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author Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
date Sun, 31 Jan 2016 17:34:49 +0900
parents afa8332a0e37
children 1172e4bd9c6f
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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:
0
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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:: gas
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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:: gas
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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
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71 .secidx _function_name
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72 ...
0
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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
77
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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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160 In the following example the symobl ``sym`` is the comdat symbol of ``.foo``
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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
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169 ELF-Dependent
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170 -------------
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171
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172 ``.section`` Directive
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173 ^^^^^^^^^^^^^^^^^^^^^^
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174
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175 In order to support creating multiple sections with the same name and comdat,
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176 it is possible to add an unique number at the end of the ``.seciton`` directive.
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177 For example, the following code creates two sections named ``.text``.
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178
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179 .. code-block:: gas
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180
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181 .section .text,"ax",@progbits,unique,1
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182 nop
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183
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184 .section .text,"ax",@progbits,unique,2
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185 nop
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186
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187
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188 The unique number is not present in the resulting object at all. It is just used
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189 in the assembler to differentiate the sections.
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190
77
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191 Target Specific Behaviour
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192 =========================
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193
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194 Windows on ARM
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195 --------------
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196
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197 Stack Probe Emission
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198 ^^^^^^^^^^^^^^^^^^^^
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199
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200 The reference implementation (Microsoft Visual Studio 2012) emits stack probes
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201 in the following fashion:
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202
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203 .. code-block:: gas
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204
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205 movw r4, #constant
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206 bl __chkstk
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207 sub.w sp, sp, r4
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208
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209 However, this has the limitation of 32 MiB (±16MiB). In order to accommodate
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210 larger binaries, LLVM supports the use of ``-mcode-model=large`` to allow a 4GiB
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211 range via a slight deviation. It will generate an indirect jump as follows:
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212
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213 .. code-block:: gas
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214
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215 movw r4, #constant
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216 movw r12, :lower16:__chkstk
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217 movt r12, :upper16:__chkstk
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218 blx r12
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219 sub.w sp, sp, r4
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220
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221 Variable Length Arrays
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222 ^^^^^^^^^^^^^^^^^^^^^^
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223
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224 The reference implementation (Microsoft Visual Studio 2012) does not permit the
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225 emission of Variable Length Arrays (VLAs).
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226
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227 The Windows ARM Itanium ABI extends the base ABI by adding support for emitting
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228 a dynamic stack allocation. When emitting a variable stack allocation, a call
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229 to ``__chkstk`` is emitted unconditionally to ensure that guard pages are setup
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230 properly. The emission of this stack probe emission is handled similar to the
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231 standard stack probe emission.
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232
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233 The MSVC environment does not emit code for VLAs currently.
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234