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annotate docs/GoldPlugin.rst @ 33:e4204d083e25 LLVM3.5

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LLVM 3.5
author Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
date Thu, 12 Dec 2013 14:32:10 +0900
parents 95c75e76d11b
children 60c9769439b8
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1 ====================
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2 The LLVM gold plugin
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3 ====================
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4
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5 Introduction
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6 ============
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7
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8 Building with link time optimization requires cooperation from
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9 the system linker. LTO support on Linux systems requires that you use the
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10 `gold linker`_ which supports LTO via plugins. This is the same mechanism
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11 used by the `GCC LTO`_ project.
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12
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13 The LLVM gold plugin implements the gold plugin interface on top of
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14 :ref:`libLTO`. The same plugin can also be used by other tools such as
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15 ``ar`` and ``nm``.
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16
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17 .. _`gold linker`: http://sourceware.org/binutils
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18 .. _`GCC LTO`: http://gcc.gnu.org/wiki/LinkTimeOptimization
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19 .. _`gold plugin interface`: http://gcc.gnu.org/wiki/whopr/driver
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20
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21 .. _lto-how-to-build:
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22
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23 How to build it
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24 ===============
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25
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26 You need to have gold with plugin support and build the LLVMgold plugin.
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27 Check whether you have gold running ``/usr/bin/ld -v``. It will report "GNU
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28 gold" or else "GNU ld" if not. If you have gold, check for plugin support
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29 by running ``/usr/bin/ld -plugin``. If it complains "missing argument" then
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30 you have plugin support. If not, such as an "unknown option" error then you
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31 will either need to build gold or install a version with plugin support.
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32
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33 * Download, configure and build gold with plugin support:
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34
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35 .. code-block:: bash
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36
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37 $ git clone --depth 1 git://sourceware.org/git/binutils-gdb.git binutils
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38 $ mkdir build
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39 $ cd build
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40 $ ../binutils/configure --enable-gold --enable-plugins --disable-werror
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41 $ make all-gold
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42
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43 That should leave you with ``build/gold/ld-new`` which supports
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44 the ``-plugin`` option. Running ``make`` will additionally build
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45 ``build/binutils/ar`` and ``nm-new`` binaries supporting plugins.
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46
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47 * Build the LLVMgold plugin: Configure LLVM with
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48 ``--with-binutils-include=/path/to/binutils/include`` and run
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49 ``make``.
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50
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51 Usage
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52 =====
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53
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54 The linker takes a ``-plugin`` option that points to the path of
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55 the plugin ``.so`` file. To find out what link command ``gcc``
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56 would run in a given situation, run ``gcc -v [...]`` and
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57 look for the line where it runs ``collect2``. Replace that with
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58 ``ld-new -plugin /path/to/LLVMgold.so`` to test it out. Once you're
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59 ready to switch to using gold, backup your existing ``/usr/bin/ld``
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60 then replace it with ``ld-new``.
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61
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62 You should produce bitcode files from ``clang`` with the option
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63 ``-flto``. This flag will also cause ``clang`` to look for the gold plugin in
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64 the ``lib`` directory under its prefix and pass the ``-plugin`` option to
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65 ``ld``. It will not look for an alternate linker, which is why you need
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66 gold to be the installed system linker in your path.
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67
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68 ``ar`` and ``nm`` also accept the ``-plugin`` option and it's possible to
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69 to install ``LLVMgold.so`` to ``/usr/lib/bfd-plugins`` for a seamless setup.
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70 If you built your own gold, be sure to install the ``ar`` and ``nm-new`` you
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71 built to ``/usr/bin``.
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72
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73
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74 Example of link time optimization
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75 ---------------------------------
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76
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77 The following example shows a worked example of the gold plugin mixing LLVM
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78 bitcode and native code.
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79
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80 .. code-block:: c
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81
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82 --- a.c ---
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83 #include <stdio.h>
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84
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85 extern void foo1(void);
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86 extern void foo4(void);
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87
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88 void foo2(void) {
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89 printf("Foo2\n");
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90 }
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91
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92 void foo3(void) {
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93 foo4();
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94 }
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95
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96 int main(void) {
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97 foo1();
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98 }
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99
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100 --- b.c ---
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101 #include <stdio.h>
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102
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103 extern void foo2(void);
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104
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105 void foo1(void) {
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106 foo2();
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107 }
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108
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109 void foo4(void) {
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110 printf("Foo4");
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111 }
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112
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113 .. code-block:: bash
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114
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115 --- command lines ---
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116 $ clang -flto a.c -c -o a.o # <-- a.o is LLVM bitcode file
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117 $ ar q a.a a.o # <-- a.a is an archive with LLVM bitcode
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118 $ clang b.c -c -o b.o # <-- b.o is native object file
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119 $ clang -flto a.a b.o -o main # <-- link with LLVMgold plugin
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120
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121 Gold informs the plugin that foo3 is never referenced outside the IR,
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122 leading LLVM to delete that function. However, unlike in the :ref:`libLTO
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123 example <libLTO-example>` gold does not currently eliminate foo4.
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124
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125 Quickstart for using LTO with autotooled projects
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126 =================================================
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127
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128 Once your system ``ld``, ``ar``, and ``nm`` all support LLVM bitcode,
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129 everything is in place for an easy to use LTO build of autotooled projects:
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130
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131 * Follow the instructions :ref:`on how to build LLVMgold.so
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132 <lto-how-to-build>`.
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133
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134 * Install the newly built binutils to ``$PREFIX``
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135
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136 * Copy ``Release/lib/LLVMgold.so`` to ``$PREFIX/lib/bfd-plugins/``
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137
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138 * Set environment variables (``$PREFIX`` is where you installed clang and
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139 binutils):
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140
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141 .. code-block:: bash
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142
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143 export CC="$PREFIX/bin/clang -flto"
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144 export CXX="$PREFIX/bin/clang++ -flto"
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145 export AR="$PREFIX/bin/ar"
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146 export NM="$PREFIX/bin/nm"
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147 export RANLIB=/bin/true #ranlib is not needed, and doesn't support .bc files in .a
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148
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149 * Or you can just set your path:
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150
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151 .. code-block:: bash
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152
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153 export PATH="$PREFIX/bin:$PATH"
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154 export CC="clang -flto"
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155 export CXX="clang++ -flto"
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156 export RANLIB=/bin/true
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157 * Configure and build the project as usual:
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158
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159 .. code-block:: bash
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160
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161 % ./configure && make && make check
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162
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163 The environment variable settings may work for non-autotooled projects too,
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164 but you may need to set the ``LD`` environment variable as well.
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165
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166 Licensing
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167 =========
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168
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169 Gold is licensed under the GPLv3. LLVMgold uses the interface file
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170 ``plugin-api.h`` from gold which means that the resulting ``LLVMgold.so``
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171 binary is also GPLv3. This can still be used to link non-GPLv3 programs
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172 just as much as gold could without the plugin.