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  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>The <tt>configure</tt> script finds the right C compiler, but it uses the
  40   LLVM linker from a previous build.  What do I do?</li>
  41   <li>When creating a dynamic library, I get a strange GLIBC error.</li>
  42   <li>I've updated my source tree from Subversion, and now my build is trying
  43   to use a file/directory that doesn't exist.</li>
  44   <li>I've modified a Makefile in my source tree, but my build tree keeps using
  45   the old version.  What do I do?</li>
  46   <li>I've upgraded to a new version of LLVM, and I get strange build
  47   errors.</li>
  48   <li>I've built LLVM and am testing it, but the tests freeze.</li>
  49   <li>Why do test results differ when I perform different types of builds?</li>
  50   <li>Compiling LLVM with GCC 3.3.2 fails, what should I do?</li>
  51   <li>When I use the test suite, all of the C Backend tests fail.  What is
  52       wrong?</li>
  53   <li>After Subversion update, rebuilding gives the error "No rule to make
  54       target".</li>
  55   <li><a href="#llvmc">The <tt>llvmc</tt> program gives me errors/doesn't
  56       work.</a></li>
  57   </ol></li>
  58
  59   <li><a href="#felangs">Source Languages</a>
  60   <ol>
  61     <li><a href="#langs">What source languages are supported?</a></li>
  62     <li><a href="#langhlsupp">What support is there for higher level source
  63       language constructs for building a compiler?</a></li>
  64     <li><a href="GetElementPtr.html">I don't understand the GetElementPtr
  65       instruction. Help!</a></li>
  66   </ol>
  67
  68   <li><a href="#cfe">Using the GCC Front End</a>
  69   <ol>
  70     <li>
  71     When I compile software that uses a configure script, the configure script
  72     thinks my system has all of the header files and libraries it is testing
  73     for.  How do I get configure to work correctly?
  74     </li>
  75
  76     <li>
  77     When I compile code using the LLVM GCC front end, it complains that it
  78     cannot find libcrtend.a.
  79     </li>
  80
  81     <li>
  82     How can I disable all optimizations when compiling code using the LLVM GCC front end?
  83     </li>
  84
  85     <li><a href="#translatec++">Can I use LLVM to convert C++ code to C code?</a></li>
  86
  87   </ol>
  88   </li>
  89
  90   <li><a href="#cfe_code">Questions about code generated by the GCC front-end</a>
  91   <ol>
  92      <li><a href="#__main">What is this <tt>__main()</tt> call that gets inserted into
  93          <tt>main()</tt>?</a></li>
  94      <li><a href="#iosinit">What is this <tt>llvm.global_ctors</tt> and
  95           <tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I
  96           #include &lt;iostream&gt;?</a></li>
  97      <li><a href="#codedce">Where did all of my code go??</a></li>
  98      <li><a href="#undef">What is this "<tt>undef</tt>" thing that shows up in my code?</a></li>
  99   </ol>
 100   </li>
 101 </ol>
 102
 103 <div class="doc_author">
 104   <p>Written by <a href="http://llvm.org">The LLVM Team</a></p>
 105 </div>
 106
 107
 108 <!-- *********************************************************************** -->
 109 <div class="doc_section">
 110   <a name="license">License</a>
 111 </div>
 112 <!-- *********************************************************************** -->
 113
 114 <div class="question">
 115 <p>Why are the LLVM source code and the front-end distributed under different
 116 licenses?</p>
 117 </div>
 118
 119 <div class="answer">
 120 <p>The C/C++ front-ends are based on GCC and must be distributed under the GPL.
 121 Our aim is to distribute LLVM source code under a <em>much less restrictive</em>
 122 license, in particular one that does not compel users who distribute tools based
 123 on modifying the source to redistribute the modified source code as well.</p>
 124 </div>
 125
 126 <div class="question">
 127 <p>Does the University of Illinois Open Source License really qualify as an
 128 "open source" license?</p>
 129 </div>
 130
 131 <div class="answer">
 132 <p>Yes, the license is <a
 133 href="http://www.opensource.org/licenses/UoI-NCSA.php">certified</a> by the Open
 134 Source Initiative (OSI).</p>
 135 </div>
 136
 137 <div class="question">
 138 <p>Can I modify LLVM source code and redistribute the modified source?</p>
 139 </div>
 140
 141 <div class="answer">
 142 <p>Yes.  The modified source distribution must retain the copyright notice and
 143 follow the three bulletted conditions listed in the <a
 144 href="http://llvm.org/releases/1.3/LICENSE.TXT">LLVM license</a>.</p>
 145 </div>
 146
 147 <div class="question">
 148 <p>Can I modify LLVM source code and redistribute binaries or other tools based
 149 on it, without redistributing the source?</p>
 150 </div>
 151
 152 <div class="answer">
 153 <p>Yes, this is why we distribute LLVM under a less restrictive license than
 154 GPL, as explained in the first question above.</p>
 155 </div>
 156
 157 <!-- *********************************************************************** -->
 158 <div class="doc_section">
 159   <a name="source">Source Code</a>
 160 </div>
 161 <!-- *********************************************************************** -->
 162
 163 <div class="question">
 164 <p>In what language is LLVM written?</p>
 165 </div>
 166
 167 <div class="answer">
 168 <p>All of the LLVM tools and libraries are written in C++ with extensive use of
 169 the STL.</p>
 170 </div>
 171
 172 <div class="question">
 173 <p>How portable is the LLVM source code?</p>
 174 </div>
 175
 176 <div class="answer">
 177 <p>The LLVM source code should be portable to most modern UNIX-like operating
 178 systems.  Most of the code is written in standard C++ with operating system
 179 services abstracted to a support library.  The tools required to build and test
 180 LLVM have been ported to a plethora of platforms.</p>
 181
 182 <p>Some porting problems may exist in the following areas:</p>
 183
 184 <ul>
 185   <li>The GCC front end code is not as portable as the LLVM suite, so it may not
 186       compile as well on unsupported platforms.</li>
 187
 188   <li>The LLVM build system relies heavily on UNIX shell tools, like the Bourne
 189       Shell and sed.  Porting to systems without these tools (MacOS 9, Plan 9)
 190       will require more effort.</li>
 191 </ul>
 192
 193 </div>
 194
 195 <!-- *********************************************************************** -->
 196 <div class="doc_section">
 197   <a name="build">Build Problems</a>
 198 </div>
 199 <!-- *********************************************************************** -->
 200
 201 <div class="question">
 202 <p>When I run configure, it finds the wrong C compiler.</p>
 203 </div>
 204
 205 <div class="answer">
 206
 207 <p>The <tt>configure</tt> script attempts to locate first <tt>gcc</tt> and then
 208 <tt>cc</tt>, unless it finds compiler paths set in <tt>CC</tt> and <tt>CXX</tt>
 209 for the C and C++ compiler, respectively.</p>
 210
 211 <p>If <tt>configure</tt> finds the wrong compiler, either adjust your
 212 <tt>PATH</tt> environment variable or set <tt>CC</tt> and <tt>CXX</tt>
 213 explicitly.</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   <li><p>Adjust your <tt>PATH</tt> environment variable so that the correct
 229       program appears first in the <tt>PATH</tt>.  This may work, but may not be
 230       convenient when you want them <i>first</i> in your path for other
 231       work.</p></li>
 232
 233   <li><p>Run <tt>configure</tt> with an alternative <tt>PATH</tt> that is
 234       correct. In a Borne compatible shell, the syntax would be:</p>
 235
 236 <div class="doc_code">
 237 <pre>
 238 % PATH=[the path without the bad program] ./configure ...
 239 </pre>
 240 </div>
 241
 242       <p>This is still somewhat inconvenient, but it allows <tt>configure</tt>
 243          to do its work without having to adjust your <tt>PATH</tt>
 244          permanently.</p></li>
 245 </ol>
 246
 247 </div>
 248
 249 <div class="question">
 250 <p>When creating a dynamic library, I get a strange GLIBC error.</p>
 251 </div>
 252
 253 <div class="answer">
 254 <p>Under some operating systems (i.e. Linux), libtool does not work correctly if
 255 GCC was compiled with the --disable-shared option.  To work around this, install
 256 your own version of GCC that has shared libraries enabled by default.</p>
 257 </div>
 258
 259 <div class="question">
 260 <p>I've updated my source tree from Subversion, and now my build is trying to
 261 use a file/directory that doesn't exist.</p>
 262 </div>
 263
 264 <div class="answer">
 265 <p>You need to re-run configure in your object directory.  When new Makefiles
 266 are added to the source tree, they have to be copied over to the object tree in
 267 order to be used by the build.</p>
 268 </div>
 269
 270 <div class="question">
 271 <p>I've modified a Makefile in my source tree, but my build tree keeps using the
 272 old version.  What do I do?</p>
 273 </div>
 274
 275 <div class="answer">
 276 <p>If the Makefile already exists in your object tree, you
 277 can just run the following command in the top level directory of your object
 278 tree:</p>
 279
 280 <div class="doc_code">
 281 <pre>% ./config.status &lt;relative path to Makefile&gt;</pre>
 282 </div>
 283
 284 <p>If the Makefile is new, you will have to modify the configure script to copy
 285 it over.</p>
 286
 287 </div>
 288
 289 <div class="question">
 290 <p>I've upgraded to a new version of LLVM, and I get strange build errors.</p>
 291 </div>
 292
 293 <div class="answer">
 294
 295 <p>Sometimes, changes to the LLVM source code alters how the build system works.
 296 Changes in libtool, autoconf, or header file dependencies are especially prone
 297 to this sort of problem.</p>
 298
 299 <p>The best thing to try is to remove the old files and re-build.  In most
 300 cases, this takes care of the problem.  To do this, just type <tt>make
 301 clean</tt> and then <tt>make</tt> in the directory that fails to build.</p>
 302
 303 </div>
 304
 305 <div class="question">
 306 <p>I've built LLVM and am testing it, but the tests freeze.</p>
 307 </div>
 308
 309 <div class="answer">
 310
 311 <p>This is most likely occurring because you built a profile or release
 312 (optimized) build of LLVM and have not specified the same information on the
 313 <tt>gmake</tt> command line.</p>
 314
 315 <p>For example, if you built LLVM with the command:</p>
 316
 317 <div class="doc_code">
 318 <pre>% gmake ENABLE_PROFILING=1</pre>
 319 </div>
 320
 321 <p>...then you must run the tests with the following commands:</p>
 322
 323 <div class="doc_code">
 324 <pre>
 325 % cd llvm/test
 326 % gmake ENABLE_PROFILING=1
 327 </pre>
 328 </div>
 329
 330 </div>
 331
 332 <div class="question">
 333 <p>Why do test results differ when I perform different types of builds?</p>
 334 </div>
 335
 336 <div class="answer">
 337
 338 <p>The LLVM test suite is dependent upon several features of the LLVM tools and
 339 libraries.</p>
 340
 341 <p>First, the debugging assertions in code are not enabled in optimized or
 342 profiling builds.  Hence, tests that used to fail may pass.</p>
 343
 344 <p>Second, some tests may rely upon debugging options or behavior that is only
 345 available in the debug build.  These tests will fail in an optimized or profile
 346 build.</p>
 347
 348 </div>
 349
 350 <div class="question">
 351 <p>Compiling LLVM with GCC 3.3.2 fails, what should I do?</p>
 352 </div>
 353
 354 <div class="answer">
 355 <p>This is <a href="http://gcc.gnu.org/PR?13392">a bug in GCC</a>, and
 356 affects projects other than LLVM.  Try upgrading or downgrading your GCC.</p>
 357 </div>
 358
 359 <div class="question">
 360 <p>After Subversion update, rebuilding gives the error "No rule to make
 361 target".</p>
 362 </div>
 363
 364 <div class="answer">
 365 <p>If the error is of the form:</p>
 366
 367 <div class="doc_code">
 368 <pre>
 369 gmake[2]: *** No rule to make target `/path/to/somefile', needed by
 370 `/path/to/another/file.d'.<br>
 371 Stop.
 372 </pre>
 373 </div>
 374
 375 <p>This may occur anytime files are moved within the Subversion repository or
 376 removed entirely.  In this case, the best solution is to erase all
 377 <tt>.d</tt> files, which list dependencies for source files, and rebuild:</p>
 378
 379 <div class="doc_code">
 380 <pre>
 381 % cd $LLVM_OBJ_DIR
 382 % rm -f `find . -name \*\.d`
 383 % gmake
 384 </pre>
 385 </div>
 386
 387 <p>In other cases, it may be necessary to run <tt>make clean</tt> before
 388 rebuilding.</p>
 389 </div>
 390
 391 <div class="question"><p><a name="llvmc">
 392 The <tt>llvmc</tt> program gives me errors/doesn't work.</a></p>
 393 </div>
 394
 395 <div class="answer">
 396 <p><tt>llvmc</tt> is experimental and isn't really supported. We suggest
 397 using <tt>llvm-gcc</tt> instead.</p>
 398 </div>
 399
 400 <!-- *********************************************************************** -->
 401 <div class="doc_section"><a name="felangs">Source Languages</a></div>
 402
 403 <div class="question"><p>
 404   <a name="langs">What source languages are supported?</a></p>
 405 </div>
 406 <div class="answer">
 407   <p>LLVM currently has full support for C and C++ source languages. These are
 408   available through a special version of GCC that LLVM calls the
 409   <a href="#cfe">C Front End</a></p>
 410   <p>There is an incomplete version of a Java front end available in the
 411   <tt>java</tt> module. There is no documentation on this yet so
 412   you'll need to download the code, compile it, and try it.</p>
 413   <p>In the <tt>stacker</tt> module is a compiler and runtime
 414   library for the Stacker language, a "toy" language loosely based on Forth.</p>
 415   <p>The PyPy developers are working on integrating LLVM into the PyPy backend
 416   so that PyPy language can translate to LLVM.</p>
 417 </div>
 418 <div class="question"><p><a name="langhlsupp">
 419   What support is there for a higher level source language constructs for
 420   building a compiler?</a></p>
 421 </div>
 422 <div class="answer">
 423   <p>Currently, there isn't much. LLVM supports an intermediate representation
 424   which is useful for code representation but will not support the high level
 425   (abstract syntax tree) representation needed by most compilers. There are no
 426   facilities for lexical nor semantic analysis. There is, however, a <i>mostly
 427     implemented</i> configuration-driven
 428   <a href="CompilerDriver.html">compiler driver</a> which simplifies the task
 429   of running optimizations, linking, and executable generation.</p>
 430 </div>
 431
 432 <div class="question"><p><a name="langhlsupp">
 433   I don't understand the GetElementPtr instruction. Help!</a></p>
 434 </div>
 435 <div class="answer">
 436   <p>See <a href="GetElementPtr.html">The Often Misunderstood GEP
 437    Instruction</a>.</p>
 438 </div>
 439
 440 <!-- *********************************************************************** -->
 441 <div class="doc_section">
 442   <a name="cfe">Using the GCC Front End</a>
 443 </div>
 444
 445 <div class="question">
 446 <p>
 447 When I compile software that uses a configure script, the configure script
 448 thinks my system has all of the header files and libraries it is testing for.
 449 How do I get configure to work correctly?
 450 </p>
 451 </div>
 452
 453 <div class="answer">
 454 <p>
 455 The configure script is getting things wrong because the LLVM linker allows
 456 symbols to be undefined at link time (so that they can be resolved during JIT
 457 or translation to the C back end).  That is why configure thinks your system
 458 "has everything."
 459 </p>
 460 <p>
 461 To work around this, perform the following steps:
 462 </p>
 463 <ol>
 464   <li>Make sure the CC and CXX environment variables contains the full path to
 465   the LLVM GCC front end.</li>
 466
 467   <li>Make sure that the regular C compiler is first in your PATH. </li>
 468
 469   <li>Add the string "-Wl,-native" to your CFLAGS environment variable.</li>
 470 </ol>
 471
 472 <p>
 473 This will allow the <tt>llvm-ld</tt> linker to create a native code executable
 474 instead of shell script that runs the JIT.  Creating native code requires
 475 standard linkage, which in turn will allow the configure script to find out if
 476 code is not linking on your system because the feature isn't available on your
 477 system.</p>
 478 </div>
 479
 480 <div class="question">
 481 <p>
 482 When I compile code using the LLVM GCC front end, it complains that it cannot
 483 find libcrtend.a.
 484 </p>
 485 </div>
 486
 487 <div class="answer">
 488 <p>
 489 The only way this can happen is if you haven't installed the runtime library. To
 490 correct this, do:</p>
 491
 492 <div class="doc_code">
 493 <pre>
 494 % cd llvm/runtime
 495 % make clean ; make install-bytecode
 496 </pre>
 497 </div>
 498 </div>
 499
 500 <div class="question">
 501 <p>
 502 How can I disable all optimizations when compiling code using the LLVM GCC front end?
 503 </p>
 504 </div>
 505
 506 <div class="answer">
 507 <p>
 508 Passing "-Wa,-disable-opt -Wl,-disable-opt" will disable *all* cleanup and
 509 optimizations done at the llvm level, leaving you with the truly horrible
 510 code that you desire.
 511 </p>
 512 </div>
 513
 514
 515 <div class="question">
 516 <p>
 517 <a name="translatec++">Can I use LLVM to convert C++ code to C code?</a>
 518 </p>
 519 </div>
 520
 521 <div class="answer">
 522 <p>Yes, you can use LLVM to convert code from any language LLVM supports to C.
 523 Note that the generated C code will be very low level (all loops are lowered
 524 to gotos, etc) and not very pretty (comments are stripped, original source
 525 formatting is totally lost, variables are renamed, expressions are regrouped),
 526 so this may not be what you're looking for.  However, this is a good way to add
 527 C++ support for a processor that does not otherwise have a C++ compiler.
 528 </p>
 529
 530 <p>Use commands like this:</p>
 531
 532 <ol>
 533   <li><p>Compile your program as normal with llvm-g++:</p>
 534
 535 <div class="doc_code">
 536 <pre>
 537 % llvm-g++ x.cpp -o program
 538 </pre>
 539 </div>
 540
 541   <p>or:</p>
 542
 543 <div class="doc_code">
 544 <pre>
 545 % llvm-g++ a.cpp -c
 546 % llvm-g++ b.cpp -c
 547 % llvm-g++ a.o b.o -o program
 548 </pre>
 549 </div>
 550
 551       <p>With llvm-gcc3, this will generate program and program.bc.  The .bc
 552          file is the LLVM version of the program all linked together.</p></li>
 553
 554   <li><p>Convert the LLVM code to C code, using the LLC tool with the C
 555       backend:</p>
 556
 557 <div class="doc_code">
 558 <pre>
 559 % llc -march=c program.bc -o program.c
 560 </pre>
 561 </div></li>
 562
 563 <li><p>Finally, compile the C file:</p>
 564
 565 <div class="doc_code">
 566 <pre>
 567 % cc x.c
 568 </pre>
 569 </div></li>
 570
 571 </ol>
 572
 573 <p>Note that, by default, the C backend does not support exception handling.  If
 574 you want/need it for a certain program, you can enable it by passing
 575 "-enable-correct-eh-support" to the llc program.  The resultant code will use
 576 setjmp/longjmp to implement exception support that is correct but relatively
 577 slow.</p>
 578
 579 <p>Also note: this specific sequence of commands won't work if you use a
 580 function defined in the C++ runtime library (or any other C++ library).  To
 581 access an external C++ library, you must manually compile libstdc++ to LLVM
 582 bitcode, statically link it into your program, then use the commands above to
 583 convert the whole result into C code.  Alternatively, you can compile the
 584 libraries and your application into two different chunks of C code and link
 585 them.</p>
 586
 587 </div>
 588
 589 <!-- *********************************************************************** -->
 590 <div class="doc_section">
 591   <a name="cfe_code">Questions about code generated by the GCC front-end</a>
 592 </div>
 593
 594 <div class="question"><p>
 595 <a name="__main"></a>
 596 What is this <tt>__main()</tt> call that gets inserted into <tt>main()</tt>?
 597 </p></div>
 598
 599 <div class="answer">
 600 <p>
 601 The <tt>__main</tt> call is inserted by the C/C++ compiler in order to guarantee
 602 that static constructors and destructors are called when the program starts up
 603 and shuts down.  In C, you can create static constructors and destructors by
 604 using GCC extensions, and in C++ you can do so by creating a global variable
 605 whose class has a ctor or dtor.
 606 </p>
 607
 608 <p>
 609 The actual implementation of <tt>__main</tt> lives in the
 610 <tt>llvm/runtime/GCCLibraries/crtend/</tt> directory in the source-base, and is
 611 linked in automatically when you link the program.
 612 </p>
 613 </div>
 614
 615 <!--=========================================================================-->
 616
 617 <div class="question">
 618 <a name="iosinit"></a>
 619 <p> What is this <tt>llvm.global_ctors</tt> and
 620 <tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I #include
 621 &lt;iostream&gt;?</p>
 622 </div>
 623
 624 <div class="answer">
 625
 626 <p>If you #include the &lt;iostream&gt; header into a C++ translation unit, the
 627 file will probably use the <tt>std::cin</tt>/<tt>std::cout</tt>/... global
 628 objects.  However, C++ does not guarantee an order of initialization between
 629 static objects in different translation units, so if a static ctor/dtor in your
 630 .cpp file used <tt>std::cout</tt>, for example, the object would not necessarily
 631 be automatically initialized before your use.</p>
 632
 633 <p>To make <tt>std::cout</tt> and friends work correctly in these scenarios, the
 634 STL that we use declares a static object that gets created in every translation
 635 unit that includes <tt>&lt;iostream&gt;</tt>.  This object has a static
 636 constructor and destructor that initializes and destroys the global iostream
 637 objects before they could possibly be used in the file.  The code that you see
 638 in the .ll file corresponds to the constructor and destructor registration code.
 639 </p>
 640
 641 <p>If you would like to make it easier to <b>understand</b> the LLVM code
 642 generated by the compiler in the demo page, consider using <tt>printf()</tt>
 643 instead of <tt>iostream</tt>s to print values.</p>
 644
 645 </div>
 646
 647 <!--=========================================================================-->
 648
 649 <div class="question"><p>
 650 <a name="codedce"></a>
 651 Where did all of my code go??
 652 </p></div>
 653
 654 <div class="answer">
 655 <p>
 656 If you are using the LLVM demo page, you may often wonder what happened to all
 657 of the code that you typed in.  Remember that the demo script is running the
 658 code through the LLVM optimizers, so if your code doesn't actually do anything
 659 useful, it might all be deleted.
 660 </p>
 661
 662 <p>
 663 To prevent this, make sure that the code is actually needed.  For example, if
 664 you are computing some expression, return the value from the function instead of
 665 leaving it in a local variable.  If you really want to constrain the optimizer,
 666 you can read from and assign to <tt>volatile</tt> global variables.
 667 </p>
 668 </div>
 669
 670 <!--=========================================================================-->
 671
 672 <div class="question"><p>
 673 <a name="undef"></a>
 674 <p>What is this "<tt>undef</tt>" thing that shows up in my code?
 675 </p></div>
 676
 677 <div class="answer">
 678 <p>
 679 <a href="LangRef.html#undef"><tt>undef</tt></a> is the LLVM way of representing
 680 a value that is not defined.  You can get these if you do not initialize a
 681 variable before you use it.  For example, the C function:</p>
 682
 683 <div class="doc_code">
 684 <pre>
 685 int X() { int i; return i; }
 686 </pre>
 687 </div>
 688
 689 <p>Is compiled to "<tt>ret i32 undef</tt>" because "<tt>i</tt>" never has
 690 a value specified for it.</p>
 691 </div>
 692
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