llvm/lib/LTO/LTO.cpp

//===-LTO.cpp - LLVM Link Time Optimizer ----------------------------------===//
//
// 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
//
//===----------------------------------------------------------------------===//
//
// This file implements functions and classes used to support LTO.
//
//===----------------------------------------------------------------------===//

#include "llvm/LTO/LTO.h"
#include "llvm/ADT/ArrayRef.h"
#include "llvm/ADT/ScopeExit.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/ADT/StableHashing.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/StringExtras.h"
#include "llvm/Analysis/OptimizationRemarkEmitter.h"
#include "llvm/Analysis/StackSafetyAnalysis.h"
#include "llvm/Analysis/TargetTransformInfo.h"
#include "llvm/Bitcode/BitcodeReader.h"
#include "llvm/Bitcode/BitcodeWriter.h"
#include "llvm/CGData/CodeGenData.h"
#include "llvm/CodeGen/Analysis.h"
#include "llvm/Config/llvm-config.h"
#include "llvm/IR/AutoUpgrade.h"
#include "llvm/IR/DiagnosticPrinter.h"
#include "llvm/IR/GlobalValue.h"
#include "llvm/IR/Intrinsics.h"
#include "llvm/IR/LLVMRemarkStreamer.h"
#include "llvm/IR/LegacyPassManager.h"
#include "llvm/IR/Mangler.h"
#include "llvm/IR/Metadata.h"
#include "llvm/IR/RuntimeLibcalls.h"
#include "llvm/LTO/LTOBackend.h"
#include "llvm/Linker/IRMover.h"
#include "llvm/MC/TargetRegistry.h"
#include "llvm/Object/IRObjectFile.h"
#include "llvm/Support/Caching.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/Error.h"
#include "llvm/Support/FileSystem.h"
#include "llvm/Support/MemoryBuffer.h"
#include "llvm/Support/Path.h"
#include "llvm/Support/Process.h"
#include "llvm/Support/SHA1.h"
#include "llvm/Support/Signals.h"
#include "llvm/Support/SourceMgr.h"
#include "llvm/Support/ThreadPool.h"
#include "llvm/Support/Threading.h"
#include "llvm/Support/TimeProfiler.h"
#include "llvm/Support/ToolOutputFile.h"
#include "llvm/Support/VCSRevision.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Target/TargetOptions.h"
#include "llvm/Transforms/IPO.h"
#include "llvm/Transforms/IPO/MemProfContextDisambiguation.h"
#include "llvm/Transforms/IPO/WholeProgramDevirt.h"
#include "llvm/Transforms/Utils/FunctionImportUtils.h"
#include "llvm/Transforms/Utils/SplitModule.h"

#include <optional>
#include <set>

using namespace llvm;
using namespace lto;
using namespace object;

#define DEBUG_TYPE "lto"

Error LTO::setupOptimizationRemarks() {
  // Setup the remark streamer according to the provided configuration.
  auto DiagFileOrErr = lto::setupLLVMOptimizationRemarks(
      RegularLTO.Ctx, Conf.RemarksFilename, Conf.RemarksPasses,
      Conf.RemarksFormat, Conf.RemarksWithHotness,
      Conf.RemarksHotnessThreshold);
  if (!DiagFileOrErr)
    return DiagFileOrErr.takeError();

  DiagnosticOutputFile = std::move(*DiagFileOrErr);

  // Create a dummy function to serve as a context for LTO-link remarks.
  // This is required because OptimizationRemark requires a valid Function,
  // and in ThinLTO we may not have any IR functions available during the
  // thin link. Host it in a private module to avoid interfering with the LTO
  // process.
  if (!LinkerRemarkFunction) {
    DummyModule = std::make_unique<Module>("remark_dummy", RegularLTO.Ctx);
    LinkerRemarkFunction = Function::Create(
        FunctionType::get(Type::getVoidTy(RegularLTO.Ctx), false),
        GlobalValue::ExternalLinkage, "thinlto_remark_dummy",
        DummyModule.get());
  }

  return Error::success();
}

void LTO::emitRemark(OptimizationRemark &Remark) {
  const Function &F = Remark.getFunction();
  OptimizationRemarkEmitter ORE(const_cast<Function *>(&F));
  ORE.emit(Remark);
}

static cl::opt<bool>
    DumpThinCGSCCs("dump-thin-cg-sccs", cl::init(false), cl::Hidden,
                   cl::desc("Dump the SCCs in the ThinLTO index's callgraph"));
namespace llvm {
extern cl::opt<bool> ForceImportAll;
extern cl::opt<bool> AlwaysRenamePromotedLocals;
} // end namespace llvm

namespace llvm {
/// Enable global value internalization in LTO.
cl::opt<bool> EnableLTOInternalization(
    "enable-lto-internalization", cl::init(true), cl::Hidden,
    cl::desc("Enable global value internalization in LTO"));

static cl::opt<bool>
    LTOKeepSymbolCopies("lto-keep-symbol-copies", cl::init(false), cl::Hidden,
                        cl::desc("Keep copies of symbols in LTO indexing"));

/// Indicate we are linking with an allocator that supports hot/cold operator
/// new interfaces.
extern cl::opt<bool> SupportsHotColdNew;

/// Enable MemProf context disambiguation for thin link.
extern cl::opt<bool> EnableMemProfContextDisambiguation;
} // namespace llvm

// Computes a unique hash for the Module considering the current list of
// export/import and other global analysis results.
// Returns the hash in its hexadecimal representation.
std::string llvm::computeLTOCacheKey(
    const Config &Conf, const ModuleSummaryIndex &Index, StringRef ModuleID,
    const FunctionImporter::ImportMapTy &ImportList,
    const FunctionImporter::ExportSetTy &ExportList,
    const std::map<GlobalValue::GUID, GlobalValue::LinkageTypes> &ResolvedODR,
    const GVSummaryMapTy &DefinedGlobals,
    const DenseSet<GlobalValue::GUID> &CfiFunctionDefs,
    const DenseSet<GlobalValue::GUID> &CfiFunctionDecls) {
  // Compute the unique hash for this entry.
  // This is based on the current compiler version, the module itself, the
  // export list, the hash for every single module in the import list, the
  // list of ResolvedODR for the module, and the list of preserved symbols.
  SHA1 Hasher;

  // Start with the compiler revision
  Hasher.update(LLVM_VERSION_STRING);
#ifdef LLVM_REVISION
  Hasher.update(LLVM_REVISION);
#endif

  // Include the parts of the LTO configuration that affect code generation.
  auto AddString = [&](StringRef Str) {
    Hasher.update(Str);
    Hasher.update(ArrayRef<uint8_t>{0});
  };
  auto AddUnsigned = [&](unsigned I) {
    uint8_t Data[4];
    support::endian::write32le(Data, I);
    Hasher.update(Data);
  };
  auto AddUint64 = [&](uint64_t I) {
    uint8_t Data[8];
    support::endian::write64le(Data, I);
    Hasher.update(Data);
  };
  auto AddUint8 = [&](const uint8_t I) {
    Hasher.update(ArrayRef<uint8_t>(&I, 1));
  };
  AddString(Conf.CPU);
  // FIXME: Hash more of Options. For now all clients initialize Options from
  // command-line flags (which is unsupported in production), but may set
  // X86RelaxRelocations. The clang driver can also pass FunctionSections,
  // DataSections and DebuggerTuning via command line flags.
  AddUnsigned(Conf.Options.MCOptions.X86RelaxRelocations);
  AddUnsigned(Conf.Options.FunctionSections);
  AddUnsigned(Conf.Options.DataSections);
  AddUnsigned((unsigned)Conf.Options.DebuggerTuning);
  for (auto &A : Conf.MAttrs)
    AddString(A);
  if (Conf.RelocModel)
    AddUnsigned(*Conf.RelocModel);
  else
    AddUnsigned(-1);
  if (Conf.CodeModel)
    AddUnsigned(*Conf.CodeModel);
  else
    AddUnsigned(-1);
  for (const auto &S : Conf.MllvmArgs)
    AddString(S);
  AddUnsigned(static_cast<int>(Conf.CGOptLevel));
  AddUnsigned(static_cast<int>(Conf.CGFileType));
  AddUnsigned(Conf.OptLevel);
  AddUnsigned(Conf.Freestanding);
  AddString(Conf.OptPipeline);
  AddString(Conf.AAPipeline);
  AddString(Conf.OverrideTriple);
  AddString(Conf.DefaultTriple);
  AddString(Conf.DwoDir);
  AddUint8(Conf.Dtlto);

  // Include the hash for the current module
  auto ModHash = Index.getModuleHash(ModuleID);
  Hasher.update(ArrayRef<uint8_t>((uint8_t *)&ModHash[0], sizeof(ModHash)));

  // TODO: `ExportList` is determined by `ImportList`. Since `ImportList` is
  // used to compute cache key, we could omit hashing `ExportList` here.
  std::vector<uint64_t> ExportsGUID;
  ExportsGUID.reserve(ExportList.size());
  for (const auto &VI : ExportList)
    ExportsGUID.push_back(VI.getGUID());

  // Sort the export list elements GUIDs.
  llvm::sort(ExportsGUID);
  for (auto GUID : ExportsGUID)
    Hasher.update(ArrayRef<uint8_t>((uint8_t *)&GUID, sizeof(GUID)));

  // Order using module hash, to be both independent of module name and
  // module order.
  auto Comp = [&](const std::pair<StringRef, GlobalValue::GUID> &L,
                  const std::pair<StringRef, GlobalValue::GUID> &R) {
    return std::make_pair(Index.getModule(L.first)->second, L.second) <
           std::make_pair(Index.getModule(R.first)->second, R.second);
  };
  FunctionImporter::SortedImportList SortedImportList(ImportList, Comp);

  // Count the number of imports for each source module.
  DenseMap<StringRef, unsigned> ModuleToNumImports;
  for (const auto &[FromModule, GUID, Type] : SortedImportList)
    ++ModuleToNumImports[FromModule];

  std::optional<StringRef> LastModule;
  for (const auto &[FromModule, GUID, Type] : SortedImportList) {
    if (LastModule != FromModule) {
      // Include the hash for every module we import functions from. The set of
      // imported symbols for each module may affect code generation and is
      // sensitive to link order, so include that as well.
      LastModule = FromModule;
      auto ModHash = Index.getModule(FromModule)->second;
      Hasher.update(ArrayRef<uint8_t>((uint8_t *)&ModHash[0], sizeof(ModHash)));
      AddUint64(ModuleToNumImports[FromModule]);
    }
    AddUint64(GUID);
    AddUint8(Type);
  }

  // Include the hash for the resolved ODR.
  for (auto &Entry : ResolvedODR) {
    Hasher.update(ArrayRef<uint8_t>((const uint8_t *)&Entry.first,
                                    sizeof(GlobalValue::GUID)));
    Hasher.update(ArrayRef<uint8_t>((const uint8_t *)&Entry.second,
                                    sizeof(GlobalValue::LinkageTypes)));
  }

  // Members of CfiFunctionDefs and CfiFunctionDecls that are referenced or
  // defined in this module.
  std::set<GlobalValue::GUID> UsedCfiDefs;
  std::set<GlobalValue::GUID> UsedCfiDecls;

  // Typeids used in this module.
  std::set<GlobalValue::GUID> UsedTypeIds;

  auto AddUsedCfiGlobal = [&](GlobalValue::GUID ValueGUID) {
    if (CfiFunctionDefs.contains(ValueGUID))
      UsedCfiDefs.insert(ValueGUID);
    if (CfiFunctionDecls.contains(ValueGUID))
      UsedCfiDecls.insert(ValueGUID);
  };

  auto AddUsedThings = [&](GlobalValueSummary *GS) {
    if (!GS) return;
    AddUnsigned(GS->getVisibility());
    AddUnsigned(GS->isLive());
    AddUnsigned(GS->canAutoHide());
    for (const ValueInfo &VI : GS->refs()) {
      AddUnsigned(VI.isDSOLocal(Index.withDSOLocalPropagation()));
      AddUsedCfiGlobal(VI.getGUID());
    }
    if (auto *GVS = dyn_cast<GlobalVarSummary>(GS)) {
      AddUnsigned(GVS->maybeReadOnly());
      AddUnsigned(GVS->maybeWriteOnly());
    }
    if (auto *FS = dyn_cast<FunctionSummary>(GS)) {
      for (auto &TT : FS->type_tests())
        UsedTypeIds.insert(TT);
      for (auto &TT : FS->type_test_assume_vcalls())
        UsedTypeIds.insert(TT.GUID);
      for (auto &TT : FS->type_checked_load_vcalls())
        UsedTypeIds.insert(TT.GUID);
      for (auto &TT : FS->type_test_assume_const_vcalls())
        UsedTypeIds.insert(TT.VFunc.GUID);
      for (auto &TT : FS->type_checked_load_const_vcalls())
        UsedTypeIds.insert(TT.VFunc.GUID);
      for (auto &ET : FS->calls()) {
        AddUnsigned(ET.first.isDSOLocal(Index.withDSOLocalPropagation()));
        AddUsedCfiGlobal(ET.first.getGUID());
      }
    }
  };

  // Sort the defined globals by GUID to be independent of the insertion order,
  // which may depend on the order that modules are added.
  SmallVector<std::pair<GlobalValue::GUID, GlobalValueSummary *>>
      SortedDefinedGlobals(DefinedGlobals.begin(), DefinedGlobals.end());
  llvm::sort(SortedDefinedGlobals, llvm::less_first());
  for (auto &GS : SortedDefinedGlobals) {
    // Include the hash for the linkage type to reflect internalization and weak
    // resolution, and collect any used type identifier resolutions.
    GlobalValue::LinkageTypes Linkage = GS.second->linkage();
    Hasher.update(
        ArrayRef<uint8_t>((const uint8_t *)&Linkage, sizeof(Linkage)));
    AddUsedCfiGlobal(GS.first);
    AddUsedThings(GS.second);
  }

  // Imported functions may introduce new uses of type identifier resolutions,
  // so we need to collect their used resolutions as well.
  for (const auto &[FromModule, GUID, Type] : SortedImportList) {
    GlobalValueSummary *S = Index.findSummaryInModule(GUID, FromModule);
    AddUsedThings(S);
    // If this is an alias, we also care about any types/etc. that the aliasee
    // may reference.
    if (auto *AS = dyn_cast_or_null<AliasSummary>(S))
      AddUsedThings(AS->getBaseObject());
  }

  auto AddTypeIdSummary = [&](StringRef TId, const TypeIdSummary &S) {
    AddString(TId);

    AddUnsigned(S.TTRes.TheKind);
    AddUnsigned(S.TTRes.SizeM1BitWidth);

    AddUint64(S.TTRes.AlignLog2);
    AddUint64(S.TTRes.SizeM1);
    AddUint64(S.TTRes.BitMask);
    AddUint64(S.TTRes.InlineBits);

    AddUint64(S.WPDRes.size());
    for (auto &WPD : S.WPDRes) {
      AddUnsigned(WPD.first);
      AddUnsigned(WPD.second.TheKind);
      AddString(WPD.second.SingleImplName);

      AddUint64(WPD.second.ResByArg.size());
      for (auto &ByArg : WPD.second.ResByArg) {
        AddUint64(ByArg.first.size());
        for (uint64_t Arg : ByArg.first)
          AddUint64(Arg);
        AddUnsigned(ByArg.second.TheKind);
        AddUint64(ByArg.second.Info);
        AddUnsigned(ByArg.second.Byte);
        AddUnsigned(ByArg.second.Bit);
      }
    }
  };

  // Include the hash for all type identifiers used by this module.
  for (GlobalValue::GUID TId : UsedTypeIds) {
    auto TidIter = Index.typeIds().equal_range(TId);
    for (const auto &I : make_range(TidIter))
      AddTypeIdSummary(I.second.first, I.second.second);
  }

  AddUnsigned(UsedCfiDefs.size());
  for (auto &V : UsedCfiDefs)
    AddUint64(V);

  AddUnsigned(UsedCfiDecls.size());
  for (auto &V : UsedCfiDecls)
    AddUint64(V);

  if (!Conf.SampleProfile.empty()) {
    auto FileOrErr = MemoryBuffer::getFile(Conf.SampleProfile);
    if (FileOrErr) {
      Hasher.update(FileOrErr.get()->getBuffer());

      if (!Conf.ProfileRemapping.empty()) {
        FileOrErr = MemoryBuffer::getFile(Conf.ProfileRemapping);
        if (FileOrErr)
          Hasher.update(FileOrErr.get()->getBuffer());
      }
    }
  }

  return toHex(Hasher.result());
}

std::string llvm::recomputeLTOCacheKey(const std::string &Key,
                                       StringRef ExtraID) {
  SHA1 Hasher;

  auto AddString = [&](StringRef Str) {
    Hasher.update(Str);
    Hasher.update(ArrayRef<uint8_t>{0});
  };
  AddString(Key);
  AddString(ExtraID);

  return toHex(Hasher.result());
}

static void thinLTOResolvePrevailingGUID(
    const Config &C, ValueInfo VI,
    DenseSet<GlobalValueSummary *> &GlobalInvolvedWithAlias,
    function_ref<bool(GlobalValue::GUID, const GlobalValueSummary *)>
        isPrevailing,
    function_ref<void(StringRef, GlobalValue::GUID, GlobalValue::LinkageTypes)>
        recordNewLinkage,
    const DenseSet<GlobalValue::GUID> &GUIDPreservedSymbols) {
  GlobalValue::VisibilityTypes Visibility =
      C.VisibilityScheme == Config::ELF ? VI.getELFVisibility()
                                        : GlobalValue::DefaultVisibility;
  for (auto &S : VI.getSummaryList()) {
    GlobalValue::LinkageTypes OriginalLinkage = S->linkage();
    // Ignore local and appending linkage values since the linker
    // doesn't resolve them.
    if (GlobalValue::isLocalLinkage(OriginalLinkage) ||
        GlobalValue::isAppendingLinkage(S->linkage()))
      continue;
    // We need to emit only one of these. The prevailing module will keep it,
    // but turned into a weak, while the others will drop it when possible.
    // This is both a compile-time optimization and a correctness
    // transformation. This is necessary for correctness when we have exported
    // a reference - we need to convert the linkonce to weak to
    // ensure a copy is kept to satisfy the exported reference.
    // FIXME: We may want to split the compile time and correctness
    // aspects into separate routines.
    if (isPrevailing(VI.getGUID(), S.get())) {
      assert(!S->wasPromoted() &&
             "promoted symbols used to be internal linkage and shouldn't have "
             "a prevailing variant");
      if (GlobalValue::isLinkOnceLinkage(OriginalLinkage)) {
        S->setLinkage(GlobalValue::getWeakLinkage(
            GlobalValue::isLinkOnceODRLinkage(OriginalLinkage)));
        // The kept copy is eligible for auto-hiding (hidden visibility) if all
        // copies were (i.e. they were all linkonce_odr global unnamed addr).
        // If any copy is not (e.g. it was originally weak_odr), then the symbol
        // must remain externally available (e.g. a weak_odr from an explicitly
        // instantiated template). Additionally, if it is in the
        // GUIDPreservedSymbols set, that means that it is visibile outside
        // the summary (e.g. in a native object or a bitcode file without
        // summary), and in that case we cannot hide it as it isn't possible to
        // check all copies.
        S->setCanAutoHide(VI.canAutoHide() &&
                          !GUIDPreservedSymbols.count(VI.getGUID()));
      }
      if (C.VisibilityScheme == Config::FromPrevailing)
        Visibility = S->getVisibility();
    }
    // Alias and aliasee can't be turned into available_externally.
    // When force-import-all is used, it indicates that object linking is not
    // supported by the target. In this case, we can't change the linkage as
    // well in case the global is converted to declaration.
    // Also, if the symbol was promoted, it wouldn't have a prevailing variant,
    // but also its linkage is set correctly (to External) already.
    else if (!isa<AliasSummary>(S.get()) &&
             !GlobalInvolvedWithAlias.count(S.get()) && !ForceImportAll &&
             !S->wasPromoted())
      S->setLinkage(GlobalValue::AvailableExternallyLinkage);

    // For ELF, set visibility to the computed visibility from summaries. We
    // don't track visibility from declarations so this may be more relaxed than
    // the most constraining one.
    if (C.VisibilityScheme == Config::ELF)
      S->setVisibility(Visibility);

    if (S->linkage() != OriginalLinkage)
      recordNewLinkage(S->modulePath(), VI.getGUID(), S->linkage());
  }

  if (C.VisibilityScheme == Config::FromPrevailing) {
    for (auto &S : VI.getSummaryList()) {
      GlobalValue::LinkageTypes OriginalLinkage = S->linkage();
      if (GlobalValue::isLocalLinkage(OriginalLinkage) ||
          GlobalValue::isAppendingLinkage(S->linkage()))
        continue;
      S->setVisibility(Visibility);
    }
  }
}

/// Resolve linkage for prevailing symbols in the \p Index.
//
// We'd like to drop these functions if they are no longer referenced in the
// current module. However there is a chance that another module is still
// referencing them because of the import. We make sure we always emit at least
// one copy.
void llvm::thinLTOResolvePrevailingInIndex(
    const Config &C, ModuleSummaryIndex &Index,
    function_ref<bool(GlobalValue::GUID, const GlobalValueSummary *)>
        isPrevailing,
    function_ref<void(StringRef, GlobalValue::GUID, GlobalValue::LinkageTypes)>
        recordNewLinkage,
    const DenseSet<GlobalValue::GUID> &GUIDPreservedSymbols) {
  // We won't optimize the globals that are referenced by an alias for now
  // Ideally we should turn the alias into a global and duplicate the definition
  // when needed.
  DenseSet<GlobalValueSummary *> GlobalInvolvedWithAlias;
  for (auto &I : Index)
    for (auto &S : I.second.getSummaryList())
      if (auto AS = dyn_cast<AliasSummary>(S.get()))
        GlobalInvolvedWithAlias.insert(&AS->getAliasee());

  for (auto &I : Index)
    thinLTOResolvePrevailingGUID(C, Index.getValueInfo(I),
                                 GlobalInvolvedWithAlias, isPrevailing,
                                 recordNewLinkage, GUIDPreservedSymbols);
}

static void thinLTOInternalizeAndPromoteGUID(
    ValueInfo VI, function_ref<bool(StringRef, ValueInfo)> isExported,
    function_ref<bool(GlobalValue::GUID, const GlobalValueSummary *)>
        isPrevailing,
    DenseSet<StringRef> *ExternallyVisibleSymbolNamesPtr) {
  // Before performing index-based internalization and promotion for this GUID,
  // the local flag should be consistent with the summary list linkage types.
  VI.verifyLocal();

  const bool SingleExternallyVisibleCopy =
      VI.getSummaryList().size() == 1 &&
      !GlobalValue::isLocalLinkage(VI.getSummaryList().front()->linkage());

  bool NameRecorded = false;
  for (auto &S : VI.getSummaryList()) {
    // First see if we need to promote an internal value because it is not
    // exported.
    if (isExported(S->modulePath(), VI)) {
      if (GlobalValue::isLocalLinkage(S->linkage())) {
        // Only the first local GlobalValue in a list of summaries does not
        // need renaming. In rare cases if there exist more than one summaries
        // in the list, the rest of them must have renaming (through promotion)
        // to avoid conflict.
        if (ExternallyVisibleSymbolNamesPtr && !NameRecorded) {
          NameRecorded = true;
          if (ExternallyVisibleSymbolNamesPtr->insert(VI.name()).second)
            S->setNoRenameOnPromotion(true);
        }

        S->promote();
      }
      continue;
    }

    // Otherwise, see if we can internalize.
    if (!EnableLTOInternalization)
      continue;

    // Non-exported values with external linkage can be internalized.
    if (GlobalValue::isExternalLinkage(S->linkage())) {
      S->setLinkage(GlobalValue::InternalLinkage);
      continue;
    }

    // Non-exported function and variable definitions with a weak-for-linker
    // linkage can be internalized in certain cases. The minimum legality
    // requirements would be that they are not address taken to ensure that we
    // don't break pointer equality checks, and that variables are either read-
    // or write-only. For functions, this is the case if either all copies are
    // [local_]unnamed_addr, or we can propagate reference edge attributes
    // (which is how this is guaranteed for variables, when analyzing whether
    // they are read or write-only).
    //
    // However, we only get to this code for weak-for-linkage values in one of
    // two cases:
    // 1) The prevailing copy is not in IR (it is in native code).
    // 2) The prevailing copy in IR is not exported from its module.
    // Additionally, at least for the new LTO API, case 2 will only happen if
    // there is exactly one definition of the value (i.e. in exactly one
    // module), as duplicate defs are result in the value being marked exported.
    // Likely, users of the legacy LTO API are similar, however, currently there
    // are llvm-lto based tests of the legacy LTO API that do not mark
    // duplicate linkonce_odr copies as exported via the tool, so we need
    // to handle that case below by checking the number of copies.
    //
    // Generally, we only want to internalize a weak-for-linker value in case
    // 2, because in case 1 we cannot see how the value is used to know if it
    // is read or write-only. We also don't want to bloat the binary with
    // multiple internalized copies of non-prevailing linkonce/weak functions.
    // Note if we don't internalize, we will convert non-prevailing copies to
    // available_externally anyway, so that we drop them after inlining. The
    // only reason to internalize such a function is if we indeed have a single
    // copy, because internalizing it won't increase binary size, and enables
    // use of inliner heuristics that are more aggressive in the face of a
    // single call to a static (local). For variables, internalizing a read or
    // write only variable can enable more aggressive optimization. However, we
    // already perform this elsewhere in the ThinLTO backend handling for
    // read or write-only variables (processGlobalForThinLTO).
    //
    // Therefore, only internalize linkonce/weak if there is a single copy, that
    // is prevailing in this IR module. We can do so aggressively, without
    // requiring the address to be insignificant, or that a variable be read or
    // write-only.
    if (!GlobalValue::isWeakForLinker(S->linkage()) ||
        GlobalValue::isExternalWeakLinkage(S->linkage()))
      continue;

    // We may have a single summary copy that is externally visible but not
    // prevailing if the prevailing copy is in a native object.
    if (SingleExternallyVisibleCopy && isPrevailing(VI.getGUID(), S.get()))
      S->setLinkage(GlobalValue::InternalLinkage);
  }
}

// Update the linkages in the given \p Index to mark exported values
// as external and non-exported values as internal.
void llvm::thinLTOInternalizeAndPromoteInIndex(
    ModuleSummaryIndex &Index,
    function_ref<bool(StringRef, ValueInfo)> isExported,
    function_ref<bool(GlobalValue::GUID, const GlobalValueSummary *)>
        isPrevailing,
    DenseSet<StringRef> *ExternallyVisibleSymbolNamesPtr) {
  assert(!Index.withInternalizeAndPromote());

  for (auto &I : Index)
    thinLTOInternalizeAndPromoteGUID(Index.getValueInfo(I), isExported,
                                     isPrevailing,
                                     ExternallyVisibleSymbolNamesPtr);
  Index.setWithInternalizeAndPromote();
}

// Requires a destructor for std::vector<InputModule>.
InputFile::~InputFile() = default;

Expected<std::unique_ptr<InputFile>> InputFile::create(MemoryBufferRef Object) {
  std::unique_ptr<InputFile> File(new InputFile);

  Expected<IRSymtabFile> FOrErr = readIRSymtab(Object);
  if (!FOrErr)
    return FOrErr.takeError();

  File->TargetTriple = FOrErr->TheReader.getTargetTriple();
  File->SourceFileName = FOrErr->TheReader.getSourceFileName();
  File->COFFLinkerOpts = FOrErr->TheReader.getCOFFLinkerOpts();
  File->DependentLibraries = FOrErr->TheReader.getDependentLibraries();
  File->ComdatTable = FOrErr->TheReader.getComdatTable();
  File->MbRef =
      Object; // Save a memory buffer reference to an input file object.

  for (unsigned I = 0; I != FOrErr->Mods.size(); ++I) {
    size_t Begin = File->Symbols.size();
    for (const irsymtab::Reader::SymbolRef &Sym :
         FOrErr->TheReader.module_symbols(I))
      // Skip symbols that are irrelevant to LTO. Note that this condition needs
      // to match the one in Skip() in LTO::addRegularLTO().
      if (Sym.isGlobal() && !Sym.isFormatSpecific())
        File->Symbols.push_back(Sym);
    File->ModuleSymIndices.push_back({Begin, File->Symbols.size()});
  }

  File->Mods = FOrErr->Mods;
  File->Strtab = std::move(FOrErr->Strtab);
  return std::move(File);
}

bool InputFile::Symbol::isLibcall(
    const TargetLibraryInfo &TLI,
    const RTLIB::RuntimeLibcallsInfo &Libcalls) const {
  if (TLI.has(TLI.getLibFunc(IRName)))
    return true;
  return Libcalls.getSupportedLibcallImpl(IRName) != RTLIB::Unsupported;
}

StringRef InputFile::getName() const {
  return Mods[0].getModuleIdentifier();
}

BitcodeModule &InputFile::getSingleBitcodeModule() {
  assert(Mods.size() == 1 && "Expect only one bitcode module");
  return Mods[0];
}

BitcodeModule &InputFile::getPrimaryBitcodeModule() { return Mods[0]; }

LTO::RegularLTOState::RegularLTOState(unsigned ParallelCodeGenParallelismLevel,
                                      const Config &Conf)
    : ParallelCodeGenParallelismLevel(ParallelCodeGenParallelismLevel),
      Ctx(Conf), CombinedModule(std::make_unique<Module>("ld-temp.o", Ctx)),
      Mover(std::make_unique<IRMover>(*CombinedModule)) {}

LTO::ThinLTOState::ThinLTOState(ThinBackend BackendParam)
    : Backend(std::move(BackendParam)), CombinedIndex(/*HaveGVs*/ false) {
  if (!Backend.isValid())
    Backend =
        createInProcessThinBackend(llvm::heavyweight_hardware_concurrency());
}

LTO::LTO(Config Conf, ThinBackend Backend,
         unsigned ParallelCodeGenParallelismLevel, LTOKind LTOMode)
    : Conf(std::move(Conf)),
      RegularLTO(ParallelCodeGenParallelismLevel, this->Conf),
      ThinLTO(std::move(Backend)),
      GlobalResolutions(
          std::make_unique<DenseMap<StringRef, GlobalResolution>>()),
      LTOMode(LTOMode) {
  if (Conf.KeepSymbolNameCopies || LTOKeepSymbolCopies) {
    Alloc = std::make_unique<BumpPtrAllocator>();
    GlobalResolutionSymbolSaver = std::make_unique<llvm::StringSaver>(*Alloc);
  }
}

// Requires a destructor for MapVector<BitcodeModule>.
LTO::~LTO() = default;

void LTO::cleanup() {
  DummyModule.reset();
  LinkerRemarkFunction = nullptr;
  consumeError(finalizeOptimizationRemarks(std::move(DiagnosticOutputFile)));
}

// Add the symbols in the given module to the GlobalResolutions map, and resolve
// their partitions.
void LTO::addModuleToGlobalRes(ArrayRef<InputFile::Symbol> Syms,
                               ArrayRef<SymbolResolution> Res,
                               unsigned Partition, bool InSummary,
                               const Triple &TT) {
  llvm::TimeTraceScope timeScope("LTO add module to global resolution");
  auto *ResI = Res.begin();
  auto *ResE = Res.end();
  (void)ResE;
  RTLIB::RuntimeLibcallsInfo Libcalls(TT);
  TargetLibraryInfoImpl TLII(TT);
  TargetLibraryInfo TLI(TLII);
  for (const InputFile::Symbol &Sym : Syms) {
    assert(ResI != ResE);
    SymbolResolution Res = *ResI++;

    StringRef SymbolName = Sym.getName();
    // Keep copies of symbols if the client of LTO says so.
    if (GlobalResolutionSymbolSaver && !GlobalResolutions->contains(SymbolName))
      SymbolName = GlobalResolutionSymbolSaver->save(SymbolName);

    auto &GlobalRes = (*GlobalResolutions)[SymbolName];
    GlobalRes.UnnamedAddr &= Sym.isUnnamedAddr();
    if (Res.Prevailing) {
      assert(!GlobalRes.Prevailing &&
             "Multiple prevailing defs are not allowed");
      GlobalRes.Prevailing = true;
      GlobalRes.IRName = std::string(Sym.getIRName());
    } else if (!GlobalRes.Prevailing && GlobalRes.IRName.empty()) {
      // Sometimes it can be two copies of symbol in a module and prevailing
      // symbol can have no IR name. That might happen if symbol is defined in
      // module level inline asm block. In case we have multiple modules with
      // the same symbol we want to use IR name of the prevailing symbol.
      // Otherwise, if we haven't seen a prevailing symbol, set the name so that
      // we can later use it to check if there is any prevailing copy in IR.
      GlobalRes.IRName = std::string(Sym.getIRName());
    }

    // In rare occasion, the symbol used to initialize GlobalRes has a different
    // IRName from the inspected Symbol. This can happen on macOS + iOS, when a
    // symbol is referenced through its mangled name, say @"\01_symbol" while
    // the IRName is @symbol (the prefix underscore comes from MachO mangling).
    // In that case, we have the same actual Symbol that can get two different
    // GUID, leading to some invalid internalization. Workaround this by marking
    // the GlobalRes external.

    // FIXME: instead of this check, it would be desirable to compute GUIDs
    // based on mangled name, but this requires an access to the Target Triple
    // and would be relatively invasive on the codebase.
    // FIXME: use the GUID member of GlobalRes.
    if (GlobalRes.IRName != Sym.getIRName()) {
      GlobalRes.Partition = GlobalResolution::External;
      GlobalRes.VisibleOutsideSummary = true;
    }

    bool IsLibcall = Sym.isLibcall(TLI, Libcalls);

    // Set the partition to external if we know it is re-defined by the linker
    // with -defsym or -wrap options, used elsewhere, e.g. it is visible to a
    // regular object, is referenced from llvm.compiler.used/llvm.used, or was
    // already recorded as being referenced from a different partition.
    if (Res.LinkerRedefined || Res.VisibleToRegularObj || Sym.isUsed() ||
        IsLibcall ||
        (GlobalRes.Partition != GlobalResolution::Unknown &&
         GlobalRes.Partition != Partition)) {
      GlobalRes.Partition = GlobalResolution::External;
    } else
      // First recorded reference, save the current partition.
      GlobalRes.Partition = Partition;

    // Flag as visible outside of summary if visible from a regular object or
    // from a module that does not have a summary.
    GlobalRes.VisibleOutsideSummary |=
        (Res.VisibleToRegularObj || Sym.isUsed() || IsLibcall || !InSummary);

    GlobalRes.ExportDynamic |= Res.ExportDynamic;
  }
}

void LTO::releaseGlobalResolutionsMemory() {
  // Release GlobalResolutions dense-map itself.
  GlobalResolutions.reset();
  // Release the string saver memory.
  GlobalResolutionSymbolSaver.reset();
  Alloc.reset();
}

static void writeToResolutionFile(raw_ostream &OS, InputFile *Input,
                                  ArrayRef<SymbolResolution> Res) {
  StringRef Path = Input->getName();
  OS << Path << '\n';
  auto ResI = Res.begin();
  for (const InputFile::Symbol &Sym : Input->symbols()) {
    assert(ResI != Res.end());
    SymbolResolution Res = *ResI++;

    OS << "-r=" << Path << ',' << Sym.getName() << ',';
    if (Res.Prevailing)
      OS << 'p';
    if (Res.FinalDefinitionInLinkageUnit)
      OS << 'l';
    if (Res.VisibleToRegularObj)
      OS << 'x';
    if (Res.LinkerRedefined)
      OS << 'r';
    OS << '\n';
  }
  OS.flush();
  assert(ResI == Res.end());
}

Error LTO::add(std::unique_ptr<InputFile> InputPtr,
               ArrayRef<SymbolResolution> Res) {
  llvm::TimeTraceScope timeScope("LTO add input", InputPtr->getName());
  assert(!CalledGetMaxTasks);

  Expected<std::shared_ptr<InputFile>> InputOrErr =
      addInput(std::move(InputPtr));
  if (!InputOrErr)
    return InputOrErr.takeError();
  InputFile *Input = (*InputOrErr).get();

  if (Conf.ResolutionFile)
    writeToResolutionFile(*Conf.ResolutionFile, Input, Res);

  if (RegularLTO.CombinedModule->getTargetTriple().empty()) {
    Triple InputTriple(Input->getTargetTriple());
    RegularLTO.CombinedModule->setTargetTriple(InputTriple);
    if (InputTriple.isOSBinFormatELF())
      Conf.VisibilityScheme = Config::ELF;
  }

  ArrayRef<SymbolResolution> InputRes = Res;
  for (unsigned I = 0; I != Input->Mods.size(); ++I) {
    if (auto Err = addModule(*Input, InputRes, I, Res).moveInto(Res))
      return Err;
  }

  assert(Res.empty());
  return Error::success();
}

void LTO::setBitcodeLibFuncs(ArrayRef<StringRef> BitcodeLibFuncs) {
  assert(this->BitcodeLibFuncs.empty() &&
         "bitcode libfuncs were set twice; maybe accidentally clobbered?");
  this->BitcodeLibFuncs.append(BitcodeLibFuncs.begin(), BitcodeLibFuncs.end());
}

Expected<ArrayRef<SymbolResolution>>
LTO::addModule(InputFile &Input, ArrayRef<SymbolResolution> InputRes,
               unsigned ModI, ArrayRef<SymbolResolution> Res) {
  llvm::TimeTraceScope timeScope("LTO add module", Input.getName());
  Expected<BitcodeLTOInfo> LTOInfo = Input.Mods[ModI].getLTOInfo();
  if (!LTOInfo)
    return LTOInfo.takeError();

  if (EnableSplitLTOUnit) {
    // If only some modules were split, flag this in the index so that
    // we can skip or error on optimizations that need consistently split
    // modules (whole program devirt and lower type tests).
    if (*EnableSplitLTOUnit != LTOInfo->EnableSplitLTOUnit)
      ThinLTO.CombinedIndex.setPartiallySplitLTOUnits();
  } else
    EnableSplitLTOUnit = LTOInfo->EnableSplitLTOUnit;

  BitcodeModule BM = Input.Mods[ModI];

  if ((LTOMode == LTOK_UnifiedRegular || LTOMode == LTOK_UnifiedThin) &&
      !LTOInfo->UnifiedLTO)
    return make_error<StringError>(
        "unified LTO compilation must use "
        "compatible bitcode modules (use -funified-lto)",
        inconvertibleErrorCode());

  if (LTOInfo->UnifiedLTO && LTOMode == LTOK_Default)
    LTOMode = LTOK_UnifiedThin;

  bool IsThinLTO = LTOInfo->IsThinLTO && (LTOMode != LTOK_UnifiedRegular);
  // If any of the modules inside of a input bitcode file was compiled with
  // ThinLTO, we assume that the whole input file also was compiled with
  // ThinLTO.
  Input.IsThinLTO |= IsThinLTO;

  auto ModSyms = Input.module_symbols(ModI);
  addModuleToGlobalRes(ModSyms, Res,
                       IsThinLTO ? ThinLTO.ModuleMap.size() + 1 : 0,
                       LTOInfo->HasSummary, Triple(Input.getTargetTriple()));

  if (IsThinLTO)
    return addThinLTO(BM, ModSyms, Res);

  RegularLTO.EmptyCombinedModule = false;
  auto ModOrErr = addRegularLTO(Input, InputRes, BM, ModSyms, Res);
  if (!ModOrErr)
    return ModOrErr.takeError();
  Res = ModOrErr->second;

  if (!LTOInfo->HasSummary) {
    if (Error Err = linkRegularLTO(std::move(ModOrErr->first),
                                   /*LivenessFromIndex=*/false))
      return Err;
    return Res;
  }

  // Regular LTO module summaries are added to a dummy module that represents
  // the combined regular LTO module.
  if (Error Err = BM.readSummary(ThinLTO.CombinedIndex, ""))
    return Err;
  RegularLTO.ModsWithSummaries.push_back(std::move(ModOrErr->first));
  return Res;
}

// Checks whether the given global value is in a non-prevailing comdat
// (comdat containing values the linker indicated were not prevailing,
// which we then dropped to available_externally), and if so, removes
// it from the comdat. This is called for all global values to ensure the
// comdat is empty rather than leaving an incomplete comdat. It is needed for
// regular LTO modules, in case we are in a mixed-LTO mode (both regular
// and thin LTO modules) compilation. Since the regular LTO module will be
// linked first in the final native link, we want to make sure the linker
// doesn't select any of these incomplete comdats that would be left
// in the regular LTO module without this cleanup.
static void
handleNonPrevailingComdat(GlobalValue &GV,
                          std::set<const Comdat *> &NonPrevailingComdats) {
  Comdat *C = GV.getComdat();
  if (!C)
    return;

  if (!NonPrevailingComdats.count(C))
    return;

  // Additionally need to drop all global values from the comdat to
  // available_externally, to satisfy the COMDAT requirement that all members
  // are discarded as a unit. The non-local linkage global values avoid
  // duplicate definition linker errors.
  GV.setLinkage(GlobalValue::AvailableExternallyLinkage);

  if (auto GO = dyn_cast<GlobalObject>(&GV))
    GO->setComdat(nullptr);
}

// Add a regular LTO object to the link.
// The resulting module needs to be linked into the combined LTO module with
// linkRegularLTO.
Expected<
    std::pair<LTO::RegularLTOState::AddedModule, ArrayRef<SymbolResolution>>>
LTO::addRegularLTO(InputFile &Input, ArrayRef<SymbolResolution> InputRes,
                   BitcodeModule BM, ArrayRef<InputFile::Symbol> Syms,
                   ArrayRef<SymbolResolution> Res) {
  llvm::TimeTraceScope timeScope("LTO add regular LTO");
  RegularLTOState::AddedModule Mod;
  Expected<std::unique_ptr<Module>> MOrErr =
      BM.getLazyModule(RegularLTO.Ctx, /*ShouldLazyLoadMetadata*/ true,
                       /*IsImporting*/ false);
  if (!MOrErr)
    return MOrErr.takeError();
  Module &M = **MOrErr;
  Mod.M = std::move(*MOrErr);

  if (Error Err = M.materializeMetadata())
    return std::move(Err);

  if (LTOMode == LTOK_UnifiedRegular) {
    // cfi.functions metadata is intended to be used with ThinLTO and may
    // trigger invalid IR transformations if they are present when doing regular
    // LTO, so delete it.
    if (NamedMDNode *CfiFunctionsMD = M.getNamedMetadata("cfi.functions"))
      M.eraseNamedMetadata(CfiFunctionsMD);
  } else if (NamedMDNode *AliasesMD = M.getNamedMetadata("aliases")) {
    // Delete aliases entries for non-prevailing symbols on the ThinLTO side of
    // this input file.
    DenseSet<StringRef> Prevailing;
    for (auto [I, R] : zip(Input.symbols(), InputRes))
      if (R.Prevailing && !I.getIRName().empty())
        Prevailing.insert(I.getIRName());
    std::vector<MDNode *> AliasGroups;
    for (MDNode *AliasGroup : AliasesMD->operands()) {
      std::vector<Metadata *> Aliases;
      for (Metadata *Alias : AliasGroup->operands()) {
        if (isa<MDString>(Alias) &&
            Prevailing.count(cast<MDString>(Alias)->getString()))
          Aliases.push_back(Alias);
      }
      if (Aliases.size() > 1)
        AliasGroups.push_back(MDTuple::get(RegularLTO.Ctx, Aliases));
    }
    AliasesMD->clearOperands();
    for (MDNode *G : AliasGroups)
      AliasesMD->addOperand(G);
  }

  UpgradeDebugInfo(M);

  ModuleSymbolTable SymTab;
  SymTab.addModule(&M);

  for (GlobalVariable &GV : M.globals())
    if (GV.hasAppendingLinkage())
      Mod.Keep.push_back(&GV);

  DenseSet<GlobalObject *> AliasedGlobals;
  for (auto &GA : M.aliases())
    if (GlobalObject *GO = GA.getAliaseeObject())
      AliasedGlobals.insert(GO);

  // In this function we need IR GlobalValues matching the symbols in Syms
  // (which is not backed by a module), so we need to enumerate them in the same
  // order. The symbol enumeration order of a ModuleSymbolTable intentionally
  // matches the order of an irsymtab, but when we read the irsymtab in
  // InputFile::create we omit some symbols that are irrelevant to LTO. The
  // Skip() function skips the same symbols from the module as InputFile does
  // from the symbol table.
  auto MsymI = SymTab.symbols().begin(), MsymE = SymTab.symbols().end();
  auto Skip = [&]() {
    while (MsymI != MsymE) {
      auto Flags = SymTab.getSymbolFlags(*MsymI);
      if ((Flags & object::BasicSymbolRef::SF_Global) &&
          !(Flags & object::BasicSymbolRef::SF_FormatSpecific))
        return;
      ++MsymI;
    }
  };
  Skip();

  std::set<const Comdat *> NonPrevailingComdats;
  SmallSet<StringRef, 2> NonPrevailingAsmSymbols;
  for (const InputFile::Symbol &Sym : Syms) {
    assert(!Res.empty());
    const SymbolResolution &R = Res.consume_front();

    assert(MsymI != MsymE);
    ModuleSymbolTable::Symbol Msym = *MsymI++;
    Skip();

    if (GlobalValue *GV = dyn_cast_if_present<GlobalValue *>(Msym)) {
      if (R.Prevailing) {
        if (Sym.isUndefined())
          continue;
        Mod.Keep.push_back(GV);
        // For symbols re-defined with linker -wrap and -defsym options,
        // set the linkage to weak to inhibit IPO. The linkage will be
        // restored by the linker.
        if (R.LinkerRedefined)
          GV->setLinkage(GlobalValue::WeakAnyLinkage);

        GlobalValue::LinkageTypes OriginalLinkage = GV->getLinkage();
        if (GlobalValue::isLinkOnceLinkage(OriginalLinkage))
          GV->setLinkage(GlobalValue::getWeakLinkage(
              GlobalValue::isLinkOnceODRLinkage(OriginalLinkage)));
      } else if (isa<GlobalObject>(GV) &&
                 (GV->hasLinkOnceODRLinkage() || GV->hasWeakODRLinkage() ||
                  GV->hasAvailableExternallyLinkage()) &&
                 !AliasedGlobals.count(cast<GlobalObject>(GV))) {
        // Any of the above three types of linkage indicates that the
        // chosen prevailing symbol will have the same semantics as this copy of
        // the symbol, so we may be able to link it with available_externally
        // linkage. We will decide later whether to do that when we link this
        // module (in linkRegularLTO), based on whether it is undefined.
        Mod.Keep.push_back(GV);
        GV->setLinkage(GlobalValue::AvailableExternallyLinkage);
        if (GV->hasComdat())
          NonPrevailingComdats.insert(GV->getComdat());
        cast<GlobalObject>(GV)->setComdat(nullptr);
      }

      // Set the 'local' flag based on the linker resolution for this symbol.
      if (R.FinalDefinitionInLinkageUnit) {
        GV->setDSOLocal(true);
        if (GV->hasDLLImportStorageClass())
          GV->setDLLStorageClass(GlobalValue::DLLStorageClassTypes::
                                 DefaultStorageClass);
      }
    } else if (auto *AS =
                   dyn_cast_if_present<ModuleSymbolTable::AsmSymbol *>(Msym)) {
      // Collect non-prevailing symbols.
      if (!R.Prevailing)
        NonPrevailingAsmSymbols.insert(AS->first);
    } else {
      llvm_unreachable("unknown symbol type");
    }

    // Common resolution: collect the maximum size/alignment over all commons.
    // We also record if we see an instance of a common as prevailing, so that
    // if none is prevailing we can ignore it later.
    if (Sym.isCommon()) {
      // FIXME: We should figure out what to do about commons defined by asm.
      // For now they aren't reported correctly by ModuleSymbolTable.
      auto &CommonRes = RegularLTO.Commons[std::string(Sym.getIRName())];
      CommonRes.Size = std::max(CommonRes.Size, Sym.getCommonSize());
      if (uint32_t SymAlignValue = Sym.getCommonAlignment()) {
        CommonRes.Alignment =
            std::max(Align(SymAlignValue), CommonRes.Alignment);
      }
      CommonRes.Prevailing |= R.Prevailing;
    }
  }

  if (!M.getComdatSymbolTable().empty())
    for (GlobalValue &GV : M.global_values())
      handleNonPrevailingComdat(GV, NonPrevailingComdats);

  // Prepend ".lto_discard <sym>, <sym>*" directive to each module inline asm
  // block.
  if (M.hasModuleInlineAsm()) {
    std::string NewIA = ".lto_discard";
    if (!NonPrevailingAsmSymbols.empty()) {
      // Don't dicard a symbol if there is a live .symver for it.
      ModuleSymbolTable::CollectAsmSymvers(
          M, [&](StringRef Name, StringRef Alias) {
            if (!NonPrevailingAsmSymbols.count(Alias))
              NonPrevailingAsmSymbols.erase(Name);
          });
      NewIA += " " + llvm::join(NonPrevailingAsmSymbols, ", ");
    }
    NewIA += "\n";
    M.prependModuleInlineAsm({NewIA, M.getModuleInlineAsm().front().Props});
  }

  assert(MsymI == MsymE);
  return std::make_pair(std::move(Mod), Res);
}

Error LTO::linkRegularLTO(RegularLTOState::AddedModule Mod,
                          bool LivenessFromIndex) {
  llvm::TimeTraceScope timeScope("LTO link regular LTO");
  std::vector<GlobalValue *> Keep;
  for (GlobalValue *GV : Mod.Keep) {
    if (LivenessFromIndex) {
      const auto GUID = GV->getGUIDOrFallback();
      if (!ThinLTO.CombinedIndex.isGUIDLive(GUID)) {
        if (Function *F = dyn_cast<Function>(GV)) {
          if (DiagnosticOutputFile) {
            if (Error Err = F->materialize())
              return Err;
            auto R = OptimizationRemark(DEBUG_TYPE, "deadfunction", F);
            R << ore::NV("Function", F) << " not added to the combined module ";
            emitRemark(R);
          }
        }
        continue;
      }
    }

    if (!GV->hasAvailableExternallyLinkage()) {
      Keep.push_back(GV);
      continue;
    }

    // Only link available_externally definitions if we don't already have a
    // definition.
    GlobalValue *CombinedGV =
        RegularLTO.CombinedModule->getNamedValue(GV->getName());
    if (CombinedGV && !CombinedGV->isDeclaration())
      continue;

    Keep.push_back(GV);
  }

  return RegularLTO.Mover->move(std::move(Mod.M), Keep, nullptr,
                                /* IsPerformingImport */ false);
}

// Add a ThinLTO module to the link.
Expected<ArrayRef<SymbolResolution>>
LTO::addThinLTO(BitcodeModule BM, ArrayRef<InputFile::Symbol> Syms,
                ArrayRef<SymbolResolution> Res) {
  llvm::TimeTraceScope timeScope("LTO add thin LTO");
  const auto BMID = BM.getModuleIdentifier();
  ArrayRef<SymbolResolution> ResTmp = Res;
  DenseSet<StringRef> Prevailing;
  for (const InputFile::Symbol &Sym : Syms) {
    assert(!ResTmp.empty());
    const SymbolResolution &R = ResTmp.consume_front();
    if (!Sym.getIRName().empty() && R.Prevailing)
      Prevailing.insert(Sym.getIRName());
  }

  // Track the GUIDs stored in the bitcode GUID table.
  StringMap<GlobalValue::GUID> IRSpecifiedGUIDs;
  if (Error Err = BM.readSummary(
          ThinLTO.CombinedIndex, BMID,
          [&](StringRef Name) { return (Prevailing.count(Name) > 0); },
          [&](ValueInfo VI) {
            auto IT = IRSpecifiedGUIDs.insert({VI.name(), VI.getGUID()});
            (void)IT;
            assert(IT.second);
            if (auto GRIt = GlobalResolutions->find(VI.name());
                GRIt != GlobalResolutions->end() &&
                Prevailing.count(VI.name())) {
              GRIt->second.setGUID(VI.getGUID());
            }
          }))
    return Err;
  LLVM_DEBUG(dbgs() << "Module " << BMID << "\n");

  for (const InputFile::Symbol &Sym : Syms) {
    assert(!Res.empty());
    const SymbolResolution &R = Res.consume_front();
    auto GUIDIter = IRSpecifiedGUIDs.find(Sym.getIRName());
    // The bitcode GUID table might not be present if this is an old bitcode
    // file. For backwards-compatibility, just compute the GUID now in that
    // case.
    auto GUID =
        GUIDIter == IRSpecifiedGUIDs.end()
            ? GlobalValue::getGUIDAssumingExternalLinkage(
                  GlobalValue::getGlobalIdentifier(
                      Sym.getIRName(), GlobalValue::ExternalLinkage, ""))
            : GUIDIter->second;
    if (!Sym.getIRName().empty() &&
        (R.Prevailing || R.FinalDefinitionInLinkageUnit)) {
      if (R.Prevailing) {
        ThinLTO.setPrevailingModuleForGUID(GUID, BMID);
        // For linker redefined symbols (via --wrap or --defsym) we want to
        // switch the linkage to `weak` to prevent IPOs from happening.
        // Find the summary in the module for this very GV and record the new
        // linkage so that we can switch it when we import the GV.
        if (R.LinkerRedefined)
          if (auto *S = ThinLTO.CombinedIndex.findSummaryInModule(GUID, BMID))
            S->setLinkage(GlobalValue::WeakAnyLinkage);
      }

      // If the linker resolved the symbol to a local definition then mark it
      // as local in the summary for the module we are adding.
      if (R.FinalDefinitionInLinkageUnit) {
        if (auto *S = ThinLTO.CombinedIndex.findSummaryInModule(GUID, BMID)) {
          S->setDSOLocal(true);
        }
      }
    }
  }

  if (!ThinLTO.ModuleMap.insert({BMID, BM}).second)
    return make_error<StringError>(
        "Expected at most one ThinLTO module per bitcode file",
        inconvertibleErrorCode());

  if (!Conf.ThinLTOModulesToCompile.empty()) {
    if (!ThinLTO.ModulesToCompile)
      ThinLTO.ModulesToCompile = ModuleMapType();
    // This is a fuzzy name matching where only modules with name containing the
    // specified switch values are going to be compiled.
    for (const std::string &Name : Conf.ThinLTOModulesToCompile) {
      if (BMID.contains(Name)) {
        ThinLTO.ModulesToCompile->insert({BMID, BM});
        LLVM_DEBUG(dbgs() << "[ThinLTO] Selecting " << BMID << " to compile\n");
        break;
      }
    }
  }

  return Res;
}

unsigned LTO::getMaxTasks() const {
  CalledGetMaxTasks = true;
  auto ModuleCount = ThinLTO.ModulesToCompile ? ThinLTO.ModulesToCompile->size()
                                              : ThinLTO.ModuleMap.size();
  return RegularLTO.ParallelCodeGenParallelismLevel + ModuleCount;
}

// If only some of the modules were split, we cannot correctly handle
// code that contains type tests or type checked loads.
Error LTO::checkPartiallySplit() {
  if (!ThinLTO.CombinedIndex.partiallySplitLTOUnits())
    return Error::success();

  const Module *Combined = RegularLTO.CombinedModule.get();
  Function *TypeTestFunc =
      Intrinsic::getDeclarationIfExists(Combined, Intrinsic::type_test);
  Function *TypeCheckedLoadFunc =
      Intrinsic::getDeclarationIfExists(Combined, Intrinsic::type_checked_load);
  Function *TypeCheckedLoadRelativeFunc = Intrinsic::getDeclarationIfExists(
      Combined, Intrinsic::type_checked_load_relative);

  // First check if there are type tests / type checked loads in the
  // merged regular LTO module IR.
  if ((TypeTestFunc && !TypeTestFunc->use_empty()) ||
      (TypeCheckedLoadFunc && !TypeCheckedLoadFunc->use_empty()) ||
      (TypeCheckedLoadRelativeFunc &&
       !TypeCheckedLoadRelativeFunc->use_empty()))
    return make_error<StringError>(
        "inconsistent LTO Unit splitting (recompile with -fsplit-lto-unit)",
        inconvertibleErrorCode());

  // Otherwise check if there are any recorded in the combined summary from the
  // ThinLTO modules.
  for (auto &P : ThinLTO.CombinedIndex) {
    for (auto &S : P.second.getSummaryList()) {
      auto *FS = dyn_cast<FunctionSummary>(S.get());
      if (!FS)
        continue;
      if (!FS->type_test_assume_vcalls().empty() ||
          !FS->type_checked_load_vcalls().empty() ||
          !FS->type_test_assume_const_vcalls().empty() ||
          !FS->type_checked_load_const_vcalls().empty() ||
          !FS->type_tests().empty())
        return make_error<StringError>(
            "inconsistent LTO Unit splitting (recompile with -fsplit-lto-unit)",
            inconvertibleErrorCode());
    }
  }
  return Error::success();
}

Error LTO::run(AddStreamFn AddStream, FileCache Cache) {
  // Call the base class cleanup() explicitly since run() may be invoked on a
  // derived LTO object.
  llvm::scope_exit CleanUp([this]() { LTO::cleanup(); });

  // Compute "dead" symbols, we don't want to import/export these!
  DenseSet<GlobalValue::GUID> GUIDPreservedSymbols;
  DenseMap<GlobalValue::GUID, PrevailingType> GUIDPrevailingResolutions;
  for (auto &Res : *GlobalResolutions) {
    // Normally resolution have IR name of symbol. We can do nothing here
    // otherwise. See comments in GlobalResolution struct for more details.
    if (Res.second.IRName.empty())
      continue;

    GlobalValue::GUID GUID = Res.second.getGUID();

    if (Res.second.VisibleOutsideSummary && Res.second.Prevailing)
      GUIDPreservedSymbols.insert(GUID);

    if (Res.second.ExportDynamic)
      DynamicExportSymbols.insert(GUID);

    GUIDPrevailingResolutions[GUID] =
        Res.second.Prevailing ? PrevailingType::Yes : PrevailingType::No;
  }

  auto isPrevailing = [&](GlobalValue::GUID G) {
    auto It = GUIDPrevailingResolutions.find(G);
    if (It == GUIDPrevailingResolutions.end())
      return PrevailingType::Unknown;
    return It->second;
  };
  computeDeadSymbolsWithConstProp(ThinLTO.CombinedIndex, GUIDPreservedSymbols,
                                  isPrevailing, Conf.OptLevel > 0);

  // Setup output file to emit statistics.
  auto StatsFileOrErr = setupStatsFile(Conf.StatsFile);
  if (!StatsFileOrErr)
    return StatsFileOrErr.takeError();
  std::unique_ptr<ToolOutputFile> StatsFile = std::move(StatsFileOrErr.get());

  if (Error Err = setupOptimizationRemarks())
    return Err;

  // TODO: Ideally this would be controlled automatically by detecting that we
  // are linking with an allocator that supports these interfaces, rather than
  // an internal option (which would still be needed for tests, however). For
  // example, if the library exported a symbol like __malloc_hot_cold the linker
  // could recognize that and set a flag in the lto::Config.
  if (SupportsHotColdNew)
    ThinLTO.CombinedIndex.setWithSupportsHotColdNew();

  Error Result = runRegularLTO(AddStream);
  if (!Result)
    // This will reset the GlobalResolutions optional once done with it to
    // reduce peak memory before importing.
    Result = runThinLTO(AddStream, Cache, GUIDPreservedSymbols);

  if (StatsFile)
    PrintStatisticsJSON(StatsFile->os());

  return Result;
}

Error LTO::runRegularLTO(AddStreamFn AddStream) {
  llvm::TimeTraceScope timeScope("Run regular LTO");
  LLVM_DEBUG(dbgs() << "Running regular LTO\n");

  // Finalize linking of regular LTO modules containing summaries now that
  // we have computed liveness information.
  {
    llvm::TimeTraceScope timeScope("Link regular LTO");
    for (auto &M : RegularLTO.ModsWithSummaries)
      if (Error Err = linkRegularLTO(std::move(M), /*LivenessFromIndex=*/true))
        return Err;
  }

  // Ensure we don't have inconsistently split LTO units with type tests.
  // FIXME: this checks both LTO and ThinLTO. It happens to work as we take
  // this path both cases but eventually this should be split into two and
  // do the ThinLTO checks in `runThinLTO`.
  if (Error Err = checkPartiallySplit())
    return Err;

  // Make sure commons have the right size/alignment: we kept the largest from
  // all the prevailing when adding the inputs, and we apply it here.
  const DataLayout &DL = RegularLTO.CombinedModule->getDataLayout();
  for (auto &I : RegularLTO.Commons) {
    if (!I.second.Prevailing)
      // Don't do anything if no instance of this common was prevailing.
      continue;
    GlobalVariable *OldGV = RegularLTO.CombinedModule->getNamedGlobal(I.first);
    if (OldGV && OldGV->getGlobalSize(DL) == I.second.Size) {
      // Don't create a new global if the type is already correct, just make
      // sure the alignment is correct.
      OldGV->setAlignment(I.second.Alignment);
      continue;
    }
    ArrayType *Ty =
        ArrayType::get(Type::getInt8Ty(RegularLTO.Ctx), I.second.Size);
    auto *GV = new GlobalVariable(*RegularLTO.CombinedModule, Ty, false,
                                  GlobalValue::CommonLinkage,
                                  ConstantAggregateZero::get(Ty), "");
    GV->setAlignment(I.second.Alignment);
    if (OldGV) {
      OldGV->replaceAllUsesWith(GV);
      GV->takeName(OldGV);
      OldGV->eraseFromParent();
    } else {
      GV->setName(I.first);
    }
  }

  bool WholeProgramVisibilityEnabledInLTO =
      Conf.HasWholeProgramVisibility &&
      // If validation is enabled, upgrade visibility only when all vtables
      // have typeinfos.
      (!Conf.ValidateAllVtablesHaveTypeInfos || Conf.AllVtablesHaveTypeInfos);

  // This returns true when the name is local or not defined. Locals are
  // expected to be handled separately.
  auto IsVisibleToRegularObj = [&](StringRef name) {
    auto It = GlobalResolutions->find(name);
    return (It == GlobalResolutions->end() ||
            It->second.VisibleOutsideSummary || !It->second.Prevailing);
  };

  // If allowed, upgrade public vcall visibility metadata to linkage unit
  // visibility before whole program devirtualization in the optimizer.
  updateVCallVisibilityInModule(
      *RegularLTO.CombinedModule, WholeProgramVisibilityEnabledInLTO,
      DynamicExportSymbols, Conf.ValidateAllVtablesHaveTypeInfos,
      IsVisibleToRegularObj);
  updatePublicTypeTestCalls(*RegularLTO.CombinedModule,
                            WholeProgramVisibilityEnabledInLTO);

  if (Conf.PreOptModuleHook &&
      !Conf.PreOptModuleHook(0, *RegularLTO.CombinedModule))
    return Error::success();

  if (!Conf.CodeGenOnly) {
    for (const auto &R : *GlobalResolutions) {
      GlobalValue *GV =
          RegularLTO.CombinedModule->getNamedValue(R.second.IRName);
      if (!R.second.isPrevailingIRSymbol())
        continue;
      if (R.second.Partition != 0 &&
          R.second.Partition != GlobalResolution::External)
        continue;

      // Ignore symbols defined in other partitions.
      // Also skip declarations, which are not allowed to have internal linkage.
      if (!GV || GV->hasLocalLinkage() || GV->isDeclaration())
        continue;

      // Symbols that are marked DLLImport or DLLExport should not be
      // internalized, as they are either externally visible or referencing
      // external symbols. Symbols that have AvailableExternally or Appending
      // linkage might be used by future passes and should be kept as is.
      // These linkages are seen in Unified regular LTO, because the process
      // of creating split LTO units introduces symbols with that linkage into
      // one of the created modules. Normally, only the ThinLTO backend would
      // compile this module, but Unified Regular LTO processes both
      // modules created by the splitting process as regular LTO modules.
      if ((LTOMode == LTOKind::LTOK_UnifiedRegular) &&
          ((GV->getDLLStorageClass() != GlobalValue::DefaultStorageClass) ||
           GV->hasAvailableExternallyLinkage() || GV->hasAppendingLinkage()))
        continue;

      GV->setUnnamedAddr(R.second.UnnamedAddr ? GlobalValue::UnnamedAddr::Global
                                              : GlobalValue::UnnamedAddr::None);
      if (EnableLTOInternalization && R.second.Partition == 0)
        GV->setLinkage(GlobalValue::InternalLinkage);
    }

    if (Conf.PostInternalizeModuleHook &&
        !Conf.PostInternalizeModuleHook(0, *RegularLTO.CombinedModule))
      return Error::success();
  }

  if (!RegularLTO.EmptyCombinedModule || Conf.AlwaysEmitRegularLTOObj) {
    if (Error Err = backend(
            Conf, AddStream, RegularLTO.ParallelCodeGenParallelismLevel,
            *RegularLTO.CombinedModule, ThinLTO.CombinedIndex, BitcodeLibFuncs))
      return Err;
  }

  return Error::success();
}

SmallVector<const char *> LTO::getRuntimeLibcallSymbols(const Triple &TT) {
  RTLIB::RuntimeLibcallsInfo Libcalls(TT);
  SmallVector<const char *> LibcallSymbols;
  LibcallSymbols.reserve(Libcalls.getNumAvailableLibcallImpls());

  for (RTLIB::LibcallImpl Impl : RTLIB::libcall_impls()) {
    if (Libcalls.isAvailable(Impl))
      LibcallSymbols.push_back(Libcalls.getLibcallImplName(Impl).data());
  }

  return LibcallSymbols;
}

SmallVector<StringRef> LTO::getLibFuncSymbols(const Triple &TT,
                                              StringSaver &Saver) {
  auto TLII = std::make_unique<TargetLibraryInfoImpl>(TT);
  TargetLibraryInfo TLI(*TLII);
  SmallVector<StringRef> LibFuncSymbols;
  LibFuncSymbols.reserve(LibFunc::NumLibFuncs);
  for (unsigned I = LibFunc::Begin_LibFunc; I != LibFunc::End_LibFunc; ++I) {
    LibFunc F = static_cast<LibFunc>(I);
    if (TLI.has(F))
      LibFuncSymbols.push_back(Saver.save(TLI.getName(F)).data());
  }
  return LibFuncSymbols;
}

Error ThinBackendProc::emitFiles(
    const FunctionImporter::ImportMapTy &ImportList, unsigned Task,
    llvm::StringRef ModulePath, const std::string &NewModulePath) const {
  return emitFiles(ImportList, Task, ModulePath, NewModulePath,
                   NewModulePath + ".thinlto.bc");
}

Error ThinBackendProc::emitFiles(
    const FunctionImporter::ImportMapTy &ImportList, unsigned Task,
    llvm::StringRef ModulePath, const std::string &NewModulePath,
    StringRef SummaryPath) const {
  ModuleToSummariesForIndexTy ModuleToSummariesForIndex;
  GVSummaryPtrSet DeclarationSummaries;

  std::error_code EC;
  gatherImportedSummariesForModule(ModulePath, ModuleToDefinedGVSummaries,
                                   ImportList, ModuleToSummariesForIndex,
                                   DeclarationSummaries);
  // Resolve the output stream (either file-backed or callback-provided) for the
  // index file.
  std::unique_ptr<raw_pwrite_stream> OS;
  if (Conf.GetSummaryIndexOutputStream) {
    OS = Conf.GetSummaryIndexOutputStream(Task);
    assert(OS && "GetSummaryIndexOutputStream returned null");
  } else {
    auto FileOS = std::make_unique<raw_fd_ostream>(SummaryPath, EC,
                                                   sys::fs::OpenFlags::OF_None);
    if (EC)
      return createFileError("cannot open " + Twine(SummaryPath), EC);
    OS = std::move(FileOS);
  }

  writeIndexToFile(CombinedIndex, *OS, &ModuleToSummariesForIndex,
                   &DeclarationSummaries);

  // Emit imports files if requested, using callback if provided.
  if (Conf.GetImportsListOutputArray) {
    std::vector<std::string> &ImportsListRef =
        Conf.GetImportsListOutputArray(Task);
    processImportsFiles(
        ModulePath, ModuleToSummariesForIndex,
        [&](StringRef M) { ImportsListRef.push_back(M.str()); });
  } else if (ShouldEmitImportsFiles) {
    if (Error E = EmitImportsFiles(ModulePath, NewModulePath + ".imports",
                                   ModuleToSummariesForIndex))
      return E;
  }
  return Error::success();
}

namespace {
/// Base class for ThinLTO backends that perform code generation and insert the
/// generated files back into the link.
class CGThinBackend : public ThinBackendProc {
protected:
  DenseSet<GlobalValue::GUID> CfiFunctionDefs;
  DenseSet<GlobalValue::GUID> CfiFunctionDecls;
  bool ShouldEmitIndexFiles;

public:
  CGThinBackend(
      const Config &Conf, ModuleSummaryIndex &CombinedIndex,
      const DenseMap<StringRef, GVSummaryMapTy> &ModuleToDefinedGVSummaries,
      lto::IndexWriteCallback OnWrite, bool ShouldEmitIndexFiles,
      bool ShouldEmitImportsFiles, ThreadPoolStrategy ThinLTOParallelism)
      : ThinBackendProc(Conf, CombinedIndex, ModuleToDefinedGVSummaries,
                        OnWrite, ShouldEmitImportsFiles, ThinLTOParallelism),
        ShouldEmitIndexFiles(ShouldEmitIndexFiles) {
    auto &Defs = CombinedIndex.cfiFunctionDefs();
    CfiFunctionDefs.insert_range(Defs.getExportedThinLTOGUIDs());
    auto &Decls = CombinedIndex.cfiFunctionDecls();
    CfiFunctionDecls.insert_range(Decls.getExportedThinLTOGUIDs());
  }
};

/// This backend performs code generation by scheduling a job to run on
/// an in-process thread when invoked for each task.
class InProcessThinBackend : public CGThinBackend {
protected:
  // Callback used to add generated native object files to the link by code
  // generating directly into the returned output stream.
  AddStreamFn AddStream;
  FileCache Cache;
  ArrayRef<StringRef> BitcodeLibFuncs;

public:
  InProcessThinBackend(
      const Config &Conf, ModuleSummaryIndex &CombinedIndex,
      ThreadPoolStrategy ThinLTOParallelism,
      const DenseMap<StringRef, GVSummaryMapTy> &ModuleToDefinedGVSummaries,
      AddStreamFn AddStream, FileCache Cache, lto::IndexWriteCallback OnWrite,
      bool ShouldEmitIndexFiles, bool ShouldEmitImportsFiles,
      ArrayRef<StringRef> BitcodeLibFuncs)
      : CGThinBackend(Conf, CombinedIndex, ModuleToDefinedGVSummaries, OnWrite,
                      ShouldEmitIndexFiles, ShouldEmitImportsFiles,
                      ThinLTOParallelism),
        AddStream(std::move(AddStream)), Cache(std::move(Cache)),
        BitcodeLibFuncs(BitcodeLibFuncs) {}

  virtual Error runThinLTOBackendThread(
      AddStreamFn AddStream, FileCache Cache, unsigned Task, BitcodeModule BM,
      ModuleSummaryIndex &CombinedIndex,
      const FunctionImporter::ImportMapTy &ImportList,
      const FunctionImporter::ExportSetTy &ExportList,
      const std::map<GlobalValue::GUID, GlobalValue::LinkageTypes> &ResolvedODR,
      const GVSummaryMapTy &DefinedGlobals,
      MapVector<StringRef, BitcodeModule> &ModuleMap) {
    auto ModuleID = BM.getModuleIdentifier();
    llvm::TimeTraceScope timeScope("Run ThinLTO backend thread (in-process)",
                                   ModuleID);
    auto RunThinBackend = [&](AddStreamFn AddStream) {
      LTOLLVMContext BackendContext(Conf);
      Expected<std::unique_ptr<Module>> MOrErr = BM.parseModule(BackendContext);
      if (!MOrErr)
        return MOrErr.takeError();

      return thinBackend(Conf, Task, AddStream, **MOrErr, CombinedIndex,
                         ImportList, DefinedGlobals, &ModuleMap,
                         Conf.CodeGenOnly, BitcodeLibFuncs);
    };
    if (ShouldEmitIndexFiles) {
      if (auto E = emitFiles(ImportList, Task, ModuleID, ModuleID.str()))
        return E;
    }

    if (!Cache.isValid() || !CombinedIndex.modulePaths().count(ModuleID) ||
        all_of(CombinedIndex.getModuleHash(ModuleID),
               [](uint32_t V) { return V == 0; }))
      // Cache disabled or no entry for this module in the combined index or
      // no module hash.
      return RunThinBackend(AddStream);

    // The module may be cached, this helps handling it.
    std::string Key = computeLTOCacheKey(
        Conf, CombinedIndex, ModuleID, ImportList, ExportList, ResolvedODR,
        DefinedGlobals, CfiFunctionDefs, CfiFunctionDecls);
    Expected<AddStreamFn> CacheAddStreamOrErr = Cache(Task, Key, ModuleID);
    if (Error Err = CacheAddStreamOrErr.takeError())
      return Err;
    AddStreamFn &CacheAddStream = *CacheAddStreamOrErr;
    if (CacheAddStream)
      return RunThinBackend(CacheAddStream);

    return Error::success();
  }

  Error start(
      unsigned Task, BitcodeModule BM,
      const FunctionImporter::ImportMapTy &ImportList,
      const FunctionImporter::ExportSetTy &ExportList,
      const std::map<GlobalValue::GUID, GlobalValue::LinkageTypes> &ResolvedODR,
      MapVector<StringRef, BitcodeModule> &ModuleMap) override {
    StringRef ModulePath = BM.getModuleIdentifier();
    assert(ModuleToDefinedGVSummaries.count(ModulePath));
    const GVSummaryMapTy &DefinedGlobals =
        ModuleToDefinedGVSummaries.find(ModulePath)->second;
    BackendThreadPool.async(
        [=](BitcodeModule BM, ModuleSummaryIndex &CombinedIndex,
            const FunctionImporter::ImportMapTy &ImportList,
            const FunctionImporter::ExportSetTy &ExportList,
            const std::map<GlobalValue::GUID, GlobalValue::LinkageTypes>
                &ResolvedODR,
            const GVSummaryMapTy &DefinedGlobals,
            MapVector<StringRef, BitcodeModule> &ModuleMap) {
          if (LLVM_ENABLE_THREADS && Conf.TimeTraceEnabled)
            timeTraceProfilerInitialize(Conf.TimeTraceGranularity,
                                        "thin backend");
          Error E = runThinLTOBackendThread(
              AddStream, Cache, Task, BM, CombinedIndex, ImportList, ExportList,
              ResolvedODR, DefinedGlobals, ModuleMap);
          if (E) {
            std::unique_lock<std::mutex> L(ErrMu);
            if (Err)
              Err = joinErrors(std::move(*Err), std::move(E));
            else
              Err = std::move(E);
          }
          if (LLVM_ENABLE_THREADS && Conf.TimeTraceEnabled)
            timeTraceProfilerFinishThread();
        },
        BM, std::ref(CombinedIndex), std::ref(ImportList), std::ref(ExportList),
        std::ref(ResolvedODR), std::ref(DefinedGlobals), std::ref(ModuleMap));

    if (OnWrite)
      OnWrite(std::string(ModulePath));
    return Error::success();
  }
};

/// This backend is utilized in the first round of a two-codegen round process.
/// It first saves optimized bitcode files to disk before the codegen process
/// begins. After codegen, it stores the resulting object files in a scratch
/// buffer. Note the codegen data stored in the scratch buffer will be extracted
/// and merged in the subsequent step.
class FirstRoundThinBackend : public InProcessThinBackend {
  AddStreamFn IRAddStream;
  FileCache IRCache;

public:
  FirstRoundThinBackend(
      const Config &Conf, ModuleSummaryIndex &CombinedIndex,
      ThreadPoolStrategy ThinLTOParallelism,
      const DenseMap<StringRef, GVSummaryMapTy> &ModuleToDefinedGVSummaries,
      AddStreamFn CGAddStream, FileCache CGCache,
      ArrayRef<StringRef> BitcodeLibFuncs, AddStreamFn IRAddStream,
      FileCache IRCache)
      : InProcessThinBackend(Conf, CombinedIndex, ThinLTOParallelism,
                             ModuleToDefinedGVSummaries, std::move(CGAddStream),
                             std::move(CGCache), /*OnWrite=*/nullptr,
                             /*ShouldEmitIndexFiles=*/false,
                             /*ShouldEmitImportsFiles=*/false, BitcodeLibFuncs),
        IRAddStream(std::move(IRAddStream)), IRCache(std::move(IRCache)) {}

  Error runThinLTOBackendThread(
      AddStreamFn CGAddStream, FileCache CGCache, unsigned Task,
      BitcodeModule BM, ModuleSummaryIndex &CombinedIndex,
      const FunctionImporter::ImportMapTy &ImportList,
      const FunctionImporter::ExportSetTy &ExportList,
      const std::map<GlobalValue::GUID, GlobalValue::LinkageTypes> &ResolvedODR,
      const GVSummaryMapTy &DefinedGlobals,
      MapVector<StringRef, BitcodeModule> &ModuleMap) override {
    auto ModuleID = BM.getModuleIdentifier();
    llvm::TimeTraceScope timeScope("Run ThinLTO backend thread (first round)",
                                   ModuleID);
    auto RunThinBackend = [&](AddStreamFn CGAddStream,
                              AddStreamFn IRAddStream) {
      LTOLLVMContext BackendContext(Conf);
      Expected<std::unique_ptr<Module>> MOrErr = BM.parseModule(BackendContext);
      if (!MOrErr)
        return MOrErr.takeError();

      return thinBackend(Conf, Task, CGAddStream, **MOrErr, CombinedIndex,
                         ImportList, DefinedGlobals, &ModuleMap,
                         Conf.CodeGenOnly, BitcodeLibFuncs, IRAddStream);
    };
    // Like InProcessThinBackend, we produce index files as needed for
    // FirstRoundThinBackend. However, these files are not generated for
    // SecondRoundThinBackend.
    if (ShouldEmitIndexFiles) {
      if (auto E = emitFiles(ImportList, Task, ModuleID, ModuleID.str()))
        return E;
    }

    assert((CGCache.isValid() == IRCache.isValid()) &&
           "Both caches for CG and IR should have matching availability");
    if (!CGCache.isValid() || !CombinedIndex.modulePaths().count(ModuleID) ||
        all_of(CombinedIndex.getModuleHash(ModuleID),
               [](uint32_t V) { return V == 0; }))
      // Cache disabled or no entry for this module in the combined index or
      // no module hash.
      return RunThinBackend(CGAddStream, IRAddStream);

    // Get CGKey for caching object in CGCache.
    std::string CGKey = computeLTOCacheKey(
        Conf, CombinedIndex, ModuleID, ImportList, ExportList, ResolvedODR,
        DefinedGlobals, CfiFunctionDefs, CfiFunctionDecls);
    Expected<AddStreamFn> CacheCGAddStreamOrErr =
        CGCache(Task, CGKey, ModuleID);
    if (Error Err = CacheCGAddStreamOrErr.takeError())
      return Err;
    AddStreamFn &CacheCGAddStream = *CacheCGAddStreamOrErr;

    // Get IRKey for caching (optimized) IR in IRCache with an extra ID.
    std::string IRKey = recomputeLTOCacheKey(CGKey, /*ExtraID=*/"IR");
    Expected<AddStreamFn> CacheIRAddStreamOrErr =
        IRCache(Task, IRKey, ModuleID);
    if (Error Err = CacheIRAddStreamOrErr.takeError())
      return Err;
    AddStreamFn &CacheIRAddStream = *CacheIRAddStreamOrErr;

    // Ideally, both CG and IR caching should be synchronized. However, in
    // practice, their availability may differ due to different expiration
    // times. Therefore, if either cache is missing, the backend process is
    // triggered.
    if (CacheCGAddStream || CacheIRAddStream) {
      LLVM_DEBUG(dbgs() << "[FirstRound] Cache Miss for "
                        << BM.getModuleIdentifier() << "\n");
      return RunThinBackend(CacheCGAddStream ? CacheCGAddStream : CGAddStream,
                            CacheIRAddStream ? CacheIRAddStream : IRAddStream);
    }

    return Error::success();
  }
};

/// This backend operates in the second round of a two-codegen round process.
/// It starts by reading the optimized bitcode files that were saved during the
/// first round. The backend then executes the codegen only to further optimize
/// the code, utilizing the codegen data merged from the first round. Finally,
/// it writes the resulting object files as usual.
class SecondRoundThinBackend : public InProcessThinBackend {
  std::unique_ptr<SmallVector<StringRef>> IRFiles;
  stable_hash CombinedCGDataHash;

public:
  SecondRoundThinBackend(
      const Config &Conf, ModuleSummaryIndex &CombinedIndex,
      ThreadPoolStrategy ThinLTOParallelism,
      const DenseMap<StringRef, GVSummaryMapTy> &ModuleToDefinedGVSummaries,
      AddStreamFn AddStream, FileCache Cache,
      ArrayRef<StringRef> BitcodeLibFuncs,
      std::unique_ptr<SmallVector<StringRef>> IRFiles,
      stable_hash CombinedCGDataHash)
      : InProcessThinBackend(Conf, CombinedIndex, ThinLTOParallelism,
                             ModuleToDefinedGVSummaries, std::move(AddStream),
                             std::move(Cache),
                             /*OnWrite=*/nullptr,
                             /*ShouldEmitIndexFiles=*/false,
                             /*ShouldEmitImportsFiles=*/false, BitcodeLibFuncs),
        IRFiles(std::move(IRFiles)), CombinedCGDataHash(CombinedCGDataHash) {}

  Error runThinLTOBackendThread(
      AddStreamFn AddStream, FileCache Cache, unsigned Task, BitcodeModule BM,
      ModuleSummaryIndex &CombinedIndex,
      const FunctionImporter::ImportMapTy &ImportList,
      const FunctionImporter::ExportSetTy &ExportList,
      const std::map<GlobalValue::GUID, GlobalValue::LinkageTypes> &ResolvedODR,
      const GVSummaryMapTy &DefinedGlobals,
      MapVector<StringRef, BitcodeModule> &ModuleMap) override {
    auto ModuleID = BM.getModuleIdentifier();
    llvm::TimeTraceScope timeScope("Run ThinLTO backend thread (second round)",
                                   ModuleID);
    auto RunThinBackend = [&](AddStreamFn AddStream) {
      LTOLLVMContext BackendContext(Conf);
      std::unique_ptr<Module> LoadedModule =
          cgdata::loadModuleForTwoRounds(BM, Task, BackendContext, *IRFiles);

      return thinBackend(Conf, Task, AddStream, *LoadedModule, CombinedIndex,
                         ImportList, DefinedGlobals, &ModuleMap,
                         /*CodeGenOnly=*/true, BitcodeLibFuncs);
    };
    if (!Cache.isValid() || !CombinedIndex.modulePaths().count(ModuleID) ||
        all_of(CombinedIndex.getModuleHash(ModuleID),
               [](uint32_t V) { return V == 0; }))
      // Cache disabled or no entry for this module in the combined index or
      // no module hash.
      return RunThinBackend(AddStream);

    // Get Key for caching the final object file in Cache with the combined
    // CGData hash.
    std::string Key = computeLTOCacheKey(
        Conf, CombinedIndex, ModuleID, ImportList, ExportList, ResolvedODR,
        DefinedGlobals, CfiFunctionDefs, CfiFunctionDecls);
    Key = recomputeLTOCacheKey(Key,
                               /*ExtraID=*/std::to_string(CombinedCGDataHash));
    Expected<AddStreamFn> CacheAddStreamOrErr = Cache(Task, Key, ModuleID);
    if (Error Err = CacheAddStreamOrErr.takeError())
      return Err;
    AddStreamFn &CacheAddStream = *CacheAddStreamOrErr;

    if (CacheAddStream) {
      LLVM_DEBUG(dbgs() << "[SecondRound] Cache Miss for "
                        << BM.getModuleIdentifier() << "\n");
      return RunThinBackend(CacheAddStream);
    }

    return Error::success();
  }
};
} // end anonymous namespace

ThinBackend lto::createInProcessThinBackend(ThreadPoolStrategy Parallelism,
                                            lto::IndexWriteCallback OnWrite,
                                            bool ShouldEmitIndexFiles,
                                            bool ShouldEmitImportsFiles) {
  auto Func =
      [=](const Config &Conf, ModuleSummaryIndex &CombinedIndex,
          const DenseMap<StringRef, GVSummaryMapTy> &ModuleToDefinedGVSummaries,
          AddStreamFn AddStream, FileCache Cache,
          ArrayRef<StringRef> BitcodeLibFuncs) {
        return std::make_unique<InProcessThinBackend>(
            Conf, CombinedIndex, Parallelism, ModuleToDefinedGVSummaries,
            AddStream, Cache, OnWrite, ShouldEmitIndexFiles,
            ShouldEmitImportsFiles, BitcodeLibFuncs);
      };
  return ThinBackend(Func, Parallelism);
}

StringLiteral lto::getThinLTODefaultCPU(const Triple &TheTriple) {
  if (!TheTriple.isOSDarwin())
    return "";
  if (TheTriple.getArch() == Triple::x86_64)
    return "core2";
  if (TheTriple.getArch() == Triple::x86)
    return "yonah";
  if (TheTriple.isArm64e())
    return "apple-a12";
  if (TheTriple.getArch() == Triple::aarch64 ||
      TheTriple.getArch() == Triple::aarch64_32)
    return "cyclone";
  return "";
}

// Given the original \p Path to an output file, replace any path
// prefix matching \p OldPrefix with \p NewPrefix. Also, create the
// resulting directory if it does not yet exist.
std::string lto::getThinLTOOutputFile(StringRef Path, StringRef OldPrefix,
                                      StringRef NewPrefix) {
  if (OldPrefix.empty() && NewPrefix.empty())
    return std::string(Path);
  SmallString<128> NewPath(Path);
  llvm::sys::path::replace_path_prefix(NewPath, OldPrefix, NewPrefix);
  StringRef ParentPath = llvm::sys::path::parent_path(NewPath.str());
  if (!ParentPath.empty()) {
    // Make sure the new directory exists, creating it if necessary.
    if (std::error_code EC = llvm::sys::fs::create_directories(ParentPath))
      llvm::errs() << "warning: could not create directory '" << ParentPath
                   << "': " << EC.message() << '\n';
  }
  return std::string(NewPath);
}

namespace {
class WriteIndexesThinBackend : public ThinBackendProc {
  std::string OldPrefix, NewPrefix, NativeObjectPrefix;
  raw_fd_ostream *LinkedObjectsFile;
  DenseSet<GlobalValue::GUID> CfiFunctionDefs;
  DenseSet<GlobalValue::GUID> CfiFunctionDecls;

public:
  WriteIndexesThinBackend(
      const Config &Conf, ModuleSummaryIndex &CombinedIndex,
      ThreadPoolStrategy ThinLTOParallelism,
      const DenseMap<StringRef, GVSummaryMapTy> &ModuleToDefinedGVSummaries,
      std::string OldPrefix, std::string NewPrefix,
      std::string NativeObjectPrefix, bool ShouldEmitImportsFiles,
      raw_fd_ostream *LinkedObjectsFile, lto::IndexWriteCallback OnWrite)
      : ThinBackendProc(Conf, CombinedIndex, ModuleToDefinedGVSummaries,
                        OnWrite, ShouldEmitImportsFiles, ThinLTOParallelism),
        OldPrefix(OldPrefix), NewPrefix(NewPrefix),
        NativeObjectPrefix(NativeObjectPrefix),
        LinkedObjectsFile(LinkedObjectsFile) {
    auto Defs = CombinedIndex.cfiFunctionDefs().getExportedThinLTOGUIDs();
    CfiFunctionDefs.insert(Defs.begin(), Defs.end());
    auto Decls = CombinedIndex.cfiFunctionDecls().getExportedThinLTOGUIDs();
    CfiFunctionDecls.insert(Decls.begin(), Decls.end());
  }

  Error start(
      unsigned Task, BitcodeModule BM,
      const FunctionImporter::ImportMapTy &ImportList,
      const FunctionImporter::ExportSetTy &ExportList,
      const std::map<GlobalValue::GUID, GlobalValue::LinkageTypes> &ResolvedODR,
      MapVector<StringRef, BitcodeModule> &ModuleMap) override {
    StringRef ModulePath = BM.getModuleIdentifier();

    // The contents of this file may be used as input to a native link, and must
    // therefore contain the processed modules in a determinstic order that
    // match the order they are provided on the command line. For that reason,
    // we cannot include this in the asynchronously executed lambda below.
    if (LinkedObjectsFile) {
      std::string ObjectPrefix =
          NativeObjectPrefix.empty() ? NewPrefix : NativeObjectPrefix;
      std::string LinkedObjectsFilePath =
          getThinLTOOutputFile(ModulePath, OldPrefix, ObjectPrefix);
      *LinkedObjectsFile << LinkedObjectsFilePath << '\n';
    }

    BackendThreadPool.async(
        [this](unsigned Task, const StringRef ModulePath,
               const FunctionImporter::ImportMapTy &ImportList,
               const FunctionImporter::ExportSetTy &ExportList,
               const std::map<GlobalValue::GUID, GlobalValue::LinkageTypes>
                   &ResolvedODR,
               const std::string &OldPrefix, const std::string &NewPrefix) {
          std::string NewModulePath =
              getThinLTOOutputFile(ModulePath, OldPrefix, NewPrefix);
          auto E = emitFiles(ImportList, Task, ModulePath, NewModulePath);
          if (E) {
            std::unique_lock<std::mutex> L(ErrMu);
            if (Err)
              Err = joinErrors(std::move(*Err), std::move(E));
            else
              Err = std::move(E);
          }
          assert(ModuleToDefinedGVSummaries.count(ModulePath));
          const GVSummaryMapTy &DefinedGlobals =
              ModuleToDefinedGVSummaries.find(ModulePath)->second;

          // DTLTO needs the per-module LTO cache key to probe the cache.
          if (Conf.GetCacheKeyOutputString) {
            std::string &CacheKey = Conf.GetCacheKeyOutputString(Task);
            CacheKey = computeLTOCacheKey(
                Conf, CombinedIndex, ModulePath, ImportList, ExportList,
                ResolvedODR, DefinedGlobals, CfiFunctionDefs, CfiFunctionDecls);
          }
        },
        Task, ModulePath, ImportList, ExportList, ResolvedODR, OldPrefix,
        NewPrefix);

    if (OnWrite)
      OnWrite(std::string(ModulePath));
    return Error::success();
  }

  bool isSensitiveToInputOrder() override {
    // The order which modules are written to LinkedObjectsFile should be
    // deterministic and match the order they are passed on the command line.
    return true;
  }
};
} // end anonymous namespace

ThinBackend lto::createWriteIndexesThinBackend(
    ThreadPoolStrategy Parallelism, std::string OldPrefix,
    std::string NewPrefix, std::string NativeObjectPrefix,
    bool ShouldEmitImportsFiles, raw_fd_ostream *LinkedObjectsFile,
    IndexWriteCallback OnWrite) {
  auto Func =
      [=](const Config &Conf, ModuleSummaryIndex &CombinedIndex,
          const DenseMap<StringRef, GVSummaryMapTy> &ModuleToDefinedGVSummaries,
          AddStreamFn AddStream, FileCache Cache,
          ArrayRef<StringRef> BitcodeLibFuncs) {
        return std::make_unique<WriteIndexesThinBackend>(
            Conf, CombinedIndex, Parallelism, ModuleToDefinedGVSummaries,
            OldPrefix, NewPrefix, NativeObjectPrefix, ShouldEmitImportsFiles,
            LinkedObjectsFile, OnWrite);
      };
  return ThinBackend(Func, Parallelism);
}

Error LTO::runThinLTO(AddStreamFn AddStream, FileCache Cache,
                      const DenseSet<GlobalValue::GUID> &GUIDPreservedSymbols) {
  llvm::TimeTraceScope timeScope("Run ThinLTO");
  LLVM_DEBUG(dbgs() << "Running ThinLTO\n");
  ThinLTO.CombinedIndex.releaseTemporaryMemory();
  timeTraceProfilerBegin("ThinLink", StringRef(""));
  llvm::scope_exit TimeTraceScopeExit([]() {
    if (llvm::timeTraceProfilerEnabled())
      llvm::timeTraceProfilerEnd();
  });
  if (ThinLTO.ModuleMap.empty())
    return Error::success();

  if (ThinLTO.ModulesToCompile && ThinLTO.ModulesToCompile->empty()) {
    llvm::errs() << "warning: [ThinLTO] No module compiled\n";
    return Error::success();
  }

  if (Conf.CombinedIndexHook &&
      !Conf.CombinedIndexHook(ThinLTO.CombinedIndex, GUIDPreservedSymbols))
    return Error::success();

  // Collect for each module the list of function it defines (GUID ->
  // Summary).
  DenseMap<StringRef, GVSummaryMapTy> ModuleToDefinedGVSummaries(
      ThinLTO.ModuleMap.size());
  ThinLTO.CombinedIndex.collectDefinedGVSummariesPerModule(
      ModuleToDefinedGVSummaries);
  // Create entries for any modules that didn't have any GV summaries
  // (either they didn't have any GVs to start with, or we suppressed
  // generation of the summaries because they e.g. had inline assembly
  // uses that couldn't be promoted/renamed on export). This is so
  // InProcessThinBackend::start can still launch a backend thread, which
  // is passed the map of summaries for the module, without any special
  // handling for this case.
  for (auto &Mod : ThinLTO.ModuleMap)
    if (!ModuleToDefinedGVSummaries.count(Mod.first))
      ModuleToDefinedGVSummaries.try_emplace(Mod.first);

  FunctionImporter::ImportListsTy ImportLists(ThinLTO.ModuleMap.size());
  DenseMap<StringRef, FunctionImporter::ExportSetTy> ExportLists(
      ThinLTO.ModuleMap.size());
  StringMap<std::map<GlobalValue::GUID, GlobalValue::LinkageTypes>> ResolvedODR;

  if (DumpThinCGSCCs)
    ThinLTO.CombinedIndex.dumpSCCs(outs());

  std::set<GlobalValue::GUID> ExportedGUIDs;

  bool WholeProgramVisibilityEnabledInLTO =
      Conf.HasWholeProgramVisibility &&
      // If validation is enabled, upgrade visibility only when all vtables
      // have typeinfos.
      (!Conf.ValidateAllVtablesHaveTypeInfos || Conf.AllVtablesHaveTypeInfos);
  if (hasWholeProgramVisibility(WholeProgramVisibilityEnabledInLTO))
    ThinLTO.CombinedIndex.setWithWholeProgramVisibility();

  // If we're validating, get the vtable symbols that should not be
  // upgraded because they correspond to typeIDs outside of index-based
  // WPD info.
  DenseSet<GlobalValue::GUID> VisibleToRegularObjSymbols;
  if (WholeProgramVisibilityEnabledInLTO &&
      Conf.ValidateAllVtablesHaveTypeInfos) {
    // This returns true when the name is local or not defined. Locals are
    // expected to be handled separately.
    auto IsVisibleToRegularObj = [&](StringRef name) {
      auto It = GlobalResolutions->find(name);
      return (It == GlobalResolutions->end() ||
              It->second.VisibleOutsideSummary || !It->second.Prevailing);
    };

    getVisibleToRegularObjVtableGUIDs(ThinLTO.CombinedIndex,
                                      VisibleToRegularObjSymbols,
                                      IsVisibleToRegularObj);
  }

  // If allowed, upgrade public vcall visibility to linkage unit visibility in
  // the summaries before whole program devirtualization below.
  updateVCallVisibilityInIndex(
      ThinLTO.CombinedIndex, WholeProgramVisibilityEnabledInLTO,
      DynamicExportSymbols, VisibleToRegularObjSymbols);

  // Perform index-based WPD. This will return immediately if there are
  // no index entries in the typeIdMetadata map (e.g. if we are instead
  // performing IR-based WPD in hybrid regular/thin LTO mode).
  std::map<ValueInfo, std::vector<VTableSlotSummary>> LocalWPDTargetsMap;
  DenseSet<StringRef> ExternallyVisibleSymbolNames;

  // Used by the promotion-time renaming logic. When non-null, this set
  // identifies symbols that should not be renamed during promotion.
  // It is non-null only when whole-program visibility is enabled and
  // renaming is not forced. Otherwise, the default renaming behavior applies.
  DenseSet<StringRef> *ExternallyVisibleSymbolNamesPtr =
      (WholeProgramVisibilityEnabledInLTO && !AlwaysRenamePromotedLocals)
          ? &ExternallyVisibleSymbolNames
          : nullptr;
  runWholeProgramDevirtOnIndex(ThinLTO.CombinedIndex, ExportedGUIDs,
                               LocalWPDTargetsMap,
                               ExternallyVisibleSymbolNamesPtr);

  auto isPrevailing = [&](GlobalValue::GUID GUID, const GlobalValueSummary *S) {
    return ThinLTO.isPrevailingModuleForGUID(GUID, S->modulePath());
  };
  if (EnableMemProfContextDisambiguation) {
    MemProfContextDisambiguation ContextDisambiguation;
    ContextDisambiguation.run(
        ThinLTO.CombinedIndex, isPrevailing, RegularLTO.Ctx,
        [&](StringRef PassName, StringRef RemarkName, const Twine &Msg) {
          auto R = OptimizationRemark(PassName.data(), RemarkName,
                                      LinkerRemarkFunction);
          R << Msg.str();
          emitRemark(R);
        });
  }

  // Figure out which symbols need to be internalized. This also needs to happen
  // at -O0 because summary-based DCE is implemented using internalization, and
  // we must apply DCE consistently with the full LTO module in order to avoid
  // undefined references during the final link.
  for (auto &Res : *GlobalResolutions) {
    // If the symbol does not have external references or it is not prevailing,
    // then not need to mark it as exported from a ThinLTO partition.
    if (Res.second.Partition != GlobalResolution::External ||
        !Res.second.isPrevailingIRSymbol())
      continue;
    auto GUID = Res.second.getGUID();
    // Mark exported unless index-based analysis determined it to be dead.
    if (ThinLTO.CombinedIndex.isGUIDLive(GUID))
      ExportedGUIDs.insert(GUID);
  }

  // Reset the GlobalResolutions to deallocate the associated memory, as there
  // are no further accesses. We specifically want to do this before computing
  // cross module importing, which adds to peak memory via the computed import
  // and export lists.
  releaseGlobalResolutionsMemory();

  if (Conf.OptLevel > 0)
    ComputeCrossModuleImport(ThinLTO.CombinedIndex, ModuleToDefinedGVSummaries,
                             isPrevailing, ImportLists, ExportLists);

  // Any functions referenced by the jump table in the regular LTO object must
  // be exported.
  auto Defs = ThinLTO.CombinedIndex.cfiFunctionDefs().getExportedThinLTOGUIDs();
  ExportedGUIDs.insert(Defs.begin(), Defs.end());
  auto Decls =
      ThinLTO.CombinedIndex.cfiFunctionDecls().getExportedThinLTOGUIDs();
  ExportedGUIDs.insert(Decls.begin(), Decls.end());

  auto isExported = [&](StringRef ModuleIdentifier, ValueInfo VI) {
    const auto &ExportList = ExportLists.find(ModuleIdentifier);
    return (ExportList != ExportLists.end() && ExportList->second.count(VI)) ||
           ExportedGUIDs.count(VI.getGUID());
  };

  // Update local devirtualized targets that were exported by cross-module
  // importing or by other devirtualizations marked in the ExportedGUIDs set.
  updateIndexWPDForExports(ThinLTO.CombinedIndex, isExported,
                           LocalWPDTargetsMap, ExternallyVisibleSymbolNamesPtr);

  if (ExternallyVisibleSymbolNamesPtr) {
    // Add to ExternallyVisibleSymbolNames the set of unique names used by all
    // externally visible symbols in the index.
    for (auto &I : ThinLTO.CombinedIndex) {
      ValueInfo VI = ThinLTO.CombinedIndex.getValueInfo(I);
      for (const auto &Summary : VI.getSummaryList()) {
        const GlobalValueSummary *Base = Summary->getBaseObject();
        if (GlobalValue::isLocalLinkage(Base->linkage()))
          continue;

        ExternallyVisibleSymbolNamesPtr->insert(VI.name());
        break;
      }
    }
  }

  thinLTOInternalizeAndPromoteInIndex(ThinLTO.CombinedIndex, isExported,
                                      isPrevailing,
                                      ExternallyVisibleSymbolNamesPtr);

  auto recordNewLinkage = [&](StringRef ModuleIdentifier,
                              GlobalValue::GUID GUID,
                              GlobalValue::LinkageTypes NewLinkage) {
    ResolvedODR[ModuleIdentifier][GUID] = NewLinkage;
  };
  thinLTOResolvePrevailingInIndex(Conf, ThinLTO.CombinedIndex, isPrevailing,
                                  recordNewLinkage, GUIDPreservedSymbols);

  thinLTOPropagateFunctionAttrs(ThinLTO.CombinedIndex, isPrevailing);

  generateParamAccessSummary(ThinLTO.CombinedIndex);

  if (llvm::timeTraceProfilerEnabled())
    llvm::timeTraceProfilerEnd();

  TimeTraceScopeExit.release();

  auto &ModuleMap =
      ThinLTO.ModulesToCompile ? *ThinLTO.ModulesToCompile : ThinLTO.ModuleMap;

  auto RunBackends = [&](ThinBackendProc *BackendProcess) -> Error {
    auto ProcessOneModule = [&](int I) -> Error {
      auto &Mod = *(ModuleMap.begin() + I);
      // Tasks 0 through ParallelCodeGenParallelismLevel-1 are reserved for
      // combined module and parallel code generation partitions.
      return BackendProcess->start(
          RegularLTO.ParallelCodeGenParallelismLevel + I, Mod.second,
          ImportLists[Mod.first], ExportLists[Mod.first],
          ResolvedODR[Mod.first], ThinLTO.ModuleMap);
    };

    BackendProcess->setup(ModuleMap.size(),
                          RegularLTO.ParallelCodeGenParallelismLevel,
                          RegularLTO.CombinedModule->getTargetTriple());

    if (BackendProcess->getThreadCount() == 1 ||
        BackendProcess->isSensitiveToInputOrder()) {
      // Process the modules in the order they were provided on the
      // command-line. It is important for this codepath to be used for
      // WriteIndexesThinBackend, to ensure the emitted LinkedObjectsFile lists
      // ThinLTO objects in the same order as the inputs, which otherwise would
      // affect the final link order.
      for (int I = 0, E = ModuleMap.size(); I != E; ++I)
        if (Error E = ProcessOneModule(I))
          return E;
    } else {
      // When executing in parallel, process largest bitsize modules first to
      // improve parallelism, and avoid starving the thread pool near the end.
      // This saves about 15 sec on a 36-core machine while link `clang.exe`
      // (out of 100 sec).
      std::vector<BitcodeModule *> ModulesVec;
      ModulesVec.reserve(ModuleMap.size());
      for (auto &Mod : ModuleMap)
        ModulesVec.push_back(&Mod.second);
      for (int I : generateModulesOrdering(ModulesVec))
        if (Error E = ProcessOneModule(I))
          return E;
    }
    return BackendProcess->wait();
  };

  if (!cgdata::thinLTOTwoRounds()) {
    std::unique_ptr<ThinBackendProc> BackendProc =
        ThinLTO.Backend(Conf, ThinLTO.CombinedIndex, ModuleToDefinedGVSummaries,
                        AddStream, Cache, BitcodeLibFuncs);
    return RunBackends(BackendProc.get());
  }

  // Perform two rounds of code generation for ThinLTO:
  // 1. First round: Perform optimization and code generation, outputting to
  // temporary scratch objects.
  // 2. Merge code generation data extracted from the temporary scratch objects.
  // 3. Second round: Execute code generation again using the merged data.
  LLVM_DEBUG(dbgs() << "[TwoRounds] Initializing ThinLTO two-codegen rounds\n");

  unsigned MaxTasks = getMaxTasks();
  auto Parallelism = ThinLTO.Backend.getParallelism();
  // Set up two additional streams and caches for storing temporary scratch
  // objects and optimized IRs, using the same cache directory as the original.
  cgdata::StreamCacheData CG(MaxTasks, Cache, "CG"), IR(MaxTasks, Cache, "IR");

  // First round: Execute optimization and code generation, outputting to
  // temporary scratch objects. Serialize the optimized IRs before initiating
  // code generation.
  LLVM_DEBUG(dbgs() << "[TwoRounds] Running the first round of codegen\n");
  auto FirstRoundLTO = std::make_unique<FirstRoundThinBackend>(
      Conf, ThinLTO.CombinedIndex, Parallelism, ModuleToDefinedGVSummaries,
      CG.AddStream, CG.Cache, BitcodeLibFuncs, IR.AddStream, IR.Cache);
  if (Error E = RunBackends(FirstRoundLTO.get()))
    return E;

  LLVM_DEBUG(dbgs() << "[TwoRounds] Merging codegen data\n");
  auto CombinedHashOrErr = cgdata::mergeCodeGenData(*CG.getResult());
  if (Error E = CombinedHashOrErr.takeError())
    return E;
  auto CombinedHash = *CombinedHashOrErr;
  LLVM_DEBUG(dbgs() << "[TwoRounds] CGData hash: " << CombinedHash << "\n");

  // Second round: Read the optimized IRs and execute code generation using the
  // merged data.
  LLVM_DEBUG(dbgs() << "[TwoRounds] Running the second round of codegen\n");
  auto SecondRoundLTO = std::make_unique<SecondRoundThinBackend>(
      Conf, ThinLTO.CombinedIndex, Parallelism, ModuleToDefinedGVSummaries,
      AddStream, Cache, BitcodeLibFuncs, IR.getResult(), CombinedHash);
  return RunBackends(SecondRoundLTO.get());
}

Expected<LLVMRemarkFileHandle> lto::setupLLVMOptimizationRemarks(
    LLVMContext &Context, StringRef RemarksFilename, StringRef RemarksPasses,
    StringRef RemarksFormat, bool RemarksWithHotness,
    std::optional<uint64_t> RemarksHotnessThreshold, int Count) {
  std::string Filename = std::string(RemarksFilename);
  // For ThinLTO, file.opt.<format> becomes
  // file.opt.<format>.thin.<num>.<format>.
  if (!Filename.empty() && Count != -1)
    Filename =
        (Twine(Filename) + ".thin." + llvm::utostr(Count) + "." + RemarksFormat)
            .str();

  auto ResultOrErr = llvm::setupLLVMOptimizationRemarks(
      Context, Filename, RemarksPasses, RemarksFormat, RemarksWithHotness,
      RemarksHotnessThreshold);
  if (Error E = ResultOrErr.takeError())
    return std::move(E);

  if (*ResultOrErr)
    (*ResultOrErr)->keep();

  return ResultOrErr;
}

Expected<std::unique_ptr<ToolOutputFile>>
lto::setupStatsFile(StringRef StatsFilename) {
  // Setup output file to emit statistics.
  if (StatsFilename.empty())
    return nullptr;

  llvm::EnableStatistics(false);
  std::error_code EC;
  auto StatsFile =
      std::make_unique<ToolOutputFile>(StatsFilename, EC, sys::fs::OF_None);
  if (EC)
    return errorCodeToError(EC);

  StatsFile->keep();
  return std::move(StatsFile);
}

// Compute the ordering we will process the inputs: the rough heuristic here
// is to sort them per size so that the largest module get schedule as soon as
// possible. This is purely a compile-time optimization.
std::vector<int> lto::generateModulesOrdering(ArrayRef<BitcodeModule *> R) {
  auto Seq = llvm::seq<int>(0, R.size());
  std::vector<int> ModulesOrdering(Seq.begin(), Seq.end());
  llvm::sort(ModulesOrdering, [&](int LeftIndex, int RightIndex) {
    auto LSize = R[LeftIndex]->getBuffer().size();
    auto RSize = R[RightIndex]->getBuffer().size();
    return LSize > RSize;
  });
  return ModulesOrdering;
}