Gitiles
Code Review Sign In
gerrit-public.fairphone.software / platform / external / llvm / 04367bfc20c021c4105abf0c33b86d55f782d1e8 / . / docs / FAQ.html
blob: 7342488193ad6d97f8ec99dec7483165a7f11642 [file] [log] [blame]
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]1<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
2 "http://www.w3.org/TR/html4/strict.dtd">
3<html>
4<head>
5 <title>LLVM: Frequently Asked Questions</title>
Misha Brukman7ce62cc2004-06-01 18:51:03 +0000[diff] [blame]6 <style type="text/css">
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]7 @import url("llvm.css");
8 .question { font-weight: bold }
9 .answer { margin-left: 2em }
10 </style>
11</head>
12<body>
John Criswellc310f622003-10-13 16:13:06 +0000[diff] [blame]13
Misha Brukmana6538852003-11-06 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 Brukmana6538852003-11-06 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>
42 <li>I've updated my source tree from CVS, and now my build is trying to use a
43 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>
Chris Lattner306acee2003-12-22 04:06:12 +0000[diff] [blame]50 <li>Compiling LLVM with GCC 3.3.2 fails, what should I do?</li>
John Criswelld1799612004-03-29 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>
Misha Brukman1739aec2004-09-09 16:36:47 +0000[diff] [blame]53 <li>After CVS update, rebuilding gives the error "No rule to make
Bill Wendling174d5782007-05-29 09:35:34 +0000[diff] [blame]54 target".</li>
55 <li><a href="#llvmc">The <tt>llvmc</tt> program gives me errors/doesn't
56 work.</li></a>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]57 </ol></li>
John Criswell76c1e382003-11-18 16:08:49 +0000[diff] [blame]58
Reid Spencer501bfee2006-04-26 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 Spencere00906f2006-08-10 20:15:58 +0000[diff] [blame]64 <li><a href="GetElementPtr.html">I don't understand the GetElementPtr
65 instruction. Help!</a></li>
Reid Spencer501bfee2006-04-26 14:52:19 +0000[diff] [blame]66 </ol>
67
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]68 <li><a href="#cfe">Using the GCC Front End</a>
John Criswell76c1e382003-11-18 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 Lattner7911ce22004-05-23 21:07:27 +0000[diff] [blame]78 cannot find libcrtend.a.
John Criswell76c1e382003-11-18 16:08:49 +0000[diff] [blame]79 </li>
Tanya Lattner14fc5c12005-04-25 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 Lattneraf7fd202006-07-19 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 Criswell76c1e382003-11-18 16:08:49 +0000[diff] [blame]87 </ol>
88 </li>
Chris Lattnercc33d702003-11-19 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 Lattner5a53c5d2005-02-25 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 Lattnerc50bbc92004-03-29 19:14:35 +0000[diff] [blame]95 <tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I
Chris Lattner5a53c5d2005-02-25 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 Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]99 </ol>
100 </li>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]101</ol>
102
Chris Lattner7911ce22004-05-23 21:07:27 +0000[diff] [blame]103<div class="doc_author">
Reid Spencer05fe4b02006-03-14 05:39:39 +0000[diff] [blame]104 <p>Written by <a href="http://llvm.org">The LLVM Team</a></p>
Chris Lattner7911ce22004-05-23 21:07:27 +0000[diff] [blame]105</div>
106
107
Misha Brukmana6538852003-11-06 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 Spencer05fe4b02006-03-14 05:39:39 +0000[diff] [blame]144href="http://llvm.org/releases/1.3/LICENSE.TXT">LLVM license</a>.</p>
Misha Brukmana6538852003-11-06 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>
185
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>
188
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]189 <li>The LLVM build system relies heavily on UNIX shell tools, like the Bourne
190 Shell and sed. Porting to systems without these tools (MacOS 9, Plan 9) will
191 require more effort.</li>
192
193</ul>
194
195</div>
196
197<!-- *********************************************************************** -->
198<div class="doc_section">
199 <a name="build">Build Problems</a>
200</div>
201<!-- *********************************************************************** -->
202
203<div class="question">
204<p>When I run configure, it finds the wrong C compiler.</p>
205</div>
206
207<div class="answer">
208
209<p>The <tt>configure</tt> script attempts to locate first <tt>gcc</tt> and then
210<tt>cc</tt>, unless it finds compiler paths set in <tt>CC</tt> and <tt>CXX</tt>
211for the C and C++ compiler, respectively.</p>
212
213<p>If <tt>configure</tt> finds the wrong compiler, either adjust your
214<tt>PATH</tt> environment variable or set <tt>CC</tt> and <tt>CXX</tt>
215explicitly.</p>
216
217</div>
218
219<div class="question">
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]220<p>The <tt>configure</tt> script finds the right C compiler, but it uses the
221LLVM linker from a previous build. What do I do?</p>
222</div>
223
224<div class="answer">
225<p>The <tt>configure</tt> script uses the <tt>PATH</tt> to find executables, so
226if it's grabbing the wrong linker/assembler/etc, there are two ways to fix
227it:</p>
228
229<ol>
230
231 <li><p>Adjust your <tt>PATH</tt> environment variable so that the correct
232 program appears first in the <tt>PATH</tt>. This may work, but may not be
233 convenient when you want them <i>first</i> in your path for other
234 work.</p></li>
235
236 <li><p>Run <tt>configure</tt> with an alternative <tt>PATH</tt> that is
237 correct. In a Borne compatible shell, the syntax would be:</p>
238
Misha Brukman7ce62cc2004-06-01 18:51:03 +0000[diff] [blame]239 <p><tt>PATH=[the path without the bad program] ./configure ...</tt></p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]240
241 <p>This is still somewhat inconvenient, but it allows <tt>configure</tt>
242 to do its work without having to adjust your <tt>PATH</tt>
243 permanently.</p></li>
244
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">
260<p>I've updated my source tree from CVS, and now my build is trying to use a
261file/directory that doesn't exist.</p>
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">
276
277<p>If the Makefile already exists in your object tree, you
278can just run the following command in the top level directory of your object
279tree:</p>
280
281<p><tt>./config.status &lt;relative path to Makefile&gt;</tt><p>
282
283<p>If the Makefile is new, you will have to modify the configure script to copy
284it over.</p>
285
286</div>
287
288<div class="question">
289<p>I've upgraded to a new version of LLVM, and I get strange build errors.</p>
290</div>
291
292<div class="answer">
293
294<p>Sometimes, changes to the LLVM source code alters how the build system works.
295Changes in libtool, autoconf, or header file dependencies are especially prone
296to this sort of problem.</p>
297
298<p>The best thing to try is to remove the old files and re-build. In most
299cases, this takes care of the problem. To do this, just type <tt>make
300clean</tt> and then <tt>make</tt> in the directory that fails to build.</p>
301
302</div>
303
304<div class="question">
305<p>I've built LLVM and am testing it, but the tests freeze.</p>
306</div>
307
308<div class="answer">
309
310<p>This is most likely occurring because you built a profile or release
311(optimized) build of LLVM and have not specified the same information on the
312<tt>gmake</tt> command line.</p>
313
314<p>For example, if you built LLVM with the command:</p>
315
Bill Wendlingd6a68eb2007-05-29 09:24:33 +0000[diff] [blame]316<div class="doc_code">
317<pre>
318% gmake ENABLE_PROFILING=1
319</pre>
320</div>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]321
322<p>...then you must run the tests with the following commands:</p>
323
Bill Wendlingd6a68eb2007-05-29 09:24:33 +0000[diff] [blame]324<div class="doc_code">
325<pre>
326% cd llvm/test
327% gmake ENABLE_PROFILING=1
328</pre>
329</div>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]330
331</div>
332
333<div class="question">
334<p>Why do test results differ when I perform different types of builds?</p>
335</div>
336
337<div class="answer">
338
339<p>The LLVM test suite is dependent upon several features of the LLVM tools and
340libraries.</p>
341
342<p>First, the debugging assertions in code are not enabled in optimized or
343profiling builds. Hence, tests that used to fail may pass.</p>
344
345<p>Second, some tests may rely upon debugging options or behavior that is only
346available in the debug build. These tests will fail in an optimized or profile
347build.</p>
348
349</div>
350
Chris Lattner8a0b9242003-12-08 05:43:19 +0000[diff] [blame]351<div class="question">
Chris Lattner306acee2003-12-22 04:06:12 +0000[diff] [blame]352<p>Compiling LLVM with GCC 3.3.2 fails, what should I do?</p>
Chris Lattner8a0b9242003-12-08 05:43:19 +0000[diff] [blame]353</div>
354
355<div class="answer">
Chris Lattner306acee2003-12-22 04:06:12 +0000[diff] [blame]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>
Chris Lattner8a0b9242003-12-08 05:43:19 +0000[diff] [blame]358</div>
359
John Criswelld1799612004-03-29 20:23:11 +0000[diff] [blame]360<div class="question">
Misha Brukman1739aec2004-09-09 16:36:47 +0000[diff] [blame]361<p>After CVS update, rebuilding gives the error "No rule to make target".</p>
362</div>
363
364<div class="answer">
365<p>If the error is of the form:</p>
366
367<div class="doc_code">
Bill Wendlingd6a68eb2007-05-29 09:24:33 +0000[diff] [blame]368<pre>
Misha Brukman1739aec2004-09-09 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 Wendlingd6a68eb2007-05-29 09:24:33 +0000[diff] [blame]372</pre>
Misha Brukman1739aec2004-09-09 16:36:47 +0000[diff] [blame]373</div>
374
375<p>This may occur anytime files are moved within the CVS repository or removed
376entirely. In this case, the best solution is to erase all <tt>.d</tt> files,
377which list dependencies for source files, and rebuild:</p>
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 Wendling174d5782007-05-29 09:35:34 +0000[diff] [blame]391<div class="question">
392 <a name="llvmc"<p>The <tt>llvmc</tt> program gives me errors/doesn't
393 work.</p></a>
394</div>
395
396<div class="answer">
397 <p><tt>llvmc</tt> is experimental and isn't really supported. We suggest
398 using <tt>llvm-gcc</tt> instead.</p>
399</div>
400
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]401<!-- *********************************************************************** -->
Reid Spencerc87f4972006-04-26 15:46:53 +0000[diff] [blame]402<div class="doc_section"><a name="felangs">Source Languages</a></div>
Reid Spencer501bfee2006-04-26 14:52:19 +0000[diff] [blame]403
Reid Spencerc87f4972006-04-26 15:46:53 +0000[diff] [blame]404<div class="question"><p>
405 <a name="langs">What source languages are supported?</a></p>
406</div>
407<div class="answer">
Reid Spencer501bfee2006-04-26 14:52:19 +0000[diff] [blame]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>llvm-java</tt> CVS repository. 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>examples/BFtoLLVM</tt> directory is a translator for the
415 BrainF*** language (2002 Language Specification).</p>
416 <p>In the <tt>projects/Stacker</tt> directory is a compiler and runtime
417 library for the Stacker language, a "toy" language loosely based on Forth.</p>
418 <p>The PyPy developers are working on integrating LLVM into the PyPy backend
419 so that PyPy language can translate to LLVM.</p>
420</div>
Reid Spencerc87f4972006-04-26 15:46:53 +0000[diff] [blame]421<div class="question"><a name="langhlsupp">
Reid Spencer501bfee2006-04-26 14:52:19 +0000[diff] [blame]422 <p>What support is there for a higher level source language constructs for
Reid Spencerc87f4972006-04-26 15:46:53 +0000[diff] [blame]423 building a compiler?</a></p>
Reid Spencer501bfee2006-04-26 14:52:19 +0000[diff] [blame]424</div>
425<div class="answer">
426 <p>Currently, there isn't much. LLVM supports an intermediate representation
427 which is useful for code representation but will not support the high level
428 (abstract syntax tree) representation needed by most compilers. There are no
Jeff Cohen7b8229a2006-04-26 21:03:17 +0000[diff] [blame]429 facilities for lexical nor semantic analysis. There is, however, a <i>mostly
Reid Spencer501bfee2006-04-26 14:52:19 +0000[diff] [blame]430 implemented</i> configuration-driven
431 <a href="CompilerDriver.html">compiler driver</a> which simplifies the task
432 of running optimizations, linking, and executable generation.</p>
Reid Spencer501bfee2006-04-26 14:52:19 +0000[diff] [blame]433</div>
Chris Lattner33bef482006-08-15 00:43:35 +0000[diff] [blame]434
435<div class="question"><a name="langhlsupp">
436 <p>I don't understand the GetElementPtr
437 instruction. Help!</a></p>
438</div>
439<div class="answer">
440 <p>See <a href="GetElementPtr.html">The Often Misunderstood GEP
441 Instruction</a>.</li>
442</div>
443
Reid Spencer501bfee2006-04-26 14:52:19 +0000[diff] [blame]444<!-- *********************************************************************** -->
445<div class="doc_section">
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]446 <a name="cfe">Using the GCC Front End</a>
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]447</div>
448
449<div class="question">
450<p>
451When I compile software that uses a configure script, the configure script
452thinks my system has all of the header files and libraries it is testing for.
453How do I get configure to work correctly?
454</p>
455</div>
456
457<div class="answer">
458<p>
459The configure script is getting things wrong because the LLVM linker allows
460symbols to be undefined at link time (so that they can be resolved during JIT
461or translation to the C back end). That is why configure thinks your system
462"has everything."
463</p>
464<p>
465To work around this, perform the following steps:
466</p>
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]467<ol>
Reid Spencer434262a2007-02-09 15:59:08 +0000[diff] [blame]468 <li>Make sure the CC and CXX environment variables contains the full path to
469 the LLVM GCC front end.</li>
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]470
Reid Spencer434262a2007-02-09 15:59:08 +0000[diff] [blame]471 <li>Make sure that the regular C compiler is first in your PATH. </li>
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]472
Reid Spencer434262a2007-02-09 15:59:08 +0000[diff] [blame]473 <li>Add the string "-Wl,-native" to your CFLAGS environment variable.</li>
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]474</ol>
475
476<p>
Reid Spencer434262a2007-02-09 15:59:08 +0000[diff] [blame]477This will allow the <tt>llvm-ld</tt> linker to create a native code executable
478instead of shell script that runs the JIT. Creating native code requires
479standard linkage, which in turn will allow the configure script to find out if
480code is not linking on your system because the feature isn't available on your
481system.</p>
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]482</div>
483
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]484<div class="question">
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]485<p>
486When I compile code using the LLVM GCC front end, it complains that it cannot
John Criswellcd161192004-03-12 18:20:15 +0000[diff] [blame]487find libcrtend.a.
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]488</p>
489</div>
490
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]491<div class="answer">
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]492<p>
Reid Spencerf96eb572004-12-15 00:14:01 +0000[diff] [blame]493The only way this can happen is if you haven't installed the runtime library. To
494correct this, do:</p>
Bill Wendlingd6a68eb2007-05-29 09:24:33 +0000[diff] [blame]495
496<div class="doc_code">
Reid Spencerf96eb572004-12-15 00:14:01 +0000[diff] [blame]497<pre>
Bill Wendlingd6a68eb2007-05-29 09:24:33 +0000[diff] [blame]498% cd llvm/runtime
499% make clean ; make install-bytecode
Reid Spencerf96eb572004-12-15 00:14:01 +0000[diff] [blame]500</pre>
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]501</div>
Bill Wendlingd6a68eb2007-05-29 09:24:33 +0000[diff] [blame]502</div>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]503
Tanya Lattner14fc5c12005-04-25 20:36:56 +0000[diff] [blame]504<div class="question">
505<p>
506How can I disable all optimizations when compiling code using the LLVM GCC front end?
507</p>
508</div>
509
510<div class="answer">
511<p>
512Passing "-Wa,-disable-opt -Wl,-disable-opt" will disable *all* cleanup and
513optimizations done at the llvm level, leaving you with the truly horrible
514code that you desire.
515</p>
516</div>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]517
Chris Lattneraf7fd202006-07-19 18:19:59 +0000[diff] [blame]518
519<div class="question">
520<p>
521<a name="translatec++">Can I use LLVM to convert C++ code to C code?</a>
522</p>
523</div>
524
525<div class="answer">
526<p>Yes, you can use LLVM to convert code from any language LLVM supports to C.
527Note that the generated C code will be very low level (all loops are lowered
528to gotos, etc) and not very pretty (comments are stripped, original source
529formatting is totally lost, variables are renamed, expressions are regrouped),
530so this may not be what you're looking for. However, this is a good way to add
531C++ support for a processor that does not otherwise have a C++ compiler.
532</p>
533
534<p>Use commands like this:</p>
535
536<ol>
537<li><p>Compile your program as normal with llvm-g++:</p></li>
538
Bill Wendlingd6a68eb2007-05-29 09:24:33 +0000[diff] [blame]539<div class="doc_code">
540<pre>
541% llvm-g++ x.cpp -o program
542</pre>
543</div>
Chris Lattneraf7fd202006-07-19 18:19:59 +0000[diff] [blame]544
545<p>or:</p>
546
547<div class="doc_code">
Bill Wendlingd6a68eb2007-05-29 09:24:33 +0000[diff] [blame]548<pre>
549% llvm-g++ a.cpp -c
550% llvm-g++ b.cpp -c
551% llvm-g++ a.o b.o -o program
552</pre>
Chris Lattneraf7fd202006-07-19 18:19:59 +0000[diff] [blame]553</div>
554
555<p>With llvm-gcc3, this will generate program and program.bc. The .bc file is
556the LLVM version of the program all linked together.</p>
557
558<li><p>Convert the LLVM code to C code, using the LLC tool with the C
559backend:</p></li>
560
Bill Wendlingd6a68eb2007-05-29 09:24:33 +0000[diff] [blame]561<div class="doc_code">
562<pre>
563% llc -march=c program.bc -o program.c
564</pre>
565</div>
Chris Lattneraf7fd202006-07-19 18:19:59 +0000[diff] [blame]566
567<li><p>Finally, compile the c file:</p></li>
568
Bill Wendlingd6a68eb2007-05-29 09:24:33 +0000[diff] [blame]569<div class="doc_code">
570<pre>
571% cc x.c
572</pre>
573</div>
Chris Lattneraf7fd202006-07-19 18:19:59 +0000[diff] [blame]574
575</ol>
576
577<p>Note that, by default, the C backend does not support exception handling.
578If you want/need it for a certain program, you can enable it by passing
579"-enable-correct-eh-support" to the llc program. The resultant code will
580use setjmp/longjmp to implement exception support that is correct but
581relatively slow.
582</p>
Chris Lattnerb495fb02006-08-31 04:26:31 +0000[diff] [blame]583
584<p>Also note: this specific sequence of commands won't work if you use a
585function defined in the C++ runtime library (or any other C++ library). To
586access an external C++ library, you must manually
Gabor Greif04367bf2007-07-06 22:07:22 +0000[diff] [blame^]587compile libstdc++ to LLVM bitcode, statically link it into your program, then
Chris Lattnerb495fb02006-08-31 04:26:31 +0000[diff] [blame]588use the commands above to convert the whole result into C code. Alternatively,
589you can compile the libraries and your application into two different chunks
590of C code and link them.</p>
591
Chris Lattneraf7fd202006-07-19 18:19:59 +0000[diff] [blame]592</div>
593
594
595
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]596<!-- *********************************************************************** -->
597<div class="doc_section">
598 <a name="cfe_code">Questions about code generated by the GCC front-end</a>
599</div>
600
Chris Lattnera28e3ce2003-12-16 22:33:55 +0000[diff] [blame]601<div class="question"><p>
Chris Lattner5a53c5d2005-02-25 20:30:21 +0000[diff] [blame]602<a name="__main"></a>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]603What is this <tt>__main()</tt> call that gets inserted into <tt>main()</tt>?
Chris Lattnera28e3ce2003-12-16 22:33:55 +0000[diff] [blame]604</p></div>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]605
606<div class="answer">
607<p>
608The <tt>__main</tt> call is inserted by the C/C++ compiler in order to guarantee
609that static constructors and destructors are called when the program starts up
610and shuts down. In C, you can create static constructors and destructors by
611using GCC extensions, and in C++ you can do so by creating a global variable
612whose class has a ctor or dtor.
613</p>
614
615<p>
616The actual implementation of <tt>__main</tt> lives in the
617<tt>llvm/runtime/GCCLibraries/crtend/</tt> directory in the source-base, and is
618linked in automatically when you link the program.
619</p>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]620</div>
621
Chris Lattnera28e3ce2003-12-16 22:33:55 +0000[diff] [blame]622<!--=========================================================================-->
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]623
Misha Brukman237dc2a2004-12-03 23:58:18 +0000[diff] [blame]624<div class="question">
Chris Lattner5a53c5d2005-02-25 20:30:21 +0000[diff] [blame]625<a name="iosinit"></a>
Misha Brukman237dc2a2004-12-03 23:58:18 +0000[diff] [blame]626<p> What is this <tt>llvm.global_ctors</tt> and
627<tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I #include
628&lt;iostream&gt;?</p>
629</div>
Chris Lattnerc50bbc92004-03-29 19:14:35 +0000[diff] [blame]630
631<div class="answer">
Misha Brukman237dc2a2004-12-03 23:58:18 +0000[diff] [blame]632
633<p>If you #include the &lt;iostream&gt; header into a C++ translation unit, the
Chris Lattnerc50bbc92004-03-29 19:14:35 +0000[diff] [blame]634file will probably use the <tt>std::cin</tt>/<tt>std::cout</tt>/... global
635objects. However, C++ does not guarantee an order of initialization between
636static objects in different translation units, so if a static ctor/dtor in your
637.cpp file used <tt>std::cout</tt>, for example, the object would not necessarily
Misha Brukman237dc2a2004-12-03 23:58:18 +0000[diff] [blame]638be automatically initialized before your use.</p>
Chris Lattnerc50bbc92004-03-29 19:14:35 +0000[diff] [blame]639
Misha Brukman237dc2a2004-12-03 23:58:18 +0000[diff] [blame]640<p>To make <tt>std::cout</tt> and friends work correctly in these scenarios, the
Chris Lattnerc50bbc92004-03-29 19:14:35 +0000[diff] [blame]641STL that we use declares a static object that gets created in every translation
Misha Brukman237dc2a2004-12-03 23:58:18 +0000[diff] [blame]642unit that includes <tt>&lt;iostream&gt;</tt>. This object has a static
643constructor and destructor that initializes and destroys the global iostream
644objects before they could possibly be used in the file. The code that you see
645in the .ll file corresponds to the constructor and destructor registration code.
Chris Lattnerc50bbc92004-03-29 19:14:35 +0000[diff] [blame]646</p>
647
Misha Brukman237dc2a2004-12-03 23:58:18 +0000[diff] [blame]648<p>If you would like to make it easier to <b>understand</b> the LLVM code
649generated by the compiler in the demo page, consider using <tt>printf()</tt>
650instead of <tt>iostream</tt>s to print values.</p>
651
Chris Lattnerc50bbc92004-03-29 19:14:35 +0000[diff] [blame]652</div>
653
Chris Lattner5a53c5d2005-02-25 20:30:21 +0000[diff] [blame]654<!--=========================================================================-->
655
656<div class="question"><p>
657<a name="codedce"></a>
658Where did all of my code go??
659</p></div>
660
661<div class="answer">
662<p>
663If you are using the LLVM demo page, you may often wonder what happened to all
664of the code that you typed in. Remember that the demo script is running the
665code through the LLVM optimizers, so if your code doesn't actually do anything
666useful, it might all be deleted.
667</p>
668
669<p>
670To prevent this, make sure that the code is actually needed. For example, if
671you are computing some expression, return the value from the function instead of
672leaving it in a local variable. If you really want to constrain the optimizer,
673you can read from and assign to <tt>volatile</tt> global variables.
674</p>
675</div>
676
677<!--=========================================================================-->
678
679<div class="question"><p>
680<a name="undef"></a>
681<p>What is this "<tt>undef</tt>" thing that shows up in my code?
682</p></div>
683
684<div class="answer">
685<p>
686<a href="LangRef.html#undef"><tt>undef</tt></a> is the LLVM way of representing
687a value that is not defined. You can get these if you do not initialize a
688variable before you use it. For example, the C function:</p>
689
690<div class="doc_code">
Bill Wendlingd6a68eb2007-05-29 09:24:33 +0000[diff] [blame]691<pre>
692int X() { int i; return i; }
693</pre>
Chris Lattner5a53c5d2005-02-25 20:30:21 +0000[diff] [blame]694</div>
695
Bill Wendlingd6a68eb2007-05-29 09:24:33 +0000[diff] [blame]696<p>Is compiled to "<tt>ret i32 undef</tt>" because "<tt>i</tt>" never has
697a value specified for it.</p>
Chris Lattner5a53c5d2005-02-25 20:30:21 +0000[diff] [blame]698</div>
699
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]700<!-- *********************************************************************** -->
John Criswellc310f622003-10-13 16:13:06 +0000[diff] [blame]701
702<hr>
Misha Brukman7ce62cc2004-06-01 18:51:03 +0000[diff] [blame]703<address>
704 <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
705 src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
706 <a href="http://validator.w3.org/check/referer"><img
707 src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!"></a>
708
Reid Spencer05fe4b02006-03-14 05:39:39 +0000[diff] [blame]709 <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]710 Last modified: $Date$
Misha Brukman7ce62cc2004-06-01 18:51:03 +0000[diff] [blame]711</address>
John Criswellf08c5d82003-10-24 22:48:20 +0000[diff] [blame]712
John Criswellc310f622003-10-13 16:13:06 +0000[diff] [blame]713</body>
714</html>