David Hopwood wrote:
Raphael Collet wrote:
David Hopwood wrote:
Hmm. I believe the following optimization is worth considering: when two
variables are either bound together or successfully tested for structural
equality, set both to point to the structure with the lower address.
(Note that this only incurs extra overhead in the slow path where the
full unification or structural equality algorithms are needed, not in the
fast paths where they either already have the same address, or one was
unbound.)
Your suggestion implies that the memory representation of a record could
be turned into an indirection to another representation of the same
record.
I explained this poorly. The intent was that no extra indirections are added;
the optimization only applies to cases where there is already an indirection.
Please read carefully my former reply:
http://www.mozart-oz.org/pipermail/mozart-users/2006/014360.html
I checked Mozart's unification procedure, and the "optimization" is
possible. A few nontrivial modifications are necessary, though. But I
am still not convinced it is worth the effort.
If the same comparison is done one more time, then the optimization will
pay off. In some cases, where it leads to a large data structure becoming
unreachable, it can pay off spectacularly, in terms of memory use and cache
behaviour. (This is the rationale behind preferring the lower address: you
want to consistently choose the same instance of the structure to survive.)
The problem is that the case you are mentioning simply never happens in
most programs!
In some implementations of GC'd languages, especially those with concurrent
GC (see <http://citeseer.csail.mit.edu/huelsbergen93concurrent.html>), and
lazy functional languages, it is already the case that any object access
might have to traverse a forwarding pointer. Then the optimization can be
applied in more cases.
We use that technique during GC, but this phase is not interleaved with
code interpretation.
Cheers,
raph
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