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4<head>
5 <title>LLVM: Frequently Asked Questions</title>
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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>
39 <li>I compile the code, and I get some error about <tt>/localhome</tt>.</li>
40 <li>The <tt>configure</tt> script finds the right C compiler, but it uses the
41 LLVM linker from a previous build. What do I do?</li>
42 <li>When creating a dynamic library, I get a strange GLIBC error.</li>
43 <li>I've updated my source tree from CVS, and now my build is trying to use a
44 file/directory that doesn't exist.</li>
45 <li>I've modified a Makefile in my source tree, but my build tree keeps using
46 the old version. What do I do?</li>
47 <li>I've upgraded to a new version of LLVM, and I get strange build
48 errors.</li>
49 <li>I've built LLVM and am testing it, but the tests freeze.</li>
50 <li>Why do test results differ when I perform different types of builds?</li>
Chris Lattner306acee2003-12-22 04:06:12 +0000[diff] [blame]51 <li>Compiling LLVM with GCC 3.3.2 fails, what should I do?</li>
John Criswelld1799612004-03-29 20:23:11 +0000[diff] [blame]52 <li>When I use the test suite, all of the C Backend tests fail. What is
53 wrong?</li>
Misha Brukman1739aec2004-09-09 16:36:47 +0000[diff] [blame]54 <li>After CVS update, rebuilding gives the error "No rule to make
55 target".</li>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]56 </ol></li>
John Criswell76c1e382003-11-18 16:08:49 +0000[diff] [blame]57
Reid Spencer501bfee2006-04-26 14:52:19 +0000[diff] [blame]58 <li><a href="#felangs">Source Languages</a>
59 <ol>
60 <li><a href="#langs">What source languages are supported?</a></li>
61 <li><a href="#langhlsupp">What support is there for higher level source
62 language constructs for building a compiler?</a></li>
Reid Spencere00906f2006-08-10 20:15:58 +0000[diff] [blame]63 <li><a href="GetElementPtr.html">I don't understand the GetElementPtr
64 instruction. Help!</a></li>
Reid Spencer501bfee2006-04-26 14:52:19 +0000[diff] [blame]65 </ol>
66
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]67 <li><a href="#cfe">Using the GCC Front End</a>
John Criswell76c1e382003-11-18 16:08:49 +0000[diff] [blame]68 <ol>
69 <li>
70 When I compile software that uses a configure script, the configure script
71 thinks my system has all of the header files and libraries it is testing
72 for. How do I get configure to work correctly?
73 </li>
74
75 <li>
76 When I compile code using the LLVM GCC front end, it complains that it
Chris Lattner7911ce22004-05-23 21:07:27 +0000[diff] [blame]77 cannot find libcrtend.a.
John Criswell76c1e382003-11-18 16:08:49 +0000[diff] [blame]78 </li>
Tanya Lattner14fc5c12005-04-25 20:36:56 +0000[diff] [blame]79
80 <li>
81 How can I disable all optimizations when compiling code using the LLVM GCC front end?
82 </li>
83
Chris Lattneraf7fd202006-07-19 18:19:59 +0000[diff] [blame]84 <li><a href="#translatec++">Can I use LLVM to convert C++ code to C code?</a></li>
85
John Criswell76c1e382003-11-18 16:08:49 +0000[diff] [blame]86 </ol>
87 </li>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]88
89 <li><a href="#cfe_code">Questions about code generated by the GCC front-end</a>
90 <ol>
Chris Lattner5a53c5d2005-02-25 20:30:21 +0000[diff] [blame]91 <li><a href="#__main">What is this <tt>__main()</tt> call that gets inserted into
92 <tt>main()</tt>?</a></li>
93 <li><a href="#iosinit">What is this <tt>llvm.global_ctors</tt> and
Chris Lattnerc50bbc92004-03-29 19:14:35 +0000[diff] [blame]94 <tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I
Chris Lattner5a53c5d2005-02-25 20:30:21 +0000[diff] [blame]95 #include &lt;iostream&gt;?</a></li>
96 <li><a href="#codedce">Where did all of my code go??</a></li>
97 <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]98 </ol>
99 </li>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]100</ol>
101
Chris Lattner7911ce22004-05-23 21:07:27 +0000[diff] [blame]102<div class="doc_author">
Reid Spencer05fe4b02006-03-14 05:39:39 +0000[diff] [blame]103 <p>Written by <a href="http://llvm.org">The LLVM Team</a></p>
Chris Lattner7911ce22004-05-23 21:07:27 +0000[diff] [blame]104</div>
105
106
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]107<!-- *********************************************************************** -->
108<div class="doc_section">
109 <a name="license">License</a>
110</div>
111<!-- *********************************************************************** -->
112
113<div class="question">
114<p>Why are the LLVM source code and the front-end distributed under different
115licenses?</p>
116</div>
117
118<div class="answer">
119<p>The C/C++ front-ends are based on GCC and must be distributed under the GPL.
120Our aim is to distribute LLVM source code under a <em>much less restrictive</em>
121license, in particular one that does not compel users who distribute tools based
122on modifying the source to redistribute the modified source code as well.</p>
123</div>
124
125<div class="question">
126<p>Does the University of Illinois Open Source License really qualify as an
127"open source" license?</p>
128</div>
129
130<div class="answer">
131<p>Yes, the license is <a
132href="http://www.opensource.org/licenses/UoI-NCSA.php">certified</a> by the Open
133Source Initiative (OSI).</p>
134</div>
135
136<div class="question">
137<p>Can I modify LLVM source code and redistribute the modified source?</p>
138</div>
139
140<div class="answer">
141<p>Yes. The modified source distribution must retain the copyright notice and
142follow the three bulletted conditions listed in the <a
Reid Spencer05fe4b02006-03-14 05:39:39 +0000[diff] [blame]143href="http://llvm.org/releases/1.3/LICENSE.TXT">LLVM license</a>.</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]144</div>
145
146<div class="question">
147<p>Can I modify LLVM source code and redistribute binaries or other tools based
148on it, without redistributing the source?</p>
149</div>
150
151<div class="answer">
152<p>Yes, this is why we distribute LLVM under a less restrictive license than
153GPL, as explained in the first question above.</p>
154</div>
155
156<!-- *********************************************************************** -->
157<div class="doc_section">
158 <a name="source">Source Code</a>
159</div>
160<!-- *********************************************************************** -->
161
162<div class="question">
163<p>In what language is LLVM written?</p>
164</div>
165
166<div class="answer">
167<p>All of the LLVM tools and libraries are written in C++ with extensive use of
168the STL.</p>
169</div>
170
171<div class="question">
172<p>How portable is the LLVM source code?</p>
173</div>
174
175<div class="answer">
176<p>The LLVM source code should be portable to most modern UNIX-like operating
177systems. Most of the code is written in standard C++ with operating system
178services abstracted to a support library. The tools required to build and test
179LLVM have been ported to a plethora of platforms.</p>
180
181<p>Some porting problems may exist in the following areas:</p>
182
183<ul>
184
185 <li>The GCC front end code is not as portable as the LLVM suite, so it may not
186 compile as well on unsupported platforms.</li>
187
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]188 <li>The LLVM build system relies heavily on UNIX shell tools, like the Bourne
189 Shell and sed. Porting to systems without these tools (MacOS 9, Plan 9) will
190 require more effort.</li>
191
192</ul>
193
194</div>
195
196<!-- *********************************************************************** -->
197<div class="doc_section">
198 <a name="build">Build Problems</a>
199</div>
200<!-- *********************************************************************** -->
201
202<div class="question">
203<p>When I run configure, it finds the wrong C compiler.</p>
204</div>
205
206<div class="answer">
207
208<p>The <tt>configure</tt> script attempts to locate first <tt>gcc</tt> and then
209<tt>cc</tt>, unless it finds compiler paths set in <tt>CC</tt> and <tt>CXX</tt>
210for the C and C++ compiler, respectively.</p>
211
212<p>If <tt>configure</tt> finds the wrong compiler, either adjust your
213<tt>PATH</tt> environment variable or set <tt>CC</tt> and <tt>CXX</tt>
214explicitly.</p>
215
216</div>
217
218<div class="question">
219<p>I compile the code, and I get some error about <tt>/localhome</tt>.</p>
220</div>
221
222<div class="answer">
223
224<p>There are several possible causes for this. The first is that you didn't set
225a pathname properly when using <tt>configure</tt>, and it defaulted to a
226pathname that we use on our research machines.</p>
227
228<p>Another possibility is that we hardcoded a path in our Makefiles. If you see
229this, please email the LLVM bug mailing list with the name of the offending
230Makefile and a description of what is wrong with it.</p>
231
232</div>
233
234<div class="question">
235<p>The <tt>configure</tt> script finds the right C compiler, but it uses the
236LLVM linker from a previous build. What do I do?</p>
237</div>
238
239<div class="answer">
240<p>The <tt>configure</tt> script uses the <tt>PATH</tt> to find executables, so
241if it's grabbing the wrong linker/assembler/etc, there are two ways to fix
242it:</p>
243
244<ol>
245
246 <li><p>Adjust your <tt>PATH</tt> environment variable so that the correct
247 program appears first in the <tt>PATH</tt>. This may work, but may not be
248 convenient when you want them <i>first</i> in your path for other
249 work.</p></li>
250
251 <li><p>Run <tt>configure</tt> with an alternative <tt>PATH</tt> that is
252 correct. In a Borne compatible shell, the syntax would be:</p>
253
Misha Brukman7ce62cc2004-06-01 18:51:03 +0000[diff] [blame]254 <p><tt>PATH=[the path without the bad program] ./configure ...</tt></p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]255
256 <p>This is still somewhat inconvenient, but it allows <tt>configure</tt>
257 to do its work without having to adjust your <tt>PATH</tt>
258 permanently.</p></li>
259
260</ol>
261
262</div>
263
264<div class="question">
265<p>When creating a dynamic library, I get a strange GLIBC error.</p>
266</div>
267
268<div class="answer">
269<p>Under some operating systems (i.e. Linux), libtool does not work correctly if
270GCC was compiled with the --disable-shared option. To work around this, install
271your own version of GCC that has shared libraries enabled by default.</p>
272</div>
273
274<div class="question">
275<p>I've updated my source tree from CVS, and now my build is trying to use a
276file/directory that doesn't exist.</p>
277</div>
278
279<div class="answer">
280<p>You need to re-run configure in your object directory. When new Makefiles
281are added to the source tree, they have to be copied over to the object tree in
282order to be used by the build.</p>
283</div>
284
285<div class="question">
286<p>I've modified a Makefile in my source tree, but my build tree keeps using the
287old version. What do I do?</p>
288</div>
289
290<div class="answer">
291
292<p>If the Makefile already exists in your object tree, you
293can just run the following command in the top level directory of your object
294tree:</p>
295
296<p><tt>./config.status &lt;relative path to Makefile&gt;</tt><p>
297
298<p>If the Makefile is new, you will have to modify the configure script to copy
299it over.</p>
300
301</div>
302
303<div class="question">
304<p>I've upgraded to a new version of LLVM, and I get strange build errors.</p>
305</div>
306
307<div class="answer">
308
309<p>Sometimes, changes to the LLVM source code alters how the build system works.
310Changes in libtool, autoconf, or header file dependencies are especially prone
311to this sort of problem.</p>
312
313<p>The best thing to try is to remove the old files and re-build. In most
314cases, this takes care of the problem. To do this, just type <tt>make
315clean</tt> and then <tt>make</tt> in the directory that fails to build.</p>
316
317</div>
318
319<div class="question">
320<p>I've built LLVM and am testing it, but the tests freeze.</p>
321</div>
322
323<div class="answer">
324
325<p>This is most likely occurring because you built a profile or release
326(optimized) build of LLVM and have not specified the same information on the
327<tt>gmake</tt> command line.</p>
328
329<p>For example, if you built LLVM with the command:</p>
330
331<p><tt>gmake ENABLE_PROFILING=1</tt>
332
333<p>...then you must run the tests with the following commands:</p>
334
335<p><tt>cd llvm/test<br>gmake ENABLE_PROFILING=1</tt></p>
336
337</div>
338
339<div class="question">
340<p>Why do test results differ when I perform different types of builds?</p>
341</div>
342
343<div class="answer">
344
345<p>The LLVM test suite is dependent upon several features of the LLVM tools and
346libraries.</p>
347
348<p>First, the debugging assertions in code are not enabled in optimized or
349profiling builds. Hence, tests that used to fail may pass.</p>
350
351<p>Second, some tests may rely upon debugging options or behavior that is only
352available in the debug build. These tests will fail in an optimized or profile
353build.</p>
354
355</div>
356
Chris Lattner8a0b9242003-12-08 05:43:19 +0000[diff] [blame]357<div class="question">
Chris Lattner306acee2003-12-22 04:06:12 +0000[diff] [blame]358<p>Compiling LLVM with GCC 3.3.2 fails, what should I do?</p>
Chris Lattner8a0b9242003-12-08 05:43:19 +0000[diff] [blame]359</div>
360
361<div class="answer">
Chris Lattner306acee2003-12-22 04:06:12 +0000[diff] [blame]362<p>This is <a href="http://gcc.gnu.org/PR?13392">a bug in GCC</a>, and
363 affects projects other than LLVM. Try upgrading or downgrading your GCC.</p>
Chris Lattner8a0b9242003-12-08 05:43:19 +0000[diff] [blame]364</div>
365
John Criswelld1799612004-03-29 20:23:11 +0000[diff] [blame]366<div class="question">
Misha Brukman1739aec2004-09-09 16:36:47 +0000[diff] [blame]367<p>After CVS update, rebuilding gives the error "No rule to make target".</p>
368</div>
369
370<div class="answer">
371<p>If the error is of the form:</p>
372
373<div class="doc_code">
374<tt>
375gmake[2]: *** No rule to make target `/path/to/somefile', needed by
376`/path/to/another/file.d'.<br>
377Stop.
378</tt>
379</div>
380
381<p>This may occur anytime files are moved within the CVS repository or removed
382entirely. In this case, the best solution is to erase all <tt>.d</tt> files,
383which list dependencies for source files, and rebuild:</p>
384
385<div class="doc_code">
386<pre>
387% cd $LLVM_OBJ_DIR
388% rm -f `find . -name \*\.d`
389% gmake
390</pre>
391</div>
392
393<p>In other cases, it may be necessary to run <tt>make clean</tt> before
394rebuilding.</p>
395</div>
396
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]397<!-- *********************************************************************** -->
Reid Spencerc87f4972006-04-26 15:46:53 +0000[diff] [blame]398<div class="doc_section"><a name="felangs">Source Languages</a></div>
Reid Spencer501bfee2006-04-26 14:52:19 +0000[diff] [blame]399
Reid Spencerc87f4972006-04-26 15:46:53 +0000[diff] [blame]400<div class="question"><p>
401 <a name="langs">What source languages are supported?</a></p>
402</div>
403<div class="answer">
Reid Spencer501bfee2006-04-26 14:52:19 +0000[diff] [blame]404 <p>LLVM currently has full support for C and C++ source languages. These are
405 available through a special version of GCC that LLVM calls the
406 <a href="#cfe">C Front End</a></p>
407 <p>There is an incomplete version of a Java front end available in the
408 <tt>llvm-java</tt> CVS repository. There is no documentation on this yet so
409 you'll need to download the code, compile it, and try it.</p>
410 <p>In the <tt>examples/BFtoLLVM</tt> directory is a translator for the
411 BrainF*** language (2002 Language Specification).</p>
412 <p>In the <tt>projects/Stacker</tt> directory is a compiler and runtime
413 library for the Stacker language, a "toy" language loosely based on Forth.</p>
414 <p>The PyPy developers are working on integrating LLVM into the PyPy backend
415 so that PyPy language can translate to LLVM.</p>
416</div>
Reid Spencerc87f4972006-04-26 15:46:53 +0000[diff] [blame]417<div class="question"><a name="langhlsupp">
Reid Spencer501bfee2006-04-26 14:52:19 +0000[diff] [blame]418 <p>What support is there for a higher level source language constructs for
Reid Spencerc87f4972006-04-26 15:46:53 +0000[diff] [blame]419 building a compiler?</a></p>
Reid Spencer501bfee2006-04-26 14:52:19 +0000[diff] [blame]420</div>
421<div class="answer">
422 <p>Currently, there isn't much. LLVM supports an intermediate representation
423 which is useful for code representation but will not support the high level
424 (abstract syntax tree) representation needed by most compilers. There are no
Jeff Cohen7b8229a2006-04-26 21:03:17 +0000[diff] [blame]425 facilities for lexical nor semantic analysis. There is, however, a <i>mostly
Reid Spencer501bfee2006-04-26 14:52:19 +0000[diff] [blame]426 implemented</i> configuration-driven
427 <a href="CompilerDriver.html">compiler driver</a> which simplifies the task
428 of running optimizations, linking, and executable generation.</p>
Reid Spencer501bfee2006-04-26 14:52:19 +0000[diff] [blame]429</div>
Chris Lattner33bef482006-08-15 00:43:35 +0000[diff] [blame^]430
431<div class="question"><a name="langhlsupp">
432 <p>I don't understand the GetElementPtr
433 instruction. Help!</a></p>
434</div>
435<div class="answer">
436 <p>See <a href="GetElementPtr.html">The Often Misunderstood GEP
437 Instruction</a>.</li>
438</div>
439
Reid Spencer501bfee2006-04-26 14:52:19 +0000[diff] [blame]440<!-- *********************************************************************** -->
441<div class="doc_section">
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]442 <a name="cfe">Using the GCC Front End</a>
John Criswell6ea30b02003-11-18 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>
463
464<ol>
465 <li>
466 Make sure the CC and CXX environment variables contains the full path to the
467 LLVM GCC front end.
468 </li>
469
470 <li>
471 Make sure that the regular C compiler is first in your PATH.
472 </li>
473
474 <li>
475 Add the string "-Wl,-native" to your CFLAGS environment variable.
476 </li>
477</ol>
478
479<p>
480This will allow the gccld linker to create a native code executable instead of
481a shell script that runs the JIT. Creating native code requires standard
482linkage, which in turn will allow the configure script to find out if code is
483not linking on your system because the feature isn't available on your system.
484</p>
485</div>
486
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]487<div class="question">
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]488<p>
489When I compile code using the LLVM GCC front end, it complains that it cannot
John Criswellcd161192004-03-12 18:20:15 +0000[diff] [blame]490find libcrtend.a.
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]491</p>
492</div>
493
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]494<div class="answer">
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]495<p>
Reid Spencerf96eb572004-12-15 00:14:01 +0000[diff] [blame]496The only way this can happen is if you haven't installed the runtime library. To
497correct this, do:</p>
498<pre>
499 % cd llvm/runtime
500 % make clean ; make install-bytecode
501</pre>
John Criswell6ea30b02003-11-18 16:05:23 +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
539<div class="doc_code">$ llvm-g++ x.cpp -o program</div>
540
541<p>or:</p>
542
543<div class="doc_code">
544 llvm-g++ a.cpp -c
545 llvm-g++ b.cpp -c
546 llvm-g++ a.o b.o -o program
547</div>
548
549<p>With llvm-gcc3, this will generate program and program.bc. The .bc file is
550the LLVM version of the program all linked together.</p>
551
552<li><p>Convert the LLVM code to C code, using the LLC tool with the C
553backend:</p></li>
554
555<div class="doc_code">$ llc -march=c program.bc -o program.c</div>
556
557<li><p>Finally, compile the c file:</p></li>
558
559<div class="doc_code">$ cc x.c</div>
560
561</ol>
562
563<p>Note that, by default, the C backend does not support exception handling.
564If you want/need it for a certain program, you can enable it by passing
565"-enable-correct-eh-support" to the llc program. The resultant code will
566use setjmp/longjmp to implement exception support that is correct but
567relatively slow.
568</p>
569</div>
570
571
572
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]573<!-- *********************************************************************** -->
574<div class="doc_section">
575 <a name="cfe_code">Questions about code generated by the GCC front-end</a>
576</div>
577
Chris Lattnera28e3ce2003-12-16 22:33:55 +0000[diff] [blame]578<div class="question"><p>
Chris Lattner5a53c5d2005-02-25 20:30:21 +0000[diff] [blame]579<a name="__main"></a>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]580What is this <tt>__main()</tt> call that gets inserted into <tt>main()</tt>?
Chris Lattnera28e3ce2003-12-16 22:33:55 +0000[diff] [blame]581</p></div>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]582
583<div class="answer">
584<p>
585The <tt>__main</tt> call is inserted by the C/C++ compiler in order to guarantee
586that static constructors and destructors are called when the program starts up
587and shuts down. In C, you can create static constructors and destructors by
588using GCC extensions, and in C++ you can do so by creating a global variable
589whose class has a ctor or dtor.
590</p>
591
592<p>
593The actual implementation of <tt>__main</tt> lives in the
594<tt>llvm/runtime/GCCLibraries/crtend/</tt> directory in the source-base, and is
595linked in automatically when you link the program.
596</p>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]597</div>
598
Chris Lattnera28e3ce2003-12-16 22:33:55 +0000[diff] [blame]599<!--=========================================================================-->
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]600
Misha Brukman237dc2a2004-12-03 23:58:18 +0000[diff] [blame]601<div class="question">
Chris Lattner5a53c5d2005-02-25 20:30:21 +0000[diff] [blame]602<a name="iosinit"></a>
Misha Brukman237dc2a2004-12-03 23:58:18 +0000[diff] [blame]603<p> What is this <tt>llvm.global_ctors</tt> and
604<tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I #include
605&lt;iostream&gt;?</p>
606</div>
Chris Lattnerc50bbc92004-03-29 19:14:35 +0000[diff] [blame]607
608<div class="answer">
Misha Brukman237dc2a2004-12-03 23:58:18 +0000[diff] [blame]609
610<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]611file will probably use the <tt>std::cin</tt>/<tt>std::cout</tt>/... global
612objects. However, C++ does not guarantee an order of initialization between
613static objects in different translation units, so if a static ctor/dtor in your
614.cpp file used <tt>std::cout</tt>, for example, the object would not necessarily
Misha Brukman237dc2a2004-12-03 23:58:18 +0000[diff] [blame]615be automatically initialized before your use.</p>
Chris Lattnerc50bbc92004-03-29 19:14:35 +0000[diff] [blame]616
Misha Brukman237dc2a2004-12-03 23:58:18 +0000[diff] [blame]617<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]618STL that we use declares a static object that gets created in every translation
Misha Brukman237dc2a2004-12-03 23:58:18 +0000[diff] [blame]619unit that includes <tt>&lt;iostream&gt;</tt>. This object has a static
620constructor and destructor that initializes and destroys the global iostream
621objects before they could possibly be used in the file. The code that you see
622in the .ll file corresponds to the constructor and destructor registration code.
Chris Lattnerc50bbc92004-03-29 19:14:35 +0000[diff] [blame]623</p>
624
Misha Brukman237dc2a2004-12-03 23:58:18 +0000[diff] [blame]625<p>If you would like to make it easier to <b>understand</b> the LLVM code
626generated by the compiler in the demo page, consider using <tt>printf()</tt>
627instead of <tt>iostream</tt>s to print values.</p>
628
Chris Lattnerc50bbc92004-03-29 19:14:35 +0000[diff] [blame]629</div>
630
Chris Lattner5a53c5d2005-02-25 20:30:21 +0000[diff] [blame]631<!--=========================================================================-->
632
633<div class="question"><p>
634<a name="codedce"></a>
635Where did all of my code go??
636</p></div>
637
638<div class="answer">
639<p>
640If you are using the LLVM demo page, you may often wonder what happened to all
641of the code that you typed in. Remember that the demo script is running the
642code through the LLVM optimizers, so if your code doesn't actually do anything
643useful, it might all be deleted.
644</p>
645
646<p>
647To prevent this, make sure that the code is actually needed. For example, if
648you are computing some expression, return the value from the function instead of
649leaving it in a local variable. If you really want to constrain the optimizer,
650you can read from and assign to <tt>volatile</tt> global variables.
651</p>
652</div>
653
654<!--=========================================================================-->
655
656<div class="question"><p>
657<a name="undef"></a>
658<p>What is this "<tt>undef</tt>" thing that shows up in my code?
659</p></div>
660
661<div class="answer">
662<p>
663<a href="LangRef.html#undef"><tt>undef</tt></a> is the LLVM way of representing
664a value that is not defined. You can get these if you do not initialize a
665variable before you use it. For example, the C function:</p>
666
667<div class="doc_code">
668 <tt>int X() { int i; return i; }</tt>
669</div>
670
671<p>Is compiled to "<tt>ret int undef</tt>" because "i" never has a value
672specified for it.
673</p>
674</div>
675
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]676<!-- *********************************************************************** -->
John Criswellc310f622003-10-13 16:13:06 +0000[diff] [blame]677
678<hr>
Misha Brukman7ce62cc2004-06-01 18:51:03 +0000[diff] [blame]679<address>
680 <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
681 src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
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684
Reid Spencer05fe4b02006-03-14 05:39:39 +0000[diff] [blame]685 <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]686 Last modified: $Date$
Misha Brukman7ce62cc2004-06-01 18:51:03 +0000[diff] [blame]687</address>
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690</html>