1 ==================== 2 The LLVM gold plugin 3 ==================== 4 5 Introduction 6 ============ 7 8 Building with link time optimization requires cooperation from 9 the system linker. LTO support on Linux systems requires that you use the 10 `gold linker`_ which supports LTO via plugins. This is the same mechanism 11 used by the `GCC LTO`_ project. 12 13 The LLVM gold plugin implements the gold plugin interface on top of 14 :ref:`libLTO`. The same plugin can also be used by other tools such as 15 ``ar`` and ``nm``. 16 17 .. _`gold linker`: http://sourceware.org/binutils 18 .. _`GCC LTO`: http://gcc.gnu.org/wiki/LinkTimeOptimization 19 .. _`gold plugin interface`: http://gcc.gnu.org/wiki/whopr/driver 20 21 .. _lto-how-to-build: 22 23 How to build it 24 =============== 25 26 You need to have gold with plugin support and build the LLVMgold plugin. 27 Check whether you have gold running ``/usr/bin/ld -v``. It will report "GNU 28 gold" or else "GNU ld" if not. If you have gold, check for plugin support 29 by running ``/usr/bin/ld -plugin``. If it complains "missing argument" then 30 you have plugin support. If not, such as an "unknown option" error then you 31 will either need to build gold or install a version with plugin support. 32 33 * To build gold with plugin support: 34 35 .. code-block:: bash 36 37 $ mkdir binutils 38 $ cd binutils 39 $ cvs -z 9 -d :pserver:anoncvs (a] sourceware.org:/cvs/src login 40 {enter "anoncvs" as the password} 41 $ cvs -z 9 -d :pserver:anoncvs (a] sourceware.org:/cvs/src co binutils 42 $ mkdir build 43 $ cd build 44 $ ../src/configure --enable-gold --enable-plugins 45 $ make all-gold 46 47 That should leave you with ``binutils/build/gold/ld-new`` which supports 48 the ``-plugin`` option. It also built would have 49 ``binutils/build/binutils/ar`` and ``nm-new`` which support plugins but 50 don't have a visible -plugin option, instead relying on the gold plugin 51 being present in ``../lib/bfd-plugins`` relative to where the binaries 52 are placed. 53 54 * Build the LLVMgold plugin: Configure LLVM with 55 ``--with-binutils-include=/path/to/binutils/src/include`` and run 56 ``make``. 57 58 Usage 59 ===== 60 61 The linker takes a ``-plugin`` option that points to the path of 62 the plugin ``.so`` file. To find out what link command ``gcc`` 63 would run in a given situation, run ``gcc -v [...]`` and 64 look for the line where it runs ``collect2``. Replace that with 65 ``ld-new -plugin /path/to/LLVMgold.so`` to test it out. Once you're 66 ready to switch to using gold, backup your existing ``/usr/bin/ld`` 67 then replace it with ``ld-new``. 68 69 You can produce bitcode files from ``clang`` using ``-emit-llvm`` or 70 ``-flto``, or the ``-O4`` flag which is synonymous with ``-O3 -flto``. 71 72 Any of these flags will also cause ``clang`` to look for the gold plugin in 73 the ``lib`` directory under its prefix and pass the ``-plugin`` option to 74 ``ld``. It will not look for an alternate linker, which is why you need 75 gold to be the installed system linker in your path. 76 77 If you want ``ar`` and ``nm`` to work seamlessly as well, install 78 ``LLVMgold.so`` to ``/usr/lib/bfd-plugins``. If you built your own gold, be 79 sure to install the ``ar`` and ``nm-new`` you built to ``/usr/bin``. 80 81 82 Example of link time optimization 83 --------------------------------- 84 85 The following example shows a worked example of the gold plugin mixing LLVM 86 bitcode and native code. 87 88 .. code-block:: c 89 90 --- a.c --- 91 #include <stdio.h> 92 93 extern void foo1(void); 94 extern void foo4(void); 95 96 void foo2(void) { 97 printf("Foo2\n"); 98 } 99 100 void foo3(void) { 101 foo4(); 102 } 103 104 int main(void) { 105 foo1(); 106 } 107 108 --- b.c --- 109 #include <stdio.h> 110 111 extern void foo2(void); 112 113 void foo1(void) { 114 foo2(); 115 } 116 117 void foo4(void) { 118 printf("Foo4"); 119 } 120 121 .. code-block:: bash 122 123 --- command lines --- 124 $ clang -flto a.c -c -o a.o # <-- a.o is LLVM bitcode file 125 $ ar q a.a a.o # <-- a.a is an archive with LLVM bitcode 126 $ clang b.c -c -o b.o # <-- b.o is native object file 127 $ clang -flto a.a b.o -o main # <-- link with LLVMgold plugin 128 129 Gold informs the plugin that foo3 is never referenced outside the IR, 130 leading LLVM to delete that function. However, unlike in the :ref:`libLTO 131 example <libLTO-example>` gold does not currently eliminate foo4. 132 133 Quickstart for using LTO with autotooled projects 134 ================================================= 135 136 Once your system ``ld``, ``ar``, and ``nm`` all support LLVM bitcode, 137 everything is in place for an easy to use LTO build of autotooled projects: 138 139 * Follow the instructions :ref:`on how to build LLVMgold.so 140 <lto-how-to-build>`. 141 142 * Install the newly built binutils to ``$PREFIX`` 143 144 * Copy ``Release/lib/LLVMgold.so`` to ``$PREFIX/lib/bfd-plugins/`` 145 146 * Set environment variables (``$PREFIX`` is where you installed clang and 147 binutils): 148 149 .. code-block:: bash 150 151 export CC="$PREFIX/bin/clang -flto" 152 export CXX="$PREFIX/bin/clang++ -flto" 153 export AR="$PREFIX/bin/ar" 154 export NM="$PREFIX/bin/nm" 155 export RANLIB=/bin/true #ranlib is not needed, and doesn't support .bc files in .a 156 export CFLAGS="-O4" 157 158 * Or you can just set your path: 159 160 .. code-block:: bash 161 162 export PATH="$PREFIX/bin:$PATH" 163 export CC="clang -flto" 164 export CXX="clang++ -flto" 165 export RANLIB=/bin/true 166 export CFLAGS="-O4" 167 * Configure and build the project as usual: 168 169 .. code-block:: bash 170 171 % ./configure && make && make check 172 173 The environment variable settings may work for non-autotooled projects too, 174 but you may need to set the ``LD`` environment variable as well. 175 176 Licensing 177 ========= 178 179 Gold is licensed under the GPLv3. LLVMgold uses the interface file 180 ``plugin-api.h`` from gold which means that the resulting ``LLVMgold.so`` 181 binary is also GPLv3. This can still be used to link non-GPLv3 programs 182 just as much as gold could without the plugin. 183
