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5 <title>LLVM: Frequently Asked Questions</title>
6 <style>
7 @import url("llvm.css");
8 .question { font-weight: bold }
9 .answer { margin-left: 2em }
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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>
52</ol>
53
54<!-- *********************************************************************** -->
55<div class="doc_section">
56 <a name="license">License</a>
57</div>
58<!-- *********************************************************************** -->
59
60<div class="question">
61<p>Why are the LLVM source code and the front-end distributed under different
62licenses?</p>
63</div>
64
65<div class="answer">
66<p>The C/C++ front-ends are based on GCC and must be distributed under the GPL.
67Our aim is to distribute LLVM source code under a <em>much less restrictive</em>
68license, in particular one that does not compel users who distribute tools based
69on modifying the source to redistribute the modified source code as well.</p>
70</div>
71
72<div class="question">
73<p>Does the University of Illinois Open Source License really qualify as an
74"open source" license?</p>
75</div>
76
77<div class="answer">
78<p>Yes, the license is <a
79href="http://www.opensource.org/licenses/UoI-NCSA.php">certified</a> by the Open
80Source Initiative (OSI).</p>
81</div>
82
83<div class="question">
84<p>Can I modify LLVM source code and redistribute the modified source?</p>
85</div>
86
87<div class="answer">
88<p>Yes. The modified source distribution must retain the copyright notice and
89follow the three bulletted conditions listed in the <a
90href="http://llvm.cs.uiuc.edu/releases/1.0/LICENSE.TXT">LLVM license</a>.</p>
91</div>
92
93<div class="question">
94<p>Can I modify LLVM source code and redistribute binaries or other tools based
95on it, without redistributing the source?</p>
96</div>
97
98<div class="answer">
99<p>Yes, this is why we distribute LLVM under a less restrictive license than
100GPL, as explained in the first question above.</p>
101</div>
102
103<!-- *********************************************************************** -->
104<div class="doc_section">
105 <a name="source">Source Code</a>
106</div>
107<!-- *********************************************************************** -->
108
109<div class="question">
110<p>In what language is LLVM written?</p>
111</div>
112
113<div class="answer">
114<p>All of the LLVM tools and libraries are written in C++ with extensive use of
115the STL.</p>
116</div>
117
118<div class="question">
119<p>How portable is the LLVM source code?</p>
120</div>
121
122<div class="answer">
123<p>The LLVM source code should be portable to most modern UNIX-like operating
124systems. Most of the code is written in standard C++ with operating system
125services abstracted to a support library. The tools required to build and test
126LLVM have been ported to a plethora of platforms.</p>
127
128<p>Some porting problems may exist in the following areas:</p>
129
130<ul>
131
132 <li>The GCC front end code is not as portable as the LLVM suite, so it may not
133 compile as well on unsupported platforms.</li>
134
135 <li>The Python test classes are more UNIX-centric than they should be, so
136 porting to non-UNIX like platforms (i.e. Windows, MacOS 9) will require some
137 effort.</li>
138
139 <li>The LLVM build system relies heavily on UNIX shell tools, like the Bourne
140 Shell and sed. Porting to systems without these tools (MacOS 9, Plan 9) will
141 require more effort.</li>
142
143</ul>
144
145</div>
146
147<!-- *********************************************************************** -->
148<div class="doc_section">
149 <a name="build">Build Problems</a>
150</div>
151<!-- *********************************************************************** -->
152
153<div class="question">
154<p>When I run configure, it finds the wrong C compiler.</p>
155</div>
156
157<div class="answer">
158
159<p>The <tt>configure</tt> script attempts to locate first <tt>gcc</tt> and then
160<tt>cc</tt>, unless it finds compiler paths set in <tt>CC</tt> and <tt>CXX</tt>
161for the C and C++ compiler, respectively.</p>
162
163<p>If <tt>configure</tt> finds the wrong compiler, either adjust your
164<tt>PATH</tt> environment variable or set <tt>CC</tt> and <tt>CXX</tt>
165explicitly.</p>
166
167</div>
168
169<div class="question">
170<p>I compile the code, and I get some error about <tt>/localhome</tt>.</p>
171</div>
172
173<div class="answer">
174
175<p>There are several possible causes for this. The first is that you didn't set
176a pathname properly when using <tt>configure</tt>, and it defaulted to a
177pathname that we use on our research machines.</p>
178
179<p>Another possibility is that we hardcoded a path in our Makefiles. If you see
180this, please email the LLVM bug mailing list with the name of the offending
181Makefile and a description of what is wrong with it.</p>
182
183</div>
184
185<div class="question">
186<p>The <tt>configure</tt> script finds the right C compiler, but it uses the
187LLVM linker from a previous build. What do I do?</p>
188</div>
189
190<div class="answer">
191<p>The <tt>configure</tt> script uses the <tt>PATH</tt> to find executables, so
192if it's grabbing the wrong linker/assembler/etc, there are two ways to fix
193it:</p>
194
195<ol>
196
197 <li><p>Adjust your <tt>PATH</tt> environment variable so that the correct
198 program appears first in the <tt>PATH</tt>. This may work, but may not be
199 convenient when you want them <i>first</i> in your path for other
200 work.</p></li>
201
202 <li><p>Run <tt>configure</tt> with an alternative <tt>PATH</tt> that is
203 correct. In a Borne compatible shell, the syntax would be:</p>
204
205 <p><tt>PATH=<the path without the bad program> ./configure ...</tt></p>
206
207 <p>This is still somewhat inconvenient, but it allows <tt>configure</tt>
208 to do its work without having to adjust your <tt>PATH</tt>
209 permanently.</p></li>
210
211</ol>
212
213</div>
214
215<div class="question">
216<p>When creating a dynamic library, I get a strange GLIBC error.</p>
217</div>
218
219<div class="answer">
220<p>Under some operating systems (i.e. Linux), libtool does not work correctly if
221GCC was compiled with the --disable-shared option. To work around this, install
222your own version of GCC that has shared libraries enabled by default.</p>
223</div>
224
225<div class="question">
226<p>I've updated my source tree from CVS, and now my build is trying to use a
227file/directory that doesn't exist.</p>
228</div>
229
230<div class="answer">
231<p>You need to re-run configure in your object directory. When new Makefiles
232are added to the source tree, they have to be copied over to the object tree in
233order to be used by the build.</p>
234</div>
235
236<div class="question">
237<p>I've modified a Makefile in my source tree, but my build tree keeps using the
238old version. What do I do?</p>
239</div>
240
241<div class="answer">
242
243<p>If the Makefile already exists in your object tree, you
244can just run the following command in the top level directory of your object
245tree:</p>
246
247<p><tt>./config.status &lt;relative path to Makefile&gt;</tt><p>
248
249<p>If the Makefile is new, you will have to modify the configure script to copy
250it over.</p>
251
252</div>
253
254<div class="question">
255<p>I've upgraded to a new version of LLVM, and I get strange build errors.</p>
256</div>
257
258<div class="answer">
259
260<p>Sometimes, changes to the LLVM source code alters how the build system works.
261Changes in libtool, autoconf, or header file dependencies are especially prone
262to this sort of problem.</p>
263
264<p>The best thing to try is to remove the old files and re-build. In most
265cases, this takes care of the problem. To do this, just type <tt>make
266clean</tt> and then <tt>make</tt> in the directory that fails to build.</p>
267
268</div>
269
270<div class="question">
271<p>I've built LLVM and am testing it, but the tests freeze.</p>
272</div>
273
274<div class="answer">
275
276<p>This is most likely occurring because you built a profile or release
277(optimized) build of LLVM and have not specified the same information on the
278<tt>gmake</tt> command line.</p>
279
280<p>For example, if you built LLVM with the command:</p>
281
282<p><tt>gmake ENABLE_PROFILING=1</tt>
283
284<p>...then you must run the tests with the following commands:</p>
285
286<p><tt>cd llvm/test<br>gmake ENABLE_PROFILING=1</tt></p>
287
288</div>
289
290<div class="question">
291<p>Why do test results differ when I perform different types of builds?</p>
292</div>
293
294<div class="answer">
295
296<p>The LLVM test suite is dependent upon several features of the LLVM tools and
297libraries.</p>
298
299<p>First, the debugging assertions in code are not enabled in optimized or
300profiling builds. Hence, tests that used to fail may pass.</p>
301
302<p>Second, some tests may rely upon debugging options or behavior that is only
303available in the debug build. These tests will fail in an optimized or profile
304build.</p>
305
306</div>
307
308<!-- *********************************************************************** -->
John Criswellc310f622003-10-13 16:13:06 +0000[diff] [blame]309
310<hr>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame^]311<div class="doc_footer">
312 <a href="http://llvm.cs.uiuc.edu">The LLVM Compiler Infrastructure</a>
313 <br>
314 Last modified: $Date$
315</div>
John Criswellf08c5d82003-10-24 22:48:20 +0000[diff] [blame]316
John Criswellc310f622003-10-13 16:13:06 +0000[diff] [blame]317</body>
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