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+.. _gold-plugin:
+
+====================
+The LLVM gold plugin
+====================
+
+.. sectionauthor:: Nick Lewycky
+
+Introduction
+============
+
+Building with link time optimization requires cooperation from
+the system linker. LTO support on Linux systems requires that you use the
+`gold linker`_ which supports LTO via plugins. This is the same mechanism
+used by the `GCC LTO`_ project.
+
+The LLVM gold plugin implements the gold plugin interface on top of
+:ref:`libLTO`.  The same plugin can also be used by other tools such as
+``ar`` and ``nm``.
+
+.. _`gold linker`: http://sourceware.org/binutils
+.. _`GCC LTO`: http://gcc.gnu.org/wiki/LinkTimeOptimization
+.. _`gold plugin interface`: http://gcc.gnu.org/wiki/whopr/driver
+
+.. _lto-how-to-build:
+
+How to build it
+===============
+
+You need to have gold with plugin support and build the LLVMgold plugin.
+Check whether you have gold running ``/usr/bin/ld -v``. It will report "GNU
+gold" or else "GNU ld" if not. If you have gold, check for plugin support
+by running ``/usr/bin/ld -plugin``. If it complains "missing argument" then
+you have plugin support. If not, such as an "unknown option" error then you
+will either need to build gold or install a version with plugin support.
+
+* To build gold with plugin support:
+
+  .. code-block:: bash
+
+     $ mkdir binutils
+     $ cd binutils
+     $ cvs -z 9 -d :pserver:anoncvs@sourceware.org:/cvs/src login
+     {enter "anoncvs" as the password}
+     $ cvs -z 9 -d :pserver:anoncvs@sourceware.org:/cvs/src co binutils
+     $ mkdir build
+     $ cd build
+     $ ../src/configure --enable-gold --enable-plugins
+     $ make all-gold
+
+  That should leave you with ``binutils/build/gold/ld-new`` which supports
+  the ``-plugin`` option. It also built would have
+  ``binutils/build/binutils/ar`` and ``nm-new`` which support plugins but
+  don't have a visible -plugin option, instead relying on the gold plugin
+  being present in ``../lib/bfd-plugins`` relative to where the binaries
+  are placed.
+
+* Build the LLVMgold plugin: Configure LLVM with
+  ``--with-binutils-include=/path/to/binutils/src/include`` and run
+  ``make``.
+
+Usage
+=====
+
+The linker takes a ``-plugin`` option that points to the path of
+the plugin ``.so`` file. To find out what link command ``gcc``
+would run in a given situation, run ``gcc -v [...]`` and
+look for the line where it runs ``collect2``. Replace that with
+``ld-new -plugin /path/to/LLVMgold.so`` to test it out. Once you're
+ready to switch to using gold, backup your existing ``/usr/bin/ld``
+then replace it with ``ld-new``.
+
+You can produce bitcode files from ``clang`` using ``-emit-llvm`` or
+``-flto``, or the ``-O4`` flag which is synonymous with ``-O3 -flto``.
+
+Any of these flags will also cause ``clang`` to look for the gold plugin in
+the ``lib`` directory under its prefix and pass the ``-plugin`` option to
+``ld``. It will not look for an alternate linker, which is why you need
+gold to be the installed system linker in your path.
+
+If you want ``ar`` and ``nm`` to work seamlessly as well, install
+``LLVMgold.so`` to ``/usr/lib/bfd-plugins``. If you built your own gold, be
+sure to install the ``ar`` and ``nm-new`` you built to ``/usr/bin``.
+
+
+Example of link time optimization
+---------------------------------
+
+The following example shows a worked example of the gold plugin mixing LLVM
+bitcode and native code.
+
+.. code-block:: c
+
+   --- a.c ---
+   #include <stdio.h>
+
+   extern void foo1(void);
+   extern void foo4(void);
+
+   void foo2(void) {
+     printf("Foo2\n");
+   }
+
+   void foo3(void) {
+     foo4();
+   }
+
+   int main(void) {
+     foo1();
+   }
+
+   --- b.c ---
+   #include <stdio.h>
+
+   extern void foo2(void);
+
+   void foo1(void) {
+     foo2();
+   }
+
+   void foo4(void) {
+     printf("Foo4");
+   }
+
+.. code-block:: bash
+
+   --- command lines ---
+   $ clang -flto a.c -c -o a.o      # <-- a.o is LLVM bitcode file
+   $ ar q a.a a.o                   # <-- a.a is an archive with LLVM bitcode
+   $ clang b.c -c -o b.o            # <-- b.o is native object file
+   $ clang -flto a.a b.o -o main    # <-- link with LLVMgold plugin
+
+Gold informs the plugin that foo3 is never referenced outside the IR,
+leading LLVM to delete that function. However, unlike in the :ref:`libLTO
+example <libLTO-example>` gold does not currently eliminate foo4.
+
+Quickstart for using LTO with autotooled projects
+=================================================
+
+Once your system ``ld``, ``ar``, and ``nm`` all support LLVM bitcode,
+everything is in place for an easy to use LTO build of autotooled projects:
+
+* Follow the instructions :ref:`on how to build LLVMgold.so
+  <lto-how-to-build>`.
+
+* Install the newly built binutils to ``$PREFIX``
+
+* Copy ``Release/lib/LLVMgold.so`` to ``$PREFIX/lib/bfd-plugins/``
+
+* Set environment variables (``$PREFIX`` is where you installed clang and
+  binutils):
+
+  .. code-block:: bash
+
+     export CC="$PREFIX/bin/clang -flto"
+     export CXX="$PREFIX/bin/clang++ -flto"
+     export AR="$PREFIX/bin/ar"
+     export NM="$PREFIX/bin/nm"
+     export RANLIB=/bin/true #ranlib is not needed, and doesn't support .bc files in .a
+     export CFLAGS="-O4"
+
+* Or you can just set your path:
+
+  .. code-block:: bash
+
+     export PATH="$PREFIX/bin:$PATH"
+     export CC="clang -flto"
+     export CXX="clang++ -flto"
+     export RANLIB=/bin/true
+     export CFLAGS="-O4"
+* Configure and build the project as usual:
+
+  .. code-block:: bash
+
+     % ./configure && make && make check
+
+The environment variable settings may work for non-autotooled projects too,
+but you may need to set the ``LD`` environment variable as well.
+
+Licensing
+=========
+
+Gold is licensed under the GPLv3. LLVMgold uses the interface file
+``plugin-api.h`` from gold which means that the resulting ``LLVMgold.so``
+binary is also GPLv3. This can still be used to link non-GPLv3 programs
+just as much as gold could without the plugin.