<!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> </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 system used by the upcoming <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 build gold with plugin support and build the LLVMgold plugin.</p> <ul> <li>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 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. <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> </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 $ 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 </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="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: 2009-01-01 23:10:51 -0800 (Thu, 01 Jan 2009) $ </address> </body> </html>