LTO.h

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//===-LTO.h - 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 declares functions and classes used to support LTO. It is intended
// to be used both by LTO classes as well as by clients (gold-plugin) that
// don't utilize the LTO code generator interfaces.
//
//===----------------------------------------------------------------------===//
 
#ifndef LLVM_LTO_LTO_H
#define LLVM_LTO_LTO_H
 
#include "llvm/IR/LLVMRemarkStreamer.h"
#include "llvm/IR/RuntimeLibcalls.h"
#include "llvm/Support/Compiler.h"
#include <memory>
 
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/MapVector.h"
#include "llvm/Bitcode/BitcodeReader.h"
#include "llvm/IR/ModuleSummaryIndex.h"
#include "llvm/LTO/Config.h"
#include "llvm/Object/IRSymtab.h"
#include "llvm/Support/Caching.h"
#include "llvm/Support/Error.h"
#include "llvm/Support/StringSaver.h"
#include "llvm/Support/ThreadPool.h"
#include "llvm/Support/thread.h"
#include "llvm/Transforms/IPO/FunctionAttrs.h"
#include "llvm/Transforms/IPO/FunctionImport.h"
 
namespace llvm {
 
class Error;
class IRMover;
class LLVMContext;
class MemoryBufferRef;
class Module;
class raw_pwrite_stream;
class ToolOutputFile;
 
/// Resolve linkage for prevailing symbols in the \p Index. Linkage changes
/// recorded in the index and the ThinLTO backends must apply the changes to
/// the module via thinLTOFinalizeInModule.
///
/// This is done for correctness (if value exported, ensure we always
/// emit a copy), and compile-time optimization (allow drop of duplicates).
LLVM_ABI void thinLTOResolvePrevailingInIndex(
    const lto::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);
 
/// Update the linkages in the given \p Index to mark exported values
/// as external and non-exported values as internal. The ThinLTO backends
/// must apply the changes to the Module via thinLTOInternalizeModule.
LLVM_ABI void thinLTOInternalizeAndPromoteInIndex(
    ModuleSummaryIndex &Index,
    function_ref<bool(StringRef, ValueInfo)> isExported,
    function_ref<bool(GlobalValue::GUID, const GlobalValueSummary *)>
        isPrevailing,
    DenseSet<StringRef> *ExternallyVisibleSymbolNamesPtr = nullptr);
 
/// Computes a unique hash for the Module considering the current list of
/// export/import and other global analysis results.
LLVM_ABI std::string computeLTOCacheKey(
    const lto::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 = {});
 
/// Recomputes the LTO cache key for a given key with an extra identifier.
LLVM_ABI std::string recomputeLTOCacheKey(const std::string &Key,
                                          StringRef ExtraID);
 
namespace lto {
 
LLVM_ABI StringLiteral getThinLTODefaultCPU(const Triple &TheTriple);
 
/// 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.
LLVM_ABI std::string getThinLTOOutputFile(StringRef Path, StringRef OldPrefix,
                                          StringRef NewPrefix);
 
/// Setup optimization remarks.
LLVM_ABI Expected<LLVMRemarkFileHandle> setupLLVMOptimizationRemarks(
    LLVMContext &Context, StringRef RemarksFilename, StringRef RemarksPasses,
    StringRef RemarksFormat, bool RemarksWithHotness,
    std::optional<uint64_t> RemarksHotnessThreshold = 0, int Count = -1);
 
/// Setups the output file for saving statistics.
LLVM_ABI Expected<std::unique_ptr<ToolOutputFile>>
setupStatsFile(StringRef StatsFilename);
 
/// Produces a container ordering for optimal multi-threaded processing. Returns
/// ordered indices to elements in the input array.
LLVM_ABI std::vector<int> generateModulesOrdering(ArrayRef<BitcodeModule *> R);
 
class LTO;
struct SymbolResolution;
 
/// An input file. This is a symbol table wrapper that only exposes the
/// information that an LTO client should need in order to do symbol resolution.
class InputFile {
public:
  struct Symbol;
 
private:
  // FIXME: Remove LTO class friendship once we have bitcode symbol tables.
  friend LTO;
  InputFile() = default;
 
  std::vector<BitcodeModule> Mods;
  SmallVector<char, 0> Strtab;
  std::vector<Symbol> Symbols;
 
  // [begin, end) for each module
  std::vector<std::pair<size_t, size_t>> ModuleSymIndices;
 
  StringRef TargetTriple, SourceFileName, COFFLinkerOpts;
  std::vector<StringRef> DependentLibraries;
  std::vector<std::pair<StringRef, Comdat::SelectionKind>> ComdatTable;
 
  MemoryBufferRef MbRef;
  bool IsFatLTOObject = false;
  // For distributed compilation, each input must exist as an individual
  // bitcode file on disk identified by its ModuleID. For archive members and
  // FatLTO objects, the input bitcode is a sub-section of a larger file. In
  // these cases we flag that the bitcode must be written to a temporary
  // standalone file. Effectively, extracted from its container.
  bool ExtractForDistribution = false;
  bool IsThinLTO = false;
  StringRef ArchivePath;
  StringRef MemberName;
 
public:
  LLVM_ABI ~InputFile();
 
  /// Create an InputFile.
  LLVM_ABI static Expected<std::unique_ptr<InputFile>>
  create(MemoryBufferRef Object);
 
  /// The purpose of this struct is to only expose the symbol information that
  /// an LTO client should need in order to do symbol resolution.
  struct Symbol : irsymtab::Symbol {
    friend LTO;
 
  public:
    Symbol(const irsymtab::Symbol &S) : irsymtab::Symbol(S) {}
 
    using irsymtab::Symbol::isUndefined;
    using irsymtab::Symbol::isCommon;
    using irsymtab::Symbol::isWeak;
    using irsymtab::Symbol::isIndirect;
    using irsymtab::Symbol::getName;
    using irsymtab::Symbol::getIRName;
    using irsymtab::Symbol::getVisibility;
    using irsymtab::Symbol::canBeOmittedFromSymbolTable;
    using irsymtab::Symbol::isTLS;
    using irsymtab::Symbol::getComdatIndex;
    using irsymtab::Symbol::getCommonSize;
    using irsymtab::Symbol::getCommonAlignment;
    using irsymtab::Symbol::getCOFFWeakExternalFallback;
    using irsymtab::Symbol::getSectionName;
    using irsymtab::Symbol::isExecutable;
    using irsymtab::Symbol::isUsed;
 
    // Returns whether this symbol is a library call that LTO code generation
    // may emit references to. Such symbols must be considered external, as
    // removing them or modifying their interfaces would invalidate the code
    // generator's knowledge about them.
    LLVM_ABI bool isLibcall(const TargetLibraryInfo &TLI,
                            const RTLIB::RuntimeLibcallsInfo &Libcalls) const;
  };
 
  /// A range over the symbols in this InputFile.
  ArrayRef<Symbol> symbols() const { return Symbols; }
 
  /// Returns linker options specified in the input file.
  StringRef getCOFFLinkerOpts() const { return COFFLinkerOpts; }
 
  /// Returns dependent library specifiers from the input file.
  ArrayRef<StringRef> getDependentLibraries() const { return DependentLibraries; }
 
  /// Returns the path to the InputFile.
  LLVM_ABI StringRef getName() const;
 
  /// Returns the input file's target triple.
  StringRef getTargetTriple() const { return TargetTriple; }
 
  /// Returns the source file path specified at compile time.
  StringRef getSourceFileName() const { return SourceFileName; }
 
  // Returns a table with all the comdats used by this file.
  ArrayRef<std::pair<StringRef, Comdat::SelectionKind>> getComdatTable() const {
    return ComdatTable;
  }
 
  // Returns the only BitcodeModule from InputFile.
  LLVM_ABI BitcodeModule &getSingleBitcodeModule();
  // Returns the primary BitcodeModule from InputFile.
  LLVM_ABI BitcodeModule &getPrimaryBitcodeModule();
  // Returns the memory buffer reference for this input file.
  MemoryBufferRef getFileBuffer() const { return MbRef; }
  // Returns true if this input should be extracted to disk for distribution.
  // See the comment on ExtractForDistribution for details.
  bool getExtractForDistribution() const { return ExtractForDistribution; }
  // Mark whether this input should be extracted to disk for distribution.
  // See the comment on ExtractForDistribution for details.
  void setExtractForDistribution(bool EFD) { ExtractForDistribution = EFD; }
  // Returns true if this bitcode came from a FatLTO object.
  bool isFatLTOObject() const { return IsFatLTOObject; }
  // Mark this bitcode as coming from a FatLTO object.
  void fatLTOObject(bool FO) { IsFatLTOObject = FO; }
 
  // Returns true if bitcode is ThinLTO.
  bool isThinLTO() const { return IsThinLTO; }
 
  // Store an archive path and a member name.
  void setArchivePathAndName(StringRef Path, StringRef Name) {
    ArchivePath = Path;
    MemberName = Name;
  }
  StringRef getArchivePath() const { return ArchivePath; }
  StringRef getMemberName() const { return MemberName; }
 
private:
  ArrayRef<Symbol> module_symbols(unsigned I) const {
    const auto &Indices = ModuleSymIndices[I];
    return {Symbols.data() + Indices.first, Symbols.data() + Indices.second};
  }
};
 
using IndexWriteCallback = std::function<void(const std::string &)>;
 
using ImportsFilesContainer = llvm::SmallVector<std::string>;
 
/// This class defines the interface to the ThinLTO backend.
class ThinBackendProc {
protected:
  const Config &Conf;
  ModuleSummaryIndex &CombinedIndex;
  const DenseMap<StringRef, GVSummaryMapTy> &ModuleToDefinedGVSummaries;
  IndexWriteCallback OnWrite;
  bool ShouldEmitImportsFiles;
  DefaultThreadPool BackendThreadPool;
  std::optional<Error> Err;
  std::mutex ErrMu;
 
public:
  ThinBackendProc(
      const Config &Conf, ModuleSummaryIndex &CombinedIndex,
      const DenseMap<StringRef, GVSummaryMapTy> &ModuleToDefinedGVSummaries,
      lto::IndexWriteCallback OnWrite, bool ShouldEmitImportsFiles,
      ThreadPoolStrategy ThinLTOParallelism)
      : Conf(Conf), CombinedIndex(CombinedIndex),
        ModuleToDefinedGVSummaries(ModuleToDefinedGVSummaries),
        OnWrite(OnWrite), ShouldEmitImportsFiles(ShouldEmitImportsFiles),
        BackendThreadPool(ThinLTOParallelism) {}
 
  virtual ~ThinBackendProc() = default;
  virtual void setup(unsigned ThinLTONumTasks, unsigned ThinLTOTaskOffset,
                     Triple Triple) {}
  virtual 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) = 0;
  virtual Error wait() {
    BackendThreadPool.wait();
    if (Err)
      return std::move(*Err);
    return Error::success();
  }
  unsigned getThreadCount() { return BackendThreadPool.getMaxConcurrency(); }
  virtual bool isSensitiveToInputOrder() { return false; }
 
  // Write sharded indices and (optionally) imports to disk
  LLVM_ABI Error emitFiles(const FunctionImporter::ImportMapTy &ImportList,
                           unsigned Task, StringRef ModulePath,
                           const std::string &NewModulePath) const;
 
  // Write sharded indices to SummaryPath, (optionally) imports to disk, and
  // (optionally) record imports in ImportsFilesList.
  LLVM_ABI Error emitFiles(const FunctionImporter::ImportMapTy &ImportList,
                           unsigned Task, StringRef ModulePath,
                           const std::string &NewModulePath,
                           StringRef SummaryPath) const;
};
 
/// This callable defines the behavior of a ThinLTO backend after the thin-link
/// phase. It accepts a configuration \p C, a combined module summary index
/// \p CombinedIndex, a map of module identifiers to global variable summaries
/// \p ModuleToDefinedGVSummaries, a function to add output streams \p
/// AddStream, and a file cache \p Cache. It returns a unique pointer to a
/// ThinBackendProc, which can be used to launch backends in parallel.
using ThinBackendFunction = std::function<std::unique_ptr<ThinBackendProc>(
    const Config &C, ModuleSummaryIndex &CombinedIndex,
    const DenseMap<StringRef, GVSummaryMapTy> &ModuleToDefinedGVSummaries,
    AddStreamFn AddStream, FileCache Cache,
    ArrayRef<StringRef> BitcodeLibFuncs)>;
 
/// This type defines the behavior following the thin-link phase during ThinLTO.
/// It encapsulates a backend function and a strategy for thread pool
/// parallelism. Clients should use one of the provided create*ThinBackend()
/// functions to instantiate a ThinBackend. Parallelism defines the thread pool
/// strategy to be used for processing.
struct ThinBackend {
  ThinBackend(ThinBackendFunction Func, ThreadPoolStrategy Parallelism)
      : Func(std::move(Func)), Parallelism(std::move(Parallelism)) {}
  ThinBackend() = default;
 
  std::unique_ptr<ThinBackendProc> operator()(
      const Config &Conf, ModuleSummaryIndex &CombinedIndex,
      const DenseMap<StringRef, GVSummaryMapTy> &ModuleToDefinedGVSummaries,
      AddStreamFn AddStream, FileCache Cache,
      ArrayRef<StringRef> BitcodeLibFuncs) {
    assert(isValid() && "Invalid backend function");
    return Func(Conf, CombinedIndex, ModuleToDefinedGVSummaries,
                std::move(AddStream), std::move(Cache), BitcodeLibFuncs);
  }
  ThreadPoolStrategy getParallelism() const { return Parallelism; }
  bool isValid() const { return static_cast<bool>(Func); }
 
private:
  ThinBackendFunction Func = nullptr;
  ThreadPoolStrategy Parallelism;
};
 
/// This ThinBackend runs the individual backend jobs in-process.
/// The default value means to use one job per hardware core (not hyper-thread).
/// OnWrite is callback which receives module identifier and notifies LTO user
/// that index file for the module (and optionally imports file) was created.
/// ShouldEmitIndexFiles being true will write sharded ThinLTO index files
/// to the same path as the input module, with suffix ".thinlto.bc"
/// ShouldEmitImportsFiles is true it also writes a list of imported files to a
/// similar path with ".imports" appended instead.
LLVM_ABI ThinBackend createInProcessThinBackend(
    ThreadPoolStrategy Parallelism, IndexWriteCallback OnWrite = nullptr,
    bool ShouldEmitIndexFiles = false, bool ShouldEmitImportsFiles = false);
 
/// This ThinBackend writes individual module indexes to files, instead of
/// running the individual backend jobs. This backend is for distributed builds
/// where separate processes will invoke the real backends.
///
/// To find the path to write the index to, the backend checks if the path has a
/// prefix of OldPrefix; if so, it replaces that prefix with NewPrefix. It then
/// appends ".thinlto.bc" and writes the index to that path. If
/// ShouldEmitImportsFiles is true it also writes a list of imported files to a
/// similar path with ".imports" appended instead.
/// LinkedObjectsFile is an output stream to write the list of object files for
/// the final ThinLTO linking. Can be nullptr.  If LinkedObjectsFile is not
/// nullptr and NativeObjectPrefix is not empty then it replaces the prefix of
/// the objects with NativeObjectPrefix instead of NewPrefix. OnWrite is
/// callback which receives module identifier and notifies LTO user that index
/// file for the module (and optionally imports file) was created.
LLVM_ABI ThinBackend createWriteIndexesThinBackend(
    ThreadPoolStrategy Parallelism, std::string OldPrefix,
    std::string NewPrefix, std::string NativeObjectPrefix,
    bool ShouldEmitImportsFiles, raw_fd_ostream *LinkedObjectsFile,
    IndexWriteCallback OnWrite);
 
/// This class implements a resolution-based interface to LLVM's LTO
/// functionality. It supports regular LTO, parallel LTO code generation and
/// ThinLTO. You can use it from a linker in the following way:
/// - Set hooks and code generation options (see lto::Config struct defined in
///   Config.h), and use the lto::Config object to create an lto::LTO object.
/// - Create lto::InputFile objects using lto::InputFile::create(), then use
///   the symbols() function to enumerate its symbols and compute a resolution
///   for each symbol (see SymbolResolution below).
/// - After the linker has visited each input file (and each regular object
///   file) and computed a resolution for each symbol, take each lto::InputFile
///   and pass it and an array of symbol resolutions to the add() function.
/// - Call the getMaxTasks() function to get an upper bound on the number of
///   native object files that LTO may add to the link.
/// - Call the run() function. This function will use the supplied AddStream
///   and Cache functions to add up to getMaxTasks() native object files to
///   the link.
class LLVM_ABI LTO {
  friend InputFile;
 
public:
  /// Unified LTO modes
  enum LTOKind {
    /// Any LTO mode without Unified LTO. The default mode.
    LTOK_Default,
 
    /// Regular LTO, with Unified LTO enabled.
    LTOK_UnifiedRegular,
 
    /// ThinLTO, with Unified LTO enabled.
    LTOK_UnifiedThin,
  };
 
  /// Create an LTO object. A default constructed LTO object has a reasonable
  /// production configuration, but you can customize it by passing arguments to
  /// this constructor.
  /// FIXME: We do currently require the DiagHandler field to be set in Conf.
  /// Until that is fixed, a Config argument is required.
  LTO(Config Conf, ThinBackend Backend = {},
      unsigned ParallelCodeGenParallelismLevel = 1,
      LTOKind LTOMode = LTOK_Default);
  virtual ~LTO();
 
  /// Add an input file to the LTO link, using the provided symbol resolutions.
  /// The symbol resolutions must appear in the enumeration order given by
  /// InputFile::symbols().
  Error add(std::unique_ptr<InputFile> Obj, ArrayRef<SymbolResolution> Res);
 
  /// Set the list of functions implemented in bitcode that were not extracted
  /// from an archive. Such functions may not be referenced, as they have
  /// lost their opportunity to be defined.
  void setBitcodeLibFuncs(ArrayRef<StringRef> BitcodeLibFuncs);
 
  /// Returns an upper bound on the number of tasks that the client may expect.
  /// This may only be called after all IR object files have been added. For a
  /// full description of tasks see LTOBackend.h.
  unsigned getMaxTasks() const;
 
  /// Runs the LTO pipeline. This function calls the supplied AddStream
  /// function to add native object files to the link.
  ///
  /// The Cache parameter is optional. If supplied, it will be used to cache
  /// native object files and add them to the link.
  ///
  /// The client will receive at most one callback (via either AddStream or
  /// Cache) for each task identifier.
  virtual Error run(AddStreamFn AddStream, FileCache Cache = {});
 
  /// Wait for cleanup work started by run() to finish.
  ///
  /// A client may delay this call to overlap asynchronous cleanup with later
  /// linking work, but must call it before finalizing time trace data because
  /// cleanup may emit time trace events. Most LTO implementations have no
  /// asynchronous cleanup.
  virtual void waitForCleanup() {}
 
  /// Static method that returns a list of libcall symbols that can be generated
  /// by LTO but might not be visible from bitcode symbol table.
  static SmallVector<const char *> getRuntimeLibcallSymbols(const Triple &TT);
 
  /// Static method that returns a list of library function symbols that can be
  /// generated by LTO but might not be visible from bitcode symbol table.
  /// Unlike the runtime libcalls, the linker can report to the code generator
  /// which of these are actually available in the link, and the code generator
  /// can then only reference that set of symbols.
  static SmallVector<StringRef> getLibFuncSymbols(const Triple &TT,
                                                  llvm::StringSaver &Saver);
 
protected:
  // Called before returning from run().
  virtual void cleanup();
 
  Config Conf;
 
  struct RegularLTOState {
    LLVM_ABI RegularLTOState(unsigned ParallelCodeGenParallelismLevel,
                             const Config &Conf);
    struct CommonResolution {
      uint64_t Size = 0;
      Align Alignment;
      /// Record if at least one instance of the common was marked as prevailing
      bool Prevailing = false;
    };
    std::map<std::string, CommonResolution> Commons;
 
    unsigned ParallelCodeGenParallelismLevel;
    LTOLLVMContext Ctx;
    std::unique_ptr<Module> CombinedModule;
    std::unique_ptr<IRMover> Mover;
 
    // This stores the information about a regular LTO module that we have added
    // to the link. It will either be linked immediately (for modules without
    // summaries) or after summary-based dead stripping (for modules with
    // summaries).
    struct AddedModule {
      std::unique_ptr<Module> M;
      std::vector<GlobalValue *> Keep;
    };
    std::vector<AddedModule> ModsWithSummaries;
    bool EmptyCombinedModule = true;
  } RegularLTO;
 
  using ModuleMapType = MapVector<StringRef, BitcodeModule>;
 
  struct ThinLTOState {
    LLVM_ABI ThinLTOState(ThinBackend Backend);
 
    ThinBackend Backend;
    ModuleSummaryIndex CombinedIndex;
    // The full set of bitcode modules in input order.
    ModuleMapType ModuleMap;
    // The bitcode modules to compile, if specified by the LTO Config.
    std::optional<ModuleMapType> ModulesToCompile;
 
    void setPrevailingModuleForGUID(GlobalValue::GUID GUID, StringRef Module) {
      PrevailingModuleForGUID[GUID] = Module;
    }
    bool isPrevailingModuleForGUID(GlobalValue::GUID GUID,
                                   StringRef Module) const {
      auto It = PrevailingModuleForGUID.find(GUID);
      return It != PrevailingModuleForGUID.end() && It->second == Module;
    }
 
  private:
    // Make this private so all accesses must go through above accessor methods
    // to avoid inadvertently creating new entries on lookups.
    DenseMap<GlobalValue::GUID, StringRef> PrevailingModuleForGUID;
  } ThinLTO;
 
private:
  // The global resolution for a particular (mangled) symbol name. This is in
  // particular necessary to track whether each symbol can be internalized.
  // Because any input file may introduce a new cross-partition reference, we
  // cannot make any final internalization decisions until all input files have
  // been added and the client has called run(). During run() we apply
  // internalization decisions either directly to the module (for regular LTO)
  // or to the combined index (for ThinLTO).
  // FIXME: Make this GlobalResolution a class, it has been becoming more than
  // just a data bag.
  struct GlobalResolution {
    /// The unmangled name of the global.
    std::string IRName;
 
    /// Keep track if the symbol is visible outside of a module with a summary
    /// (i.e. in either a regular object or a regular LTO module without a
    /// summary).
    bool VisibleOutsideSummary = false;
 
    /// The symbol was exported dynamically, and therefore could be referenced
    /// by a shared library not visible to the linker.
    bool ExportDynamic = false;
 
    bool UnnamedAddr = true;
 
    /// True if module contains the prevailing definition.
    bool Prevailing = false;
 
    /// Returns true if module contains the prevailing definition and symbol is
    /// an IR symbol. For example when module-level inline asm block is used,
    /// symbol can be prevailing in module but have no IR name.
    bool isPrevailingIRSymbol() const { return Prevailing && !IRName.empty(); }
 
    /// This field keeps track of the partition number of this global. The
    /// regular LTO object is partition 0, while each ThinLTO object has its own
    /// partition number from 1 onwards.
    ///
    /// Any global that is defined or used by more than one partition, or that
    /// is referenced externally, may not be internalized.
    ///
    /// Partitions generally have a one-to-one correspondence with tasks, except
    /// that we use partition 0 for all parallel LTO code generation partitions.
    /// Any partitioning of the combined LTO object is done internally by the
    /// LTO backend.
    unsigned Partition = Unknown;
 
  private:
    GlobalValue::GUID GUID = 0;
 
  public:
    void setGUID(GlobalValue::GUID G) {
      assert(G);
      assert(!GUID || GUID == G);
      GUID = G;
    }
 
    GlobalValue::GUID getGUID() const {
      return GUID ? GUID
                  : GlobalValue::getGUIDAssumingExternalLinkage(
                        GlobalValue::getGlobalIdentifier(
                            IRName, GlobalValue::LinkageTypes::ExternalLinkage,
                            ""));
    }
 
    /// Special partition numbers.
    enum : unsigned {
      /// A partition number has not yet been assigned to this global.
      Unknown = -1u,
 
      /// This global is either used by more than one partition or has an
      /// external reference, and therefore cannot be internalized.
      External = -2u,
 
      /// The RegularLTO partition
      RegularLTO = 0,
    };
  };
 
  // GlobalResolutionSymbolSaver allocator.
  std::unique_ptr<llvm::BumpPtrAllocator> Alloc;
 
  // Symbol saver for global resolution map.
  std::unique_ptr<llvm::StringSaver> GlobalResolutionSymbolSaver;
 
  // Global mapping from mangled symbol names to resolutions.
  // Make this an unique_ptr to guard against accessing after it has been reset
  // (to reduce memory after we're done with it).
  std::unique_ptr<llvm::DenseMap<StringRef, GlobalResolution>>
      GlobalResolutions;
 
  void releaseGlobalResolutionsMemory();
 
  void addModuleToGlobalRes(ArrayRef<InputFile::Symbol> Syms,
                            ArrayRef<SymbolResolution> Res, unsigned Partition,
                            bool InSummary, const Triple &TT);
 
  // These functions take a range of symbol resolutions and consume the
  // resolutions used by a single input module. Functions return ranges refering
  // to the resolutions for the remaining modules in the InputFile.
  Expected<ArrayRef<SymbolResolution>>
  addModule(InputFile &Input, ArrayRef<SymbolResolution> InputRes,
            unsigned ModI, ArrayRef<SymbolResolution> Res);
 
  Expected<std::pair<RegularLTOState::AddedModule, ArrayRef<SymbolResolution>>>
  addRegularLTO(InputFile &Input, ArrayRef<SymbolResolution> InputRes,
                BitcodeModule BM, ArrayRef<InputFile::Symbol> Syms,
                ArrayRef<SymbolResolution> Res);
  Error linkRegularLTO(RegularLTOState::AddedModule Mod,
                       bool LivenessFromIndex);
 
  Expected<ArrayRef<SymbolResolution>>
  addThinLTO(BitcodeModule BM, ArrayRef<InputFile::Symbol> Syms,
             ArrayRef<SymbolResolution> Res);
 
  Error runRegularLTO(AddStreamFn AddStream);
  Error runThinLTO(AddStreamFn AddStream, FileCache Cache,
                   const DenseSet<GlobalValue::GUID> &GUIDPreservedSymbols);
 
  Error checkPartiallySplit();
 
  mutable bool CalledGetMaxTasks = false;
 
protected:
  // LTO mode when using Unified LTO.
  LTOKind LTOMode;
 
private:
  // Use Optional to distinguish false from not yet initialized.
  std::optional<bool> EnableSplitLTOUnit;
 
  // Identify symbols exported dynamically, and that therefore could be
  // referenced by a shared library not visible to the linker.
  DenseSet<GlobalValue::GUID> DynamicExportSymbols;
 
  // Diagnostic optimization remarks file
  LLVMRemarkFileHandle DiagnosticOutputFile;
 
  // A dummy module to host the dummy function.
  std::unique_ptr<Module> DummyModule;
 
  // A dummy function created in a private module to provide a context for
  // LTO-link optimization remarks. This is needed for ThinLTO where we
  // may not have any IR functions available, because the optimization remark
  // handling requires a function.
  Function *LinkerRemarkFunction = nullptr;
 
  // Setup optimization remarks according to the provided configuration.
  Error setupOptimizationRemarks();
 
  // LibFuncs that were implemented in bitcode but were not extracted
  // from their libraries. Such functions cannot safely be called, since
  // they have lost their opportunity to be defined.
  SmallVector<StringRef> BitcodeLibFuncs;
 
public:
  /// Helper to emit an optimization remark during the LTO link when outside of
  /// the standard optimization pass pipeline.
  void emitRemark(OptimizationRemark &Remark);
 
  virtual Expected<std::shared_ptr<lto::InputFile>>
  addInput(std::unique_ptr<lto::InputFile> InputPtr) {
    return std::shared_ptr<lto::InputFile>(InputPtr.release());
  }
};
 
/// The resolution for a symbol. The linker must provide a SymbolResolution for
/// each global symbol based on its internal resolution of that symbol.
struct SymbolResolution {
  SymbolResolution()
      : Prevailing(0), FinalDefinitionInLinkageUnit(0), VisibleToRegularObj(0),
        ExportDynamic(0), LinkerRedefined(0) {}
 
  /// The linker has chosen this definition of the symbol.
  unsigned Prevailing : 1;
 
  /// The definition of this symbol is unpreemptable at runtime and is known to
  /// be in this linkage unit.
  unsigned FinalDefinitionInLinkageUnit : 1;
 
  /// The definition of this symbol is visible outside of the LTO unit.
  unsigned VisibleToRegularObj : 1;
 
  /// The symbol was exported dynamically, and therefore could be referenced
  /// by a shared library not visible to the linker.
  unsigned ExportDynamic : 1;
 
  /// Linker redefined version of the symbol which appeared in -wrap or -defsym
  /// linker option.
  unsigned LinkerRedefined : 1;
};
 
} // namespace lto
} // namespace llvm
 
#endif