[fpc-devel] Improving Ref Counting
jamie-junk at blueyonder.co.uk
Sun Feb 27 12:35:22 CET 2005
Rather than continuing the GC stuff which seems fruitless I thought it
might be better to improve what we have with ref counting (whilst taking
a leaf out of the GC book as well).
I note that ref counting is very fast without all the implicit
try..finally stuff so I was wondering if it was possible to implement it
without them (whilst still being able to free them even if an exception
My proposal is to do the following (its a hybrid ref count/GC):
1) Remove all implicit try..finally in ref counting.
2) Have a single linked list that contains references to all managed
objects (ansi strings, variants etc)
3) Each managed object also has a single linked list that contains
addresses of all pointers that reference it.
4) inc ref count means adding the pointer's address to that list
5) dec ref count means removing the pointer's address from that list.
When that list is empty the object is freed. (a check for eliminating
circular refs should also be done if applicable)
6) Whenever an exception is thrown, wait until its either handled or
fully propagated and then perform some garbage collection. (traverse the
single linked list of all managed objects and for each object check
whether anything that references it is still valid and delete if
Im not sure if there are any corner cases where the above would not work
but it does have the advantage of being very fast and very efficient
memory wise. It also avoids the problem of extended lifetimes that you
get with GCs. The only performance hit you will get is when an exception
occurs but considering they only occur in exceptional circumstances and
that exceptions are slow anyhow I dont think thats any loss. If no
exceptions are thrown then no GC needs to be performed ever. Can we get
away with doing this?
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