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Add debugging hints for when bugpoint does not suffice, specifically for instcombine and TargetLowering

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@155209 91177308-0d34-0410-b5e6-96231b3b80d8
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Joel Jones 2012-04-20 18:11:07 +00:00
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@ -21,6 +21,7 @@
<li><a href="#miscompilationdebug">Miscompilation debugger</a></li>
</ul></li>
<li><a href="#advice">Advice for using <tt>bugpoint</tt></a></li>
<li><a href="notEnough">What to do when <tt>bugpoint</tt> isn't enough</a></li>
</ul>
<div class="doc_author">
@ -219,6 +220,82 @@ non-obvious ways. Here are some hints and tips:<p>
kills <tt>bugpoint</tt>.
</ol>
</div>
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<h2>
<a name="notEnough">What to do when bugpoint isn't enough</a>
</h2>
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<div>
<p>Sometimes, <tt>bugpoint</tt> is not enough. In particular, InstCombine and
TargetLowering both have visitor structured code with lots of potential
transformations. If the process of using bugpoint has left you with
still too much code to figure out and the problem seems
to be in instcombine, the following steps may help. These same techniques
are useful with TargetLowering as well.</p>
<p>Turn on <tt>-debug-only=instcombine</tt> and see which transformations
within instcombine are firing by selecting out lines with "<tt>IC</tt>"
in them.</p>
<p>At this point, you have a decision to make. Is the number
of transformations small enough to step through them using a debugger?
If so, then try that.</p>
<p>If there are too many transformations, then a source modification
approach may be helpful.
In this approach, you can modify the source code of instcombine
to disable just those transformations that are being performed on your
test input and perform a binary search over the set of transformations.
One set of places to modify are the "<tt>visit*</tt>" methods of
<tt>InstCombiner</tt> (<I>e.g.</I> <tt>visitICmpInst</tt>) by adding a
"<tt>return false</tt>" as the first line of the method.</p>
<p>If that still doesn't remove enough, then change the caller of
<tt>InstCombiner::DoOneIteration</tt>, <tt>InstCombiner::runOnFunction</tt>
to limit the number of iterations.</p>
<p>You may also find it useful to use "<tt>-stats</tt>" now to see what parts
of instcombine are firing. This can guide where to put additional reporting
code.</p>
<p>At this point, if the amount of transformations is still too large, then
inserting code to limit whether or not to execute the body of the code
in the visit function can be helpful. Add a static counter which is
incremented on every invocation of the function. Then add code which
simply returns false on desired ranges. For example:</p>
<div class="doc_code">
<p><tt>static int calledCount = 0;</tt></p>
<p><tt>calledCount++;</tt></p>
<p><tt>DEBUG(if (calledCount &lt; 212) return false);</tt></p>
<p><tt>DEBUG(if (calledCount &gt; 217) return false);</tt></p>
<p><tt>DEBUG(if (calledCount == 213) return false);</tt></p>
<p><tt>DEBUG(if (calledCount == 214) return false);</tt></p>
<p><tt>DEBUG(if (calledCount == 215) return false);</tt></p>
<p><tt>DEBUG(if (calledCount == 216) return false);</tt></p>
<p><tt>DEBUG(dbgs() &lt;&lt; "visitXOR calledCount: " &lt;&lt; calledCount
&lt;&lt; "\n");</tt></p>
<p><tt>DEBUG(dbgs() &lt;&lt; "I: "; I->dump());</tt></p>
</div>
<p>could be added to <tt>visitXOR</tt> to limit <tt>visitXor</tt> to being
applied only to calls 212 and 217. This is from an actual test case and raises
an important point---a simple binary search may not be sufficient, as
transformations that interact may require isolating more than one call.
In TargetLowering, use <tt>return SDNode();</tt> instead of
<tt>return false;</tt>.</p>
<p>Now that that the number of transformations is down to a manageable
number, try examining the output to see if you can figure out which
transformations are being done. If that can be figured out, then
do the usual debugging. If which code corresponds to the transformation
being performed isn't obvious, set a breakpoint after the call count
based disabling and step through the code. Alternatively, you can use
"printf" style debugging to report waypoints.</p>
</div>
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