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<div class="section" id="writing-an-llvm-pass">
<h1>Writing an LLVM Pass<a class="headerlink" href="#writing-an-llvm-pass" title="Permalink to this headline">¶</a></h1>
<div class="contents local topic" id="contents">
<ul class="simple">
<li><p><a class="reference internal" href="#introduction-what-is-a-pass" id="id1">Introduction — What is a pass?</a></p></li>
<li><p><a class="reference internal" href="#quick-start-writing-hello-world" id="id2">Quick Start — Writing hello world</a></p>
<ul>
<li><p><a class="reference internal" href="#setting-up-the-build" id="id3">Setting up the build</a></p></li>
<li><p><a class="reference internal" href="#basic-code-required" id="id4">Basic code required</a></p></li>
<li><p><a class="reference internal" href="#running-a-pass-with-opt" id="id5">Running a pass with <code class="docutils literal notranslate"><span class="pre">opt</span></code></a></p></li>
<li><p><a class="reference internal" href="#testing-a-pass" id="id6">Testing a pass</a></p></li>
</ul>
</li>
<li><p><a class="reference internal" href="#faqs" id="id7">FAQs</a></p>
<ul>
<li><p><a class="reference internal" href="#required-passes" id="id8">Required passes</a></p></li>
</ul>
</li>
</ul>
</div>
<div class="section" id="introduction-what-is-a-pass">
<h2><a class="toc-backref" href="#id1">Introduction — What is a pass?</a><a class="headerlink" href="#introduction-what-is-a-pass" title="Permalink to this headline">¶</a></h2>
<p>The LLVM pass framework is an important part of the LLVM system, because LLVM
passes are where most of the interesting parts of the compiler exist. Passes
perform the transformations and optimizations that make up the compiler, they
build the analysis results that are used by these transformations, and they
are, above all, a structuring technique for compiler code.</p>
<p>Unlike passes under the legacy pass manager where the pass interface is
defined via inheritance, passes under the new pass manager rely on
concept-based polymorphism, meaning there is no explicit interface (see
comments in <code class="docutils literal notranslate"><span class="pre">PassManager.h</span></code> for more details). All LLVM passes inherit from
the CRTP mix-in <code class="docutils literal notranslate"><span class="pre">PassInfoMixin<PassT></span></code>. The pass should have a <code class="docutils literal notranslate"><span class="pre">run()</span></code>
method which returns a <code class="docutils literal notranslate"><span class="pre">PreservedAnalyses</span></code> and takes in some unit of IR
along with an analysis manager. For example, a function pass would have a
<code class="docutils literal notranslate"><span class="pre">PreservedAnalyses</span> <span class="pre">run(Function</span> <span class="pre">&F,</span> <span class="pre">FunctionAnalysisManager</span> <span class="pre">&AM);</span></code> method.</p>
<p>We start by showing you how to construct a pass, from setting up the build,
creating the pass, to executing and testing it. Looking at existing passes is
always a great way to learn details.</p>
<div class="admonition warning">
<p class="admonition-title">Warning</p>
<p>This document deals with the new pass manager. LLVM uses the legacy pass
manager for the codegen pipeline. For more details, see
<a class="reference internal" href="WritingAnLLVMPass.html"><span class="doc">Writing an LLVM Pass</span></a> and <a class="reference internal" href="NewPassManager.html"><span class="doc">Using the New Pass Manager</span></a>.</p>
</div>
</div>
<div class="section" id="quick-start-writing-hello-world">
<h2><a class="toc-backref" href="#id2">Quick Start — Writing hello world</a><a class="headerlink" href="#quick-start-writing-hello-world" title="Permalink to this headline">¶</a></h2>
<p>Here we describe how to write the “hello world” of passes. The “HelloWorld”
pass is designed to simply print out the name of non-external functions that
exist in the program being compiled. It does not modify the program at all,
it just inspects it.</p>
<p>The code below already exists; feel free to create a pass with a different
name alongside the HelloWorld source files.</p>
<div class="section" id="setting-up-the-build">
<span id="writing-an-llvm-npm-pass-build"></span><h3><a class="toc-backref" href="#id3">Setting up the build</a><a class="headerlink" href="#setting-up-the-build" title="Permalink to this headline">¶</a></h3>
<p>First, configure and build LLVM as described in <a class="reference internal" href="GettingStarted.html"><span class="doc">Getting Started with the LLVM System</span></a>.</p>
<p>Next, we will reuse an existing directory (creating a new directory involves
messing around with more CMake files than we want). For this example, we’ll use
<code class="docutils literal notranslate"><span class="pre">llvm/lib/Transforms/Utils/HelloWorld.cpp</span></code>, which has already been created.
If you’d like to create your own pass, add a new source file into
<code class="docutils literal notranslate"><span class="pre">llvm/lib/Transforms/Utils/CMakeLists.txt</span></code> (assuming you want your pass in
the <code class="docutils literal notranslate"><span class="pre">Transforms/Utils</span></code> directory.</p>
<p>Now that we have the build set up for a new pass, we need to write the code
for the pass itself.</p>
</div>
<div class="section" id="basic-code-required">
<span id="writing-an-llvm-npm-pass-basiccode"></span><h3><a class="toc-backref" href="#id4">Basic code required</a><a class="headerlink" href="#basic-code-required" title="Permalink to this headline">¶</a></h3>
<p>Now that the build is setup for a new pass, we just have to write it.</p>
<p>First we need to define the pass in a header file. We’ll create
<code class="docutils literal notranslate"><span class="pre">llvm/include/llvm/Transforms/Utils/HelloWorld.h</span></code>. The file should
contain the following boilerplate:</p>
<div class="highlight-c++ notranslate"><div class="highlight"><pre><span></span><span class="cp">#ifndef LLVM_TRANSFORMS_HELLONEW_HELLOWORLD_H</span>
<span class="cp">#define LLVM_TRANSFORMS_HELLONEW_HELLOWORLD_H</span>
<span class="cp">#include</span> <span class="cpf">"llvm/IR/PassManager.h"</span><span class="cp"></span>
<span class="k">namespace</span> <span class="n">llvm</span> <span class="p">{</span>
<span class="k">class</span> <span class="nc">HelloWorldPass</span> <span class="o">:</span> <span class="k">public</span> <span class="n">PassInfoMixin</span><span class="o"><</span><span class="n">HelloWorldPass</span><span class="o">></span> <span class="p">{</span>
<span class="k">public</span><span class="o">:</span>
<span class="n">PreservedAnalyses</span> <span class="n">run</span><span class="p">(</span><span class="n">Function</span> <span class="o">&</span><span class="n">F</span><span class="p">,</span> <span class="n">FunctionAnalysisManager</span> <span class="o">&</span><span class="n">AM</span><span class="p">);</span>
<span class="p">};</span>
<span class="p">}</span> <span class="c1">// namespace llvm</span>
<span class="cp">#endif </span><span class="c1">// LLVM_TRANSFORMS_HELLONEW_HELLOWORLD_H</span>
</pre></div>
</div>
<p>This creates the class for the pass with a declaration of the <code class="docutils literal notranslate"><span class="pre">run()</span></code>
method which actually runs the pass. Inheriting from <code class="docutils literal notranslate"><span class="pre">PassInfoMixin<PassT></span></code>
sets up some more boilerplate so that we don’t have to write it ourselves.</p>
<p>Our class is in the <code class="docutils literal notranslate"><span class="pre">llvm</span></code> namespace so that we don’t pollute the global
namespace.</p>
<p>Next we’ll create <code class="docutils literal notranslate"><span class="pre">llvm/lib/Transforms/Utils/HelloWorld.cpp</span></code>, starting
with</p>
<div class="highlight-c++ notranslate"><div class="highlight"><pre><span></span><span class="cp">#include</span> <span class="cpf">"llvm/Transforms/Utils/HelloWorld.h"</span><span class="cp"></span>
</pre></div>
</div>
<p>… to include the header file we just created.</p>
<div class="highlight-c++ notranslate"><div class="highlight"><pre><span></span><span class="k">using</span> <span class="k">namespace</span> <span class="n">llvm</span><span class="p">;</span>
</pre></div>
</div>
<p>… is required because the functions from the include files live in the llvm
namespace. This should only be done in non-header files.</p>
<p>Next we have the pass’s <code class="docutils literal notranslate"><span class="pre">run()</span></code> definition:</p>
<div class="highlight-c++ notranslate"><div class="highlight"><pre><span></span><span class="n">PreservedAnalyses</span> <span class="n">HelloWorldPass</span><span class="o">::</span><span class="n">run</span><span class="p">(</span><span class="n">Function</span> <span class="o">&</span><span class="n">F</span><span class="p">,</span>
<span class="n">FunctionAnalysisManager</span> <span class="o">&</span><span class="n">AM</span><span class="p">)</span> <span class="p">{</span>
<span class="n">errs</span><span class="p">()</span> <span class="o"><<</span> <span class="n">F</span><span class="p">.</span><span class="n">getName</span><span class="p">()</span> <span class="o"><<</span> <span class="s">"</span><span class="se">\n</span><span class="s">"</span><span class="p">;</span>
<span class="k">return</span> <span class="n">PreservedAnalyses</span><span class="o">::</span><span class="n">all</span><span class="p">();</span>
<span class="p">}</span>
</pre></div>
</div>
<p>… which simply prints out the name of the function to stderr. The pass
manager will ensure that the pass will be run on every function in a module.
The <code class="docutils literal notranslate"><span class="pre">PreservedAnalyses</span></code> return value says that all analyses (e.g. dominator
tree) are still valid after this pass since we didn’t modify any functions.</p>
<p>That’s it for the pass itself. Now in order to “register” the pass, we need
to add it to a couple places. Add the following to
<code class="docutils literal notranslate"><span class="pre">llvm/lib/Passes/PassRegistry.def</span></code> in the <code class="docutils literal notranslate"><span class="pre">FUNCTION_PASS</span></code> section</p>
<div class="highlight-c++ notranslate"><div class="highlight"><pre><span></span><span class="n">FUNCTION_PASS</span><span class="p">(</span><span class="s">"helloworld"</span><span class="p">,</span> <span class="n">HelloWorldPass</span><span class="p">())</span>
</pre></div>
</div>
<p>… which adds the pass under the name “helloworld”.</p>
<p><code class="docutils literal notranslate"><span class="pre">llvm/lib/Passes/PassRegistry.def</span></code> is #include’d into
<code class="docutils literal notranslate"><span class="pre">llvm/lib/Passes/PassBuilder.cpp</span></code> multiple times for various reasons. Since
it constructs our pass, we need to also add the proper #include in
<code class="docutils literal notranslate"><span class="pre">llvm/lib/Passes/PassBuilder.cpp</span></code>:</p>
<div class="highlight-c++ notranslate"><div class="highlight"><pre><span></span><span class="cp">#include</span> <span class="cpf">"llvm/Transforms/Utils/HelloWorld.h"</span><span class="cp"></span>
</pre></div>
</div>
<p>This should be all the code necessary for our pass, now it’s time to compile
and run it.</p>
</div>
<div class="section" id="running-a-pass-with-opt">
<h3><a class="toc-backref" href="#id5">Running a pass with <code class="docutils literal notranslate"><span class="pre">opt</span></code></a><a class="headerlink" href="#running-a-pass-with-opt" title="Permalink to this headline">¶</a></h3>
<p>Now that you have a brand new shiny pass, we can build <strong class="program">opt</strong> and use
it to run some LLVM IR through the pass.</p>
<div class="highlight-console notranslate"><div class="highlight"><pre><span></span><span class="gp">$</span> ninja -C build/ opt
<span class="gp">#</span> or whatever build system/build directory you are using
<span class="gp">$</span> cat /tmp/a.ll
<span class="go">define i32 @foo() {</span>
<span class="gp"> %</span><span class="nv">a</span> <span class="o">=</span> add i32 <span class="m">2</span>, <span class="m">3</span>
<span class="go"> ret i32 %a</span>
<span class="go">}</span>
<span class="go">define void @bar() {</span>
<span class="go"> ret void</span>
<span class="go">}</span>
<span class="gp">$</span> build/bin/opt -disable-output /tmp/a.ll -passes<span class="o">=</span>helloworld
<span class="go">foo</span>
<span class="go">bar</span>
</pre></div>
</div>
<p>Our pass ran and printed the names of functions as expected!</p>
</div>
<div class="section" id="testing-a-pass">
<h3><a class="toc-backref" href="#id6">Testing a pass</a><a class="headerlink" href="#testing-a-pass" title="Permalink to this headline">¶</a></h3>
<p>Testing our pass is important to prevent future regressions. We’ll add a lit
test at <code class="docutils literal notranslate"><span class="pre">llvm/test/Transforms/Utils/helloworld.ll</span></code>. See
<a class="reference internal" href="TestingGuide.html"><span class="doc">LLVM Testing Infrastructure Guide</span></a> for more information on testing.</p>
<div class="highlight-llvm notranslate"><div class="highlight"><pre><span></span>$ cat llvm/test/Transforms/Utils/helloworld.ll
; RUN: opt -disable-output -passes=helloworld %s 2>&1 | FileCheck %s
; CHECK: {{^}}foo{{$}}
define i32 @foo() {
%a = add i32 2, 3
ret i32 %a
}
; CHECK-NEXT: {{^}}bar{{$}}
define void @bar() {
ret void
}
$ ninja -C build check-llvm
# runs our new test alongside all other llvm lit tests
</pre></div>
</div>
</div>
</div>
<div class="section" id="faqs">
<h2><a class="toc-backref" href="#id7">FAQs</a><a class="headerlink" href="#faqs" title="Permalink to this headline">¶</a></h2>
<div class="section" id="required-passes">
<h3><a class="toc-backref" href="#id8">Required passes</a><a class="headerlink" href="#required-passes" title="Permalink to this headline">¶</a></h3>
<p>A pass that defines a static <code class="docutils literal notranslate"><span class="pre">isRequired()</span></code> method that returns true is a required pass. For example:</p>
<div class="highlight-c++ notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">HelloWorldPass</span> <span class="o">:</span> <span class="k">public</span> <span class="n">PassInfoMixin</span><span class="o"><</span><span class="n">HelloWorldPass</span><span class="o">></span> <span class="p">{</span>
<span class="k">public</span><span class="o">:</span>
<span class="n">PreservedAnalyses</span> <span class="n">run</span><span class="p">(</span><span class="n">Function</span> <span class="o">&</span><span class="n">F</span><span class="p">,</span> <span class="n">FunctionAnalysisManager</span> <span class="o">&</span><span class="n">AM</span><span class="p">);</span>
<span class="k">static</span> <span class="kt">bool</span> <span class="n">isRequired</span><span class="p">()</span> <span class="p">{</span> <span class="k">return</span> <span class="nb">true</span><span class="p">;</span> <span class="p">}</span>
<span class="p">};</span>
</pre></div>
</div>
<p>A required pass is a pass that may not be skipped. An example of a required
pass is <code class="docutils literal notranslate"><span class="pre">AlwaysInlinerPass</span></code>, which must always be run to preserve
<code class="docutils literal notranslate"><span class="pre">alwaysinline</span></code> semantics. Pass managers are required since they may contain
other required passes.</p>
<p>An example of how a pass can be skipped is the <code class="docutils literal notranslate"><span class="pre">optnone</span></code> function
attribute, which specifies that optimizations should not be run on the
function. Required passes will still be run on <code class="docutils literal notranslate"><span class="pre">optnone</span></code> functions.</p>
<p>For more implementation details, see
<code class="docutils literal notranslate"><span class="pre">PassInstrumentation::runBeforePass()</span></code>.</p>
</div>
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