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4<head>
5 <title>LLVM: Frequently Asked Questions</title>
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Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]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 Brukman1739aec2004-09-09 16:36:47 +0000[diff] [blame^]54 <li>After CVS update, rebuilding gives the error "No rule to make
55 target".</li>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]56 </ol></li>
John Criswell76c1e382003-11-18 16:08:49 +0000[diff] [blame]57
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]58 <li><a href="#cfe">Using the GCC Front End</a>
John Criswell76c1e382003-11-18 16:08:49 +0000[diff] [blame]59 <ol>
60 <li>
61 When I compile software that uses a configure script, the configure script
62 thinks my system has all of the header files and libraries it is testing
63 for. How do I get configure to work correctly?
64 </li>
65
66 <li>
67 When I compile code using the LLVM GCC front end, it complains that it
Chris Lattner7911ce22004-05-23 21:07:27 +0000[diff] [blame]68 cannot find libcrtend.a.
John Criswell76c1e382003-11-18 16:08:49 +0000[diff] [blame]69 </li>
70 </ol>
71 </li>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]72
73 <li><a href="#cfe_code">Questions about code generated by the GCC front-end</a>
74 <ol>
75 <li>What is this <tt>__main()</tt> call that gets inserted into
76 <tt>main()</tt>?</li>
Chris Lattnera28e3ce2003-12-16 22:33:55 +0000[diff] [blame]77 <li>Where did all of my code go??</li>
Chris Lattnerc50bbc92004-03-29 19:14:35 +0000[diff] [blame]78 <li>What is this <tt>llvm.global_ctors</tt> and
79 <tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I
80 #include &lt;iostream&gt;?</li>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]81 </ol>
82 </li>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]83</ol>
84
Chris Lattner7911ce22004-05-23 21:07:27 +0000[diff] [blame]85<div class="doc_author">
86 <p>Written by <a href="http://llvm.cs.uiuc.edu">The LLVM Team</a></p>
87</div>
88
89
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]90<!-- *********************************************************************** -->
91<div class="doc_section">
92 <a name="license">License</a>
93</div>
94<!-- *********************************************************************** -->
95
96<div class="question">
97<p>Why are the LLVM source code and the front-end distributed under different
98licenses?</p>
99</div>
100
101<div class="answer">
102<p>The C/C++ front-ends are based on GCC and must be distributed under the GPL.
103Our aim is to distribute LLVM source code under a <em>much less restrictive</em>
104license, in particular one that does not compel users who distribute tools based
105on modifying the source to redistribute the modified source code as well.</p>
106</div>
107
108<div class="question">
109<p>Does the University of Illinois Open Source License really qualify as an
110"open source" license?</p>
111</div>
112
113<div class="answer">
114<p>Yes, the license is <a
115href="http://www.opensource.org/licenses/UoI-NCSA.php">certified</a> by the Open
116Source Initiative (OSI).</p>
117</div>
118
119<div class="question">
120<p>Can I modify LLVM source code and redistribute the modified source?</p>
121</div>
122
123<div class="answer">
124<p>Yes. The modified source distribution must retain the copyright notice and
125follow the three bulletted conditions listed in the <a
John Criswellcd161192004-03-12 18:20:15 +0000[diff] [blame]126href="http://llvm.cs.uiuc.edu/releases/1.2/LICENSE.TXT">LLVM license</a>.</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]127</div>
128
129<div class="question">
130<p>Can I modify LLVM source code and redistribute binaries or other tools based
131on it, without redistributing the source?</p>
132</div>
133
134<div class="answer">
135<p>Yes, this is why we distribute LLVM under a less restrictive license than
136GPL, as explained in the first question above.</p>
137</div>
138
139<!-- *********************************************************************** -->
140<div class="doc_section">
141 <a name="source">Source Code</a>
142</div>
143<!-- *********************************************************************** -->
144
145<div class="question">
146<p>In what language is LLVM written?</p>
147</div>
148
149<div class="answer">
150<p>All of the LLVM tools and libraries are written in C++ with extensive use of
151the STL.</p>
152</div>
153
154<div class="question">
155<p>How portable is the LLVM source code?</p>
156</div>
157
158<div class="answer">
159<p>The LLVM source code should be portable to most modern UNIX-like operating
160systems. Most of the code is written in standard C++ with operating system
161services abstracted to a support library. The tools required to build and test
162LLVM have been ported to a plethora of platforms.</p>
163
164<p>Some porting problems may exist in the following areas:</p>
165
166<ul>
167
168 <li>The GCC front end code is not as portable as the LLVM suite, so it may not
169 compile as well on unsupported platforms.</li>
170
171 <li>The Python test classes are more UNIX-centric than they should be, so
172 porting to non-UNIX like platforms (i.e. Windows, MacOS 9) will require some
173 effort.</li>
174
175 <li>The LLVM build system relies heavily on UNIX shell tools, like the Bourne
176 Shell and sed. Porting to systems without these tools (MacOS 9, Plan 9) will
177 require more effort.</li>
178
179</ul>
180
181</div>
182
183<!-- *********************************************************************** -->
184<div class="doc_section">
185 <a name="build">Build Problems</a>
186</div>
187<!-- *********************************************************************** -->
188
189<div class="question">
190<p>When I run configure, it finds the wrong C compiler.</p>
191</div>
192
193<div class="answer">
194
195<p>The <tt>configure</tt> script attempts to locate first <tt>gcc</tt> and then
196<tt>cc</tt>, unless it finds compiler paths set in <tt>CC</tt> and <tt>CXX</tt>
197for the C and C++ compiler, respectively.</p>
198
199<p>If <tt>configure</tt> finds the wrong compiler, either adjust your
200<tt>PATH</tt> environment variable or set <tt>CC</tt> and <tt>CXX</tt>
201explicitly.</p>
202
203</div>
204
205<div class="question">
206<p>I compile the code, and I get some error about <tt>/localhome</tt>.</p>
207</div>
208
209<div class="answer">
210
211<p>There are several possible causes for this. The first is that you didn't set
212a pathname properly when using <tt>configure</tt>, and it defaulted to a
213pathname that we use on our research machines.</p>
214
215<p>Another possibility is that we hardcoded a path in our Makefiles. If you see
216this, please email the LLVM bug mailing list with the name of the offending
217Makefile and a description of what is wrong with it.</p>
218
219</div>
220
221<div class="question">
222<p>The <tt>configure</tt> script finds the right C compiler, but it uses the
223LLVM linker from a previous build. What do I do?</p>
224</div>
225
226<div class="answer">
227<p>The <tt>configure</tt> script uses the <tt>PATH</tt> to find executables, so
228if it's grabbing the wrong linker/assembler/etc, there are two ways to fix
229it:</p>
230
231<ol>
232
233 <li><p>Adjust your <tt>PATH</tt> environment variable so that the correct
234 program appears first in the <tt>PATH</tt>. This may work, but may not be
235 convenient when you want them <i>first</i> in your path for other
236 work.</p></li>
237
238 <li><p>Run <tt>configure</tt> with an alternative <tt>PATH</tt> that is
239 correct. In a Borne compatible shell, the syntax would be:</p>
240
Misha Brukman7ce62cc2004-06-01 18:51:03 +0000[diff] [blame]241 <p><tt>PATH=[the path without the bad program] ./configure ...</tt></p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]242
243 <p>This is still somewhat inconvenient, but it allows <tt>configure</tt>
244 to do its work without having to adjust your <tt>PATH</tt>
245 permanently.</p></li>
246
247</ol>
248
249</div>
250
251<div class="question">
252<p>When creating a dynamic library, I get a strange GLIBC error.</p>
253</div>
254
255<div class="answer">
256<p>Under some operating systems (i.e. Linux), libtool does not work correctly if
257GCC was compiled with the --disable-shared option. To work around this, install
258your own version of GCC that has shared libraries enabled by default.</p>
259</div>
260
261<div class="question">
262<p>I've updated my source tree from CVS, and now my build is trying to use a
263file/directory that doesn't exist.</p>
264</div>
265
266<div class="answer">
267<p>You need to re-run configure in your object directory. When new Makefiles
268are added to the source tree, they have to be copied over to the object tree in
269order to be used by the build.</p>
270</div>
271
272<div class="question">
273<p>I've modified a Makefile in my source tree, but my build tree keeps using the
274old version. What do I do?</p>
275</div>
276
277<div class="answer">
278
279<p>If the Makefile already exists in your object tree, you
280can just run the following command in the top level directory of your object
281tree:</p>
282
283<p><tt>./config.status &lt;relative path to Makefile&gt;</tt><p>
284
285<p>If the Makefile is new, you will have to modify the configure script to copy
286it over.</p>
287
288</div>
289
290<div class="question">
291<p>I've upgraded to a new version of LLVM, and I get strange build errors.</p>
292</div>
293
294<div class="answer">
295
296<p>Sometimes, changes to the LLVM source code alters how the build system works.
297Changes in libtool, autoconf, or header file dependencies are especially prone
298to this sort of problem.</p>
299
300<p>The best thing to try is to remove the old files and re-build. In most
301cases, this takes care of the problem. To do this, just type <tt>make
302clean</tt> and then <tt>make</tt> in the directory that fails to build.</p>
303
304</div>
305
306<div class="question">
307<p>I've built LLVM and am testing it, but the tests freeze.</p>
308</div>
309
310<div class="answer">
311
312<p>This is most likely occurring because you built a profile or release
313(optimized) build of LLVM and have not specified the same information on the
314<tt>gmake</tt> command line.</p>
315
316<p>For example, if you built LLVM with the command:</p>
317
318<p><tt>gmake ENABLE_PROFILING=1</tt>
319
320<p>...then you must run the tests with the following commands:</p>
321
322<p><tt>cd llvm/test<br>gmake ENABLE_PROFILING=1</tt></p>
323
324</div>
325
326<div class="question">
327<p>Why do test results differ when I perform different types of builds?</p>
328</div>
329
330<div class="answer">
331
332<p>The LLVM test suite is dependent upon several features of the LLVM tools and
333libraries.</p>
334
335<p>First, the debugging assertions in code are not enabled in optimized or
336profiling builds. Hence, tests that used to fail may pass.</p>
337
338<p>Second, some tests may rely upon debugging options or behavior that is only
339available in the debug build. These tests will fail in an optimized or profile
340build.</p>
341
342</div>
343
Chris Lattner8a0b9242003-12-08 05:43:19 +0000[diff] [blame]344<div class="question">
Chris Lattner306acee2003-12-22 04:06:12 +0000[diff] [blame]345<p>Compiling LLVM with GCC 3.3.2 fails, what should I do?</p>
Chris Lattner8a0b9242003-12-08 05:43:19 +0000[diff] [blame]346</div>
347
348<div class="answer">
Chris Lattner306acee2003-12-22 04:06:12 +0000[diff] [blame]349<p>This is <a href="http://gcc.gnu.org/PR?13392">a bug in GCC</a>, and
350 affects projects other than LLVM. Try upgrading or downgrading your GCC.</p>
Chris Lattner8a0b9242003-12-08 05:43:19 +0000[diff] [blame]351</div>
352
John Criswelld1799612004-03-29 20:23:11 +0000[diff] [blame]353<div class="question">
354<p>
355When I use the test suite, all of the C Backend tests fail. What is
356wrong?
357</p>
358</div>
359
360<div class="answer">
361<p>
362If you build LLVM and the C Backend tests fail in <tt>llvm/test/Programs</tt>,
363then chances are good that the directory pointed to by the LLVM_LIB_SEARCH_PATH
364environment variable does not contain the libcrtend.a library.
365</p>
366
367<p>
368To fix it, verify that LLVM_LIB_SEARCH_PATH points to the correct directory
369and that libcrtend.a is inside. For pre-built LLVM GCC front ends, this
370should be the absolute path to
371<tt>cfrontend/&lt;<i>platform</i>&gt;/llvm-gcc/bytecode-libs</tt>. If you've
372built your own LLVM GCC front end, then ensure that you've built and installed
373the libraries in <tt>llvm/runtime</tt> and have LLVM_LIB_SEARCH_PATH pointing
374to the <tt>LLVMGCCDIR/bytecode-libs</tt> subdirectory.
375</p>
376</div>
377
Misha Brukman1739aec2004-09-09 16:36:47 +0000[diff] [blame^]378<div class="question">
379<p>After CVS update, rebuilding gives the error "No rule to make target".</p>
380</div>
381
382<div class="answer">
383<p>If the error is of the form:</p>
384
385<div class="doc_code">
386<tt>
387gmake[2]: *** No rule to make target `/path/to/somefile', needed by
388`/path/to/another/file.d'.<br>
389Stop.
390</tt>
391</div>
392
393<p>This may occur anytime files are moved within the CVS repository or removed
394entirely. In this case, the best solution is to erase all <tt>.d</tt> files,
395which list dependencies for source files, and rebuild:</p>
396
397<div class="doc_code">
398<pre>
399% cd $LLVM_OBJ_DIR
400% rm -f `find . -name \*\.d`
401% gmake
402</pre>
403</div>
404
405<p>In other cases, it may be necessary to run <tt>make clean</tt> before
406rebuilding.</p>
407</div>
408
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]409<!-- *********************************************************************** -->
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]410<div class="doc_section">
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]411 <a name="cfe">Using the GCC Front End</a>
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]412</div>
413
414<div class="question">
415<p>
416When I compile software that uses a configure script, the configure script
417thinks my system has all of the header files and libraries it is testing for.
418How do I get configure to work correctly?
419</p>
420</div>
421
422<div class="answer">
423<p>
424The configure script is getting things wrong because the LLVM linker allows
425symbols to be undefined at link time (so that they can be resolved during JIT
426or translation to the C back end). That is why configure thinks your system
427"has everything."
428</p>
429<p>
430To work around this, perform the following steps:
431</p>
432
433<ol>
434 <li>
435 Make sure the CC and CXX environment variables contains the full path to the
436 LLVM GCC front end.
437 </li>
438
439 <li>
440 Make sure that the regular C compiler is first in your PATH.
441 </li>
442
443 <li>
444 Add the string "-Wl,-native" to your CFLAGS environment variable.
445 </li>
446</ol>
447
448<p>
449This will allow the gccld linker to create a native code executable instead of
450a shell script that runs the JIT. Creating native code requires standard
451linkage, which in turn will allow the configure script to find out if code is
452not linking on your system because the feature isn't available on your system.
453</p>
454</div>
455
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]456<div class="question">
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]457<p>
458When I compile code using the LLVM GCC front end, it complains that it cannot
John Criswellcd161192004-03-12 18:20:15 +0000[diff] [blame]459find libcrtend.a.
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]460</p>
461</div>
462
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]463<div class="answer">
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]464<p>
John Criswellcd161192004-03-12 18:20:15 +0000[diff] [blame]465In 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]466your LLVM_LIB_SEARCH_PATH environment variable. For the binary distribution of
467the LLVM GCC front end, this will be the full path of the bytecode-libs
468directory inside of the LLVM GCC distribution.
469</p>
470</div>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]471
472
473<!-- *********************************************************************** -->
474<div class="doc_section">
475 <a name="cfe_code">Questions about code generated by the GCC front-end</a>
476</div>
477
Chris Lattnera28e3ce2003-12-16 22:33:55 +0000[diff] [blame]478<div class="question"><p>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]479What is this <tt>__main()</tt> call that gets inserted into <tt>main()</tt>?
Chris Lattnera28e3ce2003-12-16 22:33:55 +0000[diff] [blame]480</p></div>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]481
482<div class="answer">
483<p>
484The <tt>__main</tt> call is inserted by the C/C++ compiler in order to guarantee
485that static constructors and destructors are called when the program starts up
486and shuts down. In C, you can create static constructors and destructors by
487using GCC extensions, and in C++ you can do so by creating a global variable
488whose class has a ctor or dtor.
489</p>
490
491<p>
492The actual implementation of <tt>__main</tt> lives in the
493<tt>llvm/runtime/GCCLibraries/crtend/</tt> directory in the source-base, and is
494linked in automatically when you link the program.
495</p>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]496</div>
497
Chris Lattnera28e3ce2003-12-16 22:33:55 +0000[diff] [blame]498<!--=========================================================================-->
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]499
Chris Lattnera28e3ce2003-12-16 22:33:55 +0000[diff] [blame]500<div class="question"><p>
501Where did all of my code go??
502</p></div>
503
504<div class="answer">
505<p>
506If you are using the LLVM demo page, you may often wonder what happened to all
507of the code that you typed in. Remember that the demo script is running the
John Criswell02f61c12003-12-23 22:22:10 +0000[diff] [blame]508code through the LLVM optimizers, so if your code doesn't actually do anything
Chris Lattnera28e3ce2003-12-16 22:33:55 +0000[diff] [blame]509useful, it might all be deleted.
510</p>
511
512<p>
513To prevent this, make sure that the code is actually needed. For example, if
514you are computing some expression, return the value from the function instead of
515leaving it in a local variable. If you really want to constrain the optimizer,
516you can read from and assign to <tt>volatile</tt> global variables.
517</p>
518</div>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]519
Chris Lattnerc50bbc92004-03-29 19:14:35 +0000[diff] [blame]520<!--=========================================================================-->
521
522<div class="question"><p>
523What is this <tt>llvm.global_ctors</tt> and <tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I #include &lt;iostream&gt;?
524</p></div>
525
526<div class="answer">
527<p>
528If you #include the &lt;iostream&gt; header into a C++ translation unit, the
529file will probably use the <tt>std::cin</tt>/<tt>std::cout</tt>/... global
530objects. However, C++ does not guarantee an order of initialization between
531static objects in different translation units, so if a static ctor/dtor in your
532.cpp file used <tt>std::cout</tt>, for example, the object would not necessarily
533be automatically initialized before your use.
534</p>
535
536<p>
537To make <tt>std::cout</tt> and friends work correctly in these scenarios, the
538STL that we use declares a static object that gets created in every translation
539unit that includes &lt;iostream&gt;. This object has a static constructor and
540destructor that initializes and destroys the global iostream objects before they
541could possibly be used in the file. The code that you see in the .ll file
542corresponds to the constructor and destructor registration code.
543</p>
544
545<p>
546If you would like to make it easier to <b>understand</b> the LLVM code generated
547by the compiler in the demo page, consider using printf instead of iostreams to
548print values.
549</p>
550</div>
551
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]552<!-- *********************************************************************** -->
John Criswellc310f622003-10-13 16:13:06 +0000[diff] [blame]553
554<hr>
Misha Brukman7ce62cc2004-06-01 18:51:03 +0000[diff] [blame]555<address>
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