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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>The <tt>configure</tt> script finds the right C compiler, but it uses the
40 LLVM linker from a previous build. What do I do?</li>
41 <li>When creating a dynamic library, I get a strange GLIBC error.</li>
42 <li>I've updated my source tree from Subversion, and now my build is trying
43 to use a file/directory that doesn't exist.</li>
44 <li>I've modified a Makefile in my source tree, but my build tree keeps using
45 the old version. What do I do?</li>
46 <li>I've upgraded to a new version of LLVM, and I get strange build
48 <li>I've built LLVM and am testing it, but the tests freeze.</li>
49 <li>Why do test results differ when I perform different types of builds?</li>
50 <li>Compiling LLVM with GCC 3.3.2 fails, what should I do?</li>
51 <li>When I use the test suite, all of the C Backend tests fail. What is
53 <li>After Subversion update, rebuilding gives the error "No rule to make
55 <li><a href="#llvmc">The <tt>llvmc</tt> program gives me errors/doesn't
59 <li><a href="#felangs">Source Languages</a>
61 <li><a href="#langs">What source languages are supported?</a></li>
62 <li><a href="#langhlsupp">What support is there for higher level source
63 language constructs for building a compiler?</a></li>
64 <li><a href="GetElementPtr.html">I don't understand the GetElementPtr
65 instruction. Help!</a></li>
68 <li><a href="#cfe">Using the GCC Front End</a>
71 When I compile software that uses a configure script, the configure script
72 thinks my system has all of the header files and libraries it is testing
73 for. How do I get configure to work correctly?
77 When I compile code using the LLVM GCC front end, it complains that it
78 cannot find libcrtend.a.
82 How can I disable all optimizations when compiling code using the LLVM GCC front end?
85 <li><a href="#translatec++">Can I use LLVM to convert C++ code to C code?</a></li>
90 <li><a href="#cfe_code">Questions about code generated by the GCC front-end</a>
92 <li><a href="#__main">What is this <tt>__main()</tt> call that gets inserted into
93 <tt>main()</tt>?</a></li>
94 <li><a href="#iosinit">What is this <tt>llvm.global_ctors</tt> and
95 <tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I
96 #include <iostream>?</a></li>
97 <li><a href="#codedce">Where did all of my code go??</a></li>
98 <li><a href="#undef">What is this "<tt>undef</tt>" thing that shows up in my code?</a></li>
103 <div class="doc_author">
104 <p>Written by <a href="http://llvm.org">The LLVM Team</a></p>
108 <!-- *********************************************************************** -->
109 <div class="doc_section">
110 <a name="license">License</a>
112 <!-- *********************************************************************** -->
114 <div class="question">
115 <p>Why are the LLVM source code and the front-end distributed under different
120 <p>The C/C++ front-ends are based on GCC and must be distributed under the GPL.
121 Our aim is to distribute LLVM source code under a <em>much less restrictive</em>
122 license, in particular one that does not compel users who distribute tools based
123 on modifying the source to redistribute the modified source code as well.</p>
126 <div class="question">
127 <p>Does the University of Illinois Open Source License really qualify as an
128 "open source" license?</p>
132 <p>Yes, the license is <a
133 href="http://www.opensource.org/licenses/UoI-NCSA.php">certified</a> by the Open
134 Source Initiative (OSI).</p>
137 <div class="question">
138 <p>Can I modify LLVM source code and redistribute the modified source?</p>
142 <p>Yes. The modified source distribution must retain the copyright notice and
143 follow the three bulletted conditions listed in the <a
144 href="http://llvm.org/releases/1.3/LICENSE.TXT">LLVM license</a>.</p>
147 <div class="question">
148 <p>Can I modify LLVM source code and redistribute binaries or other tools based
149 on it, without redistributing the source?</p>
153 <p>Yes, this is why we distribute LLVM under a less restrictive license than
154 GPL, as explained in the first question above.</p>
157 <!-- *********************************************************************** -->
158 <div class="doc_section">
159 <a name="source">Source Code</a>
161 <!-- *********************************************************************** -->
163 <div class="question">
164 <p>In what language is LLVM written?</p>
168 <p>All of the LLVM tools and libraries are written in C++ with extensive use of
172 <div class="question">
173 <p>How portable is the LLVM source code?</p>
177 <p>The LLVM source code should be portable to most modern UNIX-like operating
178 systems. Most of the code is written in standard C++ with operating system
179 services abstracted to a support library. The tools required to build and test
180 LLVM have been ported to a plethora of platforms.</p>
182 <p>Some porting problems may exist in the following areas:</p>
186 <li>The GCC front end code is not as portable as the LLVM suite, so it may not
187 compile as well on unsupported platforms.</li>
189 <li>The LLVM build system relies heavily on UNIX shell tools, like the Bourne
190 Shell and sed. Porting to systems without these tools (MacOS 9, Plan 9) will
191 require more effort.</li>
197 <!-- *********************************************************************** -->
198 <div class="doc_section">
199 <a name="build">Build Problems</a>
201 <!-- *********************************************************************** -->
203 <div class="question">
204 <p>When I run configure, it finds the wrong C compiler.</p>
209 <p>The <tt>configure</tt> script attempts to locate first <tt>gcc</tt> and then
210 <tt>cc</tt>, unless it finds compiler paths set in <tt>CC</tt> and <tt>CXX</tt>
211 for the C and C++ compiler, respectively.</p>
213 <p>If <tt>configure</tt> finds the wrong compiler, either adjust your
214 <tt>PATH</tt> environment variable or set <tt>CC</tt> and <tt>CXX</tt>
219 <div class="question">
220 <p>The <tt>configure</tt> script finds the right C compiler, but it uses the
221 LLVM linker from a previous build. What do I do?</p>
225 <p>The <tt>configure</tt> script uses the <tt>PATH</tt> to find executables, so
226 if it's grabbing the wrong linker/assembler/etc, there are two ways to fix
231 <li><p>Adjust your <tt>PATH</tt> environment variable so that the correct
232 program appears first in the <tt>PATH</tt>. This may work, but may not be
233 convenient when you want them <i>first</i> in your path for other
236 <li><p>Run <tt>configure</tt> with an alternative <tt>PATH</tt> that is
237 correct. In a Borne compatible shell, the syntax would be:</p>
239 <p><tt>PATH=[the path without the bad program] ./configure ...</tt></p>
241 <p>This is still somewhat inconvenient, but it allows <tt>configure</tt>
242 to do its work without having to adjust your <tt>PATH</tt>
243 permanently.</p></li>
249 <div class="question">
250 <p>When creating a dynamic library, I get a strange GLIBC error.</p>
254 <p>Under some operating systems (i.e. Linux), libtool does not work correctly if
255 GCC was compiled with the --disable-shared option. To work around this, install
256 your own version of GCC that has shared libraries enabled by default.</p>
259 <div class="question">
260 <p>I've updated my source tree from Subversion, and now my build is trying to
261 use a file/directory that doesn't exist.</p>
265 <p>You need to re-run configure in your object directory. When new Makefiles
266 are added to the source tree, they have to be copied over to the object tree in
267 order to be used by the build.</p>
270 <div class="question">
271 <p>I've modified a Makefile in my source tree, but my build tree keeps using the
272 old version. What do I do?</p>
277 <p>If the Makefile already exists in your object tree, you
278 can just run the following command in the top level directory of your object
281 <p><tt>./config.status <relative path to Makefile></tt><p>
283 <p>If the Makefile is new, you will have to modify the configure script to copy
288 <div class="question">
289 <p>I've upgraded to a new version of LLVM, and I get strange build errors.</p>
294 <p>Sometimes, changes to the LLVM source code alters how the build system works.
295 Changes in libtool, autoconf, or header file dependencies are especially prone
296 to this sort of problem.</p>
298 <p>The best thing to try is to remove the old files and re-build. In most
299 cases, this takes care of the problem. To do this, just type <tt>make
300 clean</tt> and then <tt>make</tt> in the directory that fails to build.</p>
304 <div class="question">
305 <p>I've built LLVM and am testing it, but the tests freeze.</p>
310 <p>This is most likely occurring because you built a profile or release
311 (optimized) build of LLVM and have not specified the same information on the
312 <tt>gmake</tt> command line.</p>
314 <p>For example, if you built LLVM with the command:</p>
316 <div class="doc_code">
318 % gmake ENABLE_PROFILING=1
322 <p>...then you must run the tests with the following commands:</p>
324 <div class="doc_code">
327 % gmake ENABLE_PROFILING=1
333 <div class="question">
334 <p>Why do test results differ when I perform different types of builds?</p>
339 <p>The LLVM test suite is dependent upon several features of the LLVM tools and
342 <p>First, the debugging assertions in code are not enabled in optimized or
343 profiling builds. Hence, tests that used to fail may pass.</p>
345 <p>Second, some tests may rely upon debugging options or behavior that is only
346 available in the debug build. These tests will fail in an optimized or profile
351 <div class="question">
352 <p>Compiling LLVM with GCC 3.3.2 fails, what should I do?</p>
356 <p>This is <a href="http://gcc.gnu.org/PR?13392">a bug in GCC</a>, and
357 affects projects other than LLVM. Try upgrading or downgrading your GCC.</p>
360 <div class="question">
361 <p>After Subversion update, rebuilding gives the error
362 "No rule to make target".</p>
366 <p>If the error is of the form:</p>
368 <div class="doc_code">
370 gmake[2]: *** No rule to make target `/path/to/somefile', needed by
371 `/path/to/another/file.d'.<br>
376 <p>This may occur anytime files are moved within the Subversion repository or
377 removed entirely. In this case, the best solution is to erase all
378 <tt>.d</tt> files, which list dependencies for source files, and rebuild:</p>
380 <div class="doc_code">
383 % rm -f `find . -name \*\.d`
388 <p>In other cases, it may be necessary to run <tt>make clean</tt> before
392 <div class="question">
393 <a name="llvmc"<p>The <tt>llvmc</tt> program gives me errors/doesn't
398 <p><tt>llvmc</tt> is experimental and isn't really supported. We suggest
399 using <tt>llvm-gcc</tt> instead.</p>
402 <!-- *********************************************************************** -->
403 <div class="doc_section"><a name="felangs">Source Languages</a></div>
405 <div class="question"><p>
406 <a name="langs">What source languages are supported?</a></p>
409 <p>LLVM currently has full support for C and C++ source languages. These are
410 available through a special version of GCC that LLVM calls the
411 <a href="#cfe">C Front End</a></p>
412 <p>There is an incomplete version of a Java front end available in the
413 <tt>java</tt> module. There is no documentation on this yet so
414 you'll need to download the code, compile it, and try it.</p>
415 <p>In the <tt>stacker</tt> module is a compiler and runtime
416 library for the Stacker language, a "toy" language loosely based on Forth.</p>
417 <p>The PyPy developers are working on integrating LLVM into the PyPy backend
418 so that PyPy language can translate to LLVM.</p>
420 <div class="question"><a name="langhlsupp">
421 <p>What support is there for a higher level source language constructs for
422 building a compiler?</a></p>
425 <p>Currently, there isn't much. LLVM supports an intermediate representation
426 which is useful for code representation but will not support the high level
427 (abstract syntax tree) representation needed by most compilers. There are no
428 facilities for lexical nor semantic analysis. There is, however, a <i>mostly
429 implemented</i> configuration-driven
430 <a href="CompilerDriver.html">compiler driver</a> which simplifies the task
431 of running optimizations, linking, and executable generation.</p>
434 <div class="question"><a name="langhlsupp">
435 <p>I don't understand the GetElementPtr
436 instruction. Help!</a></p>
439 <p>See <a href="GetElementPtr.html">The Often Misunderstood GEP
440 Instruction</a>.</li>
443 <!-- *********************************************************************** -->
444 <div class="doc_section">
445 <a name="cfe">Using the GCC Front End</a>
448 <div class="question">
450 When I compile software that uses a configure script, the configure script
451 thinks my system has all of the header files and libraries it is testing for.
452 How do I get configure to work correctly?
458 The configure script is getting things wrong because the LLVM linker allows
459 symbols to be undefined at link time (so that they can be resolved during JIT
460 or translation to the C back end). That is why configure thinks your system
464 To work around this, perform the following steps:
467 <li>Make sure the CC and CXX environment variables contains the full path to
468 the LLVM GCC front end.</li>
470 <li>Make sure that the regular C compiler is first in your PATH. </li>
472 <li>Add the string "-Wl,-native" to your CFLAGS environment variable.</li>
476 This will allow the <tt>llvm-ld</tt> linker to create a native code executable
477 instead of shell script that runs the JIT. Creating native code requires
478 standard linkage, which in turn will allow the configure script to find out if
479 code is not linking on your system because the feature isn't available on your
483 <div class="question">
485 When I compile code using the LLVM GCC front end, it complains that it cannot
492 The only way this can happen is if you haven't installed the runtime library. To
493 correct this, do:</p>
495 <div class="doc_code">
498 % make clean ; make install-bytecode
503 <div class="question">
505 How can I disable all optimizations when compiling code using the LLVM GCC front end?
511 Passing "-Wa,-disable-opt -Wl,-disable-opt" will disable *all* cleanup and
512 optimizations done at the llvm level, leaving you with the truly horrible
513 code that you desire.
518 <div class="question">
520 <a name="translatec++">Can I use LLVM to convert C++ code to C code?</a>
525 <p>Yes, you can use LLVM to convert code from any language LLVM supports to C.
526 Note that the generated C code will be very low level (all loops are lowered
527 to gotos, etc) and not very pretty (comments are stripped, original source
528 formatting is totally lost, variables are renamed, expressions are regrouped),
529 so this may not be what you're looking for. However, this is a good way to add
530 C++ support for a processor that does not otherwise have a C++ compiler.
533 <p>Use commands like this:</p>
536 <li><p>Compile your program as normal with llvm-g++:</p></li>
538 <div class="doc_code">
540 % llvm-g++ x.cpp -o program
546 <div class="doc_code">
550 % llvm-g++ a.o b.o -o program
554 <p>With llvm-gcc3, this will generate program and program.bc. The .bc file is
555 the LLVM version of the program all linked together.</p>
557 <li><p>Convert the LLVM code to C code, using the LLC tool with the C
560 <div class="doc_code">
562 % llc -march=c program.bc -o program.c
566 <li><p>Finally, compile the c file:</p></li>
568 <div class="doc_code">
576 <p>Note that, by default, the C backend does not support exception handling.
577 If you want/need it for a certain program, you can enable it by passing
578 "-enable-correct-eh-support" to the llc program. The resultant code will
579 use setjmp/longjmp to implement exception support that is correct but
583 <p>Also note: this specific sequence of commands won't work if you use a
584 function defined in the C++ runtime library (or any other C++ library). To
585 access an external C++ library, you must manually
586 compile libstdc++ to LLVM bitcode, statically link it into your program, then
587 use the commands above to convert the whole result into C code. Alternatively,
588 you can compile the libraries and your application into two different chunks
589 of C code and link them.</p>
595 <!-- *********************************************************************** -->
596 <div class="doc_section">
597 <a name="cfe_code">Questions about code generated by the GCC front-end</a>
600 <div class="question"><p>
601 <a name="__main"></a>
602 What is this <tt>__main()</tt> call that gets inserted into <tt>main()</tt>?
607 The <tt>__main</tt> call is inserted by the C/C++ compiler in order to guarantee
608 that static constructors and destructors are called when the program starts up
609 and shuts down. In C, you can create static constructors and destructors by
610 using GCC extensions, and in C++ you can do so by creating a global variable
611 whose class has a ctor or dtor.
615 The actual implementation of <tt>__main</tt> lives in the
616 <tt>llvm/runtime/GCCLibraries/crtend/</tt> directory in the source-base, and is
617 linked in automatically when you link the program.
621 <!--=========================================================================-->
623 <div class="question">
624 <a name="iosinit"></a>
625 <p> What is this <tt>llvm.global_ctors</tt> and
626 <tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I #include
627 <iostream>?</p>
632 <p>If you #include the <iostream> header into a C++ translation unit, the
633 file will probably use the <tt>std::cin</tt>/<tt>std::cout</tt>/... global
634 objects. However, C++ does not guarantee an order of initialization between
635 static objects in different translation units, so if a static ctor/dtor in your
636 .cpp file used <tt>std::cout</tt>, for example, the object would not necessarily
637 be automatically initialized before your use.</p>
639 <p>To make <tt>std::cout</tt> and friends work correctly in these scenarios, the
640 STL that we use declares a static object that gets created in every translation
641 unit that includes <tt><iostream></tt>. This object has a static
642 constructor and destructor that initializes and destroys the global iostream
643 objects before they could possibly be used in the file. The code that you see
644 in the .ll file corresponds to the constructor and destructor registration code.
647 <p>If you would like to make it easier to <b>understand</b> the LLVM code
648 generated by the compiler in the demo page, consider using <tt>printf()</tt>
649 instead of <tt>iostream</tt>s to print values.</p>
653 <!--=========================================================================-->
655 <div class="question"><p>
656 <a name="codedce"></a>
657 Where did all of my code go??
662 If you are using the LLVM demo page, you may often wonder what happened to all
663 of the code that you typed in. Remember that the demo script is running the
664 code through the LLVM optimizers, so if your code doesn't actually do anything
665 useful, it might all be deleted.
669 To prevent this, make sure that the code is actually needed. For example, if
670 you are computing some expression, return the value from the function instead of
671 leaving it in a local variable. If you really want to constrain the optimizer,
672 you can read from and assign to <tt>volatile</tt> global variables.
676 <!--=========================================================================-->
678 <div class="question"><p>
680 <p>What is this "<tt>undef</tt>" thing that shows up in my code?
685 <a href="LangRef.html#undef"><tt>undef</tt></a> is the LLVM way of representing
686 a value that is not defined. You can get these if you do not initialize a
687 variable before you use it. For example, the C function:</p>
689 <div class="doc_code">
691 int X() { int i; return i; }
695 <p>Is compiled to "<tt>ret i32 undef</tt>" because "<tt>i</tt>" never has
696 a value specified for it.</p>
699 <!-- *********************************************************************** -->
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