--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/lld/ELF/MarkLive.cpp	Thu Feb 13 15:10:13 2020 +0900
@@ -0,0 +1,401 @@
+//===- MarkLive.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 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 "MarkLive.h"
+#include "InputSection.h"
+#include "LinkerScript.h"
+#include "OutputSections.h"
+#include "SymbolTable.h"
+#include "Symbols.h"
+#include "SyntheticSections.h"
+#include "Target.h"
+#include "lld/Common/Memory.h"
+#include "lld/Common/Strings.h"
+#include "llvm/ADT/STLExtras.h"
+#include "llvm/Object/ELF.h"
+#include "llvm/Support/TimeProfiler.h"
+#include <functional>
+#include <vector>
+
+using namespace llvm;
+using namespace llvm::ELF;
+using namespace llvm::object;
+
+namespace endian = llvm::support::endian;
+
+namespace lld {
+namespace elf {
+namespace {
+template <class ELFT> class MarkLive {
+public:
+  MarkLive(unsigned partition) : partition(partition) {}
+
+  void run();
+  void moveToMain();
+
+private:
+  void enqueue(InputSectionBase *sec, uint64_t offset);
+  void markSymbol(Symbol *sym);
+  void mark();
+
+  template <class RelTy>
+  void resolveReloc(InputSectionBase &sec, RelTy &rel, bool isLSDA);
+
+  template <class RelTy>
+  void scanEhFrameSection(EhInputSection &eh, ArrayRef<RelTy> rels);
+
+  // The index of the partition that we are currently processing.
+  unsigned partition;
+
+  // A list of sections to visit.
+  SmallVector<InputSection *, 256> queue;
+
+  // There are normally few input sections whose names are valid C
+  // identifiers, so we just store a std::vector instead of a multimap.
+  DenseMap<StringRef, std::vector<InputSectionBase *>> cNamedSections;
+};
+} // namespace
+
+template <class ELFT>
+static uint64_t getAddend(InputSectionBase &sec,
+                          const typename ELFT::Rel &rel) {
+  return target->getImplicitAddend(sec.data().begin() + rel.r_offset,
+                                   rel.getType(config->isMips64EL));
+}
+
+template <class ELFT>
+static uint64_t getAddend(InputSectionBase &sec,
+                          const typename ELFT::Rela &rel) {
+  return rel.r_addend;
+}
+
+template <class ELFT>
+template <class RelTy>
+void MarkLive<ELFT>::resolveReloc(InputSectionBase &sec, RelTy &rel,
+                                  bool isLSDA) {
+  Symbol &sym = sec.getFile<ELFT>()->getRelocTargetSym(rel);
+
+  // If a symbol is referenced in a live section, it is used.
+  sym.used = true;
+
+  if (auto *d = dyn_cast<Defined>(&sym)) {
+    auto *relSec = dyn_cast_or_null<InputSectionBase>(d->section);
+    if (!relSec)
+      return;
+
+    uint64_t offset = d->value;
+    if (d->isSection())
+      offset += getAddend<ELFT>(sec, rel);
+
+    if (!isLSDA || !(relSec->flags & SHF_EXECINSTR))
+      enqueue(relSec, offset);
+    return;
+  }
+
+  if (auto *ss = dyn_cast<SharedSymbol>(&sym))
+    if (!ss->isWeak())
+      ss->getFile().isNeeded = true;
+
+  for (InputSectionBase *sec : cNamedSections.lookup(sym.getName()))
+    enqueue(sec, 0);
+}
+
+// The .eh_frame section is an unfortunate special case.
+// The section is divided in CIEs and FDEs and the relocations it can have are
+// * CIEs can refer to a personality function.
+// * FDEs can refer to a LSDA
+// * FDEs refer to the function they contain information about
+// The last kind of relocation cannot keep the referred section alive, or they
+// would keep everything alive in a common object file. In fact, each FDE is
+// alive if the section it refers to is alive.
+// To keep things simple, in here we just ignore the last relocation kind. The
+// other two keep the referred section alive.
+//
+// A possible improvement would be to fully process .eh_frame in the middle of
+// the gc pass. With that we would be able to also gc some sections holding
+// LSDAs and personality functions if we found that they were unused.
+template <class ELFT>
+template <class RelTy>
+void MarkLive<ELFT>::scanEhFrameSection(EhInputSection &eh,
+                                        ArrayRef<RelTy> rels) {
+  for (size_t i = 0, end = eh.pieces.size(); i < end; ++i) {
+    EhSectionPiece &piece = eh.pieces[i];
+    size_t firstRelI = piece.firstRelocation;
+    if (firstRelI == (unsigned)-1)
+      continue;
+
+    if (endian::read32<ELFT::TargetEndianness>(piece.data().data() + 4) == 0) {
+      // This is a CIE, we only need to worry about the first relocation. It is
+      // known to point to the personality function.
+      resolveReloc(eh, rels[firstRelI], false);
+      continue;
+    }
+
+    // This is a FDE. The relocations point to the described function or to
+    // a LSDA. We only need to keep the LSDA alive, so ignore anything that
+    // points to executable sections.
+    uint64_t pieceEnd = piece.inputOff + piece.size;
+    for (size_t j = firstRelI, end2 = rels.size(); j < end2; ++j)
+      if (rels[j].r_offset < pieceEnd)
+        resolveReloc(eh, rels[j], true);
+  }
+}
+
+// Some sections are used directly by the loader, so they should never be
+// garbage-collected. This function returns true if a given section is such
+// section.
+static bool isReserved(InputSectionBase *sec) {
+  switch (sec->type) {
+  case SHT_FINI_ARRAY:
+  case SHT_INIT_ARRAY:
+  case SHT_PREINIT_ARRAY:
+    return true;
+  case SHT_NOTE:
+    // SHT_NOTE sections in a group are subject to garbage collection.
+    return !sec->nextInSectionGroup;
+  default:
+    StringRef s = sec->name;
+    return s.startswith(".ctors") || s.startswith(".dtors") ||
+           s.startswith(".init") || s.startswith(".fini") ||
+           s.startswith(".jcr");
+  }
+}
+
+template <class ELFT>
+void MarkLive<ELFT>::enqueue(InputSectionBase *sec, uint64_t offset) {
+  // Skip over discarded sections. This in theory shouldn't happen, because
+  // the ELF spec doesn't allow a relocation to point to a deduplicated
+  // COMDAT section directly. Unfortunately this happens in practice (e.g.
+  // .eh_frame) so we need to add a check.
+  if (sec == &InputSection::discarded)
+    return;
+
+  // Usually, a whole section is marked as live or dead, but in mergeable
+  // (splittable) sections, each piece of data has independent liveness bit.
+  // So we explicitly tell it which offset is in use.
+  if (auto *ms = dyn_cast<MergeInputSection>(sec))
+    ms->getSectionPiece(offset)->live = true;
+
+  // Set Sec->Partition to the meet (i.e. the "minimum") of Partition and
+  // Sec->Partition in the following lattice: 1 < other < 0. If Sec->Partition
+  // doesn't change, we don't need to do anything.
+  if (sec->partition == 1 || sec->partition == partition)
+    return;
+  sec->partition = sec->partition ? 1 : partition;
+
+  // Add input section to the queue.
+  if (InputSection *s = dyn_cast<InputSection>(sec))
+    queue.push_back(s);
+}
+
+template <class ELFT> void MarkLive<ELFT>::markSymbol(Symbol *sym) {
+  if (auto *d = dyn_cast_or_null<Defined>(sym))
+    if (auto *isec = dyn_cast_or_null<InputSectionBase>(d->section))
+      enqueue(isec, d->value);
+}
+
+// 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 MarkLive<ELFT>::run() {
+  // Add GC root symbols.
+
+  // Preserve externally-visible symbols if the symbols defined by this
+  // file can interrupt other ELF file's symbols at runtime.
+  for (Symbol *sym : symtab->symbols())
+    if (sym->includeInDynsym() && sym->partition == partition)
+      markSymbol(sym);
+
+  // If this isn't the main partition, that's all that we need to preserve.
+  if (partition != 1) {
+    mark();
+    return;
+  }
+
+  markSymbol(symtab->find(config->entry));
+  markSymbol(symtab->find(config->init));
+  markSymbol(symtab->find(config->fini));
+  for (StringRef s : config->undefined)
+    markSymbol(symtab->find(s));
+  for (StringRef s : script->referencedSymbols)
+    markSymbol(symtab->find(s));
+
+  // Preserve special sections and those which are specified in linker
+  // script KEEP command.
+  for (InputSectionBase *sec : inputSections) {
+    // Mark .eh_frame sections as live because there are usually no relocations
+    // that point to .eh_frames. Otherwise, the garbage collector would drop
+    // all of them. We also want to preserve personality routines and LSDA
+    // referenced by .eh_frame sections, so we scan them for that here.
+    if (auto *eh = dyn_cast<EhInputSection>(sec)) {
+      eh->markLive();
+      if (!eh->numRelocations)
+        continue;
+
+      if (eh->areRelocsRela)
+        scanEhFrameSection(*eh, eh->template relas<ELFT>());
+      else
+        scanEhFrameSection(*eh, eh->template rels<ELFT>());
+    }
+
+    if (sec->flags & SHF_LINK_ORDER)
+      continue;
+
+    if (isReserved(sec) || script->shouldKeep(sec)) {
+      enqueue(sec, 0);
+    } else if (isValidCIdentifier(sec->name)) {
+      cNamedSections[saver.save("__start_" + sec->name)].push_back(sec);
+      cNamedSections[saver.save("__stop_" + sec->name)].push_back(sec);
+    }
+  }
+
+  mark();
+}
+
+template <class ELFT> void MarkLive<ELFT>::mark() {
+  // Mark all reachable sections.
+  while (!queue.empty()) {
+    InputSectionBase &sec = *queue.pop_back_val();
+
+    if (sec.areRelocsRela) {
+      for (const typename ELFT::Rela &rel : sec.template relas<ELFT>())
+        resolveReloc(sec, rel, false);
+    } else {
+      for (const typename ELFT::Rel &rel : sec.template rels<ELFT>())
+        resolveReloc(sec, rel, false);
+    }
+
+    for (InputSectionBase *isec : sec.dependentSections)
+      enqueue(isec, 0);
+
+    // Mark the next group member.
+    if (sec.nextInSectionGroup)
+      enqueue(sec.nextInSectionGroup, 0);
+  }
+}
+
+// Move the sections for some symbols to the main partition, specifically ifuncs
+// (because they can result in an IRELATIVE being added to the main partition's
+// GOT, which means that the ifunc must be available when the main partition is
+// loaded) and TLS symbols (because we only know how to correctly process TLS
+// relocations for the main partition).
+//
+// We also need to move sections whose names are C identifiers that are referred
+// to from __start_/__stop_ symbols because there will only be one set of
+// symbols for the whole program.
+template <class ELFT> void MarkLive<ELFT>::moveToMain() {
+  for (InputFile *file : objectFiles)
+    for (Symbol *s : file->getSymbols())
+      if (auto *d = dyn_cast<Defined>(s))
+        if ((d->type == STT_GNU_IFUNC || d->type == STT_TLS) && d->section &&
+            d->section->isLive())
+          markSymbol(s);
+
+  for (InputSectionBase *sec : inputSections) {
+    if (!sec->isLive() || !isValidCIdentifier(sec->name))
+      continue;
+    if (symtab->find(("__start_" + sec->name).str()) ||
+        symtab->find(("__stop_" + sec->name).str()))
+      enqueue(sec, 0);
+  }
+
+  mark();
+}
+
+// Before calling this function, Live bits are off for all
+// input sections. This function make some or all of them on
+// so that they are emitted to the output file.
+template <class ELFT> void markLive() {
+  llvm::TimeTraceScope timeScope("markLive");
+  // If -gc-sections is not given, no sections are removed.
+  if (!config->gcSections) {
+    for (InputSectionBase *sec : inputSections)
+      sec->markLive();
+
+    // If a DSO defines a symbol referenced in a regular object, it is needed.
+    for (Symbol *sym : symtab->symbols())
+      if (auto *s = dyn_cast<SharedSymbol>(sym))
+        if (s->isUsedInRegularObj && !s->isWeak())
+          s->getFile().isNeeded = true;
+    return;
+  }
+
+  // Otherwise, do mark-sweep GC.
+  //
+  // The -gc-sections option works only for SHF_ALLOC sections
+  // (sections that are memory-mapped at runtime). So we can
+  // unconditionally make non-SHF_ALLOC sections alive except
+  // SHF_LINK_ORDER and SHT_REL/SHT_RELA sections.
+  //
+  // Usually, non-SHF_ALLOC sections are not removed even if they are
+  // unreachable through relocations because reachability is not
+  // a good signal whether they are garbage or not (e.g. there is
+  // usually no section referring to a .comment section, but we
+  // want to keep it.).
+  //
+  // Note on SHF_LINK_ORDER: Such sections contain metadata and they
+  // have a reverse dependency on the InputSection they are linked with.
+  // We are able to garbage collect them.
+  //
+  // Note on SHF_REL{,A}: Such sections reach here only when -r
+  // or -emit-reloc were given. And they are subject of garbage
+  // collection because, if we remove a text section, we also
+  // remove its relocation section.
+  //
+  // Note on nextInSectionGroup: The ELF spec says that group sections are
+  // included or omitted as a unit. We take the interpretation that:
+  //
+  // - Group members (nextInSectionGroup != nullptr) are subject to garbage
+  //   collection.
+  // - Groups members are retained or discarded as a unit.
+  for (InputSectionBase *sec : inputSections) {
+    bool isAlloc = (sec->flags & SHF_ALLOC);
+    bool isLinkOrder = (sec->flags & SHF_LINK_ORDER);
+    bool isRel = (sec->type == SHT_REL || sec->type == SHT_RELA);
+
+    if (!isAlloc && !isLinkOrder && !isRel && !sec->nextInSectionGroup)
+      sec->markLive();
+  }
+
+  // Follow the graph to mark all live sections.
+  for (unsigned curPart = 1; curPart <= partitions.size(); ++curPart)
+    MarkLive<ELFT>(curPart).run();
+
+  // If we have multiple partitions, some sections need to live in the main
+  // partition even if they were allocated to a loadable partition. Move them
+  // there now.
+  if (partitions.size() != 1)
+    MarkLive<ELFT>(1).moveToMain();
+
+  // Report garbage-collected sections.
+  if (config->printGcSections)
+    for (InputSectionBase *sec : inputSections)
+      if (!sec->isLive())
+        message("removing unused section " + toString(sec));
+}
+
+template void markLive<ELF32LE>();
+template void markLive<ELF32BE>();
+template void markLive<ELF64LE>();
+template void markLive<ELF64BE>();
+
+} // namespace elf
+} // namespace lld