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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 testcases 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 intrastructure 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 testcase 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 exters <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 has over 30 optimization it runs.<p>
Next, <tt>bugpoint</tt> tries removing functions from the module, to reduce the
size of the testcase 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 absense 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>
<h3>OPTIONS</h3>
<ul>
<li><tt>-additional-so <library.so></tt><br>
Use this option to specify .so files which must be loaded by the program
when 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 <arguments></tt><br>
All arguments specified after <tt>-args</tt> are passed into the
executed program when the program must be executed. Note that if the
program takes an argument which starts with a '-', you should use:
<p>
<tt>bugpoint .... -args -- (the arguments here)</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>
<tt>bugpoint</tt> uses several passes internally for cleanup routines to
reduce the size of the program. If you're trying to find a bug in one
of these passes, <tt>bugpoint</tt> may crash. These options tell
<tt>bugpoint</tt> not use the specified passes.<p>
<li> <tt>-help</tt><br>
Print a summary of command line options.<p>
<a name="opt_input"><li><tt>-input <filename></tt><br>
Specify the contents of <stdin> when the program must be executed.
<p>
<li> <tt>-load <plugin.so></tt><br>
Load the dynamic object plugin.so. 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>opt -load <plugin.so> -help</tt>
<p>
<a name="opt_output"><li><tt>-output <filename></tt><br>
Specify a reference output for the <stdout> file stream.<p>
<a name="opt_run-"><li><tt>-run-(int|jit|llc|cbe)</tt><br>
Specify which code generator <tt>bugpoint</tt> should use to run the
program. You may choose the interpreter, the JIT compiler, the static
native code compiler, or the C backend.<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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