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//===- 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 *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,
std::function<void(size_t, size_t)> Fn);
void forEachClass(std::function<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 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 *S) {
return hash_combine(S->Flags, S->getSize(), S->NumRelocations);
}
// Returns true if section S is subject of ICF.
static bool isEligible(InputSection *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_EXECINSTR) &&
!(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 *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 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->IsMips64EL) == B.getType(Config->IsMips64EL) &&
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 *A, const InputSection *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->template relas<ELFT>(), B->template relas<ELFT>());
return constantEq(A->template rels<ELFT>(), B->template rels<ELFT>());
}
// 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 *A, ArrayRef<RelTy> RelsA,
const InputSection *B, ArrayRef<RelTy> RelsB) {
auto Eq = [&](const RelTy &RA, const RelTy &RB) {
// The two sections must be identical.
SymbolBody &SA = A->template getFile<ELFT>()->getRelocTargetSym(RA);
SymbolBody &SB = B->template getFile<ELFT>()->getRelocTargetSym(RB);
if (&SA == &SB)
return true;
auto *DA = dyn_cast<DefinedRegular>(&SA);
auto *DB = dyn_cast<DefinedRegular>(&SB);
if (!DA || !DB)
return false;
if (DA->Value != DB->Value)
return false;
// Either both symbols must be absolute...
if (!DA->Section || !DB->Section)
return !DA->Section && !DB->Section;
// Or the two sections must be in the same equivalence class.
auto *X = dyn_cast<InputSection>(DA->Section);
auto *Y = dyn_cast<InputSection>(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 *A, const InputSection *B) {
if (A->AreRelocsRela)
return variableEq(A, A->template relas<ELFT>(), B,
B->template relas<ELFT>());
return variableEq(A, A->template rels<ELFT>(), B, B->template rels<ELFT>());
}
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 start 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;
parallelForEachN(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 *Sec : InputSections)
if (auto *S = dyn_cast<InputSection>(Sec))
if (isEligible(S))
Sections.push_back(S);
// Initially, we use hash values to partition sections.
for (InputSection *S : Sections)
// Set MSB to 1 to avoid collisions with non-hash IDs.
S->Class[0] = getHash<ELFT>(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 *A, InputSection *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]);
}
});
// Mark ARM Exception Index table sections that refer to folded code
// sections as not live. These sections have an implict dependency
// via the link order dependency.
if (Config->EMachine == EM_ARM)
for (InputSectionBase *Sec : InputSections)
if (auto *S = dyn_cast<InputSection>(Sec))
if (S->Flags & SHF_LINK_ORDER)
S->Live = S->getLinkOrderDep()->Live;
}
// 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>();