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2 "http://www.w3.org/TR/html4/strict.dtd">
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 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
66 cannot find crtend.o.
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
83<!-- *********************************************************************** -->
84<div class="doc_section">
85 <a name="license">License</a>
86</div>
87<!-- *********************************************************************** -->
88
89<div class="question">
90<p>Why are the LLVM source code and the front-end distributed under different
91licenses?</p>
92</div>
93
94<div class="answer">
95<p>The C/C++ front-ends are based on GCC and must be distributed under the GPL.
96Our aim is to distribute LLVM source code under a <em>much less restrictive</em>
97license, in particular one that does not compel users who distribute tools based
98on modifying the source to redistribute the modified source code as well.</p>
99</div>
100
101<div class="question">
102<p>Does the University of Illinois Open Source License really qualify as an
103"open source" license?</p>
104</div>
105
106<div class="answer">
107<p>Yes, the license is <a
108href="http://www.opensource.org/licenses/UoI-NCSA.php">certified</a> by the Open
109Source Initiative (OSI).</p>
110</div>
111
112<div class="question">
113<p>Can I modify LLVM source code and redistribute the modified source?</p>
114</div>
115
116<div class="answer">
117<p>Yes. The modified source distribution must retain the copyright notice and
118follow the three bulletted conditions listed in the <a
John Criswellcd161192004-03-12 18:20:15 +0000[diff] [blame]119href="http://llvm.cs.uiuc.edu/releases/1.2/LICENSE.TXT">LLVM license</a>.</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]120</div>
121
122<div class="question">
123<p>Can I modify LLVM source code and redistribute binaries or other tools based
124on it, without redistributing the source?</p>
125</div>
126
127<div class="answer">
128<p>Yes, this is why we distribute LLVM under a less restrictive license than
129GPL, as explained in the first question above.</p>
130</div>
131
132<!-- *********************************************************************** -->
133<div class="doc_section">
134 <a name="source">Source Code</a>
135</div>
136<!-- *********************************************************************** -->
137
138<div class="question">
139<p>In what language is LLVM written?</p>
140</div>
141
142<div class="answer">
143<p>All of the LLVM tools and libraries are written in C++ with extensive use of
144the STL.</p>
145</div>
146
147<div class="question">
148<p>How portable is the LLVM source code?</p>
149</div>
150
151<div class="answer">
152<p>The LLVM source code should be portable to most modern UNIX-like operating
153systems. Most of the code is written in standard C++ with operating system
154services abstracted to a support library. The tools required to build and test
155LLVM have been ported to a plethora of platforms.</p>
156
157<p>Some porting problems may exist in the following areas:</p>
158
159<ul>
160
161 <li>The GCC front end code is not as portable as the LLVM suite, so it may not
162 compile as well on unsupported platforms.</li>
163
164 <li>The Python test classes are more UNIX-centric than they should be, so
165 porting to non-UNIX like platforms (i.e. Windows, MacOS 9) will require some
166 effort.</li>
167
168 <li>The LLVM build system relies heavily on UNIX shell tools, like the Bourne
169 Shell and sed. Porting to systems without these tools (MacOS 9, Plan 9) will
170 require more effort.</li>
171
172</ul>
173
174</div>
175
176<!-- *********************************************************************** -->
177<div class="doc_section">
178 <a name="build">Build Problems</a>
179</div>
180<!-- *********************************************************************** -->
181
182<div class="question">
183<p>When I run configure, it finds the wrong C compiler.</p>
184</div>
185
186<div class="answer">
187
188<p>The <tt>configure</tt> script attempts to locate first <tt>gcc</tt> and then
189<tt>cc</tt>, unless it finds compiler paths set in <tt>CC</tt> and <tt>CXX</tt>
190for the C and C++ compiler, respectively.</p>
191
192<p>If <tt>configure</tt> finds the wrong compiler, either adjust your
193<tt>PATH</tt> environment variable or set <tt>CC</tt> and <tt>CXX</tt>
194explicitly.</p>
195
196</div>
197
198<div class="question">
199<p>I compile the code, and I get some error about <tt>/localhome</tt>.</p>
200</div>
201
202<div class="answer">
203
204<p>There are several possible causes for this. The first is that you didn't set
205a pathname properly when using <tt>configure</tt>, and it defaulted to a
206pathname that we use on our research machines.</p>
207
208<p>Another possibility is that we hardcoded a path in our Makefiles. If you see
209this, please email the LLVM bug mailing list with the name of the offending
210Makefile and a description of what is wrong with it.</p>
211
212</div>
213
214<div class="question">
215<p>The <tt>configure</tt> script finds the right C compiler, but it uses the
216LLVM linker from a previous build. What do I do?</p>
217</div>
218
219<div class="answer">
220<p>The <tt>configure</tt> script uses the <tt>PATH</tt> to find executables, so
221if it's grabbing the wrong linker/assembler/etc, there are two ways to fix
222it:</p>
223
224<ol>
225
226 <li><p>Adjust your <tt>PATH</tt> environment variable so that the correct
227 program appears first in the <tt>PATH</tt>. This may work, but may not be
228 convenient when you want them <i>first</i> in your path for other
229 work.</p></li>
230
231 <li><p>Run <tt>configure</tt> with an alternative <tt>PATH</tt> that is
232 correct. In a Borne compatible shell, the syntax would be:</p>
233
234 <p><tt>PATH=<the path without the bad program> ./configure ...</tt></p>
235
236 <p>This is still somewhat inconvenient, but it allows <tt>configure</tt>
237 to do its work without having to adjust your <tt>PATH</tt>
238 permanently.</p></li>
239
240</ol>
241
242</div>
243
244<div class="question">
245<p>When creating a dynamic library, I get a strange GLIBC error.</p>
246</div>
247
248<div class="answer">
249<p>Under some operating systems (i.e. Linux), libtool does not work correctly if
250GCC was compiled with the --disable-shared option. To work around this, install
251your own version of GCC that has shared libraries enabled by default.</p>
252</div>
253
254<div class="question">
255<p>I've updated my source tree from CVS, and now my build is trying to use a
256file/directory that doesn't exist.</p>
257</div>
258
259<div class="answer">
260<p>You need to re-run configure in your object directory. When new Makefiles
261are added to the source tree, they have to be copied over to the object tree in
262order to be used by the build.</p>
263</div>
264
265<div class="question">
266<p>I've modified a Makefile in my source tree, but my build tree keeps using the
267old version. What do I do?</p>
268</div>
269
270<div class="answer">
271
272<p>If the Makefile already exists in your object tree, you
273can just run the following command in the top level directory of your object
274tree:</p>
275
276<p><tt>./config.status &lt;relative path to Makefile&gt;</tt><p>
277
278<p>If the Makefile is new, you will have to modify the configure script to copy
279it over.</p>
280
281</div>
282
283<div class="question">
284<p>I've upgraded to a new version of LLVM, and I get strange build errors.</p>
285</div>
286
287<div class="answer">
288
289<p>Sometimes, changes to the LLVM source code alters how the build system works.
290Changes in libtool, autoconf, or header file dependencies are especially prone
291to this sort of problem.</p>
292
293<p>The best thing to try is to remove the old files and re-build. In most
294cases, this takes care of the problem. To do this, just type <tt>make
295clean</tt> and then <tt>make</tt> in the directory that fails to build.</p>
296
297</div>
298
299<div class="question">
300<p>I've built LLVM and am testing it, but the tests freeze.</p>
301</div>
302
303<div class="answer">
304
305<p>This is most likely occurring because you built a profile or release
306(optimized) build of LLVM and have not specified the same information on the
307<tt>gmake</tt> command line.</p>
308
309<p>For example, if you built LLVM with the command:</p>
310
311<p><tt>gmake ENABLE_PROFILING=1</tt>
312
313<p>...then you must run the tests with the following commands:</p>
314
315<p><tt>cd llvm/test<br>gmake ENABLE_PROFILING=1</tt></p>
316
317</div>
318
319<div class="question">
320<p>Why do test results differ when I perform different types of builds?</p>
321</div>
322
323<div class="answer">
324
325<p>The LLVM test suite is dependent upon several features of the LLVM tools and
326libraries.</p>
327
328<p>First, the debugging assertions in code are not enabled in optimized or
329profiling builds. Hence, tests that used to fail may pass.</p>
330
331<p>Second, some tests may rely upon debugging options or behavior that is only
332available in the debug build. These tests will fail in an optimized or profile
333build.</p>
334
335</div>
336
Chris Lattner8a0b9242003-12-08 05:43:19 +0000[diff] [blame]337<div class="question">
Chris Lattner306acee2003-12-22 04:06:12 +0000[diff] [blame]338<p>Compiling LLVM with GCC 3.3.2 fails, what should I do?</p>
Chris Lattner8a0b9242003-12-08 05:43:19 +0000[diff] [blame]339</div>
340
341<div class="answer">
Chris Lattner306acee2003-12-22 04:06:12 +0000[diff] [blame]342<p>This is <a href="http://gcc.gnu.org/PR?13392">a bug in GCC</a>, and
343 affects projects other than LLVM. Try upgrading or downgrading your GCC.</p>
Chris Lattner8a0b9242003-12-08 05:43:19 +0000[diff] [blame]344</div>
345
John Criswelld1799612004-03-29 20:23:11 +0000[diff] [blame^]346<div class="question">
347<p>
348When I use the test suite, all of the C Backend tests fail. What is
349wrong?
350</p>
351</div>
352
353<div class="answer">
354<p>
355If you build LLVM and the C Backend tests fail in <tt>llvm/test/Programs</tt>,
356then chances are good that the directory pointed to by the LLVM_LIB_SEARCH_PATH
357environment variable does not contain the libcrtend.a library.
358</p>
359
360<p>
361To fix it, verify that LLVM_LIB_SEARCH_PATH points to the correct directory
362and that libcrtend.a is inside. For pre-built LLVM GCC front ends, this
363should be the absolute path to
364<tt>cfrontend/&lt;<i>platform</i>&gt;/llvm-gcc/bytecode-libs</tt>. If you've
365built your own LLVM GCC front end, then ensure that you've built and installed
366the libraries in <tt>llvm/runtime</tt> and have LLVM_LIB_SEARCH_PATH pointing
367to the <tt>LLVMGCCDIR/bytecode-libs</tt> subdirectory.
368</p>
369</div>
370
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]371<!-- *********************************************************************** -->
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]372<div class="doc_section">
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]373 <a name="cfe">Using the GCC Front End</a>
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]374</div>
375
376<div class="question">
377<p>
378When I compile software that uses a configure script, the configure script
379thinks my system has all of the header files and libraries it is testing for.
380How do I get configure to work correctly?
381</p>
382</div>
383
384<div class="answer">
385<p>
386The configure script is getting things wrong because the LLVM linker allows
387symbols to be undefined at link time (so that they can be resolved during JIT
388or translation to the C back end). That is why configure thinks your system
389"has everything."
390</p>
391<p>
392To work around this, perform the following steps:
393</p>
394
395<ol>
396 <li>
397 Make sure the CC and CXX environment variables contains the full path to the
398 LLVM GCC front end.
399 </li>
400
401 <li>
402 Make sure that the regular C compiler is first in your PATH.
403 </li>
404
405 <li>
406 Add the string "-Wl,-native" to your CFLAGS environment variable.
407 </li>
408</ol>
409
410<p>
411This will allow the gccld linker to create a native code executable instead of
412a shell script that runs the JIT. Creating native code requires standard
413linkage, which in turn will allow the configure script to find out if code is
414not linking on your system because the feature isn't available on your system.
415</p>
416</div>
417
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]418<div class="question">
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]419<p>
420When I compile code using the LLVM GCC front end, it complains that it cannot
John Criswellcd161192004-03-12 18:20:15 +0000[diff] [blame]421find libcrtend.a.
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]422</p>
423</div>
424
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]425<div class="answer">
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]426<p>
John Criswellcd161192004-03-12 18:20:15 +0000[diff] [blame]427In 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]428your LLVM_LIB_SEARCH_PATH environment variable. For the binary distribution of
429the LLVM GCC front end, this will be the full path of the bytecode-libs
430directory inside of the LLVM GCC distribution.
431</p>
432</div>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]433
434
435<!-- *********************************************************************** -->
436<div class="doc_section">
437 <a name="cfe_code">Questions about code generated by the GCC front-end</a>
438</div>
439
Chris Lattnera28e3ce2003-12-16 22:33:55 +0000[diff] [blame]440<div class="question"><p>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]441What is this <tt>__main()</tt> call that gets inserted into <tt>main()</tt>?
Chris Lattnera28e3ce2003-12-16 22:33:55 +0000[diff] [blame]442</p></div>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]443
444<div class="answer">
445<p>
446The <tt>__main</tt> call is inserted by the C/C++ compiler in order to guarantee
447that static constructors and destructors are called when the program starts up
448and shuts down. In C, you can create static constructors and destructors by
449using GCC extensions, and in C++ you can do so by creating a global variable
450whose class has a ctor or dtor.
451</p>
452
453<p>
454The actual implementation of <tt>__main</tt> lives in the
455<tt>llvm/runtime/GCCLibraries/crtend/</tt> directory in the source-base, and is
456linked in automatically when you link the program.
457</p>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]458</div>
459
Chris Lattnera28e3ce2003-12-16 22:33:55 +0000[diff] [blame]460<!--=========================================================================-->
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]461
Chris Lattnera28e3ce2003-12-16 22:33:55 +0000[diff] [blame]462<div class="question"><p>
463Where did all of my code go??
464</p></div>
465
466<div class="answer">
467<p>
468If you are using the LLVM demo page, you may often wonder what happened to all
469of the code that you typed in. Remember that the demo script is running the
John Criswell02f61c12003-12-23 22:22:10 +0000[diff] [blame]470code through the LLVM optimizers, so if your code doesn't actually do anything
Chris Lattnera28e3ce2003-12-16 22:33:55 +0000[diff] [blame]471useful, it might all be deleted.
472</p>
473
474<p>
475To prevent this, make sure that the code is actually needed. For example, if
476you are computing some expression, return the value from the function instead of
477leaving it in a local variable. If you really want to constrain the optimizer,
478you can read from and assign to <tt>volatile</tt> global variables.
479</p>
480</div>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]481
Chris Lattnerc50bbc92004-03-29 19:14:35 +0000[diff] [blame]482<!--=========================================================================-->
483
484<div class="question"><p>
485What is this <tt>llvm.global_ctors</tt> and <tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I #include &lt;iostream&gt;?
486</p></div>
487
488<div class="answer">
489<p>
490If you #include the &lt;iostream&gt; header into a C++ translation unit, the
491file will probably use the <tt>std::cin</tt>/<tt>std::cout</tt>/... global
492objects. However, C++ does not guarantee an order of initialization between
493static objects in different translation units, so if a static ctor/dtor in your
494.cpp file used <tt>std::cout</tt>, for example, the object would not necessarily
495be automatically initialized before your use.
496</p>
497
498<p>
499To make <tt>std::cout</tt> and friends work correctly in these scenarios, the
500STL that we use declares a static object that gets created in every translation
501unit that includes &lt;iostream&gt;. This object has a static constructor and
502destructor that initializes and destroys the global iostream objects before they
503could possibly be used in the file. The code that you see in the .ll file
504corresponds to the constructor and destructor registration code.
505</p>
506
507<p>
508If you would like to make it easier to <b>understand</b> the LLVM code generated
509by the compiler in the demo page, consider using printf instead of iostreams to
510print values.
511</p>
512</div>
513
514
515 <li>
516
517
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]518<!-- *********************************************************************** -->
519<!-- *********************************************************************** -->
John Criswellc310f622003-10-13 16:13:06 +0000[diff] [blame]520
521<hr>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]522<div class="doc_footer">
523 <a href="http://llvm.cs.uiuc.edu">The LLVM Compiler Infrastructure</a>
524 <br>
525 Last modified: $Date$
526</div>
John Criswellf08c5d82003-10-24 22:48:20 +0000[diff] [blame]527
John Criswellc310f622003-10-13 16:13:06 +0000[diff] [blame]528</body>
529</html>