Misha Brukman | a653885 | 2003-11-06 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 Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 5 | <meta http-equiv="Content-Type" content="text/html; charset=utf-8"> |
Misha Brukman | a653885 | 2003-11-06 21:55:44 +0000 | [diff] [blame] | 6 | <title>LLVM: Frequently Asked Questions</title> |
Misha Brukman | 7ce62cc | 2004-06-01 18:51:03 +0000 | [diff] [blame] | 7 | <style type="text/css"> |
Misha Brukman | a653885 | 2003-11-06 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 Criswell | c310f62 | 2003-10-13 16:13:06 +0000 | [diff] [blame] | 14 | |
Misha Brukman | a653885 | 2003-11-06 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 Wendling | 290235f | 2009-04-07 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 Brukman | a653885 | 2003-11-06 21:55:44 +0000 | [diff] [blame] | 32 | </ol></li> |
| 33 | |
| 34 | <li><a href="#source">Source code</a> |
| 35 | <ol> |
Bill Wendling | 290235f | 2009-04-07 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 Brukman | a653885 | 2003-11-06 21:55:44 +0000 | [diff] [blame] | 39 | </ol></li> |
| 40 | |
| 41 | <li><a href="#build">Build Problems</a> |
| 42 | <ol> |
Bill Wendling | 290235f | 2009-04-07 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> |
Misha Brukman | a653885 | 2003-11-06 21:55:44 +0000 | [diff] [blame] | 77 | </ol></li> |
John Criswell | 76c1e38 | 2003-11-18 16:08:49 +0000 | [diff] [blame] | 78 | |
Reid Spencer | 501bfee | 2006-04-26 14:52:19 +0000 | [diff] [blame] | 79 | <li><a href="#felangs">Source Languages</a> |
| 80 | <ol> |
| 81 | <li><a href="#langs">What source languages are supported?</a></li> |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 82 | |
Gordon Henriksen | e507905 | 2008-02-22 21:55:51 +0000 | [diff] [blame] | 83 | <li><a href="#langirgen">I'd like to write a self-hosting LLVM compiler. How |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 84 | should I interface with the LLVM middle-end optimizers and back-end code |
| 85 | generators?</a></li> |
| 86 | |
Reid Spencer | 501bfee | 2006-04-26 14:52:19 +0000 | [diff] [blame] | 87 | <li><a href="#langhlsupp">What support is there for higher level source |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 88 | language constructs for building a compiler?</a></li> |
| 89 | |
Reid Spencer | e00906f | 2006-08-10 20:15:58 +0000 | [diff] [blame] | 90 | <li><a href="GetElementPtr.html">I don't understand the GetElementPtr |
| 91 | instruction. Help!</a></li> |
Reid Spencer | 501bfee | 2006-04-26 14:52:19 +0000 | [diff] [blame] | 92 | </ol> |
| 93 | |
Chris Lattner | cc33d70 | 2003-11-19 05:53:12 +0000 | [diff] [blame] | 94 | <li><a href="#cfe">Using the GCC Front End</a> |
John Criswell | 76c1e38 | 2003-11-18 16:08:49 +0000 | [diff] [blame] | 95 | <ol> |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 96 | <li>When I compile software that uses a configure script, the configure |
| 97 | script thinks my system has all of the header files and libraries it is |
| 98 | testing for. How do I get configure to work correctly?</li> |
John Criswell | 76c1e38 | 2003-11-18 16:08:49 +0000 | [diff] [blame] | 99 | |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 100 | <li>When I compile code using the LLVM GCC front end, it complains that it |
| 101 | cannot find libcrtend.a?</li> |
Tanya Lattner | 14fc5c1 | 2005-04-25 20:36:56 +0000 | [diff] [blame] | 102 | |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 103 | <li>How can I disable all optimizations when compiling code using the LLVM |
| 104 | GCC front end?</li> |
Tanya Lattner | 14fc5c1 | 2005-04-25 20:36:56 +0000 | [diff] [blame] | 105 | |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 106 | <li><a href="#translatecxx">Can I use LLVM to convert C++ code to C |
| 107 | code?</a></li> |
Chris Lattner | af7fd20 | 2006-07-19 18:19:59 +0000 | [diff] [blame] | 108 | |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 109 | <li><a href="#platformindependent">Can I compile C or C++ code to |
| 110 | platform-independent LLVM bitcode?</a></li> |
John Criswell | 76c1e38 | 2003-11-18 16:08:49 +0000 | [diff] [blame] | 111 | </ol> |
| 112 | </li> |
Chris Lattner | cc33d70 | 2003-11-19 05:53:12 +0000 | [diff] [blame] | 113 | |
| 114 | <li><a href="#cfe_code">Questions about code generated by the GCC front-end</a> |
| 115 | <ol> |
Chris Lattner | 5a53c5d | 2005-02-25 20:30:21 +0000 | [diff] [blame] | 116 | <li><a href="#iosinit">What is this <tt>llvm.global_ctors</tt> and |
Chris Lattner | c50bbc9 | 2004-03-29 19:14:35 +0000 | [diff] [blame] | 117 | <tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I |
Chris Lattner | 5a53c5d | 2005-02-25 20:30:21 +0000 | [diff] [blame] | 118 | #include <iostream>?</a></li> |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 119 | |
Chris Lattner | 5a53c5d | 2005-02-25 20:30:21 +0000 | [diff] [blame] | 120 | <li><a href="#codedce">Where did all of my code go??</a></li> |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 121 | |
| 122 | <li><a href="#undef">What is this "<tt>undef</tt>" thing that shows up in |
| 123 | my code?</a></li> |
Chris Lattner | cc33d70 | 2003-11-19 05:53:12 +0000 | [diff] [blame] | 124 | </ol> |
| 125 | </li> |
Misha Brukman | a653885 | 2003-11-06 21:55:44 +0000 | [diff] [blame] | 126 | </ol> |
| 127 | |
Chris Lattner | 7911ce2 | 2004-05-23 21:07:27 +0000 | [diff] [blame] | 128 | <div class="doc_author"> |
Reid Spencer | 05fe4b0 | 2006-03-14 05:39:39 +0000 | [diff] [blame] | 129 | <p>Written by <a href="http://llvm.org">The LLVM Team</a></p> |
Chris Lattner | 7911ce2 | 2004-05-23 21:07:27 +0000 | [diff] [blame] | 130 | </div> |
| 131 | |
| 132 | |
Misha Brukman | a653885 | 2003-11-06 21:55:44 +0000 | [diff] [blame] | 133 | <!-- *********************************************************************** --> |
| 134 | <div class="doc_section"> |
| 135 | <a name="license">License</a> |
| 136 | </div> |
| 137 | <!-- *********************************************************************** --> |
| 138 | |
| 139 | <div class="question"> |
| 140 | <p>Why are the LLVM source code and the front-end distributed under different |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 141 | licenses?</p> |
Misha Brukman | a653885 | 2003-11-06 21:55:44 +0000 | [diff] [blame] | 142 | </div> |
| 143 | |
| 144 | <div class="answer"> |
| 145 | <p>The C/C++ front-ends are based on GCC and must be distributed under the GPL. |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 146 | Our aim is to distribute LLVM source code under a <em>much less |
| 147 | restrictive</em> license, in particular one that does not compel users who |
| 148 | distribute tools based on modifying the source to redistribute the modified |
| 149 | source code as well.</p> |
Misha Brukman | a653885 | 2003-11-06 21:55:44 +0000 | [diff] [blame] | 150 | </div> |
| 151 | |
| 152 | <div class="question"> |
| 153 | <p>Does the University of Illinois Open Source License really qualify as an |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 154 | "open source" license?</p> |
Misha Brukman | a653885 | 2003-11-06 21:55:44 +0000 | [diff] [blame] | 155 | </div> |
| 156 | |
| 157 | <div class="answer"> |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 158 | <p>Yes, the license |
| 159 | is <a href="http://www.opensource.org/licenses/UoI-NCSA.php">certified</a> by |
| 160 | the Open Source Initiative (OSI).</p> |
Misha Brukman | a653885 | 2003-11-06 21:55:44 +0000 | [diff] [blame] | 161 | </div> |
| 162 | |
| 163 | <div class="question"> |
| 164 | <p>Can I modify LLVM source code and redistribute the modified source?</p> |
| 165 | </div> |
| 166 | |
| 167 | <div class="answer"> |
| 168 | <p>Yes. The modified source distribution must retain the copyright notice and |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 169 | follow the three bulletted conditions listed in |
| 170 | the <a href="http://llvm.org/svn/llvm-project/llvm/trunk/LICENSE.TXT">LLVM |
| 171 | license</a>.</p> |
Misha Brukman | a653885 | 2003-11-06 21:55:44 +0000 | [diff] [blame] | 172 | </div> |
| 173 | |
| 174 | <div class="question"> |
| 175 | <p>Can I modify LLVM source code and redistribute binaries or other tools based |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 176 | on it, without redistributing the source?</p> |
Misha Brukman | a653885 | 2003-11-06 21:55:44 +0000 | [diff] [blame] | 177 | </div> |
| 178 | |
| 179 | <div class="answer"> |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 180 | <p>Yes. This is why we distribute LLVM under a less restrictive license than |
| 181 | GPL, as explained in the first question above.</p> |
Misha Brukman | a653885 | 2003-11-06 21:55:44 +0000 | [diff] [blame] | 182 | </div> |
| 183 | |
| 184 | <!-- *********************************************************************** --> |
| 185 | <div class="doc_section"> |
| 186 | <a name="source">Source Code</a> |
| 187 | </div> |
| 188 | <!-- *********************************************************************** --> |
| 189 | |
| 190 | <div class="question"> |
| 191 | <p>In what language is LLVM written?</p> |
| 192 | </div> |
| 193 | |
| 194 | <div class="answer"> |
| 195 | <p>All of the LLVM tools and libraries are written in C++ with extensive use of |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 196 | the STL.</p> |
Misha Brukman | a653885 | 2003-11-06 21:55:44 +0000 | [diff] [blame] | 197 | </div> |
| 198 | |
| 199 | <div class="question"> |
| 200 | <p>How portable is the LLVM source code?</p> |
| 201 | </div> |
| 202 | |
| 203 | <div class="answer"> |
| 204 | <p>The LLVM source code should be portable to most modern UNIX-like operating |
| 205 | systems. Most of the code is written in standard C++ with operating system |
| 206 | services abstracted to a support library. The tools required to build and test |
| 207 | LLVM have been ported to a plethora of platforms.</p> |
| 208 | |
| 209 | <p>Some porting problems may exist in the following areas:</p> |
| 210 | |
| 211 | <ul> |
Misha Brukman | a653885 | 2003-11-06 21:55:44 +0000 | [diff] [blame] | 212 | <li>The GCC front end code is not as portable as the LLVM suite, so it may not |
Bill Wendling | e9a6c35 | 2007-09-22 09:54:47 +0000 | [diff] [blame] | 213 | compile as well on unsupported platforms.</li> |
Misha Brukman | a653885 | 2003-11-06 21:55:44 +0000 | [diff] [blame] | 214 | |
Misha Brukman | a653885 | 2003-11-06 21:55:44 +0000 | [diff] [blame] | 215 | <li>The LLVM build system relies heavily on UNIX shell tools, like the Bourne |
Bill Wendling | e9a6c35 | 2007-09-22 09:54:47 +0000 | [diff] [blame] | 216 | Shell and sed. Porting to systems without these tools (MacOS 9, Plan 9) |
| 217 | will require more effort.</li> |
Misha Brukman | a653885 | 2003-11-06 21:55:44 +0000 | [diff] [blame] | 218 | </ul> |
| 219 | |
| 220 | </div> |
| 221 | |
| 222 | <!-- *********************************************************************** --> |
| 223 | <div class="doc_section"> |
| 224 | <a name="build">Build Problems</a> |
| 225 | </div> |
| 226 | <!-- *********************************************************************** --> |
| 227 | |
| 228 | <div class="question"> |
| 229 | <p>When I run configure, it finds the wrong C compiler.</p> |
| 230 | </div> |
| 231 | |
| 232 | <div class="answer"> |
Misha Brukman | a653885 | 2003-11-06 21:55:44 +0000 | [diff] [blame] | 233 | <p>The <tt>configure</tt> script attempts to locate first <tt>gcc</tt> and then |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 234 | <tt>cc</tt>, unless it finds compiler paths set in <tt>CC</tt> |
| 235 | and <tt>CXX</tt> for the C and C++ compiler, respectively.</p> |
Misha Brukman | a653885 | 2003-11-06 21:55:44 +0000 | [diff] [blame] | 236 | |
| 237 | <p>If <tt>configure</tt> finds the wrong compiler, either adjust your |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 238 | <tt>PATH</tt> environment variable or set <tt>CC</tt> and <tt>CXX</tt> |
| 239 | explicitly.</p> |
Misha Brukman | a653885 | 2003-11-06 21:55:44 +0000 | [diff] [blame] | 240 | |
| 241 | </div> |
| 242 | |
| 243 | <div class="question"> |
Misha Brukman | a653885 | 2003-11-06 21:55:44 +0000 | [diff] [blame] | 244 | <p>The <tt>configure</tt> script finds the right C compiler, but it uses the |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 245 | LLVM linker from a previous build. What do I do?</p> |
Misha Brukman | a653885 | 2003-11-06 21:55:44 +0000 | [diff] [blame] | 246 | </div> |
| 247 | |
| 248 | <div class="answer"> |
| 249 | <p>The <tt>configure</tt> script uses the <tt>PATH</tt> to find executables, so |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 250 | if it's grabbing the wrong linker/assembler/etc, there are two ways to fix |
| 251 | it:</p> |
Misha Brukman | a653885 | 2003-11-06 21:55:44 +0000 | [diff] [blame] | 252 | |
| 253 | <ol> |
Misha Brukman | a653885 | 2003-11-06 21:55:44 +0000 | [diff] [blame] | 254 | <li><p>Adjust your <tt>PATH</tt> environment variable so that the correct |
Bill Wendling | e9a6c35 | 2007-09-22 09:54:47 +0000 | [diff] [blame] | 255 | program appears first in the <tt>PATH</tt>. This may work, but may not be |
| 256 | convenient when you want them <i>first</i> in your path for other |
| 257 | work.</p></li> |
Misha Brukman | a653885 | 2003-11-06 21:55:44 +0000 | [diff] [blame] | 258 | |
| 259 | <li><p>Run <tt>configure</tt> with an alternative <tt>PATH</tt> that is |
Bill Wendling | e9a6c35 | 2007-09-22 09:54:47 +0000 | [diff] [blame] | 260 | correct. In a Borne compatible shell, the syntax would be:</p> |
| 261 | |
Misha Brukman | a54d4b2 | 2008-12-17 18:11:40 +0000 | [diff] [blame] | 262 | <pre class="doc_code"> |
Bill Wendling | e9a6c35 | 2007-09-22 09:54:47 +0000 | [diff] [blame] | 263 | % PATH=[the path without the bad program] ./configure ... |
| 264 | </pre> |
Misha Brukman | a653885 | 2003-11-06 21:55:44 +0000 | [diff] [blame] | 265 | |
| 266 | <p>This is still somewhat inconvenient, but it allows <tt>configure</tt> |
Bill Wendling | e9a6c35 | 2007-09-22 09:54:47 +0000 | [diff] [blame] | 267 | to do its work without having to adjust your <tt>PATH</tt> |
| 268 | permanently.</p></li> |
Misha Brukman | a653885 | 2003-11-06 21:55:44 +0000 | [diff] [blame] | 269 | </ol> |
Misha Brukman | a653885 | 2003-11-06 21:55:44 +0000 | [diff] [blame] | 270 | </div> |
| 271 | |
| 272 | <div class="question"> |
| 273 | <p>When creating a dynamic library, I get a strange GLIBC error.</p> |
| 274 | </div> |
| 275 | |
| 276 | <div class="answer"> |
| 277 | <p>Under some operating systems (i.e. Linux), libtool does not work correctly if |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 278 | GCC was compiled with the --disable-shared option. To work around this, |
| 279 | install your own version of GCC that has shared libraries enabled by |
| 280 | default.</p> |
Misha Brukman | a653885 | 2003-11-06 21:55:44 +0000 | [diff] [blame] | 281 | </div> |
| 282 | |
| 283 | <div class="question"> |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 284 | <p>I've updated my source tree from Subversion, and now my build is trying to |
| 285 | use a file/directory that doesn't exist.</p> |
Misha Brukman | a653885 | 2003-11-06 21:55:44 +0000 | [diff] [blame] | 286 | </div> |
| 287 | |
| 288 | <div class="answer"> |
| 289 | <p>You need to re-run configure in your object directory. When new Makefiles |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 290 | are added to the source tree, they have to be copied over to the object tree |
| 291 | in order to be used by the build.</p> |
Misha Brukman | a653885 | 2003-11-06 21:55:44 +0000 | [diff] [blame] | 292 | </div> |
| 293 | |
| 294 | <div class="question"> |
| 295 | <p>I've modified a Makefile in my source tree, but my build tree keeps using the |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 296 | old version. What do I do?</p> |
Misha Brukman | a653885 | 2003-11-06 21:55:44 +0000 | [diff] [blame] | 297 | </div> |
| 298 | |
| 299 | <div class="answer"> |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 300 | <p>If the Makefile already exists in your object tree, you can just run the |
| 301 | following command in the top level directory of your object tree:</p> |
Misha Brukman | a653885 | 2003-11-06 21:55:44 +0000 | [diff] [blame] | 302 | |
Misha Brukman | a54d4b2 | 2008-12-17 18:11:40 +0000 | [diff] [blame] | 303 | <pre class="doc_code"> |
| 304 | % ./config.status <relative path to Makefile> |
| 305 | </pre> |
Misha Brukman | a653885 | 2003-11-06 21:55:44 +0000 | [diff] [blame] | 306 | |
| 307 | <p>If the Makefile is new, you will have to modify the configure script to copy |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 308 | it over.</p> |
Misha Brukman | a653885 | 2003-11-06 21:55:44 +0000 | [diff] [blame] | 309 | </div> |
| 310 | |
| 311 | <div class="question"> |
| 312 | <p>I've upgraded to a new version of LLVM, and I get strange build errors.</p> |
| 313 | </div> |
| 314 | |
| 315 | <div class="answer"> |
| 316 | |
| 317 | <p>Sometimes, changes to the LLVM source code alters how the build system works. |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 318 | Changes in libtool, autoconf, or header file dependencies are especially |
| 319 | prone to this sort of problem.</p> |
Misha Brukman | a653885 | 2003-11-06 21:55:44 +0000 | [diff] [blame] | 320 | |
| 321 | <p>The best thing to try is to remove the old files and re-build. In most |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 322 | cases, this takes care of the problem. To do this, just type <tt>make |
| 323 | clean</tt> and then <tt>make</tt> in the directory that fails to build.</p> |
Misha Brukman | a653885 | 2003-11-06 21:55:44 +0000 | [diff] [blame] | 324 | </div> |
| 325 | |
| 326 | <div class="question"> |
| 327 | <p>I've built LLVM and am testing it, but the tests freeze.</p> |
| 328 | </div> |
| 329 | |
| 330 | <div class="answer"> |
Misha Brukman | a653885 | 2003-11-06 21:55:44 +0000 | [diff] [blame] | 331 | <p>This is most likely occurring because you built a profile or release |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 332 | (optimized) build of LLVM and have not specified the same information on the |
| 333 | <tt>gmake</tt> command line.</p> |
Misha Brukman | a653885 | 2003-11-06 21:55:44 +0000 | [diff] [blame] | 334 | |
| 335 | <p>For example, if you built LLVM with the command:</p> |
| 336 | |
Misha Brukman | a54d4b2 | 2008-12-17 18:11:40 +0000 | [diff] [blame] | 337 | <pre class="doc_code"> |
| 338 | % gmake ENABLE_PROFILING=1 |
| 339 | </pre> |
Misha Brukman | a653885 | 2003-11-06 21:55:44 +0000 | [diff] [blame] | 340 | |
| 341 | <p>...then you must run the tests with the following commands:</p> |
| 342 | |
Misha Brukman | a54d4b2 | 2008-12-17 18:11:40 +0000 | [diff] [blame] | 343 | <pre class="doc_code"> |
Bill Wendling | d6a68eb | 2007-05-29 09:24:33 +0000 | [diff] [blame] | 344 | % cd llvm/test |
| 345 | % gmake ENABLE_PROFILING=1 |
| 346 | </pre> |
Misha Brukman | a653885 | 2003-11-06 21:55:44 +0000 | [diff] [blame] | 347 | </div> |
| 348 | |
| 349 | <div class="question"> |
| 350 | <p>Why do test results differ when I perform different types of builds?</p> |
| 351 | </div> |
| 352 | |
| 353 | <div class="answer"> |
Misha Brukman | a653885 | 2003-11-06 21:55:44 +0000 | [diff] [blame] | 354 | <p>The LLVM test suite is dependent upon several features of the LLVM tools and |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 355 | libraries.</p> |
Misha Brukman | a653885 | 2003-11-06 21:55:44 +0000 | [diff] [blame] | 356 | |
| 357 | <p>First, the debugging assertions in code are not enabled in optimized or |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 358 | profiling builds. Hence, tests that used to fail may pass.</p> |
Misha Brukman | a653885 | 2003-11-06 21:55:44 +0000 | [diff] [blame] | 359 | |
| 360 | <p>Second, some tests may rely upon debugging options or behavior that is only |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 361 | available in the debug build. These tests will fail in an optimized or |
| 362 | profile build.</p> |
Misha Brukman | a653885 | 2003-11-06 21:55:44 +0000 | [diff] [blame] | 363 | </div> |
| 364 | |
Chris Lattner | 8a0b924 | 2003-12-08 05:43:19 +0000 | [diff] [blame] | 365 | <div class="question"> |
Chris Lattner | 306acee | 2003-12-22 04:06:12 +0000 | [diff] [blame] | 366 | <p>Compiling LLVM with GCC 3.3.2 fails, what should I do?</p> |
Chris Lattner | 8a0b924 | 2003-12-08 05:43:19 +0000 | [diff] [blame] | 367 | </div> |
| 368 | |
| 369 | <div class="answer"> |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 370 | <p>This is <a href="http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13392">a bug in |
| 371 | GCC</a>, and affects projects other than LLVM. Try upgrading or downgrading |
| 372 | your GCC.</p> |
Chris Lattner | 8a0b924 | 2003-12-08 05:43:19 +0000 | [diff] [blame] | 373 | </div> |
| 374 | |
John Criswell | d179961 | 2004-03-29 20:23:11 +0000 | [diff] [blame] | 375 | <div class="question"> |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 376 | <p>Compiling LLVM with GCC succeeds, but the resulting tools do not work, what |
| 377 | can be wrong?</p> |
Gabor Greif | 54820ce | 2009-03-02 19:08:05 +0000 | [diff] [blame] | 378 | </div> |
| 379 | |
| 380 | <div class="answer"> |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 381 | <p>Several versions of GCC have shown a weakness in miscompiling the LLVM |
| 382 | codebase. Please consult your compiler version (<tt>gcc --version</tt>) to |
| 383 | find out whether it is <a href="GettingStarted.html#brokengcc">broken</a>. |
| 384 | If so, your only option is to upgrade GCC to a known good version.</p> |
Gabor Greif | 54820ce | 2009-03-02 19:08:05 +0000 | [diff] [blame] | 385 | </div> |
| 386 | |
| 387 | <div class="question"> |
Bill Wendling | e9a6c35 | 2007-09-22 09:54:47 +0000 | [diff] [blame] | 388 | <p>After Subversion update, rebuilding gives the error "No rule to make |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 389 | target".</p> |
Misha Brukman | 1739aec | 2004-09-09 16:36:47 +0000 | [diff] [blame] | 390 | </div> |
| 391 | |
| 392 | <div class="answer"> |
| 393 | <p>If the error is of the form:</p> |
| 394 | |
Misha Brukman | a54d4b2 | 2008-12-17 18:11:40 +0000 | [diff] [blame] | 395 | <pre class="doc_code"> |
Misha Brukman | 1739aec | 2004-09-09 16:36:47 +0000 | [diff] [blame] | 396 | gmake[2]: *** No rule to make target `/path/to/somefile', needed by |
| 397 | `/path/to/another/file.d'.<br> |
| 398 | Stop. |
Bill Wendling | d6a68eb | 2007-05-29 09:24:33 +0000 | [diff] [blame] | 399 | </pre> |
Misha Brukman | 1739aec | 2004-09-09 16:36:47 +0000 | [diff] [blame] | 400 | |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 401 | <p>This may occur anytime files are moved within the Subversion repository or |
| 402 | removed entirely. In this case, the best solution is to erase all |
| 403 | <tt>.d</tt> files, which list dependencies for source files, and rebuild:</p> |
Misha Brukman | 1739aec | 2004-09-09 16:36:47 +0000 | [diff] [blame] | 404 | |
Misha Brukman | a54d4b2 | 2008-12-17 18:11:40 +0000 | [diff] [blame] | 405 | <pre class="doc_code"> |
Misha Brukman | 1739aec | 2004-09-09 16:36:47 +0000 | [diff] [blame] | 406 | % cd $LLVM_OBJ_DIR |
| 407 | % rm -f `find . -name \*\.d` |
| 408 | % gmake |
| 409 | </pre> |
Misha Brukman | 1739aec | 2004-09-09 16:36:47 +0000 | [diff] [blame] | 410 | |
| 411 | <p>In other cases, it may be necessary to run <tt>make clean</tt> before |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 412 | rebuilding.</p> |
Misha Brukman | 1739aec | 2004-09-09 16:36:47 +0000 | [diff] [blame] | 413 | </div> |
| 414 | |
Bill Wendling | e9a6c35 | 2007-09-22 09:54:47 +0000 | [diff] [blame] | 415 | <div class="question"><p><a name="llvmc"> |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 416 | <p>The <tt>llvmc</tt> program gives me errors/doesn't work.</a></p> |
Bill Wendling | 174d578 | 2007-05-29 09:35:34 +0000 | [diff] [blame] | 417 | </div> |
| 418 | |
| 419 | <div class="answer"> |
Bill Wendling | e9a6c35 | 2007-09-22 09:54:47 +0000 | [diff] [blame] | 420 | <p><tt>llvmc</tt> is experimental and isn't really supported. We suggest |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 421 | using <tt>llvm-gcc</tt> instead.</p> |
Bill Wendling | 174d578 | 2007-05-29 09:35:34 +0000 | [diff] [blame] | 422 | </div> |
| 423 | |
Misha Brukman | a653885 | 2003-11-06 21:55:44 +0000 | [diff] [blame] | 424 | <!-- *********************************************************************** --> |
Reid Spencer | c87f497 | 2006-04-26 15:46:53 +0000 | [diff] [blame] | 425 | <div class="doc_section"><a name="felangs">Source Languages</a></div> |
Reid Spencer | 501bfee | 2006-04-26 14:52:19 +0000 | [diff] [blame] | 426 | |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 427 | <div class="question"> |
| 428 | <p><a name="langs">What source languages are supported?</a></p> |
Reid Spencer | 501bfee | 2006-04-26 14:52:19 +0000 | [diff] [blame] | 429 | </div> |
Gordon Henriksen | 5836682 | 2008-02-22 20:58:29 +0000 | [diff] [blame] | 430 | |
Gordon Henriksen | 5836682 | 2008-02-22 20:58:29 +0000 | [diff] [blame] | 431 | <div class="answer"> |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 432 | <p>LLVM currently has full support for C and C++ source languages. These are |
| 433 | available through a special version of GCC that LLVM calls the |
| 434 | <a href="#cfe">C Front End</a></p> |
| 435 | |
| 436 | <p>There is an incomplete version of a Java front end available in the |
| 437 | <tt>java</tt> module. There is no documentation on this yet so you'll need to |
| 438 | download the code, compile it, and try it.</p> |
| 439 | |
| 440 | <p>The PyPy developers are working on integrating LLVM into the PyPy backend so |
| 441 | that PyPy language can translate to LLVM.</p> |
Gordon Henriksen | 5836682 | 2008-02-22 20:58:29 +0000 | [diff] [blame] | 442 | </div> |
| 443 | |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 444 | <div class="question"> |
| 445 | <p><a name="langirgen">I'd like to write a self-hosting LLVM compiler. How |
| 446 | should I interface with the LLVM middle-end optimizers and back-end code |
| 447 | generators?</a></p> |
Reid Spencer | 501bfee | 2006-04-26 14:52:19 +0000 | [diff] [blame] | 448 | </div> |
Chris Lattner | 33bef48 | 2006-08-15 00:43:35 +0000 | [diff] [blame] | 449 | |
Chris Lattner | 33bef48 | 2006-08-15 00:43:35 +0000 | [diff] [blame] | 450 | <div class="answer"> |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 451 | <p>Your compiler front-end will communicate with LLVM by creating a module in |
| 452 | the LLVM intermediate representation (IR) format. Assuming you want to write |
| 453 | your language's compiler in the language itself (rather than C++), there are |
| 454 | 3 major ways to tackle generating LLVM IR from a front-end:</p> |
| 455 | |
| 456 | <ul> |
| 457 | <li><strong>Call into the LLVM libraries code using your language's FFI |
| 458 | (foreign function interface).</strong> |
| 459 | |
| 460 | <ul> |
| 461 | <li><em>for:</em> best tracks changes to the LLVM IR, .ll syntax, and .bc |
| 462 | format</li> |
| 463 | |
| 464 | <li><em>for:</em> enables running LLVM optimization passes without a |
| 465 | emit/parse overhead</li> |
| 466 | |
| 467 | <li><em>for:</em> adapts well to a JIT context</li> |
| 468 | |
| 469 | <li><em>against:</em> lots of ugly glue code to write</li> |
| 470 | </ul></li> |
| 471 | |
| 472 | <li> <strong>Emit LLVM assembly from your compiler's native language.</strong> |
| 473 | <ul> |
| 474 | <li><em>for:</em> very straightforward to get started</li> |
| 475 | |
| 476 | <li><em>against:</em> the .ll parser is slower than the bitcode reader |
| 477 | when interfacing to the middle end</li> |
| 478 | |
| 479 | <li><em>against:</em> you'll have to re-engineer the LLVM IR object model |
| 480 | and asm writer in your language</li> |
| 481 | |
| 482 | <li><em>against:</em> it may be harder to track changes to the IR</li> |
| 483 | </ul></li> |
| 484 | |
| 485 | <li><strong>Emit LLVM bitcode from your compiler's native language.</strong> |
| 486 | |
| 487 | <ul> |
| 488 | <li><em>for:</em> can use the more-efficient bitcode reader when |
| 489 | interfacing to the middle end</li> |
| 490 | |
| 491 | <li><em>against:</em> you'll have to re-engineer the LLVM IR object |
| 492 | model and bitcode writer in your language</li> |
| 493 | |
| 494 | <li><em>against:</em> it may be harder to track changes to the IR</li> |
| 495 | </ul></li> |
| 496 | </ul> |
| 497 | |
| 498 | <p>If you go with the first option, the C bindings in include/llvm-c should help |
| 499 | a lot, since most languages have strong support for interfacing with C. The |
| 500 | most common hurdle with calling C from managed code is interfacing with the |
| 501 | garbage collector. The C interface was designed to require very little memory |
| 502 | management, and so is straightforward in this regard.</p> |
| 503 | </div> |
| 504 | |
| 505 | <div class="question"> |
| 506 | <p><a name="langhlsupp">What support is there for a higher level source language |
| 507 | constructs for building a compiler?</a></p> |
| 508 | </div> |
| 509 | |
| 510 | <div class="answer"> |
| 511 | <p>Currently, there isn't much. LLVM supports an intermediate representation |
| 512 | which is useful for code representation but will not support the high level |
| 513 | (abstract syntax tree) representation needed by most compilers. There are no |
| 514 | facilities for lexical nor semantic analysis. There is, however, a <i>mostly |
| 515 | implemented</i> configuration-driven |
| 516 | <a href="CompilerDriver.html">compiler driver</a> which simplifies the task |
| 517 | of running optimizations, linking, and executable generation.</p> |
| 518 | </div> |
| 519 | |
| 520 | <div class="question"> |
| 521 | <p><a name="getelementptr">I don't understand the GetElementPtr |
| 522 | instruction. Help!</a></p> |
| 523 | </div> |
| 524 | |
| 525 | <div class="answer"> |
| 526 | <p>See <a href="GetElementPtr.html">The Often Misunderstood GEP |
Bill Wendling | e9a6c35 | 2007-09-22 09:54:47 +0000 | [diff] [blame] | 527 | Instruction</a>.</p> |
Chris Lattner | 33bef48 | 2006-08-15 00:43:35 +0000 | [diff] [blame] | 528 | </div> |
| 529 | |
Reid Spencer | 501bfee | 2006-04-26 14:52:19 +0000 | [diff] [blame] | 530 | <!-- *********************************************************************** --> |
| 531 | <div class="doc_section"> |
Chris Lattner | cc33d70 | 2003-11-19 05:53:12 +0000 | [diff] [blame] | 532 | <a name="cfe">Using the GCC Front End</a> |
John Criswell | 6ea30b0 | 2003-11-18 16:05:23 +0000 | [diff] [blame] | 533 | </div> |
| 534 | |
| 535 | <div class="question"> |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 536 | <p>When I compile software that uses a configure script, the configure script |
| 537 | thinks my system has all of the header files and libraries it is testing for. |
| 538 | How do I get configure to work correctly?</p> |
John Criswell | 6ea30b0 | 2003-11-18 16:05:23 +0000 | [diff] [blame] | 539 | </div> |
| 540 | |
| 541 | <div class="answer"> |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 542 | <p>The configure script is getting things wrong because the LLVM linker allows |
| 543 | symbols to be undefined at link time (so that they can be resolved during JIT |
| 544 | or translation to the C back end). That is why configure thinks your system |
| 545 | "has everything."</p> |
| 546 | |
| 547 | <p>To work around this, perform the following steps:</p> |
| 548 | |
John Criswell | 6ea30b0 | 2003-11-18 16:05:23 +0000 | [diff] [blame] | 549 | <ol> |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 550 | <li>Make sure the CC and CXX environment variables contains the full path to |
| 551 | the LLVM GCC front end.</li> |
John Criswell | 6ea30b0 | 2003-11-18 16:05:23 +0000 | [diff] [blame] | 552 | |
Reid Spencer | 434262a | 2007-02-09 15:59:08 +0000 | [diff] [blame] | 553 | <li>Make sure that the regular C compiler is first in your PATH. </li> |
John Criswell | 6ea30b0 | 2003-11-18 16:05:23 +0000 | [diff] [blame] | 554 | |
Reid Spencer | 434262a | 2007-02-09 15:59:08 +0000 | [diff] [blame] | 555 | <li>Add the string "-Wl,-native" to your CFLAGS environment variable.</li> |
John Criswell | 6ea30b0 | 2003-11-18 16:05:23 +0000 | [diff] [blame] | 556 | </ol> |
| 557 | |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 558 | <p>This will allow the <tt>llvm-ld</tt> linker to create a native code |
| 559 | executable instead of shell script that runs the JIT. Creating native code |
| 560 | requires standard linkage, which in turn will allow the configure script to |
| 561 | find out if code is not linking on your system because the feature isn't |
| 562 | available on your system.</p> |
John Criswell | 6ea30b0 | 2003-11-18 16:05:23 +0000 | [diff] [blame] | 563 | </div> |
| 564 | |
Chris Lattner | cc33d70 | 2003-11-19 05:53:12 +0000 | [diff] [blame] | 565 | <div class="question"> |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 566 | <p>When I compile code using the LLVM GCC front end, it complains that it cannot |
| 567 | find libcrtend.a. |
John Criswell | 6ea30b0 | 2003-11-18 16:05:23 +0000 | [diff] [blame] | 568 | </p> |
| 569 | </div> |
| 570 | |
Chris Lattner | cc33d70 | 2003-11-19 05:53:12 +0000 | [diff] [blame] | 571 | <div class="answer"> |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 572 | <p>The only way this can happen is if you haven't installed the runtime |
| 573 | library. To correct this, do:</p> |
Bill Wendling | d6a68eb | 2007-05-29 09:24:33 +0000 | [diff] [blame] | 574 | |
Misha Brukman | a54d4b2 | 2008-12-17 18:11:40 +0000 | [diff] [blame] | 575 | <pre class="doc_code"> |
Bill Wendling | d6a68eb | 2007-05-29 09:24:33 +0000 | [diff] [blame] | 576 | % cd llvm/runtime |
| 577 | % make clean ; make install-bytecode |
Reid Spencer | f96eb57 | 2004-12-15 00:14:01 +0000 | [diff] [blame] | 578 | </pre> |
John Criswell | 6ea30b0 | 2003-11-18 16:05:23 +0000 | [diff] [blame] | 579 | </div> |
Chris Lattner | cc33d70 | 2003-11-19 05:53:12 +0000 | [diff] [blame] | 580 | |
Tanya Lattner | 14fc5c1 | 2005-04-25 20:36:56 +0000 | [diff] [blame] | 581 | <div class="question"> |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 582 | <p>How can I disable all optimizations when compiling code using the LLVM GCC |
| 583 | front end?</p> |
Tanya Lattner | 14fc5c1 | 2005-04-25 20:36:56 +0000 | [diff] [blame] | 584 | </div> |
| 585 | |
| 586 | <div class="answer"> |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 587 | <p>Passing "-Wa,-disable-opt -Wl,-disable-opt" will disable *all* cleanup and |
| 588 | optimizations done at the llvm level, leaving you with the truly horrible |
| 589 | code that you desire.</p> |
Tanya Lattner | 14fc5c1 | 2005-04-25 20:36:56 +0000 | [diff] [blame] | 590 | </div> |
Chris Lattner | cc33d70 | 2003-11-19 05:53:12 +0000 | [diff] [blame] | 591 | |
Chris Lattner | af7fd20 | 2006-07-19 18:19:59 +0000 | [diff] [blame] | 592 | |
| 593 | <div class="question"> |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 594 | <p><a name="translatecxx">Can I use LLVM to convert C++ code to C code?</a></p> |
Chris Lattner | af7fd20 | 2006-07-19 18:19:59 +0000 | [diff] [blame] | 595 | </div> |
| 596 | |
| 597 | <div class="answer"> |
| 598 | <p>Yes, you can use LLVM to convert code from any language LLVM supports to C. |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 599 | Note that the generated C code will be very low level (all loops are lowered |
| 600 | to gotos, etc) and not very pretty (comments are stripped, original source |
| 601 | formatting is totally lost, variables are renamed, expressions are |
| 602 | regrouped), so this may not be what you're looking for. Also, there are |
| 603 | several limitations noted below.<p> |
Chris Lattner | af7fd20 | 2006-07-19 18:19:59 +0000 | [diff] [blame] | 604 | |
| 605 | <p>Use commands like this:</p> |
| 606 | |
| 607 | <ol> |
Bill Wendling | e9a6c35 | 2007-09-22 09:54:47 +0000 | [diff] [blame] | 608 | <li><p>Compile your program as normal with llvm-g++:</p> |
Chris Lattner | af7fd20 | 2006-07-19 18:19:59 +0000 | [diff] [blame] | 609 | |
Misha Brukman | a54d4b2 | 2008-12-17 18:11:40 +0000 | [diff] [blame] | 610 | <pre class="doc_code"> |
Bill Wendling | d6a68eb | 2007-05-29 09:24:33 +0000 | [diff] [blame] | 611 | % llvm-g++ x.cpp -o program |
| 612 | </pre> |
Chris Lattner | af7fd20 | 2006-07-19 18:19:59 +0000 | [diff] [blame] | 613 | |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 614 | <p>or:</p> |
Chris Lattner | af7fd20 | 2006-07-19 18:19:59 +0000 | [diff] [blame] | 615 | |
Misha Brukman | a54d4b2 | 2008-12-17 18:11:40 +0000 | [diff] [blame] | 616 | <pre class="doc_code"> |
Bill Wendling | d6a68eb | 2007-05-29 09:24:33 +0000 | [diff] [blame] | 617 | % llvm-g++ a.cpp -c |
| 618 | % llvm-g++ b.cpp -c |
| 619 | % llvm-g++ a.o b.o -o program |
| 620 | </pre> |
Chris Lattner | af7fd20 | 2006-07-19 18:19:59 +0000 | [diff] [blame] | 621 | |
Bill Wendling | e9a6c35 | 2007-09-22 09:54:47 +0000 | [diff] [blame] | 622 | <p>With llvm-gcc3, this will generate program and program.bc. The .bc |
| 623 | file is the LLVM version of the program all linked together.</p></li> |
Chris Lattner | af7fd20 | 2006-07-19 18:19:59 +0000 | [diff] [blame] | 624 | |
Bill Wendling | e9a6c35 | 2007-09-22 09:54:47 +0000 | [diff] [blame] | 625 | <li><p>Convert the LLVM code to C code, using the LLC tool with the C |
| 626 | backend:</p> |
Chris Lattner | af7fd20 | 2006-07-19 18:19:59 +0000 | [diff] [blame] | 627 | |
Misha Brukman | a54d4b2 | 2008-12-17 18:11:40 +0000 | [diff] [blame] | 628 | <pre class="doc_code"> |
Bill Wendling | d6a68eb | 2007-05-29 09:24:33 +0000 | [diff] [blame] | 629 | % llc -march=c program.bc -o program.c |
Misha Brukman | a54d4b2 | 2008-12-17 18:11:40 +0000 | [diff] [blame] | 630 | </pre></li> |
Chris Lattner | af7fd20 | 2006-07-19 18:19:59 +0000 | [diff] [blame] | 631 | |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 632 | <li><p>Finally, compile the C file:</p> |
Chris Lattner | af7fd20 | 2006-07-19 18:19:59 +0000 | [diff] [blame] | 633 | |
Misha Brukman | a54d4b2 | 2008-12-17 18:11:40 +0000 | [diff] [blame] | 634 | <pre class="doc_code"> |
Bill Wendling | d6a68eb | 2007-05-29 09:24:33 +0000 | [diff] [blame] | 635 | % cc x.c |
Misha Brukman | a54d4b2 | 2008-12-17 18:11:40 +0000 | [diff] [blame] | 636 | </pre></li> |
Chris Lattner | af7fd20 | 2006-07-19 18:19:59 +0000 | [diff] [blame] | 637 | |
| 638 | </ol> |
| 639 | |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 640 | <p>Using LLVM does not eliminate the need for C++ library support. If you use |
| 641 | the llvm-g++ front-end, the generated code will depend on g++'s C++ support |
| 642 | libraries in the same way that code generated from g++ would. If you use |
| 643 | another C++ front-end, the generated code will depend on whatever library |
| 644 | that front-end would normally require.</p> |
Chris Lattner | b495fb0 | 2006-08-31 04:26:31 +0000 | [diff] [blame] | 645 | |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 646 | <p>If you are working on a platform that does not provide any C++ libraries, you |
| 647 | may be able to manually compile libstdc++ to LLVM bitcode, statically link it |
| 648 | into your program, then use the commands above to convert the whole result |
| 649 | into C code. Alternatively, you might compile the libraries and your |
| 650 | application into two different chunks of C code and link them.</p> |
Chris Lattner | b495fb0 | 2006-08-31 04:26:31 +0000 | [diff] [blame] | 651 | |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 652 | <p>Note that, by default, the C back end does not support exception handling. |
| 653 | If you want/need it for a certain program, you can enable it by passing |
| 654 | "-enable-correct-eh-support" to the llc program. The resultant code will use |
| 655 | setjmp/longjmp to implement exception support that is relatively slow, and |
| 656 | not C++-ABI-conforming on most platforms, but otherwise correct.</p> |
Dan Gohman | d5b455f | 2009-01-25 16:04:50 +0000 | [diff] [blame] | 657 | |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 658 | <p>Also, there are a number of other limitations of the C backend that cause it |
| 659 | to produce code that does not fully conform to the C++ ABI on most |
| 660 | platforms. Some of the C++ programs in LLVM's test suite are known to fail |
| 661 | when compiled with the C back end because of ABI incompatiblities with |
| 662 | standard C++ libraries.</p> |
Chris Lattner | af7fd20 | 2006-07-19 18:19:59 +0000 | [diff] [blame] | 663 | </div> |
| 664 | |
Dan Gohman | cfbcd59 | 2009-02-10 17:26:53 +0000 | [diff] [blame] | 665 | <div class="question"> |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 666 | <p><a name="platformindependent">Can I compile C or C++ code to |
| 667 | platform-independent LLVM bitcode?</a></p> |
Dan Gohman | cfbcd59 | 2009-02-10 17:26:53 +0000 | [diff] [blame] | 668 | </div> |
| 669 | |
| 670 | <div class="answer"> |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 671 | <p>No. C and C++ are inherently platform-dependent languages. The most obvious |
| 672 | example of this is the preprocessor. A very common way that C code is made |
| 673 | portable is by using the preprocessor to include platform-specific code. In |
| 674 | practice, information about other platforms is lost after preprocessing, so |
| 675 | the result is inherently dependent on the platform that the preprocessing was |
| 676 | targetting.</p> |
Dan Gohman | cfbcd59 | 2009-02-10 17:26:53 +0000 | [diff] [blame] | 677 | |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 678 | <p>Another example is <tt>sizeof</tt>. It's common for <tt>sizeof(long)</tt> to |
| 679 | vary between platforms. In most C front-ends, <tt>sizeof</tt> is expanded to |
| 680 | a constant immediately, thus hardwaring a platform-specific detail.</p> |
Dan Gohman | cfbcd59 | 2009-02-10 17:26:53 +0000 | [diff] [blame] | 681 | |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 682 | <p>Also, since many platforms define their ABIs in terms of C, and since LLVM is |
| 683 | lower-level than C, front-ends currently must emit platform-specific IR in |
| 684 | order to have the result conform to the platform ABI.</p> |
Dan Gohman | cfbcd59 | 2009-02-10 17:26:53 +0000 | [diff] [blame] | 685 | </div> |
| 686 | |
Chris Lattner | cc33d70 | 2003-11-19 05:53:12 +0000 | [diff] [blame] | 687 | <!-- *********************************************************************** --> |
| 688 | <div class="doc_section"> |
| 689 | <a name="cfe_code">Questions about code generated by the GCC front-end</a> |
| 690 | </div> |
| 691 | |
Misha Brukman | 237dc2a | 2004-12-03 23:58:18 +0000 | [diff] [blame] | 692 | <div class="question"> |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 693 | <p><a name="iosinit">What is this <tt>llvm.global_ctors</tt> and |
| 694 | <tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I <tt>#include |
| 695 | <iostream></tt>?</a></p> |
Misha Brukman | 237dc2a | 2004-12-03 23:58:18 +0000 | [diff] [blame] | 696 | </div> |
Chris Lattner | c50bbc9 | 2004-03-29 19:14:35 +0000 | [diff] [blame] | 697 | |
| 698 | <div class="answer"> |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 699 | <p>If you <tt>#include</tt> the <tt><iostream></tt> header into a C++ |
| 700 | translation unit, the file will probably use |
| 701 | the <tt>std::cin</tt>/<tt>std::cout</tt>/... global objects. However, C++ |
| 702 | does not guarantee an order of initialization between static objects in |
| 703 | different translation units, so if a static ctor/dtor in your .cpp file |
| 704 | used <tt>std::cout</tt>, for example, the object would not necessarily be |
| 705 | automatically initialized before your use.</p> |
Chris Lattner | c50bbc9 | 2004-03-29 19:14:35 +0000 | [diff] [blame] | 706 | |
Misha Brukman | 237dc2a | 2004-12-03 23:58:18 +0000 | [diff] [blame] | 707 | <p>To make <tt>std::cout</tt> and friends work correctly in these scenarios, the |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 708 | STL that we use declares a static object that gets created in every |
| 709 | translation unit that includes <tt><iostream></tt>. This object has a |
| 710 | static constructor and destructor that initializes and destroys the global |
| 711 | iostream objects before they could possibly be used in the file. The code |
| 712 | that you see in the .ll file corresponds to the constructor and destructor |
| 713 | registration code. |
Chris Lattner | c50bbc9 | 2004-03-29 19:14:35 +0000 | [diff] [blame] | 714 | </p> |
| 715 | |
Misha Brukman | 237dc2a | 2004-12-03 23:58:18 +0000 | [diff] [blame] | 716 | <p>If you would like to make it easier to <b>understand</b> the LLVM code |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 717 | generated by the compiler in the demo page, consider using <tt>printf()</tt> |
| 718 | instead of <tt>iostream</tt>s to print values.</p> |
Chris Lattner | c50bbc9 | 2004-03-29 19:14:35 +0000 | [diff] [blame] | 719 | </div> |
| 720 | |
Chris Lattner | 5a53c5d | 2005-02-25 20:30:21 +0000 | [diff] [blame] | 721 | <!--=========================================================================--> |
| 722 | |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 723 | <div class="question"> |
| 724 | <p><a name="codedce">Where did all of my code go??</a></p> |
| 725 | </div> |
Chris Lattner | 5a53c5d | 2005-02-25 20:30:21 +0000 | [diff] [blame] | 726 | |
| 727 | <div class="answer"> |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 728 | <p>If you are using the LLVM demo page, you may often wonder what happened to |
| 729 | all of the code that you typed in. Remember that the demo script is running |
| 730 | the code through the LLVM optimizers, so if your code doesn't actually do |
| 731 | anything useful, it might all be deleted.</p> |
Chris Lattner | 5a53c5d | 2005-02-25 20:30:21 +0000 | [diff] [blame] | 732 | |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 733 | <p>To prevent this, make sure that the code is actually needed. For example, if |
| 734 | you are computing some expression, return the value from the function instead |
| 735 | of leaving it in a local variable. If you really want to constrain the |
| 736 | optimizer, you can read from and assign to <tt>volatile</tt> global |
| 737 | variables.</p> |
Chris Lattner | 5a53c5d | 2005-02-25 20:30:21 +0000 | [diff] [blame] | 738 | </div> |
| 739 | |
| 740 | <!--=========================================================================--> |
| 741 | |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 742 | <div class="question"> |
| 743 | <p><a name="undef">What is this "<tt>undef</tt>" thing that shows up in my |
| 744 | code?</p> |
| 745 | </div> |
Chris Lattner | 5a53c5d | 2005-02-25 20:30:21 +0000 | [diff] [blame] | 746 | |
| 747 | <div class="answer"> |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 748 | <p><a href="LangRef.html#undef"><tt>undef</tt></a> is the LLVM way of |
| 749 | representing a value that is not defined. You can get these if you do not |
| 750 | initialize a variable before you use it. For example, the C function:</p> |
Chris Lattner | 5a53c5d | 2005-02-25 20:30:21 +0000 | [diff] [blame] | 751 | |
Misha Brukman | a54d4b2 | 2008-12-17 18:11:40 +0000 | [diff] [blame] | 752 | <pre class="doc_code"> |
Bill Wendling | d6a68eb | 2007-05-29 09:24:33 +0000 | [diff] [blame] | 753 | int X() { int i; return i; } |
| 754 | </pre> |
Chris Lattner | 5a53c5d | 2005-02-25 20:30:21 +0000 | [diff] [blame] | 755 | |
Bill Wendling | 290235f | 2009-04-07 18:40:56 +0000 | [diff] [blame^] | 756 | <p>Is compiled to "<tt>ret i32 undef</tt>" because "<tt>i</tt>" never has a |
| 757 | value specified for it.</p> |
Chris Lattner | 5a53c5d | 2005-02-25 20:30:21 +0000 | [diff] [blame] | 758 | </div> |
| 759 | |
John Criswell | 6ea30b0 | 2003-11-18 16:05:23 +0000 | [diff] [blame] | 760 | <!-- *********************************************************************** --> |
John Criswell | c310f62 | 2003-10-13 16:13:06 +0000 | [diff] [blame] | 761 | |
| 762 | <hr> |
Misha Brukman | 7ce62cc | 2004-06-01 18:51:03 +0000 | [diff] [blame] | 763 | <address> |
| 764 | <a href="http://jigsaw.w3.org/css-validator/check/referer"><img |
Misha Brukman | 4440870 | 2008-12-11 17:34:48 +0000 | [diff] [blame] | 765 | src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a> |
Misha Brukman | 7ce62cc | 2004-06-01 18:51:03 +0000 | [diff] [blame] | 766 | <a href="http://validator.w3.org/check/referer"><img |
Misha Brukman | 4440870 | 2008-12-11 17:34:48 +0000 | [diff] [blame] | 767 | src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a> |
Misha Brukman | 7ce62cc | 2004-06-01 18:51:03 +0000 | [diff] [blame] | 768 | |
Reid Spencer | 05fe4b0 | 2006-03-14 05:39:39 +0000 | [diff] [blame] | 769 | <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br> |
Misha Brukman | a653885 | 2003-11-06 21:55:44 +0000 | [diff] [blame] | 770 | Last modified: $Date$ |
Misha Brukman | 7ce62cc | 2004-06-01 18:51:03 +0000 | [diff] [blame] | 771 | </address> |
John Criswell | f08c5d8 | 2003-10-24 22:48:20 +0000 | [diff] [blame] | 772 | |
John Criswell | c310f62 | 2003-10-13 16:13:06 +0000 | [diff] [blame] | 773 | </body> |
| 774 | </html> |