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Small simplifier improvements #863

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Small simplifier improvements #863

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17 changes: 17 additions & 0 deletions src/main/scala/viper/silver/ast/utility/Expressions.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -200,6 +200,23 @@ object Expressions {
case _ => false
}

def isKnownWellDefined(e: Exp, program: Option[Program]): Boolean = {
e match {
case FieldAccessPredicate(FieldAccess(rcv, _), prm) =>
// Extra case for field access predicates because the contained field access does NOT require already having the field permission.
isKnownWellDefined(rcv, program) && (prm.isEmpty || isKnownWellDefined(prm.get, program))
case _: FieldAccess | _: Unfolding | _: Applying | _: Asserting => false
case _: SeqIndex | _: MapLookup => false
case _: Div | _: Mod => false
case f: FuncApp =>
program match {
case Some(p) => p.findFunction(f.funcname).pres.isEmpty && f.args.forall(isKnownWellDefined(_, program))
case None => false // conservative
}
case other => other.subExps.forall(isKnownWellDefined(_, program))
}
}

// note: dependency on program for looking up function preconditions
def proofObligations(e: Exp): (Program => Seq[Exp]) = (prog: Program) => {
e.reduceTree[Seq[Exp]] {
Expand Down
67 changes: 54 additions & 13 deletions src/main/scala/viper/silver/ast/utility/Simplifier.scala
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Expand Up @@ -24,6 +24,10 @@ object Simplifier {
*/
def simplify[N <: Node](n: N, assumeWelldefinedness: Boolean = false): N = {

def isKnownWellDefined(e: Exp): Boolean = {
assumeWelldefinedness || Expressions.isKnownWellDefined(e, None)
}

val simplifySingle: PartialFunction[Node, Node] = {
// expression simplifications
case root: Exp if root.simplified.isDefined =>
Expand All @@ -43,19 +47,26 @@ object Simplifier {
case root@And(FalseLit(), _) => FalseLit()(root.pos, root.info)
case root@And(_, FalseLit()) => FalseLit()(root.pos, root.info)

case And(p1@FieldAccessPredicate(loc1, Some(perm1)), FieldAccessPredicate(loc2, Some(perm2))) if loc1 == loc2 =>
FieldAccessPredicate(loc1, Some(PermAdd(perm1, perm2)(perm1.pos, perm1.info)))(p1.pos, p1.info)

case Or(FalseLit(), right) => right
case Or(left, FalseLit()) => left
case root@Or(TrueLit(), _) => TrueLit()(root.pos, root.info)
case root@Or(_, TrueLit()) => TrueLit()(root.pos, root.info)

case root@Implies(FalseLit(), _) => TrueLit()(root.pos, root.info)
case Implies(_, tl@TrueLit()) if assumeWelldefinedness => tl
case Implies(l1, tl@TrueLit()) if isKnownWellDefined(l1) => tl
case Implies(TrueLit(), consequent) => consequent
case root@Implies(FalseLit(), _) => TrueLit()(root.pos, root.info)
case root@Implies(l1, Implies(l2, r)) => Implies(And(l1, l2)(), r)(root.pos, root.info)
case root@Implies(l1, i2@Implies(l2, r)) =>
if (l1 == l2)
i2
else
Implies(And(l1, l2)(), r)(root.pos, root.info)

// TODO: Consider checking if Expressions.proofObligations(left) is empty (requires adding the program as parameter).
case root@EqCmp(left, right) if assumeWelldefinedness && left == right => TrueLit()(root.pos, root.info)
case root@EqCmp(left, right) if left == right && isKnownWellDefined(left) => TrueLit()(root.pos, root.info)
case root@EqCmp(BoolLit(left), BoolLit(right)) =>
BoolLit(left == right)(root.pos, root.info)
case root@EqCmp(FalseLit(), right) => Not(right)(root.pos, root.info)
Expand All @@ -76,11 +87,11 @@ object Simplifier {
BoolLit(left != right)(root.pos, root.info)
case root@NeCmp(NullLit(), NullLit()) =>
FalseLit()(root.pos, root.info)
case root@NeCmp(left, right) if assumeWelldefinedness && left == right => FalseLit()(root.pos, root.info)
case root@NeCmp(left, right) if left == right && isKnownWellDefined(left) => FalseLit()(root.pos, root.info)

case CondExp(TrueLit(), ifTrue, _) => ifTrue
case CondExp(FalseLit(), _, ifFalse) => ifFalse
case CondExp(_, ifTrue, ifFalse) if assumeWelldefinedness && ifTrue == ifFalse =>
case CondExp(condition, ifTrue, ifFalse) if ifTrue == ifFalse && isKnownWellDefined(condition) && isKnownWellDefined(ifTrue) =>
ifTrue
case root@CondExp(condition, FalseLit(), TrueLit()) =>
Not(condition)(root.pos, root.info)
Expand All @@ -98,6 +109,36 @@ object Simplifier {
case root@CondExp(condition, ifTrue, TrueLit()) =>
Implies(condition, ifTrue)(root.pos, root.info)

case root@CondExp(c1, CondExp(c2, a, _), c) if c1 == c2 =>
if (a == c && isKnownWellDefined(c1))
a
else
CondExp(c1, a, c)(root.pos, root.info)

case root@CondExp(c1, a, CondExp(c2, _, c)) if c1 == c2 =>
if (a == c && isKnownWellDefined(c1))
a
else
CondExp(c1, a, c)(root.pos, root.info)

case root@CondExp(c1, CondExp(c2, a, b), c) if a == c =>
CondExp(And(c1, Not(c2)(c2.pos, c2.info))(c1.pos, c1.info), b, a)(root.pos, root.info)

case root@CondExp(c1, CondExp(c2, a, b), c) if b == c =>
CondExp(And(c1, c2)(c1.pos, c1.info), a, b)(root.pos, root.info)

case root@CondExp(c1, a, CondExp(c2, b, c)) if a == b =>
CondExp(Or(c1, c2)(c1.pos, c1.info), a, c)(root.pos, root.info)

case root@CondExp(c1, a, CondExp(c2, b, c)) if a == c =>
CondExp(Or(c1, Not(c2)(c2.pos, c2.info))(c1.pos, c1.info), a, b)(root.pos, root.info)

case root@CondExp(c1, a, Implies(c2, b)) if a == b =>
Implies(Or(c1, c2)(c1.pos, c1.info), a)(root.pos, root.info)

case root@CondExp(c1, Implies(c2, a), b) if a == b =>
Implies(Or(Not(c1)(c1.pos, c1.info), c2)(c1.pos, c1.info), a)(root.pos, root.info)

case root@Forall(_, _, BoolLit(literal)) =>
BoolLit(literal)(root.pos, root.info)
case root@Exists(_, _, BoolLit(literal)) =>
Expand All @@ -112,16 +153,16 @@ object Simplifier {
case root@PermMul(AnyPermLiteral(a, b), AnyPermLiteral(c, d)) =>
val product = Rational(a, b) * Rational(c, d)
FractionalPerm(IntLit(product.numerator)(root.pos, root.info), IntLit(product.denominator)(root.pos, root.info))(root.pos, root.info)
case PermMul(_, np@NoPerm()) if assumeWelldefinedness => np
case PermMul(np@NoPerm(), _) if assumeWelldefinedness => np
case PermMul(p, np@NoPerm()) if isKnownWellDefined(p) => np
case PermMul(np@NoPerm(), p) if isKnownWellDefined(p) => np

case PermMul(wc@WildcardPerm(), _) if assumeWelldefinedness => wc
case PermMul(_, wc@WildcardPerm()) if assumeWelldefinedness => wc
case PermMul(wc@WildcardPerm(), p) if isKnownWellDefined(p) => wc
case PermMul(p, wc@WildcardPerm()) if isKnownWellDefined(p) => wc

case root@PermGeCmp(a, b) if assumeWelldefinedness && a == b => TrueLit()(root.pos, root.info)
case root@PermLeCmp(a, b) if assumeWelldefinedness && a == b => TrueLit()(root.pos, root.info)
case root@PermGtCmp(a, b) if assumeWelldefinedness && a == b => FalseLit()(root.pos, root.info)
case root@PermLtCmp(a, b) if assumeWelldefinedness && a == b => FalseLit()(root.pos, root.info)
case root@PermGeCmp(a, b) if a == b && isKnownWellDefined(a) => TrueLit()(root.pos, root.info)
case root@PermLeCmp(a, b) if a == b && isKnownWellDefined(a) => TrueLit()(root.pos, root.info)
case root@PermGtCmp(a, b) if a == b && isKnownWellDefined(a) => FalseLit()(root.pos, root.info)
case root@PermLtCmp(a, b) if a == b && isKnownWellDefined(a) => FalseLit()(root.pos, root.info)

case root@PermGtCmp(AnyPermLiteral(a, b), AnyPermLiteral(c, d)) =>
BoolLit(Rational(a, b) > Rational(c, d))(root.pos, root.info)
Expand Down