org.jikesrvm.mm.mminterface

Class AlignmentEncoding

java.lang.Object

org.jikesrvm.mm.mminterface.AlignmentEncoding

public class AlignmentEncoding
extends Object

Support for encoding a small amount of metadata in the alignment of a TIB. We choose the alignment of the TIB so that the pointer looks like

     31      24      16      8       0
     +-------+-------+-------+-------+
     xxxxxxxxxxxxxxxxxxxxxxxxxxxxfff00
 

where the natural alignment of the object is preserved (the low-order bits are zero), and the next least significant n bits contain the encoded metadata.

With the cooperation of MemoryManager, the idea is that we allocate 2^n additional words of memory, then offset the object within the allocated region so that the value of fff is encoded.

The current implementation specifically encodes the TIB pointer, because this is the only pointer field where this technique can give a speedup that makes it worthwhile.

Field Summary

Fields

Modifier and Type	Field and Description
static int	ALIGN_CODE_NONE
private static int	ALIGNMENT_INCREMENT How far do we need to shift the object to increment the encoded field by 1
private static int	FIELD_SHIFT First bit of the encoded field
(package private) static int	FIELD_WIDTH Bits of metadata that we encode
private static int	MAX_ALIGN_WORDS Maximum distance (in words) that we shift an object
private static int	TIB_ALIGN_MASK Bit-mask to select out the encoded field
private static boolean	VERBOSE

Constructor Summary

Constructors

Constructor and Description
AlignmentEncoding()

Method Summary

Modifier and Type	Method and Description
static Address	adjustRegion(int alignCode, Address region) Adjust a region address so that the object pointer of an object that starts at this address will be aligned so as to encode the specified value.
(package private) static void	assertSanity(int alignCode) Assert that a prospective encoded value is sane
static int	extractTibCode(Address address) Extract the encoded value from a TIB pointer, represented as a raw address.
static int	getTibCode(ObjectReference object) Extract the encoded value from an object's TIB pointer
private static int	getTibCodeForRegion(Address region)
static int	padding(int alignCode) Number of padding bytes required.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

ALIGN_CODE_NONE

public static final int ALIGN_CODE_NONE

See Also:

Constant Field Values

FIELD_WIDTH

static final int FIELD_WIDTH

Bits of metadata that we encode

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Constant Field Values

MAX_ALIGN_WORDS

private static final int MAX_ALIGN_WORDS

Maximum distance (in words) that we shift an object

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Constant Field Values

FIELD_SHIFT

private static final int FIELD_SHIFT

First bit of the encoded field

See Also:

Constant Field Values

ALIGNMENT_INCREMENT

private static final int ALIGNMENT_INCREMENT

How far do we need to shift the object to increment the encoded field by 1

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Constant Field Values

TIB_ALIGN_MASK

private static final int TIB_ALIGN_MASK

Bit-mask to select out the encoded field

See Also:

Constant Field Values

VERBOSE

private static final boolean VERBOSE

See Also:

Constant Field Values

Constructor Detail

AlignmentEncoding

public AlignmentEncoding()

Method Detail

assertSanity

static void assertSanity(int alignCode)

Assert that a prospective encoded value is sane

Parameters:

alignCode - Prospective encoded value

padding

public static int padding(int alignCode)

Number of padding bytes required.

Parameters:

alignCode - Prospective encoded value.

Returns:

the number of padding bytes required

adjustRegion

public static Address adjustRegion(int alignCode,
                                   Address region)

Adjust a region address so that the object pointer of an object that starts at this address will be aligned so as to encode the specified value.

Parameters:

alignCode - Value to encode

region - The initial region

Returns:

the aligned address

getTibCodeForRegion

private static int getTibCodeForRegion(Address region)

extractTibCode

public static int extractTibCode(Address address)

Extract the encoded value from a TIB pointer, represented as a raw address.

Parameters:

address - the TIB's address

Returns:

the encoded value from a TIB pointer

getTibCode

public static int getTibCode(ObjectReference object)

Extract the encoded value from an object's TIB pointer

Parameters:

object - the object

Returns:

the encoded value from a TIB pointer