xref: /llvm-project-15.0.7/lld/MachO/InputFiles.cpp (revision 45597316)

//===- InputFiles.cpp -----------------------------------------------------===//
//
// 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 contains functions to parse Mach-O object files. In this comment,
// we describe the Mach-O file structure and how we parse it.
//
// Mach-O is not very different from ELF or COFF. The notion of symbols,
// sections and relocations exists in Mach-O as it does in ELF and COFF.
//
// Perhaps the notion that is new to those who know ELF/COFF is "subsections".
// In ELF/COFF, sections are an atomic unit of data copied from input files to
// output files. When we merge or garbage-collect sections, we treat each
// section as an atomic unit. In Mach-O, that's not the case. Sections can
// consist of multiple subsections, and subsections are a unit of merging and
// garbage-collecting. Therefore, Mach-O's subsections are more similar to
// ELF/COFF's sections than Mach-O's sections are.
//
// A section can have multiple symbols. A symbol that does not have the
// N_ALT_ENTRY attribute indicates a beginning of a subsection. Therefore, by
// definition, a symbol is always present at the beginning of each subsection. A
// symbol with N_ALT_ENTRY attribute does not start a new subsection and can
// point to a middle of a subsection.
//
// The notion of subsections also affects how relocations are represented in
// Mach-O. All references within a section need to be explicitly represented as
// relocations if they refer to different subsections, because we obviously need
// to fix up addresses if subsections are laid out in an output file differently
// than they were in object files. To represent that, Mach-O relocations can
// refer to an unnamed location via its address. Scattered relocations (those
// with the R_SCATTERED bit set) always refer to unnamed locations.
// Non-scattered relocations refer to an unnamed location if r_extern is not set
// and r_symbolnum is zero.
//
// Without the above differences, I think you can use your knowledge about ELF
// and COFF for Mach-O.
//
//===----------------------------------------------------------------------===//

#include "InputFiles.h"
#include "Config.h"
#include "Driver.h"
#include "Dwarf.h"
#include "ExportTrie.h"
#include "InputSection.h"
#include "MachOStructs.h"
#include "ObjC.h"
#include "OutputSection.h"
#include "OutputSegment.h"
#include "SymbolTable.h"
#include "Symbols.h"
#include "Target.h"

#include "lld/Common/DWARF.h"
#include "lld/Common/ErrorHandler.h"
#include "lld/Common/Memory.h"
#include "lld/Common/Reproduce.h"
#include "llvm/ADT/iterator.h"
#include "llvm/BinaryFormat/MachO.h"
#include "llvm/LTO/LTO.h"
#include "llvm/Support/Endian.h"
#include "llvm/Support/MemoryBuffer.h"
#include "llvm/Support/Path.h"
#include "llvm/Support/TarWriter.h"
#include "llvm/TextAPI/MachO/Architecture.h"
#include "llvm/TextAPI/MachO/InterfaceFile.h"

using namespace llvm;
using namespace llvm::MachO;
using namespace llvm::support::endian;
using namespace llvm::sys;
using namespace lld;
using namespace lld::macho;

// Returns "<internal>", "foo.a(bar.o)", or "baz.o".
std::string lld::toString(const InputFile *f) {
  if (!f)
    return "<internal>";

  // Multiple dylibs can be defined in one .tbd file.
  if (auto dylibFile = dyn_cast<DylibFile>(f))
    if (f->getName().endswith(".tbd"))
      return (f->getName() + "(" + dylibFile->dylibName + ")").str();

  if (f->archiveName.empty())
    return std::string(f->getName());
  return (path::filename(f->archiveName) + "(" + path::filename(f->getName()) +
          ")")
      .str();
}

SetVector<InputFile *> macho::inputFiles;
std::unique_ptr<TarWriter> macho::tar;
int InputFile::idCount = 0;

// Open a given file path and return it as a memory-mapped file.
Optional<MemoryBufferRef> macho::readFile(StringRef path) {
  // Open a file.
  ErrorOr<std::unique_ptr<MemoryBuffer>> mbOrErr = MemoryBuffer::getFile(path);
  if (std::error_code ec = mbOrErr.getError()) {
    error("cannot open " + path + ": " + ec.message());
    return None;
  }

  std::unique_ptr<MemoryBuffer> &mb = *mbOrErr;
  MemoryBufferRef mbref = mb->getMemBufferRef();
  make<std::unique_ptr<MemoryBuffer>>(std::move(mb)); // take mb ownership

  // If this is a regular non-fat file, return it.
  const char *buf = mbref.getBufferStart();
  const auto *hdr = reinterpret_cast<const fat_header *>(buf);
  if (mbref.getBufferSize() < sizeof(uint32_t) ||
      read32be(&hdr->magic) != FAT_MAGIC) {
    if (tar)
      tar->append(relativeToRoot(path), mbref.getBuffer());
    return mbref;
  }

  // Object files and archive files may be fat files, which contains
  // multiple real files for different CPU ISAs. Here, we search for a
  // file that matches with the current link target and returns it as
  // a MemoryBufferRef.
  const auto *arch = reinterpret_cast<const fat_arch *>(buf + sizeof(*hdr));

  for (uint32_t i = 0, n = read32be(&hdr->nfat_arch); i < n; ++i) {
    if (reinterpret_cast<const char *>(arch + i + 1) >
        buf + mbref.getBufferSize()) {
      error(path + ": fat_arch struct extends beyond end of file");
      return None;
    }

    if (read32be(&arch[i].cputype) != target->cpuType ||
        read32be(&arch[i].cpusubtype) != target->cpuSubtype)
      continue;

    uint32_t offset = read32be(&arch[i].offset);
    uint32_t size = read32be(&arch[i].size);
    if (offset + size > mbref.getBufferSize())
      error(path + ": slice extends beyond end of file");
    if (tar)
      tar->append(relativeToRoot(path), mbref.getBuffer());
    return MemoryBufferRef(StringRef(buf + offset, size), path.copy(bAlloc));
  }

  error("unable to find matching architecture in " + path);
  return None;
}

InputFile::InputFile(Kind kind, const InterfaceFile &interface)
    : id(idCount++), fileKind(kind), name(saver.save(interface.getPath())) {}

void ObjFile::parseSections(ArrayRef<section_64> sections) {
  subsections.reserve(sections.size());
  auto *buf = reinterpret_cast<const uint8_t *>(mb.getBufferStart());

  for (const section_64 &sec : sections) {
    InputSection *isec = make<InputSection>();
    isec->file = this;
    isec->name =
        StringRef(sec.sectname, strnlen(sec.sectname, sizeof(sec.sectname)));
    isec->segname =
        StringRef(sec.segname, strnlen(sec.segname, sizeof(sec.segname)));
    isec->data = {isZeroFill(sec.flags) ? nullptr : buf + sec.offset,
                  static_cast<size_t>(sec.size)};
    if (sec.align >= 32)
      error("alignment " + std::to_string(sec.align) + " of section " +
            isec->name + " is too large");
    else
      isec->align = 1 << sec.align;
    isec->flags = sec.flags;

    if (!(isDebugSection(isec->flags) &&
          isec->segname == segment_names::dwarf)) {
      subsections.push_back({{0, isec}});
    } else {
      // Instead of emitting DWARF sections, we emit STABS symbols to the
      // object files that contain them. We filter them out early to avoid
      // parsing their relocations unnecessarily. But we must still push an
      // empty map to ensure the indices line up for the remaining sections.
      subsections.push_back({});
      debugSections.push_back(isec);
    }
  }
}

// Find the subsection corresponding to the greatest section offset that is <=
// that of the given offset.
//
// offset: an offset relative to the start of the original InputSection (before
// any subsection splitting has occurred). It will be updated to represent the
// same location as an offset relative to the start of the containing
// subsection.
static InputSection *findContainingSubsection(SubsectionMapping &map,
                                              uint64_t *offset) {
  auto it = std::prev(llvm::upper_bound(
      map, *offset, [](uint64_t value, SubsectionEntry subsectionEntry) {
        return value < subsectionEntry.offset;
      }));
  *offset -= it->offset;
  return it->isec;
}

static bool validateRelocationInfo(InputFile *file, const section_64 &sec,
                                   relocation_info rel) {
  const RelocAttrs &relocAttrs = target->getRelocAttrs(rel.r_type);
  bool valid = true;
  auto message = [relocAttrs, file, sec, rel, &valid](const Twine &diagnostic) {
    valid = false;
    return (relocAttrs.name + " relocation " + diagnostic + " at offset " +
            std::to_string(rel.r_address) + " of " + sec.segname + "," +
            sec.sectname + " in " + toString(file))
        .str();
  };

  if (!relocAttrs.hasAttr(RelocAttrBits::LOCAL) && !rel.r_extern)
    error(message("must be extern"));
  if (relocAttrs.hasAttr(RelocAttrBits::PCREL) != rel.r_pcrel)
    error(message(Twine("must ") + (rel.r_pcrel ? "not " : "") +
                  "be PC-relative"));
  if (isThreadLocalVariables(sec.flags) &&
      !relocAttrs.hasAttr(RelocAttrBits::UNSIGNED))
    error(message("not allowed in thread-local section, must be UNSIGNED"));
  if (rel.r_length < 2 || rel.r_length > 3 ||
      !relocAttrs.hasAttr(static_cast<RelocAttrBits>(1 << rel.r_length))) {
    static SmallVector<StringRef, 4> widths{"0", "4", "8", "4 or 8"};
    error(message("has width " + std::to_string(1 << rel.r_length) +
                  " bytes, but must be " +
                  widths[(static_cast<int>(relocAttrs.bits) >> 2) & 3] +
                  " bytes"));
  }
  return valid;
}

void ObjFile::parseRelocations(const section_64 &sec,
                               SubsectionMapping &subsecMap) {
  auto *buf = reinterpret_cast<const uint8_t *>(mb.getBufferStart());
  ArrayRef<relocation_info> relInfos(
      reinterpret_cast<const relocation_info *>(buf + sec.reloff), sec.nreloc);

  for (size_t i = 0; i < relInfos.size(); i++) {
    // Paired relocations serve as Mach-O's method for attaching a
    // supplemental datum to a primary relocation record. ELF does not
    // need them because the *_RELOC_RELA records contain the extra
    // addend field, vs. *_RELOC_REL which omit the addend.
    //
    // The {X86_64,ARM64}_RELOC_SUBTRACTOR record holds the subtrahend,
    // and the paired *_RELOC_UNSIGNED record holds the minuend. The
    // datum for each is a symbolic address. The result is the offset
    // between two addresses.
    //
    // The ARM64_RELOC_ADDEND record holds the addend, and the paired
    // ARM64_RELOC_BRANCH26 or ARM64_RELOC_PAGE21/PAGEOFF12 holds the
    // base symbolic address.
    //
    // Note: X86 does not use *_RELOC_ADDEND because it can embed an
    // addend into the instruction stream. On X86, a relocatable address
    // field always occupies an entire contiguous sequence of byte(s),
    // so there is no need to merge opcode bits with address
    // bits. Therefore, it's easy and convenient to store addends in the
    // instruction-stream bytes that would otherwise contain zeroes. By
    // contrast, RISC ISAs such as ARM64 mix opcode bits with with
    // address bits so that bitwise arithmetic is necessary to extract
    // and insert them. Storing addends in the instruction stream is
    // possible, but inconvenient and more costly at link time.

    int64_t pairedAddend = 0;
    relocation_info relInfo = relInfos[i];
    if (target->hasAttr(relInfo.r_type, RelocAttrBits::ADDEND)) {
      pairedAddend = SignExtend64<24>(relInfo.r_symbolnum);
      relInfo = relInfos[++i];
    }
    assert(i < relInfos.size());
    if (!validateRelocationInfo(this, sec, relInfo))
      continue;
    if (relInfo.r_address & R_SCATTERED)
      fatal("TODO: Scattered relocations not supported");

    int64_t embeddedAddend = target->getEmbeddedAddend(mb, sec, relInfo);
    assert(!(embeddedAddend && pairedAddend));
    int64_t totalAddend = pairedAddend + embeddedAddend;
    Reloc r;
    r.type = relInfo.r_type;
    r.pcrel = relInfo.r_pcrel;
    r.length = relInfo.r_length;
    r.offset = relInfo.r_address;
    if (relInfo.r_extern) {
      r.referent = symbols[relInfo.r_symbolnum];
      r.addend = totalAddend;
    } else {
      SubsectionMapping &referentSubsecMap =
          subsections[relInfo.r_symbolnum - 1];
      const section_64 &referentSec = sectionHeaders[relInfo.r_symbolnum - 1];
      uint64_t referentOffset;
      if (relInfo.r_pcrel) {
        // The implicit addend for pcrel section relocations is the pcrel offset
        // in terms of the addresses in the input file. Here we adjust it so
        // that it describes the offset from the start of the referent section.
        // FIXME This logic was written around x86_64 behavior -- ARM64 doesn't
        // have pcrel section relocations. We may want to factor this out into
        // the arch-specific .cpp file.
        assert(target->hasAttr(r.type, RelocAttrBits::BYTE4));
        referentOffset =
            sec.addr + relInfo.r_address + 4 + totalAddend - referentSec.addr;
      } else {
        // The addend for a non-pcrel relocation is its absolute address.
        referentOffset = totalAddend - referentSec.addr;
      }
      r.referent = findContainingSubsection(referentSubsecMap, &referentOffset);
      r.addend = referentOffset;
    }

    InputSection *subsec = findContainingSubsection(subsecMap, &r.offset);
    subsec->relocs.push_back(r);

    if (target->hasAttr(r.type, RelocAttrBits::SUBTRAHEND)) {
      relInfo = relInfos[++i];
      // SUBTRACTOR relocations should always be followed by an UNSIGNED one
      // indicating the minuend symbol.
      assert(target->hasAttr(relInfo.r_type, RelocAttrBits::UNSIGNED) &&
             relInfo.r_extern);
      Reloc p;
      p.type = relInfo.r_type;
      p.referent = symbols[relInfo.r_symbolnum];
      subsec->relocs.push_back(p);
    }
  }
}

static macho::Symbol *createDefined(const structs::nlist_64 &sym,
                                    StringRef name, InputSection *isec,
                                    uint64_t value, uint64_t size) {
  // Symbol scope is determined by sym.n_type & (N_EXT | N_PEXT):
  // N_EXT: Global symbols
  // N_EXT | N_PEXT: Linkage unit (think: dylib) scoped
  // N_PEXT: Does not occur in input files in practice,
  //         a private extern must be external.
  // 0: Translation-unit scoped. These are not in the symbol table.

  if (sym.n_type & (N_EXT | N_PEXT)) {
    assert((sym.n_type & N_EXT) && "invalid input");
    return symtab->addDefined(name, isec->file, isec, value, size,
                              sym.n_desc & N_WEAK_DEF, sym.n_type & N_PEXT);
  }
  return make<Defined>(name, isec->file, isec, value, size,
                       sym.n_desc & N_WEAK_DEF,
                       /*isExternal=*/false, /*isPrivateExtern=*/false);
}

// Checks if the version specified in `cmd` is compatible with target
// version. IOW, check if cmd's version >= config's version.
static bool hasCompatVersion(const InputFile *input,
                             const build_version_command *cmd) {

  if (config->target.Platform != static_cast<PlatformKind>(cmd->platform)) {
    error(toString(input) + " has platform " +
          getPlatformName(static_cast<PlatformKind>(cmd->platform)) +
          Twine(", which is different from target platform ") +
          getPlatformName(config->target.Platform));
    return false;
  }

  unsigned major = cmd->minos >> 16;
  unsigned minor = (cmd->minos >> 8) & 0xffu;
  unsigned subMinor = cmd->minos & 0xffu;
  VersionTuple version(major, minor, subMinor);
  if (version >= config->platformInfo.minimum)
    return true;

  error(toString(input) + " has version " + version.getAsString() +
        ", which is incompatible with target version of " +
        config->platformInfo.minimum.getAsString());
  return false;
}

// Absolute symbols are defined symbols that do not have an associated
// InputSection. They cannot be weak.
static macho::Symbol *createAbsolute(const structs::nlist_64 &sym,
                                     InputFile *file, StringRef name) {
  if (sym.n_type & (N_EXT | N_PEXT)) {
    assert((sym.n_type & N_EXT) && "invalid input");
    return symtab->addDefined(name, file, nullptr, sym.n_value, /*size=*/0,
                              /*isWeakDef=*/false, sym.n_type & N_PEXT);
  }
  return make<Defined>(name, file, nullptr, sym.n_value, /*size=*/0,
                       /*isWeakDef=*/false,
                       /*isExternal=*/false, /*isPrivateExtern=*/false);
}

macho::Symbol *ObjFile::parseNonSectionSymbol(const structs::nlist_64 &sym,
                                              StringRef name) {
  uint8_t type = sym.n_type & N_TYPE;
  switch (type) {
  case N_UNDF:
    return sym.n_value == 0
               ? symtab->addUndefined(name, this, sym.n_desc & N_WEAK_REF)
               : symtab->addCommon(name, this, sym.n_value,
                                   1 << GET_COMM_ALIGN(sym.n_desc),
                                   sym.n_type & N_PEXT);
  case N_ABS:
    return createAbsolute(sym, this, name);
  case N_PBUD:
  case N_INDR:
    error("TODO: support symbols of type " + std::to_string(type));
    return nullptr;
  case N_SECT:
    llvm_unreachable(
        "N_SECT symbols should not be passed to parseNonSectionSymbol");
  default:
    llvm_unreachable("invalid symbol type");
  }
}

void ObjFile::parseSymbols(ArrayRef<structs::nlist_64> nList,
                           const char *strtab, bool subsectionsViaSymbols) {
  // Precompute the boundaries of symbols within a section.
  // If subsectionsViaSymbols is True then the corresponding subsections will be
  // created, otherwise these boundaries are used for the calculation of symbols
  // sizes only.

  for (const structs::nlist_64 &sym : nList) {
    if ((sym.n_type & N_TYPE) == N_SECT && !(sym.n_desc & N_ALT_ENTRY) &&
        !subsections[sym.n_sect - 1].empty()) {
      SubsectionMapping &subsectionMapping = subsections[sym.n_sect - 1];
      subsectionMapping.push_back(
          {sym.n_value - sectionHeaders[sym.n_sect - 1].addr,
           subsectionMapping.front().isec});
    }
  }

  for (SubsectionMapping &subsectionMap : subsections) {
    if (subsectionMap.empty())
      continue;
    llvm::sort(subsectionMap,
               [](const SubsectionEntry &lhs, const SubsectionEntry &rhs) {
                 return lhs.offset < rhs.offset;
               });
    subsectionMap.erase(
        std::unique(subsectionMap.begin(), subsectionMap.end(),
                    [](const SubsectionEntry &lhs, const SubsectionEntry &rhs) {
                      return lhs.offset == rhs.offset;
                    }),
        subsectionMap.end());
    if (!subsectionsViaSymbols)
      continue;
    for (size_t i = 0; i < subsectionMap.size(); ++i) {
      uint32_t offset = subsectionMap[i].offset;
      InputSection *&isec = subsectionMap[i].isec;
      uint32_t end = i + 1 < subsectionMap.size() ? subsectionMap[i + 1].offset
                                                  : isec->data.size();
      isec = make<InputSection>(*isec);
      isec->data = isec->data.slice(offset, end - offset);
      // TODO: ld64 appears to preserve the original alignment as well as each
      // subsection's offset from the last aligned address. We should consider
      // emulating that behavior.
      isec->align = MinAlign(isec->align, offset);
    }
  }

  symbols.resize(nList.size());
  for (size_t i = 0, n = nList.size(); i < n; ++i) {
    const structs::nlist_64 &sym = nList[i];
    StringRef name = strtab + sym.n_strx;

    if ((sym.n_type & N_TYPE) != N_SECT) {
      symbols[i] = parseNonSectionSymbol(sym, name);
      continue;
    }

    const section_64 &sec = sectionHeaders[sym.n_sect - 1];
    SubsectionMapping &subsecMap = subsections[sym.n_sect - 1];

    // parseSections() may have chosen not to parse this section.
    if (subsecMap.empty())
      continue;

    uint64_t offset = sym.n_value - sec.addr;

    auto it = llvm::upper_bound(
        subsecMap, offset, [](uint64_t value, SubsectionEntry subsectionEntry) {
          return value < subsectionEntry.offset;
        });
    uint32_t size = it != subsecMap.end()
                        ? it->offset - offset
                        : subsecMap.front().isec->getSize() - offset;

    // If the input file does not use subsections-via-symbols, all symbols can
    // use the same subsection. Otherwise, we must split the sections along
    // symbol boundaries.
    if (!subsectionsViaSymbols) {
      symbols[i] =
          createDefined(sym, name, subsecMap.front().isec, offset, size);
      continue;
    }

    InputSection *subsec = (--it)->isec;
    symbols[i] = createDefined(sym, name, subsec, offset - it->offset, size);
  }

  if (!subsectionsViaSymbols)
    for (SubsectionMapping &subsectionMap : subsections)
      if (!subsectionMap.empty())
        subsectionMap = {subsectionMap.front()};
}

OpaqueFile::OpaqueFile(MemoryBufferRef mb, StringRef segName,
                       StringRef sectName)
    : InputFile(OpaqueKind, mb) {
  InputSection *isec = make<InputSection>();
  isec->file = this;
  isec->name = sectName.take_front(16);
  isec->segname = segName.take_front(16);
  const auto *buf = reinterpret_cast<const uint8_t *>(mb.getBufferStart());
  isec->data = {buf, mb.getBufferSize()};
  subsections.push_back({{0, isec}});
}

ObjFile::ObjFile(MemoryBufferRef mb, uint32_t modTime, StringRef archiveName)
    : InputFile(ObjKind, mb), modTime(modTime) {
  this->archiveName = std::string(archiveName);

  auto *buf = reinterpret_cast<const uint8_t *>(mb.getBufferStart());
  auto *hdr = reinterpret_cast<const mach_header_64 *>(mb.getBufferStart());

  Architecture arch = getArchitectureFromCpuType(hdr->cputype, hdr->cpusubtype);
  if (arch != config->target.Arch) {
    error(toString(this) + " has architecture " + getArchitectureName(arch) +
          " which is incompatible with target architecture " +
          getArchitectureName(config->target.Arch));
    return;
  }

  if (const auto *cmd =
          findCommand<build_version_command>(hdr, LC_BUILD_VERSION)) {
    if (!hasCompatVersion(this, cmd))
      return;
  }

  if (const load_command *cmd = findCommand(hdr, LC_LINKER_OPTION)) {
    auto *c = reinterpret_cast<const linker_option_command *>(cmd);
    StringRef data{reinterpret_cast<const char *>(c + 1),
                   c->cmdsize - sizeof(linker_option_command)};
    parseLCLinkerOption(this, c->count, data);
  }

  if (const load_command *cmd = findCommand(hdr, LC_SEGMENT_64)) {
    auto *c = reinterpret_cast<const segment_command_64 *>(cmd);
    sectionHeaders = ArrayRef<section_64>{
        reinterpret_cast<const section_64 *>(c + 1), c->nsects};
    parseSections(sectionHeaders);
  }

  // TODO: Error on missing LC_SYMTAB?
  if (const load_command *cmd = findCommand(hdr, LC_SYMTAB)) {
    auto *c = reinterpret_cast<const symtab_command *>(cmd);
    ArrayRef<structs::nlist_64> nList(
        reinterpret_cast<const structs::nlist_64 *>(buf + c->symoff), c->nsyms);
    const char *strtab = reinterpret_cast<const char *>(buf) + c->stroff;
    bool subsectionsViaSymbols = hdr->flags & MH_SUBSECTIONS_VIA_SYMBOLS;
    parseSymbols(nList, strtab, subsectionsViaSymbols);
  }

  // The relocations may refer to the symbols, so we parse them after we have
  // parsed all the symbols.
  for (size_t i = 0, n = subsections.size(); i < n; ++i)
    if (!subsections[i].empty())
      parseRelocations(sectionHeaders[i], subsections[i]);

  parseDebugInfo();
}

void ObjFile::parseDebugInfo() {
  std::unique_ptr<DwarfObject> dObj = DwarfObject::create(this);
  if (!dObj)
    return;

  auto *ctx = make<DWARFContext>(
      std::move(dObj), "",
      [&](Error err) {
        warn(toString(this) + ": " + toString(std::move(err)));
      },
      [&](Error warning) {
        warn(toString(this) + ": " + toString(std::move(warning)));
      });

  // TODO: Since object files can contain a lot of DWARF info, we should verify
  // that we are parsing just the info we need
  const DWARFContext::compile_unit_range &units = ctx->compile_units();
  // FIXME: There can be more than one compile unit per object file. See
  // PR48637.
  auto it = units.begin();
  compileUnit = it->get();
}

// The path can point to either a dylib or a .tbd file.
static Optional<DylibFile *> loadDylib(StringRef path, DylibFile *umbrella) {
  Optional<MemoryBufferRef> mbref = readFile(path);
  if (!mbref) {
    error("could not read dylib file at " + path);
    return {};
  }
  return loadDylib(*mbref, umbrella);
}

// TBD files are parsed into a series of TAPI documents (InterfaceFiles), with
// the first document storing child pointers to the rest of them. When we are
// processing a given TBD file, we store that top-level document in
// currentTopLevelTapi. When processing re-exports, we search its children for
// potentially matching documents in the same TBD file. Note that the children
// themselves don't point to further documents, i.e. this is a two-level tree.
//
// Re-exports can either refer to on-disk files, or to documents within .tbd
// files.
static Optional<DylibFile *>
findDylib(StringRef path, DylibFile *umbrella,
          const InterfaceFile *currentTopLevelTapi) {
  if (path::is_absolute(path, path::Style::posix))
    for (StringRef root : config->systemLibraryRoots)
      if (Optional<std::string> dylibPath =
              resolveDylibPath((root + path).str()))
        return loadDylib(*dylibPath, umbrella);

  // TODO: Expand @loader_path, @executable_path, @rpath etc, handle -dylib_path

  if (currentTopLevelTapi) {
    for (InterfaceFile &child :
         make_pointee_range(currentTopLevelTapi->documents())) {
      assert(child.documents().empty());
      if (path == child.getInstallName())
        return make<DylibFile>(child, umbrella);
    }
  }

  if (Optional<std::string> dylibPath = resolveDylibPath(path))
    return loadDylib(*dylibPath, umbrella);

  return {};
}

// If a re-exported dylib is public (lives in /usr/lib or
// /System/Library/Frameworks), then it is considered implicitly linked: we
// should bind to its symbols directly instead of via the re-exporting umbrella
// library.
static bool isImplicitlyLinked(StringRef path) {
  if (!config->implicitDylibs)
    return false;

  if (path::parent_path(path) == "/usr/lib")
    return true;

  // Match /System/Library/Frameworks/$FOO.framework/**/$FOO
  if (path.consume_front("/System/Library/Frameworks/")) {
    StringRef frameworkName = path.take_until([](char c) { return c == '.'; });
    return path::filename(path) == frameworkName;
  }

  return false;
}

void loadReexport(StringRef path, DylibFile *umbrella,
                  const InterfaceFile *currentTopLevelTapi) {
  Optional<DylibFile *> reexport =
      findDylib(path, umbrella, currentTopLevelTapi);
  if (!reexport)
    error("unable to locate re-export with install name " + path);
  else if (isImplicitlyLinked(path))
    inputFiles.insert(*reexport);
}

DylibFile::DylibFile(MemoryBufferRef mb, DylibFile *umbrella,
                     bool isBundleLoader)
    : InputFile(DylibKind, mb), refState(RefState::Unreferenced),
      isBundleLoader(isBundleLoader) {
  assert(!isBundleLoader || !umbrella);
  if (umbrella == nullptr)
    umbrella = this;

  auto *buf = reinterpret_cast<const uint8_t *>(mb.getBufferStart());
  auto *hdr = reinterpret_cast<const mach_header_64 *>(mb.getBufferStart());

  // Initialize dylibName.
  if (const load_command *cmd = findCommand(hdr, LC_ID_DYLIB)) {
    auto *c = reinterpret_cast<const dylib_command *>(cmd);
    currentVersion = read32le(&c->dylib.current_version);
    compatibilityVersion = read32le(&c->dylib.compatibility_version);
    dylibName = reinterpret_cast<const char *>(cmd) + read32le(&c->dylib.name);
  } else if (!isBundleLoader) {
    // macho_executable and macho_bundle don't have LC_ID_DYLIB,
    // so it's OK.
    error("dylib " + toString(this) + " missing LC_ID_DYLIB load command");
    return;
  }

  if (const build_version_command *cmd =
          findCommand<build_version_command>(hdr, LC_BUILD_VERSION)) {
    if (!hasCompatVersion(this, cmd))
      return;
  }

  // Initialize symbols.
  DylibFile *exportingFile = isImplicitlyLinked(dylibName) ? this : umbrella;
  if (const load_command *cmd = findCommand(hdr, LC_DYLD_INFO_ONLY)) {
    auto *c = reinterpret_cast<const dyld_info_command *>(cmd);
    parseTrie(buf + c->export_off, c->export_size,
              [&](const Twine &name, uint64_t flags) {
                bool isWeakDef = flags & EXPORT_SYMBOL_FLAGS_WEAK_DEFINITION;
                bool isTlv = flags & EXPORT_SYMBOL_FLAGS_KIND_THREAD_LOCAL;
                symbols.push_back(symtab->addDylib(
                    saver.save(name), exportingFile, isWeakDef, isTlv));
              });
  } else {
    error("LC_DYLD_INFO_ONLY not found in " + toString(this));
    return;
  }

  const uint8_t *p =
      reinterpret_cast<const uint8_t *>(hdr) + sizeof(mach_header_64);
  for (uint32_t i = 0, n = hdr->ncmds; i < n; ++i) {
    auto *cmd = reinterpret_cast<const load_command *>(p);
    p += cmd->cmdsize;

    if (!(hdr->flags & MH_NO_REEXPORTED_DYLIBS) &&
        cmd->cmd == LC_REEXPORT_DYLIB) {
      const auto *c = reinterpret_cast<const dylib_command *>(cmd);
      StringRef reexportPath =
          reinterpret_cast<const char *>(c) + read32le(&c->dylib.name);
      loadReexport(reexportPath, exportingFile, nullptr);
    }

    // FIXME: What about LC_LOAD_UPWARD_DYLIB, LC_LAZY_LOAD_DYLIB,
    // LC_LOAD_WEAK_DYLIB, LC_REEXPORT_DYLIB (..are reexports from dylibs with
    // MH_NO_REEXPORTED_DYLIBS loaded for -flat_namespace)?
    if (config->namespaceKind == NamespaceKind::flat &&
        cmd->cmd == LC_LOAD_DYLIB) {
      const auto *c = reinterpret_cast<const dylib_command *>(cmd);
      StringRef dylibPath =
          reinterpret_cast<const char *>(c) + read32le(&c->dylib.name);
      Optional<DylibFile *> dylib = findDylib(dylibPath, umbrella, nullptr);
      if (!dylib)
        error(Twine("unable to locate library '") + dylibPath +
              "' loaded from '" + toString(this) + "' for -flat_namespace");
    }
  }
}

DylibFile::DylibFile(const InterfaceFile &interface, DylibFile *umbrella,
                     bool isBundleLoader)
    : InputFile(DylibKind, interface), refState(RefState::Unreferenced),
      isBundleLoader(isBundleLoader) {
  // FIXME: Add test for the missing TBD code path.

  if (umbrella == nullptr)
    umbrella = this;

  dylibName = saver.save(interface.getInstallName());
  compatibilityVersion = interface.getCompatibilityVersion().rawValue();
  currentVersion = interface.getCurrentVersion().rawValue();

  if (!is_contained(interface.targets(), config->target)) {
    error(toString(this) + " is incompatible with " +
          std::string(config->target));
    return;
  }

  DylibFile *exportingFile = isImplicitlyLinked(dylibName) ? this : umbrella;
  auto addSymbol = [&](const Twine &name) -> void {
    symbols.push_back(symtab->addDylib(saver.save(name), exportingFile,
                                       /*isWeakDef=*/false,
                                       /*isTlv=*/false));
  };
  // TODO(compnerd) filter out symbols based on the target platform
  // TODO: handle weak defs, thread locals
  for (const auto *symbol : interface.symbols()) {
    if (!symbol->getArchitectures().has(config->target.Arch))
      continue;

    switch (symbol->getKind()) {
    case SymbolKind::GlobalSymbol:
      addSymbol(symbol->getName());
      break;
    case SymbolKind::ObjectiveCClass:
      // XXX ld64 only creates these symbols when -ObjC is passed in. We may
      // want to emulate that.
      addSymbol(objc::klass + symbol->getName());
      addSymbol(objc::metaclass + symbol->getName());
      break;
    case SymbolKind::ObjectiveCClassEHType:
      addSymbol(objc::ehtype + symbol->getName());
      break;
    case SymbolKind::ObjectiveCInstanceVariable:
      addSymbol(objc::ivar + symbol->getName());
      break;
    }
  }

  const InterfaceFile *topLevel =
      interface.getParent() == nullptr ? &interface : interface.getParent();

  for (InterfaceFileRef intfRef : interface.reexportedLibraries()) {
    InterfaceFile::const_target_range targets = intfRef.targets();
    if (is_contained(targets, config->target))
      loadReexport(intfRef.getInstallName(), exportingFile, topLevel);
  }
}

ArchiveFile::ArchiveFile(std::unique_ptr<object::Archive> &&f)
    : InputFile(ArchiveKind, f->getMemoryBufferRef()), file(std::move(f)) {
  for (const object::Archive::Symbol &sym : file->symbols())
    symtab->addLazy(sym.getName(), this, sym);
}

void ArchiveFile::fetch(const object::Archive::Symbol &sym) {
  object::Archive::Child c =
      CHECK(sym.getMember(), toString(this) +
                                 ": could not get the member for symbol " +
                                 toMachOString(sym));

  if (!seen.insert(c.getChildOffset()).second)
    return;

  MemoryBufferRef mb =
      CHECK(c.getMemoryBufferRef(),
            toString(this) +
                ": could not get the buffer for the member defining symbol " +
                toMachOString(sym));

  if (tar && c.getParent()->isThin())
    tar->append(relativeToRoot(CHECK(c.getFullName(), this)), mb.getBuffer());

  uint32_t modTime = toTimeT(
      CHECK(c.getLastModified(), toString(this) +
                                     ": could not get the modification time "
                                     "for the member defining symbol " +
                                     toMachOString(sym)));

  // `sym` is owned by a LazySym, which will be replace<>() by make<ObjFile>
  // and become invalid after that call. Copy it to the stack so we can refer
  // to it later.
  const object::Archive::Symbol sym_copy = sym;

  if (Optional<InputFile *> file =
          loadArchiveMember(mb, modTime, getName(), /*objCOnly=*/false)) {
    inputFiles.insert(*file);
    // ld64 doesn't demangle sym here even with -demangle. Match that, so
    // intentionally no call to toMachOString() here.
    printArchiveMemberLoad(sym_copy.getName(), *file);
  }
}

static macho::Symbol *createBitcodeSymbol(const lto::InputFile::Symbol &objSym,
                                          BitcodeFile &file) {
  StringRef name = saver.save(objSym.getName());

  // TODO: support weak references
  if (objSym.isUndefined())
    return symtab->addUndefined(name, &file, /*isWeakRef=*/false);

  assert(!objSym.isCommon() && "TODO: support common symbols in LTO");

  // TODO: Write a test demonstrating why computing isPrivateExtern before
  // LTO compilation is important.
  bool isPrivateExtern = false;
  switch (objSym.getVisibility()) {
  case GlobalValue::HiddenVisibility:
    isPrivateExtern = true;
    break;
  case GlobalValue::ProtectedVisibility:
    error(name + " has protected visibility, which is not supported by Mach-O");
    break;
  case GlobalValue::DefaultVisibility:
    break;
  }

  return symtab->addDefined(name, &file, /*isec=*/nullptr, /*value=*/0,
                            /*size=*/0, objSym.isWeak(), isPrivateExtern);
}

BitcodeFile::BitcodeFile(MemoryBufferRef mbref)
    : InputFile(BitcodeKind, mbref) {
  obj = check(lto::InputFile::create(mbref));

  // Convert LTO Symbols to LLD Symbols in order to perform resolution. The
  // "winning" symbol will then be marked as Prevailing at LTO compilation
  // time.
  for (const lto::InputFile::Symbol &objSym : obj->symbols())
    symbols.push_back(createBitcodeSymbol(objSym, *this));
}