Some quick background: CVC4 has two primary contexts: the assertion
context (which corresponds to the push/pops issued by the user) and the
decision/SAT context (which corresponds to the push/pops done by the
user and each decision made by MiniSat.
Before
2cc0e2c6a691fb6d30ed8879832b49bc1f277d77, we had an additonal
push/pop when doing the actual solving. With these removed, it could
happen that we get a wrong result when doing incremental solving:
```
...
; check-sat call returns sat, the decision level in the SAT solver is
; non-zero at this point
(check-sat)
; Solver::addClause_() determines that we cannot add the clause to the
; SAT solver directly because the decision level is non-zero (i.e. the
; clause is treated like a theory lemma), thus it is added to the lemmas
; list.
(assert false)
; The solver stores some information about the current state, including
; the value of the "ok" flag in Solver, which indicates whether the
; current constraints are unsatisfiable. Note that "ok" is true at this
; point.
(push)
; Now, in Solver::updateLemmas(), we add clauses from the lemmas list.
; The problem is that empty clauses (which "false" is after removing "false"
; literals) and unit clauses are not added like normal clauses. Instead,
; the empty clause, essentially just results in the "ok" flag being set to
; false (unit clauses are added to the decision trail).
(check-sat)
; Here, the solver restores the information stored during
; (push), including the "ok" flag, which is now true again.
(pop)
; At this point, the solver has "forgotten" about (assert false) since
; the "ok" flag is back to true and it answers sat.
(check-sat)
```
There are multiple ways to look at the problem and to fix it:
- One could argue that an input assertion should always be added
directly to the clauses in the SAT solver, i.e. the solver should
always be at decision level 0 when we are adding input clauses. The
advantage of this is that it is relatively easy to implement,
corresponds to what the original MiniSat code does (it calls
Solver::cancelUntil(0) after solving), and is no worse than what we had
before commit
2cc0e2c6a691fb6d30ed8879832b49bc1f277d77 (push/pop do a
strict superset of what resetting the decision at the current assertion
level does). The disadvantage is that we might throw away some useful work.
- One could argue that is fine that (assert false) is treated like a
theory lemma. The advantage being that it might result in more efficient
solving and the disadvantage being that it is much trickier to
implement.
Unfortunately, it is not quite clear from the code what the original
intention was but we decided to implement the first solution. This
commit exposes a method in MiniSat to reset the decisions at the current
assertion level. We call this method from SmtEngine after solving.
Resetting the decisions directly after solving while we are still in
MiniSat does not work because it causes issues with datastructures in
the SMT solver that use the decision context (e.g.
TheoryEngine::d_incomplete seems to be reset too early, resulting in us
answering sat instead of unknown).
// Fit to size
ps.shrink(i - j);
- // If we are in solve or decision level > 0
- if (minisat_busy || decisionLevel() > 0) {
+ // If we are in solve_ or propagate
+ if (minisat_busy)
+ {
Debug("pf::sat") << "Add clause adding a new lemma: ";
for (int k = 0; k < ps.size(); ++k) {
Debug("pf::sat") << ps[k] << " ";
// Debug("cores") << "lemma push " << proof_id << " " << (proof_id & 0xffffffff) << std::endl;
// lemmas_proof_id.push(proof_id);
} else {
+ assert(decisionLevel() == 0);
+
// If all false, we're in conflict
if (ps.size() == falseLiteralsCount) {
if(PROOF_ON()) {
}
}
-void Solver::popTrail() {
- cancelUntil(0);
-}
+void Solver::resetTrail() { cancelUntil(0); }
//=================================================================================================
// Major methods:
ScopedBool scoped_bool(minisat_busy, true);
- popTrail();
+ assert(decisionLevel() == 0);
model.clear();
conflict.clear();
void Solver::push()
{
assert(enable_incremental);
+ assert(decisionLevel() == 0);
- popTrail();
++assertionLevel;
Debug("minisat") << "in user push, increasing assertion level to " << assertionLevel << std::endl;
trail_ok.push(ok);
{
assert(enable_incremental);
- // Pop the trail to 0 level
- popTrail();
assert(decisionLevel() == 0);
// Pop the trail below the user level
void push ();
void pop ();
+ /*
+ * Reset the decisions in the DPLL(T) SAT solver at the current assertion
+ * level.
+ */
+ void resetTrail();
// addClause returns the ClauseId corresponding to the clause added in the
// reference parameter id.
bool addClause (const vec<Lit>& ps, bool removable, ClauseId& id); // Add a clause to the solver.
void theoryCheck (CVC4::theory::Theory::Effort effort); // Perform a theory satisfiability check. Adds lemmas.
CRef updateLemmas (); // Add the lemmas, backtraking if necessary and return a conflict if there is one
void cancelUntil (int level); // Backtrack until a certain level.
- void popTrail (); // Backtrack the trail to the previous push position
int analyze (CRef confl, vec<Lit>& out_learnt, int& out_btlevel); // (bt = backtrack)
void analyzeFinal (Lit p, vec<Lit>& out_conflict); // COULD THIS BE IMPLEMENTED BY THE ORDINARIY "analyze" BY SOME REASONABLE GENERALIZATION?
bool litRedundant (Lit p, uint32_t abstract_levels); // (helper method for 'analyze()') - true if p is redundant
d_minisat->pop();
}
+void MinisatSatSolver::resetTrail() { d_minisat->resetTrail(); }
+
/// Statistics for MinisatSatSolver
MinisatSatSolver::Statistics::Statistics(StatisticsRegistry* registry) :
void pop() override;
+ void resetTrail() override;
+
void requirePhase(SatLiteral lit) override;
bool flipDecision() override;
toDimacs();
return l_Undef;
}
- popTrail();
+ assert(decisionLevel() == 0);
vec<Var> extra_frozen;
lbool result = l_True;
Debug("prop") << "pop()" << endl;
}
+void PropEngine::resetTrail()
+{
+ d_satSolver->resetTrail();
+ Debug("prop") << "resetTrail()" << endl;
+}
+
unsigned PropEngine::getAssertionLevel() const {
return d_satSolver->getAssertionLevel();
}
*/
void pop();
+ /*
+ * Reset the decisions in the DPLL(T) SAT solver at the current assertion
+ * level.
+ */
+ void resetTrail();
+
/**
* Get the assertion level of the SAT solver.
*/
virtual void pop() = 0;
+ /*
+ * Reset the decisions in the DPLL(T) SAT solver at the current assertion
+ * level.
+ */
+ virtual void resetTrail() = 0;
+
virtual bool properExplanation(SatLiteral lit, SatLiteral expl) const = 0;
virtual void requirePhase(SatLiteral lit) = 0;
}
}
+ d_propEngine->resetTrail();
+
// Pop the context
if (didInternalPush)
{
regress0/push-pop/boolean/fuzz_48.smt2 \
regress0/push-pop/boolean/fuzz_49.smt2 \
regress0/push-pop/boolean/fuzz_50.smt2 \
+ regress0/push-pop/bug1990.smt2 \
regress0/push-pop/bug233.cvc \
regress0/push-pop/bug654-dd.smt2 \
regress0/push-pop/bug691.smt2 \
--- /dev/null
+; COMMAND-LINE: --incremental
+; EXPECT: sat
+; EXPECT: unsat
+; EXPECT: unsat
+(set-logic QF_SAT)
+(declare-fun v1 () Bool)
+(declare-fun v2 () Bool)
+(assert (or v1 v2))
+(check-sat)
+(assert false)
+(push)
+(check-sat)
+(pop)
+(check-sat)
projects / cvc5.git / commitdiff