Alloy integer comparison semantics using "Forbid Overflow: Yes" -


i have following alloy module , run command:

sig { x : set } run {all a: a| #a.x<3 , #a.x>1} 2 a, 2 int

with "forbid overflow: no" alloy analyzer 4.2 (build date: 2012-09-25) not find instance. believe reason due overflow of constant 3 run predicate reads {all a: a| #a.x<-1 , #a.x>1}.

with "forbid overflow: yes" alloy analyzer finds instance. 

---instance--- integers={-2, -1, 0, 1} univ={-1, -2, 0, 1, a$0, a$1} int={-1, -2, 0, 1} seq/int={0} string={} none={} this/a={a$0, a$1} this/a<:x={a$0->a$0, a$0->a$1, a$1->a$0, a$1->a$1}

the alloy evaluator tells me predicate {all a: a| #a.x<3 , #a.x>1} used in run command evaluates false.

could please explain behavior? there difference in sematics of integer comparisons in evaluator , analyzer?

edit: noticed behavior different in latest experimental version: alloy 4.2_2014-03-07. not find instance. behavior expected.

you provided right answers in question, can reiterate them

  • the expected behavior (no instance) "correct" behavior model;
  • version 4.2 has known bugs regarding handling overflows, that's reason finds instance (those should fixed in latest version, hence correct behavior model);
  • under "wraparound" semantics of integers (overflow detection turned off), there still no instance, reason described (-3 being treated -1);
  • the evaluator still has issues (even in latest version), use wraparound semantics.

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