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