1 <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" 2 "http://www.w3.org/TR/html4/strict.dtd"> 3 <html> 4 <head> 5 <title>LLVM gold plugin</title> 6 <link rel="stylesheet" href="llvm.css" type="text/css"> 7 </head> 8 <body> 9 10 <h1>LLVM gold plugin</h1> 11 <ol> 12 <li><a href="#introduction">Introduction</a></li> 13 <li><a href="#build">How to build it</a></li> 14 <li><a href="#usage">Usage</a> 15 <ul> 16 <li><a href="#example1">Example of link time optimization</a></li> 17 <li><a href="#lto_autotools">Quickstart for using LTO with autotooled projects</a></li> 18 </ul></li> 19 <li><a href="#licensing">Licensing</a></li> 20 </ol> 21 <div class="doc_author">Written by Nick Lewycky</div> 22 23 <!--=========================================================================--> 24 <h2><a name="introduction">Introduction</a></h2> 25 <!--=========================================================================--> 26 <div> 27 <p>Building with link time optimization requires cooperation from the 28 system linker. LTO support on Linux systems requires that you use 29 the <a href="http://sourceware.org/binutils">gold linker</a> which supports 30 LTO via plugins. This is the same mechanism used by the 31 <a href="http://gcc.gnu.org/wiki/LinkTimeOptimization">GCC LTO</a> 32 project.</p> 33 <p>The LLVM gold plugin implements the 34 <a href="http://gcc.gnu.org/wiki/whopr/driver">gold plugin interface</a> 35 on top of 36 <a href="LinkTimeOptimization.html#lto">libLTO</a>. 37 The same plugin can also be used by other tools such as <tt>ar</tt> and 38 <tt>nm</tt>. 39 </div> 40 <!--=========================================================================--> 41 <h2><a name="build">How to build it</a></h2> 42 <!--=========================================================================--> 43 <div> 44 <p>You need to have gold with plugin support and build the LLVMgold 45 plugin. Check whether you have gold running <tt>/usr/bin/ld -v</tt>. It will 46 report “GNU gold” or else “GNU ld” if not. If you have 47 gold, check for plugin support by running <tt>/usr/bin/ld -plugin</tt>. If it 48 complains “missing argument” then you have plugin support. If not, 49 such as an “unknown option” error then you will either need to 50 build gold or install a version with plugin support.</p> 51 <ul> 52 <li>To build gold with plugin support: 53 <pre class="doc_code"> 54 mkdir binutils 55 cd binutils 56 cvs -z 9 -d :pserver:anoncvs (a] sourceware.org:/cvs/src login 57 <em>{enter "anoncvs" as the password}</em> 58 cvs -z 9 -d :pserver:anoncvs (a] sourceware.org:/cvs/src co binutils 59 mkdir build 60 cd build 61 ../src/configure --enable-gold --enable-plugins 62 make all-gold 63 </pre> 64 That should leave you with <tt>binutils/build/gold/ld-new</tt> which supports the <tt>-plugin</tt> option. It also built would have 65 <tt>binutils/build/binutils/ar</tt> and <tt>nm-new</tt> which support plugins 66 but don't have a visible -plugin option, instead relying on the gold plugin 67 being present in <tt>../lib/bfd-plugins</tt> relative to where the binaries are 68 placed. 69 <li>Build the LLVMgold plugin: Configure LLVM with 70 <tt>--with-binutils-include=/path/to/binutils/src/include</tt> and run 71 <tt>make</tt>. 72 </ul> 73 </div> 74 <!--=========================================================================--> 75 <h2><a name="usage">Usage</a></h2> 76 <!--=========================================================================--> 77 <div> 78 <p>The linker takes a <tt>-plugin</tt> option that points to the path of 79 the plugin <tt>.so</tt> file. To find out what link command <tt>gcc</tt> 80 would run in a given situation, run <tt>gcc -v <em>[...]</em></tt> and look 81 for the line where it runs <tt>collect2</tt>. Replace that with 82 <tt>ld-new -plugin /path/to/LLVMgold.so</tt> to test it out. Once you're 83 ready to switch to using gold, backup your existing <tt>/usr/bin/ld</tt> 84 then replace it with <tt>ld-new</tt>.</p> 85 <p>You can produce bitcode files from <tt>llvm-gcc</tt> using 86 <tt>-emit-llvm</tt> or <tt>-flto</tt>, or the <tt>-O4</tt> flag which is 87 synonymous with <tt>-O3 -flto</tt>.</p> 88 <p><tt>llvm-gcc</tt> has a <tt>-use-gold-plugin</tt> option which looks 89 for the gold plugin in the same directories as it looks for <tt>cc1</tt> and 90 passes the <tt>-plugin</tt> option to ld. It will not look for an alternate 91 linker, which is why you need gold to be the installed system linker in your 92 path.</p> 93 <p>If you want <tt>ar</tt> and <tt>nm</tt> to work seamlessly as well, install 94 <tt>LLVMgold.so</tt> to <tt>/usr/lib/bfd-plugins</tt>. If you built your 95 own gold, be sure to install the <tt>ar</tt> and <tt>nm-new</tt> you built to 96 <tt>/usr/bin</tt>. 97 <p> 98 99 <!-- ======================================================================= --> 100 <h3> 101 <a name="example1">Example of link time optimization</a> 102 </h3> 103 104 <div> 105 <p>The following example shows a worked example of the gold plugin mixing 106 LLVM bitcode and native code. 107 <pre class="doc_code"> 108 --- a.c --- 109 #include <stdio.h> 110 111 extern void foo1(void); 112 extern void foo4(void); 113 114 void foo2(void) { 115 printf("Foo2\n"); 116 } 117 118 void foo3(void) { 119 foo4(); 120 } 121 122 int main(void) { 123 foo1(); 124 } 125 126 --- b.c --- 127 #include <stdio.h> 128 129 extern void foo2(void); 130 131 void foo1(void) { 132 foo2(); 133 } 134 135 void foo4(void) { 136 printf("Foo4"); 137 } 138 139 --- command lines --- 140 $ llvm-gcc -flto a.c -c -o a.o # <-- a.o is LLVM bitcode file 141 $ ar q a.a a.o # <-- a.a is an archive with LLVM bitcode 142 $ llvm-gcc b.c -c -o b.o # <-- b.o is native object file 143 $ llvm-gcc -use-gold-plugin a.a b.o -o main # <-- link with LLVMgold plugin 144 </pre> 145 <p>Gold informs the plugin that foo3 is never referenced outside the IR, 146 leading LLVM to delete that function. However, unlike in the 147 <a href="LinkTimeOptimization.html#example1">libLTO 148 example</a> gold does not currently eliminate foo4.</p> 149 </div> 150 151 </div> 152 153 <!--=========================================================================--> 154 <h2> 155 <a name="lto_autotools"> 156 Quickstart for using LTO with autotooled projects 157 </a> 158 </h2> 159 <!--=========================================================================--> 160 <div> 161 <p>Once your system <tt>ld</tt>, <tt>ar</tt> and <tt>nm</tt> all support LLVM 162 bitcode, everything is in place for an easy to use LTO build of autotooled 163 projects:</p> 164 <ul> 165 <li>Follow the instructions <a href="#build">on how to build LLVMgold.so</a>.</li> 166 <li>Install the newly built binutils to <tt>$PREFIX</tt></li> 167 <li>Copy <tt>Release/lib/LLVMgold.so</tt> to 168 <tt>$PREFIX/libexec/gcc/x86_64-unknown-linux-gnu/4.2.1/</tt> and 169 <tt>$PREFIX/lib/bfd-plugins/</tt></li> 170 <li>Set environment variables (<tt>$PREFIX</tt> is where you installed llvm-gcc and 171 binutils): 172 <pre class="doc_code"> 173 export CC="$PREFIX/bin/llvm-gcc -use-gold-plugin" 174 export CXX="$PREFIX/bin/llvm-g++ -use-gold-plugin" 175 export AR="$PREFIX/bin/ar" 176 export NM="$PREFIX/bin/nm" 177 export RANLIB=/bin/true #ranlib is not needed, and doesn't support .bc files in .a 178 export CFLAGS="-O4" 179 </pre> 180 </li> 181 <li>Or you can just set your path: 182 <pre class="doc_code"> 183 export PATH="$PREFIX/bin:$PATH" 184 export CC="llvm-gcc -use-gold-plugin" 185 export CXX="llvm-g++ -use-gold-plugin" 186 export RANLIB=/bin/true 187 export CFLAGS="-O4" 188 </pre> 189 </li> 190 <li>Configure & build the project as usual: <tt>./configure && make && make check</tt> </li> 191 </ul> 192 <p> The environment variable settings may work for non-autotooled projects 193 too, but you may need to set the <tt>LD</tt> environment variable as well.</p> 194 </div> 195 196 <!--=========================================================================--> 197 <h2><a name="licensing">Licensing</a></h2> 198 <!--=========================================================================--> 199 <div> 200 <p>Gold is licensed under the GPLv3. LLVMgold uses the interface file 201 <tt>plugin-api.h</tt> from gold which means that the resulting LLVMgold.so 202 binary is also GPLv3. This can still be used to link non-GPLv3 programs just 203 as much as gold could without the plugin.</p> 204 </div> 205 206 <!-- *********************************************************************** --> 207 <hr> 208 <address> 209 <a href="http://jigsaw.w3.org/css-validator/check/referer"><img 210 src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a> 211 <a href="http://validator.w3.org/check/referer"><img 212 src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a> 213 <a href="mailto:nicholas (a] metrix.on.ca">Nick Lewycky</a><br> 214 <a href="http://llvm.org/">The LLVM Compiler Infrastructure</a><br> 215 Last modified: $Date: 2010-04-16 23:58:21 -0800 (Fri, 16 Apr 2010) $ 216 </address> 217 </body> 218 </html> 219
