private static Dictionary <Node, ICollection <ILAssignmentStatement> > InsertPhiNodes(ILCompilationUnit unit)
{
var result = unit.ControlFlowGraph.Nodes.ToDictionary(
x => x,
x => (ICollection <ILAssignmentStatement>) new List <ILAssignmentStatement>());
// We try to find all variables that have more than one assignment, and therefore have multiple
// versions of it during execution of the program. This is only a problem when they have different
// values at join nodes, as depicted below. We therefore need to get to the dominance frontier of
// those nodes and insert phi nodes there.
//
// [ x1 <- value1 ] [ x2 <- value2 ]
// | |
// '------------+-----------'
// |
// [ x3 <- phi(x1, x2) ]
//
// Collect all nodes that contain a variable assignment (i.e. a new definition).
var variableBlocks = unit.Variables.ToDictionary(
x => x,
x => new HashSet <Node>(x.AssignedBy.Select(a => a.GetParentNode())));
foreach (var variable in unit.Variables.Where(x =>
x.AssignedBy.Count > 1 || // If the variable has more than one definition
GetNodesReferencingVariable(x).Count() > 1) // Or is used in multiple nodes.
)
{
var agenda = new Queue <Node>(variableBlocks[variable]);
while (agenda.Count > 0)
{
var current = agenda.Dequeue();
foreach (var frontierNode in unit.DominatorInfo.GetDominanceFrontier(current))
{
// If the frontier node does not define a phi node already for this variable, we need to add it.
if (result[frontierNode].All(x => x.Variable != variable))
{
// Check if the variable is defined in the frontier node.
bool defined = variableBlocks[variable].Contains(frontierNode);
// Build phi node.
// The number of different versions of the variable is equal to the amount of predecessors.
var phiExpression = new ILPhiExpression(Enumerable
.Repeat(variable, frontierNode.InDegree)
.Select(v => new ILVariableExpression(v)));
var phiNode = new ILAssignmentStatement(variable, phiExpression);
// Insert at top of the current block.
var block = (ILAstBlock)frontierNode.UserData[ILAstBlock.AstBlockProperty];
block.Statements.Insert(0, phiNode);
// Register phi node.
result[frontierNode].Add(phiNode);
// We might have to check this node again if we introduce a new version of this variable
// at this node.
if (!defined)
{
agenda.Enqueue(frontierNode);
}
}
}
}
}
return(result);
}
public virtual bool VisitPhiExpression(ILPhiExpression expression)
{
return(false);
}
public string VisitPhiExpression(ILPhiExpression expression) => "phi";
public void VisitPhiExpression(ILPhiExpression expression)
{
}
