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5 <title>LLVM: Frequently Asked Questions</title>
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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>
54 <li><a href="#cfe">Using the GCC Front End</a>
57 When I compile software that uses a configure script, the configure script
58 thinks my system has all of the header files and libraries it is testing
59 for. How do I get configure to work correctly?
63 When I compile code using the LLVM GCC front end, it complains that it
69 <li><a href="#cfe_code">Questions about code generated by the GCC front-end</a>
71 <li>What is this <tt>__main()</tt> call that gets inserted into
73 <li>Where did all of my code go??</li>
78 <!-- *********************************************************************** -->
79 <div class="doc_section">
80 <a name="license">License</a>
82 <!-- *********************************************************************** -->
84 <div class="question">
85 <p>Why are the LLVM source code and the front-end distributed under different
90 <p>The C/C++ front-ends are based on GCC and must be distributed under the GPL.
91 Our aim is to distribute LLVM source code under a <em>much less restrictive</em>
92 license, in particular one that does not compel users who distribute tools based
93 on modifying the source to redistribute the modified source code as well.</p>
96 <div class="question">
97 <p>Does the University of Illinois Open Source License really qualify as an
98 "open source" license?</p>
102 <p>Yes, the license is <a
103 href="http://www.opensource.org/licenses/UoI-NCSA.php">certified</a> by the Open
104 Source Initiative (OSI).</p>
107 <div class="question">
108 <p>Can I modify LLVM source code and redistribute the modified source?</p>
112 <p>Yes. The modified source distribution must retain the copyright notice and
113 follow the three bulletted conditions listed in the <a
114 href="http://llvm.cs.uiuc.edu/releases/1.0/LICENSE.TXT">LLVM license</a>.</p>
117 <div class="question">
118 <p>Can I modify LLVM source code and redistribute binaries or other tools based
119 on it, without redistributing the source?</p>
123 <p>Yes, this is why we distribute LLVM under a less restrictive license than
124 GPL, as explained in the first question above.</p>
127 <!-- *********************************************************************** -->
128 <div class="doc_section">
129 <a name="source">Source Code</a>
131 <!-- *********************************************************************** -->
133 <div class="question">
134 <p>In what language is LLVM written?</p>
138 <p>All of the LLVM tools and libraries are written in C++ with extensive use of
142 <div class="question">
143 <p>How portable is the LLVM source code?</p>
147 <p>The LLVM source code should be portable to most modern UNIX-like operating
148 systems. Most of the code is written in standard C++ with operating system
149 services abstracted to a support library. The tools required to build and test
150 LLVM have been ported to a plethora of platforms.</p>
152 <p>Some porting problems may exist in the following areas:</p>
156 <li>The GCC front end code is not as portable as the LLVM suite, so it may not
157 compile as well on unsupported platforms.</li>
159 <li>The Python test classes are more UNIX-centric than they should be, so
160 porting to non-UNIX like platforms (i.e. Windows, MacOS 9) will require some
163 <li>The LLVM build system relies heavily on UNIX shell tools, like the Bourne
164 Shell and sed. Porting to systems without these tools (MacOS 9, Plan 9) will
165 require more effort.</li>
171 <!-- *********************************************************************** -->
172 <div class="doc_section">
173 <a name="build">Build Problems</a>
175 <!-- *********************************************************************** -->
177 <div class="question">
178 <p>When I run configure, it finds the wrong C compiler.</p>
183 <p>The <tt>configure</tt> script attempts to locate first <tt>gcc</tt> and then
184 <tt>cc</tt>, unless it finds compiler paths set in <tt>CC</tt> and <tt>CXX</tt>
185 for the C and C++ compiler, respectively.</p>
187 <p>If <tt>configure</tt> finds the wrong compiler, either adjust your
188 <tt>PATH</tt> environment variable or set <tt>CC</tt> and <tt>CXX</tt>
193 <div class="question">
194 <p>I compile the code, and I get some error about <tt>/localhome</tt>.</p>
199 <p>There are several possible causes for this. The first is that you didn't set
200 a pathname properly when using <tt>configure</tt>, and it defaulted to a
201 pathname that we use on our research machines.</p>
203 <p>Another possibility is that we hardcoded a path in our Makefiles. If you see
204 this, please email the LLVM bug mailing list with the name of the offending
205 Makefile and a description of what is wrong with it.</p>
209 <div class="question">
210 <p>The <tt>configure</tt> script finds the right C compiler, but it uses the
211 LLVM linker from a previous build. What do I do?</p>
215 <p>The <tt>configure</tt> script uses the <tt>PATH</tt> to find executables, so
216 if it's grabbing the wrong linker/assembler/etc, there are two ways to fix
221 <li><p>Adjust your <tt>PATH</tt> environment variable so that the correct
222 program appears first in the <tt>PATH</tt>. This may work, but may not be
223 convenient when you want them <i>first</i> in your path for other
226 <li><p>Run <tt>configure</tt> with an alternative <tt>PATH</tt> that is
227 correct. In a Borne compatible shell, the syntax would be:</p>
229 <p><tt>PATH=<the path without the bad program> ./configure ...</tt></p>
231 <p>This is still somewhat inconvenient, but it allows <tt>configure</tt>
232 to do its work without having to adjust your <tt>PATH</tt>
233 permanently.</p></li>
239 <div class="question">
240 <p>When creating a dynamic library, I get a strange GLIBC error.</p>
244 <p>Under some operating systems (i.e. Linux), libtool does not work correctly if
245 GCC was compiled with the --disable-shared option. To work around this, install
246 your own version of GCC that has shared libraries enabled by default.</p>
249 <div class="question">
250 <p>I've updated my source tree from CVS, and now my build is trying to use a
251 file/directory that doesn't exist.</p>
255 <p>You need to re-run configure in your object directory. When new Makefiles
256 are added to the source tree, they have to be copied over to the object tree in
257 order to be used by the build.</p>
260 <div class="question">
261 <p>I've modified a Makefile in my source tree, but my build tree keeps using the
262 old version. What do I do?</p>
267 <p>If the Makefile already exists in your object tree, you
268 can just run the following command in the top level directory of your object
271 <p><tt>./config.status <relative path to Makefile></tt><p>
273 <p>If the Makefile is new, you will have to modify the configure script to copy
278 <div class="question">
279 <p>I've upgraded to a new version of LLVM, and I get strange build errors.</p>
284 <p>Sometimes, changes to the LLVM source code alters how the build system works.
285 Changes in libtool, autoconf, or header file dependencies are especially prone
286 to this sort of problem.</p>
288 <p>The best thing to try is to remove the old files and re-build. In most
289 cases, this takes care of the problem. To do this, just type <tt>make
290 clean</tt> and then <tt>make</tt> in the directory that fails to build.</p>
294 <div class="question">
295 <p>I've built LLVM and am testing it, but the tests freeze.</p>
300 <p>This is most likely occurring because you built a profile or release
301 (optimized) build of LLVM and have not specified the same information on the
302 <tt>gmake</tt> command line.</p>
304 <p>For example, if you built LLVM with the command:</p>
306 <p><tt>gmake ENABLE_PROFILING=1</tt>
308 <p>...then you must run the tests with the following commands:</p>
310 <p><tt>cd llvm/test<br>gmake ENABLE_PROFILING=1</tt></p>
314 <div class="question">
315 <p>Why do test results differ when I perform different types of builds?</p>
320 <p>The LLVM test suite is dependent upon several features of the LLVM tools and
323 <p>First, the debugging assertions in code are not enabled in optimized or
324 profiling builds. Hence, tests that used to fail may pass.</p>
326 <p>Second, some tests may rely upon debugging options or behavior that is only
327 available in the debug build. These tests will fail in an optimized or profile
332 <div class="question">
333 <p>Compiling LLVM with GCC 3.3.2 fails, what should I do?</p>
337 <p>This is <a href="http://gcc.gnu.org/PR?13392">a bug in GCC</a>, and
338 affects projects other than LLVM. Try upgrading or downgrading your GCC.</p>
341 <!-- *********************************************************************** -->
342 <div class="doc_section">
343 <a name="cfe">Using the GCC Front End</a>
346 <div class="question">
348 When I compile software that uses a configure script, the configure script
349 thinks my system has all of the header files and libraries it is testing for.
350 How do I get configure to work correctly?
356 The configure script is getting things wrong because the LLVM linker allows
357 symbols to be undefined at link time (so that they can be resolved during JIT
358 or translation to the C back end). That is why configure thinks your system
362 To work around this, perform the following steps:
367 Make sure the CC and CXX environment variables contains the full path to the
372 Make sure that the regular C compiler is first in your PATH.
376 Add the string "-Wl,-native" to your CFLAGS environment variable.
381 This will allow the gccld linker to create a native code executable instead of
382 a shell script that runs the JIT. Creating native code requires standard
383 linkage, which in turn will allow the configure script to find out if code is
384 not linking on your system because the feature isn't available on your system.
388 <div class="question">
390 When I compile code using the LLVM GCC front end, it complains that it cannot
397 In order to find crtend.o, you must have the directory in which it lives in
398 your LLVM_LIB_SEARCH_PATH environment variable. For the binary distribution of
399 the LLVM GCC front end, this will be the full path of the bytecode-libs
400 directory inside of the LLVM GCC distribution.
405 <!-- *********************************************************************** -->
406 <div class="doc_section">
407 <a name="cfe_code">Questions about code generated by the GCC front-end</a>
410 <div class="question"><p>
411 What is this <tt>__main()</tt> call that gets inserted into <tt>main()</tt>?
416 The <tt>__main</tt> call is inserted by the C/C++ compiler in order to guarantee
417 that static constructors and destructors are called when the program starts up
418 and shuts down. In C, you can create static constructors and destructors by
419 using GCC extensions, and in C++ you can do so by creating a global variable
420 whose class has a ctor or dtor.
424 The actual implementation of <tt>__main</tt> lives in the
425 <tt>llvm/runtime/GCCLibraries/crtend/</tt> directory in the source-base, and is
426 linked in automatically when you link the program.
430 <!--=========================================================================-->
432 <div class="question"><p>
433 Where did all of my code go??
438 If you are using the LLVM demo page, you may often wonder what happened to all
439 of the code that you typed in. Remember that the demo script is running the
440 code through the LLVM optimizers, so if your code doesn't actually do anything
441 useful, it might all be deleted.
445 To prevent this, make sure that the code is actually needed. For example, if
446 you are computing some expression, return the value from the function instead of
447 leaving it in a local variable. If you really want to constrain the optimizer,
448 you can read from and assign to <tt>volatile</tt> global variables.
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