1 ==================================== 2 LLVM bugpoint tool: design and usage 3 ==================================== 4 5 .. contents:: 6 :local: 7 8 Description 9 =========== 10 11 ``bugpoint`` narrows down the source of problems in LLVM tools and passes. It 12 can be used to debug three types of failures: optimizer crashes, miscompilations 13 by optimizers, or bad native code generation (including problems in the static 14 and JIT compilers). It aims to reduce large test cases to small, useful ones. 15 For example, if ``opt`` crashes while optimizing a file, it will identify the 16 optimization (or combination of optimizations) that causes the crash, and reduce 17 the file down to a small example which triggers the crash. 18 19 For detailed case scenarios, such as debugging ``opt``, or one of the LLVM code 20 generators, see :doc:`HowToSubmitABug`. 21 22 Design Philosophy 23 ================= 24 25 ``bugpoint`` is designed to be a useful tool without requiring any hooks into 26 the LLVM infrastructure at all. It works with any and all LLVM passes and code 27 generators, and does not need to "know" how they work. Because of this, it may 28 appear to do stupid things or miss obvious simplifications. ``bugpoint`` is 29 also designed to trade off programmer time for computer time in the 30 compiler-debugging process; consequently, it may take a long period of 31 (unattended) time to reduce a test case, but we feel it is still worth it. Note 32 that ``bugpoint`` is generally very quick unless debugging a miscompilation 33 where each test of the program (which requires executing it) takes a long time. 34 35 Automatic Debugger Selection 36 ---------------------------- 37 38 ``bugpoint`` reads each ``.bc`` or ``.ll`` file specified on the command line 39 and links them together into a single module, called the test program. If any 40 LLVM passes are specified on the command line, it runs these passes on the test 41 program. If any of the passes crash, or if they produce malformed output (which 42 causes the verifier to abort), ``bugpoint`` starts the `crash debugger`_. 43 44 Otherwise, if the ``-output`` option was not specified, ``bugpoint`` runs the 45 test program with the "safe" backend (which is assumed to generate good code) to 46 generate a reference output. Once ``bugpoint`` has a reference output for the 47 test program, it tries executing it with the selected code generator. If the 48 selected code generator crashes, ``bugpoint`` starts the `crash debugger`_ on 49 the code generator. Otherwise, if the resulting output differs from the 50 reference output, it assumes the difference resulted from a code generator 51 failure, and starts the `code generator debugger`_. 52 53 Finally, if the output of the selected code generator matches the reference 54 output, ``bugpoint`` runs the test program after all of the LLVM passes have 55 been applied to it. If its output differs from the reference output, it assumes 56 the difference resulted from a failure in one of the LLVM passes, and enters the 57 `miscompilation debugger`_. Otherwise, there is no problem ``bugpoint`` can 58 debug. 59 60 .. _crash debugger: 61 62 Crash debugger 63 -------------- 64 65 If an optimizer or code generator crashes, ``bugpoint`` will try as hard as it 66 can to reduce the list of passes (for optimizer crashes) and the size of the 67 test program. First, ``bugpoint`` figures out which combination of optimizer 68 passes triggers the bug. This is useful when debugging a problem exposed by 69 ``opt``, for example, because it runs over 38 passes. 70 71 Next, ``bugpoint`` tries removing functions from the test program, to reduce its 72 size. Usually it is able to reduce a test program to a single function, when 73 debugging intraprocedural optimizations. Once the number of functions has been 74 reduced, it attempts to delete various edges in the control flow graph, to 75 reduce the size of the function as much as possible. Finally, ``bugpoint`` 76 deletes any individual LLVM instructions whose absence does not eliminate the 77 failure. At the end, ``bugpoint`` should tell you what passes crash, give you a 78 bitcode file, and give you instructions on how to reproduce the failure with 79 ``opt`` or ``llc``. 80 81 .. _code generator debugger: 82 83 Code generator debugger 84 ----------------------- 85 86 The code generator debugger attempts to narrow down the amount of code that is 87 being miscompiled by the selected code generator. To do this, it takes the test 88 program and partitions it into two pieces: one piece which it compiles with the 89 "safe" backend (into a shared object), and one piece which it runs with either 90 the JIT or the static LLC compiler. It uses several techniques to reduce the 91 amount of code pushed through the LLVM code generator, to reduce the potential 92 scope of the problem. After it is finished, it emits two bitcode files (called 93 "test" [to be compiled with the code generator] and "safe" [to be compiled with 94 the "safe" backend], respectively), and instructions for reproducing the 95 problem. The code generator debugger assumes that the "safe" backend produces 96 good code. 97 98 .. _miscompilation debugger: 99 100 Miscompilation debugger 101 ----------------------- 102 103 The miscompilation debugger works similarly to the code generator debugger. It 104 works by splitting the test program into two pieces, running the optimizations 105 specified on one piece, linking the two pieces back together, and then executing 106 the result. It attempts to narrow down the list of passes to the one (or few) 107 which are causing the miscompilation, then reduce the portion of the test 108 program which is being miscompiled. The miscompilation debugger assumes that 109 the selected code generator is working properly. 110 111 Advice for using bugpoint 112 ========================= 113 114 ``bugpoint`` can be a remarkably useful tool, but it sometimes works in 115 non-obvious ways. Here are some hints and tips: 116 117 * In the code generator and miscompilation debuggers, ``bugpoint`` only works 118 with programs that have deterministic output. Thus, if the program outputs 119 ``argv[0]``, the date, time, or any other "random" data, ``bugpoint`` may 120 misinterpret differences in these data, when output, as the result of a 121 miscompilation. Programs should be temporarily modified to disable outputs 122 that are likely to vary from run to run. 123 124 * In the code generator and miscompilation debuggers, debugging will go faster 125 if you manually modify the program or its inputs to reduce the runtime, but 126 still exhibit the problem. 127 128 * ``bugpoint`` is extremely useful when working on a new optimization: it helps 129 track down regressions quickly. To avoid having to relink ``bugpoint`` every 130 time you change your optimization however, have ``bugpoint`` dynamically load 131 your optimization with the ``-load`` option. 132 133 * ``bugpoint`` can generate a lot of output and run for a long period of time. 134 It is often useful to capture the output of the program to file. For example, 135 in the C shell, you can run: 136 137 .. code-block:: console 138 139 $ bugpoint ... |& tee bugpoint.log 140 141 to get a copy of ``bugpoint``'s output in the file ``bugpoint.log``, as well 142 as on your terminal. 143 144 * ``bugpoint`` cannot debug problems with the LLVM linker. If ``bugpoint`` 145 crashes before you see its "All input ok" message, you might try ``llvm-link 146 -v`` on the same set of input files. If that also crashes, you may be 147 experiencing a linker bug. 148 149 * ``bugpoint`` is useful for proactively finding bugs in LLVM. Invoking 150 ``bugpoint`` with the ``-find-bugs`` option will cause the list of specified 151 optimizations to be randomized and applied to the program. This process will 152 repeat until a bug is found or the user kills ``bugpoint``. 153 154 What to do when bugpoint isn't enough 155 ===================================== 156 157 Sometimes, ``bugpoint`` is not enough. In particular, InstCombine and 158 TargetLowering both have visitor structured code with lots of potential 159 transformations. If the process of using bugpoint has left you with still too 160 much code to figure out and the problem seems to be in instcombine, the 161 following steps may help. These same techniques are useful with TargetLowering 162 as well. 163 164 Turn on ``-debug-only=instcombine`` and see which transformations within 165 instcombine are firing by selecting out lines with "``IC``" in them. 166 167 At this point, you have a decision to make. Is the number of transformations 168 small enough to step through them using a debugger? If so, then try that. 169 170 If there are too many transformations, then a source modification approach may 171 be helpful. In this approach, you can modify the source code of instcombine to 172 disable just those transformations that are being performed on your test input 173 and perform a binary search over the set of transformations. One set of places 174 to modify are the "``visit*``" methods of ``InstCombiner`` (*e.g.* 175 ``visitICmpInst``) by adding a "``return false``" as the first line of the 176 method. 177 178 If that still doesn't remove enough, then change the caller of 179 ``InstCombiner::DoOneIteration``, ``InstCombiner::runOnFunction`` to limit the 180 number of iterations. 181 182 You may also find it useful to use "``-stats``" now to see what parts of 183 instcombine are firing. This can guide where to put additional reporting code. 184 185 At this point, if the amount of transformations is still too large, then 186 inserting code to limit whether or not to execute the body of the code in the 187 visit function can be helpful. Add a static counter which is incremented on 188 every invocation of the function. Then add code which simply returns false on 189 desired ranges. For example: 190 191 .. code-block:: c++ 192 193 194 static int calledCount = 0; 195 calledCount++; 196 DEBUG(if (calledCount < 212) return false); 197 DEBUG(if (calledCount > 217) return false); 198 DEBUG(if (calledCount == 213) return false); 199 DEBUG(if (calledCount == 214) return false); 200 DEBUG(if (calledCount == 215) return false); 201 DEBUG(if (calledCount == 216) return false); 202 DEBUG(dbgs() << "visitXOR calledCount: " << calledCount << "\n"); 203 DEBUG(dbgs() << "I: "; I->dump()); 204 205 could be added to ``visitXOR`` to limit ``visitXor`` to being applied only to 206 calls 212 and 217. This is from an actual test case and raises an important 207 point---a simple binary search may not be sufficient, as transformations that 208 interact may require isolating more than one call. In TargetLowering, use 209 ``return SDNode();`` instead of ``return false;``. 210 211 Now that the number of transformations is down to a manageable number, try 212 examining the output to see if you can figure out which transformations are 213 being done. If that can be figured out, then do the usual debugging. If which 214 code corresponds to the transformation being performed isn't obvious, set a 215 breakpoint after the call count based disabling and step through the code. 216 Alternatively, you can use "``printf``" style debugging to report waypoints. 217
