1 <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" 2 "http://www.w3.org/TR/html4/strict.dtd"> 3 <html> 4 <head> 5 <title>LLVM bugpoint tool: design and usage</title> 6 <link rel="stylesheet" href="llvm.css" type="text/css"> 7 </head> 8 9 <h1> 10 LLVM bugpoint tool: design and usage 11 </h1> 12 13 <ul> 14 <li><a href="#desc">Description</a></li> 15 <li><a href="#design">Design Philosophy</a> 16 <ul> 17 <li><a href="#autoselect">Automatic Debugger Selection</a></li> 18 <li><a href="#crashdebug">Crash debugger</a></li> 19 <li><a href="#codegendebug">Code generator debugger</a></li> 20 <li><a href="#miscompilationdebug">Miscompilation debugger</a></li> 21 </ul></li> 22 <li><a href="#advice">Advice for using <tt>bugpoint</tt></a></li> 23 </ul> 24 25 <div class="doc_author"> 26 <p>Written by <a href="mailto:sabre (a] nondot.org">Chris Lattner</a></p> 27 </div> 28 29 <!-- *********************************************************************** --> 30 <h2> 31 <a name="desc">Description</a> 32 </h2> 33 <!-- *********************************************************************** --> 34 35 <div> 36 37 <p><tt>bugpoint</tt> narrows down the source of problems in LLVM tools and 38 passes. It can be used to debug three types of failures: optimizer crashes, 39 miscompilations by optimizers, or bad native code generation (including problems 40 in the static and JIT compilers). It aims to reduce large test cases to small, 41 useful ones. For example, if <tt>opt</tt> crashes while optimizing a 42 file, it will identify the optimization (or combination of optimizations) that 43 causes the crash, and reduce the file down to a small example which triggers the 44 crash.</p> 45 46 <p>For detailed case scenarios, such as debugging <tt>opt</tt>, 47 <tt>llvm-ld</tt>, or one of the LLVM code generators, see <a 48 href="HowToSubmitABug.html">How To Submit a Bug Report document</a>.</p> 49 50 </div> 51 52 <!-- *********************************************************************** --> 53 <h2> 54 <a name="design">Design Philosophy</a> 55 </h2> 56 <!-- *********************************************************************** --> 57 58 <div> 59 60 <p><tt>bugpoint</tt> is designed to be a useful tool without requiring any 61 hooks into the LLVM infrastructure at all. It works with any and all LLVM 62 passes and code generators, and does not need to "know" how they work. Because 63 of this, it may appear to do stupid things or miss obvious 64 simplifications. <tt>bugpoint</tt> is also designed to trade off programmer 65 time for computer time in the compiler-debugging process; consequently, it may 66 take a long period of (unattended) time to reduce a test case, but we feel it 67 is still worth it. Note that <tt>bugpoint</tt> is generally very quick unless 68 debugging a miscompilation where each test of the program (which requires 69 executing it) takes a long time.</p> 70 71 <!-- ======================================================================= --> 72 <h3> 73 <a name="autoselect">Automatic Debugger Selection</a> 74 </h3> 75 76 <div> 77 78 <p><tt>bugpoint</tt> reads each <tt>.bc</tt> or <tt>.ll</tt> file specified on 79 the command line and links them together into a single module, called the test 80 program. If any LLVM passes are specified on the command line, it runs these 81 passes on the test program. If any of the passes crash, or if they produce 82 malformed output (which causes the verifier to abort), <tt>bugpoint</tt> starts 83 the <a href="#crashdebug">crash debugger</a>.</p> 84 85 <p>Otherwise, if the <tt>-output</tt> option was not specified, 86 <tt>bugpoint</tt> runs the test program with the C backend (which is assumed to 87 generate good code) to generate a reference output. Once <tt>bugpoint</tt> has 88 a reference output for the test program, it tries executing it with the 89 selected code generator. If the selected code generator crashes, 90 <tt>bugpoint</tt> starts the <a href="#crashdebug">crash debugger</a> on the 91 code generator. Otherwise, if the resulting output differs from the reference 92 output, it assumes the difference resulted from a code generator failure, and 93 starts the <a href="#codegendebug">code generator debugger</a>.</p> 94 95 <p>Finally, if the output of the selected code generator matches the reference 96 output, <tt>bugpoint</tt> runs the test program after all of the LLVM passes 97 have been applied to it. If its output differs from the reference output, it 98 assumes the difference resulted from a failure in one of the LLVM passes, and 99 enters the <a href="#miscompilationdebug">miscompilation debugger</a>. 100 Otherwise, there is no problem <tt>bugpoint</tt> can debug.</p> 101 102 </div> 103 104 <!-- ======================================================================= --> 105 <h3> 106 <a name="crashdebug">Crash debugger</a> 107 </h3> 108 109 <div> 110 111 <p>If an optimizer or code generator crashes, <tt>bugpoint</tt> will try as hard 112 as it can to reduce the list of passes (for optimizer crashes) and the size of 113 the test program. First, <tt>bugpoint</tt> figures out which combination of 114 optimizer passes triggers the bug. This is useful when debugging a problem 115 exposed by <tt>opt</tt>, for example, because it runs over 38 passes.</p> 116 117 <p>Next, <tt>bugpoint</tt> tries removing functions from the test program, to 118 reduce its size. Usually it is able to reduce a test program to a single 119 function, when debugging intraprocedural optimizations. Once the number of 120 functions has been reduced, it attempts to delete various edges in the control 121 flow graph, to reduce the size of the function as much as possible. Finally, 122 <tt>bugpoint</tt> deletes any individual LLVM instructions whose absence does 123 not eliminate the failure. At the end, <tt>bugpoint</tt> should tell you what 124 passes crash, give you a bitcode file, and give you instructions on how to 125 reproduce the failure with <tt>opt</tt> or <tt>llc</tt>.</p> 126 127 </div> 128 129 <!-- ======================================================================= --> 130 <h3> 131 <a name="codegendebug">Code generator debugger</a> 132 </h3> 133 134 <div> 135 136 <p>The code generator debugger attempts to narrow down the amount of code that 137 is being miscompiled by the selected code generator. To do this, it takes the 138 test program and partitions it into two pieces: one piece which it compiles 139 with the C backend (into a shared object), and one piece which it runs with 140 either the JIT or the static LLC compiler. It uses several techniques to 141 reduce the amount of code pushed through the LLVM code generator, to reduce the 142 potential scope of the problem. After it is finished, it emits two bitcode 143 files (called "test" [to be compiled with the code generator] and "safe" [to be 144 compiled with the C backend], respectively), and instructions for reproducing 145 the problem. The code generator debugger assumes that the C backend produces 146 good code.</p> 147 148 </div> 149 150 <!-- ======================================================================= --> 151 <h3> 152 <a name="miscompilationdebug">Miscompilation debugger</a> 153 </h3> 154 155 <div> 156 157 <p>The miscompilation debugger works similarly to the code generator debugger. 158 It works by splitting the test program into two pieces, running the 159 optimizations specified on one piece, linking the two pieces back together, and 160 then executing the result. It attempts to narrow down the list of passes to 161 the one (or few) which are causing the miscompilation, then reduce the portion 162 of the test program which is being miscompiled. The miscompilation debugger 163 assumes that the selected code generator is working properly.</p> 164 165 </div> 166 167 </div> 168 169 <!-- *********************************************************************** --> 170 <h2> 171 <a name="advice">Advice for using bugpoint</a> 172 </h2> 173 <!-- *********************************************************************** --> 174 175 <div> 176 177 <tt>bugpoint</tt> can be a remarkably useful tool, but it sometimes works in 178 non-obvious ways. Here are some hints and tips:<p> 179 180 <ol> 181 <li>In the code generator and miscompilation debuggers, <tt>bugpoint</tt> only 182 works with programs that have deterministic output. Thus, if the program 183 outputs <tt>argv[0]</tt>, the date, time, or any other "random" data, 184 <tt>bugpoint</tt> may misinterpret differences in these data, when output, 185 as the result of a miscompilation. Programs should be temporarily modified 186 to disable outputs that are likely to vary from run to run. 187 188 <li>In the code generator and miscompilation debuggers, debugging will go 189 faster if you manually modify the program or its inputs to reduce the 190 runtime, but still exhibit the problem. 191 192 <li><tt>bugpoint</tt> is extremely useful when working on a new optimization: 193 it helps track down regressions quickly. To avoid having to relink 194 <tt>bugpoint</tt> every time you change your optimization however, have 195 <tt>bugpoint</tt> dynamically load your optimization with the 196 <tt>-load</tt> option. 197 198 <li><p><tt>bugpoint</tt> can generate a lot of output and run for a long period 199 of time. It is often useful to capture the output of the program to file. 200 For example, in the C shell, you can run:</p> 201 202 <div class="doc_code"> 203 <p><tt>bugpoint ... |& tee bugpoint.log</tt></p> 204 </div> 205 206 <p>to get a copy of <tt>bugpoint</tt>'s output in the file 207 <tt>bugpoint.log</tt>, as well as on your terminal.</p> 208 209 <li><tt>bugpoint</tt> cannot debug problems with the LLVM linker. If 210 <tt>bugpoint</tt> crashes before you see its "All input ok" message, 211 you might try <tt>llvm-link -v</tt> on the same set of input files. If 212 that also crashes, you may be experiencing a linker bug. 213 214 <li><tt>bugpoint</tt> is useful for proactively finding bugs in LLVM. 215 Invoking <tt>bugpoint</tt> with the <tt>-find-bugs</tt> option will cause 216 the list of specified optimizations to be randomized and applied to the 217 program. This process will repeat until a bug is found or the user 218 kills <tt>bugpoint</tt>. 219 </ol> 220 221 </div> 222 223 <!-- *********************************************************************** --> 224 225 <hr> 226 <address> 227 <a href="http://jigsaw.w3.org/css-validator/check/referer"><img 228 src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a> 229 <a href="http://validator.w3.org/check/referer"><img 230 src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a> 231 232 <a href="mailto:sabre (a] nondot.org">Chris Lattner</a><br> 233 <a href="http://llvm.org/">LLVM Compiler Infrastructure</a><br> 234 Last modified: $Date: 2011-08-30 14:26:11 -0400 (Tue, 30 Aug 2011) $ 235 </address> 236 237 </body> 238 </html> 239
