ELF/ICF.cpp

//===- ICF.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
//
//===----------------------------------------------------------------------===//
//
// ICF is short for Identical Code Folding. This is a size optimization to
// identify and merge two or more read-only sections (typically functions)
// that happened to have the same contents. It usually reduces output size
// by a few percent.
//
// In ICF, two sections are considered identical if they have the same
// section flags, section data, and relocations. Relocations are tricky,
// because two relocations are considered the same if they have the same
// relocation types, values, and if they point to the same sections *in
// terms of ICF*.
//
// Here is an example. If foo and bar defined below are compiled to the
// same machine instructions, ICF can and should merge the two, although
// their relocations point to each other.
//
//   void foo() { bar(); }
//   void bar() { foo(); }
//
// If you merge the two, their relocations point to the same section and
// thus you know they are mergeable, but how do you know they are
// mergeable in the first place? This is not an easy problem to solve.
//
// What we are doing in LLD is to partition sections into equivalence
// classes. Sections in the same equivalence class when the algorithm
// terminates are considered identical. Here are details:
//
// 1. First, we partition sections using their hash values as keys. Hash
//    values contain section types, section contents and numbers of
//    relocations. During this step, relocation targets are not taken into
//    account. We just put sections that apparently differ into different
//    equivalence classes.
//
// 2. Next, for each equivalence class, we visit sections to compare
//    relocation targets. Relocation targets are considered equivalent if
//    their targets are in the same equivalence class. Sections with
//    different relocation targets are put into different equivalence
//    classes.
//
// 3. If we split an equivalence class in step 2, two relocations
//    previously target the same equivalence class may now target
//    different equivalence classes. Therefore, we repeat step 2 until a
//    convergence is obtained.
//
// 4. For each equivalence class C, pick an arbitrary section in C, and
//    merge all the other sections in C with it.
//
// For small programs, this algorithm needs 3-5 iterations. For large
// programs such as Chromium, it takes more than 20 iterations.
//
// This algorithm was mentioned as an "optimistic algorithm" in [1],
// though gold implements a different algorithm than this.
//
// We parallelize each step so that multiple threads can work on different
// equivalence classes concurrently. That gave us a large performance
// boost when applying ICF on large programs. For example, MSVC link.exe
// or GNU gold takes 10-20 seconds to apply ICF on Chromium, whose output
// size is about 1.5 GB, but LLD can finish it in less than 2 seconds on a
// 2.8 GHz 40 core machine. Even without threading, LLD's ICF is still
// faster than MSVC or gold though.
//
// [1] Safe ICF: Pointer Safe and Unwinding aware Identical Code Folding
// in the Gold Linker
// http://static.googleusercontent.com/media/research.google.com/en//pubs/archive/36912.pdf
//
//===----------------------------------------------------------------------===//

#include "ICF.h"
#include "Config.h"
#include "EhFrame.h"
#include "LinkerScript.h"
#include "OutputSections.h"
#include "SymbolTable.h"
#include "Symbols.h"
#include "SyntheticSections.h"
#include "Writer.h"
#include "llvm/ADT/StringExtras.h"
#include "llvm/BinaryFormat/ELF.h"
#include "llvm/Object/ELF.h"
#include "llvm/Support/Parallel.h"
#include "llvm/Support/TimeProfiler.h"
#include "llvm/Support/xxhash.h"
#include <algorithm>
#include <atomic>

using namespace llvm;
using namespace llvm::ELF;
using namespace llvm::object;
using namespace lld;
using namespace lld::elf;

namespace {
template <class ELFT> class ICF {
public:
  void run();

private:
  void segregate(size_t begin, size_t end, uint32_t eqClassBase, bool constant);

  template <class RelTy>
  bool constantEq(const InputSection *a, ArrayRef<RelTy> relsA,
                  const InputSection *b, ArrayRef<RelTy> relsB);

  template <class RelTy>
  bool variableEq(const InputSection *a, ArrayRef<RelTy> relsA,
                  const InputSection *b, ArrayRef<RelTy> relsB);

  bool equalsConstant(const InputSection *a, const InputSection *b);
  bool equalsVariable(const InputSection *a, const InputSection *b);

  size_t findBoundary(size_t begin, size_t end);

  void forEachClassRange(size_t begin, size_t end,
                         llvm::function_ref<void(size_t, size_t)> fn);

  void forEachClass(llvm::function_ref<void(size_t, size_t)> fn);

  std::vector<InputSection *> sections;

  // We repeat the main loop while `Repeat` is true.
  std::atomic<bool> repeat;

  // The main loop counter.
  int cnt = 0;

  // We have two locations for equivalence classes. On the first iteration
  // of the main loop, Class[0] has a valid value, and Class[1] contains
  // garbage. We read equivalence classes from slot 0 and write to slot 1.
  // So, Class[0] represents the current class, and Class[1] represents
  // the next class. On each iteration, we switch their roles and use them
  // alternately.
  //
  // Why are we doing this? Recall that other threads may be working on
  // other equivalence classes in parallel. They may read sections that we
  // are updating. We cannot update equivalence classes in place because
  // it breaks the invariance that all possibly-identical sections must be
  // in the same equivalence class at any moment. In other words, the for
  // loop to update equivalence classes is not atomic, and that is
  // observable from other threads. By writing new classes to other
  // places, we can keep the invariance.
  //
  // Below, `Current` has the index of the current class, and `Next` has
  // the index of the next class. If threading is enabled, they are either
  // (0, 1) or (1, 0).
  //
  // Note on single-thread: if that's the case, they are always (0, 0)
  // because we can safely read the next class without worrying about race
  // conditions. Using the same location makes this algorithm converge
  // faster because it uses results of the same iteration earlier.
  int current = 0;
  int next = 0;
};
}

// Returns true if section S is subject of ICF.
static bool isEligible(InputSection *s) {
  if (!s->isLive() || s->keepUnique || !(s->flags & SHF_ALLOC))
    return false;

  // Don't merge writable sections. .data.rel.ro sections are marked as writable
  // but are semantically read-only.
  if ((s->flags & SHF_WRITE) && s->name != ".data.rel.ro" &&
      !s->name.startswith(".data.rel.ro."))
    return false;

  // SHF_LINK_ORDER sections are ICF'd as a unit with their dependent sections,
  // so we don't consider them for ICF individually.
  if (s->flags & SHF_LINK_ORDER)
    return false;

  // Don't merge synthetic sections as their Data member is not valid and empty.
  // The Data member needs to be valid for ICF as it is used by ICF to determine
  // the equality of section contents.
  if (isa<SyntheticSection>(s))
    return false;

  // .init and .fini contains instructions that must be executed to initialize
  // and finalize the process. They cannot and should not be merged.
  if (s->name == ".init" || s->name == ".fini")
    return false;

  // A user program may enumerate sections named with a C identifier using
  // __start_* and __stop_* symbols. We cannot ICF any such sections because
  // that could change program semantics.
  if (isValidCIdentifier(s->name))
    return false;

  return true;
}

// Split an equivalence class into smaller classes.
template <class ELFT>
void ICF<ELFT>::segregate(size_t begin, size_t end, uint32_t eqClassBase,
                          bool constant) {
  // This loop rearranges sections in [Begin, End) so that all sections
  // that are equal in terms of equals{Constant,Variable} are contiguous
  // in [Begin, End).
  //
  // The algorithm is quadratic in the worst case, but that is not an
  // issue in practice because the number of the distinct sections in
  // each range is usually very small.

  while (begin < end) {
    // Divide [Begin, End) into two. Let Mid be the start index of the
    // second group.
    auto bound =
        std::stable_partition(sections.begin() + begin + 1,
                              sections.begin() + end, [&](InputSection *s) {
                                if (constant)
                                  return equalsConstant(sections[begin], s);
                                return equalsVariable(sections[begin], s);
                              });
    size_t mid = bound - sections.begin();

    // Now we split [Begin, End) into [Begin, Mid) and [Mid, End) by
    // updating the sections in [Begin, Mid). We use Mid as the basis for
    // the equivalence class ID because every group ends with a unique index.
    // Add this to eqClassBase to avoid equality with unique IDs.
    for (size_t i = begin; i < mid; ++i)
      sections[i]->eqClass[next] = eqClassBase + mid;

    // If we created a group, we need to iterate the main loop again.
    if (mid != end)
      repeat = true;

    begin = mid;
  }
}

// Compare two lists of relocations.
template <class ELFT>
template <class RelTy>
bool ICF<ELFT>::constantEq(const InputSection *secA, ArrayRef<RelTy> ra,
                           const InputSection *secB, ArrayRef<RelTy> rb) {
  if (ra.size() != rb.size())
    return false;
  for (size_t i = 0; i < ra.size(); ++i) {
    if (ra[i].r_offset != rb[i].r_offset ||
        ra[i].getType(config->isMips64EL) != rb[i].getType(config->isMips64EL))
      return false;

    uint64_t addA = getAddend<ELFT>(ra[i]);
    uint64_t addB = getAddend<ELFT>(rb[i]);

    Symbol &sa = secA->template getFile<ELFT>()->getRelocTargetSym(ra[i]);
    Symbol &sb = secB->template getFile<ELFT>()->getRelocTargetSym(rb[i]);
    if (&sa == &sb) {
      if (addA == addB)
        continue;
      return false;
    }

    auto *da = dyn_cast<Defined>(&sa);
    auto *db = dyn_cast<Defined>(&sb);

    // Placeholder symbols generated by linker scripts look the same now but
    // may have different values later.
    if (!da || !db || da->scriptDefined || db->scriptDefined)
      return false;

    // When comparing a pair of relocations, if they refer to different symbols,
    // and either symbol is preemptible, the containing sections should be
    // considered different. This is because even if the sections are identical
    // in this DSO, they may not be after preemption.
    if (da->isPreemptible || db->isPreemptible)
      return false;

    // Relocations referring to absolute symbols are constant-equal if their
    // values are equal.
    if (!da->section && !db->section && da->value + addA == db->value + addB)
      continue;
    if (!da->section || !db->section)
      return false;

    if (da->section->kind() != db->section->kind())
      return false;

    // Relocations referring to InputSections are constant-equal if their
    // section offsets are equal.
    if (isa<InputSection>(da->section)) {
      if (da->value + addA == db->value + addB)
        continue;
      return false;
    }

    // Relocations referring to MergeInputSections are constant-equal if their
    // offsets in the output section are equal.
    auto *x = dyn_cast<MergeInputSection>(da->section);
    if (!x)
      return false;
    auto *y = cast<MergeInputSection>(db->section);
    if (x->getParent() != y->getParent())
      return false;

    uint64_t offsetA =
        sa.isSection() ? x->getOffset(addA) : x->getOffset(da->value) + addA;
    uint64_t offsetB =
        sb.isSection() ? y->getOffset(addB) : y->getOffset(db->value) + addB;
    if (offsetA != offsetB)
      return false;
  }

  return true;
}

// Compare "non-moving" part of two InputSections, namely everything
// except relocation targets.
template <class ELFT>
bool ICF<ELFT>::equalsConstant(const InputSection *a, const InputSection *b) {
  if (a->flags != b->flags || a->getSize() != b->getSize() ||
      a->data() != b->data())
    return false;

  // If two sections have different output sections, we cannot merge them.
  assert(a->getParent() && b->getParent());
  if (a->getParent() != b->getParent())
    return false;

  const RelsOrRelas<ELFT> ra = a->template relsOrRelas<ELFT>();
  const RelsOrRelas<ELFT> rb = b->template relsOrRelas<ELFT>();
  return ra.areRelocsRel() ? constantEq(a, ra.rels, b, rb.rels)
                           : constantEq(a, ra.relas, b, rb.relas);
}

// Compare two lists of relocations. Returns true if all pairs of
// relocations point to the same section in terms of ICF.
template <class ELFT>
template <class RelTy>
bool ICF<ELFT>::variableEq(const InputSection *secA, ArrayRef<RelTy> ra,
                           const InputSection *secB, ArrayRef<RelTy> rb) {
  assert(ra.size() == rb.size());

  for (size_t i = 0; i < ra.size(); ++i) {
    // The two sections must be identical.
    Symbol &sa = secA->template getFile<ELFT>()->getRelocTargetSym(ra[i]);
    Symbol &sb = secB->template getFile<ELFT>()->getRelocTargetSym(rb[i]);
    if (&sa == &sb)
      continue;

    auto *da = cast<Defined>(&sa);
    auto *db = cast<Defined>(&sb);

    // We already dealt with absolute and non-InputSection symbols in
    // constantEq, and for InputSections we have already checked everything
    // except the equivalence class.
    if (!da->section)
      continue;
    auto *x = dyn_cast<InputSection>(da->section);
    if (!x)
      continue;
    auto *y = cast<InputSection>(db->section);

    // Sections that are in the special equivalence class 0, can never be the
    // same in terms of the equivalence class.
    if (x->eqClass[current] == 0)
      return false;
    if (x->eqClass[current] != y->eqClass[current])
      return false;
  };

  return true;
}

// Compare "moving" part of two InputSections, namely relocation targets.
template <class ELFT>
bool ICF<ELFT>::equalsVariable(const InputSection *a, const InputSection *b) {
  const RelsOrRelas<ELFT> ra = a->template relsOrRelas<ELFT>();
  const RelsOrRelas<ELFT> rb = b->template relsOrRelas<ELFT>();
  return ra.areRelocsRel() ? variableEq(a, ra.rels, b, rb.rels)
                           : variableEq(a, ra.relas, b, rb.relas);
}

template <class ELFT> size_t ICF<ELFT>::findBoundary(size_t begin, size_t end) {
  uint32_t eqClass = sections[begin]->eqClass[current];
  for (size_t i = begin + 1; i < end; ++i)
    if (eqClass != sections[i]->eqClass[current])
      return i;
  return end;
}

// Sections in the same equivalence class are contiguous in Sections
// vector. Therefore, Sections vector can be considered as contiguous
// groups of sections, grouped by the class.
//
// This function calls Fn on every group within [Begin, End).
template <class ELFT>
void ICF<ELFT>::forEachClassRange(size_t begin, size_t end,
                                  llvm::function_ref<void(size_t, size_t)> fn) {
  while (begin < end) {
    size_t mid = findBoundary(begin, end);
    fn(begin, mid);
    begin = mid;
  }
}

// Call Fn on each equivalence class.
template <class ELFT>
void ICF<ELFT>::forEachClass(llvm::function_ref<void(size_t, size_t)> fn) {
  // If threading is disabled or the number of sections are
  // too small to use threading, call Fn sequentially.
  if (parallel::strategy.ThreadsRequested == 1 || sections.size() < 1024) {
    forEachClassRange(0, sections.size(), fn);
    ++cnt;
    return;
  }

  current = cnt % 2;
  next = (cnt + 1) % 2;

  // Shard into non-overlapping intervals, and call Fn in parallel.
  // The sharding must be completed before any calls to Fn are made
  // so that Fn can modify the Chunks in its shard without causing data
  // races.
  const size_t numShards = 256;
  size_t step = sections.size() / numShards;
  size_t boundaries[numShards + 1];
  boundaries[0] = 0;
  boundaries[numShards] = sections.size();

  parallelForEachN(1, numShards, [&](size_t i) {
    boundaries[i] = findBoundary((i - 1) * step, sections.size());
  });

  parallelForEachN(1, numShards + 1, [&](size_t i) {
    if (boundaries[i - 1] < boundaries[i])
      forEachClassRange(boundaries[i - 1], boundaries[i], fn);
  });
  ++cnt;
}

// Combine the hashes of the sections referenced by the given section into its
// hash.
template <class ELFT, class RelTy>
static void combineRelocHashes(unsigned cnt, InputSection *isec,
                               ArrayRef<RelTy> rels) {
  uint32_t hash = isec->eqClass[cnt % 2];
  for (RelTy rel : rels) {
    Symbol &s = isec->template getFile<ELFT>()->getRelocTargetSym(rel);
    if (auto *d = dyn_cast<Defined>(&s))
      if (auto *relSec = dyn_cast_or_null<InputSection>(d->section))
        hash += relSec->eqClass[cnt % 2];
  }
  // Set MSB to 1 to avoid collisions with unique IDs.
  isec->eqClass[(cnt + 1) % 2] = hash | (1U << 31);
}

static void print(const Twine &s) {
  if (config->printIcfSections)
    message(s);
}

// The main function of ICF.
template <class ELFT> void ICF<ELFT>::run() {
  // Compute isPreemptible early. We may add more symbols later, so this loop
  // cannot be merged with the later computeIsPreemptible() pass which is used
  // by scanRelocations().
  for (Symbol *sym : symtab->symbols())
    sym->isPreemptible = computeIsPreemptible(*sym);

  // Two text sections may have identical content and relocations but different
  // LSDA, e.g. the two functions may have catch blocks of different types. If a
  // text section is referenced by a .eh_frame FDE with LSDA, it is not
  // eligible. This is implemented by iterating over CIE/FDE and setting
  // eqClass[0] to the referenced text section from a live FDE.
  //
  // If two .gcc_except_table have identical semantics (usually identical
  // content with PC-relative encoding), we will lose folding opportunity.
  uint32_t uniqueId = 0;
  for (Partition &part : partitions)
    part.ehFrame->iterateFDEWithLSDA<ELFT>(
        [&](InputSection &s) { s.eqClass[0] = s.eqClass[1] = ++uniqueId; });

  // Collect sections to merge.
  for (InputSectionBase *sec : inputSections) {
    auto *s = cast<InputSection>(sec);
    if (s->eqClass[0] == 0) {
      if (isEligible(s))
        sections.push_back(s);
      else
        // Ineligible sections are assigned unique IDs, i.e. each section
        // belongs to an equivalence class of its own.
        s->eqClass[0] = s->eqClass[1] = ++uniqueId;
    }
  }

  // Initially, we use hash values to partition sections.
  parallelForEach(sections, [&](InputSection *s) {
    // Set MSB to 1 to avoid collisions with unique IDs.
    s->eqClass[0] = xxHash64(s->data()) | (1U << 31);
  });

  // Perform 2 rounds of relocation hash propagation. 2 is an empirical value to
  // reduce the average sizes of equivalence classes, i.e. segregate() which has
  // a large time complexity will have less work to do.
  for (unsigned cnt = 0; cnt != 2; ++cnt) {
    parallelForEach(sections, [&](InputSection *s) {
      const RelsOrRelas<ELFT> rels = s->template relsOrRelas<ELFT>();
      if (rels.areRelocsRel())
        combineRelocHashes<ELFT>(cnt, s, rels.rels);
      else
        combineRelocHashes<ELFT>(cnt, s, rels.relas);
    });
  }

  // From now on, sections in Sections vector are ordered so that sections
  // in the same equivalence class are consecutive in the vector.
  llvm::stable_sort(sections, [](const InputSection *a, const InputSection *b) {
    return a->eqClass[0] < b->eqClass[0];
  });

  // Compare static contents and assign unique equivalence class IDs for each
  // static content. Use a base offset for these IDs to ensure no overlap with
  // the unique IDs already assigned.
  uint32_t eqClassBase = ++uniqueId;
  forEachClass([&](size_t begin, size_t end) {
    segregate(begin, end, eqClassBase, true);
  });

  // Split groups by comparing relocations until convergence is obtained.
  do {
    repeat = false;
    forEachClass([&](size_t begin, size_t end) {
      segregate(begin, end, eqClassBase, false);
    });
  } while (repeat);

  log("ICF needed " + Twine(cnt) + " iterations");

  // Merge sections by the equivalence class.
  forEachClassRange(0, sections.size(), [&](size_t begin, size_t end) {
    if (end - begin == 1)
      return;
    print("selected section " + toString(sections[begin]));
    for (size_t i = begin + 1; i < end; ++i) {
      print("  removing identical section " + toString(sections[i]));
      sections[begin]->replace(sections[i]);

      // At this point we know sections merged are fully identical and hence
      // we want to remove duplicate implicit dependencies such as link order
      // and relocation sections.
      for (InputSection *isec : sections[i]->dependentSections)
        isec->markDead();
    }
  });

  // InputSectionDescription::sections is populated by processSectionCommands().
  // ICF may fold some input sections assigned to output sections. Remove them.
  for (BaseCommand *base : script->sectionCommands)
    if (auto *sec = dyn_cast<OutputSection>(base))
      for (BaseCommand *sub_base : sec->commands)
        if (auto *isd = dyn_cast<InputSectionDescription>(sub_base))
          llvm::erase_if(isd->sections,
                         [](InputSection *isec) { return !isec->isLive(); });
}

// ICF entry point function.
template <class ELFT> void elf::doIcf() {
  llvm::TimeTraceScope timeScope("ICF");
  ICF<ELFT>().run();
}

template void elf::doIcf<ELF32LE>();
template void elf::doIcf<ELF32BE>();
template void elf::doIcf<ELF64LE>();
template void elf::doIcf<ELF64BE>();