tools/llvmc/doc/LLVMC-Reference.rst

===================================
Customizing LLVMC: Reference Manual
===================================
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   <p>Written by <a href="mailto:foldr@codedgers.com">Mikhail Glushenkov</a></p>
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Introduction
============

LLVMC is a generic compiler driver, designed to be customizable and
extensible. It plays the same role for LLVM as the ``gcc`` program
does for GCC - LLVMC's job is essentially to transform a set of input
files into a set of targets depending on configuration rules and user
options. What makes LLVMC different is that these transformation rules
are completely customizable - in fact, LLVMC knows nothing about the
specifics of transformation (even the command-line options are mostly
not hard-coded) and regards the transformation structure as an
abstract graph. The structure of this graph is completely determined
by plugins, which can be either statically or dynamically linked. This
makes it possible to easily adapt LLVMC for other purposes - for
example, as a build tool for game resources.

Because LLVMC employs TableGen_ as its configuration language, you
need to be familiar with it to customize LLVMC.

.. _TableGen: http://llvm.cs.uiuc.edu/docs/TableGenFundamentals.html


Compiling with LLVMC
====================

LLVMC tries hard to be as compatible with ``gcc`` as possible,
although there are some small differences. Most of the time, however,
you shouldn't be able to notice them::

     $ # This works as expected:
     $ llvmc -O3 -Wall hello.cpp
     $ ./a.out
     hello

One nice feature of LLVMC is that one doesn't have to distinguish
between different compilers for different languages (think ``g++`` and
``gcc``) - the right toolchain is chosen automatically based on input
language names (which are, in turn, determined from file
extensions). If you want to force files ending with ".c" to compile as
C++, use the ``-x`` option, just like you would do it with ``gcc``::

      $ # hello.c is really a C++ file
      $ llvmc -x c++ hello.c
      $ ./a.out
      hello

On the other hand, when using LLVMC as a linker to combine several C++
object files you should provide the ``--linker`` option since it's
impossible for LLVMC to choose the right linker in that case::

    $ llvmc -c hello.cpp
    $ llvmc hello.o
    [A lot of link-time errors skipped]
    $ llvmc --linker=c++ hello.o
    $ ./a.out
    hello

By default, LLVMC uses ``llvm-gcc`` to compile the source code. It is
also possible to choose the work-in-progress ``clang`` compiler with
the ``-clang`` option.


Predefined options
==================

LLVMC has some built-in options that can't be overridden in the
configuration libraries:

* ``-o FILE`` - Output file name.

* ``-x LANGUAGE`` - Specify the language of the following input files
  until the next -x option.

* ``-load PLUGIN_NAME`` - Load the specified plugin DLL. Example:
  ``-load $LLVM_DIR/Release/lib/LLVMCSimple.so``.

* ``-v`` - Enable verbose mode, i.e. print out all executed commands.

* ``--view-graph`` - Show a graphical representation of the compilation
  graph. Requires that you have ``dot`` and ``gv`` programs
  installed. Hidden option, useful for debugging.

* ``--write-graph`` - Write a ``compilation-graph.dot`` file in the
  current directory with the compilation graph description in the
  Graphviz format. Hidden option, useful for debugging.

* ``--save-temps`` - Write temporary files to the current directory
  and do not delete them on exit. Hidden option, useful for debugging.

* ``--help``, ``--help-hidden``, ``--version`` - These options have
  their standard meaning.


Compiling LLVMC plugins
=======================

It's easiest to start working on your own LLVMC plugin by copying the
skeleton project which lives under ``$LLVMC_DIR/plugins/Simple``::

   $ cd $LLVMC_DIR/plugins
   $ cp -r Simple MyPlugin
   $ cd MyPlugin
   $ ls
   Makefile PluginMain.cpp Simple.td

As you can see, our basic plugin consists of only two files (not
counting the build script). ``Simple.td`` contains TableGen
description of the compilation graph; its format is documented in the
following sections. ``PluginMain.cpp`` is just a helper file used to
compile the auto-generated C++ code produced from TableGen source. It
can also contain hook definitions (see `below`__).

__ hooks_

The first thing that you should do is to change the ``LLVMC_PLUGIN``
variable in the ``Makefile`` to avoid conflicts (since this variable
is used to name the resulting library)::

   LLVMC_PLUGIN=MyPlugin

It is also a good idea to rename ``Simple.td`` to something less
generic::

   $ mv Simple.td MyPlugin.td

Note that the plugin source directory must be placed under
``$LLVMC_DIR/plugins`` to make use of the existing build
infrastructure. To build a version of the LLVMC executable called
``mydriver`` with your plugin compiled in, use the following command::

   $ cd $LLVMC_DIR
   $ make BUILTIN_PLUGINS=MyPlugin DRIVER_NAME=mydriver

To build your plugin as a dynamic library, just ``cd`` to its source
directory and run ``make``. The resulting file will be called
``LLVMC$(LLVMC_PLUGIN).$(DLL_EXTENSION)`` (in our case,
``LLVMCMyPlugin.so``). This library can be then loaded in with the
``-load`` option. Example::

    $ cd $LLVMC_DIR/plugins/Simple
    $ make
    $ llvmc -load $LLVM_DIR/Release/lib/LLVMCSimple.so

Sometimes, you will want a 'bare-bones' version of LLVMC that has no
built-in plugins. It can be compiled with the following command::

    $ cd $LLVMC_DIR
    $ make BUILTIN_PLUGINS=""


Customizing LLVMC: the compilation graph
========================================

Each TableGen configuration file should include the common
definitions::

   include "llvm/CompilerDriver/Common.td"

Internally, LLVMC stores information about possible source
transformations in form of a graph. Nodes in this graph represent
tools, and edges between two nodes represent a transformation path. A
special "root" node is used to mark entry points for the
transformations. LLVMC also assigns a weight to each edge (more on
this later) to choose between several alternative edges.

The definition of the compilation graph (see file
``plugins/Base/Base.td`` for an example) is just a list of edges::

    def CompilationGraph : CompilationGraph<[
        Edge<"root", "llvm_gcc_c">,
        Edge<"root", "llvm_gcc_assembler">,
        ...

        Edge<"llvm_gcc_c", "llc">,
        Edge<"llvm_gcc_cpp", "llc">,
        ...

        OptionalEdge<"llvm_gcc_c", "opt", (case (switch_on "opt"),
                                          (inc_weight))>,
        OptionalEdge<"llvm_gcc_cpp", "opt", (case (switch_on "opt"),
                                                  (inc_weight))>,
        ...

        OptionalEdge<"llvm_gcc_assembler", "llvm_gcc_cpp_linker",
            (case (input_languages_contain "c++"), (inc_weight),
                  (or (parameter_equals "linker", "g++"),
                      (parameter_equals "linker", "c++")), (inc_weight))>,
        ...

        ]>;

As you can see, the edges can be either default or optional, where
optional edges are differentiated by an additional ``case`` expression
used to calculate the weight of this edge. Notice also that we refer
to tools via their names (as strings). This makes it possible to add
edges to an existing compilation graph in plugins without having to
know about all tool definitions used in the graph.

The default edges are assigned a weight of 1, and optional edges get a
weight of 0 + 2*N where N is the number of tests that evaluated to
true in the ``case`` expression. It is also possible to provide an
integer parameter to ``inc_weight`` and ``dec_weight`` - in this case,
the weight is increased (or decreased) by the provided value instead
of the default 2. It is also possible to change the default weight of
an optional edge by using the ``default`` clause of the ``case``
construct.

When passing an input file through the graph, LLVMC picks the edge
with the maximum weight. To avoid ambiguity, there should be only one
default edge between two nodes (with the exception of the root node,
which gets a special treatment - there you are allowed to specify one
default edge *per language*).

When multiple plugins are loaded, their compilation graphs are merged
together. Since multiple edges that have the same end nodes are not
allowed (i.e. the graph is not a multigraph), an edge defined in
several plugins will be replaced by the definition from the plugin
that was loaded last. Plugin load order can be controlled by using the
plugin priority feature described above.

To get a visual representation of the compilation graph (useful for
debugging), run ``llvmc --view-graph``. You will need ``dot`` and
``gsview`` installed for this to work properly.

Describing options
==================

Command-line options that the plugin supports are defined by using an
``OptionList``::

    def Options : OptionList<[
    (switch_option "E", (help "Help string")),
    (alias_option "quiet", "q")
    ...
    ]>;

As you can see, the option list is just a list of DAGs, where each DAG
is an option description consisting of the option name and some
properties. A plugin can define more than one option list (they are
all merged together in the end), which can be handy if one wants to
separate option groups syntactically.

* Possible option types:

   - ``switch_option`` - a simple boolean switch, for example ``-time``.

   - ``parameter_option`` - option that takes an argument, for example
     ``-std=c99``;

   - ``parameter_list_option`` - same as the above, but more than one
     occurence of the option is allowed.

   - ``prefix_option`` - same as the parameter_option, but the option name
     and parameter value are not separated.

   - ``prefix_list_option`` - same as the above, but more than one
     occurence of the option is allowed; example: ``-lm -lpthread``.

   - ``alias_option`` - a special option type for creating
     aliases. Unlike other option types, aliases are not allowed to
     have any properties besides the aliased option name. Usage
     example: ``(alias_option "preprocess", "E")``


* Possible option properties:

   - ``help`` - help string associated with this option. Used for
     ``--help`` output.

   - ``required`` - this option is obligatory.

   - ``hidden`` - this option should not appear in the ``--help``
     output (but should appear in the ``--help-hidden`` output).

   - ``really_hidden`` - the option should not appear in any help
     output.

   - ``extern`` - this option is defined in some other plugin, see below.

External options
----------------

Sometimes, when linking several plugins together, one plugin needs to
access options defined in some other plugin. Because of the way
options are implemented, such options should be marked as
``extern``. This is what the ``extern`` option property is
for. Example::

     ...
     (switch_option "E", (extern))
     ...

See also the section on plugin `priorities`__.

__ priorities_

.. _case:

Conditional evaluation
======================

The 'case' construct is the main means by which programmability is
achieved in LLVMC. It can be used to calculate edge weights, program
actions and modify the shell commands to be executed. The 'case'
expression is designed after the similarly-named construct in
functional languages and takes the form ``(case (test_1), statement_1,
(test_2), statement_2, ... (test_N), statement_N)``. The statements
are evaluated only if the corresponding tests evaluate to true.

Examples::

    // Edge weight calculation

    // Increases edge weight by 5 if "-A" is provided on the
    // command-line, and by 5 more if "-B" is also provided.
    (case
        (switch_on "A"), (inc_weight 5),
        (switch_on "B"), (inc_weight 5))


    // Tool command line specification

    // Evaluates to "cmdline1" if the option "-A" is provided on the
    // command line; to "cmdline2" if "-B" is provided;
    // otherwise to "cmdline3".

    (case
        (switch_on "A"), "cmdline1",
        (switch_on "B"), "cmdline2",
        (default), "cmdline3")

Note the slight difference in 'case' expression handling in contexts
of edge weights and command line specification - in the second example
the value of the ``"B"`` switch is never checked when switch ``"A"`` is
enabled, and the whole expression always evaluates to ``"cmdline1"`` in
that case.

Case expressions can also be nested, i.e. the following is legal::

    (case (switch_on "E"), (case (switch_on "o"), ..., (default), ...)
          (default), ...)

You should, however, try to avoid doing that because it hurts
readability. It is usually better to split tool descriptions and/or
use TableGen inheritance instead.

* Possible tests are:

  - ``switch_on`` - Returns true if a given command-line switch is
    provided by the user. Example: ``(switch_on "opt")``.

  - ``parameter_equals`` - Returns true if a command-line parameter equals
    a given value.
    Example: ``(parameter_equals "W", "all")``.

  - ``element_in_list`` - Returns true if a command-line parameter
    list contains a given value.
    Example: ``(parameter_in_list "l", "pthread")``.

  - ``input_languages_contain`` - Returns true if a given language
    belongs to the current input language set.
    Example: ``(input_languages_contain "c++")``.

  - ``in_language`` - Evaluates to true if the input file language
    equals to the argument. At the moment works only with ``cmd_line``
    and ``actions`` (on non-join nodes).
    Example: ``(in_language "c++")``.

  - ``not_empty`` - Returns true if a given option (which should be
    either a parameter or a parameter list) is set by the
    user.
    Example: ``(not_empty "o")``.

  - ``default`` - Always evaluates to true. Should always be the last
    test in the ``case`` expression.

  - ``and`` - A standard logical combinator that returns true iff all
    of its arguments return true. Used like this: ``(and (test1),
    (test2), ... (testN))``. Nesting of ``and`` and ``or`` is allowed,
    but not encouraged.

  - ``or`` - Another logical combinator that returns true only if any
    one of its arguments returns true. Example: ``(or (test1),
    (test2), ... (testN))``.


Writing a tool description
==========================

As was said earlier, nodes in the compilation graph represent tools,
which are described separately. A tool definition looks like this
(taken from the ``include/llvm/CompilerDriver/Tools.td`` file)::

  def llvm_gcc_cpp : Tool<[
      (in_language "c++"),
      (out_language "llvm-assembler"),
      (output_suffix "bc"),
      (cmd_line "llvm-g++ -c $INFILE -o $OUTFILE -emit-llvm"),
      (sink)
      ]>;

This defines a new tool called ``llvm_gcc_cpp``, which is an alias for
``llvm-g++``. As you can see, a tool definition is just a list of
properties; most of them should be self-explanatory. The ``sink``
property means that this tool should be passed all command-line
options that aren't mentioned in the option list.

The complete list of all currently implemented tool properties follows.

* Possible tool properties:

  - ``in_language`` - input language name. Can be either a string or a
    list, in case the tool supports multiple input languages.

  - ``out_language`` - output language name. Tools are not allowed to
    have multiple output languages.

  - ``output_suffix`` - output file suffix. Can also be changed
    dynamically, see documentation on actions.

  - ``cmd_line`` - the actual command used to run the tool. You can
    use ``$INFILE`` and ``$OUTFILE`` variables, output redirection
    with ``>``, hook invocations (``$CALL``), environment variables
    (via ``$ENV``) and the ``case`` construct.

  - ``join`` - this tool is a "join node" in the graph, i.e. it gets a
    list of input files and joins them together. Used for linkers.

  - ``sink`` - all command-line options that are not handled by other
    tools are passed to this tool.

  - ``actions`` - A single big ``case`` expression that specifies how
    this tool reacts on command-line options (described in more detail
    below).

Actions
-------

A tool often needs to react to command-line options, and this is
precisely what the ``actions`` property is for. The next example
illustrates this feature::

  def llvm_gcc_linker : Tool<[
      (in_language "object-code"),
      (out_language "executable"),
      (output_suffix "out"),
      (cmd_line "llvm-gcc $INFILE -o $OUTFILE"),
      (join),
      (actions (case (not_empty "L"), (forward "L"),
                     (not_empty "l"), (forward "l"),
                     (not_empty "dummy"),
                               [(append_cmd "-dummy1"), (append_cmd "-dummy2")])
      ]>;

The ``actions`` tool property is implemented on top of the omnipresent
``case`` expression. It associates one or more different *actions*
with given conditions - in the example, the actions are ``forward``,
which forwards a given option unchanged, and ``append_cmd``, which
appends a given string to the tool execution command. Multiple actions
can be associated with a single condition by using a list of actions
(used in the example to append some dummy options). The same ``case``
construct can also be used in the ``cmd_line`` property to modify the
tool command line.

The "join" property used in the example means that this tool behaves
like a linker.

The list of all possible actions follows.

* Possible actions:

   - ``append_cmd`` - append a string to the tool invocation
     command.
     Example: ``(case (switch_on "pthread"), (append_cmd "-lpthread"))``

   - ``forward`` - forward an option unchanged.
     Example: ``(forward "Wall")``.

   - ``forward_as`` - Change the name of an option, but forward the
     argument unchanged.
     Example: ``(forward_as "O0" "--disable-optimization")``.

   - ``output_suffix`` - modify the output suffix of this
     tool.
     Example: ``(output_suffix "i")``.

   - ``stop_compilation`` - stop compilation after this tool processes
     its input. Used without arguments.

   - ``unpack_values`` - used for for splitting and forwarding
     comma-separated lists of options, e.g. ``-Wa,-foo=bar,-baz`` is
     converted to ``-foo=bar -baz`` and appended to the tool invocation
     command.
     Example: ``(unpack_values "Wa,")``.

Language map
============

If you are adding support for a new language to LLVMC, you'll need to
modify the language map, which defines mappings from file extensions
to language names. It is used to choose the proper toolchain(s) for a
given input file set. Language map definition looks like this::

    def LanguageMap : LanguageMap<
        [LangToSuffixes<"c++", ["cc", "cp", "cxx", "cpp", "CPP", "c++", "C"]>,
         LangToSuffixes<"c", ["c"]>,
         ...
        ]>;

For example, without those definitions the following command wouldn't work::

    $ llvmc hello.cpp
    llvmc: Unknown suffix: cpp

The language map entries should be added only for tools that are
linked with the root node. Since tools are not allowed to have
multiple output languages, for nodes "inside" the graph the input and
output languages should match. This is enforced at compile-time.


More advanced topics
====================

.. _hooks:

Hooks and environment variables
-------------------------------

Normally, LLVMC executes programs from the system ``PATH``. Sometimes,
this is not sufficient: for example, we may want to specify tool names
in the configuration file. This can be achieved via the mechanism of
hooks - to write your own hooks, just add their definitions to the
``PluginMain.cpp`` or drop a ``.cpp`` file into the
``$LLVMC_DIR/driver`` directory. Hooks should live in the ``hooks``
namespace and have the signature ``std::string hooks::MyHookName
(void)``. They can be used from the ``cmd_line`` tool property::

    (cmd_line "$CALL(MyHook)/path/to/file -o $CALL(AnotherHook)")

It is also possible to use environment variables in the same manner::

   (cmd_line "$ENV(VAR1)/path/to/file -o $ENV(VAR2)")

To change the command line string based on user-provided options use
the ``case`` expression (documented `above`__)::

    (cmd_line
      (case
        (switch_on "E"),
           "llvm-g++ -E -x c $INFILE -o $OUTFILE",
        (default),
           "llvm-g++ -c -x c $INFILE -o $OUTFILE -emit-llvm"))

__ case_

.. _priorities:

How plugins are loaded
----------------------

It is possible for LLVMC plugins to depend on each other. For example,
one can create edges between nodes defined in some other plugin. To
make this work, however, that plugin should be loaded first. To
achieve this, the concept of plugin priority was introduced. By
default, every plugin has priority zero; to specify the priority
explicitly, put the following line in your plugin's TableGen file::

    def Priority : PluginPriority<$PRIORITY_VALUE>;
    # Where PRIORITY_VALUE is some integer > 0

Plugins are loaded in order of their (increasing) priority, starting
with 0. Therefore, the plugin with the highest priority value will be
loaded last.

Debugging
---------

When writing LLVMC plugins, it can be useful to get a visual view of
the resulting compilation graph. This can be achieved via the command
line option ``--view-graph``. This command assumes that Graphviz_ and
Ghostview_ are installed. There is also a ``--dump-graph`` option that
creates a Graphviz source file (``compilation-graph.dot``) in the
current directory.

.. _Graphviz: http://www.graphviz.org/
.. _Ghostview: http://pages.cs.wisc.edu/~ghost/

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