1 <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
2 "http://www.w3.org/TR/html4/strict.dtd">
5 <title>LLVM gold plugin</title>
6 <link rel="stylesheet" href="llvm.css" type="text/css">
10 <div class="doc_title">LLVM gold plugin</div>
12 <li><a href="#introduction">Introduction</a></li>
13 <li><a href="#build">How to build it</a></li>
14 <li><a href="#usage">Usage</a>
16 <li><a href="#example1">Example of link time optimization</a></li>
18 <li><a href="#licensing">Licensing</a></li>
20 <div class="doc_author">Written by Nick Lewycky</div>
22 <!--=========================================================================-->
23 <div class="doc_section"><a name="introduction">Introduction</a></div>
24 <!--=========================================================================-->
25 <div class="doc_text">
26 <p>Building with link time optimization requires cooperation from the
27 system linker. LTO support on Linux systems requires that you use
28 the <a href="http://sourceware.org/binutils">gold linker</a> which supports
29 LTO via plugins. This is the same system used by the upcoming
30 <a href="http://gcc.gnu.org/wiki/LinkTimeOptimization">GCC LTO</a>
32 <p>The LLVM gold plugin implements the
33 <a href="http://gcc.gnu.org/wiki/whopr/driver">gold plugin interface</a>
35 <a href="http://llvm.org/docs/LinkTimeOptimization.html#lto">libLTO</a>.
36 The same plugin can also be used by other tools such as <tt>ar</tt> and
39 <!--=========================================================================-->
40 <div class="doc_section"><a name="build">How to build it</a></div>
41 <!--=========================================================================-->
42 <div class="doc_text">
43 <p>You need to build gold with plugin support and build the LLVMgold
46 <li>Build gold with plugin support:
47 <pre class="doc_code">
50 cvs -z 9 -d :pserver:anoncvs@sourceware.org:/cvs/src login
51 <em>{enter "anoncvs" as the password}</em>
52 cvs -z 9 -d :pserver:anoncvs@sourceware.org:/cvs/src co src
55 ../src/configure --enable-gold --enable-plugins
58 That should leave you with binutils/build/gold/ld-new which supports the
59 <tt>-plugin</tt> option.
61 <li>Build the LLVMgold plugin: Configure LLVM with
62 <tt>--with-binutils-include=/path/to/binutils/src/include</tt> and run
66 <!--=========================================================================-->
67 <div class="doc_section"><a name="usage">Usage</a></div>
68 <!--=========================================================================-->
69 <div class="doc_text">
70 <p>The linker takes a <tt>-plugin</tt> option that points to the path of
71 the plugin <tt>.so</tt> file. To find out what link command <tt>gcc</tt>
72 would run in a given situation, run <tt>gcc -v <em>[...]</em></tt> and look
73 for the line where it runs <tt>collect2</tt>. Replace that with
74 <tt>ld-new -plugin /path/to/LLVMgold.so</tt> to test it out. Once you're
75 ready to switch to using gold, backup your existing <tt>/usr/bin/ld</tt>
76 then replace it with <tt>ld-new</tt>.</p>
77 <p>You can produce bitcode files from <tt>llvm-gcc</tt> using
78 <tt>-emit-llvm</tt> or <tt>-flto</tt>, or the <tt>-O4</tt> flag which is
79 synonymous with <tt>-O3 -flto</tt>.</p>
80 <p><tt>llvm-gcc</tt> has a <tt>-use-gold-plugin</tt> option which looks
81 for the gold plugin in the same directories as it looks for <tt>cc1</tt> and
82 passes the <tt>-plugin</tt> option to ld. It will not look for an alternate
83 linker, which is why you need gold to be the installed system linker in your
87 <!-- ======================================================================= -->
88 <div class="doc_subsection">
89 <a name="example1">Example of link time optimization</a>
92 <div class="doc_text">
93 <p>The following example shows a worked example of the gold plugin mixing
94 LLVM bitcode and native code.
95 <pre class="doc_code">
97 #include <stdio.h>
99 extern void foo1(void);
100 extern void foo4(void);
115 #include <stdio.h>
117 extern void foo2(void);
127 --- command lines ---
128 $ llvm-gcc -flto a.c -c -o a.o # <-- a.o is LLVM bitcode file
129 $ llvm-gcc b.c -c -o b.o # <-- b.o is native object file
130 $ llvm-gcc -use-gold-plugin a.o b.o -o main # <-- link with LLVMgold plugin
132 <p>Gold informs the plugin that foo3 is never referenced outside the IR,
133 leading LLVM to delete that function. However, unlike in the
134 <a href="http://llvm.org/docs/LinkTimeOptimization.html#example1">libLTO
135 example</a> gold does not currently eliminate foo4.</p>
138 <!--=========================================================================-->
139 <div class="doc_section"><a name="licensing">Licensing</a></div>
140 <!--=========================================================================-->
141 <div class="doc_text">
142 <p>Gold is licensed under the GPLv3. LLVMgold uses the interface file
143 <tt>plugin-api.h</tt> from gold which means that the resulting LLVMgold.so
144 binary is also GPLv3. This can still be used to link non-GPLv3 programs just
145 as much as gold could without the plugin.</p>
148 <!-- *********************************************************************** -->
151 <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
152 src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a>
153 <a href="http://validator.w3.org/check/referer"><img
154 src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a>
155 <a href="mailto:nicholas@metrix.on.ca">Nick Lewycky</a><br>
156 <a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br>
157 Last modified: $Date: 2009-01-01 23:10:51 -0800 (Thu, 01 Jan 2009) $