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5 <title>LLVM: Frequently Asked Questions</title>
6 <style type="text/css">
7 @import url("llvm.css");
8 .question { font-weight: bold }
9 .answer { margin-left: 2em }
10 </style>
11</head>
12<body>
13
14<div class="doc_title">
15 LLVM: Frequently Asked Questions
16</div>
17
18<ol>
19 <li><a href="#license">License</a>
20 <ol>
21 <li>Why are the LLVM source code and the front-end distributed under different
22 licenses?</li>
23 <li>Does the University of Illinois Open Source License really qualify as an
24 "open source" license?</li>
25 <li>Can I modify LLVM source code and redistribute the modified source?</li>
26 <li>Can I modify LLVM source code and redistribute binaries or other tools
27 based on it, without redistributing the source?</li>
28 </ol></li>
29
30 <li><a href="#source">Source code</a>
31 <ol>
32 <li>In what language is LLVM written?</li>
33 <li>How portable is the LLVM source code?</li>
34 </ol></li>
35
36 <li><a href="#build">Build Problems</a>
37 <ol>
38 <li>When I run configure, it finds the wrong C compiler.</li>
39 <li>The <tt>configure</tt> script finds the right C compiler, but it uses the
40 LLVM linker from a previous build. What do I do?</li>
41 <li>When creating a dynamic library, I get a strange GLIBC error.</li>
42 <li>I've updated my source tree from Subversion, and now my build is trying
43 to use a file/directory that doesn't exist.</li>
44 <li>I've modified a Makefile in my source tree, but my build tree keeps using
45 the old version. What do I do?</li>
46 <li>I've upgraded to a new version of LLVM, and I get strange build
47 errors.</li>
48 <li>I've built LLVM and am testing it, but the tests freeze.</li>
49 <li>Why do test results differ when I perform different types of builds?</li>
50 <li>Compiling LLVM with GCC 3.3.2 fails, what should I do?</li>
51 <li>When I use the test suite, all of the C Backend tests fail. What is
52 wrong?</li>
53 <li>After Subversion update, rebuilding gives the error "No rule to make
54 target".</li>
55 <li><a href="#llvmc">The <tt>llvmc</tt> program gives me errors/doesn't
Bill Wendling4e05864c2007-09-22 09:54:47 +0000[diff] [blame]56 work.</a></li>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]57 </ol></li>
58
59 <li><a href="#felangs">Source Languages</a>
60 <ol>
61 <li><a href="#langs">What source languages are supported?</a></li>
Gordon Henriksendb0558b2008-02-22 21:55:51 +0000[diff] [blame]62 <li><a href="#langirgen">I'd like to write a self-hosting LLVM compiler. How
63 should I interface with the LLVM middle-end optimizers and back-end code
Dan Gohman4dfac702008-11-24 17:18:39 +0000[diff] [blame]64 generators?</a></li>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]65 <li><a href="#langhlsupp">What support is there for higher level source
66 language constructs for building a compiler?</a></li>
67 <li><a href="GetElementPtr.html">I don't understand the GetElementPtr
68 instruction. Help!</a></li>
69 </ol>
70
71 <li><a href="#cfe">Using the GCC Front End</a>
72 <ol>
73 <li>
74 When I compile software that uses a configure script, the configure script
75 thinks my system has all of the header files and libraries it is testing
76 for. How do I get configure to work correctly?
77 </li>
78
79 <li>
80 When I compile code using the LLVM GCC front end, it complains that it
81 cannot find libcrtend.a.
82 </li>
83
84 <li>
85 How can I disable all optimizations when compiling code using the LLVM GCC front end?
86 </li>
87
Dan Gohman4dfac702008-11-24 17:18:39 +0000[diff] [blame]88 <li><a href="#translatecxx">Can I use LLVM to convert C++ code to C code?</a></li>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]89
90 </ol>
91 </li>
92
93 <li><a href="#cfe_code">Questions about code generated by the GCC front-end</a>
94 <ol>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]95 <li><a href="#iosinit">What is this <tt>llvm.global_ctors</tt> and
96 <tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I
97 #include &lt;iostream&gt;?</a></li>
98 <li><a href="#codedce">Where did all of my code go??</a></li>
99 <li><a href="#undef">What is this "<tt>undef</tt>" thing that shows up in my code?</a></li>
100 </ol>
101 </li>
102</ol>
103
104<div class="doc_author">
105 <p>Written by <a href="http://llvm.org">The LLVM Team</a></p>
106</div>
107
108
109<!-- *********************************************************************** -->
110<div class="doc_section">
111 <a name="license">License</a>
112</div>
113<!-- *********************************************************************** -->
114
115<div class="question">
116<p>Why are the LLVM source code and the front-end distributed under different
117licenses?</p>
118</div>
119
120<div class="answer">
121<p>The C/C++ front-ends are based on GCC and must be distributed under the GPL.
122Our aim is to distribute LLVM source code under a <em>much less restrictive</em>
123license, in particular one that does not compel users who distribute tools based
124on modifying the source to redistribute the modified source code as well.</p>
125</div>
126
127<div class="question">
128<p>Does the University of Illinois Open Source License really qualify as an
129"open source" license?</p>
130</div>
131
132<div class="answer">
133<p>Yes, the license is <a
134href="http://www.opensource.org/licenses/UoI-NCSA.php">certified</a> by the Open
135Source Initiative (OSI).</p>
136</div>
137
138<div class="question">
139<p>Can I modify LLVM source code and redistribute the modified source?</p>
140</div>
141
142<div class="answer">
143<p>Yes. The modified source distribution must retain the copyright notice and
144follow the three bulletted conditions listed in the <a
Misha Brukman5f135db2008-12-17 18:06:53 +0000[diff] [blame]145href="http://llvm.org/svn/llvm-project/llvm/trunk/LICENSE.TXT">LLVM license</a>.</p>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]146</div>
147
148<div class="question">
149<p>Can I modify LLVM source code and redistribute binaries or other tools based
150on it, without redistributing the source?</p>
151</div>
152
153<div class="answer">
154<p>Yes, this is why we distribute LLVM under a less restrictive license than
155GPL, as explained in the first question above.</p>
156</div>
157
158<!-- *********************************************************************** -->
159<div class="doc_section">
160 <a name="source">Source Code</a>
161</div>
162<!-- *********************************************************************** -->
163
164<div class="question">
165<p>In what language is LLVM written?</p>
166</div>
167
168<div class="answer">
169<p>All of the LLVM tools and libraries are written in C++ with extensive use of
170the STL.</p>
171</div>
172
173<div class="question">
174<p>How portable is the LLVM source code?</p>
175</div>
176
177<div class="answer">
178<p>The LLVM source code should be portable to most modern UNIX-like operating
179systems. Most of the code is written in standard C++ with operating system
180services abstracted to a support library. The tools required to build and test
181LLVM have been ported to a plethora of platforms.</p>
182
183<p>Some porting problems may exist in the following areas:</p>
184
185<ul>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]186 <li>The GCC front end code is not as portable as the LLVM suite, so it may not
Bill Wendling4e05864c2007-09-22 09:54:47 +0000[diff] [blame]187 compile as well on unsupported platforms.</li>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]188
189 <li>The LLVM build system relies heavily on UNIX shell tools, like the Bourne
Bill Wendling4e05864c2007-09-22 09:54:47 +0000[diff] [blame]190 Shell and sed. Porting to systems without these tools (MacOS 9, Plan 9)
191 will require more effort.</li>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]192</ul>
193
194</div>
195
196<!-- *********************************************************************** -->
197<div class="doc_section">
198 <a name="build">Build Problems</a>
199</div>
200<!-- *********************************************************************** -->
201
202<div class="question">
203<p>When I run configure, it finds the wrong C compiler.</p>
204</div>
205
206<div class="answer">
207
208<p>The <tt>configure</tt> script attempts to locate first <tt>gcc</tt> and then
209<tt>cc</tt>, unless it finds compiler paths set in <tt>CC</tt> and <tt>CXX</tt>
210for the C and C++ compiler, respectively.</p>
211
212<p>If <tt>configure</tt> finds the wrong compiler, either adjust your
213<tt>PATH</tt> environment variable or set <tt>CC</tt> and <tt>CXX</tt>
214explicitly.</p>
215
216</div>
217
218<div class="question">
219<p>The <tt>configure</tt> script finds the right C compiler, but it uses the
220LLVM linker from a previous build. What do I do?</p>
221</div>
222
223<div class="answer">
224<p>The <tt>configure</tt> script uses the <tt>PATH</tt> to find executables, so
225if it's grabbing the wrong linker/assembler/etc, there are two ways to fix
226it:</p>
227
228<ol>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]229 <li><p>Adjust your <tt>PATH</tt> environment variable so that the correct
Bill Wendling4e05864c2007-09-22 09:54:47 +0000[diff] [blame]230 program appears first in the <tt>PATH</tt>. This may work, but may not be
231 convenient when you want them <i>first</i> in your path for other
232 work.</p></li>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]233
234 <li><p>Run <tt>configure</tt> with an alternative <tt>PATH</tt> that is
Bill Wendling4e05864c2007-09-22 09:54:47 +0000[diff] [blame]235 correct. In a Borne compatible shell, the syntax would be:</p>
236
Misha Brukman4fbcf682008-12-17 18:11:40 +0000[diff] [blame]237<pre class="doc_code">
Bill Wendling4e05864c2007-09-22 09:54:47 +0000[diff] [blame]238% PATH=[the path without the bad program] ./configure ...
239</pre>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]240
241 <p>This is still somewhat inconvenient, but it allows <tt>configure</tt>
Bill Wendling4e05864c2007-09-22 09:54:47 +0000[diff] [blame]242 to do its work without having to adjust your <tt>PATH</tt>
243 permanently.</p></li>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]244</ol>
245
246</div>
247
248<div class="question">
249<p>When creating a dynamic library, I get a strange GLIBC error.</p>
250</div>
251
252<div class="answer">
253<p>Under some operating systems (i.e. Linux), libtool does not work correctly if
254GCC was compiled with the --disable-shared option. To work around this, install
255your own version of GCC that has shared libraries enabled by default.</p>
256</div>
257
258<div class="question">
Bill Wendling4e05864c2007-09-22 09:54:47 +0000[diff] [blame]259<p>I've updated my source tree from Subversion, and now my build is trying to
260use a file/directory that doesn't exist.</p>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]261</div>
262
263<div class="answer">
264<p>You need to re-run configure in your object directory. When new Makefiles
265are added to the source tree, they have to be copied over to the object tree in
266order to be used by the build.</p>
267</div>
268
269<div class="question">
270<p>I've modified a Makefile in my source tree, but my build tree keeps using the
271old version. What do I do?</p>
272</div>
273
274<div class="answer">
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]275<p>If the Makefile already exists in your object tree, you
276can just run the following command in the top level directory of your object
277tree:</p>
278
Misha Brukman4fbcf682008-12-17 18:11:40 +0000[diff] [blame]279<pre class="doc_code">
280% ./config.status &lt;relative path to Makefile&gt;
281</pre>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]282
283<p>If the Makefile is new, you will have to modify the configure script to copy
284it over.</p>
285
286</div>
287
288<div class="question">
289<p>I've upgraded to a new version of LLVM, and I get strange build errors.</p>
290</div>
291
292<div class="answer">
293
294<p>Sometimes, changes to the LLVM source code alters how the build system works.
295Changes in libtool, autoconf, or header file dependencies are especially prone
296to this sort of problem.</p>
297
298<p>The best thing to try is to remove the old files and re-build. In most
299cases, this takes care of the problem. To do this, just type <tt>make
300clean</tt> and then <tt>make</tt> in the directory that fails to build.</p>
301
302</div>
303
304<div class="question">
305<p>I've built LLVM and am testing it, but the tests freeze.</p>
306</div>
307
308<div class="answer">
309
310<p>This is most likely occurring because you built a profile or release
311(optimized) build of LLVM and have not specified the same information on the
312<tt>gmake</tt> command line.</p>
313
314<p>For example, if you built LLVM with the command:</p>
315
Misha Brukman4fbcf682008-12-17 18:11:40 +0000[diff] [blame]316<pre class="doc_code">
317% gmake ENABLE_PROFILING=1
318</pre>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]319
320<p>...then you must run the tests with the following commands:</p>
321
Misha Brukman4fbcf682008-12-17 18:11:40 +0000[diff] [blame]322<pre class="doc_code">
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]323% cd llvm/test
324% gmake ENABLE_PROFILING=1
325</pre>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]326
327</div>
328
329<div class="question">
330<p>Why do test results differ when I perform different types of builds?</p>
331</div>
332
333<div class="answer">
334
335<p>The LLVM test suite is dependent upon several features of the LLVM tools and
336libraries.</p>
337
338<p>First, the debugging assertions in code are not enabled in optimized or
339profiling builds. Hence, tests that used to fail may pass.</p>
340
341<p>Second, some tests may rely upon debugging options or behavior that is only
342available in the debug build. These tests will fail in an optimized or profile
343build.</p>
344
345</div>
346
347<div class="question">
348<p>Compiling LLVM with GCC 3.3.2 fails, what should I do?</p>
349</div>
350
351<div class="answer">
352<p>This is <a href="http://gcc.gnu.org/PR?13392">a bug in GCC</a>, and
Bill Wendling4e05864c2007-09-22 09:54:47 +0000[diff] [blame]353affects projects other than LLVM. Try upgrading or downgrading your GCC.</p>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]354</div>
355
356<div class="question">
Bill Wendling4e05864c2007-09-22 09:54:47 +0000[diff] [blame]357<p>After Subversion update, rebuilding gives the error "No rule to make
358target".</p>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]359</div>
360
361<div class="answer">
362<p>If the error is of the form:</p>
363
Misha Brukman4fbcf682008-12-17 18:11:40 +0000[diff] [blame]364<pre class="doc_code">
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]365gmake[2]: *** No rule to make target `/path/to/somefile', needed by
366`/path/to/another/file.d'.<br>
367Stop.
368</pre>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]369
370<p>This may occur anytime files are moved within the Subversion repository or
371removed entirely. In this case, the best solution is to erase all
372<tt>.d</tt> files, which list dependencies for source files, and rebuild:</p>
373
Misha Brukman4fbcf682008-12-17 18:11:40 +0000[diff] [blame]374<pre class="doc_code">
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]375% cd $LLVM_OBJ_DIR
376% rm -f `find . -name \*\.d`
377% gmake
378</pre>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]379
380<p>In other cases, it may be necessary to run <tt>make clean</tt> before
381rebuilding.</p>
382</div>
383
Bill Wendling4e05864c2007-09-22 09:54:47 +0000[diff] [blame]384<div class="question"><p><a name="llvmc">
385The <tt>llvmc</tt> program gives me errors/doesn't work.</a></p>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]386</div>
387
388<div class="answer">
Bill Wendling4e05864c2007-09-22 09:54:47 +0000[diff] [blame]389<p><tt>llvmc</tt> is experimental and isn't really supported. We suggest
390using <tt>llvm-gcc</tt> instead.</p>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]391</div>
392
393<!-- *********************************************************************** -->
394<div class="doc_section"><a name="felangs">Source Languages</a></div>
395
396<div class="question"><p>
397 <a name="langs">What source languages are supported?</a></p>
398</div>
399<div class="answer">
400 <p>LLVM currently has full support for C and C++ source languages. These are
401 available through a special version of GCC that LLVM calls the
402 <a href="#cfe">C Front End</a></p>
403 <p>There is an incomplete version of a Java front end available in the
404 <tt>java</tt> module. There is no documentation on this yet so
405 you'll need to download the code, compile it, and try it.</p>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]406 <p>The PyPy developers are working on integrating LLVM into the PyPy backend
407 so that PyPy language can translate to LLVM.</p>
408</div>
Gordon Henriksen2c2e1482008-02-22 20:58:29 +0000[diff] [blame]409
410<div class="question"><p><a name="langirgen">
Gordon Henriksendb0558b2008-02-22 21:55:51 +0000[diff] [blame]411 I'd like to write a self-hosting LLVM compiler. How should I interface with
412 the LLVM middle-end optimizers and back-end code generators?
Gordon Henriksen2c2e1482008-02-22 20:58:29 +0000[diff] [blame]413</a></p></div>
414<div class="answer">
415 <p>Your compiler front-end will communicate with LLVM by creating a module in
Gordon Henriksendb0558b2008-02-22 21:55:51 +0000[diff] [blame]416 the LLVM intermediate representation (IR) format. Assuming you want to
417 write your language's compiler in the language itself (rather than C++),
418 there are 3 major ways to tackle generating LLVM IR from a front-end:</p>
Gordon Henriksen2c2e1482008-02-22 20:58:29 +0000[diff] [blame]419 <ul>
420 <li>
421 <strong>Call into the LLVM libraries code using your language's FFI
422 (foreign function interface).</strong>
423 <ul>
424 <li><em>for:</em> best tracks changes to the LLVM IR, .ll syntax,
425 and .bc format</li>
426 <li><em>for:</em> enables running LLVM optimization passes without a
427 emit/parse overhead</li>
428 <li><em>for:</em> adapts well to a JIT context</li>
429 <li><em>against:</em> lots of ugly glue code to write</li>
430 </ul>
431 </li>
432 <li>
433 <strong>Emit LLVM assembly from your compiler's native language.</strong>
434 <ul>
435 <li><em>for:</em> very straightforward to get started</li>
436 <li><em>against:</em> the .ll parser is slower than the bitcode reader
437 when interfacing to the middle end</li>
438 <li><em>against:</em> you'll have to re-engineer the LLVM IR object
439 model and asm writer in your language</li>
440 <li><em>against:</em> it may be harder to track changes to the IR</li>
441 </ul>
442 </li>
443 <li>
444 <strong>Emit LLVM bitcode from your compiler's native language.</strong>
445 <ul>
446 <li><em>for:</em> can use the more-efficient bitcode reader when
447 interfacing to the middle end</li>
448 <li><em>against:</em> you'll have to re-engineer the LLVM IR object
449 model and bitcode writer in your language</li>
450 <li><em>against:</em> it may be harder to track changes to the IR</li>
451 </ul>
452 </li>
453 </ul>
454 <p>If you go with the first option, the C bindings in include/llvm-c should
455 help a lot, since most languages have strong support for interfacing with
456 C. The most common hurdle with calling C from managed code is interfacing
457 with the garbage collector. The C interface was designed to require very
458 little memory management, and so is straightforward in this regard.</p>
459</div>
460
Bill Wendling4e05864c2007-09-22 09:54:47 +0000[diff] [blame]461<div class="question"><p><a name="langhlsupp">
462 What support is there for a higher level source language constructs for
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]463 building a compiler?</a></p>
464</div>
465<div class="answer">
466 <p>Currently, there isn't much. LLVM supports an intermediate representation
467 which is useful for code representation but will not support the high level
468 (abstract syntax tree) representation needed by most compilers. There are no
469 facilities for lexical nor semantic analysis. There is, however, a <i>mostly
470 implemented</i> configuration-driven
471 <a href="CompilerDriver.html">compiler driver</a> which simplifies the task
472 of running optimizations, linking, and executable generation.</p>
473</div>
474
Dan Gohman4dfac702008-11-24 17:18:39 +0000[diff] [blame]475<div class="question"><p><a name="getelementptr">
Bill Wendling4e05864c2007-09-22 09:54:47 +0000[diff] [blame]476 I don't understand the GetElementPtr instruction. Help!</a></p>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]477</div>
478<div class="answer">
479 <p>See <a href="GetElementPtr.html">The Often Misunderstood GEP
Bill Wendling4e05864c2007-09-22 09:54:47 +0000[diff] [blame]480 Instruction</a>.</p>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]481</div>
482
483<!-- *********************************************************************** -->
484<div class="doc_section">
485 <a name="cfe">Using the GCC Front End</a>
486</div>
487
488<div class="question">
489<p>
490When I compile software that uses a configure script, the configure script
491thinks my system has all of the header files and libraries it is testing for.
492How do I get configure to work correctly?
493</p>
494</div>
495
496<div class="answer">
497<p>
498The configure script is getting things wrong because the LLVM linker allows
499symbols to be undefined at link time (so that they can be resolved during JIT
500or translation to the C back end). That is why configure thinks your system
501"has everything."
502</p>
503<p>
504To work around this, perform the following steps:
505</p>
506<ol>
507 <li>Make sure the CC and CXX environment variables contains the full path to
508 the LLVM GCC front end.</li>
509
510 <li>Make sure that the regular C compiler is first in your PATH. </li>
511
512 <li>Add the string "-Wl,-native" to your CFLAGS environment variable.</li>
513</ol>
514
515<p>
516This will allow the <tt>llvm-ld</tt> linker to create a native code executable
517instead of shell script that runs the JIT. Creating native code requires
518standard linkage, which in turn will allow the configure script to find out if
519code is not linking on your system because the feature isn't available on your
520system.</p>
521</div>
522
523<div class="question">
524<p>
525When I compile code using the LLVM GCC front end, it complains that it cannot
526find libcrtend.a.
527</p>
528</div>
529
530<div class="answer">
531<p>
532The only way this can happen is if you haven't installed the runtime library. To
533correct this, do:</p>
534
Misha Brukman4fbcf682008-12-17 18:11:40 +0000[diff] [blame]535<pre class="doc_code">
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]536% cd llvm/runtime
537% make clean ; make install-bytecode
538</pre>
539</div>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]540
541<div class="question">
542<p>
543How can I disable all optimizations when compiling code using the LLVM GCC front end?
544</p>
545</div>
546
547<div class="answer">
548<p>
549Passing "-Wa,-disable-opt -Wl,-disable-opt" will disable *all* cleanup and
550optimizations done at the llvm level, leaving you with the truly horrible
551code that you desire.
552</p>
553</div>
554
555
556<div class="question">
557<p>
Dan Gohman4dfac702008-11-24 17:18:39 +0000[diff] [blame]558<a name="translatecxx">Can I use LLVM to convert C++ code to C code?</a>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]559</p>
560</div>
561
562<div class="answer">
563<p>Yes, you can use LLVM to convert code from any language LLVM supports to C.
564Note that the generated C code will be very low level (all loops are lowered
565to gotos, etc) and not very pretty (comments are stripped, original source
566formatting is totally lost, variables are renamed, expressions are regrouped),
567so this may not be what you're looking for. However, this is a good way to add
568C++ support for a processor that does not otherwise have a C++ compiler.
569</p>
570
571<p>Use commands like this:</p>
572
573<ol>
Bill Wendling4e05864c2007-09-22 09:54:47 +0000[diff] [blame]574 <li><p>Compile your program as normal with llvm-g++:</p>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]575
Misha Brukman4fbcf682008-12-17 18:11:40 +0000[diff] [blame]576<pre class="doc_code">
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]577% llvm-g++ x.cpp -o program
578</pre>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]579
Bill Wendling4e05864c2007-09-22 09:54:47 +0000[diff] [blame]580 <p>or:</p>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]581
Misha Brukman4fbcf682008-12-17 18:11:40 +0000[diff] [blame]582<pre class="doc_code">
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]583% llvm-g++ a.cpp -c
584% llvm-g++ b.cpp -c
585% llvm-g++ a.o b.o -o program
586</pre>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]587
Bill Wendling4e05864c2007-09-22 09:54:47 +0000[diff] [blame]588 <p>With llvm-gcc3, this will generate program and program.bc. The .bc
589 file is the LLVM version of the program all linked together.</p></li>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]590
Bill Wendling4e05864c2007-09-22 09:54:47 +0000[diff] [blame]591 <li><p>Convert the LLVM code to C code, using the LLC tool with the C
592 backend:</p>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]593
Misha Brukman4fbcf682008-12-17 18:11:40 +0000[diff] [blame]594<pre class="doc_code">
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]595% llc -march=c program.bc -o program.c
Misha Brukman4fbcf682008-12-17 18:11:40 +0000[diff] [blame]596</pre></li>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]597
Bill Wendling4e05864c2007-09-22 09:54:47 +0000[diff] [blame]598<li><p>Finally, compile the C file:</p>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]599
Misha Brukman4fbcf682008-12-17 18:11:40 +0000[diff] [blame]600<pre class="doc_code">
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]601% cc x.c
Misha Brukman4fbcf682008-12-17 18:11:40 +0000[diff] [blame]602</pre></li>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]603
604</ol>
605
Bill Wendling4e05864c2007-09-22 09:54:47 +0000[diff] [blame]606<p>Note that, by default, the C backend does not support exception handling. If
607you want/need it for a certain program, you can enable it by passing
608"-enable-correct-eh-support" to the llc program. The resultant code will use
609setjmp/longjmp to implement exception support that is correct but relatively
610slow.</p>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]611
Bill Wendling4e05864c2007-09-22 09:54:47 +0000[diff] [blame]612<p>Also note: this specific sequence of commands won't work if you use a
613function defined in the C++ runtime library (or any other C++ library). To
614access an external C++ library, you must manually compile libstdc++ to LLVM
615bitcode, statically link it into your program, then use the commands above to
616convert the whole result into C code. Alternatively, you can compile the
617libraries and your application into two different chunks of C code and link
618them.</p>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]619
620</div>
621
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]622<!-- *********************************************************************** -->
623<div class="doc_section">
624 <a name="cfe_code">Questions about code generated by the GCC front-end</a>
625</div>
626
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]627<div class="question">
628<a name="iosinit"></a>
629<p> What is this <tt>llvm.global_ctors</tt> and
630<tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I #include
631&lt;iostream&gt;?</p>
632</div>
633
634<div class="answer">
635
636<p>If you #include the &lt;iostream&gt; header into a C++ translation unit, the
637file will probably use the <tt>std::cin</tt>/<tt>std::cout</tt>/... global
638objects. However, C++ does not guarantee an order of initialization between
639static objects in different translation units, so if a static ctor/dtor in your
640.cpp file used <tt>std::cout</tt>, for example, the object would not necessarily
641be automatically initialized before your use.</p>
642
643<p>To make <tt>std::cout</tt> and friends work correctly in these scenarios, the
644STL that we use declares a static object that gets created in every translation
645unit that includes <tt>&lt;iostream&gt;</tt>. This object has a static
646constructor and destructor that initializes and destroys the global iostream
647objects before they could possibly be used in the file. The code that you see
648in the .ll file corresponds to the constructor and destructor registration code.
649</p>
650
651<p>If you would like to make it easier to <b>understand</b> the LLVM code
652generated by the compiler in the demo page, consider using <tt>printf()</tt>
653instead of <tt>iostream</tt>s to print values.</p>
654
655</div>
656
657<!--=========================================================================-->
658
659<div class="question"><p>
660<a name="codedce"></a>
661Where did all of my code go??
662</p></div>
663
664<div class="answer">
665<p>
666If you are using the LLVM demo page, you may often wonder what happened to all
667of the code that you typed in. Remember that the demo script is running the
668code through the LLVM optimizers, so if your code doesn't actually do anything
669useful, it might all be deleted.
670</p>
671
672<p>
673To prevent this, make sure that the code is actually needed. For example, if
674you are computing some expression, return the value from the function instead of
675leaving it in a local variable. If you really want to constrain the optimizer,
676you can read from and assign to <tt>volatile</tt> global variables.
677</p>
678</div>
679
680<!--=========================================================================-->
681
682<div class="question"><p>
683<a name="undef"></a>
684<p>What is this "<tt>undef</tt>" thing that shows up in my code?
685</p></div>
686
687<div class="answer">
688<p>
689<a href="LangRef.html#undef"><tt>undef</tt></a> is the LLVM way of representing
690a value that is not defined. You can get these if you do not initialize a
691variable before you use it. For example, the C function:</p>
692
Misha Brukman4fbcf682008-12-17 18:11:40 +0000[diff] [blame]693<pre class="doc_code">
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]694int X() { int i; return i; }
695</pre>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]696
697<p>Is compiled to "<tt>ret i32 undef</tt>" because "<tt>i</tt>" never has
698a value specified for it.</p>
699</div>
700
701<!-- *********************************************************************** -->
702
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Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]709
710 <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
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