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5 <title>The LLVM Compiler Driver (llvmc)</title>
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15 <meta name="author" content="Reid Spencer" name="author">
16 <meta name="description"
17 content="A description of the use and design of the LLVM Compiler Driver.">
18</head>
19<body>
20<div class="doc_title">The LLVM Compiler Driver (llvmc)</div>
21<p class="doc_warning">NOTE: This document is a work in progress!</p>
22<ol>
23 <li><a href="#abstract">Abstract</a></li>
24 <li><a href="#introduction">Introduction</a>
25 <ol>
26 <li><a href="#purpose">Purpose</a></li>
27 <li><a href="#operation">Operation</a></li>
28 <li><a href="#phases">Phases</a></li>
29 <li><a href="#actions">Actions</a></li>
30 </ol>
31 </li>
32 <li><a href="#details">Details</a>
33 <li><a href="#configuration">Configuration</a>
34 <li><a href="#glossary">Glossary</a>
35</ol>
36<div class="doc_author">
37<p>Written by <a href="mailto:rspencer@x10sys.com">Reid Spencer</a>
38</p>
39</div>
40
41<!-- *********************************************************************** -->
42<div class="doc_section"> <a name="abstract">Abstract</a></div>
43<!-- *********************************************************************** -->
44<div class="doc_text">
45 <p>This document describes the requirements, design, and configuration of the
46 LLVM compiler driver, <tt>llvmc</tt>. The compiler driver knows about LLVM's
47 tool set and can be configured to know about a variety of compilers for
48 source languages. It uses this knowledge to execute the tools necessary
49 to accomplish general compilation, optimization, and linking tasks. The main
50 purpose of <tt>llvmc</tt> is to provide a simple and consistent interface to
51 all compilation tasks. This reduces the burden on the end user who can just
52 learn to use <tt>llvmc</tt> instead of the entire LLVM tool set and all the
53 source language compilers compatible with LLVM.</p>
54</div>
55<!-- *********************************************************************** -->
56<div class="doc_section"> <a name="introduction">Introduction</a></div>
57<!-- *********************************************************************** -->
58<div class="doc_text">
59 <p>The <tt>llvmc</tt> <a href="def_tool">tool</a> is a configurable compiler
60 <a href="def_driver">driver</a>. As such, it isn't the compiler, optimizer,
61 or linker itself but it drives (invokes) other software that perform those
62 tasks. If you are familiar with the GNU Compiler Collection's <tt>gcc</tt>
63 tool, <tt>llvmc</tt> is very similar.</p>
64 <p>The following introductory sections will help you understand why this tool
65 is necessary and what it does.</p>
66</div>
67
68<!-- _______________________________________________________________________ -->
69<div class="doc_subsection"><a name="purpose">Purpose</a></div>
70<div class="doc_text">
71 <p><tt>llvmc</tt> was invented to make compilation with LLVM based compilers
72 easier. To accomplish this, <tt>llvmc</tt> strives to:</p>
73 <ul>
74 <li>Be the single point of access to most of the LLVM tool set.</li>
75 <li>Hide the complexities of the LLVM tools through a single interface.</li>
76 <li>Provide a consistent interface for compiling all languages.</li>
77 </ul>
78 <p>Additionally, <tt>llvmc</tt> makes it easier to write a compiler for use
79 with LLVM, because it:</p>
80 <ul>
81 <li>Makes integration of existing non-LLVM tools simple.</li>
82 <li>Extends the capabilities of minimal front ends by optimizing their
83 output.</li>
84 <li>Reduces the number of interfaces a compiler writer must know about
85 before a working compiler can be completed (essentially only the VMCore
86 interfaces need to be understood).</li>
87 <li>Supports source language translator invocation via both dynamically
88 loadable shared objects and invocation of an executable.</li>
89 </ol>
90</p>
91</div>
92
93<!-- _______________________________________________________________________ -->
94<div class="doc_subsection"><a name="operation">Operation</a></div>
95<div class="doc_text">
96 <p>At a high level, <tt>llvmc</tt> operation is very simple. The basic action
97 taken by <tt>llvmc</tt> is to simply invoke some tool or set of tools to fill
98 the user's request for compilation. Every execution of <tt>llvmc</tt>takes the
99 following sequence of steps:<br/>
100 <dl>
101 <dt><b>Collect Command Line Options</b></dt>
102 <dd>The command line options provide the marching orders to <tt>llvmc</tt>
103 on what actions it should perform. This is the request the user is making
104 of <tt>llvmc</tt> and it is interpreted first. See the <tt>llvmc</tt>
105 <a href="CommandGuide/html/llvmc.html">manual page</a> for details on the
106 options.</dd>
107 <dt><b>Read Configuration Files</b></dt>
108 <dd>Based on the options and the suffixes of the filenames presented, a set
109 of configuration files are read to configure the actions <tt>llvmc</tt> will
110 take. Configuration files are provided by either LLVM or the front end
111 compiler tools that B<llvmc> invokes. These files determine what actions
112 <tt>llvmc</tt> will take in response to the user's request. See the section
113 on <a href="#configuration">configuration</a> for more details.</dd>
114 <dt><b>Determine Phases To Execute</b></dt>
115 <dd>Based on the command line options and configuration files,
116 <tt>llvmc</tt> determines the compilation <a href="#phases">phases</a> that
117 must be executed by the user's request. This is the primary work of
118 <tt>llvmc</tt>.</dd>
119 <dt><b>Determine Actions To Execute</b></dt>
120 <dd>Each <a href="#phases">phase</a> to be executed can result in the
121 invocation of one or more <a href="#actions">actions</a>. An action is
122 either a whole program or a function in a dynamically linked shared library.
123 In this step, <tt>llvmc</tt> determines the sequence of actions that must be
124 executed. Actions will always be executed in a deterministic order.</dd>
125 <dt><b>Execute Actions</b></dt>
126 <dd>The <a href="#actions">actions</a> necessary to support the user's
127 original request are executed sequentially and deterministically. All
128 actions result in either the invocation of a whole program to perform the
129 action or the loading of a dynamically linkable shared library and invocation
130 of a standard interface function within that library.</dd>
131 <dt><b>Termination</b></dt>
132 <dd>If any action fails (returns a non-zero result code), <tt>llvmc</tt>
133 also fails and returns the result code from the failing action. If
134 everything succeeds, <tt>llvmc</tt> will return a zero result code.</dd>
135 </dl></p>
136 <p><tt>llvmc</tt>'s operation must be simple, regular and predictable.
137 Developers need to be able to rely on it to take a consistent approach to
138 compilation. For example, the invocation:</p>
139 <tt><pre>
140 llvmc -O2 x.c y.c z.c -o xyz</pre></tt>
141 <p>must produce <i>exactly</i> the same results as:</p>
142 <tt><pre>
143 llvmc -O2 x.c
144 llvmc -O2 y.c
145 llvmc -O2 z.c
146 llvmc -O2 x.o y.o z.o -o xyz</pre></tt>
147 <p>To accomplish this, <tt>llvmc</tt> uses a very simple goal oriented
148 procedure to do its work. The overall goal is to produce a functioning
149 executable. To accomplish this, <tt>llvmc</tt> always attempts to execute a
150 series of compilation <a href="#def_phase">phases</a> in the same sequence.
151 However, the user's options to <tt>llvmc</tt> can cause the sequence of phases
152 to start in the middle or finish early.</p>
153</div>
154
155<!-- _______________________________________________________________________ -->
156<div class="doc_subsection"><a name="phases"></a>Phases </div>
157<div class="doc_text">
158 <p><tt>llvmc</tt> breaks every compilation task into the following five
159 distinct phases:</p>
160 <dl><dt><b>Preprocessing</b></dt><dd>Not all languages support preprocessing;
161 but for those that do, this phase can be invoked. This phase is for
162 languages that provide combining, filtering, or otherwise altering with the
163 source language input before the translator parses it. Although C and C++
164 are the most common users of this phase, other languages may provide their
165 own preprocessor (whether its the C pre-processor or not).</dd>
166 </dl>
167 <dl><dt><b>Translation</b></dt><dd>The translation phase converts the source
168 language input into something that LLVM can interpret and use for
169 downstream phases. The translation is essentially from "non-LLVM form" to
170 "LLVM form".</dd>
171 </dl>
172 <dl><dt><b>Optimization</b></dt><dd>Once an LLVM Module has been obtained from
173 the translation phase, the program enters the optimization phase. This phase
174 attempts to optimize all of the input provided on the command line according
175 to the options provided.</dd>
176 </dl>
177 <dl><dt><b>Linking</b></dt><dd>The inputs are combined to form a complete
178 program.</dd>
179 </dl>
180 <p>The following table shows the inputs, outputs, and command line options
181 applicabe to each phase.</p>
182 <table>
183 <tr>
184 <th style="width: 10%">Phase</th>
185 <th style="width: 25%">Inputs</th>
186 <th style="width: 25%">Outputs</th>
187 <th style="width: 40%">Options</th>
188 </tr>
189 <tr><td><b>Preprocessing</b></td>
190 <td class="td_left"><ul><li>Source Language File</li></ul></td>
191 <td class="td_left"><ul><li>Source Language File</li></ul></td>
192 <td class="td_left"><dl>
193 <dt><tt>-E</tt></dt>
194 <dd>Stops the compilation after preprocessing</dd>
195 </dl></td>
196 </tr>
197 <tr>
198 <td><b>Translation</b></td>
199 <td class="td_left"><ul>
200 <li>Source Language File</li>
201 </ul></td>
202 <td class="td_left"><ul>
203 <li>LLVM Assembly</li>
204 <li>LLVM Bytecode</li>
205 <li>LLVM C++ IR</li>
206 </ul></td>
207 <td class="td_left"><dl>
208 <dt><tt>-c</tt></dt>
209 <dd>Stops the compilation after translation so that optimization and
210 linking are not done.</dd>
211 <dt><tt>-S</tt></dt>
212 <dd>Stops the compilation before object code is written so that only
213 assembly code remains.</dd>
214 </dl></td>
215 </tr>
216 <tr>
217 <td><b>Optimization</b></td>
218 <td class="td_left"><ul>
219 <li>LLVM Assembly</li>
220 <li>LLVM Bytecode</li>
221 </ul></td>
222 <td class="td_left"><ul>
223 <li>LLVM Bytecode</li>
224 </ul></td>
225 <td class="td_left"><dl>
226 <dt><tt>-Ox</tt>
227 <dd>This group of options affects the amount of optimization
228 performed.</dd>
229 </dl></td>
230 </tr>
231 <tr>
232 <td><b>Linking</b></td>
233 <td class="td_left"><ul>
234 <li>LLVM Bytecode</li>
235 <li>Native Object Code</li>
236 <li>LLVM Library</li>
237 <li>Native Library</li>
238 </ul></td>
239 <td class="td_left"><ul>
240 <li>LLVM Bytecode Executable</li>
241 <li>Native Executable</li>
242 </ul></td>
243 <td class="td_left"><dl>
244 <dt><tt>-L</tt></dt><dd>Specifies a path for library search.</dd>
245 <dt><tt>-l</tt></dt><dd>Specifies a library to link in.</dd>
246 </dl></td>
247 </tr>
248 </table>
249</div>
250
251<!-- _______________________________________________________________________ -->
252<div class="doc_subsection"><a name="actions"></a>Actions</div>
253<div class="doc_text">
254 <p>An action, with regard to <tt>llvmc</tt> is a basic operation that it takes
255 in order to fulfill the user's request. Each phase of compilation will invoke
256 zero or more actions in order to accomplish that phase.</p>
257 <p>Actions come in two forms:<ol>
258 <li>Invokable Executables</li>
259 <li>Functions in a shared library</li>
260 </ul></p>
261</div>
262
263<!-- *********************************************************************** -->
264<div class="doc_section"><a name="details">Details</a></div>
265<!-- *********************************************************************** -->
266<div class="doc_text">
267</div>
268
269<!-- *********************************************************************** -->
270<div class="doc_section"><a name="configuration">Configuration</a></div>
271<!-- *********************************************************************** -->
272<div class="doc_text">
273 <p>This section of the document describes the configuration files used by
274 <tt>llvmc</tt>. Configuration information is relatively static for a
275 given release of LLVM and a front end compiler. However, the details may
276 change from release to release of either. Users are encouraged to simply use
277 the various options of the B<llvmc> command and ignore the configuration of
278 the tool. These configuration files are for compiler writers and LLVM
279 developers. Those wishing to simply use B<llvmc> don't need to understand
280 this section but it may be instructive on how the tool works.</p>
281</div>
282
283<!-- _______________________________________________________________________ -->
284<div class="doc_subsection"><a name="overview"></a>Overview</div>
285<div class="doc_text">
286<p><tt>llvmc</tt> is highly configurable both on the command line and in
287configuration files. The options it understands are generic, consistent and
288simple by design. Furthermore, the <tt>llvmc</tt> options apply to the
289compilation of any LLVM enabled programming language. To be enabled as a
290supported source language compiler, a compiler writer must provide a
291configuration file that tells <tt>llvmc</tt> how to invoke the compiler
292and what its capabilities are. The purpose of the configuration files then
293is to allow compiler writers to specify to <tt>llvmc</tt> how the compiler
294should be invoked. Users may but are not advised to alter the compiler's
295<tt>llvmc</tt> configuration.</p>
296
297<p>Because <tt>llvmc</tt> just invokes other programs, it must deal with the
298available command line options for those programs regardless of whether they
299were written for LLVM or not. Furthermore, not all compilation front ends will
300have the same capabilities. Some front ends will simply generate LLVM assembly
301code, others will be able to generate fully optimized byte code. In general,
302<tt>llvmc</tt> doesn't make any assumptions about the capabilities or command
303line options of a sub-tool. It simply uses the details found in the configuration
304files and leaves it to the compiler writer to specify the configuration
305correctly.</p>
306
307<p>This approach means that new compiler front ends can be up and working very
308quickly. As a first cut, a front end can simply compile its source to raw
309(unoptimized) bytecode or LLVM assembly and <tt>llvmc</tt> can be configured
310to pick up the slack (translate LLVM assembly to bytecode, optimize the
311bytecode, generate native assembly, link, etc.). In fact, the front end need
312not use any LLVM libraries, and it could be written in any language (instead of
313C++). The configuration data will allow the full range of optimization,
314assembly, and linking capabilities that LLVM provides to be added to these kinds
315of tools. Enabling the rapid development of front-ends is one of the primary
316goals of <tt>llvmc</tt>.</p>
317
318<p>As a compiler front end matures, it may utilize the LLVM libraries and tools
319to more efficiently produce optimized bytecode directly in a single compilation
320and optimization program. In these cases, multiple tools would not be needed
321and the configuration data for the compiler would change.</p>
322
323<p>Configuring <tt>llvmc</tt> to the needs and capabilities of a source language
324compiler is relatively straight forward. A compiler writer must provide a
325definition of what to do for each of the five compilation phases for each of
326the optimization levels. The specification consists simply of prototypical
327command lines into which <tt>llvmc</tt> can substitute command line
328arguments and file names. Note that any given phase can be completely blank if
329the source language's compiler combines multiple phases into a single program.
330For example, quite often pre-processing, translation, and optimization are
331combined into a single program. The specification for such a compiler would have
332blank entries for pre-processing and translation but a full command line for
333optimization.</p>
334</div>
335
336<!-- _______________________________________________________________________ -->
337<div class="doc_subsection"><a name="filetypes"></a>Configuration Files</div>
338<div class="doc_text">
339 <h3>Types of Files</h3>
340 <p>There are two types of configuration files: the master configuration file
341 and the language specific configuration file. The master configuration file
342 contains the general configuration of <tt>llvmc</tt> itself and is supplied
343 with the tool. It contains information that is source language agnostic.
344 Language specific configuration files tell <tt>llvmc</tt> how to invoke the
345 language's compiler for a variety of different tasks and what other tools
346 are needed to backfill the compiler's missing features (e.g.
347 optimization).</p>
348
349 <h3>Directory Search</h3>
350 <p><tt>llvmc</tt> always looks for files of a specific name. It uses the
351 first file with the name its looking for by searching directories in the
352 following order:<br/>
353 <ol>
354 <li>Any directory specified by the <tt>--config-dir</tt> option will be
355 checked first.</li>
356 <li>If the environment variable LLVM_CONFIG_DIR is set, and it contains
357 the name of a valid directory, that directory will be searched next.</li>
358 <li>If the user's home directory (typically <tt>/home/user</tt> contains
359 a sub-directory named <tt>.llvm</tt> and that directory contains a
360 sub-directory named <tt>etc</tt> then that directory will be tried
361 next.</li>
362 <li>If the LLVM installation directory (typically <tt>/usr/local/llvm</tt>
363 contains a sub-directory named <tt>etc</tt> then that directory will be
364 tried last.</li>
365 <li>If the configuration file sought still can't be found, <tt>llvmc</tt>
366 will print an error message and exit.</li>
367 </ol>
368 The first file found in this search will be used. Other files with the same
369 name will be ignored even if they exist in one of the subsequent search
370 locations.</p>
371
372 <h3>File Names</h3>
373 <p>In the directories searched, a file named <tt>master</tt> will be
374 recognized as the master configuration file for <tt>llvmc</tt>. Note that
375 users <i>may</i> override the master file with a copy in their home directory
376 but they are advised not to. This capability is only useful for compiler
377 implementers needing to alter the master configuration while developing
378 their compiler front end. When reading the configuration files, the master
379 files are always read first.</p>
380 <p>Language specific configuration files are given specific names to foster
381 faster lookup. The name of a given language specific configuration file is
382 the same as the suffix used to identify files containing source in that
383 language. For example, a configuration file for C++ source might be named
384 <tt>cpp</tt>, <tt>C</tt>, or <tt>cxx</tt>.</p>
385
386 <h3>What Gets Read</h3>
387 <p>The master configuration file is always read. Which language specific
388 configuration files are read depends on the command line options and the
389 suffixes of the file names provided on <tt>llvmc</tt>'s command line. Note
390 that the <tt>--x LANGUAGE</tt> option alters the language that <tt>llvmc</tt>
391 uses for the subsequent files on the command line. Only the language
392 specific configuration files actually needed to complete <tt>llvmc</tt>'s
393 task are read. Other language specific files will be ignored.</p>
394</div>
395
396<!-- _______________________________________________________________________ -->
397<div class="doc_subsection"><a name="syntax"></a>Syntax</div>
398<div class="doc_text">
399 <p>The syntax of the configuration files is yet to be determined. There are
400 two viable options remaining:<br/>
401 <ul>
402 <li>XML DTD Specific To <tt>llvmc</tt></li>
403 <li>Windows .ini style file with numerous sections</li>
404 </ul></p>
405</div>
406
407<!-- _______________________________________________________________________ -->
408<div class="doc_subsection"><a name="master_items"></a>
409 Master Configuration Items
410</div>
411<div class="doc_text">
412 <pre>
413
414=head3 Section: [lang=I<LANGUAGE>]
415
416This section provides the master configuration data for a given language. The
417language specific data will be found in a file named I<LANGUAGE>.
418
419=over
420
421=item C<suffix=>I<suffix>
422
423This adds the I<suffix> specified to the list of recognized suffixes for
424the I<LANGUAGE> identified in the section. As many suffixes as are commonly used
425for source files for the I<LANGUAGE> should be specified.
426
427=back
428
429=begin html
430
431<p>For example, the following might appear for C++:
432<pre><tt>
433[lang=C++]
434suffix=.cpp
435suffix=.cxx
436suffix=.C
437</tt></pre></p>
438
439=end html
440</pre>
441</div>
442
443<!-- _______________________________________________________________________ -->
444<div class="doc_subsection"><a name="lang_items"></a>
445 Language Specific Configuration Items
446</div>
447<div class="doc_text">
448 <pre>
449=head3 Section: [general]
450
451=over
452
453=item C<hasPreProcessor=yes|no>
454
455This item specifies whether the language has a pre-processing phase or not. This
456controls whether the B<-E> option works for the language or not.
457
458=item C<output=bc|ll>
459
460This item specifies the kind of output the language's compiler generates. The
461choices are either bytecode (C<bc>) or LLVM assembly (C<ll>).
462
463=back
464
465=head3 Section: [-O0]
466
467=over
468
469=item C<preprocess=>I<commandline>
470
471This item specifies the I<commandline> to use for pre-processing the input.
472
473=over
474
475Valid substitutions for this item are:
476
477=item %in%
478
479The input source file.
480
481=item %out%
482
483The output file.
484
485=item %options%
486
487Any pre-processing specific options (e.g. B<-I>).
488
489=back
490
491=item C<translate=>I<commandline>
492
493This item specifies the I<commandline> to use for translating the source
494language input into the output format given by the C<output> item.
495
496=item C<optimize=>I<commandline>
497
498This item specifies the I<commandline> for optimizing the translator's output.
499
500=back
501</pre>
502</div>
503
504<!-- *********************************************************************** -->
505<div class="doc_section"><a name="glossary">Glossary</a></div>
506<!-- *********************************************************************** -->
507<div class="doc_text">
508 <p>This document uses precise terms in reference to the various artifacts and
509 concepts related to compilation. The terms used throughout this document are
510 defined below.</p>
511 <dl>
512 <dt><a name="def_assembly"><b>assembly</b></a></dt>
513 <dd>A compilation <a href="#def_phase">phase</a> in which LLVM bytecode or
514 LLVM assembly code is assembled to a native code format (either target
515 specific aseembly language or the platform's native object file format).
516 </dd>
517
518 <dt><a name="def_compiler"><b>compiler</b></a></dt>
519 <dd>Refers to any program that can be invoked by <tt>llvmc</tt> to accomplish
520 the work of one or more compilation <a href="#def_phase">phases</a>.</dd>
521
522 <dt><a name="def_driver"><b>driver</b></a></dt>
523 <dd>Refers to <tt>llvmc</tt> itself.</dd>
524
525 <dt><a name="def_linking"><b>linking</b></a></dt>
526 <dd>A compilation <a href="#def_phase">phase</a> in which LLVM bytecode files
527 and (optionally) native system libraries are combined to form a complete
528 executable program.</dd>
529
530 <dt><a name="def_optimization"><b>optimization</b></a></dt>
531 <dd>A compilation <a href="#def_phase">phase</a> in which LLVM bytecode is
532 optimized.</dd>
533
534 <dt><a name="def_phase"><b>phase</b></a></dt>
535 <dd>Refers to any one of the five compilation phases that that
536 <tt>llvmc</tt> supports. The five phases are:
537 <a href="#def_preprocessing">preprocessing</a>,
538 <a href="#def_translation">translation</a>,
539 <a href="#def_optimization">optimization</a>,
540 <a href="#def_assembly">assembly</a>,
541 <a href="#def_linking">linking</a>.</dd>
542
543 <dt><a name="def_sourcelanguage"><b>source language</b></a></dt>
544 <dd>Any common programming language (e.g. C, C++, Java, Stacker, ML,
545 FORTRAN). These languages are distinguished from any of the lower level
546 languages (such as LLVM or native assembly), by the fact that a
547 <a href="#def_translation">translation</a> <a href="#def_phase">phase</a>
548 is required before LLVM can be applied.</dd>
549
550 <dt><a name="def_tool"><b>tool</b></a></dt>
551 <dd>Refers to any program in the LLVM tool set.</dd>
552
553 <dt><a name="def_translation"><b>translation</b></a></dt>
554 <dd>A compilation <a href="#def_phase">phase</a> in which
555 <a href="#def_sourcelanguage">source language</a> code is translated into
556 either LLVM assembly language or LLVM bytecode.</dd>
557 </dl>
558</div>
559<!-- *********************************************************************** -->
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