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5 <title>How to submit an LLVM bug report</title>
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10 <div class="doc_title">
11 How to submit an LLVM bug report
14 <table border="0" width="100%">
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="#gccas">GCCAS bugs</a>
24 <li><a href="#gccld">GCCLD bugs</a>
25 <li><a href="#passes">Bugs in LLVM passes</a>
27 <li><a href="#miscompilations">Miscompilations</a></li>
28 <li><a href="#codegen">Incorrect code generation (JIT and LLC)</a></li>
32 <div class="doc_text">
33 <p><b>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a> and
34 <a href="http://misha.brukman.net">Misha Brukman</a></b></p>
39 <img src="img/Debugging.gif" alt="Debugging" width="444" height="314">
44 <!-- *********************************************************************** -->
45 <div class="doc_section">
46 <a name="introduction">Introduction - Got bugs?</a>
48 <!-- *********************************************************************** -->
50 <div class="doc_text">
52 <p>If you're working with LLVM and run into a bug, we definitely want to know
53 about it. This document describes what you can do to increase the odds of
54 getting it fixed quickly.</p>
56 <p>Basically you have to do two things at a minimum. First, decide whether the
57 bug <a href="#crashers">crashes the compiler</a> (or an LLVM pass), or if the
58 compiler is <a href="#miscompilations">miscompiling</a> the program. Based on
59 what type of bug it is, follow the instructions in the linked section to narrow
60 down the bug so that the person who fixes it will be able to find the problem
63 <p>Once you have a reduced test-case, go to <a
64 href="http://llvm.cs.uiuc.edu/bugs/enter_bug.cgi">the LLVM Bug Tracking
65 System</a>, select the category in which the bug falls, and fill out the form
66 with the necessary details. The bug description should contain the following
70 <li>All information necessary to reproduce the problem.</li>
71 <li>The reduced test-case that triggers the bug.</li>
72 <li>The location where you obtained LLVM (if not from our CVS
76 <p>Thanks for helping us make LLVM better!</p>
80 <!-- *********************************************************************** -->
81 <div class="doc_section">
82 <a name="crashers">Crashing Bugs</a>
84 <!-- *********************************************************************** -->
86 <div class="doc_text">
88 <p>More often than not, bugs in the compiler cause it to crash - often due to an
89 assertion failure of some sort. If you are running <tt><b>opt</b></tt> or
90 <tt><b>analyze</b></tt> directly, and something crashes, jump to the section on
91 <a href="#passes">bugs in LLVM passes</a>. Otherwise, the most important
92 piece of the puzzle is to figure out if it is the GCC-based front-end that is
93 buggy or if it's one of the LLVM tools that has problems.</p>
95 <p>To figure out which program is crashing (the front-end,
96 <tt><b>gccas</b></tt>, or <tt><b>gccld</b></tt>), run the
97 <tt><b>llvm-gcc</b></tt> command line as you were when the crash occurred, but
98 add a <tt>-v</tt> option to the command line. The compiler will print out a
99 bunch of stuff, and should end with telling you that one of
100 <tt><b>cc1</b>/<b>cc1plus</b></tt>, <tt><b>gccas</b></tt>, or
101 <tt><b>gccld</b></tt> crashed.</p>
105 <li>If <tt><b>cc1</b></tt> or <tt><b>cc1plus</b></tt> crashed, you found a
106 problem with the front-end.
107 Jump ahead to the section on <a href="#front-end">front-end bugs</a>.</li>
109 <li>If <tt><b>gccas</b></tt> crashed, you found a bug in <a href="#gccas">one
110 of the passes in <tt><b>gccas</b></tt></a>.</li>
112 <li>If <tt><b>gccld</b></tt> crashed, you found a bug in <a href="#gccld">one
113 of the passes in <tt><b>gccld</b></tt></a>.</li>
115 <li>Otherwise, something really weird happened. Email the list with what you
116 have at this point.</li>
122 <!-- ======================================================================= -->
123 <div class="doc_subsection">
124 <a name="front-end">Front-end bugs</a>
127 <div class="doc_text">
129 <p>If the problem is in the front-end, you should re-run the same
130 <tt>llvm-gcc</tt> command that resulted in the crash, but add the
131 <tt>-save-temps</tt> option. The compiler will crash again, but it will leave
132 behind a <tt><i>foo</i>.i</tt> file (containing preprocessed C source code) and
133 possibly <tt><i>foo</i>.s</tt> (containing LLVM assembly code), for each
134 compiled <tt><i>foo</i>.c</tt> file. Send us the <tt><i>foo</i>.i</tt> file,
135 along with a brief description of the error it caused.</p>
139 <!-- ======================================================================= -->
140 <div class="doc_subsection">
141 <a name="gccas">GCCAS bugs</a>
144 <div class="doc_text">
146 <p>If you find that a bug crashes in the <tt><b>gccas</b></tt> stage of
147 compilation, compile your test-case to a <tt>.s</tt> file with the
148 <tt>-save-temps</tt> option to <tt><b>llvm-gcc</b></tt>. Then run:</p>
150 <div class="doc_code">
151 <pre><b>gccas</b> -debug-pass=Arguments < /dev/null -o - > /dev/null</pre>
154 <p>... which will print a list of arguments, indicating the list of passes that
155 <tt><b>gccas</b></tt> runs. Once you have the input file and the list of
156 passes, go to the section on <a href="#passes">debugging bugs in LLVM
161 <!-- ======================================================================= -->
162 <div class="doc_subsection">
163 <a name="gccld">GCCLD bugs</a>
166 <div class="doc_text">
168 <p>If you find that a bug crashes in the <tt><b>gccld</b></tt> stage of
169 compilation, gather all of the <tt>.o</tt> bytecode files and libraries that are
170 being linked together (the "<tt><b>llvm-gcc</b> -v</tt>" output should include
171 the full list of objects linked). Then run:</p>
173 <div class="doc_code">
175 <b>llvm-as</b> < /dev/null > null.bc
176 <b>gccld</b> -debug-pass=Arguments null.bc</tt></p>
179 <p>... which will print a list of arguments, indicating the list of passes that
180 <tt><b>gccld</b></tt> runs. Once you have the input files and the list of
181 passes, go to the section on <a href="#passes">debugging bugs in LLVM
186 <!-- ======================================================================= -->
187 <div class="doc_subsection">
188 <a name="passes">Bugs in LLVM passes</a>
191 <div class="doc_text">
193 <p>At this point, you should have some number of LLVM assembly files or bytecode
194 files and a list of passes which crash when run on the specified input. In
195 order to reduce the list of passes (which is probably large) and the input to
196 something tractable, use the <tt><b>bugpoint</b></tt> tool as follows:</p>
198 <div class="doc_code">
199 <pre><b>bugpoint</b> <input files> <list of passes></pre>
202 <p><tt><b>bugpoint</b></tt> will print a bunch of output as it reduces the
203 test-case, but it should eventually print something like this:</p>
205 <div class="doc_code">
208 Emitted bytecode to 'bugpoint-reduced-simplified.bc'
210 *** You can reproduce the problem with: opt bugpoint-reduced-simplified.bc -licm
214 <p>Once you complete this, please send the LLVM bytecode file and the command
215 line to reproduce the problem to the llvmbugs mailing list.</p>
219 <!-- *********************************************************************** -->
220 <div class="doc_section">
221 <a name="miscompilations">Miscompilations</a>
223 <!-- *********************************************************************** -->
225 <div class="doc_text">
227 <p>A miscompilation occurs when a pass does not correctly transform a program,
228 thus producing errors that are only noticed during execution. This is different
229 from producing invalid LLVM code (i.e., code not in SSA form, using values
230 before defining them, etc.) which the verifier will check for after a pass
231 finishes its run.</p>
233 <p>If it looks like the LLVM compiler is miscompiling a program, the very first
234 thing to check is to make sure it is not using undefined behavior. In
235 particular, check to see if the program <a
236 href="http://valgrind.kde.org/">valgrind</a>s clean, passes purify, or some
237 other memory checker tool. Many of the "LLVM bugs" that we have chased down
238 ended up being bugs in the program being compiled, not LLVM.</p>
240 <p>Once you determine that the program itself is not buggy, you should choose
241 which code generator you wish to compile the program with (e.g. C backend, the
242 JIT, or LLC) and optionally a series of LLVM passes to run. For example:</p>
244 <div class="doc_code">
246 <b>bugpoint</b> -run-cbe [... optimization passes ...] file-to-test.bc --args -- [program arguments]</pre>
249 <p><tt>bugpoint</tt> will try to narrow down your list of passes to the one pass
250 that causes an error, and simplify the bytecode file as much as it can to assist
251 you. It will print a message letting you know how to reproduce the resulting
256 <!-- *********************************************************************** -->
257 <div class="doc_section">
258 <a name="codegen">Incorrect code generation</a>
260 <!-- *********************************************************************** -->
262 <div class="doc_text">
264 <p>Similarly to debugging incorrect compilation by mis-behaving passes, you can
265 debug incorrect code generation by either LLC or the JIT, using
266 <tt>bugpoint</tt>. The process <tt>bugpoint</tt> follows in this case is to try
267 to narrow the code down to a function that is miscompiled by one or the other
268 method, but since for correctness, the entire program must be run,
269 <tt>bugpoint</tt> will compile the code it deems to not be affected with the C
270 Backend, and then link in the shared object it generates.</p>
272 <p>To debug the JIT:</p>
274 <div class="doc_code">
276 bugpoint -run-jit -output=[correct output file] [bytecode file] \
277 --tool-args -- [arguments to pass to lli] \
278 --args -- [program arguments]
282 <p>Similarly, to debug the LLC, one would run:</p>
284 <div class="doc_code">
286 bugpoint -run-llc -output=[correct output file] [bytecode file] \
287 --tool-args -- [arguments to pass to llc] \
288 --args -- [program arguments]
292 <p><b>Special note:</b> if you are debugging MultiSource or SPEC tests that
293 already exist in the <tt>llvm/test</tt> hierarchy, there is an easier way to
294 debug the JIT, LLC, and CBE, using the pre-written Makefile targets, which
295 will pass the program options specified in the Makefiles:</p>
297 <div class="doc_code">
299 cd llvm/test/../../program
304 <p>At the end of a successful <tt>bugpoint</tt> run, you will be presented
305 with two bytecode files: a <em>safe</em> file which can be compiled with the C
306 backend and the <em>test</em> file which either LLC or the JIT
307 mis-codegenerates, and thus causes the error.</p>
309 <p>To reproduce the error that <tt>bugpoint</tt> found, it is sufficient to do
314 <li><p>Regenerate the shared object from the safe bytecode file:</p>
316 <div class="doc_code">
318 <b>llc</b> -march=c safe.bc -o safe.c
319 <b>gcc</b> -shared safe.c -o safe.so
323 <li><p>If debugging LLC, compile test bytecode native and link with the shared
326 <div class="doc_code">
328 <b>llc</b> test.bc -o test.s -f
329 <b>gcc</b> test.s safe.so -o test.llc
330 ./test.llc [program options]
334 <li><p>If debugging the JIT, load the shared object and supply the test
337 <div class="doc_code">
338 <pre><b>lli</b> -load=safe.so test.bc [program options]</pre>
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353 <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
354 <a href="http://llvm.cs.uiuc.edu">The LLVM Compiler Infrastructure</a>
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