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1 //===- Symbols.h ------------------------------------------------*- C++ -*-===//
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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 defines various types of Symbols.
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10 //
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11 //===----------------------------------------------------------------------===//
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12
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13 #ifndef LLD_ELF_SYMBOLS_H
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14 #define LLD_ELF_SYMBOLS_H
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15
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16 #include "InputFiles.h"
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17 #include "InputSection.h"
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18 #include "lld/Common/LLVM.h"
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19 #include "lld/Common/Strings.h"
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20 #include "llvm/Object/Archive.h"
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21 #include "llvm/Object/ELF.h"
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22
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23 namespace lld {
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24 std::string toString(const elf::Symbol &);
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25
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26 // There are two different ways to convert an Archive::Symbol to a string:
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27 // One for Microsoft name mangling and one for Itanium name mangling.
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28 // Call the functions toCOFFString and toELFString, not just toString.
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29 std::string toELFString(const llvm::object::Archive::Symbol &);
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30
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31 namespace elf {
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32 class CommonSymbol;
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33 class Defined;
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34 class InputFile;
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35 class LazyArchive;
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36 class LazyObject;
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37 class SharedSymbol;
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38 class Symbol;
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39 class Undefined;
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40
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41 // This is a StringRef-like container that doesn't run strlen().
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42 //
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43 // ELF string tables contain a lot of null-terminated strings. Most of them
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44 // are not necessary for the linker because they are names of local symbols,
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45 // and the linker doesn't use local symbol names for name resolution. So, we
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46 // use this class to represents strings read from string tables.
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47 struct StringRefZ {
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48 StringRefZ(const char *s) : data(s), size(-1) {}
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49 StringRefZ(StringRef s) : data(s.data()), size(s.size()) {}
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50
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51 const char *data;
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52 const uint32_t size;
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53 };
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54
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55 // The base class for real symbol classes.
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56 class Symbol {
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57 public:
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58 enum Kind {
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59 PlaceholderKind,
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60 DefinedKind,
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61 CommonKind,
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62 SharedKind,
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63 UndefinedKind,
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64 LazyArchiveKind,
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65 LazyObjectKind,
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66 };
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67
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68 Kind kind() const { return static_cast<Kind>(symbolKind); }
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69
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70 // The file from which this symbol was created.
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71 InputFile *file;
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72
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73 protected:
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74 const char *nameData;
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75 mutable uint32_t nameSize;
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76
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77 public:
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78 uint32_t dynsymIndex = 0;
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79 uint32_t gotIndex = -1;
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80 uint32_t pltIndex = -1;
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81
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82 uint32_t globalDynIndex = -1;
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83
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84 // This field is a index to the symbol's version definition.
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85 uint32_t verdefIndex = -1;
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86
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87 // Version definition index.
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88 uint16_t versionId;
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89
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90 // Symbol binding. This is not overwritten by replace() to track
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91 // changes during resolution. In particular:
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92 // - An undefined weak is still weak when it resolves to a shared library.
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93 // - An undefined weak will not fetch archive members, but we have to
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94 // remember it is weak.
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95 uint8_t binding;
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96
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97 // The following fields have the same meaning as the ELF symbol attributes.
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98 uint8_t type; // symbol type
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99 uint8_t stOther; // st_other field value
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100
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101 uint8_t symbolKind;
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102
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103 // Symbol visibility. This is the computed minimum visibility of all
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104 // observed non-DSO symbols.
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105 uint8_t visibility : 2;
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106
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107 // True if the symbol was used for linking and thus need to be added to the
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108 // output file's symbol table. This is true for all symbols except for
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109 // unreferenced DSO symbols, lazy (archive) symbols, and bitcode symbols that
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110 // are unreferenced except by other bitcode objects.
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111 uint8_t isUsedInRegularObj : 1;
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112
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113 // Used by a Defined symbol with protected or default visibility, to record
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114 // whether it is required to be exported into .dynsym. This is set when any of
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115 // the following conditions hold:
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116 //
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117 // - If there is an interposable symbol from a DSO.
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118 // - If -shared or --export-dynamic is specified, any symbol in an object
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119 // file/bitcode sets this property, unless suppressed by LTO
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120 // canBeOmittedFromSymbolTable().
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121 uint8_t exportDynamic : 1;
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122
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123 // True if the symbol is in the --dynamic-list file. A Defined symbol with
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124 // protected or default visibility with this property is required to be
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125 // exported into .dynsym.
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126 uint8_t inDynamicList : 1;
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127
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128 // False if LTO shouldn't inline whatever this symbol points to. If a symbol
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129 // is overwritten after LTO, LTO shouldn't inline the symbol because it
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130 // doesn't know the final contents of the symbol.
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131 uint8_t canInline : 1;
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132
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133 // Used by Undefined and SharedSymbol to track if there has been at least one
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134 // undefined reference to the symbol. The binding may change to STB_WEAK if
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135 // the first undefined reference from a non-shared object is weak.
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136 uint8_t referenced : 1;
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137
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138 // True if this symbol is specified by --trace-symbol option.
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139 uint8_t traced : 1;
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140
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141 inline void replace(const Symbol &newSym);
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142
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143 bool includeInDynsym() const;
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144 uint8_t computeBinding() const;
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145 bool isWeak() const { return binding == llvm::ELF::STB_WEAK; }
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146
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147 bool isUndefined() const { return symbolKind == UndefinedKind; }
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148 bool isCommon() const { return symbolKind == CommonKind; }
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149 bool isDefined() const { return symbolKind == DefinedKind; }
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150 bool isShared() const { return symbolKind == SharedKind; }
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151 bool isPlaceholder() const { return symbolKind == PlaceholderKind; }
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152
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153 bool isLocal() const { return binding == llvm::ELF::STB_LOCAL; }
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154
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155 bool isLazy() const {
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156 return symbolKind == LazyArchiveKind || symbolKind == LazyObjectKind;
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157 }
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158
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159 // True if this is an undefined weak symbol. This only works once
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160 // all input files have been added.
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161 bool isUndefWeak() const {
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162 // See comment on lazy symbols for details.
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163 return isWeak() && (isUndefined() || isLazy());
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164 }
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165
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166 StringRef getName() const {
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167 if (nameSize == (uint32_t)-1)
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168 nameSize = strlen(nameData);
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169 return {nameData, nameSize};
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170 }
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171
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172 void setName(StringRef s) {
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173 nameData = s.data();
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174 nameSize = s.size();
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175 }
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176
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177 void parseSymbolVersion();
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178
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179 bool isInGot() const { return gotIndex != -1U; }
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180 bool isInPlt() const { return pltIndex != -1U; }
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181
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182 uint64_t getVA(int64_t addend = 0) const;
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183
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184 uint64_t getGotOffset() const;
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185 uint64_t getGotVA() const;
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186 uint64_t getGotPltOffset() const;
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187 uint64_t getGotPltVA() const;
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188 uint64_t getPltVA() const;
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189 uint64_t getSize() const;
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190 OutputSection *getOutputSection() const;
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191
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192 // The following two functions are used for symbol resolution.
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193 //
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194 // You are expected to call mergeProperties for all symbols in input
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195 // files so that attributes that are attached to names rather than
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196 // indivisual symbol (such as visibility) are merged together.
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197 //
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198 // Every time you read a new symbol from an input, you are supposed
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199 // to call resolve() with the new symbol. That function replaces
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200 // "this" object as a result of name resolution if the new symbol is
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201 // more appropriate to be included in the output.
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202 //
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203 // For example, if "this" is an undefined symbol and a new symbol is
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204 // a defined symbol, "this" is replaced with the new symbol.
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205 void mergeProperties(const Symbol &other);
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206 void resolve(const Symbol &other);
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207
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208 // If this is a lazy symbol, fetch an input file and add the symbol
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209 // in the file to the symbol table. Calling this function on
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210 // non-lazy object causes a runtime error.
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211 void fetch() const;
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212
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213 private:
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214 static bool isExportDynamic(Kind k, uint8_t visibility) {
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215 if (k == SharedKind)
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216 return visibility == llvm::ELF::STV_DEFAULT;
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217 return config->shared || config->exportDynamic;
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218 }
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219
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220 void resolveUndefined(const Undefined &other);
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221 void resolveCommon(const CommonSymbol &other);
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222 void resolveDefined(const Defined &other);
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223 template <class LazyT> void resolveLazy(const LazyT &other);
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224 void resolveShared(const SharedSymbol &other);
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225
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226 int compare(const Symbol *other) const;
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227
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228 inline size_t getSymbolSize() const;
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229
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230 protected:
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231 Symbol(Kind k, InputFile *file, StringRefZ name, uint8_t binding,
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232 uint8_t stOther, uint8_t type)
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233 : file(file), nameData(name.data), nameSize(name.size), binding(binding),
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234 type(type), stOther(stOther), symbolKind(k), visibility(stOther & 3),
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235 isUsedInRegularObj(!file || file->kind() == InputFile::ObjKind),
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236 exportDynamic(isExportDynamic(k, visibility)), inDynamicList(false),
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237 canInline(false), referenced(false), traced(false), needsPltAddr(false),
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238 isInIplt(false), gotInIgot(false), isPreemptible(false),
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239 used(!config->gcSections), needsTocRestore(false),
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240 scriptDefined(false) {}
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241
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242 public:
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243 // True the symbol should point to its PLT entry.
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244 // For SharedSymbol only.
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245 uint8_t needsPltAddr : 1;
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246
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247 // True if this symbol is in the Iplt sub-section of the Plt and the Igot
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248 // sub-section of the .got.plt or .got.
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249 uint8_t isInIplt : 1;
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250
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251 // True if this symbol needs a GOT entry and its GOT entry is actually in
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252 // Igot. This will be true only for certain non-preemptible ifuncs.
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253 uint8_t gotInIgot : 1;
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254
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255 // True if this symbol is preemptible at load time.
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256 uint8_t isPreemptible : 1;
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257
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258 // True if an undefined or shared symbol is used from a live section.
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259 uint8_t used : 1;
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260
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261 // True if a call to this symbol needs to be followed by a restore of the
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262 // PPC64 toc pointer.
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263 uint8_t needsTocRestore : 1;
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264
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265 // True if this symbol is defined by a linker script.
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266 uint8_t scriptDefined : 1;
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267
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268 // The partition whose dynamic symbol table contains this symbol's definition.
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269 uint8_t partition = 1;
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270
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271 bool isSection() const { return type == llvm::ELF::STT_SECTION; }
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272 bool isTls() const { return type == llvm::ELF::STT_TLS; }
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273 bool isFunc() const { return type == llvm::ELF::STT_FUNC; }
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274 bool isGnuIFunc() const { return type == llvm::ELF::STT_GNU_IFUNC; }
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275 bool isObject() const { return type == llvm::ELF::STT_OBJECT; }
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276 bool isFile() const { return type == llvm::ELF::STT_FILE; }
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277 };
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278
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279 // Represents a symbol that is defined in the current output file.
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280 class Defined : public Symbol {
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281 public:
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282 Defined(InputFile *file, StringRefZ name, uint8_t binding, uint8_t stOther,
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283 uint8_t type, uint64_t value, uint64_t size, SectionBase *section)
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284 : Symbol(DefinedKind, file, name, binding, stOther, type), value(value),
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285 size(size), section(section) {}
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286
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287 static bool classof(const Symbol *s) { return s->isDefined(); }
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288
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289 uint64_t value;
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290 uint64_t size;
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291 SectionBase *section;
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292 };
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293
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294 // Represents a common symbol.
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295 //
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296 // On Unix, it is traditionally allowed to write variable definitions
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297 // without initialization expressions (such as "int foo;") to header
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298 // files. Such definition is called "tentative definition".
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299 //
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300 // Using tentative definition is usually considered a bad practice
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301 // because you should write only declarations (such as "extern int
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302 // foo;") to header files. Nevertheless, the linker and the compiler
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303 // have to do something to support bad code by allowing duplicate
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304 // definitions for this particular case.
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305 //
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306 // Common symbols represent variable definitions without initializations.
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307 // The compiler creates common symbols when it sees variable definitions
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308 // without initialization (you can suppress this behavior and let the
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309 // compiler create a regular defined symbol by -fno-common).
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310 //
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311 // The linker allows common symbols to be replaced by regular defined
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312 // symbols. If there are remaining common symbols after name resolution is
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313 // complete, they are converted to regular defined symbols in a .bss
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314 // section. (Therefore, the later passes don't see any CommonSymbols.)
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315 class CommonSymbol : public Symbol {
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316 public:
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317 CommonSymbol(InputFile *file, StringRefZ name, uint8_t binding,
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318 uint8_t stOther, uint8_t type, uint64_t alignment, uint64_t size)
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319 : Symbol(CommonKind, file, name, binding, stOther, type),
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320 alignment(alignment), size(size) {}
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321
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322 static bool classof(const Symbol *s) { return s->isCommon(); }
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323
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324 uint32_t alignment;
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325 uint64_t size;
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326 };
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327
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328 class Undefined : public Symbol {
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329 public:
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330 Undefined(InputFile *file, StringRefZ name, uint8_t binding, uint8_t stOther,
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331 uint8_t type, uint32_t discardedSecIdx = 0)
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332 : Symbol(UndefinedKind, file, name, binding, stOther, type),
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333 discardedSecIdx(discardedSecIdx) {}
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334
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335 static bool classof(const Symbol *s) { return s->kind() == UndefinedKind; }
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336
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337 // The section index if in a discarded section, 0 otherwise.
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338 uint32_t discardedSecIdx;
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339 };
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340
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341 class SharedSymbol : public Symbol {
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342 public:
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343 static bool classof(const Symbol *s) { return s->kind() == SharedKind; }
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344
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345 SharedSymbol(InputFile &file, StringRef name, uint8_t binding,
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346 uint8_t stOther, uint8_t type, uint64_t value, uint64_t size,
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347 uint32_t alignment, uint32_t verdefIndex)
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348 : Symbol(SharedKind, &file, name, binding, stOther, type), value(value),
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349 size(size), alignment(alignment) {
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350 this->verdefIndex = verdefIndex;
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351 // GNU ifunc is a mechanism to allow user-supplied functions to
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352 // resolve PLT slot values at load-time. This is contrary to the
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353 // regular symbol resolution scheme in which symbols are resolved just
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354 // by name. Using this hook, you can program how symbols are solved
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355 // for you program. For example, you can make "memcpy" to be resolved
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356 // to a SSE-enabled version of memcpy only when a machine running the
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357 // program supports the SSE instruction set.
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358 //
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359 // Naturally, such symbols should always be called through their PLT
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360 // slots. What GNU ifunc symbols point to are resolver functions, and
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361 // calling them directly doesn't make sense (unless you are writing a
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362 // loader).
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363 //
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364 // For DSO symbols, we always call them through PLT slots anyway.
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365 // So there's no difference between GNU ifunc and regular function
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366 // symbols if they are in DSOs. So we can handle GNU_IFUNC as FUNC.
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367 if (this->type == llvm::ELF::STT_GNU_IFUNC)
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368 this->type = llvm::ELF::STT_FUNC;
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369 }
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370
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371 SharedFile &getFile() const { return *cast<SharedFile>(file); }
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372
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373 uint64_t value; // st_value
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374 uint64_t size; // st_size
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375 uint32_t alignment;
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376 };
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377
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378 // LazyArchive and LazyObject represent a symbols that is not yet in the link,
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379 // but we know where to find it if needed. If the resolver finds both Undefined
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380 // and Lazy for the same name, it will ask the Lazy to load a file.
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381 //
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382 // A special complication is the handling of weak undefined symbols. They should
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383 // not load a file, but we have to remember we have seen both the weak undefined
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384 // and the lazy. We represent that with a lazy symbol with a weak binding. This
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385 // means that code looking for undefined symbols normally also has to take lazy
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386 // symbols into consideration.
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387
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388 // This class represents a symbol defined in an archive file. It is
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389 // created from an archive file header, and it knows how to load an
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390 // object file from an archive to replace itself with a defined
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391 // symbol.
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392 class LazyArchive : public Symbol {
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393 public:
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394 LazyArchive(InputFile &file, const llvm::object::Archive::Symbol s)
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395 : Symbol(LazyArchiveKind, &file, s.getName(), llvm::ELF::STB_GLOBAL,
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396 llvm::ELF::STV_DEFAULT, llvm::ELF::STT_NOTYPE),
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397 sym(s) {}
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398
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399 static bool classof(const Symbol *s) { return s->kind() == LazyArchiveKind; }
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400
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401 MemoryBufferRef getMemberBuffer();
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402
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403 const llvm::object::Archive::Symbol sym;
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404 };
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405
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406 // LazyObject symbols represents symbols in object files between
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407 // --start-lib and --end-lib options.
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408 class LazyObject : public Symbol {
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409 public:
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410 LazyObject(InputFile &file, StringRef name)
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411 : Symbol(LazyObjectKind, &file, name, llvm::ELF::STB_GLOBAL,
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412 llvm::ELF::STV_DEFAULT, llvm::ELF::STT_NOTYPE) {}
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413
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414 static bool classof(const Symbol *s) { return s->kind() == LazyObjectKind; }
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415 };
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416
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417 // Some linker-generated symbols need to be created as
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418 // Defined symbols.
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419 struct ElfSym {
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420 // __bss_start
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421 static Defined *bss;
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422
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423 // etext and _etext
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424 static Defined *etext1;
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425 static Defined *etext2;
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426
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427 // edata and _edata
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428 static Defined *edata1;
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429 static Defined *edata2;
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430
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431 // end and _end
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432 static Defined *end1;
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433 static Defined *end2;
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434
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435 // The _GLOBAL_OFFSET_TABLE_ symbol is defined by target convention to
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436 // be at some offset from the base of the .got section, usually 0 or
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437 // the end of the .got.
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438 static Defined *globalOffsetTable;
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439
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440 // _gp, _gp_disp and __gnu_local_gp symbols. Only for MIPS.
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441 static Defined *mipsGp;
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442 static Defined *mipsGpDisp;
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443 static Defined *mipsLocalGp;
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444
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445 // __rel{,a}_iplt_{start,end} symbols.
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446 static Defined *relaIpltStart;
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447 static Defined *relaIpltEnd;
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448
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449 // __global_pointer$ for RISC-V.
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450 static Defined *riscvGlobalPointer;
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451
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452 // _TLS_MODULE_BASE_ on targets that support TLSDESC.
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453 static Defined *tlsModuleBase;
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454 };
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455
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456 // A buffer class that is large enough to hold any Symbol-derived
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457 // object. We allocate memory using this class and instantiate a symbol
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458 // using the placement new.
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459 union SymbolUnion {
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460 alignas(Defined) char a[sizeof(Defined)];
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461 alignas(CommonSymbol) char b[sizeof(CommonSymbol)];
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462 alignas(Undefined) char c[sizeof(Undefined)];
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463 alignas(SharedSymbol) char d[sizeof(SharedSymbol)];
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464 alignas(LazyArchive) char e[sizeof(LazyArchive)];
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465 alignas(LazyObject) char f[sizeof(LazyObject)];
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466 };
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467
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468 // It is important to keep the size of SymbolUnion small for performance and
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469 // memory usage reasons. 80 bytes is a soft limit based on the size of Defined
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470 // on a 64-bit system.
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471 static_assert(sizeof(SymbolUnion) <= 80, "SymbolUnion too large");
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472
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473 template <typename T> struct AssertSymbol {
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474 static_assert(std::is_trivially_destructible<T>(),
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475 "Symbol types must be trivially destructible");
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476 static_assert(sizeof(T) <= sizeof(SymbolUnion), "SymbolUnion too small");
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477 static_assert(alignof(T) <= alignof(SymbolUnion),
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478 "SymbolUnion not aligned enough");
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479 };
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480
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481 static inline void assertSymbols() {
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482 AssertSymbol<Defined>();
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483 AssertSymbol<CommonSymbol>();
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484 AssertSymbol<Undefined>();
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485 AssertSymbol<SharedSymbol>();
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486 AssertSymbol<LazyArchive>();
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487 AssertSymbol<LazyObject>();
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488 }
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489
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490 void printTraceSymbol(const Symbol *sym);
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491
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492 size_t Symbol::getSymbolSize() const {
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493 switch (kind()) {
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494 case CommonKind:
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495 return sizeof(CommonSymbol);
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496 case DefinedKind:
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497 return sizeof(Defined);
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498 case LazyArchiveKind:
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499 return sizeof(LazyArchive);
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500 case LazyObjectKind:
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501 return sizeof(LazyObject);
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502 case SharedKind:
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503 return sizeof(SharedSymbol);
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504 case UndefinedKind:
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505 return sizeof(Undefined);
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506 case PlaceholderKind:
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507 return sizeof(Symbol);
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508 }
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509 llvm_unreachable("unknown symbol kind");
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510 }
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511
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512 // replace() replaces "this" object with a given symbol by memcpy'ing
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513 // it over to "this". This function is called as a result of name
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514 // resolution, e.g. to replace an undefind symbol with a defined symbol.
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515 void Symbol::replace(const Symbol &newSym) {
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516 using llvm::ELF::STT_TLS;
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517
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518 // Symbols representing thread-local variables must be referenced by
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519 // TLS-aware relocations, and non-TLS symbols must be reference by
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520 // non-TLS relocations, so there's a clear distinction between TLS
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521 // and non-TLS symbols. It is an error if the same symbol is defined
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522 // as a TLS symbol in one file and as a non-TLS symbol in other file.
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523 if (symbolKind != PlaceholderKind && !isLazy() && !newSym.isLazy() &&
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524 (type == STT_TLS) != (newSym.type == STT_TLS))
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525 error("TLS attribute mismatch: " + toString(*this) + "\n>>> defined in " +
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526 toString(newSym.file) + "\n>>> defined in " + toString(file));
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527
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528 Symbol old = *this;
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529 memcpy(this, &newSym, newSym.getSymbolSize());
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530
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531 // old may be a placeholder. The referenced fields must be initialized in
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532 // SymbolTable::insert.
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533 versionId = old.versionId;
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534 visibility = old.visibility;
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535 isUsedInRegularObj = old.isUsedInRegularObj;
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536 exportDynamic = old.exportDynamic;
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537 inDynamicList = old.inDynamicList;
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538 canInline = old.canInline;
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539 referenced = old.referenced;
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540 traced = old.traced;
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541 isPreemptible = old.isPreemptible;
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542 scriptDefined = old.scriptDefined;
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543 partition = old.partition;
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544
|
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545 // Symbol length is computed lazily. If we already know a symbol length,
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546 // propagate it.
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547 if (nameData == old.nameData && nameSize == 0 && old.nameSize != 0)
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548 nameSize = old.nameSize;
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549
|
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550 // Print out a log message if --trace-symbol was specified.
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|
551 // This is for debugging.
|
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552 if (traced)
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553 printTraceSymbol(this);
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554 }
|
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555
|
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556 void maybeWarnUnorderableSymbol(const Symbol *sym);
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557 bool computeIsPreemptible(const Symbol &sym);
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558
|
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559 } // namespace elf
|
|
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560 } // namespace lld
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561
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562 #endif
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