6502/llvm-6502
1
0
Fork 0
mirror of https://github.com/c64scene-ar/llvm-6502.git synced 2024-07-20 10:30:03 +00:00
Code Issues Projects Releases Wiki Activity

The textual description is gone from this manpage, only the options and a quick

Browse Source 
summary remain.  The manpage references Bugpoint.html as the repository for more
detailed info.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@18722 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Misha Brukman 2004-12-09 20:28:42 +00:00
parent 94218a7e56
commit ee3b04596d
1 changed files with 3 additions and 134 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

137
docs/CommandGuide/bugpoint.pod
View File

@ -15,140 +15,9 @@ B<bugpoint> 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 B<gccas> 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.
=head2 Design Philosophy
B<bugpoint> 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. B<bugpoint> 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 B<bugpoint> is generally very quick unless debugging a miscompilation where
each test of the program (which requires executing it) takes a long time.
=head2 Automatic Debugger Selection
B<bugpoint> reads each F<.bc> or F<.ll> 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), B<bugpoint> starts the crash debugger.
Otherwise, if the B<-output> option was not specified, B<bugpoint> runs the test
program with the C backend (which is assumed to generate good code) to generate
a reference output. Once B<bugpoint> has a reference output for the test
program, it tries executing it with the selected code generator. If the
selected code generator crashes, B<bugpoint> starts the L</Crash debugger> 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 L</Code generator debugger>.
Finally, if the output of the selected code generator matches the reference
output, B<bugpoint> 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
miscompilation debugger. Otherwise, there is no problem B<bugpoint> can debug.
=head2 Crash debugger
If an optimizer or code generator crashes, B<bugpoint> will try as hard as it
can to reduce the list of passes (for optimizer crashes) and the size of the
test program. First, B<bugpoint> figures out which combination of optimizer
passes triggers the bug. This is useful when debugging a problem exposed by
B<gccas>, for example, because it runs over 38 passes.
Next, B<bugpoint> 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, B<bugpoint>
deletes any individual LLVM instructions whose absence does not eliminate the
failure. At the end, B<bugpoint> should tell you what passes crash, give you a
bytecode file, and give you instructions on how to reproduce the failure with
B<opt>, B<analyze>, or B<llc>.
=head2 Code generator debugger
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 compiler (B<llc>). 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.
=head2 Miscompilation debugger
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.
=head2 Advice for using bugpoint
B<bugpoint> can be a remarkably useful tool, but it sometimes works in
non-obvious ways. Here are some hints and tips:
=over
=item *
In the code generator and miscompilation debuggers, B<bugpoint> only
works with programs that have deterministic output. Thus, if the program
outputs C<argv[0]>, the date, time, or any other "random" data, B<bugpoint> 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.
=item *
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.
=item *
B<bugpoint> is extremely useful when working on a new optimization: it helps
track down regressions quickly. To avoid having to relink B<bugpoint> every
time you change your optimization, make B<bugpoint> dynamically load
your optimization by using the B<-load> option.
=item *
B<bugpoint> 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:
bugpoint ... |& tee bugpoint.log
to get a copy of B<bugpoint>'s output in the file F<bugpoint.log>, as well as on
your terminal.
=item *
B<bugpoint> cannot debug problems with the LLVM linker. If B<bugpoint> crashes
before you see its C<All input ok> message, you might try running C<llvm-link
-v> on the same set of input files. If that also crashes, you may be
experiencing a linker bug.
=item *
If your program is supposed to crash, B<bugpoint> will be confused. One way to
deal with this is to cause B<bugpoint> to ignore the exit code from your
program, by giving it the B<-check-exit-code=false> option.
=back
For more information on the design and inner workings of B<bugpoint>, as well as
advice for using bugpoint, see F<llvm/docs/Bugpoint.html> in the LLVM
distribution.
=head1 OPTIONS