internal static bool TryGetFusibleTail(
BasicBlockTag head,
FlowGraph graph,
BasicBlockPredecessors predecessors,
out BasicBlockTag tail)
{
var block = graph.GetBasicBlock(head);
var jumpFlow = block.Flow as JumpFlow;
if (jumpFlow != null && graph.EntryPointTag != jumpFlow.Branch.Target)
{
var preds = predecessors.GetPredecessorsOf(jumpFlow.Branch.Target).ToArray();
if (preds.Length == 1 && preds[0] == block.Tag)
{
tail = jumpFlow.Branch.Target;
return(true);
}
}
tail = null;
return(false);
}
/// <summary>
/// Runs the SSA construction algorithm.
/// </summary>
public void Run()
{
var entryPoint = graphBuilder.GetBasicBlock(graphBuilder.EntryPointTag);
// Create a set of all entry point parameters.
// Here's why: entry point parameters have a
// very specific meaning in Flame IR. Each parameter
// corresponds to a method body parameter.
// The SSA construction algorithm will sometimes
// append block parameters to the entry point block,
// which is not something we want.
//
// By keeping track of the original entry point
// parameters, we can tell parameters inserted
// by the algorithm from preexisting parameters.
var oldEntryPointParams = entryPoint.Parameters;
// Fill the entry point first. This will also
// fill all blocks reachable from the entry
// point.
FillBlock(entryPoint);
// Fill all the garbage blocks.
foreach (var block in graphBuilder.BasicBlocks)
{
FillBlock(block);
}
// Delete the allocas we replaced with copies.
foreach (var tag in eligibleAllocas)
{
graphBuilder.RemoveInstruction(tag);
}
// Somehow eliminate additional entry point parameters.
// We will use one of two techniques, depending on the
// situation:
//
// 1. If the entry point does not have any predecessors,
// then we will simply replace all newly created
// entry point parameters with `default` instructions.
//
// 2. If the entry point has one or more predecessors,
// then we'll create a new entry point block with
// a parameter list equivalent to the original entry
// point parameters and have that block jump to the
// original entry point.
//
var epPreds = predecessors.GetPredecessorsOf(entryPoint.Tag).ToArray();
if (epPreds.Length == 0)
{
var oldParamSet = new HashSet <ValueTag>(oldEntryPointParams.Select(p => p.Tag));
var newParams = entryPoint.Parameters;
// Restore entry point parameters.
entryPoint.Parameters = oldEntryPointParams;
// Define extra parameters as `default` constant instructions.
foreach (var parameter in newParams.Reverse())
{
if (!oldParamSet.Contains(parameter.Tag))
{
entryPoint.InsertInstruction(
0,
Instruction.CreateDefaultConstant(parameter.Type),
parameter.Tag);
}
}
}
else
{
// Create a new entry point.
var newEntryPoint = graphBuilder.AddBasicBlock(
entryPoint.Tag.Name + ".thunk");
graphBuilder.EntryPointTag = newEntryPoint.Tag;
var oldToNew = new Dictionary <ValueTag, ValueTag>();
foreach (var oldParam in oldEntryPointParams)
{
var oldTag = oldParam.Tag;
var newTag = new ValueTag(oldTag.Name);
newEntryPoint.AppendParameter(
new BlockParameter(oldParam.Type, newTag));
oldToNew[oldTag] = newTag;
}
var branchArgs = new List <ValueTag>();
foreach (var parameter in entryPoint.Parameters)
{
ValueTag newTag;
if (oldToNew.TryGetValue(parameter.Tag, out newTag))
{
branchArgs.Add(newTag);
}
else
{
branchArgs.Add(
newEntryPoint.AppendInstruction(
Instruction.CreateDefaultConstant(parameter.Type)));
}
}
newEntryPoint.Flow = new JumpFlow(entryPoint.Tag, branchArgs);
}
}