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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/1.3/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 LLVM build system relies heavily on UNIX shell tools, like the Bourne
172 Shell and sed. Porting to systems without these tools (MacOS 9, Plan 9) will
173 require more effort.</li>
179 <!-- *********************************************************************** -->
180 <div class="doc_section">
181 <a name="build">Build Problems</a>
183 <!-- *********************************************************************** -->
185 <div class="question">
186 <p>When I run configure, it finds the wrong C compiler.</p>
191 <p>The <tt>configure</tt> script attempts to locate first <tt>gcc</tt> and then
192 <tt>cc</tt>, unless it finds compiler paths set in <tt>CC</tt> and <tt>CXX</tt>
193 for the C and C++ compiler, respectively.</p>
195 <p>If <tt>configure</tt> finds the wrong compiler, either adjust your
196 <tt>PATH</tt> environment variable or set <tt>CC</tt> and <tt>CXX</tt>
201 <div class="question">
202 <p>I compile the code, and I get some error about <tt>/localhome</tt>.</p>
207 <p>There are several possible causes for this. The first is that you didn't set
208 a pathname properly when using <tt>configure</tt>, and it defaulted to a
209 pathname that we use on our research machines.</p>
211 <p>Another possibility is that we hardcoded a path in our Makefiles. If you see
212 this, please email the LLVM bug mailing list with the name of the offending
213 Makefile and a description of what is wrong with it.</p>
217 <div class="question">
218 <p>The <tt>configure</tt> script finds the right C compiler, but it uses the
219 LLVM linker from a previous build. What do I do?</p>
223 <p>The <tt>configure</tt> script uses the <tt>PATH</tt> to find executables, so
224 if it's grabbing the wrong linker/assembler/etc, there are two ways to fix
229 <li><p>Adjust your <tt>PATH</tt> environment variable so that the correct
230 program appears first in the <tt>PATH</tt>. This may work, but may not be
231 convenient when you want them <i>first</i> in your path for other
234 <li><p>Run <tt>configure</tt> with an alternative <tt>PATH</tt> that is
235 correct. In a Borne compatible shell, the syntax would be:</p>
237 <p><tt>PATH=[the path without the bad program] ./configure ...</tt></p>
239 <p>This is still somewhat inconvenient, but it allows <tt>configure</tt>
240 to do its work without having to adjust your <tt>PATH</tt>
241 permanently.</p></li>
247 <div class="question">
248 <p>When creating a dynamic library, I get a strange GLIBC error.</p>
252 <p>Under some operating systems (i.e. Linux), libtool does not work correctly if
253 GCC was compiled with the --disable-shared option. To work around this, install
254 your own version of GCC that has shared libraries enabled by default.</p>
257 <div class="question">
258 <p>I've updated my source tree from CVS, and now my build is trying to use a
259 file/directory that doesn't exist.</p>
263 <p>You need to re-run configure in your object directory. When new Makefiles
264 are added to the source tree, they have to be copied over to the object tree in
265 order to be used by the build.</p>
268 <div class="question">
269 <p>I've modified a Makefile in my source tree, but my build tree keeps using the
270 old version. What do I do?</p>
275 <p>If the Makefile already exists in your object tree, you
276 can just run the following command in the top level directory of your object
279 <p><tt>./config.status <relative path to Makefile></tt><p>
281 <p>If the Makefile is new, you will have to modify the configure script to copy
286 <div class="question">
287 <p>I've upgraded to a new version of LLVM, and I get strange build errors.</p>
292 <p>Sometimes, changes to the LLVM source code alters how the build system works.
293 Changes in libtool, autoconf, or header file dependencies are especially prone
294 to this sort of problem.</p>
296 <p>The best thing to try is to remove the old files and re-build. In most
297 cases, this takes care of the problem. To do this, just type <tt>make
298 clean</tt> and then <tt>make</tt> in the directory that fails to build.</p>
302 <div class="question">
303 <p>I've built LLVM and am testing it, but the tests freeze.</p>
308 <p>This is most likely occurring because you built a profile or release
309 (optimized) build of LLVM and have not specified the same information on the
310 <tt>gmake</tt> command line.</p>
312 <p>For example, if you built LLVM with the command:</p>
314 <p><tt>gmake ENABLE_PROFILING=1</tt>
316 <p>...then you must run the tests with the following commands:</p>
318 <p><tt>cd llvm/test<br>gmake ENABLE_PROFILING=1</tt></p>
322 <div class="question">
323 <p>Why do test results differ when I perform different types of builds?</p>
328 <p>The LLVM test suite is dependent upon several features of the LLVM tools and
331 <p>First, the debugging assertions in code are not enabled in optimized or
332 profiling builds. Hence, tests that used to fail may pass.</p>
334 <p>Second, some tests may rely upon debugging options or behavior that is only
335 available in the debug build. These tests will fail in an optimized or profile
340 <div class="question">
341 <p>Compiling LLVM with GCC 3.3.2 fails, what should I do?</p>
345 <p>This is <a href="http://gcc.gnu.org/PR?13392">a bug in GCC</a>, and
346 affects projects other than LLVM. Try upgrading or downgrading your GCC.</p>
349 <div class="question">
351 When I use the test suite, all of the C Backend tests fail. What is
358 If you build LLVM and the C Backend tests fail in <tt>llvm/test/Programs</tt>,
359 then chances are good that the directory pointed to by the LLVM_LIB_SEARCH_PATH
360 environment variable does not contain the libcrtend.a library.
364 To fix it, verify that LLVM_LIB_SEARCH_PATH points to the correct directory
365 and that libcrtend.a is inside. For pre-built LLVM GCC front ends, this
366 should be the absolute path to
367 <tt>cfrontend/<<i>platform</i>>/llvm-gcc/bytecode-libs</tt>. If you've
368 built your own LLVM GCC front end, then ensure that you've built and installed
369 the libraries in <tt>llvm/runtime</tt> and have LLVM_LIB_SEARCH_PATH pointing
370 to the <tt>LLVMGCCDIR/bytecode-libs</tt> subdirectory.
374 <div class="question">
375 <p>After CVS update, rebuilding gives the error "No rule to make target".</p>
379 <p>If the error is of the form:</p>
381 <div class="doc_code">
383 gmake[2]: *** No rule to make target `/path/to/somefile', needed by
384 `/path/to/another/file.d'.<br>
389 <p>This may occur anytime files are moved within the CVS repository or removed
390 entirely. In this case, the best solution is to erase all <tt>.d</tt> files,
391 which list dependencies for source files, and rebuild:</p>
393 <div class="doc_code">
396 % rm -f `find . -name \*\.d`
401 <p>In other cases, it may be necessary to run <tt>make clean</tt> before
405 <!-- *********************************************************************** -->
406 <div class="doc_section">
407 <a name="cfe">Using the GCC Front End</a>
410 <div class="question">
412 When I compile software that uses a configure script, the configure script
413 thinks my system has all of the header files and libraries it is testing for.
414 How do I get configure to work correctly?
420 The configure script is getting things wrong because the LLVM linker allows
421 symbols to be undefined at link time (so that they can be resolved during JIT
422 or translation to the C back end). That is why configure thinks your system
426 To work around this, perform the following steps:
431 Make sure the CC and CXX environment variables contains the full path to the
436 Make sure that the regular C compiler is first in your PATH.
440 Add the string "-Wl,-native" to your CFLAGS environment variable.
445 This will allow the gccld linker to create a native code executable instead of
446 a shell script that runs the JIT. Creating native code requires standard
447 linkage, which in turn will allow the configure script to find out if code is
448 not linking on your system because the feature isn't available on your system.
452 <div class="question">
454 When I compile code using the LLVM GCC front end, it complains that it cannot
461 In order to find libcrtend.a, you must have the directory in which it lives in
462 your LLVM_LIB_SEARCH_PATH environment variable. For the binary distribution of
463 the LLVM GCC front end, this will be the full path of the bytecode-libs
464 directory inside of the LLVM GCC distribution.
469 <!-- *********************************************************************** -->
470 <div class="doc_section">
471 <a name="cfe_code">Questions about code generated by the GCC front-end</a>
474 <div class="question"><p>
475 What is this <tt>__main()</tt> call that gets inserted into <tt>main()</tt>?
480 The <tt>__main</tt> call is inserted by the C/C++ compiler in order to guarantee
481 that static constructors and destructors are called when the program starts up
482 and shuts down. In C, you can create static constructors and destructors by
483 using GCC extensions, and in C++ you can do so by creating a global variable
484 whose class has a ctor or dtor.
488 The actual implementation of <tt>__main</tt> lives in the
489 <tt>llvm/runtime/GCCLibraries/crtend/</tt> directory in the source-base, and is
490 linked in automatically when you link the program.
494 <!--=========================================================================-->
496 <div class="question"><p>
497 Where did all of my code go??
502 If you are using the LLVM demo page, you may often wonder what happened to all
503 of the code that you typed in. Remember that the demo script is running the
504 code through the LLVM optimizers, so if your code doesn't actually do anything
505 useful, it might all be deleted.
509 To prevent this, make sure that the code is actually needed. For example, if
510 you are computing some expression, return the value from the function instead of
511 leaving it in a local variable. If you really want to constrain the optimizer,
512 you can read from and assign to <tt>volatile</tt> global variables.
516 <!--=========================================================================-->
518 <div class="question">
519 <p> What is this <tt>llvm.global_ctors</tt> and
520 <tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I #include
521 <iostream>?</p>
526 <p>If you #include the <iostream> header into a C++ translation unit, the
527 file will probably use the <tt>std::cin</tt>/<tt>std::cout</tt>/... global
528 objects. However, C++ does not guarantee an order of initialization between
529 static objects in different translation units, so if a static ctor/dtor in your
530 .cpp file used <tt>std::cout</tt>, for example, the object would not necessarily
531 be automatically initialized before your use.</p>
533 <p>To make <tt>std::cout</tt> and friends work correctly in these scenarios, the
534 STL that we use declares a static object that gets created in every translation
535 unit that includes <tt><iostream></tt>. This object has a static
536 constructor and destructor that initializes and destroys the global iostream
537 objects before they could possibly be used in the file. The code that you see
538 in the .ll file corresponds to the constructor and destructor registration code.
541 <p>If you would like to make it easier to <b>understand</b> the LLVM code
542 generated by the compiler in the demo page, consider using <tt>printf()</tt>
543 instead of <tt>iostream</tt>s to print values.</p>
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