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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<title>How to submit an LLVM bug report</title>
<link rel="stylesheet" href="llvm.css" type="text/css">
</head>
<body>
<div class="doc_title">
How to submit an LLVM bug report
</div>
<table border="0" width="100%">
<tr>
<td valign="top">
<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_text">
<p><b>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a> and
<a href="http://misha.brukman.net">Misha Brukman</a></b></p>
</div>
</td>
<td align="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
<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">
<pre>
...
Emitted bytecode to 'bugpoint-reduced-simplified.bc'
*** You can reproduce the problem with: opt bugpoint-reduced-simplified.bc -licm
</pre>
</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 [... optimization 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">
<p><tt><b>bugpoint</b> -run-jit -output=[correct output file] [bytecode file]
--args -- [program arguments]</tt></p>
</div>
<p>Similarly, to debug the LLC, one would run:</p>
<div class="doc_code">
<p><tt><b>bugpoint</b> -run-llc -output=[correct output file] [bytecode file]
--args -- [program arguments]</tt></p>
</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>
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<a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
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