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gerrit-public.fairphone.software / fp2-dev / platform / external / llvm / 43f51aeca8367ea35adad963c00bd2bc5b8d1391 / . / docs / FAQ.html
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Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]5 <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]6 <title>LLVM: Frequently Asked Questions</title>
Misha Brukman7ce62cc2004-06-01 18:51:03 +0000[diff] [blame]7 <style type="text/css">
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]8 @import url("llvm.css");
9 .question { font-weight: bold }
10 .answer { margin-left: 2em }
11 </style>
12</head>
13<body>
John Criswellc310f622003-10-13 16:13:06 +0000[diff] [blame]14
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]15<h1>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]16 LLVM: Frequently Asked Questions
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]17</h1>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]18
19<ol>
20 <li><a href="#license">License</a>
21 <ol>
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]22 <li>Why are the LLVM source code and the front-end distributed under
23 different licenses?</li>
24
25 <li>Does the University of Illinois Open Source License really qualify as an
26 "open source" license?</li>
27
28 <li>Can I modify LLVM source code and redistribute the modified source?</li>
29
30 <li>Can I modify LLVM source code and redistribute binaries or other tools
31 based on it, without redistributing the source?</li>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]32 </ol></li>
33
34 <li><a href="#source">Source code</a>
35 <ol>
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]36 <li>In what language is LLVM written?</li>
37
38 <li>How portable is the LLVM source code?</li>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]39 </ol></li>
40
41 <li><a href="#build">Build Problems</a>
42 <ol>
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]43 <li>When I run configure, it finds the wrong C compiler.</li>
44
45 <li>The <tt>configure</tt> script finds the right C compiler, but it uses
46 the LLVM linker from a previous build. What do I do?</li>
47
48 <li>When creating a dynamic library, I get a strange GLIBC error.</li>
49
50 <li>I've updated my source tree from Subversion, and now my build is trying
51 to use a file/directory that doesn't exist.</li>
52
53 <li>I've modified a Makefile in my source tree, but my build tree keeps
54 using the old version. What do I do?</li>
55
56 <li>I've upgraded to a new version of LLVM, and I get strange build
57 errors.</li>
58
59 <li>I've built LLVM and am testing it, but the tests freeze.</li>
60
61 <li>Why do test results differ when I perform different types of
62 builds?</li>
63
64 <li>Compiling LLVM with GCC 3.3.2 fails, what should I do?</li>
65
66 <li>Compiling LLVM with GCC succeeds, but the resulting tools do not work,
67 what can be wrong?</li>
68
69 <li>When I use the test suite, all of the C Backend tests fail. What is
70 wrong?</li>
71
72 <li>After Subversion update, rebuilding gives the error "No rule to make
73 target".</li>
74
75 <li><a href="#llvmc">The <tt>llvmc</tt> program gives me errors/doesn't
76 work.</a></li>
Bill Wendlingf1594022009-04-07 18:51:13 +0000[diff] [blame]77
78 <li><a href="#srcdir-objdir">When I compile LLVM-GCC with srcdir == objdir,
79 it fails. Why?</a></li>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]80 </ol></li>
John Criswell76c1e382003-11-18 16:08:49 +0000[diff] [blame]81
Reid Spencer501bfee2006-04-26 14:52:19 +0000[diff] [blame]82 <li><a href="#felangs">Source Languages</a>
83 <ol>
84 <li><a href="#langs">What source languages are supported?</a></li>
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]85
Gordon Henriksene5079052008-02-22 21:55:51 +0000[diff] [blame]86 <li><a href="#langirgen">I'd like to write a self-hosting LLVM compiler. How
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]87 should I interface with the LLVM middle-end optimizers and back-end code
88 generators?</a></li>
89
Reid Spencer501bfee2006-04-26 14:52:19 +0000[diff] [blame]90 <li><a href="#langhlsupp">What support is there for higher level source
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]91 language constructs for building a compiler?</a></li>
92
Reid Spencere00906f2006-08-10 20:15:58 +0000[diff] [blame]93 <li><a href="GetElementPtr.html">I don't understand the GetElementPtr
94 instruction. Help!</a></li>
Reid Spencer501bfee2006-04-26 14:52:19 +0000[diff] [blame]95 </ol>
96
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]97 <li><a href="#cfe">Using the GCC Front End</a>
John Criswell76c1e382003-11-18 16:08:49 +0000[diff] [blame]98 <ol>
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]99 <li>When I compile software that uses a configure script, the configure
100 script thinks my system has all of the header files and libraries it is
101 testing for. How do I get configure to work correctly?</li>
John Criswell76c1e382003-11-18 16:08:49 +0000[diff] [blame]102
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]103 <li>When I compile code using the LLVM GCC front end, it complains that it
104 cannot find libcrtend.a?</li>
Tanya Lattner14fc5c12005-04-25 20:36:56 +0000[diff] [blame]105
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]106 <li>How can I disable all optimizations when compiling code using the LLVM
107 GCC front end?</li>
Tanya Lattner14fc5c12005-04-25 20:36:56 +0000[diff] [blame]108
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]109 <li><a href="#translatecxx">Can I use LLVM to convert C++ code to C
110 code?</a></li>
Chris Lattneraf7fd202006-07-19 18:19:59 +0000[diff] [blame]111
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]112 <li><a href="#platformindependent">Can I compile C or C++ code to
113 platform-independent LLVM bitcode?</a></li>
John Criswell76c1e382003-11-18 16:08:49 +0000[diff] [blame]114 </ol>
115 </li>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]116
117 <li><a href="#cfe_code">Questions about code generated by the GCC front-end</a>
118 <ol>
Chris Lattner5a53c5d2005-02-25 20:30:21 +0000[diff] [blame]119 <li><a href="#iosinit">What is this <tt>llvm.global_ctors</tt> and
Chris Lattnerc50bbc92004-03-29 19:14:35 +0000[diff] [blame]120 <tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I
Chris Lattner5a53c5d2005-02-25 20:30:21 +0000[diff] [blame]121 #include &lt;iostream&gt;?</a></li>
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]122
Chris Lattner5a53c5d2005-02-25 20:30:21 +0000[diff] [blame]123 <li><a href="#codedce">Where did all of my code go??</a></li>
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]124
125 <li><a href="#undef">What is this "<tt>undef</tt>" thing that shows up in
126 my code?</a></li>
Chris Lattner2c6f9f72009-06-30 17:10:19 +0000[diff] [blame]127
128 <li><a href="#callconvwrong">Why does instcombine + simplifycfg turn
129 a call to a function with a mismatched calling convention into "unreachable"?
130 Why not make the verifier reject it?</a></li>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]131 </ol>
132 </li>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]133</ol>
134
Chris Lattner7911ce22004-05-23 21:07:27 +0000[diff] [blame]135<div class="doc_author">
NAKAMURA Takumib9a33632011-04-09 02:13:37 +0000[diff] [blame]136 <p>Written by <a href="http://llvm.org/">The LLVM Team</a></p>
Chris Lattner7911ce22004-05-23 21:07:27 +0000[diff] [blame]137</div>
138
139
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]140<!-- *********************************************************************** -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]141<h2>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]142 <a name="license">License</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]143</h2>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]144<!-- *********************************************************************** -->
145
146<div class="question">
147<p>Why are the LLVM source code and the front-end distributed under different
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]148 licenses?</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]149</div>
150
151<div class="answer">
152<p>The C/C++ front-ends are based on GCC and must be distributed under the GPL.
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]153 Our aim is to distribute LLVM source code under a <em>much less
154 restrictive</em> license, in particular one that does not compel users who
155 distribute tools based on modifying the source to redistribute the modified
156 source code as well.</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]157</div>
158
159<div class="question">
160<p>Does the University of Illinois Open Source License really qualify as an
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]161 "open source" license?</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]162</div>
163
164<div class="answer">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]165<p>Yes, the license
166 is <a href="http://www.opensource.org/licenses/UoI-NCSA.php">certified</a> by
167 the Open Source Initiative (OSI).</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]168</div>
169
170<div class="question">
171<p>Can I modify LLVM source code and redistribute the modified source?</p>
172</div>
173
174<div class="answer">
175<p>Yes. The modified source distribution must retain the copyright notice and
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]176 follow the three bulletted conditions listed in
177 the <a href="http://llvm.org/svn/llvm-project/llvm/trunk/LICENSE.TXT">LLVM
178 license</a>.</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]179</div>
180
181<div class="question">
182<p>Can I modify LLVM source code and redistribute binaries or other tools based
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]183 on it, without redistributing the source?</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]184</div>
185
186<div class="answer">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]187<p>Yes. This is why we distribute LLVM under a less restrictive license than
188 GPL, as explained in the first question above.</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]189</div>
190
191<!-- *********************************************************************** -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]192<h2>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]193 <a name="source">Source Code</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]194</h2>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]195<!-- *********************************************************************** -->
196
197<div class="question">
198<p>In what language is LLVM written?</p>
199</div>
200
201<div class="answer">
202<p>All of the LLVM tools and libraries are written in C++ with extensive use of
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]203 the STL.</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]204</div>
205
206<div class="question">
207<p>How portable is the LLVM source code?</p>
208</div>
209
210<div class="answer">
211<p>The LLVM source code should be portable to most modern UNIX-like operating
212systems. Most of the code is written in standard C++ with operating system
213services abstracted to a support library. The tools required to build and test
214LLVM have been ported to a plethora of platforms.</p>
215
216<p>Some porting problems may exist in the following areas:</p>
217
218<ul>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]219 <li>The GCC front end code is not as portable as the LLVM suite, so it may not
Bill Wendlinge9a6c352007-09-22 09:54:47 +0000[diff] [blame]220 compile as well on unsupported platforms.</li>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]221
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]222 <li>The LLVM build system relies heavily on UNIX shell tools, like the Bourne
Bill Wendlinge9a6c352007-09-22 09:54:47 +0000[diff] [blame]223 Shell and sed. Porting to systems without these tools (MacOS 9, Plan 9)
224 will require more effort.</li>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]225</ul>
226
227</div>
228
229<!-- *********************************************************************** -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]230<h2>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]231 <a name="build">Build Problems</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]232</h2>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]233<!-- *********************************************************************** -->
234
235<div class="question">
236<p>When I run configure, it finds the wrong C compiler.</p>
237</div>
238
239<div class="answer">
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]240<p>The <tt>configure</tt> script attempts to locate first <tt>gcc</tt> and then
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]241 <tt>cc</tt>, unless it finds compiler paths set in <tt>CC</tt>
242 and <tt>CXX</tt> for the C and C++ compiler, respectively.</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]243
244<p>If <tt>configure</tt> finds the wrong compiler, either adjust your
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]245 <tt>PATH</tt> environment variable or set <tt>CC</tt> and <tt>CXX</tt>
246 explicitly.</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]247
248</div>
249
250<div class="question">
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]251<p>The <tt>configure</tt> script finds the right C compiler, but it uses the
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]252 LLVM linker from a previous build. What do I do?</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]253</div>
254
255<div class="answer">
256<p>The <tt>configure</tt> script uses the <tt>PATH</tt> to find executables, so
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]257 if it's grabbing the wrong linker/assembler/etc, there are two ways to fix
258 it:</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]259
260<ol>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]261 <li><p>Adjust your <tt>PATH</tt> environment variable so that the correct
Bill Wendlinge9a6c352007-09-22 09:54:47 +0000[diff] [blame]262 program appears first in the <tt>PATH</tt>. This may work, but may not be
263 convenient when you want them <i>first</i> in your path for other
264 work.</p></li>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]265
266 <li><p>Run <tt>configure</tt> with an alternative <tt>PATH</tt> that is
Bill Wendlinge9a6c352007-09-22 09:54:47 +0000[diff] [blame]267 correct. In a Borne compatible shell, the syntax would be:</p>
268
Misha Brukmana54d4b22008-12-17 18:11:40 +0000[diff] [blame]269<pre class="doc_code">
Bill Wendlinge9a6c352007-09-22 09:54:47 +0000[diff] [blame]270% PATH=[the path without the bad program] ./configure ...
271</pre>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]272
273 <p>This is still somewhat inconvenient, but it allows <tt>configure</tt>
Bill Wendlinge9a6c352007-09-22 09:54:47 +0000[diff] [blame]274 to do its work without having to adjust your <tt>PATH</tt>
275 permanently.</p></li>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]276</ol>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]277</div>
278
279<div class="question">
280<p>When creating a dynamic library, I get a strange GLIBC error.</p>
281</div>
282
283<div class="answer">
284<p>Under some operating systems (i.e. Linux), libtool does not work correctly if
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]285 GCC was compiled with the --disable-shared option. To work around this,
286 install your own version of GCC that has shared libraries enabled by
287 default.</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]288</div>
289
290<div class="question">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]291<p>I've updated my source tree from Subversion, and now my build is trying to
292 use a file/directory that doesn't exist.</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]293</div>
294
295<div class="answer">
296<p>You need to re-run configure in your object directory. When new Makefiles
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]297 are added to the source tree, they have to be copied over to the object tree
298 in order to be used by the build.</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]299</div>
300
301<div class="question">
302<p>I've modified a Makefile in my source tree, but my build tree keeps using the
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]303 old version. What do I do?</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]304</div>
305
306<div class="answer">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]307<p>If the Makefile already exists in your object tree, you can just run the
308 following command in the top level directory of your object tree:</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]309
Misha Brukmana54d4b22008-12-17 18:11:40 +0000[diff] [blame]310<pre class="doc_code">
311% ./config.status &lt;relative path to Makefile&gt;
312</pre>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]313
314<p>If the Makefile is new, you will have to modify the configure script to copy
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]315 it over.</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]316</div>
317
318<div class="question">
319<p>I've upgraded to a new version of LLVM, and I get strange build errors.</p>
320</div>
321
322<div class="answer">
323
324<p>Sometimes, changes to the LLVM source code alters how the build system works.
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]325 Changes in libtool, autoconf, or header file dependencies are especially
326 prone to this sort of problem.</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]327
328<p>The best thing to try is to remove the old files and re-build. In most
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]329 cases, this takes care of the problem. To do this, just type <tt>make
330 clean</tt> and then <tt>make</tt> in the directory that fails to build.</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]331</div>
332
333<div class="question">
334<p>I've built LLVM and am testing it, but the tests freeze.</p>
335</div>
336
337<div class="answer">
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]338<p>This is most likely occurring because you built a profile or release
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]339 (optimized) build of LLVM and have not specified the same information on the
340 <tt>gmake</tt> command line.</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]341
342<p>For example, if you built LLVM with the command:</p>
343
Misha Brukmana54d4b22008-12-17 18:11:40 +0000[diff] [blame]344<pre class="doc_code">
345% gmake ENABLE_PROFILING=1
346</pre>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]347
348<p>...then you must run the tests with the following commands:</p>
349
Misha Brukmana54d4b22008-12-17 18:11:40 +0000[diff] [blame]350<pre class="doc_code">
Bill Wendlingd6a68eb2007-05-29 09:24:33 +0000[diff] [blame]351% cd llvm/test
352% gmake ENABLE_PROFILING=1
353</pre>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]354</div>
355
356<div class="question">
357<p>Why do test results differ when I perform different types of builds?</p>
358</div>
359
360<div class="answer">
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]361<p>The LLVM test suite is dependent upon several features of the LLVM tools and
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]362 libraries.</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]363
364<p>First, the debugging assertions in code are not enabled in optimized or
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]365 profiling builds. Hence, tests that used to fail may pass.</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]366
367<p>Second, some tests may rely upon debugging options or behavior that is only
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]368 available in the debug build. These tests will fail in an optimized or
369 profile build.</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]370</div>
371
Chris Lattner8a0b9242003-12-08 05:43:19 +0000[diff] [blame]372<div class="question">
Chris Lattner306acee2003-12-22 04:06:12 +0000[diff] [blame]373<p>Compiling LLVM with GCC 3.3.2 fails, what should I do?</p>
Chris Lattner8a0b9242003-12-08 05:43:19 +0000[diff] [blame]374</div>
375
376<div class="answer">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]377<p>This is <a href="http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13392">a bug in
378 GCC</a>, and affects projects other than LLVM. Try upgrading or downgrading
379 your GCC.</p>
Chris Lattner8a0b9242003-12-08 05:43:19 +0000[diff] [blame]380</div>
381
John Criswelld1799612004-03-29 20:23:11 +0000[diff] [blame]382<div class="question">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]383<p>Compiling LLVM with GCC succeeds, but the resulting tools do not work, what
384 can be wrong?</p>
Gabor Greif54820ce2009-03-02 19:08:05 +0000[diff] [blame]385</div>
386
387<div class="answer">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]388<p>Several versions of GCC have shown a weakness in miscompiling the LLVM
389 codebase. Please consult your compiler version (<tt>gcc --version</tt>) to
390 find out whether it is <a href="GettingStarted.html#brokengcc">broken</a>.
391 If so, your only option is to upgrade GCC to a known good version.</p>
Gabor Greif54820ce2009-03-02 19:08:05 +0000[diff] [blame]392</div>
393
394<div class="question">
Bill Wendlinge9a6c352007-09-22 09:54:47 +0000[diff] [blame]395<p>After Subversion update, rebuilding gives the error "No rule to make
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]396 target".</p>
Misha Brukman1739aec2004-09-09 16:36:47 +0000[diff] [blame]397</div>
398
399<div class="answer">
400<p>If the error is of the form:</p>
401
Misha Brukmana54d4b22008-12-17 18:11:40 +0000[diff] [blame]402<pre class="doc_code">
Misha Brukman1739aec2004-09-09 16:36:47 +0000[diff] [blame]403gmake[2]: *** No rule to make target `/path/to/somefile', needed by
404`/path/to/another/file.d'.<br>
405Stop.
Bill Wendlingd6a68eb2007-05-29 09:24:33 +0000[diff] [blame]406</pre>
Misha Brukman1739aec2004-09-09 16:36:47 +0000[diff] [blame]407
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]408<p>This may occur anytime files are moved within the Subversion repository or
409 removed entirely. In this case, the best solution is to erase all
410 <tt>.d</tt> files, which list dependencies for source files, and rebuild:</p>
Misha Brukman1739aec2004-09-09 16:36:47 +0000[diff] [blame]411
Misha Brukmana54d4b22008-12-17 18:11:40 +0000[diff] [blame]412<pre class="doc_code">
Misha Brukman1739aec2004-09-09 16:36:47 +0000[diff] [blame]413% cd $LLVM_OBJ_DIR
414% rm -f `find . -name \*\.d`
415% gmake
416</pre>
Misha Brukman1739aec2004-09-09 16:36:47 +0000[diff] [blame]417
418<p>In other cases, it may be necessary to run <tt>make clean</tt> before
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]419 rebuilding.</p>
Misha Brukman1739aec2004-09-09 16:36:47 +0000[diff] [blame]420</div>
421
Bill Wendlingf1594022009-04-07 18:51:13 +0000[diff] [blame]422<div class="question">
423<p><a name="llvmc">The <tt>llvmc</tt> program gives me errors/doesn't
424 work.</a></p>
Bill Wendling174d5782007-05-29 09:35:34 +0000[diff] [blame]425</div>
426
427<div class="answer">
Bill Wendlinge9a6c352007-09-22 09:54:47 +0000[diff] [blame]428<p><tt>llvmc</tt> is experimental and isn't really supported. We suggest
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]429 using <tt>llvm-gcc</tt> instead.</p>
Bill Wendling174d5782007-05-29 09:35:34 +0000[diff] [blame]430</div>
431
Bill Wendlingf1594022009-04-07 18:51:13 +0000[diff] [blame]432<div class="question">
Bill Wendlinga0bd8102009-04-07 18:54:06 +0000[diff] [blame]433<p><a name="srcdir-objdir">When I compile LLVM-GCC with srcdir == objdir, it
Bill Wendlingf1594022009-04-07 18:51:13 +0000[diff] [blame]434 fails. Why?</a></p>
435</div>
436
437<div class="answer">
438<p>The <tt>GNUmakefile</tt> in the top-level directory of LLVM-GCC is a special
439 <tt>Makefile</tt> used by Apple to invoke the <tt>build_gcc</tt> script after
Dan Gohmanabbf7cb2010-02-25 23:41:41 +0000[diff] [blame]440 setting up a special environment. This has the unfortunate side-effect that
Bill Wendlingf1594022009-04-07 18:51:13 +0000[diff] [blame]441 trying to build LLVM-GCC with srcdir == objdir in a "non-Apple way" invokes
442 the <tt>GNUmakefile</tt> instead of <tt>Makefile</tt>. Because the
443 environment isn't set up correctly to do this, the build fails.</p>
444
445<p>People not building LLVM-GCC the "Apple way" need to build LLVM-GCC with
446 srcdir != objdir, or simply remove the GNUmakefile entirely.</p>
447
448<p>We regret the inconvenience.</p>
449</div>
450
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]451<!-- *********************************************************************** -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]452<h2>
453 <a name="felangs">Source Languages</a>
454</h2>
Reid Spencer501bfee2006-04-26 14:52:19 +0000[diff] [blame]455
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]456<div class="question">
457<p><a name="langs">What source languages are supported?</a></p>
Reid Spencer501bfee2006-04-26 14:52:19 +0000[diff] [blame]458</div>
Gordon Henriksen58366822008-02-22 20:58:29 +0000[diff] [blame]459
Gordon Henriksen58366822008-02-22 20:58:29 +0000[diff] [blame]460<div class="answer">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]461<p>LLVM currently has full support for C and C++ source languages. These are
462 available through a special version of GCC that LLVM calls the
463 <a href="#cfe">C Front End</a></p>
464
465<p>There is an incomplete version of a Java front end available in the
466 <tt>java</tt> module. There is no documentation on this yet so you'll need to
467 download the code, compile it, and try it.</p>
468
469<p>The PyPy developers are working on integrating LLVM into the PyPy backend so
470 that PyPy language can translate to LLVM.</p>
Gordon Henriksen58366822008-02-22 20:58:29 +0000[diff] [blame]471</div>
472
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]473<div class="question">
474<p><a name="langirgen">I'd like to write a self-hosting LLVM compiler. How
475 should I interface with the LLVM middle-end optimizers and back-end code
476 generators?</a></p>
Reid Spencer501bfee2006-04-26 14:52:19 +0000[diff] [blame]477</div>
Chris Lattner33bef482006-08-15 00:43:35 +0000[diff] [blame]478
Chris Lattner33bef482006-08-15 00:43:35 +0000[diff] [blame]479<div class="answer">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]480<p>Your compiler front-end will communicate with LLVM by creating a module in
481 the LLVM intermediate representation (IR) format. Assuming you want to write
482 your language's compiler in the language itself (rather than C++), there are
483 3 major ways to tackle generating LLVM IR from a front-end:</p>
484
485<ul>
486 <li><strong>Call into the LLVM libraries code using your language's FFI
487 (foreign function interface).</strong>
488
489 <ul>
490 <li><em>for:</em> best tracks changes to the LLVM IR, .ll syntax, and .bc
491 format</li>
492
493 <li><em>for:</em> enables running LLVM optimization passes without a
494 emit/parse overhead</li>
495
496 <li><em>for:</em> adapts well to a JIT context</li>
497
498 <li><em>against:</em> lots of ugly glue code to write</li>
499 </ul></li>
500
501 <li> <strong>Emit LLVM assembly from your compiler's native language.</strong>
502 <ul>
503 <li><em>for:</em> very straightforward to get started</li>
504
505 <li><em>against:</em> the .ll parser is slower than the bitcode reader
506 when interfacing to the middle end</li>
507
508 <li><em>against:</em> you'll have to re-engineer the LLVM IR object model
509 and asm writer in your language</li>
510
511 <li><em>against:</em> it may be harder to track changes to the IR</li>
512 </ul></li>
513
514 <li><strong>Emit LLVM bitcode from your compiler's native language.</strong>
515
516 <ul>
517 <li><em>for:</em> can use the more-efficient bitcode reader when
518 interfacing to the middle end</li>
519
520 <li><em>against:</em> you'll have to re-engineer the LLVM IR object
521 model and bitcode writer in your language</li>
522
523 <li><em>against:</em> it may be harder to track changes to the IR</li>
524 </ul></li>
525</ul>
526
527<p>If you go with the first option, the C bindings in include/llvm-c should help
528 a lot, since most languages have strong support for interfacing with C. The
529 most common hurdle with calling C from managed code is interfacing with the
530 garbage collector. The C interface was designed to require very little memory
531 management, and so is straightforward in this regard.</p>
532</div>
533
534<div class="question">
535<p><a name="langhlsupp">What support is there for a higher level source language
536 constructs for building a compiler?</a></p>
537</div>
538
539<div class="answer">
540<p>Currently, there isn't much. LLVM supports an intermediate representation
541 which is useful for code representation but will not support the high level
542 (abstract syntax tree) representation needed by most compilers. There are no
543 facilities for lexical nor semantic analysis. There is, however, a <i>mostly
544 implemented</i> configuration-driven
545 <a href="CompilerDriver.html">compiler driver</a> which simplifies the task
546 of running optimizations, linking, and executable generation.</p>
547</div>
548
549<div class="question">
550<p><a name="getelementptr">I don't understand the GetElementPtr
551 instruction. Help!</a></p>
552</div>
553
554<div class="answer">
555<p>See <a href="GetElementPtr.html">The Often Misunderstood GEP
Bill Wendlinge9a6c352007-09-22 09:54:47 +0000[diff] [blame]556 Instruction</a>.</p>
Chris Lattner33bef482006-08-15 00:43:35 +0000[diff] [blame]557</div>
558
Reid Spencer501bfee2006-04-26 14:52:19 +0000[diff] [blame]559<!-- *********************************************************************** -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]560<h2>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]561 <a name="cfe">Using the GCC Front End</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]562</h2>
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]563
564<div class="question">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]565<p>When I compile software that uses a configure script, the configure script
566 thinks my system has all of the header files and libraries it is testing for.
567 How do I get configure to work correctly?</p>
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]568</div>
569
570<div class="answer">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]571<p>The configure script is getting things wrong because the LLVM linker allows
572 symbols to be undefined at link time (so that they can be resolved during JIT
573 or translation to the C back end). That is why configure thinks your system
574 "has everything."</p>
575
576<p>To work around this, perform the following steps:</p>
577
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]578<ol>
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]579 <li>Make sure the CC and CXX environment variables contains the full path to
580 the LLVM GCC front end.</li>
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]581
Reid Spencer434262a2007-02-09 15:59:08 +0000[diff] [blame]582 <li>Make sure that the regular C compiler is first in your PATH. </li>
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]583
Reid Spencer434262a2007-02-09 15:59:08 +0000[diff] [blame]584 <li>Add the string "-Wl,-native" to your CFLAGS environment variable.</li>
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]585</ol>
586
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]587<p>This will allow the <tt>llvm-ld</tt> linker to create a native code
588 executable instead of shell script that runs the JIT. Creating native code
589 requires standard linkage, which in turn will allow the configure script to
590 find out if code is not linking on your system because the feature isn't
591 available on your system.</p>
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]592</div>
593
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]594<div class="question">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]595<p>When I compile code using the LLVM GCC front end, it complains that it cannot
596 find libcrtend.a.
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]597</p>
598</div>
599
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]600<div class="answer">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]601<p>The only way this can happen is if you haven't installed the runtime
602 library. To correct this, do:</p>
Bill Wendlingd6a68eb2007-05-29 09:24:33 +0000[diff] [blame]603
Misha Brukmana54d4b22008-12-17 18:11:40 +0000[diff] [blame]604<pre class="doc_code">
Bill Wendlingd6a68eb2007-05-29 09:24:33 +0000[diff] [blame]605% cd llvm/runtime
606% make clean ; make install-bytecode
Reid Spencerf96eb572004-12-15 00:14:01 +0000[diff] [blame]607</pre>
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]608</div>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]609
Tanya Lattner14fc5c12005-04-25 20:36:56 +0000[diff] [blame]610<div class="question">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]611<p>How can I disable all optimizations when compiling code using the LLVM GCC
612 front end?</p>
Tanya Lattner14fc5c12005-04-25 20:36:56 +0000[diff] [blame]613</div>
614
615<div class="answer">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]616<p>Passing "-Wa,-disable-opt -Wl,-disable-opt" will disable *all* cleanup and
617 optimizations done at the llvm level, leaving you with the truly horrible
618 code that you desire.</p>
Tanya Lattner14fc5c12005-04-25 20:36:56 +0000[diff] [blame]619</div>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]620
Chris Lattneraf7fd202006-07-19 18:19:59 +0000[diff] [blame]621
622<div class="question">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]623<p><a name="translatecxx">Can I use LLVM to convert C++ code to C code?</a></p>
Chris Lattneraf7fd202006-07-19 18:19:59 +0000[diff] [blame]624</div>
625
626<div class="answer">
627<p>Yes, you can use LLVM to convert code from any language LLVM supports to C.
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]628 Note that the generated C code will be very low level (all loops are lowered
629 to gotos, etc) and not very pretty (comments are stripped, original source
630 formatting is totally lost, variables are renamed, expressions are
631 regrouped), so this may not be what you're looking for. Also, there are
632 several limitations noted below.<p>
Chris Lattneraf7fd202006-07-19 18:19:59 +0000[diff] [blame]633
634<p>Use commands like this:</p>
635
636<ol>
Chris Lattner860e0b42010-05-04 18:15:33 +0000[diff] [blame]637 <li><p>Compile your program with llvm-g++:</p>
Chris Lattneraf7fd202006-07-19 18:19:59 +0000[diff] [blame]638
Misha Brukmana54d4b22008-12-17 18:11:40 +0000[diff] [blame]639<pre class="doc_code">
Chris Lattner860e0b42010-05-04 18:15:33 +0000[diff] [blame]640% llvm-g++ -emit-llvm x.cpp -o program.bc -c
Bill Wendlingd6a68eb2007-05-29 09:24:33 +0000[diff] [blame]641</pre>
Chris Lattneraf7fd202006-07-19 18:19:59 +0000[diff] [blame]642
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]643 <p>or:</p>
Chris Lattneraf7fd202006-07-19 18:19:59 +0000[diff] [blame]644
Misha Brukmana54d4b22008-12-17 18:11:40 +0000[diff] [blame]645<pre class="doc_code">
Chris Lattner860e0b42010-05-04 18:15:33 +0000[diff] [blame]646% llvm-g++ a.cpp -c -emit-llvm
647% llvm-g++ b.cpp -c -emit-llvm
648% llvm-ld a.o b.o -o program
Bill Wendlingd6a68eb2007-05-29 09:24:33 +0000[diff] [blame]649</pre>
Chris Lattneraf7fd202006-07-19 18:19:59 +0000[diff] [blame]650
Chris Lattner860e0b42010-05-04 18:15:33 +0000[diff] [blame]651 <p>This will generate program and program.bc. The .bc
652 file is the LLVM version of the program all linked together.</p></li>
Chris Lattneraf7fd202006-07-19 18:19:59 +0000[diff] [blame]653
Bill Wendlinge9a6c352007-09-22 09:54:47 +0000[diff] [blame]654 <li><p>Convert the LLVM code to C code, using the LLC tool with the C
655 backend:</p>
Chris Lattneraf7fd202006-07-19 18:19:59 +0000[diff] [blame]656
Misha Brukmana54d4b22008-12-17 18:11:40 +0000[diff] [blame]657<pre class="doc_code">
Bill Wendlingd6a68eb2007-05-29 09:24:33 +0000[diff] [blame]658% llc -march=c program.bc -o program.c
Misha Brukmana54d4b22008-12-17 18:11:40 +0000[diff] [blame]659</pre></li>
Chris Lattneraf7fd202006-07-19 18:19:59 +0000[diff] [blame]660
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]661 <li><p>Finally, compile the C file:</p>
Chris Lattneraf7fd202006-07-19 18:19:59 +0000[diff] [blame]662
Misha Brukmana54d4b22008-12-17 18:11:40 +0000[diff] [blame]663<pre class="doc_code">
Chris Lattnercd465012010-05-04 18:16:00 +0000[diff] [blame]664% cc x.c -lstdc++
Misha Brukmana54d4b22008-12-17 18:11:40 +0000[diff] [blame]665</pre></li>
Chris Lattneraf7fd202006-07-19 18:19:59 +0000[diff] [blame]666
667</ol>
668
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]669<p>Using LLVM does not eliminate the need for C++ library support. If you use
670 the llvm-g++ front-end, the generated code will depend on g++'s C++ support
671 libraries in the same way that code generated from g++ would. If you use
672 another C++ front-end, the generated code will depend on whatever library
673 that front-end would normally require.</p>
Chris Lattnerb495fb02006-08-31 04:26:31 +0000[diff] [blame]674
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]675<p>If you are working on a platform that does not provide any C++ libraries, you
676 may be able to manually compile libstdc++ to LLVM bitcode, statically link it
677 into your program, then use the commands above to convert the whole result
678 into C code. Alternatively, you might compile the libraries and your
679 application into two different chunks of C code and link them.</p>
Chris Lattnerb495fb02006-08-31 04:26:31 +0000[diff] [blame]680
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]681<p>Note that, by default, the C back end does not support exception handling.
682 If you want/need it for a certain program, you can enable it by passing
683 "-enable-correct-eh-support" to the llc program. The resultant code will use
684 setjmp/longjmp to implement exception support that is relatively slow, and
685 not C++-ABI-conforming on most platforms, but otherwise correct.</p>
Dan Gohmand5b455f2009-01-25 16:04:50 +0000[diff] [blame]686
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]687<p>Also, there are a number of other limitations of the C backend that cause it
688 to produce code that does not fully conform to the C++ ABI on most
689 platforms. Some of the C++ programs in LLVM's test suite are known to fail
Benjamin Kramer8040cd32009-10-12 14:46:08 +0000[diff] [blame]690 when compiled with the C back end because of ABI incompatibilities with
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]691 standard C++ libraries.</p>
Chris Lattneraf7fd202006-07-19 18:19:59 +0000[diff] [blame]692</div>
693
Dan Gohmancfbcd592009-02-10 17:26:53 +0000[diff] [blame]694<div class="question">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]695<p><a name="platformindependent">Can I compile C or C++ code to
696 platform-independent LLVM bitcode?</a></p>
Dan Gohmancfbcd592009-02-10 17:26:53 +0000[diff] [blame]697</div>
698
699<div class="answer">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]700<p>No. C and C++ are inherently platform-dependent languages. The most obvious
701 example of this is the preprocessor. A very common way that C code is made
702 portable is by using the preprocessor to include platform-specific code. In
703 practice, information about other platforms is lost after preprocessing, so
704 the result is inherently dependent on the platform that the preprocessing was
Benjamin Kramer8040cd32009-10-12 14:46:08 +0000[diff] [blame]705 targeting.</p>
Dan Gohmancfbcd592009-02-10 17:26:53 +0000[diff] [blame]706
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]707<p>Another example is <tt>sizeof</tt>. It's common for <tt>sizeof(long)</tt> to
708 vary between platforms. In most C front-ends, <tt>sizeof</tt> is expanded to
Misha Brukman7e0fc8a2009-04-10 20:48:27 +0000[diff] [blame]709 a constant immediately, thus hard-wiring a platform-specific detail.</p>
Dan Gohmancfbcd592009-02-10 17:26:53 +0000[diff] [blame]710
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]711<p>Also, since many platforms define their ABIs in terms of C, and since LLVM is
712 lower-level than C, front-ends currently must emit platform-specific IR in
713 order to have the result conform to the platform ABI.</p>
Dan Gohmancfbcd592009-02-10 17:26:53 +0000[diff] [blame]714</div>
715
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]716<!-- *********************************************************************** -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]717<h2>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]718 <a name="cfe_code">Questions about code generated by the GCC front-end</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]719</h2>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]720
Misha Brukman237dc2a2004-12-03 23:58:18 +0000[diff] [blame]721<div class="question">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]722<p><a name="iosinit">What is this <tt>llvm.global_ctors</tt> and
723 <tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I <tt>#include
724 &lt;iostream&gt;</tt>?</a></p>
Misha Brukman237dc2a2004-12-03 23:58:18 +0000[diff] [blame]725</div>
Chris Lattnerc50bbc92004-03-29 19:14:35 +0000[diff] [blame]726
727<div class="answer">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]728<p>If you <tt>#include</tt> the <tt>&lt;iostream&gt;</tt> header into a C++
729 translation unit, the file will probably use
730 the <tt>std::cin</tt>/<tt>std::cout</tt>/... global objects. However, C++
731 does not guarantee an order of initialization between static objects in
732 different translation units, so if a static ctor/dtor in your .cpp file
733 used <tt>std::cout</tt>, for example, the object would not necessarily be
734 automatically initialized before your use.</p>
Chris Lattnerc50bbc92004-03-29 19:14:35 +0000[diff] [blame]735
Misha Brukman237dc2a2004-12-03 23:58:18 +0000[diff] [blame]736<p>To make <tt>std::cout</tt> and friends work correctly in these scenarios, the
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]737 STL that we use declares a static object that gets created in every
738 translation unit that includes <tt>&lt;iostream&gt;</tt>. This object has a
739 static constructor and destructor that initializes and destroys the global
740 iostream objects before they could possibly be used in the file. The code
741 that you see in the .ll file corresponds to the constructor and destructor
742 registration code.
Chris Lattnerc50bbc92004-03-29 19:14:35 +0000[diff] [blame]743</p>
744
Misha Brukman237dc2a2004-12-03 23:58:18 +0000[diff] [blame]745<p>If you would like to make it easier to <b>understand</b> the LLVM code
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]746 generated by the compiler in the demo page, consider using <tt>printf()</tt>
747 instead of <tt>iostream</tt>s to print values.</p>
Chris Lattnerc50bbc92004-03-29 19:14:35 +0000[diff] [blame]748</div>
749
Chris Lattner5a53c5d2005-02-25 20:30:21 +0000[diff] [blame]750<!--=========================================================================-->
751
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]752<div class="question">
753<p><a name="codedce">Where did all of my code go??</a></p>
754</div>
Chris Lattner5a53c5d2005-02-25 20:30:21 +0000[diff] [blame]755
756<div class="answer">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]757<p>If you are using the LLVM demo page, you may often wonder what happened to
758 all of the code that you typed in. Remember that the demo script is running
759 the code through the LLVM optimizers, so if your code doesn't actually do
760 anything useful, it might all be deleted.</p>
Chris Lattner5a53c5d2005-02-25 20:30:21 +0000[diff] [blame]761
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]762<p>To prevent this, make sure that the code is actually needed. For example, if
763 you are computing some expression, return the value from the function instead
764 of leaving it in a local variable. If you really want to constrain the
765 optimizer, you can read from and assign to <tt>volatile</tt> global
766 variables.</p>
Chris Lattner5a53c5d2005-02-25 20:30:21 +0000[diff] [blame]767</div>
768
769<!--=========================================================================-->
770
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]771<div class="question">
772<p><a name="undef">What is this "<tt>undef</tt>" thing that shows up in my
Bill Wendlingb1a61bd2009-04-07 18:52:30 +0000[diff] [blame]773 code?</a></p>
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]774</div>
Chris Lattner5a53c5d2005-02-25 20:30:21 +0000[diff] [blame]775
776<div class="answer">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]777<p><a href="LangRef.html#undef"><tt>undef</tt></a> is the LLVM way of
778 representing a value that is not defined. You can get these if you do not
779 initialize a variable before you use it. For example, the C function:</p>
Chris Lattner5a53c5d2005-02-25 20:30:21 +0000[diff] [blame]780
Misha Brukmana54d4b22008-12-17 18:11:40 +0000[diff] [blame]781<pre class="doc_code">
Bill Wendlingd6a68eb2007-05-29 09:24:33 +0000[diff] [blame]782int X() { int i; return i; }
783</pre>
Chris Lattner5a53c5d2005-02-25 20:30:21 +0000[diff] [blame]784
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]785<p>Is compiled to "<tt>ret i32 undef</tt>" because "<tt>i</tt>" never has a
786 value specified for it.</p>
Chris Lattner5a53c5d2005-02-25 20:30:21 +0000[diff] [blame]787</div>
788
Chris Lattner2c6f9f72009-06-30 17:10:19 +0000[diff] [blame]789<!--=========================================================================-->
790
791<div class="question">
792<p><a name="callconvwrong">Why does instcombine + simplifycfg turn
793 a call to a function with a mismatched calling convention into "unreachable"?
794 Why not make the verifier reject it?</a></p>
795</div>
796
797<div class="answer">
798<p>This is a common problem run into by authors of front-ends that are using
799custom calling conventions: you need to make sure to set the right calling
800convention on both the function and on each call to the function. For example,
801this code:</p>
802
803<pre class="doc_code">
804define fastcc void @foo() {
805 ret void
806}
807define void @bar() {
Dan Gohman3dfb3cf2010-05-28 17:07:41 +0000[diff] [blame]808 call void @foo()
Chris Lattner2c6f9f72009-06-30 17:10:19 +0000[diff] [blame]809 ret void
810}
811</pre>
812
813<p>Is optimized to:</p>
814
815<pre class="doc_code">
816define fastcc void @foo() {
817 ret void
818}
819define void @bar() {
820 unreachable
821}
822</pre>
823
824<p>... with "opt -instcombine -simplifycfg". This often bites people because
825"all their code disappears". Setting the calling convention on the caller and
826callee is required for indirect calls to work, so people often ask why not make
827the verifier reject this sort of thing.</p>
828
829<p>The answer is that this code has undefined behavior, but it is not illegal.
830If we made it illegal, then every transformation that could potentially create
831this would have to ensure that it doesn't, and there is valid code that can
832create this sort of construct (in dead code). The sorts of things that can
833cause this to happen are fairly contrived, but we still need to accept them.
834Here's an example:</p>
835
836<pre class="doc_code">
837define fastcc void @foo() {
838 ret void
839}
840define internal void @bar(void()* %FP, i1 %cond) {
841 br i1 %cond, label %T, label %F
842T:
843 call void %FP()
844 ret void
845F:
846 call fastcc void %FP()
847 ret void
848}
849define void @test() {
850 %X = or i1 false, false
851 call void @bar(void()* @foo, i1 %X)
852 ret void
853}
854</pre>
855
856<p>In this example, "test" always passes @foo/false into bar, which ensures that
857 it is dynamically called with the right calling conv (thus, the code is
858 perfectly well defined). If you run this through the inliner, you get this
859 (the explicit "or" is there so that the inliner doesn't dead code eliminate
860 a bunch of stuff):
861</p>
862
863<pre class="doc_code">
864define fastcc void @foo() {
865 ret void
866}
867define void @test() {
868 %X = or i1 false, false
869 br i1 %X, label %T.i, label %F.i
870T.i:
871 call void @foo()
872 br label %bar.exit
873F.i:
874 call fastcc void @foo()
875 br label %bar.exit
876bar.exit:
877 ret void
878}
879</pre>
880
881<p>Here you can see that the inlining pass made an undefined call to @foo with
882 the wrong calling convention. We really don't want to make the inliner have
883 to know about this sort of thing, so it needs to be valid code. In this case,
884 dead code elimination can trivially remove the undefined code. However, if %X
885 was an input argument to @test, the inliner would produce this:
886</p>
887
888<pre class="doc_code">
889define fastcc void @foo() {
890 ret void
891}
892
893define void @test(i1 %X) {
894 br i1 %X, label %T.i, label %F.i
895T.i:
896 call void @foo()
897 br label %bar.exit
898F.i:
899 call fastcc void @foo()
900 br label %bar.exit
901bar.exit:
902 ret void
903}
904</pre>
905
906<p>The interesting thing about this is that %X <em>must</em> be false for the
907code to be well-defined, but no amount of dead code elimination will be able to
908delete the broken call as unreachable. However, since instcombine/simplifycfg
909turns the undefined call into unreachable, we end up with a branch on a
910condition that goes to unreachable: a branch to unreachable can never happen, so
911"-inline -instcombine -simplifycfg" is able to produce:</p>
912
913<pre class="doc_code">
914define fastcc void @foo() {
915 ret void
916}
917define void @test(i1 %X) {
918F.i:
919 call fastcc void @foo()
920 ret void
921}
922</pre>
923
924</div>
925
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]926<!-- *********************************************************************** -->
John Criswellc310f622003-10-13 16:13:06 +0000[diff] [blame]927
928<hr>
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