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3<html>
4<head>
5 <title>LLVM: Frequently Asked Questions</title>
6 <style>
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 Lattner8a0b9242003-12-08 05:43:19 +0000[diff] [blame]51 <li>Compiling LLVM with GCC 3.3 on SuSE 9 fails, what should I do?</li>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]52 </ol></li>
John Criswell76c1e382003-11-18 16:08:49 +0000[diff] [blame]53
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]54 <li><a href="#cfe">Using the GCC Front End</a>
John Criswell76c1e382003-11-18 16:08:49 +0000[diff] [blame]55 <ol>
56 <li>
57 When I compile software that uses a configure script, the configure script
58 thinks my system has all of the header files and libraries it is testing
59 for. How do I get configure to work correctly?
60 </li>
61
62 <li>
63 When I compile code using the LLVM GCC front end, it complains that it
64 cannot find crtend.o.
65 </li>
66 </ol>
67 </li>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]68
69 <li><a href="#cfe_code">Questions about code generated by the GCC front-end</a>
70 <ol>
71 <li>What is this <tt>__main()</tt> call that gets inserted into
72 <tt>main()</tt>?</li>
Chris Lattnera28e3ce2003-12-16 22:33:55 +0000[diff] [blame]73 <li>Where did all of my code go??</li>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]74 </ol>
75 </li>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]76</ol>
77
78<!-- *********************************************************************** -->
79<div class="doc_section">
80 <a name="license">License</a>
81</div>
82<!-- *********************************************************************** -->
83
84<div class="question">
85<p>Why are the LLVM source code and the front-end distributed under different
86licenses?</p>
87</div>
88
89<div class="answer">
90<p>The C/C++ front-ends are based on GCC and must be distributed under the GPL.
91Our aim is to distribute LLVM source code under a <em>much less restrictive</em>
92license, in particular one that does not compel users who distribute tools based
93on modifying the source to redistribute the modified source code as well.</p>
94</div>
95
96<div class="question">
97<p>Does the University of Illinois Open Source License really qualify as an
98"open source" license?</p>
99</div>
100
101<div class="answer">
102<p>Yes, the license is <a
103href="http://www.opensource.org/licenses/UoI-NCSA.php">certified</a> by the Open
104Source Initiative (OSI).</p>
105</div>
106
107<div class="question">
108<p>Can I modify LLVM source code and redistribute the modified source?</p>
109</div>
110
111<div class="answer">
112<p>Yes. The modified source distribution must retain the copyright notice and
113follow the three bulletted conditions listed in the <a
114href="http://llvm.cs.uiuc.edu/releases/1.0/LICENSE.TXT">LLVM license</a>.</p>
115</div>
116
117<div class="question">
118<p>Can I modify LLVM source code and redistribute binaries or other tools based
119on it, without redistributing the source?</p>
120</div>
121
122<div class="answer">
123<p>Yes, this is why we distribute LLVM under a less restrictive license than
124GPL, as explained in the first question above.</p>
125</div>
126
127<!-- *********************************************************************** -->
128<div class="doc_section">
129 <a name="source">Source Code</a>
130</div>
131<!-- *********************************************************************** -->
132
133<div class="question">
134<p>In what language is LLVM written?</p>
135</div>
136
137<div class="answer">
138<p>All of the LLVM tools and libraries are written in C++ with extensive use of
139the STL.</p>
140</div>
141
142<div class="question">
143<p>How portable is the LLVM source code?</p>
144</div>
145
146<div class="answer">
147<p>The LLVM source code should be portable to most modern UNIX-like operating
148systems. Most of the code is written in standard C++ with operating system
149services abstracted to a support library. The tools required to build and test
150LLVM have been ported to a plethora of platforms.</p>
151
152<p>Some porting problems may exist in the following areas:</p>
153
154<ul>
155
156 <li>The GCC front end code is not as portable as the LLVM suite, so it may not
157 compile as well on unsupported platforms.</li>
158
159 <li>The Python test classes are more UNIX-centric than they should be, so
160 porting to non-UNIX like platforms (i.e. Windows, MacOS 9) will require some
161 effort.</li>
162
163 <li>The LLVM build system relies heavily on UNIX shell tools, like the Bourne
164 Shell and sed. Porting to systems without these tools (MacOS 9, Plan 9) will
165 require more effort.</li>
166
167</ul>
168
169</div>
170
171<!-- *********************************************************************** -->
172<div class="doc_section">
173 <a name="build">Build Problems</a>
174</div>
175<!-- *********************************************************************** -->
176
177<div class="question">
178<p>When I run configure, it finds the wrong C compiler.</p>
179</div>
180
181<div class="answer">
182
183<p>The <tt>configure</tt> script attempts to locate first <tt>gcc</tt> and then
184<tt>cc</tt>, unless it finds compiler paths set in <tt>CC</tt> and <tt>CXX</tt>
185for the C and C++ compiler, respectively.</p>
186
187<p>If <tt>configure</tt> finds the wrong compiler, either adjust your
188<tt>PATH</tt> environment variable or set <tt>CC</tt> and <tt>CXX</tt>
189explicitly.</p>
190
191</div>
192
193<div class="question">
194<p>I compile the code, and I get some error about <tt>/localhome</tt>.</p>
195</div>
196
197<div class="answer">
198
199<p>There are several possible causes for this. The first is that you didn't set
200a pathname properly when using <tt>configure</tt>, and it defaulted to a
201pathname that we use on our research machines.</p>
202
203<p>Another possibility is that we hardcoded a path in our Makefiles. If you see
204this, please email the LLVM bug mailing list with the name of the offending
205Makefile and a description of what is wrong with it.</p>
206
207</div>
208
209<div class="question">
210<p>The <tt>configure</tt> script finds the right C compiler, but it uses the
211LLVM linker from a previous build. What do I do?</p>
212</div>
213
214<div class="answer">
215<p>The <tt>configure</tt> script uses the <tt>PATH</tt> to find executables, so
216if it's grabbing the wrong linker/assembler/etc, there are two ways to fix
217it:</p>
218
219<ol>
220
221 <li><p>Adjust your <tt>PATH</tt> environment variable so that the correct
222 program appears first in the <tt>PATH</tt>. This may work, but may not be
223 convenient when you want them <i>first</i> in your path for other
224 work.</p></li>
225
226 <li><p>Run <tt>configure</tt> with an alternative <tt>PATH</tt> that is
227 correct. In a Borne compatible shell, the syntax would be:</p>
228
229 <p><tt>PATH=<the path without the bad program> ./configure ...</tt></p>
230
231 <p>This is still somewhat inconvenient, but it allows <tt>configure</tt>
232 to do its work without having to adjust your <tt>PATH</tt>
233 permanently.</p></li>
234
235</ol>
236
237</div>
238
239<div class="question">
240<p>When creating a dynamic library, I get a strange GLIBC error.</p>
241</div>
242
243<div class="answer">
244<p>Under some operating systems (i.e. Linux), libtool does not work correctly if
245GCC was compiled with the --disable-shared option. To work around this, install
246your own version of GCC that has shared libraries enabled by default.</p>
247</div>
248
249<div class="question">
250<p>I've updated my source tree from CVS, and now my build is trying to use a
251file/directory that doesn't exist.</p>
252</div>
253
254<div class="answer">
255<p>You need to re-run configure in your object directory. When new Makefiles
256are added to the source tree, they have to be copied over to the object tree in
257order to be used by the build.</p>
258</div>
259
260<div class="question">
261<p>I've modified a Makefile in my source tree, but my build tree keeps using the
262old version. What do I do?</p>
263</div>
264
265<div class="answer">
266
267<p>If the Makefile already exists in your object tree, you
268can just run the following command in the top level directory of your object
269tree:</p>
270
271<p><tt>./config.status &lt;relative path to Makefile&gt;</tt><p>
272
273<p>If the Makefile is new, you will have to modify the configure script to copy
274it over.</p>
275
276</div>
277
278<div class="question">
279<p>I've upgraded to a new version of LLVM, and I get strange build errors.</p>
280</div>
281
282<div class="answer">
283
284<p>Sometimes, changes to the LLVM source code alters how the build system works.
285Changes in libtool, autoconf, or header file dependencies are especially prone
286to this sort of problem.</p>
287
288<p>The best thing to try is to remove the old files and re-build. In most
289cases, this takes care of the problem. To do this, just type <tt>make
290clean</tt> and then <tt>make</tt> in the directory that fails to build.</p>
291
292</div>
293
294<div class="question">
295<p>I've built LLVM and am testing it, but the tests freeze.</p>
296</div>
297
298<div class="answer">
299
300<p>This is most likely occurring because you built a profile or release
301(optimized) build of LLVM and have not specified the same information on the
302<tt>gmake</tt> command line.</p>
303
304<p>For example, if you built LLVM with the command:</p>
305
306<p><tt>gmake ENABLE_PROFILING=1</tt>
307
308<p>...then you must run the tests with the following commands:</p>
309
310<p><tt>cd llvm/test<br>gmake ENABLE_PROFILING=1</tt></p>
311
312</div>
313
314<div class="question">
315<p>Why do test results differ when I perform different types of builds?</p>
316</div>
317
318<div class="answer">
319
320<p>The LLVM test suite is dependent upon several features of the LLVM tools and
321libraries.</p>
322
323<p>First, the debugging assertions in code are not enabled in optimized or
324profiling builds. Hence, tests that used to fail may pass.</p>
325
326<p>Second, some tests may rely upon debugging options or behavior that is only
327available in the debug build. These tests will fail in an optimized or profile
328build.</p>
329
330</div>
331
Chris Lattner8a0b9242003-12-08 05:43:19 +0000[diff] [blame]332<div class="question">
333<p>Compiling LLVM with GCC 3.3 on SuSE 9 fails, what should I do?</p>
334</div>
335
336<div class="answer">
Misha Brukmanc99ba9f2003-12-08 06:22:05 +0000[diff] [blame]337<p>This is a bug in the customized version of GCC shipped with SuSE, and affects
Chris Lattner8a0b9242003-12-08 05:43:19 +0000[diff] [blame]338 projects other than LLVM. Complain loudly to SuSE. :)</p>
339</div>
340
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]341<!-- *********************************************************************** -->
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]342<div class="doc_section">
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]343 <a name="cfe">Using the GCC Front End</a>
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]344</div>
345
346<div class="question">
347<p>
348When I compile software that uses a configure script, the configure script
349thinks my system has all of the header files and libraries it is testing for.
350How do I get configure to work correctly?
351</p>
352</div>
353
354<div class="answer">
355<p>
356The configure script is getting things wrong because the LLVM linker allows
357symbols to be undefined at link time (so that they can be resolved during JIT
358or translation to the C back end). That is why configure thinks your system
359"has everything."
360</p>
361<p>
362To work around this, perform the following steps:
363</p>
364
365<ol>
366 <li>
367 Make sure the CC and CXX environment variables contains the full path to the
368 LLVM GCC front end.
369 </li>
370
371 <li>
372 Make sure that the regular C compiler is first in your PATH.
373 </li>
374
375 <li>
376 Add the string "-Wl,-native" to your CFLAGS environment variable.
377 </li>
378</ol>
379
380<p>
381This will allow the gccld linker to create a native code executable instead of
382a shell script that runs the JIT. Creating native code requires standard
383linkage, which in turn will allow the configure script to find out if code is
384not linking on your system because the feature isn't available on your system.
385</p>
386</div>
387
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]388<div class="question">
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]389<p>
390When I compile code using the LLVM GCC front end, it complains that it cannot
391find crtend.o.
392</p>
393</div>
394
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]395<div class="answer">
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]396<p>
397In order to find crtend.o, you must have the directory in which it lives in
398your LLVM_LIB_SEARCH_PATH environment variable. For the binary distribution of
399the LLVM GCC front end, this will be the full path of the bytecode-libs
400directory inside of the LLVM GCC distribution.
401</p>
402</div>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]403
404
405<!-- *********************************************************************** -->
406<div class="doc_section">
407 <a name="cfe_code">Questions about code generated by the GCC front-end</a>
408</div>
409
Chris Lattnera28e3ce2003-12-16 22:33:55 +0000[diff] [blame]410<div class="question"><p>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]411What is this <tt>__main()</tt> call that gets inserted into <tt>main()</tt>?
Chris Lattnera28e3ce2003-12-16 22:33:55 +0000[diff] [blame]412</p></div>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]413
414<div class="answer">
415<p>
416The <tt>__main</tt> call is inserted by the C/C++ compiler in order to guarantee
417that static constructors and destructors are called when the program starts up
418and shuts down. In C, you can create static constructors and destructors by
419using GCC extensions, and in C++ you can do so by creating a global variable
420whose class has a ctor or dtor.
421</p>
422
423<p>
424The actual implementation of <tt>__main</tt> lives in the
425<tt>llvm/runtime/GCCLibraries/crtend/</tt> directory in the source-base, and is
426linked in automatically when you link the program.
427</p>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]428</div>
429
Chris Lattnera28e3ce2003-12-16 22:33:55 +0000[diff] [blame]430<!--=========================================================================-->
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]431
Chris Lattnera28e3ce2003-12-16 22:33:55 +0000[diff] [blame]432<div class="question"><p>
433Where did all of my code go??
434</p></div>
435
436<div class="answer">
437<p>
438If you are using the LLVM demo page, you may often wonder what happened to all
439of the code that you typed in. Remember that the demo script is running the
440code through the LLVM optimizers, so if you code doesn't actually do anything
441useful, it might all be deleted.
442</p>
443
444<p>
445To prevent this, make sure that the code is actually needed. For example, if
446you are computing some expression, return the value from the function instead of
447leaving it in a local variable. If you really want to constrain the optimizer,
448you can read from and assign to <tt>volatile</tt> global variables.
449</p>
450</div>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]451
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]452<!-- *********************************************************************** -->
453<!-- *********************************************************************** -->
John Criswellc310f622003-10-13 16:13:06 +0000[diff] [blame]454
455<hr>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]456<div class="doc_footer">
457 <a href="http://llvm.cs.uiuc.edu">The LLVM Compiler Infrastructure</a>
458 <br>
459 Last modified: $Date$
460</div>
John Criswellf08c5d82003-10-24 22:48:20 +0000[diff] [blame]461
John Criswellc310f622003-10-13 16:13:06 +0000[diff] [blame]462</body>
463</html>