1 <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
2 "http://www.w3.org/TR/html4/strict.dtd">
5 <title>How to submit an LLVM bug report</title>
6 <link rel="stylesheet" href="llvm.css" type="text/css">
10 <div class="doc_title">
11 How to submit an LLVM bug report
14 <table class="layout" style="width: 90%" >
18 <li><a href="#introduction">Introduction - Got bugs?</a></li>
19 <li><a href="#crashers">Crashing Bugs</a>
21 <li><a href="#front-end">Front-end bugs</a>
22 <li><a href="#gccas">GCCAS bugs</a>
23 <li><a href="#gccld">GCCLD bugs</a>
24 <li><a href="#passes">Bugs in LLVM passes</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 <!-- *********************************************************************** -->
41 <div class="doc_section">
42 <a name="introduction">Introduction - Got bugs?</a>
44 <!-- *********************************************************************** -->
46 <div class="doc_text">
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. Based on
55 what type of bug it is, follow the instructions in the linked section to narrow
56 down the bug so that the person who fixes it will be able to find the problem
59 <p>Once you have a reduced test-case, go to <a
60 href="http://llvm.org/bugs/enter_bug.cgi">the LLVM Bug Tracking
61 System</a>, select the category in which the bug falls, and fill out the form
62 with the necessary details. The bug description should contain the following
66 <li>All information necessary to reproduce the problem.</li>
67 <li>The reduced test-case that triggers the bug.</li>
68 <li>The location where you obtained LLVM (if not from our CVS
72 <p>Thanks for helping us make LLVM better!</p>
76 <!-- *********************************************************************** -->
77 <div class="doc_section">
78 <a name="crashers">Crashing Bugs</a>
80 <!-- *********************************************************************** -->
82 <div class="doc_text">
84 <p>More often than not, bugs in the compiler cause it to crash - often due to an
85 assertion failure of some sort. If you are running <tt><b>opt</b></tt>
86 directly, and something crashes, jump to the section on
87 <a href="#passes">bugs in LLVM passes</a>. Otherwise, the most important
88 piece of the puzzle is to figure out if it is the GCC-based front-end that is
89 buggy or if it's one of the LLVM tools that has problems.</p>
91 <p>To figure out which program is crashing (the front-end,
92 <tt><b>gccas</b></tt>, or <tt><b>gccld</b></tt>), run the
93 <tt><b>llvm-gcc</b></tt> command line as you were when the crash occurred, but
94 add a <tt>-v</tt> option to the command line. The compiler will print out a
95 bunch of stuff, and should end with telling you that one of
96 <tt><b>cc1</b>/<b>cc1plus</b></tt>, <tt><b>gccas</b></tt>, or
97 <tt><b>gccld</b></tt> crashed.</p>
101 <li>If <tt><b>cc1</b></tt> or <tt><b>cc1plus</b></tt> crashed, you found a
102 problem with the front-end.
103 Jump ahead to the section on <a href="#front-end">front-end bugs</a>.</li>
105 <li>If <tt><b>gccas</b></tt> crashed, you found a bug in <a href="#gccas">one
106 of the passes in <tt><b>gccas</b></tt></a>.</li>
108 <li>If <tt><b>gccld</b></tt> crashed, you found a bug in <a href="#gccld">one
109 of the passes in <tt><b>gccld</b></tt></a>.</li>
111 <li>Otherwise, something really weird happened. Email the list with what you
112 have at this point.</li>
118 <!-- ======================================================================= -->
119 <div class="doc_subsection">
120 <a name="front-end">Front-end bugs</a>
123 <div class="doc_text">
125 <p>If the problem is in the front-end, you should re-run the same
126 <tt>llvm-gcc</tt> command that resulted in the crash, but add the
127 <tt>-save-temps</tt> option. The compiler will crash again, but it will leave
128 behind a <tt><i>foo</i>.i</tt> file (containing preprocessed C source code) and
129 possibly <tt><i>foo</i>.s</tt> (containing LLVM assembly code), for each
130 compiled <tt><i>foo</i>.c</tt> file. Send us the <tt><i>foo</i>.i</tt> file,
131 along with a brief description of the error it caused. A tool that might help
132 you reduce a front-end testcase to a more manageable size is
133 <a href="http://delta.tigris.org/">delta</a>.
138 <!-- ======================================================================= -->
139 <div class="doc_subsection">
140 <a name="gccas">GCCAS bugs</a>
143 <div class="doc_text">
145 <p>If you find that a bug crashes in the <tt><b>gccas</b></tt> stage of
146 compilation, compile your test-case to a <tt>.s</tt> file with the
147 <tt>-save-temps</tt> option to <tt><b>llvm-gcc</b></tt>. Then run:</p>
149 <div class="doc_code">
150 <p><tt><b>gccas</b> -debug-pass=Arguments < /dev/null -o - > /dev/null</tt></p>
153 <p>... which will print a list of arguments, indicating the list of passes that
154 <tt><b>gccas</b></tt> runs. Once you have the input file and the list of
155 passes, go to the section on <a href="#passes">debugging bugs in LLVM
160 <!-- ======================================================================= -->
161 <div class="doc_subsection">
162 <a name="gccld">GCCLD bugs</a>
165 <div class="doc_text">
167 <p>If you find that a bug crashes in the <tt><b>gccld</b></tt> stage of
168 compilation, gather all of the <tt>.o</tt> bytecode files and libraries that are
169 being linked together (the "<tt><b>llvm-gcc</b> -v</tt>" output should include
170 the full list of objects linked). Then run:</p>
172 <div class="doc_code">
173 <p><tt><b>llvm-as</b> < /dev/null > null.bc<br>
174 <b>gccld</b> -debug-pass=Arguments null.bc</tt>
178 <p>... which will print a list of arguments, indicating the list of passes that
179 <tt><b>gccld</b></tt> runs. Once you have the input files and the list of
180 passes, go to the section on <a href="#passes">debugging bugs in LLVM
185 <!-- ======================================================================= -->
186 <div class="doc_subsection">
187 <a name="passes">Bugs in LLVM passes</a>
190 <div class="doc_text">
192 <p>At this point, you should have some number of LLVM assembly files or bytecode
193 files and a list of passes which crash when run on the specified input. In
194 order to reduce the list of passes (which is probably large) and the input to
195 something tractable, use the <tt><b>bugpoint</b></tt> tool as follows:</p>
197 <div class="doc_code">
198 <p><tt><b>bugpoint</b> <input files> <list of passes></tt></p>
201 <p><tt><b>bugpoint</b></tt> will print a bunch of output as it reduces the
202 test-case, but it should eventually print something like this:</p>
204 <div class="doc_code">
207 Emitted bytecode to 'bugpoint-reduced-simplified.bc'<br>
209 *** You can reproduce the problem with: opt bugpoint-reduced-simplified.bc -licm<br>
213 <p>Once you complete this, please send the LLVM bytecode file and the command
214 line to reproduce the problem to the llvmbugs mailing list.</p>
218 <!-- *********************************************************************** -->
219 <div class="doc_section">
220 <a name="miscompilations">Miscompilations</a>
222 <!-- *********************************************************************** -->
224 <div class="doc_text">
226 <p>A miscompilation occurs when a pass does not correctly transform a program,
227 thus producing errors that are only noticed during execution. This is different
228 from producing invalid LLVM code (i.e., code not in SSA form, using values
229 before defining them, etc.) which the verifier will check for after a pass
230 finishes its run.</p>
232 <p>If it looks like the LLVM compiler is miscompiling a program, the very first
233 thing to check is to make sure it is not using undefined behavior. In
234 particular, check to see if the program <a
235 href="http://valgrind.kde.org/">valgrind</a>s clean, passes purify, or some
236 other memory checker tool. Many of the "LLVM bugs" that we have chased down
237 ended up being bugs in the program being compiled, not LLVM.</p>
239 <p>Once you determine that the program itself is not buggy, you should choose
240 which code generator you wish to compile the program with (e.g. C backend, the
241 JIT, or LLC) and optionally a series of LLVM passes to run. For example:</p>
243 <div class="doc_code">
245 <b>bugpoint</b> -run-cbe [... optzn passes ...] file-to-test.bc --args -- [program arguments]</tt></p>
248 <p><tt>bugpoint</tt> will try to narrow down your list of passes to the one pass
249 that causes an error, and simplify the bytecode file as much as it can to assist
250 you. It will print a message letting you know how to reproduce the resulting
255 <!-- *********************************************************************** -->
256 <div class="doc_section">
257 <a name="codegen">Incorrect code generation</a>
259 <!-- *********************************************************************** -->
261 <div class="doc_text">
263 <p>Similarly to debugging incorrect compilation by mis-behaving passes, you can
264 debug incorrect code generation by either LLC or the JIT, using
265 <tt>bugpoint</tt>. The process <tt>bugpoint</tt> follows in this case is to try
266 to narrow the code down to a function that is miscompiled by one or the other
267 method, but since for correctness, the entire program must be run,
268 <tt>bugpoint</tt> will compile the code it deems to not be affected with the C
269 Backend, and then link in the shared object it generates.</p>
271 <p>To debug the JIT:</p>
273 <div class="doc_code">
275 bugpoint -run-jit -output=[correct output file] [bytecode file] \
276 --tool-args -- [arguments to pass to lli] \
277 --args -- [program arguments]
281 <p>Similarly, to debug the LLC, one would run:</p>
283 <div class="doc_code">
285 bugpoint -run-llc -output=[correct output file] [bytecode file] \
286 --tool-args -- [arguments to pass to llc] \
287 --args -- [program arguments]
291 <p><b>Special note:</b> if you are debugging MultiSource or SPEC tests that
292 already exist in the <tt>llvm/test</tt> hierarchy, there is an easier way to
293 debug the JIT, LLC, and CBE, using the pre-written Makefile targets, which
294 will pass the program options specified in the Makefiles:</p>
296 <div class="doc_code">
298 cd llvm/test/../../program<br>
303 <p>At the end of a successful <tt>bugpoint</tt> run, you will be presented
304 with two bytecode files: a <em>safe</em> file which can be compiled with the C
305 backend and the <em>test</em> file which either LLC or the JIT
306 mis-codegenerates, and thus causes the error.</p>
308 <p>To reproduce the error that <tt>bugpoint</tt> found, it is sufficient to do
313 <li><p>Regenerate the shared object from the safe bytecode file:</p>
315 <div class="doc_code">
317 <b>llc</b> -march=c safe.bc -o safe.c<br>
318 <b>gcc</b> -shared safe.c -o safe.so
322 <li><p>If debugging LLC, compile test bytecode native and link with the shared
325 <div class="doc_code">
327 <b>llc</b> test.bc -o test.s -f<br>
328 <b>gcc</b> test.s safe.so -o test.llc<br>
329 ./test.llc [program options]
333 <li><p>If debugging the JIT, load the shared object and supply the test
336 <div class="doc_code">
337 <p><tt><b>lli</b> -load=safe.so test.bc [program options]</tt></p>
344 <!-- *********************************************************************** -->
347 <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
348 src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
349 <a href="http://validator.w3.org/check/referer"><img
350 src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!" /></a>
352 <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
353 <a href="http://llvm.org">The LLVM Compiler Infrastructure</a>
355 Last modified: $Date$