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150
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1 ===============
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2 LLVM Extensions
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3 ===============
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4
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5 .. contents::
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6 :local:
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7
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8 .. toctree::
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9 :hidden:
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10
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11 Introduction
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12 ============
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13
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14 This document describes extensions to tools and formats LLVM seeks compatibility
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15 with.
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16
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17 General Assembly Syntax
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18 ===========================
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19
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20 C99-style Hexadecimal Floating-point Constants
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21 ----------------------------------------------
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22
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23 LLVM's assemblers allow floating-point constants to be written in C99's
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24 hexadecimal format instead of decimal if desired.
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25
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26 .. code-block:: gas
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27
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28 .section .data
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29 .float 0x1c2.2ap3
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30
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31 Machine-specific Assembly Syntax
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32 ================================
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33
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34 X86/COFF-Dependent
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35 ------------------
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36
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37 Relocations
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38 ^^^^^^^^^^^
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39
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40 The following additional relocation types are supported:
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41
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42 **@IMGREL** (AT&T syntax only) generates an image-relative relocation that
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43 corresponds to the COFF relocation types ``IMAGE_REL_I386_DIR32NB`` (32-bit) or
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44 ``IMAGE_REL_AMD64_ADDR32NB`` (64-bit).
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45
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46 .. code-block:: text
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47
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48 .text
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49 fun:
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50 mov foo@IMGREL(%ebx, %ecx, 4), %eax
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51
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52 .section .pdata
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53 .long fun@IMGREL
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54 .long (fun@imgrel + 0x3F)
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55 .long $unwind$fun@imgrel
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56
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57 **.secrel32** generates a relocation that corresponds to the COFF relocation
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58 types ``IMAGE_REL_I386_SECREL`` (32-bit) or ``IMAGE_REL_AMD64_SECREL`` (64-bit).
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59
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60 **.secidx** relocation generates an index of the section that contains
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61 the target. It corresponds to the COFF relocation types
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62 ``IMAGE_REL_I386_SECTION`` (32-bit) or ``IMAGE_REL_AMD64_SECTION`` (64-bit).
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63
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64 .. code-block:: none
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65
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66 .section .debug$S,"rn"
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67 .long 4
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68 .long 242
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69 .long 40
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70 .secrel32 _function_name + 0
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71 .secidx _function_name
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72 ...
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73
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74 ``.linkonce`` Directive
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75 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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76
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77 Syntax:
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78
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79 ``.linkonce [ comdat type ]``
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80
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81 Supported COMDAT types:
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82
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83 ``discard``
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84 Discards duplicate sections with the same COMDAT symbol. This is the default
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85 if no type is specified.
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86
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87 ``one_only``
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88 If the symbol is defined multiple times, the linker issues an error.
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89
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90 ``same_size``
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91 Duplicates are discarded, but the linker issues an error if any have
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92 different sizes.
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93
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94 ``same_contents``
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95 Duplicates are discarded, but the linker issues an error if any duplicates
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96 do not have exactly the same content.
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97
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98 ``largest``
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99 Links the largest section from among the duplicates.
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100
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101 ``newest``
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102 Links the newest section from among the duplicates.
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103
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104
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105 .. code-block:: gas
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106
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107 .section .text$foo
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108 .linkonce
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109 ...
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110
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111 ``.section`` Directive
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112 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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113
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114 MC supports passing the information in ``.linkonce`` at the end of
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115 ``.section``. For example, these two codes are equivalent
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116
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117 .. code-block:: gas
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118
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119 .section secName, "dr", discard, "Symbol1"
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120 .globl Symbol1
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121 Symbol1:
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122 .long 1
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123
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124 .. code-block:: gas
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125
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126 .section secName, "dr"
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127 .linkonce discard
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128 .globl Symbol1
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129 Symbol1:
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130 .long 1
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131
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132 Note that in the combined form the COMDAT symbol is explicit. This
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133 extension exists to support multiple sections with the same name in
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134 different COMDATs:
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135
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136
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137 .. code-block:: gas
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138
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139 .section secName, "dr", discard, "Symbol1"
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140 .globl Symbol1
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141 Symbol1:
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142 .long 1
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143
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144 .section secName, "dr", discard, "Symbol2"
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145 .globl Symbol2
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146 Symbol2:
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147 .long 1
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148
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149 In addition to the types allowed with ``.linkonce``, ``.section`` also accepts
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150 ``associative``. The meaning is that the section is linked if a certain other
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151 COMDAT section is linked. This other section is indicated by the comdat symbol
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152 in this directive. It can be any symbol defined in the associated section, but
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153 is usually the associated section's comdat.
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154
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155 The following restrictions apply to the associated section:
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156
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157 1. It must be a COMDAT section.
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158 2. It cannot be another associative COMDAT section.
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159
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173
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160 In the following example the symbol ``sym`` is the comdat symbol of ``.foo``
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150
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161 and ``.bar`` is associated to ``.foo``.
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162
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163 .. code-block:: gas
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164
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165 .section .foo,"bw",discard, "sym"
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166 .section .bar,"rd",associative, "sym"
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167
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168 MC supports these flags in the COFF ``.section`` directive:
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169
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170 - ``b``: BSS section (``IMAGE_SCN_CNT_INITIALIZED_DATA``)
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171 - ``d``: Data section (``IMAGE_SCN_CNT_UNINITIALIZED_DATA``)
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172 - ``n``: Section is not loaded (``IMAGE_SCN_LNK_REMOVE``)
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173 - ``r``: Read-only
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174 - ``s``: Shared section
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175 - ``w``: Writable
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176 - ``x``: Executable section
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177 - ``y``: Not readable
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178 - ``D``: Discardable (``IMAGE_SCN_MEM_DISCARDABLE``)
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179
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180 These flags are all compatible with gas, with the exception of the ``D`` flag,
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181 which gnu as does not support. For gas compatibility, sections with a name
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182 starting with ".debug" are implicitly discardable.
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183
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184
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185 ARM64/COFF-Dependent
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186 --------------------
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187
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188 Relocations
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189 ^^^^^^^^^^^
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190
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191 The following additional symbol variants are supported:
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192
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193 **:secrel_lo12:** generates a relocation that corresponds to the COFF relocation
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194 types ``IMAGE_REL_ARM64_SECREL_LOW12A`` or ``IMAGE_REL_ARM64_SECREL_LOW12L``.
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195
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196 **:secrel_hi12:** generates a relocation that corresponds to the COFF relocation
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197 type ``IMAGE_REL_ARM64_SECREL_HIGH12A``.
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198
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199 .. code-block:: gas
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200
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201 add x0, x0, :secrel_hi12:symbol
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202 ldr x0, [x0, :secrel_lo12:symbol]
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203
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204 add x1, x1, :secrel_hi12:symbol
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205 add x1, x1, :secrel_lo12:symbol
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206 ...
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207
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208
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209 ELF-Dependent
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210 -------------
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211
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212 ``.section`` Directive
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213 ^^^^^^^^^^^^^^^^^^^^^^
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214
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215 In order to support creating multiple sections with the same name and comdat,
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216 it is possible to add an unique number at the end of the ``.section`` directive.
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217 For example, the following code creates two sections named ``.text``.
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218
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219 .. code-block:: gas
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220
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221 .section .text,"ax",@progbits,unique,1
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222 nop
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223
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224 .section .text,"ax",@progbits,unique,2
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225 nop
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226
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227
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228 The unique number is not present in the resulting object at all. It is just used
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229 in the assembler to differentiate the sections.
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230
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231 The 'o' flag is mapped to SHF_LINK_ORDER. If it is present, a symbol
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232 must be given that identifies the section to be placed is the
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233 .sh_link.
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234
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235 .. code-block:: gas
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236
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237 .section .foo,"a",@progbits
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238 .Ltmp:
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239 .section .bar,"ao",@progbits,.Ltmp
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240
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241 which is equivalent to just
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242
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243 .. code-block:: gas
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244
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245 .section .foo,"a",@progbits
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246 .section .bar,"ao",@progbits,.foo
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247
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248 ``.linker-options`` Section (linker options)
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249 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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250
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251 In order to support passing linker options from the frontend to the linker, a
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252 special section of type ``SHT_LLVM_LINKER_OPTIONS`` (usually named
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253 ``.linker-options`` though the name is not significant as it is identified by
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254 the type). The contents of this section is a simple pair-wise encoding of
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255 directives for consideration by the linker. The strings are encoded as standard
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256 null-terminated UTF-8 strings. They are emitted inline to avoid having the
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257 linker traverse the object file for retrieving the value. The linker is
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258 permitted to not honour the option and instead provide a warning/error to the
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259 user that the requested option was not honoured.
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260
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261 The section has type ``SHT_LLVM_LINKER_OPTIONS`` and has the ``SHF_EXCLUDE``
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262 flag to ensure that the section is treated as opaque by linkers which do not
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263 support the feature and will not be emitted into the final linked binary.
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264
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265 This would be equivalent to the follow raw assembly:
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266
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267 .. code-block:: gas
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268
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269 .section ".linker-options","e",@llvm_linker_options
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270 .asciz "option 1"
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271 .asciz "value 1"
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272 .asciz "option 2"
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273 .asciz "value 2"
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274
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275 The following directives are specified:
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276
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277 - lib
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278
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279 The parameter identifies a library to be linked against. The library will
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280 be looked up in the default and any specified library search paths
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281 (specified to this point).
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282
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283 - libpath
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284
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173
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285 The parameter identifies an additional library search path to be considered
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150
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286 when looking up libraries after the inclusion of this option.
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287
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288 ``SHT_LLVM_DEPENDENT_LIBRARIES`` Section (Dependent Libraries)
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289 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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290
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291 This section contains strings specifying libraries to be added to the link by
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292 the linker.
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293
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294 The section should be consumed by the linker and not written to the output.
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295
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296 The strings are encoded as standard null-terminated UTF-8 strings.
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297
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298 For example:
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299
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300 .. code-block:: gas
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301
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302 .section ".deplibs","MS",@llvm_dependent_libraries,1
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303 .asciz "library specifier 1"
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304 .asciz "library specifier 2"
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305
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306 The interpretation of the library specifiers is defined by the consuming linker.
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307
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308 ``SHT_LLVM_CALL_GRAPH_PROFILE`` Section (Call Graph Profile)
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309 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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310
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311 This section is used to pass a call graph profile to the linker which can be
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312 used to optimize the placement of sections. It contains a sequence of
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313 (from symbol, to symbol, weight) tuples.
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314
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315 It shall have a type of ``SHT_LLVM_CALL_GRAPH_PROFILE`` (0x6fff4c02), shall
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316 have the ``SHF_EXCLUDE`` flag set, the ``sh_link`` member shall hold the section
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317 header index of the associated symbol table, and shall have a ``sh_entsize`` of
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318 16. It should be named ``.llvm.call-graph-profile``.
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319
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320 The contents of the section shall be a sequence of ``Elf_CGProfile`` entries.
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321
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322 .. code-block:: c
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323
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324 typedef struct {
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325 Elf_Word cgp_from;
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326 Elf_Word cgp_to;
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327 Elf_Xword cgp_weight;
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328 } Elf_CGProfile;
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329
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330 cgp_from
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331 The symbol index of the source of the edge.
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332
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333 cgp_to
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334 The symbol index of the destination of the edge.
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335
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336 cgp_weight
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337 The weight of the edge.
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338
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339 This is represented in assembly as:
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340
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341 .. code-block:: gas
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342
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343 .cg_profile from, to, 42
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344
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345 ``.cg_profile`` directives are processed at the end of the file. It is an error
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346 if either ``from`` or ``to`` are undefined temporary symbols. If either symbol
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347 is a temporary symbol, then the section symbol is used instead. If either
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348 symbol is undefined, then that symbol is defined as if ``.weak symbol`` has been
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349 written at the end of the file. This forces the symbol to show up in the symbol
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350 table.
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351
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352 ``SHT_LLVM_ADDRSIG`` Section (address-significance table)
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353 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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354
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355 This section is used to mark symbols as address-significant, i.e. the address
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356 of the symbol is used in a comparison or leaks outside the translation unit. It
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357 has the same meaning as the absence of the LLVM attributes ``unnamed_addr``
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358 and ``local_unnamed_addr``.
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359
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360 Any sections referred to by symbols that are not marked as address-significant
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361 in any object file may be safely merged by a linker without breaking the
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362 address uniqueness guarantee provided by the C and C++ language standards.
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363
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364 The contents of the section are a sequence of ULEB128-encoded integers
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365 referring to the symbol table indexes of the address-significant symbols.
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366
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367 There are two associated assembly directives:
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368
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369 .. code-block:: gas
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370
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371 .addrsig
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372
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373 This instructs the assembler to emit an address-significance table. Without
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374 this directive, all symbols are considered address-significant.
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375
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376 .. code-block:: gas
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377
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378 .addrsig_sym sym
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379
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380 This marks ``sym`` as address-significant.
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381
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382 ``SHT_LLVM_SYMPART`` Section (symbol partition specification)
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383 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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384
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385 This section is used to mark symbols with the `partition`_ that they
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386 belong to. An ``.llvm_sympart`` section consists of a null-terminated string
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387 specifying the name of the partition followed by a relocation referring to
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388 the symbol that belongs to the partition. It may be constructed as follows:
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389
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390 .. code-block:: gas
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391
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392 .section ".llvm_sympart","",@llvm_sympart
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393 .asciz "libpartition.so"
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394 .word symbol_in_partition
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395
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396 .. _partition: https://lld.llvm.org/Partitions.html
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397
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398 CodeView-Dependent
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399 ------------------
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400
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401 ``.cv_file`` Directive
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402 ^^^^^^^^^^^^^^^^^^^^^^
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403 Syntax:
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404 ``.cv_file`` *FileNumber FileName* [ *checksum* ] [ *checksumkind* ]
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405
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406 ``.cv_func_id`` Directive
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407 ^^^^^^^^^^^^^^^^^^^^^^^^^
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408 Introduces a function ID that can be used with ``.cv_loc``.
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409
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410 Syntax:
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411 ``.cv_func_id`` *FunctionId*
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412
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413 ``.cv_inline_site_id`` Directive
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414 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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415 Introduces a function ID that can be used with ``.cv_loc``. Includes
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416 ``inlined at`` source location information for use in the line table of the
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417 caller, whether the caller is a real function or another inlined call site.
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418
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419 Syntax:
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173
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420 ``.cv_inline_site_id`` *FunctionId* ``within`` *Function* ``inlined_at`` *FileNumber Line* [ *Column* ]
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150
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421
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422 ``.cv_loc`` Directive
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423 ^^^^^^^^^^^^^^^^^^^^^
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424 The first number is a file number, must have been previously assigned with a
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425 ``.file`` directive, the second number is the line number and optionally the
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426 third number is a column position (zero if not specified). The remaining
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427 optional items are ``.loc`` sub-directives.
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428
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429 Syntax:
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430 ``.cv_loc`` *FunctionId FileNumber* [ *Line* ] [ *Column* ] [ *prologue_end* ] [ ``is_stmt`` *value* ]
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431
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432 ``.cv_linetable`` Directive
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433 ^^^^^^^^^^^^^^^^^^^^^^^^^^^
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434 Syntax:
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435 ``.cv_linetable`` *FunctionId* ``,`` *FunctionStart* ``,`` *FunctionEnd*
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436
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437 ``.cv_inline_linetable`` Directive
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438 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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439 Syntax:
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440 ``.cv_inline_linetable`` *PrimaryFunctionId* ``,`` *FileNumber Line FunctionStart FunctionEnd*
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441
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442 ``.cv_def_range`` Directive
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443 ^^^^^^^^^^^^^^^^^^^^^^^^^^^
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444 The *GapStart* and *GapEnd* options may be repeated as needed.
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445
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446 Syntax:
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447 ``.cv_def_range`` *RangeStart RangeEnd* [ *GapStart GapEnd* ] ``,`` *bytes*
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448
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449 ``.cv_stringtable`` Directive
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450 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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451
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452 ``.cv_filechecksums`` Directive
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453 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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454
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455 ``.cv_filechecksumoffset`` Directive
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456 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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457 Syntax:
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458 ``.cv_filechecksumoffset`` *FileNumber*
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459
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460 ``.cv_fpo_data`` Directive
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461 ^^^^^^^^^^^^^^^^^^^^^^^^^^
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462 Syntax:
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463 ``.cv_fpo_data`` *procsym*
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464
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465 Target Specific Behaviour
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466 =========================
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467
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468 X86
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469 ---
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470
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471 Relocations
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472 ^^^^^^^^^^^
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473
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474 ``@ABS8`` can be applied to symbols which appear as immediate operands to
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475 instructions that have an 8-bit immediate form for that operand. It causes
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476 the assembler to use the 8-bit form and an 8-bit relocation (e.g. ``R_386_8``
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477 or ``R_X86_64_8``) for the symbol.
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478
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479 For example:
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480
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481 .. code-block:: gas
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482
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483 cmpq $foo@ABS8, %rdi
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484
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485 This causes the assembler to select the form of the 64-bit ``cmpq`` instruction
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486 that takes an 8-bit immediate operand that is sign extended to 64 bits, as
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487 opposed to ``cmpq $foo, %rdi`` which takes a 32-bit immediate operand. This
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488 is also not the same as ``cmpb $foo, %dil``, which is an 8-bit comparison.
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489
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490 Windows on ARM
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491 --------------
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492
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493 Stack Probe Emission
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494 ^^^^^^^^^^^^^^^^^^^^
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495
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496 The reference implementation (Microsoft Visual Studio 2012) emits stack probes
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497 in the following fashion:
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498
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499 .. code-block:: gas
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500
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501 movw r4, #constant
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502 bl __chkstk
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503 sub.w sp, sp, r4
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504
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505 However, this has the limitation of 32 MiB (±16MiB). In order to accommodate
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173
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506 larger binaries, LLVM supports the use of ``-mcmodel=large`` to allow a 4GiB
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150
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507 range via a slight deviation. It will generate an indirect jump as follows:
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508
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509 .. code-block:: gas
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510
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511 movw r4, #constant
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512 movw r12, :lower16:__chkstk
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513 movt r12, :upper16:__chkstk
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514 blx r12
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515 sub.w sp, sp, r4
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516
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517 Variable Length Arrays
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518 ^^^^^^^^^^^^^^^^^^^^^^
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519
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520 The reference implementation (Microsoft Visual Studio 2012) does not permit the
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521 emission of Variable Length Arrays (VLAs).
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522
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523 The Windows ARM Itanium ABI extends the base ABI by adding support for emitting
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524 a dynamic stack allocation. When emitting a variable stack allocation, a call
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525 to ``__chkstk`` is emitted unconditionally to ensure that guard pages are setup
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526 properly. The emission of this stack probe emission is handled similar to the
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527 standard stack probe emission.
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528
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529 The MSVC environment does not emit code for VLAs currently.
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530
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531 Windows on ARM64
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532 ----------------
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533
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534 Stack Probe Emission
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|
535 ^^^^^^^^^^^^^^^^^^^^
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536
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537 The reference implementation (Microsoft Visual Studio 2017) emits stack probes
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|
|
538 in the following fashion:
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539
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|
540 .. code-block:: gas
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|
541
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|
542 mov x15, #constant
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|
543 bl __chkstk
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|
544 sub sp, sp, x15, lsl #4
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545
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|
546 However, this has the limitation of 256 MiB (±128MiB). In order to accommodate
|
|
173
|
547 larger binaries, LLVM supports the use of ``-mcmodel=large`` to allow a 8GiB
|
|
150
|
548 (±4GiB) range via a slight deviation. It will generate an indirect jump as
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|
|
549 follows:
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|
550
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|
551 .. code-block:: gas
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|
|
552
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|
|
553 mov x15, #constant
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|
|
554 adrp x16, __chkstk
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|
|
555 add x16, x16, :lo12:__chkstk
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|
|
556 blr x16
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|
|
557 sub sp, sp, x15, lsl #4
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558
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