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