Islands : Aliasing Protection In ObjectOriented Languages

1
Islands Aliasing Protection In Object-Oriented
Languages

• By John Hogg
• OOPSLA 91

Aharon Abadi
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Islands

• The main contribution of the paper.
• Provide alias protection.
• Alias definition
• aliased object two pointer access paths

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Aliases problem

• Aliased objects allow changing the state
  through
• different access paths

Perform operation x x 1
Harmless aliasing y is unaffected
Perform operation x .increment
Harm aliasing y is affected
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Aliasing Types

• Dynamic Aliasing
• at least one of the access paths has a prefix
  consisting of temporary variables or parameters
• Class A xB
• void f() B yx

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Aliasing Types

• Static Aliasing
• an object is aliased statically if two
  different access paths are both composed entirely
  of chains of instance variables.
• Class A C x
• Class B C y

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Static Aliasing Problems

• Dynamic alias
• has no effect beyond the
• scope in which it occurs
• Static alias
• problems scope arbitrarily
• point of the execution
• The paths length may be
• arbitrarily long
• function f may be affected by
• function g even though they
• share no variables
• Islands only prevents static aliasing

. g(x) . .
f(y) . .
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Island Motivation

• In practice aliasing tends to be local
• Programmers understand aliases complexity
• Islands permit aliasing only on small groups of
  object
• Each group called island

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Island Definition

• Island
• the transitive closure of a set of objects
  accessible from a bridge object
• Bridge
• the unique access point to a set of instances
  that make up an island

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Island Requirements

• It must be possible to pass a structure
• into an island with a guarantee that no other
  references to it are held

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Island Requirements

• It must be possible to retrieve structure
• from an island with a guarantee that no other
  references to it are held

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Island Requirements

• Ability invoking external functions and
  procedures.

A
B
C
D
Dynamic aliasing
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Island Benefits and Disadvantages

• Benefits
• Static references cannot cross Island boundary
• Dynamic references that cross Island boundary
  are visible and controlled by the bridge
• An island provides a true encapsulation of its
  components.
• Disadvantages
• Dynamic aliasing prevents information hiding is
  not supported (not in the paper)
• Need to construct a proof about intra-island
  behavior.

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Island Implementation Language Extension

• One new operation destructive read
• Two more access modes unique and free which
  mutually exclusive to read

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Destructive Read

• Atomic operation that returns the value of a
  variable and sets the variable to nil.
• Only an instance variable may be destructively
  read.

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Unique Access Mode

• Indicates that the object has only one static
  reference in the entire system.
• Rules
• A unique variable may only be assigned the result
  of a free expression.
• A unique expression may not be assigned to
  anything.
• A unique expression may only be exported as
  unique.
• If a method receiver is unique, then every
  parameter and the result must be read or unique
  or free.

A
A
Unique object B
Unique object B
C
C
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Free Access Mode

• Indicates that no other static references to the
  variable exist anywhere in the system.
• Definition
• A free expression is
• destructive read of a unique instance variable
• destructive read of a free variable
• result of new
• result of free valued function
• Rules
• A free variable may only be accessed via a
  destructive
• read.

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Additional Rules

• Bridge class - in every method every parameter
• and function result is
  read, unique or free
• A read expression may not be the right side of an
  assignment.
• A unique expression may not be assigned to
  anything
• Free is not harmful
• Disadvantage internal object can be exported as
  read, no
• information hiding.

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Bridge Example

• class name DictionaryBuffer
• instance variable names head
• tail
• initialize size read do end
• insertKey newKeyfree , value newValuefree do
  end
• read Find searchKeyread read do end

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Conclusion

• Islands allow a set of objects to be nicely
  encapsulated.
• Bridge may used as true black box.
• Future work extension to multiple threads