mirror of
https://github.com/c64scene-ar/llvm-6502.git
synced 2024-12-14 11:32:34 +00:00
a9498a2ee6
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@155213 91177308-0d34-0410-b5e6-96231b3b80d8
317 lines
14 KiB
HTML
317 lines
14 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
|
|
"http://www.w3.org/TR/html4/strict.dtd">
|
|
<html>
|
|
<head>
|
|
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
|
|
<title>LLVM bugpoint tool: design and usage</title>
|
|
<link rel="stylesheet" href="_static/llvm.css" type="text/css">
|
|
</head>
|
|
|
|
<h1>
|
|
LLVM bugpoint tool: design and usage
|
|
</h1>
|
|
|
|
<ul>
|
|
<li><a href="#desc">Description</a></li>
|
|
<li><a href="#design">Design Philosophy</a>
|
|
<ul>
|
|
<li><a href="#autoselect">Automatic Debugger Selection</a></li>
|
|
<li><a href="#crashdebug">Crash debugger</a></li>
|
|
<li><a href="#codegendebug">Code generator debugger</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>
|
|
|
|
<div class="doc_author">
|
|
<p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
<h2>
|
|
<a name="desc">Description</a>
|
|
</h2>
|
|
<!-- *********************************************************************** -->
|
|
|
|
<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>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>opt</tt>, or one of the
|
|
LLVM code generators, see <a href="HowToSubmitABug.html">How To Submit a Bug
|
|
Report document</a>.</p>
|
|
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
<h2>
|
|
<a name="design">Design Philosophy</a>
|
|
</h2>
|
|
<!-- *********************************************************************** -->
|
|
|
|
<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
|
|
passes and code generators, and does not need to "know" how they work. Because
|
|
of this, it may appear to do stupid things or miss obvious
|
|
simplifications. <tt>bugpoint</tt> is also designed to trade off programmer
|
|
time for computer time in the compiler-debugging process; consequently, it may
|
|
take a long period of (unattended) time to reduce a test case, but we feel it
|
|
is still worth it. Note that <tt>bugpoint</tt> is generally very quick unless
|
|
debugging a miscompilation where each test of the program (which requires
|
|
executing it) takes a long time.</p>
|
|
|
|
<!-- ======================================================================= -->
|
|
<h3>
|
|
<a name="autoselect">Automatic Debugger Selection</a>
|
|
</h3>
|
|
|
|
<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
|
|
program. If any LLVM passes are specified on the command line, it runs these
|
|
passes on the test program. If any of the passes crash, or if they produce
|
|
malformed output (which causes the verifier to abort), <tt>bugpoint</tt> starts
|
|
the <a href="#crashdebug">crash debugger</a>.</p>
|
|
|
|
<p>Otherwise, if the <tt>-output</tt> option was not specified,
|
|
<tt>bugpoint</tt> runs the test program with the C backend (which is assumed to
|
|
generate good code) to generate a reference output. Once <tt>bugpoint</tt> has
|
|
a reference output for the test program, it tries executing it with the
|
|
selected code generator. If the selected code generator crashes,
|
|
<tt>bugpoint</tt> starts the <a href="#crashdebug">crash debugger</a> on the
|
|
code generator. Otherwise, if the resulting output differs from the reference
|
|
output, it assumes the difference resulted from a code generator failure, and
|
|
starts the <a href="#codegendebug">code generator debugger</a>.</p>
|
|
|
|
<p>Finally, if the output of the selected code generator matches the reference
|
|
output, <tt>bugpoint</tt> runs the test program after all of the LLVM passes
|
|
have been applied to it. If its output differs from the reference output, it
|
|
assumes the difference resulted from a failure in one of the LLVM passes, and
|
|
enters the <a href="#miscompilationdebug">miscompilation debugger</a>.
|
|
Otherwise, there is no problem <tt>bugpoint</tt> can debug.</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<h3>
|
|
<a name="crashdebug">Crash debugger</a>
|
|
</h3>
|
|
|
|
<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>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
|
|
function, when debugging intraprocedural optimizations. Once the number of
|
|
functions has been reduced, it attempts to delete various edges in the control
|
|
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 bitcode file, and give you instructions on how to
|
|
reproduce the failure with <tt>opt</tt> or <tt>llc</tt>.</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<h3>
|
|
<a name="codegendebug">Code generator debugger</a>
|
|
</h3>
|
|
|
|
<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
|
|
test program and partitions it into two pieces: one piece which it compiles
|
|
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 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
|
|
good code.</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<h3>
|
|
<a name="miscompilationdebug">Miscompilation debugger</a>
|
|
</h3>
|
|
|
|
<div>
|
|
|
|
<p>The miscompilation debugger works similarly to the code generator debugger.
|
|
It works by splitting the test program into two pieces, running the
|
|
optimizations specified on one piece, linking the two pieces back together, and
|
|
then executing the result. It attempts to narrow down the list of passes to
|
|
the one (or few) which are causing the miscompilation, then reduce the portion
|
|
of the test program which is being miscompiled. The miscompilation debugger
|
|
assumes that the selected code generator is working properly.</p>
|
|
|
|
</div>
|
|
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
<h2>
|
|
<a name="advice">Advice for using bugpoint</a>
|
|
</h2>
|
|
<!-- *********************************************************************** -->
|
|
|
|
<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>
|
|
|
|
<ol>
|
|
<li>In the code generator and miscompilation debuggers, <tt>bugpoint</tt> only
|
|
works with programs that have deterministic output. Thus, if the program
|
|
outputs <tt>argv[0]</tt>, the date, time, or any other "random" data,
|
|
<tt>bugpoint</tt> may misinterpret differences in these data, when output,
|
|
as the result of a miscompilation. Programs should be temporarily modified
|
|
to disable outputs that are likely to vary from run to run.
|
|
|
|
<li>In the code generator and miscompilation debuggers, debugging will go
|
|
faster if you manually modify the program or its inputs to reduce the
|
|
runtime, but still exhibit the problem.
|
|
|
|
<li><tt>bugpoint</tt> is extremely useful when working on a new optimization:
|
|
it helps track down regressions quickly. To avoid having to relink
|
|
<tt>bugpoint</tt> every time you change your optimization however, have
|
|
<tt>bugpoint</tt> dynamically load your optimization with the
|
|
<tt>-load</tt> option.
|
|
|
|
<li><p><tt>bugpoint</tt> can generate a lot of output and run for a long period
|
|
of time. It is often useful to capture the output of the program to file.
|
|
For example, in the C shell, you can run:</p>
|
|
|
|
<div class="doc_code">
|
|
<p><tt>bugpoint ... |& tee bugpoint.log</tt></p>
|
|
</div>
|
|
|
|
<p>to get a copy of <tt>bugpoint</tt>'s output in the file
|
|
<tt>bugpoint.log</tt>, as well as on your terminal.</p>
|
|
|
|
<li><tt>bugpoint</tt> cannot debug problems with the LLVM linker. If
|
|
<tt>bugpoint</tt> crashes before you see its "All input ok" message,
|
|
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><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>
|
|
|
|
<!-- *********************************************************************** -->
|
|
|
|
<hr>
|
|
<address>
|
|
<a href="http://jigsaw.w3.org/css-validator/check/referer"><img
|
|
src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a>
|
|
<a href="http://validator.w3.org/check/referer"><img
|
|
src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a>
|
|
|
|
<a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
|
|
<a href="http://llvm.org/">LLVM Compiler Infrastructure</a><br>
|
|
Last modified: $Date$
|
|
</address>
|
|
|
|
</body>
|
|
</html>
|