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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 <program arguments>...</tt>
<img src="../Debugging.gif" width=444 height=314 align=right>
<h3>DESCRIPTION</h3>
The <tt>bugpoint</tt> tool is a generally useful tool for narrowing down
problems in LLVM tools and passes. It can be used to debug three types of
failures: optimizer crashes, miscompilations by optimizers, or invalid native
code generation. It aims to reduce test cases to something useful. 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>
<tt>bugpoint</tt> has been 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 a lot of stupid things or miss obvious
simplifications. Remember, however, that computer time is much cheaper than
programmer time, so if it takes a long time to reduce a test case it is still
worth it. :)<p>
<a name="crashdebug">
<h4>Automatic Mode Selection</h4>
<tt>bugpoint</tt> reads the specified list of <tt>.bc</tt> or <tt>.ll</tt> files
specified on the command-line and links them together. If any LLVM passes are
specified on the command line, it runs these passes on the resultant module. If
any of the passes crash, or if they produce a malformed LLVM module,
<tt>bugpoint</tt> enters <a href="#crashdebug">crash debugging mode</a>.<p>
Otherwise, if the <a href="#opt_output"><tt>-output</tt></a> option was not
specified, <tt>bugpoint</tt> runs the initial 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 to match, it tries executing the
original program with the <a href="#opt_run-">selected</a> code generator. If
the resultant output is different than the reference output, it enters <a
href="#codegendebug">code generator debugging mode</a>.<p>
Otherwise, <tt>bugpoint</tt> runs the LLVM program after all of the LLVM passes
have been applied to it. If the executed program matches the reference output,
there is no problem <tt>bugpoint</tt> can debug. Otherwise, it enters <a
href="#miscompilationdebug">miscompilation debugging mode</a>.<p>
<a name="crashdebug">
<h4>Crash debugging mode</h4>
If an optimizer crashes, <tt>bugpoint</tt> will try a variety of techniques to
narrow down the list of passes and the code to a more manageable amount. First,
<tt>bugpoint</tt> figures out which combination of passes trigger the bug. This
is useful when debugging a problem exposed by <tt>gccas</tt> for example,
because it runs over 30 optimizations.<p>
Next, <tt>bugpoint</tt> tries removing functions from the module, to reduce the
size of the test case to a reasonable amount. Usually it is able to get it down
to a single function for 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> or
<tt><a href="analyze.html">analyze</a></tt>.<p>
<a name="codegendebug">
<h4>Code generator debugging mode</h4>
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 LLVM 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 (the "test" [to be compiled with the code generator] and
"safe" [to be compiled with the C backend] modules), and instructions for
reproducing the problem. This module assume the C backend produces good
code.<p>
If you are using this mode and get an error message that says "Non-instruction
is using an external function!", try using the <tt>-run-llc</tt> option instead
of the <tt>-run-jit</tt> option. This is due to an unimplemented feature in the
code generator debugging mode.<p>
<a name="miscompilationdebug">
<h4>Miscompilation debugging mode</h4>
The miscompilation debugging mode works similarly to the code generator
debugging mode. It works by splitting the program into two pieces, running the
optimizations specified on one piece, relinking the program, then executing it.
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 program which is
being miscompiled. This module 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 code generator and miscompilation debugging modes, <tt>bugpoint</tt> only
works with programs that have deterministic output. Thus, if the program
outputs 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 code generator and miscompilation debugging modes, 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 ..... |& 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.<p>
</ol>
<h3>OPTIONS</h3>
<ul>
<li><tt>-additional-so <library.so></tt><br>
Load <tt><library.so></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 <program args></tt><br>
Pass all arguments specified after <tt>-args</tt> to the
test program whenever it runs. Note that if any of
the <tt><program args></tt> start with a '-', you should use:
<p>
<tt>bugpoint <bugpoint args> -args -- <program args></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>-disable-{adce,dce,final-cleanup,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 <filename></tt><br>
Open <tt><filename></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 <plugin.so></tt><br>
Load the dynamic object <tt><plugin.so></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 <plugin.so> -help</tt>
<p>
<a name="opt_output"><li><tt>-output <filename></tt><br>
Whenever the test program produces output on its standard output
stream, it should match the contents of <tt><filename></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>
<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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