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14 <div class="doc_title">
15 LLVM: Frequently Asked Questions
19 <li><a href="#license">License</a>
21 <li>Why are the LLVM source code and the front-end distributed under different
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>
30 <li><a href="#source">Source code</a>
32 <li>In what language is LLVM written?</li>
33 <li>How portable is the LLVM source code?</li>
36 <li><a href="#build">Build Problems</a>
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
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 <li>Compiling LLVM with GCC 3.3.2 fails, what should I do?</li>
52 <li>When I use the test suite, all of the C Backend tests fail. What is
56 <li><a href="#cfe">Using the GCC Front End</a>
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?
65 When I compile code using the LLVM GCC front end, it complains that it
66 cannot find libcrtend.a.
71 <li><a href="#cfe_code">Questions about code generated by the GCC front-end</a>
73 <li>What is this <tt>__main()</tt> call that gets inserted into
75 <li>Where did all of my code go??</li>
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 <iostream>?</li>
83 <div class="doc_author">
84 <p>Written by <a href="http://llvm.cs.uiuc.edu">The LLVM Team</a></p>
88 <!-- *********************************************************************** -->
89 <div class="doc_section">
90 <a name="license">License</a>
92 <!-- *********************************************************************** -->
94 <div class="question">
95 <p>Why are the LLVM source code and the front-end distributed under different
100 <p>The C/C++ front-ends are based on GCC and must be distributed under the GPL.
101 Our aim is to distribute LLVM source code under a <em>much less restrictive</em>
102 license, in particular one that does not compel users who distribute tools based
103 on modifying the source to redistribute the modified source code as well.</p>
106 <div class="question">
107 <p>Does the University of Illinois Open Source License really qualify as an
108 "open source" license?</p>
112 <p>Yes, the license is <a
113 href="http://www.opensource.org/licenses/UoI-NCSA.php">certified</a> by the Open
114 Source Initiative (OSI).</p>
117 <div class="question">
118 <p>Can I modify LLVM source code and redistribute the modified source?</p>
122 <p>Yes. The modified source distribution must retain the copyright notice and
123 follow the three bulletted conditions listed in the <a
124 href="http://llvm.cs.uiuc.edu/releases/1.2/LICENSE.TXT">LLVM license</a>.</p>
127 <div class="question">
128 <p>Can I modify LLVM source code and redistribute binaries or other tools based
129 on it, without redistributing the source?</p>
133 <p>Yes, this is why we distribute LLVM under a less restrictive license than
134 GPL, as explained in the first question above.</p>
137 <!-- *********************************************************************** -->
138 <div class="doc_section">
139 <a name="source">Source Code</a>
141 <!-- *********************************************************************** -->
143 <div class="question">
144 <p>In what language is LLVM written?</p>
148 <p>All of the LLVM tools and libraries are written in C++ with extensive use of
152 <div class="question">
153 <p>How portable is the LLVM source code?</p>
157 <p>The LLVM source code should be portable to most modern UNIX-like operating
158 systems. Most of the code is written in standard C++ with operating system
159 services abstracted to a support library. The tools required to build and test
160 LLVM have been ported to a plethora of platforms.</p>
162 <p>Some porting problems may exist in the following areas:</p>
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>
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
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>
181 <!-- *********************************************************************** -->
182 <div class="doc_section">
183 <a name="build">Build Problems</a>
185 <!-- *********************************************************************** -->
187 <div class="question">
188 <p>When I run configure, it finds the wrong C compiler.</p>
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>
195 for the C and C++ compiler, respectively.</p>
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>
203 <div class="question">
204 <p>I compile the code, and I get some error about <tt>/localhome</tt>.</p>
209 <p>There are several possible causes for this. The first is that you didn't set
210 a pathname properly when using <tt>configure</tt>, and it defaulted to a
211 pathname that we use on our research machines.</p>
213 <p>Another possibility is that we hardcoded a path in our Makefiles. If you see
214 this, please email the LLVM bug mailing list with the name of the offending
215 Makefile and a description of what is wrong with it.</p>
219 <div class="question">
220 <p>The <tt>configure</tt> script finds the right C compiler, but it uses the
221 LLVM linker from a previous build. What do I do?</p>
225 <p>The <tt>configure</tt> script uses the <tt>PATH</tt> to find executables, so
226 if it's grabbing the wrong linker/assembler/etc, there are two ways to fix
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
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>
239 <p><tt>PATH=[the path without the bad program] ./configure ...</tt></p>
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>
249 <div class="question">
250 <p>When creating a dynamic library, I get a strange GLIBC error.</p>
254 <p>Under some operating systems (i.e. Linux), libtool does not work correctly if
255 GCC was compiled with the --disable-shared option. To work around this, install
256 your own version of GCC that has shared libraries enabled by default.</p>
259 <div class="question">
260 <p>I've updated my source tree from CVS, and now my build is trying to use a
261 file/directory that doesn't exist.</p>
265 <p>You need to re-run configure in your object directory. When new Makefiles
266 are added to the source tree, they have to be copied over to the object tree in
267 order to be used by the build.</p>
270 <div class="question">
271 <p>I've modified a Makefile in my source tree, but my build tree keeps using the
272 old version. What do I do?</p>
277 <p>If the Makefile already exists in your object tree, you
278 can just run the following command in the top level directory of your object
281 <p><tt>./config.status <relative path to Makefile></tt><p>
283 <p>If the Makefile is new, you will have to modify the configure script to copy
288 <div class="question">
289 <p>I've upgraded to a new version of LLVM, and I get strange build errors.</p>
294 <p>Sometimes, changes to the LLVM source code alters how the build system works.
295 Changes in libtool, autoconf, or header file dependencies are especially prone
296 to this sort of problem.</p>
298 <p>The best thing to try is to remove the old files and re-build. In most
299 cases, this takes care of the problem. To do this, just type <tt>make
300 clean</tt> and then <tt>make</tt> in the directory that fails to build.</p>
304 <div class="question">
305 <p>I've built LLVM and am testing it, but the tests freeze.</p>
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>
314 <p>For example, if you built LLVM with the command:</p>
316 <p><tt>gmake ENABLE_PROFILING=1</tt>
318 <p>...then you must run the tests with the following commands:</p>
320 <p><tt>cd llvm/test<br>gmake ENABLE_PROFILING=1</tt></p>
324 <div class="question">
325 <p>Why do test results differ when I perform different types of builds?</p>
330 <p>The LLVM test suite is dependent upon several features of the LLVM tools and
333 <p>First, the debugging assertions in code are not enabled in optimized or
334 profiling builds. Hence, tests that used to fail may pass.</p>
336 <p>Second, some tests may rely upon debugging options or behavior that is only
337 available in the debug build. These tests will fail in an optimized or profile
342 <div class="question">
343 <p>Compiling LLVM with GCC 3.3.2 fails, what should I do?</p>
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>
351 <div class="question">
353 When I use the test suite, all of the C Backend tests fail. What is
360 If you build LLVM and the C Backend tests fail in <tt>llvm/test/Programs</tt>,
361 then chances are good that the directory pointed to by the LLVM_LIB_SEARCH_PATH
362 environment variable does not contain the libcrtend.a library.
366 To fix it, verify that LLVM_LIB_SEARCH_PATH points to the correct directory
367 and that libcrtend.a is inside. For pre-built LLVM GCC front ends, this
368 should be the absolute path to
369 <tt>cfrontend/<<i>platform</i>>/llvm-gcc/bytecode-libs</tt>. If you've
370 built your own LLVM GCC front end, then ensure that you've built and installed
371 the libraries in <tt>llvm/runtime</tt> and have LLVM_LIB_SEARCH_PATH pointing
372 to the <tt>LLVMGCCDIR/bytecode-libs</tt> subdirectory.
376 <!-- *********************************************************************** -->
377 <div class="doc_section">
378 <a name="cfe">Using the GCC Front End</a>
381 <div class="question">
383 When I compile software that uses a configure script, the configure script
384 thinks my system has all of the header files and libraries it is testing for.
385 How do I get configure to work correctly?
391 The configure script is getting things wrong because the LLVM linker allows
392 symbols to be undefined at link time (so that they can be resolved during JIT
393 or translation to the C back end). That is why configure thinks your system
397 To work around this, perform the following steps:
402 Make sure the CC and CXX environment variables contains the full path to the
407 Make sure that the regular C compiler is first in your PATH.
411 Add the string "-Wl,-native" to your CFLAGS environment variable.
416 This will allow the gccld linker to create a native code executable instead of
417 a shell script that runs the JIT. Creating native code requires standard
418 linkage, which in turn will allow the configure script to find out if code is
419 not linking on your system because the feature isn't available on your system.
423 <div class="question">
425 When I compile code using the LLVM GCC front end, it complains that it cannot
432 In order to find libcrtend.a, you must have the directory in which it lives in
433 your LLVM_LIB_SEARCH_PATH environment variable. For the binary distribution of
434 the LLVM GCC front end, this will be the full path of the bytecode-libs
435 directory inside of the LLVM GCC distribution.
440 <!-- *********************************************************************** -->
441 <div class="doc_section">
442 <a name="cfe_code">Questions about code generated by the GCC front-end</a>
445 <div class="question"><p>
446 What is this <tt>__main()</tt> call that gets inserted into <tt>main()</tt>?
451 The <tt>__main</tt> call is inserted by the C/C++ compiler in order to guarantee
452 that static constructors and destructors are called when the program starts up
453 and shuts down. In C, you can create static constructors and destructors by
454 using GCC extensions, and in C++ you can do so by creating a global variable
455 whose class has a ctor or dtor.
459 The actual implementation of <tt>__main</tt> lives in the
460 <tt>llvm/runtime/GCCLibraries/crtend/</tt> directory in the source-base, and is
461 linked in automatically when you link the program.
465 <!--=========================================================================-->
467 <div class="question"><p>
468 Where did all of my code go??
473 If you are using the LLVM demo page, you may often wonder what happened to all
474 of the code that you typed in. Remember that the demo script is running the
475 code through the LLVM optimizers, so if your code doesn't actually do anything
476 useful, it might all be deleted.
480 To prevent this, make sure that the code is actually needed. For example, if
481 you are computing some expression, return the value from the function instead of
482 leaving it in a local variable. If you really want to constrain the optimizer,
483 you can read from and assign to <tt>volatile</tt> global variables.
487 <!--=========================================================================-->
489 <div class="question"><p>
490 What is this <tt>llvm.global_ctors</tt> and <tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I #include <iostream>?
495 If you #include the <iostream> header into a C++ translation unit, the
496 file will probably use the <tt>std::cin</tt>/<tt>std::cout</tt>/... global
497 objects. However, C++ does not guarantee an order of initialization between
498 static 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
500 be automatically initialized before your use.
504 To make <tt>std::cout</tt> and friends work correctly in these scenarios, the
505 STL that we use declares a static object that gets created in every translation
506 unit that includes <iostream>. This object has a static constructor and
507 destructor that initializes and destroys the global iostream objects before they
508 could possibly be used in the file. The code that you see in the .ll file
509 corresponds to the constructor and destructor registration code.
513 If you would like to make it easier to <b>understand</b> the LLVM code generated
514 by the compiler in the demo page, consider using printf instead of iostreams to
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