Re: [isabelle] Efficient Integers?



Hello,

although it is generally unsound, is there still a simple way to get machine integers?

I have an application that uses only positive integers between 0 and 2^12 and the only operation performed on these numbers is bitwise disjunction (which leaves them in this range). In the logic, I used the nat type and define the bit-or using div/mod, but, of course, this is very slow when executed (proof checking takes almost half an hour since there is a lot computation involved). Since the range is bounded, there is a "meta-theoretic" argument that machine integers would not introduce unsoundness, so I wonder if it is possible to "cheat" and use them and instruct the code generator to call the target-language bit-or in place of my div/mod defined bit-or (I tried something like code_const "bitor" (SML "IntInf.orb ((_), (_))") (Haskell infixl 6 "(.|.)"), but it did not give expected efficiency since big integers are used). Evaluations are used within a "proof-by-evaluation" paradigm and I use the method "by eval".

I suppose the other option would be not to use nats but machine words in the logic level, but then the formalization would be a bit more complicated, and I not sure what theory I should start from (I assume there is such a formalization in the Library)?

Thank you very much,
Filip

On Fri, 10 Feb 2012, Tobias Nipkow wrote:

PS No, you don't get machine integers, that would be unsound. You get
big integers, which are language specific. I assume that in most of the
languages we support big integers are like machine integers plus some
fixed overhead, as long as you stay in the machine integer range.






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