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| <title>How to submit an LLVM bug report</title> |
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| |
| <h1> |
| How to submit an LLVM bug report |
| </h1> |
| |
| <table class="layout" style="width: 90%" > |
| <tr class="layout"> |
| <td class="left"> |
| <ol> |
| <li><a href="#introduction">Introduction - Got bugs?</a></li> |
| <li><a href="#crashers">Crashing Bugs</a> |
| <ul> |
| <li><a href="#front-end">Front-end bugs</a> |
| <li><a href="#ct_optimizer">Compile-time optimization bugs</a> |
| <li><a href="#ct_codegen">Code generator bugs</a> |
| </ul></li> |
| <li><a href="#miscompilations">Miscompilations</a></li> |
| <li><a href="#codegen">Incorrect code generation (JIT and LLC)</a></li> |
| </ol> |
| <div class="doc_author"> |
| <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a> and |
| <a href="http://misha.brukman.net">Misha Brukman</a></p> |
| </div> |
| </td> |
| </tr> |
| </table> |
| |
| <!-- *********************************************************************** --> |
| <h2> |
| <a name="introduction">Introduction - Got bugs?</a> |
| </h2> |
| <!-- *********************************************************************** --> |
| |
| <div> |
| |
| <p>If you're working with LLVM and run into a bug, we definitely want to know |
| about it. This document describes what you can do to increase the odds of |
| getting it fixed quickly.</p> |
| |
| <p>Basically you have to do two things at a minimum. First, decide whether the |
| bug <a href="#crashers">crashes the compiler</a> (or an LLVM pass), or if the |
| compiler is <a href="#miscompilations">miscompiling</a> the program (i.e., the |
| compiler successfully produces an executable, but it doesn't run right). Based |
| on |
| what type of bug it is, follow the instructions in the linked section to narrow |
| down the bug so that the person who fixes it will be able to find the problem |
| more easily.</p> |
| |
| <p>Once you have a reduced test-case, go to <a |
| href="http://llvm.org/bugs/enter_bug.cgi">the LLVM Bug Tracking |
| System</a> and fill out the form with the necessary details (note that you don't |
| need to pick a category, just use the "new-bugs" category if you're not sure). |
| The bug description should contain the following |
| information:</p> |
| |
| <ul> |
| <li>All information necessary to reproduce the problem.</li> |
| <li>The reduced test-case that triggers the bug.</li> |
| <li>The location where you obtained LLVM (if not from our Subversion |
| repository).</li> |
| </ul> |
| |
| <p>Thanks for helping us make LLVM better!</p> |
| |
| </div> |
| |
| <!-- *********************************************************************** --> |
| <h2> |
| <a name="crashers">Crashing Bugs</a> |
| </h2> |
| <!-- *********************************************************************** --> |
| |
| <div> |
| |
| <p>More often than not, bugs in the compiler cause it to crash—often due |
| to an assertion failure of some sort. The most important |
| piece of the puzzle is to figure out if it is crashing in the GCC front-end |
| or if it is one of the LLVM libraries (e.g. the optimizer or code generator) |
| that has problems.</p> |
| |
| <p>To figure out which component is crashing (the front-end, |
| optimizer or code generator), run the |
| <tt><b>llvm-gcc</b></tt> command line as you were when the crash occurred, but |
| with the following extra command line options:</p> |
| |
| <ul> |
| <li><tt><b>-O0 -emit-llvm</b></tt>: If <tt>llvm-gcc</tt> still crashes when |
| passed these options (which disable the optimizer and code generator), then |
| the crash is in the front-end. Jump ahead to the section on <a |
| href="#front-end">front-end bugs</a>.</li> |
| |
| <li><tt><b>-emit-llvm</b></tt>: If <tt>llvm-gcc</tt> crashes with this option |
| (which disables the code generator), you found an optimizer bug. Jump ahead |
| to <a href="#ct_optimizer"> compile-time optimization bugs</a>.</li> |
| |
| <li>Otherwise, you have a code generator crash. Jump ahead to <a |
| href="#ct_codegen">code generator bugs</a>.</li> |
| |
| </ul> |
| |
| <!-- ======================================================================= --> |
| <h3> |
| <a name="front-end">Front-end bugs</a> |
| </h3> |
| |
| <div> |
| |
| <p>If the problem is in the front-end, you should re-run the same |
| <tt>llvm-gcc</tt> command that resulted in the crash, but add the |
| <tt>-save-temps</tt> option. The compiler will crash again, but it will leave |
| behind a <tt><i>foo</i>.i</tt> file (containing preprocessed C source code) and |
| possibly <tt><i>foo</i>.s</tt> for each |
| compiled <tt><i>foo</i>.c</tt> file. Send us the <tt><i>foo</i>.i</tt> file, |
| along with the options you passed to llvm-gcc, and a brief description of the |
| error it caused.</p> |
| |
| <p>The <a href="http://delta.tigris.org/">delta</a> tool helps to reduce the |
| preprocessed file down to the smallest amount of code that still replicates the |
| problem. You're encouraged to use delta to reduce the code to make the |
| developers' lives easier. <a |
| href="http://gcc.gnu.org/wiki/A_guide_to_testcase_reduction">This website</a> |
| has instructions on the best way to use delta.</p> |
| |
| </div> |
| |
| <!-- ======================================================================= --> |
| <h3> |
| <a name="ct_optimizer">Compile-time optimization bugs</a> |
| </h3> |
| |
| <div> |
| |
| <p>If you find that a bug crashes in the optimizer, compile your test-case to a |
| <tt>.bc</tt> file by passing "<tt><b>-emit-llvm -O0 -c -o foo.bc</b></tt>". |
| Then run:</p> |
| |
| <div class="doc_code"> |
| <p><tt><b>opt</b> -std-compile-opts -debug-pass=Arguments foo.bc |
| -disable-output</tt></p> |
| </div> |
| |
| <p>This command should do two things: it should print out a list of passes, and |
| then it should crash in the same way as llvm-gcc. If it doesn't crash, please |
| follow the instructions for a <a href="#front-end">front-end bug</a>.</p> |
| |
| <p>If this does crash, then you should be able to debug this with the following |
| bugpoint command:</p> |
| |
| <div class="doc_code"> |
| <p><tt><b>bugpoint</b> foo.bc <list of passes printed by |
| <b>opt</b>></tt></p> |
| </div> |
| |
| <p>Please run this, then file a bug with the instructions and reduced .bc files |
| that bugpoint emits. If something goes wrong with bugpoint, please submit the |
| "foo.bc" file and the list of passes printed by <b>opt</b>.</p> |
| |
| </div> |
| |
| <!-- ======================================================================= --> |
| <h3> |
| <a name="ct_codegen">Code generator bugs</a> |
| </h3> |
| |
| <div> |
| |
| <p>If you find a bug that crashes llvm-gcc in the code generator, compile your |
| source file to a .bc file by passing "<tt><b>-emit-llvm -c -o foo.bc</b></tt>" |
| to llvm-gcc (in addition to the options you already pass). Once your have |
| foo.bc, one of the following commands should fail:</p> |
| |
| <ol> |
| <li><tt><b>llc</b> foo.bc</tt></li> |
| <li><tt><b>llc</b> foo.bc -relocation-model=pic</tt></li> |
| <li><tt><b>llc</b> foo.bc -relocation-model=static</tt></li> |
| </ol> |
| |
| <p>If none of these crash, please follow the instructions for a |
| <a href="#front-end">front-end bug</a>. If one of these do crash, you should |
| be able to reduce this with one of the following bugpoint command lines (use |
| the one corresponding to the command above that failed):</p> |
| |
| <ol> |
| <li><tt><b>bugpoint</b> -run-llc foo.bc</tt></li> |
| <li><tt><b>bugpoint</b> -run-llc foo.bc --tool-args |
| -relocation-model=pic</tt></li> |
| <li><tt><b>bugpoint</b> -run-llc foo.bc --tool-args |
| -relocation-model=static</tt></li> |
| </ol> |
| |
| <p>Please run this, then file a bug with the instructions and reduced .bc file |
| that bugpoint emits. If something goes wrong with bugpoint, please submit the |
| "foo.bc" file and the option that llc crashes with.</p> |
| |
| </div> |
| |
| </div> |
| |
| <!-- *********************************************************************** --> |
| <h2> |
| <a name="miscompilations">Miscompilations</a> |
| </h2> |
| <!-- *********************************************************************** --> |
| |
| <div> |
| |
| <p>If llvm-gcc successfully produces an executable, but that executable doesn't |
| run right, this is either a bug in the code or a bug in the |
| compiler. The first thing to check is to make sure it is not using undefined |
| behavior (e.g. reading a variable before it is defined). In particular, check |
| to see if the program <a href="http://valgrind.org/">valgrind</a>s clean, |
| passes purify, or some other memory checker tool. Many of the "LLVM bugs" that |
| we have chased down ended up being bugs in the program being compiled, not |
| LLVM.</p> |
| |
| <p>Once you determine that the program itself is not buggy, you should choose |
| which code generator you wish to compile the program with (e.g. C backend, the |
| JIT, or LLC) and optionally a series of LLVM passes to run. For example:</p> |
| |
| <div class="doc_code"> |
| <p><tt> |
| <b>bugpoint</b> -run-cbe [... optzn passes ...] file-to-test.bc --args -- [program arguments]</tt></p> |
| </div> |
| |
| <p><tt>bugpoint</tt> will try to narrow down your list of passes to the one pass |
| that causes an error, and simplify the bitcode file as much as it can to assist |
| you. It will print a message letting you know how to reproduce the resulting |
| error.</p> |
| |
| </div> |
| |
| <!-- *********************************************************************** --> |
| <h2> |
| <a name="codegen">Incorrect code generation</a> |
| </h2> |
| <!-- *********************************************************************** --> |
| |
| <div> |
| |
| <p>Similarly to debugging incorrect compilation by mis-behaving passes, you can |
| debug incorrect code generation by either LLC or the JIT, using |
| <tt>bugpoint</tt>. The process <tt>bugpoint</tt> follows in this case is to try |
| to narrow the code down to a function that is miscompiled by one or the other |
| method, but since for correctness, the entire program must be run, |
| <tt>bugpoint</tt> will compile the code it deems to not be affected with the C |
| Backend, and then link in the shared object it generates.</p> |
| |
| <p>To debug the JIT:</p> |
| |
| <div class="doc_code"> |
| <pre> |
| bugpoint -run-jit -output=[correct output file] [bitcode file] \ |
| --tool-args -- [arguments to pass to lli] \ |
| --args -- [program arguments] |
| </pre> |
| </div> |
| |
| <p>Similarly, to debug the LLC, one would run:</p> |
| |
| <div class="doc_code"> |
| <pre> |
| bugpoint -run-llc -output=[correct output file] [bitcode file] \ |
| --tool-args -- [arguments to pass to llc] \ |
| --args -- [program arguments] |
| </pre> |
| </div> |
| |
| <p><b>Special note:</b> if you are debugging MultiSource or SPEC tests that |
| already exist in the <tt>llvm/test</tt> hierarchy, there is an easier way to |
| debug the JIT, LLC, and CBE, using the pre-written Makefile targets, which |
| will pass the program options specified in the Makefiles:</p> |
| |
| <div class="doc_code"> |
| <p><tt> |
| cd llvm/test/../../program<br> |
| make bugpoint-jit |
| </tt></p> |
| </div> |
| |
| <p>At the end of a successful <tt>bugpoint</tt> run, you will be presented |
| with two bitcode files: a <em>safe</em> file which can be compiled with the C |
| backend and the <em>test</em> file which either LLC or the JIT |
| mis-codegenerates, and thus causes the error.</p> |
| |
| <p>To reproduce the error that <tt>bugpoint</tt> found, it is sufficient to do |
| the following:</p> |
| |
| <ol> |
| |
| <li><p>Regenerate the shared object from the safe bitcode file:</p> |
| |
| <div class="doc_code"> |
| <p><tt> |
| <b>llc</b> -march=c safe.bc -o safe.c<br> |
| <b>gcc</b> -shared safe.c -o safe.so |
| </tt></p> |
| </div></li> |
| |
| <li><p>If debugging LLC, compile test bitcode native and link with the shared |
| object:</p> |
| |
| <div class="doc_code"> |
| <p><tt> |
| <b>llc</b> test.bc -o test.s<br> |
| <b>gcc</b> test.s safe.so -o test.llc<br> |
| ./test.llc [program options] |
| </tt></p> |
| </div></li> |
| |
| <li><p>If debugging the JIT, load the shared object and supply the test |
| bitcode:</p> |
| |
| <div class="doc_code"> |
| <p><tt><b>lli</b> -load=safe.so test.bc [program options]</tt></p> |
| </div></li> |
| |
| </ol> |
| |
| </div> |
| |
| <!-- *********************************************************************** --> |
| <hr> |
| <address> |
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| <a href="mailto:sabre@nondot.org">Chris Lattner</a><br> |
| <a href="http://llvm.org/">The LLVM Compiler Infrastructure</a> |
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