private ValueDispatchNode GatherValueDispatchNodes(
BoundDecisionDagNode node,
HashSet <BoundDecisionDagNode> loweredNodes,
BoundDagTemp input)
{
IValueSetFactory fac = ValueSetFactory.ForType(input.Type);
return(GatherValueDispatchNodes(node, loweredNodes, input, fac));
}
/// <summary>
/// Given a decision dag and a constant-valued input, produce a simplified decision dag that has removed all the
/// tests that are unnecessary due to that constant value. This simplification affects flow analysis (reachability
/// and definite assignment) and permits us to simplify the generated code.
/// </summary>
public BoundDecisionDag SimplifyDecisionDagIfConstantInput(BoundExpression input)
{
if (input.ConstantValue == null)
{
return(this);
}
else
{
ConstantValue inputConstant = input.ConstantValue;
return(Rewrite(makeReplacement));
// Make a replacement for a given node, using the precomputed replacements for its successors.
BoundDecisionDagNode makeReplacement(BoundDecisionDagNode dag, Func <BoundDecisionDagNode, BoundDecisionDagNode> replacement)
{
if (dag is BoundTestDecisionDagNode p)
{
// This is the key to the optimization. The result of a top-level test might be known if the input is constant.
switch (knownResult(p.Test))
{
case true:
return(replacement(p.WhenTrue));
case false:
return(replacement(p.WhenFalse));
}
}
return(TrivialReplacement(dag, replacement));
}
// Is the decision's result known because the input is a constant?
bool?knownResult(BoundDagTest choice)
{
if (!choice.Input.IsOriginalInput)
{
// This is a test of something other than the main input; result unknown
return(null);
}
switch (choice)
{
case BoundDagExplicitNullTest d:
return(inputConstant.IsNull);
case BoundDagNonNullTest d:
return(!inputConstant.IsNull);
case BoundDagValueTest d:
return(d.Value == inputConstant);
case BoundDagTypeTest d:
return(inputConstant.IsNull ? (bool?)false : null);
case BoundDagRelationalTest d:
var f = ValueSetFactory.ForType(input.Type);
if (f is null)
{
return(null);
}
// TODO: When ValueSetFactory has a method for comparing two values, use it.
var set = f.Related(d.Relation.Operator(), d.Value);
return(set.Any(BinaryOperatorKind.Equal, inputConstant));
default:
throw ExceptionUtilities.UnexpectedValue(choice);
}
}
}
}