1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
|
===================================
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.
.. contents::
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
Predefined options
==================
LLVMC has some built-in options that can't be overridden in the
configuration files:
* ``-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.
* ``--view-graph`` - Show a graphical representation of the compilation
graph. Requires that you have ``dot`` and ``gv`` programs
installed. Hidden option, useful for debugging.
* ``--write-graph`` - Write a ``compilation-graph.dot`` file in the
current directory with the compilation graph description in the
Graphviz format. Hidden option, useful for debugging.
* ``--save-temps`` - Write temporary files to the current directory
and do not delete them on exit. Hidden option, useful for debugging.
* ``--help``, ``--help-hidden``, ``--version`` - These options have
their standard meaning.
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.
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 $OUTFILE -emit-llvm"),
(sink)
]>;
This defines a new tool called ``llvm_gcc_cpp``, which is an alias for
``llvm-g++``. As you can see, a tool definition is just a list of
properties; most of them should be self-explanatory. The ``sink``
property means that this tool should be passed all command-line
options that lack explicit descriptions.
The complete list of the currently implemented tool properties follows:
* Possible tool properties:
- ``in_language`` - input language name. Can be either a string or a
list, in case the tool supports multiple input languages.
- ``out_language`` - output language name.
- ``output_suffix`` - output file suffix.
- ``cmd_line`` - the actual command used to run the tool. You can
use ``$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``.
- ``alias_option`` - a special option type for creating
aliases. Unlike other option types, aliases are not allowed to
have any properties besides the aliased option name. Usage
example: ``(alias_option "preprocess", "E")``
* Possible option properties:
- ``append_cmd`` - append a string to the tool invocation command.
- ``forward`` - forward this option unchanged.
- ``forward_as`` - Change the name of this option, but forward the
argument unchanged. Example: ``(forward_as "--disable-optimize")``.
- ``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.
Option list - specifying all options in a single place
======================================================
It can be handy to have all information about options gathered in a
single place to provide an overview. This can be achieved by using a
so-called ``OptionList``::
def Options : OptionList<[
(switch_option "E", (help "Help string")),
(alias_option "quiet", "q")
...
]>;
``OptionList`` is also a good place to specify option aliases.
Tool-specific option properties like ``append_cmd`` have (obviously)
no meaning in the context of ``OptionList``, so the only properties
allowed there are ``help`` and ``required``.
Option lists are used at the file scope. See file
``examples/Clang.td`` for an example of ``OptionList`` usage.
Using hooks and environment variables in the ``cmd_line`` property
==================================================================
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 (documented below)::
(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"))
Conditional evaluation: 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",
(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 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++")``.
- ``in_language`` - Evaluates to true if the language of the input
file equals to the argument. At the moment works only with
``cmd_line`` property on non-join nodes. Example: ``(in_language
"c++")``.
- ``not_empty`` - Returns true if a given option (which should be
either a parameter or a parameter list) is set by the
user. Example: ``(not_empty "o")``.
- ``default`` - Always evaluates to true. Should always be the last
test in the ``case`` expression.
- ``and`` - A standard logical combinator that returns true iff all
of its arguments return true. Used like this: ``(and (test1),
(test2), ... (testN))``. Nesting of ``and`` and ``or`` is allowed,
but not encouraged.
- ``or`` - Another logical combinator that returns true only if any
one of its arguments returns true. Example: ``(or (test1),
(test2), ... (testN))``.
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"]>,
...
]>;
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 [2]_ and
Ghostview [3]_ are installed. There is also a ``--dump-graph`` option that
creates a Graphviz source file(``compilation-graph.dot``) in the
current directory.
References
==========
.. [1] TableGen Fundamentals
http://llvm.cs.uiuc.edu/docs/TableGenFundamentals.html
.. [2] Graphviz
http://www.graphviz.org/
.. [3] Ghostview
http://pages.cs.wisc.edu/~ghost/
|