docs/HowToSubmitABug.html

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html><head><title>How to submit an LLVM bug report</title></head>

<body bgcolor=white>

<table width="100%" bgcolor="#330077" border=0 cellpadding=4 cellspacing=0>
<tr><td>&nbsp; <font size=+3 color="#EEEEFF" face="Georgia,Palatino,Times,Roman"><b>How to submit an LLVM bug report</b></font></td>
</tr></table>

<table border=0 width=100%>
<tr><td valign=top>
<p><font size=+1>
<ol>
  <li><a href="#introduction">Introduction - Got bugs?</a>
  <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><a href="#miscompilations">Miscompilations</a>
  <li><a href="#codegen">Incorrect code generation (JIT and LLC)</a>

  <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>
</ol><p></font>
</td><td valign=top align=right>
<img src="Debugging.gif" width=444 height=314>
</td></tr>
</table>


<!-- *********************************************************************** -->
<table width="100%" bgcolor="#330077" border=0 cellpadding=4 cellspacing=0>
<tr><td align=center><font color="#EEEEFF" size=+2 face="Georgia,Palatino"><b>
<a name="introduction">Introduction - Got bugs?
</b></font></td></tr></table><ul>
<!-- *********************************************************************** -->

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>

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>

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 catagory in which the bug falls, and fill out the form with the
necessary details.  The bug description should contain the following
information:
<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>


<!-- *********************************************************************** -->
</ul><table width="100%" bgcolor="#330077" border=0 cellpadding=4 cellspacing=0>
<tr><td align=center><font color="#EEEEFF" size=+2 face="Georgia,Palatino"><b>
<a name="crashers">Crashing Bugs
</b></font></td></tr></table><ul>
<!-- *********************************************************************** -->

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>

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></tt>, <tt><b>gccas</b></tt>, or <tt><b>gccld</b></tt>
crashed.<p>

<ul>
<li>If <tt><b>cc1</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>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>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>Otherwise, something really weird happened.  Email the list with what you
have at this point.
</ul><p>


<!-- ======================================================================= -->
</ul><table width="100%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0>
<tr><td>&nbsp;</td><td width="100%">&nbsp; 
<font color="#EEEEFF" face="Georgia,Palatino"><b>
<a name="front-end">Front-end bugs
</b></font></td></tr></table><ul>

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>

<!-- ======================================================================= -->
</ul><table width="100%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0>
<tr><td>&nbsp;</td><td width="100%">&nbsp; 
<font color="#EEEEFF" face="Georgia,Palatino"><b>
<a name="gccas">GCCAS bugs
</b></font></td></tr></table><ul>

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>

<pre>
  <b>gccas</b> -debug-pass=Arguments &lt; /dev/null -o - &gt; /dev/null
</pre><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>


<!-- ======================================================================= -->
</ul><table width="100%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0>
<tr><td>&nbsp;</td><td width="100%">&nbsp; 
<font color="#EEEEFF" face="Georgia,Palatino"><b>
<a name="gccld">GCCLD bugs
</b></font></td></tr></table><ul>

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>

<pre>
  <b>llvm-as</b> &lt; /dev/null &gt; null.bc
  <b>gccld</b> -debug-pass=Arguments null.bc
</pre><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>

<!-- ======================================================================= -->
</ul><table width="100%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0>
<tr><td>&nbsp;</td><td width="100%">&nbsp; 
<font color="#EEEEFF" face="Georgia,Palatino"><b>
<a name="passes">Bugs in LLVM passes
</b></font></td></tr></table><ul>

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>

<pre>
  <b>bugpoint</b> &lt;input files&gt; &lt;list of passes&gt;
</pre><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>

<pre>
  ...
  Emitted bytecode to 'bugpoint-reduced-simplified.bc'

  *** You can reproduce the problem with: opt bugpoint-reduced-simplified.bc -licm
</pre><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>


<!-- *********************************************************************** -->
</ul><table width="100%" bgcolor="#330077" border=0 cellpadding=4 cellspacing=0>
<tr><td align=center><font color="#EEEEFF" size=+2 face="Georgia,Palatino"><b>
<a name="miscompilations">Miscompilations</a>
</b></font></td></tr></table><ul>
<!-- *********************************************************************** -->

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>

To debug a miscompilation, you should choose which program you wish to run the
output through, e.g. C backend, the JIT, or LLC, and a selection of passes, one
of which may be causing the error, and run, for example:

<pre>
  <b>bugpoint</b> -run-cbe [... optimization passes ...] file-to-test.bc
</pre>

<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.

<!-- *********************************************************************** -->
</ul><table width="100%" bgcolor="#330077" border=0 cellpadding=4 cellspacing=0>
<tr><td align=center><font color="#EEEEFF" size=+2 face="Georgia,Palatino"><b>
<a name="codegen">Incorrect code generation</a>
</b></font></td></tr></table><ul>
<!-- *********************************************************************** -->

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>

To debug the JIT:
<pre>
  <b>bugpoint</b> -run-jit -output=[correct output file] [bytecodefile]
</pre>

Similarly, to debug the LLC, one would run:
<pre>
  <b>bugpoint</b> -run-llc -output=[correct output file] [bytecodefile]
</pre>

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>

To reproduce the error that <tt>bugpoint</tt> found, it is sufficient to do the
following:

<ol>
  <li>Regenerate the shared object from the safe bytecode file:<br>
<pre>
  <b>llvm-dis</b> -c safe.bc -o safe.c<br>
  <b>gcc</b> -shared safe.c -o safe.so
</pre></li>
  <li>If debugging LLC, compile test bytecode native and link with the shared object:<br>
<pre>
  <b>llc</b> test.bc -o test.s -f<br>
  gcc test.s safe.so -o test.llc<br>
  ./test.llc [program options]
</pre></li>
      <p>
     If debugging the JIT, load the shared object and supply the test bytecode:<br>
<pre>
  <b>lli</b> -load=safe.so test.bc [program options]
</pre></li>  
</ol>

<!-- *********************************************************************** -->
</ul>
<!-- *********************************************************************** -->

<hr><font size-1>
<address><a href="mailto:sabre@nondot.org">Chris Lattner</a></address>
<a href="http://llvm.cs.uiuc.edu">The LLVM Compiler Infrastructure</a>
<br>
<!-- Created: Tue Aug  6 15:00:33 CDT 2002 -->
<!-- hhmts start -->
Last modified: Tue Oct 14 15:57:47 CDT 2003
<!-- hhmts end -->
</font></body></html>