1. 11th Nov 2004PLDI 20041 Region Inference for an Object-Oriented Language Wei Ngan Chin 1,2 Joint work with Florin Craciun 1, Shengchao Qin 1,2, Martin Rinard 3 1 National University of Singapore 2 Singapore-MIT Alliance 3 MIT

2. 2 Background Problem: objects have different lifetimes garbage collection can be expensive One Solution: Use regions with different lifetimes Avoid dangling references. Previous Works : Region checkers for Java [DIKU98], Real-Time Java : [Boyapati et al PLDI03], C [Cyclone PLDI02]. Our Goal: auto. region inference for OO paradigm

3. 3 Assumptions Lexically-Scoped Regions: LIFO behaviour construct : letreg r in e No-Dangling References: Regions of fields must outlive the region of its enclosing object. top (younger) bottom (older)

4. 4 Main Contributions Region Inference for Core-Java Constraint-Based Type System. Region Polymorphism & Region Subtyping Handles Inheritance, Overriding, Downcast Implementation

5. 5 Core-Java

6. 6 Region-Annotated Core-Java region types, parameters, constraints, letreg

7. 7 Inferring for Class

8. 8 Key Principles : Classes Region type : c h r 1,…, r n i r 1 : (region for current object of this type) r 2,…,r n (regions for components/fields) r k º r 1, 8 k 2 2..n (no dangling references) First region is special. Keep regions of components distinct.

9. 9 List Example

10. 10 Key Principles : Methods Region polymorphism from the region types of parameters and output t mn h r * i ((t v) * ) where rc e r * captures region polymorphism Keep region constraints of classes and methods separate.

11. 11 Inference of Methods class invariant method precondition

12. 12 Outline Background Key Principles Region Subtyping Region Inference Method Overriding + Downcast Experimental Results

13. 13 No Region Subtyping [Boyapati et al PLDI03] supports class subtyping but not region subtyping

14. 14 (Object) Region Subtyping concept from [Cyclone PLDI02] A variable location v::c can be passed an object of type c provided r  s

15. 15 Example Variables a and b are assigned to tmp. Without region subtyping, their regions are equal. With region subtyping, their regions may be distinct.

16. 16 Problem – Recursive Fields Recursive nodes being forced into the same region Some programs permit recursive nodes to be in different regions …..

17. 17 Example - Reynolds3 Escape Analysis [Deutsch:POPL97] allows the List(x,p) object to be build on runtime stack. This effect is equivalent to local region at each recursive call.

18. 18 (Field) Region Subtyping Our Extension : Covariance for Read-Only Fields.

19. 19 Outline Background Key Principles Region Subtyping Region Inference Method Overriding + Downcast Experimental Results

20. 20 Region Inference Format of type inference rules Flow-insensitive but context sensitive. Modular inference according to dependency graph.

21. 21 Inference Rule for Block rs: the set of regions that do not escape the block They are coalesced into a single localised region r

22. 22 Example – Localised Region

23. 23 Example – Localised Region

24. 24 Example – Cyclic Structure p1p2

25. 25 Outline Background Key Principles Region Subtyping Region Inference Overriding + Downcast Experimental Results

26. 26 Method Overriding class B h r 1.. r n i extends A h r 1.. r m i where C B  In class A: Y mn h r’ 1,..,r’ p i (X) where M A {  } In class B: Y mn h r’ 1,..,r’ p i (X) where M B {  } Function Subtyping Rule: Method Override Rule: C B Æ M A ) M B argumentresultselection

27. 27 Override Conflict Resolution If method override rule do not hold for: C B M A M B Can strengthen C B, M A to C’ B, M’ A using: Until:C’ B Æ M’ A ) M B C B, M A, M B ` C’ B, M’ A

28. 28 Downcast Safety In our framework : regions may be lost during upcast e.g.Object o = new Pair (…) lost regions must be recovered during downcast. e.g.Pair p = (Pair) o Key technique : maintain extra regions that were supposedly lost during upcasting! e.g.Object [r2,r3] p = new Pair (…) Solution : Flow analysis

29. 29 Experiments Region annotation needed in 12% of the code. Comparable space reuse for all programs. Inference is as good as hand-annotation. On Olden benchmark, inference takes < 5s.

30. 30 Concluding Remarks Automatic region inference for CoreJava. Support classes and methods with region polymorphism. Supports region subtyping. Supports OO features : class subtyping, method overriding, and downcast safety. Implementation

31. 31 Future Work Better Lifetime Precision variable liveness analysis flow sensitivity via SSA treat null as a primitive value Memory Efficiency : Sized Regions + Reuse Extensions : Genericity + RTSJ

32. 32 Inference Rule for Sequence Inference rule is flow-insensitive. Improvement possible via SSA.

33. 33 Correctness

34. 34 Example : Fixed-Point Analysis

35. 35 Inference of Methods

36. 36 Experiments Region inference is fast for large programs too. Our prototype was built using Glasgow Haskell Compiler.