ELF/ICF.cpp

//===- ICF.cpp ------------------------------------------------------------===//
//
//                             The LLVM Linker
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// 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
//    clases.
//
// 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 "SymbolTable.h"
#include "Threads.h"

#include "llvm/ADT/Hashing.h"
#include "llvm/Object/ELF.h"
#include "llvm/Support/ELF.h"
#include <algorithm>
#include <atomic>

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

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

private:
  void segregate(size_t Begin, size_t End, bool Constant);

  template <class RelTy>
  bool constantEq(ArrayRef<RelTy> RelsA, ArrayRef<RelTy> RelsB);

  template <class RelTy>
  bool variableEq(const InputSection<ELFT> *A, ArrayRef<RelTy> RelsA,
                  const InputSection<ELFT> *B, ArrayRef<RelTy> RelsB);

  bool equalsConstant(const InputSection<ELFT> *A, const InputSection<ELFT> *B);
  bool equalsVariable(const InputSection<ELFT> *A, const InputSection<ELFT> *B);

  size_t findBoundary(size_t Begin, size_t End);

  void forEachClassRange(size_t Begin, size_t End,
                         std::function<void(size_t, size_t)> Fn);

  void forEachClass(std::function<void(size_t, size_t)> Fn);

  std::vector<InputSection<ELFT> *> 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 a hash value for S. Note that the information about
// relocation targets is not included in the hash value.
template <class ELFT> static uint32_t getHash(InputSection<ELFT> *S) {
  return hash_combine(S->Flags, S->getSize(), S->NumRelocations);
}

// Returns true if section S is subject of ICF.
template <class ELFT> static bool isEligible(InputSection<ELFT> *S) {
  // .init and .fini contains instructions that must be executed to
  // initialize and finalize the process. They cannot and should not
  // be merged.
  return S->Live && (S->Flags & SHF_ALLOC) && !(S->Flags & SHF_WRITE) &&
         S->Name != ".init" && S->Name != ".fini";
}

// Split an equivalence class into smaller classes.
template <class ELFT>
void ICF<ELFT>::segregate(size_t Begin, size_t End, 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<ELFT> *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, End). We use Mid as an equivalence
    // class ID because every group ends with a unique index.
    for (size_t I = Begin; I < Mid; ++I)
      Sections[I]->Class[Next] = 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(ArrayRef<RelTy> RelsA, ArrayRef<RelTy> RelsB) {
  auto Eq = [](const RelTy &A, const RelTy &B) {
    return A.r_offset == B.r_offset &&
           A.getType(Config->Mips64EL) == B.getType(Config->Mips64EL) &&
           getAddend<ELFT>(A) == getAddend<ELFT>(B);
  };

  return RelsA.size() == RelsB.size() &&
         std::equal(RelsA.begin(), RelsA.end(), RelsB.begin(), Eq);
}

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

  if (A->AreRelocsRela)
    return constantEq(A->relas(), B->relas());
  return constantEq(A->rels(), B->rels());
}

// 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<ELFT> *A, ArrayRef<RelTy> RelsA,
                           const InputSection<ELFT> *B, ArrayRef<RelTy> RelsB) {
  auto Eq = [&](const RelTy &RA, const RelTy &RB) {
    // The two sections must be identical.
    SymbolBody &SA = A->getFile()->getRelocTargetSym(RA);
    SymbolBody &SB = B->getFile()->getRelocTargetSym(RB);
    if (&SA == &SB)
      return true;

    // Or, the two sections must be in the same equivalence class.
    auto *DA = dyn_cast<DefinedRegular<ELFT>>(&SA);
    auto *DB = dyn_cast<DefinedRegular<ELFT>>(&SB);
    if (!DA || !DB)
      return false;
    if (DA->Value != DB->Value)
      return false;

    auto *X = dyn_cast<InputSection<ELFT>>(DA->Section);
    auto *Y = dyn_cast<InputSection<ELFT>>(DB->Section);
    if (!X || !Y)
      return false;

    // Ineligible sections are in the special equivalence class 0.
    // They can never be the same in terms of the equivalence class.
    if (X->Class[Current] == 0)
      return false;

    return X->Class[Current] == Y->Class[Current];
  };

  return std::equal(RelsA.begin(), RelsA.end(), RelsB.begin(), Eq);
}

// Compare "moving" part of two InputSections, namely relocation targets.
template <class ELFT>
bool ICF<ELFT>::equalsVariable(const InputSection<ELFT> *A,
                               const InputSection<ELFT> *B) {
  if (A->AreRelocsRela)
    return variableEq(A, A->relas(), B, B->relas());
  return variableEq(A, A->rels(), B, B->rels());
}

template <class ELFT> size_t ICF<ELFT>::findBoundary(size_t Begin, size_t End) {
  uint32_t Class = Sections[Begin]->Class[Current];
  for (size_t I = Begin + 1; I < End; ++I)
    if (Class != Sections[I]->Class[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 that starts within [Begin, End).
// Note that a group must starts in that range but doesn't necessarily
// have to end before End.
template <class ELFT>
void ICF<ELFT>::forEachClassRange(size_t Begin, size_t End,
                                  std::function<void(size_t, size_t)> Fn) {
  if (Begin > 0)
    Begin = findBoundary(Begin - 1, End);

  while (Begin < End) {
    size_t Mid = findBoundary(Begin, Sections.size());
    Fn(Begin, Mid);
    Begin = Mid;
  }
}

// Call Fn on each equivalence class.
template <class ELFT>
void ICF<ELFT>::forEachClass(std::function<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 (!Config->Threads || Sections.size() < 1024) {
    forEachClassRange(0, Sections.size(), Fn);
    ++Cnt;
    return;
  }

  Current = Cnt % 2;
  Next = (Cnt + 1) % 2;

  // Split sections into 256 shards and call Fn in parallel.
  size_t NumShards = 256;
  size_t Step = Sections.size() / NumShards;
  forLoop(0, NumShards,
          [&](size_t I) { forEachClassRange(I * Step, (I + 1) * Step, Fn); });
  forEachClassRange(Step * NumShards, Sections.size(), Fn);
  ++Cnt;
}

// The main function of ICF.
template <class ELFT> void ICF<ELFT>::run() {
  // Collect sections to merge.
  for (InputSectionBase<ELFT> *Sec : Symtab<ELFT>::X->Sections)
    if (auto *S = dyn_cast<InputSection<ELFT>>(Sec))
      if (isEligible(S))
        Sections.push_back(S);

  // Initially, we use hash values to partition sections.
  for (InputSection<ELFT> *S : Sections)
    // Set MSB to 1 to avoid collisions with non-hash IDs.
    S->Class[0] = getHash(S) | (1 << 31);

  // From now on, sections in Sections vector are ordered so that sections
  // in the same equivalence class are consecutive in the vector.
  std::stable_sort(Sections.begin(), Sections.end(),
                   [](InputSection<ELFT> *A, InputSection<ELFT> *B) {
                     return A->Class[0] < B->Class[0];
                   });

  // Compare static contents and assign unique IDs for each static content.
  forEachClass([&](size_t Begin, size_t End) { segregate(Begin, End, true); });

  // Split groups by comparing relocations until convergence is obtained.
  do {
    Repeat = false;
    forEachClass(
        [&](size_t Begin, size_t End) { segregate(Begin, End, false); });
  } while (Repeat);

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

  // Merge sections by the equivalence class.
  forEachClass([&](size_t Begin, size_t End) {
    if (End - Begin == 1)
      return;

    log("selected " + Sections[Begin]->Name);
    for (size_t I = Begin + 1; I < End; ++I) {
      log("  removed " + Sections[I]->Name);
      Sections[Begin]->replace(Sections[I]);
    }
  });
}

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

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