LTOCodeGenerator.cpp

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//===-LTOCodeGenerator.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 the Link Time Optimization library. This library is
// intended to be used by linker to optimize code at link time.
//
//===----------------------------------------------------------------------===//
 
#include "llvm/LTO/legacy/LTOCodeGenerator.h"
 
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/StringExtras.h"
#include "llvm/Analysis/Passes.h"
#include "llvm/Analysis/TargetLibraryInfo.h"
#include "llvm/Analysis/TargetTransformInfo.h"
#include "llvm/Bitcode/BitcodeWriter.h"
#include "llvm/CodeGen/ParallelCG.h"
#include "llvm/CodeGen/TargetSubtargetInfo.h"
#include "llvm/Config/config.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/DebugInfo.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/DiagnosticInfo.h"
#include "llvm/IR/DiagnosticPrinter.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/LLVMRemarkStreamer.h"
#include "llvm/IR/LegacyPassManager.h"
#include "llvm/IR/Mangler.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/PassTimingInfo.h"
#include "llvm/IR/Verifier.h"
#include "llvm/InitializePasses.h"
#include "llvm/LTO/LTO.h"
#include "llvm/LTO/LTOBackend.h"
#include "llvm/LTO/legacy/LTOModule.h"
#include "llvm/LTO/legacy/UpdateCompilerUsed.h"
#include "llvm/Linker/Linker.h"
#include "llvm/MC/MCAsmInfo.h"
#include "llvm/MC/MCContext.h"
#include "llvm/MC/SubtargetFeature.h"
#include "llvm/MC/TargetRegistry.h"
#include "llvm/Remarks/HotnessThresholdParser.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/FileSystem.h"
#include "llvm/Support/Host.h"
#include "llvm/Support/MemoryBuffer.h"
#include "llvm/Support/Signals.h"
#include "llvm/Support/TargetSelect.h"
#include "llvm/Support/ToolOutputFile.h"
#include "llvm/Support/YAMLTraits.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Target/TargetOptions.h"
#include "llvm/Transforms/IPO.h"
#include "llvm/Transforms/IPO/Internalize.h"
#include "llvm/Transforms/IPO/PassManagerBuilder.h"
#include "llvm/Transforms/IPO/WholeProgramDevirt.h"
#include "llvm/Transforms/ObjCARC.h"
#include "llvm/Transforms/Utils/ModuleUtils.h"
#include <system_error>
using namespace llvm;
 
const char* LTOCodeGenerator::getVersionString() {
  return PACKAGE_NAME " version " PACKAGE_VERSION;
}
 
namespace llvm {
cl::opt<bool> LTODiscardValueNames(
    "lto-discard-value-names",
    cl::desc("Strip names from Value during LTO (other than GlobalValue)."),
#ifdef NDEBUG
    cl::init(true),
#else
    cl::init(false),
#endif
    cl::Hidden);
 
cl::opt<bool> RemarksWithHotness(
    "lto-pass-remarks-with-hotness",
    cl::desc("With PGO, include profile count in optimization remarks"),
    cl::Hidden);
 
cl::opt<Optional<uint64_t>, false, remarks::HotnessThresholdParser>
    RemarksHotnessThreshold(
        "lto-pass-remarks-hotness-threshold",
        cl::desc("Minimum profile count required for an "
                 "optimization remark to be output."
                 " Use 'auto' to apply the threshold from profile summary."),
        cl::value_desc("uint or 'auto'"), cl::init(0), cl::Hidden);
 
cl::opt<std::string>
    RemarksFilename("lto-pass-remarks-output",
                    cl::desc("Output filename for pass remarks"),
                    cl::value_desc("filename"));
 
cl::opt<std::string>
    RemarksPasses("lto-pass-remarks-filter",
                  cl::desc("Only record optimization remarks from passes whose "
                           "names match the given regular expression"),
                  cl::value_desc("regex"));
 
cl::opt<std::string> RemarksFormat(
    "lto-pass-remarks-format",
    cl::desc("The format used for serializing remarks (default: YAML)"),
    cl::value_desc("format"), cl::init("yaml"));
 
cl::opt<std::string> LTOStatsFile(
    "lto-stats-file",
    cl::desc("Save statistics to the specified file"),
    cl::Hidden);
}
 
LTOCodeGenerator::LTOCodeGenerator(LLVMContext &Context)
    : Context(Context), MergedModule(new Module("ld-temp.o", Context)),
      TheLinker(new Linker(*MergedModule)) {
  Context.setDiscardValueNames(LTODiscardValueNames);
  Context.enableDebugTypeODRUniquing();
 
  Config.CodeModel = None;
  Config.StatsFile = LTOStatsFile;
  Config.PreCodeGenPassesHook = [](legacy::PassManager &PM) {
    PM.add(createObjCARCContractPass());
  };
}
 
LTOCodeGenerator::~LTOCodeGenerator() = default;
 
void LTOCodeGenerator::setAsmUndefinedRefs(LTOModule *Mod) {
  for (const StringRef &Undef : Mod->getAsmUndefinedRefs())
    AsmUndefinedRefs.insert(Undef);
}
 
bool LTOCodeGenerator::addModule(LTOModule *Mod) {
  assert(&Mod->getModule().getContext() == &Context &&
         "Expected module in same context");
 
  bool ret = TheLinker->linkInModule(Mod->takeModule());
  setAsmUndefinedRefs(Mod);
 
  // We've just changed the input, so let's make sure we verify it.
  HasVerifiedInput = false;
 
  return !ret;
}
 
void LTOCodeGenerator::setModule(std::unique_ptr<LTOModule> Mod) {
  assert(&Mod->getModule().getContext() == &Context &&
         "Expected module in same context");
 
  AsmUndefinedRefs.clear();
 
  MergedModule = Mod->takeModule();
  TheLinker = std::make_unique<Linker>(*MergedModule);
  setAsmUndefinedRefs(&*Mod);
 
  // We've just changed the input, so let's make sure we verify it.
  HasVerifiedInput = false;
}
 
void LTOCodeGenerator::setTargetOptions(const TargetOptions &Options) {
  Config.Options = Options;
}
 
void LTOCodeGenerator::setDebugInfo(lto_debug_model Debug) {
  switch (Debug) {
  case LTO_DEBUG_MODEL_NONE:
    EmitDwarfDebugInfo = false;
    return;
 
  case LTO_DEBUG_MODEL_DWARF:
    EmitDwarfDebugInfo = true;
    return;
  }
  llvm_unreachable("Unknown debug format!");
}
 
void LTOCodeGenerator::setOptLevel(unsigned Level) {
  Config.OptLevel = Level;
  Config.PTO.LoopVectorization = Config.OptLevel > 1;
  Config.PTO.SLPVectorization = Config.OptLevel > 1;
  switch (Config.OptLevel) {
  case 0:
    Config.CGOptLevel = CodeGenOpt::None;
    return;
  case 1:
    Config.CGOptLevel = CodeGenOpt::Less;
    return;
  case 2:
    Config.CGOptLevel = CodeGenOpt::Default;
    return;
  case 3:
    Config.CGOptLevel = CodeGenOpt::Aggressive;
    return;
  }
  llvm_unreachable("Unknown optimization level!");
}
 
bool LTOCodeGenerator::writeMergedModules(StringRef Path) {
  if (!determineTarget())
    return false;
 
  // We always run the verifier once on the merged module.
  verifyMergedModuleOnce();
 
  // mark which symbols can not be internalized
  applyScopeRestrictions();
 
  // create output file
  std::error_code EC;
  ToolOutputFile Out(Path, EC, sys::fs::OF_None);
  if (EC) {
    std::string ErrMsg = "could not open bitcode file for writing: ";
    ErrMsg += Path.str() + ": " + EC.message();
    emitError(ErrMsg);
    return false;
  }
 
  // write bitcode to it
  WriteBitcodeToFile(*MergedModule, Out.os(), ShouldEmbedUselists);
  Out.os().close();
 
  if (Out.os().has_error()) {
    std::string ErrMsg = "could not write bitcode file: ";
    ErrMsg += Path.str() + ": " + Out.os().error().message();
    emitError(ErrMsg);
    Out.os().clear_error();
    return false;
  }
 
  Out.keep();
  return true;
}
 
bool LTOCodeGenerator::compileOptimizedToFile(const char **Name) {
  // make unique temp output file to put generated code
  SmallString<128> Filename;
 
  auto AddStream = [&](size_t Task) -> std::unique_ptr<CachedFileStream> {
    StringRef Extension(Config.CGFileType == CGFT_AssemblyFile ? "s" : "o");
 
    int FD;
    std::error_code EC =
        sys::fs::createTemporaryFile("lto-llvm", Extension, FD, Filename);
    if (EC)
      emitError(EC.message());
 
    return std::make_unique<CachedFileStream>(
        std::make_unique<llvm::raw_fd_ostream>(FD, true));
  };
 
  bool genResult = compileOptimized(AddStream, 1);
 
  if (!genResult) {
    sys::fs::remove(Twine(Filename));
    return false;
  }
 
  // If statistics were requested, save them to the specified file or
  // print them out after codegen.
  if (StatsFile)
    PrintStatisticsJSON(StatsFile->os());
  else if (AreStatisticsEnabled())
    PrintStatistics();
 
  NativeObjectPath = Filename.c_str();
  *Name = NativeObjectPath.c_str();
  return true;
}
 
std::unique_ptr<MemoryBuffer>
LTOCodeGenerator::compileOptimized() {
  const char *name;
  if (!compileOptimizedToFile(&name))
    return nullptr;
 
  // read .o file into memory buffer
  ErrorOr<std::unique_ptr<MemoryBuffer>> BufferOrErr = MemoryBuffer::getFile(
      name, /*IsText=*/false, /*RequiresNullTerminator=*/false);
  if (std::error_code EC = BufferOrErr.getError()) {
    emitError(EC.message());
    sys::fs::remove(NativeObjectPath);
    return nullptr;
  }
 
  // remove temp files
  sys::fs::remove(NativeObjectPath);
 
  return std::move(*BufferOrErr);
}
 
bool LTOCodeGenerator::compile_to_file(const char **Name) {
  if (!optimize())
    return false;
 
  return compileOptimizedToFile(Name);
}
 
std::unique_ptr<MemoryBuffer> LTOCodeGenerator::compile() {
  if (!optimize())
    return nullptr;
 
  return compileOptimized();
}
 
bool LTOCodeGenerator::determineTarget() {
  if (TargetMach)
    return true;
 
  TripleStr = MergedModule->getTargetTriple();
  if (TripleStr.empty()) {
    TripleStr = sys::getDefaultTargetTriple();
    MergedModule->setTargetTriple(TripleStr);
  }
  llvm::Triple Triple(TripleStr);
 
  // create target machine from info for merged modules
  std::string ErrMsg;
  MArch = TargetRegistry::lookupTarget(TripleStr, ErrMsg);
  if (!MArch) {
    emitError(ErrMsg);
    return false;
  }
 
  // Construct LTOModule, hand over ownership of module and target. Use MAttr as
  // the default set of features.
  SubtargetFeatures Features(join(Config.MAttrs, ""));
  Features.getDefaultSubtargetFeatures(Triple);
  FeatureStr = Features.getString();
  // Set a default CPU for Darwin triples.
  if (Config.CPU.empty() && Triple.isOSDarwin()) {
    if (Triple.getArch() == llvm::Triple::x86_64)
      Config.CPU = "core2";
    else if (Triple.getArch() == llvm::Triple::x86)
      Config.CPU = "yonah";
    else if (Triple.isArm64e())
      Config.CPU = "apple-a12";
    else if (Triple.getArch() == llvm::Triple::aarch64 ||
             Triple.getArch() == llvm::Triple::aarch64_32)
      Config.CPU = "cyclone";
  }
 
  TargetMach = createTargetMachine();
  assert(TargetMach && "Unable to create target machine");
 
  return true;
}
 
std::unique_ptr<TargetMachine> LTOCodeGenerator::createTargetMachine() {
  assert(MArch && "MArch is not set!");
  return std::unique_ptr<TargetMachine>(MArch->createTargetMachine(
      TripleStr, Config.CPU, FeatureStr, Config.Options, Config.RelocModel,
      None, Config.CGOptLevel));
}
 
// If a linkonce global is present in the MustPreserveSymbols, we need to make
// sure we honor this. To force the compiler to not drop it, we add it to the
// "llvm.compiler.used" global.
void LTOCodeGenerator::preserveDiscardableGVs(
    Module &TheModule,
    llvm::function_ref<bool(const GlobalValue &)> mustPreserveGV) {
  std::vector<GlobalValue *> Used;
  auto mayPreserveGlobal = [&](GlobalValue &GV) {
    if (!GV.isDiscardableIfUnused() || GV.isDeclaration() ||
        !mustPreserveGV(GV))
      return;
    if (GV.hasAvailableExternallyLinkage())
      return emitWarning(
          (Twine("Linker asked to preserve available_externally global: '") +
           GV.getName() + "'").str());
    if (GV.hasInternalLinkage())
      return emitWarning((Twine("Linker asked to preserve internal global: '") +
                   GV.getName() + "'").str());
    Used.push_back(&GV);
  };
  for (auto &GV : TheModule)
    mayPreserveGlobal(GV);
  for (auto &GV : TheModule.globals())
    mayPreserveGlobal(GV);
  for (auto &GV : TheModule.aliases())
    mayPreserveGlobal(GV);
 
  if (Used.empty())
    return;
 
  appendToCompilerUsed(TheModule, Used);
}
 
void LTOCodeGenerator::applyScopeRestrictions() {
  if (ScopeRestrictionsDone)
    return;
 
  // Declare a callback for the internalize pass that will ask for every
  // candidate GlobalValue if it can be internalized or not.
  Mangler Mang;
  SmallString<64> MangledName;
  auto mustPreserveGV = [&](const GlobalValue &GV) -> bool {
    // Unnamed globals can't be mangled, but they can't be preserved either.
    if (!GV.hasName())
      return false;
 
    // Need to mangle the GV as the "MustPreserveSymbols" StringSet is filled
    // with the linker supplied name, which on Darwin includes a leading
    // underscore.
    MangledName.clear();
    MangledName.reserve(GV.getName().size() + 1);
    Mang.getNameWithPrefix(MangledName, &GV, /*CannotUsePrivateLabel=*/false);
    return MustPreserveSymbols.count(MangledName);
  };
 
  // Preserve linkonce value on linker request
  preserveDiscardableGVs(*MergedModule, mustPreserveGV);
 
  if (!ShouldInternalize)
    return;
 
  if (ShouldRestoreGlobalsLinkage) {
    // Record the linkage type of non-local symbols so they can be restored
    // prior
    // to module splitting.
    auto RecordLinkage = [&](const GlobalValue &GV) {
      if (!GV.hasAvailableExternallyLinkage() && !GV.hasLocalLinkage() &&
          GV.hasName())
        ExternalSymbols.insert(std::make_pair(GV.getName(), GV.getLinkage()));
    };
    for (auto &GV : *MergedModule)
      RecordLinkage(GV);
    for (auto &GV : MergedModule->globals())
      RecordLinkage(GV);
    for (auto &GV : MergedModule->aliases())
      RecordLinkage(GV);
  }
 
  // Update the llvm.compiler_used globals to force preserving libcalls and
  // symbols referenced from asm
  updateCompilerUsed(*MergedModule, *TargetMach, AsmUndefinedRefs);
 
  internalizeModule(*MergedModule, mustPreserveGV);
 
  ScopeRestrictionsDone = true;
}
 
/// Restore original linkage for symbols that may have been internalized
void LTOCodeGenerator::restoreLinkageForExternals() {
  if (!ShouldInternalize || !ShouldRestoreGlobalsLinkage)
    return;
 
  assert(ScopeRestrictionsDone &&
         "Cannot externalize without internalization!");
 
  if (ExternalSymbols.empty())
    return;
 
  auto externalize = [this](GlobalValue &GV) {
    if (!GV.hasLocalLinkage() || !GV.hasName())
      return;
 
    auto I = ExternalSymbols.find(GV.getName());
    if (I == ExternalSymbols.end())
      return;
 
    GV.setLinkage(I->second);
  };
 
  llvm::for_each(MergedModule->functions(), externalize);
  llvm::for_each(MergedModule->globals(), externalize);
  llvm::for_each(MergedModule->aliases(), externalize);
}
 
void LTOCodeGenerator::verifyMergedModuleOnce() {
  // Only run on the first call.
  if (HasVerifiedInput)
    return;
  HasVerifiedInput = true;
 
  bool BrokenDebugInfo = false;
  if (verifyModule(*MergedModule, &dbgs(), &BrokenDebugInfo))
    report_fatal_error("Broken module found, compilation aborted!");
  if (BrokenDebugInfo) {
    emitWarning("Invalid debug info found, debug info will be stripped");
    StripDebugInfo(*MergedModule);
  }
}
 
void LTOCodeGenerator::finishOptimizationRemarks() {
  if (DiagnosticOutputFile) {
    DiagnosticOutputFile->keep();
    // FIXME: LTOCodeGenerator dtor is not invoked on Darwin
    DiagnosticOutputFile->os().flush();
  }
}
 
/// Optimize merged modules using various IPO passes
bool LTOCodeGenerator::optimize() {
  if (!this->determineTarget())
    return false;
 
  auto DiagFileOrErr = lto::setupLLVMOptimizationRemarks(
      Context, RemarksFilename, RemarksPasses, RemarksFormat,
      RemarksWithHotness, RemarksHotnessThreshold);
  if (!DiagFileOrErr) {
    errs() << "Error: " << toString(DiagFileOrErr.takeError()) << "\n";
    report_fatal_error("Can't get an output file for the remarks");
  }
  DiagnosticOutputFile = std::move(*DiagFileOrErr);
 
  // Setup output file to emit statistics.
  auto StatsFileOrErr = lto::setupStatsFile(LTOStatsFile);
  if (!StatsFileOrErr) {
    errs() << "Error: " << toString(StatsFileOrErr.takeError()) << "\n";
    report_fatal_error("Can't get an output file for the statistics");
  }
  StatsFile = std::move(StatsFileOrErr.get());
 
  // Currently there is no support for enabling whole program visibility via a
  // linker option in the old LTO API, but this call allows it to be specified
  // via the internal option. Must be done before WPD invoked via the optimizer
  // pipeline run below.
  updateVCallVisibilityInModule(*MergedModule,
                                /* WholeProgramVisibilityEnabledInLTO */ false,
                                // FIXME: This needs linker information via a
                                // TBD new interface.
                                /* DynamicExportSymbols */ {});
 
  // We always run the verifier once on the merged module, the `DisableVerify`
  // parameter only applies to subsequent verify.
  verifyMergedModuleOnce();
 
  // Mark which symbols can not be internalized
  this->applyScopeRestrictions();
 
  // Write LTOPostLink flag for passes that require all the modules.
  MergedModule->addModuleFlag(Module::Error, "LTOPostLink", 1);
 
  // Add an appropriate DataLayout instance for this module...
  MergedModule->setDataLayout(TargetMach->createDataLayout());
 
  ModuleSummaryIndex CombinedIndex(false);
  TargetMach = createTargetMachine();
  if (!opt(Config, TargetMach.get(), 0, *MergedModule, /*IsThinLTO=*/false,
           /*ExportSummary=*/&CombinedIndex, /*ImportSummary=*/nullptr,
           /*CmdArgs*/ std::vector<uint8_t>())) {
    emitError("LTO middle-end optimizations failed");
    return false;
  }
 
  return true;
}
 
bool LTOCodeGenerator::compileOptimized(AddStreamFn AddStream,
                                        unsigned ParallelismLevel) {
  if (!this->determineTarget())
    return false;
 
  // We always run the verifier once on the merged module.  If it has already
  // been called in optimize(), this call will return early.
  verifyMergedModuleOnce();
 
  // Re-externalize globals that may have been internalized to increase scope
  // for splitting
  restoreLinkageForExternals();
 
  ModuleSummaryIndex CombinedIndex(false);
 
  Config.CodeGenOnly = true;
  Error Err = backend(Config, AddStream, ParallelismLevel, *MergedModule,
                      CombinedIndex);
  assert(!Err && "unexpected code-generation failure");
  (void)Err;
 
  // If statistics were requested, save them to the specified file or
  // print them out after codegen.
  if (StatsFile)
    PrintStatisticsJSON(StatsFile->os());
  else if (AreStatisticsEnabled())
    PrintStatistics();
 
  reportAndResetTimings();
 
  finishOptimizationRemarks();
 
  return true;
}
 
void LTOCodeGenerator::setCodeGenDebugOptions(ArrayRef<StringRef> Options) {
  for (StringRef Option : Options)
    CodegenOptions.push_back(Option.str());
}
 
void LTOCodeGenerator::parseCodeGenDebugOptions() {
  if (!CodegenOptions.empty())
    llvm::parseCommandLineOptions(CodegenOptions);
}
 
void llvm::parseCommandLineOptions(std::vector<std::string> &Options) {
  if (!Options.empty()) {
    // ParseCommandLineOptions() expects argv[0] to be program name.
    std::vector<const char *> CodegenArgv(1, "libLLVMLTO");
    for (std::string &Arg : Options)
      CodegenArgv.push_back(Arg.c_str());
    cl::ParseCommandLineOptions(CodegenArgv.size(), CodegenArgv.data());
  }
}
 
void LTOCodeGenerator::DiagnosticHandler(const DiagnosticInfo &DI) {
  // Map the LLVM internal diagnostic severity to the LTO diagnostic severity.
  lto_codegen_diagnostic_severity_t Severity;
  switch (DI.getSeverity()) {
  case DS_Error:
    Severity = LTO_DS_ERROR;
    break;
  case DS_Warning:
    Severity = LTO_DS_WARNING;
    break;
  case DS_Remark:
    Severity = LTO_DS_REMARK;
    break;
  case DS_Note:
    Severity = LTO_DS_NOTE;
    break;
  }
  // Create the string that will be reported to the external diagnostic handler.
  std::string MsgStorage;
  raw_string_ostream Stream(MsgStorage);
  DiagnosticPrinterRawOStream DP(Stream);
  DI.print(DP);
  Stream.flush();
 
  // If this method has been called it means someone has set up an external
  // diagnostic handler. Assert on that.
  assert(DiagHandler && "Invalid diagnostic handler");
  (*DiagHandler)(Severity, MsgStorage.c_str(), DiagContext);
}
 
namespace {
struct LTODiagnosticHandler : public DiagnosticHandler {
  LTOCodeGenerator *CodeGenerator;
  LTODiagnosticHandler(LTOCodeGenerator *CodeGenPtr)
      : CodeGenerator(CodeGenPtr) {}
  bool handleDiagnostics(const DiagnosticInfo &DI) override {
    CodeGenerator->DiagnosticHandler(DI);
    return true;
  }
};
}
 
void
LTOCodeGenerator::setDiagnosticHandler(lto_diagnostic_handler_t DiagHandler,
                                       void *Ctxt) {
  this->DiagHandler = DiagHandler;
  this->DiagContext = Ctxt;
  if (!DiagHandler)
    return Context.setDiagnosticHandler(nullptr);
  // Register the LTOCodeGenerator stub in the LLVMContext to forward the
  // diagnostic to the external DiagHandler.
  Context.setDiagnosticHandler(std::make_unique<LTODiagnosticHandler>(this),
                               true);
}
 
namespace {
class LTODiagnosticInfo : public DiagnosticInfo {
  const Twine &Msg;
public:
  LTODiagnosticInfo(const Twine &DiagMsg, DiagnosticSeverity Severity=DS_Error)
      : DiagnosticInfo(DK_Linker, Severity), Msg(DiagMsg) {}
  void print(DiagnosticPrinter &DP) const override { DP << Msg; }
};
}
 
void LTOCodeGenerator::emitError(const std::string &ErrMsg) {
  if (DiagHandler)
    (*DiagHandler)(LTO_DS_ERROR, ErrMsg.c_str(), DiagContext);
  else
    Context.diagnose(LTODiagnosticInfo(ErrMsg));
}
 
void LTOCodeGenerator::emitWarning(const std::string &ErrMsg) {
  if (DiagHandler)
    (*DiagHandler)(LTO_DS_WARNING, ErrMsg.c_str(), DiagContext);
  else
    Context.diagnose(LTODiagnosticInfo(ErrMsg, DS_Warning));
}