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+<h1>
LLVM bugpoint tool: design and usage
-</div>
+</h1>
<ul>
<li><a href="#desc">Description</a></li>
<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>
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+<h2>
<a name="desc">Description</a>
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+</h2>
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-<div class="doc_text">
+<div>
<p><tt>bugpoint</tt> narrows down the source of problems in LLVM tools and
passes. It can be used to debug three types of failures: optimizer crashes,
miscompilations by optimizers, or bad native code generation (including problems
in the static and JIT compilers). It aims to reduce large test cases to small,
-useful ones. For example, if <tt>gccas</tt> crashes while optimizing a
+useful ones. For example, if <tt>opt</tt> crashes while optimizing a
file, it will identify the optimization (or combination of optimizations) that
causes the crash, and reduce the file down to a small example which triggers the
crash.</p>
-<p>For detailed case scenarios, such as debugging <tt>gccas</tt>,
-<tt>gccld</tt>, or one of the LLVM code generators, see <a
-href="HowToSubmitABug.html">How To Submit a Bug Report document</a>.</p>
+<p>For detailed case scenarios, such as debugging <tt>opt</tt>, or one of the
+LLVM code generators, see <a href="HowToSubmitABug.html">How To Submit a Bug
+Report document</a>.</p>
</div>
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-<div class="doc_section">
+<h2>
<a name="design">Design Philosophy</a>
-</div>
+</h2>
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-<div class="doc_text">
+<div>
<p><tt>bugpoint</tt> is designed to be a useful tool without requiring any
hooks into the LLVM infrastructure at all. It works with any and all LLVM
debugging a miscompilation where each test of the program (which requires
executing it) takes a long time.</p>
-</div>
-
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-<div class="doc_subsection">
+<h3>
<a name="autoselect">Automatic Debugger Selection</a>
-</div>
+</h3>
-<div class="doc_text">
+<div>
<p><tt>bugpoint</tt> reads each <tt>.bc</tt> or <tt>.ll</tt> file specified on
the command line and links them together into a single module, called the test
</div>
<!-- ======================================================================= -->
-<div class="doc_subsection">
+<h3>
<a name="crashdebug">Crash debugger</a>
-</div>
+</h3>
-<div class="doc_text">
+<div>
<p>If an optimizer or code generator crashes, <tt>bugpoint</tt> will try as hard
as it can to reduce the list of passes (for optimizer crashes) and the size of
the test program. First, <tt>bugpoint</tt> figures out which combination of
optimizer passes triggers the bug. This is useful when debugging a problem
-exposed by <tt>gccas</tt>, for example, because it runs over 38 passes.</p>
+exposed by <tt>opt</tt>, for example, because it runs over 38 passes.</p>
<p>Next, <tt>bugpoint</tt> tries removing functions from the test program, to
reduce its size. Usually it is able to reduce a test program to a single
flow graph, to reduce the size of the function as much as possible. Finally,
<tt>bugpoint</tt> deletes any individual LLVM instructions whose absence does
not eliminate the failure. At the end, <tt>bugpoint</tt> should tell you what
-passes crash, give you a bytecode file, and give you instructions on how to
-reproduce the failure with <tt>opt</tt>, <tt>analyze</tt>, or <tt>llc</tt>.</p>
+passes crash, give you a bitcode file, and give you instructions on how to
+reproduce the failure with <tt>opt</tt> or <tt>llc</tt>.</p>
</div>
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-<div class="doc_subsection">
+<h3>
<a name="codegendebug">Code generator debugger</a>
-</div>
+</h3>
-<div class="doc_text">
+<div>
<p>The code generator debugger attempts to narrow down the amount of code that
is being miscompiled by the selected code generator. To do this, it takes the
with the C backend (into a shared object), and one piece which it runs with
either the JIT or the static LLC compiler. It uses several techniques to
reduce the amount of code pushed through the LLVM code generator, to reduce the
-potential scope of the problem. After it is finished, it emits two bytecode
+potential scope of the problem. After it is finished, it emits two bitcode
files (called "test" [to be compiled with the code generator] and "safe" [to be
compiled with the C backend], respectively), and instructions for reproducing
the problem. The code generator debugger assumes that the C backend produces
</div>
<!-- ======================================================================= -->
-<div class="doc_subsection">
+<h3>
<a name="miscompilationdebug">Miscompilation debugger</a>
-</div>
+</h3>
-<div class="doc_text">
+<div>
<p>The miscompilation debugger works similarly to the code generator debugger.
It works by splitting the test program into two pieces, running the
</div>
+</div>
+
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-<div class="doc_section">
+<h2>
<a name="advice">Advice for using bugpoint</a>
-</div>
+</h2>
<!-- *********************************************************************** -->
-<div class="doc_text">
+<div>
<tt>bugpoint</tt> can be a remarkably useful tool, but it sometimes works in
non-obvious ways. Here are some hints and tips:<p>
you might try <tt>llvm-link -v</tt> on the same set of input files. If
that also crashes, you may be experiencing a linker bug.
-<li>If your program is <b>supposed</b> to crash, <tt>bugpoint</tt> will be
- confused. One way to deal with this is to cause bugpoint to ignore the exit
- code from your program, by giving it the <tt>-check-exit-code=false</tt>
- option.
-
+<li><tt>bugpoint</tt> is useful for proactively finding bugs in LLVM.
+ Invoking <tt>bugpoint</tt> with the <tt>-find-bugs</tt> option will cause
+ the list of specified optimizations to be randomized and applied to the
+ program. This process will repeat until a bug is found or the user
+ kills <tt>bugpoint</tt>.
</ol>
+</div>
+<!-- *********************************************************************** -->
+<h2>
+ <a name="notEnough">What to do when bugpoint isn't enough</a>
+</h2>
+<!-- *********************************************************************** -->
+
+<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 < 212) return false);</tt></p>
+<p><tt>DEBUG(if (calledCount > 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() << "visitXOR calledCount: " << calledCount
+ << "\n");</tt></p>
+<p><tt>DEBUG(dbgs() << "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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