Specialized Reference Counting Garbage Collection using Data Structure Annotations By Eric Watkins...
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Transcript of Specialized Reference Counting Garbage Collection using Data Structure Annotations By Eric Watkins...
Specialized Reference Counting Garbage Collection using Data
Structure AnnotationsBy 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
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 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
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
BugManager Grid
Bug
Secondary refs Primary refs
• Programs can contain complicated, interconnected relationships between objects
Garbage Example #2
Bug
BugManager Grid
Bug
Secondary refs Primary 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