xref: /external/llvm/tools/llvmc/doc/LLVMC-Reference.rst

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

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 described in
high-level TableGen code, from which an efficient C++ representation is
automatically derived. This makes it possible to 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.org/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++`` vs.  ``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 ``clang`` compiler with the ``-clang`` option.


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

LLVMC has some built-in options that can't be overridden in the TableGen code:

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

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

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

* ``--save-temps`` - Write temporary files to the current directory and do not
  delete them on exit. This option can also take an argument: the
  ``--save-temps=obj`` switch will write files into the directory specified with
  the ``-o`` option. The ``--save-temps=cwd`` and ``--save-temps`` switches are
  both synonyms for the default behaviour.

* ``--temp-dir DIRECTORY`` - Store temporary files in the given directory. This
  directory is deleted on exit unless ``--save-temps`` is specified. If
  ``--save-temps=obj`` is also specified, ``--temp-dir`` is given the
  precedence.

* ``--check-graph`` - Check the compilation for common errors like mismatched
  output/input language names, multiple default edges and cycles. Exit with code
  zero if no errors were found, and return the number of found errors
  otherwise. Hidden option, useful for debugging.

* ``--view-graph`` - Show a graphical representation of the compilation graph
  and exit. 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 Graphviz format (identical
  to the file used by the ``--view-graph`` option). The ``-o`` option can be
  used to set the output file name. Hidden option, useful for debugging.

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

Compiling LLVMC-based drivers
=============================

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

   $ cd $LLVMC_DIR/examples
   $ cp -r Skeleton MyDriver
   $ cd MyDriver
   $ ls
   AutoGenerated.td  Hooks.cpp  Main.cpp  Makefile

As you can see, our basic driver consists of only three files (not counting the
build script). ``AutoGenerated.td`` contains TableGen description of the
compilation graph; its format is documented in the following
sections. ``Hooks.cpp`` is an empty file that should be used for hook
definitions (see `below`__). ``Main.cpp`` is just a helper used to compile the
auto-generated C++ code produced from TableGen source.

__ hooks_

The first thing that you should do is to change the ``LLVMC_BASED_DRIVER``
variable in the ``Makefile``::

   LLVMC_BASED_DRIVER=MyDriver

It can also be a good idea to put your TableGen code into a file with a less
generic name::

   $ touch MyDriver.td
   $ vim AutoGenerated.td
   [...]
   include "MyDriver.td"

If you have more than one TableGen source file, they all should be included from
``AutoGenerated.td``, since this file is used by the build system to generate
C++ code.

To build your driver, just ``cd`` to its source directory and run ``make``. The
resulting executable will be put into ``$LLVM_OBJ_DIR/$(BuildMode)/bin``.

If you're compiling LLVM with different source and object directories, then you
must perform the following additional steps before running ``make``::

    # LLVMC_SRC_DIR = $LLVM_SRC_DIR/tools/llvmc/
    # LLVMC_OBJ_DIR = $LLVM_OBJ_DIR/tools/llvmc/
    $ mkdir $LLVMC_OBJ_DIR/examples/MyDriver/
    $ cp $LLVMC_SRC_DIR/examples/MyDriver/Makefile \
      $LLVMC_OBJ_DIR/examples/MyDriver/
    $ cd $LLVMC_OBJ_DIR/examples/MyDriver
    $ make


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 ``llvmc/src/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
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. Default weight of an
optional edge can be changed 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 compilation graphs are defined, they are merged together. Multiple
edges with the same end nodes are not allowed (i.e. the graph is not a
multigraph), and will lead to a compile-time error.

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 supported by the driver 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. More than
one option list can be defined (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 without arguments, for example
     ``-O2`` or ``-time``. At most one occurrence is allowed by default.

   - ``parameter_option`` - option that takes one argument, for example
     ``-std=c99``. It is also allowed to use spaces instead of the equality
     sign: ``-std c99``. At most one occurrence is allowed.

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

   - ``prefix_option`` - same as the parameter_option, but the option name and
     argument do not have to be separated. Example: ``-ofile``. This can be also
     specified as ``-o file``; however, ``-o=file`` will be parsed incorrectly
     (``=file`` will be interpreted as option value). At most one occurrence is
     allowed.

   - ``prefix_list_option`` - same as the above, but more than one occurrence 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")``

   - ``switch_list_option`` - like ``switch_option`` with the ``zero_or_more``
     property, but remembers how many times the switch was turned on. Useful
     mostly for forwarding. Example: when ``-foo`` is a switch option (with the
     ``zero_or_more`` property), the command ``driver -foo -foo`` is forwarded
     as ``some-tool -foo``, but when ``-foo`` is a switch list, the same command
     is forwarded as ``some-tool -foo -foo``.


* Possible option properties:

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

   - ``required`` - this option must be specified exactly once (or, in case of
     the list options without the ``multi_val`` property, at least
     once). Incompatible with ``optional`` and ``one_or_more``.

   - ``optional`` - the option can be specified either zero times or exactly
     once. The default for switch options. Useful only for list options in
     conjunction with ``multi_val``. Incompatible with ``required``,
     ``zero_or_more`` and ``one_or_more``.

   - ``one_or_more`` - the option must be specified at least once. Can be useful
     to allow switch options be both obligatory and be specified multiple
     times. For list options is useful only in conjunction with ``multi_val``;
     for ordinary it is synonymous with ``required``. Incompatible with
     ``required``, ``optional`` and ``zero_or_more``.

   - ``zero_or_more`` - the option can be specified zero or more times. Useful
     to allow a single switch option to be specified more than
     once. Incompatible with ``required``, ``optional`` and ``one_or_more``.

   - ``hidden`` - the description of this option will not appear in
     the ``--help`` output (but will appear in the ``--help-hidden``
     output).

   - ``really_hidden`` - the option will not be mentioned in any help
     output.

   - ``comma_separated`` - Indicates that any commas specified for an option's
     value should be used to split the value up into multiple values for the
     option. This property is valid only for list options. In conjunction with
     ``forward_value`` can be used to implement option forwarding in style of
     gcc's ``-Wa,``.

   - ``multi_val n`` - this option takes *n* arguments (can be useful in some
     special cases). Usage example: ``(parameter_list_option "foo", (multi_val
     3))``; the command-line syntax is '-foo a b c'. Only list options can have
     this attribute; you can, however, use the ``one_or_more``, ``optional``
     and ``required`` properties.

   - ``init`` - this option has a default value, either a string (if it is a
     parameter), or a boolean (if it is a switch; as in C++, boolean constants
     are called ``true`` and ``false``). List options can't have ``init``
     attribute.
     Usage examples: ``(switch_option "foo", (init true))``; ``(prefix_option
     "bar", (init "baz"))``.

.. _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. Can be given multiple arguments, in that case ``(switch_on "foo",
    "bar", "baz")`` is equivalent to ``(and (switch_on "foo"), (switch_on
    "bar"), (switch_on "baz"))``.
    Example: ``(switch_on "opt")``.

  - ``any_switch_on`` - Given a number of switch options, returns true if any of
    the switches is turned on.
    Example: ``(any_switch_on "foo", "bar", "baz")`` is equivalent to ``(or
    (switch_on "foo"), (switch_on "bar"), (switch_on "baz"))``.

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

  - ``element_in_list`` - Returns true if a command-line parameter list (first
    argument) contains a given value (second argument).
    Example: ``(element_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 is equal to
    the argument. At the moment works only with ``command`` 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. Like ``switch_on``, can
    be also given multiple arguments.
    Examples: ``(not_empty "o")``, ``(not_empty "o", "l")``.

  - ``any_not_empty`` - Returns true if ``not_empty`` returns true for any of
    the provided options.
    Example: ``(any_not_empty "foo", "bar", "baz")`` is equivalent to ``(or
    (not_empty "foo"), (not_empty "bar"), (not_empty "baz"))``.

  - ``empty`` - The opposite of ``not_empty``. Equivalent to ``(not (not_empty
    X))``. Can be given multiple arguments.

  - ``any_not_empty`` - Returns true if ``not_empty`` returns true for any of
    the provided options.
    Example: ``(any_empty "foo", "bar", "baz")`` is equivalent to ``(or
    (not_empty "foo"), (not_empty "bar"), (not_empty "baz"))``.

  - ``single_input_file`` - Returns true if there was only one input file
    provided on the command-line. Used without arguments:
    ``(single_input_file)``.

  - ``multiple_input_files`` - Equivalent to ``(not (single_input_file))`` (the
    case of zero input files is considered an error).

  - ``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`` - A logical combinator that returns true iff any of its arguments
    return true.
    Example: ``(or (test1), (test2), ... (testN))``.

  - ``not`` - Standard unary logical combinator that negates its
    argument.
    Example: ``(not (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
``llvmc/src/Base.td`` file)::

  def llvm_gcc_cpp : Tool<[
      (in_language "c++"),
      (out_language "llvm-assembler"),
      (output_suffix "bc"),
      (command "llvm-g++ -c -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 given multiple arguments, in
    case the tool supports multiple input languages. Used for typechecking and
    mapping file extensions to tools.

  - ``out_language`` - output language name. Multiple output languages are
    allowed. Used for typechecking the compilation graph.

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

__ actions_

  - ``command`` - the actual command used to run the tool. You can use 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_

  - ``out_file_option``, ``in_file_option`` - Options appended to the
    ``command`` string to designate output and input files. Default values are
    ``"-o"`` and ``""``, respectively.

.. _actions:

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"),
      (command "llvm-gcc"),
      (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"))``.

   - ``error`` - Exit with error.
     Example: ``(error "Mixing -c and -S is not allowed!")``.

   - ``warning`` - Print a warning.
     Example: ``(warning "Specifying both -O1 and -O2 is meaningless!")``.

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

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

   - ``forward_value`` - Forward only option's value. Cannot be used with switch
     options (since they don't have values), but works fine with lists.
     Example: ``(forward_value "Wa,")``.

   - ``forward_transformed_value`` - As above, but applies a hook to the
     option's value before forwarding (see `below`__). When
     ``forward_transformed_value`` is applied to a list
     option, the hook must have signature
     ``std::string hooks::HookName (const std::vector<std::string>&)``.
     Example: ``(forward_transformed_value "m", "ConvertToMAttr")``.

     __ hooks_

   - ``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.
     Example: ``(stop_compilation)``.


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 are needed only for the tools that are linked from the
root node. A tool can have multiple output languages.

Option preprocessor
===================

It is sometimes useful to run error-checking code before processing the
compilation graph. For example, if optimization options "-O1" and "-O2" are
implemented as switches, we might want to output a warning if the user invokes
the driver with both of these options enabled.

The ``OptionPreprocessor`` feature is reserved specially for these
occasions. Example (adapted from ``llvm/src/Base.td.in``)::


    def Preprocess : OptionPreprocessor<
    (case (not (any_switch_on "O0", "O1", "O2", "O3")),
               (set_option "O2"),
          (and (switch_on "O3"), (any_switch_on "O0", "O1", "O2")),
               (unset_option "O0", "O1", "O2"),
          (and (switch_on "O2"), (any_switch_on "O0", "O1")),
               (unset_option "O0", "O1"),
          (and (switch_on "O1"), (switch_on "O0")),
               (unset_option "O0"))
    >;

Here, ``OptionPreprocessor`` is used to unset all spurious ``-O`` options so
that they are not forwarded to the compiler. If no optimization options are
specified, ``-O2`` is enabled.

``OptionPreprocessor`` is basically a single big ``case`` expression, which is
evaluated only once right after the driver is started. The only allowed actions
in ``OptionPreprocessor`` are ``error``, ``warning``, and two special actions:
``unset_option`` and ``set_option``. As their names suggest, they can be used to
set or unset a given option. To set an option with ``set_option``, use the
two-argument form: ``(set_option "parameter", VALUE)``. Here, ``VALUE`` can be
either a string, a string list, or a boolean constant.

For convenience, ``set_option`` and ``unset_option`` also work with multiple
arguments. That is, instead of ``[(unset_option "A"), (unset_option "B")]`` you
can use ``(unset_option "A", "B")``. Obviously, ``(set_option "A", "B")`` is
only valid if both ``A`` and ``B`` are switches.


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

.. _hooks:

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

Normally, LLVMC searches for programs in the system ``PATH``. Sometimes, this is
not sufficient: for example, we may want to specify tool paths or names in the
configuration file. This can be achieved via the hooks mechanism. To write your
own hooks, add their definitions to the ``Hooks.cpp`` or drop a ``.cpp`` file
into your driver directory. Hooks should live in the ``hooks`` namespace and
have the signature ``std::string hooks::MyHookName ([const char* Arg0 [ const
char* Arg2 [, ...]]])``. They can be used from the ``command`` tool property::

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

To pass arguments to hooks, use the following syntax::

    (command "$CALL(MyHook, 'Arg1', 'Arg2', 'Arg # 3')/path/to/file -o1 -o2")

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

   (command "$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`__)::

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

__ case_

Debugging
---------

When writing LLVMC-based drivers, 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`` (which assumes that Graphviz_ and Ghostview_ are
installed). There is also a ``--write-graph`` option that creates a Graphviz
source file (``compilation-graph.dot``) in the current directory.

Another useful ``llvmc`` option is ``--check-graph``. It checks the compilation
graph for common errors like mismatched output/input language names, multiple
default edges and cycles. When invoked with ``--check-graph``, ``llvmc`` doesn't
perform any compilation tasks and returns the number of encountered errors as
its status code. In the future, these checks will be performed at compile-time
and this option will disappear.

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

Conditioning on the executable name
-----------------------------------

For now, the executable name (the value passed to the driver in ``argv[0]``) is
accessible only in the C++ code (i.e. hooks). Use the following code::

    namespace llvmc {
    extern const char* ProgramName;
    }

    namespace hooks {

    std::string MyHook() {
    //...
    if (strcmp(ProgramName, "mydriver") == 0) {
       //...

    }

    } // end namespace hooks

In general, you're encouraged not to make the behaviour dependent on the
executable file name, and use command-line switches instead. See for example how
the ``llvmc`` program behaves when it needs to choose the correct linker options
(think ``g++`` vs. ``gcc``).

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   <a href="mailto:foldr@codedgers.com">Mikhail Glushenkov</a><br />
   <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br />

   Last modified: $Date: 2008-12-11 11:34:48 -0600 (Thu, 11 Dec 2008) $
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