LLVM: bugpoint tool

NAME

SYNOPSIS

DESCRIPTION

bugpoint has been 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 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 test case it is still worth it. :)

Automatic Mode Selection

Otherwise, if the -output option was not specified, bugpoint runs the initial program with the C backend (which is assumed to generate good code) to generate a reference output. Once bugpoint has a reference output to match, it tries executing the original program with the selected code generator. If the resultant output is different than the reference output, it enters code generator debugging mode.

Otherwise, bugpoint 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 bugpoint can debug. Otherwise, it enters miscompilation debugging mode.

Crash debugging mode

Next, bugpoint tries removing functions from the module, to reduce the size of the test case 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, 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 or analyze.

Code generator debugging mode

If you are using this mode and get an error message that says "Non-instruction is using an external function!", try using the -run-llc option instead of the -run-jit option. This is due to an unimplemented feature in the code generator debugging mode.

Miscompilation debugging mode

Advice for using bugpoint

1. In code generator and miscompilation debugging modes, bugpoint only works with programs that have deterministic output. Thus, if the program outputs 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 code generator and miscompilation debugging modes, 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.

OPTIONS

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

• -disable-{adce,dce,final-cleanup,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.so>
  Load the dynamic object <plugin.so> 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.so> -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.

• -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.