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6 <title>How to submit an LLVM bug report</title>
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12 How to submit an LLVM bug report
15 <table class="layout" style="width: 90%" >
19 <li><a href="#introduction">Introduction - Got bugs?</a></li>
20 <li><a href="#crashers">Crashing Bugs</a>
22 <li><a href="#front-end">Front-end bugs</a>
23 <li><a href="#ct_optimizer">Compile-time optimization bugs</a>
24 <li><a href="#ct_codegen">Code generator bugs</a>
26 <li><a href="#miscompilations">Miscompilations</a></li>
27 <li><a href="#codegen">Incorrect code generation (JIT and LLC)</a></li>
29 <div class="doc_author">
30 <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a> and
31 <a href="http://misha.brukman.net">Misha Brukman</a></p>
35 <img src="img/Debugging.gif" alt="Debugging" width="444" height="314">
40 <!-- *********************************************************************** -->
42 <a name="introduction">Introduction - Got bugs?</a>
44 <!-- *********************************************************************** -->
48 <p>If you're working with LLVM and run into a bug, we definitely want to know
49 about it. This document describes what you can do to increase the odds of
50 getting it fixed quickly.</p>
52 <p>Basically you have to do two things at a minimum. First, decide whether the
53 bug <a href="#crashers">crashes the compiler</a> (or an LLVM pass), or if the
54 compiler is <a href="#miscompilations">miscompiling</a> the program (i.e., the
55 compiler successfully produces an executable, but it doesn't run right). Based
57 what type of bug it is, follow the instructions in the linked section to narrow
58 down the bug so that the person who fixes it will be able to find the problem
61 <p>Once you have a reduced test-case, go to <a
62 href="http://llvm.org/bugs/enter_bug.cgi">the LLVM Bug Tracking
63 System</a> and fill out the form with the necessary details (note that you don't
64 need to pick a category, just use the "new-bugs" category if you're not sure).
65 The bug description should contain the following
69 <li>All information necessary to reproduce the problem.</li>
70 <li>The reduced test-case that triggers the bug.</li>
71 <li>The location where you obtained LLVM (if not from our Subversion
75 <p>Thanks for helping us make LLVM better!</p>
79 <!-- *********************************************************************** -->
81 <a name="crashers">Crashing Bugs</a>
83 <!-- *********************************************************************** -->
87 <p>More often than not, bugs in the compiler cause it to crash—often due
88 to an assertion failure of some sort. The most important
89 piece of the puzzle is to figure out if it is crashing in the GCC front-end
90 or if it is one of the LLVM libraries (e.g. the optimizer or code generator)
91 that has problems.</p>
93 <p>To figure out which component is crashing (the front-end,
94 optimizer or code generator), run the
95 <tt><b>llvm-gcc</b></tt> command line as you were when the crash occurred, but
96 with the following extra command line options:</p>
99 <li><tt><b>-O0 -emit-llvm</b></tt>: If <tt>llvm-gcc</tt> still crashes when
100 passed these options (which disable the optimizer and code generator), then
101 the crash is in the front-end. Jump ahead to the section on <a
102 href="#front-end">front-end bugs</a>.</li>
104 <li><tt><b>-emit-llvm</b></tt>: If <tt>llvm-gcc</tt> crashes with this option
105 (which disables the code generator), you found an optimizer bug. Jump ahead
106 to <a href="#ct_optimizer"> compile-time optimization bugs</a>.</li>
108 <li>Otherwise, you have a code generator crash. Jump ahead to <a
109 href="#ct_codegen">code generator bugs</a>.</li>
113 <!-- ======================================================================= -->
115 <a name="front-end">Front-end bugs</a>
120 <p>If the problem is in the front-end, you should re-run the same
121 <tt>llvm-gcc</tt> command that resulted in the crash, but add the
122 <tt>-save-temps</tt> option. The compiler will crash again, but it will leave
123 behind a <tt><i>foo</i>.i</tt> file (containing preprocessed C source code) and
124 possibly <tt><i>foo</i>.s</tt> for each
125 compiled <tt><i>foo</i>.c</tt> file. Send us the <tt><i>foo</i>.i</tt> file,
126 along with the options you passed to llvm-gcc, and a brief description of the
129 <p>The <a href="http://delta.tigris.org/">delta</a> tool helps to reduce the
130 preprocessed file down to the smallest amount of code that still replicates the
131 problem. You're encouraged to use delta to reduce the code to make the
132 developers' lives easier. <a
133 href="http://gcc.gnu.org/wiki/A_guide_to_testcase_reduction">This website</a>
134 has instructions on the best way to use delta.</p>
138 <!-- ======================================================================= -->
140 <a name="ct_optimizer">Compile-time optimization bugs</a>
145 <p>If you find that a bug crashes in the optimizer, compile your test-case to a
146 <tt>.bc</tt> file by passing "<tt><b>-emit-llvm -O0 -c -o foo.bc</b></tt>".
149 <div class="doc_code">
150 <p><tt><b>opt</b> -std-compile-opts -debug-pass=Arguments foo.bc
151 -disable-output</tt></p>
154 <p>This command should do two things: it should print out a list of passes, and
155 then it should crash in the same way as llvm-gcc. If it doesn't crash, please
156 follow the instructions for a <a href="#front-end">front-end bug</a>.</p>
158 <p>If this does crash, then you should be able to debug this with the following
159 bugpoint command:</p>
161 <div class="doc_code">
162 <p><tt><b>bugpoint</b> foo.bc <list of passes printed by
163 <b>opt</b>></tt></p>
166 <p>Please run this, then file a bug with the instructions and reduced .bc files
167 that bugpoint emits. If something goes wrong with bugpoint, please submit the
168 "foo.bc" file and the list of passes printed by <b>opt</b>.</p>
172 <!-- ======================================================================= -->
174 <a name="ct_codegen">Code generator bugs</a>
179 <p>If you find a bug that crashes llvm-gcc in the code generator, compile your
180 source file to a .bc file by passing "<tt><b>-emit-llvm -c -o foo.bc</b></tt>"
181 to llvm-gcc (in addition to the options you already pass). Once your have
182 foo.bc, one of the following commands should fail:</p>
185 <li><tt><b>llc</b> foo.bc</tt></li>
186 <li><tt><b>llc</b> foo.bc -relocation-model=pic</tt></li>
187 <li><tt><b>llc</b> foo.bc -relocation-model=static</tt></li>
190 <p>If none of these crash, please follow the instructions for a
191 <a href="#front-end">front-end bug</a>. If one of these do crash, you should
192 be able to reduce this with one of the following bugpoint command lines (use
193 the one corresponding to the command above that failed):</p>
196 <li><tt><b>bugpoint</b> -run-llc foo.bc</tt></li>
197 <li><tt><b>bugpoint</b> -run-llc foo.bc --tool-args
198 -relocation-model=pic</tt></li>
199 <li><tt><b>bugpoint</b> -run-llc foo.bc --tool-args
200 -relocation-model=static</tt></li>
203 <p>Please run this, then file a bug with the instructions and reduced .bc file
204 that bugpoint emits. If something goes wrong with bugpoint, please submit the
205 "foo.bc" file and the option that llc crashes with.</p>
211 <!-- *********************************************************************** -->
213 <a name="miscompilations">Miscompilations</a>
215 <!-- *********************************************************************** -->
219 <p>If llvm-gcc successfully produces an executable, but that executable doesn't
220 run right, this is either a bug in the code or a bug in the
221 compiler. The first thing to check is to make sure it is not using undefined
222 behavior (e.g. reading a variable before it is defined). In particular, check
223 to see if the program <a href="http://valgrind.org/">valgrind</a>s clean,
224 passes purify, or some other memory checker tool. Many of the "LLVM bugs" that
225 we have chased down ended up being bugs in the program being compiled, not
228 <p>Once you determine that the program itself is not buggy, you should choose
229 which code generator you wish to compile the program with (e.g. C backend, the
230 JIT, or LLC) and optionally a series of LLVM passes to run. For example:</p>
232 <div class="doc_code">
234 <b>bugpoint</b> -run-cbe [... optzn passes ...] file-to-test.bc --args -- [program arguments]</tt></p>
237 <p><tt>bugpoint</tt> will try to narrow down your list of passes to the one pass
238 that causes an error, and simplify the bitcode file as much as it can to assist
239 you. It will print a message letting you know how to reproduce the resulting
244 <!-- *********************************************************************** -->
246 <a name="codegen">Incorrect code generation</a>
248 <!-- *********************************************************************** -->
252 <p>Similarly to debugging incorrect compilation by mis-behaving passes, you can
253 debug incorrect code generation by either LLC or the JIT, using
254 <tt>bugpoint</tt>. The process <tt>bugpoint</tt> follows in this case is to try
255 to narrow the code down to a function that is miscompiled by one or the other
256 method, but since for correctness, the entire program must be run,
257 <tt>bugpoint</tt> will compile the code it deems to not be affected with the C
258 Backend, and then link in the shared object it generates.</p>
260 <p>To debug the JIT:</p>
262 <div class="doc_code">
264 bugpoint -run-jit -output=[correct output file] [bitcode file] \
265 --tool-args -- [arguments to pass to lli] \
266 --args -- [program arguments]
270 <p>Similarly, to debug the LLC, one would run:</p>
272 <div class="doc_code">
274 bugpoint -run-llc -output=[correct output file] [bitcode file] \
275 --tool-args -- [arguments to pass to llc] \
276 --args -- [program arguments]
280 <p><b>Special note:</b> if you are debugging MultiSource or SPEC tests that
281 already exist in the <tt>llvm/test</tt> hierarchy, there is an easier way to
282 debug the JIT, LLC, and CBE, using the pre-written Makefile targets, which
283 will pass the program options specified in the Makefiles:</p>
285 <div class="doc_code">
287 cd llvm/test/../../program<br>
292 <p>At the end of a successful <tt>bugpoint</tt> run, you will be presented
293 with two bitcode files: a <em>safe</em> file which can be compiled with the C
294 backend and the <em>test</em> file which either LLC or the JIT
295 mis-codegenerates, and thus causes the error.</p>
297 <p>To reproduce the error that <tt>bugpoint</tt> found, it is sufficient to do
302 <li><p>Regenerate the shared object from the safe bitcode file:</p>
304 <div class="doc_code">
306 <b>llc</b> -march=c safe.bc -o safe.c<br>
307 <b>gcc</b> -shared safe.c -o safe.so
311 <li><p>If debugging LLC, compile test bitcode native and link with the shared
314 <div class="doc_code">
316 <b>llc</b> test.bc -o test.s<br>
317 <b>gcc</b> test.s safe.so -o test.llc<br>
318 ./test.llc [program options]
322 <li><p>If debugging the JIT, load the shared object and supply the test
325 <div class="doc_code">
326 <p><tt><b>lli</b> -load=safe.so test.bc [program options]</tt></p>
333 <!-- *********************************************************************** -->
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341 <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
342 <a href="http://llvm.org/">The LLVM Compiler Infrastructure</a>
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