-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
projects / oota-llvm.git / blobdiff