Region Inference
for an Object-Oriented Language
Wei Ngan Chin1,2
Joint work with
Florin Craciun1, Shengchao Qin1,2, Martin Rinard3
National University of Singapore
2 Singapore-MIT Alliance
3 MIT
1
11th June 2004
PLDI 2004
1
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
2
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
top (younger)
• Lexically-Scoped Regions:
Stack-like lifetimes
construct for local region :
letreg r in e
• No Dangling References:
r4
r3
r2
Regions of components should have
longer lifetimes than its object.
r1
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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Region-Annotated Core-Java
region parameters for polymorphism
class invariant
method
precondition
region constraints
constraint
abstractions
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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
method precondition
Step 1 : Region parameters
from method signature.
Step 2 : Gather constraints
from method body.
Step 3 : Localise regions that do not escape.
Step 4 : Fix-point analysis for recursion.
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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 Subtyping
concept 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:
mutable
read-only
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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:
selection
argument
result
Method Override Rule:
CB Æ MA ) MB
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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
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