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3<html>
4<head>
5 <title>LLVM gold plugin</title>
6 <link rel="stylesheet" href="llvm.css" type="text/css">
7</head>
8<body>
9
10<div class="doc_title">LLVM gold plugin</div>
11<ol>
12 <li><a href="#introduction">Introduction</a></li>
13 <li><a href="#build">How to build it</a></li>
Nick Lewyckyf3888ba2009-03-01 21:55:10 +0000[diff] [blame]14 <li><a href="#usage">Usage</a>
15 <ul>
16 <li><a href="#example1">Example of link time optimization</a></li>
Torok Edwin5641f8d2009-06-03 15:06:19 +0000[diff] [blame]17 <li><a href="#lto_autotools">Quickstart for using LTO with autotooled projects</a></li>
Nick Lewyckyf3888ba2009-03-01 21:55:10 +0000[diff] [blame]18 </ul></li>
Nick Lewyckyebc67652009-03-01 09:38:29 +0000[diff] [blame]19 <li><a href="#licensing">Licensing</a></li>
20</ol>
21<div class="doc_author">Written by Nick Lewycky</div>
22
23<!--=========================================================================-->
24<div class="doc_section"><a name="introduction">Introduction</a></div>
25<!--=========================================================================-->
26<div class="doc_text">
Duncan Sands10f06752009-03-01 15:19:03 +0000[diff] [blame]27 <p>Building with link time optimization requires cooperation from the
Nick Lewyckyfd97aa22009-03-01 09:51:07 +0000[diff] [blame]28system linker. LTO support on Linux systems requires that you use
Duncan Sands10f06752009-03-01 15:19:03 +0000[diff] [blame]29the <a href="http://sourceware.org/binutils">gold linker</a> which supports
Nick Lewyckyebc67652009-03-01 09:38:29 +0000[diff] [blame]30LTO via plugins. This is the same system used by the upcoming
31<a href="http://gcc.gnu.org/wiki/LinkTimeOptimization">GCC LTO</a>
Duncan Sands10f06752009-03-01 15:19:03 +0000[diff] [blame]32project.</p>
33 <p>The LLVM gold plugin implements the
34<a href="http://gcc.gnu.org/wiki/whopr/driver">gold plugin interface</a>
35on top of
Nick Lewyckyebc67652009-03-01 09:38:29 +0000[diff] [blame]36<a href="http://llvm.org/docs/LinkTimeOptimization.html#lto">libLTO</a>.
37The same plugin can also be used by other tools such as <tt>ar</tt> and
38<tt>nm</tt>.
39</div>
40<!--=========================================================================-->
41<div class="doc_section"><a name="build">How to build it</a></div>
42<!--=========================================================================-->
43<div class="doc_text">
44 <p>You need to build gold with plugin support and build the LLVMgold
45plugin.</p>
46<ul>
47 <li>Build gold with plugin support:
48 <pre class="doc_code">
49mkdir binutils
50cd binutils
51cvs -z 9 -d :pserver:anoncvs@sourceware.org:/cvs/src login
52<em>{enter "anoncvs" as the password}</em>
53cvs -z 9 -d :pserver:anoncvs@sourceware.org:/cvs/src co src
54mkdir build
55cd build
56../src/configure --enable-gold --enable-plugins
57make all-gold
58</pre>
59 That should leave you with binutils/build/gold/ld-new which supports the
Nick Lewycky017730c2009-03-01 21:06:42 +0000[diff] [blame]60<tt>-plugin</tt> option.
Nick Lewyckyebc67652009-03-01 09:38:29 +0000[diff] [blame]61
Duncan Sands10f06752009-03-01 15:19:03 +0000[diff] [blame]62 <li>Build the LLVMgold plugin: Configure LLVM with
Nick Lewyckyebc67652009-03-01 09:38:29 +0000[diff] [blame]63 <tt>--with-binutils-include=/path/to/binutils/src/include</tt> and run
64 <tt>make</tt>.
65</ul>
66</div>
67<!--=========================================================================-->
68<div class="doc_section"><a name="usage">Usage</a></div>
69<!--=========================================================================-->
70<div class="doc_text">
71 <p>The linker takes a <tt>-plugin</tt> option that points to the path of
72 the plugin <tt>.so</tt> file. To find out what link command <tt>gcc</tt>
73 would run in a given situation, run <tt>gcc -v <em>[...]</em></tt> and look
74 for the line where it runs <tt>collect2</tt>. Replace that with
75 <tt>ld-new -plugin /path/to/LLVMgold.so</tt> to test it out. Once you're
76 ready to switch to using gold, backup your existing <tt>/usr/bin/ld</tt>
77 then replace it with <tt>ld-new</tt>.</p>
78 <p>You can produce bitcode files from <tt>llvm-gcc</tt> using
Nick Lewyckyd66ff0f2009-03-01 20:58:07 +0000[diff] [blame]79 <tt>-emit-llvm</tt> or <tt>-flto</tt>, or the <tt>-O4</tt> flag which is
80 synonymous with <tt>-O3 -flto</tt>.</p>
Nick Lewyckyebc67652009-03-01 09:38:29 +0000[diff] [blame]81 <p><tt>llvm-gcc</tt> has a <tt>-use-gold-plugin</tt> option which looks
Nick Lewycky050147c2009-03-01 18:48:53 +0000[diff] [blame]82 for the gold plugin in the same directories as it looks for <tt>cc1</tt> and
83 passes the <tt>-plugin</tt> option to ld. It will not look for an alternate
84 linker, which is why you need gold to be the installed system linker in your
85 path.</p>
Nick Lewyckyebc67652009-03-01 09:38:29 +0000[diff] [blame]86</div>
Nick Lewyckyf3888ba2009-03-01 21:55:10 +0000[diff] [blame]87
88<!-- ======================================================================= -->
89<div class="doc_subsection">
90 <a name="example1">Example of link time optimization</a>
91</div>
92
93<div class="doc_text">
94 <p>The following example shows a worked example of the gold plugin mixing
95 LLVM bitcode and native code.
96<pre class="doc_code">
97--- a.c ---
98#include &lt;stdio.h&gt;
99
100extern void foo1(void);
101extern void foo4(void);
102
103void foo2(void) {
104 printf("Foo2\n");
105}
106
107void foo3(void) {
108 foo4();
109}
110
111int main(void) {
112 foo1();
113}
114
115--- b.c ---
116#include &lt;stdio.h&gt;
117
118extern void foo2(void);
119
120void foo1(void) {
121 foo2();
122}
123
124void foo4(void) {
125 printf("Foo4");
126}
127
128--- command lines ---
129$ llvm-gcc -flto a.c -c -o a.o # &lt;-- a.o is LLVM bitcode file
130$ llvm-gcc b.c -c -o b.o # &lt;-- b.o is native object file
131$ llvm-gcc -use-gold-plugin a.o b.o -o main # &lt;-- link with LLVMgold plugin
132</pre>
133 <p>Gold informs the plugin that foo3 is never referenced outside the IR,
134 leading LLVM to delete that function. However, unlike in the
135 <a href="http://llvm.org/docs/LinkTimeOptimization.html#example1">libLTO
136 example</a> gold does not currently eliminate foo4.</p>
137</div>
138
Nick Lewyckyebc67652009-03-01 09:38:29 +0000[diff] [blame]139<!--=========================================================================-->
Torok Edwin5641f8d2009-06-03 15:06:19 +0000[diff] [blame]140<div class="doc_section"><a name="lto_autotools">Quickstart for using LTO with autotooled projects</a></div>
141<!--=========================================================================-->
142<div class="doc_text">
143 <p><tt>gold</tt>, <tt>ar</tt> and <tt>nm</tt> all support plugins now, so everything should be
144 in place for an easy to use LTO build of autotooled projects:</p>
145 <ul>
146 <li>Follow the instructions <a href="#build">on how to build libLLVMgold.so</a>.</li>
147 <li>Install the newly built binutils to <tt>$PREFIX</tt></li>
148 <li>Copy <tt>Release/lib/libLLVMgold.so</tt> to
149 <tt>$PREFIX/libexec/gcc/x86_64-unknown-linux-gnu/4.2.1/</tt> and
150 <tt>$PREFIX/lib/bfd-plugins/</tt></li>
151 <li>Set environment variables (<tt>$PREFIX</tt> is where you installed llvm-gcc and
152 binutils):
153 <pre class="doc_code">
154export CC="$PREFIX/bin/llvm-gcc -use-gold-plugin"
Torok Edwin4734ed82009-06-03 15:42:26 +0000[diff] [blame^]155export CXX="$PREFIX/bin/llvm-g++ -use-gold-plugin"
Torok Edwin5641f8d2009-06-03 15:06:19 +0000[diff] [blame]156export AR="$PREFIX/bin/ar --plugin libLLVMgold.so"
157export NM="$PREFIX/bin/nm --plugin libLLVMgold.so"
158export RANLIB=/bin/true #ranlib is not needed, and doesn't support .bc files in .a
159export CFLAGS="-O4"
160 </pre>
161 </li>
162 <li>Configure &amp; build the project as usual: <tt>./configure &amp;&amp; make &amp;&amp; make check</tt> </li>
163 </ul>
164 <p> The environment variable settings may work for non-autotooled projects
165 too, but you may need to set the <tt>LD</tt> environment variable as well.</p>
166</div>
167
168<!--=========================================================================-->
Nick Lewyckyebc67652009-03-01 09:38:29 +0000[diff] [blame]169<div class="doc_section"><a name="licensing">Licensing</a></div>
170<!--=========================================================================-->
171<div class="doc_text">
Nick Lewyckyef5facd2009-04-13 02:03:40 +0000[diff] [blame]172 <p>Gold is licensed under the GPLv3. LLVMgold uses the interface file
Nick Lewyckyebc67652009-03-01 09:38:29 +0000[diff] [blame]173<tt>plugin-api.h</tt> from gold which means that the resulting LLVMgold.so
174binary is also GPLv3. This can still be used to link non-GPLv3 programs just
Nick Lewyckyef5facd2009-04-13 02:03:40 +0000[diff] [blame]175as much as gold could without the plugin.</p>
Nick Lewyckyebc67652009-03-01 09:38:29 +0000[diff] [blame]176</div>
177
178<!-- *********************************************************************** -->
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Nick Lewyckyebc67652009-03-01 09:38:29 +0000[diff] [blame]185 <a href="mailto:nicholas@metrix.on.ca">Nick Lewycky</a><br>
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