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<div class="section" id="how-to-submit-an-llvm-bug-report">
<h1>How to submit an LLVM bug report<a class="headerlink" href="#how-to-submit-an-llvm-bug-report" title="Permalink to this headline">¶</a></h1>
<div class="section" id="introduction-got-bugs">
<h2>Introduction - Got bugs?<a class="headerlink" href="#introduction-got-bugs" title="Permalink to this headline">¶</a></h2>
<p>If you’re working with LLVM and run into a bug, we definitely want to know
about it. This document describes what you can do to increase the odds of
getting it fixed quickly.</p>
<p>Basically you have to do two things at a minimum. First, decide whether
the bug <a class="reference internal" href="#crashes-the-compiler">crashes the compiler</a> (or an LLVM pass), or if the
compiler is <a class="reference internal" href="#miscompiling">miscompiling</a> the program (i.e., the
compiler successfully produces an executable, but it doesn’t run right).
Based on what type of bug it is, follow the instructions in the linked
section to narrow down the bug so that the person who fixes it will be able
to find the problem more easily.</p>
<p>Once you have a reduced test-case, go to <a class="reference external" href="https://bugs.llvm.org/enter_bug.cgi">the LLVM Bug Tracking System</a> and fill out the form with the
necessary details (note that you don’t need to pick a category, just use
the “new-bugs” category if you’re not sure). The bug description should
contain the following information:</p>
<ul class="simple">
<li>All information necessary to reproduce the problem.</li>
<li>The reduced test-case that triggers the bug.</li>
<li>The location where you obtained LLVM (if not from our Subversion
repository).</li>
</ul>
<p>Thanks for helping us make LLVM better!</p>
</div>
<div class="section" id="crashing-bugs">
<span id="crashes-the-compiler"></span><h2>Crashing Bugs<a class="headerlink" href="#crashing-bugs" title="Permalink to this headline">¶</a></h2>
<p>More often than not, bugs in the compiler cause it to crash—often due to
an assertion failure of some sort. The most important piece of the puzzle
is to figure out if it is crashing in the Clang front-end or if it is one of
the LLVM libraries (e.g. the optimizer or code generator) that has
problems.</p>
<p>To figure out which component is crashing (the front-end, optimizer or code
generator), run the <code class="docutils literal notranslate"><span class="pre">clang</span></code> command line as you were when the crash
occurred, but with the following extra command line options:</p>
<ul class="simple">
<li><code class="docutils literal notranslate"><span class="pre">-O0</span> <span class="pre">-emit-llvm</span></code>: If <code class="docutils literal notranslate"><span class="pre">clang</span></code> still crashes when passed these
options (which disable the optimizer and code generator), then the crash
is in the front-end. Jump ahead to the section on <a class="reference internal" href="#front-end"><span class="std std-ref">front-end bugs</span></a>.</li>
<li><code class="docutils literal notranslate"><span class="pre">-emit-llvm</span></code>: If <code class="docutils literal notranslate"><span class="pre">clang</span></code> crashes with this option (which disables
the code generator), you found an optimizer bug. Jump ahead to
<a class="reference internal" href="#compile-time-optimization-bugs">compile-time optimization bugs</a>.</li>
<li>Otherwise, you have a code generator crash. Jump ahead to <a class="reference internal" href="#code-generator-bugs">code
generator bugs</a>.</li>
</ul>
<div class="section" id="front-end-bugs">
<span id="front-end"></span><span id="front-end-bug"></span><h3>Front-end bugs<a class="headerlink" href="#front-end-bugs" title="Permalink to this headline">¶</a></h3>
<p>If the problem is in the front-end, you should re-run the same <code class="docutils literal notranslate"><span class="pre">clang</span></code>
command that resulted in the crash, but add the <code class="docutils literal notranslate"><span class="pre">-save-temps</span></code> option.
The compiler will crash again, but it will leave behind a <code class="docutils literal notranslate"><span class="pre">foo.i</span></code> file
(containing preprocessed C source code) and possibly <code class="docutils literal notranslate"><span class="pre">foo.s</span></code> for each
compiled <code class="docutils literal notranslate"><span class="pre">foo.c</span></code> file. Send us the <code class="docutils literal notranslate"><span class="pre">foo.i</span></code> file, along with the options
you passed to <code class="docutils literal notranslate"><span class="pre">clang</span></code>, and a brief description of the error it caused.</p>
<p>The <a class="reference external" href="http://delta.tigris.org/">delta</a> tool helps to reduce the
preprocessed file down to the smallest amount of code that still replicates
the problem. You’re encouraged to use delta to reduce the code to make the
developers’ lives easier. <a class="reference external" href="http://gcc.gnu.org/wiki/A_guide_to_testcase_reduction">This website</a> has instructions
on the best way to use delta.</p>
</div>
<div class="section" id="compile-time-optimization-bugs">
<span id="id1"></span><h3>Compile-time optimization bugs<a class="headerlink" href="#compile-time-optimization-bugs" title="Permalink to this headline">¶</a></h3>
<p>If you find that a bug crashes in the optimizer, compile your test-case to a
<code class="docutils literal notranslate"><span class="pre">.bc</span></code> file by passing “<code class="docutils literal notranslate"><span class="pre">-emit-llvm</span> <span class="pre">-O0</span> <span class="pre">-c</span> <span class="pre">-o</span> <span class="pre">foo.bc</span></code>”.
Then run:</p>
<div class="highlight-bash notranslate"><div class="highlight"><pre><span></span>opt -O3 -debug-pass<span class="o">=</span>Arguments foo.bc -disable-output
</pre></div>
</div>
<p>This command should do two things: it should print out a list of passes, and
then it should crash in the same way as clang. If it doesn’t crash, please
follow the instructions for a <a class="reference internal" href="#front-end-bug">front-end bug</a>.</p>
<p>If this does crash, then you should be able to debug this with the following
bugpoint command:</p>
<div class="highlight-bash notranslate"><div class="highlight"><pre><span></span>bugpoint foo.bc <list of passes printed by opt>
</pre></div>
</div>
<p>Please run this, then file a bug with the instructions and reduced .bc
files that bugpoint emits. If something goes wrong with bugpoint, please
submit the “foo.bc” file and the list of passes printed by <code class="docutils literal notranslate"><span class="pre">opt</span></code>.</p>
</div>
<div class="section" id="code-generator-bugs">
<span id="id2"></span><h3>Code generator bugs<a class="headerlink" href="#code-generator-bugs" title="Permalink to this headline">¶</a></h3>
<p>If you find a bug that crashes clang in the code generator, compile your
source file to a .bc file by passing “<code class="docutils literal notranslate"><span class="pre">-emit-llvm</span> <span class="pre">-c</span> <span class="pre">-o</span> <span class="pre">foo.bc</span></code>” to
clang (in addition to the options you already pass). Once your have
foo.bc, one of the following commands should fail:</p>
<ol class="arabic simple">
<li><code class="docutils literal notranslate"><span class="pre">llc</span> <span class="pre">foo.bc</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">llc</span> <span class="pre">foo.bc</span> <span class="pre">-relocation-model=pic</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">llc</span> <span class="pre">foo.bc</span> <span class="pre">-relocation-model=static</span></code></li>
</ol>
<p>If none of these crash, please follow the instructions for a <a class="reference internal" href="#front-end-bug">front-end
bug</a>. If one of these do crash, you should be able to reduce this with
one of the following bugpoint command lines (use the one corresponding to
the command above that failed):</p>
<ol class="arabic simple">
<li><code class="docutils literal notranslate"><span class="pre">bugpoint</span> <span class="pre">-run-llc</span> <span class="pre">foo.bc</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">bugpoint</span> <span class="pre">-run-llc</span> <span class="pre">foo.bc</span> <span class="pre">--tool-args</span> <span class="pre">-relocation-model=pic</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">bugpoint</span> <span class="pre">-run-llc</span> <span class="pre">foo.bc</span> <span class="pre">--tool-args</span> <span class="pre">-relocation-model=static</span></code></li>
</ol>
<p>Please run this, then file a bug with the instructions and reduced .bc file
that bugpoint emits. If something goes wrong with bugpoint, please submit
the “foo.bc” file and the option that llc crashes with.</p>
</div>
</div>
<div class="section" id="miscompilations">
<span id="miscompiling"></span><h2>Miscompilations<a class="headerlink" href="#miscompilations" title="Permalink to this headline">¶</a></h2>
<p>If clang successfully produces an executable, but that executable
doesn’t run right, this is either a bug in the code or a bug in the
compiler. The first thing to check is to make sure it is not using
undefined behavior (e.g. reading a variable before it is defined). In
particular, check to see if the program <a class="reference external" href="http://valgrind.org/">valgrind</a>’s clean, passes purify, or some other memory
checker tool. Many of the “LLVM bugs” that we have chased down ended up
being bugs in the program being compiled, not LLVM.</p>
<p>Once you determine that the program itself is not buggy, you should choose
which code generator you wish to compile the program with (e.g. LLC or the JIT)
and optionally a series of LLVM passes to run. For example:</p>
<div class="highlight-bash notranslate"><div class="highlight"><pre><span></span>bugpoint -run-llc <span class="o">[</span>... optzn passes ...<span class="o">]</span> file-to-test.bc --args -- <span class="o">[</span>program arguments<span class="o">]</span>
</pre></div>
</div>
<p>bugpoint will try to narrow down your list of passes to the one pass that
causes an error, and simplify the bitcode file as much as it can to assist
you. It will print a message letting you know how to reproduce the
resulting error.</p>
</div>
<div class="section" id="incorrect-code-generation">
<h2>Incorrect code generation<a class="headerlink" href="#incorrect-code-generation" title="Permalink to this headline">¶</a></h2>
<p>Similarly to debugging incorrect compilation by mis-behaving passes, you
can debug incorrect code generation by either LLC or the JIT, using
<code class="docutils literal notranslate"><span class="pre">bugpoint</span></code>. The process <code class="docutils literal notranslate"><span class="pre">bugpoint</span></code> follows in this case is to try to
narrow the code down to a function that is miscompiled by one or the other
method, but since for correctness, the entire program must be run,
<code class="docutils literal notranslate"><span class="pre">bugpoint</span></code> will compile the code it deems to not be affected with the C
Backend, and then link in the shared object it generates.</p>
<p>To debug the JIT:</p>
<div class="highlight-bash notranslate"><div class="highlight"><pre><span></span>bugpoint -run-jit -output<span class="o">=[</span>correct output file<span class="o">]</span> <span class="o">[</span>bitcode file<span class="o">]</span> <span class="se">\</span>
--tool-args -- <span class="o">[</span>arguments to pass to lli<span class="o">]</span> <span class="se">\</span>
--args -- <span class="o">[</span>program arguments<span class="o">]</span>
</pre></div>
</div>
<p>Similarly, to debug the LLC, one would run:</p>
<div class="highlight-bash notranslate"><div class="highlight"><pre><span></span>bugpoint -run-llc -output<span class="o">=[</span>correct output file<span class="o">]</span> <span class="o">[</span>bitcode file<span class="o">]</span> <span class="se">\</span>
--tool-args -- <span class="o">[</span>arguments to pass to llc<span class="o">]</span> <span class="se">\</span>
--args -- <span class="o">[</span>program arguments<span class="o">]</span>
</pre></div>
</div>
<p><strong>Special note:</strong> if you are debugging MultiSource or SPEC tests that
already exist in the <code class="docutils literal notranslate"><span class="pre">llvm/test</span></code> hierarchy, there is an easier way to
debug the JIT, LLC, and CBE, using the pre-written Makefile targets, which
will pass the program options specified in the Makefiles:</p>
<div class="highlight-bash notranslate"><div class="highlight"><pre><span></span><span class="nb">cd</span> llvm/test/../../program
make bugpoint-jit
</pre></div>
</div>
<p>At the end of a successful <code class="docutils literal notranslate"><span class="pre">bugpoint</span></code> run, you will be presented
with two bitcode files: a <em>safe</em> file which can be compiled with the C
backend and the <em>test</em> file which either LLC or the JIT
mis-codegenerates, and thus causes the error.</p>
<p>To reproduce the error that <code class="docutils literal notranslate"><span class="pre">bugpoint</span></code> found, it is sufficient to do
the following:</p>
<ol class="arabic">
<li><p class="first">Regenerate the shared object from the safe bitcode file:</p>
<div class="highlight-bash notranslate"><div class="highlight"><pre><span></span>llc -march<span class="o">=</span>c safe.bc -o safe.c
gcc -shared safe.c -o safe.so
</pre></div>
</div>
</li>
<li><p class="first">If debugging LLC, compile test bitcode native and link with the shared
object:</p>
<div class="highlight-bash notranslate"><div class="highlight"><pre><span></span>llc test.bc -o test.s
gcc test.s safe.so -o test.llc
./test.llc <span class="o">[</span>program options<span class="o">]</span>
</pre></div>
</div>
</li>
<li><p class="first">If debugging the JIT, load the shared object and supply the test
bitcode:</p>
<div class="highlight-bash notranslate"><div class="highlight"><pre><span></span>lli -load<span class="o">=</span>safe.so test.bc <span class="o">[</span>program options<span class="o">]</span>
</pre></div>
</div>
</li>
</ol>
</div>
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