docs/GoldPlugin.html - llvm - Git at Google

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   <title>LLVM gold plugin</title>
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 <body>

 <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>

 <!-- *********************************************************************** -->
 <hr>
 <address>
   <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
   src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a>
   <a href="http://validator.w3.org/check/referer"><img
   src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a>
   <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) $
 </address>
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	<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
	"http://www.w3.org/TR/html4/strict.dtd">
	<html>
	<head>
	<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
	<title>LLVM gold plugin</title>
	<link rel="stylesheet" href="llvm.css" type="text/css">
	</head>
	<body>

	<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 “GNU gold” or else &#8220GNU ld” 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 “unknown option” 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 <stdio.h>

	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 <stdio.h>

	extern void foo2(void);

	void foo1(void) {
	foo2();
	}

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

	--- command lines ---
	$ clang -flto a.c -c -o a.o # <-- a.o is LLVM bitcode file
	$ ar q a.a a.o # <-- a.a is an archive with LLVM bitcode
	$ clang b.c -c -o b.o # <-- b.o is native object file
	$ clang -flto a.a b.o -o main # <-- 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 & build the project as usual:
	<pre class="doc_code">
	% ./configure && make && 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>

	<!-- *********************************************************************** -->
	<hr>
	<address>
	<a href="http://jigsaw.w3.org/css-validator/check/referer"><img
	src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a>
	<a href="http://validator.w3.org/check/referer"><img
	src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a>
	<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) $
	</address>
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