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