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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="#interfaces">Interfaces for user programs</a>
+ <li><a href="#quickstart">Getting started</a>
<ul>
- <li><a href="#roots">Identifying GC roots on the stack: <tt>llvm.gcroot</tt></a></li>
- <li><a href="#allocate">Allocating memory from the GC</a></li>
- <li><a href="#barriers">Reading and writing references to the heap</a></li>
- <li><a href="#explicit">Explicit invocation of the garbage 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>
- <li><a href="#gcimpl">Implementing a garbage collector</a>
+ <li><a href="#core">Core support</a>
<ul>
- <li><a href="#llvm_gc_readwrite">Implementing <tt>llvm_gc_read</tt> and <tt>llvm_gc_write</tt></a></li>
- <li><a href="#callbacks">Callback functions used to implement the garbage collector</a></li>
+ <li><a href="#gcattr">Specifying GC code generation:
+ <tt>gc "..."</tt></a></li>
+ <li><a href="#gcroot">Identifying GC roots on the stack:
+ <tt>llvm.gcroot</tt></a></li>
+ <li><a href="#barriers">Reading and writing references in the heap</a>
+ <ul>
+ <li><a href="#gcwrite">Write barrier: <tt>llvm.gcwrite</tt></a></li>
+ <li><a href="#gcread">Read barrier: <tt>llvm.gcread</tt></a></li>
+ </ul>
+ </li>
</ul>
</li>
- <li><a href="#gcimpls">GC implementations available</a>
+
+ <li><a href="#plugin">Compiler plugin interface</a>
<ul>
- <li><a href="#semispace">SemiSpace - A simple copying garbage collector</a></li>
+ <li><a href="#collector-algos">Overview of available features</a></li>
+ <li><a href="#stack-map">Computing stack maps</a></li>
+ <li><a href="#init-roots">Initializing roots to null:
+ <tt>InitRoots</tt></a></li>
+ <li><a href="#custom">Custom lowering of intrinsics: <tt>CustomRoots</tt>,
+ <tt>CustomReadBarriers</tt>, and <tt>CustomWriteBarriers</tt></a></li>
+ <li><a href="#safe-points">Generating safe points:
+ <tt>NeededSafePoints</tt></a></li>
+ <li><a href="#assembly">Emitting assembly code:
+ <tt>GCMetadataPrinter</tt></a></li>
</ul>
</li>
-<!--
- <li><a href="#codegen">Implementing GC support in a code generator</a></li>
--->
+ <li><a href="#runtime-impl">Implementing a collector runtime</a>
+ <ul>
+ <li><a href="#gcdescriptors">Tracing GC pointers from heap
+ objects</a></li>
+ </ul>
+ </li>
+
+ <li><a href="#references">References</a></li>
+
</ol>
<div class="doc_author">
- <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
+ <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a> and
+ Gordon Henriksen</p>
</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 life-times 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:
+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
+information from the compiler. The
<a 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
+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 href="#roots">processor stack and registers</a>.</p>
+<a href="#gcroot">processor stack and registers</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
+<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 <i>know</i> 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>
+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,
+<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 allocation
-techniques dominates any low-level losses.
-</p>
+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>
-<p>
-This document describes the mechanisms and interfaces provided by LLVM to
-support accurate garbage collection.
-</p>
+<!-- ======================================================================= -->
+<h3>
+ <a name="feature">Goals and non-goals</a>
+</h3>
+
+<div>
+
+<p>LLVM's intermediate representation provides <a href="#intrinsics">garbage
+collection intrinsics</a> that offer support for a broad class of
+collector models. For instance, the intrinsics permit:</p>
+
+<ul>
+ <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 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>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>
+
+<!-- *********************************************************************** -->
+<h2>
+ <a name="quickstart">Getting started</a>
+</h2>
+<!-- *********************************************************************** -->
+
+<div>
+
+<p>Using a GC with LLVM implies many things, for example:</p>
+
+<ul>
+ <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>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="feature">GC features provided and algorithms supported</a>
+<h3>
+ <a name="quickstart-compiler">In your compiler</a>
+</h3>
+
+<div>
+
+<p>To turn the shadow stack on for your functions, first call:</p>
+
+<div class="doc_code"><pre
+>F.setGC("shadow-stack");</pre></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>@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>
+
+<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>
-<div class="doc_text">
-
-<p>
-LLVM provides support for a broad class of garbage collection algorithms,
-including compacting semi-space collectors, mark-sweep collectors, generational
-collectors, and even reference counting implementations. It includes support
-for <a href="#barriers">read and write barriers</a>, and associating <a
-href="#roots">meta-data with stack objects</a> (used for tagless garbage
-collection). All LLVM code generators support garbage collection, including the
-C backend.
-</p>
-
-<p>
-We hope that the primitive support built into LLVM is sufficient to support a
-broad class of garbage collected languages, including Scheme, ML, scripting
-languages, Java, C#, etc. That said, the implemented garbage collectors may
-need to be extended to support language-specific features such as finalization,
-weak references, or other features. As these needs are identified and
-implemented, they should be added to this specification.
-</p>
-
-<p>
-LLVM does not currently support garbage collection of multi-threaded programs or
-GC-safe points other than function calls, but these will be added in the future
-as there is interest.
-</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_section">
- <a name="interfaces">Interfaces for user programs</a>
+<!-- ======================================================================= -->
+<h3>
+ <a name="shadow-stack">About the shadow stack</a>
+</h3>
+
+<div>
+
+<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 tradeoff for this simplicity and portability is:</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>
+
+<!-- *********************************************************************** -->
+<h2>
+ <a name="core">IR features</a><a name="intrinsics"></a>
+</h2>
<!-- *********************************************************************** -->
-<div class="doc_text">
+<div>
-<p>This section describes the interfaces provided by LLVM and by the garbage
-collector run-time that should be used by user programs. As such, this is the
-interface that front-end authors should generate code for.
-</p>
+<p>This section describes the garbage collection facilities provided by the
+<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>
-</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">
- <a name="roots">Identifying GC roots on the stack: <tt>llvm.gcroot</tt></a>
-</div>
+<h3>
+ <a name="gcattr">Specifying GC code generation: <tt>gc "..."</tt></a>
+</h3>
-<div class="doc_text">
+<div>
<div class="doc_code"><tt>
- void %llvm.gcroot(<ty>** %ptrloc, <ty2>* %metadata)
+ define <i>ty</i> @<i>name</i>(...) <span style="text-decoration: underline">gc "<i>name</i>"</span> { ...
</tt></div>
-<p>
-The <tt>llvm.gcroot</tt> intrinsic is used to inform LLVM of a pointer variable
-on the stack. The first argument contains the address of the variable on the
-stack, and the second contains a pointer to metadata that should be associated
-with the pointer (which <b>must</b> be a constant or global value address). At
-runtime, the <tt>llvm.gcroot</tt> intrinsic stores a null pointer into the
-specified location to initialize the pointer.</p>
+<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>
-Consider the following fragment of Java code:
-</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 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>
-<pre>
+<!-- ======================================================================= -->
+<h3>
+ <a name="gcroot">Identifying GC roots on the stack: <tt>llvm.gcroot</tt></a>
+</h3>
+
+<div>
+
+<div class="doc_code"><tt>
+ void @llvm.gcroot(i8** %ptrloc, i8* %metadata)
+</tt></div>
+
+<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 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 class="doc_code">
{
Object X; // A null-initialized reference to an object
...
}
</pre>
-<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>
+<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.
%X = alloca %Object*
+
+ ;; 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** %tmp, i8* null)
...
;; "CodeBlock" is the block corresponding to the start
;; of the scope above.
CodeBlock:
- ;; Initialize the object, telling LLVM that it is now live.
- ;; Java has type-tags on objects, so it doesn't need any
- ;; metadata.
- call void %llvm.gcroot(%Object** %X, sbyte* null)
+ ;; Java null-initializes pointers.
+ store %Object* null, %Object** %X
+
...
;; As the pointer goes out of scope, store a null value into
</div>
<!-- ======================================================================= -->
-<div class="doc_subsection">
- <a name="allocate">Allocating memory from the GC</a>
-</div>
+<h3>
+ <a name="barriers">Reading and writing references in the heap</a>
+</h3>
+
+<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
+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>
-<div class="doc_text">
+<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>%object</tt> is the object pointer, and
+<tt>%derived</tt> is the derived pointer:</p>
-<div class="doc_code"><tt>
- sbyte *%llvm_gc_allocate(unsigned %Size)
-</tt></div>
+<blockquote><pre>
+ ;; An array type.
+ %class.Array = type { %class.Object, i32, [0 x %class.Object*] }
+ ...
-<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 appropriate size.</p>
+ ;; Load the object pointer from a gcroot.
+ %object = load %class.Array** %object_addr
-</div>
+ ;; Compute the derived pointer.
+ %derived = getelementptr %object, i32 0, i32 2, i32 %n</pre></blockquote>
+
+<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_subsection">
- <a name="barriers">Reading and writing references to the heap</a>
-</div>
+<h4>
+ <a name="gcwrite">Write barrier: <tt>llvm.gcwrite</tt></a>
+</h4>
-<div class="doc_text">
+<div>
<div class="doc_code"><tt>
- sbyte *%llvm.gcread(sbyte *, sbyte **)<br>
- void %llvm.gcwrite(sbyte*, sbyte*, sbyte**)
+void @llvm.gcwrite(i8* %value, i8* %object, i8** %derived)
</tt></div>
-<p>Several of the more interesting garbage collectors (e.g., generational
-collectors) need to be informed when the mutator (the program that needs garbage
-collection) reads or writes object references into the heap. In the case of a
-generational collector, it needs to keep track of which "old" generation objects
-have references stored into them. The amount of code that typically needs to be
-executed is usually quite small (and not on the critical path of any
-computation), so the overall performance impact of the inserted code is
-tolerable.</p>
+<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). The exact code generated is specified
+by a <a href="#plugin">compiler plugin</a>.</p>
-<p>To support garbage collectors that use read or write barriers, LLVM provides
-the <tt>llvm.gcread</tt> and <tt>llvm.gcwrite</tt> intrinsics. The first
-intrinsic has exactly the same semantics as a non-volatile LLVM load and the
-second has the same semantics as a non-volatile LLVM store, with the
-additions that they also take a pointer to the start of the memory
-object as an argument. At code generation
-time, these intrinsics are replaced with calls into the garbage collector
-(<tt><a href="#llvm_gc_readwrite">llvm_gc_read</a></tt> and <tt><a
-href="#llvm_gc_readwrite">llvm_gc_write</a></tt> respectively), which are then
-inlined into the code.
-</p>
-
-<p>
-If you are writing a front-end for a garbage collected language, every load or
-store of a reference from or to the heap should use these intrinsics instead of
-normal LLVM loads/stores.</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="doc_subsection">
- <a name="initialize">Garbage collector startup and initialization</a>
-</div>
+<h4>
+ <a name="gcread">Read barrier: <tt>llvm.gcread</tt></a>
+</h4>
-<div class="doc_text">
+<div>
<div class="doc_code"><tt>
- void %llvm_gc_initialize(unsigned %InitialHeapSize)
+i8* @llvm.gcread(i8* %object, i8** %derived)<br>
</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 spaces. The initial heap size
-to allocate should be specified as an argument.
-</p>
+<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). The exact code generated is specified by
+a <a href="#plugin">compiler plugin</a>.</p>
-</div>
+<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 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>
<!-- *********************************************************************** -->
-<div class="doc_section">
- <a name="gcimpl">Implementing a garbage collector</a>
-</div>
+<h2>
+ <a name="plugin">Implementing a collector plugin</a>
+</h2>
<!-- *********************************************************************** -->
-<div class="doc_text">
-
-<p>
-Implementing a garbage collector for LLVM is fairly straight-forward. 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, and it must implement
-the <a href="#llvm_gc_readwrite">read/write barrier</a> functions as well. 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="#gcimpls">example
-implementations</a> available.
-</p>
-</div>
-
+<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
+<tt>Function</tt>.</p>
+
+<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
+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 href="#stack-map">stack map</a>.</p>
+
+<p>To subclass <tt>llvm::GCStrategy</tt> and register it with the compiler:</p>
+
+<blockquote><pre>// lib/MyGC/MyGC.cpp - Example LLVM GC plugin
+
+#include "llvm/CodeGen/GCStrategy.h"
+#include "llvm/CodeGen/GCMetadata.h"
+#include "llvm/Support/Compiler.h"
+
+using namespace llvm;
+
+namespace {
+ class LLVM_LIBRARY_VISIBILITY MyGC : public GCStrategy {
+ public:
+ MyGC() {}
+ };
+
+ GCRegistry::Add<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 code can be compiled as a plugin using a simple
+makefile:</p>
+
+<blockquote><pre
+># lib/MyGC/Makefile
+
+LEVEL := ../..
+LIBRARYNAME = <var>MyGC</var>
+LOADABLE_MODULE = 1
+
+include $(LEVEL)/Makefile.common</pre></blockquote>
+
+<p>Once the plugin is compiled, code using it may be compiled using <tt>llc
+-load=<var>MyGC.so</var></tt> (though <var>MyGC.so</var> may have some other
+platform-specific extension):</p>
+
+<blockquote><pre
+>$ cat sample.ll
+define void @f() gc "mygc" {
+entry:
+ ret void
+}
+$ llvm-as < sample.ll | llc -load=MyGC.so</pre></blockquote>
+
+<p>It is also possible to statically link the collector plugin into tools, such
+as a language-specific compiler front-end.</p>
<!-- ======================================================================= -->
-<div class="doc_subsection">
- <a name="llvm_gc_readwrite">Implementing <tt>llvm_gc_read</tt> and <tt>llvm_gc_write</tt></a>
-</div>
-
-<div class="doc_text">
- <div class="doc_code"><tt>
- void *llvm_gc_read(void*, void **)<br>
- void llvm_gc_write(void*, void *, void**)
- </tt></div>
-
-<p>
-These functions <i>must</i> be implemented in every garbage collector, even if
-they do not need read/write barriers. In this case, just load or store the
-pointer, then return.
-</p>
-
-<p>
-If an actual read or write barrier is needed, it should be straight-forward to
-implement it.
-</p>
+<h3>
+ <a name="collector-algos">Overview of available features</a>
+</h3>
+
+<div>
+
+<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>
+ <th>Algorithm</th>
+ <th>Done</th>
+ <th>shadow stack</th>
+ <th>refcount</th>
+ <th>mark-sweep</th>
+ <th>copying</th>
+ <th>incremental</th>
+ <th>threaded</th>
+ <th>concurrent</th>
+ </tr>
+ <tr>
+ <th class="rowhead"><a href="#stack-map">stack map</a></th>
+ <td>✔</td>
+ <td></td>
+ <td></td>
+ <td>✘</td>
+ <td>✘</td>
+ <td>✘</td>
+ <td>✘</td>
+ <td>✘</td>
+ </tr>
+ <tr>
+ <th class="rowhead"><a href="#init-roots">initialize roots</a></th>
+ <td>✔</td>
+ <td>✘</td>
+ <td>✘</td>
+ <td>✘</td>
+ <td>✘</td>
+ <td>✘</td>
+ <td>✘</td>
+ <td>✘</td>
+ </tr>
+ <tr class="doc_warning">
+ <th class="rowhead">derived pointers</th>
+ <td>NO</td>
+ <td></td>
+ <td></td>
+ <td></td>
+ <td></td>
+ <td></td>
+ <td>✘*</td>
+ <td>✘*</td>
+ </tr>
+ <tr>
+ <th class="rowhead"><em><a href="#custom">custom lowering</a></em></th>
+ <td>✔</td>
+ <th></th>
+ <th></th>
+ <th></th>
+ <th></th>
+ <th></th>
+ <th></th>
+ <th></th>
+ </tr>
+ <tr>
+ <th class="rowhead indent">gcroot</th>
+ <td>✔</td>
+ <td>✘</td>
+ <td>✘</td>
+ <td></td>
+ <td></td>
+ <td></td>
+ <td></td>
+ <td></td>
+ </tr>
+ <tr>
+ <th class="rowhead indent">gcwrite</th>
+ <td>✔</td>
+ <td></td>
+ <td>✘</td>
+ <td></td>
+ <td></td>
+ <td>✘</td>
+ <td></td>
+ <td>✘</td>
+ </tr>
+ <tr>
+ <th class="rowhead indent">gcread</th>
+ <td>✔</td>
+ <td></td>
+ <td></td>
+ <td></td>
+ <td></td>
+ <td></td>
+ <td></td>
+ <td>✘</td>
+ </tr>
+ <tr>
+ <th class="rowhead"><em><a href="#safe-points">safe points</a></em></th>
+ <td></td>
+ <th></th>
+ <th></th>
+ <th></th>
+ <th></th>
+ <th></th>
+ <th></th>
+ <th></th>
+ </tr>
+ <tr>
+ <th class="rowhead indent">in calls</th>
+ <td>✔</td>
+ <td></td>
+ <td></td>
+ <td>✘</td>
+ <td>✘</td>
+ <td>✘</td>
+ <td>✘</td>
+ <td>✘</td>
+ </tr>
+ <tr>
+ <th class="rowhead indent">before calls</th>
+ <td>✔</td>
+ <td></td>
+ <td></td>
+ <td></td>
+ <td></td>
+ <td></td>
+ <td>✘</td>
+ <td>✘</td>
+ </tr>
+ <tr class="doc_warning">
+ <th class="rowhead indent">for loops</th>
+ <td>NO</td>
+ <td></td>
+ <td></td>
+ <td></td>
+ <td></td>
+ <td></td>
+ <td>✘</td>
+ <td>✘</td>
+ </tr>
+ <tr>
+ <th class="rowhead indent">before escape</th>
+ <td>✔</td>
+ <td></td>
+ <td></td>
+ <td></td>
+ <td></td>
+ <td></td>
+ <td>✘</td>
+ <td>✘</td>
+ </tr>
+ <tr class="doc_warning">
+ <th class="rowhead">emit code at safe points</th>
+ <td>NO</td>
+ <td></td>
+ <td></td>
+ <td></td>
+ <td></td>
+ <td></td>
+ <td>✘</td>
+ <td>✘</td>
+ </tr>
+ <tr>
+ <th class="rowhead"><em>output</em></th>
+ <td></td>
+ <th></th>
+ <th></th>
+ <th></th>
+ <th></th>
+ <th></th>
+ <th></th>
+ <th></th>
+ </tr>
+ <tr>
+ <th class="rowhead indent"><a href="#assembly">assembly</a></th>
+ <td>✔</td>
+ <td></td>
+ <td></td>
+ <td>✘</td>
+ <td>✘</td>
+ <td>✘</td>
+ <td>✘</td>
+ <td>✘</td>
+ </tr>
+ <tr class="doc_warning">
+ <th class="rowhead indent">JIT</th>
+ <td>NO</td>
+ <td></td>
+ <td></td>
+ <td class="optl">✘</td>
+ <td class="optl">✘</td>
+ <td class="optl">✘</td>
+ <td class="optl">✘</td>
+ <td class="optl">✘</td>
+ </tr>
+ <tr class="doc_warning">
+ <th class="rowhead indent">obj</th>
+ <td>NO</td>
+ <td></td>
+ <td></td>
+ <td class="optl">✘</td>
+ <td class="optl">✘</td>
+ <td class="optl">✘</td>
+ <td class="optl">✘</td>
+ <td class="optl">✘</td>
+ </tr>
+ <tr class="doc_warning">
+ <th class="rowhead">live analysis</th>
+ <td>NO</td>
+ <td></td>
+ <td></td>
+ <td class="optl">✘</td>
+ <td class="optl">✘</td>
+ <td class="optl">✘</td>
+ <td class="optl">✘</td>
+ <td class="optl">✘</td>
+ </tr>
+ <tr class="doc_warning">
+ <th class="rowhead">register map</th>
+ <td>NO</td>
+ <td></td>
+ <td></td>
+ <td class="optl">✘</td>
+ <td class="optl">✘</td>
+ <td class="optl">✘</td>
+ <td class="optl">✘</td>
+ <td class="optl">✘</td>
+ </tr>
+ <tr>
+ <td colspan="10">
+ <div><span class="doc_warning">*</span> Derived pointers only pose a
+ hazard to copying collectors.</div>
+ <div><span class="optl">✘</span> in gray denotes a feature which
+ could be utilized if available.</div>
+ </td>
+ </tr>
+</table>
+
+<p>To be clear, the collection techniques above are defined as:</p>
+
+<dl>
+ <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="doc_subsection">
- <a name="callbacks">Callback functions used to implement the garbage collector</a>
-</div>
-
-<div class="doc_text">
-<p>
-Garbage collector implementations make use of call-back functions that are
-implemented by other parts of the LLVM system.
-</p>
-</div>
-
-<!--_________________________________________________________________________-->
-<div class="doc_subsubsection">
- <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="#roots"><tt>llvm.gcroot</tt></a> intrinsic) are provided.
-</p>
-</div>
+<h3>
+ <a name="stack-map">Computing stack maps</a>
+</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>
+
+<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) {
+ GCFunctionInfo *FI = *I;
+ unsigned FrameSize = FI->getFrameSize();
+ size_t RootCount = FI->roots_size();
+
+ for (GCFunctionInfo::roots_iterator RI = FI->roots_begin(),
+ RE = FI->roots_end();
+ RI != RE; ++RI) {
+ int RootNum = RI->Num;
+ int RootStackOffset = RI->StackOffset;
+ Constant *RootMetadata = RI->Metadata;
+ }
+}</pre></blockquote>
+
+<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 class="doc_subsubsection">
- <a name="staticroots">Tracing GC pointers from static roots</a>
</div>
-<div class="doc_text">
-TODO
-</div>
-
-
-<!--_________________________________________________________________________-->
-<div class="doc_subsubsection">
- <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>
+<!-- ======================================================================= -->
+<h3>
+ <a name="init-roots">Initializing roots to null: <tt>InitRoots</tt></a>
+</h3>
-<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="#roots">stack roots</a>, and the heap tracing routines can propagate the
-information. In addition, LLVM allows the front-end to extract GC information
-from in any form from a specific object pointer (this supports situations #1 and
-#3).
-</p>
+<div>
-<p><b>Making this efficient</b></p>
+<blockquote><pre
+>MyGC::MyGC() {
+ InitRoots = true;
+}</pre></blockquote>
+<p>When set, LLVM will automatically initialize each root to <tt>null</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>
+<!-- ======================================================================= -->
+<h3>
+ <a name="custom">Custom lowering of intrinsics: <tt>CustomRoots</tt>,
+ <tt>CustomReadBarriers</tt>, and <tt>CustomWriteBarriers</tt></a>
+</h3>
+
+<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
+IR:</p>
+
+<blockquote><pre
+>class MyGC : public GCStrategy {
+public:
+ MyGC() {
+ CustomRoots = true;
+ CustomReadBarriers = true;
+ CustomWriteBarriers = true;
+ }
+
+ virtual bool initializeCustomLowering(Module &M);
+ virtual bool performCustomLowering(Function &F);
+};</pre></blockquote>
+
+<p>If any of these flags are set, then LLVM suppresses its default lowering for
+the corresponding intrinsics and instead calls
+<tt>performCustomLowering</tt>.</p>
+
+<p>LLVM's default action for each intrinsic is as follows:</p>
+
+<ul>
+ <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>
+
+<p>If <tt>CustomReadBarriers</tt> or <tt>CustomWriteBarriers</tt> are specified,
+then <tt>performCustomLowering</tt> <strong>must</strong> eliminate the
+corresponding barriers.</p>
+
+<p><tt>performCustomLowering</tt> must comply with the same restrictions as <a
+href="WritingAnLLVMPass.html#runOnFunction"><tt
+>FunctionPass::runOnFunction</tt></a>.
+Likewise, <tt>initializeCustomLowering</tt> has the same semantics as <a
+href="WritingAnLLVMPass.html#doInitialization_mod"><tt
+>Pass::doInitialization(Module&)</tt></a>.</p>
+
+<p>The following can be used as a template:</p>
+
+<blockquote><pre
+>#include "llvm/Module.h"
+#include "llvm/IntrinsicInst.h"
+
+bool MyGC::initializeCustomLowering(Module &M) {
+ return false;
+}
+
+bool MyGC::performCustomLowering(Function &F) {
+ bool MadeChange = false;
+
+ for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
+ for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; )
+ if (IntrinsicInst *CI = dyn_cast<IntrinsicInst>(II++))
+ if (Function *F = CI->getCalledFunction())
+ switch (F->getIntrinsicID()) {
+ case Intrinsic::gcwrite:
+ // Handle llvm.gcwrite.
+ CI->eraseFromParent();
+ MadeChange = true;
+ break;
+ case Intrinsic::gcread:
+ // Handle llvm.gcread.
+ CI->eraseFromParent();
+ MadeChange = true;
+ break;
+ case Intrinsic::gcroot:
+ // Handle llvm.gcroot.
+ CI->eraseFromParent();
+ MadeChange = true;
+ break;
+ }
+
+ return MadeChange;
+}</pre></blockquote>
-<!-- *********************************************************************** -->
-<div class="doc_section">
- <a name="gcimpls">GC implementations available</a>
</div>
-<!-- *********************************************************************** -->
-
-<div class="doc_text">
-
-<p>
-To make this more concrete, the currently implemented LLVM garbage collectors
-all live in the <tt>llvm/runtime/GC/*</tt> directories in the LLVM source-base.
-If you are interested in implementing an algorithm, there are many interesting
-possibilities (mark/sweep, a generational collector, a reference counting
-collector, etc), or you could choose to improve one of the existing algorithms.
-</p>
-</div>
<!-- ======================================================================= -->
-<div class="doc_subsection">
- <a name="semispace">SemiSpace - A simple copying garbage collector</a>
-</div>
-
-<div class="doc_text">
-<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>
+<h3>
+ <a name="safe-points">Generating safe points: <tt>NeededSafePoints</tt></a>
+</h3>
+
+<div>
+
+<p>LLVM can compute four kinds of safe points:</p>
+
+<blockquote><pre
+>namespace GC {
+ /// PointKind - The type of a collector-safe point.
+ ///
+ enum PointKind {
+ Loop, //< Instr is a loop (backwards branch).
+ Return, //< Instr is a return instruction.
+ PreCall, //< Instr is a call instruction.
+ PostCall //< Instr is the return address of a call.
+ };
+}</pre></blockquote>
+
+<p>A collector can request any combination of the four by setting the
+<tt>NeededSafePoints</tt> mask:</p>
+
+<blockquote><pre
+>MyGC::MyGC() {
+ NeededSafePoints = 1 << GC::Loop
+ | 1 << GC::Return
+ | 1 << GC::PreCall
+ | 1 << GC::PostCall;
+}</pre></blockquote>
+
+<p>It can then use the following routines to access safe points.</p>
+
+<blockquote><pre
+>for (iterator I = begin(), E = end(); I != E; ++I) {
+ GCFunctionInfo *MD = *I;
+ size_t PointCount = MD->size();
+
+ for (GCFunctionInfo::iterator PI = MD->begin(),
+ PE = MD->end(); PI != PE; ++PI) {
+ GC::PointKind PointKind = PI->Kind;
+ unsigned PointNum = PI->Num;
+ }
+}
+</pre></blockquote>
+
+<p>Almost every collector requires <tt>PostCall</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>Loop</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>Return</tt> and <tt>PreCall</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="doc_subsubsection">
- Possible Improvements
-</div>
-<div class="doc_text">
+<!-- ======================================================================= -->
+<h3>
+ <a name="assembly">Emitting assembly code: <tt>GCMetadataPrinter</tt></a>
+</h3>
+
+<div>
+
+<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>GCMetadataPrinterRegistry</tt>. The AsmWriter
+will look for such a subclass if the <tt>GCStrategy</tt> sets
+<tt>UsesMetadata</tt>:</p>
+
+<blockquote><pre
+>MyGC::MyGC() {
+ 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>
+
+<blockquote><pre
+>// lib/MyGC/MyGCPrinter.cpp - Example LLVM GC printer
+
+#include "llvm/CodeGen/GCMetadataPrinter.h"
+#include "llvm/Support/Compiler.h"
+
+using namespace llvm;
+
+namespace {
+ class LLVM_LIBRARY_VISIBILITY MyGCPrinter : public GCMetadataPrinter {
+ public:
+ virtual void beginAssembly(std::ostream &OS, AsmPrinter &AP,
+ const TargetAsmInfo &TAI);
+
+ virtual void finishAssembly(std::ostream &OS, AsmPrinter &AP,
+ const TargetAsmInfo &TAI);
+ };
+
+ GCMetadataPrinterRegistry::Add<MyGCPrinter>
+ X("mygc", "My bespoke garbage collector.");
+}</pre></blockquote>
+
+<p>The collector should use <tt>AsmPrinter</tt> and <tt>TargetAsmInfo</tt> to
+print portable assembly code to the <tt>std::ostream</tt>. The collector itself
+contains the stack map for the entire module, and may access the
+<tt>GCFunctionInfo</tt> using its own <tt>begin()</tt> and <tt>end()</tt>
+methods. Here's a realistic example:</p>
+
+<blockquote><pre
+>#include "llvm/CodeGen/AsmPrinter.h"
+#include "llvm/Function.h"
+#include "llvm/Target/TargetMachine.h"
+#include "llvm/DataLayout.h"
+#include "llvm/Target/TargetAsmInfo.h"
+
+void MyGCPrinter::beginAssembly(std::ostream &OS, AsmPrinter &AP,
+ const TargetAsmInfo &TAI) {
+ // Nothing to do.
+}
+
+void MyGCPrinter::finishAssembly(std::ostream &OS, AsmPrinter &AP,
+ const TargetAsmInfo &TAI) {
+ // Set up for emitting addresses.
+ const char *AddressDirective;
+ int AddressAlignLog;
+ if (AP.TM.getDataLayout()->getPointerSize() == sizeof(int32_t)) {
+ AddressDirective = TAI.getData32bitsDirective();
+ AddressAlignLog = 2;
+ } else {
+ AddressDirective = TAI.getData64bitsDirective();
+ AddressAlignLog = 3;
+ }
+
+ // Put this in the data section.
+ AP.SwitchToDataSection(TAI.getDataSection());
+
+ // For each function...
+ for (iterator FI = begin(), FE = end(); FI != FE; ++FI) {
+ GCFunctionInfo &MD = **FI;
+
+ // Emit this data structure:
+ //
+ // struct {
+ // int32_t PointCount;
+ // struct {
+ // void *SafePointAddress;
+ // int32_t LiveCount;
+ // int32_t LiveOffsets[LiveCount];
+ // } Points[PointCount];
+ // } __gcmap_<FUNCTIONNAME>;
+
+ // Align to address width.
+ AP.EmitAlignment(AddressAlignLog);
+
+ // Emit the symbol by which the stack map entry can be found.
+ std::string Symbol;
+ Symbol += TAI.getGlobalPrefix();
+ Symbol += "__gcmap_";
+ Symbol += MD.getFunction().getName();
+ if (const char *GlobalDirective = TAI.getGlobalDirective())
+ OS << GlobalDirective << Symbol << "\n";
+ OS << TAI.getGlobalPrefix() << Symbol << ":\n";
+
+ // Emit PointCount.
+ AP.EmitInt32(MD.size());
+ AP.EOL("safe point count");
+
+ // And each safe point...
+ for (GCFunctionInfo::iterator PI = MD.begin(),
+ PE = MD.end(); PI != PE; ++PI) {
+ // Align to address width.
+ AP.EmitAlignment(AddressAlignLog);
+
+ // Emit the address of the safe point.
+ OS << AddressDirective
+ << TAI.getPrivateGlobalPrefix() << "label" << PI->Num;
+ AP.EOL("safe point address");
+
+ // Emit the stack frame size.
+ AP.EmitInt32(MD.getFrameSize());
+ AP.EOL("stack frame size");
+
+ // Emit the number of live roots in the function.
+ AP.EmitInt32(MD.live_size(PI));
+ AP.EOL("live root count");
+
+ // And for each live root...
+ for (GCFunctionInfo::live_iterator LI = MD.live_begin(PI),
+ LE = MD.live_end(PI);
+ LI != LE; ++LI) {
+ // Print its offset within the stack frame.
+ AP.EmitInt32(LI->StackOffset);
+ AP.EOL("stack offset");
+ }
+ }
+ }
+}
+</pre></blockquote>
-<p>
-If a collection cycle happens and the heap is not compacted very much (say less
-than 25% of the allocated memory was freed), the memory regions should be
-doubled in size.</p>
+</div>
</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>
<p><a name="goldberg91">[Goldberg91]</a> Tag-free garbage collection for
-strongly typed programming languages. Benjamin Goldberg. ACM SIGPLAN
+strongly typed programming languages. Benjamin Goldberg. ACM SIGPLAN
PLDI'91.</p>
<p><a name="tolmach94">[Tolmach94]</a> Tag-free garbage collection using
-explicit type parameters. Andrew Tolmach. Proceedings of the 1994 ACM
+explicit type parameters. Andrew Tolmach. Proceedings of the 1994 ACM
conference on LISP and functional programming.</p>
+<p><a name="henderson02">[Henderson2002]</a> <a
+href="http://citeseer.ist.psu.edu/henderson02accurate.html">
+Accurate Garbage Collection in an Uncooperative Environment</a>.
+Fergus Henderson. International Symposium on Memory Management 2002.</p>
+
</div>
+
<!-- *********************************************************************** -->
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