llvm-project/lld/COFF/ICF.cpp

318 lines
11 KiB
C++

//===- 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. That 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.
//
// On Windows, ICF is enabled by default.
//
// See ELF/ICF.cpp for the details about the algortihm.
//
//===----------------------------------------------------------------------===//
#include "ICF.h"
#include "Chunks.h"
#include "Symbols.h"
#include "lld/Common/ErrorHandler.h"
#include "lld/Common/Threads.h"
#include "lld/Common/Timer.h"
#include "llvm/ADT/Hashing.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/Parallel.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Support/xxhash.h"
#include <algorithm>
#include <atomic>
#include <vector>
using namespace llvm;
namespace lld {
namespace coff {
static Timer ICFTimer("ICF", Timer::root());
class ICF {
public:
void run(ArrayRef<Chunk *> V);
private:
void segregate(size_t Begin, size_t End, bool Constant);
bool assocEquals(const SectionChunk *A, const SectionChunk *B);
bool equalsConstant(const SectionChunk *A, const SectionChunk *B);
bool equalsVariable(const SectionChunk *A, const SectionChunk *B);
bool isEligible(SectionChunk *C);
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<SectionChunk *> Chunks;
int Cnt = 0;
std::atomic<bool> Repeat = {false};
};
// Returns true if section S is subject of ICF.
//
// Microsoft's documentation
// (https://msdn.microsoft.com/en-us/library/bxwfs976.aspx; visited April
// 2017) says that /opt:icf folds both functions and read-only data.
// Despite that, the MSVC linker folds only functions. We found
// a few instances of programs that are not safe for data merging.
// Therefore, we merge only functions just like the MSVC tool. However, we also
// merge read-only sections in a couple of cases where the address of the
// section is insignificant to the user program and the behaviour matches that
// of the Visual C++ linker.
bool ICF::isEligible(SectionChunk *C) {
// Non-comdat chunks, dead chunks, and writable chunks are not elegible.
bool Writable = C->getOutputCharacteristics() & llvm::COFF::IMAGE_SCN_MEM_WRITE;
if (!C->isCOMDAT() || !C->Live || Writable)
return false;
// Code sections are eligible.
if (C->getOutputCharacteristics() & llvm::COFF::IMAGE_SCN_MEM_EXECUTE)
return true;
// .pdata and .xdata unwind info sections are eligible.
StringRef OutSecName = C->getSectionName().split('$').first;
if (OutSecName == ".pdata" || OutSecName == ".xdata")
return true;
// So are vtables.
if (C->Sym && C->Sym->getName().startswith("??_7"))
return true;
// Anything else not in an address-significance table is eligible.
return !C->KeepUnique;
}
// Split an equivalence class into smaller classes.
void ICF::segregate(size_t Begin, size_t End, bool Constant) {
while (Begin < End) {
// Divide [Begin, End) into two. Let Mid be the start index of the
// second group.
auto Bound = std::stable_partition(
Chunks.begin() + Begin + 1, Chunks.begin() + End, [&](SectionChunk *S) {
if (Constant)
return equalsConstant(Chunks[Begin], S);
return equalsVariable(Chunks[Begin], S);
});
size_t Mid = Bound - Chunks.begin();
// Split [Begin, End) into [Begin, Mid) and [Mid, End). We use Mid as an
// equivalence class ID because every group ends with a unique index.
for (size_t I = Begin; I < Mid; ++I)
Chunks[I]->Class[(Cnt + 1) % 2] = Mid;
// If we created a group, we need to iterate the main loop again.
if (Mid != End)
Repeat = true;
Begin = Mid;
}
}
// Returns true if two sections' associative children are equal.
bool ICF::assocEquals(const SectionChunk *A, const SectionChunk *B) {
auto ChildClasses = [&](const SectionChunk *SC) {
std::vector<uint32_t> Classes;
for (const SectionChunk &C : SC->children())
if (!C.getSectionName().startswith(".debug") &&
C.getSectionName() != ".gfids$y" && C.getSectionName() != ".gljmp$y")
Classes.push_back(C.Class[Cnt % 2]);
return Classes;
};
return ChildClasses(A) == ChildClasses(B);
}
// Compare "non-moving" part of two sections, namely everything
// except relocation targets.
bool ICF::equalsConstant(const SectionChunk *A, const SectionChunk *B) {
if (A->RelocsSize != B->RelocsSize)
return false;
// Compare relocations.
auto Eq = [&](const coff_relocation &R1, const coff_relocation &R2) {
if (R1.Type != R2.Type ||
R1.VirtualAddress != R2.VirtualAddress) {
return false;
}
Symbol *B1 = A->File->getSymbol(R1.SymbolTableIndex);
Symbol *B2 = B->File->getSymbol(R2.SymbolTableIndex);
if (B1 == B2)
return true;
if (auto *D1 = dyn_cast<DefinedRegular>(B1))
if (auto *D2 = dyn_cast<DefinedRegular>(B2))
return D1->getValue() == D2->getValue() &&
D1->getChunk()->Class[Cnt % 2] == D2->getChunk()->Class[Cnt % 2];
return false;
};
if (!std::equal(A->getRelocs().begin(), A->getRelocs().end(),
B->getRelocs().begin(), Eq))
return false;
// Compare section attributes and contents.
return A->getOutputCharacteristics() == B->getOutputCharacteristics() &&
A->getSectionName() == B->getSectionName() &&
A->Header->SizeOfRawData == B->Header->SizeOfRawData &&
A->Checksum == B->Checksum && A->getContents() == B->getContents() &&
assocEquals(A, B);
}
// Compare "moving" part of two sections, namely relocation targets.
bool ICF::equalsVariable(const SectionChunk *A, const SectionChunk *B) {
// Compare relocations.
auto Eq = [&](const coff_relocation &R1, const coff_relocation &R2) {
Symbol *B1 = A->File->getSymbol(R1.SymbolTableIndex);
Symbol *B2 = B->File->getSymbol(R2.SymbolTableIndex);
if (B1 == B2)
return true;
if (auto *D1 = dyn_cast<DefinedRegular>(B1))
if (auto *D2 = dyn_cast<DefinedRegular>(B2))
return D1->getChunk()->Class[Cnt % 2] == D2->getChunk()->Class[Cnt % 2];
return false;
};
return std::equal(A->getRelocs().begin(), A->getRelocs().end(),
B->getRelocs().begin(), Eq) &&
assocEquals(A, B);
}
// Find the first Chunk after Begin that has a different class from Begin.
size_t ICF::findBoundary(size_t Begin, size_t End) {
for (size_t I = Begin + 1; I < End; ++I)
if (Chunks[Begin]->Class[Cnt % 2] != Chunks[I]->Class[Cnt % 2])
return I;
return End;
}
void ICF::forEachClassRange(size_t Begin, size_t End,
std::function<void(size_t, size_t)> Fn) {
while (Begin < End) {
size_t Mid = findBoundary(Begin, End);
Fn(Begin, Mid);
Begin = Mid;
}
}
// Call Fn on each class group.
void ICF::forEachClass(std::function<void(size_t, size_t)> Fn) {
// If the number of sections are too small to use threading,
// call Fn sequentially.
if (Chunks.size() < 1024) {
forEachClassRange(0, Chunks.size(), Fn);
++Cnt;
return;
}
// 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 = Chunks.size() / NumShards;
size_t Boundaries[NumShards + 1];
Boundaries[0] = 0;
Boundaries[NumShards] = Chunks.size();
parallelForEachN(1, NumShards, [&](size_t I) {
Boundaries[I] = findBoundary((I - 1) * Step, Chunks.size());
});
parallelForEachN(1, NumShards + 1, [&](size_t I) {
if (Boundaries[I - 1] < Boundaries[I]) {
forEachClassRange(Boundaries[I - 1], Boundaries[I], Fn);
}
});
++Cnt;
}
// Merge identical COMDAT sections.
// Two sections are considered the same if their section headers,
// contents and relocations are all the same.
void ICF::run(ArrayRef<Chunk *> Vec) {
ScopedTimer T(ICFTimer);
// Collect only mergeable sections and group by hash value.
uint32_t NextId = 1;
for (Chunk *C : Vec) {
if (auto *SC = dyn_cast<SectionChunk>(C)) {
if (isEligible(SC))
Chunks.push_back(SC);
else
SC->Class[0] = NextId++;
}
}
// Make sure that ICF doesn't merge sections that are being handled by string
// tail merging.
for (MergeChunk *MC : MergeChunk::Instances)
if (MC)
for (SectionChunk *SC : MC->Sections)
SC->Class[0] = NextId++;
// Initially, we use hash values to partition sections.
parallelForEach(Chunks, [&](SectionChunk *SC) {
SC->Class[0] = xxHash64(SC->getContents());
});
// Combine the hashes of the sections referenced by each section into its
// hash.
for (unsigned Cnt = 0; Cnt != 2; ++Cnt) {
parallelForEach(Chunks, [&](SectionChunk *SC) {
uint32_t Hash = SC->Class[Cnt % 2];
for (Symbol *B : SC->symbols())
if (auto *Sym = dyn_cast_or_null<DefinedRegular>(B))
Hash += Sym->getChunk()->Class[Cnt % 2];
// Set MSB to 1 to avoid collisions with non-hash classs.
SC->Class[(Cnt + 1) % 2] = Hash | (1U << 31);
});
}
// From now on, sections in Chunks are ordered so that sections in
// the same group are consecutive in the vector.
llvm::stable_sort(Chunks, [](const SectionChunk *A, const SectionChunk *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 in the same classs.
forEachClass([&](size_t Begin, size_t End) {
if (End - Begin == 1)
return;
log("Selected " + Chunks[Begin]->getDebugName());
for (size_t I = Begin + 1; I < End; ++I) {
log(" Removed " + Chunks[I]->getDebugName());
Chunks[Begin]->replace(Chunks[I]);
}
});
}
// Entry point to ICF.
void doICF(ArrayRef<Chunk *> Chunks) { ICF().run(Chunks); }
} // namespace coff
} // namespace lld