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<!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>

<div class="doc_title">
  Accurate Garbage Collection with LLVM
</div>

<ol>
  <li><a href="#introduction">Introduction</a>
    <ul>
    <li><a href="#feature">GC features provided and algorithms
      supported</a></li>
    </ul>
  </li>

  <li><a href="#usage">Using the collectors</a>
    <ul>
    <li><a href="#shadow-stack">ShadowStack -
      A highly portable collector</a></li>
    <li><a href="#semispace">SemiSpace -
      A simple copying collector runtime</a></li>
    <li><a href="#ocaml">Ocaml -
      An Objective Caml-compatible collector</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="#runtime">Recommended runtime interface</a>
    <ul>
    <li><a href="#initialize">Garbage collector startup and
    initialization</a></li>
    <li><a href="#allocate">Allocating memory from the GC</a></li>
    <li><a href="#explicit">Explicit invocation of the garbage
    collector</a></li>
    <li><a href="#traceroots">Tracing GC pointers from the program
    stack</a></li>
    <li><a href="#staticroots">Tracing GC pointers from static roots</a></li>
    </ul>
  </li>

  <li><a href="#plugin">Implementing a collector plugin</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>beginAssembly</tt> and <tt>finishAssembly</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>

<!-- *********************************************************************** -->
<div class="doc_section">
  <a name="introduction">Introduction</a>
</div>
<!-- *********************************************************************** -->

<div class="doc_text">

<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 allocation
techniques dominates any low-level losses.</p>

<p>This document describes the mechanisms and interfaces provided by LLVM to
support accurate garbage collection.</p>

</div>

<!-- ======================================================================= -->
<div class="doc_subsection">
  <a name="feature">GC features provided and algorithms supported</a>
</div>

<div class="doc_text">

<p>LLVM's intermediate representation provides <a href="#intrinsics">garbage
collection intrinsics</a> which 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 implement a garbage collector. This is because
collectors are tightly coupled to object models, and LLVM is agnostic to object
models. Since LLVM is agnostic to object models, it would be inappropriate for
LLVM to dictate any particular collector. Instead, LLVM provides a framework for
garbage collector implementations in two manners:</p>

<ul>
  <li><b>At compile time</b> with <a href="#plugin">collector plugins</a> for
  the compiler. Collector plugins have ready access to important garbage
  collector algorithms. Leveraging these tools, it is straightforward to
  emit type-accurate stack maps for your runtime in as little as ~100 lines of
  C++ code.</li>

  <li><b>At runtime</b> with <a href="#runtime">suggested runtime
  interfaces</a>, which allow front-end compilers to support a range of
  collection runtimes.</li>
</ul>

</div>

<!-- *********************************************************************** -->
<div class="doc_section">
  <a name="usage">Using the collectors</a>
</div>
<!-- *********************************************************************** -->

<div class="doc_text">

<p>In general, using a collector implies:</p>

<ul>
  <li>Emitting compatible code, including initialization in the main
      program if necessary.</li>
  <li>Loading a compiler plugin if the collector is not statically linked with
      your compiler. For <tt>llc</tt>, use the <tt>-load</tt> option.</li>
  <li>Selecting the collection algorithm by applying the <tt>gc "..."</tt> 
      attribute to your garbage collected functions, or equivalently with
      the <tt>setCollector</tt> method.</li>
  <li>Linking your final executable with the garbage collector runtime.</li>
</ul>

<p>This table summarizes the available runtimes.</p>

<table>
  <tr>
    <th>Collector</th>
    <th><tt>gc</tt> attribute</th>
    <th>Linkage</th>
    <th><tt>gcroot</tt></th>
    <th><tt>gcread</tt></th>
    <th><tt>gcwrite</tt></th>
  </tr>
  <tr valign="baseline">
    <td><a href="#semispace">SemiSpace</a></td>
    <td><tt>gc "shadow-stack"</tt></td>
    <td>TODO FIXME</td>
    <td>required</td>
    <td>optional</td>
    <td>optional</td>
  </tr>
  <tr valign="baseline">
    <td><a href="#ocaml">Ocaml</a></td>
    <td><tt>gc "ocaml"</tt></td>
    <td><i>provided by ocamlopt</i></td>
    <td>required</td>
    <td>optional</td>
    <td>optional</td>
  </tr>
</table>

<p>The sections for <a href="#intrinsics">Collection intrinsics</a> and
<a href="#runtime">Recommended runtime interface</a> detail the interfaces that
collectors may require user programs to utilize.</p>

</div>

<!-- ======================================================================= -->
<div class="doc_subsection">
  <a name="shadow-stack">ShadowStack - A highly portable collector</a>
</div>

<div class="doc_code"><tt>
  Collector *llvm::createShadowStackCollector();
</tt></div>

<div class="doc_text">

<p>The ShadowStack backend is invoked with the <tt>gc "shadow-stack"</tt>
function attribute.
Unlike many collectors which rely on a cooperative code generator to generate
stack maps, this algorithm carefully maintains a linked list of stack root
descriptors [<a href="#henderson02">Henderson2002</a>]. This so-called "shadow
stack" mirrors the machine stack. Maintaining this data structure is slower
than using stack maps, but has a significant portability advantage because it
requires no special support from the target code generator.</p>

<p>The ShadowStack collector does not use read or write barriers, so the user
program may use <tt>load</tt> and <tt>store</tt> instead of <tt>llvm.gcread</tt>
and <tt>llvm.gcwrite</tt>.</p>

<p>ShadowStack is a code generator plugin only. It must be paired with a
compatible runtime.</p>

</div>

<!-- ======================================================================= -->
<div class="doc_subsection">
  <a name="semispace">SemiSpace - A simple copying collector runtime</a>
</div>

<div class="doc_text">

<p>The SemiSpace runtime implements with the <a href="runtime">suggested
runtime interface</a> and is compatible the ShadowStack backend.</p>

<p>SemiSpace is a very simple copying collector. When it starts up, it
allocates two blocks of memory for the heap. It uses a simple bump-pointer
allocator to allocate memory from the first block until it runs out of space.
When it runs out of space, it traces through all of the roots of the program,
copying blocks to the other half of the memory space.</p>

<p>This runtime is highly experimental and has not been used in a real project.
Enhancements would be welcomed.</p>

</div>

<!-- ======================================================================= -->
<div class="doc_subsection">
  <a name="ocaml">Ocaml - An Objective Caml-compatible collector</a>
</div>

<div class="doc_code"><tt>
  Collector *llvm::createOcamlCollector();
</tt></div>

<div class="doc_text">

<p>The ocaml backend is invoked with the <tt>gc "ocaml"</tt> function attribute.
It supports the
<a href="http://caml.inria.fr/">Objective Caml</a> language runtime by emitting
a type-accurate stack map in the form of an ocaml 3.10.0-compatible frametable.
The linkage requirements are satisfied automatically by the <tt>ocamlopt</tt>
compiler when linking an executable.</p>

<p>The ocaml collector does not use read or write barriers, so the user program
may use <tt>load</tt> and <tt>store</tt> instead of <tt>llvm.gcread</tt> and
<tt>llvm.gcwrite</tt>.</p>

</div>


<!-- *********************************************************************** -->
<div class="doc_section">
  <a name="core">Core support</a>
</div>
<!-- *********************************************************************** -->

<div class="doc_text">

<p>This section describes the garbage collection facilities provided by the
<a href="LangRef.html">LLVM intermediate representation</a>.</p>

<p>These facilities are limited to those strictly necessary for compilation.
They are not intended to be a complete interface to any garbage collector.
Notably, heap allocation is not among the supplied primitives. A user program
will also need to interface with the runtime, using either the
<a href="#runtime">suggested runtime interface</a> or another interface
specified by the runtime.</p>

</div>

<!-- ======================================================================= -->
<div class="doc_subsection">
  <a name="gcattr">Specifying GC code generation: <tt>gc "..."</tt></a>
</div>

<div class="doc_code"><tt>
  define <i>ty</i> @<i>name</i>(...) <u>gc "<i>collector</i>"</u> { ...
</tt></div>

<div class="doc_text">

<p>The <tt>gc</tt> function attribute is used to specify the desired collector
algorithm to the compiler. It is equivalent to specify the collector name
programmatically using the <tt>setCollector</tt> method of
<tt>Function</tt>.</p>

<p>Specifying the collector on a per-function basis allows LLVM to link together
programs which use different garbage collection algorithms.</p>

</div>

<!-- ======================================================================= -->
<div class="doc_subsection">
  <a name="gcroot">Identifying GC roots on the stack: <tt>llvm.gcroot</tt></a>
</div>

<div class="doc_code"><tt>
  void %llvm.gcroot(i8** %ptrloc, i8* %metadata)
</tt></div>

<div class="doc_text">

<p>The <tt>llvm.gcroot</tt> intrinsic is used to inform LLVM of a pointer
variable on the stack. The first argument <b>must</b> be 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>Consider the following fragment of Java code:</p>

<pre>
       {
         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>
Entry:
   ;; In the entry block for the function, allocate the
   ;; stack space for X, which is an LLVM pointer.
   %X = alloca %Object*
   
   ;; Tell LLVM that the stack space is a stack root.
   ;; Java has type-tags on objects, so we pass null as metadata.
   %tmp = bitcast %Object** %X to i8**
   call void %llvm.gcroot(%i8** %X, i8* null)
   ...

   ;; "CodeBlock" is the block corresponding to the start
   ;;  of the scope above.
CodeBlock:
   ;; Java null-initializes pointers.
   store %Object* null, %Object** %X

   ...

   ;; As the pointer goes out of scope, store a null value into
   ;; it, to indicate that the value is no longer live.
   store %Object* null, %Object** %X
   ...
</pre>

</div>

<!-- ======================================================================= -->
<div class="doc_subsection">
  <a name="barriers">Reading and writing references in the heap</a>
</div>

<div class="doc_text">

<p>Some collectors need to be informed when the mutator (the program that needs
garbage collection) either reads a pointer from or writes a pointer to a field
of a heap object. The code fragments inserted at these points are called
<em>read barriers</em> and <em>write barriers</em>, respectively. The amount of
code that needs to be executed is usually quite small and not on the critical
path of any computation, so the overall performance impact of the barrier is
tolerable.</p>

<p>Barriers often require access to the <em>object pointer</em> rather than the
<em>derived pointer</em> (which is a pointer to the field within the
object). Accordingly, these intrinsics take both pointers as separate arguments
for completeness. In this snippet, <tt>%object</tt> is the object pointer, and 
<tt>%derived</tt> is the derived pointer:</p>

<blockquote><pre
>    ;; An array type.
    %class.Array = type { %class.Object, i32, [0 x %class.Object*] }
...

    ;; Load the object pointer from a gcroot.
    %object = load %class.Array** %object_addr

    ;; Compute the derived pointer.
    %derived = getelementptr %obj, i32 0, i32 2, i32 %n</pre></blockquote>

</div>

<!-- ======================================================================= -->
<div class="doc_subsubsection">
  <a name="gcwrite">Write barrier: <tt>llvm.gcwrite</tt></a>
</div>

<div class="doc_code"><tt>
void @llvm.gcwrite(i8* %value, i8* %object, i8** %derived)
</tt></div>

<div class="doc_text">

<p>For write barriers, LLVM provides the <tt>llvm.gcwrite</tt> intrinsic
function. It has exactly the same semantics as a non-volatile <tt>store</tt> to
the derived pointer (the third argument).</p>

<p>Many important algorithms require write barriers, including generational
and concurrent collectors. Additionally, write barriers could be used to
implement reference counting.</p>

<p>The use of this intrinsic is optional if the target collector does use
write barriers. If so, the collector will replace it with the corresponding
<tt>store</tt>.</p>

</div>

<!-- ======================================================================= -->
<div class="doc_subsubsection">
  <a name="gcread">Read barrier: <tt>llvm.gcread</tt></a>
</div>

<div class="doc_code"><tt>
i8* @llvm.gcread(i8* %object, i8** %derived)<br>
</tt></div>

<div class="doc_text">

<p>For read barriers, LLVM provides the <tt>llvm.gcread</tt> intrinsic function.
It has exactly the same semantics as a non-volatile <tt>load</tt> from the
derived pointer (the second argument).</p>

<p>Read barriers are needed by fewer algorithms than write barriers, and may
have a greater performance impact since pointer reads are more frequent than
writes.</p>

<p>As with <tt>llvm.gcwrite</tt>, a target collector might not require the use
of this intrinsic.</p>

</div>

<!-- *********************************************************************** -->
<div class="doc_section">
  <a name="runtime">Recommended runtime interface</a>
</div>
<!-- *********************************************************************** -->

<div class="doc_text">

<p>LLVM specifies the following recommended runtime interface to the garbage
collection at runtime. A program should use these interfaces to accomplish the
tasks not supported by the intrinsics.</p>

<p>Unlike the intrinsics, which are integral to LLVM's code generator, there is
nothing unique about these interfaces; a front-end compiler and runtime are free
to agree to a different specification.</p>

<p class="doc_warning">Note: This interface is a work in progress.</p>

</div>

<!-- ======================================================================= -->
<div class="doc_subsection">
  <a name="initialize">Garbage collector startup and initialization</a>
</div>

<div class="doc_text">

<div class="doc_code"><tt>
  void llvm_gc_initialize(unsigned InitialHeapSize);
</tt></div>

<p>
The <tt>llvm_gc_initialize</tt> function should be called once before any other
garbage collection functions are called. This gives the garbage collector the
chance to initialize itself and allocate the heap. The initial heap size to
allocate should be specified as an argument.
</p>

</div>

<!-- ======================================================================= -->
<div class="doc_subsection">
  <a name="allocate">Allocating memory from the GC</a>
</div>

<div class="doc_text">

<div class="doc_code"><tt>
  void *llvm_gc_allocate(unsigned Size);
</tt></div>

<p>The <tt>llvm_gc_allocate</tt> function is a global function defined by the
garbage collector implementation to allocate memory. It returns a
zeroed-out block of memory of the specified size, sufficiently aligned to store
any object.</p>

</div>

<!-- ======================================================================= -->
<div class="doc_subsection">
  <a name="explicit">Explicit invocation of the garbage collector</a>
</div>

<div class="doc_text">

<div class="doc_code"><tt>
  void llvm_gc_collect();
</tt></div>

<p>
The <tt>llvm_gc_collect</tt> function is exported by the garbage collector
implementations to provide a full collection, even when the heap is not
exhausted. This can be used by end-user code as a hint, and may be ignored by
the garbage collector.
</p>

</div>

<!-- ======================================================================= -->
<div class="doc_subsection">
  <a name="traceroots">Tracing GC pointers from the program stack</a>
</div>

<div class="doc_text">
  <div class="doc_code"><tt>
     void llvm_cg_walk_gcroots(void (*FP)(void **Root, void *Meta));
  </tt></div>

<p>
The <tt>llvm_cg_walk_gcroots</tt> function is a function provided by the code
generator that iterates through all of the GC roots on the stack, calling the
specified function pointer with each record. For each GC root, the address of
the pointer and the meta-data (from the <a
href="#roots"><tt>llvm.gcroot</tt></a> intrinsic) are provided.
</p>
</div>

<!-- ======================================================================= -->
<div class="doc_subsection">
  <a name="staticroots">Tracing GC pointers from static roots</a>
</div>

<div class="doc_text">
TODO
</div>


<!-- *********************************************************************** -->
<div class="doc_section">
  <a name="plugin">Implementing a collector plugin</a>
</div>
<!-- *********************************************************************** -->

<div class="doc_text">

<p>User code specifies which collector plugin to use with the <tt>gc</tt>
function attribute or, equivalently, with the <tt>setCollector</tt> method of
<tt>Function</tt>.</p>

<p>To implement a collector plugin, it is necessary to subclass
<tt>llvm::Collector</tt>, which can be accomplished in a few lines of
boilerplate code. LLVM's infrastructure provides access to several important
algorithms. For an uncontroversial collector, all that remains may be to emit
the assembly code for the collector's unique stack map data structure, which
might be accomplished in as few as 100 LOC.</p>

<p>To subclass <tt>llvm::Collector</tt> and register a collector:</p>

<blockquote><pre>// lib/MyGC/MyGC.cpp - Example LLVM collector plugin

#include "llvm/CodeGen/Collector.h"
#include "llvm/CodeGen/Collectors.h"
#include "llvm/CodeGen/CollectorMetadata.h"
#include "llvm/Support/Compiler.h"

using namespace llvm;

namespace {
  class VISIBILITY_HIDDEN MyCollector : public Collector {
  public:
    MyCollector() {}
  };
  
  CollectorRegistry::Add&lt;MyCollector&gt;
  X("mygc", "My bespoke garbage collector.");
}</pre></blockquote>

<p>Using the LLVM makefiles (like the <a
href="http://llvm.org/viewvc/llvm-project/llvm/trunk/projects/sample/">sample
project</a>), this can be built into a plugin using a simple makefile:</p>

<blockquote><pre
># lib/MyGC/Makefile

LEVEL := ../..
LIBRARYNAME = <var>MyGC</var>
LOADABLE_MODULE = 1

include $(LEVEL)/Makefile.common</pre></blockquote>

<p>Once the plugin is compiled, code using it may be compiled using <tt>llc
-load=<var>MyGC.so</var></tt> (though <var>MyGC.so</var> may have some other
platform-specific extension):</p>

<blockquote><pre
>$ cat sample.ll
define void @f() gc "mygc" {
entry:
        ret void
}
$ llvm-as &lt; sample.ll | llc -load=MyGC.so</pre></blockquote>

<p>It is also possible to statically link the collector plugin into tools, such
as a language-specific compiler front-end.</p>

</div>

<!-- ======================================================================= -->
<div class="doc_subsection">
  <a name="collector-algos">Overview of available features</a>
</div>

<div class="doc_text">

<p>The boilerplate collector above does nothing. More specifically:</p>

<ul>
  <li><tt>llvm.gcread</tt> calls are replaced with the corresponding
      <tt>load</tt> instruction.</li>
  <li><tt>llvm.gcwrite</tt> calls are replaced with the corresponding
      <tt>store</tt> instruction.</li>
  <li>No stack map is emitted, and no safe points are added.</li>
</ul>

<p><tt>Collector</tt> provides a range of features through which a plugin
collector may do useful work. This matrix summarizes the supported (and planned)
features and correlates them with the collection techniques which typically
require them.</p>

<table>
  <tr>
    <th>Algorithm</th>
    <th>Done</th>
    <th>shadow stack</th>
    <th>refcount</th>
    <th>mark-sweep</th>
    <th>copying</th>
    <th>incremental</th>
    <th>threaded</th>
    <th>concurrent</th>
  </tr>
  <tr>
    <th class="rowhead"><a href="#stack-map">stack map</a></th>
    <td>&#10004;</td>
    <td></td>
    <td></td>
    <td>&#10008;</td>
    <td>&#10008;</td>
    <td>&#10008;</td>
    <td>&#10008;</td>
    <td>&#10008;</td>
  </tr>
  <tr>
    <th class="rowhead"><a href="#init-roots">initialize roots</a></th>
    <td>&#10004;</td>
    <td>&#10008;</td>
    <td>&#10008;</td>
    <td>&#10008;</td>
    <td>&#10008;</td>
    <td>&#10008;</td>
    <td>&#10008;</td>
    <td>&#10008;</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>&#10008;*</td>
    <td>&#10008;*</td>
  </tr>
  <tr>
    <th class="rowhead"><em><a href="#custom">custom lowering</a></em></th>
    <td>&#10004;</td>
    <th></th>
    <th></th>
    <th></th>
    <th></th>
    <th></th>
    <th></th>
    <th></th>
  </tr>
  <tr>
    <th class="rowhead indent">gcroot</th>
    <td>&#10004;</td>
    <td>&#10008;</td>
    <td>&#10008;</td>
    <td></td>
    <td></td>
    <td></td>
    <td></td>
    <td></td>
  </tr>
  <tr>
    <th class="rowhead indent">gcwrite</th>
    <td>&#10004;</td>
    <td></td>
    <td>&#10008;</td>
    <td></td>
    <td></td>
    <td>&#10008;</td>
    <td></td>
    <td>&#10008;</td>
  </tr>
  <tr>
    <th class="rowhead indent">gcread</th>
    <td>&#10004;</td>
    <td></td>
    <td></td>
    <td></td>
    <td></td>
    <td></td>
    <td></td>
    <td>&#10008;</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>&#10004;</td>
    <td></td>
    <td></td>
    <td>&#10008;</td>
    <td>&#10008;</td>
    <td>&#10008;</td>
    <td>&#10008;</td>
    <td>&#10008;</td>
  </tr>
  <tr>
    <th class="rowhead indent">before calls</th>
    <td>&#10004;</td>
    <td></td>
    <td></td>
    <td></td>
    <td></td>
    <td></td>
    <td>&#10008;</td>
    <td>&#10008;</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>&#10008;</td>
    <td>&#10008;</td>
  </tr>
  <tr>
    <th class="rowhead indent">before escape</th>
    <td>&#10004;</td>
    <td></td>
    <td></td>
    <td></td>
    <td></td>
    <td></td>
    <td>&#10008;</td>
    <td>&#10008;</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>&#10008;</td>
    <td>&#10008;</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>&#10004;</td>
    <td></td>
    <td></td>
    <td>&#10008;</td>
    <td>&#10008;</td>
    <td>&#10008;</td>
    <td>&#10008;</td>
    <td>&#10008;</td>
  </tr>
  <tr class="doc_warning">
    <th class="rowhead indent">JIT</th>
    <td>NO</td>
    <td></td>
    <td></td>
    <td class="optl">&#10008;</td>
    <td class="optl">&#10008;</td>
    <td class="optl">&#10008;</td>
    <td class="optl">&#10008;</td>
    <td class="optl">&#10008;</td>
  </tr>
  <tr class="doc_warning">
    <th class="rowhead indent">obj</th>
    <td>NO</td>
    <td></td>
    <td></td>
    <td class="optl">&#10008;</td>
    <td class="optl">&#10008;</td>
    <td class="optl">&#10008;</td>
    <td class="optl">&#10008;</td>
    <td class="optl">&#10008;</td>
  </tr>
  <tr class="doc_warning">
    <th class="rowhead">live analysis</th>
    <td>NO</td>
    <td></td>
    <td></td>
    <td class="optl">&#10008;</td>
    <td class="optl">&#10008;</td>
    <td class="optl">&#10008;</td>
    <td class="optl">&#10008;</td>
    <td class="optl">&#10008;</td>
  </tr>
  <tr class="doc_warning">
    <th class="rowhead">register map</th>
    <td>NO</td>
    <td></td>
    <td></td>
    <td class="optl">&#10008;</td>
    <td class="optl">&#10008;</td>
    <td class="optl">&#10008;</td>
    <td class="optl">&#10008;</td>
    <td class="optl">&#10008;</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">&#10008;</span> in gray denotes a feature which
           could be utilized if available.</div>
    </td>
  </tr>
</table>

<p>To be clear, the collection techniques above are defined as:</p>

<dl>
  <dt>Shadow Stack</dt>
  <dd>The mutator carefully maintains a linked list of stack root
      descriptors.</dd>
  <dt>Reference Counting</dt>
  <dd>The mutator maintains a reference count for each object and frees an
      object when its count falls to zero.</dd>
  <dt>Mark-Sweep</dt>
  <dd>When the heap is exhausted, the collector marks reachable objects starting
      from the roots, then deallocates unreachable objects in a sweep
      phase.</dd>
  <dt>Copying</dt>
  <dd>As reachability analysis proceeds, the collector copies objects from one
      heap area to another, compacting them in the process. Copying collectors
      enable highly efficient "bump pointer" allocation and can improve locality
      of reference.</dd>
  <dt>Incremental</dt>
  <dd>(Including generational collectors.) Incremental collectors generally have
      all the properties of a copying collector (regardless of whether the
      mature heap is compacting), but bring the added complexity of requiring
      write barriers.</dd>
  <dt>Threaded</dt>
  <dd>Denotes a multithreaded mutator; the collector must still stop the mutator
      ("stop the world") before beginning reachability analysis. Stopping a
      multithreaded mutator is a complicated problem. It generally requires
      highly platform specific code in the runtime, and the production of
      carefully designed machine code at safe points.</dd>
  <dt>Concurrent</dt>
  <dd>In this technique, the mutator and the collector run concurrently, with
      the goal of eliminating pause times. In a <em>cooperative</em> collector,
      the mutator further aids with collection should a pause occur, allowing
      collection to take advantage of multiprocessor hosts. The "stop the world"
      problem of threaded collectors is generally still present to a limited
      extent. Sophisticated marking algorithms are necessary. Read barriers may
      be necessary.</dd>
</dl>

<p>As the matrix indicates, LLVM's garbage collection infrastructure is already
suitable for a wide variety of collectors, but does not currently extend to
multithreaded programs. This will be added in the future as there is
interest.</p>

</div>

<!-- ======================================================================= -->
<div class="doc_subsection">
  <a name="stack-map">Computing stack maps</a>
</div>

<div class="doc_text">

<blockquote><pre
>for (iterator I = begin(), E = end(); I != E; ++I) {
  CollectorMetadata *MD = *I;
  unsigned FrameSize = MD-&gt;getFrameSize();
  size_t RootCount = MD-&gt;roots_size();

  for (CollectorMetadata::roots_iterator RI = MD-&gt;roots_begin(),
                                         RE = MD-&gt;roots_end();
                                         RI != RE; ++RI) {
    int RootNum = RI->Num;
    int RootStackOffset = RI->StackOffset;
    Constant *RootMetadata = RI->Metadata;
  }
}</pre></blockquote>

<p>LLVM automatically computes a stack map. All a <tt>Collector</tt> needs to do
is access it using <tt>CollectorMetadata::roots_begin()</tt> and
-<tt>end()</tt>. If the <tt>llvm.gcroot</tt> intrinsic is eliminated before code
generation by a custom lowering pass, LLVM's stack map will be empty.</p>

</div>


<!-- ======================================================================= -->
<div class="doc_subsection">
  <a name="init-roots">Initializing roots to null: <tt>InitRoots</tt></a>
</div>

<div class="doc_text">

<blockquote><pre
>MyCollector::MyCollector() {
  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 reachability analysis from finding
uninitialized values in stack roots at runtime, which will almost certainly
cause it to segfault. This initialization occurs before custom lowering, so the
two may be used together.</p>

<p>Since LLVM does not yet compute liveness information, this feature should be
used by all collectors which do not custom lower <tt>llvm.gcroot</tt>, and even
some that do.</p>

</div>


<!-- ======================================================================= -->
<div class="doc_subsection">
  <a name="custom">Custom lowering of intrinsics: <tt>CustomRoots</tt>, 
    <tt>CustomReadBarriers</tt>, and <tt>CustomWriteBarriers</tt></a>
</div>

<div class="doc_text">

<p>For collectors with barriers or unusual treatment of stack roots, these
flags allow the collector to perform any required transformation on the LLVM
IR:</p>

<blockquote><pre
>class MyCollector : public Collector {
public:
  MyCollector() {
    CustomRoots = true;
    CustomReadBarriers = true;
    CustomWriteBarriers = true;
  }
  
  virtual bool initializeCustomLowering(Module &amp;M);
  virtual bool performCustomLowering(Function &amp;F);
};</pre></blockquote>

<p>If any of these flags are set, then LLVM suppresses its default lowering for
the corresponding intrinsics and instead passes them on to a custom lowering
pass specified by the collector.</p>

<p>LLVM's default action for each intrinsic is as follows:</p>

<ul>
  <li><tt>llvm.gcroot</tt>: Pass through to the code generator to generate a
                            stack map.</li>
  <li><tt>llvm.gcread</tt>: Substitute a <tt>load</tt> instruction.</li>
  <li><tt>llvm.gcwrite</tt>: Substitute a <tt>store</tt> instruction.</li>
</ul>

<p>If <tt>CustomReadBarriers</tt> or <tt>CustomWriteBarriers</tt> are specified,
then <tt>performCustomLowering</tt> <strong>must</strong> eliminate the
corresponding barriers.</p>

<p><tt>performCustomLowering</tt>, must comply with the same restrictions as <a
href="WritingAnLLVMPass.html#runOnFunction"><tt>runOnFunction</tt></a>, and
that <tt>initializeCustomLowering</tt> has the same semantics as <a
href="WritingAnLLVMPass.html#doInitialization_mod"><tt>doInitialization(Module
&amp;)</tt></a>.</p>

<p>The following can be used as a template:</p>

<blockquote><pre
>#include "llvm/Module.h"
#include "llvm/IntrinsicInst.h"

bool MyCollector::initializeCustomLowering(Module &amp;M) {
  return false;
}

bool MyCollector::performCustomLowering(Function &amp;F) {
  bool MadeChange = false;
  
  for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
    for (BasicBlock::iterator II = BB-&gt;begin(), E = BB-&gt;end(); II != E; )
      if (IntrinsicInst *CI = dyn_cast&lt;IntrinsicInst&gt;(II++))
        if (Function *F = CI-&gt;getCalledFunction())
          switch (F-&gt;getIntrinsicID()) {
          case Intrinsic::gcwrite:
            // Handle llvm.gcwrite.
            CI-&gt;eraseFromParent();
            MadeChange = true;
            break;
          case Intrinsic::gcread:
            // Handle llvm.gcread.
            CI-&gt;eraseFromParent();
            MadeChange = true;
            break;
          case Intrinsic::gcroot:
            // Handle llvm.gcroot.
            CI-&gt;eraseFromParent();
            MadeChange = true;
            break;
          }
  
  return MadeChange;
}</pre></blockquote>

</div>


<!-- ======================================================================= -->
<div class="doc_subsection">
  <a name="safe-points">Generating safe points: <tt>NeededSafePoints</tt></a>
</div>

<div class="doc_text">

<p>LLVM can compute four kinds of safe points:</p>

<blockquote><pre
>namespace GC {
  /// PointKind - The type of a collector-safe point.
  /// 
  enum PointKind {
    Loop,    //&lt; Instr is a loop (backwards branch).
    Return,  //&lt; Instr is a return instruction.
    PreCall, //&lt; Instr is a call instruction.
    PostCall //&lt; 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
>MyCollector::MyCollector() {
  NeededSafePoints = 1 &lt;&lt; GC::Loop
                   | 1 &lt;&lt; GC::Return
                   | 1 &lt;&lt; GC::PreCall
                   | 1 &lt;&lt; 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) {
  CollectorMetadata *MD = *I;
  size_t PointCount = MD-&gt;size();

  for (CollectorMetadata::iterator PI = MD-&gt;begin(),
                                   PE = MD-&gt;end(); PI != PE; ++PI) {
    GC::PointKind PointKind = PI-&gt;Kind;
    unsigned PointNum = PI-&gt;Num;
  }
}
</pre></blockquote>

<p>Almost every collector requires <tt>PostCall</tt> safe points, since these
correspond to the moments when the function is suspended during a call to a
subroutine.</p>

<p>Threaded programs generally require <tt>Loop</tt> safe points to guarantee
that the application will reach a safe point within a bounded amount of time,
even if it is executing a long-running loop which contains no function
calls.</p>

<p>Threaded collectors may also require <tt>Return</tt> and <tt>PreCall</tt>
safe points to implement "stop the world" techniques using self-modifying code,
where it is important that the program not exit the function without reaching a
safe point (because only the topmost function has been patched).</p>

</div>


<!-- ======================================================================= -->
<div class="doc_subsection">
  <a name="assembly">Emitting assembly code:
    <tt>beginAssembly</tt> and <tt>finishAssembly</tt></a>
</div>

<div class="doc_text">

<p>LLVM allows a collector to print arbitrary assembly code before and after
the rest of a module's assembly code. From the latter callback, the collector
can print stack maps built by the code generator.</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
>class MyCollector : public Collector {
public:
  virtual void beginAssembly(std::ostream &amp;OS, AsmPrinter &amp;AP,
                             const TargetAsmInfo &amp;TAI);

  virtual void finishAssembly(std::ostream &amp;OS, AsmPrinter &amp;AP,
                              const TargetAsmInfo &amp;TAI);
}</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>CollectorMetadata</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 MyCollector::beginAssembly(std::ostream &amp;OS, AsmPrinter &amp;AP,
                                const TargetAsmInfo &amp;TAI) {
  // Nothing to do.
}

void MyCollector::finishAssembly(std::ostream &amp;OS, AsmPrinter &amp;AP,
                                 const TargetAsmInfo &amp;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) {
    CollectorMetadata &amp;MD = **FI;
    
    // Emit this data structure:
    // 
    // struct {
    //   int32_t PointCount;
    //   struct {
    //     void *SafePointAddress;
    //     int32_t LiveCount;
    //     int32_t LiveOffsets[LiveCount];
    //   } Points[PointCount];
    // } __gcmap_&lt;FUNCTIONNAME&gt;;
    
    // Align to address width.
    AP.EmitAlignment(AddressAlignLog);
    
    // Emit the symbol by which the stack map can be found.
    std::string Symbol;
    Symbol += TAI.getGlobalPrefix();
    Symbol += "__gcmap_";
    Symbol += MD.getFunction().getName();
    if (const char *GlobalDirective = TAI.getGlobalDirective())
      OS &lt;&lt; GlobalDirective &lt;&lt; Symbol &lt;&lt; "\n";
    OS &lt;&lt; TAI.getGlobalPrefix() &lt;&lt; Symbol &lt;&lt; ":\n";
    
    // Emit PointCount.
    AP.EmitInt32(MD.size());
    AP.EOL("safe point count");
    
    // And each safe point...
    for (CollectorMetadata::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 &lt;&lt; AddressDirective
         &lt;&lt; TAI.getPrivateGlobalPrefix() &lt;&lt; "label" &lt;&lt; PI-&gt;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 (CollectorMetadata::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-&gt;StackOffset);
        AP.EOL("stack offset");
      }
    }
  }
}
</pre></blockquote>

</div>


<!-- *********************************************************************** -->
<div class="doc_section">
  <a name="runtime-impl">Implementing a collector runtime</a>
</div>
<!-- *********************************************************************** -->

<div class="doc_text">

<p>Implementing a garbage collector for LLVM is fairly straightforward. The
LLVM garbage collectors are provided in a form that makes them easy to link into
the language-specific runtime that a language front-end would use. They require
functionality from the language-specific runtime to get information about <a
href="#gcdescriptors">where pointers are located in heap objects</a>.</p>

<p>The implementation must include the
<a href="#allocate"><tt>llvm_gc_allocate</tt></a> and
<a href="#explicit"><tt>llvm_gc_collect</tt></a> functions. To do this, it will
probably have to <a href="#traceroots">trace through the roots
from the stack</a> and understand the <a href="#gcdescriptors">GC descriptors
for heap objects</a>. Luckily, there are some <a href="#gcimpls">example
implementations</a> available.
</p>
</div>


<!-- ======================================================================= -->
<div class="doc_subsection">
  <a name="gcdescriptors">Tracing GC pointers from heap objects</a>
</div>

<div class="doc_text">
<p>
The three most common ways to keep track of where pointers live in heap objects
are (listed in order of space overhead required):</p>

<ol>
<li>In languages with polymorphic objects, pointers from an object header are
usually used to identify the GC pointers in the heap object. This is common for
object-oriented languages like Self, Smalltalk, Java, or C#.</li>

<li>If heap objects are not polymorphic, often the "shape" of the heap can be
determined from the roots of the heap or from some other meta-data [<a
href="#appel89">Appel89</a>, <a href="#goldberg91">Goldberg91</a>, <a
href="#tolmach94">Tolmach94</a>]. In this case, the garbage collector can
propagate the information around from meta data stored with the roots. This
often eliminates the need to have a header on objects in the heap. This is
common in the ML family.</li>

<li>If all heap objects have pointers in the same locations, or pointers can be
distinguished just by looking at them (e.g., the low order bit is clear), no
book-keeping is needed at all. This is common for Lisp-like languages.</li>
</ol>

<p>The LLVM garbage collectors are capable of supporting all of these styles of
language, including ones that mix various implementations. To do this, it
allows the source-language to associate meta-data with the <a
href="#roots">stack roots</a>, and the heap tracing routines can propagate the
information. In addition, LLVM allows the front-end to extract GC information
in any form from a specific object pointer (this supports situations #1 and #3).
</p>

</div>


<!-- *********************************************************************** -->
<div class="doc_section">
  <a name="references">References</a>
</div>
<!-- *********************************************************************** -->

<div class="doc_text">

<p><a name="appel89">[Appel89]</a> Runtime Tags Aren't Necessary. Andrew
W. Appel. Lisp and Symbolic Computation 19(7):703-705, July 1989.</p>

<p><a name="goldberg91">[Goldberg91]</a> Tag-free garbage collection for
strongly typed programming languages. Benjamin Goldberg. ACM SIGPLAN
PLDI'91.</p>

<p><a name="tolmach94">[Tolmach94]</a> Tag-free garbage collection using
explicit type parameters. Andrew Tolmach. Proceedings of the 1994 ACM
conference on LISP and functional programming.</p>

<p><a name="henderson02">[Henderson2002]</a> <a
href="http://citeseer.ist.psu.edu/henderson02accurate.html">
Accurate Garbage Collection in an Uncooperative Environment</a>.
Fergus Henderson. International Symposium on Memory Management 2002.</p>

</div>


<!-- *********************************************************************** -->

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