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| <title>LLVM gold plugin</title> |
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| |
| <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><tt>Clang</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 <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 -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="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/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 clang and |
| binutils): |
| <pre class="doc_code"> |
| export CC="$PREFIX/bin/clang -use-gold-plugin" |
| export CXX="$PREFIX/bin/clang++ -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="clang -use-gold-plugin" |
| export CXX="clang++ -use-gold-plugin" |
| 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> |
| |
| <!-- *********************************************************************** --> |
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| <address> |
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| <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) $ |
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