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534 lines
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
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"http://www.w3.org/TR/html4/strict.dtd">
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<html>
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<head>
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<title>Accurate Garbage Collection with LLVM</title>
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<link rel="stylesheet" href="llvm.css" type="text/css">
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</head>
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<body>
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<div class="doc_title">
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Accurate Garbage Collection with LLVM
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</div>
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<ol>
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<li><a href="#introduction">Introduction</a>
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<ul>
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<li><a href="#feature">GC features provided and algorithms supported</a></li>
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</ul>
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</li>
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<li><a href="#interfaces">Interfaces for user programs</a>
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<ul>
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<li><a href="#roots">Identifying GC roots on the stack: <tt>llvm.gcroot</tt></a></li>
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<li><a href="#allocate">Allocating memory from the GC</a></li>
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<li><a href="#barriers">Reading and writing references to the heap</a></li>
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<li><a href="#explicit">Explicit invocation of the garbage collector</a></li>
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</ul>
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</li>
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<li><a href="#gcimpl">Implementing a garbage collector</a>
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<ul>
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<li><a href="#llvm_gc_readwrite">Implementing <tt>llvm_gc_read</tt> and <tt>llvm_gc_write</tt></a></li>
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<li><a href="#callbacks">Callback functions used to implement the garbage collector</a></li>
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</ul>
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</li>
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<li><a href="#gcimpls">GC implementations available</a>
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<ul>
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<li><a href="#semispace">SemiSpace - A simple copying garbage collector</a></li>
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</ul>
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</li>
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<!--
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<li><a href="#codegen">Implementing GC support in a code generator</a></li>
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-->
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</ol>
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<div class="doc_author">
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<p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section">
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<a name="introduction">Introduction</a>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_text">
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<p>Garbage collection is a widely used technique that frees the programmer from
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having to know the life-times of heap objects, making software easier to produce
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and maintain. Many programming languages rely on garbage collection for
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automatic memory management. There are two primary forms of garbage collection:
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conservative and accurate.</p>
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<p>Conservative garbage collection often does not require any special support
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from either the language or the compiler: it can handle non-type-safe
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programming languages (such as C/C++) and does not require any special
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information from the compiler. The [LINK] Boehm collector is an example of a
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state-of-the-art conservative collector.</p>
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<p>Accurate garbage collection requires the ability to identify all pointers in
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the program at run-time (which requires that the source-language be type-safe in
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most cases). Identifying pointers at run-time requires compiler support to
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locate all places that hold live pointer variables at run-time, including the
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<a href="#roots">processor stack and registers</a>.</p>
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<p>
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Conservative garbage collection is attractive because it does not require any
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special compiler support, but it does have problems. In particular, because the
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conservative garbage collector cannot <i>know</i> that a particular word in the
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machine is a pointer, it cannot move live objects in the heap (preventing the
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use of compacting and generational GC algorithms) and it can occasionally suffer
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from memory leaks due to integer values that happen to point to objects in the
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program. In addition, some aggressive compiler transformations can break
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conservative garbage collectors (though these seem rare in practice).
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</p>
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<p>
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Accurate garbage collectors do not suffer from any of these problems, but they
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can suffer from degraded scalar optimization of the program. In particular,
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because the runtime must be able to identify and update all pointers active in
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the program, some optimizations are less effective. In practice, however, the
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locality and performance benefits of using aggressive garbage allocation
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techniques dominates any low-level losses.
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</p>
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<p>
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This document describes the mechanisms and interfaces provided by LLVM to
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support accurate garbage collection.
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</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="feature">GC features provided and algorithms supported</a>
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</div>
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<div class="doc_text">
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<p>
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LLVM provides support for a broad class of garbage collection algorithms,
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including compacting semi-space collectors, mark-sweep collectors, generational
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collectors, and even reference counting implementations. It includes support
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for <a href="#barriers">read and write barriers</a>, and associating <a
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href="#roots">meta-data with stack objects</a> (used for tagless garbage
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collection). All LLVM code generators support garbage collection, including the
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C backend.
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</p>
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<p>
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We hope that the primitive support built into LLVM is sufficient to support a
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broad class of garbage collected languages, including Scheme, ML, scripting
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languages, Java, C#, etc. That said, the implemented garbage collectors may
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need to be extended to support language-specific features such as finalization,
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weak references, or other features. As these needs are identified and
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implemented, they should be added to this specification.
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</p>
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<p>
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LLVM does not currently support garbage collection of multi-threaded programs or
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GC-safe points other than function calls, but these will be added in the future
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as there is interest.
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</p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section">
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<a name="interfaces">Interfaces for user programs</a>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_text">
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<p>This section describes the interfaces provided by LLVM and by the garbage
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collector run-time that should be used by user programs. As such, this is the
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interface that front-end authors should generate code for.
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</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="roots">Identifying GC roots on the stack: <tt>llvm.gcroot</tt></a>
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</div>
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<div class="doc_text">
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<div class="doc_code"><tt>
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void %llvm.gcroot(<ty>** %ptrloc, <ty2>* %metadata)
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</tt></div>
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<p>
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The <tt>llvm.gcroot</tt> intrinsic is used to inform LLVM of a pointer variable
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on the stack. The first argument contains the address of the variable on the
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stack, and the second contains a pointer to metadata that should be associated
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with the pointer (which <b>must</b> be a constant or global value address). At
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runtime, the <tt>llvm.gcroot</tt> intrinsic stores a null pointer into the
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specified location to initialize the pointer.</p>
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<p>
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Consider the following fragment of Java code:
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</p>
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<pre>
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{
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Object X; // A null-initialized reference to an object
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...
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}
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</pre>
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<p>
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This block (which may be located in the middle of a function or in a loop nest),
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could be compiled to this LLVM code:
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</p>
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<pre>
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Entry:
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;; In the entry block for the function, allocate the
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;; stack space for X, which is an LLVM pointer.
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%X = alloca %Object*
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...
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;; "CodeBlock" is the block corresponding to the start
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;; of the scope above.
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CodeBlock:
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;; Initialize the object, telling LLVM that it is now live.
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;; Java has type-tags on objects, so it doesn't need any
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;; metadata.
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call void %llvm.gcroot(%Object** %X, sbyte* null)
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...
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;; As the pointer goes out of scope, store a null value into
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;; it, to indicate that the value is no longer live.
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store %Object* null, %Object** %X
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...
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</pre>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="allocate">Allocating memory from the GC</a>
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</div>
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<div class="doc_text">
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<div class="doc_code"><tt>
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sbyte *%llvm_gc_allocate(unsigned %Size)
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</tt></div>
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<p>The <tt>llvm_gc_allocate</tt> function is a global function defined by the
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garbage collector implementation to allocate memory. It returns a
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zeroed-out block of memory of the appropriate size.</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="barriers">Reading and writing references to the heap</a>
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</div>
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<div class="doc_text">
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<div class="doc_code"><tt>
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sbyte *%llvm.gcread(sbyte *, sbyte **)<br>
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void %llvm.gcwrite(sbyte*, sbyte*, sbyte**)
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</tt></div>
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<p>Several of the more interesting garbage collectors (e.g., generational
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collectors) need to be informed when the mutator (the program that needs garbage
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collection) reads or writes object references into the heap. In the case of a
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generational collector, it needs to keep track of which "old" generation objects
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have references stored into them. The amount of code that typically needs to be
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executed is usually quite small (and not on the critical path of any
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computation), so the overall performance impact of the inserted code is
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tolerable.</p>
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<p>To support garbage collectors that use read or write barriers, LLVM provides
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the <tt>llvm.gcread</tt> and <tt>llvm.gcwrite</tt> intrinsics. The first
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intrinsic has exactly the same semantics as a non-volatile LLVM load and the
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second has the same semantics as a non-volatile LLVM store, with the
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additions that they also take a pointer to the start of the memory
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object as an argument. At code generation
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time, these intrinsics are replaced with calls into the garbage collector
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(<tt><a href="#llvm_gc_readwrite">llvm_gc_read</a></tt> and <tt><a
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href="#llvm_gc_readwrite">llvm_gc_write</a></tt> respectively), which are then
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inlined into the code.
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</p>
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<p>
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If you are writing a front-end for a garbage collected language, every load or
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store of a reference from or to the heap should use these intrinsics instead of
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normal LLVM loads/stores.</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="initialize">Garbage collector startup and initialization</a>
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</div>
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<div class="doc_text">
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<div class="doc_code"><tt>
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void %llvm_gc_initialize(unsigned %InitialHeapSize)
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</tt></div>
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<p>
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The <tt>llvm_gc_initialize</tt> function should be called once before any other
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garbage collection functions are called. This gives the garbage collector the
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chance to initialize itself and allocate the heap spaces. The initial heap size
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to allocate should be specified as an argument.
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</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="explicit">Explicit invocation of the garbage collector</a>
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</div>
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<div class="doc_text">
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<div class="doc_code"><tt>
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void %llvm_gc_collect()
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</tt></div>
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<p>
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The <tt>llvm_gc_collect</tt> function is exported by the garbage collector
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implementations to provide a full collection, even when the heap is not
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exhausted. This can be used by end-user code as a hint, and may be ignored by
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the garbage collector.
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</p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section">
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<a name="gcimpl">Implementing a garbage collector</a>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_text">
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<p>
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Implementing a garbage collector for LLVM is fairly straight-forward. The LLVM
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garbage collectors are provided in a form that makes them easy to link into the
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language-specific runtime that a language front-end would use. They require
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functionality from the language-specific runtime to get information about <a
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href="#gcdescriptors">where pointers are located in heap objects</a>.
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</p>
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<p>The
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implementation must include the <a
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href="#allocate"><tt>llvm_gc_allocate</tt></a> and <a
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href="#explicit"><tt>llvm_gc_collect</tt></a> functions, and it must implement
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the <a href="#llvm_gc_readwrite">read/write barrier</a> functions as well. To
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do this, it will probably have to <a href="#traceroots">trace through the roots
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from the stack</a> and understand the <a href="#gcdescriptors">GC descriptors
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for heap objects</a>. Luckily, there are some <a href="#gcimpls">example
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implementations</a> available.
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</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="llvm_gc_readwrite">Implementing <tt>llvm_gc_read</tt> and <tt>llvm_gc_write</tt></a>
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</div>
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<div class="doc_text">
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<div class="doc_code"><tt>
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void *llvm_gc_read(void*, void **)<br>
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void llvm_gc_write(void*, void *, void**)
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</tt></div>
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<p>
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These functions <i>must</i> be implemented in every garbage collector, even if
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they do not need read/write barriers. In this case, just load or store the
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pointer, then return.
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</p>
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<p>
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If an actual read or write barrier is needed, it should be straight-forward to
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implement it.
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</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="callbacks">Callback functions used to implement the garbage collector</a>
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</div>
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<div class="doc_text">
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<p>
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Garbage collector implementations make use of call-back functions that are
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implemented by other parts of the LLVM system.
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</p>
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</div>
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<!--_________________________________________________________________________-->
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<div class="doc_subsubsection">
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<a name="traceroots">Tracing GC pointers from the program stack</a>
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</div>
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<div class="doc_text">
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<div class="doc_code"><tt>
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void llvm_cg_walk_gcroots(void (*FP)(void **Root, void *Meta));
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</tt></div>
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<p>
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The <tt>llvm_cg_walk_gcroots</tt> function is a function provided by the code
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generator that iterates through all of the GC roots on the stack, calling the
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specified function pointer with each record. For each GC root, the address of
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the pointer and the meta-data (from the <a
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href="#roots"><tt>llvm.gcroot</tt></a> intrinsic) are provided.
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</p>
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</div>
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<!--_________________________________________________________________________-->
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<div class="doc_subsubsection">
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<a name="staticroots">Tracing GC pointers from static roots</a>
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</div>
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<div class="doc_text">
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TODO
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</div>
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<!--_________________________________________________________________________-->
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<div class="doc_subsubsection">
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<a name="gcdescriptors">Tracing GC pointers from heap objects</a>
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</div>
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<div class="doc_text">
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<p>
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The three most common ways to keep track of where pointers live in heap objects
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are (listed in order of space overhead required):</p>
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<ol>
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<li>In languages with polymorphic objects, pointers from an object header are
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usually used to identify the GC pointers in the heap object. This is common for
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object-oriented languages like Self, Smalltalk, Java, or C#.</li>
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<li>If heap objects are not polymorphic, often the "shape" of the heap can be
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determined from the roots of the heap or from some other meta-data [<a
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href="#appel89">Appel89</a>, <a href="#goldberg91">Goldberg91</a>, <a
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href="#tolmach94">Tolmach94</a>]. In this case, the garbage collector can
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propagate the information around from meta data stored with the roots. This
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often eliminates the need to have a header on objects in the heap. This is
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common in the ML family.</li>
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<li>If all heap objects have pointers in the same locations, or pointers can be
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distinguished just by looking at them (e.g., the low order bit is clear), no
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book-keeping is needed at all. This is common for Lisp-like languages.</li>
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</ol>
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<p>The LLVM garbage collectors are capable of supporting all of these styles of
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language, including ones that mix various implementations. To do this, it
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allows the source-language to associate meta-data with the <a
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href="#roots">stack roots</a>, and the heap tracing routines can propagate the
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information. In addition, LLVM allows the front-end to extract GC information
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from in any form from a specific object pointer (this supports situations #1 and
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#3).
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</p>
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<p><b>Making this efficient</b></p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section">
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<a name="gcimpls">GC implementations available</a>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_text">
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<p>
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To make this more concrete, the currently implemented LLVM garbage collectors
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all live in the <tt>llvm/runtime/GC/*</tt> directories in the LLVM source-base.
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If you are interested in implementing an algorithm, there are many interesting
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possibilities (mark/sweep, a generational collector, a reference counting
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collector, etc), or you could choose to improve one of the existing algorithms.
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</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
|
|
<a name="semispace">SemiSpace - A simple copying garbage collector</a>
|
|
</div>
|
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|
|
<div class="doc_text">
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<p>
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SemiSpace is a very simple copying collector. When it starts up, it allocates
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two blocks of memory for the heap. It uses a simple bump-pointer allocator to
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allocate memory from the first block until it runs out of space. When it runs
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out of space, it traces through all of the roots of the program, copying blocks
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to the other half of the memory space.
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</p>
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</div>
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<!--_________________________________________________________________________-->
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<div class="doc_subsubsection">
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Possible Improvements
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</div>
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<div class="doc_text">
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<p>
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If a collection cycle happens and the heap is not compacted very much (say less
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than 25% of the allocated memory was freed), the memory regions should be
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doubled in size.</p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section">
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<a name="references">References</a>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_text">
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<p><a name="appel89">[Appel89]</a> Runtime Tags Aren't Necessary. Andrew
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W. Appel. Lisp and Symbolic Computation 19(7):703-705, July 1989.</p>
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<p><a name="goldberg91">[Goldberg91]</a> Tag-free garbage collection for
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strongly typed programming languages. Benjamin Goldberg. ACM SIGPLAN
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PLDI'91.</p>
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<p><a name="tolmach94">[Tolmach94]</a> Tag-free garbage collection using
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explicit type parameters. Andrew Tolmach. Proceedings of the 1994 ACM
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conference on LISP and functional programming.</p>
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