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5 <title>LLVM: Frequently Asked Questions</title>
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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 Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]54 </ol></li>
John Criswell76c1e382003-11-18 16:08:49 +0000[diff] [blame]55
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]56 <li><a href="#cfe">Using the GCC Front End</a>
John Criswell76c1e382003-11-18 16:08:49 +0000[diff] [blame]57 <ol>
58 <li>
59 When I compile software that uses a configure script, the configure script
60 thinks my system has all of the header files and libraries it is testing
61 for. How do I get configure to work correctly?
62 </li>
63
64 <li>
65 When I compile code using the LLVM GCC front end, it complains that it
Chris Lattner7911ce22004-05-23 21:07:27 +0000[diff] [blame]66 cannot find libcrtend.a.
John Criswell76c1e382003-11-18 16:08:49 +0000[diff] [blame]67 </li>
68 </ol>
69 </li>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]70
71 <li><a href="#cfe_code">Questions about code generated by the GCC front-end</a>
72 <ol>
73 <li>What is this <tt>__main()</tt> call that gets inserted into
74 <tt>main()</tt>?</li>
Chris Lattnera28e3ce2003-12-16 22:33:55 +0000[diff] [blame]75 <li>Where did all of my code go??</li>
Chris Lattnerc50bbc92004-03-29 19:14:35 +0000[diff] [blame]76 <li>What is this <tt>llvm.global_ctors</tt> and
77 <tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I
78 #include &lt;iostream&gt;?</li>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]79 </ol>
80 </li>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]81</ol>
82
Chris Lattner7911ce22004-05-23 21:07:27 +0000[diff] [blame]83<div class="doc_author">
84 <p>Written by <a href="http://llvm.cs.uiuc.edu">The LLVM Team</a></p>
85</div>
86
87
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]88<!-- *********************************************************************** -->
89<div class="doc_section">
90 <a name="license">License</a>
91</div>
92<!-- *********************************************************************** -->
93
94<div class="question">
95<p>Why are the LLVM source code and the front-end distributed under different
96licenses?</p>
97</div>
98
99<div class="answer">
100<p>The C/C++ front-ends are based on GCC and must be distributed under the GPL.
101Our aim is to distribute LLVM source code under a <em>much less restrictive</em>
102license, in particular one that does not compel users who distribute tools based
103on modifying the source to redistribute the modified source code as well.</p>
104</div>
105
106<div class="question">
107<p>Does the University of Illinois Open Source License really qualify as an
108"open source" license?</p>
109</div>
110
111<div class="answer">
112<p>Yes, the license is <a
113href="http://www.opensource.org/licenses/UoI-NCSA.php">certified</a> by the Open
114Source Initiative (OSI).</p>
115</div>
116
117<div class="question">
118<p>Can I modify LLVM source code and redistribute the modified source?</p>
119</div>
120
121<div class="answer">
122<p>Yes. The modified source distribution must retain the copyright notice and
123follow the three bulletted conditions listed in the <a
John Criswellcd161192004-03-12 18:20:15 +0000[diff] [blame]124href="http://llvm.cs.uiuc.edu/releases/1.2/LICENSE.TXT">LLVM license</a>.</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]125</div>
126
127<div class="question">
128<p>Can I modify LLVM source code and redistribute binaries or other tools based
129on it, without redistributing the source?</p>
130</div>
131
132<div class="answer">
133<p>Yes, this is why we distribute LLVM under a less restrictive license than
134GPL, as explained in the first question above.</p>
135</div>
136
137<!-- *********************************************************************** -->
138<div class="doc_section">
139 <a name="source">Source Code</a>
140</div>
141<!-- *********************************************************************** -->
142
143<div class="question">
144<p>In what language is LLVM written?</p>
145</div>
146
147<div class="answer">
148<p>All of the LLVM tools and libraries are written in C++ with extensive use of
149the STL.</p>
150</div>
151
152<div class="question">
153<p>How portable is the LLVM source code?</p>
154</div>
155
156<div class="answer">
157<p>The LLVM source code should be portable to most modern UNIX-like operating
158systems. Most of the code is written in standard C++ with operating system
159services abstracted to a support library. The tools required to build and test
160LLVM have been ported to a plethora of platforms.</p>
161
162<p>Some porting problems may exist in the following areas:</p>
163
164<ul>
165
166 <li>The GCC front end code is not as portable as the LLVM suite, so it may not
167 compile as well on unsupported platforms.</li>
168
169 <li>The Python test classes are more UNIX-centric than they should be, so
170 porting to non-UNIX like platforms (i.e. Windows, MacOS 9) will require some
171 effort.</li>
172
173 <li>The LLVM build system relies heavily on UNIX shell tools, like the Bourne
174 Shell and sed. Porting to systems without these tools (MacOS 9, Plan 9) will
175 require more effort.</li>
176
177</ul>
178
179</div>
180
181<!-- *********************************************************************** -->
182<div class="doc_section">
183 <a name="build">Build Problems</a>
184</div>
185<!-- *********************************************************************** -->
186
187<div class="question">
188<p>When I run configure, it finds the wrong C compiler.</p>
189</div>
190
191<div class="answer">
192
193<p>The <tt>configure</tt> script attempts to locate first <tt>gcc</tt> and then
194<tt>cc</tt>, unless it finds compiler paths set in <tt>CC</tt> and <tt>CXX</tt>
195for the C and C++ compiler, respectively.</p>
196
197<p>If <tt>configure</tt> finds the wrong compiler, either adjust your
198<tt>PATH</tt> environment variable or set <tt>CC</tt> and <tt>CXX</tt>
199explicitly.</p>
200
201</div>
202
203<div class="question">
204<p>I compile the code, and I get some error about <tt>/localhome</tt>.</p>
205</div>
206
207<div class="answer">
208
209<p>There are several possible causes for this. The first is that you didn't set
210a pathname properly when using <tt>configure</tt>, and it defaulted to a
211pathname that we use on our research machines.</p>
212
213<p>Another possibility is that we hardcoded a path in our Makefiles. If you see
214this, please email the LLVM bug mailing list with the name of the offending
215Makefile and a description of what is wrong with it.</p>
216
217</div>
218
219<div class="question">
220<p>The <tt>configure</tt> script finds the right C compiler, but it uses the
221LLVM linker from a previous build. What do I do?</p>
222</div>
223
224<div class="answer">
225<p>The <tt>configure</tt> script uses the <tt>PATH</tt> to find executables, so
226if it's grabbing the wrong linker/assembler/etc, there are two ways to fix
227it:</p>
228
229<ol>
230
231 <li><p>Adjust your <tt>PATH</tt> environment variable so that the correct
232 program appears first in the <tt>PATH</tt>. This may work, but may not be
233 convenient when you want them <i>first</i> in your path for other
234 work.</p></li>
235
236 <li><p>Run <tt>configure</tt> with an alternative <tt>PATH</tt> that is
237 correct. In a Borne compatible shell, the syntax would be:</p>
238
Misha Brukman7ce62cc2004-06-01 18:51:03 +0000[diff] [blame^]239 <p><tt>PATH=[the path without the bad program] ./configure ...</tt></p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]240
241 <p>This is still somewhat inconvenient, but it allows <tt>configure</tt>
242 to do its work without having to adjust your <tt>PATH</tt>
243 permanently.</p></li>
244
245</ol>
246
247</div>
248
249<div class="question">
250<p>When creating a dynamic library, I get a strange GLIBC error.</p>
251</div>
252
253<div class="answer">
254<p>Under some operating systems (i.e. Linux), libtool does not work correctly if
255GCC was compiled with the --disable-shared option. To work around this, install
256your own version of GCC that has shared libraries enabled by default.</p>
257</div>
258
259<div class="question">
260<p>I've updated my source tree from CVS, and now my build is trying to use a
261file/directory that doesn't exist.</p>
262</div>
263
264<div class="answer">
265<p>You need to re-run configure in your object directory. When new Makefiles
266are added to the source tree, they have to be copied over to the object tree in
267order to be used by the build.</p>
268</div>
269
270<div class="question">
271<p>I've modified a Makefile in my source tree, but my build tree keeps using the
272old version. What do I do?</p>
273</div>
274
275<div class="answer">
276
277<p>If the Makefile already exists in your object tree, you
278can just run the following command in the top level directory of your object
279tree:</p>
280
281<p><tt>./config.status &lt;relative path to Makefile&gt;</tt><p>
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
316<p><tt>gmake ENABLE_PROFILING=1</tt>
317
318<p>...then you must run the tests with the following commands:</p>
319
320<p><tt>cd llvm/test<br>gmake ENABLE_PROFILING=1</tt></p>
321
322</div>
323
324<div class="question">
325<p>Why do test results differ when I perform different types of builds?</p>
326</div>
327
328<div class="answer">
329
330<p>The LLVM test suite is dependent upon several features of the LLVM tools and
331libraries.</p>
332
333<p>First, the debugging assertions in code are not enabled in optimized or
334profiling builds. Hence, tests that used to fail may pass.</p>
335
336<p>Second, some tests may rely upon debugging options or behavior that is only
337available in the debug build. These tests will fail in an optimized or profile
338build.</p>
339
340</div>
341
Chris Lattner8a0b9242003-12-08 05:43:19 +0000[diff] [blame]342<div class="question">
Chris Lattner306acee2003-12-22 04:06:12 +0000[diff] [blame]343<p>Compiling LLVM with GCC 3.3.2 fails, what should I do?</p>
Chris Lattner8a0b9242003-12-08 05:43:19 +0000[diff] [blame]344</div>
345
346<div class="answer">
Chris Lattner306acee2003-12-22 04:06:12 +0000[diff] [blame]347<p>This is <a href="http://gcc.gnu.org/PR?13392">a bug in GCC</a>, and
348 affects projects other than LLVM. Try upgrading or downgrading your GCC.</p>
Chris Lattner8a0b9242003-12-08 05:43:19 +0000[diff] [blame]349</div>
350
John Criswelld1799612004-03-29 20:23:11 +0000[diff] [blame]351<div class="question">
352<p>
353When I use the test suite, all of the C Backend tests fail. What is
354wrong?
355</p>
356</div>
357
358<div class="answer">
359<p>
360If you build LLVM and the C Backend tests fail in <tt>llvm/test/Programs</tt>,
361then chances are good that the directory pointed to by the LLVM_LIB_SEARCH_PATH
362environment variable does not contain the libcrtend.a library.
363</p>
364
365<p>
366To fix it, verify that LLVM_LIB_SEARCH_PATH points to the correct directory
367and that libcrtend.a is inside. For pre-built LLVM GCC front ends, this
368should be the absolute path to
369<tt>cfrontend/&lt;<i>platform</i>&gt;/llvm-gcc/bytecode-libs</tt>. If you've
370built your own LLVM GCC front end, then ensure that you've built and installed
371the libraries in <tt>llvm/runtime</tt> and have LLVM_LIB_SEARCH_PATH pointing
372to the <tt>LLVMGCCDIR/bytecode-libs</tt> subdirectory.
373</p>
374</div>
375
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]376<!-- *********************************************************************** -->
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]377<div class="doc_section">
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]378 <a name="cfe">Using the GCC Front End</a>
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]379</div>
380
381<div class="question">
382<p>
383When I compile software that uses a configure script, the configure script
384thinks my system has all of the header files and libraries it is testing for.
385How do I get configure to work correctly?
386</p>
387</div>
388
389<div class="answer">
390<p>
391The configure script is getting things wrong because the LLVM linker allows
392symbols to be undefined at link time (so that they can be resolved during JIT
393or translation to the C back end). That is why configure thinks your system
394"has everything."
395</p>
396<p>
397To work around this, perform the following steps:
398</p>
399
400<ol>
401 <li>
402 Make sure the CC and CXX environment variables contains the full path to the
403 LLVM GCC front end.
404 </li>
405
406 <li>
407 Make sure that the regular C compiler is first in your PATH.
408 </li>
409
410 <li>
411 Add the string "-Wl,-native" to your CFLAGS environment variable.
412 </li>
413</ol>
414
415<p>
416This will allow the gccld linker to create a native code executable instead of
417a shell script that runs the JIT. Creating native code requires standard
418linkage, which in turn will allow the configure script to find out if code is
419not linking on your system because the feature isn't available on your system.
420</p>
421</div>
422
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]423<div class="question">
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]424<p>
425When I compile code using the LLVM GCC front end, it complains that it cannot
John Criswellcd161192004-03-12 18:20:15 +0000[diff] [blame]426find libcrtend.a.
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]427</p>
428</div>
429
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]430<div class="answer">
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]431<p>
John Criswellcd161192004-03-12 18:20:15 +0000[diff] [blame]432In order to find libcrtend.a, you must have the directory in which it lives in
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]433your LLVM_LIB_SEARCH_PATH environment variable. For the binary distribution of
434the LLVM GCC front end, this will be the full path of the bytecode-libs
435directory inside of the LLVM GCC distribution.
436</p>
437</div>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]438
439
440<!-- *********************************************************************** -->
441<div class="doc_section">
442 <a name="cfe_code">Questions about code generated by the GCC front-end</a>
443</div>
444
Chris Lattnera28e3ce2003-12-16 22:33:55 +0000[diff] [blame]445<div class="question"><p>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]446What is this <tt>__main()</tt> call that gets inserted into <tt>main()</tt>?
Chris Lattnera28e3ce2003-12-16 22:33:55 +0000[diff] [blame]447</p></div>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]448
449<div class="answer">
450<p>
451The <tt>__main</tt> call is inserted by the C/C++ compiler in order to guarantee
452that static constructors and destructors are called when the program starts up
453and shuts down. In C, you can create static constructors and destructors by
454using GCC extensions, and in C++ you can do so by creating a global variable
455whose class has a ctor or dtor.
456</p>
457
458<p>
459The actual implementation of <tt>__main</tt> lives in the
460<tt>llvm/runtime/GCCLibraries/crtend/</tt> directory in the source-base, and is
461linked in automatically when you link the program.
462</p>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]463</div>
464
Chris Lattnera28e3ce2003-12-16 22:33:55 +0000[diff] [blame]465<!--=========================================================================-->
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]466
Chris Lattnera28e3ce2003-12-16 22:33:55 +0000[diff] [blame]467<div class="question"><p>
468Where did all of my code go??
469</p></div>
470
471<div class="answer">
472<p>
473If you are using the LLVM demo page, you may often wonder what happened to all
474of the code that you typed in. Remember that the demo script is running the
John Criswell02f61c12003-12-23 22:22:10 +0000[diff] [blame]475code through the LLVM optimizers, so if your code doesn't actually do anything
Chris Lattnera28e3ce2003-12-16 22:33:55 +0000[diff] [blame]476useful, it might all be deleted.
477</p>
478
479<p>
480To prevent this, make sure that the code is actually needed. For example, if
481you are computing some expression, return the value from the function instead of
482leaving it in a local variable. If you really want to constrain the optimizer,
483you can read from and assign to <tt>volatile</tt> global variables.
484</p>
485</div>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]486
Chris Lattnerc50bbc92004-03-29 19:14:35 +0000[diff] [blame]487<!--=========================================================================-->
488
489<div class="question"><p>
490What is this <tt>llvm.global_ctors</tt> and <tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I #include &lt;iostream&gt;?
491</p></div>
492
493<div class="answer">
494<p>
495If you #include the &lt;iostream&gt; header into a C++ translation unit, the
496file will probably use the <tt>std::cin</tt>/<tt>std::cout</tt>/... global
497objects. However, C++ does not guarantee an order of initialization between
498static objects in different translation units, so if a static ctor/dtor in your
499.cpp file used <tt>std::cout</tt>, for example, the object would not necessarily
500be automatically initialized before your use.
501</p>
502
503<p>
504To make <tt>std::cout</tt> and friends work correctly in these scenarios, the
505STL that we use declares a static object that gets created in every translation
506unit that includes &lt;iostream&gt;. This object has a static constructor and
507destructor that initializes and destroys the global iostream objects before they
508could possibly be used in the file. The code that you see in the .ll file
509corresponds to the constructor and destructor registration code.
510</p>
511
512<p>
513If you would like to make it easier to <b>understand</b> the LLVM code generated
514by the compiler in the demo page, consider using printf instead of iostreams to
515print values.
516</p>
517</div>
518
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]519<!-- *********************************************************************** -->
John Criswellc310f622003-10-13 16:13:06 +0000[diff] [blame]520
521<hr>
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528 <a href="http://llvm.cs.uiuc.edu">LLVM Compiler Infrastructure</a><br>
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