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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="#felangs">Source Languages</a>
60 <li><a href="#langs">What source languages are supported?</a></li>
61 <li><a href="#langhlsupp">What support is there for higher level source
62 language constructs for building a compiler?</a></li>
65 <li><a href="#cfe">Using the GCC Front End</a>
68 When I compile software that uses a configure script, the configure script
69 thinks my system has all of the header files and libraries it is testing
70 for. How do I get configure to work correctly?
74 When I compile code using the LLVM GCC front end, it complains that it
75 cannot find libcrtend.a.
79 How can I disable all optimizations when compiling code using the LLVM GCC front end?
85 <li><a href="#cfe_code">Questions about code generated by the GCC front-end</a>
87 <li><a href="#__main">What is this <tt>__main()</tt> call that gets inserted into
88 <tt>main()</tt>?</a></li>
89 <li><a href="#iosinit">What is this <tt>llvm.global_ctors</tt> and
90 <tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I
91 #include <iostream>?</a></li>
92 <li><a href="#codedce">Where did all of my code go??</a></li>
93 <li><a href="#undef">What is this "<tt>undef</tt>" thing that shows up in my code?</a></li>
98 <div class="doc_author">
99 <p>Written by <a href="http://llvm.org">The LLVM Team</a></p>
103 <!-- *********************************************************************** -->
104 <div class="doc_section">
105 <a name="license">License</a>
107 <!-- *********************************************************************** -->
109 <div class="question">
110 <p>Why are the LLVM source code and the front-end distributed under different
115 <p>The C/C++ front-ends are based on GCC and must be distributed under the GPL.
116 Our aim is to distribute LLVM source code under a <em>much less restrictive</em>
117 license, in particular one that does not compel users who distribute tools based
118 on modifying the source to redistribute the modified source code as well.</p>
121 <div class="question">
122 <p>Does the University of Illinois Open Source License really qualify as an
123 "open source" license?</p>
127 <p>Yes, the license is <a
128 href="http://www.opensource.org/licenses/UoI-NCSA.php">certified</a> by the Open
129 Source Initiative (OSI).</p>
132 <div class="question">
133 <p>Can I modify LLVM source code and redistribute the modified source?</p>
137 <p>Yes. The modified source distribution must retain the copyright notice and
138 follow the three bulletted conditions listed in the <a
139 href="http://llvm.org/releases/1.3/LICENSE.TXT">LLVM license</a>.</p>
142 <div class="question">
143 <p>Can I modify LLVM source code and redistribute binaries or other tools based
144 on it, without redistributing the source?</p>
148 <p>Yes, this is why we distribute LLVM under a less restrictive license than
149 GPL, as explained in the first question above.</p>
152 <!-- *********************************************************************** -->
153 <div class="doc_section">
154 <a name="source">Source Code</a>
156 <!-- *********************************************************************** -->
158 <div class="question">
159 <p>In what language is LLVM written?</p>
163 <p>All of the LLVM tools and libraries are written in C++ with extensive use of
167 <div class="question">
168 <p>How portable is the LLVM source code?</p>
172 <p>The LLVM source code should be portable to most modern UNIX-like operating
173 systems. Most of the code is written in standard C++ with operating system
174 services abstracted to a support library. The tools required to build and test
175 LLVM have been ported to a plethora of platforms.</p>
177 <p>Some porting problems may exist in the following areas:</p>
181 <li>The GCC front end code is not as portable as the LLVM suite, so it may not
182 compile as well on unsupported platforms.</li>
184 <li>The LLVM build system relies heavily on UNIX shell tools, like the Bourne
185 Shell and sed. Porting to systems without these tools (MacOS 9, Plan 9) will
186 require more effort.</li>
192 <!-- *********************************************************************** -->
193 <div class="doc_section">
194 <a name="build">Build Problems</a>
196 <!-- *********************************************************************** -->
198 <div class="question">
199 <p>When I run configure, it finds the wrong C compiler.</p>
204 <p>The <tt>configure</tt> script attempts to locate first <tt>gcc</tt> and then
205 <tt>cc</tt>, unless it finds compiler paths set in <tt>CC</tt> and <tt>CXX</tt>
206 for the C and C++ compiler, respectively.</p>
208 <p>If <tt>configure</tt> finds the wrong compiler, either adjust your
209 <tt>PATH</tt> environment variable or set <tt>CC</tt> and <tt>CXX</tt>
214 <div class="question">
215 <p>I compile the code, and I get some error about <tt>/localhome</tt>.</p>
220 <p>There are several possible causes for this. The first is that you didn't set
221 a pathname properly when using <tt>configure</tt>, and it defaulted to a
222 pathname that we use on our research machines.</p>
224 <p>Another possibility is that we hardcoded a path in our Makefiles. If you see
225 this, please email the LLVM bug mailing list with the name of the offending
226 Makefile and a description of what is wrong with it.</p>
230 <div class="question">
231 <p>The <tt>configure</tt> script finds the right C compiler, but it uses the
232 LLVM linker from a previous build. What do I do?</p>
236 <p>The <tt>configure</tt> script uses the <tt>PATH</tt> to find executables, so
237 if it's grabbing the wrong linker/assembler/etc, there are two ways to fix
242 <li><p>Adjust your <tt>PATH</tt> environment variable so that the correct
243 program appears first in the <tt>PATH</tt>. This may work, but may not be
244 convenient when you want them <i>first</i> in your path for other
247 <li><p>Run <tt>configure</tt> with an alternative <tt>PATH</tt> that is
248 correct. In a Borne compatible shell, the syntax would be:</p>
250 <p><tt>PATH=[the path without the bad program] ./configure ...</tt></p>
252 <p>This is still somewhat inconvenient, but it allows <tt>configure</tt>
253 to do its work without having to adjust your <tt>PATH</tt>
254 permanently.</p></li>
260 <div class="question">
261 <p>When creating a dynamic library, I get a strange GLIBC error.</p>
265 <p>Under some operating systems (i.e. Linux), libtool does not work correctly if
266 GCC was compiled with the --disable-shared option. To work around this, install
267 your own version of GCC that has shared libraries enabled by default.</p>
270 <div class="question">
271 <p>I've updated my source tree from CVS, and now my build is trying to use a
272 file/directory that doesn't exist.</p>
276 <p>You need to re-run configure in your object directory. When new Makefiles
277 are added to the source tree, they have to be copied over to the object tree in
278 order to be used by the build.</p>
281 <div class="question">
282 <p>I've modified a Makefile in my source tree, but my build tree keeps using the
283 old version. What do I do?</p>
288 <p>If the Makefile already exists in your object tree, you
289 can just run the following command in the top level directory of your object
292 <p><tt>./config.status <relative path to Makefile></tt><p>
294 <p>If the Makefile is new, you will have to modify the configure script to copy
299 <div class="question">
300 <p>I've upgraded to a new version of LLVM, and I get strange build errors.</p>
305 <p>Sometimes, changes to the LLVM source code alters how the build system works.
306 Changes in libtool, autoconf, or header file dependencies are especially prone
307 to this sort of problem.</p>
309 <p>The best thing to try is to remove the old files and re-build. In most
310 cases, this takes care of the problem. To do this, just type <tt>make
311 clean</tt> and then <tt>make</tt> in the directory that fails to build.</p>
315 <div class="question">
316 <p>I've built LLVM and am testing it, but the tests freeze.</p>
321 <p>This is most likely occurring because you built a profile or release
322 (optimized) build of LLVM and have not specified the same information on the
323 <tt>gmake</tt> command line.</p>
325 <p>For example, if you built LLVM with the command:</p>
327 <p><tt>gmake ENABLE_PROFILING=1</tt>
329 <p>...then you must run the tests with the following commands:</p>
331 <p><tt>cd llvm/test<br>gmake ENABLE_PROFILING=1</tt></p>
335 <div class="question">
336 <p>Why do test results differ when I perform different types of builds?</p>
341 <p>The LLVM test suite is dependent upon several features of the LLVM tools and
344 <p>First, the debugging assertions in code are not enabled in optimized or
345 profiling builds. Hence, tests that used to fail may pass.</p>
347 <p>Second, some tests may rely upon debugging options or behavior that is only
348 available in the debug build. These tests will fail in an optimized or profile
353 <div class="question">
354 <p>Compiling LLVM with GCC 3.3.2 fails, what should I do?</p>
358 <p>This is <a href="http://gcc.gnu.org/PR?13392">a bug in GCC</a>, and
359 affects projects other than LLVM. Try upgrading or downgrading your GCC.</p>
362 <div class="question">
363 <p>After CVS update, rebuilding gives the error "No rule to make target".</p>
367 <p>If the error is of the form:</p>
369 <div class="doc_code">
371 gmake[2]: *** No rule to make target `/path/to/somefile', needed by
372 `/path/to/another/file.d'.<br>
377 <p>This may occur anytime files are moved within the CVS repository or removed
378 entirely. In this case, the best solution is to erase all <tt>.d</tt> files,
379 which list dependencies for source files, and rebuild:</p>
381 <div class="doc_code">
384 % rm -f `find . -name \*\.d`
389 <p>In other cases, it may be necessary to run <tt>make clean</tt> before
393 <!-- *********************************************************************** -->
394 <div class="doc_section"><a name="felangs">Source Languages</a></div>
396 <div class="question"><p>
397 <a name="langs">What source languages are supported?</a></p>
400 <p>LLVM currently has full support for C and C++ source languages. These are
401 available through a special version of GCC that LLVM calls the
402 <a href="#cfe">C Front End</a></p>
403 <p>There is an incomplete version of a Java front end available in the
404 <tt>llvm-java</tt> CVS repository. There is no documentation on this yet so
405 you'll need to download the code, compile it, and try it.</p>
406 <p>In the <tt>examples/BFtoLLVM</tt> directory is a translator for the
407 BrainF*** language (2002 Language Specification).</p>
408 <p>In the <tt>projects/Stacker</tt> directory is a compiler and runtime
409 library for the Stacker language, a "toy" language loosely based on Forth.</p>
410 <p>The PyPy developers are working on integrating LLVM into the PyPy backend
411 so that PyPy language can translate to LLVM.</p>
413 <div class="question"><a name="langhlsupp">
414 <p>What support is there for a higher level source language constructs for
415 building a compiler?</a></p>
418 <p>Currently, there isn't much. LLVM supports an intermediate representation
419 which is useful for code representation but will not support the high level
420 (abstract syntax tree) representation needed by most compilers. There are no
421 facilities for lexical nor semantic analysis. There is, however, a <i>mostly
422 implemented</i> configuration-driven
423 <a href="CompilerDriver.html">compiler driver</a> which simplifies the task
424 of running optimizations, linking, and executable generation.</p>
426 <!-- *********************************************************************** -->
427 <div class="doc_section">
428 <a name="cfe">Using the GCC Front End</a>
431 <div class="question">
433 When I compile software that uses a configure script, the configure script
434 thinks my system has all of the header files and libraries it is testing for.
435 How do I get configure to work correctly?
441 The configure script is getting things wrong because the LLVM linker allows
442 symbols to be undefined at link time (so that they can be resolved during JIT
443 or translation to the C back end). That is why configure thinks your system
447 To work around this, perform the following steps:
452 Make sure the CC and CXX environment variables contains the full path to the
457 Make sure that the regular C compiler is first in your PATH.
461 Add the string "-Wl,-native" to your CFLAGS environment variable.
466 This will allow the gccld linker to create a native code executable instead of
467 a shell script that runs the JIT. Creating native code requires standard
468 linkage, which in turn will allow the configure script to find out if code is
469 not linking on your system because the feature isn't available on your system.
473 <div class="question">
475 When I compile code using the LLVM GCC front end, it complains that it cannot
482 The only way this can happen is if you haven't installed the runtime library. To
483 correct this, do:</p>
486 % make clean ; make install-bytecode
490 <div class="question">
492 How can I disable all optimizations when compiling code using the LLVM GCC front end?
498 Passing "-Wa,-disable-opt -Wl,-disable-opt" will disable *all* cleanup and
499 optimizations done at the llvm level, leaving you with the truly horrible
500 code that you desire.
504 <!-- *********************************************************************** -->
505 <div class="doc_section">
506 <a name="cfe_code">Questions about code generated by the GCC front-end</a>
509 <div class="question"><p>
510 <a name="__main"></a>
511 What is this <tt>__main()</tt> call that gets inserted into <tt>main()</tt>?
516 The <tt>__main</tt> call is inserted by the C/C++ compiler in order to guarantee
517 that static constructors and destructors are called when the program starts up
518 and shuts down. In C, you can create static constructors and destructors by
519 using GCC extensions, and in C++ you can do so by creating a global variable
520 whose class has a ctor or dtor.
524 The actual implementation of <tt>__main</tt> lives in the
525 <tt>llvm/runtime/GCCLibraries/crtend/</tt> directory in the source-base, and is
526 linked in automatically when you link the program.
530 <!--=========================================================================-->
532 <div class="question">
533 <a name="iosinit"></a>
534 <p> What is this <tt>llvm.global_ctors</tt> and
535 <tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I #include
536 <iostream>?</p>
541 <p>If you #include the <iostream> header into a C++ translation unit, the
542 file will probably use the <tt>std::cin</tt>/<tt>std::cout</tt>/... global
543 objects. However, C++ does not guarantee an order of initialization between
544 static objects in different translation units, so if a static ctor/dtor in your
545 .cpp file used <tt>std::cout</tt>, for example, the object would not necessarily
546 be automatically initialized before your use.</p>
548 <p>To make <tt>std::cout</tt> and friends work correctly in these scenarios, the
549 STL that we use declares a static object that gets created in every translation
550 unit that includes <tt><iostream></tt>. This object has a static
551 constructor and destructor that initializes and destroys the global iostream
552 objects before they could possibly be used in the file. The code that you see
553 in the .ll file corresponds to the constructor and destructor registration code.
556 <p>If you would like to make it easier to <b>understand</b> the LLVM code
557 generated by the compiler in the demo page, consider using <tt>printf()</tt>
558 instead of <tt>iostream</tt>s to print values.</p>
562 <!--=========================================================================-->
564 <div class="question"><p>
565 <a name="codedce"></a>
566 Where did all of my code go??
571 If you are using the LLVM demo page, you may often wonder what happened to all
572 of the code that you typed in. Remember that the demo script is running the
573 code through the LLVM optimizers, so if your code doesn't actually do anything
574 useful, it might all be deleted.
578 To prevent this, make sure that the code is actually needed. For example, if
579 you are computing some expression, return the value from the function instead of
580 leaving it in a local variable. If you really want to constrain the optimizer,
581 you can read from and assign to <tt>volatile</tt> global variables.
585 <!--=========================================================================-->
587 <div class="question"><p>
589 <p>What is this "<tt>undef</tt>" thing that shows up in my code?
594 <a href="LangRef.html#undef"><tt>undef</tt></a> is the LLVM way of representing
595 a value that is not defined. You can get these if you do not initialize a
596 variable before you use it. For example, the C function:</p>
598 <div class="doc_code">
599 <tt>int X() { int i; return i; }</tt>
602 <p>Is compiled to "<tt>ret int undef</tt>" because "i" never has a value
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