1 <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" 2 "http://www.w3.org/TR/html4/strict.dtd"> 3 <html> 4 <head> 5 <meta http-equiv="Content-Type" content="text/html; charset=utf-8"> 6 <title>LLVM gold plugin</title> 7 <link rel="stylesheet" href="llvm.css" type="text/css"> 8 </head> 9 <body> 10 11 <h1>LLVM gold plugin</h1> 12 <ol> 13 <li><a href="#introduction">Introduction</a></li> 14 <li><a href="#build">How to build it</a></li> 15 <li><a href="#usage">Usage</a> 16 <ul> 17 <li><a href="#example1">Example of link time optimization</a></li> 18 <li><a href="#lto_autotools">Quickstart for using LTO with autotooled projects</a></li> 19 </ul></li> 20 <li><a href="#licensing">Licensing</a></li> 21 </ol> 22 <div class="doc_author">Written by Nick Lewycky</div> 23 24 <!--=========================================================================--> 25 <h2><a name="introduction">Introduction</a></h2> 26 <!--=========================================================================--> 27 <div> 28 <p>Building with link time optimization requires cooperation from the 29 system linker. LTO support on Linux systems requires that you use 30 the <a href="http://sourceware.org/binutils">gold linker</a> which supports 31 LTO via plugins. This is the same mechanism used by the 32 <a href="http://gcc.gnu.org/wiki/LinkTimeOptimization">GCC LTO</a> 33 project.</p> 34 <p>The LLVM gold plugin implements the 35 <a href="http://gcc.gnu.org/wiki/whopr/driver">gold plugin interface</a> 36 on top of 37 <a href="LinkTimeOptimization.html#lto">libLTO</a>. 38 The same plugin can also be used by other tools such as <tt>ar</tt> and 39 <tt>nm</tt>. 40 </div> 41 <!--=========================================================================--> 42 <h2><a name="build">How to build it</a></h2> 43 <!--=========================================================================--> 44 <div> 45 <p>You need to have gold with plugin support and build the LLVMgold 46 plugin. Check whether you have gold running <tt>/usr/bin/ld -v</tt>. It will 47 report “GNU gold” or else “GNU ld” if not. If you have 48 gold, check for plugin support by running <tt>/usr/bin/ld -plugin</tt>. If it 49 complains “missing argument” then you have plugin support. If not, 50 such as an “unknown option” error then you will either need to 51 build gold or install a version with plugin support.</p> 52 <ul> 53 <li>To build gold with plugin support: 54 <pre class="doc_code"> 55 mkdir binutils 56 cd binutils 57 cvs -z 9 -d :pserver:anoncvs (a] sourceware.org:/cvs/src login 58 <em>{enter "anoncvs" as the password}</em> 59 cvs -z 9 -d :pserver:anoncvs (a] sourceware.org:/cvs/src co binutils 60 mkdir build 61 cd build 62 ../src/configure --enable-gold --enable-plugins 63 make all-gold 64 </pre> 65 That should leave you with <tt>binutils/build/gold/ld-new</tt> which supports the <tt>-plugin</tt> option. It also built would have 66 <tt>binutils/build/binutils/ar</tt> and <tt>nm-new</tt> which support plugins 67 but don't have a visible -plugin option, instead relying on the gold plugin 68 being present in <tt>../lib/bfd-plugins</tt> relative to where the binaries are 69 placed. 70 <li>Build the LLVMgold plugin: Configure LLVM with 71 <tt>--with-binutils-include=/path/to/binutils/src/include</tt> and run 72 <tt>make</tt>. 73 </ul> 74 </div> 75 <!--=========================================================================--> 76 <h2><a name="usage">Usage</a></h2> 77 <!--=========================================================================--> 78 <div> 79 80 <p>The linker takes a <tt>-plugin</tt> option that points to the path of 81 the plugin <tt>.so</tt> file. To find out what link command <tt>gcc</tt> 82 would run in a given situation, run <tt>gcc -v <em>[...]</em></tt> and look 83 for the line where it runs <tt>collect2</tt>. Replace that with 84 <tt>ld-new -plugin /path/to/LLVMgold.so</tt> to test it out. Once you're 85 ready to switch to using gold, backup your existing <tt>/usr/bin/ld</tt> 86 then replace it with <tt>ld-new</tt>.</p> 87 88 <p>You can produce bitcode files from <tt>clang</tt> using 89 <tt>-emit-llvm</tt> or <tt>-flto</tt>, or the <tt>-O4</tt> flag which is 90 synonymous with <tt>-O3 -flto</tt>.</p> 91 92 <p>Any of these flags will also cause <tt>clang</tt> to look for the 93 gold plugin in the <tt>lib</tt> directory under its prefix and pass the 94 <tt>-plugin</tt> option to <tt>ld</tt>. It will not look for an alternate 95 linker, which is why you need gold to be the installed system linker in 96 your path.</p> 97 98 <p>If you want <tt>ar</tt> and <tt>nm</tt> to work seamlessly as well, install 99 <tt>LLVMgold.so</tt> to <tt>/usr/lib/bfd-plugins</tt>. If you built your 100 own gold, be sure to install the <tt>ar</tt> and <tt>nm-new</tt> you built to 101 <tt>/usr/bin</tt>.<p> 102 103 <!-- ======================================================================= --> 104 <h3> 105 <a name="example1">Example of link time optimization</a> 106 </h3> 107 108 <div> 109 <p>The following example shows a worked example of the gold plugin mixing 110 LLVM bitcode and native code. 111 <pre class="doc_code"> 112 --- a.c --- 113 #include <stdio.h> 114 115 extern void foo1(void); 116 extern void foo4(void); 117 118 void foo2(void) { 119 printf("Foo2\n"); 120 } 121 122 void foo3(void) { 123 foo4(); 124 } 125 126 int main(void) { 127 foo1(); 128 } 129 130 --- b.c --- 131 #include <stdio.h> 132 133 extern void foo2(void); 134 135 void foo1(void) { 136 foo2(); 137 } 138 139 void foo4(void) { 140 printf("Foo4"); 141 } 142 143 --- command lines --- 144 $ clang -flto a.c -c -o a.o # <-- a.o is LLVM bitcode file 145 $ ar q a.a a.o # <-- a.a is an archive with LLVM bitcode 146 $ clang b.c -c -o b.o # <-- b.o is native object file 147 $ clang -flto a.a b.o -o main # <-- link with LLVMgold plugin 148 </pre> 149 150 <p>Gold informs the plugin that foo3 is never referenced outside the IR, 151 leading LLVM to delete that function. However, unlike in the 152 <a href="LinkTimeOptimization.html#example1">libLTO 153 example</a> gold does not currently eliminate foo4.</p> 154 </div> 155 156 </div> 157 158 <!--=========================================================================--> 159 <h2> 160 <a name="lto_autotools"> 161 Quickstart for using LTO with autotooled projects 162 </a> 163 </h2> 164 <!--=========================================================================--> 165 <div> 166 <p>Once your system <tt>ld</tt>, <tt>ar</tt>, and <tt>nm</tt> all support LLVM 167 bitcode, everything is in place for an easy to use LTO build of autotooled 168 projects:</p> 169 170 <ul> 171 <li>Follow the instructions <a href="#build">on how to build LLVMgold.so</a>.</li> 172 <li>Install the newly built binutils to <tt>$PREFIX</tt></li> 173 <li>Copy <tt>Release/lib/LLVMgold.so</tt> to 174 <tt>$PREFIX/lib/bfd-plugins/</tt></li> 175 <li>Set environment variables (<tt>$PREFIX</tt> is where you installed clang and 176 binutils): 177 <pre class="doc_code"> 178 export CC="$PREFIX/bin/clang -flto" 179 export CXX="$PREFIX/bin/clang++ -flto" 180 export AR="$PREFIX/bin/ar" 181 export NM="$PREFIX/bin/nm" 182 export RANLIB=/bin/true #ranlib is not needed, and doesn't support .bc files in .a 183 export CFLAGS="-O4" 184 </pre> 185 </li> 186 <li>Or you can just set your path: 187 <pre class="doc_code"> 188 export PATH="$PREFIX/bin:$PATH" 189 export CC="clang -flto" 190 export CXX="clang++ -flto" 191 export RANLIB=/bin/true 192 export CFLAGS="-O4" 193 </pre></li> 194 <li>Configure & build the project as usual: 195 <pre class="doc_code"> 196 % ./configure && make && make check 197 </pre></li> 198 </ul> 199 200 <p>The environment variable settings may work for non-autotooled projects 201 too, but you may need to set the <tt>LD</tt> environment variable as 202 well.</p> 203 </div> 204 205 <!--=========================================================================--> 206 <h2><a name="licensing">Licensing</a></h2> 207 <!--=========================================================================--> 208 <div> 209 <p>Gold is licensed under the GPLv3. LLVMgold uses the interface file 210 <tt>plugin-api.h</tt> from gold which means that the resulting LLVMgold.so 211 binary is also GPLv3. This can still be used to link non-GPLv3 programs just 212 as much as gold could without the plugin.</p> 213 </div> 214 215 <!-- *********************************************************************** --> 216 <hr> 217 <address> 218 <a href="http://jigsaw.w3.org/css-validator/check/referer"><img 219 src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a> 220 <a href="http://validator.w3.org/check/referer"><img 221 src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a> 222 <a href="mailto:nicholas (a] metrix.on.ca">Nick Lewycky</a><br> 223 <a href="http://llvm.org/">The LLVM Compiler Infrastructure</a><br> 224 Last modified: $Date: 2010-04-16 23:58:21 -0800 (Fri, 16 Apr 2010) $ 225 </address> 226 </body> 227 </html> 228
