1==================================== 2LLVM bugpoint tool: design and usage 3==================================== 4 5.. contents:: 6 :local: 7 8Description 9=========== 10 11``bugpoint`` narrows down the source of problems in LLVM tools and passes. It 12can be used to debug three types of failures: optimizer crashes, miscompilations 13by optimizers, or bad native code generation (including problems in the static 14and JIT compilers). It aims to reduce large test cases to small, useful ones. 15For example, if ``opt`` crashes while optimizing a file, it will identify the 16optimization (or combination of optimizations) that causes the crash, and reduce 17the file down to a small example which triggers the crash. 18 19For detailed case scenarios, such as debugging ``opt``, or one of the LLVM code 20generators, see :doc:`HowToSubmitABug`. 21 22Design Philosophy 23================= 24 25``bugpoint`` is designed to be a useful tool without requiring any hooks into 26the LLVM infrastructure at all. It works with any and all LLVM passes and code 27generators, and does not need to "know" how they work. Because of this, it may 28appear to do stupid things or miss obvious simplifications. ``bugpoint`` is 29also designed to trade off programmer time for computer time in the 30compiler-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 32that ``bugpoint`` is generally very quick unless debugging a miscompilation 33where each test of the program (which requires executing it) takes a long time. 34 35Automatic Debugger Selection 36---------------------------- 37 38``bugpoint`` reads each ``.bc`` or ``.ll`` file specified on the command line 39and links them together into a single module, called the test program. If any 40LLVM passes are specified on the command line, it runs these passes on the test 41program. If any of the passes crash, or if they produce malformed output (which 42causes the verifier to abort), ``bugpoint`` starts the `crash debugger`_. 43 44Otherwise, if the ``-output`` option was not specified, ``bugpoint`` runs the 45test program with the "safe" backend (which is assumed to generate good code) to 46generate a reference output. Once ``bugpoint`` has a reference output for the 47test program, it tries executing it with the selected code generator. If the 48selected code generator crashes, ``bugpoint`` starts the `crash debugger`_ on 49the code generator. Otherwise, if the resulting output differs from the 50reference output, it assumes the difference resulted from a code generator 51failure, and starts the `code generator debugger`_. 52 53Finally, if the output of the selected code generator matches the reference 54output, ``bugpoint`` runs the test program after all of the LLVM passes have 55been applied to it. If its output differs from the reference output, it assumes 56the difference resulted from a failure in one of the LLVM passes, and enters the 57`miscompilation debugger`_. Otherwise, there is no problem ``bugpoint`` can 58debug. 59 60.. _crash debugger: 61 62Crash debugger 63-------------- 64 65If an optimizer or code generator crashes, ``bugpoint`` will try as hard as it 66can to reduce the list of passes (for optimizer crashes) and the size of the 67test program. First, ``bugpoint`` figures out which combination of optimizer 68passes triggers the bug. This is useful when debugging a problem exposed by 69``opt``, for example, because it runs over 38 passes. 70 71Next, ``bugpoint`` tries removing functions from the test program, to reduce its 72size. Usually it is able to reduce a test program to a single function, when 73debugging intraprocedural optimizations. Once the number of functions has been 74reduced, it attempts to delete various edges in the control flow graph, to 75reduce the size of the function as much as possible. Finally, ``bugpoint`` 76deletes any individual LLVM instructions whose absence does not eliminate the 77failure. At the end, ``bugpoint`` should tell you what passes crash, give you a 78bitcode file, and give you instructions on how to reproduce the failure with 79``opt`` or ``llc``. 80 81.. _code generator debugger: 82 83Code generator debugger 84----------------------- 85 86The code generator debugger attempts to narrow down the amount of code that is 87being miscompiled by the selected code generator. To do this, it takes the test 88program 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 90the JIT or the static LLC compiler. It uses several techniques to reduce the 91amount of code pushed through the LLVM code generator, to reduce the potential 92scope 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 94the "safe" backend], respectively), and instructions for reproducing the 95problem. The code generator debugger assumes that the "safe" backend produces 96good code. 97 98.. _miscompilation debugger: 99 100Miscompilation debugger 101----------------------- 102 103The miscompilation debugger works similarly to the code generator debugger. It 104works by splitting the test program into two pieces, running the optimizations 105specified on one piece, linking the two pieces back together, and then executing 106the result. It attempts to narrow down the list of passes to the one (or few) 107which are causing the miscompilation, then reduce the portion of the test 108program which is being miscompiled. The miscompilation debugger assumes that 109the selected code generator is working properly. 110 111Advice for using bugpoint 112========================= 113 114``bugpoint`` can be a remarkably useful tool, but it sometimes works in 115non-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* ``bugpoint`` can produce IR which contains long names. Run ``opt 155 -metarenamer`` over the IR to rename everything using easy-to-read, 156 metasyntactic names. Alternatively, run ``opt -strip -instnamer`` to rename 157 everything with very short (often purely numeric) names. 158 159What to do when bugpoint isn't enough 160===================================== 161 162Sometimes, ``bugpoint`` is not enough. In particular, InstCombine and 163TargetLowering both have visitor structured code with lots of potential 164transformations. If the process of using bugpoint has left you with still too 165much code to figure out and the problem seems to be in instcombine, the 166following steps may help. These same techniques are useful with TargetLowering 167as well. 168 169Turn on ``-debug-only=instcombine`` and see which transformations within 170instcombine are firing by selecting out lines with "``IC``" in them. 171 172At this point, you have a decision to make. Is the number of transformations 173small enough to step through them using a debugger? If so, then try that. 174 175If there are too many transformations, then a source modification approach may 176be helpful. In this approach, you can modify the source code of instcombine to 177disable just those transformations that are being performed on your test input 178and perform a binary search over the set of transformations. One set of places 179to modify are the "``visit*``" methods of ``InstCombiner`` (*e.g.* 180``visitICmpInst``) by adding a "``return false``" as the first line of the 181method. 182 183If that still doesn't remove enough, then change the caller of 184``InstCombiner::DoOneIteration``, ``InstCombiner::runOnFunction`` to limit the 185number of iterations. 186 187You may also find it useful to use "``-stats``" now to see what parts of 188instcombine are firing. This can guide where to put additional reporting code. 189 190At this point, if the amount of transformations is still too large, then 191inserting code to limit whether or not to execute the body of the code in the 192visit function can be helpful. Add a static counter which is incremented on 193every invocation of the function. Then add code which simply returns false on 194desired ranges. For example: 195 196.. code-block:: c++ 197 198 199 static int calledCount = 0; 200 calledCount++; 201 LLVM_DEBUG(if (calledCount < 212) return false); 202 LLVM_DEBUG(if (calledCount > 217) return false); 203 LLVM_DEBUG(if (calledCount == 213) return false); 204 LLVM_DEBUG(if (calledCount == 214) return false); 205 LLVM_DEBUG(if (calledCount == 215) return false); 206 LLVM_DEBUG(if (calledCount == 216) return false); 207 LLVM_DEBUG(dbgs() << "visitXOR calledCount: " << calledCount << "\n"); 208 LLVM_DEBUG(dbgs() << "I: "; I->dump()); 209 210could be added to ``visitXOR`` to limit ``visitXor`` to being applied only to 211calls 212 and 217. This is from an actual test case and raises an important 212point---a simple binary search may not be sufficient, as transformations that 213interact may require isolating more than one call. In TargetLowering, use 214``return SDNode();`` instead of ``return false;``. 215 216Now that the number of transformations is down to a manageable number, try 217examining the output to see if you can figure out which transformations are 218being done. If that can be figured out, then do the usual debugging. If which 219code corresponds to the transformation being performed isn't obvious, set a 220breakpoint after the call count based disabling and step through the code. 221Alternatively, you can use "``printf``" style debugging to report waypoints. 222