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5 <title>LLVM: Frequently Asked Questions</title>
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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>
63 </ol>
64
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]65 <li><a href="#cfe">Using the GCC Front End</a>
John Criswell76c1e382003-11-18 16:08:49 +0000[diff] [blame]66 <ol>
67 <li>
68 When I compile software that uses a configure script, the configure script
69 thinks my system has all of the header files and libraries it is testing
70 for. How do I get configure to work correctly?
71 </li>
72
73 <li>
74 When I compile code using the LLVM GCC front end, it complains that it
Chris Lattner7911ce22004-05-23 21:07:27 +0000[diff] [blame]75 cannot find libcrtend.a.
John Criswell76c1e382003-11-18 16:08:49 +0000[diff] [blame]76 </li>
Tanya Lattner14fc5c12005-04-25 20:36:56 +0000[diff] [blame]77
78 <li>
79 How can I disable all optimizations when compiling code using the LLVM GCC front end?
80 </li>
81
Chris Lattneraf7fd202006-07-19 18:19:59 +0000[diff] [blame^]82 <li><a href="#translatec++">Can I use LLVM to convert C++ code to C code?</a></li>
83
John Criswell76c1e382003-11-18 16:08:49 +0000[diff] [blame]84 </ol>
85 </li>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]86
87 <li><a href="#cfe_code">Questions about code generated by the GCC front-end</a>
88 <ol>
Chris Lattner5a53c5d2005-02-25 20:30:21 +0000[diff] [blame]89 <li><a href="#__main">What is this <tt>__main()</tt> call that gets inserted into
90 <tt>main()</tt>?</a></li>
91 <li><a href="#iosinit">What is this <tt>llvm.global_ctors</tt> and
Chris Lattnerc50bbc92004-03-29 19:14:35 +0000[diff] [blame]92 <tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I
Chris Lattner5a53c5d2005-02-25 20:30:21 +0000[diff] [blame]93 #include &lt;iostream&gt;?</a></li>
94 <li><a href="#codedce">Where did all of my code go??</a></li>
95 <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]96 </ol>
97 </li>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]98</ol>
99
Chris Lattner7911ce22004-05-23 21:07:27 +0000[diff] [blame]100<div class="doc_author">
Reid Spencer05fe4b02006-03-14 05:39:39 +0000[diff] [blame]101 <p>Written by <a href="http://llvm.org">The LLVM Team</a></p>
Chris Lattner7911ce22004-05-23 21:07:27 +0000[diff] [blame]102</div>
103
104
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]105<!-- *********************************************************************** -->
106<div class="doc_section">
107 <a name="license">License</a>
108</div>
109<!-- *********************************************************************** -->
110
111<div class="question">
112<p>Why are the LLVM source code and the front-end distributed under different
113licenses?</p>
114</div>
115
116<div class="answer">
117<p>The C/C++ front-ends are based on GCC and must be distributed under the GPL.
118Our aim is to distribute LLVM source code under a <em>much less restrictive</em>
119license, in particular one that does not compel users who distribute tools based
120on modifying the source to redistribute the modified source code as well.</p>
121</div>
122
123<div class="question">
124<p>Does the University of Illinois Open Source License really qualify as an
125"open source" license?</p>
126</div>
127
128<div class="answer">
129<p>Yes, the license is <a
130href="http://www.opensource.org/licenses/UoI-NCSA.php">certified</a> by the Open
131Source Initiative (OSI).</p>
132</div>
133
134<div class="question">
135<p>Can I modify LLVM source code and redistribute the modified source?</p>
136</div>
137
138<div class="answer">
139<p>Yes. The modified source distribution must retain the copyright notice and
140follow the three bulletted conditions listed in the <a
Reid Spencer05fe4b02006-03-14 05:39:39 +0000[diff] [blame]141href="http://llvm.org/releases/1.3/LICENSE.TXT">LLVM license</a>.</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]142</div>
143
144<div class="question">
145<p>Can I modify LLVM source code and redistribute binaries or other tools based
146on it, without redistributing the source?</p>
147</div>
148
149<div class="answer">
150<p>Yes, this is why we distribute LLVM under a less restrictive license than
151GPL, as explained in the first question above.</p>
152</div>
153
154<!-- *********************************************************************** -->
155<div class="doc_section">
156 <a name="source">Source Code</a>
157</div>
158<!-- *********************************************************************** -->
159
160<div class="question">
161<p>In what language is LLVM written?</p>
162</div>
163
164<div class="answer">
165<p>All of the LLVM tools and libraries are written in C++ with extensive use of
166the STL.</p>
167</div>
168
169<div class="question">
170<p>How portable is the LLVM source code?</p>
171</div>
172
173<div class="answer">
174<p>The LLVM source code should be portable to most modern UNIX-like operating
175systems. Most of the code is written in standard C++ with operating system
176services abstracted to a support library. The tools required to build and test
177LLVM have been ported to a plethora of platforms.</p>
178
179<p>Some porting problems may exist in the following areas:</p>
180
181<ul>
182
183 <li>The GCC front end code is not as portable as the LLVM suite, so it may not
184 compile as well on unsupported platforms.</li>
185
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]186 <li>The LLVM build system relies heavily on UNIX shell tools, like the Bourne
187 Shell and sed. Porting to systems without these tools (MacOS 9, Plan 9) will
188 require more effort.</li>
189
190</ul>
191
192</div>
193
194<!-- *********************************************************************** -->
195<div class="doc_section">
196 <a name="build">Build Problems</a>
197</div>
198<!-- *********************************************************************** -->
199
200<div class="question">
201<p>When I run configure, it finds the wrong C compiler.</p>
202</div>
203
204<div class="answer">
205
206<p>The <tt>configure</tt> script attempts to locate first <tt>gcc</tt> and then
207<tt>cc</tt>, unless it finds compiler paths set in <tt>CC</tt> and <tt>CXX</tt>
208for the C and C++ compiler, respectively.</p>
209
210<p>If <tt>configure</tt> finds the wrong compiler, either adjust your
211<tt>PATH</tt> environment variable or set <tt>CC</tt> and <tt>CXX</tt>
212explicitly.</p>
213
214</div>
215
216<div class="question">
217<p>I compile the code, and I get some error about <tt>/localhome</tt>.</p>
218</div>
219
220<div class="answer">
221
222<p>There are several possible causes for this. The first is that you didn't set
223a pathname properly when using <tt>configure</tt>, and it defaulted to a
224pathname that we use on our research machines.</p>
225
226<p>Another possibility is that we hardcoded a path in our Makefiles. If you see
227this, please email the LLVM bug mailing list with the name of the offending
228Makefile and a description of what is wrong with it.</p>
229
230</div>
231
232<div class="question">
233<p>The <tt>configure</tt> script finds the right C compiler, but it uses the
234LLVM linker from a previous build. What do I do?</p>
235</div>
236
237<div class="answer">
238<p>The <tt>configure</tt> script uses the <tt>PATH</tt> to find executables, so
239if it's grabbing the wrong linker/assembler/etc, there are two ways to fix
240it:</p>
241
242<ol>
243
244 <li><p>Adjust your <tt>PATH</tt> environment variable so that the correct
245 program appears first in the <tt>PATH</tt>. This may work, but may not be
246 convenient when you want them <i>first</i> in your path for other
247 work.</p></li>
248
249 <li><p>Run <tt>configure</tt> with an alternative <tt>PATH</tt> that is
250 correct. In a Borne compatible shell, the syntax would be:</p>
251
Misha Brukman7ce62cc2004-06-01 18:51:03 +0000[diff] [blame]252 <p><tt>PATH=[the path without the bad program] ./configure ...</tt></p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]253
254 <p>This is still somewhat inconvenient, but it allows <tt>configure</tt>
255 to do its work without having to adjust your <tt>PATH</tt>
256 permanently.</p></li>
257
258</ol>
259
260</div>
261
262<div class="question">
263<p>When creating a dynamic library, I get a strange GLIBC error.</p>
264</div>
265
266<div class="answer">
267<p>Under some operating systems (i.e. Linux), libtool does not work correctly if
268GCC was compiled with the --disable-shared option. To work around this, install
269your own version of GCC that has shared libraries enabled by default.</p>
270</div>
271
272<div class="question">
273<p>I've updated my source tree from CVS, and now my build is trying to use a
274file/directory that doesn't exist.</p>
275</div>
276
277<div class="answer">
278<p>You need to re-run configure in your object directory. When new Makefiles
279are added to the source tree, they have to be copied over to the object tree in
280order to be used by the build.</p>
281</div>
282
283<div class="question">
284<p>I've modified a Makefile in my source tree, but my build tree keeps using the
285old version. What do I do?</p>
286</div>
287
288<div class="answer">
289
290<p>If the Makefile already exists in your object tree, you
291can just run the following command in the top level directory of your object
292tree:</p>
293
294<p><tt>./config.status &lt;relative path to Makefile&gt;</tt><p>
295
296<p>If the Makefile is new, you will have to modify the configure script to copy
297it over.</p>
298
299</div>
300
301<div class="question">
302<p>I've upgraded to a new version of LLVM, and I get strange build errors.</p>
303</div>
304
305<div class="answer">
306
307<p>Sometimes, changes to the LLVM source code alters how the build system works.
308Changes in libtool, autoconf, or header file dependencies are especially prone
309to this sort of problem.</p>
310
311<p>The best thing to try is to remove the old files and re-build. In most
312cases, this takes care of the problem. To do this, just type <tt>make
313clean</tt> and then <tt>make</tt> in the directory that fails to build.</p>
314
315</div>
316
317<div class="question">
318<p>I've built LLVM and am testing it, but the tests freeze.</p>
319</div>
320
321<div class="answer">
322
323<p>This is most likely occurring because you built a profile or release
324(optimized) build of LLVM and have not specified the same information on the
325<tt>gmake</tt> command line.</p>
326
327<p>For example, if you built LLVM with the command:</p>
328
329<p><tt>gmake ENABLE_PROFILING=1</tt>
330
331<p>...then you must run the tests with the following commands:</p>
332
333<p><tt>cd llvm/test<br>gmake ENABLE_PROFILING=1</tt></p>
334
335</div>
336
337<div class="question">
338<p>Why do test results differ when I perform different types of builds?</p>
339</div>
340
341<div class="answer">
342
343<p>The LLVM test suite is dependent upon several features of the LLVM tools and
344libraries.</p>
345
346<p>First, the debugging assertions in code are not enabled in optimized or
347profiling builds. Hence, tests that used to fail may pass.</p>
348
349<p>Second, some tests may rely upon debugging options or behavior that is only
350available in the debug build. These tests will fail in an optimized or profile
351build.</p>
352
353</div>
354
Chris Lattner8a0b9242003-12-08 05:43:19 +0000[diff] [blame]355<div class="question">
Chris Lattner306acee2003-12-22 04:06:12 +0000[diff] [blame]356<p>Compiling LLVM with GCC 3.3.2 fails, what should I do?</p>
Chris Lattner8a0b9242003-12-08 05:43:19 +0000[diff] [blame]357</div>
358
359<div class="answer">
Chris Lattner306acee2003-12-22 04:06:12 +0000[diff] [blame]360<p>This is <a href="http://gcc.gnu.org/PR?13392">a bug in GCC</a>, and
361 affects projects other than LLVM. Try upgrading or downgrading your GCC.</p>
Chris Lattner8a0b9242003-12-08 05:43:19 +0000[diff] [blame]362</div>
363
John Criswelld1799612004-03-29 20:23:11 +0000[diff] [blame]364<div class="question">
Misha Brukman1739aec2004-09-09 16:36:47 +0000[diff] [blame]365<p>After CVS update, rebuilding gives the error "No rule to make target".</p>
366</div>
367
368<div class="answer">
369<p>If the error is of the form:</p>
370
371<div class="doc_code">
372<tt>
373gmake[2]: *** No rule to make target `/path/to/somefile', needed by
374`/path/to/another/file.d'.<br>
375Stop.
376</tt>
377</div>
378
379<p>This may occur anytime files are moved within the CVS repository or removed
380entirely. In this case, the best solution is to erase all <tt>.d</tt> files,
381which list dependencies for source files, and rebuild:</p>
382
383<div class="doc_code">
384<pre>
385% cd $LLVM_OBJ_DIR
386% rm -f `find . -name \*\.d`
387% gmake
388</pre>
389</div>
390
391<p>In other cases, it may be necessary to run <tt>make clean</tt> before
392rebuilding.</p>
393</div>
394
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]395<!-- *********************************************************************** -->
Reid Spencerc87f4972006-04-26 15:46:53 +0000[diff] [blame]396<div class="doc_section"><a name="felangs">Source Languages</a></div>
Reid Spencer501bfee2006-04-26 14:52:19 +0000[diff] [blame]397
Reid Spencerc87f4972006-04-26 15:46:53 +0000[diff] [blame]398<div class="question"><p>
399 <a name="langs">What source languages are supported?</a></p>
400</div>
401<div class="answer">
Reid Spencer501bfee2006-04-26 14:52:19 +0000[diff] [blame]402 <p>LLVM currently has full support for C and C++ source languages. These are
403 available through a special version of GCC that LLVM calls the
404 <a href="#cfe">C Front End</a></p>
405 <p>There is an incomplete version of a Java front end available in the
406 <tt>llvm-java</tt> CVS repository. There is no documentation on this yet so
407 you'll need to download the code, compile it, and try it.</p>
408 <p>In the <tt>examples/BFtoLLVM</tt> directory is a translator for the
409 BrainF*** language (2002 Language Specification).</p>
410 <p>In the <tt>projects/Stacker</tt> directory is a compiler and runtime
411 library for the Stacker language, a "toy" language loosely based on Forth.</p>
412 <p>The PyPy developers are working on integrating LLVM into the PyPy backend
413 so that PyPy language can translate to LLVM.</p>
414</div>
Reid Spencerc87f4972006-04-26 15:46:53 +0000[diff] [blame]415<div class="question"><a name="langhlsupp">
Reid Spencer501bfee2006-04-26 14:52:19 +0000[diff] [blame]416 <p>What support is there for a higher level source language constructs for
Reid Spencerc87f4972006-04-26 15:46:53 +0000[diff] [blame]417 building a compiler?</a></p>
Reid Spencer501bfee2006-04-26 14:52:19 +0000[diff] [blame]418</div>
419<div class="answer">
420 <p>Currently, there isn't much. LLVM supports an intermediate representation
421 which is useful for code representation but will not support the high level
422 (abstract syntax tree) representation needed by most compilers. There are no
Jeff Cohen7b8229a2006-04-26 21:03:17 +0000[diff] [blame]423 facilities for lexical nor semantic analysis. There is, however, a <i>mostly
Reid Spencer501bfee2006-04-26 14:52:19 +0000[diff] [blame]424 implemented</i> configuration-driven
425 <a href="CompilerDriver.html">compiler driver</a> which simplifies the task
426 of running optimizations, linking, and executable generation.</p>
Reid Spencer501bfee2006-04-26 14:52:19 +0000[diff] [blame]427</div>
428<!-- *********************************************************************** -->
429<div class="doc_section">
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]430 <a name="cfe">Using the GCC Front End</a>
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]431</div>
432
433<div class="question">
434<p>
435When I compile software that uses a configure script, the configure script
436thinks my system has all of the header files and libraries it is testing for.
437How do I get configure to work correctly?
438</p>
439</div>
440
441<div class="answer">
442<p>
443The configure script is getting things wrong because the LLVM linker allows
444symbols to be undefined at link time (so that they can be resolved during JIT
445or translation to the C back end). That is why configure thinks your system
446"has everything."
447</p>
448<p>
449To work around this, perform the following steps:
450</p>
451
452<ol>
453 <li>
454 Make sure the CC and CXX environment variables contains the full path to the
455 LLVM GCC front end.
456 </li>
457
458 <li>
459 Make sure that the regular C compiler is first in your PATH.
460 </li>
461
462 <li>
463 Add the string "-Wl,-native" to your CFLAGS environment variable.
464 </li>
465</ol>
466
467<p>
468This will allow the gccld linker to create a native code executable instead of
469a shell script that runs the JIT. Creating native code requires standard
470linkage, which in turn will allow the configure script to find out if code is
471not linking on your system because the feature isn't available on your system.
472</p>
473</div>
474
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]475<div class="question">
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]476<p>
477When I compile code using the LLVM GCC front end, it complains that it cannot
John Criswellcd161192004-03-12 18:20:15 +0000[diff] [blame]478find libcrtend.a.
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]479</p>
480</div>
481
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]482<div class="answer">
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]483<p>
Reid Spencerf96eb572004-12-15 00:14:01 +0000[diff] [blame]484The only way this can happen is if you haven't installed the runtime library. To
485correct this, do:</p>
486<pre>
487 % cd llvm/runtime
488 % make clean ; make install-bytecode
489</pre>
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]490</div>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]491
Tanya Lattner14fc5c12005-04-25 20:36:56 +0000[diff] [blame]492<div class="question">
493<p>
494How can I disable all optimizations when compiling code using the LLVM GCC front end?
495</p>
496</div>
497
498<div class="answer">
499<p>
500Passing "-Wa,-disable-opt -Wl,-disable-opt" will disable *all* cleanup and
501optimizations done at the llvm level, leaving you with the truly horrible
502code that you desire.
503</p>
504</div>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]505
Chris Lattneraf7fd202006-07-19 18:19:59 +0000[diff] [blame^]506
507<div class="question">
508<p>
509<a name="translatec++">Can I use LLVM to convert C++ code to C code?</a>
510</p>
511</div>
512
513<div class="answer">
514<p>Yes, you can use LLVM to convert code from any language LLVM supports to C.
515Note that the generated C code will be very low level (all loops are lowered
516to gotos, etc) and not very pretty (comments are stripped, original source
517formatting is totally lost, variables are renamed, expressions are regrouped),
518so this may not be what you're looking for. However, this is a good way to add
519C++ support for a processor that does not otherwise have a C++ compiler.
520</p>
521
522<p>Use commands like this:</p>
523
524<ol>
525<li><p>Compile your program as normal with llvm-g++:</p></li>
526
527<div class="doc_code">$ llvm-g++ x.cpp -o program</div>
528
529<p>or:</p>
530
531<div class="doc_code">
532 llvm-g++ a.cpp -c
533 llvm-g++ b.cpp -c
534 llvm-g++ a.o b.o -o program
535</div>
536
537<p>With llvm-gcc3, this will generate program and program.bc. The .bc file is
538the LLVM version of the program all linked together.</p>
539
540<li><p>Convert the LLVM code to C code, using the LLC tool with the C
541backend:</p></li>
542
543<div class="doc_code">$ llc -march=c program.bc -o program.c</div>
544
545<li><p>Finally, compile the c file:</p></li>
546
547<div class="doc_code">$ cc x.c</div>
548
549</ol>
550
551<p>Note that, by default, the C backend does not support exception handling.
552If you want/need it for a certain program, you can enable it by passing
553"-enable-correct-eh-support" to the llc program. The resultant code will
554use setjmp/longjmp to implement exception support that is correct but
555relatively slow.
556</p>
557</div>
558
559
560
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]561<!-- *********************************************************************** -->
562<div class="doc_section">
563 <a name="cfe_code">Questions about code generated by the GCC front-end</a>
564</div>
565
Chris Lattnera28e3ce2003-12-16 22:33:55 +0000[diff] [blame]566<div class="question"><p>
Chris Lattner5a53c5d2005-02-25 20:30:21 +0000[diff] [blame]567<a name="__main"></a>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]568What is this <tt>__main()</tt> call that gets inserted into <tt>main()</tt>?
Chris Lattnera28e3ce2003-12-16 22:33:55 +0000[diff] [blame]569</p></div>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]570
571<div class="answer">
572<p>
573The <tt>__main</tt> call is inserted by the C/C++ compiler in order to guarantee
574that static constructors and destructors are called when the program starts up
575and shuts down. In C, you can create static constructors and destructors by
576using GCC extensions, and in C++ you can do so by creating a global variable
577whose class has a ctor or dtor.
578</p>
579
580<p>
581The actual implementation of <tt>__main</tt> lives in the
582<tt>llvm/runtime/GCCLibraries/crtend/</tt> directory in the source-base, and is
583linked in automatically when you link the program.
584</p>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]585</div>
586
Chris Lattnera28e3ce2003-12-16 22:33:55 +0000[diff] [blame]587<!--=========================================================================-->
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]588
Misha Brukman237dc2a2004-12-03 23:58:18 +0000[diff] [blame]589<div class="question">
Chris Lattner5a53c5d2005-02-25 20:30:21 +0000[diff] [blame]590<a name="iosinit"></a>
Misha Brukman237dc2a2004-12-03 23:58:18 +0000[diff] [blame]591<p> What is this <tt>llvm.global_ctors</tt> and
592<tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I #include
593&lt;iostream&gt;?</p>
594</div>
Chris Lattnerc50bbc92004-03-29 19:14:35 +0000[diff] [blame]595
596<div class="answer">
Misha Brukman237dc2a2004-12-03 23:58:18 +0000[diff] [blame]597
598<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]599file will probably use the <tt>std::cin</tt>/<tt>std::cout</tt>/... global
600objects. However, C++ does not guarantee an order of initialization between
601static objects in different translation units, so if a static ctor/dtor in your
602.cpp file used <tt>std::cout</tt>, for example, the object would not necessarily
Misha Brukman237dc2a2004-12-03 23:58:18 +0000[diff] [blame]603be automatically initialized before your use.</p>
Chris Lattnerc50bbc92004-03-29 19:14:35 +0000[diff] [blame]604
Misha Brukman237dc2a2004-12-03 23:58:18 +0000[diff] [blame]605<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]606STL that we use declares a static object that gets created in every translation
Misha Brukman237dc2a2004-12-03 23:58:18 +0000[diff] [blame]607unit that includes <tt>&lt;iostream&gt;</tt>. This object has a static
608constructor and destructor that initializes and destroys the global iostream
609objects before they could possibly be used in the file. The code that you see
610in the .ll file corresponds to the constructor and destructor registration code.
Chris Lattnerc50bbc92004-03-29 19:14:35 +0000[diff] [blame]611</p>
612
Misha Brukman237dc2a2004-12-03 23:58:18 +0000[diff] [blame]613<p>If you would like to make it easier to <b>understand</b> the LLVM code
614generated by the compiler in the demo page, consider using <tt>printf()</tt>
615instead of <tt>iostream</tt>s to print values.</p>
616
Chris Lattnerc50bbc92004-03-29 19:14:35 +0000[diff] [blame]617</div>
618
Chris Lattner5a53c5d2005-02-25 20:30:21 +0000[diff] [blame]619<!--=========================================================================-->
620
621<div class="question"><p>
622<a name="codedce"></a>
623Where did all of my code go??
624</p></div>
625
626<div class="answer">
627<p>
628If you are using the LLVM demo page, you may often wonder what happened to all
629of the code that you typed in. Remember that the demo script is running the
630code through the LLVM optimizers, so if your code doesn't actually do anything
631useful, it might all be deleted.
632</p>
633
634<p>
635To prevent this, make sure that the code is actually needed. For example, if
636you are computing some expression, return the value from the function instead of
637leaving it in a local variable. If you really want to constrain the optimizer,
638you can read from and assign to <tt>volatile</tt> global variables.
639</p>
640</div>
641
642<!--=========================================================================-->
643
644<div class="question"><p>
645<a name="undef"></a>
646<p>What is this "<tt>undef</tt>" thing that shows up in my code?
647</p></div>
648
649<div class="answer">
650<p>
651<a href="LangRef.html#undef"><tt>undef</tt></a> is the LLVM way of representing
652a value that is not defined. You can get these if you do not initialize a
653variable before you use it. For example, the C function:</p>
654
655<div class="doc_code">
656 <tt>int X() { int i; return i; }</tt>
657</div>
658
659<p>Is compiled to "<tt>ret int undef</tt>" because "i" never has a value
660specified for it.
661</p>
662</div>
663
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]664<!-- *********************************************************************** -->
John Criswellc310f622003-10-13 16:13:06 +0000[diff] [blame]665
666<hr>
Misha Brukman7ce62cc2004-06-01 18:51:03 +0000[diff] [blame]667<address>
668 <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
669 src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
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672
Reid Spencer05fe4b02006-03-14 05:39:39 +0000[diff] [blame]673 <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]674 Last modified: $Date$
Misha Brukman7ce62cc2004-06-01 18:51:03 +0000[diff] [blame]675</address>
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John Criswellc310f622003-10-13 16:13:06 +0000[diff] [blame]677</body>
678</html>