docs/GoldPlugin.html

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   5   <title>LLVM gold plugin</title>
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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   </ul></li>
  18   <li><a href="#licensing">Licensing</a></li>
  19 </ol>
  20 <div class="doc_author">Written by Nick Lewycky</div>
  21
  22 <!--=========================================================================-->
  23 <div class="doc_section"><a name="introduction">Introduction</a></div>
  24 <!--=========================================================================-->
  25 <div class="doc_text">
  26   <p>Building with link time optimization requires cooperation from the
  27 system linker. LTO support on Linux systems requires that you use
  28 the <a href="http://sourceware.org/binutils">gold linker</a> which supports
  29 LTO via plugins. This is the same system used by the upcoming
  30 <a href="http://gcc.gnu.org/wiki/LinkTimeOptimization">GCC LTO</a>
  31 project.</p>
  32   <p>The LLVM gold plugin implements the
  33 <a href="http://gcc.gnu.org/wiki/whopr/driver">gold plugin interface</a>
  34 on top of
  35 <a href="http://llvm.org/docs/LinkTimeOptimization.html#lto">libLTO</a>.
  36 The same plugin can also be used by other tools such as <tt>ar</tt> and
  37 <tt>nm</tt>.
  38 </div>
  39 <!--=========================================================================-->
  40 <div class="doc_section"><a name="build">How to build it</a></div>
  41 <!--=========================================================================-->
  42 <div class="doc_text">
  43   <p>You need to build gold with plugin support and build the LLVMgold
  44 plugin.</p>
  45 <ul>
  46   <li>Build gold with plugin support:
  47     <pre class="doc_code">
  48 mkdir binutils
  49 cd binutils
  50 cvs -z 9 -d :pserver:anoncvs@sourceware.org:/cvs/src login
  51 <em>{enter "anoncvs" as the password}</em>
  52 cvs -z 9 -d :pserver:anoncvs@sourceware.org:/cvs/src co src
  53 mkdir build
  54 cd build
  55 ../src/configure --enable-gold --enable-plugins
  56 make all-gold
  57 </pre>
  58     That should leave you with binutils/build/gold/ld-new which supports the
  59 <tt>-plugin</tt> option.
  60
  61     <li>Build the LLVMgold plugin: Configure LLVM with
  62     <tt>--with-binutils-include=/path/to/binutils/src/include</tt> and run
  63     <tt>make</tt>.
  64 </ul>
  65 </div>
  66 <!--=========================================================================-->
  67 <div class="doc_section"><a name="usage">Usage</a></div>
  68 <!--=========================================================================-->
  69 <div class="doc_text">
  70   <p>The linker takes a <tt>-plugin</tt> option that points to the path of
  71   the plugin <tt>.so</tt> file. To find out what link command <tt>gcc</tt>
  72   would run in a given situation, run <tt>gcc -v <em>[...]</em></tt> and look
  73   for the line where it runs <tt>collect2</tt>. Replace that with
  74   <tt>ld-new -plugin /path/to/LLVMgold.so</tt> to test it out. Once you're
  75   ready to switch to using gold, backup your existing <tt>/usr/bin/ld</tt>
  76   then replace it with <tt>ld-new</tt>.</p>
  77   <p>You can produce bitcode files from <tt>llvm-gcc</tt> using
  78   <tt>-emit-llvm</tt> or <tt>-flto</tt>, or the <tt>-O4</tt> flag which is
  79   synonymous with <tt>-O3 -flto</tt>.</p>
  80   <p><tt>llvm-gcc</tt> has a <tt>-use-gold-plugin</tt> option which looks
  81   for the gold plugin in the same directories as it looks for <tt>cc1</tt> and
  82   passes the <tt>-plugin</tt> option to ld. It will not look for an alternate
  83   linker, which is why you need gold to be the installed system linker in your
  84   path.</p>
  85 </div>
  86
  87 <!-- ======================================================================= -->
  88 <div class="doc_subsection">
  89   <a name="example1">Example of link time optimization</a>
  90 </div>
  91
  92 <div class="doc_text">
  93   <p>The following example shows a worked example of the gold plugin mixing
  94   LLVM bitcode and native code.
  95 <pre class="doc_code">
  96 --- a.c ---
  97 #include &lt;stdio.h&gt;
  98
  99 extern void foo1(void);
 100 extern void foo4(void);
 101
 102 void foo2(void) {
 103   printf("Foo2\n");
 104 }
 105
 106 void foo3(void) {
 107   foo4();
 108 }
 109
 110 int main(void) {
 111   foo1();
 112 }
 113
 114 --- b.c ---
 115 #include &lt;stdio.h&gt;
 116
 117 extern void foo2(void);
 118
 119 void foo1(void) {
 120   foo2();
 121 }
 122
 123 void foo4(void) {
 124   printf("Foo4");
 125 }
 126
 127 --- command lines ---
 128 $ llvm-gcc -flto a.c -c -o a.o              # &lt;-- a.o is LLVM bitcode file
 129 $ llvm-gcc b.c -c -o b.o                    # &lt;-- b.o is native object file
 130 $ llvm-gcc -use-gold-plugin a.o b.o -o main # &lt;-- link with LLVMgold plugin
 131 </pre>
 132   <p>Gold informs the plugin that foo3 is never referenced outside the IR,
 133   leading LLVM to delete that function. However, unlike in the
 134   <a href="http://llvm.org/docs/LinkTimeOptimization.html#example1">libLTO
 135   example</a> gold does not currently eliminate foo4.</p>
 136 </div>
 137
 138 <!--=========================================================================-->
 139 <div class="doc_section"><a name="licensing">Licensing</a></div>
 140 <!--=========================================================================-->
 141 <div class="doc_text">
 142 Gold is licensed under the GPLv3. LLVMgold uses the interface file
 143 <tt>plugin-api.h</tt> from gold which means that the resulting LLVMgold.so
 144 binary is also GPLv3. This can still be used to link non-GPLv3 programs just
 145 as much as gold could without the plugin.
 146 </div>
 147
 148 <!-- *********************************************************************** -->
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 155   <a href="mailto:nicholas@metrix.on.ca">Nick Lewycky</a><br>
 156   <a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br>
 157   Last modified: $Date: 2009-01-01 23:10:51 -0800 (Thu, 01 Jan 2009) $
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