Region Inference for an ObjectOriented Language

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Region Inference for an Object-Oriented Language

• Wei Ngan Chin1,2
• Joint work with
• Florin Craciun1, Shengchao Qin1,2, Martin Rinard3
• 1 National University of Singapore
• 2 Singapore-MIT Alliance 3 MIT

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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

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Assumptions
top (younger)

• Lexically-Scoped Regions
• LIFO behaviour
• construct letreg r in e
• No-Dangling References
• Regions of fields must outlive
• the region of its enclosing object.

bottom (older)
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Main Contributions

• Region Inference for Core-Java
• Constraint-Based Type System.
• Region Polymorphism Region Subtyping
• Handles Inheritance, Overriding, Downcast
• Implementation

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Core-Java
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Region-Annotated Core-Java
region types, parameters, constraints, letreg
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Inferring for Class
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Key Principles Classes

• Region type chr1,, rni
• r1 (region for current object of this type)
• r2,,rn (regions for components/fields)
• rk º r1, 8 k 2 2..n (no dangling references)
• First region is special.
• Keep regions of components distinct.

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List Example
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Key Principles Methods

• Region polymorphism from the region types of
  parameters and output
• t mn hri ((t v)) where rc e
• r captures region polymorphism
• Keep region constraints of classes and methods
  separate.

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Inference of Methods
class invariant
method precondition
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Outline

• Background
• Key Principles
• Region Subtyping
• Region Inference
• Method Overriding Downcast
• Experimental Results

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No Region Subtyping
Boyapati et al PLDI03 supports class subtyping
but not region subtyping
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(Object) Region Subtyping
concept from Cyclone PLDI02 A variable
location vclts,..gt can be passed an object of
type cltr,..gt provided r ? s
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Example
Variables a and b are assigned to tmp. Without
region subtyping, their regions are equal. With
region subtyping, their regions may be distinct.
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Problem Recursive Fields
Recursive nodes being forced into the same region
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Example - Reynolds3
Escape Analysis DeutschPOPL97 allows the
List(x,p) object to be build on runtime stack.
This effect is equivalent to local region at each
recursive call.
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(Field) Region Subtyping
Our Extension Covariance for Read-Only Fields.
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Outline

• Background
• Key Principles
• Region Subtyping
• Region Inference
• Method Overriding Downcast
• Experimental Results

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Region Inference

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

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Inference Rule for Block
rs the set of regions that do not escape the
block They are coalesced into a single localised
region r
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Example Localised Region
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Example Localised Region
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Example Cyclic Structure
p1
p2
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Outline

• Background
• Key Principles
• Region Subtyping
• Region Inference
• Overriding Downcast
• Experimental Results

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Method Overriding

• class Bhr1 .. rni extends Ahr1 .. rmi where CB ?
  
• In class A Y mnhr1,..,rpi (X) where MA
  ?
• In class B Y mnhr1,..,rpi (X) where MB
  ?
• Function Subtyping Rule
• Method Override Rule
• CB Æ MA ) MB

argument
result
selection
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Override Conflict Resolution

• If method override rule do not hold for
• CB MA MB
• Can strengthen CB , MA to CB , MA using
• Until CB Æ MA ) MB

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Downcast Safety

• In our framework
• regions may be lost during upcast
• e.g. Objectltr1gt o new Pairlts1,s2,s3gt ()
• lost regions must be recovered during downcast.
• e.g. Pairltt1,t2,t3gt p (Pair) o
• Key technique maintain extra regions that were
  supposedly lost during upcasting!
• e.g. Objectltr1gtr2,r3 p new Pairlts1,s2,s3gt ()
• Solution Flow analysis

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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 lt 5s.

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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

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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

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Inference Rule for Sequence
Inference rule is flow-insensitive. Improvement
possible via SSA.
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Correctness
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Example Fixed-Point Analysis
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Inference of Methods
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Experiments

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