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c9c0b59402
Also, drop reference to the half-baked runtime interface. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@65802 91177308-0d34-0410-b5e6-96231b3b80d8
1343 lines
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1343 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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<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">Goals and non-goals</a></li>
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</ul>
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</li>
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<li><a href="#quickstart">Getting started</a>
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<ul>
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<li><a href="quickstart-compiler">In your compiler</a></li>
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<li><a href="quickstart-runtime">In your runtime library</a></li>
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<li><a href="shadow-stack">About the shadow stack</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="#plugin">Compiler plugin interface</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">Goals and non-goals</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 provide a garbage collector—this should
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be part of your language's runtime library. LLVM provides a framework for
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compile time <a href="#plugin">code generation plugins</a>. The role of these
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plugins is to generate code and data structures which conforms to the <em>binary
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interface</em> specified by the <em>runtime library</em>. This is similar to the
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relationship between LLVM and DWARF debugging info, for example. The
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difference primarily lies in the lack of an established standard in the domain
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of garbage collection—thus the plugins.</p>
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<p>The aspects of the binary interface with which LLVM's GC support is
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concerned are:</p>
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<ul>
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<li>Creation of GC-safe points within code where collection is allowed to
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execute safely.</li>
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<li>Definition of a stack frame descriptor. For each safe point in the code,
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a frame descriptor maps where object references are located within the
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frame so that the GC may traverse and perhaps update them.</li>
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<li>Write barriers when storing object references within the heap. These
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are commonly used to optimize incremental scans.</li>
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<li>Emission of read barriers when loading object references. These are
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useful for interoperating with concurrent collectors.</li>
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</ul>
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<p>There are additional areas that LLVM does not directly address:</p>
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<ul>
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<li>Registration of global roots.</li>
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<li>Discovery or registration of stack frame descriptors.</li>
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<li>The functions used by the program to allocate memory, trigger a
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collection, etc.</li>
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</ul>
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<p>In general, LLVM's support for GC does not include features which can be
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adequately addressed with other features of the IR and does not specify a
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particular binary interface. On the plus side, this means that you should be
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able to integrate LLVM with an existing runtime. On the other hand, it leaves
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a lot of work for the developer of a novel language. However, it's easy to get
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started quickly and scale up to a more sophisticated implementation as your
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compiler matures.</p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section">
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<a name="quickstart">Getting started</a>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_text">
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<p>Using a GC with LLVM implies many things, for example:</p>
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<ul>
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<li>Write a runtime library or find an existing one which implements a GC
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heap.<ol>
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<li>Implement a memory allocator.</li>
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<li>Design a binary interface for frame descriptors, used to identify
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references within a stack frame.*</li>
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<li>Implement a stack crawler to discover functions on the call stack.*</li>
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<li>Implement a registry for global roots.</li>
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<li>Design a binary interface for type descriptors, used to map references
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within heap objects.</li>
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<li>Implement a collection routine bringing together all of the above.</li>
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</ol></li>
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<li>Emit compatible code from your compiler.<ul>
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<li>Initialization in the main function.</li>
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<li>Use the <tt>gc "..."</tt> attribute to enable GC code generation
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(or <tt>F.setGC("...")</tt>).</li>
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<li>Use <tt>@llvm.gcroot</tt> to mark stack roots.</li>
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<li>Use <tt>@llvm.gcread</tt> and/or <tt>@llvm.gcwrite</tt> to
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manipulate GC references, if necessary.</li>
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<li>Allocate memory using the GC allocation routine provided by the
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runtime library.</li>
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<li>Generate type descriptors according to your runtime's binary interface.</li>
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</ul></li>
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<li>Write a compiler plugin to interface LLVM with the runtime library.*<ul>
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<li>Lower <tt>@llvm.gcread</tt> and <tt>@llvm.gcwrite</tt> to appropriate
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code sequences.*</li>
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<li>Generate stack maps according to the runtime's binary interface.*</li>
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</ul></li>
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<li>Load the plugin into the compiler. Use <tt>llc -load</tt> or link the
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plugin statically with your language's compiler.*</li>
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<li>Link program executables with the runtime.</li>
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</ul>
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<p>To help with several of these tasks (those indicated with a *), LLVM
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includes a highly portable, built-in ShadowStack code generator. It is compiled
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into <tt>llc</tt> and works even with the interpreter and C backends.</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="quickstart-compiler">In your compiler</a>
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</div>
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<div class="doc_text">
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<p>To turn the shadow stack on for your functions, first call:</p>
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<div class="doc_code"><pre
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>F.setGC("shadow-stack");</pre></div>
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<p>for each function your compiler emits. Since the shadow stack is built into
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LLVM, you do not need to load a plugin.</p>
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<p>Your compiler must also use <tt>@llvm.gcroot</tt> as documented.
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Don't forget to create a root for each intermediate value that is generated
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when evaluating an expression. In <tt>h(f(), g())</tt>, the result of
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<tt>f()</tt> could easily be collected if evaluating <tt>g()</tt> triggers a
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collection.</p>
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<p>There's no need to use <tt>@llvm.gcread</tt> and <tt>@llvm.gcwrite</tt> over
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plain <tt>load</tt> and <tt>store</tt> for now. You will need them when
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switching to a more advanced GC.</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="quickstart-runtime">In your runtime</a>
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</div>
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<div class="doc_text">
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<p>The shadow stack doesn't imply a memory allocation algorithm. A semispace
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collector or building atop <tt>malloc</tt> are great places to start, and can
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be implemented with very little code.</p>
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<p>When it comes time to collect, however, your runtime needs to traverse the
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stack roots, and for this it needs to integrate with the shadow stack. Luckily,
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doing so is very simple. (This code is heavily commented to help you
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understand the data structure, but there are only 20 lines of meaningful
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code.)</p>
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</div>
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<div class="doc_code"><pre
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>/// @brief A constant shadow stack frame descriptor. The compiler emits one of
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/// these for each function.
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///
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/// Storage of metadata values is elided if the %meta parameter to @llvm.gcroot
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/// is null.
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struct FrameMap {
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int32_t NumRoots; //< Number of roots in stack frame.
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int32_t NumMeta; //< Number of metadata descriptors. May be < NumRoots.
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const void *Meta[0]; //< Metadata for each root.
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};
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/// @brief A link in the dynamic shadow stack. One of these is embedded in the
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/// stack frame of each function on the call stack.
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struct StackEntry {
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StackEntry *Next; //< Link to next stack entry (the caller's).
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const FrameMap *Map; //< Pointer to constant FrameMap.
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void *Roots[0]; //< Stack roots (in-place array).
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};
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/// @brief The head of the singly-linked list of StackEntries. Functions push
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/// and pop onto this in their prologue and epilogue.
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///
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/// Since there is only a global list, this technique is not threadsafe.
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StackEntry *llvm_gc_root_chain;
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/// @brief Calls Visitor(root, meta) for each GC root on the stack.
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/// root and meta are exactly the values passed to
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/// <tt>@llvm.gcroot</tt>.
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///
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/// Visitor could be a function to recursively mark live objects. Or it
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/// might copy them to another heap or generation.
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///
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/// @param Visitor A function to invoke for every GC root on the stack.
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void visitGCRoots(void (*Visitor)(void **Root, const void *Meta)) {
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for (StackEntry *R = llvm_gc_root_chain; R; R = R->Next) {
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unsigned i = 0;
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// For roots [0, NumMeta), the metadata pointer is in the FrameMap.
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for (unsigned e = R->Map->NumMeta; i != e; ++i)
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Visitor(&R->Roots[i], R->Map->Meta[i]);
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// For roots [NumMeta, NumRoots), the metadata pointer is null.
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for (unsigned e = R->Map->NumRoots; i != e; ++i)
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Visitor(&R->Roots[i], NULL);
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}
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}</pre></div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="shadow-stack">About the shadow stack</a>
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</div>
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<div class="doc_text">
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<p>Unlike many GC algorithms which rely on a cooperative code generator to
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generate stack maps, this algorithm carefully maintains a linked list of stack
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root descriptors [<a href="#henderson02">Henderson2002</a>]. This so-called
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"shadow stack" mirrors the machine stack. Maintaining this data structure is
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slower than using stack maps, but has a significant portability advantage
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because it requires no special support from the target code generator.</p>
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<p>The tradeoff for this simplicity and portability is:</p>
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<ul>
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<li>High overhead per function call.</li>
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<li>Not thread-safe.</li>
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</ul>
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<p>Still, it's an easy way to get started.</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">IR features</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>. The exact behavior
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of these IR features is specified by the binary interface implemented by a
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<a href="#plugin">code generation plugin</a>, not by this document.</p>
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<p>These facilities are limited to those strictly necessary; they are not
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intended to be a complete interface to any garbage collector. A program will
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need to interface with the GC library using the facilities provided by that
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program.</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>name</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 GC style
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to the compiler. Its programmatic equivalent is the <tt>setGC</tt> method of
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<tt>Function</tt>.</p>
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<p>Setting <tt>gc "<i>name</i>"</tt> on a function triggers a search for a
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matching code generation plugin "<i>name</i>"; it is that plugin which defines
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the exact nature of the code generated to support GC. If none is found, the
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compiler will raise an error.</p>
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<p>Specifying the GC style on a per-function basis allows LLVM to link together
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programs that use different garbage collection algorithms (or none at all).</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 that a stack
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variable references an object on the heap and is to be tracked for garbage
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collection. The exact impact on generated code is specified by a <a
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href="#plugin">compiler plugin</a>.</p>
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<p>A compiler which uses mem2reg to raise imperative code using <tt>alloca</tt>
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into SSA form need only add a call to <tt>@llvm.gcroot</tt> for those variables
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which a pointers into the GC heap.</p>
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<p>It is also important to mark intermediate values with <tt>llvm.gcroot</tt>.
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For example, consider <tt>h(f(), g())</tt>. Beware leaking the result of
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<tt>f()</tt> in the case that <tt>g()</tt> triggers a collection.</p>
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<p>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>The <tt>%metadata</tt> argument can be used to avoid requiring heap objects
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to have 'isa' pointers or tag bits. [<a href="#appel89">Appel89</a>, <a
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href="#goldberg91">Goldberg91</a>, <a href="#tolmach94">Tolmach94</a>] If
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specified, its value will be tracked along with the location of the pointer in
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the stack frame.</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>
|
|
Entry:
|
|
;; In the entry block for the function, allocate the
|
|
;; stack space for X, which is an LLVM pointer.
|
|
%X = alloca %Object*
|
|
|
|
;; Tell LLVM that the stack space is a stack root.
|
|
;; Java has type-tags on objects, so we pass null as metadata.
|
|
%tmp = bitcast %Object** %X to i8**
|
|
call void @llvm.gcroot(i8** %X, i8* null)
|
|
...
|
|
|
|
;; "CodeBlock" is the block corresponding to the start
|
|
;; of the scope above.
|
|
CodeBlock:
|
|
;; Java null-initializes pointers.
|
|
store %Object* null, %Object** %X
|
|
|
|
...
|
|
|
|
;; As the pointer goes out of scope, store a null value into
|
|
;; it, to indicate that the value is no longer live.
|
|
store %Object* null, %Object** %X
|
|
...
|
|
</pre>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="barriers">Reading and writing references in the heap</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>Some collectors need to be informed when the mutator (the program that needs
|
|
garbage collection) either reads a pointer from or writes a pointer to a field
|
|
of a heap object. The code fragments inserted at these points are called
|
|
<em>read barriers</em> and <em>write barriers</em>, respectively. The amount of
|
|
code that needs to be executed is usually quite small and not on the critical
|
|
path of any computation, so the overall performance impact of the barrier is
|
|
tolerable.</p>
|
|
|
|
<p>Barriers often require access to the <em>object pointer</em> rather than the
|
|
<em>derived pointer</em> (which is a pointer to the field within the
|
|
object). Accordingly, these intrinsics take both pointers as separate arguments
|
|
for completeness. In this snippet, <tt>%object</tt> is the object pointer, and
|
|
<tt>%derived</tt> is the derived pointer:</p>
|
|
|
|
<blockquote><pre>
|
|
;; An array type.
|
|
%class.Array = type { %class.Object, i32, [0 x %class.Object*] }
|
|
...
|
|
|
|
;; Load the object pointer from a gcroot.
|
|
%object = load %class.Array** %object_addr
|
|
|
|
;; Compute the derived pointer.
|
|
%derived = getelementptr %object, i32 0, i32 2, i32 %n</pre></blockquote>
|
|
|
|
<p>The use of these intrinsics is naturally optional if the target GC does
|
|
require the corresponding barrier. If so, the GC plugin will replace the
|
|
intrinsic calls with the corresponding <tt>load</tt> or <tt>store</tt>
|
|
instruction if they are used.</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsubsection">
|
|
<a name="gcwrite">Write barrier: <tt>llvm.gcwrite</tt></a>
|
|
</div>
|
|
|
|
<div class="doc_code"><tt>
|
|
void @llvm.gcwrite(i8* %value, i8* %object, i8** %derived)
|
|
</tt></div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>For write barriers, LLVM provides the <tt>llvm.gcwrite</tt> intrinsic
|
|
function. It has exactly the same semantics as a non-volatile <tt>store</tt> to
|
|
the derived pointer (the third argument). The exact code generated is specified
|
|
by a <a href="#plugin">compiler plugin</a>.</p>
|
|
|
|
<p>Many important algorithms require write barriers, including generational
|
|
and concurrent collectors. Additionally, write barriers could be used to
|
|
implement reference counting.</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsubsection">
|
|
<a name="gcread">Read barrier: <tt>llvm.gcread</tt></a>
|
|
</div>
|
|
|
|
<div class="doc_code"><tt>
|
|
i8* @llvm.gcread(i8* %object, i8** %derived)<br>
|
|
</tt></div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>For read barriers, LLVM provides the <tt>llvm.gcread</tt> intrinsic function.
|
|
It has exactly the same semantics as a non-volatile <tt>load</tt> from the
|
|
derived pointer (the second argument). The exact code generated is specified by
|
|
a <a href="#plugin">compiler plugin</a>.</p>
|
|
|
|
<p>Read barriers are needed by fewer algorithms than write barriers, and may
|
|
have a greater performance impact since pointer reads are more frequent than
|
|
writes.</p>
|
|
|
|
</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
|
|
conforms to the binary interface defined by library, most essentially the
|
|
<a href="stack-map">stack map</a>.</p>
|
|
|
|
<p>To subclass <tt>llvm::GCStrategy</tt> and register it with the compiler:</p>
|
|
|
|
<blockquote><pre>// lib/MyGC/MyGC.cpp - Example LLVM GC plugin
|
|
|
|
#include "llvm/CodeGen/GCStrategy.h"
|
|
#include "llvm/CodeGen/GCMetadata.h"
|
|
#include "llvm/Support/Compiler.h"
|
|
|
|
using namespace llvm;
|
|
|
|
namespace {
|
|
class 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>GCMetadataPrinterRegistry</tt>. The AsmWriter
|
|
will look for such a subclass if the <tt>GCStrategy</tt> sets
|
|
<tt>UsesMetadata</tt>:</p>
|
|
|
|
<blockquote><pre
|
|
>MyGC::MyGC() {
|
|
UsesMetadata = true;
|
|
}</pre></blockquote>
|
|
|
|
<p>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="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>
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|
|
|
<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>
|