<!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>