public static IEnumerable <BitSet> Choose(this BitSet b, int choose)
{
if (choose == 0)
{
// Choosing zero elements from any set gives the empty set.
yield return(BitSet.Empty);
}
else if (b.Any())
{
// We are choosing at least one element from a set that has
// a first element. Get the first element, and the set
// lacking the first element.
int first = b.First();
BitSet rest = b.Remove(first);
// These are the permutations that contain the first element:
foreach (BitSet r in rest.Choose(choose - 1))
{
yield return(r.Add(first));
}
// These are the permutations that do not contain the first element:
foreach (BitSet r in rest.Choose(choose))
{
yield return(r);
}
}
}
/// <summary>
/// Recursive function that lifts the specified instruction.
/// The input instruction is expected to a subexpression of the trueInst
/// (so that all nullableVars are guaranteed non-null within this expression).
///
/// Creates a new lifted instruction without modifying the input instruction.
/// On success, returns (new lifted instruction, bitset).
/// If lifting fails, returns (null, null).
///
/// The returned bitset specifies which nullableVars were considered "relevant" for this instruction.
/// bitSet[i] == true means nullableVars[i] was relevant.
///
/// The new lifted instruction will have equivalent semantics to the input instruction
/// if all relevant variables are non-null [except that the result will be wrapped in a Nullable{T} struct].
/// If any relevant variable is null, the new instruction is guaranteed to evaluate to <c>null</c>
/// without having any other effect.
/// </summary>
(ILInstruction, BitSet) DoLift(ILInstruction inst)
{
if (MatchGetValueOrDefault(inst, out ILVariable inputVar))
{
// n.GetValueOrDefault() lifted => n.
BitSet foundIndices = new BitSet(nullableVars.Count);
for (int i = 0; i < nullableVars.Count; i++)
{
if (nullableVars[i] == inputVar)
{
foundIndices[i] = true;
}
}
if (foundIndices.Any())
{
return(new LdLoc(inputVar)
{
ILRange = inst.ILRange
}, foundIndices);
}
else
{
return(null, null);
}
}
else if (inst is Conv conv)
{
var(arg, bits) = DoLift(conv.Argument);
if (arg != null)
{
if (conv.HasFlag(InstructionFlags.MayThrow) && !bits.All(0, nullableVars.Count))
{
// Cannot execute potentially-throwing instruction unless all
// the nullableVars are arguments to the instruction
// (thus causing it not to throw when any of them is null).
return(null, null);
}
var newInst = new Conv(arg, conv.InputType, conv.InputSign, conv.TargetType, conv.CheckForOverflow, isLifted: true)
{
ILRange = conv.ILRange
};
return(newInst, bits);
}
}
else if (inst is BitNot bitnot)
{
var(arg, bits) = DoLift(bitnot.Argument);
if (arg != null)
{
var newInst = new BitNot(arg, isLifted: true, stackType: bitnot.ResultType)
{
ILRange = bitnot.ILRange
};
return(newInst, bits);
}
}
else if (inst is BinaryNumericInstruction binary)
{
var(left, right, bits) = DoLiftBinary(binary.Left, binary.Right);
if (left != null && right != null)
{
if (binary.HasFlag(InstructionFlags.MayThrow) && !bits.All(0, nullableVars.Count))
{
// Cannot execute potentially-throwing instruction unless all
// the nullableVars are arguments to the instruction
// (thus causing it not to throw when any of them is null).
return(null, null);
}
var newInst = new BinaryNumericInstruction(
binary.Operator, left, right,
binary.LeftInputType, binary.RightInputType,
binary.CheckForOverflow, binary.Sign,
isLifted: true
)
{
ILRange = binary.ILRange
};
return(newInst, bits);
}
}
else if (inst is Comp comp && !comp.IsLifted && comp.Kind == ComparisonKind.Equality &&
MatchGetValueOrDefault(comp.Left, out ILVariable v) &&
NullableType.GetUnderlyingType(v.Type).IsKnownType(KnownTypeCode.Boolean) &&
comp.Right.MatchLdcI4(0)
)
{
// C# doesn't support ComparisonLiftingKind.ThreeValuedLogic,
// except for operator! on bool?.
var(arg, bits) = DoLift(comp.Left);
Debug.Assert(arg != null);
var newInst = new Comp(comp.Kind, ComparisonLiftingKind.ThreeValuedLogic, comp.InputType, comp.Sign, arg, comp.Right.Clone())
{
ILRange = comp.ILRange
};
return(newInst, bits);
}
/// <summary>
/// Recursive function that lifts the specified instruction.
/// The input instruction is expected to a subexpression of the trueInst
/// (so that all nullableVars are guaranteed non-null within this expression).
///
/// Creates a new lifted instruction without modifying the input instruction.
/// On success, returns (new lifted instruction, bitset).
/// If lifting fails, returns (null, null).
///
/// The returned bitset specifies which nullableVars were considered "relevant" for this instruction.
/// bitSet[i] == true means nullableVars[i] was relevant.
///
/// The new lifted instruction will have equivalent semantics to the input instruction
/// if all relevant variables are non-null [except that the result will be wrapped in a Nullable{T} struct].
/// If any relevant variable is null, the new instruction is guaranteed to evaluate to <c>null</c>
/// without having any other effect.
/// </summary>
(ILInstruction, BitSet) DoLift(ILInstruction inst)
{
if (MatchGetValueOrDefault(inst, out ILVariable inputVar))
{
// n.GetValueOrDefault() lifted => n.
BitSet foundIndices = new BitSet(nullableVars.Count);
for (int i = 0; i < nullableVars.Count; i++)
{
if (nullableVars[i] == inputVar)
{
foundIndices[i] = true;
}
}
if (foundIndices.Any())
{
return(new LdLoc(inputVar)
{
ILRange = inst.ILRange
}, foundIndices);
}
else
{
return(null, null);
}
}
else if (inst is Conv conv)
{
var(arg, bits) = DoLift(conv.Argument);
if (arg != null)
{
if (conv.HasFlag(InstructionFlags.MayThrow) && !bits.All(0, nullableVars.Count))
{
// Cannot execute potentially-throwing instruction unless all
// the nullableVars are arguments to the instruction
// (thus causing it not to throw when any of them is null).
return(null, null);
}
var newInst = new Conv(arg, conv.InputType, conv.InputSign, conv.TargetType, conv.CheckForOverflow, isLifted: true)
{
ILRange = conv.ILRange
};
return(newInst, bits);
}
}
else if (inst is BinaryNumericInstruction binary)
{
var(left, leftBits) = DoLift(binary.Left);
var(right, rightBits) = DoLift(binary.Right);
if (left != null && right == null && SemanticHelper.IsPure(binary.Right.Flags))
{
// Embed non-nullable pure expression in lifted expression.
right = binary.Right.Clone();
}
if (left == null && right != null && SemanticHelper.IsPure(binary.Left.Flags))
{
// Embed non-nullable pure expression in lifted expression.
left = binary.Left.Clone();
}
if (left != null && right != null)
{
var bits = leftBits ?? rightBits;
if (rightBits != null)
{
bits.UnionWith(rightBits);
}
if (binary.HasFlag(InstructionFlags.MayThrow) && !bits.All(0, nullableVars.Count))
{
// Cannot execute potentially-throwing instruction unless all
// the nullableVars are arguments to the instruction
// (thus causing it not to throw when any of them is null).
return(null, null);
}
var newInst = new BinaryNumericInstruction(
binary.Operator, left, right,
binary.LeftInputType, binary.RightInputType,
binary.CheckForOverflow, binary.Sign,
isLifted: true
)
{
ILRange = binary.ILRange
};
return(newInst, bits);
}
}
return(null, null);
}