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   <title>Accurate Garbage Collection with LLVM</title>
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 </head>
 <body>

 <div class="doc_title">
   Accurate Garbage Collection with LLVM
 </div>

 <ol>
   <li><a href="#introduction">Introduction</a>
     <ul>
     <li><a href="#feature">GC features provided and algorithms supported</a></li>
     </ul>
   </li>

   <li><a href="#interfaces">Interfaces for user programs</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>
     </ul>
   </li>

   <li><a href="#gcimpl">Implementing a garbage collector</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>
     </ul>
   </li>
   <li><a href="#gcimpls">GC implementations available</a>
     <ul>
     <li><a href="#semispace">SemiSpace - A simple copying garbage collector</a></li>
     </ul>
   </li>

 <!--
   <li><a href="#codegen">Implementing GC support in a code generator</a></li>
 -->
 </ol>

 <div class="doc_author">
   <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
 </div>

 <!-- *********************************************************************** -->
 <div class="doc_section">
   <a name="introduction">Introduction</a>
 </div>
 <!-- *********************************************************************** -->

 <div class="doc_text">

 <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:
 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 [LINK] Boehm collector is an example of a
 state-of-the-art conservative collector.</p>

 <p>Accurate garbage collection requires the ability to identify all pointers in
 the program at run-time (which requires that the source-language be type-safe in
 most cases).  Identifying pointers at run-time requires compiler support to
 locate all places that hold live pointer variables at run-time, including the
 <a href="#roots">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
 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>

 <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>

 <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>

 <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>

 </div>

 <!-- *********************************************************************** -->
 <div class="doc_section">
   <a name="interfaces">Interfaces for user programs</a>
 </div>
 <!-- *********************************************************************** -->

 <div class="doc_text">

 <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>

 </div>

 <!-- ======================================================================= -->
 <div class="doc_subsection">
   <a name="roots">Identifying GC roots on the stack: <tt>llvm.gcroot</tt></a>
 </div>

 <div class="doc_text">

 <div class="doc_code"><tt>
   void %llvm.gcroot(&lt;ty&gt;** %ptrloc, &lt;ty2&gt;* %metadata)
 </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>
 Consider the following fragment of Java code:
 </p>

 <pre>
        {
          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>

 <pre>
 Entry:
    ;; In the entry block for the function, allocate the
    ;; stack space for X, which is an LLVM pointer.
    %X = alloca %Object*
    ...

    ;; "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)
    ...

    ;; As the pointer goes out of scope, store a null value into
    ;; it, to indicate that the value is no longer live.
    store %Object* null, %Object** %X
    ...
 </pre>

 </div>

 <!-- ======================================================================= -->
 <div class="doc_subsection">
   <a name="allocate">Allocating memory from the GC</a>
 </div>

 <div class="doc_text">

 <div class="doc_code"><tt>
   sbyte *%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 appropriate size.</p>

 </div>

 <!-- ======================================================================= -->
 <div class="doc_subsection">
   <a name="barriers">Reading and writing references to the heap</a>
 </div>

 <div class="doc_text">

 <div class="doc_code"><tt>
   sbyte *%llvm.gcread(sbyte *, sbyte **)<br>
   void %llvm.gcwrite(sbyte*, sbyte*, sbyte**)
 </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>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>

 </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 spaces.  The initial heap size
 to allocate should be specified as an argument.
 </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_section">
   <a name="gcimpl">Implementing a garbage collector</a>
 </div>
 <!-- *********************************************************************** -->

 <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 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>

 </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="#gcroot"><tt>llvm.gcroot</tt></a> intrinsic) are provided.
 </p>
 </div>

 <!--_________________________________________________________________________-->
 <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>

 <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>

 <p><b>Making this efficient</b></p>


 </div>


 <!-- *********************************************************************** -->
 <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>

 </div>

 <!--_________________________________________________________________________-->
 <div class="doc_subsubsection">
   Possible Improvements
 </div>

 <div class="doc_text">

 <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 class="doc_section">
   <a name="references">References</a>
 </div>
 <!-- *********************************************************************** -->

 <div class="doc_text">

 <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
 PLDI'91.</p>

 <p><a name="tolmach94">[Tolmach94]</a> Tag-free garbage collection using
 explicit type parameters.  Andrew Tolmach.  Proceedings of the 1994 ACM
 conference on LISP and functional programming.</p>

 </div>

 <!-- *********************************************************************** -->

 <hr>
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   <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
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   <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
   <a href="http://llvm.cs.uiuc.edu">LLVM Compiler Infrastructure</a><br>
   Last modified: $Date$
 </address>

 </body>
 </html>
	<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
	"http://www.w3.org/TR/html4/strict.dtd">
	<html>
	<head>
	<title>Accurate Garbage Collection with LLVM</title>
	<link rel="stylesheet" href="llvm.css" type="text/css">
	</head>
	<body>

	<div class="doc_title">
	Accurate Garbage Collection with LLVM
	</div>

	<ol>
	<li><a href="#introduction">Introduction</a>
	<ul>
	<li><a href="#feature">GC features provided and algorithms supported</a></li>
	</ul>
	</li>

	<li><a href="#interfaces">Interfaces for user programs</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>
	</ul>
	</li>

	<li><a href="#gcimpl">Implementing a garbage collector</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>
	</ul>
	</li>
	<li><a href="#gcimpls">GC implementations available</a>
	<ul>
	<li><a href="#semispace">SemiSpace - A simple copying garbage collector</a></li>
	</ul>
	</li>

	<!--
	<li><a href="#codegen">Implementing GC support in a code generator</a></li>
	-->
	</ol>

	<div class="doc_author">
	<p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
	</div>

	<!-- *********************************************************************** -->
	<div class="doc_section">
	<a name="introduction">Introduction</a>
	</div>
	<!-- *********************************************************************** -->

	<div class="doc_text">

	<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:
	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 [LINK] Boehm collector is an example of a
	state-of-the-art conservative collector.</p>

	<p>Accurate garbage collection requires the ability to identify all pointers in
	the program at run-time (which requires that the source-language be type-safe in
	most cases). Identifying pointers at run-time requires compiler support to
	locate all places that hold live pointer variables at run-time, including the
	<a href="#roots">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
	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>

	<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>

	<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>

	<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>

	</div>

	<!-- *********************************************************************** -->
	<div class="doc_section">
	<a name="interfaces">Interfaces for user programs</a>
	</div>
	<!-- *********************************************************************** -->

	<div class="doc_text">

	<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>

	</div>

	<!-- ======================================================================= -->
	<div class="doc_subsection">
	<a name="roots">Identifying GC roots on the stack: <tt>llvm.gcroot</tt></a>
	</div>

	<div class="doc_text">

	<div class="doc_code"><tt>
	void %llvm.gcroot(<ty>** %ptrloc, <ty2>* %metadata)
	</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>
	Consider the following fragment of Java code:
	</p>

	<pre>
	{
	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>

	<pre>
	Entry:
	;; In the entry block for the function, allocate the
	;; stack space for X, which is an LLVM pointer.
	%X = alloca %Object*
	...

	;; "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)
	...

	;; As the pointer goes out of scope, store a null value into
	;; it, to indicate that the value is no longer live.
	store %Object* null, %Object** %X
	...
	</pre>

	</div>

	<!-- ======================================================================= -->
	<div class="doc_subsection">
	<a name="allocate">Allocating memory from the GC</a>
	</div>

	<div class="doc_text">

	<div class="doc_code"><tt>
	sbyte *%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 appropriate size.</p>

	</div>

	<!-- ======================================================================= -->
	<div class="doc_subsection">
	<a name="barriers">Reading and writing references to the heap</a>
	</div>

	<div class="doc_text">

	<div class="doc_code"><tt>
	sbyte %llvm.gcread(sbyte , sbyte **)<br>
	void %llvm.gcwrite(sbyte, sbyte, sbyte**)
	</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>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>

	</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 spaces. The initial heap size
	to allocate should be specified as an argument.
	</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_section">
	<a name="gcimpl">Implementing a garbage collector</a>
	</div>
	<!-- *********************************************************************** -->

	<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 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>

	</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="#gcroot"><tt>llvm.gcroot</tt></a> intrinsic) are provided.
	</p>
	</div>

	<!--_________________________________________________________________________-->
	<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>

	<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>

	<p><b>Making this efficient</b></p>



	</div>



	<!-- *********************************************************************** -->
	<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>

	</div>

	<!--_________________________________________________________________________-->
	<div class="doc_subsubsection">
	Possible Improvements
	</div>

	<div class="doc_text">

	<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 class="doc_section">
	<a name="references">References</a>
	</div>
	<!-- *********************************************************************** -->

	<div class="doc_text">

	<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
	PLDI'91.</p>

	<p><a name="tolmach94">[Tolmach94]</a> Tag-free garbage collection using
	explicit type parameters. Andrew Tolmach. Proceedings of the 1994 ACM
	conference on LISP and functional programming.</p>

	</div>

	<!-- *********************************************************************** -->

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