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5 <title>LLVM: Frequently Asked Questions</title>
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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
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 <!-- *********************************************************************** -->
84 <div class="doc_section">
85 <a name="license">License</a>
87 <!-- *********************************************************************** -->
89 <div class="question">
90 <p>Why are the LLVM source code and the front-end distributed under different
95 <p>The C/C++ front-ends are based on GCC and must be distributed under the GPL.
96 Our aim is to distribute LLVM source code under a <em>much less restrictive</em>
97 license, in particular one that does not compel users who distribute tools based
98 on modifying the source to redistribute the modified source code as well.</p>
101 <div class="question">
102 <p>Does the University of Illinois Open Source License really qualify as an
103 "open source" license?</p>
107 <p>Yes, the license is <a
108 href="http://www.opensource.org/licenses/UoI-NCSA.php">certified</a> by the Open
109 Source Initiative (OSI).</p>
112 <div class="question">
113 <p>Can I modify LLVM source code and redistribute the modified source?</p>
117 <p>Yes. The modified source distribution must retain the copyright notice and
118 follow the three bulletted conditions listed in the <a
119 href="http://llvm.cs.uiuc.edu/releases/1.2/LICENSE.TXT">LLVM license</a>.</p>
122 <div class="question">
123 <p>Can I modify LLVM source code and redistribute binaries or other tools based
124 on it, without redistributing the source?</p>
128 <p>Yes, this is why we distribute LLVM under a less restrictive license than
129 GPL, as explained in the first question above.</p>
132 <!-- *********************************************************************** -->
133 <div class="doc_section">
134 <a name="source">Source Code</a>
136 <!-- *********************************************************************** -->
138 <div class="question">
139 <p>In what language is LLVM written?</p>
143 <p>All of the LLVM tools and libraries are written in C++ with extensive use of
147 <div class="question">
148 <p>How portable is the LLVM source code?</p>
152 <p>The LLVM source code should be portable to most modern UNIX-like operating
153 systems. Most of the code is written in standard C++ with operating system
154 services abstracted to a support library. The tools required to build and test
155 LLVM have been ported to a plethora of platforms.</p>
157 <p>Some porting problems may exist in the following areas:</p>
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>
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
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>
176 <!-- *********************************************************************** -->
177 <div class="doc_section">
178 <a name="build">Build Problems</a>
180 <!-- *********************************************************************** -->
182 <div class="question">
183 <p>When I run configure, it finds the wrong C compiler.</p>
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>
190 for the C and C++ compiler, respectively.</p>
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>
198 <div class="question">
199 <p>I compile the code, and I get some error about <tt>/localhome</tt>.</p>
204 <p>There are several possible causes for this. The first is that you didn't set
205 a pathname properly when using <tt>configure</tt>, and it defaulted to a
206 pathname that we use on our research machines.</p>
208 <p>Another possibility is that we hardcoded a path in our Makefiles. If you see
209 this, please email the LLVM bug mailing list with the name of the offending
210 Makefile and a description of what is wrong with it.</p>
214 <div class="question">
215 <p>The <tt>configure</tt> script finds the right C compiler, but it uses the
216 LLVM linker from a previous build. What do I do?</p>
220 <p>The <tt>configure</tt> script uses the <tt>PATH</tt> to find executables, so
221 if it's grabbing the wrong linker/assembler/etc, there are two ways to fix
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
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>
234 <p><tt>PATH=<the path without the bad program> ./configure ...</tt></p>
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>
244 <div class="question">
245 <p>When creating a dynamic library, I get a strange GLIBC error.</p>
249 <p>Under some operating systems (i.e. Linux), libtool does not work correctly if
250 GCC was compiled with the --disable-shared option. To work around this, install
251 your own version of GCC that has shared libraries enabled by default.</p>
254 <div class="question">
255 <p>I've updated my source tree from CVS, and now my build is trying to use a
256 file/directory that doesn't exist.</p>
260 <p>You need to re-run configure in your object directory. When new Makefiles
261 are added to the source tree, they have to be copied over to the object tree in
262 order to be used by the build.</p>
265 <div class="question">
266 <p>I've modified a Makefile in my source tree, but my build tree keeps using the
267 old version. What do I do?</p>
272 <p>If the Makefile already exists in your object tree, you
273 can just run the following command in the top level directory of your object
276 <p><tt>./config.status <relative path to Makefile></tt><p>
278 <p>If the Makefile is new, you will have to modify the configure script to copy
283 <div class="question">
284 <p>I've upgraded to a new version of LLVM, and I get strange build errors.</p>
289 <p>Sometimes, changes to the LLVM source code alters how the build system works.
290 Changes in libtool, autoconf, or header file dependencies are especially prone
291 to this sort of problem.</p>
293 <p>The best thing to try is to remove the old files and re-build. In most
294 cases, this takes care of the problem. To do this, just type <tt>make
295 clean</tt> and then <tt>make</tt> in the directory that fails to build.</p>
299 <div class="question">
300 <p>I've built LLVM and am testing it, but the tests freeze.</p>
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>
309 <p>For example, if you built LLVM with the command:</p>
311 <p><tt>gmake ENABLE_PROFILING=1</tt>
313 <p>...then you must run the tests with the following commands:</p>
315 <p><tt>cd llvm/test<br>gmake ENABLE_PROFILING=1</tt></p>
319 <div class="question">
320 <p>Why do test results differ when I perform different types of builds?</p>
325 <p>The LLVM test suite is dependent upon several features of the LLVM tools and
328 <p>First, the debugging assertions in code are not enabled in optimized or
329 profiling builds. Hence, tests that used to fail may pass.</p>
331 <p>Second, some tests may rely upon debugging options or behavior that is only
332 available in the debug build. These tests will fail in an optimized or profile
337 <div class="question">
338 <p>Compiling LLVM with GCC 3.3.2 fails, what should I do?</p>
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>
346 <div class="question">
348 When I use the test suite, all of the C Backend tests fail. What is
355 If you build LLVM and the C Backend tests fail in <tt>llvm/test/Programs</tt>,
356 then chances are good that the directory pointed to by the LLVM_LIB_SEARCH_PATH
357 environment variable does not contain the libcrtend.a library.
361 To fix it, verify that LLVM_LIB_SEARCH_PATH points to the correct directory
362 and that libcrtend.a is inside. For pre-built LLVM GCC front ends, this
363 should be the absolute path to
364 <tt>cfrontend/<<i>platform</i>>/llvm-gcc/bytecode-libs</tt>. If you've
365 built your own LLVM GCC front end, then ensure that you've built and installed
366 the libraries in <tt>llvm/runtime</tt> and have LLVM_LIB_SEARCH_PATH pointing
367 to the <tt>LLVMGCCDIR/bytecode-libs</tt> subdirectory.
371 <!-- *********************************************************************** -->
372 <div class="doc_section">
373 <a name="cfe">Using the GCC Front End</a>
376 <div class="question">
378 When I compile software that uses a configure script, the configure script
379 thinks my system has all of the header files and libraries it is testing for.
380 How do I get configure to work correctly?
386 The configure script is getting things wrong because the LLVM linker allows
387 symbols to be undefined at link time (so that they can be resolved during JIT
388 or translation to the C back end). That is why configure thinks your system
392 To work around this, perform the following steps:
397 Make sure the CC and CXX environment variables contains the full path to the
402 Make sure that the regular C compiler is first in your PATH.
406 Add the string "-Wl,-native" to your CFLAGS environment variable.
411 This will allow the gccld linker to create a native code executable instead of
412 a shell script that runs the JIT. Creating native code requires standard
413 linkage, which in turn will allow the configure script to find out if code is
414 not linking on your system because the feature isn't available on your system.
418 <div class="question">
420 When I compile code using the LLVM GCC front end, it complains that it cannot
427 In order to find libcrtend.a, you must have the directory in which it lives in
428 your LLVM_LIB_SEARCH_PATH environment variable. For the binary distribution of
429 the LLVM GCC front end, this will be the full path of the bytecode-libs
430 directory inside of the LLVM GCC distribution.
435 <!-- *********************************************************************** -->
436 <div class="doc_section">
437 <a name="cfe_code">Questions about code generated by the GCC front-end</a>
440 <div class="question"><p>
441 What is this <tt>__main()</tt> call that gets inserted into <tt>main()</tt>?
446 The <tt>__main</tt> call is inserted by the C/C++ compiler in order to guarantee
447 that static constructors and destructors are called when the program starts up
448 and shuts down. In C, you can create static constructors and destructors by
449 using GCC extensions, and in C++ you can do so by creating a global variable
450 whose class has a ctor or dtor.
454 The actual implementation of <tt>__main</tt> lives in the
455 <tt>llvm/runtime/GCCLibraries/crtend/</tt> directory in the source-base, and is
456 linked in automatically when you link the program.
460 <!--=========================================================================-->
462 <div class="question"><p>
463 Where did all of my code go??
468 If you are using the LLVM demo page, you may often wonder what happened to all
469 of the code that you typed in. Remember that the demo script is running the
470 code through the LLVM optimizers, so if your code doesn't actually do anything
471 useful, it might all be deleted.
475 To prevent this, make sure that the code is actually needed. For example, if
476 you are computing some expression, return the value from the function instead of
477 leaving it in a local variable. If you really want to constrain the optimizer,
478 you can read from and assign to <tt>volatile</tt> global variables.
482 <!--=========================================================================-->
484 <div class="question"><p>
485 What is this <tt>llvm.global_ctors</tt> and <tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I #include <iostream>?
490 If you #include the <iostream> header into a C++ translation unit, the
491 file will probably use the <tt>std::cin</tt>/<tt>std::cout</tt>/... global
492 objects. However, C++ does not guarantee an order of initialization between
493 static 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
495 be automatically initialized before your use.
499 To make <tt>std::cout</tt> and friends work correctly in these scenarios, the
500 STL that we use declares a static object that gets created in every translation
501 unit that includes <iostream>. This object has a static constructor and
502 destructor that initializes and destroys the global iostream objects before they
503 could possibly be used in the file. The code that you see in the .ll file
504 corresponds to the constructor and destructor registration code.
508 If you would like to make it easier to <b>understand</b> the LLVM code generated
509 by the compiler in the demo page, consider using printf instead of iostreams to
518 <!-- *********************************************************************** -->
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