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-<div class="doc_title">
+<h1>
Accurate Garbage Collection with LLVM
-</div>
+</h1>
<ol>
<li><a href="#introduction">Introduction</a>
<ul>
- <li><a href="#feature">GC features provided and algorithms
- supported</a></li>
+ <li><a href="#feature">Goals and non-goals</a></li>
</ul>
</li>
- <li><a href="#usage">Using the collectors</a>
+ <li><a href="#quickstart">Getting started</a>
<ul>
- <li><a href="#shadow-stack">ShadowStack -
- A highly portable collector</a></li>
- <li><a href="#semispace">SemiSpace -
- A simple copying collector runtime</a></li>
- <li><a href="#ocaml">Ocaml -
- An Objective Caml-compatible collector</a></li>
+ <li><a href="#quickstart-compiler">In your compiler</a></li>
+ <li><a href="#quickstart-runtime">In your runtime library</a></li>
+ <li><a href="#shadow-stack">About the shadow stack</a></li>
</ul>
</li>
</ul>
</li>
- <li><a href="#runtime">Recommended runtime interface</a>
- <ul>
- <li><a href="#initialize">Garbage collector startup and
- initialization</a></li>
- <li><a href="#allocate">Allocating memory from the GC</a></li>
- <li><a href="#explicit">Explicit invocation of the garbage
- collector</a></li>
- <li><a href="#traceroots">Tracing GC pointers from the program
- stack</a></li>
- <li><a href="#staticroots">Tracing GC pointers from static roots</a></li>
- </ul>
- </li>
-
- <li><a href="#plugin">Implementing a collector plugin</a>
+ <li><a href="#plugin">Compiler plugin interface</a>
<ul>
<li><a href="#collector-algos">Overview of available features</a></li>
<li><a href="#stack-map">Computing stack maps</a></li>
</div>
<!-- *********************************************************************** -->
-<div class="doc_section">
+<h2>
<a name="introduction">Introduction</a>
-</div>
+</h2>
<!-- *********************************************************************** -->
-<div class="doc_text">
+<div>
<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
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 allocation
+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>
-
<!-- ======================================================================= -->
-<div class="doc_subsection">
- <a name="feature">GC features provided and algorithms supported</a>
-</div>
+<h3>
+ <a name="feature">Goals and non-goals</a>
+</h3>
-<div class="doc_text">
+<div>
<p>LLVM's intermediate representation provides <a href="#intrinsics">garbage
collection intrinsics</a> that offer support for a broad class of
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 implement a garbage collector. This is because
-collectors are tightly coupled to object models, and LLVM is agnostic to object
-models. Since LLVM is agnostic to object models, it would be inappropriate for
-LLVM to dictate any particular collector. Instead, LLVM provides a framework for
-garbage collector implementations in two manners:</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 href="#plugin">code generation plugins</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>
+ <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>
- <li><b>At compile time</b> with <a href="#plugin">collector plugins</a> for
- the compiler. Collector plugins have ready access to important garbage
- collector algorithms. Leveraging these tools, it is straightforward to
- emit type-accurate stack maps for your runtime in as little as ~100 lines of
- C++ code.</li>
-
- <li><b>At runtime</b> with <a href="#runtime">suggested runtime
- interfaces</a>, which allow front-end compilers to support a range of
- collection runtimes.</li>
+ <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 class="doc_section">
- <a name="usage">Using the collectors</a>
</div>
+
+<!-- *********************************************************************** -->
+<h2>
+ <a name="quickstart">Getting started</a>
+</h2>
<!-- *********************************************************************** -->
-<div class="doc_text">
+<div>
-<p>In general, using a collector implies:</p>
+<p>Using a GC with LLVM implies many things, for example:</p>
<ul>
- <li>Emitting compatible code, including initialization in the main
- program if necessary.</li>
- <li>Loading a compiler plugin if the collector is not statically linked with
- your compiler. For <tt>llc</tt>, use the <tt>-load</tt> option.</li>
- <li>Selecting the collection algorithm by applying the <tt>gc "..."</tt>
- attribute to your garbage collected functions, or equivalently with
- the <tt>setGC</tt> method.</li>
- <li>Linking your final executable with the garbage collector runtime.</li>
+ <li>Write a runtime library or find an existing one which implements a GC
+ heap.<ol>
+ <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>gc "..."</tt> attribute to enable GC code generation
+ (or <tt>F.setGC("...")</tt>).</li>
+ <li>Use <tt>@llvm.gcroot</tt> to mark stack roots.</li>
+ <li>Use <tt>@llvm.gcread</tt> and/or <tt>@llvm.gcwrite</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>@llvm.gcread</tt> and <tt>@llvm.gcwrite</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>llc -load</tt> or link the
+ plugin statically with your language's compiler.*</li>
+ <li>Link program executables with the runtime.</li>
</ul>
-<p>This table summarizes the available runtimes.</p>
-
-<table>
- <tr>
- <th>Collector</th>
- <th><tt>gc</tt> attribute</th>
- <th>Linkage</th>
- <th><tt>gcroot</tt></th>
- <th><tt>gcread</tt></th>
- <th><tt>gcwrite</tt></th>
- </tr>
- <tr valign="baseline">
- <td><a href="#semispace">SemiSpace</a></td>
- <td><tt>gc "shadow-stack"</tt></td>
- <td>TODO FIXME</td>
- <td>required</td>
- <td>optional</td>
- <td>optional</td>
- </tr>
- <tr valign="baseline">
- <td><a href="#ocaml">Ocaml</a></td>
- <td><tt>gc "ocaml"</tt></td>
- <td><i>provided by ocamlopt</i></td>
- <td>required</td>
- <td>optional</td>
- <td>optional</td>
- </tr>
-</table>
-
-<p>The sections for <a href="#intrinsics">Collection intrinsics</a> and
-<a href="#runtime">Recommended runtime interface</a> detail the interfaces that
-collectors may require user programs to utilize.</p>
-
-</div>
+<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>llc</tt> and works even with the interpreter and C backends.</p>
<!-- ======================================================================= -->
-<div class="doc_subsection">
- <a name="shadow-stack">ShadowStack - A highly portable collector</a>
-</div>
+<h3>
+ <a name="quickstart-compiler">In your compiler</a>
+</h3>
-<div class="doc_code"><tt>
- Collector *llvm::createShadowStackCollector();
-</tt></div>
+<div>
-<div class="doc_text">
+<p>To turn the shadow stack on for your functions, first call:</p>
-<p>The ShadowStack backend is invoked with the <tt>gc "shadow-stack"</tt>
-function attribute.
-Unlike many collectors which rely on a cooperative code generator to generate
-stack maps, this algorithm carefully maintains a linked list of stack root
-descriptors [<a href="#henderson02">Henderson2002</a>]. This so-called "shadow
-stack" mirrors the machine stack. Maintaining this data structure is slower
-than using stack maps, but has a significant portability advantage because it
-requires no special support from the target code generator.</p>
+<div class="doc_code"><pre
+>F.setGC("shadow-stack");</pre></div>
-<p>The ShadowStack collector does not use read or write barriers, so the user
-program may use <tt>load</tt> and <tt>store</tt> instead of <tt>llvm.gcread</tt>
-and <tt>llvm.gcwrite</tt>.</p>
+<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>ShadowStack is a code generator plugin only. It must be paired with a
-compatible runtime.</p>
+<p>Your compiler must also use <tt>@llvm.gcroot</tt> as documented.
+Don't forget to create a root for each intermediate value that is generated
+when evaluating an expression. In <tt>h(f(), g())</tt>, the result of
+<tt>f()</tt> could easily be collected if evaluating <tt>g()</tt> triggers a
+collection.</p>
-</div>
+<p>There's no need to use <tt>@llvm.gcread</tt> and <tt>@llvm.gcwrite</tt> over
+plain <tt>load</tt> and <tt>store</tt> for now. You will need them when
+switching to a more advanced GC.</p>
-<!-- ======================================================================= -->
-<div class="doc_subsection">
- <a name="semispace">SemiSpace - A simple copying collector runtime</a>
</div>
-<div class="doc_text">
-
-<p>The SemiSpace runtime implements the <a href="runtime">suggested
-runtime interface</a> and is compatible with the ShadowStack backend.</p>
-
-<p>SemiSpace is a very simple copying collector. When it starts up, it
-allocates two blocks of memory for the heap. It uses a simple bump-pointer
-allocator to allocate memory from the first block until it runs out of space.
-When it runs out of space, it traces through all of the roots of the program,
-copying blocks to the other half of the memory space.</p>
-
-<p>This runtime is highly experimental and has not been used in a real project.
-Enhancements would be welcomed.</p>
+<!-- ======================================================================= -->
+<h3>
+ <a name="quickstart-runtime">In your runtime</a>
+</h3>
+
+<div>
+
+<p>The shadow stack doesn't imply a memory allocation algorithm. A semispace
+collector or building atop <tt>malloc</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>
+
+<pre class="doc_code">
+/// @brief The map for a single function's stack frame. One of these is
+/// compiled as constant data into the executable for each function.
+///
+/// Storage of metadata values is elided if the %metadata parameter to
+/// @llvm.gcroot is null.
+struct FrameMap {
+ int32_t NumRoots; //< Number of roots in stack frame.
+ int32_t NumMeta; //< Number of metadata entries. May be < NumRoots.
+ const void *Meta[0]; //< Metadata for each root.
+};
+
+/// @brief A link in the dynamic shadow stack. One of these is embedded in the
+/// stack frame of each function on the call stack.
+struct StackEntry {
+ StackEntry *Next; //< Link to next stack entry (the caller's).
+ const FrameMap *Map; //< Pointer to constant FrameMap.
+ void *Roots[0]; //< Stack roots (in-place array).
+};
+
+/// @brief The head of the singly-linked list of StackEntries. Functions push
+/// and pop onto this in their prologue and epilogue.
+///
+/// Since there is only a global list, this technique is not threadsafe.
+StackEntry *llvm_gc_root_chain;
+
+/// @brief Calls Visitor(root, meta) for each GC root on the stack.
+/// root and meta are exactly the values passed to
+/// <tt>@llvm.gcroot</tt>.
+///
+/// Visitor could be a function to recursively mark live objects. Or it
+/// might copy them to another heap or generation.
+///
+/// @param Visitor A function to invoke for every GC root on the stack.
+void visitGCRoots(void (*Visitor)(void **Root, const void *Meta)) {
+ for (StackEntry *R = llvm_gc_root_chain; R; R = R->Next) {
+ unsigned i = 0;
+
+ // For roots [0, NumMeta), the metadata pointer is in the FrameMap.
+ for (unsigned e = R->Map->NumMeta; i != e; ++i)
+ Visitor(&R->Roots[i], R->Map->Meta[i]);
+
+ // For roots [NumMeta, NumRoots), the metadata pointer is null.
+ for (unsigned e = R->Map->NumRoots; i != e; ++i)
+ Visitor(&R->Roots[i], NULL);
+ }
+}</pre>
</div>
<!-- ======================================================================= -->
-<div class="doc_subsection">
- <a name="ocaml">Ocaml - An Objective Caml-compatible collector</a>
-</div>
+<h3>
+ <a name="shadow-stack">About the shadow stack</a>
+</h3>
-<div class="doc_code"><tt>
- Collector *llvm::createOcamlCollector();
-</tt></div>
+<div>
-<div class="doc_text">
+<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 href="#henderson02">Henderson2002</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 ocaml backend is invoked with the <tt>gc "ocaml"</tt> function attribute.
-It supports the
-<a href="http://caml.inria.fr/">Objective Caml</a> language runtime by emitting
-a type-accurate stack map in the form of an ocaml 3.10.0-compatible frametable.
-The linkage requirements are satisfied automatically by the <tt>ocamlopt</tt>
-compiler when linking an executable.</p>
+<p>The tradeoff for this simplicity and portability is:</p>
-<p>The ocaml collector does not use read or write barriers, so the user program
-may use <tt>load</tt> and <tt>store</tt> instead of <tt>llvm.gcread</tt> and
-<tt>llvm.gcwrite</tt>.</p>
+<ul>
+ <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 href="#plugin">plugin</a> will allow you to take
+advantage of <a href="#collector-algos">more advanced GC features</a> of LLVM
+in order to improve performance.</p>
</div>
+</div>
<!-- *********************************************************************** -->
-<div class="doc_section">
- <a name="core">
Core support</a><a name="intrinsics"></a>
-</div>
+<h2>
+ <a name="core">
IR features</a><a name="intrinsics"></a>
+</h2>
<!-- *********************************************************************** -->
-<div class="doc_text">
+<div>
<p>This section describes the garbage collection facilities provided by the
-<a href="LangRef.html">LLVM intermediate representation</a>.</p>
+<a href="LangRef.html">LLVM intermediate representation</a>. The exact behavior
+of these IR features is specified by the binary interface implemented by a
+<a href="#plugin">code generation plugin</a>, not by this document.</p>
-<p>These facilities are limited to those strictly necessary for compilation.
-They are not intended to be a complete interface to any garbage collector.
-Notably, heap allocation is not among the supplied primitives. A user program
-will also need to interface with the runtime, using either the
-<a href="#runtime">suggested runtime interface</a> or another interface
-specified by the runtime.</p>
-
-</div>
+<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="doc_subsection">
+<h3>
<a name="gcattr">Specifying GC code generation: <tt>gc "..."</tt></a>
-</div>
+</h3>
+
+<div>
<div class="doc_code"><tt>
- define <i>ty</i> @<i>name</i>(...) <u>gc "<i>collector</i>"</u> { ...
+ define <i>ty</i> @<i>name</i>(...) <span style="text-decoration: underline">gc "<i>name</i>"</span> { ...
</tt></div>
-<div class="doc_text">
+<p>The <tt>gc</tt> function attribute is used to specify the desired GC style
+to the compiler. Its programmatic equivalent is the <tt>setGC</tt> method of
+<tt>Function</tt>.</p>
-<p>The <tt>gc</tt> function attribute is used to specify the desired collector
-algorithm to the compiler. It is equivalent to specifying the collector name
-programmatically using the <tt>setGC</tt> method of <tt>Function</tt>.</p>
+<p>Setting <tt>gc "<i>name</i>"</tt> on a function triggers a search for a
+matching code generation plugin "<i>name</i>"; 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 collector on a per-function basis allows LLVM to link together
-programs that use different garbage collection algorithms.</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="doc_subsection">
+<h3>
<a name="gcroot">Identifying GC roots on the stack: <tt>llvm.gcroot</tt></a>
-</div>
+</h3>
+
+<div>
<div class="doc_code"><tt>
void @llvm.gcroot(i8** %ptrloc, i8* %metadata)
</tt></div>
-<div class="doc_text">
+<p>The <tt>llvm.gcroot</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
+href="#plugin">compiler plugin</a>. All calls to <tt>llvm.gcroot</tt> <b>must</b> reside
+ inside the first basic block.</p>
+
+<p>A compiler which uses mem2reg to raise imperative code using <tt>alloca</tt>
+into SSA form need only add a call to <tt>@llvm.gcroot</tt> for those variables
+which a pointers into the GC heap.</p>
+
+<p>It is also important to mark intermediate values with <tt>llvm.gcroot</tt>.
+For example, consider <tt>h(f(), g())</tt>. Beware leaking the result of
+<tt>f()</tt> in the case that <tt>g()</tt> triggers a collection. Note, that
+stack variables must be initialized and marked with <tt>llvm.gcroot</tt> in
+function's prologue.</p>
-<p>The <tt>llvm.gcroot</tt> intrinsic is used to inform LLVM of a pointer
-variable on the stack. The first argument <b>must</b> be a value referring to an alloca instruction
+<p>The first argument <b>must</b> 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 <b>must</b> be a constant or global
value address. If your target collector uses tags, use a null pointer for
metadata.</p>
+<p>The <tt>%metadata</tt> argument can be used to avoid requiring heap objects
+to have 'isa' pointers or tag bits. [<a href="#appel89">Appel89</a>, <a
+href="#goldberg91">Goldberg91</a>, <a 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>
-<pre>
+<pre class="doc_code">
{
Object X; // A null-initialized reference to an object
...
<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>
-<pre>
+<pre class="doc_code">
Entry:
;; In the entry block for the function, allocate the
;; stack space for X, which is an LLVM pointer.
;; Tell LLVM that the stack space is a stack root.
;; Java has type-tags on objects, so we pass null as metadata.
%tmp = bitcast %Object** %X to i8**
- call void @llvm.gcroot(i8** %X, i8* null)
+ call void @llvm.gcroot(i8** %tmp, i8* null)
...
;; "CodeBlock" is the block corresponding to the start
</div>
<!-- ======================================================================= -->
-<div class="doc_subsection">
+<h3>
<a name="barriers">Reading and writing references in the heap</a>
-</div>
+</h3>
-<div class="doc_text">
+<div>
<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
;; Compute the derived pointer.
%derived = getelementptr %object, i32 0, i32 2, i32 %n</pre></blockquote>
-</div>
+<p>LLVM does not enforce this relationship between the object and derived
+pointer (although a <a href="#plugin">plugin</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>load</tt> or <tt>store</tt> instruction if they
+are used.</p>
<!-- ======================================================================= -->
-<div class="doc_subsubsection">
+<h4>
<a name="gcwrite">Write barrier: <tt>llvm.gcwrite</tt></a>
-</div>
+</h4>
+
+<div>
<div class="doc_code"><tt>
void @llvm.gcwrite(i8* %value, i8* %object, i8** %derived)
</tt></div>
-<div class="doc_text">
-
<p>For write barriers, LLVM provides the <tt>llvm.gcwrite</tt> intrinsic
function. It has exactly the same semantics as a non-volatile <tt>store</tt> to
-the derived pointer (the third argument).</p>
+the derived pointer (the third argument). The exact code generated is specified
+by a <a href="#plugin">compiler plugin</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>
-<p>The use of this intrinsic is optional if the target collector does use
-write barriers. If so, the collector will replace it with the corresponding
-<tt>store</tt>.</p>
-
</div>
<!-- ======================================================================= -->
-<div class="doc_subsubsection">
+<h4>
<a name="gcread">Read barrier: <tt>llvm.gcread</tt></a>
-</div>
+</h4>
+
+<div>
<div class="doc_code"><tt>
i8* @llvm.gcread(i8* %object, i8** %derived)<br>
</tt></div>
-<div class="doc_text">
-
<p>For read barriers, LLVM provides the <tt>llvm.gcread</tt> intrinsic function.
It has exactly the same semantics as a non-volatile <tt>load</tt> from the
-derived pointer (the second argument).</p>
+derived pointer (the second argument). The exact code generated is specified by
+a <a href="#plugin">compiler plugin</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>
-<p>As with <tt>llvm.gcwrite</tt>, a target collector might not require the use
-of this intrinsic.</p>
-
-</div>
-
-<!-- *********************************************************************** -->
-<div class="doc_section">
- <a name="runtime">Recommended runtime interface</a>
-</div>
-<!-- *********************************************************************** -->
-
-<div class="doc_text">
-
-<p>LLVM specifies the following recommended runtime interface to the garbage
-collection at runtime. A program should use these interfaces to accomplish the
-tasks not supported by the intrinsics.</p>
-
-<p>Unlike the intrinsics, which are integral to LLVM's code generator, there is
-nothing unique about these interfaces; a front-end compiler and runtime are free
-to agree to a different specification.</p>
-
-<p class="doc_warning">Note: This interface is a work in progress.</p>
-
-</div>
-
-<!-- ======================================================================= -->
-<div class="doc_subsection">
- <a name="initialize">Garbage collector startup and initialization</a>
-</div>
-
-<div class="doc_text">
-
-<div class="doc_code"><tt>
- void llvm_gc_initialize(unsigned InitialHeapSize);
-</tt></div>
-
-<p>
-The <tt>llvm_gc_initialize</tt> function should be called once before any other
-garbage collection functions are called. This gives the garbage collector the
-chance to initialize itself and allocate the heap. The initial heap size to
-allocate should be specified as an argument.
-</p>
-
-</div>
-
-<!-- ======================================================================= -->
-<div class="doc_subsection">
- <a name="allocate">Allocating memory from the GC</a>
-</div>
-
-<div class="doc_text">
-
-<div class="doc_code"><tt>
- void *llvm_gc_allocate(unsigned Size);
-</tt></div>
-
-<p>The <tt>llvm_gc_allocate</tt> function is a global function defined by the
-garbage collector implementation to allocate memory. It returns a
-zeroed-out block of memory of the specified size, sufficiently aligned to store
-any object.</p>
-
</div>
-<!-- ======================================================================= -->
-<div class="doc_subsection">
- <a name="explicit">Explicit invocation of the garbage collector</a>
</div>
-<div class="doc_text">
-
-<div class="doc_code"><tt>
- void llvm_gc_collect();
-</tt></div>
-
-<p>
-The <tt>llvm_gc_collect</tt> function is exported by the garbage collector
-implementations to provide a full collection, even when the heap is not
-exhausted. This can be used by end-user code as a hint, and may be ignored by
-the garbage collector.
-</p>
-
</div>
-<!-- ======================================================================= -->
-<div class="doc_subsection">
- <a name="traceroots">Tracing GC pointers from the program stack</a>
-</div>
-
-<div class="doc_text">
- <div class="doc_code"><tt>
- void llvm_cg_walk_gcroots(void (*FP)(void **Root, void *Meta));
- </tt></div>
-
-<p>
-The <tt>llvm_cg_walk_gcroots</tt> function is a function provided by the code
-generator that iterates through all of the GC roots on the stack, calling the
-specified function pointer with each record. For each GC root, the address of
-the pointer and the meta-data (from the <a
-href="#gcroot"><tt>llvm.gcroot</tt></a> intrinsic) are provided.
-</p>
-</div>
-
-<!-- ======================================================================= -->
-<div class="doc_subsection">
- <a name="staticroots">Tracing GC pointers from static roots</a>
-</div>
-
-<div class="doc_text">
-TODO
-</div>
-
-
<!-- *********************************************************************** -->
-<div class="doc_section">
+<h2>
<a name="plugin">Implementing a collector plugin</a>
-</div>
+</h2>
<!-- *********************************************************************** -->
-<div class="doc_text">
+<div>
<p>User code specifies which GC code generation to use with the <tt>gc</tt>
function attribute or, equivalently, with the <tt>setGC</tt> method of
<p>To implement a GC plugin, it is necessary to subclass
<tt>llvm::GCStrategy</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 emit
-the assembly code for the collector's unique stack map data structure, which
-might be accomplished in as few as 100 LOC.</p>
+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 is
-compatible with that runtime library.</p>
+library. The compiler plugin is responsible for generating code which
+conforms to the binary interface defined by library, most essentially the
+<a href="#stack-map">stack map</a>.</p>
<p>To subclass <tt>llvm::GCStrategy</tt> and register it with the compiler:</p>
using namespace llvm;
namespace {
- class VISIBILITY_HIDDEN MyGC : public GCStrategy {
+ class LLVM_LIBRARY_VISIBILITY MyGC : public GCStrategy {
public:
MyGC() {}
};
X("mygc", "My bespoke garbage collector.");
}</pre></blockquote>
+<p>This boilerplate collector does nothing. More specifically:</p>
+
+<ul>
+ <li><tt>llvm.gcread</tt> calls are replaced with the corresponding
+ <tt>load</tt> instruction.</li>
+ <li><tt>llvm.gcwrite</tt> calls are replaced with the corresponding
+ <tt>store</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
href="http://llvm.org/viewvc/llvm-project/llvm/trunk/projects/sample/">sample
-project</a>), this can be built into a plugin using a simple makefile:</p>
+project</a>), this code can be compiled as a plugin using a simple
+makefile:</p>
<blockquote><pre
># lib/MyGC/Makefile
<p>It is also possible to statically link the collector plugin into tools, such
as a language-specific compiler front-end.</p>
-</div>
-
<!-- ======================================================================= -->
-<div class="doc_subsection">
+<h3>
<a name="collector-algos">Overview of available features</a>
-</div>
+</h3>
-<div class="doc_text">
+<div>
-<p>The boilerplate collector above does nothing. More specifically:</p>
-
-<ul>
- <li><tt>llvm.gcread</tt> calls are replaced with the corresponding
- <tt>load</tt> instruction.</li>
- <li><tt>llvm.gcwrite</tt> calls are replaced with the corresponding
- <tt>store</tt> instruction.</li>
- <li>No stack map is emitted, and no safe points are added.</li>
-</ul>
-
-<p><tt>Collector</tt> provides a range of features through which a plugin
-collector may do useful work. This matrix summarizes the supported (and planned)
-features and correlates them with the collection techniques which typically
-require them.</p>
+<p><tt>GCStrategy</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>
<tr>
<dl>
<dt>Shadow Stack</dt>
- <dd>The mutator carefully maintains a linked list of stack root
- descriptors.</dd>
+ <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>
</div>
<!-- ======================================================================= -->
-<div class="doc_subsection">
+<h3>
<a name="stack-map">Computing stack maps</a>
-</div>
+</h3>
+
+<div>
+
+<p>LLVM automatically computes a stack map. One of the most important features
+of a <tt>GCStrategy</tt> is to compile this information into the executable in
+the binary representation expected by the runtime library.</p>
-<div class="doc_text">
+<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>
+ <li><tt>RootNum</tt>: The index of the root.</li>
+ <li><tt>StackOffset</tt>: The offset of the object relative to the frame
+ pointer.</li>
+ <li><tt>RootMetadata</tt>: The value passed as the <tt>%metadata</tt>
+ parameter to the <a href="#gcroot"><tt>@llvm.gcroot</tt></a> intrinsic.</li>
+</ul>
+
+<p>Also, for the function as a whole:</p>
+
+<ul>
+ <li><tt>getFrameSize()</tt>: The overall size of the function's initial
+ stack frame, not accounting for any dynamic allocation.</li>
+ <li><tt>roots_size()</tt>: The count of roots in the function.</li>
+</ul>
+
+<p>To access the stack map, use <tt>GCFunctionMetadata::roots_begin()</tt> and
+-<tt>end()</tt> from the <tt><a
+href="#assembly">GCMetadataPrinter</a></tt>:</p>
<blockquote><pre
>for (iterator I = begin(), E = end(); I != E; ++I) {
}
}</pre></blockquote>
-<p>LLVM automatically computes a stack map. All a <tt>GCStrategy</tt> needs to do
-is access it using <tt>GCFunctionMetadata::roots_begin()</tt> and
--<tt>end()</tt>. If the <tt>llvm.gcroot</tt> intrinsic is eliminated before code
-generation by a custom lowering pass, LLVM's stack map will be empty.</p>
+<p>If the <tt>llvm.gcroot</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="doc_subsection">
+<h3>
<a name="init-roots">Initializing roots to null: <tt>InitRoots</tt></a>
-</div>
+</h3>
-<div class="doc_text">
+<div>
<blockquote><pre
>MyGC::MyGC() {
<!-- ======================================================================= -->
-<div class="doc_subsection">
+<h3>
<a name="custom">Custom lowering of intrinsics: <tt>CustomRoots</tt>,
<tt>CustomReadBarriers</tt>, and <tt>CustomWriteBarriers</tt></a>
-</div>
+</h3>
-<div class="doc_text">
+<div>
<p>For GCs which use barriers or unusual treatment of stack roots, these
flags allow the collector to perform arbitrary transformations of the LLVM
<p>LLVM's default action for each intrinsic is as follows:</p>
<ul>
- <li><tt>llvm.gcroot</tt>: Pass through to the code generator to generate a
- stack map.</li>
+ <li><tt>llvm.gcroot</tt>: Leave it alone. The code generator must see it
+ or the stack map will not be computed.</li>
<li><tt>llvm.gcread</tt>: Substitute a <tt>load</tt> instruction.</li>
<li><tt>llvm.gcwrite</tt>: Substitute a <tt>store</tt> instruction.</li>
</ul>
<!-- ======================================================================= -->
-<div class="doc_subsection">
+<h3>
<a name="safe-points">Generating safe points: <tt>NeededSafePoints</tt></a>
-</div>
+</h3>
-<div class="doc_text">
+<div>
<p>LLVM can compute four kinds of safe points:</p>
<!-- ======================================================================= -->
-<div class="doc_subsection">
+<h3>
<a name="assembly">Emitting assembly code: <tt>GCMetadataPrinter</tt></a>
-</div>
+</h3>
-<div class="doc_text">
+<div>
-<p>LLVM allows a GC 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 print stack
-maps built by the code generator. (At the beginning, this information is not
+<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>GCMetadaPrinter</tt> and <tt>GCMetadaPrinterRegistry</tt>. The AsmWriter
+<tt>GCMetadaPrinter</tt> and <tt>GCMetadataPrinterRegistry</tt>. The AsmWriter
will look for such a subclass if the <tt>GCStrategy</tt> sets
<tt>UsesMetadata</tt>:</p>
UsesMetadata = true;
}</pre></blockquote>
+<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>
using namespace llvm;
namespace {
- class VISIBILITY_HIDDEN MyGCPrinter : public GCMetadataPrinter {
+ class LLVM_LIBRARY_VISIBILITY MyGCPrinter : public GCMetadataPrinter {
public:
virtual void beginAssembly(std::ostream &OS, AsmPrinter &AP,
const TargetAsmInfo &TAI);
>#include "llvm/CodeGen/AsmPrinter.h"
#include "llvm/Function.h"
#include "llvm/Target/TargetMachine.h"
-#include "llvm/Target/TargetData.h"
+#include "llvm/DataLayout.h"
#include "llvm/Target/TargetAsmInfo.h"
void MyGCPrinter::beginAssembly(std::ostream &OS, AsmPrinter &AP,
// Set up for emitting addresses.
const char *AddressDirective;
int AddressAlignLog;
- if (AP.TM.getTargetData()->getPointerSize() == sizeof(int32_t)) {
+ if (AP.TM.getDataLayout()->getPointerSize() == sizeof(int32_t)) {
AddressDirective = TAI.getData32bitsDirective();
AddressAlignLog = 2;
} else {
// Align to address width.
AP.EmitAlignment(AddressAlignLog);
- // Emit the symbol by which the stack map can be found.
+ // Emit the symbol by which the stack map entry can be found.
std::string Symbol;
Symbol += TAI.getGlobalPrefix();
Symbol += "__gcmap_";
</div>
-
-<!-- *********************************************************************** -->
-<div class="doc_section">
- <a name="runtime-impl">Implementing a collector runtime</a>
</div>
-<!-- *********************************************************************** -->
-
-<div class="doc_text">
-
-<p>Implementing a garbage collector for LLVM is fairly straightforward. The
-LLVM garbage collectors are provided in a form that makes them easy to link into
-the language-specific runtime that a language front-end would use. They require
-functionality from the language-specific runtime to get information about <a
-href="#gcdescriptors">where pointers are located in heap objects</a>.</p>
-
-<p>The implementation must include the
-<a href="#allocate"><tt>llvm_gc_allocate</tt></a> and
-<a href="#explicit"><tt>llvm_gc_collect</tt></a> functions. To do this, it will
-probably have to <a href="#traceroots">trace through the roots
-from the stack</a> and understand the <a href="#gcdescriptors">GC descriptors
-for heap objects</a>. Luckily, there are some <a href="#usage">example
-implementations</a> available.
-</p>
-</div>
-
-
-<!-- ======================================================================= -->
-<div class="doc_subsection">
- <a name="gcdescriptors">Tracing GC pointers from heap objects</a>
-</div>
-
-<div class="doc_text">
-<p>
-The three most common ways to keep track of where pointers live in heap objects
-are (listed in order of space overhead required):</p>
-
-<ol>
-<li>In languages with polymorphic objects, pointers from an object header are
-usually used to identify the GC pointers in the heap object. This is common for
-object-oriented languages like Self, Smalltalk, Java, or C#.</li>
-
-<li>If heap objects are not polymorphic, often the "shape" of the heap can be
-determined from the roots of the heap or from some other meta-data [<a
-href="#appel89">Appel89</a>, <a href="#goldberg91">Goldberg91</a>, <a
-href="#tolmach94">Tolmach94</a>]. In this case, the garbage collector can
-propagate the information around from meta data stored with the roots. This
-often eliminates the need to have a header on objects in the heap. This is
-common in the ML family.</li>
-
-<li>If all heap objects have pointers in the same locations, or pointers can be
-distinguished just by looking at them (e.g., the low order bit is clear), no
-book-keeping is needed at all. This is common for Lisp-like languages.</li>
-</ol>
-
-<p>The LLVM garbage collectors are capable of supporting all of these styles of
-language, including ones that mix various implementations. To do this, it
-allows the source-language to associate meta-data with the <a
-href="#gcroot">stack roots</a>, and the heap tracing routines can propagate the
-information. In addition, LLVM allows the front-end to extract GC information
-in any form from a specific object pointer (this supports situations #1 and #3).
-</p>
-
-</div>
-
<!-- *********************************************************************** -->
-<div class="doc_section">
+<h2>
<a name="references">References</a>
-</div>
+</h2>
<!-- *********************************************************************** -->
-<div class="doc_text">
+<div>
<p><a name="appel89">[Appel89]</a> Runtime Tags Aren't Necessary. Andrew
W. Appel. Lisp and Symbolic Computation 19(7):703-705, July 1989.</p>
src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a>
<a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
- <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
+ <a href="http://llvm.org/">LLVM Compiler Infrastructure</a><br>
Last modified: $Date$
</address>
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