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250 lines
10 KiB
Plaintext
250 lines
10 KiB
Plaintext
=pod
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=head1 NAME
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bugpoint - automatic test case reduction tool
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=head1 SYNOPSIS
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B<bugpoint> [I<options>] [I<input LLVM ll/bc files>] [I<LLVM passes>] B<--args>
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I<program arguments>
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=head1 DESCRIPTION
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B<bugpoint> narrows down the source of problems in LLVM tools and passes. It
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can be used to debug three types of failures: optimizer crashes, miscompilations
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by optimizers, or bad native code generation (including problems in the static
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and JIT compilers). It aims to reduce large test cases to small, useful ones.
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For example, if B<gccas> crashes while optimizing a file, it will identify the
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optimization (or combination of optimizations) that causes the crash, and reduce
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the file down to a small example which triggers the crash.
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=head2 Design Philosophy
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B<bugpoint> is designed to be a useful tool without requiring any hooks into the
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LLVM infrastructure at all. It works with any and all LLVM passes and code
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generators, and does not need to "know" how they work. Because of this, it may
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appear to do stupid things or miss obvious simplifications. B<bugpoint> is also
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designed to trade off programmer time for computer time in the
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compiler-debugging process; consequently, it may take a long period of
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(unattended) time to reduce a test case, but we feel it is still worth it. Note
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that B<bugpoint> is generally very quick unless debugging a miscompilation where
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each test of the program (which requires executing it) takes a long time.
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=head2 Automatic Debugger Selection
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B<bugpoint> reads each F<.bc> or F<.ll> file specified on the command line and
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links them together into a single module, called the test program. If any LLVM
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passes are specified on the command line, it runs these passes on the test
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program. If any of the passes crash, or if they produce malformed output (which
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causes the verifier to abort), B<bugpoint> starts the crash debugger.
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Otherwise, if the B<-output> option was not specified, B<bugpoint> runs the test
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program with the C backend (which is assumed to generate good code) to generate
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a reference output. Once B<bugpoint> has a reference output for the test
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program, it tries executing it with the selected code generator. If the
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selected code generator crashes, B<bugpoint> starts the L</Crash debugger> on
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the code generator. Otherwise, if the resulting output differs from the
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reference output, it assumes the difference resulted from a code generator
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failure, and starts the L</Code generator debugger>.
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Finally, if the output of the selected code generator matches the reference
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output, B<bugpoint> runs the test program after all of the LLVM passes have been
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applied to it. If its output differs from the reference output, it assumes the
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difference resulted from a failure in one of the LLVM passes, and enters the
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miscompilation debugger. Otherwise, there is no problem B<bugpoint> can debug.
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=head2 Crash debugger
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If an optimizer or code generator crashes, B<bugpoint> will try as hard as it
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can to reduce the list of passes (for optimizer crashes) and the size of the
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test program. First, B<bugpoint> figures out which combination of optimizer
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passes triggers the bug. This is useful when debugging a problem exposed by
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B<gccas>, for example, because it runs over 38 passes.
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Next, B<bugpoint> tries removing functions from the test program, to reduce its
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size. Usually it is able to reduce a test program to a single function, when
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debugging intraprocedural optimizations. Once the number of functions has been
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reduced, it attempts to delete various edges in the control flow graph, to
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reduce the size of the function as much as possible. Finally, B<bugpoint>
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deletes any individual LLVM instructions whose absence does not eliminate the
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failure. At the end, B<bugpoint> should tell you what passes crash, give you a
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bytecode file, and give you instructions on how to reproduce the failure with
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B<opt>, B<analyze>, or B<llc>.
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=head2 Code generator debugger
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The code generator debugger attempts to narrow down the amount of code that is
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being miscompiled by the selected code generator. To do this, it takes the test
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program and partitions it into two pieces: one piece which it compiles with the
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C backend (into a shared object), and one piece which it runs with either the
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JIT or the static compiler (B<llc>). It uses several techniques to reduce the
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amount of code pushed through the LLVM code generator, to reduce the potential
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scope of the problem. After it is finished, it emits two bytecode files (called
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"test" [to be compiled with the code generator] and "safe" [to be compiled with
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the C backend], respectively), and instructions for reproducing the problem.
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The code generator debugger assumes that the C backend produces good code.
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=head2 Miscompilation debugger
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The miscompilation debugger works similarly to the code generator debugger. It
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works by splitting the test program into two pieces, running the optimizations
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specified on one piece, linking the two pieces back together, and then executing
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the result. It attempts to narrow down the list of passes to the one (or few)
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which are causing the miscompilation, then reduce the portion of the test
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program which is being miscompiled. The miscompilation debugger assumes that
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the selected code generator is working properly.
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=head2 Advice for using bugpoint
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B<bugpoint> can be a remarkably useful tool, but it sometimes works in
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non-obvious ways. Here are some hints and tips:
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=over
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=item *
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In the code generator and miscompilation debuggers, B<bugpoint> only
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works with programs that have deterministic output. Thus, if the program
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outputs C<argv[0]>, the date, time, or any other "random" data, B<bugpoint> may
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misinterpret differences in these data, when output, as the result of a
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miscompilation. Programs should be temporarily modified to disable outputs that
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are likely to vary from run to run.
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=item *
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In the code generator and miscompilation debuggers, debugging will go faster if
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you manually modify the program or its inputs to reduce the runtime, but still
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exhibit the problem.
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=item *
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B<bugpoint> is extremely useful when working on a new optimization: it helps
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track down regressions quickly. To avoid having to relink B<bugpoint> every
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time you change your optimization, make B<bugpoint> dynamically load
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your optimization by using the B<-load> option.
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=item *
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B<bugpoint> can generate a lot of output and run for a long period of time. It
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is often useful to capture the output of the program to file. For example, in
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the C shell, you can type:
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bugpoint ... |& tee bugpoint.log
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to get a copy of B<bugpoint>'s output in the file F<bugpoint.log>, as well as on
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your terminal.
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=item *
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B<bugpoint> cannot debug problems with the LLVM linker. If B<bugpoint> crashes
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before you see its C<All input ok> message, you might try running C<llvm-link
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-v> on the same set of input files. If that also crashes, you may be
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experiencing a linker bug.
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=item *
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If your program is supposed to crash, B<bugpoint> will be confused. One way to
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deal with this is to cause B<bugpoint> to ignore the exit code from your
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program, by giving it the B<-check-exit-code=false> option.
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=back
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=head1 OPTIONS
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=over
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=item B<--additional-so> F<library>
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Load the dynamic shared object F<library> into the test program whenever it is
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run. This is useful if you are debugging programs which depend on non-LLVM
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libraries (such as the X or curses libraries) to run.
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=item B<--args> I<program args>
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Pass all arguments specified after -args to the test program whenever it runs.
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Note that if any of the I<program args> start with a '-', you should use:
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bugpoint [bugpoint args] --args -- [program args]
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The "--" right after the B<--args> option tells B<bugpoint> to consider any
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options starting with C<-> to be part of the B<--args> option, not as options to
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B<bugpoint> itself.
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=item B<--tool-args> I<tool args>
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Pass all arguments specified after --tool-args to the LLVM tool under test
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(B<llc>, B<lli>, etc.) whenever it runs. You should use this option in the
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following way:
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bugpoint [bugpoint args] --tool-args -- [tool args]
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The "--" right after the B<--tool-args> option tells B<bugpoint> to consider any
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options starting with C<-> to be part of the B<--tool-args> option, not as
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options to B<bugpoint> itself. (See B<--args>, above.)
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=item B<--check-exit-code>=I<{true,false}>
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Assume a non-zero exit code or core dump from the test program is a failure.
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Defaults to true.
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=item B<--disable-{dce,simplifycfg}>
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Do not run the specified passes to clean up and reduce the size of the test
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program. By default, B<bugpoint> uses these passes internally when attempting to
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reduce test programs. If you're trying to find a bug in one of these passes,
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B<bugpoint> may crash.
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=item B<--help>
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Print a summary of command line options.
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=item B<--input> F<filename>
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Open F<filename> and redirect the standard input of the test program, whenever
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it runs, to come from that file.
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=item B<--load> F<plugin>
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Load the dynamic object F<plugin> into B<bugpoint> itself. This object should
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register new optimization passes. Once loaded, the object will add new command
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line options to enable various optimizations. To see the new complete list of
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optimizations, use the B<--help> and B<--load> options together; for example:
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bugpoint --load myNewPass.so --help
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=item B<--output> F<filename>
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Whenever the test program produces output on its standard output stream, it
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should match the contents of F<filename> (the "reference output"). If you
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do not use this option, B<bugpoint> will attempt to generate a reference output
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by compiling the program with the C backend and running it.
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=item B<--profile-info-file> F<filename>
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Profile file loaded by B<--profile-loader>.
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=item B<--run-{int,jit,llc,cbe}>
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Whenever the test program is compiled, B<bugpoint> should generate code for it
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using the specified code generator. These options allow you to choose the
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interpreter, the JIT compiler, the static native code compiler, or the C
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backend, respectively.
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=back
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=head1 EXIT STATUS
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If B<bugpoint> succeeds in finding a problem, it will exit with 0. Otherwise,
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if an error occurs, it will exit with a non-zero value.
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=head1 SEE ALSO
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L<opt|opt>, L<analyze|analyze>
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=head1 AUTHOR
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Maintained by the LLVM Team (L<http://llvm.cs.uiuc.edu>).
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=cut
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