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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 How to submit an LLVM bug report
3 ================================
4
5 Introduction - Got bugs?
6 ========================
7
8
9 If you're working with LLVM and run into a bug, we definitely want to know
10 about it. This document describes what you can do to increase the odds of
11 getting it fixed quickly.
12
13 Basically you have to do two things at a minimum. First, decide whether
14 the bug `crashes the compiler`_ (or an LLVM pass), or if the
15 compiler is `miscompiling`_ the program (i.e., the
16 compiler successfully produces an executable, but it doesn't run right).
17 Based on what type of bug it is, follow the instructions in the linked
18 section to narrow down the bug so that the person who fixes it will be able
19 to find the problem more easily.
20
21 Once you have a reduced test-case, go to `the LLVM Bug Tracking System
22 <http://llvm.org/bugs/enter_bug.cgi>`_ and fill out the form with the
23 necessary details (note that you don't need to pick a category, just use
24 the "new-bugs" category if you're not sure). The bug description should
25 contain the following information:
26
27 * All information necessary to reproduce the problem.
28 * The reduced test-case that triggers the bug.
29 * The location where you obtained LLVM (if not from our Subversion
30 repository).
31
32 Thanks for helping us make LLVM better!
33
34 .. _crashes the compiler:
35
36 Crashing Bugs
37 =============
38
39 More often than not, bugs in the compiler cause it to crash---often due to
40 an assertion failure of some sort. The most important piece of the puzzle
41 is to figure out if it is crashing in the GCC front-end or if it is one of
42 the LLVM libraries (e.g. the optimizer or code generator) that has
43 problems.
44
45 To figure out which component is crashing (the front-end, optimizer or code
46 generator), run the ``llvm-gcc`` command line as you were when the crash
47 occurred, but with the following extra command line options:
48
49 * ``-O0 -emit-llvm``: If ``llvm-gcc`` still crashes when passed these
50 options (which disable the optimizer and code generator), then the crash
51 is in the front-end. Jump ahead to the section on :ref:`front-end bugs
52 <front-end>`.
53
54 * ``-emit-llvm``: If ``llvm-gcc`` crashes with this option (which disables
55 the code generator), you found an optimizer bug. Jump ahead to
56 `compile-time optimization bugs`_.
57
58 * Otherwise, you have a code generator crash. Jump ahead to `code
59 generator bugs`_.
60
61 .. _front-end bug:
62 .. _front-end:
63
64 Front-end bugs
65 --------------
66
67 If the problem is in the front-end, you should re-run the same ``llvm-gcc``
68 command that resulted in the crash, but add the ``-save-temps`` option.
69 The compiler will crash again, but it will leave behind a ``foo.i`` file
70 (containing preprocessed C source code) and possibly ``foo.s`` for each
71 compiled ``foo.c`` file. Send us the ``foo.i`` file, along with the options
72 you passed to ``llvm-gcc``, and a brief description of the error it caused.
73
74 The `delta <http://delta.tigris.org/>`_ tool helps to reduce the
75 preprocessed file down to the smallest amount of code that still replicates
76 the problem. You're encouraged to use delta to reduce the code to make the
77 developers' lives easier. `This website
78 <http://gcc.gnu.org/wiki/A_guide_to_testcase_reduction>`_ has instructions
79 on the best way to use delta.
80
81 .. _compile-time optimization bugs:
82
83 Compile-time optimization bugs
84 ------------------------------
85
86 If you find that a bug crashes in the optimizer, compile your test-case to a
87 ``.bc`` file by passing "``-emit-llvm -O0 -c -o foo.bc``".
88 Then run:
89
90 .. code-block:: bash
91
92 opt -std-compile-opts -debug-pass=Arguments foo.bc -disable-output
93
94 This command should do two things: it should print out a list of passes, and
95 then it should crash in the same way as llvm-gcc. If it doesn't crash, please
96 follow the instructions for a `front-end bug`_.
97
98 If this does crash, then you should be able to debug this with the following
99 bugpoint command:
100
101 .. code-block:: bash
102
103 bugpoint foo.bc <list of passes printed by opt>
104
105 Please run this, then file a bug with the instructions and reduced .bc
106 files that bugpoint emits. If something goes wrong with bugpoint, please
107 submit the "foo.bc" file and the list of passes printed by ``opt``.
108
109 .. _code generator bugs:
110
111 Code generator bugs
112 -------------------
113
114 If you find a bug that crashes llvm-gcc in the code generator, compile your
115 source file to a .bc file by passing "``-emit-llvm -c -o foo.bc``" to
116 llvm-gcc (in addition to the options you already pass). Once your have
117 foo.bc, one of the following commands should fail:
118
119 #. ``llc foo.bc``
120 #. ``llc foo.bc -relocation-model=pic``
121 #. ``llc foo.bc -relocation-model=static``
122
123 If none of these crash, please follow the instructions for a `front-end
124 bug`_. If one of these do crash, you should be able to reduce this with
125 one of the following bugpoint command lines (use the one corresponding to
126 the command above that failed):
127
128 #. ``bugpoint -run-llc foo.bc``
129 #. ``bugpoint -run-llc foo.bc --tool-args -relocation-model=pic``
130 #. ``bugpoint -run-llc foo.bc --tool-args -relocation-model=static``
131
132 Please run this, then file a bug with the instructions and reduced .bc file
133 that bugpoint emits. If something goes wrong with bugpoint, please submit
134 the "foo.bc" file and the option that llc crashes with.
135
136 .. _miscompiling:
137
138 Miscompilations
139 ===============
140
141 If llvm-gcc successfully produces an executable, but that executable
142 doesn't run right, this is either a bug in the code or a bug in the
143 compiler. The first thing to check is to make sure it is not using
144 undefined behavior (e.g. reading a variable before it is defined). In
145 particular, check to see if the program `valgrind
146 <http://valgrind.org/>`_'s clean, passes purify, or some other memory
147 checker tool. Many of the "LLVM bugs" that we have chased down ended up
148 being bugs in the program being compiled, not LLVM.
149
150 Once you determine that the program itself is not buggy, you should choose
151 which code generator you wish to compile the program with (e.g. LLC or the JIT)
152 and optionally a series of LLVM passes to run. For example:
153
154 .. code-block:: bash
155
156 bugpoint -run-llc [... optzn passes ...] file-to-test.bc --args -- [program arguments]
157
158 bugpoint will try to narrow down your list of passes to the one pass that
159 causes an error, and simplify the bitcode file as much as it can to assist
160 you. It will print a message letting you know how to reproduce the
161 resulting error.
162
163 Incorrect code generation
164 =========================
165
166 Similarly to debugging incorrect compilation by mis-behaving passes, you
167 can debug incorrect code generation by either LLC or the JIT, using
168 ``bugpoint``. The process ``bugpoint`` follows in this case is to try to
169 narrow the code down to a function that is miscompiled by one or the other
170 method, but since for correctness, the entire program must be run,
171 ``bugpoint`` will compile the code it deems to not be affected with the C
172 Backend, and then link in the shared object it generates.
173
174 To debug the JIT:
175
176 .. code-block:: bash
177
178 bugpoint -run-jit -output=[correct output file] [bitcode file] \
179 --tool-args -- [arguments to pass to lli] \
180 --args -- [program arguments]
181
182 Similarly, to debug the LLC, one would run:
183
184 .. code-block:: bash
185
186 bugpoint -run-llc -output=[correct output file] [bitcode file] \
187 --tool-args -- [arguments to pass to llc] \
188 --args -- [program arguments]
189
190 **Special note:** if you are debugging MultiSource or SPEC tests that
191 already exist in the ``llvm/test`` hierarchy, there is an easier way to
192 debug the JIT, LLC, and CBE, using the pre-written Makefile targets, which
193 will pass the program options specified in the Makefiles:
194
195 .. code-block:: bash
196
197 cd llvm/test/../../program
198 make bugpoint-jit
199
200 At the end of a successful ``bugpoint`` run, you will be presented
201 with two bitcode files: a *safe* file which can be compiled with the C
202 backend and the *test* file which either LLC or the JIT
203 mis-codegenerates, and thus causes the error.
204
205 To reproduce the error that ``bugpoint`` found, it is sufficient to do
206 the following:
207
208 #. Regenerate the shared object from the safe bitcode file:
209
210 .. code-block:: bash
211
212 llc -march=c safe.bc -o safe.c
213 gcc -shared safe.c -o safe.so
214
215 #. If debugging LLC, compile test bitcode native and link with the shared
216 object:
217
218 .. code-block:: bash
219
220 llc test.bc -o test.s
221 gcc test.s safe.so -o test.llc
222 ./test.llc [program options]
223
224 #. If debugging the JIT, load the shared object and supply the test
225 bitcode:
226
227 .. code-block:: bash
228
229 lli -load=safe.so test.bc [program options]