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diff --git a/docs/GoldPlugin.html b/docs/GoldPlugin.html new file mode 100644 index 00000000000..1e99a5a3d6a --- /dev/null +++ b/docs/GoldPlugin.html @@ -0,0 +1,227 @@ +<!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="_static/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> |