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gerrit-public.fairphone.software / platform / external / llvm / adea497673dab74b08fa2546e69619f1abe55662 / . / docs / FAQ.html
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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">
Daniel Dunbaradea4972012-04-19 20:20:34 +0000[diff] [blame^]8 @import url("_static/llvm.css");
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]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>Does the University of Illinois Open Source License really qualify as an
23 "open source" license?</li>
24
25 <li>Can I modify LLVM source code and redistribute the modified source?</li>
26
27 <li>Can I modify LLVM source code and redistribute binaries or other tools
28 based on it, without redistributing the source?</li>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]29 </ol></li>
30
31 <li><a href="#source">Source code</a>
32 <ol>
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]33 <li>In what language is LLVM written?</li>
34
35 <li>How portable is the LLVM source code?</li>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]36 </ol></li>
37
38 <li><a href="#build">Build Problems</a>
39 <ol>
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]40 <li>When I run configure, it finds the wrong C compiler.</li>
41
42 <li>The <tt>configure</tt> script finds the right C compiler, but it uses
43 the LLVM linker from a previous build. What do I do?</li>
44
45 <li>When creating a dynamic library, I get a strange GLIBC error.</li>
46
47 <li>I've updated my source tree from Subversion, and now my build is trying
48 to use a file/directory that doesn't exist.</li>
49
50 <li>I've modified a Makefile in my source tree, but my build tree keeps
51 using the old version. What do I do?</li>
52
53 <li>I've upgraded to a new version of LLVM, and I get strange build
54 errors.</li>
55
56 <li>I've built LLVM and am testing it, but the tests freeze.</li>
57
58 <li>Why do test results differ when I perform different types of
59 builds?</li>
60
61 <li>Compiling LLVM with GCC 3.3.2 fails, what should I do?</li>
62
63 <li>Compiling LLVM with GCC succeeds, but the resulting tools do not work,
64 what can be wrong?</li>
65
66 <li>When I use the test suite, all of the C Backend tests fail. What is
67 wrong?</li>
68
69 <li>After Subversion update, rebuilding gives the error "No rule to make
70 target".</li>
71
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]72 </ol></li>
John Criswell76c1e382003-11-18 16:08:49 +0000[diff] [blame]73
Reid Spencer501bfee2006-04-26 14:52:19 +0000[diff] [blame]74 <li><a href="#felangs">Source Languages</a>
75 <ol>
76 <li><a href="#langs">What source languages are supported?</a></li>
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]77
Gordon Henriksene5079052008-02-22 21:55:51 +0000[diff] [blame]78 <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]79 should I interface with the LLVM middle-end optimizers and back-end code
80 generators?</a></li>
81
Reid Spencer501bfee2006-04-26 14:52:19 +0000[diff] [blame]82 <li><a href="#langhlsupp">What support is there for higher level source
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]83 language constructs for building a compiler?</a></li>
84
Reid Spencere00906f2006-08-10 20:15:58 +0000[diff] [blame]85 <li><a href="GetElementPtr.html">I don't understand the GetElementPtr
86 instruction. Help!</a></li>
Reid Spencer501bfee2006-04-26 14:52:19 +0000[diff] [blame]87 </ol>
88
Michael J. Spencer75338092012-04-19 19:27:54 +0000[diff] [blame]89 <li><a href="#cfe">Using the C and C++ Front Ends</a>
John Criswell76c1e382003-11-18 16:08:49 +0000[diff] [blame]90 <ol>
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]91 <li><a href="#platformindependent">Can I compile C or C++ code to
92 platform-independent LLVM bitcode?</a></li>
John Criswell76c1e382003-11-18 16:08:49 +0000[diff] [blame]93 </ol>
94 </li>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]95
Michael J. Spencer75338092012-04-19 19:27:54 +0000[diff] [blame]96 <li><a href="#cfe_code">Questions about code generated by the demo page</a>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]97 <ol>
Chris Lattner5a53c5d2005-02-25 20:30:21 +0000[diff] [blame]98 <li><a href="#iosinit">What is this <tt>llvm.global_ctors</tt> and
Michael J. Spencer75338092012-04-19 19:27:54 +0000[diff] [blame]99 <tt>_GLOBAL__I_a...</tt> stuff that happens when I
Chris Lattner5a53c5d2005-02-25 20:30:21 +0000[diff] [blame]100 #include &lt;iostream&gt;?</a></li>
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]101
Chris Lattner5a53c5d2005-02-25 20:30:21 +0000[diff] [blame]102 <li><a href="#codedce">Where did all of my code go??</a></li>
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]103
104 <li><a href="#undef">What is this "<tt>undef</tt>" thing that shows up in
105 my code?</a></li>
Chris Lattner2c6f9f72009-06-30 17:10:19 +0000[diff] [blame]106
107 <li><a href="#callconvwrong">Why does instcombine + simplifycfg turn
108 a call to a function with a mismatched calling convention into "unreachable"?
109 Why not make the verifier reject it?</a></li>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]110 </ol>
111 </li>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]112</ol>
113
Chris Lattner7911ce22004-05-23 21:07:27 +0000[diff] [blame]114<div class="doc_author">
NAKAMURA Takumib9a33632011-04-09 02:13:37 +0000[diff] [blame]115 <p>Written by <a href="http://llvm.org/">The LLVM Team</a></p>
Chris Lattner7911ce22004-05-23 21:07:27 +0000[diff] [blame]116</div>
117
118
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]119<!-- *********************************************************************** -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]120<h2>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]121 <a name="license">License</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]122</h2>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]123<!-- *********************************************************************** -->
124
NAKAMURA Takumi9c55f592012-03-27 11:25:16 +0000[diff] [blame]125<div>
126
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]127<div class="question">
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]128<p>Does the University of Illinois Open Source License really qualify as an
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]129 "open source" license?</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]130</div>
131
132<div class="answer">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]133<p>Yes, the license
134 is <a href="http://www.opensource.org/licenses/UoI-NCSA.php">certified</a> by
135 the Open Source Initiative (OSI).</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]136</div>
137
138<div class="question">
139<p>Can I modify LLVM source code and redistribute the modified source?</p>
140</div>
141
142<div class="answer">
143<p>Yes. The modified source distribution must retain the copyright notice and
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]144 follow the three bulletted conditions listed in
145 the <a href="http://llvm.org/svn/llvm-project/llvm/trunk/LICENSE.TXT">LLVM
146 license</a>.</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]147</div>
148
149<div class="question">
150<p>Can I modify LLVM source code and redistribute binaries or other tools based
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]151 on it, without redistributing the source?</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]152</div>
153
154<div class="answer">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]155<p>Yes. This is why we distribute LLVM under a less restrictive license than
156 GPL, as explained in the first question above.</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]157</div>
158
NAKAMURA Takumi9c55f592012-03-27 11:25:16 +0000[diff] [blame]159</div>
160
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]161<!-- *********************************************************************** -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]162<h2>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]163 <a name="source">Source Code</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]164</h2>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]165<!-- *********************************************************************** -->
166
NAKAMURA Takumi9c55f592012-03-27 11:25:16 +0000[diff] [blame]167<div>
168
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]169<div class="question">
170<p>In what language is LLVM written?</p>
171</div>
172
173<div class="answer">
174<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]175 the STL.</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]176</div>
177
178<div class="question">
179<p>How portable is the LLVM source code?</p>
180</div>
181
182<div class="answer">
183<p>The LLVM source code should be portable to most modern UNIX-like operating
184systems. Most of the code is written in standard C++ with operating system
185services abstracted to a support library. The tools required to build and test
186LLVM have been ported to a plethora of platforms.</p>
187
188<p>Some porting problems may exist in the following areas:</p>
189
190<ul>
Michael J. Spencer75338092012-04-19 19:27:54 +0000[diff] [blame]191 <li>The autoconf/makefile build system relies heavily on UNIX shell tools,
192 like the Bourne Shell and sed. Porting to systems without these tools
193 (MacOS 9, Plan 9) Will require more effort.</li>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]194</ul>
195
196</div>
197
NAKAMURA Takumi9c55f592012-03-27 11:25:16 +0000[diff] [blame]198</div>
199
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]200<!-- *********************************************************************** -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]201<h2>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]202 <a name="build">Build Problems</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]203</h2>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]204<!-- *********************************************************************** -->
205
NAKAMURA Takumi9c55f592012-03-27 11:25:16 +0000[diff] [blame]206<div>
207
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]208<div class="question">
209<p>When I run configure, it finds the wrong C compiler.</p>
210</div>
211
212<div class="answer">
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]213<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]214 <tt>cc</tt>, unless it finds compiler paths set in <tt>CC</tt>
215 and <tt>CXX</tt> for the C and C++ compiler, respectively.</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]216
217<p>If <tt>configure</tt> finds the wrong compiler, either adjust your
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]218 <tt>PATH</tt> environment variable or set <tt>CC</tt> and <tt>CXX</tt>
219 explicitly.</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]220
221</div>
222
223<div class="question">
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]224<p>The <tt>configure</tt> script finds the right C compiler, but it uses the
Michael J. Spencer75338092012-04-19 19:27:54 +0000[diff] [blame]225 LLVM tools from a previous build. What do I do?</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]226</div>
227
228<div class="answer">
229<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]230 if it's grabbing the wrong linker/assembler/etc, there are two ways to fix
231 it:</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]232
233<ol>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]234 <li><p>Adjust your <tt>PATH</tt> environment variable so that the correct
Bill Wendlinge9a6c352007-09-22 09:54:47 +0000[diff] [blame]235 program appears first in the <tt>PATH</tt>. This may work, but may not be
236 convenient when you want them <i>first</i> in your path for other
237 work.</p></li>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]238
239 <li><p>Run <tt>configure</tt> with an alternative <tt>PATH</tt> that is
Bill Wendlinge9a6c352007-09-22 09:54:47 +0000[diff] [blame]240 correct. In a Borne compatible shell, the syntax would be:</p>
241
Misha Brukmana54d4b22008-12-17 18:11:40 +0000[diff] [blame]242<pre class="doc_code">
Bill Wendlinge9a6c352007-09-22 09:54:47 +0000[diff] [blame]243% PATH=[the path without the bad program] ./configure ...
244</pre>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]245
246 <p>This is still somewhat inconvenient, but it allows <tt>configure</tt>
Bill Wendlinge9a6c352007-09-22 09:54:47 +0000[diff] [blame]247 to do its work without having to adjust your <tt>PATH</tt>
248 permanently.</p></li>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]249</ol>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]250</div>
251
252<div class="question">
253<p>When creating a dynamic library, I get a strange GLIBC error.</p>
254</div>
255
256<div class="answer">
257<p>Under some operating systems (i.e. Linux), libtool does not work correctly if
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]258 GCC was compiled with the --disable-shared option. To work around this,
259 install your own version of GCC that has shared libraries enabled by
260 default.</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]261</div>
262
263<div class="question">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]264<p>I've updated my source tree from Subversion, and now my build is trying to
265 use a file/directory that doesn't exist.</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]266</div>
267
268<div class="answer">
269<p>You need to re-run configure in your object directory. When new Makefiles
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]270 are added to the source tree, they have to be copied over to the object tree
271 in order to be used by the build.</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]272</div>
273
274<div class="question">
275<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]276 old version. What do I do?</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]277</div>
278
279<div class="answer">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]280<p>If the Makefile already exists in your object tree, you can just run the
281 following command in the top level directory of your object tree:</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]282
Misha Brukmana54d4b22008-12-17 18:11:40 +0000[diff] [blame]283<pre class="doc_code">
284% ./config.status &lt;relative path to Makefile&gt;
285</pre>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]286
287<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]288 it over.</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]289</div>
290
291<div class="question">
292<p>I've upgraded to a new version of LLVM, and I get strange build errors.</p>
293</div>
294
295<div class="answer">
296
297<p>Sometimes, changes to the LLVM source code alters how the build system works.
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]298 Changes in libtool, autoconf, or header file dependencies are especially
299 prone to this sort of problem.</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]300
301<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]302 cases, this takes care of the problem. To do this, just type <tt>make
303 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]304</div>
305
306<div class="question">
307<p>I've built LLVM and am testing it, but the tests freeze.</p>
308</div>
309
310<div class="answer">
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]311<p>This is most likely occurring because you built a profile or release
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]312 (optimized) build of LLVM and have not specified the same information on the
313 <tt>gmake</tt> command line.</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]314
315<p>For example, if you built LLVM with the command:</p>
316
Misha Brukmana54d4b22008-12-17 18:11:40 +0000[diff] [blame]317<pre class="doc_code">
318% gmake ENABLE_PROFILING=1
319</pre>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]320
321<p>...then you must run the tests with the following commands:</p>
322
Misha Brukmana54d4b22008-12-17 18:11:40 +0000[diff] [blame]323<pre class="doc_code">
Bill Wendlingd6a68eb2007-05-29 09:24:33 +0000[diff] [blame]324% cd llvm/test
325% gmake ENABLE_PROFILING=1
326</pre>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]327</div>
328
329<div class="question">
330<p>Why do test results differ when I perform different types of builds?</p>
331</div>
332
333<div class="answer">
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]334<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]335 libraries.</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]336
337<p>First, the debugging assertions in code are not enabled in optimized or
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]338 profiling builds. Hence, tests that used to fail may pass.</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]339
340<p>Second, some tests may rely upon debugging options or behavior that is only
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]341 available in the debug build. These tests will fail in an optimized or
342 profile build.</p>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]343</div>
344
Chris Lattner8a0b9242003-12-08 05:43:19 +0000[diff] [blame]345<div class="question">
Chris Lattner306acee2003-12-22 04:06:12 +0000[diff] [blame]346<p>Compiling LLVM with GCC 3.3.2 fails, what should I do?</p>
Chris Lattner8a0b9242003-12-08 05:43:19 +0000[diff] [blame]347</div>
348
349<div class="answer">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]350<p>This is <a href="http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13392">a bug in
351 GCC</a>, and affects projects other than LLVM. Try upgrading or downgrading
352 your GCC.</p>
Chris Lattner8a0b9242003-12-08 05:43:19 +0000[diff] [blame]353</div>
354
John Criswelld1799612004-03-29 20:23:11 +0000[diff] [blame]355<div class="question">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]356<p>Compiling LLVM with GCC succeeds, but the resulting tools do not work, what
357 can be wrong?</p>
Gabor Greif54820ce2009-03-02 19:08:05 +0000[diff] [blame]358</div>
359
360<div class="answer">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]361<p>Several versions of GCC have shown a weakness in miscompiling the LLVM
362 codebase. Please consult your compiler version (<tt>gcc --version</tt>) to
363 find out whether it is <a href="GettingStarted.html#brokengcc">broken</a>.
364 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]365</div>
366
367<div class="question">
Bill Wendlinge9a6c352007-09-22 09:54:47 +0000[diff] [blame]368<p>After Subversion update, rebuilding gives the error "No rule to make
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]369 target".</p>
Misha Brukman1739aec2004-09-09 16:36:47 +0000[diff] [blame]370</div>
371
372<div class="answer">
373<p>If the error is of the form:</p>
374
Misha Brukmana54d4b22008-12-17 18:11:40 +0000[diff] [blame]375<pre class="doc_code">
Misha Brukman1739aec2004-09-09 16:36:47 +0000[diff] [blame]376gmake[2]: *** No rule to make target `/path/to/somefile', needed by
377`/path/to/another/file.d'.<br>
378Stop.
Bill Wendlingd6a68eb2007-05-29 09:24:33 +0000[diff] [blame]379</pre>
Misha Brukman1739aec2004-09-09 16:36:47 +0000[diff] [blame]380
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]381<p>This may occur anytime files are moved within the Subversion repository or
382 removed entirely. In this case, the best solution is to erase all
383 <tt>.d</tt> files, which list dependencies for source files, and rebuild:</p>
Misha Brukman1739aec2004-09-09 16:36:47 +0000[diff] [blame]384
Misha Brukmana54d4b22008-12-17 18:11:40 +0000[diff] [blame]385<pre class="doc_code">
Misha Brukman1739aec2004-09-09 16:36:47 +0000[diff] [blame]386% cd $LLVM_OBJ_DIR
387% rm -f `find . -name \*\.d`
388% gmake
389</pre>
Misha Brukman1739aec2004-09-09 16:36:47 +0000[diff] [blame]390
391<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]392 rebuilding.</p>
Misha Brukman1739aec2004-09-09 16:36:47 +0000[diff] [blame]393</div>
394
NAKAMURA Takumi9c55f592012-03-27 11:25:16 +0000[diff] [blame]395</div>
396
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]397<!-- *********************************************************************** -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]398<h2>
399 <a name="felangs">Source Languages</a>
400</h2>
Reid Spencer501bfee2006-04-26 14:52:19 +0000[diff] [blame]401
NAKAMURA Takumi9c55f592012-03-27 11:25:16 +0000[diff] [blame]402<div>
403
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]404<div class="question">
405<p><a name="langs">What source languages are supported?</a></p>
Reid Spencer501bfee2006-04-26 14:52:19 +0000[diff] [blame]406</div>
Gordon Henriksen58366822008-02-22 20:58:29 +0000[diff] [blame]407
Gordon Henriksen58366822008-02-22 20:58:29 +0000[diff] [blame]408<div class="answer">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]409<p>LLVM currently has full support for C and C++ source languages. These are
Michael J. Spencer75338092012-04-19 19:27:54 +0000[diff] [blame]410 available through both <a href="http://clang.llvm.org/">Clang</a> and
411 <a href="http://dragonegg.llvm.org/">DragonEgg</a>.</p>
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]412
413<p>The PyPy developers are working on integrating LLVM into the PyPy backend so
414 that PyPy language can translate to LLVM.</p>
Gordon Henriksen58366822008-02-22 20:58:29 +0000[diff] [blame]415</div>
416
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]417<div class="question">
418<p><a name="langirgen">I'd like to write a self-hosting LLVM compiler. How
419 should I interface with the LLVM middle-end optimizers and back-end code
420 generators?</a></p>
Reid Spencer501bfee2006-04-26 14:52:19 +0000[diff] [blame]421</div>
Chris Lattner33bef482006-08-15 00:43:35 +0000[diff] [blame]422
Chris Lattner33bef482006-08-15 00:43:35 +0000[diff] [blame]423<div class="answer">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]424<p>Your compiler front-end will communicate with LLVM by creating a module in
425 the LLVM intermediate representation (IR) format. Assuming you want to write
426 your language's compiler in the language itself (rather than C++), there are
427 3 major ways to tackle generating LLVM IR from a front-end:</p>
428
429<ul>
430 <li><strong>Call into the LLVM libraries code using your language's FFI
431 (foreign function interface).</strong>
432
433 <ul>
434 <li><em>for:</em> best tracks changes to the LLVM IR, .ll syntax, and .bc
435 format</li>
436
437 <li><em>for:</em> enables running LLVM optimization passes without a
438 emit/parse overhead</li>
439
440 <li><em>for:</em> adapts well to a JIT context</li>
441
442 <li><em>against:</em> lots of ugly glue code to write</li>
443 </ul></li>
444
445 <li> <strong>Emit LLVM assembly from your compiler's native language.</strong>
446 <ul>
447 <li><em>for:</em> very straightforward to get started</li>
448
449 <li><em>against:</em> the .ll parser is slower than the bitcode reader
450 when interfacing to the middle end</li>
451
452 <li><em>against:</em> you'll have to re-engineer the LLVM IR object model
453 and asm writer in your language</li>
454
455 <li><em>against:</em> it may be harder to track changes to the IR</li>
456 </ul></li>
457
458 <li><strong>Emit LLVM bitcode from your compiler's native language.</strong>
459
460 <ul>
461 <li><em>for:</em> can use the more-efficient bitcode reader when
462 interfacing to the middle end</li>
463
464 <li><em>against:</em> you'll have to re-engineer the LLVM IR object
465 model and bitcode writer in your language</li>
466
467 <li><em>against:</em> it may be harder to track changes to the IR</li>
468 </ul></li>
469</ul>
470
471<p>If you go with the first option, the C bindings in include/llvm-c should help
472 a lot, since most languages have strong support for interfacing with C. The
473 most common hurdle with calling C from managed code is interfacing with the
474 garbage collector. The C interface was designed to require very little memory
475 management, and so is straightforward in this regard.</p>
476</div>
477
478<div class="question">
479<p><a name="langhlsupp">What support is there for a higher level source language
480 constructs for building a compiler?</a></p>
481</div>
482
483<div class="answer">
484<p>Currently, there isn't much. LLVM supports an intermediate representation
485 which is useful for code representation but will not support the high level
486 (abstract syntax tree) representation needed by most compilers. There are no
Eric Christophereae5a132011-09-20 00:34:27 +0000[diff] [blame]487 facilities for lexical nor semantic analysis.</p>
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]488</div>
489
490<div class="question">
491<p><a name="getelementptr">I don't understand the GetElementPtr
492 instruction. Help!</a></p>
493</div>
494
495<div class="answer">
496<p>See <a href="GetElementPtr.html">The Often Misunderstood GEP
Bill Wendlinge9a6c352007-09-22 09:54:47 +0000[diff] [blame]497 Instruction</a>.</p>
Chris Lattner33bef482006-08-15 00:43:35 +0000[diff] [blame]498</div>
499
NAKAMURA Takumi9c55f592012-03-27 11:25:16 +0000[diff] [blame]500</div>
501
Reid Spencer501bfee2006-04-26 14:52:19 +0000[diff] [blame]502<!-- *********************************************************************** -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]503<h2>
Michael J. Spencer75338092012-04-19 19:27:54 +0000[diff] [blame]504 <a name="cfe">Using the C and C++ Front Ends</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]505</h2>
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]506
NAKAMURA Takumi9c55f592012-03-27 11:25:16 +0000[diff] [blame]507<div>
508
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]509<div class="question">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]510<p><a name="platformindependent">Can I compile C or C++ code to
511 platform-independent LLVM bitcode?</a></p>
Dan Gohmancfbcd592009-02-10 17:26:53 +0000[diff] [blame]512</div>
513
514<div class="answer">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]515<p>No. C and C++ are inherently platform-dependent languages. The most obvious
516 example of this is the preprocessor. A very common way that C code is made
517 portable is by using the preprocessor to include platform-specific code. In
518 practice, information about other platforms is lost after preprocessing, so
519 the result is inherently dependent on the platform that the preprocessing was
Benjamin Kramer8040cd32009-10-12 14:46:08 +0000[diff] [blame]520 targeting.</p>
Dan Gohmancfbcd592009-02-10 17:26:53 +0000[diff] [blame]521
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]522<p>Another example is <tt>sizeof</tt>. It's common for <tt>sizeof(long)</tt> to
523 vary between platforms. In most C front-ends, <tt>sizeof</tt> is expanded to
Misha Brukman7e0fc8a2009-04-10 20:48:27 +0000[diff] [blame]524 a constant immediately, thus hard-wiring a platform-specific detail.</p>
Dan Gohmancfbcd592009-02-10 17:26:53 +0000[diff] [blame]525
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]526<p>Also, since many platforms define their ABIs in terms of C, and since LLVM is
527 lower-level than C, front-ends currently must emit platform-specific IR in
528 order to have the result conform to the platform ABI.</p>
Dan Gohmancfbcd592009-02-10 17:26:53 +0000[diff] [blame]529</div>
530
NAKAMURA Takumi9c55f592012-03-27 11:25:16 +0000[diff] [blame]531</div>
532
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]533<!-- *********************************************************************** -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]534<h2>
Michael J. Spencer75338092012-04-19 19:27:54 +0000[diff] [blame]535 <a name="cfe_code">Questions about code generated by the demo page</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]536</h2>
Chris Lattnercc33d702003-11-19 05:53:12 +0000[diff] [blame]537
NAKAMURA Takumi9c55f592012-03-27 11:25:16 +0000[diff] [blame]538<div>
539
Misha Brukman237dc2a2004-12-03 23:58:18 +0000[diff] [blame]540<div class="question">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]541<p><a name="iosinit">What is this <tt>llvm.global_ctors</tt> and
Michael J. Spencer75338092012-04-19 19:27:54 +0000[diff] [blame]542 <tt>_GLOBAL__I_a...</tt> stuff that happens when I <tt>#include
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]543 &lt;iostream&gt;</tt>?</a></p>
Misha Brukman237dc2a2004-12-03 23:58:18 +0000[diff] [blame]544</div>
Chris Lattnerc50bbc92004-03-29 19:14:35 +0000[diff] [blame]545
546<div class="answer">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]547<p>If you <tt>#include</tt> the <tt>&lt;iostream&gt;</tt> header into a C++
548 translation unit, the file will probably use
549 the <tt>std::cin</tt>/<tt>std::cout</tt>/... global objects. However, C++
550 does not guarantee an order of initialization between static objects in
551 different translation units, so if a static ctor/dtor in your .cpp file
552 used <tt>std::cout</tt>, for example, the object would not necessarily be
553 automatically initialized before your use.</p>
Chris Lattnerc50bbc92004-03-29 19:14:35 +0000[diff] [blame]554
Misha Brukman237dc2a2004-12-03 23:58:18 +0000[diff] [blame]555<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]556 STL that we use declares a static object that gets created in every
557 translation unit that includes <tt>&lt;iostream&gt;</tt>. This object has a
558 static constructor and destructor that initializes and destroys the global
559 iostream objects before they could possibly be used in the file. The code
560 that you see in the .ll file corresponds to the constructor and destructor
561 registration code.
Chris Lattnerc50bbc92004-03-29 19:14:35 +0000[diff] [blame]562</p>
563
Misha Brukman237dc2a2004-12-03 23:58:18 +0000[diff] [blame]564<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]565 generated by the compiler in the demo page, consider using <tt>printf()</tt>
566 instead of <tt>iostream</tt>s to print values.</p>
Chris Lattnerc50bbc92004-03-29 19:14:35 +0000[diff] [blame]567</div>
568
Chris Lattner5a53c5d2005-02-25 20:30:21 +0000[diff] [blame]569<!--=========================================================================-->
570
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]571<div class="question">
572<p><a name="codedce">Where did all of my code go??</a></p>
573</div>
Chris Lattner5a53c5d2005-02-25 20:30:21 +0000[diff] [blame]574
575<div class="answer">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]576<p>If you are using the LLVM demo page, you may often wonder what happened to
577 all of the code that you typed in. Remember that the demo script is running
578 the code through the LLVM optimizers, so if your code doesn't actually do
579 anything useful, it might all be deleted.</p>
Chris Lattner5a53c5d2005-02-25 20:30:21 +0000[diff] [blame]580
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]581<p>To prevent this, make sure that the code is actually needed. For example, if
582 you are computing some expression, return the value from the function instead
583 of leaving it in a local variable. If you really want to constrain the
584 optimizer, you can read from and assign to <tt>volatile</tt> global
585 variables.</p>
Chris Lattner5a53c5d2005-02-25 20:30:21 +0000[diff] [blame]586</div>
587
588<!--=========================================================================-->
589
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]590<div class="question">
591<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]592 code?</a></p>
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]593</div>
Chris Lattner5a53c5d2005-02-25 20:30:21 +0000[diff] [blame]594
595<div class="answer">
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]596<p><a href="LangRef.html#undef"><tt>undef</tt></a> is the LLVM way of
597 representing a value that is not defined. You can get these if you do not
598 initialize a variable before you use it. For example, the C function:</p>
Chris Lattner5a53c5d2005-02-25 20:30:21 +0000[diff] [blame]599
Misha Brukmana54d4b22008-12-17 18:11:40 +0000[diff] [blame]600<pre class="doc_code">
Bill Wendlingd6a68eb2007-05-29 09:24:33 +0000[diff] [blame]601int X() { int i; return i; }
602</pre>
Chris Lattner5a53c5d2005-02-25 20:30:21 +0000[diff] [blame]603
Bill Wendling290235f2009-04-07 18:40:56 +0000[diff] [blame]604<p>Is compiled to "<tt>ret i32 undef</tt>" because "<tt>i</tt>" never has a
605 value specified for it.</p>
Chris Lattner5a53c5d2005-02-25 20:30:21 +0000[diff] [blame]606</div>
607
Chris Lattner2c6f9f72009-06-30 17:10:19 +0000[diff] [blame]608<!--=========================================================================-->
609
610<div class="question">
611<p><a name="callconvwrong">Why does instcombine + simplifycfg turn
612 a call to a function with a mismatched calling convention into "unreachable"?
613 Why not make the verifier reject it?</a></p>
614</div>
615
616<div class="answer">
617<p>This is a common problem run into by authors of front-ends that are using
618custom calling conventions: you need to make sure to set the right calling
619convention on both the function and on each call to the function. For example,
620this code:</p>
621
622<pre class="doc_code">
623define fastcc void @foo() {
624 ret void
625}
626define void @bar() {
Dan Gohman3dfb3cf2010-05-28 17:07:41 +0000[diff] [blame]627 call void @foo()
Chris Lattner2c6f9f72009-06-30 17:10:19 +0000[diff] [blame]628 ret void
629}
630</pre>
631
632<p>Is optimized to:</p>
633
634<pre class="doc_code">
635define fastcc void @foo() {
636 ret void
637}
638define void @bar() {
639 unreachable
640}
641</pre>
642
643<p>... with "opt -instcombine -simplifycfg". This often bites people because
644"all their code disappears". Setting the calling convention on the caller and
645callee is required for indirect calls to work, so people often ask why not make
646the verifier reject this sort of thing.</p>
647
648<p>The answer is that this code has undefined behavior, but it is not illegal.
649If we made it illegal, then every transformation that could potentially create
650this would have to ensure that it doesn't, and there is valid code that can
651create this sort of construct (in dead code). The sorts of things that can
652cause this to happen are fairly contrived, but we still need to accept them.
653Here's an example:</p>
654
655<pre class="doc_code">
656define fastcc void @foo() {
657 ret void
658}
659define internal void @bar(void()* %FP, i1 %cond) {
660 br i1 %cond, label %T, label %F
661T:
662 call void %FP()
663 ret void
664F:
665 call fastcc void %FP()
666 ret void
667}
668define void @test() {
669 %X = or i1 false, false
670 call void @bar(void()* @foo, i1 %X)
671 ret void
672}
673</pre>
674
675<p>In this example, "test" always passes @foo/false into bar, which ensures that
676 it is dynamically called with the right calling conv (thus, the code is
677 perfectly well defined). If you run this through the inliner, you get this
678 (the explicit "or" is there so that the inliner doesn't dead code eliminate
679 a bunch of stuff):
680</p>
681
682<pre class="doc_code">
683define fastcc void @foo() {
684 ret void
685}
686define void @test() {
687 %X = or i1 false, false
688 br i1 %X, label %T.i, label %F.i
689T.i:
690 call void @foo()
691 br label %bar.exit
692F.i:
693 call fastcc void @foo()
694 br label %bar.exit
695bar.exit:
696 ret void
697}
698</pre>
699
700<p>Here you can see that the inlining pass made an undefined call to @foo with
701 the wrong calling convention. We really don't want to make the inliner have
702 to know about this sort of thing, so it needs to be valid code. In this case,
703 dead code elimination can trivially remove the undefined code. However, if %X
704 was an input argument to @test, the inliner would produce this:
705</p>
706
707<pre class="doc_code">
708define fastcc void @foo() {
709 ret void
710}
711
712define void @test(i1 %X) {
713 br i1 %X, label %T.i, label %F.i
714T.i:
715 call void @foo()
716 br label %bar.exit
717F.i:
718 call fastcc void @foo()
719 br label %bar.exit
720bar.exit:
721 ret void
722}
723</pre>
724
725<p>The interesting thing about this is that %X <em>must</em> be false for the
726code to be well-defined, but no amount of dead code elimination will be able to
727delete the broken call as unreachable. However, since instcombine/simplifycfg
728turns the undefined call into unreachable, we end up with a branch on a
729condition that goes to unreachable: a branch to unreachable can never happen, so
730"-inline -instcombine -simplifycfg" is able to produce:</p>
731
732<pre class="doc_code">
733define fastcc void @foo() {
734 ret void
735}
736define void @test(i1 %X) {
737F.i:
738 call fastcc void @foo()
739 ret void
740}
741</pre>
742
743</div>
744
NAKAMURA Takumi9c55f592012-03-27 11:25:16 +0000[diff] [blame]745</div>
746
John Criswell6ea30b02003-11-18 16:05:23 +0000[diff] [blame]747<!-- *********************************************************************** -->
John Criswellc310f622003-10-13 16:13:06 +0000[diff] [blame]748
749<hr>
Misha Brukman7ce62cc2004-06-01 18:51:03 +0000[diff] [blame]750<address>
751 <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
Misha Brukman44408702008-12-11 17:34:48 +0000[diff] [blame]752 src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a>
Misha Brukman7ce62cc2004-06-01 18:51:03 +0000[diff] [blame]753 <a href="http://validator.w3.org/check/referer"><img
Misha Brukman44408702008-12-11 17:34:48 +0000[diff] [blame]754 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]755
NAKAMURA Takumib9a33632011-04-09 02:13:37 +0000[diff] [blame]756 <a href="http://llvm.org/">LLVM Compiler Infrastructure</a><br>
Misha Brukmana6538852003-11-06 21:55:44 +0000[diff] [blame]757 Last modified: $Date$
Misha Brukman7ce62cc2004-06-01 18:51:03 +0000[diff] [blame]758</address>
John Criswellf08c5d82003-10-24 22:48:20 +0000[diff] [blame]759
John Criswellc310f622003-10-13 16:13:06 +0000[diff] [blame]760</body>
761</html>