--- layout: fc_discuss_archives title: Message 12 from Frama-C-discuss on October 2013 ---
On Mon, Oct 7, 2013 at 9:10 AM, Claude March? <Claude.Marche at inria.fr>wrote: > > Pascal, I am sure you know that the default model in Jessie rules out > special values (infinities and NaNs). Ahem. Yes, of course, I know the large and the small of it. But for the sake of everyone else on this list, please explain it as if I wasn't such an expert. > PS: just for the braves who want to play with special values, Jessie has > a model with special values > > #pragma JessieFloatModel(full) > So what happens with the ACSL formula a == b, when the program variable b contains a copy of the program variable a (that contain NaN), in this ?full? float model, then? Because == is still the (reflexive) mathematical equality, not the IEEE equality between doubles that can also be introduced in ACSL as a convenient additional predicate ieee754_eq of double arguments that would match the semantics of == in C, right? And, incidentally, a==b is typed as an equality between reals in this case, isn't it? So the formula is in a way equivalent to: (real)NaN == (real)NaN And the above formula is not dissimilar to 1 / 0 == 1 / 0, in that neither side can be evaluated further (but ACSL, as a first-order logic, is total, so these terms exist). And, like 1/0 == 1/0, it is an instance of \forall x, x == x, so it is correct for a prover to infer that this formula is true? Pascal -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.gforge.inria.fr/pipermail/frama-c-discuss/attachments/20131007/bc4bc509/attachment.html>