lld/ELF/OutputSections.cpp - toolchain/llvm-project - Gitiles

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

 #include "OutputSections.h"
 #include "Config.h"
 #include "LinkerScript.h"
 #include "Memory.h"
 #include "Strings.h"
 #include "SymbolTable.h"
 #include "SyntheticSections.h"
 #include "Target.h"
 #include "Threads.h"
 #include "llvm/Support/Compression.h"
 #include "llvm/Support/Dwarf.h"
 #include "llvm/Support/MD5.h"
 #include "llvm/Support/MathExtras.h"
 #include "llvm/Support/SHA1.h"

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

 using namespace lld;
 using namespace lld::elf;

 uint8_t Out::First;
 OutputSection *Out::Opd;
 uint8_t *Out::OpdBuf;
 PhdrEntry *Out::TlsPhdr;
 OutputSection *Out::DebugInfo;
 OutputSection *Out::ElfHeader;
 OutputSection *Out::ProgramHeaders;
 OutputSection *Out::PreinitArray;
 OutputSection *Out::InitArray;
 OutputSection *Out::FiniArray;

 uint32_t OutputSection::getPhdrFlags() const {
   uint32_t Ret = PF_R;
   if (Flags & SHF_WRITE)
     Ret |= PF_W;
   if (Flags & SHF_EXECINSTR)
     Ret |= PF_X;
   return Ret;
 }

 template <class ELFT>
 void OutputSection::writeHeaderTo(typename ELFT::Shdr *Shdr) {
   Shdr->sh_entsize = Entsize;
   Shdr->sh_addralign = Alignment;
   Shdr->sh_type = Type;
   Shdr->sh_offset = Offset;
   Shdr->sh_flags = Flags;
   Shdr->sh_info = Info;
   Shdr->sh_link = Link;
   Shdr->sh_addr = Addr;
   Shdr->sh_size = Size;
   Shdr->sh_name = ShName;
 }

 OutputSection::OutputSection(StringRef Name, uint32_t Type, uint64_t Flags)
     : SectionBase(Output, Name, Flags, /*Entsize*/ 0, /*Alignment*/ 1, Type,
                   /*Info*/ 0,
                   /*Link*/ 0),
       SectionIndex(INT_MAX) {}

 static bool compareByFilePosition(InputSection *A, InputSection *B) {
   // Synthetic doesn't have link order dependecy, stable_sort will keep it last
   if (A->kind() == InputSectionBase::Synthetic ||
       B->kind() == InputSectionBase::Synthetic)
     return false;
   auto *LA = cast<InputSection>(A->getLinkOrderDep());
   auto *LB = cast<InputSection>(B->getLinkOrderDep());
   OutputSection *AOut = LA->OutSec;
   OutputSection *BOut = LB->OutSec;
   if (AOut != BOut)
     return AOut->SectionIndex < BOut->SectionIndex;
   return LA->OutSecOff < LB->OutSecOff;
 }

 // Compress section contents if this section contains debug info.
 template <class ELFT> void OutputSection::maybeCompress() {
   typedef typename ELFT::Chdr Elf_Chdr;

   // Compress only DWARF debug sections.
   if (!Config->CompressDebugSections || (Flags & SHF_ALLOC) ||
       !Name.startswith(".debug_"))
     return;

   // Create a section header.
   ZDebugHeader.resize(sizeof(Elf_Chdr));
   auto *Hdr = reinterpret_cast<Elf_Chdr *>(ZDebugHeader.data());
   Hdr->ch_type = ELFCOMPRESS_ZLIB;
   Hdr->ch_size = Size;
   Hdr->ch_addralign = Alignment;

   // Write section contents to a temporary buffer and compress it.
   std::vector<uint8_t> Buf(Size);
   writeTo<ELFT>(Buf.data());
   if (Error E = zlib::compress(toStringRef(Buf), CompressedData))
     fatal("compress failed: " + llvm::toString(std::move(E)));

   // Update section headers.
   Size = sizeof(Elf_Chdr) + CompressedData.size();
   Flags |= SHF_COMPRESSED;
 }

 template <class ELFT> void OutputSection::finalize() {
   if ((this->Flags & SHF_LINK_ORDER) && !this->Sections.empty()) {
     std::sort(Sections.begin(), Sections.end(), compareByFilePosition);
     assignOffsets();

     // We must preserve the link order dependency of sections with the
     // SHF_LINK_ORDER flag. The dependency is indicated by the sh_link field. We
     // need to translate the InputSection sh_link to the OutputSection sh_link,
     // all InputSections in the OutputSection have the same dependency.
     if (auto *D = this->Sections.front()->getLinkOrderDep())
       this->Link = D->OutSec->SectionIndex;
   }

   uint32_t Type = this->Type;
   if (!Config->CopyRelocs || (Type != SHT_RELA && Type != SHT_REL))
     return;

   InputSection *First = Sections[0];
   if (isa<SyntheticSection>(First))
     return;

   this->Link = In<ELFT>::SymTab->OutSec->SectionIndex;
   // sh_info for SHT_REL[A] sections should contain the section header index of
   // the section to which the relocation applies.
   InputSectionBase *S = First->getRelocatedSection();
   this->Info = S->OutSec->SectionIndex;
 }

 static uint64_t updateOffset(uint64_t Off, InputSection *S) {
   Off = alignTo(Off, S->Alignment);
   S->OutSecOff = Off;
   return Off + S->getSize();
 }

 void OutputSection::addSection(InputSection *S) {
   assert(S->Live);
   Sections.push_back(S);
   S->OutSec = this;
   this->updateAlignment(S->Alignment);

   // The actual offsets will be computed by assignAddresses. For now, use
   // crude approximation so that it is at least easy for other code to know the
   // section order. It is also used to calculate the output section size early
   // for compressed debug sections.
   this->Size = updateOffset(Size, S);

   // If this section contains a table of fixed-size entries, sh_entsize
   // holds the element size. Consequently, if this contains two or more
   // input sections, all of them must have the same sh_entsize. However,
   // you can put different types of input sections into one output
   // sectin by using linker scripts. I don't know what to do here.
   // Probably we sholuld handle that as an error. But for now we just
   // pick the largest sh_entsize.
   this->Entsize = std::max(this->Entsize, S->Entsize);
 }

 // This function is called after we sort input sections
 // and scan relocations to setup sections' offsets.
 void OutputSection::assignOffsets() {
   uint64_t Off = 0;
   for (InputSection *S : Sections)
     Off = updateOffset(Off, S);
   this->Size = Off;
 }

 void OutputSection::sort(std::function<int(InputSectionBase *S)> Order) {
   typedef std::pair<unsigned, InputSection *> Pair;
   auto Comp = [](const Pair &A, const Pair &B) { return A.first < B.first; };

   std::vector<Pair> V;
   for (InputSection *S : Sections)
     V.push_back({Order(S), S});
   std::stable_sort(V.begin(), V.end(), Comp);
   Sections.clear();
   for (Pair &P : V)
     Sections.push_back(P.second);
 }

 // Sorts input sections by section name suffixes, so that .foo.N comes
 // before .foo.M if N < M. Used to sort .{init,fini}_array.N sections.
 // We want to keep the original order if the priorities are the same
 // because the compiler keeps the original initialization order in a
 // translation unit and we need to respect that.
 // For more detail, read the section of the GCC's manual about init_priority.
 void OutputSection::sortInitFini() {
   // Sort sections by priority.
   sort([](InputSectionBase *S) { return getPriority(S->Name); });
 }

 // Returns true if S matches /Filename.?\.o$/.
 static bool isCrtBeginEnd(StringRef S, StringRef Filename) {
   if (!S.endswith(".o"))
     return false;
   S = S.drop_back(2);
   if (S.endswith(Filename))
     return true;
   return !S.empty() && S.drop_back().endswith(Filename);
 }

 static bool isCrtbegin(StringRef S) { return isCrtBeginEnd(S, "crtbegin"); }
 static bool isCrtend(StringRef S) { return isCrtBeginEnd(S, "crtend"); }

 // .ctors and .dtors are sorted by this priority from highest to lowest.
 //
 //  1. The section was contained in crtbegin (crtbegin contains
 //     some sentinel value in its .ctors and .dtors so that the runtime
 //     can find the beginning of the sections.)
 //
 //  2. The section has an optional priority value in the form of ".ctors.N"
 //     or ".dtors.N" where N is a number. Unlike .{init,fini}_array,
 //     they are compared as string rather than number.
 //
 //  3. The section is just ".ctors" or ".dtors".
 //
 //  4. The section was contained in crtend, which contains an end marker.
 //
 // In an ideal world, we don't need this function because .init_array and
 // .ctors are duplicate features (and .init_array is newer.) However, there
 // are too many real-world use cases of .ctors, so we had no choice to
 // support that with this rather ad-hoc semantics.
 static bool compCtors(const InputSection *A, const InputSection *B) {
   bool BeginA = isCrtbegin(A->File->getName());
   bool BeginB = isCrtbegin(B->File->getName());
   if (BeginA != BeginB)
     return BeginA;
   bool EndA = isCrtend(A->File->getName());
   bool EndB = isCrtend(B->File->getName());
   if (EndA != EndB)
     return EndB;
   StringRef X = A->Name;
   StringRef Y = B->Name;
   assert(X.startswith(".ctors") || X.startswith(".dtors"));
   assert(Y.startswith(".ctors") || Y.startswith(".dtors"));
   X = X.substr(6);
   Y = Y.substr(6);
   if (X.empty() && Y.empty())
     return false;
   return X < Y;
 }

 // Sorts input sections by the special rules for .ctors and .dtors.
 // Unfortunately, the rules are different from the one for .{init,fini}_array.
 // Read the comment above.
 void OutputSection::sortCtorsDtors() {
   std::stable_sort(Sections.begin(), Sections.end(), compCtors);
 }

 // Fill [Buf, Buf + Size) with Filler.
 // This is used for linker script "=fillexp" command.
 static void fill(uint8_t *Buf, size_t Size, uint32_t Filler) {
   size_t I = 0;
   for (; I + 4 < Size; I += 4)
     memcpy(Buf + I, &Filler, 4);
   memcpy(Buf + I, &Filler, Size - I);
 }

 uint32_t OutputSection::getFiller() {
   // Determine what to fill gaps between InputSections with, as specified by the
   // linker script. If nothing is specified and this is an executable section,
   // fall back to trap instructions to prevent bad diassembly and detect invalid
   // jumps to padding.
   if (Optional<uint32_t> Filler = Script->getFiller(this))
     return *Filler;
   if (Flags & SHF_EXECINSTR)
     return Target->TrapInstr;
   return 0;
 }

 template <class ELFT> void OutputSection::writeTo(uint8_t *Buf) {
   Loc = Buf;

   // We may have already rendered compressed content when using
   // -compress-debug-sections option. Write it together with header.
   if (!CompressedData.empty()) {
     memcpy(Buf, ZDebugHeader.data(), ZDebugHeader.size());
     memcpy(Buf + ZDebugHeader.size(), CompressedData.data(),
            CompressedData.size());
     return;
   }

   // Write leading padding.
   uint32_t Filler = getFiller();
   if (Filler)
     fill(Buf, Sections.empty() ? Size : Sections[0]->OutSecOff, Filler);

   parallelForEachN(0, Sections.size(), [=](size_t I) {
     InputSection *Sec = Sections[I];
     Sec->writeTo<ELFT>(Buf);

     // Fill gaps between sections.
     if (Filler) {
       uint8_t *Start = Buf + Sec->OutSecOff + Sec->getSize();
       uint8_t *End;
       if (I + 1 == Sections.size())
         End = Buf + Size;
       else
         End = Buf + Sections[I + 1]->OutSecOff;
       fill(Start, End - Start, Filler);
     }
   });

   // Linker scripts may have BYTE()-family commands with which you
   // can write arbitrary bytes to the output. Process them if any.
   Script->writeDataBytes(this, Buf);
 }

 static uint64_t getOutFlags(InputSectionBase *S) {
   return S->Flags & ~SHF_GROUP & ~SHF_COMPRESSED;
 }

 static SectionKey createKey(InputSectionBase *C, StringRef OutsecName) {
   //  The ELF spec just says
   // ----------------------------------------------------------------
   // In the first phase, input sections that match in name, type and
   // attribute flags should be concatenated into single sections.
   // ----------------------------------------------------------------
   //
   // However, it is clear that at least some flags have to be ignored for
   // section merging. At the very least SHF_GROUP and SHF_COMPRESSED have to be
   // ignored. We should not have two output .text sections just because one was
   // in a group and another was not for example.
   //
   // It also seems that that wording was a late addition and didn't get the
   // necessary scrutiny.
   //
   // Merging sections with different flags is expected by some users. One
   // reason is that if one file has
   //
   // int *const bar __attribute__((section(".foo"))) = (int *)0;
   //
   // gcc with -fPIC will produce a read only .foo section. But if another
   // file has
   //
   // int zed;
   // int *const bar __attribute__((section(".foo"))) = (int *)&zed;
   //
   // gcc with -fPIC will produce a read write section.
   //
   // Last but not least, when using linker script the merge rules are forced by
   // the script. Unfortunately, linker scripts are name based. This means that
   // expressions like *(.foo*) can refer to multiple input sections with
   // different flags. We cannot put them in different output sections or we
   // would produce wrong results for
   //
   // start = .; *(.foo.*) end = .; *(.bar)
   //
   // and a mapping of .foo1 and .bar1 to one section and .foo2 and .bar2 to
   // another. The problem is that there is no way to layout those output
   // sections such that the .foo sections are the only thing between the start
   // and end symbols.
   //
   // Given the above issues, we instead merge sections by name and error on
   // incompatible types and flags.

   uint32_t Alignment = 0;
   uint64_t Flags = 0;
   if (Config->Relocatable && (C->Flags & SHF_MERGE)) {
     Alignment = std::max<uint64_t>(C->Alignment, C->Entsize);
     Flags = C->Flags & (SHF_MERGE | SHF_STRINGS);
   }

   return SectionKey{OutsecName, Flags, Alignment};
 }

 OutputSectionFactory::OutputSectionFactory(
     std::vector<OutputSection *> &OutputSections)
     : OutputSections(OutputSections) {}

 static uint64_t getIncompatibleFlags(uint64_t Flags) {
   return Flags & (SHF_ALLOC | SHF_TLS);
 }

 // We allow sections of types listed below to merged into a
 // single progbits section. This is typically done by linker
 // scripts. Merging nobits and progbits will force disk space
 // to be allocated for nobits sections. Other ones don't require
 // any special treatment on top of progbits, so there doesn't
 // seem to be a harm in merging them.
 static bool canMergeToProgbits(unsigned Type) {
   return Type == SHT_NOBITS || Type == SHT_PROGBITS || Type == SHT_INIT_ARRAY ||
          Type == SHT_PREINIT_ARRAY || Type == SHT_FINI_ARRAY ||
          Type == SHT_NOTE;
 }

 static void reportDiscarded(InputSectionBase *IS) {
   if (!Config->PrintGcSections)
     return;
   message("removing unused section from '" + IS->Name + "' in file '" +
           IS->File->getName());
 }

 void OutputSectionFactory::addInputSec(InputSectionBase *IS,
                                        StringRef OutsecName) {
   SectionKey Key = createKey(IS, OutsecName);
   OutputSection *&Sec = Map[Key];
   return addInputSec(IS, OutsecName, Sec);
 }

 void OutputSectionFactory::addInputSec(InputSectionBase *IS,
                                        StringRef OutsecName,
                                        OutputSection *&Sec) {
   if (!IS->Live) {
     reportDiscarded(IS);
     return;
   }

   uint64_t Flags = getOutFlags(IS);
   if (Sec) {
     if (getIncompatibleFlags(Sec->Flags) != getIncompatibleFlags(IS->Flags))
       error("incompatible section flags for " + Sec->Name +
             "\n>>> " + toString(IS) + ": 0x" + utohexstr(IS->Flags) +
             "\n>>> output section " + Sec->Name + ": 0x" +
             utohexstr(Sec->Flags));
     if (Sec->Type != IS->Type) {
       if (canMergeToProgbits(Sec->Type) && canMergeToProgbits(IS->Type))
         Sec->Type = SHT_PROGBITS;
       else
         error("section type mismatch for " + IS->Name +
               "\n>>> " + toString(IS) + ": " +
               getELFSectionTypeName(Config->EMachine, IS->Type) +
               "\n>>> output section " + Sec->Name + ": " +
               getELFSectionTypeName(Config->EMachine, Sec->Type));
     }
     Sec->Flags |= Flags;
   } else {
     Sec = make<OutputSection>(OutsecName, IS->Type, Flags);
     OutputSections.push_back(Sec);
   }

   Sec->addSection(cast<InputSection>(IS));
 }

 OutputSectionFactory::~OutputSectionFactory() {}

 SectionKey DenseMapInfo<SectionKey>::getEmptyKey() {
   return SectionKey{DenseMapInfo<StringRef>::getEmptyKey(), 0, 0};
 }

 SectionKey DenseMapInfo<SectionKey>::getTombstoneKey() {
   return SectionKey{DenseMapInfo<StringRef>::getTombstoneKey(), 0, 0};
 }

 unsigned DenseMapInfo<SectionKey>::getHashValue(const SectionKey &Val) {
   return hash_combine(Val.Name, Val.Flags, Val.Alignment);
 }

 bool DenseMapInfo<SectionKey>::isEqual(const SectionKey &LHS,
                                        const SectionKey &RHS) {
   return DenseMapInfo<StringRef>::isEqual(LHS.Name, RHS.Name) &&
          LHS.Flags == RHS.Flags && LHS.Alignment == RHS.Alignment;
 }

 uint64_t elf::getHeaderSize() {
   if (Config->OFormatBinary)
     return 0;
   return Out::ElfHeader->Size + Out::ProgramHeaders->Size;
 }

 template void OutputSection::writeHeaderTo<ELF32LE>(ELF32LE::Shdr *Shdr);
 template void OutputSection::writeHeaderTo<ELF32BE>(ELF32BE::Shdr *Shdr);
 template void OutputSection::writeHeaderTo<ELF64LE>(ELF64LE::Shdr *Shdr);
 template void OutputSection::writeHeaderTo<ELF64BE>(ELF64BE::Shdr *Shdr);

 template void OutputSection::finalize<ELF32LE>();
 template void OutputSection::finalize<ELF32BE>();
 template void OutputSection::finalize<ELF64LE>();
 template void OutputSection::finalize<ELF64BE>();

 template void OutputSection::maybeCompress<ELF32LE>();
 template void OutputSection::maybeCompress<ELF32BE>();
 template void OutputSection::maybeCompress<ELF64LE>();
 template void OutputSection::maybeCompress<ELF64BE>();

 template void OutputSection::writeTo<ELF32LE>(uint8_t *Buf);
 template void OutputSection::writeTo<ELF32BE>(uint8_t *Buf);
 template void OutputSection::writeTo<ELF64LE>(uint8_t *Buf);
 template void OutputSection::writeTo<ELF64BE>(uint8_t *Buf);
	//===- OutputSections.cpp -------------------------------------------------===//
	//
	// The LLVM Linker
	//
	// This file is distributed under the University of Illinois Open Source
	// License. See LICENSE.TXT for details.
	//
	//===----------------------------------------------------------------------===//

	#include "OutputSections.h"
	#include "Config.h"
	#include "LinkerScript.h"
	#include "Memory.h"
	#include "Strings.h"
	#include "SymbolTable.h"
	#include "SyntheticSections.h"
	#include "Target.h"
	#include "Threads.h"
	#include "llvm/Support/Compression.h"
	#include "llvm/Support/Dwarf.h"
	#include "llvm/Support/MD5.h"
	#include "llvm/Support/MathExtras.h"
	#include "llvm/Support/SHA1.h"

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

	using namespace lld;
	using namespace lld::elf;

	uint8_t Out::First;
	OutputSection *Out::Opd;
	uint8_t *Out::OpdBuf;
	PhdrEntry *Out::TlsPhdr;
	OutputSection *Out::DebugInfo;
	OutputSection *Out::ElfHeader;
	OutputSection *Out::ProgramHeaders;
	OutputSection *Out::PreinitArray;
	OutputSection *Out::InitArray;
	OutputSection *Out::FiniArray;

	uint32_t OutputSection::getPhdrFlags() const {
	uint32_t Ret = PF_R;
	if (Flags & SHF_WRITE)
	Ret \|= PF_W;
	if (Flags & SHF_EXECINSTR)
	Ret \|= PF_X;
	return Ret;
	}

	template <class ELFT>
	void OutputSection::writeHeaderTo(typename ELFT::Shdr *Shdr) {
	Shdr->sh_entsize = Entsize;
	Shdr->sh_addralign = Alignment;
	Shdr->sh_type = Type;
	Shdr->sh_offset = Offset;
	Shdr->sh_flags = Flags;
	Shdr->sh_info = Info;
	Shdr->sh_link = Link;
	Shdr->sh_addr = Addr;
	Shdr->sh_size = Size;
	Shdr->sh_name = ShName;
	}

	OutputSection::OutputSection(StringRef Name, uint32_t Type, uint64_t Flags)
	: SectionBase(Output, Name, Flags, /Entsize/ 0, /Alignment/ 1, Type,
	/Info/ 0,
	/Link/ 0),
	SectionIndex(INT_MAX) {}

	static bool compareByFilePosition(InputSection A, InputSection B) {
	// Synthetic doesn't have link order dependecy, stable_sort will keep it last
	if (A->kind() == InputSectionBase::Synthetic \|\|
	B->kind() == InputSectionBase::Synthetic)
	return false;
	auto *LA = cast<InputSection>(A->getLinkOrderDep());
	auto *LB = cast<InputSection>(B->getLinkOrderDep());
	OutputSection *AOut = LA->OutSec;
	OutputSection *BOut = LB->OutSec;
	if (AOut != BOut)
	return AOut->SectionIndex < BOut->SectionIndex;
	return LA->OutSecOff < LB->OutSecOff;
	}

	// Compress section contents if this section contains debug info.
	template <class ELFT> void OutputSection::maybeCompress() {
	typedef typename ELFT::Chdr Elf_Chdr;

	// Compress only DWARF debug sections.
	if (!Config->CompressDebugSections \|\| (Flags & SHF_ALLOC) \|\|
	!Name.startswith(".debug_"))
	return;

	// Create a section header.
	ZDebugHeader.resize(sizeof(Elf_Chdr));
	auto Hdr = reinterpret_cast<Elf_Chdr >(ZDebugHeader.data());
	Hdr->ch_type = ELFCOMPRESS_ZLIB;
	Hdr->ch_size = Size;
	Hdr->ch_addralign = Alignment;

	// Write section contents to a temporary buffer and compress it.
	std::vector<uint8_t> Buf(Size);
	writeTo<ELFT>(Buf.data());
	if (Error E = zlib::compress(toStringRef(Buf), CompressedData))
	fatal("compress failed: " + llvm::toString(std::move(E)));

	// Update section headers.
	Size = sizeof(Elf_Chdr) + CompressedData.size();
	Flags \|= SHF_COMPRESSED;
	}

	template <class ELFT> void OutputSection::finalize() {
	if ((this->Flags & SHF_LINK_ORDER) && !this->Sections.empty()) {
	std::sort(Sections.begin(), Sections.end(), compareByFilePosition);
	assignOffsets();

	// We must preserve the link order dependency of sections with the
	// SHF_LINK_ORDER flag. The dependency is indicated by the sh_link field. We
	// need to translate the InputSection sh_link to the OutputSection sh_link,
	// all InputSections in the OutputSection have the same dependency.
	if (auto *D = this->Sections.front()->getLinkOrderDep())
	this->Link = D->OutSec->SectionIndex;
	}

	uint32_t Type = this->Type;
	if (!Config->CopyRelocs \|\| (Type != SHT_RELA && Type != SHT_REL))
	return;

	InputSection *First = Sections[0];
	if (isa<SyntheticSection>(First))
	return;

	this->Link = In<ELFT>::SymTab->OutSec->SectionIndex;
	// sh_info for SHT_REL[A] sections should contain the section header index of
	// the section to which the relocation applies.
	InputSectionBase *S = First->getRelocatedSection();
	this->Info = S->OutSec->SectionIndex;
	}

	static uint64_t updateOffset(uint64_t Off, InputSection *S) {
	Off = alignTo(Off, S->Alignment);
	S->OutSecOff = Off;
	return Off + S->getSize();
	}

	void OutputSection::addSection(InputSection *S) {
	assert(S->Live);
	Sections.push_back(S);
	S->OutSec = this;
	this->updateAlignment(S->Alignment);

	// The actual offsets will be computed by assignAddresses. For now, use
	// crude approximation so that it is at least easy for other code to know the
	// section order. It is also used to calculate the output section size early
	// for compressed debug sections.
	this->Size = updateOffset(Size, S);

	// If this section contains a table of fixed-size entries, sh_entsize
	// holds the element size. Consequently, if this contains two or more
	// input sections, all of them must have the same sh_entsize. However,
	// you can put different types of input sections into one output
	// sectin by using linker scripts. I don't know what to do here.
	// Probably we sholuld handle that as an error. But for now we just
	// pick the largest sh_entsize.
	this->Entsize = std::max(this->Entsize, S->Entsize);
	}

	// This function is called after we sort input sections
	// and scan relocations to setup sections' offsets.
	void OutputSection::assignOffsets() {
	uint64_t Off = 0;
	for (InputSection *S : Sections)
	Off = updateOffset(Off, S);
	this->Size = Off;
	}

	void OutputSection::sort(std::function<int(InputSectionBase *S)> Order) {
	typedef std::pair<unsigned, InputSection *> Pair;
	auto Comp = [](const Pair &A, const Pair &B) { return A.first < B.first; };

	std::vector<Pair> V;
	for (InputSection *S : Sections)
	V.push_back({Order(S), S});
	std::stable_sort(V.begin(), V.end(), Comp);
	Sections.clear();
	for (Pair &P : V)
	Sections.push_back(P.second);
	}

	// Sorts input sections by section name suffixes, so that .foo.N comes
	// before .foo.M if N < M. Used to sort .{init,fini}_array.N sections.
	// We want to keep the original order if the priorities are the same
	// because the compiler keeps the original initialization order in a
	// translation unit and we need to respect that.
	// For more detail, read the section of the GCC's manual about init_priority.
	void OutputSection::sortInitFini() {
	// Sort sections by priority.
	sort([](InputSectionBase *S) { return getPriority(S->Name); });
	}

	// Returns true if S matches /Filename.?\.o$/.
	static bool isCrtBeginEnd(StringRef S, StringRef Filename) {
	if (!S.endswith(".o"))
	return false;
	S = S.drop_back(2);
	if (S.endswith(Filename))
	return true;
	return !S.empty() && S.drop_back().endswith(Filename);
	}

	static bool isCrtbegin(StringRef S) { return isCrtBeginEnd(S, "crtbegin"); }
	static bool isCrtend(StringRef S) { return isCrtBeginEnd(S, "crtend"); }

	// .ctors and .dtors are sorted by this priority from highest to lowest.
	//
	// 1. The section was contained in crtbegin (crtbegin contains
	// some sentinel value in its .ctors and .dtors so that the runtime
	// can find the beginning of the sections.)
	//
	// 2. The section has an optional priority value in the form of ".ctors.N"
	// or ".dtors.N" where N is a number. Unlike .{init,fini}_array,
	// they are compared as string rather than number.
	//
	// 3. The section is just ".ctors" or ".dtors".
	//
	// 4. The section was contained in crtend, which contains an end marker.
	//
	// In an ideal world, we don't need this function because .init_array and
	// .ctors are duplicate features (and .init_array is newer.) However, there
	// are too many real-world use cases of .ctors, so we had no choice to
	// support that with this rather ad-hoc semantics.
	static bool compCtors(const InputSection A, const InputSection B) {
	bool BeginA = isCrtbegin(A->File->getName());
	bool BeginB = isCrtbegin(B->File->getName());
	if (BeginA != BeginB)
	return BeginA;
	bool EndA = isCrtend(A->File->getName());
	bool EndB = isCrtend(B->File->getName());
	if (EndA != EndB)
	return EndB;
	StringRef X = A->Name;
	StringRef Y = B->Name;
	assert(X.startswith(".ctors") \|\| X.startswith(".dtors"));
	assert(Y.startswith(".ctors") \|\| Y.startswith(".dtors"));
	X = X.substr(6);
	Y = Y.substr(6);
	if (X.empty() && Y.empty())
	return false;
	return X < Y;
	}

	// Sorts input sections by the special rules for .ctors and .dtors.
	// Unfortunately, the rules are different from the one for .{init,fini}_array.
	// Read the comment above.
	void OutputSection::sortCtorsDtors() {
	std::stable_sort(Sections.begin(), Sections.end(), compCtors);
	}

	// Fill [Buf, Buf + Size) with Filler.
	// This is used for linker script "=fillexp" command.
	static void fill(uint8_t *Buf, size_t Size, uint32_t Filler) {
	size_t I = 0;
	for (; I + 4 < Size; I += 4)
	memcpy(Buf + I, &Filler, 4);
	memcpy(Buf + I, &Filler, Size - I);
	}

	uint32_t OutputSection::getFiller() {
	// Determine what to fill gaps between InputSections with, as specified by the
	// linker script. If nothing is specified and this is an executable section,
	// fall back to trap instructions to prevent bad diassembly and detect invalid
	// jumps to padding.
	if (Optional<uint32_t> Filler = Script->getFiller(this))
	return *Filler;
	if (Flags & SHF_EXECINSTR)
	return Target->TrapInstr;
	return 0;
	}

	template <class ELFT> void OutputSection::writeTo(uint8_t *Buf) {
	Loc = Buf;

	// We may have already rendered compressed content when using
	// -compress-debug-sections option. Write it together with header.
	if (!CompressedData.empty()) {
	memcpy(Buf, ZDebugHeader.data(), ZDebugHeader.size());
	memcpy(Buf + ZDebugHeader.size(), CompressedData.data(),
	CompressedData.size());
	return;
	}

	// Write leading padding.
	uint32_t Filler = getFiller();
	if (Filler)
	fill(Buf, Sections.empty() ? Size : Sections[0]->OutSecOff, Filler);

	parallelForEachN(0, Sections.size(), [=](size_t I) {
	InputSection *Sec = Sections[I];
	Sec->writeTo<ELFT>(Buf);

	// Fill gaps between sections.
	if (Filler) {
	uint8_t *Start = Buf + Sec->OutSecOff + Sec->getSize();
	uint8_t *End;
	if (I + 1 == Sections.size())
	End = Buf + Size;
	else
	End = Buf + Sections[I + 1]->OutSecOff;
	fill(Start, End - Start, Filler);
	}
	});

	// Linker scripts may have BYTE()-family commands with which you
	// can write arbitrary bytes to the output. Process them if any.
	Script->writeDataBytes(this, Buf);
	}

	static uint64_t getOutFlags(InputSectionBase *S) {
	return S->Flags & ~SHF_GROUP & ~SHF_COMPRESSED;
	}

	static SectionKey createKey(InputSectionBase *C, StringRef OutsecName) {
	// The ELF spec just says
	// ----------------------------------------------------------------
	// In the first phase, input sections that match in name, type and
	// attribute flags should be concatenated into single sections.
	// ----------------------------------------------------------------
	//
	// However, it is clear that at least some flags have to be ignored for
	// section merging. At the very least SHF_GROUP and SHF_COMPRESSED have to be
	// ignored. We should not have two output .text sections just because one was
	// in a group and another was not for example.
	//
	// It also seems that that wording was a late addition and didn't get the
	// necessary scrutiny.
	//
	// Merging sections with different flags is expected by some users. One
	// reason is that if one file has
	//
	// int const bar __attribute__((section(".foo"))) = (int )0;
	//
	// gcc with -fPIC will produce a read only .foo section. But if another
	// file has
	//
	// int zed;
	// int const bar __attribute__((section(".foo"))) = (int )&zed;
	//
	// gcc with -fPIC will produce a read write section.
	//
	// Last but not least, when using linker script the merge rules are forced by
	// the script. Unfortunately, linker scripts are name based. This means that
	// expressions like (.foo) can refer to multiple input sections with
	// different flags. We cannot put them in different output sections or we
	// would produce wrong results for
	//
	// start = .; (.foo.) end = .; *(.bar)
	//
	// and a mapping of .foo1 and .bar1 to one section and .foo2 and .bar2 to
	// another. The problem is that there is no way to layout those output
	// sections such that the .foo sections are the only thing between the start
	// and end symbols.
	//
	// Given the above issues, we instead merge sections by name and error on
	// incompatible types and flags.

	uint32_t Alignment = 0;
	uint64_t Flags = 0;
	if (Config->Relocatable && (C->Flags & SHF_MERGE)) {
	Alignment = std::max<uint64_t>(C->Alignment, C->Entsize);
	Flags = C->Flags & (SHF_MERGE \| SHF_STRINGS);
	}

	return SectionKey{OutsecName, Flags, Alignment};
	}

	OutputSectionFactory::OutputSectionFactory(
	std::vector<OutputSection *> &OutputSections)
	: OutputSections(OutputSections) {}

	static uint64_t getIncompatibleFlags(uint64_t Flags) {
	return Flags & (SHF_ALLOC \| SHF_TLS);
	}

	// We allow sections of types listed below to merged into a
	// single progbits section. This is typically done by linker
	// scripts. Merging nobits and progbits will force disk space
	// to be allocated for nobits sections. Other ones don't require
	// any special treatment on top of progbits, so there doesn't
	// seem to be a harm in merging them.
	static bool canMergeToProgbits(unsigned Type) {
	return Type == SHT_NOBITS \|\| Type == SHT_PROGBITS \|\| Type == SHT_INIT_ARRAY \|\|
	Type == SHT_PREINIT_ARRAY \|\| Type == SHT_FINI_ARRAY \|\|
	Type == SHT_NOTE;
	}

	static void reportDiscarded(InputSectionBase *IS) {
	if (!Config->PrintGcSections)
	return;
	message("removing unused section from '" + IS->Name + "' in file '" +
	IS->File->getName());
	}

	void OutputSectionFactory::addInputSec(InputSectionBase *IS,
	StringRef OutsecName) {
	SectionKey Key = createKey(IS, OutsecName);
	OutputSection *&Sec = Map[Key];
	return addInputSec(IS, OutsecName, Sec);
	}

	void OutputSectionFactory::addInputSec(InputSectionBase *IS,
	StringRef OutsecName,
	OutputSection *&Sec) {
	if (!IS->Live) {
	reportDiscarded(IS);
	return;
	}

	uint64_t Flags = getOutFlags(IS);
	if (Sec) {
	if (getIncompatibleFlags(Sec->Flags) != getIncompatibleFlags(IS->Flags))
	error("incompatible section flags for " + Sec->Name +
	"\n>>> " + toString(IS) + ": 0x" + utohexstr(IS->Flags) +
	"\n>>> output section " + Sec->Name + ": 0x" +
	utohexstr(Sec->Flags));
	if (Sec->Type != IS->Type) {
	if (canMergeToProgbits(Sec->Type) && canMergeToProgbits(IS->Type))
	Sec->Type = SHT_PROGBITS;
	else
	error("section type mismatch for " + IS->Name +
	"\n>>> " + toString(IS) + ": " +
	getELFSectionTypeName(Config->EMachine, IS->Type) +
	"\n>>> output section " + Sec->Name + ": " +
	getELFSectionTypeName(Config->EMachine, Sec->Type));
	}
	Sec->Flags \|= Flags;
	} else {
	Sec = make<OutputSection>(OutsecName, IS->Type, Flags);
	OutputSections.push_back(Sec);
	}

	Sec->addSection(cast<InputSection>(IS));
	}

	OutputSectionFactory::~OutputSectionFactory() {}

	SectionKey DenseMapInfo<SectionKey>::getEmptyKey() {
	return SectionKey{DenseMapInfo<StringRef>::getEmptyKey(), 0, 0};
	}

	SectionKey DenseMapInfo<SectionKey>::getTombstoneKey() {
	return SectionKey{DenseMapInfo<StringRef>::getTombstoneKey(), 0, 0};
	}

	unsigned DenseMapInfo<SectionKey>::getHashValue(const SectionKey &Val) {
	return hash_combine(Val.Name, Val.Flags, Val.Alignment);
	}

	bool DenseMapInfo<SectionKey>::isEqual(const SectionKey &LHS,
	const SectionKey &RHS) {
	return DenseMapInfo<StringRef>::isEqual(LHS.Name, RHS.Name) &&
	LHS.Flags == RHS.Flags && LHS.Alignment == RHS.Alignment;
	}

	uint64_t elf::getHeaderSize() {
	if (Config->OFormatBinary)
	return 0;
	return Out::ElfHeader->Size + Out::ProgramHeaders->Size;
	}

	template void OutputSection::writeHeaderTo<ELF32LE>(ELF32LE::Shdr *Shdr);
	template void OutputSection::writeHeaderTo<ELF32BE>(ELF32BE::Shdr *Shdr);
	template void OutputSection::writeHeaderTo<ELF64LE>(ELF64LE::Shdr *Shdr);
	template void OutputSection::writeHeaderTo<ELF64BE>(ELF64BE::Shdr *Shdr);

	template void OutputSection::finalize<ELF32LE>();
	template void OutputSection::finalize<ELF32BE>();
	template void OutputSection::finalize<ELF64LE>();
	template void OutputSection::finalize<ELF64BE>();

	template void OutputSection::maybeCompress<ELF32LE>();
	template void OutputSection::maybeCompress<ELF32BE>();
	template void OutputSection::maybeCompress<ELF64LE>();
	template void OutputSection::maybeCompress<ELF64BE>();

	template void OutputSection::writeTo<ELF32LE>(uint8_t *Buf);
	template void OutputSection::writeTo<ELF32BE>(uint8_t *Buf);
	template void OutputSection::writeTo<ELF64LE>(uint8_t *Buf);
	template void OutputSection::writeTo<ELF64BE>(uint8_t *Buf);