From: Pascal Cuoq <Pascal.Cuoq@cea.fr>
To: caml-list@yquem.inria.fr
Subject: Re: [Caml-list] Physical counterpart to Pervasives.compare?
Date: Mon, 24 Aug 2009 14:30:40 +0200 [thread overview]
Message-ID: <67BF878D-BA80-4CE8-87BA-760AE8082517@cea.fr> (raw)
In-Reply-To: <20090824100004.6DBBDBBAF@yquem.inria.fr>
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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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next parent reply other threads:[~2009-08-24 12:32 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <20090824100004.6DBBDBBAF@yquem.inria.fr>
2009-08-24 12:30 ` Pascal Cuoq [this message]
2009-08-25 18:03 ` Christophe Raffalli
2009-08-26 10:25 ` Damien Doligez
[not found] <20090729030710.BB3C6BC5C@yquem.inria.fr>
2009-07-29 7:04 ` Pascal Cuoq
2009-08-24 7:01 ` Jean-Christophe Filliâtre
2009-07-29 1:25 Elnatan Reisner
2009-07-29 3:06 ` [Caml-list] " Edgar Friendly
2009-07-29 3:58 ` Edgar Friendly
2009-07-29 6:13 ` Alain Frisch
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