docs/GoldPlugin.html

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  10 <div class="doc_title">LLVM gold plugin</div>
  11 <ol>
  12   <li><a href="#introduction">Introduction</a></li>
  13   <li><a href="#build">How to build it</a></li>
  14   <li><a href="#usage">Usage</a>
  15   <ul>
  16     <li><a href="#example1">Example of link time optimization</a></li>
  17     <li><a href="#lto_autotools">Quickstart for using LTO with autotooled projects</a></li>
  18   </ul></li>
  19   <li><a href="#licensing">Licensing</a></li>
  20 </ol>
  21 <div class="doc_author">Written by Nick Lewycky</div>
  22
  23 <!--=========================================================================-->
  24 <div class="doc_section"><a name="introduction">Introduction</a></div>
  25 <!--=========================================================================-->
  26 <div class="doc_text">
  27   <p>Building with link time optimization requires cooperation from the
  28 system linker. LTO support on Linux systems requires that you use
  29 the <a href="http://sourceware.org/binutils">gold linker</a> which supports
  30 LTO via plugins. This is the same system used by the upcoming
  31 <a href="http://gcc.gnu.org/wiki/LinkTimeOptimization">GCC LTO</a>
  32 project.</p>
  33   <p>The LLVM gold plugin implements the
  34 <a href="http://gcc.gnu.org/wiki/whopr/driver">gold plugin interface</a>
  35 on top of
  36 <a href="http://llvm.org/docs/LinkTimeOptimization.html#lto">libLTO</a>.
  37 The same plugin can also be used by other tools such as <tt>ar</tt> and
  38 <tt>nm</tt>.
  39 </div>
  40 <!--=========================================================================-->
  41 <div class="doc_section"><a name="build">How to build it</a></div>
  42 <!--=========================================================================-->
  43 <div class="doc_text">
  44   <p>You need to build gold with plugin support and build the LLVMgold
  45 plugin.</p>
  46 <ul>
  47   <li>Build gold with plugin support:
  48     <pre class="doc_code">
  49 mkdir binutils
  50 cd binutils
  51 cvs -z 9 -d :pserver:anoncvs@sourceware.org:/cvs/src login
  52 <em>{enter "anoncvs" as the password}</em>
  53 cvs -z 9 -d :pserver:anoncvs@sourceware.org:/cvs/src co src
  54 mkdir build
  55 cd build
  56 ../src/configure --enable-gold --enable-plugins
  57 make all-gold
  58 </pre>
  59     That should leave you with binutils/build/gold/ld-new which supports the
  60 <tt>-plugin</tt> option.
  61
  62     <li>Build the LLVMgold plugin: Configure LLVM with
  63     <tt>--with-binutils-include=/path/to/binutils/src/include</tt> and run
  64     <tt>make</tt>.
  65 </ul>
  66 </div>
  67 <!--=========================================================================-->
  68 <div class="doc_section"><a name="usage">Usage</a></div>
  69 <!--=========================================================================-->
  70 <div class="doc_text">
  71   <p>The linker takes a <tt>-plugin</tt> option that points to the path of
  72   the plugin <tt>.so</tt> file. To find out what link command <tt>gcc</tt>
  73   would run in a given situation, run <tt>gcc -v <em>[...]</em></tt> and look
  74   for the line where it runs <tt>collect2</tt>. Replace that with
  75   <tt>ld-new -plugin /path/to/LLVMgold.so</tt> to test it out. Once you're
  76   ready to switch to using gold, backup your existing <tt>/usr/bin/ld</tt>
  77   then replace it with <tt>ld-new</tt>.</p>
  78   <p>You can produce bitcode files from <tt>llvm-gcc</tt> using
  79   <tt>-emit-llvm</tt> or <tt>-flto</tt>, or the <tt>-O4</tt> flag which is
  80   synonymous with <tt>-O3 -flto</tt>.</p>
  81   <p><tt>llvm-gcc</tt> has a <tt>-use-gold-plugin</tt> option which looks
  82   for the gold plugin in the same directories as it looks for <tt>cc1</tt> and
  83   passes the <tt>-plugin</tt> option to ld. It will not look for an alternate
  84   linker, which is why you need gold to be the installed system linker in your
  85   path.</p>
  86 </div>
  87
  88 <!-- ======================================================================= -->
  89 <div class="doc_subsection">
  90   <a name="example1">Example of link time optimization</a>
  91 </div>
  92
  93 <div class="doc_text">
  94   <p>The following example shows a worked example of the gold plugin mixing
  95   LLVM bitcode and native code.
  96 <pre class="doc_code">
  97 --- a.c ---
  98 #include &lt;stdio.h&gt;
  99
 100 extern void foo1(void);
 101 extern void foo4(void);
 102
 103 void foo2(void) {
 104   printf("Foo2\n");
 105 }
 106
 107 void foo3(void) {
 108   foo4();
 109 }
 110
 111 int main(void) {
 112   foo1();
 113 }
 114
 115 --- b.c ---
 116 #include &lt;stdio.h&gt;
 117
 118 extern void foo2(void);
 119
 120 void foo1(void) {
 121   foo2();
 122 }
 123
 124 void foo4(void) {
 125   printf("Foo4");
 126 }
 127
 128 --- command lines ---
 129 $ llvm-gcc -flto a.c -c -o a.o              # &lt;-- a.o is LLVM bitcode file
 130 $ ar q a.a a.o                              # &lt;-- a.a is an archive with LLVM bitcode
 131 $ llvm-gcc b.c -c -o b.o                    # &lt;-- b.o is native object file
 132 $ llvm-gcc -use-gold-plugin a.a b.o -o main # &lt;-- link with LLVMgold plugin
 133 </pre>
 134   <p>Gold informs the plugin that foo3 is never referenced outside the IR,
 135   leading LLVM to delete that function. However, unlike in the
 136   <a href="http://llvm.org/docs/LinkTimeOptimization.html#example1">libLTO
 137   example</a> gold does not currently eliminate foo4.</p>
 138 </div>
 139
 140 <!--=========================================================================-->
 141 <div class="doc_section"><a name="lto_autotools">Quickstart for using LTO with autotooled projects</a></div>
 142 <!--=========================================================================-->
 143 <div class="doc_text">
 144   <p><tt>gold</tt>, <tt>ar</tt> and <tt>nm</tt> all support plugins now, so everything should be
 145   in place for an easy to use LTO build of autotooled projects:</p>
 146   <ul>
 147     <li>Follow the instructions <a href="#build">on how to build libLLVMgold.so</a>.</li>
 148     <li>Install the newly built binutils to <tt>$PREFIX</tt></li>
 149     <li>Copy <tt>Release/lib/libLLVMgold.so</tt> to
 150     <tt>$PREFIX/libexec/gcc/x86_64-unknown-linux-gnu/4.2.1/</tt> and
 151     <tt>$PREFIX/lib/bfd-plugins/</tt></li>
 152     <li>Set environment variables (<tt>$PREFIX</tt> is where you installed llvm-gcc and
 153     binutils):
 154     <pre class="doc_code">
 155 export CC="$PREFIX/bin/llvm-gcc -use-gold-plugin"
 156 export CXX="$PREFIX/bin/llvm-g++ -use-gold-plugin"
 157 export AR="$PREFIX/bin/ar"
 158 export NM="$PREFIX/bin/nm"
 159 export RANLIB=/bin/true #ranlib is not needed, and doesn't support .bc files in .a
 160 export CFLAGS="-O4"
 161 </pre>
 162      </li>
 163      <li>Or you can just set your path:
 164     <pre class="doc_code">
 165 export PATH="$PREFIX/bin:$PATH"
 166 export CC="llvm-gcc -use-gold-plugin"
 167 export CXX="llvm-g++ -use-gold-plugin"
 168 export RANLIB=/bin/true
 169 export CFLAGS="-O4"
 170 </pre>
 171      </li>
 172      <li>Configure &amp; build the project as usual: <tt>./configure &amp;&amp; make &amp;&amp; make check</tt> </li>
 173    </ul>
 174    <p> The environment variable settings may work for non-autotooled projects
 175    too, but you may need to set the <tt>LD</tt> environment variable as well.</p>
 176 </div>
 177
 178 <!--=========================================================================-->
 179 <div class="doc_section"><a name="licensing">Licensing</a></div>
 180 <!--=========================================================================-->
 181 <div class="doc_text">
 182   <p>Gold is licensed under the GPLv3. LLVMgold uses the interface file
 183 <tt>plugin-api.h</tt> from gold which means that the resulting LLVMgold.so
 184 binary is also GPLv3. This can still be used to link non-GPLv3 programs just
 185 as much as gold could without the plugin.</p>
 186 </div>
 187
 188 <!-- *********************************************************************** -->
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 195   <a href="mailto:nicholas@metrix.on.ca">Nick Lewycky</a><br>
 196   <a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br>
 197   Last modified: $Date: 2009-01-01 23:10:51 -0800 (Thu, 01 Jan 2009) $
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