lld/ELF/MarkLive.cpp

//===- MarkLive.cpp -------------------------------------------------------===//
//
//                             The LLVM Linker
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements --gc-sections, which is a feature to remove unused
// sections from output. Unused sections are sections that are not reachable
// from known GC-root symbols or sections. Naturally the feature is
// implemented as a mark-sweep garbage collector.
//
// Here's how it works. Each InputSectionBase has a "Live" bit. The bit is off
// by default. Starting with GC-root symbols or sections, markLive function
// defined in this file visits all reachable sections to set their Live
// bits. Writer will then ignore sections whose Live bits are off, so that
// such sections are removed from output.
//
//===----------------------------------------------------------------------===//

#include "InputSection.h"
#include "OutputSections.h"
#include "SymbolTable.h"
#include "Symbols.h"
#include "Writer.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/Object/ELF.h"
#include <functional>
#include <vector>

using namespace llvm;
using namespace llvm::ELF;
using namespace llvm::object;

using namespace lld;
using namespace lld::elf2;

// Calls Fn for each section that Sec refers to.
template <class ELFT>
static void forEachSuccessor(InputSection<ELFT> *Sec,
                             std::function<void(InputSectionBase<ELFT> *)> Fn) {
  typedef typename ELFFile<ELFT>::Elf_Rel Elf_Rel;
  typedef typename ELFFile<ELFT>::Elf_Rela Elf_Rela;
  typedef typename ELFFile<ELFT>::Elf_Shdr Elf_Shdr;

  ELFFile<ELFT> &Obj = Sec->getFile()->getObj();
  for (const Elf_Shdr *RelSec : Sec->RelocSections) {
    if (RelSec->sh_type == SHT_RELA) {
      for (const Elf_Rela &RI : Obj.relas(RelSec))
        if (InputSectionBase<ELFT> *Succ = Sec->getRelocTarget(RI))
          Fn(Succ);
    } else {
      for (const Elf_Rel &RI : Obj.rels(RelSec))
        if (InputSectionBase<ELFT> *Succ = Sec->getRelocTarget(RI))
          Fn(Succ);
    }
  }
}

// Sections listed below are special because they are used by the loader
// just by being in an ELF file. They should not be garbage-collected.
template <class ELFT> static bool isReserved(InputSectionBase<ELFT> *Sec) {
  switch (Sec->getSectionHdr()->sh_type) {
  case SHT_FINI_ARRAY:
  case SHT_INIT_ARRAY:
  case SHT_NOTE:
  case SHT_PREINIT_ARRAY:
    return true;
  default:
    StringRef S = Sec->getSectionName();
    return S.startswith(".ctors") || S.startswith(".dtors") ||
           S.startswith(".init") || S.startswith(".fini") ||
           S.startswith(".jcr");
  }
}

// This is the main function of the garbage collector.
// Starting from GC-root sections, this function visits all reachable
// sections to set their "Live" bits.
template <class ELFT> void lld::elf2::markLive(SymbolTable<ELFT> *Symtab) {
  SmallVector<InputSection<ELFT> *, 256> Q;

  auto Enqueue = [&](InputSectionBase<ELFT> *Sec) {
    if (!Sec || Sec->Live)
      return;
    Sec->Live = true;
    if (InputSection<ELFT> *S = dyn_cast<InputSection<ELFT>>(Sec))
      Q.push_back(S);
  };

  auto MarkSymbol = [&](SymbolBody *Sym) {
    if (Sym)
      if (auto *D = dyn_cast<DefinedRegular<ELFT>>(Sym->repl()))
        Enqueue(&D->Section);
  };

  // Add GC root symbols.
  MarkSymbol(Config->EntrySym);
  MarkSymbol(Symtab->find(Config->Init));
  MarkSymbol(Symtab->find(Config->Fini));
  for (StringRef S : Config->Undefined)
    MarkSymbol(Symtab->find(S));

  // Preserve externally-visible symbols if the symbols defined by this
  // file could override other ELF file's symbols at runtime.
  if (Config->Shared || Config->ExportDynamic) {
    for (const std::pair<StringRef, Symbol *> &P : Symtab->getSymbols()) {
      SymbolBody *B = P.second->Body;
      if (B->getVisibility() == STV_DEFAULT)
        MarkSymbol(B);
    }
  }

  // Preserve special sections.
  for (const std::unique_ptr<ObjectFile<ELFT>> &F : Symtab->getObjectFiles()) {
    for (InputSectionBase<ELFT> *Sec : F->getSections()) {
      if (!Sec || Sec == &InputSection<ELFT>::Discarded)
        continue;
      if (isReserved(Sec))
        Enqueue(Sec);
      else if (Sec->getSectionName() == ".eh_frame")
        // .eh_frame is special. It should be marked live so that we don't
        // drop it, but it should not keep any section alive.
        Sec->Live = true;
    }
  }

  // Mark all reachable sections.
  while (!Q.empty())
    forEachSuccessor<ELFT>(Q.pop_back_val(), Enqueue);
}

template void lld::elf2::markLive<ELF32LE>(SymbolTable<ELF32LE> *);
template void lld::elf2::markLive<ELF32BE>(SymbolTable<ELF32BE> *);
template void lld::elf2::markLive<ELF64LE>(SymbolTable<ELF64LE> *);
template void lld::elf2::markLive<ELF64BE>(SymbolTable<ELF64BE> *);