A Principled Approach to Nondeferred Reference Counting Garbage Collection

1. A Principled Approach to Nondeferred Reference-Counting Garbage Collection† Pramod G. Joisha HP Labs, Palo Alto †This work was done when the author was at Microsoft Research. VEE’08 (ACM SIGPLAN/SIGOPS International Conference on Virtual Execution Environments) March 7, 2008

2. Classic RC Instrumentation local references global reference

3. Unique Advantages • Prompt reuse • Cache effect gives performance benefits • Low footprint • Can run in a smaller heap • Useful in memory-constrained environments • Incremental in time and space • No repeated traversals of long-lived data • Spatial locality usually no worse than mutator • Can present simpler overall designs • No stack maps needed, at least in single-thread case

4. What is “Nondeferred” RC Collection? • Three invariants should always hold: • All live data have positive reference counts • Reference count is zero when last reference disappears • Zero reference count implies dead data • Classic reference counting is nondeferred • Deutsch & Bobrow’s reference counting is not • There are subtleties in the definition • “Last reference” is from mutator’s standpoint • Reference count could be zero even before

5. Approximate reference counts Only heap references tallied Zero-Count Table (ZCT) Threads paused to reconcile Stacks scanned to purge ZCT Remaining entries garbage Nondeferred invariants might not always hold .g 1 a f x b 0 stack c y 1 z heap Deutsch & Bobrow’s Reference Counting globals

6. The Spectrum of RC Techniques

7. What is Different from Previous Efforts? • Past work addressed optimizations: • Given a nondeferred app, how to make it faster? • 2006 & 2007 ACM International Symposium on Memory Management • This work addresses a different obstacle: • How to convert object-oriented apps to use nondeferred? • The line of work has an overarching goal: • Making nondeferred RC garbage collection practical • Making general reference counting more efficient

8. Contributions of This Work • Addresses main problems to conversion: • How to handle object-oriented instruction sets uniformly? • How to insert RC updates to achieve early reclamation? • How to do this in the midst of modern language features? • Exceptions, object pinning, interior pointers • How to treat the run-time system? • Interaction of conversion with other phases • Impact on downstream phases should be minimal • Conversion has implications on inlining

9. Talk Outline • The RC update insertion phase • High-level process • Code templates • Trading code quality with eagerness • The scope of the insertion phase • How does the technique fare? • Summary • Some Further Thoughts

10. The RC Update Insertion Phase

11. array object 1 2 3 4 5 6 7 8 9 10 interior ptr shadow Preprocessing Stage • Motivation: Normalize the IR • Key step: Subsume interior pointers • Interior pointers similar to conventional pointers • But can only be used in a few well-defined ways • Should be honored by the garbage collector • Approach: Shadow interior pointer accesses

12. Analysis Stage • Find dying references using liveness analysis • Supplementary decrements may be needed • If references are both defined and live on entry to the statement s • If other references could be defined in s • Liveness modified for pinned references

13. Injection Stage • Three kinds of RC updates inserted • RC increment, standard and eager RC decrement • Operates in three steps • Insert RC updates against definitions and deaths • Insert RC increments against explicit intrafunction throws • Insert standard RC decrements into exception headers • Injection guided by code templates • Based on classifying a statement as call or “non-call”

14. Non-Call Statements

15. An Example

16. Issue with Representing the Eager Decrement

17. Trading Code Quality with Eagerness

18. Everything Shouldn’t be Converted! • All code in the run-time isn’t converted • Presently, C# attributes flag special code • [PreInitRefCounts] • Bootstrap memory allocation, and static initializers • [RecursiveRefCounts] • Code transitively reachable from addref and release • [ManualRefCounts] • Methods that directly manipulate reference counts • [ZombieRefCounts] • Code accessing zombie objects • Affixation rules can be mechanized

19. Related Work • The use of liveness in GC isn’t new • But past work considers it for a tracing collector • This work considers reference deaths • Important distinction: Arises from a basic difference between tracing and reference counting • One looks at live matter and the other at matter that is dead (see Bacon et al. OOPSLA’04). • Certain complications are unique here • E.g. exceptions

20. How does Nondeferred Fare? Xlisp interpreter (SPEC CINT95 port)

21. Summary • A systematic conversion algorithm • Instructions are treated in a uniform manner • Modern language features handled • Degree of eagerness can be varied • Conversion has implications on other parts • Has been implemented in a compiler • Many large programs have been successfully compiled

22. Some Further Thoughts • What remains for primetime? • Optimizing single-thread heap reference counting • Concurrent nondeferred reference counting • Cycle reclamation • Fragmentation • An ecosystem of supporting tools required • Presently, a memory leak verifier and profiler exist • But more needed! decreasing importance

23. Questions?