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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>
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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://llvm.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
60 <li>All information necessary to reproduce the problem.</li>
61 <li>The reduced test-case that triggers the bug.</li>
62 <li>The location where you obtained LLVM (if not from our CVS
67 Thanks for helping us make LLVM better!<p>
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71 </ul><table width="100%" bgcolor="#330077" border=0 cellpadding=4 cellspacing=0>
72 <tr><td align=center><font color="#EEEEFF" size=+2 face="Georgia,Palatino"><b>
73 <a name="crashers">Crashing Bugs
74 </b></font></td></tr></table><ul>
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77 More often than not, bugs in the compiler cause it to crash - often due to an
78 assertion failure of some sort. If you are running <tt><b>opt</b></tt> or
79 <tt><b>analyze</b></tt> directly, and something crashes, jump to the section on
80 <a href="#passes">bugs in LLVM passes</a>. Otherwise, the most important
81 piece of the puzzle is to figure out if it is the GCC-based front-end that is
82 buggy or if it's one of the LLVM tools that has problems.<p>
84 To figure out which program is crashing (the front-end, <tt><b>gccas</b></tt>,
85 or <tt><b>gccld</b></tt>), run the <tt><b>llvm-gcc</b></tt> command line as you
86 were when the crash occurred, but add a <tt>-v</tt> option to the command line.
87 The compiler will print out a bunch of stuff, and should end with telling you
88 that one of <tt><b>cc1</b></tt>, <tt><b>gccas</b></tt>, or <tt><b>gccld</b></tt>
92 <li>If <tt><b>cc1</b></tt> crashed, you found a problem with the front-end.
93 Jump ahead to the section on <a href="#front-end">front-end bugs</a>.
94 <li>If <tt><b>gccas</b></tt> crashed, you found a bug in <a href="#gccas">one of
95 the passes in <tt><b>gccas</b></tt></a>.
96 <li>If <tt><b>gccld</b></tt> crashed, you found a bug in <a href="#gccld">one
97 of the passes in <tt><b>gccld</b></tt></a>.
98 <li>Otherwise, something really weird happened. Email the list with what you
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104 </ul><table width="100%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0>
105 <tr><td> </td><td width="100%">
106 <font color="#EEEEFF" face="Georgia,Palatino"><b>
107 <a name="front-end">Front-end bugs
108 </b></font></td></tr></table><ul>
110 If the problem is in the front-end, you should re-run the same
111 <tt>llvm-gcc</tt> command that resulted in the crash, but add the
112 <tt>-save-temps</tt> option. The compiler will crash again, but it
113 will leave behind a <tt><i>foo</i>.i</tt> file (containing preprocessed
114 C source code) and possibly <tt><i>foo</i>.s</tt> (containing LLVM
115 assembly code), for each compiled <tt><i>foo</i>.c</tt> file. Send us
116 the <tt><i>foo</i>.i</tt> file, along with a brief description of the
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120 </ul><table width="100%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0>
121 <tr><td> </td><td width="100%">
122 <font color="#EEEEFF" face="Georgia,Palatino"><b>
123 <a name="gccas">GCCAS bugs
124 </b></font></td></tr></table><ul>
126 If you find that a bug crashes in the <tt><b>gccas</b></tt> stage of
127 compilation, compile your test-case to a <tt>.s</tt> file with the
128 <tt>-save-temps</tt> option to <tt><b>llvm-gcc</b></tt>. Then run:<p>
131 <b>gccas</b> -debug-pass=Arguments < /dev/null -o - > /dev/null
134 ... which will print a list of arguments, indicating the list of passes that
135 <tt><b>gccas</b></tt> runs. Once you have the input file and the list of
136 passes, go to the section on <a href="#passes">debugging bugs in LLVM
140 <!-- ======================================================================= -->
141 </ul><table width="100%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0>
142 <tr><td> </td><td width="100%">
143 <font color="#EEEEFF" face="Georgia,Palatino"><b>
144 <a name="gccld">GCCLD bugs
145 </b></font></td></tr></table><ul>
147 If you find that a bug crashes in the <tt><b>gccld</b></tt> stage of
148 compilation, gather all of the <tt>.o</tt> bytecode files and libraries that are
149 being linked together (the "<tt><b>llvm-gcc</b> -v</tt>" output should include
150 the full list of objects linked). Then run:<p>
153 <b>llvm-as</b> < /dev/null > null.bc
154 <b>gccld</b> -debug-pass=Arguments null.bc
157 ... which will print a list of arguments, indicating the list of passes that
158 <tt><b>gccld</b></tt> runs. Once you have the input files and the list of
159 passes, go to the section on <a href="#passes">debugging bugs in LLVM
162 <!-- ======================================================================= -->
163 </ul><table width="100%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0>
164 <tr><td> </td><td width="100%">
165 <font color="#EEEEFF" face="Georgia,Palatino"><b>
166 <a name="passes">Bugs in LLVM passes
167 </b></font></td></tr></table><ul>
169 At this point, you should have some number of LLVM assembly files or bytecode
170 files and a list of passes which crash when run on the specified input. In
171 order to reduce the list of passes (which is probably large) and the input to
172 something tractable, use the <tt><b>bugpoint</b></tt> tool as follows:<p>
175 <b>bugpoint</b> <input files> <list of passes>
178 <tt><b>bugpoint</b></tt> will print a bunch of output as it reduces the
179 test-case, but it should eventually print something like this:<p>
183 Emitted bytecode to 'bugpoint-reduced-simplified.bc'
185 *** You can reproduce the problem with: opt bugpoint-reduced-simplified.bc -licm
188 Once you complete this, please send the LLVM bytecode file and the command line
189 to reproduce the problem to the llvmbugs mailing list.<p>
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193 </ul><table width="100%" bgcolor="#330077" border=0 cellpadding=4 cellspacing=0>
194 <tr><td align=center><font color="#EEEEFF" size=+2 face="Georgia,Palatino"><b>
195 <a name="miscompilations">Miscompilations</a>
196 </b></font></td></tr></table><ul>
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199 A miscompilation occurs when a pass does not correctly transform a program, thus
200 producing errors that are only noticed during execution. This is different from
201 producing invalid LLVM code (i.e., code not in SSA form, using values before
202 defining them, etc.) which the verifier will check for after a pass finishes its
205 To debug a miscompilation, you should choose which program you wish to run the
206 output through, e.g. C backend, the JIT, or LLC, and a selection of passes, one
207 of which may be causing the error, and run, for example:
210 <b>bugpoint</b> -run-cbe [... optimization passes ...] file-to-test.bc
213 <tt>bugpoint</tt> will try to narrow down your list of passes to the one pass
214 that causes an error, and simplify the bytecode file as much as it can to assist
215 you. It will print a message letting you know how to reproduce the resulting
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219 </ul><table width="100%" bgcolor="#330077" border=0 cellpadding=4 cellspacing=0>
220 <tr><td align=center><font color="#EEEEFF" size=+2 face="Georgia,Palatino"><b>
221 <a name="codegen">Incorrect code generation</a>
222 </b></font></td></tr></table><ul>
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225 Similarly to debugging incorrect compilation by mis-behaving passes, you can
226 debug incorrect code generation by either LLC or the JIT, using
227 <tt>bugpoint</tt>. The process <tt>bugpoint</tt> follows in this case is to try
228 to narrow the code down to a function that is miscompiled by one or the other
229 method, but since for correctness, the entire program must be run,
230 <tt>bugpoint</tt> will compile the code it deems to not be affected with the C
231 Backend, and then link in the shared object it generates.<p>
235 <b>bugpoint</b> -run-jit -output=[correct output file] [bytecodefile]
238 Similarly, to debug the LLC, one would run:
240 <b>bugpoint</b> -run-llc -output=[correct output file] [bytecodefile]
243 At the end of a successful <tt>bugpoint</tt> run, you will be presented
244 with two bytecode files: a <em>safe</em> file which can be compiled with the C
245 backend and the <em>test</em> file which either LLC or the JIT
246 mis-codegenerates, and thus causes the error.<p>
248 To reproduce the error that <tt>bugpoint</tt> found, it is sufficient to do the
252 <li>Regenerate the shared object from the safe bytecode file:<br>
254 <b>llvm-dis</b> -c safe.bc -o safe.c<br>
255 <b>gcc</b> -shared safe.c -o safe.so
257 <li>If debugging LLC, compile test bytecode native and link with the shared object:<br>
259 <b>llc</b> test.bc -o test.s -f<br>
260 gcc test.s safe.so -o test.llc<br>
261 ./test.llc [program options]
264 If debugging the JIT, load the shared object and supply the test bytecode:<br>
266 <b>lli</b> -load=safe.so test.bc [program options]
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275 <address><a href="mailto:sabre@nondot.org">Chris Lattner</a></address>
276 <!-- Created: Tue Aug 6 15:00:33 CDT 2002 -->
278 Last modified: Tue Oct 14 15:57:47 CDT 2003
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