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  <title>LLVM gold plugin</title>
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<h1>LLVM gold plugin</h1>
<ol>
  <li><a href="#introduction">Introduction</a></li>
  <li><a href="#build">How to build it</a></li>
  <li><a href="#usage">Usage</a>
  <ul>
    <li><a href="#example1">Example of link time optimization</a></li>
    <li><a href="#lto_autotools">Quickstart for using LTO with autotooled projects</a></li>
  </ul></li>
  <li><a href="#licensing">Licensing</a></li>
</ol>
<div class="doc_author">Written by Nick Lewycky</div>

<!--=========================================================================-->
<h2><a name="introduction">Introduction</a></h2>
<!--=========================================================================-->
<div>
  <p>Building with link time optimization requires cooperation from the
system linker. LTO support on Linux systems requires that you use
the <a href="http://sourceware.org/binutils">gold linker</a> which supports
LTO via plugins. This is the same mechanism used by the
<a href="http://gcc.gnu.org/wiki/LinkTimeOptimization">GCC LTO</a>
project.</p>
  <p>The LLVM gold plugin implements the
<a href="http://gcc.gnu.org/wiki/whopr/driver">gold plugin interface</a>
on top of
<a href="LinkTimeOptimization.html#lto">libLTO</a>.
The same plugin can also be used by other tools such as <tt>ar</tt> and
<tt>nm</tt>.
</div>
<!--=========================================================================-->
<h2><a name="build">How to build it</a></h2>
<!--=========================================================================-->
<div>
  <p>You need to have gold with plugin support and build the LLVMgold
plugin. Check whether you have gold running <tt>/usr/bin/ld -v</tt>. It will
report &#8220;GNU gold&#8221; or else &#8220GNU ld&#8221; if not. If you have
gold, check for plugin support by running <tt>/usr/bin/ld -plugin</tt>. If it
complains &#8220missing argument&#8221 then you have plugin support. If not,
such as an &#8220;unknown option&#8221; error then you will either need to
build gold or install a version with plugin support.</p>
<ul>
  <li>To build gold with plugin support:
    <pre class="doc_code">
mkdir binutils
cd binutils
cvs -z 9 -d :pserver:anoncvs@sourceware.org:/cvs/src login
<em>{enter "anoncvs" as the password}</em>
cvs -z 9 -d :pserver:anoncvs@sourceware.org:/cvs/src co binutils
mkdir build
cd build
../src/configure --enable-gold --enable-plugins
make all-gold
</pre>
    That should leave you with <tt>binutils/build/gold/ld-new</tt> which supports the <tt>-plugin</tt> option. It also built would have
<tt>binutils/build/binutils/ar</tt> and <tt>nm-new</tt> which support plugins
but don't have a visible -plugin option, instead relying on the gold plugin
being present in <tt>../lib/bfd-plugins</tt> relative to where the binaries are
placed.
    <li>Build the LLVMgold plugin: Configure LLVM with
    <tt>--with-binutils-include=/path/to/binutils/src/include</tt> and run
    <tt>make</tt>.
</ul>
</div>
<!--=========================================================================-->
<h2><a name="usage">Usage</a></h2>
<!--=========================================================================-->
<div>

  <p>The linker takes a <tt>-plugin</tt> option that points to the path of
  the plugin <tt>.so</tt> file. To find out what link command <tt>gcc</tt>
  would run in a given situation, run <tt>gcc -v <em>[...]</em></tt> and look
  for the line where it runs <tt>collect2</tt>. Replace that with
  <tt>ld-new -plugin /path/to/LLVMgold.so</tt> to test it out. Once you're
  ready to switch to using gold, backup your existing <tt>/usr/bin/ld</tt>
  then replace it with <tt>ld-new</tt>.</p>

  <p>You can produce bitcode files from <tt>clang</tt> using
  <tt>-emit-llvm</tt> or <tt>-flto</tt>, or the <tt>-O4</tt> flag which is
  synonymous with <tt>-O3 -flto</tt>.</p>

  <p>Any of these flags will also cause <tt>clang</tt> to look for the
  gold plugin in the <tt>lib</tt> directory under its prefix and pass the
  <tt>-plugin</tt> option to <tt>ld</tt>. It will not look for an alternate
  linker, which is why you need gold to be the installed system linker in
  your path.</p>

  <p>If you want <tt>ar</tt> and <tt>nm</tt> to work seamlessly as well, install
  <tt>LLVMgold.so</tt> to <tt>/usr/lib/bfd-plugins</tt>. If you built your
  own gold, be sure to install the <tt>ar</tt> and <tt>nm-new</tt> you built to
  <tt>/usr/bin</tt>.<p>

<!-- ======================================================================= -->
<h3>
  <a name="example1">Example of link time optimization</a>
</h3>

<div>
  <p>The following example shows a worked example of the gold plugin mixing
  LLVM bitcode and native code.
<pre class="doc_code">
--- a.c ---
#include &lt;stdio.h&gt;

extern void foo1(void);
extern void foo4(void);

void foo2(void) {
  printf("Foo2\n");
}

void foo3(void) {
  foo4();
}

int main(void) {
  foo1();
}

--- b.c ---
#include &lt;stdio.h&gt;

extern void foo2(void);

void foo1(void) {
  foo2();
}

void foo4(void) {
  printf("Foo4");
}

--- command lines ---
$ clang -flto a.c -c -o a.o      # &lt;-- a.o is LLVM bitcode file
$ ar q a.a a.o                   # &lt;-- a.a is an archive with LLVM bitcode
$ clang b.c -c -o b.o            # &lt;-- b.o is native object file
$ clang -flto a.a b.o -o main    # &lt;-- link with LLVMgold plugin
</pre>

  <p>Gold informs the plugin that foo3 is never referenced outside the IR,
  leading LLVM to delete that function. However, unlike in the
  <a href="LinkTimeOptimization.html#example1">libLTO
  example</a> gold does not currently eliminate foo4.</p>
</div>

</div>

<!--=========================================================================-->
<h2>
  <a name="lto_autotools">
    Quickstart for using LTO with autotooled projects
  </a>
</h2>
<!--=========================================================================-->
<div>
  <p>Once your system <tt>ld</tt>, <tt>ar</tt>, and <tt>nm</tt> all support LLVM
     bitcode, everything is in place for an easy to use LTO build of autotooled
     projects:</p>

  <ul>
    <li>Follow the instructions <a href="#build">on how to build LLVMgold.so</a>.</li>
    <li>Install the newly built binutils to <tt>$PREFIX</tt></li>
    <li>Copy <tt>Release/lib/LLVMgold.so</tt> to
        <tt>$PREFIX/lib/bfd-plugins/</tt></li>
    <li>Set environment variables (<tt>$PREFIX</tt> is where you installed clang and
        binutils):
<pre class="doc_code">
export CC="$PREFIX/bin/clang -flto"
export CXX="$PREFIX/bin/clang++ -flto"
export AR="$PREFIX/bin/ar"
export NM="$PREFIX/bin/nm"
export RANLIB=/bin/true #ranlib is not needed, and doesn't support .bc files in .a
export CFLAGS="-O4"
</pre>
     </li>
     <li>Or you can just set your path:
<pre class="doc_code">
export PATH="$PREFIX/bin:$PATH"
export CC="clang -flto"
export CXX="clang++ -flto"
export RANLIB=/bin/true
export CFLAGS="-O4"
</pre></li>
     <li>Configure &amp; build the project as usual:
<pre class="doc_code">
% ./configure &amp;&amp; make &amp;&amp; make check
</pre></li>
   </ul>

   <p>The environment variable settings may work for non-autotooled projects
      too, but you may need to set the <tt>LD</tt> environment variable as
      well.</p>
</div>

<!--=========================================================================-->
<h2><a name="licensing">Licensing</a></h2>
<!--=========================================================================-->
<div>
  <p>Gold is licensed under the GPLv3. LLVMgold uses the interface file
<tt>plugin-api.h</tt> from gold which means that the resulting LLVMgold.so
binary is also GPLv3. This can still be used to link non-GPLv3 programs just
as much as gold could without the plugin.</p>
</div>

<!-- *********************************************************************** -->
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  <a href="mailto:nicholas@metrix.on.ca">Nick Lewycky</a><br>
  <a href="http://llvm.org/">The LLVM Compiler Infrastructure</a><br>
  Last modified: $Date: 2010-04-16 23:58:21 -0800 (Fri, 16 Apr 2010) $
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