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diff --git a/docs/GarbageCollection.html b/docs/GarbageCollection.html deleted file mode 100644 index 5bc70f1bb0..0000000000 --- a/docs/GarbageCollection.html +++ /dev/null @@ -1,1389 +0,0 @@ -<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" - "http://www.w3.org/TR/html4/strict.dtd"> -<html> -<head> - <meta http-equiv="Content-Type" Content="text/html; charset=UTF-8" > - <title>Accurate Garbage Collection with LLVM</title> - <link rel="stylesheet" href="_static/llvm.css" type="text/css"> - <style type="text/css"> - .rowhead { text-align: left; background: inherit; } - .indent { padding-left: 1em; } - .optl { color: #BFBFBF; } - </style> -</head> -<body> - -<h1> - Accurate Garbage Collection with LLVM -</h1> - -<ol> - <li><a href="#introduction">Introduction</a> - <ul> - <li><a href="#feature">Goals and non-goals</a></li> - </ul> - </li> - - <li><a href="#quickstart">Getting started</a> - <ul> - <li><a href="#quickstart-compiler">In your compiler</a></li> - <li><a href="#quickstart-runtime">In your runtime library</a></li> - <li><a href="#shadow-stack">About the shadow stack</a></li> - </ul> - </li> - - <li><a href="#core">Core support</a> - <ul> - <li><a href="#gcattr">Specifying GC code generation: - <tt>gc "..."</tt></a></li> - <li><a href="#gcroot">Identifying GC roots on the stack: - <tt>llvm.gcroot</tt></a></li> - <li><a href="#barriers">Reading and writing references in the heap</a> - <ul> - <li><a href="#gcwrite">Write barrier: <tt>llvm.gcwrite</tt></a></li> - <li><a href="#gcread">Read barrier: <tt>llvm.gcread</tt></a></li> - </ul> - </li> - </ul> - </li> - - <li><a href="#plugin">Compiler plugin interface</a> - <ul> - <li><a href="#collector-algos">Overview of available features</a></li> - <li><a href="#stack-map">Computing stack maps</a></li> - <li><a href="#init-roots">Initializing roots to null: - <tt>InitRoots</tt></a></li> - <li><a href="#custom">Custom lowering of intrinsics: <tt>CustomRoots</tt>, - <tt>CustomReadBarriers</tt>, and <tt>CustomWriteBarriers</tt></a></li> - <li><a href="#safe-points">Generating safe points: - <tt>NeededSafePoints</tt></a></li> - <li><a href="#assembly">Emitting assembly code: - <tt>GCMetadataPrinter</tt></a></li> - </ul> - </li> - - <li><a href="#runtime-impl">Implementing a collector runtime</a> - <ul> - <li><a href="#gcdescriptors">Tracing GC pointers from heap - objects</a></li> - </ul> - </li> - - <li><a href="#references">References</a></li> - -</ol> - -<div class="doc_author"> - <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a> and - Gordon Henriksen</p> -</div> - -<!-- *********************************************************************** --> -<h2> - <a name="introduction">Introduction</a> -</h2> -<!-- *********************************************************************** --> - -<div> - -<p>Garbage collection is a widely used technique that frees the programmer from -having to know the lifetimes of heap objects, making software easier to produce -and maintain. Many programming languages rely on garbage collection for -automatic memory management. There are two primary forms of garbage collection: -conservative and accurate.</p> - -<p>Conservative garbage collection often does not require any special support -from either the language or the compiler: it can handle non-type-safe -programming languages (such as C/C++) and does not require any special -information from the compiler. The -<a href="http://www.hpl.hp.com/personal/Hans_Boehm/gc/">Boehm collector</a> is -an example of a state-of-the-art conservative collector.</p> - -<p>Accurate garbage collection requires the ability to identify all pointers in -the program at run-time (which requires that the source-language be type-safe in -most cases). Identifying pointers at run-time requires compiler support to -locate all places that hold live pointer variables at run-time, including the -<a href="#gcroot">processor stack and registers</a>.</p> - -<p>Conservative garbage collection is attractive because it does not require any -special compiler support, but it does have problems. In particular, because the -conservative garbage collector cannot <i>know</i> that a particular word in the -machine is a pointer, it cannot move live objects in the heap (preventing the -use of compacting and generational GC algorithms) and it can occasionally suffer -from memory leaks due to integer values that happen to point to objects in the -program. In addition, some aggressive compiler transformations can break -conservative garbage collectors (though these seem rare in practice).</p> - -<p>Accurate garbage collectors do not suffer from any of these problems, but -they can suffer from degraded scalar optimization of the program. In particular, -because the runtime must be able to identify and update all pointers active in -the program, some optimizations are less effective. In practice, however, the -locality and performance benefits of using aggressive garbage collection -techniques dominates any low-level losses.</p> - -<p>This document describes the mechanisms and interfaces provided by LLVM to -support accurate garbage collection.</p> - -<!-- ======================================================================= --> -<h3> - <a name="feature">Goals and non-goals</a> -</h3> - -<div> - -<p>LLVM's intermediate representation provides <a href="#intrinsics">garbage -collection intrinsics</a> that offer support for a broad class of -collector models. For instance, the intrinsics permit:</p> - -<ul> - <li>semi-space collectors</li> - <li>mark-sweep collectors</li> - <li>generational collectors</li> - <li>reference counting</li> - <li>incremental collectors</li> - <li>concurrent collectors</li> - <li>cooperative collectors</li> -</ul> - -<p>We hope that the primitive support built into the LLVM IR is sufficient to -support a broad class of garbage collected languages including Scheme, ML, Java, -C#, Perl, Python, Lua, Ruby, other scripting languages, and more.</p> - -<p>However, LLVM does not itself provide a garbage collector—this should -be part of your language's runtime library. LLVM provides a framework for -compile time <a href="#plugin">code generation plugins</a>. The role of these -plugins is to generate code and data structures which conforms to the <em>binary -interface</em> specified by the <em>runtime library</em>. This is similar to the -relationship between LLVM and DWARF debugging info, for example. The -difference primarily lies in the lack of an established standard in the domain -of garbage collection—thus the plugins.</p> - -<p>The aspects of the binary interface with which LLVM's GC support is -concerned are:</p> - -<ul> - <li>Creation of GC-safe points within code where collection is allowed to - execute safely.</li> - <li>Computation of the stack map. For each safe point in the code, object - references within the stack frame must be identified so that the - collector may traverse and perhaps update them.</li> - <li>Write barriers when storing object references to the heap. These are - commonly used to optimize incremental scans in generational - collectors.</li> - <li>Emission of read barriers when loading object references. These are - useful for interoperating with concurrent collectors.</li> -</ul> - -<p>There are additional areas that LLVM does not directly address:</p> - -<ul> - <li>Registration of global roots with the runtime.</li> - <li>Registration of stack map entries with the runtime.</li> - <li>The functions used by the program to allocate memory, trigger a - collection, etc.</li> - <li>Computation or compilation of type maps, or registration of them with - the runtime. These are used to crawl the heap for object - references.</li> -</ul> - -<p>In general, LLVM's support for GC does not include features which can be -adequately addressed with other features of the IR and does not specify a -particular binary interface. On the plus side, this means that you should be -able to integrate LLVM with an existing runtime. On the other hand, it leaves -a lot of work for the developer of a novel language. However, it's easy to get -started quickly and scale up to a more sophisticated implementation as your -compiler matures.</p> - -</div> - -</div> - -<!-- *********************************************************************** --> -<h2> - <a name="quickstart">Getting started</a> -</h2> -<!-- *********************************************************************** --> - -<div> - -<p>Using a GC with LLVM implies many things, for example:</p> - -<ul> - <li>Write a runtime library or find an existing one which implements a GC - heap.<ol> - <li>Implement a memory allocator.</li> - <li>Design a binary interface for the stack map, used to identify - references within a stack frame on the machine stack.*</li> - <li>Implement a stack crawler to discover functions on the call stack.*</li> - <li>Implement a registry for global roots.</li> - <li>Design a binary interface for type maps, used to identify references - within heap objects.</li> - <li>Implement a collection routine bringing together all of the above.</li> - </ol></li> - <li>Emit compatible code from your compiler.<ul> - <li>Initialization in the main function.</li> - <li>Use the <tt>gc "..."</tt> attribute to enable GC code generation - (or <tt>F.setGC("...")</tt>).</li> - <li>Use <tt>@llvm.gcroot</tt> to mark stack roots.</li> - <li>Use <tt>@llvm.gcread</tt> and/or <tt>@llvm.gcwrite</tt> to - manipulate GC references, if necessary.</li> - <li>Allocate memory using the GC allocation routine provided by the - runtime library.</li> - <li>Generate type maps according to your runtime's binary interface.</li> - </ul></li> - <li>Write a compiler plugin to interface LLVM with the runtime library.*<ul> - <li>Lower <tt>@llvm.gcread</tt> and <tt>@llvm.gcwrite</tt> to appropriate - code sequences.*</li> - <li>Compile LLVM's stack map to the binary form expected by the - runtime.</li> - </ul></li> - <li>Load the plugin into the compiler. Use <tt>llc -load</tt> or link the - plugin statically with your language's compiler.*</li> - <li>Link program executables with the runtime.</li> -</ul> - -<p>To help with several of these tasks (those indicated with a *), LLVM -includes a highly portable, built-in ShadowStack code generator. It is compiled -into <tt>llc</tt> and works even with the interpreter and C backends.</p> - -<!-- ======================================================================= --> -<h3> - <a name="quickstart-compiler">In your compiler</a> -</h3> - -<div> - -<p>To turn the shadow stack on for your functions, first call:</p> - -<div class="doc_code"><pre ->F.setGC("shadow-stack");</pre></div> - -<p>for each function your compiler emits. Since the shadow stack is built into -LLVM, you do not need to load a plugin.</p> - -<p>Your compiler must also use <tt>@llvm.gcroot</tt> as documented. -Don't forget to create a root for each intermediate value that is generated -when evaluating an expression. In <tt>h(f(), g())</tt>, the result of -<tt>f()</tt> could easily be collected if evaluating <tt>g()</tt> triggers a -collection.</p> - -<p>There's no need to use <tt>@llvm.gcread</tt> and <tt>@llvm.gcwrite</tt> over -plain <tt>load</tt> and <tt>store</tt> for now. You will need them when -switching to a more advanced GC.</p> - -</div> - -<!-- ======================================================================= --> -<h3> - <a name="quickstart-runtime">In your runtime</a> -</h3> - -<div> - -<p>The shadow stack doesn't imply a memory allocation algorithm. A semispace -collector or building atop <tt>malloc</tt> are great places to start, and can -be implemented with very little code.</p> - -<p>When it comes time to collect, however, your runtime needs to traverse the -stack roots, and for this it needs to integrate with the shadow stack. Luckily, -doing so is very simple. (This code is heavily commented to help you -understand the data structure, but there are only 20 lines of meaningful -code.)</p> - -<pre class="doc_code"> -/// @brief The map for a single function's stack frame. One of these is -/// compiled as constant data into the executable for each function. -/// -/// Storage of metadata values is elided if the %metadata parameter to -/// @llvm.gcroot is null. -struct FrameMap { - int32_t NumRoots; //< Number of roots in stack frame. - int32_t NumMeta; //< Number of metadata entries. May be < NumRoots. - const void *Meta[0]; //< Metadata for each root. -}; - -/// @brief A link in the dynamic shadow stack. One of these is embedded in the -/// stack frame of each function on the call stack. -struct StackEntry { - StackEntry *Next; //< Link to next stack entry (the caller's). - const FrameMap *Map; //< Pointer to constant FrameMap. - void *Roots[0]; //< Stack roots (in-place array). -}; - -/// @brief The head of the singly-linked list of StackEntries. Functions push -/// and pop onto this in their prologue and epilogue. -/// -/// Since there is only a global list, this technique is not threadsafe. -StackEntry *llvm_gc_root_chain; - -/// @brief Calls Visitor(root, meta) for each GC root on the stack. -/// root and meta are exactly the values passed to -/// <tt>@llvm.gcroot</tt>. -/// -/// Visitor could be a function to recursively mark live objects. Or it -/// might copy them to another heap or generation. -/// -/// @param Visitor A function to invoke for every GC root on the stack. -void visitGCRoots(void (*Visitor)(void **Root, const void *Meta)) { - for (StackEntry *R = llvm_gc_root_chain; R; R = R->Next) { - unsigned i = 0; - - // For roots [0, NumMeta), the metadata pointer is in the FrameMap. - for (unsigned e = R->Map->NumMeta; i != e; ++i) - Visitor(&R->Roots[i], R->Map->Meta[i]); - - // For roots [NumMeta, NumRoots), the metadata pointer is null. - for (unsigned e = R->Map->NumRoots; i != e; ++i) - Visitor(&R->Roots[i], NULL); - } -}</pre> - -</div> - -<!-- ======================================================================= --> -<h3> - <a name="shadow-stack">About the shadow stack</a> -</h3> - -<div> - -<p>Unlike many GC algorithms which rely on a cooperative code generator to -compile stack maps, this algorithm carefully maintains a linked list of stack -roots [<a href="#henderson02">Henderson2002</a>]. This so-called "shadow stack" -mirrors the machine stack. Maintaining this data structure is slower than using -a stack map compiled into the executable as constant data, but has a significant -portability advantage because it requires no special support from the target -code generator, and does not require tricky platform-specific code to crawl -the machine stack.</p> - -<p>The tradeoff for this simplicity and portability is:</p> - -<ul> - <li>High overhead per function call.</li> - <li>Not thread-safe.</li> -</ul> - -<p>Still, it's an easy way to get started. After your compiler and runtime are -up and running, writing a <a href="#plugin">plugin</a> will allow you to take -advantage of <a href="#collector-algos">more advanced GC features</a> of LLVM -in order to improve performance.</p> - -</div> - -</div> - -<!-- *********************************************************************** --> -<h2> - <a name="core">IR features</a><a name="intrinsics"></a> -</h2> -<!-- *********************************************************************** --> - -<div> - -<p>This section describes the garbage collection facilities provided by the -<a href="LangRef.html">LLVM intermediate representation</a>. The exact behavior -of these IR features is specified by the binary interface implemented by a -<a href="#plugin">code generation plugin</a>, not by this document.</p> - -<p>These facilities are limited to those strictly necessary; they are not -intended to be a complete interface to any garbage collector. A program will -need to interface with the GC library using the facilities provided by that -program.</p> - -<!-- ======================================================================= --> -<h3> - <a name="gcattr">Specifying GC code generation: <tt>gc "..."</tt></a> -</h3> - -<div> - -<div class="doc_code"><tt> - define <i>ty</i> @<i>name</i>(...) <span style="text-decoration: underline">gc "<i>name</i>"</span> { ... -</tt></div> - -<p>The <tt>gc</tt> function attribute is used to specify the desired GC style -to the compiler. Its programmatic equivalent is the <tt>setGC</tt> method of -<tt>Function</tt>.</p> - -<p>Setting <tt>gc "<i>name</i>"</tt> on a function triggers a search for a -matching code generation plugin "<i>name</i>"; it is that plugin which defines -the exact nature of the code generated to support GC. If none is found, the -compiler will raise an error.</p> - -<p>Specifying the GC style on a per-function basis allows LLVM to link together -programs that use different garbage collection algorithms (or none at all).</p> - -</div> - -<!-- ======================================================================= --> -<h3> - <a name="gcroot">Identifying GC roots on the stack: <tt>llvm.gcroot</tt></a> -</h3> - -<div> - -<div class="doc_code"><tt> - void @llvm.gcroot(i8** %ptrloc, i8* %metadata) -</tt></div> - -<p>The <tt>llvm.gcroot</tt> intrinsic is used to inform LLVM that a stack -variable references an object on the heap and is to be tracked for garbage -collection. The exact impact on generated code is specified by a <a -href="#plugin">compiler plugin</a>. All calls to <tt>llvm.gcroot</tt> <b>must</b> reside - inside the first basic block.</p> - -<p>A compiler which uses mem2reg to raise imperative code using <tt>alloca</tt> -into SSA form need only add a call to <tt>@llvm.gcroot</tt> for those variables -which a pointers into the GC heap.</p> - -<p>It is also important to mark intermediate values with <tt>llvm.gcroot</tt>. -For example, consider <tt>h(f(), g())</tt>. Beware leaking the result of -<tt>f()</tt> in the case that <tt>g()</tt> triggers a collection. Note, that -stack variables must be initialized and marked with <tt>llvm.gcroot</tt> in -function's prologue.</p> - -<p>The first argument <b>must</b> be a value referring to an alloca instruction -or a bitcast of an alloca. The second contains a pointer to metadata that -should be associated with the pointer, and <b>must</b> be a constant or global -value address. If your target collector uses tags, use a null pointer for -metadata.</p> - -<p>The <tt>%metadata</tt> argument can be used to avoid requiring heap objects -to have 'isa' pointers or tag bits. [<a href="#appel89">Appel89</a>, <a -href="#goldberg91">Goldberg91</a>, <a href="#tolmach94">Tolmach94</a>] If -specified, its value will be tracked along with the location of the pointer in -the stack frame.</p> - -<p>Consider the following fragment of Java code:</p> - -<pre class="doc_code"> - { - Object X; // A null-initialized reference to an object - ... - } -</pre> - -<p>This block (which may be located in the middle of a function or in a loop -nest), could be compiled to this LLVM code:</p> - -<pre class="doc_code"> -Entry: - ;; In the entry block for the function, allocate the - ;; stack space for X, which is an LLVM pointer. - %X = alloca %Object* - - ;; Tell LLVM that the stack space is a stack root. - ;; Java has type-tags on objects, so we pass null as metadata. - %tmp = bitcast %Object** %X to i8** - call void @llvm.gcroot(i8** %tmp, i8* null) - ... - - ;; "CodeBlock" is the block corresponding to the start - ;; of the scope above. -CodeBlock: - ;; 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> - -<!-- ======================================================================= --> -<h3> - <a name="barriers">Reading and writing references in the heap</a> -</h3> - -<div> - -<p>Some collectors need to be informed when the mutator (the program that needs -garbage collection) either reads a pointer from or writes a pointer to a field -of a heap object. The code fragments inserted at these points are called -<em>read barriers</em> and <em>write barriers</em>, respectively. The amount of -code that needs to be executed is usually quite small and not on the critical -path of any computation, so the overall performance impact of the barrier is -tolerable.</p> - -<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>LLVM does not enforce this relationship between the object and derived -pointer (although a <a href="#plugin">plugin</a> might). However, it would be -an unusual collector that violated it.</p> - -<p>The use of these intrinsics is naturally optional if the target GC does -require the corresponding barrier. Such a GC plugin will replace the intrinsic -calls with the corresponding <tt>load</tt> or <tt>store</tt> instruction if they -are used.</p> - -<!-- ======================================================================= --> -<h4> - <a name="gcwrite">Write barrier: <tt>llvm.gcwrite</tt></a> -</h4> - -<div> - -<div class="doc_code"><tt> -void @llvm.gcwrite(i8* %value, i8* %object, i8** %derived) -</tt></div> - -<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> - -<!-- ======================================================================= --> -<h4> - <a name="gcread">Read barrier: <tt>llvm.gcread</tt></a> -</h4> - -<div> - -<div class="doc_code"><tt> -i8* @llvm.gcread(i8* %object, i8** %derived)<br> -</tt></div> - -<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> - -</div> - -<!-- *********************************************************************** --> -<h2> - <a name="plugin">Implementing a collector plugin</a> -</h2> -<!-- *********************************************************************** --> - -<div> - -<p>User code specifies which GC code generation to use with the <tt>gc</tt> -function attribute or, equivalently, with the <tt>setGC</tt> method of -<tt>Function</tt>.</p> - -<p>To implement a GC plugin, it is necessary to subclass -<tt>llvm::GCStrategy</tt>, which can be accomplished in a few lines of -boilerplate code. LLVM's infrastructure provides access to several important -algorithms. For an uncontroversial collector, all that remains may be to -compile LLVM's computed stack map to assembly code (using the binary -representation expected by the runtime library). This can be accomplished in -about 100 lines of code.</p> - -<p>This is not the appropriate place to implement a garbage collected heap or a -garbage collector itself. That code should exist in the language's runtime -library. The compiler plugin is responsible for generating code which -conforms to the binary interface defined by library, most essentially the -<a href="#stack-map">stack map</a>.</p> - -<p>To subclass <tt>llvm::GCStrategy</tt> and register it with the compiler:</p> - -<blockquote><pre>// lib/MyGC/MyGC.cpp - Example LLVM GC plugin - -#include "llvm/CodeGen/GCStrategy.h" -#include "llvm/CodeGen/GCMetadata.h" -#include "llvm/Support/Compiler.h" - -using namespace llvm; - -namespace { - class LLVM_LIBRARY_VISIBILITY MyGC : public GCStrategy { - public: - MyGC() {} - }; - - GCRegistry::Add<MyGC> - X("mygc", "My bespoke garbage collector."); -}</pre></blockquote> - -<p>This boilerplate collector does nothing. More specifically:</p> - -<ul> - <li><tt>llvm.gcread</tt> calls are replaced with the corresponding - <tt>load</tt> instruction.</li> - <li><tt>llvm.gcwrite</tt> calls are replaced with the corresponding - <tt>store</tt> instruction.</li> - <li>No safe points are added to the code.</li> - <li>The stack map is not compiled into the executable.</li> -</ul> - -<p>Using the LLVM makefiles (like the <a -href="http://llvm.org/viewvc/llvm-project/llvm/trunk/projects/sample/">sample -project</a>), this code can be compiled as a plugin using a simple -makefile:</p> - -<blockquote><pre -># lib/MyGC/Makefile - -LEVEL := ../.. -LIBRARYNAME = <var>MyGC</var> -LOADABLE_MODULE = 1 - -include $(LEVEL)/Makefile.common</pre></blockquote> - -<p>Once the plugin is compiled, code using it may be compiled using <tt>llc --load=<var>MyGC.so</var></tt> (though <var>MyGC.so</var> may have some other -platform-specific extension):</p> - -<blockquote><pre ->$ cat sample.ll -define void @f() gc "mygc" { -entry: - ret void -} -$ llvm-as < sample.ll | llc -load=MyGC.so</pre></blockquote> - -<p>It is also possible to statically link the collector plugin into tools, such -as a language-specific compiler front-end.</p> - -<!-- ======================================================================= --> -<h3> - <a name="collector-algos">Overview of available features</a> -</h3> - -<div> - -<p><tt>GCStrategy</tt> provides a range of features through which a plugin -may do useful work. Some of these are callbacks, some are algorithms that can -be enabled, disabled, or customized. This matrix summarizes the supported (and -planned) features and correlates them with the collection techniques which -typically require them.</p> - -<table> - <tr> - <th>Algorithm</th> - <th>Done</th> - <th>shadow stack</th> - <th>refcount</th> - <th>mark-sweep</th> - <th>copying</th> - <th>incremental</th> - <th>threaded</th> - <th>concurrent</th> - </tr> - <tr> - <th class="rowhead"><a href="#stack-map">stack map</a></th> - <td>✔</td> - <td></td> - <td></td> - <td>✘</td> - <td>✘</td> - <td>✘</td> - <td>✘</td> - <td>✘</td> - </tr> - <tr> - <th class="rowhead"><a href="#init-roots">initialize roots</a></th> - <td>✔</td> - <td>✘</td> - <td>✘</td> - <td>✘</td> - <td>✘</td> - <td>✘</td> - <td>✘</td> - <td>✘</td> - </tr> - <tr class="doc_warning"> - <th class="rowhead">derived pointers</th> - <td>NO</td> - <td></td> - <td></td> - <td></td> - <td></td> - <td></td> - <td>✘*</td> - <td>✘*</td> - </tr> - <tr> - <th class="rowhead"><em><a href="#custom">custom lowering</a></em></th> - <td>✔</td> - <th></th> - <th></th> - <th></th> - <th></th> - <th></th> - <th></th> - <th></th> - </tr> - <tr> - <th class="rowhead indent">gcroot</th> - <td>✔</td> - <td>✘</td> - <td>✘</td> - <td></td> - <td></td> - <td></td> - <td></td> - <td></td> - </tr> - <tr> - <th class="rowhead indent">gcwrite</th> - <td>✔</td> - <td></td> - <td>✘</td> - <td></td> - <td></td> - <td>✘</td> - <td></td> - <td>✘</td> - </tr> - <tr> - <th class="rowhead indent">gcread</th> - <td>✔</td> - <td></td> - <td></td> - <td></td> - <td></td> - <td></td> - <td></td> - <td>✘</td> - </tr> - <tr> - <th class="rowhead"><em><a href="#safe-points">safe points</a></em></th> - <td></td> - <th></th> - <th></th> - <th></th> - <th></th> - <th></th> - <th></th> - <th></th> - </tr> - <tr> - <th class="rowhead indent">in calls</th> - <td>✔</td> - <td></td> - <td></td> - <td>✘</td> - <td>✘</td> - <td>✘</td> - <td>✘</td> - <td>✘</td> - </tr> - <tr> - <th class="rowhead indent">before calls</th> - <td>✔</td> - <td></td> - <td></td> - <td></td> - <td></td> - <td></td> - <td>✘</td> - <td>✘</td> - </tr> - <tr class="doc_warning"> - <th class="rowhead indent">for loops</th> - <td>NO</td> - <td></td> - <td></td> - <td></td> - <td></td> - <td></td> - <td>✘</td> - <td>✘</td> - </tr> - <tr> - <th class="rowhead indent">before escape</th> - <td>✔</td> - <td></td> - <td></td> - <td></td> - <td></td> - <td></td> - <td>✘</td> - <td>✘</td> - </tr> - <tr class="doc_warning"> - <th class="rowhead">emit code at safe points</th> - <td>NO</td> - <td></td> - <td></td> - <td></td> - <td></td> - <td></td> - <td>✘</td> - <td>✘</td> - </tr> - <tr> - <th class="rowhead"><em>output</em></th> - <td></td> - <th></th> - <th></th> - <th></th> - <th></th> - <th></th> - <th></th> - <th></th> - </tr> - <tr> - <th class="rowhead indent"><a href="#assembly">assembly</a></th> - <td>✔</td> - <td></td> - <td></td> - <td>✘</td> - <td>✘</td> - <td>✘</td> - <td>✘</td> - <td>✘</td> - </tr> - <tr class="doc_warning"> - <th class="rowhead indent">JIT</th> - <td>NO</td> - <td></td> - <td></td> - <td class="optl">✘</td> - <td class="optl">✘</td> - <td class="optl">✘</td> - <td class="optl">✘</td> - <td class="optl">✘</td> - </tr> - <tr class="doc_warning"> - <th class="rowhead indent">obj</th> - <td>NO</td> - <td></td> - <td></td> - <td class="optl">✘</td> - <td class="optl">✘</td> - <td class="optl">✘</td> - <td class="optl">✘</td> - <td class="optl">✘</td> - </tr> - <tr class="doc_warning"> - <th class="rowhead">live analysis</th> - <td>NO</td> - <td></td> - <td></td> - <td class="optl">✘</td> - <td class="optl">✘</td> - <td class="optl">✘</td> - <td class="optl">✘</td> - <td class="optl">✘</td> - </tr> - <tr class="doc_warning"> - <th class="rowhead">register map</th> - <td>NO</td> - <td></td> - <td></td> - <td class="optl">✘</td> - <td class="optl">✘</td> - <td class="optl">✘</td> - <td class="optl">✘</td> - <td class="optl">✘</td> - </tr> - <tr> - <td colspan="10"> - <div><span class="doc_warning">*</span> Derived pointers only pose a - hazard to copying collectors.</div> - <div><span class="optl">✘</span> in gray denotes a feature which - could be utilized if available.</div> - </td> - </tr> -</table> - -<p>To be clear, the collection techniques above are defined as:</p> - -<dl> - <dt>Shadow Stack</dt> - <dd>The mutator carefully maintains a linked list of stack roots.</dd> - <dt>Reference Counting</dt> - <dd>The mutator maintains a reference count for each object and frees an - object when its count falls to zero.</dd> - <dt>Mark-Sweep</dt> - <dd>When the heap is exhausted, the collector marks reachable objects starting - from the roots, then deallocates unreachable objects in a sweep - phase.</dd> - <dt>Copying</dt> - <dd>As reachability analysis proceeds, the collector copies objects from one - heap area to another, compacting them in the process. Copying collectors - enable highly efficient "bump pointer" allocation and can improve locality - of reference.</dd> - <dt>Incremental</dt> - <dd>(Including generational collectors.) Incremental collectors generally have - all the properties of a copying collector (regardless of whether the - mature heap is compacting), but bring the added complexity of requiring - write barriers.</dd> - <dt>Threaded</dt> - <dd>Denotes a multithreaded mutator; the collector must still stop the mutator - ("stop the world") before beginning reachability analysis. Stopping a - multithreaded mutator is a complicated problem. It generally requires - highly platform specific code in the runtime, and the production of - carefully designed machine code at safe points.</dd> - <dt>Concurrent</dt> - <dd>In this technique, the mutator and the collector run concurrently, with - the goal of eliminating pause times. In a <em>cooperative</em> collector, - the mutator further aids with collection should a pause occur, allowing - collection to take advantage of multiprocessor hosts. The "stop the world" - problem of threaded collectors is generally still present to a limited - extent. Sophisticated marking algorithms are necessary. Read barriers may - be necessary.</dd> -</dl> - -<p>As the matrix indicates, LLVM's garbage collection infrastructure is already -suitable for a wide variety of collectors, but does not currently extend to -multithreaded programs. This will be added in the future as there is -interest.</p> - -</div> - -<!-- ======================================================================= --> -<h3> - <a name="stack-map">Computing stack maps</a> -</h3> - -<div> - -<p>LLVM automatically computes a stack map. One of the most important features -of a <tt>GCStrategy</tt> is to compile this information into the executable in -the binary representation expected by the runtime library.</p> - -<p>The stack map consists of the location and identity of each GC root in the -each function in the module. For each root:</p> - -<ul> - <li><tt>RootNum</tt>: The index of the root.</li> - <li><tt>StackOffset</tt>: The offset of the object relative to the frame - pointer.</li> - <li><tt>RootMetadata</tt>: The value passed as the <tt>%metadata</tt> - parameter to the <a href="#gcroot"><tt>@llvm.gcroot</tt></a> intrinsic.</li> -</ul> - -<p>Also, for the function as a whole:</p> - -<ul> - <li><tt>getFrameSize()</tt>: The overall size of the function's initial - stack frame, not accounting for any dynamic allocation.</li> - <li><tt>roots_size()</tt>: The count of roots in the function.</li> -</ul> - -<p>To access the stack map, use <tt>GCFunctionMetadata::roots_begin()</tt> and --<tt>end()</tt> from the <tt><a -href="#assembly">GCMetadataPrinter</a></tt>:</p> - -<blockquote><pre ->for (iterator I = begin(), E = end(); I != E; ++I) { - GCFunctionInfo *FI = *I; - unsigned FrameSize = FI->getFrameSize(); - size_t RootCount = FI->roots_size(); - - for (GCFunctionInfo::roots_iterator RI = FI->roots_begin(), - RE = FI->roots_end(); - RI != RE; ++RI) { - int RootNum = RI->Num; - int RootStackOffset = RI->StackOffset; - Constant *RootMetadata = RI->Metadata; - } -}</pre></blockquote> - -<p>If the <tt>llvm.gcroot</tt> intrinsic is eliminated before code generation by -a custom lowering pass, LLVM will compute an empty stack map. This may be useful -for collector plugins which implement reference counting or a shadow stack.</p> - -</div> - - -<!-- ======================================================================= --> -<h3> - <a name="init-roots">Initializing roots to null: <tt>InitRoots</tt></a> -</h3> - -<div> - -<blockquote><pre ->MyGC::MyGC() { - InitRoots = true; -}</pre></blockquote> - -<p>When set, LLVM will automatically initialize each root to <tt>null</tt> upon -entry to the function. This prevents the GC's sweep phase from visiting -uninitialized pointers, which will almost certainly cause it to crash. This -initialization occurs before custom lowering, so the two may be used -together.</p> - -<p>Since LLVM does not yet compute liveness information, there is no means of -distinguishing an uninitialized stack root from an initialized one. Therefore, -this feature should be used by all GC plugins. It is enabled by default.</p> - -</div> - - -<!-- ======================================================================= --> -<h3> - <a name="custom">Custom lowering of intrinsics: <tt>CustomRoots</tt>, - <tt>CustomReadBarriers</tt>, and <tt>CustomWriteBarriers</tt></a> -</h3> - -<div> - -<p>For GCs which use barriers or unusual treatment of stack roots, these -flags allow the collector to perform arbitrary transformations of the LLVM -IR:</p> - -<blockquote><pre ->class MyGC : public GCStrategy { -public: - MyGC() { - CustomRoots = true; - CustomReadBarriers = true; - CustomWriteBarriers = true; - } - - virtual bool initializeCustomLowering(Module &M); - virtual bool performCustomLowering(Function &F); -};</pre></blockquote> - -<p>If any of these flags are set, then LLVM suppresses its default lowering for -the corresponding intrinsics and instead calls -<tt>performCustomLowering</tt>.</p> - -<p>LLVM's default action for each intrinsic is as follows:</p> - -<ul> - <li><tt>llvm.gcroot</tt>: Leave it alone. The code generator must see it - or the stack map will not be computed.</li> - <li><tt>llvm.gcread</tt>: Substitute a <tt>load</tt> instruction.</li> - <li><tt>llvm.gcwrite</tt>: Substitute a <tt>store</tt> instruction.</li> -</ul> - -<p>If <tt>CustomReadBarriers</tt> or <tt>CustomWriteBarriers</tt> are specified, -then <tt>performCustomLowering</tt> <strong>must</strong> eliminate the -corresponding barriers.</p> - -<p><tt>performCustomLowering</tt> must comply with the same restrictions as <a -href="WritingAnLLVMPass.html#runOnFunction"><tt ->FunctionPass::runOnFunction</tt></a>. -Likewise, <tt>initializeCustomLowering</tt> has the same semantics as <a -href="WritingAnLLVMPass.html#doInitialization_mod"><tt ->Pass::doInitialization(Module&)</tt></a>.</p> - -<p>The following can be used as a template:</p> - -<blockquote><pre ->#include "llvm/Module.h" -#include "llvm/IntrinsicInst.h" - -bool MyGC::initializeCustomLowering(Module &M) { - return false; -} - -bool MyGC::performCustomLowering(Function &F) { - bool MadeChange = false; - - for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) - for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ) - if (IntrinsicInst *CI = dyn_cast<IntrinsicInst>(II++)) - if (Function *F = CI->getCalledFunction()) - switch (F->getIntrinsicID()) { - case Intrinsic::gcwrite: - // Handle llvm.gcwrite. - CI->eraseFromParent(); - MadeChange = true; - break; - case Intrinsic::gcread: - // Handle llvm.gcread. - CI->eraseFromParent(); - MadeChange = true; - break; - case Intrinsic::gcroot: - // Handle llvm.gcroot. - CI->eraseFromParent(); - MadeChange = true; - break; - } - - return MadeChange; -}</pre></blockquote> - -</div> - - -<!-- ======================================================================= --> -<h3> - <a name="safe-points">Generating safe points: <tt>NeededSafePoints</tt></a> -</h3> - -<div> - -<p>LLVM can compute four kinds of safe points:</p> - -<blockquote><pre ->namespace GC { - /// PointKind - The type of a collector-safe point. - /// - enum PointKind { - Loop, //< Instr is a loop (backwards branch). - Return, //< Instr is a return instruction. - PreCall, //< Instr is a call instruction. - PostCall //< Instr is the return address of a call. - }; -}</pre></blockquote> - -<p>A collector can request any combination of the four by setting the -<tt>NeededSafePoints</tt> mask:</p> - -<blockquote><pre ->MyGC::MyGC() { - NeededSafePoints = 1 << GC::Loop - | 1 << GC::Return - | 1 << GC::PreCall - | 1 << GC::PostCall; -}</pre></blockquote> - -<p>It can then use the following routines to access safe points.</p> - -<blockquote><pre ->for (iterator I = begin(), E = end(); I != E; ++I) { - GCFunctionInfo *MD = *I; - size_t PointCount = MD->size(); - - for (GCFunctionInfo::iterator PI = MD->begin(), - PE = MD->end(); PI != PE; ++PI) { - GC::PointKind PointKind = PI->Kind; - unsigned PointNum = PI->Num; - } -} -</pre></blockquote> - -<p>Almost every collector requires <tt>PostCall</tt> safe points, since these -correspond to the moments when the function is suspended during a call to a -subroutine.</p> - -<p>Threaded programs generally require <tt>Loop</tt> safe points to guarantee -that the application will reach a safe point within a bounded amount of time, -even if it is executing a long-running loop which contains no function -calls.</p> - -<p>Threaded collectors may also require <tt>Return</tt> and <tt>PreCall</tt> -safe points to implement "stop the world" techniques using self-modifying code, -where it is important that the program not exit the function without reaching a -safe point (because only the topmost function has been patched).</p> - -</div> - - -<!-- ======================================================================= --> -<h3> - <a name="assembly">Emitting assembly code: <tt>GCMetadataPrinter</tt></a> -</h3> - -<div> - -<p>LLVM allows a plugin to print arbitrary assembly code before and after the -rest of a module's assembly code. At the end of the module, the GC can compile -the LLVM stack map into assembly code. (At the beginning, this information is not -yet computed.)</p> - -<p>Since AsmWriter and CodeGen are separate components of LLVM, a separate -abstract base class and registry is provided for printing assembly code, the -<tt>GCMetadaPrinter</tt> and <tt>GCMetadataPrinterRegistry</tt>. The AsmWriter -will look for such a subclass if the <tt>GCStrategy</tt> sets -<tt>UsesMetadata</tt>:</p> - -<blockquote><pre ->MyGC::MyGC() { - UsesMetadata = true; -}</pre></blockquote> - -<p>This separation allows JIT-only clients to be smaller.</p> - -<p>Note that LLVM does not currently have analogous APIs to support code -generation in the JIT, nor using the object writers.</p> - -<blockquote><pre ->// lib/MyGC/MyGCPrinter.cpp - Example LLVM GC printer - -#include "llvm/CodeGen/GCMetadataPrinter.h" -#include "llvm/Support/Compiler.h" - -using namespace llvm; - -namespace { - class LLVM_LIBRARY_VISIBILITY MyGCPrinter : public GCMetadataPrinter { - public: - virtual void beginAssembly(std::ostream &OS, AsmPrinter &AP, - const TargetAsmInfo &TAI); - - virtual void finishAssembly(std::ostream &OS, AsmPrinter &AP, - const TargetAsmInfo &TAI); - }; - - GCMetadataPrinterRegistry::Add<MyGCPrinter> - X("mygc", "My bespoke garbage collector."); -}</pre></blockquote> - -<p>The collector should use <tt>AsmPrinter</tt> and <tt>TargetAsmInfo</tt> to -print portable assembly code to the <tt>std::ostream</tt>. The collector itself -contains the stack map for the entire module, and may access the -<tt>GCFunctionInfo</tt> using its own <tt>begin()</tt> and <tt>end()</tt> -methods. Here's a realistic example:</p> - -<blockquote><pre ->#include "llvm/CodeGen/AsmPrinter.h" -#include "llvm/Function.h" -#include "llvm/Target/TargetMachine.h" -#include "llvm/DataLayout.h" -#include "llvm/Target/TargetAsmInfo.h" - -void MyGCPrinter::beginAssembly(std::ostream &OS, AsmPrinter &AP, - const TargetAsmInfo &TAI) { - // Nothing to do. -} - -void MyGCPrinter::finishAssembly(std::ostream &OS, AsmPrinter &AP, - const TargetAsmInfo &TAI) { - // Set up for emitting addresses. - const char *AddressDirective; - int AddressAlignLog; - if (AP.TM.getDataLayout()->getPointerSize() == sizeof(int32_t)) { - AddressDirective = TAI.getData32bitsDirective(); - AddressAlignLog = 2; - } else { - AddressDirective = TAI.getData64bitsDirective(); - AddressAlignLog = 3; - } - - // Put this in the data section. - AP.SwitchToDataSection(TAI.getDataSection()); - - // For each function... - for (iterator FI = begin(), FE = end(); FI != FE; ++FI) { - GCFunctionInfo &MD = **FI; - - // Emit this data structure: - // - // struct { - // int32_t PointCount; - // struct { - // void *SafePointAddress; - // int32_t LiveCount; - // int32_t LiveOffsets[LiveCount]; - // } Points[PointCount]; - // } __gcmap_<FUNCTIONNAME>; - - // Align to address width. - AP.EmitAlignment(AddressAlignLog); - - // Emit the symbol by which the stack map entry can be found. - std::string Symbol; - Symbol += TAI.getGlobalPrefix(); - Symbol += "__gcmap_"; - Symbol += MD.getFunction().getName(); - if (const char *GlobalDirective = TAI.getGlobalDirective()) - OS << GlobalDirective << Symbol << "\n"; - OS << TAI.getGlobalPrefix() << Symbol << ":\n"; - - // Emit PointCount. - AP.EmitInt32(MD.size()); - AP.EOL("safe point count"); - - // And each safe point... - for (GCFunctionInfo::iterator PI = MD.begin(), - PE = MD.end(); PI != PE; ++PI) { - // Align to address width. - AP.EmitAlignment(AddressAlignLog); - - // Emit the address of the safe point. - OS << AddressDirective - << TAI.getPrivateGlobalPrefix() << "label" << PI->Num; - AP.EOL("safe point address"); - - // Emit the stack frame size. - AP.EmitInt32(MD.getFrameSize()); - AP.EOL("stack frame size"); - - // Emit the number of live roots in the function. - AP.EmitInt32(MD.live_size(PI)); - AP.EOL("live root count"); - - // And for each live root... - for (GCFunctionInfo::live_iterator LI = MD.live_begin(PI), - LE = MD.live_end(PI); - LI != LE; ++LI) { - // Print its offset within the stack frame. - AP.EmitInt32(LI->StackOffset); - AP.EOL("stack offset"); - } - } - } -} -</pre></blockquote> - -</div> - -</div> - -<!-- *********************************************************************** --> -<h2> - <a name="references">References</a> -</h2> -<!-- *********************************************************************** --> - -<div> - -<p><a name="appel89">[Appel89]</a> Runtime Tags Aren't Necessary. Andrew -W. Appel. Lisp and Symbolic Computation 19(7):703-705, July 1989.</p> - -<p><a name="goldberg91">[Goldberg91]</a> Tag-free garbage collection for -strongly typed programming languages. Benjamin Goldberg. ACM SIGPLAN -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 - src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a> - <a href="http://validator.w3.org/check/referer"><img - 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> - -</body> -</html> |