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<div class="section" id="accurate-garbage-collection-with-llvm">
<h1>Accurate Garbage Collection with LLVM<a class="headerlink" href="#accurate-garbage-collection-with-llvm" title="Permalink to this headline">¶</a></h1>
<div class="contents local topic" id="contents">
<ul class="simple">
<li><a class="reference internal" href="#introduction" id="id1">Introduction</a><ul>
<li><a class="reference internal" href="#goals-and-non-goals" id="id2">Goals and non-goals</a></li>
</ul>
</li>
<li><a class="reference internal" href="#getting-started" id="id3">Getting started</a><ul>
<li><a class="reference internal" href="#in-your-compiler" id="id4">In your compiler</a></li>
<li><a class="reference internal" href="#in-your-runtime" id="id5">In your runtime</a></li>
<li><a class="reference internal" href="#about-the-shadow-stack" id="id6">About the shadow stack</a></li>
</ul>
</li>
<li><a class="reference internal" href="#ir-features" id="id7">IR features</a><ul>
<li><a class="reference internal" href="#specifying-gc-code-generation-gc" id="id8">Specifying GC code generation: <tt class="docutils literal"><span class="pre">gc</span> <span class="pre">"..."</span></tt></a></li>
<li><a class="reference internal" href="#gcroot" id="id9">Identifying GC roots on the stack: <tt class="docutils literal"><span class="pre">llvm.gcroot</span></tt></a></li>
<li><a class="reference internal" href="#reading-and-writing-references-in-the-heap" id="id10">Reading and writing references in the heap</a><ul>
<li><a class="reference internal" href="#write-barrier-llvm-gcwrite" id="id11">Write barrier: <tt class="docutils literal"><span class="pre">llvm.gcwrite</span></tt></a></li>
<li><a class="reference internal" href="#read-barrier-llvm-gcread" id="id12">Read barrier: <tt class="docutils literal"><span class="pre">llvm.gcread</span></tt></a></li>
</ul>
</li>
</ul>
</li>
<li><a class="reference internal" href="#implementing-a-collector-plugin" id="id13">Implementing a collector plugin</a><ul>
<li><a class="reference internal" href="#overview-of-available-features" id="id14">Overview of available features</a></li>
<li><a class="reference internal" href="#computing-stack-maps" id="id15">Computing stack maps</a></li>
<li><a class="reference internal" href="#initializing-roots-to-null-initroots" id="id16">Initializing roots to null: <tt class="docutils literal"><span class="pre">InitRoots</span></tt></a></li>
<li><a class="reference internal" href="#custom-lowering-of-intrinsics-customroots-customreadbarriers-and-customwritebarriers" id="id17">Custom lowering of intrinsics: <tt class="docutils literal"><span class="pre">CustomRoots</span></tt>, <tt class="docutils literal"><span class="pre">CustomReadBarriers</span></tt>, and <tt class="docutils literal"><span class="pre">CustomWriteBarriers</span></tt></a></li>
<li><a class="reference internal" href="#generating-safe-points-neededsafepoints" id="id18">Generating safe points: <tt class="docutils literal"><span class="pre">NeededSafePoints</span></tt></a></li>
<li><a class="reference internal" href="#emitting-assembly-code-gcmetadataprinter" id="id19">Emitting assembly code: <tt class="docutils literal"><span class="pre">GCMetadataPrinter</span></tt></a></li>
</ul>
</li>
<li><a class="reference internal" href="#references" id="id20">References</a></li>
</ul>
</div>
<div class="section" id="introduction">
<h2><a class="toc-backref" href="#id1">Introduction</a><a class="headerlink" href="#introduction" title="Permalink to this headline">¶</a></h2>
<p>Garbage collection is a widely used technique that frees the programmer from
having to know the lifetimes of heap objects, making software easier to produce
and maintain. Many programming languages rely on garbage collection for
automatic memory management. There are two primary forms of garbage collection:
conservative and accurate.</p>
<p>Conservative garbage collection often does not require any special support from
either the language or the compiler: it can handle non-type-safe programming
languages (such as C/C++) and does not require any special information from the
compiler. The <a class="reference external" href="http://www.hpl.hp.com/personal/Hans_Boehm/gc/">Boehm collector</a> is an example of a
state-of-the-art conservative collector.</p>
<p>Accurate garbage collection requires the ability to identify all pointers in the
program at run-time (which requires that the source-language be type-safe in
most cases). Identifying pointers at run-time requires compiler support to
locate all places that hold live pointer variables at run-time, including the
<a class="reference internal" href="#gcroot"><em>processor stack and registers</em></a>.</p>
<p>Conservative garbage collection is attractive because it does not require any
special compiler support, but it does have problems. In particular, because the
conservative garbage collector cannot <em>know</em> that a particular word in the
machine is a pointer, it cannot move live objects in the heap (preventing the
use of compacting and generational GC algorithms) and it can occasionally suffer
from memory leaks due to integer values that happen to point to objects in the
program. In addition, some aggressive compiler transformations can break
conservative garbage collectors (though these seem rare in practice).</p>
<p>Accurate garbage collectors do not suffer from any of these problems, but they
can suffer from degraded scalar optimization of the program. In particular,
because the runtime must be able to identify and update all pointers active in
the program, some optimizations are less effective. In practice, however, the
locality and performance benefits of using aggressive garbage collection
techniques dominates any low-level losses.</p>
<p>This document describes the mechanisms and interfaces provided by LLVM to
support accurate garbage collection.</p>
<div class="section" id="goals-and-non-goals">
<h3><a class="toc-backref" href="#id2">Goals and non-goals</a><a class="headerlink" href="#goals-and-non-goals" title="Permalink to this headline">¶</a></h3>
<p>LLVM’s intermediate representation provides <a class="reference internal" href="#gc-intrinsics"><em>garbage collection intrinsics</em></a> that offer support for a broad class of collector models. For
instance, the intrinsics permit:</p>
<ul class="simple">
<li>semi-space collectors</li>
<li>mark-sweep collectors</li>
<li>generational collectors</li>
<li>reference counting</li>
<li>incremental collectors</li>
<li>concurrent collectors</li>
<li>cooperative collectors</li>
</ul>
<p>We hope that the primitive support built into the LLVM IR is sufficient to
support a broad class of garbage collected languages including Scheme, ML, Java,
C#, Perl, Python, Lua, Ruby, other scripting languages, and more.</p>
<p>However, LLVM does not itself provide a garbage collector — this should be
part of your language’s runtime library. LLVM provides a framework for compile
time <a class="reference internal" href="#plugin"><em>code generation plugins</em></a>. The role of these plugins is to
generate code and data structures which conforms to the <em>binary interface</em>
specified by the <em>runtime library</em>. This is similar to the relationship between
LLVM and DWARF debugging info, for example. The difference primarily lies in
the lack of an established standard in the domain of garbage collection — thus
the plugins.</p>
<p>The aspects of the binary interface with which LLVM’s GC support is
concerned are:</p>
<ul class="simple">
<li>Creation of GC-safe points within code where collection is allowed to execute
safely.</li>
<li>Computation of the stack map. For each safe point in the code, object
references within the stack frame must be identified so that the collector may
traverse and perhaps update them.</li>
<li>Write barriers when storing object references to the heap. These are commonly
used to optimize incremental scans in generational collectors.</li>
<li>Emission of read barriers when loading object references. These are useful
for interoperating with concurrent collectors.</li>
</ul>
<p>There are additional areas that LLVM does not directly address:</p>
<ul class="simple">
<li>Registration of global roots with the runtime.</li>
<li>Registration of stack map entries with the runtime.</li>
<li>The functions used by the program to allocate memory, trigger a collection,
etc.</li>
<li>Computation or compilation of type maps, or registration of them with the
runtime. These are used to crawl the heap for object references.</li>
</ul>
<p>In general, LLVM’s support for GC does not include features which can be
adequately addressed with other features of the IR and does not specify a
particular binary interface. On the plus side, this means that you should be
able to integrate LLVM with an existing runtime. On the other hand, it leaves a
lot of work for the developer of a novel language. However, it’s easy to get
started quickly and scale up to a more sophisticated implementation as your
compiler matures.</p>
</div>
</div>
<div class="section" id="getting-started">
<h2><a class="toc-backref" href="#id3">Getting started</a><a class="headerlink" href="#getting-started" title="Permalink to this headline">¶</a></h2>
<p>Using a GC with LLVM implies many things, for example:</p>
<ul class="simple">
<li>Write a runtime library or find an existing one which implements a GC heap.<ol class="arabic">
<li>Implement a memory allocator.</li>
<li>Design a binary interface for the stack map, used to identify references
within a stack frame on the machine stack.*</li>
<li>Implement a stack crawler to discover functions on the call stack.*</li>
<li>Implement a registry for global roots.</li>
<li>Design a binary interface for type maps, used to identify references
within heap objects.</li>
<li>Implement a collection routine bringing together all of the above.</li>
</ol>
</li>
<li>Emit compatible code from your compiler.<ul>
<li>Initialization in the main function.</li>
<li>Use the <tt class="docutils literal"><span class="pre">gc</span> <span class="pre">"..."</span></tt> attribute to enable GC code generation (or
<tt class="docutils literal"><span class="pre">F.setGC("...")</span></tt>).</li>
<li>Use <tt class="docutils literal"><span class="pre">@llvm.gcroot</span></tt> to mark stack roots.</li>
<li>Use <tt class="docutils literal"><span class="pre">@llvm.gcread</span></tt> and/or <tt class="docutils literal"><span class="pre">@llvm.gcwrite</span></tt> to manipulate GC references,
if necessary.</li>
<li>Allocate memory using the GC allocation routine provided by the runtime
library.</li>
<li>Generate type maps according to your runtime’s binary interface.</li>
</ul>
</li>
<li>Write a compiler plugin to interface LLVM with the runtime library.*<ul>
<li>Lower <tt class="docutils literal"><span class="pre">@llvm.gcread</span></tt> and <tt class="docutils literal"><span class="pre">@llvm.gcwrite</span></tt> to appropriate code
sequences.*</li>
<li>Compile LLVM’s stack map to the binary form expected by the runtime.</li>
</ul>
</li>
<li>Load the plugin into the compiler. Use <tt class="docutils literal"><span class="pre">llc</span> <span class="pre">-load</span></tt> or link the plugin
statically with your language’s compiler.*</li>
<li>Link program executables with the runtime.</li>
</ul>
<p>To help with several of these tasks (those indicated with a *), LLVM includes a
highly portable, built-in ShadowStack code generator. It is compiled into
<tt class="docutils literal"><span class="pre">llc</span></tt> and works even with the interpreter and C backends.</p>
<div class="section" id="in-your-compiler">
<h3><a class="toc-backref" href="#id4">In your compiler</a><a class="headerlink" href="#in-your-compiler" title="Permalink to this headline">¶</a></h3>
<p>To turn the shadow stack on for your functions, first call:</p>
<div class="highlight-c++"><div class="highlight"><pre><span class="n">F</span><span class="p">.</span><span class="n">setGC</span><span class="p">(</span><span class="s">"shadow-stack"</span><span class="p">);</span>
</pre></div>
</div>
<p>for each function your compiler emits. Since the shadow stack is built into
LLVM, you do not need to load a plugin.</p>
<p>Your compiler must also use <tt class="docutils literal"><span class="pre">@llvm.gcroot</span></tt> as documented. Don’t forget to
create a root for each intermediate value that is generated when evaluating an
expression. In <tt class="docutils literal"><span class="pre">h(f(),</span> <span class="pre">g())</span></tt>, the result of <tt class="docutils literal"><span class="pre">f()</span></tt> could easily be collected
if evaluating <tt class="docutils literal"><span class="pre">g()</span></tt> triggers a collection.</p>
<p>There’s no need to use <tt class="docutils literal"><span class="pre">@llvm.gcread</span></tt> and <tt class="docutils literal"><span class="pre">@llvm.gcwrite</span></tt> over plain
<tt class="docutils literal"><span class="pre">load</span></tt> and <tt class="docutils literal"><span class="pre">store</span></tt> for now. You will need them when switching to a more
advanced GC.</p>
</div>
<div class="section" id="in-your-runtime">
<h3><a class="toc-backref" href="#id5">In your runtime</a><a class="headerlink" href="#in-your-runtime" title="Permalink to this headline">¶</a></h3>
<p>The shadow stack doesn’t imply a memory allocation algorithm. A semispace
collector or building atop <tt class="docutils literal"><span class="pre">malloc</span></tt> are great places to start, and can be
implemented with very little code.</p>
<p>When it comes time to collect, however, your runtime needs to traverse the stack
roots, and for this it needs to integrate with the shadow stack. Luckily, doing
so is very simple. (This code is heavily commented to help you understand the
data structure, but there are only 20 lines of meaningful code.)</p>
<div class="highlight-c++"><div class="highlight"><pre><span class="c1">/// @brief The map for a single function's stack frame. One of these is</span>
<span class="c1">/// compiled as constant data into the executable for each function.</span>
<span class="c1">///</span>
<span class="c1">/// Storage of metadata values is elided if the %metadata parameter to</span>
<span class="c1">/// @llvm.gcroot is null.</span>
<span class="k">struct</span> <span class="n">FrameMap</span> <span class="p">{</span>
<span class="kt">int32_t</span> <span class="n">NumRoots</span><span class="p">;</span> <span class="c1">//< Number of roots in stack frame.</span>
<span class="kt">int32_t</span> <span class="n">NumMeta</span><span class="p">;</span> <span class="c1">//< Number of metadata entries. May be < NumRoots.</span>
<span class="k">const</span> <span class="kt">void</span> <span class="o">*</span><span class="n">Meta</span><span class="p">[</span><span class="mi">0</span><span class="p">];</span> <span class="c1">//< Metadata for each root.</span>
<span class="p">};</span>
<span class="c1">/// @brief A link in the dynamic shadow stack. One of these is embedded in</span>
<span class="c1">/// the stack frame of each function on the call stack.</span>
<span class="k">struct</span> <span class="n">StackEntry</span> <span class="p">{</span>
<span class="n">StackEntry</span> <span class="o">*</span><span class="n">Next</span><span class="p">;</span> <span class="c1">//< Link to next stack entry (the caller's).</span>
<span class="k">const</span> <span class="n">FrameMap</span> <span class="o">*</span><span class="n">Map</span><span class="p">;</span> <span class="c1">//< Pointer to constant FrameMap.</span>
<span class="kt">void</span> <span class="o">*</span><span class="n">Roots</span><span class="p">[</span><span class="mi">0</span><span class="p">];</span> <span class="c1">//< Stack roots (in-place array).</span>
<span class="p">};</span>
<span class="c1">/// @brief The head of the singly-linked list of StackEntries. Functions push</span>
<span class="c1">/// and pop onto this in their prologue and epilogue.</span>
<span class="c1">///</span>
<span class="c1">/// Since there is only a global list, this technique is not threadsafe.</span>
<span class="n">StackEntry</span> <span class="o">*</span><span class="n">llvm_gc_root_chain</span><span class="p">;</span>
<span class="c1">/// @brief Calls Visitor(root, meta) for each GC root on the stack.</span>
<span class="c1">/// root and meta are exactly the values passed to</span>
<span class="c1">/// @llvm.gcroot.</span>
<span class="c1">///</span>
<span class="c1">/// Visitor could be a function to recursively mark live objects. Or it</span>
<span class="c1">/// might copy them to another heap or generation.</span>
<span class="c1">///</span>
<span class="c1">/// @param Visitor A function to invoke for every GC root on the stack.</span>
<span class="kt">void</span> <span class="nf">visitGCRoots</span><span class="p">(</span><span class="kt">void</span> <span class="p">(</span><span class="o">*</span><span class="n">Visitor</span><span class="p">)(</span><span class="kt">void</span> <span class="o">**</span><span class="n">Root</span><span class="p">,</span> <span class="k">const</span> <span class="kt">void</span> <span class="o">*</span><span class="n">Meta</span><span class="p">))</span> <span class="p">{</span>
<span class="k">for</span> <span class="p">(</span><span class="n">StackEntry</span> <span class="o">*</span><span class="n">R</span> <span class="o">=</span> <span class="n">llvm_gc_root_chain</span><span class="p">;</span> <span class="n">R</span><span class="p">;</span> <span class="n">R</span> <span class="o">=</span> <span class="n">R</span><span class="o">-></span><span class="n">Next</span><span class="p">)</span> <span class="p">{</span>
<span class="kt">unsigned</span> <span class="n">i</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span>
<span class="c1">// For roots [0, NumMeta), the metadata pointer is in the FrameMap.</span>
<span class="k">for</span> <span class="p">(</span><span class="kt">unsigned</span> <span class="n">e</span> <span class="o">=</span> <span class="n">R</span><span class="o">-></span><span class="n">Map</span><span class="o">-></span><span class="n">NumMeta</span><span class="p">;</span> <span class="n">i</span> <span class="o">!=</span> <span class="n">e</span><span class="p">;</span> <span class="o">++</span><span class="n">i</span><span class="p">)</span>
<span class="n">Visitor</span><span class="p">(</span><span class="o">&</span><span class="n">R</span><span class="o">-></span><span class="n">Roots</span><span class="p">[</span><span class="n">i</span><span class="p">],</span> <span class="n">R</span><span class="o">-></span><span class="n">Map</span><span class="o">-></span><span class="n">Meta</span><span class="p">[</span><span class="n">i</span><span class="p">]);</span>
<span class="c1">// For roots [NumMeta, NumRoots), the metadata pointer is null.</span>
<span class="k">for</span> <span class="p">(</span><span class="kt">unsigned</span> <span class="n">e</span> <span class="o">=</span> <span class="n">R</span><span class="o">-></span><span class="n">Map</span><span class="o">-></span><span class="n">NumRoots</span><span class="p">;</span> <span class="n">i</span> <span class="o">!=</span> <span class="n">e</span><span class="p">;</span> <span class="o">++</span><span class="n">i</span><span class="p">)</span>
<span class="n">Visitor</span><span class="p">(</span><span class="o">&</span><span class="n">R</span><span class="o">-></span><span class="n">Roots</span><span class="p">[</span><span class="n">i</span><span class="p">],</span> <span class="nb">NULL</span><span class="p">);</span>
<span class="p">}</span>
<span class="p">}</span>
</pre></div>
</div>
</div>
<div class="section" id="about-the-shadow-stack">
<h3><a class="toc-backref" href="#id6">About the shadow stack</a><a class="headerlink" href="#about-the-shadow-stack" title="Permalink to this headline">¶</a></h3>
<p>Unlike many GC algorithms which rely on a cooperative code generator to compile
stack maps, this algorithm carefully maintains a linked list of stack roots
[<a class="reference internal" href="#henderson02"><em>Henderson2002</em></a>]. This so-called “shadow stack” mirrors the
machine stack. Maintaining this data structure is slower than using a stack map
compiled into the executable as constant data, but has a significant portability
advantage because it requires no special support from the target code generator,
and does not require tricky platform-specific code to crawl the machine stack.</p>
<p>The tradeoff for this simplicity and portability is:</p>
<ul class="simple">
<li>High overhead per function call.</li>
<li>Not thread-safe.</li>
</ul>
<p>Still, it’s an easy way to get started. After your compiler and runtime are up
and running, writing a <a class="reference internal" href="#plugin"><em>plugin</em></a> will allow you to take advantage
of <a class="reference internal" href="#collector-algos"><em>more advanced GC features</em></a> of LLVM in order to
improve performance.</p>
</div>
</div>
<div class="section" id="ir-features">
<span id="gc-intrinsics"></span><h2><a class="toc-backref" href="#id7">IR features</a><a class="headerlink" href="#ir-features" title="Permalink to this headline">¶</a></h2>
<p>This section describes the garbage collection facilities provided by the
<a class="reference internal" href="LangRef.html"><em>LLVM intermediate representation</em></a>. The exact behavior of these
IR features is specified by the binary interface implemented by a <a class="reference internal" href="#plugin"><em>code
generation plugin</em></a>, not by this document.</p>
<p>These facilities are limited to those strictly necessary; they are not intended
to be a complete interface to any garbage collector. A program will need to
interface with the GC library using the facilities provided by that program.</p>
<div class="section" id="specifying-gc-code-generation-gc">
<h3><a class="toc-backref" href="#id8">Specifying GC code generation: <tt class="docutils literal"><span class="pre">gc</span> <span class="pre">"..."</span></tt></a><a class="headerlink" href="#specifying-gc-code-generation-gc" title="Permalink to this headline">¶</a></h3>
<div class="highlight-llvm"><div class="highlight"><pre>define ty @name(...) gc "name" { ...
</pre></div>
</div>
<p>The <tt class="docutils literal"><span class="pre">gc</span></tt> function attribute is used to specify the desired GC style to the
compiler. Its programmatic equivalent is the <tt class="docutils literal"><span class="pre">setGC</span></tt> method of <tt class="docutils literal"><span class="pre">Function</span></tt>.</p>
<p>Setting <tt class="docutils literal"><span class="pre">gc</span> <span class="pre">"name"</span></tt> on a function triggers a search for a matching code
generation plugin “<em>name</em>”; it is that plugin which defines the exact nature of
the code generated to support GC. If none is found, the compiler will raise an
error.</p>
<p>Specifying the GC style on a per-function basis allows LLVM to link together
programs that use different garbage collection algorithms (or none at all).</p>
</div>
<div class="section" id="gcroot">
<span id="identifying-gc-roots-on-the-stack-llvm-gcroot"></span><h3><a class="toc-backref" href="#id9">Identifying GC roots on the stack: <tt class="docutils literal"><span class="pre">llvm.gcroot</span></tt></a><a class="headerlink" href="#gcroot" title="Permalink to this headline">¶</a></h3>
<div class="highlight-llvm"><div class="highlight"><pre><span class="kt">void</span> <span class="vg">@llvm.gcroot</span><span class="p">(</span><span class="k">i8</span><span class="p">**</span> <span class="nv">%ptrloc</span><span class="p">,</span> <span class="k">i8</span><span class="p">*</span> <span class="nv">%metadata</span><span class="p">)</span>
</pre></div>
</div>
<p>The <tt class="docutils literal"><span class="pre">llvm.gcroot</span></tt> intrinsic is used to inform LLVM that a stack variable
references an object on the heap and is to be tracked for garbage collection.
The exact impact on generated code is specified by a <a class="reference internal" href="#plugin"><em>compiler plugin</em></a>. All calls to <tt class="docutils literal"><span class="pre">llvm.gcroot</span></tt> <strong>must</strong> reside inside the first basic
block.</p>
<p>A compiler which uses mem2reg to raise imperative code using <tt class="docutils literal"><span class="pre">alloca</span></tt> into SSA
form need only add a call to <tt class="docutils literal"><span class="pre">@llvm.gcroot</span></tt> for those variables which a
pointers into the GC heap.</p>
<p>It is also important to mark intermediate values with <tt class="docutils literal"><span class="pre">llvm.gcroot</span></tt>. For
example, consider <tt class="docutils literal"><span class="pre">h(f(),</span> <span class="pre">g())</span></tt>. Beware leaking the result of <tt class="docutils literal"><span class="pre">f()</span></tt> in the
case that <tt class="docutils literal"><span class="pre">g()</span></tt> triggers a collection. Note, that stack variables must be
initialized and marked with <tt class="docutils literal"><span class="pre">llvm.gcroot</span></tt> in function’s prologue.</p>
<p>The first argument <strong>must</strong> be a value referring to an alloca instruction or a
bitcast of an alloca. The second contains a pointer to metadata that should be
associated with the pointer, and <strong>must</strong> be a constant or global value
address. If your target collector uses tags, use a null pointer for metadata.</p>
<p>The <tt class="docutils literal"><span class="pre">%metadata</span></tt> argument can be used to avoid requiring heap objects to have
‘isa’ pointers or tag bits. [<a class="reference internal" href="#appel89">Appel89</a>, <a class="reference internal" href="#goldberg91">Goldberg91</a>, <a class="reference internal" href="#tolmach94">Tolmach94</a>] If specified,
its value will be tracked along with the location of the pointer in the stack
frame.</p>
<p>Consider the following fragment of Java code:</p>
<div class="highlight-java"><div class="highlight"><pre><span class="o">{</span>
<span class="n">Object</span> <span class="n">X</span><span class="o">;</span> <span class="c1">// A null-initialized reference to an object</span>
<span class="o">...</span>
<span class="o">}</span>
</pre></div>
</div>
<p>This block (which may be located in the middle of a function or in a loop nest),
could be compiled to this LLVM code:</p>
<div class="highlight-llvm"><div class="highlight"><pre><span class="nl">Entry:</span>
<span class="c">;; In the entry block for the function, allocate the</span>
<span class="c">;; stack space for X, which is an LLVM pointer.</span>
<span class="nv">%X</span> <span class="p">=</span> <span class="k">alloca</span> <span class="nv">%Object</span><span class="p">*</span>
<span class="c">;; Tell LLVM that the stack space is a stack root.</span>
<span class="c">;; Java has type-tags on objects, so we pass null as metadata.</span>
<span class="nv">%tmp</span> <span class="p">=</span> <span class="k">bitcast</span> <span class="nv">%Object</span><span class="p">**</span> <span class="nv">%X</span> <span class="k">to</span> <span class="k">i8</span><span class="p">**</span>
<span class="k">call</span> <span class="kt">void</span> <span class="vg">@llvm.gcroot</span><span class="p">(</span><span class="k">i8</span><span class="p">**</span> <span class="nv">%tmp</span><span class="p">,</span> <span class="k">i8</span><span class="p">*</span> <span class="k">null</span><span class="p">)</span>
<span class="p">...</span>
<span class="c">;; "CodeBlock" is the block corresponding to the start</span>
<span class="c">;; of the scope above.</span>
<span class="nl">CodeBlock:</span>
<span class="c">;; Java null-initializes pointers.</span>
<span class="k">store</span> <span class="nv">%Object</span><span class="p">*</span> <span class="k">null</span><span class="p">,</span> <span class="nv">%Object</span><span class="p">**</span> <span class="nv">%X</span>
<span class="p">...</span>
<span class="c">;; As the pointer goes out of scope, store a null value into</span>
<span class="c">;; it, to indicate that the value is no longer live.</span>
<span class="k">store</span> <span class="nv">%Object</span><span class="p">*</span> <span class="k">null</span><span class="p">,</span> <span class="nv">%Object</span><span class="p">**</span> <span class="nv">%X</span>
<span class="p">...</span>
</pre></div>
</div>
</div>
<div class="section" id="reading-and-writing-references-in-the-heap">
<h3><a class="toc-backref" href="#id10">Reading and writing references in the heap</a><a class="headerlink" href="#reading-and-writing-references-in-the-heap" title="Permalink to this headline">¶</a></h3>
<p>Some collectors need to be informed when the mutator (the program that needs
garbage collection) either reads a pointer from or writes a pointer to a field
of a heap object. The code fragments inserted at these points are called <em>read
barriers</em> and <em>write barriers</em>, respectively. The amount of code that needs to
be executed is usually quite small and not on the critical path of any
computation, so the overall performance impact of the barrier is tolerable.</p>
<p>Barriers often require access to the <em>object pointer</em> rather than the <em>derived
pointer</em> (which is a pointer to the field within the object). Accordingly,
these intrinsics take both pointers as separate arguments for completeness. In
this snippet, <tt class="docutils literal"><span class="pre">%object</span></tt> is the object pointer, and <tt class="docutils literal"><span class="pre">%derived</span></tt> is the derived
pointer:</p>
<div class="highlight-llvm"><div class="highlight"><pre><span class="c">;; An array type.</span>
<span class="nv">%class.Array</span> <span class="p">=</span> <span class="k">type</span> <span class="p">{</span> <span class="nv">%class.Object</span><span class="p">,</span> <span class="k">i32</span><span class="p">,</span> <span class="p">[</span><span class="m">0</span> <span class="k">x</span> <span class="nv">%class.Object</span><span class="p">*]</span> <span class="p">}</span>
<span class="p">...</span>
<span class="c">;; Load the object pointer from a gcroot.</span>
<span class="nv">%object</span> <span class="p">=</span> <span class="k">load</span> <span class="nv">%class.Array</span><span class="p">**</span> <span class="nv">%object_addr</span>
<span class="c">;; Compute the derived pointer.</span>
<span class="nv">%derived</span> <span class="p">=</span> <span class="k">getelementptr</span> <span class="nv">%object</span><span class="p">,</span> <span class="k">i32</span> <span class="m">0</span><span class="p">,</span> <span class="k">i32</span> <span class="m">2</span><span class="p">,</span> <span class="k">i32</span> <span class="nv">%n</span>
</pre></div>
</div>
<p>LLVM does not enforce this relationship between the object and derived pointer
(although a <a class="reference internal" href="#plugin"><em>plugin</em></a> might). However, it would be an unusual
collector that violated it.</p>
<p>The use of these intrinsics is naturally optional if the target GC does require
the corresponding barrier. Such a GC plugin will replace the intrinsic calls
with the corresponding <tt class="docutils literal"><span class="pre">load</span></tt> or <tt class="docutils literal"><span class="pre">store</span></tt> instruction if they are used.</p>
<div class="section" id="write-barrier-llvm-gcwrite">
<h4><a class="toc-backref" href="#id11">Write barrier: <tt class="docutils literal"><span class="pre">llvm.gcwrite</span></tt></a><a class="headerlink" href="#write-barrier-llvm-gcwrite" title="Permalink to this headline">¶</a></h4>
<div class="highlight-llvm"><div class="highlight"><pre><span class="kt">void</span> <span class="vg">@llvm.gcwrite</span><span class="p">(</span><span class="k">i8</span><span class="p">*</span> <span class="nv">%value</span><span class="p">,</span> <span class="k">i8</span><span class="p">*</span> <span class="nv">%object</span><span class="p">,</span> <span class="k">i8</span><span class="p">**</span> <span class="nv">%derived</span><span class="p">)</span>
</pre></div>
</div>
<p>For write barriers, LLVM provides the <tt class="docutils literal"><span class="pre">llvm.gcwrite</span></tt> intrinsic function. It
has exactly the same semantics as a non-volatile <tt class="docutils literal"><span class="pre">store</span></tt> to the derived
pointer (the third argument). The exact code generated is specified by a
compiler <a class="reference internal" href="#plugin"><em>plugin</em></a>.</p>
<p>Many important algorithms require write barriers, including generational and
concurrent collectors. Additionally, write barriers could be used to implement
reference counting.</p>
</div>
<div class="section" id="read-barrier-llvm-gcread">
<h4><a class="toc-backref" href="#id12">Read barrier: <tt class="docutils literal"><span class="pre">llvm.gcread</span></tt></a><a class="headerlink" href="#read-barrier-llvm-gcread" title="Permalink to this headline">¶</a></h4>
<div class="highlight-llvm"><div class="highlight"><pre><span class="k">i8</span><span class="p">*</span> <span class="vg">@llvm.gcread</span><span class="p">(</span><span class="k">i8</span><span class="p">*</span> <span class="nv">%object</span><span class="p">,</span> <span class="k">i8</span><span class="p">**</span> <span class="nv">%derived</span><span class="p">)</span>
</pre></div>
</div>
<p>For read barriers, LLVM provides the <tt class="docutils literal"><span class="pre">llvm.gcread</span></tt> intrinsic function. It has
exactly the same semantics as a non-volatile <tt class="docutils literal"><span class="pre">load</span></tt> from the derived pointer
(the second argument). The exact code generated is specified by a
<a class="reference internal" href="#plugin"><em>compiler plugin</em></a>.</p>
<p>Read barriers are needed by fewer algorithms than write barriers, and may have a
greater performance impact since pointer reads are more frequent than writes.</p>
</div>
</div>
</div>
<div class="section" id="implementing-a-collector-plugin">
<span id="plugin"></span><h2><a class="toc-backref" href="#id13">Implementing a collector plugin</a><a class="headerlink" href="#implementing-a-collector-plugin" title="Permalink to this headline">¶</a></h2>
<p>User code specifies which GC code generation to use with the <tt class="docutils literal"><span class="pre">gc</span></tt> function
attribute or, equivalently, with the <tt class="docutils literal"><span class="pre">setGC</span></tt> method of <tt class="docutils literal"><span class="pre">Function</span></tt>.</p>
<p>To implement a GC plugin, it is necessary to subclass <tt class="docutils literal"><span class="pre">llvm::GCStrategy</span></tt>,
which can be accomplished in a few lines of boilerplate code. LLVM’s
infrastructure provides access to several important algorithms. For an
uncontroversial collector, all that remains may be to compile LLVM’s computed
stack map to assembly code (using the binary representation expected by the
runtime library). This can be accomplished in about 100 lines of code.</p>
<p>This is not the appropriate place to implement a garbage collected heap or a
garbage collector itself. That code should exist in the language’s runtime
library. The compiler plugin is responsible for generating code which conforms
to the binary interface defined by library, most essentially the <a class="reference internal" href="#stack-map"><em>stack map</em></a>.</p>
<p>To subclass <tt class="docutils literal"><span class="pre">llvm::GCStrategy</span></tt> and register it with the compiler:</p>
<div class="highlight-c++"><div class="highlight"><pre><span class="c1">// lib/MyGC/MyGC.cpp - Example LLVM GC plugin</span>
<span class="cp">#include "llvm/CodeGen/GCStrategy.h"</span>
<span class="cp">#include "llvm/CodeGen/GCMetadata.h"</span>
<span class="cp">#include "llvm/Support/Compiler.h"</span>
<span class="k">using</span> <span class="k">namespace</span> <span class="n">llvm</span><span class="p">;</span>
<span class="k">namespace</span> <span class="p">{</span>
<span class="k">class</span> <span class="nc">LLVM_LIBRARY_VISIBILITY</span> <span class="n">MyGC</span> <span class="o">:</span> <span class="k">public</span> <span class="n">GCStrategy</span> <span class="p">{</span>
<span class="nl">public:</span>
<span class="n">MyGC</span><span class="p">()</span> <span class="p">{}</span>
<span class="p">};</span>
<span class="n">GCRegistry</span><span class="o">::</span><span class="n">Add</span><span class="o"><</span><span class="n">MyGC</span><span class="o">></span>
<span class="n">X</span><span class="p">(</span><span class="s">"mygc"</span><span class="p">,</span> <span class="s">"My bespoke garbage collector."</span><span class="p">);</span>
<span class="p">}</span>
</pre></div>
</div>
<p>This boilerplate collector does nothing. More specifically:</p>
<ul class="simple">
<li><tt class="docutils literal"><span class="pre">llvm.gcread</span></tt> calls are replaced with the corresponding <tt class="docutils literal"><span class="pre">load</span></tt>
instruction.</li>
<li><tt class="docutils literal"><span class="pre">llvm.gcwrite</span></tt> calls are replaced with the corresponding <tt class="docutils literal"><span class="pre">store</span></tt>
instruction.</li>
<li>No safe points are added to the code.</li>
<li>The stack map is not compiled into the executable.</li>
</ul>
<p>Using the LLVM makefiles (like the <a class="reference external" href="http://llvm.org/viewvc/llvm-project/llvm/trunk/projects/sample/">sample project</a>), this code
can be compiled as a plugin using a simple makefile:</p>
<div class="highlight-make"><div class="highlight"><pre><span class="c"># lib/MyGC/Makefile</span>
<span class="nv">LEVEL</span> <span class="o">:=</span> ../..
<span class="nv">LIBRARYNAME</span> <span class="o">=</span> MyGC
<span class="nv">LOADABLE_MODULE</span> <span class="o">=</span> 1
<span class="cp">include $(LEVEL)/Makefile.common</span>
</pre></div>
</div>
<p>Once the plugin is compiled, code using it may be compiled using <tt class="docutils literal"><span class="pre">llc</span>
<span class="pre">-load=MyGC.so</span></tt> (though MyGC.so may have some other platform-specific
extension):</p>
<div class="highlight-python"><div class="highlight"><pre>$ cat sample.ll
define void @f() gc "mygc" {
entry:
ret void
}
$ llvm-as < sample.ll | llc -load=MyGC.so
</pre></div>
</div>
<p>It is also possible to statically link the collector plugin into tools, such as
a language-specific compiler front-end.</p>
<div class="section" id="overview-of-available-features">
<span id="collector-algos"></span><h3><a class="toc-backref" href="#id14">Overview of available features</a><a class="headerlink" href="#overview-of-available-features" title="Permalink to this headline">¶</a></h3>
<p><tt class="docutils literal"><span class="pre">GCStrategy</span></tt> provides a range of features through which a plugin may do useful
work. Some of these are callbacks, some are algorithms that can be enabled,
disabled, or customized. This matrix summarizes the supported (and planned)
features and correlates them with the collection techniques which typically
require them.</p>
<table border="1" class="docutils">
<colgroup>
<col width="14%" />
<col width="7%" />
<col width="9%" />
<col width="11%" />
<col width="8%" />
<col width="10%" />
<col width="15%" />
<col width="11%" />
<col width="14%" />
</colgroup>
<thead valign="bottom">
<tr class="row-odd"><th class="head">Algorithm</th>
<th class="head">Done</th>
<th class="head">Shadow
stack</th>
<th class="head">refcount</th>
<th class="head">mark-
sweep</th>
<th class="head">copying</th>
<th class="head">incremental</th>
<th class="head">threaded</th>
<th class="head">concurrent</th>
</tr>
</thead>
<tbody valign="top">
<tr class="row-even"><td>stack map</td>
<td>✔</td>
<td> </td>
<td> </td>
<td>✘</td>
<td>✘</td>
<td>✘</td>
<td>✘</td>
<td>✘</td>
</tr>
<tr class="row-odd"><td>initialize
roots</td>
<td>✔</td>
<td>✘</td>
<td>✘</td>
<td>✘</td>
<td>✘</td>
<td>✘</td>
<td>✘</td>
<td>✘</td>
</tr>
<tr class="row-even"><td>derived
pointers</td>
<td>NO</td>
<td> </td>
<td> </td>
<td> </td>
<td> </td>
<td> </td>
<td><strong>N</strong>*</td>
<td><strong>N</strong>*</td>
</tr>
<tr class="row-odd"><td><strong>custom
lowering</strong></td>
<td>✔</td>
<td> </td>
<td> </td>
<td> </td>
<td> </td>
<td> </td>
<td> </td>
<td> </td>
</tr>
<tr class="row-even"><td><em>gcroot</em></td>
<td>✔</td>
<td>✘</td>
<td>✘</td>
<td> </td>
<td> </td>
<td> </td>
<td> </td>
<td> </td>
</tr>
<tr class="row-odd"><td><em>gcwrite</em></td>
<td>✔</td>
<td> </td>
<td>✘</td>
<td> </td>
<td> </td>
<td>✘</td>
<td> </td>
<td>✘</td>
</tr>
<tr class="row-even"><td><em>gcread</em></td>
<td>✔</td>
<td> </td>
<td> </td>
<td> </td>
<td> </td>
<td> </td>
<td> </td>
<td>✘</td>
</tr>
<tr class="row-odd"><td><strong>safe
points</strong></td>
<td> </td>
<td> </td>
<td> </td>
<td> </td>
<td> </td>
<td> </td>
<td> </td>
<td> </td>
</tr>
<tr class="row-even"><td><em>in
calls</em></td>
<td>✔</td>
<td> </td>
<td> </td>
<td>✘</td>
<td>✘</td>
<td>✘</td>
<td>✘</td>
<td>✘</td>
</tr>
<tr class="row-odd"><td><em>before
calls</em></td>
<td>✔</td>
<td> </td>
<td> </td>
<td> </td>
<td> </td>
<td> </td>
<td>✘</td>
<td>✘</td>
</tr>
<tr class="row-even"><td><em>for
loops</em></td>
<td>NO</td>
<td> </td>
<td> </td>
<td> </td>
<td> </td>
<td> </td>
<td><strong>N</strong></td>
<td><strong>N</strong></td>
</tr>
<tr class="row-odd"><td><em>before
escape</em></td>
<td>✔</td>
<td> </td>
<td> </td>
<td> </td>
<td> </td>
<td> </td>
<td>✘</td>
<td>✘</td>
</tr>
<tr class="row-even"><td>emit code
at safe
points</td>
<td>NO</td>
<td> </td>
<td> </td>
<td> </td>
<td> </td>
<td> </td>
<td><strong>N</strong></td>
<td><strong>N</strong></td>
</tr>
<tr class="row-odd"><td><strong>output</strong></td>
<td> </td>
<td> </td>
<td> </td>
<td> </td>
<td> </td>
<td> </td>
<td> </td>
<td> </td>
</tr>
<tr class="row-even"><td><em>assembly</em></td>
<td>✔</td>
<td> </td>
<td> </td>
<td>✘</td>
<td>✘</td>
<td>✘</td>
<td>✘</td>
<td>✘</td>
</tr>
<tr class="row-odd"><td><em>JIT</em></td>
<td>NO</td>
<td> </td>
<td> </td>
<td><strong>?</strong></td>
<td><strong>?</strong></td>
<td><strong>?</strong></td>
<td><strong>?</strong></td>
<td><strong>?</strong></td>
</tr>
<tr class="row-even"><td><em>obj</em></td>
<td>NO</td>
<td> </td>
<td> </td>
<td><strong>?</strong></td>
<td><strong>?</strong></td>
<td><strong>?</strong></td>
<td><strong>?</strong></td>
<td><strong>?</strong></td>
</tr>
<tr class="row-odd"><td>live
analysis</td>
<td>NO</td>
<td> </td>
<td> </td>
<td><strong>?</strong></td>
<td><strong>?</strong></td>
<td><strong>?</strong></td>
<td><strong>?</strong></td>
<td><strong>?</strong></td>
</tr>
<tr class="row-even"><td>register
map</td>
<td>NO</td>
<td> </td>
<td> </td>
<td><strong>?</strong></td>
<td><strong>?</strong></td>
<td><strong>?</strong></td>
<td><strong>?</strong></td>
<td><strong>?</strong></td>
</tr>
<tr class="row-odd"><td colspan="9">* Derived pointers only pose a hasard to copying collections.</td>
</tr>
<tr class="row-even"><td colspan="9"><strong>?</strong> denotes a feature which could be utilized if available.</td>
</tr>
</tbody>
</table>
<p>To be clear, the collection techniques above are defined as:</p>
<dl class="docutils">
<dt>Shadow Stack</dt>
<dd>The mutator carefully maintains a linked list of stack roots.</dd>
<dt>Reference Counting</dt>
<dd>The mutator maintains a reference count for each object and frees an object
when its count falls to zero.</dd>
<dt>Mark-Sweep</dt>
<dd>When the heap is exhausted, the collector marks reachable objects starting
from the roots, then deallocates unreachable objects in a sweep phase.</dd>
<dt>Copying</dt>
<dd>As reachability analysis proceeds, the collector copies objects from one heap
area to another, compacting them in the process. Copying collectors enable
highly efficient “bump pointer” allocation and can improve locality of
reference.</dd>
<dt>Incremental</dt>
<dd>(Including generational collectors.) Incremental collectors generally have all
the properties of a copying collector (regardless of whether the mature heap
is compacting), but bring the added complexity of requiring write barriers.</dd>
<dt>Threaded</dt>
<dd>Denotes a multithreaded mutator; the collector must still stop the mutator
(“stop the world”) before beginning reachability analysis. Stopping a
multithreaded mutator is a complicated problem. It generally requires highly
platform specific code in the runtime, and the production of carefully
designed machine code at safe points.</dd>
<dt>Concurrent</dt>
<dd>In this technique, the mutator and the collector run concurrently, with the
goal of eliminating pause times. In a <em>cooperative</em> collector, the mutator
further aids with collection should a pause occur, allowing collection to take
advantage of multiprocessor hosts. The “stop the world” problem of threaded
collectors is generally still present to a limited extent. Sophisticated
marking algorithms are necessary. Read barriers may be necessary.</dd>
</dl>
<p>As the matrix indicates, LLVM’s garbage collection infrastructure is already
suitable for a wide variety of collectors, but does not currently extend to
multithreaded programs. This will be added in the future as there is
interest.</p>
</div>
<div class="section" id="computing-stack-maps">
<span id="stack-map"></span><h3><a class="toc-backref" href="#id15">Computing stack maps</a><a class="headerlink" href="#computing-stack-maps" title="Permalink to this headline">¶</a></h3>
<p>LLVM automatically computes a stack map. One of the most important features
of a <tt class="docutils literal"><span class="pre">GCStrategy</span></tt> is to compile this information into the executable in
the binary representation expected by the runtime library.</p>
<p>The stack map consists of the location and identity of each GC root in the
each function in the module. For each root:</p>
<ul class="simple">
<li><tt class="docutils literal"><span class="pre">RootNum</span></tt>: The index of the root.</li>
<li><tt class="docutils literal"><span class="pre">StackOffset</span></tt>: The offset of the object relative to the frame pointer.</li>
<li><tt class="docutils literal"><span class="pre">RootMetadata</span></tt>: The value passed as the <tt class="docutils literal"><span class="pre">%metadata</span></tt> parameter to the
<tt class="docutils literal"><span class="pre">@llvm.gcroot</span></tt> intrinsic.</li>
</ul>
<p>Also, for the function as a whole:</p>
<ul>
<li><dl class="first docutils">
<dt><tt class="docutils literal"><span class="pre">getFrameSize()</span></tt>: The overall size of the function’s initial stack frame,</dt>
<dd><p class="first last">not accounting for any dynamic allocation.</p>
</dd>
</dl>
</li>
<li><p class="first"><tt class="docutils literal"><span class="pre">roots_size()</span></tt>: The count of roots in the function.</p>
</li>
</ul>
<p>To access the stack map, use <tt class="docutils literal"><span class="pre">GCFunctionMetadata::roots_begin()</span></tt> and
-<tt class="docutils literal"><span class="pre">end()</span></tt> from the <a class="reference internal" href="#assembly"><em>GCMetadataPrinter</em></a>:</p>
<div class="highlight-c++"><div class="highlight"><pre><span class="k">for</span> <span class="p">(</span><span class="n">iterator</span> <span class="n">I</span> <span class="o">=</span> <span class="n">begin</span><span class="p">(),</span> <span class="n">E</span> <span class="o">=</span> <span class="n">end</span><span class="p">();</span> <span class="n">I</span> <span class="o">!=</span> <span class="n">E</span><span class="p">;</span> <span class="o">++</span><span class="n">I</span><span class="p">)</span> <span class="p">{</span>
<span class="n">GCFunctionInfo</span> <span class="o">*</span><span class="n">FI</span> <span class="o">=</span> <span class="o">*</span><span class="n">I</span><span class="p">;</span>
<span class="kt">unsigned</span> <span class="n">FrameSize</span> <span class="o">=</span> <span class="n">FI</span><span class="o">-></span><span class="n">getFrameSize</span><span class="p">();</span>
<span class="kt">size_t</span> <span class="n">RootCount</span> <span class="o">=</span> <span class="n">FI</span><span class="o">-></span><span class="n">roots_size</span><span class="p">();</span>
<span class="k">for</span> <span class="p">(</span><span class="n">GCFunctionInfo</span><span class="o">::</span><span class="n">roots_iterator</span> <span class="n">RI</span> <span class="o">=</span> <span class="n">FI</span><span class="o">-></span><span class="n">roots_begin</span><span class="p">(),</span>
<span class="n">RE</span> <span class="o">=</span> <span class="n">FI</span><span class="o">-></span><span class="n">roots_end</span><span class="p">();</span>
<span class="n">RI</span> <span class="o">!=</span> <span class="n">RE</span><span class="p">;</span> <span class="o">++</span><span class="n">RI</span><span class="p">)</span> <span class="p">{</span>
<span class="kt">int</span> <span class="n">RootNum</span> <span class="o">=</span> <span class="n">RI</span><span class="o">-></span><span class="n">Num</span><span class="p">;</span>
<span class="kt">int</span> <span class="n">RootStackOffset</span> <span class="o">=</span> <span class="n">RI</span><span class="o">-></span><span class="n">StackOffset</span><span class="p">;</span>
<span class="n">Constant</span> <span class="o">*</span><span class="n">RootMetadata</span> <span class="o">=</span> <span class="n">RI</span><span class="o">-></span><span class="n">Metadata</span><span class="p">;</span>
<span class="p">}</span>
<span class="p">}</span>
</pre></div>
</div>
<p>If the <tt class="docutils literal"><span class="pre">llvm.gcroot</span></tt> intrinsic is eliminated before code generation by a
custom lowering pass, LLVM will compute an empty stack map. This may be useful
for collector plugins which implement reference counting or a shadow stack.</p>
</div>
<div class="section" id="initializing-roots-to-null-initroots">
<span id="init-roots"></span><h3><a class="toc-backref" href="#id16">Initializing roots to null: <tt class="docutils literal"><span class="pre">InitRoots</span></tt></a><a class="headerlink" href="#initializing-roots-to-null-initroots" title="Permalink to this headline">¶</a></h3>
<div class="highlight-c++"><div class="highlight"><pre><span class="n">MyGC</span><span class="o">::</span><span class="n">MyGC</span><span class="p">()</span> <span class="p">{</span>
<span class="n">InitRoots</span> <span class="o">=</span> <span class="nb">true</span><span class="p">;</span>
<span class="p">}</span>
</pre></div>
</div>
<p>When set, LLVM will automatically initialize each root to <tt class="docutils literal"><span class="pre">null</span></tt> upon entry to
the function. This prevents the GC’s sweep phase from visiting uninitialized
pointers, which will almost certainly cause it to crash. This initialization
occurs before custom lowering, so the two may be used together.</p>
<p>Since LLVM does not yet compute liveness information, there is no means of
distinguishing an uninitialized stack root from an initialized one. Therefore,
this feature should be used by all GC plugins. It is enabled by default.</p>
</div>
<div class="section" id="custom-lowering-of-intrinsics-customroots-customreadbarriers-and-customwritebarriers">
<h3><a class="toc-backref" href="#id17">Custom lowering of intrinsics: <tt class="docutils literal"><span class="pre">CustomRoots</span></tt>, <tt class="docutils literal"><span class="pre">CustomReadBarriers</span></tt>, and <tt class="docutils literal"><span class="pre">CustomWriteBarriers</span></tt></a><a class="headerlink" href="#custom-lowering-of-intrinsics-customroots-customreadbarriers-and-customwritebarriers" title="Permalink to this headline">¶</a></h3>
<p>For GCs which use barriers or unusual treatment of stack roots, these flags
allow the collector to perform arbitrary transformations of the LLVM IR:</p>
<div class="highlight-c++"><div class="highlight"><pre><span class="k">class</span> <span class="nc">MyGC</span> <span class="o">:</span> <span class="k">public</span> <span class="n">GCStrategy</span> <span class="p">{</span>
<span class="nl">public:</span>
<span class="n">MyGC</span><span class="p">()</span> <span class="p">{</span>
<span class="n">CustomRoots</span> <span class="o">=</span> <span class="nb">true</span><span class="p">;</span>
<span class="n">CustomReadBarriers</span> <span class="o">=</span> <span class="nb">true</span><span class="p">;</span>
<span class="n">CustomWriteBarriers</span> <span class="o">=</span> <span class="nb">true</span><span class="p">;</span>
<span class="p">}</span>
<span class="k">virtual</span> <span class="kt">bool</span> <span class="n">initializeCustomLowering</span><span class="p">(</span><span class="n">Module</span> <span class="o">&</span><span class="n">M</span><span class="p">);</span>
<span class="k">virtual</span> <span class="kt">bool</span> <span class="nf">performCustomLowering</span><span class="p">(</span><span class="n">Function</span> <span class="o">&</span><span class="n">F</span><span class="p">);</span>
<span class="p">};</span>
</pre></div>
</div>
<p>If any of these flags are set, then LLVM suppresses its default lowering for the
corresponding intrinsics and instead calls <tt class="docutils literal"><span class="pre">performCustomLowering</span></tt>.</p>
<p>LLVM’s default action for each intrinsic is as follows:</p>
<ul class="simple">
<li><tt class="docutils literal"><span class="pre">llvm.gcroot</span></tt>: Leave it alone. The code generator must see it or the stack
map will not be computed.</li>
<li><tt class="docutils literal"><span class="pre">llvm.gcread</span></tt>: Substitute a <tt class="docutils literal"><span class="pre">load</span></tt> instruction.</li>
<li><tt class="docutils literal"><span class="pre">llvm.gcwrite</span></tt>: Substitute a <tt class="docutils literal"><span class="pre">store</span></tt> instruction.</li>
</ul>
<p>If <tt class="docutils literal"><span class="pre">CustomReadBarriers</span></tt> or <tt class="docutils literal"><span class="pre">CustomWriteBarriers</span></tt> are specified, then
<tt class="docutils literal"><span class="pre">performCustomLowering</span></tt> <strong>must</strong> eliminate the corresponding barriers.</p>
<p><tt class="docutils literal"><span class="pre">performCustomLowering</span></tt> must comply with the same restrictions as
<a class="reference internal" href="WritingAnLLVMPass.html#writing-an-llvm-pass-runonfunction"><em>FunctionPass::runOnFunction</em></a>
Likewise, <tt class="docutils literal"><span class="pre">initializeCustomLowering</span></tt> has the same semantics as
<a class="reference internal" href="WritingAnLLVMPass.html#writing-an-llvm-pass-doinitialization-mod"><em>Pass::doInitialization(Module&)</em></a></p>
<p>The following can be used as a template:</p>
<div class="highlight-c++"><div class="highlight"><pre><span class="cp">#include "llvm/IR/Module.h"</span>
<span class="cp">#include "llvm/IR/IntrinsicInst.h"</span>
<span class="kt">bool</span> <span class="n">MyGC</span><span class="o">::</span><span class="n">initializeCustomLowering</span><span class="p">(</span><span class="n">Module</span> <span class="o">&</span><span class="n">M</span><span class="p">)</span> <span class="p">{</span>
<span class="k">return</span> <span class="nb">false</span><span class="p">;</span>
<span class="p">}</span>
<span class="kt">bool</span> <span class="n">MyGC</span><span class="o">::</span><span class="n">performCustomLowering</span><span class="p">(</span><span class="n">Function</span> <span class="o">&</span><span class="n">F</span><span class="p">)</span> <span class="p">{</span>
<span class="kt">bool</span> <span class="n">MadeChange</span> <span class="o">=</span> <span class="nb">false</span><span class="p">;</span>
<span class="k">for</span> <span class="p">(</span><span class="n">Function</span><span class="o">::</span><span class="n">iterator</span> <span class="n">BB</span> <span class="o">=</span> <span class="n">F</span><span class="p">.</span><span class="n">begin</span><span class="p">(),</span> <span class="n">E</span> <span class="o">=</span> <span class="n">F</span><span class="p">.</span><span class="n">end</span><span class="p">();</span> <span class="n">BB</span> <span class="o">!=</span> <span class="n">E</span><span class="p">;</span> <span class="o">++</span><span class="n">BB</span><span class="p">)</span>
<span class="k">for</span> <span class="p">(</span><span class="n">BasicBlock</span><span class="o">::</span><span class="n">iterator</span> <span class="n">II</span> <span class="o">=</span> <span class="n">BB</span><span class="o">-></span><span class="n">begin</span><span class="p">(),</span> <span class="n">E</span> <span class="o">=</span> <span class="n">BB</span><span class="o">-></span><span class="n">end</span><span class="p">();</span> <span class="n">II</span> <span class="o">!=</span> <span class="n">E</span><span class="p">;</span> <span class="p">)</span>
<span class="k">if</span> <span class="p">(</span><span class="n">IntrinsicInst</span> <span class="o">*</span><span class="n">CI</span> <span class="o">=</span> <span class="n">dyn_cast</span><span class="o"><</span><span class="n">IntrinsicInst</span><span class="o">></span><span class="p">(</span><span class="n">II</span><span class="o">++</span><span class="p">))</span>
<span class="k">if</span> <span class="p">(</span><span class="n">Function</span> <span class="o">*</span><span class="n">F</span> <span class="o">=</span> <span class="n">CI</span><span class="o">-></span><span class="n">getCalledFunction</span><span class="p">())</span>
<span class="k">switch</span> <span class="p">(</span><span class="n">F</span><span class="o">-></span><span class="n">getIntrinsicID</span><span class="p">())</span> <span class="p">{</span>
<span class="k">case</span> <span class="n">Intrinsic</span>:<span class="o">:</span><span class="n">gcwrite</span><span class="o">:</span>
<span class="c1">// Handle llvm.gcwrite.</span>
<span class="n">CI</span><span class="o">-></span><span class="n">eraseFromParent</span><span class="p">();</span>
<span class="n">MadeChange</span> <span class="o">=</span> <span class="nb">true</span><span class="p">;</span>
<span class="k">break</span><span class="p">;</span>
<span class="k">case</span> <span class="n">Intrinsic</span>:<span class="o">:</span><span class="n">gcread</span><span class="o">:</span>
<span class="c1">// Handle llvm.gcread.</span>
<span class="n">CI</span><span class="o">-></span><span class="n">eraseFromParent</span><span class="p">();</span>
<span class="n">MadeChange</span> <span class="o">=</span> <span class="nb">true</span><span class="p">;</span>
<span class="k">break</span><span class="p">;</span>
<span class="k">case</span> <span class="n">Intrinsic</span>:<span class="o">:</span><span class="n">gcroot</span><span class="o">:</span>
<span class="c1">// Handle llvm.gcroot.</span>
<span class="n">CI</span><span class="o">-></span><span class="n">eraseFromParent</span><span class="p">();</span>
<span class="n">MadeChange</span> <span class="o">=</span> <span class="nb">true</span><span class="p">;</span>
<span class="k">break</span><span class="p">;</span>
<span class="p">}</span>
<span class="k">return</span> <span class="n">MadeChange</span><span class="p">;</span>
<span class="p">}</span>
</pre></div>
</div>
</div>
<div class="section" id="generating-safe-points-neededsafepoints">
<span id="safe-points"></span><h3><a class="toc-backref" href="#id18">Generating safe points: <tt class="docutils literal"><span class="pre">NeededSafePoints</span></tt></a><a class="headerlink" href="#generating-safe-points-neededsafepoints" title="Permalink to this headline">¶</a></h3>
<p>LLVM can compute four kinds of safe points:</p>
<div class="highlight-c++"><div class="highlight"><pre><span class="k">namespace</span> <span class="n">GC</span> <span class="p">{</span>
<span class="c1">/// PointKind - The type of a collector-safe point.</span>
<span class="c1">///</span>
<span class="k">enum</span> <span class="n">PointKind</span> <span class="p">{</span>
<span class="n">Loop</span><span class="p">,</span> <span class="c1">//< Instr is a loop (backwards branch).</span>
<span class="n">Return</span><span class="p">,</span> <span class="c1">//< Instr is a return instruction.</span>
<span class="n">PreCall</span><span class="p">,</span> <span class="c1">//< Instr is a call instruction.</span>
<span class="n">PostCall</span> <span class="c1">//< Instr is the return address of a call.</span>
<span class="p">};</span>
<span class="p">}</span>
</pre></div>
</div>
<p>A collector can request any combination of the four by setting the
<tt class="docutils literal"><span class="pre">NeededSafePoints</span></tt> mask:</p>
<div class="highlight-c++"><div class="highlight"><pre><span class="n">MyGC</span><span class="o">::</span><span class="n">MyGC</span><span class="p">()</span> <span class="p">{</span>
<span class="n">NeededSafePoints</span> <span class="o">=</span> <span class="mi">1</span> <span class="o"><<</span> <span class="n">GC</span><span class="o">::</span><span class="n">Loop</span>
<span class="o">|</span> <span class="mi">1</span> <span class="o"><<</span> <span class="n">GC</span><span class="o">::</span><span class="n">Return</span>
<span class="o">|</span> <span class="mi">1</span> <span class="o"><<</span> <span class="n">GC</span><span class="o">::</span><span class="n">PreCall</span>
<span class="o">|</span> <span class="mi">1</span> <span class="o"><<</span> <span class="n">GC</span><span class="o">::</span><span class="n">PostCall</span><span class="p">;</span>
<span class="p">}</span>
</pre></div>
</div>
<p>It can then use the following routines to access safe points.</p>
<div class="highlight-c++"><div class="highlight"><pre><span class="k">for</span> <span class="p">(</span><span class="n">iterator</span> <span class="n">I</span> <span class="o">=</span> <span class="n">begin</span><span class="p">(),</span> <span class="n">E</span> <span class="o">=</span> <span class="n">end</span><span class="p">();</span> <span class="n">I</span> <span class="o">!=</span> <span class="n">E</span><span class="p">;</span> <span class="o">++</span><span class="n">I</span><span class="p">)</span> <span class="p">{</span>
<span class="n">GCFunctionInfo</span> <span class="o">*</span><span class="n">MD</span> <span class="o">=</span> <span class="o">*</span><span class="n">I</span><span class="p">;</span>
<span class="kt">size_t</span> <span class="n">PointCount</span> <span class="o">=</span> <span class="n">MD</span><span class="o">-></span><span class="n">size</span><span class="p">();</span>
<span class="k">for</span> <span class="p">(</span><span class="n">GCFunctionInfo</span><span class="o">::</span><span class="n">iterator</span> <span class="n">PI</span> <span class="o">=</span> <span class="n">MD</span><span class="o">-></span><span class="n">begin</span><span class="p">(),</span>
<span class="n">PE</span> <span class="o">=</span> <span class="n">MD</span><span class="o">-></span><span class="n">end</span><span class="p">();</span> <span class="n">PI</span> <span class="o">!=</span> <span class="n">PE</span><span class="p">;</span> <span class="o">++</span><span class="n">PI</span><span class="p">)</span> <span class="p">{</span>
<span class="n">GC</span><span class="o">::</span><span class="n">PointKind</span> <span class="n">PointKind</span> <span class="o">=</span> <span class="n">PI</span><span class="o">-></span><span class="n">Kind</span><span class="p">;</span>
<span class="kt">unsigned</span> <span class="n">PointNum</span> <span class="o">=</span> <span class="n">PI</span><span class="o">-></span><span class="n">Num</span><span class="p">;</span>
<span class="p">}</span>
<span class="p">}</span>
</pre></div>
</div>
<p>Almost every collector requires <tt class="docutils literal"><span class="pre">PostCall</span></tt> safe points, since these correspond
to the moments when the function is suspended during a call to a subroutine.</p>
<p>Threaded programs generally require <tt class="docutils literal"><span class="pre">Loop</span></tt> safe points to guarantee that the
application will reach a safe point within a bounded amount of time, even if it
is executing a long-running loop which contains no function calls.</p>
<p>Threaded collectors may also require <tt class="docutils literal"><span class="pre">Return</span></tt> and <tt class="docutils literal"><span class="pre">PreCall</span></tt> safe points to
implement “stop the world” techniques using self-modifying code, where it is
important that the program not exit the function without reaching a safe point
(because only the topmost function has been patched).</p>
</div>
<div class="section" id="emitting-assembly-code-gcmetadataprinter">
<span id="assembly"></span><h3><a class="toc-backref" href="#id19">Emitting assembly code: <tt class="docutils literal"><span class="pre">GCMetadataPrinter</span></tt></a><a class="headerlink" href="#emitting-assembly-code-gcmetadataprinter" title="Permalink to this headline">¶</a></h3>
<p>LLVM allows a plugin to print arbitrary assembly code before and after the rest
of a module’s assembly code. At the end of the module, the GC can compile the
LLVM stack map into assembly code. (At the beginning, this information is not
yet computed.)</p>
<p>Since AsmWriter and CodeGen are separate components of LLVM, a separate abstract
base class and registry is provided for printing assembly code, the
<tt class="docutils literal"><span class="pre">GCMetadaPrinter</span></tt> and <tt class="docutils literal"><span class="pre">GCMetadataPrinterRegistry</span></tt>. The AsmWriter will look
for such a subclass if the <tt class="docutils literal"><span class="pre">GCStrategy</span></tt> sets <tt class="docutils literal"><span class="pre">UsesMetadata</span></tt>:</p>
<div class="highlight-c++"><div class="highlight"><pre><span class="n">MyGC</span><span class="o">::</span><span class="n">MyGC</span><span class="p">()</span> <span class="p">{</span>
<span class="n">UsesMetadata</span> <span class="o">=</span> <span class="nb">true</span><span class="p">;</span>
<span class="p">}</span>
</pre></div>
</div>
<p>This separation allows JIT-only clients to be smaller.</p>
<p>Note that LLVM does not currently have analogous APIs to support code generation
in the JIT, nor using the object writers.</p>
<div class="highlight-c++"><div class="highlight"><pre><span class="c1">// lib/MyGC/MyGCPrinter.cpp - Example LLVM GC printer</span>
<span class="cp">#include "llvm/CodeGen/GCMetadataPrinter.h"</span>
<span class="cp">#include "llvm/Support/Compiler.h"</span>
<span class="k">using</span> <span class="k">namespace</span> <span class="n">llvm</span><span class="p">;</span>
<span class="k">namespace</span> <span class="p">{</span>
<span class="k">class</span> <span class="nc">LLVM_LIBRARY_VISIBILITY</span> <span class="n">MyGCPrinter</span> <span class="o">:</span> <span class="k">public</span> <span class="n">GCMetadataPrinter</span> <span class="p">{</span>
<span class="nl">public:</span>
<span class="k">virtual</span> <span class="kt">void</span> <span class="n">beginAssembly</span><span class="p">(</span><span class="n">AsmPrinter</span> <span class="o">&</span><span class="n">AP</span><span class="p">);</span>
<span class="k">virtual</span> <span class="kt">void</span> <span class="nf">finishAssembly</span><span class="p">(</span><span class="n">AsmPrinter</span> <span class="o">&</span><span class="n">AP</span><span class="p">);</span>
<span class="p">};</span>
<span class="n">GCMetadataPrinterRegistry</span><span class="o">::</span><span class="n">Add</span><span class="o"><</span><span class="n">MyGCPrinter</span><span class="o">></span>
<span class="n">X</span><span class="p">(</span><span class="s">"mygc"</span><span class="p">,</span> <span class="s">"My bespoke garbage collector."</span><span class="p">);</span>
<span class="p">}</span>
</pre></div>
</div>
<p>The collector should use <tt class="docutils literal"><span class="pre">AsmPrinter</span></tt> to print portable assembly code. The
collector itself contains the stack map for the entire module, and may access
the <tt class="docutils literal"><span class="pre">GCFunctionInfo</span></tt> using its own <tt class="docutils literal"><span class="pre">begin()</span></tt> and <tt class="docutils literal"><span class="pre">end()</span></tt> methods. Here’s
a realistic example:</p>
<div class="highlight-c++"><div class="highlight"><pre><span class="cp">#include "llvm/CodeGen/AsmPrinter.h"</span>
<span class="cp">#include "llvm/IR/Function.h"</span>
<span class="cp">#include "llvm/IR/DataLayout.h"</span>
<span class="cp">#include "llvm/Target/TargetAsmInfo.h"</span>
<span class="cp">#include "llvm/Target/TargetMachine.h"</span>
<span class="kt">void</span> <span class="n">MyGCPrinter</span><span class="o">::</span><span class="n">beginAssembly</span><span class="p">(</span><span class="n">AsmPrinter</span> <span class="o">&</span><span class="n">AP</span><span class="p">)</span> <span class="p">{</span>
<span class="c1">// Nothing to do.</span>
<span class="p">}</span>
<span class="kt">void</span> <span class="n">MyGCPrinter</span><span class="o">::</span><span class="n">finishAssembly</span><span class="p">(</span><span class="n">AsmPrinter</span> <span class="o">&</span><span class="n">AP</span><span class="p">)</span> <span class="p">{</span>
<span class="n">MCStreamer</span> <span class="o">&</span><span class="n">OS</span> <span class="o">=</span> <span class="n">AP</span><span class="p">.</span><span class="n">OutStreamer</span><span class="p">;</span>
<span class="kt">unsigned</span> <span class="n">IntPtrSize</span> <span class="o">=</span> <span class="n">AP</span><span class="p">.</span><span class="n">TM</span><span class="p">.</span><span class="n">getDataLayout</span><span class="p">()</span><span class="o">-></span><span class="n">getPointerSize</span><span class="p">();</span>
<span class="c1">// Put this in the data section.</span>
<span class="n">OS</span><span class="p">.</span><span class="n">SwitchSection</span><span class="p">(</span><span class="n">AP</span><span class="p">.</span><span class="n">getObjFileLowering</span><span class="p">().</span><span class="n">getDataSection</span><span class="p">());</span>
<span class="c1">// For each function...</span>
<span class="k">for</span> <span class="p">(</span><span class="n">iterator</span> <span class="n">FI</span> <span class="o">=</span> <span class="n">begin</span><span class="p">(),</span> <span class="n">FE</span> <span class="o">=</span> <span class="n">end</span><span class="p">();</span> <span class="n">FI</span> <span class="o">!=</span> <span class="n">FE</span><span class="p">;</span> <span class="o">++</span><span class="n">FI</span><span class="p">)</span> <span class="p">{</span>
<span class="n">GCFunctionInfo</span> <span class="o">&</span><span class="n">MD</span> <span class="o">=</span> <span class="o">**</span><span class="n">FI</span><span class="p">;</span>
<span class="c1">// A compact GC layout. Emit this data structure:</span>
<span class="c1">//</span>
<span class="c1">// struct {</span>
<span class="c1">// int32_t PointCount;</span>
<span class="c1">// void *SafePointAddress[PointCount];</span>
<span class="c1">// int32_t StackFrameSize; // in words</span>
<span class="c1">// int32_t StackArity;</span>
<span class="c1">// int32_t LiveCount;</span>
<span class="c1">// int32_t LiveOffsets[LiveCount];</span>
<span class="c1">// } __gcmap_<FUNCTIONNAME>;</span>
<span class="c1">// Align to address width.</span>
<span class="n">AP</span><span class="p">.</span><span class="n">EmitAlignment</span><span class="p">(</span><span class="n">IntPtrSize</span> <span class="o">==</span> <span class="mi">4</span> <span class="o">?</span> <span class="mi">2</span> <span class="o">:</span> <span class="mi">3</span><span class="p">);</span>
<span class="c1">// Emit PointCount.</span>
<span class="n">OS</span><span class="p">.</span><span class="n">AddComment</span><span class="p">(</span><span class="s">"safe point count"</span><span class="p">);</span>
<span class="n">AP</span><span class="p">.</span><span class="n">EmitInt32</span><span class="p">(</span><span class="n">MD</span><span class="p">.</span><span class="n">size</span><span class="p">());</span>
<span class="c1">// And each safe point...</span>
<span class="k">for</span> <span class="p">(</span><span class="n">GCFunctionInfo</span><span class="o">::</span><span class="n">iterator</span> <span class="n">PI</span> <span class="o">=</span> <span class="n">MD</span><span class="p">.</span><span class="n">begin</span><span class="p">(),</span>
<span class="n">PE</span> <span class="o">=</span> <span class="n">MD</span><span class="p">.</span><span class="n">end</span><span class="p">();</span> <span class="n">PI</span> <span class="o">!=</span> <span class="n">PE</span><span class="p">;</span> <span class="o">++</span><span class="n">PI</span><span class="p">)</span> <span class="p">{</span>
<span class="c1">// Emit the address of the safe point.</span>
<span class="n">OS</span><span class="p">.</span><span class="n">AddComment</span><span class="p">(</span><span class="s">"safe point address"</span><span class="p">);</span>
<span class="n">MCSymbol</span> <span class="o">*</span><span class="n">Label</span> <span class="o">=</span> <span class="n">PI</span><span class="o">-></span><span class="n">Label</span><span class="p">;</span>
<span class="n">AP</span><span class="p">.</span><span class="n">EmitLabelPlusOffset</span><span class="p">(</span><span class="n">Label</span><span class="cm">/*Hi*/</span><span class="p">,</span> <span class="mi">0</span><span class="cm">/*Offset*/</span><span class="p">,</span> <span class="mi">4</span><span class="cm">/*Size*/</span><span class="p">);</span>
<span class="p">}</span>
<span class="c1">// Stack information never change in safe points! Only print info from the</span>
<span class="c1">// first call-site.</span>
<span class="n">GCFunctionInfo</span><span class="o">::</span><span class="n">iterator</span> <span class="n">PI</span> <span class="o">=</span> <span class="n">MD</span><span class="p">.</span><span class="n">begin</span><span class="p">();</span>
<span class="c1">// Emit the stack frame size.</span>
<span class="n">OS</span><span class="p">.</span><span class="n">AddComment</span><span class="p">(</span><span class="s">"stack frame size (in words)"</span><span class="p">);</span>
<span class="n">AP</span><span class="p">.</span><span class="n">EmitInt32</span><span class="p">(</span><span class="n">MD</span><span class="p">.</span><span class="n">getFrameSize</span><span class="p">()</span> <span class="o">/</span> <span class="n">IntPtrSize</span><span class="p">);</span>
<span class="c1">// Emit stack arity, i.e. the number of stacked arguments.</span>
<span class="kt">unsigned</span> <span class="n">RegisteredArgs</span> <span class="o">=</span> <span class="n">IntPtrSize</span> <span class="o">==</span> <span class="mi">4</span> <span class="o">?</span> <span class="mi">5</span> <span class="o">:</span> <span class="mi">6</span><span class="p">;</span>
<span class="kt">unsigned</span> <span class="n">StackArity</span> <span class="o">=</span> <span class="n">MD</span><span class="p">.</span><span class="n">getFunction</span><span class="p">().</span><span class="n">arg_size</span><span class="p">()</span> <span class="o">></span> <span class="n">RegisteredArgs</span> <span class="o">?</span>
<span class="n">MD</span><span class="p">.</span><span class="n">getFunction</span><span class="p">().</span><span class="n">arg_size</span><span class="p">()</span> <span class="o">-</span> <span class="n">RegisteredArgs</span> <span class="o">:</span> <span class="mi">0</span><span class="p">;</span>
<span class="n">OS</span><span class="p">.</span><span class="n">AddComment</span><span class="p">(</span><span class="s">"stack arity"</span><span class="p">);</span>
<span class="n">AP</span><span class="p">.</span><span class="n">EmitInt32</span><span class="p">(</span><span class="n">StackArity</span><span class="p">);</span>
<span class="c1">// Emit the number of live roots in the function.</span>
<span class="n">OS</span><span class="p">.</span><span class="n">AddComment</span><span class="p">(</span><span class="s">"live root count"</span><span class="p">);</span>
<span class="n">AP</span><span class="p">.</span><span class="n">EmitInt32</span><span class="p">(</span><span class="n">MD</span><span class="p">.</span><span class="n">live_size</span><span class="p">(</span><span class="n">PI</span><span class="p">));</span>
<span class="c1">// And for each live root...</span>
<span class="k">for</span> <span class="p">(</span><span class="n">GCFunctionInfo</span><span class="o">::</span><span class="n">live_iterator</span> <span class="n">LI</span> <span class="o">=</span> <span class="n">MD</span><span class="p">.</span><span class="n">live_begin</span><span class="p">(</span><span class="n">PI</span><span class="p">),</span>
<span class="n">LE</span> <span class="o">=</span> <span class="n">MD</span><span class="p">.</span><span class="n">live_end</span><span class="p">(</span><span class="n">PI</span><span class="p">);</span>
<span class="n">LI</span> <span class="o">!=</span> <span class="n">LE</span><span class="p">;</span> <span class="o">++</span><span class="n">LI</span><span class="p">)</span> <span class="p">{</span>
<span class="c1">// Emit live root's offset within the stack frame.</span>
<span class="n">OS</span><span class="p">.</span><span class="n">AddComment</span><span class="p">(</span><span class="s">"stack index (offset / wordsize)"</span><span class="p">);</span>
<span class="n">AP</span><span class="p">.</span><span class="n">EmitInt32</span><span class="p">(</span><span class="n">LI</span><span class="o">-></span><span class="n">StackOffset</span><span class="p">);</span>
<span class="p">}</span>
<span class="p">}</span>
<span class="p">}</span>
</pre></div>
</div>
</div>
</div>
<div class="section" id="references">
<h2><a class="toc-backref" href="#id20">References</a><a class="headerlink" href="#references" title="Permalink to this headline">¶</a></h2>
<p id="appel89">[Appel89] Runtime Tags Aren’t Necessary. Andrew W. Appel. Lisp and Symbolic
Computation 19(7):703-705, July 1989.</p>
<p id="goldberg91">[Goldberg91] Tag-free garbage collection for strongly typed programming
languages. Benjamin Goldberg. ACM SIGPLAN PLDI‘91.</p>
<p id="tolmach94">[Tolmach94] Tag-free garbage collection using explicit type parameters. Andrew
Tolmach. Proceedings of the 1994 ACM conference on LISP and functional
programming.</p>
<p id="henderson02">[Henderson2002] <a class="reference external" href="http://citeseer.ist.psu.edu/henderson02accurate.html">Accurate Garbage Collection in an Uncooperative Environment</a></p>
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