1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
|
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<title>LLVM: Frequently Asked Questions</title>
<style type="text/css">
@import url("llvm.css");
.question { font-weight: bold }
.answer { margin-left: 2em }
</style>
</head>
<body>
<div class="doc_title">
LLVM: Frequently Asked Questions
</div>
<ol>
<li><a href="#license">License</a>
<ol>
<li>Why are the LLVM source code and the front-end distributed under different
licenses?</li>
<li>Does the University of Illinois Open Source License really qualify as an
"open source" license?</li>
<li>Can I modify LLVM source code and redistribute the modified source?</li>
<li>Can I modify LLVM source code and redistribute binaries or other tools
based on it, without redistributing the source?</li>
</ol></li>
<li><a href="#source">Source code</a>
<ol>
<li>In what language is LLVM written?</li>
<li>How portable is the LLVM source code?</li>
</ol></li>
<li><a href="#build">Build Problems</a>
<ol>
<li>When I run configure, it finds the wrong C compiler.</li>
<li>I compile the code, and I get some error about <tt>/localhome</tt>.</li>
<li>The <tt>configure</tt> script finds the right C compiler, but it uses the
LLVM linker from a previous build. What do I do?</li>
<li>When creating a dynamic library, I get a strange GLIBC error.</li>
<li>I've updated my source tree from CVS, and now my build is trying to use a
file/directory that doesn't exist.</li>
<li>I've modified a Makefile in my source tree, but my build tree keeps using
the old version. What do I do?</li>
<li>I've upgraded to a new version of LLVM, and I get strange build
errors.</li>
<li>I've built LLVM and am testing it, but the tests freeze.</li>
<li>Why do test results differ when I perform different types of builds?</li>
<li>Compiling LLVM with GCC 3.3.2 fails, what should I do?</li>
<li>When I use the test suite, all of the C Backend tests fail. What is
wrong?</li>
<li>After CVS update, rebuilding gives the error "No rule to make
target".</li>
</ol></li>
<li><a href="#cfe">Using the GCC Front End</a>
<ol>
<li>
When I compile software that uses a configure script, the configure script
thinks my system has all of the header files and libraries it is testing
for. How do I get configure to work correctly?
</li>
<li>
When I compile code using the LLVM GCC front end, it complains that it
cannot find libcrtend.a.
</li>
<li>
How can I disable all optimizations when compiling code using the LLVM GCC front end?
</li>
</ol>
</li>
<li><a href="#cfe_code">Questions about code generated by the GCC front-end</a>
<ol>
<li><a href="#__main">What is this <tt>__main()</tt> call that gets inserted into
<tt>main()</tt>?</a></li>
<li><a href="#iosinit">What is this <tt>llvm.global_ctors</tt> and
<tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I
#include <iostream>?</a></li>
<li><a href="#codedce">Where did all of my code go??</a></li>
<li><a href="#undef">What is this "<tt>undef</tt>" thing that shows up in my code?</a></li>
</ol>
</li>
</ol>
<div class="doc_author">
<p>Written by <a href="http://llvm.cs.uiuc.edu">The LLVM Team</a></p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section">
<a name="license">License</a>
</div>
<!-- *********************************************************************** -->
<div class="question">
<p>Why are the LLVM source code and the front-end distributed under different
licenses?</p>
</div>
<div class="answer">
<p>The C/C++ front-ends are based on GCC and must be distributed under the GPL.
Our aim is to distribute LLVM source code under a <em>much less restrictive</em>
license, in particular one that does not compel users who distribute tools based
on modifying the source to redistribute the modified source code as well.</p>
</div>
<div class="question">
<p>Does the University of Illinois Open Source License really qualify as an
"open source" license?</p>
</div>
<div class="answer">
<p>Yes, the license is <a
href="http://www.opensource.org/licenses/UoI-NCSA.php">certified</a> by the Open
Source Initiative (OSI).</p>
</div>
<div class="question">
<p>Can I modify LLVM source code and redistribute the modified source?</p>
</div>
<div class="answer">
<p>Yes. The modified source distribution must retain the copyright notice and
follow the three bulletted conditions listed in the <a
href="http://llvm.cs.uiuc.edu/releases/1.3/LICENSE.TXT">LLVM license</a>.</p>
</div>
<div class="question">
<p>Can I modify LLVM source code and redistribute binaries or other tools based
on it, without redistributing the source?</p>
</div>
<div class="answer">
<p>Yes, this is why we distribute LLVM under a less restrictive license than
GPL, as explained in the first question above.</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section">
<a name="source">Source Code</a>
</div>
<!-- *********************************************************************** -->
<div class="question">
<p>In what language is LLVM written?</p>
</div>
<div class="answer">
<p>All of the LLVM tools and libraries are written in C++ with extensive use of
the STL.</p>
</div>
<div class="question">
<p>How portable is the LLVM source code?</p>
</div>
<div class="answer">
<p>The LLVM source code should be portable to most modern UNIX-like operating
systems. Most of the code is written in standard C++ with operating system
services abstracted to a support library. The tools required to build and test
LLVM have been ported to a plethora of platforms.</p>
<p>Some porting problems may exist in the following areas:</p>
<ul>
<li>The GCC front end code is not as portable as the LLVM suite, so it may not
compile as well on unsupported platforms.</li>
<li>The LLVM build system relies heavily on UNIX shell tools, like the Bourne
Shell and sed. Porting to systems without these tools (MacOS 9, Plan 9) will
require more effort.</li>
</ul>
</div>
<!-- *********************************************************************** -->
<div class="doc_section">
<a name="build">Build Problems</a>
</div>
<!-- *********************************************************************** -->
<div class="question">
<p>When I run configure, it finds the wrong C compiler.</p>
</div>
<div class="answer">
<p>The <tt>configure</tt> script attempts to locate first <tt>gcc</tt> and then
<tt>cc</tt>, unless it finds compiler paths set in <tt>CC</tt> and <tt>CXX</tt>
for the C and C++ compiler, respectively.</p>
<p>If <tt>configure</tt> finds the wrong compiler, either adjust your
<tt>PATH</tt> environment variable or set <tt>CC</tt> and <tt>CXX</tt>
explicitly.</p>
</div>
<div class="question">
<p>I compile the code, and I get some error about <tt>/localhome</tt>.</p>
</div>
<div class="answer">
<p>There are several possible causes for this. The first is that you didn't set
a pathname properly when using <tt>configure</tt>, and it defaulted to a
pathname that we use on our research machines.</p>
<p>Another possibility is that we hardcoded a path in our Makefiles. If you see
this, please email the LLVM bug mailing list with the name of the offending
Makefile and a description of what is wrong with it.</p>
</div>
<div class="question">
<p>The <tt>configure</tt> script finds the right C compiler, but it uses the
LLVM linker from a previous build. What do I do?</p>
</div>
<div class="answer">
<p>The <tt>configure</tt> script uses the <tt>PATH</tt> to find executables, so
if it's grabbing the wrong linker/assembler/etc, there are two ways to fix
it:</p>
<ol>
<li><p>Adjust your <tt>PATH</tt> environment variable so that the correct
program appears first in the <tt>PATH</tt>. This may work, but may not be
convenient when you want them <i>first</i> in your path for other
work.</p></li>
<li><p>Run <tt>configure</tt> with an alternative <tt>PATH</tt> that is
correct. In a Borne compatible shell, the syntax would be:</p>
<p><tt>PATH=[the path without the bad program] ./configure ...</tt></p>
<p>This is still somewhat inconvenient, but it allows <tt>configure</tt>
to do its work without having to adjust your <tt>PATH</tt>
permanently.</p></li>
</ol>
</div>
<div class="question">
<p>When creating a dynamic library, I get a strange GLIBC error.</p>
</div>
<div class="answer">
<p>Under some operating systems (i.e. Linux), libtool does not work correctly if
GCC was compiled with the --disable-shared option. To work around this, install
your own version of GCC that has shared libraries enabled by default.</p>
</div>
<div class="question">
<p>I've updated my source tree from CVS, and now my build is trying to use a
file/directory that doesn't exist.</p>
</div>
<div class="answer">
<p>You need to re-run configure in your object directory. When new Makefiles
are added to the source tree, they have to be copied over to the object tree in
order to be used by the build.</p>
</div>
<div class="question">
<p>I've modified a Makefile in my source tree, but my build tree keeps using the
old version. What do I do?</p>
</div>
<div class="answer">
<p>If the Makefile already exists in your object tree, you
can just run the following command in the top level directory of your object
tree:</p>
<p><tt>./config.status <relative path to Makefile></tt><p>
<p>If the Makefile is new, you will have to modify the configure script to copy
it over.</p>
</div>
<div class="question">
<p>I've upgraded to a new version of LLVM, and I get strange build errors.</p>
</div>
<div class="answer">
<p>Sometimes, changes to the LLVM source code alters how the build system works.
Changes in libtool, autoconf, or header file dependencies are especially prone
to this sort of problem.</p>
<p>The best thing to try is to remove the old files and re-build. In most
cases, this takes care of the problem. To do this, just type <tt>make
clean</tt> and then <tt>make</tt> in the directory that fails to build.</p>
</div>
<div class="question">
<p>I've built LLVM and am testing it, but the tests freeze.</p>
</div>
<div class="answer">
<p>This is most likely occurring because you built a profile or release
(optimized) build of LLVM and have not specified the same information on the
<tt>gmake</tt> command line.</p>
<p>For example, if you built LLVM with the command:</p>
<p><tt>gmake ENABLE_PROFILING=1</tt>
<p>...then you must run the tests with the following commands:</p>
<p><tt>cd llvm/test<br>gmake ENABLE_PROFILING=1</tt></p>
</div>
<div class="question">
<p>Why do test results differ when I perform different types of builds?</p>
</div>
<div class="answer">
<p>The LLVM test suite is dependent upon several features of the LLVM tools and
libraries.</p>
<p>First, the debugging assertions in code are not enabled in optimized or
profiling builds. Hence, tests that used to fail may pass.</p>
<p>Second, some tests may rely upon debugging options or behavior that is only
available in the debug build. These tests will fail in an optimized or profile
build.</p>
</div>
<div class="question">
<p>Compiling LLVM with GCC 3.3.2 fails, what should I do?</p>
</div>
<div class="answer">
<p>This is <a href="http://gcc.gnu.org/PR?13392">a bug in GCC</a>, and
affects projects other than LLVM. Try upgrading or downgrading your GCC.</p>
</div>
<div class="question">
<p>After CVS update, rebuilding gives the error "No rule to make target".</p>
</div>
<div class="answer">
<p>If the error is of the form:</p>
<div class="doc_code">
<tt>
gmake[2]: *** No rule to make target `/path/to/somefile', needed by
`/path/to/another/file.d'.<br>
Stop.
</tt>
</div>
<p>This may occur anytime files are moved within the CVS repository or removed
entirely. In this case, the best solution is to erase all <tt>.d</tt> files,
which list dependencies for source files, and rebuild:</p>
<div class="doc_code">
<pre>
% cd $LLVM_OBJ_DIR
% rm -f `find . -name \*\.d`
% gmake
</pre>
</div>
<p>In other cases, it may be necessary to run <tt>make clean</tt> before
rebuilding.</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section">
<a name="cfe">Using the GCC Front End</a>
</div>
<div class="question">
<p>
When I compile software that uses a configure script, the configure script
thinks my system has all of the header files and libraries it is testing for.
How do I get configure to work correctly?
</p>
</div>
<div class="answer">
<p>
The configure script is getting things wrong because the LLVM linker allows
symbols to be undefined at link time (so that they can be resolved during JIT
or translation to the C back end). That is why configure thinks your system
"has everything."
</p>
<p>
To work around this, perform the following steps:
</p>
<ol>
<li>
Make sure the CC and CXX environment variables contains the full path to the
LLVM GCC front end.
</li>
<li>
Make sure that the regular C compiler is first in your PATH.
</li>
<li>
Add the string "-Wl,-native" to your CFLAGS environment variable.
</li>
</ol>
<p>
This will allow the gccld linker to create a native code executable instead of
a shell script that runs the JIT. Creating native code requires standard
linkage, which in turn will allow the configure script to find out if code is
not linking on your system because the feature isn't available on your system.
</p>
</div>
<div class="question">
<p>
When I compile code using the LLVM GCC front end, it complains that it cannot
find libcrtend.a.
</p>
</div>
<div class="answer">
<p>
The only way this can happen is if you haven't installed the runtime library. To
correct this, do:</p>
<pre>
% cd llvm/runtime
% make clean ; make install-bytecode
</pre>
</div>
<div class="question">
<p>
How can I disable all optimizations when compiling code using the LLVM GCC front end?
</p>
</div>
<div class="answer">
<p>
Passing "-Wa,-disable-opt -Wl,-disable-opt" will disable *all* cleanup and
optimizations done at the llvm level, leaving you with the truly horrible
code that you desire.
</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section">
<a name="cfe_code">Questions about code generated by the GCC front-end</a>
</div>
<div class="question"><p>
<a name="__main"></a>
What is this <tt>__main()</tt> call that gets inserted into <tt>main()</tt>?
</p></div>
<div class="answer">
<p>
The <tt>__main</tt> call is inserted by the C/C++ compiler in order to guarantee
that static constructors and destructors are called when the program starts up
and shuts down. In C, you can create static constructors and destructors by
using GCC extensions, and in C++ you can do so by creating a global variable
whose class has a ctor or dtor.
</p>
<p>
The actual implementation of <tt>__main</tt> lives in the
<tt>llvm/runtime/GCCLibraries/crtend/</tt> directory in the source-base, and is
linked in automatically when you link the program.
</p>
</div>
<!--=========================================================================-->
<div class="question">
<a name="iosinit"></a>
<p> What is this <tt>llvm.global_ctors</tt> and
<tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I #include
<iostream>?</p>
</div>
<div class="answer">
<p>If you #include the <iostream> header into a C++ translation unit, the
file will probably use the <tt>std::cin</tt>/<tt>std::cout</tt>/... global
objects. However, C++ does not guarantee an order of initialization between
static objects in different translation units, so if a static ctor/dtor in your
.cpp file used <tt>std::cout</tt>, for example, the object would not necessarily
be automatically initialized before your use.</p>
<p>To make <tt>std::cout</tt> and friends work correctly in these scenarios, the
STL that we use declares a static object that gets created in every translation
unit that includes <tt><iostream></tt>. This object has a static
constructor and destructor that initializes and destroys the global iostream
objects before they could possibly be used in the file. The code that you see
in the .ll file corresponds to the constructor and destructor registration code.
</p>
<p>If you would like to make it easier to <b>understand</b> the LLVM code
generated by the compiler in the demo page, consider using <tt>printf()</tt>
instead of <tt>iostream</tt>s to print values.</p>
</div>
<!--=========================================================================-->
<div class="question"><p>
<a name="codedce"></a>
Where did all of my code go??
</p></div>
<div class="answer">
<p>
If you are using the LLVM demo page, you may often wonder what happened to all
of the code that you typed in. Remember that the demo script is running the
code through the LLVM optimizers, so if your code doesn't actually do anything
useful, it might all be deleted.
</p>
<p>
To prevent this, make sure that the code is actually needed. For example, if
you are computing some expression, return the value from the function instead of
leaving it in a local variable. If you really want to constrain the optimizer,
you can read from and assign to <tt>volatile</tt> global variables.
</p>
</div>
<!--=========================================================================-->
<div class="question"><p>
<a name="undef"></a>
<p>What is this "<tt>undef</tt>" thing that shows up in my code?
</p></div>
<div class="answer">
<p>
<a href="LangRef.html#undef"><tt>undef</tt></a> is the LLVM way of representing
a value that is not defined. You can get these if you do not initialize a
variable before you use it. For example, the C function:</p>
<div class="doc_code">
<tt>int X() { int i; return i; }</tt>
</div>
<p>Is compiled to "<tt>ret int undef</tt>" because "i" never has a value
specified for it.
</p>
</div>
<!-- *********************************************************************** -->
<hr>
<address>
<a href="http://jigsaw.w3.org/css-validator/check/referer"><img
src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
<a href="http://validator.w3.org/check/referer"><img
src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!"></a>
<a href="http://llvm.cs.uiuc.edu">LLVM Compiler Infrastructure</a><br>
Last modified: $Date$
</address>
</body>
</html>
|
