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150
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1 //===- MarkLive.cpp -------------------------------------------------------===//
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2 //
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3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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4 // See https://llvm.org/LICENSE.txt for license information.
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5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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6 //
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7 //===----------------------------------------------------------------------===//
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8 //
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9 // This file implements --gc-sections, which is a feature to remove unused
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10 // sections from output. Unused sections are sections that are not reachable
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11 // from known GC-root symbols or sections. Naturally the feature is
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12 // implemented as a mark-sweep garbage collector.
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13 //
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14 // Here's how it works. Each InputSectionBase has a "Live" bit. The bit is off
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15 // by default. Starting with GC-root symbols or sections, markLive function
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16 // defined in this file visits all reachable sections to set their Live
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17 // bits. Writer will then ignore sections whose Live bits are off, so that
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18 // such sections are not included into output.
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19 //
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20 //===----------------------------------------------------------------------===//
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21
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22 #include "MarkLive.h"
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23 #include "InputSection.h"
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24 #include "LinkerScript.h"
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25 #include "OutputSections.h"
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26 #include "SymbolTable.h"
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27 #include "Symbols.h"
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28 #include "SyntheticSections.h"
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29 #include "Target.h"
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30 #include "lld/Common/Memory.h"
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31 #include "lld/Common/Strings.h"
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32 #include "llvm/ADT/STLExtras.h"
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33 #include "llvm/Object/ELF.h"
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34 #include "llvm/Support/TimeProfiler.h"
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35 #include <functional>
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36 #include <vector>
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37
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38 using namespace llvm;
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39 using namespace llvm::ELF;
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40 using namespace llvm::object;
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173
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41 using namespace llvm::support::endian;
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42 using namespace lld;
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43 using namespace lld::elf;
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150
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44
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45 namespace {
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46 template <class ELFT> class MarkLive {
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47 public:
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48 MarkLive(unsigned partition) : partition(partition) {}
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49
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50 void run();
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51 void moveToMain();
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52
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53 private:
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54 void enqueue(InputSectionBase *sec, uint64_t offset);
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55 void markSymbol(Symbol *sym);
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56 void mark();
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57
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58 template <class RelTy>
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59 void resolveReloc(InputSectionBase &sec, RelTy &rel, bool isLSDA);
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60
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61 template <class RelTy>
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62 void scanEhFrameSection(EhInputSection &eh, ArrayRef<RelTy> rels);
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63
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64 // The index of the partition that we are currently processing.
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65 unsigned partition;
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66
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67 // A list of sections to visit.
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68 SmallVector<InputSection *, 256> queue;
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69
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70 // There are normally few input sections whose names are valid C
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71 // identifiers, so we just store a std::vector instead of a multimap.
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72 DenseMap<StringRef, std::vector<InputSectionBase *>> cNamedSections;
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73 };
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74 } // namespace
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75
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76 template <class ELFT>
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77 static uint64_t getAddend(InputSectionBase &sec,
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78 const typename ELFT::Rel &rel) {
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79 return target->getImplicitAddend(sec.data().begin() + rel.r_offset,
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80 rel.getType(config->isMips64EL));
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81 }
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82
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83 template <class ELFT>
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84 static uint64_t getAddend(InputSectionBase &sec,
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85 const typename ELFT::Rela &rel) {
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86 return rel.r_addend;
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87 }
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88
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89 template <class ELFT>
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90 template <class RelTy>
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91 void MarkLive<ELFT>::resolveReloc(InputSectionBase &sec, RelTy &rel,
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92 bool isLSDA) {
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93 Symbol &sym = sec.getFile<ELFT>()->getRelocTargetSym(rel);
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94
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95 // If a symbol is referenced in a live section, it is used.
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96 sym.used = true;
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97
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98 if (auto *d = dyn_cast<Defined>(&sym)) {
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99 auto *relSec = dyn_cast_or_null<InputSectionBase>(d->section);
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100 if (!relSec)
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101 return;
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102
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103 uint64_t offset = d->value;
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104 if (d->isSection())
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105 offset += getAddend<ELFT>(sec, rel);
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106
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107 if (!isLSDA || !(relSec->flags & SHF_EXECINSTR))
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108 enqueue(relSec, offset);
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109 return;
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110 }
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111
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112 if (auto *ss = dyn_cast<SharedSymbol>(&sym))
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113 if (!ss->isWeak())
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114 ss->getFile().isNeeded = true;
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115
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116 for (InputSectionBase *sec : cNamedSections.lookup(sym.getName()))
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117 enqueue(sec, 0);
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118 }
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119
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120 // The .eh_frame section is an unfortunate special case.
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121 // The section is divided in CIEs and FDEs and the relocations it can have are
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122 // * CIEs can refer to a personality function.
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123 // * FDEs can refer to a LSDA
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124 // * FDEs refer to the function they contain information about
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125 // The last kind of relocation cannot keep the referred section alive, or they
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126 // would keep everything alive in a common object file. In fact, each FDE is
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127 // alive if the section it refers to is alive.
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128 // To keep things simple, in here we just ignore the last relocation kind. The
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129 // other two keep the referred section alive.
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130 //
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131 // A possible improvement would be to fully process .eh_frame in the middle of
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132 // the gc pass. With that we would be able to also gc some sections holding
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133 // LSDAs and personality functions if we found that they were unused.
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134 template <class ELFT>
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135 template <class RelTy>
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136 void MarkLive<ELFT>::scanEhFrameSection(EhInputSection &eh,
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137 ArrayRef<RelTy> rels) {
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138 for (size_t i = 0, end = eh.pieces.size(); i < end; ++i) {
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139 EhSectionPiece &piece = eh.pieces[i];
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140 size_t firstRelI = piece.firstRelocation;
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141 if (firstRelI == (unsigned)-1)
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142 continue;
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143
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173
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144 if (read32<ELFT::TargetEndianness>(piece.data().data() + 4) == 0) {
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150
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145 // This is a CIE, we only need to worry about the first relocation. It is
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146 // known to point to the personality function.
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147 resolveReloc(eh, rels[firstRelI], false);
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148 continue;
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149 }
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150
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151 // This is a FDE. The relocations point to the described function or to
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152 // a LSDA. We only need to keep the LSDA alive, so ignore anything that
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153 // points to executable sections.
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154 uint64_t pieceEnd = piece.inputOff + piece.size;
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155 for (size_t j = firstRelI, end2 = rels.size(); j < end2; ++j)
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156 if (rels[j].r_offset < pieceEnd)
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157 resolveReloc(eh, rels[j], true);
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158 }
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159 }
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160
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161 // Some sections are used directly by the loader, so they should never be
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162 // garbage-collected. This function returns true if a given section is such
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163 // section.
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164 static bool isReserved(InputSectionBase *sec) {
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165 switch (sec->type) {
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166 case SHT_FINI_ARRAY:
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167 case SHT_INIT_ARRAY:
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168 case SHT_PREINIT_ARRAY:
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169 return true;
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170 case SHT_NOTE:
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171 // SHT_NOTE sections in a group are subject to garbage collection.
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172 return !sec->nextInSectionGroup;
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173 default:
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174 StringRef s = sec->name;
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175 return s.startswith(".ctors") || s.startswith(".dtors") ||
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176 s.startswith(".init") || s.startswith(".fini") ||
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177 s.startswith(".jcr");
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178 }
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179 }
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180
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181 template <class ELFT>
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182 void MarkLive<ELFT>::enqueue(InputSectionBase *sec, uint64_t offset) {
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183 // Skip over discarded sections. This in theory shouldn't happen, because
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184 // the ELF spec doesn't allow a relocation to point to a deduplicated
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185 // COMDAT section directly. Unfortunately this happens in practice (e.g.
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186 // .eh_frame) so we need to add a check.
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187 if (sec == &InputSection::discarded)
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188 return;
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189
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190 // Usually, a whole section is marked as live or dead, but in mergeable
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191 // (splittable) sections, each piece of data has independent liveness bit.
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192 // So we explicitly tell it which offset is in use.
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193 if (auto *ms = dyn_cast<MergeInputSection>(sec))
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194 ms->getSectionPiece(offset)->live = true;
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195
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196 // Set Sec->Partition to the meet (i.e. the "minimum") of Partition and
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197 // Sec->Partition in the following lattice: 1 < other < 0. If Sec->Partition
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198 // doesn't change, we don't need to do anything.
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199 if (sec->partition == 1 || sec->partition == partition)
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200 return;
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201 sec->partition = sec->partition ? 1 : partition;
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202
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203 // Add input section to the queue.
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204 if (InputSection *s = dyn_cast<InputSection>(sec))
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205 queue.push_back(s);
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206 }
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207
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208 template <class ELFT> void MarkLive<ELFT>::markSymbol(Symbol *sym) {
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209 if (auto *d = dyn_cast_or_null<Defined>(sym))
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210 if (auto *isec = dyn_cast_or_null<InputSectionBase>(d->section))
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211 enqueue(isec, d->value);
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212 }
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213
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214 // This is the main function of the garbage collector.
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215 // Starting from GC-root sections, this function visits all reachable
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216 // sections to set their "Live" bits.
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217 template <class ELFT> void MarkLive<ELFT>::run() {
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218 // Add GC root symbols.
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219
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220 // Preserve externally-visible symbols if the symbols defined by this
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221 // file can interrupt other ELF file's symbols at runtime.
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222 for (Symbol *sym : symtab->symbols())
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223 if (sym->includeInDynsym() && sym->partition == partition)
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224 markSymbol(sym);
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225
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226 // If this isn't the main partition, that's all that we need to preserve.
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227 if (partition != 1) {
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228 mark();
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229 return;
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230 }
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231
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232 markSymbol(symtab->find(config->entry));
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233 markSymbol(symtab->find(config->init));
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234 markSymbol(symtab->find(config->fini));
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235 for (StringRef s : config->undefined)
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236 markSymbol(symtab->find(s));
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237 for (StringRef s : script->referencedSymbols)
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238 markSymbol(symtab->find(s));
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239
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240 // Preserve special sections and those which are specified in linker
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241 // script KEEP command.
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242 for (InputSectionBase *sec : inputSections) {
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243 // Mark .eh_frame sections as live because there are usually no relocations
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244 // that point to .eh_frames. Otherwise, the garbage collector would drop
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245 // all of them. We also want to preserve personality routines and LSDA
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246 // referenced by .eh_frame sections, so we scan them for that here.
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247 if (auto *eh = dyn_cast<EhInputSection>(sec)) {
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248 eh->markLive();
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249 if (!eh->numRelocations)
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250 continue;
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251
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252 if (eh->areRelocsRela)
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253 scanEhFrameSection(*eh, eh->template relas<ELFT>());
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254 else
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255 scanEhFrameSection(*eh, eh->template rels<ELFT>());
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256 }
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257
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258 if (sec->flags & SHF_LINK_ORDER)
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259 continue;
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260
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261 if (isReserved(sec) || script->shouldKeep(sec)) {
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262 enqueue(sec, 0);
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263 } else if (isValidCIdentifier(sec->name)) {
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264 cNamedSections[saver.save("__start_" + sec->name)].push_back(sec);
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265 cNamedSections[saver.save("__stop_" + sec->name)].push_back(sec);
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266 }
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267 }
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268
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269 mark();
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270 }
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271
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272 template <class ELFT> void MarkLive<ELFT>::mark() {
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273 // Mark all reachable sections.
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274 while (!queue.empty()) {
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275 InputSectionBase &sec = *queue.pop_back_val();
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276
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277 if (sec.areRelocsRela) {
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278 for (const typename ELFT::Rela &rel : sec.template relas<ELFT>())
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279 resolveReloc(sec, rel, false);
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280 } else {
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281 for (const typename ELFT::Rel &rel : sec.template rels<ELFT>())
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282 resolveReloc(sec, rel, false);
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283 }
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284
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285 for (InputSectionBase *isec : sec.dependentSections)
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286 enqueue(isec, 0);
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287
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288 // Mark the next group member.
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289 if (sec.nextInSectionGroup)
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290 enqueue(sec.nextInSectionGroup, 0);
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291 }
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292 }
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293
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294 // Move the sections for some symbols to the main partition, specifically ifuncs
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295 // (because they can result in an IRELATIVE being added to the main partition's
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296 // GOT, which means that the ifunc must be available when the main partition is
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297 // loaded) and TLS symbols (because we only know how to correctly process TLS
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298 // relocations for the main partition).
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299 //
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300 // We also need to move sections whose names are C identifiers that are referred
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301 // to from __start_/__stop_ symbols because there will only be one set of
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302 // symbols for the whole program.
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303 template <class ELFT> void MarkLive<ELFT>::moveToMain() {
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304 for (InputFile *file : objectFiles)
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305 for (Symbol *s : file->getSymbols())
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306 if (auto *d = dyn_cast<Defined>(s))
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307 if ((d->type == STT_GNU_IFUNC || d->type == STT_TLS) && d->section &&
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308 d->section->isLive())
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309 markSymbol(s);
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310
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311 for (InputSectionBase *sec : inputSections) {
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312 if (!sec->isLive() || !isValidCIdentifier(sec->name))
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313 continue;
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314 if (symtab->find(("__start_" + sec->name).str()) ||
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315 symtab->find(("__stop_" + sec->name).str()))
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316 enqueue(sec, 0);
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317 }
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318
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319 mark();
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320 }
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321
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322 // Before calling this function, Live bits are off for all
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323 // input sections. This function make some or all of them on
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324 // so that they are emitted to the output file.
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173
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325 template <class ELFT> void elf::markLive() {
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150
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326 llvm::TimeTraceScope timeScope("markLive");
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327 // If -gc-sections is not given, no sections are removed.
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328 if (!config->gcSections) {
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329 for (InputSectionBase *sec : inputSections)
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330 sec->markLive();
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331
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332 // If a DSO defines a symbol referenced in a regular object, it is needed.
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333 for (Symbol *sym : symtab->symbols())
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334 if (auto *s = dyn_cast<SharedSymbol>(sym))
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335 if (s->isUsedInRegularObj && !s->isWeak())
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336 s->getFile().isNeeded = true;
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337 return;
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338 }
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339
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340 // Otherwise, do mark-sweep GC.
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341 //
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342 // The -gc-sections option works only for SHF_ALLOC sections
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343 // (sections that are memory-mapped at runtime). So we can
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344 // unconditionally make non-SHF_ALLOC sections alive except
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345 // SHF_LINK_ORDER and SHT_REL/SHT_RELA sections.
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346 //
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347 // Usually, non-SHF_ALLOC sections are not removed even if they are
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348 // unreachable through relocations because reachability is not
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349 // a good signal whether they are garbage or not (e.g. there is
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350 // usually no section referring to a .comment section, but we
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351 // want to keep it.).
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352 //
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353 // Note on SHF_LINK_ORDER: Such sections contain metadata and they
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354 // have a reverse dependency on the InputSection they are linked with.
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355 // We are able to garbage collect them.
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356 //
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357 // Note on SHF_REL{,A}: Such sections reach here only when -r
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358 // or -emit-reloc were given. And they are subject of garbage
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359 // collection because, if we remove a text section, we also
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360 // remove its relocation section.
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361 //
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362 // Note on nextInSectionGroup: The ELF spec says that group sections are
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363 // included or omitted as a unit. We take the interpretation that:
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364 //
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365 // - Group members (nextInSectionGroup != nullptr) are subject to garbage
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366 // collection.
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367 // - Groups members are retained or discarded as a unit.
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368 for (InputSectionBase *sec : inputSections) {
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369 bool isAlloc = (sec->flags & SHF_ALLOC);
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370 bool isLinkOrder = (sec->flags & SHF_LINK_ORDER);
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371 bool isRel = (sec->type == SHT_REL || sec->type == SHT_RELA);
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372
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373 if (!isAlloc && !isLinkOrder && !isRel && !sec->nextInSectionGroup)
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374 sec->markLive();
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375 }
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376
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377 // Follow the graph to mark all live sections.
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378 for (unsigned curPart = 1; curPart <= partitions.size(); ++curPart)
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379 MarkLive<ELFT>(curPart).run();
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380
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381 // If we have multiple partitions, some sections need to live in the main
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382 // partition even if they were allocated to a loadable partition. Move them
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383 // there now.
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384 if (partitions.size() != 1)
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385 MarkLive<ELFT>(1).moveToMain();
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386
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387 // Report garbage-collected sections.
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388 if (config->printGcSections)
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389 for (InputSectionBase *sec : inputSections)
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390 if (!sec->isLive())
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391 message("removing unused section " + toString(sec));
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392 }
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393
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173
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394 template void elf::markLive<ELF32LE>();
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395 template void elf::markLive<ELF32BE>();
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396 template void elf::markLive<ELF64LE>();
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397 template void elf::markLive<ELF64BE>();
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