/// <inheritdoc/>
public ValueLiveness Analyze(FlowGraph graph)
{
var liveness = new Dictionary <BasicBlockTag, BlockLiveness>();
// First create liveness data for blocks in isolation.
foreach (var block in graph.BasicBlocks)
{
liveness[block.Tag] = BlockLiveness.Create(block);
}
var preds = graph.GetAnalysisResult <BasicBlockPredecessors>();
// Then propagate imports until a fixpoint is reached.
// The idea is to create a worklist of basic blocks that
// still need to be processed.
var worklist = new Queue <BasicBlockTag>(graph.BasicBlockTags);
var workset = new HashSet <BasicBlockTag>(graph.BasicBlockTags);
while (worklist.Count > 0)
{
// Dequeue a block from the worklist.
var blockTag = worklist.Dequeue();
workset.Remove(blockTag);
var blockData = liveness[blockTag];
// Propagate imports to predecessors.
foreach (var predTag in preds.GetPredecessorsOf(blockTag))
{
// Every predecessor must export each and every one of the
// block's imports.
var predData = liveness[predTag];
foreach (var import in blockData.Imports)
{
// Have the predecessor export the imported value.
if (predData.Export(import))
{
// The predecessor block doesn't define the imported value
// and hence also had to import the imported value. Add the
// predecessor block to the worklist.
if (workset.Add(predTag))
{
worklist.Enqueue(predTag);
}
}
}
}
}
return(new ValueLiveness(liveness));
}
/// <summary>
/// Creates block liveness data for a particular block.
/// Live and dead positions are recorded for all definitions
/// and uses in the block.
/// </summary>
/// <param name="block">The block to analyze.</param>
/// <returns>
/// Block liveness data for a single block.
/// </returns>
internal static BlockLiveness Create(BasicBlock block)
{
var result = new BlockLiveness(block);
// Mark parameters as defined here.
foreach (var parameter in block.Parameters)
{
result.livePositions.Add(parameter.Tag, ParameterIndex);
result.deadPositions.Add(parameter.Tag, ParameterIndex);
}
// Mark instructions as defined here, use their arguments.
foreach (var selection in block.NamedInstructions)
{
result.livePositions.Add(selection.Tag, selection.InstructionIndex);
result.deadPositions.Add(selection.Tag, selection.InstructionIndex);
foreach (var argTag in selection.Instruction.Arguments)
{
result.UseAt(argTag, selection.InstructionIndex);
}
}
int insnCount = block.InstructionTags.Count;
foreach (var flowInsn in block.Flow.Instructions)
{
foreach (var argTag in flowInsn.Arguments)
{
result.UseAt(argTag, insnCount);
}
insnCount++;
}
foreach (var branch in block.Flow.Branches)
{
foreach (var arg in branch.Arguments)
{
if (arg.IsValue)
{
result.Export(arg.ValueOrNull);
}
}
}
return(result);
}
