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5 <title>LLVM: Frequently Asked Questions</title>
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9 .answer { margin-left: 2em }
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11</head>
12<body>
John Criswell8af73202003-10-13 16:13:06 +0000[diff] [blame]13
Misha Brukman6c91a332003-11-06 21:55:44 +0000[diff] [blame]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>
Misha Brukman6c91a332003-11-06 21:55:44 +0000[diff] [blame]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>
Reid Spencerc7f87f22007-07-09 08:04:31 +0000[diff] [blame]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>
Misha Brukman6c91a332003-11-06 21:55:44 +0000[diff] [blame]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>
Chris Lattner5abdc6f2003-12-22 04:06:12 +0000[diff] [blame]50 <li>Compiling LLVM with GCC 3.3.2 fails, what should I do?</li>
John Criswellfa2f3682004-03-29 20:23:11 +0000[diff] [blame]51 <li>When I use the test suite, all of the C Backend tests fail. What is
52 wrong?</li>
Reid Spencerc7f87f22007-07-09 08:04:31 +0000[diff] [blame]53 <li>After Subversion update, rebuilding gives the error "No rule to make
Bill Wendling0a303eb2007-05-29 09:35:34 +0000[diff] [blame]54 target".</li>
55 <li><a href="#llvmc">The <tt>llvmc</tt> program gives me errors/doesn't
Bill Wendlingbeee6222007-09-22 09:54:47 +0000[diff] [blame^]56 work.</a></li>
Misha Brukman6c91a332003-11-06 21:55:44 +0000[diff] [blame]57 </ol></li>
John Criswell956cf452003-11-18 16:08:49 +0000[diff] [blame]58
Reid Spencer9aa244e2006-04-26 14:52:19 +0000[diff] [blame]59 <li><a href="#felangs">Source Languages</a>
60 <ol>
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>
Reid Spencer4bded9c2006-08-10 20:15:58 +0000[diff] [blame]64 <li><a href="GetElementPtr.html">I don't understand the GetElementPtr
65 instruction. Help!</a></li>
Reid Spencer9aa244e2006-04-26 14:52:19 +0000[diff] [blame]66 </ol>
67
Chris Lattner23353312003-11-19 05:53:12 +0000[diff] [blame]68 <li><a href="#cfe">Using the GCC Front End</a>
John Criswell956cf452003-11-18 16:08:49 +0000[diff] [blame]69 <ol>
70 <li>
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?
74 </li>
75
76 <li>
77 When I compile code using the LLVM GCC front end, it complains that it
Chris Lattner020e1fc2004-05-23 21:07:27 +0000[diff] [blame]78 cannot find libcrtend.a.
John Criswell956cf452003-11-18 16:08:49 +0000[diff] [blame]79 </li>
Tanya Lattneraf0ac272005-04-25 20:36:56 +0000[diff] [blame]80
81 <li>
82 How can I disable all optimizations when compiling code using the LLVM GCC front end?
83 </li>
84
Chris Lattner7b1d3c02006-07-19 18:19:59 +0000[diff] [blame]85 <li><a href="#translatec++">Can I use LLVM to convert C++ code to C code?</a></li>
86
John Criswell956cf452003-11-18 16:08:49 +0000[diff] [blame]87 </ol>
88 </li>
Chris Lattner23353312003-11-19 05:53:12 +0000[diff] [blame]89
90 <li><a href="#cfe_code">Questions about code generated by the GCC front-end</a>
91 <ol>
Chris Lattner37d55472005-02-25 20:30:21 +0000[diff] [blame]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
Chris Lattner87d6b402004-03-29 19:14:35 +0000[diff] [blame]95 <tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I
Chris Lattner37d55472005-02-25 20:30:21 +0000[diff] [blame]96 #include &lt;iostream&gt;?</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>
Chris Lattner23353312003-11-19 05:53:12 +0000[diff] [blame]99 </ol>
100 </li>
Misha Brukman6c91a332003-11-06 21:55:44 +0000[diff] [blame]101</ol>
102
Chris Lattner020e1fc2004-05-23 21:07:27 +0000[diff] [blame]103<div class="doc_author">
Reid Spencerca058542006-03-14 05:39:39 +0000[diff] [blame]104 <p>Written by <a href="http://llvm.org">The LLVM Team</a></p>
Chris Lattner020e1fc2004-05-23 21:07:27 +0000[diff] [blame]105</div>
106
107
Misha Brukman6c91a332003-11-06 21:55:44 +0000[diff] [blame]108<!-- *********************************************************************** -->
109<div class="doc_section">
110 <a name="license">License</a>
111</div>
112<!-- *********************************************************************** -->
113
114<div class="question">
115<p>Why are the LLVM source code and the front-end distributed under different
116licenses?</p>
117</div>
118
119<div class="answer">
120<p>The C/C++ front-ends are based on GCC and must be distributed under the GPL.
121Our aim is to distribute LLVM source code under a <em>much less restrictive</em>
122license, in particular one that does not compel users who distribute tools based
123on modifying the source to redistribute the modified source code as well.</p>
124</div>
125
126<div class="question">
127<p>Does the University of Illinois Open Source License really qualify as an
128"open source" license?</p>
129</div>
130
131<div class="answer">
132<p>Yes, the license is <a
133href="http://www.opensource.org/licenses/UoI-NCSA.php">certified</a> by the Open
134Source Initiative (OSI).</p>
135</div>
136
137<div class="question">
138<p>Can I modify LLVM source code and redistribute the modified source?</p>
139</div>
140
141<div class="answer">
142<p>Yes. The modified source distribution must retain the copyright notice and
143follow the three bulletted conditions listed in the <a
Reid Spencerca058542006-03-14 05:39:39 +0000[diff] [blame]144href="http://llvm.org/releases/1.3/LICENSE.TXT">LLVM license</a>.</p>
Misha Brukman6c91a332003-11-06 21:55:44 +0000[diff] [blame]145</div>
146
147<div class="question">
148<p>Can I modify LLVM source code and redistribute binaries or other tools based
149on it, without redistributing the source?</p>
150</div>
151
152<div class="answer">
153<p>Yes, this is why we distribute LLVM under a less restrictive license than
154GPL, as explained in the first question above.</p>
155</div>
156
157<!-- *********************************************************************** -->
158<div class="doc_section">
159 <a name="source">Source Code</a>
160</div>
161<!-- *********************************************************************** -->
162
163<div class="question">
164<p>In what language is LLVM written?</p>
165</div>
166
167<div class="answer">
168<p>All of the LLVM tools and libraries are written in C++ with extensive use of
169the STL.</p>
170</div>
171
172<div class="question">
173<p>How portable is the LLVM source code?</p>
174</div>
175
176<div class="answer">
177<p>The LLVM source code should be portable to most modern UNIX-like operating
178systems. Most of the code is written in standard C++ with operating system
179services abstracted to a support library. The tools required to build and test
180LLVM have been ported to a plethora of platforms.</p>
181
182<p>Some porting problems may exist in the following areas:</p>
183
184<ul>
Misha Brukman6c91a332003-11-06 21:55:44 +0000[diff] [blame]185 <li>The GCC front end code is not as portable as the LLVM suite, so it may not
Bill Wendlingbeee6222007-09-22 09:54:47 +0000[diff] [blame^]186 compile as well on unsupported platforms.</li>
Misha Brukman6c91a332003-11-06 21:55:44 +0000[diff] [blame]187
Misha Brukman6c91a332003-11-06 21:55:44 +0000[diff] [blame]188 <li>The LLVM build system relies heavily on UNIX shell tools, like the Bourne
Bill Wendlingbeee6222007-09-22 09:54:47 +0000[diff] [blame^]189 Shell and sed. Porting to systems without these tools (MacOS 9, Plan 9)
190 will require more effort.</li>
Misha Brukman6c91a332003-11-06 21:55:44 +0000[diff] [blame]191</ul>
192
193</div>
194
195<!-- *********************************************************************** -->
196<div class="doc_section">
197 <a name="build">Build Problems</a>
198</div>
199<!-- *********************************************************************** -->
200
201<div class="question">
202<p>When I run configure, it finds the wrong C compiler.</p>
203</div>
204
205<div class="answer">
206
207<p>The <tt>configure</tt> script attempts to locate first <tt>gcc</tt> and then
208<tt>cc</tt>, unless it finds compiler paths set in <tt>CC</tt> and <tt>CXX</tt>
209for the C and C++ compiler, respectively.</p>
210
211<p>If <tt>configure</tt> finds the wrong compiler, either adjust your
212<tt>PATH</tt> environment variable or set <tt>CC</tt> and <tt>CXX</tt>
213explicitly.</p>
214
215</div>
216
217<div class="question">
Misha Brukman6c91a332003-11-06 21:55:44 +0000[diff] [blame]218<p>The <tt>configure</tt> script finds the right C compiler, but it uses the
219LLVM linker from a previous build. What do I do?</p>
220</div>
221
222<div class="answer">
223<p>The <tt>configure</tt> script uses the <tt>PATH</tt> to find executables, so
224if it's grabbing the wrong linker/assembler/etc, there are two ways to fix
225it:</p>
226
227<ol>
Misha Brukman6c91a332003-11-06 21:55:44 +0000[diff] [blame]228 <li><p>Adjust your <tt>PATH</tt> environment variable so that the correct
Bill Wendlingbeee6222007-09-22 09:54:47 +0000[diff] [blame^]229 program appears first in the <tt>PATH</tt>. This may work, but may not be
230 convenient when you want them <i>first</i> in your path for other
231 work.</p></li>
Misha Brukman6c91a332003-11-06 21:55:44 +0000[diff] [blame]232
233 <li><p>Run <tt>configure</tt> with an alternative <tt>PATH</tt> that is
Bill Wendlingbeee6222007-09-22 09:54:47 +0000[diff] [blame^]234 correct. In a Borne compatible shell, the syntax would be:</p>
235
236<div class="doc_code">
237<pre>
238% PATH=[the path without the bad program] ./configure ...
239</pre>
240</div>
Misha Brukman6c91a332003-11-06 21:55:44 +0000[diff] [blame]241
242 <p>This is still somewhat inconvenient, but it allows <tt>configure</tt>
Bill Wendlingbeee6222007-09-22 09:54:47 +0000[diff] [blame^]243 to do its work without having to adjust your <tt>PATH</tt>
244 permanently.</p></li>
Misha Brukman6c91a332003-11-06 21:55:44 +0000[diff] [blame]245</ol>
246
247</div>
248
249<div class="question">
250<p>When creating a dynamic library, I get a strange GLIBC error.</p>
251</div>
252
253<div class="answer">
254<p>Under some operating systems (i.e. Linux), libtool does not work correctly if
255GCC was compiled with the --disable-shared option. To work around this, install
256your own version of GCC that has shared libraries enabled by default.</p>
257</div>
258
259<div class="question">
Bill Wendlingbeee6222007-09-22 09:54:47 +0000[diff] [blame^]260<p>I've updated my source tree from Subversion, and now my build is trying to
261use a file/directory that doesn't exist.</p>
Misha Brukman6c91a332003-11-06 21:55:44 +0000[diff] [blame]262</div>
263
264<div class="answer">
265<p>You need to re-run configure in your object directory. When new Makefiles
266are added to the source tree, they have to be copied over to the object tree in
267order to be used by the build.</p>
268</div>
269
270<div class="question">
271<p>I've modified a Makefile in my source tree, but my build tree keeps using the
272old version. What do I do?</p>
273</div>
274
275<div class="answer">
Misha Brukman6c91a332003-11-06 21:55:44 +0000[diff] [blame]276<p>If the Makefile already exists in your object tree, you
277can just run the following command in the top level directory of your object
278tree:</p>
279
Bill Wendlingbeee6222007-09-22 09:54:47 +0000[diff] [blame^]280<div class="doc_code">
281<pre>% ./config.status &lt;relative path to Makefile&gt;</pre>
282</div>
Misha Brukman6c91a332003-11-06 21:55:44 +0000[diff] [blame]283
284<p>If the Makefile is new, you will have to modify the configure script to copy
285it over.</p>
286
287</div>
288
289<div class="question">
290<p>I've upgraded to a new version of LLVM, and I get strange build errors.</p>
291</div>
292
293<div class="answer">
294
295<p>Sometimes, changes to the LLVM source code alters how the build system works.
296Changes in libtool, autoconf, or header file dependencies are especially prone
297to this sort of problem.</p>
298
299<p>The best thing to try is to remove the old files and re-build. In most
300cases, this takes care of the problem. To do this, just type <tt>make
301clean</tt> and then <tt>make</tt> in the directory that fails to build.</p>
302
303</div>
304
305<div class="question">
306<p>I've built LLVM and am testing it, but the tests freeze.</p>
307</div>
308
309<div class="answer">
310
311<p>This is most likely occurring because you built a profile or release
312(optimized) build of LLVM and have not specified the same information on the
313<tt>gmake</tt> command line.</p>
314
315<p>For example, if you built LLVM with the command:</p>
316
Bill Wendling2c22fcb2007-05-29 09:24:33 +0000[diff] [blame]317<div class="doc_code">
Bill Wendlingbeee6222007-09-22 09:54:47 +0000[diff] [blame^]318<pre>% gmake ENABLE_PROFILING=1</pre>
Bill Wendling2c22fcb2007-05-29 09:24:33 +0000[diff] [blame]319</div>
Misha Brukman6c91a332003-11-06 21:55:44 +0000[diff] [blame]320
321<p>...then you must run the tests with the following commands:</p>
322
Bill Wendling2c22fcb2007-05-29 09:24:33 +0000[diff] [blame]323<div class="doc_code">
324<pre>
325% cd llvm/test
326% gmake ENABLE_PROFILING=1
327</pre>
328</div>
Misha Brukman6c91a332003-11-06 21:55:44 +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
Chris Lattner408916b2003-12-08 05:43:19 +0000[diff] [blame]350<div class="question">
Chris Lattner5abdc6f2003-12-22 04:06:12 +0000[diff] [blame]351<p>Compiling LLVM with GCC 3.3.2 fails, what should I do?</p>
Chris Lattner408916b2003-12-08 05:43:19 +0000[diff] [blame]352</div>
353
354<div class="answer">
Chris Lattner5abdc6f2003-12-22 04:06:12 +0000[diff] [blame]355<p>This is <a href="http://gcc.gnu.org/PR?13392">a bug in GCC</a>, and
Bill Wendlingbeee6222007-09-22 09:54:47 +0000[diff] [blame^]356affects projects other than LLVM. Try upgrading or downgrading your GCC.</p>
Chris Lattner408916b2003-12-08 05:43:19 +0000[diff] [blame]357</div>
358
John Criswellfa2f3682004-03-29 20:23:11 +0000[diff] [blame]359<div class="question">
Bill Wendlingbeee6222007-09-22 09:54:47 +0000[diff] [blame^]360<p>After Subversion update, rebuilding gives the error "No rule to make
361target".</p>
Misha Brukman46572352004-09-09 16:36:47 +0000[diff] [blame]362</div>
363
364<div class="answer">
365<p>If the error is of the form:</p>
366
367<div class="doc_code">
Bill Wendling2c22fcb2007-05-29 09:24:33 +0000[diff] [blame]368<pre>
Misha Brukman46572352004-09-09 16:36:47 +0000[diff] [blame]369gmake[2]: *** No rule to make target `/path/to/somefile', needed by
370`/path/to/another/file.d'.<br>
371Stop.
Bill Wendling2c22fcb2007-05-29 09:24:33 +0000[diff] [blame]372</pre>
Misha Brukman46572352004-09-09 16:36:47 +0000[diff] [blame]373</div>
374
Reid Spencerc7f87f22007-07-09 08:04:31 +0000[diff] [blame]375<p>This may occur anytime files are moved within the Subversion repository or
376removed entirely. In this case, the best solution is to erase all
377<tt>.d</tt> files, which list dependencies for source files, and rebuild:</p>
Misha Brukman46572352004-09-09 16:36:47 +0000[diff] [blame]378
379<div class="doc_code">
380<pre>
381% cd $LLVM_OBJ_DIR
382% rm -f `find . -name \*\.d`
383% gmake
384</pre>
385</div>
386
387<p>In other cases, it may be necessary to run <tt>make clean</tt> before
388rebuilding.</p>
389</div>
390
Bill Wendlingbeee6222007-09-22 09:54:47 +0000[diff] [blame^]391<div class="question"><p><a name="llvmc">
392The <tt>llvmc</tt> program gives me errors/doesn't work.</a></p>
Bill Wendling0a303eb2007-05-29 09:35:34 +0000[diff] [blame]393</div>
394
395<div class="answer">
Bill Wendlingbeee6222007-09-22 09:54:47 +0000[diff] [blame^]396<p><tt>llvmc</tt> is experimental and isn't really supported. We suggest
397using <tt>llvm-gcc</tt> instead.</p>
Bill Wendling0a303eb2007-05-29 09:35:34 +0000[diff] [blame]398</div>
399
Misha Brukman6c91a332003-11-06 21:55:44 +0000[diff] [blame]400<!-- *********************************************************************** -->
Reid Spencer4d3171f2006-04-26 15:46:53 +0000[diff] [blame]401<div class="doc_section"><a name="felangs">Source Languages</a></div>
Reid Spencer9aa244e2006-04-26 14:52:19 +0000[diff] [blame]402
Reid Spencer4d3171f2006-04-26 15:46:53 +0000[diff] [blame]403<div class="question"><p>
404 <a name="langs">What source languages are supported?</a></p>
405</div>
406<div class="answer">
Reid Spencer9aa244e2006-04-26 14:52:19 +0000[diff] [blame]407 <p>LLVM currently has full support for C and C++ source languages. These are
408 available through a special version of GCC that LLVM calls the
409 <a href="#cfe">C Front End</a></p>
410 <p>There is an incomplete version of a Java front end available in the
Reid Spencerc7f87f22007-07-09 08:04:31 +0000[diff] [blame]411 <tt>java</tt> module. There is no documentation on this yet so
Reid Spencer9aa244e2006-04-26 14:52:19 +0000[diff] [blame]412 you'll need to download the code, compile it, and try it.</p>
Reid Spencerc7f87f22007-07-09 08:04:31 +0000[diff] [blame]413 <p>In the <tt>stacker</tt> module is a compiler and runtime
Reid Spencer9aa244e2006-04-26 14:52:19 +0000[diff] [blame]414 library for the Stacker language, a "toy" language loosely based on Forth.</p>
415 <p>The PyPy developers are working on integrating LLVM into the PyPy backend
416 so that PyPy language can translate to LLVM.</p>
417</div>
Bill Wendlingbeee6222007-09-22 09:54:47 +0000[diff] [blame^]418<div class="question"><p><a name="langhlsupp">
419 What support is there for a higher level source language constructs for
Reid Spencer4d3171f2006-04-26 15:46:53 +0000[diff] [blame]420 building a compiler?</a></p>
Reid Spencer9aa244e2006-04-26 14:52:19 +0000[diff] [blame]421</div>
422<div class="answer">
423 <p>Currently, there isn't much. LLVM supports an intermediate representation
424 which is useful for code representation but will not support the high level
425 (abstract syntax tree) representation needed by most compilers. There are no
Jeff Cohen027fbc22006-04-26 21:03:17 +0000[diff] [blame]426 facilities for lexical nor semantic analysis. There is, however, a <i>mostly
Reid Spencer9aa244e2006-04-26 14:52:19 +0000[diff] [blame]427 implemented</i> configuration-driven
428 <a href="CompilerDriver.html">compiler driver</a> which simplifies the task
429 of running optimizations, linking, and executable generation.</p>
Reid Spencer9aa244e2006-04-26 14:52:19 +0000[diff] [blame]430</div>
Chris Lattner667c9012006-08-15 00:43:35 +0000[diff] [blame]431
Bill Wendlingbeee6222007-09-22 09:54:47 +0000[diff] [blame^]432<div class="question"><p><a name="langhlsupp">
433 I don't understand the GetElementPtr instruction. Help!</a></p>
Chris Lattner667c9012006-08-15 00:43:35 +0000[diff] [blame]434</div>
435<div class="answer">
436 <p>See <a href="GetElementPtr.html">The Often Misunderstood GEP
Bill Wendlingbeee6222007-09-22 09:54:47 +0000[diff] [blame^]437 Instruction</a>.</p>
Chris Lattner667c9012006-08-15 00:43:35 +0000[diff] [blame]438</div>
439
Reid Spencer9aa244e2006-04-26 14:52:19 +0000[diff] [blame]440<!-- *********************************************************************** -->
441<div class="doc_section">
Chris Lattner23353312003-11-19 05:53:12 +0000[diff] [blame]442 <a name="cfe">Using the GCC Front End</a>
John Criswell5e0f07e2003-11-18 16:05:23 +0000[diff] [blame]443</div>
444
445<div class="question">
446<p>
447When I compile software that uses a configure script, the configure script
448thinks my system has all of the header files and libraries it is testing for.
449How do I get configure to work correctly?
450</p>
451</div>
452
453<div class="answer">
454<p>
455The configure script is getting things wrong because the LLVM linker allows
456symbols to be undefined at link time (so that they can be resolved during JIT
457or translation to the C back end). That is why configure thinks your system
458"has everything."
459</p>
460<p>
461To work around this, perform the following steps:
462</p>
John Criswell5e0f07e2003-11-18 16:05:23 +0000[diff] [blame]463<ol>
Reid Spencera7974cc2007-02-09 15:59:08 +0000[diff] [blame]464 <li>Make sure the CC and CXX environment variables contains the full path to
465 the LLVM GCC front end.</li>
John Criswell5e0f07e2003-11-18 16:05:23 +0000[diff] [blame]466
Reid Spencera7974cc2007-02-09 15:59:08 +0000[diff] [blame]467 <li>Make sure that the regular C compiler is first in your PATH. </li>
John Criswell5e0f07e2003-11-18 16:05:23 +0000[diff] [blame]468
Reid Spencera7974cc2007-02-09 15:59:08 +0000[diff] [blame]469 <li>Add the string "-Wl,-native" to your CFLAGS environment variable.</li>
John Criswell5e0f07e2003-11-18 16:05:23 +0000[diff] [blame]470</ol>
471
472<p>
Reid Spencera7974cc2007-02-09 15:59:08 +0000[diff] [blame]473This will allow the <tt>llvm-ld</tt> linker to create a native code executable
474instead of shell script that runs the JIT. Creating native code requires
475standard linkage, which in turn will allow the configure script to find out if
476code is not linking on your system because the feature isn't available on your
477system.</p>
John Criswell5e0f07e2003-11-18 16:05:23 +0000[diff] [blame]478</div>
479
Chris Lattner23353312003-11-19 05:53:12 +0000[diff] [blame]480<div class="question">
John Criswell5e0f07e2003-11-18 16:05:23 +0000[diff] [blame]481<p>
482When I compile code using the LLVM GCC front end, it complains that it cannot
John Criswelld89dbd02004-03-12 18:20:15 +0000[diff] [blame]483find libcrtend.a.
John Criswell5e0f07e2003-11-18 16:05:23 +0000[diff] [blame]484</p>
485</div>
486
Chris Lattner23353312003-11-19 05:53:12 +0000[diff] [blame]487<div class="answer">
John Criswell5e0f07e2003-11-18 16:05:23 +0000[diff] [blame]488<p>
Reid Spencer360d6b72004-12-15 00:14:01 +0000[diff] [blame]489The only way this can happen is if you haven't installed the runtime library. To
490correct this, do:</p>
Bill Wendling2c22fcb2007-05-29 09:24:33 +0000[diff] [blame]491
492<div class="doc_code">
Reid Spencer360d6b72004-12-15 00:14:01 +0000[diff] [blame]493<pre>
Bill Wendling2c22fcb2007-05-29 09:24:33 +0000[diff] [blame]494% cd llvm/runtime
495% make clean ; make install-bytecode
Reid Spencer360d6b72004-12-15 00:14:01 +0000[diff] [blame]496</pre>
John Criswell5e0f07e2003-11-18 16:05:23 +0000[diff] [blame]497</div>
Bill Wendling2c22fcb2007-05-29 09:24:33 +0000[diff] [blame]498</div>
Chris Lattner23353312003-11-19 05:53:12 +0000[diff] [blame]499
Tanya Lattneraf0ac272005-04-25 20:36:56 +0000[diff] [blame]500<div class="question">
501<p>
502How can I disable all optimizations when compiling code using the LLVM GCC front end?
503</p>
504</div>
505
506<div class="answer">
507<p>
508Passing "-Wa,-disable-opt -Wl,-disable-opt" will disable *all* cleanup and
509optimizations done at the llvm level, leaving you with the truly horrible
510code that you desire.
511</p>
512</div>
Chris Lattner23353312003-11-19 05:53:12 +0000[diff] [blame]513
Chris Lattner7b1d3c02006-07-19 18:19:59 +0000[diff] [blame]514
515<div class="question">
516<p>
517<a name="translatec++">Can I use LLVM to convert C++ code to C code?</a>
518</p>
519</div>
520
521<div class="answer">
522<p>Yes, you can use LLVM to convert code from any language LLVM supports to C.
523Note that the generated C code will be very low level (all loops are lowered
524to gotos, etc) and not very pretty (comments are stripped, original source
525formatting is totally lost, variables are renamed, expressions are regrouped),
526so this may not be what you're looking for. However, this is a good way to add
527C++ support for a processor that does not otherwise have a C++ compiler.
528</p>
529
530<p>Use commands like this:</p>
531
532<ol>
Bill Wendlingbeee6222007-09-22 09:54:47 +0000[diff] [blame^]533 <li><p>Compile your program as normal with llvm-g++:</p>
Chris Lattner7b1d3c02006-07-19 18:19:59 +0000[diff] [blame]534
Bill Wendling2c22fcb2007-05-29 09:24:33 +0000[diff] [blame]535<div class="doc_code">
536<pre>
537% llvm-g++ x.cpp -o program
538</pre>
539</div>
Chris Lattner7b1d3c02006-07-19 18:19:59 +0000[diff] [blame]540
Bill Wendlingbeee6222007-09-22 09:54:47 +0000[diff] [blame^]541 <p>or:</p>
Chris Lattner7b1d3c02006-07-19 18:19:59 +0000[diff] [blame]542
543<div class="doc_code">
Bill Wendling2c22fcb2007-05-29 09:24:33 +0000[diff] [blame]544<pre>
545% llvm-g++ a.cpp -c
546% llvm-g++ b.cpp -c
547% llvm-g++ a.o b.o -o program
548</pre>
Chris Lattner7b1d3c02006-07-19 18:19:59 +0000[diff] [blame]549</div>
550
Bill Wendlingbeee6222007-09-22 09:54:47 +0000[diff] [blame^]551 <p>With llvm-gcc3, this will generate program and program.bc. The .bc
552 file is the LLVM version of the program all linked together.</p></li>
Chris Lattner7b1d3c02006-07-19 18:19:59 +0000[diff] [blame]553
Bill Wendlingbeee6222007-09-22 09:54:47 +0000[diff] [blame^]554 <li><p>Convert the LLVM code to C code, using the LLC tool with the C
555 backend:</p>
Chris Lattner7b1d3c02006-07-19 18:19:59 +0000[diff] [blame]556
Bill Wendling2c22fcb2007-05-29 09:24:33 +0000[diff] [blame]557<div class="doc_code">
558<pre>
559% llc -march=c program.bc -o program.c
560</pre>
Bill Wendlingbeee6222007-09-22 09:54:47 +0000[diff] [blame^]561</div></li>
Chris Lattner7b1d3c02006-07-19 18:19:59 +0000[diff] [blame]562
Bill Wendlingbeee6222007-09-22 09:54:47 +0000[diff] [blame^]563<li><p>Finally, compile the C file:</p>
Chris Lattner7b1d3c02006-07-19 18:19:59 +0000[diff] [blame]564
Bill Wendling2c22fcb2007-05-29 09:24:33 +0000[diff] [blame]565<div class="doc_code">
566<pre>
567% cc x.c
568</pre>
Bill Wendlingbeee6222007-09-22 09:54:47 +0000[diff] [blame^]569</div></li>
Chris Lattner7b1d3c02006-07-19 18:19:59 +0000[diff] [blame]570
571</ol>
572
Bill Wendlingbeee6222007-09-22 09:54:47 +0000[diff] [blame^]573<p>Note that, by default, the C backend does not support exception handling. If
574you want/need it for a certain program, you can enable it by passing
575"-enable-correct-eh-support" to the llc program. The resultant code will use
576setjmp/longjmp to implement exception support that is correct but relatively
577slow.</p>
Chris Lattnerb0961902006-08-31 04:26:31 +0000[diff] [blame]578
Bill Wendlingbeee6222007-09-22 09:54:47 +0000[diff] [blame^]579<p>Also note: this specific sequence of commands won't work if you use a
580function defined in the C++ runtime library (or any other C++ library). To
581access an external C++ library, you must manually compile libstdc++ to LLVM
582bitcode, statically link it into your program, then use the commands above to
583convert the whole result into C code. Alternatively, you can compile the
584libraries and your application into two different chunks of C code and link
585them.</p>
Chris Lattnerb0961902006-08-31 04:26:31 +0000[diff] [blame]586
Chris Lattner7b1d3c02006-07-19 18:19:59 +0000[diff] [blame]587</div>
588
Chris Lattner23353312003-11-19 05:53:12 +0000[diff] [blame]589<!-- *********************************************************************** -->
590<div class="doc_section">
591 <a name="cfe_code">Questions about code generated by the GCC front-end</a>
592</div>
593
Chris Lattnere7670a02003-12-16 22:33:55 +0000[diff] [blame]594<div class="question"><p>
Chris Lattner37d55472005-02-25 20:30:21 +0000[diff] [blame]595<a name="__main"></a>
Chris Lattner23353312003-11-19 05:53:12 +0000[diff] [blame]596What is this <tt>__main()</tt> call that gets inserted into <tt>main()</tt>?
Chris Lattnere7670a02003-12-16 22:33:55 +0000[diff] [blame]597</p></div>
Chris Lattner23353312003-11-19 05:53:12 +0000[diff] [blame]598
599<div class="answer">
600<p>
601The <tt>__main</tt> call is inserted by the C/C++ compiler in order to guarantee
602that static constructors and destructors are called when the program starts up
603and shuts down. In C, you can create static constructors and destructors by
604using GCC extensions, and in C++ you can do so by creating a global variable
605whose class has a ctor or dtor.
606</p>
607
608<p>
609The actual implementation of <tt>__main</tt> lives in the
610<tt>llvm/runtime/GCCLibraries/crtend/</tt> directory in the source-base, and is
611linked in automatically when you link the program.
612</p>
Chris Lattner23353312003-11-19 05:53:12 +0000[diff] [blame]613</div>
614
Chris Lattnere7670a02003-12-16 22:33:55 +0000[diff] [blame]615<!--=========================================================================-->
Chris Lattner23353312003-11-19 05:53:12 +0000[diff] [blame]616
Misha Brukman8d9a83a2004-12-03 23:58:18 +0000[diff] [blame]617<div class="question">
Chris Lattner37d55472005-02-25 20:30:21 +0000[diff] [blame]618<a name="iosinit"></a>
Misha Brukman8d9a83a2004-12-03 23:58:18 +0000[diff] [blame]619<p> What is this <tt>llvm.global_ctors</tt> and
620<tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I #include
621&lt;iostream&gt;?</p>
622</div>
Chris Lattner87d6b402004-03-29 19:14:35 +0000[diff] [blame]623
624<div class="answer">
Misha Brukman8d9a83a2004-12-03 23:58:18 +0000[diff] [blame]625
626<p>If you #include the &lt;iostream&gt; header into a C++ translation unit, the
Chris Lattner87d6b402004-03-29 19:14:35 +0000[diff] [blame]627file will probably use the <tt>std::cin</tt>/<tt>std::cout</tt>/... global
628objects. However, C++ does not guarantee an order of initialization between
629static objects in different translation units, so if a static ctor/dtor in your
630.cpp file used <tt>std::cout</tt>, for example, the object would not necessarily
Misha Brukman8d9a83a2004-12-03 23:58:18 +0000[diff] [blame]631be automatically initialized before your use.</p>
Chris Lattner87d6b402004-03-29 19:14:35 +0000[diff] [blame]632
Misha Brukman8d9a83a2004-12-03 23:58:18 +0000[diff] [blame]633<p>To make <tt>std::cout</tt> and friends work correctly in these scenarios, the
Chris Lattner87d6b402004-03-29 19:14:35 +0000[diff] [blame]634STL that we use declares a static object that gets created in every translation
Misha Brukman8d9a83a2004-12-03 23:58:18 +0000[diff] [blame]635unit that includes <tt>&lt;iostream&gt;</tt>. This object has a static
636constructor and destructor that initializes and destroys the global iostream
637objects before they could possibly be used in the file. The code that you see
638in the .ll file corresponds to the constructor and destructor registration code.
Chris Lattner87d6b402004-03-29 19:14:35 +0000[diff] [blame]639</p>
640
Misha Brukman8d9a83a2004-12-03 23:58:18 +0000[diff] [blame]641<p>If you would like to make it easier to <b>understand</b> the LLVM code
642generated by the compiler in the demo page, consider using <tt>printf()</tt>
643instead of <tt>iostream</tt>s to print values.</p>
644
Chris Lattner87d6b402004-03-29 19:14:35 +0000[diff] [blame]645</div>
646
Chris Lattner37d55472005-02-25 20:30:21 +0000[diff] [blame]647<!--=========================================================================-->
648
649<div class="question"><p>
650<a name="codedce"></a>
651Where did all of my code go??
652</p></div>
653
654<div class="answer">
655<p>
656If you are using the LLVM demo page, you may often wonder what happened to all
657of the code that you typed in. Remember that the demo script is running the
658code through the LLVM optimizers, so if your code doesn't actually do anything
659useful, it might all be deleted.
660</p>
661
662<p>
663To prevent this, make sure that the code is actually needed. For example, if
664you are computing some expression, return the value from the function instead of
665leaving it in a local variable. If you really want to constrain the optimizer,
666you can read from and assign to <tt>volatile</tt> global variables.
667</p>
668</div>
669
670<!--=========================================================================-->
671
672<div class="question"><p>
673<a name="undef"></a>
674<p>What is this "<tt>undef</tt>" thing that shows up in my code?
675</p></div>
676
677<div class="answer">
678<p>
679<a href="LangRef.html#undef"><tt>undef</tt></a> is the LLVM way of representing
680a value that is not defined. You can get these if you do not initialize a
681variable before you use it. For example, the C function:</p>
682
683<div class="doc_code">
Bill Wendling2c22fcb2007-05-29 09:24:33 +0000[diff] [blame]684<pre>
685int X() { int i; return i; }
686</pre>
Chris Lattner37d55472005-02-25 20:30:21 +0000[diff] [blame]687</div>
688
Bill Wendling2c22fcb2007-05-29 09:24:33 +0000[diff] [blame]689<p>Is compiled to "<tt>ret i32 undef</tt>" because "<tt>i</tt>" never has
690a value specified for it.</p>
Chris Lattner37d55472005-02-25 20:30:21 +0000[diff] [blame]691</div>
692
John Criswell5e0f07e2003-11-18 16:05:23 +0000[diff] [blame]693<!-- *********************************************************************** -->
John Criswell8af73202003-10-13 16:13:06 +0000[diff] [blame]694
695<hr>
Misha Brukman5363e382004-06-01 18:51:03 +0000[diff] [blame]696<address>
697 <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
698 src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
699 <a href="http://validator.w3.org/check/referer"><img
700 src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!"></a>
701
Reid Spencerca058542006-03-14 05:39:39 +0000[diff] [blame]702 <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
Misha Brukman6c91a332003-11-06 21:55:44 +0000[diff] [blame]703 Last modified: $Date$
Misha Brukman5363e382004-06-01 18:51:03 +0000[diff] [blame]704</address>
John Criswellebe140b2003-10-24 22:48:20 +0000[diff] [blame]705
John Criswell8af73202003-10-13 16:13:06 +0000[diff] [blame]706</body>
707</html>