(Record[6])
                                : DINode::FlagZero;
    uint32_t NumExtraInhabitants = (Record.size() > 7) ? Record[7] : 0;
 
    Metadata *SizeInBits = getMetadataOrConstant(SizeIsMetadata, Record[3]);
 
    MetadataList.assignValue(
        GET_OR_DISTINCT(DIBasicType,
                        (Context, Record[1], getMDString(Record[2]), SizeInBits,
                         Record[4], Record[5], NumExtraInhabitants, Flags)),
        NextMetadataNo);
    NextMetadataNo++;
    break;
  }
  case bitc::METADATA_FIXED_POINT_TYPE: {
    if (Record.size() < 11)
      return error("Invalid record");
 
    IsDistinct = Record[0] & 1;
    bool SizeIsMetadata = Record[0] & 2;
    DINode::DIFlags Flags = static_cast<DINode::DIFlags>(Record[6]);
 
    Metadata *SizeInBits = getMetadataOrConstant(SizeIsMetadata, Record[3]);
 
    size_t Offset = 9;
 
    auto ReadWideInt = [&]() {
      uint64_t Encoded = Record[Offset++];
      unsigned NumWords = Encoded >> 32;
      unsigned BitWidth = Encoded & 0xffffffff;
      auto Value = readWideAPInt(ArrayRef(&Record[Offset], NumWords), BitWidth);
      Offset += NumWords;
      return Value;
    };
 
    APInt Numerator = ReadWideInt();
    APInt Denominator = ReadWideInt();
 
    if (Offset != Record.size())
      return error("Invalid record");
 
    MetadataList.assignValue(
        GET_OR_DISTINCT(DIFixedPointType,
                        (Context, Record[1], getMDString(Record[2]), SizeInBits,
                         Record[4], Record[5], Flags, Record[7], Record[8],
                         Numerator, Denominator)),
        NextMetadataNo);
    NextMetadataNo++;
    break;
  }
  case bitc::METADATA_STRING_TYPE: {
    if (Record.size() > 9 || Record.size() < 8)
      return error("Invalid record");
 
    IsDistinct = Record[0] & 1;
    bool SizeIsMetadata = Record[0] & 2;
    bool SizeIs8 = Record.size() == 8;
    // StringLocationExp (i.e. Record[5]) is added at a later time
    // than the other fields. The code here enables backward compatibility.
    Metadata *StringLocationExp = SizeIs8 ? nullptr : getMDOrNull(Record[5]);
    unsigned Offset = SizeIs8 ? 5 : 6;
    Metadata *SizeInBits =
        getMetadataOrConstant(SizeIsMetadata, Record[Offset]);
 
    MetadataList.assignValue(
        GET_OR_DISTINCT(DIStringType,
                        (Context, Record[1], getMDString(Record[2]),
                         getMDOrNull(Record[3]), getMDOrNull(Record[4]),
                         StringLocationExp, SizeInBits, Record[Offset + 1],
                         Record[Offset + 2])),
        NextMetadataNo);
    NextMetadataNo++;
    break;
  }
  case bitc::METADATA_DERIVED_TYPE: {
    if (Record.size() < 12 || Record.size() > 15)
      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.
    std::optional<unsigned> DWARFAddressSpace;
    if (Record.size() > 12 && Record[12])
      DWARFAddressSpace = Record[12] - 1;
 
    Metadata *Annotations = nullptr;
    std::optional<DIDerivedType::PtrAuthData> PtrAuthData;
 
    // Only look for annotations/ptrauth if both are allocated.
    // If not, we can't tell which was intended to be embedded, as both ptrauth
    // and annotations have been expected at Record[13] at various times.
    if (Record.size() > 14) {
      if (Record[13])
        Annotations = getMDOrNull(Record[13]);
      if (Record[14])
        PtrAuthData.emplace(Record[14]);
    }
 
    IsDistinct = Record[0] & 1;
    bool SizeIsMetadata = Record[0] & 2;
    DINode::DIFlags Flags = static_cast<DINode::DIFlags>(Record[10]);
 
    Metadata *SizeInBits = getMetadataOrConstant(SizeIsMetadata, Record[7]);
    Metadata *OffsetInBits = getMetadataOrConstant(SizeIsMetadata, Record[9]);
 
    MetadataList.assignValue(
        GET_OR_DISTINCT(DIDerivedType,
                        (Context, Record[1], getMDString(Record[2]),
                         getMDOrNull(Record[3]), Record[4],
                         getDITypeRefOrNull(Record[5]),
                         getDITypeRefOrNull(Record[6]), SizeInBits, Record[8],
                         OffsetInBits, DWARFAddressSpace, PtrAuthData, Flags,
                         getDITypeRefOrNull(Record[11]), Annotations)),
        NextMetadataNo);
    NextMetadataNo++;
    break;
  }
  case bitc::METADATA_SUBRANGE_TYPE: {
    if (Record.size() != 13)
      return error("Invalid record");
 
    IsDistinct = Record[0] & 1;
    bool SizeIsMetadata = Record[0] & 2;
    DINode::DIFlags Flags = static_cast<DINode::DIFlags>(Record[7]);
 
    Metadata *SizeInBits = getMetadataOrConstant(SizeIsMetadata, Record[5]);
 
    MetadataList.assignValue(
        GET_OR_DISTINCT(DISubrangeType,
                        (Context, getMDString(Record[1]),
                         getMDOrNull(Record[2]), Record[3],
                         getMDOrNull(Record[4]), SizeInBits, Record[6], Flags,
                         getDITypeRefOrNull(Record[8]), getMDOrNull(Record[9]),
                         getMDOrNull(Record[10]), getMDOrNull(Record[11]),
                         getMDOrNull(Record[12]))),
        NextMetadataNo);
    NextMetadataNo++;
    break;
  }
  case bitc::METADATA_COMPOSITE_TYPE: {
    if (Record.size() < 16 || Record.size() > 26)
      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;
    bool SizeIsMetadata = Record[0] & 4;
    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;
    if (Record[8] > (uint64_t)std::numeric_limits<uint32_t>::max())
      return error("Alignment value is too large");
    uint32_t AlignInBits = Record[8];
    Metadata *OffsetInBits = nullptr;
    uint32_t NumExtraInhabitants = (Record.size() > 22) ? Record[22] : 0;
    DINode::DIFlags Flags = static_cast<DINode::DIFlags>(Record[10]);
    Metadata *Elements = nullptr;
    unsigned RuntimeLang = Record[12];
    std::optional<uint32_t> EnumKind;
 
    Metadata *VTableHolder = nullptr;
    Metadata *TemplateParams = nullptr;
    Metadata *Discriminator = nullptr;
    Metadata *DataLocation = nullptr;
    Metadata *Associated = nullptr;
    Metadata *Allocated = nullptr;
    Metadata *Rank = nullptr;
    Metadata *Annotations = nullptr;
    Metadata *Specification = nullptr;
    Metadata *BitStride = 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 are requested).
    // Create type declarations up front to save memory. This is only
    // done for types which have an Identifier, and are therefore
    // subject to the ODR.
    //
    // 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.
    //
    // We always import full definitions for anonymous composite types,
    // as without a name, debuggers cannot easily resolve a declaration
    // to its definition.
    if (IsImporting && !ImportFullTypeDefinitions && Identifier && Name &&
        (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;
      // This is a hack around preserving template parameters for simplified
      // template names - it should probably be replaced with a
      // DICompositeType flag specifying whether template parameters are
      // required on declarations of this type.
      StringRef NameStr = Name->getString();
      if (!NameStr.contains('<') || NameStr.starts_with("_STN|"))
        TemplateParams = getMDOrNull(Record[14]);
    } else {
      BaseType = getDITypeRefOrNull(Record[6]);
 
      OffsetInBits = getMetadataOrConstant(SizeIsMetadata, Record[9]);
 
      Elements = getMDOrNull(Record[11]);
      VTableHolder = getDITypeRefOrNull(Record[13]);
      TemplateParams = getMDOrNull(Record[14]);
      if (Record.size() > 16)
        Discriminator = getMDOrNull(Record[16]);
      if (Record.size() > 17)
        DataLocation = getMDOrNull(Record[17]);
      if (Record.size() > 19) {
        Associated = getMDOrNull(Record[18]);
        Allocated = getMDOrNull(Record[19]);
      }
      if (Record.size() > 20) {
        Rank = getMDOrNull(Record[20]);
      }
      if (Record.size() > 21) {
        Annotations = getMDOrNull(Record[21]);
      }
      if (Record.size() > 23) {
        Specification = getMDOrNull(Record[23]);
      }
      if (Record.size() > 25)
        BitStride = getMDOrNull(Record[25]);
    }
 
    if (Record.size() > 24 && Record[24] != dwarf::DW_APPLE_ENUM_KIND_invalid)
      EnumKind = Record[24];
 
    Metadata *SizeInBits = getMetadataOrConstant(SizeIsMetadata, Record[7]);
 
    DICompositeType *CT = nullptr;
    if (Identifier)
      CT = DICompositeType::buildODRType(
          Context, *Identifier, Tag, Name, File, Line, Scope, BaseType,
          SizeInBits, AlignInBits, OffsetInBits, Specification,
          NumExtraInhabitants, Flags, Elements, RuntimeLang, EnumKind,
          VTableHolder, TemplateParams, Discriminator, DataLocation, Associated,
          Allocated, Rank, Annotations, BitStride);
 
    // 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, EnumKind,
           VTableHolder, TemplateParams, Identifier, Discriminator,
           DataLocation, Associated, Allocated, Rank, Annotations,
           Specification, NumExtraInhabitants, BitStride));
    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() < 5 || Record.size() > 9)
      return error("Invalid record");
 
    unsigned Offset = Record.size() >= 8 ? 2 : 1;
    IsDistinct = Record[0];
    MetadataList.assignValue(
        GET_OR_DISTINCT(
            DIModule,
            (Context, Record.size() >= 8 ? getMDOrNull(Record[1]) : nullptr,
             getMDOrNull(Record[0 + Offset]), getMDString(Record[1 + Offset]),
             getMDString(Record[2 + Offset]), getMDString(Record[3 + Offset]),
             getMDString(Record[4 + Offset]),
             Record.size() <= 7 ? 0 : Record[7],
             Record.size() <= 8 ? false : Record[8])),
        NextMetadataNo);
    NextMetadataNo++;
    break;
  }
 
  case bitc::METADATA_FILE: {
    if (Record.size() != 3 && Record.size() != 5 && Record.size() != 6)
      return error("Invalid record");
 
    IsDistinct = Record[0];
    std::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 ? getMDString(Record[5]) : nullptr)),
        NextMetadataNo);
    NextMetadataNo++;
    break;
  }
  case bitc::METADATA_COMPILE_UNIT: {
    if (Record.size() < 14 || Record.size() > 22)
      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 ? false : Record[19],
        Record.size() <= 20 ? nullptr : getMDString(Record[20]),
        Record.size() <= 21 ? nullptr : getMDString(Record[21]));
 
    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() > 22)
      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],
          /*IsMainSubprogram=*/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;
    bool HasAnnotations = false;
    bool HasTargetFuncName = false;
    unsigned OffsetA = 0;
    unsigned OffsetB = 0;
    // Key instructions won't be enabled in old-format bitcode, so only
    // check it if HasSPFlags is true.
    bool UsesKeyInstructions = false;
    if (!HasSPFlags) {
      OffsetA = 2;
      OffsetB = 2;
      if (Record.size() >= 19) {
        HasFn = !HasUnit;
        OffsetB++;
      }
      HasThisAdj = Record.size() >= 20;
      HasThrownTypes = Record.size() >= 21;
    } else {
      HasAnnotations = Record.size() >= 19;
      HasTargetFuncName = Record.size() >= 20;
      UsesKeyInstructions = Record.size() >= 21 ? Record[20] : 0;
    }
 
    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
         HasAnnotations ? getMDOrNull(Record[18 + OffsetB])
                        : nullptr, // annotations
         HasTargetFuncName ? getMDString(Record[19 + OffsetB])
                           : nullptr, // targetFuncName
         UsesKeyInstructions));
    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 || Record.size() > 4)
      return error("Invalid record");
 
    IsDistinct = Record[0];
    MetadataList.assignValue(
        GET_OR_DISTINCT(DITemplateTypeParameter,
                        (Context, getMDString(Record[1]),
                         getDITypeRefOrNull(Record[2]),
                         (Record.size() == 4) ? getMDOrNull(Record[3])
                                              : getMDOrNull(false))),
        NextMetadataNo);
    NextMetadataNo++;
    break;
  }
  case bitc::METADATA_TEMPLATE_VALUE: {
    if (Record.size() < 5 || Record.size() > 6)
      return error("Invalid record");
 
    IsDistinct = Record[0];
 
    MetadataList.assignValue(
        GET_OR_DISTINCT(
            DITemplateValueParameter,
            (Context, Record[1], getMDString(Record[2]),
             getDITypeRefOrNull(Record[3]),
             (Record.size() == 6) ? getMDOrNull(Record[4]) : getMDOrNull(false),
             (Record.size() == 6) ? getMDOrNull(Record[5])
                                  : 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) {
      Metadata *Annotations = nullptr;
      if (Record.size() > 12)
        Annotations = getMDOrNull(Record[12]);
 
      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], Annotations)),
          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], nullptr)),
          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, nullptr));
 
      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_ASSIGN_ID: {
    if (Record.size() != 1)
      return error("Invalid DIAssignID record.");
 
    IsDistinct = Record[0] & 1;
    if (!IsDistinct)
      return error("Invalid DIAssignID record. Must be distinct");
 
    MetadataList.assignValue(DIAssignID::getDistinct(Context), NextMetadataNo);
    NextMetadataNo++;
    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;
    Metadata *Annotations = nullptr;
    if (HasAlignment) {
      if (Record[8] > (uint64_t)std::numeric_limits<uint32_t>::max())
        return error("Alignment value is too large");
      AlignInBits = Record[8];
      if (Record.size() > 9)
        Annotations = getMDOrNull(Record[9]);
    }
 
    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, Annotations)),
        NextMetadataNo);
    NextMetadataNo++;
    break;
  }
  case bitc::METADATA_LABEL: {
    if (Record.size() < 5 || Record.size() > 7)
      return error("Invalid record");
 
    IsDistinct = Record[0] & 1;
    uint64_t Line = Record[4];
    uint64_t Column = Record.size() > 5 ? Record[5] : 0;
    bool IsArtificial = Record[0] & 2;
    std::optional<unsigned> CoroSuspendIdx;
    if (Record.size() > 6) {
      uint64_t RawSuspendIdx = Record[6];
      if (RawSuspendIdx != std::numeric_limits<uint64_t>::max()) {
        if (RawSuspendIdx > (uint64_t)std::numeric_limits<unsigned>::max())
          return error("CoroSuspendIdx value is too large");
        CoroSuspendIdx = RawSuspendIdx;
      }
    }
 
    MetadataList.assignValue(
        GET_OR_DISTINCT(DILabel,
                        (Context, getMDOrNull(Record[1]),
                         getMDString(Record[2]), getMDOrNull(Record[3]), Line,
                         Column, IsArtificial, CoroSuspendIdx)),
        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() > 8)
      return error("Invalid DIImportedEntity record");
 
    IsDistinct = Record[0];
    bool HasFile = (Record.size() >= 7);
    bool HasElements = (Record.size() >= 8);
    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]),
                         HasElements ? getMDOrNull(Record[7]) : nullptr)),
        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;
  }
  case bitc::METADATA_ARG_LIST: {
    SmallVector<ValueAsMetadata *, 4> Elts;
    Elts.reserve(Record.size());
    for (uint64_t Elt : Record) {
      Metadata *MD = getMD(Elt);
      if (isa<MDNode>(MD) && cast<MDNode>(MD)->isTemporary())
        return error(
            "Invalid record: DIArgList should not contain forward refs");
      if (!isa<ValueAsMetadata>(MD))
        return error("Invalid record");
      Elts.push_back(cast<ValueAsMetadata>(MD));
    }
 
    MetadataList.assignValue(DIArgList::get(Context, Elts), NextMetadataNo);
    NextMetadataNo++;
    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");
 
    uint32_t Size;
    if (Error E = R.ReadVBR(6).moveInto(Size))
      return E;
    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 =
        dyn_cast_or_null<MDNode>(getMetadataFwdRefOrLoad(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, ArrayRef<Instruction *> InstructionList) {
  if (Error Err = Stream.EnterSubBlock(bitc::METADATA_ATTACHMENT_ID))
    return Err;
 
  SmallVector<uint64_t, 64> Record;
  PlaceholderQueue Placeholders;
 
  while (true) {
    BitstreamEntry Entry;
    if (Error E = Stream.advanceSkippingSubblocks().moveInto(Entry))
      return E;
 
    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) {
    BitstreamEntry Entry;
    if (Error E = Stream.advanceSkippingSubblocks().moveInto(Entry))
      return E;
 
    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,
                               MetadataLoaderCallbacks Callbacks)
    : Pimpl(std::make_unique<MetadataLoaderImpl>(
          Stream, TheModule, ValueList, std::move(Callbacks), 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, ArrayRef<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);
}