LLVM: bugpoint tool

NAME

SYNOPSIS

DESCRIPTION

bugpoint has been designed to be a useful tool without requiring any hooks into the LLVM intrastructure 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 testcase 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 exters 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 testcase 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 absense 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, it should be masked out.
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 optization 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:
   bugpoint ..... |& tee bugpoint.log

OPTIONS

• -additional-so <library.so>
  Use this option to specify .so files which must be loaded by the program when 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 <arguments>
  All arguments specified after -args are passed into the executed program when the program must be executed. Note that if the program takes an argument which starts with a '-', you should use:

  bugpoint .... -args -- (the arguments here)

  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)
  bugpoint uses several passes internally for cleanup routines to reduce the size of the program. If you're trying to find a bug in one of these passes, bugpoint may crash. These options tell bugpoint not use the specified passes.

• -help
  Print a summary of command line options.

• -input <filename>
  Specify the contents of <stdin> when the program must be executed.

• -load <plugin.so>
  Load the dynamic object plugin.so. 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:

  opt -load <plugin.so> -help

• -output <filename>
  Specify a reference output for the <stdout> file stream.

• -run-(int|jit|llc|cbe)
  Specify which code generator bugpoint should use to run the program. You may choose the interpreter, the JIT compiler, the static native code compiler, or the C backend.