xref: /llvm-project/llvm/lib/MC/MachObjectWriter.cpp (revision d73d5c8c9b56fadcbd89dd1dab71dca2c8f5e38d)

//===- lib/MC/MachObjectWriter.cpp - Mach-O 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
//
//===----------------------------------------------------------------------===//

#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/Twine.h"
#include "llvm/BinaryFormat/MachO.h"
#include "llvm/MC/MCAsmBackend.h"
#include "llvm/MC/MCAsmInfoDarwin.h"
#include "llvm/MC/MCAssembler.h"
#include "llvm/MC/MCContext.h"
#include "llvm/MC/MCDirectives.h"
#include "llvm/MC/MCExpr.h"
#include "llvm/MC/MCFixupKindInfo.h"
#include "llvm/MC/MCFragment.h"
#include "llvm/MC/MCMachObjectWriter.h"
#include "llvm/MC/MCObjectFileInfo.h"
#include "llvm/MC/MCObjectWriter.h"
#include "llvm/MC/MCSection.h"
#include "llvm/MC/MCSectionMachO.h"
#include "llvm/MC/MCSymbol.h"
#include "llvm/MC/MCSymbolMachO.h"
#include "llvm/MC/MCValue.h"
#include "llvm/Support/Alignment.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/MathExtras.h"
#include "llvm/Support/raw_ostream.h"
#include <algorithm>
#include <cassert>
#include <cstdint>
#include <string>
#include <utility>
#include <vector>

using namespace llvm;

#define DEBUG_TYPE "mc"

void MachObjectWriter::reset() {
  Relocations.clear();
  IndirectSymBase.clear();
  IndirectSymbols.clear();
  DataRegions.clear();
  SectionAddress.clear();
  SectionOrder.clear();
  StringTable.clear();
  LocalSymbolData.clear();
  ExternalSymbolData.clear();
  UndefinedSymbolData.clear();
  LOHContainer.reset();
  VersionInfo.Major = 0;
  VersionInfo.SDKVersion = VersionTuple();
  TargetVariantVersionInfo.Major = 0;
  TargetVariantVersionInfo.SDKVersion = VersionTuple();
  LinkerOptions.clear();
  MCObjectWriter::reset();
}

bool MachObjectWriter::doesSymbolRequireExternRelocation(const MCSymbol &S) {
  // Undefined symbols are always extern.
  if (S.isUndefined())
    return true;

  // References to weak definitions require external relocation entries; the
  // definition may not always be the one in the same object file.
  if (cast<MCSymbolMachO>(S).isWeakDefinition())
    return true;

  // Otherwise, we can use an internal relocation.
  return false;
}

bool MachObjectWriter::
MachSymbolData::operator<(const MachSymbolData &RHS) const {
  return Symbol->getName() < RHS.Symbol->getName();
}

bool MachObjectWriter::isFixupKindPCRel(const MCAssembler &Asm, unsigned Kind) {
  const MCFixupKindInfo &FKI = Asm.getBackend().getFixupKindInfo(
    (MCFixupKind) Kind);

  return FKI.Flags & MCFixupKindInfo::FKF_IsPCRel;
}

uint64_t
MachObjectWriter::getFragmentAddress(const MCAssembler &Asm,
                                     const MCFragment *Fragment) const {
  return getSectionAddress(Fragment->getParent()) +
         Asm.getFragmentOffset(*Fragment);
}

uint64_t MachObjectWriter::getSymbolAddress(const MCSymbol &S,
                                            const MCAssembler &Asm) const {
  // If this is a variable, then recursively evaluate now.
  if (S.isVariable()) {
    if (const MCConstantExpr *C =
          dyn_cast<const MCConstantExpr>(S.getVariableValue()))
      return C->getValue();

    MCValue Target;
    if (!S.getVariableValue()->evaluateAsRelocatable(Target, &Asm, nullptr))
      report_fatal_error("unable to evaluate offset for variable '" +
                         S.getName() + "'");

    // Verify that any used symbols are defined.
    if (Target.getSymA() && Target.getSymA()->getSymbol().isUndefined())
      report_fatal_error("unable to evaluate offset to undefined symbol '" +
                         Target.getSymA()->getSymbol().getName() + "'");
    if (Target.getSymB() && Target.getSymB()->getSymbol().isUndefined())
      report_fatal_error("unable to evaluate offset to undefined symbol '" +
                         Target.getSymB()->getSymbol().getName() + "'");

    uint64_t Address = Target.getConstant();
    if (Target.getSymA())
      Address += getSymbolAddress(Target.getSymA()->getSymbol(), Asm);
    if (Target.getSymB())
      Address += getSymbolAddress(Target.getSymB()->getSymbol(), Asm);
    return Address;
  }

  return getSectionAddress(S.getFragment()->getParent()) +
         Asm.getSymbolOffset(S);
}

uint64_t MachObjectWriter::getPaddingSize(const MCAssembler &Asm,
                                          const MCSection *Sec) const {
  uint64_t EndAddr = getSectionAddress(Sec) + Asm.getSectionAddressSize(*Sec);
  unsigned Next = cast<MCSectionMachO>(Sec)->getLayoutOrder() + 1;
  if (Next >= SectionOrder.size())
    return 0;

  const MCSection &NextSec = *SectionOrder[Next];
  if (NextSec.isVirtualSection())
    return 0;
  return offsetToAlignment(EndAddr, NextSec.getAlign());
}

static bool isSymbolLinkerVisible(const MCSymbol &Symbol) {
  // Non-temporary labels should always be visible to the linker.
  if (!Symbol.isTemporary())
    return true;

  if (Symbol.isUsedInReloc())
    return true;

  return false;
}

const MCSymbol *MachObjectWriter::getAtom(const MCSymbol &S) const {
  // Linker visible symbols define atoms.
  if (isSymbolLinkerVisible(S))
    return &S;

  // Absolute and undefined symbols have no defining atom.
  if (!S.isInSection())
    return nullptr;

  // Non-linker visible symbols in sections which can't be atomized have no
  // defining atom.
  if (!MCAsmInfoDarwin::isSectionAtomizableBySymbols(
          *S.getFragment()->getParent()))
    return nullptr;

  // Otherwise, return the atom for the containing fragment.
  return S.getFragment()->getAtom();
}

void MachObjectWriter::writeHeader(MachO::HeaderFileType Type,
                                   unsigned NumLoadCommands,
                                   unsigned LoadCommandsSize,
                                   bool SubsectionsViaSymbols) {
  uint32_t Flags = 0;

  if (SubsectionsViaSymbols)
    Flags |= MachO::MH_SUBSECTIONS_VIA_SYMBOLS;

  // struct mach_header (28 bytes) or
  // struct mach_header_64 (32 bytes)

  uint64_t Start = W.OS.tell();
  (void) Start;

  W.write<uint32_t>(is64Bit() ? MachO::MH_MAGIC_64 : MachO::MH_MAGIC);

  W.write<uint32_t>(TargetObjectWriter->getCPUType());

  uint32_t Cpusubtype = TargetObjectWriter->getCPUSubtype();

  // Promote arm64e subtypes to always be ptrauth-ABI-versioned, at version 0.
  // We never need to emit unversioned binaries.
  // And we don't support arbitrary ABI versions (or the kernel flag) yet.
  if (TargetObjectWriter->getCPUType() == MachO::CPU_TYPE_ARM64 &&
      Cpusubtype == MachO::CPU_SUBTYPE_ARM64E)
    Cpusubtype = MachO::CPU_SUBTYPE_ARM64E_WITH_PTRAUTH_VERSION(
        /*PtrAuthABIVersion=*/0, /*PtrAuthKernelABIVersion=*/false);

  W.write<uint32_t>(Cpusubtype);

  W.write<uint32_t>(Type);
  W.write<uint32_t>(NumLoadCommands);
  W.write<uint32_t>(LoadCommandsSize);
  W.write<uint32_t>(Flags);
  if (is64Bit())
    W.write<uint32_t>(0); // reserved

  assert(W.OS.tell() - Start == (is64Bit() ? sizeof(MachO::mach_header_64)
                                           : sizeof(MachO::mach_header)));
}

void MachObjectWriter::writeWithPadding(StringRef Str, uint64_t Size) {
  assert(Size >= Str.size());
  W.OS << Str;
  W.OS.write_zeros(Size - Str.size());
}

/// writeSegmentLoadCommand - Write a segment load command.
///
/// \param NumSections The number of sections in this segment.
/// \param SectionDataSize The total size of the sections.
void MachObjectWriter::writeSegmentLoadCommand(
    StringRef Name, unsigned NumSections, uint64_t VMAddr, uint64_t VMSize,
    uint64_t SectionDataStartOffset, uint64_t SectionDataSize, uint32_t MaxProt,
    uint32_t InitProt) {
  // struct segment_command (56 bytes) or
  // struct segment_command_64 (72 bytes)

  uint64_t Start = W.OS.tell();
  (void) Start;

  unsigned SegmentLoadCommandSize =
    is64Bit() ? sizeof(MachO::segment_command_64):
    sizeof(MachO::segment_command);
  W.write<uint32_t>(is64Bit() ? MachO::LC_SEGMENT_64 : MachO::LC_SEGMENT);
  W.write<uint32_t>(SegmentLoadCommandSize +
          NumSections * (is64Bit() ? sizeof(MachO::section_64) :
                         sizeof(MachO::section)));

  writeWithPadding(Name, 16);
  if (is64Bit()) {
    W.write<uint64_t>(VMAddr);                 // vmaddr
    W.write<uint64_t>(VMSize); // vmsize
    W.write<uint64_t>(SectionDataStartOffset); // file offset
    W.write<uint64_t>(SectionDataSize); // file size
  } else {
    W.write<uint32_t>(VMAddr);                 // vmaddr
    W.write<uint32_t>(VMSize); // vmsize
    W.write<uint32_t>(SectionDataStartOffset); // file offset
    W.write<uint32_t>(SectionDataSize); // file size
  }
  // maxprot
  W.write<uint32_t>(MaxProt);
  // initprot
  W.write<uint32_t>(InitProt);
  W.write<uint32_t>(NumSections);
  W.write<uint32_t>(0); // flags

  assert(W.OS.tell() - Start == SegmentLoadCommandSize);
}

void MachObjectWriter::writeSection(const MCAssembler &Asm,
                                    const MCSection &Sec, uint64_t VMAddr,
                                    uint64_t FileOffset, unsigned Flags,
                                    uint64_t RelocationsStart,
                                    unsigned NumRelocations) {
  uint64_t SectionSize = Asm.getSectionAddressSize(Sec);
  const MCSectionMachO &Section = cast<MCSectionMachO>(Sec);

  // The offset is unused for virtual sections.
  if (Section.isVirtualSection()) {
    assert(Asm.getSectionFileSize(Sec) == 0 && "Invalid file size!");
    FileOffset = 0;
  }

  // struct section (68 bytes) or
  // struct section_64 (80 bytes)

  uint64_t Start = W.OS.tell();
  (void) Start;

  writeWithPadding(Section.getName(), 16);
  writeWithPadding(Section.getSegmentName(), 16);
  if (is64Bit()) {
    W.write<uint64_t>(VMAddr);      // address
    W.write<uint64_t>(SectionSize); // size
  } else {
    W.write<uint32_t>(VMAddr);      // address
    W.write<uint32_t>(SectionSize); // size
  }
  assert(isUInt<32>(FileOffset) && "Cannot encode offset of section");
  W.write<uint32_t>(FileOffset);

  W.write<uint32_t>(Log2(Section.getAlign()));
  assert((!NumRelocations || isUInt<32>(RelocationsStart)) &&
         "Cannot encode offset of relocations");
  W.write<uint32_t>(NumRelocations ? RelocationsStart : 0);
  W.write<uint32_t>(NumRelocations);
  W.write<uint32_t>(Flags);
  W.write<uint32_t>(IndirectSymBase.lookup(&Sec)); // reserved1
  W.write<uint32_t>(Section.getStubSize()); // reserved2
  if (is64Bit())
    W.write<uint32_t>(0); // reserved3

  assert(W.OS.tell() - Start ==
         (is64Bit() ? sizeof(MachO::section_64) : sizeof(MachO::section)));
}

void MachObjectWriter::writeSymtabLoadCommand(uint32_t SymbolOffset,
                                              uint32_t NumSymbols,
                                              uint32_t StringTableOffset,
                                              uint32_t StringTableSize) {
  // struct symtab_command (24 bytes)

  uint64_t Start = W.OS.tell();
  (void) Start;

  W.write<uint32_t>(MachO::LC_SYMTAB);
  W.write<uint32_t>(sizeof(MachO::symtab_command));
  W.write<uint32_t>(SymbolOffset);
  W.write<uint32_t>(NumSymbols);
  W.write<uint32_t>(StringTableOffset);
  W.write<uint32_t>(StringTableSize);

  assert(W.OS.tell() - Start == sizeof(MachO::symtab_command));
}

void MachObjectWriter::writeDysymtabLoadCommand(uint32_t FirstLocalSymbol,
                                                uint32_t NumLocalSymbols,
                                                uint32_t FirstExternalSymbol,
                                                uint32_t NumExternalSymbols,
                                                uint32_t FirstUndefinedSymbol,
                                                uint32_t NumUndefinedSymbols,
                                                uint32_t IndirectSymbolOffset,
                                                uint32_t NumIndirectSymbols) {
  // struct dysymtab_command (80 bytes)

  uint64_t Start = W.OS.tell();
  (void) Start;

  W.write<uint32_t>(MachO::LC_DYSYMTAB);
  W.write<uint32_t>(sizeof(MachO::dysymtab_command));
  W.write<uint32_t>(FirstLocalSymbol);
  W.write<uint32_t>(NumLocalSymbols);
  W.write<uint32_t>(FirstExternalSymbol);
  W.write<uint32_t>(NumExternalSymbols);
  W.write<uint32_t>(FirstUndefinedSymbol);
  W.write<uint32_t>(NumUndefinedSymbols);
  W.write<uint32_t>(0); // tocoff
  W.write<uint32_t>(0); // ntoc
  W.write<uint32_t>(0); // modtaboff
  W.write<uint32_t>(0); // nmodtab
  W.write<uint32_t>(0); // extrefsymoff
  W.write<uint32_t>(0); // nextrefsyms
  W.write<uint32_t>(IndirectSymbolOffset);
  W.write<uint32_t>(NumIndirectSymbols);
  W.write<uint32_t>(0); // extreloff
  W.write<uint32_t>(0); // nextrel
  W.write<uint32_t>(0); // locreloff
  W.write<uint32_t>(0); // nlocrel

  assert(W.OS.tell() - Start == sizeof(MachO::dysymtab_command));
}

MachObjectWriter::MachSymbolData *
MachObjectWriter::findSymbolData(const MCSymbol &Sym) {
  for (auto *SymbolData :
       {&LocalSymbolData, &ExternalSymbolData, &UndefinedSymbolData})
    for (MachSymbolData &Entry : *SymbolData)
      if (Entry.Symbol == &Sym)
        return &Entry;

  return nullptr;
}

const MCSymbol &MachObjectWriter::findAliasedSymbol(const MCSymbol &Sym) const {
  const MCSymbol *S = &Sym;
  while (S->isVariable()) {
    const MCExpr *Value = S->getVariableValue();
    const auto *Ref = dyn_cast<MCSymbolRefExpr>(Value);
    if (!Ref)
      return *S;
    S = &Ref->getSymbol();
  }
  return *S;
}

void MachObjectWriter::writeNlist(MachSymbolData &MSD, const MCAssembler &Asm) {
  const MCSymbol *Symbol = MSD.Symbol;
  const auto &Data = cast<MCSymbolMachO>(*Symbol);
  const MCSymbol *AliasedSymbol = &findAliasedSymbol(*Symbol);
  uint8_t SectionIndex = MSD.SectionIndex;
  uint8_t Type = 0;
  uint64_t Address = 0;
  bool IsAlias = Symbol != AliasedSymbol;

  const MCSymbol &OrigSymbol = *Symbol;
  MachSymbolData *AliaseeInfo;
  if (IsAlias) {
    AliaseeInfo = findSymbolData(*AliasedSymbol);
    if (AliaseeInfo)
      SectionIndex = AliaseeInfo->SectionIndex;
    Symbol = AliasedSymbol;
    // FIXME: Should this update Data as well?
  }

  // Set the N_TYPE bits. See <mach-o/nlist.h>.
  //
  // FIXME: Are the prebound or indirect fields possible here?
  if (IsAlias && Symbol->isUndefined())
    Type = MachO::N_INDR;
  else if (Symbol->isUndefined())
    Type = MachO::N_UNDF;
  else if (Symbol->isAbsolute())
    Type = MachO::N_ABS;
  else
    Type = MachO::N_SECT;

  // FIXME: Set STAB bits.

  if (Data.isPrivateExtern())
    Type |= MachO::N_PEXT;

  // Set external bit.
  if (Data.isExternal() || (!IsAlias && Symbol->isUndefined()))
    Type |= MachO::N_EXT;

  // Compute the symbol address.
  if (IsAlias && Symbol->isUndefined())
    Address = AliaseeInfo->StringIndex;
  else if (Symbol->isDefined())
    Address = getSymbolAddress(OrigSymbol, Asm);
  else if (Symbol->isCommon()) {
    // Common symbols are encoded with the size in the address
    // field, and their alignment in the flags.
    Address = Symbol->getCommonSize();
  }

  // struct nlist (12 bytes)

  W.write<uint32_t>(MSD.StringIndex);
  W.OS << char(Type);
  W.OS << char(SectionIndex);

  // The Mach-O streamer uses the lowest 16-bits of the flags for the 'desc'
  // value.
  bool EncodeAsAltEntry =
    IsAlias && cast<MCSymbolMachO>(OrigSymbol).isAltEntry();
  W.write<uint16_t>(cast<MCSymbolMachO>(Symbol)->getEncodedFlags(EncodeAsAltEntry));
  if (is64Bit())
    W.write<uint64_t>(Address);
  else
    W.write<uint32_t>(Address);
}

void MachObjectWriter::writeLinkeditLoadCommand(uint32_t Type,
                                                uint32_t DataOffset,
                                                uint32_t DataSize) {
  uint64_t Start = W.OS.tell();
  (void) Start;

  W.write<uint32_t>(Type);
  W.write<uint32_t>(sizeof(MachO::linkedit_data_command));
  W.write<uint32_t>(DataOffset);
  W.write<uint32_t>(DataSize);

  assert(W.OS.tell() - Start == sizeof(MachO::linkedit_data_command));
}

static unsigned ComputeLinkerOptionsLoadCommandSize(
  const std::vector<std::string> &Options, bool is64Bit)
{
  unsigned Size = sizeof(MachO::linker_option_command);
  for (const std::string &Option : Options)
    Size += Option.size() + 1;
  return alignTo(Size, is64Bit ? 8 : 4);
}

void MachObjectWriter::writeLinkerOptionsLoadCommand(
  const std::vector<std::string> &Options)
{
  unsigned Size = ComputeLinkerOptionsLoadCommandSize(Options, is64Bit());
  uint64_t Start = W.OS.tell();
  (void) Start;

  W.write<uint32_t>(MachO::LC_LINKER_OPTION);
  W.write<uint32_t>(Size);
  W.write<uint32_t>(Options.size());
  uint64_t BytesWritten = sizeof(MachO::linker_option_command);
  for (const std::string &Option : Options) {
    // Write each string, including the null byte.
    W.OS << Option << '\0';
    BytesWritten += Option.size() + 1;
  }

  // Pad to a multiple of the pointer size.
  W.OS.write_zeros(
      offsetToAlignment(BytesWritten, is64Bit() ? Align(8) : Align(4)));

  assert(W.OS.tell() - Start == Size);
}

static bool isFixupTargetValid(const MCValue &Target) {
  // Target is (LHS - RHS + cst).
  // We don't support the form where LHS is null: -RHS + cst
  if (!Target.getSymA() && Target.getSymB())
    return false;
  return true;
}

void MachObjectWriter::recordRelocation(MCAssembler &Asm,
                                        const MCFragment *Fragment,
                                        const MCFixup &Fixup, MCValue Target,
                                        uint64_t &FixedValue) {
  if (!isFixupTargetValid(Target)) {
    Asm.getContext().reportError(Fixup.getLoc(),
                                 "unsupported relocation expression");
    return;
  }

  TargetObjectWriter->recordRelocation(this, Asm, Fragment, Fixup, Target,
                                       FixedValue);
}

void MachObjectWriter::bindIndirectSymbols(MCAssembler &Asm) {
  // This is the point where 'as' creates actual symbols for indirect symbols
  // (in the following two passes). It would be easier for us to do this sooner
  // when we see the attribute, but that makes getting the order in the symbol
  // table much more complicated than it is worth.
  //
  // FIXME: Revisit this when the dust settles.

  // Report errors for use of .indirect_symbol not in a symbol pointer section
  // or stub section.
  for (IndirectSymbolData &ISD : IndirectSymbols) {
    const MCSectionMachO &Section = cast<MCSectionMachO>(*ISD.Section);

    if (Section.getType() != MachO::S_NON_LAZY_SYMBOL_POINTERS &&
        Section.getType() != MachO::S_LAZY_SYMBOL_POINTERS &&
        Section.getType() != MachO::S_THREAD_LOCAL_VARIABLE_POINTERS &&
        Section.getType() != MachO::S_SYMBOL_STUBS) {
      MCSymbol &Symbol = *ISD.Symbol;
      report_fatal_error("indirect symbol '" + Symbol.getName() +
                         "' not in a symbol pointer or stub section");
    }
  }

  // Bind non-lazy symbol pointers first.
  for (auto [IndirectIndex, ISD] : enumerate(IndirectSymbols)) {
    const auto &Section = cast<MCSectionMachO>(*ISD.Section);

    if (Section.getType() != MachO::S_NON_LAZY_SYMBOL_POINTERS &&
        Section.getType() !=  MachO::S_THREAD_LOCAL_VARIABLE_POINTERS)
      continue;

    // Initialize the section indirect symbol base, if necessary.
    IndirectSymBase.insert(std::make_pair(ISD.Section, IndirectIndex));

    Asm.registerSymbol(*ISD.Symbol);
  }

  // Then lazy symbol pointers and symbol stubs.
  for (auto [IndirectIndex, ISD] : enumerate(IndirectSymbols)) {
    const auto &Section = cast<MCSectionMachO>(*ISD.Section);

    if (Section.getType() != MachO::S_LAZY_SYMBOL_POINTERS &&
        Section.getType() != MachO::S_SYMBOL_STUBS)
      continue;

    // Initialize the section indirect symbol base, if necessary.
    IndirectSymBase.insert(std::make_pair(ISD.Section, IndirectIndex));

    // Set the symbol type to undefined lazy, but only on construction.
    //
    // FIXME: Do not hardcode.
    if (Asm.registerSymbol(*ISD.Symbol))
      cast<MCSymbolMachO>(ISD.Symbol)->setReferenceTypeUndefinedLazy(true);
  }
}

/// computeSymbolTable - Compute the symbol table data
void MachObjectWriter::computeSymbolTable(
    MCAssembler &Asm, std::vector<MachSymbolData> &LocalSymbolData,
    std::vector<MachSymbolData> &ExternalSymbolData,
    std::vector<MachSymbolData> &UndefinedSymbolData) {
  // Build section lookup table.
  DenseMap<const MCSection*, uint8_t> SectionIndexMap;
  unsigned Index = 1;
  for (MCSection &Sec : Asm)
    SectionIndexMap[&Sec] = Index++;
  assert(Index <= 256 && "Too many sections!");

  // Build the string table.
  for (const MCSymbol &Symbol : Asm.symbols()) {
    if (!cast<MCSymbolMachO>(Symbol).isSymbolLinkerVisible())
      continue;

    StringTable.add(Symbol.getName());
  }
  StringTable.finalize();

  // Build the symbol arrays but only for non-local symbols.
  //
  // The particular order that we collect and then sort the symbols is chosen to
  // match 'as'. Even though it doesn't matter for correctness, this is
  // important for letting us diff .o files.
  for (const MCSymbol &Symbol : Asm.symbols()) {
    // Ignore non-linker visible symbols.
    if (!cast<MCSymbolMachO>(Symbol).isSymbolLinkerVisible())
      continue;

    if (!Symbol.isExternal() && !Symbol.isUndefined())
      continue;

    MachSymbolData MSD;
    MSD.Symbol = &Symbol;
    MSD.StringIndex = StringTable.getOffset(Symbol.getName());

    if (Symbol.isUndefined()) {
      MSD.SectionIndex = 0;
      UndefinedSymbolData.push_back(MSD);
    } else if (Symbol.isAbsolute()) {
      MSD.SectionIndex = 0;
      ExternalSymbolData.push_back(MSD);
    } else {
      MSD.SectionIndex = SectionIndexMap.lookup(&Symbol.getSection());
      assert(MSD.SectionIndex && "Invalid section index!");
      ExternalSymbolData.push_back(MSD);
    }
  }

  // Now add the data for local symbols.
  for (const MCSymbol &Symbol : Asm.symbols()) {
    // Ignore non-linker visible symbols.
    if (!cast<MCSymbolMachO>(Symbol).isSymbolLinkerVisible())
      continue;

    if (Symbol.isExternal() || Symbol.isUndefined())
      continue;

    MachSymbolData MSD;
    MSD.Symbol = &Symbol;
    MSD.StringIndex = StringTable.getOffset(Symbol.getName());

    if (Symbol.isAbsolute()) {
      MSD.SectionIndex = 0;
      LocalSymbolData.push_back(MSD);
    } else {
      MSD.SectionIndex = SectionIndexMap.lookup(&Symbol.getSection());
      assert(MSD.SectionIndex && "Invalid section index!");
      LocalSymbolData.push_back(MSD);
    }
  }

  // External and undefined symbols are required to be in lexicographic order.
  llvm::sort(ExternalSymbolData);
  llvm::sort(UndefinedSymbolData);

  // Set the symbol indices.
  Index = 0;
  for (auto *SymbolData :
       {&LocalSymbolData, &ExternalSymbolData, &UndefinedSymbolData})
    for (MachSymbolData &Entry : *SymbolData)
      Entry.Symbol->setIndex(Index++);

  for (const MCSection &Section : Asm) {
    for (RelAndSymbol &Rel : Relocations[&Section]) {
      if (!Rel.Sym)
        continue;

      // Set the Index and the IsExtern bit.
      unsigned Index = Rel.Sym->getIndex();
      assert(isInt<24>(Index));
      if (W.Endian == llvm::endianness::little)
        Rel.MRE.r_word1 = (Rel.MRE.r_word1 & (~0U << 24)) | Index | (1 << 27);
      else
        Rel.MRE.r_word1 = (Rel.MRE.r_word1 & 0xff) | Index << 8 | (1 << 4);
    }
  }
}

void MachObjectWriter::computeSectionAddresses(const MCAssembler &Asm) {
  // Assign layout order indices to sections.
  unsigned i = 0;
  // Compute the section layout order. Virtual sections must go last.
  for (MCSection &Sec : Asm) {
    if (!Sec.isVirtualSection()) {
      SectionOrder.push_back(&Sec);
      cast<MCSectionMachO>(Sec).setLayoutOrder(i++);
    }
  }
  for (MCSection &Sec : Asm) {
    if (Sec.isVirtualSection()) {
      SectionOrder.push_back(&Sec);
      cast<MCSectionMachO>(Sec).setLayoutOrder(i++);
    }
  }

  uint64_t StartAddress = 0;
  for (const MCSection *Sec : SectionOrder) {
    StartAddress = alignTo(StartAddress, Sec->getAlign());
    SectionAddress[Sec] = StartAddress;
    StartAddress += Asm.getSectionAddressSize(*Sec);

    // Explicitly pad the section to match the alignment requirements of the
    // following one. This is for 'gas' compatibility, it shouldn't
    /// strictly be necessary.
    StartAddress += getPaddingSize(Asm, Sec);
  }
}

void MachObjectWriter::executePostLayoutBinding(MCAssembler &Asm) {
  computeSectionAddresses(Asm);

  // Create symbol data for any indirect symbols.
  bindIndirectSymbols(Asm);
}

bool MachObjectWriter::isSymbolRefDifferenceFullyResolvedImpl(
    const MCAssembler &Asm, const MCSymbol &SymA, const MCFragment &FB,
    bool InSet, bool IsPCRel) const {
  if (InSet)
    return true;

  // The effective address is
  //     addr(atom(A)) + offset(A)
  //   - addr(atom(B)) - offset(B)
  // and the offsets are not relocatable, so the fixup is fully resolved when
  //  addr(atom(A)) - addr(atom(B)) == 0.
  const MCSymbol &SA = findAliasedSymbol(SymA);
  const MCSection &SecA = SA.getSection();
  const MCSection &SecB = *FB.getParent();

  if (IsPCRel) {
    // The simple (Darwin, except on x86_64) way of dealing with this was to
    // assume that any reference to a temporary symbol *must* be a temporary
    // symbol in the same atom, unless the sections differ. Therefore, any PCrel
    // relocation to a temporary symbol (in the same section) is fully
    // resolved. This also works in conjunction with absolutized .set, which
    // requires the compiler to use .set to absolutize the differences between
    // symbols which the compiler knows to be assembly time constants, so we
    // don't need to worry about considering symbol differences fully resolved.
    //
    // If the file isn't using sub-sections-via-symbols, we can make the
    // same assumptions about any symbol that we normally make about
    // assembler locals.

    bool hasReliableSymbolDifference = isX86_64();
    if (!hasReliableSymbolDifference) {
      if (!SA.isInSection() || &SecA != &SecB ||
          (!SA.isTemporary() && FB.getAtom() != SA.getFragment()->getAtom() &&
           SubsectionsViaSymbols))
        return false;
      return true;
    }
  }

  // If they are not in the same section, we can't compute the diff.
  if (&SecA != &SecB)
    return false;

  // If the atoms are the same, they are guaranteed to have the same address.
  return SA.getFragment()->getAtom() == FB.getAtom();
}

static MachO::LoadCommandType getLCFromMCVM(MCVersionMinType Type) {
  switch (Type) {
  case MCVM_OSXVersionMin:     return MachO::LC_VERSION_MIN_MACOSX;
  case MCVM_IOSVersionMin:     return MachO::LC_VERSION_MIN_IPHONEOS;
  case MCVM_TvOSVersionMin:    return MachO::LC_VERSION_MIN_TVOS;
  case MCVM_WatchOSVersionMin: return MachO::LC_VERSION_MIN_WATCHOS;
  }
  llvm_unreachable("Invalid mc version min type");
}

void MachObjectWriter::populateAddrSigSection(MCAssembler &Asm) {
  MCSection *AddrSigSection =
      Asm.getContext().getObjectFileInfo()->getAddrSigSection();
  unsigned Log2Size = is64Bit() ? 3 : 2;
  for (const MCSymbol *S : getAddrsigSyms()) {
    if (!S->isRegistered())
      continue;
    MachO::any_relocation_info MRE;
    MRE.r_word0 = 0;
    MRE.r_word1 = (Log2Size << 25) | (MachO::GENERIC_RELOC_VANILLA << 28);
    addRelocation(S, AddrSigSection, MRE);
  }
}

uint64_t MachObjectWriter::writeObject(MCAssembler &Asm) {
  uint64_t StartOffset = W.OS.tell();
  auto NumBytesWritten = [&] { return W.OS.tell() - StartOffset; };

  populateAddrSigSection(Asm);

  // Compute symbol table information and bind symbol indices.
  computeSymbolTable(Asm, LocalSymbolData, ExternalSymbolData,
                     UndefinedSymbolData);

  if (!CGProfile.empty()) {
    MCSection *CGProfileSection = Asm.getContext().getMachOSection(
        "__LLVM", "__cg_profile", 0, SectionKind::getMetadata());
    auto &Frag = cast<MCDataFragment>(*CGProfileSection->begin());
    Frag.getContents().clear();
    raw_svector_ostream OS(Frag.getContents());
    for (const MCObjectWriter::CGProfileEntry &CGPE : CGProfile) {
      uint32_t FromIndex = CGPE.From->getSymbol().getIndex();
      uint32_t ToIndex = CGPE.To->getSymbol().getIndex();
      support::endian::write(OS, FromIndex, W.Endian);
      support::endian::write(OS, ToIndex, W.Endian);
      support::endian::write(OS, CGPE.Count, W.Endian);
    }
  }

  unsigned NumSections = Asm.end() - Asm.begin();

  // The section data starts after the header, the segment load command (and
  // section headers) and the symbol table.
  unsigned NumLoadCommands = 1;
  uint64_t LoadCommandsSize = is64Bit() ?
    sizeof(MachO::segment_command_64) + NumSections * sizeof(MachO::section_64):
    sizeof(MachO::segment_command) + NumSections * sizeof(MachO::section);

  // Add the deployment target version info load command size, if used.
  if (VersionInfo.Major != 0) {
    ++NumLoadCommands;
    if (VersionInfo.EmitBuildVersion)
      LoadCommandsSize += sizeof(MachO::build_version_command);
    else
      LoadCommandsSize += sizeof(MachO::version_min_command);
  }

  // Add the target variant version info load command size, if used.
  if (TargetVariantVersionInfo.Major != 0) {
    ++NumLoadCommands;
    assert(TargetVariantVersionInfo.EmitBuildVersion &&
           "target variant should use build version");
    LoadCommandsSize += sizeof(MachO::build_version_command);
  }

  // Add the data-in-code load command size, if used.
  unsigned NumDataRegions = DataRegions.size();
  if (NumDataRegions) {
    ++NumLoadCommands;
    LoadCommandsSize += sizeof(MachO::linkedit_data_command);
  }

  // Add the loh load command size, if used.
  uint64_t LOHRawSize = LOHContainer.getEmitSize(Asm, *this);
  uint64_t LOHSize = alignTo(LOHRawSize, is64Bit() ? 8 : 4);
  if (LOHSize) {
    ++NumLoadCommands;
    LoadCommandsSize += sizeof(MachO::linkedit_data_command);
  }

  // Add the symbol table load command sizes, if used.
  unsigned NumSymbols = LocalSymbolData.size() + ExternalSymbolData.size() +
    UndefinedSymbolData.size();
  if (NumSymbols) {
    NumLoadCommands += 2;
    LoadCommandsSize += (sizeof(MachO::symtab_command) +
                         sizeof(MachO::dysymtab_command));
  }

  // Add the linker option load commands sizes.
  for (const auto &Option : LinkerOptions) {
    ++NumLoadCommands;
    LoadCommandsSize += ComputeLinkerOptionsLoadCommandSize(Option, is64Bit());
  }

  // Compute the total size of the section data, as well as its file size and vm
  // size.
  uint64_t SectionDataStart = (is64Bit() ? sizeof(MachO::mach_header_64) :
                               sizeof(MachO::mach_header)) + LoadCommandsSize;
  uint64_t SectionDataSize = 0;
  uint64_t SectionDataFileSize = 0;
  uint64_t VMSize = 0;
  for (const MCSection &Sec : Asm) {
    uint64_t Address = getSectionAddress(&Sec);
    uint64_t Size = Asm.getSectionAddressSize(Sec);
    uint64_t FileSize = Asm.getSectionFileSize(Sec);
    FileSize += getPaddingSize(Asm, &Sec);

    VMSize = std::max(VMSize, Address + Size);

    if (Sec.isVirtualSection())
      continue;

    SectionDataSize = std::max(SectionDataSize, Address + Size);
    SectionDataFileSize = std::max(SectionDataFileSize, Address + FileSize);
  }

  // The section data is padded to pointer size bytes.
  //
  // FIXME: Is this machine dependent?
  unsigned SectionDataPadding =
      offsetToAlignment(SectionDataFileSize, is64Bit() ? Align(8) : Align(4));
  SectionDataFileSize += SectionDataPadding;

  // Write the prolog, starting with the header and load command...
  writeHeader(MachO::MH_OBJECT, NumLoadCommands, LoadCommandsSize,
              SubsectionsViaSymbols);
  uint32_t Prot =
      MachO::VM_PROT_READ | MachO::VM_PROT_WRITE | MachO::VM_PROT_EXECUTE;
  writeSegmentLoadCommand("", NumSections, 0, VMSize, SectionDataStart,
                          SectionDataSize, Prot, Prot);

  // ... and then the section headers.
  uint64_t RelocTableEnd = SectionDataStart + SectionDataFileSize;
  for (const MCSection &Section : Asm) {
    const auto &Sec = cast<MCSectionMachO>(Section);
    std::vector<RelAndSymbol> &Relocs = Relocations[&Sec];
    unsigned NumRelocs = Relocs.size();
    uint64_t SectionStart = SectionDataStart + getSectionAddress(&Sec);
    unsigned Flags = Sec.getTypeAndAttributes();
    if (Sec.hasInstructions())
      Flags |= MachO::S_ATTR_SOME_INSTRUCTIONS;
    if (!cast<MCSectionMachO>(Sec).isVirtualSection() &&
        !isUInt<32>(SectionStart)) {
      Asm.getContext().reportError(
          SMLoc(), "cannot encode offset of section; object file too large");
      return NumBytesWritten();
    }
    if (NumRelocs && !isUInt<32>(RelocTableEnd)) {
      Asm.getContext().reportError(
          SMLoc(),
          "cannot encode offset of relocations; object file too large");
      return NumBytesWritten();
    }
    writeSection(Asm, Sec, getSectionAddress(&Sec), SectionStart, Flags,
                 RelocTableEnd, NumRelocs);
    RelocTableEnd += NumRelocs * sizeof(MachO::any_relocation_info);
  }

  // Write out the deployment target information, if it's available.
  auto EmitDeploymentTargetVersion =
      [&](const VersionInfoType &VersionInfo) {
        auto EncodeVersion = [](VersionTuple V) -> uint32_t {
          assert(!V.empty() && "empty version");
          unsigned Update = V.getSubminor().value_or(0);
          unsigned Minor = V.getMinor().value_or(0);
          assert(Update < 256 && "unencodable update target version");
          assert(Minor < 256 && "unencodable minor target version");
          assert(V.getMajor() < 65536 && "unencodable major target version");
          return Update | (Minor << 8) | (V.getMajor() << 16);
        };
        uint32_t EncodedVersion = EncodeVersion(VersionTuple(
            VersionInfo.Major, VersionInfo.Minor, VersionInfo.Update));
        uint32_t SDKVersion = !VersionInfo.SDKVersion.empty()
                                  ? EncodeVersion(VersionInfo.SDKVersion)
                                  : 0;
        if (VersionInfo.EmitBuildVersion) {
          // FIXME: Currently empty tools. Add clang version in the future.
          W.write<uint32_t>(MachO::LC_BUILD_VERSION);
          W.write<uint32_t>(sizeof(MachO::build_version_command));
          W.write<uint32_t>(VersionInfo.TypeOrPlatform.Platform);
          W.write<uint32_t>(EncodedVersion);
          W.write<uint32_t>(SDKVersion);
          W.write<uint32_t>(0); // Empty tools list.
        } else {
          MachO::LoadCommandType LCType =
              getLCFromMCVM(VersionInfo.TypeOrPlatform.Type);
          W.write<uint32_t>(LCType);
          W.write<uint32_t>(sizeof(MachO::version_min_command));
          W.write<uint32_t>(EncodedVersion);
          W.write<uint32_t>(SDKVersion);
        }
      };
  if (VersionInfo.Major != 0)
    EmitDeploymentTargetVersion(VersionInfo);
  if (TargetVariantVersionInfo.Major != 0)
    EmitDeploymentTargetVersion(TargetVariantVersionInfo);

  // Write the data-in-code load command, if used.
  uint64_t DataInCodeTableEnd = RelocTableEnd + NumDataRegions * 8;
  if (NumDataRegions) {
    uint64_t DataRegionsOffset = RelocTableEnd;
    uint64_t DataRegionsSize = NumDataRegions * 8;
    writeLinkeditLoadCommand(MachO::LC_DATA_IN_CODE, DataRegionsOffset,
                             DataRegionsSize);
  }

  // Write the loh load command, if used.
  uint64_t LOHTableEnd = DataInCodeTableEnd + LOHSize;
  if (LOHSize)
    writeLinkeditLoadCommand(MachO::LC_LINKER_OPTIMIZATION_HINT,
                             DataInCodeTableEnd, LOHSize);

  // Write the symbol table load command, if used.
  if (NumSymbols) {
    unsigned FirstLocalSymbol = 0;
    unsigned NumLocalSymbols = LocalSymbolData.size();
    unsigned FirstExternalSymbol = FirstLocalSymbol + NumLocalSymbols;
    unsigned NumExternalSymbols = ExternalSymbolData.size();
    unsigned FirstUndefinedSymbol = FirstExternalSymbol + NumExternalSymbols;
    unsigned NumUndefinedSymbols = UndefinedSymbolData.size();
    unsigned NumIndirectSymbols = IndirectSymbols.size();
    unsigned NumSymTabSymbols =
      NumLocalSymbols + NumExternalSymbols + NumUndefinedSymbols;
    uint64_t IndirectSymbolSize = NumIndirectSymbols * 4;
    uint64_t IndirectSymbolOffset = 0;

    // If used, the indirect symbols are written after the section data.
    if (NumIndirectSymbols)
      IndirectSymbolOffset = LOHTableEnd;

    // The symbol table is written after the indirect symbol data.
    uint64_t SymbolTableOffset = LOHTableEnd + IndirectSymbolSize;

    // The string table is written after symbol table.
    uint64_t StringTableOffset =
      SymbolTableOffset + NumSymTabSymbols * (is64Bit() ?
                                              sizeof(MachO::nlist_64) :
                                              sizeof(MachO::nlist));
    writeSymtabLoadCommand(SymbolTableOffset, NumSymTabSymbols,
                           StringTableOffset, StringTable.getSize());

    writeDysymtabLoadCommand(FirstLocalSymbol, NumLocalSymbols,
                             FirstExternalSymbol, NumExternalSymbols,
                             FirstUndefinedSymbol, NumUndefinedSymbols,
                             IndirectSymbolOffset, NumIndirectSymbols);
  }

  // Write the linker options load commands.
  for (const auto &Option : LinkerOptions)
    writeLinkerOptionsLoadCommand(Option);

  // Write the actual section data.
  for (const MCSection &Sec : Asm) {
    Asm.writeSectionData(W.OS, &Sec);

    uint64_t Pad = getPaddingSize(Asm, &Sec);
    W.OS.write_zeros(Pad);
  }

  // Write the extra padding.
  W.OS.write_zeros(SectionDataPadding);

  // Write the relocation entries.
  for (const MCSection &Sec : Asm) {
    // Write the section relocation entries, in reverse order to match 'as'
    // (approximately, the exact algorithm is more complicated than this).
    std::vector<RelAndSymbol> &Relocs = Relocations[&Sec];
    for (const RelAndSymbol &Rel : llvm::reverse(Relocs)) {
      W.write<uint32_t>(Rel.MRE.r_word0);
      W.write<uint32_t>(Rel.MRE.r_word1);
    }
  }

  // Write out the data-in-code region payload, if there is one.
  for (DataRegionData Data : DataRegions) {
    uint64_t Start = getSymbolAddress(*Data.Start, Asm);
    uint64_t End;
    if (Data.End)
      End = getSymbolAddress(*Data.End, Asm);
    else
      report_fatal_error("Data region not terminated");

    LLVM_DEBUG(dbgs() << "data in code region-- kind: " << Data.Kind
                      << "  start: " << Start << "(" << Data.Start->getName()
                      << ")" << "  end: " << End << "(" << Data.End->getName()
                      << ")" << "  size: " << End - Start << "\n");
    W.write<uint32_t>(Start);
    W.write<uint16_t>(End - Start);
    W.write<uint16_t>(Data.Kind);
  }

  // Write out the loh commands, if there is one.
  if (LOHSize) {
#ifndef NDEBUG
    unsigned Start = W.OS.tell();
#endif
    LOHContainer.emit(Asm, *this);
    // Pad to a multiple of the pointer size.
    W.OS.write_zeros(
        offsetToAlignment(LOHRawSize, is64Bit() ? Align(8) : Align(4)));
    assert(W.OS.tell() - Start == LOHSize);
  }

  // Write the symbol table data, if used.
  if (NumSymbols) {
    // Write the indirect symbol entries.
    for (auto &ISD : IndirectSymbols) {
      // Indirect symbols in the non-lazy symbol pointer section have some
      // special handling.
      const MCSectionMachO &Section =
          static_cast<const MCSectionMachO &>(*ISD.Section);
      if (Section.getType() == MachO::S_NON_LAZY_SYMBOL_POINTERS) {
        // If this symbol is defined and internal, mark it as such.
        if (ISD.Symbol->isDefined() && !ISD.Symbol->isExternal()) {
          uint32_t Flags = MachO::INDIRECT_SYMBOL_LOCAL;
          if (ISD.Symbol->isAbsolute())
            Flags |= MachO::INDIRECT_SYMBOL_ABS;
          W.write<uint32_t>(Flags);
          continue;
        }
      }

      W.write<uint32_t>(ISD.Symbol->getIndex());
    }

    // FIXME: Check that offsets match computed ones.

    // Write the symbol table entries.
    for (auto *SymbolData :
         {&LocalSymbolData, &ExternalSymbolData, &UndefinedSymbolData})
      for (MachSymbolData &Entry : *SymbolData)
        writeNlist(Entry, Asm);

    // Write the string table.
    StringTable.write(W.OS);
  }

  return NumBytesWritten();
}