bugpoint - automatic test case reduction tool

bugpoint [options] [input LLVM ll/bc files] [LLVM passes] --args program arguments

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 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.

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. 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 bugpoint is generally very quick unless debugging a miscompilation where each test of the program (which requires executing it) takes a long time.

bugpoint reads each .bc or .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), bugpoint starts the crash debugger.

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.

If an optimizer or code generator crashes, 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, bugpoint figures out which combination of optimizer passes triggers the bug. This is useful when debugging a problem exposed by gccas, for example, because it runs over 38 passes.

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.

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 (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.

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.

bugpoint can be a remarkably useful tool, but it sometimes works in non-obvious ways. Here are some hints and tips:

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In the code generator and miscompilation debuggers, bugpoint only works with programs that have deterministic output. Thus, if the program outputs CWargv[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.

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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.

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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, make bugpoint dynamically load your optimization by using the -load option.

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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.

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bugpoint cannot debug problems with the LLVM linker. If bugpoint crashes before you see its CWAll input ok message, you might try running CWllvm-link -v on the same set of input files. If that also crashes, you may be experiencing a linker bug.

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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.

--additional-so library

Load the dynamic shared object 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 CW- to be part of the --args option, not as options to bugpoint itself.

--tool-args tool args

Pass all arguments specified after --tool-args to the LLVM tool under test (llc, lli, etc.) whenever it runs. You should use this option in the following way:

    bugpoint [bugpoint args] --tool-args -- [tool args]

The -- right after the --tool-args option tells bugpoint to consider any options starting with CW- to be part of the --tool-args option, not as options to bugpoint itself. (See --args, above.)

--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; for example:

    bugpoint --load myNewPass.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.

--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.

If bugpoint succeeds in finding a problem, it will exit with 0. Otherwise, if an error occurs, it will exit with a non-zero value.

opt, analyze

Maintained by the LLVM Team (<http://llvm.cs.uiuc.edu>).