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author Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
date Wed, 15 May 2013 06:43:32 +0900
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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 `How To Submit a Bug Report document <HowToSubmitABug.html>`_.
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 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.