mirror of
https://github.com/RPCS3/llvm-mirror.git
synced 2024-12-03 08:51:43 +00:00
535 lines
18 KiB
HTML
535 lines
18 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
|
|
"http://www.w3.org/TR/html4/strict.dtd">
|
|
<html>
|
|
<head>
|
|
<title>Accurate Garbage Collection with LLVM</title>
|
|
<link rel="stylesheet" href="llvm.css" type="text/css">
|
|
</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="#interfaces">Interfaces for user programs</a>
|
|
<ul>
|
|
<li><a href="#roots">Identifying GC roots on the stack: <tt>llvm.gcroot</tt></a></li>
|
|
<li><a href="#allocate">Allocating memory from the GC</a></li>
|
|
<li><a href="#barriers">Reading and writing references to the heap</a></li>
|
|
<li><a href="#explicit">Explicit invocation of the garbage collector</a></li>
|
|
</ul>
|
|
</li>
|
|
|
|
<li><a href="#gcimpl">Implementing a garbage collector</a>
|
|
<ul>
|
|
<li><a href="#llvm_gc_readwrite">Implementing <tt>llvm_gc_read</tt> and <tt>llvm_gc_write</tt></a></li>
|
|
<li><a href="#callbacks">Callback functions used to implement the garbage collector</a></li>
|
|
</ul>
|
|
</li>
|
|
<li><a href="#gcimpls">GC implementations available</a>
|
|
<ul>
|
|
<li><a href="#semispace">SemiSpace - A simple copying garbage collector</a></li>
|
|
</ul>
|
|
</li>
|
|
|
|
<!--
|
|
<li><a href="#codegen">Implementing GC support in a code generator</a></li>
|
|
-->
|
|
</ol>
|
|
|
|
<div class="doc_author">
|
|
<p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></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 life-times 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="#roots">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 provides support for a broad class of garbage collection algorithms,
|
|
including compacting semi-space collectors, mark-sweep collectors, generational
|
|
collectors, and even reference counting implementations. It includes support
|
|
for <a href="#barriers">read and write barriers</a>, and associating <a
|
|
href="#roots">meta-data with stack objects</a> (used for tagless garbage
|
|
collection). All LLVM code generators support garbage collection, including the
|
|
C backend.
|
|
</p>
|
|
|
|
<p>
|
|
We hope that the primitive support built into LLVM is sufficient to support a
|
|
broad class of garbage collected languages, including Scheme, ML, scripting
|
|
languages, Java, C#, etc. That said, the implemented garbage collectors may
|
|
need to be extended to support language-specific features such as finalization,
|
|
weak references, or other features. As these needs are identified and
|
|
implemented, they should be added to this specification.
|
|
</p>
|
|
|
|
<p>
|
|
LLVM does not currently support garbage collection of multi-threaded programs or
|
|
GC-safe points other than function calls, but these will be added in the future
|
|
as there is interest.
|
|
</p>
|
|
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
<div class="doc_section">
|
|
<a name="interfaces">Interfaces for user programs</a>
|
|
</div>
|
|
<!-- *********************************************************************** -->
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>This section describes the interfaces provided by LLVM and by the garbage
|
|
collector run-time that should be used by user programs. As such, this is the
|
|
interface that front-end authors should generate code for.
|
|
</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="roots">Identifying GC roots on the stack: <tt>llvm.gcroot</tt></a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<div class="doc_code"><tt>
|
|
void %llvm.gcroot(<ty>** %ptrloc, <ty2>* %metadata)
|
|
</tt></div>
|
|
|
|
<p>
|
|
The <tt>llvm.gcroot</tt> intrinsic is used to inform LLVM of a pointer variable
|
|
on the stack. The first argument contains the address of the variable on the
|
|
stack, and the second contains a pointer to metadata that should be associated
|
|
with the pointer (which <b>must</b> be a constant or global value address). At
|
|
runtime, the <tt>llvm.gcroot</tt> intrinsic stores a null pointer into the
|
|
specified location to initialize the pointer.</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*
|
|
...
|
|
|
|
;; "CodeBlock" is the block corresponding to the start
|
|
;; of the scope above.
|
|
CodeBlock:
|
|
;; Initialize the object, telling LLVM that it is now live.
|
|
;; Java has type-tags on objects, so it doesn't need any
|
|
;; metadata.
|
|
call void %llvm.gcroot(%Object** %X, sbyte* null)
|
|
...
|
|
|
|
;; 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="allocate">Allocating memory from the GC</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<div class="doc_code"><tt>
|
|
sbyte *%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 appropriate size.</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="barriers">Reading and writing references to the heap</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<div class="doc_code"><tt>
|
|
sbyte *%llvm.gcread(sbyte *, sbyte **)<br>
|
|
void %llvm.gcwrite(sbyte*, sbyte*, sbyte**)
|
|
</tt></div>
|
|
|
|
<p>Several of the more interesting garbage collectors (e.g., generational
|
|
collectors) need to be informed when the mutator (the program that needs garbage
|
|
collection) reads or writes object references into the heap. In the case of a
|
|
generational collector, it needs to keep track of which "old" generation objects
|
|
have references stored into them. The amount of code that typically needs to be
|
|
executed is usually quite small (and not on the critical path of any
|
|
computation), so the overall performance impact of the inserted code is
|
|
tolerable.</p>
|
|
|
|
<p>To support garbage collectors that use read or write barriers, LLVM provides
|
|
the <tt>llvm.gcread</tt> and <tt>llvm.gcwrite</tt> intrinsics. The first
|
|
intrinsic has exactly the same semantics as a non-volatile LLVM load and the
|
|
second has the same semantics as a non-volatile LLVM store, with the
|
|
additions that they also take a pointer to the start of the memory
|
|
object as an argument. At code generation
|
|
time, these intrinsics are replaced with calls into the garbage collector
|
|
(<tt><a href="#llvm_gc_readwrite">llvm_gc_read</a></tt> and <tt><a
|
|
href="#llvm_gc_readwrite">llvm_gc_write</a></tt> respectively), which are then
|
|
inlined into the code.
|
|
</p>
|
|
|
|
<p>
|
|
If you are writing a front-end for a garbage collected language, every load or
|
|
store of a reference from or to the heap should use these intrinsics instead of
|
|
normal LLVM loads/stores.</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 spaces. The initial heap size
|
|
to allocate should be specified as an argument.
|
|
</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_section">
|
|
<a name="gcimpl">Implementing a garbage collector</a>
|
|
</div>
|
|
<!-- *********************************************************************** -->
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>
|
|
Implementing a garbage collector for LLVM is fairly straight-forward. 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, and it must implement
|
|
the <a href="#llvm_gc_readwrite">read/write barrier</a> functions as well. 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="llvm_gc_readwrite">Implementing <tt>llvm_gc_read</tt> and <tt>llvm_gc_write</tt></a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
<div class="doc_code"><tt>
|
|
void *llvm_gc_read(void*, void **)<br>
|
|
void llvm_gc_write(void*, void *, void**)
|
|
</tt></div>
|
|
|
|
<p>
|
|
These functions <i>must</i> be implemented in every garbage collector, even if
|
|
they do not need read/write barriers. In this case, just load or store the
|
|
pointer, then return.
|
|
</p>
|
|
|
|
<p>
|
|
If an actual read or write barrier is needed, it should be straight-forward to
|
|
implement it.
|
|
</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="callbacks">Callback functions used to implement the garbage collector</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
<p>
|
|
Garbage collector implementations make use of call-back functions that are
|
|
implemented by other parts of the LLVM system.
|
|
</p>
|
|
</div>
|
|
|
|
<!--_________________________________________________________________________-->
|
|
<div class="doc_subsubsection">
|
|
<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_subsubsection">
|
|
<a name="staticroots">Tracing GC pointers from static roots</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
TODO
|
|
</div>
|
|
|
|
|
|
<!--_________________________________________________________________________-->
|
|
<div class="doc_subsubsection">
|
|
<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
|
|
from in any form from a specific object pointer (this supports situations #1 and
|
|
#3).
|
|
</p>
|
|
|
|
<p><b>Making this efficient</b></p>
|
|
|
|
|
|
|
|
</div>
|
|
|
|
|
|
|
|
<!-- *********************************************************************** -->
|
|
<div class="doc_section">
|
|
<a name="gcimpls">GC implementations available</a>
|
|
</div>
|
|
<!-- *********************************************************************** -->
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>
|
|
To make this more concrete, the currently implemented LLVM garbage collectors
|
|
all live in the <tt>llvm/runtime/GC/*</tt> directories in the LLVM source-base.
|
|
If you are interested in implementing an algorithm, there are many interesting
|
|
possibilities (mark/sweep, a generational collector, a reference counting
|
|
collector, etc), or you could choose to improve one of the existing algorithms.
|
|
</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="semispace">SemiSpace - A simple copying garbage collector</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
<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>
|
|
|
|
</div>
|
|
|
|
<!--_________________________________________________________________________-->
|
|
<div class="doc_subsubsection">
|
|
Possible Improvements
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>
|
|
If a collection cycle happens and the heap is not compacted very much (say less
|
|
than 25% of the allocated memory was freed), the memory regions should be
|
|
doubled in size.</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>
|
|
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
|
|
<hr>
|
|
<address>
|
|
<a href="http://jigsaw.w3.org/css-validator/check/referer"><img
|
|
src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
|
|
<a href="http://validator.w3.org/check/referer"><img
|
|
src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!"></a>
|
|
|
|
<a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
|
|
<a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
|
|
Last modified: $Date$
|
|
</address>
|
|
|
|
</body>
|
|
</html>
|