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//===- 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 not included into output.
//
//===----------------------------------------------------------------------===//
#include "InputSection.h"
#include "LinkerScript.h"
#include "OutputSections.h"
#include "SymbolTable.h"
#include "Symbols.h"
#include "Target.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::elf;
// A resolved relocation. The Sec and Offset fields are set if the relocation
// was resolved to an offset within a section.
template <class ELFT>
struct ResolvedReloc {
InputSectionBase<ELFT> *Sec;
typename ELFT::uint Offset;
};
template <class ELFT>
static typename ELFT::uint getAddend(InputSectionBase<ELFT> *Sec,
const typename ELFT::Rel &Rel) {
return Target->getImplicitAddend(Sec->getSectionData().begin(),
Rel.getType(Config->Mips64EL));
}
template <class ELFT>
static typename ELFT::uint getAddend(InputSectionBase<ELFT> *Sec,
const typename ELFT::Rela &Rel) {
return Rel.r_addend;
}
template <class ELFT, class RelT>
static ResolvedReloc<ELFT> resolveReloc(InputSectionBase<ELFT> *Sec,
RelT &Rel) {
SymbolBody &B = Sec->getFile()->getRelocTargetSym(Rel);
auto *D = dyn_cast<DefinedRegular<ELFT>>(&B);
if (!D || !D->Section)
return {nullptr, 0};
typename ELFT::uint Offset = D->Value;
if (D->isSection())
Offset += getAddend(Sec, Rel);
return {D->Section->Repl, Offset};
}
template <class ELFT, class Elf_Shdr>
static void run(ELFFile<ELFT> &Obj, InputSectionBase<ELFT> *Sec,
Elf_Shdr *RelSec, std::function<void(ResolvedReloc<ELFT>)> Fn) {
if (RelSec->sh_type == SHT_RELA) {
for (const typename ELFT::Rela &RI : Obj.relas(RelSec))
Fn(resolveReloc(Sec, RI));
} else {
for (const typename ELFT::Rel &RI : Obj.rels(RelSec))
Fn(resolveReloc(Sec, RI));
}
}
// Calls Fn for each section that Sec refers to via relocations.
template <class ELFT>
static void forEachSuccessor(InputSection<ELFT> *Sec,
std::function<void(ResolvedReloc<ELFT>)> Fn) {
ELFFile<ELFT> &Obj = Sec->getFile()->getObj();
for (const typename ELFT::Shdr *RelSec : Sec->RelocSections)
run(Obj, Sec, RelSec, Fn);
}
template <class ELFT> static void scanEhFrameSection(EHInputSection<ELFT> &EH) {
if (!EH.RelocSection)
return;
ELFFile<ELFT> &EObj = EH.getFile()->getObj();
run<ELFT>(EObj, &EH, EH.RelocSection, [&](ResolvedReloc<ELFT> R) {
if (!R.Sec || R.Sec == &InputSection<ELFT>::Discarded)
return;
if (R.Sec->getSectionHdr()->sh_flags & SHF_EXECINSTR)
return;
R.Sec->Live = true;
});
}
// 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();
// We do not want to reclaim sections if they can be referred
// by __start_* and __stop_* symbols.
if (isValidCIdentifier(S))
return true;
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 elf::markLive() {
typedef typename ELFT::uint uintX_t;
SmallVector<InputSection<ELFT> *, 256> Q;
auto Enqueue = [&](ResolvedReloc<ELFT> R) {
if (!R.Sec)
return;
if (auto *MS = dyn_cast<MergeInputSection<ELFT>>(R.Sec)) {
std::pair<std::pair<uintX_t, uintX_t> *, uintX_t> T =
MS->getRangeAndSize(R.Offset);
T.first->second = 0;
}
if (R.Sec->Live)
return;
R.Sec->Live = true;
if (InputSection<ELFT> *S = dyn_cast<InputSection<ELFT>>(R.Sec))
Q.push_back(S);
};
auto MarkSymbol = [&](const SymbolBody *Sym) {
if (auto *D = dyn_cast_or_null<DefinedRegular<ELFT>>(Sym))
Enqueue({D->Section, D->Value});
};
// Add GC root symbols.
if (Config->EntrySym)
MarkSymbol(Config->EntrySym->body());
MarkSymbol(Symtab<ELFT>::X->find(Config->Init));
MarkSymbol(Symtab<ELFT>::X->find(Config->Fini));
for (StringRef S : Config->Undefined)
MarkSymbol(Symtab<ELFT>::X->find(S));
// Preserve externally-visible symbols if the symbols defined by this
// file can interrupt other ELF file's symbols at runtime.
for (const Symbol *S : Symtab<ELFT>::X->getSymbols())
if (S->includeInDynsym())
MarkSymbol(S->body());
// Preserve special sections and those which are specified in linker
// script KEEP command.
for (const std::unique_ptr<ObjectFile<ELFT>> &F :
Symtab<ELFT>::X->getObjectFiles())
for (InputSectionBase<ELFT> *Sec : F->getSections())
if (Sec && Sec != &InputSection<ELFT>::Discarded) {
// .eh_frame is always marked as live now, but also it can reference to
// sections that contain personality. We preserve all non-text sections
// referred by .eh_frame here.
if (auto *EH = dyn_cast_or_null<EHInputSection<ELFT>>(Sec))
scanEhFrameSection<ELFT>(*EH);
if (isReserved(Sec) || Script<ELFT>::X->shouldKeep(Sec))
Enqueue({Sec, 0});
}
// Mark all reachable sections.
while (!Q.empty())
forEachSuccessor<ELFT>(Q.pop_back_val(), Enqueue);
}
template void elf::markLive<ELF32LE>();
template void elf::markLive<ELF32BE>();
template void elf::markLive<ELF64LE>();
template void elf::markLive<ELF64BE>();
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