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| <title>How to submit an LLVM bug report</title> |
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| |
| <div class="doc_title"> |
| How to submit an LLVM bug report |
| </div> |
| |
| <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="#gccas">GCCAS bugs</a> |
| <li><a href="#gccld">GCCLD bugs</a> |
| <li><a href="#passes">Bugs in LLVM passes</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> |
| <td class="right"> |
| <img src="img/Debugging.gif" alt="Debugging" width="444" height="314"> |
| </td> |
| </tr> |
| </table> |
| |
| <!-- *********************************************************************** --> |
| <div class="doc_section"> |
| <a name="introduction">Introduction - Got bugs?</a> |
| </div> |
| <!-- *********************************************************************** --> |
| |
| <div class="doc_text"> |
| |
| <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. 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.cs.uiuc.edu/bugs/enter_bug.cgi">the LLVM Bug Tracking |
| System</a>, select the category in which the bug falls, and fill out the form |
| with the necessary details. 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 CVS |
| repository).</li> |
| </ul> |
| |
| <p>Thanks for helping us make LLVM better!</p> |
| |
| </div> |
| |
| <!-- *********************************************************************** --> |
| <div class="doc_section"> |
| <a name="crashers">Crashing Bugs</a> |
| </div> |
| <!-- *********************************************************************** --> |
| |
| <div class="doc_text"> |
| |
| <p>More often than not, bugs in the compiler cause it to crash - often due to an |
| assertion failure of some sort. If you are running <tt><b>opt</b></tt> or |
| <tt><b>analyze</b></tt> directly, and something crashes, jump to the section on |
| <a href="#passes">bugs in LLVM passes</a>. Otherwise, the most important |
| piece of the puzzle is to figure out if it is the GCC-based front-end that is |
| buggy or if it's one of the LLVM tools that has problems.</p> |
| |
| <p>To figure out which program is crashing (the front-end, |
| <tt><b>gccas</b></tt>, or <tt><b>gccld</b></tt>), run the |
| <tt><b>llvm-gcc</b></tt> command line as you were when the crash occurred, but |
| add a <tt>-v</tt> option to the command line. The compiler will print out a |
| bunch of stuff, and should end with telling you that one of |
| <tt><b>cc1</b>/<b>cc1plus</b></tt>, <tt><b>gccas</b></tt>, or |
| <tt><b>gccld</b></tt> crashed.</p> |
| |
| <ul> |
| |
| <li>If <tt><b>cc1</b></tt> or <tt><b>cc1plus</b></tt> crashed, you found a |
| problem with the front-end. |
| Jump ahead to the section on <a href="#front-end">front-end bugs</a>.</li> |
| |
| <li>If <tt><b>gccas</b></tt> crashed, you found a bug in <a href="#gccas">one |
| of the passes in <tt><b>gccas</b></tt></a>.</li> |
| |
| <li>If <tt><b>gccld</b></tt> crashed, you found a bug in <a href="#gccld">one |
| of the passes in <tt><b>gccld</b></tt></a>.</li> |
| |
| <li>Otherwise, something really weird happened. Email the list with what you |
| have at this point.</li> |
| |
| </ul> |
| |
| </div> |
| |
| <!-- ======================================================================= --> |
| <div class="doc_subsection"> |
| <a name="front-end">Front-end bugs</a> |
| </div> |
| |
| <div class="doc_text"> |
| |
| <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> (containing LLVM assembly code), for each |
| compiled <tt><i>foo</i>.c</tt> file. Send us the <tt><i>foo</i>.i</tt> file, |
| along with a brief description of the error it caused.</p> |
| |
| </div> |
| |
| <!-- ======================================================================= --> |
| <div class="doc_subsection"> |
| <a name="gccas">GCCAS bugs</a> |
| </div> |
| |
| <div class="doc_text"> |
| |
| <p>If you find that a bug crashes in the <tt><b>gccas</b></tt> stage of |
| compilation, compile your test-case to a <tt>.s</tt> file with the |
| <tt>-save-temps</tt> option to <tt><b>llvm-gcc</b></tt>. Then run:</p> |
| |
| <div class="doc_code"> |
| <p><tt><b>gccas</b> -debug-pass=Arguments < /dev/null -o - > /dev/null</tt></p> |
| </div> |
| |
| <p>... which will print a list of arguments, indicating the list of passes that |
| <tt><b>gccas</b></tt> runs. Once you have the input file and the list of |
| passes, go to the section on <a href="#passes">debugging bugs in LLVM |
| passes</a>.</p> |
| |
| </div> |
| |
| <!-- ======================================================================= --> |
| <div class="doc_subsection"> |
| <a name="gccld">GCCLD bugs</a> |
| </div> |
| |
| <div class="doc_text"> |
| |
| <p>If you find that a bug crashes in the <tt><b>gccld</b></tt> stage of |
| compilation, gather all of the <tt>.o</tt> bytecode files and libraries that are |
| being linked together (the "<tt><b>llvm-gcc</b> -v</tt>" output should include |
| the full list of objects linked). Then run:</p> |
| |
| <div class="doc_code"> |
| <p><tt><b>llvm-as</b> < /dev/null > null.bc<br> |
| <b>gccld</b> -debug-pass=Arguments null.bc</tt> |
| </p> |
| </div> |
| |
| <p>... which will print a list of arguments, indicating the list of passes that |
| <tt><b>gccld</b></tt> runs. Once you have the input files and the list of |
| passes, go to the section on <a href="#passes">debugging bugs in LLVM |
| passes</a>.</p> |
| |
| </div> |
| |
| <!-- ======================================================================= --> |
| <div class="doc_subsection"> |
| <a name="passes">Bugs in LLVM passes</a> |
| </div> |
| |
| <div class="doc_text"> |
| |
| <p>At this point, you should have some number of LLVM assembly files or bytecode |
| files and a list of passes which crash when run on the specified input. In |
| order to reduce the list of passes (which is probably large) and the input to |
| something tractable, use the <tt><b>bugpoint</b></tt> tool as follows:</p> |
| |
| <div class="doc_code"> |
| <p><tt><b>bugpoint</b> <input files> <list of passes></tt></p> |
| </div> |
| |
| <p><tt><b>bugpoint</b></tt> will print a bunch of output as it reduces the |
| test-case, but it should eventually print something like this:</p> |
| |
| <div class="doc_code"> |
| <p><tt> |
| ...<br> |
| Emitted bytecode to 'bugpoint-reduced-simplified.bc'<br> |
| <br> |
| *** You can reproduce the problem with: opt bugpoint-reduced-simplified.bc -licm<br> |
| </tt></p> |
| </div> |
| |
| <p>Once you complete this, please send the LLVM bytecode file and the command |
| line to reproduce the problem to the llvmbugs mailing list.</p> |
| |
| </div> |
| |
| <!-- *********************************************************************** --> |
| <div class="doc_section"> |
| <a name="miscompilations">Miscompilations</a> |
| </div> |
| <!-- *********************************************************************** --> |
| |
| <div class="doc_text"> |
| |
| <p>A miscompilation occurs when a pass does not correctly transform a program, |
| thus producing errors that are only noticed during execution. This is different |
| from producing invalid LLVM code (i.e., code not in SSA form, using values |
| before defining them, etc.) which the verifier will check for after a pass |
| finishes its run.</p> |
| |
| <p>If it looks like the LLVM compiler is miscompiling a program, the very first |
| thing to check is to make sure it is not using undefined behavior. In |
| particular, check to see if the program <a |
| href="http://valgrind.kde.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 bytecode 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> |
| |
| <!-- *********************************************************************** --> |
| <div class="doc_section"> |
| <a name="codegen">Incorrect code generation</a> |
| </div> |
| <!-- *********************************************************************** --> |
| |
| <div class="doc_text"> |
| |
| <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] [bytecode 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] [bytecode 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 bytecode 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 bytecode 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 bytecode native and link with the shared |
| object:</p> |
| |
| <div class="doc_code"> |
| <p><tt> |
| <b>llc</b> test.bc -o test.s -f<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 |
| bytecode:</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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