| Brian Gaeke | 74f470b | 2004-07-01 20:10:40 +0000 | [diff] [blame] | 1 | <html> |
| 2 | <title>LLVM: bugpoint tool</title> |
| 3 | |
| 4 | <body bgcolor=white> |
| 5 | |
| 6 | <center><h1>LLVM: <tt>bugpoint</tt> tool</h1></center> |
| 7 | <HR> |
| 8 | |
| 9 | <h3>NAME</h3> |
| 10 | <tt>bugpoint</tt> |
| 11 | |
| 12 | <h3>SYNOPSIS</h3> |
| 13 | <tt>bugpoint [options] [input LLVM ll/bc files] [LLVM passes] --args <program arguments>...</tt> |
| 14 | |
| Chris Lattner | 5706f25 | 2004-07-01 20:41:43 +0000 | [diff] [blame] | 15 | <img src="img/Debugging.gif" width=444 height=314 align=right> |
| Brian Gaeke | 74f470b | 2004-07-01 20:10:40 +0000 | [diff] [blame] | 16 | <h3>DESCRIPTION</h3> |
| 17 | |
| 18 | The <tt>bugpoint</tt> tool narrows down the source of |
| 19 | problems in LLVM tools and passes. It can be used to debug three types of |
| 20 | failures: optimizer crashes, miscompilations by optimizers, or bad native |
| 21 | code generation (including problems in the static and JIT compilers). It aims |
| 22 | to reduce large test cases to small, useful ones. For example, |
| Chris Lattner | 5706f25 | 2004-07-01 20:41:43 +0000 | [diff] [blame] | 23 | if <tt><a href="CommandGuide/gccas.html">gccas</a></tt> crashes while optimizing a file, it |
| Brian Gaeke | 74f470b | 2004-07-01 20:10:40 +0000 | [diff] [blame] | 24 | will identify the optimization (or combination of optimizations) that causes the |
| 25 | crash, and reduce the file down to a small example which triggers the crash.<p> |
| 26 | |
| 27 | <a name="designphilosophy"> |
| 28 | <h4>Design Philosophy</h4> |
| 29 | |
| 30 | <tt>bugpoint</tt> is designed to be a useful tool without requiring any |
| 31 | hooks into the LLVM infrastructure at all. It works with any and all LLVM |
| 32 | passes and code generators, and does not need to "know" how they work. Because |
| 33 | of this, it may appear to do stupid things or miss obvious |
| 34 | simplifications. <tt>bugpoint</tt> is also designed to trade off programmer |
| 35 | time for computer time in the compiler-debugging process; consequently, it may |
| 36 | take a long period of (unattended) time to reduce a test case, but we feel it |
| 37 | is still worth it. Note that <tt>bugpoint</tt> is generally very quick unless |
| 38 | debugging a miscompilation where each test of the program (which requires |
| 39 | executing it) takes a long time.<p> |
| 40 | |
| 41 | <a name="automaticdebuggerselection"> |
| 42 | <h4>Automatic Debugger Selection</h4> |
| 43 | |
| 44 | <tt>bugpoint</tt> reads each <tt>.bc</tt> or <tt>.ll</tt> file |
| 45 | specified on the command line and links them together into a single module, |
| 46 | called the test program. If any LLVM passes are |
| 47 | specified on the command line, it runs these passes on the test program. If |
| 48 | any of the passes crash, or if they produce malformed output (which causes the |
| 49 | verifier to abort), |
| 50 | <tt>bugpoint</tt> starts the <a href="#crashdebug">crash debugger</a>.<p> |
| 51 | |
| 52 | Otherwise, if the <a href="#opt_output"><tt>-output</tt></a> option was not |
| 53 | specified, <tt>bugpoint</tt> runs the test program with the C backend (which is |
| 54 | assumed to generate good code) to generate a reference output. Once |
| 55 | <tt>bugpoint</tt> has a reference output for the test program, it tries |
| 56 | executing it with the <a href="#opt_run-">selected</a> code generator. If the |
| 57 | selected code generator crashes, <tt>bugpoint</tt> starts the <a |
| 58 | href="#crashdebug">crash debugger</a> on the code generator. Otherwise, if the |
| 59 | resulting output differs from the reference output, it assumes the difference |
| 60 | resulted from a code generator failure, and starts the <a |
| 61 | href="#codegendebug">code generator debugger</a>.<p> |
| 62 | |
| 63 | Finally, if the output of the selected code generator matches the reference |
| 64 | output, <tt>bugpoint</tt> runs the test program after all of the LLVM passes |
| 65 | have been applied to it. If its output differs from the reference output, it |
| 66 | assumes the difference resulted from a failure in one of the LLVM passes, and |
| 67 | enters the <a href="#miscompilationdebug">miscompilation |
| 68 | debugger</a>. Otherwise, there is no problem <tt>bugpoint</tt> can debug.<p> |
| 69 | |
| 70 | <a name="crashdebug"> |
| 71 | <h4>Crash debugger</h4> |
| 72 | |
| 73 | If an optimizer or code generator crashes, <tt>bugpoint</tt> will try as hard as |
| 74 | it can to reduce the list of passes (for optimizer crashes) and the size of the |
| 75 | test program. First, <tt>bugpoint</tt> figures out which combination of |
| 76 | optimizer passes triggers the bug. This is useful when debugging a problem |
| 77 | exposed by <tt>gccas</tt>, for example, because it runs over 38 passes.<p> |
| 78 | |
| 79 | Next, <tt>bugpoint</tt> tries removing functions from the test program, to |
| 80 | reduce its size. Usually it is able to reduce a test program to a single |
| 81 | function, when debugging intraprocedural optimizations. Once the number of |
| 82 | functions has been reduced, it attempts to delete various edges in the control |
| 83 | flow graph, to reduce the size of the function as much as possible. Finally, |
| 84 | <tt>bugpoint</tt> deletes any individual LLVM instructions whose absence does |
| 85 | not eliminate the failure. At the end, <tt>bugpoint</tt> should tell you what |
| 86 | passes crash, give you a bytecode file, and give you instructions on how to |
| Chris Lattner | 5706f25 | 2004-07-01 20:41:43 +0000 | [diff] [blame] | 87 | reproduce the failure with <tt><a href="CommandGuide/opt.html">opt</a></tt>, <tt><a |
| 88 | href="CommandGuide/analyze.html">analyze</a></tt>, or <tt><a href="CommandGuide/llc.html">llc</a></tt>.<p> |
| Brian Gaeke | 74f470b | 2004-07-01 20:10:40 +0000 | [diff] [blame] | 89 | |
| 90 | <a name="codegendebug"> |
| 91 | <h4>Code generator debugger</h4> |
| 92 | |
| 93 | <p>The code generator debugger attempts to narrow down the amount of code that |
| 94 | is being miscompiled by the <a href="#opt_run-">selected</a> code generator. To |
| 95 | do this, it takes the test program and partitions it into two pieces: one piece |
| 96 | which it compiles with the C backend (into a shared object), and one piece which |
| 97 | it runs with either the JIT or the static LLC compiler. It uses several |
| 98 | techniques to reduce the amount of code pushed through the LLVM code generator, |
| 99 | to reduce the potential scope of the problem. After it is finished, it emits |
| 100 | two bytecode files (called "test" [to be compiled with the code generator] and |
| 101 | "safe" [to be compiled with the C backend], respectively), and instructions for |
| 102 | reproducing the problem. The code generator debugger assumes that the C backend |
| 103 | produces good code.</p> |
| 104 | |
| 105 | <a name="miscompilationdebug"> |
| 106 | <h4>Miscompilation debugger</h4> |
| 107 | |
| 108 | The miscompilation debugger works similarly to the code generator |
| 109 | debugger. It works by splitting the test program into two pieces, running the |
| 110 | optimizations specified on one piece, linking the two pieces back together, |
| 111 | and then executing the result. |
| 112 | It attempts to narrow down the list of passes to the one (or few) which are |
| 113 | causing the miscompilation, then reduce the portion of the test program which is |
| 114 | being miscompiled. The miscompilation debugger assumes that the selected |
| 115 | code generator is working properly.<p> |
| 116 | |
| 117 | <a name="bugpoint notes"> |
| 118 | <h4>Advice for using <tt>bugpoint</tt></h4> |
| 119 | |
| 120 | <tt>bugpoint</tt> can be a remarkably useful tool, but it sometimes works in |
| 121 | non-obvious ways. Here are some hints and tips:<p> |
| 122 | |
| 123 | <ol> |
| 124 | <li>In the code generator and miscompilation debuggers, <tt>bugpoint</tt> only |
| 125 | works with programs that have deterministic output. Thus, if the program |
| 126 | outputs <tt>argv[0]</tt>, the date, time, or any other "random" data, <tt>bugpoint</tt> may |
| 127 | misinterpret differences in these data, when output, as the result of a |
| 128 | miscompilation. Programs should be temporarily modified to disable |
| 129 | outputs that are likely to vary from run to run. |
| 130 | |
| 131 | <li>In the code generator and miscompilation debuggers, debugging will go |
| 132 | faster if you manually modify the program or its inputs to reduce the |
| 133 | runtime, but still exhibit the problem. |
| 134 | |
| 135 | <li><tt>bugpoint</tt> is extremely useful when working on a new optimization: |
| 136 | it helps track down regressions quickly. To avoid having to relink |
| 137 | <tt>bugpoint</tt> every time you change your optimization however, have |
| 138 | <tt>bugpoint</tt> dynamically load your optimization with the <a |
| 139 | href="#opt_load"><tt>-load</tt></a> option. |
| 140 | |
| 141 | <li><tt>bugpoint</tt> can generate a lot of output and run for a long period of |
| 142 | time. It is often useful to capture the output of the program to file. For |
| 143 | example, in the C shell, you can type:<br> |
| 144 | <tt>bugpoint ..... |& tee bugpoint.log</tt> |
| 145 | <br>to get a copy of <tt>bugpoint</tt>'s output in the file |
| 146 | <tt>bugpoint.log</tt>, as well as on your terminal. |
| 147 | |
| 148 | <li><tt>bugpoint</tt> cannot debug problems with the LLVM linker. If |
| 149 | <tt>bugpoint</tt> crashes before you see its "All input ok" message, |
| 150 | you might try <tt>llvm-link -v</tt> on the same set of input files. If |
| 151 | that also crashes, you may be experiencing a linker bug. |
| 152 | |
| 153 | <li>If your program is <b>supposed</b> to crash, <tt>bugpoint</tt> will be |
| 154 | confused. One way to deal with this is to cause bugpoint to ignore the exit |
| 155 | code from your program, by giving it the <tt>-check-exit-code=false</tt> |
| 156 | option. |
| 157 | |
| 158 | </ol> |
| 159 | |
| 160 | <h3>OPTIONS</h3> |
| 161 | |
| 162 | <ul> |
| 163 | <li><tt>-additional-so <library></tt><br> |
| 164 | Load <tt><library></tt> into the test program whenever it is run. |
| 165 | This is useful if you are debugging programs which depend on non-LLVM |
| 166 | libraries (such as the X or curses libraries) to run.<p> |
| 167 | |
| 168 | <li><tt>-args <program args></tt><br> |
| 169 | Pass all arguments specified after <tt>-args</tt> to the |
| 170 | test program whenever it runs. Note that if any of |
| 171 | the <tt><program args></tt> start with a '-', you should use: |
| 172 | <p> |
| 173 | <tt>bugpoint <bugpoint args> -args -- <program args></tt> |
| 174 | <p> |
| 175 | The "<tt>--</tt>" right after the <tt>-args</tt> option tells |
| 176 | <tt>bugpoint</tt> to consider any options starting with <tt>-</tt> to be |
| 177 | part of the <tt>-args</tt> option, not as options to <tt>bugpoint</tt> |
| 178 | itself.<p> |
| 179 | |
| 180 | <li><tt>-tool-args <tool args></tt><br> |
| 181 | Pass all arguments specified after <tt>-tool-args</tt> to the |
| 182 | LLVM tool under test (llc, lli, etc.) whenever it runs. |
| 183 | You should use this option in the following way: |
| 184 | <p> |
| 185 | <tt>bugpoint <bugpoint args> -tool-args -- <tool args></tt> |
| 186 | <p> |
| 187 | The "<tt>--</tt>" right after the <tt>-tool-args</tt> option tells |
| 188 | <tt>bugpoint</tt> to consider any options starting with <tt>-</tt> to be |
| 189 | part of the <tt>-tool-args</tt> option, not as options to |
| 190 | <tt>bugpoint</tt> itself. (See <tt>-args</tt>, above.)<p> |
| 191 | |
| 192 | <li><tt>-check-exit-code={true,false}</tt><br> |
| 193 | Assume a non-zero exit code or core dump from the test program is |
| 194 | a failure. Defaults to true.<p> |
| 195 | |
| 196 | <li><tt>-disable-{dce,simplifycfg}</tt><br> |
| 197 | Do not run the specified passes to clean up and reduce the size of the |
| 198 | test program. By default, <tt>bugpoint</tt> uses these passes internally |
| 199 | when attempting to reduce test programs. If you're trying to find |
| 200 | a bug in one of these passes, <tt>bugpoint</tt> may crash.<p> |
| 201 | |
| 202 | <li> <tt>-help</tt><br> |
| 203 | Print a summary of command line options.<p> |
| 204 | |
| 205 | <a name="opt_input"><li><tt>-input <filename></tt><br> |
| 206 | Open <tt><filename></tt> and redirect the standard input of the |
| 207 | test program, whenever it runs, to come from that file. |
| 208 | <p> |
| 209 | |
| 210 | <a name="opt_load"><li> <tt>-load <plugin></tt><br> |
| 211 | Load the dynamic object <tt><plugin></tt> into <tt>bugpoint</tt> |
| 212 | itself. This object should register new |
| 213 | optimization passes. Once loaded, the object will add new command line |
| 214 | options to enable various optimizations. To see the new complete list |
| 215 | of optimizations, use the -help and -load options together: |
| 216 | <p> |
| 217 | <tt>bugpoint -load <plugin> -help</tt> |
| 218 | <p> |
| 219 | |
| 220 | <a name="opt_output"><li><tt>-output <filename></tt><br> |
| 221 | Whenever the test program produces output on its standard output |
| 222 | stream, it should match the contents of <tt><filename></tt> |
| 223 | (the "reference output"). If you do not use this option, |
| 224 | <tt>bugpoint</tt> will attempt to generate a reference output by |
| 225 | compiling the program with the C backend and running it.<p> |
| 226 | |
| 227 | <li><tt>-profile-info-file <filename></tt><br> |
| 228 | Profile file loaded by -profile-loader.<p> |
| 229 | |
| 230 | <a name="opt_run-"><li><tt>-run-{int,jit,llc,cbe}</tt><br> |
| 231 | Whenever the test program is compiled, <tt>bugpoint</tt> should generate |
| 232 | code for it using the specified code generator. These options allow |
| 233 | you to choose the interpreter, the JIT compiler, the static native |
| 234 | code compiler, or the C backend, respectively.<p> |
| 235 | </ul> |
| 236 | |
| 237 | <h3>EXIT STATUS</h3> |
| 238 | |
| 239 | If <tt>bugpoint</tt> succeeds in finding a problem, it will exit with 0. |
| 240 | Otherwise, if an error occurs, it will exit with a non-zero value. |
| 241 | |
| Brian Gaeke | 74f470b | 2004-07-01 20:10:40 +0000 | [diff] [blame] | 242 | <HR> |
| 243 | Maintained by the <a href="http://llvm.cs.uiuc.edu">LLVM Team</a>. |
| 244 | </body> |
| 245 | </html> |