So, I've been thinking for a while that the implementation of GC that
Apple seem to have settled upon is, to be polite, a complete
disaster. The need for weak pointers means that it has no advantages
over ref counting and lots of disadvantages.
One thing that Apple seem to have forgotten is that reference counting
is a form of GC - replacing it with a tracing collector doesn't make
much sense, especially if it's a very poor tracing collector, and
adding GC to the compiler, rather than the library, goes against the
spirit of Objective-C.
Reference counting works quite well, and doesn't have very large
overheads (it also plays nicely with swapping, which tracing doesn't
and leads to some quite pathological behaviour if you have low
memory), but it has one significant flaw - it doesn't handle cycles.
Adding a cycle detector to NSObject seemed like a better solution.
I recently came across a paper from 2001 which suggests exactly this.
They observe that you only need to scan for cycles when -release is
called and the retain count is > 0. They propose a very efficient
algorithm for doing this, which we can make slightly more efficient
(they suggest deferring the cycle detection. If we defer it until the
current autorelease pool is popped then we can delete a load of
objects that might have been cycles and carry on).
I have started implementing this, and it shows potential. For now, I
will develop it in an external framework, and move it into GNUstep
base later if other people are interested in it.
One other thing I have been pondering is altering the behaviour of
NSAutoreleasePools so that:
1) +alloc adds the newly-created object to the autorelease pool.
2) -autorelease is a no-op
3) Objects are only free'd when their ref count is 0 and the top
autorelease pool is pop'd.
This would make interacting with GNUstep from other languages (which
expect full GC) simpler, and would simplify memory management since
every created object would have a refcount of 1 (irrespective of how
it was created) and autorelease would never need to be explicitly
called.
Thoughts?
David
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