[LinkerWrapper] Remove in-house handling of LTO

[jhuber6]

Summary:
This should be the linker's job if the user creates any bitcode files,
then passing -flto to the linker for the toolchain should be able to
handle it. Right now this path is only used in the case where someone
does LTO w/ ld.gold targeting a CPU so I think we are safe here as that
will still be forwarded, for bfd it'll be an error as it would on the
host. I think I talked the SYCL team out of using this as well so I
should be good to delete it.

[llvmbot]

@llvm/pr-subscribers-clang

Author: Joseph Huber (jhuber6)

Changes

Summary:
This should be the linker's job if the user creates any bitcode files,
then passing -flto to the linker for the toolchain should be able to
handle it. Right now this path is only used in the case where someone
does LTO w/ ld.gold targeting a CPU so I think we are safe here as that
will still be forwarded, for bfd it'll be an error as it would on the
host. I think I talked the SYCL team out of using this as well so I
should be good to delete it.

---

Full diff: https://github.com/llvm/llvm-project/pull/113715.diff

1 Files Affected:

• (modified) clang/tools/clang-linker-wrapper/ClangLinkerWrapper.cpp (+2-343)

diff --git a/clang/tools/clang-linker-wrapper/ClangLinkerWrapper.cpp b/clang/tools/clang-linker-wrapper/ClangLinkerWrapper.cpp
index 9fea1fdcd5fb46..804514d9c9777a 100644
--- a/clang/tools/clang-linker-wrapper/ClangLinkerWrapper.cpp
+++ b/clang/tools/clang-linker-wrapper/ClangLinkerWrapper.cpp
@@ -140,9 +140,6 @@ static std::list<SmallString<128>> TempFiles;
 /// Codegen flags for LTO backend.
 static codegen::RegisterCodeGenFlags CodeGenFlags;
 
-/// Global flag to indicate that the LTO pipeline threw an error.
-static std::atomic<bool> LTOError;
-
 using OffloadingImage = OffloadBinary::OffloadingImage;
 
 namespace llvm {
@@ -293,12 +290,10 @@ Expected<std::string> findProgram(StringRef Name, ArrayRef<StringRef> Paths) {
   return *Path;
 }
 
-/// We will defer LTO to the target's linker if we are not doing JIT and it is
-/// supported by the toolchain.
 bool linkerSupportsLTO(const ArgList &Args) {
   llvm::Triple Triple(Args.getLastArgValue(OPT_triple_EQ));
   return Triple.isNVPTX() || Triple.isAMDGPU() ||
-         Args.getLastArgValue(OPT_linker_path_EQ).ends_with("ld.lld");
+         Args.getLastArgValue(OPT_linker_path_EQ).ends_with("lld");
 }
 
 /// Returns the hashed value for a constant string.
@@ -653,7 +648,6 @@ void diagnosticHandler(const DiagnosticInfo &DI) {
   switch (DI.getSeverity()) {
   case DS_Error:
     WithColor::error(errs(), LinkerExecutable) << ErrStorage << "\n";
-    LTOError = true;
     break;
   case DS_Warning:
     WithColor::warning(errs(), LinkerExecutable) << ErrStorage << "\n";
@@ -667,334 +661,6 @@ void diagnosticHandler(const DiagnosticInfo &DI) {
   }
 }
 
-// Get the list of target features from the input file and unify them such that
-// if there are multiple +xxx or -xxx features we only keep the last one.
-std::vector<std::string> getTargetFeatures(ArrayRef<OffloadFile> InputFiles) {
-  SmallVector<StringRef> Features;
-  for (const OffloadFile &File : InputFiles) {
-    for (auto Arg : llvm::split(File.getBinary()->getString("feature"), ","))
-      Features.emplace_back(Arg);
-  }
-
-  // Only add a feature if it hasn't been seen before starting from the end.
-  std::vector<std::string> UnifiedFeatures;
-  DenseSet<StringRef> UsedFeatures;
-  for (StringRef Feature : llvm::reverse(Features)) {
-    if (UsedFeatures.insert(Feature.drop_front()).second)
-      UnifiedFeatures.push_back(Feature.str());
-  }
-
-  return UnifiedFeatures;
-}
-
-template <typename ModuleHook = function_ref<bool(size_t, const Module &)>>
-std::unique_ptr<lto::LTO> createLTO(
-    const ArgList &Args, const std::vector<std::string> &Features,
-    ModuleHook Hook = [](size_t, const Module &) { return true; }) {
-  const llvm::Triple Triple(Args.getLastArgValue(OPT_triple_EQ));
-  // We need to remove AMD's target-id from the processor if present.
-  StringRef TargetID = Args.getLastArgValue(OPT_arch_EQ);
-  StringRef Arch = clang::getProcessorFromTargetID(Triple, TargetID);
-  lto::Config Conf;
-  lto::ThinBackend Backend;
-  // TODO: Handle index-only thin-LTO
-  Backend =
-      lto::createInProcessThinBackend(llvm::heavyweight_hardware_concurrency());
-
-  Conf.CPU = Arch.str();
-  Conf.Options = codegen::InitTargetOptionsFromCodeGenFlags(Triple);
-
-  Conf.RemarksFilename = RemarksFilename;
-  Conf.RemarksPasses = RemarksPasses;
-  Conf.RemarksWithHotness = RemarksWithHotness;
-  Conf.RemarksHotnessThreshold = RemarksHotnessThreshold;
-  Conf.RemarksFormat = RemarksFormat;
-
-  StringRef OptLevel = Args.getLastArgValue(OPT_opt_level, "O2");
-  Conf.MAttrs = Features;
-  std::optional<CodeGenOptLevel> CGOptLevelOrNone =
-      CodeGenOpt::parseLevel(OptLevel[1]);
-  assert(CGOptLevelOrNone && "Invalid optimization level");
-  Conf.CGOptLevel = *CGOptLevelOrNone;
-  Conf.OptLevel = OptLevel[1] - '0';
-  Conf.DefaultTriple = Triple.getTriple();
-
-  // TODO: Should we complain about combining --opt-level and -passes, as opt
-  // does?  That might be too limiting in clang-linker-wrapper, so for now we
-  // just warn in the help entry for -passes that the default<O?> corresponding
-  // to --opt-level=O? should be included there.  The problem is that
-  // --opt-level produces effects in clang-linker-wrapper beyond what -passes
-  // appears to be able to achieve, so rejecting the combination of --opt-level
-  // and -passes would apparently make it impossible to combine those effects
-  // with a custom pass pipeline.
-  Conf.OptPipeline = PassPipeline;
-  Conf.PassPlugins = PassPlugins;
-
-  LTOError = false;
-  Conf.DiagHandler = diagnosticHandler;
-
-  Conf.PTO.LoopVectorization = Conf.OptLevel > 1;
-  Conf.PTO.SLPVectorization = Conf.OptLevel > 1;
-
-  if (SaveTemps) {
-    std::string TempName = (sys::path::filename(ExecutableName) + "." +
-                            Triple.getTriple() + "." + TargetID)
-                               .str();
-    Conf.PostInternalizeModuleHook = [=](size_t Task, const Module &M) {
-      std::string File =
-          !Task ? TempName + ".postlink.bc"
-                : TempName + "." + std::to_string(Task) + ".postlink.bc";
-      error_code EC;
-      raw_fd_ostream LinkedBitcode(File, EC, sys::fs::OF_None);
-      if (EC)
-        reportError(errorCodeToError(EC));
-      WriteBitcodeToFile(M, LinkedBitcode);
-      return true;
-    };
-    Conf.PreCodeGenModuleHook = [=](size_t Task, const Module &M) {
-      std::string File =
-          !Task ? TempName + ".postopt.bc"
-                : TempName + "." + std::to_string(Task) + ".postopt.bc";
-      error_code EC;
-      raw_fd_ostream LinkedBitcode(File, EC, sys::fs::OF_None);
-      if (EC)
-        reportError(errorCodeToError(EC));
-      WriteBitcodeToFile(M, LinkedBitcode);
-      return true;
-    };
-  }
-  Conf.PostOptModuleHook = Hook;
-  Conf.CGFileType = (Triple.isNVPTX() || SaveTemps)
-                        ? CodeGenFileType::AssemblyFile
-                        : CodeGenFileType::ObjectFile;
-
-  // TODO: Handle remark files
-  Conf.HasWholeProgramVisibility = Args.hasArg(OPT_whole_program);
-
-  return std::make_unique<lto::LTO>(std::move(Conf), Backend);
-}
-
-// Returns true if \p S is valid as a C language identifier and will be given
-// `__start_` and `__stop_` symbols.
-bool isValidCIdentifier(StringRef S) {
-  return !S.empty() && (isAlpha(S[0]) || S[0] == '_') &&
-         llvm::all_of(llvm::drop_begin(S),
-                      [](char C) { return C == '_' || isAlnum(C); });
-}
-
-Error linkBitcodeFiles(SmallVectorImpl<OffloadFile> &InputFiles,
-                       SmallVectorImpl<StringRef> &OutputFiles,
-                       const ArgList &Args) {
-  llvm::TimeTraceScope TimeScope("Link bitcode files");
-  const llvm::Triple Triple(Args.getLastArgValue(OPT_triple_EQ));
-  StringRef Arch = Args.getLastArgValue(OPT_arch_EQ);
-
-  SmallVector<OffloadFile, 4> BitcodeInputFiles;
-  DenseSet<StringRef> StrongResolutions;
-  DenseSet<StringRef> UsedInRegularObj;
-  DenseSet<StringRef> UsedInSharedLib;
-  BumpPtrAllocator Alloc;
-  StringSaver Saver(Alloc);
-
-  // Search for bitcode files in the input and create an LTO input file. If
-  // it is not a bitcode file, scan its symbol table for symbols we need to
-  // save.
-  for (OffloadFile &File : InputFiles) {
-    MemoryBufferRef Buffer = MemoryBufferRef(File.getBinary()->getImage(), "");
-
-    file_magic Type = identify_magic(Buffer.getBuffer());
-    switch (Type) {
-    case file_magic::bitcode: {
-      Expected<IRSymtabFile> IRSymtabOrErr = readIRSymtab(Buffer);
-      if (!IRSymtabOrErr)
-        return IRSymtabOrErr.takeError();
-
-      // Check for any strong resolutions we need to preserve.
-      for (unsigned I = 0; I != IRSymtabOrErr->Mods.size(); ++I) {
-        for (const auto &Sym : IRSymtabOrErr->TheReader.module_symbols(I)) {
-          if (!Sym.isFormatSpecific() && Sym.isGlobal() && !Sym.isWeak() &&
-              !Sym.isUndefined())
-            StrongResolutions.insert(Saver.save(Sym.Name));
-        }
-      }
-      BitcodeInputFiles.emplace_back(std::move(File));
-      continue;
-    }
-    case file_magic::elf_relocatable:
-    case file_magic::elf_shared_object: {
-      Expected<std::unique_ptr<ObjectFile>> ObjFile =
-          ObjectFile::createObjectFile(Buffer);
-      if (!ObjFile)
-        continue;
-
-      for (SymbolRef Sym : (*ObjFile)->symbols()) {
-        Expected<StringRef> Name = Sym.getName();
-        if (!Name)
-          return Name.takeError();
-
-        // Record if we've seen these symbols in any object or shared
-        // libraries.
-        if ((*ObjFile)->isRelocatableObject())
-          UsedInRegularObj.insert(Saver.save(*Name));
-        else
-          UsedInSharedLib.insert(Saver.save(*Name));
-      }
-      continue;
-    }
-    default:
-      continue;
-    }
-  }
-
-  if (BitcodeInputFiles.empty())
-    return Error::success();
-
-  // Remove all the bitcode files that we moved from the original input.
-  llvm::erase_if(InputFiles, [](OffloadFile &F) { return !F.getBinary(); });
-
-  // LTO Module hook to output bitcode without running the backend.
-  SmallVector<StringRef> BitcodeOutput;
-  auto OutputBitcode = [&](size_t, const Module &M) {
-    auto TempFileOrErr = createOutputFile(sys::path::filename(ExecutableName) +
-                                              "-jit-" + Triple.getTriple(),
-                                          "bc");
-    if (!TempFileOrErr)
-      reportError(TempFileOrErr.takeError());
-
-    std::error_code EC;
-    raw_fd_ostream LinkedBitcode(*TempFileOrErr, EC, sys::fs::OF_None);
-    if (EC)
-      reportError(errorCodeToError(EC));
-    WriteBitcodeToFile(M, LinkedBitcode);
-    BitcodeOutput.push_back(*TempFileOrErr);
-    return false;
-  };
-
-  // We assume visibility of the whole program if every input file was
-  // bitcode.
-  auto Features = getTargetFeatures(BitcodeInputFiles);
-  auto LTOBackend = Args.hasArg(OPT_embed_bitcode) ||
-                            Args.hasArg(OPT_builtin_bitcode_EQ) ||
-                            Args.hasArg(OPT_clang_backend)
-                        ? createLTO(Args, Features, OutputBitcode)
-                        : createLTO(Args, Features);
-
-  // We need to resolve the symbols so the LTO backend knows which symbols
-  // need to be kept or can be internalized. This is a simplified symbol
-  // resolution scheme to approximate the full resolution a linker would do.
-  uint64_t Idx = 0;
-  DenseSet<StringRef> PrevailingSymbols;
-  for (auto &BitcodeInput : BitcodeInputFiles) {
-    // Get a semi-unique buffer identifier for Thin-LTO.
-    StringRef Identifier = Saver.save(
-        std::to_string(Idx++) + "." +
-        BitcodeInput.getBinary()->getMemoryBufferRef().getBufferIdentifier());
-    MemoryBufferRef Buffer =
-        MemoryBufferRef(BitcodeInput.getBinary()->getImage(), Identifier);
-    Expected<std::unique_ptr<lto::InputFile>> BitcodeFileOrErr =
-        llvm::lto::InputFile::create(Buffer);
-    if (!BitcodeFileOrErr)
-      return BitcodeFileOrErr.takeError();
-
-    // Save the input file and the buffer associated with its memory.
-    const auto Symbols = (*BitcodeFileOrErr)->symbols();
-    SmallVector<lto::SymbolResolution, 16> Resolutions(Symbols.size());
-    size_t Idx = 0;
-    for (auto &Sym : Symbols) {
-      lto::SymbolResolution &Res = Resolutions[Idx++];
-
-      // We will use this as the prevailing symbol definition in LTO unless
-      // it is undefined or another definition has already been used.
-      Res.Prevailing =
-          !Sym.isUndefined() &&
-          !(Sym.isWeak() && StrongResolutions.contains(Sym.getName())) &&
-          PrevailingSymbols.insert(Saver.save(Sym.getName())).second;
-
-      // We need LTO to preseve the following global symbols:
-      // 1) Symbols used in regular objects.
-      // 2) Sections that will be given a __start/__stop symbol.
-      // 3) Prevailing symbols that are needed visible to external
-      // libraries.
-      Res.VisibleToRegularObj =
-          UsedInRegularObj.contains(Sym.getName()) ||
-          isValidCIdentifier(Sym.getSectionName()) ||
-          (Res.Prevailing &&
-           (Sym.getVisibility() != GlobalValue::HiddenVisibility &&
-            !Sym.canBeOmittedFromSymbolTable()));
-
-      // Identify symbols that must be exported dynamically and can be
-      // referenced by other files.
-      Res.ExportDynamic =
-          Sym.getVisibility() != GlobalValue::HiddenVisibility &&
-          (UsedInSharedLib.contains(Sym.getName()) ||
-           !Sym.canBeOmittedFromSymbolTable());
-
-      // The final definition will reside in this linkage unit if the symbol
-      // is defined and local to the module. This only checks for bitcode
-      // files, full assertion will require complete symbol resolution.
-      Res.FinalDefinitionInLinkageUnit =
-          Sym.getVisibility() != GlobalValue::DefaultVisibility &&
-          (!Sym.isUndefined() && !Sym.isCommon());
-
-      // We do not support linker redefined symbols (e.g. --wrap) for device
-      // image linking, so the symbols will not be changed after LTO.
-      Res.LinkerRedefined = false;
-    }
-
-    // Add the bitcode file with its resolved symbols to the LTO job.
-    if (Error Err = LTOBackend->add(std::move(*BitcodeFileOrErr), Resolutions))
-      return Err;
-  }
-
-  // Run the LTO job to compile the bitcode.
-  size_t MaxTasks = LTOBackend->getMaxTasks();
-  SmallVector<StringRef> Files(MaxTasks);
-  auto AddStream =
-      [&](size_t Task,
-          const Twine &ModuleName) -> std::unique_ptr<CachedFileStream> {
-    int FD = -1;
-    auto &TempFile = Files[Task];
-    StringRef Extension = (Triple.isNVPTX() || SaveTemps) ? "s" : "o";
-    std::string TaskStr = Task ? "." + std::to_string(Task) : "";
-    auto TempFileOrErr =
-        createOutputFile(sys::path::filename(ExecutableName) + "." +
-                             Triple.getTriple() + "." + Arch + TaskStr,
-                         Extension);
-    if (!TempFileOrErr)
-      reportError(TempFileOrErr.takeError());
-    TempFile = *TempFileOrErr;
-    if (std::error_code EC = sys::fs::openFileForWrite(TempFile, FD))
-      reportError(errorCodeToError(EC));
-    return std::make_unique<CachedFileStream>(
-        std::make_unique<llvm::raw_fd_ostream>(FD, true));
-  };
-
-  if (Error Err = LTOBackend->run(AddStream))
-    return Err;
-
-  if (LTOError)
-    return createStringError("Errors encountered inside the LTO pipeline.");
-
-  // If we are embedding bitcode we only need the intermediate output.
-  bool SingleOutput = Files.size() == 1;
-  if (Args.hasArg(OPT_embed_bitcode)) {
-    if (BitcodeOutput.size() != 1 || !SingleOutput)
-      return createStringError("Cannot embed bitcode with multiple files.");
-    OutputFiles.push_back(Args.MakeArgString(BitcodeOutput.front()));
-    return Error::success();
-  }
-
-  // Append the new inputs to the device linker input. If the user requested
-  // an internalizing link we need to pass the bitcode to clang.
-  for (StringRef File :
-       Args.hasArg(OPT_clang_backend) || Args.hasArg(OPT_builtin_bitcode_EQ)
-           ? BitcodeOutput
-           : Files)
-    OutputFiles.push_back(File);
-
-  return Error::success();
-}
-
 Expected<StringRef> writeOffloadFile(const OffloadFile &File) {
   const OffloadBinary &Binary = *File.getBinary();
 
@@ -1325,15 +991,8 @@ Expected<SmallVector<StringRef>> linkAndWrapDeviceFiles(
       if (File.getBinary()->getOffloadKind() != OFK_None)
         ActiveOffloadKinds.insert(File.getBinary()->getOffloadKind());
 
-    // First link and remove all the input files containing bitcode if
-    // the target linker does not support it natively.
+    // Write any remaining device inputs to an output file.
     SmallVector<StringRef> InputFiles;
-    if (!linkerSupportsLTO(LinkerArgs))
-      if (Error Err = linkBitcodeFiles(Input, InputFiles, LinkerArgs))
-        return Err;
-
-    // Write any remaining device inputs to an output file for the
-    // linker.
     for (const OffloadFile &File : Input) {
       auto FileNameOrErr = writeOffloadFile(File);
       if (!FileNameOrErr)

[jhuber6]

@asudarsa