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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<title>LLVM bugpoint tool: design and usage</title>
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<link rel="stylesheet" href="_static/llvm.css" type="text/css">
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</head>
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<h1>
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LLVM bugpoint tool: design and usage
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</h1>
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<ul>
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<li><a href="#desc">Description</a></li>
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<li><a href="#design">Design Philosophy</a>
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<ul>
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<li><a href="#autoselect">Automatic Debugger Selection</a></li>
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<li><a href="#crashdebug">Crash debugger</a></li>
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<li><a href="#codegendebug">Code generator debugger</a></li>
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<li><a href="#miscompilationdebug">Miscompilation debugger</a></li>
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</ul></li>
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<li><a href="#advice">Advice for using <tt>bugpoint</tt></a></li>
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<li><a href="#notEnough">What to do when <tt>bugpoint</tt> isn't enough</a></li>
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</ul>
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<div class="doc_author">
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<p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
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</div>
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<!-- *********************************************************************** -->
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<h2>
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<a name="desc">Description</a>
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</h2>
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<!-- *********************************************************************** -->
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<div>
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<p><tt>bugpoint</tt> narrows down the source of problems in LLVM tools and
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passes. It can be used to debug three types of failures: optimizer crashes,
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miscompilations by optimizers, or bad native code generation (including problems
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in the static and JIT compilers). It aims to reduce large test cases to small,
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useful ones. For example, if <tt>opt</tt> crashes while optimizing a
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file, it will identify the optimization (or combination of optimizations) that
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causes the crash, and reduce the file down to a small example which triggers the
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crash.</p>
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<p>For detailed case scenarios, such as debugging <tt>opt</tt>, or one of the
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LLVM code generators, see <a href="HowToSubmitABug.html">How To Submit a Bug
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Report document</a>.</p>
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</div>
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<!-- *********************************************************************** -->
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<h2>
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<a name="design">Design Philosophy</a>
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</h2>
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<!-- *********************************************************************** -->
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<div>
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<p><tt>bugpoint</tt> is designed to be a useful tool without requiring any
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hooks into the LLVM infrastructure at all. It works with any and all LLVM
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passes and code generators, and does not need to "know" how they work. Because
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of this, it may appear to do stupid things or miss obvious
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simplifications. <tt>bugpoint</tt> is also designed to trade off programmer
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time for computer time in the compiler-debugging process; consequently, it may
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take a long period of (unattended) time to reduce a test case, but we feel it
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is still worth it. Note that <tt>bugpoint</tt> is generally very quick unless
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debugging a miscompilation where each test of the program (which requires
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executing it) takes a long time.</p>
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<!-- ======================================================================= -->
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<h3>
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<a name="autoselect">Automatic Debugger Selection</a>
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</h3>
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<div>
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<p><tt>bugpoint</tt> reads each <tt>.bc</tt> or <tt>.ll</tt> file specified on
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the command line and links them together into a single module, called the test
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program. If any LLVM passes are specified on the command line, it runs these
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passes on the test program. If any of the passes crash, or if they produce
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malformed output (which causes the verifier to abort), <tt>bugpoint</tt> starts
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the <a href="#crashdebug">crash debugger</a>.</p>
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<p>Otherwise, if the <tt>-output</tt> option was not specified,
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<tt>bugpoint</tt> runs the test program with the C backend (which is assumed to
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generate good code) to generate a reference output. Once <tt>bugpoint</tt> has
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a reference output for the test program, it tries executing it with the
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selected code generator. If the selected code generator crashes,
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<tt>bugpoint</tt> starts the <a href="#crashdebug">crash debugger</a> on the
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code generator. Otherwise, if the resulting output differs from the reference
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output, it assumes the difference resulted from a code generator failure, and
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starts the <a href="#codegendebug">code generator debugger</a>.</p>
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<p>Finally, if the output of the selected code generator matches the reference
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output, <tt>bugpoint</tt> runs the test program after all of the LLVM passes
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have been applied to it. If its output differs from the reference output, it
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assumes the difference resulted from a failure in one of the LLVM passes, and
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enters the <a href="#miscompilationdebug">miscompilation debugger</a>.
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Otherwise, there is no problem <tt>bugpoint</tt> can debug.</p>
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</div>
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<!-- ======================================================================= -->
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<h3>
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<a name="crashdebug">Crash debugger</a>
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</h3>
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<div>
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<p>If an optimizer or code generator crashes, <tt>bugpoint</tt> will try as hard
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as it can to reduce the list of passes (for optimizer crashes) and the size of
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the test program. First, <tt>bugpoint</tt> figures out which combination of
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optimizer passes triggers the bug. This is useful when debugging a problem
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exposed by <tt>opt</tt>, for example, because it runs over 38 passes.</p>
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<p>Next, <tt>bugpoint</tt> tries removing functions from the test program, to
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reduce its size. Usually it is able to reduce a test program to a single
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function, when debugging intraprocedural optimizations. Once the number of
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functions has been reduced, it attempts to delete various edges in the control
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flow graph, to reduce the size of the function as much as possible. Finally,
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<tt>bugpoint</tt> deletes any individual LLVM instructions whose absence does
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not eliminate the failure. At the end, <tt>bugpoint</tt> should tell you what
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passes crash, give you a bitcode file, and give you instructions on how to
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reproduce the failure with <tt>opt</tt> or <tt>llc</tt>.</p>
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</div>
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<!-- ======================================================================= -->
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<h3>
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<a name="codegendebug">Code generator debugger</a>
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</h3>
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<div>
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<p>The code generator debugger attempts to narrow down the amount of code that
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is being miscompiled by the selected code generator. To do this, it takes the
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test program and partitions it into two pieces: one piece which it compiles
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with the C backend (into a shared object), and one piece which it runs with
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either the JIT or the static LLC compiler. It uses several techniques to
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reduce the amount of code pushed through the LLVM code generator, to reduce the
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potential scope of the problem. After it is finished, it emits two bitcode
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files (called "test" [to be compiled with the code generator] and "safe" [to be
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compiled with the C backend], respectively), and instructions for reproducing
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the problem. The code generator debugger assumes that the C backend produces
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good code.</p>
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</div>
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<!-- ======================================================================= -->
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<h3>
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<a name="miscompilationdebug">Miscompilation debugger</a>
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</h3>
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<div>
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<p>The miscompilation debugger works similarly to the code generator debugger.
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It works by splitting the test program into two pieces, running the
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optimizations specified on one piece, linking the two pieces back together, and
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then executing the result. It attempts to narrow down the list of passes to
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the one (or few) which are causing the miscompilation, then reduce the portion
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of the test program which is being miscompiled. The miscompilation debugger
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assumes that the selected code generator is working properly.</p>
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</div>
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</div>
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<!-- *********************************************************************** -->
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<h2>
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<a name="advice">Advice for using bugpoint</a>
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</h2>
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<!-- *********************************************************************** -->
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<div>
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<tt>bugpoint</tt> can be a remarkably useful tool, but it sometimes works in
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non-obvious ways. Here are some hints and tips:<p>
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<ol>
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<li>In the code generator and miscompilation debuggers, <tt>bugpoint</tt> only
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works with programs that have deterministic output. Thus, if the program
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outputs <tt>argv[0]</tt>, the date, time, or any other "random" data,
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<tt>bugpoint</tt> may misinterpret differences in these data, when output,
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as the result of a miscompilation. Programs should be temporarily modified
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to disable outputs that are likely to vary from run to run.
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<li>In the code generator and miscompilation debuggers, debugging will go
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faster if you manually modify the program or its inputs to reduce the
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runtime, but still exhibit the problem.
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<li><tt>bugpoint</tt> is extremely useful when working on a new optimization:
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it helps track down regressions quickly. To avoid having to relink
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<tt>bugpoint</tt> every time you change your optimization however, have
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<tt>bugpoint</tt> dynamically load your optimization with the
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<tt>-load</tt> option.
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<li><p><tt>bugpoint</tt> can generate a lot of output and run for a long period
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of time. It is often useful to capture the output of the program to file.
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For example, in the C shell, you can run:</p>
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<div class="doc_code">
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<p><tt>bugpoint ... |& tee bugpoint.log</tt></p>
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</div>
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<p>to get a copy of <tt>bugpoint</tt>'s output in the file
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<tt>bugpoint.log</tt>, as well as on your terminal.</p>
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<li><tt>bugpoint</tt> cannot debug problems with the LLVM linker. If
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<tt>bugpoint</tt> crashes before you see its "All input ok" message,
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you might try <tt>llvm-link -v</tt> on the same set of input files. If
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that also crashes, you may be experiencing a linker bug.
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<li><tt>bugpoint</tt> is useful for proactively finding bugs in LLVM.
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Invoking <tt>bugpoint</tt> with the <tt>-find-bugs</tt> option will cause
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the list of specified optimizations to be randomized and applied to the
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program. This process will repeat until a bug is found or the user
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kills <tt>bugpoint</tt>.
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</ol>
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</div>
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<!-- *********************************************************************** -->
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<h2>
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<a name="notEnough">What to do when bugpoint isn't enough</a>
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</h2>
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<!-- *********************************************************************** -->
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<div>
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<p>Sometimes, <tt>bugpoint</tt> is not enough. In particular, InstCombine and
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TargetLowering both have visitor structured code with lots of potential
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transformations. If the process of using bugpoint has left you with
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still too much code to figure out and the problem seems
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to be in instcombine, the following steps may help. These same techniques
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are useful with TargetLowering as well.</p>
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<p>Turn on <tt>-debug-only=instcombine</tt> and see which transformations
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within instcombine are firing by selecting out lines with "<tt>IC</tt>"
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in them.</p>
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<p>At this point, you have a decision to make. Is the number
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of transformations small enough to step through them using a debugger?
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If so, then try that.</p>
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<p>If there are too many transformations, then a source modification
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approach may be helpful.
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In this approach, you can modify the source code of instcombine
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to disable just those transformations that are being performed on your
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test input and perform a binary search over the set of transformations.
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One set of places to modify are the "<tt>visit*</tt>" methods of
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<tt>InstCombiner</tt> (<I>e.g.</I> <tt>visitICmpInst</tt>) by adding a
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"<tt>return false</tt>" as the first line of the method.</p>
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<p>If that still doesn't remove enough, then change the caller of
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<tt>InstCombiner::DoOneIteration</tt>, <tt>InstCombiner::runOnFunction</tt>
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to limit the number of iterations.</p>
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<p>You may also find it useful to use "<tt>-stats</tt>" now to see what parts
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of instcombine are firing. This can guide where to put additional reporting
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code.</p>
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<p>At this point, if the amount of transformations is still too large, then
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inserting code to limit whether or not to execute the body of the code
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in the visit function can be helpful. Add a static counter which is
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incremented on every invocation of the function. Then add code which
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simply returns false on desired ranges. For example:</p>
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<div class="doc_code">
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<p><tt>static int calledCount = 0;</tt></p>
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<p><tt>calledCount++;</tt></p>
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<p><tt>DEBUG(if (calledCount < 212) return false);</tt></p>
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<p><tt>DEBUG(if (calledCount > 217) return false);</tt></p>
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<p><tt>DEBUG(if (calledCount == 213) return false);</tt></p>
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<p><tt>DEBUG(if (calledCount == 214) return false);</tt></p>
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<p><tt>DEBUG(if (calledCount == 215) return false);</tt></p>
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<p><tt>DEBUG(if (calledCount == 216) return false);</tt></p>
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<p><tt>DEBUG(dbgs() << "visitXOR calledCount: " << calledCount
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<< "\n");</tt></p>
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<p><tt>DEBUG(dbgs() << "I: "; I->dump());</tt></p>
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</div>
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<p>could be added to <tt>visitXOR</tt> to limit <tt>visitXor</tt> to being
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applied only to calls 212 and 217. This is from an actual test case and raises
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an important point---a simple binary search may not be sufficient, as
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transformations that interact may require isolating more than one call.
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In TargetLowering, use <tt>return SDNode();</tt> instead of
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<tt>return false;</tt>.</p>
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<p>Now that that the number of transformations is down to a manageable
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number, try examining the output to see if you can figure out which
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transformations are being done. If that can be figured out, then
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do the usual debugging. If which code corresponds to the transformation
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being performed isn't obvious, set a breakpoint after the call count
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based disabling and step through the code. Alternatively, you can use
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"printf" style debugging to report waypoints.</p>
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</div>
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<!-- *********************************************************************** -->
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<hr>
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<address>
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<a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
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<a href="http://llvm.org/">LLVM Compiler Infrastructure</a><br>
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Last modified: $Date$
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</address>
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