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author Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
date Sun, 31 Jan 2016 17:34:49 +0900
parents 60c9769439b8
children 803732b1fca8
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1 ================================
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2 How to submit an LLVM bug report
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3 ================================
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4
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5 Introduction - Got bugs?
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6 ========================
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9 If you're working with LLVM and run into a bug, we definitely want to know
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10 about it. This document describes what you can do to increase the odds of
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11 getting it fixed quickly.
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12
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13 Basically you have to do two things at a minimum. First, decide whether
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14 the bug `crashes the compiler`_ (or an LLVM pass), or if the
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15 compiler is `miscompiling`_ the program (i.e., the
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16 compiler successfully produces an executable, but it doesn't run right).
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17 Based on what type of bug it is, follow the instructions in the linked
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18 section to narrow down the bug so that the person who fixes it will be able
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19 to find the problem more easily.
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20
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21 Once you have a reduced test-case, go to `the LLVM Bug Tracking System
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22 <http://llvm.org/bugs/enter_bug.cgi>`_ and fill out the form with the
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23 necessary details (note that you don't need to pick a category, just use
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24 the "new-bugs" category if you're not sure). The bug description should
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25 contain the following information:
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26
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27 * All information necessary to reproduce the problem.
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28 * The reduced test-case that triggers the bug.
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29 * The location where you obtained LLVM (if not from our Subversion
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30 repository).
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31
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32 Thanks for helping us make LLVM better!
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33
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34 .. _crashes the compiler:
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35
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36 Crashing Bugs
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37 =============
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38
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39 More often than not, bugs in the compiler cause it to crash---often due to
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40 an assertion failure of some sort. The most important piece of the puzzle
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41 is to figure out if it is crashing in the GCC front-end or if it is one of
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42 the LLVM libraries (e.g. the optimizer or code generator) that has
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43 problems.
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44
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45 To figure out which component is crashing (the front-end, optimizer or code
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46 generator), run the ``clang`` command line as you were when the crash
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47 occurred, but with the following extra command line options:
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48
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49 * ``-O0 -emit-llvm``: If ``clang`` still crashes when passed these
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50 options (which disable the optimizer and code generator), then the crash
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51 is in the front-end. Jump ahead to the section on :ref:`front-end bugs
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52 <front-end>`.
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53
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54 * ``-emit-llvm``: If ``clang`` crashes with this option (which disables
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55 the code generator), you found an optimizer bug. Jump ahead to
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56 `compile-time optimization bugs`_.
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57
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58 * Otherwise, you have a code generator crash. Jump ahead to `code
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59 generator bugs`_.
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60
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61 .. _front-end bug:
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62 .. _front-end:
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63
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64 Front-end bugs
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65 --------------
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66
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67 If the problem is in the front-end, you should re-run the same ``clang``
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68 command that resulted in the crash, but add the ``-save-temps`` option.
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69 The compiler will crash again, but it will leave behind a ``foo.i`` file
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70 (containing preprocessed C source code) and possibly ``foo.s`` for each
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71 compiled ``foo.c`` file. Send us the ``foo.i`` file, along with the options
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72 you passed to ``clang``, and a brief description of the error it caused.
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73
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74 The `delta <http://delta.tigris.org/>`_ tool helps to reduce the
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75 preprocessed file down to the smallest amount of code that still replicates
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76 the problem. You're encouraged to use delta to reduce the code to make the
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77 developers' lives easier. `This website
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78 <http://gcc.gnu.org/wiki/A_guide_to_testcase_reduction>`_ has instructions
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79 on the best way to use delta.
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80
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81 .. _compile-time optimization bugs:
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82
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83 Compile-time optimization bugs
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84 ------------------------------
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85
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86 If you find that a bug crashes in the optimizer, compile your test-case to a
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87 ``.bc`` file by passing "``-emit-llvm -O0 -c -o foo.bc``".
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88 Then run:
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89
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90 .. code-block:: bash
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91
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92 opt -O3 -debug-pass=Arguments foo.bc -disable-output
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93
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94 This command should do two things: it should print out a list of passes, and
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95 then it should crash in the same way as clang. If it doesn't crash, please
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96 follow the instructions for a `front-end bug`_.
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97
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98 If this does crash, then you should be able to debug this with the following
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99 bugpoint command:
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100
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101 .. code-block:: bash
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102
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103 bugpoint foo.bc <list of passes printed by opt>
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104
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105 Please run this, then file a bug with the instructions and reduced .bc
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106 files that bugpoint emits. If something goes wrong with bugpoint, please
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107 submit the "foo.bc" file and the list of passes printed by ``opt``.
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108
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109 .. _code generator bugs:
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110
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111 Code generator bugs
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112 -------------------
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113
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114 If you find a bug that crashes clang in the code generator, compile your
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115 source file to a .bc file by passing "``-emit-llvm -c -o foo.bc``" to
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116 clang (in addition to the options you already pass). Once your have
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117 foo.bc, one of the following commands should fail:
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118
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119 #. ``llc foo.bc``
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120 #. ``llc foo.bc -relocation-model=pic``
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121 #. ``llc foo.bc -relocation-model=static``
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122
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123 If none of these crash, please follow the instructions for a `front-end
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124 bug`_. If one of these do crash, you should be able to reduce this with
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125 one of the following bugpoint command lines (use the one corresponding to
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126 the command above that failed):
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127
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128 #. ``bugpoint -run-llc foo.bc``
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129 #. ``bugpoint -run-llc foo.bc --tool-args -relocation-model=pic``
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130 #. ``bugpoint -run-llc foo.bc --tool-args -relocation-model=static``
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131
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132 Please run this, then file a bug with the instructions and reduced .bc file
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133 that bugpoint emits. If something goes wrong with bugpoint, please submit
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134 the "foo.bc" file and the option that llc crashes with.
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135
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136 .. _miscompiling:
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137
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138 Miscompilations
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139 ===============
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140
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141 If clang successfully produces an executable, but that executable
0
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142 doesn't run right, this is either a bug in the code or a bug in the
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143 compiler. The first thing to check is to make sure it is not using
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144 undefined behavior (e.g. reading a variable before it is defined). In
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145 particular, check to see if the program `valgrind
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146 <http://valgrind.org/>`_'s clean, passes purify, or some other memory
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147 checker tool. Many of the "LLVM bugs" that we have chased down ended up
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148 being bugs in the program being compiled, not LLVM.
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149
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150 Once you determine that the program itself is not buggy, you should choose
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151 which code generator you wish to compile the program with (e.g. LLC or the JIT)
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152 and optionally a series of LLVM passes to run. For example:
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153
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154 .. code-block:: bash
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155
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156 bugpoint -run-llc [... optzn passes ...] file-to-test.bc --args -- [program arguments]
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157
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158 bugpoint will try to narrow down your list of passes to the one pass that
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159 causes an error, and simplify the bitcode file as much as it can to assist
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160 you. It will print a message letting you know how to reproduce the
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161 resulting error.
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162
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163 Incorrect code generation
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164 =========================
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165
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166 Similarly to debugging incorrect compilation by mis-behaving passes, you
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167 can debug incorrect code generation by either LLC or the JIT, using
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168 ``bugpoint``. The process ``bugpoint`` follows in this case is to try to
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169 narrow the code down to a function that is miscompiled by one or the other
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170 method, but since for correctness, the entire program must be run,
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171 ``bugpoint`` will compile the code it deems to not be affected with the C
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172 Backend, and then link in the shared object it generates.
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173
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174 To debug the JIT:
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175
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176 .. code-block:: bash
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177
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178 bugpoint -run-jit -output=[correct output file] [bitcode file] \
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179 --tool-args -- [arguments to pass to lli] \
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180 --args -- [program arguments]
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181
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182 Similarly, to debug the LLC, one would run:
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183
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184 .. code-block:: bash
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185
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186 bugpoint -run-llc -output=[correct output file] [bitcode file] \
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187 --tool-args -- [arguments to pass to llc] \
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188 --args -- [program arguments]
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189
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190 **Special note:** if you are debugging MultiSource or SPEC tests that
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191 already exist in the ``llvm/test`` hierarchy, there is an easier way to
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192 debug the JIT, LLC, and CBE, using the pre-written Makefile targets, which
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193 will pass the program options specified in the Makefiles:
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194
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195 .. code-block:: bash
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196
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197 cd llvm/test/../../program
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198 make bugpoint-jit
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199
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200 At the end of a successful ``bugpoint`` run, you will be presented
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201 with two bitcode files: a *safe* file which can be compiled with the C
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202 backend and the *test* file which either LLC or the JIT
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203 mis-codegenerates, and thus causes the error.
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204
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205 To reproduce the error that ``bugpoint`` found, it is sufficient to do
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206 the following:
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207
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208 #. Regenerate the shared object from the safe bitcode file:
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209
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210 .. code-block:: bash
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211
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212 llc -march=c safe.bc -o safe.c
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213 gcc -shared safe.c -o safe.so
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214
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215 #. If debugging LLC, compile test bitcode native and link with the shared
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216 object:
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217
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218 .. code-block:: bash
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219
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220 llc test.bc -o test.s
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221 gcc test.s safe.so -o test.llc
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222 ./test.llc [program options]
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223
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224 #. If debugging the JIT, load the shared object and supply the test
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225 bitcode:
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226
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227 .. code-block:: bash
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228
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229 lli -load=safe.so test.bc [program options]