7 .. sectionauthor:: Nick Lewycky
12 Building with link time optimization requires cooperation from
13 the system linker. LTO support on Linux systems requires that you use the
14 `gold linker`_ which supports LTO via plugins. This is the same mechanism
15 used by the `GCC LTO`_ project.
17 The LLVM gold plugin implements the gold plugin interface on top of
18 :ref:`libLTO`. The same plugin can also be used by other tools such as
21 .. _`gold linker`: http://sourceware.org/binutils
22 .. _`GCC LTO`: http://gcc.gnu.org/wiki/LinkTimeOptimization
23 .. _`gold plugin interface`: http://gcc.gnu.org/wiki/whopr/driver
30 You need to have gold with plugin support and build the LLVMgold plugin.
31 Check whether you have gold running ``/usr/bin/ld -v``. It will report "GNU
32 gold" or else "GNU ld" if not. If you have gold, check for plugin support
33 by running ``/usr/bin/ld -plugin``. If it complains "missing argument" then
34 you have plugin support. If not, such as an "unknown option" error then you
35 will either need to build gold or install a version with plugin support.
37 * To build gold with plugin support:
43 $ cvs -z 9 -d :pserver:anoncvs@sourceware.org:/cvs/src login
44 {enter "anoncvs" as the password}
45 $ cvs -z 9 -d :pserver:anoncvs@sourceware.org:/cvs/src co binutils
48 $ ../src/configure --enable-gold --enable-plugins
51 That should leave you with ``binutils/build/gold/ld-new`` which supports
52 the ``-plugin`` option. It also built would have
53 ``binutils/build/binutils/ar`` and ``nm-new`` which support plugins but
54 don't have a visible -plugin option, instead relying on the gold plugin
55 being present in ``../lib/bfd-plugins`` relative to where the binaries
58 * Build the LLVMgold plugin: Configure LLVM with
59 ``--with-binutils-include=/path/to/binutils/src/include`` and run
65 The linker takes a ``-plugin`` option that points to the path of
66 the plugin ``.so`` file. To find out what link command ``gcc``
67 would run in a given situation, run ``gcc -v [...]`` and
68 look for the line where it runs ``collect2``. Replace that with
69 ``ld-new -plugin /path/to/LLVMgold.so`` to test it out. Once you're
70 ready to switch to using gold, backup your existing ``/usr/bin/ld``
71 then replace it with ``ld-new``.
73 You can produce bitcode files from ``clang`` using ``-emit-llvm`` or
74 ``-flto``, or the ``-O4`` flag which is synonymous with ``-O3 -flto``.
76 Any of these flags will also cause ``clang`` to look for the gold plugin in
77 the ``lib`` directory under its prefix and pass the ``-plugin`` option to
78 ``ld``. It will not look for an alternate linker, which is why you need
79 gold to be the installed system linker in your path.
81 If you want ``ar`` and ``nm`` to work seamlessly as well, install
82 ``LLVMgold.so`` to ``/usr/lib/bfd-plugins``. If you built your own gold, be
83 sure to install the ``ar`` and ``nm-new`` you built to ``/usr/bin``.
86 Example of link time optimization
87 ---------------------------------
89 The following example shows a worked example of the gold plugin mixing LLVM
90 bitcode and native code.
97 extern void foo1(void);
98 extern void foo4(void);
115 extern void foo2(void);
127 --- command lines ---
128 $ clang -flto a.c -c -o a.o # <-- a.o is LLVM bitcode file
129 $ ar q a.a a.o # <-- a.a is an archive with LLVM bitcode
130 $ clang b.c -c -o b.o # <-- b.o is native object file
131 $ clang -flto a.a b.o -o main # <-- link with LLVMgold plugin
133 Gold informs the plugin that foo3 is never referenced outside the IR,
134 leading LLVM to delete that function. However, unlike in the :ref:`libLTO
135 example <libLTO-example>` gold does not currently eliminate foo4.
137 Quickstart for using LTO with autotooled projects
138 =================================================
140 Once your system ``ld``, ``ar``, and ``nm`` all support LLVM bitcode,
141 everything is in place for an easy to use LTO build of autotooled projects:
143 * Follow the instructions :ref:`on how to build LLVMgold.so
146 * Install the newly built binutils to ``$PREFIX``
148 * Copy ``Release/lib/LLVMgold.so`` to ``$PREFIX/lib/bfd-plugins/``
150 * Set environment variables (``$PREFIX`` is where you installed clang and
155 export CC="$PREFIX/bin/clang -flto"
156 export CXX="$PREFIX/bin/clang++ -flto"
157 export AR="$PREFIX/bin/ar"
158 export NM="$PREFIX/bin/nm"
159 export RANLIB=/bin/true #ranlib is not needed, and doesn't support .bc files in .a
162 * Or you can just set your path:
166 export PATH="$PREFIX/bin:$PATH"
167 export CC="clang -flto"
168 export CXX="clang++ -flto"
169 export RANLIB=/bin/true
171 * Configure and build the project as usual:
175 % ./configure && make && make check
177 The environment variable settings may work for non-autotooled projects too,
178 but you may need to set the ``LD`` environment variable as well.
183 Gold is licensed under the GPLv3. LLVMgold uses the interface file
184 ``plugin-api.h`` from gold which means that the resulting ``LLVMgold.so``
185 binary is also GPLv3. This can still be used to link non-GPLv3 programs
186 just as much as gold could without the plugin.