forked from OSchip/llvm-project
[BOLT] Support .plt.got section
Summary: We decode the regular .plt section and we are able to perform optimizations on it with -plt=hot or -plt=all, however, .plt.got sections are not decoded by BOLT. This patch teaches BOLT how to read them. They are created by the bfd linker whenever there is no need for the dynamic linker to lazy-bind the symbol (when they are eagerly resolved at binary load time). These entries are 8-byte sized instead of 16-byte sized like the regular PLT, and contain a single indirect call instruction with 7 bytes and a nop. (cherry picked from FBD17060515)
This commit is contained in:
Rafael Auler
Maksim Panchenko
parent
243507db99
commit
1f6564f117
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@ -1351,8 +1351,9 @@ void RewriteInstance::discoverFileObjects() {
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std::string UniqueName;
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std::string AlternativeName;
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if (Name.empty()) {
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if (PLTSection && PLTSection->getAddress() == Address) {
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// Don't register BOLT_PLT_PSEUDO twice.
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// Symbols that will be registered by disassemblePLT()
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if ((PLTSection && PLTSection->getAddress() == Address) ||
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(PLTGOTSection && PLTGOTSection->getAddress() == Address)) {
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continue;
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}
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UniqueName = "ANONYMOUS." + std::to_string(AnonymousId++);
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@ -1660,88 +1661,92 @@ void RewriteInstance::discoverFileObjects() {
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}
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void RewriteInstance::disassemblePLT() {
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if (!PLTSection)
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return;
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// Used to analyze both the .plt section (most common) and the less common
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// .plt.got created by the BFD linker.
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auto analyzeOnePLTSection = [&](BinarySection &Section,
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BinarySection *RelocsSection,
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uint64_t RelocType, uint64_t EntrySize) {
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const auto PLTAddress = Section.getAddress();
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StringRef PLTContents = Section.getContents();
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ArrayRef<uint8_t> PLTData(
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reinterpret_cast<const uint8_t *>(PLTContents.data()),
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Section.getSize());
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const auto PLTAddress = PLTSection->getAddress();
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StringRef PLTContents = PLTSection->getContents();
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ArrayRef<uint8_t> PLTData(
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reinterpret_cast<const uint8_t *>(PLTContents.data()),
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PLTSection->getSize());
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for (uint64_t Offset = 0; Offset < Section.getSize();
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Offset += EntrySize) {
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uint64_t InstrSize;
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MCInst Instruction;
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const uint64_t InstrAddr = PLTAddress + Offset;
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if (!BC->DisAsm->getInstruction(Instruction, InstrSize,
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PLTData.slice(Offset), InstrAddr, nulls(),
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nulls())) {
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errs() << "BOLT-ERROR: unable to disassemble instruction in PLT "
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"section "
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<< Section.getName() << " at offset 0x"
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<< Twine::utohexstr(Offset) << '\n';
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exit(1);
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}
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// Pseudo function for the start of PLT. The table could have a matching
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// FDE that we want to match to pseudo function.
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BC->createBinaryFunction("__BOLT_PLT_PSEUDO", *PLTSection, PLTAddress, 0,
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false, PLTSize, PLTAlignment);
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for (uint64_t Offset = 0; Offset < PLTSection->getSize(); Offset += PLTSize) {
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uint64_t InstrSize;
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MCInst Instruction;
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const uint64_t InstrAddr = PLTAddress + Offset;
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if (!BC->DisAsm->getInstruction(Instruction,
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InstrSize,
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PLTData.slice(Offset),
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InstrAddr,
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nulls(),
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nulls())) {
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errs() << "BOLT-ERROR: unable to disassemble instruction in .plt "
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<< "at offset 0x" << Twine::utohexstr(Offset) << '\n';
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exit(1);
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}
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if (!BC->MIB->isIndirectBranch(Instruction))
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continue;
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if (!BC->MIB->isIndirectBranch(Instruction))
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continue;
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uint64_t TargetAddress;
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if (!BC->MIB->evaluateMemOperandTarget(Instruction, TargetAddress,
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InstrAddr, InstrSize)) {
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errs() << "BOLT-ERROR: error evaluating PLT instruction at offset 0x"
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<< Twine::utohexstr(InstrAddr) << '\n';
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exit(1);
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}
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uint64_t TargetAddress;
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if (!BC->MIB->evaluateMemOperandTarget(Instruction,
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TargetAddress,
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InstrAddr,
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InstrSize)) {
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errs() << "BOLT-ERROR: error evaluating PLT instruction at offset 0x"
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<< Twine::utohexstr(InstrAddr) << '\n';
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exit(1);
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}
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// To get the name we have to read a relocation against the address.
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if (RelaPLTSection) {
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for (const auto &Rel : RelaPLTSection->getSectionRef().relocations()) {
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if (Rel.getType() != ELF::R_X86_64_JUMP_SLOT)
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continue;
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if (Rel.getOffset() == TargetAddress) {
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const auto SymbolIter = Rel.getSymbol();
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assert(SymbolIter != InputFile->symbol_end() &&
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"non-null symbol expected");
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const auto SymbolName = cantFail((*SymbolIter).getName());
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std::string Name = SymbolName.str() + "@PLT";
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const auto PtrSize = BC->AsmInfo->getCodePointerSize();
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auto *BF = BC->createBinaryFunction(Name,
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*PLTSection,
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InstrAddr,
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0,
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/*IsSimple=*/false,
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PLTSize,
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PLTAlignment);
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auto TargetSymbol =
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BC->registerNameAtAddress(SymbolName.str() + "@GOT",
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TargetAddress,
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PtrSize,
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PLTAlignment);
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BF->setPLTSymbol(TargetSymbol);
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break;
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// To get the name we have to read a relocation against the address.
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if (RelocsSection) {
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for (const auto &Rel : RelocsSection->getSectionRef().relocations()) {
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if (Rel.getType() != RelocType)
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continue;
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if (Rel.getOffset() == TargetAddress) {
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const auto SymbolIter = Rel.getSymbol();
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assert(SymbolIter != InputFile->symbol_end() &&
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"non-null symbol expected");
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const auto SymbolName = cantFail((*SymbolIter).getName());
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std::string Name = SymbolName.str() + "@PLT";
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const auto PtrSize = BC->AsmInfo->getCodePointerSize();
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auto *BF = BC->createBinaryFunction(Name, Section, InstrAddr, 0,
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/*IsSimple=*/false, EntrySize,
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PLTAlignment);
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auto TargetSymbol =
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BC->registerNameAtAddress(SymbolName.str() + "@GOT",
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TargetAddress, PtrSize, PLTAlignment);
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BF->setPLTSymbol(TargetSymbol);
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break;
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}
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}
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}
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}
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};
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if (PLTSection) {
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// Pseudo function for the start of PLT. The table could have a matching
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// FDE that we want to match to pseudo function.
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BC->createBinaryFunction("__BOLT_PLT_PSEUDO", *PLTSection,
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PLTSection->getAddress(), 0, false, PLTSize,
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PLTAlignment);
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analyzeOnePLTSection(*PLTSection,
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RelaPLTSection ? &*RelaPLTSection : nullptr,
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ELF::R_X86_64_JUMP_SLOT, PLTSize);
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}
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if (PLTGOTSection) {
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// Check if we need to create a function for .plt.got. Some linkers
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// (depending on the version) would mark it with FDE while others wouldn't.
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if (!BC->getBinaryFunctionAtAddress(PLTGOTSection->getAddress())) {
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BC->createBinaryFunction("__BOLT_PLT_GOT_PSEUDO",
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*PLTGOTSection,
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PLTGOTSection->getAddress(),
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0,
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false,
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PLTAlignment);
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analyzeOnePLTSection(*PLTGOTSection,
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RelaDynSection ? &*RelaDynSection : nullptr,
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ELF::R_X86_64_GLOB_DAT,
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/*Size=*/8);
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// If we did not register any function at PLTGOT start, we may be missing
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// relocs. Add a function at the start to mark this section.
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if (BC->getBinaryFunctions().find(PLTGOTSection->getAddress()) ==
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BC->getBinaryFunctions().end()) {
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BC->createBinaryFunction("__BOLT_PLTGOT_PSEUDO", *PLTGOTSection,
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PLTGOTSection->getAddress(), 0, false,
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/*Size*/ 8, PLTAlignment);
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}
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}
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}
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@ -1763,7 +1768,7 @@ void RewriteInstance::adjustFunctionBoundaries() {
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static bool PrintedWarning = false;
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if (BC->HasRelocations && !PrintedWarning) {
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errs() << "BOLT-WARNING: split function detected on input : "
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<< *FragName <<". The support is limited in relocation mode.\n";
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<< *FragName << ". The support is limited in relocation mode.\n";
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PrintedWarning = true;
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}
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Function.IsFragment = true;
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@ -1810,8 +1815,8 @@ void RewriteInstance::adjustFunctionBoundaries() {
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}
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// Or till the next function not marked by a symbol.
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if (NextFunction) {
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NextObjectAddress = std::min(NextFunction->getAddress(),
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NextObjectAddress);
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NextObjectAddress =
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std::min(NextFunction->getAddress(), NextObjectAddress);
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}
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const auto MaxSize = NextObjectAddress - Function.getAddress();
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@ -1827,8 +1832,8 @@ void RewriteInstance::adjustFunctionBoundaries() {
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// Some assembly functions have their size set to 0, use the max
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// size as their real size.
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if (opts::Verbosity >= 1) {
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outs() << "BOLT-INFO: setting size of function " << Function
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<< " to " << Function.getMaxSize() << " (was 0)\n";
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outs() << "BOLT-INFO: setting size of function " << Function << " to "
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<< Function.getMaxSize() << " (was 0)\n";
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}
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Function.setSize(Function.getMaxSize());
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}
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@ -1935,6 +1940,7 @@ void RewriteInstance::readSpecialSections() {
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GOTPLTSection = BC->getUniqueSectionByName(".got.plt");
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PLTGOTSection = BC->getUniqueSectionByName(".plt.got");
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RelaPLTSection = BC->getUniqueSectionByName(".rela.plt");
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RelaDynSection = BC->getUniqueSectionByName(".rela.dyn");
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BuildIDSection = BC->getUniqueSectionByName(".note.gnu.build-id");
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SDTSection = BC->getUniqueSectionByName(".note.stapsdt");
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@ -409,6 +409,7 @@ private:
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///
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/// Contains relocations against .got.plt.
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ErrorOr<BinarySection &> RelaPLTSection{std::errc::bad_address};
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ErrorOr<BinarySection &> RelaDynSection{std::errc::bad_address};
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/// .note.gnu.build-id section.
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ErrorOr<BinarySection &> BuildIDSection{std::errc::bad_address};
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