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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>
63 <li><a href="GetElementPtr.html">I don't understand the GetElementPtr
64 instruction. Help!</a></li>
67 <li><a href="#cfe">Using the GCC Front End</a>
70 When I compile software that uses a configure script, the configure script
71 thinks my system has all of the header files and libraries it is testing
72 for. How do I get configure to work correctly?
76 When I compile code using the LLVM GCC front end, it complains that it
77 cannot find libcrtend.a.
81 How can I disable all optimizations when compiling code using the LLVM GCC front end?
84 <li><a href="#translatec++">Can I use LLVM to convert C++ code to C code?</a></li>
89 <li><a href="#cfe_code">Questions about code generated by the GCC front-end</a>
91 <li><a href="#__main">What is this <tt>__main()</tt> call that gets inserted into
92 <tt>main()</tt>?</a></li>
93 <li><a href="#iosinit">What is this <tt>llvm.global_ctors</tt> and
94 <tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I
95 #include <iostream>?</a></li>
96 <li><a href="#codedce">Where did all of my code go??</a></li>
97 <li><a href="#undef">What is this "<tt>undef</tt>" thing that shows up in my code?</a></li>
102 <div class="doc_author">
103 <p>Written by <a href="http://llvm.org">The LLVM Team</a></p>
107 <!-- *********************************************************************** -->
108 <div class="doc_section">
109 <a name="license">License</a>
111 <!-- *********************************************************************** -->
113 <div class="question">
114 <p>Why are the LLVM source code and the front-end distributed under different
119 <p>The C/C++ front-ends are based on GCC and must be distributed under the GPL.
120 Our aim is to distribute LLVM source code under a <em>much less restrictive</em>
121 license, in particular one that does not compel users who distribute tools based
122 on modifying the source to redistribute the modified source code as well.</p>
125 <div class="question">
126 <p>Does the University of Illinois Open Source License really qualify as an
127 "open source" license?</p>
131 <p>Yes, the license is <a
132 href="http://www.opensource.org/licenses/UoI-NCSA.php">certified</a> by the Open
133 Source Initiative (OSI).</p>
136 <div class="question">
137 <p>Can I modify LLVM source code and redistribute the modified source?</p>
141 <p>Yes. The modified source distribution must retain the copyright notice and
142 follow the three bulletted conditions listed in the <a
143 href="http://llvm.org/releases/1.3/LICENSE.TXT">LLVM license</a>.</p>
146 <div class="question">
147 <p>Can I modify LLVM source code and redistribute binaries or other tools based
148 on it, without redistributing the source?</p>
152 <p>Yes, this is why we distribute LLVM under a less restrictive license than
153 GPL, as explained in the first question above.</p>
156 <!-- *********************************************************************** -->
157 <div class="doc_section">
158 <a name="source">Source Code</a>
160 <!-- *********************************************************************** -->
162 <div class="question">
163 <p>In what language is LLVM written?</p>
167 <p>All of the LLVM tools and libraries are written in C++ with extensive use of
171 <div class="question">
172 <p>How portable is the LLVM source code?</p>
176 <p>The LLVM source code should be portable to most modern UNIX-like operating
177 systems. Most of the code is written in standard C++ with operating system
178 services abstracted to a support library. The tools required to build and test
179 LLVM have been ported to a plethora of platforms.</p>
181 <p>Some porting problems may exist in the following areas:</p>
185 <li>The GCC front end code is not as portable as the LLVM suite, so it may not
186 compile as well on unsupported platforms.</li>
188 <li>The LLVM build system relies heavily on UNIX shell tools, like the Bourne
189 Shell and sed. Porting to systems without these tools (MacOS 9, Plan 9) will
190 require more effort.</li>
196 <!-- *********************************************************************** -->
197 <div class="doc_section">
198 <a name="build">Build Problems</a>
200 <!-- *********************************************************************** -->
202 <div class="question">
203 <p>When I run configure, it finds the wrong C compiler.</p>
208 <p>The <tt>configure</tt> script attempts to locate first <tt>gcc</tt> and then
209 <tt>cc</tt>, unless it finds compiler paths set in <tt>CC</tt> and <tt>CXX</tt>
210 for the C and C++ compiler, respectively.</p>
212 <p>If <tt>configure</tt> finds the wrong compiler, either adjust your
213 <tt>PATH</tt> environment variable or set <tt>CC</tt> and <tt>CXX</tt>
218 <div class="question">
219 <p>I compile the code, and I get some error about <tt>/localhome</tt>.</p>
224 <p>There are several possible causes for this. The first is that you didn't set
225 a pathname properly when using <tt>configure</tt>, and it defaulted to a
226 pathname that we use on our research machines.</p>
228 <p>Another possibility is that we hardcoded a path in our Makefiles. If you see
229 this, please email the LLVM bug mailing list with the name of the offending
230 Makefile and a description of what is wrong with it.</p>
234 <div class="question">
235 <p>The <tt>configure</tt> script finds the right C compiler, but it uses the
236 LLVM linker from a previous build. What do I do?</p>
240 <p>The <tt>configure</tt> script uses the <tt>PATH</tt> to find executables, so
241 if it's grabbing the wrong linker/assembler/etc, there are two ways to fix
246 <li><p>Adjust your <tt>PATH</tt> environment variable so that the correct
247 program appears first in the <tt>PATH</tt>. This may work, but may not be
248 convenient when you want them <i>first</i> in your path for other
251 <li><p>Run <tt>configure</tt> with an alternative <tt>PATH</tt> that is
252 correct. In a Borne compatible shell, the syntax would be:</p>
254 <p><tt>PATH=[the path without the bad program] ./configure ...</tt></p>
256 <p>This is still somewhat inconvenient, but it allows <tt>configure</tt>
257 to do its work without having to adjust your <tt>PATH</tt>
258 permanently.</p></li>
264 <div class="question">
265 <p>When creating a dynamic library, I get a strange GLIBC error.</p>
269 <p>Under some operating systems (i.e. Linux), libtool does not work correctly if
270 GCC was compiled with the --disable-shared option. To work around this, install
271 your own version of GCC that has shared libraries enabled by default.</p>
274 <div class="question">
275 <p>I've updated my source tree from CVS, and now my build is trying to use a
276 file/directory that doesn't exist.</p>
280 <p>You need to re-run configure in your object directory. When new Makefiles
281 are added to the source tree, they have to be copied over to the object tree in
282 order to be used by the build.</p>
285 <div class="question">
286 <p>I've modified a Makefile in my source tree, but my build tree keeps using the
287 old version. What do I do?</p>
292 <p>If the Makefile already exists in your object tree, you
293 can just run the following command in the top level directory of your object
296 <p><tt>./config.status <relative path to Makefile></tt><p>
298 <p>If the Makefile is new, you will have to modify the configure script to copy
303 <div class="question">
304 <p>I've upgraded to a new version of LLVM, and I get strange build errors.</p>
309 <p>Sometimes, changes to the LLVM source code alters how the build system works.
310 Changes in libtool, autoconf, or header file dependencies are especially prone
311 to this sort of problem.</p>
313 <p>The best thing to try is to remove the old files and re-build. In most
314 cases, this takes care of the problem. To do this, just type <tt>make
315 clean</tt> and then <tt>make</tt> in the directory that fails to build.</p>
319 <div class="question">
320 <p>I've built LLVM and am testing it, but the tests freeze.</p>
325 <p>This is most likely occurring because you built a profile or release
326 (optimized) build of LLVM and have not specified the same information on the
327 <tt>gmake</tt> command line.</p>
329 <p>For example, if you built LLVM with the command:</p>
331 <p><tt>gmake ENABLE_PROFILING=1</tt>
333 <p>...then you must run the tests with the following commands:</p>
335 <p><tt>cd llvm/test<br>gmake ENABLE_PROFILING=1</tt></p>
339 <div class="question">
340 <p>Why do test results differ when I perform different types of builds?</p>
345 <p>The LLVM test suite is dependent upon several features of the LLVM tools and
348 <p>First, the debugging assertions in code are not enabled in optimized or
349 profiling builds. Hence, tests that used to fail may pass.</p>
351 <p>Second, some tests may rely upon debugging options or behavior that is only
352 available in the debug build. These tests will fail in an optimized or profile
357 <div class="question">
358 <p>Compiling LLVM with GCC 3.3.2 fails, what should I do?</p>
362 <p>This is <a href="http://gcc.gnu.org/PR?13392">a bug in GCC</a>, and
363 affects projects other than LLVM. Try upgrading or downgrading your GCC.</p>
366 <div class="question">
367 <p>After CVS update, rebuilding gives the error "No rule to make target".</p>
371 <p>If the error is of the form:</p>
373 <div class="doc_code">
375 gmake[2]: *** No rule to make target `/path/to/somefile', needed by
376 `/path/to/another/file.d'.<br>
381 <p>This may occur anytime files are moved within the CVS repository or removed
382 entirely. In this case, the best solution is to erase all <tt>.d</tt> files,
383 which list dependencies for source files, and rebuild:</p>
385 <div class="doc_code">
388 % rm -f `find . -name \*\.d`
393 <p>In other cases, it may be necessary to run <tt>make clean</tt> before
397 <!-- *********************************************************************** -->
398 <div class="doc_section"><a name="felangs">Source Languages</a></div>
400 <div class="question"><p>
401 <a name="langs">What source languages are supported?</a></p>
404 <p>LLVM currently has full support for C and C++ source languages. These are
405 available through a special version of GCC that LLVM calls the
406 <a href="#cfe">C Front End</a></p>
407 <p>There is an incomplete version of a Java front end available in the
408 <tt>llvm-java</tt> CVS repository. There is no documentation on this yet so
409 you'll need to download the code, compile it, and try it.</p>
410 <p>In the <tt>examples/BFtoLLVM</tt> directory is a translator for the
411 BrainF*** language (2002 Language Specification).</p>
412 <p>In the <tt>projects/Stacker</tt> directory is a compiler and runtime
413 library for the Stacker language, a "toy" language loosely based on Forth.</p>
414 <p>The PyPy developers are working on integrating LLVM into the PyPy backend
415 so that PyPy language can translate to LLVM.</p>
417 <div class="question"><a name="langhlsupp">
418 <p>What support is there for a higher level source language constructs for
419 building a compiler?</a></p>
422 <p>Currently, there isn't much. LLVM supports an intermediate representation
423 which is useful for code representation but will not support the high level
424 (abstract syntax tree) representation needed by most compilers. There are no
425 facilities for lexical nor semantic analysis. There is, however, a <i>mostly
426 implemented</i> configuration-driven
427 <a href="CompilerDriver.html">compiler driver</a> which simplifies the task
428 of running optimizations, linking, and executable generation.</p>
431 <div class="question"><a name="langhlsupp">
432 <p>I don't understand the GetElementPtr
433 instruction. Help!</a></p>
436 <p>See <a href="GetElementPtr.html">The Often Misunderstood GEP
437 Instruction</a>.</li>
440 <!-- *********************************************************************** -->
441 <div class="doc_section">
442 <a name="cfe">Using the GCC Front End</a>
445 <div class="question">
447 When I compile software that uses a configure script, the configure script
448 thinks my system has all of the header files and libraries it is testing for.
449 How do I get configure to work correctly?
455 The configure script is getting things wrong because the LLVM linker allows
456 symbols to be undefined at link time (so that they can be resolved during JIT
457 or translation to the C back end). That is why configure thinks your system
461 To work around this, perform the following steps:
466 Make sure the CC and CXX environment variables contains the full path to the
471 Make sure that the regular C compiler is first in your PATH.
475 Add the string "-Wl,-native" to your CFLAGS environment variable.
480 This will allow the gccld linker to create a native code executable instead of
481 a shell script that runs the JIT. Creating native code requires standard
482 linkage, which in turn will allow the configure script to find out if code is
483 not linking on your system because the feature isn't available on your system.
487 <div class="question">
489 When I compile code using the LLVM GCC front end, it complains that it cannot
496 The only way this can happen is if you haven't installed the runtime library. To
497 correct this, do:</p>
500 % make clean ; make install-bytecode
504 <div class="question">
506 How can I disable all optimizations when compiling code using the LLVM GCC front end?
512 Passing "-Wa,-disable-opt -Wl,-disable-opt" will disable *all* cleanup and
513 optimizations done at the llvm level, leaving you with the truly horrible
514 code that you desire.
519 <div class="question">
521 <a name="translatec++">Can I use LLVM to convert C++ code to C code?</a>
526 <p>Yes, you can use LLVM to convert code from any language LLVM supports to C.
527 Note that the generated C code will be very low level (all loops are lowered
528 to gotos, etc) and not very pretty (comments are stripped, original source
529 formatting is totally lost, variables are renamed, expressions are regrouped),
530 so this may not be what you're looking for. However, this is a good way to add
531 C++ support for a processor that does not otherwise have a C++ compiler.
534 <p>Use commands like this:</p>
537 <li><p>Compile your program as normal with llvm-g++:</p></li>
539 <div class="doc_code">$ llvm-g++ x.cpp -o program</div>
543 <div class="doc_code">
544 llvm-g++ a.cpp -c<br>
545 llvm-g++ b.cpp -c<br>
546 llvm-g++ a.o b.o -o program
549 <p>With llvm-gcc3, this will generate program and program.bc. The .bc file is
550 the LLVM version of the program all linked together.</p>
552 <li><p>Convert the LLVM code to C code, using the LLC tool with the C
555 <div class="doc_code">$ llc -march=c program.bc -o program.c</div>
557 <li><p>Finally, compile the c file:</p></li>
559 <div class="doc_code">$ cc x.c</div>
563 <p>Note that, by default, the C backend does not support exception handling.
564 If you want/need it for a certain program, you can enable it by passing
565 "-enable-correct-eh-support" to the llc program. The resultant code will
566 use setjmp/longjmp to implement exception support that is correct but
570 <p>Also note: this specific sequence of commands won't work if you use a
571 function defined in the C++ runtime library (or any other C++ library). To
572 access an external C++ library, you must manually
573 compile libstdc++ to LLVM bytecode, statically link it into your program, then
574 use the commands above to convert the whole result into C code. Alternatively,
575 you can compile the libraries and your application into two different chunks
576 of C code and link them.</p>
582 <!-- *********************************************************************** -->
583 <div class="doc_section">
584 <a name="cfe_code">Questions about code generated by the GCC front-end</a>
587 <div class="question"><p>
588 <a name="__main"></a>
589 What is this <tt>__main()</tt> call that gets inserted into <tt>main()</tt>?
594 The <tt>__main</tt> call is inserted by the C/C++ compiler in order to guarantee
595 that static constructors and destructors are called when the program starts up
596 and shuts down. In C, you can create static constructors and destructors by
597 using GCC extensions, and in C++ you can do so by creating a global variable
598 whose class has a ctor or dtor.
602 The actual implementation of <tt>__main</tt> lives in the
603 <tt>llvm/runtime/GCCLibraries/crtend/</tt> directory in the source-base, and is
604 linked in automatically when you link the program.
608 <!--=========================================================================-->
610 <div class="question">
611 <a name="iosinit"></a>
612 <p> What is this <tt>llvm.global_ctors</tt> and
613 <tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I #include
614 <iostream>?</p>
619 <p>If you #include the <iostream> header into a C++ translation unit, the
620 file will probably use the <tt>std::cin</tt>/<tt>std::cout</tt>/... global
621 objects. However, C++ does not guarantee an order of initialization between
622 static objects in different translation units, so if a static ctor/dtor in your
623 .cpp file used <tt>std::cout</tt>, for example, the object would not necessarily
624 be automatically initialized before your use.</p>
626 <p>To make <tt>std::cout</tt> and friends work correctly in these scenarios, the
627 STL that we use declares a static object that gets created in every translation
628 unit that includes <tt><iostream></tt>. This object has a static
629 constructor and destructor that initializes and destroys the global iostream
630 objects before they could possibly be used in the file. The code that you see
631 in the .ll file corresponds to the constructor and destructor registration code.
634 <p>If you would like to make it easier to <b>understand</b> the LLVM code
635 generated by the compiler in the demo page, consider using <tt>printf()</tt>
636 instead of <tt>iostream</tt>s to print values.</p>
640 <!--=========================================================================-->
642 <div class="question"><p>
643 <a name="codedce"></a>
644 Where did all of my code go??
649 If you are using the LLVM demo page, you may often wonder what happened to all
650 of the code that you typed in. Remember that the demo script is running the
651 code through the LLVM optimizers, so if your code doesn't actually do anything
652 useful, it might all be deleted.
656 To prevent this, make sure that the code is actually needed. For example, if
657 you are computing some expression, return the value from the function instead of
658 leaving it in a local variable. If you really want to constrain the optimizer,
659 you can read from and assign to <tt>volatile</tt> global variables.
663 <!--=========================================================================-->
665 <div class="question"><p>
667 <p>What is this "<tt>undef</tt>" thing that shows up in my code?
672 <a href="LangRef.html#undef"><tt>undef</tt></a> is the LLVM way of representing
673 a value that is not defined. You can get these if you do not initialize a
674 variable before you use it. For example, the C function:</p>
676 <div class="doc_code">
677 <tt>int X() { int i; return i; }</tt>
680 <p>Is compiled to "<tt>ret int undef</tt>" because "i" never has a value
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