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<!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">
<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>
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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: 2009-01-01 23:10:51 -0800 (Thu, 01 Jan 2009) $
</address>
</body>
</html>
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