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      6   <title>How to submit an LLVM bug report</title>
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     10 
     11 <h1>
     12   How to submit an LLVM bug report
     13 </h1>
     14 
     15 <table class="layout" style="width: 90%" >
     16 <tr class="layout">
     17   <td class="left">
     18 <ol>
     19   <li><a href="#introduction">Introduction - Got bugs?</a></li>
     20   <li><a href="#crashers">Crashing Bugs</a>
     21     <ul>
     22     <li><a href="#front-end">Front-end bugs</a>
     23     <li><a href="#ct_optimizer">Compile-time optimization bugs</a>
     24     <li><a href="#ct_codegen">Code generator bugs</a>
     25     </ul></li>
     26   <li><a href="#miscompilations">Miscompilations</a></li>
     27   <li><a href="#codegen">Incorrect code generation (JIT and LLC)</a></li>
     28 </ol>
     29 <div class="doc_author">
     30   <p>Written by <a href="mailto:sabre (a] nondot.org">Chris Lattner</a> and
     31                 <a href="http://misha.brukman.net">Misha Brukman</a></p>
     32 </div>
     33 </td>
     34 <td class="right">
     35   <img src="img/Debugging.gif" alt="Debugging" width="444" height="314">
     36 </td>
     37 </tr>
     38 </table>
     39 
     40 <!-- *********************************************************************** -->
     41 <h2>
     42   <a name="introduction">Introduction - Got bugs?</a>
     43 </h2>
     44 <!-- *********************************************************************** -->
     45 
     46 <div>
     47 
     48 <p>If you're working with LLVM and run into a bug, we definitely want to know
     49 about it.  This document describes what you can do to increase the odds of
     50 getting it fixed quickly.</p>
     51 
     52 <p>Basically you have to do two things at a minimum.  First, decide whether the
     53 bug <a href="#crashers">crashes the compiler</a> (or an LLVM pass), or if the
     54 compiler is <a href="#miscompilations">miscompiling</a> the program (i.e., the
     55 compiler successfully produces an executable, but it doesn't run right).  Based
     56 on
     57 what type of bug it is, follow the instructions in the linked section to narrow
     58 down the bug so that the person who fixes it will be able to find the problem
     59 more easily.</p>
     60 
     61 <p>Once you have a reduced test-case, go to <a
     62 href="http://llvm.org/bugs/enter_bug.cgi">the LLVM Bug Tracking
     63 System</a> and fill out the form with the necessary details (note that you don't
     64 need to pick a category, just use the "new-bugs" category if you're not sure).
     65 The bug description should contain the following
     66 information:</p>
     67 
     68 <ul>
     69   <li>All information necessary to reproduce the problem.</li>
     70   <li>The reduced test-case that triggers the bug.</li>
     71   <li>The location where you obtained LLVM (if not from our Subversion
     72   repository).</li>
     73 </ul>
     74 
     75 <p>Thanks for helping us make LLVM better!</p>
     76 
     77 </div>
     78 
     79 <!-- *********************************************************************** -->
     80 <h2>
     81   <a name="crashers">Crashing Bugs</a>
     82 </h2>
     83 <!-- *********************************************************************** -->
     84 
     85 <div>
     86 
     87 <p>More often than not, bugs in the compiler cause it to crash&mdash;often due
     88 to an assertion failure of some sort. The most important
     89 piece of the puzzle is to figure out if it is crashing in the GCC front-end
     90 or if it is one of the LLVM libraries (e.g. the optimizer or code generator)
     91 that has problems.</p>
     92 
     93 <p>To figure out which component is crashing (the front-end,
     94 optimizer or code generator), run the
     95 <tt><b>llvm-gcc</b></tt> command line as you were when the crash occurred, but
     96 with the following extra command line options:</p>
     97 
     98 <ul>
     99   <li><tt><b>-O0 -emit-llvm</b></tt>: If <tt>llvm-gcc</tt> still crashes when
    100   passed these options (which disable the optimizer and code generator), then
    101   the crash is in the front-end.  Jump ahead to the section on <a
    102   href="#front-end">front-end bugs</a>.</li>
    103 
    104   <li><tt><b>-emit-llvm</b></tt>: If <tt>llvm-gcc</tt> crashes with this option
    105   (which disables the code generator), you found an optimizer bug.  Jump ahead
    106   to <a href="#ct_optimizer"> compile-time optimization bugs</a>.</li>
    107 
    108   <li>Otherwise, you have a code generator crash.  Jump ahead to <a
    109   href="#ct_codegen">code generator bugs</a>.</li>
    110 
    111 </ul>
    112 
    113 <!-- ======================================================================= -->
    114 <h3>
    115   <a name="front-end">Front-end bugs</a>
    116 </h3>
    117 
    118 <div>
    119 
    120 <p>If the problem is in the front-end, you should re-run the same
    121 <tt>llvm-gcc</tt> command that resulted in the crash, but add the
    122 <tt>-save-temps</tt> option.  The compiler will crash again, but it will leave
    123 behind a <tt><i>foo</i>.i</tt> file (containing preprocessed C source code) and
    124 possibly <tt><i>foo</i>.s</tt> for each
    125 compiled <tt><i>foo</i>.c</tt> file. Send us the <tt><i>foo</i>.i</tt> file,
    126 along with the options you passed to llvm-gcc, and a brief description of the
    127 error it caused.</p>
    128 
    129 <p>The <a href="http://delta.tigris.org/">delta</a> tool helps to reduce the
    130 preprocessed file down to the smallest amount of code that still replicates the
    131 problem. You're encouraged to use delta to reduce the code to make the
    132 developers' lives easier. <a
    133 href="http://gcc.gnu.org/wiki/A_guide_to_testcase_reduction">This website</a>
    134 has instructions on the best way to use delta.</p>
    135 
    136 </div>
    137 
    138 <!-- ======================================================================= -->
    139 <h3>
    140   <a name="ct_optimizer">Compile-time optimization bugs</a>
    141 </h3>
    142 
    143 <div>
    144 
    145 <p>If you find that a bug crashes in the optimizer, compile your test-case to a
    146 <tt>.bc</tt> file by passing "<tt><b>-emit-llvm -O0 -c -o foo.bc</b></tt>".
    147 Then run:</p>
    148 
    149 <div class="doc_code">
    150 <p><tt><b>opt</b> -std-compile-opts -debug-pass=Arguments foo.bc
    151     -disable-output</tt></p>
    152 </div>
    153 
    154 <p>This command should do two things: it should print out a list of passes, and
    155 then it should crash in the same way as llvm-gcc.  If it doesn't crash, please
    156 follow the instructions for a <a href="#front-end">front-end bug</a>.</p>
    157 
    158 <p>If this does crash, then you should be able to debug this with the following
    159 bugpoint command:</p>
    160 
    161 <div class="doc_code">
    162 <p><tt><b>bugpoint</b> foo.bc &lt;list of passes printed by 
    163 <b>opt</b>&gt;</tt></p>
    164 </div>
    165 
    166 <p>Please run this, then file a bug with the instructions and reduced .bc files
    167 that bugpoint emits.  If something goes wrong with bugpoint, please submit the
    168 "foo.bc" file and the list of passes printed by <b>opt</b>.</p>
    169 
    170 </div>
    171 
    172 <!-- ======================================================================= -->
    173 <h3>
    174   <a name="ct_codegen">Code generator bugs</a>
    175 </h3>
    176 
    177 <div>
    178 
    179 <p>If you find a bug that crashes llvm-gcc in the code generator, compile your
    180 source file to a .bc file by passing "<tt><b>-emit-llvm -c -o foo.bc</b></tt>"
    181 to llvm-gcc (in addition to the options you already pass).  Once your have
    182 foo.bc, one of the following commands should fail:</p>
    183 
    184 <ol>
    185 <li><tt><b>llc</b> foo.bc</tt></li>
    186 <li><tt><b>llc</b> foo.bc -relocation-model=pic</tt></li>
    187 <li><tt><b>llc</b> foo.bc -relocation-model=static</tt></li>
    188 </ol>
    189 
    190 <p>If none of these crash, please follow the instructions for a
    191 <a href="#front-end">front-end bug</a>.  If one of these do crash, you should
    192 be able to reduce this with one of the following bugpoint command lines (use
    193 the one corresponding to the command above that failed):</p>
    194 
    195 <ol>
    196 <li><tt><b>bugpoint</b> -run-llc foo.bc</tt></li>
    197 <li><tt><b>bugpoint</b> -run-llc foo.bc --tool-args
    198            -relocation-model=pic</tt></li>
    199 <li><tt><b>bugpoint</b> -run-llc foo.bc --tool-args
    200            -relocation-model=static</tt></li>
    201 </ol>
    202 
    203 <p>Please run this, then file a bug with the instructions and reduced .bc file
    204 that bugpoint emits.  If something goes wrong with bugpoint, please submit the
    205 "foo.bc" file and the option that llc crashes with.</p>
    206 
    207 </div>
    208 
    209 </div>
    210 
    211 <!-- *********************************************************************** -->
    212 <h2>
    213   <a name="miscompilations">Miscompilations</a>
    214 </h2>
    215 <!-- *********************************************************************** -->
    216 
    217 <div>
    218 
    219 <p>If llvm-gcc successfully produces an executable, but that executable doesn't
    220 run right, this is either a bug in the code or a bug in the
    221 compiler.  The first thing to check is to make sure it is not using undefined
    222 behavior (e.g. reading a variable before it is defined).  In particular, check
    223 to see if the program <a href="http://valgrind.org/">valgrind</a>s clean,
    224 passes purify, or some other memory checker tool.  Many of the "LLVM bugs" that
    225 we have chased down ended up being bugs in the program being compiled, not
    226  LLVM.</p>
    227 
    228 <p>Once you determine that the program itself is not buggy, you should choose 
    229 which code generator you wish to compile the program with (e.g. C backend, the 
    230 JIT, or LLC) and optionally a series of LLVM passes to run.  For example:</p>
    231 
    232 <div class="doc_code">
    233 <p><tt>
    234 <b>bugpoint</b> -run-cbe [... optzn passes ...] file-to-test.bc --args -- [program arguments]</tt></p>
    235 </div>
    236 
    237 <p><tt>bugpoint</tt> will try to narrow down your list of passes to the one pass
    238 that causes an error, and simplify the bitcode file as much as it can to assist
    239 you. It will print a message letting you know how to reproduce the resulting
    240 error.</p>
    241 
    242 </div>
    243 
    244 <!-- *********************************************************************** -->
    245 <h2>
    246   <a name="codegen">Incorrect code generation</a>
    247 </h2>
    248 <!-- *********************************************************************** -->
    249 
    250 <div>
    251 
    252 <p>Similarly to debugging incorrect compilation by mis-behaving passes, you can
    253 debug incorrect code generation by either LLC or the JIT, using
    254 <tt>bugpoint</tt>. The process <tt>bugpoint</tt> follows in this case is to try
    255 to narrow the code down to a function that is miscompiled by one or the other
    256 method, but since for correctness, the entire program must be run,
    257 <tt>bugpoint</tt> will compile the code it deems to not be affected with the C
    258 Backend, and then link in the shared object it generates.</p>
    259 
    260 <p>To debug the JIT:</p>
    261 
    262 <div class="doc_code">
    263 <pre>
    264 bugpoint -run-jit -output=[correct output file] [bitcode file]  \
    265          --tool-args -- [arguments to pass to lli]              \
    266          --args -- [program arguments]
    267 </pre>
    268 </div>
    269 
    270 <p>Similarly, to debug the LLC, one would run:</p>
    271 
    272 <div class="doc_code">
    273 <pre>
    274 bugpoint -run-llc -output=[correct output file] [bitcode file]  \
    275          --tool-args -- [arguments to pass to llc]              \
    276          --args -- [program arguments]
    277 </pre>
    278 </div>
    279 
    280 <p><b>Special note:</b> if you are debugging MultiSource or SPEC tests that
    281 already exist in the <tt>llvm/test</tt> hierarchy, there is an easier way to
    282 debug the JIT, LLC, and CBE, using the pre-written Makefile targets, which
    283 will pass the program options specified in the Makefiles:</p>
    284 
    285 <div class="doc_code">
    286 <p><tt>
    287 cd llvm/test/../../program<br>
    288 make bugpoint-jit
    289 </tt></p>
    290 </div>
    291 
    292 <p>At the end of a successful <tt>bugpoint</tt> run, you will be presented
    293 with two bitcode files: a <em>safe</em> file which can be compiled with the C
    294 backend and the <em>test</em> file which either LLC or the JIT
    295 mis-codegenerates, and thus causes the error.</p>
    296 
    297 <p>To reproduce the error that <tt>bugpoint</tt> found, it is sufficient to do
    298 the following:</p>
    299 
    300 <ol>
    301 
    302 <li><p>Regenerate the shared object from the safe bitcode file:</p>
    303 
    304 <div class="doc_code">
    305 <p><tt>
    306 <b>llc</b> -march=c safe.bc -o safe.c<br>
    307 <b>gcc</b> -shared safe.c -o safe.so
    308 </tt></p>
    309 </div></li>
    310 
    311 <li><p>If debugging LLC, compile test bitcode native and link with the shared
    312     object:</p>
    313 
    314 <div class="doc_code">
    315 <p><tt>
    316 <b>llc</b> test.bc -o test.s<br>
    317 <b>gcc</b> test.s safe.so -o test.llc<br>
    318 ./test.llc [program options]
    319 </tt></p>
    320 </div></li>
    321     
    322 <li><p>If debugging the JIT, load the shared object and supply the test
    323     bitcode:</p>
    324 
    325 <div class="doc_code">
    326 <p><tt><b>lli</b> -load=safe.so test.bc [program options]</tt></p>
    327 </div></li>  
    328 
    329 </ol>
    330 
    331 </div>
    332 
    333 <!-- *********************************************************************** -->
    334 <hr>
    335 <address>
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    341   <a href="mailto:sabre (a] nondot.org">Chris Lattner</a><br>
    342   <a href="http://llvm.org/">The LLVM Compiler Infrastructure</a>
    343   <br>
    344   Last modified: $Date$
    345 </address>
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