Re: [Caml-list] Physical counterpart to Pervasives.compare?
But you should still do the comparison with a unique C function written for this purpose. If you tried to use a convert address to int function, you would have a race condition between the conversion of each address and garbage collection. Or, if all major GCs are compacting and the minor heap is at a higher adress than the major heap, then OCaml's could also preserve the adress order between GC ... How much slower is the compacting major GC comparer to the standard one ? Cheers, Christophe signature.asc Description: OpenPGP digital signature ___ Caml-list mailing list. Subscription management: http://yquem.inria.fr/cgi-bin/mailman/listinfo/caml-list Archives: http://caml.inria.fr Beginner's list: http://groups.yahoo.com/group/ocaml_beginners Bug reports: http://caml.inria.fr/bin/caml-bugs
Re: [Caml-list] Physical counterpart to Pervasives.compare?
Pascal Cuoq a écrit : Elnatan Reisner wrote: Is there something that can complete this analogy: (=) is to (==) as Pervasives.compare is to ___? The simple solution is to number at creation the objects that you want to physically compare, using an additional field. You can do that while hash-consing your values, which has many other benefits than allowing physical comparison, as explained in this paper: http://www.lri.fr/~filliatr/ftp/publis/hash-consing2.pdf Ocaml code for hash-consing can be found on that page: http://www.lri.fr/~filliatr/software.en.html Hope this helps, -- Jean-Christophe ___ Caml-list mailing list. Subscription management: http://yquem.inria.fr/cgi-bin/mailman/listinfo/caml-list Archives: http://caml.inria.fr Beginner's list: http://groups.yahoo.com/group/ocaml_beginners Bug reports: http://caml.inria.fr/bin/caml-bugs
Re: [Caml-list] Physical counterpart to Pervasives.compare?
Pascal Cuoq a écrit :
Elnatan Reisner wrote:
Is there something that can complete this analogy:
(=) is to (==) as Pervasives.compare is to ___?
The simple solution is to number at creation the objects that you
want to
physically compare, using an additional field.
Since people are still participating in this topic,
I have a remark. It is more a what not to do kind of
remark, but after replying last time, I remembered
that the current algorithm used by OCaml's GC
for compaction does not change the order of blocks
in memory (byterun/compact.c). Therefore, with the
current version, if you make sure in some way
that the values that you want to compare
are allocated directly in the major heap
(there are a couple of ways to do that),
you can theoretically compare their addresses
as unsigned longs: their order will not change
during execution.
But you should still do the comparison
with a unique C function written for this purpose.
If you tried to use a convert address to int function,
you would have a race condition between the conversion
of each address and garbage collection.
This is all in very poor taste, even more so than
the usual discussions about == on this list.
Do I get some kind of prize for breaking the record?
Pascal
__
PS: I wrote a convert address to int function for
another purpose once. In order to increase my chances
for the prize, I will provide it here. Someone might be
interested in it. Perhaps someone who maintains a small
library for computing the size of an ML value...
external address_of_value: 'a - int = address_of_value
value address_of_value(value v)
{
return (Val_long(((unsigned long)v)/sizeof(long)));
}
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Re: [Caml-list] Physical counterpart to Pervasives.compare?
On 7/29/2009 3:25 AM, Elnatan Reisner wrote: Is there something that can complete this analogy: (=) is to (==) as Pervasives.compare is to ___? That is, is there a polymorphic total ordering with respect to *physical* entities, rather than to their structure? Not really. The physical location of heap values is not stable (because of the GC), so you cannot use it as a total ordering. It may be useful to know that the generic structural comparison has a physical behavior for OCaml objects (it compares their unique ids). So wrapping your values inside objects can be a good trick to get physical comparison (and hashing), as in: class ['a] physical_reference init = object val mutable current : 'a = init method get = current method set x = current - x end -- Alain ___ Caml-list mailing list. Subscription management: http://yquem.inria.fr/cgi-bin/mailman/listinfo/caml-list Archives: http://caml.inria.fr Beginner's list: http://groups.yahoo.com/group/ocaml_beginners Bug reports: http://caml.inria.fr/bin/caml-bugs
[Caml-list] Physical counterpart to Pervasives.compare?
Is there something that can complete this analogy: (=) is to (==) as Pervasives.compare is to ___? That is, is there a polymorphic total ordering with respect to *physical* entities, rather than to their structure? I'm afraid of getting into trouble with Obj.magic, but what would this do: let f (x:'a) (y:'a) = compare (Obj.magic x) (Obj.magic y) ? Or would annotations make any difference: let f (x:'a) (y:'a) = compare (Obj.magic x : int) (Obj.magic y : int) -Elnatan ___ Caml-list mailing list. Subscription management: http://yquem.inria.fr/cgi-bin/mailman/listinfo/caml-list Archives: http://caml.inria.fr Beginner's list: http://groups.yahoo.com/group/ocaml_beginners Bug reports: http://caml.inria.fr/bin/caml-bugs
Re: [Caml-list] Physical counterpart to Pervasives.compare?
Elnatan Reisner wrote: Is there something that can complete this analogy: (=) is to (==) as Pervasives.compare is to ___? That is, is there a polymorphic total ordering with respect to *physical* entities, rather than to their structure? No, but it'd be pretty trivial to implement through the C interface. I'm afraid of getting into trouble with Obj.magic, but what would this do: let f (x:'a) (y:'a) = compare (Obj.magic x) (Obj.magic y) ? Or would annotations make any difference: let f (x:'a) (y:'a) = compare (Obj.magic x : int) (Obj.magic y : int) -Elnatan Nope, Obj.magic and casting only have compile-time effects, the code given compiles exactly the same as [let f x y = compare x y]. If you had to stay in the OCaml realm, you might be able to do [let phys_comp (x:'a) (y:'a) = (Obj.magic x) - (Obj.magic y)], but it depends on the exact implementation of (-) on your architecture, as it may produce a value that's not an OCaml int when given non-ints as input. E ___ Caml-list mailing list. Subscription management: http://yquem.inria.fr/cgi-bin/mailman/listinfo/caml-list Archives: http://caml.inria.fr Beginner's list: http://groups.yahoo.com/group/ocaml_beginners Bug reports: http://caml.inria.fr/bin/caml-bugs
Re: [Caml-list] Physical counterpart to Pervasives.compare?
Edgar Friendly wrote: Elnatan Reisner wrote: I'm afraid of getting into trouble with Obj.magic, but what would this do: let f (x:'a) (y:'a) = compare (Obj.magic x) (Obj.magic y) ? Or would annotations make any difference: let f (x:'a) (y:'a) = compare (Obj.magic x : int) (Obj.magic y : int) -Elnatan Nope, Obj.magic and casting only have compile-time effects, the code given compiles exactly the same as [let f x y = compare x y]. It looks like I'm wrong on this - there's versions of compare specialized for floats, strings and ints. Maybe you *could* trigger the use of the caml_int_compare by this kind of type system manipulation. It looks quite safe to use on non-ints. Testing on your platform is possible, by trying to find store pointers to ints, so that caml_compare compares the ints, but the tricked caml_int_compare compares the pointers, and gets a different result. E ___ Caml-list mailing list. Subscription management: http://yquem.inria.fr/cgi-bin/mailman/listinfo/caml-list Archives: http://caml.inria.fr Beginner's list: http://groups.yahoo.com/group/ocaml_beginners Bug reports: http://caml.inria.fr/bin/caml-bugs
