/// <summary>
/// The last Rule is often a 'fallback' rule with an empty condition. If this is the case,
/// we can sometimes perform an optimisation that reduces the overall code size.
/// </summary>
/// <returns>Root of the generated tree.</returns>
private Node?BuildFallbackSequence()
{
// sanity check
if (ruleSet.NumRules <= 1)
{
return(null);
}
// check that there is a fallback rule
RuleSetEntry lastRule = ruleSet[ruleSet.NumRules - 1];
if (!lastRule.EffectiveCondition.IsEmpty)
{
return(null);
}
// check that sequence nodes are permitted
if (!spec.Config.AllowSequence)
{
return(null);
}
if (spec.Config.Verbose)
{
Console.WriteLine("Performing fallback sequence optimisation.");
}
// remove the fallback rule and build the tree
RuleSet rSet = ruleSet.DeriveExcludingLast();
var builder = new TreeBuilder(spec, rSet, this);
Node node = builder.Build();
// ensure that there is a sequence node including the root
// of the tree
SequenceNode seq;
if (node is SequenceNode)
{
seq = (SequenceNode)node;
}
else
{
seq = new SequenceNode(spec);
seq.Append(node);
}
// now paste the fallback rule onto the end of the sequence
seq.Append(lastRule.Rule);
return(seq);
}
/// <summary>
/// The sole publicly-visible entry point. Generates a tree, given a Specification.
/// </summary>
/// <param name="spec">The decoder specification.</param>
/// <returns>The root of the decoder tree.</returns>
public static Node BuildTree(Specification spec, TristateBitArray?flags)
{
if (flags == null)
{
flags = new TristateBitArray(spec.NumFlags);
}
// initialise a TreeBuilder with an empty condition and a RuleSet
// including all rules.
var initialCondition = new Condition(spec, new TristateBitArray(spec.NumBits), flags);
RuleSet ruleSet = new RuleSet(spec).Derive(initialCondition);
var builder = new TreeBuilder(spec, ruleSet);
return(builder.Build());
}
/// <summary>
/// Build a decoder subtree for the given switch case.
/// </summary>
/// <param name="node">The enclosing switch node.</param>
/// <param name="value">Value of case (eg., case 0x0 has value 0).</param>
/// <returns>Decoder subtree.</returns>
private Node BuildSwitchCase(SwitchNode node, int value)
{
Debug.Assert(value >= 0 && value < node.Expression.NumValues);
if (spec.Config.Verbose)
{
Console.WriteLine("Switch case {0}", value);
}
// generate a derived RuleSet for this switch case
var cond = new Condition(spec,
node.Expression.GetBitsForValue(value),
new TristateBitArray(spec.NumFlags));
RuleSet derivedSet = ruleSet.Derive(cond);
// generate a corresponding TreeBuilder and update its totals for
// switch bits & nesting depth
var builder = new TreeBuilder(spec, derivedSet, this);
builder.totalSwitchBits += node.Expression.NumBits;
builder.switchNestingDepth++;
// use it to build tree for case label
Node child = builder.Build();
// search previously-generated switch cases for functionally identical subtrees
for (int previousValue = 0; previousValue < value; previousValue++)
{
if (node[previousValue].Equals(child))
{
if (spec.Config.Verbose)
{
Console.WriteLine("Pruning child {0} => {1}", value, previousValue);
}
// replace generated subtree with a reference to the (identical) subtree
child = new ChildReferenceNode(spec, previousValue);
break;
}
}
return(child);
}
/// <summary>
/// Try to improve efficiency by 'lifting' a test higher up the decoder tree. This is
/// only done if the test is identical for every member of a rule set.
/// </summary>
private Node?BuildLift(string name, Func <Condition, TristateBitArray> extract, Func <TristateBitArray, Condition> init)
{
if (ruleSet.NumRules < 2)
{
return(null);
}
// extract component from 1st rule & verify that flags are specified
TristateBitArray component = extract(ruleSet[0].EffectiveCondition);
if (component.IsEmpty)
{
return(null);
}
// check that all other rules have the same component spec
for (int i = 1; i < ruleSet.NumRules; i++)
{
if (!extract(ruleSet[i].EffectiveCondition).IsEqual(component))
{
return(null);
}
}
if (spec.Config.Verbose)
{
Console.WriteLine($"Performing {name} lifting optimisation.");
}
// build subtree without flags
Condition cond = init(component);
RuleSet rSet = ruleSet.Derive(cond);
var builder = new TreeBuilder(spec, rSet, this);
Node node = builder.Build();
// and attach it to an if-node
return(new IfElseNode(spec, cond, node, new EmptyNode(spec)));
}
