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Improve trigger for acc(s) when s is a slice #471

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Jun 23, 2022
Merged

Improve trigger for acc(s) when s is a slice #471

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9757219
Improve trigger for acc(s) when s is a slice
jcp19 Jun 23, 2022
7dcee0c
Update src/main/scala/viper/gobra/translator/encodings/slices/SliceEn…
jcp19 Jun 23, 2022
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4 changes: 2 additions & 2 deletions src/main/scala/viper/gobra/ast/internal/Program.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -488,7 +488,7 @@ case class Exists(vars: Vector[BoundVar], triggers: Vector[Trigger], body: Expr)
}

sealed trait Permission extends Expr {
override val typ: Type = PermissionT(Addressability.rValue)
override def typ: Type = PermissionT(Addressability.rValue)
}

case class FullPerm(info: Source.Parser.Info) extends Permission
Expand Down Expand Up @@ -1025,7 +1025,7 @@ case class IntLit(v: BigInt, kind: IntegerKind = UnboundedInteger, base: NumBase
override def typ: Type = IntT(Addressability.literal, kind)
}

case class PermLit(dividend: BigInt, divisor: BigInt)(val info: Source.Parser.Info) extends Lit {
case class PermLit(dividend: BigInt, divisor: BigInt)(val info: Source.Parser.Info) extends Lit with Permission {
require(divisor != 0)
override def typ: Type = PermissionT(Addressability.literal)
}
Expand Down
1 change: 1 addition & 0 deletions src/main/scala/viper/gobra/ast/internal/utility/Nodes.scala
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Expand Up @@ -171,6 +171,7 @@ object Nodes {
case c: CurrentPerm => Seq(c.acc)
case PermMinus(exp) => Seq(exp)
case BinaryExpr(left, _, right, _) => Seq(left, right)
case _: PermLit => Seq.empty
}
case l: Lit => l match {
case IntLit(_, _, _) => Seq.empty
Expand Down
85 changes: 44 additions & 41 deletions src/main/scala/viper/gobra/translator/encodings/slices/SliceEncoding.scala
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Expand Up @@ -47,26 +47,17 @@ class SliceEncoding(arrayEmb : SharedArrayEmbedding) extends LeafTypeEncoding {
case Shared => vpr.Ref
}
}

/**
* Encodes assertions.
* [acc(m: []T, perm)] -> [forall i int :: 0 <= i && i < len(m) ==> acc(&m[i], perm)]
* [acc(m: []T, perm)] -> getCellPerms(m, perm, SliceBound.Len)
*/
override def assertion(ctx: Context): in.Assertion ==> CodeWriter[vpr.Exp] = {
default(super.assertion(ctx)) {
case n@ in.Access(in.Accessible.ExprAccess(exp :: ctx.Slice(elem)), perm) =>
val iterVar = in.BoundVar(ctx.freshNames.next(), in.IntT(Addressability.Exclusive))(n.info)
val underlyingType = in.SliceT(elem, Addressability.exprInAcc)
val quantifiedAssert = in.SepForall(
vars = Vector(iterVar),
triggers = Vector(in.Trigger(Vector(in.IndexedExp(exp, iterVar, underlyingType)(n.info)))(n.info)),
body = in.Implication(
in.And(
in.AtMostCmp(in.IntLit(0)(n.info), iterVar)(n.info),
in.LessCmp(iterVar, in.Length(exp)(n.info))(n.info))(n.info),
in.Access(in.Accessible.Address(in.IndexedExp(exp, iterVar, underlyingType)(n.info)), perm)(n.info)
)(n.info)
)(n.info)
ctx.assertion(quantifiedAssert)
case in.Access(in.Accessible.ExprAccess(exp :: ctx.Slice(_)), perm :: ctx.Perm()) =>
// in practice, requiring permissions to all elements within the length of the slice
// seems to be more common than requiring permissions to all elements within the capacity
getCellPerms(ctx)(exp, perm, SliceBound.Len)
}
}

Expand Down Expand Up @@ -185,7 +176,7 @@ class SliceEncoding(arrayEmb : SharedArrayEmbedding) extends LeafTypeEncoding {
}

// inhale forall i: int :: {loc(a, i)} 0 <= i && i < [cap] ==> Footprint[ a[i] ]
footprintAssertion <- getCellPerms(ctx)(slice, in.FullPerm(slice.info))
footprintAssertion <- getCellPerms(ctx)(slice, in.FullPerm(slice.info), SliceBound.Cap)
_ <- write(vpr.Inhale(footprintAssertion)(pos, info, errT))

lenExpr = in.Length(slice)(makeStmt.info)
Expand All @@ -201,7 +192,7 @@ class SliceEncoding(arrayEmb : SharedArrayEmbedding) extends LeafTypeEncoding {

// inhale forall i: int :: {loc(a, i)} 0 <= i && i < [len] ==> [ a[i] == dfltVal(T) ]
eqValueAssertion <- boundedQuant(
length = vprLength,
bound = vprLength,
trigger = (idx: vpr.LocalVar) =>
Seq(vpr.Trigger(Seq(ctx.slice.loc(vprSlice.localVar, idx)(pos, info, errT)))(pos, info, errT)),
body = (x: in.BoundVar) =>
Expand All @@ -215,31 +206,43 @@ class SliceEncoding(arrayEmb : SharedArrayEmbedding) extends LeafTypeEncoding {
}
}


/**
* Obtains permission to all cells of a slice
* getCellPerms[loc: []T] -> forall idx :: {loc(a, idx) 0 <= idx < cap(l) ==> Footprint[ loc[idx] ]
* getCellPerms[loc: []T] ->
* forall idx :: { loc(a, idx) } 0 <= idx < bound ==> Footprint[ loc[idx] ]
* where bound is len(l) if sliceBound = SliceBound.Len, and cap(l) otherwise.
*/
def getCellPerms(ctx: Context)(expr: in.Location, perm: in.Permission): CodeWriter[vpr.Exp] = expr match {
case loc :: ctx.Slice(_) / Exclusive =>
val (pos, info, errT) = loc.vprMeta

val cap = in.Capacity(loc)(loc.info)
val vprCap = ctx.expression(cap).res
val vprLoc = ctx.expression(loc).res
val trigger = (idx: vpr.LocalVar) =>
Seq(vpr.Trigger(Seq(ctx.slice.loc(vprLoc, idx)(pos, info, errT)))(pos, info, errT))
val underlyingBaseTyp = underlyingType(loc.typ)(ctx)
val body = (idx: in.BoundVar) => ctx.footprint(in.IndexedExp(loc, idx, underlyingBaseTyp)(loc.info), perm)
boundedQuant(vprCap, trigger, body)(loc)(ctx).map(forall =>
// to eliminate nested quantified permissions, which are not supported by the silver ast.
vu.bigAnd(viper.silver.ast.utility.QuantifiedPermissions.desugarSourceQuantifiedPermissionSyntax(forall))(pos, info, errT)
)
case c => Violation.violation(s"getCellPerm should only be called with exclusive slices, but got $c")
private def getCellPerms(ctx: Context)(expr: in.Expr, perm: in.Expr, sliceBound: SliceBound): CodeWriter[vpr.Exp] =
(expr, perm) match {
case (loc :: ctx.Slice(_), perm :: ctx.Perm()) =>
val (pos, info, errT) = loc.vprMeta
val bound = sliceBound match {
case SliceBound.Cap => in.Capacity(loc)(loc.info)
case SliceBound.Len => in.Length(loc)(loc.info)
}
val vprBound = ctx.expression(bound).res
val vprLoc = ctx.expression(loc).res
val trigger = (idx: vpr.LocalVar) =>
Seq(vpr.Trigger(Seq(ctx.slice.loc(vprLoc, idx)(pos, info, errT)))(pos, info, errT))
val underlyingBaseTyp = underlyingType(loc.typ)(ctx)
val body = (idx: in.BoundVar) => ctx.footprint(in.IndexedExp(loc, idx, underlyingBaseTyp)(loc.info), perm)
boundedQuant(vprBound, trigger, body)(loc)(ctx).map{ forall =>
import viper.silver.ast.utility.QuantifiedPermissions
// to eliminate nested quantified permissions, which are not supported by the silver ast.
vu.bigAnd(QuantifiedPermissions.desugarSourceQuantifiedPermissionSyntax(forall))(pos, info, errT)
}
case (p1, p2) =>
Violation.violation(s"getCellPerm expected a slice and a perm expression, but instead got a $p1 and $p2")
}

private sealed trait SliceBound
private object SliceBound {
case object Len extends SliceBound
case object Cap extends SliceBound
}

/** Returns: Forall idx :: {'trigger'(idx)} 0 <= idx && idx < 'length' => ['body'(idx)] */
private def boundedQuant(length: vpr.Exp, trigger: vpr.LocalVar => Seq[vpr.Trigger], body: in.BoundVar => CodeWriter[vpr.Exp])
/** Returns: Forall idx :: {'trigger'(idx)} 0 <= idx && idx < 'bound' => ['body'(idx)] */
private def boundedQuant(bound: vpr.Exp, trigger: vpr.LocalVar => Seq[vpr.Trigger], body: in.BoundVar => CodeWriter[vpr.Exp])
(src: in.Node)(ctx: Context)
: CodeWriter[vpr.Forall] = {

Expand All @@ -253,18 +256,18 @@ class SliceEncoding(arrayEmb : SharedArrayEmbedding) extends LeafTypeEncoding {
forall = vpr.Forall(
variables = Vector(vIdx),
triggers = trigger(vIdx.localVar),
exp = vpr.Implies(boundaryCondition(vIdx.localVar, length)(src), vBody)(pos, info, errT)
exp = vpr.Implies(boundaryCondition(vIdx.localVar, bound)(src), vBody)(pos, info, errT)
)(pos, info, errT)
} yield forall
}

/** Returns: 0 <= 'base' && 'base' < 'length'. */
private def boundaryCondition(base: vpr.Exp, length: vpr.Exp)(src : in.Node) : vpr.Exp = {
/** Returns: 0 <= 'base' && 'base' < 'bound'. */
private def boundaryCondition(base: vpr.Exp, bound: vpr.Exp)(src : in.Node) : vpr.Exp = {
val (pos, info, errT) = src.vprMeta

vpr.And(
vpr.LeCmp(vpr.IntLit(0)(pos, info, errT), base)(pos, info, errT),
vpr.LtCmp(base, length)(pos, info, errT)
vpr.LtCmp(base, bound)(pos, info, errT)
)(pos, info, errT)
}

Expand Down