forked from OSchip/llvm-project
276 lines
9.1 KiB
C++
276 lines
9.1 KiB
C++
//===- ICF.cpp ------------------------------------------------------------===//
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//
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// The LLVM Linker
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// ICF is short for Identical Code Folding. That is a size optimization to
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// identify and merge two or more read-only sections (typically functions)
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// that happened to have the same contents. It usually reduces output size
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// by a few percent.
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//
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// On Windows, ICF is enabled by default.
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//
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// See ELF/ICF.cpp for the details about the algortihm.
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//
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//===----------------------------------------------------------------------===//
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#include "ICF.h"
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#include "Chunks.h"
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#include "Symbols.h"
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#include "lld/Common/ErrorHandler.h"
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#include "lld/Common/Timer.h"
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#include "llvm/ADT/Hashing.h"
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#include "llvm/Support/Debug.h"
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#include "llvm/Support/Parallel.h"
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#include "llvm/Support/raw_ostream.h"
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#include <algorithm>
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#include <atomic>
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#include <vector>
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using namespace llvm;
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namespace lld {
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namespace coff {
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static Timer ICFTimer("ICF", Timer::root());
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class ICF {
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public:
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void run(ArrayRef<Chunk *> V);
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private:
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void segregate(size_t Begin, size_t End, bool Constant);
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bool equalsConstant(const SectionChunk *A, const SectionChunk *B);
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bool equalsVariable(const SectionChunk *A, const SectionChunk *B);
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uint32_t getHash(SectionChunk *C);
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bool isEligible(SectionChunk *C);
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size_t findBoundary(size_t Begin, size_t End);
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void forEachClassRange(size_t Begin, size_t End,
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std::function<void(size_t, size_t)> Fn);
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void forEachClass(std::function<void(size_t, size_t)> Fn);
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std::vector<SectionChunk *> Chunks;
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int Cnt = 0;
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std::atomic<bool> Repeat = {false};
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};
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// Returns a hash value for S.
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uint32_t ICF::getHash(SectionChunk *C) {
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return hash_combine(C->getPermissions(), C->SectionName, C->NumRelocs,
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C->Alignment, uint32_t(C->Header->SizeOfRawData),
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C->Checksum, C->getContents());
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}
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// Returns true if section S is subject of ICF.
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//
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// Microsoft's documentation
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// (https://msdn.microsoft.com/en-us/library/bxwfs976.aspx; visited April
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// 2017) says that /opt:icf folds both functions and read-only data.
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// Despite that, the MSVC linker folds only functions. We found
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// a few instances of programs that are not safe for data merging.
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// Therefore, we merge only functions just like the MSVC tool. However, we merge
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// identical .xdata sections, because the address of unwind information is
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// insignificant to the user program and the Visual C++ linker does this.
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bool ICF::isEligible(SectionChunk *C) {
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// Non-comdat chunks, dead chunks, and writable chunks are not elegible.
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bool Writable = C->getPermissions() & llvm::COFF::IMAGE_SCN_MEM_WRITE;
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if (!C->isCOMDAT() || !C->isLive() || Writable)
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return false;
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// Code sections are eligible.
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if (C->getPermissions() & llvm::COFF::IMAGE_SCN_MEM_EXECUTE)
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return true;
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// .xdata unwind info sections are eligble.
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return C->getSectionName().split('$').first == ".xdata";
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}
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// Split an equivalence class into smaller classes.
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void ICF::segregate(size_t Begin, size_t End, bool Constant) {
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while (Begin < End) {
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// Divide [Begin, End) into two. Let Mid be the start index of the
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// second group.
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auto Bound = std::stable_partition(
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Chunks.begin() + Begin + 1, Chunks.begin() + End, [&](SectionChunk *S) {
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if (Constant)
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return equalsConstant(Chunks[Begin], S);
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return equalsVariable(Chunks[Begin], S);
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});
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size_t Mid = Bound - Chunks.begin();
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// Split [Begin, End) into [Begin, Mid) and [Mid, End). We use Mid as an
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// equivalence class ID because every group ends with a unique index.
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for (size_t I = Begin; I < Mid; ++I)
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Chunks[I]->Class[(Cnt + 1) % 2] = Mid;
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// If we created a group, we need to iterate the main loop again.
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if (Mid != End)
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Repeat = true;
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Begin = Mid;
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}
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}
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// Compare "non-moving" part of two sections, namely everything
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// except relocation targets.
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bool ICF::equalsConstant(const SectionChunk *A, const SectionChunk *B) {
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if (A->NumRelocs != B->NumRelocs)
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return false;
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// Compare relocations.
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auto Eq = [&](const coff_relocation &R1, const coff_relocation &R2) {
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if (R1.Type != R2.Type ||
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R1.VirtualAddress != R2.VirtualAddress) {
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return false;
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}
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Symbol *B1 = A->File->getSymbol(R1.SymbolTableIndex);
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Symbol *B2 = B->File->getSymbol(R2.SymbolTableIndex);
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if (B1 == B2)
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return true;
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if (auto *D1 = dyn_cast<DefinedRegular>(B1))
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if (auto *D2 = dyn_cast<DefinedRegular>(B2))
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return D1->getValue() == D2->getValue() &&
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D1->getChunk()->Class[Cnt % 2] == D2->getChunk()->Class[Cnt % 2];
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return false;
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};
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if (!std::equal(A->Relocs.begin(), A->Relocs.end(), B->Relocs.begin(), Eq))
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return false;
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// Compare section attributes and contents.
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return A->getPermissions() == B->getPermissions() &&
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A->SectionName == B->SectionName && A->Alignment == B->Alignment &&
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A->Header->SizeOfRawData == B->Header->SizeOfRawData &&
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A->Checksum == B->Checksum && A->getContents() == B->getContents();
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}
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// Compare "moving" part of two sections, namely relocation targets.
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bool ICF::equalsVariable(const SectionChunk *A, const SectionChunk *B) {
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// Compare relocations.
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auto Eq = [&](const coff_relocation &R1, const coff_relocation &R2) {
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Symbol *B1 = A->File->getSymbol(R1.SymbolTableIndex);
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Symbol *B2 = B->File->getSymbol(R2.SymbolTableIndex);
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if (B1 == B2)
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return true;
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if (auto *D1 = dyn_cast<DefinedRegular>(B1))
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if (auto *D2 = dyn_cast<DefinedRegular>(B2))
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return D1->getChunk()->Class[Cnt % 2] == D2->getChunk()->Class[Cnt % 2];
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return false;
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};
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return std::equal(A->Relocs.begin(), A->Relocs.end(), B->Relocs.begin(), Eq);
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}
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size_t ICF::findBoundary(size_t Begin, size_t End) {
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for (size_t I = Begin + 1; I < End; ++I)
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if (Chunks[Begin]->Class[Cnt % 2] != Chunks[I]->Class[Cnt % 2])
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return I;
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return End;
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}
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void ICF::forEachClassRange(size_t Begin, size_t End,
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std::function<void(size_t, size_t)> Fn) {
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if (Begin > 0)
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Begin = findBoundary(Begin - 1, End);
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while (Begin < End) {
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size_t Mid = findBoundary(Begin, Chunks.size());
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Fn(Begin, Mid);
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Begin = Mid;
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}
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}
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// Call Fn on each class group.
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void ICF::forEachClass(std::function<void(size_t, size_t)> Fn) {
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// If the number of sections are too small to use threading,
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// call Fn sequentially.
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if (Chunks.size() < 1024) {
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forEachClassRange(0, Chunks.size(), Fn);
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++Cnt;
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return;
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}
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// Split sections into 256 shards and call Fn in parallel.
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size_t NumShards = 256;
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size_t Step = Chunks.size() / NumShards;
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for_each_n(parallel::par, size_t(0), NumShards, [&](size_t I) {
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size_t End = (I == NumShards - 1) ? Chunks.size() : (I + 1) * Step;
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forEachClassRange(I * Step, End, Fn);
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});
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++Cnt;
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}
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// Merge identical COMDAT sections.
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// Two sections are considered the same if their section headers,
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// contents and relocations are all the same.
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void ICF::run(ArrayRef<Chunk *> Vec) {
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ScopedTimer T(ICFTimer);
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// Collect only mergeable sections and group by hash value.
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uint32_t NextId = 1;
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for (Chunk *C : Vec) {
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if (auto *SC = dyn_cast<SectionChunk>(C)) {
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if (isEligible(SC))
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Chunks.push_back(SC);
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else
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SC->Class[0] = NextId++;
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}
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}
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// Make sure that ICF doesn't merge sections that are being handled by string
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// tail merging.
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for (auto &P : MergeChunk::Instances)
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for (SectionChunk *SC : P.second->Sections)
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SC->Class[0] = NextId++;
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// Initially, we use hash values to partition sections.
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for_each(parallel::par, Chunks.begin(), Chunks.end(), [&](SectionChunk *SC) {
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// Set MSB to 1 to avoid collisions with non-hash classs.
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SC->Class[0] = getHash(SC) | (1 << 31);
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});
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// From now on, sections in Chunks are ordered so that sections in
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// the same group are consecutive in the vector.
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std::stable_sort(Chunks.begin(), Chunks.end(),
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[](SectionChunk *A, SectionChunk *B) {
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return A->Class[0] < B->Class[0];
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});
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// Compare static contents and assign unique IDs for each static content.
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forEachClass([&](size_t Begin, size_t End) { segregate(Begin, End, true); });
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// Split groups by comparing relocations until convergence is obtained.
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do {
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Repeat = false;
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forEachClass(
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[&](size_t Begin, size_t End) { segregate(Begin, End, false); });
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} while (Repeat);
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log("ICF needed " + Twine(Cnt) + " iterations");
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// Merge sections in the same classs.
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forEachClass([&](size_t Begin, size_t End) {
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if (End - Begin == 1)
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return;
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log("Selected " + Chunks[Begin]->getDebugName());
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for (size_t I = Begin + 1; I < End; ++I) {
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log(" Removed " + Chunks[I]->getDebugName());
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Chunks[Begin]->replace(Chunks[I]);
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}
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});
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}
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// Entry point to ICF.
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void doICF(ArrayRef<Chunk *> Chunks) { ICF().run(Chunks); }
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} // namespace coff
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} // namespace lld
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