Combining Garbage Collection and Safe Manual Memory Management Michael Hicks University of Maryland, College Park Joint work with Greg Morrisett - Harvard, Dan Grossman - Uwash, and Trevor Jim - AT&T

Cyclone Derived from C, having similar goals –Exposes low-level data representations, provides fine-grained operations But memory safe –Restrictions to C (e.g., (int *)1 not allowed) –Additions and types to regain flexibility

Goal: Programmer Control Many reasonable MM choices –Garbage collection –Stack allocation – malloc/free –Reference counting Linux, COM –Arenas (individual allocation, bulk free) Apache, LCC Depends on the application

Unifying Theme: Region types Conceptually divide memory into regions –Different kinds of regions (e.g., not just bulk-free) Associate every pointer with a region Prevent dereferencing pointers into dead regions int *`r x; // x points into region `r *x = 3; // deref allowed if `r is live (inference often obviates annotations `r)

Regions Summary (PLDI 02) Region Variety Allocation (objects) Deallocation (what) (when) Aliasing (objects) Stackstaticwhole region exit of scope free Lexicaldynamic Heapsingle objects GC

Regions Summary (now) Region Variety Allocation (objects) Deallocation (what) (when) Aliasing (objects) Stackstaticwhole region exit of scope free Lexicaldynamic Dynamicmanual Heapsingle objects GC Uniquemanual restricted Refcounted

Enabled by Linearity (Affinity) Pointers whose state is carefully tracked To simplify programming: –Polymorphism –Temporary aliasing –Atomic swap (e.g., destructive reads) Main ideas close to Walker & Watkins Key contribution: extension and integration into realistic low-level language

Programming Experience Optimize for memory use –Important for embedded systems, OSs Optimize for speed –Servers, OSs, etc. Applications –Event-based webserver (only unique pointers) –MediaNet: Streaming data overlay network All six region varieties; packet data is unique or reference-counted

Memory Usage: webserver

MediaNet: gc (4 KB packets)

MediaNet: gc+free (4 KB packets)

MediaNet: throughput

Future Work Further generalization –Type-safe Reaps (Berger et al.) More programming experience Better inference (e.g. for alias) Formal model …

Conclusions High degree of control, safely: Sound mechanisms for programmer- controlled memory management –Region-based vs. object-based deallocation –Manual vs. automatic reclamation Region-annotated pointers within a simple framework –Lexical regions as unifying theme (alias,open) –Region polymorphism, for code reuse

More Information Cyclone homepage – Has papers and free distribution –Read about it, write some code!

Related Work (incomplete) Regions –ML-Kit (foundation for Cyclone’s type system) –RC –Reaps –Walker/Watkins Uniqueness –Wadler, Walker/Watkins, Clean –Alias types, Calculus of Capabilities, Vault –Destructive reads (e.g., Boyland)