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<html>
<title>LLVM: bugpoint tool</title>
<body bgcolor=white>
<center><h1>LLVM: <tt>bugpoint</tt> tool</h1></center>
<HR>
<h3>NAME</h3>
<tt>bugpoint</tt>
<h3>SYNOPSIS</h3>
<tt>bugpoint [options] [input LLVM ll/bc files] [LLVM passes] --args &lt;program arguments&gt;...</tt>
<img src="../img/Debugging.gif" width=444 height=314 align=right>
<h3>DESCRIPTION</h3>
The <tt>bugpoint</tt> tool 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><a href="gccas.html">gccas</a></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>
<a name="designphilosophy">
<h4>Design Philosophy</h4>
<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>
<a name="automaticdebuggerselection">
<h4>Automatic Debugger Selection</h4>
<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>
Otherwise, if the <a href="#opt_output"><tt>-output</tt></a> 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 <a href="#opt_run-">selected</a> 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>
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>
<a name="crashdebug">
<h4>Crash debugger</h4>
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>
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><a href="opt.html">opt</a></tt>, <tt><a
href="analyze.html">analyze</a></tt>, or <tt><a href="llc.html">llc</a></tt>.<p>
<a name="codegendebug">
<h4>Code generator debugger</h4>
<p>The code generator debugger attempts to narrow down the amount of code that
is being miscompiled by the <a href="#opt_run-">selected</a> 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>
<a name="miscompilationdebug">
<h4>Miscompilation debugger</h4>
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>
<a name="bugpoint notes">
<h4>Advice for using <tt>bugpoint</tt></h4>
<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 <a
href="#opt_load"><tt>-load</tt></a> option.
<li><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 type:<br>
<tt>bugpoint ..... |&amp; tee bugpoint.log</tt>
<br>to get a copy of <tt>bugpoint</tt>'s output in the file
<tt>bugpoint.log</tt>, as well as on your terminal.
<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>
<h3>OPTIONS</h3>
<ul>
<li><tt>-additional-so &lt;library&gt;</tt><br>
Load <tt>&lt;library&gt;</tt> into the test program whenever it is run.
This is useful if you are debugging programs which depend on non-LLVM
libraries (such as the X or curses libraries) to run.<p>
<li><tt>-args &lt;program args&gt;</tt><br>
Pass all arguments specified after <tt>-args</tt> to the
test program whenever it runs. Note that if any of
the <tt>&lt;program args&gt;</tt> start with a '-', you should use:
<p>
<tt>bugpoint &lt;bugpoint args&gt; -args -- &lt;program args&gt;</tt>
<p>
The "<tt>--</tt>" right after the <tt>-args</tt> option tells
<tt>bugpoint</tt> to consider any options starting with <tt>-</tt> to be
part of the <tt>-args</tt> option, not as options to <tt>bugpoint</tt>
itself.<p>
<li><tt>-tool-args &lt;tool args&gt;</tt><br>
Pass all arguments specified after <tt>-tool-args</tt> to the
LLVM tool under test (llc, lli, etc.) whenever it runs.
You should use this option in the following way:
<p>
<tt>bugpoint &lt;bugpoint args&gt; -tool-args -- &lt;tool args&gt;</tt>
<p>
The "<tt>--</tt>" right after the <tt>-tool-args</tt> option tells
<tt>bugpoint</tt> to consider any options starting with <tt>-</tt> to be
part of the <tt>-tool-args</tt> option, not as options to
<tt>bugpoint</tt> itself. (See <tt>-args</tt>, above.)<p>
<li><tt>-check-exit-code={true,false}</tt><br>
Assume a non-zero exit code or core dump from the test program is
a failure. Defaults to true.<p>
<li><tt>-disable-{dce,simplifycfg}</tt><br>
Do not run the specified passes to clean up and reduce the size of the
test program. By default, <tt>bugpoint</tt> uses these passes internally
when attempting to reduce test programs. If you're trying to find
a bug in one of these passes, <tt>bugpoint</tt> may crash.<p>
<li> <tt>-help</tt><br>
Print a summary of command line options.<p>
<a name="opt_input"><li><tt>-input &lt;filename&gt;</tt><br>
Open <tt>&lt;filename&gt;</tt> and redirect the standard input of the
test program, whenever it runs, to come from that file.
<p>
<a name="opt_load"><li> <tt>-load &lt;plugin&gt;</tt><br>
Load the dynamic object <tt>&lt;plugin&gt;</tt> into <tt>bugpoint</tt>
itself. This object should register new
optimization passes. Once loaded, the object will add new command line
options to enable various optimizations. To see the new complete list
of optimizations, use the -help and -load options together:
<p>
<tt>bugpoint -load &lt;plugin&gt; -help</tt>
<p>
<a name="opt_output"><li><tt>-output &lt;filename&gt;</tt><br>
Whenever the test program produces output on its standard output
stream, it should match the contents of <tt>&lt;filename&gt;</tt>
(the "reference output"). If you do not use this option,
<tt>bugpoint</tt> will attempt to generate a reference output by
compiling the program with the C backend and running it.<p>
<li><tt>-profile-info-file &lt;filename&gt;</tt><br>
Profile file loaded by -profile-loader.<p>
<a name="opt_run-"><li><tt>-run-{int,jit,llc,cbe}</tt><br>
Whenever the test program is compiled, <tt>bugpoint</tt> should generate
code for it using the specified code generator. These options allow
you to choose the interpreter, the JIT compiler, the static native
code compiler, or the C backend, respectively.<p>
</ul>
<h3>EXIT STATUS</h3>
If <tt>bugpoint</tt> succeeds in finding a problem, it will exit with 0.
Otherwise, if an error occurs, it will exit with a non-zero value.
<h3>SEE ALSO</h3>
<a href="opt.html"><tt>opt</tt></a>,
<a href="analyze.html"><tt>analyze</tt></a>
<HR>
Maintained by the <a href="http://llvm.cs.uiuc.edu">LLVM Team</a>.
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