tools/llvmc2/doc/LLVMC-Reference.rst

Customizing LLVMC: Reference Manual
===================================

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. This makes it possible to adapt LLVMC for other
purposes - for example, as a build tool for game resources.

Because LLVMC employs TableGen [1]_ as its configuration language, you
need to be familiar with it to customize LLVMC.

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:
     $ llvmc2 -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``::

      $ llvmc2 -x c hello.cpp
      $ # hello.cpp is really a C file
      $ ./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::

    $ llvmc2 -c hello.cpp
    $ llvmc2 hello.o
    [A lot of link-time errors skipped]
    $ llvmc2 --linker=c++ hello.o
    $ ./a.out
    hello


Customizing LLVMC: the compilation graph
----------------------------------------

At the time of writing LLVMC does not support on-the-fly reloading of
configuration, so to customize LLVMC you'll have to recompile the
source code (which lives under ``$LLVM_DIR/tools/llvmc2``). The
default configuration files are ``Common.td`` (contains common
definitions, don't forget to ``include`` it in your configuration
files), ``Tools.td`` (tool descriptions) and ``Graph.td`` (compilation
graph definition).

To compile LLVMC with your own configuration file (say,``MyGraph.td``),
run ``make`` like this::

    $ cd $LLVM_DIR/tools/llvmc2
    $ make GRAPH=MyGraph.td TOOLNAME=my_llvmc

This will build an executable named ``my_llvmc``. There are also
several sample configuration files in the ``llvmc2/examples``
subdirectory that should help to get you started.

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 ``Graph.td``) 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, [(switch_on "opt")]>,
        OptionalEdge<llvm_gcc_cpp, opt, [(switch_on "opt")]>,
        ...

        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 sporting a ``case`` expression
used to calculate the edge's weight.

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.

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*).

To get a visual representation of the compilation graph (useful for
debugging), run ``llvmc2 --view-graph``. You will need ``dot`` and
``gsview`` installed for this to work properly.


The 'case' expression
---------------------

The 'case' construct can be used to calculate weights of the optional
edges and to choose between several alternative command line strings
in the ``cmd_line`` tool property. It 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::

    // 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))

    // Evaluates to "cmdline1" if option "-A" is provided on the
    // command line, otherwise to "cmdline2"
    (case
        (switch_on "A"), ("cmdline1"),
        (default), ("cmdline2"))


* Possible tests are:

  - ``switch_on`` - Returns true if a given command-line option is
    provided by the user. Example: ``(switch_on "opt")``. Note that
    you have to define all possible command-line options separately in
    the tool descriptions. See the next section for the discussion of
    different kinds of command-line options.

  - ``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
    includes 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++")``.

  - ``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 ``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 lack explicit descriptions.

The complete list of the currently implemented tool properties follows:

* Possible tool properties:

  - ``in_language`` - input language name.

  - ``out_language`` - output language name.

  - ``output_suffix`` - output file suffix.

  - ``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 (more on this below).

  - ``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.

The next tool definition is slightly more complex::

  def llvm_gcc_linker : Tool<[
      (in_language "object-code"),
      (out_language "executable"),
      (output_suffix "out"),
      (cmd_line "llvm-gcc $INFILE -o $OUTFILE"),
      (join),
      (prefix_list_option "L", (forward),
                          (help "add a directory to link path")),
      (prefix_list_option "l", (forward),
                          (help "search a library when linking")),
      (prefix_list_option "Wl", (unpack_values),
                          (help "pass options to linker"))
      ]>;

This tool has a "join" property, which means that it behaves like a
linker. This tool also defines several command-line options: ``-l``,
``-L`` and ``-Wl`` which have their usual meaning. An option has two
attributes: a name and a (possibly empty) list of properties. All
currently implemented option types and properties are described below:

* 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``.


* Possible option properties:

   - ``append_cmd`` - append a string to the tool invocation command.

   - ``forward`` - forward this option unchanged.

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

   - ``stop_compilation`` - stop compilation after this phase.

   - ``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.

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

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


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 compile LLVMC with your hooks, just drop a .cpp file into
``tools/llvmc2`` 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 (which we have already seen before)::

    (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"))


Language map
------------

One last thing that you will need to modify when adding support for a
new language to LLVMC is 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 is
located in the file ``Tools.td`` and looks like this::

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


References
==========

.. [1] TableGen Fundamentals
       http://llvm.cs.uiuc.edu/docs/TableGenFundamentals.html