xref: /freebsd-src/contrib/llvm-project/llvm/lib/MC/ELFObjectWriter.cpp (revision 0fca6ea1d4eea4c934cfff25ac9ee8ad6fe95583)

//===- lib/MC/ELFObjectWriter.cpp - ELF File Writer -----------------------===//
//
// 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 ELF object file writer information.
//
//===----------------------------------------------------------------------===//

#include "llvm/ADT/ArrayRef.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/StringExtras.h"
#include "llvm/ADT/StringRef.h"
#include "llvm/ADT/Twine.h"
#include "llvm/ADT/iterator.h"
#include "llvm/BinaryFormat/ELF.h"
#include "llvm/MC/MCAsmBackend.h"
#include "llvm/MC/MCAsmInfo.h"
#include "llvm/MC/MCAssembler.h"
#include "llvm/MC/MCContext.h"
#include "llvm/MC/MCELFExtras.h"
#include "llvm/MC/MCELFObjectWriter.h"
#include "llvm/MC/MCExpr.h"
#include "llvm/MC/MCFixup.h"
#include "llvm/MC/MCFixupKindInfo.h"
#include "llvm/MC/MCFragment.h"
#include "llvm/MC/MCObjectWriter.h"
#include "llvm/MC/MCSection.h"
#include "llvm/MC/MCSectionELF.h"
#include "llvm/MC/MCSymbol.h"
#include "llvm/MC/MCSymbolELF.h"
#include "llvm/MC/MCTargetOptions.h"
#include "llvm/MC/MCValue.h"
#include "llvm/MC/StringTableBuilder.h"
#include "llvm/Support/Alignment.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Compression.h"
#include "llvm/Support/Endian.h"
#include "llvm/Support/EndianStream.h"
#include "llvm/Support/Error.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/LEB128.h"
#include "llvm/Support/MathExtras.h"
#include "llvm/Support/SMLoc.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/TargetParser/Host.h"
#include <algorithm>
#include <cassert>
#include <cstddef>
#include <cstdint>
#include <map>
#include <memory>
#include <string>
#include <utility>
#include <vector>

using namespace llvm;

#undef  DEBUG_TYPE
#define DEBUG_TYPE "reloc-info"

namespace {

struct ELFWriter;

bool isDwoSection(const MCSectionELF &Sec) {
  return Sec.getName().ends_with(".dwo");
}

class SymbolTableWriter {
  ELFWriter &EWriter;
  bool Is64Bit;

  // indexes we are going to write to .symtab_shndx.
  std::vector<uint32_t> ShndxIndexes;

  // The numbel of symbols written so far.
  unsigned NumWritten;

  void createSymtabShndx();

  template <typename T> void write(T Value);

public:
  SymbolTableWriter(ELFWriter &EWriter, bool Is64Bit);

  void writeSymbol(uint32_t name, uint8_t info, uint64_t value, uint64_t size,
                   uint8_t other, uint32_t shndx, bool Reserved);

  ArrayRef<uint32_t> getShndxIndexes() const { return ShndxIndexes; }
};

struct ELFWriter {
  ELFObjectWriter &OWriter;
  support::endian::Writer W;

  enum DwoMode {
    AllSections,
    NonDwoOnly,
    DwoOnly,
  } Mode;

  static uint64_t symbolValue(const MCAssembler &Asm, const MCSymbol &Sym);
  static bool isInSymtab(const MCAssembler &Asm, const MCSymbolELF &Symbol,
                         bool Used, bool Renamed);

  /// Helper struct for containing some precomputed information on symbols.
  struct ELFSymbolData {
    const MCSymbolELF *Symbol;
    StringRef Name;
    uint32_t SectionIndex;
    uint32_t Order;
  };

  /// @}
  /// @name Symbol Table Data
  /// @{

  StringTableBuilder StrTabBuilder{StringTableBuilder::ELF};

  /// @}

  // This holds the symbol table index of the last local symbol.
  unsigned LastLocalSymbolIndex = ~0u;
  // This holds the .strtab section index.
  unsigned StringTableIndex = ~0u;
  // This holds the .symtab section index.
  unsigned SymbolTableIndex = ~0u;

  // Sections in the order they are to be output in the section table.
  std::vector<MCSectionELF *> SectionTable;
  unsigned addToSectionTable(MCSectionELF *Sec);

  // TargetObjectWriter wrappers.
  bool is64Bit() const;

  uint64_t align(Align Alignment);

  bool maybeWriteCompression(uint32_t ChType, uint64_t Size,
                             SmallVectorImpl<uint8_t> &CompressedContents,
                             Align Alignment);

public:
  ELFWriter(ELFObjectWriter &OWriter, raw_pwrite_stream &OS,
            bool IsLittleEndian, DwoMode Mode)
      : OWriter(OWriter), W(OS, IsLittleEndian ? llvm::endianness::little
                                               : llvm::endianness::big),
        Mode(Mode) {}

  void WriteWord(uint64_t Word) {
    if (is64Bit())
      W.write<uint64_t>(Word);
    else
      W.write<uint32_t>(Word);
  }

  template <typename T> void write(T Val) {
    W.write(Val);
  }

  void writeHeader(const MCAssembler &Asm);

  void writeSymbol(const MCAssembler &Asm, SymbolTableWriter &Writer,
                   uint32_t StringIndex, ELFSymbolData &MSD);

  // Map from a signature symbol to the group section index
  using RevGroupMapTy = DenseMap<const MCSymbol *, unsigned>;

  /// Compute the symbol table data
  ///
  /// \param Asm - The assembler.
  /// \param RevGroupMap - Maps a signature symbol to the group section.
  void computeSymbolTable(MCAssembler &Asm, const RevGroupMapTy &RevGroupMap);

  void writeAddrsigSection();

  MCSectionELF *createRelocationSection(MCContext &Ctx,
                                        const MCSectionELF &Sec);

  void writeSectionHeader(const MCAssembler &Asm);

  void writeSectionData(const MCAssembler &Asm, MCSection &Sec);

  void WriteSecHdrEntry(uint32_t Name, uint32_t Type, uint64_t Flags,
                        uint64_t Address, uint64_t Offset, uint64_t Size,
                        uint32_t Link, uint32_t Info, MaybeAlign Alignment,
                        uint64_t EntrySize);

  void writeRelocations(const MCAssembler &Asm, const MCSectionELF &Sec);

  uint64_t writeObject(MCAssembler &Asm);
  void writeSection(uint32_t GroupSymbolIndex, uint64_t Offset, uint64_t Size,
                    const MCSectionELF &Section);
};
} // end anonymous namespace

uint64_t ELFWriter::align(Align Alignment) {
  uint64_t Offset = W.OS.tell();
  uint64_t NewOffset = alignTo(Offset, Alignment);
  W.OS.write_zeros(NewOffset - Offset);
  return NewOffset;
}

unsigned ELFWriter::addToSectionTable(MCSectionELF *Sec) {
  SectionTable.push_back(Sec);
  StrTabBuilder.add(Sec->getName());
  return SectionTable.size();
}

void SymbolTableWriter::createSymtabShndx() {
  if (!ShndxIndexes.empty())
    return;

  ShndxIndexes.resize(NumWritten);
}

template <typename T> void SymbolTableWriter::write(T Value) {
  EWriter.write(Value);
}

SymbolTableWriter::SymbolTableWriter(ELFWriter &EWriter, bool Is64Bit)
    : EWriter(EWriter), Is64Bit(Is64Bit), NumWritten(0) {}

void SymbolTableWriter::writeSymbol(uint32_t name, uint8_t info, uint64_t value,
                                    uint64_t size, uint8_t other,
                                    uint32_t shndx, bool Reserved) {
  bool LargeIndex = shndx >= ELF::SHN_LORESERVE && !Reserved;

  if (LargeIndex)
    createSymtabShndx();

  if (!ShndxIndexes.empty()) {
    if (LargeIndex)
      ShndxIndexes.push_back(shndx);
    else
      ShndxIndexes.push_back(0);
  }

  uint16_t Index = LargeIndex ? uint16_t(ELF::SHN_XINDEX) : shndx;

  if (Is64Bit) {
    write(name);  // st_name
    write(info);  // st_info
    write(other); // st_other
    write(Index); // st_shndx
    write(value); // st_value
    write(size);  // st_size
  } else {
    write(name);            // st_name
    write(uint32_t(value)); // st_value
    write(uint32_t(size));  // st_size
    write(info);            // st_info
    write(other);           // st_other
    write(Index);           // st_shndx
  }

  ++NumWritten;
}

bool ELFWriter::is64Bit() const {
  return OWriter.TargetObjectWriter->is64Bit();
}

// Emit the ELF header.
void ELFWriter::writeHeader(const MCAssembler &Asm) {
  // ELF Header
  // ----------
  //
  // Note
  // ----
  // emitWord method behaves differently for ELF32 and ELF64, writing
  // 4 bytes in the former and 8 in the latter.

  W.OS << ELF::ElfMagic; // e_ident[EI_MAG0] to e_ident[EI_MAG3]

  W.OS << char(is64Bit() ? ELF::ELFCLASS64 : ELF::ELFCLASS32); // e_ident[EI_CLASS]

  // e_ident[EI_DATA]
  W.OS << char(W.Endian == llvm::endianness::little ? ELF::ELFDATA2LSB
                                                    : ELF::ELFDATA2MSB);

  W.OS << char(ELF::EV_CURRENT);        // e_ident[EI_VERSION]
  // e_ident[EI_OSABI]
  uint8_t OSABI = OWriter.TargetObjectWriter->getOSABI();
  W.OS << char(OSABI == ELF::ELFOSABI_NONE && OWriter.seenGnuAbi()
                   ? int(ELF::ELFOSABI_GNU)
                   : OSABI);
  // e_ident[EI_ABIVERSION]
  W.OS << char(OWriter.OverrideABIVersion
                   ? *OWriter.OverrideABIVersion
                   : OWriter.TargetObjectWriter->getABIVersion());

  W.OS.write_zeros(ELF::EI_NIDENT - ELF::EI_PAD);

  W.write<uint16_t>(ELF::ET_REL);             // e_type

  W.write<uint16_t>(OWriter.TargetObjectWriter->getEMachine()); // e_machine = target

  W.write<uint32_t>(ELF::EV_CURRENT);         // e_version
  WriteWord(0);                    // e_entry, no entry point in .o file
  WriteWord(0);                    // e_phoff, no program header for .o
  WriteWord(0);                     // e_shoff = sec hdr table off in bytes

  // e_flags = whatever the target wants
  W.write<uint32_t>(OWriter.getELFHeaderEFlags());

  // e_ehsize = ELF header size
  W.write<uint16_t>(is64Bit() ? sizeof(ELF::Elf64_Ehdr)
                              : sizeof(ELF::Elf32_Ehdr));

  W.write<uint16_t>(0);                  // e_phentsize = prog header entry size
  W.write<uint16_t>(0);                  // e_phnum = # prog header entries = 0

  // e_shentsize = Section header entry size
  W.write<uint16_t>(is64Bit() ? sizeof(ELF::Elf64_Shdr)
                              : sizeof(ELF::Elf32_Shdr));

  // e_shnum     = # of section header ents
  W.write<uint16_t>(0);

  // e_shstrndx  = Section # of '.strtab'
  assert(StringTableIndex < ELF::SHN_LORESERVE);
  W.write<uint16_t>(StringTableIndex);
}

uint64_t ELFWriter::symbolValue(const MCAssembler &Asm, const MCSymbol &Sym) {
  if (Sym.isCommon())
    return Sym.getCommonAlignment()->value();

  uint64_t Res;
  if (!Asm.getSymbolOffset(Sym, Res))
    return 0;

  if (Asm.isThumbFunc(&Sym))
    Res |= 1;

  return Res;
}

static uint8_t mergeTypeForSet(uint8_t origType, uint8_t newType) {
  uint8_t Type = newType;

  // Propagation rules:
  // IFUNC > FUNC > OBJECT > NOTYPE
  // TLS_OBJECT > OBJECT > NOTYPE
  //
  // dont let the new type degrade the old type
  switch (origType) {
  default:
    break;
  case ELF::STT_GNU_IFUNC:
    if (Type == ELF::STT_FUNC || Type == ELF::STT_OBJECT ||
        Type == ELF::STT_NOTYPE || Type == ELF::STT_TLS)
      Type = ELF::STT_GNU_IFUNC;
    break;
  case ELF::STT_FUNC:
    if (Type == ELF::STT_OBJECT || Type == ELF::STT_NOTYPE ||
        Type == ELF::STT_TLS)
      Type = ELF::STT_FUNC;
    break;
  case ELF::STT_OBJECT:
    if (Type == ELF::STT_NOTYPE)
      Type = ELF::STT_OBJECT;
    break;
  case ELF::STT_TLS:
    if (Type == ELF::STT_OBJECT || Type == ELF::STT_NOTYPE ||
        Type == ELF::STT_GNU_IFUNC || Type == ELF::STT_FUNC)
      Type = ELF::STT_TLS;
    break;
  }

  return Type;
}

static bool isIFunc(const MCSymbolELF *Symbol) {
  while (Symbol->getType() != ELF::STT_GNU_IFUNC) {
    const MCSymbolRefExpr *Value;
    if (!Symbol->isVariable() ||
        !(Value = dyn_cast<MCSymbolRefExpr>(Symbol->getVariableValue())) ||
        Value->getKind() != MCSymbolRefExpr::VK_None ||
        mergeTypeForSet(Symbol->getType(), ELF::STT_GNU_IFUNC) != ELF::STT_GNU_IFUNC)
      return false;
    Symbol = &cast<MCSymbolELF>(Value->getSymbol());
  }
  return true;
}

void ELFWriter::writeSymbol(const MCAssembler &Asm, SymbolTableWriter &Writer,
                            uint32_t StringIndex, ELFSymbolData &MSD) {
  const auto &Symbol = cast<MCSymbolELF>(*MSD.Symbol);
  const MCSymbolELF *Base =
      cast_or_null<MCSymbolELF>(Asm.getBaseSymbol(Symbol));

  // This has to be in sync with when computeSymbolTable uses SHN_ABS or
  // SHN_COMMON.
  bool IsReserved = !Base || Symbol.isCommon();

  // Binding and Type share the same byte as upper and lower nibbles
  uint8_t Binding = Symbol.getBinding();
  uint8_t Type = Symbol.getType();
  if (isIFunc(&Symbol))
    Type = ELF::STT_GNU_IFUNC;
  if (Base) {
    Type = mergeTypeForSet(Type, Base->getType());
  }
  uint8_t Info = (Binding << 4) | Type;

  // Other and Visibility share the same byte with Visibility using the lower
  // 2 bits
  uint8_t Visibility = Symbol.getVisibility();
  uint8_t Other = Symbol.getOther() | Visibility;

  uint64_t Value = symbolValue(Asm, *MSD.Symbol);
  uint64_t Size = 0;

  const MCExpr *ESize = MSD.Symbol->getSize();
  if (!ESize && Base) {
    // For expressions like .set y, x+1, if y's size is unset, inherit from x.
    ESize = Base->getSize();

    // For `.size x, 2; y = x; .size y, 1; z = y; z1 = z; .symver y, y@v1`, z,
    // z1, and y@v1's st_size equals y's. However, `Base` is `x` which will give
    // us 2. Follow the MCSymbolRefExpr assignment chain, which covers most
    // needs. MCBinaryExpr is not handled.
    const MCSymbolELF *Sym = &Symbol;
    while (Sym->isVariable()) {
      if (auto *Expr =
              dyn_cast<MCSymbolRefExpr>(Sym->getVariableValue(false))) {
        Sym = cast<MCSymbolELF>(&Expr->getSymbol());
        if (!Sym->getSize())
          continue;
        ESize = Sym->getSize();
      }
      break;
    }
  }

  if (ESize) {
    int64_t Res;
    if (!ESize->evaluateKnownAbsolute(Res, Asm))
      report_fatal_error("Size expression must be absolute.");
    Size = Res;
  }

  // Write out the symbol table entry
  Writer.writeSymbol(StringIndex, Info, Value, Size, Other, MSD.SectionIndex,
                     IsReserved);
}

bool ELFWriter::isInSymtab(const MCAssembler &Asm, const MCSymbolELF &Symbol,
                           bool Used, bool Renamed) {
  if (Symbol.isVariable()) {
    const MCExpr *Expr = Symbol.getVariableValue();
    // Target Expressions that are always inlined do not appear in the symtab
    if (const auto *T = dyn_cast<MCTargetExpr>(Expr))
      if (T->inlineAssignedExpr())
        return false;
    if (const MCSymbolRefExpr *Ref = dyn_cast<MCSymbolRefExpr>(Expr)) {
      if (Ref->getKind() == MCSymbolRefExpr::VK_WEAKREF)
        return false;
    }
  }

  if (Used)
    return true;

  if (Renamed)
    return false;

  if (Symbol.isVariable() && Symbol.isUndefined()) {
    // FIXME: this is here just to diagnose the case of a var = commmon_sym.
    Asm.getBaseSymbol(Symbol);
    return false;
  }

  if (Symbol.isTemporary())
    return false;

  if (Symbol.getType() == ELF::STT_SECTION)
    return false;

  return true;
}

void ELFWriter::computeSymbolTable(MCAssembler &Asm,
                                   const RevGroupMapTy &RevGroupMap) {
  MCContext &Ctx = Asm.getContext();
  SymbolTableWriter Writer(*this, is64Bit());

  // Symbol table
  unsigned EntrySize = is64Bit() ? ELF::SYMENTRY_SIZE64 : ELF::SYMENTRY_SIZE32;
  MCSectionELF *SymtabSection =
      Ctx.getELFSection(".symtab", ELF::SHT_SYMTAB, 0, EntrySize);
  SymtabSection->setAlignment(is64Bit() ? Align(8) : Align(4));
  SymbolTableIndex = addToSectionTable(SymtabSection);

  uint64_t SecStart = align(SymtabSection->getAlign());

  // The first entry is the undefined symbol entry.
  Writer.writeSymbol(0, 0, 0, 0, 0, 0, false);

  std::vector<ELFSymbolData> LocalSymbolData;
  std::vector<ELFSymbolData> ExternalSymbolData;
  MutableArrayRef<std::pair<std::string, size_t>> FileNames =
      OWriter.getFileNames();
  for (const std::pair<std::string, size_t> &F : FileNames)
    StrTabBuilder.add(F.first);

  // Add the data for the symbols.
  bool HasLargeSectionIndex = false;
  for (auto It : llvm::enumerate(Asm.symbols())) {
    const auto &Symbol = cast<MCSymbolELF>(It.value());
    bool Used = Symbol.isUsedInReloc();
    bool WeakrefUsed = Symbol.isWeakrefUsedInReloc();
    bool isSignature = Symbol.isSignature();

    if (!isInSymtab(Asm, Symbol, Used || WeakrefUsed || isSignature,
                    OWriter.Renames.count(&Symbol)))
      continue;

    if (Symbol.isTemporary() && Symbol.isUndefined()) {
      Ctx.reportError(SMLoc(), "Undefined temporary symbol " + Symbol.getName());
      continue;
    }

    ELFSymbolData MSD;
    MSD.Symbol = cast<MCSymbolELF>(&Symbol);
    MSD.Order = It.index();

    bool Local = Symbol.getBinding() == ELF::STB_LOCAL;
    assert(Local || !Symbol.isTemporary());

    if (Symbol.isAbsolute()) {
      MSD.SectionIndex = ELF::SHN_ABS;
    } else if (Symbol.isCommon()) {
      if (Symbol.isTargetCommon()) {
        MSD.SectionIndex = Symbol.getIndex();
      } else {
        assert(!Local);
        MSD.SectionIndex = ELF::SHN_COMMON;
      }
    } else if (Symbol.isUndefined()) {
      if (isSignature && !Used) {
        MSD.SectionIndex = RevGroupMap.lookup(&Symbol);
        if (MSD.SectionIndex >= ELF::SHN_LORESERVE)
          HasLargeSectionIndex = true;
      } else {
        MSD.SectionIndex = ELF::SHN_UNDEF;
      }
    } else {
      const MCSectionELF &Section =
          static_cast<const MCSectionELF &>(Symbol.getSection());

      // We may end up with a situation when section symbol is technically
      // defined, but should not be. That happens because we explicitly
      // pre-create few .debug_* sections to have accessors.
      // And if these sections were not really defined in the code, but were
      // referenced, we simply error out.
      if (!Section.isRegistered()) {
        assert(static_cast<const MCSymbolELF &>(Symbol).getType() ==
               ELF::STT_SECTION);
        Ctx.reportError(SMLoc(),
                        "Undefined section reference: " + Symbol.getName());
        continue;
      }

      if (Mode == NonDwoOnly && isDwoSection(Section))
        continue;
      MSD.SectionIndex = Section.getOrdinal();
      assert(MSD.SectionIndex && "Invalid section index!");
      if (MSD.SectionIndex >= ELF::SHN_LORESERVE)
        HasLargeSectionIndex = true;
    }

    // Temporary symbols generated for certain assembler features (.eh_frame,
    // .debug_line) of an empty name may be referenced by relocations due to
    // linker relaxation. Rename them to ".L0 " to match the gas fake label name
    // and allow ld/objcopy --discard-locals to discard such symbols.
    StringRef Name = Symbol.getName();
    if (Name.empty())
      Name = ".L0 ";

    // Sections have their own string table
    if (Symbol.getType() != ELF::STT_SECTION) {
      MSD.Name = Name;
      StrTabBuilder.add(Name);
    }

    if (Local)
      LocalSymbolData.push_back(MSD);
    else
      ExternalSymbolData.push_back(MSD);
  }

  // This holds the .symtab_shndx section index.
  unsigned SymtabShndxSectionIndex = 0;

  if (HasLargeSectionIndex) {
    MCSectionELF *SymtabShndxSection =
        Ctx.getELFSection(".symtab_shndx", ELF::SHT_SYMTAB_SHNDX, 0, 4);
    SymtabShndxSectionIndex = addToSectionTable(SymtabShndxSection);
    SymtabShndxSection->setAlignment(Align(4));
  }

  StrTabBuilder.finalize();

  // Make the first STT_FILE precede previous local symbols.
  unsigned Index = 1;
  auto FileNameIt = FileNames.begin();
  if (!FileNames.empty())
    FileNames[0].second = 0;

  for (ELFSymbolData &MSD : LocalSymbolData) {
    // Emit STT_FILE symbols before their associated local symbols.
    for (; FileNameIt != FileNames.end() && FileNameIt->second <= MSD.Order;
         ++FileNameIt) {
      Writer.writeSymbol(StrTabBuilder.getOffset(FileNameIt->first),
                         ELF::STT_FILE | ELF::STB_LOCAL, 0, 0, ELF::STV_DEFAULT,
                         ELF::SHN_ABS, true);
      ++Index;
    }

    unsigned StringIndex = MSD.Symbol->getType() == ELF::STT_SECTION
                               ? 0
                               : StrTabBuilder.getOffset(MSD.Name);
    MSD.Symbol->setIndex(Index++);
    writeSymbol(Asm, Writer, StringIndex, MSD);
  }
  for (; FileNameIt != FileNames.end(); ++FileNameIt) {
    Writer.writeSymbol(StrTabBuilder.getOffset(FileNameIt->first),
                       ELF::STT_FILE | ELF::STB_LOCAL, 0, 0, ELF::STV_DEFAULT,
                       ELF::SHN_ABS, true);
    ++Index;
  }

  // Write the symbol table entries.
  LastLocalSymbolIndex = Index;

  for (ELFSymbolData &MSD : ExternalSymbolData) {
    unsigned StringIndex = StrTabBuilder.getOffset(MSD.Name);
    MSD.Symbol->setIndex(Index++);
    writeSymbol(Asm, Writer, StringIndex, MSD);
    assert(MSD.Symbol->getBinding() != ELF::STB_LOCAL);
  }

  uint64_t SecEnd = W.OS.tell();
  SymtabSection->setOffsets(SecStart, SecEnd);

  ArrayRef<uint32_t> ShndxIndexes = Writer.getShndxIndexes();
  if (ShndxIndexes.empty()) {
    assert(SymtabShndxSectionIndex == 0);
    return;
  }
  assert(SymtabShndxSectionIndex != 0);

  SecStart = W.OS.tell();
  MCSectionELF *SymtabShndxSection = SectionTable[SymtabShndxSectionIndex - 1];
  for (uint32_t Index : ShndxIndexes)
    write(Index);
  SecEnd = W.OS.tell();
  SymtabShndxSection->setOffsets(SecStart, SecEnd);
}

void ELFWriter::writeAddrsigSection() {
  for (const MCSymbol *Sym : OWriter.getAddrsigSyms())
    if (Sym->getIndex() != 0)
      encodeULEB128(Sym->getIndex(), W.OS);
}

MCSectionELF *ELFWriter::createRelocationSection(MCContext &Ctx,
                                                 const MCSectionELF &Sec) {
  if (OWriter.Relocations[&Sec].empty())
    return nullptr;

  unsigned Flags = ELF::SHF_INFO_LINK;
  if (Sec.getFlags() & ELF::SHF_GROUP)
    Flags = ELF::SHF_GROUP;

  const StringRef SectionName = Sec.getName();
  const MCTargetOptions *TO = Ctx.getTargetOptions();
  if (TO && TO->Crel) {
    MCSectionELF *RelaSection =
        Ctx.createELFRelSection(".crel" + SectionName, ELF::SHT_CREL, Flags,
                                /*EntrySize=*/1, Sec.getGroup(), &Sec);
    return RelaSection;
  }

  const bool Rela = OWriter.usesRela(TO, Sec);
  unsigned EntrySize;
  if (Rela)
    EntrySize = is64Bit() ? sizeof(ELF::Elf64_Rela) : sizeof(ELF::Elf32_Rela);
  else
    EntrySize = is64Bit() ? sizeof(ELF::Elf64_Rel) : sizeof(ELF::Elf32_Rel);

  MCSectionELF *RelaSection =
      Ctx.createELFRelSection(((Rela ? ".rela" : ".rel") + SectionName),
                              Rela ? ELF::SHT_RELA : ELF::SHT_REL, Flags,
                              EntrySize, Sec.getGroup(), &Sec);
  RelaSection->setAlignment(is64Bit() ? Align(8) : Align(4));
  return RelaSection;
}

// Include the debug info compression header.
bool ELFWriter::maybeWriteCompression(
    uint32_t ChType, uint64_t Size,
    SmallVectorImpl<uint8_t> &CompressedContents, Align Alignment) {
  uint64_t HdrSize =
      is64Bit() ? sizeof(ELF::Elf64_Chdr) : sizeof(ELF::Elf32_Chdr);
  if (Size <= HdrSize + CompressedContents.size())
    return false;
  // Platform specific header is followed by compressed data.
  if (is64Bit()) {
    // Write Elf64_Chdr header.
    write(static_cast<ELF::Elf64_Word>(ChType));
    write(static_cast<ELF::Elf64_Word>(0)); // ch_reserved field.
    write(static_cast<ELF::Elf64_Xword>(Size));
    write(static_cast<ELF::Elf64_Xword>(Alignment.value()));
  } else {
    // Write Elf32_Chdr header otherwise.
    write(static_cast<ELF::Elf32_Word>(ChType));
    write(static_cast<ELF::Elf32_Word>(Size));
    write(static_cast<ELF::Elf32_Word>(Alignment.value()));
  }
  return true;
}

void ELFWriter::writeSectionData(const MCAssembler &Asm, MCSection &Sec) {
  MCSectionELF &Section = static_cast<MCSectionELF &>(Sec);
  StringRef SectionName = Section.getName();
  auto &Ctx = Asm.getContext();
  const DebugCompressionType CompressionType =
      Ctx.getTargetOptions() ? Ctx.getTargetOptions()->CompressDebugSections
                             : DebugCompressionType::None;
  if (CompressionType == DebugCompressionType::None ||
      !SectionName.starts_with(".debug_")) {
    Asm.writeSectionData(W.OS, &Section);
    return;
  }

  SmallVector<char, 128> UncompressedData;
  raw_svector_ostream VecOS(UncompressedData);
  Asm.writeSectionData(VecOS, &Section);
  ArrayRef<uint8_t> Uncompressed =
      ArrayRef(reinterpret_cast<uint8_t *>(UncompressedData.data()),
               UncompressedData.size());

  SmallVector<uint8_t, 128> Compressed;
  uint32_t ChType;
  switch (CompressionType) {
  case DebugCompressionType::None:
    llvm_unreachable("has been handled");
  case DebugCompressionType::Zlib:
    ChType = ELF::ELFCOMPRESS_ZLIB;
    break;
  case DebugCompressionType::Zstd:
    ChType = ELF::ELFCOMPRESS_ZSTD;
    break;
  }
  compression::compress(compression::Params(CompressionType), Uncompressed,
                        Compressed);
  if (!maybeWriteCompression(ChType, UncompressedData.size(), Compressed,
                             Sec.getAlign())) {
    W.OS << UncompressedData;
    return;
  }

  Section.setFlags(Section.getFlags() | ELF::SHF_COMPRESSED);
  // Alignment field should reflect the requirements of
  // the compressed section header.
  Section.setAlignment(is64Bit() ? Align(8) : Align(4));
  W.OS << toStringRef(Compressed);
}

void ELFWriter::WriteSecHdrEntry(uint32_t Name, uint32_t Type, uint64_t Flags,
                                 uint64_t Address, uint64_t Offset,
                                 uint64_t Size, uint32_t Link, uint32_t Info,
                                 MaybeAlign Alignment, uint64_t EntrySize) {
  W.write<uint32_t>(Name);        // sh_name: index into string table
  W.write<uint32_t>(Type);        // sh_type
  WriteWord(Flags);     // sh_flags
  WriteWord(Address);   // sh_addr
  WriteWord(Offset);    // sh_offset
  WriteWord(Size);      // sh_size
  W.write<uint32_t>(Link);        // sh_link
  W.write<uint32_t>(Info);        // sh_info
  WriteWord(Alignment ? Alignment->value() : 0); // sh_addralign
  WriteWord(EntrySize); // sh_entsize
}

template <bool Is64>
static void encodeCrel(ArrayRef<ELFRelocationEntry> Relocs, raw_ostream &OS) {
  using uint = std::conditional_t<Is64, uint64_t, uint32_t>;
  ELF::encodeCrel<Is64>(OS, Relocs, [&](const ELFRelocationEntry &R) {
    uint32_t SymIdx = R.Symbol ? R.Symbol->getIndex() : 0;
    return ELF::Elf_Crel<Is64>{static_cast<uint>(R.Offset), SymIdx, R.Type,
                               std::make_signed_t<uint>(R.Addend)};
  });
}

void ELFWriter::writeRelocations(const MCAssembler &Asm,
                                       const MCSectionELF &Sec) {
  std::vector<ELFRelocationEntry> &Relocs = OWriter.Relocations[&Sec];
  const MCTargetOptions *TO = Asm.getContext().getTargetOptions();
  const bool Rela = OWriter.usesRela(TO, Sec);

  // Sort the relocation entries. MIPS needs this.
  OWriter.TargetObjectWriter->sortRelocs(Asm, Relocs);

  if (OWriter.TargetObjectWriter->getEMachine() == ELF::EM_MIPS) {
    for (const ELFRelocationEntry &Entry : Relocs) {
      uint32_t SymIdx = Entry.Symbol ? Entry.Symbol->getIndex() : 0;
      if (is64Bit()) {
        write(Entry.Offset);
        write(uint32_t(SymIdx));
        write(OWriter.TargetObjectWriter->getRSsym(Entry.Type));
        write(OWriter.TargetObjectWriter->getRType3(Entry.Type));
        write(OWriter.TargetObjectWriter->getRType2(Entry.Type));
        write(OWriter.TargetObjectWriter->getRType(Entry.Type));
        if (Rela)
          write(Entry.Addend);
      } else {
        write(uint32_t(Entry.Offset));
        ELF::Elf32_Rela ERE32;
        ERE32.setSymbolAndType(SymIdx, Entry.Type);
        write(ERE32.r_info);
        if (Rela)
          write(uint32_t(Entry.Addend));
        if (uint32_t RType =
                OWriter.TargetObjectWriter->getRType2(Entry.Type)) {
          write(uint32_t(Entry.Offset));
          ERE32.setSymbolAndType(0, RType);
          write(ERE32.r_info);
          write(uint32_t(0));
        }
        if (uint32_t RType =
                OWriter.TargetObjectWriter->getRType3(Entry.Type)) {
          write(uint32_t(Entry.Offset));
          ERE32.setSymbolAndType(0, RType);
          write(ERE32.r_info);
          write(uint32_t(0));
        }
      }
    }
  } else if (TO && TO->Crel) {
    if (is64Bit())
      encodeCrel<true>(Relocs, W.OS);
    else
      encodeCrel<false>(Relocs, W.OS);
  } else {
    for (const ELFRelocationEntry &Entry : Relocs) {
      uint32_t Symidx = Entry.Symbol ? Entry.Symbol->getIndex() : 0;
      if (is64Bit()) {
        write(Entry.Offset);
        ELF::Elf64_Rela ERE;
        ERE.setSymbolAndType(Symidx, Entry.Type);
        write(ERE.r_info);
        if (Rela)
          write(Entry.Addend);
      } else {
        write(uint32_t(Entry.Offset));
        ELF::Elf32_Rela ERE;
        ERE.setSymbolAndType(Symidx, Entry.Type);
        write(ERE.r_info);
        if (Rela)
          write(uint32_t(Entry.Addend));
      }
    }
  }
}

void ELFWriter::writeSection(uint32_t GroupSymbolIndex, uint64_t Offset,
                             uint64_t Size, const MCSectionELF &Section) {
  uint64_t sh_link = 0;
  uint64_t sh_info = 0;

  switch(Section.getType()) {
  default:
    // Nothing to do.
    break;

  case ELF::SHT_DYNAMIC:
    llvm_unreachable("SHT_DYNAMIC in a relocatable object");

  case ELF::SHT_REL:
  case ELF::SHT_RELA:
  case ELF::SHT_CREL: {
    sh_link = SymbolTableIndex;
    assert(sh_link && ".symtab not found");
    const MCSection *InfoSection = Section.getLinkedToSection();
    sh_info = InfoSection->getOrdinal();
    break;
  }

  case ELF::SHT_SYMTAB:
    sh_link = StringTableIndex;
    sh_info = LastLocalSymbolIndex;
    break;

  case ELF::SHT_SYMTAB_SHNDX:
  case ELF::SHT_LLVM_CALL_GRAPH_PROFILE:
  case ELF::SHT_LLVM_ADDRSIG:
    sh_link = SymbolTableIndex;
    break;

  case ELF::SHT_GROUP:
    sh_link = SymbolTableIndex;
    sh_info = GroupSymbolIndex;
    break;
  }

  if (Section.getFlags() & ELF::SHF_LINK_ORDER) {
    // If the value in the associated metadata is not a definition, Sym will be
    // undefined. Represent this with sh_link=0.
    const MCSymbol *Sym = Section.getLinkedToSymbol();
    if (Sym && Sym->isInSection())
      sh_link = Sym->getSection().getOrdinal();
  }

  WriteSecHdrEntry(StrTabBuilder.getOffset(Section.getName()),
                   Section.getType(), Section.getFlags(), 0, Offset, Size,
                   sh_link, sh_info, Section.getAlign(),
                   Section.getEntrySize());
}

void ELFWriter::writeSectionHeader(const MCAssembler &Asm) {
  const unsigned NumSections = SectionTable.size();

  // Null section first.
  uint64_t FirstSectionSize =
      (NumSections + 1) >= ELF::SHN_LORESERVE ? NumSections + 1 : 0;
  WriteSecHdrEntry(0, 0, 0, 0, 0, FirstSectionSize, 0, 0, std::nullopt, 0);

  for (const MCSectionELF *Section : SectionTable) {
    uint32_t GroupSymbolIndex;
    unsigned Type = Section->getType();
    if (Type != ELF::SHT_GROUP)
      GroupSymbolIndex = 0;
    else
      GroupSymbolIndex = Section->getGroup()->getIndex();

    std::pair<uint64_t, uint64_t> Offsets = Section->getOffsets();
    uint64_t Size;
    if (Type == ELF::SHT_NOBITS)
      Size = Asm.getSectionAddressSize(*Section);
    else
      Size = Offsets.second - Offsets.first;

    writeSection(GroupSymbolIndex, Offsets.first, Size, *Section);
  }
}

uint64_t ELFWriter::writeObject(MCAssembler &Asm) {
  uint64_t StartOffset = W.OS.tell();

  MCContext &Ctx = Asm.getContext();
  MCSectionELF *StrtabSection =
      Ctx.getELFSection(".strtab", ELF::SHT_STRTAB, 0);
  StringTableIndex = addToSectionTable(StrtabSection);

  RevGroupMapTy RevGroupMap;

  // Write out the ELF header ...
  writeHeader(Asm);

  // ... then the sections ...
  SmallVector<std::pair<MCSectionELF *, SmallVector<unsigned>>, 0> Groups;
  // Map from group section index to group
  SmallVector<unsigned, 0> GroupMap;
  SmallVector<MCSectionELF *> Relocations;
  for (MCSection &Sec : Asm) {
    MCSectionELF &Section = static_cast<MCSectionELF &>(Sec);
    if (Mode == NonDwoOnly && isDwoSection(Section))
      continue;
    if (Mode == DwoOnly && !isDwoSection(Section))
      continue;

    // Remember the offset into the file for this section.
    const uint64_t SecStart = align(Section.getAlign());

    const MCSymbolELF *SignatureSymbol = Section.getGroup();
    writeSectionData(Asm, Section);

    uint64_t SecEnd = W.OS.tell();
    Section.setOffsets(SecStart, SecEnd);

    MCSectionELF *RelSection = createRelocationSection(Ctx, Section);

    unsigned *GroupIdxEntry = nullptr;
    if (SignatureSymbol) {
      GroupIdxEntry = &RevGroupMap[SignatureSymbol];
      if (!*GroupIdxEntry) {
        MCSectionELF *Group =
            Ctx.createELFGroupSection(SignatureSymbol, Section.isComdat());
        *GroupIdxEntry = addToSectionTable(Group);
        Group->setAlignment(Align(4));

        GroupMap.resize(*GroupIdxEntry + 1);
        GroupMap[*GroupIdxEntry] = Groups.size();
        Groups.emplace_back(Group, SmallVector<unsigned>{});
      }
    }

    Section.setOrdinal(addToSectionTable(&Section));
    if (RelSection) {
      RelSection->setOrdinal(addToSectionTable(RelSection));
      Relocations.push_back(RelSection);
    }

    if (GroupIdxEntry) {
      auto &Members = Groups[GroupMap[*GroupIdxEntry]];
      Members.second.push_back(Section.getOrdinal());
      if (RelSection)
        Members.second.push_back(RelSection->getOrdinal());
    }

    OWriter.TargetObjectWriter->addTargetSectionFlags(Ctx, Section);
  }

  for (auto &[Group, Members] : Groups) {
    // Remember the offset into the file for this section.
    const uint64_t SecStart = align(Group->getAlign());

    write(uint32_t(Group->isComdat() ? unsigned(ELF::GRP_COMDAT) : 0));
    W.write<unsigned>(Members);

    uint64_t SecEnd = W.OS.tell();
    Group->setOffsets(SecStart, SecEnd);
  }

  if (Mode == DwoOnly) {
    // dwo files don't have symbol tables or relocations, but they do have
    // string tables.
    StrTabBuilder.finalize();
  } else {
    MCSectionELF *AddrsigSection;
    if (OWriter.getEmitAddrsigSection()) {
      AddrsigSection = Ctx.getELFSection(".llvm_addrsig", ELF::SHT_LLVM_ADDRSIG,
                                         ELF::SHF_EXCLUDE);
      addToSectionTable(AddrsigSection);
    }

    // Compute symbol table information.
    computeSymbolTable(Asm, RevGroupMap);

    for (MCSectionELF *RelSection : Relocations) {
      // Remember the offset into the file for this section.
      const uint64_t SecStart = align(RelSection->getAlign());

      writeRelocations(Asm,
                       cast<MCSectionELF>(*RelSection->getLinkedToSection()));

      uint64_t SecEnd = W.OS.tell();
      RelSection->setOffsets(SecStart, SecEnd);
    }

    if (OWriter.getEmitAddrsigSection()) {
      uint64_t SecStart = W.OS.tell();
      writeAddrsigSection();
      uint64_t SecEnd = W.OS.tell();
      AddrsigSection->setOffsets(SecStart, SecEnd);
    }
  }

  {
    uint64_t SecStart = W.OS.tell();
    StrTabBuilder.write(W.OS);
    StrtabSection->setOffsets(SecStart, W.OS.tell());
  }

  const uint64_t SectionHeaderOffset = align(is64Bit() ? Align(8) : Align(4));

  // ... then the section header table ...
  writeSectionHeader(Asm);

  uint16_t NumSections = support::endian::byte_swap<uint16_t>(
      (SectionTable.size() + 1 >= ELF::SHN_LORESERVE) ? (uint16_t)ELF::SHN_UNDEF
                                                      : SectionTable.size() + 1,
      W.Endian);
  unsigned NumSectionsOffset;

  auto &Stream = static_cast<raw_pwrite_stream &>(W.OS);
  if (is64Bit()) {
    uint64_t Val =
        support::endian::byte_swap<uint64_t>(SectionHeaderOffset, W.Endian);
    Stream.pwrite(reinterpret_cast<char *>(&Val), sizeof(Val),
                  offsetof(ELF::Elf64_Ehdr, e_shoff));
    NumSectionsOffset = offsetof(ELF::Elf64_Ehdr, e_shnum);
  } else {
    uint32_t Val =
        support::endian::byte_swap<uint32_t>(SectionHeaderOffset, W.Endian);
    Stream.pwrite(reinterpret_cast<char *>(&Val), sizeof(Val),
                  offsetof(ELF::Elf32_Ehdr, e_shoff));
    NumSectionsOffset = offsetof(ELF::Elf32_Ehdr, e_shnum);
  }
  Stream.pwrite(reinterpret_cast<char *>(&NumSections), sizeof(NumSections),
                NumSectionsOffset);

  return W.OS.tell() - StartOffset;
}

ELFObjectWriter::ELFObjectWriter(std::unique_ptr<MCELFObjectTargetWriter> MOTW,
                                 raw_pwrite_stream &OS, bool IsLittleEndian)
    : TargetObjectWriter(std::move(MOTW)), OS(OS),
      IsLittleEndian(IsLittleEndian) {}
ELFObjectWriter::ELFObjectWriter(std::unique_ptr<MCELFObjectTargetWriter> MOTW,
                                 raw_pwrite_stream &OS,
                                 raw_pwrite_stream &DwoOS, bool IsLittleEndian)
    : TargetObjectWriter(std::move(MOTW)), OS(OS), DwoOS(&DwoOS),
      IsLittleEndian(IsLittleEndian) {}

void ELFObjectWriter::reset() {
  ELFHeaderEFlags = 0;
  SeenGnuAbi = false;
  OverrideABIVersion.reset();
  Relocations.clear();
  Renames.clear();
  Symvers.clear();
  MCObjectWriter::reset();
}

bool ELFObjectWriter::hasRelocationAddend() const {
  return TargetObjectWriter->hasRelocationAddend();
}

void ELFObjectWriter::executePostLayoutBinding(MCAssembler &Asm) {
  // The presence of symbol versions causes undefined symbols and
  // versions declared with @@@ to be renamed.
  for (const Symver &S : Symvers) {
    StringRef AliasName = S.Name;
    const auto &Symbol = cast<MCSymbolELF>(*S.Sym);
    size_t Pos = AliasName.find('@');
    assert(Pos != StringRef::npos);

    StringRef Prefix = AliasName.substr(0, Pos);
    StringRef Rest = AliasName.substr(Pos);
    StringRef Tail = Rest;
    if (Rest.starts_with("@@@"))
      Tail = Rest.substr(Symbol.isUndefined() ? 2 : 1);

    auto *Alias =
        cast<MCSymbolELF>(Asm.getContext().getOrCreateSymbol(Prefix + Tail));
    Asm.registerSymbol(*Alias);
    const MCExpr *Value = MCSymbolRefExpr::create(&Symbol, Asm.getContext());
    Alias->setVariableValue(Value);

    // Aliases defined with .symvar copy the binding from the symbol they alias.
    // This is the first place we are able to copy this information.
    Alias->setBinding(Symbol.getBinding());
    Alias->setVisibility(Symbol.getVisibility());
    Alias->setOther(Symbol.getOther());

    if (!Symbol.isUndefined() && S.KeepOriginalSym)
      continue;

    if (Symbol.isUndefined() && Rest.starts_with("@@") &&
        !Rest.starts_with("@@@")) {
      Asm.getContext().reportError(S.Loc, "default version symbol " +
                                              AliasName + " must be defined");
      continue;
    }

    if (Renames.count(&Symbol) && Renames[&Symbol] != Alias) {
      Asm.getContext().reportError(S.Loc, Twine("multiple versions for ") +
                                              Symbol.getName());
      continue;
    }

    Renames.insert(std::make_pair(&Symbol, Alias));
  }

  for (const MCSymbol *&Sym : AddrsigSyms) {
    if (const MCSymbol *R = Renames.lookup(cast<MCSymbolELF>(Sym)))
      Sym = R;
    if (Sym->isInSection() && Sym->getName().starts_with(".L"))
      Sym = Sym->getSection().getBeginSymbol();
    Sym->setUsedInReloc();
  }
}

// It is always valid to create a relocation with a symbol. It is preferable
// to use a relocation with a section if that is possible. Using the section
// allows us to omit some local symbols from the symbol table.
bool ELFObjectWriter::shouldRelocateWithSymbol(const MCAssembler &Asm,
                                               const MCValue &Val,
                                               const MCSymbolELF *Sym,
                                               uint64_t C,
                                               unsigned Type) const {
  const MCSymbolRefExpr *RefA = Val.getSymA();
  // A PCRel relocation to an absolute value has no symbol (or section). We
  // represent that with a relocation to a null section.
  if (!RefA)
    return false;

  MCSymbolRefExpr::VariantKind Kind = RefA->getKind();
  switch (Kind) {
  default:
    break;
  // The .odp creation emits a relocation against the symbol ".TOC." which
  // create a R_PPC64_TOC relocation. However the relocation symbol name
  // in final object creation should be NULL, since the symbol does not
  // really exist, it is just the reference to TOC base for the current
  // object file. Since the symbol is undefined, returning false results
  // in a relocation with a null section which is the desired result.
  case MCSymbolRefExpr::VK_PPC_TOCBASE:
    return false;

  // These VariantKind cause the relocation to refer to something other than
  // the symbol itself, like a linker generated table. Since the address of
  // symbol is not relevant, we cannot replace the symbol with the
  // section and patch the difference in the addend.
  case MCSymbolRefExpr::VK_GOT:
  case MCSymbolRefExpr::VK_PLT:
  case MCSymbolRefExpr::VK_GOTPCREL:
  case MCSymbolRefExpr::VK_GOTPCREL_NORELAX:
  case MCSymbolRefExpr::VK_PPC_GOT_LO:
  case MCSymbolRefExpr::VK_PPC_GOT_HI:
  case MCSymbolRefExpr::VK_PPC_GOT_HA:
    return true;
  }

  // An undefined symbol is not in any section, so the relocation has to point
  // to the symbol itself.
  assert(Sym && "Expected a symbol");
  if (Sym->isUndefined())
    return true;

  // For memory-tagged symbols, ensure that the relocation uses the symbol. For
  // tagged symbols, we emit an empty relocation (R_AARCH64_NONE) in a special
  // section (SHT_AARCH64_MEMTAG_GLOBALS_STATIC) to indicate to the linker that
  // this global needs to be tagged. In addition, the linker needs to know
  // whether to emit a special addend when relocating `end` symbols, and this
  // can only be determined by the attributes of the symbol itself.
  if (Sym->isMemtag())
    return true;

  unsigned Binding = Sym->getBinding();
  switch(Binding) {
  default:
    llvm_unreachable("Invalid Binding");
  case ELF::STB_LOCAL:
    break;
  case ELF::STB_WEAK:
    // If the symbol is weak, it might be overridden by a symbol in another
    // file. The relocation has to point to the symbol so that the linker
    // can update it.
    return true;
  case ELF::STB_GLOBAL:
  case ELF::STB_GNU_UNIQUE:
    // Global ELF symbols can be preempted by the dynamic linker. The relocation
    // has to point to the symbol for a reason analogous to the STB_WEAK case.
    return true;
  }

  // Keep symbol type for a local ifunc because it may result in an IRELATIVE
  // reloc that the dynamic loader will use to resolve the address at startup
  // time.
  if (Sym->getType() == ELF::STT_GNU_IFUNC)
    return true;

  // If a relocation points to a mergeable section, we have to be careful.
  // If the offset is zero, a relocation with the section will encode the
  // same information. With a non-zero offset, the situation is different.
  // For example, a relocation can point 42 bytes past the end of a string.
  // If we change such a relocation to use the section, the linker would think
  // that it pointed to another string and subtracting 42 at runtime will
  // produce the wrong value.
  if (Sym->isInSection()) {
    auto &Sec = cast<MCSectionELF>(Sym->getSection());
    unsigned Flags = Sec.getFlags();
    if (Flags & ELF::SHF_MERGE) {
      if (C != 0)
        return true;

      // gold<2.34 incorrectly ignored the addend for R_386_GOTOFF (9)
      // (http://sourceware.org/PR16794).
      if (TargetObjectWriter->getEMachine() == ELF::EM_386 &&
          Type == ELF::R_386_GOTOFF)
        return true;

      // ld.lld handles R_MIPS_HI16/R_MIPS_LO16 separately, not as a whole, so
      // it doesn't know that an R_MIPS_HI16 with implicit addend 1 and an
      // R_MIPS_LO16 with implicit addend -32768 represents 32768, which is in
      // range of a MergeInputSection. We could introduce a new RelExpr member
      // (like R_RISCV_PC_INDIRECT for R_RISCV_PCREL_HI20 / R_RISCV_PCREL_LO12)
      // but the complexity is unnecessary given that GNU as keeps the original
      // symbol for this case as well.
      if (TargetObjectWriter->getEMachine() == ELF::EM_MIPS &&
          !hasRelocationAddend())
        return true;
    }

    // Most TLS relocations use a got, so they need the symbol. Even those that
    // are just an offset (@tpoff), require a symbol in gold versions before
    // 5efeedf61e4fe720fd3e9a08e6c91c10abb66d42 (2014-09-26) which fixed
    // http://sourceware.org/PR16773.
    if (Flags & ELF::SHF_TLS)
      return true;
  }

  // If the symbol is a thumb function the final relocation must set the lowest
  // bit. With a symbol that is done by just having the symbol have that bit
  // set, so we would lose the bit if we relocated with the section.
  // FIXME: We could use the section but add the bit to the relocation value.
  if (Asm.isThumbFunc(Sym))
    return true;

  if (TargetObjectWriter->needsRelocateWithSymbol(Val, *Sym, Type))
    return true;
  return false;
}

bool ELFObjectWriter::checkRelocation(MCContext &Ctx, SMLoc Loc,
                                      const MCSectionELF *From,
                                      const MCSectionELF *To) {
  if (DwoOS) {
    if (isDwoSection(*From)) {
      Ctx.reportError(Loc, "A dwo section may not contain relocations");
      return false;
    }
    if (To && isDwoSection(*To)) {
      Ctx.reportError(Loc, "A relocation may not refer to a dwo section");
      return false;
    }
  }
  return true;
}

void ELFObjectWriter::recordRelocation(MCAssembler &Asm,
                                       const MCFragment *Fragment,
                                       const MCFixup &Fixup, MCValue Target,
                                       uint64_t &FixedValue) {
  MCAsmBackend &Backend = Asm.getBackend();
  bool IsPCRel = Backend.getFixupKindInfo(Fixup.getKind()).Flags &
                 MCFixupKindInfo::FKF_IsPCRel;
  const MCSectionELF &FixupSection = cast<MCSectionELF>(*Fragment->getParent());
  uint64_t C = Target.getConstant();
  uint64_t FixupOffset = Asm.getFragmentOffset(*Fragment) + Fixup.getOffset();
  MCContext &Ctx = Asm.getContext();
  const MCTargetOptions *TO = Ctx.getTargetOptions();

  if (const MCSymbolRefExpr *RefB = Target.getSymB()) {
    const auto &SymB = cast<MCSymbolELF>(RefB->getSymbol());
    if (SymB.isUndefined()) {
      Ctx.reportError(Fixup.getLoc(),
                      Twine("symbol '") + SymB.getName() +
                          "' can not be undefined in a subtraction expression");
      return;
    }

    assert(!SymB.isAbsolute() && "Should have been folded");
    const MCSection &SecB = SymB.getSection();
    if (&SecB != &FixupSection) {
      Ctx.reportError(Fixup.getLoc(),
                      "Cannot represent a difference across sections");
      return;
    }

    assert(!IsPCRel && "should have been folded");
    IsPCRel = true;
    C += FixupOffset - Asm.getSymbolOffset(SymB);
  }

  // We either rejected the fixup or folded B into C at this point.
  const MCSymbolRefExpr *RefA = Target.getSymA();
  const auto *SymA = RefA ? cast<MCSymbolELF>(&RefA->getSymbol()) : nullptr;

  bool ViaWeakRef = false;
  if (SymA && SymA->isVariable()) {
    const MCExpr *Expr = SymA->getVariableValue();
    if (const auto *Inner = dyn_cast<MCSymbolRefExpr>(Expr)) {
      if (Inner->getKind() == MCSymbolRefExpr::VK_WEAKREF) {
        SymA = cast<MCSymbolELF>(&Inner->getSymbol());
        ViaWeakRef = true;
      }
    }
  }

  const MCSectionELF *SecA = (SymA && SymA->isInSection())
                                 ? cast<MCSectionELF>(&SymA->getSection())
                                 : nullptr;
  if (!checkRelocation(Ctx, Fixup.getLoc(), &FixupSection, SecA))
    return;

  unsigned Type = TargetObjectWriter->getRelocType(Ctx, Target, Fixup, IsPCRel);
  const auto *Parent = cast<MCSectionELF>(Fragment->getParent());
  // Emiting relocation with sybmol for CG Profile to  help with --cg-profile.
  bool RelocateWithSymbol =
      shouldRelocateWithSymbol(Asm, Target, SymA, C, Type) ||
      (Parent->getType() == ELF::SHT_LLVM_CALL_GRAPH_PROFILE);
  uint64_t Addend = 0;

  FixedValue = !RelocateWithSymbol && SymA && !SymA->isUndefined()
                   ? C + Asm.getSymbolOffset(*SymA)
                   : C;
  if (usesRela(TO, FixupSection)) {
    Addend = FixedValue;
    FixedValue = 0;
  }

  if (!RelocateWithSymbol) {
    const auto *SectionSymbol =
        SecA ? cast<MCSymbolELF>(SecA->getBeginSymbol()) : nullptr;
    if (SectionSymbol)
      SectionSymbol->setUsedInReloc();
    ELFRelocationEntry Rec(FixupOffset, SectionSymbol, Type, Addend, SymA, C);
    Relocations[&FixupSection].push_back(Rec);
    return;
  }

  const MCSymbolELF *RenamedSymA = SymA;
  if (SymA) {
    if (const MCSymbolELF *R = Renames.lookup(SymA))
      RenamedSymA = R;

    if (ViaWeakRef)
      RenamedSymA->setIsWeakrefUsedInReloc();
    else
      RenamedSymA->setUsedInReloc();
  }
  ELFRelocationEntry Rec(FixupOffset, RenamedSymA, Type, Addend, SymA, C);
  Relocations[&FixupSection].push_back(Rec);
}

bool ELFObjectWriter::usesRela(const MCTargetOptions *TO,
                               const MCSectionELF &Sec) const {
  return (hasRelocationAddend() &&
          Sec.getType() != ELF::SHT_LLVM_CALL_GRAPH_PROFILE) ||
         (TO && TO->Crel);
}

bool ELFObjectWriter::isSymbolRefDifferenceFullyResolvedImpl(
    const MCAssembler &Asm, const MCSymbol &SA, const MCFragment &FB,
    bool InSet, bool IsPCRel) const {
  const auto &SymA = cast<MCSymbolELF>(SA);
  if (IsPCRel) {
    assert(!InSet);
    if (SymA.getBinding() != ELF::STB_LOCAL ||
        SymA.getType() == ELF::STT_GNU_IFUNC)
      return false;
  }
  return &SymA.getSection() == FB.getParent();
}

uint64_t ELFObjectWriter::writeObject(MCAssembler &Asm) {
  uint64_t Size =
      ELFWriter(*this, OS, IsLittleEndian,
                DwoOS ? ELFWriter::NonDwoOnly : ELFWriter::AllSections)
          .writeObject(Asm);
  if (DwoOS)
    Size += ELFWriter(*this, *DwoOS, IsLittleEndian, ELFWriter::DwoOnly)
                .writeObject(Asm);
  return Size;
}