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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
54 <li>After CVS update, rebuilding gives the error "No rule to make
58 <li><a href="#cfe">Using the GCC Front End</a>
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?
67 When I compile code using the LLVM GCC front end, it complains that it
68 cannot find libcrtend.a.
73 <li><a href="#cfe_code">Questions about code generated by the GCC front-end</a>
75 <li>What is this <tt>__main()</tt> call that gets inserted into
77 <li>Where did all of my code go??</li>
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 <iostream>?</li>
85 <div class="doc_author">
86 <p>Written by <a href="http://llvm.cs.uiuc.edu">The LLVM Team</a></p>
90 <!-- *********************************************************************** -->
91 <div class="doc_section">
92 <a name="license">License</a>
94 <!-- *********************************************************************** -->
96 <div class="question">
97 <p>Why are the LLVM source code and the front-end distributed under different
102 <p>The C/C++ front-ends are based on GCC and must be distributed under the GPL.
103 Our aim is to distribute LLVM source code under a <em>much less restrictive</em>
104 license, in particular one that does not compel users who distribute tools based
105 on modifying the source to redistribute the modified source code as well.</p>
108 <div class="question">
109 <p>Does the University of Illinois Open Source License really qualify as an
110 "open source" license?</p>
114 <p>Yes, the license is <a
115 href="http://www.opensource.org/licenses/UoI-NCSA.php">certified</a> by the Open
116 Source Initiative (OSI).</p>
119 <div class="question">
120 <p>Can I modify LLVM source code and redistribute the modified source?</p>
124 <p>Yes. The modified source distribution must retain the copyright notice and
125 follow the three bulletted conditions listed in the <a
126 href="http://llvm.cs.uiuc.edu/releases/1.2/LICENSE.TXT">LLVM license</a>.</p>
129 <div class="question">
130 <p>Can I modify LLVM source code and redistribute binaries or other tools based
131 on it, without redistributing the source?</p>
135 <p>Yes, this is why we distribute LLVM under a less restrictive license than
136 GPL, as explained in the first question above.</p>
139 <!-- *********************************************************************** -->
140 <div class="doc_section">
141 <a name="source">Source Code</a>
143 <!-- *********************************************************************** -->
145 <div class="question">
146 <p>In what language is LLVM written?</p>
150 <p>All of the LLVM tools and libraries are written in C++ with extensive use of
154 <div class="question">
155 <p>How portable is the LLVM source code?</p>
159 <p>The LLVM source code should be portable to most modern UNIX-like operating
160 systems. Most of the code is written in standard C++ with operating system
161 services abstracted to a support library. The tools required to build and test
162 LLVM have been ported to a plethora of platforms.</p>
164 <p>Some porting problems may exist in the following areas:</p>
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>
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
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>
183 <!-- *********************************************************************** -->
184 <div class="doc_section">
185 <a name="build">Build Problems</a>
187 <!-- *********************************************************************** -->
189 <div class="question">
190 <p>When I run configure, it finds the wrong C compiler.</p>
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>
197 for the C and C++ compiler, respectively.</p>
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>
205 <div class="question">
206 <p>I compile the code, and I get some error about <tt>/localhome</tt>.</p>
211 <p>There are several possible causes for this. The first is that you didn't set
212 a pathname properly when using <tt>configure</tt>, and it defaulted to a
213 pathname that we use on our research machines.</p>
215 <p>Another possibility is that we hardcoded a path in our Makefiles. If you see
216 this, please email the LLVM bug mailing list with the name of the offending
217 Makefile and a description of what is wrong with it.</p>
221 <div class="question">
222 <p>The <tt>configure</tt> script finds the right C compiler, but it uses the
223 LLVM linker from a previous build. What do I do?</p>
227 <p>The <tt>configure</tt> script uses the <tt>PATH</tt> to find executables, so
228 if it's grabbing the wrong linker/assembler/etc, there are two ways to fix
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
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>
241 <p><tt>PATH=[the path without the bad program] ./configure ...</tt></p>
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>
251 <div class="question">
252 <p>When creating a dynamic library, I get a strange GLIBC error.</p>
256 <p>Under some operating systems (i.e. Linux), libtool does not work correctly if
257 GCC was compiled with the --disable-shared option. To work around this, install
258 your own version of GCC that has shared libraries enabled by default.</p>
261 <div class="question">
262 <p>I've updated my source tree from CVS, and now my build is trying to use a
263 file/directory that doesn't exist.</p>
267 <p>You need to re-run configure in your object directory. When new Makefiles
268 are added to the source tree, they have to be copied over to the object tree in
269 order to be used by the build.</p>
272 <div class="question">
273 <p>I've modified a Makefile in my source tree, but my build tree keeps using the
274 old version. What do I do?</p>
279 <p>If the Makefile already exists in your object tree, you
280 can just run the following command in the top level directory of your object
283 <p><tt>./config.status <relative path to Makefile></tt><p>
285 <p>If the Makefile is new, you will have to modify the configure script to copy
290 <div class="question">
291 <p>I've upgraded to a new version of LLVM, and I get strange build errors.</p>
296 <p>Sometimes, changes to the LLVM source code alters how the build system works.
297 Changes in libtool, autoconf, or header file dependencies are especially prone
298 to this sort of problem.</p>
300 <p>The best thing to try is to remove the old files and re-build. In most
301 cases, this takes care of the problem. To do this, just type <tt>make
302 clean</tt> and then <tt>make</tt> in the directory that fails to build.</p>
306 <div class="question">
307 <p>I've built LLVM and am testing it, but the tests freeze.</p>
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>
316 <p>For example, if you built LLVM with the command:</p>
318 <p><tt>gmake ENABLE_PROFILING=1</tt>
320 <p>...then you must run the tests with the following commands:</p>
322 <p><tt>cd llvm/test<br>gmake ENABLE_PROFILING=1</tt></p>
326 <div class="question">
327 <p>Why do test results differ when I perform different types of builds?</p>
332 <p>The LLVM test suite is dependent upon several features of the LLVM tools and
335 <p>First, the debugging assertions in code are not enabled in optimized or
336 profiling builds. Hence, tests that used to fail may pass.</p>
338 <p>Second, some tests may rely upon debugging options or behavior that is only
339 available in the debug build. These tests will fail in an optimized or profile
344 <div class="question">
345 <p>Compiling LLVM with GCC 3.3.2 fails, what should I do?</p>
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>
353 <div class="question">
355 When I use the test suite, all of the C Backend tests fail. What is
362 If you build LLVM and the C Backend tests fail in <tt>llvm/test/Programs</tt>,
363 then chances are good that the directory pointed to by the LLVM_LIB_SEARCH_PATH
364 environment variable does not contain the libcrtend.a library.
368 To fix it, verify that LLVM_LIB_SEARCH_PATH points to the correct directory
369 and that libcrtend.a is inside. For pre-built LLVM GCC front ends, this
370 should be the absolute path to
371 <tt>cfrontend/<<i>platform</i>>/llvm-gcc/bytecode-libs</tt>. If you've
372 built your own LLVM GCC front end, then ensure that you've built and installed
373 the libraries in <tt>llvm/runtime</tt> and have LLVM_LIB_SEARCH_PATH pointing
374 to the <tt>LLVMGCCDIR/bytecode-libs</tt> subdirectory.
378 <div class="question">
379 <p>After CVS update, rebuilding gives the error "No rule to make target".</p>
383 <p>If the error is of the form:</p>
385 <div class="doc_code">
387 gmake[2]: *** No rule to make target `/path/to/somefile', needed by
388 `/path/to/another/file.d'.<br>
393 <p>This may occur anytime files are moved within the CVS repository or removed
394 entirely. In this case, the best solution is to erase all <tt>.d</tt> files,
395 which list dependencies for source files, and rebuild:</p>
397 <div class="doc_code">
400 % rm -f `find . -name \*\.d`
405 <p>In other cases, it may be necessary to run <tt>make clean</tt> before
409 <!-- *********************************************************************** -->
410 <div class="doc_section">
411 <a name="cfe">Using the GCC Front End</a>
414 <div class="question">
416 When I compile software that uses a configure script, the configure script
417 thinks my system has all of the header files and libraries it is testing for.
418 How do I get configure to work correctly?
424 The configure script is getting things wrong because the LLVM linker allows
425 symbols to be undefined at link time (so that they can be resolved during JIT
426 or translation to the C back end). That is why configure thinks your system
430 To work around this, perform the following steps:
435 Make sure the CC and CXX environment variables contains the full path to the
440 Make sure that the regular C compiler is first in your PATH.
444 Add the string "-Wl,-native" to your CFLAGS environment variable.
449 This will allow the gccld linker to create a native code executable instead of
450 a shell script that runs the JIT. Creating native code requires standard
451 linkage, which in turn will allow the configure script to find out if code is
452 not linking on your system because the feature isn't available on your system.
456 <div class="question">
458 When I compile code using the LLVM GCC front end, it complains that it cannot
465 In order to find libcrtend.a, you must have the directory in which it lives in
466 your LLVM_LIB_SEARCH_PATH environment variable. For the binary distribution of
467 the LLVM GCC front end, this will be the full path of the bytecode-libs
468 directory inside of the LLVM GCC distribution.
473 <!-- *********************************************************************** -->
474 <div class="doc_section">
475 <a name="cfe_code">Questions about code generated by the GCC front-end</a>
478 <div class="question"><p>
479 What is this <tt>__main()</tt> call that gets inserted into <tt>main()</tt>?
484 The <tt>__main</tt> call is inserted by the C/C++ compiler in order to guarantee
485 that static constructors and destructors are called when the program starts up
486 and shuts down. In C, you can create static constructors and destructors by
487 using GCC extensions, and in C++ you can do so by creating a global variable
488 whose class has a ctor or dtor.
492 The actual implementation of <tt>__main</tt> lives in the
493 <tt>llvm/runtime/GCCLibraries/crtend/</tt> directory in the source-base, and is
494 linked in automatically when you link the program.
498 <!--=========================================================================-->
500 <div class="question"><p>
501 Where did all of my code go??
506 If you are using the LLVM demo page, you may often wonder what happened to all
507 of the code that you typed in. Remember that the demo script is running the
508 code through the LLVM optimizers, so if your code doesn't actually do anything
509 useful, it might all be deleted.
513 To prevent this, make sure that the code is actually needed. For example, if
514 you are computing some expression, return the value from the function instead of
515 leaving it in a local variable. If you really want to constrain the optimizer,
516 you can read from and assign to <tt>volatile</tt> global variables.
520 <!--=========================================================================-->
522 <div class="question"><p>
523 What is this <tt>llvm.global_ctors</tt> and <tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I #include <iostream>?
528 If you #include the <iostream> header into a C++ translation unit, the
529 file will probably use the <tt>std::cin</tt>/<tt>std::cout</tt>/... global
530 objects. However, C++ does not guarantee an order of initialization between
531 static 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
533 be automatically initialized before your use.
537 To make <tt>std::cout</tt> and friends work correctly in these scenarios, the
538 STL that we use declares a static object that gets created in every translation
539 unit that includes <iostream>. This object has a static constructor and
540 destructor that initializes and destroys the global iostream objects before they
541 could possibly be used in the file. The code that you see in the .ll file
542 corresponds to the constructor and destructor registration code.
546 If you would like to make it easier to <b>understand</b> the LLVM code generated
547 by the compiler in the demo page, consider using printf instead of iostreams to
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