docs/Bugpoint.html - platform/external/llvm - Gitiles

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   <title>LLVM bugpoint tool: design and usage</title>
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 <div class="doc_title">
   LLVM bugpoint tool: design and usage
 </div>

 <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>
 </ul>

 <div class="doc_author">
 <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
 </div>

 <!-- *********************************************************************** -->
 <div class="doc_section">
 <a name="desc">Description</a>
 </div>
 <!-- *********************************************************************** -->

 <div class="doc_text">

 <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
 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>

 </div>

 <!-- *********************************************************************** -->
 <div class="doc_section">
 <a name="design">Design Philosophy</a>
 </div>
 <!-- *********************************************************************** -->

 <div class="doc_text">

 <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>

 </div>

 <!-- ======================================================================= -->
 <div class="doc_subsection">
   <a name="autoselect">Automatic Debugger Selection</a>
 </div>

 <div class="doc_text">

 <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>

 <!-- ======================================================================= -->
 <div class="doc_subsection">
   <a name="crashdebug">Crash debugger</a>
 </div>

 <div class="doc_text">

 <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>

 <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 bytecode file, and give you instructions on how to
 reproduce the failure with <tt>opt</tt>, <tt>analyze</tt>, or <tt>llc</tt>.</p>

 </div>

 <!-- ======================================================================= -->
 <div class="doc_subsection">
   <a name="codegendebug">Code generator debugger</a>
 </div>

 <div class="doc_text">

 <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 bytecode
 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>

 <!-- ======================================================================= -->
 <div class="doc_subsection">
   <a name="miscompilationdebug">Miscompilation debugger</a>
 </div>

 <div class="doc_text">

 <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 class="doc_section">
   <a name="advice">Advice for using bugpoint</a>
 </div>
 <!-- *********************************************************************** -->

 <div class="doc_text">

 <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  ... |&amp; 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>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.

 </ol>

 </div>

 <!-- *********************************************************************** -->

 <hr>
 <address>
   <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
   src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
   <a href="http://validator.w3.org/check/referer"><img
   src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!"></a>

   <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
   <a href="http://llvm.cs.uiuc.edu">LLVM Compiler Infrastructure</a><br>
   Last modified: $Date$
 </address>

 </body>
 </html>
	<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
	"http://www.w3.org/TR/html4/strict.dtd">
	<html>
	<head>
	<title>LLVM bugpoint tool: design and usage</title>
	<link rel="stylesheet" href="llvm.css" type="text/css">
	</head>

	<div class="doc_title">
	LLVM bugpoint tool: design and usage
	</div>

	<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>
	</ul>

	<div class="doc_author">
	<p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
	</div>

	<!-- *********************************************************************** -->
	<div class="doc_section">
	<a name="desc">Description</a>
	</div>
	<!-- *********************************************************************** -->

	<div class="doc_text">

	<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
	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>

	</div>

	<!-- *********************************************************************** -->
	<div class="doc_section">
	<a name="design">Design Philosophy</a>
	</div>
	<!-- *********************************************************************** -->

	<div class="doc_text">

	<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>

	</div>

	<!-- ======================================================================= -->
	<div class="doc_subsection">
	<a name="autoselect">Automatic Debugger Selection</a>
	</div>

	<div class="doc_text">

	<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>

	<!-- ======================================================================= -->
	<div class="doc_subsection">
	<a name="crashdebug">Crash debugger</a>
	</div>

	<div class="doc_text">

	<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>

	<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 bytecode file, and give you instructions on how to
	reproduce the failure with <tt>opt</tt>, <tt>analyze</tt>, or <tt>llc</tt>.</p>

	</div>

	<!-- ======================================================================= -->
	<div class="doc_subsection">
	<a name="codegendebug">Code generator debugger</a>
	</div>

	<div class="doc_text">

	<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 bytecode
	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>

	<!-- ======================================================================= -->
	<div class="doc_subsection">
	<a name="miscompilationdebug">Miscompilation debugger</a>
	</div>

	<div class="doc_text">

	<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 class="doc_section">
	<a name="advice">Advice for using bugpoint</a>
	</div>
	<!-- *********************************************************************** -->

	<div class="doc_text">

	<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>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.

	</ol>

	</div>

	<!-- *********************************************************************** -->

	<hr>
	<address>
	<a href="http://jigsaw.w3.org/css-validator/check/referer"><img
	src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
	<a href="http://validator.w3.org/check/referer"><img
	src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!"></a>

	<a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
	<a href="http://llvm.cs.uiuc.edu">LLVM Compiler Infrastructure</a><br>
	Last modified: $Date$
	</address>

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