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

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

3. 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. 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. 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. 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. 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. 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. 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. 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. 11 Ability invoking external functions and procedures. Island Requirements D B A C Dynamic aliasing

12. 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. 13 Island Implementation Language Extension One new operation: destructive read Two more access modes: unique and free which mutually exclusive to read

14. 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. 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. 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. 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. 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. 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