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===================================
Customizing LLVMC: Reference Manual
===================================
..
This file was automatically generated by rst2html.
Please do not edit directly!
The ReST source lives in the directory 'tools/llvmc/doc'.
.. contents::
.. raw:: html
<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 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.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
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 $LLVM_DIR/Release/lib/LLVMCSimple.so``.
* ``-v`` - Enable verbose mode, i.e. print out all executed commands.
* ``--save-temps`` - Write temporary files to the current directory and do not
delete them on exit. This option can also take an argument: the
``--save-temps=obj`` switch will write files into the directory specified with
the ``-o`` option. The ``--save-temps=cwd`` and ``--save-temps`` switches are
both synonyms for the default behaviour.
* ``--temp-dir DIRECTORY`` - Store temporary files in the given directory. This
directory is deleted on exit unless ``--save-temps`` is specified. If
``--save-temps=obj`` is also specified, ``--temp-dir`` is given the
precedence.
* ``--check-graph`` - Check the compilation for common errors like mismatched
output/input language names, multiple default edges and cycles. Because of
plugins, these checks can't be performed at compile-time. Exit with code zero
if no errors were found, and return the number of found errors
otherwise. Hidden option, useful for debugging LLVMC plugins.
* ``--view-graph`` - Show a graphical representation of the compilation graph
and exit. Requires that you have ``dot`` and ``gv`` programs installed. Hidden
option, useful for debugging LLVMC plugins.
* ``--write-graph`` - Write a ``compilation-graph.dot`` file in the current
directory with the compilation graph description in Graphviz format (identical
to the file used by the ``--view-graph`` option). The ``-o`` option can be
used to set the output file name. Hidden option, useful for debugging LLVMC
plugins.
* ``--help``, ``--help-hidden``, ``--version`` - These options have
their standard meaning.
Compiling LLVMC plugins
=======================
It's easiest to start working on your own LLVMC plugin by copying the
skeleton project which lives under ``$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
To build your plugin as a dynamic library, just ``cd`` to its source
directory and run ``make``. The resulting file will be called
``plugin_llvmc_$(LLVMC_PLUGIN).$(DLL_EXTENSION)`` (in our case,
``plugin_llvmc_MyPlugin.so``). This library can be then loaded in with the
``-load`` option. Example::
$ cd $LLVMC_DIR/plugins/Simple
$ make
$ llvmc -load $LLVM_DIR/Release/lib/plugin_llvmc_Simple.so
Compiling standalone LLVMC-based drivers
========================================
By default, the ``llvmc`` executable consists of a driver core plus several
statically linked plugins (``Base`` and ``Clang`` at the moment). You can
produce a standalone LLVMC-based driver executable by linking the core with your
own plugins. The recommended way to do this is by starting with the provided
``Skeleton`` example (``$LLVMC_DIR/example/Skeleton``)::
$ cd $LLVMC_DIR/example/
$ cp -r Skeleton mydriver
$ cd mydriver
$ vim Makefile
[...]
$ make
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/
$ cp $LLVMC_SRC_DIR/example/mydriver/Makefile \
$LLVMC_OBJ_DIR/example/mydriver/
$ cd $LLVMC_OBJ_DIR/example/mydriver
$ make
Another way to do the same thing is by using the following command::
$ cd $LLVMC_DIR
$ make LLVMC_BUILTIN_PLUGINS=MyPlugin LLVMC_BASED_DRIVER_NAME=mydriver
This works with both srcdir == objdir and srcdir != objdir, but assumes that the
plugin source directory was placed under ``$LLVMC_DIR/plugins``.
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 LLVMC_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 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"))``.
- ``extern`` - this option is defined in some other plugin, see `below`__.
__ extern_
.. _extern:
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 must be marked as
``extern``. This is what the ``extern`` option property is
for. Example::
...
(switch_option "E", (extern))
...
If an external option has additional attributes besides 'extern', they are
ignored. 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. 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 ``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 $OUTFILE -emit-llvm"),
(sink)
]>;
This defines a new tool called ``llvm_gcc_cpp``, which is an alias for
``llvm-g++``. As you can see, a tool definition is just a list of
properties; most of them should be self-explanatory. The ``sink``
property means that this tool should be passed all command-line
options that aren't mentioned in the option list.
The complete list of all currently implemented tool properties follows.
* Possible tool properties:
- ``in_language`` - input language name. Can be given multiple arguments, in
case the tool supports multiple input languages.
- ``out_language`` - output language name. Multiple output languages are
allowed.
- ``output_suffix`` - output file suffix. Can also be changed
dynamically, see documentation on actions.
- ``cmd_line`` - the actual command used to run the tool. You can
use ``$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_
.. _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"),
(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"))``.
- ``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. Since a tool can't have multiple output languages, for inner nodes of
the graph the input and output languages should match. This is enforced at
compile-time.
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 the built-in Base plugin)::
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 plugin is loaded. 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 executes programs from 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 easily achieved via
the hooks mechanism. To write your own hooks, just add their
definitions to the ``PluginMain.cpp`` or drop a ``.cpp`` file into the
your plugin 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
``cmd_line`` tool property::
(cmd_line "$CALL(MyHook)/path/to/file -o $CALL(AnotherHook)")
To pass arguments to hooks, use the following syntax::
(cmd_line "$CALL(MyHook, 'Arg1', 'Arg2', 'Arg # 3')/path/to/file -o1 -o2")
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 ``--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. These checks can't
be performed at compile-time because the plugins can load code
dynamically. When invoked with ``--check-graph``, ``llvmc`` doesn't
perform any compilation tasks and returns the number of encountered
errors as its status code.
.. _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 ``Base`` plugin 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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