forked from OSchip/llvm-project
214 lines
8.7 KiB
HTML
214 lines
8.7 KiB
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>
|
|
<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>
|
|
|
|
<!--=========================================================================-->
|
|
<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 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="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 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 “GNU ld” if not. If you have
|
|
gold, check for plugin support by running <tt>/usr/bin/ld -plugin</tt>. If it
|
|
complains “missing argument” 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>
|
|
<!--=========================================================================-->
|
|
<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>
|
|
<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>
|
|
</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
|
|
$ ar q a.a a.o # <-- a.a is an archive with LLVM bitcode
|
|
$ llvm-gcc b.c -c -o b.o # <-- b.o is native object file
|
|
$ llvm-gcc -use-gold-plugin 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="http://llvm.org/docs/LinkTimeOptimization.html#example1">libLTO
|
|
example</a> gold does not currently eliminate foo4.</p>
|
|
</div>
|
|
|
|
<!--=========================================================================-->
|
|
<div class="doc_section"><a name="lto_autotools">Quickstart for using LTO with autotooled projects</a></div>
|
|
<!--=========================================================================-->
|
|
<div class="doc_text">
|
|
<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/libexec/gcc/x86_64-unknown-linux-gnu/4.2.1/</tt> and
|
|
<tt>$PREFIX/lib/bfd-plugins/</tt></li>
|
|
<li>Set environment variables (<tt>$PREFIX</tt> is where you installed llvm-gcc and
|
|
binutils):
|
|
<pre class="doc_code">
|
|
export CC="$PREFIX/bin/llvm-gcc -use-gold-plugin"
|
|
export CXX="$PREFIX/bin/llvm-g++ -use-gold-plugin"
|
|
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="llvm-gcc -use-gold-plugin"
|
|
export CXX="llvm-g++ -use-gold-plugin"
|
|
export RANLIB=/bin/true
|
|
export CFLAGS="-O4"
|
|
</pre>
|
|
</li>
|
|
<li>Configure & build the project as usual: <tt>./configure && make && make check</tt> </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>
|
|
|
|
<!--=========================================================================-->
|
|
<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>
|
|
<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>
|
|
</body>
|
|
</html>
|