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2 <html><head><title>How to submit an LLVM bug report</title></head>
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7 <tr><td> <font size=+3 color="#EEEEFF" face="Georgia,Palatino,Times,Roman"><b>How to submit an LLVM bug report</b></font></td>
10 <table border=0 width=100%>
14 <li><a href="#introduction">Introduction - Got bugs?</a>
15 <li><a href="#crashers">Crashing Bugs</a>
17 <li><a href="#front-end">Front-end bugs</a>
18 <li><a href="#gccas">GCCAS bugs</a>
19 <li><a href="#gccld">GCCLD bugs</a>
20 <li><a href="#passes">Bugs in LLVM passes</a>
22 <li><a href="#miscompilations">Miscompilations</a>
23 <li><a href="#codegen">Incorrect code generation (JIT and LLC)</a>
25 <p><b>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a> and
26 <a href="http://misha.brukman.net">Misha Brukman</a></b><p>
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29 <img src="Debugging.gif" width=444 height=314>
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35 <table width="100%" bgcolor="#330077" border=0 cellpadding=4 cellspacing=0>
36 <tr><td align=center><font color="#EEEEFF" size=+2 face="Georgia,Palatino"><b>
37 <a name="introduction">Introduction - Got bugs?
38 </b></font></td></tr></table><ul>
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41 If you're working with LLVM and run into a bug, we definitely want to know about
42 it. This document describes what you can do to increase the odds of getting it
45 Basically you have to do two things at a minimum. First, decide whether the bug
46 <a href="#crashers">crashes the compiler</a> (or an LLVM pass), or if the
47 compiler is <a href="#miscompilations">miscompiling</a> the program. Based on
48 what type of bug it is, follow the instructions in the linked section to narrow
49 down the bug so that the person who fixes it will be able to find the problem
52 Once you have a reduced test-case, go to
53 <a href="http://zion.cs.uiuc.edu/bugs/enter_bug.cgi">
54 the LLVM Bug Tracking System,
56 select the catagory in which the bug falls, and fill out the form with the
57 necessary details. The bug description should contain the following
61 All information necessary to reproduce the problem.
64 The reduced test-case that triggers the bug.
67 The location where you obtained LLVM (if not from our CVS repository).
71 Thanks for helping us make LLVM better!<p>
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75 </ul><table width="100%" bgcolor="#330077" border=0 cellpadding=4 cellspacing=0>
76 <tr><td align=center><font color="#EEEEFF" size=+2 face="Georgia,Palatino"><b>
77 <a name="crashers">Crashing Bugs
78 </b></font></td></tr></table><ul>
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81 More often than not, bugs in the compiler cause it to crash - often due to an
82 assertion failure of some sort. If you are running <tt><b>opt</b></tt> or
83 <tt><b>analyze</b></tt> directly, and something crashes, jump to the section on
84 <a href="#passes">bugs in LLVM passes</a>. Otherwise, the most important
85 piece of the puzzle is to figure out if it is the GCC-based front-end that is
86 buggy or if it's one of the LLVM tools that has problems.<p>
88 To figure out which program is crashing (the front-end, <tt><b>gccas</b></tt>,
89 or <tt><b>gccld</b></tt>), run the <tt><b>llvm-gcc</b></tt> command line as you
90 were when the crash occurred, but add a <tt>-v</tt> option to the command line.
91 The compiler will print out a bunch of stuff, and should end with telling you
92 that one of <tt><b>cc1</b></tt>, <tt><b>gccas</b></tt>, or <tt><b>gccld</b></tt>
96 <li>If <tt><b>cc1</b></tt> crashed, you found a problem with the front-end.
97 Jump ahead to the section on <a href="#front-end">front-end bugs</a>.
98 <li>If <tt><b>gccas</b></tt> crashed, you found a bug in <a href="#gccas">one of
99 the passes in <tt><b>gccas</b></tt></a>.
100 <li>If <tt><b>gccld</b></tt> crashed, you found a bug in <a href="#gccld">one
101 of the passes in <tt><b>gccld</b></tt></a>.
102 <li>Otherwise, something really weird happened. Email the list with what you
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108 </ul><table width="100%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0>
109 <tr><td> </td><td width="100%">
110 <font color="#EEEEFF" face="Georgia,Palatino"><b>
111 <a name="front-end">Front-end bugs
112 </b></font></td></tr></table><ul>
114 If the problem is in the front-end, you should re-run the same
115 <tt>llvm-gcc</tt> command that resulted in the crash, but add the
116 <tt>-save-temps</tt> option. The compiler will crash again, but it
117 will leave behind a <tt><i>foo</i>.i</tt> file (containing preprocessed
118 C source code) and possibly <tt><i>foo</i>.s</tt> (containing LLVM
119 assembly code), for each compiled <tt><i>foo</i>.c</tt> file. Send us
120 the <tt><i>foo</i>.i</tt> file, along with a brief description of the
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124 </ul><table width="100%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0>
125 <tr><td> </td><td width="100%">
126 <font color="#EEEEFF" face="Georgia,Palatino"><b>
127 <a name="gccas">GCCAS bugs
128 </b></font></td></tr></table><ul>
130 If you find that a bug crashes in the <tt><b>gccas</b></tt> stage of
131 compilation, compile your test-case to a <tt>.s</tt> file with the
132 <tt>-save-temps</tt> option to <tt><b>llvm-gcc</b></tt>. Then run:<p>
135 <b>gccas</b> -debug-pass=Arguments < /dev/null -o - > /dev/null
138 ... which will print a list of arguments, indicating the list of passes that
139 <tt><b>gccas</b></tt> runs. Once you have the input file and the list of
140 passes, go to the section on <a href="#passes">debugging bugs in LLVM
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145 </ul><table width="100%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0>
146 <tr><td> </td><td width="100%">
147 <font color="#EEEEFF" face="Georgia,Palatino"><b>
148 <a name="gccld">GCCLD bugs
149 </b></font></td></tr></table><ul>
151 If you find that a bug crashes in the <tt><b>gccld</b></tt> stage of
152 compilation, gather all of the <tt>.o</tt> bytecode files and libraries that are
153 being linked together (the "<tt><b>llvm-gcc</b> -v</tt>" output should include
154 the full list of objects linked). Then run:<p>
157 <b>llvm-as</b> < /dev/null > null.bc
158 <b>gccld</b> -debug-pass=Arguments null.bc
161 ... which will print a list of arguments, indicating the list of passes that
162 <tt><b>gccld</b></tt> runs. Once you have the input files and the list of
163 passes, go to the section on <a href="#passes">debugging bugs in LLVM
166 <!-- ======================================================================= -->
167 </ul><table width="100%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0>
168 <tr><td> </td><td width="100%">
169 <font color="#EEEEFF" face="Georgia,Palatino"><b>
170 <a name="passes">Bugs in LLVM passes
171 </b></font></td></tr></table><ul>
173 At this point, you should have some number of LLVM assembly files or bytecode
174 files and a list of passes which crash when run on the specified input. In
175 order to reduce the list of passes (which is probably large) and the input to
176 something tractable, use the <tt><b>bugpoint</b></tt> tool as follows:<p>
179 <b>bugpoint</b> <input files> <list of passes>
182 <tt><b>bugpoint</b></tt> will print a bunch of output as it reduces the
183 test-case, but it should eventually print something like this:<p>
187 Emitted bytecode to 'bugpoint-reduced-simplified.bc'
189 *** You can reproduce the problem with: opt bugpoint-reduced-simplified.bc -licm
192 Once you complete this, please send the LLVM bytecode file and the command line
193 to reproduce the problem to the llvmbugs mailing list.<p>
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197 </ul><table width="100%" bgcolor="#330077" border=0 cellpadding=4 cellspacing=0>
198 <tr><td align=center><font color="#EEEEFF" size=+2 face="Georgia,Palatino"><b>
199 <a name="miscompilations">Miscompilations</a>
200 </b></font></td></tr></table><ul>
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203 A miscompilation occurs when a pass does not correctly transform a program, thus
204 producing errors that are only noticed during execution. This is different from
205 producing invalid LLVM code (i.e., code not in SSA form, using values before
206 defining them, etc.) which the verifier will check for after a pass finishes its
209 To debug a miscompilation, you should choose which program you wish to run the
210 output through, e.g. C backend, the JIT, or LLC, and a selection of passes, one
211 of which may be causing the error, and run, for example:
214 <b>bugpoint</b> -run-cbe [... optimization passes ...] file-to-test.bc
217 <tt>bugpoint</tt> will try to narrow down your list of passes to the one pass
218 that causes an error, and simplify the bytecode file as much as it can to assist
219 you. It will print a message letting you know how to reproduce the resulting
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223 </ul><table width="100%" bgcolor="#330077" border=0 cellpadding=4 cellspacing=0>
224 <tr><td align=center><font color="#EEEEFF" size=+2 face="Georgia,Palatino"><b>
225 <a name="codegen">Incorrect code generation</a>
226 </b></font></td></tr></table><ul>
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229 Similarly to debugging incorrect compilation by mis-behaving passes, you can
230 debug incorrect code generation by either LLC or the JIT, using
231 <tt>bugpoint</tt>. The process <tt>bugpoint</tt> follows in this case is to try
232 to narrow the code down to a function that is miscompiled by one or the other
233 method, but since for correctness, the entire program must be run,
234 <tt>bugpoint</tt> will compile the code it deems to not be affected with the C
235 Backend, and then link in the shared object it generates.<p>
239 <b>bugpoint</b> -run-jit -output=[correct output file] [bytecodefile]
242 Similarly, to debug the LLC, one would run:
244 <b>bugpoint</b> -run-llc -output=[correct output file] [bytecodefile]
247 At the end of a successful <tt>bugpoint</tt> run, you will be presented
248 with two bytecode files: a <em>safe</em> file which can be compiled with the C
249 backend and the <em>test</em> file which either LLC or the JIT
250 mis-codegenerates, and thus causes the error.<p>
252 To reproduce the error that <tt>bugpoint</tt> found, it is sufficient to do the
256 <li>Regenerate the shared object from the safe bytecode file:<br>
258 <b>llvm-dis</b> -c safe.bc -o safe.c<br>
259 <b>gcc</b> -shared safe.c -o safe.so
261 <li>If debugging LLC, compile test bytecode native and link with the shared object:<br>
263 <b>llc</b> test.bc -o test.s -f<br>
264 gcc test.s safe.so -o test.llc<br>
265 ./test.llc [program options]
268 If debugging the JIT, load the shared object and supply the test bytecode:<br>
270 <b>lli</b> -load=safe.so test.bc [program options]
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279 <address><a href="mailto:sabre@nondot.org">Chris Lattner</a></address>
280 <!-- Created: Tue Aug 6 15:00:33 CDT 2002 -->
282 Last modified: Tue Oct 14 15:57:47 CDT 2003
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