1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
|
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>LLVM gold plugin</title>
<link rel="stylesheet" href="llvm.css" type="text/css">
</head>
<body>
<h1>LLVM gold plugin</h1>
<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>
<!--=========================================================================-->
<h2><a name="introduction">Introduction</a></h2>
<!--=========================================================================-->
<div>
<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="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>
<!--=========================================================================-->
<h2><a name="build">How to build it</a></h2>
<!--=========================================================================-->
<div>
<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>
<!--=========================================================================-->
<h2><a name="usage">Usage</a></h2>
<!--=========================================================================-->
<div>
<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>clang</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>Any of these flags will also cause <tt>clang</tt> to look for the
gold plugin in the <tt>lib</tt> directory under its prefix and pass the
<tt>-plugin</tt> option to <tt>ld</tt>. 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>
<!-- ======================================================================= -->
<h3>
<a name="example1">Example of link time optimization</a>
</h3>
<div>
<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 ---
$ clang -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
$ clang b.c -c -o b.o # <-- b.o is native object file
$ clang -flto 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="LinkTimeOptimization.html#example1">libLTO
example</a> gold does not currently eliminate foo4.</p>
</div>
</div>
<!--=========================================================================-->
<h2>
<a name="lto_autotools">
Quickstart for using LTO with autotooled projects
</a>
</h2>
<!--=========================================================================-->
<div>
<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/lib/bfd-plugins/</tt></li>
<li>Set environment variables (<tt>$PREFIX</tt> is where you installed clang and
binutils):
<pre class="doc_code">
export CC="$PREFIX/bin/clang -flto"
export CXX="$PREFIX/bin/clang++ -flto"
export AR="$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="clang -flto"
export CXX="clang++ -flto"
export RANLIB=/bin/true
export CFLAGS="-O4"
</pre></li>
<li>Configure & build the project as usual:
<pre class="doc_code">
% ./configure && make && make check
</pre></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>
<!--=========================================================================-->
<h2><a name="licensing">Licensing</a></h2>
<!--=========================================================================-->
<div>
<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>
|
