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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.
72 How can I disable all optimizations when compiling code using the LLVM GCC front end?
78 <li><a href="#cfe_code">Questions about code generated by the GCC front-end</a>
80 <li><a href="#__main">What is this <tt>__main()</tt> call that gets inserted into
81 <tt>main()</tt>?</a></li>
82 <li><a href="#iosinit">What is this <tt>llvm.global_ctors</tt> and
83 <tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I
84 #include <iostream>?</a></li>
85 <li><a href="#codedce">Where did all of my code go??</a></li>
86 <li><a href="#undef">What is this "<tt>undef</tt>" thing that shows up in my code?</a></li>
91 <div class="doc_author">
92 <p>Written by <a href="http://llvm.org">The LLVM Team</a></p>
96 <!-- *********************************************************************** -->
97 <div class="doc_section">
98 <a name="license">License</a>
100 <!-- *********************************************************************** -->
102 <div class="question">
103 <p>Why are the LLVM source code and the front-end distributed under different
108 <p>The C/C++ front-ends are based on GCC and must be distributed under the GPL.
109 Our aim is to distribute LLVM source code under a <em>much less restrictive</em>
110 license, in particular one that does not compel users who distribute tools based
111 on modifying the source to redistribute the modified source code as well.</p>
114 <div class="question">
115 <p>Does the University of Illinois Open Source License really qualify as an
116 "open source" license?</p>
120 <p>Yes, the license is <a
121 href="http://www.opensource.org/licenses/UoI-NCSA.php">certified</a> by the Open
122 Source Initiative (OSI).</p>
125 <div class="question">
126 <p>Can I modify LLVM source code and redistribute the modified source?</p>
130 <p>Yes. The modified source distribution must retain the copyright notice and
131 follow the three bulletted conditions listed in the <a
132 href="http://llvm.org/releases/1.3/LICENSE.TXT">LLVM license</a>.</p>
135 <div class="question">
136 <p>Can I modify LLVM source code and redistribute binaries or other tools based
137 on it, without redistributing the source?</p>
141 <p>Yes, this is why we distribute LLVM under a less restrictive license than
142 GPL, as explained in the first question above.</p>
145 <!-- *********************************************************************** -->
146 <div class="doc_section">
147 <a name="source">Source Code</a>
149 <!-- *********************************************************************** -->
151 <div class="question">
152 <p>In what language is LLVM written?</p>
156 <p>All of the LLVM tools and libraries are written in C++ with extensive use of
160 <div class="question">
161 <p>How portable is the LLVM source code?</p>
165 <p>The LLVM source code should be portable to most modern UNIX-like operating
166 systems. Most of the code is written in standard C++ with operating system
167 services abstracted to a support library. The tools required to build and test
168 LLVM have been ported to a plethora of platforms.</p>
170 <p>Some porting problems may exist in the following areas:</p>
174 <li>The GCC front end code is not as portable as the LLVM suite, so it may not
175 compile as well on unsupported platforms.</li>
177 <li>The LLVM build system relies heavily on UNIX shell tools, like the Bourne
178 Shell and sed. Porting to systems without these tools (MacOS 9, Plan 9) will
179 require more effort.</li>
185 <!-- *********************************************************************** -->
186 <div class="doc_section">
187 <a name="build">Build Problems</a>
189 <!-- *********************************************************************** -->
191 <div class="question">
192 <p>When I run configure, it finds the wrong C compiler.</p>
197 <p>The <tt>configure</tt> script attempts to locate first <tt>gcc</tt> and then
198 <tt>cc</tt>, unless it finds compiler paths set in <tt>CC</tt> and <tt>CXX</tt>
199 for the C and C++ compiler, respectively.</p>
201 <p>If <tt>configure</tt> finds the wrong compiler, either adjust your
202 <tt>PATH</tt> environment variable or set <tt>CC</tt> and <tt>CXX</tt>
207 <div class="question">
208 <p>I compile the code, and I get some error about <tt>/localhome</tt>.</p>
213 <p>There are several possible causes for this. The first is that you didn't set
214 a pathname properly when using <tt>configure</tt>, and it defaulted to a
215 pathname that we use on our research machines.</p>
217 <p>Another possibility is that we hardcoded a path in our Makefiles. If you see
218 this, please email the LLVM bug mailing list with the name of the offending
219 Makefile and a description of what is wrong with it.</p>
223 <div class="question">
224 <p>The <tt>configure</tt> script finds the right C compiler, but it uses the
225 LLVM linker from a previous build. What do I do?</p>
229 <p>The <tt>configure</tt> script uses the <tt>PATH</tt> to find executables, so
230 if it's grabbing the wrong linker/assembler/etc, there are two ways to fix
235 <li><p>Adjust your <tt>PATH</tt> environment variable so that the correct
236 program appears first in the <tt>PATH</tt>. This may work, but may not be
237 convenient when you want them <i>first</i> in your path for other
240 <li><p>Run <tt>configure</tt> with an alternative <tt>PATH</tt> that is
241 correct. In a Borne compatible shell, the syntax would be:</p>
243 <p><tt>PATH=[the path without the bad program] ./configure ...</tt></p>
245 <p>This is still somewhat inconvenient, but it allows <tt>configure</tt>
246 to do its work without having to adjust your <tt>PATH</tt>
247 permanently.</p></li>
253 <div class="question">
254 <p>When creating a dynamic library, I get a strange GLIBC error.</p>
258 <p>Under some operating systems (i.e. Linux), libtool does not work correctly if
259 GCC was compiled with the --disable-shared option. To work around this, install
260 your own version of GCC that has shared libraries enabled by default.</p>
263 <div class="question">
264 <p>I've updated my source tree from CVS, and now my build is trying to use a
265 file/directory that doesn't exist.</p>
269 <p>You need to re-run configure in your object directory. When new Makefiles
270 are added to the source tree, they have to be copied over to the object tree in
271 order to be used by the build.</p>
274 <div class="question">
275 <p>I've modified a Makefile in my source tree, but my build tree keeps using the
276 old version. What do I do?</p>
281 <p>If the Makefile already exists in your object tree, you
282 can just run the following command in the top level directory of your object
285 <p><tt>./config.status <relative path to Makefile></tt><p>
287 <p>If the Makefile is new, you will have to modify the configure script to copy
292 <div class="question">
293 <p>I've upgraded to a new version of LLVM, and I get strange build errors.</p>
298 <p>Sometimes, changes to the LLVM source code alters how the build system works.
299 Changes in libtool, autoconf, or header file dependencies are especially prone
300 to this sort of problem.</p>
302 <p>The best thing to try is to remove the old files and re-build. In most
303 cases, this takes care of the problem. To do this, just type <tt>make
304 clean</tt> and then <tt>make</tt> in the directory that fails to build.</p>
308 <div class="question">
309 <p>I've built LLVM and am testing it, but the tests freeze.</p>
314 <p>This is most likely occurring because you built a profile or release
315 (optimized) build of LLVM and have not specified the same information on the
316 <tt>gmake</tt> command line.</p>
318 <p>For example, if you built LLVM with the command:</p>
320 <p><tt>gmake ENABLE_PROFILING=1</tt>
322 <p>...then you must run the tests with the following commands:</p>
324 <p><tt>cd llvm/test<br>gmake ENABLE_PROFILING=1</tt></p>
328 <div class="question">
329 <p>Why do test results differ when I perform different types of builds?</p>
334 <p>The LLVM test suite is dependent upon several features of the LLVM tools and
337 <p>First, the debugging assertions in code are not enabled in optimized or
338 profiling builds. Hence, tests that used to fail may pass.</p>
340 <p>Second, some tests may rely upon debugging options or behavior that is only
341 available in the debug build. These tests will fail in an optimized or profile
346 <div class="question">
347 <p>Compiling LLVM with GCC 3.3.2 fails, what should I do?</p>
351 <p>This is <a href="http://gcc.gnu.org/PR?13392">a bug in GCC</a>, and
352 affects projects other than LLVM. Try upgrading or downgrading your GCC.</p>
355 <div class="question">
356 <p>After CVS update, rebuilding gives the error "No rule to make target".</p>
360 <p>If the error is of the form:</p>
362 <div class="doc_code">
364 gmake[2]: *** No rule to make target `/path/to/somefile', needed by
365 `/path/to/another/file.d'.<br>
370 <p>This may occur anytime files are moved within the CVS repository or removed
371 entirely. In this case, the best solution is to erase all <tt>.d</tt> files,
372 which list dependencies for source files, and rebuild:</p>
374 <div class="doc_code">
377 % rm -f `find . -name \*\.d`
382 <p>In other cases, it may be necessary to run <tt>make clean</tt> before
386 <!-- *********************************************************************** -->
387 <div class="doc_section">
388 <a name="cfe">Using the GCC Front End</a>
391 <div class="question">
393 When I compile software that uses a configure script, the configure script
394 thinks my system has all of the header files and libraries it is testing for.
395 How do I get configure to work correctly?
401 The configure script is getting things wrong because the LLVM linker allows
402 symbols to be undefined at link time (so that they can be resolved during JIT
403 or translation to the C back end). That is why configure thinks your system
407 To work around this, perform the following steps:
412 Make sure the CC and CXX environment variables contains the full path to the
417 Make sure that the regular C compiler is first in your PATH.
421 Add the string "-Wl,-native" to your CFLAGS environment variable.
426 This will allow the gccld linker to create a native code executable instead of
427 a shell script that runs the JIT. Creating native code requires standard
428 linkage, which in turn will allow the configure script to find out if code is
429 not linking on your system because the feature isn't available on your system.
433 <div class="question">
435 When I compile code using the LLVM GCC front end, it complains that it cannot
442 The only way this can happen is if you haven't installed the runtime library. To
443 correct this, do:</p>
446 % make clean ; make install-bytecode
450 <div class="question">
452 How can I disable all optimizations when compiling code using the LLVM GCC front end?
458 Passing "-Wa,-disable-opt -Wl,-disable-opt" will disable *all* cleanup and
459 optimizations done at the llvm level, leaving you with the truly horrible
460 code that you desire.
464 <!-- *********************************************************************** -->
465 <div class="doc_section">
466 <a name="cfe_code">Questions about code generated by the GCC front-end</a>
469 <div class="question"><p>
470 <a name="__main"></a>
471 What is this <tt>__main()</tt> call that gets inserted into <tt>main()</tt>?
476 The <tt>__main</tt> call is inserted by the C/C++ compiler in order to guarantee
477 that static constructors and destructors are called when the program starts up
478 and shuts down. In C, you can create static constructors and destructors by
479 using GCC extensions, and in C++ you can do so by creating a global variable
480 whose class has a ctor or dtor.
484 The actual implementation of <tt>__main</tt> lives in the
485 <tt>llvm/runtime/GCCLibraries/crtend/</tt> directory in the source-base, and is
486 linked in automatically when you link the program.
490 <!--=========================================================================-->
492 <div class="question">
493 <a name="iosinit"></a>
494 <p> What is this <tt>llvm.global_ctors</tt> and
495 <tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I #include
496 <iostream>?</p>
501 <p>If you #include the <iostream> header into a C++ translation unit, the
502 file will probably use the <tt>std::cin</tt>/<tt>std::cout</tt>/... global
503 objects. However, C++ does not guarantee an order of initialization between
504 static objects in different translation units, so if a static ctor/dtor in your
505 .cpp file used <tt>std::cout</tt>, for example, the object would not necessarily
506 be automatically initialized before your use.</p>
508 <p>To make <tt>std::cout</tt> and friends work correctly in these scenarios, the
509 STL that we use declares a static object that gets created in every translation
510 unit that includes <tt><iostream></tt>. This object has a static
511 constructor and destructor that initializes and destroys the global iostream
512 objects before they could possibly be used in the file. The code that you see
513 in the .ll file corresponds to the constructor and destructor registration code.
516 <p>If you would like to make it easier to <b>understand</b> the LLVM code
517 generated by the compiler in the demo page, consider using <tt>printf()</tt>
518 instead of <tt>iostream</tt>s to print values.</p>
522 <!--=========================================================================-->
524 <div class="question"><p>
525 <a name="codedce"></a>
526 Where did all of my code go??
531 If you are using the LLVM demo page, you may often wonder what happened to all
532 of the code that you typed in. Remember that the demo script is running the
533 code through the LLVM optimizers, so if your code doesn't actually do anything
534 useful, it might all be deleted.
538 To prevent this, make sure that the code is actually needed. For example, if
539 you are computing some expression, return the value from the function instead of
540 leaving it in a local variable. If you really want to constrain the optimizer,
541 you can read from and assign to <tt>volatile</tt> global variables.
545 <!--=========================================================================-->
547 <div class="question"><p>
549 <p>What is this "<tt>undef</tt>" thing that shows up in my code?
554 <a href="LangRef.html#undef"><tt>undef</tt></a> is the LLVM way of representing
555 a value that is not defined. You can get these if you do not initialize a
556 variable before you use it. For example, the C function:</p>
558 <div class="doc_code">
559 <tt>int X() { int i; return i; }</tt>
562 <p>Is compiled to "<tt>ret int undef</tt>" because "i" never has a value
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