--- layout: fc_discuss_archives title: Message 19 from Frama-C-discuss on October 2013 ---
I am happy to assume that the inputs to the function are not NaNs (Jessie's default interpretation), so the contract should hold, right? Then it is just a question of why Z3 can't prove it. Is there a way to see the actual query that is submitted to Z3 (or any other prover)? As to Claude's comment that he has seen many VCs that Z3 could not discharge, I have seen this too, but only when using Frama-C+Jessie+Why. When I have used Z3, CVC3, etc., by hand, it is usually the opposite -- Z3 wins. Also Z3 wins many competitions. So it makes me wonder if there is a problem in the way the translation to Z3 is done. If it is really a shortcoming of Z3, it would be interesting to understand why. -Steve On Oct 8, 2013, at 4:16 AM, Guillaume Melquiond <Guillaume.Melquiond at inria.fr> wrote: > On 07/10/2013 22:35, Pascal Cuoq wrote: > >> 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? > > I am not able to test it in practice, so I will give the theoretical answer. > > A prover will be able to prove the formula, as long as your two NaNs come from the exact same place. So, in your example where b is truly a copy of a, then a == b will hold. Otherwise, the non-determinism that occurs while creating NaN data will cause the equality to fail. > > Best regards, > > Guillaume > > _______________________________________________ > Frama-c-discuss mailing list > Frama-c-discuss at lists.gforge.inria.fr > http://lists.gforge.inria.fr/cgi-bin/mailman/listinfo/frama-c-discuss