1 Islands : Aliasing Protection In Object-Oriented Languages By : John Hogg OOPSLA 91 Aharon Abadi

2 Islands The main contribution of the paper. Provide alias protection. Alias definition: aliased object two pointer access paths Aliased Object

3 Aliases problem x y 3 Perform operation x = x+ 1 x y 3 4 Harmless aliasing: y is unaffected x y z 3 Harm aliasing: y is affected Perform operation x.increment x y z 4 Aliased objects allow changing the state through different access paths

4 Aliasing Types Dynamic Aliasing: at least one of the access paths has a prefix consisting of temporary variables or parameters Class A{ x:B; void f(){ B & y=x; } } Within f object pointed by x is dynamic aliased y stack A object B object x

5 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; } A object C object B object x y

6 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 x y z 3 heap x y z 5 heap Problem. g(x);. f(y);.

7 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 G1 G2 G3 G4

8 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 bridge Static references Are disallowed

9 Island Requirements It must be possible to pass a structure into an island with a guarantee that no other references to it are held Insert an object with no external references

10 Island Requirements It must be possible to retrieve structure from an island with a guarantee that no other references to it are held Retrieve an object with no internal references

11 Ability invoking external functions and procedures. Island Requirements D B A C Dynamic aliasing

12 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.

13 Island Implementation Language Extension One new operation: destructive read Two more access modes: unique and free which mutually exclusive to read

14 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. y = x yx object yx nill

15 Unique Access Mode Indicates that the object has only one static reference in the entire system. Rules: 1.A unique variable may only be assigned the result of a free expression. 2.A unique expression may not be assigned to anything. 3.A unique expression may only be exported as unique. 4.If a method receiver is unique, then every parameter and the result must be read or unique or free. A Unique object B C Unprotected object within unique object A Unique object B C Unprotected object within unique object

16 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.

17 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.

18 Bridge Example class name DictionaryBuffer instance variable names head tail initialize: size% read do … end insertKey: newKey%free, value: newValue%free do … end %read Find: searchKey%read :%read do … end DictionaryBuffer

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