<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
+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
<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>
+ <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>Build gold with plugin support:
+ <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 src
+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 binutils/build/gold/ld-new which supports the
-<tt>-plugin</tt> option.
-
+ 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>.
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>
<!-- ======================================================================= -->
--- 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.o b.o -o main # <-- link with LLVMgold plugin
+$ 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
<div class="doc_section"><a name="lto_autotools">Quickstart for using LTO with autotooled projects</a></div>
<!--=========================================================================-->
<div class="doc_text">
- <p><tt>gold</tt>, <tt>ar</tt> and <tt>nm</tt> all support plugins now, so everything should be
- in place for an easy to use LTO build of autotooled projects:</p>
+ <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 libLLVMgold.so</a>.</li>
+ <li>Follow the instructions <a href="#build">on how to build LLVMgold.so</a>.</li>
<li>Install the newly built binutils to <tt>$PREFIX</tt></li>
- <li>Copy <tt>Release/lib/libLLVMgold.so</tt> to
+ <li>Copy <tt>Release/lib/LLVMgold.so</tt> to
<tt>$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
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: 2009-01-01 23:10:51 -0800 (Thu, 01 Jan 2009) $
+ Last modified: $Date: 2010-04-16 23:58:21 -0800 (Fri, 16 Apr 2010) $
</address>
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