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5 <title>LLVM: Frequently Asked Questions</title>
6 <style type="text/css">
7 @import url("llvm.css");
8 .question { font-weight: bold }
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11</head>
12<body>
13
14<div class="doc_title">
15 LLVM: Frequently Asked Questions
16</div>
17
18<ol>
19 <li><a href="#license">License</a>
20 <ol>
21 <li>Why are the LLVM source code and the front-end distributed under different
22 licenses?</li>
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>
28 </ol></li>
29
30 <li><a href="#source">Source code</a>
31 <ol>
32 <li>In what language is LLVM written?</li>
33 <li>How portable is the LLVM source code?</li>
34 </ol></li>
35
36 <li><a href="#build">Build Problems</a>
37 <ol>
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
47 errors.</li>
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>
Gabor Greif5b6141c2009-03-02 19:08:05 +0000[diff] [blame]51 <li>Compiling LLVM with GCC succeeds, but the resulting tools do not work, what can be wrong?</li>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]52 <li>When I use the test suite, all of the C Backend tests fail. What is
53 wrong?</li>
54 <li>After Subversion update, rebuilding gives the error "No rule to make
55 target".</li>
56 <li><a href="#llvmc">The <tt>llvmc</tt> program gives me errors/doesn't
Bill Wendling4e05864c2007-09-22 09:54:47 +0000[diff] [blame]57 work.</a></li>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]58 </ol></li>
59
60 <li><a href="#felangs">Source Languages</a>
61 <ol>
62 <li><a href="#langs">What source languages are supported?</a></li>
Gordon Henriksendb0558b2008-02-22 21:55:51 +0000[diff] [blame]63 <li><a href="#langirgen">I'd like to write a self-hosting LLVM compiler. How
64 should I interface with the LLVM middle-end optimizers and back-end code
Dan Gohman4dfac702008-11-24 17:18:39 +0000[diff] [blame]65 generators?</a></li>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]66 <li><a href="#langhlsupp">What support is there for higher level source
67 language constructs for building a compiler?</a></li>
68 <li><a href="GetElementPtr.html">I don't understand the GetElementPtr
69 instruction. Help!</a></li>
70 </ol>
71
72 <li><a href="#cfe">Using the GCC Front End</a>
73 <ol>
74 <li>
75 When I compile software that uses a configure script, the configure script
76 thinks my system has all of the header files and libraries it is testing
77 for. How do I get configure to work correctly?
78 </li>
79
80 <li>
81 When I compile code using the LLVM GCC front end, it complains that it
82 cannot find libcrtend.a.
83 </li>
84
85 <li>
86 How can I disable all optimizations when compiling code using the LLVM GCC front end?
87 </li>
88
Dan Gohman4dfac702008-11-24 17:18:39 +0000[diff] [blame]89 <li><a href="#translatecxx">Can I use LLVM to convert C++ code to C code?</a></li>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]90
Dan Gohmanb8e23ab2009-02-10 17:26:53 +0000[diff] [blame]91 <li><a href="#platformindependent">Can I compile C or C++ code to platform-independent LLVM bitcode?</a></li>
92
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]93 </ol>
94 </li>
95
96 <li><a href="#cfe_code">Questions about code generated by the GCC front-end</a>
97 <ol>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]98 <li><a href="#iosinit">What is this <tt>llvm.global_ctors</tt> and
99 <tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I
100 #include &lt;iostream&gt;?</a></li>
101 <li><a href="#codedce">Where did all of my code go??</a></li>
102 <li><a href="#undef">What is this "<tt>undef</tt>" thing that shows up in my code?</a></li>
103 </ol>
104 </li>
105</ol>
106
107<div class="doc_author">
108 <p>Written by <a href="http://llvm.org">The LLVM Team</a></p>
109</div>
110
111
112<!-- *********************************************************************** -->
113<div class="doc_section">
114 <a name="license">License</a>
115</div>
116<!-- *********************************************************************** -->
117
118<div class="question">
119<p>Why are the LLVM source code and the front-end distributed under different
120licenses?</p>
121</div>
122
123<div class="answer">
124<p>The C/C++ front-ends are based on GCC and must be distributed under the GPL.
125Our aim is to distribute LLVM source code under a <em>much less restrictive</em>
126license, in particular one that does not compel users who distribute tools based
127on modifying the source to redistribute the modified source code as well.</p>
128</div>
129
130<div class="question">
131<p>Does the University of Illinois Open Source License really qualify as an
132"open source" license?</p>
133</div>
134
135<div class="answer">
136<p>Yes, the license is <a
137href="http://www.opensource.org/licenses/UoI-NCSA.php">certified</a> by the Open
138Source Initiative (OSI).</p>
139</div>
140
141<div class="question">
142<p>Can I modify LLVM source code and redistribute the modified source?</p>
143</div>
144
145<div class="answer">
146<p>Yes. The modified source distribution must retain the copyright notice and
147follow the three bulletted conditions listed in the <a
Misha Brukman5f135db2008-12-17 18:06:53 +0000[diff] [blame]148href="http://llvm.org/svn/llvm-project/llvm/trunk/LICENSE.TXT">LLVM license</a>.</p>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]149</div>
150
151<div class="question">
152<p>Can I modify LLVM source code and redistribute binaries or other tools based
153on it, without redistributing the source?</p>
154</div>
155
156<div class="answer">
157<p>Yes, this is why we distribute LLVM under a less restrictive license than
158GPL, as explained in the first question above.</p>
159</div>
160
161<!-- *********************************************************************** -->
162<div class="doc_section">
163 <a name="source">Source Code</a>
164</div>
165<!-- *********************************************************************** -->
166
167<div class="question">
168<p>In what language is LLVM written?</p>
169</div>
170
171<div class="answer">
172<p>All of the LLVM tools and libraries are written in C++ with extensive use of
173the STL.</p>
174</div>
175
176<div class="question">
177<p>How portable is the LLVM source code?</p>
178</div>
179
180<div class="answer">
181<p>The LLVM source code should be portable to most modern UNIX-like operating
182systems. Most of the code is written in standard C++ with operating system
183services abstracted to a support library. The tools required to build and test
184LLVM have been ported to a plethora of platforms.</p>
185
186<p>Some porting problems may exist in the following areas:</p>
187
188<ul>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]189 <li>The GCC front end code is not as portable as the LLVM suite, so it may not
Bill Wendling4e05864c2007-09-22 09:54:47 +0000[diff] [blame]190 compile as well on unsupported platforms.</li>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]191
192 <li>The LLVM build system relies heavily on UNIX shell tools, like the Bourne
Bill Wendling4e05864c2007-09-22 09:54:47 +0000[diff] [blame]193 Shell and sed. Porting to systems without these tools (MacOS 9, Plan 9)
194 will require more effort.</li>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]195</ul>
196
197</div>
198
199<!-- *********************************************************************** -->
200<div class="doc_section">
201 <a name="build">Build Problems</a>
202</div>
203<!-- *********************************************************************** -->
204
205<div class="question">
206<p>When I run configure, it finds the wrong C compiler.</p>
207</div>
208
209<div class="answer">
210
211<p>The <tt>configure</tt> script attempts to locate first <tt>gcc</tt> and then
212<tt>cc</tt>, unless it finds compiler paths set in <tt>CC</tt> and <tt>CXX</tt>
213for the C and C++ compiler, respectively.</p>
214
215<p>If <tt>configure</tt> finds the wrong compiler, either adjust your
216<tt>PATH</tt> environment variable or set <tt>CC</tt> and <tt>CXX</tt>
217explicitly.</p>
218
219</div>
220
221<div class="question">
222<p>The <tt>configure</tt> script finds the right C compiler, but it uses the
223LLVM linker from a previous build. What do I do?</p>
224</div>
225
226<div class="answer">
227<p>The <tt>configure</tt> script uses the <tt>PATH</tt> to find executables, so
228if it's grabbing the wrong linker/assembler/etc, there are two ways to fix
229it:</p>
230
231<ol>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]232 <li><p>Adjust your <tt>PATH</tt> environment variable so that the correct
Bill Wendling4e05864c2007-09-22 09:54:47 +0000[diff] [blame]233 program appears first in the <tt>PATH</tt>. This may work, but may not be
234 convenient when you want them <i>first</i> in your path for other
235 work.</p></li>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]236
237 <li><p>Run <tt>configure</tt> with an alternative <tt>PATH</tt> that is
Bill Wendling4e05864c2007-09-22 09:54:47 +0000[diff] [blame]238 correct. In a Borne compatible shell, the syntax would be:</p>
239
Misha Brukman4fbcf682008-12-17 18:11:40 +0000[diff] [blame]240<pre class="doc_code">
Bill Wendling4e05864c2007-09-22 09:54:47 +0000[diff] [blame]241% PATH=[the path without the bad program] ./configure ...
242</pre>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]243
244 <p>This is still somewhat inconvenient, but it allows <tt>configure</tt>
Bill Wendling4e05864c2007-09-22 09:54:47 +0000[diff] [blame]245 to do its work without having to adjust your <tt>PATH</tt>
246 permanently.</p></li>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]247</ol>
248
249</div>
250
251<div class="question">
252<p>When creating a dynamic library, I get a strange GLIBC error.</p>
253</div>
254
255<div class="answer">
256<p>Under some operating systems (i.e. Linux), libtool does not work correctly if
257GCC was compiled with the --disable-shared option. To work around this, install
258your own version of GCC that has shared libraries enabled by default.</p>
259</div>
260
261<div class="question">
Bill Wendling4e05864c2007-09-22 09:54:47 +0000[diff] [blame]262<p>I've updated my source tree from Subversion, and now my build is trying to
263use a file/directory that doesn't exist.</p>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]264</div>
265
266<div class="answer">
267<p>You need to re-run configure in your object directory. When new Makefiles
268are added to the source tree, they have to be copied over to the object tree in
269order to be used by the build.</p>
270</div>
271
272<div class="question">
273<p>I've modified a Makefile in my source tree, but my build tree keeps using the
274old version. What do I do?</p>
275</div>
276
277<div class="answer">
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]278<p>If the Makefile already exists in your object tree, you
279can just run the following command in the top level directory of your object
280tree:</p>
281
Misha Brukman4fbcf682008-12-17 18:11:40 +0000[diff] [blame]282<pre class="doc_code">
283% ./config.status &lt;relative path to Makefile&gt;
284</pre>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]285
286<p>If the Makefile is new, you will have to modify the configure script to copy
287it over.</p>
288
289</div>
290
291<div class="question">
292<p>I've upgraded to a new version of LLVM, and I get strange build errors.</p>
293</div>
294
295<div class="answer">
296
297<p>Sometimes, changes to the LLVM source code alters how the build system works.
298Changes in libtool, autoconf, or header file dependencies are especially prone
299to this sort of problem.</p>
300
301<p>The best thing to try is to remove the old files and re-build. In most
302cases, this takes care of the problem. To do this, just type <tt>make
303clean</tt> and then <tt>make</tt> in the directory that fails to build.</p>
304
305</div>
306
307<div class="question">
308<p>I've built LLVM and am testing it, but the tests freeze.</p>
309</div>
310
311<div class="answer">
312
313<p>This is most likely occurring because you built a profile or release
314(optimized) build of LLVM and have not specified the same information on the
315<tt>gmake</tt> command line.</p>
316
317<p>For example, if you built LLVM with the command:</p>
318
Misha Brukman4fbcf682008-12-17 18:11:40 +0000[diff] [blame]319<pre class="doc_code">
320% gmake ENABLE_PROFILING=1
321</pre>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]322
323<p>...then you must run the tests with the following commands:</p>
324
Misha Brukman4fbcf682008-12-17 18:11:40 +0000[diff] [blame]325<pre class="doc_code">
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]326% cd llvm/test
327% gmake ENABLE_PROFILING=1
328</pre>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]329
330</div>
331
332<div class="question">
333<p>Why do test results differ when I perform different types of builds?</p>
334</div>
335
336<div class="answer">
337
338<p>The LLVM test suite is dependent upon several features of the LLVM tools and
339libraries.</p>
340
341<p>First, the debugging assertions in code are not enabled in optimized or
342profiling builds. Hence, tests that used to fail may pass.</p>
343
344<p>Second, some tests may rely upon debugging options or behavior that is only
345available in the debug build. These tests will fail in an optimized or profile
346build.</p>
347
348</div>
349
350<div class="question">
351<p>Compiling LLVM with GCC 3.3.2 fails, what should I do?</p>
352</div>
353
354<div class="answer">
Gabor Greif32ad7562009-03-02 19:11:53 +0000[diff] [blame]355<p>This is <a href="http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13392">a bug in GCC</a>, and
Bill Wendling4e05864c2007-09-22 09:54:47 +0000[diff] [blame]356affects projects other than LLVM. Try upgrading or downgrading your GCC.</p>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]357</div>
358
359<div class="question">
Gabor Greif5b6141c2009-03-02 19:08:05 +0000[diff] [blame]360<p>Compiling LLVM with GCC succeeds, but the resulting tools do not work, what can be wrong?</p>
361</div>
362
363<div class="answer">
364<p>Several versions of GCC have shown a weakness in miscompiling the LLVM codebase. Please
365consult your compiler version (<tt>gcc --version</tt>) to find out whether it is
366<a href="GettingStarted.html#brokengcc">broken</a>. If so, your only option is to upgrade
367GCC to a known good version.</p>
368</div>
369
370<div class="question">
Bill Wendling4e05864c2007-09-22 09:54:47 +0000[diff] [blame]371<p>After Subversion update, rebuilding gives the error "No rule to make
372target".</p>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]373</div>
374
375<div class="answer">
376<p>If the error is of the form:</p>
377
Misha Brukman4fbcf682008-12-17 18:11:40 +0000[diff] [blame]378<pre class="doc_code">
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]379gmake[2]: *** No rule to make target `/path/to/somefile', needed by
380`/path/to/another/file.d'.<br>
381Stop.
382</pre>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]383
384<p>This may occur anytime files are moved within the Subversion repository or
385removed entirely. In this case, the best solution is to erase all
386<tt>.d</tt> files, which list dependencies for source files, and rebuild:</p>
387
Misha Brukman4fbcf682008-12-17 18:11:40 +0000[diff] [blame]388<pre class="doc_code">
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]389% cd $LLVM_OBJ_DIR
390% rm -f `find . -name \*\.d`
391% gmake
392</pre>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]393
394<p>In other cases, it may be necessary to run <tt>make clean</tt> before
395rebuilding.</p>
396</div>
397
Bill Wendling4e05864c2007-09-22 09:54:47 +0000[diff] [blame]398<div class="question"><p><a name="llvmc">
399The <tt>llvmc</tt> program gives me errors/doesn't work.</a></p>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]400</div>
401
402<div class="answer">
Bill Wendling4e05864c2007-09-22 09:54:47 +0000[diff] [blame]403<p><tt>llvmc</tt> is experimental and isn't really supported. We suggest
404using <tt>llvm-gcc</tt> instead.</p>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]405</div>
406
407<!-- *********************************************************************** -->
408<div class="doc_section"><a name="felangs">Source Languages</a></div>
409
410<div class="question"><p>
411 <a name="langs">What source languages are supported?</a></p>
412</div>
413<div class="answer">
414 <p>LLVM currently has full support for C and C++ source languages. These are
415 available through a special version of GCC that LLVM calls the
416 <a href="#cfe">C Front End</a></p>
417 <p>There is an incomplete version of a Java front end available in the
418 <tt>java</tt> module. There is no documentation on this yet so
419 you'll need to download the code, compile it, and try it.</p>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]420 <p>The PyPy developers are working on integrating LLVM into the PyPy backend
421 so that PyPy language can translate to LLVM.</p>
422</div>
Gordon Henriksen2c2e1482008-02-22 20:58:29 +0000[diff] [blame]423
424<div class="question"><p><a name="langirgen">
Gordon Henriksendb0558b2008-02-22 21:55:51 +0000[diff] [blame]425 I'd like to write a self-hosting LLVM compiler. How should I interface with
426 the LLVM middle-end optimizers and back-end code generators?
Gordon Henriksen2c2e1482008-02-22 20:58:29 +0000[diff] [blame]427</a></p></div>
428<div class="answer">
429 <p>Your compiler front-end will communicate with LLVM by creating a module in
Gordon Henriksendb0558b2008-02-22 21:55:51 +0000[diff] [blame]430 the LLVM intermediate representation (IR) format. Assuming you want to
431 write your language's compiler in the language itself (rather than C++),
432 there are 3 major ways to tackle generating LLVM IR from a front-end:</p>
Gordon Henriksen2c2e1482008-02-22 20:58:29 +0000[diff] [blame]433 <ul>
434 <li>
435 <strong>Call into the LLVM libraries code using your language's FFI
436 (foreign function interface).</strong>
437 <ul>
438 <li><em>for:</em> best tracks changes to the LLVM IR, .ll syntax,
439 and .bc format</li>
440 <li><em>for:</em> enables running LLVM optimization passes without a
441 emit/parse overhead</li>
442 <li><em>for:</em> adapts well to a JIT context</li>
443 <li><em>against:</em> lots of ugly glue code to write</li>
444 </ul>
445 </li>
446 <li>
447 <strong>Emit LLVM assembly from your compiler's native language.</strong>
448 <ul>
449 <li><em>for:</em> very straightforward to get started</li>
450 <li><em>against:</em> the .ll parser is slower than the bitcode reader
451 when interfacing to the middle end</li>
452 <li><em>against:</em> you'll have to re-engineer the LLVM IR object
453 model and asm writer in your language</li>
454 <li><em>against:</em> it may be harder to track changes to the IR</li>
455 </ul>
456 </li>
457 <li>
458 <strong>Emit LLVM bitcode from your compiler's native language.</strong>
459 <ul>
460 <li><em>for:</em> can use the more-efficient bitcode reader when
461 interfacing to the middle end</li>
462 <li><em>against:</em> you'll have to re-engineer the LLVM IR object
463 model and bitcode writer in your language</li>
464 <li><em>against:</em> it may be harder to track changes to the IR</li>
465 </ul>
466 </li>
467 </ul>
468 <p>If you go with the first option, the C bindings in include/llvm-c should
469 help a lot, since most languages have strong support for interfacing with
470 C. The most common hurdle with calling C from managed code is interfacing
471 with the garbage collector. The C interface was designed to require very
472 little memory management, and so is straightforward in this regard.</p>
473</div>
474
Bill Wendling4e05864c2007-09-22 09:54:47 +0000[diff] [blame]475<div class="question"><p><a name="langhlsupp">
476 What support is there for a higher level source language constructs for
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]477 building a compiler?</a></p>
478</div>
479<div class="answer">
480 <p>Currently, there isn't much. LLVM supports an intermediate representation
481 which is useful for code representation but will not support the high level
482 (abstract syntax tree) representation needed by most compilers. There are no
483 facilities for lexical nor semantic analysis. There is, however, a <i>mostly
484 implemented</i> configuration-driven
485 <a href="CompilerDriver.html">compiler driver</a> which simplifies the task
486 of running optimizations, linking, and executable generation.</p>
487</div>
488
Dan Gohman4dfac702008-11-24 17:18:39 +0000[diff] [blame]489<div class="question"><p><a name="getelementptr">
Bill Wendling4e05864c2007-09-22 09:54:47 +0000[diff] [blame]490 I don't understand the GetElementPtr instruction. Help!</a></p>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]491</div>
492<div class="answer">
493 <p>See <a href="GetElementPtr.html">The Often Misunderstood GEP
Bill Wendling4e05864c2007-09-22 09:54:47 +0000[diff] [blame]494 Instruction</a>.</p>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]495</div>
496
497<!-- *********************************************************************** -->
498<div class="doc_section">
499 <a name="cfe">Using the GCC Front End</a>
500</div>
501
502<div class="question">
503<p>
504When I compile software that uses a configure script, the configure script
505thinks my system has all of the header files and libraries it is testing for.
506How do I get configure to work correctly?
507</p>
508</div>
509
510<div class="answer">
511<p>
512The configure script is getting things wrong because the LLVM linker allows
513symbols to be undefined at link time (so that they can be resolved during JIT
514or translation to the C back end). That is why configure thinks your system
515"has everything."
516</p>
517<p>
518To work around this, perform the following steps:
519</p>
520<ol>
521 <li>Make sure the CC and CXX environment variables contains the full path to
522 the LLVM GCC front end.</li>
523
524 <li>Make sure that the regular C compiler is first in your PATH. </li>
525
526 <li>Add the string "-Wl,-native" to your CFLAGS environment variable.</li>
527</ol>
528
529<p>
530This will allow the <tt>llvm-ld</tt> linker to create a native code executable
531instead of shell script that runs the JIT. Creating native code requires
532standard linkage, which in turn will allow the configure script to find out if
533code is not linking on your system because the feature isn't available on your
534system.</p>
535</div>
536
537<div class="question">
538<p>
539When I compile code using the LLVM GCC front end, it complains that it cannot
540find libcrtend.a.
541</p>
542</div>
543
544<div class="answer">
545<p>
546The only way this can happen is if you haven't installed the runtime library. To
547correct this, do:</p>
548
Misha Brukman4fbcf682008-12-17 18:11:40 +0000[diff] [blame]549<pre class="doc_code">
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]550% cd llvm/runtime
551% make clean ; make install-bytecode
552</pre>
553</div>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]554
555<div class="question">
556<p>
557How can I disable all optimizations when compiling code using the LLVM GCC front end?
558</p>
559</div>
560
561<div class="answer">
562<p>
563Passing "-Wa,-disable-opt -Wl,-disable-opt" will disable *all* cleanup and
564optimizations done at the llvm level, leaving you with the truly horrible
565code that you desire.
566</p>
567</div>
568
569
570<div class="question">
571<p>
Dan Gohman4dfac702008-11-24 17:18:39 +0000[diff] [blame]572<a name="translatecxx">Can I use LLVM to convert C++ code to C code?</a>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]573</p>
574</div>
575
576<div class="answer">
577<p>Yes, you can use LLVM to convert code from any language LLVM supports to C.
578Note that the generated C code will be very low level (all loops are lowered
579to gotos, etc) and not very pretty (comments are stripped, original source
580formatting is totally lost, variables are renamed, expressions are regrouped),
djga0237ec2009-01-25 16:04:50 +0000[diff] [blame]581so this may not be what you're looking for. Also, there are several
582limitations noted below.<p>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]583
584<p>Use commands like this:</p>
585
586<ol>
Bill Wendling4e05864c2007-09-22 09:54:47 +0000[diff] [blame]587 <li><p>Compile your program as normal with llvm-g++:</p>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]588
Misha Brukman4fbcf682008-12-17 18:11:40 +0000[diff] [blame]589<pre class="doc_code">
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]590% llvm-g++ x.cpp -o program
591</pre>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]592
Bill Wendling4e05864c2007-09-22 09:54:47 +0000[diff] [blame]593 <p>or:</p>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]594
Misha Brukman4fbcf682008-12-17 18:11:40 +0000[diff] [blame]595<pre class="doc_code">
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]596% llvm-g++ a.cpp -c
597% llvm-g++ b.cpp -c
598% llvm-g++ a.o b.o -o program
599</pre>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]600
Bill Wendling4e05864c2007-09-22 09:54:47 +0000[diff] [blame]601 <p>With llvm-gcc3, this will generate program and program.bc. The .bc
602 file is the LLVM version of the program all linked together.</p></li>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]603
Bill Wendling4e05864c2007-09-22 09:54:47 +0000[diff] [blame]604 <li><p>Convert the LLVM code to C code, using the LLC tool with the C
605 backend:</p>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]606
Misha Brukman4fbcf682008-12-17 18:11:40 +0000[diff] [blame]607<pre class="doc_code">
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]608% llc -march=c program.bc -o program.c
Misha Brukman4fbcf682008-12-17 18:11:40 +0000[diff] [blame]609</pre></li>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]610
Bill Wendling4e05864c2007-09-22 09:54:47 +0000[diff] [blame]611<li><p>Finally, compile the C file:</p>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]612
Misha Brukman4fbcf682008-12-17 18:11:40 +0000[diff] [blame]613<pre class="doc_code">
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]614% cc x.c
Misha Brukman4fbcf682008-12-17 18:11:40 +0000[diff] [blame]615</pre></li>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]616
617</ol>
618
djga0237ec2009-01-25 16:04:50 +0000[diff] [blame]619<p>Using LLVM does not eliminate the need for C++ library support.
620If you use the llvm-g++ front-end, the generated code will depend on
621g++'s C++ support libraries in the same way that code generated from
622g++ would. If you use another C++ front-end, the generated code will
623depend on whatever library that front-end would normally require.</p>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]624
djga0237ec2009-01-25 16:04:50 +0000[diff] [blame]625<p>If you are working on a platform that does not provide any C++
626libraries, you may be able to manually compile libstdc++ to LLVM
Bill Wendling4e05864c2007-09-22 09:54:47 +0000[diff] [blame]627bitcode, statically link it into your program, then use the commands above to
djga0237ec2009-01-25 16:04:50 +0000[diff] [blame]628convert the whole result into C code. Alternatively, you might compile the
Bill Wendling4e05864c2007-09-22 09:54:47 +0000[diff] [blame]629libraries and your application into two different chunks of C code and link
630them.</p>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]631
djga0237ec2009-01-25 16:04:50 +0000[diff] [blame]632<p>Note that, by default, the C back end does not support exception handling. If
633you want/need it for a certain program, you can enable it by passing
634"-enable-correct-eh-support" to the llc program. The resultant code will use
635setjmp/longjmp to implement exception support that is relatively slow, and
636not C++-ABI-conforming on most platforms, but otherwise correct.</p>
637
638<p>Also, there are a number of other limitations of the C backend that
639cause it to produce code that does not fully conform to the C++ ABI on
640most platforms. Some of the C++ programs in LLVM's test suite are known
641to fail when compiled with the C back end because of ABI incompatiblities
642with standard C++ libraries.</p>
643
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]644</div>
645
Dan Gohmanb8e23ab2009-02-10 17:26:53 +0000[diff] [blame]646<div class="question">
647<p>
648<a name="platformindependent">Can I compile C or C++ code to platform-independent LLVM bitcode?</a>
649</p>
650</div>
651
652<div class="answer">
653
654<p>No. C and C++ are inherently platform-dependent languages. The most
655obvious example of this is the preprocessor. A very common way that C code
656is made portable is by using the preprocessor to include platform-specific
657code. In practice, information about other platforms is lost after
658preprocessing, so the result is inherently dependent on the platform that
659the preprocessing was targetting.</p>
660
661<p>Another example is <tt>sizeof</tt>. It's common for <tt>sizeof(long)</tt>
662to vary between platforms. In most C front-ends, <tt>sizeof</tt> is expanded
663to a constant immediately, thus hardwaring a platform-specific detail.</p>
664
665<p>Also, since many platforms define their ABIs in terms of C, and since
666LLVM is lower-level than C, front-ends currently must emit platform-specific
667IR in order to have the result conform to the platform ABI.</p>
668
669</div>
670
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]671<!-- *********************************************************************** -->
672<div class="doc_section">
673 <a name="cfe_code">Questions about code generated by the GCC front-end</a>
674</div>
675
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]676<div class="question">
677<a name="iosinit"></a>
678<p> What is this <tt>llvm.global_ctors</tt> and
679<tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I #include
680&lt;iostream&gt;?</p>
681</div>
682
683<div class="answer">
684
685<p>If you #include the &lt;iostream&gt; header into a C++ translation unit, the
686file will probably use the <tt>std::cin</tt>/<tt>std::cout</tt>/... global
687objects. However, C++ does not guarantee an order of initialization between
688static objects in different translation units, so if a static ctor/dtor in your
689.cpp file used <tt>std::cout</tt>, for example, the object would not necessarily
690be automatically initialized before your use.</p>
691
692<p>To make <tt>std::cout</tt> and friends work correctly in these scenarios, the
693STL that we use declares a static object that gets created in every translation
694unit that includes <tt>&lt;iostream&gt;</tt>. This object has a static
695constructor and destructor that initializes and destroys the global iostream
696objects before they could possibly be used in the file. The code that you see
697in the .ll file corresponds to the constructor and destructor registration code.
698</p>
699
700<p>If you would like to make it easier to <b>understand</b> the LLVM code
701generated by the compiler in the demo page, consider using <tt>printf()</tt>
702instead of <tt>iostream</tt>s to print values.</p>
703
704</div>
705
706<!--=========================================================================-->
707
708<div class="question"><p>
709<a name="codedce"></a>
710Where did all of my code go??
711</p></div>
712
713<div class="answer">
714<p>
715If you are using the LLVM demo page, you may often wonder what happened to all
716of the code that you typed in. Remember that the demo script is running the
717code through the LLVM optimizers, so if your code doesn't actually do anything
718useful, it might all be deleted.
719</p>
720
721<p>
722To prevent this, make sure that the code is actually needed. For example, if
723you are computing some expression, return the value from the function instead of
724leaving it in a local variable. If you really want to constrain the optimizer,
725you can read from and assign to <tt>volatile</tt> global variables.
726</p>
727</div>
728
729<!--=========================================================================-->
730
731<div class="question"><p>
732<a name="undef"></a>
733<p>What is this "<tt>undef</tt>" thing that shows up in my code?
734</p></div>
735
736<div class="answer">
737<p>
738<a href="LangRef.html#undef"><tt>undef</tt></a> is the LLVM way of representing
739a value that is not defined. You can get these if you do not initialize a
740variable before you use it. For example, the C function:</p>
741
Misha Brukman4fbcf682008-12-17 18:11:40 +0000[diff] [blame]742<pre class="doc_code">
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]743int X() { int i; return i; }
744</pre>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]745
746<p>Is compiled to "<tt>ret i32 undef</tt>" because "<tt>i</tt>" never has
747a value specified for it.</p>
748</div>
749
750<!-- *********************************************************************** -->
751
752<hr>
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Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]758
759 <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
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