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5 <title>LLVM: Frequently Asked Questions</title>
Misha Brukman7ce62cc2004-06-01 18:51:03 +0000[diff] [blame]6 <style type="text/css">
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]7 @import url("llvm.css");
8 .question { font-weight: bold }
9 .answer { margin-left: 2em }
10 </style>
11</head>
12<body>
John Criswellc310f622003-10-13 16:13:06 +0000[diff] [blame]13
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]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>I compile the code, and I get some error about <tt>/localhome</tt>.</li>
40 <li>The <tt>configure</tt> script finds the right C compiler, but it uses the
41 LLVM linker from a previous build. What do I do?</li>
42 <li>When creating a dynamic library, I get a strange GLIBC error.</li>
43 <li>I've updated my source tree from CVS, and now my build is trying to use a
44 file/directory that doesn't exist.</li>
45 <li>I've modified a Makefile in my source tree, but my build tree keeps using
46 the old version. What do I do?</li>
47 <li>I've upgraded to a new version of LLVM, and I get strange build
48 errors.</li>
49 <li>I've built LLVM and am testing it, but the tests freeze.</li>
50 <li>Why do test results differ when I perform different types of builds?</li>
Chris Lattner306acee2003-12-22 04:06:12 +0000[diff] [blame]51 <li>Compiling LLVM with GCC 3.3.2 fails, what should I do?</li>
John Criswelld1799612004-03-29 20:23:11 +0000[diff] [blame]52 <li>When I use the test suite, all of the C Backend tests fail. What is
53 wrong?</li>
Misha Brukman1739aec2004-09-09 16:36:47 +0000[diff] [blame]54 <li>After CVS update, rebuilding gives the error "No rule to make
55 target".</li>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]56 </ol></li>
John Criswell76c1e382003-11-18 16:08:49 +0000[diff] [blame]57
Reid Spencer501bfee2006-04-26 14:52:19 +0000[diff] [blame^]58 <li><a href="#felangs">Source Languages</a>
59 <ol>
60 <li><a href="#langs">What source languages are supported?</a></li>
61 <li><a href="#langhlsupp">What support is there for higher level source
62 language constructs for building a compiler?</a></li>
63 </ol>
64
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]65 <li><a href="#cfe">Using the GCC Front End</a>
John Criswell76c1e382003-11-18 16:08:49 +0000[diff] [blame]66 <ol>
67 <li>
68 When I compile software that uses a configure script, the configure script
69 thinks my system has all of the header files and libraries it is testing
70 for. How do I get configure to work correctly?
71 </li>
72
73 <li>
74 When I compile code using the LLVM GCC front end, it complains that it
Chris Lattner7911ce22004-05-23 21:07:27 +0000[diff] [blame]75 cannot find libcrtend.a.
John Criswell76c1e382003-11-18 16:08:49 +0000[diff] [blame]76 </li>
Tanya Lattner14fc5c12005-04-25 20:36:56 +0000[diff] [blame]77
78 <li>
79 How can I disable all optimizations when compiling code using the LLVM GCC front end?
80 </li>
81
John Criswell76c1e382003-11-18 16:08:49 +0000[diff] [blame]82 </ol>
83 </li>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]84
85 <li><a href="#cfe_code">Questions about code generated by the GCC front-end</a>
86 <ol>
Chris Lattner5a53c5d2005-02-25 20:30:21 +0000[diff] [blame]87 <li><a href="#__main">What is this <tt>__main()</tt> call that gets inserted into
88 <tt>main()</tt>?</a></li>
89 <li><a href="#iosinit">What is this <tt>llvm.global_ctors</tt> and
Chris Lattnerc50bbc92004-03-29 19:14:35 +0000[diff] [blame]90 <tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I
Chris Lattner5a53c5d2005-02-25 20:30:21 +0000[diff] [blame]91 #include &lt;iostream&gt;?</a></li>
92 <li><a href="#codedce">Where did all of my code go??</a></li>
93 <li><a href="#undef">What is this "<tt>undef</tt>" thing that shows up in my code?</a></li>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]94 </ol>
95 </li>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]96</ol>
97
Chris Lattner7911ce22004-05-23 21:07:27 +0000[diff] [blame]98<div class="doc_author">
Reid Spencer05fe4b02006-03-14 05:39:39 +0000[diff] [blame]99 <p>Written by <a href="http://llvm.org">The LLVM Team</a></p>
Chris Lattner7911ce22004-05-23 21:07:27 +0000[diff] [blame]100</div>
101
102
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]103<!-- *********************************************************************** -->
104<div class="doc_section">
105 <a name="license">License</a>
106</div>
107<!-- *********************************************************************** -->
108
109<div class="question">
110<p>Why are the LLVM source code and the front-end distributed under different
111licenses?</p>
112</div>
113
114<div class="answer">
115<p>The C/C++ front-ends are based on GCC and must be distributed under the GPL.
116Our aim is to distribute LLVM source code under a <em>much less restrictive</em>
117license, in particular one that does not compel users who distribute tools based
118on modifying the source to redistribute the modified source code as well.</p>
119</div>
120
121<div class="question">
122<p>Does the University of Illinois Open Source License really qualify as an
123"open source" license?</p>
124</div>
125
126<div class="answer">
127<p>Yes, the license is <a
128href="http://www.opensource.org/licenses/UoI-NCSA.php">certified</a> by the Open
129Source Initiative (OSI).</p>
130</div>
131
132<div class="question">
133<p>Can I modify LLVM source code and redistribute the modified source?</p>
134</div>
135
136<div class="answer">
137<p>Yes. The modified source distribution must retain the copyright notice and
138follow the three bulletted conditions listed in the <a
Reid Spencer05fe4b02006-03-14 05:39:39 +0000[diff] [blame]139href="http://llvm.org/releases/1.3/LICENSE.TXT">LLVM license</a>.</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]140</div>
141
142<div class="question">
143<p>Can I modify LLVM source code and redistribute binaries or other tools based
144on it, without redistributing the source?</p>
145</div>
146
147<div class="answer">
148<p>Yes, this is why we distribute LLVM under a less restrictive license than
149GPL, as explained in the first question above.</p>
150</div>
151
152<!-- *********************************************************************** -->
153<div class="doc_section">
154 <a name="source">Source Code</a>
155</div>
156<!-- *********************************************************************** -->
157
158<div class="question">
159<p>In what language is LLVM written?</p>
160</div>
161
162<div class="answer">
163<p>All of the LLVM tools and libraries are written in C++ with extensive use of
164the STL.</p>
165</div>
166
167<div class="question">
168<p>How portable is the LLVM source code?</p>
169</div>
170
171<div class="answer">
172<p>The LLVM source code should be portable to most modern UNIX-like operating
173systems. Most of the code is written in standard C++ with operating system
174services abstracted to a support library. The tools required to build and test
175LLVM have been ported to a plethora of platforms.</p>
176
177<p>Some porting problems may exist in the following areas:</p>
178
179<ul>
180
181 <li>The GCC front end code is not as portable as the LLVM suite, so it may not
182 compile as well on unsupported platforms.</li>
183
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]184 <li>The LLVM build system relies heavily on UNIX shell tools, like the Bourne
185 Shell and sed. Porting to systems without these tools (MacOS 9, Plan 9) will
186 require more effort.</li>
187
188</ul>
189
190</div>
191
192<!-- *********************************************************************** -->
193<div class="doc_section">
194 <a name="build">Build Problems</a>
195</div>
196<!-- *********************************************************************** -->
197
198<div class="question">
199<p>When I run configure, it finds the wrong C compiler.</p>
200</div>
201
202<div class="answer">
203
204<p>The <tt>configure</tt> script attempts to locate first <tt>gcc</tt> and then
205<tt>cc</tt>, unless it finds compiler paths set in <tt>CC</tt> and <tt>CXX</tt>
206for the C and C++ compiler, respectively.</p>
207
208<p>If <tt>configure</tt> finds the wrong compiler, either adjust your
209<tt>PATH</tt> environment variable or set <tt>CC</tt> and <tt>CXX</tt>
210explicitly.</p>
211
212</div>
213
214<div class="question">
215<p>I compile the code, and I get some error about <tt>/localhome</tt>.</p>
216</div>
217
218<div class="answer">
219
220<p>There are several possible causes for this. The first is that you didn't set
221a pathname properly when using <tt>configure</tt>, and it defaulted to a
222pathname that we use on our research machines.</p>
223
224<p>Another possibility is that we hardcoded a path in our Makefiles. If you see
225this, please email the LLVM bug mailing list with the name of the offending
226Makefile and a description of what is wrong with it.</p>
227
228</div>
229
230<div class="question">
231<p>The <tt>configure</tt> script finds the right C compiler, but it uses the
232LLVM linker from a previous build. What do I do?</p>
233</div>
234
235<div class="answer">
236<p>The <tt>configure</tt> script uses the <tt>PATH</tt> to find executables, so
237if it's grabbing the wrong linker/assembler/etc, there are two ways to fix
238it:</p>
239
240<ol>
241
242 <li><p>Adjust your <tt>PATH</tt> environment variable so that the correct
243 program appears first in the <tt>PATH</tt>. This may work, but may not be
244 convenient when you want them <i>first</i> in your path for other
245 work.</p></li>
246
247 <li><p>Run <tt>configure</tt> with an alternative <tt>PATH</tt> that is
248 correct. In a Borne compatible shell, the syntax would be:</p>
249
Misha Brukman7ce62cc2004-06-01 18:51:03 +0000[diff] [blame]250 <p><tt>PATH=[the path without the bad program] ./configure ...</tt></p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]251
252 <p>This is still somewhat inconvenient, but it allows <tt>configure</tt>
253 to do its work without having to adjust your <tt>PATH</tt>
254 permanently.</p></li>
255
256</ol>
257
258</div>
259
260<div class="question">
261<p>When creating a dynamic library, I get a strange GLIBC error.</p>
262</div>
263
264<div class="answer">
265<p>Under some operating systems (i.e. Linux), libtool does not work correctly if
266GCC was compiled with the --disable-shared option. To work around this, install
267your own version of GCC that has shared libraries enabled by default.</p>
268</div>
269
270<div class="question">
271<p>I've updated my source tree from CVS, and now my build is trying to use a
272file/directory that doesn't exist.</p>
273</div>
274
275<div class="answer">
276<p>You need to re-run configure in your object directory. When new Makefiles
277are added to the source tree, they have to be copied over to the object tree in
278order to be used by the build.</p>
279</div>
280
281<div class="question">
282<p>I've modified a Makefile in my source tree, but my build tree keeps using the
283old version. What do I do?</p>
284</div>
285
286<div class="answer">
287
288<p>If the Makefile already exists in your object tree, you
289can just run the following command in the top level directory of your object
290tree:</p>
291
292<p><tt>./config.status &lt;relative path to Makefile&gt;</tt><p>
293
294<p>If the Makefile is new, you will have to modify the configure script to copy
295it over.</p>
296
297</div>
298
299<div class="question">
300<p>I've upgraded to a new version of LLVM, and I get strange build errors.</p>
301</div>
302
303<div class="answer">
304
305<p>Sometimes, changes to the LLVM source code alters how the build system works.
306Changes in libtool, autoconf, or header file dependencies are especially prone
307to this sort of problem.</p>
308
309<p>The best thing to try is to remove the old files and re-build. In most
310cases, this takes care of the problem. To do this, just type <tt>make
311clean</tt> and then <tt>make</tt> in the directory that fails to build.</p>
312
313</div>
314
315<div class="question">
316<p>I've built LLVM and am testing it, but the tests freeze.</p>
317</div>
318
319<div class="answer">
320
321<p>This is most likely occurring because you built a profile or release
322(optimized) build of LLVM and have not specified the same information on the
323<tt>gmake</tt> command line.</p>
324
325<p>For example, if you built LLVM with the command:</p>
326
327<p><tt>gmake ENABLE_PROFILING=1</tt>
328
329<p>...then you must run the tests with the following commands:</p>
330
331<p><tt>cd llvm/test<br>gmake ENABLE_PROFILING=1</tt></p>
332
333</div>
334
335<div class="question">
336<p>Why do test results differ when I perform different types of builds?</p>
337</div>
338
339<div class="answer">
340
341<p>The LLVM test suite is dependent upon several features of the LLVM tools and
342libraries.</p>
343
344<p>First, the debugging assertions in code are not enabled in optimized or
345profiling builds. Hence, tests that used to fail may pass.</p>
346
347<p>Second, some tests may rely upon debugging options or behavior that is only
348available in the debug build. These tests will fail in an optimized or profile
349build.</p>
350
351</div>
352
Chris Lattner8a0b9242003-12-08 05:43:19 +0000[diff] [blame]353<div class="question">
Chris Lattner306acee2003-12-22 04:06:12 +0000[diff] [blame]354<p>Compiling LLVM with GCC 3.3.2 fails, what should I do?</p>
Chris Lattner8a0b9242003-12-08 05:43:19 +0000[diff] [blame]355</div>
356
357<div class="answer">
Chris Lattner306acee2003-12-22 04:06:12 +0000[diff] [blame]358<p>This is <a href="http://gcc.gnu.org/PR?13392">a bug in GCC</a>, and
359 affects projects other than LLVM. Try upgrading or downgrading your GCC.</p>
Chris Lattner8a0b9242003-12-08 05:43:19 +0000[diff] [blame]360</div>
361
John Criswelld1799612004-03-29 20:23:11 +0000[diff] [blame]362<div class="question">
Misha Brukman1739aec2004-09-09 16:36:47 +0000[diff] [blame]363<p>After CVS update, rebuilding gives the error "No rule to make target".</p>
364</div>
365
366<div class="answer">
367<p>If the error is of the form:</p>
368
369<div class="doc_code">
370<tt>
371gmake[2]: *** No rule to make target `/path/to/somefile', needed by
372`/path/to/another/file.d'.<br>
373Stop.
374</tt>
375</div>
376
377<p>This may occur anytime files are moved within the CVS repository or removed
378entirely. In this case, the best solution is to erase all <tt>.d</tt> files,
379which list dependencies for source files, and rebuild:</p>
380
381<div class="doc_code">
382<pre>
383% cd $LLVM_OBJ_DIR
384% rm -f `find . -name \*\.d`
385% gmake
386</pre>
387</div>
388
389<p>In other cases, it may be necessary to run <tt>make clean</tt> before
390rebuilding.</p>
391</div>
392
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]393<!-- *********************************************************************** -->
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]394<div class="doc_section">
Reid Spencer501bfee2006-04-26 14:52:19 +0000[diff] [blame^]395 <a name="felangs">Source Languages</a>
396</div>
397
398<div class="question"><p>What source languages are supported?</p></div>
399<div name="langs" 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>llvm-java</tt> CVS repository. There is no documentation on this yet so
405 you'll need to download the code, compile it, and try it.</p>
406 <p>In the <tt>examples/BFtoLLVM</tt> directory is a translator for the
407 BrainF*** language (2002 Language Specification).</p>
408 <p>In the <tt>projects/Stacker</tt> directory is a compiler and runtime
409 library for the Stacker language, a "toy" language loosely based on Forth.</p>
410 <p>The PyPy developers are working on integrating LLVM into the PyPy backend
411 so that PyPy language can translate to LLVM.</p>
412</div>
413<div name="langhlsupp" class="question">
414 <p>What support is there for a higher level source language constructs for
415 building a compiler?</p>
416</div>
417<div class="answer">
418 <p>Currently, there isn't much. LLVM supports an intermediate representation
419 which is useful for code representation but will not support the high level
420 (abstract syntax tree) representation needed by most compilers. There are no
421 facilities for lexical nor semantica analysis. There is, however, a <i>mostly
422 implemented</i> configuration-driven
423 <a href="CompilerDriver.html">compiler driver</a> which simplifies the task
424 of running optimizations, linking, and executable generation.</p>
425 <p>You might be interested in following the progress of the <a
426 href="http://hlvm.org">HLVM Project</a> which is attempting to address these
427 issues.</p>
428</div>
429<!-- *********************************************************************** -->
430<div class="doc_section">
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]431 <a name="cfe">Using the GCC Front End</a>
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]432</div>
433
434<div class="question">
435<p>
436When I compile software that uses a configure script, the configure script
437thinks my system has all of the header files and libraries it is testing for.
438How do I get configure to work correctly?
439</p>
440</div>
441
442<div class="answer">
443<p>
444The configure script is getting things wrong because the LLVM linker allows
445symbols to be undefined at link time (so that they can be resolved during JIT
446or translation to the C back end). That is why configure thinks your system
447"has everything."
448</p>
449<p>
450To work around this, perform the following steps:
451</p>
452
453<ol>
454 <li>
455 Make sure the CC and CXX environment variables contains the full path to the
456 LLVM GCC front end.
457 </li>
458
459 <li>
460 Make sure that the regular C compiler is first in your PATH.
461 </li>
462
463 <li>
464 Add the string "-Wl,-native" to your CFLAGS environment variable.
465 </li>
466</ol>
467
468<p>
469This will allow the gccld linker to create a native code executable instead of
470a shell script that runs the JIT. Creating native code requires standard
471linkage, which in turn will allow the configure script to find out if code is
472not linking on your system because the feature isn't available on your system.
473</p>
474</div>
475
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]476<div class="question">
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]477<p>
478When I compile code using the LLVM GCC front end, it complains that it cannot
John Criswellcd161192004-03-12 18:20:15 +0000[diff] [blame]479find libcrtend.a.
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]480</p>
481</div>
482
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]483<div class="answer">
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]484<p>
Reid Spencerf96eb572004-12-15 00:14:01 +0000[diff] [blame]485The only way this can happen is if you haven't installed the runtime library. To
486correct this, do:</p>
487<pre>
488 % cd llvm/runtime
489 % make clean ; make install-bytecode
490</pre>
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]491</div>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]492
Tanya Lattner14fc5c12005-04-25 20:36:56 +0000[diff] [blame]493<div class="question">
494<p>
495How can I disable all optimizations when compiling code using the LLVM GCC front end?
496</p>
497</div>
498
499<div class="answer">
500<p>
501Passing "-Wa,-disable-opt -Wl,-disable-opt" will disable *all* cleanup and
502optimizations done at the llvm level, leaving you with the truly horrible
503code that you desire.
504</p>
505</div>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]506
507<!-- *********************************************************************** -->
508<div class="doc_section">
509 <a name="cfe_code">Questions about code generated by the GCC front-end</a>
510</div>
511
Chris Lattnera28e3ce2003-12-16 22:33:55 +0000[diff] [blame]512<div class="question"><p>
Chris Lattner5a53c5d2005-02-25 20:30:21 +0000[diff] [blame]513<a name="__main"></a>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]514What is this <tt>__main()</tt> call that gets inserted into <tt>main()</tt>?
Chris Lattnera28e3ce2003-12-16 22:33:55 +0000[diff] [blame]515</p></div>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]516
517<div class="answer">
518<p>
519The <tt>__main</tt> call is inserted by the C/C++ compiler in order to guarantee
520that static constructors and destructors are called when the program starts up
521and shuts down. In C, you can create static constructors and destructors by
522using GCC extensions, and in C++ you can do so by creating a global variable
523whose class has a ctor or dtor.
524</p>
525
526<p>
527The actual implementation of <tt>__main</tt> lives in the
528<tt>llvm/runtime/GCCLibraries/crtend/</tt> directory in the source-base, and is
529linked in automatically when you link the program.
530</p>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]531</div>
532
Chris Lattnera28e3ce2003-12-16 22:33:55 +0000[diff] [blame]533<!--=========================================================================-->
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]534
Misha Brukman237dc2a2004-12-03 23:58:18 +0000[diff] [blame]535<div class="question">
Chris Lattner5a53c5d2005-02-25 20:30:21 +0000[diff] [blame]536<a name="iosinit"></a>
Misha Brukman237dc2a2004-12-03 23:58:18 +0000[diff] [blame]537<p> What is this <tt>llvm.global_ctors</tt> and
538<tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I #include
539&lt;iostream&gt;?</p>
540</div>
Chris Lattnerc50bbc92004-03-29 19:14:35 +0000[diff] [blame]541
542<div class="answer">
Misha Brukman237dc2a2004-12-03 23:58:18 +0000[diff] [blame]543
544<p>If you #include the &lt;iostream&gt; header into a C++ translation unit, the
Chris Lattnerc50bbc92004-03-29 19:14:35 +0000[diff] [blame]545file will probably use the <tt>std::cin</tt>/<tt>std::cout</tt>/... global
546objects. However, C++ does not guarantee an order of initialization between
547static objects in different translation units, so if a static ctor/dtor in your
548.cpp file used <tt>std::cout</tt>, for example, the object would not necessarily
Misha Brukman237dc2a2004-12-03 23:58:18 +0000[diff] [blame]549be automatically initialized before your use.</p>
Chris Lattnerc50bbc92004-03-29 19:14:35 +0000[diff] [blame]550
Misha Brukman237dc2a2004-12-03 23:58:18 +0000[diff] [blame]551<p>To make <tt>std::cout</tt> and friends work correctly in these scenarios, the
Chris Lattnerc50bbc92004-03-29 19:14:35 +0000[diff] [blame]552STL that we use declares a static object that gets created in every translation
Misha Brukman237dc2a2004-12-03 23:58:18 +0000[diff] [blame]553unit that includes <tt>&lt;iostream&gt;</tt>. This object has a static
554constructor and destructor that initializes and destroys the global iostream
555objects before they could possibly be used in the file. The code that you see
556in the .ll file corresponds to the constructor and destructor registration code.
Chris Lattnerc50bbc92004-03-29 19:14:35 +0000[diff] [blame]557</p>
558
Misha Brukman237dc2a2004-12-03 23:58:18 +0000[diff] [blame]559<p>If you would like to make it easier to <b>understand</b> the LLVM code
560generated by the compiler in the demo page, consider using <tt>printf()</tt>
561instead of <tt>iostream</tt>s to print values.</p>
562
Chris Lattnerc50bbc92004-03-29 19:14:35 +0000[diff] [blame]563</div>
564
Chris Lattner5a53c5d2005-02-25 20:30:21 +0000[diff] [blame]565<!--=========================================================================-->
566
567<div class="question"><p>
568<a name="codedce"></a>
569Where did all of my code go??
570</p></div>
571
572<div class="answer">
573<p>
574If you are using the LLVM demo page, you may often wonder what happened to all
575of the code that you typed in. Remember that the demo script is running the
576code through the LLVM optimizers, so if your code doesn't actually do anything
577useful, it might all be deleted.
578</p>
579
580<p>
581To prevent this, make sure that the code is actually needed. For example, if
582you are computing some expression, return the value from the function instead of
583leaving it in a local variable. If you really want to constrain the optimizer,
584you can read from and assign to <tt>volatile</tt> global variables.
585</p>
586</div>
587
588<!--=========================================================================-->
589
590<div class="question"><p>
591<a name="undef"></a>
592<p>What is this "<tt>undef</tt>" thing that shows up in my code?
593</p></div>
594
595<div class="answer">
596<p>
597<a href="LangRef.html#undef"><tt>undef</tt></a> is the LLVM way of representing
598a value that is not defined. You can get these if you do not initialize a
599variable before you use it. For example, the C function:</p>
600
601<div class="doc_code">
602 <tt>int X() { int i; return i; }</tt>
603</div>
604
605<p>Is compiled to "<tt>ret int undef</tt>" because "i" never has a value
606specified for it.
607</p>
608</div>
609
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]610<!-- *********************************************************************** -->
John Criswellc310f622003-10-13 16:13:06 +0000[diff] [blame]611
612<hr>
Misha Brukman7ce62cc2004-06-01 18:51:03 +0000[diff] [blame]613<address>
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618
Reid Spencer05fe4b02006-03-14 05:39:39 +0000[diff] [blame]619 <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
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