1 ===================================
2 Customizing LLVMC: Reference Manual
3 ===================================
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13 <div class="doc_author">
14 <p>Written by <a href="mailto:foldr@codedgers.com">Mikhail Glushenkov</a></p>
20 LLVMC is a generic compiler driver, designed to be customizable and
21 extensible. It plays the same role for LLVM as the ``gcc`` program
22 does for GCC - LLVMC's job is essentially to transform a set of input
23 files into a set of targets depending on configuration rules and user
24 options. What makes LLVMC different is that these transformation rules
25 are completely customizable - in fact, LLVMC knows nothing about the
26 specifics of transformation (even the command-line options are mostly
27 not hard-coded) and regards the transformation structure as an
28 abstract graph. The structure of this graph is completely determined
29 by plugins, which can be either statically or dynamically linked. This
30 makes it possible to easily adapt LLVMC for other purposes - for
31 example, as a build tool for game resources.
33 Because LLVMC employs TableGen_ as its configuration language, you
34 need to be familiar with it to customize LLVMC.
36 .. _TableGen: http://llvm.org/docs/TableGenFundamentals.html
42 LLVMC tries hard to be as compatible with ``gcc`` as possible,
43 although there are some small differences. Most of the time, however,
44 you shouldn't be able to notice them::
46 $ # This works as expected:
47 $ llvmc -O3 -Wall hello.cpp
51 One nice feature of LLVMC is that one doesn't have to distinguish between
52 different compilers for different languages (think ``g++`` vs. ``gcc``) - the
53 right toolchain is chosen automatically based on input language names (which
54 are, in turn, determined from file extensions). If you want to force files
55 ending with ".c" to compile as C++, use the ``-x`` option, just like you would
58 $ # hello.c is really a C++ file
59 $ llvmc -x c++ hello.c
63 On the other hand, when using LLVMC as a linker to combine several C++
64 object files you should provide the ``--linker`` option since it's
65 impossible for LLVMC to choose the right linker in that case::
69 [A lot of link-time errors skipped]
70 $ llvmc --linker=c++ hello.o
74 By default, LLVMC uses ``llvm-gcc`` to compile the source code. It is also
75 possible to choose the ``clang`` compiler with the ``-clang`` option.
81 LLVMC has some built-in options that can't be overridden in the
82 configuration libraries:
84 * ``-o FILE`` - Output file name.
86 * ``-x LANGUAGE`` - Specify the language of the following input files
87 until the next -x option.
89 * ``-load PLUGIN_NAME`` - Load the specified plugin DLL. Example:
90 ``-load $LLVM_DIR/Release/lib/LLVMCSimple.so``.
92 * ``-v`` - Enable verbose mode, i.e. print out all executed commands.
94 * ``--save-temps`` - Write temporary files to the current directory and do not
95 delete them on exit. This option can also take an argument: the
96 ``--save-temps=obj`` switch will write files into the directory specified with
97 the ``-o`` option. The ``--save-temps=cwd`` and ``--save-temps`` switches are
98 both synonyms for the default behaviour.
100 * ``--check-graph`` - Check the compilation for common errors like mismatched
101 output/input language names, multiple default edges and cycles. Because of
102 plugins, these checks can't be performed at compile-time. Exit with code zero
103 if no errors were found, and return the number of found errors
104 otherwise. Hidden option, useful for debugging LLVMC plugins.
106 * ``--view-graph`` - Show a graphical representation of the compilation graph
107 and exit. Requires that you have ``dot`` and ``gv`` programs installed. Hidden
108 option, useful for debugging LLVMC plugins.
110 * ``--write-graph`` - Write a ``compilation-graph.dot`` file in the current
111 directory with the compilation graph description in Graphviz format (identical
112 to the file used by the ``--view-graph`` option). The ``-o`` option can be
113 used to set the output file name. Hidden option, useful for debugging LLVMC
116 * ``--help``, ``--help-hidden``, ``--version`` - These options have
117 their standard meaning.
119 Compiling LLVMC plugins
120 =======================
122 It's easiest to start working on your own LLVMC plugin by copying the
123 skeleton project which lives under ``$LLVMC_DIR/plugins/Simple``::
125 $ cd $LLVMC_DIR/plugins
126 $ cp -r Simple MyPlugin
129 Makefile PluginMain.cpp Simple.td
131 As you can see, our basic plugin consists of only two files (not
132 counting the build script). ``Simple.td`` contains TableGen
133 description of the compilation graph; its format is documented in the
134 following sections. ``PluginMain.cpp`` is just a helper file used to
135 compile the auto-generated C++ code produced from TableGen source. It
136 can also contain hook definitions (see `below`__).
140 The first thing that you should do is to change the ``LLVMC_PLUGIN``
141 variable in the ``Makefile`` to avoid conflicts (since this variable
142 is used to name the resulting library)::
144 LLVMC_PLUGIN=MyPlugin
146 It is also a good idea to rename ``Simple.td`` to something less
149 $ mv Simple.td MyPlugin.td
151 To build your plugin as a dynamic library, just ``cd`` to its source
152 directory and run ``make``. The resulting file will be called
153 ``plugin_llvmc_$(LLVMC_PLUGIN).$(DLL_EXTENSION)`` (in our case,
154 ``plugin_llvmc_MyPlugin.so``). This library can be then loaded in with the
155 ``-load`` option. Example::
157 $ cd $LLVMC_DIR/plugins/Simple
159 $ llvmc -load $LLVM_DIR/Release/lib/plugin_llvmc_Simple.so
161 Compiling standalone LLVMC-based drivers
162 ========================================
164 By default, the ``llvmc`` executable consists of a driver core plus several
165 statically linked plugins (``Base`` and ``Clang`` at the moment). You can
166 produce a standalone LLVMC-based driver executable by linking the core with your
167 own plugins. The recommended way to do this is by starting with the provided
168 ``Skeleton`` example (``$LLVMC_DIR/example/Skeleton``)::
170 $ cd $LLVMC_DIR/example/
171 $ cp -r Skeleton mydriver
177 If you're compiling LLVM with different source and object directories, then you
178 must perform the following additional steps before running ``make``::
180 # LLVMC_SRC_DIR = $LLVM_SRC_DIR/tools/llvmc/
181 # LLVMC_OBJ_DIR = $LLVM_OBJ_DIR/tools/llvmc/
182 $ cp $LLVMC_SRC_DIR/example/mydriver/Makefile \
183 $LLVMC_OBJ_DIR/example/mydriver/
184 $ cd $LLVMC_OBJ_DIR/example/mydriver
187 Another way to do the same thing is by using the following command::
190 $ make LLVMC_BUILTIN_PLUGINS=MyPlugin LLVMC_BASED_DRIVER_NAME=mydriver
192 This works with both srcdir == objdir and srcdir != objdir, but assumes that the
193 plugin source directory was placed under ``$LLVMC_DIR/plugins``.
195 Sometimes, you will want a 'bare-bones' version of LLVMC that has no
196 built-in plugins. It can be compiled with the following command::
199 $ make LLVMC_BUILTIN_PLUGINS=""
202 Customizing LLVMC: the compilation graph
203 ========================================
205 Each TableGen configuration file should include the common
208 include "llvm/CompilerDriver/Common.td"
210 Internally, LLVMC stores information about possible source
211 transformations in form of a graph. Nodes in this graph represent
212 tools, and edges between two nodes represent a transformation path. A
213 special "root" node is used to mark entry points for the
214 transformations. LLVMC also assigns a weight to each edge (more on
215 this later) to choose between several alternative edges.
217 The definition of the compilation graph (see file
218 ``plugins/Base/Base.td`` for an example) is just a list of edges::
220 def CompilationGraph : CompilationGraph<[
221 Edge<"root", "llvm_gcc_c">,
222 Edge<"root", "llvm_gcc_assembler">,
225 Edge<"llvm_gcc_c", "llc">,
226 Edge<"llvm_gcc_cpp", "llc">,
229 OptionalEdge<"llvm_gcc_c", "opt", (case (switch_on "opt"),
231 OptionalEdge<"llvm_gcc_cpp", "opt", (case (switch_on "opt"),
235 OptionalEdge<"llvm_gcc_assembler", "llvm_gcc_cpp_linker",
236 (case (input_languages_contain "c++"), (inc_weight),
237 (or (parameter_equals "linker", "g++"),
238 (parameter_equals "linker", "c++")), (inc_weight))>,
243 As you can see, the edges can be either default or optional, where
244 optional edges are differentiated by an additional ``case`` expression
245 used to calculate the weight of this edge. Notice also that we refer
246 to tools via their names (as strings). This makes it possible to add
247 edges to an existing compilation graph in plugins without having to
248 know about all tool definitions used in the graph.
250 The default edges are assigned a weight of 1, and optional edges get a
251 weight of 0 + 2*N where N is the number of tests that evaluated to
252 true in the ``case`` expression. It is also possible to provide an
253 integer parameter to ``inc_weight`` and ``dec_weight`` - in this case,
254 the weight is increased (or decreased) by the provided value instead
255 of the default 2. It is also possible to change the default weight of
256 an optional edge by using the ``default`` clause of the ``case``
259 When passing an input file through the graph, LLVMC picks the edge
260 with the maximum weight. To avoid ambiguity, there should be only one
261 default edge between two nodes (with the exception of the root node,
262 which gets a special treatment - there you are allowed to specify one
263 default edge *per language*).
265 When multiple plugins are loaded, their compilation graphs are merged
266 together. Since multiple edges that have the same end nodes are not
267 allowed (i.e. the graph is not a multigraph), an edge defined in
268 several plugins will be replaced by the definition from the plugin
269 that was loaded last. Plugin load order can be controlled by using the
270 plugin priority feature described above.
272 To get a visual representation of the compilation graph (useful for
273 debugging), run ``llvmc --view-graph``. You will need ``dot`` and
274 ``gsview`` installed for this to work properly.
279 Command-line options that the plugin supports are defined by using an
282 def Options : OptionList<[
283 (switch_option "E", (help "Help string")),
284 (alias_option "quiet", "q")
288 As you can see, the option list is just a list of DAGs, where each DAG
289 is an option description consisting of the option name and some
290 properties. A plugin can define more than one option list (they are
291 all merged together in the end), which can be handy if one wants to
292 separate option groups syntactically.
294 * Possible option types:
296 - ``switch_option`` - a simple boolean switch without arguments, for example
297 ``-O2`` or ``-time``. At most one occurrence is allowed.
299 - ``parameter_option`` - option that takes one argument, for example
300 ``-std=c99``. It is also allowed to use spaces instead of the equality
301 sign: ``-std c99``. At most one occurrence is allowed.
303 - ``parameter_list_option`` - same as the above, but more than one option
304 occurence is allowed.
306 - ``prefix_option`` - same as the parameter_option, but the option name and
307 argument do not have to be separated. Example: ``-ofile``. This can be also
308 specified as ``-o file``; however, ``-o=file`` will be parsed incorrectly
309 (``=file`` will be interpreted as option value). At most one occurrence is
312 - ``prefix_list_option`` - same as the above, but more than one occurence of
313 the option is allowed; example: ``-lm -lpthread``.
315 - ``alias_option`` - a special option type for creating aliases. Unlike other
316 option types, aliases are not allowed to have any properties besides the
317 aliased option name. Usage example: ``(alias_option "preprocess", "E")``
320 * Possible option properties:
322 - ``help`` - help string associated with this option. Used for ``--help``
325 - ``required`` - this option must be specified exactly once (or, in case of
326 the list options without the ``multi_val`` property, at least
327 once). Incompatible with ``zero_or_one`` and ``one_or_more``.
329 - ``one_or_more`` - the option must be specified at least one time. Useful
330 only for list options in conjunction with ``multi_val``; for ordinary lists
331 it is synonymous with ``required``. Incompatible with ``required`` and
334 - ``zero_or_one`` - the option can be specified zero or one times. Useful
335 only for list options in conjunction with ``multi_val``. Incompatible with
336 ``required`` and ``one_or_more``.
338 - ``hidden`` - the description of this option will not appear in
339 the ``--help`` output (but will appear in the ``--help-hidden``
342 - ``really_hidden`` - the option will not be mentioned in any help
345 - ``multi_val n`` - this option takes *n* arguments (can be useful in some
346 special cases). Usage example: ``(parameter_list_option "foo", (multi_val
347 3))``. Only list options can have this attribute; you can, however, use
348 the ``one_or_more`` and ``zero_or_one`` properties.
350 - ``init`` - this option has a default value, either a string (if it is a
351 parameter), or a boolean (if it is a switch; boolean constants are called
352 ``true`` and ``false``). List options can't have this attribute. Usage
353 examples: ``(switch_option "foo", (init true))``; ``(prefix_option "bar",
356 - ``extern`` - this option is defined in some other plugin, see below.
361 Sometimes, when linking several plugins together, one plugin needs to
362 access options defined in some other plugin. Because of the way
363 options are implemented, such options must be marked as
364 ``extern``. This is what the ``extern`` option property is
368 (switch_option "E", (extern))
371 See also the section on plugin `priorities`__.
377 Conditional evaluation
378 ======================
380 The 'case' construct is the main means by which programmability is
381 achieved in LLVMC. It can be used to calculate edge weights, program
382 actions and modify the shell commands to be executed. The 'case'
383 expression is designed after the similarly-named construct in
384 functional languages and takes the form ``(case (test_1), statement_1,
385 (test_2), statement_2, ... (test_N), statement_N)``. The statements
386 are evaluated only if the corresponding tests evaluate to true.
390 // Edge weight calculation
392 // Increases edge weight by 5 if "-A" is provided on the
393 // command-line, and by 5 more if "-B" is also provided.
395 (switch_on "A"), (inc_weight 5),
396 (switch_on "B"), (inc_weight 5))
399 // Tool command line specification
401 // Evaluates to "cmdline1" if the option "-A" is provided on the
402 // command line; to "cmdline2" if "-B" is provided;
403 // otherwise to "cmdline3".
406 (switch_on "A"), "cmdline1",
407 (switch_on "B"), "cmdline2",
408 (default), "cmdline3")
410 Note the slight difference in 'case' expression handling in contexts
411 of edge weights and command line specification - in the second example
412 the value of the ``"B"`` switch is never checked when switch ``"A"`` is
413 enabled, and the whole expression always evaluates to ``"cmdline1"`` in
416 Case expressions can also be nested, i.e. the following is legal::
418 (case (switch_on "E"), (case (switch_on "o"), ..., (default), ...)
421 You should, however, try to avoid doing that because it hurts
422 readability. It is usually better to split tool descriptions and/or
423 use TableGen inheritance instead.
425 * Possible tests are:
427 - ``switch_on`` - Returns true if a given command-line switch is
428 provided by the user. Example: ``(switch_on "opt")``.
430 - ``parameter_equals`` - Returns true if a command-line parameter equals
432 Example: ``(parameter_equals "W", "all")``.
434 - ``element_in_list`` - Returns true if a command-line parameter
435 list contains a given value.
436 Example: ``(parameter_in_list "l", "pthread")``.
438 - ``input_languages_contain`` - Returns true if a given language
439 belongs to the current input language set.
440 Example: ``(input_languages_contain "c++")``.
442 - ``in_language`` - Evaluates to true if the input file language
443 equals to the argument. At the moment works only with ``cmd_line``
444 and ``actions`` (on non-join nodes).
445 Example: ``(in_language "c++")``.
447 - ``not_empty`` - Returns true if a given option (which should be
448 either a parameter or a parameter list) is set by the
450 Example: ``(not_empty "o")``.
452 - ``empty`` - The opposite of ``not_empty``. Equivalent to ``(not (not_empty
453 X))``. Provided for convenience.
455 - ``default`` - Always evaluates to true. Should always be the last
456 test in the ``case`` expression.
458 - ``and`` - A standard logical combinator that returns true iff all
459 of its arguments return true. Used like this: ``(and (test1),
460 (test2), ... (testN))``. Nesting of ``and`` and ``or`` is allowed,
463 - ``or`` - Another logical combinator that returns true only if any
464 one of its arguments returns true. Example: ``(or (test1),
465 (test2), ... (testN))``.
468 Writing a tool description
469 ==========================
471 As was said earlier, nodes in the compilation graph represent tools,
472 which are described separately. A tool definition looks like this
473 (taken from the ``include/llvm/CompilerDriver/Tools.td`` file)::
475 def llvm_gcc_cpp : Tool<[
477 (out_language "llvm-assembler"),
478 (output_suffix "bc"),
479 (cmd_line "llvm-g++ -c $INFILE -o $OUTFILE -emit-llvm"),
483 This defines a new tool called ``llvm_gcc_cpp``, which is an alias for
484 ``llvm-g++``. As you can see, a tool definition is just a list of
485 properties; most of them should be self-explanatory. The ``sink``
486 property means that this tool should be passed all command-line
487 options that aren't mentioned in the option list.
489 The complete list of all currently implemented tool properties follows.
491 * Possible tool properties:
493 - ``in_language`` - input language name. Can be either a string or a
494 list, in case the tool supports multiple input languages.
496 - ``out_language`` - output language name. Tools are not allowed to
497 have multiple output languages.
499 - ``output_suffix`` - output file suffix. Can also be changed
500 dynamically, see documentation on actions.
502 - ``cmd_line`` - the actual command used to run the tool. You can
503 use ``$INFILE`` and ``$OUTFILE`` variables, output redirection
504 with ``>``, hook invocations (``$CALL``), environment variables
505 (via ``$ENV``) and the ``case`` construct.
507 - ``join`` - this tool is a "join node" in the graph, i.e. it gets a
508 list of input files and joins them together. Used for linkers.
510 - ``sink`` - all command-line options that are not handled by other
511 tools are passed to this tool.
513 - ``actions`` - A single big ``case`` expression that specifies how
514 this tool reacts on command-line options (described in more detail
520 A tool often needs to react to command-line options, and this is
521 precisely what the ``actions`` property is for. The next example
522 illustrates this feature::
524 def llvm_gcc_linker : Tool<[
525 (in_language "object-code"),
526 (out_language "executable"),
527 (output_suffix "out"),
528 (cmd_line "llvm-gcc $INFILE -o $OUTFILE"),
530 (actions (case (not_empty "L"), (forward "L"),
531 (not_empty "l"), (forward "l"),
533 [(append_cmd "-dummy1"), (append_cmd "-dummy2")])
536 The ``actions`` tool property is implemented on top of the omnipresent
537 ``case`` expression. It associates one or more different *actions*
538 with given conditions - in the example, the actions are ``forward``,
539 which forwards a given option unchanged, and ``append_cmd``, which
540 appends a given string to the tool execution command. Multiple actions
541 can be associated with a single condition by using a list of actions
542 (used in the example to append some dummy options). The same ``case``
543 construct can also be used in the ``cmd_line`` property to modify the
546 The "join" property used in the example means that this tool behaves
549 The list of all possible actions follows.
553 - ``append_cmd`` - append a string to the tool invocation
555 Example: ``(case (switch_on "pthread"), (append_cmd
558 - ``error` - exit with error.
559 Example: ``(error "Mixing -c and -S is not allowed!")``.
561 - ``forward`` - forward an option unchanged.
562 Example: ``(forward "Wall")``.
564 - ``forward_as`` - Change the name of an option, but forward the
566 Example: ``(forward_as "O0", "--disable-optimization")``.
568 - ``output_suffix`` - modify the output suffix of this
570 Example: ``(output_suffix "i")``.
572 - ``stop_compilation`` - stop compilation after this tool processes
573 its input. Used without arguments.
575 - ``unpack_values`` - used for for splitting and forwarding
576 comma-separated lists of options, e.g. ``-Wa,-foo=bar,-baz`` is
577 converted to ``-foo=bar -baz`` and appended to the tool invocation
579 Example: ``(unpack_values "Wa,")``.
584 If you are adding support for a new language to LLVMC, you'll need to
585 modify the language map, which defines mappings from file extensions
586 to language names. It is used to choose the proper toolchain(s) for a
587 given input file set. Language map definition looks like this::
589 def LanguageMap : LanguageMap<
590 [LangToSuffixes<"c++", ["cc", "cp", "cxx", "cpp", "CPP", "c++", "C"]>,
591 LangToSuffixes<"c", ["c"]>,
595 For example, without those definitions the following command wouldn't work::
598 llvmc: Unknown suffix: cpp
600 The language map entries should be added only for tools that are
601 linked with the root node. Since tools are not allowed to have
602 multiple output languages, for nodes "inside" the graph the input and
603 output languages should match. This is enforced at compile-time.
611 Hooks and environment variables
612 -------------------------------
614 Normally, LLVMC executes programs from the system ``PATH``. Sometimes,
615 this is not sufficient: for example, we may want to specify tool paths
616 or names in the configuration file. This can be easily achieved via
617 the hooks mechanism. To write your own hooks, just add their
618 definitions to the ``PluginMain.cpp`` or drop a ``.cpp`` file into the
619 your plugin directory. Hooks should live in the ``hooks`` namespace
620 and have the signature ``std::string hooks::MyHookName ([const char*
621 Arg0 [ const char* Arg2 [, ...]]])``. They can be used from the
622 ``cmd_line`` tool property::
624 (cmd_line "$CALL(MyHook)/path/to/file -o $CALL(AnotherHook)")
626 To pass arguments to hooks, use the following syntax::
628 (cmd_line "$CALL(MyHook, 'Arg1', 'Arg2', 'Arg # 3')/path/to/file -o1 -o2")
630 It is also possible to use environment variables in the same manner::
632 (cmd_line "$ENV(VAR1)/path/to/file -o $ENV(VAR2)")
634 To change the command line string based on user-provided options use
635 the ``case`` expression (documented `above`__)::
640 "llvm-g++ -E -x c $INFILE -o $OUTFILE",
642 "llvm-g++ -c -x c $INFILE -o $OUTFILE -emit-llvm"))
648 How plugins are loaded
649 ----------------------
651 It is possible for LLVMC plugins to depend on each other. For example,
652 one can create edges between nodes defined in some other plugin. To
653 make this work, however, that plugin should be loaded first. To
654 achieve this, the concept of plugin priority was introduced. By
655 default, every plugin has priority zero; to specify the priority
656 explicitly, put the following line in your plugin's TableGen file::
658 def Priority : PluginPriority<$PRIORITY_VALUE>;
659 # Where PRIORITY_VALUE is some integer > 0
661 Plugins are loaded in order of their (increasing) priority, starting
662 with 0. Therefore, the plugin with the highest priority value will be
668 When writing LLVMC plugins, it can be useful to get a visual view of
669 the resulting compilation graph. This can be achieved via the command
670 line option ``--view-graph``. This command assumes that Graphviz_ and
671 Ghostview_ are installed. There is also a ``--write-graph`` option that
672 creates a Graphviz source file (``compilation-graph.dot``) in the
675 Another useful ``llvmc`` option is ``--check-graph``. It checks the
676 compilation graph for common errors like mismatched output/input
677 language names, multiple default edges and cycles. These checks can't
678 be performed at compile-time because the plugins can load code
679 dynamically. When invoked with ``--check-graph``, ``llvmc`` doesn't
680 perform any compilation tasks and returns the number of encountered
681 errors as its status code.
683 .. _Graphviz: http://www.graphviz.org/
684 .. _Ghostview: http://pages.cs.wisc.edu/~ghost/
686 Conditioning on the executable name
687 -----------------------------------
689 For now, the executable name (the value passed to the driver in ``argv[0]``) is
690 accessible only in the C++ code (i.e. hooks). Use the following code::
693 extern const char* ProgramName;
696 std::string MyHook() {
698 if (strcmp(ProgramName, "mydriver") == 0) {
703 In general, you're encouraged not to make the behaviour dependent on the
704 executable file name, and use command-line switches instead. See for example how
705 the ``Base`` plugin behaves when it needs to choose the correct linker options
706 (think ``g++`` vs. ``gcc``).
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719 <a href="mailto:foldr@codedgers.com">Mikhail Glushenkov</a><br />
720 <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br />
722 Last modified: $Date: 2008-12-11 11:34:48 -0600 (Thu, 11 Dec 2008) $