LLVM: bugpoint tool

NAME

SYNOPSIS

DESCRIPTION

Design Philosophy

Automatic Debugger Selection

Otherwise, if the -output option was not specified, bugpoint runs the test program with the C backend (which is assumed to generate good code) to generate a reference output. Once bugpoint has a reference output for the test program, it tries executing it with the selected code generator. If the selected code generator crashes, bugpoint starts the 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 code generator debugger.

Finally, if the output of the selected code generator matches the reference output, 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 bugpoint can debug.

Crash debugger

Next, 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, bugpoint deletes any individual LLVM instructions whose absence does not eliminate the failure. At the end, bugpoint should tell you what passes crash, give you a bytecode file, and give you instructions on how to reproduce the failure with opt, analyze, or llc.

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 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 (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.

Advice for using bugpoint

1. In the code generator and miscompilation debuggers, bugpoint only works with programs that have deterministic output. Thus, if the program outputs argv[0], the date, time, or any other "random" data, 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.
2. 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.
3. bugpoint is extremely useful when working on a new optimization: it helps track down regressions quickly. To avoid having to relink bugpoint every time you change your optimization however, have bugpoint dynamically load your optimization with the -load option.
4. 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 bugpoint's output in the file bugpoint.log, as well as on your terminal.
5. bugpoint cannot debug problems with the LLVM linker. If bugpoint crashes before you see its "All input ok" message, you might try llvm-link -v on the same set of input files. If that also crashes, you may be experiencing a linker bug.
6. If your program is supposed to crash, bugpoint will be confused. One way to deal with this is to cause bugpoint to ignore the exit code from your program, by giving it the -check-exit-code=false option.

OPTIONS

• -additional-so <library>
  Load <library> 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.

• -args <program args>
  Pass all arguments specified after -args to the test program whenever it runs. Note that if any of the <program args> start with a '-', you should use:

  bugpoint <bugpoint args> -args -- <program args>

  The "--" right after the -args option tells bugpoint to consider any options starting with - to be part of the -args option, not as options to bugpoint itself.

• -check-exit-code={true,false}
  Assume a non-zero exit code or core dump from the test program is a failure. Defaults to true.

• -disable-{dce,simplifycfg}
  Do not run the specified passes to clean up and reduce the size of the test program. By default, bugpoint uses these passes internally when attempting to reduce test programs. If you're trying to find a bug in one of these passes, bugpoint may crash.

• -help
  Print a summary of command line options.

• -input <filename>
  Open <filename> and redirect the standard input of the test program, whenever it runs, to come from that file.

• -load <plugin>
  Load the dynamic object <plugin> into bugpoint 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:

  bugpoint -load <plugin> -help

• -output <filename>
  Whenever the test program produces output on its standard output stream, it should match the contents of <filename> (the "reference output"). If you do not use this option, bugpoint will attempt to generate a reference output by compiling the program with the C backend and running it.

• -profile-info-file <filename>
  Profile file loaded by -profile-loader.

• -run-{int,jit,llc,cbe}
  Whenever the test program is compiled, bugpoint 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.