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<div class="section" id="bisecting-llvm-code">
<h1>Bisecting LLVM code<a class="headerlink" href="#bisecting-llvm-code" title="Permalink to this headline">¶</a></h1>
<div class="section" id="introduction">
<h2>Introduction<a class="headerlink" href="#introduction" title="Permalink to this headline">¶</a></h2>
<p><code class="docutils literal notranslate"><span class="pre">git</span> <span class="pre">bisect</span></code> is a useful tool for finding which revision caused a bug.</p>
<p>This document describes how to use <code class="docutils literal notranslate"><span class="pre">git</span> <span class="pre">bisect</span></code>. In particular, while LLVM
has a mostly linear history, it has a few merge commits that added projects –
and these merged the linear history of those projects. As a consequence, the
LLVM repository has multiple roots: One “normal” root, and then one for each
toplevel project that was developed out-of-tree and then merged later.
As of early 2020, the only such merged project is MLIR, but flang will likely
be merged in a similar way soon.</p>
</div>
<div class="section" id="basic-operation">
<h2>Basic operation<a class="headerlink" href="#basic-operation" title="Permalink to this headline">¶</a></h2>
<p>See <a class="reference external" href="https://git-scm.com/docs/git-bisect">https://git-scm.com/docs/git-bisect</a> for a good overview. In summary:</p>
<blockquote>
<div><div class="highlight-bash notranslate"><div class="highlight"><pre><span></span>git bisect start
git bisect bad main
git bisect good f00ba
</pre></div>
</div>
</div></blockquote>
<p>git will check out a revision in between. Try to reproduce your problem at
that revision, and run <code class="docutils literal notranslate"><span class="pre">git</span> <span class="pre">bisect</span> <span class="pre">good</span></code> or <code class="docutils literal notranslate"><span class="pre">git</span> <span class="pre">bisect</span> <span class="pre">bad</span></code>.</p>
<p>If you can’t repro at the current commit (maybe the build is broken), run
<code class="docutils literal notranslate"><span class="pre">git</span> <span class="pre">bisect</span> <span class="pre">skip</span></code> and git will pick a nearby alternate commit.</p>
<p>(To abort a bisect, run <code class="docutils literal notranslate"><span class="pre">git</span> <span class="pre">bisect</span> <span class="pre">reset</span></code>, and if git complains about not
being able to reset, do the usual <code class="docutils literal notranslate"><span class="pre">git</span> <span class="pre">checkout</span> <span class="pre">-f</span> <span class="pre">main;</span> <span class="pre">git</span> <span class="pre">reset</span> <span class="pre">--hard</span>
<span class="pre">origin/main</span></code> dance and try again).</p>
</div>
<div class="section" id="git-bisect-run">
<h2><code class="docutils literal notranslate"><span class="pre">git</span> <span class="pre">bisect</span> <span class="pre">run</span></code><a class="headerlink" href="#git-bisect-run" title="Permalink to this headline">¶</a></h2>
<p>A single bisect step often requires first building clang, and then compiling
a large code base with just-built clang. This can take a long time, so it’s
good if it can happen completely automatically. <code class="docutils literal notranslate"><span class="pre">git</span> <span class="pre">bisect</span> <span class="pre">run</span></code> can do
this for you if you write a run script that reproduces the problem
automatically. Writing the script can take 10-20 minutes, but it’s almost
always worth it – you can do something else while the bisect runs (such
as writing this document).</p>
<p>Here’s an example run script. It assumes that you’re in <code class="docutils literal notranslate"><span class="pre">llvm-project</span></code> and
that you have a sibling <code class="docutils literal notranslate"><span class="pre">llvm-build-project</span></code> build directory where you
configured CMake to use Ninja. You have a file <code class="docutils literal notranslate"><span class="pre">repro.c</span></code> in the current
directory that makes clang crash at trunk, but it worked fine at revision
<code class="docutils literal notranslate"><span class="pre">f00ba</span></code>.</p>
<blockquote>
<div><div class="highlight-bash notranslate"><div class="highlight"><pre><span></span><span class="c1"># Build clang. If the build fails, `exit 125` causes this</span>
<span class="c1"># revision to be skipped</span>
ninja -C ../llvm-build-project clang <span class="o">||</span> <span class="nb">exit</span> <span class="m">125</span>
../llvm-build-project/bin/clang repro.c
</pre></div>
</div>
</div></blockquote>
<p>To make sure your run script works, it’s a good idea to run <code class="docutils literal notranslate"><span class="pre">./run.sh</span></code> by
hand and tweak the script until it works, then run <code class="docutils literal notranslate"><span class="pre">git</span> <span class="pre">bisect</span> <span class="pre">good</span></code> or
<code class="docutils literal notranslate"><span class="pre">git</span> <span class="pre">bisect</span> <span class="pre">bad</span></code> manually once based on the result of the script
(check <code class="docutils literal notranslate"><span class="pre">echo</span> <span class="pre">$?</span></code> after your script ran), and only then run <code class="docutils literal notranslate"><span class="pre">git</span> <span class="pre">bisect</span> <span class="pre">run</span>
<span class="pre">./run.sh</span></code>. Don’t forget to mark your run script as executable – <code class="docutils literal notranslate"><span class="pre">git</span> <span class="pre">bisect</span>
<span class="pre">run</span></code> doesn’t check for that, it just assumes the run script failed each time.</p>
<p>Once your run script works, run <code class="docutils literal notranslate"><span class="pre">git</span> <span class="pre">bisect</span> <span class="pre">run</span> <span class="pre">./run.sh</span></code> and a few hours
later you’ll know which commit caused the regression.</p>
<p>(This is a very simple run script. Often, you want to use just-built clang
to build a different project and then run a built executable of that project
in the run script.)</p>
</div>
<div class="section" id="bisecting-across-multiple-roots">
<h2>Bisecting across multiple roots<a class="headerlink" href="#bisecting-across-multiple-roots" title="Permalink to this headline">¶</a></h2>
<p>Here’s how LLVM’s history currently looks:</p>
<blockquote>
<div><div class="highlight-none notranslate"><div class="highlight"><pre><span></span>A-o-o-......-o-D-o-o-HEAD
/
B-o-...-o-C-
</pre></div>
</div>
</div></blockquote>
<p><code class="docutils literal notranslate"><span class="pre">A</span></code> is the first commit in LLVM ever, <code class="docutils literal notranslate"><span class="pre">97724f18c79c</span></code>.</p>
<p><code class="docutils literal notranslate"><span class="pre">B</span></code> is the first commit in MLIR, <code class="docutils literal notranslate"><span class="pre">aed0d21a62db</span></code>.</p>
<p><code class="docutils literal notranslate"><span class="pre">D</span></code> is the merge commit that merged MLIR into the main LLVM repository,
<code class="docutils literal notranslate"><span class="pre">0f0d0ed1c78f</span></code>.</p>
<p><code class="docutils literal notranslate"><span class="pre">C</span></code> is the last commit in MLIR before it got merged, <code class="docutils literal notranslate"><span class="pre">0f0d0ed1c78f^2</span></code>. (The
<code class="docutils literal notranslate"><span class="pre">^n</span></code> modifier selects the n’th parent of a merge commit.)</p>
<p><code class="docutils literal notranslate"><span class="pre">git</span> <span class="pre">bisect</span></code> goes through all parent revisions. Due to the way MLIR was
merged, at every revision at <code class="docutils literal notranslate"><span class="pre">C</span></code> or earlier, <em>only</em> the <code class="docutils literal notranslate"><span class="pre">mlir/</span></code> directory
exists, and nothing else does.</p>
<p>As of early 2020, there is no flag to <code class="docutils literal notranslate"><span class="pre">git</span> <span class="pre">bisect</span></code> to tell it to not
descend into all reachable commits. Ideally, we’d want to tell it to only
follow the first parent of <code class="docutils literal notranslate"><span class="pre">D</span></code>.</p>
<p>The best workaround is to pass a list of directories to <code class="docutils literal notranslate"><span class="pre">git</span> <span class="pre">bisect</span></code>:
If you know the bug is due to a change in llvm, clang, or compiler-rt, use</p>
<blockquote>
<div><div class="highlight-bash notranslate"><div class="highlight"><pre><span></span>git bisect start -- clang llvm compiler-rt
</pre></div>
</div>
</div></blockquote>
<p>That way, the commits in <code class="docutils literal notranslate"><span class="pre">mlir</span></code> are never evaluated.</p>
<p>Alternatively, <code class="docutils literal notranslate"><span class="pre">git</span> <span class="pre">bisect</span> <span class="pre">skip</span> <span class="pre">aed0d21a6</span> <span class="pre">aed0d21a6..0f0d0ed1c78f</span></code> explicitly
skips all commits on that branch. It takes 1.5 minutes to run on a fast
machine, and makes <code class="docutils literal notranslate"><span class="pre">git</span> <span class="pre">bisect</span> <span class="pre">log</span></code> output unreadable. (<code class="docutils literal notranslate"><span class="pre">aed0d21a6</span></code> is
listed twice because git ranges exclude the revision listed on the left,
so it needs to be ignored explicitly.)</p>
</div>
<div class="section" id="more-resources">
<h2>More Resources<a class="headerlink" href="#more-resources" title="Permalink to this headline">¶</a></h2>
<p><a class="reference external" href="https://git-scm.com/book/en/v2/Git-Tools-Revision-Selection">https://git-scm.com/book/en/v2/Git-Tools-Revision-Selection</a></p>
</div>
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