llvm/docs/GoldPlugin.html - toolchain/llvm-project - Gitiles

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 <head>
   <title>LLVM gold plugin</title>
   <link rel="stylesheet" href="llvm.css" type="text/css">
 </head>
 <body>

 <div class="doc_title">LLVM gold plugin</div>
 <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>
   </ul></li>
   <li><a href="#licensing">Licensing</a></li>
 </ol>
 <div class="doc_author">Written by Nick Lewycky</div>

 <!--=========================================================================-->
 <div class="doc_section"><a name="introduction">Introduction</a></div>
 <!--=========================================================================-->
 <div class="doc_text">
   <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 system used by the upcoming
 <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="http://llvm.org/docs/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>
 <!--=========================================================================-->
 <div class="doc_section"><a name="build">How to build it</a></div>
 <!--=========================================================================-->
 <div class="doc_text">
   <p>You need to build gold with plugin support and build the LLVMgold
 plugin.</p>
 <ul>
   <li>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 src
 mkdir build
 cd build
 ../src/configure --enable-gold --enable-plugins
 make all-gold
 </pre>
     That should leave you with binutils/build/gold/ld-new which supports the
 <tt>-plugin</tt> option.

     <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>
 <!--=========================================================================-->
 <div class="doc_section"><a name="usage">Usage</a></div>
 <!--=========================================================================-->
 <div class="doc_text">
   <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>llvm-gcc</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><tt>llvm-gcc</tt> has a <tt>-use-gold-plugin</tt> option which looks
   for the gold plugin in the same directories as it looks for <tt>cc1</tt> and
   passes the <tt>-plugin</tt> option to ld. It will not look for an alternate
   linker, which is why you need gold to be the installed system linker in your
   path.</p>
 </div>

 <!-- ======================================================================= -->
 <div class="doc_subsection">
   <a name="example1">Example of link time optimization</a>
 </div>

 <div class="doc_text">
   <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 ---
 $ llvm-gcc -flto a.c -c -o a.o              # &lt;-- a.o is LLVM bitcode file
 $ llvm-gcc b.c -c -o b.o                    # &lt;-- b.o is native object file
 $ llvm-gcc -use-gold-plugin a.o 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="http://llvm.org/docs/LinkTimeOptimization.html#example1">libLTO
   example</a> gold does not currently eliminate foo4.</p>
 </div>

 <!--=========================================================================-->
 <div class="doc_section"><a name="licensing">Licensing</a></div>
 <!--=========================================================================-->
 <div class="doc_text">
 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.
 </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: 2009-01-01 23:10:51 -0800 (Thu, 01 Jan 2009) $
 </address>
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 </html>
	<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
	"http://www.w3.org/TR/html4/strict.dtd">
	<html>
	<head>
	<title>LLVM gold plugin</title>
	<link rel="stylesheet" href="llvm.css" type="text/css">
	</head>
	<body>

	<div class="doc_title">LLVM gold plugin</div>
	<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>
	</ul></li>
	<li><a href="#licensing">Licensing</a></li>
	</ol>
	<div class="doc_author">Written by Nick Lewycky</div>

	<!--=========================================================================-->
	<div class="doc_section"><a name="introduction">Introduction</a></div>
	<!--=========================================================================-->
	<div class="doc_text">
	<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 system used by the upcoming
	<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="http://llvm.org/docs/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>
	<!--=========================================================================-->
	<div class="doc_section"><a name="build">How to build it</a></div>
	<!--=========================================================================-->
	<div class="doc_text">
	<p>You need to build gold with plugin support and build the LLVMgold
	plugin.</p>
	<ul>
	<li>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 src
	mkdir build
	cd build
	../src/configure --enable-gold --enable-plugins
	make all-gold
	</pre>
	That should leave you with binutils/build/gold/ld-new which supports the
	<tt>-plugin</tt> option.

	<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>
	<!--=========================================================================-->
	<div class="doc_section"><a name="usage">Usage</a></div>
	<!--=========================================================================-->
	<div class="doc_text">
	<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>llvm-gcc</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><tt>llvm-gcc</tt> has a <tt>-use-gold-plugin</tt> option which looks
	for the gold plugin in the same directories as it looks for <tt>cc1</tt> and
	passes the <tt>-plugin</tt> option to ld. It will not look for an alternate
	linker, which is why you need gold to be the installed system linker in your
	path.</p>
	</div>

	<!-- ======================================================================= -->
	<div class="doc_subsection">
	<a name="example1">Example of link time optimization</a>
	</div>

	<div class="doc_text">
	<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 ---
	$ llvm-gcc -flto a.c -c -o a.o # <-- a.o is LLVM bitcode file
	$ llvm-gcc b.c -c -o b.o # <-- b.o is native object file
	$ llvm-gcc -use-gold-plugin a.o 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="http://llvm.org/docs/LinkTimeOptimization.html#example1">libLTO
	example</a> gold does not currently eliminate foo4.</p>
	</div>

	<!--=========================================================================-->
	<div class="doc_section"><a name="licensing">Licensing</a></div>
	<!--=========================================================================-->
	<div class="doc_text">
	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.
	</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: 2009-01-01 23:10:51 -0800 (Thu, 01 Jan 2009) $
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