LLVM bugpoint tool: design and usage

- Description
- Design Philosophy
  - Automatic Debugger Selection
  - Crash debugger
  - Code generator debugger
  - Miscompilation debugger
- Advice for using `bugpoint`

Written by Chris Lattner

Description

`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.

For detailed case scenarios, such as debugging `gccas`, `gccld`, or one of the LLVM code generators, see How To Submit a Bug Report document.

Design Philosophy

`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.

Automatic Debugger Selection

`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.

Crash debugger

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` 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.

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.

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

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 run:

   `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.
7. `bugpoint` is useful for proactively finding bugs in LLVM. Invoking `bugpoint` with the `-find-bugs` option will cause the list of specified optimizations to be randomized and applied to the program. This process will repeat until a bug is found or the user kills `bugpoint`.