<!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="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>.</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.</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** %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> | |
<!-- ======================================================================= --> | |
<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/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 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: 2011-08-12 02:17:17 -0400 (Fri, 12 Aug 2011) $ | |
</address> | |
</body> | |
</html> |