private static LiveVariables ComputeLiveness(ControlFlowGraph function, Edges edges)
{
var blocks = new Queue <BasicBlock>();
blocks.EnqueueRange(function.ExitBlocks);
var liveVariables = new LiveVariables(function);
var numberOfVariables = function.VariableDeclarations.Count;
while (blocks.TryDequeue(out var block))
{
var liveBeforeBlock = new BitArray(liveVariables.Before(block.Statements.First()));
var liveAfterBlock = new BitArray(numberOfVariables);
foreach (var successor in edges.From(block))
{
liveAfterBlock.Or(liveVariables.Before(successor.Statements.Last()));
}
var liveAfterStatement = liveAfterBlock;
foreach (var statement in block.Statements.Reverse())
{
var liveSet = liveVariables.Before(statement);
liveSet.Or(liveAfterStatement);
switch (statement)
{
case AssignmentStatement assignment:
KillVariables(liveSet, assignment.Place);
EnlivenVariables(liveSet, assignment.Value);
break;
case ActionStatement action:
EnlivenVariables(liveSet, action.Value);
break;
case DeleteStatement _:
throw new InvalidOperationException("Delete statements are supposed to be added after liveness analysis");
//liveSet[deleteStatement.Place.CoreVariable()] = true;
//break;
case IfStatement _:
case GotoStatement _:
// We already or'ed together the live variables from successor blocks
break;
case ReturnStatement _:
// No effect on variables?
// TODO should we check the liveSet is empty?
break;
default:
throw NonExhaustiveMatchException.For(statement);
}
// For the next statement
liveAfterStatement = liveSet;
}
if (!liveBeforeBlock.ValuesEqual(liveVariables.Before(block.Statements.First())))
{
foreach (var basicBlock in edges.To(block)
.Where(fromBlock => !blocks.Contains(fromBlock)).ToList())
{
blocks.Enqueue(basicBlock);
}
}
}
return(liveVariables);
}
