docs/GoldPlugin.html

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  10 <h1>LLVM gold plugin</h1>
  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 <h2><a name="introduction">Introduction</a></h2>
  25 <!--=========================================================================-->
  26 <div>
  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 mechanism used by the
  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="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 <h2><a name="build">How to build it</a></h2>
  42 <!--=========================================================================-->
  43 <div>
  44   <p>You need to have gold with plugin support and build the LLVMgold
  45 plugin. Check whether you have gold running <tt>/usr/bin/ld -v</tt>. It will
  46 report &#8220;GNU gold&#8221; or else &#8220GNU ld&#8221; if not. If you have
  47 gold, check for plugin support by running <tt>/usr/bin/ld -plugin</tt>. If it
  48 complains &#8220missing argument&#8221 then you have plugin support. If not,
  49 such as an &#8220;unknown option&#8221; error then you will either need to
  50 build gold or install a version with plugin support.</p>
  51 <ul>
  52   <li>To build gold with plugin support:
  53     <pre class="doc_code">
  54 mkdir binutils
  55 cd binutils
  56 cvs -z 9 -d :pserver:anoncvs@sourceware.org:/cvs/src login
  57 <em>{enter "anoncvs" as the password}</em>
  58 cvs -z 9 -d :pserver:anoncvs@sourceware.org:/cvs/src co binutils
  59 mkdir build
  60 cd build
  61 ../src/configure --enable-gold --enable-plugins
  62 make all-gold
  63 </pre>
  64     That should leave you with <tt>binutils/build/gold/ld-new</tt> which supports the <tt>-plugin</tt> option. It also built would have
  65 <tt>binutils/build/binutils/ar</tt> and <tt>nm-new</tt> which support plugins
  66 but don't have a visible -plugin option, instead relying on the gold plugin
  67 being present in <tt>../lib/bfd-plugins</tt> relative to where the binaries are
  68 placed.
  69     <li>Build the LLVMgold plugin: Configure LLVM with
  70     <tt>--with-binutils-include=/path/to/binutils/src/include</tt> and run
  71     <tt>make</tt>.
  72 </ul>
  73 </div>
  74 <!--=========================================================================-->
  75 <h2><a name="usage">Usage</a></h2>
  76 <!--=========================================================================-->
  77 <div>
  78   <p>The linker takes a <tt>-plugin</tt> option that points to the path of
  79   the plugin <tt>.so</tt> file. To find out what link command <tt>gcc</tt>
  80   would run in a given situation, run <tt>gcc -v <em>[...]</em></tt> and look
  81   for the line where it runs <tt>collect2</tt>. Replace that with
  82   <tt>ld-new -plugin /path/to/LLVMgold.so</tt> to test it out. Once you're
  83   ready to switch to using gold, backup your existing <tt>/usr/bin/ld</tt>
  84   then replace it with <tt>ld-new</tt>.</p>
  85   <p>You can produce bitcode files from <tt>llvm-gcc</tt> using
  86   <tt>-emit-llvm</tt> or <tt>-flto</tt>, or the <tt>-O4</tt> flag which is
  87   synonymous with <tt>-O3 -flto</tt>.</p>
  88   <p><tt>llvm-gcc</tt> has a <tt>-use-gold-plugin</tt> option which looks
  89   for the gold plugin in the same directories as it looks for <tt>cc1</tt> and
  90   passes the <tt>-plugin</tt> option to ld. It will not look for an alternate
  91   linker, which is why you need gold to be the installed system linker in your
  92   path.</p>
  93   <p>If you want <tt>ar</tt> and <tt>nm</tt> to work seamlessly as well, install
  94   <tt>LLVMgold.so</tt> to <tt>/usr/lib/bfd-plugins</tt>. If you built your
  95   own gold, be sure to install the <tt>ar</tt> and <tt>nm-new</tt> you built to
  96   <tt>/usr/bin</tt>.
  97   <p>
  98
  99 <!-- ======================================================================= -->
 100 <h3>
 101   <a name="example1">Example of link time optimization</a>
 102 </h3>
 103
 104 <div>
 105   <p>The following example shows a worked example of the gold plugin mixing
 106   LLVM bitcode and native code.
 107 <pre class="doc_code">
 108 --- a.c ---
 109 #include &lt;stdio.h&gt;
 110
 111 extern void foo1(void);
 112 extern void foo4(void);
 113
 114 void foo2(void) {
 115   printf("Foo2\n");
 116 }
 117
 118 void foo3(void) {
 119   foo4();
 120 }
 121
 122 int main(void) {
 123   foo1();
 124 }
 125
 126 --- b.c ---
 127 #include &lt;stdio.h&gt;
 128
 129 extern void foo2(void);
 130
 131 void foo1(void) {
 132   foo2();
 133 }
 134
 135 void foo4(void) {
 136   printf("Foo4");
 137 }
 138
 139 --- command lines ---
 140 $ llvm-gcc -flto a.c -c -o a.o              # &lt;-- a.o is LLVM bitcode file
 141 $ ar q a.a a.o                              # &lt;-- a.a is an archive with LLVM bitcode
 142 $ llvm-gcc b.c -c -o b.o                    # &lt;-- b.o is native object file
 143 $ llvm-gcc -use-gold-plugin a.a b.o -o main # &lt;-- link with LLVMgold plugin
 144 </pre>
 145   <p>Gold informs the plugin that foo3 is never referenced outside the IR,
 146   leading LLVM to delete that function. However, unlike in the
 147   <a href="LinkTimeOptimization.html#example1">libLTO
 148   example</a> gold does not currently eliminate foo4.</p>
 149 </div>
 150
 151 </div>
 152
 153 <!--=========================================================================-->
 154 <h2>
 155   <a name="lto_autotools">
 156     Quickstart for using LTO with autotooled projects
 157   </a>
 158 </h2>
 159 <!--=========================================================================-->
 160 <div>
 161   <p>Once your system <tt>ld</tt>, <tt>ar</tt> and <tt>nm</tt> all support LLVM
 162   bitcode, everything is in place for an easy to use LTO build of autotooled
 163   projects:</p>
 164   <ul>
 165     <li>Follow the instructions <a href="#build">on how to build LLVMgold.so</a>.</li>
 166     <li>Install the newly built binutils to <tt>$PREFIX</tt></li>
 167     <li>Copy <tt>Release/lib/LLVMgold.so</tt> to
 168     <tt>$PREFIX/libexec/gcc/x86_64-unknown-linux-gnu/4.2.1/</tt> and
 169     <tt>$PREFIX/lib/bfd-plugins/</tt></li>
 170     <li>Set environment variables (<tt>$PREFIX</tt> is where you installed llvm-gcc and
 171     binutils):
 172     <pre class="doc_code">
 173 export CC="$PREFIX/bin/llvm-gcc -use-gold-plugin"
 174 export CXX="$PREFIX/bin/llvm-g++ -use-gold-plugin"
 175 export AR="$PREFIX/bin/ar"
 176 export NM="$PREFIX/bin/nm"
 177 export RANLIB=/bin/true #ranlib is not needed, and doesn't support .bc files in .a
 178 export CFLAGS="-O4"
 179 </pre>
 180      </li>
 181      <li>Or you can just set your path:
 182     <pre class="doc_code">
 183 export PATH="$PREFIX/bin:$PATH"
 184 export CC="llvm-gcc -use-gold-plugin"
 185 export CXX="llvm-g++ -use-gold-plugin"
 186 export RANLIB=/bin/true
 187 export CFLAGS="-O4"
 188 </pre>
 189      </li>
 190      <li>Configure &amp; build the project as usual: <tt>./configure &amp;&amp; make &amp;&amp; make check</tt> </li>
 191    </ul>
 192    <p> The environment variable settings may work for non-autotooled projects
 193    too, but you may need to set the <tt>LD</tt> environment variable as well.</p>
 194 </div>
 195
 196 <!--=========================================================================-->
 197 <h2><a name="licensing">Licensing</a></h2>
 198 <!--=========================================================================-->
 199 <div>
 200   <p>Gold is licensed under the GPLv3. LLVMgold uses the interface file
 201 <tt>plugin-api.h</tt> from gold which means that the resulting LLVMgold.so
 202 binary is also GPLv3. This can still be used to link non-GPLv3 programs just
 203 as much as gold could without the plugin.</p>
 204 </div>
 205
 206 <!-- *********************************************************************** -->
 207 <hr>
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 213   <a href="mailto:nicholas@metrix.on.ca">Nick Lewycky</a><br>
 214   <a href="http://llvm.org/">The LLVM Compiler Infrastructure</a><br>
 215   Last modified: $Date: 2010-04-16 23:58:21 -0800 (Fri, 16 Apr 2010) $
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