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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 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="#ct_optimizer">Compile-time optimization bugs</a>
23 <li><a href="#ct_codegen">Code generator bugs</a>
25 <li><a href="#miscompilations">Miscompilations</a></li>
26 <li><a href="#codegen">Incorrect code generation (JIT and LLC)</a></li>
28 <div class="doc_author">
29 <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a> and
30 <a href="http://misha.brukman.net">Misha Brukman</a></p>
34 <img src="img/Debugging.gif" alt="Debugging" width="444" height="314">
39 <!-- *********************************************************************** -->
40 <div class="doc_section">
41 <a name="introduction">Introduction - Got bugs?</a>
43 <!-- *********************************************************************** -->
45 <div class="doc_text">
47 <p>If you're working with LLVM and run into a bug, we definitely want to know
48 about it. This document describes what you can do to increase the odds of
49 getting it fixed quickly.</p>
51 <p>Basically you have to do two things at a minimum. First, decide whether the
52 bug <a href="#crashers">crashes the compiler</a> (or an LLVM pass), or if the
53 compiler is <a href="#miscompilations">miscompiling</a> the program (i.e., the
54 compiler successfully produces an executable, but it doesn't run right). Based
56 what type of bug it is, follow the instructions in the linked section to narrow
57 down the bug so that the person who fixes it will be able to find the problem
60 <p>Once you have a reduced test-case, go to <a
61 href="http://llvm.org/bugs/enter_bug.cgi">the LLVM Bug Tracking
62 System</a> and fill out the form with the necessary details (note that you don't
63 need to pick a catagory, just use the "new-bugs" catagory if you're not sure).
64 The bug description should contain the following
68 <li>All information necessary to reproduce the problem.</li>
69 <li>The reduced test-case that triggers the bug.</li>
70 <li>The location where you obtained LLVM (if not from our Subversion
74 <p>Thanks for helping us make LLVM better!</p>
78 <!-- *********************************************************************** -->
79 <div class="doc_section">
80 <a name="crashers">Crashing Bugs</a>
82 <!-- *********************************************************************** -->
84 <div class="doc_text">
86 <p>More often than not, bugs in the compiler cause it to crash—often due
87 to an assertion failure of some sort. The most important
88 piece of the puzzle is to figure out if it is crashing in the GCC front-end
89 or if it is one of the LLVM libraries (e.g. the optimizer or code generator)
90 that has problems.</p>
92 <p>To figure out which component is crashing (the front-end,
93 optimizer or code generator), run the
94 <tt><b>llvm-gcc</b></tt> command line as you were when the crash occurred, but
95 with the following extra command line options:</p>
98 <li><tt><b>-O0 -emit-llvm</b></tt>: If <tt>llvm-gcc</tt> still crashes when
99 passed these options (which disable the optimizer and code generator), then
100 the crash is in the front-end. Jump ahead to the section on <a
101 href="#front-end">front-end bugs</a>.</li>
103 <li><tt><b>-emit-llvm</b></tt>: If <tt>llvm-gcc</tt> crashes with this option
104 (which disables the code generator), you found an optimizer bug. Jump ahead
105 to <a href="#ct_optimizer"> compile-time optimization bugs</a>.</li>
107 <li>Otherwise, you have a code generator crash. Jump ahead to <a
108 href="#ct_codegen">code generator bugs</a>.</li>
114 <!-- ======================================================================= -->
115 <div class="doc_subsection">
116 <a name="front-end">Front-end bugs</a>
119 <div class="doc_text">
121 <p>If the problem is in the front-end, you should re-run the same
122 <tt>llvm-gcc</tt> command that resulted in the crash, but add the
123 <tt>-save-temps</tt> option. The compiler will crash again, but it will leave
124 behind a <tt><i>foo</i>.i</tt> file (containing preprocessed C source code) and
125 possibly <tt><i>foo</i>.s</tt> for each
126 compiled <tt><i>foo</i>.c</tt> file. Send us the <tt><i>foo</i>.i</tt> file,
127 along with the options you passed to llvm-gcc, and a brief description of the
130 <p>The <a href="http://delta.tigris.org/">delta</a> tool helps to reduce the
131 preprocessed file down to the smallest amount of code that still replicates the
132 problem. You're encouraged to use delta to reduce the code to make the
133 developers' lives easier. <a
134 href="http://gcc.gnu.org/wiki/A_guide_to_testcase_reduction">This website</a>
135 has instructions on the best way to use delta.</p>
139 <!-- ======================================================================= -->
140 <div class="doc_subsection">
141 <a name="ct_optimizer">Compile-time optimization bugs</a>
144 <div class="doc_text">
146 <p>If you find that a bug crashes in the optimizer, compile your test-case to a
147 <tt>.bc</tt> file by passing "<tt><b>-emit-llvm -O0 -c -o foo.bc</b></tt>".
150 <div class="doc_code">
151 <p><tt><b>opt</b> -std-compile-opts -debug-pass=Arguments foo.bc
152 -disable-output</tt></p>
155 <p>This command should do two things: it should print out a list of passes, and
156 then it should crash in the same was as llvm-gcc. If it doesn't crash, please
157 follow the instructions for a <a href="#front-end">front-end bug</a>.</p>
159 <p>If this does crash, then you should be able to debug this with the following
160 bugpoint command:</p>
162 <div class="doc_code">
163 <p><tt><b>bugpoint</b> foo.bc <list of passes printed by
164 <b>opt</b>></tt></p>
167 <p>Please run this, then file a bug with the instructions and reduced .bc files
168 that bugpoint emits. If something goes wrong with bugpoint, please submit the
169 "foo.bc" file and the list of passes printed by <b>opt</b>.</p>
173 <!-- ======================================================================= -->
174 <div class="doc_subsection">
175 <a name="ct_codegen">Code generator bugs</a>
178 <div class="doc_text">
180 <p>If you find a bug that crashes llvm-gcc in the code generator, compile your
181 source file to a .bc file by passing "<tt><b>-emit-llvm -c -o foo.bc</b></tt>"
182 to llvm-gcc (in addition to the options you already pass). Once your have
183 foo.bc, one of the following commands should fail:</p>
186 <li><tt><b>llc</b> foo.bc -f</tt></li>
187 <li><tt><b>llc</b> foo.bc -f -relocation-model=pic</tt></li>
188 <li><tt><b>llc</b> foo.bc -f -relocation-model=static</tt></li>
189 <li><tt><b>llc</b> foo.bc -f -enable-eh</tt></li>
190 <li><tt><b>llc</b> foo.bc -f -relocation-model=pic -enable-eh</tt></li>
191 <li><tt><b>llc</b> foo.bc -f -relocation-model=static -enable-eh</tt></li>
194 <p>If none of these crash, please follow the instructions for a
195 <a href="#front-end">front-end bug</a>. If one of these do crash, you should
196 be able to reduce this with one of the following bugpoint command lines (use
197 the one corresponding to the command above that failed):</p>
200 <li><tt><b>bugpoint</b> -run-llc foo.bc</tt></li>
201 <li><tt><b>bugpoint</b> -run-llc foo.bc --tool-args
202 -relocation-model=pic</tt></li>
203 <li><tt><b>bugpoint</b> -run-llc foo.bc --tool-args
204 -relocation-model=static</tt></li>
205 <li><tt><b>bugpoint</b> -run-llc foo.bc --tool-args -enable-eh</tt></li>
206 <li><tt><b>bugpoint</b> -run-llc foo.bc --tool-args
207 -relocation-model=pic -enable-eh</tt></li>
208 <li><tt><b>bugpoint</b> -run-llc foo.bc --tool-args
209 -relocation-model=static -enable-eh</tt></li>
212 <p>Please run this, then file a bug with the instructions and reduced .bc file
213 that bugpoint emits. If something goes wrong with bugpoint, please submit the
214 "foo.bc" file and the option that llc crashes with.</p>
218 <!-- *********************************************************************** -->
219 <div class="doc_section">
220 <a name="miscompilations">Miscompilations</a>
222 <!-- *********************************************************************** -->
224 <div class="doc_text">
226 <p>If llvm-gcc successfully produces an executable, but that executable doesn't
227 run right, this is either a bug in the code or a bug in the
228 compiler. The first thing to check is to make sure it is not using undefined
229 behavior (e.g. reading a variable before it is defined). In particular, check
230 to see if the program <a href="http://valgrind.org/">valgrind</a>s clean,
231 passes purify, or some other memory checker tool. Many of the "LLVM bugs" that
232 we have chased down ended up being bugs in the program being compiled, not
235 <p>Once you determine that the program itself is not buggy, you should choose
236 which code generator you wish to compile the program with (e.g. C backend, the
237 JIT, or LLC) and optionally a series of LLVM passes to run. For example:</p>
239 <div class="doc_code">
241 <b>bugpoint</b> -run-cbe [... optzn passes ...] file-to-test.bc --args -- [program arguments]</tt></p>
244 <p><tt>bugpoint</tt> will try to narrow down your list of passes to the one pass
245 that causes an error, and simplify the bitcode file as much as it can to assist
246 you. It will print a message letting you know how to reproduce the resulting
251 <!-- *********************************************************************** -->
252 <div class="doc_section">
253 <a name="codegen">Incorrect code generation</a>
255 <!-- *********************************************************************** -->
257 <div class="doc_text">
259 <p>Similarly to debugging incorrect compilation by mis-behaving passes, you can
260 debug incorrect code generation by either LLC or the JIT, using
261 <tt>bugpoint</tt>. The process <tt>bugpoint</tt> follows in this case is to try
262 to narrow the code down to a function that is miscompiled by one or the other
263 method, but since for correctness, the entire program must be run,
264 <tt>bugpoint</tt> will compile the code it deems to not be affected with the C
265 Backend, and then link in the shared object it generates.</p>
267 <p>To debug the JIT:</p>
269 <div class="doc_code">
271 bugpoint -run-jit -output=[correct output file] [bitcode file] \
272 --tool-args -- [arguments to pass to lli] \
273 --args -- [program arguments]
277 <p>Similarly, to debug the LLC, one would run:</p>
279 <div class="doc_code">
281 bugpoint -run-llc -output=[correct output file] [bitcode file] \
282 --tool-args -- [arguments to pass to llc] \
283 --args -- [program arguments]
287 <p><b>Special note:</b> if you are debugging MultiSource or SPEC tests that
288 already exist in the <tt>llvm/test</tt> hierarchy, there is an easier way to
289 debug the JIT, LLC, and CBE, using the pre-written Makefile targets, which
290 will pass the program options specified in the Makefiles:</p>
292 <div class="doc_code">
294 cd llvm/test/../../program<br>
299 <p>At the end of a successful <tt>bugpoint</tt> run, you will be presented
300 with two bitcode files: a <em>safe</em> file which can be compiled with the C
301 backend and the <em>test</em> file which either LLC or the JIT
302 mis-codegenerates, and thus causes the error.</p>
304 <p>To reproduce the error that <tt>bugpoint</tt> found, it is sufficient to do
309 <li><p>Regenerate the shared object from the safe bitcode file:</p>
311 <div class="doc_code">
313 <b>llc</b> -march=c safe.bc -o safe.c<br>
314 <b>gcc</b> -shared safe.c -o safe.so
318 <li><p>If debugging LLC, compile test bitcode native and link with the shared
321 <div class="doc_code">
323 <b>llc</b> test.bc -o test.s -f<br>
324 <b>gcc</b> test.s safe.so -o test.llc<br>
325 ./test.llc [program options]
329 <li><p>If debugging the JIT, load the shared object and supply the test
332 <div class="doc_code">
333 <p><tt><b>lli</b> -load=safe.so test.bc [program options]</tt></p>
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348 <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
349 <a href="http://llvm.org">The LLVM Compiler Infrastructure</a>
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