1. Programming with Regions
Chris Durham
CSCI 297
June 16th, 2005

2. Paper
Language Support for Regions, David Gay and Alex Aiken; UC Berkeley, 2001

3. Memory Management Issues
Allocation / Deallocation mismatch
malloc() / free()
Dereference ‘dangling’ pointers
Heap fragmentation, boundary issues
Sizing, when to allocate
Remembering to free() !

4. Garbage Collection Questions
When to scan?
How often?
Rearranging?
Paper states that they did some work with gcc and the Boehm-Weiser garbage collector

5. Regions Concept to address some of these issues
All allocations of a particular ‘type’ allocated to same ‘region’
Reference count kept to the ‘region’
Can only deallocate region after no references exist; deallocates all ‘objects’ within the region
Best used for complex data structures

6. Example (1)
reference count = 1

7. Example (2)
reference count = 2

8. Example (3)
reference count = 2

9. Region Benefits
Region can only be de-allocated when reference count == 0; attempts abort program when checking enabled
Rely on region library to optimize heap usage and make garbage collection easier - presumably, implementation of regions is well structured and uses knowledge of allocator
Programmer doesn’t necessarily have to focus on memory management to same degree as with traditional malloc()/free()
In some ways, this concept is similar to resource pooling as in Apache, but is unstructured

10. RC Compiler
A library and compiler (rcc), compiles annotated code to C code. Introduces several annotations
Also:
Cyclone has its own region sub-system

11. Code snippet struct rlist { struct rlist *sameregion next;
struct finfo *sameregion data;
} *rl, *last = NULL;
region r = newregion();
while (…) {
rl = ralloc(r,struct rlist);
rl->data = ralloc(r,struct rlist);
/* fill in data */
rl->next = last; last = rl; }
output_rlist(last);
deleteregion(r);

12. Pointer annotations et al
sameregion: in same region
traditional: not in a region at all
parentptr: you can have sub-regions, this means this pointer exists in the region one level higher
struct Foo *sameregion foo;
struct Bar *traditional bar;
struct Parent *parentptr parent;
struct region has a member ‘rc’ for reference count
regionof() to determine what region an identifier is in
Also: a constraint inference system that proves some type safety situations was built into rcc, enables them to remove some runtime checks that verify the type constraints to speed things up

13. Results

14. Conclusions
Regions can help make memory management easier by eliminating the need for the programmer to concentrate on some memory management issues and it even speeds up things a bit over traditional methods
As we discussed previously, benchmarks and real life are two different things
- no real test of the robustness of the region library itself

15. Original Problems? / New Problems
Of the original problems, which ones do regions address?
What new challenges are there for the programmer?
What are the security implications here?

16. References
Language Support for Regions; David Gay and Alex Aiken; UC Berkeley, 2001
Region-Based Memory Mapping in Cyclone, Dan Grossman, Greg Morrisett, Trevor Jim, Michael Hicks, Yanling Wang, James Cheney; Cornell University 2002