11th June 2004 PLDI 2004 1

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 Singapore2 Singapore-MIT Alliance 3 MIT

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Background

• Basic problem:• how to recover memory of dead objects?

• Two Approaches• garbage collection• explicit memory management

• Our Focus: • Explicit memory regions with different lifetimes

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Current Status

Region Inference for a functional language • -Calculus [Tofte-Talpin POPL94]

Previous Works on Region Type Checkers: • Java [DIKU98], • Real-Time Java : [MIT-PLDI03], • Cyclone [Cornell-PLDI02].

Our Goal: automatic region inference for object-oriented programs

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Two Major Assumptions

• Lexically-Scoped Regions: Stack-like lifetimes construct for local region :

letreg r in e

top (younger)

bottom (older)

r1

r2

r3

r4

• No Dangling References: Regions of components should have longer lifetimes than its object.

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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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Region-Annotated Core-Javaregion parameters for

polymorphism

constraint abstractions

class invariant

method precondition

region constraints

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Inferring for Class

Step 1 : Reserve first region for the receiver object

(this).Step 2 : Keep regions of fields

distinct for precision.

Step 3 : No dangling property as class invariant.

Step 4 : Inherit constraint of superclass.

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Inferring for Class

Step 1 : Reserve first region for the receiver object

(this).Step 2 : Keep regions of fields

distinct for precision.

Step 3 : No dangling requirement as class invariant.

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Inferring for Methods

Step 3 : Localise regions that do not escape.

Step 4 : Fix-point analysis for recursion.

method precondition

Step 2 : Gather constraints from method body.

Step 1 : Region parameters from method

signature.

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Outline

• Background• Key Techniques

• Inferring Class• Inferring Methods• Region Subtyping

• Region Inference• Method Overriding + Downcast• Experimental Results

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(Object) Region Subtypingconcept pioneered in Cyclone [PLDI02]

an object w of type c<r,..> can be passed to a location v of type c<s,..> provided r º s (e.g. in assignment and parameter/result passing)

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Example

Without region subtyping, regions of a and b are forced to be equal.

With region subtyping, these regions may be distinct.

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Problem – Recursive Fields

Recursive nodes being forced into the same region

Some programs work better with recursive nodes in different regions …

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(Field) Region Subtyping

SOLUTION : Covariance for Read-Only Fields.

Example:

read-onlymutable

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Outline

• Background

• Key Techniques

• Region Inference

• Method Overriding + Downcast

• Experimental Results

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Region Inference

• Main type inference rules

• Apply inference according to global dependency graph (class,call,override dependencies).

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Inference Rule : Sequence

Inference is flow-insensitive but can be improved by SSA form

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Inference Rule : Expression Block

rs : the set of regions that do not escape the block

Regions of rs coalesced into a single localised region r

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Example – Localised Region

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Example – Localised Region

localised region r

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Example : Recursive Function

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Outline

• Background

• Key Techniques

• Region Inference

• Overriding + Downcast

• Experimental Results

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Method Overriding

class Bhr1 .. rni extends Ahr1 .. rmi where CB

In class A: Y mnhr’1,..,r’pi (X) where MA { }

In class B: Y mnhr’1,..,r’pi (X) where MB { }

Dynamic Dispatch : Allows B.mn may be invoked where A.mn is expected

Method Override Rule: CB Æ MA ) MB

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Override Conflict Resolution

Derived constraints CB MA MB may conflict with method override rule.

Solution : strengthen CB , MA to C’B , M’A

Until: C’B Æ M’A ) MB

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Downcast Safety

• Compile-time guarantee to downcast safety.

• Problem : regions are lost during upcast that have to be recovered for downcast.

Our Solution:• Selectively maintain extra regions that are lost

during upcast and may be needed by downcast.• Use flow analysis

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Experiments

• Prototype was built using Haskell.• Inference is fast and takes <5s for the

Olden benchmark.

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Experiments

• Inference is competitive to hand-annotation (RegJava benchmark)• similar localised regions • same space recovery

• Region subtyping gives better space recovery for some examples.

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Concluding Remarks

• Constraint-based region inference system:• Region subtyping.

• Region polymorphism.• Polymorphic recursion

• Handles OO features:• class subtyping• method overriding• downcast safety

• Implementation is fast and precise.

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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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Correctness

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Core-Java

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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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No Region Subtyping

[Boyapati et al PLDI03] supports class subtyping but not region subtyping

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No Region Subtyping

[Boyapati et al PLDI03] supports class subtyping but not region subtyping

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Key Principles : Methods

• Region polymorphism from the region types of parameters and output • t mn hr*i ((t v)*) where rc e• r* captures region polymorphism

• Keep region constraints of classes and methods separate.

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Example - Reynolds3

With field region subtyping, the List(x,p) object can be allocated in a local region of the recursive call.

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Method Overriding

class Bhr1 .. rni extends Ahr1 .. rmi where CB

In class A: Y mnhr’1,..,r’pi (X) where MA { }

In class B: Y mnhr’1,..,r’pi (X) where MB { }

Function Subtyping Rule:

Method Override Rule: CB Æ MA ) MB

argument resultselection

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Downcast Safety

• In our framework :• regions may be lost during upcast

e.g. Object<r1> o = new Pair<s1,s2,s3> (…)

• lost regions must be recovered during downcast.e.g. Pair<a1,a2,a3> p = (Pair) o

• Key technique : maintain extra regions that were lost during upcasting and may be needed!

e.g. Object<r1>[r2,r3] p = new Pair<s1,s2,s3> (…)

• Solution : Flow analysis

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Region Subtyping

concept from [Cyclone PLDI02]

allows v=w where location v::c<s,..> can be passed an object w of type c<r,..> provided r

s

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Experiments

• Inference is competitive hand-annotation.• Prototype is fast (< 5s for upto 1000 lines)• http://loris-4.ddns.comp.nus.edu.sg/~craciunm

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Experiments

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Inference of Methods

class invariant

method precondition

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Inferring for Class

class invariant to prevent dangling references

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List Example

Key Principles• First region is for only object of the class.• Keep regions of fields distinct for precision.

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Inference of Methods

Key Principles• Region polymorphism from parameters/result• Method precondition is applicable to only

those objects that may invoke the method.

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Example – Cyclic Structure

p1 p2

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Inference of Methods

method precondition