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<div class="section" id="jitlink-and-orc-s-objectlinkinglayer">
<h1>JITLink and ORC’s ObjectLinkingLayer<a class="headerlink" href="#jitlink-and-orc-s-objectlinkinglayer" 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" id="id10">Introduction</a></p></li>
<li><p><a class="reference internal" href="#jitlink-and-objectlinkinglayer" id="id11">JITLink and ObjectLinkingLayer</a></p>
<ul>
<li><p><a class="reference internal" href="#objectlinkinglayer-plugins" id="id12">ObjectLinkingLayer Plugins</a></p></li>
</ul>
</li>
<li><p><a class="reference internal" href="#linkgraph" id="id13">LinkGraph</a></p></li>
<li><p><a class="reference internal" href="#generic-link-algorithm" id="id14">Generic Link Algorithm</a></p>
<ul>
<li><p><a class="reference internal" href="#passes" id="id15">Passes</a></p></li>
<li><p><a class="reference internal" href="#memory-management-with-jitlinkmemorymanager" id="id16">Memory Management with JITLinkMemoryManager</a></p></li>
<li><p><a class="reference internal" href="#jitlinkmemorymanager-and-security" id="id17">JITLinkMemoryManager and Security</a></p></li>
<li><p><a class="reference internal" href="#error-handling" id="id18">Error Handling</a></p></li>
</ul>
</li>
<li><p><a class="reference internal" href="#connection-to-the-orc-runtime" id="id19">Connection to the ORC Runtime</a></p></li>
<li><p><a class="reference internal" href="#constructing-linkgraphs" id="id20">Constructing LinkGraphs</a></p></li>
<li><p><a class="reference internal" href="#jit-linking" id="id21">JIT Linking</a></p>
<ul>
<li><p><a class="reference internal" href="#runtimedyld" id="id22">RuntimeDyld</a></p></li>
</ul>
</li>
<li><p><a class="reference internal" href="#the-llvm-jitlink-tool" id="id23">The llvm-jitlink tool</a></p>
<ul>
<li><p><a class="reference internal" href="#basic-usage" id="id24">Basic usage</a></p></li>
<li><p><a class="reference internal" href="#llvm-jitlink-as-a-regression-testing-utility" id="id25">llvm-jitlink as a regression testing utility</a></p></li>
<li><p><a class="reference internal" href="#remote-execution-via-llvm-jitlink-executor" id="id26">Remote execution via llvm-jitlink-executor</a></p></li>
<li><p><a class="reference internal" href="#harness-mode" id="id27">Harness mode</a></p></li>
<li><p><a class="reference internal" href="#tips-for-jitlink-backend-developers" id="id28">Tips for JITLink backend developers</a></p></li>
</ul>
</li>
<li><p><a class="reference internal" href="#roadmap" id="id29">Roadmap</a></p>
<ul>
<li><p><a class="reference internal" href="#jitlink-availability-and-feature-status" id="id30">JITLink Availability and Feature Status</a></p></li>
</ul>
</li>
</ul>
</div>
<div class="section" id="introduction">
<h2><a class="toc-backref" href="#id10">Introduction</a><a class="headerlink" href="#introduction" title="Permalink to this headline">¶</a></h2>
<p>This document aims to provide a high-level overview of the design and API
of the JITLink library. It assumes some familiarity with linking and
relocatable object files, but should not require deep expertise. If you know
what a section, symbol, and relocation are you should find this document
accessible. If it is not, please submit a patch (<a class="reference internal" href="Contributing.html"><span class="doc">Contributing to LLVM</span></a>) or file a
bug (<a class="reference internal" href="HowToSubmitABug.html"><span class="doc">How to submit an LLVM bug report</span></a>).</p>
<p>JITLink is a library for <a class="reference internal" href="#jit-linking"><span class="std std-ref">JIT Linking</span></a>. It was built to support the ORC JIT
APIs and is most commonly accessed via ORC’s ObjectLinkingLayer API. JITLink was
developed with the aim of supporting the full set of features provided by each
object format; including static initializers, exception handling, thread local
variables, and language runtime registration. Supporting these features enables
ORC to execute code generated from source languages which rely on these features
(e.g. C++ requires object format support for static initializers to support
static constructors, eh-frame registration for exceptions, and TLV support for
thread locals; Swift and Objective-C require language runtime registration for
many features). For some object format features support is provided entirely
within JITLink, and for others it is provided in cooperation with the
(prototype) ORC runtime.</p>
<p>JITLink aims to support the following features, some of which are still under
development:</p>
<ol class="arabic simple">
<li><p>Cross-process and cross-architecture linking of single relocatable objects
into a target <em>executor</em> process.</p></li>
<li><p>Support for all object format features.</p></li>
<li><p>Open linker data structures (<code class="docutils literal notranslate"><span class="pre">LinkGraph</span></code>) and pass system.</p></li>
</ol>
</div>
<div class="section" id="jitlink-and-objectlinkinglayer">
<h2><a class="toc-backref" href="#id11">JITLink and ObjectLinkingLayer</a><a class="headerlink" href="#jitlink-and-objectlinkinglayer" title="Permalink to this headline">¶</a></h2>
<p><code class="docutils literal notranslate"><span class="pre">ObjectLinkingLayer</span></code> is ORCs wrapper for JITLink. It is an ORC layer that
allows objects to be added to a <code class="docutils literal notranslate"><span class="pre">JITDylib</span></code>, or emitted from some higher level
program representation. When an object is emitted, <code class="docutils literal notranslate"><span class="pre">ObjectLinkingLayer</span></code> uses
JITLink to construct a <code class="docutils literal notranslate"><span class="pre">LinkGraph</span></code> (see <a class="reference internal" href="#constructing-linkgraphs"><span class="std std-ref">Constructing LinkGraphs</span></a>) and
calls JITLink’s <code class="docutils literal notranslate"><span class="pre">link</span></code> function to link the graph into the executor process.</p>
<p>The <code class="docutils literal notranslate"><span class="pre">ObjectLinkingLayer</span></code> class provides a plugin API,
<code class="docutils literal notranslate"><span class="pre">ObjectLinkingLayer::Plugin</span></code>, which users can subclass in order to inspect and
modify <code class="docutils literal notranslate"><span class="pre">LinkGraph</span></code> instances at link time, and react to important JIT events
(such as an object being emitted into target memory). This enables many features
and optimizations that were not possible under MCJIT or RuntimeDyld.</p>
<div class="section" id="objectlinkinglayer-plugins">
<h3><a class="toc-backref" href="#id12">ObjectLinkingLayer Plugins</a><a class="headerlink" href="#objectlinkinglayer-plugins" title="Permalink to this headline">¶</a></h3>
<p>The <code class="docutils literal notranslate"><span class="pre">ObjectLinkingLayer::Plugin</span></code> class provides the following methods:</p>
<ul>
<li><p><code class="docutils literal notranslate"><span class="pre">modifyPassConfig</span></code> is called each time a LinkGraph is about to be linked. It
can be overridden to install JITLink <em>Passes</em> to run during the link process.</p>
<div class="highlight-c++ notranslate"><div class="highlight"><pre><span></span><span class="kt">void</span> <span class="n">modifyPassConfig</span><span class="p">(</span><span class="n">MaterializationResponsibility</span> <span class="o">&</span><span class="n">MR</span><span class="p">,</span>
<span class="k">const</span> <span class="n">Triple</span> <span class="o">&</span><span class="n">TT</span><span class="p">,</span>
<span class="n">jitlink</span><span class="o">::</span><span class="n">PassConfiguration</span> <span class="o">&</span><span class="n">Config</span><span class="p">)</span>
</pre></div>
</div>
</li>
<li><p><code class="docutils literal notranslate"><span class="pre">notifyLoaded</span></code> is called before the link begins, and can be overridden to
set up any initial state for the given <code class="docutils literal notranslate"><span class="pre">MaterializationResponsibility</span></code> if
needed.</p>
<div class="highlight-c++ notranslate"><div class="highlight"><pre><span></span><span class="kt">void</span> <span class="n">notifyLoaded</span><span class="p">(</span><span class="n">MaterializationResponsibility</span> <span class="o">&</span><span class="n">MR</span><span class="p">)</span>
</pre></div>
</div>
</li>
<li><p><code class="docutils literal notranslate"><span class="pre">notifyEmitted</span></code> is called after the link is complete and code has been
emitted to the executor process. It can be overridden to finalize state
for the <code class="docutils literal notranslate"><span class="pre">MaterializationResponsibility</span></code> if needed.</p>
<div class="highlight-c++ notranslate"><div class="highlight"><pre><span></span><span class="n">Error</span> <span class="n">notifyEmitted</span><span class="p">(</span><span class="n">MaterializationResponsibility</span> <span class="o">&</span><span class="n">MR</span><span class="p">)</span>
</pre></div>
</div>
</li>
<li><p><code class="docutils literal notranslate"><span class="pre">notifyFailed</span></code> is called if the link fails at any point. It can be
overridden to react to the failure (e.g. to deallocate any already allocated
resources).</p>
<div class="highlight-c++ notranslate"><div class="highlight"><pre><span></span><span class="n">Error</span> <span class="n">notifyFailed</span><span class="p">(</span><span class="n">MaterializationResponsibility</span> <span class="o">&</span><span class="n">MR</span><span class="p">)</span>
</pre></div>
</div>
</li>
<li><p><code class="docutils literal notranslate"><span class="pre">notifyRemovingResources</span></code> is called when a request is made to remove any
resources associated with the <code class="docutils literal notranslate"><span class="pre">ResourceKey</span></code> <em>K</em> for the
<code class="docutils literal notranslate"><span class="pre">MaterializationResponsibility</span></code>.</p>
<div class="highlight-c++ notranslate"><div class="highlight"><pre><span></span><span class="n">Error</span> <span class="n">notifyRemovingResources</span><span class="p">(</span><span class="n">ResourceKey</span> <span class="n">K</span><span class="p">)</span>
</pre></div>
</div>
</li>
<li><p><code class="docutils literal notranslate"><span class="pre">notifyTransferringResources</span></code> is called if/when a request is made to
transfer tracking of any resources associated with <code class="docutils literal notranslate"><span class="pre">ResourceKey</span></code>
<em>SrcKey</em> to <em>DstKey</em>.</p>
<div class="highlight-c++ notranslate"><div class="highlight"><pre><span></span><span class="kt">void</span> <span class="n">notifyTransferringResources</span><span class="p">(</span><span class="n">ResourceKey</span> <span class="n">DstKey</span><span class="p">,</span>
<span class="n">ResourceKey</span> <span class="n">SrcKey</span><span class="p">)</span>
</pre></div>
</div>
</li>
</ul>
<p>Plugin authors are required to implement the <code class="docutils literal notranslate"><span class="pre">notifyFailed</span></code>,
<code class="docutils literal notranslate"><span class="pre">notifyRemovingResources</span></code>, and <code class="docutils literal notranslate"><span class="pre">notifyTransferringResources</span></code> methods in
order to safely manage resources in the case of resource removal or transfer,
or link failure. If no resources are managed by the plugin then these methods
can be implemented as no-ops returning <code class="docutils literal notranslate"><span class="pre">Error::success()</span></code>.</p>
<p>Plugin instances are added to an <code class="docutils literal notranslate"><span class="pre">ObjectLinkingLayer</span></code> by
calling the <code class="docutils literal notranslate"><span class="pre">addPlugin</span></code> method <a class="footnote-reference brackets" href="#id7" id="id1">1</a>. E.g.</p>
<div class="highlight-c++ notranslate"><div class="highlight"><pre><span></span><span class="c1">// Plugin class to print the set of defined symbols in an object when that</span>
<span class="c1">// object is linked.</span>
<span class="k">class</span> <span class="nc">MyPlugin</span> <span class="o">:</span> <span class="k">public</span> <span class="n">ObjectLinkingLayer</span><span class="o">::</span><span class="n">Plugin</span> <span class="p">{</span>
<span class="k">public</span><span class="o">:</span>
<span class="c1">// Add passes to print the set of defined symbols after dead-stripping.</span>
<span class="kt">void</span> <span class="n">modifyPassConfig</span><span class="p">(</span><span class="n">MaterializationResponsibility</span> <span class="o">&</span><span class="n">MR</span><span class="p">,</span>
<span class="k">const</span> <span class="n">Triple</span> <span class="o">&</span><span class="n">TT</span><span class="p">,</span>
<span class="n">jitlink</span><span class="o">::</span><span class="n">PassConfiguration</span> <span class="o">&</span><span class="n">Config</span><span class="p">)</span> <span class="k">override</span> <span class="p">{</span>
<span class="n">Config</span><span class="p">.</span><span class="n">PostPrunePasses</span><span class="p">.</span><span class="n">push_back</span><span class="p">([</span><span class="k">this</span><span class="p">](</span><span class="n">jitlink</span><span class="o">::</span><span class="n">LinkGraph</span> <span class="o">&</span><span class="n">G</span><span class="p">)</span> <span class="p">{</span>
<span class="k">return</span> <span class="nf">printAllSymbols</span><span class="p">(</span><span class="n">G</span><span class="p">);</span>
<span class="p">});</span>
<span class="p">}</span>
<span class="c1">// Implement mandatory overrides:</span>
<span class="n">Error</span> <span class="n">notifyFailed</span><span class="p">(</span><span class="n">MaterializationResponsibility</span> <span class="o">&</span><span class="n">MR</span><span class="p">)</span> <span class="k">override</span> <span class="p">{</span>
<span class="k">return</span> <span class="n">Error</span><span class="o">::</span><span class="n">success</span><span class="p">();</span>
<span class="p">}</span>
<span class="n">Error</span> <span class="n">notifyRemovingResources</span><span class="p">(</span><span class="n">ResourceKey</span> <span class="n">K</span><span class="p">)</span> <span class="k">override</span> <span class="p">{</span>
<span class="k">return</span> <span class="n">Error</span><span class="o">::</span><span class="n">success</span><span class="p">();</span>
<span class="p">}</span>
<span class="kt">void</span> <span class="n">notifyTransferringResources</span><span class="p">(</span><span class="n">ResourceKey</span> <span class="n">DstKey</span><span class="p">,</span>
<span class="n">ResourceKey</span> <span class="n">SrcKey</span><span class="p">)</span> <span class="k">override</span> <span class="p">{}</span>
<span class="c1">// JITLink pass to print all defined symbols in G.</span>
<span class="n">Error</span> <span class="n">printAllSymbols</span><span class="p">(</span><span class="n">LinkGraph</span> <span class="o">&</span><span class="n">G</span><span class="p">)</span> <span class="p">{</span>
<span class="k">for</span> <span class="p">(</span><span class="k">auto</span> <span class="o">*</span><span class="nl">Sym</span> <span class="p">:</span> <span class="n">G</span><span class="p">.</span><span class="n">defined_symbols</span><span class="p">())</span>
<span class="k">if</span> <span class="p">(</span><span class="n">Sym</span><span class="o">-></span><span class="n">hasName</span><span class="p">())</span>
<span class="n">dbgs</span><span class="p">()</span> <span class="o"><<</span> <span class="n">Sym</span><span class="o">-></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">Error</span><span class="o">::</span><span class="n">success</span><span class="p">();</span>
<span class="p">}</span>
<span class="p">};</span>
<span class="c1">// Create our LLJIT instance using a custom object linking layer setup.</span>
<span class="c1">// This gives us a chance to install our plugin.</span>
<span class="k">auto</span> <span class="n">J</span> <span class="o">=</span> <span class="n">ExitOnErr</span><span class="p">(</span><span class="n">LLJITBuilder</span><span class="p">()</span>
<span class="p">.</span><span class="n">setObjectLinkingLayerCreator</span><span class="p">(</span>
<span class="p">[](</span><span class="n">ExecutionSession</span> <span class="o">&</span><span class="n">ES</span><span class="p">,</span> <span class="k">const</span> <span class="n">Triple</span> <span class="o">&</span><span class="n">T</span><span class="p">)</span> <span class="p">{</span>
<span class="c1">// Manually set up the ObjectLinkingLayer for our LLJIT</span>
<span class="c1">// instance.</span>
<span class="k">auto</span> <span class="n">OLL</span> <span class="o">=</span> <span class="n">std</span><span class="o">::</span><span class="n">make_unique</span><span class="o"><</span><span class="n">ObjectLinkingLayer</span><span class="o">></span><span class="p">(</span>
<span class="n">ES</span><span class="p">,</span> <span class="n">std</span><span class="o">::</span><span class="n">make_unique</span><span class="o"><</span><span class="n">jitlink</span><span class="o">::</span><span class="n">InProcessMemoryManager</span><span class="o">></span><span class="p">());</span>
<span class="c1">// Install our plugin:</span>
<span class="n">OLL</span><span class="o">-></span><span class="n">addPlugin</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">make_unique</span><span class="o"><</span><span class="n">MyPlugin</span><span class="o">></span><span class="p">());</span>
<span class="k">return</span> <span class="n">OLL</span><span class="p">;</span>
<span class="p">})</span>
<span class="p">.</span><span class="n">create</span><span class="p">());</span>
<span class="c1">// Add an object to the JIT. Nothing happens here: linking isn't triggered</span>
<span class="c1">// until we look up some symbol in our object.</span>
<span class="n">ExitOnErr</span><span class="p">(</span><span class="n">J</span><span class="o">-></span><span class="n">addObject</span><span class="p">(</span><span class="n">loadFromDisk</span><span class="p">(</span><span class="s">"main.o"</span><span class="p">)));</span>
<span class="c1">// Plugin triggers here when our lookup of main triggers linking of main.o</span>
<span class="k">auto</span> <span class="n">MainSym</span> <span class="o">=</span> <span class="n">J</span><span class="o">-></span><span class="n">lookup</span><span class="p">(</span><span class="s">"main"</span><span class="p">);</span>
</pre></div>
</div>
</div>
</div>
<div class="section" id="linkgraph">
<h2><a class="toc-backref" href="#id13">LinkGraph</a><a class="headerlink" href="#linkgraph" title="Permalink to this headline">¶</a></h2>
<p>JITLink maps all relocatable object formats to a generic <code class="docutils literal notranslate"><span class="pre">LinkGraph</span></code> type
that is designed to make linking fast and easy (<code class="docutils literal notranslate"><span class="pre">LinkGraph</span></code> instances can
also be created manually. See <a class="reference internal" href="#constructing-linkgraphs"><span class="std std-ref">Constructing LinkGraphs</span></a>).</p>
<p>Relocatable object formats (e.g. COFF, ELF, MachO) differ in their details,
but share a common goal: to represent machine level code and data with
annotations that allow them to be relocated in a virtual address space. To
this end they usually contain names (symbols) for content defined inside the
file or externally, chunks of content that must be moved as a unit (sections
or subsections, depending on the format), and annotations describing how to
patch content based on the final address of some target symbol/section
(relocations).</p>
<p>At a high level, the <code class="docutils literal notranslate"><span class="pre">LinkGraph</span></code> type represents these concepts as a decorated
graph. Nodes in the graph represent symbols and content, and edges represent
relocations. Each of the elements of the graph is listed here:</p>
<ul>
<li><p><code class="docutils literal notranslate"><span class="pre">Addressable</span></code> – A node in the link graph that can be assigned an address
in the executor process’s virtual address space.</p>
<p>Absolute and external symbols are represented using plain <code class="docutils literal notranslate"><span class="pre">Addressable</span></code>
instances. Content defined inside the object file is represented using the
<code class="docutils literal notranslate"><span class="pre">Block</span></code> subclass.</p>
</li>
<li><p><code class="docutils literal notranslate"><span class="pre">Block</span></code> – An <code class="docutils literal notranslate"><span class="pre">Addressable</span></code> node that has <code class="docutils literal notranslate"><span class="pre">Content</span></code> (or is marked as
zero-filled), a parent <code class="docutils literal notranslate"><span class="pre">Section</span></code>, a <code class="docutils literal notranslate"><span class="pre">Size</span></code>, an <code class="docutils literal notranslate"><span class="pre">Alignment</span></code> (and an
<code class="docutils literal notranslate"><span class="pre">AlignmentOffset</span></code>), and a list of <code class="docutils literal notranslate"><span class="pre">Edge</span></code> instances.</p>
<p>Blocks provide a container for binary content which must remain contiguous in
the target address space (a <em>layout unit</em>). Many interesting low level
operations on <code class="docutils literal notranslate"><span class="pre">LinkGraph</span></code> instances involve inspecting or mutating block
content or edges.</p>
<ul class="simple">
<li><p><code class="docutils literal notranslate"><span class="pre">Content</span></code> is represented as an <code class="docutils literal notranslate"><span class="pre">llvm::StringRef</span></code>, and accessible via
the <code class="docutils literal notranslate"><span class="pre">getContent</span></code> method. Content is only available for content blocks,
and not for zero-fill blocks (use <code class="docutils literal notranslate"><span class="pre">isZeroFill</span></code> to check, and prefer
<code class="docutils literal notranslate"><span class="pre">getSize</span></code> when only the block size is needed as it works for both
zero-fill and content blocks).</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">Section</span></code> is represented as a <code class="docutils literal notranslate"><span class="pre">Section&</span></code> reference, and accessible via
the <code class="docutils literal notranslate"><span class="pre">getSection</span></code> method. The <code class="docutils literal notranslate"><span class="pre">Section</span></code> class is described in more detail
below.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">Size</span></code> is represented as a <code class="docutils literal notranslate"><span class="pre">size_t</span></code>, and is accessible via the
<code class="docutils literal notranslate"><span class="pre">getSize</span></code> method for both content and zero-filled blocks.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">Alignment</span></code> is represented as a <code class="docutils literal notranslate"><span class="pre">uint64_t</span></code>, and available via the
<code class="docutils literal notranslate"><span class="pre">getAlignment</span></code> method. It represents the minimum alignment requirement (in
bytes) of the start of the block.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">AlignmentOffset</span></code> is represented as a <code class="docutils literal notranslate"><span class="pre">uint64_t</span></code>, and accessible via the
<code class="docutils literal notranslate"><span class="pre">getAlignmentOffset</span></code> method. It represents the offset from the alignment
required for the start of the block. This is required to support blocks
whose minimum alignment requirement comes from data at some non-zero offset
inside the block. E.g. if a block consists of a single byte (with byte
alignment) followed by a uint64_t (with 8-byte alignment), then the block
will have 8-byte alignment with an alignment offset of 7.</p></li>
<li><p>list of <code class="docutils literal notranslate"><span class="pre">Edge</span></code> instances. An iterator range for this list is returned by
the <code class="docutils literal notranslate"><span class="pre">edges</span></code> method. The <code class="docutils literal notranslate"><span class="pre">Edge</span></code> class is described in more detail below.</p></li>
</ul>
</li>
<li><p><code class="docutils literal notranslate"><span class="pre">Symbol</span></code> – An offset from an <code class="docutils literal notranslate"><span class="pre">Addressable</span></code> (often a <code class="docutils literal notranslate"><span class="pre">Block</span></code>), with an
optional <code class="docutils literal notranslate"><span class="pre">Name</span></code>, a <code class="docutils literal notranslate"><span class="pre">Linkage</span></code>, a <code class="docutils literal notranslate"><span class="pre">Scope</span></code>, a <code class="docutils literal notranslate"><span class="pre">Callable</span></code> flag, and a
<code class="docutils literal notranslate"><span class="pre">Live</span></code> flag.</p>
<p>Symbols make it possible to name content (blocks and addressables are
anonymous), or target content with an <code class="docutils literal notranslate"><span class="pre">Edge</span></code>.</p>
<ul class="simple">
<li><p><code class="docutils literal notranslate"><span class="pre">Name</span></code> is represented as an <code class="docutils literal notranslate"><span class="pre">llvm::StringRef</span></code> (equal to
<code class="docutils literal notranslate"><span class="pre">llvm::StringRef()</span></code> if the symbol has no name), and accessible via the
<code class="docutils literal notranslate"><span class="pre">getName</span></code> method.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">Linkage</span></code> is one of <em>Strong</em> or <em>Weak</em>, and is accessible via the
<code class="docutils literal notranslate"><span class="pre">getLinkage</span></code> method. The <code class="docutils literal notranslate"><span class="pre">JITLinkContext</span></code> can use this flag to determine
whether this symbol definition should be kept or dropped.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">Scope</span></code> is one of <em>Default</em>, <em>Hidden</em>, or <em>Local</em>, and is accessible via
the <code class="docutils literal notranslate"><span class="pre">getScope</span></code> method. The <code class="docutils literal notranslate"><span class="pre">JITLinkContext</span></code> can use this to determine
who should be able to see the symbol. A symbol with default scope should be
globally visible. A symbol with hidden scope should be visible to other
definitions within the same simulated dylib (e.g. ORC <code class="docutils literal notranslate"><span class="pre">JITDylib</span></code>) or
executable, but not from elsewhere. A symbol with local scope should only be
visible within the current <code class="docutils literal notranslate"><span class="pre">LinkGraph</span></code>.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">Callable</span></code> is a boolean which is set to true if this symbol can be called,
and is accessible via the <code class="docutils literal notranslate"><span class="pre">isCallable</span></code> method. This can be used to
automate the introduction of call-stubs for lazy compilation.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">Live</span></code> is a boolean that can be set to mark this symbol as root for
dead-stripping purposes (see <a class="reference internal" href="#generic-link-algorithm"><span class="std std-ref">Generic Link Algorithm</span></a>). JITLink’s
dead-stripping algorithm will propagate liveness flags through the graph to
all reachable symbols before deleting any symbols (and blocks) that are not
marked live.</p></li>
</ul>
</li>
<li><p><code class="docutils literal notranslate"><span class="pre">Edge</span></code> – A quad of an <code class="docutils literal notranslate"><span class="pre">Offset</span></code> (implicitly from the start of the
containing <code class="docutils literal notranslate"><span class="pre">Block</span></code>), a <code class="docutils literal notranslate"><span class="pre">Kind</span></code> (describing the relocation type), a
<code class="docutils literal notranslate"><span class="pre">Target</span></code>, and an <code class="docutils literal notranslate"><span class="pre">Addend</span></code>.</p>
<p>Edges represent relocations, and occasionally other relationships, between
blocks and symbols.</p>
<ul class="simple">
<li><p><code class="docutils literal notranslate"><span class="pre">Offset</span></code>, accessible via <code class="docutils literal notranslate"><span class="pre">getOffset</span></code>, is an offset from the start of the
<code class="docutils literal notranslate"><span class="pre">Block</span></code> containing the <code class="docutils literal notranslate"><span class="pre">Edge</span></code>.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">Kind</span></code>, accessible via <code class="docutils literal notranslate"><span class="pre">getKind</span></code> is a relocation type – it describes
what kinds of changes (if any) should be made to block content at the given
<code class="docutils literal notranslate"><span class="pre">Offset</span></code> based on the address of the <code class="docutils literal notranslate"><span class="pre">Target</span></code>.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">Target</span></code>, accessible via <code class="docutils literal notranslate"><span class="pre">getTarget</span></code>, is a pointer to a <code class="docutils literal notranslate"><span class="pre">Symbol</span></code>,
representing whose address is relevant to the fixup calculation specified by
the edge’s <code class="docutils literal notranslate"><span class="pre">Kind</span></code>.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">Addend</span></code>, accessible via <code class="docutils literal notranslate"><span class="pre">getAddend</span></code>, is a constant whose interpretation
is determined by the edge’s <code class="docutils literal notranslate"><span class="pre">Kind</span></code>.</p></li>
</ul>
</li>
<li><p><code class="docutils literal notranslate"><span class="pre">Section</span></code> – A set of <code class="docutils literal notranslate"><span class="pre">Symbol</span></code> instances, plus a set of <code class="docutils literal notranslate"><span class="pre">Block</span></code>
instances, with a <code class="docutils literal notranslate"><span class="pre">Name</span></code>, a set of <code class="docutils literal notranslate"><span class="pre">ProtectionFlags</span></code>, and an <code class="docutils literal notranslate"><span class="pre">Ordinal</span></code>.</p>
<p>Sections make it easy to iterate over the symbols or blocks associated with
a particular section in the source object file.</p>
<ul class="simple">
<li><p><code class="docutils literal notranslate"><span class="pre">blocks()</span></code> returns an iterator over the set of blocks defined in the
section (as <code class="docutils literal notranslate"><span class="pre">Block*</span></code> pointers).</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">symbols()</span></code> returns an iterator over the set of symbols defined in the
section (as <code class="docutils literal notranslate"><span class="pre">Symbol*</span></code> pointers).</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">Name</span></code> is represented as an <code class="docutils literal notranslate"><span class="pre">llvm::StringRef</span></code>, and is accessible via the
<code class="docutils literal notranslate"><span class="pre">getName</span></code> method.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">ProtectionFlags</span></code> are represented as a sys::Memory::ProtectionFlags enum,
and accessible via the <code class="docutils literal notranslate"><span class="pre">getProtectionFlags</span></code> method. These flags describe
whether the section is readable, writable, executable, or some combination
of these. The most common combinations are <code class="docutils literal notranslate"><span class="pre">RW-</span></code> for writable data,
<code class="docutils literal notranslate"><span class="pre">R--</span></code> for constant data, and <code class="docutils literal notranslate"><span class="pre">R-X</span></code> for code.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">SectionOrdinal</span></code>, accessible via <code class="docutils literal notranslate"><span class="pre">getOrdinal</span></code>, is a number used to order
the section relative to others. It is usually used to preserve section
order within a segment (a set of sections with the same memory protections)
when laying out memory.</p></li>
</ul>
</li>
</ul>
<p>For the graph-theorists: The <code class="docutils literal notranslate"><span class="pre">LinkGraph</span></code> is bipartite, with one set of
<code class="docutils literal notranslate"><span class="pre">Symbol</span></code> nodes and one set of <code class="docutils literal notranslate"><span class="pre">Addressable</span></code> nodes. Each <code class="docutils literal notranslate"><span class="pre">Symbol</span></code> node has
one (implicit) edge to its target <code class="docutils literal notranslate"><span class="pre">Addressable</span></code>. Each <code class="docutils literal notranslate"><span class="pre">Block</span></code> has a set of
edges (possibly empty, represented as <code class="docutils literal notranslate"><span class="pre">Edge</span></code> instances) back to elements of
the <code class="docutils literal notranslate"><span class="pre">Symbol</span></code> set. For convenience and performance of common algorithms,
symbols and blocks are further grouped into <code class="docutils literal notranslate"><span class="pre">Sections</span></code>.</p>
<p>The <code class="docutils literal notranslate"><span class="pre">LinkGraph</span></code> itself provides operations for constructing, removing, and
iterating over sections, symbols, and blocks. It also provides metadata
and utilities relevant to the linking process:</p>
<ul class="simple">
<li><p>Graph element operations</p>
<ul>
<li><p><code class="docutils literal notranslate"><span class="pre">sections</span></code> returns an iterator over all sections in the graph.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">findSectionByName</span></code> returns a pointer to the section with the given
name (as a <code class="docutils literal notranslate"><span class="pre">Section*</span></code>) if it exists, otherwise returns a nullptr.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">blocks</span></code> returns an iterator over all blocks in the graph (across all
sections).</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">defined_symbols</span></code> returns an iterator over all defined symbols in the
graph (across all sections).</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">external_symbols</span></code> returns an iterator over all external symbols in the
graph.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">absolute_symbols</span></code> returns an iterator over all absolute symbols in the
graph.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">createSection</span></code> creates a section with a given name and protection flags.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">createContentBlock</span></code> creates a block with the given initial content,
parent section, address, alignment, and alignment offset.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">createZeroFillBlock</span></code> creates a zero-fill block with the given size,
parent section, address, alignment, and alignment offset.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">addExternalSymbol</span></code> creates a new addressable and symbol with a given
name, size, and linkage.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">addAbsoluteSymbol</span></code> creates a new addressable and symbol with a given
name, address, size, linkage, scope, and liveness.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">addCommonSymbol</span></code> convenience function for creating a zero-filled block
and weak symbol with a given name, scope, section, initial address, size,
alignment and liveness.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">addAnonymousSymbol</span></code> creates a new anonymous symbol for a given block,
offset, size, callable-ness, and liveness.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">addDefinedSymbol</span></code> creates a new symbol for a given block with a name,
offset, size, linkage, scope, callable-ness and liveness.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">makeExternal</span></code> transforms a formerly defined symbol into an external one
by creating a new addressable and pointing the symbol at it. The existing
block is not deleted, but can be manually removed (if unreferenced) by
calling <code class="docutils literal notranslate"><span class="pre">removeBlock</span></code>. All edges to the symbol remain valid, but the
symbol must now be defined outside this <code class="docutils literal notranslate"><span class="pre">LinkGraph</span></code>.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">removeExternalSymbol</span></code> removes an external symbol and its target
addressable. The target addressable must not be referenced by any other
symbols.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">removeAbsoluteSymbol</span></code> removes an absolute symbol and its target
addressable. The target addressable must not be referenced by any other
symbols.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">removeDefinedSymbol</span></code> removes a defined symbol, but <em>does not</em> remove
its target block.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">removeBlock</span></code> removes the given block.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">splitBlock</span></code> split a given block in two at a given index (useful where
it is known that a block contains decomposable records, e.g. CFI records
in an eh-frame section).</p></li>
</ul>
</li>
<li><p>Graph utility operations</p>
<ul>
<li><p><code class="docutils literal notranslate"><span class="pre">getName</span></code> returns the name of this graph, which is usually based on the
name of the input object file.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">getTargetTriple</span></code> returns an <cite>llvm::Triple</cite> for the executor process.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">getPointerSize</span></code> returns the size of a pointer (in bytes) in the executor
process.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">getEndinaness</span></code> returns the endianness of the executor process.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">allocateString</span></code> copies data from a given <code class="docutils literal notranslate"><span class="pre">llvm::Twine</span></code> into the
link graph’s internal allocator. This can be used to ensure that content
created inside a pass outlives that pass’s execution.</p></li>
</ul>
</li>
</ul>
</div>
<div class="section" id="generic-link-algorithm">
<span id="id2"></span><h2><a class="toc-backref" href="#id14">Generic Link Algorithm</a><a class="headerlink" href="#generic-link-algorithm" title="Permalink to this headline">¶</a></h2>
<p>JITLink provides a generic link algorithm which can be extended / modified at
certain points by the introduction of JITLink <a class="reference internal" href="#passes"><span class="std std-ref">Passes</span></a>:</p>
<ol class="arabic">
<li><p>Phase 1</p>
<p>This phase is called immediately by the <code class="docutils literal notranslate"><span class="pre">link</span></code> function as soon as the
initial configuration (including the pass pipeline setup) is complete.</p>
<ol class="arabic">
<li><p>Run pre-prune passes.</p>
<p>These passes are called on the graph before it is pruned. At this stage
<code class="docutils literal notranslate"><span class="pre">LinkGraph</span></code> nodes still have their original vmaddrs. A mark-live pass
(supplied by the <code class="docutils literal notranslate"><span class="pre">JITLinkContext</span></code>) will be run at the end of this
sequence to mark the initial set of live symbols.</p>
<p>Notable use cases: marking nodes live, accessing/copying graph data that
will be pruned (e.g. metadata that’s important for the JIT, but not needed
for the link process).</p>
</li>
<li><p>Prune (dead-strip) the <code class="docutils literal notranslate"><span class="pre">LinkGraph</span></code>.</p>
<p>Removes all symbols and blocks not reachable from the initial set of live
symbols.</p>
<p>This allows JITLink to remove unreachable symbols / content, including
overridden weak and redundant ODR definitions.</p>
</li>
<li><p>Run post-prune passes.</p>
<p>These passes are run on the graph after dead-stripping, but before memory
is allocated or nodes assigned their final target vmaddrs.</p>
<p>Passes run at this stage benefit from pruning, as dead functions and data
have been stripped from the graph. However new content can still be added
to the graph, as target and working memory have not been allocated yet.</p>
<p>Notable use cases: Building Global Offset Table (GOT), Procedure Linkage
Table (PLT), and Thread Local Variable (TLV) entries.</p>
</li>
<li><p>Sort blocks into segments.</p>
<p>Sorts all blocks by ordinal and then address. Collects sections with
matching permissions into segments and computes the size of these
segments for memory allocation.</p>
</li>
<li><p>Allocate segment memory, update node addresses.</p>
<p>Calls the <code class="docutils literal notranslate"><span class="pre">JITLinkContext</span></code>’s <code class="docutils literal notranslate"><span class="pre">JITLinkMemoryManager</span></code> to allocate both
working and target memory for the graph, then updates all node addresses
to their assigned target address.</p>
<p>Note: This step only updates the addresses of nodes defined in this graph.
External symbols will still have null addresses.</p>
</li>
<li><p>Run post-allocation passes.</p>
<p>These passes are run on the graph after working and target memory have
been allocated, but before the <code class="docutils literal notranslate"><span class="pre">JITLinkContext</span></code> is notified of the
final addresses of the symbols in the graph. This gives these passes a
chance to set up data structures associated with target addresses before
any JITLink clients (especially ORC queries for symbol resolution) can
attempt to access them.</p>
<p>Notable use cases: Setting up mappings between target addresses and
JIT data structures, such as a mapping between <code class="docutils literal notranslate"><span class="pre">__dso_handle</span></code> and
<code class="docutils literal notranslate"><span class="pre">JITDylib*</span></code>.</p>
</li>
<li><p>Notify the <code class="docutils literal notranslate"><span class="pre">JITLinkContext</span></code> of the assigned symbol addresses.</p>
<p>Calls <code class="docutils literal notranslate"><span class="pre">JITLinkContext::notifyResolved</span></code> on the link graph, allowing
clients to react to the symbol address assignments made for this graph.
In ORC this is used to notify any pending queries for <em>resolved</em> symbols,
including pending queries from concurrently running JITLink instances that
have reached the next step and are waiting on the address of a symbol in
this graph to proceed with their link.</p>
</li>
<li><p>Identify external symbols and resolve their addresses asynchronously.</p>
<p>Calls the <code class="docutils literal notranslate"><span class="pre">JITLinkContext</span></code> to resolve the target address of any external
symbols in the graph. This step is asynchronous – JITLink will pack the
link state into a <em>continuation</em> to be run once the symbols are resolved.</p>
<p>This is the final step of Phase 1.</p>
</li>
</ol>
</li>
<li><p>Phase 2</p>
<p>This phase is called by the continuation constructed at the end of the
external symbol resolution step above.</p>
<ol class="arabic">
<li><p>Apply external symbol resolution results.</p>
<p>This updates the addresses of all external symbols. At this point all
nodes in the graph have their final target addresses, however node
content still points back to the original data in the object file.</p>
</li>
<li><p>Run pre-fixup passes.</p>
<p>These passes are called on the graph after all nodes have been assigned
their final target addresses, but before node content is copied into
working memory and fixed up. Passes run at this stage can make late
optimizations to the graph and content based on address layout.</p>
<p>Notable use cases: GOT and PLT relaxation, where GOT and PLT accesses are
bypassed for fixup targets that are directly accessible under the assigned
memory layout.</p>
</li>
<li><p>Copy block content to working memory and apply fixups.</p>
<p>Copies all block content into allocated working memory (following the
target layout) and applies fixups. Graph blocks are updated to point at
the fixed up content.</p>
</li>
<li><p>Run post-fixup passes.</p>
<p>These passes are called on the graph after fixups have been applied and
blocks updated to point to the fixed up content.</p>
<p>Post-fixup passes can inspect blocks contents to see the exact bytes that
will be copied to the assigned target addresses.</p>
</li>
<li><p>Finalize memory asynchronously.</p>
<p>Calls the <code class="docutils literal notranslate"><span class="pre">JITLinkMemoryManager</span></code> to copy working memory to the executor
process and apply the requested permissions. This step is asynchronous –
JITLink will pack the link state into a <em>continuation</em> to be run once
memory has been copied and protected.</p>
<p>This is the final step of Phase 2.</p>
</li>
</ol>
</li>
<li><p>Phase 3.</p>
<p>This phase is called by the continuation constructed at the end of the
memory finalization step above.</p>
<ol class="arabic">
<li><p>Notify the context that the graph has been emitted.</p>
<p>Calls <code class="docutils literal notranslate"><span class="pre">JITLinkContext::notifyFinalized</span></code> and hands off the
<code class="docutils literal notranslate"><span class="pre">JITLinkMemoryManager::Allocation</span></code> object for this graph’s memory
allocation. This allows the context to track/hold memory allocations and
react to the newly emitted definitions. In ORC this is used to update the
<code class="docutils literal notranslate"><span class="pre">ExecutionSession</span></code> instance’s dependence graph, which may result in
these symbols (and possibly others) becoming <em>Ready</em> if all of their
dependencies have also been emitted.</p>
</li>
</ol>
</li>
</ol>
<div class="section" id="passes">
<span id="id3"></span><h3><a class="toc-backref" href="#id15">Passes</a><a class="headerlink" href="#passes" title="Permalink to this headline">¶</a></h3>
<p>JITLink passes are <code class="docutils literal notranslate"><span class="pre">std::function<Error(LinkGraph&)></span></code> instances. They are free
to inspect and modify the given <code class="docutils literal notranslate"><span class="pre">LinkGraph</span></code> subject to the constraints of
whatever phase they are running in (see <a class="reference internal" href="#generic-link-algorithm"><span class="std std-ref">Generic Link Algorithm</span></a>). If a
pass returns <code class="docutils literal notranslate"><span class="pre">Error::success()</span></code> then linking continues. If a pass returns
a failure value then linking is stopped and the <code class="docutils literal notranslate"><span class="pre">JITLinkContext</span></code> is notified
that the link failed.</p>
<p>Passes may be used by both JITLink backends (e.g. MachO/x86-64 implements GOT
and PLT construction as a pass), and external clients like
<code class="docutils literal notranslate"><span class="pre">ObjectLinkingLayer::Plugin</span></code>.</p>
<p>In combination with the open <code class="docutils literal notranslate"><span class="pre">LinkGraph</span></code> API, JITLink passes enable the
implementation of powerful new features. For example:</p>
<ul>
<li><p>Relaxation optimizations – A pre-fixup pass can inspect GOT accesses and PLT
calls and identify situations where the addresses of the entry target and the
access are close enough to be accessed directly. In this case the pass can
rewrite the instruction stream of the containing block and update the fixup
edges to make the access direct.</p>
<p>Code for this looks like:</p>
</li>
</ul>
<div class="highlight-c++ notranslate"><div class="highlight"><pre><span></span><span class="n">Error</span> <span class="nf">relaxGOTEdges</span><span class="p">(</span><span class="n">LinkGraph</span> <span class="o">&</span><span class="n">G</span><span class="p">)</span> <span class="p">{</span>
<span class="k">for</span> <span class="p">(</span><span class="k">auto</span> <span class="o">*</span><span class="nl">B</span> <span class="p">:</span> <span class="n">G</span><span class="p">.</span><span class="n">blocks</span><span class="p">())</span>
<span class="k">for</span> <span class="p">(</span><span class="k">auto</span> <span class="o">&</span><span class="nl">E</span> <span class="p">:</span> <span class="n">B</span><span class="o">-></span><span class="n">edges</span><span class="p">())</span>
<span class="k">if</span> <span class="p">(</span><span class="n">E</span><span class="p">.</span><span class="n">getKind</span><span class="p">()</span> <span class="o">==</span> <span class="n">x86_64</span><span class="o">::</span><span class="n">GOTLoad</span><span class="p">)</span> <span class="p">{</span>
<span class="k">auto</span> <span class="o">&</span><span class="n">GOTTarget</span> <span class="o">=</span> <span class="n">getGOTEntryTarget</span><span class="p">(</span><span class="n">E</span><span class="p">.</span><span class="n">getTarget</span><span class="p">());</span>
<span class="k">if</span> <span class="p">(</span><span class="n">isInRange</span><span class="p">(</span><span class="n">B</span><span class="p">.</span><span class="n">getFixupAddress</span><span class="p">(</span><span class="n">E</span><span class="p">),</span> <span class="n">GOTTarget</span><span class="p">))</span> <span class="p">{</span>
<span class="c1">// Rewrite B.getContent() at fixup address from</span>
<span class="c1">// MOVQ to LEAQ</span>
<span class="c1">// Update edge target and kind.</span>
<span class="n">E</span><span class="p">.</span><span class="n">setTarget</span><span class="p">(</span><span class="n">GOTTarget</span><span class="p">);</span>
<span class="n">E</span><span class="p">.</span><span class="n">setKind</span><span class="p">(</span><span class="n">x86_64</span><span class="o">::</span><span class="n">PCRel32</span><span class="p">);</span>
<span class="p">}</span>
<span class="p">}</span>
<span class="k">return</span> <span class="n">Error</span><span class="o">::</span><span class="n">success</span><span class="p">();</span>
<span class="p">}</span>
</pre></div>
</div>
<ul class="simple">
<li><p>Metadata registration – Post allocation passes can be used to record the
address range of sections in the target. This can be used to register the
metadata (e.g exception handling frames, language metadata) in the target
once memory has been finalized.</p></li>
</ul>
<div class="highlight-c++ notranslate"><div class="highlight"><pre><span></span><span class="n">Error</span> <span class="nf">registerEHFrameSection</span><span class="p">(</span><span class="n">LinkGraph</span> <span class="o">&</span><span class="n">G</span><span class="p">)</span> <span class="p">{</span>
<span class="k">if</span> <span class="p">(</span><span class="k">auto</span> <span class="o">*</span><span class="n">Sec</span> <span class="o">=</span> <span class="n">G</span><span class="p">.</span><span class="n">findSectionByName</span><span class="p">(</span><span class="s">"__eh_frame"</span><span class="p">))</span> <span class="p">{</span>
<span class="n">SectionRange</span> <span class="n">SR</span><span class="p">(</span><span class="o">*</span><span class="n">Sec</span><span class="p">);</span>
<span class="n">registerEHFrameSection</span><span class="p">(</span><span class="n">SR</span><span class="p">.</span><span class="n">getStart</span><span class="p">(),</span> <span class="n">SR</span><span class="p">.</span><span class="n">getEnd</span><span class="p">());</span>
<span class="p">}</span>
<span class="k">return</span> <span class="n">Error</span><span class="o">::</span><span class="n">success</span><span class="p">();</span>
<span class="p">}</span>
</pre></div>
</div>
<ul class="simple">
<li><p>Record call sites for later mutation – A post-allocation pass can record
the call sites of all calls to a particular function, allowing those call
sites to be updated later at runtime (e.g. for instrumentation, or to
enable the function to be lazily compiled but still called directly after
compilation).</p></li>
</ul>
<div class="highlight-c++ notranslate"><div class="highlight"><pre><span></span><span class="n">StringRef</span> <span class="n">FunctionName</span> <span class="o">=</span> <span class="s">"foo"</span><span class="p">;</span>
<span class="n">std</span><span class="o">::</span><span class="n">vector</span><span class="o"><</span><span class="n">JITTargetAddress</span><span class="o">></span> <span class="n">CallSitesForFunction</span><span class="p">;</span>
<span class="k">auto</span> <span class="n">RecordCallSites</span> <span class="o">=</span>
<span class="p">[</span><span class="o">&</span><span class="p">](</span><span class="n">LinkGraph</span> <span class="o">&</span><span class="n">G</span><span class="p">)</span> <span class="o">-></span> <span class="n">Error</span> <span class="p">{</span>
<span class="k">for</span> <span class="p">(</span><span class="k">auto</span> <span class="o">*</span><span class="nl">B</span> <span class="p">:</span> <span class="n">G</span><span class="p">.</span><span class="n">blocks</span><span class="p">())</span>
<span class="k">for</span> <span class="p">(</span><span class="k">auto</span> <span class="o">&</span><span class="nl">E</span> <span class="p">:</span> <span class="n">B</span><span class="p">.</span><span class="n">edges</span><span class="p">())</span>
<span class="k">if</span> <span class="p">(</span><span class="n">E</span><span class="p">.</span><span class="n">getKind</span><span class="p">()</span> <span class="o">==</span> <span class="n">CallEdgeKind</span> <span class="o">&&</span>
<span class="n">E</span><span class="p">.</span><span class="n">getTarget</span><span class="p">().</span><span class="n">hasName</span><span class="p">()</span> <span class="o">&&</span>
<span class="n">E</span><span class="p">.</span><span class="n">getTraget</span><span class="p">().</span><span class="n">getName</span><span class="p">()</span> <span class="o">==</span> <span class="n">FunctionName</span><span class="p">)</span>
<span class="n">CallSitesForFunction</span><span class="p">.</span><span class="n">push_back</span><span class="p">(</span><span class="n">B</span><span class="p">.</span><span class="n">getFixupAddress</span><span class="p">(</span><span class="n">E</span><span class="p">));</span>
<span class="k">return</span> <span class="n">Error</span><span class="o">::</span><span class="n">success</span><span class="p">();</span>
<span class="p">};</span>
</pre></div>
</div>
</div>
<div class="section" id="memory-management-with-jitlinkmemorymanager">
<h3><a class="toc-backref" href="#id16">Memory Management with JITLinkMemoryManager</a><a class="headerlink" href="#memory-management-with-jitlinkmemorymanager" title="Permalink to this headline">¶</a></h3>
<p>JIT linking requires allocation of two kinds of memory: working memory in the
JIT process and target memory in the execution process (these processes and
memory allocations may be one and the same, depending on how the user wants
to build their JIT). It also requires that these allocations conform to the
requested code model in the target process (e.g. MachO/x86-64’s Small code
model requires that all code and data for a simulated dylib is allocated within
4Gb). Finally, it is natural to make the memory manager responsible for
transferring memory to the target address space and applying memory protections,
since the memory manager must know how to communicate with the executor, and
since sharing and protection assignment can often be efficiently managed (in
the common case of running across processes on the same machine for security)
via the host operating system’s virtual memory management APIs.</p>
<p>To satisfy these requirements <code class="docutils literal notranslate"><span class="pre">JITLinkMemoryManager</span></code> adopts the following
design: The memory manager itself has just one virtual method that returns a
<code class="docutils literal notranslate"><span class="pre">JITLinkMemoryManager::Allocation</span></code>:</p>
<div class="highlight-c++ notranslate"><div class="highlight"><pre><span></span><span class="k">virtual</span> <span class="n">Expected</span><span class="o"><</span><span class="n">std</span><span class="o">::</span><span class="n">unique_ptr</span><span class="o"><</span><span class="n">Allocation</span><span class="o">>></span>
<span class="n">allocate</span><span class="p">(</span><span class="k">const</span> <span class="n">JITLinkDylib</span> <span class="o">*</span><span class="n">JD</span><span class="p">,</span> <span class="k">const</span> <span class="n">SegmentsRequestMap</span> <span class="o">&</span><span class="n">Request</span><span class="p">)</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span>
</pre></div>
</div>
<p>This method takes a <code class="docutils literal notranslate"><span class="pre">JITLinkDylib*</span></code> representing the target simulated
dylib, and the full set of sections that must be allocated for this object.
<code class="docutils literal notranslate"><span class="pre">JITLinkMemoryManager</span></code> implementations can (optionally) use the <code class="docutils literal notranslate"><span class="pre">JD</span></code>
argument to manage a per-simulated-dylib memory pool (since code model
constraints are typically imposed on a per-dylib basis, and not across
dylibs) <a class="footnote-reference brackets" href="#id8" id="id4">2</a>. The <code class="docutils literal notranslate"><span class="pre">Request</span></code> argument, by describing all sections in the current
object up-front, allows the implementer to allocate those sections as a
single slab, either within a pre-allocated per-jitdylib pool or directly
from system memory.</p>
<p>All subsequent operations are provided by the
<code class="docutils literal notranslate"><span class="pre">JITLinkMemoryManager::Allocation</span></code> interface:</p>
<ul>
<li><p><code class="docutils literal notranslate"><span class="pre">virtual</span> <span class="pre">MutableArrayRef<char></span> <span class="pre">getWorkingMemory(ProtectionFlags</span> <span class="pre">Seg)</span></code></p>
<p>Should be overridden to return the address in working memory of the segment
with the given protection flags.</p>
</li>
<li><p><code class="docutils literal notranslate"><span class="pre">virtual</span> <span class="pre">JITTargetAddress</span> <span class="pre">getTargetMemory(ProtectionFlags</span> <span class="pre">Seg)</span></code></p>
<p>Should be overridden to return the address in the executor’s address space of
the segment with the given protection flags.</p>
</li>
<li><p><code class="docutils literal notranslate"><span class="pre">virtual</span> <span class="pre">void</span> <span class="pre">finalizeAsync(FinalizeContinuation</span> <span class="pre">OnFinalize)</span></code></p>
<p>Should be overridden to copy the contents of working memory to the target
address space and apply memory protections for all segments. Where working
memory and target memory are separate, this method should deallocate the
working memory.</p>
</li>
<li><p><code class="docutils literal notranslate"><span class="pre">virtual</span> <span class="pre">Error</span> <span class="pre">deallocate()</span></code></p>
<p>Should be overridden to deallocate memory in the target address space.</p>
</li>
</ul>
<p>JITLink provides a simple in-process implementation of this interface:
<code class="docutils literal notranslate"><span class="pre">InProcessMemoryManager</span></code>. It allocates pages once and re-uses them as both
working and target memory.</p>
<p>ORC provides a cross-process <code class="docutils literal notranslate"><span class="pre">JITLinkMemoryManager</span></code> based on an ORC-RPC-based
implementation of the <code class="docutils literal notranslate"><span class="pre">orc::TargetProcessControl</span></code> API:
<code class="docutils literal notranslate"><span class="pre">OrcRPCTPCJITLinkMemoryManager</span></code>. This API uses TargetProcessControl API calls
to allocate and manage memory in a remote process. The underlying communication
channel is determined by the ORC-RPC channel type. Common options include unix
sockets or TCP.</p>
</div>
<div class="section" id="jitlinkmemorymanager-and-security">
<h3><a class="toc-backref" href="#id17">JITLinkMemoryManager and Security</a><a class="headerlink" href="#jitlinkmemorymanager-and-security" title="Permalink to this headline">¶</a></h3>
<p>JITLink’s ability to link JIT’d code for a separate executor process can be
used to improve the security of a JIT system: The executor process can be
sandboxed, run within a VM, or even run on a fully separate machine.</p>
<p>JITLink’s memory manager interface is flexible enough to allow for a range of
trade-offs between performance and security. For example, on a system where code
pages must be signed (preventing code from being updated), the memory manager
can deallocate working memory pages after linking to free memory in the process
running JITLink. Alternatively, on a system that allows RWX pages, the memory
manager may use the same pages for both working and target memory by marking
them as RWX, allowing code to be modified in place without further overhead.
Finally, if RWX pages are not permitted but dual-virtual-mappings of
physical memory pages are, then the memory manager can dual map physical pages
as RW- in the JITLink process and R-X in the executor process, allowing
modification from the JITLink process but not from the executor (at the cost of
extra administrative overhead for the dual mapping).</p>
</div>
<div class="section" id="error-handling">
<h3><a class="toc-backref" href="#id18">Error Handling</a><a class="headerlink" href="#error-handling" title="Permalink to this headline">¶</a></h3>
<p>JITLink makes extensive use of the <code class="docutils literal notranslate"><span class="pre">llvm::Error</span></code> type (see the error handling
section of <a class="reference internal" href="ProgrammersManual.html"><span class="doc">LLVM Programmer’s Manual</span></a> for details). The link process itself, all
passes, the memory manager interface, and operations on the <code class="docutils literal notranslate"><span class="pre">JITLinkContext</span></code>
are all permitted to fail. Link graph construction utilities (especially parsers
for object formats) are encouraged to validate input, and validate fixups
(e.g. with range checks) before application.</p>
<p>Any error will halt the link process and notify the context of failure. In ORC,
reported failures are propagated to queries pending on definitions provided by
the failing link, and also through edges of the dependence graph to any queries
waiting on dependent symbols.</p>
</div>
</div>
<div class="section" id="connection-to-the-orc-runtime">
<span id="connection-to-orc-runtime"></span><h2><a class="toc-backref" href="#id19">Connection to the ORC Runtime</a><a class="headerlink" href="#connection-to-the-orc-runtime" title="Permalink to this headline">¶</a></h2>
<p>The ORC Runtime (currently under development) aims to provide runtime support
for advanced JIT features, including object format features that require
non-trivial action in the executor (e.g. running initializers, managing thread
local storage, registering with language runtimes, etc.).</p>
<p>ORC Runtime support for object format features typically requires cooperation
between the runtime (which executes in the executor process) and JITLink (which
runs in the JIT process and can inspect LinkGraphs to determine what actions
must be taken in the executor). For example: Execution of MachO static
initializers in the ORC runtime is performed by the <code class="docutils literal notranslate"><span class="pre">jit_dlopen</span></code> function,
which calls back to the JIT process to ask for the list of address ranges of
<code class="docutils literal notranslate"><span class="pre">__mod_init</span></code> sections to walk. This list is collated by the
<code class="docutils literal notranslate"><span class="pre">MachOPlatformPlugin</span></code>, which installs a pass to record this information for
each object as it is linked into the target.</p>
</div>
<div class="section" id="constructing-linkgraphs">
<span id="id5"></span><h2><a class="toc-backref" href="#id20">Constructing LinkGraphs</a><a class="headerlink" href="#constructing-linkgraphs" title="Permalink to this headline">¶</a></h2>
<p>Clients usually access and manipulate <code class="docutils literal notranslate"><span class="pre">LinkGraph</span></code> instances that were created
for them by an <code class="docutils literal notranslate"><span class="pre">ObjectLinkingLayer</span></code> instance, but they can be created manually:</p>
<ol class="arabic simple">
<li><p>By directly constructing and populating a <code class="docutils literal notranslate"><span class="pre">LinkGraph</span></code> instance.</p></li>
<li><p>By using the <code class="docutils literal notranslate"><span class="pre">createLinkGraph</span></code> family of functions to create a
<code class="docutils literal notranslate"><span class="pre">LinkGraph</span></code> from an in-memory buffer containing an object file. This is how
<code class="docutils literal notranslate"><span class="pre">ObjectLinkingLayer</span></code> usually creates <code class="docutils literal notranslate"><span class="pre">LinkGraphs</span></code>.</p></li>
</ol>
<blockquote>
<div><ol class="arabic simple">
<li><dl class="simple">
<dt><code class="docutils literal notranslate"><span class="pre">createLinkGraph_<Object-Format>_<Architecture></span></code> can be used when</dt><dd><p>both the object format and architecture are known ahead of time.</p>
</dd>
</dl>
</li>
<li><p><code class="docutils literal notranslate"><span class="pre">createLinkGraph_<Object-Format></span></code> can be used when the object format is
known ahead of time, but the architecture is not. In this case the
architecture will be determined by inspection of the object header.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">createLinkGraph</span></code> can be used when neither the object format nor
the architecture are known ahead of time. In this case the object header
will be inspected to determine both the format and architecture.</p></li>
</ol>
</div></blockquote>
</div>
<div class="section" id="jit-linking">
<span id="id6"></span><h2><a class="toc-backref" href="#id21">JIT Linking</a><a class="headerlink" href="#jit-linking" title="Permalink to this headline">¶</a></h2>
<p>The JIT linker concept was introduced in LLVM’s earlier generation of JIT APIs,
MCJIT. In MCJIT the <em>RuntimeDyld</em> component enabled re-use of LLVM as an
in-memory compiler by adding an in-memory link step to the end of the usual
compiler pipeline. Rather than dumping relocatable objects to disk as a compiler
usually would, MCJIT passed them to RuntimeDyld to be linked into a target
process.</p>
<p>This approach to linking differs from standard <em>static</em> or <em>dynamic</em> linking:</p>
<p>A <em>static linker</em> takes one or more relocatable object files as input and links
them into an executable or dynamic library on disk.</p>
<p>A <em>dynamic linker</em> applies relocations to executables and dynamic libraries that
have been loaded into memory.</p>
<p>A <em>JIT linker</em> takes a single relocatable object file at a time and links it
into a target process, usually using a context object to allow the linked code
to resolve symbols in the target.</p>
<div class="section" id="runtimedyld">
<h3><a class="toc-backref" href="#id22">RuntimeDyld</a><a class="headerlink" href="#runtimedyld" title="Permalink to this headline">¶</a></h3>
<p>In order to keep RuntimeDyld’s implementation simple MCJIT imposed some
restrictions on compiled code:</p>
<ol class="arabic simple">
<li><p>It had to use the Large code model, and often restricted available relocation
models in order to limit the kinds of relocations that had to be supported.</p></li>
<li><p>It required strong linkage and default visibility on all symbols – behavior
for other linkages/visibilities was not well defined.</p></li>
<li><p>It constrained and/or prohibited the use of features requiring runtime
support, e.g. static initializers or thread local storage.</p></li>
</ol>
<p>As a result of these restrictions not all language features supported by LLVM
worked under MCJIT, and objects to be loaded under the JIT had to be compiled to
target it (precluding the use of precompiled code from other sources under the
JIT).</p>
<p>RuntimeDyld also provided very limited visibility into the linking process
itself: Clients could access conservative estimates of section size
(RuntimeDyld bundled stub size and padding estimates into the section size
value) and the final relocated bytes, but could not access RuntimeDyld’s
internal object representations.</p>
<p>Eliminating these restrictions and limitations was one of the primary motivations
for the development of JITLink.</p>
</div>
</div>
<div class="section" id="the-llvm-jitlink-tool">
<h2><a class="toc-backref" href="#id23">The llvm-jitlink tool</a><a class="headerlink" href="#the-llvm-jitlink-tool" title="Permalink to this headline">¶</a></h2>
<p>The <code class="docutils literal notranslate"><span class="pre">llvm-jitlink</span></code> tool is a command line wrapper for the JITLink library.
It loads some set of relocatable object files and then links them using
JITLink. Depending on the options used it will then execute them, or validate
the linked memory.</p>
<p>The <code class="docutils literal notranslate"><span class="pre">llvm-jitlink</span></code> tool was originally designed to aid JITLink development by
providing a simple environment for testing.</p>
<div class="section" id="basic-usage">
<h3><a class="toc-backref" href="#id24">Basic usage</a><a class="headerlink" href="#basic-usage" title="Permalink to this headline">¶</a></h3>
<p>By default, <code class="docutils literal notranslate"><span class="pre">llvm-jitlink</span></code> will link the set of objects passed on the command
line, then search for a “main” function and execute it:</p>
<div class="highlight-sh notranslate"><div class="highlight"><pre><span></span>% cat hello-world.c
<span class="c1">#include <stdio.h></span>
int main<span class="o">(</span>int argc, char *argv<span class="o">[])</span> <span class="o">{</span>
printf<span class="o">(</span><span class="s2">"hello, world!\n"</span><span class="o">)</span><span class="p">;</span>
<span class="k">return</span> <span class="m">0</span><span class="p">;</span>
<span class="o">}</span>
% clang -c -o hello-world.o hello-world.c
% llvm-jitlink hello-world.o
Hello, World!
</pre></div>
</div>
<p>Multiple objects may be specified, and arguments may be provided to the JIT’d
main function using the -args option:</p>
<div class="highlight-sh notranslate"><div class="highlight"><pre><span></span>% cat print-args.c
<span class="c1">#include <stdio.h></span>
void print_args<span class="o">(</span>int argc, char *argv<span class="o">[])</span> <span class="o">{</span>
<span class="k">for</span> <span class="o">(</span>int <span class="nv">i</span> <span class="o">=</span> <span class="m">0</span><span class="p">;</span> i !<span class="o">=</span> argc<span class="p">;</span> ++i<span class="o">)</span>
printf<span class="o">(</span><span class="s2">"arg %i is \"%s\"\n"</span>, i, argv<span class="o">[</span>i<span class="o">])</span><span class="p">;</span>
<span class="o">}</span>
% cat print-args-main.c
void print_args<span class="o">(</span>int argc, char *argv<span class="o">[])</span><span class="p">;</span>
int main<span class="o">(</span>int argc, char *argv<span class="o">[])</span> <span class="o">{</span>
print_args<span class="o">(</span>argc, argv<span class="o">)</span><span class="p">;</span>
<span class="k">return</span> <span class="m">0</span><span class="p">;</span>
<span class="o">}</span>
% clang -c -o print-args.o print-args.c
% clang -c -o print-args-main.o print-args-main.c
% llvm-jitlink print-args.o print-args-main.o -args a b c
arg <span class="m">0</span> is <span class="s2">"a"</span>
arg <span class="m">1</span> is <span class="s2">"b"</span>
arg <span class="m">2</span> is <span class="s2">"c"</span>
</pre></div>
</div>
<p>Alternative entry points may be specified using the <code class="docutils literal notranslate"><span class="pre">-entry</span> <span class="pre"><entry</span> <span class="pre">point</span>
<span class="pre">name></span></code> option.</p>
<p>Other options can be found by calling <code class="docutils literal notranslate"><span class="pre">llvm-jitlink</span> <span class="pre">-help</span></code>.</p>
</div>
<div class="section" id="llvm-jitlink-as-a-regression-testing-utility">
<h3><a class="toc-backref" href="#id25">llvm-jitlink as a regression testing utility</a><a class="headerlink" href="#llvm-jitlink-as-a-regression-testing-utility" title="Permalink to this headline">¶</a></h3>
<p>One of the primary aims of <code class="docutils literal notranslate"><span class="pre">llvm-jitlink</span></code> was to enable readable regression
tests for JITLink. To do this it supports two options:</p>
<p>The <code class="docutils literal notranslate"><span class="pre">-noexec</span></code> option tells llvm-jitlink to stop after looking up the entry
point, and before attempting to execute it. Since the linked code is not
executed, this can be used to link for other targets even if you do not have
access to the target being linked (the <code class="docutils literal notranslate"><span class="pre">-define-abs</span></code> or <code class="docutils literal notranslate"><span class="pre">-phony-externals</span></code>
options can be used to supply any missing definitions in this case).</p>
<p>The <code class="docutils literal notranslate"><span class="pre">-check</span> <span class="pre"><check-file></span></code> option can be used to run a set of <code class="docutils literal notranslate"><span class="pre">jitlink-check</span></code>
expressions against working memory. It is typically used in conjunction with
<code class="docutils literal notranslate"><span class="pre">-noexec</span></code>, since the aim is to validate JIT’d memory rather than to run the
code and <code class="docutils literal notranslate"><span class="pre">-noexec</span></code> allows us to link for any supported target architecture
from the current process. In <code class="docutils literal notranslate"><span class="pre">-check</span></code> mode, <code class="docutils literal notranslate"><span class="pre">llvm-jitlink</span></code> will scan the
given check-file for lines of the form <code class="docutils literal notranslate"><span class="pre">#</span> <span class="pre">jitlink-check:</span> <span class="pre"><expr></span></code>. See
examples of this usage in <code class="docutils literal notranslate"><span class="pre">llvm/test/ExecutionEngine/JITLink</span></code>.</p>
</div>
<div class="section" id="remote-execution-via-llvm-jitlink-executor">
<h3><a class="toc-backref" href="#id26">Remote execution via llvm-jitlink-executor</a><a class="headerlink" href="#remote-execution-via-llvm-jitlink-executor" title="Permalink to this headline">¶</a></h3>
<p>By default <code class="docutils literal notranslate"><span class="pre">llvm-jitlink</span></code> will link the given objects into its own process,
but this can be overridden by two options:</p>
<p>The <code class="docutils literal notranslate"><span class="pre">-oop-executor[=/path/to/executor]</span></code> option tells <code class="docutils literal notranslate"><span class="pre">llvm-jitlink</span></code> to
execute the given executor (which defaults to <code class="docutils literal notranslate"><span class="pre">llvm-jitlink-executor</span></code>) and
communicate with it via file descriptors which it passes to the executor
as the first argument with the format <code class="docutils literal notranslate"><span class="pre">filedescs=<in-fd>,<out-fd></span></code>.</p>
<p>The <code class="docutils literal notranslate"><span class="pre">-oop-executor-connect=<host>:<port></span></code> option tells <code class="docutils literal notranslate"><span class="pre">llvm-jitlink</span></code> to
connect to an already running executor via TCP on the given host and port. To
use this option you will need to start <code class="docutils literal notranslate"><span class="pre">llvm-jitlink-executor</span></code> manually with
<code class="docutils literal notranslate"><span class="pre">listen=<host>:<port></span></code> as the first argument.</p>
</div>
<div class="section" id="harness-mode">
<h3><a class="toc-backref" href="#id27">Harness mode</a><a class="headerlink" href="#harness-mode" title="Permalink to this headline">¶</a></h3>
<p>The <code class="docutils literal notranslate"><span class="pre">-harness</span></code> option allows a set of input objects to be designated as a test
harness, with the regular object files implicitly treated as objects to be
tested. Definitions of symbols in the harness set override definitions in the
test set, and external references from the harness cause automatic scope
promotion of local symbols in the test set (these modifications to the usual
linker rules are accomplished via an <code class="docutils literal notranslate"><span class="pre">ObjectLinkingLayer::Plugin</span></code> installed by
<code class="docutils literal notranslate"><span class="pre">llvm-jitlink</span></code> when it sees the <code class="docutils literal notranslate"><span class="pre">-harness</span></code> option).</p>
<p>With these modifications in place we can selectively test functions in an object
file by mocking those function’s callees. For example, suppose we have an object
file, <code class="docutils literal notranslate"><span class="pre">test_code.o</span></code>, compiled from the following C source (which we need not
have access to):</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="kt">void</span> <span class="nf">irrelevant_function</span><span class="p">()</span> <span class="p">{</span> <span class="n">irrelevant_external</span><span class="p">();</span> <span class="p">}</span>
<span class="kt">int</span> <span class="nf">function_to_mock</span><span class="p">(</span><span class="kt">int</span> <span class="n">X</span><span class="p">)</span> <span class="p">{</span>
<span class="k">return</span> <span class="cm">/* some function of X */</span><span class="p">;</span>
<span class="p">}</span>
<span class="k">static</span> <span class="kt">void</span> <span class="n">function_to_test</span><span class="p">()</span> <span class="p">{</span>
<span class="p">...</span>
<span class="kt">int</span> <span class="n">Y</span> <span class="o">=</span> <span class="n">function_to_mock</span><span class="p">();</span>
<span class="n">printf</span><span class="p">(</span><span class="s">"Y is %i</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span> <span class="n">Y</span><span class="p">);</span>
<span class="p">}</span>
</pre></div>
</div>
<p>If we want to know how <code class="docutils literal notranslate"><span class="pre">function_to_test</span></code> behaves when we change the behavior
of <code class="docutils literal notranslate"><span class="pre">function_to_mock</span></code> we can test it by writing a test harness:</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="kt">void</span> <span class="nf">function_to_test</span><span class="p">();</span>
<span class="kt">int</span> <span class="nf">function_to_mock</span><span class="p">(</span><span class="kt">int</span> <span class="n">X</span><span class="p">)</span> <span class="p">{</span>
<span class="n">printf</span><span class="p">(</span><span class="s">"used mock utility function</span><span class="se">\n</span><span class="s">"</span><span class="p">);</span>
<span class="k">return</span> <span class="mi">42</span><span class="p">;</span>
<span class="p">}</span>
<span class="kt">int</span> <span class="nf">main</span><span class="p">(</span><span class="kt">int</span> <span class="n">argc</span><span class="p">,</span> <span class="kt">char</span> <span class="o">*</span><span class="n">argv</span><span class="p">[])</span> <span class="p">{</span>
<span class="n">function_to_test</span><span class="p">()</span><span class="o">:</span>
<span class="k">return</span> <span class="mi">0</span><span class="p">;</span>
<span class="p">}</span>
</pre></div>
</div>
<p>Under normal circumstances these objects could not be linked together:
<code class="docutils literal notranslate"><span class="pre">function_to_test</span></code> is static and could not be resolved outside
<code class="docutils literal notranslate"><span class="pre">test_code.o</span></code>, the two <code class="docutils literal notranslate"><span class="pre">function_to_mock</span></code> functions would result in a
duplicate definition error, and <code class="docutils literal notranslate"><span class="pre">irrelevant_external</span></code> is undefined.
However, using <code class="docutils literal notranslate"><span class="pre">-harness</span></code> and <code class="docutils literal notranslate"><span class="pre">-phony-externals</span></code> we can run this code
with:</p>
<div class="highlight-sh notranslate"><div class="highlight"><pre><span></span>% clang -c -o test_code_harness.o test_code_harness.c
% llvm-jitlink -phony-externals test_code.o -harness test_code_harness.o
used mock utility <span class="k">function</span>
Y is <span class="m">42</span>
</pre></div>
</div>
<p>The <code class="docutils literal notranslate"><span class="pre">-harness</span></code> option may be of interest to people who want to perform some
very late testing on build products to verify that compiled code behaves as
expected. On basic C test cases this is relatively straightforward. Mocks for
more complicated languages (e.g. C++) are much trickier: Any code involving
classes tends to have a lot of non-trivial surface area (e.g. vtables) that
would require great care to mock.</p>
</div>
<div class="section" id="tips-for-jitlink-backend-developers">
<h3><a class="toc-backref" href="#id28">Tips for JITLink backend developers</a><a class="headerlink" href="#tips-for-jitlink-backend-developers" title="Permalink to this headline">¶</a></h3>
<ol class="arabic simple">
<li><p>Make liberal use of assert and <code class="docutils literal notranslate"><span class="pre">llvm::Error</span></code>. Do <em>not</em> assume that the input
object is well formed: Return any errors produced by libObject (or your own
object parsing code) and validate as you construct. Think carefully about the
distinction between contract (which should be validated with asserts and
llvm_unreachable) and environmental errors (which should generate
<code class="docutils literal notranslate"><span class="pre">llvm::Error</span></code> instances).</p></li>
<li><p>Don’t assume you’re linking in-process. Use libSupport’s sized,
endian-specific types when reading/writing content in the <code class="docutils literal notranslate"><span class="pre">LinkGraph</span></code>.</p></li>
</ol>
<p>As a “minimum viable” JITLink wrapper, the <code class="docutils literal notranslate"><span class="pre">llvm-jitlink</span></code> tool is an
invaluable resource for developers bringing in a new JITLink backend. A standard
workflow is to start by throwing an unsupported object at the tool and seeing
what error is returned, then fixing that (you can often make a reasonable guess
at what should be done based on existing code for other formats or
architectures).</p>
<p>In debug builds of LLVM, the <code class="docutils literal notranslate"><span class="pre">-debug-only=jitlink</span></code> option dumps logs from the
JITLink library during the link process. These can be useful for spotting some bugs at
a glance. The <code class="docutils literal notranslate"><span class="pre">-debug-only=llvm_jitlink</span></code> option dumps logs from the <code class="docutils literal notranslate"><span class="pre">llvm-jitlink</span></code>
tool, which can be useful for debugging both testcases (it is often less verbose than
<code class="docutils literal notranslate"><span class="pre">-debug-only=jitlink</span></code>) and the tool itself.</p>
<p>The <code class="docutils literal notranslate"><span class="pre">-oop-executor</span></code> and <code class="docutils literal notranslate"><span class="pre">-oop-executor-connect</span></code> options are helpful for testing
handling of cross-process and cross-architecture use cases.</p>
</div>
</div>
<div class="section" id="roadmap">
<h2><a class="toc-backref" href="#id29">Roadmap</a><a class="headerlink" href="#roadmap" title="Permalink to this headline">¶</a></h2>
<p>JITLink is under active development. Work so far has focused on the MachO
implementation. In LLVM 12 there is limited support for ELF on x86-64.</p>
<p>Major outstanding projects include:</p>
<ul>
<li><p>Refactor architecture support to maximize sharing across formats.</p>
<p>All formats should be able to share the bulk of the architecture specific
code (especially relocations) for each supported architecture.</p>
</li>
<li><p>Refactor ELF link graph construction.</p>
<p>ELF’s link graph construction is currently implemented in the <cite>ELF_x86_64.cpp</cite>
file, and tied to the x86-64 relocation parsing code. The bulk of the code is
generic and should be split into an ELFLinkGraphBuilder base class along the
same lines as the existing generic MachOLinkGraphBuilder.</p>
</li>
<li><p>Implement ELF support for arm64.</p>
<p>Once the architecture support code has been refactored to enable sharing and
ELF link graph construction has been refactored to allow re-use we should be
able to construct an ELF / arm64 JITLink implementation by combining
these existing pieces.</p>
</li>
<li><p>Implement support for new architectures.</p></li>
<li><p>Implement support for COFF.</p>
<p>There is no COFF implementation of JITLink yet. Such an implementation should
follow the MachO and ELF paths: a generic COFFLinkGraphBuilder base class that
can be specialized for each architecture.</p>
</li>
<li><p>Design and implement a shared-memory based JITLinkMemoryManager.</p>
<p>One use-case that is expected to be common is out-of-process linking targeting
another process on the same machine. This allows JITs to sandbox JIT’d code.
For this use case a shared-memory based JITLinkMemoryManager would provide the
most efficient form of allocation. Creating one will require designing a
generic API for shared memory though, as LLVM does not currently have one.</p>
</li>
</ul>
<div class="section" id="jitlink-availability-and-feature-status">
<h3><a class="toc-backref" href="#id30">JITLink Availability and Feature Status</a><a class="headerlink" href="#jitlink-availability-and-feature-status" title="Permalink to this headline">¶</a></h3>
<table class="colwidths-given docutils align-default" id="id9">
<caption><span class="caption-text">Availability and Status</span><a class="headerlink" href="#id9" title="Permalink to this table">¶</a></caption>
<colgroup>
<col style="width: 10%" />
<col style="width: 30%" />
<col style="width: 30%" />
<col style="width: 30%" />
</colgroup>
<thead>
<tr class="row-odd"><th class="head"><p>Architecture</p></th>
<th class="head"><p>ELF</p></th>
<th class="head"><p>COFF</p></th>
<th class="head"><p>MachO</p></th>
</tr>
</thead>
<tbody>
<tr class="row-even"><td><p>arm64</p></td>
<td></td>
<td></td>
<td><p>Partial (small code model, PIC relocation model only)</p></td>
</tr>
<tr class="row-odd"><td><p>x86-64</p></td>
<td><p>Partial</p></td>
<td></td>
<td><p>Full (except TLV and debugging)</p></td>
</tr>
</tbody>
</table>
<dl class="footnote brackets">
<dt class="label" id="id7"><span class="brackets"><a class="fn-backref" href="#id1">1</a></span></dt>
<dd><p>See <code class="docutils literal notranslate"><span class="pre">llvm/examples/OrcV2Examples/LLJITWithObjectLinkingLayerPlugin</span></code> for
a full worked example.</p>
</dd>
<dt class="label" id="id8"><span class="brackets"><a class="fn-backref" href="#id4">2</a></span></dt>
<dd><p>If not for <em>hidden</em> scoped symbols we could eliminate the
<code class="docutils literal notranslate"><span class="pre">JITLinkDylib*</span></code> argument to <code class="docutils literal notranslate"><span class="pre">JITLinkMemoryManager::allocate</span></code> and
treat every object as a separate simulated dylib for the purposes of
memory layout. Hidden symbols break this by generating in-range accesses
to external symbols, requiring the access and symbol to be allocated
within range of one another. That said, providing a pre-reserved address
range pool for each simulated dylib guarantees that the relaxation
optimizations will kick in for all intra-dylib references, which is good
for performance (at the cost of whatever overhead is introduced by
reserving the address-range up-front).</p>
</dd>
</dl>
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