Programming with Regions

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

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Example (1)
reference count 1
7
Example (2)
reference count 2
8
Example (3)
reference count 2
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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 doesnt 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
• http//www.cs.berkeley.edu/dgay/rc
• Also
• Cyclone has its own region sub-system

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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-gtdata ralloc(r,struct rlist)
• / fill in data /
• rl-gtnext last last rl
• output_rlist(last)
• deleteregion(r)

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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

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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