Specialized Reference Counting Garbage Collection using Data Structure Annotations By Eric Watkins and Dzin Avots for CS 343 Spring 2002

Motivation Improve use of ref counting for C++ Problem: cycles can’t be freed automatically, expensive to determine what is unreachable Solution: Use user specified data structure annotations to improve performance

Goals & Techniques Avoid unnecessary mark & sweeps –Acyclic Data Structure Optimizations Avoid some memory leaks –Reference Removal Invariant Model

Reference Counting using Smart Pointers

Garbage Example #1 Doubly Linked List Everything is strongly connected Nothing can be freed until all external refs are gone How can we recognize this data structure? ext ptr

Abstract Description of Data Structures The compiler can’t differentiate these two even though they will obviously be connected in different ways. ADDS can tell us how to customize smart pointer usage for certain data structures

Doubly Linked List ADDS

Binary Tree w/ Parents ADDS

ADDS spec Dimensions (1-d, 2-d, etc.) Directionality (forward and backward) Uniqueness of forward references (DAGs vs. lists) Cyclicity

Linked List Optimization The entire dimension is strongly connected, forwards and backwards Summarize with single ref counter for all external references to dimension

Dimensional Independence Dimensions X and Y are dependent if one traversal along X and another traversal along Y may lead to a common node by different paths

Other ADDS issues What about describing relations between different acyclic data structures? Could avoid marking & sweeping this structure etc.

Primary & Secondary Pointers Primary pointers – if these point to an object, then object is still in use Secondary pointers – point to object, but don’t affect use status Memory leak – Primary pointers are all removed, but a secondary pointer remains

Garbage Example #2 Bug BugManagerGrid Bug Secondary refsPrimary refs Programs can contain complicated, interconnected relationships between objects

Garbage Example #2 Bug BugManagerGrid Bug Secondary refsPrimary ref If the user forgets to remove the secondary reference, the Bug won’t be deleted.

Primary & Secondary Pointers User specifies primary & secondary pointers in ADDS specification Primary & Secondary pointer invariant can be expressed as instrumentation predicate in 3-valued logic Generate shape analysis to detect leak condition

Plan of Action Investigate use of ADDS specs to improve refcount performance Generate interprocedural shape analysis from primary & secondary edge specs Look into other improvements from user specifications

Related Work ADDS work: L. Hendren and J. Hummel, Abstractions for Recursive Pointer Data Structures: Improving the Analysis and Transformation of Imperative Programs Shape Analysis: M. Sagiv, T. Reps and R. Wilhelm, Parametric Shape Analysis via 3- Valued Logic