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annotate docs/Bugpoint.rst @ 107:a03ddd01be7e

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author Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
date Sun, 31 Jan 2016 17:34:49 +0900
parents afa8332a0e37
children
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1 ====================================
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2 LLVM bugpoint tool: design and usage
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3 ====================================
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4
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5 .. contents::
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6 :local:
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7
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8 Description
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9 ===========
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10
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11 ``bugpoint`` narrows down the source of problems in LLVM tools and passes. It
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12 can be used to debug three types of failures: optimizer crashes, miscompilations
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13 by optimizers, or bad native code generation (including problems in the static
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14 and JIT compilers). It aims to reduce large test cases to small, useful ones.
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15 For example, if ``opt`` crashes while optimizing a file, it will identify the
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16 optimization (or combination of optimizations) that causes the crash, and reduce
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17 the file down to a small example which triggers the crash.
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18
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19 For detailed case scenarios, such as debugging ``opt``, or one of the LLVM code
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20 generators, see :doc:`HowToSubmitABug`.
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21
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22 Design Philosophy
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23 =================
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24
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25 ``bugpoint`` is designed to be a useful tool without requiring any hooks into
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26 the LLVM infrastructure at all. It works with any and all LLVM passes and code
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27 generators, and does not need to "know" how they work. Because of this, it may
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28 appear to do stupid things or miss obvious simplifications. ``bugpoint`` is
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29 also designed to trade off programmer time for computer time in the
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30 compiler-debugging process; consequently, it may take a long period of
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31 (unattended) time to reduce a test case, but we feel it is still worth it. Note
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32 that ``bugpoint`` is generally very quick unless debugging a miscompilation
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33 where each test of the program (which requires executing it) takes a long time.
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34
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35 Automatic Debugger Selection
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36 ----------------------------
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37
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38 ``bugpoint`` reads each ``.bc`` or ``.ll`` file specified on the command line
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39 and links them together into a single module, called the test program. If any
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40 LLVM passes are specified on the command line, it runs these passes on the test
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41 program. If any of the passes crash, or if they produce malformed output (which
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42 causes the verifier to abort), ``bugpoint`` starts the `crash debugger`_.
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43
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44 Otherwise, if the ``-output`` option was not specified, ``bugpoint`` runs the
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45 test program with the "safe" backend (which is assumed to generate good code) to
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46 generate a reference output. Once ``bugpoint`` has a reference output for the
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47 test program, it tries executing it with the selected code generator. If the
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48 selected code generator crashes, ``bugpoint`` starts the `crash debugger`_ on
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49 the code generator. Otherwise, if the resulting output differs from the
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50 reference output, it assumes the difference resulted from a code generator
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51 failure, and starts the `code generator debugger`_.
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52
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53 Finally, if the output of the selected code generator matches the reference
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54 output, ``bugpoint`` runs the test program after all of the LLVM passes have
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55 been applied to it. If its output differs from the reference output, it assumes
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56 the difference resulted from a failure in one of the LLVM passes, and enters the
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57 `miscompilation debugger`_. Otherwise, there is no problem ``bugpoint`` can
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58 debug.
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59
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60 .. _crash debugger:
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61
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62 Crash debugger
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63 --------------
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64
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65 If an optimizer or code generator crashes, ``bugpoint`` will try as hard as it
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66 can to reduce the list of passes (for optimizer crashes) and the size of the
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67 test program. First, ``bugpoint`` figures out which combination of optimizer
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68 passes triggers the bug. This is useful when debugging a problem exposed by
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69 ``opt``, for example, because it runs over 38 passes.
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70
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71 Next, ``bugpoint`` tries removing functions from the test program, to reduce its
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72 size. Usually it is able to reduce a test program to a single function, when
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73 debugging intraprocedural optimizations. Once the number of functions has been
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74 reduced, it attempts to delete various edges in the control flow graph, to
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75 reduce the size of the function as much as possible. Finally, ``bugpoint``
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76 deletes any individual LLVM instructions whose absence does not eliminate the
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77 failure. At the end, ``bugpoint`` should tell you what passes crash, give you a
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78 bitcode file, and give you instructions on how to reproduce the failure with
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79 ``opt`` or ``llc``.
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80
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81 .. _code generator debugger:
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82
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83 Code generator debugger
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84 -----------------------
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85
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86 The code generator debugger attempts to narrow down the amount of code that is
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87 being miscompiled by the selected code generator. To do this, it takes the test
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88 program and partitions it into two pieces: one piece which it compiles with the
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89 "safe" backend (into a shared object), and one piece which it runs with either
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90 the JIT or the static LLC compiler. It uses several techniques to reduce the
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91 amount of code pushed through the LLVM code generator, to reduce the potential
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92 scope of the problem. After it is finished, it emits two bitcode files (called
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93 "test" [to be compiled with the code generator] and "safe" [to be compiled with
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94 the "safe" backend], respectively), and instructions for reproducing the
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95 problem. The code generator debugger assumes that the "safe" backend produces
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96 good code.
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97
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98 .. _miscompilation debugger:
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99
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100 Miscompilation debugger
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101 -----------------------
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102
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103 The miscompilation debugger works similarly to the code generator debugger. It
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104 works by splitting the test program into two pieces, running the optimizations
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105 specified on one piece, linking the two pieces back together, and then executing
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106 the result. It attempts to narrow down the list of passes to the one (or few)
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107 which are causing the miscompilation, then reduce the portion of the test
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108 program which is being miscompiled. The miscompilation debugger assumes that
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109 the selected code generator is working properly.
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110
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111 Advice for using bugpoint
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112 =========================
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113
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114 ``bugpoint`` can be a remarkably useful tool, but it sometimes works in
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115 non-obvious ways. Here are some hints and tips:
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116
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117 * In the code generator and miscompilation debuggers, ``bugpoint`` only works
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118 with programs that have deterministic output. Thus, if the program outputs
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119 ``argv[0]``, the date, time, or any other "random" data, ``bugpoint`` may
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120 misinterpret differences in these data, when output, as the result of a
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121 miscompilation. Programs should be temporarily modified to disable outputs
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122 that are likely to vary from run to run.
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123
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124 * In the code generator and miscompilation debuggers, debugging will go faster
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125 if you manually modify the program or its inputs to reduce the runtime, but
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126 still exhibit the problem.
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127
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128 * ``bugpoint`` is extremely useful when working on a new optimization: it helps
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129 track down regressions quickly. To avoid having to relink ``bugpoint`` every
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130 time you change your optimization however, have ``bugpoint`` dynamically load
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131 your optimization with the ``-load`` option.
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132
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133 * ``bugpoint`` can generate a lot of output and run for a long period of time.
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134 It is often useful to capture the output of the program to file. For example,
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135 in the C shell, you can run:
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136
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137 .. code-block:: console
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138
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139 $ bugpoint ... |& tee bugpoint.log
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140
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141 to get a copy of ``bugpoint``'s output in the file ``bugpoint.log``, as well
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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142 as on your terminal.
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143
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144 * ``bugpoint`` cannot debug problems with the LLVM linker. If ``bugpoint``
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145 crashes before you see its "All input ok" message, you might try ``llvm-link
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146 -v`` on the same set of input files. If that also crashes, you may be
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147 experiencing a linker bug.
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148
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149 * ``bugpoint`` is useful for proactively finding bugs in LLVM. Invoking
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150 ``bugpoint`` with the ``-find-bugs`` option will cause the list of specified
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151 optimizations to be randomized and applied to the program. This process will
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152 repeat until a bug is found or the user kills ``bugpoint``.
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153
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154 What to do when bugpoint isn't enough
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155 =====================================
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156
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157 Sometimes, ``bugpoint`` is not enough. In particular, InstCombine and
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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158 TargetLowering both have visitor structured code with lots of potential
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parents:
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159 transformations. If the process of using bugpoint has left you with still too
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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160 much code to figure out and the problem seems to be in instcombine, the
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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161 following steps may help. These same techniques are useful with TargetLowering
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162 as well.
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163
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164 Turn on ``-debug-only=instcombine`` and see which transformations within
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165 instcombine are firing by selecting out lines with "``IC``" in them.
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166
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167 At this point, you have a decision to make. Is the number of transformations
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168 small enough to step through them using a debugger? If so, then try that.
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169
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170 If there are too many transformations, then a source modification approach may
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171 be helpful. In this approach, you can modify the source code of instcombine to
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172 disable just those transformations that are being performed on your test input
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173 and perform a binary search over the set of transformations. One set of places
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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174 to modify are the "``visit*``" methods of ``InstCombiner`` (*e.g.*
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parents:
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175 ``visitICmpInst``) by adding a "``return false``" as the first line of the
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parents:
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176 method.
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177
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178 If that still doesn't remove enough, then change the caller of
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parents:
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179 ``InstCombiner::DoOneIteration``, ``InstCombiner::runOnFunction`` to limit the
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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180 number of iterations.
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181
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182 You may also find it useful to use "``-stats``" now to see what parts of
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parents:
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183 instcombine are firing. This can guide where to put additional reporting code.
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184
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185 At this point, if the amount of transformations is still too large, then
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parents:
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186 inserting code to limit whether or not to execute the body of the code in the
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parents:
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187 visit function can be helpful. Add a static counter which is incremented on
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parents:
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188 every invocation of the function. Then add code which simply returns false on
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189 desired ranges. For example:
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190
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191 .. code-block:: c++
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192
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193
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194 static int calledCount = 0;
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195 calledCount++;
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196 DEBUG(if (calledCount < 212) return false);
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197 DEBUG(if (calledCount > 217) return false);
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198 DEBUG(if (calledCount == 213) return false);
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199 DEBUG(if (calledCount == 214) return false);
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200 DEBUG(if (calledCount == 215) return false);
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201 DEBUG(if (calledCount == 216) return false);
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diff changeset
202 DEBUG(dbgs() << "visitXOR calledCount: " << calledCount << "\n");
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parents:
diff changeset
203 DEBUG(dbgs() << "I: "; I->dump());
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parents:
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204
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parents:
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205 could be added to ``visitXOR`` to limit ``visitXor`` to being applied only to
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parents:
diff changeset
206 calls 212 and 217. This is from an actual test case and raises an important
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
207 point---a simple binary search may not be sufficient, as transformations that
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parents:
diff changeset
208 interact may require isolating more than one call. In TargetLowering, use
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parents:
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209 ``return SDNode();`` instead of ``return false;``.
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parents:
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210
95
afa8332a0e37 LLVM 3.8
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parents: 77
diff changeset
211 Now that the number of transformations is down to a manageable number, try
0
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parents:
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212 examining the output to see if you can figure out which transformations are
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parents:
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213 being done. If that can be figured out, then do the usual debugging. If which
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
214 code corresponds to the transformation being performed isn't obvious, set a
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
215 breakpoint after the call count based disabling and step through the code.
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parents:
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216 Alternatively, you can use "``printf``" style debugging to report waypoints.