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The Maxine Project: Object representation in the Maxine VM

This page describes how objects are represented in the Maxine VM runtime. This aspect of the VM is especially interesting for a variety of reasons.

As a metacircular VM, most of Maxine's internal data structures, including those used to implement the representation of objects (e.g. actors, layouts), are themselves represented uniformly as objects. Some of those objects, however, (see hubs) are of an extended type that is not expressible in the Java language, and one kind of object (static tuples) cannot be expressed by any type.

As a compiled-only VM, Maxine can only start ("bootstrap") from a synthesized binary boot image (see Garden of Eden configurations). The boot heap in particular is a translation of an object graph created during boot image generation, a process complicated by the fact that generation is hosted on a standard VM but must deal with objects that cannot be represented in the standard language.

Heap

The general behavior of the heap (memory management, object allocation, garbage collection, etc.) is configurable, defined by a binding to the VM's heap scheme.

The runtime heap contains multiple heap segments. The first heap segment is the boot heap, which is part of the VM's boot image and which is pre-populated with the objects that the VM needs in order to begin operation. The boot heap is represented in the standard heap format and is exceptional only in that its objects never move, although they may become permanent garbage.

Object layout

The lowest level details of memory layout of objects (in particular with respect to headers, contents, and pointers) is configured by a binding to the VM's layout scheme. The scheme describes layout information for the three kinds of object representation described below: tuples, arrays, and hybrids. All memory access to the various parts of an object takes place via a layout scheme.

Of the two runtime layout schemes currently implemented, described below, OHM is the default binding for CISC architectures (like x86) and HOM is the default for RISC architectures such as SPARC. A third layout scheme ("Hosted") is specialized for representing objects being prototyped during boot image generation, especially important for the kinds of VM objects that cannot be directly represented as standard Java objects.

OHM layout

The OHM object layout scheme, described as Origin-Header-Mixed, is implemented by class OhmLayoutScheme in package com.sun.max.vm.layout.ohm.

Click for OHM layout details

The OHM layout packs tuple objects for minimal space consumption, observing alignment restrictions. Tuple objects have a 2 word header and are laid out as shown below.

 cell/origin --> +-------------+
                 |    class    |  // reference to dynamic hub of class
                 +-------------+
                 |    misc     |  // monitor and hashcode
        data --> +=============+
                 |             |
                 :   fields    :  // mixed reference and scalar data
                 |             |
                 +-------------+

OHM array objects have a 3 word header and are laid out as shown below (OHM hybrid objects have a similar header):

 cell/origin --> +-------------+
                 |    class    |  // reference to dynamic hub of class
                 +-------------+
                 |    misc     |  // monitor and hashcode
                 +-------------+
                 |   length    |
        data --> +=============+
                 |             |
                 :  elements   :
                 |             |
                 +-------------+

HOM layout

The HOM object layout scheme, described as Header-Origin-Mixed, is implemented by class HomLayoutScheme in the package com.sun.max.vm.layout.hom.

This layout enables more optimized code on SPARC for accessing array elements smaller than a word. The origin points at array element 0, so there is no need to perform address arithmetic to skip over the header. On the other hand, this layout requires reading memory for converting between cell and origin addresses, since they are not the same (as they are for OHM layout).

Click for HOM layout details

The HOM layout packs tuple objects for minimal space consumption, observing alignment restrictions. Tuple objects have a 2 word header and are laid out as shown below.

        cell --> +-------------+
                 |    misc     |  // monitor and hashcode
                 +-------------+
                 |   class     |  // reference to dynamic hub of class 
 origin/data --> +=============+
                 |             |
                 :   fields    :  // mixed reference and scalar data
                 |             |
                 +-------------+

Array objects have a 3 word header and are laid out as shown below (HOM hybrid objects have a similar header):

        cell --> +-------------+
                 |   length    |
                 +-------------+
                 |    misc     |  // monitor and hashcode
                 +-------------+
                 |   class     |  // reference to dynamic hub of class
 origin/data --> +=============+
                 |             |
                 :  elements   :
                 |             |
                 +-------------+


Hosted layout

The Hosted object layout scheme is not designed for VM runtime, but rather for the object prototyping phase of boot image generation. The generation machinery runs hosted on a standard Java VM and creates a prototype boot heap that will eventually be translated into the binary format of the target platform, and written into the boot image. This layout scheme is implemented by class HostedLayoutScheme in the package com.sun.max.vm.layout.hosted.

Object representation

There are exactly three low-level memory representations in the Maxine heap: tuples (for Java object instances), arrays (for Java array instances), and hybrids (for Maxine hubs). Memory access to the parts of these three representations is mediated through a layout scheme. Types and other aspects of object contents are defined by the ClassActor instance that represents type of the object being represented.

Tuple representation

A Maxine tuple is a memory representation that combines a two-word header plus a collection of named values (fields). The names, types, and locations of the values are defined by an instance of class TupleClassActor.

As with all Maxine object representations, the first word of the tuple header points at the dynamic hub for the class. The second (misc) word is used for a variety of purposes, including hash code and locking information.

The tuple memory representation is used to represent standard Java class instances in the heap. Note that static tuples are also represented this way, even they are not ordinary class instances and have no type.

Array representation

A Maxine array is a memory representation that combines a three-word header plus some fixed number of values of identical type. The type of the array elements is defined by an instance of class ArrayClassActor.

As with all Maxine object representations, the first word of the array header points at the dynamic hub for the class. The second (misc) word is used for a variety of purposes, including hash code and locking information. The third word holds the number of elements contained in the array.

The array memory representation is used to represent standard Java arrays in the heap.

Hybrid representation

A Maxine hybrid is a memory representation that combines a three-word header, a collection of named values (fields), and an array of words. The names, types, and location of the field values, together with information about the arrays, are defined by an instance of class HybridClassActor. Although hybrids are represented uniformly as instances of a class, they are classes that cannot be expressed in standard Java.

As with all Maxine object representations, the first word of the hybrid header points at the dynamic hub for the class. The second (misc) word is used for a variety of purposes, including hash code and locking information. The third word holds the number of words contained in the array.

The hybrid memory representation is used to represent Maxine hubs in the heap, even though hubs are not standard Java class instances and cannot be described with standard Java types.

Actors

Specific information about the contents of heap instances (tuples, arrays, and hybrids) is represented uniformly using Java type information, represented in the form of class actors. Class actors are themselves instances (represented as tuples) in the heap of the three types TupleClassActor, ArrayClassActor, and HybridClassActor respectively.

Hubs

A hub is a hybrid instance holding information, derived from a class actor, that must be immediately accessible (one memory hop) from each class instance. That is, a hub is what is pointed to from the (logical) class word of an object's header. A hub corresponds to a TIB in the Jikes RVM.

Hubs hold the vtables and itables used for efficient method dispatch. They also hold all the information needed when a garbage collector visits each instance, for example the size and reference map for the instance, avoiding the need to reference any further objects, which could themselves be subject to collection.

Hubs contains both named fields and embedded arrays and thus cannot be represented as a ordinary Java objects. They are instead represented as hybrids, and their contents are described uniformly with a class actor (describing a class not expressible in standard Java) of type HybridClassActor.

There are two kinds of hubs, distinguished by the context of their use: dynamic hubs are pointed to by class instances, and static hubs are pointed to by static tuples.

Dynamic hubs

Every ClassActor in the VM holds a reference to the dynamic hub (an instance of class com.sun.max.vm.actor.holder.DynamicHub) for the class it represents. Every instance of that class in the heap contains (in its header) a reference to that dynamic hub.

The following figure depicts the relationships among a class instance, the dynamic hub for the class, and the ClassActor for the type. The figure also demonstrates the three kinds of representation in the heap: tuples, arrays, and hybrids.

Click for figure showing the role played by dynamic hubs

The following screen snapshot shows how the dynamic hub for class com.sun.max.vm.type.BootClassLoader appears in an Object Inspector View in the Maxine Inspector. It is displayed as a hybrid object, with special display machinery for viewing the embedded arrays. The inspector's frame header identifies it as DynamicHub{BootClassLoader}, meaning "the DynamicHub associated with class BootClassLoader".

Note also that the hub pointer for this instance of DynamicHub points to another DynamicHub which is described as "the DynamicHub associated with class DynamicHub". In other words, the hub pointer of that hub points at itself: it participates in its own implementation and closes the hub recursion loop.

Click here to see an Inspector Object View of a dynamic hub

Static hubs

There is exactly one kind of instance, represented as a tuple in the heap, that cannot be treated uniformly by the VM's type information: a static tuple. A static tuple is unique in that cannot be described by a type, so it has no ClassActor that describes its type and must be treated exceptionally wherever types matter.

Every ClassActor in the VM holds a reference to a static tuple, which holds values of the class (static) variables for the class. Each ClassActor also holds a reference to the static hub, an instance of class com.sun.max.vm.actor.holder.StaticHub, to which the header of the static tuple points. This specialized hub, to which only the static tuple points, allows uniform treatment by GC.

The following screen snapshot shows how the static hub for class com.sun.max.vm.type.BootClassLoader appears in an Object Inspector View in the Maxine Inspector. It is displayed as a hybrid object, with special display machinery for viewing the embedded arrays. The inspector's frame header identifies it as StaticHub{BootClassLoader}, meaning "the StaticHub associated with class BootClassLoader".

Click here to see an Inspector Object View of a static hub

Static tuples

A static tuple is special kind of heap object that holds the class variables (static fields) for a class in the VM. Although it appears superficially as an ordinary tuple, with named fields, it is unique within the heap in that it has no type at all: think of it as a singularity in the VM's type system (a byproduct of the VM's metacircularity). In practice, this means that there is no ClassActor describing any static tuple; they must be treated using implicit knowledge of their structure.

On the other hand, static tuples are represented the same as other tuples in heap memory, and are amenable to ordinary garbage collection without special handling. This is done by having a special kind of static hub to which they point.

The following screen snapshot shows how the static tuple for class com.sun.max.vm.type.BootClassLoader appears in an Object Inspector View in the Maxine Inspector. It is displayed as an ordinary tuple, but the frame header identifies it as StaticTuple{BootClassLoader} and its hub pointer refers to an instance of StaticHub.

Click here to see an Inspector Object View of a static tuple

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