docs/HowToSubmitABug.html

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