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47 KiB
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
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<html>
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<head>
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<meta http-equiv="Content-Type" Content="text/html; charset=UTF-8" >
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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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<style type="text/css">
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.rowhead { text-align: left; background: inherit; }
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.indent { padding-left: 1em; }
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.optl { color: #BFBFBF; }
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</style>
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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
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supported</a></li>
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</ul>
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</li>
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<li><a href="#usage">Using the collectors</a>
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<ul>
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<li><a href="#shadow-stack">ShadowStack -
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A highly portable collector</a></li>
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<li><a href="#semispace">SemiSpace -
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A simple copying collector runtime</a></li>
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<li><a href="#ocaml">Ocaml -
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An Objective Caml-compatible collector</a></li>
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</ul>
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</li>
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<li><a href="#core">Core support</a>
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<ul>
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<li><a href="#gcattr">Specifying GC code generation:
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<tt>gc "..."</tt></a></li>
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<li><a href="#gcroot">Identifying GC roots on the stack:
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<tt>llvm.gcroot</tt></a></li>
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<li><a href="#barriers">Reading and writing references in the heap</a>
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<ul>
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<li><a href="#gcwrite">Write barrier: <tt>llvm.gcwrite</tt></a></li>
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<li><a href="#gcread">Read barrier: <tt>llvm.gcread</tt></a></li>
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</ul>
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</li>
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</ul>
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</li>
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<li><a href="#runtime">Recommended runtime interface</a>
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<ul>
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<li><a href="#initialize">Garbage collector startup and
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initialization</a></li>
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<li><a href="#allocate">Allocating memory from the GC</a></li>
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<li><a href="#explicit">Explicit invocation of the garbage
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collector</a></li>
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<li><a href="#traceroots">Tracing GC pointers from the program
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stack</a></li>
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<li><a href="#staticroots">Tracing GC pointers from static roots</a></li>
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</ul>
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</li>
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<li><a href="#plugin">Implementing a collector plugin</a>
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<ul>
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<li><a href="#collector-algos">Overview of available features</a></li>
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<li><a href="#stack-map">Computing stack maps</a></li>
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<li><a href="#init-roots">Initializing roots to null:
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<tt>InitRoots</tt></a></li>
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<li><a href="#custom">Custom lowering of intrinsics: <tt>CustomRoots</tt>,
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<tt>CustomReadBarriers</tt>, and <tt>CustomWriteBarriers</tt></a></li>
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<li><a href="#safe-points">Generating safe points:
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<tt>NeededSafePoints</tt></a></li>
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<li><a href="#assembly">Emitting assembly code:
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<tt>GCMetadataPrinter</tt></a></li>
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</ul>
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</li>
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<li><a href="#runtime-impl">Implementing a collector runtime</a>
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<ul>
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<li><a href="#gcdescriptors">Tracing GC pointers from heap
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objects</a></li>
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</ul>
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</li>
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<li><a href="#references">References</a></li>
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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> and
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Gordon Henriksen</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 lifetimes 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
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<a href="http://www.hpl.hp.com/personal/Hans_Boehm/gc/">Boehm collector</a> is
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an example of a 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="#gcroot">processor stack and registers</a>.</p>
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<p>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).</p>
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<p>Accurate garbage collectors do not suffer from any of these problems, but
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they 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.</p>
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<p>This document describes the mechanisms and interfaces provided by LLVM to
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support accurate garbage collection.</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>LLVM's intermediate representation provides <a href="#intrinsics">garbage
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collection intrinsics</a> that offer support for a broad class of
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collector models. For instance, the intrinsics permit:</p>
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<ul>
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<li>semi-space collectors</li>
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<li>mark-sweep collectors</li>
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<li>generational collectors</li>
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<li>reference counting</li>
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<li>incremental collectors</li>
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<li>concurrent collectors</li>
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<li>cooperative collectors</li>
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</ul>
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<p>We hope that the primitive support built into the LLVM IR is sufficient to
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support a broad class of garbage collected languages including Scheme, ML, Java,
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C#, Perl, Python, Lua, Ruby, other scripting languages, and more.</p>
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<p>However, LLVM does not itself implement a garbage collector. This is because
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collectors are tightly coupled to object models, and LLVM is agnostic to object
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models. Since LLVM is agnostic to object models, it would be inappropriate for
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LLVM to dictate any particular collector. Instead, LLVM provides a framework for
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garbage collector implementations in two manners:</p>
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<ul>
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<li><b>At compile time</b> with <a href="#plugin">collector plugins</a> for
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the compiler. Collector plugins have ready access to important garbage
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collector algorithms. Leveraging these tools, it is straightforward to
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emit type-accurate stack maps for your runtime in as little as ~100 lines of
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C++ code.</li>
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<li><b>At runtime</b> with <a href="#runtime">suggested runtime
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interfaces</a>, which allow front-end compilers to support a range of
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collection runtimes.</li>
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</ul>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section">
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<a name="usage">Using the collectors</a>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_text">
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<p>In general, using a collector implies:</p>
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<ul>
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<li>Emitting compatible code, including initialization in the main
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program if necessary.</li>
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<li>Loading a compiler plugin if the collector is not statically linked with
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your compiler. For <tt>llc</tt>, use the <tt>-load</tt> option.</li>
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<li>Selecting the collection algorithm by applying the <tt>gc "..."</tt>
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attribute to your garbage collected functions, or equivalently with
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the <tt>setGC</tt> method.</li>
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<li>Linking your final executable with the garbage collector runtime.</li>
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</ul>
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<p>This table summarizes the available runtimes.</p>
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<table>
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<tr>
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<th>Collector</th>
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<th><tt>gc</tt> attribute</th>
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<th>Linkage</th>
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<th><tt>gcroot</tt></th>
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<th><tt>gcread</tt></th>
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<th><tt>gcwrite</tt></th>
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</tr>
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<tr valign="baseline">
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<td><a href="#semispace">SemiSpace</a></td>
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<td><tt>gc "shadow-stack"</tt></td>
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<td>TODO FIXME</td>
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<td>required</td>
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<td>optional</td>
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<td>optional</td>
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</tr>
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<tr valign="baseline">
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<td><a href="#ocaml">Ocaml</a></td>
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<td><tt>gc "ocaml"</tt></td>
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<td><i>provided by ocamlopt</i></td>
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<td>required</td>
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<td>optional</td>
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<td>optional</td>
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</tr>
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</table>
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<p>The sections for <a href="#intrinsics">Collection intrinsics</a> and
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<a href="#runtime">Recommended runtime interface</a> detail the interfaces that
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collectors may require user programs to utilize.</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="shadow-stack">ShadowStack - A highly portable collector</a>
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</div>
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<div class="doc_code"><tt>
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Collector *llvm::createShadowStackCollector();
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</tt></div>
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<div class="doc_text">
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<p>The ShadowStack backend is invoked with the <tt>gc "shadow-stack"</tt>
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function attribute.
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Unlike many collectors which rely on a cooperative code generator to generate
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stack maps, this algorithm carefully maintains a linked list of stack root
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descriptors [<a href="#henderson02">Henderson2002</a>]. This so-called "shadow
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stack" mirrors the machine stack. Maintaining this data structure is slower
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than using stack maps, but has a significant portability advantage because it
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requires no special support from the target code generator.</p>
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<p>The ShadowStack collector does not use read or write barriers, so the user
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program may use <tt>load</tt> and <tt>store</tt> instead of <tt>llvm.gcread</tt>
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and <tt>llvm.gcwrite</tt>.</p>
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<p>ShadowStack is a code generator plugin only. It must be paired with a
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compatible runtime.</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="semispace">SemiSpace - A simple copying collector runtime</a>
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</div>
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<div class="doc_text">
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<p>The SemiSpace runtime implements the <a href="runtime">suggested
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runtime interface</a> and is compatible with the ShadowStack backend.</p>
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<p>SemiSpace is a very simple copying collector. When it starts up, it
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allocates two blocks of memory for the heap. It uses a simple bump-pointer
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allocator to allocate memory from the first block until it runs out of space.
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When it runs out of space, it traces through all of the roots of the program,
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copying blocks to the other half of the memory space.</p>
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<p>This runtime is highly experimental and has not been used in a real project.
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Enhancements would be welcomed.</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="ocaml">Ocaml - An Objective Caml-compatible collector</a>
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</div>
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<div class="doc_code"><tt>
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Collector *llvm::createOcamlCollector();
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</tt></div>
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<div class="doc_text">
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<p>The ocaml backend is invoked with the <tt>gc "ocaml"</tt> function attribute.
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It supports the
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<a href="http://caml.inria.fr/">Objective Caml</a> language runtime by emitting
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a type-accurate stack map in the form of an ocaml 3.10.0-compatible frametable.
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The linkage requirements are satisfied automatically by the <tt>ocamlopt</tt>
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compiler when linking an executable.</p>
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<p>The ocaml collector does not use read or write barriers, so the user program
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may use <tt>load</tt> and <tt>store</tt> instead of <tt>llvm.gcread</tt> and
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<tt>llvm.gcwrite</tt>.</p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section">
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<a name="core">Core support</a><a name="intrinsics"></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 garbage collection facilities provided by the
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<a href="LangRef.html">LLVM intermediate representation</a>.</p>
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<p>These facilities are limited to those strictly necessary for compilation.
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They are not intended to be a complete interface to any garbage collector.
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Notably, heap allocation is not among the supplied primitives. A user program
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will also need to interface with the runtime, using either the
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<a href="#runtime">suggested runtime interface</a> or another interface
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specified by the runtime.</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="gcattr">Specifying GC code generation: <tt>gc "..."</tt></a>
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</div>
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<div class="doc_code"><tt>
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define <i>ty</i> @<i>name</i>(...) <u>gc "<i>collector</i>"</u> { ...
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</tt></div>
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<div class="doc_text">
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<p>The <tt>gc</tt> function attribute is used to specify the desired collector
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algorithm to the compiler. It is equivalent to specifying the collector name
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programmatically using the <tt>setGC</tt> method of <tt>Function</tt>.</p>
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<p>Specifying the collector on a per-function basis allows LLVM to link together
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programs that use different garbage collection algorithms.</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="gcroot">Identifying GC roots on the stack: <tt>llvm.gcroot</tt></a>
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</div>
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<div class="doc_code"><tt>
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void @llvm.gcroot(i8** %ptrloc, i8* %metadata)
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</tt></div>
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<div class="doc_text">
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<p>The <tt>llvm.gcroot</tt> intrinsic is used to inform LLVM of a pointer
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variable on the stack. The first argument <b>must</b> be a value referring to an alloca instruction
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or a bitcast of an alloca. The second contains a pointer to metadata that
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should be associated with the pointer, and <b>must</b> be a constant or global
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value address. If your target collector uses tags, use a null pointer for
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metadata.</p>
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<p>Consider the following fragment of Java code:</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>This block (which may be located in the middle of a function or in a loop
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nest), could be compiled to this LLVM code:</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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;; Tell LLVM that the stack space is a stack root.
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;; Java has type-tags on objects, so we pass null as metadata.
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%tmp = bitcast %Object** %X to i8**
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call void @llvm.gcroot(i8** %X, i8* null)
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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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;; Java null-initializes pointers.
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store %Object* null, %Object** %X
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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="barriers">Reading and writing references in the heap</a>
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</div>
|
|
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<div class="doc_text">
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<p>Some collectors need to be informed when the mutator (the program that needs
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garbage collection) either reads a pointer from or writes a pointer to a field
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of a heap object. The code fragments inserted at these points are called
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<em>read barriers</em> and <em>write barriers</em>, respectively. The amount of
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code that needs to be executed is usually quite small and not on the critical
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path of any computation, so the overall performance impact of the barrier is
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tolerable.</p>
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<p>Barriers often require access to the <em>object pointer</em> rather than the
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<em>derived pointer</em> (which is a pointer to the field within the
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object). Accordingly, these intrinsics take both pointers as separate arguments
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for completeness. In this snippet, <tt>%object</tt> is the object pointer, and
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<tt>%derived</tt> is the derived pointer:</p>
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<blockquote><pre>
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;; An array type.
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%class.Array = type { %class.Object, i32, [0 x %class.Object*] }
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...
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;; Load the object pointer from a gcroot.
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%object = load %class.Array** %object_addr
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;; Compute the derived pointer.
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%derived = getelementptr %object, i32 0, i32 2, i32 %n</pre></blockquote>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsubsection">
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<a name="gcwrite">Write barrier: <tt>llvm.gcwrite</tt></a>
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|
</div>
|
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<div class="doc_code"><tt>
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void @llvm.gcwrite(i8* %value, i8* %object, i8** %derived)
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</tt></div>
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<div class="doc_text">
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<p>For write barriers, LLVM provides the <tt>llvm.gcwrite</tt> intrinsic
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function. It has exactly the same semantics as a non-volatile <tt>store</tt> to
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the derived pointer (the third argument).</p>
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<p>Many important algorithms require write barriers, including generational
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and concurrent collectors. Additionally, write barriers could be used to
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implement reference counting.</p>
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<p>The use of this intrinsic is optional if the target collector does use
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write barriers. If so, the collector will replace it with the corresponding
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<tt>store</tt>.</p>
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</div>
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|
|
<!-- ======================================================================= -->
|
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<div class="doc_subsubsection">
|
|
<a name="gcread">Read barrier: <tt>llvm.gcread</tt></a>
|
|
</div>
|
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|
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<div class="doc_code"><tt>
|
|
i8* @llvm.gcread(i8* %object, i8** %derived)<br>
|
|
</tt></div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>For read barriers, LLVM provides the <tt>llvm.gcread</tt> intrinsic function.
|
|
It has exactly the same semantics as a non-volatile <tt>load</tt> from the
|
|
derived pointer (the second argument).</p>
|
|
|
|
<p>Read barriers are needed by fewer algorithms than write barriers, and may
|
|
have a greater performance impact since pointer reads are more frequent than
|
|
writes.</p>
|
|
|
|
<p>As with <tt>llvm.gcwrite</tt>, a target collector might not require the use
|
|
of this intrinsic.</p>
|
|
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
<div class="doc_section">
|
|
<a name="runtime">Recommended runtime interface</a>
|
|
</div>
|
|
<!-- *********************************************************************** -->
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>LLVM specifies the following recommended runtime interface to the garbage
|
|
collection at runtime. A program should use these interfaces to accomplish the
|
|
tasks not supported by the intrinsics.</p>
|
|
|
|
<p>Unlike the intrinsics, which are integral to LLVM's code generator, there is
|
|
nothing unique about these interfaces; a front-end compiler and runtime are free
|
|
to agree to a different specification.</p>
|
|
|
|
<p class="doc_warning">Note: This interface is a work in progress.</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="initialize">Garbage collector startup and initialization</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<div class="doc_code"><tt>
|
|
void llvm_gc_initialize(unsigned InitialHeapSize);
|
|
</tt></div>
|
|
|
|
<p>
|
|
The <tt>llvm_gc_initialize</tt> function should be called once before any other
|
|
garbage collection functions are called. This gives the garbage collector the
|
|
chance to initialize itself and allocate the heap. The initial heap size to
|
|
allocate should be specified as an argument.
|
|
</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="allocate">Allocating memory from the GC</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<div class="doc_code"><tt>
|
|
void *llvm_gc_allocate(unsigned Size);
|
|
</tt></div>
|
|
|
|
<p>The <tt>llvm_gc_allocate</tt> function is a global function defined by the
|
|
garbage collector implementation to allocate memory. It returns a
|
|
zeroed-out block of memory of the specified size, sufficiently aligned to store
|
|
any object.</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="explicit">Explicit invocation of the garbage collector</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<div class="doc_code"><tt>
|
|
void llvm_gc_collect();
|
|
</tt></div>
|
|
|
|
<p>
|
|
The <tt>llvm_gc_collect</tt> function is exported by the garbage collector
|
|
implementations to provide a full collection, even when the heap is not
|
|
exhausted. This can be used by end-user code as a hint, and may be ignored by
|
|
the garbage collector.
|
|
</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="traceroots">Tracing GC pointers from the program stack</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
<div class="doc_code"><tt>
|
|
void llvm_cg_walk_gcroots(void (*FP)(void **Root, void *Meta));
|
|
</tt></div>
|
|
|
|
<p>
|
|
The <tt>llvm_cg_walk_gcroots</tt> function is a function provided by the code
|
|
generator that iterates through all of the GC roots on the stack, calling the
|
|
specified function pointer with each record. For each GC root, the address of
|
|
the pointer and the meta-data (from the <a
|
|
href="#gcroot"><tt>llvm.gcroot</tt></a> intrinsic) are provided.
|
|
</p>
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="staticroots">Tracing GC pointers from static roots</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
TODO
|
|
</div>
|
|
|
|
|
|
<!-- *********************************************************************** -->
|
|
<div class="doc_section">
|
|
<a name="plugin">Implementing a collector plugin</a>
|
|
</div>
|
|
<!-- *********************************************************************** -->
|
|
|
|
<div class="doc_text">
|
|
|
|
<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 emit
|
|
the assembly code for the collector's unique stack map data structure, which
|
|
might be accomplished in as few as 100 LOC.</p>
|
|
|
|
<p>This is not the appropriate place to implement a garbage collected heap or a
|
|
garbage collector itself. That code should exist in the language's runtime
|
|
library. The compiler plugin is responsible for generating code which is
|
|
compatible with that runtime library.</p>
|
|
|
|
<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 VISIBILITY_HIDDEN MyGC : public GCStrategy {
|
|
public:
|
|
MyGC() {}
|
|
};
|
|
|
|
GCRegistry::Add<MyGC>
|
|
X("mygc", "My bespoke garbage collector.");
|
|
}</pre></blockquote>
|
|
|
|
<p>Using the LLVM makefiles (like the <a
|
|
href="http://llvm.org/viewvc/llvm-project/llvm/trunk/projects/sample/">sample
|
|
project</a>), this can be built into a plugin using a simple makefile:</p>
|
|
|
|
<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>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="collector-algos">Overview of available features</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>The boilerplate collector above does nothing. More specifically:</p>
|
|
|
|
<ul>
|
|
<li><tt>llvm.gcread</tt> calls are replaced with the corresponding
|
|
<tt>load</tt> instruction.</li>
|
|
<li><tt>llvm.gcwrite</tt> calls are replaced with the corresponding
|
|
<tt>store</tt> instruction.</li>
|
|
<li>No stack map is emitted, and no safe points are added.</li>
|
|
</ul>
|
|
|
|
<p><tt>Collector</tt> provides a range of features through which a plugin
|
|
collector may do useful work. This matrix summarizes the supported (and planned)
|
|
features and correlates them with the collection techniques which typically
|
|
require them.</p>
|
|
|
|
<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 root
|
|
descriptors.</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="stack-map">Computing stack maps</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<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>LLVM automatically computes a stack map. All a <tt>GCStrategy</tt> needs to do
|
|
is access it using <tt>GCFunctionMetadata::roots_begin()</tt> and
|
|
-<tt>end()</tt>. If the <tt>llvm.gcroot</tt> intrinsic is eliminated before code
|
|
generation by a custom lowering pass, LLVM's stack map will be empty.</p>
|
|
|
|
</div>
|
|
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="init-roots">Initializing roots to null: <tt>InitRoots</tt></a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<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>
|
|
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="custom">Custom lowering of intrinsics: <tt>CustomRoots</tt>,
|
|
<tt>CustomReadBarriers</tt>, and <tt>CustomWriteBarriers</tt></a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<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>: Pass through to the code generator to generate a
|
|
stack map.</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>
|
|
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="safe-points">Generating safe points: <tt>NeededSafePoints</tt></a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<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_subsection">
|
|
<a name="assembly">Emitting assembly code: <tt>GCMetadataPrinter</tt></a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>LLVM allows a GC to print arbitrary assembly code before and after the rest
|
|
of a module's assembly code. At the end of the module, the GC can print stack
|
|
maps built by the code generator. (At the beginning, this information is not
|
|
yet computed.)</p>
|
|
|
|
<p>Since AsmWriter and CodeGen are separate components of LLVM, a separate
|
|
abstract base class and registry is provided for printing assembly code, the
|
|
<tt>GCMetadaPrinter</tt> and <tt>GCMetadaPrinterRegistry</tt>. The AsmWriter
|
|
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>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 VISIBILITY_HIDDEN 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/Target/TargetData.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.getTargetData()->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 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>
|
|
|
|
</div>
|
|
|
|
|
|
<!-- *********************************************************************** -->
|
|
<div class="doc_section">
|
|
<a name="runtime-impl">Implementing a collector runtime</a>
|
|
</div>
|
|
<!-- *********************************************************************** -->
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>Implementing a garbage collector for LLVM is fairly straightforward. The
|
|
LLVM garbage collectors are provided in a form that makes them easy to link into
|
|
the language-specific runtime that a language front-end would use. They require
|
|
functionality from the language-specific runtime to get information about <a
|
|
href="#gcdescriptors">where pointers are located in heap objects</a>.</p>
|
|
|
|
<p>The implementation must include the
|
|
<a href="#allocate"><tt>llvm_gc_allocate</tt></a> and
|
|
<a href="#explicit"><tt>llvm_gc_collect</tt></a> functions. To do this, it will
|
|
probably have to <a href="#traceroots">trace through the roots
|
|
from the stack</a> and understand the <a href="#gcdescriptors">GC descriptors
|
|
for heap objects</a>. Luckily, there are some <a href="#usage">example
|
|
implementations</a> available.
|
|
</p>
|
|
</div>
|
|
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="gcdescriptors">Tracing GC pointers from heap objects</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
<p>
|
|
The three most common ways to keep track of where pointers live in heap objects
|
|
are (listed in order of space overhead required):</p>
|
|
|
|
<ol>
|
|
<li>In languages with polymorphic objects, pointers from an object header are
|
|
usually used to identify the GC pointers in the heap object. This is common for
|
|
object-oriented languages like Self, Smalltalk, Java, or C#.</li>
|
|
|
|
<li>If heap objects are not polymorphic, often the "shape" of the heap can be
|
|
determined from the roots of the heap or from some other meta-data [<a
|
|
href="#appel89">Appel89</a>, <a href="#goldberg91">Goldberg91</a>, <a
|
|
href="#tolmach94">Tolmach94</a>]. In this case, the garbage collector can
|
|
propagate the information around from meta data stored with the roots. This
|
|
often eliminates the need to have a header on objects in the heap. This is
|
|
common in the ML family.</li>
|
|
|
|
<li>If all heap objects have pointers in the same locations, or pointers can be
|
|
distinguished just by looking at them (e.g., the low order bit is clear), no
|
|
book-keeping is needed at all. This is common for Lisp-like languages.</li>
|
|
</ol>
|
|
|
|
<p>The LLVM garbage collectors are capable of supporting all of these styles of
|
|
language, including ones that mix various implementations. To do this, it
|
|
allows the source-language to associate meta-data with the <a
|
|
href="#gcroot">stack roots</a>, and the heap tracing routines can propagate the
|
|
information. In addition, LLVM allows the front-end to extract GC information
|
|
in any form from a specific object pointer (this supports situations #1 and #3).
|
|
</p>
|
|
|
|
</div>
|
|
|
|
|
|
<!-- *********************************************************************** -->
|
|
<div class="doc_section">
|
|
<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>
|
|
|
|
<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>
|
|
|
|
|
|
<!-- *********************************************************************** -->
|
|
|
|
<hr>
|
|
<address>
|
|
<a href="http://jigsaw.w3.org/css-validator/check/referer"><img
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src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a>
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src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a>
|
|
|
|
<a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
|
|
<a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
|
|
Last modified: $Date$
|
|
</address>
|
|
|
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</body>
|
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</html>
|