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> |
| 5 | <title>LLVM: Frequently Asked Questions</title> |
Misha Brukman | 7ce62cc | 2004-06-01 18:51:03 +0000 | [diff] [blame] | 6 | <style type="text/css"> |
Misha Brukman | a653885 | 2003-11-06 21:55:44 +0000 | [diff] [blame] | 7 | @import url("llvm.css"); |
| 8 | .question { font-weight: bold } |
| 9 | .answer { margin-left: 2em } |
| 10 | </style> |
| 11 | </head> |
| 12 | <body> |
John Criswell | c310f62 | 2003-10-13 16:13:06 +0000 | [diff] [blame] | 13 | |
Misha Brukman | a653885 | 2003-11-06 21:55:44 +0000 | [diff] [blame] | 14 | <div class="doc_title"> |
| 15 | LLVM: Frequently Asked Questions |
| 16 | </div> |
| 17 | |
| 18 | <ol> |
| 19 | <li><a href="#license">License</a> |
| 20 | <ol> |
| 21 | <li>Why are the LLVM source code and the front-end distributed under different |
| 22 | licenses?</li> |
| 23 | <li>Does the University of Illinois Open Source License really qualify as an |
| 24 | "open source" license?</li> |
| 25 | <li>Can I modify LLVM source code and redistribute the modified source?</li> |
| 26 | <li>Can I modify LLVM source code and redistribute binaries or other tools |
| 27 | based on it, without redistributing the source?</li> |
| 28 | </ol></li> |
| 29 | |
| 30 | <li><a href="#source">Source code</a> |
| 31 | <ol> |
| 32 | <li>In what language is LLVM written?</li> |
| 33 | <li>How portable is the LLVM source code?</li> |
| 34 | </ol></li> |
| 35 | |
| 36 | <li><a href="#build">Build Problems</a> |
| 37 | <ol> |
| 38 | <li>When I run configure, it finds the wrong C compiler.</li> |
| 39 | <li>I compile the code, and I get some error about <tt>/localhome</tt>.</li> |
| 40 | <li>The <tt>configure</tt> script finds the right C compiler, but it uses the |
| 41 | LLVM linker from a previous build. What do I do?</li> |
| 42 | <li>When creating a dynamic library, I get a strange GLIBC error.</li> |
| 43 | <li>I've updated my source tree from CVS, and now my build is trying to use a |
| 44 | file/directory that doesn't exist.</li> |
| 45 | <li>I've modified a Makefile in my source tree, but my build tree keeps using |
| 46 | the old version. What do I do?</li> |
| 47 | <li>I've upgraded to a new version of LLVM, and I get strange build |
| 48 | errors.</li> |
| 49 | <li>I've built LLVM and am testing it, but the tests freeze.</li> |
| 50 | <li>Why do test results differ when I perform different types of builds?</li> |
Chris Lattner | 306acee | 2003-12-22 04:06:12 +0000 | [diff] [blame] | 51 | <li>Compiling LLVM with GCC 3.3.2 fails, what should I do?</li> |
John Criswell | d179961 | 2004-03-29 20:23:11 +0000 | [diff] [blame] | 52 | <li>When I use the test suite, all of the C Backend tests fail. What is |
| 53 | wrong?</li> |
Misha Brukman | 1739aec | 2004-09-09 16:36:47 +0000 | [diff] [blame] | 54 | <li>After CVS update, rebuilding gives the error "No rule to make |
| 55 | target".</li> |
Misha Brukman | a653885 | 2003-11-06 21:55:44 +0000 | [diff] [blame] | 56 | </ol></li> |
John Criswell | 76c1e38 | 2003-11-18 16:08:49 +0000 | [diff] [blame] | 57 | |
Chris Lattner | cc33d70 | 2003-11-19 05:53:12 +0000 | [diff] [blame] | 58 | <li><a href="#cfe">Using the GCC Front End</a> |
John Criswell | 76c1e38 | 2003-11-18 16:08:49 +0000 | [diff] [blame] | 59 | <ol> |
| 60 | <li> |
| 61 | When I compile software that uses a configure script, the configure script |
| 62 | thinks my system has all of the header files and libraries it is testing |
| 63 | for. How do I get configure to work correctly? |
| 64 | </li> |
| 65 | |
| 66 | <li> |
| 67 | When I compile code using the LLVM GCC front end, it complains that it |
Chris Lattner | 7911ce2 | 2004-05-23 21:07:27 +0000 | [diff] [blame] | 68 | cannot find libcrtend.a. |
John Criswell | 76c1e38 | 2003-11-18 16:08:49 +0000 | [diff] [blame] | 69 | </li> |
| 70 | </ol> |
| 71 | </li> |
Chris Lattner | cc33d70 | 2003-11-19 05:53:12 +0000 | [diff] [blame] | 72 | |
| 73 | <li><a href="#cfe_code">Questions about code generated by the GCC front-end</a> |
| 74 | <ol> |
| 75 | <li>What is this <tt>__main()</tt> call that gets inserted into |
| 76 | <tt>main()</tt>?</li> |
Chris Lattner | a28e3ce | 2003-12-16 22:33:55 +0000 | [diff] [blame] | 77 | <li>Where did all of my code go??</li> |
Chris Lattner | c50bbc9 | 2004-03-29 19:14:35 +0000 | [diff] [blame] | 78 | <li>What is this <tt>llvm.global_ctors</tt> and |
| 79 | <tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I |
| 80 | #include <iostream>?</li> |
Chris Lattner | cc33d70 | 2003-11-19 05:53:12 +0000 | [diff] [blame] | 81 | </ol> |
| 82 | </li> |
Misha Brukman | a653885 | 2003-11-06 21:55:44 +0000 | [diff] [blame] | 83 | </ol> |
| 84 | |
Chris Lattner | 7911ce2 | 2004-05-23 21:07:27 +0000 | [diff] [blame] | 85 | <div class="doc_author"> |
| 86 | <p>Written by <a href="http://llvm.cs.uiuc.edu">The LLVM Team</a></p> |
| 87 | </div> |
| 88 | |
| 89 | |
Misha Brukman | a653885 | 2003-11-06 21:55:44 +0000 | [diff] [blame] | 90 | <!-- *********************************************************************** --> |
| 91 | <div class="doc_section"> |
| 92 | <a name="license">License</a> |
| 93 | </div> |
| 94 | <!-- *********************************************************************** --> |
| 95 | |
| 96 | <div class="question"> |
| 97 | <p>Why are the LLVM source code and the front-end distributed under different |
| 98 | licenses?</p> |
| 99 | </div> |
| 100 | |
| 101 | <div class="answer"> |
| 102 | <p>The C/C++ front-ends are based on GCC and must be distributed under the GPL. |
| 103 | Our aim is to distribute LLVM source code under a <em>much less restrictive</em> |
| 104 | license, in particular one that does not compel users who distribute tools based |
| 105 | on modifying the source to redistribute the modified source code as well.</p> |
| 106 | </div> |
| 107 | |
| 108 | <div class="question"> |
| 109 | <p>Does the University of Illinois Open Source License really qualify as an |
| 110 | "open source" license?</p> |
| 111 | </div> |
| 112 | |
| 113 | <div class="answer"> |
| 114 | <p>Yes, the license is <a |
| 115 | href="http://www.opensource.org/licenses/UoI-NCSA.php">certified</a> by the Open |
| 116 | Source Initiative (OSI).</p> |
| 117 | </div> |
| 118 | |
| 119 | <div class="question"> |
| 120 | <p>Can I modify LLVM source code and redistribute the modified source?</p> |
| 121 | </div> |
| 122 | |
| 123 | <div class="answer"> |
| 124 | <p>Yes. The modified source distribution must retain the copyright notice and |
| 125 | follow the three bulletted conditions listed in the <a |
Misha Brukman | 237dc2a | 2004-12-03 23:58:18 +0000 | [diff] [blame] | 126 | href="http://llvm.cs.uiuc.edu/releases/1.3/LICENSE.TXT">LLVM license</a>.</p> |
Misha Brukman | a653885 | 2003-11-06 21:55:44 +0000 | [diff] [blame] | 127 | </div> |
| 128 | |
| 129 | <div class="question"> |
| 130 | <p>Can I modify LLVM source code and redistribute binaries or other tools based |
| 131 | on it, without redistributing the source?</p> |
| 132 | </div> |
| 133 | |
| 134 | <div class="answer"> |
| 135 | <p>Yes, this is why we distribute LLVM under a less restrictive license than |
| 136 | GPL, as explained in the first question above.</p> |
| 137 | </div> |
| 138 | |
| 139 | <!-- *********************************************************************** --> |
| 140 | <div class="doc_section"> |
| 141 | <a name="source">Source Code</a> |
| 142 | </div> |
| 143 | <!-- *********************************************************************** --> |
| 144 | |
| 145 | <div class="question"> |
| 146 | <p>In what language is LLVM written?</p> |
| 147 | </div> |
| 148 | |
| 149 | <div class="answer"> |
| 150 | <p>All of the LLVM tools and libraries are written in C++ with extensive use of |
| 151 | the STL.</p> |
| 152 | </div> |
| 153 | |
| 154 | <div class="question"> |
| 155 | <p>How portable is the LLVM source code?</p> |
| 156 | </div> |
| 157 | |
| 158 | <div class="answer"> |
| 159 | <p>The LLVM source code should be portable to most modern UNIX-like operating |
| 160 | systems. Most of the code is written in standard C++ with operating system |
| 161 | services abstracted to a support library. The tools required to build and test |
| 162 | LLVM have been ported to a plethora of platforms.</p> |
| 163 | |
| 164 | <p>Some porting problems may exist in the following areas:</p> |
| 165 | |
| 166 | <ul> |
| 167 | |
| 168 | <li>The GCC front end code is not as portable as the LLVM suite, so it may not |
| 169 | compile as well on unsupported platforms.</li> |
| 170 | |
Misha Brukman | a653885 | 2003-11-06 21:55:44 +0000 | [diff] [blame] | 171 | <li>The LLVM build system relies heavily on UNIX shell tools, like the Bourne |
| 172 | Shell and sed. Porting to systems without these tools (MacOS 9, Plan 9) will |
| 173 | require more effort.</li> |
| 174 | |
| 175 | </ul> |
| 176 | |
| 177 | </div> |
| 178 | |
| 179 | <!-- *********************************************************************** --> |
| 180 | <div class="doc_section"> |
| 181 | <a name="build">Build Problems</a> |
| 182 | </div> |
| 183 | <!-- *********************************************************************** --> |
| 184 | |
| 185 | <div class="question"> |
| 186 | <p>When I run configure, it finds the wrong C compiler.</p> |
| 187 | </div> |
| 188 | |
| 189 | <div class="answer"> |
| 190 | |
| 191 | <p>The <tt>configure</tt> script attempts to locate first <tt>gcc</tt> and then |
| 192 | <tt>cc</tt>, unless it finds compiler paths set in <tt>CC</tt> and <tt>CXX</tt> |
| 193 | for the C and C++ compiler, respectively.</p> |
| 194 | |
| 195 | <p>If <tt>configure</tt> finds the wrong compiler, either adjust your |
| 196 | <tt>PATH</tt> environment variable or set <tt>CC</tt> and <tt>CXX</tt> |
| 197 | explicitly.</p> |
| 198 | |
| 199 | </div> |
| 200 | |
| 201 | <div class="question"> |
| 202 | <p>I compile the code, and I get some error about <tt>/localhome</tt>.</p> |
| 203 | </div> |
| 204 | |
| 205 | <div class="answer"> |
| 206 | |
| 207 | <p>There are several possible causes for this. The first is that you didn't set |
| 208 | a pathname properly when using <tt>configure</tt>, and it defaulted to a |
| 209 | pathname that we use on our research machines.</p> |
| 210 | |
| 211 | <p>Another possibility is that we hardcoded a path in our Makefiles. If you see |
| 212 | this, please email the LLVM bug mailing list with the name of the offending |
| 213 | Makefile and a description of what is wrong with it.</p> |
| 214 | |
| 215 | </div> |
| 216 | |
| 217 | <div class="question"> |
| 218 | <p>The <tt>configure</tt> script finds the right C compiler, but it uses the |
| 219 | LLVM linker from a previous build. What do I do?</p> |
| 220 | </div> |
| 221 | |
| 222 | <div class="answer"> |
| 223 | <p>The <tt>configure</tt> script uses the <tt>PATH</tt> to find executables, so |
| 224 | if it's grabbing the wrong linker/assembler/etc, there are two ways to fix |
| 225 | it:</p> |
| 226 | |
| 227 | <ol> |
| 228 | |
| 229 | <li><p>Adjust your <tt>PATH</tt> environment variable so that the correct |
| 230 | program appears first in the <tt>PATH</tt>. This may work, but may not be |
| 231 | convenient when you want them <i>first</i> in your path for other |
| 232 | work.</p></li> |
| 233 | |
| 234 | <li><p>Run <tt>configure</tt> with an alternative <tt>PATH</tt> that is |
| 235 | correct. In a Borne compatible shell, the syntax would be:</p> |
| 236 | |
Misha Brukman | 7ce62cc | 2004-06-01 18:51:03 +0000 | [diff] [blame] | 237 | <p><tt>PATH=[the path without the bad program] ./configure ...</tt></p> |
Misha Brukman | a653885 | 2003-11-06 21:55:44 +0000 | [diff] [blame] | 238 | |
| 239 | <p>This is still somewhat inconvenient, but it allows <tt>configure</tt> |
| 240 | to do its work without having to adjust your <tt>PATH</tt> |
| 241 | permanently.</p></li> |
| 242 | |
| 243 | </ol> |
| 244 | |
| 245 | </div> |
| 246 | |
| 247 | <div class="question"> |
| 248 | <p>When creating a dynamic library, I get a strange GLIBC error.</p> |
| 249 | </div> |
| 250 | |
| 251 | <div class="answer"> |
| 252 | <p>Under some operating systems (i.e. Linux), libtool does not work correctly if |
| 253 | GCC was compiled with the --disable-shared option. To work around this, install |
| 254 | your own version of GCC that has shared libraries enabled by default.</p> |
| 255 | </div> |
| 256 | |
| 257 | <div class="question"> |
| 258 | <p>I've updated my source tree from CVS, and now my build is trying to use a |
| 259 | file/directory that doesn't exist.</p> |
| 260 | </div> |
| 261 | |
| 262 | <div class="answer"> |
| 263 | <p>You need to re-run configure in your object directory. When new Makefiles |
| 264 | are added to the source tree, they have to be copied over to the object tree in |
| 265 | order to be used by the build.</p> |
| 266 | </div> |
| 267 | |
| 268 | <div class="question"> |
| 269 | <p>I've modified a Makefile in my source tree, but my build tree keeps using the |
| 270 | old version. What do I do?</p> |
| 271 | </div> |
| 272 | |
| 273 | <div class="answer"> |
| 274 | |
| 275 | <p>If the Makefile already exists in your object tree, you |
| 276 | can just run the following command in the top level directory of your object |
| 277 | tree:</p> |
| 278 | |
| 279 | <p><tt>./config.status <relative path to Makefile></tt><p> |
| 280 | |
| 281 | <p>If the Makefile is new, you will have to modify the configure script to copy |
| 282 | it over.</p> |
| 283 | |
| 284 | </div> |
| 285 | |
| 286 | <div class="question"> |
| 287 | <p>I've upgraded to a new version of LLVM, and I get strange build errors.</p> |
| 288 | </div> |
| 289 | |
| 290 | <div class="answer"> |
| 291 | |
| 292 | <p>Sometimes, changes to the LLVM source code alters how the build system works. |
| 293 | Changes in libtool, autoconf, or header file dependencies are especially prone |
| 294 | to this sort of problem.</p> |
| 295 | |
| 296 | <p>The best thing to try is to remove the old files and re-build. In most |
| 297 | cases, this takes care of the problem. To do this, just type <tt>make |
| 298 | clean</tt> and then <tt>make</tt> in the directory that fails to build.</p> |
| 299 | |
| 300 | </div> |
| 301 | |
| 302 | <div class="question"> |
| 303 | <p>I've built LLVM and am testing it, but the tests freeze.</p> |
| 304 | </div> |
| 305 | |
| 306 | <div class="answer"> |
| 307 | |
| 308 | <p>This is most likely occurring because you built a profile or release |
| 309 | (optimized) build of LLVM and have not specified the same information on the |
| 310 | <tt>gmake</tt> command line.</p> |
| 311 | |
| 312 | <p>For example, if you built LLVM with the command:</p> |
| 313 | |
| 314 | <p><tt>gmake ENABLE_PROFILING=1</tt> |
| 315 | |
| 316 | <p>...then you must run the tests with the following commands:</p> |
| 317 | |
| 318 | <p><tt>cd llvm/test<br>gmake ENABLE_PROFILING=1</tt></p> |
| 319 | |
| 320 | </div> |
| 321 | |
| 322 | <div class="question"> |
| 323 | <p>Why do test results differ when I perform different types of builds?</p> |
| 324 | </div> |
| 325 | |
| 326 | <div class="answer"> |
| 327 | |
| 328 | <p>The LLVM test suite is dependent upon several features of the LLVM tools and |
| 329 | libraries.</p> |
| 330 | |
| 331 | <p>First, the debugging assertions in code are not enabled in optimized or |
| 332 | profiling builds. Hence, tests that used to fail may pass.</p> |
| 333 | |
| 334 | <p>Second, some tests may rely upon debugging options or behavior that is only |
| 335 | available in the debug build. These tests will fail in an optimized or profile |
| 336 | build.</p> |
| 337 | |
| 338 | </div> |
| 339 | |
Chris Lattner | 8a0b924 | 2003-12-08 05:43:19 +0000 | [diff] [blame] | 340 | <div class="question"> |
Chris Lattner | 306acee | 2003-12-22 04:06:12 +0000 | [diff] [blame] | 341 | <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] | 342 | </div> |
| 343 | |
| 344 | <div class="answer"> |
Chris Lattner | 306acee | 2003-12-22 04:06:12 +0000 | [diff] [blame] | 345 | <p>This is <a href="http://gcc.gnu.org/PR?13392">a bug in GCC</a>, and |
| 346 | affects projects other than LLVM. Try upgrading or downgrading your GCC.</p> |
Chris Lattner | 8a0b924 | 2003-12-08 05:43:19 +0000 | [diff] [blame] | 347 | </div> |
| 348 | |
John Criswell | d179961 | 2004-03-29 20:23:11 +0000 | [diff] [blame] | 349 | <div class="question"> |
| 350 | <p> |
| 351 | When I use the test suite, all of the C Backend tests fail. What is |
| 352 | wrong? |
| 353 | </p> |
| 354 | </div> |
| 355 | |
| 356 | <div class="answer"> |
| 357 | <p> |
| 358 | If you build LLVM and the C Backend tests fail in <tt>llvm/test/Programs</tt>, |
| 359 | then chances are good that the directory pointed to by the LLVM_LIB_SEARCH_PATH |
| 360 | environment variable does not contain the libcrtend.a library. |
| 361 | </p> |
| 362 | |
| 363 | <p> |
| 364 | To fix it, verify that LLVM_LIB_SEARCH_PATH points to the correct directory |
| 365 | and that libcrtend.a is inside. For pre-built LLVM GCC front ends, this |
| 366 | should be the absolute path to |
Reid Spencer | 201ddef | 2004-12-14 22:07:29 +0000 | [diff] [blame] | 367 | <tt>cfrontend/<<i>platform</i>>/llvm-gcc/lib</tt>. If you've |
John Criswell | d179961 | 2004-03-29 20:23:11 +0000 | [diff] [blame] | 368 | built your own LLVM GCC front end, then ensure that you've built and installed |
| 369 | the libraries in <tt>llvm/runtime</tt> and have LLVM_LIB_SEARCH_PATH pointing |
Reid Spencer | 201ddef | 2004-12-14 22:07:29 +0000 | [diff] [blame] | 370 | to the <tt>$LLVMGCCDIR/lib</tt> subdirectory. |
John Criswell | d179961 | 2004-03-29 20:23:11 +0000 | [diff] [blame] | 371 | </p> |
| 372 | </div> |
| 373 | |
Misha Brukman | 1739aec | 2004-09-09 16:36:47 +0000 | [diff] [blame] | 374 | <div class="question"> |
| 375 | <p>After CVS update, rebuilding gives the error "No rule to make target".</p> |
| 376 | </div> |
| 377 | |
| 378 | <div class="answer"> |
| 379 | <p>If the error is of the form:</p> |
| 380 | |
| 381 | <div class="doc_code"> |
| 382 | <tt> |
| 383 | gmake[2]: *** No rule to make target `/path/to/somefile', needed by |
| 384 | `/path/to/another/file.d'.<br> |
| 385 | Stop. |
| 386 | </tt> |
| 387 | </div> |
| 388 | |
| 389 | <p>This may occur anytime files are moved within the CVS repository or removed |
| 390 | entirely. In this case, the best solution is to erase all <tt>.d</tt> files, |
| 391 | which list dependencies for source files, and rebuild:</p> |
| 392 | |
| 393 | <div class="doc_code"> |
| 394 | <pre> |
| 395 | % cd $LLVM_OBJ_DIR |
| 396 | % rm -f `find . -name \*\.d` |
| 397 | % gmake |
| 398 | </pre> |
| 399 | </div> |
| 400 | |
| 401 | <p>In other cases, it may be necessary to run <tt>make clean</tt> before |
| 402 | rebuilding.</p> |
| 403 | </div> |
| 404 | |
Misha Brukman | a653885 | 2003-11-06 21:55:44 +0000 | [diff] [blame] | 405 | <!-- *********************************************************************** --> |
John Criswell | 6ea30b0 | 2003-11-18 16:05:23 +0000 | [diff] [blame] | 406 | <div class="doc_section"> |
Chris Lattner | cc33d70 | 2003-11-19 05:53:12 +0000 | [diff] [blame] | 407 | <a name="cfe">Using the GCC Front End</a> |
John Criswell | 6ea30b0 | 2003-11-18 16:05:23 +0000 | [diff] [blame] | 408 | </div> |
| 409 | |
| 410 | <div class="question"> |
| 411 | <p> |
| 412 | When I compile software that uses a configure script, the configure script |
| 413 | thinks my system has all of the header files and libraries it is testing for. |
| 414 | How do I get configure to work correctly? |
| 415 | </p> |
| 416 | </div> |
| 417 | |
| 418 | <div class="answer"> |
| 419 | <p> |
| 420 | The configure script is getting things wrong because the LLVM linker allows |
| 421 | symbols to be undefined at link time (so that they can be resolved during JIT |
| 422 | or translation to the C back end). That is why configure thinks your system |
| 423 | "has everything." |
| 424 | </p> |
| 425 | <p> |
| 426 | To work around this, perform the following steps: |
| 427 | </p> |
| 428 | |
| 429 | <ol> |
| 430 | <li> |
| 431 | Make sure the CC and CXX environment variables contains the full path to the |
| 432 | LLVM GCC front end. |
| 433 | </li> |
| 434 | |
| 435 | <li> |
| 436 | Make sure that the regular C compiler is first in your PATH. |
| 437 | </li> |
| 438 | |
| 439 | <li> |
| 440 | Add the string "-Wl,-native" to your CFLAGS environment variable. |
| 441 | </li> |
| 442 | </ol> |
| 443 | |
| 444 | <p> |
| 445 | This will allow the gccld linker to create a native code executable instead of |
| 446 | a shell script that runs the JIT. Creating native code requires standard |
| 447 | linkage, which in turn will allow the configure script to find out if code is |
| 448 | not linking on your system because the feature isn't available on your system. |
| 449 | </p> |
| 450 | </div> |
| 451 | |
Chris Lattner | cc33d70 | 2003-11-19 05:53:12 +0000 | [diff] [blame] | 452 | <div class="question"> |
John Criswell | 6ea30b0 | 2003-11-18 16:05:23 +0000 | [diff] [blame] | 453 | <p> |
| 454 | When I compile code using the LLVM GCC front end, it complains that it cannot |
John Criswell | cd16119 | 2004-03-12 18:20:15 +0000 | [diff] [blame] | 455 | find libcrtend.a. |
John Criswell | 6ea30b0 | 2003-11-18 16:05:23 +0000 | [diff] [blame] | 456 | </p> |
| 457 | </div> |
| 458 | |
Chris Lattner | cc33d70 | 2003-11-19 05:53:12 +0000 | [diff] [blame] | 459 | <div class="answer"> |
John Criswell | 6ea30b0 | 2003-11-18 16:05:23 +0000 | [diff] [blame] | 460 | <p> |
John Criswell | cd16119 | 2004-03-12 18:20:15 +0000 | [diff] [blame] | 461 | In order to find libcrtend.a, you must have the directory in which it lives in |
John Criswell | 6ea30b0 | 2003-11-18 16:05:23 +0000 | [diff] [blame] | 462 | your LLVM_LIB_SEARCH_PATH environment variable. For the binary distribution of |
Reid Spencer | 201ddef | 2004-12-14 22:07:29 +0000 | [diff] [blame] | 463 | the LLVM GCC front end, this will be the full path of the <tt>lib</tt> |
John Criswell | 6ea30b0 | 2003-11-18 16:05:23 +0000 | [diff] [blame] | 464 | directory inside of the LLVM GCC distribution. |
| 465 | </p> |
| 466 | </div> |
Chris Lattner | cc33d70 | 2003-11-19 05:53:12 +0000 | [diff] [blame] | 467 | |
| 468 | |
| 469 | <!-- *********************************************************************** --> |
| 470 | <div class="doc_section"> |
| 471 | <a name="cfe_code">Questions about code generated by the GCC front-end</a> |
| 472 | </div> |
| 473 | |
Chris Lattner | a28e3ce | 2003-12-16 22:33:55 +0000 | [diff] [blame] | 474 | <div class="question"><p> |
Chris Lattner | cc33d70 | 2003-11-19 05:53:12 +0000 | [diff] [blame] | 475 | What is this <tt>__main()</tt> call that gets inserted into <tt>main()</tt>? |
Chris Lattner | a28e3ce | 2003-12-16 22:33:55 +0000 | [diff] [blame] | 476 | </p></div> |
Chris Lattner | cc33d70 | 2003-11-19 05:53:12 +0000 | [diff] [blame] | 477 | |
| 478 | <div class="answer"> |
| 479 | <p> |
| 480 | The <tt>__main</tt> call is inserted by the C/C++ compiler in order to guarantee |
| 481 | that static constructors and destructors are called when the program starts up |
| 482 | and shuts down. In C, you can create static constructors and destructors by |
| 483 | using GCC extensions, and in C++ you can do so by creating a global variable |
| 484 | whose class has a ctor or dtor. |
| 485 | </p> |
| 486 | |
| 487 | <p> |
| 488 | The actual implementation of <tt>__main</tt> lives in the |
| 489 | <tt>llvm/runtime/GCCLibraries/crtend/</tt> directory in the source-base, and is |
| 490 | linked in automatically when you link the program. |
| 491 | </p> |
Chris Lattner | cc33d70 | 2003-11-19 05:53:12 +0000 | [diff] [blame] | 492 | </div> |
| 493 | |
Chris Lattner | a28e3ce | 2003-12-16 22:33:55 +0000 | [diff] [blame] | 494 | <!--=========================================================================--> |
Chris Lattner | cc33d70 | 2003-11-19 05:53:12 +0000 | [diff] [blame] | 495 | |
Chris Lattner | a28e3ce | 2003-12-16 22:33:55 +0000 | [diff] [blame] | 496 | <div class="question"><p> |
| 497 | Where did all of my code go?? |
| 498 | </p></div> |
| 499 | |
| 500 | <div class="answer"> |
| 501 | <p> |
| 502 | If you are using the LLVM demo page, you may often wonder what happened to all |
| 503 | of the code that you typed in. Remember that the demo script is running the |
John Criswell | 02f61c1 | 2003-12-23 22:22:10 +0000 | [diff] [blame] | 504 | code through the LLVM optimizers, so if your code doesn't actually do anything |
Chris Lattner | a28e3ce | 2003-12-16 22:33:55 +0000 | [diff] [blame] | 505 | useful, it might all be deleted. |
| 506 | </p> |
| 507 | |
| 508 | <p> |
| 509 | To prevent this, make sure that the code is actually needed. For example, if |
| 510 | you are computing some expression, return the value from the function instead of |
| 511 | leaving it in a local variable. If you really want to constrain the optimizer, |
| 512 | you can read from and assign to <tt>volatile</tt> global variables. |
| 513 | </p> |
| 514 | </div> |
Chris Lattner | cc33d70 | 2003-11-19 05:53:12 +0000 | [diff] [blame] | 515 | |
Chris Lattner | c50bbc9 | 2004-03-29 19:14:35 +0000 | [diff] [blame] | 516 | <!--=========================================================================--> |
| 517 | |
Misha Brukman | 237dc2a | 2004-12-03 23:58:18 +0000 | [diff] [blame] | 518 | <div class="question"> |
| 519 | <p> What is this <tt>llvm.global_ctors</tt> and |
| 520 | <tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I #include |
| 521 | <iostream>?</p> |
| 522 | </div> |
Chris Lattner | c50bbc9 | 2004-03-29 19:14:35 +0000 | [diff] [blame] | 523 | |
| 524 | <div class="answer"> |
Misha Brukman | 237dc2a | 2004-12-03 23:58:18 +0000 | [diff] [blame] | 525 | |
| 526 | <p>If you #include the <iostream> header into a C++ translation unit, the |
Chris Lattner | c50bbc9 | 2004-03-29 19:14:35 +0000 | [diff] [blame] | 527 | file will probably use the <tt>std::cin</tt>/<tt>std::cout</tt>/... global |
| 528 | objects. However, C++ does not guarantee an order of initialization between |
| 529 | static objects in different translation units, so if a static ctor/dtor in your |
| 530 | .cpp file used <tt>std::cout</tt>, for example, the object would not necessarily |
Misha Brukman | 237dc2a | 2004-12-03 23:58:18 +0000 | [diff] [blame] | 531 | be automatically initialized before your use.</p> |
Chris Lattner | c50bbc9 | 2004-03-29 19:14:35 +0000 | [diff] [blame] | 532 | |
Misha Brukman | 237dc2a | 2004-12-03 23:58:18 +0000 | [diff] [blame] | 533 | <p>To make <tt>std::cout</tt> and friends work correctly in these scenarios, the |
Chris Lattner | c50bbc9 | 2004-03-29 19:14:35 +0000 | [diff] [blame] | 534 | STL that we use declares a static object that gets created in every translation |
Misha Brukman | 237dc2a | 2004-12-03 23:58:18 +0000 | [diff] [blame] | 535 | unit that includes <tt><iostream></tt>. This object has a static |
| 536 | constructor and destructor that initializes and destroys the global iostream |
| 537 | objects before they could possibly be used in the file. The code that you see |
| 538 | in the .ll file corresponds to the constructor and destructor registration code. |
Chris Lattner | c50bbc9 | 2004-03-29 19:14:35 +0000 | [diff] [blame] | 539 | </p> |
| 540 | |
Misha Brukman | 237dc2a | 2004-12-03 23:58:18 +0000 | [diff] [blame] | 541 | <p>If you would like to make it easier to <b>understand</b> the LLVM code |
| 542 | generated by the compiler in the demo page, consider using <tt>printf()</tt> |
| 543 | instead of <tt>iostream</tt>s to print values.</p> |
| 544 | |
Chris Lattner | c50bbc9 | 2004-03-29 19:14:35 +0000 | [diff] [blame] | 545 | </div> |
| 546 | |
John Criswell | 6ea30b0 | 2003-11-18 16:05:23 +0000 | [diff] [blame] | 547 | <!-- *********************************************************************** --> |
John Criswell | c310f62 | 2003-10-13 16:13:06 +0000 | [diff] [blame] | 548 | |
| 549 | <hr> |
Misha Brukman | 7ce62cc | 2004-06-01 18:51:03 +0000 | [diff] [blame] | 550 | <address> |
| 551 | <a href="http://jigsaw.w3.org/css-validator/check/referer"><img |
| 552 | src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a> |
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| 554 | src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!"></a> |
| 555 | |
| 556 | <a href="http://llvm.cs.uiuc.edu">LLVM Compiler Infrastructure</a><br> |
Misha Brukman | a653885 | 2003-11-06 21:55:44 +0000 | [diff] [blame] | 557 | Last modified: $Date$ |
Misha Brukman | 7ce62cc | 2004-06-01 18:51:03 +0000 | [diff] [blame] | 558 | </address> |
John Criswell | f08c5d8 | 2003-10-24 22:48:20 +0000 | [diff] [blame] | 559 | |
John Criswell | c310f62 | 2003-10-13 16:13:06 +0000 | [diff] [blame] | 560 | </body> |
| 561 | </html> |