xref: /llvm-project-15.0.7/lld/ELF/InputSection.cpp (revision 0ac158a5)

//===- InputSection.cpp ---------------------------------------------------===//
//
//                             The LLVM Linker
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//

#include "InputSection.h"
#include "Config.h"
#include "EhFrame.h"
#include "Error.h"
#include "InputFiles.h"
#include "LinkerScript.h"
#include "OutputSections.h"
#include "Target.h"
#include "Thunks.h"

#include "llvm/Support/Compression.h"
#include "llvm/Support/Endian.h"

using namespace llvm;
using namespace llvm::ELF;
using namespace llvm::object;
using namespace llvm::support;
using namespace llvm::support::endian;

using namespace lld;
using namespace lld::elf;

template <class ELFT>
static ArrayRef<uint8_t> getSectionContents(elf::ObjectFile<ELFT> *File,
                                            const typename ELFT::Shdr *Hdr) {
  if (!File || Hdr->sh_type == SHT_NOBITS)
    return makeArrayRef<uint8_t>(nullptr, Hdr->sh_size);
  return check(File->getObj().getSectionContents(Hdr));
}

// ELF supports ZLIB-compressed section. Returns true if the section
// is compressed.
template <class ELFT>
static bool isCompressed(typename ELFT::uint Flags, StringRef Name) {
  return (Flags & SHF_COMPRESSED) || Name.startswith(".zdebug");
}

template <class ELFT>
InputSectionBase<ELFT>::InputSectionBase(elf::ObjectFile<ELFT> *File,
                                         uintX_t Flags, uint32_t Type,
                                         uintX_t Entsize, uint32_t Link,
                                         uint32_t Info, uintX_t Addralign,
                                         ArrayRef<uint8_t> Data, StringRef Name,
                                         Kind SectionKind)
    : InputSectionData(SectionKind, Name, Data, isCompressed<ELFT>(Flags, Name),
                       !Config->GcSections || !(Flags & SHF_ALLOC)),
      File(File), Flags(Flags), Entsize(Entsize), Type(Type), Link(Link),
      Info(Info), Repl(this) {
  // The ELF spec states that a value of 0 means the section has
  // no alignment constraits.
  uint64_t V = std::max<uint64_t>(Addralign, 1);
  if (!isPowerOf2_64(V))
    fatal(getFilename(File) + ": section sh_addralign is not a power of 2");

  // We reject object files having insanely large alignments even though
  // they are allowed by the spec. I think 4GB is a reasonable limitation.
  // We might want to relax this in the future.
  if (V > UINT32_MAX)
    fatal(getFilename(File) + ": section sh_addralign is too large");
  Alignment = V;
}

template <class ELFT>
InputSectionBase<ELFT>::InputSectionBase(elf::ObjectFile<ELFT> *File,
                                         const Elf_Shdr *Hdr, StringRef Name,
                                         Kind SectionKind)
    : InputSectionBase(File, Hdr->sh_flags, Hdr->sh_type, Hdr->sh_entsize,
                       Hdr->sh_link, Hdr->sh_info, Hdr->sh_addralign,
                       getSectionContents(File, Hdr), Name, SectionKind) {}

template <class ELFT> size_t InputSectionBase<ELFT>::getSize() const {
  if (auto *D = dyn_cast<InputSection<ELFT>>(this))
    if (D->getThunksSize() > 0)
      return D->getThunkOff() + D->getThunksSize();
  return Data.size();
}

// Returns a string for an error message.
template <class SectionT> static std::string getName(SectionT *Sec) {
  return (Sec->getFile()->getName() + "(" + Sec->Name + ")").str();
}

template <class ELFT>
typename ELFT::uint InputSectionBase<ELFT>::getOffset(uintX_t Offset) const {
  switch (kind()) {
  case Regular:
    return cast<InputSection<ELFT>>(this)->OutSecOff + Offset;
  case EHFrame:
    // The file crtbeginT.o has relocations pointing to the start of an empty
    // .eh_frame that is known to be the first in the link. It does that to
    // identify the start of the output .eh_frame.
    return Offset;
  case Merge:
    return cast<MergeInputSection<ELFT>>(this)->getOffset(Offset);
  case MipsReginfo:
  case MipsOptions:
  case MipsAbiFlags:
    // MIPS .reginfo, .MIPS.options, and .MIPS.abiflags sections are consumed
    // by the linker, and the linker produces a single output section. It is
    // possible that input files contain section symbol points to the
    // corresponding input section. Redirect it to the produced output section.
    if (Offset != 0)
      fatal(getName(this) + ": unsupported reference to the middle of '" +
            Name + "' section");
    return this->OutSec->getVA();
  }
  llvm_unreachable("invalid section kind");
}

// Returns compressed data and its size when uncompressed.
template <class ELFT>
std::pair<ArrayRef<uint8_t>, uint64_t>
InputSectionBase<ELFT>::getElfCompressedData(ArrayRef<uint8_t> Data) {
  // Compressed section with Elf_Chdr is the ELF standard.
  if (Data.size() < sizeof(Elf_Chdr))
    fatal(getName(this) + ": corrupted compressed section");
  auto *Hdr = reinterpret_cast<const Elf_Chdr *>(Data.data());
  if (Hdr->ch_type != ELFCOMPRESS_ZLIB)
    fatal(getName(this) + ": unsupported compression type");
  return {Data.slice(sizeof(*Hdr)), Hdr->ch_size};
}

// Returns compressed data and its size when uncompressed.
template <class ELFT>
std::pair<ArrayRef<uint8_t>, uint64_t>
InputSectionBase<ELFT>::getRawCompressedData(ArrayRef<uint8_t> Data) {
  // Compressed sections without Elf_Chdr header contain this header
  // instead. This is a GNU extension.
  struct ZlibHeader {
    char Magic[4]; // Should be "ZLIB"
    char Size[8];  // Uncompressed size in big-endian
  };

  if (Data.size() < sizeof(ZlibHeader))
    fatal(getName(this) + ": corrupted compressed section");
  auto *Hdr = reinterpret_cast<const ZlibHeader *>(Data.data());
  if (memcmp(Hdr->Magic, "ZLIB", 4))
    fatal(getName(this) + ": broken ZLIB-compressed section");
  return {Data.slice(sizeof(*Hdr)), read64be(Hdr->Size)};
}

template <class ELFT> void InputSectionBase<ELFT>::uncompress() {
  if (!zlib::isAvailable())
    fatal(getName(this) +
          ": build lld with zlib to enable compressed sections support");

  // This section is compressed. Here we decompress it. Ideally, all
  // compressed sections have SHF_COMPRESSED bit and their contents
  // start with headers of Elf_Chdr type. However, sections whose
  // names start with ".zdebug_" don't have the bit and contains a raw
  // ZLIB-compressed data (which is a bad thing because section names
  // shouldn't be significant in ELF.) We need to be able to read both.
  ArrayRef<uint8_t> Buf; // Compressed data
  size_t Size;           // Uncompressed size
  if (Flags & SHF_COMPRESSED)
    std::tie(Buf, Size) = getElfCompressedData(Data);
  else
    std::tie(Buf, Size) = getRawCompressedData(Data);

  // Uncompress Buf.
  UncompressedData.reset(new uint8_t[Size]);
  if (zlib::uncompress(toStringRef(Buf), (char *)UncompressedData.get(),
                       Size) != zlib::StatusOK)
    fatal(getName(this) + ": error while uncompressing section");
  Data = ArrayRef<uint8_t>(UncompressedData.get(), Size);
}

template <class ELFT>
typename ELFT::uint
InputSectionBase<ELFT>::getOffset(const DefinedRegular<ELFT> &Sym) const {
  return getOffset(Sym.Value);
}

template <class ELFT>
InputSectionBase<ELFT> *InputSectionBase<ELFT>::getLinkOrderDep() const {
  if ((Flags & SHF_LINK_ORDER) && Link != 0)
    return getFile()->getSections()[Link];
  return nullptr;
}

template <class ELFT>
InputSection<ELFT>::InputSection(uintX_t Flags, uint32_t Type,
                                 uintX_t Addralign, ArrayRef<uint8_t> Data,
                                 StringRef Name)
    : InputSectionBase<ELFT>(nullptr, Flags, Type,
                             /*Entsize*/ 0, /*Link*/ 0, /*Info*/ 0, Addralign,
                             Data, Name, Base::Regular) {}

template <class ELFT>
InputSection<ELFT>::InputSection(elf::ObjectFile<ELFT> *F,
                                 const Elf_Shdr *Header, StringRef Name)
    : InputSectionBase<ELFT>(F, Header, Name, Base::Regular) {}

template <class ELFT>
bool InputSection<ELFT>::classof(const InputSectionData *S) {
  return S->kind() == Base::Regular;
}

template <class ELFT>
InputSectionBase<ELFT> *InputSection<ELFT>::getRelocatedSection() {
  assert(this->Type == SHT_RELA || this->Type == SHT_REL);
  ArrayRef<InputSectionBase<ELFT> *> Sections = this->File->getSections();
  return Sections[this->Info];
}

template <class ELFT> void InputSection<ELFT>::addThunk(const Thunk<ELFT> *T) {
  Thunks.push_back(T);
}

template <class ELFT> uint64_t InputSection<ELFT>::getThunkOff() const {
  return this->Data.size();
}

template <class ELFT> uint64_t InputSection<ELFT>::getThunksSize() const {
  uint64_t Total = 0;
  for (const Thunk<ELFT> *T : Thunks)
    Total += T->size();
  return Total;
}

// This is used for -r. We can't use memcpy to copy relocations because we need
// to update symbol table offset and section index for each relocation. So we
// copy relocations one by one.
template <class ELFT>
template <class RelTy>
void InputSection<ELFT>::copyRelocations(uint8_t *Buf, ArrayRef<RelTy> Rels) {
  InputSectionBase<ELFT> *RelocatedSection = getRelocatedSection();

  for (const RelTy &Rel : Rels) {
    uint32_t Type = Rel.getType(Config->Mips64EL);
    SymbolBody &Body = this->File->getRelocTargetSym(Rel);

    Elf_Rela *P = reinterpret_cast<Elf_Rela *>(Buf);
    Buf += sizeof(RelTy);

    if (Config->Rela)
      P->r_addend = getAddend<ELFT>(Rel);
    P->r_offset = RelocatedSection->getOffset(Rel.r_offset);
    P->setSymbolAndType(Body.DynsymIndex, Type, Config->Mips64EL);
  }
}

// Page(Expr) is the page address of the expression Expr, defined
// as (Expr & ~0xFFF). (This applies even if the machine page size
// supported by the platform has a different value.)
static uint64_t getAArch64Page(uint64_t Expr) {
  return Expr & (~static_cast<uint64_t>(0xFFF));
}

template <class ELFT>
static typename ELFT::uint getSymVA(uint32_t Type, typename ELFT::uint A,
                                    typename ELFT::uint P,
                                    const SymbolBody &Body, RelExpr Expr) {
  switch (Expr) {
  case R_HINT:
  case R_TLSDESC_CALL:
    llvm_unreachable("cannot relocate hint relocs");
  case R_TLSLD:
    return Out<ELFT>::Got->getTlsIndexOff() + A - Out<ELFT>::Got->getSize();
  case R_TLSLD_PC:
    return Out<ELFT>::Got->getTlsIndexVA() + A - P;
  case R_THUNK_ABS:
    return Body.getThunkVA<ELFT>() + A;
  case R_THUNK_PC:
  case R_THUNK_PLT_PC:
    return Body.getThunkVA<ELFT>() + A - P;
  case R_PPC_TOC:
    return getPPC64TocBase() + A;
  case R_TLSGD:
    return Out<ELFT>::Got->getGlobalDynOffset(Body) + A -
           Out<ELFT>::Got->getSize();
  case R_TLSGD_PC:
    return Out<ELFT>::Got->getGlobalDynAddr(Body) + A - P;
  case R_TLSDESC:
    return Out<ELFT>::Got->getGlobalDynAddr(Body) + A;
  case R_TLSDESC_PAGE:
    return getAArch64Page(Out<ELFT>::Got->getGlobalDynAddr(Body) + A) -
           getAArch64Page(P);
  case R_PLT:
    return Body.getPltVA<ELFT>() + A;
  case R_PLT_PC:
  case R_PPC_PLT_OPD:
    return Body.getPltVA<ELFT>() + A - P;
  case R_SIZE:
    return Body.getSize<ELFT>() + A;
  case R_GOTREL:
    return Body.getVA<ELFT>(A) - Out<ELFT>::Got->getVA();
  case R_GOTREL_FROM_END:
    return Body.getVA<ELFT>(A) - Out<ELFT>::Got->getVA() -
           Out<ELFT>::Got->getSize();
  case R_RELAX_TLS_GD_TO_IE_END:
  case R_GOT_FROM_END:
    return Body.getGotOffset<ELFT>() + A - Out<ELFT>::Got->getSize();
  case R_RELAX_TLS_GD_TO_IE_ABS:
  case R_GOT:
    return Body.getGotVA<ELFT>() + A;
  case R_RELAX_TLS_GD_TO_IE_PAGE_PC:
  case R_GOT_PAGE_PC:
    return getAArch64Page(Body.getGotVA<ELFT>() + A) - getAArch64Page(P);
  case R_RELAX_TLS_GD_TO_IE:
  case R_GOT_PC:
    return Body.getGotVA<ELFT>() + A - P;
  case R_GOTONLY_PC:
    return Out<ELFT>::Got->getVA() + A - P;
  case R_GOTONLY_PC_FROM_END:
    return Out<ELFT>::Got->getVA() + A - P + Out<ELFT>::Got->getSize();
  case R_RELAX_TLS_LD_TO_LE:
  case R_RELAX_TLS_IE_TO_LE:
  case R_RELAX_TLS_GD_TO_LE:
  case R_TLS:
    // A weak undefined TLS symbol resolves to the base of the TLS
    // block, i.e. gets a value of zero. If we pass --gc-sections to
    // lld and .tbss is not referenced, it gets reclaimed and we don't
    // create a TLS program header. Therefore, we resolve this
    // statically to zero.
    if (Body.isTls() && (Body.isLazy() || Body.isUndefined()) &&
        Body.symbol()->isWeak())
      return 0;
    if (Target->TcbSize)
      return Body.getVA<ELFT>(A) +
             alignTo(Target->TcbSize, Out<ELFT>::TlsPhdr->p_align);
    return Body.getVA<ELFT>(A) - Out<ELFT>::TlsPhdr->p_memsz;
  case R_RELAX_TLS_GD_TO_LE_NEG:
  case R_NEG_TLS:
    return Out<ELF32LE>::TlsPhdr->p_memsz - Body.getVA<ELFT>(A);
  case R_ABS:
  case R_RELAX_GOT_PC_NOPIC:
    return Body.getVA<ELFT>(A);
  case R_GOT_OFF:
    return Body.getGotOffset<ELFT>() + A;
  case R_MIPS_GOT_LOCAL_PAGE:
    // If relocation against MIPS local symbol requires GOT entry, this entry
    // should be initialized by 'page address'. This address is high 16-bits
    // of sum the symbol's value and the addend.
    return Out<ELFT>::Got->getMipsLocalPageOffset(Body.getVA<ELFT>(A));
  case R_MIPS_GOT_OFF:
  case R_MIPS_GOT_OFF32:
    // In case of MIPS if a GOT relocation has non-zero addend this addend
    // should be applied to the GOT entry content not to the GOT entry offset.
    // That is why we use separate expression type.
    return Out<ELFT>::Got->getMipsGotOffset(Body, A);
  case R_MIPS_TLSGD:
    return Out<ELFT>::Got->getGlobalDynOffset(Body) +
           Out<ELFT>::Got->getMipsTlsOffset() - MipsGPOffset;
  case R_MIPS_TLSLD:
    return Out<ELFT>::Got->getTlsIndexOff() +
           Out<ELFT>::Got->getMipsTlsOffset() - MipsGPOffset;
  case R_PPC_OPD: {
    uint64_t SymVA = Body.getVA<ELFT>(A);
    // If we have an undefined weak symbol, we might get here with a symbol
    // address of zero. That could overflow, but the code must be unreachable,
    // so don't bother doing anything at all.
    if (!SymVA)
      return 0;
    if (Out<ELF64BE>::Opd) {
      // If this is a local call, and we currently have the address of a
      // function-descriptor, get the underlying code address instead.
      uint64_t OpdStart = Out<ELF64BE>::Opd->getVA();
      uint64_t OpdEnd = OpdStart + Out<ELF64BE>::Opd->getSize();
      bool InOpd = OpdStart <= SymVA && SymVA < OpdEnd;
      if (InOpd)
        SymVA = read64be(&Out<ELF64BE>::OpdBuf[SymVA - OpdStart]);
    }
    return SymVA - P;
  }
  case R_PC:
  case R_RELAX_GOT_PC:
    return Body.getVA<ELFT>(A) - P;
  case R_PLT_PAGE_PC:
  case R_PAGE_PC:
    return getAArch64Page(Body.getVA<ELFT>(A)) - getAArch64Page(P);
  }
  llvm_unreachable("Invalid expression");
}

// This function applies relocations to sections without SHF_ALLOC bit.
// Such sections are never mapped to memory at runtime. Debug sections are
// an example. Relocations in non-alloc sections are much easier to
// handle than in allocated sections because it will never need complex
// treatement such as GOT or PLT (because at runtime no one refers them).
// So, we handle relocations for non-alloc sections directly in this
// function as a performance optimization.
template <class ELFT>
template <class RelTy>
void InputSection<ELFT>::relocateNonAlloc(uint8_t *Buf, ArrayRef<RelTy> Rels) {
  for (const RelTy &Rel : Rels) {
    uint32_t Type = Rel.getType(Config->Mips64EL);
    uintX_t Offset = this->getOffset(Rel.r_offset);
    uint8_t *BufLoc = Buf + Offset;
    uintX_t Addend = getAddend<ELFT>(Rel);
    if (!RelTy::IsRela)
      Addend += Target->getImplicitAddend(BufLoc, Type);

    SymbolBody &Sym = this->File->getRelocTargetSym(Rel);
    if (Target->getRelExpr(Type, Sym) != R_ABS) {
      error(getName(this) + " has non-ABS reloc");
      return;
    }

    uintX_t AddrLoc = this->OutSec->getVA() + Offset;
    uint64_t SymVA = SignExtend64<sizeof(uintX_t) * 8>(
        getSymVA<ELFT>(Type, Addend, AddrLoc, Sym, R_ABS));
    Target->relocateOne(BufLoc, Type, SymVA);
  }
}

template <class ELFT>
void InputSectionBase<ELFT>::relocate(uint8_t *Buf, uint8_t *BufEnd) {
  // scanReloc function in Writer.cpp constructs Relocations
  // vector only for SHF_ALLOC'ed sections. For other sections,
  // we handle relocations directly here.
  auto *IS = dyn_cast<InputSection<ELFT>>(this);
  if (IS && !(IS->Flags & SHF_ALLOC)) {
    for (const Elf_Shdr *RelSec : IS->RelocSections) {
      if (RelSec->sh_type == SHT_RELA)
        IS->relocateNonAlloc(Buf, IS->File->getObj().relas(RelSec));
      else
        IS->relocateNonAlloc(Buf, IS->File->getObj().rels(RelSec));
    }
    return;
  }

  const unsigned Bits = sizeof(uintX_t) * 8;
  for (const Relocation &Rel : Relocations) {
    uintX_t Offset = getOffset(Rel.Offset);
    uint8_t *BufLoc = Buf + Offset;
    uint32_t Type = Rel.Type;
    uintX_t A = Rel.Addend;

    uintX_t AddrLoc = OutSec->getVA() + Offset;
    RelExpr Expr = Rel.Expr;
    uint64_t SymVA =
        SignExtend64<Bits>(getSymVA<ELFT>(Type, A, AddrLoc, *Rel.Sym, Expr));

    switch (Expr) {
    case R_RELAX_GOT_PC:
    case R_RELAX_GOT_PC_NOPIC:
      Target->relaxGot(BufLoc, SymVA);
      break;
    case R_RELAX_TLS_IE_TO_LE:
      Target->relaxTlsIeToLe(BufLoc, Type, SymVA);
      break;
    case R_RELAX_TLS_LD_TO_LE:
      Target->relaxTlsLdToLe(BufLoc, Type, SymVA);
      break;
    case R_RELAX_TLS_GD_TO_LE:
    case R_RELAX_TLS_GD_TO_LE_NEG:
      Target->relaxTlsGdToLe(BufLoc, Type, SymVA);
      break;
    case R_RELAX_TLS_GD_TO_IE:
    case R_RELAX_TLS_GD_TO_IE_ABS:
    case R_RELAX_TLS_GD_TO_IE_PAGE_PC:
    case R_RELAX_TLS_GD_TO_IE_END:
      Target->relaxTlsGdToIe(BufLoc, Type, SymVA);
      break;
    case R_PPC_PLT_OPD:
      // Patch a nop (0x60000000) to a ld.
      if (BufLoc + 8 <= BufEnd && read32be(BufLoc + 4) == 0x60000000)
        write32be(BufLoc + 4, 0xe8410028); // ld %r2, 40(%r1)
    // fallthrough
    default:
      Target->relocateOne(BufLoc, Type, SymVA);
      break;
    }
  }
}

template <class ELFT> void InputSection<ELFT>::writeTo(uint8_t *Buf) {
  if (this->Type == SHT_NOBITS)
    return;

  // If -r is given, then an InputSection may be a relocation section.
  if (this->Type == SHT_RELA) {
    copyRelocations(Buf + OutSecOff, this->template getDataAs<Elf_Rela>());
    return;
  }
  if (this->Type == SHT_REL) {
    copyRelocations(Buf + OutSecOff, this->template getDataAs<Elf_Rel>());
    return;
  }

  // Copy section contents from source object file to output file.
  ArrayRef<uint8_t> Data = this->Data;
  memcpy(Buf + OutSecOff, Data.data(), Data.size());

  // Iterate over all relocation sections that apply to this section.
  uint8_t *BufEnd = Buf + OutSecOff + Data.size();
  this->relocate(Buf, BufEnd);

  // The section might have a data/code generated by the linker and need
  // to be written after the section. Usually these are thunks - small piece
  // of code used to jump between "incompatible" functions like PIC and non-PIC
  // or if the jump target too far and its address does not fit to the short
  // jump istruction.
  if (!Thunks.empty()) {
    Buf += OutSecOff + getThunkOff();
    for (const Thunk<ELFT> *T : Thunks) {
      T->writeTo(Buf);
      Buf += T->size();
    }
  }
}

template <class ELFT>
void InputSection<ELFT>::replace(InputSection<ELFT> *Other) {
  assert(Other->Alignment <= this->Alignment);
  Other->Repl = this->Repl;
  Other->Live = false;
}

template <class ELFT>
EhInputSection<ELFT>::EhInputSection(elf::ObjectFile<ELFT> *F,
                                     const Elf_Shdr *Header, StringRef Name)
    : InputSectionBase<ELFT>(F, Header, Name, InputSectionBase<ELFT>::EHFrame) {
  // Mark .eh_frame sections as live by default because there are
  // usually no relocations that point to .eh_frames. Otherwise,
  // the garbage collector would drop all .eh_frame sections.
  this->Live = true;
}

template <class ELFT>
bool EhInputSection<ELFT>::classof(const InputSectionData *S) {
  return S->kind() == InputSectionBase<ELFT>::EHFrame;
}

// Returns the index of the first relocation that points to a region between
// Begin and Begin+Size.
template <class IntTy, class RelTy>
static unsigned getReloc(IntTy Begin, IntTy Size, const ArrayRef<RelTy> &Rels,
                         unsigned &RelocI) {
  // Start search from RelocI for fast access. That works because the
  // relocations are sorted in .eh_frame.
  for (unsigned N = Rels.size(); RelocI < N; ++RelocI) {
    const RelTy &Rel = Rels[RelocI];
    if (Rel.r_offset < Begin)
      continue;

    if (Rel.r_offset < Begin + Size)
      return RelocI;
    return -1;
  }
  return -1;
}

// .eh_frame is a sequence of CIE or FDE records.
// This function splits an input section into records and returns them.
template <class ELFT> void EhInputSection<ELFT>::split() {
  // Early exit if already split.
  if (!this->Pieces.empty())
    return;

  if (RelocSection) {
    ELFFile<ELFT> &Obj = this->File->getObj();
    if (RelocSection->sh_type == SHT_RELA)
      split(Obj.relas(RelocSection));
    else
      split(Obj.rels(RelocSection));
    return;
  }
  split(makeArrayRef<typename ELFT::Rela>(nullptr, nullptr));
}

template <class ELFT>
template <class RelTy>
void EhInputSection<ELFT>::split(ArrayRef<RelTy> Rels) {
  ArrayRef<uint8_t> Data = this->Data;
  unsigned RelI = 0;
  for (size_t Off = 0, End = Data.size(); Off != End;) {
    size_t Size = readEhRecordSize<ELFT>(Data.slice(Off));
    this->Pieces.emplace_back(Off, Data.slice(Off, Size),
                              getReloc(Off, Size, Rels, RelI));
    // The empty record is the end marker.
    if (Size == 4)
      break;
    Off += Size;
  }
}

static size_t findNull(ArrayRef<uint8_t> A, size_t EntSize) {
  // Optimize the common case.
  StringRef S((const char *)A.data(), A.size());
  if (EntSize == 1)
    return S.find(0);

  for (unsigned I = 0, N = S.size(); I != N; I += EntSize) {
    const char *B = S.begin() + I;
    if (std::all_of(B, B + EntSize, [](char C) { return C == 0; }))
      return I;
  }
  return StringRef::npos;
}

// Split SHF_STRINGS section. Such section is a sequence of
// null-terminated strings.
template <class ELFT>
std::vector<SectionPiece>
MergeInputSection<ELFT>::splitStrings(ArrayRef<uint8_t> Data, size_t EntSize) {
  std::vector<SectionPiece> V;
  size_t Off = 0;
  bool IsAlloca = this->Flags & SHF_ALLOC;
  while (!Data.empty()) {
    size_t End = findNull(Data, EntSize);
    if (End == StringRef::npos)
      fatal(getName(this) + ": string is not null terminated");
    size_t Size = End + EntSize;
    V.emplace_back(Off, !IsAlloca);
    Hashes.push_back(hash_value(toStringRef(Data.slice(0, Size))));
    Data = Data.slice(Size);
    Off += Size;
  }
  return V;
}

template <class ELFT>
ArrayRef<uint8_t> MergeInputSection<ELFT>::getData(
    std::vector<SectionPiece>::const_iterator I) const {
  auto Next = I + 1;
  size_t End = Next == Pieces.end() ? this->Data.size() : Next->InputOff;
  return this->Data.slice(I->InputOff, End - I->InputOff);
}

// Split non-SHF_STRINGS section. Such section is a sequence of
// fixed size records.
template <class ELFT>
std::vector<SectionPiece>
MergeInputSection<ELFT>::splitNonStrings(ArrayRef<uint8_t> Data,
                                         size_t EntSize) {
  std::vector<SectionPiece> V;
  size_t Size = Data.size();
  assert((Size % EntSize) == 0);
  bool IsAlloca = this->Flags & SHF_ALLOC;
  for (unsigned I = 0, N = Size; I != N; I += EntSize) {
    Hashes.push_back(hash_value(toStringRef(Data.slice(I, EntSize))));
    V.emplace_back(I, !IsAlloca);
  }
  return V;
}

template <class ELFT>
MergeInputSection<ELFT>::MergeInputSection(elf::ObjectFile<ELFT> *F,
                                           const Elf_Shdr *Header,
                                           StringRef Name)
    : InputSectionBase<ELFT>(F, Header, Name, InputSectionBase<ELFT>::Merge) {}

template <class ELFT> void MergeInputSection<ELFT>::splitIntoPieces() {
  ArrayRef<uint8_t> Data = this->Data;
  uintX_t EntSize = this->Entsize;
  if (this->Flags & SHF_STRINGS)
    this->Pieces = splitStrings(Data, EntSize);
  else
    this->Pieces = splitNonStrings(Data, EntSize);

  if (Config->GcSections && (this->Flags & SHF_ALLOC))
    for (uintX_t Off : LiveOffsets)
      this->getSectionPiece(Off)->Live = true;
}

template <class ELFT>
bool MergeInputSection<ELFT>::classof(const InputSectionData *S) {
  return S->kind() == InputSectionBase<ELFT>::Merge;
}

// Do binary search to get a section piece at a given input offset.
template <class ELFT>
SectionPiece *MergeInputSection<ELFT>::getSectionPiece(uintX_t Offset) {
  auto *This = static_cast<const MergeInputSection<ELFT> *>(this);
  return const_cast<SectionPiece *>(This->getSectionPiece(Offset));
}

template <class It, class T, class Compare>
static It fastUpperBound(It First, It Last, const T &Value, Compare Comp) {
  size_t Size = std::distance(First, Last);
  assert(Size != 0);
  while (Size != 1) {
    size_t H = Size / 2;
    const It MI = First + H;
    Size -= H;
    First = Comp(Value, *MI) ? First : First + H;
  }
  return Comp(Value, *First) ? First : First + 1;
}

template <class ELFT>
const SectionPiece *
MergeInputSection<ELFT>::getSectionPiece(uintX_t Offset) const {
  uintX_t Size = this->Data.size();
  if (Offset >= Size)
    fatal(getName(this) + ": entry is past the end of the section");

  // Find the element this offset points to.
  auto I = fastUpperBound(
      Pieces.begin(), Pieces.end(), Offset,
      [](const uintX_t &A, const SectionPiece &B) { return A < B.InputOff; });
  --I;
  return &*I;
}

// Returns the offset in an output section for a given input offset.
// Because contents of a mergeable section is not contiguous in output,
// it is not just an addition to a base output offset.
template <class ELFT>
typename ELFT::uint MergeInputSection<ELFT>::getOffset(uintX_t Offset) const {
  auto It = OffsetMap.find(Offset);
  if (It != OffsetMap.end())
    return It->second;

  if (!this->Live)
    return 0;

  // If Offset is not at beginning of a section piece, it is not in the map.
  // In that case we need to search from the original section piece vector.
  const SectionPiece &Piece = *this->getSectionPiece(Offset);
  if (!Piece.Live)
    return 0;

  uintX_t Addend = Offset - Piece.InputOff;
  return Piece.OutputOff + Addend;
}

// Create a map from input offsets to output offsets for all section pieces.
// It is called after finalize().
template <class ELFT> void MergeInputSection<ELFT>::finalizePieces() {
  OffsetMap.reserve(this->Pieces.size());
  auto HashI = Hashes.begin();
  for (auto I = Pieces.begin(), E = Pieces.end(); I != E; ++I) {
    uint32_t Hash = *HashI;
    ++HashI;
    SectionPiece &Piece = *I;
    if (!Piece.Live)
      continue;
    if (Piece.OutputOff == -1) {
      // Offsets of tail-merged strings are computed lazily.
      auto *OutSec = static_cast<MergeOutputSection<ELFT> *>(this->OutSec);
      ArrayRef<uint8_t> D = this->getData(I);
      StringRef S((const char *)D.data(), D.size());
      CachedHashStringRef V(S, Hash);
      Piece.OutputOff = OutSec->getOffset(V);
    }
    OffsetMap[Piece.InputOff] = Piece.OutputOff;
  }
}

template <class ELFT>
MipsReginfoInputSection<ELFT>::MipsReginfoInputSection(elf::ObjectFile<ELFT> *F,
                                                       const Elf_Shdr *Hdr,
                                                       StringRef Name)
    : InputSectionBase<ELFT>(F, Hdr, Name,
                             InputSectionBase<ELFT>::MipsReginfo) {
  ArrayRef<uint8_t> Data = this->Data;
  // Initialize this->Reginfo.
  if (Data.size() != sizeof(Elf_Mips_RegInfo<ELFT>)) {
    error(getName(this) + ": invalid size of .reginfo section");
    return;
  }
  Reginfo = reinterpret_cast<const Elf_Mips_RegInfo<ELFT> *>(Data.data());
  if (Config->Relocatable && Reginfo->ri_gp_value)
    error(getName(this) + ": unsupported non-zero ri_gp_value");
}

template <class ELFT>
bool MipsReginfoInputSection<ELFT>::classof(const InputSectionData *S) {
  return S->kind() == InputSectionBase<ELFT>::MipsReginfo;
}

template <class ELFT>
MipsOptionsInputSection<ELFT>::MipsOptionsInputSection(elf::ObjectFile<ELFT> *F,
                                                       const Elf_Shdr *Hdr,
                                                       StringRef Name)
    : InputSectionBase<ELFT>(F, Hdr, Name,
                             InputSectionBase<ELFT>::MipsOptions) {
  // Find ODK_REGINFO option in the section's content.
  ArrayRef<uint8_t> D = this->Data;
  while (!D.empty()) {
    if (D.size() < sizeof(Elf_Mips_Options<ELFT>)) {
      error(getName(this) + ": invalid size of .MIPS.options section");
      break;
    }
    auto *O = reinterpret_cast<const Elf_Mips_Options<ELFT> *>(D.data());
    if (O->kind == ODK_REGINFO) {
      Reginfo = &O->getRegInfo();
      if (Config->Relocatable && Reginfo->ri_gp_value)
        error(getName(this) + ": unsupported non-zero ri_gp_value");
      break;
    }
    if (!O->size)
      fatal(getName(this) + ": zero option descriptor size");
    D = D.slice(O->size);
  }
}

template <class ELFT>
bool MipsOptionsInputSection<ELFT>::classof(const InputSectionData *S) {
  return S->kind() == InputSectionBase<ELFT>::MipsOptions;
}

template <class ELFT>
MipsAbiFlagsInputSection<ELFT>::MipsAbiFlagsInputSection(
    elf::ObjectFile<ELFT> *F, const Elf_Shdr *Hdr, StringRef Name)
    : InputSectionBase<ELFT>(F, Hdr, Name,
                             InputSectionBase<ELFT>::MipsAbiFlags) {
  // Initialize this->Flags.
  ArrayRef<uint8_t> Data = this->Data;
  if (Data.size() != sizeof(Elf_Mips_ABIFlags<ELFT>)) {
    error("invalid size of .MIPS.abiflags section");
    return;
  }
  Flags = reinterpret_cast<const Elf_Mips_ABIFlags<ELFT> *>(Data.data());
}

template <class ELFT>
bool MipsAbiFlagsInputSection<ELFT>::classof(const InputSectionData *S) {
  return S->kind() == InputSectionBase<ELFT>::MipsAbiFlags;
}

template <class ELFT>
InputSection<ELFT> InputSection<ELFT>::createCommonInputSection(
    std::vector<DefinedCommon *> Syms) {
  // Sort the common symbols by alignment as an heuristic to pack them better.
  std::stable_sort(Syms.begin(), Syms.end(),
                   [](const DefinedCommon *A, const DefinedCommon *B) {
                     return A->Alignment > B->Alignment;
                   });

  size_t Size = 0;
  uintX_t Alignment = 1;
  for (DefinedCommon *Sym : Syms) {
    Alignment = std::max<uintX_t>(Alignment, Sym->Alignment);
    Size = alignTo(Size, Sym->Alignment);

    // Compute symbol offset relative to beginning of input section.
    Sym->Offset = Size;
    Size += Sym->Size;
  }
  ArrayRef<uint8_t> Data = makeArrayRef<uint8_t>(nullptr, Size);
  InputSection Ret(SHF_ALLOC | SHF_WRITE, SHT_NOBITS, Alignment, Data, "");
  Ret.Live = true;
  return Ret;
}

template class elf::InputSectionBase<ELF32LE>;
template class elf::InputSectionBase<ELF32BE>;
template class elf::InputSectionBase<ELF64LE>;
template class elf::InputSectionBase<ELF64BE>;

template class elf::InputSection<ELF32LE>;
template class elf::InputSection<ELF32BE>;
template class elf::InputSection<ELF64LE>;
template class elf::InputSection<ELF64BE>;

template class elf::EhInputSection<ELF32LE>;
template class elf::EhInputSection<ELF32BE>;
template class elf::EhInputSection<ELF64LE>;
template class elf::EhInputSection<ELF64BE>;

template class elf::MergeInputSection<ELF32LE>;
template class elf::MergeInputSection<ELF32BE>;
template class elf::MergeInputSection<ELF64LE>;
template class elf::MergeInputSection<ELF64BE>;

template class elf::MipsReginfoInputSection<ELF32LE>;
template class elf::MipsReginfoInputSection<ELF32BE>;
template class elf::MipsReginfoInputSection<ELF64LE>;
template class elf::MipsReginfoInputSection<ELF64BE>;

template class elf::MipsOptionsInputSection<ELF32LE>;
template class elf::MipsOptionsInputSection<ELF32BE>;
template class elf::MipsOptionsInputSection<ELF64LE>;
template class elf::MipsOptionsInputSection<ELF64BE>;

template class elf::MipsAbiFlagsInputSection<ELF32LE>;
template class elf::MipsAbiFlagsInputSection<ELF32BE>;
template class elf::MipsAbiFlagsInputSection<ELF64LE>;
template class elf::MipsAbiFlagsInputSection<ELF64BE>;