Gitiles
Code Review Sign In
gerrit-public.fairphone.software / platform / external / llvm / 5bea822a0c21d46b2ee539a38474735b2617365d / . / docs / FAQ.html
blob: 7e15df21f38c97175e807d4b11d0108e9b772a73 [file] [log] [blame]
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]1<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
2 "http://www.w3.org/TR/html4/strict.dtd">
3<html>
4<head>
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
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]15<div class="doc_title">
16 LLVM: Frequently Asked Questions
17</div>
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 Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]127 </ol>
128 </li>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]129</ol>
130
Chris Lattner7911ce22004-05-23 21:07:27 +0000[diff] [blame]131<div class="doc_author">
Reid Spencer05fe4b02006-03-14 05:39:39 +0000[diff] [blame]132 <p>Written by <a href="http://llvm.org">The LLVM Team</a></p>
Chris Lattner7911ce22004-05-23 21:07:27 +0000[diff] [blame]133</div>
134
135
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]136<!-- *********************************************************************** -->
137<div class="doc_section">
138 <a name="license">License</a>
139</div>
140<!-- *********************************************************************** -->
141
142<div class="question">
143<p>Why are the LLVM source code and the front-end distributed under different
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]144 licenses?</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]145</div>
146
147<div class="answer">
148<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]149 Our aim is to distribute LLVM source code under a <em>much less
150 restrictive</em> license, in particular one that does not compel users who
151 distribute tools based on modifying the source to redistribute the modified
152 source code as well.</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]153</div>
154
155<div class="question">
156<p>Does the University of Illinois Open Source License really qualify as an
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]157 "open source" license?</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]158</div>
159
160<div class="answer">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]161<p>Yes, the license
162 is <a href="http://www.opensource.org/licenses/UoI-NCSA.php">certified</a> by
163 the Open Source Initiative (OSI).</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]164</div>
165
166<div class="question">
167<p>Can I modify LLVM source code and redistribute the modified source?</p>
168</div>
169
170<div class="answer">
171<p>Yes. The modified source distribution must retain the copyright notice and
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]172 follow the three bulletted conditions listed in
173 the <a href="http://llvm.org/svn/llvm-project/llvm/trunk/LICENSE.TXT">LLVM
174 license</a>.</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]175</div>
176
177<div class="question">
178<p>Can I modify LLVM source code and redistribute binaries or other tools based
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]179 on it, without redistributing the source?</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]180</div>
181
182<div class="answer">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]183<p>Yes. This is why we distribute LLVM under a less restrictive license than
184 GPL, as explained in the first question above.</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]185</div>
186
187<!-- *********************************************************************** -->
188<div class="doc_section">
189 <a name="source">Source Code</a>
190</div>
191<!-- *********************************************************************** -->
192
193<div class="question">
194<p>In what language is LLVM written?</p>
195</div>
196
197<div class="answer">
198<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]199 the STL.</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]200</div>
201
202<div class="question">
203<p>How portable is the LLVM source code?</p>
204</div>
205
206<div class="answer">
207<p>The LLVM source code should be portable to most modern UNIX-like operating
208systems. Most of the code is written in standard C++ with operating system
209services abstracted to a support library. The tools required to build and test
210LLVM have been ported to a plethora of platforms.</p>
211
212<p>Some porting problems may exist in the following areas:</p>
213
214<ul>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]215 <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]216 compile as well on unsupported platforms.</li>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]217
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]218 <li>The LLVM build system relies heavily on UNIX shell tools, like the Bourne
Bill Wendlinge9a6c352007-09-22 09:54:47 +0000[diff] [blame]219 Shell and sed. Porting to systems without these tools (MacOS 9, Plan 9)
220 will require more effort.</li>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]221</ul>
222
223</div>
224
225<!-- *********************************************************************** -->
226<div class="doc_section">
227 <a name="build">Build Problems</a>
228</div>
229<!-- *********************************************************************** -->
230
231<div class="question">
232<p>When I run configure, it finds the wrong C compiler.</p>
233</div>
234
235<div class="answer">
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]236<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]237 <tt>cc</tt>, unless it finds compiler paths set in <tt>CC</tt>
238 and <tt>CXX</tt> for the C and C++ compiler, respectively.</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]239
240<p>If <tt>configure</tt> finds the wrong compiler, either adjust your
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]241 <tt>PATH</tt> environment variable or set <tt>CC</tt> and <tt>CXX</tt>
242 explicitly.</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]243
244</div>
245
246<div class="question">
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]247<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]248 LLVM linker from a previous build. What do I do?</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]249</div>
250
251<div class="answer">
252<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]253 if it's grabbing the wrong linker/assembler/etc, there are two ways to fix
254 it:</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]255
256<ol>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]257 <li><p>Adjust your <tt>PATH</tt> environment variable so that the correct
Bill Wendlinge9a6c352007-09-22 09:54:47 +0000[diff] [blame]258 program appears first in the <tt>PATH</tt>. This may work, but may not be
259 convenient when you want them <i>first</i> in your path for other
260 work.</p></li>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]261
262 <li><p>Run <tt>configure</tt> with an alternative <tt>PATH</tt> that is
Bill Wendlinge9a6c352007-09-22 09:54:47 +0000[diff] [blame]263 correct. In a Borne compatible shell, the syntax would be:</p>
264
Misha Brukmana54d4b22008-12-17 18:11:40 +0000[diff] [blame]265<pre class="doc_code">
Bill Wendlinge9a6c352007-09-22 09:54:47 +0000[diff] [blame]266% PATH=[the path without the bad program] ./configure ...
267</pre>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]268
269 <p>This is still somewhat inconvenient, but it allows <tt>configure</tt>
Bill Wendlinge9a6c352007-09-22 09:54:47 +0000[diff] [blame]270 to do its work without having to adjust your <tt>PATH</tt>
271 permanently.</p></li>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]272</ol>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]273</div>
274
275<div class="question">
276<p>When creating a dynamic library, I get a strange GLIBC error.</p>
277</div>
278
279<div class="answer">
280<p>Under some operating systems (i.e. Linux), libtool does not work correctly if
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]281 GCC was compiled with the --disable-shared option. To work around this,
282 install your own version of GCC that has shared libraries enabled by
283 default.</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]284</div>
285
286<div class="question">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]287<p>I've updated my source tree from Subversion, and now my build is trying to
288 use a file/directory that doesn't exist.</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]289</div>
290
291<div class="answer">
292<p>You need to re-run configure in your object directory. When new Makefiles
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]293 are added to the source tree, they have to be copied over to the object tree
294 in order to be used by the build.</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]295</div>
296
297<div class="question">
298<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]299 old version. What do I do?</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]300</div>
301
302<div class="answer">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]303<p>If the Makefile already exists in your object tree, you can just run the
304 following command in the top level directory of your object tree:</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]305
Misha Brukmana54d4b22008-12-17 18:11:40 +0000[diff] [blame]306<pre class="doc_code">
307% ./config.status &lt;relative path to Makefile&gt;
308</pre>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]309
310<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]311 it over.</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]312</div>
313
314<div class="question">
315<p>I've upgraded to a new version of LLVM, and I get strange build errors.</p>
316</div>
317
318<div class="answer">
319
320<p>Sometimes, changes to the LLVM source code alters how the build system works.
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]321 Changes in libtool, autoconf, or header file dependencies are especially
322 prone to this sort of problem.</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]323
324<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]325 cases, this takes care of the problem. To do this, just type <tt>make
326 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]327</div>
328
329<div class="question">
330<p>I've built LLVM and am testing it, but the tests freeze.</p>
331</div>
332
333<div class="answer">
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]334<p>This is most likely occurring because you built a profile or release
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]335 (optimized) build of LLVM and have not specified the same information on the
336 <tt>gmake</tt> command line.</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]337
338<p>For example, if you built LLVM with the command:</p>
339
Misha Brukmana54d4b22008-12-17 18:11:40 +0000[diff] [blame]340<pre class="doc_code">
341% gmake ENABLE_PROFILING=1
342</pre>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]343
344<p>...then you must run the tests with the following commands:</p>
345
Misha Brukmana54d4b22008-12-17 18:11:40 +0000[diff] [blame]346<pre class="doc_code">
Bill Wendlingd6a68eb2007-05-29 09:24:33 +0000[diff] [blame]347% cd llvm/test
348% gmake ENABLE_PROFILING=1
349</pre>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]350</div>
351
352<div class="question">
353<p>Why do test results differ when I perform different types of builds?</p>
354</div>
355
356<div class="answer">
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]357<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]358 libraries.</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]359
360<p>First, the debugging assertions in code are not enabled in optimized or
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]361 profiling builds. Hence, tests that used to fail may pass.</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]362
363<p>Second, some tests may rely upon debugging options or behavior that is only
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]364 available in the debug build. These tests will fail in an optimized or
365 profile build.</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]366</div>
367
Chris Lattner8a0b9242003-12-08 05:43:19 +0000[diff] [blame]368<div class="question">
Chris Lattner306acee2003-12-22 04:06:12 +0000[diff] [blame]369<p>Compiling LLVM with GCC 3.3.2 fails, what should I do?</p>
Chris Lattner8a0b9242003-12-08 05:43:19 +0000[diff] [blame]370</div>
371
372<div class="answer">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]373<p>This is <a href="http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13392">a bug in
374 GCC</a>, and affects projects other than LLVM. Try upgrading or downgrading
375 your GCC.</p>
Chris Lattner8a0b9242003-12-08 05:43:19 +0000[diff] [blame]376</div>
377
John Criswelld1799612004-03-29 20:23:11 +0000[diff] [blame]378<div class="question">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]379<p>Compiling LLVM with GCC succeeds, but the resulting tools do not work, what
380 can be wrong?</p>
Gabor Greif54820ce2009-03-02 19:08:05 +0000[diff] [blame]381</div>
382
383<div class="answer">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]384<p>Several versions of GCC have shown a weakness in miscompiling the LLVM
385 codebase. Please consult your compiler version (<tt>gcc --version</tt>) to
386 find out whether it is <a href="GettingStarted.html#brokengcc">broken</a>.
387 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]388</div>
389
390<div class="question">
Bill Wendlinge9a6c352007-09-22 09:54:47 +0000[diff] [blame]391<p>After Subversion update, rebuilding gives the error "No rule to make
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]392 target".</p>
Misha Brukman1739aec2004-09-09 16:36:47 +0000[diff] [blame]393</div>
394
395<div class="answer">
396<p>If the error is of the form:</p>
397
Misha Brukmana54d4b22008-12-17 18:11:40 +0000[diff] [blame]398<pre class="doc_code">
Misha Brukman1739aec2004-09-09 16:36:47 +0000[diff] [blame]399gmake[2]: *** No rule to make target `/path/to/somefile', needed by
400`/path/to/another/file.d'.<br>
401Stop.
Bill Wendlingd6a68eb2007-05-29 09:24:33 +0000[diff] [blame]402</pre>
Misha Brukman1739aec2004-09-09 16:36:47 +0000[diff] [blame]403
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]404<p>This may occur anytime files are moved within the Subversion repository or
405 removed entirely. In this case, the best solution is to erase all
406 <tt>.d</tt> files, which list dependencies for source files, and rebuild:</p>
Misha Brukman1739aec2004-09-09 16:36:47 +0000[diff] [blame]407
Misha Brukmana54d4b22008-12-17 18:11:40 +0000[diff] [blame]408<pre class="doc_code">
Misha Brukman1739aec2004-09-09 16:36:47 +0000[diff] [blame]409% cd $LLVM_OBJ_DIR
410% rm -f `find . -name \*\.d`
411% gmake
412</pre>
Misha Brukman1739aec2004-09-09 16:36:47 +0000[diff] [blame]413
414<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]415 rebuilding.</p>
Misha Brukman1739aec2004-09-09 16:36:47 +0000[diff] [blame]416</div>
417
Bill Wendlingf1594022009-04-07 18:51:13 +0000[diff] [blame]418<div class="question">
419<p><a name="llvmc">The <tt>llvmc</tt> program gives me errors/doesn't
420 work.</a></p>
Bill Wendling174d5782007-05-29 09:35:34 +0000[diff] [blame]421</div>
422
423<div class="answer">
Bill Wendlinge9a6c352007-09-22 09:54:47 +0000[diff] [blame]424<p><tt>llvmc</tt> is experimental and isn't really supported. We suggest
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]425 using <tt>llvm-gcc</tt> instead.</p>
Bill Wendling174d5782007-05-29 09:35:34 +0000[diff] [blame]426</div>
427
Bill Wendlingf1594022009-04-07 18:51:13 +0000[diff] [blame]428<div class="question">
Bill Wendlinga0bd8102009-04-07 18:54:06 +0000[diff] [blame]429<p><a name="srcdir-objdir">When I compile LLVM-GCC with srcdir == objdir, it
Bill Wendlingf1594022009-04-07 18:51:13 +0000[diff] [blame]430 fails. Why?</a></p>
431</div>
432
433<div class="answer">
434<p>The <tt>GNUmakefile</tt> in the top-level directory of LLVM-GCC is a special
435 <tt>Makefile</tt> used by Apple to invoke the <tt>build_gcc</tt> script after
436 setting up a special environment. This has the unforunate side-effect that
437 trying to build LLVM-GCC with srcdir == objdir in a "non-Apple way" invokes
438 the <tt>GNUmakefile</tt> instead of <tt>Makefile</tt>. Because the
439 environment isn't set up correctly to do this, the build fails.</p>
440
441<p>People not building LLVM-GCC the "Apple way" need to build LLVM-GCC with
442 srcdir != objdir, or simply remove the GNUmakefile entirely.</p>
443
444<p>We regret the inconvenience.</p>
445</div>
446
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]447<!-- *********************************************************************** -->
Reid Spencerc87f4972006-04-26 15:46:53 +0000[diff] [blame]448<div class="doc_section"><a name="felangs">Source Languages</a></div>
Reid Spencer501bfee2006-04-26 14:52:19 +0000[diff] [blame]449
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]450<div class="question">
451<p><a name="langs">What source languages are supported?</a></p>
Reid Spencer501bfee2006-04-26 14:52:19 +0000[diff] [blame]452</div>
Gordon Henriksen58366822008-02-22 20:58:29 +0000[diff] [blame]453
Gordon Henriksen58366822008-02-22 20:58:29 +0000[diff] [blame]454<div class="answer">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]455<p>LLVM currently has full support for C and C++ source languages. These are
456 available through a special version of GCC that LLVM calls the
457 <a href="#cfe">C Front End</a></p>
458
459<p>There is an incomplete version of a Java front end available in the
460 <tt>java</tt> module. There is no documentation on this yet so you'll need to
461 download the code, compile it, and try it.</p>
462
463<p>The PyPy developers are working on integrating LLVM into the PyPy backend so
464 that PyPy language can translate to LLVM.</p>
Gordon Henriksen58366822008-02-22 20:58:29 +0000[diff] [blame]465</div>
466
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]467<div class="question">
468<p><a name="langirgen">I'd like to write a self-hosting LLVM compiler. How
469 should I interface with the LLVM middle-end optimizers and back-end code
470 generators?</a></p>
Reid Spencer501bfee2006-04-26 14:52:19 +0000[diff] [blame]471</div>
Chris Lattner33bef482006-08-15 00:43:35 +0000[diff] [blame]472
Chris Lattner33bef482006-08-15 00:43:35 +0000[diff] [blame]473<div class="answer">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]474<p>Your compiler front-end will communicate with LLVM by creating a module in
475 the LLVM intermediate representation (IR) format. Assuming you want to write
476 your language's compiler in the language itself (rather than C++), there are
477 3 major ways to tackle generating LLVM IR from a front-end:</p>
478
479<ul>
480 <li><strong>Call into the LLVM libraries code using your language's FFI
481 (foreign function interface).</strong>
482
483 <ul>
484 <li><em>for:</em> best tracks changes to the LLVM IR, .ll syntax, and .bc
485 format</li>
486
487 <li><em>for:</em> enables running LLVM optimization passes without a
488 emit/parse overhead</li>
489
490 <li><em>for:</em> adapts well to a JIT context</li>
491
492 <li><em>against:</em> lots of ugly glue code to write</li>
493 </ul></li>
494
495 <li> <strong>Emit LLVM assembly from your compiler's native language.</strong>
496 <ul>
497 <li><em>for:</em> very straightforward to get started</li>
498
499 <li><em>against:</em> the .ll parser is slower than the bitcode reader
500 when interfacing to the middle end</li>
501
502 <li><em>against:</em> you'll have to re-engineer the LLVM IR object model
503 and asm writer in your language</li>
504
505 <li><em>against:</em> it may be harder to track changes to the IR</li>
506 </ul></li>
507
508 <li><strong>Emit LLVM bitcode from your compiler's native language.</strong>
509
510 <ul>
511 <li><em>for:</em> can use the more-efficient bitcode reader when
512 interfacing to the middle end</li>
513
514 <li><em>against:</em> you'll have to re-engineer the LLVM IR object
515 model and bitcode writer in your language</li>
516
517 <li><em>against:</em> it may be harder to track changes to the IR</li>
518 </ul></li>
519</ul>
520
521<p>If you go with the first option, the C bindings in include/llvm-c should help
522 a lot, since most languages have strong support for interfacing with C. The
523 most common hurdle with calling C from managed code is interfacing with the
524 garbage collector. The C interface was designed to require very little memory
525 management, and so is straightforward in this regard.</p>
526</div>
527
528<div class="question">
529<p><a name="langhlsupp">What support is there for a higher level source language
530 constructs for building a compiler?</a></p>
531</div>
532
533<div class="answer">
534<p>Currently, there isn't much. LLVM supports an intermediate representation
535 which is useful for code representation but will not support the high level
536 (abstract syntax tree) representation needed by most compilers. There are no
537 facilities for lexical nor semantic analysis. There is, however, a <i>mostly
538 implemented</i> configuration-driven
539 <a href="CompilerDriver.html">compiler driver</a> which simplifies the task
540 of running optimizations, linking, and executable generation.</p>
541</div>
542
543<div class="question">
544<p><a name="getelementptr">I don't understand the GetElementPtr
545 instruction. Help!</a></p>
546</div>
547
548<div class="answer">
549<p>See <a href="GetElementPtr.html">The Often Misunderstood GEP
Bill Wendlinge9a6c352007-09-22 09:54:47 +0000[diff] [blame]550 Instruction</a>.</p>
Chris Lattner33bef482006-08-15 00:43:35 +0000[diff] [blame]551</div>
552
Reid Spencer501bfee2006-04-26 14:52:19 +0000[diff] [blame]553<!-- *********************************************************************** -->
554<div class="doc_section">
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]555 <a name="cfe">Using the GCC Front End</a>
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]556</div>
557
558<div class="question">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]559<p>When I compile software that uses a configure script, the configure script
560 thinks my system has all of the header files and libraries it is testing for.
561 How do I get configure to work correctly?</p>
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]562</div>
563
564<div class="answer">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]565<p>The configure script is getting things wrong because the LLVM linker allows
566 symbols to be undefined at link time (so that they can be resolved during JIT
567 or translation to the C back end). That is why configure thinks your system
568 "has everything."</p>
569
570<p>To work around this, perform the following steps:</p>
571
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]572<ol>
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]573 <li>Make sure the CC and CXX environment variables contains the full path to
574 the LLVM GCC front end.</li>
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]575
Reid Spencer434262a2007-02-09 15:59:08 +0000[diff] [blame]576 <li>Make sure that the regular C compiler is first in your PATH. </li>
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]577
Reid Spencer434262a2007-02-09 15:59:08 +0000[diff] [blame]578 <li>Add the string "-Wl,-native" to your CFLAGS environment variable.</li>
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]579</ol>
580
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]581<p>This will allow the <tt>llvm-ld</tt> linker to create a native code
582 executable instead of shell script that runs the JIT. Creating native code
583 requires standard linkage, which in turn will allow the configure script to
584 find out if code is not linking on your system because the feature isn't
585 available on your system.</p>
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]586</div>
587
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]588<div class="question">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]589<p>When I compile code using the LLVM GCC front end, it complains that it cannot
590 find libcrtend.a.
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]591</p>
592</div>
593
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]594<div class="answer">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]595<p>The only way this can happen is if you haven't installed the runtime
596 library. To correct this, do:</p>
Bill Wendlingd6a68eb2007-05-29 09:24:33 +0000[diff] [blame]597
Misha Brukmana54d4b22008-12-17 18:11:40 +0000[diff] [blame]598<pre class="doc_code">
Bill Wendlingd6a68eb2007-05-29 09:24:33 +0000[diff] [blame]599% cd llvm/runtime
600% make clean ; make install-bytecode
Reid Spencerf96eb572004-12-15 00:14:01 +0000[diff] [blame]601</pre>
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]602</div>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]603
Tanya Lattner14fc5c12005-04-25 20:36:56 +0000[diff] [blame]604<div class="question">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]605<p>How can I disable all optimizations when compiling code using the LLVM GCC
606 front end?</p>
Tanya Lattner14fc5c12005-04-25 20:36:56 +0000[diff] [blame]607</div>
608
609<div class="answer">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]610<p>Passing "-Wa,-disable-opt -Wl,-disable-opt" will disable *all* cleanup and
611 optimizations done at the llvm level, leaving you with the truly horrible
612 code that you desire.</p>
Tanya Lattner14fc5c12005-04-25 20:36:56 +0000[diff] [blame]613</div>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]614
Chris Lattneraf7fd202006-07-19 18:19:59 +0000[diff] [blame]615
616<div class="question">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]617<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]618</div>
619
620<div class="answer">
621<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]622 Note that the generated C code will be very low level (all loops are lowered
623 to gotos, etc) and not very pretty (comments are stripped, original source
624 formatting is totally lost, variables are renamed, expressions are
625 regrouped), so this may not be what you're looking for. Also, there are
626 several limitations noted below.<p>
Chris Lattneraf7fd202006-07-19 18:19:59 +0000[diff] [blame]627
628<p>Use commands like this:</p>
629
630<ol>
Bill Wendlinge9a6c352007-09-22 09:54:47 +0000[diff] [blame]631 <li><p>Compile your program as normal with llvm-g++:</p>
Chris Lattneraf7fd202006-07-19 18:19:59 +0000[diff] [blame]632
Misha Brukmana54d4b22008-12-17 18:11:40 +0000[diff] [blame]633<pre class="doc_code">
Bill Wendlingd6a68eb2007-05-29 09:24:33 +0000[diff] [blame]634% llvm-g++ x.cpp -o program
635</pre>
Chris Lattneraf7fd202006-07-19 18:19:59 +0000[diff] [blame]636
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]637 <p>or:</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">
Bill Wendlingd6a68eb2007-05-29 09:24:33 +0000[diff] [blame]640% llvm-g++ a.cpp -c
641% llvm-g++ b.cpp -c
642% llvm-g++ a.o b.o -o program
643</pre>
Chris Lattneraf7fd202006-07-19 18:19:59 +0000[diff] [blame]644
Bill Wendlinge9a6c352007-09-22 09:54:47 +0000[diff] [blame]645 <p>With llvm-gcc3, this will generate program and program.bc. The .bc
646 file is the LLVM version of the program all linked together.</p></li>
Chris Lattneraf7fd202006-07-19 18:19:59 +0000[diff] [blame]647
Bill Wendlinge9a6c352007-09-22 09:54:47 +0000[diff] [blame]648 <li><p>Convert the LLVM code to C code, using the LLC tool with the C
649 backend:</p>
Chris Lattneraf7fd202006-07-19 18:19:59 +0000[diff] [blame]650
Misha Brukmana54d4b22008-12-17 18:11:40 +0000[diff] [blame]651<pre class="doc_code">
Bill Wendlingd6a68eb2007-05-29 09:24:33 +0000[diff] [blame]652% llc -march=c program.bc -o program.c
Misha Brukmana54d4b22008-12-17 18:11:40 +0000[diff] [blame]653</pre></li>
Chris Lattneraf7fd202006-07-19 18:19:59 +0000[diff] [blame]654
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]655 <li><p>Finally, compile the C file:</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% cc x.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
661</ol>
662
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]663<p>Using LLVM does not eliminate the need for C++ library support. If you use
664 the llvm-g++ front-end, the generated code will depend on g++'s C++ support
665 libraries in the same way that code generated from g++ would. If you use
666 another C++ front-end, the generated code will depend on whatever library
667 that front-end would normally require.</p>
Chris Lattnerb495fb02006-08-31 04:26:31 +0000[diff] [blame]668
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]669<p>If you are working on a platform that does not provide any C++ libraries, you
670 may be able to manually compile libstdc++ to LLVM bitcode, statically link it
671 into your program, then use the commands above to convert the whole result
672 into C code. Alternatively, you might compile the libraries and your
673 application into two different chunks of C code and link them.</p>
Chris Lattnerb495fb02006-08-31 04:26:31 +0000[diff] [blame]674
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]675<p>Note that, by default, the C back end does not support exception handling.
676 If you want/need it for a certain program, you can enable it by passing
677 "-enable-correct-eh-support" to the llc program. The resultant code will use
678 setjmp/longjmp to implement exception support that is relatively slow, and
679 not C++-ABI-conforming on most platforms, but otherwise correct.</p>
Dan Gohmand5b455f2009-01-25 16:04:50 +0000[diff] [blame]680
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]681<p>Also, there are a number of other limitations of the C backend that cause it
682 to produce code that does not fully conform to the C++ ABI on most
683 platforms. Some of the C++ programs in LLVM's test suite are known to fail
684 when compiled with the C back end because of ABI incompatiblities with
685 standard C++ libraries.</p>
Chris Lattneraf7fd202006-07-19 18:19:59 +0000[diff] [blame]686</div>
687
Dan Gohmancfbcd592009-02-10 17:26:53 +0000[diff] [blame]688<div class="question">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]689<p><a name="platformindependent">Can I compile C or C++ code to
690 platform-independent LLVM bitcode?</a></p>
Dan Gohmancfbcd592009-02-10 17:26:53 +0000[diff] [blame]691</div>
692
693<div class="answer">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]694<p>No. C and C++ are inherently platform-dependent languages. The most obvious
695 example of this is the preprocessor. A very common way that C code is made
696 portable is by using the preprocessor to include platform-specific code. In
697 practice, information about other platforms is lost after preprocessing, so
698 the result is inherently dependent on the platform that the preprocessing was
699 targetting.</p>
Dan Gohmancfbcd592009-02-10 17:26:53 +0000[diff] [blame]700
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]701<p>Another example is <tt>sizeof</tt>. It's common for <tt>sizeof(long)</tt> to
702 vary between platforms. In most C front-ends, <tt>sizeof</tt> is expanded to
Misha Brukman7e0fc8a2009-04-10 20:48:27 +0000[diff] [blame]703 a constant immediately, thus hard-wiring a platform-specific detail.</p>
Dan Gohmancfbcd592009-02-10 17:26:53 +0000[diff] [blame]704
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]705<p>Also, since many platforms define their ABIs in terms of C, and since LLVM is
706 lower-level than C, front-ends currently must emit platform-specific IR in
707 order to have the result conform to the platform ABI.</p>
Dan Gohmancfbcd592009-02-10 17:26:53 +0000[diff] [blame]708</div>
709
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]710<!-- *********************************************************************** -->
711<div class="doc_section">
712 <a name="cfe_code">Questions about code generated by the GCC front-end</a>
713</div>
714
Misha Brukman237dc2a2004-12-03 23:58:18 +0000[diff] [blame]715<div class="question">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]716<p><a name="iosinit">What is this <tt>llvm.global_ctors</tt> and
717 <tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I <tt>#include
718 &lt;iostream&gt;</tt>?</a></p>
Misha Brukman237dc2a2004-12-03 23:58:18 +0000[diff] [blame]719</div>
Chris Lattnerc50bbc92004-03-29 19:14:35 +0000[diff] [blame]720
721<div class="answer">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]722<p>If you <tt>#include</tt> the <tt>&lt;iostream&gt;</tt> header into a C++
723 translation unit, the file will probably use
724 the <tt>std::cin</tt>/<tt>std::cout</tt>/... global objects. However, C++
725 does not guarantee an order of initialization between static objects in
726 different translation units, so if a static ctor/dtor in your .cpp file
727 used <tt>std::cout</tt>, for example, the object would not necessarily be
728 automatically initialized before your use.</p>
Chris Lattnerc50bbc92004-03-29 19:14:35 +0000[diff] [blame]729
Misha Brukman237dc2a2004-12-03 23:58:18 +0000[diff] [blame]730<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]731 STL that we use declares a static object that gets created in every
732 translation unit that includes <tt>&lt;iostream&gt;</tt>. This object has a
733 static constructor and destructor that initializes and destroys the global
734 iostream objects before they could possibly be used in the file. The code
735 that you see in the .ll file corresponds to the constructor and destructor
736 registration code.
Chris Lattnerc50bbc92004-03-29 19:14:35 +0000[diff] [blame]737</p>
738
Misha Brukman237dc2a2004-12-03 23:58:18 +0000[diff] [blame]739<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]740 generated by the compiler in the demo page, consider using <tt>printf()</tt>
741 instead of <tt>iostream</tt>s to print values.</p>
Chris Lattnerc50bbc92004-03-29 19:14:35 +0000[diff] [blame]742</div>
743
Chris Lattner5a53c5d2005-02-25 20:30:21 +0000[diff] [blame]744<!--=========================================================================-->
745
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]746<div class="question">
747<p><a name="codedce">Where did all of my code go??</a></p>
748</div>
Chris Lattner5a53c5d2005-02-25 20:30:21 +0000[diff] [blame]749
750<div class="answer">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]751<p>If you are using the LLVM demo page, you may often wonder what happened to
752 all of the code that you typed in. Remember that the demo script is running
753 the code through the LLVM optimizers, so if your code doesn't actually do
754 anything useful, it might all be deleted.</p>
Chris Lattner5a53c5d2005-02-25 20:30:21 +0000[diff] [blame]755
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]756<p>To prevent this, make sure that the code is actually needed. For example, if
757 you are computing some expression, return the value from the function instead
758 of leaving it in a local variable. If you really want to constrain the
759 optimizer, you can read from and assign to <tt>volatile</tt> global
760 variables.</p>
Chris Lattner5a53c5d2005-02-25 20:30:21 +0000[diff] [blame]761</div>
762
763<!--=========================================================================-->
764
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]765<div class="question">
766<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]767 code?</a></p>
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]768</div>
Chris Lattner5a53c5d2005-02-25 20:30:21 +0000[diff] [blame]769
770<div class="answer">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]771<p><a href="LangRef.html#undef"><tt>undef</tt></a> is the LLVM way of
772 representing a value that is not defined. You can get these if you do not
773 initialize a variable before you use it. For example, the C function:</p>
Chris Lattner5a53c5d2005-02-25 20:30:21 +0000[diff] [blame]774
Misha Brukmana54d4b22008-12-17 18:11:40 +0000[diff] [blame]775<pre class="doc_code">
Bill Wendlingd6a68eb2007-05-29 09:24:33 +0000[diff] [blame]776int X() { int i; return i; }
777</pre>
Chris Lattner5a53c5d2005-02-25 20:30:21 +0000[diff] [blame]778
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]779<p>Is compiled to "<tt>ret i32 undef</tt>" because "<tt>i</tt>" never has a
780 value specified for it.</p>
Chris Lattner5a53c5d2005-02-25 20:30:21 +0000[diff] [blame]781</div>
782
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]783<!-- *********************************************************************** -->
John Criswellc310f622003-10-13 16:13:06 +0000[diff] [blame]784
785<hr>
Misha Brukman7ce62cc2004-06-01 18:51:03 +0000[diff] [blame]786<address>
787 <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
Misha Brukman44408702008-12-11 17:34:48 +0000[diff] [blame]788 src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a>
Misha Brukman7ce62cc2004-06-01 18:51:03 +0000[diff] [blame]789 <a href="http://validator.w3.org/check/referer"><img
Misha Brukman44408702008-12-11 17:34:48 +0000[diff] [blame]790 src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a>
Misha Brukman7ce62cc2004-06-01 18:51:03 +0000[diff] [blame]791
Reid Spencer05fe4b02006-03-14 05:39:39 +0000[diff] [blame]792 <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]793 Last modified: $Date$
Misha Brukman7ce62cc2004-06-01 18:51:03 +0000[diff] [blame]794</address>
John Criswellf08c5d82003-10-24 22:48:20 +0000[diff] [blame]795
John Criswellc310f622003-10-13 16:13:06 +0000[diff] [blame]796</body>
797</html>