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| <title>LLVM: bugpoint tool</title> |
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| <center><h1>LLVM: <tt>bugpoint</tt> tool</h1></center> |
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| <h3>NAME</h3> |
| <tt>bugpoint</tt> |
| |
| <h3>SYNOPSIS</h3> |
| <tt>bugpoint [options] [input LLVM ll/bc files] [LLVM passes] --args <program arguments>...</tt> |
| |
| <img src="../Debugging.gif" width=444 height=314 align=right> |
| <h3>DESCRIPTION</h3> |
| |
| The <tt>bugpoint</tt> tool 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 invalid native |
| code generation. It aims to reduce large test cases to small, useful ones. |
| For example, |
| if <tt><a href="gccas.html">gccas</a></tt> 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.<p> |
| |
| <a name="designphilosophy"> |
| <h4>Design Philosophy</h4> |
| |
| <tt>bugpoint</tt> 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 a lot of stupid things or miss obvious |
| simplifications. <tt>bugpoint</tt> 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. :-) <p> |
| |
| <a name="automaticdebuggerselection"> |
| <h4>Automatic Debugger Selection</h4> |
| |
| <tt>bugpoint</tt> reads each <tt>.bc</tt> or <tt>.ll</tt> 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, |
| <tt>bugpoint</tt> starts the <a href="#crashdebug">crash debugger</a>.<p> |
| |
| Otherwise, if the <a href="#opt_output"><tt>-output</tt></a> option was not |
| specified, <tt>bugpoint</tt> runs the test program with the C backend (which |
| is assumed to generate good code) to generate a reference output. Once |
| <tt>bugpoint</tt> has a reference output for the test program, it tries |
| executing it |
| with the <a href="#opt_run-">selected</a> code generator. If |
| the resulting output differs from the reference output, it assumes the |
| difference resulted from a code generator failure, and starts the |
| <a href="#codegendebug">code generator debugger</a>.<p> |
| |
| Otherwise, <tt>bugpoint</tt> 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 |
| <a href="#miscompilationdebug">miscompilation debugger</a>. Otherwise, |
| there is no problem <tt>bugpoint</tt> can debug.<p> |
| |
| <a name="crashdebug"> |
| <h4>Crash debugger</h4> |
| |
| If an optimizer crashes, <tt>bugpoint</tt> will try as hard as it can to |
| reduce the list of passes and the size of the test program. First, |
| <tt>bugpoint</tt> figures out which combination of passes triggers the bug. This |
| is useful when debugging a problem exposed by <tt>gccas</tt>, for example, |
| because it runs over 25 optimizations.<p> |
| |
| Next, <tt>bugpoint</tt> 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, |
| <tt>bugpoint</tt> deletes any individual LLVM instructions whose absence does |
| not eliminate the failure. At the end, <tt>bugpoint</tt> should tell you what |
| passes crash, give you a bytecode file, and give you instructions on how to |
| reproduce the failure with <tt><a href="opt.html">opt</a></tt> or |
| <tt><a href="analyze.html">analyze</a></tt>.<p> |
| |
| <a name="codegendebug"> |
| <h4>Code generator debugger</h4> |
| |
| The code generator debugger attempts to narrow down the amount of code that is |
| being miscompiled by the <a href="#opt_run-">selected</a> 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.<p> |
| |
| If you are using the code generator debugger and get an error message that |
| says "UNSUPPORTED: external function used as a global initializer!", try using |
| the <tt>-run-llc</tt> option instead of the <tt>-run-jit</tt> option. This is |
| due to an unimplemented feature in the code generator debugger.<p> |
| |
| <a name="miscompilationdebug"> |
| <h4>Miscompilation debugger</h4> |
| |
| 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.<p> |
| |
| <a name="bugpoint notes"> |
| <h4>Advice for using <tt>bugpoint</tt></h4> |
| |
| <tt>bugpoint</tt> can be a remarkably useful tool, but it sometimes works in |
| non-obvious ways. Here are some hints and tips:<p> |
| |
| <ol> |
| <li>In the code generator and miscompilation debuggers, <tt>bugpoint</tt> only |
| works with programs that have deterministic output. Thus, if the program |
| outputs the date, time, or any other "random" data, <tt>bugpoint</tt> 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. |
| |
| <li>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. |
| |
| <li><tt>bugpoint</tt> is extremely useful when working on a new optimization: |
| it helps track down regressions quickly. To avoid having to relink |
| <tt>bugpoint</tt> every time you change your optimization however, have |
| <tt>bugpoint</tt> dynamically load your optimization with the <a |
| href="#opt_load"><tt>-load</tt></a> option. |
| |
| <li><tt>bugpoint</tt> 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:<br> |
| <tt>bugpoint ..... |& tee bugpoint.log</tt> |
| <br>to get a copy of <tt>bugpoint</tt>'s output in the file |
| <tt>bugpoint.log</tt>, as well as on your terminal. |
| |
| <li><tt>bugpoint</tt> cannot debug problems with the linker. If |
| <tt>bugpoint</tt> crashes before you see its "All input ok" message, |
| you might try <tt>llvm-link -v</tt> on the same set of input files. If |
| that also crashes, you may be experiencing a linker bug. |
| |
| </ol> |
| |
| <h3>OPTIONS</h3> |
| |
| <ul> |
| <li><tt>-additional-so <library.so></tt><br> |
| Load <tt><library.so></tt> 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.<p> |
| |
| <li><tt>-args <program args></tt><br> |
| Pass all arguments specified after <tt>-args</tt> to the |
| test program whenever it runs. Note that if any of |
| the <tt><program args></tt> start with a '-', you should use: |
| <p> |
| <tt>bugpoint <bugpoint args> -args -- <program args></tt> |
| <p> |
| The "<tt>--</tt>" right after the <tt>-args</tt> option tells |
| <tt>bugpoint</tt> to consider any options starting with <tt>-</tt> to be |
| part of the <tt>-args</tt> option, not as options to <tt>bugpoint</tt> |
| itself.<p> |
| |
| <li><tt>-disable-{adce,dce,final-cleanup,simplifycfg}</tt><br> |
| Do not run the specified passes to clean up and reduce the size of the |
| test program. By default, <tt>bugpoint</tt> uses these passes internally |
| when attempting to reduce test programs. If you're trying to find |
| a bug in one of these passes, <tt>bugpoint</tt> may crash.<p> |
| |
| <li> <tt>-help</tt><br> |
| Print a summary of command line options.<p> |
| |
| <a name="opt_input"><li><tt>-input <filename></tt><br> |
| Open <tt><filename></tt> and redirect the standard input of the |
| test program, whenever it runs, to come from that file. |
| <p> |
| |
| <a name="opt_load"><li> <tt>-load <plugin.so></tt><br> |
| Load the dynamic object <tt><plugin.so></tt> into <tt>bugpoint</tt> |
| 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: |
| <p> |
| <tt>bugpoint -load <plugin.so> -help</tt> |
| <p> |
| |
| <a name="opt_output"><li><tt>-output <filename></tt><br> |
| Whenever the test program produces output on its standard output |
| stream, it should match the contents of <tt><filename></tt> |
| (the "reference output"). If you do not use this option, |
| <tt>bugpoint</tt> will attempt to generate a reference output by |
| compiling the program with the C backend and running it.<p> |
| |
| <a name="opt_run-"><li><tt>-run-{int,jit,llc,cbe}</tt><br> |
| Whenever the test program is compiled, <tt>bugpoint</tt> 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.<p> |
| </ul> |
| |
| <h3>EXIT STATUS</h3> |
| |
| If <tt>bugpoint</tt> succeeds in finding a problem, it will exit with 0. |
| Otherwise, if an error occurs, it will exit with a non-zero value. |
| |
| <h3>SEE ALSO</h3> |
| <a href="opt.html"><tt>opt</tt></a>, |
| <a href="analyze.html"><tt>analyze</tt></a> |
| |
| <HR> |
| Maintained by the <a href="http://llvm.cs.uiuc.edu">LLVM Team</a>. |
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