On Sat, 2 Jul 2011 16:51:53 +0100, you wrote:
Apple recently announced a new static analysis in Clang called ARC (Automatic Reference Counting). The idea is to provide what GC provides (zero memory management code by the programmer), but not to incur the runtime penalty of having to have the GC run. It seems to be extremely effective in objective-C land.
I was wondering if any of the compiler gurus out there could comment on the applicability of this kind of analysis to Haskell. Dropping the GC and hence stopping it blocking all threads and killing parallel performance seems like nirvana.
Reference counting as a means of lifetime management has been around for a long time in Microsoft's Component Object Model. And in both (Microsoft) Visual Basic and (Borland/CodeGear/Embarcadero) Delphi, reference counting is automatic, in a way that appears to be essentially identical to what Apple is describing. So, the concept is nothing new; the new part is that it is being offerred in a C dialect.
The one major problem that's still left with ARC is that it does not solve the retain cycle problem of reference counting. Programmers must insert a "weak" keyword to break cycles in their data graphs. Could even this analysis be automated statically? Could we put up with a language extension that added this annotation?
For handling cycles, there are alternatives to weak references, but I don't know that any of them is better than weak references. Excluding weak references, I don't know of a way to deal with cycles that isn't computationally equivalent to what full-blown garbage collectors already do, so there's really nothing to be gained there. If there were a way to do static analysis, there would be no need for garbage collection as it is currently implemented--the compiler would be able to insert explicit object lifetime management directly into the code. -Steve Schafer