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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="#langirgen">I'd like to write a self-hosting LLVM compiler. How
63 should I interface with the LLVM middle-end optimizers and back-end code
65 <li><a href="#langhlsupp">What support is there for higher level source
66 language constructs for building a compiler?</a></li>
67 <li><a href="GetElementPtr.html">I don't understand the GetElementPtr
68 instruction. Help!</a></li>
71 <li><a href="#cfe">Using the GCC Front End</a>
74 When I compile software that uses a configure script, the configure script
75 thinks my system has all of the header files and libraries it is testing
76 for. How do I get configure to work correctly?
80 When I compile code using the LLVM GCC front end, it complains that it
81 cannot find libcrtend.a.
85 How can I disable all optimizations when compiling code using the LLVM GCC front end?
88 <li><a href="#translatec++">Can I use LLVM to convert C++ code to C code?</a></li>
93 <li><a href="#cfe_code">Questions about code generated by the GCC front-end</a>
95 <li><a href="#iosinit">What is this <tt>llvm.global_ctors</tt> and
96 <tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I
97 #include <iostream>?</a></li>
98 <li><a href="#codedce">Where did all of my code go??</a></li>
99 <li><a href="#undef">What is this "<tt>undef</tt>" thing that shows up in my code?</a></li>
104 <div class="doc_author">
105 <p>Written by <a href="http://llvm.org">The LLVM Team</a></p>
109 <!-- *********************************************************************** -->
110 <div class="doc_section">
111 <a name="license">License</a>
113 <!-- *********************************************************************** -->
115 <div class="question">
116 <p>Why are the LLVM source code and the front-end distributed under different
121 <p>The C/C++ front-ends are based on GCC and must be distributed under the GPL.
122 Our aim is to distribute LLVM source code under a <em>much less restrictive</em>
123 license, in particular one that does not compel users who distribute tools based
124 on modifying the source to redistribute the modified source code as well.</p>
127 <div class="question">
128 <p>Does the University of Illinois Open Source License really qualify as an
129 "open source" license?</p>
133 <p>Yes, the license is <a
134 href="http://www.opensource.org/licenses/UoI-NCSA.php">certified</a> by the Open
135 Source Initiative (OSI).</p>
138 <div class="question">
139 <p>Can I modify LLVM source code and redistribute the modified source?</p>
143 <p>Yes. The modified source distribution must retain the copyright notice and
144 follow the three bulletted conditions listed in the <a
145 href="http://llvm.org/releases/1.3/LICENSE.TXT">LLVM license</a>.</p>
148 <div class="question">
149 <p>Can I modify LLVM source code and redistribute binaries or other tools based
150 on it, without redistributing the source?</p>
154 <p>Yes, this is why we distribute LLVM under a less restrictive license than
155 GPL, as explained in the first question above.</p>
158 <!-- *********************************************************************** -->
159 <div class="doc_section">
160 <a name="source">Source Code</a>
162 <!-- *********************************************************************** -->
164 <div class="question">
165 <p>In what language is LLVM written?</p>
169 <p>All of the LLVM tools and libraries are written in C++ with extensive use of
173 <div class="question">
174 <p>How portable is the LLVM source code?</p>
178 <p>The LLVM source code should be portable to most modern UNIX-like operating
179 systems. Most of the code is written in standard C++ with operating system
180 services abstracted to a support library. The tools required to build and test
181 LLVM have been ported to a plethora of platforms.</p>
183 <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)
191 will 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>The <tt>configure</tt> script finds the right C compiler, but it uses the
220 LLVM linker from a previous build. What do I do?</p>
224 <p>The <tt>configure</tt> script uses the <tt>PATH</tt> to find executables, so
225 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 <div class="doc_code">
239 % PATH=[the path without the bad program] ./configure ...
243 <p>This is still somewhat inconvenient, but it allows <tt>configure</tt>
244 to do its work without having to adjust your <tt>PATH</tt>
245 permanently.</p></li>
250 <div class="question">
251 <p>When creating a dynamic library, I get a strange GLIBC error.</p>
255 <p>Under some operating systems (i.e. Linux), libtool does not work correctly if
256 GCC was compiled with the --disable-shared option. To work around this, install
257 your own version of GCC that has shared libraries enabled by default.</p>
260 <div class="question">
261 <p>I've updated my source tree from Subversion, and now my build is trying to
262 use a file/directory that doesn't exist.</p>
266 <p>You need to re-run configure in your object directory. When new Makefiles
267 are added to the source tree, they have to be copied over to the object tree in
268 order to be used by the build.</p>
271 <div class="question">
272 <p>I've modified a Makefile in my source tree, but my build tree keeps using the
273 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 <div class="doc_code">
282 <pre>% ./config.status <relative path to Makefile></pre>
285 <p>If the Makefile is new, you will have to modify the configure script to copy
290 <div class="question">
291 <p>I've upgraded to a new version of LLVM, and I get strange build errors.</p>
296 <p>Sometimes, changes to the LLVM source code alters how the build system works.
297 Changes in libtool, autoconf, or header file dependencies are especially prone
298 to this sort of problem.</p>
300 <p>The best thing to try is to remove the old files and re-build. In most
301 cases, this takes care of the problem. To do this, just type <tt>make
302 clean</tt> and then <tt>make</tt> in the directory that fails to build.</p>
306 <div class="question">
307 <p>I've built LLVM and am testing it, but the tests freeze.</p>
312 <p>This is most likely occurring because you built a profile or release
313 (optimized) build of LLVM and have not specified the same information on the
314 <tt>gmake</tt> command line.</p>
316 <p>For example, if you built LLVM with the command:</p>
318 <div class="doc_code">
319 <pre>% gmake ENABLE_PROFILING=1</pre>
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 "No rule to make
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"><p><a name="llvmc">
393 The <tt>llvmc</tt> program gives me errors/doesn't work.</a></p>
397 <p><tt>llvmc</tt> is experimental and isn't really supported. We suggest
398 using <tt>llvm-gcc</tt> instead.</p>
401 <!-- *********************************************************************** -->
402 <div class="doc_section"><a name="felangs">Source Languages</a></div>
404 <div class="question"><p>
405 <a name="langs">What source languages are supported?</a></p>
408 <p>LLVM currently has full support for C and C++ source languages. These are
409 available through a special version of GCC that LLVM calls the
410 <a href="#cfe">C Front End</a></p>
411 <p>There is an incomplete version of a Java front end available in the
412 <tt>java</tt> module. There is no documentation on this yet so
413 you'll need to download the code, compile it, and try it.</p>
414 <p>In the <tt>stacker</tt> module is a compiler and runtime
415 library for the Stacker language, a "toy" language loosely based on Forth.</p>
416 <p>The PyPy developers are working on integrating LLVM into the PyPy backend
417 so that PyPy language can translate to LLVM.</p>
420 <div class="question"><p><a name="langirgen">
421 I'd like to write a self-hosting LLVM compiler. How should I interface with
422 the LLVM middle-end optimizers and back-end code generators?
425 <p>Your compiler front-end will communicate with LLVM by creating a module in
426 the LLVM intermediate representation (IR) format. Assuming you want to
427 write your language's compiler in the language itself (rather than C++),
428 there are 3 major ways to tackle generating LLVM IR from a front-end:</p>
431 <strong>Call into the LLVM libraries code using your language's FFI
432 (foreign function interface).</strong>
434 <li><em>for:</em> best tracks changes to the LLVM IR, .ll syntax,
436 <li><em>for:</em> enables running LLVM optimization passes without a
437 emit/parse overhead</li>
438 <li><em>for:</em> adapts well to a JIT context</li>
439 <li><em>against:</em> lots of ugly glue code to write</li>
443 <strong>Emit LLVM assembly from your compiler's native language.</strong>
445 <li><em>for:</em> very straightforward to get started</li>
446 <li><em>against:</em> the .ll parser is slower than the bitcode reader
447 when interfacing to the middle end</li>
448 <li><em>against:</em> you'll have to re-engineer the LLVM IR object
449 model and asm writer in your language</li>
450 <li><em>against:</em> it may be harder to track changes to the IR</li>
454 <strong>Emit LLVM bitcode from your compiler's native language.</strong>
456 <li><em>for:</em> can use the more-efficient bitcode reader when
457 interfacing to the middle end</li>
458 <li><em>against:</em> you'll have to re-engineer the LLVM IR object
459 model and bitcode writer in your language</li>
460 <li><em>against:</em> it may be harder to track changes to the IR</li>
464 <p>If you go with the first option, the C bindings in include/llvm-c should
465 help a lot, since most languages have strong support for interfacing with
466 C. The most common hurdle with calling C from managed code is interfacing
467 with the garbage collector. The C interface was designed to require very
468 little memory management, and so is straightforward in this regard.</p>
471 <div class="question"><p><a name="langhlsupp">
472 What support is there for a higher level source language constructs for
473 building a compiler?</a></p>
476 <p>Currently, there isn't much. LLVM supports an intermediate representation
477 which is useful for code representation but will not support the high level
478 (abstract syntax tree) representation needed by most compilers. There are no
479 facilities for lexical nor semantic analysis. There is, however, a <i>mostly
480 implemented</i> configuration-driven
481 <a href="CompilerDriver.html">compiler driver</a> which simplifies the task
482 of running optimizations, linking, and executable generation.</p>
485 <div class="question"><p><a name="langhlsupp">
486 I don't understand the GetElementPtr instruction. Help!</a></p>
489 <p>See <a href="GetElementPtr.html">The Often Misunderstood GEP
493 <!-- *********************************************************************** -->
494 <div class="doc_section">
495 <a name="cfe">Using the GCC Front End</a>
498 <div class="question">
500 When I compile software that uses a configure script, the configure script
501 thinks my system has all of the header files and libraries it is testing for.
502 How do I get configure to work correctly?
508 The configure script is getting things wrong because the LLVM linker allows
509 symbols to be undefined at link time (so that they can be resolved during JIT
510 or translation to the C back end). That is why configure thinks your system
514 To work around this, perform the following steps:
517 <li>Make sure the CC and CXX environment variables contains the full path to
518 the LLVM GCC front end.</li>
520 <li>Make sure that the regular C compiler is first in your PATH. </li>
522 <li>Add the string "-Wl,-native" to your CFLAGS environment variable.</li>
526 This will allow the <tt>llvm-ld</tt> linker to create a native code executable
527 instead of shell script that runs the JIT. Creating native code requires
528 standard linkage, which in turn will allow the configure script to find out if
529 code is not linking on your system because the feature isn't available on your
533 <div class="question">
535 When I compile code using the LLVM GCC front end, it complains that it cannot
542 The only way this can happen is if you haven't installed the runtime library. To
543 correct this, do:</p>
545 <div class="doc_code">
548 % make clean ; make install-bytecode
553 <div class="question">
555 How can I disable all optimizations when compiling code using the LLVM GCC front end?
561 Passing "-Wa,-disable-opt -Wl,-disable-opt" will disable *all* cleanup and
562 optimizations done at the llvm level, leaving you with the truly horrible
563 code that you desire.
568 <div class="question">
570 <a name="translatec++">Can I use LLVM to convert C++ code to C code?</a>
575 <p>Yes, you can use LLVM to convert code from any language LLVM supports to C.
576 Note that the generated C code will be very low level (all loops are lowered
577 to gotos, etc) and not very pretty (comments are stripped, original source
578 formatting is totally lost, variables are renamed, expressions are regrouped),
579 so this may not be what you're looking for. However, this is a good way to add
580 C++ support for a processor that does not otherwise have a C++ compiler.
583 <p>Use commands like this:</p>
586 <li><p>Compile your program as normal with llvm-g++:</p>
588 <div class="doc_code">
590 % llvm-g++ x.cpp -o program
596 <div class="doc_code">
600 % llvm-g++ a.o b.o -o program
604 <p>With llvm-gcc3, this will generate program and program.bc. The .bc
605 file is the LLVM version of the program all linked together.</p></li>
607 <li><p>Convert the LLVM code to C code, using the LLC tool with the C
610 <div class="doc_code">
612 % llc -march=c program.bc -o program.c
616 <li><p>Finally, compile the C file:</p>
618 <div class="doc_code">
626 <p>Note that, by default, the C backend does not support exception handling. If
627 you want/need it for a certain program, you can enable it by passing
628 "-enable-correct-eh-support" to the llc program. The resultant code will use
629 setjmp/longjmp to implement exception support that is correct but relatively
632 <p>Also note: this specific sequence of commands won't work if you use a
633 function defined in the C++ runtime library (or any other C++ library). To
634 access an external C++ library, you must manually compile libstdc++ to LLVM
635 bitcode, statically link it into your program, then use the commands above to
636 convert the whole result into C code. Alternatively, you can compile the
637 libraries and your application into two different chunks of C code and link
642 <!-- *********************************************************************** -->
643 <div class="doc_section">
644 <a name="cfe_code">Questions about code generated by the GCC front-end</a>
647 <div class="question">
648 <a name="iosinit"></a>
649 <p> What is this <tt>llvm.global_ctors</tt> and
650 <tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I #include
651 <iostream>?</p>
656 <p>If you #include the <iostream> header into a C++ translation unit, the
657 file will probably use the <tt>std::cin</tt>/<tt>std::cout</tt>/... global
658 objects. However, C++ does not guarantee an order of initialization between
659 static objects in different translation units, so if a static ctor/dtor in your
660 .cpp file used <tt>std::cout</tt>, for example, the object would not necessarily
661 be automatically initialized before your use.</p>
663 <p>To make <tt>std::cout</tt> and friends work correctly in these scenarios, the
664 STL that we use declares a static object that gets created in every translation
665 unit that includes <tt><iostream></tt>. This object has a static
666 constructor and destructor that initializes and destroys the global iostream
667 objects before they could possibly be used in the file. The code that you see
668 in the .ll file corresponds to the constructor and destructor registration code.
671 <p>If you would like to make it easier to <b>understand</b> the LLVM code
672 generated by the compiler in the demo page, consider using <tt>printf()</tt>
673 instead of <tt>iostream</tt>s to print values.</p>
677 <!--=========================================================================-->
679 <div class="question"><p>
680 <a name="codedce"></a>
681 Where did all of my code go??
686 If you are using the LLVM demo page, you may often wonder what happened to all
687 of the code that you typed in. Remember that the demo script is running the
688 code through the LLVM optimizers, so if your code doesn't actually do anything
689 useful, it might all be deleted.
693 To prevent this, make sure that the code is actually needed. For example, if
694 you are computing some expression, return the value from the function instead of
695 leaving it in a local variable. If you really want to constrain the optimizer,
696 you can read from and assign to <tt>volatile</tt> global variables.
700 <!--=========================================================================-->
702 <div class="question"><p>
704 <p>What is this "<tt>undef</tt>" thing that shows up in my code?
709 <a href="LangRef.html#undef"><tt>undef</tt></a> is the LLVM way of representing
710 a value that is not defined. You can get these if you do not initialize a
711 variable before you use it. For example, the C function:</p>
713 <div class="doc_code">
715 int X() { int i; return i; }
719 <p>Is compiled to "<tt>ret i32 undef</tt>" because "<tt>i</tt>" never has
720 a value specified for it.</p>
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